Effects of hydronium intercalation and cation substitution on the photocatalytic performance of layered titanium oxide

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Abstract

We have investigated the effects of hydronium (H3O+) intercalation and transition metal substitution on the photocatalytic activity of layered titanate. The basal spacings of the cesium titanate and Fe-/Ni-substituted titanates are notably expanded by 1 M HCl treatment, indicating the intercalation of hydronium ion. Diffuse reflectance UV–vis spectroscopic results clearly demonstrate that, in contrast to the hydronium intercalation, the substitution of transition metal ions gives rise to a remarkable narrowing of bandgap energy down to ∼1.9–2.3 eV. According to Fe K- and Ni K-edge X-ray absorption spectroscopic analysis, the substituted iron and nickel ions are stabilized in octahedral titanium sites with trivalent and divalent oxidation states, respectively. The photodegradation tests for organic dye molecules reveal that the hydronium intercalation enhances remarkably the photocatalytic performance of cesium titanate under UV irradiation and the substitution of Fe or Ni endows wide bandgap titanate with visible light driven photocatalytic activity.

Introduction

Over the last decades, photocatalysts have attracted special attention as an economic and environmentally safe option for air and water pollutions [1]. Among various semiconductors showing photocatalytic activity, TiO2 is one of the most investigated materials because of its low price, low toxicity, high stability, and high activity for photo-induced redox reactions [2], [3]. However, this compound suffers from a severe drawback of wide bandgap energy (Eg) of ∼3.2 eV, which prevents the harvesting of visible light and hence limits its photoefficiency with respect to solar energy. In order to overcome this barrier, tremendous research efforts have been made to modify the band structure of TiO2 as well as to optimize its photocatalytic performance. Many researchers reported that the substitution of titanium ions with other transition metal ions would be effective for the bandgap engineering of TiO2 [4], [5]. Recently, layered titanium oxide has received research interest as a promising photocatalyst because it has an ability to form intercalation complexes with narrow bandgap semiconductors [6]. It is highly feasible that the intercalation of water layer into the layered titanate lattice would enhance the photodegradation of organic molecules through the introduction into the interlayer space. Up to date, however, there have been no systematic studies on the effects of hydronium (H3O+) intercalation and transition metal substitution on the photocatalytic activity of the layered titanium oxide.

In this work, we have synthesized the cesium titanate and Fe-/Ni-substituted titanates, and their H3O+-intercalates. Also, their photocatalytic activity and their chemical bonding natures have been systematically characterized.

Section snippets

Experimental

The layered cesium titanate Cs0.67Ti1.830.17O4 and the Fe- and Ni-substituted layered titanates, K0.8Fe0.8Ti1.2O4 and K0.8Ni0.4Ti1.6O4, were prepared by conventional solid-state reactions, as reported previously [7]. The intercalation of H3O+ ions was achieved by the reaction between the pristine titanates and 1 M HCl aqueous solution at room temperature for 72 h. In the course of acid treatment, the HCl solution was replaced with a fresh one everyday. The resultant powders were thoroughly

Results and discussion

According to powder ND and XRD analyses, all the present titanates and their acid-treated derivatives display well-developed Bragg reflections, which are well indexed with lepidocrocite structure [7]. In contrast to negligible changes of the in-plane lattice parameters (a and c) of the layered titanates, the acid treatment induces remarkable increases of basal spacing from 8.63 to 9.41 Å (Cs0.67Ti1.830.17O4), from 7.73 to 8.35 Å (K0.8Fe0.8Ti1.2O4), and from 7.63 to 8.54 Å (K0.8Ni0.4Ti1.6O4),

Conclusion

Based on the experimental findings presented here, we are able to conclude that both the Fe- and Ni-substitutions are effective not only in decreasing the bandgap energy of the layered titanate but also in providing visible light driven photocatalytic activity. Also, it is certain that the intercalation of hydronium ions can improve the photocatalyst performance of cesium titanate but its application for cation-substituted titanates is not so fruitful due to the dissolution of the substituent

Acknowledgments

This work was performed by the financial support of National R&D Programs of the Ministry of Science and Technology (MOST), Republic of Korea, and supported partly by the SRC/ERC Program of MOST/KOSEF (Grant R11-2005-008-00302-0). The experiments at PAL were supported in part by MOST and POSTECH.

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This paper was presented in ISIC14 meeting on June 2007.

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