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The Fe3O4 origin of the “Biphase” reconstruction on α-Fe2O3(0001)
Abstract
The so-called Biphase termination on α-Fe2O3 has been widely accepted to be a structure with a ∼40 Å unit supercell composed of coexisting islands of Fe1−xO and α-Fe2O3. Based on thermodynamic arguments and experimental evidence, including transmission electron diffraction, imaging, magnetic and spectroscopic information, it is found that the previously proposed structure model is inaccurate. The actual Biphase structure is instead a layered structure related to the reduction of α-Fe2O3 to Fe3O4. A model for the Biphase termination is proposed which does not contain islands of Fe1−xO but instead consists of bulk α-Fe2O3 and a Fe3O4-derived overlayer. The proposed model is consistent with all current and previously reported experimental findings.
... Later, DFT calculations with a Hubbard-type on-site Coulomb repulsion (DFT+U) have suggested that Fe-O 3 -Feis almost the only termination appearing in the whole environment [16,17]. Experimental studies of α-Fe 2 O 3 (0001) have also shown various surface termination structures depending on the preparation process [18][19][20][21][22][23][24][25][26][27]. X-ray photoelectron diffraction studies on the epitaxial film grown on a sapphire substrate have shown that the stacking sequence of the surface is Fe-O 3 -Fetermination regardless of the oxygen pressure [18,19]. ...
... FeO(111) phases, so-called biphase reconstruction, has often been reported on α-Fe 2 O 3 (0001) after repeated Ar sputtering followed by annealing in an oxygen-poor environment [20][21][22][23]. ...
... Thus, the mirror symmetry in Fig. 3(b) is caused by the coexistence of symmetrical layers that are energetically equivalent. The possible stacking sequences of the α- According to previous studies on the surface structure of α-Fe 2 O 3 (0001), however, the Fe-bilayer termination is predicted in most theoretical reports to not appear [14][15][16][17] and also is rarely observed in experiments [18][19][20][21][22][23][24][25][26][27]. Instead, ferryl termination [see Fig. 4(a)] has been reported to be a possible termination in both theoretical and experimental studies [15,16,25,26]. ...
We analyze the surface structure of an α-Fe2O3(0001) film grown on a c-plane sapphire substrate by mist chemical vapor deposition (CVD), which has been recently developed as a simple, safe, and cost-effective film growth method. Using coaxial impact-collision ion scattering spectroscopy, we found that the atomic-layer sequence of the surface termination of an α-Fe2O3(0001) film grown by mist CVD was Fe–O3–Fe– from the top layer. This surface termination is predicted to form in an oxygen-poor environment by density functional theory combined with a thermodynamical approach despite that the mist CVD process is performed with atmospheric-pressure air. The surface structure markedly changes after annealing above 600 °C in ultrahigh vacuum. We found that only a couple of layers from the top layer transform into Fe3O4(111) after 650 °C annealing, which would be so-called biphase reconstruction. Complete transformation into a Fe3O4(111) film occurs at 700 °C, whose atomic-layer sequence is determined to be Fe–O4–Fe3– from the top layer.
... Langmuir model is used in this study over the BET model due to the simplicity as well as hematite's surface area and interaction with oxygen as it is reoxidized at low pressure. Langmuir provides a means of determining surface area based on a monolayer coverage of the solid surface by the absorptive [26]. Relative pressure (pressure/initial pressure) range of 0.01-0.995 ...
... This process opens up lath structures between large and growing grains as small grains are consumed. Growth of the lath structure within the grains is slowed as surface phase change and volume expansion on the oxide surface of the grain occur [13,20,26]. Hayes holds a similar explanation in that phase of surface and structure formations change with temperature and bulk gas content [23]. ...
... Additionally, thermal expansion and stresses may also cause a shift in these crystal structures and cut off the forming lath structures within the grain. The material goes through a biphase change of Fe 2 O 3 and Fe 3 O 4 between the temperature of 650 C and 810 C until stabilized at 850 C [26]. This may create a lag in formation of a lath structure into the hematite grain. ...
For chemical looping processes to become an economically viable technology, an inexpensive carrier that can endure repeated reduction and oxidation cycles needs to be identified or developed. Unfortunately, the reduction of hematite ore with methane in both batch and fluidized beds has revealed that the performance (methane conversion) decreases with time. Previous analysis had shown that the grains within the particle grew with the net effect of reducing the surface area of the particles and thereby reducing the rate and net conversion for a fixed reduction time. To improve the lifespan of hematite ore, it is hypothesized that if the grain size could be stabilized, then the conversion could be stabilized. In this work, series of tests were conducted in an electrically heated fluidized bed. The hematite ore was first pretreated at a temperature higher than the subsequent reduction temperatures. After pretreatment, the hematite ore was subjected to a series of cyclic reduction/oxidation experiments. The results show that the ore can be stabilized for cycles at different conditions up to the pretreatment temperature without any degradation. Details of the pretreatment process and the test results will be presented.
... 21 Sub-surface Fe atoms which have low coordination numbers are frequently missing, 22 and phase transformations or coexistence of multiple phases also occurs under appropriate conditions. 19,23,24 Despite these difficulties, various experimental and computational studies have accumulated an extensive body of data of the surface stability, especially for the well-studied surfaces such as hematite (001). However, the data from separate studies have been obtained using different methodologies and computational settings, and have different inherent approximations. ...
... The morphologically important surfaces of hematite are (001), (012), (100), (110), (104), (018), and (113), 1,67 among which (001) was the most extensively studied in the literature. While various terminations of (001) have been studied both experimentally 14,18,19,23,24,[68][69][70][71][72][73][74][75][76][77] and computationally, 13,78-84 studies of other hematite surfaces are relatively rare, e.g. (012), 83,85 and a few low index surfaces. ...
... Considerable experimental and theoretical studies showed that both the Ferich and O-rich terminations are present on hematite (001) surfaces, under ambient or vacuum conditions, coexisting with the stoichiometric variation. 14,19,23,24,78,81 However, under atmospheric conditions the O-rich surface is expected to be passivated by hydroxyl groups. 67,75,77,80 Our calculations showed the hydroxyl termination lowered the surface energy to 0.782 J m À2 , although it has been suggested that inhomogeneous coverages of oxygen or hydroxyl layer can lower the surface energy even further. ...
Iron oxide and oxyhydroxidenanoparticles are among the most important mobile and catalytic agents in a variety of biogeochemical environments, and are being increasingly synthesized for energy, electronic, catalyst, environmental and medical applications. The morphologies at nanoscale are relevant to the control of shapes and sizes, surface chemistry, and performance of these nanoparticles, as well as our understanding of naturally occurring processes. Therefore, we have begun to develop this understanding by studying the relationship between size, shape, and thermodynamic stability of unpassivated hematite (α-Fe2O3) and goethite (α-FeOOH) nanoparticles, using a robust thermodynamic morphology model with input parameters from reliable first-principles calculations and thermochemical data. The results revealed the thermodynamic stable shapes of hematite and goethitenanoparticles, and demonstrated that the phase transformation from goethite to hematite is highly dependent on the particle size and temperature. Goethitenanoparticles are thermodynamically stable with small sizes, compared to hematite, but the equilibrium transformation temperature increases rapidly with decreasing particle size. The morphology sensitive phase transformation predicted by our model is a step further towards a nanophase diagram of iron oxides and oxyhydroxides.
... As the present study focuses on thermodynamics of the surfaces, we will not go in details about the oxidation states of individual surfaces, but direct Readers to Refs. [30,82,53,26]. ...
... The structures and stability of nonhydroxylated surfaces have been investigated by a vast number of studies in the past. The stoichiometric clean termination has been consistently proposed to be thermodynamically stable with low-pressure background oxygen [4] and may co-exist with magnetite (1 1 1) or wustite (1 1 1) in minute concentrations of oxygen [26,30,40,[82][83][84]. Hematite (0 0 1) surface is coherent with magnetite (1 1 1) and wustite (1 1 1) surfaces. ...
... The predicted hydroxylation configurations in aqueous environments agree well with previous studies [85,86,19,18]. Whereas in anhydrous environments, it appears that the (0 0 1) surface displays several types of terminations depending on the partial pressure of oxygen and temperature [28,38], including the stoichiometric Fe-termination [37,36] and in coexistence with Otermination [39,30,26,31,29,34,32], and exclusively O-terminations [28]. Because of the low P H 2 O to hydroxylate the hematite (0 0 1) surface [16,41], the surface hydroxyl groups may form spontaneously during sample transfer or characterization. ...
... This surface termination has a very unfavourable free energy, which decreases fast with increasing m O . The O-terminated surface can become more stable, however, upon creation of oxygen vacancies (see Fig. 2 37 It was proposed that the effect of the metal substrate, often used for preparation of epitaxial thin films, cannot be ignored even in the case of rather thick (10-50 nm) iron oxide layers. 37,38 When growing a-Fe 2 O 3 as films on Pt(111), the coexistence of two different domains (a single-metal termination and a ferryl termination) was observed at intermediate pressures, while higher and lower pressures led to one or the other of these domains becoming dominant. ...
... The O-terminated surface can become more stable, however, upon creation of oxygen vacancies (see Fig. 2 37 It was proposed that the effect of the metal substrate, often used for preparation of epitaxial thin films, cannot be ignored even in the case of rather thick (10-50 nm) iron oxide layers. 37,38 When growing a-Fe 2 O 3 as films on Pt(111), the coexistence of two different domains (a single-metal termination and a ferryl termination) was observed at intermediate pressures, while higher and lower pressures led to one or the other of these domains becoming dominant. 39 Contrary to the observations made when growing a-Fe 2 O 3 films on metal surfaces, the clean a-Fe 2 O 3 (0001) surface grown epitaxially (E35 nm thick) on a-Al 2 O 3 (0001) is single-Fe-terminated and, in this case, the surface structure of a-Fe 2 O 3 (0001) is similar to that of a-Al 2 O 3 (0001) (1 Â 1). ...
The stability and reactivity of the hematite, Fe2O3(0001) surface are studied by density functional theory including an on-site Coulomb term (DFT+U). Even under oxygen rich conditions, the metal-terminated surface is shown to be stable. On this surface termination, the isolated water molecule forms a heterolytically dissociated structure with the OH− group attached to a surface Fe3+ ion and the proton to a surface O2− ion. Dissociative adsorption is strongly enhanced at oxygen vacancy sites. Here, the OH− group fills the oxygen vacancy site. Dehydrogenation accompanied by defect healing is favoured compared to water desorption (178 kJ mol−1 compared to 236 kJ mol−1). The water adsorption energies (at 0 K) for the clean and defective surfaces are 100 kJ mol−1 and 288 kJ mol−1, respectively.
... 15 Therefore, thermodynamics and chemical reactions on the substrate are the key for the growth of Fe3O4 films, as also indicated by many studies on the transitions between iron oxide phases using surface characterizations. [20][21][22][23][24][25][26][27][28][29][30] The 3 surface structures, including termination, reconstruction, and morphology, have been the focus of study. [20][21][22][23]27 On the other hand, the kinetics of the transitions (time scale as a function of conditions) have seldom been systematically carried out, although the important energetic information can be extracted from the kinetics and the time scale itself is a critical factor both for studying or applying these transitions. ...
... The boundary between the α-Fe2O3 and the Fe3O4 phases is found by setting = 0 and solving the relation between and . As shown in Fig. 4(c), the solid line is the calculated phase boundary of the Fe2O3 and Fe3O4, consistent with the phase boundary calculated previously 29,38 . ...
We have studied the growth of Fe3O4 (111) epitaxial films on Al2O3 (001) substrates using a pulsed laser deposition / thermal reduction cycle using an {\alpha}-Fe2O3 target. While direct deposition onto the Al2O3 (001) substrates results in an {\alpha}-Fe2O3 epilayer, deposition on the Fe3O4 (111) surface results in a {\gamma}-Fe2O3 epilayer. The kinetics of the transitions between Fe2O3 and Fe3O4 were studied by measuring the time constants of the transitions. The transition from {\alpha}-Fe2O3 to Fe3O4 via thermal reduction turns out to be very slow, due to the high activation energy. Despite the significant grain boundaries due to the mismatch between the unit cells of the film and the substrate, the Fe3O4 (111) films grown from deposition/thermal reduction show high crystallinity.
... Films with mixed Fe 3 O 4 (111) and α-Fe 2 O 3 (0001) phases were prepared following the procedure recently described in ref. [13]. Under our preparation conditions, the α-Fe 2 O 3 (0001) surface is always biphase terminated [41][42][43]. A uniform γ-Fe 2 O 3 (111) phase was obtained by low temperature oxidation (T b 620 K, p O2 = 3 10 −5 mbar for 10 min) of the Fe 3 O 4 film with subsequent cooling down, during which the oxygen pressure was not reduced before reaching 450 K [13,40]. ...
... The two spinel oxides, in the (111) orientation, exhibit an approximate (2 × 2) superstructure with respect to the Pt(111) and Ag(111) surface structure. In contrast, the α-Fe 2 O 3 (0001) surface is characterized by an approximate (√3 × √3)R30°superstructure [13,29] with extra satellite spots due to the biphase [41][42][43]. These differences can be utilized to separate the phases in dark field LEEM imaging ( Fig. 2b-c), visualizing selectively the sample regions contributing to individual LEED spots. ...
... Magnetite is present in . Experimental work has demonstrated that hematite reduces to magnetite when heated (Lanier, et al., 2009). Hematite signals can come from the minerals the rocks are made of, the burial environment, or the hafting adhesive. ...
Evidence of different compound resin-based adhesives is present in South Africa from at least 77000 years ago. Ancient glue production is considered one of the oldest known highly complex technologies, requiring advanced technological and mental abilities. However, our current knowledge of adhesive materials, recipes, and uses in South Africa is limited by the lack of in-depth analysis and molecular characterization of residues. To deepen our knowledge of past adhesive technology, we performed a detailed multi-analytical analysis (use-wear, XRD, μ-CT, IR spectroscopy, GC-MS) of 30 Later Stone Age tools with adhesive remains from Steenbokfontein Cave, South Africa. At the site, tools made of various rocks were hafted with compound adhesives, and we identified three recipes: 1) resin/tar of Widdringtonia or Podocarpus species combined with hematite; 2) resin/tar of Widdringtonia or Podocarpus species mixed with hematite and another plant exudate; 3) resin/tar without hematite. The studied scrapers were used in hide-working activities, and the studied cutting tools were used to work animal and soft plant matters. All scrapers display evidence of intense resharpening and were discarded when no longer useable. The combination of different methods for residue analysis reveals the flexibility of adhesive technology at Steenbokfontein. Despite the consistent use of conifer resin/tar throughout the sequence, we observed that other ingredients were added or excluded independently of the tools' raw materials and functions. Our results highlight the long-lasting tradition of using adhesive material from conifer species but also the adaptability and flexibility of adhesive traditions. The systematic application of this multi-analytical approach to Pleistocene adhesives will be useful to better characterise adhesive traditions and enhance the debate on the technological, cognitive, and behavioural implications of this technology.
... Using scanning tunneling microscopy (STM), Condon et al. observed surface reconstruction of a reduced Fe3O4 and revealed biphase ordering of the surface consisting of superlattices of Fe3O4 and Fe1-xO islands. 31,32 Later, Lanier et al. 33 conversion in UHV which was observed only for the films grown on Pt substrate, is driven by the diffusion of Fe from the surface into the bulk of the film indicating the important role of the substrate on the formation of the bi-phase. These studies raise an interesting, fundamental question regarding the effect of phase coexistence (FM Fe3O4 and AFM -Fe2O3 phases) on the thermally driven spin transport in such iron oxide systems, which was not examined in the previous studies [28][29][30]35 . ...
Understanding impacts of phase transition, phase coexistence, and surface magnetism on the longitudinal spin Seebeck effect (LSSE) in a magnetic system is essential to manipulate the spin to charge current conversion efficiency for spincaloritronic applications. We aim to elucidate these effects by performing a comprehensive study of the temperature dependence of LSSE in biphase iron oxide (BPIO = alpha-Fe2O3 + Fe3O4) thin films grown on Si (100) and Al2O3 (111) substrates. A combination of temperature-dependent anomalous Nernst effect (ANE) and electrical resistivity measurements show that the contribution of ANE from the BPIO layer is negligible compared to the intrinsic LSSE in the Si/BPIO/Pt heterostructure even at room temperature. Below the Verwey transition of the Fe3O4 phase, the total signal across BPIO/Pt is dominated by the LSSE. Noticeable changes in the intrinsic LSSE signal for both Si/BPIO/Pt and Al2O3/BPIO/Pt heterostructures around the Verwey transition of the Fe3O4 phase and the antiferromagnetic (AFM) Morin transition of the alpha-Fe2O3 phase are observed. The LSSE signal for Si/BPIO/Pt is found to be almost two times greater than that for Al2O3/BPIO/Pt, an opposite trend is observed for the saturation magnetization though. Magnetic force microscopy reveals the higher density of surface magnetic moments of the Si/BPIO film compared to the Al2O3/BPIO film, which underscores a dominant role of interfacial magnetism on the LSSE signal and thereby explains the larger LSSE for Si/BPIO/Pt.
... These findings are in stark contrast with reports of formation of NPs under higher O 2 pressure which are usually core-shell J Mater Sci [19,20,22] and have a significant number of structural defects including antidomain phase boundaries that originate at the core-shell interface, and are responsible for complex magnetic behaviors of the nanoparticles. It is interesting also to note that according to the calculated structural phase stability for iron oxides with respect to temperature and O 2 pressure [37] for bulk systems, at both 600°C and 700°C and at O 2 pressure of 10 -4 Pa, only the a-Fe 2 O 3 phase is energetically favorable, while the Fe 3 O 4 phase is expected above 800°C. These calculations do not correspond to NPs; our observations clearly show that, at the nanoscale, the transformation into Fe 3 O 4 happens at lower temperature (700°C), and it is preceded not from a-Fe 2 O 3 NPs, as discussed above. ...
In this work, we present an in situ transmission electron microscopy (TEM) study of Fe thin films to Fe nanoparticle formation and their oxidation to single-crystal magnetite nanoparticles. Amorphous Fe thin films were prepared by sputtering on TEM carbon grids. The thin Fe films were continuously heated in situ from room temperature to 700 °C under vacuum (4 × 10–4 Pa). With the increase in temperature, the continuity of the thin film starts breaking, and Fe nanoparticle nucleation centers are formed. At 600 °C, the thin film transforms into metallic Fe nanoparticles (NPs) with a small presence of different Fe oxide NPs. Further increase in the temperature to 700 °C resulted in the full oxidation of the NPs (i.e., no core–shell were found). Zero-loss energy filtered diffraction and HRTEM analysis of the lattice spacing reveals that all NPs have fully transformed into single-phase magnetite NPs. The structural study of the magnetite NPs shows that magnetite NPs are free of antiphase domain boundary defects. This work demonstrates that under low partial pressure of oxygen at elevated temperatures a complete oxidation of Fe NPs into magnetite single-crystal nanoparticles can be achieved.
... Additionally, the (1 1 1) surface can exhibit several types of hexagonal reconstruction with different atomic structures and periodicities in a 5 nm range, termed a biphase [12,13]. The explanation of the biphase superstructures that were observed on magnetite [13][14][15][16], as well as on hematite [17][18][19][20], involves a surface atomic layer with modified stoichiometry, and detailed studies of the biphase nature as a function of the surface composition were reported recently for magnetite [16]. ...
We present studies on CO adsorption on an Fe 3 O 4 (1 1 1) surface with a regular tetrahedral termination (Fe tet1) and with different hexagonal superstructures called 'biphase' structures. All these terminations were precisely controlled on a 70 Å magnetite layer epitaxially grown on a Pt(1 1 1) single crystal. The experiments, including low energy electron diffraction, scanning tunnelling microscopy (STM) and temperature-programmed deso-rption (TPD), were carried out in situ under ultrahigh vacuum conditions. During the TPD measurements, the Fe 3 O 4 (1 1 1) surfaces were exposed to 0.1-20 L of CO at 105 K, and then the samples were heated with a linear temperature rise to 600 K. We found that as the Fe tet1-terminated area decreases, as confirmed by STM observations , the surface becomes less active in CO adsorption. This means there are fewer adsorption states on the biphase structure than on Fe 3 O 4 (1 1 1) with regular termination.
... Thereafter the name "biphase" has been coined for this particular surface termination and for a similar one found on the natural and Pt-supported Fe3O4(111) [19][20][21] . Whereas several works use the biphase concept to explain microscopy and spectroscopy data 15,[22][23][24][25][26][27] , other research suggests the existence of a thin Fe3O4(111) layer on the bulk 28 , or states that the surface is oxygen-terminated 29 . Moreover, despite a large number of density functional theory (DFT) calculations, currently there is no atomic model that comprehensively explains the various experimental observations 23,30-33 . ...
Iron oxides are among the most abundant compounds on Earth and have been exploited and researched extensively. Knowing the atomic structure of their surfaces is essential for the understanding and control of their catalytic properties, electronic character and spin arrangement. By a combination of atomically resolved microscopy, electron diffraction and surface-sensitive spectroscopies, we examine the oxygen-rich superstructure grown on ɑ-Fe2O3(0001) hematite surface and reveal a continuous two-dimensional layer of iron dioxide, structurally analogous to transition metal dichalcogenides. Using total-energy density functional simulation to optimize an atomic model of the superstructure, we identify it as antiferromagnetic and conductive 1T-FeO2 attached on half-metal terminated bulk. These results open the way to the identification of epitaxial 2D layers on other similar metal-oxide surfaces and to a better understanding of their catalytic activity.
... Thereafter the name "biphase" has been coined for this particular surface reconstruction 19,20 . Whereas several works use the biphase concept to explain microscopy and spectroscopy data 15,[21][22][23][24][25][26] , other research somewhat counterintuitively suggests the existence of a thin Fe3O4(111) layer on the bulk, 27 or states that the surface is oxygen-terminated 28 . Moreover, despite a large number of density functional theory (DFT) calculations, currently there is no atomic model that comprehensively explains the various experimental observations 22,[29][30][31][32][33] . ...
Iron oxides are among the most abundant compounds on Earth and have consequently been studied and used extensively in industrial processes. Despite these efforts, concrete understanding of some of their surface phase structures has remained elusive, in particular the oxidized ${\alpha}-Fe_2O_3(0001)$ hematite surface. We detail an optimized recipe to produce this phase over the entire hematite surface and study the geometrical parameters and composition of its complex structure by means of atomically resolved microscopy, electron diffraction and surface-sensitive spectroscopies. We conclude that the oxidized ${\alpha}-Fe_2O_3(0001)$ surface is terminated by a two-dimensional iron oxide with structure, lattice parameters, and orientation different from the bulk substrate. Using total-energy density functional theory for simulation of a large-scale atomic model, we identify the structure of the surface layer as antiferromagnetic, conductive $1T-FeO_2$ attached on half-metal terminated bulk. The model succeeds in reproducing the characteristic modulations observed in the atomically resolved images and electron diffraction patterns.
... Hematite is the thermodynamically most stable iron oxide, and its interaction with water is promising for photocatalytic (cheap) solar H 2 production. [24][25][26][27][28][29][30][31] Several experimental [32][33][34][35][36][37][38] and theoretical [39][40][41][42] studies have been reported for single crystal surfaces. There are likely to exist six possible surface terminations of hematite 39 which can be classified into two categories, oxygen and iron terminations. ...
We report on electronic structure measurements of the interface between hematite nanoparticles (6 nm diameter) and aqueous solutions. Using soft X-ray photoelectron spectroscopy from a liquid microjet we detect valence and core-level photoelectrons as well as Auger-electrons from liquid water, from the nanoparticle – water interface, and from the interior of the aqueous-phase nanoparticles. Most noteworthy, the method is shown to be sufficiently sensitive for the detection of adsorbed hydroxyl species, resulting from H2O dissociation at the nanoparticle surface in aqueous solution. We obtain signal from surface OH from resonant, non-resonant, and from so-called partial-electron-yield X-ray absorption (PEY-XA) spectra. In addition, we report resonant photoelectron measurements at the iron 2p excitation. The respective Fe iron 2p3/2 edge (L3-edge) PEY-XA spectra exhibit two main absorption peaks with their energies being sensitive to the chemical environment of the Fe³⁺ ions at the nanoparticle–solution interface. This manifests in the 10Dq value which is a measure of the ligand-field strength. Furthermore, an observed intensity variation of the pre-peak, when comparing the PEY-XA spectra for different iron Auger-decay channels, can be assigned to different extents of electron delocalization. From the experimental fraction of local versus non-local autoionization signals we then find a very fast, approximately 1 fs, charge transfer time from interfacial Fe³⁺ into the environment. The present study, which is complementary to ambient-pressure photoemission studies on solid – electrolyte systems, also highlights the multiple aspects of photoemission that need to be explored for a full characterization of the transition-metal-oxide nanoparticle surface in aqueous phase.
... The XRD diffractogram shows a R3cH polycrystalline BiFeO 3 pure phase, indexed with JCD188396 card, without any traces of the common spurious phases Bi 25 FeO 39 , Bi 2 Fe 4 O 9 and iron oxides [14,15]. No significant amorphous phase signature can be detected in the diffractogram as expected considering the Fe-O and Bi 2 O 3 -Fe 2 O 3 phase diagrams [13,16,17] The estimated particle size was calculated from Debye-Scherrer equation, considering the peaks overlap and the instrumental contributions, giving a mean size of $30 ± 8 nm (Table 1). ...
We have investigated the structural and magnetic properties of BiFeO3 (BFO) thin films grown over (1 0 0)-oriented Si substrates by rf magnetron sputtering in a new route under O2 free low pressure Ar atmosphere. Single-phase BFO films were deposited in a heated substrate and post-annealed in situ. The new routed allows high deposition rate and produce polycrystalline BFO pure phase, confirmed by high resolution X-ray diffraction. Scanning electron and atomic force microscopy reveal very low surface roughness and mean particle size of 33 nm. The BFO phase and composition were confirmed by transmission electron microscopy and line scanning energy-dispersive X-ray spectroscopy in transmission electron microscopy mode. The surface chemistry of the thin film, analyzed by X-ray photoelectron spectroscopy, reveals the presence of Fe³⁺ and Fe²⁺ in a 2:1 ratio, a strong indication that the film contains oxygen vacancies. An hysteretic ferromagnetic behavior with room temperature high saturation magnetization ∼165 × 10³ A/m was measured along the film perpendicular and parallel directions. Such high magnetization, deriving from this new route, is explained in the scope of oxygen vacancies, the break of the antiferromagnetic cycloidal order and the increase of spin canting by change in the surface/volume ratio. Understanding the magnetic behavior of a multiferroic thin films is a key for the development of heterogeneous layered structures and multilayered devices and the production of multiferroic materials over Si substrates opens new possibilities in the development of materials that can be directly integrated into the existent semiconductor and spintronic technologies.
... 1, 15-16 Unfortunately, it has proven extremely difficult to prepare and measure a stoichiometric α-Fe2O3(0001) surface under UHV conditions 16 and the termination is still debated, 1 as are those of several reduced phases that have also been reported. [16][17][18][19][20][21][22][23] The non-polar α-Fe2O3(11 ̅ 02) surface (see Fig. 1) has attracted considerably less attention, despite the fact that it is prevalent on nano-hematite, 1, 24 and it is reported that a (1×1) surface can be easily prepared by annealing a single-crystal sample in ≈10 -6 mbar O2. 17, 25-29 A reduced (2×1) termination is formed upon annealing in UHV, and it is possible to cycle reproducibly backward and forward between the two terminations. To date, adsorption studies have primarily focused on water, and both terminations of α-Fe2O3(11 ̅ 02) have been shown to be active for dissociative adsorption. ...
The α-Fe2O3(1 ̅102) surface (also known as the hematite r-cut or (012) surface) was studied using low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunnelling microscopy (STM), non-contact atomic force microscopy (nc-AFM), and ab initio Density Functional Theory (DFT)+U calculations. Two surface structures are stable under ultra-high vacuum (UHV) conditions; a stoichiometric (1×1) surface can be prepared by annealing at 450 °C in ≈10⁻⁶ mbar O2, and a reduced (2×1) reconstruction is formed by UHV annealing at 540 °C. The (1×1) surface is close to an ideal bulk termination, and the undercoordinated surface Fe atoms reduce the surface bandgap by ≈0.2 eV with respect to the bulk. The work function is measured to be 5.7 ± 0.2 eV, and the VBM is located 1.5 ± 0.1 eV below EF. The images obtained from the (2×1) reconstruction cannot be reconciled with previously proposed models, and a new “alternating trench” structure is proposed based on an ordered removal of lattice oxygen atoms. DFT+U calculations show that this surface is favoured in reducing conditions, and that fourfold-coordinated Fe²⁺ cations at the surface introduce gap states approximately 1 eV below EF. The work function on the (2×1) termination is 5.4 ± 0.2 eV.
... This long-range order produces a "floreted" or "rosette" superstructure low-energy electron diffraction (LEED) pattern that had been reported previously 25−27 but was interpreted by double scattering between an Fe 3 O 4 (111) overlayer and the α-Fe 2 O 3 (0001) substrate, an interpretation also adopted in a later study. 28 In more recent low-energy electron microscopy (LEEM) studies of surface reactions on the α-Fe 2 O 3 (0001) surface 29 and of the α-Fe 2 O 3 ↔ Fe 3 O 4 surface phase transition, 30,31 this LEED pattern has become the signature of the α-Fe 2 O 3 (0001) surface. Long-range superstructures occur also on the Fe 3 O 4 (111) surface. ...
Polar ionic surfaces with bulk termination are inherently unstable due to their diverging electrostatic surface energy. Nevertheless they are frequently observed in nature, mainly due to charge neutralization by adsorbates, but occur also under atomically clean conditions. Several mechanisms have been invoked to explain the stability of atomically clean polar surfaces but the frequently observed periodic nanoscale pattern formation has not been explained yet. Here we propose that long-range interactions between alternating electropositive and electronegative regions of different surface terminations minimize the electrostatic energy of the surface and thus stabilize the nanoscale pattern. This is illustrated using the example of polar Fe oxide surfaces by combining scanning tunneling microscopy and spectroscopy with results from density functional theory based calculations and dipole-dipole interaction models
... Both patterns match well with the rhombohedral α-Fe 2 O 3 structure and show no characteristic peaks for any other phases or impurities, indicating the high purity of the products. From the phase diagram calculated for the iron-oxygen system [23], the reduced product of α-Fe 2 O 3 under the conditions in the present study remains in the hematite phase, which is consistent with our XRD results. ...
Bicrystalline α-Fe2O3 nanoblades (NBs) synthesized by thermal oxidation of iron foils were reduced in vacuum, to study the effect of reduction treatment on microstructural changes and photocatalytic properties. After the vacuum reduction, most bicrystalline α-Fe2O3 NBs transform into single-layered NBs, which contain more defects such as oxygen vacancies, perfect dislocations and dense pores. By comparing the photodegradation capability of non-reduced and reduced α-Fe2O3 NBs over model dye rhodamine B (RhB) in the presence of hydrogen peroxide, we find that vacuum-reduction induced microstructural defects can significantly enhance the photocatalytic efficiency. Even after 10 cycles, the reduced α-Fe2O3 NBs still show a very high photocatalytic activity. Our results demonstrate that defect engineering is a powerful tool to enhance the photocatalytic performance of nanomaterials.
... In 2009, Lanier et al. [337] challenged the Fe 1 À x O(111)/α-Fe 2 O 3 (0001) interpretation of the bi-phase surface, and instead suggested that the reconstruction is related to a surface transformed to a Fe 3 O 4 (111)-like structure (Fig. 55). Essentially, the authors discount the possibility of the Fe 1 À x O(111)/ α-Fe 2 O 3 (0001) bi-phase model [326] on the basis of TEM diffraction measurements, suggesting that a complex diffraction pattern would be observed for such a structure. ...
The current status of knowledge regarding the surfaces of the iron oxides, magnetite (Fe3O4), maghemite (γ-Fe2O3), haematite (α-Fe2O3), and wüstite (Fe1−xO) is reviewed. The paper starts with a summary of applications where iron oxide surfaces play a major role, including corrosion, catalysis, spintronics, magnetic nanoparticles (MNPs), biomedicine, photoelectrochemical water splitting and groundwater remediation. The bulk structure and properties are then briefly presented; each compound is based on a close-packed anion lattice, with a different distribution and oxidation state of the Fe cations in interstitial sites. The bulk defect chemistry is dominated by cation vacancies and interstitials (not oxygen vacancies) and this provides the context to understand iron oxide surfaces, which represent the front line in reduction and oxidation processes. Fe diffuses in and out from the bulk in response to the O2 chemical potential, forming sometimes complex intermediate phases at the surface. For example, α-Fe2O3 adopts Fe3O4-like surfaces in reducing conditions, and Fe3O4 adopts Fe1−xO-like structures in further reducing conditions still. It is argued that known bulk defect structures are an excellent starting point in building models for iron oxide surfaces.
... For the hematite (001) surface, the O-rich (surface layer sequence: O 3 -Fe-Fe-R) and Fe-rich (Fe-Fe-O 3 -R) terminations depend on surface energies on the chemical potential of oxygen (Figure 12.3) which indicates both terminations are unstable compared with stoichiometric surface. Considerable experimental and theoretical studies showed that both the Fe-rich and O-rich terminations are present on hematite (001) surfaces under ambient or vacuum conditions, mostly coexisting with the stoichiometric termination [85,93,94,96,98,102]. However, since in ambient conditions the O-rich surface develops hydroxyl groups, which lower the surface energies significantly, the real surface termination may be hydroxyl groups [81,97,101]. ...
In recent years, computational modeling has opened up another potential way to solve the pending questions about the relative stability of different iron oxides and oxyhydroxides. Modeling of iron oxides and oxyhydroxides is challenging, due to the different magnetization states, the small energy differences between different states and solid phases, and the need for more advanced implementations to cope with the strong correlation effects and weak hydrogen bonds and van der Waals forces. Despite the difficulties, density functional theory (DFT) calculations have been successfully applied to some iron oxides and oxyhydroxides in recent years. In this chapter we will explore the relative stability of five iron oxides and oxyhydroxides, including magnetite, hematite, maghemite, goethite, and lepidocrocite, from the macro- to the nanoscale. In this context a general method for modeling the impact of size, shape, temperature, and chemical environment on the morphology and polymorphic stability (known as thermodynamic cartography) will also be described.
... Treated with Ar ion sputtering followed by annealing at~700°C for 30 min in UHV, the α-Fe 2 O 3 (0001) surface shows a hexagonal structure with the periodicity of 6.0 ± 0.5 Å, as shown in Fig. 2. It is the characteristic of the Fe-terminated Fe 3 O 4 (111)-(1 × 1) lattice and has been commonly observed on the surface of single crystalline Fe 3 O 4 (111) [7,[29][30][31][32][33][40][41][42][43]. In the following, we refer to this surface as the "regular" (R) phase. ...
By using scanning tunneling microscopy (STM), we study the structure of the (0001) surface of hematite (α-Fe2O3) pre.pared by ultra-high vacuum treatment. The surface is reduced into a Fe3O4(111) phase by Ar ion sputtering followed by annealing in vacuum, while it is oxidized to a honeycomb superstructure by annealing in O2. High-resolution STM images reveal that the O-terminated FeO(111), Fe- and O-terminated Fe2O3(0001) domains coexist with each other in the superstructure. Unlike the reduction of Fe2O3(0001) whose depth increases with repeated annealing in vacuum, the oxidation of Fe3O4(111) occurs only partially on the top layers by annealing in O2.
... The basal plane surface of hematite (α-Fe 2 O 3 ) finds importance in research aimed at better understanding such phenomena as mineral chemistry [1][2][3][4][5][6][7][8][9][10], catalysis [11][12][13][14][15][16]and photoconversion [17,18]. Substantial effort has been investing in characterizing the surface structure of α-Fe 2 O 3 (0001) [7,8,12,[19][20][21][22][23][24][25][26][27][28][29][30], and in preparing films of this crystal orientation on metal or metal oxide single crystal substrates [11,[13][14][15][16]19,[31][32][33][34][35][36][37][38][39][40][41][42][43][44][45]. In particular, the interfaces between Cr oxides and the α-Fe 2 O 3 (0001) surface have been studied by several groups to better understand anticorrosion films, magnetic materials, photochemistry and mineralogy [32,[46][47][48][49][50][51][52][53][54][55]. ...
The chemistry of Cr(CO)6 on the Fe3O4(111) surface termination of α-Fe2O3(0001) was explored using temperature programmed desorption (TPD), Auger electron spectroscopy (AES), static secondary ion mass spectrometry (SSIMS) and low energy electron diffraction (LEED) both with and without activation from an oxygen plasma source. No thermal decomposition of Cr(CO)6 was detected on the surface in the absence of O2 plasma treatment, with first layer molecules desorbing in TPD at 215 K from a close-packed overlayer. The interaction of first layer Cr(CO)6 with the Fe3O4(111)-termination was weak, desorbing only ∼ 30 K above the leading edge of the multilayer state. Activation of multilayer coverages of Cr(CO)6 with the O2 plasma source at 100 K resulted in complete conversion of the outer Cr(CO)6 layers, presumably to a disordered Cr oxide film, with Cr(CO)6 molecules near the surface left unaffected. Absence of CO or CO2 desorption states suggests that all carbonyl ligands are liberated for each Cr(CO)6 molecule activated by the plasma. AES and SSIMS both show that O2 plasma activation of Cr(CO)6 results in a carbon-free surface (after desorption of unreacted Cr(CO)6). LEED, however, shows that the Cr oxide film was disordered at 600 K and likely O-terminated based on subsequent water TPD. Attempts to order the film at temperatures above 650 K resulted in dissolution of Cr into the α-Fe2O3(0001) crystal based on SSIMS, an observation linked to the Fe3O4(111) termination of the surface and not to the properties of α-Cr2O3/α-Fe2O3 corundum interface. Nevertheless, this study shows that O2 plasma activation of Cr(CO)6 is an effective means of depositing Cr oxide films on surfaces without accompanying carbon contamination.
... Normally, corundum type compounds, such as α-Al 2 O 3 , have octahedral coordination of metal ions in the bulk, 84 due to the slightly distorted surrounding oxygen atoms octahedra. 85 Fe 3+ has a 3d 5 electron configuration and there is no crystal field splitting energy (CFSE) difference between octahedral and tetrahedral coordination, whereas Cr 3+ has a 3d 3 electron configuration with CFSE favoring octahedral coordination. Thus, the splitting of the band located at 6 eV (Figure 3d Table S2 Supporting Information for total and spin difference. ...
Metal oxide based minerals naturally contain transition metal impurities isomorphically substituted into the structure that can alter the structural and electronic properties as well as the reactivity of these metal oxides. Natural α-Al(2)O(3) (corundum) can contain up to 9.17% (w/w) Fe(2)O(3) and 1.81% (w/w) of Cr(2)O(3.) Here we report on changes in the structural and electronic properties of undoped and doped α-Al(2)O(3) (0001) surfaces using periodic density functional theory (DFT) methods with spin unrestricted B3LYP functional and a local atomic basis set. Both structural and electronic properties are altered upon doping. Implications for doping effects on photochemical processes are discussed.As metal oxides are major components of the environment, including atmospheric mineral aerosol, DFT was also used to study the effect of transition metal impurities on gas/surface interactions of a model acidic atmospheric gas molecule, carbon monoxide (CO). The theoretical results indicated that the presence of Fe(3+) and Cr(3+) impurities substituted on the outer layer of natural corundum surfaces reduces the propensity toward CO adsorption relative to the undoped surface. However, CO-surface interactions resemble that of bulk α-Al(2)O(3) when the impurity is substituted below the first surface layer. The presence and location of the mineral dopant was found to significantly alter the structural and electronic properties and gas/surface interactions studied here.
... Some of these studies 83,85 indicated the creation of Fe 3 O 4 (111) at the surface, or even the coexistence of Fe 2 O 3 (0001) and FeO (111) phases, 86 although later studies do appear to have identified preparation conditions that lead to the expected α-Fe 2 O 3 (0001)(1 × 1) terminations and have provided some clarification of the existence of "biphase" surfaces. 89 The first attempt to achieve an experimental quantitative surface structure determination, using quantitative LEED, 90 based on epitaxial films grown on Pt (111), sought to determine the influence of the partial pressure of oxygen on the surface termination. As shown in Figure 5, a theoretical determination of the equilibrium surface phase diagram 81 indicates that at low pressures (larger negative oxygen chemical potentials) a halfmetal termination is predicted, whereas at higher oxygen partial pressures (smaller negative chemical potentials) a ferryl (Fe O) termination is expected. ...
A number of distinctly different experimental techniques have been developed to determine surface structures in a quantitative fashion, and as the information gained is specific to the method, some understanding of these methods and their complementary aspects is essential to evaluate the data that emerges. The dependence of the EXAFS amplitude on the direction of the polarization vector of the incident radiation also provides some information on the directions of the nearest-neighbor scatterers. In photoelectron diffraction, particularly in the energy-scan (PhD) mode, this interference occurs at the detector, and the (much larger) modulations of intensity with photon energy are also direction-dependent, providing a method to determine the complete local geometry. One important feature of all three of these local structural probes is that, because they involve measurements of core electron binding energies that are characteristic of the photo-absorbing atom, they are element specific.
Hematite is a common iron oxide found in nature, and the α ‐Fe 2 O 3 (0001) plane is prevalent on the nanomaterial utilized in photo‐ and electrocatalytic applications. The atomic‐scale structure of the surface remains controversial despite decades of study, partly because it depends on sample history as well as the preparation conditions. Here, a comprehensive study is performed using an arsenal of surface techniques (non‐contact atomic force microscopy, scanning tunneling microscopy, low‐energy electron diffraction, and X‐ray photoemission spectroscopy) complemented by analyses of the near surface region by high‐resolution transmission electron microscopy and electron energy loss spectroscopy. The results show that the so‐called “bi‐phase” termination forms even under highly oxidizing conditions; a (1 × 1) surface is only observed in the presence of impurities. Furthermore, it is shown that the biphase is actually a continuous layer distorted due to a mismatch with the subsurface layers, and thus not the proposed mixture of FeO(111) and α ‐Fe 2 O 3 (0001) phases. Overall, the results show how combining surface and cross‐sectional imaging provides a full view that can be essential for understanding the role of the near‐surface region on oxide surface properties.
Phase transition between iron oxides practically defines their functionalities in both physical and chemical applications. Direct observation of the atomic rearrangement and a quantitative description of the dynamic behavior of the phase transition, however, are rare. Here, we monitored the structure evolution from a rod-shaped hematite nanoparticle to magnetite during H2 reduction at elevated temperatures. Environmental transmission electron microscopy observations, along with selected area electron diffraction experiments, identified that the reduction preferentially commenced with Fe3O4 nucleation on the surface defective sites, followed by laterally growing into a Fe3O4 film until fully covering the particle surface. The Fe3O4 phase then propagated toward the bulk particle via a Fe3O4/α-Fe2O3 interface with the relationship α-Fe2O3(0001)//Fe3O4(111) in an aligned orientation of [112]Fe3O4||[112̅0]α-Fe2O3. Upon this Fe3O4/α-Fe2O3 interface, the Fe-O octahedra in Fe3O4(111) (as layer A) matches that of α-Fe2O3(0001) at a rotation angle of 30°, and the reduction proceeds in such a pattern that two-thirds of the FeOh in the adjacent layer (layer B) is transformed into FeTe.
We have developed a phase mapping method based on machine learning analysis of reflection high-energy electron diffraction (RHEED) images. RHEED produces diffraction patterns containing a wealth of static and dynamic information and is commonly used to determine the growth rate, the growth mode, and the surface morphology of epitaxial thin films. However, the ability to extract quantitative structural information from the RHEED patterns that appear during film growth is limited by the lack of versatile and automated analysis techniques. We have created a deep learning-based analysis method for automating the identification of different RHEED pattern types that occur during the growth of a material. Our approach combines several supervised and unsupervised machine learning techniques and permits the extraction of quantitative phase composition information. We applied this method to the mapping of the structural phase diagram of FexOy thin films grown by pulsed laser deposition as a function of growth temperature and oxygen pressure close to the hematite-magnetite phase boundary. The in situ RHEED-based mapping method produces results that are qualitatively similar to postsynthesis x-ray diffraction analysis.
Experimental scanning tunneling microscopy (STM) images of the hematite (0001) surface exhibit long-range superstructures formed by coexisting domains consisting of different terminations (α, β, γ) of the α-Fe2O3(0001) surface. In this work extensive simulations of STM images of different terminations of hematite (0001) surfaces are performed and compared with measured ones in order to identify the nature of the superstructure unit meshes. Based on DFT calculations of α-Fe2O3(0001) surfaces, the STM images are simulated using Chen’s derivative rules approach. Different bias voltages and tip–sample distances as well as the role of the tip type are considered and discussed. For some terminations an extreme dependence of the simulated image on the distance and bias voltage is found. The difference between simulations of metal and insulator/semiconductor surfaces is discussed. The condition of simultaneous agreement between simulation and experiment for all terminations within a narrow STM parameter range leads to the assignment of the α, β and γ terminations to the ferryl, Fe-Fe–O3– or Fe–O3–Fe–, and O3–Fe–Fe– terminations, respectively.
We studied magnetite–hematite–magnetite transformations in an ultrathin epitaxial film on Pt(111) using surface sensitive and bulk sensitive methods. At initial oxidation stages at elevated temperature (810 K), a 5-nm thick magnetite Fe3O4(111) film became non-stoichiometric toward maghemite and then showed the first signs of hematite phase formation under an oxygen exposure of 3 × 10³ L. Finally, under 2 × 10⁴ L, the film fully transformed to hematite α-Fe2O3(0001), maintaining a high single-crystal quality. A comparison of the conversion electron Mössbauer spectra and low energy electron diffraction pattern showed that at intermediate oxidation stages, hematite dominated at the surface, whereas a spinel phase was still observed in the deeper layers. The magnetite–hematite conversion was fully reversed by annealing under ultra-high vacuum at temperatures exceeding 600 K, and despite a change in morphology, the magnetite film preserved the original crystal structure and orientation.
The preparation of α-Fe2O3(0001) single crystal surface by argon ion sputtering followed by ozone-annealing is demonstrated to achieve a stoichiometric surface termination not attainable with molecular oxygen. Both methanol and ethanol thermally react on the stoichiometric α-Fe2O3(0001) and three different desorption states were found. At temperatures above 600 K a dehydrogenative disproportionation pathway to yield the respective aldehyde was found, while two peaks at lower temperatures were observed. The peak at 380 K is assigned as dissociative adsorption and recombination, while the less strongly bound peak at 285 K is attributed to molecular adsorption. The mechanism is attributed to a dissociative adsorption mediated by extra atomic oxygen species on top of iron surface sites from the ozone preparation. The absence of photochemical reaction for either methanol nor ethanol shows that no hole-driven chemistry occurs on stoichiometric α-Fe2O3(0001).
CO adsorption and oxidation on Au-covered 'O-poor' biphase α-Fe2O3(0001) have been studied with HREELS and TDS. We found that the amount of CO that the surface can bind at room temperature increases with the CO dose, indicating that the CO-surface interaction produces new adsorption sites. Surface reduction via carbon dioxide formation was identified as the mechanism responsible for this. Reduction does probably already occur during dosing, since the CO molecules detected at the surface after dosing just occupy the produced sites, but are not oxidized towards CO2. CO oxidation does not occur without the gold clusters at the surface under the given experimental conditions. According to a theoretical study by Hoh et al [Res. Chem. Intermed 2015, 41, 9587] gold clusters weaken the bond of oxygen at the oxide surface, which might facilitate the consumption of these atoms for CO oxidation. Spectroscopic data provide evidence that the reduction induces electron charge accumulation in the oxide near to the Fermi level. The reduced surface is active for CO oxidation in a Mars-van Krevelen type mechanism at room temperature: oxygen bound to the sample surface reacts with subsequently dosed CO towards CO2.
The choice of oxygen carrier particles is a key parameter for the process development of chemical looping combustion (CLC) to allow for minimization of cost. This chapter discusses in depth the oxygen carrier structure as affected by thermal and chemical stresses encountered in chemical looping systems. It focuses on hematite and hematite‐based oxygen carriers, understanding how thermal and chemical stresses affect the structure of the oxygen carriers, and its importance to activity and mechanical stability in chemical looping processes. Among various reasons why attrition is undesirable in a CLC system, the greatest is the cost required to replace the attrited oxygen carrier. To complement the attrition modeling approaches, experimental techniques to quantitatively and qualitatively compare materials are needed. Most experimental test rigs for attrition are intended to isolate either abrasion‐ or impact‐based attrition mechanisms.
Tungsten oxide hexagonal prisms with biphase of h-WO3 and o-WO3•0.33H2O were prepared by a facile hydrothermal method with aid of Fe3+ cations. The combination of instrumental characterization and software simulation proved that two phases coexist in one nanoparticle with same morphology. The ratio of two phases could be changed by adjusting the concentration of Fe3+ cations. On the basis of controlled experiments, a mechanism was proposed to illustrate the formation of such a biphase WO3 structure and it was also proved that the self-growth of Fe species was unfavorable to the coexistence of the two phases. .
Gold deposited on iron oxide surfaces can catalyze the oxidation of carbon monoxide. The adsorption of gold subnano-structures on the Fe-rich termination of the magnetite(111) surface has been investigated using density functional theory. The structural, energetic, and electronic properties of gold/magnetite systems have been examined for vertical and flattened configurations of adsorbed Aun (n = 1-4) species. Single gold adatoms strongly bonded to the iron atoms of the Fe3O4(111) surface appear to be negatively charged, and consequently increase the work function. For a more stable class of larger, flattened Aun structures the adsorption binding energy per adatom is substantially increased. The structures exhibit a net positive charge, with the Au atoms binding with the oxide having distinctly cationic character. A charge transfer from the larger gold structures to the substrate is consistent with the lowering of the work function. The bonding of a CO molecule to a Au monomer on the Fe3O4(111) surface has been found nearly as strong as that to the iron site of the bare Fe-terminated surface. However, CO bonding to larger, oxide supported Aun structures is distinctly stronger than that to the bare oxide surface. Upon CO adsorption all Aun structures are cationic and CO shows a tendency to bind to the most cationic atom of the Aun cluster.
The initial thermal reduction of biphase Fe2O3(0001) films grown on Pt(111) has been studied with HREELS, LEED, TDS, and synchrotron-based valence band photoelectron spectroscopy. Ab initio calculations of the electronic excitation energies of Fe2 + and Fe3 + ions in different oxidic environments were carried out to support the experimental studies. Annealing the biphase Fe2O3(0001) at 1000 K results in the desorption of oxygen and a concomitant significant change of the electronic excitation spectra measured with HREELS. On the other hand, studies employing more surface sensitive methods like LEED, vibrational spectroscopy of adsorbates, and surface-sensitive valence band photoelectron spectroscopy reveal barely any changes induced by the desorption of oxygen. Based on these experimental findings we propose that the thermal reduction of biphase Fe2O3(0001) occurs mostly below the surface under the chosen conditions.
Iron oxides play an increasingly prominent role in heterogeneous catalysis, hydrogen production, spintronics, and drug delivery. The surface or material interface can be performance-limiting in these applications, so it is vital to determine accurate atomic-scale structures for iron oxides and understand why they form. Using a combination of quantitative low-energy electron diffraction, scanning tunneling microscopy, and density functional theory calculations, we show that an ordered array of subsurface iron vacancies and interstitials underlies the well-known (√2 × √2)R45° reconstruction of Fe3O4(001). This hitherto unobserved stabilization mechanism occurs because the iron oxides prefer to redistribute cations in the lattice in response to oxidizing or reducing environments. Many other metal oxides also achieve stoichiometry variation in this way, so such surface structures are likely commonplace.
Copyright © 2014, American Association for the Advancement of Science.
The reversible transformations of thin magnetite (Fe3O4) and hematite (α-
Fe2O3) films grown on Pt(111) and Ag(111) single crystals as support have been
investigated by a combined low energy electron microscopy (LEEM) and low-energy
electron diffraction (LEED) study. The conversions were driven by oxidation, annealing in
ultrahigh vacuum (UHV), or Fe deposition with subsequent annealing. As expected, the
oxidation of a Fe3O4
film yielded an α-Fe2O3 structure. Unexpectedly, the annealing in UHV
also led to a transformation from Fe3O4 into α-Fe2O3, but only if Pt(111) was used as
substrate. In contrast, on a Ag(111) substrate the inverse reaction, a slow transformation
from α-Fe2O3 into Fe3O4, was observed, as expected for oxygen desorption. Fe deposition on
α-Fe2O3 and subsequent annealing in UHV transformed the film into Fe3O4. As the most
probable explanation we propose that the UHV conversion on Pt(111) supports proceeds by
Fe cation diffusion through the film and Fe atom dissolution in the substrate, decreasing the
Fe concentration within the iron oxide film. This process is not possible for a Ag(111)
substrate. The interconversions, which were best observable in mixed films containing
domains of both oxides, occurred by growth of one domain type with well-defined boundaries and growth rates.
The structure and electronic properties of different terminations of hematite (0001) and magnetite (111) surfaces upon submonolayer Fe adsorption were studied using the spin-polarized density functional theory (DFT) including the Hubbard correction term U (DFT+U). On both oxides the Fe atoms were adsorbed on the most stable iron and oxygen terminated surfaces. The results show that Fe atoms bind strongly both to hematite and magnetite surfaces, however, the binding is distinctly stronger at the oxygen than at the iron terminated surfaces. For both oxides and surface terminations the binding energy of the Fe decreases with increasing coverage, which indicates substantial repulsive interactions between Fe adatoms. On the hematite surface, the most stable sites for Fe adsorption are bulk continuation sites which result in formation of the Fe-rich terminations. On the magnetite surface, the bulk continuation site is favored only for Fe adsorption on the oxygen terminated surface while on the iron terminated one Fe adsorbs in a position closer to the surface iron layer. Submonolayer coverages of Fe modify substantially the surface electronic structure of the oxides and, depending on the termination, can change its character from half-metallic to insulating one, and vice versa.
The detailed structure of hematite (0001) surfaces is both of fundamental interest and of crucial meaning in understanding the reactivity of the surfaces with respect to different adsorbates. The structure and electronic properties of mixed terminations of the α-Fe2O3(0001) surface were studied with the spin-polarized density functional theory (DFT) and the DFT+U methods in order to explore possibilities of their stable coexistence. The DFT+U results show that the mixed terminated slabs consisting of Fe- and O-terminated domains of large periodicity are energetically more stable than those resulting from the combination of pure Fe- and O-terminated fragments.
Density functional theory (DFT) calculations are used to explore water adsorption and activation on different α-Ga2O3 surfaces, namely (001), (100), (110), and (012). The geometries and binding energies of molecular and dissociative adsorption are studied as a function of coverage. The simulations reveal that dissociative water adsorption on all the studied low-index surfaces are thermodynamically favorable. Analysis of surface energies suggests that the most preferentially exposed surface is (012). The contribution of surface relaxation to the respective surface energies is significant. Calculations of electron local density of states indicate that the electron-energy band gaps for the four investigated surfaces appears to be less related to the difference in coordinative unsaturation of the surface atoms, but rather to changes in the ionicity of the surface chemical bonds. The electrochemical computation is used to investigate the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) on α-Ga2O3 surfaces. Our results indicate that the (100) and (110) surfaces, which have low stability, are the most favorable ones for HER and OER, respectively.
Direct hydroxylation of toluene and related aromatics over Fe/activated carbon catalyst using hydrogen peroxide as oxidant in acetonitrile was studied. The catalysts were characterized by ICP-AES, N2 adsorption, FTIR, XPS and ion exchange experiment. It was found that iron was anchored on the surface of activated carbon forming iron carboxylate like species acting as active phase for toluene hydroxylation. 13.4% yield (TOF, 34.4 × 10−2 h−1) and 74.3% selectivity to cresols were obtained under the optimal conditions, and o-cresol was dominant with 56% selectivity. The interaction between benzene ring and activated carbon might be responsible for the selective ring CH bonds activation. The ortho-selectivity might be due to the synergetic effect by activated carbon and high-valent iron-oxo species. The structure of the key intermediate for the titled reaction was proposed and confirmed by theoretical modeling using Gaussian 09 program package.
The structure/morphology-reactivity relations of a variety of thin film systems, including mainly binary oxides and the effect of defects, as well as oxide supported oxide nanoparticles, is reviewed. The N1s spectrum of NO on NiO(100) exhibits two well-resolved peaks, while the O1s level cannot be separated from the substrate level. Because the N1s states of NO on NiO(100) and epitaxially grown NiO(100)/Ni(100) are rather similar and the coverages do not differ very much, it may be concluded that the adsorption on the epitaxial layer is not dominated by adsorption on defects. Structural characterization of the catalysts, performed primarily using Raman and UV/vis spectroscopy, has been used to postulate that vanadia catalysts consist of isolated and polymer structures that wet the supporting oxide.
The adsorption, desorption, and reaction of gas phase methyl radicals were studied on the (0001)-oriented α-Fe2O3 surface in ultrahigh vacuum. Two different surface terminations were compared: An Fe3O4 (111) layer and the so-called “biphase” surface thought to be a mixture of FeO and Fe2O3 terminations. Gas phase methyl radicals were prepared by pyrolysis of azomethane. On Fe3O4 (111) methyl radical adsorption forms surface methoxide species as determined by the C(1s) XPS binding energy. Temperature programmed reaction spectroscopy produced direct desorption of methyl radicals at all coverages and the formation of ethane at high coverages in two desorption peaks at 331 and 439 K. The activation energies for desorption were 84 and 133 kJ/mol in the two regimes. The two surface terminations exhibit saturation coverages that differ by ca., 30×: 1.5 × 1014 and 5.2 × 1012 per cm2 for the Fe3O4(111) and “biphase” terminations, respectively. These results are interpreted in terms of bonding models and differences in atomic structure for the two terminations.
Fe3O4 nanoparticles and thin films were prepared on the Au(1 1 1) surface and characterized using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Fe3O4 was formed by annealing α-Fe2O3(0 0 0 1) structures on Au(1 1 1) at 750 K in ultrahigh vacuum (UHV) for 60 min. Transformation of the α-Fe2O3(0 0 0 1) structures into Fe3O4 nanoparticles and thin films was supported by XPS. STM images show that during the growth procedure used, Fe3O4 initially appears as nanoparticles at low coverages, and forms thin films at ∼2 monolayer equivalents (MLE) of iron. Two types of ordered superstructures were observed on the Fe3O4 particles with periodicities of ∼50 and ∼42 Å, respectively. As the Fe3O4 particles form more continuous films, the ∼50 Å feature was the predominant superstructure observed. The Fe3O4 structures at all coverages show a hexagonal unit cell with a ∼3 Å periodicity in the atomically resolved STM images.
The structure and electronic properties of different terminations of the α-Fe(2)O(3)(0001) surface were studied with spin-polarized density functional theory (DFT) and the DFT + U method accounting for the effect of strong on-site Coulomb correlations. The results for lattice relaxation, electronic and magnetic properties are presented and discussed. Though the DFT and DFT + U methods provide qualitatively very similar surface geometries they differ very much in the prediction of the electronic and magnetic properties, and the surface energetics of the clean α-Fe(2)O(3)(0001). The most stable single iron terminated (0001) surface and the oxygen-rich termination were chosen to study Au and Pd atom adsorption. The results show that both Au and Pd bind strongly to hematite surfaces and induce large changes in their geometry. The DFT + U bonding is weaker by 0.3-0.6 eV than DFT on the iron terminated surface and about 2 eV stronger on the oxygen terminated one. The binding is stronger for Pd than Au and for both adsorbates is distinctly stronger at the oxygen than at the iron terminated surface. On the iron terminated surface the adsorption binding energy per adatom increases both with Au and Pd coverage, whereas for the oxygen terminated one the opposite trend is observed.
Ordered iron oxide ultrathin films were fabricated on a single-crystal Mo(110) substrate under ultrahigh vacuum conditions by either depositing Fe in ambient oxygen or oxidizing preprepared Fe(110) films. The surface structure and electronic structure of the iron oxide films were investigated by various surface analytical techniques. The results indicate surface structural transformations from metastable FeO(111) and O-terminated Fe(2)O(3)(0001) to Fe(3)O(4)(111) films, respectively. The former depends strongly on the oxygen pressure and substrate temperature, and the latter relies mostly upon the annealing temperature. Our experimental observations are helpful in understanding the mechanisms of surface structural evolution in iron oxides. The model surfaces of Fe-oxide films, particularly O-terminated surfaces, can be used for further investigation in chemical reactions (e.g., in catalysis).
Abstract--The surface microtopography,of hematite over the course of dissolution in oxalic and citric acids was examined by in-situ and ex-situ atomic-force microscopy, ln-situ imaging of the basal-plane surface of a centimeter-scale natural hematite sample immersed,in 2 mM,citric acid demonstrated,that the basal-plane surface was relatively unreactive; rather, dissolution occurred along step edges and via etch-pit formation. Ex-situ imaging of synthetic hematite particles following batch dissolution in 1 mM oxalic acid showed similar dissolution features on basal-plane surfaces; in addition, etching along particle edges was apparent. The presence of etch features is consistent with a surface-controlled dissolution reaction. The results are in agreement with previous investigations suggesting that the basal-plane surface is relatively unreactive with respect to ligand exchange. Both in-situ and ex-situ imaging of particle surfaces can provide valuable information on the roles of surface structures and microtopographic features in mineral dissolution. Key Words--Atomic force microscope, Clay mineral surfaces, Dissolution, Hematite, Organic acids.
The atomic scale structure of crystalline surfaces plays an important role in the overall properties of materials systems, especially those relating to heterogeneous catalysis, thin film growth, and the increasingly miniaturized world of micro electrical mechanical systems (MEMS). Nanoscale engineering of materials has become commonplace and as technologies begin to emerge on smaller and smaller length scales, surface properties become increasingly more important relative to those of the bulk. Only through knowledge of the surface structure of materials can one truly understand the nature of the processes that play out on them. The research described in this manuscript involves a combination of computational (Direct Methods) and experimental (TEM) methods for the investigation of the surface structure and thermodynamics of two model heterogeneous catalytic oxides, alpha-Fe2O 3 (the mineral hematite) and SrTiO3. A new flux system has been developed for the growth of single crystal alpha-Fe 2O3. The effect of ion bombardment and annealing on the (0001) surface of hematite was investigated, and the differences between pure and impure samples are presented. Finally, methodologies are discussed for obtaining the "biphase" surface structure on a TEM-ready sample. Several new reconstructions were observed on the (111) surface of SrTiO 3, including (3x3), (9/5x9/5), (6x6), and (4x4) unit cells. The structure of the (3x3) surface has been solved and it is proposed that all of the herein observed reconstruct ions will be scalable structures from a general motif, with the different unit cell sizes being accommodated by differences in the occupancy of surface sites. The regions of phase stability have been accurately determined for the reconstructions in time, temperature and oxygen partial pressure, and a metastable surface phase diagram has been constructed. A region of metastable three-phase coexistence has been observed over a wide range of temperatures and oxygen partial pressures.
A brief review is given of the status of crystallographic direct methods for solving surface structure problems. The basic concepts of using the a priori information that the scattering comes from atoms to generate self-consistent equations and statistical constraints upon the phases are described. The use of a genetic algorithm to perform a search over possible phases is then discussed. Finally, solved cases in projection as well as the status of three-dimensional solutions for surfaces with a representative example are detailed.
A brief overview of transmission electron microscopy as it applies specifically to obtaining surface crystallographic information is presented. This review will encompass many of the practical aspects of obtaining surface crystal information from a transmission electron microscope, including equipment requirements, experimental techniques, sample preparation methods, data extraction and image processing, and complimentary techniques.
The first comprehensive survey of the principles and applications of heterogeneous catalysis! With contributions from more than two hundred leading scientists, this book is indispensable for every scientist concerned with heterogeneous catalysis.
We report success in applying direct phasing methods to produce images of surface structures at the atomic scale from intensity data collected using transmission electron diffraction.
Nonstoichiometry and point defect structure of magnetite have been studied experimentally by thermogravimetry covering the stability range of magnetite between 900 and 1400 degree C. A cation deficit is observed at high and a cation excess at low oxygen activities corresponding to cation vacancies and iron interstitials as the predominant point defects, respectively. An overexponential temperature dependence occurs for the defect concentrations in stoichiometric magnetite.
The surface stoichiometry and structure of α-Fe2O3(0001) and (10ovbar|12) natural growth faces has been studied with X-ray photoemission and LEED following room temperature Ar ion bombardment and subsequent annealing in partial pressures of O2 between 400°C and 1000°C. Disordered, oxygen-deficient ion bombardment surfaces were annealed to produce surface structures and stoichiometries approaching the ideal truncation of bulk α-Fe2O3, but only after long, high temperature annealing treatments in 10−6 Torr O2. LEED suggests that a Fe3O4(111) layer initially forms on the (0001) face for annealing temperatures up to 775°C. At higher temperatures, extra diffraction beams are observed which are attributed to multiple scattering across a Fe1−xO(111)/α-Fe2O3(0001) interface. The extra beams disappear after long annealing times at 900°C and are replaced by a (1 × 1) pattern characteristic of α-Fe2O3(0001). On the (10ovbar|12) surface, a (1 × 1) LEED pattern is observed when the surface is annealed in 10−6 Torr O2, but a (1 × 2) reconstruction similar to that observed on the surface of α-Al2O3 is evident in lower O2 partial pressures. The surface phases produced by ion bombardment and annealing treatments are quite different from those predicted by the bulk FeO phase diagram.
Ion-beam-induced microstructural modifications in the alpha-Fe2O3 matrix are investigated by using conversion-electron Mössbauer spectroscopy and small-angle x-ray diffraction techniques. It is shown that with an increase in ion dose the system tends to precipitate into spinel ferrite structures. It is also shown that the effect of heating during implantation is distinctly different as compared to the postannealing treatment.
Transition–metal–oxide particles comprise a small but important fraction of atmospheric aerosols as they are among the few particles in the troposphere having band gaps less than the cutoff of solar radiation (about 4.3 eV), thus allowing photoexcited charge-transfer excitations. We have used single-crystal α-Fe2O3(0001) to study photoinduced charge-transfer processes and chemisorption of SO2, an atmospheric pollutant. Changes in electronic structure as a result of preparation method are presented which complement previous studies. Ultraviolet photoelectron spectroscopy (UPS) was used to study changes in the electronic structure of α-Fe2O3(0001) surfaces due to ultraviolet (UV) irradiation, and to differentiate them from thermal excitations. Intense UV irradiation of the surface by a Hg(Xe) arc lamp results in an increased density-of-states near EF similar to that produced by reduction of the surface; the increase is reversible when the irradiation is terminated. In addition, the upper edge of the valence band is observed to shift upon both UV irradiation and temperature change; however, the band edge shifts to higher binding energy upon UV irradiation, but to lower energy with increased temperature. UPS results show that photoexcited α-Fe2O3(0001) surfaces chemisorb much larger amounts of SO2 than does that surface in the dark; however, adsorbate molecular-orbital peaks were found at similar positions in both cases. X-ray photoelectron spectroscopy (XPS) showed that more SO2 chemisorbed on surfaces at - - 267 K than at 300 K, and that photoexcitation increased chemisorption at both temperatures, especially at low SO2 exposures. Based upon UPS and XPS results, the adsorbed species is identified as SO3 or SO4. © 1998 American Vacuum Society.
The stoichiometric recovery and accompanying changes of subsurface atomic structure of Ar ion sputtered, oxygen deficient α-Fe2O3(0001) are studied with X-ray scattering. Although oxygen annealing up to 735°C results in a stoichiometric top surface layer manifested as a (1×1) surface structure, the X-ray scattering reveals the existence of a non-stoichiometric phase in the subsurface region. The non-stoichiometric region is characterized as a remnant of Fe3O4 or γ-Fe2O3. The stoichiometric outmost surface layers act as a barrier for propagation of oxidation to underneath remnant phase. The remnant phase can be removed after annealing at a higher temperature.
Using spin-density functional theory we investigated various possible structures of the hematite (0001) surface. Depending on the ambient oxygen partial pressure, two geometries are found to be particularly stable under thermal equilibrium: one being terminated by iron and the other by oxygen. Both exhibit huge surface relaxations ( -57% for the Fe and -79% for the O termination) with important consequences for the surface electronic and magnetic properties. With scanning tunneling microscopy we observe two different surface terminations coexisting on single crystalline alpha- Fe2O3 (0001) films, which were prepared in high oxygen pressures.
It is shown that the principal topotaxial relationship between hematite α-Fe2O3 and magnetite Fe3O4 during cycles of reduction and oxidation can be considered as crystallographically reversible. The precision of the mutual orientation of the two lattices decreases as the number of cycles of reduction and oxidation increases and there is a correlation between crystallographic orientation and the rate of oxidation and of reduction.
SO2 is an atmospheric pollutant whose oxidation leads to acid rain, while α-Fe2O3 (hematite, a naturally occurring component of atmospheric aerosol particles) has a charge-transfer band gap (about 2.2 eV) smaller than the cutoff of solar radiation in the troposphere (about 4.3 eV) and is thus able to participate in photochemical reactions. The interaction of SO2 with UV-irradiated, single-crystal α-Fe2O3 was examined by using UPS, XPS, and Auger spectroscopies. Between 261 and 331 K, SO2 adsorbs on the α-Fe2O3(0001) surface with a very low sticking coefficient in the absence of UV irradiation. The adsorbed species resembles SO42-; its heat of adsorption is estimated to be 2.4 eV. UV irradiation of the α-Fe2O3 substrate during SO2 exposure leads to a significantly increased rate of adsorption. All of the cations in stoichiometric α-Fe2O3 are Fe3+; UV irradiation produces Fe2+ cations through the creation of electron−hole pairs. The Fe2+ sites are found to be much more reactive to SO2 adsorption than are Fe3+ sites. A model is proposed in which SO2 adsorbs only at Fe2+ sites. According to this model, the small number of adsorption sites which are present in thermal equilibrium are greatly enhanced by UV irradiation, leading to an increased rate of SO2 adsorption.
Following Ar+ ion bombardment, an epitaxial spinel phase forms on the near-surface of hematite specimens that contain impurity species. This near-surface spinel phase can readily be distinguished in a transmission electron diffraction pattern, and is stable under a much broader range of oxygen partial pressures than has been previously observed in the literature. Conversely, hematite samples which are impurity free show no evidence of an epitaxial spinel phase. Regardless of the impurity concentration, samples annealed at high temperature in oxygen-rich environments show (1×1) diffraction patterns indicative of a bulk hematite termination.
We describe ultrahigh vacuum Auger electron spectrometric measurements of the uptake of chlorine following the room temperature exposure of single crystal hematite, α-Fe2O3, to CCl4. We compare the surface chemistry of two specific surface phases formed on the basal plane of α-Fe2O3: the Fe3O4(111)-(2×2) “selvedge” and the α-Fe2O3/Fe1−xO “biphase.” For Fe3O4(111)-(2×2) an estimated saturation level of Cl of ∼75% of a monolayer is readily attained. Carbon uptake is well below that expected for simple stoichiometric dissociative chemisorption, consistent with desorption of organic products during the surface reaction. Low energy electron diffraction measurements suggest that, dependent upon preparation procedures, at least two types of α-Fe2O3/Fe1−xO biphase structures can be formed. Surprisingly, upon exposure to CCl4, Cl uptake does not occur on either of these biphase surfaces, despite the fact that these surfaces are thought to have the same surface concentrations of iron and oxygen as Fe3O4(111). The dramatic difference between the reactivity of the Fe3O4 and biphase surfaces suggests that the active site for the dissociative adsorption of CCl4 on Fe3O4(111)-(2×2) comprises both an iron cation and an oxygen anion with a surface-normal-oriented dangling bond that is uncapped by iron cations. Electron stimulated and thermal desorption of Cl from the saturated Fe3O4(111)-(2×2) selvedge is also reported.
Scanning tunnelling microscopy (STM) images of two different reconstructions of an α-Fe2O3(0001) crystal are presented. Annealing the sample to 1000 K creates a selvedge stabilised by a thin film of Fe3O4, with its (111) plane parallel to the basal plane of the underlying substrate. The STM images confirm that this surface is structurally equivalent to that previously reported for the surface of Fe3O4(111) single crystals, in that two coexisting terminations, denoted A and B, are present separated by alternate steps. Termination A has been identified with 14ML of O atoms capping 34ML of Fe atoms, while termination B consists of 12ML of Fe atoms overlaying a close-packed O layer. Some regions of the sample are disordered but contain small triangular islands of termination A. This structure is attributed to Ar ion induced sputter damage. A different termination, created by annealing the sample at 1100 K in 1 × 10−6 mbar O2, has a distinctive hexagonal LEED pattern, with all the main beams floreted, being surrounded by a hexagon of smaller spots. The STM results show that this surface is stabilized by coexisting α-Fe2O3(0001) and Fe1−xO(111) phases, with each phase existing in atomically well ordered islands of mesoscopic dimensions. The islands themselves are arranged to form a superlattice. The formation of this superlattice can be explained in terms of the lattice mismatch between the two types of oxygen sub-lattices.
Results are reported using UHV electron microscopy to determine the role of background gases in influencing surface damage experiments and on the gold (001) surface prepared by ion-beam cleaning/thinning and annealing. In maximum valence oxides the end product is a higher-symmetry oxide or metal in UHV, but in a non-UHV environment secondary reactions take place. No evidence is found for electronic damage of non-maximal valence oxides, only sputtering and electron-stimulated reactions. For the gold (001) surface we have reproduced conventional surface-science preparations using ion-beam cleaning and annealing to produce the known reconstructed hexagonal monolayer on the surface.
Scanning tunneling microscopy and low-energy electron diffraction have been used to study the effect of reduction on Fe3O4(111). The results show that the selvedge can exist as a superlattice of coexisting Fe3O4(111) and Fe1-xO(111) islands, offering another example of the recently discovered phenomenon of biphase ordering. Two different structures that are able to coexist with this superlattice and increase in concentration as the surface is oxidized have been identified as resulting from a 'simple' termination of the Fe3O4(111) lattice. A more reduced selvedge is identified with islands of Fe1-xO(111).
Selected values for the entropy (So), molar volume (Vo), and for the enthalpy and Gibbs energy of formation (ΔfHo and ΔfGo) are given for 50 reference elements, 55 sulfides, 93 oxides and hydroxides, 141 silicates, and 154 other minerals and related substances at 298.15 K. For those minerals for which high-temperature heat-capacity or heat-content data are also available (HTo - Ho298)T-1, SoT, (GoT - Ho298)T-1, Cpo, ΔfHTo, ΔfGoT and log Kf,T are tabulated at 100 K intervals for temperatures up to 1800 K. -Authors
Results concerning the calibration and use of a new ultrahigh
vacuum (UHV) surface preparation and analysis system are reported.
This Sample Preparation Evaluation Analysis and Reaction (SPEAR)
side chamber system replaces an older surface side chamber that
was attached to a Hitachi UHV H-9000 microscope. The system
combines the ability to prepare clean surfaces using sample
heating, cooling, ion milling, or thin film growth with surface
analytical tools such as Auger electron spectroscopy (AES),
X-ray photoelectron spectroscopy (XPS), and scanning electron
microscopy (SEM), along with atomic surface structure information
available from high-resolution transmission electron microscopy
(HREM). The chemical sensitivity of the XPS and AES are
demonstrated in preliminary studies of catalytic and semiconductor
samples. In addition, the surface preparation capabilities are
also demonstrated for the Si(100) and Ge(100) surfaces, including
the ability to acquire secondary electron images during milling.
During operation, the entire system is capable of maintaining
the UHV conditions necessary for surface studies.
Evidence is presented for the surface mechanism of molecular recognition between 2-aminophenol and hematite in a process involving aminophenol −OH and −NH2 groups. The aim of the present study was to correlate the adsorption of some aminophenols on hematite with the degradation features observed in the dark or in the light. The hydroxyl group in the ortho position was observed to be a preferred position for chelation compared to the meta or para positions. Chelation was detected by diffuse reflectance Fourier transform infrared spectroscopy (DRIFT). Both the −NH2 and −OH groups participate in the adsorption of aminophenol onto hematite. The bridging bidentate formation during the adsorption of 2-aminophenol is supported by the simultaneous shifts of the vibrational frequencies of CC from 1513 to 1501 cm-1 and from 1403 to 1395 cm-1. The matching of atomic distances between Fe−Fe bonds in the α-Fe2O3 crystal and the N−O bond in 2-AP allows for the formation of the bridged bidentate structure. Evidence was found that adsorption enhances degradation in dark processes. The degradation of aminophenols in the dark produced long-lived intermediates that precluded further degradation. Acceleration of the degradation was observed during a photochemically induced charge-transfer process. Highly oxidative radicals generated only under light significantly increased the degradation efficiency of 2-AP and 4-AP. The degradation of 2-aminophenol on hematite proceeded more favorably than the degradations of 3- and 4-aminophenol because of the formation of a strong surface complex between 2-aminophenol and hematite that facilitates charge transfer to the oxide surface.
The α-Fe2O3(0001) biphase surface consists of an ordered arrangement of FeO(111) and α-Fe2O3(0001) surface domains on a Fe2O3 bulk [N.G. Condon et al., Phys. Rev. Lett. 75, 1961, 1995]. The interaction of atomic hydrogen Hat with this surface has been investigated at room temperature (RT, 300 K) by means of low-energy electron diffraction, X-ray photoelectron spectroscopy, and thermal desorption mass spectroscopy. The surface is easily hydroxylated by Hat. Upon heating, the OH groups react to produce hydrogen and water, the latter of which results in the partial reduction of the surface. In a parallel but slower process, bulk reduction proceeds already during exposure at RT. First, Fe3O4(111) domains, embedded in the α-Fe2O3(0001) matrix, are formed. Finally, the film is completely reduced to Fe3O4. Further reduction toward FeO or metallic Fe appears kinetically hindered and is not observed under the low exposures used in our experiments.
Large single crystals (Φ 5 × L 33 mm) of hematite (α-Fe2O3) have been grown using a CaFe4O7-based solvent and 8 bar of oxygen in a four-mirror optical floating zone furnace in the traveling solvent zone configuration. The crystals grow along the [001] direction, are of excellent quality, and contain no residual Ca contamination from the solvent.
Nonstoichiometry and point defect structure of magnetite have been studied experimentally by thermogravimetry covering the stability range of magnetite between 900 and 1400 °C. A cation deficit is observed at high and a cation excess at low oxygen activities corresponding to cation vacancies and iron interstitials as the predominant point defects, respectively. An overexponential temperature dependence occurs for the defect concentrations in stoichiometric magnetite.
A critical evaluation of diffusion data and measurements of the nonstoichiometry as a function of the oxygen potential in magnetite allows the following conclusions to be drawn: At high oxygen potential, magnetite exists with a cation deficit (cation vacancies); at low oxygen potential, magnetite exists with a cation excess (interstitial cations). Assuming the point defects to form an ideal solution with the crystal, the point defect thermodynamics describes the known data on diffusion and nonstoichiometry in magnetite quantitatively. The mobility of interstitial iron cations in magnetite (at T = 1200°C) is approximately 15 times faster than the cation vacancy mobility.Die kritische Auswertung von Diffusionsmessungen und Messungen der Abweichung von der stöchiometrischen Zusammensetzung als Funktion des Sauerstoffpotentials am Magnetit erlaubt folgende Schlüsse: Magnetit tritt bei hohem Sauerstoffpotential mit Kationendefizit (Kationenleerstellen), bei niedrigem Sauerstoffpotential mit Kationenüberschuß (Zwischengitterkationen) auf. Unter der Annahme ideal gelöster Kationenpunktfehlstellen beschreibt die Fehlstellenthermodynamik die bisher vorliegenden Befunde über Nichtstöchiometrie und Kationendiffusion quantitativ. Die Beweglichkeit der Eisenionen im Zwischengitter ist etwa 15mal größer als die der Eisenionenleerstellen (T = 1200°C).
The interactions of D2O with the Fe3O4(111) and the biphase ordered structures on α-Fe2O3(0001) were studied with X-ray photoelectron spectroscopy (XPS), low energy electron diffraction, thermal desorption, and mass spectroscopy of recoiled ions. These studies indicate that D2O is weakly bound to the surface and that virtually none exists at the surface after a 10 Langmuir dose at 300 K. However, XPS indicates that when the sample is dosed with 3 Langmuirs at 85 K, a significant amount of molecular D2O is bound to the surface, with a lesser amount of hydroxyls present as well.
The dependence of the α-Fe2O3(0001) hematite surface structure on ambient oxygen gas pressure was investigated with scanning tunneling microscopy and low energy electron diffraction. For this, thin epitaxial α-Fe2O3(0001) films grown onto a Pt(111) substrate were prepared in oxygen partial pressures between 10−6 and 1 mbar at temperatures around 830°C. In high pressures of 1 mbar an oxygen-terminated surface structure covers almost the whole sample surface. At pressures between 10−4 and 10−1 mbar comparable amounts of oxygen and iron-terminated surface areas coexist in neighboring domains. The lateral size of these domains decreases from values of 200–900 Å for pressures between 10−3 and 10−1 mbar to values around 30 Å at 10−4 mbar. At 10−5 mbar the oxygen-terminated surface areas completely vanish and an iron-terminated α-Fe2O3(0001) surface structure is formed, which is partly covered by disordered patches with lateral sizes of 10–20 Å. Further decreasing the oxygen pressure to 10−6 mbar results in a partial reduction of the surface region and the formation of coexisting α-Fe2O3(0001) and FeO1−x(111) domains that are arranged in an ordered manner forming a superstructure known as ‘biphase structure’.
Chemically disordered B3–7-nm-diameter FePt nanoparticles are synthesized using the airless operation technique based on decomposition of iron pentacarbonyl and reduction of platinum acetylacetonate. The particle solution is then washed and subsequently deposited onto a thermally oxidized Si substrate. Nanoparticle assemblies are formed after solvent evaporation. The samples are heat-treated using rapid thermal annealing at 650 C for 30 min, in an atmosphere of Ar gas with less than 1 ppm of O 2 . Oxidation has to be avoided to obtain the FePt L1 0 phase. Other crystalline phases such as FePt 3 (L1 2), magnetite Fe 3 O 4 , hematite Fe 2 O 3 and FCC Pt are obtained if oxygen is present.
We have studied magnetic properties of ultrathin Fe films grown on Pt(111) substrate by using the in situ surface magneto-optic Kerr effect (SMOKE), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). SMOKE measurements show that the Fe layers are not ferromagnetic when the film is thinner than approximately 4.5 ML (monolayers), but the in-plane magnetization is present for a 4.1-ML Fe film on Pt(111) annealed at 550 K. Upon post-annealing at 770 K, a 9.2-ML Fe film does not show any Kerr signal, while a 5.1-ML Fe film has the in-plane Kerr signal with increased coercivity. A diffuse 2 × 2 LEED pattern was observed for both cases, and the average Fe concentration of the intermixed layers caused by the interdiffusion of Fe atoms into the Pt substrate is estimated to be about 50 and 61 at.% Fe for the 9.2-and 5.1-ML Fe films, respectively. Underlying reasons are discussed on the basis of the XPS results.
The atomic structure of the Fe3O4(111) surface was determined by means of dynamical low-energy electron diffraction (LEED) after being prepared in two different ways. In a first experiment up to 10 monolayers of well-ordered iron oxide films were grown epitaxially onto Pt(111) substrates. A 1 monolayer thick film forms a hexagonal lattice with a lateral repeat distance of 3.2 Å, 15% larger than the lateral periodicity of Pt(111). Above 1 monolayer coverage the LEED pattern reveals a lateral repeat distance of 3.0 Å, indicating a contraction of the oxide lattice with respect to the first monolayer. This new LEED pattern shows half-order spots and is compatible with (2 × 2) reconstructed FeO(111) and bulk terminated Fe3O4(111) surfaces. By applying automated tensor LEED to many possible surface structures of these two iron oxides, 8 monolayer thick films were identified to be magnetite, Fe3O4. Auger electron spectroscopy (AES) measurements on these films also reveal a stoichiometry close to that of Fe3O4. In a second experiment the (111) surface of an α-Fe2O3 single crystal was prepared by Ar+ ion bombardment and subsequent annealing. Brief annealing to 900, 1000 and 1200 K in 10−10 and 10−6 mbar oxygen creates three different LEED patterns indicating structural transformations occurring in the surface region of this crystal. Prolonged annealing to temperatures between 900 and 1200 K stabilizes the same LEED pattern and gives identical intensity-voltage curves as obtained on the 8 monolayer thick films. Therefore the crystal surface region has been reduced to Fe3O4 and has the same surface structure as the epitaxially grown films. X-ray photoelectron spectroscopy (XPS) measurements on this surface also reveal a stoichiometry dose to that of Fe3O4. The best fit structure for both preparations corresponds to an unreconstructed, but strongly relaxed, polar (111) surface termination of magnetite that exposes monolayer of Fe ions over a distorted hexagonal close-packed oxygen layer and minimizes the number of dangling bonds. The surface relaxations are probably driven by electrostatic forces. Our results indicate that minimization of both the number of dangling bonds and the electrostatic surface energy are important in determining the termination and relaxations of this polar metal oxide surface. The electrostatic surface energetics is qualitatively discussed within general, simple concepts applicable to all ionic crystals.
Electrokinetic experiments were conducted on three different types of soil: glacial till, kaolin and Na-montmorillonite, in order to investigate the effect of soil mineralogy and naturally occurring hematite (Fe2O3) on the removal of chromium from these soils. Batch tests were also performed to characterize Cr(VI) adsorption onto these soils. This study has shown that soils which contain high carbonate buffers, such as the glacial till, hinder the development of an acid front, which results in alkaline conditions throughout the soil during electrokinetic remediation. However, soils possessing low buffering capacity, such as kaolin and Na-montmorillonite, favor the development of an acid front which results in a distinct pH gradient with pH values varying from 2 near the anode to over 11 near the cathode. The results from the adsorption tests showed that Cr(VI) adsorption onto soils depends on the soil type and soil pH. The adsorption of Cr(VI) was found to be governed by soil surface complexation reactions and was significant in Na-montmorillonite, moderate in kaolin and low in glacial till. The Cr(VI) adsorption was found to be pH dependent, with low adsorption occurring at high pH values and high adsorption occurring at low pH values. The low adsorption of Cr(VI) under alkaline conditions in the glacial till resulted in high Cr(VI) removal during electrokinetics. Moderate Cr(VI) adsorption in the acidic regions in kaolin resulted in lower Cr(VI) removal than in the glacial till. High Cr(VI) adsorption in acidic regions of Na-montmorillonite resulted in low Cr(VI) migration. The presence of hematite or iron oxide in soils on the removal of Cr(VI) by electrokinetics depends on the soil mineralogical composition. In soils such as glacial till, the presence of iron oxide creates complex geochemistry and retards Cr(VI) removal. However, in homogeneous clays such as kaolin and Na-montmorillonite, the presence of iron oxide does not significantly affect Cr(VI) removal by electrokinetics.
In the course of X-ray photoemission studies on the oxidation of metals and alloys and on bulk oxides, we found that in addition to physical sputtering Ar+ ion bombardment can in many cases reduce an oxide to a mixture of the original oxide, lower oxides and metal. The effect is even more pronounced in systems containing hydroxyl groups which are readily destroyed by the ion beam. Specific examples for oxidized cobalt, nickel and iron surfaces and their bulk oxides and hydroxides are given. The relative reduction rates of CoII and FeIII in CoFe2O4 are also examined. From these observations, it is clear that any depth compositional profiling using ion sputtering in conjunction with Auger or X-ray photoelectron spectroscopy should be treated with extreme caution. The mechanism for the chemical changes induced by ion bombardment is briefly discussed.
ESCA has been used to monitor alterations of catalytically and electrochemically important metal-oxygen surfaces following exposure to Ar+ and O2+ ion bombardment. This treatment resulted not only in sputtering, but also, in many cases, in reduction to the corresponding metal or lower oxide. A model based on bulk thermodynamic free energy considerations Is proposed to explain this phenomenon. We have also exploited this approach to obtain an in-depth concentration profile of various oxidation states of an element, to selectively prepare desired surface oxide compositio and to aid in interpreting complex O ls spectra. Results obtained from metal-oxygen surfaces for Ni, Ru and Mo are presented. Ni2O3 and RuO3, which are gross defect structures of the bulk species, are present on NiO and RuO2 respectively, with the former being confined to the surface layers. The MoO2, on the other hand, is covered with a surface layer of MoO3 present as a regular crystal structure.
Nanoparticle iron oxide catalyst was studied to determine the phase changes that occur during catalysis experiments and to determine if these changes could explain the oxidative catalysis and deactivation mechanisms. The starting material was characterized as a mixture of glassy material, poorly crystalline iron hydroxides, and a small amount of highly crystalline γ-Fe2O3. Under oxidative (with respect to phase changes in iron oxide) heating conditions (3% oxygen and higher) this material transformed to a much coarser α-Fe2O3, γ-Fe2O3, or Fe3O4 phase depending on the conditions of the experiment. When the material was pre-heated to intentionally transform it to α-Fe2O3, and then later exposed to catechol vapors below 450 °C, the α-Fe2O3 was further transformed to either an oxygen deficient γ-phase or to magnetite even though input oxygen tension remained well above the magnetite (Fe3O4), hematite (α-Fe2O3) phase boundary. These two phases (γ-Fe2O3 and magnetite), both of which are spinels, do not seem to be catalytically active for either the oxidation of CO or the oxidative degradation of catechol. It is proposed that the appearance of these phases are evidence of an intermediate reduction step in the oxidative catalysis mechanism performed by iron oxide. Specifically, this provides ex situ analytical evidence that a mechanism similar to the Mars–van-Krevelan mechanism operated for the catalysis by bulk crystalline iron oxide. Subsequent oxidation of the reduced iron oxide would normally regenerate the α-phase completing the Mars–van-Krevelan cycle. However, in cases where the rate of reduction exceeds the rate of oxidation, the iron oxide permanently transforms to a spinel and catalysis halts even though one of these spinels, maghemite (γ-Fe2O3), has no stability field in P–T space with respect to magnetite and hematite. The appearance of magnetite is similar to the results expected for reducing conditions and further attests to the role of α-Fe2O3 as an oxygen source during catalysis rather than as an electron acceptor.
The combination of an XPS/UPS surface analysis instrument with a microreactor allowed the investigation of the surface composition of catalysts characterized by varying activities and selectivities. The active surface is a potassium iron oxide with a 1 : I atomic ratio of K : Fe, whereby iron is only in its trivalent state. Conversion of oxidic oxygen to OH groups is detrimental to the activity. No significant amount of promotor additives is present in the active surface. The process of regeneration with steam removes carbonaceous deposits but cannot reoxidize iron from Fe2+ to Fe3+. A constant but small amount of potassium carbonate that cannot be increased by addition of CO2 to the feed of the working catalyst is present at the surface. Catalysts are precursors, active materials, and irreversibly deactivated samples were studied by SEM and TEM. The surface morphology as well as the microstructure clearly indicates a solid as the active phase. This phase is generated and maintained through solid-state reactions during operation. A potassium-rich liquid film with a thickness exceeding one monolayer can be ruled out for the catalyst performance. Formation of droplets of KOH in certain regions of the catalyst signals bulk structural desintegration of the active material.
The core and valence level XPS spectra of FexO (x ~ 0.90-0.95); Fe2O3 ([alpha] and [gamma]); Fe3O4; and FeOOH have been studied under a variety of sample surface conditions. The oxides may be characterized by a combination of valence level differences and core-level effects (chemical shifts, multiplet splittings, and shake-up structure). FeII and FeIII states are distinguishable, but octahedral and tetrahedral sites are not. The O 1 s BE cannot be used to distinghuish between the oxides since it has a nearly constant value. Fe 3d valence level structure spreads some 10 eV below EF, much broader than suggested by previous UPS and photoelectron-spin-polarization (ESP) measurements for FexO and Fe3O4. Fe surfaces (films, foils, (100) face) yield predominantly FeIII species when exposed to high exposures of oxygen or air, though there is evidence for some FeII also. At low exposures the FeII/FeIII ratio increases.
Aufgrund ihres enormen wirtschaftlichen Potenzials ist die heterogene Katalyse ein wichtiges Thema für Industrie und Forschung. Das Handbuch ist konkurrenzlos und unentbehrlich für jeden, der sich mit der Anwendung des Verfahrens befasst, die zweite Auflage wurde lange erwartet. ––– Now in 8 volumes, the completely revised and expanded second edition of this much-cited handbook collates the knowledge available on heterogeneous catalysis, providing easy-to-find yet comprehensive information. The new edition contains some 80% more material and takes into account the latest developments in the field, making it still the most up-to-date compendium in heterogeneous catalysis. More than 300 leading experts -- a veritable "Who's Who" in catalysis -- contributed to this unrivalled masterpiece, covering all aspects of the subject, from the physico-chemical foundations to large-scale industrial applications. With its straightforward presentation, this is an essential and indispensable tool for every scientist working in this area. Preparation of Solid Catalysts – Characterization of Solid Catalysts – Model Systems – Elementary Steps and Mechanisms – Kinetics and Transport Processes – Deactivation and Regeneration – Special Catalytic Systems – Laboratory Reactors – Reaction Engineering – Environmental Catalysis – Inorganic Reactions – Energy-related Catalysis – Organic Reactions
The conversion of methane to its partially oxidized products, such as methanol and formaldehyde, remains one of the most challenging processes in heterogeneous catalysis. Direct transformation of methane in these products is thermodynamically favored, but hindered by the selectivity. There has been an increase in evidence suggesting that the selectivity of this conversion is controlled by the surface reaction of methyl radicals. However, there has been no report of studying a methyl radical surface reaction on metal oxide surfaces using ultrahigh vacuum (UHV) techniques, even though the partial oxidation of methane conversion is carried out on metal oxide based catalysts. In this research, methyl radical chemistry was studied on two model samples in an UHV chamber. One was a single crystal hematite mineral sample oriented along the (0001) direction. The other was UO3 supported on a crystalline thin film of hematite. TPD spectra showed that methyl radicals adsorbed on the Fe3O4 (111)-terminated hematite (0001) surface and desorbed at higher temperatures. At the saturated methyl radical coverage, the XPS C(1s) line position indicated the formation of methoxide ions on the surface, and the carbon-surface bond strength calculated by the threshold TPD analysis agreed with the carbon-oxygen bond strength of surface methoxide ions. On the other hand, methyl radicals displayed tiny desorption features on the biphase-terminated hematite (0001) surface. A TPD spectrum quantification technique was developed in this study. This quantification proved to be very important not only for probing surface adsorption, desorption, and reaction mechanisms, but also for relating the surface reactivity to the surface structure. The quantification results showed that methyl radicals adsorbed on the regular surface sites of the Fe 3O4 (111)-terminated surface, but on the defect sites of the biphase terminated surface. Based on the surface structure differences between these two surfaces, it is proposed that methyl radicals adsorbed on surface oxygen atoms with a dangling bond perpendicular to the surface plane. The methyl radical sticking probability on the Fe3O4 (111)-terminated surface was measured at various coverages. The sticking probability-coverage profile follows a linear form, which suggests that methyl radical adsorption is governed by a site-blocking mechanism. On the hematite-supported UO3 surface, partial oxidation products, such as methanol, formaldehyde, and CO were identified by TPD as the methyl radical surface reaction products. XPS quantification results indicated that UO3 formed a monolayer structure on the hematite support. A surface methoxide ion was the proposed reaction intermediate. Source: Dissertation Abstracts International, Volume: 66-06, Section: B, page: 3149. Adviser: Peter C. Stair. Thesis (Ph.D.)--Northwestern University, 2005.
Deuterated water adsorption on epitaxially grown FeO(111), Fe3O4(111) and Fe2O3
(biphase) films was investigated in the range 110–320 K by infrared reflection–absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) spectroscopy. At 110 K, a first water layer forms on Fe3O4(111) and Fe2O3
(biphase) before the second and higher layers develop. The first half layer on Fe3O4 adsorbs dissociatively. The second half layer develops features characteristic for hydrogen bonding and the formation of dimers is concluded. Also on Fe2O3(biphase), initial water adsorption is dissociative. A strongly bound minority species is observed. Heating to 169 K causes formation of ice clusters. On FeO(111) adsorption is molecular and weak. On all studied surfaces, thick ice layers grown at 110 K are amorphous. On Fe3O4(111) they transform at 170 K into hexagonal ice (IceH) while up to 10 L on FeO(111) remain amorphous. The mechanisms for adsorption and ice formation correlate with structure and termination of the different oxide surfaces.
Epitaxial Fe3O4(111) films were grown onto a
Pt(111) substrate by repeated cycles of iron deposition and subsequent
oxidation in 10-6 mbar oxygen. A previous low energy electron
diffraction (LEED) intensity analysis revealed the regular
Fe3O4(111) surface to expose 14 monolayer Fe atoms
over a close-packed oxygen layer underneath. With scanning tunneling
microscopy (STM) a hexagonal lattice of protrusions with a 6 Å
periodicity is observed. The protrusions are assigned to the topmost
layer Fe atoms, which agrees with the dominating Fe3d electron density
of states near the Fermi level related to these surface atoms, as
revealed by ab initio spin-density-functional theory calculations. The
most abundant type of point defects observed by STM are attributed to
iron vacancies in the topmost layer, which was confirmed by LEED
intensity calculations where different types of vacancy defects have
been simulated. For oxidation temperatures around 870 K the regular
Fe3O4(111) surface coexists with several different
surface structures covering about 5% of the films, which expose 34 ML
iron atoms or close-packed iron and oxygen layers, resulting in surface
domains that are FeO(111) and Fe3O4(111) in
nature. These domains are arranged periodically on the surface and form
ordered biphase superstructures. At 1000 K oxidation temperature they
vanish and only the regular Fe3O4(111) surface
remains.
Low-energy electron diffraction and scanning tunneling microscopy studies of FeO(111) films on Ru(0001) show formation of coincidence structures with a Moir\'e pattern up to a thickness of four monolayers. In the four monolayer thick film, strained conducting ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)$ nanodomains nucleate in the insulating FeO(111) matrix and form an ordered inverse biphase superstructure. Further oxidation causes these domains to grow and to coalesce into a closed ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)$ film.
Thermodynamic stability ranges of different iron oxides were calculated as a function of the ambient oxygen or water gas phase pressure (p⩽1 bar) and temperature by use of the computer program EquiTherm. The phase diagram for Fe–H2O is almost completely determined by the O2 pressure due to the H2O dissociation equilibrium. The formation of epitaxially grown iron oxide films on platinum and ruthenium substrates agrees very well with the calculated phase diagrams. Thin films exhibit the advantage over single crystals that bulk diffusion has only limited influence on the establishment of equilibrium phases. Near the phase boundary Fe3O4–Fe2O3, surface structures are observed consisting of biphase ordered domains of FeO(111) on
both oxides. They are formed due to kinetic effects in the course of the oxidation to hematite or reduction to magnetite, respectively. Annealing a Fe3O4(111) film in 5 × 10−5
mbar oxygen at 920–1000 K results in a new γ-Fe2O3(111)-like
intermediate surface phase during the oxidation to α-Fe2O3(0001). A model is suggested for the growth
of iron oxides and for redox processes involving iron oxides. The formation of several equilibrium surface phases is discussed.
Scanning tunneling microscopy and low energy electron diffraction have been used to study the alpha-Fe2O3\(0001\) surface in an ultrahigh vacuum. Our results show that this surface can be stabilized by coexisting alpha-Fe2O3\(0001\) and FeO(111) phases, with each phase existing in atomically well-ordered islands of mesoscopic dimensions. Furthermore, the islands themselves are arranged to form a superlattice. The formation of this superlattice can be explained in terms of the lattice mismatch between two types of oxygen sublattices.
Surface structure analysis is an important area of research, and in recent years notable advances have been made in this field, both in improved techniques for studying surfaces and in methods of analyzing them. This review aims to summarize the techniques available, particularly those relating to electron microscopy, and also to outline one of the newest areas of development, the application of direct methods to surface structure analysis. Microsc. Res. Tech. 46:160-177, 1999. (C) 1999 Wiley-Liss, Inc.
The aim of this study was to examine the catalyzed decomposition of hydrogen peroxide and 2-chlorophenol (2-CP) in the presence of iron oxides. Granular ferrihydrite, goethite, and hematite were selected as catalysts in this study. 2-CP was used as the model compound because it is a typical toxic compound and has not been investigated in the catalytic decomposition by iron oxides. The catalytic activity for hydrogen peroxide decomposition followed the sequence: granular ferrihydrite > goethite > hematite. However, hematite exhibited the highest activity in catalyzing 2-CP oxidation. The oxidation efficiency of 2-CP corresponded with the inverse sequence of specific area and pHpzc of the iron oxides. The catalytic activity of granular ferrihydrite was affected significantly by the mixing speed and particle size for its large value of Thiele modulus (phi) and Damkohler number (Da). The strong diffusion resistance for granular ferrihydrite was attributed either to its microporous structure or to the formation of oxygen in the pores of the iron oxide leading to the unexpected catalytic activity of granular ferrihydrite to hydrogen peroxide and 2-CP.
The study of atomic structure of surfaces is fundamental to the understanding of electronic, chemical and mechanical properties of surfaces and numerous techniques have been developed to this end. Transmission Electron Microscopy techniques, namely transmission electron imaging (TEM) and diffraction (TED), due to their ability to provide structural information at very high resolutions, have emerged as powerful tools for the study of surface structure. In this article we review the experimental method alongside the various post-processing routines that are necessary to extract vital structural information from experimental data.