where D is the grain size, λ is the X-ray wavelength (λ =1.54ÅforCu-Kα), and Wis the full-width at half-maximum in radian. The calculated grain sizes are shown in Table 1. It clearly shows that a high applied voltage and a low flow rate lead to films with a large grain size, which can be well explained by the high temperature and long retention time under such a condition [51,52].