摘要
When a proton reduction cocatalyst is loaded on an n-type semiconductor for photocatalytic overall water splitting(POWS),the location of water oxidation sites is generally considered at the surface of the semiconductor due to upward band-bending of n-type semiconductor which may ease the transfer of the photogenerated holes to the surface.However,this is not the case for Pt/SrTiO_(3),a model semiconductor based photocatalyst for POWS.It was found that the photogenerated holes are more readily accumulated at the interface between Pt cocatalyst and SrTiO_(3) photocatalyst as probed by photo-oxidative deposition of PbO_(2),indicating that the water oxidation sites are located at the interface between Pt and SrTiO_(3).Electron paramagnetic resonance and scanning transmission electron microscope studies suggest that the interfacial oxygen atoms between Pt and SrTiO_(3) in Pt/SrTiO_(3) after POWS are more readily lost to form oxygen vacancies upon vacuum heat treatment,regardless of Pt loading by photodeposition or impregnation methods,which may serve as additional support for the location of the active sites for water oxidation at the interface.Density functional theory calculations also suggest that the oxygen evolution reaction more readily occurs at the interfacial sites with the lowest overpotential.These experimental and theoretical studies reveal that the more active sites for water oxidation are located at the interface between Pt and SrTiO_(3),rather than on the surface of SrTiO_(3).Hence,the tailor design and control of the interfacial properties are extremely important for the achievement or improvement of the POWS on cocatalyst loaded semiconductor photocatalyst.
光催化全分解水制氢是转换太阳能的理想途径之一.目前,实现光催化全分解水的半导体光催化剂多为n型半导体,并且需要担载助催化剂.当n型半导体担载产氢助催化剂时,由于能带弯曲,空穴更容易迁移至半导体的表面.因此,n型半导体的表面被认为是产氧活性位点.光催化全分解水过程中,水氧化半反应被认为是速率决定步骤,因此,深入认识水氧化活性位点意义重大.SrTiO_(3)是一种能够高效光催化全分解水的n型半导体光催化剂,Pt是一种常见的产氢助催化剂.本文以Pt/SrTiO_(3)为模型体系,对光催化全分解水过程中水氧化活性位点进行了研究.研究表明,光催化全分解水过程中水氧化活性位点主要位于Pt与SrTiO_(3)的界面处.首先,利用光氧化沉积实验研究了水氧化活性位点.光生空穴可以将Pb^(2+)氧化为PbO_(2),因此,可以利用电镜观察PbO_(2)的沉积位置,并推测出水氧化活性位点位置.扫描透射电镜结果表明,更多的PbO_(2)沉积在Pt与SrTiO_(3)的界面处.电子顺磁共振、热分析以及扫描透射电镜等结果表明,真空热处理Pt/SrTiO_(3)样品时,Pt与SrTiO_(3)界面处的氧原子更容易失去,同时伴随着氧空位的生成,该界面氧空位的生成,与Pt/SrTiO_(3)在真空热处理前的光催化全分解水过程密切相关,与助催化剂的担载方式无关.只有先经历光催化全分解水反应的Pt/SrTiO_(3),才更易生成界面氧空位.利用密度泛函理论对水氧化活性位点进行了理论计算研究,结果发现,当水氧化反应发生在SrTiO_(3)的表面时,第一个质子移除步骤是速率决定步骤,过电势为2.17 V;当水氧化反应发生在Pt与SrTiO_(3)的界面时,第三步是速率决定步骤,过电势仅为0.62 V.Pt与SrTiO_(3)界面处发生水氧化反应的过电势,远低于SrTiO_(3)表面发生水氧化反应的过电势.这表明水氧化活性位点主要位于界面处,理论计算结果也与实验结果一致.本文揭示了当n型半导体SrTiO_(3)担载产氢助催化剂Pt时,光催化全分解水过程中水氧化活性位点主要位于Pt与SrTiO_(3)的界面处.该结果加深了人们对产氢助催化与半导体界面的认识.界面不仅可以调控光生载流子的分离、迁移,也可提供光催化水氧化的活性位点.本文结果有助于设计和构建高效的全分解水光催化剂.
基金
国家重点研发计划(2017YFA0204800)
国家自然科学基金(21761142018,22088102).
