The role of redox states and junctions in photocatalytic hydrogen generation of MoS2-TiO2-rGO and CeO2-Ce2Ti3O8.7-TiO2-rGO composites

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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.

Original languageEnglish
Article number105185
JournalMaterials Science in Semiconductor Processing
Volume118
DOIs
StatePublished - 1 Nov 2020

Keywords

  • Hydrogen evolution
  • Interstate defects
  • Mixed phases
  • Oxygen vacancies
  • Photocatalysis
  • Reduced graphene oxide
  • Semiconductor heterojunction

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