**4. Conclusions**

The WO3 thin films were successfully prepared on ITO glass substrate by spin coating technique using the glycerol-modified ammonium tungstate precursor solution. For better surface wetting, the sol-gel free ammonium tungstate solution with the 25% volume fraction of glycerol was selected to promote the growth of WO3 thin films. It is concluded that the ammonium tungstate transformed to WO3 range from 220 ◦C to 300 ◦C, and the crystalline structure of the WO3 films was observed when films were annealed above 300 ◦C.

Additionally, as the annealing temperature increases, the component content of WO3 within the film increased. The WO3 films prepared in this work were smoother and more homogeneous than the sol-gel-prepared films. The optical band gap of the WO3 films was variational from 3.48 eV to 3.37 eV with the increase in the annealing temperature above 350 ◦C. Furthermore, the coloration e fficiency of the WO3 films attained 46.3 cm2·C−1.

**Author Contributions:** Conceptualization, J.L. and H.N.; methodology, R.Y. and D.G.; validation, M.S., X.F. and J.C.; resources, J.P. and R.Y.; data curation, G.Z., T.Q. and K.G.; writing—Original draft preparation, J.L. and G.Z.; writing—Review and editing, J.P. and H.N.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by Key-Area Research and Development Program of Guangdong Province (No.2019B010934001), National Natural Science Foundation of China (Grant No.51771074, 61804029 and 61574061), the Major Integrated Projects of National Natural Science Foundation of China (Grant No.U1601651), the Basic and Applied Basic Research Major Program of Guangdong Province (No.2019B030302007), the Guangdong Natural Science Foundation (No.2018A030310353), Science and Technology Project of Guangzhou (No.201904010344), the Fundamental Research Funds for the Central Universities (No.2019MS012), the Open Project of Guangdong Province Key Lab of Display Material and Technology (No.2017B030314031), 2019 Guangdong University Student Science and Technology Innovation Special Fund ("Climbing Plan" Special Fund) (No.pdjh2019a0028 and pdjh2019b0041), National College Students' Innovation and Entrepreneurship Training Program (No.201910561005 and 201910561007), South China University of Technology 100 Step Ladder Climbing Plan Research Project (No.j2tw201902475, j2tw201902203 and j2tw202004095).

**Conflicts of Interest:** The authors declare no conflict of interest.
