Paper
NJC
migrate to the surface of the material and oxidize the adsorbed sufficient redox potential to promote O2 reduction, and thus
pollutant. In indirect oxidation, the electron–hole pair acts to improve ZHS photoactivity.
form free radicals from surface groups, such as M–OH, O2, and
The cyclic stability results are shown in Fig. 6. These results
ꢀ
adsorbed water. The formed radical species are generally OH, indicated that no significant deactivation was observed with ZHS
HO2 and O2 ꢂ, which are able to oxidize organic compounds cubes even after four successive re-uses for RhB photodegradation.
ꢀ
ꢀ
in solution.23,31
Therefore, the prepared ZHS cubes exhibit stable photocatalytic
Therefore, to investigate the photodegradation mechanism, properties and an insignificant photocorrosion process, which is a
the rate of OH radical formation over a photocatalyst surface problem generally associated with Zn compounds.34,35
ꢀ
during UV radiation was tracked for all the samples studied,
and the results are depicted in Fig. 5(c) and (d). Considering a
zero order kinetic ([ꢀOH]t = kOH t) for the reaction of hydroxyl 4. Conclusion
ꢀ
radical formation over a semiconductor surface, the plot of the
In summary, zinc based hydroxide/oxide and zinc hydroxy stannate
intensity at 425 nm against the irradiation time should be
structures were synthesized by a facile aqueous solution process at a
linear and the slope of the curve should be directly proportional to
low temperature. It was investigated that by controlling the reaction
the rate of hydroxyl radical formation (Table 1).23 Interestingly, the
parameters, mixed phases of Zn(OH)2 and ZnO, pure ZnO and
analysis followed the same trend as compared to the photoactivity
ZnSn(OH)6 structures can be prepared. All the synthesized materials
of the samples, showing the relation between the photodegrada-
were used as photocatalysts for the photocatalytic degradation of
RhB under UV-light irradiation. The most active photocatalyst,
tion of the dye and hydroxyl radical formation. Finally, it can be
observed that both kinetic constants (k0 and kOH) follow the same
among all the prepared samples, was found to be ZnSn(OH)6.
trend. Therefore, we can conclude that attack by ꢀOH radical and
Furthermore, the RhB photodegradation mechanism could be
dye oxidation, i.e. an indirect photodegradation mechanism, is
correlated to the attack of hydroxyl radicals formed during UV
the main photodegradation process.23 This result is in agreement
irradiation, instead of the direct oxidation of dye over the photo-
with the studies reported by Kumari et al., who evaluated the
photodegradation mechanism of RhB dye by zinc stannate
catalyst surface. Finally, the as-prepared ZnSn(OH)6 cubes presented
themselves as potential material for the efficient photodegradation
(Zn2SnO4 and ZnSnO3) under UV light.32
of organic dyes and pigments with high stability.
From these results, it is possible to infer that the zinc hydroxy
stannate structure influences the reactivity of zinc sites with
adsorbed OH groups because the FTIR results demonstrate the
linkage of OH to Sn sites, and thus it is possible that both Zn and
Acknowledgements
Sn sites were active in the photocatalytic process. This is a remark-
The authors thank FAPESP (project no. 13/13888-0 and 2009/
able observation because Sn sites are generally inactive. It is known
54216-9), CNPq, CAPES and FINEP for the financial support.
that SnO2 sites do not exhibit significant photocatalytic activity
We are also grateful to LCE-DEMa and LIEC/UFSCar Brazil for
most likely due to the relative position of (reduction potential) its
providing HRTEM and Raman spectroscopy facilities, respec-
tively. R. Kumar would like to thank the Council of Scientific and
Industrial Research (CSIR), New Delhi, India for the awarding
Senior Research Fellowship.
conduction band, which is insufficient for reducing molecular
oxygen (O2 + eꢂ
-
ꢂO2 E = ꢂ0.33 eV).31 In addition, it is well
ꢀ
known that Sn and Sn oxides when coupled with the other oxides
can improve photoactivity by acting as dopant or forming
heterostructures.33 However, it is possible that in perovskite-
structured ZHS, Sn sites can be active, i.e., the Sn sites can have
Notes and references
˜
1 H. A. J. L. Mourao, W. Avansi, J. E. Oliveira, E. S. Firmiano
and C. Ribeiro, Sci. Adv. Mater., 2013, 5, 71.
˜
2 T. M. Milao, V. R. de Mendonça, V. D. Arau´jo, W. Avansi,
C. Ribeiro, E. Longo and M. I. Bernardi, Sci. Adv. Mater.,
2012, 4, 54.
3 M. A. Gondal, M. A. Dastageer, S. G. Rashid, S. M. Zubair,
M. A. Ali, D. H. Anjum, J. H. Lienhard, G. H. McKinley and
K. Varanasi, Sci. Adv. Mater., 2013, 5, 2007.
4 M. Wang, Y. Gao, L. Dai, C. Cao, Z. Chen and X. Guo,
Sci. Adv. Mater., 2013, 5, 1867.
5 G. R. Chaudhary, P. Saharan, A. Umar, S. K. Mehta and
S. Mor, Sci. Adv. Mater., 2013, 5, 1886.
6 K. Nakata and A. Fujishima, J. Photochem. Photobiol., C,
2012, 13, 169.
7 S. S. Arbuj, R. R. Hawaldar, S. Varma, S. B. Waghmode and
B. N. Wani, Sci. Adv. Mater., 2012, 4, 568.
Fig. 6 Degradation percentage of RhB, i.e., photocatalytic efficiency for
the ZHS sample during the four cyclic stability tests.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015