Critical factors for photoelectrochemical and photocatalytic H₂ evolution from gray anatase (001) nanosheets

Abstract

In recent years, defect engineering of titania via reduction treatments has shown a high promise for enabling efficient and co-catalyst-free photocatalytic H2 generation from methanol/water solutions. However defect engineering alters several properties of TiO2 simultaneously. Here we use pristine (white) and hydrogenated (grey) anatase nanosheets with dominant (001) facets and by comparing electrical conductivity, photocurrent spectra, transient photocurrent response, and photocatalytic H2 evolution, we show that the increased conductivity or broad visible light absorption of grey titania are not responsible for its increased activity. Instead, the true bottleneck is the hole transfer rate that is significantly accelerated when using the grey instead of the white modification. Moreover, the hole transfer reaction leads to accumulation of the reaction products in pure water, which causes hindering of the photocatalytic H2 evolution over time. These combined factors explain the superior performance of grey titania over white titania in photoelectrochemical or photocatalytic water splitting.

Metastable Ni(I)-TiO2–x Photocatalysts: Self-Amplifying H2 Evolution from Plain Water without Noble Metal Co-Catalyst and Sacrificial Agent

“…Consequently, the three forms of titania herein studied exhibit different density and nature of oxygen vacancies (Ti 3+ -O V ) and, thus, different electronic properties . We characterized these materials in view of their electronic and defect structure in previous work. ,,− Unlike low temperature treatments (e.g., 500 °C) that maintain anatase as the only crystalline phase, hydrogenation at 700 °C (to form the over-reduced black TiO 2 ) results in a mixed anatase–rutile composition . Among different “shades” of gray titania, i.e., thermally treated in H 2 at different reduction temperatures (300–700 °C), the powder treated at 500 °C for 1 h ( gray TiO 2 ) shows the highest photocatalytic activity (shown in our previous work).…”

Long‐Term Stability of Light‐Induced Ti3+ Defects in TiO2 Nanotubes for Amplified Photoelectrochemical Water Splitting

“…However, the surface amorphization become a matter of debate. For example, titania commercial powders with a rather rough surface exhibit surface amorphization, while single crystals [3,12] and hydrothermal powders with well‐defined facets do not exhibit, but only point defect formation [13] . Additionally, it was confirmed that different titania polymorphs show varied behaviors towards reductive treatments, i. e., brookite is more readily reduced compared to anatase and rutile [3] .…”

Pattern-illumination time-resolved phase microscopy and its applications for photocatalytic and photovoltaic materials

“…The dynamics of these photo-excited charge carriers are typically studied using techniques like time-resolved photoluminescence (TP), 40–45 transient absorption (TA), 20,33,45–58 and time-resolved microwave conductivity (TMC). 59,60 TP is sensitive to the emission of excited carriers but overlooks non-emissive processes.…”

“…38 In the process of water splitting in hematite charge carriers show a non-exponential decay with long lifetimes, which are dependent on the applied bias voltage. 39 The dynamics of these photo-excited charge carriers are typically studied using techniques like time-resolved photoluminescence (TP), [40][41][42][43][44][45] transient absorption (TA), 20,33,[45][46][47][48][49][50][51][52][53][54][55][56][57][58] and time-resolved microwave conductivity (TMC). 59,60 TP is sensitive to the emission of excited carriers but overlooks non-emissive processes.…”