Abstract
Octahedral anatase particles (OAPs) were prepared by an ultrasonication (US)-hydrothermal (HT) reaction of partially proton-exchanged potassium titanate nanowires (TNWs). The structural/physical properties of OAP-containing samples, including specific surface area, crystallinity, crystallite size, particle aspect ratio, composition and total OAP content, were analyzed. Photocatalytic activities of samples were measured under irradiation (>290 nm) for oxidative decomposition of acetic acid (CO2 system) and dehydrogenation of methanol (H2 system) under aerobic and deaerated conditions, respectively. Total density of electron traps (ETs) was measured by double-beam photoacoustic spectroscopy (DB-PAS). Mobility and lifetime of charge carriers (electrons) were investigated by the time-resolved microwave conductivity (TRMC) method. The effects of synthesis parameters, i.e., HT duration, HT temperature and US duration, on properties and photocatalytic activities of final products were examined in detail. The sample prepared with 1 h US duration and 6 h HT duration at 433 K using 267 mg of TNWs in 80 mL of Milli-Q water exhibited the highest photocatalytic activity. It was found that change in HT duration or HT temperature while keeping the other conditions the same resulted in changes in all properties and photocatalytic activity. On the other hand, duration of US treatment, before HT reaction, influenced the morphology of both the reagent (by TNWs breaking) and final products (change in total OAP content); samples prepared with various US durations exhibited almost the same structural/physical properties evaluated in this study but were different in morphology and photocatalytic activity. This enabled clarification of the correlation between morphology and photocatalytic activity, i.e., the higher the total OAP content was, the higher was the level of photocatalytic activity, especially in the CO2 system. Although the decay after maximum TRMC signal intensity (Imax) was almost constant for all samples used in this study, photocatalytic activities were roughly proportional to Imax, which tended to be proportional to total OAP content. Assuming that Imax corresponds to the product of density of electrons in mobile shallow ETs and their mobility, the results suggest that OAP particles have beneficial shallow ETs in higher density and thereby the OAP content governs the photocatalytic activities. Thus, morphology-dependent photocatalytic activity of OAP-containing particles was reasonably interpreted by density of ETs presumably located on the exposed {101} facets.
Highlights
ACatalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan.France cCNRS, Laboratory of Physical Chemistry, UMR 8000, 91405 Orsay, France † Electronic supplementary information (ESI) available
We have found that OAPcontaining products obtained with various durations of US prior to HT treatment showed variations in morphology and photocatalytic activity with all of the other properties remaining almost unchanged, and we concluded that their photocatalytic activities depend solely on morphology; the higher the Octahedral anatase particles (OAPs) content is, the higher is the photocatalytic activity
The composition of titanate nanowires (TNWs) was determined to be H1.32K0.68Ti8O17·4.0H2O, corresponding to partially proton-exchanged potassium titanate (PEPT), by calculation based on the assumption that TNWs consisted of H2−xKxTi8O17·yH2O without any titania (TiO2) component using water content (11.1% corresponding to 18.02(y + (2 − x)/2)/ {1.01(2 − x) + 39.10x + 8 × 47.87 + 17 × 16.00 + 18.02y}) and 12394 | Nanoscale, 2015, 7, 12392–12404
Summary
ACatalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan.France cCNRS, Laboratory of Physical Chemistry, UMR 8000, 91405 Orsay, France † Electronic supplementary information (ESI) available. Titanium(IV) oxide (titania) has been widely used for many years as a photocatalyst, presumably due to its advantages including high redox ability, stability and nontoxicity.[1,2,3] Another hidden reason for the popular use of titania as a photocatalyst is that titania shows appreciable (detectable) photocatalytic activity when purchased from any supplier or prepared through any procedure. Paper conversion,[4,5] have been studied for at least 40 years, there are still many issues which should be explained and clarified.[6,7] The key property of high photocatalytic activity is one of those non-clarified issues, but there seems to be no effective way to clarify this issue since it is almost impossible to obtain a series of photocatalyst samples with a difference in only one structural/physical property and with no change in the other properties. The balance between different properties has often been suggested as a reason for the high level of photocatalytic activity
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