Regular Article
A building blocks strategy for preparing photocatalytically active anatase TiO2/rutile SnO2 heterostructures by hydrothermal annealing

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Abstract

Effective heterostructures for photocatalysis need to present good electron-hole mobility among phases, a feature that is only attained with satisfactory interface quality. For very small sizes and with a stable colloidal state, the oriented attachment mechanism can be used to prepare suitable structures by means of a building blocks strategy, whereby preformed nanoparticles are used to control the desired amount of each phase in the heterostructure. Here, we show the success of this strategy for anatase TiO2/rutile SnO2 heterostructures, applying conventional hydrothermal annealing, where successive collisions among particles increase the probability of oriented attachment. The photodegradation of Rhodamine B in water under UV radiation was used as a probe reaction to evaluate formation of the heterostructure, as indicated by an increase in photoactivity. Increased heterostructure photoactivity was related to heterojunction formation and charge separation. The increased lifetimes of the photogenerated charges, due to heterojunction formation, enabled them to reach the oxides surface and promote photocatalytic reactions. The insights presented here may be used in a rationalized synthesis method to obtain heterostructures from preformed nanocrystals.

Introduction

In recent years, a number of strategies have been developed aimed at improving the photocatalytic performance of semiconductors [1]. Among these, heterostructures have attracted much interest due to their promising properties, especially those related to the increase in the photogenerated charge lifetime [2]. This aspect is considered crucial in order to improve the photocatalytic performance of semiconductors [3]. Among the various semiconductors that have been studied, titanium dioxide (TiO2), in the form of nanoparticles, aerogels, or thin films, has been employed to produce heterostructures in combination with different metals or metal oxides [4], [5], [6], [7], [8], [9]. Among these materials, tin oxide (SnO2) is of particular interest, because its electronic structure is perfectly matched to that of TiO2, resulting in materials with superior photocatalytic properties [4], [10], [11].

Several studies have reported the growth, in reactive steps, of rutile SnO2 over preformed anatase TiO2 crystals [4], [12], [13]. In this procedure, the SnO2 is formed directly over TiO2 and epitaxial growth is expected. The arrangement in the interface depends on the synthesis parameters, such as the reagents concentrations and temperature. These factors influence characteristics including the coverage density and morphology of the materials, which directly affect photocatalytic performance [12], [14]. However, it is difficult to control the simultaneous crystallization processes of two different components, or the modification of the surface of one crystalline material with another one, such as a semiconductor or a metal [13]. These difficulties could be overcome by the building of heterostructures using preformed nanoparticles with defined composition and properties as building blocks, although suitable technological procedures are still required to perform this process.

Previously, we proposed a kinetic model to describe the growth of heterostructures from preformed nanoparticles under microwave-driven hydrothermal conditions [15]. It was shown that the formation of heterojunctions was time-dependent and that the rate of hydroxyl radical formation under UV radiation should be directly related to the number of heterojunctions formed. In the proposed model, the rate of heterojunction formation could be described by:d(TiO2/SnO2)HSdt=k·[TiO2][SnO2],where (TiO2/SnO2)HS represents heterojunctions. Since a diffusion-limited process was considered, the rate constant, k, could be described by:k=23NAkBTη2+RTiO2RSnO2+RSnO2RTiO2,where NA is Avogadro’s constant, kB is the Boltzmann constant, T is the annealing temperature, η is the coefficient of viscosity, and R is the radius of the nanoparticles.

This model was based on an interpretation of the oriented attachment mechanism, whereby particle growth occurs by directly attachment among locally oriented particles. In the colloidal state, the driving force is the particle boundary migration during effective collisions [16]. However, important points to consider are the faster kinetics of microwave-driven annealing, compared to conventional hydrothermal annealing, as well as the possible interference of electromagnetic fields in particle orientation [17].

In this work, we describe a photocatalytic study of TiO2/SnO2 heterostructures obtained by conventional hydrothermal annealing of preformed nanocrystals under different conditions. The main goal was to establish the relation between the hydrothermal conditions and photocatalytic performance, assuming that heterostructure formation was only affected by the rate of particle collisions [18]. TiO2 and SnO2 were considered suitable materials for use in this experiment due to their very low water solubility, so other crystal growth effects were not expected to occur [15]. The photoactivities of the heterostructures and pure TiO2 were compared using the photodegradation of Rhodamine B (RhB) under UV radiation, before and after the hydrothermal treatments of the materials, as a probe of effective heterostructure formation. The results showed that oriented attachment should be considered in heterostructure formation, and that understanding of this mechanism could provide valuable insights for the technological production of these advanced materials.

Section snippets

Experimental section

The first step was the preparation of the TiO2 and SnO2 nanocrystals to be used as precursors for heterostructure formation. The oxides were synthesized according to the methods described by Ribeiro et al. [19] and Leite et al. [20], respectively, and were denoted PRE-TiO2 and PRE-SnO2.

For TiO2, in a typical procedure, 250 mg of metallic Ti (99.7%, Aldrich) was added to 80 mL of a 3:1 H2O2/NH3 solution (both 29.0%, Synth). After around 6 h, a transparent yellow solution was obtained, indicating

Results and discussion

The as-synthesized preformed nanoparticles used in the heterostructure assembly were studied in detail previously [15], [19], [20]. As expected, PRE-TiO2 and PRE-SnO2 presented the XRD patterns indexed to the anatase (PDF#21-1272) and rutile (PDF#41-1445) crystalline phases, respectively.

The specific surface areas of PRE-TiO2 and PRE-SnO2 were 87.5 and 160 m2⋅g−1, as determined by N2 adsorption analyses. Considering near-spherical shapes in both cases, the average sizes were around 18 and 5.5 

Conclusions

This work provides information about the photocatalytic properties of TiO2/SnO2 heterostructures obtained from preformed nanocrystals used as building blocks in a hydrothermal annealing strategy. The heterostructures showed better performance in the photodegradation of RhB, compared to pristine TiO2, which could be attributed to charge migration and a longer photogenerated charge lifetime. The synthesis process was relatively simple and could be explained based on the previously proposed

Acknowledgements

The authors gratefully acknowledge the financial support provided by the Brazilian research funding agencies FAPESP and CNPq. The HRTEM analyses were conducted in the Laboratorio de Microscopias Avanzadas at the Instituto de Nanociencia de Aragon, Universidad de Zaragoza (Spain). Some of the research leading to these results received funding from the European Union Seventh Framework Program, under grant agreement 312483 ESTEEM2 (Integrated Infrastructure Initiative – I3). R.A. gratefully

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