Thin films of tungsten trioxide prepared under an equal-flow argon and oxygen sputtering environment achieved an 86.18 percent sensitivity response toward 100 parts per million of hydrogen gas at an operating temperature of 150 degrees Celsius, outperforming films fabricated with added helium. The experimental investigation, conducted by researchers across institutions in India including UPES in Dehradun, the University of Delhi, MM Mahila College in Ara, and Samrat Ashok Rajkiya Engineering College in Mirzapur, examined how systematically varying the composition of reactive sputtering gas atmospheres directly alters the crystallite size, surface morphology, electronic band structure, and gas detection capabilities of tungsten trioxide thin films. The authors noted that identifying hydrogen at low concentrations is vital for early leak detection across hydrogen storage, transportation infrastructure, and fuel-cell systems, where combustible mixtures forming above 4,000 parts per million in air present severe safety and explosion hazards due to hydrogen’s low ignition energy of 0.017 millijoules.
To investigate these material changes, the team fabricated tungsten trioxide films onto indium tin oxide-coated glass and silicon (100) substrates using direct current reactive magnetron sputtering with an applied target power of 100 watts for 60 minutes. Sputtering was performed at a working chamber pressure of 0.02 Torr and a fixed substrate temperature of 400 degrees Celsius, which provided sufficient thermal energy to promote surface mobility and crystalline growth. Three distinct gas flow environments were maintained: the baseline sample, labeled W1, utilized 15 standard cubic centimeters per minute of argon and 15 standard cubic centimeters per minute of oxygen. In sample W2, 5 standard cubic centimeters per minute of helium was introduced while reducing argon to 10 standard cubic centimeters per minute. For sample W3, argon was entirely replaced, using 15 standard cubic centimeters per minute of helium alongside 15 standard cubic centimeters per minute of oxygen.
Structural evaluation using grazing incidence X-ray diffraction confirmed that all synthesized films crystallized in a polycrystalline monoclinic phase, showing characteristic diffraction peaks along the (020), (202), and (420) crystallographic planes without extraneous impurity phases. The film grown purely in argon and oxygen exhibited sharp diffraction reflections, with full width at half maximum of 0.8209 degrees for the principal (020) plane, corresponding to a Debye-Scherrer crystallite size of 9.23 nanometers. By contrast, replacing argon with helium caused significant peak broadening and a slight shift toward higher angles; the full width at half maximum expanded to 2.9048 degrees for W2 and 3.0727 degrees for W3, which yielded sharply diminished crystallite sizes of 2.96 nanometers and 2.57 nanometers. The researchers attributed this crystallite degradation to the significantly lower momentum transfer of light helium ions colliding with the tungsten target relative to heavy argon ions, which substantially lowered the kinetic energy and mobility of adatoms arriving at the substrate.
Field emission scanning electron microscopy and cross-sectional profiling further demonstrated stark morphological and physical differences governed by the working gas. Cross-sectional measurements established film thicknesses of roughly 650 nanometers for W1, 337 nanometers for W2, and 222 nanometers for W3, corresponding to deposition rates of 10.83, 5.61, and 3.7 nanometers per minute. This sharp drop in deposition rate aligns with the calculated energy-transfer factor at the tungsten target, which drops from approximately 0.586 for argon-tungsten collisions down to approximately 0.083 for helium-tungsten collisions. Furthermore, helium’s high ionization potential of 24.6 electron volts and its long-lived 19.8 electron volt metastable states dissociated molecular oxygen through Penning ionization, substantially elevating atomic oxygen flux while tungsten arrival decreased. As confirmed by energy-dispersive X-ray spectroscopy, tungsten content declined from 16.37 atomic percent in W1 to 7.56 atomic percent in W3, while oxygen increased from 83.63 to 92.44 atomic percent, producing over-oxidized, disordered networks.
Statistical evaluation of grain size distributions via ImageJ image processing revealed that the argon-grown W1 film developed coarse surface nanostructures with average grain diameters of 108 plus or minus 33 nanometers, approximately five times larger than the helium-deposited films. Sample W2 exhibited an average grain size of 19 plus or minus 4 nanometers, while sample W3 measured 24 plus or minus 6 nanometers, both approaching the measurement resolution limit of the electron microscope. In all samples, the scanning electron microscopy grain measurements exceeded the crystallite dimensions determined by X-ray diffraction, demonstrating that visible surface grains were composed of agglomerated coherent diffraction domains. Complementary atomic force microscopy confirmed that the W1 film had a surface roughness value of approximately 2.5 nanometers, noticeably rougher than the values of 0.3 nanometers recorded for W2 and 0.8 nanometers recorded for W3.
Optical and electronic band properties were evaluated using ultraviolet-visible spectroscopy and X-ray photoelectron spectroscopy. Optical absorption spectra analyzed via Tauc plots established that the monoclinic tungsten trioxide films exhibited indirect band gaps measuring 3.03 electron volts for W1, 3.31 electron volts for W2, and 3.15 electron volts for W3, with an experimental uncertainty of plus or minus 0.1 electron volts. X-ray photoelectron spectroscopy of sample W1 showed core W 4f peaks at 35.7 and 37.8 electron volts with a 2.1 electron volt spin-orbit splitting, confirming fully oxidized hexavalent tungsten without lower valence states. The O 1s spectrum revealed lattice oxygen at 530.1 electron volts and a secondary peak at 531.8 electron volts comprising 44 percent of total area, representing chemisorbed oxygen, hydroxyl species, and oxygen vacancies. Linear extrapolation placed the valence band maximum at 2.87 electron volts below the Fermi level, situating the conduction band minimum 0.16 electron volts above the Fermi level and validating n-type semiconducting behavior.
Gas sensing experiments were performed in a computer-controlled stainless steel chamber using real-time four-probe electrical measurements across silver contact electrodes that exhibited linear ohmic behavior between 2 and 10 volts. Testing across temperatures of 100, 150, and 200 degrees Celsius under 100 parts per million of hydrogen established 150 degrees Celsius as the optimal working temperature for the W1 film, which delivered a maximum sensitivity of approximately 86.18 percent. Sample W2 reached a lower sensitivity of 77.89 percent at 150 degrees Celsius, but maintained higher relative efficiency at 200 degrees Celsius, while sample W3 demonstrated the lowest sensing response across the tested temperature range. The drop in sensing performance above 150 degrees Celsius for W1 was attributed to accelerated thermal desorption of surface-bound oxygen and hydrogen molecules, which curtailed their active residence time on the metal oxide surface.
The sensing mechanism of the tungsten trioxide films depends on chemi-resistive resistance modulation. When exposed to ambient air, atmospheric oxygen molecules adsorb onto the film surface and extract conduction-band electrons to form ionized oxygen species, producing an electron depletion layer at grain boundaries that raises baseline electrical resistance. Upon the introduction of reducing hydrogen gas, hydrogen atoms react with these chemisorbed oxygen species to form water as a reaction byproduct, releasing trapped electrons back into the conduction band and decreasing film resistance. The superior response of the argon-grown W1 film was attributed to its higher density of surface-adsorbed oxygen ions, larger grain boundaries offering active reaction sites, higher surface area, and optimized crystalline charge transport characteristics compared to the helium-modified specimens.
Dynamic performance testing of the optimized W1 film demonstrated high selectivity toward hydrogen when evaluated against 100 parts per million of carbon monoxide, carbon dioxide, and methane at 150 degrees Celsius, with response and recovery times recorded at 22 seconds and 108 seconds, respectively. Extended concentration trials demonstrated reliable hydrogen detection across lower concentrations of 75, 50, and down to 25 parts per million. Sensor stability and repeatability assessments over three consecutive 100 parts per million hydrogen cycles at 150 degrees Celsius recorded sensitivities of 86.93 percent, 86.18 percent, and 88.40 percent, yielding an average sensitivity of 87.17 plus or minus 1.13 percent with consistent signal amplitudes. When contextualized against earlier literature involving radio frequency sputtering, hydrothermal synthesis, and chemical calcination methods, the direct current reactive magnetron sputtering technique demonstrated favorable reaction speeds and sensitivity, establishing gas-atmosphere tailoring as an effective methodology for engineering solid-state metal oxide hydrogen leak sensors.
Subject of Research: Hydrogen gas sensing performance of reactive magnetron sputtered tungsten trioxide thin films
Article Title: Tailoring surface and structural properties of WO3 thin films via mixed-gas plasma deposition for optimized H2 gas sensing performance
Article References: Kumar, S., Kharb, A. S., Bist, S., Yadav, H., Chawla, A. K., Awasthi, S., Singh, A., & Pandey, R. K. (2026). Tailoring surface and structural properties of WO3 thin films via mixed-gas plasma deposition for optimized H2 gas sensing performance. Frontiers in Materials, 13, Article 1942932. https://doi.org/10.3389/fmats.2026.1942932
Image Credits: AI Generated
DOI: 10.3389/fmats.2026.1942932
Keywords: tungsten trioxide, hydrogen gas sensing, reactive magnetron sputtering, thin films, gas sensors, semiconducting oxides, surface morphology, hydrogen safety, Tailoring, surface, structural, properties
News Source: Denise Maddox. (November 8, 2026). Argon Plasma Sputtering Optimizes Tungsten Oxide Thin Films for Hydrogen Sensing. Scienmag.
