Effects of Defects on Photocatalytic Activity of Hydrogen-Treated Titanium Oxide Nanobelts

Abstract

Previous studies have shown that hydrogen treatment leads to the formation of blue to black TiO2, which exhibits photocatalytic activity different from that of white pristine TiO2. However, the underlying mechanism remains poorly understood. Herein, density functional theory is combined with comprehensive analytical approaches such as X-ray absorption near edge structure spectroscopy and transient absorption spectroscopy to gain fundamental understanding of the correlation among the oxygen vacancy, electronic band structure, charge separation, charge carrier lifetime, reactive oxygen species (ROS) generation, and photocatalytic activity. The present work reveals that hydrogen treatment results in chemical reduction of TiO2, inducing surface and subsurface oxygen vacancies, which create shallow and deep sub-band gap Ti­(III) states below the conduction band. This leads to a blue color but limited enhancement of visible light photocatalytic activity up to 440 nm at the cost of reduced ultraviolet photocatalytic activity. The extended light absorption spectral range for reduced TiO2 is ascribed to both the defect-to-conduction band transitions and the valence band-to-defect transitions. The photogenerated charge carriers from the defect states to the conduction band have lifetimes too short to drive photocatalysis. The Ti­(III) deep and shallow trap states below the conduction band are also found to reduce the lifetime of photogenerated charge carriers under ultraviolet light irradiation. The ROS generated by the reduced TiO2 are less than those generated by pristine TiO2. Consequently, the reduced TiO2 exhibits ultraviolet-responsive photocatalytic activity worse than that of pristine TiO2. This report shows that increasing the light absorption spectral range of a semiconductor by doping or introduction of defects does not necessarily guarantee an increase in photocatalytic activity.

Engineering oxygen vacancy on rutile TiO2 for efficient electron-hole separation and high solar-driven photocatalytic hydrogen evolution

“…Because the rutile TiO 2 is extremely stable thermodynamically and has less surface defects than the anatase TiO 2 , constructing rutile TiO 2 with surface defects is promising to have enhanced photocatalytic activity. Surface oxygen vacancy (V O ), as one of the most important surface defects of TiO 2 , receives extensive attention [31][32][33][34][35]. Too low or high content of V O defects are not conducive to narrowing the bandgap and improving the separation efficiency of photogenerated electron-hole pairs.…”

Noble Metal Free, Visible Light Driven Photocatalysis Using TiO2 Nanotube Arrays Sensitized by P‐Doped C3N4 Quantum Dots

“…However, the presence of defects and mid‐gap states that originated in the anodization process and were not completely healed during annealing, enables longer wavelength absorption which is evident from the absorption band at 450 nm in the UV–vis spectrum of TNA 43a,50. However, these traps are detrimental to the water splitting process due to the annihilation of photogenerated charge carriers via trap assisted recombination . This explained the low observed photocurrent density and low ABPE for TNA samples under irradiation from the 450 nm LED (blue curve in Figure b,e).…”

“…Recently plasmonic materials coupled with semiconductor or graphenic materials has shown wide potential in plasmon‐exciton (collectively called “plexciton”) co‐driven surface catalytic reaction due to a synergistic improvement in plasmon‐to‐electron conversion efficiency . In this venture, plasmonic nanoparticles (NPs) integrated with semiconductor photocatalysts such as Cu 2 O, MoS 2 , TiO 2 ,50b,51 graphitic carbon nitride (g‐C 3 N 4 ), doped and undoped graphene,53d,57 etc. have been explored for the surface photocatalytic conversion of aromatic thiols into azo compounds .…”

SnS2 Nanosheets/H‐TiO2 Nanotube Arrays as a Type II Heterojunctioned Photoanode for Photoelectrochemical Water Splitting

“…Electron-hole recombination was suppressed with the semiconductor heterojunction of discontinuous band edges at the interface to improves eparation of photoexcited chargec arriers. In comparison, the adsorbed hydrogen impurity in H-TiO 2 createdd elocalized electron carriers with Ti 3d character at the bottom of the conductionb and, [36] which led to ac onduction-band tail in H-TiO 2 . [33] The CPD of the SnS 2 nanosheet was highert han that of TiO 2 ,i mplying that the work function of TiO 2 was larger than that of SnS 2 .T he values for the valence-band edge and the band gap were À7.4 and 3.33 eV for TiO 2 ,r espectively, [34] and À6.44 and 2.26 eV for SnS 2 .…”