The diffraction patterns were plotted only in the 2θ range of 9°–15° to clearly show the size difference of the peaks for each micro-sized and nano-sized WO3. As shown in Fig. 8, the broad peaks belonged to nanometer-sized WO3 crystallite whereas the well-defined crystalline peaks belonged to micrometer-sized WO3. The crystallite size determined from the Scherrer equation for nano-sized WO3 was 51.5 nm. These nano-crystallites were significantly smaller than the particle sizes provided by Sigma-Aldrich, thus indicating that at least 2 crystallites were present in each WO3 particle of 100 nm in size.