Influence of Oxygen Vacancies on the Frictional Properties of

16 Aug 2017 - Oxygen vacancy is the most studied point defect and has been found to significantly influence the physical properties of zinc oxide (ZnO...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/Langmuir

Influence of Oxygen Vacancies on the Frictional Properties of Nanocrystalline Zinc Oxide Thin Films in Ambient Conditions Huan-Pu Chang,† En-De Chu,† Yu-Ting Yeh,† Yueh-Chun Wu,‡ Fang-Yuh Lo,† Wei-Hua Wang,‡ Ming-Yau Chern,§ and Hsiang-Chih Chiu*,† †

Department of Physics, National Taiwan Normal University, Taipei City 11677, Taiwan Institute of Atomic and Molecular Science, Academic Sinica, Taipei City 10617, Taiwan § Department of Physics, National Taiwan University, Taipei City 10617, Taiwan ‡

S Supporting Information *

ABSTRACT: Oxygen vacancy is the most studied point defect and has been found to significantly influence the physical properties of zinc oxide (ZnO). By using atomic force microscopy (AFM), we show that the frictional properties on the ZnO surface at the nanoscale greatly depend on the amount of oxygen vacancies present in the surface layer and the ambient humidity. The photocatalytic effect (PCE) is used to qualitatively control the amount of oxygen vacancies in the surface layer of ZnO and reversibly switch the surface wettability between hydrophobic and superhydrophilic states. Because oxygen vacancies in the ZnO surface can attract ambient water molecules, during the AFM friction measurement, water meniscus can form between the asperities at the AFM tip−ZnO contact due to the capillary condensation, leading to negative dependence of friction on the logarithm of tip sliding velocity. Such dependence is found to be a strong function of relative humidity and can be reversibly manipulated by the PCE. Our results indicate that it is possible to control the frictional properties of ZnO surface at the nanoscale using optical approaches.

■

INTRODUCTION Understanding the frictional properties of materials at the nanoscale is critical to the development of nanoelectromechanical systems (NEMS). For miniature components in NEMS, the friction and adhesion between moving surfaces may have a substantial impact on the functionality and performance of NEMS. However, the frictional properties of materials at the nanoscale can be immensely influenced by the universal presence of defects.1−3 To achieve an optimal NEMS operation, a full understanding about how defects, either intrinsic or intentionally introduced, modulate the frictional properties of a material employed in NEMS is imperative. Among materials that can be potentially used in NEMS, zinc oxide (ZnO) is probably the most promising for multifunctional applications due to its exceptional physical and chemical properties.4 Besides being a semiconductor with a wide band gap of 3.37 eV, ZnO also exhibits piezoelectric and pyroelectric properties that have great applications in optoelectronic devices, sensors, energy generators, actuators, and surface acoustic wave devices. By controlling the growth conditions, ZnO can also be tailored into different forms of nanostructures such as nanorod, nanowire, nanobelts, and nanofilms, which will have versatile applications in NEMS.5,6 In addition, the friction coefficient of pulsed laser deposited (PLD) ZnO thin films grown on a substrate temperature around 500 °C has been found to be around 0.2 in ambient, which is much smaller than 0.6 of bulk ZnO.7 Such improved tribological properties are attributed to the © XXXX American Chemical Society

nanocrystalline structures that are more ductile under surface shear.8 Highly crystalline ZnO surface prepared by PLD can thus be beneficial for NEMS applications,9−11 especially for lubrication coatings. However, as-grown ZnO usually has defects such as oxygen and zinc vacancies or interstitials. Among them, oxygen vacancies are the most mentioned and studied point defects and in general have a significant impact on the physical properties of ZnO.12 In fact, experiments have shown that the elastic modulus of ZnO nanobelts can be reduced by an order of magnitude due to the presence of oxygen vacancies and stacking faults.13,14 Additionally, oxygen vacancies can be used to induce band gap narrowing in ZnO nanostructures without dopants,15 and ferromagnetism in ZnO single crystal.16 The green luminescence and n-type semiconducting behavior of ZnO have generally been attributed to the presence of oxygen vacancies,17 but the exact origins are still under debate.18−20 ZnO nanostructures that have larger polar planes usually generate oxygen vacancies easily and thus become more catalytically active.21 Furthermore, water molecules have been found to dissociatively absorb on the sites of oxygen vacancies, resulting in surface hydroxylation and water adsorption on the ZnO surface.22−25 Such dissociative adsorption of water has been experimentally observed on both oxygen-terminated22,23 and Received: April 11, 2017 Revised: August 3, 2017

A

DOI: 10.1021/acs.langmuir.7b01242 Langmuir XXXX, XXX, XXX−XXX

Langmuir

■

zinc-terminated ZnO surface24 in ultrahigh vacuum (UHV) conditions. Although the frictional properties of ZnO surface have been investigated in the last decades by either the ball-onflat tribometer7,26−28 or atomic force microscopy (AFM),29−32 how the presence of oxygen vacancies influences the frictional properties of ZnO surface in ambient has not been systematically explored. Generally speaking, the nanoscopic sliding friction is governed by the superposition of two competitive friction mechanisms: the thermally activated capillary condensation and the stick−slip effect, which result in negative and positive dependences of friction on the logarithm of velocity, respectively.33−37 The capillary condensation, in particular, plays a major role in the determination of nanoscopic friction in ambient because water meniscus can condense within the nanosize gaps between asperities of two sliding surfaces, resulting in a negative dependence of friction on the logarithm of velocity.35−37 Because most ZnO-related applications are likely to function in atmospheric conditions, it is important to understand the influence of oxygen vacancy-assisted water adsorption on the frictional properties of ZnO surfaces in ambient conditions. Here, by using AFM-based techniques, we show that the nanoscopic sliding friction on ZnO surface can be significantly influenced by the interplay between the amount of oxygen vacancies in the ZnO surface and the relative humidity (RH). Photocatalytic effect (PCE)38−43 and argon plasma sputtering39 are methods known to create oxygen vacancies in ZnO after growth. While the former generates oxygen vacancies through the interaction of photoinduced holes and lattice oxygen in the ZnO surface using ultraviolet (UV) light, the later does so by physical ablation with argon ions. In our experiment, to avoid structural damage of ZnO surface, PCE is used to generate excess oxygen vacancies via UV illumination and converts the ZnO surface from being hydrophobic to superhydrophilic. Although UV treatment has been used to investigate the relationship between surface wettability and frictional properties of ZnO prepared by sol−gel method31 at the nanoscale, the influence of oxygen vacancies and ambient humidity on the velocitydependent friction, which definitely has a significant impact on the applications of ZnO-based NEMS in ambient, has never been studied in detail. Through the AFM friction measurements on ZnO, we find that the nanoscopic friction acquired on the asgrown ZnO surface at low sliding velocities exhibits a positive trend with the logarithm of velocity at RH = 4%, but shows a negative one when RH is larger than 41% due to the water molecules absorbed on the intrinsic oxygen vacancies and water bridges formed within the gap at the tip−ZnO contact. PCE is used to generate an excess amount of oxygen vacancies in the ZnO surface for wettability conversion. Much more negative dependence between friction and velocity is found on the UVtreated ZnO surface due to the enhanced capillary condensation at the tip−ZnO contact. At higher sliding velocities, the velocitydependent friction found on the as-grown and UV-treated ZnO surfaces demonstrates similar behavior dictated by the stick−slip effect. Furthermore, while the wettability conversion on the ZnO surface is reversible by the alternation of UV exposure and dark storage,42 we also find that the frictional behavior on the ZnO surface of each wettability state is also reproducible. Our results indicate that it is possible to manipulate the nanoscopic sliding friction on the ZnO surface via the PCE.

Article

MATERIALS AND METHODS

Preparation of ZnO Thin Films. The ZnO thin films studied in this work are grown on high-quality c-sapphire (α-Al2O3) substrates by using the pulse laser deposition (PLD) technique. During the thin film growth, the background pressure is kept lower than 8 × 10−7 mbar, while the deposition is carried out at an oxygen pressure of 3 × 10−1 mbar. It is known that a substrate temperature around 500 °C is the optimal condition for crystalline ZnO growth.9,10,44 Therefore, two different substrate temperatures, 200 and 550 °C, are used to prepare ZnO thin films with different crystalline film structures for comparison. In fact, our X-ray diffraction data shown in Figure 1 indicate that the as-grown ZnO

Figure 1. X-ray diffraction data of ZnO thin films grown at substrate temperatures of 200 and 550 °C. While ZnO550 shows highly crystalline (0002) structure, ZnO200 exhibits a mixture of both (0002) and (101̅1) crystalline planes. thin film at 550 °C, ZnO550, has predominantly (0002) crystalline structure with a c-axis lattice constant of 5.2 Å, while for the ZnO thin film deposited at 200 °C, ZnO200, its film structure exhibits both (0002) and (101̅1) crystalline planes. Determined by using AFM, the thicknesses of both ZnO films are found to be around 110 nm. Topography and Adhesion Measurements by AFM. The surface topography and adhesive properties of the as-grown ZnO films are investigated simultaneously by using the Peak-Force Quantitative Nanoscale Mechanical Characterization (PF-QNM) technique with a Bruker Multimode 8 AFM. During PF-QNM operation, the AFM cantilever taps on the ZnO surface at a frequency of 0.25 kHz, and the approaching and retracting force−distance curves are recorded for each pixel of the scanned area. Feedback loop is used to maintain a constant “peak force” exerted by the AFM tip on each pixel of the image to obtain topography information. The adhesion force corresponding to each pixel of the topography image is obtained from the retracting force− distance curve at the same time. The AFM cantilever (SCANASYSTAir) that has a typical tip radius of 2 nm is used. Thus, the topography and adhesion images can be obtained concurrently with high spatial resolution. During this measurement, the AFM system is placed inside a tight acrylic box with two inlets for dry and moist nitrogen gas flows to control the RH inside. When the target RH is reached, the whole system is stabilized for 30 min before the experiment starts. The humidity sensor is calibrated using saturated salt solutions (Supporting Information).45 The fluctuation of each target RH during the experiment is within ±2%. Wettability Characterization on the ZnO Surface. The surface wettability of freshly grown ZnO film is characterized by using a homebuilt contact angle (CA) goniometer. The CA is evaluated by placing a 2 μL droplet of ultrapure water on the ZnO surface. The CAs of as-grown ZnO200 and ZnO550 are measured to be 98.4 ± 6.0° and 94.1 ± 8.5°, respectively. Photocatalytic Effect and Wettability Conversion of the ZnO Surface. Photocatalytic effect (PCE) can be used to generate a considerable amount of oxygen vacancies in the surface layer of ZnO or titanium oxide (TiO2), further converting the surface from being hydrophobic to superhydrophilic if UV light with a wavelength of 254 nm38 or 365 nm is chosen.39−43 The generation process of UV-induced B

DOI: 10.1021/acs.langmuir.7b01242 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 2. (a) The evolution of water CA on ZnO200 and ZnO550 as a function of UV illumination time. The solid lines are a guide to the eyes. The inserted images show water droplets on the respective surfaces used for CA measurements. (b) Wettability conversion of both ZnO surfaces by alternation of UV treatment and dark storage. The averaged minimum water CAs on ZnO200 and ZnO550 after UV treatment are 39.0 ± 3.4° and 11.8 ± 2.2°, respectively.

Figure 3. (a) Raman spectrum of both ZnO thin films before and after UV treatment; (b) Zoom-in spectrum of (a) in the region between 415 and 500 cm−1. A clear increase of the peak intensity at 483 cm−1 is found on ZnO550. (c) Zoom-in spectrum of (a) in the region between 550 and 590 cm−1. Only the Eg mode of c-sapphire is clearly observed. CA is at first 98.4 ± 6.0° and eventually becomes 34.2 ± 8.3° after 3 h of UV exposure. In addition, the hydrophobicity of ZnO can be retained after leaving the sample in a dark desiccator for approximately 50 h. Through this process, the adsorbed hydroxyl groups on the defect sites of UV-treated ZnO will be slowly replaced by ambient oxygen, and subsequently the surface hydrophobicity can be recovered.41,42 During the course of our experiment, the photoinduced wettability conversion can be repeated for at least 5 cycles on both ZnO surfaces, as presented in Figure 2b, and the averaged minimum CAs after 3 h of exposure are 39.0 ± 3.4° and 11.8 ± 2.2° for ZnO200 and ZnO550, respectively. There is a concern that such a change in wettability might result from the removal of surface absorbed hydrocarbon, not the increased oxygen vacancies in the ZnO surface,48 because UV light is known to destroy the hydrocarbon,49 and possibly reduce surface hydrophobicity. However, if this is true in our experiment, then the minimum CAs found on both ZnO surfaces shown in Figure 2a and b should be very similar after a long period of UV treatment, and yet after several conversion cycles, the minimum CAs found on ZnO200 and ZnO550 are consistently around 39.0° and 11.8°, respectively. Such a difference in PCE response of ZnO surfaces demonstrated in Figure 2 cannot simply be explained by the UV-removal of surface absorbed hydrocarbon molecules. The surface of PLD ZnO thin films grown on the substrate temperature around 500 °C has been found to be predominantly oxygen-terminated.10,44 With decreasing substrate temperature, the ZnO thin films will become mixed with other facets, as evidenced by our X-ray diffraction results shown in Figure 1, and possibly exhibit both zinc and oxygen surface terminations. Therefore, as compared to ZnO200, the surface of ZnO550 may have more exposed oxygen anions and therefore

oxygen vacancies in the surface layer of ZnO is summarized in eqs 1−4:39

ZnO + 2hν → 2h+ + 2e−

(1)

O2 − + h+ → O1− (surface trapped hole)

(2)

O1− + h+ →

1 O2 + □ (formation of oxygen vacancy) 2

Zn 2 + + e− → Zn+S (surface trapped electron)

(3) (4)

Upon UV illumination, electron−hole pairs can be generated in the surface layer of ZnO. Some of the holes may react with the oxygen in the ZnO lattice, resulting in oxygen vacancies (□), where water and oxygen may dissociatively adsorb. Such water adsorption on ZnO or TiO2 surfaces due to the presence of oxygen vacancies has been experimentally confirmed by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectrometry (FTIR).46,47 Moreover, these defect sites are thermodynamically more favorable for hydroxyl than oxygen adsorption, subsequently leading to enhanced water adsorption and thus a superhydrophilic surface.43 In our experiment, an ultraviolet light source (Pen-Ray 11SC-2, UVP LLC) with a wavelength of 365 nm is used to illuminate the ZnO surface. The ZnO films are kept at 2 cm away from the light source for all tests. Figure 2a illustrates the CA evolutions on ZnO surfaces as a function of the UV illumination time. For the as-grown ZnO550, the water CA is initially 94.1 ± 8.5°, but quickly dropped to 16.6 ± 5.0° after 30 min UV exposure, and becomes only 5.9 ± 0.1° after 3 h of UV treatment. For the as-grown ZnO200, the C

DOI: 10.1021/acs.langmuir.7b01242 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 4. (a) AFM topography image of the as-grown ZnO550 surface acquired at RH = 4%. (b) Zoom-in topography (top) and adhesion (bottom) images of the area within the green dashed box indicated in (a) at RH = 4%. (c) Topography (top) and adhesion (bottom) images of the similar location in (b) at RH = 70%. In (a−c), the green stars in the topography images indicate the same surface location to guide the eyes. (d) Variations of the height profile (top) as well as the surface adhesions measured at RH = 4% and 70% (bottom) along the green lines shown in (b) and (c). (e) Histograms of adhesion distributions on the as-grown ZnO550 surface shown in (a) acquired at RH = 4% and 70% using PF-QNM. The solid lines are the Gaussian fit to the data. The fitted centers of the adhesion distribution are 0.8 ± 0.2 and 5.4 ± 2.6 nN for RH = 4% and 70%, respectively. cm−1, assigned to the E1LO mode, has been previously correlated to surface defects such as oxygen vacancy or zinc interstitials.54 However, in our experiment, the c-sapphire substrate chosen for optimal ZnO growth has a Raman peak at 578 cm−1, which is very close to the peak position of E1LO mode of ZnO. Therefore, as shown in the zoom-in spectra in Figure 3c, we are not able to identify the presence of this mode in our experiments probably due to weak Raman signals from ZnO and strong background from c-sapphire. On the basis of the evidence above, we believe that the observed wettability conversion found on the PLD ZnO surfaces must originate from the UV-induced oxygen vacancies. The surface absorbed hydrocarbon plays a negligible role in the observed phenomena. To further understand how oxygen vacancies influence the frictional properties of ZnO thin films in ambient, the optimal and most photoresponsive ZnO550 surface will be used for the following friction and adhesion investigations. Friction Measurements by AFM. The frictional properties of the PLD ZnO550 thin film before and after the UV illumination are studied by using AFM. For the following friction tests, the ZnO550 surface is treated with UV for 2 h for consistency. The friction force is recorded through the torsional deflection of the AFM cantilever. The normal and lateral force constants of the silicon AFM cantilever (Nanosensors, PPPCONTSCAuD) are calibrated using the thermal tune method55 and the improved wedge method,56 respectively. The normal force constant of the cantilever is determined to be 0.23 N/m. The lateral sensitivity is found to be 48.9 nN/V. The tip radius, RTip, is frequently checked and found to be 119.3 ± 6.4 nm during the course of the experiment (Figure S1). Two friction studies are conducted. First, we measure the friction coefficient between the silicon AFM probe and ZnO550 surface. The applied normal force is less than 25 nN for this measurement to avoid surface wear. The obtained friction versus normal load curves are found to be linear due to the high stiffness of both surfaces, that is, silicon and ZnO, which results in negligible surface deformations (Figure S2). Larger surface deformation at contact usually leads to nonlinear dependence between nanoscopic friction and normal load.2 Therefore, the classical Amonton’s law will be used to fit our data and extract the

is more prone to oxygen vacancy formation upon UV illumination. Similar PCE results have been found on different faces of ZnO50 and TiO246 due to distinct surface structures. Therefore, the observed wettability conversion in our experiment can mainly be attributed to the UV-induced oxygen vacancy generation in the ZnO surface. Raman Spectroscopy. We perform Raman spectroscopy on both ZnO samples before and after UV treatment to identify possible traces of photogenerated oxygen vacancies. The RH during our Raman measurement is around 40%. The obtained Raman spectra are shown in Figure 3a. The Raman spectrum of bare c-sapphire substrate is also plotted for comparison. The Raman band modes identified for csapphire are A1g at 419.6 cm−1 and Eg at 432.5, 450.8, 578.5, and 752.4 cm−1. For the as-grown ZnO thin films without any UV treatment, the Raman spectra obtained on ZnO200 and ZnO550 are very similar, and Raman mode E2High at 439.9 cm−1 of ZnO can be identified for both surfaces. However, very interestingly, if we zoom in at the region between 415 and 500 cm−1 in Figure 3a, as depicted in Figure 3b, a clear increase of the peak intensity at 483 cm−1 can be observed on the UVtreated ZnO550 as compared to that on the hydrophobic ZnO550 surface. Actually, the Raman peak position at 483 cm−1 for ZnO has been assigned to the disorder-induced surface phonon mode (2LA).51,52 The peak intensity of this particular mode has been found to be weak in undoped ZnO and mildly increases with doping concentration due to the change of surface morphology or local polarizability induced by the dopants.52 In addition, the intensity of this particular band (2LA) has also been related to the formation of hydroxyl group on the ZnO surface due to dissociatively absorbed water molecule on the defect sites.53 Thus, the enhanced intensity at 483 cm−1 observed in our experiment strongly suggests the occurrence of increased oxygen vacancies and water adsorption on the ZnO surface after UV illumination. On the other hand, no obvious change in the peak intensity at 483 cm−1 on ZnO200 is observed after UV treatment, indicating that there are less oxygen vacancies generated after UV treatment, and hence the wettability conversion on ZnO200 is not as dramatic as ZnO550 shown in Figure 2. We remark that for ZnO, a Raman peak position at 584 D

DOI: 10.1021/acs.langmuir.7b01242 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

ZnO550 thin films, the trapped air due to surface roughness may result in the observed large water CA. To get further insight into the effect of roughness on CA, we repeat the CA measurements on two single-crystal c-ZnO films that have oxygen and zinc terminated surfaces (MTI Corp., U.S.) with surface roughness of only 0.2 and 0.8 nm, respectively. The CAs are found to be 72.3 ± 2.1° and 53.8 ± 2.0° on O−ZnO and on Zn−ZnO, respectively (Figure S6). Actually, a recent theoretical calculation indicates that the surface energies of O−ZnO and Zn−ZnO are 0.96 and 2.39 J/m2, respectively.61 Therefore, the surface of Zn−ZnO can be more hydrophilic than that of O−ZnO. For our PLD ZnO550 surface, which is most likely to be oxygen-terminated,10,44 surface hydroxylation of the surface might take place in ambient if the intrinsic oxygen vacancies are present.22 Along the grain boundaries on the ZnO surface, oxygen vacancies and other facets of ZnO might occur, giving rise to enhanced water adsorption and stronger adhesion force, as seen in the adhesion map presented in Figure 4b and c. In general, the adhesion force FAdh between two solid surfaces in a humid environment can be considered as

friction coefficient and adhesion force between the silicon AFM tip apex and the ZnO550 surface. Second, we measure the velocity-dependent friction by sliding an AFM probe on the ZnO550 surface with different sliding velocities ranging from 0.3 to 17 μm/s. The normal force FN is kept at 5 nN for all velocity-dependent friction measurements. The scan size is kept at 1 μm2 for all friction tests.

■

RESULTS AND DISCUSSION

Topography and Adhesion Images of the ZnO Surface Acquired by AFM. We first measure the surface morphology and adhesive properties of the as-grown PLD ZnO550 thin film simultaneously by using PF-QNM. Figure 4a shows the typical topography image of our ZnO550 surface acquired at RH = 4%. The root-mean-square surface roughness within 1 μm2 area on ZnO550 is found to be 10.0 ± 0.9 nm, which is much larger than 4.0 ± 0.2 nm of ZnO200 (Figure S3). Highly crystalline ZnO films will have a larger grain size that gives rise to larger surface roughness.9 The measurements by PF-QNM performed at RH = 4% and 70%, and on nearly the same surface location, are presented in Figure 4b and c to observe the change in adhesion. The topography and adhesion images at 4% and 70% are shown in the top and bottom panels of Figure 4b and c, respectively. The fluctuations of surface topography and adhesion along the green lines indicated in Figure 4b and c are shown in Figure 4d. A direct comparison between the surface topography and adhesion can be observed clearly. Interestingly, it seems that the adhesion force is larger at the boundaries between surface grain structures, as compared to the top of grain features. The histograms of surface adhesion distribution within 3 μm2 on the ZnO550 surface at RH = 4% and 70% are plotted in Figure 4e (Figure S4). From the Gaussian fit, the center of the adhesion distribution on the asgrown ZnO550 surface increases from 0.8 ± 0.2 to 5.4 ± 2.6 nN when the RH is elevated from 4% to 70%, which is a strong indication of water adsorption on the surface, probably due to the presence of intrinsic oxygen vacancies. The standard deviation in the measured adhesion at RH = 70% becomes much larger than that at RH = 4% due to the inhomogeneous water adsorption presented in the bottom panel of Figure 4c. Next, the as-grown PLD ZnO550 surface is irradiated by UV light (365 nm) for the wettability conversion. The adhesion on the UV-treated ZnO550 measured by PF-QNM is found to increase from 3.9 ± 1.9 to 5.4 ± 2.6 nN (Figure S5). Increased surface density of oxygen vacancies on the ZnO550 surface due to UV illumination will easily attract additional ambient water molecules and lead to the enhanced surface adhesion.43 The wettability of a surface is in general goverend by the material-dependent interaction between water molecules and the surface as well as the surface topography.57 Although the ZnO surfaces are usually considered to be hydrophobic with CA ≈ 90°, experiments have shown that water molecules can be either dissociatively absorbed onto oxygen vacancies22 or bound to zinc sites24 on a ZnO surface even in UHV conditions. Energetically speaking, if the interaction energy between water molecules and a surface is greater than the heat of water liquefaction, 44 kJ/mol, this surface can be considered to be hydrophilic.58 In fact, the adsorption heat of water molecule on the zinc site of a ZnO surface has been measured to be 46 kJ/mol59 and 89 kJ/mol.60 A recent molecular dynamics simulation also shows that the energy of a water molecule−zinc bonding on a ZnO surface is 53 and 56.0 kJ/mol for surfaces with and without oxygen defects, respectively.43 Therefore, ambient water molecules can be easily attracted onto a ZnO surface at the nanoscale and result in partial water coverage, as found by our adhesion measurement. For our

FAdh = Fss + FC

(5)

where Fss is the direct interaction force of the surfaces in liquid, and FC represents the capillary force between two flat surfaces. For a spherical AFM probe in contact with a flat sample surface, the capillary force can be expressed by FC = 2πRTipγ ·(cos θs + cos θTip)

(6)

where γ is the surface tension of water, and θs and θTip are the water CAs of the smooth surface and an AFM tip, respectively.62 In this ideal situation, FC is independent of RH and is only a function of the surface wettability and AFM tip radius. However, in real experimental conditions, the AFM tip apex may not be spherical and has nanoscale protrusions. The sample surface may have roughness and defects such as oxygen vacancies in our ZnO550 surface that attract ambient water molecules. Thus, when the AFM tip is in contact with a rough ZnO550 surface for a sufficiently long time, usually in the order of milliseconds,37 ambient water molecules can condense and form water bridges within the interstitials of the tip−ZnO550 contact, giving rise to a RH-dependent capillary force.35 We also measure the magnitude of Fss between the AFM tip (SCANASYST-Air) and ZnO550 surface directly in ultrapure water. The average Fss is found to be 0.3 ± 0.03 nN within 1 μm2 area on the ZnO550 surface (Figure S7). Comparing the obtained Fss with the adhesion forces shown in Figure 4e, from eq 5, it is evident that the capillary force FC is the major contribution to the measured change of adhesions. Friction Coefficient and Adhesion Force Measured by AFM. From the classical Amonton’s law, the fundamental relationship between the friction force FF and normal load FN between two sliding surfaces is FF = μ·(FN + FAdh)

(7)

where μ is the coefficient of friction between sliding surfaces. Because FAdh is a strong function of the RH and capillary effect between two surfaces in contact, which has been found to play an important role in the nanoscopic friction,31,35,36,63,64 it is reasonable to expect that the frictional behavior of ZnO550 surfaces should be significantly influenced by the water adsorption due to the presence of oxygen vacancies at the nanoscale. First, we measure the FF versus FN at a sliding velocity v = 0.3 μm/s and extract the adhesion FAdh and friction coefficient μ between the AFM tip apex and the ZnO550 surface through eq E

DOI: 10.1021/acs.langmuir.7b01242 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

nearly doubled after the UV treatment. There is a concern that UV treatment might change the grain structure of metal oxide surface,68 and the resulting morphology change might have potential influence on the nanoscopic friction. However, due to the surface roughness and random size of surface features, it is not possible to observe the same location of the ZnO surface before and after UV illumination for morphology change using our AFM system. Thus, at present, we are not able to observe the influence of UV-induced grain size change on the nanoscopic friction. Velocity-Dependent Friction on ZnO Surface. Next, we perform the velocity-dependent friction measurements on the ZnO550 surface. The experiments are first conducted at RH = 4%, and then without moving the relative position between the AFM tip and ZnO surface, the RH is raised to 70% to observe how variations in RH influence the velocity-dependent friction on the same surface location of ZnO. Later, a separate friction experiment is carried out at RH = 41%. The obtained sliding friction, FF, versus the logarithmic of the tip sliding velocity, ln(v), on the as-grown and UV-treated ZnO550 thin films is presented in Figure 5a and b, respectively. In Figure 5a, at RH = 4%, the FF on the as-grown ZnO550 shows a logarithmic increase with the tip sliding velocity, which can be understood by the thermally activated stick−slip process during which the atoms at the tip−surface contact are hopping across a corrugated tip− surface interaction potential as described by the Prandtl− Tomlinson model.33,34 If the environmental thermal energy is larger than the energy barrier that the moving tip apex has to overcome, the sliding friction can be greatly reduced, especially for low sliding velocities. With increasing velocity, the effect of thermally activated hopping process becomes less important, hence the observed logarithmic increase of friction on velocity. However, very interestingly, at RH = 41% and 70%, the FF does

7. The experiments are performed at the same surface location on ZnO550 at RH = 4% and 70%, respectively. The obtained μ and FAdh are summarized in Table 1. The magnitudes of FAdh found Table 1. Summary of the Measured Friction Coefficients μ and Adhesion FAdh on the As-Grown and UV-Treated ZnO550 Surfaces at Different RH Values ZnO550

RH

μ

FAdh (nN)

as-grown

4% 41% 70% 4% 41% 70%

0.20 ± 0.03 N/A 0.35 ± 0.02 0.24 ± 0.03 N/A 0.41 ± 0.04

7.7 ± 2.7

UV-treated

12.4 ± 1.0 14.8 ± 3.0 22.6 ± 2.8

here are different from those presented in Figure 4d due to different RTip values used in the experiments. For the as-grown ZnO550 surface, when the RH increases from 4% to 70%, the FAdh increases from 7.7 ± 2.7 to 12.4 ± 1.0 nN, and μ is enhanced from 0.20 ± 0.03 to 0.35 ± 0.02, respectively. The measured μ is close to the value obtained on single-crystal O−ZnO (Figure S2) and in the literature.7 In addition to the enhanced FAdh, the increase of μ is an indication of increased water adsorption on the ZnO550 surface. When water is confined between a nanosize gap formed by the AFM tip and a rough surface, the structure of adsorbed water layer can be more structurally ordered, resulting in an enhanced water viscosity64−66 that provides additional viscos drag between two sliding surfaces, hence the observed increase in μ. In fact, such an increase of μ with elevated RH has already been observed on the hydrophilic mica.67 Similarly, for the UV-treated ZnO550 surface, the FAdh changes from 14.8 ± 3.0 to 22.6 ± 2.8 nN, while the value of μ increases from 0.24 ± 0.03 at RH = 4% to 0.41 ± 0.04 at RH = 70%. The magnitudes of FAdh are found to be

Figure 5. (a and b) FF versus ln(v) on the as-grown and UV-treated ZnO550 surfaces at different RH values, respectively. The error bars are determined from friction measurements on at least three different locations on the ZnO550 surface. Black arrows indicate the transition velocity between regimes with negative and positive slopes in the FF versus ln(v) curves. (c) Schematics (not to scale) showing water bridges forming at the interstices of AFM tip− ZnO contact. (d) Repetition of the slope in the FF versus ln(v) plot when v < vC for approximately three cycles of wettability conversion via the alternation of PCE and dark storage. F

DOI: 10.1021/acs.langmuir.7b01242 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Table 2. Obtained Characteristic Velocity vC and Estimated Nucleation Time τ of the Formation of Water Meniscus

not increase monotonically with ln(v) anymore. Two different regimes in the FF versus ln(v) plots in Figure 5a can be identified. At low velocities, FF first decreases linearly with increasing ln(v) until a characteristic sliding velocity, vC, at which the slope of FF versus ln(v) begins to change sign, is attained, and FF starts to increase linearly again with ln(v). This observed nonlinear friction behavior on ZnO550 surface can be immediately understood by the water meniscus formed at the tip−ZnO550 contact at low sliding velocities, due to the interplay of surface roughness and presence of oxygen vacancies. Slow sliding velocity provides a longer tip−ZnO550 contact time that permits the formation of water meniscus at the tip−ZnO550 contact, leading to larger FAdh and thus larger FF. With the increasing sliding velocity, the FF gradually decreases until vC is reached, and then the effect of capillary condensation becomes negligible, such that FF becomes positively dependent on ln(v) again. At RH = 41% and 70%, the magnitudes of measured FF are very similar probably due to the limited existence of intrinsic oxygen vacancies that can act as binding sites for ambient water molecules such that the increase of RH has no obvious effect on the magnitude of FF. It is also possible that the change of slope in FF versus ln(v) with increasing RH is beyond the sensitivity of our experiment due to the low concentration of oxygen vacancies in the as-grown ZnO550 surface. On the other hand, the FF versus ln(v) curves found on the UV-treated ZnO550 presented in Figure 5b demonstrate very different frictional phenomena. At the lowest RH = 4%, the FF already shows negative dependence with ln(v). After the UV treatment, usually performed at RH ≈ 40%, it is possible that the increased oxygen vacancies in the ZnO550 surface will immediately attract ambient water molecules, which may stick tightly to the defect sites on ZnO550 even at RH = 4%, because such water adsorption has been observed even in UHV conditions.22−24 Therefore, increased water coverage and capillary condensation between the AFM tip and the UV-treated surface at RH = 4% can be expected, giving rise to enhanced adhesion and negative dependence between FF and ln(v) reported in Figure 5b. With increasing RH, the measured FF values on UV-treated ZnO550 surface are consistently much larger than those found on the as-grown ZnO550 due to a stronger capillary effect. In the low velocity regime, the slopes of FF versus ln(v) are becoming more negative as RH is increasing from 4% to 70%. When the sliding velocity exceeds vC, the slopes of FF versus ln(v) curves become positive again. For both the as-grown and the UV-treated ZnO550 surfaces, it can be seen that at the largest sliding velocity, 17 μm/s, the magnitude of FF approaches similar values when the capillary effect no longer has a vital influence on the nanoscopic friction. Next, by performing linear fittings to both the low and the high velocity regimes of FF versus ln(v) shown in Figure 5a and b, we can determine the characteristic velocity, vC, that defines that transition of two regimes in each curve by finding the intersections of the fitted lines, at positions indicated by black arrows in Figure 5a and b. The values of vC, which are summarized in Table 2, mark the onset of the transition between the low and high velocity regimes of each investigated RH. The value of vC seems to be larger when the RH is higher. It is also obvious that at RH = 70%, the vC is larger on the UV-treated ZnO550 than the as-grown one. This can be attributed to the higher surface density of oxygen vacancies on the UV-treated ZnO550 surface,43 which assist the adsorption of ambient water molecules and promote the formation of nanoscale water meniscus. Therefore, the effect of capillary condensation on

ZnO550

RH

vC (μm/s)

as-grown

4% 41% 70% 4% 41% 70%

N/A 3.0 ± 1.1 3.5 ± 1.3 1.7 ± 1.0 2.9 ± 1.0 4.7 ± 1.4

UV-treated

τ (ms) 1.5 ± 0.5 1.3 ± 0.5 2.7 ± 0.1 1.7 ± 0.6 1.1 ± 0.3

the friction begins at larger sliding velocities on the UV-treated surface. Because vC denotes the beginning of water bridge formation, we can further estimate the nucleation time, τ, of the water meniscus at the tip−ZnO contact, if we know the distance that AFM tip travels during which the meniscus is formed. As schematically shown in Figure 5c, water bridges can condense between the nanosize gaps within the contact region of two nanorough surfaces. The distance d that AFM tip travels on the ZnO550 surface can be approximated to be the diameter of the apparent tip−ZnO contact area, and can be estimated using the modified Hertz contact theory.2,37 For a hard contact as in our case, the contact diameter can be estimated by using ⎡ RTip ⎤1/3 d = 2 × ⎣ E * (FN + FAdh)⎦ , where E* is the effective Young modulus of the tip−ZnO system. We then can estimate the nucleation time of water bridges simply by using τ = d/vC, and the results are summarized in Table 2 (Tables S1 and S2). With increasing RH, the nucleation time of water bridges seem to decrease accordingly. In fact, the observed difference in the nucleation time τ at different RH can be understood by the Arrhenius law, which has been applied to explain the mechanisms of nanoscale water nucleation.37 The nucleation time τ can be represented as τ = τ0·exp(ΔE/(kBT)), where ΔE is the nucleation energy barrier and 1/τ0 is the attempt frequency of the formation of water meniscus.37 For a gap height h, as illustrated in Figure 5c, the formation of the condensed water bridge within the gap requires one to overcome an energy barrier of ΔE(h) = kBT ln(1/RH)ρAh, where A is the cross-sectional area of the water bridge depicted in Figure 5c, and ρ is the molecular density of water.69 Evidently, the energy barrier ΔE is smaller at higher RH, subsequently leading to a smaller τ as observed in our experiment. For a given RH, the values of τ found on the as-grown and UV-treated ZnO550 are very similar. To further understand the behavior of FF versus ln(v) presented in Figure 5a and b, we compare our data with the friction model developed by Riedo et al.35 that takes both the thermally activated capillary effect and the stick−slip process into account. Because of the surface roughness as presented in Figure 5c, the height of nanosize gaps within the contact region will have a broad distribution. Consequently, depending on the surface contact time t, only a fraction f(t) of total water meniscus can form at the interstices, and eq 5 will now become FAdh = Fss + FC·f (t )

(8)

with f (t ) =

1 1 RH

( )

λAρ ·ln

⎛t ⎞ ·ln⎜ ⎟ ⎝τ⎠

(9)

where λ is the full width half-maximum of the interstitial height distribution at the tip−surface contact,35,69 which may vary when G

DOI: 10.1021/acs.langmuir.7b01242 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir the AFM tip is in contact with different locations on the ZnO550 surface due to large surface roughness and inhomogeneity as shown in Figure 4a (see the Supporting Information for further details). The FC here depends only on the wettability of the ZnO550 surface, which in our case can be manipulated by the PCE. Combing eqs 7, 8, and 9, we arrive at the friction equation that considers the combined thermally activated effects of the capillary condensation and the stick−slip process: ⎛v⎞ FF = μ·(FN + Fss) + μ·FC·f (t ) + m ·ln⎜ ⎟ ⎝ v0 ⎠ ⎛ FC ⎜ = μ·(FN + Fss) − μ·⎜ ⎝ λAρ ·ln ⎛v⎞ + m ·ln⎜ ⎟ ⎝ v0 ⎠

( RH1 )

⎞ ⎟ ⎛v⎞ ⎟ ·ln⎜⎝ v ⎟⎠ C ⎠

Table 3. Comparison of the Velocity-Dependent Friction Data on the As-Grown and UV-Treated ZnO550 Surface with Equation 12

( RH1 )

⎞ ⎟ ⎟+m ⎠

FC/λAρ (nN)

m

0.81 ± 0.10 1.79 ± 0.22

0.23 ± 0.11 −0.18 ± 0.11

treated ZnO550 surface. The friction process described in the Prandtl−Tomlinson model only considers the direct interaction energy between atoms at the tip−surface contact. In real experimental conditions, the dangling bonds such as hydroxyl species on the silicon AFM tip apex may interact covalently with the oxygen vacancies present on the ZnO550 surface, providing an additional strong interaction at low sliding velocities even at the absence of ambient moisture. Such interactions may not occur at large sliding velocities and thus possibly result in the negative value of m on the UV-treated ZnO550 surface. Finally, because the photoinduced wettability conversion is reversible on the ZnO550 surface, we repeat the velocity-dependent friction measurements on the ZnO550 surface of each wettability state for several conversion cycles, and the results are shown in Figure 5d. We find that, at RH = 4%, the slopes of FF versus ln(v) in the low velocity regime (for v < vC) can be consecutively switched between 0.16 ± 0.05 and −0.3 ± 0.1. At RH = 70%, the slopes are found to vary back and forth between −3.0 ± 0.3 and −0.8 ± 0.2. These results indicate that PCE can be an effective method to manipulate the velocity-dependent friction for a certain velocity range on the ZnO surface.

(10)

(11)

The second and third terms in eq 11 represent the effect of capillary condensation and thermal activated stick−slip effects on FF, respectively. In the third term, the slope m is a positive value due to the stick−slip effect in the Prandtl−Tomlinson model. The parameters vC and v0 are two characteristic velocities. While vC denotes the transition in FF versus ln(v) when the capillary effects no longer have an impact on the velocity-dependent friction, v0 represents the velocity beyond which the friction becomes independent of ln(v).33−35 Obviously, the second term and the third term in eq 11, respectively, give rise to the negative and positive slopes in the FF versus ln(v) curves. The overall nonlinear behavior of the FF versus ln(v) curve is in fact a competition between the capillary condensation and stick−slip effect. Next, we compare our results with the aforementioned theoretical model to gain more insights into the observed friction phenomena on ZnO550. From eq 11, for a given RH, the slope of linear parts in FF versus ln(v) curves shown in Figure 5a and b can be expressed as ⎛ FC ⎜ slope = − μ·⎜ ⎝ λAρ ·ln

ZnO as-grown UV-treated

■

CONCLUSIONS By means of AFM-based techniques, we have investigated the frictional and adhesive properties of nanocrystalline ZnO thin films prepared by PLD in a controlled ambient environment. We find that there is in general a nonlinear relationship between the sliding friction and the logarithm of sliding velocities on the ZnO surface, which significantly depends on the amount of oxygen vacancies and ambient humidity. Oxygen vacancies in ZnO surface layer can act as binding sites for ambient water molecules and result in water coverage on the ZnO surface. Therefore, when the AFM tip is slowly sliding on the ZnO surface with nanoscale surface roughness, water meniscus can condense within the gap between the asperities at the tip−ZnO contact due to the capillary condensation, and result in a negative dependence of friction on the logarithm of velocity. The influence of capillary condensation on nanoscopic sliding friction on ZnO surface is found to be humidity dependent and can be further manipulated via UV illumination, by which the density of oxygen vacancies in the ZnO surface can be greatly promoted, and the ZnO surface can be converted from being hydrophobic to superhydrophilic. Furthermore, because the wettability conversion of the ZnO surface is reversible by alternation of UV exposure and dark storage, we also find that the velocitydependent frictional behavior on ZnO of each wettability state is reproduced. The observed friction phenomena on our PLD ZnO surface should also be found on those prepared by other methods such as atomic layer deposition,28 magnetron sputter deposition,30 and sol−gel method,31 or ZnO films prepared using nanomaterials such as nanorods,41 due to the inevitable presence of defects and surface roughness. Thus, it is in general possible to control the frictional behavior of ZnO surface via PCE. Photoresponsive surfaces such as titanium dioxide and tungsten oxide may also exhibit similar frictional properties and are worth

(12)

While the slope and μ can be experimentally determined for a certain RH, FC and m are in fact RH-independent and inherently related to the surface properties of ZnO550. The parameter λ is unknown and may vary when the AFM probe contacts with different surface locations. Because in our velocity-dependent measurements at RH = 4% and 70%, the FF versus ln(v) is measured at almost the same surface location, we can assume λ to be a constant in these experiments. By inserting the slopes of FF versus ln(v) for v < vC and μ acquired, respectively, at RH = 4% and 70% on the as-grown ZnO550 into eq 12, we can solve for the parameters FC/λAρ and m for this surface. The same procedure can be applied to the data acquired from the UV-treated ZnO550 surface. Detailed calculations can be found in the Supporting Information, and the obtained FC/λAρ and m for both as-grown and UV-treated ZnO550 surfaces are summarized in Table 3. Assuming the cross-sectional area, A, of the formed water bridge to be constant, the values of FC/λAρ are found to increase by a factor of 2.2 after the UV treatment, which is close to the increase of FAdh listed in Table 1. In addition, the value of m, which originates from the thermally activated stick−slip effect, and is usually found to be positive, is found to be negative on the UVH

DOI: 10.1021/acs.langmuir.7b01242 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir further investigation.40 Our results might provide a feasible route to manipulate nanoscopic friction in NEMS employed photoresponsive materials using optical approaches.

■

(12) Anderson, J.; Chris, G. V. d. W. Fundamentals of zinc oxide as a semiconductor. Rep. Prog. Phys. 2009, 72 (12), 126501. (13) Lucas, M.; Wang, Z. L.; Riedo, E. Combined polarized Raman and atomic force microscopy: In situ study of point defects and mechanical properties in individual ZnO nanobelts. Appl. Phys. Lett. 2009, 95 (5), 051904. (14) Lucas, M.; Mai, W.; Yang, R.; Wang, Z. L.; Riedo, E. Aspect Ratio Dependence of the Elastic Properties of ZnO Nanobelts. Nano Lett. 2007, 7 (5), 1314−1317. (15) Ansari, S. A.; Khan, M. M.; Kalathil, S.; Nisar, A.; Lee, J.; Cho, M. H. Oxygen vacancy induced band gap narrowing of ZnO nanostructures by an electrochemically active biofilm. Nanoscale 2013, 5 (19), 9238− 9246. (16) Zhan, P.; Xie, Z.; Li, Z.; Wang, W.; Zhang, Z.; Li, Z.; Cheng, G.; Zhang, P.; Wang, B.; Cao, X. Origin of the defects-induced ferromagnetism in un-doped ZnO single crystals. Appl. Phys. Lett. 2013, 102 (7), 071914. (17) Wu, X. L.; Siu, G. G.; Fu, C. L.; Ong, H. C. Photoluminescence and cathodoluminescence studies of stoichiometric and oxygendeficient ZnO films. Appl. Phys. Lett. 2001, 78 (16), 2285−2287. (18) Janotti, A.; Van de Walle, C. G. Oxygen vacancies in ZnO. Appl. Phys. Lett. 2005, 87 (12), 122102. (19) Fabbri, F.; Villani, M.; Catellani, A.; Calzolari, A.; Cicero, G.; Calestani, D.; Calestani, G.; Zappettini, A.; Dierre, B.; Sekiguchi, T.; Salviati, G. Zn vacancy induced green luminescence on non-polar surfaces in ZnO nanostructures. Sci. Rep. 2015, 4, 5158. (20) Liu, L.; Mei, Z.; Tang, A.; Azarov, A.; Kuznetsov, A.; Xue, Q.-K.; Du, X. Oxygen vacancies: The origin of n-type conductivity in ZnO. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93 (23), 235305. (21) Li, G. R.; Hu, T.; Pan, G. L.; Yan, T. Y.; Gao, X. P.; Zhu, H. Y. Morphology−Function Relationship of ZnO: Polar Planes, Oxygen Vacancies, and Activity. J. Phys. Chem. C 2008, 112 (31), 11859−11864. (22) Kunat, M.; Girol, S. G.; Burghaus, U.; Wöll, C. The Interaction of Water with the Oxygen-Terminated, Polar Surface of ZnO. J. Phys. Chem. B 2003, 107 (51), 14350−14356. (23) Schiek, M.; Al-Shamery, K.; Kunat, M.; Traeger, F.; Woll, C. Water adsorption on the hydroxylated H-(1 × 1) O-ZnO(0001̅) surface. Phys. Chem. Chem. Phys. 2006, 8 (13), 1505−1512. (24) Ö nsten, A.; Stoltz, D.; Palmgren, P.; Yu, S.; Göthelid, M.; Karlsson, U. O. Water Adsorption on ZnO(0001): Transition from Triangular Surface Structures to a Disordered Hydroxyl Terminated phase. J. Phys. Chem. C 2010, 114 (25), 11157−11161. (25) Ye, H.; Chen, G.; Niu, H.; Zhu, Y.; Shao, L.; Qiao, Z. Structures and Mechanisms of Water Adsorption on ZnO(0001) and GaN(0001) Surface. J. Phys. Chem. C 2013, 117 (31), 15976−15983. (26) Zabinski, J. S.; Corneille, J.; Prasad, S. V.; Mc Devitt, N. T.; Bultman, J. B. Lubricious zinc oxide films: synthesis, characterization and tribological behaviour. J. Mater. Sci. 1997, 32 (20), 5313−5319. (27) Prasad, S. V.; Walck, S. D.; Zabinski, J. S. Microstructural evolution in lubricious ZnO films grown by pulsed laser deposition. Thin Solid Films 2000, 360, 107−117. (28) Chai, Z.; Lu, X.; He, D. Friction mechanism of zinc oxide films prepared by atomic layer deposition. RSC Adv. 2015, 5 (68), 55411− 55418. (29) Nainaparampil, J. J.; Zabinski, J. S.; Prasad, S. V. Nanotribology of single crystal ZnO surfaces: Restructuring at high temperature annealing. J. Vac. Sci. Technol., A 1999, 17 (4), 1787−1792. (30) Masahiro, G.; Akira, K.; Youko, K.; Tetsuo, O.; Masahiro, T.; Kazuhiro, Y. Frictional Property of Zinc Oxide Coating Films Observed by Lateral Force Microscopy. J. J. Appl. Phys. 2003, 42 (7S), 4834−4836. (31) Rhee, T. H.; Shin, M. W.; Jang, H. Effects of humidity and substrate hydrophilicity on nanoscale friction. Tribol. Int. 2016, 94, 234−239. (32) Chai, Z.; Liu, Y.; Lu, X.; He, D. Reducing Friction Force of Si Material by Means of Atomic Layer-Deposited ZnO Films. Tribol. Lett. 2014, 56 (1), 67−75. (33) Gnecco, E.; Bennewitz, R.; Gyalog, T.; Loppacher, C.; Bammerlin, M.; Meyer, E.; Güntherodt, H. J. Velocity Dependence of Atomic Friction. Phys. Rev. Lett. 2000, 84 (6), 1172−1175.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01242. Experimental details about pulse laser deposition, X-ray diffraction, and Raman spectroscopy; complementary friction data and AFM images; discussion about the variations in λ; and calculations of contact diameter d, FC/ λAρ, and m (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hsiang-Chih Chiu: 0000-0003-2815-9307 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS H.-P.C., E.-D.C., and H.-C.C. acknowledge the financial support from the Ministry of Science and Technology (MoST), Taiwan, under contract numbers MOST 102-2112-M-003-018-MY3 and MOST 105-2112-M-003-001-MY3. Y.-T.Y. and F.-Y.L. are grateful for the support from MoST under grant number MOST 103-2112-M-003-004. H.-C.C. acknowledges the fruitful discussion with Prof. I.-S. Hwang at the Institute of Physics, Academia Sinica, Taiwan.

■

REFERENCES

(1) Kis, A.; Jensen, K.; Aloni, S.; Mickelson, W.; Zettl, A. Interlayer Forces and Ultralow Sliding Friction in Multiwalled Carbon Nanotubes. Phys. Rev. Lett. 2006, 97 (2), 025501. (2) Chiu, H.-C.; Ritz, B.; Kim, S.; Tosatti, E.; Klinke, C.; Riedo, E. Nanotubes: Sliding on Nanotube: Interplay of Friction, Deformations and Structure. Adv. Mater. 2012, 24 (21), 2879−2884. (3) Kwon, S.; Ko, J.-H.; Jeon, K.-J.; Kim, Y.-H.; Park, J. Y. Enhanced Nanoscale Friction on Fluorinated Graphene. Nano Lett. 2012, 12 (12), 6043−6048. (4) Kołodziejczak-Radzimska, A.; Jesionowski, T. Zinc OxideFrom Synthesis to Application: A Review. Materials 2014, 7 (4), 2833−2881. (5) Zhong Lin, W. Zinc oxide nanostructures: growth, properties and applications. J. Phys.: Condens. Matter 2004, 16 (25), R829−R858. (6) Djurisic, A. B.; Chen, X.; Leung, Y. H.; Man Ching Ng, A. ZnO nanostructures: growth, properties and applications. J. Mater. Chem. 2012, 22 (14), 6526−6535. (7) Prasad, S. V.; Zabinski, J. S. Tribological behavior of nanocrystalline zinc oxide films. Wear 1997, 203−204 (0), 498−506. (8) Karch, J.; Birringer, R.; Gleiter, H. Ceramics ductile at low temperature. Nature 1987, 330 (6148), 556−558. (9) Shan, F. K.; L, G. X.; Lee, W. J.; Shin, B. C. The role of oxygen vacancies in epitaxial deposited ZnO thin films. J. Appl. Phys. 2007, 101, 053106. (10) Sasaki, A.; Hara, W.; Matsuda, A.; Tateda, N.; Otaka, S.; Akiba, S.; Saito, K.; Yodo, T.; Yoshimoto, M. Buffer-enhanced room-temperature growth and characterization of epitaxial ZnO thin films. Appl. Phys. Lett. 2005, 86 (23), 231911. (11) King, S. L.; Gardeniers, J. G. E.; Boyd, I. W. Pulsed-laser deposited ZnO for device applications. Appl. Surf. Sci. 1996, 96, 811−818. I

DOI: 10.1021/acs.langmuir.7b01242 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

(56) Varenberg, M.; Etsion, I.; Halperin, G. An improved wedge calibration method for lateral force in atomic force microscopy. Rev. Sci. Instrum. 2003, 74 (7), 3362−3367. (57) Pandey, P. R.; Roy, S. Is it Possible to Change Wettability of Hydrophilic Surface by Changing Its Roughness? J. Phys. Chem. Lett. 2013, 4 (21), 3692−3697. (58) Bolis, V.; Fubini, B.; Marchese, L.; Martra, G.; Costa, D. Hydrophilic and hydrophobic sites on dehydrated crystalline and amorphous silicas. J. Chem. Soc., Faraday Trans. 1991, 87 (3), 497−505. (59) Nagao, M.; Kumashiro, R.; Matsuda, T.; Kuroda, Y. Calorimetric study of water two-dimensionally condensed on the homogeneous surface of a solid. Thermochim. Acta 1995, 253, 221−233. (60) Nakazawa, M.; Somorjai, G. A. Coadsorption of water and selected aromatic molecules to model the adhesion of epoxy resins on hydrated surfaces of zinc oxide and iron oxide. Appl. Surf. Sci. 1995, 84 (3), 309−323. (61) Tang, C.; Spencer, M. J. S.; Barnard, A. S. Activity of ZnO polar surfaces: an insight from surface energies. Phys. Chem. Chem. Phys. 2014, 16 (40), 22139−22144. (62) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 2011. (63) Riedo, E.; Palaci, I.; Boragno, C.; Brune, H. The 2/3 Power Law Dependence of Capillary Force on Normal Load in Nanoscopic Friction. J. Phys. Chem. B 2004, 108 (17), 5324−5328. (64) Jinesh, K. B.; Frenken, J. W. M. Capillary Condensation in Atomic Scale Friction: How Water Acts like a Glue. Phys. Rev. Lett. 2006, 96 (16), 166103. (65) Li, T. D.; Gao, J. P.; Szoszkiewicz, R.; Landman, U.; Riedo, E. Structured and viscous water in subnanometer gaps. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 115415. (66) Ortiz-Young, D.; Chiu, H.-C.; Kim, S.; Voïtchovsky, K.; Riedo, E. The interplay between apparent viscosity and wettability in nanoconfined water. Nat. Commun. 2013, 4, 2482. (67) Schumacher, A.; Kruse, N.; Prins, R.; Meyer, E.; Lüthi, R.; Howald, L.; Güntherodt, H. J.; Scandella, L. Influence of humidity on friction measurements of supported MoS2 single layers. J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 1996, 14 (2), 1264−1267. (68) Shirasawa, T.; Voegeli, W.; Arakawa, E.; Takahashi, T.; Matsushita, T. Structural Change of the Rutile−TiO2(110) Surface During the Photoinduced Wettability Conversion. J. Phys. Chem. C 2016, 120 (51), 29107−29115. (69) Bocquet, L.; Charlaix, E.; Ciliberto, S.; Crassous, J. Moistureinduced ageing in granular media and the kinetics of capillary condensation. Nature 1998, 396 (6713), 735−737.

(34) Riedo, E.; Gnecco, E.; Bennewitz, R.; Meyer, E.; Brune, H. Interaction Potential and Hopping Dynamics Governing Sliding Friction. Phys. Rev. Lett. 2003, 91 (8), 084502. (35) Riedo, E.; Lévy, F.; Brune, H. Kinetics of Capillary Condensation in Nanoscopic Sliding Friction. Phys. Rev. Lett. 2002, 88 (18), 185505. (36) Greiner, C.; Felts, J. R.; Dai, Z.; King, W. P.; Carpick, R. W. Controlling Nanoscale Friction through the Competition between Capillary Adsorption and Thermally Activated Sliding. ACS Nano 2012, 6 (5), 4305−4313. (37) Szoszkiewicz, R.; Riedo, E. Nucleation Time of Nanoscale Water Bridges. Phys. Rev. Lett. 2005, 95 (13), 135502. (38) Khranovskyy, V.; Ekblad, T.; Yakimova, R.; Hultman, L. Surface morphology effects on the light-controlled wettability of ZnO nanostructures. Appl. Surf. Sci. 2012, 258 (20), 8146−8152. (39) Sun, R.-D.; Nakajima, A.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Photoinduced Surface Wettability Conversion of ZnO and TiO2 Thin Films. J. Phys. Chem. B 2001, 105 (10), 1984−1990. (40) Wang, S.; Song, Y.; Jiang, L. Photoresponsive surfaces with controllable wettability. J. Photochem. Photobiol., C 2007, 8 (1), 18−29. (41) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. Reversible Super-hydrophobicity to Super-hydrophilicity Transition of Aligned ZnO Nanorod Films. J. Am. Chem. Soc. 2004, 126 (1), 62−63. (42) Liu, H.; Feng, L.; Zhai, J.; Jiang, L.; Zhu, D. Reversible Wettability of a Chemical Vapor Deposition Prepared ZnO Film between Superhydrophobicity and Superhydrophilicity. Langmuir 2004, 20 (14), 5659−5661. (43) Hu, H.; Ji, H.-F.; Sun, Y. The effect of oxygen vacancies on water wettability of a ZnO surface. Phys. Chem. Chem. Phys. 2013, 15 (39), 16557−16565. (44) Ohnishi, T.; Ohtomo, A.; Kawasaki, M.; Takahashi, K.; Yoshimoto, M.; Koinuma, H. Determination of surface polarity of caxis oriented ZnO films by coaxial impact-collision ion scattering spectroscopy. Appl. Phys. Lett. 1998, 72 (7), 824−826. (45) Greenspan, L. Humidity fixed points of binary saturated aqueous solutions. J. Res. Natl. Bur. Stand., Sect. A 1977, 81A, 89−96. (46) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Studies of Surface Wettability Conversion on TiO2 Single-Crystal Surfaces. J. Phys. Chem. B 1999, 103 (12), 2188−2194. (47) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Photogeneration of Highly Amphiphilic TiO2 Surfaces. Adv. Mater. 1998, 10 (2), 135−138. (48) Ohtsu, N.; Masahashi, N.; Mizukoshi, Y.; Wagatsuma, K. Hydrocarbon Decomposition on a Hydrophilic TiO2 Surface by UV Irradiation: Spectral and Quantitative Analysis Using in-Situ XPS Technique. Langmuir 2009, 25 (19), 11586−11591. (49) Vig, John R. UV/ozone cleaning of surfaces. J. Vac. Sci. Technol., A 1985, 3 (3), 1027−1034. (50) Miyauchi, M.; Shimai, A.; Tsuru, Y. Photoinduced Hydrophilicity of Heteroepitaxially Grown ZnO Thin Films. J. Phys. Chem. B 2005, 109 (27), 13307−13311. (51) Cuscó, R.; Alarcón-Lladó, E.; Ibáñez, J.; Artús, L.; Jiménez, J.; Wang, B.; Callahan, M. J. Temperature dependence of Raman scattering in ZnO. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75 (16), 165202. (52) Hadri, A.; Taibi, M.; loghmarti, M.; Nassiri, C.; Slimani Tlemcani, T.; Mzerd, A. Development of transparent conductive indium and fluorine co-doped ZnO thin films: Effect of F concentration and postannealing temperature. Thin Solid Films 2016, 601, 7−12. (53) Wong, K. W. J.; Field, M. R.; Ou, J. Z.; Latham, K.; Spencer, M. J. S.; Yarovsky, I.; Kalantar-zadeh, K. Interaction of hydrogen with ZnO nanopowdersevidence of hydroxyl group formation. Nanotechnology 2012, 23 (1), 015705. (54) Šćepanović, M.; Grujić-Brojčin, M.; Vojisavljević, K.; Bernik, S.; Srećković, T. Raman study of structural disorder in ZnO nanopowders. J. Raman Spectrosc. 2010, 41 (9), 914−921. (55) Hutter, J. L.; Bechhoefer, J. Calibration of atomic-force microscope tips. Rev. Sci. Instrum. 1993, 64 (7), 1868−1873. J

DOI: 10.1021/acs.langmuir.7b01242 Langmuir XXXX, XXX, XXX−XXX