4 eV (515 nm) band gap energy, it is a most promising candidate among II--VI compounds for detecting visible radiation.
Estimation of the Optical Band Gap Energy and the Average Particle Size of PVA/CdS Nanocomposite Films
The optical band gap energy was estimated from the absorption coefficient data as a function of wavelength using the Tauc relation as shown in the following equation [12, 13]:
Among the advantages of using these photocatalysts mention can be made of high photocatalytic activity, reasonable price, non-toxicity and band gap energy
The absorption data were manipulated for the determination of the band gap energy.
Band gap energy and transition type can be derived from mathematical treatment of data obtained from optical absorbance versus wavelength with Stern  relationship of near-edge absorption (Equation 1):
80 eV for the wurtzite phase in the bulk form (13)--and high refractive indices (14); its band gap energy
has been found to increase as it goes from bulk to nano form, which makes it a potentially good material for reflective coatings.
Among the semiconductors, notably zinc oxide and titanium dioxide are known to be photosensitisers or photocatalysts (Joshi, 2006) These oxides are illuminated by photons having an energy level that exceeds their band gap energy
excites electrons from the valence band to the conduction band and holes are produced in the valence band.
As a result, silicon nanocrystals exhibit a significant blue shift in optical properties from the bulk infrared band gap energy
to the visible range.
The photon energy at the luminescence maximum is considerably larger than the band gap energy
7, the band gap energy has the same trend to meet the experimental results, because the UV--vis absorption onset wavelength of PCzBT is at 575 nm, which corresponds to a band gap ([E.
In fact, the introduction of the charge carriers ethynylene or vinylene groups into the backbones of copolymers lowers the optical band gap energy in modulating the HOMO level energy of the acceptor and results in the highly planar conformations.