Effect of polyvinylpyrrolidone on morphology and structure of In 2 O 3 nanorods by hydrothermal synthesis Tzu-Tsung Tseng, Wenjea J. Tseng * Department of Materials Science and Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 402, Taiwan Received 19 December 2008; received in revised form 2 March 2009; accepted 18 March 2009 Available online 15 April 2009 Abstract Indium oxide (In 2 O 3 ) nanorods were hydrothermally synthesized from aqueous InCl 3 solution in urea with addition of polyvinylpyrrolidone (PVP) as a steric stabilizer. Indium hydroxide, In(OH) 3 , was precipitated at 60 8C and was changed into a transient InOOH phase upon calcination at 250 8C in air. X-ray diffractometry revealed that the existence of PVP delays the phase transformation of InOOH. Cubic-structured In 2 O 3 phase was then formed when temperature was raised to 350 8C, regardless of the PVP concentration. The In(OH) 3 phase without the PVP showed a rod- based, flower-like morphology of polycrystalline character. Minor addition of the PVP, i.e., 0.1–2 wt.%, resulted in a pronounced evolution in morphology from the three-dimensional, flower-like form to discrete, one-dimensional nanorods aligned in planar form. Both the flower-like and discrete nanorod morphologies were preserved after heat treatments at 250 and 350 8C. This reveals that the morphological change is attributable to preferential adsorption of the PVP molecules on the In(OH) 3 crystallite surface, so that the aggregate attachment responsible for the multipod growth is inhibited. # 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Indium oxide (In 2 O 3 ); Nanorod; Structure 1. Introduction Semiconductive indium oxide (In 2 O 3 ) with a wide bandgap (direct bandgap around 3.6 eV at room temperature) has attracted much attention in recent years, owing mainly to its application in solar cell [1], field-emission display [2], lithium- ion battery [3], nanoscale biosensor [4], gas sensor [5], optoelectronics [6], and photocatalysis [7]. In 2 O 3 nanorods and nanowires with tailored morphology, architecture and surface functionality, in particular, present unique anisotropic proper- ties that respond readily to minute changes in specific external stimuli, giving rise to a selective recognition capability and an improved device stability [4,8–10]. In view of the literature, In 2 O 3 crystals of various morphologies have been prepared by different physical and chemical routes. For example, Pan et al. first synthesized one-dimensional In 2 O 3 nanobelts through thermal evaporation [11]. Highly oriented In 2 O 3 nanowires and branched nanostructures were grown by vapor–liquid–solid (VLS) mechanism [12] and by chemical vapor deposition (CVD) [13,14]. The CVD-grown In 2 O 3 nanostructures with morphologies such as octahedral nanoparticle, nanochain, nanobelt, nanosheet, and nanowire were obtained by properly tuning the synthesis variables [13]. In addition, arrays of vertically aligned In 2 O 3 nanorods and nanowires have been fabricated by physical means via template-assisted growth, within which the growth direction of In 2 O 3 crystals is virtually confined to pre-defined pore channels with specific orientations [6,15,16]. Apart from the physical methods, In 2 O 3 nanorods have been prepared via chemical means, such as sol–gel [17], chemical precipitation [18,19], forced hydrolysis [20], solvothermal synthesis [21,22], sonohydrolysis [23], and hydrothermal synthesis [24–26]. Preparation of one-dimensional In 2 O 3 crystals with desired nanostructure and morphology is advantageous from both time- and cost-perspectives when chemical routes are employed. Use of sacrificial templates and organic surfactants to facilitate the anisotropic growth of In 2 O 3 nanorods in solutions, in particular, has received much attention recently [10,17,22,25,27–29]. For example, Cheng et al. [17] used polyethylene octylphenylether as a directing surfactant for www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 35 (2009) 2837–2844 * Corresponding author. Fax: +886 4 2285 7017. E-mail address: wenjea@dragon.nchu.edu.tw (W.J. Tseng). 0272-8842/$36.00 # 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2009.03.028