Synthesis, luminescence, and photocatalytic activity of KLa 2 Ti 3 O 9.5 :Er 3+ nanocrystals for water decomposition to
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KLa2Ti3O9.5 and KLa2Ti3O9.5:Er31 nanocrystals were successfully synthesized using a hydrothermal method and a subsequent calcination treatment. The band gap (Eg) of the KLa2Ti3O9.5 nanocrystals was calculated to be about 2.56 eV by means of the reflectance diffusion technique. Under 980-nm excitation, the KLa2Ti3O9.5:Er31 nanocrystals emitted intense green (2H11/2/4S3/2 ! 4I15/2) and red (4F9/2 ! 4I15/2) upconversion (UC) luminescence. In comparison with pure KLa2Ti3O9.5, the KLa2Ti3O9.5:Er31 nanocrystals exhibited a higher activity for water splitting into H2 under simulated solar light irradiation. We suggest that the enhancement of photocatalytic activity is related to the Brunauer-Emmett-Teller (BET) surface area and UC luminescence of KLa2Ti3O9.5:Er31. I. INTRODUCTION
Controllable synthesis of micro/nanocrystals has received considerable attention driven primarily by the fact that the shape and size of micro/nanocrystals have tremendous effects on their properties.1–3 The reduction of particle size in a crystalline system can result in remarkable modifications of some of their bulk properties because of a high surface-to-volume ratio and the quantum confinement effect.4–6 So far, a number of synthesis techniques such as coprecipitation process, hydrothermal process, and sol–gel process have been developed to synthesize submicrocrystals and nanocrystals with a controlled size and shape.7–12 Lanthanide-doped nanomaterials have become a specific topic of interest owing to their unique luminescence properities.13–15 Compared with conventional luminescent materials, such as organic fluorescent dyes and semiconductor quantum dots, lanthanide-doped materials can present low photobleaching, narrow emission bands, and longer luminescent lifetimes.15 Among all the trivalent lanthanide ions, Er31 has attracted considerable attention due to the richest spectrum in the infrared–ultraviolet (UV) range that makes it ideally suited for energy UC studies. So far, a range of lanthanide-doped fluoride, oxide, hydroxide, orthovanadate, thioxide, borate, and phosphate nanocrystals have been successfully prepared.1,15 Solar-driven photocatalytic hydrogen production from water using a semiconductor catalyst has attracted a tremendous amount of interest since the discovery of water splitting on a TiO2 electrode.16,17 Thermodynamically, water splitting into H2 and O2 is an uphill reaction, accompanied by a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.322 J. Mater. Res., Vol. 27, No. 22, Nov 28, 2012
a large positive change in the Gibbs free energy. The efficiency of water splitting is determined by the band gap and band structure of the semiconductor and the electron transfer process.18,19 In recent years, numerous attempts have been made to enhance the activity of photocatalysts.20,21 Among various materials, layered perovskite-type oxides have attracted considerable attention due to their unique optical properties and excellent photocatalytic activity.22–24 KLa2Ti3O9.5 belongs to layered p
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