Solid-State Synthesis of Hexagonal Tungsten Trioxide via Ammonium Metatungstate

Tungsten trioxide (WO3) is a common n-type semiconductor. WO3 has a wide bandgap semiconductor between 2.5–3.6 eV, which makes being widely used in the fields of catalysis, photocatalytic and electro-chromic systems to produce smart windows, chemical sensors for detection of polluting gases such as H2S, NOx, NH3, and monitoring H2 gas concentration to ovoid explosions in embedded system. WO3 has the advantages of structural polymorphism, chemical stability, and good biocompatibility It is Hexagonal tungsten trioxide (h-WO3) possesses an open-tunnel structure and exhibits intercalation chemistry and other favorable properties, which has been used for lithium batteries, optoelectronic device, and ion exchanger.

Hydrothermal method is the most route to synthesis h-WO3, but requires prolonged reaction times and highly controlled reaction conditions, including temperature, pH, and templates. Solid-state methods have been considered effective for synthetizing a variety of nanomaterials with short-processing time, producing large quantity and reduced materials and process costs, which has been used to synthesize metal oxides.

Hexagonal tungsten trioxide has been produced with ammonium metatungstate using a solid-state method. The effect of temperature on the morphology and particle size had been investigated. The narrowest band gap can be achieved at a reaction temperature of 900 °C. The preparation procedures of hexagonal tungsten trioxide are as following steps:

Ammonium metatungstate ((NH4)6H2W12O4·xH2O) was used as tungsten source. (NH4)6H2W12O40·xH2O was firstly dispersed on a quartz plate, which was then loaded into the center of a horizontal tube furnace. The temperature was increased to 500, 600, 700, 800 or 900 °C, at which point a flow of Ar gas was commenced and retained for 3 h. The crystal structure, morphology, chemical composition, and absorption properties of the products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and ultraviolet-visible diffuse reflectance spectrophotometry, respectively.

In summary, hexagonal tungsten trioxide has been produced with ammonium metatungstate using a solid-state method, the narrowest band gap can be achieved at a reaction temperature of 900 °C.  The morphology and crystal phase could be effectively controlled by adjustment the temperature in reaction process. Hexagonal phase nanorods of width 300–600 nm and length 10–30 µm were obtained at a reaction temperature of 900 °C, and possessed the narrowest band gap among the obtained samples. This method is suitable for the large-scale production of h-WO3.

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