pH-dependent assembly of tungsten oxide three-dimensional architectures and their application in photocatalysis

Remarkable Enhancement of Photocatalytic Activity of High-Energy TiO2 Nanocrystals for NO Oxidation through Surface Defluorination

The superior photocatalytic activity of TiO2 nanocrystals with exposed high-energy (001) facets, achieved through the use of hydrofluoric acid as a shape-directing reagent, is widely reported. However, in this study, we report for the first time the detrimental effect of surface fluorination on the photoreactivity of high-energy faceted TiO2 nanocrystals towards NO oxidation (resulting in a NO removal rate of only 5.9%). This study aims to overcome this limitation by exploring surface defluorination as an effective strategy to enhance the photocatalytic oxidation of NO on TiO2 nanocrystals enclosed with (001) facets. We found that surface defluorination, achieved through either NaOH washing (resulting in an improved NO removal rate of 23.2%) or calcination (yielding an enhanced NO removal rate of 52%), leads to a large increase in the photocatalytic oxidation of NO on TiO2 nanocrystals with enclosed (001) facets. Defluorination processes stimulate charge separation, effectively retarding recombination and significantly promoting the production of reactive oxygen species, including superoxide radicals (·O2-), singlet oxygen (1O2), and hydroxyl radicals (·OH). Both in situ diffuse reflectance infrared Fourier-transform spectroscopy and density functional theory calculations confirm the higher adsorption of NO after defluorination, thus facilitating the oxidation of NO on TiO2 nanocrystals.

Enhanced photoluminescence of potassium-doped tungsten oxide by acetone exposure

Studies of optical properties of doped nanocrystals of tungsten trioxide can elucidate new information about the material. A novel molecule-enhanced photoluminescence (PL) of potassium-doped tungsten trioxide (K _x WO) was explored in the presence of different gases to understand charge transfer between molecules and K _x WO on the properties of the material. We performed Raman spectroscopy and PL experiments in the presence of gaseous acetone or ethanol mixed with other gases (N₂ and O₂). PL at 630 nm from K _x WO was observed and further enhanced when the sample was continuously irradiated with a 532 nm CW laser in acetone. A mechanism of strong emission of the PL induced by the charge transfer between the acetone and the K _x WO is proposed.

An efficient strategy to boost the directed migration of photogenerated holes by introducing phthalocyanine as a hole extraction layer

Phthalocyanine with adjustable band energy and a binding group acts as a hole extraction layer to accelerate hole transfer from Ti-Fe2O3 to CoPi, and thus improves the PEC water oxidation performance of Ti-Fe2O3.

Z-scheme heterojunction based on NiWO4/WO3 microspheres with enhanced photocatalytic performance under visible light

The green treatment of dye wastewater has always been a research hotspot in the environmental field. The photocatalytic technology is considered to be a simple and effective strategy to remove dyes in wastewater. A new type of NiWO₄/WO₃ Z-scheme heterojunction microspheres were synthesized by a simple hydrothermal method and impregnation-calcination process. The crystal structure, microscopic morphology, optical and electrochemical properties of the samples were systematically characterized. The photocatalytic activity of methylene blue (MB) was studied by visible light irradiation. The results show that the direct Z-scheme heterojunction formed by NiWO₄/WO₃ effectively reduces the transfer resistance of photogenerated carriers and improves the separation efficiency of photogenerated carriers. The degradation rates of NiWO₄/WO₃-4 Z-scheme heterojunction microspheres to MB dye are 1.8 and 3.2 times higher than that of pure WO₃·2H₂O and WO₃ microspheres, respectively. Combined with the Mott-Schottky curve and the active species capture experiments, a possible Z-scheme photogenerated carrier transfer mechanism is proposed. This study provides a method for the development and design of Z-scheme heterojunction photocatalysts in the field of wastewater purification.

Ferrihydrite-Modified Ti-Fe2 O3 as an Effective Photoanode: The Role of Interface Interactions in Enhancing the Photocatalytic Activity of Water Oxidation

Semiconductor electrodes integrated with cocatalysts are key components of photoelectrochemistry (PEC)-based solar-energy conversion. However, efforts to optimize the PEC device have been limited by an inadequate understanding of the interface interactions between the semiconductor-cocatalyst (sem|cat) and cocatalyst-electrolyte (cat|ele) interface. In our work, we used ferrihydrite (Fh)-modified Ti-Fe₂ O₃ as a model to explore the transfer process of photogenerated charge carriers between the Ti-Fe₂ O₃ -Fh (Ti-Fe₂ O₃ |Fh) interface and Fh-electrolyte (Fh|ele) interface. The results demonstrate that the biphasic structure (Fh/Ti-Fe₂ O₃ ) possesses the advantage that the minority hole transfer from Ti-Fe₂ O₃ to Fh is driven by the interfacial electric field at the Ti-Fe₂ O₃ |Fh interface; meanwhile, the holes reached at the surface of Fh can rapidly inject into the electrolyte across the Fh|ele interface. As a benefit from the improved charge transfer at the Ti-Fe₂ O₃ |Fh and Fh|ele interface, the photocurrent density obtained by Fh/Ti-Fe₂ O₃ can reach 2.32 mA cm^-2 at 1.23 V versus RHE, which is three times higher than that of Ti-Fe₂ O₃ .

Hierarchical Flowerlike Highly Synergistic Three-Dimensional Iron Tungsten Oxide Nanostructure-Anchored Nitrogen-Doped Graphene as an Efficient and Durable Electrocatalyst for Oxygen Reduction Reaction

A unique and novel structural morphology with high specific surface area, highly synergistic, remarkable porous conductive networks with outstanding catalytic performance, and durability of oxygen reduction electrocatalyst are typical promising properties in fuel cell application; however, exploring and interpreting this fundamental topic is still a challenging task in the whole world. Herein, we have demonstrated a simple and inexpensive synthesis strategy to design three-dimensional (3D) iron tungsten oxide nanoflower-anchored nitrogen-doped graphene (3D Fe-WO₃ NF/NG) hybrid for a highly efficient synergistic catalyst for oxygen reduction reaction (ORR). The construction of flowerlike Fe-WO₃ nanostructures, based on synthesis parameters, and their ORR performances are systematically investigated. Although pristine 3D Fe-WO₃ NF or reduced graphene oxides show poor catalytic performance and even their hybrid shows unsatisfactory results, impressively, the excellent ORR activity and its outstanding durability are further improved by N doping, especially due to pyridinic and graphitic nitrogen moieties into a graphene sheet. Remarkably, 3D Fe-WO₃ NF/NG hybrid nanoarchitecture reveals an outstanding electrocatalytic performance with a remarkable onset potential value (∼0.98 V), a half-wave potential (∼0.85 V) versus relative hydrogen electrode, significant methanol tolerance, and extraordinary durability of ∼95% current retention, even after 15 000 potential cycles, which is superior to a commercial Pt/C. The exclusive porous architecture, excellent electrical conductivity, and the high synergistic interaction between 3D Fe-WO₃ NF and NG sheets are the beneficial phenomena for such admirable catalytic performance. Therefore, this finding endows design of a highly efficient and durable nonprecious metal-based electrocatalyst for high-performance ORR in alkaline media.

Imaging photogenerated charge carriers on surfaces and interfaces of photocatalysts with surface photovoltage microscopy

Recent advances in imaging and characterizing charge separation on surfaces and interfaces of photocatalysts by surface photovoltage spectroscopy were reviewed and highlighted.

One-Pot Hydrothermal Synthesis of Hexagonal WO3 Nanorods/Graphene Composites as High-Performance Electrodes for Supercapacitors

Tungsten oxide (WO₃ ) as an electrode material for supercapacitors has always suffered from low capacitance and poor rate capability. In this work, a series of WO₃ nanorods/graphene composites with different weight ratios of tungsten oxide nanorods (WO₃ NRs) and reduced graphene oxide (rGO) are synthesized successfully through a facile one-pot electrostatic adsorptive hydrothermal method. In these composites, rGO enhances the conductivity, transporting electrons and protons to the WO₃ NRs. In addition, rGO can reinforce the structure of the WO₃ NRs during frequently occurring redox reactions. At a graphene weight ratio of 1 wt %, the specific capacitance of the WO₃ NRs/rGO composite is 343 F g^-1 at a current density of 0.2 A g^-1 . Compared with pure WO₃ as electrodes, the WO₃ NRs/rGO composite shows excellent specific capacity, and superior rate performance and cycling stability owing to the combined action of the double layer and pseudocapacitor. This study provides a new convenient approach to promote the electrochemical performance of tungsten oxide.

Metal Oxide Nanowire Preparation and Their Integration into Chemical Sensing Devices at the SENSOR Lab in Brescia

Metal oxide 1D nanowires are probably the most promising structures to develop cheap stable and selective chemical sensors. The purpose of this contribution is to review almost two-decades of research activity at the Sensor Lab Brescia on their preparation during by vapor solid (n-type In₂O₃, ZnO), vapor liquid solid (n-type SnO₂ and p-type NiO) and thermal evaporation and oxidation (n-type ZnO, WO₃ and p-type CuO) methods. For each material we've assessed the chemical sensing performance in relation to the preparation conditions and established a rank in the detection of environmental and industrial pollutants: SnO₂ nanowires were effective in DMMP detection, ZnO nanowires in NO₂, acetone and ethanol detection, WO₃ for ammonia and CuO for ozone.

Cobalt Phosphide Modified Titanium Oxide Nanophotocatalysts with Significantly Enhanced Photocatalytic Hydrogen Evolution from Water Splitting

Production of hydrogen from photocatalytic water splitting holds promise as an alternative energy source with superiority of cleanliness, environment friendliness, low price, and sustainability. Perfectly constructing the noble-metal-free and stable hybrid structure photocatalyst is quite essential; herein, for the first time the authors aim to use cobalt phosphide as the cocatalyst on titanium oxide to form a novel hybrid structure to enhance the utilization of the photoexcited electrons in redox reactions for improved photocatalytic H₂ evolution activity. Thus, the achieved significantly increased photocatalytic H₂ -evolution rate on the optimized CoP/TiO₂ (8350 µmol h^-1 g^-1 ) is 11 times higher than that of the pristine TiO₂ . Moreover, this work is expected to spur more insight into synthesizing such novel photofunctional systems, achieving high photocatalytic H₂ evolution activity and sufficient stability for solar-to-chemical conversion and utilization.

Hexagonal tungsten oxide nanoflowers as enzymatic mimetics and electrocatalysts

Tungsten oxide (WO_x) has been widely studied for versatile applications based on its photocatalytic, intrinsic catalytic, and electrocatalytic properties. Among the several nanostructures, we focused on the flower-like structures to increase the catalytic efficiency on the interface with both increased substrate interaction capacities due to their large surface area and efficient electron transportation. Therefore, improved WO_x nanoflowers (WONFs) with large surface areas were developed through a simple hydrothermal method using sodium tungstate and hydrogen chloride solution at low temperature, without any additional surfactant, capping agent, or reducing agent. Structural determination and electrochemical analyses revealed that the WONFs have hexagonal Na_0.17WO_3.085·0.17H₂O structure and exhibit peroxidase-like activity, turning from colorless to blue by catalyzing the oxidation of a peroxidase substrate, such as 3,3′,5,5′-tetramethylbenzidine, in the presence of H₂O₂. Additionally, a WONF-modified glassy carbon electrode was adopted to monitor the electrocatalytic reduction of H₂O₂. To verify the catalytic efficiency enhancement by the unique shape and structure of the WONFs, they were compared with calcinated WONFs, cesium WO_x nanoparticles, and other peroxidase-like nanomaterials. The results indicated that the WONFs showed a low Michaelis-Menten constant (k_m), high maximal reaction velocity (v_max), and large surface area.

On the nature of defect states in tungstate nanoflake arrays as promising photoanodes in solar fuel cells

An electrochemical method is presented to study the nature of the defect states in sub-stoichiometric tungsten oxide nanoflake photoanodes used in water splitting.

WO3 nanorolls self-assembled as thin films by hydrothermal synthesis

We report a novel type of WO3 nanostructure, i.e. nanorolls obtained as a self-assembled thin film on a transparent conductive substrate. The mild conditions of preparation, avoiding the use of HCl, result in an eco-friendly hydrothermal method with reduced crystallization time. FESEM and HR-TEM show that WO3 nanocrystals are made of rolled nanoflakes with a telescope-like appearance at their tip. For their nano-porosity, electrochemical accessibility, good adhesion to substrates and the envisaged presence of nanocavities between the WO3 layers, these materials hold tremendous promise in nano-electronics, electrochromic devices, water photo-splitting cells, Li-ion batteries and nano-templated filters for UV radiation.

Dopant-controlled morphology evolution of WO3 polyhedra synthesized by RF thermal plasma and their sensing properties

In this paper, a simple way is developed for the synthesis of Cr-doped WO3 polyhedra controlled by tailoring intrinsic thermodynamic properties in RF thermal plasma. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy are used to characterize the detail structures and surface/near-surface chemical compositions of the as-prepared products. Kinetic factors showed little effects on the equilibrium morphology of Cr-doped WO3 polyhedra, while equilibrium morphologies of WO3 polyhedra can be controlled by the thermodynamic factor (Cr doping). Set crystal growth habits of pure WO3 as an initial condition, coeffects of distortions introduced by Cr into the WO3 matrix, and a chromate layer on the crystal surface could reduce the growth rates along [001], [010], and [100] directions. The morphology evolution was turning out as the following order with increasing Cr dopants: octahedron-truncated octahedron-cuboid. 2.5 at. % Cr-doped WO3 polyhedra exhibit the highest sensing response due to coeffects of exposed crystal facets, activation energy, catalytic effects of Cr, and particle size on the surface reaction and electron transport units. By simply decorating Au on Cr-doped WO3 polyhedra, the sensing responses, detection limit, and response-recovery properties were significantly improved.

Degradation of methylene blue using porous WO3, SiO2-WO3, and their Au-loaded analogs: adsorption and photocatalytic studies

A facile sonochemical approach was used to deposit 3-5 nm monodisperse gold nanoparticles on porous SiO2-WO3 composite spheres, as confirmed by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). High-resolution TEM (HR-TEM) and energy dispersive X-ray spectroscopy (EDS) further characterized the supported Au nanoparticles within the Au-SiO2-WO3 composite. These analyses showed isolated Au nanoparticles within both SiO2- and WO3-containing regions. Selective etching of the SiO2 matrix from Au-SiO2-WO3 yielded a pure Au-WO3 material with well-dispersed 10 nm Au nanoparticles and moderate porosity. This combined sonochemical-nanocasting technique has not been previously used to synthesize Au-WO3 photocatalysts. Methylene blue (MB) served as a probe for the adsorption capacity and visible light photocatalytic activity of these WO3-containing catalysts. Extensive MB demethylation (azures A, B, C, and thionine) and polymerization of these products occurred over WO3 under dark conditions, as confirmed by electrospray ionization mass spectrometry (ESI-MS). Photoirradiation of these suspensions led to further degradation primarily through demethylation and polymerization pathways, regardless of the presence of Au nanoparticles. Ring-opening sulfur oxidation to the sulfone was a secondary photocatalytic pathway. According to UV-vis spectroscopy, pure WO3 materials showed superior MB adsorption compared to SiO2-WO3 composites. Compared to their respective nonloaded catalysts, Au-SiO2-WO3 and Au-WO3 catalysts exhibited enhanced visible light photocatalytic activity toward the degradation of MB. Specifically, the rates of MB degradation over Au-WO3 and Au-SiO2-WO3 during 300 min of irradiation were faster than those over their nonloaded counterparts (WO3 and SiO2-WO3). These studies highlight the ability of Au-WO3 to serve as an excellent adsorbant and photodegradation catalyst toward MB.

Micro-wheels composed of self-assembled tungsten oxide nanorods for highly sensitive detection of low level toxic chlorine gas

Gas nanosensor based on micro-wheels composed of self-assembled tungsten oxide nanorods exhibited excellent sensing performance to ppb level Cl₂.

Exposed facet and crystal phase tuning of hierarchical tungsten oxide nanostructures and their enhanced visible-light-driven photocatalytic performance

Tungsten oxide hierarchical nanostructures controllably assembled with one dimensional nanostructures which exhibit different exposed facets and crystal phases were synthesized via a facile hydrothermal reaction assisted by urea.

High-bandwidth, high-sampling-rate, low-noise, two-probe transient photovoltage measuring system

In this article, we present a two-probe configuration for measuring transient photovoltage (TPV) signals from photo-electronic semiconductor devices. Unlike in a conventional one-probe system, the two electrodes of the devices under test in this study are both monitored in our new measuring system, giving rise to a significantly enhanced signal-to-noise ratio. Tentative experimental data ob tained from N, N'-Di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine-based organic semiconductor devices show that the bandwidth and the sampling rate of the system reach 1.5 GHz and 50 GS/s, respectively, without degradation of the noise level. In addition, the study of TPV signals on each individual electrode is allowed. The TPV values measured by the two individual probes are not identically equal to half of the differential TPV and will not cancel each other out as expected. This abnormal phenomenon is due to the photoelectric response of the photo-electronic material. This novel two-probe TPV measuring technique and abnormal TPV behavior might be useful for studying more dynamic processes in photo-electronic semiconductors.

WO3 nanorods created by self-assembly of highly crystalline nanowires under hydrothermal conditions

WO3 nanorods and wires were obtained via hydrothermal synthesis using sodium tungstate as a precursor and either oxalic acid, citric acid, or poly(methacrylic acid) as a stabilizing agent. Transmission electron microscopy images showed that the organic acids with different numbers of carboxylic groups per molecule influence the final sizes and stacking nanostructures of WO3 wires. Three-dimensional electron diffraction tomography of a single nanocrystal revealed a hexagonal WO3 structure with preferential growth along the c-axis, which was confirmed by high-resolution transmission electron microscopy. WO3 nanowires were also spin-coated onto an indium tin oxide/glass conducting substrate, resulting in the formation of a film that was characterized by scanning electron microscopy. Finally, cyclic voltammetry measurements performed on the WO3 thin film showed voltammograms typical for the WO3 redox process.