Kinetics-Driven Dual Hydrogen Spillover Effects for Ultrasensitive Hydrogen Sensing

Palladium (Pd)-modified metal oxide semiconductors (MOSs) gas sensors often exhibit unexpected hydrogen (H₂) sensing activity through a spillover effect. However, sluggish kinetics over a limited Pd-MOS surface seriously restrict the sensing process. Here, a hollow Pd-NiO/SnO₂ buffered nanocavity is engineered to kinetically drive the H₂ spillover over dual yolk-shell surface for the ultrasensitive H₂ sensing. This unique nanocavity is found and can induce more H₂ absorption and markedly improve kinetical H₂ ab/desorption rates. Meanwhile, the limited buffer-room allows the H₂ molecules to adequately spillover in the inside-layer surface and thus realize dual H₂ spillover effect. Ex situ XPS, in situ Raman, and density functional theory (DFT) analysis further confirm that the Pd species can effectively combine H₂ to form Pd-H bonds and then dissociate the hydrogen species to NiO/SnO₂ surface. The final Pd-NiO/SnO₂ sensors exhibit an ultrasensitive response (0.1–1000 ppm H₂) and low actual detection limit (100 ppb) at the operating temperature of 230 °C, which surpass that of most reported H₂ sensors.     

Template-free to fabricate highly sensitive and selective acetone gas sensor based on WO3 microspheres

In this paper, we described how WO₃ microspheres have been synthesized by a simple hydrothermal treatment from the precursor of Na₂WO₄ and CO (NH₂)₂ at 160 °C for 3 h. The sample was characterized by X-ray diffraction (XRD), energy dispersive spectrum (EDS), scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), high resolution transmission electronic microscopy (HRTEM), selected area electron diffraction (SAED), and X-ray photoelectron spectroscopy (XPS). Obtained results indicated that the average diameter of the as-prepared WO₃ microspheres is ∼1 μm with orthorhombic phase. The WO₃ microspheres are constructed of polycrystalline WO₃ nanoparticles. The thickness of the WO₃ coating of the gas sensor is ∼2 μm. The WO₃-based gas sensor exhibited a good sensitivity and high selectivity to acetone vapor at 200 °C. A possible mechanism for the acetone vapor gas sensing was proposed.     

Nanocrystalline Ho3+-doped WO3: a promising material for acetone detection

In this article, we describe how Ho³⁺-doped WO₃ nanoparticles have been synthesised by sol–gel treatment from the precursors of W, H₂O₂, Ho₂O₃ and HNO₃ at room temperature, followed by calcinations at 500°C for 1?h. The samples were characterised by X-ray diffraction, scanning electronic microscopy and X-ray photoelectron spectroscopy. Obtained results indicated that the as-synthesised particle diameter is ～60?nm under different Ho₂O₃-doped concentrations. In comparison with pure WO₃ sensor, all of the Ho³⁺-doped sensors showed better sensing performance in respect of sensitivity, selectivity and optimum operating temperature. The effects of Ho₂O₃ content, acetone vapour concentration and operating temperature on the sensing characteristics of the Ho³⁺-doped WO₃ sensors were also investigated. The sensor containing 3.0?mol% Ho₂O₃ exhibited the maximum sensitivity to acetone vapour at 200°C. A possible mechanism for the influence of Ho₂O₃ on the acetone-sensing properties of Ho₂O₃-doped WO₃ sensors was proposed.     

A General and Straightforward Route to Noble Metal‐Decorated Mesoporous Transition‐Metal Oxides with Enhanced Gas Sensing Performance

Controllable and efficient synthesis of noble metal/transition‐metal oxide (TMO) composites with tailored nanostructures and precise components is essential for their application. Herein, a general mercaptosilane‐assisted one‐pot coassembly approach is developed to synthesize ordered mesoporous TMOs with agglomerated‐free noble metal nanoparticles, including Au/WO₃, Au/TiO₂, Au/NbO_x, and Pt/WO₃. 3‐mercaptopropyl trimethoxysilane is applied as a bridge agent to cohydrolyze with metal oxide precursors by alkoxysilane moieties and interact with the noble metal source (e.g., HAuCl₄ and H₂PtCl₄) by mercapto (? SH) groups, resulting in coassembly with poly(ethylene oxide)‐b‐polystyrene. The noble metal decorated TMO materials exhibit highly ordered mesoporous structure, large pore size (≈14–20 nm), high specific surface area (61–138 m² g^?1), and highly dispersed noble metal (e.g., Au and Pt) nanoparticles. In the system of Au/WO₃, in situ generated SiO₂ incorporation not only enhances their thermal stability but also induces the formation of ε‐phase WO₃ promoting gas sensing performance. Owning to its specific compositions and structure, the gas sensor based on Au/WO₃ materials possess enhanced ethanol sensing performance with a good response (R_air/R_gas = 36–50 ppm of ethanol), high selectivity, and excellent low‐concentration detection capability (down to 50 ppb) at low working temperature (200 °C).     

Magnetron sputtered tungsten oxide films activated by dip-coated platinum for ppm-level hydrogen detection

Platinum-activated tungsten oxide (Pt-WO₃) films are prepared for hydrogen (H₂) sensing applications. In this study, WO₃ films were fabricated by reactive magnetron sputtering and Pt clusters were deposited on them by dip-coating. The microstructure, chemical composition and phase structure of Pt-WO₃ films were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction, respectively. It was observed that a reconditioning period is required after a rest period of the sensor for obtaining a stable signal. A thermal treatment at 450 °C for 24 h is proposed to solve this problem.     

Thin film deposition and characterization of pure and iron-doped electron-beam evaporated tungsten oxide for gas sensors

Pure tungsten oxide (WO₃) and iron-doped (10 at.%) tungsten oxide (WO₃:Fe) nanostructured thin films were prepared using a dual crucible Electron Beam Evaporation (EBE) technique. The films were deposited at room temperature under high vacuum onto glass as well as alumina substrates and post-heat treated at 300 °C for 1 h. Using Raman spectroscopy the as-deposited WO₃ and WO₃:Fe films were found to be amorphous, however their crystallinity increased after annealing. The estimated surface roughness of the films was similar (of the order of 3 nm) to that determined using Atomic Force Microscopy (AFM). As observed by AFM, the WO₃:Fe film appeared to have a more compact surface as compared to the more porous WO₃ film. X-ray photoelectron spectroscopy analysis showed that the elemental stoichiometry of the tungsten oxide films was consistent with WO₃. A slight difference in optical band gap energies was found between the as-deposited WO₃ (3.22 eV) and WO₃:Fe (3.12 eV) films. The differences in the band gap energies of the annealed films were significantly higher, having values of 3.12 eV and 2.61 eV for the WO₃ and WO₃:Fe films respectively. The heat treated films were investigated for gas sensing applications using noise spectroscopy. It was found that doping of Fe to WO₃ produced gas selectivity but a reduced gas sensitivity as compared to the WO₃ sensor.     

Electrospun SiO2/WO3/NiWO4 decorated carbon nanofibers for an efficient electrocatalytic hydrogen evolution

The SiO₂/WO₃/NiWO₄ composites modified carbon nanofibers (SiWNi-CNFs) were prepared by a facile electrospinning method with following carbonization process under nitrogen atmosphere. The as-obtained SiWNi-CNFs were characterized by transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray photoelectron spectra (XPS), X-ray powder diffraction (XRD), FT-IR spectroscopy and Raman spectroscopy. As revealed by the electrochemical measurement, the SiWNi-CNFs prepared with SiW₁₂/NiAc₂ molar ratio of 1:1 presented best hydrogen evolution activity with a small Tafel slope (48?mV dec^?1) among all the as-prepared samples. Notably, the as-prepared catalysts exhibit a small onset potential (0.29?V vs. reversible hydrogen electrode), high current density and excellent stability. The experimental results pointed that the SiWNi-CNFs processes more efficient hydrogen evolution properties than that other contrast samples. This is due to the SiO₂/WO₃/NiWO₄ composite modified on the surface of carbon nanofibers can generate numerous active sites from the synergistic effect of each component. At the same time, the intimate combination of ternary oxide and carbon nanofibers can accelerate the electron transfer, enhance the stability and hinder the aggregation of active components during the carbonization. Moreover, the net-like structure stacked by carbon nanofibers should render the exposure of active sites and facilitate the mass transport for the HER process.     

Pulsed laser deposition of (WO3)1 − x(Nb2O5)x thin films: Characterization and gasochromic studies

A series of (WO₃)_{1 − x}(Nb₂O₅)_x (x = 0, 0.05, 0.1 and 0.15) mixed oxide films were fabricated by pulsed laser deposition (PLD) at 27 Pa oxygen partial pressure on ITO glass substrates. The thickness of the (WO₃)_{1 − x}(Nb₂O₅)_x thin films is about 350 ± 30 nm and their surface has a uniform morphology. A layer of platinum (Pt) was then sputtered onto the surface of the film. The hydrogen gas sensing performance of Pt catalyst activated (WO₃)_{1 − x}(Nb₂O₅)_x thin films were investigated. The cycling of the coloration was obtained from UV–vis spectra. Gasochromic coloration of (WO₃)_{1 − x}(Nb₂O₅)_x thin films were investigated at room temperature in H₂–N₂ mixtures containing 1–100 mol% of H₂. The results show that the shortest response time of (WO₃)_{1 − x}(Nb₂O₅)_x/Pt hydrogen sensor is within 30 s and the highest transmittance change (ΔT) varies from 20% to 30%.     

Interstitial Hydrogen Atom to Boost Intrinsic Catalytic Activity of Tungsten Oxide for Hydrogen Evolution Reaction

Tungsten oxide (WO₃) is an appealing electrocatalyst for the hydrogen evolution reaction (HER) owing to its cost-effectiveness and structural adjustability. However, the WO₃ electrocatalyst displays undesirable intrinsic activity for the HER, which originates from the strong hydrogen adsorption energy. Herein, for effective defect engineering, a hydrogen atom inserted into the interstitial lattice site of tungsten oxide (H_0.23WO₃) is proposed to enhance the catalytic activity by adjusting the surface electronic structure and weakening the hydrogen adsorption energy. Experimentally, the H_0.23WO₃ electrocatalyst is successfully prepared on reduced graphene oxide. It exhibits significantly improved electrocatalytic activity for HER, with a low overpotential of 33 mV to drive a current density of 10 mA cm⁻² and ultra-long catalytic stability at high-throughput hydrogen output (200 000 s, 90 mA cm⁻²) in acidic media. Theoretically, density functional theory calculations indicate that strong interactions between interstitial hydrogen and lattice oxygen lower the electron density distributions of the d-orbitals of the active tungsten (W) centers to weaken the adsorption of hydrogen intermediates on W-sites, thereby sufficiently promoting fast desorption from the catalyst surface. This work enriches defect engineering to modulate the electron structure and provides a new pathway for the rational design of efficient catalysts for HER.     

Cobalt Nanoparticles and Atomic Sites in Nitrogen‐Doped Carbon Frameworks for Highly Sensitive Sensing of Hydrogen Peroxide

In situ monitoring of hydrogen peroxide (H₂O₂) during its production process is needed. Here, an electrochemical H₂O₂ sensor with a wide linear current response range (concentration: 5 × 10^?8 to 5 × 10^?2 m ), a low detection limit (32.4 × 10^?9 m ), and a high sensitivity (568.47 µA mm ^?1 cm^?2) is developed. The electrocatalyst of the sensor consists of cobalt nanoparticles and atomic Co‐N_x moieties anchored on nitrogen doped carbon nanotube arrays (Co‐N/CNT), which is obtained through the pyrolysis of the sandwich‐like urea@ZIF‐67 complex. More cobalt nanoparticles and atomic Co‐N_x as active sites are exposed during pyrolysis, contributing to higher electrocatalytic activity. Moreover, a portable screen‐printed electrode sensor is constructed and demonstrated for rapidly detecting (cost ≈40 s) H₂O₂ produced in microbial fuel cells with only 50 µL solution. Both the synthesis strategy and sensor design can be applied to other energy and environmental fields.     

Response kinetics of a fiber-optic gas sensor using Pt/WO3 thin film to hydrogen

Response kinetics of a fiber-optic hydrogen gas sensor in air- and inert-atmosphere were characterized. The sensor is mainly based on the evanescent field interaction in hydrogen sensitive cladding which is used Platinum-supported tungsten trioxide (Pt/WO₃). When the sensor was exposed to 1 vol.% H₂/air and H₂/N₂ gas, the changes in optical power propagating through the fiber were about 30% and 50%, respectively. The detection limit was about 0.1 vol.% in air-atmosphere. The humidity dependence of the response kinetics was also evaluated. While the response speed in N₂-atmosphere was accelerated, the speed in air-atmosphere was suppressed by the humidity.     

Preparation of hexagonal WO3 from hexagonal ammonium tungsten bronze for sensing NH3

Hexagonal tungsten oxide (h-WO₃) was prepared by annealing hexagonal ammonium tungsten bronze, (NH₄)_0.07(NH₃)_0.04(H₂O)_0.09WO_2.95. The structure, composition and morphology of h-WO₃ were studied by XRD, XPS, Raman, ¹H MAS (magic angle spinning) NMR, scanning electron microscopy (SEM), and BET-N₂ specific surface area measurement, while its thermal stability was investigated by in situ XRD. The h-WO₃ sample was built up by 50-100 nm particles, had an average specific surface area of 8.3 m²/g and was thermally stable up to 450 °C. Gas sensing tests showed that h-WO₃ was sensitive to various levels (10-50 ppm) of NH₃, with the shortest response and recovery times (1.3 and 3.8 min, respectively) to 50 ppm NH₃. To this NH₃ concentration, the sensor had significantly higher sensitivity than h-WO₃ samples prepared by wet chemical methods.     

Ion sensing properties of vanadium/tungsten mixed oxides

Vanadium/tungsten mixed oxide (V₂O₅/WO₃) sensing membranes were deposited on glassy carbon substrates and used as the H⁺ sensor of the extended gate field effect transistor (EGFET) device. X-ray diffractograms indicated a decrease of the interplanar spacing of V₂O₅ after the insertion of WO₃ revealing that the lamellar structure is under compressive stress. The crystallinity increases with increasing WO₃ molar ratio. The film is not homogeneous, with more WO₃ material sitting at the surface. This influences the response of pH sensors using the EGFET configuration. The maximum sensitivity of 68 mV pH⁻¹ was obtained for the sample with 5% WO₃ molar ratio. For higher WO₃ molar ratios, the behavior is not linear. It can be concluded that V₂O₅ dominates for acidic solutions while WO₃ dominates for basic solutions. Therefore, the mixed oxide with low amount of WO₃ is the main candidate for further use as biosensor.     

Ar + H2 plasma etching for improved adhesion of PVD coatings on steel substrates

Ar + H₂ plasma cleaning has been described for the surface modification of the steel substrates, which removes oxides and other contaminants from substrate surface effectively leading to a better adhesion of the physical vapor deposited (PVD) coatings. Approximately 1.1-1.3 μm thick TiAlN coatings were deposited on plasma treated (Ar and Ar + H₂) and untreated mild steel (MS) substrates. A mechanism has been put forward to explain the effect of plasma treatment on the substrate surface based upon the data obtained from X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The XPS measurements on untreated and Ar + H₂ plasma etched MS substrates indicated that the untreated substrate surface mainly consisted of Fe₃O₄, whereas, after etching the concentration of oxides decreased considerably. The FESEM and the AFM results showed changes in the surface morphology and an increase in the substrate roughness as a result of Ar + H₂ plasma etching. Removal of oxide/contaminants, formation of coarser surface and increased substrate surface roughness as a result of Ar + H₂ plasma etching facilitate good mechanical interlocking at the substrate surface, leading to a better adhesion of the deposited PVD coatings. The adhesion of TiAlN coating could be increased further by incorporating a very thin Ti interlayer.     

Two-dimensional Aluminum Oxide Nanosheets Decorated with Palladium Oxide Nanodots for Highly Stable and Selective Hydrogen Sensing

Hydrogen (H₂) sensing materials such as semiconductor metal oxides may suffer from poor long-term stability against humidity and unsatisfactory selectivity against other interfering gases. To address the above issues, highly stable and selective H₂ sensing built with palladium oxide nanodots decorating aluminum oxide nanosheets (PdO NDs//Al₂O₃ NSs) has been achieved via combined template synthesis, photochemical deposition, and oxidation. Typically, the PdO NDs//Al₂O₃ NSs are observed with thin NSs (≈17 nm thick) decorated with nanodots (≈3.3 nm in diameter). Beneficially, the sensor prototypes built with PdO NDs//Al₂O₃ NSs show excellent long-term stability for 278 days, high selectivity against interfering gases, and outstanding stability against humidity at 300 °C. Remarkably, the sensor prototypes enable detection of a wide-range of 20 ppm – 6 V/V% H₂, and the response and recovery times are ≈5 and 16 s to 1 V/V% H₂, respectively. Theoretically, the heterojunctions of PdO NDs-Al₂O₃ NSs with a large specific surface ratio and Al₂O₃ NSs as the support exhibit excellent stability and selective H₂ sensing. Practically, a sensing device integrated with the PdO NDs//Al₂O₃ NSs sensor prototype is simulated for detecting H₂ with reliable sensing response.     

Hierarchical pseudo-cubic hematite nanoparticle as formaldehyde sensor

To develop a low cost and scalable gas, sensor for the detection of toxic and flammable gases with fast response and high sensitivity is extremely important for monitoring environmental pollution. This work reports a facile method for preparing pseudo-cubic hierarchical α-Fe₂O₃ nanostructured materials as well as their implementation in gas sensor application. The α-Fe₂O₃ is developed using Fe(NO₃)₃ and ethylene glycol followed by a facile and one-step solvo-thermal reaction without subsequent heat treatment. The pseudo-cubic nanostructures were having an average edge length of 5–10 nm. The solvent played the role of ligand and synergistically affected olation and oxolation process along with dehydration to form final product. The sensor performance of α-Fe₂O₃ in the detection of toxic and flammable gases such as formaldehyde (HCHO), ethanol (C₂H₅OH), and carbon monoxide (CO) was evaluated. As-synthesized nanostructured hematite showed better sensing performance towards formaldehyde. The fabricated gas sensor showed temperature sensitivity sensing performance for the same gas. In addition, ethanol, formaldehyde vapours, and carbon monoxide gas-sensing properties were tested and the sensing performance of the synthesized material was found to be in the order of HCHO > C₂H₅OH > CO. This sensing performance is attributed to the large specific surface area of the pseudo-cubic nanoparticles.     

Microwave-assisted synthesis and humidity sensing of nanostructured α-Fe2O3

Nanocrystalline α-Fe₂O₃ has been prepared on a large-scale by a facile microwave-assisted hydrothermal route from a solution of Fe(NO₃)₃·9H₂O and pentaerythritol. A systematic study of the morphology, crystallinity and oxidation state of Fe using different characterization techniques, such as transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy was performed. It reveals that nanostructured α-Fe₂O₃ comprises bundles of nanorods with a rhombohedral crystalline structure. The individual nanorod has 8-10 nm diameter and ∼50 nm length. The as-prepared nanostructured α-Fe₂O₃ (sensor) gives selective response towards humidity. The sensor shows high sensitivity, fast linear response to change in the humidity with almost 100% reproducibility. The sensor works at room temperature and rejuvenates without heat treatment. The as-prepared nanostructured α-Fe₂O₃ appears to be a promising humidity sensing material with the potential for commercialization.     

Facile preparation and enhanced photocatalytic H2-production activity of Cu(OH)2 nanospheres modified porous g-C3N4

A facile precipitation approach for the preparation of Cu(OH)₂/g-C₃N₄ composite photocatalysts with good porous structure was developed for the first time. The as-synthesized samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet–visible light (UV–vis) absorbance spectra, photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). A photocatalytic water splitting reaction on the as-prepared photocatalysts were carried out under visible light irradiation. The results revealed that the prepared samples showed significantly enhanced photocatalytic activity. The optimal Cu(OH)₂ loading content was found to be 0.34 mol%, giving an H₂-production rate of 48.7 μmol h⁻¹ g⁻¹, which is higher 16.5 times than that of pure g-C₃N₄. This high photocatalytic H₂-production activity is attributed to the presence of Cu(OH)₂ clusters on the surface of the porous g-C₃N₄, which efficiently promotes the visible light absorption and separation of photogenerated electron–hole pairs.     

Influence of effective surface area on gas sensing properties of WO3 sputtered thin films

WO₃ thin films having different effective surface areas were deposited under various discharge gas pressures at room temperature by using reactive magnetron sputtering. The microstructure of WO₃ thin films was investigated by X-ray diffraction, scanning electron microscopy, and by the measurement of physical adsorption isotherms. The effective surface area and pore volume of WO₃ thin films increase with increasing discharge gas pressure from 0.4 to 12 Pa. Gas sensors based on WO₃ thin films show reversible response to NO₂ gas and H₂ gas at an operating temperature of 50-300 °C. The peak sensitivity is found at 200 °C for NO₂ gas and the peak sensitivity appears at 300 °C for H₂ gas. For both kinds of detected gases, the sensor sensitivity increases linearly with an increase of effective surface area of WO₃ thin films. The results demonstrate the importance of achieving high effective surface area on improving the gas sensing performance.     

Surface treatments of indium tin oxide films by using high density plasma

We investigated the effects of various surface treatments on the work function and chemical composition of an indium tin oxide (ITO) surface. Ultraviolet photoelectron spectroscopy (UPS) was used to measure the work function of ITO. X-ray photoelectron spectroscopy (XPS) was used to study the electron structures of ITO surface. We performed surface treatments on ITO using O₂ plasma and HCl solution. Our UPS/XPS analysis indicates increases in the work functions by O₂ plasma treatments. It is known that the Fermi energy level is controlled by the donor concentration, and thus the Fermi energy level is shifted toward the valence band minimum.