WO3 Thin Films Research Articles

Effect of thermal treatment on thin films of NiO:WO3 for optoelectronic applications

Nickel oxide (NiO), a semiconducting metal oxide, with its exceptional optical, electrical, and magnetic properties make it highly sought after for use in optical, optoelectronic, and electrochromic devices. Recent advancements in NiO-optical devices relies on the optical parameters like intrinsic optical nonlinearities. The current study involves the deposition of NiO:WO3 thin films utilizing radio frequency (RF) sputtering at various substrate temperature (Ts) of 100, 200, and 300 °C. An investigation was carried out to analyse the significance of Ts on the structural, optical, morphological, & vibrational characteristics. The X-ray diffraction investigation revealed the amorphous nature of the films. Scanning electron microscopy confirms the compact pinhole free surface of the films. The results of energy dispersive X-ray spectroscopy (EDS) spectra verified the existence of nickel (Ni), oxygen (O), and tungsten (W) components in the films. XPS analysis confirms the presence of both Ni2+ and Ni3+ states. The optical transmittance increased, and the band gap decreased with increase in Ts. The impact of Ts on various optical parameters such as refractive index, extinction co-efficient, dielectric constants, optical non-linear susceptibility, and dispersion energy parameters obtained by the Wemple-DiDomenico theory has been calculated and extensively investigated. The results indicate that NiO:WO3 films can be used to fabricate electrochromic and optoelectronic devices.

Simultaneous enhancement of visible light absorption and energy storage performances of W5+ implanted WO3-x thin film electrodes

Herein, we report the implantation of oxygen vacancy dopants in electro-coated WO3 thin film electrodes for enhanced energy and environmental applications. The implantation method involved partial de-oxidation in sodium borohydride to introduce oxygen vacancies into the WO3 electrodes. The doped WO3-x demonstrated a significantly lowered average band energy of 2.11 eV, indicating its ability to absorb over 75 % of the visible light spectrum. Moreover, a thin film asymmetric supercapacitor assembled with WO3-x negatrode demonstrated an improved areal capacitance of 1.15 mFcm−2 and energy density of 0.33 μWhcm−2 while consuming power at 17.50 μWcm−2 when cycled at 25 μAcm−2. Our solution-processed technique allows for controllable vacancy implantation and binder-free coating of WO3-x on various substrates. It significantly reduces the processing time and requires low energy consumption, making it a practical and efficient method for enhancing the overall photo-conversion and energy storage performances of WO3-based electrodes.

Influence of Swift Heavy Ion Beam Irradiation on Optical, Structural, and Surface Morphological Properties of WO3 Thin Films Grown by RF Sputtering Method

Influence of Swift Heavy Ion Beam Irradiation on Optical, Structural, and Surface Morphological Properties of WO3 Thin Films Grown by RF Sputtering Method

Semiconductor WO3 thin films deposited by pulsed reactive magnetron sputtering

A pulsed reactive magnetron sputtering system with a tungsten target and a gas mixture of argon and oxygen was investigated as a source for the deposition of semiconductor WO3 thin films on soda lime glass substrates and on the glass with transparent conductive SnO2:F (FTO) electrode. The reactive sputtering process was performed in HiPIMS mode with low pulse repetition frequency fp ≈ 50–100 Hz and short pulse duration in HiPIMS discharge Ton = 100 μs. The second mode investigated was the mid-frequency (MF) magnetron discharge with pulse frequency fp = 40 kHz and pulse length Ton = 15 μs. The plasma parameters were investigated for both HiPIMS and MF modes using the planar RF probe operating at the frequency fprobe = 350 kHz and the grid QCM with biased collector electrode. Ion density ni and tail electron temperature (Te) were determined in both pulsed reactive magnetron sputtering discharge modes with time resolution. The maximum value ni ≈ 5 · 1017 m−3 was found in the reactive HiPIMS mode, and the maximum value ni ≈ 7 · 1016 m−3 was found in the reactive MF (40 kHz) mode. The degree of ionization of sputtered particles in reactive HiPIMS was determined for different values of (QO2) and was found to be in the range of ri ≈ 0.1–0.3. The deposition rate determined by QCM in reactive HiPIMS was practically independent on (QO2), but in the case of reactive MF, the measured deposition rate decreased significantly with increasing (QO2). The WO3 films deposited in both modes have a predominantly monoclinic crystal structure. The light and dark conductivity and the light/dark conductivity ratio (Ld) were measured under dark conditions and UV light illumination. At higher (QO2), the maximum value of Ld ≈ 300 was found for MF deposited WO3 and the maximum value of Ld ≈ 30 was found for HiPIMS deposited WO3. The photoelectrochemical measurement of WO3 deposited on FTO electrodes confirmed the n-type conductivity, and these films functioned as photoanodes in photoelectrochemical cells. MF deposited WO3 films systematically exhibited slightly higher photocurrents than HiPIMS deposited WO3. It was shown that these optimum photocurrents for HiPIMS and MF were found at QO2 ≈ 80 sccm and could not be improved by further increasing of (QO2).

Durability improvement of electrochromic WO3 thin films by deposition of an ultra-thin Al2O3 layer via atomic layer deposition

Introduction of an ultra-thin Al2O3 protective layer via atomic layer deposition (ALD) significantly enhances the durability and performance of amorphous tungsten trioxide (a-WO3) thin films for electrochromic devices. The Al2O3 layer, despite being less than 4 nm thick, effectively suppresses the dissolution of WO3 that typically occurs through hydration reactions forming WO3·H2O and tungstic acid (H2WO4). XRD and XPS analyses confirmed the presence of Al2O3 and its minimal impact on the amorphous structure of WO3 thin films. Optical transmittance measurements showed that the Al2O3-protected a-WO3 thin films maintained a higher transmittance modulation (ΔT) compared to bare a-WO3 thin films, with the ALD30 sample retaining 87.9 % of its initial transmittance modulation after 1000 cycles, in contrast to the rapid degradation observed in unprotected films. Microstructural analysis revealed significantly fewer surface cracks and reduced thickness loss in Al2O3-coated films after 100 cycles. Additionally, ICP-MS analysis indicated a much lower W concentration in the electrolyte for Al2O3-protected samples, confirming reduced dissolution.

Revealing Enhanced Optical Modulation and Coloration Efficiency in Nanogranular WO3 Thin Films Through Precursor Concentration Modifications

Electrochromic (EC) materials allow for dynamic tuning of optical properties via an applied electric field, presenting great potential in energy-efficient technologies, such as smart windows for effective light and temperature regulation. The precise control of precursor concentration has proven to be a powerful approach in tailoring the physicochemical properties of semiconducting metal oxides. In this study, we employed a one-step electrodeposition technique to fabricate tungsten oxide (WO3) thin films, systematically exploring how varying precursor concentrations influence the material’s characteristics. X-ray diffraction analysis revealed significant changes in diffraction patterns, reflecting subtle structural modifications due to concentration variations. Additionally, scanning electron microscopy revealed significant changes in the microstructure, showing a progression from small nanogranules to larger agglomerations within the film matrix. The W-25 mM thin film delivered exceptional EC performance, efficiently accommodating lithium ions while showcasing superior EC properties. The optimized electrode, denoted as W-25 mM, showcased exceptional EC metrics, featuring the highest optical modulation at 82.66%, outstanding reversibility at 99%, and a notably high coloring efficiency of 83.01 cm2/C. These findings emphasize the importance of precursor concentration optimization in enhancing the EC properties of WO3 thin films, contributing to the advancement of high-performance, energy-efficient materials.

Remarkable NH3 gas sensing performance of spray deposited Tb doped WO3 thin films at room temperature

To evade potential health hazards associated with exposure to NH3, there has been an increasing demand for efficient gas sensors operating at room temperature (RT). In this study, thin films of Tb-doped (0–5 wt%) WO3 synthesized by spray pyrolysis technique are used as a novel material for sensing NH3 gas. The X-ray diffraction (XRD) pattern identified the hexagonal crystal system of WO3 films and increased crystallinity for the 2 wt% Tb-doing concentration. Field emission scanning electron microscopy (FE-SEM) unveiled the distinct surface morphology of mesh-like porous structures for Tb-doped films suitable for the target gas adsorption/desorption process. Multiple photoluminescence (PL) emission peaks indicate the presence of defect states, including defect energy levels created by oxygen vacancies (Ov). Optical analysis indicated shrinkage of the bandgap of WO3 thin films for doping levels up to 2 wt%. Among all gas sensors, 2 wt% Tb-doped WO3 exhibited exceptionally high response and low response time at 250 ppm NH3 concentrations measured at room temperature (RT). The sensor’s performance for NH3 gas sensing is compared with previous reports on WO3-based NH3 sensors. The gas sensing mechanism in WO3 is also briefly discussed.

Silver doping effect on the electrochromic performance of the WO3 thin films produced by thermal evaporation in vacuum

Ag doped WO3 thin films are successfully fabricated by thermal evaporation method in vacuum as precursor, and then annealed at 450 °C in nitrogen gases atmosphere. Cyclic voltammetry, repeating chronoamperometry, chronocolumetry and optical transmittance are recorded in 1 M LiClO4/propylene carbonate electrolyte. The color of the WO3 thin films changes from transparent to the blue under the applied potential of -1.8 and +1.9 V. XRD studies show that all the films have a polycrystalline structure of WO3. The molecular formation is also confirmed by Raman and X-Ray photoelectron spectroscopy (XPS). The EDS elemental mappings clearly confirm the homogeneous distribution of Ag, W and O elements into the WO3 network. From the optical studies, indirect band gap energy of the WO3 decreases as Ag doping level increases. The best coloration and bleaching times (2.33 and 1.25 s) are obtained for the Ag(2 %):WO3 thin films, compared to the pure WO3 (3.79 and 1.96 s). The produced WO3 films exhibit high reversibility (39.6/69.1 %), high coloration efficiency values (26.3/141.2 cm2/C), and good cycling stability. From the EIS measurements, the charge-transfer resistance is Ag(2 %):WO3 < the pure WO3 < Ag(5 %):WO3 < Ag(8 %):WO3, which means that Ag(2 %):WO3 has the biggest electrochemically active surface area. Electronic energy levels of the pure WO3 and the Ag:WO3 thin films are also given. From the Mott-Schottky studies, it is determined that the donor concentration of the materials is of the order of 1014–1015 cm−3. In the literature, there are very limited studies related to electrochromic thin films by thermal evaporation in vacuum. In this sense, the reported results in this study are promising for the electrochromic applications.

Introducing oxygen vacancies into WO3 thin film for improving hydrogen sensing performance of Pd/WO3-x/AAO sensors

Introducing oxygen vacancy into semiconducting metal oxides (SMOs) is a strategic approach to promote their gas sensing performances. In this paper, the WO3-x thin films were meticulously deposited onto an anodic aluminum oxide (AAO) substrate, and the oxygen vacancies concentrations of WO3-x thin films were changed by adjusting the ratio of flux of Ar to O2 in magnetron sputtering process. Sequentially, a layer of palladium (Pd) was deposited on the WO3-x/AAO films, creating a nano-mesh structured hydrogen sensor with Pd as the catalytic element. The resulted amorphous WO3-x samples with varying concentration of oxygen vacancies were thoroughly analyzed using XPS and ESR, which confirmed that the oxygen vacancies concentration escalated with an increase in the ratio of flux of Ar to O2. The hydrogen sensitivity of prepared Pd/WO3-x/AAO sensors were examined. Our findings revealed an enhancement in hydrogen sensitivity for the sensors with an optimal concentration of oxygen vacancies. However, an excess of these vacancies was found to deteriorate their hydrogen sensing performance. Notably, the optimized Pd/WO3-x/AAO sensor exhibited a marked response even at a hydrogen concentration as low as 100 ppb, demonstrating its high sensitivity and selectivity towards hydrogen detection.

Opto-electronic responses of large area WSe2 layers synthesized by chemical conversion of WO3 thin films

Atomically thin tungsten diselenide (WSe2) has drawn much interest due to its remarkable optical properties and potential applications in next-generation optoelectronics and energy harvesting devices. High-quality, wafer-scale synthesis of such two-dimensional (2D) semiconductors is necessary for their practical applications. Chemical vapor deposition (CVD) is a promising method for synthesizing large-area thin films of 2D materials where the constituent elements are of widely different vapor pressure. Herein, we demonstrate large-area synthesis of the 2H phase of WSe2 with a precise control over the 2D film thickness. The first step of this process is the deposition of tungsten trioxide (WO3) films of a well-defined thickness by thermal evaporation over large areas. The subsequent selenization of these films in an atmospheric pressure CVD reactor, yields high-quality uniform films of WSe2. The quality of these films is ascertained by measuring their characteristic Raman active vibrational modes and layer thickness dependent optical absorption and photoluminescence. The photoconductivity (PC) of these films has been measured with 405, 532 and 915 nm laser excitation. We note an increase in the PC of the films with their thickness. The best photoresponsivity and detectivity obtained are 11.3 mA/W and 1.42 × 108 Jones respectively. These results are promising to create photo-transistor arrays over large areas using such WO3 processed films.

Optimizations of Liquid Phase Deposition Processes for Enhanced Photoelectrocatalytic Activities of Tungsten Oxide Thin Films.

This study focuses on the preparation of tungsten oxide (WO3) as the photoanode for water oxidations by the liquid phase deposition (LPD) technique and its optimizations to improve the photoelectrochemical performance. The alternative precursor large stock solution process was achieved to simplify the LPD process for WO3 thin film preparation. The effect of boric acid in the precursor solutions on the physicochemical properties of the deposited WO3 thin films was investigated. As a result, we found that the optimized concentration of boric acid realized the highest photoelectrochemical performance. Through the optimizations of reaction conditions and surface analyses, we concluded that the preparations of a semiconductor film via the LPD technique had the potential to obtain high-performance photoelectrocatalytic applications.

Enhanced hydrogen gas sensing performance with Ag-doped WO3 thin film

The paper reports the fabrication of a device, based on the thin film of Ag-doped WO3 nanoparticles, to enhance the hydrogen gas sensing performance. The synthesis of thin films of pristine and Ag-doped WO3 nanoparticles were carried out using the single-step hydrothermal method, followed by the spin coating deposition process. The characterization of the samples was carried out with XRD, FESEM, TEM, FTIR and UV–vis spectroscopy. The consistent pattern observed from the microstructure analysis of data from XRD, UV–vis spectrometer and FTIR validates effective incorporation of Ag into the WO₃ structure. The XRD patterns confirmed the formation of the monoclinic phase of WO3 nanoparticles. The crystallite size and the value of band gap decreased on increasing the Ag doping percentage in WO3. This reduction with Ag-doping could be attributed to the proper substitution of the W with Ag due to their similar sizes. The gas sensing performances of the fabricated sensing devices were analyzed by exposing different concentrations of hydrogen gas ranging from 12.5 ppm to 75 ppm. The device based on the thin film of Ag-doped WO3 performed much better than the device based on the thin film of pristine WO3 nanoparticles. The device with 4% Ag-doped WO3 showed the best sensor response of 98 % for 75 ppm H2 gas concentration with response and recovery time of 76 and 100 s, respectively at 100 °C. The improved response of the device with Ag doping compared to the pristine device could be attributed to the better charge transfer, more defect creation, and the ability of Ag to work as a catalyst to make reaction kinetics faster. The selectivity of the device was tested using the oxidizing and reducing gasses of NO2, H2, and NH3 at 100 °C for 75 ppm gas concentration. The recorded response of the device for NO2, H2, NH3 were 38%, 98%, and 43%, respectively.

Raman Study of Novel Nanostructured WO3 Thin Films Grown by Spray Deposition.

The present communication reports on the effect of the sprayed solution volume variation (as a thickness variation element) on the detailed Raman spectroscopy for WO3 thin films with different thicknesses grown from precursor solutions with two different concentrations. Walls-like structured monoclinic WO3 thin films were obtained by the spray deposition method for further integration in gas sensors. A detailed analysis of the two series of samples shows that the increase in thickness strongly affects the films' morphology, while their crystalline structure is only slightly affected. The Raman analysis contributes to refining the structural feature clarifications. It was observed that, for 0.05 M precursor concentration series, thinner films (lower volume) show less intense peaks, indicating more defects and lower crystallinity, while thicker films (higher volume) exhibit sharper and more intense peaks, suggesting improved crystallinity and structural order. For higher precursor concentration 0.1 M series, films at higher precursor concentrations show overall more intense and sharper peaks across all thicknesses, indicating higher crystallinity and fewer defects. Differences in peak intensity and presence reflect variations in film morphology and structural properties due to increased precursor concentration. Further studies are ongoing.

Improving the photoelectrochemical performance of WO3 photoelectrodes by controlling the pH and Na+ ion concentration through varying the number of H2WO4 precipitate washing treatments

In this study, WO3 thin-film photoelectrodes were grown on fluorine-doped tin oxide (FTO) substrates using the spin coating method. The H2WO4 precipitate underwent various washing treatments to prepare the Na2WO4 precursor solution, which was subsequently used for growing photoelectrodes. We found that washing with the H2WO4 precipitate changed the Na + ion concentration and pH of the precursor solution containing the H2WO4 precipitate and subsequently affected the thickness, X-ray diffraction (XRD) peak intensity, optical energy bandgap, flat-band potential, and photoelectrochemical (PEC) properties of the WO3 photoelectrode. In particular, the Na + ions in the H2WO4 precipitate solution negatively affected the PEC properties by causing photogenerated electron–hole recombination and worsening the adhesion between the substrate and the WO3 thin film. Therefore, H2WO4 precipitate washing treatment is crucial for reducing the Na + ion concentration in solutions containing H2WO4 precipitate. Consequently, among the samples grown using different numbers of washing treatments, the sample grown using 12 precipitate washing treatment cycles exhibited the highest photocurrent density of 1.37 mA/cm2 and a photostability of 97.5 %. In this study, the morphological, structural, optical, electrical, and PEC properties of WO3 photoelectrodes subjected to H2WO4 precipitate washing treatment were systematically analyzed.

Enhancement of gasochromic response to hydrogen of WO3 thin films by post-process modification and catalyst selection

The gasochromic properties of WO3 thin films deposited by electron beam evaporation were investigated, focusing on their structural, morphology and optical changes caused by heat treatment. Moreover, the influence of the thickness and type of catalyst on the gasochromic properties of WO3 was examined. X-ray diffraction measurements revealed that WO3 thin films annealed up to 473 K remained amorphous, whereas annealing at 673 K and above, initiated crystallization into a monoclinic structure. Gasochromic behaviour was observed in response to hydrogen exposure, with the most significant changes occurring in films annealed at 673 K with a thin adlayer of palladium catalyst. A gasochromic response of 1.84 was achieved for H2 concentration as low as 25 ppm. The process of gasochromic colouration was correlated with the changes of free charge carrier concentration calculated based on Drude free electron theory. XPS analysis confirms WO3 reduction under hydrogen exposure, validating proposed colouration mechanisms. These comprehensive findings offer insights into optimizing gasochromic thin films for various sensing applications.

Enhanced activity of electrodeposited WO3 thin films as bi-functional electrocatalysts for water splitting

The ultimate goal of the hydrogen economy is to develop an efficient and cost-effective electrocatalyst that can accelerate hydrogen synthesis from water without or with little additional energy. This study describes a unique surface modified electrodeposited nano-sized tungsten oxide (WO3) as an intriguing bi-functional electrocatalyst for OER in alkaline and HER in acidic conditions. The nano-WO3 was synthesized hydrothermally and electrodeposited on a fluorinated tin oxide (FTO) electrode. The highly uniform distribution of the WO3@FTO catalyst results in negligible charge transfer resistance, a large electroactive surface area, and increased water-splitting potential. During oxygen evolution reaction (OER), electrodeposited WO3@FTO initiates water splitting at an overpotential (η) of just 240 mV and represents a turnover frequency (TOF) of 0.59 sec−1. These results are comparable to previously reported electrocatalysts. Under an alkaline electrolyte, a current density of 15 mA/cm2 remained constant for several hours, indicating the high stability and durability of the electrodeposited WO3@FTO electrode. The electrode also performed well in hydrogen evolution reactions (HER). A Tafel slope of 46 mV/dec and −28 mV/dec was found for OER and HER, respectively, indicating an enhanced kinetics rate of reaction taking place at the electrode surface. Furthermore, in acidic conditions, the electrode represents a lower HER overpotential of 104 mV. The work provides a significant understanding of electrodeposited WO3@FTO electrodes and their role in electrochemical water splitting.

Temperature-dependent wetting and other physical characteristics of sputtered grown WO3 thin films

Temperature-dependent wetting and other physical characteristics of sputtered grown WO3 thin films

DFT and experimental investigations on Pr substituted WO3 for electronic, thermoelectric and optical applications

The properties of Pr-doped WO3 thin films were studied to examine the optical, thermoelectric, and electrical response for optoelectronic and photovoltaic applications. The thin films were synthesized using the spin coating technique. The various properties were explored by employing the DFT approach. The X-ray diffraction analysis showed the cubic phase crystalline structure of pure and Pr-doped compositions. The morphology contained uniform rod-like uniform features of WO3 thin films which changed to the un-even and non-uniform geometry with increased width in maximum Pr-doped compositions. The projected density of states revealed the major contribution of O-p in valance and W-d in the conduction band while Pr-f orbital showed an additional significant contribution in the Pr-doped compositions. The thermoelectric properties were significantly changed with the increase in Pr doping contents. The highest values of real epsilon and refractive index were found approximately 8.8 and 3.5 for maximum Pr-doped contents. The absorption coefficient and optical conductivity displayed an increasing trend with increment of Pr-doping contents suggesting the suitability with enhanced photovoltaic and optoelectronic properties. A reduction in the band gap of WO3 was observed with Pr-doping content.

Electrochromic properties of sputter-grown Nb18W16O93 thin films

Large-area electrochromic films with high stability under high temperatures and humid conditions are essential for the commercialization of smart windows. In this paper, we introduce a new cathodic coloration Nb18W16O93 thin film produced using sputtering that exhibits excellent electrochromic modulation, superior chemical stability, and cyclic durability owing to its stable host structure for ion intercalation. The electrochemical properties of the as-grown Nb18W16O93 thin films, including charge density and transmittance modulation, were enhanced with increasing deposition temperatures. The annealed Nb18W16O93 thin films (grown at room temperature) exhibited comparable electrochromic performances to films grown at high temperatures. Electrochromic devices composed of annealed and high-temperature-grown Nb18W16O93 thin films exhibited excellent durability and nearly identical transmittance modulation, even after 10,000 test cycles. Furthermore, degradation of the surface morphology of the Nb18W16O93 thin films after etching in heated water was alleviated, suggesting high stability under hot and humid conditions, whereas the flat surface of the room-temperature-grown WO3 thin films degraded significantly.

Sustainable kiwi fruit derived carbon dots architected WO3 (KF-CDs@WO3) thin films based flexible electrochromic devices with improved stability for next generation smart windows applications

The design and development of new advanced materials or technologies are essential for the construction of high performance smart windows for the next generation of energy efficient buildings. Tungsten trioxide (WO3) based flexible electro-chromic devices f-ECDs have received extensive attention for the development of next generation f-ECDs. Here, we demonstrated the design and fabrication of kiwi fruit peel derived carbon dots (KF-CDs) decorated WO3 films under benign approaches. Furthermore f-ECDs were developed and their performance have been examined in terms of transmittance and coloration efficiency. The amount of KF-CDs in to the WO3 was varied and optimized f-ECDs exhibited the excellent optical contrast (ΔT) of 83.74 %, coloration efficiency of 86.2 cm2/C. The optimized f-ECDs exhibited excellent cyclic stability of 25000th cycles and retained more than 80 % optical contrast compared to the initial performance. This improved stability may be attributed to the presence of strong hydrogen bonding between the functional groups of KF-CDs and WO3.