Enhanced photocatalytic activity and upconversion luminescence of flowerlike hierarchical Bi 2 MoO 6 microspheres by Er

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Flowerlike hierarchical Bi2MoO6 and Bi2MoO6:Er3+ microspheres were synthesized by a hydrothermal method. The crystalline size of microspheres decreases with increasing Er3+ concentration. The incorporation of Er3+ has no evident influence on the morphology of Bi2MoO6. The photocatalytic activity of microspheres was evaluated by the degradation of rhodamine B (RhB) aqueous solution under simulated solar light. The best photocatalytic performance was observed when the Er3+ concentration was 0.5%. In addition to the aforementioned high photocatalytic activity, the Bi2MoO6:Er3+ microspheres can emit pure green upconversion (UC) luminescence (2H11/2/4S3/2 ! 4I15/2) under 980 nm excitation. We suggest that the enhancement of photocatalytic activity of Bi2MoO6:Er3+(0.5%) is related to the UC luminescence of Er3+ ions. In addition, the BET surface areas of samples increased with increasing Er3+ concentration, which is also benefit for RhB adsorption.

I. INTRODUCTION

Controllable synthesis of micro/nanocrystals has sparked considerable interest driven primarily by the fact that the shape and size of micro/nanocrystals have tremendous effects on their properties.1–3 Among the micro/nanocrystals reported, hierarchical architectures, as a series of structures composed of low-dimensional nanostructured building blocks, have been extensively investigated for their novel properties and potential applications in many fields.4,5 Semiconductor photocatalysis has received increasing attention since the discovery of water splitting on a TiO2 electrode.6–9 Among many candidates for photocatalysts, Bi2MoO6 has attracted a great deal of attention for its good photocatalytic performance.10–13 In this process, only the light whose energy matches or exceeds the band gap energy of the semiconductor can excite the photocatalyst to generate electron–hole pairs, which oxidizes toxic organic pollutants and/or reduces toxic metal ions. And thus, the low usage of sunlight has restrained the photocatalytic efficiency of photocatalysts in the environmental remediation. The upconversion (UC) of RE ions, which can emit higher-energy photons when excited by lower-energy photons, brought a new opportunity for the utilization of long-wave length sunlight. Among all the trivalent lanthanide ions, Er3+ has attracted considerable attention due to the richest spectrum in the infrared–ultraviolet (UV) range that makes it ideally suited for energy UC studies.14–17 a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.89 J. Mater. Res., Vol. 27, No. 11, Jun 14, 2012

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Herein, enhanced photocatalytic activity was successfully achieved in Bi2MoO6 by tridoping with Er3+ ions. We suggest that the enhancement of photocatalytic activity of Bi2MoO6:Er3+(0.5%) is related to the UC luminescence of Er3+ ions. In addition, the UC luminescence properties of Bi2MoO6:Er3+ microspheres were studied in detail. II. EXPERIMENTAL

In a typical synthesis, 2 mmol of Bi(NO3)35H2O and 1 mmol