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Tuning Nanoscale Friction on Pt Nanoparticles with Engineering of Organic Capping Layer Jeong Young Park* Graduate School of EEWS (WCU) and NanoCentury KI, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea ABSTRACT: Nanoscale friction and adhesion on Pt colloid nanoparticles coated with different organic capping layers were probed with atomic/friction force microscopy. Platinum colloid nanoparticles with four types of capping layers have been synthesized and used as model lubricant systems: TTAB (tetradecyltrimethylammonium bromide), HDA (hexadecylamine), HDT (hexadecylthiol), and PVP (poly(vinylpyrrolidone)). Two-dimensional arrays of colloid nanoparticles were prepared using the Langmuir-Blodgett method. We found that the friction and adhesion properties on colloid nanoparticles are lower than those on a silicon surface. The variation of friction when changing the capping layers is ∼30%, and it appears that the friction depends on the packing and ordering of the capping layers. Partial removal of the capping layers using ultraviolet light (UV)-ozone surface treatment resulted in increased friction. These results suggest a new method of tuning nanometer scale friction and adhesion by engineering organic capping layers on nanoparticles.
I. INTRODUCTION Controlling friction and adhesion at the nanometer scale are important issues for enhanced efficiency of nanoscale moving parts.1-3 Significant efforts have been made to minimize stiction and reduce adhesion and friction in micro- or nano-electromechanical systems (MEMS/NEMS). The strategies for this approach include vapor-phase lubrication4 and passivating surfaces with self-assembled monolayers (SAMs)5-12 or hard coating.13 In order to utilize the lubrication scheme at the nanometer scale, it is important to develop nanometer scale building blocks with lubrication. Colloid nanoparticles with well-controlled size and shape14,15 can be an interesting nanoscale lubricant system that can be utilized in NEMS. Friction and nanomechanical properties on colloid nanoparticles have drawn interest for applications such as nanometer scale moving parts, and extensive studies have been performed with atomic force microscopy and surface force apparatus.16-20 The colloid nanoparticles are capped with stabilizing agents to prevent aggregation while in the liquid suspension. Nanomechanical properties can be dominated by the organic capping layers present on the surface of nanoparticles. Therefore, changing the organic capping layers on nanoparticles will have a significant influence on nanometer scale friction and adhesion properties of the nanoparticles. In this paper, we investigated nanoscale friction and adhesion on Pt colloid nanoparticles synthesized with four different organic capping layers: TTAB (tetradecyltrimethylammonium bromide), HDA (hexadecylamine), HDT (hexadecylthiol), and PVP (poly(vinylpyrrolidone))21 with atomic/friction force microscopy as shown Figure 1a. We found that the friction of these nanoparticles is r 2011 American Chemical Society
lower than that of silicon and that there was a significant variation of friction depending on the type of organic capping layers. We studied the change of friction after UV-ozone treatment22 that appears to partially remove capping layers.
II. EXPERIMENTAL APPROACH In this study, we used platinum colloid nanoparticles with four types of capping layers: TTAB, HDA, HDT, and PVP. TTAB capped nanoparticles were synthesized as reported previously.23 Briefly, 1 mM aqueous K2PtCl4 in 100 mM TTAB was reduced by 30 mM NaBH4 at 50 °C. Excess H2 evolving from the reacting solution was released by inserting a needle into the septum. After 7 h, the reaction was allowed to cool to room temperature and left overnight to decompose the remaining NaBH4 in water. The Pt nanoparticles were collected and washed by repeated centrifugation and sonication. TTAB-stabilized Pt nanoparticles have cubic shapes and an average size of 12.3 ( 1.4 nm obtained from the size distribution of 500 nanoparticles. The organic capping layer was exchanged with HDA or HDT. 8 mL of TTAB capped nanoparticles was redispersed in 2 mL of deionized water after washing, and then 10 mg of HDA or 20 μL of HDT was added to the washed nanoparticles. The solution was refluxed overnight at 50 °C, and the residual HDA or HDT was washed off with ethanol. The nanoparticles were further washed by dispersing in chloroform and precipitating with hexane. Finally, the nanoparticles were dispersed in chloroform and deposited on a silicon wafer. Since the exchanging capping layers do not change the size and shape of the nanoparticles, HDA and HDT capped nanoparticles had an average size of 12.3 ( 1.4 nm with cubic shapes. Received: November 1, 2010 Revised: January 4, 2011 Published: February 02, 2011 2509
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Figure 1. (a) Schematic of an AFM experiment on Pt nanoparticles. The colloid nanoparticles are capped with stabilizing agents in order to prevent aggregation. AFM images of (b) PVP, (c) TTAB, (d) HDT, and (e) HDA capped Pt nanoparticles. Image size is 500 nm 500 nm. The effective applied load during imaging is 10-15 nN. For PVP capped Pt nanoparticles, 0.1 mmol of chloroplatinic acid hexahydrate (H2PtCl6, ACS reagent, Sigma-Aldrich), 4 mmol of tetramethylammonium bromide ((CH3)4NBr, >98%, Sigma-Aldrich), and 2 mmol of poly(vinylpyrrolidone) (PVP, Mw = 24 000, Sigma-Aldrich) (in terms of the repeating unit) were added to 20 mL of ethylene glycol (>98%, EMD) in a 50 mL three-necked flask at room temperature. PVP capped Pt nanoparticles have an average size of 9.5 ( 0.8 nm obtained from the size distribution of 150 particles in TEM images. Commercial AFM (molecular imaging) was used to measure the adhesion force of the surface and obtain a topographical and friction image of the surface in air. Figure 1a shows the schematic of the AFM experiment on Pt nanoparticles. A silicon nitride tip with a nominal spring constant of 0.27 N/m was used in our measurements. The radii of the tips were 20-30 nm, as measured by scanning electron microscopy. To determine these forces, the cantilever spring constant was calibrated using the resonance-damping method of Sader et al.,24 while the lateral force was calibrated with the wedge method of Ogletree et al.25 It was crucial to carry out the experiment in the low load regime so that there was no damage to the surface. This was confirmed by inspection of the images with angstrom depth sensitivity as well as by the reproducibility of the friction and adhesion measurements. If the measured friction force did not change at constant load and did not show time-dependent behavior in the elastic regime, we can assume that the tip experiences minimal changes during subsequent contact measurements.
III. RESULTS AND DISCUSSION The two-dimensional array of Pt nanoparticles was generated by placing drops of Pt nanoparticles in chloroform solution onto the water subphase of a Langmuir-Blodgett trough (Nima Technology, M611) at room temperature. The surface pressure was monitored with a Wilhelmy plate and was adjusted to zero before spreading the nanocrystals.26 The Pt nanoparticles were deposited onto Si wafers (0.5 cm 1 cm) by liftup of the substrates at a rate of 1 mm/min. Contact mode AFM was used to image the array of Pt nanoparticles.
Figure 1 shows AFM images of (b) PVP, (c) TTAB, (d) HDT, and (e) HDA capped Pt nanoparticles. The effective applied load (the sum of apparent applied load and adhesion force) during imaging is 10-15 nN. AFM images show bumplike features that result from the morphology of nanoparticles. Since the Langmuir-Blodgett film is well packed, the AFM images show the top view of the monolayer surface, and the height corrugation is lower than the size of nanoparticles for well-packed surfaces. An AFM image of TTAB capped Pt nanoparticles, as shown in Figure 1c, exhibits the partially filled two-dimensional array, resulting in exposure of the silicon surface. The AFM image of Pt nanoparticles and silicon surface allows us to obtain the height of the nanoparticles and compare the friction on nanoparticles with that on the silicon surface. Figures 2a and 2b show the contact mode AFM image and friction image of TTAB coated nanoparticles on silicon surfaces, respectively. The height of the nanoparticle is determined to be 10.5 ( 1.2 nm, which is slightly smaller than the size of the Pt nanoparticles (12 nm), as measured with TEM. The lateral size of the nanoparticles can be determined by the full width of the halfmaximum of the height profile, as shown in Figure 2c, and the lateral size was measured as 40 ( 10 nm. This value is larger than the size of the nanoparticles (∼12 nm) because of the convolution between the AFM tip with a radius of 20 nm and the Pt nanoparticles. The line profile of friction across the Pt nanoparticles and silicon substrate (as shown in Figure 2c) reveals higher friction on the silicon substrate compared to that on Pt nanoparticles. At the applied load of 0 nN (or the effective load of 20 nN), the friction on silicon is 2.5 times higher than that on nanoparticles. The lower friction on nanoparticle surfaces is mainly associated with the organic capping layers that act as a lubricant. Figure 3 shows a typical force-distance curve measured on TTAB capped Pt nanoparticles and silicon surfaces. The adhesion force between the silicon nitride tip and the silicon substrate is 24 ( 4 nN, higher than that of TTAB capped Pt nanoparticles 2510
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Figure 2. (a) 300 nm 300 nm AFM topography. (b) Friction image of TTAB coated NP on silicon surfaces. Effective applied load is 10 nN. (c) Line profile of height and friction across the nanoparticles and silicon surfaces revealing lower friction on nanoparticle surfaces compared to that of silicon.
Figure 3. Force-distance curve measured on TTAB coated Pt nanoparticles and silicon surfaces.
(14 ( 3 nN). The average and error scale of the adhesion value was determined by five independent measurements. The lower friction and adhesion on organic self-assembled organic molecules are due to the lubricating function and low surface energy of organic molecules, respectively, and a number of experimental results showing this trend were reported. For example, Park et al. observed friction on alkanesilane molecule islands that is ∼3 times lower than that on the surrounding silicon surface.9 Qi et al. showed lower friction on alkanethiol islands compared to the Au (111) surface.27 Figure 4 shows the plot of friction measured on PVP coated Pt nanoparticles and silicon surfaces as a function of applied load. Both the friction and adhesion of PVP capped Pt nanoparticles are lower than those of the silicon surface. The lines that are superposed with friction data of silicon and nanoparticles surface represent DMT (Derjaguin-Muller-Toporov)28 and JKR (JohnsonKendall-Roberts) fitting,29 respectively. JKR and DMT models approximate elastic behavior in two opposite extremes. The DMT
Figure 4. Friction measured on PVP coated Pt nanoparticles and silicon surfaces as a function of applied load. The solid black and blue lines represent DMT and JKR fits, respectively.
model describes hard and poorly adhesive materials, while JKR describes soft and adhesive materials. Table 1 shows the summary of friction and adhesion values on various capping layers including TTAB, PVP, HDA, and HDT. Work of adhesion was obtained with JKR and DMT models for Pt nanoparticles surfaces and the silicon surface, respectively. The variation of friction when changing the capping layers is ∼30%. Friction forces on HDT and HDA capped nanoparticles were lower than that on PVP or TTAB capped nanoparticles. The result implies that the friction on nanoparticles is influenced by the compactness and ordering of organic capping layers.10,11,30 Friction on a disordered and rough surface is usually higher than a well-packed surface because of additional channels for energy dissipation on the disordered surface. HDT and HDA molecules have strong NH2 and sulfur bonds, respectively, with Pt forming compact capping layers on Pt nanoparticles. On the other hand, PVP and TTAB have relatively loose and disordered layers because of weaker bonding between TTAB or PVP and the Pt 2511
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Table 1. Friction Force and Adhesion Force Measured on Silicon Surface and Pt Nanoparticles Capped with Various Capping Layers Including PVP, TTAB, HDT, and HADa capping molecules
friction force (nN)
adhesion force (nN)
work of adhesion (mJ/m)
HDT
0.95 ( 0.08
14
120
HDA PVP
1.01 ( 0.07 1.28 ( 0.11
13 14
110 120
TTAB
1.21 ( 0.13
12
100
silicon
2.3 ( 0.3
24
150
a
The friction was measured at the effective applied load (apparent applied load plus adhesion force) of 20 nN. Work of adhesion was obtained with the JKR model for Pt nanoparticles and DMT for the silicon surface. The tip radius (R) of 25 nm was used to obtain the work of adhesion.
materials. Therefore, this study suggests an intriguing way to control nanometer scale friction with possible applications such as nanometer scale moving parts.
Figure 5. Plot of friction of PVP coated nanoparticles before and after UV-ozone process (2 h) as a function of the applied load.
atoms.31 Therefore, we can conclude that the lower friction on HDT and HDA capped Pt nanoparticles is associated with the well-packed surface of HDT and HDA. The adhesion forces of Pt nanoparticles with the four capping layers remain almost constant, ranging between 12 and 14 nN. These correspond to 100-120 mJ/m in work of adhesion using the JKR model. We investigated the change of adhesion and friction of the nanoparticle surface after UV-ozone treatment of organic capping layers. Because the friction and adhesion properties are dominated by the presence of capping layers, chemical modification of the capping layer can lead to significant changes of friction and adhesion. The UV-ozone oxidation process involves the simultaneous action of ozone and ultraviolet light, which are responsible for the oxidation of carbon-containing compounds into carbon dioxide and water. Ultraviolet radiation can also be used for polymer surface modification with applications including photolithography, nanocatalysts, and bioengineering.22,32 Figure 5 shows the plot of friction of PVP coated nanoparticles before and after UV-ozone process (2 h) as a function of the applied load. Indeed, the friction increased by 20-30% after UVozone treatment while the adhesion increase is not prominent. Therefore, this method suggests that the chemical treatment of nanoparticles can be an effective way to change nanoscale friction. Organic self-assembled layers have been widely used for lubricant layers to reduce friction and adhesion on the surface. In this work, it was observed that the engineering of capping layers on colloid nanoparticles is an effective way to change the nanometer scale friction and adhesion. Tuning friction and adhesion on nanoparticles is a broad concept, since engineering of capping layers can be extended into many different types of nanoparticles, nanorods, or to more complicated structures such as tetrapods.33 The materials of nanostructure can be varied in a broad range covering transition metals, metal oxides, and hard
IV. SUMMARY AND OUTLOOK We have shown that nanoscale friction and adhesion of Pt colloid nanoparticles are dominated by the capping layers present on the colloid nanoparticles. Friction and adhesion on nanoparticles were revealed with atomic/friction force microscopy. The friction on Pt nanoparticles was lower than that of silicon by a factor of 2 or 3, depending on the types of organic capping molecules. HDT and HDA exhibited lower friction due to the well-packed molecular structure compared to TTAB and PVP which are loosely bound to the Pt surface. Partial removal of capping layers using ultraviolet light-ozone surface treatment resulted in increasing both friction and adhesion. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The author thanks Gabor Somorjai for his valuable comments and Hyunjoo Lee for synthesis of nanoparticles. The work was supported by WCU (World Class University) program (R-312008-000-10055-0) and KRF-2010-0005390 through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology and by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. ’ REFERENCES (1) Carpick, R. W. Science 2006, 313, 184. (2) Socoliuc, A.; Gnecco, E.; Maier, S.; Pfeiffer, O.; Baratoff, A.; Bennewitz, R.; Meyer, E. Science 2006, 313, 207. (3) Park, J. Y.; Ogletree, D. F.; Thiel, P. A.; Salmeron, M. Science 2006, 313, 186. (4) Anselmetti, D.; Baratoff, A.; Guntherodt, H. J.; Gerber, C.; Michel, B.; Rohrer, H. J. Vac. Sci. Technol. B 1994, 12, 1677. (5) Barrena, E.; Kopta, S.; Ogletree, D. F.; Charych, D. H.; Salmeron, M. Phys. Rev. Lett. 1999, 82, 2880. (6) Bhushan, B.; Liu, H. W. Phys. Rev. B 2001, 63, 245412. (7) Flater, E. E.; Ashurst, W. R.; Carpick, R. W. Langmuir 2007, 23, 9242. (8) Liu, H.; Bhushan, B. Ultramicroscopy 2002, 91, 185. (9) Park, J. Y.; Qi, Y. B.; Ashby, P. D.; Hendriksen, B. L. M.; Salmeron, M. J. Chem. Phys. 2009, 130, 114705. 2512
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