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Effect of Hydrogen Pressure on the Size of Nickel Nanoparticles Formed during Dewetting and Reduction of Thin Nickel Films Alexandre Geissler,†,‡ Maoshuai He,§ Jean-Michel Benoit,| and Pierre Petit*,† Institut Charles Sadron, CNRS - UdS, BP 84047, 23 rue du Loess, 67034 Strasbourg, France, Institut de Science des Mate´riaux de Mulhouse, C.N.R.S - UHA, 15 rue Jean Starcky, 68057 Mulhouse, France, Laboratory of Industrial Chemistry, Helsinki UniVersity of Technology, P.O. Box 5100, Puumiehenkuja 2, FIN-02150 Espoo, Finland, and Laboratoire de Physique de la Matie`re Condense´e et Nanostructures, UniVersite´ Lyon 1, CNRS, UMR 5586, Domaine Scientifique de la Doua, 69622 Villeurbanne, France ReceiVed: September 1, 2009; ReVised Manuscript ReceiVed: NoVember 12, 2009
We report a study of the formation of nickel nanoparticles by the dewetting of nickel metal films under different reduction conditions. The effects of both film thickness and temperature of dewetting on the size distributions of Ni nanoparticles are in agreement with previously reported studies. However, our work evidences that the hydrogen pressure applied during the reduction process has a drastic influence on the size of the formed Ni nanoparticles, the hydrogen pressure effect being all the more important that the film thickness is small. We interpret this effect by the formation of nickel metal hydride displacing the surface free energy balance of the system in favor of the dewetting. Moreover, the replacement of hydrogen by an inert gas allows us to rule out Ostwald’s ripening as the mechanism originating the sintering of the nanoparticles. Introduction Presently, it is generally admitted that the diameter of single or multiwalled carbon nanotubes synthesized by chemical vapor deposition (CVD), is correlated to the size of the metal nanoparticles which catalyze their growth.1-4 The challenge is then to find a way to synthesize metal nanoparticles with a very narrow size distribution which will hopefully catalyze the growth of monosize distribution of carbon nanotubes. As a consequence, a great deal of research is devoted to the synthesis of very small nanoparticles because, from simple statistics, the smaller the average size of the particles, the narrower their size distributions. One route used for the formation of supported catalytic particles is the dewetting of thin films of transition metals (Ni, Co, Fe, etc.) by thermal treatments. This route is widely used for high yield synthesis of multiwalled carbon nanotubes that are used for examples in polymer composite materials5 or field emission devices.6 Generally, thin metal films are obtained deposing metals onto substrates such as silicon wafers covered by an oxide layer, using more or less sophisticated techniques such as sputtering, physical vapor deposition, or molecular beam epitaxy (MBE). Samples are then transferred into a reactor in which dewetting and CVD are performed. During the transfer, thin metal films are contaminated by ambient oxygen, and a reduction step is always necessary to transform the metal oxide nanoparticles into their metallic catalytic active composition. Upon heating, thin metal films dewet principally because of the modification of the surface free energy whose minimization leads to the formation of small islands, which sinter due to ripening or diffusion. Under reducing conditions, thin film dewetting is favored because metals have a weaker interaction * To whom correspondence should be addressed. E-mail: petit@ ics.u-strasbg.fr; tel: + 33 3 88 41 41 53; fax: + 33 3 88 41 40 99. † Institut Charles Sadron. ‡ Institut de Science des Mate´riaux de Mulhouse. § Helsinki University of Technology. | Universite´ Lyon 1.
with oxidized substrates than with metal oxides.7 This effect has recently been shown and discussed in the case of subnanometer Fe and Al/Fe/Al thin films.8 As reduction is more efficient at high temperature, and in order to stabilize nanoparticles against excessive sintering for CVD synthesis, dewetting and reduction of the metal films are often realized at the same time and thus at the same temperature. Experimental Methods We used silicon wafers SiO2/Si with an oxide thickness of 25 nm. Thin Ni films have been prepared by vapor deposition in an ultra high vacuum MBE system at 10-9 Torr at room temperature. The voltage and current of the electron gun were of 8 kV and 50 mA, respectively. The deposition rate was 0.05 Å · s-1 controlled using a quartz microbalance. Ni films of various thicknesses were realized, from a monatomic layer to 3 nm. The Ni nanoparticles were formed by placing the sample in a quartz tube inserted in a temperature-controlled furnace under primary vacuum (10-2 Torr). At the start, the sample sits in the nonheated part of the tube. A hydrogen flux of 300 mL/min was established, and the pressure regulated at the desired value for the reduction. Once the hydrogen pressure was stabilized, the sample was pushed into the furnace with an in-tube movement. Reduction reactions were performed at 600, 750, and 900 °C for each Ni film thickness, while for each temperature two hydrogen pressures (2.5 and 20 Torr) were applied. The reduction treatments were performed during 10 min for all runs. All surfaces were investigated using a Nanoscope III atomic force microscope (AFM, Digital Instruments) in tapping mode. The root-mean-square roughness of the bare wafers measured by AFM on a 1 × 1 micrometer image was found to be about 0.2 nm. The lateral size of the particle is convoluted by the AFM tip, which has a typical radius of 10 nm. Thus, the apparent diameter of the particles is overestimated. For those
10.1021/jp908427r 2010 American Chemical Society Published on Web 12/02/2009
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Figure 1. AFM images of Ni nanoparticles formed from a Ni film of 1.5 nm thickness at PH2 ) 20 Torr, at different temperatures: (a) 600 °C, (b) 750 °C, (c) 900 °C. Image dimensions: 1 µm × 1 µm; z-scale: 15 nm (a), 20 nm (b), 60 nm (c).
Figure 2. AFM images of Ni nanoparticles formed at 750 °C from thin films of different thicknesses: (a) 1 nm, (b) 1.5 nm, (c) 2 nm, and (d) 3 nm. Image dimensions: 1 µm × 1 µm; z-scale: 20 nm (a,b), 30 nm (c), 120 nm (d).
Figure 3. Heights (a) and apparent diameters (b) of Ni nanoparticles formed from Ni films of different thicknesses at 750 °C and PH2 ) 20 Torr.
whose diameters are comparable to the tip radius or smaller, only their heights are accurate. For each sample, the uniformity of the dewetting was checked by imaging the samples in three different spots separated by macroscopic distances (typically a few millimeters) and on two different scales. The reproducibility of the results was checked by repeating the same experiments and AFM analysis on two sets of samples prepared separately. Each image shown is representative of a collection of six images. Results To investigate the effects of temperature and film thickness on the dewetting of thin films under partial pressure of hydrogen (PH2), reductions were performed at PH2 ) 20 Torr, which is the partial pressure of hydrogen generally used for the reduction process of thin metallic films.9-11 AFM images of Ni nanoparticles formed by dewetting of a Ni thin film under hydrogen display a collection of dense, individual or aggregated, small islands of circular shape with a relatively broad size distribution. The effects of the reduction temperature and film thickness on the formation of Ni nanoparticles are shown in Figures 1 and 2, respectively. AFM images were analyzed to estimate both particle diameters and height distributions for each reduction temperature and pristine film thickness (Figure 3a,b). The above experiments and image analysis show that the increase of both reduction temperature and film thickness leads
Figure 4. AFM images of Ni particles formed from a monatomic thin film treated at 600 °C at different hydrogen pressures: (a) PH2 ) 2.5 Torr, (b) PH2 ) 20 Torr. Image dimensions: 1 µm × 1 µm (a), 10 µm × 10 µm (b); z-scale: 10 nm (a), 40 nm (b).
to a noticeable and continuous increase in particle sizes accompanied by a decrease in their spatial density. These results are in perfect agreement with previously reported results on the reduction temperature effect12 and film thickness effect.10,11 The observed increase in the particle size is accompanied by an increase in their distribution width, suggesting that the same will occur for the distribution width of carbon nanotubes grown from these particles. In order to form nanoparticles of smaller sizes and narrower distribution width, reduction treatments have been performed on very thin Ni films of one monatomic layer and of 0.5 nm thickness. For both films, opposite to what is expected, we observed that the reduction treatment at 600 °C and PH2 ) 20 Torr leads to a very small density of very large particles. To check if this result is due to the pressure of hydrogen during the reduction treatment, the same experiment has been carried out at 600 °C and PH2 ) 2.5 Torr.13 Comparison of AFM images of both experiments (Figure 4) shows that the change in partial pressure of hydrogen has a drastic effect on the size of the formed nanoparticles. Indeed, at PH2 ) 2.5 Torr a high spatial density of small particles is observed, whereas at PH2 ) 20 Torr a low denstity of very large particles is observed. The same experiment was carried on using argon to check whether the observed difference was caused by a thermodynamic effect. Figure 5 shows that, if a small increase in the particle mean size is observed by changing the pressure of an inert gas, it is far less than the result observed using hydrogen, suggesting that the effect observed using hydrogen is not purely due to a difference of gas pressure.
Effect of H2 Pressure on the Size of Ni Nanoparticles
Figure 5. AFM images of Ni particles formed from a monatomic thin film treated at 600 °C at different argon pressures: (a) PAr ) 2.5 Torr, (b) PAr ) 20 Torr. Image dimensions: 1 µm × 1 µm; z-scale: 15 nm.
J. Phys. Chem. C, Vol. 114, No. 1, 2010 91 features of different sizes: adatoms diffusing from particles of large curvature to those of small curvature. In other words, large particles grow to the detriment of small ones, which progressively disappear. Particle diffusion occurs because of the fluctuations of their shape, which lead to a random displacement of their center of mass. These fluctuations are induced by the evaporation/ condensation of atoms onto the particle, migration of vacancies inside the particle, and atom migration onto the surface. The temperature dependence of the coefficient of diffusion of a particle can be expressed as
( )
D ≈ exp These experiments at PH2 ) 2.5 Torr have also been performed on thin films of 0.5, 1, 1.5, 2, and 3 nm, and we observed that the effect of the partial pressure of hydrogen on the size of the particles decreases with increasing the film thickness, becoming negligible above 1 nm. Discussion The wetting condition for an adsorbate on a substrate is expressed by the spreading parameter S, the sign of which determines whether the interface is thermodynamically stable (S > 0, wetting) or not (S < 0, dewetting):14
S ) σs-(σa + σi) where σs, σa, and σi are, respectively, the free energy of the substrate/vapor, adsorbate/vapor, and of the adsorbate/substrate interfaces. Thus, wetting occurs if the free energy of the system is decreased with respect to that of the substrate alone. The effect of temperature on the size of the nanoparticles shows that their formation process is thermally activated, in agreement with the two mechanisms generally invoked to explain the growth of particles from a thin metal film.8,11,12,15 These mechanisms are Ostwald’s ripening and particle diffusion. In the case of Ostwald’s ripening, mobile atoms evaporate from small particles to condense on larger ones because of the difference in their curvature. Indeed, the vapor pressure of a metal in equilibrium with its condensed phase depends on the radius of curvature r of the particles and is expressed by the Gibbs-Thomson equation:
( )
eq Peq r ) P∞ exp
σaΩ rkBT
where P∞eq is the vapor pressure of a plane interface at equilibrium, Ω is the volume occupied by an atom in the condensed phase, and kB is Boltzmann’s constant. Without going into the details, this relation allows one to anticipate the general behavior of a system. Features of larger curvature (smaller radius of curvature) have the larger adatom vapor pressure at equilibrium, and a gradient of concentration is established between
-Ea -Γ N kBT
where Ea is the activation energy, N is the number of atoms of the particle, and Γ is a constant depending of the mechanism originating the diffusion.16 From this relation, one can note that, whatever the mechanism inducing the diffusion, the coefficient of diffusion is all the more important as the number of atoms of the particle is small. Hu¨ttig and Tamman temperatures are indicative of the temperature at which the diffusion of entire aggregates occurs.17 These temperatures are defined by semiempirical relations (in K): THu¨ttig ) 0.3Tmelting; TTamman ) 0.5Tmelting (for Ni, Tmelting ) 1453 °C, TTamman ) 590 °C, THu¨ttig ) 302 °C). When Hu¨ttig’s temperature is reached, atoms at the defects become mobile, whereas above Tamman’s temperature atoms of the volume are the more mobile. Finally, at the melting temperature, the mobility is so high that liquid phase behavior occurs. Actually, the temperature at which solid particles become mobile depends on their size, texture, and crystallinity, and for small particles their diffusion can be activated for temperatures lower than TTamman and THu¨ttig.18-20 In a typical setup, the gas (hydrogen or argon) flows into the reactor, and the surface vapor pressure of the particles is certainly not at equilibrium. Thus, the overall system cannot be considered at equilibrium, rendering the estimate of the real contribution of Ostwald’s ripening complicated. However, from our results, Ostwald’s ripening can be ruled out, as an increase of the gas pressure surrounding the system induces an increase in the number of collisions between the evaporated atoms and the molecules of the gas. As a consequence, the increase of the gas pressure should lead to a slowdown of the evaporation/ condensation mechanism and thus to a decrease of the size of the particles, opposite to what is observed. The difference in the formation of nanoparticles by thermal treatment of very thin films using hydrogen or argon flux (Figures 4 and 5) indicates that the difference in nanoparticle sizes at different partial pressures of gas is not due to a pure thermodynamic effect. A subnanometer untreated thin film is actually constituted of very small clusters of matter (Figure 6) instead of a uniform flat metal layer, because of the Volmer-Weber
Figure 6. AFM images of a bare substrate (a), and a monatomic Ni film before (b) and after reduction at 600 °C at PH2 ) 2.5 Torr (c). Image dimensions: 1 µm × 1 µm; z-scale: 6 nm (a,b); 10 nm (c).
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growth mode of subnanometer thin films on SiO2.12,21 Such a structure obviously exhibits a very high specific surface area, and the effect of hydrogen pressure on these aggregates is to be compared to the one observed for Raney (or skeleton) nickels,22,23 which are known for decades to dissociate and chemisorb hydrogen due to their very high specific surface areas.24 By analogy, we explain the increase in particle sizes when increasing PH2 by the high ability of nickel to chemisorb hydrogen to form nickel metal hydride NixHy, with the ratio y/x increasing with increasing PH2.25 This change in the chemical composition of the system naturally induces a modification of the free energy of the nanoparticle/vapor and of the nanoparticle/ substrate interfaces. From our results, we deduce that the increase of y/x in NixHy increases the free energy of the NixHy/ SiO2 system (σa + σi) and displaces the total energy balance in favor of the dewetting (S < 0), facilitating particle diffusion over the substrate. Moreover, the observation of a monatomic thin film before and after reduction (Figure 6b,c) clearly indicates an increase of the volume of the matter onto the substrate. This supports our interpretation of the formation of nickel metal hydride and is in agreement with what is observed for Raney nickels.24,25 This result accounts for the weak efficiency of nanotube nucleation on such materials.8 Conclusions The analysis of the thermal evolution of the size of Ni nanoparticles at different gas pressures shows that the sintering of the nanoparticles is not originated by Ostwald’s ripening but by the diffusion of the particles over the substrate. For subnanometer thin films, the nanoparticle size drastically increases with the increase of the partial pressure of hydrogen during the reduction treatment. By analogy with Raney nickels, this result strongly suggests a modification in the chemical composition of the nanoparticles with respect to that of the pristine film, changing from nickel metal to nickel metal hydride. This change leads to a modification of the surface free energy balance in favor of the dewetting of the films. Our results underline the necessity to take into account this parameter with the aim of improving the control of the catalyst nanoparticle size for the growth of carbon nanotubes. Acknowledgment. J. Arabski is thanked for the preparation of Ni films by MBE, and C. Contal is acknowledged for the AFM experiments. This work was supported by a CNRS
Geissler et al. postdoctoral fellowship (M.H.) and the Agence Nationale de la Recherche (Grant No. ANR-06-NANO-06-0025) and performed within the GDRI 2756 “Nano_E: Science and applications of nanotubes”. References and Notes (1) Ivanov, V.; Nagy, J. B.; Lambin, Ph.; Lucas, A.; Zhang, X. B.; Zhang, X. F.; Bemaerts, D.; Van Tendeloo, G.; Amelinckx, S.; Van Landuyt, J. Chem. Phys. Lett. 1994, 223, 329. (2) Li, V.; Kim, W.; Zhang, Y.; Rolandi, M.; Wang, D.; Dai, H. J. Phys. Chem. B 2001, 105, 11424–1143. (3) Cheung, C. L.; Kurtz, A.; Park, H.; Lieber, C. M. J. Phys. Chem. B 2002, 106, 2429–2433. (4) Nasibulin, A. G.; Pkhista, P.; Jiang, H.; Kauppinen, E. I. Carbon 2005, 43, 2251–2257. (5) Moniruzzaman, M.; I.; Winey, K. Macromolecules 2006, 39, 5194– 5202, and the references therein. (6) Cheng, Y.; Zhou, O. C. R. Phys. 2003, 4, 1021–1033, and the references therein. (7) Sushumma, I.; Ruckenstein, E. J. Catal. 1985, 94, 239. (8) Cantoro, M.; Hofmann, S.; Pisana, S.; Scardaci, V.; Parvez, A.; Ducati, C.; Ferrari, A. C.; Blackburn, A. M.; Wang, K.-Y.; Robertson, J. Nano Lett. 2006, 6, 1107–1112. (9) Ren, Z. F.; Huang, Z. P.; Xu, J. W.; Wang, J. H.; Bush, P.; Siegal, M. P.; Provencio, P. N. Science 1998, 282, 1105–1107. (10) Bower, C.; Zhou, O.; Zhu, W.; Werder, D. J.; Jin, S. Appl. Phys. Lett. 2000, 77, 2767–2769. (11) Chhowalla, M.; Teo, K. B. K.; Ducati, D.; Rupesinghe, N. L.; Amaratunga, G. A. J.; Ferrari, A. C.; Roy, D.; Robertson, J.; Milne, W. I. J. Appl. Phys. 2001, 90, 5308–5317. (12) Carey, J. D.; Ong, L. L.; Silva, S. R. P. Nanotechnology 2003, 14, 1223–1227. (13) Seidel, R.; Duesberg, G. S.; Unger, E.; Graham, A. P.; Liebau, M.; Kreupl, F. J. Phys. Chem. B 2004, 108, 1888–1893. (14) de Gennes, P. G. ReV. Mod. Phys. 1985, 57, 827–863. (15) Mougin, K.; Zheng, Z.; Piazzon, N.; Gnecco, E.; Haidara, H. J. Colloid Interface Sci. 2009, 333, 719–724. (16) Bogicevic, A.; Liu, S.; Jacobsen, J.; Lundqvist, B.; Metiu, H. Phys. ReV. B 1998, 57, R9459–R9462. (17) Moulijn, J. A.; van Diepen, A. E.; Kapteijn, F. Appl. Catal. A: Gen. 2001, 212, 3–16. (18) Smith, D. J. J. Vac. Sci. Technol. 1985, 3, 1563–1567. (19) Buffat, Ph.; Borel, J.-P. Phys. ReV. A 1976, 13, 2287–2298. (20) Iijima, S.; Ichihashi, T. Phys. ReV. Lett. 1986, 56, 616–619. (21) Somorjai, G. A. In Introduction to Surface Chemistry and Catalysis; Wiley & Sons: New York, 1994; Chapter 1. (22) Raney, M. US Patent 1.563.787, 1925. (23) Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, Germany, 1997. (24) Fouilloux, P. Appl. Catalysis 1983, 8, 1–42. (25) Renouprez, A. J.; Fouilloux, P.; Coudurier, G.; Tochetti, D.; Stockmeyer, R. J. Chem. Soc., Faraday Trans. I 1977, 73, 1–10.
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