2022年4月27日星期三

Preparation of Nano Tungsten Trioxide Gas Sensing Film by Gas Phase Method

Nano tungsten trioxide is a typical N-type semiconductor gas sensing material, it has a large surface area, stable chemical properties, a wide band gap, a variety of toxic gases have good sensitivity, can effectively improve gas sensor sensitivity and response speed, is a good gas-sensitive film material.

tungsten trioxide gas sensitive film image

The preparation and synthesis of nano tungsten trioxide thin film by gas phase method is mainly to convert the synthesized tungsten trioxide crystal into the state of gas phase of tungsten trioxide through steps of sublimation, evaporation and decomposition. By setting the experimental conditions, the tungsten trioxide is transformed into tungsten trioxide saturated vapor. Finally, as the temperature decreases, saturated steam will crystallize in cold to grow as nanostructured tungsten trioxide crystals.

The preparation of nano tungsten trioxide gas-sensitive film gas method includes two kinds of chemical and physical methods.

Chemical vapor deposition (CVD) principle is the use of gas raw materials in the gas phase to form a chemical reaction and after two stages of nucleation and growth of the final preparation of the desired products (such as films, particles, whiskers). The use of chemical vapor deposition is also a method of preparing WO3 films. W(CO)6 as raw material, heated to 60 ~ 100 ℃, generate steam, and then N2 carrier gas generated steam at 300cm3min-1 flow rate of loading, W(CO)6 in the reaction chamber decomposition, the WO3 deposition to the base. Chemical vapor deposition (CVD) has the characteristics of continuous process, controllable and high purity. At the same time, multi-functional composite films can be prepared continuously by this method.

Physical vapor deposition (PVD) includes many basic particle growth processes such as the nucleation of nanostructured tungsten trioxide particles and the growth and aggregation of WO3 nucleus. The method of completing the particle transfer and transferring the basic unit to a specific location through a basic physical process is called a physical vapor deposition method. The typical method is vacuum evaporation, the principle is under high vacuum or high purity inert atmosphere (Ar, He), the evaporation of material under vacuum heating, WO3 vapor condensation in an inert gas medium to form a thin film. In recent years, they have also developed electron beam evaporation, ion beam assisted evaporation technology.

Regardless of the chemical deposition method or the physical deposition method to prepare the tungsten trioxide film, the prepared film has the advantages of high purity, stable performance and even particle distribution, and is suitable for the preparation of low melting point and single component films, but the shortcomings are also obvious, that is, the high cost, the preparation conditions are strict, the process is complex, and it is not suitable for large area preparation.

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Preparation of Tungsten Trioxide Thin Films by Sol - Gel Method

Sol gel method is widely used abroad, a kind of nano material preparation method, its principle is the prepared precursor (such as inorganic salt or metal alkoxide) dissolved in small molecule solvent in solution by mixing made uniform, solute and solvent hydrolysis reaction, hydrolysis condensation the reaction of nano particles and the formation of integrated sol.

tungsten trioxide film image

On the basis of the prepared sol, different processes can be used to prepare nano thin films and ultrafine powders with different properties. There are several ways to prepare tungsten trioxide thin films by sol-gel method: 1. Tungstic acid hydrochloric acid method; 2. Tungsten powder peroxide polytungstic acid method; 3. Tungstic acid colloid solution is prepared by ion exchange; 4. Polymer method.

The traditional sol-gel method mostly controls the particle size of powders by controlling pH value, temperature and adding surfactants. Recently, some scholars have successfully controlled the particle size of WO3 powders by increasing the viscosity of the reaction medium. After aging, filtration, drying and crystallization at 60 ℃, WO3 between 20 and 30 nm was obtained. Compared with the traditional gel method, CMC increased the viscosity of the hydrolyzate and inhibited the growth of colloidal particles , so using this method can be made of small size nanoparticles.

The sol-gel method has the advantages of low reaction temperature, easy control of product composition, smaller particle size, narrow and uniform particle size distribution and high purity of powder. At the same time, there are also many problems, such as the price of raw materials more expensive, the product is easy to agglomerate in the drying process, the synthesis period is very long, so time is money in the moment, even if the gas sensing properties of tungsten oxide films prepared by the sol-gel method is excellent, but the cost is difficult to control, so it is difficult to scale production.

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Preparation of Nano Tungsten Trioxide Thin Films by Electrodeposition

Nano tungsten trioxide is an excellent N-type semiconductor gas sensor, compared with the traditional materials, nano-tungsten trioxide gas sensor, high sensitivity, faster response speed and faster recovery is the most studied in recent years gas sensitive raw materials.


tungsten trioxide electrodeposition method image

There are methods for the synthesis of nano tungsten oxide films or gas phase method and solid phase method, liquid phase method. Electrodeposition is a gas phase method for preparing nano tungsten trioxide thin film, it is in a certain electrolyte and operating conditions, the metal or alloy from the compound solution, non aqueous solution or molten salt electrochemical deposition.

The basic process of electrodeposition in preparing WO3 thin films is: first, dissolve the W powder in a certain proportion with H2O2, then remove the excess H2O2, get the electrolyte solution and deposit it naturally, then use Pt as the working electrode and the other as the counter electrode. The WO3 thin film can be obtained on the Pt electrode by the electrodeposition of the current. Some scholars have used this method to prepare WO3 films on the surface of Ti matrix. The basic process is as follows: first, add 2G tungsten powder to H2Oof 10 m L 30%, and then add 100mL distilled water and 30m L propanol after the exothermic reaction is finished, then get the electrolyte solution needed to prepare WO3 film. Using a saturated calomel electrode as the counter electrode at room temperature to 450 m V voltage electrolysis for 1 hours, the product repeatedly washed with distilled water and ethanol, and after appropriate heat treatment, you can get WO3 thin film on a substrate.

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Tungsten Trioxide Thin Films Prepared by Sputtering Method

Synthetic nano tungsten trioxide or thin film methods are usually gas phase method, solid phase method, liquid phase method. Sputtering method is a preparation of nano tungsten trioxide film gas phase preparation method.

tungsten trioxide thin films prepared by sputtering method image

The most widely used sputtering method is magnetron sputtering. Some foreign scholars use metal W as a target, and use a mixed gas of Ar and O2 to prepare a WO3 thin film which is more gas-sensitive to NOx by magnetron sputtering. In addition, there are scholars using DC magnetron sputtering device, the target is 99.95% W, in a vacuum degree of 2 × 10-6mbar reaction chamber, Ar (50%) - O2 (50%) the mixed gas is subjected to direct current radio frequency excitation and discharge to deposit the WO3 thinly on the substrate to obtain a WO3 thin film. The electrochromic properties are also studied.

Compared with other coating methods, the sputtering coating method has the following advantages: (1) Any substance can be spattered, especially high melting point, low vapor pressure element and compound. (2) The adhesion between the sputtering film and the substrate is good. High energy particles are deposited on the substrate for energy conversion, producing higher heat energy and enhancing the adhesion between the sputtering atom and the substrate. (3) The sputtering density is high, the pinholes are few, and the purity of the films is high, because there is no such phenomenon as the crucible pollution in the process of sputtering. (4) The thickness of the film is controllable and the reproducibility is good. In addition, a thin film with uniform thickness can be obtained on a large area by the sputtering coating.

In recent years, radio frequency sputtering and magnetron sputtering technology have made great progress in China, and have made some progress in improving sputtering speed and reducing substrate temperature. Sputtering method has also become the main method of preparation of nano tungsten trioxide film, but it needs to be explained that the equipment price of sputtering method is more expensive and the investment in the early stage is larger.

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