Water Wettability of an Ultrathin Layer of Silicon Oxide Epitaxially

Article pubs.acs.org/JPCC

Water Wettability of an Ultrathin Layer of Silicon Oxide Epitaxially Grown on a Rutile Titanium Dioxide (110) Surface Tu Tran Uyen Le, Akira Sasahara, and Masahiko Tomitori* Japan Advanced Institute of Science and Technology, Nomi, Ishikawa 923-1292, Japan ABSTRACT: Water wettability of a SiO2 layer fabricated on a rutile TiO2(110) surface was examined. The TiO2 wafer was annealed at 1273 K in a quartz case used as a silicon oxide vapor source. X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and frequency modulation atomic force microscopy (FM-AFM) analysis indicated that the monolayer of the stishovite SiO2 with a rutile structure was formed on the TiO2 surface. The water contact angle on the SiO2 monolayer was approximately 0° when the layer was cooled in the laboratory air for 6 h to room temperature and reached 15° after a further 24 h exposure to the laboratory air. The fused SiO2 surface and the SiO2-free TiO2(110) surface showed water contact angles of 37° and 32°, respectively, after being annealed at 1273 K and subsequently cooled for 6 h in the laboratory air. The high hydrophilicity of the SiO2 monolayer was attributed to the high density of hydrophilic surface OH groups. The O 1s XPS spectra showed that the density of the OH groups on the SiO2 monolayer was four times as high as that on the SiO2-free TiO2(110) surface. The formation of OH groups was probably promoted by the surface O vacancies originating from 9% lattice mismatch in the rutile structure between SiO2 and TiO2.

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their superhydrophilicity in the dark longer than pure TiO2.15 Tricoli et al. examined the SiO2−TiO2 film prepared by flame spray pyrolysis and attributed the superhydrophilicity of the film to the superhydrophilic nature of flame-made SiO2 and photocatalysis of TiO2.16 This work examined the wettability of the SiO2 layer fabricated on a rutile TiO2(110) surface by a vapor deposition. Surface sensitive analysis by X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and frequency modulation atomic force microscopy (FM-AFM) techniques indicated that the SiO2 layer had a rutile structure and a monolayer thickness. The rutile form of SiO2 is known as stishovite and is a high-pressure phase of SiO2. The unique stishovite SiO2 monolayer enhanced the hydrophilicity of the surface. The surface O vacancies originating from lattice mismatch between SiO2 and TiO2 probably promoted the dissociation of H2O molecules to hydrophilic OH groups.

INTRODUCTION Water wettability of titanium dioxide (TiO2) has been intensively studied because it is closely related to the major industrial applications of TiO2, such as antifog coatings,1,2 antifouling coatings,1,2 and biocompatible coatings.3,4 TiO2 surface shows superhydrophilicity (water contact angle: ∼0°) under UV light irradiation,5,6 which promises a long-lasting performance of TiO2 as a hydrophilic material used under sunlight. UV irradiation induces the generation of electron− hole pairs in TiO2. The photogenerated holes decompose hydrophobic surface organic contaminants7 and create O vacancies that promote the dissociation of H2O molecules to form hydrophilic OH groups.8 A part of the holes break the surface Ti−O bonds and further provide the sites for the dissociation of H2O molecules.9 Mixing with silicon dioxide (SiO2) is an effective method to enhance the hydrophilicity of the TiO2.10−16 The pioneering study of wettability of SiO2−TiO2 mixed oxide was reported in 1999 by Machida et al.10 The SiO2−TiO2 film prepared from the mixed sols showed superhydrophilicity when the film included 10−30 mol % of the SiO2. Contribution of the SiO2 to maintaining high surface area of the TiO2 in anatase form was concluded on the basis of X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis. Langlet and his co-workers attributed the superhydrophilicity of the sol−gel prepared SiO2−TiO2 film to the acidic sites formed at the interface between the SiO2 and the TiO2.11−13 The acidic sites arise from the Si−O−Ti hetero linkages due to a charge imbalance14 and promote the dissociation of the H2O molecules. The contribution of the acidic sites to the hydrophilicity of the SiO2−TiO2 film was also mentioned by Fateh et al., who reported that the SiO2−TiO2 films maintained © 2013 American Chemical Society

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EXPERIMENTAL METHOD Rutile TiO2(110) wafers (Shinkosha) were cleaned prior to use by ultrasonic degreasing in acetone, etching in an aqueous HF solution (10%), and rinsing in Milli-Q water. The wafers were placed in a quartz case and annealed at 1273 K in an electric furnace to form the SiO2 layer.17 In control experiments, the fused SiO2 disks were annealed in the quartz case at 1273 K for 6 h. The SiO2-free TiO2(110) surfaces were prepared by annealing the TiO2 wafers in a sapphire case at 1273 K for 6 h. Received: April 12, 2013 Revised: September 6, 2013 Published: September 19, 2013 23621

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X-ray photoelectron spectroscopy analysis was conducted at room temperature by a commercial system (S-probe, Fisons Instruments) with a monochromatic Al Kα source. The photoemission angle θ with respect to surface normal was set to 35°. The surface charge of the insulating TiO2 substrates was reduced by a neutralizer. The binding energy of the spectra was calibrated so that the C 1s peak became at 284.6 eV. Optics for LEED (SPECTALEED, Omicron) were installed in a homemade ultrahigh vacuum (UHV) chamber. FM-AFM imaging was performed in Milli-Q water by using a multipurpose scanning probe microscope (SPM 5500, Agilent technologies) and a phase-locked-loop detector (easy PLL plus, Nanosurf AG).18 The resonant frequency and the quality factor of a Si FM-AFM cantilever in water was ∼70 kHz and ∼70, respectively. The peak-to-peak amplitude of the cantilever oscillation was set to ∼4 nm. Contact angle measurement was conducted in a laboratory environment using a homemade contact angle meter consisting of a precision rotation-tilt stage and a hand-operated microsyringe. The annealed wafers were cooled in the laboratory air for 6 h, and a droplet of 2 μL MilliQ water was placed on horizontal surfaces. The contact angles were determined on the side-view photos.

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RESULTS AND DISCUSSION Figure 1a shows a wide-scan XPS spectrum taken from the asreceived TiO2(110) surface. The intense peaks at about 460

Figure 2. Narrow-scan XPS spectra of (a) Ti 2p, (b,c) O 1s, and (d) Si 2p regions obtained on the identical TiO2 surface annealed in the quartz case. (e) Dependence of the Si/OTiO2 atom ratio on the annealing time. (f) Dependence of the Si/OSiOx atom ratio on the Si/ OTiO2 atom ratio.

Figure 1. Wide-scan XPS spectra of rutile TiO2(110) surfaces (a) in as-received state, (b) annealed in a sapphire case at 1273 K for 6 h, and (c) annealed in a quartz case at 1273 K for 6 h. Peak assignment is shown in (a) and (c).

increasing annealing time. The spectrum of the O 1s region in Figure 2b showed a peak with a high binding energy shoulder. The shoulder became more intense with the annealing time. The peak was deconvoluted into three components as shown in Figure 2c: the major peak from bulk TiO2 at 529.9 eV (hereinafter denoted by OTiO2), the second component at 530.9 eV, and the third component at 531.9 eV. The second and the third components are assigned to OH group19 and silicon oxide SiOx (x ≤ 2),20 respectively. The O giving rise to the peak at 531.9 eV is designated hereafter as OSiOx. The growth of the shoulder indicates the increase of the OH groups and the SiOx on the surface. The intensity ratio of the component from the OH group with respect to the total O 1s peak was 0.10 on the nonannealed surface and increased to 0.38 on the 72 h annealed surface. The intensity of the Si 2p peak at 102.4 eV increased with the annealing time up to 72 h and showed no appreciable change by further annealing as shown in Figure 2d.

and 530 eV correspond to Ti 2p and O 1s, respectively. In addition to the Ti 2p and the O 1s peaks, peaks for C, Na, Si, P, Ca, and Zn were observed. The impurity peaks were not observed in the spectrum of the TiO2 surface annealed in the sapphire case as shown in Figure 1b. The impurities are removed by the HF pretreatment except for C. The presence of C is attributed to organic contaminants from the laboratory air. When the HF treated TiO2 surface was annealed in the quartz case, Si 2p peak appeared in the spectrum as shown in Figure 1c. This indicates that Si was provided from the quartz case, probably in a SiOx form, onto the TiO2 surface. Figure 2a-d shows narrow scans of Ti 2p, O 1s, and Si 2p regions on the TiO2 surfaces annealed in the quartz case. In the Ti 2p region spectra in Figure 2a, the Ti 2p3/2 and Ti 2p1/2 peaks were observed at 458.7 and 464.6 eV, respectively. The binding energy and the shape of the peaks did not change with 23622

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The binding energy of the OSiOx peak was higher than that of the OTiO2 peak by 2.0 eV. While the O 1s binding energy of the quartz is higher than that of the rutile TiO2 by 2.9 eV,21 the O 1s peak of the stishovite SiO2 (SiO2 in the rutile structure) appears at the binding energy lower than that of the quartz by 0.8 eV.22 The binding energy of the OSiOx peak was almost identical to that of the O atoms in the stishovite SiO2, which indicates the formation of the stishovite SiO2 layer on the TiO2(110) surface. Note that the stishovite is formed under the pressure as high as 10 GPa at temperatures higher than 1273 K in the natural world. The dependence of the Si/OTiO2 atom ratio on the annealing time is shown in Figure 2e. The Si/OTiO2 peak intensity ratio, ISi/IO(TiOx), is related to the densities of Si and OTiO2 atoms, nSi and nO(TiOx), by the following equation: ISi IO(TiOx)

=

nSiσSiλSi nO(TiOx)σOλO

Figure 3. LEED patterns of (a) the SiO2-free TiO2(110) surface and (b) the stishovite SiO2 monolayer. Incident electron energy: (a) 120 eV, (b) 140 eV.

substrate. The substrate-related periodicity suggests the epitaxial growth of the SiO2 on the TiO2(110) surface, which reinforces the formation of the stishovite SiO2. The length of the Si−O bond in the stishovite is 0.18 nm and is smaller than that of the Ti−O bond in the rutile TiO2 by 9%.22 The stishovite SiO2 layer is probably strained due to the lattice mismatch, which causes the limited growth. The strain may be reduced by a periodic relaxation of the surface atoms in the [110̅ ] direction to show the (1 × 2) periodicity. Figure 4a shows the FM-AFM images of the TiO2(110) surface annealed in the sapphire case for 6 h. The surface

(1)

σSi and σO are the photoemission cross sections for Si 2p3/2 and O 1s states and are 0.54 and 2.9, respectively.23 λSi and λO are the inelastic mean free paths (IMFPs) of Si 2p3/2 and O 1s photoelectrons in SiO2 and are 3.8 and 2.8 nm, respectively.24 The 2/3 of the ISi was assumed to be from the Si 2p3/2. The Si/OTiO2 atom ratio increased with the annealing time, and remained at 0.019 on the surfaces annealed longer than 72 h. This indicates that the growth of the stishovite SiO2 layer on the TiO2(110) surface is self-limiting, as that on the TiO2(100) surface.17 Figure 2f shows the plots of the Si/OSiOx atom ratio as a function of the Si/OTiO2 atom ratio. The Si/OSiOx atom ratio was 0.31 independently of the Si/OTiO2 atom ratio. The obtained Si/OSiOx atom ratio is an acceptable value for the SiO2 within the accuracy of the curve fitting processes. Combined with the fact that the binding energy of the OSiOx peak was independent of the Si coverage, the stishovite SiO2 is formed from the early stage of the SiOx evaporation. The thickness of the stishovite SiO2 layer was estimated on the assumption that the intensity of the photoelectron peak was attenuated in the uniformly grown layer according to the Beer− Lambert law:25 ISi I {1 − exp( −d /λSi cos θ )} = 0Si ITi I0Ti exp( −d /λ Ti cos θ )

Figure 4. FM-AFM images of the TiO2(110) surfaces (2000 × 2000 nm2). (a) The SiO2-free TiO2(110) surface. Frequency shift (Δf): +193 Hz. (b-d) The surfaces annealed in the quartz case for (b) 6, (c) 24, and (d) 48 h. (b) Δf: +170 Hz; (c) Δf: +356 Hz; (d) Δf: +248 Hz.

(2)

Here, ISi and ITi are the peak intensities of Si 2p and Ti 2p3/2. I0Si and I0Ti are the intensities of the Si 2p peak from bulk SiO2 and the Ti 2p3/2 peak from bulk TiO2. The factory-provided relative sensitivity factors modified by the atom densities and the IMFPs, 1.9 for the Si 2p and 2.1 for the Ti 2p3/2, were used in place of the I0Si and the I0Ti. λTi is the IMFP of the Ti 2p3/2 photoelectrons in SiO2 and is 3.0 nm from ref 24. The thickness d of the SiO2 layer was calculated to be 0.20 nm for the TiO2 wafer annealed for 72 h. The single step height on the stishovite SiO2(110) surface is expected to be 0.30 nm from the crystal structure. Hence, the estimated d indicates that the growth of the stishovite SiO2 layer is limited to a monolayer. Figure 3a shows the LEED pattern of the SiO2-free TiO2 surface. A bulk-plane related (1 × 1) pattern was observed. Figure 3b shows the LEED pattern observed on the stishovite SiO2 monolayer prepared by 72 h annealing of the TiO2 substrate. A sharp (1 × 2) pattern was observed, which indicated long-range order of the SiO2 surface atoms with the double periodicity along the [11̅0] direction of the TiO2

consisted of flat terraces separated by steps nearly parallel to the [11̅0] direction. The height of the steps was 0.3 nm and was roughly consistent with that of the single step of the (110) surface, 0.32 nm. No appreciable change was observed in surface topography by extending the annealing time up to 48 h. Figure 4b-d shows the FM-AFM images of the TiO2 surfaces annealed in the quartz case for 6, 24, and 48 h, respectively. Patches with a height of 0.2 nm appeared after 6 h annealing as shown in Figure 4b. The patches were attached to the step edges and appeared to be growing from the steps. The area of the patches increased after 24 h annealing as shown in Figure 4c. The surface appeared completely covered by the rectangular patches elongated to the [001] direction after 48 h annealing as shown in Figure 4d. The patches are assigned to the stishovite SiO2 layer grown on the TiO2 surface. The saturation of the Si/OTiO2 XPS peak 23623

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intensity ratio and the clear (1 × 2) LEED pattern were obtained on the patch-covered surfaces. The patch height of 0.2 nm was consistent with the thickness of the stishovite SiO2 monolayer estimated from the XPS peak intensity. Thus, the stishovite SiO2 layers are epitaxially grown as rectangular patches on the TiO2(110) surfaces by vapor transport of the SiOx from the quartz case. The SiOx species diffusing on the surface are expected to be stabilized at the step edges. The step edges of the TiO2(110) surface have the lower-coordinated Ti atoms and act as preferential growth sites for adsorbates.26,27 Figure 5a-d shows the pictures of the water droplets on the TiO2(110) surfaces annealed in the quartz case. The water

the stishovite SiO2 monolayer exhibits superhydrophilicity. The hydrophilicity is attributed to the OH groups observed in the XPS spectra in Figure 2c. The SiO2 layer grows on TiO2(110) surface with a large lattice mismatch of 9%, and therefore is expected to have a number of the O vacancies to reduce the lattice strain. The O vacancies act as the dissociation sites for H2O molecules to form hydrophilic OH groups. When the SiO2 partially covered the TiO2 surface, Lewis acid sites derived by the Si−O−Ti linkage14 might have contributed to the dissociation of H2O molecules. It is possible that the SiO2 units were incorporated into the TiO2 lattice during the SiOx evaporation. A comparison with the bulk SiO2 surface and the SiO2-free TiO2 surface highlights the high hydrophilicity of the stishovite SiO2 monolayer. Figure 7a,b shows the pictures of the water

Figure 5. Water contact angle measurements of the TiO2(110) surfaces annealed in the quartz case for (a) 6, (b) 24, (c) 48, and (d) 72 h. (d) shows a near top-view of the surface.

contact angles after 3, 6, 12, 24, 48, and 72 h annealing were, respectively, 20°, 16°, 13°, 11°, 5°, and 0°. Figure 6 shows the

Figure 7. (a,b) Water contact angle measurements of the fused SiO2 surfaces: (a) after etching in HF solution; (b) after annealing in the quartz case. (c,d) Water contact angle measurement of the SiO2-free TiO2(110) surfaces: (c) after etching in HF solution, (d) after annealing in the sapphire case.

droplets on the fused SiO2 surfaces. The water contact angle was 10° after the HF treatment as shown in Figure 7a and increased to 37° after subsequent annealing in the quartz case as shown in Figure 7b. The hydrophilicity of the HF-treated surface is attributed to the increase of the surface OH groups (silanol, Si−OH).28 The surface is terminated by hydrophobic siloxane group (Si−O−Si) when annealed at temperatures higher than 873 K.28 Thus, the fused SiO2 surfaces have less hydrophilicity than the stishovite SiO2 monolayer. A similar low hydrophilicity was observed on the SiO2-free TiO2 surfaces. The water contact angle on the HF-treated TiO2 surface was 23° as shown in Figure 7c, and increased to 32° by annealing in the sapphire case. It is likely that the surface is terminated by the Ti−O−Ti structure analogous to the siloxane group with almost no O vacancies.

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Figure 6. (a) Water contact angles, (b) Si/OTiO2 atom ratios, and (c) patch area ratios in the AFM images plotted as a function of the annealing time.

CONCLUSION The stishovite SiO2 monolayer, epitaxially grown on the rutile TiO2(110) surface, exhibited superhydrophilicity without UV light irradiation. The hydrophilicity of the SiO2 monolayer was attributed to the high density of surface OH groups. The OH groups were probably formed at the surface O vacancies originating from the lattice mismatch between SiO2 and TiO2. The wettability study of the unique SiO2 monolayer unequivocally proved the contribution of the atomistic structure to its hydrophilicity. Heteroepitaxial growth of oxide

dependence of the water contact angles, the Si/OTiO2 atom ratios evaluated from XPS peak intensity, and the coverage of the patches on the annealing time. The water contact angles decreased with the increase of the Si/OTiO2 atom ratio and the coverage of the patches. The surface became superhydrophilic after the annealing for 72 h and above. The hydrophilicity of the surface obviously arises from the stishovite SiO2 layer, and 23624

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is a potential approach for the fabrication of hydrophilic materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-761-51-1501. Fax: +81761-51-1149. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research from the Japanese Society for the Promotion of Science.

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