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PL images of an annealed particle imaged through a 610 nm long-pass filter (a) and a green band-pass filter (b). Image (a) corresponds to the red trace spectrum in (c) and image (b) corresponds to the blue trace spectrum in (c). The white lines in (a) and (b) indicate the sample location for plotting PL intensity versus distance in (d). The ratio of the remote PL 1.5 μm away from the maximum to that at the maximum intensity is approximately 0.16 for red PL and 0.11 for green PL. Note that 19 pixels is approximately 2 μm.
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The spatial distribution of intra band gap traps in micrometer-sized single crystals of anatase TiO2 was explored using single-particle photoluminescence (PL) spectroscopy and imaging. The PL from microcrystals with well-defined {001} and {101} facets was imaged for the same particle before and after annealing to explore the influence of fluorine,...
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... we turn to a discussion of our hypothesis that the broad anatase PL is composed of two distinct types of transitions, namely, trapped electrons recombining with valence band holes to produce yellow/red PL and trapped holes recombining with conduction band electrons to produce green PL. On the basis of this model, and from the assumptions that the PL spectrum from anatase microsheets (Figure 4) has three components 20 and that the diffusion lengths of holes and electrons are different, we expect to observe a difference in the spatial distribution between the PL at the green edge of the spectrum and that on the red side. To test this, we compared single- particle PL imaged through a 610 nm long-pass filter to that imaged through a green band-pass filter (450−600 nm). The results in Figure 7 show that, for excitation in the center of an annealed microsheet, the contribution of the green component to remote PL near the particle edge is approximately 30% less than the contribution of the red component. Similar results were observed for green and red components of PL from unannealed particles (see Figure S5). The results here are suggestive of red and green PL components resulting from distinct mechanisms. It should be noted that there is possible overlap between the components of anatase PL 20 and thus the separation into green and red shown here is only an approximate sampling. Furthermore, the 1 s integration time used to obtain images of Figure 7a,b is very long compared to transport times, making it difficult to observe transport dynamics. To further understand the differences between charge carrier transport in capped and clean anatase particles, it would be useful to apply time-resolved photoluminescence imaging. A dynamics study may also elucidate the recombina- tion mechanisms responsible for the different components of anatase ...
Context 2
... we turn to a discussion of our hypothesis that the broad anatase PL is composed of two distinct types of transitions, namely, trapped electrons recombining with valence band holes to produce yellow/red PL and trapped holes recombining with conduction band electrons to produce green PL. On the basis of this model, and from the assumptions that the PL spectrum from anatase microsheets (Figure 4) has three components 20 and that the diffusion lengths of holes and electrons are different, we expect to observe a difference in the spatial distribution between the PL at the green edge of the spectrum and that on the red side. To test this, we compared single- particle PL imaged through a 610 nm long-pass filter to that imaged through a green band-pass filter (450−600 nm). The results in Figure 7 show that, for excitation in the center of an annealed microsheet, the contribution of the green component to remote PL near the particle edge is approximately 30% less than the contribution of the red component. Similar results were observed for green and red components of PL from unannealed particles (see Figure S5). The results here are suggestive of red and green PL components resulting from distinct mechanisms. It should be noted that there is possible overlap between the components of anatase PL 20 and thus the separation into green and red shown here is only an approximate sampling. Furthermore, the 1 s integration time used to obtain images of Figure 7a,b is very long compared to transport times, making it difficult to observe transport dynamics. To further understand the differences between charge carrier transport in capped and clean anatase particles, it would be useful to apply time-resolved photoluminescence imaging. A dynamics study may also elucidate the recombina- tion mechanisms responsible for the different components of anatase ...
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Citations
... Transfer of electrons from the acetone molecules to the surface oxygen vacancies, in a way, can be understood considering the Lewis acid and Lewis base pair interactions of oxygen vacancies and the acetone molecules. Although HfO 2 surface is highly insulating, currents of a few hundreds of nanoampere to a few microampere on close packed nanocrystal surfaces can be realized considering carrier hoping mechanism [31,32]. The acetone gas sensing properties of our HfO 2 nanocrystals have been compared with that of other materials reported earlier, as sown in Table 1. ...
Hafnium oxide nanocrystalline powders were synthesized by a low cost sol-gel technique using hafnium tetrachloride as the main precursor. The crystal structures and the particle sizes were investigated by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). The morphologies of the powders were studied by Field Emission Scanning Electron Microscopy (FESM). The powder contained crystalline HfO2 nanoparticles having sizes from 50–100 nm which were highly stable against high temperature treatment. The chemical states of the hafnium and oxygen in HfO2 were identified by X-ray photoelectron spectroscopy (XPS). Optical properties of HfO2 nanocrystals were studied by photoluminescence spectroscopy. Intense green luminescence at an energy of 2.5 eV was emitted from HfO2 nanocrystalline powder under ultraviolet excitation. The room temperature acetone gas sensing properties of HfO2 nanocrystals were evaluated by exposing HfO2 pellets to acetone gas inside a sealed glass chamber. Due to sintering at a temperature of more than 1000 °C, a nanocrystalline, porous HfO2 pellet surface evolved that exhibited an enhanced response towards a few hundreds of ppm of acetone gas in dry air.
Fig. (a) Surface morphology and (b) dynamic acetone sensing property of HfO2 pellet after sintering at 1300 °C for 5 h.
... The above PL spectroscopical study proposed that the primary localization of the electrons and the holes occurs on {001} facets at Ti 5c sites and on {101} at V O sites, respectively [324]. In recent research [332], imaging PL emissions of single m-scaled a-TiO 2 crystals was used to study the spatial-distribution of trapped sites for electrons and holes. The a-TiO 2 microcrystal (Fig. 47(A)) with both exposed {101} and {001} surfaces were synthesized by a HF assistant hydrothermal method [333]. ...
... The passivation of these traps by fluorine cations on the as-prepared samples effectively prevents these sites from trapping the electrons. The post-annealing removed the surface capped fluorine cations and may induce the (1×4) reconstruction to {001} surfaces; this may create more gap Reproduced with permission from Ref. [332]. Copyright 2015 American Chemical Society. ...
Titanium dioxide (TiO 2 ) is regarded as an important prototype photocatalytic material for several decades. The charge carrier kinetics determines the photocatalytic properties of TiO 2 materials; this is found to be greatly dependent on electronic structures. It has been revealed that the intrinsic intermediate gap states (intrinsic GSs) play a significant role in charge carrier kinetics that drive the photocatalytic processes of TiO 2 materials, which are not well summarized until now. Motivated by this thought, the purpose of this review focuses on physiochemical science of the intrinsic GSs of TiO 2 materials and their important role in charge carrier kinetics. We first give a summary on the chemical resources of the intrinsic GSs in TiO 2 and their physiochemical nature. Their general energy distribution, charge carrier population, and the associated thermodynamic properties are also elaborated from an overall viewpoint. We further carefully summarize and compare the experimental studies on the energy and the density distribution of the intrinsic GSs and discuss the associated chemical resources and charge carrier localizations. Trapping is the dominant function of intrinsic GSs in the charge carrier kinetics of TiO 2 materials. The significant effect of trapping on the transport, recombination, and interfacial transfer of charge carriers are also comprehensive summarized. Furthermore, the effects of charge carrier kinetics on photocatalytic performances are also discussed to some extents. Because of the importance of intrinsic GSs in modulating charge carrier kinetics, it is expected to increase the photocatalytic activity by engineering the intrinsic GSs, not only for TiO 2 materials, but also for the other semiconductor photocatalysts.
... 363 The passivation of surface defect states on microcrystals of anatase TiO 2 was experimentally demonstrated by Rex et al. recently (2015). 364 A pivotal work in this regard is that by Ma et al. where they took both adsorption by F − and doping in anatase TiO 2 into account and conclusively explained the experimental knowledge amassed in this field so far. ...
Over the years, scientists have identified various synthetic “handles” while developing wet chemical protocols for achieving a high level of shape and compositional complexity in colloidal nanomaterials. Halide ions have emerged as one such handle which serve as important surface active species that regulate nanocrystal (NC) growth and concomitant physicochemical properties. Halide ions affect the NC growth kinetics through several means, including selective binding on crystal facets, complexation with the precursors, and oxidative etching. On the other hand, their presence on the surfaces of semiconducting NCs stimulates interesting changes in the intrinsic electronic structure and interparticle communication in the NC solids eventually assembled from them. Then again, halide ions also induce optoelectronic tunability in NCs where they form part of the core, through sheer composition variation. In this review, we describe these roles of halide ions in the growth of nanostructures and the physical changes introduced by them and thereafter demonstrate the commonality of these effects across different classes of nanomaterials.
... The shape and intensity of the PL emission of pure TiO 2 has been investigated mainly in relation to its crystalline phase composition 9−11 or morphology. 12,13 TiO 2 PL was found to be sensitive to both the synthesis method and to specific post synthesis treatments under aerobic or anaerobic conditions, introducing near-surface defect states, 14,15 as well as to electron donor or acceptor species adsorbed on the oxide surface. 16 −18 In a previous investigation, 19 we demonstrated that doping TiO 2 with fluorine and nitrogen (N,F-doping) plays a key role in introducing new components in the PL emission of TiO 2 and affects the dynamics of the charge carriers generated upon band gap excitation under conditions similar to those adopted in photocatalytic reactions. ...
The time-resolved photoluminescence (PL) in the nanosecond time scale of TiO2 materials undoped or co-doped with nitrogen and fluorine (N,F-doping) and modified by noble metals (NM, i.e. Au or Pt) nanoparticles (NPs) photodeposition has been systematically investigated in relation to their photocatalytic activity in hydrogen production. Main aim of the study is to elucidate the origin of the NM-dependent synergistic effects in photoactivity produced by N,F-doping of TiO2 and NM NPs deposition on the oxide surface. While TiO2 doping with fluorine and nitrogen introduces new stabilized luminescent defective trap states below the conduction band revealed by long-living PL components, the presence of NM NPs on the TiO2 surface produces a PL intensity suppression, which is more relevant for Au- rather than for Pt-NPs containing materials. Time resolved PL analysis indicates that the electron transfer occurring at the TiO2/metal interface is affected by both the defective structure of anatase N,F-doped TiO2 and the type of NM (Au or Pt). In particular, Au rather than Pt NPs appear to strongly interact with the charge carriers trapped at surface defect sites, gold NPs being expected to preferentially grow on such sites during photodeposition. Furthermore, plasmonic gold NPs excitation upon PL light absorption is evidenced by the PL spectral shape variation observed only in the case of Au/TiO2. Thus the larger synergistic effect on photocatalytic hydrogen production observed upon Au NPs photodeposition on N,F-doped TiO2 results from the opening of a new efficient electron transfer path from luminescent defective trap states on doped TiO2 to Au NPs, where proton reduction occurs.
... The photoluminescence spectra of pure NMTO, 5.00Au/ NMTO, 12.6Ag/NMTO and 5.55Pt/NMTO are shown figure 6. The intensities of composites are lower than that of pure NMTO, implying that less recombination of electron −hole pairs occurs after loading with Au, Ag or Pt [37]. Then, the photoproduced electron−hole pairs were effectively separated in Au/NMTO, Ag/NMTO and Au/NMTO, which is very favorable for improving the photocatalytic activity of NMTO. ...
The noble metal Au, Ag or Pt was loaded on Na0.9Mg0.45Ti3.55O8 (NMTO) by a chemical bath deposition method, which was obtained in our recent work for the first time. The composite photocatalysts exhibit higher photodegradative ability for degradation of methylene blue due to the Schottky barrier built between NMTO and noble metal. Hot electrons generated during localized surface plasmon process in metal nanoparticles transfer to semiconductor, which exhibited as depression of surface potential directly detected by scanning Kelvin probe microscopy. The charge separation is the key factor responsible for the improved ability of semiconductor-based photocatalysts. The best weight concentrations of Au, Ag and Pt loaded on NMTO are 5.00%, 12.6% and 5.55%, respectively. The NMTO loaded with noble metal shows high photostability and recyclability for the degradation of methylene blue. A possible mechanism for the photodegradation of methylene blue over NMTO loaded with noble metal is proposed. This work highlights the potential application of NMTO-based photocatalysts and provides an effective method to detect the localized surface plasmon.
... Then again, works on defects in TiO 2 have been scarce. While some works have associated visible luminescence with specific defects [67], the obtained emission spectra are very broad, covering the entire visible spectral range. ...
Fluorescence properties of crystallographic point defects within different morphologies of titanium dioxide were investigated. For the first time, room-temperature single-photon emission in titanium dioxide optical defects was discovered in thin films and commercial nanoparticles. Three-level defects were identified because the g
(2)
correlation data featured prominent shoulders around the antibunching dip. Stable and blinking photodynamics were observed for the single-photon emitters. These results reveal a new room-temperature single-photon source within a wide bandgap semiconductor.
... Ag-Pt/NMTO samples exhibit lower intensity than that of pure NMTO, implying that fewer recombination of electron-hole pairs occurs in Ag-Pt/NMTO. 36 Then, the photoproduced electronic-hole pairs are effectively separated in Ag-Pt/NMTO, which is favorable for improving the photocatalytic activity of NMTO. Thus, the charge separation is the main factor for enhancing the photocatalytic performance of NMTO loaded with Ag and Pt. ...
Noble metals Ag and Pt were loaded on Na0.9Mg0.45Ti3.55O8 (NMTO) by chemical bath deposition method, which was synthesized firstly in our recent work. The scanning electron microscopy and transmission electron microscopy results show that Ag and Pt nanoparticles were distributed on the different surfaces of NMTO. NMTO loaded with Ag and Pt can efficiently enhance the separation of photogenerated electron-hole pairs, exhibiting much higher photodegradative ability for methylene blue and rhodamine B than the pure NMTO. The best weight concentrations of Ag and Pt are 9.00% and 3.70%, respectively. The electrostatic field is built in the Schottky barrier between NMTO and noble metals, leading to the energy band bending. Then, photogenerated electrons and holes are efficiently separated to enhance the photocatalytic activity.
... 161 Rex et al. explored the spatial distribution of intra band gap traps in anatase TiO 2 crystal by using PL spectroscopy. 167 Fluorine was used as a capping agent to anneal anatase TiO 2 crystal. The PL spectra revealed stronger photoluminescence in the annealed crystal, indicating that there were fewer surface defects in the capped crystal. ...
... PL images showed that the remaining surface defects in the fluorine-capped particles were concentrated on the edges and corners of the crystal (Figure 7g−i). 167 In addition to the direct characterization of TiO 2 , PL can also be used to study the formation of photocatalytic active OH• radicals, which is another route to measure the photocatalytic property of TiO 2 . 168 However, the short lifetime and high activity of OH• create difficulties in the measurement. ...
TiO2 is the most investigated photocatalyst because of its non-toxicity, low cost, chemical stability, and strong photooxidative ability. Due to the morphology and structure dependent photocatalytic properties of TiO2, accurate characterization of the crystal and electronic structures of TiO2-based materials and their performance during the photocatalytic process is crucial not only for understanding the photocatalytic mechanism but also for providing experimental guidelines as well as a theoretical framework for the synthesis of high performance photocatalysts. In this review, we focused on the advanced characterization techniques that are utilized in the studies on the TiO2 structures and photocatalytic performance of TiO2 and TiO2-based materials. It is therefore anticipated that this review can provide a novel perspective to understand the fundamental aspects of photocatalysis and inspire the development of new photocatalysts with superior performances.
First principles modeling of anatase TiO2 surfaces and their interfacial contacts shows that defect-induced trap states within the band gap arise from intrinsic structural distortions, and these can be corrected by modification with Zr(IV) ions. Experimental testing of these predictions has been undertaken using anatase nanocrystals modified with a range of Zr precursors and characterized using structural and spectroscopic methods. Continuous-wave electron paramagnetic resonance (EPR) spectroscopy revealed that under illumination, nanoparticle–nanoparticle interfacial hole trap states dominate, which are significantly reduced after optimizing the Zr doping. Fabrication of nanoporous films of these materials and charge injection using electrochemical methods shows that Zr doping also leads to improved electron conductivity and mobility in these nanocrystalline systems. The simple methodology described here to reduce the concentration of interfacial defects may have wider application to improving the efficiency of systems incorporating metal oxide powders and films including photocatalysts, photovoltaics, fuel cells, and related energy applications.
Anatase TiO2 nanosheets with {0 0 1}-{1 0 1} surface heterojunction is employed as the typical photocatalyst to study surface plasmon resonance (SPR) enhanced photocatalytic Cr(VI) reduction with the help of selectively deposited Au and Pt nanoparticles. By employing an UV LED with central wavelength of 365 nm and a green LED with central wavelength of 530 nm as the light sources, results indicate the single green LED has little positive effect on driving the photocatalytic Cr(VI) reduction. In contrast, Au SPR can significantly improve the photocatalytic Cr(VI) reduction efficiency when both the UV LED and green LED are simultaneously irradiated. The {0 0 1}-{1 0 1} surface heterojunction and Pt nanoparticles can further improve the Cr(VI) reduction efficiency because of the facilitated hot electrons’ transfer. Our findings suggest that the synergistic effect among {0 0 1}-{1 0 1} surface heterojunction, Au/Pt selective deposition, and excitation wavelength is important for SPR enhanced photocatalytic Cr(VI) reduction activity.
