A novel heterostructured RuS2–titanate nanohybrid

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Abstract

A novel heterostructured RuS2–titanate nanohybrid has been prepared by reacting the exfoliated titanate sheets with the monodispersed RuS2 nano-sol particles. At first the stable colloidal nano-sheets could be prepared by intercalating tetrabutylamine into layered protonic titanate, HxTi2−x/4□x/4O4·H2O (x=0.67) with a lepidocrocite-like structure, and by successive exfoliating process in an aqueous solution. And then, the monodispersed RuS2 nano-sol particles capped with 1-thioglycerol were hybridized with the colloidal titanate nanosheets. The final product was then heated at 400 °C for 1 h under N2 gas atmosphere in order to complete the grafting reaction of intercalated RuS2 nano-sol on the interlayer surface of layered titanate. X-ray diffraction indicated that the RuS2–titanate nanohybrid has a pillar height of ∼1 nm. From UV–vis absorption spectra, it was found that the present nanohybrid showed visible light sensitization by RuS2 nanoparticles with narrow band gap.

Introduction

Recently, various nanohybrid materials have attracted considerable research interests due to their unusual and synergetic physicochemical properties which cannot be achieved by each component material itself [1], [2]. Among the various physico-chemical approaches to realize this kind of materials, the intercalation reaction—that is, the reversible insertion of guest species into two-dimensional host lattice—is expected to be one of the most effective way of preparing noble materials with desired functionality [3], [4].

Among the various two-dimensional layered materials, layered titanates (K2Ti2O5, Na2Ti3O7, K2Ti4O9, and CsxTi2−x/4□x/4O4; where □ expresses vacancy) have remarkable photocatalytic activity such as an evolution of hydrogen gas from water and an oxidative degradation of organic pollutants [5]. The layered titanates are consisted of negatively charged TiO6 sheets and the various cations stabilized between the layers in order to compensate the charges. These interlayer cations can be readily exchanged by other inorganic or organic cations through ion-exchange reaction [6].

Ruthenium sulfide belongs to the family of transition metal dichalcogenides with pyrite structure [7]. RuS2, a narrow band gap semiconductor, can be used as an inorganic sensitizer as well as a photocatalyst in the visible light region [8]. Other transition metal sulfides such as CdS and ZnS are known as a sensitizer or photocatalyst but these materials show a marked photochemical instability in aqueous solution due to self-oxidation by photo-generated holes (photocorrosion). However, RuS2 is highly stable against the photocorrosion [7].

Layered titanates are hardly swelled because of their high layer charge density. This non-swelling property prevents RuS2 nanoparticles from being intercalated effectively into the host layers. In this paper, we have developed a novel route to generate a well-ordered RuS2–titanate nanohybrid, which could only be realized by exfoliation–restacking combining two notable steps, namely the exfoliation of host titanate layer and the restacking in the presence of well-developed RuS2 guest nanoparticles. Such exfoliation–restacking route offers great potentials for preparing wide range of new hybrid materials, since it allows a way of accessing quest species especially bulky ones, to access freely into the interlayer space of the host lattice without any steric hindrance [5], [9].

Section snippets

Sample preparation

The host cesium titanate, CsxTi2−x/4□x/4O4 (x=0.67), was prepared by heating a stoichiometric mixture of Cs2CO3 and TiO2 at 800 °C for 20 h. The corresponding protonic form, HxTi2−x/4□x/4O4·H2O (x=0.67), was obtained by reacting the cesium titanate powder with 1 M HCl aqueous solution at room temperature for 3 days. During the proton exchange reaction, the HCl solution was refreshed every 24 h. The layered protonic titanate was exfoliated into single titanate sheets by intercalating TBA

Results and discussion

As listed in Table 1, the present ICP results indicate that 0.67 mol of cesium ions is incorporated into the crystal lattice, and cesium ions are successfully exchanged with protons by acid-treatment. It was also observed that 0.62 mol of RuS2 is stabilized in the titanate lattice by hybridizing the exfoliated titanate with RuS2 nanoparticles. The CHNS analysis data of the hybridizing process are summarized in Table 2. No trace of nitrogen was detected for the sample of RuS2–titanate nanohybrid

Conclusion

A new RuS2–titanate nanohybrid was successfully prepared through an exfoliation–restacking route. It was found that the RuS2–titanate nanohybrid showed a pillar height of ∼1 nm and the most exceptional thermal stability up to 400 °C. By reacting with the RuS2 nanoparticles, the layered protonic titanate band was shifted to a longer wavelength region corresponding to the visible range. This is attributed to the sensitization of the RuS2 nanoparticles with narrow band gap between the titanate

Acknowledgements

This work was supported by the Korean Research Foundation (grant: KRF-2004-041-C00187), and in part by the SRC program of the Korea Science and Engineering Foundation (KOSEF) through the Center for Intelligent Nano-Bio Materials at Ewha Womans University (grant: R11-2005-008-01001-0). Authors thank to the Ministry of Education for the Brain Korea 21 fellowship and the Science and Technology Amicable Research (STAR) Program of the Ministry of Science and Technology.

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