One-Step Combustion Synthesis of Carbon-Coated Nanoparticles using Multiple-Diffusion Flames

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1 Paper # 070HE-0031 Topic: Heterogeneous Combustion, Sprays & Droplets 8 th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute and hosted by the University of Utah May 19-22, 2013 One-Step Combustion Synthesis of Carbon-Coated Nanoparticles using Multiple-Diffusion Flames Nasir K. Memon 1, Mohamed A. Ismail 1, Dalaver H. Anjum 2, Suk Ho Chung 1 1 Clean Combustion Research Center 2 Advanced Nanofabrication Imaging and Characterization Lab King Abdullah University of Science and Technology Thuwal , Saudi Arabia Carbon-coated nanoparticles, such as silica (SiO 2 ) and titania (TiO 2 ) can be used in a wide variety of applications: water splitting, polymer fillers, pigments, precursors for carbide formation, and as an electrode for Li-ion batteries. We propose a novel process for the synthesis of carbon-coated nanoparticles based on the use of multiple-diffusion flames, also known as a multi-element diffusion burner (MEDB, Hencken Burner). Ethylene (C 2 H 4 ) is used as the precursor carrier gas, which, in a one-step process, enables the growth of carbon-coated nanostructures. The global equivalence ratio is maintained at 0.5, thus providing an oxygen-rich environment. The nanoparticles investigated using this setup are silica and titania, where hexamethyldisiloxane (HMDSO) and titanium tetraisopropoxide (TTIP) are used as these nanoparticles precursors, respectively. The crystal phase and size of the silica and titania nanoparticles are determined using x-ray diffraction (XRD). The nanoparticles are further characterized using a Raman microspectrometer, where the patterns obtained from the spectrometer are also used to validate the growth of carbon on the nanoparticles. Thermogravimetric analysis is performed to determine the percentage of carbon in the samples. The morphology and crystal structure of the samples are characterized using high-resolution transmission electron microscopy (HRTEM), with elemental mapping. The titania particle size ranged from 30 to 50 nm with a uniform carbon coating of 3 to 5 nm, as observed by HRTEM. The Raman pattern confirmed the growth of a graphitic structure in the coated particles, with the carbon content measured at 25% using TGA. The use of MEDB to produce carbon-coated nanoparticles is scalable, and this process could possibly be extended to carbon-coat a wide range of nanoparticles. 1. Introduction Flame synthesis is a scalable method that is widely used in the global production of nanoparticles (Teoh, Amal et al. 2010). Flame synthesis offers several advantages for metal-oxide production, since it readily provides both the high temperature and oxygen species necessary for growth. Furthermore, in a one-step process a flame can also serve as a carbon source and result in the production of carbon-coated nanoparticles. Most metal-oxides have low electrical conductivity, thus limit their use in a number of applications. However, when coated with carbon the electrical conductivity of metal oxide increases. Such carbon-coated material could potentially be used as an electrode material in numerous applications. Specific nanoparticles that are the focus of this work are silica (SiO 2 ) and titania (TiO 2 ). Carbon coated silica and titania can be used in a variety of applications such as water splitting (Khan, Al-Shahry et al. 2002), polymer fillers, pigments, precursors for carbide formation, and as an electrode for Li-ion batteries. A particularly exciting burner for the flame synthesis of nanomaterial is the use of a multi-element diffusion flame burner (MEDB, Hencken Burner). The burner consists of a number of needle tubes which are aligned using a honeycomb. The needle tubes are usually used for delivering fuel. This burner configuration offers several advantages such as a uniform temperature profile above the burner, the avoidance of unwanted chemical reactions since the mixing of the fuel and oxidizer occurs external to the burner, and limitations related to a premixed flame (flashback and flame speed) are avoided. Such burner confirmation has been used for the synthesis of a number of nanomaterials, such as

2 silica (Wooldridge, Torek et al. 2002), iron-oxide (Rao and Zheng 2009), and graphene (Memon, Tse et al. 2011, Memon, Tse et al. 2013). 2. Experimental Methods Carbon coated titania and silica synthesis are performed using an MEDB at atmospheric pressure. Hexamethyldisiloxane (HMDSO) and titanium tetraisopropoxide (TTIP) from Sigma-Aldrich are used as the precursor, where a syringe pump is used to inject the precursor into an evaporator. A combination of nitrogen (N 2 ) or argon (Ar), and ethylene (C 2 H 4 ) are used as the precursor carrier gas, which flows through a center tube (Fig. 1). The evaporator and precursor lines are heated to prevent any precursor condensation. CH 4 or H 2 is used as the fuel and oxygen (O 2 ) (with Ar) or air is used as the oxidizer for the MEDB (Fig. 1). The global equivalence ratio is kept at 0.5, thus providing a highly oxidizing environment. The silica and titania nanoparticles are collected on an aluminum plate downstream from the burner. The gas-phase temperature in the absence of the precursor, at 3.5 cm height above the burner is 600 o C. Figure 1: Schematic of the experimental setup. Crystal structure of the titania and silica nanoparticles are determined using X-Ray diffraction (XRD), Bruker D8 Advance diffractometer, which operates using Cu(K α ) radiation. The nanoparticles are further characterized using a Raman microspectrometer, Horiba LabRAM HR Visible. The Raman pattern obtained from the spectrometer is also used to validate the growth of carbon alongside the silica and titania nanoparticles. Thermogravimetric analysis (TGA), Mettler Toledo TGA/DSC 1, is performed to determine the carbon weight percentage of the samples. Finally the samples are analyzed using transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Both TEM and HRTEM images are obtained using FEI Company s Titan G electron microscope equipped with an EDS detector from EDAX and a GIF energy filter Gatan, Inc. Images are recorded on a 2k by 2k pixel CCD camera of model US1000 from Gatan, Inc. 3. Results and Discussion XRD is first used to determine the crystal structure of the carbon-coated titania and silica nanoparticles. From the XRD pattern in Fig. 2a, the growth of anatase titania nanoparticles is confirmed (Ohsaka, Izumi et al. 2005). All the peaks in Fig. 2a correspond to the anatase phase, while no peak related to the rutile phase is observed. From Fig. 2b, the growth of amorphous silica nanoparticles are observed. 2

3 Intensity (a.u.) Intensity (a.u.) Theta (degree) 2 Theta (degree) Figure 2: XRD spectrum of the nanoparticles titania and silica Next, Raman spectroscopy is conducted to further determine the crystal structure of the nanoparticles. Based on the factor group analysis, anatase titania has 15 optical Raman modes (Zhang, Fu et al. 2008). Fig. 3a depicts the Raman spectrum of the titania particles, where the peaks correspond to the Raman modes of the anatase phase. Additionally, Raman spectroscopy is commonly utilized to confirm the growth of carbon nanostructures (Escribano, Sloan et al. 2001). In addition to the anatase titania Raman peaks (Fig. 3a), two additional peaks are present for titania. These two peaks observed in the spectrum are: (i) the G peak at ~ 1580 cm -1, which is related to sp 2 bonded carbon atoms and is commonly observed in graphitic type structure, and (ii) the D peak, which is related to defects within the graphitic structure. In the case of silica, no signature peak related to SiO 2 is observed, however the D and G peak related to the growth of carbon nanostructures is observed. D G D G Raman Shift (cm -1 ) Raman Shift (cm -1 ) Figure 3: Raman spectrum of the nanoparticles titania and silica TGA is performed to determine the carbon content of the titania and silica samples (Fig. 4). The process involves heating the nanoparticles to 800 o C in a nitrogen atmosphere followed by changing the nitrogen with air, while keeping the temperature at 800 o C (Kammler and Pratsinis 2003). In the nitrogen environment no weight loss is detected, while in the oxygen environment, a 25% drop in the weight of the titania particles occurs (Fig. 4a). This drop corresponds to the carbon content of the particles, which happens due to the oxidation of carbon (removal as CO or CO 2 ). In the case of silica, the carbon content is estimated to be at 4% (Fig. 4b). 3

4 Mass (a.u.) Time (min) Figure 4: TGA analysis of the nanoparticles titania and silica Time (min) Finally the growth of titania and silica nanoparticles are examined using TEM. In the case of titania, Fig. 5a, the growth of nanoparticles in the range of 30 to 50 nm is observed. Additionally, a coating of 3 to 5nm is uniformly observed across the nanoparticle. For silica, Fig. 5b, a much smaller primary particle size (<10nm) is observed, however no carbon coating occurs. Furthermore, using electron energy loss spectroscopy (EELS), it is determined that the carbon for silica is present within the nanoparticles. 50 nm 50 nm 20 nm Figure 5: TEM analysis of the nanoparticles titania and silica 4

5 4. Conclusions Carbon-coated nanoparticles can be used in a wide variety of applications. A novel process for the synthesis of carboncoated nanoparticles based on the use of multiple-diffusion flames is demonstrated. Ethylene (C 2 H 4 ) is used as the precursor carrier gas, which, in a one-step process, enables the growth of carbon-coated nanostructures. The nanoparticles investigated using this setup are silica and titania. The growth of anatase titania nanoparticles is confirmed using XRD. In the case of silica, the XRD spectrum confirms the growth of an amorphous structure. Raman spectroscopy confirms the presence of graphitic carbon on both the titania and silica nanoparticles. The carbon content of the samples is determined using TGA and is around 25% and 4%, for titania and silica, respectively. TEM confirms the titania particle size range from 30 to 50 nm and the size of silica nanoparticles is below 10 nm in length. TEM also showed that titania nanoparticles have a uniform carbon coating of 3 to 5 nm. On the other hand, no carbon coating is observed for the silica nanoparticles. Instead EELS showed that the carbon was doped in the silica nanoparticles. Acknowledgements We are grateful to Dr. Yang Yang for his assistance with the Raman measurements. References Escribano, R., J. J. Sloan, N. Siddique, N. Sze and T. Dudev (2001). "Raman spectroscopy of carbon-containing particles." Vibrational Spectroscopy 26(2): Kammler, H. K. and S. E. Pratsinis (2003). "Carbon-coated titania nanostructured particles: Continuous, one-step flamesynthesis." Journal of materials research 18(11): Khan, S. U. M., M. Al-Shahry and W. B. Ingler Jr (2002). "Efficient photochemical water splitting by a chemically modified n-tio2." Science 297(5590): Memon, N. K., S. D. Tse, J. F. Al-Sharab, H. Yamaguchi, A. M. B. Goncalves, B. H. Kear, Y. Jaluria, E. Y. Andrei and M. Chhowalla (2011). "Flame synthesis of graphene films in open environments." Carbon 49(15): Memon, N. K., S. D. Tse, M. Chhowalla and B. H. Kear (2013). "Role of substrate, temperature, and hydrogen on the flame synthesis of graphene films." Proceedings of the Combustion Institute 34(2): Ohsaka, T., F. Izumi and Y. Fujiki (2005). "Raman spectrum of anatase, TiO2." Journal of Raman Spectroscopy 7(6): Rao, P. M. and X. Zheng (2009). "Rapid Catalyst-Free Flame Synthesis of Dense, Aligned α-fe2o3 Nanoflake and CuO Nanoneedle Arrays." Nano letters 9(8): Teoh, W. Y., R. Amal and L. Madler (2010). "Flame spray pyrolysis: An enabling technology for nanoparticles design and fabrication." Nanoscale 2(8): Wooldridge, M. S., P. V. Torek, M. T. Donovan, D. L. Hall, T. A. Miller, T. R. Palmer and C. R. Schrock (2002). "An experimental investigation of gas-phase combustion synthesis of SiO2 nanoparticles using a multi-element diffusion flame burner." Combustion and Flame 131(1 2): Zhang, L. W., H. B. Fu and Y. F. Zhu (2008). "Efficient TiO2 Photocatalysts from Surface Hybridization of TiO2 Particles with Graphite like Carbon." Advanced Functional Materials 18(15):