Simultaneously Enhanced Charge Separation and Transfer in Cocatalyst-Free Hematite Photoanode by Mo/Sn Codoping

It is well-known that sluggish surface charge transfer on a cocatalyst-free hematite photoanode limits its solar conversion efficiency in photoelectrochemical (PEC) water splitting. Here, Mo is used as a codopant synergistically with Sn in α-Fe2O3 to solve this problem. Besides causing a slight narrowing of bandgap, morphological change, and even sublimation loss of α-Fe2O3, Mo doping also imports low-valent Fe due to charge compensation with MoV/VI as indicated by X-ray photoelectron spectroscopy and Bader charge computation, which increases the densities of donor and surfaces states. Although rate law analysis demonstrates slight retardation of surface reaction kinetics, strongly inhibited charge recombination in surface states by Mo doping still contributes to improving the photocurrent density and reducing the onset potential of α-Fe2O3 and Sn–Fe2O3 photoanodes. An optimized Mo/Sn–Fe2O3 photoanode can realize a low onset potential of 0.68 V vs a reversible hydrogen electrode (VRHE) and a photocurrent density of 1.97 mA cm–2 at 1.23 VRHE, enhanced by 58% and 20 times compared to Sn–Fe2O3 and α-Fe2O3, respectively. It is demonstrated that Mo doping promotes charge transfer which differs from most traditional n-type dopants that facilitate charge separation but inhibit charge transfer. This report expands the n-type dopant family of α-Fe2O3 for efficient PEC water splitting.众所周知，无助催化剂的赤铁矿光阳极上迟缓的表面电荷转移，限制了其在光电化学（PEC）水分解中的太阳能转换效率。本研究采用钼（Mo）与锡（Sn）在α-三氧化二铁（α-Fe₂O₃）中进行协同共掺杂，以解决这一难题。除了会使α-Fe₂O₃的禁带宽度略微窄化、引发形貌变化，甚至造成升华损耗外，通过X射线光电子能谱（XPS）与巴德电荷计算可知，钼掺杂还会因与Mo(V)/Mo(VI)发生电荷补偿作用而引入低价态铁，进而提升施主态与表面态的密度。尽管速率定律分析表明，表面反应动力学略有延缓，但钼掺杂对表面态处电荷复合的强烈抑制作用，仍提升了α-Fe₂O₃与Sn掺杂α-Fe₂O₃（Sn–Fe₂O₃）光阳极的光电流密度，并降低了其起始电位。经优化的钼锡共掺杂α-Fe₂O₃（Mo/Sn–Fe₂O₃）光阳极可实现0.68 V 相对于可逆氢电极（VRHE）的低起始电位，以及在1.23 VRHE下1.97 mA·cm⁻²的光电流密度，分别较Sn–Fe₂O₃与纯α-Fe₂O₃提升了58%与20倍。研究表明，与大多数仅促进电荷分离却抑制电荷转移的传统n型掺杂剂不同，钼掺杂可促进电荷转移。本研究拓展了可用于高效光电化学水分解的α-Fe₂O₃的n型掺杂剂家族。