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Preparation and characterization of layer-by-layer self-assembled polyelectrolyte multilayer films doped with surface-capped SiO2 nanoparticles
Journal of Colloid and Interface Science 333 (2009) 776–781
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Preparation and characterization of layer-by-layer self-assembled polyelectrolyte multilayer films doped with surface-capped SiO2 nanoparticles Guangbin Yang, Hongxia Ma, Laigui Yu, Pingyu Zhang ∗ Laboratory for Special Functional Materials of Ministry of Education, Henan University, Kaifeng 475004, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 20 November 2008 Accepted 11 February 2009 Available online 13 February 2009
SiO2 nanoparticles capped with γ -aminopropyltrimethoxysilane were doped into polyelectrolyte (poly(allylamine hydrochloride), PAH, and poly(acrylic acid), PAA) multilayer films via spin-assisted layer-bylayer self-assembly. The resulting as-prepared multilayer films were heated at a proper temperature to generate cross-linked composite films with increased adhesion to substrates. The tribological behavior of the multilayer films was evaluated on a microtribometer. It was found that SiO2 -doped composite films had better wear resistance than pure polyelectrolyte multilayers, possibly because doped SiO2 nanoparticles were capable of enhancing load-carrying capacity and had “miniature ball bearings” effect. Moreover, heat-treatment had significant effect on the morphology of the composite films. Namely, heat-treated (SiO2 /PAA)9 film had a larger roughness than the as-prepared one, due to heattreatment-induced agglomeration of SiO2 nanoparticles and initiation of defects. However, heat-treated (PAH/PAA)3 /(SiO2 /PAA)3 (PAH/PAA)3 film had greatly reduced roughness than the as-prepared one, and it showed considerably improved wear resistance as well. This could be closely related to the “sandwichlike” structure of the composite multilayer film. Namely, the outermost strata of composite multilayer film were able to eliminate defects associated with the middle strata, allowing nanoparticles therein to maintain strength and robustness while keeping soft and fluid-like exposed surface. And the inner strata were well anchored to substrate and acted as an initial “bed” for SiO2 nanoparticles to be inhabited, resulting in good antiwear ability. © 2009 Elsevier Inc. All rights reserved.
Keywords: Polyelectrolyte SiO2 nanoparticles Spin-assisted layer-by-layer self-assembly Composite multilayer film Preparation Friction and wear behavior
1. Introduction Modern magnetic storage systems and microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) require a protective coating which is crucial for providing lower friction and higher antiwear life for inherently brittle electronic materials to ensure performance, efficiency and reliability of MEMS/NEMS devices [1,2]. Langmuir–Blodgett films, self-assembled monolayers and polyelectrolyte multilayers (PEMs) have drawn much attention in those aspects [3]. And in particular, layer-bylayer (LBL) assembly as an important and widely utilized tool to form nanocomposite thin films is advantageous in terms of incorporating multiple components to impart specific properties, while spin-assisted layer-by-layer (SA-LBL) self-assembly is timeand cost-efficient for fabrication of multilayer films [4,5]. Recently, multilayer films doped with SiO2 nanoparticles have been increasingly focused on, since the introduction of SiO2 nanoparticles helps to endow them with multifunctional features. For example, Rubner and co-workers [6,7], for the purpose of creat-
*
Corresponding author. Fax: +86 378 3881358. E-mail address: [email protected] (P. Zhang).
0021-9797/$ – see front matter doi:10.1016/j.jcis.2009.02.024
© 2009
Elsevier Inc. All rights reserved.
ing an ultrahydrophobic surface, fabricated a microporous polyelectrolyte multilayer surface over-coated with silica nanoparticles to increase the roughness. Shiratori and co-workers [8] constructed superhydrophobic SiO2 films through high temperature treatment to remove polyelectrolytes of poly(allylamine hydrochloride)/poly(acrylic acid) doped with SiO2 nanoparticles. And SiO2 was also used with poly(diallyldimethylammonium chloride) to fabricate broad-band superhydrophobic antireflection coatings in near-infrared region [9]. Kim and co-workers found that multilayer composite film of SiO2 /Ag nanoparticles possesses the optical properties [10]. We found in our previous work that in situ surfacecapped nano-SiO2 as oil additive contributed to greatly increase the anti-wear and friction-reducing ability of lubricants [11]. In the present research, nano-SiO2 particulates were surfacecapped with aminopropyltrimethoxysilane and doped into polyelectrolyte (poly(allylamine hydrochloride), PAH, and poly(acrylic acid), PAA) multilayer films to prepare nano-SiO2 doped composite films. Then cross-linked nylon-like multilayer films of PAH– PAA doped with surface-capped SiO2 nanoparticles were fabricated on silicon wafers [Si(100)] by heating at a proper temperature to transform –NH2 and –COO− groups in the polyelectrolyte molecules to –CONH–, which is described elsewhere [12].
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of bilayers. The composite films with different bilayer numbers are denoted as (PAH/PAA)9 , (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 , and (SiO2 /PAA)9 , where subscript numerals refer to the bilayer number. The multilayer films were finally heated at 215 ◦ C for 2 h under N2 protection to initiate cross-linking, which is described in Ref. [12]. 2.3. Characterization of multilayer films
Fig. 1. The ideal structure model of nano-SiO2 modified with aminopropyltrimethoxysilane.
It is anticipated that such a kind of conversion from –NH2 and –COO− groups to –CONH– would help to significantly improve the strength of the multilayer films and hence increase their endurability to loading, which is essential to the application of the multilayer films for MEMS and NEMS. An atomic force microscope (AFM) was used to observe the morphologies of the as-prepared and heat-treated films and estimate their surface roughness as well. The friction and wear behavior of the composite multilayer films was evaluated on a UMT-2 microtribometer, and the wear mechanisms of the multilayer films were discussed. 2. Materials and methods 2.1. Materials PAA (M w ∼ 100,000) and PAH (M w ∼ 70,000) were used asreceived from Aldrich. Nano-SiO2 modified with γ -aminopropyltrimethoxysilane, with an average particle diameter of 10–20 nm, was prepared at Henan Nanomaterial Engineering Technology and Research Center. The formation mechanism of nano-SiO2 particulates is described elsewhere [11], and Fig. 1 shows an ideal structure model of nano-SiO2 modified with aminopropyltrimethoxysilane. 2.2. Preparation of multilayer films PAH and PAA were dissolved in deionized water at a concentration of 20 mM. The pH value of the PAA solution was adjusted to 7.5 using 0.1 M NaOH solution, while that of PAH solution was adjusted to 3.5 with 0.1 M HCl solution. Aqueous solution of 0.03 wt% SiO2 with a pH value of 4.0 was prepared for incorporating the nanoparticles into the multiplayer films. Deionized water (>18 M cm, Millipore Milli-Q) was used for preparation of all aqueous solutions, and for rinsing as well. Before preparing composite multiplayer films doped with nanoSiO2 , polished silicon wafers (100) were cleaned and hydroxylated by immersing in a piranha solution (H2 O2 :H2 SO4 = 3:7 v/v) at 80 ◦ C for 1 h, followed by thorough rinsing and sonication in deionized water to remove any residue acidic solution before being stored in water. Prior to deposition, bare substrates were spun at 3000 rpm and heated at 110 ◦ C for 1 min to remove adsorbed water. The composite films were constructed by SA-LBL deposition at room temperature, using a KW-4A Spin Coater at a fixed spinning speed of 3000 rpm. First, PAH or SiO2 solution was pipetted onto a negative charged substrate and spun for 1 min to generate spin-assembled PAH film or nano-SiO2 film. The resulting spinassembled film was then rinsed with a few drops of deionized water and heated at 110 ◦ C for 1 min, followed by cooling in air for 1 min. Then PAA solution was deposited onto the PAH film or nano-SiO2 film and spun for 1 min, followed by rinsing, heating, and cooling as above mentioned. The deposition of the multiplayer films was repeated with respect to a predetermined number
An SPA400 atomic force microscope was employed to observe the morphology of the multilayer films deposited on silicon wafer and evaluate their surface roughness as well. The AFM analysis was carried out in tapping mode with commercial silicon microcantilever probes under ambient conditions (24 ± 2 ◦ C). The probe tip radius and probe spring constant are less than 20 nm and 2.0 N/m, respectively. The chemical states of the elements on the surface of the multilayer films were investigated by means of X-ray photoelectron spectroscopy (XPS). Al-Kα radiation, with a pass energy of 40 eV, was used as the excitation source, and the binding energy of carbon contaminant (C1s: 284.8 eV) was used as reference. 2.4. Friction and wear behavior of multilayer films Friction and wear tests were carried out on a UMT-2 multifunctional microtribometer, in a ball-on-plate contact configuration. The lower plate of silicon wafer with SA-LBL multilayer film was driven by a motor to slide against 440C stainless steel ball counterpart (4 mm in diameter) for a stroke of 3 mm. Before each test, the stainless steel ball was cleaned with acetone in a supersonic bath, and all the sliding tests were conducted at a room temperature of about 25 ◦ C and relative humidity of about 50%. The friction coefficient and sliding time were recorded automatically. It was assumed that lubrication failure of the films occurred when the friction coefficients rose sharply to a higher and stable value similar to that of a cleaned silicon wafer against the same counterpart (about 0.65). The sliding time at this point was recorded as the antiwear life of the multilayer films. Scratch tests were also carried out on the UMT-2 tester to determine the adhesion of the films to the substrate. The scratch tests were performed under a linearly increased load from 50 mN to 1000 mN. A composite diamond microcutting blade with a tip radius of 0.4 mm was driven to move across the sample at 0.6 mm/s for a distance of 3.0 mm. The load at which the film was peeled off completely from the substrate was recorded as the critical load, at which a sharp increase in friction force (F x ) or acoustic emission (AE) was detected. The morphologies of the wear tracks of the multilayer films were observed using a JSM-5600LV scanning electron microscope (SEM) with a resolution of 3.5 nm at high vacuum mode and an acceleration voltage of 20 kV. 3. Results and discussion 3.1. XPS and AFM analysis of the multilayer films Fig. 2 shows the XPS survey spectra of (PAH/PAA)9 , (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 , and (SiO2 /PAA)9 after heat-treatment. The peaks at 284.8 eV and 532.8 eV are assigned to C1s and O1s, respectively, while the N1s peak at 400.3 eV was assigned to amide group (–CONH–). Besides, the binding energies of Si2p and Si2s were detected as 99.8 eV and 152.8 eV, respectively. Interestingly, no Si signal was detected on the surface of (PAH/PAA)9 film, indicating that the Si substrate was entirely covered with nanocomposite film of (PAH/PAA)9 . Moreover, N was hardly detected on the surface of (SiO2 /PAA)9 film, possibly because in this case
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Fig. 2. XPS spectra of various multilayer films.
very little proportion of –NH2 group was present on SiO2 particulates. Contrary to the above, N of a high intensity was detected on the surface of (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 film, owing to the presence of N in PAH. Fig. 3 presents typical tapping-mode three-dimensional AFM images of the as-prepared and heat-treated composite films (taken over a scanning area of 5.0 μm × 5.0 μm). The root-mean-square (RMS) roughness values of the composite films before and after heat-treatment are given in Table 1. As shown in Figs. 3a and 3d, the as-prepared and heat-treated (PAH/PAA)9 films are flat and compact, and they have small worm-like appearance [13] and RMS roughness of 3.8 nm and 2.0 nm, respectively. After being doped with surface-capped SiO2 nanoparticles, the as-prepared and heat-treated multilayer films (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 (Figs. 3b and 3e) show morphologies similar as that of (PAH/PAA)9 film, and the heat-treated multilayer film doped with nano-SiO2 shows more distinctive worm-like appearance. At the same time, heat-treated (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 composite film has an RMS roughness of 0.8 nm, considerably smaller than 4.5 nm, that of the as-prepared same film. The reason could lie in that the outermost three bilayers, i.e., (PAH/PAA)3 , of the composite film (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 could well eliminate the defects associated with the three intermediate bilayers, (SiO2 /PAA)3 . Different from (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 composite films, as-prepared and heat-treated (SiO2 /PAA)9 films show randomly distributed grains or clusters on surface, and in this case heattreatment leads to increase of RMS roughness of the multilayer film from 0.26 nm to 0.33 nm (Figs. 3c and 3f), which could be attributed to heat-treatment-induced agglomeration of SiO2 nanoparticles and defects as well. 3.2. Friction and wear behavior Fig. 4 shows friction coefficient-time curves of heat-treated composite films (PAH/PAA)9 , (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 and (SiO2 /PAA)9 sliding against 440C stainless steel ball at a velocity of 180 mm/min and normal load of 0.5 N. It is seen that (PAH/PAA)9 film has an antiwear life of only 1711 s, (SiO2 /PAA)9 film has an increased antiwear life of 2570 s, while (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 composite film registers an antiwear life of 3028 s. In one word, the multilayer films doped with nano-SiO2 particulates have a longer antiwear life than pure polyelectrolyte film, (PAH/PAA)9 , which could be because the surface-capped SiO2 nanoparticles acted to enhance the load-carrying capacity of the multilayer films [14,15]. Furthermore, amide bond (–CONH–) was formed between not only PAH and PAA but also SiO2 and PAA after heating, which would also contribute to increasing the stability
and strength of the composite films doped with nano-SiO2 particulates [12,16]. Unsurprisingly, (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 film has the longest antiwear life and shows potential in protection of MEMS. This could be because the doped multilayer film (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 has “sandwich-like” structure with increased inter-layer adhesion [17]. Namely, the first three strata of (PAH/PAA)3 are not only anchored with silicon substrate but also serve as an initial “bed” for SiO2 nanoparticles to be inhabited [18]. The next three stratum containing alternative SiO2 nanoparticles (which bear a positive surface charge) and PAA is capable of enhancing the load-carrying capacity of the films. And the final stratum, also composed of three bilayers (PAH/PAA)3 , allows doped nanoparticles to maintain their strength and robustness while ensuring that their exposed surface is soft and fluid-like [19], which is helpful to eliminate defects associated with the middle stratum. Fig. 5 shows the friction coefficient–velocity curves for heattreated multilayer films (PAH/PAA)9 , (PAH/PAA)3 (SiO2 /PAA)3 (PAH/ PAA)3 and (SiO2 /PAA)9 sliding against stainless steel counterpart at a normal load 0.5 N respectively. (SiO2 /PAA)9 film has the lowest friction coefficient among the three kinds of tested multilayer films, and its friction coefficient keeps almost unchanged (0.15) with increasing sliding velocity. This might be attributed to the presence of SiO2 nanoparticles which act as “miniature ball bearings” [20,21] and transform the sliding between the multilayer film and stainless steel to rolling, resulting in lowered friction coefficient. However, (SiO2 /PAA)9 film has poor antiwear ability, largely because it has poor adhesion to the silicon substrate when SiO2 nanoparticles are directly assembled onto the Si substrate. Besides, (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 multilayer film with a “sandwich-like” structure has a friction coefficient of 0.25–0.35, depending on varying sliding velocity. In general, the two types of multilayer films containing nano-SiO2 particulates had lower friction coefficient than the pure polyelectrolyte film (PAH/PAA)9 , possibly because SiO2 nanoparticles acted to reduce the stiction of pure polyelectrolyte film and hence considerably reduced friction coefficient [22]. Furthermore, the friction coefficient of (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 film is higher than that of (SiO2 /PAA)9 film. This could be because the outermost strata (PAH/PAA)3 of the (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 film has a higher stiction than (SiO2 /PAA)9 , which results in increased friction coefficient. Fig. 6 shows representative SEM images of wear tracks of the multilayer films sliding against 440C stainless steel at a normal load of 0.5 N and sliding velocity of 180 mm/min. After sliding for 1680 s, the worn surface of (PAH/PAA)9 film showed signs of mild plastic deformation (Fig. 6a). When sliding against stainless steel ball for 2950 s and 2470 s, respectively, both (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 and (SiO2 /PAA)9 films had wear tracks narrower than that of (PAH/PAA)9 film (Figs. 6b and 6c), corresponding to increased wear resistance of the multilayer films doped with nano-SiO2 particulates and confirming that the introduction of SiO2 nanoparticles helped to effectively improving the antiwear ability of the SA-LBL polyelectrolyte multilayer films. Fig. 7 shows the typical scratch test results for (PAH/PAA)9 film, where F x , normal force (F z ), and AE were recorded as functions of time in seconds. In the scratch test, as the microblade moved slowly against the film, materials removal progressively occurred, and a higher critical load corresponded to a higher adhesion of the film to the substrate. (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 film has an adhesion of about 628 mN, which is higher than 550 mN and 460 mN, that of (PAH/PAA)9 film and (SiO2 /PAA)9 film. The scratch test results are also summarized in Table 1. The difference in the critical load of the three types of films could be highly dependent on their different composition and microstructures leading to different porosity and stress [23].
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Fig. 3. AFM images of various as-prepared and heat-treated multilayer films: as-prepared (a) (PAH/PAA)9 , (b) (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 , (c) (SiO2 /PAA)9 and heattreated (d) (PAH/PAA)9 , (e) (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 , (f) (SiO2 /PAA)9 .
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Table 1 Surface roughness determined by AFM and critical load determined by scratch tests of the multilayer films. Structure
RMS (nm)
Critical load (mN)
Before heating After heating After heating (PAH/PAA)9 3.831 (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 4.532 0.2601 (SiO2 /PAA)9
2.004 0.8050 0.3253
550 628 460
Fig. 4. Friction coefficient as a function of sliding time at a normal load of 0.5 N, sliding velocity of 180 mm/min.
Fig. 5. Friction coefficient as a function of sliding velocity at a normal load 0.5 N.
4. Summary SiO2 nanoparticles surface-capped with γ -aminopropyltrimethoxysilane were doped in polyelectrolyte multilayer films via spinassisted layer-by-layer deposition. The resulting nano-SiO2 -doped composite films (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 and (SiO2 /PAA)9 on Si substrates had cross-linking structures, owing to the generation of amide bond (–CONH–) between not only PAH and PAA but also SiO2 and PAA. However, (SiO2 /PAA)9 film had poor adhesion to the silicon substrate, since SiO2 nanoparticles were directly assembled onto Si substrate resulting in increased porosity and stress in SiO2 layer and hence reducing the adhesion. Both (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 and (SiO2 /PAA)9 multilayer films had better wear resistance than pure polyelectrolyte multilayer film (PAH/PAA)9 , which was related to the ability to enhance load-carrying capacity and “miniature ball bearings” effect as well of SiO2 nanoparticles. Heat-treatment had significant effect on the morphology of the composite multilayer films doped with nano-SiO2 particulates. Namely, heat-treated (SiO2 /PAA)9 film had a larger RMS roughness
Fig. 6. SEM micrographs of the wear tracks of the multilayer films sliding against 440C stainless steel at a normal load of 0.5 N and sliding velocity of 180 mm/min, (a) (PAH/PAA)9 sliding for 1680 s, (b) (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 sliding for 2950 s and (c) (SiO2 /PAA)9 sliding for 2470 s.
than the as-prepared one, possibly due to agglomeration of SiO2 nanoparticles and initiation of defects during heat-treatment. Contrary to the above, heat-treated (PAH/PAA)3 /(SiO2 /PAA)3 (PAH/PAA)3 film had greatly reduced RMS roughness than the as-prepared one, indicating that the “sandwich-like” structure was beneficial to retaining desired morphology and surface quality of the composite multilayer film. Such a kind of “sandwich-like” structure also contributed to increasing the wear resistance of multilayer film (PAH/PAA)3 /(SiO2 /PAA)3 (PAH/PAA)3 . Namely, the outermost strata of (PAH/PAA)3 in composite multilayer film (PAH/PAA)3 / (SiO2 /PAA)3 (PAH/PAA)3 were able to eliminate defects associated with the middle strata, allowing the nanoparticles therein to maintain strength and robustness while keeping soft and fluid-like exposed surface and hence decreasing shearing stress during sliding. And the inner strata of (PAH/PAA)3 well anchored to Si substrate
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Henan Innovation Project for University Prominent Research Talents (2006KYCX001) is also acknowledged. We gratefully acknowledge Beibei Chen and Xiaohong Li for helpful material preparations. References [1] [2] [3] [4] [5]
Fig. 7. The curves of scratch test for (PAH/PAA)9 film.
acted as an initial “bed” for SiO2 nanoparticles to be inhabited, resulting in better antiwear ability associated with good loadcarrying capacity and “miniature ball bearings” effect as well of SiO2 nanoparticles. Acknowledgments We are grateful to the Ministry of Science and Technology of China (“973” plan grant No. 2007CB607606) and National Natural Science Foundation of China (grant No. 20571024) for financial support to this research. The financial support by
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Preparation and characterization of layer-by-layer self-assembled polyelectrolyte multilayer films doped with surface-capped SiO2 nanoparticles Guangbin Yang, Hongxia Ma, Laigui Yu, Pingyu Zhang ∗ Laboratory for Special Functional Materials of Ministry of Education, Henan University, Kaifeng 475004, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 20 November 2008 Accepted 11 February 2009 Available online 13 February 2009
SiO2 nanoparticles capped with γ -aminopropyltrimethoxysilane were doped into polyelectrolyte (poly(allylamine hydrochloride), PAH, and poly(acrylic acid), PAA) multilayer films via spin-assisted layer-bylayer self-assembly. The resulting as-prepared multilayer films were heated at a proper temperature to generate cross-linked composite films with increased adhesion to substrates. The tribological behavior of the multilayer films was evaluated on a microtribometer. It was found that SiO2 -doped composite films had better wear resistance than pure polyelectrolyte multilayers, possibly because doped SiO2 nanoparticles were capable of enhancing load-carrying capacity and had “miniature ball bearings” effect. Moreover, heat-treatment had significant effect on the morphology of the composite films. Namely, heat-treated (SiO2 /PAA)9 film had a larger roughness than the as-prepared one, due to heattreatment-induced agglomeration of SiO2 nanoparticles and initiation of defects. However, heat-treated (PAH/PAA)3 /(SiO2 /PAA)3 (PAH/PAA)3 film had greatly reduced roughness than the as-prepared one, and it showed considerably improved wear resistance as well. This could be closely related to the “sandwichlike” structure of the composite multilayer film. Namely, the outermost strata of composite multilayer film were able to eliminate defects associated with the middle strata, allowing nanoparticles therein to maintain strength and robustness while keeping soft and fluid-like exposed surface. And the inner strata were well anchored to substrate and acted as an initial “bed” for SiO2 nanoparticles to be inhabited, resulting in good antiwear ability. © 2009 Elsevier Inc. All rights reserved.
Keywords: Polyelectrolyte SiO2 nanoparticles Spin-assisted layer-by-layer self-assembly Composite multilayer film Preparation Friction and wear behavior
1. Introduction Modern magnetic storage systems and microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) require a protective coating which is crucial for providing lower friction and higher antiwear life for inherently brittle electronic materials to ensure performance, efficiency and reliability of MEMS/NEMS devices [1,2]. Langmuir–Blodgett films, self-assembled monolayers and polyelectrolyte multilayers (PEMs) have drawn much attention in those aspects [3]. And in particular, layer-bylayer (LBL) assembly as an important and widely utilized tool to form nanocomposite thin films is advantageous in terms of incorporating multiple components to impart specific properties, while spin-assisted layer-by-layer (SA-LBL) self-assembly is timeand cost-efficient for fabrication of multilayer films [4,5]. Recently, multilayer films doped with SiO2 nanoparticles have been increasingly focused on, since the introduction of SiO2 nanoparticles helps to endow them with multifunctional features. For example, Rubner and co-workers [6,7], for the purpose of creat-
*
Corresponding author. Fax: +86 378 3881358. E-mail address: [email protected] (P. Zhang).
0021-9797/$ – see front matter doi:10.1016/j.jcis.2009.02.024
© 2009
Elsevier Inc. All rights reserved.
ing an ultrahydrophobic surface, fabricated a microporous polyelectrolyte multilayer surface over-coated with silica nanoparticles to increase the roughness. Shiratori and co-workers [8] constructed superhydrophobic SiO2 films through high temperature treatment to remove polyelectrolytes of poly(allylamine hydrochloride)/poly(acrylic acid) doped with SiO2 nanoparticles. And SiO2 was also used with poly(diallyldimethylammonium chloride) to fabricate broad-band superhydrophobic antireflection coatings in near-infrared region [9]. Kim and co-workers found that multilayer composite film of SiO2 /Ag nanoparticles possesses the optical properties [10]. We found in our previous work that in situ surfacecapped nano-SiO2 as oil additive contributed to greatly increase the anti-wear and friction-reducing ability of lubricants [11]. In the present research, nano-SiO2 particulates were surfacecapped with aminopropyltrimethoxysilane and doped into polyelectrolyte (poly(allylamine hydrochloride), PAH, and poly(acrylic acid), PAA) multilayer films to prepare nano-SiO2 doped composite films. Then cross-linked nylon-like multilayer films of PAH– PAA doped with surface-capped SiO2 nanoparticles were fabricated on silicon wafers [Si(100)] by heating at a proper temperature to transform –NH2 and –COO− groups in the polyelectrolyte molecules to –CONH–, which is described elsewhere [12].
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of bilayers. The composite films with different bilayer numbers are denoted as (PAH/PAA)9 , (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 , and (SiO2 /PAA)9 , where subscript numerals refer to the bilayer number. The multilayer films were finally heated at 215 ◦ C for 2 h under N2 protection to initiate cross-linking, which is described in Ref. [12]. 2.3. Characterization of multilayer films
Fig. 1. The ideal structure model of nano-SiO2 modified with aminopropyltrimethoxysilane.
It is anticipated that such a kind of conversion from –NH2 and –COO− groups to –CONH– would help to significantly improve the strength of the multilayer films and hence increase their endurability to loading, which is essential to the application of the multilayer films for MEMS and NEMS. An atomic force microscope (AFM) was used to observe the morphologies of the as-prepared and heat-treated films and estimate their surface roughness as well. The friction and wear behavior of the composite multilayer films was evaluated on a UMT-2 microtribometer, and the wear mechanisms of the multilayer films were discussed. 2. Materials and methods 2.1. Materials PAA (M w ∼ 100,000) and PAH (M w ∼ 70,000) were used asreceived from Aldrich. Nano-SiO2 modified with γ -aminopropyltrimethoxysilane, with an average particle diameter of 10–20 nm, was prepared at Henan Nanomaterial Engineering Technology and Research Center. The formation mechanism of nano-SiO2 particulates is described elsewhere [11], and Fig. 1 shows an ideal structure model of nano-SiO2 modified with aminopropyltrimethoxysilane. 2.2. Preparation of multilayer films PAH and PAA were dissolved in deionized water at a concentration of 20 mM. The pH value of the PAA solution was adjusted to 7.5 using 0.1 M NaOH solution, while that of PAH solution was adjusted to 3.5 with 0.1 M HCl solution. Aqueous solution of 0.03 wt% SiO2 with a pH value of 4.0 was prepared for incorporating the nanoparticles into the multiplayer films. Deionized water (>18 M cm, Millipore Milli-Q) was used for preparation of all aqueous solutions, and for rinsing as well. Before preparing composite multiplayer films doped with nanoSiO2 , polished silicon wafers (100) were cleaned and hydroxylated by immersing in a piranha solution (H2 O2 :H2 SO4 = 3:7 v/v) at 80 ◦ C for 1 h, followed by thorough rinsing and sonication in deionized water to remove any residue acidic solution before being stored in water. Prior to deposition, bare substrates were spun at 3000 rpm and heated at 110 ◦ C for 1 min to remove adsorbed water. The composite films were constructed by SA-LBL deposition at room temperature, using a KW-4A Spin Coater at a fixed spinning speed of 3000 rpm. First, PAH or SiO2 solution was pipetted onto a negative charged substrate and spun for 1 min to generate spin-assembled PAH film or nano-SiO2 film. The resulting spinassembled film was then rinsed with a few drops of deionized water and heated at 110 ◦ C for 1 min, followed by cooling in air for 1 min. Then PAA solution was deposited onto the PAH film or nano-SiO2 film and spun for 1 min, followed by rinsing, heating, and cooling as above mentioned. The deposition of the multiplayer films was repeated with respect to a predetermined number
An SPA400 atomic force microscope was employed to observe the morphology of the multilayer films deposited on silicon wafer and evaluate their surface roughness as well. The AFM analysis was carried out in tapping mode with commercial silicon microcantilever probes under ambient conditions (24 ± 2 ◦ C). The probe tip radius and probe spring constant are less than 20 nm and 2.0 N/m, respectively. The chemical states of the elements on the surface of the multilayer films were investigated by means of X-ray photoelectron spectroscopy (XPS). Al-Kα radiation, with a pass energy of 40 eV, was used as the excitation source, and the binding energy of carbon contaminant (C1s: 284.8 eV) was used as reference. 2.4. Friction and wear behavior of multilayer films Friction and wear tests were carried out on a UMT-2 multifunctional microtribometer, in a ball-on-plate contact configuration. The lower plate of silicon wafer with SA-LBL multilayer film was driven by a motor to slide against 440C stainless steel ball counterpart (4 mm in diameter) for a stroke of 3 mm. Before each test, the stainless steel ball was cleaned with acetone in a supersonic bath, and all the sliding tests were conducted at a room temperature of about 25 ◦ C and relative humidity of about 50%. The friction coefficient and sliding time were recorded automatically. It was assumed that lubrication failure of the films occurred when the friction coefficients rose sharply to a higher and stable value similar to that of a cleaned silicon wafer against the same counterpart (about 0.65). The sliding time at this point was recorded as the antiwear life of the multilayer films. Scratch tests were also carried out on the UMT-2 tester to determine the adhesion of the films to the substrate. The scratch tests were performed under a linearly increased load from 50 mN to 1000 mN. A composite diamond microcutting blade with a tip radius of 0.4 mm was driven to move across the sample at 0.6 mm/s for a distance of 3.0 mm. The load at which the film was peeled off completely from the substrate was recorded as the critical load, at which a sharp increase in friction force (F x ) or acoustic emission (AE) was detected. The morphologies of the wear tracks of the multilayer films were observed using a JSM-5600LV scanning electron microscope (SEM) with a resolution of 3.5 nm at high vacuum mode and an acceleration voltage of 20 kV. 3. Results and discussion 3.1. XPS and AFM analysis of the multilayer films Fig. 2 shows the XPS survey spectra of (PAH/PAA)9 , (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 , and (SiO2 /PAA)9 after heat-treatment. The peaks at 284.8 eV and 532.8 eV are assigned to C1s and O1s, respectively, while the N1s peak at 400.3 eV was assigned to amide group (–CONH–). Besides, the binding energies of Si2p and Si2s were detected as 99.8 eV and 152.8 eV, respectively. Interestingly, no Si signal was detected on the surface of (PAH/PAA)9 film, indicating that the Si substrate was entirely covered with nanocomposite film of (PAH/PAA)9 . Moreover, N was hardly detected on the surface of (SiO2 /PAA)9 film, possibly because in this case
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Fig. 2. XPS spectra of various multilayer films.
very little proportion of –NH2 group was present on SiO2 particulates. Contrary to the above, N of a high intensity was detected on the surface of (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 film, owing to the presence of N in PAH. Fig. 3 presents typical tapping-mode three-dimensional AFM images of the as-prepared and heat-treated composite films (taken over a scanning area of 5.0 μm × 5.0 μm). The root-mean-square (RMS) roughness values of the composite films before and after heat-treatment are given in Table 1. As shown in Figs. 3a and 3d, the as-prepared and heat-treated (PAH/PAA)9 films are flat and compact, and they have small worm-like appearance [13] and RMS roughness of 3.8 nm and 2.0 nm, respectively. After being doped with surface-capped SiO2 nanoparticles, the as-prepared and heat-treated multilayer films (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 (Figs. 3b and 3e) show morphologies similar as that of (PAH/PAA)9 film, and the heat-treated multilayer film doped with nano-SiO2 shows more distinctive worm-like appearance. At the same time, heat-treated (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 composite film has an RMS roughness of 0.8 nm, considerably smaller than 4.5 nm, that of the as-prepared same film. The reason could lie in that the outermost three bilayers, i.e., (PAH/PAA)3 , of the composite film (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 could well eliminate the defects associated with the three intermediate bilayers, (SiO2 /PAA)3 . Different from (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 composite films, as-prepared and heat-treated (SiO2 /PAA)9 films show randomly distributed grains or clusters on surface, and in this case heattreatment leads to increase of RMS roughness of the multilayer film from 0.26 nm to 0.33 nm (Figs. 3c and 3f), which could be attributed to heat-treatment-induced agglomeration of SiO2 nanoparticles and defects as well. 3.2. Friction and wear behavior Fig. 4 shows friction coefficient-time curves of heat-treated composite films (PAH/PAA)9 , (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 and (SiO2 /PAA)9 sliding against 440C stainless steel ball at a velocity of 180 mm/min and normal load of 0.5 N. It is seen that (PAH/PAA)9 film has an antiwear life of only 1711 s, (SiO2 /PAA)9 film has an increased antiwear life of 2570 s, while (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 composite film registers an antiwear life of 3028 s. In one word, the multilayer films doped with nano-SiO2 particulates have a longer antiwear life than pure polyelectrolyte film, (PAH/PAA)9 , which could be because the surface-capped SiO2 nanoparticles acted to enhance the load-carrying capacity of the multilayer films [14,15]. Furthermore, amide bond (–CONH–) was formed between not only PAH and PAA but also SiO2 and PAA after heating, which would also contribute to increasing the stability
and strength of the composite films doped with nano-SiO2 particulates [12,16]. Unsurprisingly, (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 film has the longest antiwear life and shows potential in protection of MEMS. This could be because the doped multilayer film (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 has “sandwich-like” structure with increased inter-layer adhesion [17]. Namely, the first three strata of (PAH/PAA)3 are not only anchored with silicon substrate but also serve as an initial “bed” for SiO2 nanoparticles to be inhabited [18]. The next three stratum containing alternative SiO2 nanoparticles (which bear a positive surface charge) and PAA is capable of enhancing the load-carrying capacity of the films. And the final stratum, also composed of three bilayers (PAH/PAA)3 , allows doped nanoparticles to maintain their strength and robustness while ensuring that their exposed surface is soft and fluid-like [19], which is helpful to eliminate defects associated with the middle stratum. Fig. 5 shows the friction coefficient–velocity curves for heattreated multilayer films (PAH/PAA)9 , (PAH/PAA)3 (SiO2 /PAA)3 (PAH/ PAA)3 and (SiO2 /PAA)9 sliding against stainless steel counterpart at a normal load 0.5 N respectively. (SiO2 /PAA)9 film has the lowest friction coefficient among the three kinds of tested multilayer films, and its friction coefficient keeps almost unchanged (0.15) with increasing sliding velocity. This might be attributed to the presence of SiO2 nanoparticles which act as “miniature ball bearings” [20,21] and transform the sliding between the multilayer film and stainless steel to rolling, resulting in lowered friction coefficient. However, (SiO2 /PAA)9 film has poor antiwear ability, largely because it has poor adhesion to the silicon substrate when SiO2 nanoparticles are directly assembled onto the Si substrate. Besides, (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 multilayer film with a “sandwich-like” structure has a friction coefficient of 0.25–0.35, depending on varying sliding velocity. In general, the two types of multilayer films containing nano-SiO2 particulates had lower friction coefficient than the pure polyelectrolyte film (PAH/PAA)9 , possibly because SiO2 nanoparticles acted to reduce the stiction of pure polyelectrolyte film and hence considerably reduced friction coefficient [22]. Furthermore, the friction coefficient of (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 film is higher than that of (SiO2 /PAA)9 film. This could be because the outermost strata (PAH/PAA)3 of the (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 film has a higher stiction than (SiO2 /PAA)9 , which results in increased friction coefficient. Fig. 6 shows representative SEM images of wear tracks of the multilayer films sliding against 440C stainless steel at a normal load of 0.5 N and sliding velocity of 180 mm/min. After sliding for 1680 s, the worn surface of (PAH/PAA)9 film showed signs of mild plastic deformation (Fig. 6a). When sliding against stainless steel ball for 2950 s and 2470 s, respectively, both (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 and (SiO2 /PAA)9 films had wear tracks narrower than that of (PAH/PAA)9 film (Figs. 6b and 6c), corresponding to increased wear resistance of the multilayer films doped with nano-SiO2 particulates and confirming that the introduction of SiO2 nanoparticles helped to effectively improving the antiwear ability of the SA-LBL polyelectrolyte multilayer films. Fig. 7 shows the typical scratch test results for (PAH/PAA)9 film, where F x , normal force (F z ), and AE were recorded as functions of time in seconds. In the scratch test, as the microblade moved slowly against the film, materials removal progressively occurred, and a higher critical load corresponded to a higher adhesion of the film to the substrate. (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 film has an adhesion of about 628 mN, which is higher than 550 mN and 460 mN, that of (PAH/PAA)9 film and (SiO2 /PAA)9 film. The scratch test results are also summarized in Table 1. The difference in the critical load of the three types of films could be highly dependent on their different composition and microstructures leading to different porosity and stress [23].
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Fig. 3. AFM images of various as-prepared and heat-treated multilayer films: as-prepared (a) (PAH/PAA)9 , (b) (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 , (c) (SiO2 /PAA)9 and heattreated (d) (PAH/PAA)9 , (e) (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 , (f) (SiO2 /PAA)9 .
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Table 1 Surface roughness determined by AFM and critical load determined by scratch tests of the multilayer films. Structure
RMS (nm)
Critical load (mN)
Before heating After heating After heating (PAH/PAA)9 3.831 (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 4.532 0.2601 (SiO2 /PAA)9
2.004 0.8050 0.3253
550 628 460
Fig. 4. Friction coefficient as a function of sliding time at a normal load of 0.5 N, sliding velocity of 180 mm/min.
Fig. 5. Friction coefficient as a function of sliding velocity at a normal load 0.5 N.
4. Summary SiO2 nanoparticles surface-capped with γ -aminopropyltrimethoxysilane were doped in polyelectrolyte multilayer films via spinassisted layer-by-layer deposition. The resulting nano-SiO2 -doped composite films (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 and (SiO2 /PAA)9 on Si substrates had cross-linking structures, owing to the generation of amide bond (–CONH–) between not only PAH and PAA but also SiO2 and PAA. However, (SiO2 /PAA)9 film had poor adhesion to the silicon substrate, since SiO2 nanoparticles were directly assembled onto Si substrate resulting in increased porosity and stress in SiO2 layer and hence reducing the adhesion. Both (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 and (SiO2 /PAA)9 multilayer films had better wear resistance than pure polyelectrolyte multilayer film (PAH/PAA)9 , which was related to the ability to enhance load-carrying capacity and “miniature ball bearings” effect as well of SiO2 nanoparticles. Heat-treatment had significant effect on the morphology of the composite multilayer films doped with nano-SiO2 particulates. Namely, heat-treated (SiO2 /PAA)9 film had a larger RMS roughness
Fig. 6. SEM micrographs of the wear tracks of the multilayer films sliding against 440C stainless steel at a normal load of 0.5 N and sliding velocity of 180 mm/min, (a) (PAH/PAA)9 sliding for 1680 s, (b) (PAH/PAA)3 (SiO2 /PAA)3 (PAH/PAA)3 sliding for 2950 s and (c) (SiO2 /PAA)9 sliding for 2470 s.
than the as-prepared one, possibly due to agglomeration of SiO2 nanoparticles and initiation of defects during heat-treatment. Contrary to the above, heat-treated (PAH/PAA)3 /(SiO2 /PAA)3 (PAH/PAA)3 film had greatly reduced RMS roughness than the as-prepared one, indicating that the “sandwich-like” structure was beneficial to retaining desired morphology and surface quality of the composite multilayer film. Such a kind of “sandwich-like” structure also contributed to increasing the wear resistance of multilayer film (PAH/PAA)3 /(SiO2 /PAA)3 (PAH/PAA)3 . Namely, the outermost strata of (PAH/PAA)3 in composite multilayer film (PAH/PAA)3 / (SiO2 /PAA)3 (PAH/PAA)3 were able to eliminate defects associated with the middle strata, allowing the nanoparticles therein to maintain strength and robustness while keeping soft and fluid-like exposed surface and hence decreasing shearing stress during sliding. And the inner strata of (PAH/PAA)3 well anchored to Si substrate
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Henan Innovation Project for University Prominent Research Talents (2006KYCX001) is also acknowledged. We gratefully acknowledge Beibei Chen and Xiaohong Li for helpful material preparations. References [1] [2] [3] [4] [5]
Fig. 7. The curves of scratch test for (PAH/PAA)9 film.
acted as an initial “bed” for SiO2 nanoparticles to be inhabited, resulting in better antiwear ability associated with good loadcarrying capacity and “miniature ball bearings” effect as well of SiO2 nanoparticles. Acknowledgments We are grateful to the Ministry of Science and Technology of China (“973” plan grant No. 2007CB607606) and National Natural Science Foundation of China (grant No. 20571024) for financial support to this research. The financial support by
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