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Computed Pourbaix diagram of the reconstructed (001) surface. Atomic structures show the different surface states, where pink and white atoms represent the adsorbate oxygen and hydrogen atoms respectively.
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Surface reconstruction of polar NaTaO 3 (001) surfaces is shown to strongly affect reaction pathways and catalytic activities.
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... we determine the surface adsorbate coverage on the reconstructed (001) surface as a function of the potential by computing Pourbaix diagrams as shown in Fig. 6. We explored covering the TaO 2 and NaO terraces with O and OH adsorbates separately at coverages ranging from 1/4 to 1 monolayer (ML) as well as congurations where both terraces are covered simultaneously. The clean surface is only exposed at potentials below 0.76 V at pH 0, before it becomes OH covered on the TaO 2 terrace and ...
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... terraces for potentials greater than 1.32 V. For the (113) surface, 19 we found a similar sequence of transitions, however at slightly lower potentials (0.66 and 0.99 V) and with an intermediate 1 ML O coverage on just the Ta sites. Interestingly, we observe a recombination of adjacent O adsorbed on Na sites for both surfaces (see state S3 in Fig. 6 and 7a). Solar irradiation is expected to yield potentials above 1.32 V and we thus consider state S3 in Fig. 6 as relevant under application conditions. Given that the O 2 formed on the NaO terrace is not bound to the surface, we remove it and consider a full O coverage only on the TaO 2 terrace as shown in the side and top views in Fig. 7b ...
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... however at slightly lower potentials (0.66 and 0.99 V) and with an intermediate 1 ML O coverage on just the Ta sites. Interestingly, we observe a recombination of adjacent O adsorbed on Na sites for both surfaces (see state S3 in Fig. 6 and 7a). Solar irradiation is expected to yield potentials above 1.32 V and we thus consider state S3 in Fig. 6 as relevant under application conditions. Given that the O 2 formed on the NaO terrace is not bound to the surface, we remove it and consider a full O coverage only on the TaO 2 terrace as shown in the side and top views in Fig. 7b and c. We compute the OER on Ta sites, initially considering the conventional mechanism (Fig. 3a) ...
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... in a later study. 9 We also consider the OER on this reconstruction (shown in Fig. 8a), which experimentally coexists with the one considered in the previous section. 16 We however assume that under photocatalytic conditions, the hydroxyls are deprotonated as shown in Fig. 8b and c. While this surface would not appear in the Pourbaix diagram (Fig. 6), its experimentally observed stabilisation is likely due to favorable interactions with explicit water 9 that we do not consider here. Given the stepped structure of the surface, the conventional mechanism starting from an O at the step edge, can also be considered equivalent to the above LOER pathway on the NaO terrace. For this ...
Citations
... [54][55][56][71][72][73][74] Due to the minimal surface disorder seen in rutile crystals and DFT, most studies of rutile, 75-77 both (110) and other surfaces, focus on oxygen vacancies and surface coverages. 78,79 When adsorbed species are more stable than bare surfaces, this type of surface coverage can be thought of as a surface reconstruction. Typically, the dissociation of species on surfaces in a water environment are adsorbed oxygen species like hydroxyl-bridges. ...
... [54][55][56][71][72][73][74] Due to the minimal surface disorder seen in rutile crystals and DFT, most studies of rutile, 75-77 both (110) and other surfaces, focus on oxygen vacancies and surface coverages. 78,79 When adsorbed species are more stable than bare surfaces, this type of surface coverage can be thought of as a surface reconstruction. Typically, the dissociation of species on surfaces in a water environment are adsorbed oxygen species like hydroxyl-bridges. ...
Computational modeling of metal oxide surfaces provides an important tool to help untangle complex spectroscopy and measured catalytic reactivity. There are many material properties that make rational catalytic design challenging, and computational methods provide a way to evaluate possible structural factors, like surface structure, individually. The mechanism of water oxidation or oxygen evolution is well studied on some anatase surfaces and the rutile TiO 2 (110) surface but has not yet been mapped on other low-index Miller rutile surfaces that are present in most experimental nano-titania catalysts. Here first principles calculations provide new insights into water oxidation mechanisms and reactivity of the most common low-index Miller facets of rutile TiO 2. The reactivity of three surfaces, (101), (010), and (001), are explored for the first time and the product selectivity of multistep electron transfer on each surface is compared to the well-studied (110) surface. Density functional theory shows that a peroxo, O (p) , intermediate is more favorable for water oxidation on all facets. The OH radical formation is favored on the (001) facet resulting in a high overpotential for oxygen evolution reaction (OER). The (101) and (110) facets have low overpotentials, B0.3 V, and favor two-electron proton-coupled electron transfer to produce H 2 O 2. The only facet that prefers direct OER is (001), leading to O 2 evolution in a four-electron process with an overpotential of 0.53 V. A volcano plot predicts the selectivity and activity of low-index Miller facets of rutile TiO 2 , revealing the high activity of the peroxo OER mechanism on the (010) facet.
... Thus, it is necessary to apply a more complex model that can represent the real surface reactions due to some reasons. First, the surface reaction mechanisms for water oxidation could be different at various facets [26,27], sites [28] and terraces [29], which are generally identified by different energy barrier. However, there are few ideal catalysts with single catalytic sites in practical experiments. ...
For artificial photosynthesis, the lower photon conversion efficiencies of the photogenerated charges hinder the practical application in solar energy harvesting. The challenges are commonly ascribed to the sluggish surface redox reactions and significant surface charge recombination. Although there are several transient techniques applied for monitoring the surface charges evolution, how the experimental spectroscopy data is interpreted still involves ambiguous explanations on the complicated reactions. Here, we firstly developed a parallel 1st–nth order reaction model, which could be applied to quantitatively analyze the surface charge reactions. The microkinetic calculations were carried out by varying the reaction order, rate constants as well as the contribution of the competitive ones. Interestingly, the simplified reaction model can successfully demonstrate the charges react rate changes, which can be further applied in mechanism study complementary with those experimental methods.
Graphic Abstract
... The piezoelectric properties of this material have also been investigated, showing a very promising potential for application in energy storage devices [7]. NaTaO 3 consists of Na cations located at the corners of a pseudocubic unit cell and smaller Ta cations at the center of the cell and six-fold coordinated by oxygen, giving rise to corner-linked TaO 6 octahedra [8]. NaTaO 3 typically crystallizes in an orthorhombic unit cell while further crystal systems, such as tetragonal and cubic, closer to an ideal perovskite, can be observed at high temperatures (i.e., above 600 • C) [9,10]. ...
Combinatorial approach has been widely recognized as a powerful strategy to develop new-higher performance materials and shed the light on the stoichiometry-dependent properties of known systems. Herein, we take advantage of the unique features of chemical beam vapor deposition to fabricate compositionally graded Na1+xTaO3±δ thin films with −0.6 < x < 0.5. Such a varied composition was enabled by the ability of the employed technique to deliver and combine an extensive range of precursors flows over the same deposition area. The film growth occurred in a complex process, where precursor absolute flows, flow ratios, and substrate temperature played a role. The deviation of the measured Na/Ta ratios from those predicted by flow simulations suggests that a chemical-reaction limited regime underlies the growth mechanism and highlights the importance of the Ta precursor in assisting the decomposition of the Na one. The crystallinity was observed to be strongly dependent on its stoichiometry. High under-stoichiometries (e.g., Na0.5TaO3−δ) compared to NaTaO3 were detrimental for the formation of a perovskite framework, owing to the excessive amount of sodium vacancies and oxygen vacancies. Conversely, a well-crystallized orthorhombic perovskite structure peculiar of NaTaO3 was observed from mildly under-stoichiometric (e.g., Na0.9TaO3−δ) to highly over-stoichiometric (e.g., Na1.5TaO3+δ) compositions.
... Strong charge localization, such as the one observed in the case of excess holes in both tantalates, can affect charge mobility and their photocatalytic properties. The most stable surfaces of NaTaO 3 and KTaO 3 are (001)-oriented [40,41], therefore, we assess the activation energy for hole hopping along the (001) direction between the most stable configurations through nudged elastic band (NEB) calculations [21]. We note that in principle the energy barrier calculated from NEB could be affected by finite-size corrections. ...
Perovskite tantalates have become potential candidates for water splitting photocatalysts. Therefore, it is of importance to understand the behavior of the photoinduced excess charges in these materials. Herein, we investigate the formation of electron and hole polarons in NaTaO3 and KTaO3. We perform Perdew-Burke-Ernzerhof hybrid density functional PBE0(α) calculations, in which we define the fraction α of the Fock exchange by enforcing the Koopmans' condition, to properly account for self-interaction corrections in these calculations. We find that the hole polaron mainly localizes on one oxygen site in both materials, leading to a structural distortion where two Ta–O bonds are elongated. The electron polaron, on the other hand, localizes within one atomic plane and exhibits a two-dimensional electron gas nature. Finally, we find that the strong localization of holes leads to a low hole mobility at room temperature ∼2.94×10−6cm2/Vs and ∼1.87×10−4cm2/Vs for KTaO3 and NaTaO3, respectively.
... NaNbO 3 (NN), for example, can provide ample opportunities for catalytic-tuning since its surface layer can be altered by applying the chemical gradient, which results in the migration of alkali ions towards the surface from the bulk [33]. Particularly, polar (1 0 0) surface, formed by the stacking of charged NbO 2 + and NaOlayers, is more prone to surface reconstruction due to its diverging electrostatic energy [34]. Migration of alkali ions in NaNbO 3 leads to modification in the chemical composition, crystal structure and roughness of the surface layer. ...
... The reason suggested for the higher activity of the reconstructed (1 0 0) surfaces is that it shows the coupling mechanism during OER & ORR instead of the conventional mechanism (Nb +5 as an active centre). In the coupling mechanism, adjacent NaOand NbO 2 + rows are found to be most active with a small overpotential [34]. Fig. 11a & b show the schematic representation of conventional and coupling mechanisms for OER, respectively [34,50]. ...
... In the coupling mechanism, adjacent NaOand NbO 2 + rows are found to be most active with a small overpotential [34]. Fig. 11a & b show the schematic representation of conventional and coupling mechanisms for OER, respectively [34,50]. So, the contribution of the active sites come from both cations Na + and Nb +5 . ...
In this work, we establish that surface reconstruction, oxygen vacancies —specifically those located on the catalyst surface, and high crystallinity can be effective in tuning the catalytic activity of perovskite oxides. We report a high-performance electrocatalyst with an orthorhombic perovskite structure (NaNbO3), having an anisotropic surface layer and high crystallinity that exhibits superior activity and durability for the bi-functional oxygen electrochemistry compared to that of a similar perovskite composition with an isotropic surface layer and low crystallinity. The improvement in the electrocatalytic activity for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is mainly attributed to the formation of an easy diffusion path on the surface layer due to the ionic movements, enhanced intrinsic activity of the catalytic sites resulting from the higher crystallinity, high oxygen vacancies and a large electrochemically active surface area. The sole key parameter in achieving all the acquired characteristics is the annealing temperature. We believe that the straightforward method of bringing the desired combination of properties by just tweaking the annealing temperature is handy and energy-efficient, and hence easily adoptable.
... When a diamond is terminated with oxygen, which is more electronegative than carbon, the band is bent downward [37,38]. Surface termination in materials of two or three-element also changes the surface dipole, Fermi level at the surface, and overpotentials for the catalytic reaction [39][40][41][42]. ...
... For example, in SrTiO 3 , the photogenerated hole is preferred to be accumulated on the (110) facets to oxidize the adsorbed chemical species, while the (001) facets prefer electron extraction. This difference results from exposed elements on the surface inducing the surface dipoles, which leads to specific charge carrier transfer and accumulation on the surface [40,45]. The surface dipole induced by the surface dangling bonds can be modulated by surface passivation with other atomic species [46][47][48][49]. ...
Water oxidation and reduction reactions play vital roles in highly efficient hydrogen production conducted by an electrolyzer, in which the enhanced efficiency of the system is apparently accompanied by the development of active electrocatalysts. Solar energy, a sustainable and clean energy source, can supply the kinetic energy to increase the rates of catalytic reactions. In this regard, understanding of the underlying fundamental mechanisms of the photo/electrochemical process is critical for future development. Combining light-absorbing materials with catalysts has become essential to maximizing the efficiency of hydrogen production. To fabricate an efficient absorber-catalysts system, it is imperative to fully understand the vital role of surface/interface modulation for enhanced charge transfer/separation and catalytic activity for a specific reaction. The electronic and chemical structures at the interface are directly correlated to charge carrier movements and subsequent chemical adsorption and reaction of the reactants. Therefore, rational surface modulation can indeed enhance the catalytic efficiency by preventing charge recombination and prompting transfer, increasing the reactant concentration, and ultimately boosting the catalytic reaction. Herein, the authors review recent progress on the surface modification of nanomaterials as photo/electrochemical catalysts for water reduction and oxidation, considering two successive photogenerated charge transfer/separation and catalytic chemical reactions. It is expected that this review paper will be helpful for the future development of photo/electrocatalysts.
