It is essential to know the bandgap energy to determine the right light source for activating photocatalysis. The photon energy should be higher than the bandgap energy of the photocatalyst to generate electron-hole pairs. The DRS is often used to determine the bandgap energy of photocatalysts. DRS results are plotted with reflectance or absorbance as the vertical axis vs. wavelength as the horizontal axis. To determine bandgap, the eqn.