光电化学(PEC)分解水是一种清洁可持续的获取氢燃料的方法,其中产氧半反应(OER)是制约整个水分解过程效率的关键步骤.因此,光阳极的性能是决定太阳能到氢能转化效率的关键因素.在各种水氧化光阳极材料中,赤铁矿(α-Fe2O3)因具有良好的化学稳定性、合适的带隙(～2.1 eV)、无毒、储量丰富等优点而成为最有前途的光阳极材料之一.然而,α-Fe2O3丰富的受体表面态和缓慢的水氧化动力学导致光生电荷复合严重,限制了其在光电化学中的实际应用.因此,有必要对α-Fe2O3进行表面工程设计以提高水氧化效率.本文提出了一种新方法,以金属有机框架(Ti-MOFs)为模板,在Ti-Fe2O3表面煅烧合成TiO2层,然后将富活性位点的ZIF-67加载在TiO2/Ti-Fe2O3上作为助催化剂,制备出具有较好光电化学性能的ZIF-67/TiO2/Ti-Fe2O3复合光阳极.X射线衍射、高分辨透射电镜、X射线光电子能谱和拉曼光谱等表征结果证实成功合成了ZIF-67/TiO2/Ti-Fe2O3.同时,氮气等温吸附脱附曲线和表面接触角测试结果表明,MOFs衍生的TiO2为介孔材料.采用表面光伏技术、光致发光光谱、飞秒-瞬态吸收光谱和电化学阻抗谱分析,研究了光生电荷的分离和复合行为.结果表明,MOFs衍生的TiO2不仅可以作为钝化层有效抑制了表面复合,还作为Ti-Fe2O3的电子阻挡层,显著减少了电子向表面的流失,从而大大提高了Ti-Fe2O3表面和体相的电荷分离效率.进一步的累积电荷量测试、电化学阻抗谱和Bode图分析显示,负载MOFs衍生TiO2后,可以明显促进光生空穴向电解质的注入,其多孔结构也可以增加反应接触面积,这有利于光生电荷在固液界面传输.此外,理论计算结果表明,Ti-Fe2O3水氧化速控步骤的能垒(△G=3.38 eV)明显高于TiO2(△G=1.67 eV),说明OER更容易在TiO2/Ti-Fe2O3表面发生,这与其光电流密度结果一致.为进一步提高反应活性和加快水氧化动力学,负载助催化剂ZIF-67后,ZIF-67/TiO2/Ti-Fe2O3复合光阳极实现了较好的光电化学性能,其在1.23 V vs.RHE时光电流密度高达4.04 mA cm-2,是Ti-Fe2O3的9.3倍,并且复合光阳极的入射光子电流转换效率和空穴注入效率分别达到93％(390 nm)和91％.综上所述,本研究通过MOFs衍生的TiO2和ZIF-67助催化剂改性α-Fe2O3光阳极,显著提升了其光电化学水氧化性能.其中,MOFs衍生TiO2不仅优化了电荷分离,还促进了光生空穴的注入,从而显著提高其光电化学水氧化性能.本研究为构筑高性能的有机-无机杂化光阳极提供了新思路.
Unveiling the multiple effects of MOF-derived TiO2 on Ti-Fe2O3 photoanodes for efficient and stable photoelectrochemical water oxidation
α-Fe2O3 is a promising photoanode that is limited by its high surface charge recombination and slow water oxidation kinetics.In this study,we synthesized a TiO2 layer on Ti-Fe2O3 by annealing Ti-MOFs,followed by ZIF-67 as a co-catalyst,to fabricate a ZIF-67/TiO2/Ti-Fe2O3 photoanode for photoelectrochemical(PEC)water splitting.The systematic experimental and theoretical results revealed that the improvement in performance was due to multiple effects of the MOF-derived TiO2.This molecule not only passivates the acceptor surface states of Ti-Fe2O3,thereby reducing the number of surface recombination centers,but also acts as an electron barrier to promote charge separation in the Ti-Fe2O3 bulk.Moreover,MOF-derived TiO2 can dramatically reduce the energy barrier for the OER of Ti-Fe2O3,thus promoting the conversion of the intermediate*OH into*O.The synergistic improvement in the bulk and surface properties effectively enhanced the water oxida-tion performance of Ti-Fe2O3.The ZIF-67/TiO2/Ti-Fe2O3 photoanode exhibits a photocurrent densi-ty of up to 4.04 mA cm-2 at 1.23 V vs.RHE,which is 9.4 times as that of pure Ti-Fe2O3,and has long-term stability.Our work provides a feasible strategy for constructing efficient organ-ic-inorganic hybrid photoelectrodes.
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