Growth of coatings on nanoparticles by photoinduced chemical vapor ...
J Nanopart Res (2008) 10:173–178 DOI 10.1007/s11051-007-9238-2
RESEARCH PAPER
Growth of coatings on nanoparticles by photoinduced chemical vapor deposition Bin Zhang Æ Ying-Chih Liao Æ Steven L. Girshick Æ Jeffrey T. Roberts
Received: 6 September 2006 / Accepted: 2 April 2007 / Published online: 28 April 2007 Ó Springer Science+Business Media B.V. 2007
Abstract Photoinduced chemical vapor deposition was used to grow organic coatings on NaCl nanoparticles. Aerosolized nanoparticles were mixed with a vapor-phase coating reactant and introduced into a room-temperature, atmospheric-pressure cell, where the mixture was exposed to 172-nm radiation from a Xe2* excimer lamp. Several coating reactants were investigated; the most successful was methyl methacrylate (MMA). Tandem differential mobility analysis (TDMA) was used to determine coating thicknesses as a function of initial particle size. For NaCl particles ranging from 20 to 60 nm in mobility diameter, the thicknesses ranged from sub-nm to 20 nm depending on MMA flow rate and initial particle size.
B. Zhang S. L. Girshick Department of Chemical Engineering, University of Minnesota, 111 Church St. SE, Minneapolis, MN 55455, USA Y.-C. Liao Hewlett Packard Corporation, 1000 NE Circle Blvd., Corvallis, OR 97330, USA J. T. Roberts (&) Department of Chemistry, University of Minnesota, 207 Pleasant St. SE, Minneapolis, MN 55455, USA e-mail: [email protected]
Keywords Chemical vapor deposition Aerosol Tandem differential mobility analysis Nanoparticle Photochemistry Coatings Nanocomposites
Introduction For nanoparticles to be useful in a wide variety of applications, methods must be developed to control their surface properties. In some cases the goal is to stabilize or passivate the nanoparticle surfaces, in other cases to impart some desired functionality. The former can be accomplished by coating the nanoparticle with a thin film, producing a ‘‘core-shell’’ structure, and the latter can be achieved by attaching chemical functional groups to the nanoparticle surface. A variety of methods have been developed for coating or modifying nanoparticle surfaces. While most work has involved liquid-based chemistry, gasphase (i.e., aerosol) methods are also being explored. Gas-phase methods allow greater purity, as solutionbased methods usually require a pre-activation step, typically with a solvent or catalyst, leaving unwanted trace compounds or elements on the nanoparticle surfaces. Furthermore, gas-phase methods can be run as continuous rather than batch processes, do not require management and disposal of environmentally hazardous solvents, and are obviously more compatible with systems in which the core nanoparticles are themselves synthesized in gas phase. Gas-phase
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methods for coating nanoparticles that have been reported include heated flow tubes (Jain et al. 1997; Powell et al. 1997; Fotou et al. 2000; Lee et al. 2003; Zhang et al. 2004; Bai and Wang 2005; Enz et al. 2006; Nienow and Roberts), flames (McMillin et al. 1996; Biswas et al. 1997; Yu et al. 2005), spray pyrolysis (Yu et al. 2005), microwave plasma (Vollath and Szabo´ 1999), and RF plasma (Schallehn et al. 2003; Kim et al. 2006). We here report the growth of thin films on nanoparticles by means of photoinduced chemical vapor deposition (photo-CVD), driven by vacuum ultraviolet (VUV) radiation from an excimer lamp. Photo-CVD has been used extensively to deposit thin films and coatings on macroscopic substrates (Kogelschatz et al. 2000), but has not, to our knowledge, been previously applied to coat nanoparticles. In addition to the advantages noted above for gas-phase coating methods, photo-CVD has other potential advantages, including the fact that it can be achieved in a room-temperature gas under atmospheric pressure, and that the addition of excimer lamps requires only minor modifications of gas-phase nanoparticle synthesis systems. In this contribution, we show that amorphous organic coatings can be controllably deposited on NaCl nanoparticles by photo-CVD. While this system is not itself of practical interest, NaCl particles provide a convenient test system for performing initial experiments to study photo-CVD. Hydrogenated organic carbon films on nanoparticles of many types may potentially serve as oxygen diffusion barriers, as is done for plastic beverage bottles (Boutroy et al. 2006).
Experimental A schematic of the atmospheric pressure coating cell used to conduct these experiments is shown in Fig. 1. NaCl nanoparticles were aerosolized in N2 using a nebulizer followed by a dryer. Tandem differential mobility analysis (TDMA) (Liu et al. 1978) was used to determine coating thickness for nanoparticles of well-defined sizes. The aerosolized particles were first charged by a bipolar diffusion charger, and then entered a differential mobility analyzer (TSI model 3085), whose operating voltage was set so as to pass only particles of a specified mobility diameter dp. The
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Fig. 1 Schematic of nanoparticle coating reactor and TDMA experiment
size-selected particle streams then entered the coating cell at a total flow rate of 1 L min 1, where they were mixed with a flow of gas-phase coating reactant. Several coating reactants were investigated, including CH4, C2H2, C2H4, C2H6, styrene, and methyl methacrylate (MMA) [CH2 = C(CH3)CO2CH3, MMA]. Gaeous reactants (CH4, CH2H2, C2H4, and C2H6) were introduced into the coating cell directly through a calibrated flowmeter. Liquid reactants (styrene and MMA) were introduced as N2 + reactant mixtures through a calibrated flowmeter. The styrene and MMA flow rates cited in this work were calculated assuming that N2 became saturated in reactant as it passed through the bubbler. The particle + N2 + reactant mixtures were exposed to 172-nm radiation from a Xe2* excimer lamp (USHIO model UER20H-172). The lamp was mounted end-on to the flow tube. The lamp output was collimated by a CaF2 lens, forming a beam that filled the flow tube. The lens was kept clean of particles by a purge flow of Ar. Aerosol exiting the coating reactor was delivered to a second DMA. This, in series with an ultrafine condensation particle counter (CPC, TSI model 3025A), provided on-line measurements of particle size distributions. By comparing measurements of the size distributions with and without the addition of coating reactants, the extent of nanoparticle growth during coating could be determined. Aerosol samples were collected for Fourier transform infrared spectroscopy (FTIR) and transmission
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electron microscopy (TEM). The FTIR samples were collected on a stainless steel grid in a hybrid gas cell. Spectra were recorded in transmission mode using a Magna–IR spectrometer (Nicolet, Model No. 550). Polydisperse samples were collected on carbon grids for TEM analysis by impaction and electrostatic sampling. TEM images were acquired using a Tecnai T12 transmission electron microscope.
that either the growth of a carbon coating on the NaCl particles increases their quantum efficiency with respect to photoemission, or that positive ions from photoionization or photodissociation of C2H2 have attached to some of the negatively charged particles. When the size distributions in Fig. 2 are normalized to their peak values, as in Fig. 3, it is evident that the distribution has shifted to larger sizes by *2 nm compared to that for the bare particles with nearly the same the full width at half-maximum of 5 nm. An ‘‘effective coating thickness’’ is defined as one-half the peak shift, where each of the size distributions is fit with a Gaussian. One effect that can potentially compete with photo-CVD is photoinduced nucleation of particles from the coating reactant or residual gases in the reactor. The possible occurrence of photoinduced nucleation was tested for each coating reactant by running experiments where only the coating reactant, without NaCl nanoparticles, was introduced into the coating reactor with the excimer lamp turned on. Figure 4 shows results for CH4 and C2H2, each at two different flow rates. Figure 4 also shows a background scan, which was recorded with the excimer lamp on but no hydrocarbon flow into the photlysis cell. The background scan shows that some particles are produced even without a coating reactant, either from gas-to-particle conversion or photon-initiated particle release from the reactor walls. However, the peak particle concentration (ca. 400 cm 3) was over two orders of magnitude below the measured peak concentrations of coated NaCl particles. Background
Results and discussion Figure 2 shows particle size distributions when NaCl particles with selected mobility diameters of *40 nm were introduced into the coating reactor. The polarities of both DMAs were set to pass only negatively charged particles. When the UV lamp was on but no coating reactant was introduced, the apparent particle concentration dropped to 31% of its value without UV radiation, presumably a result of photoemission removing electrons from and neutralizing the NaCl particles. In addition, two small satellite peaks appeared at *29 and 59 nm, which are ascribable to doubly charged particles. Under the conditions of the DMA, a doubly charged 41-nm (assumed spherical) particle has the same mobility diameter as a singly charged 29-nm particle, while a doubly charged 59-nm particle has the same mobility diameter as a 41-nm particle. When 0.5 sccm C2H2 was introduced, the peak diameter increased to *43 nm, and the particle concentration further decreased, to 18% of its value without UV radiation. This suggests
Fig. 2 TDMA measurements of 40-nm NaCl particles with C2H2 as coating reactant
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Fig. 3 Same measurements as in Fig. 2, normalized to peak values
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Fig. 4 Measured size distributions of particles produced by photoinduced nucleation from either CH4 or C2H2
contrast, C2H4, C2H6, and MMA showed no tendency to undergo photoinduced nucleation, except for MMA at high flow rates (>12.8 sccm). All of the coating reactants, with the exception of C2H4, produced measurable particle growth due to photo-CVD. However, for CH4, C2H6, and styrene these increases were small, corresponding to effective coating thicknesses of *1 nm, and were unaffected by the reactant flow rate. For C2H2 the effective coating thickness increased to *3 nm as the C2H2 flow rate increased, until declining as the flow rate increased above *1 sccm. This decline is likely due to the competition with gas-phase nucleation. Only MMA, among the gases tested, produced a monotonic trend for coating thickness versus reactant flow rate. Figure 5 shows the measured effective coating thickness for various MMA flow rates and for core particles of different initial mobility diameters, ranging from 20 to 60 nm. No gas-phase nucleation was detected for MMA in the range of flow rates shown here. For any given initial particle size, the coating thickness increased monatonically, from subnm to 20 nm, with increasing MMA flow rates. Each of the data points in the figure represents the average of several TDMA size distribution scans. The results were reproducible, with effective coating thicknesses varying from run to run by 105 cm 3) at flow rates as low as 0.004 sccm. In
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Fig. 5 Coating thickness versus MMA flow rate for NaCl particles of various initial mobility diameters
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O-H Absorbance (a.u.)
supersaturated vapor, where the form of the ‘‘growth law’’ is determined by the particle Knudsen number, Kn : k/rp, where k is the mean free path for collisions between molecules in the gas and rp is particle radius (Friedlander 2000). For Knudsen numbers greater than about 10 (free molecule regime) the particle growth rate d(dp)/dt is independent of particle size. For values of Kn less than about 0.1 (continuum regime), the particle develops a diffusive boundary layer, and d(dp)/dt is proportional to 1/dp. For the transition regime that lies in between, various heuristic interpolation formulas have been proposed. While particle growth by CVD is more complicated than simple vapor condensation, a reasonable hypothesis is that the dependence of coating growth rate on Knudsen number is qualitatively similar. The data in Fig. 5 support this hypothesis. As the Ar–N2 flow rate in the coating cell was *103 times higher than the MMA flow rate, the residence times in the coating cell were essentially the same for all the experiments represented in Fig. 4. Thus ‘‘coating thickness’’ in the figure is a surrogate for 1/2 d(rp)/dt. For the Ar–N2 mixture in the coating cell at standard temperature and pressure, the mean free path equals *62 nm. The particle sizes all therefore lie in the transition regime, with values of Kn varying from about 1.75 to 6.2. By inspection of Fig. 5, the coating growth rate declines as particle size increases, but more slowly than 1/dp, consistent with theory for particle growth by condensation in the transition regime. Figure 6 shows two representative TEM images of particles that were coated by photo-CVD of MMA. The images establish that the particles are roughly cubic in shape. The relationship between the mobility diameter and the physical size of a particle depends on many variables, including the dynamic shape
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Fig. 7 Diffuse reflectance FTIR spectra of NaCl nanoparticles, with and without photo-CVD coating using MMA as coating reactant. Also shown for reference are spectra of MMA and PMMA
factor (DeCarlo et al. 2004). Although the dynamic shape factors of a sphere and a cube are not equal, they are very close: one for a sphere (by definition) and *1.08 for a cube in the free-molecular regime (Zelenyuk et al. 2006). What this means is that the effective coating thickness defined above probably overestimates the physical coating thickness, but not by more than 10%, provided that the particle coatings are uniform. The TEM images establish that MMAderived coatings are indeed quite uniform, from which we conclude that the effective coating thickness is a reasonable approximation to the physical coating thickness. Analogous images were not obtained of particles coated by the other reactants because NaCl particles have limited stability under the beam currents necessary to image them. Infrared absorption spectra were obtained of coated and uncoated particles (Fig. 7) in order to investigate the structure and composition of deposited films. By analogy to other studies in which PMA was used as a photo-CVD precursor, we anticipated that
Fig. 6 TEM images of two NaCl particles coated by photo-CVD using MMA as the coating reactant. The coatings are apparent in the images as low contrast regions near the particle edges. The coatings are ca. 3 nm thick and quite uniform
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MMA-derived films are composed mostly of polymethyl methacrylate (PMMA), which is the polymeric form of MMA. Figure 7 also shows FTIR spectra of authentic sample of MMA and PMMA. Both the coated and uncoated particle spectra show peaks that may be attributed to water or adsorbed OH. (NaCl readily absorbs water during sample preparation.) The spectrum of coated particles also shows a broad C–H stretching band in the 2,900–3,000 cm 1 region, and a C = O band at 1,740 cm 1. The conditions under which particles were collected for these experiments were not sufficiently well controlled to make it meaningful to compare relative intensities of the OH stretching regions of the different particle spectra. What is clear, however, is that coatings grow on NaCl particles, and that the C– H and C = O stretching regions of the FTIR spectra of these films are very similar to those of authentic MMA and PMMA samples. Acknowledgment This work was supported in part by the Defense-University Research Initiative in NanoTechnology (DURINT) of the US Army Research Laboratory and the US Army Research Office under agreement number DAAD190110503, in part by the Minnesota Supercomputing Institute, and in part by the National Science Foundation under Grant No. CHE–0094911. The authors thank J. Holm and H. Ajo for their assistance in FTIR measurements, and Y.C. He for her assistance in TEM measurements.
References Bai J, Wang J-P (2005) High-magnetic-moment core-shell-type FeCo–Au/Ag nanoparticles. Appl Phys Lett 87:152502– 152504 Biswas P, Wu CY, Zachariah MR, McMillin B (1997) Characterization of iron oxide-silica nanocomposites in flames: Part II. Comparison of discrete-sectional model predictions to experimental data. J Mater Res 12:714–723 Boutroy N, Pernel Y, Rius JM, Auger F, von Bardeleben HJ, Cantin JL, Abel F, Zeinert A, Casiraghi C, Ferrari AC, Robertson J (2006) Hydrogenated amorphous carbon film coating of PET bottles for gas diffusion barriers. Diamond Relat Mater 15:921–927 DeCarlo P, Slowik J, Worsnop D, Davidovits P, Jimenez J (2004) Particle morphology and density characterization by combined mobility and aerodynamic diameter measurements. Part 1: theory. Aerosol Sci Technol 38:1185– 1205
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J Nanopart Res (2008) 10:173–178 Enz T, Winterer M, Stahl B, Bhattacharya S, Miehe G, Fasel C, Hahn H (2006) Structure and magnetic properties of iron nanoparticles stabilized in carbon. J Appl Phys 99:044306–0443013 Fotou GP, Kodas TT, Anderson BM (2000) Coating titania aerosol particles with ZrO2, Al2O3/ZrO2, and SiO2/ZrO2 in a gas-phase process. Aerosol Sci Technol 33:557–571 Friedlander SK (2000) Smoke, dust and haze: fundamentals of aerosol dynamics. Oxford University Press, New York Jain S, Fotou GP, Kodas TT (1997) A theoretical study on gasphase coating of aerosol particles. J Coll Interface Sci 185:26–38 Kim K-S, Kim D-J, Zhao QQ (2006) Numerical analysis on particle coating by the pulsed plasma process. Chem Eng Sci 61:3278–3289 Kogelschatz U, Esrom H, Zhang J-Y, Boyd IW (2000) Highintensity sources of incoherent UV and VUV excimer radiation for low-temperature materials processing. Appl Surf Sci 6419:1–8 Lee B-S, Kang D-J, Kim S-G (2003) Properties of binary TiO2SiO2 composite particles with various structures prepared by vapor phase hydrolysis. J Mater Sci 38:3545–3552 Liu BYH, Pui DYH, Whitby KT, Kittelson DB, Kousaka Y, McKenzie RL (1978) The aerosol mobility chromatograph: a new detector for sulfuric acid aerosols. Atmos Environ 12:99–104 McMillin BK, Biswas P, Zachariah MR (1996) In situ characterization of vapor phase growth of iron oxide-silica nanocomposites: Part I. 2-D planar laser-induced fluorescence and Mie imaging. J Mater Res 11:1552–1561 Nienow AM, Roberts JT (2006) Chemical vapor deposition of zirconium oxide on aerosolized silicon nanoparticles. Chem Mater 18:5571–5577 Powell QH, Fotou GP, Kodas TT, Anderson BM (1997) Synthesis of alumina- and alumina/silica-coated titania particles in an aerosol flow reactor. Chem Mater 9:685–693 Schallehn M, Winterer M, Weirich TE, Keiderling U, Hahn H (2003) In-situ preparation of polymer-coated alumina nanopowders by chemical vapor synthesis. Chem Vap Deposition 9:40–44 Vollath D, Szabo´ DV (1999) Coated nanoparticles: a new way to improved nanocomposites. J Nanoparticle Res 1:235– 242 Yu F, Wang JN, Sheng ZM, Su LF (2005) Synthesis of carbonencapsulated magnetic nanoparticles by spray pyrolysis of iron carbonyl and ethanol. Carbon 43:3018–3021 Zelenyuk A, Cai Y, Imre D (2006) From agglomerates of spheres to irregularly shaped particles: determination of dynamic shape factors from measurements of mobility and vacuum aerodynamic diameters. Aerosol Sci Technol 40:197–217 Zhang L, Ranade MB, Gentry JW (2004) Formation of organic coating on ultrafine silver particles using a gas-phase process. J Aerosol Sci 35:457–471
RESEARCH PAPER
Growth of coatings on nanoparticles by photoinduced chemical vapor deposition Bin Zhang Æ Ying-Chih Liao Æ Steven L. Girshick Æ Jeffrey T. Roberts
Received: 6 September 2006 / Accepted: 2 April 2007 / Published online: 28 April 2007 Ó Springer Science+Business Media B.V. 2007
Abstract Photoinduced chemical vapor deposition was used to grow organic coatings on NaCl nanoparticles. Aerosolized nanoparticles were mixed with a vapor-phase coating reactant and introduced into a room-temperature, atmospheric-pressure cell, where the mixture was exposed to 172-nm radiation from a Xe2* excimer lamp. Several coating reactants were investigated; the most successful was methyl methacrylate (MMA). Tandem differential mobility analysis (TDMA) was used to determine coating thicknesses as a function of initial particle size. For NaCl particles ranging from 20 to 60 nm in mobility diameter, the thicknesses ranged from sub-nm to 20 nm depending on MMA flow rate and initial particle size.
B. Zhang S. L. Girshick Department of Chemical Engineering, University of Minnesota, 111 Church St. SE, Minneapolis, MN 55455, USA Y.-C. Liao Hewlett Packard Corporation, 1000 NE Circle Blvd., Corvallis, OR 97330, USA J. T. Roberts (&) Department of Chemistry, University of Minnesota, 207 Pleasant St. SE, Minneapolis, MN 55455, USA e-mail: [email protected]
Keywords Chemical vapor deposition Aerosol Tandem differential mobility analysis Nanoparticle Photochemistry Coatings Nanocomposites
Introduction For nanoparticles to be useful in a wide variety of applications, methods must be developed to control their surface properties. In some cases the goal is to stabilize or passivate the nanoparticle surfaces, in other cases to impart some desired functionality. The former can be accomplished by coating the nanoparticle with a thin film, producing a ‘‘core-shell’’ structure, and the latter can be achieved by attaching chemical functional groups to the nanoparticle surface. A variety of methods have been developed for coating or modifying nanoparticle surfaces. While most work has involved liquid-based chemistry, gasphase (i.e., aerosol) methods are also being explored. Gas-phase methods allow greater purity, as solutionbased methods usually require a pre-activation step, typically with a solvent or catalyst, leaving unwanted trace compounds or elements on the nanoparticle surfaces. Furthermore, gas-phase methods can be run as continuous rather than batch processes, do not require management and disposal of environmentally hazardous solvents, and are obviously more compatible with systems in which the core nanoparticles are themselves synthesized in gas phase. Gas-phase
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methods for coating nanoparticles that have been reported include heated flow tubes (Jain et al. 1997; Powell et al. 1997; Fotou et al. 2000; Lee et al. 2003; Zhang et al. 2004; Bai and Wang 2005; Enz et al. 2006; Nienow and Roberts), flames (McMillin et al. 1996; Biswas et al. 1997; Yu et al. 2005), spray pyrolysis (Yu et al. 2005), microwave plasma (Vollath and Szabo´ 1999), and RF plasma (Schallehn et al. 2003; Kim et al. 2006). We here report the growth of thin films on nanoparticles by means of photoinduced chemical vapor deposition (photo-CVD), driven by vacuum ultraviolet (VUV) radiation from an excimer lamp. Photo-CVD has been used extensively to deposit thin films and coatings on macroscopic substrates (Kogelschatz et al. 2000), but has not, to our knowledge, been previously applied to coat nanoparticles. In addition to the advantages noted above for gas-phase coating methods, photo-CVD has other potential advantages, including the fact that it can be achieved in a room-temperature gas under atmospheric pressure, and that the addition of excimer lamps requires only minor modifications of gas-phase nanoparticle synthesis systems. In this contribution, we show that amorphous organic coatings can be controllably deposited on NaCl nanoparticles by photo-CVD. While this system is not itself of practical interest, NaCl particles provide a convenient test system for performing initial experiments to study photo-CVD. Hydrogenated organic carbon films on nanoparticles of many types may potentially serve as oxygen diffusion barriers, as is done for plastic beverage bottles (Boutroy et al. 2006).
Experimental A schematic of the atmospheric pressure coating cell used to conduct these experiments is shown in Fig. 1. NaCl nanoparticles were aerosolized in N2 using a nebulizer followed by a dryer. Tandem differential mobility analysis (TDMA) (Liu et al. 1978) was used to determine coating thickness for nanoparticles of well-defined sizes. The aerosolized particles were first charged by a bipolar diffusion charger, and then entered a differential mobility analyzer (TSI model 3085), whose operating voltage was set so as to pass only particles of a specified mobility diameter dp. The
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Fig. 1 Schematic of nanoparticle coating reactor and TDMA experiment
size-selected particle streams then entered the coating cell at a total flow rate of 1 L min 1, where they were mixed with a flow of gas-phase coating reactant. Several coating reactants were investigated, including CH4, C2H2, C2H4, C2H6, styrene, and methyl methacrylate (MMA) [CH2 = C(CH3)CO2CH3, MMA]. Gaeous reactants (CH4, CH2H2, C2H4, and C2H6) were introduced into the coating cell directly through a calibrated flowmeter. Liquid reactants (styrene and MMA) were introduced as N2 + reactant mixtures through a calibrated flowmeter. The styrene and MMA flow rates cited in this work were calculated assuming that N2 became saturated in reactant as it passed through the bubbler. The particle + N2 + reactant mixtures were exposed to 172-nm radiation from a Xe2* excimer lamp (USHIO model UER20H-172). The lamp was mounted end-on to the flow tube. The lamp output was collimated by a CaF2 lens, forming a beam that filled the flow tube. The lens was kept clean of particles by a purge flow of Ar. Aerosol exiting the coating reactor was delivered to a second DMA. This, in series with an ultrafine condensation particle counter (CPC, TSI model 3025A), provided on-line measurements of particle size distributions. By comparing measurements of the size distributions with and without the addition of coating reactants, the extent of nanoparticle growth during coating could be determined. Aerosol samples were collected for Fourier transform infrared spectroscopy (FTIR) and transmission
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electron microscopy (TEM). The FTIR samples were collected on a stainless steel grid in a hybrid gas cell. Spectra were recorded in transmission mode using a Magna–IR spectrometer (Nicolet, Model No. 550). Polydisperse samples were collected on carbon grids for TEM analysis by impaction and electrostatic sampling. TEM images were acquired using a Tecnai T12 transmission electron microscope.
that either the growth of a carbon coating on the NaCl particles increases their quantum efficiency with respect to photoemission, or that positive ions from photoionization or photodissociation of C2H2 have attached to some of the negatively charged particles. When the size distributions in Fig. 2 are normalized to their peak values, as in Fig. 3, it is evident that the distribution has shifted to larger sizes by *2 nm compared to that for the bare particles with nearly the same the full width at half-maximum of 5 nm. An ‘‘effective coating thickness’’ is defined as one-half the peak shift, where each of the size distributions is fit with a Gaussian. One effect that can potentially compete with photo-CVD is photoinduced nucleation of particles from the coating reactant or residual gases in the reactor. The possible occurrence of photoinduced nucleation was tested for each coating reactant by running experiments where only the coating reactant, without NaCl nanoparticles, was introduced into the coating reactor with the excimer lamp turned on. Figure 4 shows results for CH4 and C2H2, each at two different flow rates. Figure 4 also shows a background scan, which was recorded with the excimer lamp on but no hydrocarbon flow into the photlysis cell. The background scan shows that some particles are produced even without a coating reactant, either from gas-to-particle conversion or photon-initiated particle release from the reactor walls. However, the peak particle concentration (ca. 400 cm 3) was over two orders of magnitude below the measured peak concentrations of coated NaCl particles. Background
Results and discussion Figure 2 shows particle size distributions when NaCl particles with selected mobility diameters of *40 nm were introduced into the coating reactor. The polarities of both DMAs were set to pass only negatively charged particles. When the UV lamp was on but no coating reactant was introduced, the apparent particle concentration dropped to 31% of its value without UV radiation, presumably a result of photoemission removing electrons from and neutralizing the NaCl particles. In addition, two small satellite peaks appeared at *29 and 59 nm, which are ascribable to doubly charged particles. Under the conditions of the DMA, a doubly charged 41-nm (assumed spherical) particle has the same mobility diameter as a singly charged 29-nm particle, while a doubly charged 59-nm particle has the same mobility diameter as a 41-nm particle. When 0.5 sccm C2H2 was introduced, the peak diameter increased to *43 nm, and the particle concentration further decreased, to 18% of its value without UV radiation. This suggests
Fig. 2 TDMA measurements of 40-nm NaCl particles with C2H2 as coating reactant
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Fig. 4 Measured size distributions of particles produced by photoinduced nucleation from either CH4 or C2H2
contrast, C2H4, C2H6, and MMA showed no tendency to undergo photoinduced nucleation, except for MMA at high flow rates (>12.8 sccm). All of the coating reactants, with the exception of C2H4, produced measurable particle growth due to photo-CVD. However, for CH4, C2H6, and styrene these increases were small, corresponding to effective coating thicknesses of *1 nm, and were unaffected by the reactant flow rate. For C2H2 the effective coating thickness increased to *3 nm as the C2H2 flow rate increased, until declining as the flow rate increased above *1 sccm. This decline is likely due to the competition with gas-phase nucleation. Only MMA, among the gases tested, produced a monotonic trend for coating thickness versus reactant flow rate. Figure 5 shows the measured effective coating thickness for various MMA flow rates and for core particles of different initial mobility diameters, ranging from 20 to 60 nm. No gas-phase nucleation was detected for MMA in the range of flow rates shown here. For any given initial particle size, the coating thickness increased monatonically, from subnm to 20 nm, with increasing MMA flow rates. Each of the data points in the figure represents the average of several TDMA size distribution scans. The results were reproducible, with effective coating thicknesses varying from run to run by 105 cm 3) at flow rates as low as 0.004 sccm. In
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Fig. 5 Coating thickness versus MMA flow rate for NaCl particles of various initial mobility diameters
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O-H Absorbance (a.u.)
supersaturated vapor, where the form of the ‘‘growth law’’ is determined by the particle Knudsen number, Kn : k/rp, where k is the mean free path for collisions between molecules in the gas and rp is particle radius (Friedlander 2000). For Knudsen numbers greater than about 10 (free molecule regime) the particle growth rate d(dp)/dt is independent of particle size. For values of Kn less than about 0.1 (continuum regime), the particle develops a diffusive boundary layer, and d(dp)/dt is proportional to 1/dp. For the transition regime that lies in between, various heuristic interpolation formulas have been proposed. While particle growth by CVD is more complicated than simple vapor condensation, a reasonable hypothesis is that the dependence of coating growth rate on Knudsen number is qualitatively similar. The data in Fig. 5 support this hypothesis. As the Ar–N2 flow rate in the coating cell was *103 times higher than the MMA flow rate, the residence times in the coating cell were essentially the same for all the experiments represented in Fig. 4. Thus ‘‘coating thickness’’ in the figure is a surrogate for 1/2 d(rp)/dt. For the Ar–N2 mixture in the coating cell at standard temperature and pressure, the mean free path equals *62 nm. The particle sizes all therefore lie in the transition regime, with values of Kn varying from about 1.75 to 6.2. By inspection of Fig. 5, the coating growth rate declines as particle size increases, but more slowly than 1/dp, consistent with theory for particle growth by condensation in the transition regime. Figure 6 shows two representative TEM images of particles that were coated by photo-CVD of MMA. The images establish that the particles are roughly cubic in shape. The relationship between the mobility diameter and the physical size of a particle depends on many variables, including the dynamic shape
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C-H
NaCl particles coated w/ MMA
C=O
water
C=O C-H neat MMA
C=O
C-H O-H
neat PMMA
water bare NaCl particles
Wave number (cm-1 )
Fig. 7 Diffuse reflectance FTIR spectra of NaCl nanoparticles, with and without photo-CVD coating using MMA as coating reactant. Also shown for reference are spectra of MMA and PMMA
factor (DeCarlo et al. 2004). Although the dynamic shape factors of a sphere and a cube are not equal, they are very close: one for a sphere (by definition) and *1.08 for a cube in the free-molecular regime (Zelenyuk et al. 2006). What this means is that the effective coating thickness defined above probably overestimates the physical coating thickness, but not by more than 10%, provided that the particle coatings are uniform. The TEM images establish that MMAderived coatings are indeed quite uniform, from which we conclude that the effective coating thickness is a reasonable approximation to the physical coating thickness. Analogous images were not obtained of particles coated by the other reactants because NaCl particles have limited stability under the beam currents necessary to image them. Infrared absorption spectra were obtained of coated and uncoated particles (Fig. 7) in order to investigate the structure and composition of deposited films. By analogy to other studies in which PMA was used as a photo-CVD precursor, we anticipated that
Fig. 6 TEM images of two NaCl particles coated by photo-CVD using MMA as the coating reactant. The coatings are apparent in the images as low contrast regions near the particle edges. The coatings are ca. 3 nm thick and quite uniform
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MMA-derived films are composed mostly of polymethyl methacrylate (PMMA), which is the polymeric form of MMA. Figure 7 also shows FTIR spectra of authentic sample of MMA and PMMA. Both the coated and uncoated particle spectra show peaks that may be attributed to water or adsorbed OH. (NaCl readily absorbs water during sample preparation.) The spectrum of coated particles also shows a broad C–H stretching band in the 2,900–3,000 cm 1 region, and a C = O band at 1,740 cm 1. The conditions under which particles were collected for these experiments were not sufficiently well controlled to make it meaningful to compare relative intensities of the OH stretching regions of the different particle spectra. What is clear, however, is that coatings grow on NaCl particles, and that the C– H and C = O stretching regions of the FTIR spectra of these films are very similar to those of authentic MMA and PMMA samples. Acknowledgment This work was supported in part by the Defense-University Research Initiative in NanoTechnology (DURINT) of the US Army Research Laboratory and the US Army Research Office under agreement number DAAD190110503, in part by the Minnesota Supercomputing Institute, and in part by the National Science Foundation under Grant No. CHE–0094911. The authors thank J. Holm and H. Ajo for their assistance in FTIR measurements, and Y.C. He for her assistance in TEM measurements.
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