Facile sonochemical synthesis of zinc oxide nanoflakes at room temperature Saptarshi Ghosh, Deblina Majumder, Amarnath Sen, Somenath Roy n Sensor and Actuator Division, CSIR-Central Glass and Ceramic Research Institute, Kolkata 700032, India article info Article history: Received 13 March 2014 Accepted 18 May 2014 Available online 27 May 2014 Keywords: Sonochemical Zinc oxide Nanocrystalline materials Electron microscopy abstract Zinc oxide (ZnO) nanostructures with controlled morphology have been synthesized by a facile sonochemical method using cetyltrimethylammonium bromide (cationic surfactant) as a structure directing agent. The influence of surfactant concentration on the evolution of morphology, from starlet- like 3D structures to 2D flakes, has been systematically investigated. Room temperature formation of phase-pure ZnO crystals has been confirmed by X-ray diffraction (XRD). Field emission scanning electron microscopy (FESEM) reveals the dimensions of ZnO flakes, which are 200–400 nm wide and a few nanometre thick. Molecular fingerprints of the synthesized materials have been obtained by Fourier transform infrared spectroscopy (FTIR), while UV–vis spectroscopy estimates the bandgap of ZnO as 3.37 eV. & 2014 Published by Elsevier B.V. 1. Introduction Semiconducting oxide nanomaterials with exotic morphologies have shown tremendous potential in a diversified field of engi- neering and medicine [1–3]. Among them zinc oxide (ZnO) is a material of special interest due to its unique optical, semiconduct- ing and piezoelectric properties. While, a direct wide band gap of ZnO (  3.3 eV) is favourable for making ultra-violet photodetec- tors and optical switches, the electron mobility as high as 1000 cm 2 V 1 s 1 is ideal for fabricating nanoscale field-effect transistors [4]. Such favourable properties of ZnO are better manifested when the nanostructures are synthesized with con- trolled morphology for specific applications. For example, a dye- sensitized solar cell with ZnO nanoflowers as photoanode exhib- ited 90% superior power conversion efficiency compared to that based on ZnO nanorod arrays [5]. In another study, clusters of ZnO nanosheets in a flowerlike configuration have been found suitable for chemical sensor applications [6]. In this article, we report on the synthesis of zinc oxide nanoflakes in a catalyst-free and environmentally benign condi- tion under ultrasonication at room temperature. Compared to the conventional synthesis methods like hydrothermal or chemical vapour deposition (CVD), sonochemical synthesis is energy effi- cient, offering up to 92% energy conservation [7]. Additionally, sonochemical method renders controllable reaction conditions and rapid reaction rate [8]. In conjunction with ultrasonic energy, a cationic surfactant was used to obtain the desired morphologies. This entails in the formation of ZnO nanoflakes with large specific surface area, within minutes, at room temperature. The crystal- linity, morphological and optical properties of the synthesized ZnO nanostructures has been investigated. 2. Experimental Clusters of well-defined ZnO nanostructures were prepared at room temperature by surfactant-assisted sonochemical method. Zinc nitrate hexahydrate [Zn(NO 3 ) 2  6H 2 O], potassium hydroxide (KOH) and cetyltrimethylammonium bromide (CTAB) were pro- cured from Sigma Aldrich. In a typical process, equal amounts (0.5 g) of Zn(NO) 3  6H 2 O and KOH were dissolved in 250 ml of deionised water, to which CTAB was added in different concentra- tions (5–15 mM) and stirred for 10 min. The solution was then sonicated using an ultrasonic processor (Hielscher UP200S, 200 W, 24 kHz) with an intermittent pulse. Following a sonochemical reaction at room temperature for a definite time (3–25 min), a white precipitate was formed, which was then washed with deionised water. The as-synthesized materials were dried at room temperature in vacuum and subsequently analysed for phase purity and crystallinity by X-ray diffraction (Bruker D8 Advance, Cu K α line). The electron microscopy studies examining the morphology were carried out on Carl Zeiss-Supra 35VP FESEM system. Optical properties of the material in the UV and IR range at room temperature were investigated by a Shimadzu UV 2450 Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters http://dx.doi.org/10.1016/j.matlet.2014.05.112 0167-577X/& 2014 Published by Elsevier B.V. n Corresponding author. Tel.: þ91 33 2322 3527. E-mail address: sroy@cgcri.res.in (S. Roy). Materials Letters 130 (2014) 215–217