Sputtering onto Liquids: From Thin Films to Nanoparticles - The

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Sputtering onto Liquids: From Thin Films to Nanoparticles Heberton Wender,†,‡,* Renato V. Gonc-alves,† Adriano F. Feil,† Pedro Migowski,§ Fernanda S. Poletto,§ Adriana R. Pohlmann,§ Jairton Dupont,§ and Sergio R. Teixeira†,* †

Universidade Federal do Rio Grande do Sul, Instituto de Física, Av. Bento Gonc-alves, 9500, P.O. Box 15051, CEP 91501-970, Porto Alegre, RS, Brazil ‡ Brazilian Syncrotron Light Laboratory (LNLS), R. Giuseppe Maximo Scolfaro, 10.000, P.O. Box 6192, CEP 13083-970, Campinas, Brazil § Universidade Federal do Rio Grande do Sul, Instituto de Química, Av. Bento Gonc-alves, 9500, P.O.Box 15003, CEP 91501-970, Porto Alegre, RS, Brazil ABSTRACT: Herein, we describe the influence of discharge voltage on the formation of colloidal silver nanoparticles and/or thin films during sputtering onto castor and canola oil, two wellknown vegetable oils, and also onto the synthetic medium chain caprylic/capric triglyceride oil. Stable spherical AgNPs of small sizes and size distributions were formed on castor oil in a large discharge voltage interval. The size of the formed NPs was shown to increase with discharge voltage increment. However, by performing sputtering under the same conditions (320 V, 150 s), metallic thin films were formed on canola and caprylic/ capric triglyceride oils; and spherical nanoparticles on castor oil. The increase in the discharge voltage stopped film formation and started nanoparticle formation. Thin films were predominant for low coordinating oils and low discharge voltages. The mechanisms for the formation of nanoparticles or thin films by means of sputtering onto liquid substrates are discussed in details for the first time.

1. INTRODUCTION Colloidal metal nanoparticles (MNPs) have recently attracted significant attention from the scientific community as they promise to play an important role in developing new nanotechnology approaches.1
4 Because of their unique physicochemical properties, they are potential materials for a variety of applications such as nonlinear optics,5,6 luminescence,7 catalysis,8,9 solar energy conversion,10,11 electronics and optoelectronics,12,13 and biomedicine.14
16 In particular, the use of MNPs synthesized and stabilized in environmentally and/or biocompatible fluids is essential for special applications such as in the fields of biology, biomedicine, cosmetology, or the varnish and paint industry.17,18 In most cases, MNP solutions are obtained chemically by decomposition or reduction of metal compounds by employing external reducing agents and solvents, which may be associated with environmental and biological risks.19
22 Except for a few reports, 23
25 it is difficult to find fully environmentally friendly methods for MNP synthesis, stabilization, and their use in solution. In this view, a new route to synthesize biocompatible MNPs by direct sputtering deposition onto a liquid was described.26 In past years, sputtering was basically focused on the synthesis of thin films onto solid substrates such as silicon or glasses.27 The use of liquid substrates for sputtering only started after 1996 with silicone oil28 subject mainly to the formation of flexible thin films (TFs). This procedure was extended to ionic liquids (ILs)29
32 r 2011 American Chemical Society

and vegetable and biocompatible oils26 to the formation of stabilizer-free MNPs. However, as the sputtering process is performed under low pressures, a liquid needs to possess low vapor pressures to be introduced inside sputter chambers. Furthermore, sputter deposition has been shown to be a versatile process to prepare NPs without the addition of any external chemical reagent such as reducing or stabilizing agents.26,30,33
35 Since the first reports on the formation of thin films15,36
40 and NPs41,42 by sputtering onto liquids, no comparison or insights into the phenomenon involving their formation has been carried out. Therefore, we report herein the synthesis of biocompatible colloidal silver nanoparticles (AgNPs) and/or Ag thin films (AgTFs) through direct sputtering deposition onto castor, canola, and caprylic/capric triglyceride (CCT) oils. The influence of the discharge voltage and liquid surface coordination ability on the formation of either NPs or TFs is discussed in detail.

2. EXPERIMENTAL SECTION 2.1. NPs Preparation. All samples were obtained by dc-magnetron sputtering of an Ag target separated by 50 mm from Received: June 8, 2011 Revised: July 18, 2011 Published: July 20, 2011 16362

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The Journal of Physical Chemistry C Scheme 1. General Structure and Composition of the Vegetable Oils Applied As the Liquid Substrate for the Sputtering of Ag

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2.3. SAXS Experiments and Data Analysis. SAXS experiments were also performed in the as-prepared AgNP-oil solutions at the D02B-SAXS2 beamline of the Brazilian Synchrotron Light Laboratory (LNLS), with λ = 1.488 Å and 0.05 < q < 3.33 nm
1, where q = 4π.sin(2θ)/λ; q is the scattering vector, θ is the scattering angle, and λ is the X-ray wavelength. The colloidal solutions were injected through a syringe into a cell with mica windows specifically designed for liquids. The fitting procedures of the experimental data were made using the SASfit program, which uses the least-squares fitting approach, consisting of minimizing the squared chi (χ2). The SASfit software package was developed by J. Kohlbrecher and is available free of charge.44 In a typical scattering of X-rays, the absolute scattering intensity I(q) of a multiparticle system can be written as follow (eq 1):45

IðqÞ ¼

a cylindrical glass Petri plate (3 cm diameter  1 cm height) containing 1 mL of oil. The sputtering machine, parameters, and materials were almost the same as previously reported by our group.26,30,43 Briefly, the camera was evacuated to ∼10
2 Pa, and Ar 0 gas was admitted in until the system stabilized at a pressure of 2 Pa. Different discharge voltages ranging from 240 (2.4 W) to 490 V (54 W) were applied between the cathode and the anode for 150 s. Scheme 1 shows the main fatty acid composition of the three oils studied herein, all of pharmaceutical grade and pre-evacuated for ∼3 h at room temperature before sputtering. It is important to note that these oils demonstrated sufficiently low vapor pressure (up to 10
2 Pa) allowing sputtering to be performed and did not show structural changes after sputtering, as observed by Fourier transform infrared spectroscopy (FTIR) and high-performance liquid chromatography (HPLC) (results not shown). 2.2. General Characterization. UV
vis spectroscopy was used to monitor the formation and, qualitatively, the size of AgNPs by observation of the localized surface plasmon resonance (LSPR) peak position. The UV
vis extinction spectra were measured in a Varian Cary 100 spectrophotometer in the asprepared AgNP-oil colloidal samples using 1 mm quartz cuvettes. The NP shape, size and size distribution were finely characterized by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Samples for TEM were prepared by dispersion of the as-synthesized colloids in acetone at room temperature. After dispersion, a few drops of the suspension were deposited on a 400 mesh carbon-coated copper grid. The histograms of the AgNPs were obtained from the measurement of more than 500 particles and were reproduced in different regions of the Cu grid, found in arbitrarily chosen areas of enlarged micrographs. The thin films were observed by an electronic loupe and photographed right after the sputtering procedure.

N 2 2 Δ V SðqÞ 3 PðqÞ V 3 F3 P3

ð1Þ

In this equation, N/V is the number of dispersed particles per unit volume in the sample. ΔF is the excess electron density which is defined as the difference between the electron density of the particles and that of the surrounding medium. VP is the volume of the particle, P(q) is the form factor which characterizes single particle scattering, and S(q) is the structure factor that describes the interactions between the particles. Therefore, the measured intensity represents the sum of the scattering intensity from particles of various sizes. For monodispersed spheres, the form factor can be calculated as in eq 2, where R is the radius of the particle.45 " #2 sinðqRÞ
qR cos ðqRÞ PðqÞ ¼ 3 ð2Þ ðqRÞ3 However, in practice, most colloidal suspensions are not monodispersed and it is necessity to take into account a distribution function. In our case, all fittings were performed supposing spherical particles with a log-normal distribution (eq 3), as showed previously for colloidal AgNPs.46,47 logNormðRÞ ¼

2 2 N expðln R
ln μÞ =2ln p R

σ

ð3Þ

For highly diluted suspensions of nonaggregated particles, S(q) can be taken as unity. When it not holds, that is, when particles interact with each other, a structure factor needs to be considered. There are several theoretical ways to account S(q). Herein, all fittings have considered a hard spheres structure factor by using the monodisperse approach, where it is assumed that the interaction potential between particles are spherical symmetric and independent of the particle size, so that the integral is carried out simply by multiplying the size averaged form factor with the structure factor. In the hard sphere model, the potential energy is considered to be infinite for r < a and zero for r > a. The two parameters which were considered for P(q) were the hard sphere repulsion radius (RHS), which is linked with the structure correlation of the particles, and the volume fraction (fp).44 If the NPs do form aggregates, a new gyration radius, in addition to the radius of the particles, will be necessary to fit the scattering intensity. These aggregates will account in the low q region and small particles in the high q region. Here, it was used a mass fractal named MassFractGauss to adjust the low q region of 16363

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Figure 1. UV
vis absorption spectrum (a), representative TEM image (b), and size distribution (c) of the AgNPs containing a colloidal solution obtained by sputtering at 320 V (13 W) for 150 s onto castor oil.

the scattering curves. Additional information’s and equations can be seemed in the SASfit manual free of charge.44

3. RESULTS After sputtering of Ag at 320 V (13 W) onto the castor oil, AgNPs were formed as observed by the LSPR peak at 450 nm in the UV
vis extinction spectrum, characteristic for AgNPs of few nanometers in size, part a of Figure 1. TEM analysis showed that the NPs are spherical with mean diameter of 5.5 ( 1.0 nm, as seen in part b of Figure 1. However, it was not observed by performing sputtering under the same condition on canola and CCT oils. In these cases, AgTFs were formed on the surface of the two oils (parts a and b of Figure 2). Furthermore, very small quantities of AgNPs were found below the AgTF layer in the canola oil, as can be seen in the TEM and UV
vis analysis (parts c and d of Figure 2); meanwhile, no detectable NPs were observed in CCT. Although AgNPs could be observed by TEM in the canola oil, the absolute quantity of NPs was negligible when compared to the amount that formed in the castor oil under the same sputtering conditions (absorption jumps in the UV
vis spectra). The obtained Ag films showed significant differences between them. Both films presented cracks; however, these were of different sizes, because the AgTFs built up on the canola oil in a less homogeneous fashion than those obtained with the CCT oil. These films were probably formed by the coalescence of atomic Ag clusters during and immediately after the sputtering process, as shown before in the formation of Fe disk-shaped patterns on silicone oil.48 To better understand this interesting phenomenon, sputtering was performed using discharge voltages higher and lower than 320 V. For castor oil, the AgTFs were only formed when very low discharge voltages were used (240 V, 2.4 W), otherwise (from 280 to 490 V), AgNPs were observed, as indicated in the UV
vis absorption spectra and SAXS analysis (Figure 3). The fitting of the SAXS curves showed that the AgNPs were mainly spherical, presenting a log-normal size distribution with

Figure 2. Top-view optical photographs in bright field of the AgTFs formed by sputtering at 320 V for 150 s on the surface of canola (a) or CCT oil (b). TEM image (c) and UV
vis extinction spectrum (d) of the small quantity of AgNPs observed in the canola oil below the AgTF layer that covered its surface. (a) and (b) are in the same scale.

mean diameters of 3.8 nm (σ = 1.5), 3.7 nm (σ = 1.4), 6.2 nm (σ = 1.1), and 6.0 nm (σ = 1.4) nm for sputtering at 280, 338, 400, and 490 V, respectively (part c of Figure 3). In addition, the maximum of the LSPR peak characteristic of AgNPs was slightly shifted from 452 to 463 nm with an increase in the discharge voltage, what can only correspond to an increase in the mean size of the AgNPs, because the other parameters governing the LSPR peak were all the same. Therefore, it was clear that the mean size of the AgNPs augmented with an increase in discharge voltage. Similar behavior was recently described for gold nanoparticles 16364

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Figure 3. AgNPs obtained in the castor oil after sputtering of Ag for 150 s at different discharge voltages. Extinction spectra (a), SAXS data and fittings (b), and size distributions (c). The mean size of the formed AgNPs showed a tendency to increase as the discharge voltage increased.

Table 1. Summary of AgNPs or AgTFs Formation As a Function of the Discharge Voltage and of the Oil Chosen As the Liquid Substrate oil

Figure 4. Extinction spectra of canola oil after sputtering of an Ag target at 350 and 420 V for 150 s (a). TEM image and histogram of AgNPs obtained by sputtering at 420 V for 150 s onto canola oil (b).

(AuNPs) prepared by sputtering of Au onto castor oil.26 Moreover, it should be noted that, as opposed to previously reported

AgTFs formation (V)

AgNPs formation (V)

castor

240

g280

canola

240 to 320

g420

CCT

240 to 420

g490

AuNPs, the SAXS curves of the AgNPs stabilized into castor oil clearly showed the presence of a structure factor, that is the AgNPs were significantly correlated in solution as also could be seem by TEM (part a of Figure 1). This indicates that castor oil could not stabilize AgNPs as it could AuNPs,26 showing that the stabilization mechanisms for the two metals are significantly different. The AuNPs were well dispersed and much more stable on castor oil without precipitation for ∼8 months, whereas AgNPs started to precipitate after ∼3 months. In a contrasting way, after increasing the discharge voltage from 240 to 350 V, the canola oil surface was always covered by an AgTF layer. However, by sputtering at 420 V (38 W), spherical AgNPs with a bimodal size distribution and mean diameters of 1.7 nm (σ = 0.3 nm) and 5.5 nm (σ = 1.6 nm) were observed, as can be seen by TEM in Figure 4. A mechanism for the formation of bimodal size distributions was recently discussed in the literature suggesting aggregation of smaller particles to form the bigger ones.49 By varying the discharge voltage onto CCT oil, similar results as those from canola oil were observed and the AgNPs were only formed when potentials slightly higher (g490 V) than in the canola oil were applied and, even so, a considerable amount of AgTF was formed at the same time. These results are summarized in Table 1. 16365

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Figure 5. Schematic illustration for the formation of AgNPs or AgTFs by sputtering deposition of Ag onto liquids.

The different behavior for sputtering of Ag onto these three oils could be explained by the different fatty chain compositions of these compounds (Scheme 1). After being sputtered from the target, the Ag atoms arrive onto the liquid surface and interact with the functional groups present in the liquid/gas phase (Figure 5), exactly as showed recently for the formation of Au nanodisks by sputtering onto a nitrile functionalized ionic liquid.43 The surface composition of the liquid phase is dependent on the polarity of the gas phase and the bulk chemical composition. It is expected that the most apolar moieties of the oils, that is the fatty acid-derived chains, will predominantly populate the outmost surface of the oil. Because of the presence of hydroxyl groups on ricinoleic acid-derived fatty chains, the coordination ability of the surface of the castor oil should be the greatest of all three used oils, followed by canola oil, which is composed predominantly of unsaturated aliphatic chains, and by CCT oil, which is almost uncoordinating because it is composed solely of saturated alkyl chains. Moreover, the more coordinating the liquid surface, the less the diffusion of the atoms/nucleus/ nanoclusters on the oil surface will be due to the binding between the Ag species and the functional groups. If the formed nuclei or NPs can easily diffuse across the liquid surface, their probability of colliding with each other and to grow forming a thin film increases. Therefore, poorly coordinating surfaces might lead to TFs and strongly coordinating surfaces will probably stabilize the NPs against coalescence. A schematic illustration of the formation of AgNPs or AgTFs can be seen in Figure 5. Actually, the surface composition is not the only factor that governs the formation of NPs or TFs. The discharge voltage and the composition of the sputtered atoms also determine the formation of stable colloidal NPs or TFs. It was recently reported that sputtering of Ag onto the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate (EMI.EtSO3) formed stable NPs a few tens of nanometers in diameter, whereas sputtering of chromium tended to form a film layer.50 Moreover, the increase in the discharge voltage enhances the diffusivity of the atoms at the oil

surface, a parameter that is complementary to the surface coordination ability of the oils. More specifically, the increase in the discharge voltage enhances the diffusivity of the sputtered atoms (red arrows in Figure 5) on the oil surface, accelerating the kinetics of nucleation and growth and directing the formation of colloidal AgNPs. In this case, after a certain critical concentration of metallic atoms, the formed NPs continuously precipitated to the bulk liquid phase, renewing the atomic nucleation and growth of NPs on the oil surface. This last hypothesis is in agreement with previous reports on the formation of thin films on liquid surfaces both by sputtering or thermal evaporation of metals.48,51,52 In others words, when the atomic diffusion rate at the oil surface is relatively low, the dominant parameter is the surface coordination ability of the liquid, and for high diffusion rates, a balance between the two parameters needs to be taken into account. This explains why the discharge voltage necessary to form colloidal NPs must be higher as the coordination ability of the oils surface decreases. Additionally, the morphology of the films was dependent on the oil surface composition, reinforcing the importance of the liquid surface composition when performing sputtering onto liquids, as predicted for sputtering onto ionic liquids.30,43 Moreover, under the same sputtering conditions, colloidal NPs or TFs can be formed just by tuning the liquid surface composition. Much beyond that, special functionalities can be easily added to ionic liquids and even to synthetic oils by means of functionalization with acid, alcohol, nitrile, or thiol groups for example, so that anisotropies are induced on the NPs, as observed earlier.43

’ CONCLUSIONS Stable spherical AgNPs of mean sizes smaller than 6 nm were formed by sputtering onto castor oil. The mean size of the NPs was shown to augment with increase of discharge voltage, both confirmed by UV
vis and SAXS analyses. For the less coordinating oils, AgTFs instead of AgNPs were formed; however, it depended not only on the oil’s surface coordination ability but 16366

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The Journal of Physical Chemistry C also on the discharge voltage. Because of the presence of hydroxyl groups in the castor oil, AgTFs were formed only for discharge voltages less than 280 V; meanwhile, for canola and CCT oils, films were observed even at 350 V. Moreover, the lower the oil surface coordination ability, the higher the discharge voltage necessary to form colloidal NPs. The final morphology of the AgTFs was shown to be dependent on the composition of the oil surface, reinforcing its importance when performing sputtering onto liquids. This study has provided the first description and discussion on the formation of NPs or TFs by means of physical vapor deposition onto liquids.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (H.W.), [email protected] (S.R.T.).

’ ACKNOWLEDGMENT Thanks are due to the CNPq and CAPES funding agencies and also to LNLS for the D02B-SAXS2 beamline (proposal 9368 and 11041) and LME-UFRGS for the TEM and sputtering machine. ’ REFERENCES (1) Schmid, G. Endvr 1990, 14, 172. (2) Dupont, J.; Scholten, J. D. Chem. Soc. Rev. 2010, 39, 1780. (3) Roucoux, A.; Schulz, J.; Patin, H. Chem. Rev. 2002, 102, 3757. (4) Wender, H.; Andreazza, M. L.; Correia, R. R. B.; Teixeira, S. R.; Dupont, J. Nanoscale 2011, 3, 1240. (5) Trindade, T.; O’Brien, P.; Pickett, N. L. Chem. Mater. 2001, 13, 3843. (6) Cui, F.; Hua, Z.; He, Q.; Li, J.; Guo, L.; Cui, X.; Jiang, P.; Wei, C.; Huang, W.; Bu, W.; Shi, J. J. Opt. Soc. Am. B 2009, 26, 107. (7) Chen, Y. F.; Rosenzweig, Z. Anal. Chem. 2002, 74, 5132. (8) Castro, E. G.; Salvatierra, R. V.; Schreiner, W. H.; Oliveira, M. M.; Zarbin, A. J. G. Chem. Mater. 2010, 22, 360. (9) Migowski, P.; Zanchet, D.; Machado, G.; Gelesky, M. A.; Teixeira, S. R.; Dupont, J. Phys. Chem. Chem. Phys. 2010, 12, 6826. (10) Feil, A. F.; Migowski, P.; Scheffer, F. R.; Pierozan, M. D.; Corsetti, R. R.; Rodrigues, M.; Pezzi, R. P.; Machado, G.; Amaral, L.; Teixeira, S. R.; Weibel, D. E.; Dupont, J. J. Braz. Chem. Soc. 2010, 21, 1359. (11) Wender, H.; Feil, A. F.; Diaz, L. B.; Ribeiro, C. S.; Machado, G. J.; Migowski, P.; Weibel, D. E.; Dupont, J.; Teixeira, S. R. ACS Appl. Mater. Interfaces 2011, 3, 1359. (12) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (13) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18. (14) Gomes, A. J.; Faustino, A. S.; Lunardi, C. N.; Lunardi, L. O.; Machado, A. E. H. Int. J. Pharm. 2007, 332, 153. (15) Pohlmann, A. R.; Mezzalira, G.; Venturini, C. D.; Cruza, L.; Bernardi, A.; Jager, E.; Battastini, A. M. O.; da Silveira, N. P.; Guterres, S. S. Int. J. Pharm. 2008, 359, 288. (16) Bernardi, A.; Braganhol, E.; Jager, E.; Figueiro, F.; Edelweiss, M. I.; Pohlmann, A. R.; Guterres, S. S.; Battastini, A. M. O. Cancer Lett. 2009, 281, 53. (17) Sperling, R. A.; Rivera Gil, P.; Zhang, F.; Zanella, M.; Parak, W. J. Chem. Soc. Rev. 2008, 37, 1896. (18) Rosi, N. L.; Mirkin, C. A. Chem. Rev. 2005, 105, 1547. (19) Bardaji, M.; Uznanski, P.; Aniens, C.; Chaudret, B.; Laguna, A. Chem. Commun. 2002, 598. (20) Dash, P.; Scott, R. W. J. Chem. Commun. 2009, 812. (21) Redel, E.; Walter, M.; Thomann, R.; Hussein, L.; Kruger, M.; Janiak, C. Chem. Commun. 46, 1159.

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