Accepting Excited High-Energy-Level Electrons and Catalyzing H₂ Evolution of Dual-Functional Ag-TiO₂ Modifier for Promoting Visible-Light Photocatalytic Activities of Nanosized Oxides

Abstract

To improve the photocatalytic activities of narrow band gap oxide semiconductors for H 2 evolution under solar irradiation, it is highly desired to develop effective acceptors for visible light-excited high-energy-level electrons. Herein, we have successfully fabricated Ag-modified TiO 2 /BiVO 4 nanocomposites by putting nanosized BiVO 4 into the Ag modified TiO 2 sol. Both steady-state and transient-statesurface photovoltage spectra demonstrate that photogenerated charge separation and lifetime of nanosized BiVO 4 is promoted when coupling with TiO 2 and modifying an appropriate amount of Ag, while the lifetime of photogenerated electrons got prolonged. Interestingly, the resulting Ag-TiO 2 /BiVO 4 nanocomposites exhibit excellent visible light activities for H 2 evolution, although the visible light activities of TiO 2 /BiVO 4 one, Ag/BiVO 4 and bare BiVO 4 are neglectable, indicating that AgTiO 2 could be utilized as effective acceptors for hydrogen production. It is suggested based on the experimental data that the effective acceptors be attributed to the used TiO 2 for accepting the high-energy-level electrons generated from BiVO 4 and to the modified Ag for being reduced then to catalyze H 2 -evolution reactions. The developed strategy is versatile for other narrow band gap semiconductors, like WO 3 and Fe 2 O 3 . ■ INTRODUCTIONPhotocatalytic hydrogen evolution is regarded as a potential technique that can solve the current increasingly serious energy crisis through effective solar utilization. 1 Despite a great deal of effort made, the quantum efficiencies of state-of-art works are still low, far from the practical requirement for industrialization. 2,3 Photocatalytic reactions for H 2 evolution are mainly involved with complex thermodynamics and kinetic processes. In general, efficient photocatalysts are needed to possess strong solar-light absorption, high charge separation and superior catalytic capacity. It is well-known that cheap, stable, and environment friendly narrow band gap oxide semiconductors, such as BiVO 4 , Fe 2 O 3 , and WO 3 , can greatly absorb solar light, and the electrons will be excited from valence band (VB) to conduction band (CB) bottom and even over the level under solar irradiation. 4−6 The excited electrons at much higher energy level than the CB bottom usually relax to the CB bottom quite quickly (within picosecond time scale). 7,8 Since their CB bottom levels are usually located below 0 eV versus NHE, 9 so that the excited electrons are not suitable for energetic H 2 evolution, it is understandable that such narrowband gap oxides always exhibit low photocatalytic activities. However, it is worth noting that those visible-light-excited highenergy-level (HEL) electrons would naturally possess strong capacity to induce reduction reactions with water molecules thermodynamically. 10,11 For this, the key would be how to prolong the lifetimes of HEL electrons by altering the fast relaxation process. Unfortunately, it has seldom been reported until now.Constructing a heterojuntional ...

Synthesis of Silicate‐Bridged Heterojunctional SnO2/BiVO4 Nanoplates as Efficient Photocatalysts to Convert CO2 and Degrade 2,4‐Dichlorophenol

“…It is clear that 5Si‐10S/BV‐NP6 exhibits a detectable SS‐SPS response in N 2 as shown in Figure A and Figure S13 (Supporting Information), much higher than 10S/BV‐NP6. The detected SS‐SPS response in N 2 should be attributed to the spatial transfer of photogenerated energetic electrons from BiVO 4 to SnO 2 , according to the fundamental of SS‐SPS and previous work . By comparison, it is suggested that the silicate bridges are favorable to promote electron transfer, which could be attributed to the silicate bridges acting as a favorable charge transport channel in the fabricated nanocomposite.…”

“…Except for the surface area, another vital factor to affect the photocatalytic activity of BiVO 4 in nature is its photogenerated charge separation situation . Based on our previous work, it is proposed that wide bandgap semiconductors like TiO 2 , ZnO, and SnO 2 with high conduction band (CB) potentials could act as an appropriate platform to accept the high‐energy‐level electrons generated by BiVO 4 under visible‐light irradiation, so as to facilitate the charge separation and prolong the lifetime of charge carriers . Compared with TiO 2 or ZnO, due to the relatively lower CB potential, SnO 2 is capable of accepting more photogenerated electrons as already proved in the case of Fe 2 O 3 /SnO 2 nano‐photocatalyst, therefore, suitable to be adopted to integrate with BiVO 4 .…”

Synthesis of Large Surface‐Area g‐C3N4 Comodified with MnOx and Au‐TiO2 as Efficient Visible‐Light Photocatalysts for Fuel Production

“…[42,52] Moreover, the surroundings would affect the charge proper ties on the surfaces of semiconductors. [42,52] Moreover, the surroundings would affect the charge proper ties on the surfaces of semiconductors.…”

“…As a result, the photocatalytic performance for CO 2 conversion, H 2 evolution, and pollutant degradation of gC 3 N 4 could be significantly improved. [42] However, related attempts have seldom been made to modify gC 3 N 4 up to date. It is well demonstrated that noble metals, like Pt, Au, and Ag are taken as effective cocatalysts.…”

Increased photocatalytic activity induced by TiO 2 /Pt/SnO 2 heterostructured films

“…To avoid the recombination of photogenerated charges, some of the methods employed include the formation of composites via the incorporation of some metals into the semiconductor [18,19] [24], and the decoration of semiconductor with metallic nanoparticles [25]. All these methods have been reported to have led to an increase in photocatalytic efficiency by providing a greater charge separation and consequently higher availability of active sites compared to the use of pure TiO 2 [26].…”