Time-resolved infrared spectroscopic investigation of Ga_(2)O_(3) photocatalysts loaded with Cr_(2)O_(3)-Rh cocatalysts for photocatalytic water splitting

Cr_(2)O_(3)-Rh/Ga_(2)O_(3)光催化分解水的时间分辨红外光谱研究

Investigation of the charge dynamics and roles of cocatalysts is crucial for understanding the reaction of photocatalytic water splitting on semiconductor photocatalysts.In this work,the dynamics of photogenerated electrons in Ga_(2)O_(3) loaded with Cr_(2)O_(3)-Rh cocatalysts was studied using time-resolved mid-infrared spectroscopy.The structure of these Cr_(2)O_(3)-Rh cocatalysts was identified with high-resolution transmission electron microscopy and CO adsorption Fourier-transform infrared spectroscopy,as Rh particles partly covered with Cr_(2)O_(3).The decay dynamics of photogenerated electrons reveals that only the electrons trapped by the Rh particles efficiently participate in the H2 evolution reaction.The loaded Cr_(2)O_(3) promotes electron transfer from Ga_(2)O_(3) to Rh,which accelerates the electron-consuming reaction for H2 evolution.Based on these observations,a photocatalytic water-splitting mechanism for Cr_(2)O_(3)-Rh/Ga_(2)O_(3) photocatalysts has been proposed.The elucidation of the roles of the Cr_(2)O_(3)-Rh cocatalysts aids in further understanding the reaction mechanisms of photocatalytic water splitting and guiding the development of improved photocatalysts.

研究半导体光催化分解水反应中光生电荷动力学和助催化剂的作用对理解其反应机理至关重要.一般来说,助催化剂不仅可以促进半导体/助催化剂界面处的光生电荷高效分离,而且可以作为反应活性中心来直接催化表面氧化或还原反应.Cr_(2)O_(3)-Rh是一种重要的产氢助催化剂,通过担载Cr_(2)O_(3)-Rh助催化剂来提高光催化分解水的策略被应用到许多光催化分解水体系中.已有研究发现,Rh/Cr_(2)O_(3)核壳结构助催化剂的产氢活性位仍然在Rh纳米粒子表面,而Cr_(2)O_(3)壳层阻止O2到达Rh核从而抑制生成水的逆反应.此外,在(Rh_(2-y)Cr_(y)O_(3))/(Ga_(1-x)Zn_(x))(N_(1-x)O_(x))光催化剂中,CrO_(x)促进了从半导体光催化剂到活性位RhOx的电子转移.然而,Cr_(2)O_(3)-Rh助催化剂的作用本质(包括Cr_(2)O_(3)所起的作用)仍然是一个悬而未决的问题,特别是Cr_(2)O_(3)-Rh助催化剂的担载对半导体催化剂中光生电荷动力学影响的研究还非常少.本文采用原位光沉积的方法制备了Ga_(2)O_(3)、Rh/Ga_(2)O_(3)、Cr_(2)O_(3)/Ga_(2)O_(3)和Cr_(2)O_(3)-Rh/Ga_(2)O_(3)等一系列光催化剂;采用紫外可见漫反射光谱(UV-Vis DRS)、X射线光电子能谱(XPS)、CO吸附红外光谱和高分辨透射电镜(HRTEM)等表征手段研究了Cr_(2)O_(3)-Rh助催化剂的结构和形貌;采用时间分辨红外光谱(TR-MIR)研究了这些光催化剂在真空或者反应物(水汽或者氧气)存在条件下的光生电子的衰减动力学过程.UV-Vis DRS结果表明,Ga_(2)O_(3)的带隙基本上不受Rh或者Cr_(2)O_(3)-Rh助催化剂担载的影响.XPS结果表明,Cr_(2)O_(3)和Rh成功地担载在Ga_(2)O_(3)表面上.CO吸附红外和HRTEM结果表明,在Cr_(2)O_(3)-Rh助催化剂中Rh纳米粒子被Cr_(2)O_(3)部分覆盖.光生电子的衰减动力学研究结果显示,Ga_(2)O_(3)中光生电子很难直接参与质子还原反应,只有被Rh捕获后的电子才能高效地参与产氢反应;在水汽存在条件下Ga_(2)O_(3)、Rh/Ga_(2)O_(3)和Cr_(2)O_(3)-Rh/Ga_(2)O_(3)中光生电子的衰减速率随着它们光催化产氢活性的升高而增加;与Cr_(2)O_(3)/Ga_(2)O_(3)和Rh/Ga_(2)O_(3)相比,Cr_(2)O_(3)-Rh/Ga_(2)O_(3)中光生电子的初始吸光度和寿命均减小,说明Cr_(2)O_(3)对Rh/Ga_(2)O_(3)的结构修饰促进了电子从Ga_(2)O_(3)向Rh的转移过程,从而加速了质子还原反应.最后,基于这些结果提出了Cr_(2)O_(3)-Rh/Ga_(2)O_(3)光催化剂上的光催化分解水机理.本文的研究结果有利于更加深入地认识半导体光催化分解水反应机理,并为高效半导体光催化剂的合成提供一定的理论支持和指导.