Nanomaterials synthesized by gas combustion flames:Morphology and structure

Particuology 8(2010)556-562Contents lists available at Science DirectPARTICUOLOGYParticuologyELSEVIERjournalhomepagewww.elsevier.com/locate/particNanomaterials synthesized by gas combustion flames: Morphology and structureChunzhong Lia, b, Yanjie Hua, Weikang Yuan bKey laboratory for Ultrafine Materials of Ministry of education, School of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong road,b State Key laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, ChinaARTICLE NFOA BSTRACTrticle history.The flame technology has been employed broadly for large-scale manufacture of carbon blacks, fumedReceived 29 May 2010ica, pigmentary titania, and also ceramic commodities such as Sio2, TiO2, and Al2O3. A deeperunderstanding of the process also made it possible for production of novel nanomaterials with highfunctionality-various novel nanomaterials such as nanorods, nanowires, nanotubes, nanocoils, andnanocomposites with core/shell, hollow and ball-in-shell structures, have been synthesized recently viagas combustion technology, while the mechanisms of the material formation were investigated based onthe nucleation-growth and chemical engineering principles. Studies of the fluid flow and mass mixing,Chemical engineeringsupported by principles of chemical reaction engineering, could provide knowledge for better underStructurestanding of the process, and thus make rational manipulation of the products possibleo 2010 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy ofiences. Published by Elsevier B v. All rights reserved1. Introductionto its continuous nature and the ability for scale up Thereforethis technology has been regarded as a well-established methodThe gas combustion flame technology refers to the formation of for production of nanomaterials with high purity and controllablenanomaterials from gases in flames. this technology has been ini- structurestially developed for preparation of carbon black since prehistoricUlrich(1984)pioneered the investigation of flame synthesis oftimes as depicted in Chinese ink artwork. In the late 19th century, ceramic powders by making SiO2 powders through SiCla oxidationfine carbon black used as fillers in rubbers was commercially pro- in premixed flames. Since the early 1990s, the pace of research hasduced through pyrolyzing the undesirable by-product natural gas been further intensified with a renewed interest in flame technolfrom oil fields(Stark Pratsinis, 2002). Industrial laboratories led ogy for manufacturing advanced materials mainly for nanosizedthe research in this field in the mid-20th century, motivated from particles( Pratsinis, 1998 ) In 1996, fullerenic nanostructures werethe industrial importance of fumed silica which was first made in synthesized in flames( Das Chowdhury, Howard, Vandersandethe 1940s and was used mainly as fillers for silicon rubbers. In 1996), and single-wall carbon nanotubes were prepared in thethe late 1970s, the flame technology contributed decisively to the C2H2/O2 Ar flames ( richter et al., 1996). More recently, an arramanufacture of ultrapure silica fiber that became the basic mate- of nanoscaled, sophisticated products such as catalysts, sensors,rial for lightguides in telecommunications. Nowadays, the flame dental and bone replacement composites, phosphors, fuel cell andtechnology is employed routinely in large scale manufacturing of battery materials, and even nutritional supplements were madecarbon blacks and ceramic commodities such as fumed silica and with flames(Strobel Pratsinis, 2007). In addition, many noveltitania and. to a lesser extent fomaterials with potentialapplication in electsuch as zinc oxide and alumina powders. The flame technology ics, biotechnology and catalysis are also synthesized by gas flameappears to possess many advantages over the conventional wet combustion(Li, 2010). However, it is still difficult to produce multiprocesses because the product can be collected easily by filters, component materials with controlled morphology and structures,so the postheating treatment is no longer required, especially due because of the nonuniformity of temperature and gas compositionin the flameFlame synthesis includes combustion of volatilesors in adrocarbon, hydrogen or halide flame( Strobel Pratsinis, 2007)Corresponding author at: Key laboratory for Ultrafine Materials of Ministry of Precursor is injectEducation, School of Materials Science and Engineering, East China University ofScience and Technology, 130 Meilong Road Shanghai, Chinaform of droplets中国煤化 IS phasor intheE-mailaddress:czli@ecust.edu.cn(CLi).mediate and prodiCNMHGroducts form via674-2001/s- see front matter o 2010 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B V. All rights reserveddoi:10.1016/ 1. partic.2010.08009C Li et al. Particuology 8(2010) 556-562557nucleation, surface reaction and or coagulation and subsequent centric tubes, Jan and Kim synthesized TiO2 nanoparticles rangingcoalescence into larger aggregates As the aerosol stream leaves from 10 to 30 nm by oxidation of titanium tetrachloride (ticl4 )in athe high temperature zone, particle growth continues mostly by diffusion flame reactor ang Kim, 2001), and the mass fraction ofcoagulation to the complete particles. The flame synthesis for the the synthesized anatase reached 40% to 80% in their experimentsnanomaterials involves processes such as rapid high-temperatureAmong various photocatalysts, TiO2 has outstanding photocatvapor reaction, nucleation, particle growth, and agglomeration. alytic properties and broad applications in many fields due to itsMeanwhile these processes correlate and interact, leadingn- proper bandgap energy, large surface area, high stability low cost,plexity of the nanomaterial formation. Therefore, preparation of and non-toxicity. However, when taking TiO2 as a photocatalyst,nanomaterials and corresponding growth mechanism in this com- the low photon quantum efficiency and the utilization of UV light asplex flame environment have become the focus of the study in the excitation source would limit its application. By using a multi-he past years. For such a complex system, it would be prudent jet flame to control the distribution of temperature and residenceto study the system only to set up the ties between the so-called time and also the mixing of reactants, a modified diffusion flamedominating-scale structure, and certain holistic performance of the method was established to fabricate TiO2 nanoparticles by intro-ystem that interests people and then to manipulate the found ducing diluting gas in the central tube( zhao, Li, Liu, Gu, 2007dominating-scale structure to achieve our target(Yuan, 2007). For Zhao, Li, Liu, Gu, et al, 2007; Zhao, Li, Liu, Gu, Du, 2008). Thisynthesis of nanomaterials by the flame technology, the key target method partially overcomes the disadvantages of the traditionalis to control the micro/ meso-structures of the products, because the methods, which would result in the formation of Tioz with largerapplication of nanomaterials is determined mainly by their chemi- particle sizes and inhomogeneous distribution. The as-preparedcal composition, together with their micro / meso-structures Aided TiOz nanoparticles show much better photocatalytic activity(threeby the chemical engineering principles and developed methods, times)compared to the p-25 TiO when degrading rhodamine bmany novel nanomaterials have been synthesized by our labora- under visible light irradiation( Fig. 1), that can be attributed totory, using the flame technology. Equipments for SiO2 and TiO2 well-dispersion characteristics of the TiO2 nanoparticles preparednanoparticles have been scaled up successfully.by modified diffusion flame process(zhao, Li, Liu, Gu, 2007). Forthe Zn doped TiO2 nanoparticles prepared by the modified diffu2. Nanoparticlession flame method, most part of dopant Zn2* ions supposedly locateon the surface of TiO2 nanoparticles to form small Zno nuclei ran-The gas combustion flame synthesis is most suitable for synthe- domly dispersed on the anatase TiO2 surface. The doped TiO2 showssizing nanoparticles of high-purity, high specific surface area and better photocatalytic properties than the pure Tioz, because of thecontrolled particle size distribution. Many kinds of nanoparticles discrepancy of the energy band position of TiO2 and Zno(Zhao, Li,have been synthesized commercially for applications in electron-& Gu, 2008 ) The properly Fe 3+doped TiO 2 nanoparticles exhibitnanosized Al2O3 and SnO were prepared by oxidation of doped TiO2, since Fe* helps to separate photogenerated electrons andke Al(CH3)3 or Sn(CH3 )3 in lowholes by trapping them temporarily and shallowly( zhao, Li, liu,sure premixed H2/02/Ar flames(Lindackers, Janzen, Ellinghaus, 2007 ) However, high-level Fe 3* doping would lead to charge paiWassermann,&Roth, 1998). Amorphous spherical Al2O3 particles recombination and thus decreasing the photocatalytic activities ofwere found in a size range from 4.7 to 8.44 nm, while SnO2 particles TiO2 nanoparticles. Tian et al. prepared V-doped TiOz nanoparti-anged between 2. 7 and 8.3 nm. A theoretical model was developed cles by a simple one-step flame spray pyrolysis(FSP)techniqueincluding the full H2/02 reaction kinetics as well as the transport (Tian, Li, Gu, Jiang, Hu, et al, 2009). Benefiting from the short resiproperties of a burner stabilized flame. With an external electric dence time and high quenching rate during the flame spray process,field, needle and plate electrodes could be used to synthesize fumed V4 ions are successfully incorporated into the TiO2 crystal latticesilica with controlled particle size distribution from HMDs inIt reveals that V-doping favors the primary particle size growthcoflow double diffusion flame at atmospheric pressure(Kammler as well as the increase of rutile content in the products, and thusPratsinis, 2000). The average primary particle diameter could be V-doping enhances the photocatalytic activity under both UV andneedle electrodes was increased from 0 to1.5 kV/cm. Based on theBased on self-assembly of TiO2 nanoparticles synthesizeddesign of a multiport diffusion type burner composed of five con- by H2/O2 combustion flames, self-cleaaning films were prepareds bas-prepared TiO2P-25 TiO8飞中国煤化工CNMHG1. The morphology(a)and the transmittance of as-prepared TiOz and TiOz P-25(b). The insets show the images of RhB/TiO2 P-25 system(left), RhB/as-prepared Tiocm(middle), and rhB solution (right).C Li et al/ Particuology 8(2010)556-562ab50 nmNMA MaAl5(101)d0. 26Inm01I200)0.239mmFig. 2. TEM image of 2.5 at.% Fe-doped SnO2 nanorods(a), and their EDS analysis(b), HRTEM image (c)and corresponding SAED pattern(d)taken from the white box in (a).showing the preferred [00 1]orientationfrom multilayer deposition of poly(sodium 4-styrene sul- thesis of one-dimensional nanostructures for further promotingnate)on the treated TiO2 nanoparticles and Sio2 nanopar- their applications. Recently a continuous and scalable iron-assistedticles with electrostatic interaction by adsorbing positively flame approach was developed(liu, Gu, Hu, Li, 2010), andharged poly diallyldimethylammonium chloride) via layer-by- well-crystalline SnO2 nanorods were primarily synthesized withlayer assembly processes. The films of TiO2 SiOz assembleda production rate up to 50 g/h in the laboratory-scale(Fig. 2). The10 cycles provide effective plas-prepaimum transmittance of 99.3%, and can shorten the water droplet diameter around 20 nm are of single crystal rutile structures, growspreading time down to 0. 29 s The multilayer films assembled for ing along the [00 1]direction. The morphology and structure can beten cycles are four times more active than films assembled with easily controlled by introducing Fe dopant and adjusting the flamefive cycles, indicating that flame-synthesized Tioz with good crys- residence time. Meanwhile, the photoluminescence( Pl)spectrumtallinity can be used to fabricate high transparent self-cleaning of SnO2 nanorods exhibits a broad, stronger orange-emission peakfilms under proper assembly conditions(Wang, Hu, Zhang, Li, around 620 nm, suggesting their potential applications in optoele2010tronICsA large-scale composite method, premixed atmospheric flatflame deposition, combining advantages of both flame synthe3. Nanorods and nanowiressis and thermal evaporation, has been successfully developed toconstruct Sno2 nanowires with novel arrow-like tips by controlFlame-made metal oxides are always of spherical particles and ling the reaction conditions. SnO2 nanowires with special tips arechainlike agglomerates instead of the nanostructures with different different from the previously mentioned nanowires. They are struc-lapes, particularly one-dimensional semiconductor nanomaterturally uniform single crystals, growing along the [001] direction,als with unique electronic and optical properties for nanodevice lengthing up to 4 um(Fig 3). The arrow-like nanowire of this kindapplications. Zno nanorods were prepared via a flame spray pyrol- exhibits a much stronger emission peak at 620 nm, allowing forsis by introducing indium and tin dopants which selectively potential applications in optoelectronics. The synthesis of complex,crystal plane(Height, Madler, Pratarrow-like nanowires will provide building blocks for the future2006). SnO2 nanorods were prepared via the flame at atmo- architecture of functional nanodevicesheric pressure using a multi-element diffusion flame burnerwith a gas-phase precursor for SnO2 and solid-phase precursoror metal additives(Bakrania, Perez, Wooldridge, 2007). Cobalt 4. Nanotubes and nanocoilsnanowires were prepared with magnetic fields as an effectivestructure directing agents from a metal ferromagnetic nanopartiSince the dicle gas stream(Athanassiou, Grossmann, Grass, Stark, 2007). This ture and physicV中国煤化工atHmtemplate free, continuous and rapid production method afforded synthesis is corCNMHGhcient process:ananowires with an aspect ratio of over 1000 (length/diameter )at portion of theto leave the resta production rate of over 30 g/h. Therefore, it is still a challenge, as the reactant. Therefore, flame synthesis was tentatively usedvia the flame technology to realize morphology-controlled syn- for synthesizing carbon nanotubes. However, due to the complexC Li et al. Particuology 8(2010) 556-562559100nm100nmb0-33nm 0.3Inms nmFig 3. Low magnification TEM image of the middle part of a randomly selectede and its HRTEM image and the corresponding SAEd pattern(b) taken from Box 1in(a), showing the preferential growth direction is [001: TEM image of nanowires tips(c)and their HRTEM images(d)and (e)taken, respectively, from Boxes 2 and 3(c), displaying the structure characteristic of the wire tipschemical and temperature environment, flame synthesis of carbon Carbon nanotubes were also formed in the methane diffusion flamenanotubes has been regarded as unsuccessful. Yuan et al. (Yuan, by using of flower-shaped nio architectures as catalysts(zhou, gu,Saito, Hu, Chen, 2001; Yuan, Saito, Pan, Williams, Gordon, 2001) Li, 2009; Zhou, Li, Gu, Wang, 2008). The NiO 3D architectured carbon nanotubes (MWCNTs)by immers- exhibited good catalytic characteristics for carbon nanotube for-ng metallic substrates in a co-flow diffusion flame composed of mation; nanotubes grew along the surface of nanosheets, resultingmethane and ethylene Single-walled carbon nanotubes (SWNTs) in patterning growth Most CNTs are estimated to be of 15 nm outerwere prepared in a hydrocarbon (acetylene or ethylene ) air diffu- diameter and 7 nm inner diameter. The graphite crystal structuresion flame(Vander Wal, Ticich, Curtis, 2000). Merchan-Merchan, has a characteristic peak at 1580 cm-l while multicrystal or nonSaveliev, and Nguyen(2009)prepared carbon and metal-oxide crystal carbon materials have a peak at 1345 cm-1. The low valuenanostructures on molybdenum probes inserted in a counterflow of 11345/1580 indicates a good graphitization degree of multiwalloxy-fuel flame. Flame position and probe diameter were varied carbon nanotubes. Carbon nanotubes(CNTs)with ultrafine innerto achieve a controlled growth of carbon and metal-oxide nanos diameter have also been synthesized successfully through a sim-tructures in the fuel and oxygen-rich flame zones,, Molybdenum ple ethanol flame method (Wang, Li, Gu, Zhou, 2008; Wang, Li,probes of 1 mm diameter were introduced in the flame at vari- Zhou, Gu, 2007). The inner diameter as well as the crystallinityus heights, starting from the upper hydrocarbon-rich zone on the of the CNts can be altered greatly by controlling the experimentalfuel side of the flame to the oxygen-rich zone on the oxidizer side. conditions. The whole process has been divided into three stepsHigh density layers of carbon nanocoils(CNCs)and filamentous (1) pyrolysis of ethanol and formation of catalyst particles; (2)sur-structures containing ribbon shape and straight nanofibers were face adsorption of pyrolyzed products on the catalyst particles; (3)formed in the upper hydrocarbon-rich flame zone. MoO2 microchannel structures were formed on the oxidizer side in the viciniof the flame front. The micro-channels appeared as rectangulaand square-framed shapes; they were completely hollow, closed,and semi-open with a small circular cavity at their tips. However,these approaches are very complicated and the gaseous fuels cannot be safely and carefully controlled. therefore, it still remains achallenge to develop simpler and safer approaches to synthesizeCNTSFor preparing carbon nanotubes, a methane diffusion flame wasestablished in a tube-like burner with a fuel tube in the centre(Zhou, Li, Gu, Du, 2008 ) With a high yield of carbon nanotubes.less carbon impurities were formed. Yields and purities of car-bon nanotubes could be enhanced obviously in a suitable flameironment and over a proper catalyst Multi-walled carbon nan-中国煤化工bes grew directly on the stainless steel mesh in a controllablemethane diffusion flame On a HCl pre-etched mesh, high densityCNMHG分arbon nanotubes were synthesized with uniform outer diameteFig 4. Typical SEM image of the carbon nanotubes on HCl-etched mesh Inset showsof about 60 nm and a large inner diameter of about 50 nm(Fig 4). a high-magnification SEM image of the carbon nanotubeC Li et al/Paigy8(2010)556-562人Fig. 5. SEM image(a)and XRD pattern(b) of the CNCs synthesized by using ethanol flame over Sno2Molecules or clustersPrimary particlesaggregationAi4ir·To2oSio2SiCl(AirGas-phaseNuclearation Coagulationchemical reaction GrowthSurface ractionSintering meltingFig. 6. Illustration for the formation mechanism of the dispersing TiO2/SiO2 nanostgrowth of CNTs on the catalyst particle surface(including diffusion and Stark(2007) prepared carbon-coated copper nanoparticlesand precipitation of carbon through catalyst particles and forma- with different carbon layers; the core/shell geometry of thesetion of CNTs at the other side of the particles ) during the formation bon/ metal composites afforded two distinctly different electricalof MWCNTS, it should be noted that the morphology, especially the behaviors depending on the carbon layer properties. Grapheneinner diameter of the tubes, is strongly dependent on the burning layers with a predominant sp2 character showed an ill-definedratebandgap structure as evidenced by uv-vis diffuse reflectance specCarbonils(cncs)were first synthesized by using tioxide nanoparticles as catalyst formed in situ from stannic chlo-Based on controlling the mixing state of reactants of TiCl4 andride precursor in an ethanol flame(Wang, Li, Gu, Zhang, 2009). SiCl4, TiO2/SiO2 nanocomposite particles were prepared in a pre-The obtained CNCs of a mesoporous character have tight coil mixed flame in form of dispersing structure of Tio2 nanoparticlesches, and the average fiber and coil diameters are approximately depositing in amorphous Sio2 matrix and core/shell structure of0-80 nm and 80-100 nm, respectively(fig. 5. The anisotropic TiO2 nanoparticles encapsulated by amorphous Sio2(Hu, Li, Cong,deposition rates of carbon among tin dioxide crystal planes give rise Jiang, Zhao, 2006; Hu, Li, Cong, Jiang, Zhao, 2007; Hu, Li, Gu,to the driving force for the coiling of carbon fibers. The crystal plane Zhao, 2007). In the dispersing structure the size of the crystalline(101)of SnO2, which is the most favorable face for carbon precip- TiO2 particles was about 1-2nm, and would be changed along withitation, is situated on the outer side of the CNCs, while the crystal the ti/Si ratio in the composites. Formation of such a dispersingplane(1 10)with the lowest carbon extrusion speed is situated on structure is attributed to the heterogeneous nucleation of Sioz onthe inner side. The CNCs present excellent specific capacitance of the tiO2 surface and the short residence time in the high temperca 40 F/g while used as polarized electrodes, considerably higher ature region( Fig. 6)(Hu et al., 2006). Thickness of the Sio2 layerthan that of the micro-coiled carbon fibers or carbon nanofibers, in the core/shell structure was about 3-5 nm and can be tuned byand possibly a candidate for super capacitorschanging the processing parameters. The formation of core/shellTiO2/SiO2 nanostructures is explained by heterogeneous nucle5. Novel nanostructuresation and growth of Sio2 on the surface of Tio2 particles(Fig. 7)(Hu, Li, Gu, Zhao, 2007). With addition of SiOz into the matrix, thepare many novel nanostructures, such as nanoflakes, nanoneedles, surface modification by Sioz. The luminescence has been enhancedore/shell, hollow and ball-in-shell structures. Single and bicrys-emarkably by adding Sio2, mostly due to surface modification oftal a-Fe2O3 nanoflakes and Cuo nanoneedles were grown in the Sio2 around TiO2 nanoparticlespostflame region by a solid diffusion mechanism. The a-Fe2OWhen the Joule-Thomson throttle cooling and high speednanoflakes reached lengths exceeding 20 um after only 20 min of jet entrainment phenomena in the multi-jet reactor were takengrowth( Rao& Zheng, 2009). This rapid growth rate is attributed advantage of, a novel y-Al2O3 hollow nanospheres as well as theto a large initial heating rate of the metal substrate in the flame A203/TiO2 and Alz siOz hollow nanocomposites were successand to the presencefully prepared(hr中国煤化工 ticle size of these00 nm and a shellwhich greatly enhance the diffusion of the deficient metal to thickness of 10-30CNM Ted by small pathe nanostructure growth site and enable growth at higher tem- ticles of 5-10 nm. It is desirable to state that, even for the veryperatures than previously demonstrated. Athanassiou, Mensing, small spheres existing in the samples, hollow interior still existsC Li et al. Particuology 8(2010) 556-562H2- AirMolecules or clusters●To2Sio2Sintering&MeltingGas-phase reactionCoagulation SurfaceAggregationSiCL AirFig. 7. Illustration for the formation mechanism of the core/shell TiOz/SiOz nanostructures200nmFig 8. TEM and SEM images, SAED pattern, and EDS analysis of ball-in-shell TiO2 spheres.with the shell thickness of 5 nm, which is rare when prepared with 6. Summaryconventional methods the formation mechanism of such hollenanostructures conforms to the One-Droplet-to-One-Particle theThe flame technology has been employed widely for large-scaleory(oDOP), where hydrolysis and nucleation occur on the surface manufacture of powdery products, such as fumed silica, pigmenof droplet when the surface concentration of droplet is greater tary titania, and also ceramic commodities lick Sio2, TiO2, andthan critical degree of super-saturation and its inner concentration Al2O3. An in-depth understanding of the process makes it possibleis less than the equilibrium concentration. Introducing TiO2 can to play its role in synthesis of simple oxides nanoparticles as wellimprove Al2O3 crystallization to form Al2TiO5, whose better crys- as more complex, functional novel nanomaterials. Recently, manyllinity can reduce formation of defects such as oxygen vacancies, kinds of novel nanomaterials such as nanorods, nanowires, nanmaking the luminescent intensity weakerotubes, nanocoils, and nanocomposites with core/shell structures,Ball-in-shell structured TiO2 nanospheres with good crys- hollow structure and ball-in-shell structures, were successfullytalline nature and thermal stability were formed by feeding a synthesized by gas combustion flames, and the formation mechdiffusion flame(Liu, Hu, Gu, L, 2009). The resultant ball-in- chemical engineering principle on the nucleation-growth andmixture of titanium tetrachloride and alcohol vapor to a facile anism were investigated basedshell spheres were composed of nanocrystallites, with their shellontrolling gas concentration, as well as temperature distrithickness and void space width of 30-50 nm and 10-30 nm, respec- bution, is very important for synthesis of nanomaterials becausetively( Fig 8). The formation mechanism of ball-in-shell spheres their characteristics are often determined bv temperature and condepended on the relative rate between chemical reaction and difcentration in中国煤化工 erature distributionfusion. This structure results in the increased light absorbency of and concentratin the residence timehe spheres, and thus offers increased potential for designing and distribution inCNMHGtives to manippreparing novel materials with enhanced photocatalytic activi- late complex and functional nanomaterials Micromixing of thetiesreactants is a key to control flame temperature, reaction andC Li et al/Particuology 8(2010) 556-562nucleation-growth rates, and subsequently the morphology, com- Liu, J, Hu, Y Gu, F& Li, C (2009). Flame synthesis of ball-in-gpictured ticposition and structure of the products. Fluid flow and mixingnanospheres Industrial S Engineering Chemistry Research, 48, 735-735studies based on principles of chemical reaction engineering pro-Merchan-Merchan, W, Saveliev, A. V,& nguyen, v(2009).flame synthesis of carbon and oxide nanostructures on molybvide the tools for deeper understanding and better controlling ofproduct characteristics. Also helpful are the chemical engineering Pratsinis, S E(1998). Flame aerosol synthesis of ceramic powders. Progress in Energyprinciples and methods that give us more ideas in the preparation Rao, p.M., &Zheng. X L(2009). Rapid catalyst-ame synthesis of dense, alignedRin,a-Fe2O3 nanoflake and Cuo nanoneedle arrays. Nano Letter, 9(8),3001-3000Acknowledgments(1996). Formation of nanotubes in low pressure hydrocarbon flames, carbeStark, W.J.,& Pratsinis, S E(2002). Aerosol flame reactors for manufacture ofThis paper is written under the support of the National Naturaparticles. Powder Tech26,103-10Science Foundation of China(20925621, 20906027, 20706015), theProgram of Shanghai Subject Chief Scientist(O8XD1401500), the Tian, B, Li, c, Gu, F, Jiang, H, Hu, Y, Zhang. J(2009).FlameShanghai Shuguang Scholars Tracking Program(08GG09), the Spe-s with enhanced photocatalytic activity uncial Projects for Key Laboratories in Shanghai(09DZ2202000), the Ulrich, G. 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