Article pubs.acs.org/Langmuir
Simple Physical Approach to Reducing Frictional and Adhesive Forces on a TiO2 Surface via Creating Heterogeneous Nanopores Rong An,† Qiuming Yu,‡ Luzheng Zhang,§ Yudan Zhu,† Xiaojing Guo,† Shuangqin Fu,† Licheng Li,† Changsong Wang,† Ximing Wu,† Chang Liu,† and Xiaohua Lu*,† †
State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, 5 Xinmofan Road, Nanjing 210009, PR China ‡ Department of Chemical Engineering, University of Washington, Seattle, Washington 98195, United States § Illinois State Geological Survey, University of Illinois at Urbana−Champaign, 615 East Peabody Drive, Champaign, Illinois 61820, United States S Supporting Information *
ABSTRACT: A simple physical strategy to reduce the frictional and adhesive forces on TiO2 films was proposed by constructing mesoporous TiO2 films with heterogeneously distributed nanopores on the film surfaces. In comparison, TiO2 films with densely packed nanoparticles were also prepared. The crystal structure and morphology of the films were characterized with Raman spectroscopy, field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). It was found that the TiO2(B) phase exists in the mesoporuos TiO2 films but not in the densely packed films. The existence of TiO2(B) plays a significant role in creating and maintaining the nanopores in the mesoporous TiO2 films. The frictional and adhesive forces were measured on both films using AFM. The mesoporous films exhibit two typical adhesion forces of around 3 and 12 nN in the force distribution profile whereas the densely packed films show only one around 12 nN. The frictional coefficients were 2.6 × 10−3 and 6.7 × 10−2 for the mesoporous and densely packed TiO2 films, respectively. A model based on the atomic structures of a thin film of water molecules adsorbed on TiO2 surfaces leading to hydrophobic effects was proposed to understand the lower frictional and adhesive forces observed on the mesoporous TiO2 films. This simple physical approach to reducing the frictional and adhesive forces on TiO2 films could have broad applications to a variety of surface coatings. organic films composed of functional groups depend on the film packing density, structure, and crystallinity of the alkyl chains from surface modification.8,11 The grafted thin film on surfaces cannot provide reliable and long-lasting protection in sliding or impacting contacts in fluidic systems.12 More crucially, the functional groups of the thin film grafted on the TiO2 surfaces could lead to poor biocompatibility because the functional groups are susceptible to oxidation, hydrolyzation, and thermal degradation.13−16 However, by means of physical strategies, increasing the roughness of the TiO2 film will unavoidably lead to the retention of fluid flow, nonspecific protein adsorption, and poor hemocompatibility of titanium surface.17,18 Recent studies of roughness-induced superhydrophobic surfaces19 showed that low frictional and adhesive forces can be achieved by forming a multiple roughness scales on
1. INTRODUCTION Titanium (Ti) has been widely used for many applications ranging from heat exchangers1 to microelectromechanical systems (MEMS)2 and medical implants3 because of its excellent chemical stability. A native dense titanium dioxide (TiO2) layer3−5 is usually formed on the Ti surface, which not only prevents Ti from further oxidation but also improves the biocompatibility of Ti because the TiO2 surface can adsorb water molecules.5,6 However, the native dense TiO2 surface has high friction that is not favorable to fluid flow for long-term exposure to severe media.1,5,7 In addition, it can cause falling heat exchanger efficiency and the nonspecific adsorption of proteins, cells, thrombus, osteoarthritis, and so on. Surface modification with chemical functional groups such as designed spiroalkanedithiols-based self-assembled monolayers (SAMs),8 mixed monolayers consisting of octadecyltrichlorosilane SAMs infused with 3-phenyl-1-propanol,9 SAMs derived from 16,16,16-trifluorohexadecanethiol, and a normal alkanethiol10 was utilized to provide a heterogeneous, stable TiO2 surface with low friction. The frictional properties of such thin © 2012 American Chemical Society
Received: July 20, 2012 Revised: September 18, 2012 Published: October 9, 2012 15270
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Figure 1. FESEM images of (a) a mesoporous and (b) a dense TiO2 film. AFM phase images of (c) a mesoporous and (d) a dense TiO2 film. The vertical scales are 72° in c and d. solution (pH 3). Finally, the mesoporous TiO2 films were formed by calcination at 500 °C for 2 h.26,27 In comparison, the densely packed TiO2 films were synthesized via the sol−gel method.28 The detailed synthesis procedures are provided in the Supporting Information. 2.2. Raman Spectroscopy. The crystalline phases existing in the synthesized films were characterized using a Horiba Labram HR 800 Raman spectrometer. A 514 nm He−Cd laser was used to excite the Raman scattering. The laser power on the sample surface was 20 mW. The scattered light was detected using a thermoelectrically cooled charge-coupled device (CCD). 2.3. Field Emission Scanning Electron Microscopy (FESEM). The surface morphology of the synthesized TiO2 films was characterized using an Hitachi S-4800 (FEI, Japan) FESEM system at room temperature (20 ± 2 °C). 2.4. Atomic Force Microscopy (AFM). The frictional and adhesive forces of the mesoporous and dense TiO2 film surfaces were measured using AFM (Autoprobe CP-Reaserch, Bruker, USA) under ambient conditions at room temperature. Gold-coated silicon nitride cantilevers with spring constants of 0.01−0.6 N/m were used. The scan speed was 1.00 Hz. The images underwent second-order flattening using ProScan (version 2.1) software. AFM images were taken, and force measurements were conducted on more than three areas per sample. The frictional and adhesive forces were measured in contact mode. Frictional loops of the two TiO2 surfaces were recorded in air, at 25 °C and a relative humidity of ∼50%, and analyzed as a function of normal load. The lateral force increases linearly with the normal force for the Si3N4 tip and the TiO2 films. Measured values were reproducible for three identical samples (over five independent measurements for each sample, in air at 25 °C at a relative humidity of ∼50%). The adhesive force was measured using the force−distance curve approach. In this technique, the AFM tip is brought into contact with the sample by extending the piezo vertically. Then the piezo is retracted, and the force required to separate the tip from the sample is calculated.21,29−34 Typical force separation curves for the Si3N4 tip approaching the mesoporous and dense TiO2 surfaces were obtained. Measured values were reproducible for five identical samples (over eight independent positions for each sample, in air at 25 °C at a relative humidity of ∼50%; only the retracted part of the force curves is shown here). 2.5. Nitrogen Adsorption−Desorption Isotherm Analysis. Nitrogen adsorption−desorption isotherms were recorded at liquid
surfaces.20,21 For example, a model surface for superhydrophobicity is water-repellent lotus leaves.22 In addition to the microscale roughness, the surface of the papillae is also rough with nanoscale asperities composed of 3D epicuticular waxes that are beneficial to sliding.21 Atomic force microscopy (AFM) has been employed to explore the molecular-level factors governing friction in various systems with nanoscale sensitivity.8,11 Previous investigations using both AFM23 and molecular dynamics (MD) simulations24 show that the presence of long-range interactions related to hydrophobic effects25 makes only a small contribution to friction.21,22 To lower surface frictional and adhesive forces, a simple approach should be achieved by constructing geometrically roughnessinduced and heterogeneously nanopatterned surfaces that lead to hydrophobic effects. The effects make the surfaces possess lower frictional and adhesive forces and could further confer adsorbate resistance and, in the meantime, maintain the surface biocompatibility. In this study, we present a simple physical strategy for providing hydrophobic effects to reduce the surface friction based on titanate by constructing a roughness-induced and heterogeneously nanopatterned mesoporous TiO2 films. This simple physical strategy for reducing the friction on the mesoporous TiO2 films benefits the fluid flow. It could further block the propensity of various adsorbates that may stay on the interfaces. Meanwhile, it maintains the TiO2 biocompatibility for potential biomedical applications. The densely packed TiO2 films were also synthesized and analyzed for comparison.
2. EXPERIMENTAL SECTION 2.1. Preparation of Mesoporous and Dense TiO2 Films. TiO2 films with heterogeneously nanostructured pores, herein called mesoporous TiO2 films, were synthesized on a fresh Ti substrate. Briefly, the potassium dititanate (K2Ti2O5) films were formed on Ti surfaces using sol−gel and dip-coating methods followed by calcination. (FESEM image and Raman spectra are shown in Figure S1.) The K2Ti2O5 films were then treated with water vapor at 150 °C for 8 h, followed by exchanging K+ ions with H+ ions in an acidic 15271
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TiO2,39,40 were observed in the mesoporous TiO2 films. The existence of the TiO2(B) phase is beneficial to the thermal stability of the mesoporous anatase TiO2.41 The weight percentage of the TiO2(B) phase in the mesoporous TiO2 films was determined by calculating the peak intensity of the TiO2(B) phase to that of the anatase phase.42 It was found that the weight percentage of TiO2(B) reached as high as 59.7% in the mesoporous TiO2 films. (Detailed calculations are given in Figure S3 in the Supporting Information.) The X-ray diffraction of mesoporous TiO2 films displayed in Figure S4 offers solid proof of the coexistence of TiO2(B) and anatase phases. The AFM topographic images of the mesoporous and the dense TiO2 film surfaces are shown in Figure 3a,b, respectively. It is shown that particles and rods that are composed of smaller
nitrogen boiling temperature and the relative pressure (P/P0) interval between 0.032 and 0.986 on the equipment supplied by Micromeritics Tristar II3020. Samples were degassed at 60 °C for 12 h under 0.1 mbar prior to the measurements. 2.6. Contact Angle Measurements. Contact angle measurements were performed using an SL200B (Solon Technology Science Co., China). Six water drops were analyzed on each sample. At least three samples were prepared and tested for both mesoporous and dense TiO2 films.
3. RESULTS AND DISCUSSION The surface morphology of both mesoporous and dense TiO2 films was examined using FESEM and AFM. The typical surface morphology of mesoporous and dense TiO2 films is shown in Figure 1. The FESEM image in Figure 1a shows that nanostructured pores are created on the entire mesoporous TiO2 surface. By N2 adsorption−desorption isotherms as displayed in Figure S2 in the Supporting Information, the surface area and pore size distribution for mesoporous TiO2 films were determined. It shows a typical IV isotherm, further indicating the presence of heterogeneously distributed mesoporosity in the films, which gave a high BET surface area (83 ± 4 m2·g−1) and a very narrow pore-size distribution of around 7 nm. The FESEM image in Figure 1b shows a very different surface morphology for the dense TiO2 film. It is clearly seen that the surface of the dense TiO2 film is formed by particles of about 20−50 nm and gaps of around 15−30 nm between particles. A lower BET surface area of 22 ± 3 m2/g was obtained on this surface. Different surface morphologies for these two types of surfaces were further demonstrated by the phase images acquired using tapping mode AFM. It is known that the contrast in phase images is mainly caused by the difference in viscoelasticity and surface energy of the materials existing on the surfaces.35−37 The heterogeneously distributed nanopores are clearly observed on the mesoporous TiO2 film surface in Figure 1c whereas the homogeneously distributed nanoparticles are seen on the dense TiO2 film surface as shown in Figure 1d. The crystalline phases existing in the mesoporous and dense TiO2 films were examined using Raman spectroscopy. Figure 2
Figure 2. Raman spectra of mesoporous and dense TiO2 films. The Raman vibrational bands due to the anatase phase are marked with solid squares whereas those due to the TiO2(B) phase are marked with solid diamonds.
Figure 3. (a, b) AFM topographic images of the mesoporous and dense TiO2 films, respectively. (c, d) Line profiles along the TM−TM line in a and the TD−TD line in b, respectively. (e) Adhesive forces measured at points A and B on the mesoporous TiO2 film surface and point C on the dense TiO2 film surface, where A and C are on top of the nanoparticles and B is to the right at the dip point. (f, g) Histogram of adhesive forces measured at ∼80 different positions over an area of 2 μm × 2 μm for the mesoporous and the dense TiO2 film surfaces, respectively. The vertical scales in a and b are 30 nm.
shows the Raman spectra of mesoporous and dense TiO2 films. It was found that the dense TiO2 films have the strong peaks at 143.8, 195.4, 394.9, 513.1, and 638.2 cm−1 as a result of the anatase phase38 whereas two additional weak peaks at 119.4 and 242.2 cm−1, corresponding to the TiO2(B) phase,38 which is different from the phases of anatase, rutile, and brookite 15272
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normal load. The lateral forces increase linearly with the normal load within the measured ranges for both surfaces. The friction coefficient is determined from the slope of the line43,48,49 and is 2.6 × 10−3 for the mesoporous TiO2 film surface, which is about 26 times lower than that of the dense TiO2 film surface (6.7 × 10−2). The simulation results from our group6,50 and experimental results from Anpo’s group51 showed that the bound water52 molecules as part of the TiO2 surface have very strong interactions with the TiO2 surface, which is the reason that TiO2 is biocompatible.53,54 The desorption signal in the temperature-programmed desorption (TPD) spectra55 at 259 °C and the band at 1137 cm−1 in the infrared (IR) spectra56 in Figure S6 indicate the existence and strong interactions of the bound water on the TiO2 surfaces whereas the desorption signal in the TPD spectra at 127 °C corresponds to free water. The larger difference in the friction coefficient (26-fold) is due to the nanostructured pores on the mesoporous TiO2 surface as shown in Figure 5a. The tip (radius = 10−40 nm) readily sits
heterogeneous nanostructures found in Figure 1c randomly distribute over the entire surface of the mesoporous TiO2 film surface whereas the dense TiO2 film surface is homogeneously covered by particles with a narrow range of sizes from 20 to 70 nm. The line profiles (Figure 3c,d) show that the mesoporous TiO2 film surface is much rougher than the dense TiO2 film surface. The resulting average roughness (Ra) is 4.10 and 0.39 nm for the mesoporous and the dense TiO2 film surfaces, respectively. The adhesive force measured at point A (i.e., at the top of a particle on the mesoporous TiO2 film surface) is 12.6 nN whereas the adhesive force measured at point B (i.e., a deep void) is only 3.2 nN. At point C for a particle on the dense TiO2 film surface, the adhesive force was measured to be 13.2 nN, which is close to that obtained from the particle on the mesoporous TiO2 film surface. The heterogeneity of the mesoporous TiO2 film surface is further demonstrated by the variation of the measured adhesive forces. The statistic histograms of the adhesive forces43 for the mesoporous and the dense TiO2 film surfaces are shown in Figure 3f,g, respectively. The distribution of adhesive forces measured on the mesoporous TiO2 film surface shows a bimodal peak with one around 4 nN and another around 12 nN representing the adhesives forces measured in the deep voids and the particles, respectively. However, for the dense TiO2 film surface, the histogram shows a single peak at around 12 nN, indicating the surface homogeneity.44 Clearly, the deep nanovoids on the mesoporous TiO2 film surface have lower adhesive forces than the particles on the mesoporous and dense TiO2 film surfaces. The frictional forces on these two surfaces were also investigated. The friction coefficient was first measured on a mica surface, and a value of 7.6 × 10−5 (shown in Figure S5) was obtained, which is close to the value of 8.0 × 10−5 reported in the literature,45 verifying the reliability of our method. The mica friction coefficient decreases gradually as the humidity increases. Ohnishi et al. suggest that layering adsorbed water on mica is important.46 Hu et al. point out that water functions as a lubricant for silicon nitride and mica by experiments at high humidity and in water.45 However, mica is atomically flat, although not from the view of other different surface properties including hydrophobicity, roughness, and surface charge.47 Mesoporous and dense TiO2 films in our work possess rougher surfaces than atomically flat mica, so the influence of humidity on them is varied. More importantly, mica is first utilized to check the validity of AFM measurements in our work. The same reliable method was used to determine the friction coefficients of all of the samples in this study. Figure 4 shows the lateral forces on these two TiO2 surfaces as a function of the
Figure 5. Illustrations of a Si3N4 tip on (a) the mesoporous and (b) the dense TiO2 film surfaces in the presence of water molecules. Bound water means the first layer water molecules on the surfaces whereas free water means those beyond the first layer.
on the free water layer on the apex of nanostructured pores (
