Abstract
Cadmium sulfide (CdS) as one of the most common visible-light-responsive photocatalysts has been widely investigated for hydrogen generation. However, its low solar–hydrogen conversion efficiency caused by fast carrier recombination and poor catalytic activity hinders its practical applications. To address this issue, we develop a novel and highly efficient nickel–cobalt phosphide and phosphate cocatalyst-modified CdS (NiCoP/CdS/NiCoPi) photocatalyst for hydrogen evolution. The dual-cocatalysts were simultaneously deposited on CdS during one phosphating step by using sodium hypophosphate as the phosphorus source. After the loading of the dual-cocatalysts, the photocurrent of CdS significantly increased, while its electrical impedance and photoluminescence emission dramatically decreased, which indicates the enhancement of charge carrier separation. It was proposed that the NiCoP cocatalyst accepts electrons and promotes hydrogen evolution, while the NiCoPi cocatalyst donates electrons and accelerates the oxidation of sacrificial agents (e.g., lactic acid). Consequently, the visible-light-driven hydrogen evolution of this composite photocatalyst greatly improved. The dual-cocatalyst-modified CdS with a loading content of 5 mol % showed a high hydrogen evolution rate of 80.8 mmol·g–1·h–1, which was 202 times higher than that of bare CdS (0.4 mmol·g–1·h–1). This is the highest enhancement factor for metal phosphide-modified CdS photocatalysts. It also exhibited remarkable stability in a continuous photocatalytic test with a total reaction time of 24 h.
Highlights
Using semiconductor materials to directly convert solar energy into hydrogen energy is regarded as a promising approach to the storage of renewable energy.[1−3] The practical application of this technology requires the design and development of lowcost and highly active photocatalysts, which usually consist of a semiconductor as a solar light absorber and a noble metal as a hydrogen evolution cocatalyst.[4−6] During the photocatalytic process, the semiconductor harvests and converts solar light and generates electron−hole pairs that can be spatially separated by the in-built electric field at the metal−semiconductor heterojunction.[2]
It has previously been reported that sodium hypophosphite can decompose into low valence state products and high valence state products at an elevated temperature of ≥150 °C via disproportionation reactions.[37]
Shi et al showed that pure metal phosphides (MP) only formed in a reaction temperature range of 150−200 °C and/or at a high molar ratio (≥3) of hypophosphate to metal precursor.[34,37]
Summary
Using semiconductor materials to directly convert solar energy into hydrogen energy is regarded as a promising approach to the storage of renewable energy.[1−3] The practical application of this technology requires the design and development of lowcost and highly active photocatalysts, which usually consist of a semiconductor as a solar light absorber and a noble metal (e.g., gold, platinum, or palladium) as a hydrogen evolution cocatalyst.[4−6] During the photocatalytic process, the semiconductor harvests and converts solar light and generates electron−hole pairs (i.e., charge carriers) that can be spatially separated by the in-built electric field at the metal−semiconductor heterojunction.[2]. It has recently been demonstrated that a double-heterojunction photocatalytic system containing both reduction and oxidation cocatalysts exhibited better photocatalytic hydrogen evolution performance than their single-cocatalyst counterparts.[42−45] For example, Yang et al reported a dual-cocatalyst Pt/CdS/PdS system with Pt and PdS as reduction and oxidation cocatalysts, respectively, which showed activity 1.76 times higher than that of Pt/CdS.[42] More recently, Wei et al developed a new dual-cocatalyst system by replacing precious metal-containing redox cocatalysts with low-cost carbon dots and NiS, which exhibited 5.4 time activity enhancement, compared with pristine CdS.[44]. The resulting photocatalyst showed a hydrogen evolution rate of 80.8 mmol·g−1·h−1 under visible light irradiation (>420 nm), 202 times and 17 times higher than those of pristine CdS (0.4 mmol·g−1·h−1) and Pt/CdS (7.6 mmol·g−1·h−1), respectively
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