Morphology-dependent photocatalytic activity of octahedral anatase particles prepared by ultrasonication–hydrothermal reaction of titanates

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.

Decahedral-shaped anatase titania photocatalyst particles: Synthesis in a newly developed coaxial-flow gas-phase reactor

“…In the present study, platinum particles were deposited on both facets of DAPs ( Figure S9, supplementary data) and their photocatalytic activity in H 2 system was significantly higher than OAPs prepared according to ref. 53. The results of present study, however, suggested that the activity dependence cannot be simply interpreted by the exposure of facets and/or platinum deposit location, considering the influence of the content of particles with mounds.…”

“…It is thought that alcohol molecules would be more easily adsorbed on the {001} facet (more reactive one with 100% five-coordinated Ti (Ti 5c ) atoms) [24] or/and better separation of charge carriers (by simultaneous transfer of electrons and holes to different facets) could facilitate redox reactions, as was reported for DAPs possessing naked {001} and Pt-modified {101} facets [52]. It was reported that DAPs possessing Pt nanoparticles uniformly deposited on both facets showed much lower photocatalytic activity than that of DAPs with Pt nanoparticles selectively deposited on {101} facets [52], similar to less active OAPs possessing only {101} facets [53]. In the present study, platinum particles were deposited on both facets of DAPs ( Figure S9, supplementary data) and their photocatalytic activity in H 2 system was significantly higher than OAPs prepared according to ref.…”

Noble metal-modified octahedral anatase titania particles with enhanced activity for decomposition of chemical and microbiological pollutants

“…Octahedral anatase particles (OAPs), which expose eight equivalent and thermodynamically stable (1 0 1) facets, exhibit a very high level of photocatalytic activity, probably due to the preferential distribution of shallow electron traps (ETs) rather than deep ETs that results in a low rate of recombination of charge carriers [55], [56]. Although OAPs have a high level of photocatalytic activity, they are inactive under visible light (as is bare titania).…”

On the Origin of Enhanced Photocatalytic Activity of Copper-Modified Titania in the Oxidative Reaction Systems

“…The photocatalytic performance of Cu2O@TiO2 polyhedra was different with that of pure Cu2O polyhedra without TiO2 shells, which could be attributed to the different driving forces for the charge carrier separation [105]. It is well known that controlled morphology by application of exposed facets (faceted semiconductors) significantly enhances photocatalytic performance, due to excellent crystallinity and low content of defects, e.g., octahedral and decahedral anatase particles [103,104]. Therefore, it is not surprising that similar concept was applied for copper oxide-titania heterojunction systems.…”

“…After shutting off the irradiation, these trapped electrons could be gradually released from TiO 2 to react with O 2 producing reactive radicals, which imparted the photocatalytic memory effect to Cu 2 O-NS/TiO 2 -NI photocatalyst ( Figure 15) [102]. It is well known that controlled morphology by application of exposed facets (faceted semiconductors) significantly enhances photocatalytic performance, due to excellent crystallinity and low content of defects, e.g., octahedral and decahedral anatase particles [103,104]. Therefore, it is not surprising that similar concept was applied for copper oxide-titania heterojunction systems.…”