Fe-doped and ZnO-pillared titanates as visible-light-driven photocatalysts

doi:10.1016/j.jcis.2011.03.021

Journal of Colloid and Interface Science

Volume 358, Issue 2, 15 June 2011, Pages 360-368

https://doi.org/10.1016/j.jcis.2011.03.021 Get rights and content

Abstract

Fe-doped cesium titanate was obtained by a solid state reaction with a mixture of Cs₂CO₃, TiO₂, and Fe₂O₃. ZnO-pillared doped titanate nanocomposite was successfully fabricated by exfoliating doped titanate and restacking its nanosheets with ZnO nanoparticles. The resulting nanocomposite was characterized by powder X-ray diffraction, scanning electron microscope, X-ray photoelectron spectroscopy, N₂ adsorption–desorption measurement, thermogravimetric analysis and UV–vis spectroscopy. It was revealed that the present nanocomposite exhibits greatly increased specific surface area with mesoporous texture and that there exists an electronic coupling between the host sheets and the guest nanoparticles in the pillared system. The results of degradation of methylene blue under visible light radiation suggest that doping iron ions improves the material spectral response region and that hybridizing with ZnO nanopillars can suppress the recombination of photogenerated electron–hole pairs.

Graphical abstract

Doping to extend its absorption response region and fabricating a heterojunction structure to suppress the recombination of photogenerated electron–hole pairs, the materials exhibited good visible-light-driven photocatalytic activities.

Highlights

► Doping titanate with Fe by a solid state reaction shifts the absorption edge to the visible region. ► Fabricating a heterojunction structure by an exfoliation-restacking route to suppress the recombination of the photogenerated carriers. ► ZnO-pillared Fe-doped titanate nanocomposite processes a mesoporous texture. ► ZnO-pillared titanates exhibit an excellent visible-light-driven photocatalytic activity.

Introduction

An increasing amount of attention has been paid to semiconductor photocatalysis applied in the abatement of environmental pollutants [1], [2], [3]. Recently, layered semiconductor materials have been widely used as catalysts and catalyst supports, derived from their good chemical stability, high specific surface areas, unique cation exchange, semiconducting and swelling properties [4], [5]. Of these compounds, lepidocrocite titanates are a popular layered transition metal oxide material with versatile composition, in which cesium titanate can be delaminated into 2D molecular single sheets (i.e., titanate nanosheets) and has been used as a starting material to prepare pillared nanocomposites [6], [7], [8], [9]. It has been demonstrated that cesium titanate has similar physical and chemical properties to TiO₂ such as high resistance to photoinduced corrosion [2]. However, the titanate possesses a relatively large band gap (E_g ∼ 3.5 eV) [10], indicating that it can only be excited under UV light illumination with the wavelength equal to or less than 350 nm. To expand the light absorption spectrum and to improve the solar energy utilization efficiency, many strategies have been employed to modify oxide semiconductors sensitive to visible light [3], [11], [12], [13], [14]. Among them, introduction of transition metal doping states will lead to an improved visible light photocatalytic activity due to the band-gap narrowing, resulting from the creation of dopant energy levels below the conduction band [13], [15], [16]. On the other hand, it has been demonstrated that hybridizing two different semiconductors can greatly decrease the photogenerated charge carriers’ (electron–hole pairs) recombination probability and increase the lifetime of charge carriers, thus promoting photocatalytic efficiency [17], [18]. One of the effective methodologies is to insert other semiconductor guest particles into the interlayer spacing of the 2-D semiconductor host lattices to fabricate a heterojunction structure. Such pillared nanocomposites exhibit porous textures, high specific surface areas, and improved photocatalytic activities, predominantly attributed to the effective spatial separation of photogenerated electron–hole pairs between guest and host [10], [19], [20], [21], [22].

Zinc oxide (ZnO) is a semiconductor with high photosensitivity, non-toxic nature, low cost, and environmentally friendly features for photocatalytic applications [18], [23]. In this paper, Fe-doped cesium titanate was prepared by a solid state reaction and exhibits an extended absorption edge up to the visible light region. Subsequently, ZnO-pillared doped titanate nanocomposite was fabricated via an exfoliation-restacking route. The chemical stability of the ZnO guest nanoparticles in the as-prepared composite under acidic conditions has been improved. The photocatalytic activity of the material for the degradation of organic pollutants was investigated using methylene blue (MB) as a model substrate under visible light irradiation.

Section snippets

Sample preparation

All chemicals were of analytical grade and were used as received. Layered cesium titanate doped with Fe(III), denoted as Cs_0.68+_xTi_1.83−_xFe_xO₄, was obtained by a solid state reaction. A mixture of Cs₂CO₃, TiO₂, and Fe₂O₃ with a certainty molar ratio was placed in an alumina crucible and pre-heated in air at 650 °C for 8 h in order to remove carbonate [24]. After cooling, the decarbonated powders was ground and heated at 900 °C for 40 h. The corresponding protonic form, H_0.68+_xTi_1.83−_xFe_xO₄·H₂O, was

Characterization of Fe-doped titanate

The pristine Cs_0.68Ti_1.83O₄ possesses the lepidocrocite structure of FeO(OH) [26]. As demonstrated in Fig. 1, the XRD patterns of Fe-doped cesium titanates with different x are similar to those of Cs_0.68Ti_1.83O₄, which indicates that the as-prepared doped titanates maintain the same layered structure. The reflections of iron oxide are not found from the XRD patterns. Compared with those of Cs_0.68Ti_1.83O₄, the positions of corresponding diffraction peaks of Fe-doped titanates are slightly

Conclusions

In this paper, Fe-doped cesium titanate was obtained by a solid state reaction with a mixture of Cs₂CO₃, TiO₂ and Fe₂O₃. Introducing iron ions extended the titanate spectral response to the visible light range. ZnO-pillared doped titanate nanocomposite was successfully fabricated from H_0.68+_xTi_1.83−_xFe_xO₄ nanosheets and ZnO nanoparticles via an exfoliation-restacking route. The as-prepared material has a mesoporous texture with a high specific surface area. It exhibited a high photocatalytic

Acknowledgments

This work was supported by The National Natural Science Foundation of China (50872037), The Natural Science Foundation of Fujian Province (2010J01040), and The Program for New Century Excellent Talents in Fujian Province University (06FJR01).

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Fe-doped and ZnO-pillared titanates as visible-light-driven photocatalysts

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Sample preparation

Characterization of Fe-doped titanate

Conclusions

Acknowledgments

J. Photochem. Photobiol., C

J. Solid State Chem.

J. Hazard. Mater.

J. Catal.

Appl. Catal., B

J. Phys. Chem. Solids

Mater. Lett.

Dyes Pigm.

Appl. Surf. Sci.

Mater. Chem. Phys.

J. Photochem. Photobiol., A

Chem. Rev.

Environ. Sci. Technol.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Mater. Chem.

J. Am. Chem. Soc.

J. Phys. Chem.

Nature

Science

Angew. Chem., Int. Ed. Engl.