An innovative synthesis strategy for high-efficiency and defects-switchable-hydrogenated TiO2 photocatalysts

Hydrogenated TiO2 (H-TiO2) is one of the most promising TiO2-based photocatalysts. Through effectively narrowing the band gap of TiO2 and restraining the recombination of photogenerated carriers, in the January issue of Matter, Cui et al. reported a new and facile one-pot wet-chemistry strategy to prepare H-TiO2 for highly efficient photocatalytic hydrogen (H2) evolution.1Jia G. Wang Y. Cui X. Zhang H. Zhao J. Li L.H. Gu L. Zhang Q. Zheng L. Wu J. et al.Wet-chemistry hydrogen doped TiO2 with switchable defects control for photocatalytic hydrogen evolution.Matter. 2021; 5: 206-218Abstract Full Text Full Text PDF Scopus (35) Google Scholar Hydrogenated TiO2 (H-TiO2) is one of the most promising TiO2-based photocatalysts. Through effectively narrowing the band gap of TiO2 and restraining the recombination of photogenerated carriers, in the January issue of Matter, Cui et al. reported a new and facile one-pot wet-chemistry strategy to prepare H-TiO2 for highly efficient photocatalytic hydrogen (H2) evolution.1Jia G. Wang Y. Cui X. Zhang H. Zhao J. Li L.H. Gu L. Zhang Q. Zheng L. Wu J. et al.Wet-chemistry hydrogen doped TiO2 with switchable defects control for photocatalytic hydrogen evolution.Matter. 2021; 5: 206-218Abstract Full Text Full Text PDF Scopus (35) Google Scholar Efficiently transforming solar energy to hydrogen energy has achieved growing attention for sustainable society.2Liu G. Ma L. Yin L.-M. Wan G.D. Zhu H.Z. Zhen C. Yang Y.Q. Liang Y. Tan J. Chen H.-M. Selective chemical epitaxial growth of TiO2 islands on ferroelectric PbTiO3 crystals to boost photocatalytic activity.Joule. 2018; 2: 1095-1107Abstract Full Text Full Text PDF Scopus (86) Google Scholar, 3Liao G.F. Gong Y. Zhang L. Gao H.Y. Yang G.-J. Fang B.Z. Semiconductor polymeric graphitic carbon nitride photocatalysts: the “holy grail” for the photocatalytic hydrogen evolution reaction under visible light.Energy Environ. Sci. 2019; 12: 2080-2147Crossref Google Scholar, 4Liao G.F. Li C.X. Li X.Z. Fang B.Z. Emerging polymeric carbon nitride Z-scheme systems for photocatalysis.Cell Rep. Phys. Sci. 2021; 2: 100355Abstract Full Text Full Text PDF Scopus (102) Google Scholar As a widely used semiconductor, TiO2 has exhibited many distinctive advantages including excellent stability, moderate cost, and low toxicity. However, TiO2 also possesses some inherent disadvantages such as wide band gap, rapid recombination of the photogenerated electron-hole, etc., which result in an ultraviolet-light-driven photocatalyst with low activity.5Cai J.S. Shen J.L. Zhang S.N. Ng Y.H. Huang J.Y. Guo W.X. Lin C.J. Lai Y.K. Light-driven sustainable hydrogen production utilizing TiO2 nanostructures: A review.Small Methods. 2019; 3: 1800184Crossref Scopus (138) Google Scholar Therefore, sustained efforts are urgently necessary for the development and exploration of some effective regulation technologies of materials/structures/properties and achieving in-depth understanding of structure-activity relationship. Defect engineering (e.g., doping and vacancies) is a promising strategy for adjusting the band structure, light absorption, and photocatalytic activity of TiO2.6Zhang W. He H. Li H. Duan L. Zu L. Zhai Y. Li W. Wang L. Fu H. Zhao D.Y. Visible-light responsive TiO2-based materials for efficient solar energy utilization.Adv. Energy Mater. 2021; 11: 2003303Crossref Scopus (87) Google Scholar An in-depth exploration and analysis is beneficial to build fundamental aspects and design guidance of charge generation, transfer, and recombination in TiO2 photocatalyst. Hydrogen doping of TiO2 (H-TiO2) has achieved worldwide attention for adjusting light absorption, band structure, and photocatalytic activity. However, it often needs harsh reaction conditions (e.g., high pressure and high temperature) and noble metal catalysts to prepare H-TiO2. In addition, the hydrogen sources are also difficult to obtain because of the high energy barrier of dissociating H2 molecules into H atoms.7Xie L. Zhu Q. Zhang G. Ye K. Zou C. Prezhdo O.V. Wang Z. Luo Y. Jiang J. Tunable hydrogen doping of metal oxide semiconductors with acid–metal treatment at ambient conditions.J. Am. Chem. Soc. 2020; 142: 4136-4140Crossref PubMed Scopus (48) Google Scholar,8Selcuk S. Zhao X. Selloni A. Structural evolution of titanium dioxide during reduction in high-pressure hydrogen.Nat. Mater. 2018; 17: 923-928Crossref PubMed Scopus (84) Google Scholar Consequently, it is of great importance and a grand challenge to develop H-TiO2 with controllable defects through a facile and economical strategy under mild conditions. Among different synthetic methods for H-TiO2, the in situ wet-chemistry method is highly worth considering due to its high simplicity, effectiveness, versatility, low cost, low toxicity, and mild condition.9Jin Q. Wen W. Zheng S. Jiang R. Wu J.-M. Branching TiO2 nanowire arrays for enhanced ethanol sensing.Nanotechnology. 2021; 32: 295501Crossref Scopus (14) Google Scholar Inspiringly, Cui et al. reported a facile in situ wet-chemistry strategy to fabricate H-TiO2 with switchable defects control, which demonstrated very high photocatalytic H2 production activity1Jia G. Wang Y. Cui X. Zhang H. Zhao J. Li L.H. Gu L. Zhang Q. Zheng L. Wu J. et al.Wet-chemistry hydrogen doped TiO2 with switchable defects control for photocatalytic hydrogen evolution.Matter. 2021; 5: 206-218Abstract Full Text Full Text PDF Scopus (35) Google Scholar (Figure 1A), wherein the H doping of H-TiO2 can be switched into O vacancies with Ar annealing and conversely recovered into Ti-O with O2 annealing. These H doping and O vacancies are confirmed by nuclear magnetic resonance (NMR) data (Figure 1B). Raman spectra illustrated that the Eg peaks of H-TiO2 and H-TiO2-Ar shift and broaden, suggesting that the original symmetry of TiO2 lattice was broken because of the O vacancy in H-TiO2-Ar and the H doping in H-TiO2 (Figure 1C). Switchable defects control (switching from the H doping to O vacancies) can be confirmed by aberration-corrected annular dark field scanning transmission electron microscopy (ADF-STEM) (Figure 1D). Electron energy loss spectroscopy (EELS) indicated that H atoms inside the lattice would be removed after high temperature treatment (Figure 1E).10Guo Y. Chen S. Yu Y. Tian H. Zhao Y. Ren J.-C. Huang C. Bian H. Huang M. An L. et al.Hydrogen-location-sensitive modulation of the redox reactivity for oxygen-deficient TiO2.J. Am. Chem. Soc. 2019; 141: 8407-8411Crossref PubMed Scopus (50) Google Scholar All of these data confirmed the successful fabrication of H-TiO2 with switchable defects control, which was expected to have excellent photocatalytic performance. Photocatalytic performance including activity, stability, and light absorption ability was further investigated under UV-vis irradiation (380–780 nm) with 20 vol% MeOH and 1.78 wt% Pt co-catalyst. The photocatalytic H2 evolution rate of H-TiO2 was as high as 142.9 μmol h−1, exceeding 60 times that of C-rutile (2.4 μmol h−1) and 2.7 times that of P25 (53.3 μmol h−1) (Figure 1G), which outperforms the majority of the previously reported TiO2-based photocatalysts. Moreover, their activity remained 99.4% of the initial value after 3 months of storage and eight cycles, implying outstanding photocatalytic stability (Figure 1H). In addition, H-TiO2 exhibited the apparent quantum yield (AQY) of 22.3% (380 nm) and also showed excellent photocatalytic performance in the visible region with AQY of 2.2% at 600 nm (Figure 1I). All of the data illustrated excellent activity, stability, and light absorption ability of H-TiO2. In order to further explain the enhanced mechanism of H doping, density functional theory (DFT) calculations were proposed. H-TiO2 displayed a stronger charge distribution asymmetry, resulting in the mismatch between the positive and negative charge centers, which indicated that the charge density for H-TiO2 was redistributed after H doping (Figure 1J), and thus charge separation was accelerated. A new intermediate bandgap state appeared at 1.15 eV lower than the conduction band minimum (Figure 1K), and a hybridized characteristic was also found in calculated density of states (DOS) (Figures 1L and 1M). This hybridized feature is very beneficial for the migration of photogenerated charges. Therefore, the modified electronic and band structures of H-TiO2 can significantly boost charge separation and further promote H2 production. This work not only presents the uniqueness and superiority (switchable defects control) of H-TiO2 in photocatalytic H2 production, but more importantly provides a new and facile one-pot wet-chemistry strategy for controllable construction of H-TiO2. The formation of metal-H bonds and the controllable defect engineering of metal oxides7Xie L. Zhu Q. Zhang G. Ye K. Zou C. Prezhdo O.V. Wang Z. Luo Y. Jiang J. Tunable hydrogen doping of metal oxide semiconductors with acid–metal treatment at ambient conditions.J. Am. Chem. Soc. 2020; 142: 4136-4140Crossref PubMed Scopus (48) Google Scholar,10Guo Y. Chen S. Yu Y. Tian H. Zhao Y. Ren J.-C. Huang C. Bian H. Huang M. An L. et al.Hydrogen-location-sensitive modulation of the redox reactivity for oxygen-deficient TiO2.J. Am. Chem. Soc. 2019; 141: 8407-8411Crossref PubMed Scopus (50) Google Scholar are very promising for effectively regulating the electronic and band structures of oxide semiconductors for future high-performance photocatalysts. Although this work obtains a low-cost and high-activity H-TiO2 photocatalyst, the issue of wide band gap still remains unresolved. Therefore, in addition to the regulation of electronic and band structures, more efforts need to be made to explore visible-light-driven H-TiO2 photocatalysts through some promising strategies, especially heterojunction construction, in the future. In order to enhance visible-light absorption, some sensitizers such as Eosin Y, Erythrosin B, Zinc phthalocyanine, etc. are highly worth considering for fabricating H-TiO2/sensitizers heterojunction, which can also simultaneously improve photocatalytic activity. Meanwhile, substituting the cocatalysts and sacrificial agents with other materials and adjusting the surface active sites are also very important to increase the photocatalytic efficiency. Inspired by this work, more experimental and theoretical investigations on H-TiO2 are expected to improve the understanding of the regulation of electronic and band structures and building-up of the fundamentals of charge transfer regulation in H-TiO2. This work was supported by the Startup Funding for Scientific Research of China University of Geosciences (Wuhan). The authors declare no competing interests. Wet-chemistry hydrogen doped TiO2 with switchable defects control for photocatalytic hydrogen evolutionJia et al.MatterNovember 16, 2021In BriefHydrogenated TiO2 (H-TiO2) has been verified as a promising photocatalytic material. The conventional H-TiO2 is mainly prepared by posttreatment. A new wet-chemistry strategy is developed by synthesizing a hydrogenated TiO2 by one-step chemical method in alcoholic solution. The H-TiO2 shows a 60-fold improvement compared with commercial rutile. Also, the H-TiO2 exhibits unprecedented defects engineering ability, that is, TiO2 with or without oxygen vacancies could be separately obtained by annealing in Ar or O2 atmosphere, respectively. Full-Text PDF Open Archive