The role of pH variation on the growth of zinc oxide nanostructures Rizwan Wahab, S.G. Ansari, Young Soon Kim, Minwu Song, Hyung-Shik Shin * Thin Film Technology Laboratory, School of Chemical Engineering, Chonbuk National University, 664-14 Duckjin-dong 1ga, Chonju 561-756, Republic of Korea 1. Introduction Zinc oxide (ZnO) has received considerable attention because of its unique optical, semiconducting, piezoelectric, magnetic and gas sensing properties. ZnO nanostructures exhibit interesting proper- ties including high catalytic efficiency and strong adsorption ability. Recently, the most investigated application of ZnO is in biosensing because of its high isoelectric point (9.5), biocompat- ibility, and fast electron transfer kinetics. Such features advocate nanostructured synthesis and use of this exciting material for such novel unexplored applications [1–7]. Many deposition techniques have been employed to synthesize nanostructures of zinc oxide such as nanowires, nanobelts, nanobridges, nanonails, nanoribbons, nanorods, nanotubes, and whiskers using thermal evaporation, hydrothermal synthesis, metal organic chemical vapour deposition (MOCVD), spray pyrolysis, ion beam assisted deposition, laser-ablation, sputter deposition, tem- plate assisted growth and chemical vapour deposition, sol–gel, etc. [8–20]. Among these techniques, sol–gel process offers many advantages with excellent control over stoichiometry, composi- tional modification, microstructure control using capping mole- cules, controlled doping, with inexpensive equipments. As we know that the zinc oxide is a polar crystal, and has octahedral geometry where the zinc and oxygen atoms are arranged alternatively along the c-axis direction with hexagonal phase. The properties of zinc oxide strongly depend on the synthesis method and conditions during processing. The growth habit of zinc oxide is greatly affected by the external conditions such as reaction temperature, reaction concentration, pH value of the solution [21–22]. It is well known that the morphology of the sol–gel synthesized metal oxides strongly depend on the amount of H + or OH  ions in the sol that effectively determines the polymerization of the metal–oxygen bonds. Precursor solution pH variation affects the hydrolysis and con- densation behaviour of the solution during gel formation, and hence influences the morphology. Literature survey indicates that the effect of pH on the morphology/properties of ZnO is rare being an important factor to determine the quality. The pH of solution appears to be critical parameter for the phase formation, particles size and morphology of the final film during sol–gel processing. Pal et al., presented the synthesis of various types of zinc oxide nanostructures by using ethylenediamine, zinc acetate dihydrate and sodium hydroxide and water at two different pH values by the hydrothermal method [23]. Hochepied et al., obtained the pompom like zinc oxide nanostructures by using zinc nitrate hexahydrate and ammonia. The solution was heated in a thermostat reactor and chacked pH evolution [24]. Bai et al., presented the rose like zinc oxide nanostructures by using ZnCl 2 and ammonia (25%) synthesized through a hydrothermal decom- position method on a copper plate substrate [25]. In all the results reported above, the nanostructures were grown either at higher temperature or they need, sophisticated instruments and complex reaction procedure for the growth of zinc oxide nanostructures. In this paper we report a systematic study on the morphological variation and quality of ZnO by controlling the pH of the precursor solution using solution method and a possible growth mechanism is proposed. Applied Surface Science 255 (2009) 4891–4896 ARTICLE INFO Article history: Received 2 May 2008 Received in revised form 17 November 2008 Accepted 12 December 2008 Available online 24 December 2008 Keywords: ZnO Solution method Nanostructures pH variation Hydroxyl ion ABSTRACT In this paper we present a systematic study on the morphological variation of ZnO nanostructure by varying the pH of precursor solution via solution method. Zinc acetate dihydrate and sodium hydroxide were used as a precursor, which was refluxed at 90 8C for an hour. The pH of the precursor solution (zinc acetate di hydrate) was increased from 6 to 12 by the controlled addition of sodium hydroxide (NaOH). Morphology of ZnO nanorods markedly varies from sheet-like (at pH 6) to rod-like structure of zinc oxide (pH 10–12). Diffraction patterns match well with standard ZnO at all pH values. Crystallinity and nanostructures were confirmed by high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) pattern, which indicates structure grew along [0 0 0 1] direction with an ideal lattice fringes distance 0.52 nm. FTIR spectroscopic measurement showed a standard peak of zinc oxide at 464 cm 1 . Amount of H + and OH  ions are found key to the structure control of studied material, as discussed in the growth mechanism. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Fax: +82 63 270 4465. E-mail address: hsshin@chonbuk.ac.kr (H.-S. Shin). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.12.037