Porous Silicon-based Tungsten Oxide Nanorod Composite Gas Sensing Material

The results show that the one-dimensional tungsten oxide nanostructures have large specific surface area, surface activity and strong gas adsorption capacity, which can accelerate the reaction between gases, further improve the sensitivity and effectively lower the working temperature. In order to obtain such a high performance gas sensor, the first step is to prepare nanomaterials that can meet these requirements.

In addition to the preparation of one-dimensional nano-tungsten oxide, we need to find a suitable carrier to maximize its performance. Porous silicon is a good choice. It is known that Silicon-based Porous Silicon is also a potential new gas sensing material with adjustable pore size, pore depth and porosity on the surface of silicon wafer. Some scholars have applied one-dimensional nano-tungsten oxide and porous silicon. The porous silicon-based tungsten oxide nanorod composites were prepared by bonding porous silicon. The main steps are as follows:

(1) Preparation of porous silicon
Porous silicon layer was prepared on polished surface of step (1) by double cell electrochemical etching. The electrolyte consisted of hydrofluoric acid with 40% mass concentration and dimethylmethylamine with 40% mass concentration. The volume ratio was 1:2. The applied current density was 64 mA/cm² at room temperature and without light. The etching time was 8 minutes, and the forming area of porous silicon was 1.6 cm *0.4 cm.

(2) Preparation of seed solution
Sodium tungstate 1.65g was dissolved in 100 ml deionized water by magnetic stirrer, then dilute hydrochloric acid was added drop by drop until no white precipitation was produced. Then the mixture was placed for 1 h. After the supernatant was poured out, the precipitation was centrifuged by low-speed centrifuge, and then dissolved in a proper amount of hydrogen peroxide to form a yellow transparent seed solution with a concentration of 1M.

(3) Preparation of seed layer
The seed solution prepared in step (3) was spin-coated on the porous silicon prepared in step (2), spin-coated with five layers, then annealed in a muffle furnace. The annealing temperature was 650 ℃, the holding time was 2 h, and the heating rate was 2.5 ℃/min.

(4) Hydrothermal synthesis of porous silicon-based ordered tungsten oxide nanorods
Firstly, the reaction liquid is allocated, and 8.25g sodium tungstate is weighed. It is dissolved in 25 ml deionized water by magnetic stirrer. Then, the pH value of the reaction liquid is adjusted to 2.0 by dilute hydrochloric acid. Then, the above solution is diluted to 250 ml, and oxalic acid is added to control the pH value of the solution to 2.5. Then, 70 ml of the reaction liquid is transferred to the lining of polytetrafluoroethylene (PTFE) in 100 ml hydrothermal reactor. Then (4) the porous silicon substrate with seed layer is inserted on the sample rack and emptied into the lining. Finally, the reactor is placed in the constant temperature drying chamber and reacted at 180 ℃ for 2 hours.

(5) Cleaning of porous silicon substrates after hydrothermal reaction
In step 5, porous silicon substrates after hydrothermal reaction were repeatedly soaked in deionized water and anhydrous ethanol, then dried in a vacuum drying chamber at 60 ℃ for 8 hours to obtain porous silicon-based tungsten oxide nanorod composite gas sensitive materials.

The porous silicon-based tungsten oxide nanorod composite gas-sensing material can be scanned under an electron microscope, and the tungsten oxide nanorod can grow almost perpendicular to the porous silicon substrate. The average diameter and length of the nanorod are 50 nm and 900 nm. The nanorod has a large specific surface area, and the reaction temperature is reduced to about 60 C while greatly improving the gas-sensing performance.

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