WO3 Films Research Articles

A sight of view on electrical impacts, structural properties and surface roughness of tungsten trioxide thin film: effect of substrate temperatures in WO3/Si device fabrication

Monoclinic WO3 thin films have been effectively deposited by a simple spray pyrolysis technique at a molar concentration of 0.01 M on a glass substrate in the temperature range of 473 to 673 K. These WO3 films were used as an interlayer between the metal and the semiconductor, which formed the basic structure of the photodetector. Effect of substrate temperature on WO3 films during the process of the deposition was systematically interpreted with respect to the structural, morphological, optical and electrical properties of the WO3 films. The x-ray diffraction pattern revealed the polycrystalline nature of the prepared films with monoclinic phases. At the substrate temperature of 623 K, the nano-thin films were strongly bonded to each other as observed from the FE-SEM images. Visible and ultraviolet spectroscopies indicated the band gap (Eg) of the WO3 thin film is 3.30 eV. The dc electrical study recorded a sharp increase in the electrical conductivity of the prepared film at substrate temperature of 623 K for tungsten trioxide. It is worth noting that all diodes showed a positive photoresponse under illumination. In particular, the photodetector with the thickness of 300 nm showed higher responsivity 0.02 A/W and detection specificity 8.29 × 1010 Jones.

Optical Hydrogen Sensing Properties of e-Beam WO3 Films Decorated with Gold Nanoparticles.

Tungsten oxide thin films with different thicknesses, crystallinity and morphology were synthesized by e-beam deposition followed by thermal treatment and acid boiling. The films with different surface morphologies were coated with gold nanoparticles and tested as optical sensing materials towards hydrogen. X-ray diffraction, scanning electron microscopy, ellipsometry and UV-VIS spectroscopy were employed to characterize the structural, morphological and optical properties of the film. We demonstrated a good response towards hydrogen in air, reaching a good selectivity among other common reducing gases, such as ammonia and carbon monoxide. The sensitivity has been proven to be highly dependent on the thickness and crystallinity of the samples.

Process optimization and effect of sputtering pressure on electrochromic properties of flexible WO3 films prepared by DC magnetron sputtering

In this study, WO3 films were prepared on flexible indium tin oxide/polyethylene terephthalate substrates through DC magnetron sputtering at approximately 40 °C, and the process parameters were optimized. The effects of the sputtering pressure on the microstructure, morphology, and electrochromic properties of the films were investigated using Raman spectroscopy, scanning electron microscopy, x-ray diffraction, atomic force microscopy, and electrochemical techniques. The results showed that the thickness, surface roughness, and grain size of the WO3 films changed with increasing pressure, and the optimal values of these parameters for excellent electrochromic properties of the WO3 films were 731, 53.5, and 1.79 nm, respectively, at 2.0 Pa. The optical modulation amplitude reached 84.66% at 455 nm, and the coloring and bleaching response times were 7.64 s and 6.76 s, respectively. The electrochemical impedance spectroscopy results showed that the films prepared at 2.0 Pa had the smallest Rct (41.63 Ω).

Effect of growth pressure on sulfur content of RF-magnetron sputtered WS2 films and thermal oxidation properties of them toward using Pd decorated WO3 based H2 gas sensor

Obtaining near stoichiometric WS2 films with sputtering is challenging because of high sulfur deficiency during sputtering, which limits the application of these films in different solid-state device applications. In this study, in order to understand the growth pressure effect on the sulfur/tungsten ratio, WS2 films were grown at four different growth pressures, 5, 25, 50, and 100 mTorr, by RF-magnetron sputtering. With increasing growth pressure from 5 to 100 mTorr, it is found that films change from relatively metallic to near stoichiometric WS2 films. The thermal oxidation process of as-deposited WS2 films in the presence of O2 gas was also studied. The study showed that WS2 films were turned into WO3 films without significant change in morphology at a temperature of 400 ℃. The obtained WO3 films were then decorated by Palladium catalysis deposited by RFMS to activate the WO3 films toward H2 gas. The high response of 586-fold at an operating temperature of 200 ℃ for 1000 ppm H2 gas response and recovery times of 360 and 90 s, respectively, has been achieved for the sensor deposited at a growth pressure of 50 mTorr. In addition, a detection limit of 10 ppm was achieved for the sensor.

Revealing the nature of the graphene on-off control of the electrolyte gating effect in WO3 film phase transition

Electrolyte gating-induced metallization has been broadly observed in various transition metal oxides for decades, while the gating mechanism is still under debate. In this work, we realize the Insulator-Metal Transition (IMT) control of WO3 film by inserting a few-layer graphene. It's found that a sharp IMT process in WO3 film emerges if applying an electrolyte gating treatment, while this IMT behavior will be strikingly suppressed if inserting a graphene layer between the WO3 film and ionic liquid. Interestingly, the pronounced IMT behavior can be always observed if substituting the ionic liquid by acid solution, regardless of graphene layer insertion or not. According to the first-principles calculations, the diffusion energy barriers for the hydrogen or oxygen propagating graphene layer are extracted, and then the mechanism of electrolyte gating in WO3 films is proposed. Moreover, the roles of hydrogen and oxygen vacancy on the WO3 IMT process are further discussed based on the band structure calculations. Our current study will boost the development of electrolyte gating technique and bring deeper understanding of the electrolyte gating treatment for other transition metal oxides.

Morphology-Dependent Near-Infrared Electrochromic Properties of Tungsten Oxide

Vertically aligned WO3 nano-architectures on the transparent conducting substrate are produced via a hydrothermal method. The results revealed that different WO3 nano-architectures, including “nanoblocks”, “nanosheet” and “nanobelt” structures, can be obtained by adjusting the composition of the precursor solution, that shows that solvent composition plays an important role in the adjustment of the prepared material morphology. The nanostructured array films with thicknesses of about 600 nm show remarkable enhancement of the electrochromic properties in the infrared region. The obtained WO3 films have good electrochromic properties in the near-infrared range. In particular, significant optical modulation of the WO3 nanosheets (72% at 1200 nm), fast switching speed (6.5 s for colored and 8.7 s for bleached), and excellent cycling stability (maintained 90.2% of the initial optical modulation after 1000 cycles). The improved electrochromic performance is mainly due to the vertically arranged structure, which makes it easier for ions to diffuse in the nanoarray, and also provides a larger surface area for charge transfer reaction. The research results provide a certain reference value for the research of electrochromic near-infrared performance.

Cobalt-Phosphate (Co-Pi)-Modified WO3 Photoanodes for Performance-Enhanced Photoelectrochemical Wastewater Degradation.

Photocatalytic technology, with features of wide applicability, mild reaction conditions and sunlight availability, satisfies the requirements of "green chemistry". As the star photoanode material for photoelectrochemical catalysis, WO3 has a suitable band gap of 2.8 eV and a strong oxidation capacity, as well as displaying great potential in organic wastewater degradation. However, its performance is usually hindered by competition with water oxidation to generate peroxides, rapid charge complexation caused by surface defect sites, and so on. Herein, WO3 films modified with cobalt-phosphate (Co-Pi/WO3) film were prepared and involved in photocatalytic organic wastewater degradation. A degradation rate constant of 0.63311 h-1 was obtained for Co-Pi/WO3, which was much higher than that of WO3, 10.23 times that of direct photocatalysis (DP) and 23.99 times that of electrocatalysis (EC). After three cycles of degradation, the film can maintain a relatively good level of stability and a degradation efficiency of 93.79%.

Boosting coloration efficiency in an electrochromic device using an ITO/Ag/ITO multilayered electrode and porous WO3 chromic layer

ABSTRACT High-performance electrochromic devices (ECDs) were fabricated by introducing indium tin oxide/silver/indium tin oxide (IAI) multilayers as a transparent conducting oxide (TCO) electrode in conjunction with microstructural tailoring of tungsten trioxide (WO3) as an electrochromic (EC) layer. A 15-nm-thick Ag film with a preferential orientation of (111) and smooth morphology resulted in a high optical transmittance of 90.9% and sheet resistance 16.7 Ω/sq, making it a suitable TCO. In addition, the open network WO3 film used as an EC layer was deposited by carefully controlling the chamber pressure during the sputtering process. A high chamber pressure (2.0 Pa) resulted in a WO3 film with the lowest mass density (5.72 g/cm3) and a rough morphology. Half-cell ECDs consisting of a TCO bottom electrode (BE)/WO3 stack immersed in LiClO4 electrolyte were fabricated to verify the impact of using the IAI multi-layer stack and tailored WO3 film as a TCO and EC layer, respectively. Significant enhancement in terms of coloration efficiency was achieved for the ECDs by adopting the IAI TCO and open network WO3 EC layer compared to counterpart devices with an ITO TCO or dense WO3 EC film. This can be explained by the facile carrier extraction (injection) and transport of Li ions.

Nanoparticulate films of WO3 and MoO3 composites for enhancing UV light electrochromic transmittance variation and energy storage applications

Green technologies for the manufacture of optical shutters retain a sustainable development. In this work, the electro-exploding wires and spray coating technologies are employed to deposit nanoparticulate (WO3)1-x(MoO3)x composite films. MoO3 NPs are introduced to enhance electrochromic (EC) transmittance variation of violet and UV light with wavelengths ranging from 330 to 450 nm. Measurements of EC performances of (WO3)1-x(MoO3)x composite films in a potentiostat are carried out to discern beneficial from MoO3 addition. It is observed that the diffusion coefficient and the charge density increase with an increment of MoO3 concentration. The coloration efficiency at 430 nm raises apparently whereas that at 633 nm does not show a significant variation with increasing MoO3 concentrations. In addition, the EC device with ∼12.5% of MoO3 introduced reveals a much higher optical modulation at 430 nm. From the measurements of galvanostatic charge-discharge curves of EC films in the potentiostat, the areal capacitances of 12.9 and 14.0 mF/cm2 are estimated for pure and ∼12.5% MoO3 introduced WO3 EC films. The EC batteries are demonstrated and the enlarged energy storage capability with an addition of MoO3 is observed. The simple introduction of MoO3 into WO3 nanoparticulate films compensates drawbacks of WO3 films and enhances EC performances and energy storage capabilities.

The Effect of Transparent Conducting Oxide Films on WO3-Based Electrochromic Devices with Conducting Polymer Electrolytes

Over the past few decades, electrochromism has been a prominent topic in energy-saving applications, which is based on the mechanism of altering the optical transmittance of EC materials under the effect of a small applied voltage. Thus, tungsten oxide (WO3) is a significant chemical compound typically applied in electrochromic devices (ECDs) as it is responsible for the optical transmittance variation. In this work, the WO3 films were produced through a sol-gel spin-coating method. The effect of various transparent conducting oxides (TCOs, which are indium-doped tin oxide (ITO), fluorine-doped tin oxide (FTO) glass substrates, and aluminum-doped zinc oxide (AZO)) was investigated in the construction of ECDs. Based on a conducting polymer polypyrene carbonate electrolyte, ITO and aluminum-doped zinc oxide (AZO)-coated glasses were also examined as counter electrodes. The electrode combination employing FTO and ITO as the TCO and counter electrode, respectively, exhibited the most significant coloration efficiency of 72.53 cm2/C. It had coloring and bleaching transmittance of 14% and 56%, respectively, with a large optical modulation of 42%. In addition to that, ECDs with the AZO counter electrode have the advantage of lower intercalation charges compared to ITO and FTO. Hence, this research offers a new avenue for understanding the role of common TCO and counter electrodes in the development of WO3-based ECDs with conducting polymer electrolytes.

Review on recent progress in WO3-based electrochromic films: preparation methods and performance enhancement strategies.

Transition metal oxides have drawn tremendous interest due to their unique physical and chemical properties. As one of the most promising electrochromic (EC) materials, tungsten trioxide (WO3) has attracted great attention due to its exceptional EC characteristics. This review summarizes the background and general concept of EC devices, and key criteria for evaluation of WO3-based EC materials. Special focus is placed on preparation techniques and performance enhancement of WO3 EC films. Specifically, four methods - nanostructuring, regulating crystallinity, fabricating hybrid films, and preparing multilayer composite structures - have been developed to enhance the EC performance of WO3 films. Finally, we offer some important recommendations and perspectives on potential research directions for further study.

Seed layer formation determines photocurrent response of hydrothermally-grown WO3 photoanodes

We reveal that the preparation conditions of seed layers by spin coating have a very strong effect on the resulting photoelectrochemical performance of hydrothermally-grown WO3 films for water splitting, thus their preparation conditions can be tailored for maximum photocurrents.

Giant saturation absorption of tungsten trioxide film prepared based on the seedless layer hydrothermal method

Nonlinear materials have gained wide interest as saturable absorbers and pulse compression for pulsed laser applications due to their unique optical properties. This work investigates the third-order nonlinear phenomenon of tungsten trioxide (WO3) thin films. The giant nonlinear absorption and nonlinear refractive index of WO3 thin films were characterized by Z-scan method at 800 nm. We experimentally observed the giant saturable absorption (SA) and nonlinear refractive index of WO3 thin films prepared by the seedless layer hydrothermal method, with SA coefficient being as high as –2.59 × 105 cm⋅GW−1. The SA coefficient is at least one order of magnitude larger than those of the conventional semiconductors. The nonlinear refractive index n 2 of WO3 film has been observed for the first time in recent studies and the corresponding coefficient can be up to 1.793 cm2⋅GW−1. The large third-order nonlinear optical (NLO) response enables WO3 thin films to be promising candidates for optoelectronic and photonic applications in the near-infrared domain.

UV–Vis–NIR broad spectral photodetectors facilely fabricated with nonmetal plasmonic WO3−x nanosheets

Plasmonic metal nanostructures have been widely applied in photodetectors for the enhanced light response range and sensitivity. In contrast, photodetection based on surface plasmon effect of the emerging plasmonic nonmetals has not been investigated. Here, single nonmetal plasmonic WO3−x nanosheets were used as the sensing material for UV–Vis–NIR broad spectral photodetectors. The plasmonic WO3−x nanosheets were synthesized by solvothermal and follow-up thermal treatment in a hydrogen-containing atmosphere, which exhibited a localized surface plasmon resonance (LSPR) band centered at 899 nm with broad spectral absorption spanned from UV to NIR. Then photodetectors fabricated facilely by depositing Au electrodes on a film of WO3−x nanosheets showed sensitive response for the regulation of conductance through the plasmonic hot free charge carriers. The responsivity and detectivity were 52 mA/W and 1.46 × 108 Jones under an incident light with a wavelength of 980 nm with an ultralow bias of 0.01 V, and went up to 538 mA/W and 4.75 × 108 Jones under 0.1 V. The results demonstrate the great potential of nonmetal plasmonic materials for photodetection.

Two-color electrochromic devices using a tungsten oxide and nickel oxide double layer

Tungsten oxide (WO3) and nickel oxide (NiO) are typical inorganic electrochromic (EC) materials. The color of WO3 changes from transparent to blue by reduction, whereas that of NiO changes from transparent to brown by oxidation. This study fabricated EC devices with an indium tin oxide (ITO)/WO3/NiO/ITO/Nafion/ITO structure. The WO3/NiO double layer and Nafion were used as the EC layer and electrolyte, respectively. The color of the device changed from blue to transparent and then to brown by varying the applied potential from −2.5 to +1.0 V and then to +2.0 V. The WO3 and NiO films were reduced at −2.5 V; the WO3 film was oxidized at +1.0 V, and both were oxidized at +2.0 V. The transmittance in the visible and near-infrared regions changed independently. Hence, the devices are considered applicable for dual-band EC smart windows.

Facile Synthesis of Pure and Cr-Doped WO3 Thin Films for the Detection of Xylene at Room Temperature.

This paper focused on the preparation of pure and Cr-doped tungsten trioxide (WO3) thin films using the spray pyrolysis method. Different techniques were adopted to analyze these films' structural and morphological properties. The X-ray detection analysis showed that the average crystallite size of the WO3-nanostructured thin films increased as the Cr doping concentration increased. The atomic force microscopy results showed that the root-mean-square roughness of the films increased with Cr doping concentration up to 3 wt % and then decreased. The increased roughness is favorable for gas-sensing applications. Surface morphology and elemental analysis of the films were studied by field emission scanning electron microscopy with energy-dispersive X-ray spectroscopy measurements. The 3 wt % Cr-WO3 has a large nanoflake-like structure with high surface roughness and porous morphology. Gas-sensing characteristics of undoped and Cr-doped WO3 thin films were investigated with various gases at room temperature. The results showed that 3 wt % Cr-doped WO3 film performed the maximum response toward 50 ppm of xylene with excellent selectivity at room temperature. We believe that increased lattice defects, surface morphology, and roughness due to Cr doping in the WO3 crystal matrix might be responsible for increased xylene sensitivity.

Preparation of a Biodegradable Mulch Film Exhibiting a High Photothermal Conversion Efficiency and Bioactive Effects

AbstractIn this study, chitosan (CS), polyvinylpyrrolidone (PVP), polydopamine (PDA), and tungsten trioxide are compounded via melt blending to prepare a CS/PVP/PDA/WO3 composite mulch. The morphological characteristics, thermogravimetric analysis, transmission performance, photothermal performance, and degradation performance of the mulch film and the effect of the CS/PVP/PDA/WO3 composite mulch film on the growth of Dianthus chinensis L. and cucumber seeds are studied. The results show that the CS/PVP/PDA/WO3 film exhibits a good thermal stability, biodegradability, UV resistance, photothermal conversion ability, and transmittance. Evaluation of the effect of this mulch film on the growth of crops shows that the film can promote the germination of seeds and increase the seedling growth rate.

Large area planar UV–Visible photodetectors using wide bandgap WO3−δ films

A simple synthesis technique of semiconducting monoclinic phase tungsten oxide (WO3−δ) films is demonstrated. In this article, we report the effect of oxidation temperature on oxygen vacancy gradient of WO3 films and their photo sensing properties. The WO3−δ films were grown on Si/SiO2 substrates by argon ion-beam sputtering of a high purity W (tungsten) target followed by thermal oxidization in the tubular furnace at three different oxidation temperatures at 500, 600 and 700 °C with controlled oxygen gas pressure. Detailed crystal structure, microscopic and spectroscopic characterizations reveal that the oxidation at temperature 700 °C produces reasonably good crystalline quality of the film exhibiting UV–Visible photo sensing properties. The effect of oxidation temperature towards an enhancement in crystallinity which in turn gives rise to sharp Raman peak is discussed. The fabricated device exhibits a very low dark current of 7.5 × 10 −8 A at ±0.5 V bias for WO3−δ films oxidized at 700 °C. The photodetection measurements show linear current-voltage (I–V) behaviour with light-to-dark current (ILight/IDark) ratio ∼102 at 365 nm illumination (∼0.82 mW cm−2) at 2 V applied bias. The responsivity (Rλ), specific detectivity (D*) and external quantum efficiency (EQE) were found to be ∼0.12 AW−1, ∼2.1 × 1010 Jones (cm Hz1/2 W−1) and 40%, respectively, at 365 nm illumination (∼0.82 mW cm−2) with an applied bias of 2 V for the films oxidized at 700 °C. While upon visible light illuminations, the Rλ, D* and EQE were calculated to be ∼0.78 AW−1, ∼1.4 × 1011 Jones and 174%, respectively, upon 555 nm illumination (∼0.21 mW cm−2), with 2 V applied bias. A faster photocurrent relaxation time was observed from the films oxidized at 700 °C. The ultraviolet photoelectron spectroscopy study shows a surface band gap of 3.7 eV with absolutely no density of states in the near Fermi-level, validating fully stoichiometric surface. The effects of oxidation temperatures causing the large bandgap are discussed in terms of the device performances.

Swift Photochromic Smart Window Based on Plasmonic Yolk‐Shell Nanophosphors

AbstractHerein, the fabrication of eccentric Ag@Void@Y2SiO5:Pr3+ plasmonic yolk‐shell nanophosphors (PYSNPs) with high up‐conversion efficiency, and their incorporation on photochromic WO3 layers, is demonstrated to fabricate photochromic smart windows with high coloration contrast and swift color change. The up‐conversion efficiency of the Pr3+ ion‐doped Y2SiO5 (Y2SiO5:Pr3+) nanoshells is enhanced by about 50% through the incorporation of carefully designed plasmonic Ag cores placed inside their hollow cavities. Introduction of these geometry‐optimized plasmonic nanophosphors at the surface of photochromic WO3 films results in about 24‐fold and tenfold enhancements of their coloration rate and kinetic constant, respectively. The strategy adopted for designing and fabricating the Ag@Void@Y2SiO5:Pr3+ PYSNPs and their subsequent incorporation in conventional photochromic films offers an exciting approach for fabricating photochromic smart windows with dynamically controlled transmission of solar radiation, which is a promising technology for reducing energy consumption associated with heating, ventilation, and air‐conditioning of buildings.

Enhancement reversibility and electrochromic efficiency of porous WO3 films by sparking method under electric/magnetic field

• The film prepared under an electric field provided extremely fluffy morphology. • The film prepared under combined electric/magnetic fields revealed denser morphology. • Dense/porous double-layer WO 3 films were designed and fabricated by sparking method. • The reversibility has been improved from 16.1% to 83.7%. • Enhanced 7-fold optical modulation and 9-fold coloration efficiency. This article reports on the morphological design to enhance reversibility and electrochromic performance of porous WO 3 films synthesized by sparking method using electric field (E = 6 kV/cm) and magnetic field (B = 500 mT). The excellent H + ion exchange between liquid electrolyte and the WO 3 films was influenced by extremely fluffy morphology as observed by sparking under an electric field— WO 3 with E sample. The morphology of the films becomes denser when sparked under a combination of electric and magnetic fields—WO 3 with (E + B) sample—which reduces the diffusion pathway and improves electron transfer within the films. Finally, we designed and fabricated dense/porous double-layer WO 3 films (WO 3 with (E + B)/E sample) to integrate their advantages to enhance 7-fold optical modulation and 9-fold coloration efficiency. Furthermore, the reversible efficiency increased from 16.1 % to 83.7 % compared to WO 3 with E sample.