The experiment found that the color of the product is closely related to the O/W ratio. Obviously, the lower limit of the O/W ratio of brown tungsten oxide is about 2.60, and the upper limit is difficult to determine because there are not enough experimental facts. But it is certain that it will not be greater than 2.90, because tungsten oxide with an O/W ratio of 2.90 is blue. Considering that the product has a certain mass fraction of NH+4 within the test range, a certain mass fraction of NH+4 is also one of the factors that affect the color of the product. In any case, the O/W ratio of the prepared brown tungsten oxide is close to 2.72, which is exactly consistent with W18O49, and the XRD pattern confirms that the product mainly contains W18O49, so it is certain that the superfine particle W18O49 is black, which is different from the commonly known purple tungsten . It is worth noting that the mass fraction of carbon in brown tungsten oxide is as high as 3%, the contribution of residual carbon to black cannot be ignored, and the presence of residual carbon makes the measured value of O/W ratio low, and the O/W ratio of real tungsten oxide W is relatively high. In conclusion, residual carbon obscures the perception of the nature of brown tungsten oxide. The elemental composition results of the relevant precursors were determined as follows: W(N)=18.69%, W(C)=4.39%, W(H)=2.55%, W(W)=45.78%, W(O)= 28.59%. The mass fraction of carbon in the precursor is only 4.39%, and CO2 and CO are generated during thermal decomposition of oxalate, so the brown tungsten oxide prepared in this work does not contain carbon, or contains very low carbon.
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