Trapping-Induced Enhancement of Photocatalytic Activity on Brookite TiO<inf>2</inf> Powders: Comparison with Anatase and Rutile TiO<inf>2</inf> Powders

Brookite TiO2 is a promising material for active photocatalysts. However, the principal mechanism that determines the distinctive photocatalytic activity between anatase, rutile, and brookite TiO2 has not yet been fully elucidated. Therefore, in this work, we studied the behavior of photogenerated electrons and holes in these TiO2 powders by using femtosecond to millisecond time-resolved visible to mid-IR absorption spectroscopy. We found that most of the photogenerated electrons in brookite TiO2 are trapped at powder defects within a few ps. This electron trapping decreases the number of surviving free electrons, but it extends the lifetime of holes as well as the trapped electrons because the probability of electrons to encounter holes is decreased by this electron-trapping. As a result, the number of surviving holes increases, which is beneficial for photocatalytic oxidation. In contrast, the reactivity of electrons is decreased to some extent by trapping, but they still remain active for photocatalytic reductions. Electron trapping also takes place on anatase and rutile TiO2 powders, but the trap-depth in anatase is too shallow to extend the lifetime of holes and that of rutile is too deep than the thermal energy (kT) at room temperature for the electron-consuming reactions. As a result of the moderate depth of the electron trap in brookite, both electrons and holes are reactive for photocatalytic reductions and oxidations. These results have clearly demonstrated that the presence of an appropriate depth of the electron trap can effectively contribute to enhance the overall photocatalytic activity

The contrasting effect of the Ta/Nb ratio in (111)-layered B-site deficient hexagonal perovskite Ba5Nb4-xTaxO15 crystals on visible-light-induced photocatalytic water oxidation activity of their oxynitride derivatives

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The effect of the Ta/Nb ratio in the (111)-layered B-site deficient hexagonal perovskite Ba5Nb4-xTaxO15 (0 <= x <= 4) crystals grown by a KCl flux method on visible-light-induced photocatalytic water oxidation activity of their oxynitride derivatives BaNb1-xTaxO2N (0 <= x <= 1) was investigated. The Rietveld refinement of X-ray data revealed that all Ba5Nb4-xTaxO15 samples were well crystallized in the space group P (3) over bar m1 (no. 164). Phase-pure BaNb1-xTaxO2N (0 <= x <= 1) porous structures were obtained by nitridation of the flux-grown oxide crystals at 950 degrees C for 20, 25, 30, 35, and 40 h, respectively. The absorption edge of BaNb1-xTaxO2N (0 <= x <= 1) was slightly shifted from 720 to 690 nm with the increasing Ta/Nb ratio. The O-2 evolution rate gradually progressed and reached the highest value (127.24 mu mol in the first 2 h) with the Ta content up to 50 mol% but decreased at 75 and 100 mol% presumably due to the reduced specific surface area and high density of structural defects, such as grain boundaries acting as recombination centers, originated from high-temperature nitridation for prolonged periods. Transient absorption spectroscopy provided evidence for the effect of the Ta/Nb ratio on the behavior and energy states of photogenerated charge carriers, indicating a direct correlation with photocatalytic water oxidation activity of BaNb1-xTaxO2N

Fabrication of robust TiO2 thin films by atomized spray pyrolysis deposition for photoelectrochemical water oxidation

Photoelectrodes are highly essential for the photoelectrochemical water splitting process and development of novel fabrication techniques is vital for further enhancement of activity. In this study, we successfully fabricated highly active TiO2 thin films by using novel atomized spray pyrolysis deposition (ASPD) technique. The ASPD technique utilizes a unique atomization process to produce highly fine aerosols which resulted in a highly crystalline TiO2 nanostructure. The deposition process was optimized by controlling deposition temperatures and precursor amounts. XRD and SEM studies confirmed the formation of anatase TiO2 phase and a highly interconnected nano-flakes on FM substrate at 550 degrees C. The photoelectrochemical activity of the optimized thin films showed a photocurrent density of similar to 5 mA cm(-2) at 1.0 V (vs. Ag/AgCl) in 0.1 M Na2SO4 (aq) under 375 nm (150 mW cm(-2)) illumination. This photocurrent was much higher than the two other anatases TiO2 thin films fabricated by conventional spray pyrolysis deposition (SPD) using the same precursor and anatase TiO2 powder (particle size similar to 21 nm). Transient IR absorption study revealed that the SPD powder based thin films have deeply trapped electrons, whereas ASPD thin films consisted with only free and/or shallowly trapped electrons. Higher crystallinity and enhanced electron conductivity of the TiO2 thin films fabricated by ASPD are responsible for this stable and high photoelectrochemical activity. (C) 2017 Elsevier B.V. All rights reserved

Behavior and Energy State of Photogenerated Charge Carriers in Single-Crystalline and Polycrystalline Powder SrTiO₃ Studied by Time-Resolved Absorption Spectroscopy in the Visible to Mid-Infrared Region

The effects of defects on the behavior of photogenerated charge carriers in SrTiO₃ (STO) are studied by time-resolved absorption spectroscopy from the visible to mid-IR region. In the case of defect-free single-crystalline STO, free and shallowly trapped electrons are dominant, but they recombine within 50 ns. By contrast, in the case of defect-rich powder STO, the electron lifetime is much longer than 1 ms. The transient absorption spectra show that most of the charge carriers in powder STO are trapped in the defects, which elongates their lifetime. We found that these trapped carriers are nevertheless reactive toward O₂ or CH₃OH that depends on the trap depth. The steady-state photocatalytic activity is strongly correlated with the lifetime and the reactivity of the trapped charge carriers: the energy state of electrons can be deduced from the spectral shape, especially in the mid-IR region

Identification of individual electron- and hole-transfer kinetics at CoOx/BiVO4/SnO2 double heterojunctions

The fabrication of heterojunctions with different band gap semiconductors is a promising approach to increase photoelectrochemical (PEC) activity. The PEC activity is determined by the charge separation; hence, the behaviors of charge carriers at the junctions should be elucidated. However, it has been quite challenging since the distinction of carriers located in different layers has been extremely hard. In this work, we succeeded in the identification of the individual electron- and hole-transfer kinetics at CoO/BiVO/SnO double heterojunctions by measuring transient absorption (TA) from the visible to mid-IR region: we found that the absorption peaks of electrons and holes depend on the materials. From the change in spectral shape after the selective photoexcitation of BiVO, it was confirmed that electrons excited in the BiVO rapidly transferred to the SnO layer after ∼3 ps, but the holes remained in the BiVO and further transferred to CoO in a few picoseconds. As a result, recombination of charge carriers was suppressed and 2.4 and 3.6 times a large amount of carriers are surviving at 5 μs on BiVO/SnO and CoO/BiVO/SnO, respectively, compared to bare BiVO. For such picosecond-rapid and effective charge separation, the previously well proposed sole intralayer or interlayer charge separation mechanism is not enough. Hence the synergetic effect of these two mechanisms, the band-bending-assisted charge transfer across the heterojunction, is proposed. The enhanced PEC activity of CoO/BiVO/SnO electrodes was reasonably explained by this synergistic charge separation kinetics. This fundamental knowledge of charge carrier dynamics will be beneficial for the design of superior solar energy conversion systems

Effect of CuFe2O4 ferrite on photocatalysis and carrier dynamics of electrospun alpha-Fe2O3 nanofibers by time-resolved transient absorption spectroscopy

Pristine and Cu-doped α-Fe2O3 nanofibrous photocatalysts were prepared facilely by electrospinning followed by thermal calcination for removal of the polymeric template. The addition of Cu resulted in formation of copper ferrite (CuFe2O4) in the α-Fe2O3 matrix. Photogenerated electrons and holes in the photocatalysts were found trapped in semiconducting midgaps by the time-resolved transient absorption spectroscopy under pulsed light irradiations (355 nm wavelength). The Cu-doped α-Fe2O3 showed an increased carriers population when compared to the pristine counterpart. The finding was in good agreement with the methylene-blue (MB) dye degradation under ultraviolet-light irradiations, in which an enhanced MB removal was found for the Cu-doped α-Fe2O3 photocatalyst. The MB dye removal was directly correlated with the presence of CuFe2O4 ferrite in the α-Fe2O3 matrix; from which, the semiconducting heterojunction at the interface led to a prolonged lifetime of the carriers and a resultant increase on the carriers population and MB photodegradation

How g-C3N4 Works and Is Different from TiO2 as an Environmental Photocatalyst: Mechanistic View

Graphitic carbon nitride (CN) as a popular visible light photocatalyst needs to be better understood for environmental applications. The behaviors of CN as an environmental photocatalyst were systematically studied in comparison with a well-known TiO2 photocatalyst. The two photocatalysts exhibit different photocatalytic oxidation (PCO) behaviors and dependences on the experimental conditions (e.g., pH, Pt loading, and the kind of organic substrate and scavenger). The PCO of organic substrates was significantly enhanced by loading Pt on TiO2 under UV light (lambda > 320 nm), whereas Pt-CN exhibited a lower PCO activity than bare CN under visible light (lambda > 420 nm). While the presence of Pt enhances the charge separation in both TiO2/UV and CN/visible light systems (confirmed by transient IR absorption spectroscopic analysis), the opposite effects of Pt are ascribed to the different mechanisms of center dot OH generation in the two photocatalytic systems. The negative effect of Pt on CN is ascribed to the fact that Pt catalytically decomposes in situ-generated H2O2 (a main precursor of OH radical), which hinders center dot OH production. The production of OH radicals on CN is favored only at acidic pH but O-1(2) generation is dominant in alkaline pH. The pH-dependent behaviors of reactive oxygen species generation on CN were confirmed by electron paramagnetic resonance spin trap measurements.11Nsciescopu

Clear and transparent nanocrystals for infrared-responsive carrier transfer

赤外光を電気エネルギーや信号に変換する無色透明な材料の開発に成功 --見えない電子デバイスの開発へ道--. 京都大学プレスリリース. 2019-02-13.An Author Correction to this article was published on 17 April 2019. https://doi.org/10.1038/s41467-019-09888-2Infrared-light-induced carrier transfer is a key technology for ‘invisible’ optical devices for information communication systems and energy devices. However, clear and colourless photo-induced carrier transfer has not yet been demonstrated in the field of photochemistry, to the best of our knowledge. Here, we resolve this problem by employing short-wavelength-infrared (1400–4000 nm) localized surface plasmon resonance-induced electron injection from indium tin oxide nanocrystals to transparent metal oxides. The time-resolved infrared measurements visualize the dynamics of the carrier in this invisible system. Selective excitation of localized surface plasmon resonances causes hot electron injection with high efficiency (33%) and long-lived charge separation (~ 2–200 μs). We anticipate our study not only provides a breakthrough for plasmonic carrier transfer systems but may also stimulate the invention of state-of-the-art invisible optical devices