Nitrogen dioxide (NO₂), often produced in coal, coal oil or natural gas manufacturing, is a toxic gas that harms the environment and human health even at extremely low concentrations and a sub-ppm level. When the concentration of NO₂ is higher than 20 ppm, it is dangerous to the human body and may cause death. Reliable NO₂ sensor with high sensitivity, selectivity and low energy consumption is highly demanded for environmental safety and industrial control. In recent years, one-dimensional metal oxide nanocomposites have attracted enormous attention due to their unique electronic properties, flexible structures and excellent adsorptions. Many recent studies have shown that the important sensing parameters of one-dimensional metal oxide sensors can be improved by making them composite ones (e.g., dissimilar metal oxide,^[1–6] organics,^[7] and other semiconductors^[8–11]), which opens a new door to realize smart sensor systems based on nanostructured materials in the NO₂ gas sensing industry.^[12–15]

Tungsten oxide (WO₃), a typical n-type metal oxide semiconductor with a relatively large indirect band gap of 2.8 eV exhibits promising application potentials in catalytic/photocatalytic processing, humidity and temperature sensors, biosensors and photonic crystals^[16–19] in comparison with their diverse crystal types and also has unique electrochromic, optochromic and gaschromic properties. In the research of gas sensor applications, the gas sensitivity of WO₃ heavily depends on film parameters, such as composition, morphology (e.g., grain size), nanostructure and microstructure (e.g., porosity, surface-to-volume ratio).^[20,21] Since one-dimensional tungsten oxides, including nanowires and nanorods, exhibit high crystallinities and large surface-to-volume ratios^[22–24] compared with their bulk materials, extensive efforts have been made synthesize the tungsten oxide nanostructures and assembled ordered superstructures for detecting NO₂ gas. Porous silicon (PS), fabricated simply by electrochemical etching of silicon resulting in periodically arranged arrays of pores, has been widely used as a scaffold to synthesize nanowires and nanotubes.^[25–28] It is facile to tune pore diameters and pore spacing by adjusting parameters such as the type of electrolyte, the PH, and the applied voltage. Therefore, the composites of one-dimensional nanomaterials/PS may have enhancing effects.

The functional properties of the metal oxide-based gas sensors can be improved not only by tailoring the crystal size of metal oxides but also by adding noble metal (Pt, Pd, Ag, etc.).^[29–32] It was observed that the surface modification of nanocrystalline metal oxide by noble metal sensitizers reduced the operating temperatures appreciably and improved the sensing properties towards target gas species. Liu et al.^[33] reported a shape-controlled synthesis of Pd nanocubes on WO₃ nanoplates by a solvothermal method, which presented a high and fast response at room temperature. Liu et al.^[34] also prepared a Pt-WO₃ sensor which exhibited a fast response and recovery as well as a high sensitivity compared with the undecorated sensor. Many studies have been performed to fabricate metal oxide semiconductor materials and to optimize their detection capability and optimum working conditions. Also, though WO₃ and Au are investigated intensively in the literature, not many studies of their properties after mixing and subsequent annealing have been conducted. What we have done for the first time is to successfully synthesize the Au-loaded WO₃ nanowires on PS substrates.

In this paper, we report a novel synthesis of Au-loaded WO₃ nanowires/PS composite, which presents good sensitivity and selectivity. In addition, the NO₂ sensing characteristics of this composite are compared with those of unloaded WO₃ nanowires/PS. The Au-loaded WO₃ nanowires/PS composite exhibiting an enhanced NO₂ response may be attributed to the excellent dispersion of Au nanoparticles on WO₃ nanowires having a very large effective surface area, leading to highly effective spillover of molecular nitrogen dioxide through Au nanoparticles onto the WO₃ nanowires surface. More importantly, this work suggests that seeking methods to form composite structures between noble metal and WO₃ nanowires/PS is a promising strategy to improve the performance of gas-sensing.

The Au-loaded WO₃ nanowires/PS composites were prepared by the following process. First, the PS substrate with large pore size (1 μm–1.5 μm) was prepared by an electrochemical etching method in a Teflon double-cell configuration, which was described in detail in our previous work.^[35] Silicon substrate was designed to be 22 mm × 9 mm after cutting from a p-type (100)-orientation monocrystalline silicon wafer with a resistivity of 10 Ω·cm–15 Ω ·cm. The wafer was placed in an electrolyte solution, which was composed of a 1:2 volume mixture of 40-wt% hydrofluoric acid and 99.5-wt% N,N-dimethyl formamide (DMF). The etching current density and etching time were 100 mA·cm⁻² and 8 min, respectively.^[36] Before the etching process, the Si substrate was cleaned necessarily and immersed in hydrofluoric acid to remove the oxide layer. The etching process was carried out at room temperature (RT). After etching, all PS samples were rinsed with ethanol and dried in air. In order to obtain WO₃ nanowires/PS composite, a metallic tungsten film with a thickness of 150 nm was pre-deposited on the PS substrate by DC magnetron sputtering. Subsequently, the sputtered PS substrates were placed in the hot zone of a horizontal tube furnace. After the quartz tube was pumped to a vacuum of 5 × 10⁻² Torr (1 = 1.33322 × 10² Pa), a mixture of argon and oxygen was introduced at rates of 80 sccm and 0.5 sccm, respectively. Then the tube furnace temperature increased gradually from room temperature to 700 °C and was maintained at this temperature for 1 h. The schematic diagram for synthesizing the WO₃ nanowires/PS composite is shown in Fig. 1. Then the furnace cooled naturally to room temperature. Next, the WO₃ nanowires/PS composites were transferred into an ion sputtering chamber. A 3-inch-diameter (1 = 2.54 cm) gold disc with 99.995% purity was used as a sputtering target. The specimen prepared by loading Au layers with thickness values of 10, 20, and 30 Å are referred to as “10Au-WO₃NWs/PS”, “20Au-WO₃NWs/PS”, and “30Au-WO₃NWs/PS”, respectively. Finally, the obtained samples of Au-loaded WO₃ nanowires/porous silicon were further annealed in a muffle furnace at 400 °C for 1 h with a temperature ramp up/down of 5 °C/min in order to stabilize the WO₃ nanowires structure.

The surface morphologies of the samples were investigated by field emission scanning electron microscopy (FESEM, Hitachi S4800). The phase structures of unloaded and Au-loaded WO₃ nanowires were examined by x-ray diffraction (XRD, RIGAKU D/MAX 2500V/PC, Cu Kα radiation). High-resolution transmission electron microscopy (HRTEM, Tecnai G2 F20) and energy dispersive x-ray spectroscopy (EDXS) were also used to confirm the presence of Au nanoparticles on WO₃ nanowires.

Fig. 1.

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	Fig. 1. Schematic diagram for synthesizing WO3 nanowires/porous silicon composite.

The gas sensing properties of the unloaded and Au-loaded WO₃ nanowires/porous silicon were measured in a home-made testing system consisting of a sealed 30-L test chamber, a heating plate with an adjustment range of temperature from RT to 300 °C, a professional digital multimeter (NUI-T UT70D), a data acquisition PC, and a stirring mini-fan to promote the diffusion of the detected gas. Pure target gas was injected into the test chamber directly to obtain the desired concentration. The ambient relative humidity is about 35% during the entire measurement. The resistance change of the sensors during the whole measurement process was continuously monitored by the multimeter.^[37] The gas sensing response (S) is given by S = R_g/R_a, where R_g and R_a are the resistances of the samples in oxidizing gas and air, respectively.

Figure 7 shows gas responses of the sensors based on 10Au-WO₃NWs/PS to 2-ppm NO₂, 50-ppm NH₃, 100-ppm ethanol, and 100-ppm acetone at 25 °C. It is clearly seen that the sensors show the highest responses to NO₂. Thus, Au-WO₃NWs/PS may be a promising sensitive material for NO₂ detection.

Fig. 7.

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	Fig. 7. Selectivities of Au-WO3NWs/PS at room temperature.

In this work Au loaded WO₃ nanowires/PS composites with Au layer thickness values ranging from 10 Å to 30 Å, are prepared by the sputtering method. The structures and morphologies of the WO₃ nanowires are investigated by using XRD, FE-SEM, and TEM. The XRD results show that the WO₃ nanowires present a well orthorhombic system after the thermal treatment at 400 °C for 1 h. SEM and TEM analyses show that Au nanoparticles disperse evenly on the surfaces of the prepared products. The 10Au-WO₃NWs/PS sensor shows a high response (3.357) and good selectivity to 2-ppm NO₂ at room temperature. Therefore the sensor based on WO₃ with a proper Au loading demonstrates a promising process in the field of gas detection.