Charge Carrier Dynamics in TiO₂ Mesocrystals with Oxygen Vacancies for Photocatalytic Hydrogen Generation under Solar Light Irradiation

Abstract

There is a great concern about black TiO 2 prepared by H 2 treatment because of its ability to enhance light harvesting of TiO 2 . Black TiO 2 shows different photocatalytic activities compared with white TiO 2 . However, the mechanism of photocatalytic reaction has not been clearly understood. Here, femtosecond time-resolved diffuse reflectance (fs-TRDR) spectroscopy and single-particle photoluminescence measurements were applied to gain better understanding about the relation between oxygen vacancy, charge transfer, lifetime of photogenerated charge, and photocatalytic activity. We prepared reduced TiO 2 mesocrystals (R-TMC) through simple solid-state chemical reduction at moderate temperature 350 °C. R-TMC has nearly two times higher photocatalytic activity for H 2 production under solar light irradiation. The presence of oxygen vacancies and Ti 3+ was studied by electro paramagnetic resonance and Xray photoelectron spectroscopy. This work confirms the findings of previous studies that enhancement of light absorption by formation of surface defects does not always lead to high photocatalytic activity. Femtosecond time-resolved diffuse reflectance (fs-TRDR) spectra reveal that proper concentration of oxygen vacancies enhances the charge separation of the photogenerated carriers, leading to high photocatalytic activity.

Titanium Dioxide (TiO2) Mesocrystals: Synthesis, Growth Mechanisms and Photocatalytic Properties

“…Additionally, by thermally treating the anatase TiO2 mesocrystals precipitated from the PEG-400/TiCl3 mixed solution in vacuum, our group successfully synthesized Ti 3+ self-doped, singlecrystal-like, spindle-shaped, anatase TiO2 mesocrystals [150]. Moreover, by reducing the presynthesized TiO2 mesocrystals with NaBH4, oxygen-deficient sheet-like anatase TiO2 mesocrystals were also synthesized [168]. Recently, the introduction of oxygen vacancies or Ti 3+ ions into TiO 2 to produce oxygen-deficient/Ti 3+ self-doped TiO 2 mesostructures has been well accepted to be one of the most efficient ways to extend the light absorption region of TiO 2 to visible light [162][163][164][165][166].…”

“…At the same time, this kind of doping also improves the electroconductivity of TiO 2 , thereby facilitating charge transportation within TiO 2 particles [162,164,167]. In this regard, great efforts have been made toward preparing oxygen-deficient/Ti 3+ self-doped TiO 2 mesocrystals [65,136,150,168]. A good example in this area is that Zhao's group reported a facile evaporation-driven oriented assembly route combined with post thermal treatment in N 2 atmosphere to fabricate ultrathin-nanosheet-assembled olive-shaped mesoporous anatase TiO 2 mesocrystals ( Figure 13) [65].…”

“…Additionally, by thermally treating the anatase TiO 2 mesocrystals precipitated from the PEG-400/TiCl 3 mixed solution in vacuum, our group successfully synthesized Ti 3+ self-doped, single-crystal-like, spindle-shaped, anatase TiO 2 mesocrystals [150]. Moreover, by reducing the pre-synthesized TiO 2 mesocrystals with NaBH 4 , oxygen-deficient sheet-like anatase TiO 2 mesocrystals were also synthesized [168]. mesocrystals first, and then hydrothermally treated them in aqueous niobium oxalate solution.…”

High photoresponse and fast carrier mobility: Two‐dimensional rGO‐AgBr/Ag composite based on Z‐scheme heterointerface with plasma for hydrogen evolution

“…21,22 Based on these problems, lots of works aim to improve the performance of traditional photocatalysts, such as heterostructure fabrication, [23][24][25] metal/nonmetal doping, 26,27 plasma introduction, 28,29 composite with conductive materials, 30,31 crystal regulation, 32,33 surface modification, 34,35 defect formation 36,37 and so on. 21,22 Based on these problems, lots of works aim to improve the performance of traditional photocatalysts, such as heterostructure fabrication, [23][24][25] metal/nonmetal doping, 26,27 plasma introduction, 28,29 composite with conductive materials, 30,31 crystal regulation, 32,33 surface modification, 34,35 defect formation 36,37 and so on.…”

“…However, there are some limitations for traditional photocatalysts due to the fast combination of photoinduced carriers and poor optical response in long wavelength region. 21,22 Based on these problems, lots of works aim to improve the performance of traditional photocatalysts, such as heterostructure fabrication, [23][24][25] metal/nonmetal doping, 26,27 plasma introduction, 28,29 composite with conductive materials, 30,31 crystal regulation, 32,33 surface modification, 34,35 defect formation 36,37 and so on. For instance, Jia et al 38 designed a threecomponent Z-scheme (BiO) 2 CO 3 -BiO 2 -x-GO composite with good light absorption performance and efficient electron hole separation performance.…”