*3.5. UV–Vis Analysis*

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UV-Vis measurements in the absorption mode were carried out to reveal the electronic structure and size effect of as prepared nanoparticles. The optical band gap energy for synthesized nanoparticles has been calculated using the Tauc's relation: (α*h*ν) *<sup>n</sup>* = *A h*ν − *E<sup>g</sup>* , where, α is the absorption coefficient, hν is incident photon energy of light, A is a constant, E<sup>g</sup> denotes the band gap energy and n is constant that depends on the nature of optical transition (n = 2 and 0.5 for direct and indirect transition respectively) [32]. Figure 5b shows the plot of (αhν) <sup>2</sup> versus hν for Zn doped α-Fe2O<sup>3</sup> nanoparticles that exhibits a direct band gap with n = 2. The charge transfer in α-Fe2O<sup>3</sup> takes place between occupied O2<sup>−</sup> 2*p* state to empty Fe3<sup>+</sup> 3*d* upper state that is responsible for direct band gap transition in Fe2O3. It is found that pure Fe2O<sup>3</sup> nanoparticles has band gap of 2.66 eV which is higher than the reported band gap of 2.1 eV for pure Fe2O<sup>3</sup> [1]. This indicates existence of Fe3<sup>+</sup> in lower spin state that results in higher value of band gap for pure Fe2O3. Moreover, the obtained results show reduced band gap from 2.66 eV for pure Fe2O<sup>3</sup> to 2.31 eV for Zn 4% and then increases for higher Zn concentration. This decrease in band gap may be ascribed to an increase in structural disorder or defects with increase in Zn doping up to 4% concentration. In addition, this decrease in band gap may also be due to partial hybridization between Zn t2g and O 2p states to empty Fe t\*2g 3d orbitals. A report by Mashiko et al. explained the decrease in band gap on the basis of decrease in residual in-plane strain [33]. Based on the above considerations, the sequence of band gap for synthesized samples is Zn 6% > Pure Fe2O<sup>3</sup> > Zn 2% > Zn 4% which agrees well with experimental data and measured band gap values are given in Table 2.


**Table 2.** The calculated band gap, valence band and conduction band positions corresponding to pure Fe2O<sup>3</sup> , Zn 2%, Zn 4% and Zn 6% nanoparticles.

Band edge positions, bandgap as well as the overall band structure of semiconductors play an important role in photocatalytic applications. The energy position of the band edge level can be controlled by the electronegativity of the dopants, as well as by the quantum confinement effects. The valence band and conduction band edge potential of a semiconductor can be deduced from the relation [34,35],

$$E\_{VB} = \chi - E\_{\varepsilon} + 0.5 \, E\_{\mathcal{g}} \tag{3}$$

$$E\_{\rm CB} = E\_{\rm VB} - E\_{\rm g} \tag{4}$$

where, EVB and ECB are the valence band and conduction band edge potential, respectively, χ is the absolute electronegativity of a semiconductor oxide and its value for Fe2O<sup>3</sup> is 5.87 eV, E<sup>e</sup> represents the energy of free electrons, which is about 4.5 eV on hydrogen scale. The calculated valence and conduction band edge position for synthesized samples are given in Table 2.
