Nanoscale ZnO/CdS heterostructures with engineered interfaces for high photocatalytic activity under solar radiation† Paromita Kundu, a Parag A. Deshpande, b Giridhar Madras b and N. Ravishankar * a Received 16th September 2010, Accepted 9th December 2010 DOI: 10.1039/c0jm03116j Semiconductor based nanoscale heterostructures are promising candidates for photocatalytic and photovoltaic applications with the sensitization of a wide bandgap semiconductor with a narrow bandgap material being the most viable strategy to maximize the utilization of the solar spectrum. Here, we present a simple wet chemical route to obtain nanoscale heterostructures of ZnO/CdS without using any molecular linker. Our method involves the nucleation of a Cd-precursor on ZnO nanorods with a subsequent sulfidation step leading to the formation of the ZnO/CdS nanoscale heterostructures. Excellent control over the loading of CdS and the microstructure is realized by merely changing the initial concentration of the sulfiding agent. We show that the heterostructures with the lowest CdS loading exhibit an exceptionally high activity for the degradation of methylene blue (MB) under solar irradiation conditions; microstructural and surface analysis reveals that the higher activity in this case is related to the dispersion of the CdS nanoparticles on the ZnO nanorod surface and to the higher concentration of surface hydroxyl species. Detailed analysis of the mechanism of formation of the nanoscale heterostructures reveals that it is possible to obtain deterministic control over the nature of the interfaces. Our synthesis method is general and applicable for other heterostructures where the interfaces need to be engineered for optimal properties. In particular, the absence of any molecular linker at the interface makes our method appealing for photovoltaic applications where faster rates of electron transfer at the heterojunctions are highly desirable. 1. Introduction Clean energy and pollutant-free water/air are among the most important challenges that we currently face; a common solution for these seemingly different challenges lies in designing new materials for the maximal harvesting of solar radiation. In terms of high activity and chemical stability, TiO 2 is an excellent photocatalyst that can remove a wide range of organic pollutants from air and water, as summarized in various reviews. 1–3 However, owing to its large bandgap (3.2 eV), its activity is largely restricted to the ultraviolet (UV) region, which only contributes to about 4% of the entire solar spectrum. Several strategies have been adopted to extend the spectral response of TiO 2 to develop viable visible light-driven photocatalysts. 4–13 The doping of transition metals in TiO 2 has had limited success. 4–6 The addition of a second metal oxide like SiO 2 , ZrO 2 or Al 2 O 3 has also been used to enhance the photocatalytic activity of TiO 2 . 14 However, both these strategies have had only limited success and, therefore the development of new materials is of paramount importance. Sensitization of a wide bandgap semiconducting material with dyes has been used both in dye-sensitized solar cells 4–6,14–16 as well as in the visible-light photocatalytic degradation of organics. 7–10 Sensitization using visible-light active materials (narrow bandgap semiconductors, for instance) is another very common strategy. 17–20 Besides being photocatalysts that are both UV and visible-light active, these hybrid materials are good candidates for photovoltaic applications. 17,21 Sensitized ZnO and TiO 2 have been extensively investigated in this regard. The sensitization process is primarily limited by the relative positions of the conduction bands (CB) of the wide and narrow bandgap semi- conductors and also by the nature of the interfaces in the system. 22,23 While the first factor could be controlled by the appropriate choice of material and by tuning the bandgap of the sensitizer, 24 the second factor still remains a challenge in terms of a Materials Research Centre, Indian Institute of Science, Bangalore, 560012, India. E-mail: nravi@mrc.iisc.ernet.in; Fax: +91-80-2360 7316; Tel: +91-80-2293 2566 b Department of Chemical Engineering, Indian Institute of Science, Bangalore, 560012, India † Electronic supplementary information (ESI) available: Figure S1: Variation of the average solar intensity during the different periods of the day from April 01 to April 12, 2010. Figure S2: XRD pattern obtained from the Cd-precursor coated ZnO before sulfidation. Figure S3. XPS core-level spectra of Zn2p in ZnO nanorods. Figure S4. Bright Field TEM image of ZnO–CdS nanohybrid, ZC-3, showing fine particle clusters of CdS attached to ZnO nanorods. Figure S5. (A) XRD pattern of the CdS synthesized by precipitation method using same conditions as ZC-3 and heated to 150  C for 30 mins and (B) degradation rate of methylene blue (MB) under solar radiation in presence of CdS (cubic phase + hexagonal phase) is similar to ZC-3. See DOI: 10.1039/c0jm03116j This journal is ª The Royal Society of Chemistry 2011 J. Mater. Chem., 2011, 21, 4209–4216 | 4209 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. Chem., 2011, 21, 4209 www.rsc.org/materials PAPER Published on 09 February 2011. Downloaded by Forschungszentrum Julich Gmbh on 08/08/2014 10:31:35. View Article Online / Journal Homepage / Table of Contents for this issue