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Materials Letters 62 (20

Effect of atmosphere on photo-induced friction force variation on (001) rutile

Akira Nakajima ⁎, Aya Nakada, Naoki Arimitsu, Yoshikazu Kameshima, Kiyoshi Okada

Department of Metallurgy and Ceramic Science, Graduate School of Science and Engineering,Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo, 152-8552, Japan

Received 24 April 2007; accepted 11 August 2007Available online 21 August 2007

Abstract

Variations of friction force and surface potential were evaluated using friction force microscopy (FFM) and Kelvin force microscopy (KFM) on(100) and (001) faces of rutile single crystals under UV illumination in three different atmospheres: dry nitrogen, dry air, and ambient air. Frictionforce and surface roughness for the (100) face did not exhibit remarkable dependence on either UV illumination or on the atmosphere. However,the friction force of the (001) face increased by UV illumination in dry air without roughness variation. The surface potential of the (001) facechanged negatively and more remarkably than that of the (100) face by UV illumination.© 2007 Elsevier B.V. All rights reserved.

Keywords: Rutile; Titanium dioxide; Orientation; Friction force; UV; Kelvin force

1. Introduction

The photo-induced reaction of titanium dioxide has been wellstudied from the discovery of water breakdown of TiO2 [1].Especially, the strong oxidation power of photogenerated radicalspecies has been well investigated by many researchers from thestandpoint of application for water and air purification [2]. Asidefrom these conventional applications, another intriguing phe-nomenon, i.e., the generation of a highly hydrophilic TiO2 surfaceby UV illumination [3], was reported in the 1990s. This uniquesurface exhibits excellent antifogging and self-cleaning proper-ties. Using these properties, thin film TiO2 coatings have alreadybeen applied for various industrial items.

Rutile is a polymorph of TiO2 and is the most stable phase ofthis material. High purity single crystals are obtainable usingconventional methods. Various studies have been conducted onrutile single crystals from the perspective of surface science [4–6].However, the majority of those studies have specifically ad-dressed the (110) and (100) faces; studies of the (001) face are few.Hotsnpiller et al. examined the photoreduction ofAg+ toAgmetalfrom an aqueous solution onto various rutile faces, finding that the(100) and (110) faces have lower photoreduction rates than the

⁎ Corresponding author. Tel.: +81 3 5734 2525; fax: +81 3 5734 3355.E-mail address: anakajim@ceram.titech.ac.jp (A. Nakajima).

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.08.036

(001) face. [7]. Wang et al. investigated the crystal-face depen-dence of photo-induced wettability conversion using rutile singlecrystal and reported that the (110) and (100) faces, which havebridging site oxygen, exhibit a higher hydrophilic conversion ratethan that of the (001) face [8].

Those results imply that a strong surface orientationdependence exists in the photochemical reaction rate of rutile.This orientation dependence might reflect the difference of othersurface properties. In this study, under UV illumination in threeatmospheres (dry nitrogen, dry air, and ambient air), we usedfriction force microscopy (FFM) and Kelvin force microscopy(KFM) to investigate the friction force and surface potential of therutile (001) face in comparison to those of the (100) face, whichpossesses high bridging site oxygen concentration.

2. Experimental

Polished rutile single crystals with a (001) face or (100) face(1 cm (height)×1 cm (width)×0.5 mm (thickness); NakazumiCrystal Laboratory Co., Osaka, Japan) were used for this study.The surface roughness was evaluated using tapping mode AFM(JSPM-5200; JEOL, Tokyo, Japan) with a silicon cantilever(OMCL-AC160TS-C2, spring constant: 42 N/m, OlympusOptical Co. Ltd., Tokyo Japan) in the 1μm×1μm region. Thismeasurement was performed in ambient air.

Fig. 1. Friction force ratio and surface roughness on (100) face under ambient air, dry air, and dry nitrogen: (a) friction force ratio, (b) surface roughness.■, Ambientair; ▲, dry air; ○, dry nitrogen. The hatched area shows the UV illumination period.

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Friction force evaluation was carried out using the followingprocedure. A BK7 glass cover was set on the rutile single crystalon the stage of the AFM to control the sample chamber atmo-sphere (volume ca. 1.4 L). First, under the ambient air condition,friction force measurements were carried out without introducingany gas using silicon nitride cantilever (spring constant, 0.05N/m,OMCL-RC800PSA-1; Olympus Optical Co. Ltd., Tokyo Japan).The measurement area was a 1 μm×1 μm region; the reducedindentation pressure for measurements was 9.996 nN. Then, UV(4.5 mW/cm2) was illuminated using a Hg–Xe lamp with anoptical-fiber coupler on the crystal surface over the glass cover.TheUVilluminationwas carried out for 60min. The friction forcewas measured at 15 min and 30 min intervals from 30 min beforethe UV illumination period to 60 min after the UV illuminationperiod. During friction force evaluation, the surface roughnesswas also evaluated simultaneously.

Alternatively, under the dry gas condition, dry air (H2Ocontent about 0.5 ppm; TaiyoNippon Sanso Corp., Tokyo, Japan)or dry nitrogen (H2O content about 7 ppm; Toho Sanso Co. Ltd.,Kanagawa, Japan) was flowed into the sample chamber for30min (flow rate, 2 L/min) beforeUVillumination. Then, friction

Fig. 2. Friction force ratio and surface roughness on (001) face under ambient air, dryair; ▲, dry air; ○, dry nitrogen. The hatched area shows the UV illumination period

force measurements were carried out similarly to those in ambientair. Atmosphere dependence of friction force was evaluated in thefollowing order: dry nitrogen→dry air→ambient air.

The contact angles of water on the single crystal before andafter the friction force measurement were also evaluated. Thesessile drop method was used for contact angle measurementswith a commercial contact angle meter (CA-X; Kyowa InterfaceScience Co. Ltd., Saitama, Japan). The droplet size used for themeasurements was 2.0μL. Three different points were evalu-ated for each measurement, then averaged. The surface wasblown with ionized air to eliminate static electricity on thesurface before each measurement.

Surface potential evaluation was carried out using the Kelvinforce microscopy (KFM) mode of the same AFM with a siliconcantilever (spring constant of 0.60 N/m, Ultrasharp NSC36/C;Micromash Co.). A modulation bias voltage (25 kHz, 5 V) wasapplied between the probe and the sample during KFM mea-surement. Themeasurement areawas a 1μm×1μm(data number,256×256) region. Scanning time required for each measurementarea was approximately 200 s. The atmosphere control, UVillumination, andmeasurement interval were identical to those for

air, and dry nitrogen: (a) friction force ratio, (b) surface roughness.■, Ambient.

Fig. 3. Results of surface potential measurements by Kelvin force microscopy:▲, the (100) face; ○, the (001) face.

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friction force measurements. The KFM measurement wasconducted only in dry air atmosphere.

3. Results and discussion

Results of AFM observations showed that the variation of surfaceroughness (Ra) values of (001) and (100) faces were negligible beforeUV illumination: less than 1 nm (practical values, 0.10 nm for (001),0.12 nm for (100)).

Fig. 1 shows the variation of the friction force ratio (Fig. 1(a)) andsurface roughness (Fig. 1(b)) on the (100) face under ambient air, dryair, and dry nitrogen.

In this experiment, the friction force was evaluated according to thevariation ratio because the initial friction value fluctuated on eachexperimental condition. The hatched areas in Figs. 1(a) and 1(b) respec-tively depict the UV illumination periods. Both the friction force andsurface roughness exhibited remarkable dependence on neither UVillumination nor on the atmosphere. The contact angle changes before andafter these experiments in ambient air, dry air, and dry nitrogen were,respectively, 48°→37°, 39°→44°, and 39°→54°. The hydrophobic tohydrophilic wettability conversion appeared slightly only at ambient airatmosphere.

Fig. 2 shows variation of the friction force ratio (Fig. 2(a)) andsurface roughness (Fig. 2(b)) on the (001) face under three differentatmospheres.

Although the surface roughness did not exhibit remarkabledependence on either UV illumination or on the atmosphere, thefriction force in dry air increased about three times by UV illumination.The reproducibility of this phenomenon was confirmed at differentpoints, but the degree of variation of the friction force ratio dependedon the observation point at each experiment. Generally, a 2–3 timesincrease was observed. The contact angle change before and after theseexperiments in ambient air, dry air, and dry nitrogen are, respectively,46°→33°, 39°→54°, and 44°→59°. As with the (100) face, hydro-philic conversion appeared only slightly with ambient air atmosphere.

Friction force variation corresponds to UV illumination. Consequent-ly, it is related with photochemical reaction under a dry air atmosphere.Surface roughness was dependent neither on UV illumination nor on theatmosphere. Therefore, this phenomenon is not attributable to theincrease of friction resistance between the cantilever and surface by thephysical roughness increase. The contact angle change suggests thatthe probability of a relationship between this phenomenon and photo-induced hydrophilic conversion is small.

Fig. 3 shows the surface potential variation on the (100) and (001)faces under UV illumination in dry air. On the (001) face, the surfacepotential was decreased more markedly than on the (100) face.Photoexcited electrons are trapped on the Ti sites. Holes are localizedon the surface oxygen atoms [6]. On the (100) face, bridging siteoxygens, which are higher in position than their surrounding atoms,exist. In contrast, on the (001) face, there exists no bridging siteoxygen; all oxygen atoms are in an in-plane position. Therefore, it isexpected that accessibility of oxygen molecules in the atmosphere tothe Ti sites on the surface is easier for the (001) face than for the (100)face. Based on the KFM result and surface structure, one candidateexplanation for photo-induced negative polarization of the (001) face isa larger amount of oxygen adsorption to the surface because of thiseasy accessibility and the resultant electron attraction by the adsorbedoxygen molecules. Moreover, electron mobility in the [001] direction ismaximum in rutile [4]; high photoreduction activity was reported onAg production in solution [7]. This photo-induced negative polariza-tion might induce additional interaction between the solid surface andcantilever; then, the apparent friction force might be increased by UVillumination. A similar reaction is expected, even in ambient air.However, ambient air includes a huge amount of water and organicmolecules. Consequently, various other electron consumption pathsexist, such as H2O2 formation [9], and surface polarization will dis-appear in a short period. Practical relationship between absolute valuesof the surface potential and the surface friction force might be morecomplex and is unclear from only results in this study. Furtherexperiments and comprehensive discussion are required for definitemodeling of this relationship.

4. Conclusions

Friction force and surface roughness for the (100) faceexhibited remarkable dependence on neither UV illuminationnor on the atmosphere. However, the friction force of the (001)surface increased by UV illumination in dry air withoutroughness variation. The surface potential of the (001) facedecreased more remarkably than the (100) face by UVillumination in dry air.

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