J. Phys. Chem. C 2009, 113, 8273–8277
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Ultraviolet Irradiation Suppresses Adhesion on TiO2 R. Jribi,† E. Barthel,† H. Bluhm,‡ M. Grunze,§ P. Koelsch,§,⊥ D. Verreault,§,⊥ and E. Søndergård*,† Laboratoire Surface du Verre et Interfaces, UMR 125 CNRS/Saint-Gobain, 39 Quai Lucien Lefranc, 93303 AuberVilliers, France, Lawrence Berkeley National Laboratory, Chemical Sciences DiVision, One Cyclotron Road, Berkeley, California 94720, and Angewandte Physikalische Chemie, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany ReceiVed: October 29, 2008; ReVised Manuscript ReceiVed: February 11, 2009
Environmental concerns have recently spurred a quest for materials that stay clean, such as TiO2, when subjected to the combined action of sunlight irradiation and exposure to rain. However, the fundamental mechanism that governs the self-cleaning properties of TiO2 still needs to be elucidated. TiO2 is known to be photocatalytic as well as to decompose organic adsorbents, but these properties do not explain its capacity to eliminate mineral contaminants. In the present paper, we report that hydrophilic UV-irradiated TiO2 layers are nonadhesive in the presence of water, thus preventing adhesion of mineral particles. Surface force measurements done using atomic force microscopy reveal the presence of an additional short-range repulsive force, which screens the van der Waals attractive forces, while long-range interactions are preserved. This additional short-range force does not originate from UV-induced trapping of surface charges or OH group creation, as we demonstrate by second harmonic generation and ambient pressure X-ray photoelectron spectroscopy investigations. This short-range repulsive force, which appears to be intrinsic to the TiO2 surface, is certainly a key phenomenon for a strong self-cleaning capacity. Introduction Titanium dioxide (TiO2)-based materials have been widely investigated since the 1970s for their water hydrolysis and photocatalytic properties.1,2 Lately it has been demonstrated that TiO2 is capable of decomposing different organic compounds under UV illumination in air.3-5 The discovery of a reversible UV-induced hydrophilic conversion6-9 of TiO2 surfaces has widened their application field to self-cleaning, antifogging, and antibacterial materials.10 Intensive studies have demonstrated that the hydrophilic transition of TiO2 surfaces in air is at the root of the self-cleaning process.10 A controversy still exists on how the interaction with UV light creates a hydrophilic surface. Several papers suggest that clean TiO2 is intrinsically hydrophilic. In the ambient, however, the surface is contaminated but hydrophilicity can be restoredbythephotocatalyticoxidationoforganiccontaminants.11-13 Others have proposed a photoinduced chemical modification, such as an increase of the density of surface OH groups,14-18 as the origin of the favorable interaction with water. In this paper we aim to address the TiO2 surface emulating the ambient in order to gain insight into the role of water in the self-cleaning process. The relation between the hydrophilic state of the surface and the self-cleaning efficiency of TiO2 is often evoked. It is well documented that TiO2 is capable of eliminating most organic contamination, but there is little knowledge about how it prevents mineral contamination. In order to probe the interaction of a mineral particle with the surface in the presence * To whom correspondence should be addressed. E-mail: elin.
[email protected]. † Laboratoire Surface du Verre et Interfaces. ‡ Lawrence Berkeley National Laboratory. § Angewandte Physikalische Chemie. ⊥ Present address: Forschungszentrum Karlsruhe, Institute of Toxicology and Genetics, H. v. Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany.
of water, we have carried out surface force measurements between a colloidal probe and polycrystalline anatase TiO2 in water. This technique allows one to probe the long-range and short-range interactions as well as the adhesion between a mineral particle and a surface. The experiments were carried out at different levels of UV exposure of the sample and revealed the appearance of short-range repulsive surface forces. In order to understand the origin of this phenomenon, we investigated the chemical state and the electronic properties of the surface under UV irradiation using ambient pressure X-ray photoelectron spectroscopy (APXPS)19 and second harmonic generation (SHG).20,21 Experimental Section Transparent TiO2 thin films were prepared on silicon wafers by magnetron-enhanced sputtering, using a TiOx cathode and a 2 kW power discharge in a reactive gas mixture (Ar and O2) at 2 µbar. The sample was annealed after deposition at 620 °C for 15 min. The film structure and composition were investigated by X-ray diffraction (XRD). The XRD spectrum reveals a polycrystalline anatase phase with an average grain size of about 200 nm. The thickness of the film was about 130 nm, controlled by ellipsometric measurements. The density of the films, characterized by the measurement of the optical index by ellipsometry, is not distinguishable from the anatase single crystal. The roughness, controlled by AFM imaging, is about 1.5 nm over 1 µm2. The as-deposited films show low photocatalytic efficiency due to the low surface area while the hydrophilic conversion still occurs. This choice was made to limit the influence of roughness on the interpretation of measurements. Colloidal probe AFM force measurements were carried out in aqueous environment. The colloidal probe was a silica microsphere with a diameter of 5 µm glued on a tipless
10.1021/jp809607b CCC: $40.75 2009 American Chemical Society Published on Web 04/20/2009
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cantilever (Veeco-Instruments), which is here used as a model system for a mineral particle. The exact diameter of the probe was measured by SEM imaging. The stiffness of the cantilever (0.03 N m-1) was calibrated using a rectangular cantilever based on the method described in ref 22. The silica microsphere was cleaned once at the beginning of the experiment using UV ozone. The UV exposure was done using a Phillips lamp (365 nm). The average power on the sample was 5.3 mW cm-2. The distance between the sample and the UV lamp was about 5 cm. The superhydrophilic state was obtained for water contact angles below 10°, where the samples exhibit an optically transparent aspect as well as a smooth triple line. The combination of both properties is in general used to define a clean state for selfcleaning systems. All AFM measurements were carried out in MilliQ water. The adhesion between the colloidal probe and the TiO2 surface was measured for different stages of UV exposure. Every adhesion value is an average of the adhesion in four different locations and five measurements in each location on the sample surface. The work of adhesion (W) was calculated from the pull-out force (Fadh), accessible during the retract regime, with the Derjaguin approximation using the following formula: W ) |Fadh|/2πR.23,24 All SHG experiments were performed in air at room temperature. An output beam from a mode-locked Ti:sapphire oscillator (Vitesse 800-2, Coherent, U.S.) at 805 nm was amplified in a regenerative amplifier (Titan-DQ, Quantronix, U.S.) driven by a frequency-doubled Q-switched Nd:YLF laser (Falcon 527DQ, Quantronix) to give 3 mJ pulses with duration of 120 fs and repetition rate of 1 kHz. This fundamental frequency was used to pump an OPA/OPG (TOPAS 800, Light Conversion, Lithuania). The subsequent signal was frequencydoubled to get tuneable beams from 600 to 900 nm (2.06-1.38 eV) and an energy up to 100 µJ/pulse. The tuned light pulses were directed through a half-wave plate and a rotating polarizer and were focused onto the sample surface within a spot size of about 1 mm2 in diameter at an incidence angle of 50°. The reflected SHG light beam was filtered and detected by a photomultiplier (R4220, Hamamatsu, U.S.) and analyzed by an image-intensified high-resolution CCD camera (PIMAX:1K, Roper Scientific, U.S.). Typically, signals were collected from 30 to 120 s depending on the wavelength used to obtain spectra with a reasonable signal-to-noise ratio. The UV exposure was done in situ using the same conditions as for the AFM experiment. APXPS under UV light was carried out at beam line 11.0.2 at the Advanced Light Source at Lawrence Berkeley National Laboratory.19,25 Incident beam energies of 690 eV (O1s, Ti2p), and 450 eV (C1s) were used. The relative C1s/O1s sensitivity under these conditions (taking into account the incident photon flux intensity and the photoelectron emission cross sections for C1s and O1s) is about 4/3. Before the experiment the base pressure of the chamber was about 1.5 × 10-8 torr. The total partial pressure in the chamber was varied between 0.4 and 0.8 torr, at H2O:O2 ratios of 1 to 1. The UV irradiation was done through a transparent quartz window at a distance of about 15 cm using an UV lamp with a power of 5 W for 120 min. Before the transfer the contact angles of the samples were g10°. Contact angles between droplets of water and the surface of TiO2 were controlled using a pocket goniometer (PGX, FIBRO system ab). Results and Discussion Figure 1 shows the evolution of the adhesion energy between the probe and the surface as a function of the UV dose. Every
Jribi et al.
Figure 1. Time dependence of the calibrated adhesion energy W ) |Fadh|2πR between a silica microsphere and a TiO2 surface upon UV exposure. The light intensity was 5.3 mW/cm2 in ambient conditions. The inset shows a SEM image of the silica microsphere glued on a tipless cantilever.
Figure 2. Interaction energy as a function of the tip-surface separation for different UV doses (from bottom to top: before UV, 5, 15, 30, and 50 min UV). The inset shows the time dependence of the jump-tocontact distance upon UV exposure.
data point is an average of 20 measurements. Adhesion decreases gradually when the sample is exposed to UV light. The surface reaches a nonadhesive state at doses around 1.6 × 104 W m-2, which coincides with the observation of a macroscopic hydrophilic state of the surface, as revealed by contact angle measurements. In the absence of UV exposure, the adhesion between the silica probe and the surface remains constant over time, which validates that the suppression of adhesion is induced by UV exposure of the surface and not by measurement cycling. This progressive suppression of adhesion is consistent with the evolution of the short-range interactions between the mineral SiO2 sphere and the TiO2 surface. Figure 2 presents the approach curves for different UV doses. Two regions are apparent: the long-range repulsion (separation > 15 nm) and the jump-tocontact instability (
