摘要
Electrochemical treatment is a popular and efficient method for improving the photoelectrochemical performance of water‐splitting photoelectrodes.In our previous study,the electrochemical activation of Mo‐doped BiVO_(4) electrodes was ascribed to the removal of MoO_(x) segregations,which are considered to be surface recombination centers for photoinduced electrons and holes.However,this proposed mechanism cannot explain why activated Mo‐doped BiVO_(4) electrodes gradually lose their activity when exposed to air.In this study,based on various characterizations,it is suggested that electrochemical treatment not only removes partial MoO_(x) segregations but also initiates the formation of H_(y)MoO_(x) surface defects,which provide charge transfer channels for photogenerated holes.The charge separation of the Mo‐doped BiVO_(4) electrode was significantly enhanced by these charge transfer channels.This study offers a new insight into the electrochemical activation of Mo‐doped BiVO_(4) photoanodes,and the new concept of surface charge transfer channels,a long overlooked factor,will be valuable for the development of other(photo)electrocatalytic systems.
理解和调控材料表面性质是研究非均相反应的重点.对于电化学和光电化学驱动的表面催化反应,构建表面活性位点并理解其相关的反应机理是众多研究人员关注的重点.除表面活性位点外,表面电荷传输通道的存在对于(光)电化学催化反应也同样重要,因为其决定了传输到(光)电极表面有效电荷的数量.然而,目前关于如何构建表面电荷传输通道的研究较少.本文以经典的钼掺杂钒酸铋光阳极为例,通过简单的电化学还原处理在其表面原位构建了电荷传输通道,进而实现了水氧化光电流密度约8.5倍的提升.通过对比电化学处理前后钼掺杂钒酸铋及氧化钼电极的X射线衍射图谱、拉曼光谱、X射线光电子能谱发现,电化学处理诱导了电极表面的羟基化反应并且产生了Mo^(5+)组分.这表明电化学处理诱导的氢钼青铜相(H_(y)MoO_(x))可能在钼掺杂钒酸铋光阳极表面形成了高效的电荷传输通道.包括电荷分离和注入效率、开路电压、阻抗谱以及稳态和瞬态荧光光谱等(光)电化学测试结果表明,这些表面电荷传输通道的存在大幅度提高了钼掺杂钒酸铋光阳极的体相电荷分离和传输效率.进一步通过构建电化学处理前后钼掺杂钒酸铋和氧化钼电极的能带结构,本文提出了电化学激活钼掺杂钒酸铋光阳极的内在机理,即:钼掺杂钒酸铋电极表面存在氧化钼偏析相,并严重阻碍了空穴的有效传输,电化学处理实现了氧化钼空穴阻挡层向H_(y)MoO_(x)空穴传导层的转变,其中H_(y)MoO_(x)的缺陷能级构成了光阳极和电解液之间的有效电荷传输通道.针对该体系,本课题组曾报道(J.Phys.Chem.C,2012,116,5076–5081)电化学处理消除了充当表面光生载流子复合中心的氧化钼偏析相,本文则进一步丰富并完善了之前的理解.此外,电化学处理构建的H_(y)MoO_(x)基表面电荷传输通道对于富氧环境敏感,在富氧环境中会发生氧化及去质子化反应,进而逐渐转变成阻碍空穴有效传输的氧化钼层,使得电化学激活的钼掺杂钒酸铋光电极失去活性.综上,本文明确指出构建表面电荷传输通道对实现高效(光)电化学催化反应是不可或缺的,而进一步理解和优化表面电荷传输通道也将为储能、传感、催化等其它领域的发展提供新机遇.
基金
国家重点研发计划(2018YFA0209303)
国家自然科学基金(51902153,U1663228,51972165)
江苏省自然科学基金(BK20202003).
