conductivity is enhanced by the formation of bulk oxygen vacancies. Also in comparison to other semiconductors SnO2 synthesized Nps have many advantages over others. Commercial SnO2 and TiO2 Nps were chosen for comparison with the synthesized SnO2 to evaluate the absorb capacity of the dye with all the practical parameters for the three types were kept the same and stable. The synthesized SnO2 Nps were shown better removal ability than the commercial materials (SnO2 and TiO2). This was characterised to the very minute diameter and high surface area of the synthesized SnO2. Also due to their higher recombination resistances, faster charge transport, and more efficient charge separation. All the previous factors with the particle size, surface to volume ratio and crystallinity of the nanorods, increased the physical properties of the metal oxide [182, 183]. In addition, thin SnO2 nanowires, can also be applied in a universal and low cost method to build field emitters with long term stability [184, 185].
2.2
Photocatalytic activity of SnO2 in aqueous solutionThe surface of SnO2 is characterized by O vacancies acts as a sink. O deficiency in SnO2
is important because it is an electron donor which enhance its properties as an n-type semiconductor [186]. A study had shown that energetically favourable reconstructions of the SnO2 (110) and (101) surfaces result in surface oxygen deficiency [88, 187].
These oxygen vacancies bind with electrons forming an excitation energy level below the CB of SnO2. SnO2 has a BG of 3.6 eV, resembles to activation energy to irradiation with UV light. This BG corresponds to photons of a photoreactor of 350 nm which sits in the UV-A range. The SnO2 material is well suited as a photocatalyst for removing of hazardous compounds from wastewater [169].
In an aqueous environment, pollution adsorbed to the catalyst surface. When the catalyst with the pollution exposed to irradiation of sufficient energy, the photogenerated e- - h+ pairs are generated to activate the oxidative degradation of organic molecules. The adsorbed solution broken down under the light and the final products desorbed. Due to the chemical stability, low toxicity, high photosensitivity, cheap, and preferable material for AOPs the insoluble SnO2, metal oxide can be applied in the photodegradation of
environmentally toxic elements and harmful chemicals in wastewater. The processes of heterogenous photocatalysis on different semiconductors were explored during the last decades. Recently, scientists have improved maintainable procedures for the photodegradation of phenols and many other toxins by using different irradiation sources [71, 188].
The overall process is explained in the following reactions which involve different steps as shown below
2.2.1 Electron hole pairs generation
The path originates with the activation of the photocatalyst and the formation of e- - h+ pairs (Eq. 2).
2.2.2 Traps for holes
The e- - h+ pairs which generated above in (eq1) travel to the surface of the semiconductor. With the effect of the light, water molecules generate highly reactive radical holes h+. The h+ migrate to the pollutant adsorbed to the surface (water molecules H2Oads) and react with the catalyst. Thus, the hole in the VB h+ is capable of producing
•OH by 2 ways, either through absorbed water (Eq.3) or through the reaction of hydroxyl anions (Eq.4).
2.2.3 Traps for electrons
The generated electrons e-, which migrated to the surface and the molecular dissolved oxygen, acts as acceptor species in the oxidation-reduction reaction. The e- activate the production peroxy radical anions (superoxide anions) (and hydroperoxyl anions (Eq. 5) upon reaction with dissolved oxygen [189].
2.2 Photocatalytic activity of SnO2 in aqueous solution 53
In addition, the resulted free radicals have strong oxidation ability are then protonated and formed highly reactive hydroperoxyl radicals in the presence of H+ (Eq. 6)
or in the presence of H2O (Eq. 7) [190]. The free radicals can oxygenate or degrade the organic compounds. •OH has no selectivity to reactant, makes critical effect in photocatalyzing, but high active electron O2•− super oxide anions has strong oxidation ability also can mineralize phenol.
Eventually, all the starting compounds and intermediates are converted into CO2 and H2O upon reaction with the above mentioned reactive species, as it stated in the previous equations.
Figure 4: Schematic representation of the oxidation process taking place on the crystal surface [115, 191].
Following the observations made from the previous reactions above and from many literature reviews and reports. The overall process is schematically outlined in Figure 4 above [91, 318].
In Figure 4 above, shows that the pollutant adsorbed to the surface and the light broken the adsorbed pollutant into e- - h+. The formation •OH of the reactive species are formed by the reactions of holes (h+) with absorbed H2O or OHˉ, and via H2O2 from the superoxide ion O2ˉ [192]. Reduction of oxygen in aerated solution to form O2ˉ and H2O2
enables in maintaining of the charge neutrality and driving the photocatalytic activity [135]. Later, H2O2 would photodecompose to •OH and react with the substrate [193].
As it was explained, the O vacancies in SnO2, can donate O and play an important role in photocatalysis [194, 195].
Many researchers showed that the main products formed from phenol photodegradation led to the formation of a mixture of byproducts such as Cat, BQ, Res and HQ as shown in Figure 5. These photoproducts further react with •OH and could be mineralize to CO2
and H2O as it shown in the Figure below [196-198].
Figure 5: Different chemical photoproducts expected from phenol photodegradation [196-198]
2.3 Application of undoped SnO2 in the photocatalytic degradation of organic