Simple Crystal Phase Control of TiO2Nanoparticles via Pulsed Laser Ablation in Nitric Acid

Abstract

It has long been known that anatase, brookite, and rutile are the major crystalline structures of TiO2, of which the rutile phase is the most stable; whereas anatase and brookite phases are metastable and easily transformed to the rutile phase by heating above about 600-800 C. A number of investigations have been made on controlling the crystal structure and morphology of TiO2 using a variety of synthetic methods, such as by hydrolysis of Ti ions, hydrolysis of titanium alkoxides or titanium tetrachloride in the gas phase, sol-gel, hydrothermal hydrolysis, and precipitation. Furthermore, a variety of attempts have been made to enhance the photocatalytic effects of TiO2, including metallic and non-metallic doping, nano-size reduction, and its use in bi-metallic catalysis. However, there is no report on the selective synthesis of the different phases of TiO2 nanoparticles, involving different ratios of anatase and rutile, prepared in an ambient environment, i.e., through the variation of HNO3 concentration, without extreme temperature and pressure conditions. Here, we present an approach that differs from the previously mentioned methods for the synthesis of TiO2 nanoparticles. In our case we have employed a pulsed laser to ablate a Ti plate immersed in water. Pulsed laser ablation in liquid (PLAL) has been demonstrated to be a simple, versatile, and clean method for preparing metal and metal oxide nanomaterials due to the use of high purity starting materials, no requirement for a catalyst, and reduced byproducts, provided an appropriate metal and liquid is chosen. Recent studies have demonstrated that PLAL of metal plates employing different parameters, such as laser wavelength, power, and the presence of surfactants, can lead to different products, sizes, and/or morphologies. Nonetheless, there are few reports of the formation of TiO2 via PLAL. Furthermore, most studies have focused on the preparation of anatase and rutile TiO2 in the absence/presence of surfactants; however, the systematic preparation employing different ratios of anatase/rutile TiO2 has not been studied before. In this work, we have controlled the ratios of anatase/ rutile TiO2 nanoparticles by changing the concentration of HNO3. XRD measurements of the samples were employed in order to determine the crystal structure and the crystallinity of the prepared TiO2 samples prepared via PLAL. The XRD patterns illustrated in Figure 1 correspond to the crystalline phases of the TiO2 samples prepared via PLAL using various concentrated HNO3 solutions. Figure 1(a), (b), and (c) present XRD patterns over a scan interval from 10 to 90 for the as-is sample of TiO2 via PLA in DI water (a), 10 −3 M (b), and 1 M solution of HNO3 (c), respectively. It is seen that the XRD patterns for samples, (a) and (c), obtained via PLA in DI water and 1 M HNO3 solution match the standard anatase (JCPDS number: 00-021-1272) and rutile (JCPDS number: 00-021-1276) patterns, respectively. The XRD pattern shown in Figure 1 (b) indicates that mixed phases of TiO2, anatase and rutile are formed in 10 M HNO3 solution. While it is generally the case that rutile TiO2 is fabricated from calcination of anatase TiO2 above 600-800 C; in this work the rutile phase is simply and selectively obtained in 1 M HNO3 solution at ambient conditions. According to the XRD patterns shown in (a), (b), and (c), the peaks are broader than those in (d), (e), and (f). From the Scherrer equation, peak broadness is inversely related to the full width at half maximum of an individual peak; the broader peak, the smaller crystallite size. Thus, it is apparent that the TiO2 nanoparticles prepared at low concentrations of HNO3 (Figure 1(a)-(c)) were smaller than the particles produced at higher