Shell Thickness Dependent Photocatalytic Properties of ZnO/CdS Core-Shell Nanorods Sunita Khanchandani, † Simanta Kundu, ‡ Amitava Patra, ‡ and Ashok K. Ganguli* ,† † Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India ‡ Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata, 700 032, India * S Supporting Information ABSTRACT: Core/shell nanorod arrays of ZnO/CdS have been synthesized with varying shell thickness and their shell thickness dependent photocatalytic properties have been investigated. Core/ shell nanorod arrays of core diameter of 100 nm with variable shell thickness (10-30 nm) are synthesized by varying the concentration of the citric acid. XRD analysis reveals that tensile strain is obtained for ZnO nanorods and the compressive strain is obtained for core/shell nanorods. The UV-visible absorption spectra of the core/shell nanorod arrays show a red shift of the band edge of uncoated ZnO with shell growth. Steady-state photoluminescence (PL) spectra of the core/shell nanorod arrays show red shift of emission band with the increase in shell thickness. Decay kinetics indicate that the average lifetime (⟨τ⟩) of the core/shell nanorod arrays is larger than that of the uncoated ZnO nanorods due to charge separation. I-V studies show a 16-fold enhancement in current using the ZnO/CdS core/shell nanorod arrays having CdS shell thickness of 30 nm as compared to bare ZnO nanorods. The photocatalytic studies confirmed that the ZnO/CdS core/shell nanorod arrays exhibit improved degradation efficiency compared to bare ZnO and CdS under simulated solar radiation. The core/shell nanorods having shell (CdS) thickness of 30 nm displays the highest photocatalytic efficiency for the degradation of rhodamine B under simulated solar radiation, indicating efficient separation of electron-hole pairs. The mechanism of the photodegradation of RhB is given to elucidate the efficiency enhancement of ZnO/ CdS photocatalysts. These results demonstrate that the ZnO/CdS core/shell nanorod arrays provide a facile and compatible frame for potential applications in nanorod-based solar cells and as efficient photocatalysts. ■ INTRODUCTION Semiconductor nanostructures 1 have been of interest due to their wide ranging applications. Among various semiconductor nanomaterials, metal-oxides such as ZnO (bulk band gap of 3.37 eV) and TiO 2 (bulk band gap of 3.2 eV) have been studied intensively as photocatalysts because of their suitable band gap, high photocatalytic activity, and stability against photo- corrosion. 1-10 In particular, ZnO nanomaterials exhibit a few distinct advantages over TiO 2 . The direct band gap of ZnO (3.37 eV), simple tailoring of structures, ease of crystallization, anisotropic growth, higher exciton binding energy of 60 meV (compared to 4 meV of TiO 2 ) and higher electron mobility (200 cm 2 V -1 s -1 compared to 30 cm 2 V -1 s -1 for TiO 2 ) gives it an edge over TiO 2 . 4-8 However, bare ZnO is known to have a wide band gap, which is disadvantageous for the absorption and use of the visible range of solar energy. To use visible light and the enhancement of photocatalytic efficiency of these metal- oxide semiconductors, it is necessary to couple them with a lattice matched photosensitizer. 11 Narrow/mid band gap semiconductor nanocrystals, dye molecules, and metal nano- particles are widely used as sensitizers on the surface of photocatalysts to capture additional visible light and con- sequently enhance the photocatalytic efficiency. Semiconduc- tors such as CdS, 12 CdSe, 13 PbS, 14 InP, 15 Ag 2 S, 16 and Bi 2 S 3 , 17 which absorb light in the visible region, can serve as sensitizers, as they are able to transfer electrons to large band gap semiconductors. For an efficient electron transfer between the sensitizer and the photocatalyst, the energy level of the conduction band of the photocatalyst must be lower than that of the sensitizer. Thus, the electrons created in sensitizers are subsequently injected into the photocatalyst conduction band to perform a reduction reaction. The charge injection from narrow/mid band gap semiconductor into a wide band gap semiconductor can lead to efficient and longer charge separation by decreasing the recombination. 18 Among the most widely used inorganic semiconductor sensitizers, CdS (Bulk band gap 2.42 eV) is considered to be the most suitable visible sensitizer for ZnO; in particular, it has a high optical absorption coefficient and a similar lattice as of ZnO, which could facilitate a close interaction between the two semiconductors. 19 Another way to improve the photocatalytic efficiency is to create one- dimensional ZnO nanostructures that provide an ideal Received: August 22, 2012 Revised: October 16, 2012 Published: October 18, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 23653 dx.doi.org/10.1021/jp3083419 | J. Phys. Chem. C 2012, 116, 23653-23662