Preparation and electrical properties of Ga-doped ZnO nanoparticles by a polymer pyrolysis method Yuan-Qing Li a,b, ⁎, Kang Yong a , Hong-Mei Xiao c , Wang-Jing Ma c , Guang-Lei Zhang a , Shao-Yun Fu c, ⁎ a School of Materials Science and Engineering, Shijiazhuang Railway Institute, Shijiazhuang 050043, China b School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 639798, Singapore c Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China abstract article info Article history: Received 1 February 2010 Accepted 13 April 2010 Available online 23 April 2010 Keywords: Ga-doped zinc oxide Nanomaterials Polymer pyrolysis Electrical properties In this article, preparation of Ga-doped zinc oxide (GZO) nanoparticles by a polymer pyrolysis method is reported. The pyrolysis behaviors of the polymer precursors prepared via the in situ polymerization of metal salts and acrylic acid are analyzed using thermalgravity-differential scanning calorimetry (TG-DSC) techniques. Then, the structural characteristics of the products are studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It is revealed by the results that the GZO nanoparticles calcined at 600 °C show good crystallinity with the wurtzite structure. GZO nanoparticles doped with 4 mol% Ga have a mean particle size of 26 nm with spherical-like morphology. Electrical resistivity measurement shows that, before and after high temperature annealing under H 2 atmosphere, the resistivity of the GZO nanoparticles is decreased by one and four orders in magnitude, respectively, compared with the pure ZnO nanoparticles. In addition, due to its versatility, this in situ polymer pyrolysis method can be easily extended to synthesis of other n-type doped semiconductor, such as In and Al doped ZnO or Sb doped SnO 2 . © 2010 Elsevier B.V. All rights reserved. 1. Introduction ZnO is a well know n-type semiconductor with a wide-band gap of 3.4 eV [1]. Besides its outstanding performance in optoelectronics [2,3], more attention has also been paid to doping of ZnO for obtaining transparent electro-conductive materials that are used as fillers to polymers, papers for electrostatic prevention or electromagnetic shielding [4,5]. The most attractive features of ZnO-based materials are their low cost and low toxicity, compared with other transparent conductors such as SnO 2 :Sb, In 2 O 3 :Sn, and Cd 2 SnO 4 [6]. For applications as transparent electro-conductive materials, the electrical conductivity of ZnO should be improved, which may be achieved by replacing Zn 2+ ions by other ions with higher valence (acting as efficient shallow donors) such as In 3+ , Al 3+ and Ga 3+ [7–9]. Among the metal dopants, Ga seems to be the best for ZnO due to fact that atomic radius of Ga 3+ is similar to Zn 2+ , and its lower reactivity to oxygen [9]. The rapid development of transparent electro-conductive materi- als has led to the exploitation of several widely used methods, such as microwave heating process [1], co-precipitation [6], chemical vapor synthesis [7], sol–gel [10], and hydrothermal method [11] for preparation of doped ZnO nanoparticles. Although the aforemen- tioned methods can be employed to produce doped ZnO nanoparti- cles, they have some drawbacks, such as requirement of a large quantity of solution and organic materials, long processing time, and unsuitable for large scale production or ill crystallinity and wide particle size distribution. Thus, it is of practical importance to seek for a novel method that does not have those drawbacks for preparation of doped ZnO nanoparticles as transparent electro-conductive materials. Very recently, the polymer pyrolysis method has been developed by us for preparation of ferrite nanoparticles such as MnFe 2 O 4 , NiFe 2 O 4 , and ZnFe 2 O 4 [12]. This method is easily operated and versatile for various metals and thus should also be suitable for preparation of metallic ion doped n-type semiconductors. In the present work, the polymer pyrolysis method was used to synthesize Ga-doped ZnO (GZO) nanoparticles. First, the Ga–Zn polyacrylate precursor, which reflects the precise stoichiometry of the end product, was made via the in situ polymerization method. Subsequently, the GZO nanoparticles were prepared by pyrolyzing the polymer precursor at a moderate temper- ature. To control the doping amount of GZO, different Ga/Zn molar ratios were designed. Finally, the effects of Sb dopant on the particle size, structural properties and electrical resistivity were discussed. 2. Experimental 2.1. Preparation of materials In a typical experiment, Ga(NO 3 ) 3 ·9H 2 O and Zn(CH 3 COO) 2 ·2H 2 O were dissolved in 20 g of acrylic acid aqueous solution (acrylic acid: H 2 O = 70: 30 wt.%) under stirring. The molar ratio of Ga:Zn varied from 0:100,1:100 to 8:100 . Afterwards, a small amount (1 g) of 5 wt.% Materials Letters 64 (2010) 1735–1737 ⁎ Corresponding author. School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 639798, Singapore. Tel.: + 65 6514 1027. E-mail addresses: yqli@ntu.edu.sg (Y.-Q. Li), syfu@mail.ipc.ac.cn (S.-Y. Fu). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.04.026 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet