pubs.acs.org/crystal Published on Web 12/30/2009 r 2009 American Chemical Society DOI: 10.1021/cg9012944 2010, Vol. 10 983–987 Synthesis of Cu-Doped TiO 2 Nanorods with Various Aspect Ratios and Dopant Concentrations Minkyu You, † Tae Geun Kim, ‡ and Yun-Mo Sung* ,† † Nano Materials Chemistry Laboratory, Department of Materials Science and Engineering, Korea University, Seoul 136-713, South Korea and ‡ Department of Electronic Engineering, Korea University, Seoul 136-713, South Korea Received October 17, 2009; Revised Manuscript Received December 16, 2009 ABSTRACT: Colloidal TiO 2 nanorods were synthesized with different aspect ratios and successfully doped with copper via a controlled hydrolysis method. Inductively coupled plasma (ICP) atomic emission spectroscopy analyses showed that the as- prepared TiO 2 nanorods contained ∼1.7-3.2 at% Cu. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) analyses revealed that the doped nanorods are in a highly crystalline anatase structure and their crystal growth orientation is preferably [001]. The real doping of Ti lattices with Cu ions was evidenced by the analyses of surface compositions and chemical states of the nanorods using X-ray photoelectron spectroscopy (XPS). Through magnetic investigation using vibration sample magnetometry (VSM), it was verified that the Ti 1-x Cu x O 2 nanorods maintain apparent ferromagnetic ordering at room temperature (300 K). The origin of the ferromagnetic property was explained based on the concentration of oxygen vacancies increased by Cu doping, which was also identified by the XPS analyses. The saturation magnetization showed strong dependency on the aspect ratio of nanorods as well as the Cu content in TiO 2 nanorods. 1. Introduction Dilute magnetic semiconductors (DMSs) have shown a great potential because they have not only semiconducting properties but also magnetic properties. 1-3 Semiconducting properties permit electrical devices to be operated effectively and promptly, and magnetic properties make enormous data storage possible. Therefore, the materials having both char- acteristics can be used for the rapid process and superior storage of information at the same time. 4-6 A lot of attention has been paid to synthesize DMSs by doping relatively wide bandgap semiconductors such as ZnO, GaN, GaAs, ZnSe, InAs, and TiO 2 with transition metals such as Fe, Ni, Co, Mn, and Cu. 7-13 Among them, transition metal-doped TiO 2 has attracted great attention because it reveals room-temperature ferromagnetism (RTFM). The first report related to TiO 2 -based DMSs was demonstrated by Matsumoto et al. who showed that cobalt-doped TiO 2 thin films hold ferromagnetism above 300 K. 14 Since they reported this result, there have been many studies related to TiO 2 thin films doped with other elements. Also, some research was conducted to obtain DMSs based on TiO 2 nanowires or nanorods. 15-21 How- ever, the microscopic origins of this room-temperature ferromagnetism remain poorly understood. A detailed understanding of room-temperature ferromagnetism in wide-bandgap DMSs is required to harness functionality for device applications. The development of such an understanding has emerged as one of the most important challenges in modern magnetism. 3,4 Moreover, TiO 2 has been known as one of the most important oxide semicon- ductors showing unique properties such as high refractive index, superior optical transmittance, electron transport, photochemical activity, and water splitting capability, which comes from its distinct energy bandgap character- istics. 14 Thus, it has found broad applications in optical coating, photocatalysis, photovoltaics, etc. 22-26 Single crystalline nanorods are very advantageous com- pared to bulk and polycrystalline materials because the possibility of defect formation is low in this nanostructure and the agglomeration of dopants can be prevented, which typically occurs in bulk DMS materials. In this paper, we synthesized single crystalline copper-doped TiO 2 nanorods by a controlled hydrolysis route 21,27 and demonstrate some im- portant results. First, for the first time, we successfully doped TiO 2 nanorods with Cu ions. Doping Cu ions instead of other elements gives several merits to DMS materials. The oxygen vacancies near Cu are more stable than those near the Ti sites. The energy required to form an oxygen vacancy decreases near the Cu impurity. Cu ions have lower vacancy formation energy than other dopants which can be applied to DMSs and they allow semiconductors to form vacancy sites more easily. These vacancy sites help structure relaxation in DMSs, and therefore the magnetic properties can be enhanced compared to other systems. 16-19 Cu atoms have no clustering tendency, and Cu-based compounds are not ferromagnetic. 20 Therefore, Cu doping overcomes the critical problem of magnetic pre- cipitates in a DMS. Second, we proved the ferromagnetic properties depending on the aspect ratios in copper-doped TiO 2 nanorods. Not only the influence of Cu itself in DMSs but also the influence resulting from the length to diameter ratio was verified. By controlling the concentration of dopant, Cu, and the aspect ratio of TiO 2 nanorods, it may be possible to put the right DMS in the right place for real applications. 2. Experimental Procedures Titanium tetraisopropoxide (Ti(OPri) 4 or TTIP, 99.999%), copper(II) acetate monohydrate (99.99%), oleic acid (C 18 H 33 CO 2 H or OA, 90%), and trimethylamino-N-oxide dihydrate or anhydrous ((CH 3 ) 3 NO or TMAO, 98%) (Sigma Aldrich Chemical, Milwaukee, WI) were used for the synthesis of Cu-doped TiO 2 nanorods. *To whom correspondence should be addressed. E-mail: ymsung@ korea.ac.kr; tel: 82-2-3290-3286; fax: 82-2-928-3584.