Abstract Photocatalytic generation of hydrogen (H2) using semiconductor junctions is an effective and sustainable way to produce H2. One challenge in photocatalytic systems is to promote an effective separation of charges and reduce charge recombination to enhance overall performance of the photocatalytic system. In the present work, MoS2- TiO2-reduced graphene oxide (MTG) and CeO2-Ce2Ti3O8.7-TiO2-reduced graphene oxide (CTG) composites were synthesized through hydrothermal route using a KOH pretreatment on the TiO2 nanoparticles before Mo or Ce incorporation, in order to increase reactivity and defects such as oxygen vacancies. The structural analysis done by X-ray diffraction and Raman spectroscopy reveals the formation of KTi8O16.5 traces, TiO2 and rGO in MTG as well as rGO, CeO2, Ce2Ti3O8.7 and TiO2 phases in CTG. From high resolution transmission electron micrographs, the composite phases were identified. In MTG, well defined graphene, TiO2 and MoS2 phases were observed. In CTG, CeO2 nanoparticles nucleated onto Ce2Ti3O8.7. XPS analysis reveals the presence of oxygen vacancies and Ti3+ in TiO2 both in CTG and MTG, Mo2+ and Mo4+ in MTG, and Ce4+ and Ce3+ in CTG, respectively. From optical absorption, band gap of 3.3 eV and 3.4 eV was found for CTG and MTG respectively. CTG shows an extended absorption tail that would arise from Ce3+ induced defects within the band gap. Photoluminescence confirmed the presence of MoS2 and defect states both in MTG and CTG. From the structural, chemical and optical data, electronic band diagrams are proposed to explain the mechanisms of H2 evolution in the composites. The homojunction due to the presence of rutile and anatase in the commercial TiO2 nanoparticles improves charge separation in TiO2. The oxygen vacancies in Ce2Ti3O8.7 and Ti3+ in TiO2 as well as the sulphur vacancies in MoS2 create interband defect states below conduction bands of the respective semiconductors that trap the photoelectron, which leads to prolonging the lifetime of charge carriers, resulting in reduced charge recombination. The presence of defect states in the TiO2–Ce2Ti3O8.7-CeO2 interfaces create extended absorption below the TiO2 bandgap. The presence of the graphene boosted charge transport in composites and acted as a co-catalyst to photogenerate the H2, presumably because the work function value of rGO with respect to that of H2 evolution reaction as well as to its electron donor character. Observed H2 evolution rates in MTG and CTG were 363.83 μmolg−1h−1and 355.9 μmolg−1h−1 respectively, under 254 nm illumination. The photocatalytic activity of the CTG composite was reported for the first time. The KOH pretreatment on TiO2 nanoparticles effectively increased the H2-photogeneration with respect to previous reports.

中文翻译：

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氧化还原态和结在 MoS2-TiO2-rGO 和 CeO2-Ce2Ti3O8.7-TiO2-rGO 复合材料光催化产氢中的作用

摘要 利用半导体结光催化制氢是一种有效且可持续的制氢方式。光催化系统中的一个挑战是促进电荷的有效分离并减少电荷复合以提高光催化系统的整体性能。在目前的工作中，MoS2-TiO2 还原氧化石墨烯 (MTG) 和 CeO2-Ce2Ti3O8.7-TiO2 还原氧化石墨烯 (CTG) 复合材料是通过水热法合成的，在 Mo 或 Ce 掺入之前对 TiO2 纳米颗粒进行 KOH 预处理，以增加反应性和缺陷，如氧空位。通过 X 射线衍射和拉曼光谱进行的结构分析表明，在 MTG 中形成了 KTi8O16.5 痕量、TiO2 和 rGO，以及在 CTG 中形成了 rGO、CeO2、Ce2Ti3O8.7 和 TiO2 相。从高分辨率透射电子显微照片中，确定了复合相。在 MTG 中，观察到明确的石墨烯、TiO2 和 MoS2 相。在 CTG 中，CeO2 纳米颗粒在 Ce2Ti3O8.7 上成核。XPS 分析表明，CTG 和 MTG 中的 TiO2 中存在氧空位和 Ti3+，MTG 中分别存在 Mo2+ 和 Mo4+，CTG 中分别存在 Ce4+ 和 Ce3+。从光吸收来看，CTG 和 MTG 的带隙分别为 3.3 eV 和 3.4 eV。CTG 显示了由 Ce3+ 引起的带隙内缺陷引起的扩展吸收尾。光致发光证实了 MTG 和 CTG 中 MoS2 和缺陷态的存在。根据结构、化学和光学数据，提出了电子能带图来解释复合材料中 H2 的演化机制。由于商业二氧化钛纳米粒子中金红石和锐钛矿的存在，同质结改善了二氧化钛中的电荷分离。Ce2Ti3O8.7 中的氧空位和 TiO2 中的 Ti3+ 以及 MoS2 中的硫空位在各自半导体的导带下方产生带间缺陷态，捕获光电子，从而延长载流子的寿命，从而减少电荷复合. TiO2-Ce2Ti3O8.7-CeO2 界面中缺陷态的存在会在 TiO2 带隙以下产生扩展的吸收。石墨烯的存在促进了复合材料中的电荷传输，并充当了光生成 H2 的助催化剂，这可能是因为 rGO 的功函数值相对于 H2 演化反应的功函数值及其电子供体特征。在 254 nm 光照下，MTG 和 CTG 中观察到的 H2 释放速率分别为 363.83 μmolg-1h-1 和 355.9 μmolg-1h-1。首次报道了CTG复合材料的光催化活性。与之前的报道相比，TiO2 纳米颗粒上的 KOH 预处理有效地增加了 H2 光生。