Display omitted
•Two-faceted (DAP) ― more favorable configuration than single-faceted (OAP) under UV irradiation.•Bare and modified DAP ― efficient charge carriers’ separation under UV ...irradiation.•Noble metal-modified OAP ― higher activity than respective DAP under vis irradiation.•“Hot” (plasmonic) electron transfer via shallow electron traps in OAP.•Localization of noble metal on DAP – key factor of vis activity.
Octahedral anatase particles (OAP, with eight equivalent {101} facets) and decahedral anatase particles (DAP, with two additional {001} facets) were modified with nanoparticles of noble metals (Au, Ag, Cu). The titania morphology, expressed by the presence of different arrangements of exposed crystal facets, played a key role in the photocatalytic properties of metal-modified faceted titania. In the UV/vis systems, two-faceted configuration of DAP was more favorable for the reaction efficiency than single-faceted OAP because of an efficient charge separation described by the transfer of electrons to {101} facets and holes to {001} facets. Time-resolved microwave conductivity (TRMC) and reversed double-beam photoacoustic spectroscopy (RDB-PAS) confirmed that distribution of electron traps (ET) and mobility of electrons were key-factors of photocatalytic activity. In contrast, metal-modified OAP samples had higher photocatalytic activity than metal-modified DAP and metal-modified commercial titania samples under visible light irradiation. This indicates that the presence of single type of facets ({101}) is favorable for efficient electron transfer via shallow ET, whereas intrinsic properties of DAP result in fast charge carriers’ recombination when gold is deposited on {101} facets (migration of “hot” electrons: Au→{101}→Au).
The electronic structure of N-TiO2 samples prepared by a sol-gel method was investigated by EPR (Electronic Paramagnetic Resonance) measurements and the energy-resolved distribution of electron ...traps. In EPR spectra, some of the resonance lines assigned to paramagnetic species of nitrogen and Ti3+ were detected. Sample prepared at 300 °C revealed the highest intensity line of the nitrogen paramagnetic centers, whereas that prepared at 400 °C showed a paramagnetic line for Ti3+. Measurements of the electron trap distribution showed higher density of electron traps for sample prepared at 400 °C than that at 300 °C. Sample prepared at 300 °C, which revealed the highest amount of nitrogen built in the titania in the interstitial position was the most active under visible light. It was evidenced that photocatalytic decomposition of acetaldehyde was dependent strongly on the BET surface area and electrokinetic potential of the photocatalyst surface. The UV content in the fluorescent lamp affected the yield of acetaldehyde decomposition.
Decahedral anatase particles (DAPs) have been prepared by the gas-phase method, characterized, and analyzed for property-governed photocatalytic activity. It has been found that depending on the ...reaction systems, different properties control the photocatalytic activity, that is, the particle aspect ratio, the density of electron traps and the morphology seem to be responsible for the efficiency of water oxidation, methanol dehydrogenation and oxidative decomposition of acetic acid, respectively. For the discussion on the dependence of the photocatalytic activity on the morphology and/or the symmetry other titania-based photocatalysts have also been analyzed, that is, octahedral anatase particles (OAP), commercial titania P25, inverse opal titania with and without incorporated gold NPs in void spaces and plasmonic photocatalysts (titania with deposits of gold). It has been concluded that though the morphology governs photocatalytic activity, the symmetry (despite its importance in many cases) rather does not control the photocatalytic performance.
Display omitted
•Identification of metal-oxide powders by electron-trap (ET) density is proposed.•Degree of coincidence of ET-density patterns is introduced for analysis.•Arrays of the patterns with ...the order of photocatalytic activity were shown.
Here we report identification of powders of titania, one of the most important metal oxides, with their energy-resolved distribution of electron traps (ERDT), as a fingerprint, measured by reversed double-beam photoacoustic spectroscopy (RDB-PAS). The ERDT patterns combined with conduction-band bottom (CBB) position data measured by ordinary PAS for various titania powders and other metal-oxide powders were different depending on the kind of sample. The degree of coincidence (ζ) of ERDT/CBB patterns was evaluated for a given pair of samples as a product of each degree of coincidence for ERDT-pattern matching, total density of electron traps and CBB position. Titania samples collected from close positions in a container exhibited high values of ζ, while samples with different code names showed low values of ζ, except for pairs of samples prepared in the same way but coded differently. It was shown that the higher was the values of ζ, the higher was the degree of coincidence for photocatalytic activity of titania samples. ERDT/CBB-pattern arrays arranged in the order of photocatalytic activities in three reaction systems suggested preferable patterns for each photocatalytic reaction.
A novel instrumental methodology, reversed double-beam photoacoustic spectroscopy (RDB-PAS), was developed for measuring the energy-resolved distribution of electron traps (ERDT) of metal-oxide ...powders. In the RDB-PAS measurement, electrons in the valence band of a powder sample are directly excited to electron traps (ETs) and accumulated in the ETs from the deeper (anodic) side to the shallower (cathodic) side by wavelength-scanned continuous light, and the increase in photoabsorption of electron-filled ETs is measured by modulated light at a fixed wavelength by PAS. It was shown that ERDT/CBB (conduction-band bottom) patterns measured by RDB-PAS can be used for not only identification and detailed characterization of a wide range of metal-oxide powder samples but also estimation of difference in apparent valence-band top (VBT) position.
Display omitted
•Energy-resolved distribution of electron traps (ERDT) in metal-oxide powders was measured.•Reversed double-beam photoacoustic spectroscopy (RDB-PAS) was developed for ERDT analysis.•ERDT analysis enabled estimation of valence-band top energy of titania samples.
Here we propose a method for identification and characterization of metal-oxide powders with patterns of energy-resolved density of electron traps (ERDT) and conduction-band bottom (CBB) position, as ...a fingerprint, measured by newly developed reversed double-beam photoacoustic spectroscopy (RDB-PAS). Degrees of coincidence (ζ) of ERDT/CBB patterns could be quantitatively evaluated for a given pair of titania samples as a product of coincidence of (a) ERDT-pattern matching, (b) total density of electron traps, and (c) CBB position estimated by ordinary PAS. It was also shown that the higher the ζ is increased, the higher the degree of coincidence for photocatalytic activities of the pair of titania samples becomes regardless of three different photocatalytic reaction systems, presumably because photocatalytic activities are regulated by ERDT and CBB of titania powders.
Display omitted
•Bismuth vanadate/bismuth tungstate (BVO/BWO) composites were synthesized by various BVO fractions.•Photocatalytic oxygen liberation was depended on BVO fractions and calcination.•BVO ...particles were stabilized on BWO even calcination.•Calcined BVO/BWO showed photocatalytic enhancement under visible light over BVO.•Conduction band-bottom position of BVO/BWO was similar to BVO.
Bismuth vanadate/bismuth tungstate (BVO/BWO) composites in various BVO fractions (f(V)) were synthesized through hydrothermal reaction. Morphology analysis of the composites showed that rod-like BVO structures were partially deposited on flake-ball (FB) BWO particles. In addition, surface structures of BWO, BVO and BVO/BWO were studied in energy-resolved distribution of electron traps and conduction band-bottom (CBB) position by household reversed double-beam photoacoustic spectroscopy (PAS) and single-beam PAS, respectively. Apparent CBB of BVO/BWO was ca. 0.4 eV anodically shifted from BWO and almost similar to BVO. Moreover, photocatalytic oxygen liberation under irradiation of UV–vis and visible lights demonstrated that photocatalytic activities of the BVO/BWO composites were depended on f(V) and calcined BVO/BWO composites in which BVO was stabilized on BWO showed enhancement of photocatalytic activity comparing to a pristine BVO. The prolonged and the highest active photocatalyst under visible light was calcined 0.80 f(V)due to BVO stabilization on FB-BWO particles.
Photocatalytic activities and electronic properties of famous commercial photocatalyst - P25 has been investigated in detail. First, the active components have been isolated, i.e., anatase and ...rutile, characterized and tested. Although physical methods of phase separation are more recommended to keep original properties of polymorphs and to obtain the original amorphous phase, vibration and sonication-assisted vibration show to be ineffective for the phase separation. In contrast, chemical dissolution is effective, but resulting in impurification of titania surface, i.e., anatase with nitrogen and hydrogen peroxide, and rutile with fluorine. Therefore, purification methods must be applied, including thermal treatment and washing. Indeed, these post-treatment operations have resulted in preparation of almost pure anatase and rutile phases. However, annealing results in the particles’ aggregation and might activate impurities adsorbed on the titania surface. Accordingly, the isolated rutile samples have additionally been washed with NaOH (to remove adsorbed HF) and then annealed. It has been confirmed that the high photocatalytic efficiency of P25 could be explained by the intrinsic properties of anatase and rutile, i.e., high activity in oxidation and reduction reactions, respectively. The mobility of charges and lifetime of electrons, studied by time-resolved microwave conductivity (TRMC), as well as density of electron traps, investigated by reverse double beam photoacoustic spectroscopy (RDB-PAS), allow to explain the common higher activity of anatase than rutile. Accordingly, it is thought that the higher activity of rutile for hydrogen evolution might be caused either by the more negative position of conduction band level or the more efficient light harvesting ability. Interestingly, the isolated samples have exhibited the visible-light response, especially those washed with NaOH and annealed, and thus suggesting that sodium (and/or its compounds), despite being not-recommended for UV activity, might initiate vis response of titania. At the same, it should be concluded that up to now the perfect method of isolation of active phases from P25 does not exist, as each method (even followed by purification) results in some changes in the composition and/or the surface properties of anatase, rutile and amorphous phase
PDF
