Continuous Synthesis of Freestanding ZnO Nanorods in a Flame Reactor

ZnO can be made into many nanostructures that have unique properties for advanced applications. This Letter reports a process to synthesize ZnO nanorods in a counterflow diffusion flame reactor. Unlike the previous work on flame syntheses, our work shows that pure ZnO nanorods can be made in a freestanding form. It was demonstrated that ZnO nanorods with different aspect ratios can be obtained by adjusting the synthetic conditions of the reactor. Further, we showed that the nanorods have a unique crystalline orientation. INTRODUCTION ZnO is a II-VI semiconductor with a large bandgap (3.37 eV) that can be excited using short wavelength emissions such as UV radiation at room temperature. ZnO has a wurtzite structure and belongs to the non-centrosymmetric point group 6mm. 1 This property makes ZnO a highly suitable material for piezoelectric and pyroelectric applications. 1 ZnO has also been demonstrated to be a promising anode material in dye-sensitized solar cells. 1 Based on the applications for which it is being used, ZnO can be made into various nanostrucutures such as nanoparticles, nanorods, nanobelts, nanocombs, and nanocages. 1 Due to its high photon emission efficiency ZnO can be considered as a substitute material for TiO2 which is used in most of the photocatalytic applications. 2,3 Also, by addition of suitable dopants to ZnO the band gap can be changed and hence its photonic properties can be altered. 4,5 ZnO and ZnO composites have been used for gas sensing applications. 6 In addition to the above discussed applications, ZnO also finds it use as field emission transistors (FET), chemical and biological sensors, nanoresonators, and nanocantilevers. 1 ZnO nanorods have been produced in various gas-phase processes such as flame spray pyrolysis (FSP), laser ablation plume, metalorganic vapor phase epitaxy (MOVPE), chemical vapor deposition (CVD) and counterflow diffusion flames (CDF). 7,8,9,10,11 Apart from the above mentioned gas-phase processes, ZnO have also been synthesized using thermal plasma techniques with and without substrates. 12,13 The ZnO nanorods that were grown in the various gas-phase synthesis routes such as FSP, laser ablation plume, MOVPE and CVD used a reactor furnace for depositing ZnO on the surface of a substrate. These processes are batch type, and hence cannot be used for large-scale production. FSP and CDF processes are continuous and hence, they are pretty easy for scale-up. Initial work done by Akhtar et al. 14 and Tani et al. 15 showed that ZnO could be synthesized using FSP. But, they could not get controlled formation of the product. Hence to rectify this drawback Height et al. 7 added dopants like In and Sn to promote the growth of ZnO nanorods in a specific direction in the absence of a substrate. Addition of dopants makes the resultant product impure and as discussed earlier, purity of ZnO is an important factor for its usability in the semiconductor industry. 16 Peng et al. 13 synthesized ZnO nanorods in large quantities without substrate by using a radio frequency thermal plasma system. By varying the powder-feeding rate, they have produced ZnO nanorods with aspect ratios of 2-14. The diameter of the nanorods produced was 50 nm. Xu et.al 11 used the CDF to synthesize various nanostructures of ZnO. But, a Zn-plated metal substrate was used for the preferential growth of ZnO nanorods. Use of a substrate limits the large-scale production of ZnO nanorods. 13 The current work proposes a substrate-free and catalyst-free continuous production of ZnO nanorods. It has also been shown that by varying the flow rate of the precursor and the gas flow rates, ZnO nanorods of various aspect ratios can be obtained. Unlike the previous work done on ZnO nanarods, the current work proposes that the diameter and the length of the nanorods can be controlled easily by just varying the process parameters. EXPERIMENTAL The CDF reactor developed by Katz et al. 17 was used to study the nucleation of refractory compounds. The CDF reactor was further developed by Xing et al. 18 to produce nanoparticles of different morphologies. The CDF reactor shown in Fig.1 has a unique configuration, which consists of two vertical channels of rectangular cross sections that are positioned opposite to each other. The rectangular channels have a high aspect ratio, which makes the dimension of one of the sides negligible, and nanoparticles evolve along the third direction. In one of the channels hydrogen diluted with nitrogen and in the other channel oxygen diluted with nitrogen are introduced. Adjusting the flow rates of the gases can control the flame location. The flame established using the combustion gases produces a temperature gradient between the mouth of the burner and the flame. Particle morphology can be easily controlled using the CDF reactor because of the different chemical environment present on either sides of the flame. The important point that needs to be considered is that the flame should be nearly flat for producing nanoparticles of uniform size and shape. This flat flame can be obtained by using a honeycomb mesh on the rectangular cross sections. The setup is connected to a vacuum pump, which helps in collecting the nanoparticles. To produce ZnO, Diethyl Zinc (DEZ) was used as the precursor. DEZ is highly volatile and burns in air. Hence, DEZ was diluted with hexane in a glove box in the ratio of 1:1 to eliminate the chances of burning DEZ. The solution is then filled in gas tight syringe and then introduced into the reactor using a syringe pump. This setup helps in adjusting the flow rate with precision. The ZnO powder produced was collected using a vacuum pump. X-ray diffraction (XRD) studies were carried out using the Philips X’Pert x-ray diffractometer with a 2 varying from 6 to 90 O at a scan rate of 0.025 deg s -1 using a Cu K- radiation. The operating conditions for the diffractometer are 45kV and 40 mA. The experimental conditions used to produce ZnO are shown in Table 1. Table 1: Experimental conditions used to produce ZnO powder Precursor flow rate (ml/hr) H2 gas flow rate (l/min) N2 in H2 gas flow rate (l/min) O2 gas flow rate (l/min) N2 in O2 gas flow rate (l/min)