Transient IR absorption study of charge carriers photogenerated in sulfur-doped TiO2

Sulfur-doped TiO2 was prepared by two methods; one was simple oxidation annealing of TiS2, the other was mixing of titanium isopropoxide and thiourea. These two sulfur-doped TiO2 preparations showed fairly different photocatalytic activity under visible light. The dynamics of photogenerated charge carriers were studied by the transient absorption measurement in the region of mid-IR. In both samples, excitation by 532 nm pulse led to photocarrier generation to the same extent. Nevertheless, the reactivity of the photocarriers was totally different. Photogenerated electrons and holes transferred to reactant gas in the latter sample, whereas they did not in the former sample. We attributed the different carrier behavior to the difference in the distribution of S atoms or particle size. These observations can explain the difference in capability of photocatalysis under visible light

An unexplored role of the CrOx shell in an elaborated Rh/CrOx core–shell cocatalyst for photocatalytic water splitting: a selective electron transport pathway from semiconductors to core metals, boosting charge separation and H₂ evolution

A core–shell structured Rh/CrOx cocatalyst has endowed various semiconductors with high efficiency in water-splitting photocatalysis, where thin CrOx layers on Rh have been assumed to be physical blockers of O₂ to the metal surface to suppress unfavorable reverse reactions (e.g., catalytic H₂O formation from H₂ and O₂). Herein, we propose another unexplored but favorable function of CrOx layers: a selective electron transport pathway from photocatalysts to the Rh core boosting charge separation and H₂ production. The subsequent loading of CrOx layers onto Rh increased the rate of visible light H₂ evolution of a Bi₄NbO₈Cl photocatalyst, even in a half reaction with a hole scavenger where O₂ does not evolve. Transient absorption spectroscopy revealed that the CrOx layer increases the electron path from Bi₄NbO₈Cl to Rh. Importantly, the highest H₂-evolution activity was obtained by simultaneous photodeposition using CrIII and RhIII precursors, which had not yet been examined. In this sample, Rh nanoparticles were enclosed by an amorphous CrOx shell, where Rh particles were less directly attached to the semiconductor. Therein, CrOx inserted between Bi₄NbO₈Cl and Rh effectively suppresses undesirable hole transfer from Bi₄NbO₈Cl to Rh, while such hole transfer partially occurs when they are in direct contact. These results indicated that CrOx functions as a selective electron transport pathway and improves the H₂ evolution activity. Although the development strategy of cocatalysts has so far focused on surface redox reactions, this study offers a new approach for the design of highly efficient cocatalysts based on the carrier transfer process, especially at semiconductor–cocatalyst interfaces

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

An emissive charge-transfer excited-state at the well-defined hetero-nanostructure interface of an organic conjugated molecule and two-dimensional inorganic nanosheet

Precise engineering of excited-state interactions between an organic conjugated molecule and a two-dimensional semiconducting inorganic nanosheet, specifically the manipulation of charge-transfer excited (CTE) states, still remains a challenge for state-of-the-art photochemistry. Herein, we report a long-lived, highly emissive CTE state at structurally well-defined hetero-nanostructure interfaces of photoactive pyrene and two-dimensional MoS2 nanosheets via an N-benzylsuccinimide bridge (Py-Bn-MoS2). Spectroscopic measurements reveal that no charge-transfer state is formed in the ground state, but the locally-excited (LE) state of pyrene in Py-Bn-MoS2 efficiently generates an unusual emissive CTE state. Theoretical studies elucidate the interaction of MoS2 vacant orbitals with the pyrene LE state to form a CTE state that shows a distinct solvent dependence of the emission energy. This is the first example of organic-inorganic 2D hetero-nanostructures displaying mixed luminescence properties by an accurate design of the bridge structure, and therefore represents an important step in their applications for energy conversion and optoelectronic devices and sensors

Manipulation of charge carrier flow in Bi₄NbO₈Cl nanoplate photocatalyst with metal loading

Separation of photoexcited charge carriers in semiconductors is important for efficient solar energy conversion and yet the control strategies and underlying mechanisms are not fully established. Although layered compounds have been widely studied as photocatalysts, spatial separation between oxidation and reduction reaction sites is a challenging issue due to the parallel flow of photoexcited carriers along the layers. Here we demonstrate orthogonal carrier flow in layered Bi₄NbO₈Cl by depositing a Rh cocatalyst at the edges of nanoplates, resulting in spatial charge separation and significant enhancement of the photocatalytic activity. Combined experimental and theoretical studies revealed that lighter photogenerated electrons, due to a greater in-plane dispersion of the conduction band (vs. valence band), can travel along the plane and are readily trapped by the cocatalyst, whereas the remaining holes hop perpendicular to the plane because of the anisotropic crystal geometry. Our results propose manipulating carrier flow via cocatalyst deposition to achieve desirable carrier dynamics for photocatalytic reactions in layered compounds

Atomically dispersed antimony on carbon nitride for the artificial photosynthesis of hydrogen peroxide

Artificial photosynthesis offers a promising strategy to produce hydrogen peroxide (H2O2)—an environmentally friendly oxidant and a clean fuel. However, the low activity and selectivity of the two-electron oxygen reduction reaction (ORR) in the photocatalytic process greatly restricts the H2O2 production efficiency. Here we show a robust antimony single-atom photocatalyst (Sb-SAPC, single Sb atoms dispersed on carbon nitride) for the synthesis of H2O2 in a simple water and oxygen mixture under visible light irradiation. An apparent quantum yield of 17.6% at 420 nm together with a solar-to-chemical conversion efficiency of 0.61% for H2O2 synthesis was achieved. On the basis of time-dependent density function theory calculations, isotopic experiments and advanced spectroscopic characterizations, the photocatalytic performance is ascribed to the notably promoted two-electron ORR by forming μ-peroxide at the Sb sites and highly concentrated holes at the neighbouring N atoms. The in situ generated O2 via water oxidation is rapidly consumed by ORR, leading to boosted overall reaction kinetics

Enhancement of UV-responsive photocatalysts aided by visible-light responsive photocatalysts : Role of WO3 for H2 evolution on CuCl

WO3 is one of the most popular materials for visible-light photocatalysts. However, its conduction band minimum is too low for water reduction. Here, we found that WO3 can assist water reduction by using visible light in a CuCl2 aqueous solution. Photoirradiation of WO3 in CuCl2 reduces Cu2+ to form indissoluble CuCl adducts, and as-produced CuCl/WO3 was active for H2 evolution under UV-light. This composite has very low reactivity under visible light (>400 nm), but visible-light assisted H2 evolution was observed with simultaneous irradiation with UV light: the activity was increased ∼1.7 fold. Transient absorption measurements revealed that Z-schematic recombination initially takes place between photogenerated electrons in WO3 and holes in CuCl. As a result, the lifetime of electrons in CuCl was increased, enhancing H2 evolution. These results demonstrate that inactive narrow-band gap materials can be used to enhance the activity of wide-band gap materials under sunlight illumination

Effect of Photoexcited Electron Dynamics on Photocatalytic Efficiency of Bismuth Tungstate

Photoexcited carrier dynamics of bismuth tungstate (Bi2WO6) photocatalysts was investigated by time-resolved infrared (IR) absorption spectroscopy. Monotonic absorption at the mid-IR region, which is attributable to absorption by photoexcited electrons, was monitored as a function of time delay from the microsecond to millisecond range after photoexcitation. Bi2WO6 particles with different crystalline content were prepared by hydrothermal reaction at several temperatures and used to elucidate the relation between density of photoexcited carriers and steadystate photocatalytic efficiency. Photocatalytic efficiency was tested using two reactions: oxidative decomposition of acetic acid in an aqueous solution (reaction 1) and oxidative decomposition of acetaldehyde in air (reaction 2). Crystallization of Bi2WO6 particles suppressed the fast recombination of photoexcited electrons and holes within 1 μs. In the case of crystallized particles, the density of the photoexcited electron increased with an increase in the crystalline content, and the photocatalytic efficiency for reaction 1 strongly depended on the crystalline content, indicating that photoexcited electrons remaining in the submillisecond time range significantly affect the reaction rate. On the other hand, photocatalytic efficiency for reaction 2 showed a proportional relation with specific surface area rather than crystalline content. The difference in a decisive factor depending on reaction condition is considered to be the slower rate of reaction of photoexcited electrons with molecular oxygen, which might occur within a time range between 200 μs and 3 ms over Bi2WO6

Structural changes of water molecules during photoelectrochemical water oxidation on TiO2 thin film electrodes

Behaviors of photogenerated charge carriers and structural changes of water molecules on TiO2 photoelectrodes were investigated by using time-resolved visible to mid-IR absorption spectroscopy. From the spectra measured in the visible to NIR region, it was shown that the lifetime of trapped electrons and holes becomes longer upon applying more positive potentials. This result was reasonably explained by the enhancement of the upward band bending at the water/TiO2 interface. On the other hand, from the spectra measured in the mid-IR region, structural changes of the water molecules were observed. When a TiO2 electrode was photoexcited at the potential where the water oxidation starts, a new absorption peak appeared at 3620 cm−1 with a slight decrease in the intensity of hydrogen-bonded water. This new peak was assigned to the isolated O–H band of water molecules. Usually, TiO2 surfaces exhibit super-hydrophilic properties with strong hydrogen-bonding; however, the obtained result was opposite. Therefore, the appearance of this isolated O–H band was ascribed to the cleavage of the hydrogen-bonding networks resulting from the production of reaction intermediates such as OH radicals or H2O2. The intensity of the isolated O–H decreases when applying more positive potentials, where the O2 evolution proceeds more efficiently. This could be ascribed to the rapid consumption of the reaction intermediates. At these potentials, the intensity of hydrogen-bonded water was also further decreased