Continuous Synthesis of Freestanding ZnO Nanorods in a Flame Reactor

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1 Continuous Synthesis of Freestanding ZnO Nanorods in a Flame Reactor Abstract: 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 TiO 2 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 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

2 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 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) H 2 gas flow rate (l/min) N 2 in H 2 gas flow rate (l/min) O 2 gas flow rate (l/min) N 2 in O 2 gas flow rate (l/min) A thermophoretic probe was used to collect the sample for Transmission Electron Microscope (TEM) analysis. Thermophoretic sampling consists of a probe and a TEM grid is attached to it using duct tape. The probe is then inserted and retraced from flame at a very fast speed. Since, the grid is at the room temperature, there would be condensation of particles formed in the flame onto the grid. A Tecnai F20 TEM was used to characterize the morphology and structure of the nanorods. The images obtained were then analyzed using Image J and Digital micrograph. Fast Fourier Transform (FFT) in the digital micrograph software was used to obtain the selected area electron diffraction (SAED) patterns.

3 The electron diffraction patterns obtained were analyzed using Crystal Maker software. The software has an in-built database for various types of crystal structures or we can build our own crystal structure. The standard diffraction pattern for a single crystal is obtained and then compared to the diffraction pattern obtained from the TEM image. To match the diffraction patterns some instrument dependent parameters like camera length and spot size have to be known. The simulated diffraction patterns obtained from different directions are then compared to existing diffraction pattern to determine the orientation of the crystal, which in this case is the growth direction of the nanorod. Fig. 1 Counterflow Diffusion Flame Reactor RESULTS AND DISCUSSIONS The ZnO powder from the collector was then characterized by using XRD to determine crystallinity and crystal planes. Fig.2 shows obtained peaks in the XRD spectrum which have been well indexed to the hexagonal wutrzite structure of ZnO with lattice constants of a=0.325 nm and c= nm within the experimental error (Joint Committee on Powder Diffraction Standards, Powder Diffraction File No ; a = nm and c = nm). The sharps peaks in the XRD pattern show that the ZnO powder obtained is highly crystalline without any amorphous phases. No peaks due to undecomposed Zn or carbon (from burning hexane in the flame) and the high intensity of the peaks indicate that the ZnO powder obtained is highly pure. As discussed earlier, ZnO is a II-VI semiconductor and finds a lot of applications in the semiconductor industry where purity is an important factor. It can be seen that using our process we can produce highly pure ZnO without any impurities. Fig.3 shows a low magnification image of ZnO produced. It can be clearly seen that the powder produced has rod like nanostructures and the peak broadening observed in Fig.2 can be attributed to these nano sized structures. A high-resolution TEM image shown in Fig.4 proves our previous conclusion that the ZnO nanorods produced are highly crystalline without any trace of amorphous structures. Calculations have been made using Image J and we found out that ZnO nanorods are approximately 10 nm in diameter. The SAED pattern is shown as an inset in Fig.4, the circular

4 spot on (002) indicates the nanorod growth is parallel with the in plane direction 20. From the XRD and the SAED pattern in Fig. 4, it can be concluded that the growth direction of nanorods is [001]. The highest intensity peak in XRD pattern and the TEM image prove that the growth direction is [001]. Our studies conform to the previous studies by Wang et al 1., Peng et al. 13, Vayssieres et. al 19 who have shown that [001] direction is the preferential and thermodynamically most stable direction for growth of ZnO nanords. The relative velocities for the growth of ZnO nanorods along different directions have been reported elsewhere 19. Fig.2 XRD pattern of ZnO powder showing high crystallinity and purity Fig.3 Low magnification TEM image showing rod like morhphology of ZnO powder. Head on image of the ZnO nanorod along with its SAED pattern as an inset is shown in Fig. 5. We can clearly see that the shape of the base of the nanorod is a typical hexagon with a

5 width of 19 nm and a angle. The SAED pattern shown in Fig. 5 has its zone axis along the [001] direction which is also the growth direction of nanorods. From the diffraction pattern and the image in Fig.5, it can inferred that the hexagonal shape seen in Fig.5 is actually the basal plane of the hexagonal shaped wurtzite structure and the growth of the nanorod is along the [001] direction. Fig.4 High Magnification TEM image with its SAED pattern showing the normal growth direction and the surface growth direction of the ZnO nanorod. Fig.5 High Magnification TEM image of a ZnO nanorod with its SAED pattern in the inset with its zone axis along the [001] direction. Wang et. al 1 have reported the various growth direction of ZnO nanorods. Other than the {001} facets, the other facets along which the nanorods can grow are {010} and {210} due to

6 their lower surface energies. Fig. 4 shows the image of a nanorod that is on its surface. From the SAED pattern and the TEM image it can be concluded that the nanorods grow along the [010] direction and the surface grows along the [010] direction. The results that we have obtained correspond to the previous work done on ZnO nanorods 1, 21. Lattice spacing calculations have been done using Image J and they were found to be 0.52 nm and 0.28 nm, which correspond to the lattice spacing between the (001) planes and the (010) planes in the hexagonal crystal. CONCLUSIONS Pure ZnO nanorods have been synthesized using the CDF reactor which are highly crystalline without any traces of amorphous phases. XRD studies and TEM images along with their SAED patterns have shown that the ZnO grow along [001] direction. Based on the SAED patterns, it was also shown that the surface of the ZnO nanord grows along [010] direction. Lattice spacing has been calculated to be 0.52 nm and 0.28 nm, which correspond to the (001) and (010) interplanar spacing in the hexagonal crystal. REFERENCES 1. Wang Z. L, J. Phys.: Condens. Matter, 2004 (16) R829-R Sakthivela S, Neppolian B, Shankar M.V, Arabindoob B, Palanichamy M, Murugesan V, Sol. Energy Mater. Sol. Cells (77), Hao Y, Yang M, Li W, Qiao X, Zhang L, Cai S, Sol. Energy Mater. Sol. Cells 2000 (60), Sun X.C, Zhang H.Z, Xu J, Zhao Q, Wang R.M, Yu D.P, Solid State Commun (129), Kong X.Y, Ding Y, Yang R, Wang Z.L., Science 2004 (303), Tamaekong N, Liewhiran C,Wisitsoraat A, Phanichphant S, Sensor 2010 (10), Height M.J, Madler.L, Pratsinis S.E., Chem. Mater (18), Hartanto A.B., Ning X, Nakata Y, Okada T, Appl. Phys. A 2004 (78), Park W.I, Kim D. H, Jung S.W, Yi G.C., Appl. Phys. Lett (80), Wu J.J, Liu, S.C., Adv. Mater (14), Xu F, D Esposito C, Liu X, Kear B, Tse S.D., Mater. Res. Soc. Symp. Proc., 2009 (1142) JJ Hong Y. C, Kim, J.H, Uhm H.S., Jap. J. App. Phys (45), Peng H, Fangli Y, Liuyang B, Jinlin L, Yunfa C., J. Phys. Chem. C 2007 (111) Akhtar M.K, Pratsinis S.E, Mastrangelo S.V.R. J. Am. Ceram. Soc. 1992, 75(12), Tani T, Madler, L, Pratsinis S.E. J. Nanopart. Res. 2002, 4(4), Yi G-C, Wang C, Park W.I. Semicond. Sci. Technol. 2005, 20, S22-S Chung SL, Katz J.L, Combustion and Flame 1985 (61) Xing Y, Koylu U, Rosner D.E, Combustion and Flame 1996 (107) Vayssieres L, Keis K, Hagfeldt A, Lindquist S E, Chem. Mater. 2001, 13(12), Kim D C, Kong B H, Cho H K, J. Phys. D: Appl. Phys. 2009, 42, Ye C, Fang X, Hao Y, Teng X, Zhang L, J. Phys. Chem. B 2005, 109,