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Structural Stabilization of Metal Nanoparticles by Chemical Vapor Deposition-Applied Silica Coatings M. Seipenbusch* and A. Binder Institut fu¨r Mechanische Verfahrenstechnik und Mechanik, UniVersita¨t Karlsruhe (TH), Am Forum 8, 76131 Karlsruhe, Germany ReceiVed: July 7, 2009; ReVised Manuscript ReceiVed: October 16, 2009
The structure of aerosol Pd nanoparticle ensembles was stabilized against a loss of surface area during tempering by application of a thin silica coating by chemical vapor deposition in the gas-borne state. The coating was applied to the surfaces of particles prior to ensemble formation and after, yielding two different types of structures. The pure Pd and the stabilized structures were tempered at varied residence times and temperatures to analyze the sintering kinetics as a function of the type of structure. The sintering kinetics of clean Pd particles could be approximated using the general power law expression model. Stabilization of the nanoparticles was accomplished for both coating approaches. A stabilization of the super ordinate structure, however, was only possible by a coating of the ensemble as a whole. Introduction The functionality of nanoscale materials is closely associated with their dispersion state. As a result of the high surface energy of nanostructures, they have a strong tendency to reduce their free surface area and, concurrently, the surface free energy. There are two potential mechanisms that lead to the reduction of the surface free energy by sintering: coalescence of particles in contact by diffusion or viscous flow, and the transport of molecular or atomic species between particles by surface migration and vapor diffusion.1,2 For nanoparticles isolated on a surface, the first mechanism requires the mobility of particles to allow migration over the support. The second mechanism, known as Ostwald ripening, depends on the increased release rate of molecular species for smaller particles compared to larger ones and a preferred attachment of these species to larger particles. The molecular species can be of gaseous nature, desorb, and be transported through the gas phase, and eventually readsorb on the particles. An example for such a mechanism is given in the growth of Nickel nanoparticles in a CO atmosphere due to the formation of Nickel carbonyl.3 Another possibility is the formation of species adsorbed on the support retaining their mobility until they are readsorbed by a metal particle. Notably, the mechanism does not require mobility of the particles themselves. Ostwald ripening eventually leads to a growth of the larger particles, while the small particles simultaneously decrease in size. Apart from the surface free energy, the Gibbs free energy of nanoparticles has another component related to the dispersion state. The coordination number of individual particles within a structure determines their degree of freedom and is connected to the entropy of the system. The minimum energy is thus linked to the maximum coordination number.4 There is thus a driving force for the maximization of interparticle contacts, which can lead to the restructuring of agglomerates toward more compact structures or the coagulation of nanoparticles moving on surfaces.4,5 * To whom correspondence should be addressed. Phone: +49 721 6082416. Fax: +49 721 608 6563. E-mail:
[email protected].
The increase of particle size and the loss of surface area of nanostructured systems are problematic whenever the retention of these characteristics is necessary to maintain their functionality. Heterogeneous catalysis is a prominent example, where the activity of a catalyst depends on the specific surface area, but, in the case of a structurally sensitive reaction, also inherently depends on the particle size. Particle growth and the loss of surface area following sintering therefore lead to a loss in activity and eventually to a deactivation of the catalyst1,2,6,7 Another case of industrial importance is the control of particle growth mechanisms in the production of nanostructured metallic materials. The established processes in this area, powder metallurgy and powder injection molding, require tempering steps, setting high demands on process control to retain nanocrystallinity. In light of this, the stabilization of the nanostructure of particulate systems has received increased attention in recent years.7-9 Experimental work has been done on the stabilization of oxides with respect to particle size and crystal phase, for materials such as titania, zirconia, and alumina, used in photocatalytic applications or as catalytic support materials.9-11 It was shown that the doping of oxides with other oxides has a good potential in the prevention of sintering at elevated temperatures. The stabilization of Pd catalysts using silica coatings has been investigated by Park et al. using core shell structures with porous coatings of about 10 nm. Stabilization of the particle size could be demonstrated up to temperatures of 700 °C.12 For the oxidation of CO, the activity of untempered catalysts was lowered for the coated catalyst particles relative to a Pd@SiO2 control sample, presumably due to transport limitations in the porous layer. Surprisingly, however, for the hydrogenation of acetylene, the catalytic activity of the particles was not compromised by the coating. Ma et al. successfully stabilized gold nanoparticles on titania surfaces by applying a SiO2 coating to the supported particles using an elaborate multistep gas-phase atomic layer deposition (ALD) processes.13 The strategy was to deposit SiO2 on the surface of the titania support embedding the gold particles in this way to reduce their mobility on the support surface. A partial loss of the catalytic activity of the particles could not, however, be avoided. In unsupported metal nanostructures, the presence of minute
10.1021/jp9063998 CCC: $40.75 2009 American Chemical Society Published on Web 11/04/2009
Structural Stabilization via CVD-Applied Silica Coatings amounts of impurities on the surfaces of silver and nickel particles was shown to have a very strong effect on the kinetics of sintering as well as on the restructuring of agglomerates.4,5 In bulk solids, the introduction of poorly soluble doping materials in alloys has been used for a long time to induce the formation of a dispersed phase in the matrix. This dispersed phase limits the diffusion processes and hinders the movement of displacements in the matrix and can thus prevent the grain coarsening in polycrystalline materials and improve their mechanical properties.14 In the coarsening of nanostructures by sintering, these principles are assumed to have a stabilizing effect by hindering diffusive transport between individual particles in the presence of impurities in the contact zones. Additionally, it is assumed that the movement of particles against each other in an agglomerated structure is hindered, which is necessary for restructuring.4 In this work, the stabilization of noble metal nanostructures was studied by the deposition of small amounts of impurities on the particle surfaces. The aim was the exploration of possibilities for the structural stabilization avoiding a loss in functionality due to a diffusional limitation of mass transfer by thick coating layers. Aerosol nanoparticle agglomerates were chosen as model structures because of several unique properties. Aerosol synthesis of nanoparticles allows for a good control over particle size in controlled atmospheres, with a much higher degree of purity compared to solvent-based synthesis routes. The coating process can be carried out without leaving the system. Online methods are available to control every step of the synthesis and coating process. In conventional supported catalysts, the experimental study of the mobility of nanoparticles on a surface using online imaging is very limited in terms of temperatures and gas atmospheres.6 In the aerosol system, however, restructuring of nanoparticles agglomerates, which can be viewed as a model for the mobility of particles on a surface, can be investigated at relevant reaction conditions and monitored using online aerosol methods. Two experimental approaches were taken in this work. In the first set of experiments, the doping material was deposited on the particle surfaces before they were brought into contact, thus preventing direct tangency of the metal particles. In the second set of experiments, a coating was deposited on agglomerated nanoparticle structures. In this case, there was a direct contact between the metal nanoparticles while the coating material provided a scaffold for the structure. In the first case, it could be expected that particle growth would be prevented while the movement of particles against each other by sliding or rolling was still possible. In the second case, the stabilizing effect on the particle size was harder to foresee, while it could be expected, that the agglomerate structure would be supported. The two different experimental approaches yield different dispersion states of the metal nanoparticles, which are representative of different systems, e.g., supported and isolated catalyst particles for the first case versus porous metal nanostructures for the latter. Experimental Section Nanoparticles were generated using a spark discharge generator (SDG).15 The SDG consists of a high voltage power supply in a parallel circuit with a capacitor and a set of electrodes spaced at a distance of a few millimeters apart (see Figure 1). The charging of the capacitor leads to an increase of the potential difference between the electrodes until the breakthrough voltage is reached. The energy of the capacitor is then released into a spark plasma, which provides the energy for evaporation of
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Figure 1. Schematic of the SDG and experimental setup for approach I, the coating of nonagglomerated particles.
material from the electrode surfaces.15 The particle generation subsequently occurs by condensation of the metal atoms in the cold carrier gas. The particle size depends mainly on the material and lies in the range between 2 and 6 nm.5 The particles are crystalline, of spherical shape, and the size distributions are rather narrow. The generator produces high concentrations of nanoparticles and has an internal residence time allowing for the formation of agglomerates containing approximately 100 primary particles.5 To obtain nonagglomerated nanoparticles, a sintering furnace (tube furnace 1) was set in-line with particle generation, producing solid spherical particles with a median diameter of 10 nm by complete coalescence at 500 °C. Approach I: Coating of Nonagglomerated Particles. The spherical particles from tube furnace 1 were mixed with a flow of nitrogen laden with the precursor for the doping material [tetraethyl orthosilicate (TEOS), 8.3 × 10-6 mol/L] before entering tube furnace 2. In this second furnace, heated to 800 °C, the adsorbed precursor decomposes on the particle surfaces in a chemical vapor deposition (CVD) process and leaves the desired oxide behind. The aerosol was then given sufficient time to form agglomerates with a median diameter of 33 nm at room temperature in an agglomeration tube. These agglomerates consisted of Pd particles that were separated by the oxide used as doping material. The sintering experiments were conducted in a third tube furnace at varied temperature and residence time. The sintering of these particles was then monitored by analyzing primary particle growth using transmission electron microscopic (TEM) imaging on collected samples. The mobility of primary particles against each other in the agglomerate was investigated by monitoring the change of the agglomerate diameter due to restructuring. This was done employing electrical mobility analysis, a technique widely used in aerosol technology. In the experiments, a differential mobility analyzer (DMA, TSI model 3071A) was used in combination with a condensation particle counter (CPC, TSI model 3022). The two instruments together form a scanning mobility particle sizer (SMPS). The DMA is an electrostatic classifier, allowing the selection of a narrow cut from the particle size distribution, and the median selected particle size can be varied between approximately 5 and 750 nm.16 The CPC enables the determination of the particle concentration for each selected particle size and thereby allows the measurement of particle number distributions. Approach II: Coating of Agglomerated Nanoparticles. In contrast to the method described above the second experimental approach led to the formation of coated agglomerates instead of individually coated primary particles. For this, the agglomeration tube was set directly downstream of tube furnace 1, to allow agglomeration of pure metal particles to a median size of 35 nm (see Figure 2). The agglomerates generated in this way were then exposed to the precursor vapor (TEOS, 1.6 × 10-5 mol/L) and entered tube furnace 3. In the sintering furnace, the decomposition of the adsorbed precursor took place simultaneously with the first stages of sintering.
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Seipenbusch and Binder
Figure 2. Experimental setup for experimental approach II, the coating of agglomerates of nanoparticles.
Figure 4. Sintering of pure Pd nanoparticle agglomerates at 400 °C: experimental data, and GPLE fit using m ) 2. Primary particle size initially: 10 nm. Also shown: TEM images of Pd agglomerates after different sintering times (furnace 1: 500 °C; agglomeration at room temperature).
Figure 3. Size distributions of Pd nanoparticles (from TEM analysis) with pure surfaces, as produced in the experimental setup according to Figure 1 and after sintering for 10 s at 600 °C (furnace 1: 500 °C; furnace 2: 800 °C; agglomeration at room temperature).
Results and Discussion The sintering and restructuring of Pd nanoparticle agglomerates with pure surfaces was investigated as a reference for the later structural stabilization by oxide doping. Size distributions of the primary particles were determined by TEM analysis for five temperatures between room temperature and 800 °C, two of which are exemplarily shown in Figure 3. All size distributions could be approximated by log-normal distributions. The shape of the size distribution thus did not change during the sintering process. In the literature, the shape of the size distribution is considered to be indicative of the sintering mechanism by some authors.17,18 According to the Lifshitz-SlyozovWagner (LSW) theory, sintering occurring by Ostwald ripening leads to an asymmetric size distribution skewed to larger particles and a cut off in the size distribution at a diameter smaller than twice the mean diameter.1 Sintering occurring by particle surface migration, and coalescence on the other hand is predicted to generate size distributions following a log-normal distribution. The distinction of the sintering mechanism by the shape of the size distribution, however, is contested by others theoretically and experimentally.6,19 We are thus careful in drawing conclusions from the fact that our data can be approximated by a log-normal fit. The kinetics of the sintering process at a constant temperature of 400 °C was investigated for sintering times below 1 s (see Figure 4). For pure particle surfaces, the sintering of agglomerated Pd nanoparticles is a very rapid process: the particle size changes from an initial value of 10 nm to the final size of the fully coalesced sphere with a diameter of 25 nm in less than 0.5 s. A fit to the sintering data was carried out using the general power law expression (GPLE), introduced by Fuentes, which was proven to give good approximations to experimental data from a large number of laboratories.2,20 For our data, the usual form of the GPLE with the dispersion D as a variable (eq 1) had to be converted into an equation including the particle diameter (eqs 2 and 3). The value of 2 for the sintering order m
in the GPLE was shown to quantitatively address the influence of catalyst properties and reaction conditions on sintering.2 The change of particle size with time due to sintering can thus be expressed by the differential equation, which can be solved analytically. The solution was fitted to the data by variation of the activation energy Ea (56 kJ/mol) and k0 (ks@400 °C: 3 10-2 h-1) and is plotted as a dashed line with the data points in Figure 4.
-
(
)
d(D/D0) DEq m D ; ) ks dt D0 D0
D ) A/V )
⇒ -
ks ) k0 · exp{-Ea /RT}
(1) πd2 6 ) π 3 d d 6
( (
(2)
) )
d(D/D0) d0 d(d0 /d) d0 m for )) ks dt dt d dEq d(d0 /d) m ) 2: ) ksdt (3) d0 d0 2 d dEq
Because of its exponential influence on the kinetics of sintering, experiments were conducted at varied sintering temperature. Figure 5 shows the primary particle diameter and the agglomerate size in relation to the initial size of the primary particles d0. The primary particle size of the agglomerates increased by 20% at a residence time of 13 s and a temperature of 400 °C. For 600 °C, complete coalescence was observed at sintering times of less than 10 s (see Figure 5). For higher temperatures, the particle size was found to drop again, which is due to partial evaporation and recondensation of the particles. The GPLE model was plotted for varied temperature using the fit parameters determined from the sintering at 400 °C for varied sintering time, yielding a fair approximation of the experimental data (see Figure 5). The Pd nanoparticles were now coated with silica according to approach I, leading to agglomerates of individually coated particles. A stabilizing effect for the primary particle size is clearly visible in Figure 5. While at 600 °C the pure Pd nanoparticles are completely coalesced, there is only an increase
Structural Stabilization via CVD-Applied Silica Coatings
Figure 5. Sintering and agglomerate restructuring of clean and silicacoated Pd nanoparticle agglomerates: experimental results and model fit (GPLE) (furnace 1: 500 °C; furnace 2: 800 °C; agglomeration at room temperature).
Figure 6. Sintering of Pd agglomerates as produced and coated in the agglomerated state (approach II) (furnace 1: 500 °C; agglomeration at room temperature).
of about 20% in the particle size for silica coated nanoparticles. However, the mobility of the particles within the structure was not affected by the coating, as could be seen in the restructuring
J. Phys. Chem. C, Vol. 113, No. 48, 2009 20609 of the agglomerates induced by the tempering. The agglomerate diameter decreased monotonously with increasing temperature for both types of particle surfaces, reaching a stabile size at 600 °C. Apparently, the surface migration of Pd particles is not affected by the silica in the contact zone. For higher temperatures, partial evaporation and recondensation might be responsible for the slight decrease of the particle size at higher temperatures and a corresponding increase in the agglomerate size. The structural stability of agglomerates generated according to approach II was analyzed under the same conditions as above by variation of the sintering temperature. For uncoated Pd particles, the decrease of the agglomerate size is paralleled by an increase of the primary particle size due to sintering (see Figure 6). It can thus not be distinguished between the two processes influencing the particle size during tempering, agglomerate restructuring and primary particle sintering. At 600 °C the equilibrium particle size of complete coalescence is reached. At higher temperatures again evaporation sets in, leading to a decrease of the primary particle size with recondensation after cooling down. At the same time an increase of the agglomerate size occurs due to the coagulation of the newly formed particles at room temperature after leaving the high temperature zone. The coated agglomerates however are effectively stabilized both against restructuring and primary particle coalescence. Only a slight increase of the primary particle size was observed at 800 °C. To analyze the SiO2-coating on the particles, electron dispersive X-ray analysis in a TEM (EDX, Phillips CM200 FEG/ ST) and Fourier transform infrared spectroscopy (FTIR, Vector22) on powder samples in KBr-tablets were employed. Figure 7 shows the EDX- and infrared spectra of silica coated Pd nanoparticles. The presence of Si on the particle surfaces is indicated clearly by a peak at 1.72 keV in the EDX spectrum. The infrared spectrum contains bands characteristic of silicate between 1200 and 400 cm-1, mainly those corresponding to Si-O bonds. At 470 cm-1 Si-O-Si in-plane vibration could be observed.21 A second and third band at 1200 and 1100 cm-1 was linked to the asymmetric vibration of the SiO4 tetrahedron.22
Figure 7. EDX and FTIR spectra for silica-coated Pd nanoparticles and TEM images of coated (left) and uncoated particles (right).
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TABLE 1: Catalytic Activity of Clean and Silica Coated Pd Expressed As TOFs Pd SiO2-Pd
d0/nm
TOF/s-1
16.1 14.3
3.6 2.7
Symmetric bond stretching of SiO2 causes an absorption peak at 800 cm-1, while the peak located at 950 cm-1 is believed to originate from the vibration of terminal Si-O(H) groups.22 Thus, qualitatively, the SiO2 coating can be verified. The small amounts of material, however, make the quantification of silica very challenging. According to an estimation of the coating thickness by the evaluation of TEM images (see Figure 7), the mass ratio of SiO2 to Pd in the particles was determined to be in a range between 3 and 9%. The coating thickness was found to vary between 2 and 4 nm. An important question remains regarding the possibility of a negative effect of a coating on the function of a metallic nanostructure. This is particularly important in the field of catalysis, where accessibility of the metallic surface is of key importance. Testing the catalytic activity of clean and silicacoated Pd particles was thus a way of indirect characterization of the coating by determining its influence on the functionality. Particles produced following approach II, with and without the addition of the silica precursor, were collected on a polycarbonate filter. The sample size was in the microgram range to limit the turnover of the reaction. The filter sample was introduced into a simple catalytic reactor heated to a constant temperature of 20 °C.23 Under these conditions, sintering of the pure Pd particles does not occur, thus ensuring similar particle sizes of both catalysts and good comparability of the turnover frequency (TOF) determined. The hydrogenation of ethene was chosen as a test reaction. The reacting gases hydrogen and ethene were kept at concentrations of 2% in nitrogen as a balance gas, flowing through the filter. The conversion rate was determined by gas analysis and a TOF was calculated based on the available surface area of the sample (see Table 1). The specific surface area was determined from TEM image analysis, and the total surface area was calculated from the catalyst mass. The TOF was reduced by 23% for the silica coated particles compared to the clean Pd surface. Considering the enormous loss of surface area at higher reaction temperatures without structural stabilization, this seems tolerable. The TEM images show that a closed layer of silica formed on the particles. This indicates that diffusion of ethylene and hydrogen is possible through the coating, allowing access to the Pd surface. This observation is supported by the literature, where diffusion across much thicker layers of silica was observed.12
Seipenbusch and Binder Conclusions Structural stabilization of Pd nanoparticle ensembles was shown to be possible using silica coatings. While stabilization against coalescence of the primary particles was possible by coating the primary particles before ensemble formation, stabilization of the structure against rearrangement of the primary particles required the coating of the structure as a whole. A fit of the GPLE model at 400 °C and varied residence time showed good agreement with the experimental data. Although the time scale of the aerosol experiments was limited, the kinetics derived thereof could be extrapolated to higher temperatures and particle sizes. The structural stabilization by surface coating leads to a loss in available metal surface area, as could be shown in catalytic experiments. This loss of about one-fourth of the catalytic activity, however, is small compared to the potential loss in surface area at higher reaction temperatures by sintering, which is prevented by the coating. Acknowledgment. The authors acknowledge the experimental support by Hongyan Shi and the help of Wilfried Send with the EDX analysis. References and Notes (1) Wynblatt, P.; Gjostein, N. A. Acta Metall. 1976, 24, 1165. (2) Bartholomew, C. H. Appl. Catal., A 2001, 212, 17. (3) Defay, R.; Prigogine, I. Surface Tension and Adsorption; Wiley: New York, 1966. (4) Weber, A. P.; Friedlander, S. K. J. Aerosol Sci. 1997, 28, 179. (5) Seipenbusch, M.; Weber, A. P.; Schiel, A.; Kasper, G. J. Aerosol Sci. 2003, 34, 1699. (6) Datye, A. K.; Xu, Q.; Kharas, K. C.; McCarty, J. M. Catal. Today 2006, 111, 59. (7) Sault, A. G.; Tikare, V. J. Catal. 2002, 211, 19. (8) Gabaldon, J. P.; Bore, M.; Datye, A. K. Top. Catal. 2007, 44, 253. (9) Hayashi, K.; Horiuchi, T.; Suzuki, K.; Mori, T. Catal. Lett. 2002, 78, 43. (10) Gennari, F. C.; Pasquevich, D. M. J Mater. Sci. 1998, 33, 1571. (11) Ghosh, S. K.; Vasudevan, A. K.; Prabhakar Rao, P.; Warrier, K. G. K. Br. Ceram. Trans. 2001, 100, 151. (12) Park, J.-N.; Forman, A. J.; Tang, W.; Cheng, J.; Hu, Y.-S.; Lin, H.; McFarland, E. W. Small 2008, 4, 1694. (13) Ma, Z.; Brown, S.; Howe, J. Y.; Overbury, S. H.; Dai, S. J. Phys. Chem. C 2008, 112, 9448. (14) Kingery, W. D. Introduction to Ceramics; Wiley: New York, 1960. (15) Schwyn, S.; Garwin, E.; Schmidt-Ott, A. J. Aerosol Sci. 1988, 19, 639. (16) Fissan, H. J.; Helsper, C.; Thielen, H. J. J. Aerosol Sci. 1983, 14, 354. (17) Finsy, R. Langmuir 2004, 20, 2975. (18) Granqvist, C. G.; Buhrman, R. A. J. Catal. 1976, 42, 477. (19) Wanke, S. E. J. Catal. 1977, 46, 234. 1980, 13, 141. (20) Fuentes, G. A. Appl. Catal. 1985, 15, 33. (21) Vicente-Rodriguez, M. A.; Suarez, M.; Banares-Munoz, M. A.; Lopez-Gonzalez, J. D. Spectrochim. Acta, Part A 1996, 52, 1685. (22) Ricol, S.; Vernaz, E.; Barboux, P. J. Sol-Gel Sci. Technol. 1995, 8, 229. (23) Binder, A.; Seipenbusch, M.; Muhler, M.; Kasper, G. J. Catal., 2009, 268, 150.
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