Elsevier

Materials Research Bulletin

Volume 42, Issue 11, 6 November 2007, Pages 1914-1920
Materials Research Bulletin

Substitution effect of pentavalent bismuth ions on the electronic structure and physicochemical properties of perovskite-structured Ba(In0.5Ta0.5−xBix)O3 semiconductors

https://doi.org/10.1016/j.materresbull.2006.12.010Get rights and content

Abstract

We have investigated the substitution effect of pentavalent bismuth ions on the electronic structure and physicochemical properties of barium indium tantalate. X-ray diffraction, X-ray absorption spectroscopic, and energy dispersive spectroscopic microprobe analyses reveal that, under oxygen atmosphere of 1 atm, pentavalent Bi ions are successfully stabilized in the octahedral site of the perovskite tantalate lattice. According to diffuse reflectance UV–vis spectroscopic analysis, the Bi substitution gives rise to the significant narrowing of band gap of barium indium tantalate even at a low Bi content of ∼5%, underscoring a high efficiency of Bi substitution in the band gap engineering. Such an effective narrowing of the band gap upon the Bi substitution would be attributable to the lowering of conduction band position due to the high electronegativity of BiV substituent. As a result of band gap engineering, the Ba(In0.5Ta0.5−xBix)O3 compounds with x  0.03 can generate photocurrents under visible light irradiation (λ > 420 nm). Based on the present experimental findings, it becomes clear that the substitution of highly electronegative p-block element like BiV ion can provide a very powerful tool for tailoring the electronic structure and physicochemical properties of wide band gap semiconductors.

Introduction

Over the past decades, semiconducting metal oxides have attracted special attention due to their versatile applicability as electrodes for solar cells, photocatalysts, etc. [1], [2], [3]. Among various semiconducting materials, titanium oxide has been the most intensively investigated compound due to its many advantages such as high stability for photocorrosion, low toxicity and low price of the material, and so on [1], [2], [3], [4]. However, the wide band gap of TiO2 (Eg  3.2 eV) limits severely the photoefficiency of this material since it prevents TiO2 from harvesting photonic energy in visible light region, where most of solar energy is concentrated. In this regard, many attempts have been made to control the band structure of TiO2 [5], [6], [7], [8], [9]. Recently chemical substitution has been reported as a useful tool for band gap modification of wide band gap semiconductors [10], [11], [12], [13]. In particular, a perovskite-structured metal oxide is a good base material for the chemical substitution, since this structure can accommodate diverse cations and anions into the lattice to form stable solid solutions. In this regard, there have been many reports on the effect of cation substitution on the electronic structure and catalytic activity of perovskite-structured niobate or tantalate [14], [15], [16], [17], [18]. Up to date, most of substitution studies have been concentrated on transition metal ions (i.e. d-block element). However, instead of the low energy shift of the absorption edge, the substitution of transition metal ions gives rise to the appearance of a low-energy absorption peak related to the d–d transition of transition metal ions below the absorption edge [10].

In this work, we have investigated the substitution effect of highly electronegative p-block element (i.e. BiV cation) on the electronic structure and physicochemical properties of the perovskite-structured tantalate. The crystal and band structures of the obtained Ba(In0.5Ta0.5−xBix)O3 compounds with 0  x  0.05 have been studied by powder X-ray diffraction (XRD) and diffuse reflectance UV–vis spectroscopy. The oxidation states and local geometries of the substituted bismuth ions were examined with Bi LIII-edge X-ray absorption near-edge structure (XANES) spectroscopy. The crystallite morphology and chemical composition of the Ba(In0.5Ta0.5−xBix)O3 compounds were probed with field emission-scanning electron microscopy (FE-SEM) and energy dispersive spectroscopic analyses (EDS). Also, we have investigated the effect of BiV content on the generation of photocurrent by the Ba(In0.5Ta0.5−xBix)O3 compounds.

Section snippets

Experimental

Polycrystalline Ba(In0.5Ta0.5−xBix)O3 samples with 0  x  0.05 were prepared by the conventional solid-state reaction with the stoichiometric mixture of BaCO3, In2O3, Ta2O5, and Bi2O3. High purity samples could be obtained by calcining the mixture at 800 °C and subsequently sintering at 1000–1100 °C for 4 days with intermittent grindings, and finally slow-cooling to room temperature. All of the heat-treatments were done in oxygen atmosphere to stabilize the higher oxidation state of BiV and to avoid

Results and discussion

Fig. 1 represents the powder XRD patterns of the quinternary metal oxides Ba(In0.5Ta0.5−xBix)O3 with 0  x  0.05. All the intense diffraction peaks could be well indexed on the basis of perovskite structure with the cubic symmetry. According to least-square fitting analyses, the lattice parameter a increases as the Bi content becomes larger, which is in good agreement with the relative size of octahedral site (i.e. B-site) cations (BiV(6) = 0.90 Å, TaV(6) = 0.78 Å, InIII(6) = 0.94 Å, where the number in

Conclusion

We have investigated systematically the effect of BiV substitution on the electronic structure and physicochemical properties of barium indium tantalate. According to the XRD, XANES, EDS, and diffuse reflectance UV–vis spectroscopic analyses, pentavalent Bi ions are successfully incorporated into the octahedral site of the perovskite lattice, resulting in a significant narrowing of band gap to 1.7 eV even at a low Bi content of ∼5%. As a consequence of band gap narrowing, the Bi-substituted

Acknowledgments

This work was supported by the financial support by Ministry of Environment (Grant No.: 022-061-023) and partly by the SRC/ERC program of the MOST/KOSEF (grant: R11-2005-008-03002-0). The experiments at Pohang Accelerator Laboratory (PAL) were supported in part by MOST and POSTECH.

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