The wide BG of SnO2 requires an excitation photoreactor, limiting its role because it reduces the absorption ability of the high energy UV portion of solar light, which accounts for its relatively low efficiency.
SnO2 utilizes only about of~5% UV light, the rest~43% visible, and~52% infrared radiation to complete the photocatalytic process, and is not responsive to visible light where 400 nm [325]. To find a solution for this limitation, extending the absorption of SnO2 into the visible region enables it to utilize as much as 50% of the total sun light reaching the catalyst surface.
2.3.4.1 Iodine doped tin oxide
Wen used TiO2 Nps codoped with iodine (I) and fluorine (F) to improve the degradation of Methylene blue under visible light irradiation. After prolonged sunlight illumination a complete removal of dye colour was noticed with the disappearance of some byproducts [326].
TiO2/I were found to show improved photocatalytic properties for the oxidative degradation of phenol under UV and visible light irradiation more than the pure. When calcination temperature was 673 K, TiO2/I Nps demonstrated stronger absorption in the 400-550 nm range through a red shift in the BG transition and were therefore able to efficiency oxidize pollutants at a longer [327].
The photocatalytic efficiency of TiO2/I materials was also increased when it modified with SnO2 Nps and used for photodegradation of 2-chlorophenol. The improved photocatalytic activity is obtained from the effect between the SnO2 and TiO2/I which
helped the efficiency of migration of the photogenerated e- - h+ pairs of the catalyst [196].
SnO2/I isolated gap states located within the BG below the Fermi level above the valence band consisted of the oxygen 2P and tin 5S orbitals, which narrowed the optical BG to 3.5 eV. I 5p states appeared above the Fermi level and did not contribute to the gap states [291].
SnO2/F coatings have been prepared using the mid-frequency pulsed DC closed field unbalanced magnetron sputtering technique in an Ar/O2 atmosphere showed high chemical, structural stability, good electronic conductivity and a shift in the BG [328].
The shift in the BG due to the energy gap between the VB and the lowest energy state in the CB which found to increase in the carrier concentration [329]. The density of electronic states increased at the Fermi level with an increase in F concentration incorporated into the main SnO2 matrix, due to the increase of the CB in F/SnO2 [330].
SnO2/F are characterized by O vacancies which further examined in the decomposition of the dye under UV illumination, to show the photocatalytic properties of the material.
As a consequence, SnO2/F showed a very high photocatalytic activity for the degradation of Rhodamine B compared to the pure SnO2 [331].
In another study, when I was doped with TiO2 the results showed that the concentration of the I is mainly located to the surface of the TiO2, and it rapidly decreased within the crystal structure because of un-favourable I-O interactions as I atoms preferred doping near the TiO2 surface due to strong I-O repulsion [332]. In TiO2/I Nps the recombination of the e- - h+ pairs is inhibited because of the doped I sites will not only catch electrons but also direct them to the surface of the adsorbed species (material) thereby enhancing the photocatalytic activity [204]. Continuous states at the site of TiO2/I consists of 2p and or 5s orbitals of I5 and oxygen. The resulted 2p orbitals of the VB are favourable for the efficient trapping of e- - h+ pairs at the TiO2/I particles. The Sn interstitials and O vacancies in SnO2 were found to have low formation energies and a strong mutual attraction. The stability of the defects is due to the multi-valence of Sn, which donates O to the CB [333]. Elucidating the high conductivity and nonstoichiometric nature of
2.3 Application of undoped SnO2 in the photocatalytic degradation of organic pollutants
77 SnO2 as pointed in a previous section In the application of all these materials, charge carrier concentration and conductivity is further increased by extrinsic dopants, as has been demonstrated for SnO2 [88]. It have been reported that Sb can act as a cation dopant but F can act as an anion dopant when doped and improved the conductivity of SnO2
[334]. While SnO2 powders have been utilized on a large scale in various fields of science and technology, there are not so many reports on preparation techniques. Thus, performance enhancement of SnO2 remains a challenge, and it is extremely desirable to develop new, simplified methods for synthesizing SnO2 Nps [279]. SnO2 catalyst activity and selectivity could be significantly improved by incorporating of different hetero-elements [331, 333]. Additives are often mixed with the SnO2 matrix to modify its microstructure and defect chemistry, which may enhance sensor response and selectivity to different target gases [88].
Seema and colleagues prepared a graphene (RGO)/SnO2 composite [335] synthesized through a redox reaction and exhibited excellent electrical conductivity which improved the photocatalytic degradation of pollutants. When Methylene blue illuminated under solar light, the organic dye was rapidly and completely degraded compared to the control. The results showed that the composite might be also used in photodegradation of other dyes [335].
The results discussed above are summarized in Table below.
Table 11: Comparison of some non-metal ion doped SnO2
Catalyst Light
Abbreviations; Rhodamine B: RhB; Methylene blue: MB; concentration catalyst: C; concentration of pollutant: P.