Self-assembly and tribological properties of a novel organic–inorganic nanocomposite film on silicon using polydopamine as the adhesion layer

Abstract

Utilizing the strong coordination interactions between phenolic hydroxyls of polydopamine (Pdop) and silver oxide nanoparticles (Ag₂O NPs), Ag₂O NPs were successfully assembled on a dual-layer film composed of a polydopamine outer layer and 3-aminopropyltriethoxy silane (APS) sub-layer, on silicon (Si) substrate. The morphologies, structures and chemical compositions of Ag₂O NPs and the tri-layer film were confirmed by various instruments, including ultraviolet absorption spectrum (UV), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Furthermore, the micro adhesion forces and macro tribological performances were evaluated using AFM and UMT-2M tribometer (CETR), respectively. The results indicated that the tri-layer film displays favourable friction reduction and wear resistance ability. Hence, this renders the self-assembly attractive for constructing the multilayer-structure films with favourable tribological performances for nano/microelectromechanical systems (NEMS/MEMS).

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Introduction

Adhesion, friction and wear are the three key problems in moving parts of NEMS/MEMS devices due to the strong interfacial forces resulting from the large surface area to volume ratio, including van der Waals forces, electrostatic interactions and dangling bonds etc., when the dimensions of devices shrink to nano- and micro-scales.¹ They severely affect the efficiency, power output and reliability of NEMS/MEMS systems,^2,3 for instance, adhesion is the main reason for the failure of micromirrors in commercial digital cameras.^4,5 Consequently, it is imperative for NEMS/MEMS systems to exploit molecularly thick lubricants with favorable tribological properties in practical applications.^6,7 When conventional lubrications used in macro-scale systems are invalid in NEMS/MEMS systems, self-assembly, as an effective method for preparing the novel nanolubricants to solve these problems, has been extensively researched.^8–10 Through condensation reactions of Si–OH between hydroxylated silicon and hydrolysed APS, and followed chemical reaction between amino (–NH₂) groups of APS and carboxyls (–COOH) of aliphatic acid, a dual-layer film is assembled on silicon, exhibiting good tribological performances.^11,12 Carbon nanomaterials containing fullerene (C₆₀) and graphene with excellent mechanical and tribological behaviors, which are easy to form oxygenic groups by oxidation, have been grafted to silicon as outer or inter layer in self-assembled films.^13–17

Dopamine, forming polydopamine by self-polymerization in basic solution, has been proved to be a strong adhesive for almost all substrates,^18–20 and is an effective modifier for metallic oxides and organic functional groups due to the numerous ayzls and hydroxyls.²¹ To develop its potential as modifier and anchor, scientists conduct immense amounts of research and a large number of promising results have been acquired in many fields. Polydopamine (Pdop) modified polytetrafluoroethylene micro powders as additives in water show good friction reduction and wear resistance ability even under low concentration.²² Iron oxide nanoparticles modified by dopamine can link functional groups to the iron oxide shell, possessing well-established bio-compatibility.²³ Modification of TiO₂ nanoparticles with dopamine causes the spatial separation of the photogenerated charges, holes localize on dopamine and electrons localize within the lattice of TiO₂, resulting in suppressed recombination of charges.²⁴ The covalently chelated ZnO/polydopamine hybrid nanoparticles present special optical properties.²⁵ Carbon nanotubes coated with polydopamine was not only chemically active, but also the sidewalls was not be ruined, and thus this renders convenient construction for organic ad-layers and electroless metallization.²⁶

Meanwhile, polydopamine provides a platform as connection layer for self-assembly, which has stronger binding forces than APS, and consequently endue assembled multi-layer films the high interface strength. Polydopamine as the connection liner for the assembled graphene oxide (GO)^16,27 and aliphatic chloride^28,29 to enhance the tribological properties has been reported. Unfortunately, no work has been reported on tribological performance of the self-assembled metallic oxide nanoparticles with polydopamine as the adhesive layer so far.

Herein, a novel tri-layer film consisting of APS, polydopamine as adhesion layer between APS and Ag₂O, and Ag₂O nanoparticles has been prepared by multi-step procedures. First, APS was assembled on silicon (labeled as Si-APS) through the condensation reaction of Si–OH between hydroxylated silicon and hydrolysed APS.³⁰ Then, dopamine formed polydopamine coating via self-polymerization on Si-APS film (labeled as Si-APS-Pdop), and it is convinced that dopamine can react with APS by special chemical reaction to form the firm coating.¹⁹ At last, Ag₂O nanoparticles were assembled on Si-APS-Pdop dual-layer film via coordination bonds³¹ (labeled as Si-APS-Pdop-Ag₂O NPs). The micro adhesion behaviors and macro tribological performances of the various films were evaluated by AFM and UMT-2M tribometer (CETR), respectively. Despite the previous studies point out that silver oxide is only a good high temperature lubricant,^32,33 our experimental results indicate silver oxide nanoparticles have potential to be used in self-assembled films for boundary lubrication at ambient temperature.

Experimental

Materials

P-type polished silicon (111) wafers were provided by GRINM Semiconductor Materials Co. (Beijing, China). 3-Aminopropyltriethoxy silane (99%, coded as APS), 3-hydroxytyramine hydrochloride (dopamine hydrochloride, 99.5%), and tris(hydroxymethyl)aminomethane (99.5%, coded as Tris) were obtained from Sigma-Aldrich. Other chemicals were analytical grades and used without further treatment.

Synthesis of Ag₂O nanoparticles

Water sol of silver oxide nanoparticles were synthesized by direct mixing of 100 mL 0.8 mM silver nitrate solution and 100 mL 0.8 mM sodium hydroxide solution under vigorous stirring. The ultraviolet (UV) absorption spectrum of silver oxide nanoparticles was detected by a Cary 60 UV-Vis spectrophotometer of Agilent Technologies Co. Ltd., and the morphologies were obtained by JEM-1200EX field emission transmission electron microscopy (TEM).

Fabrication of the tri-layer film

Silicon (111) wafers with size of 1 × 1 cm were ultrasonic cleaned by acetone for 10 min, ethanol for 10 min, and ultrapure water for 10 min in turns and then dried under nitrogen flow. To realize the hydroxylation of silicon wafers, they were immersed in piranha solution (a mixed solution of 7 : 3 (v/v) 98% concentrated sulfuric acid and 30% hydrogen peroxide) at 90 °C for 1 h. After removing the silicon wafers from piranha solution, they were cleaned thoroughly by vast ultrapure water. The hydroxylated silicon wafers (referred as Si–OH) were then immersed in APS solution of acetone and water (5 : 1,by volume) for 30 min to form APS film (referred as Si-APS) and cleaned by ultrapure water for 5 times, and also dried under nitrogen flow. After that, the Si-APS were immersed in Tris–HCl solution (10 mM) of dopamine (2 mg mL⁻¹) for 24 h and cleaned by ethanol, acetone and ultrapure water in turns to finish chemical adsorption and self-polymerization of dopamine, thus, the dual-layer film of APS and Pdop (referred as Si-APS-Pdop) is formed. Taking out them from ultrapure water and dried under nitrogen flow, the Si-APS-Pdop films were successively immersed in Ag₂O nanoparticles sol for 48 h to construct the tri-layer film (referred as Si-APS-Pdop-Ag₂O NPs). The schematic diagram for the fabrication of the target film is shown in Fig. 1. The morphologies of different films were scanned by an atomic force microscope (AFM) of AIST-NT smartSTP (America).

Fig. 1 Schematic diagram for preparation processes of the Si-APS-Pdop-Ag₂O NPs film.

Test of micro adhesion forces

An atomic force microscope (AFM) of AIST-NT smartSTP (America) was employed for micro adhesion forces investigation in contact mode. Si₃N₄ cantilever for AFM measurement was coated aluminum on its back, and the force constant and radius of needle are 0.3 N m⁻¹ and 20–25 nm, respectively. The adhesion force tests for each sample were conducted five times and the value of the adhesion force was the mean value of the five times.

Test of macro tribological behaviors

To characterize the macro tribological behaviors of the prepared films, UMT-2M tribometer was applied in reciprocatory motion mode. Commercial steel balls with diameter of 3 mm were used as upper counterparts, and the various films were used as under counterparts. Additionally, steel balls were ultrasonic cleaned by acetone for 30 min before test. The sliding distance and rate were 5 mm and 5 mm s⁻¹ under different applied loads, respectively.

Results and discussion

UV absorption spectrum and TEM morphology of silver oxide nanoparticles

Ag₂O nanoparticles were synthesized by a facile method. The UV absorption spectrum and TEM image are shown in Fig. 2. It is seen from Fig. 2a that the UV spectrum of the nanoparticles exhibits a strong and peaked absorption at 210 nm, which suggests a narrow size distribution for Ag₂O nanoparticles. Fig. 2b presents TEM image of Ag₂O nanoparticles. From the inset in Fig. 2b, it can be seen that the average size of the Ag₂O nanoparticles is about 5–8 nm.

Fig. 2 UV absorption spectrum (a) and TEM morphology (b) for silver oxide nanoparticles.

AFM images of Si-APS-Pdop and Si-APS-Pdop-Ag₂O NPs films

Fig. 3 shows AFM morphologies of Si-APS-Pdop and Si-APS-Pdop-Ag₂O NPs films. The two films are characterized by particulate-like structure. What is more, the RMS micro-roughnesses of 12.2 nm for Si-APS-Pdop-Ag₂O NPs is higher than that of 7.8 nm for Si-APS-Pdop.

Fig. 3 AFM images of (a) Si-APS-Pdop and (b) Si-APS-Pdop-Ag₂O NPs. The scanning area is 2 μm × 2 μm. The average root-mean-square (RMS) micro-roughnesses for Si-APS-Pdop and Si-APS-Pdop-Ag₂O NPs are 7.8 nm and 12.2 nm, respectively.

XPS spectra analysis for Si-APS-Pdop and Si-APS-Pdop-Ag₂O NPs films

In order to further confirm the chemical constituents of the assembled films, XPS spectrum surveys were carried out. Fig. 4 provides the XPS spectra for Si-APS-Pdop and Si-APS-Pdop-Ag₂O NPs, and all the binding energies are referenced to C1s (284.5 eV). XPS is a sensitively and quickly analytical method for surficial elements with capable of measuring thickness less than 10 nm on the films.³⁴ When Ag₂O nanoparticles were assembled on Si-APS-Pdop film, the relative content of carbon within the XPS detection range decreased because that the surface was covered by Ag₂O nanoparticles with a diameter of 5–8 nm. For that reason, the relative intensity for C1s of Si-APS-Pdop-Ag₂O NPs (Fig. 4b) is weaker than that of Si-APS-Pdop (Fig. 4a). As shown in Fig. 4b, the peaks for silver are also appeared in full spectrum of Si-APS-Pdop-Ag₂O NPs. For fine spectrum of silver in Fig. 4c, two peaks at 368.2 eV and 373.9 eV can be fitted into Ag 3d5/2 and Ag 3d3/2 spectra.³⁵ The peak at 532.3 eV in Fig. 4d is assigned to O 1s electron emission spectrum.

Fig. 4 XPS full spectra for Si-APS-Pdop (a) and Si-APS-Pdop-Ag₂O NPs (b), and fine spectra for silver (c) and oxygen (d) of Si-APS-Pdop-Ag₂O NPs.

Therefore, it can be concluded that Ag₂O nanoparticles were prepared successfully and compactly arranged on the surface of the tri-layer film via chemical adsorption.

Micro adhesion forces

It is well established that friction and wear behaviors are affected by adhesion, abrasion, delamination, fatigue and so on.³⁶ But it is clear that when the size of material downsizes to nano and micro dimensions, adhesion is the key factor as a result of many enhanced interfacial forces, such as van der Waals forces, electrostatic interactions and dangling bonds etc. So, the micro adhesion forces were detected by AFM to indirectly predict the macro tribological behaviors. The adhesive forces of materials were examined by measuring the pull-off forces between the tip and sample surface at the force calibration mode of AFM.³⁷

Fig. 5 shows the micro adhesion forces of various films. Owing to the higher surface energies of Si-APS and Si-APS-Pdop, their adhesion forces are larger than that of Si-APS-Pdop-Ag₂O NPs. After the Si-APS-Pdop film is coated by Ag₂O nanoparticles, the adhesion force of the film decreases from 57.3 nN to 15.3 nN, which displays that the Si-APS-Pdop-Ag₂O NPs tri-layer film has good adhesion–resistance ability. This good adhesion resistance ability affects its macro tribological performances greatly, which will be discussed in detail following.

Fig. 5 Micro adhesion forces of the various films.

Macro tribological behaviors

To investigate the macro tribological performances of the various films, UMT-2M tribometer was employed. The friction coefficient–time curves are shown in Fig. 6. It is seen that the friction coefficient for Si-APS (Fig. 6a) increases from preliminary 0.2 to 0.8 at 60 s, indicating Si-APS fails, while the friction coefficient of Si-APS-Pdop fluctuates between 0.2 and 0.4 from the original period to 128 s and then soon increases to 0.7 at 140 s (Fig. 6b). In other words, Si-APS and Si-APS-Pdop can decrease the friction of silicon wafers, but their wear lives are short. The friction coefficients of Si-APS-Pdop-Ag₂O NPs film under different load are shown in Fig. 4c–e. After the run-period of 100 s, the friction coefficient keep at about 0.25 under the load of 10 g during the whole test period, and the curve is smooth, which indicates Si-APS-Pdop-Ag₂O NPs tri-layer film possesses good tribological performance (Fig. 6c). The friction coefficient–time curve slightly fluctuates under 15 g, but the friction coefficient still keeps at an average value of 0.26 and do not fail after sliding time of 30 min (Fig. 6d). When the applied load increases from 15 g to 20 g, the fluctuation of friction coefficient–time curve is aggravated and the friction coefficient abruptly increases to 0.4 at 1200 s, and then quickly rises to 0.9 in 200 s (Fig. 6e). From Fig. 6f, it is known that the average friction coefficient of Si-APS-Pdop-Ag₂O NPs increases with the increase of the applied load. It is obvious that the change trend of macro tribological properties of the three kinds of films are adverse to that of micro adhesion forces, namely, Si-APS-Pdop-Ag₂O NPs film has the best friction reduction and wear resistance abilities and the lowest adhesion forces among the three kinds of films. This phenomenon is easy to be understood, because the adhesion force has a significant influence on tribological behaviors when the sizes of bulk material shrink to micro and nano scales. The low adhesion forces weaken the interfacial shear strength and decrease friction and wear. Additionally, the decrease of friction coefficient for Si-APS-Pdop-Ag₂O NPs film could be attributed to the decreased surface energy and the formation of Ag₂O lubricious transfer layer on the counterpart surface during sliding.^32,33

Fig. 6 (a), (b), and (c) are the friction coefficient–time curves for Si-APS, Si-APS-Pdop and Si-APS-Pdop-Ag₂O NPs under 10 g, respectively; (d) and (e) are the friction coefficient–time curves for Si-APS-Pdop-Ag₂O NPs under 15 g and 20 g, respectively; (f) is the friction coefficient–load curve of Si-APS-Pdop-Ag₂O NPs. FC is the abbreviation of friction coefficient.

Why do the Si-APS-Pdop-Ag₂O NPs films possess good tribological properties? Except the characteristics of Ag₂O, the other key reason is the polydopamine as adhesion layer provides a strong binding force between APS and Ag₂O, and improves the interface strength of the tri-layer film, leading to the good wear resistance ability. Consequently, this renders the self-assembly attractive for constructing the multilayer-structure films with the favorable tribological performance for NEMS/MEMS systems.

Conclusions

In the present article, Ag₂O nanoparticles fabricated by a mild method were successfully assembled on Si-APS-Pdop dual-layer film to construct the Si-APS-Pdop-Ag₂O NPs tri-layer film via the coordination bonds between silver oxide nanoparticles and polydopamine. The tribological tests demonstrated that the tri-layer film has good friction decrease and wear resistance ability under very low load. Several conclusions can be drawn:

(1) The synthesized Ag₂O nanoparticles have uniform size of 5–8 nm and narrow size distribution, which can be seen in the UV absorption spectrum and TEM image;

(2) The acting forces of the self-assembly of silver oxide nanoparticles are strong coordination interactions between polydopamine and Ag₂O;

(3) The mechanisms of the friction decrease and wear resistance of the Si-APS-Pdop-Ag₂O NPs tri-layer film are supposed to be the synergetic functions of strong binding forces from the interface provided by polydopamine, the decreased surface energy of Ag₂O nanoparticles and the characteristic of Ag₂O nanoparticles as a solid lubricant.

What's more, it is well known that Ag₂O is a high temperature resistance material,^32,33,38 which is effective at 300–400, and 600–700 °C acting as solid lubricant. Compared to many organic and carbon coatings developed in NEMS/MEMS systems recent years, it is believed that this metallic oxide nanoparticles coating will adapt more harsh situations, such as high-temperature environment in NEMS/MEMS systems.

Acknowledgements

The work is financially supported by the “Western Light Talent Culture” Project and the “Top Hundred Talents” Program of Chinese Academy of Sciences.

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