Photocatalytic activity of Sn-doped ZnO synthesized via peroxide route

Highlights

• ZnO₂*ZnO mixture and two sets of samples prepared in presence of Sn²⁺ and Sn⁴⁺ ions were synthesized by a peroxo route.

• Differences in morphology was observed between the two sets of the samples.

• SnO₂ was present as an additional phase in most of the samples.

• Samples prepared in presence of Sn²⁺ are more photocatalytically active than the samples prepared in presence of Sn⁴⁺ salt.

• Highest photocatalytic activity was observed for samples annealed at 600 °C.

Abstract

ZnO*ZnO₂ mixture was prepared by thermal hydrolysis of a zinc peroxo-complex precursor in presence of Sn²⁺ and Sn⁴⁺ ions. The effect of different tin valence states on structure and photocatalytic properties of the resulting products was investigated. All samples before annealing contained ZnO₂ and except one sample also SnO₂. Microstructural analysis showed a difference in morphology between the two sets of samples. A difference was also observed for the specific surface area. The photocatalytic decolorization of Orange II dye was employed to determine the photocatalytic activity of the samples; Sn²⁺ doped samples were more active than Sn⁴⁺ doped ZnO samples. Temperature 600 °C was optimal for annealing - the sample ZnP150Sn2 annealed at 600 °C had the highest rate constant (0.0730 min⁻¹) among the samples. Additional increase of Sn²⁺ amount led to further increase in rate constant (0.0937 min⁻¹ for the sample ZnP200Sn2) up to a certain point.

Introduction

ZnO is a wide-band semiconductor with a variety of applications including photocatalysis. It is well known that metal doping can improve ZnO photocatalytic properties [1]; Several authors reported increased photocatalytic activity under UV irradiation for Sn-doped ZnO prepared by various methods. Li et al. used a parallel flow precipitation to prepare Sn-doped ZnO photocatalyst with different molar ratios; Sn-doped ZnO with 1% Sn exhibited the best photocatalytic activity for degradation of methyl orange under UV irradiation [2]. Morphology changed with doping from irregular lump and particle to irregular honeycomb.

Jia et al. also observed increased methyl orange degradation under UV light irradiation for crystalline Sn-doped ZnO samples prepared via a room temperature solid-state reaction compared to the undoped ZnO [3]. Morphology changed with doping from irregular aggregates of sheets for undoped ZnO to cluster of rods and to flake-like morphology for the doped samples.

Karunakaran et al. [4] synthesized cocoon-shaped Sn⁴⁺ doped ZnO nanoparticles by a solvothermal method with carbon microspheres as a template. The Sn⁴⁺ doped ZnO sample with 3% showed the best photocatalytic activity for degradation of Rhodamine B under UV light. Improved degradation of methyl orange under UV irradiation observed Wu et al. [5] as well as Beura et al. [6] for hydrothermally synthesized Sn⁴⁺ doped ZnO with rod-like morphology.

Moreover, some authors investigated Sn-doped ZnO for photocatalysis under sunlight irradiation. Siva et al. [7] prepared Sn-doped ZnO by precipitation method. Methylene blue degradation increased for the Sn-doped ZnO. Beura et al. [8] prepared Sn doped ZnO-graphene by a hydrothermal method. They observed maximum degradation of methyl orange 89.3% under UV and 99.1% under sunlight irradiation. It was 9 and 8% higher than by undoped ZnO-graphene composite.

Improved photocatalytic activity was also observed in ZnO/SnO₂ nanostructured composites due to formation of nano-heterojunctions [9,10]. Markovic et al. [11] prepared a ZnO/SnO₂ composite by mechanical milling and annealing. Enhanced photocatalytic activity was explained by the synergetic effect of the surface defects and the ZnO/SnO₂ heterojunction particles which facilitated charge-separation.

In this paper, ZnO*ZnO₂ mixture was synthesized by a low-cost peroxide method in presence of tin salts in different valence states (Sn²⁺ and Sn⁴⁺) and their structure, morphology and photocatalytic activity under UV light irradiation was compared. The influence of Sn²⁺ and Sn⁴⁺ presence has been so far compared for sonochemically synthesized Sn-doped ZnO quantum dots, where different optical defects were observed [12]. According to our knowledge, there are no publications focusing on comparison tin salts in different valence states on photocatalytic properties of resulting Sn doped ZnO. Although Sn²⁺ was likely oxidized to Sn⁴⁺ and the samples were contaminated by a secondary phase, we observed difference in photocatalytic activity between the two series of samples.