ARTICLE pubs.acs.org/JPCC
Chemical Kinetics of Photoinduced Chemical Vapor Deposition: Silica Coating of Gas-Phase Nanoparticles Adam M. Boies,*,† Steven Calder,‡ Pulkit Agarwal,§ Pingyan Lei,§ and Steven L. Girshick§ †
Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, U.K. Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States § Department of Mechanical Engineering, University of Minnesota, 111 Church Street, Minneapolis, Minnesota 55455, United States ‡
bS Supporting Information ABSTRACT: Experimental studies of gas-phase nanoparticle coating by photoinduced chemical vapor deposition (photoCVD) have shown that silica coatings can be produced with controllable thicknesses on different nanoparticle cores for a variety of applications. This study presents a chemical reaction sequence for the photo-CVD process to describe the production of silica coatings from the decomposition of tetraethyl orthosilicate (TEOS). The model incorporates photochemical reactions into known thermal reaction sequences involving gas-phase and surface reactions to describe the nanoparticle coating process. Modeled results of the photo-CVD process indicate that the dominant reactions for the production of silica coatings on the surface of the nanoparticles are the photodecomposition of TEOS and the removal of surface ethyl groups from adsorbed TEOS species. Relative concentrations of gas-phase and surface species are compared for different model configurations and system parameters. Modeled coating thicknesses agree well with experimental findings and demonstrate that coating thickness increases with increasing TEOS concentration and increased residence time within the reaction chamber.
a reacting flow stream. While the approach highlighted the production of silica for a specific geometry, it provided a general reaction sequence that relied on experimental15 and theoretical17 studies of the TEOSsilica system. The Coltrin et al. reaction sequence included four gas-phase reactions and eight surface reactions, none of which considered oxygen as a separate reactant. The primary gas-phase reaction involved in the production process was reported as the β-hydride elimination of ethylene, which is the least endothermic gas-phase reaction (∼10 kcal/mol) for TEOS.18 The primary surface reactions responsible for film growth were found to be the adsorption of the TEOS radical, triethoxysilanol [Si(OH)(OC2H5)3], onto the surface and then subsequent removal of the remaining ethyl groups. Modeled growth rates from Coltrin et al. provide adequate agreement with molecular beam experiments and CVD experiments performed by Kalidindi and Desu.19 Their study concluded that at lower temperatures the gas-phase TEOS reaction is the rate-limiting reaction while at higher temperatures the ethyl removal at the surface is rate limiting.14 Work by Romet et al. focused on the production of silica from TEOS by examining ozone related reactions.13 Their sequence included the gas-phase reactions of TEOS with monatomic
1. INTRODUCTION The synthesis of coreshell nanoparticles has received much interest in recent years as such structures can enhance nanoparticle properties, such as thermal stability,1 plasmon resonance,2 catalytic activity3 and surface functionality.4 There are currently a wide variety of gas-,5 liquid-6 and solid-phase7 synthesis techniques available to produce composite particle structures. While all approaches have inherent advantages, the production of composite nanoparticles by gas-phase methods allows for particles to be produced at high throughputs in inert or nonreacting environments with little or no surface impurities. Photoinduced chemical vapor deposition (photo-CVD) is a gas-phase approach that allows for the production of a variety of coreshell compositions, including organic8 and inorganic coatings9 of nanoparticles. While the approach has been shown to work experimentally, a fundamental study of the chemical reaction sequence involved in the coating process has not been presented. Silica coatings are particularly important at both the macroand nanoscale because of silica’s thermal stability,10 high electrical resistivity,11 and surface that is easily functionalized through ligand attachment.12 Several research groups have examined the chemical reaction sequences involved in the thermal decomposition of tetraethyl orthosilicate (TEOS) onto macroscopic surfaces for purposes of producing films relevant to the semiconductor industry.1316 Coltrin et al. examined the thermal decomposition of TEOS to produce silica films on a series of wafers placed within r 2011 American Chemical Society
Received: July 27, 2011 Revised: November 7, 2011 Published: November 16, 2011 104
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is assumed to be sufficiently large such that diffusion fluxes in the axial direction can be neglected.22 Gas-phase coagulation is omitted as previous experimental studies9 have shown no indication of significant particle agglomeration when concentrations are less than 108 cm3. While the homogeneous nucleation of gas-phase reactants is neglected, the inclusion of oligomerization reactions gives an indication whether macromolecules are likely to be produced within the reaction chamber. Thermodynamic data for all species come from the NASA Glenn Research Center with a reference state of 298.15 K and 1 bar.27 2.2. Photochemical Model. As of this writing, no other known studies have modeled gas-photon and particle-photon reactions for a photochemical vapor deposition reaction involving these species. To demonstrate how the photochemical reactions are modeled in this study, the photodecomposition of O2 is shown as an example. As discussed by Wayne,28 the photodecomposition of O2 is described by the following relation,
oxygen and ozone, among others. In all, their sequence consisted of 33 gas-phase reactions, which included TEOS decomposition reactions as well as oligomer formation. The reaction of TEOS with monatomic oxygen, an important reaction in this study, was estimated from experimental work by Sanogo and Zachariah.15 Like Coltrin et al., Romet et al. found that the gas-phase decomposition of TEOS to form triethoxysilanol was the dominant reaction when ozone was used as a precursor. Additionally, they showed that under certain conditions it was possible to form oligomers. These oligomers can grow large enough to be detected as particles, which have been observed previously under specific conditions in our experimental studies. Romet et al. included a simplified surface reaction sequence whereby the remaining ethyl groups on adsorbed triethoxysilanol were removed in a single step to form another receptor site for further growth. This assumption caused the total reaction sequence to be gasphase-reaction limited under all conditions. The Romet et al. model accurately predicts the growth of films at low temperatures but begins to deviate at high temperatures.13 While previously developed models accurately describe the production of thin films on macroscopic substrates from thermal decomposition or reaction with active oxygen, no previous studies have included photochemical reactions of TEOS. The purpose of this study is to develop a chemical reaction sequence that includes photochemical reactions in order to model the growth of silica films on nanoparticle surfaces from photo-CVD.9 The previous reaction sequences proposed for the thermal decomposition of TEOS serve as a starting point for this study. Others have experimentally investigated the production of thin films with TEOS on macroscopic substrates by photo-CVD but did not explicitly examine the reaction kinetics.20,21 To our knowledge, this work represents the first study of the photo-CVD chemical reaction sequence. The present study advances the understanding of the production of engineered nanoparticles by examining both photochemical TEOS reactions and film growth on gas-phase nanoparticles.
O2 þ hν f 2O
ð1Þ
where hν denotes photons, which in this study have energies of 7.2 eV/ photon (corresponding to a wavelength of 172 nm). The reaction rate between the photons and the O2 depends on the photonmolecule interaction as the photons pass through the volume. The rate of O2 molecular destruction can be determined according to the relation d½O2 1 dðΓi σ O2 ϕNO2 xÞ ¼ ð2Þ Q_ D ¼ dt NA dx where Q_ D (mol cm3 s1) is the destruction rate, [O2] (mol cm3) is the molar concentration of O2, t (s) is time, NA is Avogadro’s number, and x (cm) is the axial distance in the tube. The differentiated term, ΓiσO2ϕNO2x, represents the number of dissociated molecules as a result of photonO2 interactions, where Γi (cm2 s1) denotes the incident flux of photons, σ (cm2) is the radiation absorption cross-section, ϕ is the probability that an absorbed photon will cause a dissociating reaction, and NO2 (cm3) is the concentration of oxygen molecules. By differentiating eq 2, the resulting destruction rate is given as ϕ dðNO2 Þ dðΓi Þ _ x þ σ O2 N O2 x Γi σO2 NO2 þ Γi σ O2 QD ¼ NA dx dx
2. PHOTO-CVD MODEL 2.1. Model Background. A chemical kinetics model was developed within the framework of a commercially available kinetics software package, CHEMKIN-PRO (Reaction Design, San Diego, CA) to numerically solve the series of differential equations that represent the reaction sequence within the photo-CVD system. The existing software platform allowed for the direct implementation of gas-phase, gas-surface, and surface reactions. To model reactions that occur by gas to particle conversion and on nanoparticle surfaces, an additional software package, the Particle Tracking Module, is used. The Particle Tracking Module uses the method of moments to describe a size distribution of particles in terms of their average characteristics.22 The Particle Tracking Module is based on the work of Frenklach and Harris which approximates the general dynamics equation to account for particle nucleation, coagulation, and surface growth.26 Variations of this method are frequently used in numerical simulations and are accurate, so long as the aerosol has a well-behaved monomodal distribution.2325 The photo-CVD reactor is modeled as a plug-flow reactor whereby the gas-phase composition is assumed to be kinetically limited. All gas and aerosol properties are assumed to be uniform on the cross-flow plane, and therefore there is no mass or energy transfer in the transverse direction. Additionally, the gas velocity
ð3Þ which can be used to determine the rate of the O2 dissociation within each discrete element in the numerical model. In all cases in this study the incident radiation flux within each discrete element is treated as a constant; thus, d(Γi)/dx = 0. The radiant flux is held constant for all axially distributed model elements in the initial analyses and is then varied in accordance with a diverging light source, as discussed in section 3.3. Absorption of photons was not considered within the model. The gradient of NO2 with respect to x, d(NO2)/dx, is assumed to be zero within each differential element. The omission of the gradient of NO2 in the destruction rate was found to have little effect on the modeled photodecomposition of O2 (see Supporting Information for further details). 2.3. Model Inputs. The kinetics model simulates the photoCVD coating of particles with silica using TEOS as a precursor for conditions that match previous experimental studies.9 The experimental schematic shown in Figure 1 depicts the conditions that the model simulates, where a presynthesized aerosol enters the reaction chamber along with TEOS and purge flow near an ultraviolet lamp and then flows down a cylindrical chamber, away 105
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by the relation q ¼ J½O2 ¼ ϕΓσ O2
N O2 NA
ð4Þ
For all photochemical reactions considered within this study, ϕ is assumed to equal unity because of the high energy associated with radiation at the 172 nm wavelength.28 Therefore the rate coefficient, J, is found from eq 4, resulting in J = ΓσO2. The photochemical rate coefficient is incorporated within the modeling framework by setting the Arrhenius preexponential factor equal to the rate coefficient, A = J with Ea = 0 and β = 0. By incorporating the rate coefficient in this manner, the photon flux only appears in the rate coefficient J as shown in eq 4. The same assumptions that resulted in eq 4 are used for all other species in the model; namely, that over each differential element the concentration of species and radiation flux are constant, resulting in a rate constant that is a function of incident radiation flux and radiation absorption cross-section. A variation in the radiation flux term is incorporated in section 3.3, and therefore the results discussed in that section do not use the values of J listed in Tables 1 and 2. The radiation absorption cross-section of diatomic oxygen at 172 nm30 is σ = 6 1019 cm2. The absorption cross-sections for other gas-phase species are not well-known and were used as variable parameters within the model. The radiation absorption cross-sections for surface reactions are determined by the projected area of the particles multiplied by the relative concentration of each species on the particle surface. The resulting absorption cross-section formulation for surface species is σs,i = πr2θi, where σs,i is the absorption cross-section of surface species i, r is the particle radius, and θ is the ratio of the concentration of i surface species to the total concentration of surface species.
Figure 1. Experimental schematic of nanoparticle silica coating by photo-CVD with TEOS as a precursor.
from the lamp, as particle growth occurs due to CVD reactions. While a variety of experimental configurations were investigated, this model simulates one configuration for conditions that were used experimentally.9 The chamber is modeled as a plug-flow reactor with an inner diameter of 3.4 cm and a length of 122 cm. The lamp emits vacuum-ultraviolet (vacuum-UV) photons with a power of 50 mW cm2 across a 7.07 cm2 area, which corresponds to a photon flux of 3.4 1016 photons s1 cm2. Flow rates of nitrogen purge gas, TEOS, and oxygen are varied to match experimental conditions. The reactor is modeled at 400 °C and atmospheric pressure, which corresponds to the experimental conditions that produced the best silica films. The energy equations are not solved. The model structure requires that the core particle and coating be composed of the same material, i.e., that particles are homogeneous, and therefore both the modeled particle core and coating are silica. Therefore, the core particles introduced at the inlet of the reactor are defined as silica particles with a concentration of 107 cm3 and an initial diameter of 30 nm. While the core differs from the experimental study to which the results are compared (silver nanoparticle cores), the studies are similar since the core particle composition is expected to affect only the first monolayer of growth. Different rates of growth for the first monolayer are not included in this study and are only expected to significantly affect results for thin coatings (90% of available sites of the low TEOS flow rate case, Figure 8d. These results indicate that as TEOS flow rates increase, the availability of surface sites becomes increasingly important in determining the modeled coating thickness within the chamber. While the model only includes production of pure silica coatings by requiring hydrocarbons to be removed before subsequent adsorption can occur, actual coating growth may proceed fast enough for hydrocarbons to be incorporated in the film. These results indicate that increased TEOS flow rates may lead to increased opportunities for hydrocarbon inclusion as surface removal of ethoxy groups becomes the rate limiting reaction.
4. SUMMARY AND CONCLUSIONS Gas-phase coating of nanoparticles by photo-CVD was shown previously to be an effective way to produce silica coatings with controllable thickness for a variety of different particle cores. While the experimental studies were able to elucidate some of the underlying phenomena occurring during the photo-CVD process, a chemical kinetics model was developed to better describe 112
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Figure 8. Modeled gas-phase mole fraction of critical species within the photo-CVD system with 1 cm3 min1 O2 and (a) high TEOS flow (1.6 cm3 min1) and (b) low TEOS flow (0.05 cm3 min1); corresponding surface-site fraction of species on the nanoparticle surfaces undergoing coating with (c) high and (d) low TEOS flow rates. These conditions correspond to the maximum and minimum TEOS flow rates of the 1 cm3 min1 O2 curve in c.
the photochemical reactions occurring during the production of silica coatings on nanoparticles. This study is the first to examine a photochemical reaction of TEOS and is the first to model the production of silica films on nanoparticle surfaces. The chemical reaction sequence includes previous reactions from thermal CVD studies for TEOS. Model results indicate that a reaction sequence that includes only oxygen photodecomposition and then subsequent thermal reactions results in thinner than expected coatings because of the slow generation of sub-TEOS species, i.e., triethoxysilanol, and the slow removal of surface ethoxy groups to produce glass bonds. Thus, it is surmised that the photochemical reactions do not proceed solely through oxygen, but the photodecomposition of TEOS is also significant. Better results are achieved when the reaction sequence includes photochemical reactions involving gas-phase TEOS and sub-TEOS species, as well as photochemical surface reactions. The radiation absorption cross-sections of TEOS and triethoxysilanol are unknown parameters within the system. As the modeled absorption cross-section of TEOS and triethoxysilanol increases, the photodecomposition of TEOS begins to dominate the gas-phase reactions, thus causing the introduction of oxygen to have little effect. The inclusion of a radiation flux subroutine that models the flux from a diverging light source results in slightly thinner coatings, as less radiation at points farther from the radiation source slows the decomposition of TEOS and surface reactions. The model is in qualitative agreement with experimental results, showing an increase in coating thickness with respect to increased TEOS flow and decreased
nitrogen flow. The chemical kinetics of the silica coating process depend on the relative concentrations of gas-phase precursors. For low TEOS flow rates the production of sub-TEOS species is the rate limiting step, while at higher TEOS flow rates the surface removal of ethoxy groups begins to play an increasingly important role in determining the overall coating thickness. The modeled growth trends of coating thickness as a function of TEOS flow rates most closely match experimental results in the case of variable radiation and the greater TEOS radiation absorption crosssections (σTEOS = σsub‑TEOS = 6 1018 cm2). These modeled results show a trend similar to experimental findings and are within a factor of 4 of the experimentally measured growth rates, which represents reasonable agreement given uncertainties of many parameters including absorption cross-sections and reaction rates. These results give insights into the coating process and show that for high TEOS flow rates there is an increased likelihood of hydrocarbon inclusion within the films as a result of the relatively slow surface hydrocarbon removal compared to the rate of adsorption of available sub-TEOS species.
’ ASSOCIATED CONTENT
bS
Supporting Information. Text detailing an investigation of modeled photochemical reaction kinetics (section SI 1) and intensity of divergent and attenuated radiation (section SI 2) and figures showing comparison of analytically determined O2 concentration (Figure SI-1) and transmitted radiative flux (Figure SI-2).
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This material is available free of charge via the Internet at http:// pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: +44 1223 746972.
’ ACKNOWLEDGMENT This research was primarily supported by the National Science Foundation under Award Nos. CBET-0730184 and CBET1066343), by the MRSEC Program of the NSF under Award No. DMR-0819885, and by the Minnesota Futures Grant Program. This work was partially carried out using computing resources at the University of Minnesota Supercomputing Institute. ’ REFERENCES (1) Dick, K.; Dhanasekaran, T.; Zhang, Z.; Meisel, D. J. Am. Chem. Soc. 2002, 124, 2312. (2) Qiao, Z.; Jianping, G.; James, G.; Yongxing, H.; Yugang, S.; Yadong, Y. Adv. Mater. 2010, 22, 1905. (3) Luo, J.; Wang, L.; Mott, D.; Njoki, P. N.; Lin, Y.; He, T.; Xu, Z.; Wanjana, B. N.; Lim, I.-I. S.; Zhong, C.-J. Adv. Mater. 2008, 20, 4342. (4) Kim, D. K.; Zhang, Y.; Voit, W.; Rao, K. V.; Muhammed, M. J. Magn. Magn. Mater. 2001, 225, 30. (5) Teleki, A.; Suter, M.; Kidambi, P. R.; Ergeneman, O.; Krumeich, F.; Nelson, B. J.; Pratsinis, S. E. Chem. Mater. 2009, 21, 2094. (6) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (7) Sardar, R.; Heap, T. B.; Shumaker-Parry, J. S. J. Am. Chem. Soc. 2007, 129, 5356. (8) Zhang, B.; Liao, Y. C.; Girshick, S.; Roberts, J. J. Nanopart. Res. 2008, 10, 173. (9) Boies, A. M.; Roberts, J. T.; Girshick, S. L.; Zhang, B.; Nakamura, T.; Mochizuki, A. Nanotechnology 2009, 295604. (10) Radloff, C.; Halas, N. J. Appl. Phys. Lett. 2001, 79, 674. (11) Chang, Ee. W.; Yew Cheong, K. Phys. B (Amsterdam, Neth.) 2008, 403, 611. (12) Schroedter, A.; Weller, H.; Eritja, R.; Ford, W. E.; Wessels, J. M. Nano Lett. 2002, 2, 1363. (13) Romet, S.; Couturier, M. F.; Whidden, T. K. J. Electrochem. Soc. 2001, 148, G82. (14) Coltrin, M. E.; Ho, P.; Moffat, H. K.; Buss, R. J. Thin Solid Films 2000, 365, 251. (15) Sanogo, O.; Zachariah, M. R. J. Electrochem. Soc. 1997, 144, 2919. (16) Crowell, J. E.; Tedder, L. L.; Cho, H. C.; Cascarano, F. M.; Logan, M. A. J. Vac. Sci. Technol., A 1990, 8, 1864. (17) Allendorf, M. D.; Melius, C. F.; Ho, P.; Zachariah, M. R. J. Phys. Chem. 1995, 99, 15285. (18) Chu, J. C. S.; Soller, R.; Lin, M. C.; Melius, C. F. J. Phys. Chem. 1995, 99, 663. (19) Kalidindi, S. R.; Desu, S. B. J. Electrochem. Soc. 1990, 137, 624. (20) Motoyama, Y.; Kurosawa, K.; Yokotani, A. Electron. Commun. Jpn. (Part II: Electron.) 2005, 88, 36. (21) Motoyama, Y.; Miyano, J.; Toshikawa, K.; Yagi, Y.; Yanagida, H.; Kurosawa, K.; Yokotani, A. J. Phys. IV 2001, 11, 1131. (22) Kee, R. J.; Rupley, F. M.; Miller, J. A.; Coltrin, M. E.; Grcar, J. F.; Meeks, E.; Moffat, H. K.; Lutz, A. E.; Dixon-Lewis, G.; Smooke, M. D.; Warnatz, J.; Evans, G. H.; Larson, R. S.; Mitchell, R. E.; Petzold, L. R.; Reynolds, W. C.; Caracotsios, M.; Stewart, W. E.; Glarborg, P.; Wang, C.; McLellan, C. L.; Adigun, O.; Houf, W. G.; Chou, C. P.; Miller, S. F.; Ho, P.; Young, P. D.; Young, D. J.; Hodgson, D. W.; Petrova, M. V.; Puduppakkam, K. V. Theory Manual. CHEMKIN-PRO, Release 15101; Reaction Design: San Diego, CA, USA, 2010; p 360. 114
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