Correlation of Optical Properties, Electronic Structure, and Photocatalytic Activity in Nanostructured Tungsten Oxide

Abstract

cause a negative effect on the photocatalytic activity by acting as charge-recombination centers. [9] Therefore, controlling the level of V O plays an important role in defect engineering. The amount of V O in tungsten oxide was recently reported to be controlled through air-annealing at various temperatures (300-600 °C) and times (0.5-8 h), giving improvements in photocatalytic activity and electrochemical performance. [11] Recently, we have investigated the growth and properties of chemical vapor deposited (CVD) nanostructured tungsten oxide. [12] The appearance of defects in the direction perpendicular to NR growth was the driving force for formation of the 1D structure via a planar-defect-driven growth mechanism. [13] Herein, we study the growth of tungsten oxide nanorod (NR) array thin films, their 1D nanostructure growth mechanism and the variable concentration of V O , and their influence on the photocatalytic properties of the deposited nanomaterial. We find that as the NR grow, the cross-section increases due to the coalescence of adjacent NRs and a change in the V O concentration is observed. Hence, the V O concentration in 1D tungsten oxide deposited via aerosol assisted chemical vapor deposition (AACVD) is a function of the deposition time. The photocatalytic activity Tungsten trioxide nanorod arrays are deposited using aerosol assisted chemical vapor deposition. The electronic structure, defect chemistry, optical bandgap, and photocatalytic activity are found to vary progressively with nanorod length. Nanorods less than 1 µm in length show a widening of the optical bandgap (up to 3.1 eV), more disorder states within the bandgap, an absence of reduced tungsten cation states, and increased photocatalytic activity for destruction of a test organic pollutant (stearic acid) compared to nanorods of 2 µm length or greater which possessed bandgaps close to the bulk value for tungsten oxide (2.6-2.8 eV), the presence of reduced tungsten states (W 4+ ), and lower photocatalytic activity. The results indicate that for maximum photocatalytic performance in organic pollutant degradation, tungsten oxide should be engineered such that the bandgap is widened relative to bulk WO 3 to a value above 3 eV; although less photons are expected be absorbed, increases in the overpotential for oxidation reactions appear to more than offset this loss. It is also desirable to ensure the material remains defect free, or the defect concentration minimized, to minimize carrier recombination.

Evidence and Effect of Photogenerated Charge Transfer for Enhanced Photocatalysis in WO3/TiO2 Heterojunction Films: A Computational and Experimental Study

“…By contrast, the plain WO 3 nanorods showed an absorption edge higher than expected for a WO 3 film, which can be explained in terms of the synthesis method employed. It has been recently found by Ling et al that aerosol‐assisted CVD can be used to deposit WO 3 nanorods containing oxygen vacancies, which induce a quantum confinement effect. The existence of oxygen vacancies was inferred from the presence of a small amount of reduced tungsten species (W 5+/4+ ) in the as‐deposited WO 3 film, as observed by XPS analysis (Figure B) and the requirement for charge neutrality.…”

Effect of oxygen deficiency on the excited state kinetics of WO3 and implications for photocatalysis

“…7 To overcome this restraint, oxygen vacancies can be introduced into metal oxide lattices to extend the light absorption of these systems into the visible range. Numerous studies have explored this strategy and showed improved light absorption as well as enhanced photocatalytic activities for oxygen-decient TiO 2 [8][9][10][11][12][13][14][15] and other metal oxides such as WO 3 , [16][17][18] ZnO, 7,19,20 and SnO 2 . 21 However, enhanced performance is typically only found in the case of moderate oxygen deciency, whereas highly oxygen-decient metal oxides with strong coloration counter-intuitively exhibit signicantly lower photocatalytic activity than their stoichiometric analogues.…”

Gas-phase synthesis of hybrid nanostructured materials

“…3f in blue colour) suggested that the presence of oxygen deficient in surface. 44 The metal NPs were deposited on the as-synthesized WO 3 NR array, with the size controlled by the deposition time (0.5 to 36 min) during AACVD. The obtained metal NPs/WO 3 NRs hybrid nanostructure thin films are denoted as M/WO 3 ('t' minutes), where 't' is the deposition time for the metal precursor during AACVD.…”

“…The sample Au/WO 3 (1 min, with particle mean size 3.0 nm) and sample Pt/WO 3 (10 min, 3.0 nm) showed enhanced activity for photocatalytic degradation of SA with FQE = 11.0 × 10 −5 and 11.3 × 10 −5 respectively compared to an undecorated WO 3 NR array thin film (deposition time of 5 minutes, FQE = 8.7 × 10 −5 which was the best photocatalytic activity for WO 3 NRs array thin films according to our previous study). 44 Whereas, when the WO 3 NR array was functionalised by Pd and Ru NPs (sample Pd/WO 3 (5 min, 1.9 nm) and Ru/WO 3 (36 min, 2.8 nm)) a lower activity for photocatalytic degradation of SA was observed. After annealing, the sample PdO/WO 3 (5 min, 5.6 nm) had the highest activity for photocatalytic degradation of SA with FQE = 13.2 × 10 −5 , though RuO 2 /WO 3 (36 min, 5.1 nm) still showed lower activity (FQE = 6.0 × 10 −5 ).…”

“…In addition the tungsten oxide NRs synthesized by AACVD are typically substoichiometric (WO x where 2 < x < 3), 42,43 with annealing in air at 500°C required to obtain fully oxidized WO 3 . 44 This makes it impossible to fabricate, for instance, Pd/WO 3 or PdO/WO x due to the annealing process simultaneously oxidizing both the metal NPs and the substrate. Here we fabricate the 1D nanostructured substrate and the decorating NPs separately, to form TYPE I structures (TYPE III and IV have been reported previously) 37 of Pd/WO 3 , Ru/WO 3 and RuO 2 /WO 3 , controlling the decorating NP size (1.6 to 7.3 nm) simply by altering the deposition time, to provide a robust method for producing a wide range of heterojunction nanomaterials.…”