Three-Layer PdO/CuWO4/CuO System for Hydrogen Gas Sensing with Reduced Humidity Interference

The growing hydrogen industry is stimulating an ongoing search for new materials not only for hydrogen production or storage but also for hydrogen sensing. These materials have to be sensitive to hydrogen, but additionally, their synthesis should be compatible with the microcircuit industry to enable seamless integration into various devices. In addition, the interference of air humidity remains an issue for hydrogen sensing materials. We approach these challenges using conventional reactive sputter deposition. Using three consequential processes, we synthesized multilayer structures. A basic two-layer system composed of a base layer of cupric oxide (CuO) overlayered with a nanostructured copper tungstate (CuWO4) exhibits higher sensitivity than individual materials. This is explained by the formation of microscopic heterojunctions. The addition of a third layer of palladium oxide (PdO) in forms of thin film and particles resulted in a reduction in humidity interference. As a result, a sensing three-layer system working at 150 °C with an equalized response in dry/humid air was developed.

Synthesis and Characterization of Zn-doped CuWO4Nanoparticles and Their Opto-structural Properties

CuWO4 and Zn-doped CuWO4 nanoparticles were prepared by a solid-state reaction method. The XRD study confirms the triclinic crystal structure for both samples and the peak shift is noticed for Zn-doped CuWO4 particles with high crystallinity. The FTIR spectra show metal oxide vibration which arose from the CuWO4 and Zn-doped CuWO4 particles. The optical absorption spectra exhibit strong absorption in the visible region and the band gap of Zn-doped CuWO4 is found to be increased to 2.44 eV compared to that of CuWO4 (2.36 eV), which is due to the elevated conduction band levels after Zn-doping. The SEM images of both CuWO4 and Zn-doped CuWO4 nanoparticles show densely aggregated particles. Keywords: Copper tungstate, Zn-doped CuWO4, Absorption, Nanoparticles.

Tungsten Oxide Based Hydrogen Gas Sensor Prepared by Advanced Magnetron Sputtering

In this study, we demonstrate the advantages of two advanced sputtering techniques for the preparation of a thin-film conductometric gas sensor. We combined tungsten oxide (WO3) thin films with other materials to achieve enhanced sensorial behavior towards hydrogen. Thin films of WO3 were prepared using the DC and HiPIMS technique, which allowed us to tune the phase composition and crystallinity of the oxide by changing the deposition parameters. The second material was then added on-top of these films. We used the copper tungstate CuWO4 in the form of nano-islands deposited by reactive rf sputtering and Pd particles formed during conventional dc sputtering. The specimens were tested for their response to a time-varied hydrogen concentration in synthetic air at various temperatures. The sensitivity and response time were evaluated. The performance of the individual films is presented as well as the details of the synthesis. Advanced magnetron techniques (such as HiPIMS) allowed us to tune the property of the film to improve its sensorial behavior. The method is compatible with the silicon electronics industry and consists of a few steps that do not require any wet technique, and the films can be used in an as-deposited state. Therefore, sensorial nanostructured materials prepared using magnetron sputtering are very suitable for use in miniaturized electronic devices.

Nanostructured Materials Based on Thin Films and Nanoclusters for Hydrogen Gas Sensing

In this paper, we present two approaches to synthesize nanostructured metal oxide semiconductors in a form of multi-layer thin films later assembled as a conductometric gas-sensors. The first approach produces a combination of thin solid film of tungsten trioxide (WO3) with nanoclusters of cupric oxide (CuO) prepared by a magnetron-based gas aggregation cluster source (GAS). The second method is a two-step reactive magnetron sputtering forming a nanostructured copper tungstate (CuWO4) on-top of a WO3 film. Both methods lead to synthesis of nanosized hetero-junctions. These greatly improve the sensorial response to hydrogen in comparison with a WO3 thin film alone.

Heterogeneous catalysis for thioanisole oxidation using hydrogen peroxide and Copper, Nickel and Zinc tungstates prepared by the polymeric precursor method

The oxidation of sulfides to sulfoxides and sulfones provides valuable sulfur-containing chemical compounds that are used in the pharmaceutical agrochemical industry. Although some tungsten catalytic systems have been applied to sulfide oxidation, the most desirable heterogeneous catalytic protocols have not been described. The copper, nickel and zinc tungstates powders obtained by the polymeric precursor method and the evaluation of their catalytic activity in thioanisole oxidation were investigated. Thioanisole was oxidized by hydrogen peroxide to sulfoxides and sulfones and the presence of catalyst accelerates the conversion. Of the three catalysts, copper tungstate was the most efficient in the conversion process followed by nickel tungstate and finally zinc tungstate. The copper tungstate has higher hydrogen consumption, indicating a higher oxygen content on the surface and the ability to increase surface mobility, which increases the conversion and selectivity of the process. The addition of 0.1 mL of hydrogen peroxide enhanced the conversion and increased the amount of sulfone produced. The ideal reaction time was 12 hours and the optimum temperature was 75 °C.