Tungsten Oxide / Vanadium Oxide Heterojunction Nanowire Array Gas Sensing Materials

In the 21st century, the level of industrialization has developed rapidly, but the natural environment and ecology on which human beings depend for it have been severely damaged. There are a lot of toxic and harmful gases, smog, acid rain, PM2.5, etc. in the air. Investigating NO_x^-based toxic gases, the market demands sensors that efficiently and accurately detect and prevent toxic and harmful gases.

In order to obtain high-performance nano-sensors, the first step is to prepare nano-materials that can provide these high-performance possibilities. WO₃, as a metal oxide of n-type semiconductor material, has the advantages of low cost, high sensitivity, easy control and operation. It has become a research hotspot at home and abroad.

The atomic properties of WO₃ crystal surface are active and easy to adsorb gas molecules. When gas molecules are adsorbed on the crystal surface, the carrier concentration in WO₃ crystal will change correspondingly, which is represented by the resistance change of the sensor. Because the active atoms of tungsten oxide are located on the surface of the crystal, the contact area between the crystal surface and the gas can be greatly enlarged, and the gas sensing performance can be effectively improved. But this still can not meet the requirements of marketization and integrated application. In order to obtain gas sensors with high selectivity, high sensitivity, low working temperature and high stability, gas sensing properties are mainly improved by modifying gas sensing materials.

Improving the gas sensing properties of tungsten trioxide can be modified by constructing heterostructure. Heterojunction structures are mainly used in semiconductor lasers, light-emitting devices, solar cells and other scientific fields. It is a potential direction for heterojunction to be used in the field of gas sensing. Some scholars use a tungsten oxide/vanadium oxide heterojunction nanowire array gas sensing material to detect nitrogen dioxide. The preparation process of the material is as follows:

Step 1: Tungsten thin film material was deposited on polished silicon wafer by magnetron sputtering. Tungsten metal was used as target material, argon was used as sputtering gas, sputtering pressure was 2.0 Pa, sputtering power was 80 W, sputtering time was 20 minutes.

Step 2: Tungsten oxide nanowires were grown by crystallization of tungsten thin films prepared in step 1 in a vacuum high temperature tubular furnace. The ambient atmosphere was a mixture of oxygen and argon. During the growth of tungsten oxide nanowires, the flow rates of oxygen and argon were controlled to be 0.1 SCCM and 35 sccm, respectively. The growth pressure in the furnace was controlled to be 140 Pa, and the tubular furnace slave chamber was controlled. The temperature rises from 25 degrees Celsius to 700 degrees Celsius, the heating rate is 5 degrees Celsius/min, the temperature is kept at 700 degrees Celsius for 1 hour, then the temperature is lowered from 1 hour to 400 degrees Celsius, and finally the temperature is naturally cooled to 25 degrees Celsius.

Step 3: After annealing, the tungsten oxide nanowires prepared in step 2 were annealed at 500 ℃ and in air atmosphere for 1 hour to further stabilize the crystal orientation.

Step 4: Vanadium films were deposited on tungsten oxide nanowire layers prepared by magnetron sputtering on target after step 3. Vanadium metal was used as target, argon as sputtering gas, inert gas flow rate was 50 sccm, sputtering pressure was 2.0 Pa, sputtering power was 110 W, sputtering time was 5 min.

Fifth step, annealing treatment of vanadium is carried out. The substrate of the deposited metal vanadium film obtained by step 4 is annealed at 500 ℃ and in air atmosphere for 1 hour.

Tungsten oxide and vanadium oxide both have excellent gas sensing properties and are excellent gas sensitive semiconductor materials. Gas experiments were carried out using the above-mentioned tungsten oxide/vanadium oxide nanowires. The experimental results show that the response sensitivity of the tungsten oxide/vanadium oxide heterostructure prepared by the above-mentioned method to 5ppm-level NO₂ gas at room temperature is 7-9 times higher than that of pure tungsten oxide nanowires, and the response time is less than 3s.

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