Tungsten Trioxide Nanostructures by Hydrothermal Method Preparation

Tungsten trioxide nano-multistage structure refers to nanowires, nanorods, nanosheets and other nanostructures formed by the combination of two or more components. At present, some scholars have prepared corresponding materials by thermal evaporation, electrospinning and other processes, but these preparation methods will involve high temperature, high vacuum, high technical difficulties and other issues, which are contrary to the development of low power consumption.

Some scholars believe that tungsten trioxide nanostructures prepared by traditional hydrothermal method can control the reaction temperature below 200 ℃, and do not need complex reaction instruments. Moreover, the morphology of products can be adjusted by changing the reactant concentration, pH value, hydrothermal time and hydrothermal temperature in hydrothermal reaction. Sensitive nanomaterials suitable for gas sensors can be prepared through various adjustments. It is realized through the following technical schemes:

(1) Cleaning Alumina Base
The alumina substrates were washed successively in acetone solvent, absolute ethanol and deionized water to remove oil and organic impurities on the surface, and then dried thoroughly in an infrared oven.

(2) Preparation of seed solution
Sodium tungstate was dissolved in 15 ml deionized water by magnetic stirring. Dilute hydrochloric acid was added drop by drop until no precipitation was produced. Then, the solution was heated to 40 ℃, and 2 ml of hydrogen peroxide was added into the solution. The solution was stirred until the precipitation was dissolved, forming a yellow transparent sodium tungstate seed solution with a concentration of 0.2M-0.5M.

(3) Preparation of seed layer
Sodium tungstate seed solution prepared in step (2) was coated on the cleaned alumina substrate in step (1), then placed in an annealing furnace and annealed in air. The annealing temperature was 500-600 ℃, the holding time was 2-3 h, and the heating rate was 2-3 ℃/min.

(4) Preparation of hydrothermal reaction solution
Sodium tungstate solution of 0.06M-0.1M was prepared. Sodium tungstate was dissolved in deionized water by magnetic stirring until it was completely dissolved. Potassium chloride of 0.08M-0.15M was added, and model agent P123, i.e. triblock copolymer, was added to form a uniform colloidal solution. Then dilute hydrochloric acid was added one by one to control the pH of the solution at 2.1-2.5. Finally, a uniform latex-white sodium tungstate sol solution was formed.

(5) Preparation of tungsten trioxide one-dimensional nanowires and multistage nanostructures
The alumina substrate covered with sodium tungstate seed layer in step (3) was placed in a stainless steel hydrothermal reactor lined with polytetrafluoroethylene (PTFE). Meanwhile, the sodium tungstate solution prepared in step (4) was transferred to the reactor and sealed. Then, tungsten trioxide nanowires and multilevel nanostructures were synthesized directly on the surface of alumina substrate by hydrothermal method at 160-200 ℃. The hydrothermal reaction time is 6 to 12 hours. After the reaction, the reactor naturally cooled to room temperature.

(6) Cleaning alumina substrates after hydrothermal reaction
Tungsten trioxide nanostructured materials with different morphologies were prepared by soaking and cleaning alumina substrates after hydrothermal reaction in deionized water and anhydrous ethanol repeatedly and drying them in a vacuum drying chamber at 60 ℃.

Hydrothermal method is a low-cost method to control the preparation of tungsten trioxide nanomaterials from one-dimensional nanowires to multi-level nanostructures. Compared with existing methods such as thermal evaporation and electrodeposition, hydrothermal method has the advantages of simple equipment, easy operation, easy control of process parameters and low cost. Moreover, the prepared multistage nanostructures exhibit continuous, porous and porous microstructures with high specific surface area. The growth of nanowire arrays is more conducive to the free diffusion of gases in and out. It provides great application and research space in reducing the working temperature of gas sensors and improving the sensitivity and response speed of sensors.

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