Controlled growth of zinc oxide microrods by hydrothermal process on porous ceramic supports for catalytic application Supamas Danwittayakul a,b , Joydeep Dutta b,c,⇑ a National Metal and Materials Technology Center, 114 Thailand Science Park, Paholyothin Rd., Klong Luang, Pathumthani, Thailand b Center of Excellence in Nanotechnology, Asian Institute of Technology, P.O. Box 4, Klong Luang, Pathumthani 12120, Thailand c Chair in Nanotechnology, Water Research Center, Sultan Qaboos University, P.O. Box 33, 123 Al Khoud, Oman article info Article history: Received 16 July 2013 Received in revised form 1 October 2013 Accepted 3 October 2013 Available online 14 October 2013 Keywords: ZnO microrods Hydrothermal process Porous ceramic substrates Catalytic application abstract The growth of zinc oxide (ZnO) microrods on porous ceramic substrates by mild hydrothermal process was studied. One-dimensional ZnO microrods were grown on ZnO nanoparticle seeded substrates by using equimolar concentration of zinc nitrate and hexamethylenetetramine at temperatures lower than 100 °C. We found that the growth of ZnO microrods on alumina and diatomite substrates were affected due to hydrolysis of substrate surfaces. Stunted ZnO microrod growth on c-alumina and diatomite sub- strates were attributed to arise due to the degradation of hexamine molecules in the growth solution. Adjusting the pH prior to the growth of ZnO microrods on both alumina and diatomite lead to the growth of ZnO microrods similar to what is observed on flat glass substrates. Cordierite does not hydrolyze easily and hence ZnO microrods with aspect ratio as high as 24, were obtained without any pH control of the growth solution. Copper nanoparticles deposited on ZnO microrods were utilized as a catalyst for meth- anol steam reforming and about 14% hydrogen yield was obtained with almost 90% methanol conversion at reforming temperature of 350 °C. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Porous ceramics are important for many industries where high surface area, chemical physical and thermally resistant materials are a requirement [1,2]. Applications of porous ceramics extends from filtration to use in mechanical seals and as catalyst supports. Zinc oxide (ZnO) nanostructures have recently attracted consid- erable attention due to diverse potential applications [3]. ZnO crys- tals have anisotropic structure with polar and non-polar surfaces that lead to the possibility of growing nanorod or nanowire struc- tures [4]. One-dimensional structures like nanorods or nanowires of ZnO have been used in a wide range of applications from gas sensors, solar cells, optoelectronics to piezoelectric devices. It is generally agreed that varying the morphology suits different appli- cations of ZnO nanomaterials [4]. For gas sensor applications, high surface area of the sensing parts in devices can improve sensitivity and allow the possibility to miniaturize devices [5]. While highly dense ZnO microrods on a substrates are preferred for gas sensor and solar cell applications [6], ZnO based nanogenerators require gap between nanorods for bending during the energy harvesting processes [7–9]. ZnO as a component in catalysts for hydrogen production through methanol reforming process is also being pur- sued by several groups including ours [10–14]. Copper–zinc oxide– alumina (Cu/ZnO/Al 2 O 3 ) has been used as a catalyst for methanol synthesis and it was demonstrated that the use of catalyst support enhanced catalytic stability due to its ability to withstand higher temperatures [14,15]. Several techniques have been utilized to synthesize ZnO nano- rod arrays such as chemical vapor deposition, sol–gel and hydro- thermal processes, etc. Chemical vapor deposition is an expensive technique while sol–gel and hydrothermal processes are relatively straight-forward as they do not require any compli- cated systems during synthesis. Hydrothermal process is a simple and low temperature technique for the growth of zinc oxide single crystals [16]. Vayssieres et al. first proposed the low temperature epitaxial growth of ZnO microrods on various substrates from equimolar concentrations of zinc nitrate ((Zn(NO 3 ) 2 ) and hexa- methylenetetramine (hexamine, C 6 H 12 N 4 ) precursors [17]. Since this early work, ZnO microrods were successfully grown on flat substrates such as glass [4], transparent conducting oxide film [18], stainless steel [14], paper and flexible polymeric fibers [19,20], amongst others. Though many groups have worked on ZnO microrod growth on solid substrates, there still is a gap in the understanding of hydrothermal growth of ZnO microrods on porous ceramic substrates. In this study, we compare the growth of ZnO microrods on popular ceramic substrates specifically, alumina, calcined 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.019 ⇑ Corresponding author at: Chair in Nanotechnology, Water Research Center, Sultan Qaboos University, P.O. Box 33, 123 Al Khoud, Oman. Tel.: +968 24143266; fax: +968 24413532. E-mail address: dutta@squ.edu.om (J. Dutta). Journal of Alloys and Compounds 586 (2014) 169–175 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom