ZnSe Sensitized and Co-Pi Catalyzed TiO₂Nanowire Array Photoanode for Solar-Driven Water Splitting

Abstract

Cobalt-phosphate water oxidation catalysts were successfully deposited onto TiO 2 /ZnSe core/shell nanowire arrays by photo-assisted electrodeposition methods. Photoelectrochemical (PEC) properties of the resulting composite photoanodes were studied for solardriven water splitting under back side illumination in 0.5 M Na 2 SO 3 electrolyte. Compared with pristine TiO 2 and TiO 2 /ZnSe nanowire arrays, the photoconversion efficiency in TiO 2 /ZnSe/Co-Pi composite photoanode was increased by 1.5 and 3.8 times at a bias voltage of 0.6 V vs. Ag/AgCl. More attractive, this triple TiO 2 /ZnSe/Co-Pi photoanodes show significantly improved PEC stability compared with the TiO 2 /ZnSe electrode, remaining 56% initial photocurrent after 4000 seconds persistent light irradiation. By using the electrochemical measurements, the reason for the improved performance of TiO 2 /ZnSe/Co-Pi is discussed in detail. It is proposed that the TiO 2 provided direct channel for electron transport, ZnSe photosensitizer extended the spectral response to the visible region, and the Co-Pi acts as hole transfer relay to accelerate the surface water oxidation reaction. The strategy for designing photoanode in this work is low-cost but effective, and could be extended to prepare other hybrid photoanode with elaborate textural characteristics.To cope with the ever increasing global energy crisis and environmental concerns, it is becoming urgent to seek clean and sustainable fuels. Hydrogen is one of the most promising candidates due to its carbon-neutral, high energy density and easy to portable properties. It releases energy after combustion with oxygen and leaves the only water by-product. Particularly, it is meaningful to extract hydrogen from water using solar light through photoelectrochemical (PEC) devices, which enables recyclable use of hydrogen fuel in a clean and renewable way. The PEC performances is heavily dependent upon light absorption, charge separation/migration, charge recombination and stability in aqueous solution of the photocatalysts. 1,2 Frustratingly, considerable efforts have proven to be impossible to optimize a single semiconductor material for all these processes so far. Titanium oxide (TiO 2 ) is one of the most commonly used wide bandgap semiconductors for PEC application due to its high catalytic activity, good stability and low cost. However, the large bandgap of TiO 2 limits its light absorption within the UV part of the solar spectrum, which strictly prevents the efficient utilization of the solar energy in the visible region. 3,4 Lots of efforts have been made to enhance the visible light harvesting ability of TiO 2 . For example, transition metal or nonmetal ions doping can extend its light absorption into the visible light region. [5][6][7][8] Alternatively, by combining short bandgap semiconductors such as ZnSe, CdS, CdSe, Zn x Cd 1-x Se with TiO 2 can also enhance the visible light harvest ability, which is widely studied in quantum dot sensitized solar cells (QDSC) and solar water splitting. 9-13,14-16 More i...

Designing Co-Pi Modified One-Dimensional n–p TiO2/ZnCo2O4 Nanoheterostructure Photoanode with Reduced Electron–Hole Pair Recombination and Excellent Photoconversion Efficiency (>3%)

“…However, the identical absorption spectrum profiles of the TO/ZCO and TO/ZCO@Co-Pi NHs clearly indicate that the Co-Pi does not take part in the light absorption, as also reported by others. 39,40 The band gap energies of the nanostructures estimated from the Tauc's plot (Figure S3) indicate that the as-prepared anatase TiO 2 NTs is a direct band gap material 41−45 with band gap energy of 3.26 eV, and the band gap energies of TO/ZCO and TO/ZCO@Co-Pi NHs are 3.32 and 3.34 eV, respectively. Moreover, the absorption edges around 2.48, 2.57, and 2.58 eV for pristine TiO 2 NTs, TO/ ZCO NHs, and TO/ZCO@Co-Pi NHs, respectively, can be related to the band tail state, which arises as the midband gap states/defect states form a continuum extending to and overlapping with the conduction band edge.…”

Recent Advances in Self‐Supported Semiconductor Heterojunction Nanoarrays as Efficient Photoanodes for Photoelectrochemical Water Splitting

“…[57] This issue can be alleviated in semiconductor heterojunction systems, as the built-in electric field at the heterogeneous interface enables efficient charge carrier separation. [91] To this end, many narrow bandgap semiconductors, such as In 2 S 3 (2.1 eV), [57] Cu 2 O (2.17 eV), [53,80] Fe 2 O 3 (2.2 eV), [68] CeO 2 (2.4 eV), [46] CdS (2.42 eV), [70,73,74,92] graphyne (2.65 eV), [69] ZnSe (2.6 eV), [78] C 3 N 4 (2.7 eV) [48,61,79,93] and PbTiO 3 (2.75 eV), [76] have been hybridized with TiO 2 , leading to large enhancements in PEC OER performance.…”