8702
J. Phys. Chem. C 2008, 112, 8702–8707
SiOx Ultrathin Layer Coverage Effect on the (Photo)catalytic Activities of Rutile TiO2 Naoyuki Mukaihata,† Hideo Matsui,† Tetsuro Kawahara,‡ Hiroshi Fukui,§ and Hiroaki Tada*,† Department of Applied Chemistry, Faculty of Science and Engineering, Kinki UniVersity, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan, Building Products R & D Japan, Nippon Sheet Glass Co. Ltd., 2-13-12, Konoike, Itami, Hyogo 664-8520, Japan, and Shiseido Co. Ltd., 1-6-2, Higashi-Shinbashi, Minato-Ku, Tokyo 105-8310, Japan ReceiVed: February 3, 2008; ReVised Manuscript ReceiVed: March 19, 2008
A two-step method consisting of 1,3,5,7-tetramethycyclotetrasiloxane chemisorption and the postheating has formed a uniform and dense silica-like SiOx ultrathin layer (SiOx-UTL) on the surface of rutile TiO2. Diffuse reflectance infrared Fourier transform and solid-state 29Si NMR spectroscopy showed that three-dimensional Si-O-Si networks grow to become rigid by repeating the treatment. The coverage with the SiOx-UTL eliminates the surface acid sites, leading to an almost complete loss of the photocatalytic activity at the thicknesses above 0.36 ( 0.03 nm. On the basis of a tunneling model, the significant restriction of the photocatalytic activity can be accounted for by the excellent barrier effect of the SiOx-UTL for the photogenerated charge carrier transfer from TiO2 to acceptors in solution. I. Introduction Anatase TiO2 (A-TiO2) has attracted much interest for the last few decades as an environmental purification photocatalyst because of its ability to decompose harmful organic pollutants.1–3 A promising approach to increase the photocatalytic activity is the coupling with metal nanoparticles,4–6 different semiconductors,7–9 and both10 due to the charge separation through interfacial electron transfer. Hybridization with insulators such as SiO2 is also effective in accelerating some kinds of photocatalytic reactions, which can result from the quantum size effect of A-TiO2 nanoparticles11 or the enhancement of adsorption on the catalyst surfaces.12–15 Further, SiOx ultrathin layer (SiOx-UTL) coating improves hydrophilicity of the surfaces of A-TiO216,17 and polymers.18 On the other hand, rutile TiO2 (R-TiO2), having a large refractive index and no absorption in the visible region, is used in large quantities as white pigments of paintings for buildings, automobiles, etc. In these cases, reducing the (photo)catalytic activities is needed to improve the stability of products in the dark and under exposition to the sunlight. To this end, commercial R-TiO2 is usually coated with 5-10 mass % SiO2 and/or Al2O3.19 However, to our knowledge, fundamental studies on the effect of insulator coating on the (photo)catalytic activities of R-TiO2 have been quite limited.20 Sol-gel methods21 usually used for preparing SiOx films are incapable of controlling the film thickness at a nanometer level. We have reported the stepwise growth of the SiOxUTL on A-TiO2 using a method consisting of adsorption of 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) and the subsequent photocatalytic oxidation.22,23 The advantage of such organosilicon hydrides over other silane coupling agents is to provide a clean reaction environment because the byproduct is only H2,24 which improves the film uniformity and makes the surface modification technique more reproduc* To whom correspondence should be addressed. Tel.: +81-6-6721-5219. Fax: +81-6-6727-2024. E-mail:
[email protected]. † Kinki University. ‡ Nippon Sheet Glass Co. Ltd. § Shiseido Co. Ltd.
ible.25 However, the TMCTS chemisorption/photocatalytic oxidation method is difficult to apply for coating R-TiO2, because the photocatalytic activity of rutile is much lower than that of anatase. Brunner et al. reported stepwise growth of SiOx-UTL on Si(110) surfaces through sequential adsorption/heat oxidation cycles of alkylsiloxane monolayers.26 In this study, we prepared SiOx layers with their thickness controlled at a nanometer level on R-TiO2 by TMCTS adsorption/heat oxidation cycles and studied the SiOx-coverage effect on the (photo)catalytic activities. II. Experimental Section II.A. Preparation of SiOx/TiO2 Particles. A SiOx-UTL was formed on the surfaces of R-TiO2 particles with a mean size of 80 nm (MT-700B, Tayca Co.) by a two-step method consisting of TMCTS chemisorption and the subsequent heating. TMCTS was allowed to react with the TiO2 (1 g) particles placed in a vacuum chamber under 0.8 ( 0.1 hPa at 353 K for 0.5 h. Then the temperature was raised to 373 K, while evacuating for an additional 0.5 h to remove the physisorbed TMCTS. The particles obtained (TMCTS/R-TiO2) were heated at 773 K for 1 h in air. The same adsorption postheating processes were repeated for controlling the SiOx-UTL thickness (SiOx(n)/RTiO2): n denotes the repeated number. II.B. Characterization of SiOx/TiO2 Particles. Diffuse reflectance infrared Fourier transform (DRIFT) spectra of the samples were obtained with a JASCO FT/IR-470Plus spectrometer equipped with a diffuse reflectance attachment (Spectra Tech). The spectra were recorded in the 4000-400 cm-1 range at a resolution of 4 cm-1 with 256 coadded scans using KBr as a reference. Solid-state 29Si NMR measurements were run on a Bruker Advance DSX-400 spectrometer operating at 79.49 MHz. Experiments were performed using a Bruker 7 mm widebore MAS probe. The MAS speed was 5 kHz, and the 90° pulse time was 5.5 µs. Spectra were recorded using the standard pulse and collection sequence with a recycle delay of 30 s and highpower proton decoupling during the acquisition. The spectral width was 32.7 kHz, and over 8192 data points were collected. In processing data, a line broadening of 75 Hz was used.
10.1021/jp801022m CCC: $40.75 2008 American Chemical Society Published on Web 05/16/2008
SiOx Ultrathin Layer on TiO2
J. Phys. Chem. C, Vol. 112, No. 23, 2008 8703
Electrophoretic measurements were carried out for R-TiO2 and SiOx(n)/R-TiO2 particle suspensions containing a 1 mmol dm-3 KCl as a supporting electrolyte on a ζ-potential analyzer (ZEECOM, MICROTEC) at 298 K. The pH of the suspensions were controlled using an aqueous solution of NaOH or HCl. An electrophoresis cell was made from a Pyrex glass with a width of 0.45 mm, height of 10 mm, and length of 90 mm (electrode distance). ζ-potential was calculated from eq 127
ζ ) 6πηV/εE
(1)
where η is the viscosity of the solvent, is the dielectric constant of the solvent, E is the electric field, and V is the velocity of the particle under the electric field E. II.C. Surface Acidity Measurements. Surface acidity of R-TiO2 and SiOx/R-TiO2 was characterized by pyridine adsorption experiments. Pyridine (300 µL) was adsorbed on the particles placed in a vacuum chamber under 0.8 ( 0.1 hPa at 423 K for 0.5 h. Then the temperature was raised to 473 K, while evacuating for an additional 0.5 h to remove the physisorbed pyridine. The difference DRIFT spectra before and after adsorption of pyridine were obtained with a JASCO FT/ IR-470Plus spectrometer equipped with a diffuse reflectance attachment (Spectra Tech). II.D. Photocatalytic Activity Evaluation. To study the photocatalytic activity, liquid-phase decomposition of 2-naphthol (2-NAP) and gas-phase decomposition of acetaldehyde (CH3CHO) were carried out. A 1.0 × 10-5 M solution of 2-NAP (solvent, acetonitrile/water ) 1:99 v/v) was irradiated with a 500 W Xe lamp (Wacom HX-500) in the presence of each photocatalyst. Two pieces of SnO2 film-coated plate glass were used as a cutoff filter (λex > 320 nm, I 320-400 ) 2.0 mW cm-2). The electronic absorption spectra of the reaction solutions were measured using a spectrometer (Hitachi, U-4000) to determine the concentrations of 2-NAP from the absorption peak at 224 nm. A 594 ppm standard CH3CHO gas (CH3CHO/N2) was introduced into a reaction chamber (0.64 L) to be diluted with air such that its initial concentration was kept within the 350 ppm range. After the adsorption equilibrium of CH3CHO on R-TiO2 or SiOx/R-TiO2 (0.05 g) had been achieved under dark conditions, irradiation (λex > 300 nm, light intensity integrated between 320 and 400 nm (I320-400) ) 2.0 mW cm-2) was carried out using a 500 W Xe lamp (Wacom, HX-500) at room temperature. The concentration of CH3CHO was determined as a function of time by gas chromatography (Shimadzu, GC-9A) with a SHINCARBON A fid column (3 mm φ × 3 m): injection and column temperatures were 70 °C, and N2 was used as a carrier gas. In the photoelectrochemical measurements, an HZ5000 automatic polarization system (Hokuto Denko) was employed. R-TiO2/SnO2 samples were prepared by coating a slurry containing 18% R-TiO2 particles (MT-700B), 95% water, and 5% polyethylene glycol (MW ) 200) on SnO2 film-coated glass substrates (AGC) by a squeeze method to be heated at 773 K for 2 h (R-TiO2/SnO2). Then SiOx-UTLs were formed on the R-TiO2/SnO2 samples by the same way as in the particulate system (SiOx/R-TiO2/SnO2). Photocurrents of SiOx/ R-TiO2/SnO2 (apparent surface area ) 10 cm2) were measured at a rest potential in a regular three-electrode electrochemical cell filled with a solution. Light from a Xe lamp was filtered by two pieces of SnO2/glass plates (λex > 320 nm, I320-400 ) 2.0 mW cm-2), and irradiated from the back side of the R-TiO2 film. III. Results and Discussion The morphology of the SiOx layer was directly observed by transmission electron microscopy (TEM). Figure 1 shows a high-
Figure 1. HRTEM image of SiOx(n ) 4)/R-TiO2.
Figure 2. pH dependence of ζ-potentials: a, R-TiO2; b, SiOx(n ) 1)/ R-TiO2; c, SiOx(n ) 4)/R-TiO2.
resolution TEM (HRTEM) image of SiOx(n ) 4)/R-TiO2, in which the lattice spacing agrees with the distance between the (001) planes of R-TiO2.28 Also, the R-TiO2 particle is uniformly covered with an amorphous SiOx-UTL. The ζ-potential of the particles reflects the coverage state of all the particles, whereas HRTEM observation provides the information on a spatially limited region. Figure 2 shows ζ-potentials of R-TiO2 and SiOx/ R-TiO2 particles as a function of pH. The pH at ζ-potential ) 0 gives the point of zero charge (pzc) of each particle. The pzc value decreases from ca. 5 to 2.3 even at n ) 1 and to 2.0 at n ) 4, which is in good agreement with the value reported for SiO2.29 These results indicate that a uniform silica-like SiOxUTL is formed by this TMCTS chemisorption/heating oxidation method. Figure 3A shows the change in weight of TiO2 particles (∆w) as a function of n: a, initial weight of TiO2 (w0) ) 1.0227 g; b, w0 ) 2.0000 g. The weight increases during each adsorption step (A), whereas it is almost invariant before and after the following heating step (H). Also, the ∆w doubles when the w0 increases twice. Thus, the assumption that spherical TiO2 particles are covered with uniform and dense SiOx layers enables us to estimate its thickness (L) using eq 2
L (nm) ) (x/(100 - x))(0.33d1/d2)r
(2)
where x is the loading amount of SiOx (mass %), d1 and d2 are the densities of SiO2 (2.196 g cm-3) and R-TiO2 (4.250 g cm-3), respectively, and r is the mean radius of TiO2 particles (40 nm).
8704 J. Phys. Chem. C, Vol. 112, No. 23, 2008
Mukaihata et al.
Figure 3. (A) Plots of ∆w vs n: a, w0 ) 1.0227 g; b, w0 ) 2.0000 g. (B) Film thickness of the SiOx-UTL as a function of n.
The value of L at n ) 4 was calculated to be 0.49 nm, which is near to the thickness of the SiOx-UTL observed in Figure 1. Also, as shown in Figure 3B, the L value increases in proportion to n at n e 2, whereas the increment decreases somewhat at n > 2. DRIFT22,23 and solid-state 29Si NMR spectroscopic measurements were performed in order to obtain the information on the SiOx structure. Figure 4A, curve a, shows a difference spectrum before and after TMCTS adsorption on TiO2 (TMCTS/ R-TiO2 - R-TiO2). The positive signals at 2970, 2912, 2168, and 1267 cm-1 are assignable to the asymmetric (νas(Si-CH3)), symmetric stretching vibrations (νs(Si-CH3)) of Si-CH3 groups, the stretching vibration of Si-H groups (ν(Si-H)), and the bending vibration of Si-CH3 group (δ(Si-CH3)), respectively. Also, the positive signals at 1100 and 1061 cm-1 can be assigned to the Si-O-Si (ν(Si-O-Si)) and Ti-O-Si (ν(Tis-O-Si)) stretching vibrations, respectively: the subscript s denotes a surface atom. Further, a broad negative peak due to the stretching vibrations of Ti-OH groups is observed near 3690 cm-1. Figure 4A, curves b-e, shows difference spectra before and after n cycle treatments (SiOx(n)/R-TiO2 - R-TiO2). Upon heating at 773 K, a new peak due to isolated Sis-OH groups (ν(Sis-OH)) appears at 3740 cm-1, while the signals of Si-H and Si-CH3 groups disappear. Figure 4B shows the wavenumber of the ν(Si-O-Si) absorption peak (νmax) as a function of n. As n increases, the νmax shifts toward higher wavenumber concurrently with the increase in the signal intensity. This finding indicates that the Si-O-Si network grows to become rigid with increasing n. Figure 5 shows solid-state 29Si NMR spectra of various samples. Spectrum a for TMCTS-adsorbed TiO2 (TMCTS/RTiO2) has three signals at δ ) -34, -56, and -65 ppm due to Si(OSi)2HCH3 (D2), Si(OSi)2CH3OX (X ) H or Ti, T2), and Si(OSi)3CH3 (T3), respectively.30 In spectrum b for SiOx(n )
Figure 4. Difference DRIFT spectra: a, TMCTS/R-TiO2 - R-TiO2; b, SiOx(n ) 1)/R-TiO2 - R-TiO2; c, SiOx(n ) 2)/R-TiO2 - R-TiO2 ; d, SiOx(n ) 3)/R-TiO2 - R-TiO2; e, SiOx(n ) 4)/R-TiO2 - R-TiO2.
1)/R-TiO2, three signals appear at δ ) -110, -100, and -96 ppm, while the low-field signals observed for TMCTS/R-TiO2 disappear. These new signals are assignable to Si(OSi)4 (Q4), Si(OSi)3OX (Q3), and Si(OSi)2(OX)2 (Q2), respectively.31 The signal intensities of Q2 and Q3 are comparable at n ) 1, whereas Q3 and Q4 become dominant at n ) 2 and 4, respectively. These results indicate that 3D Si-O-Si networks in the SiOx-UTL develop by repeating the treatment. Acid properties of solid surfaces were examined by DRIFT spectroscopy using pyridine as a probe molecule. Figure 6 shows difference DRIFT spectra before and after pyridine adsorption on R-TiO2 and SiOx/R-TiO2. Spectrum a has a weak absorption at 1516 [V19b(B)] and a strong absorption at 1421 cm-1 [V19b(L)], which are characteristic of pyridinium ion adsorbed on Brønsted (B)-acid sites and pyridine coordinated to Lewis (L)-acid sites, respectively.32 The fact that the signal intensity of the latter is larger than that of the former, regardless of their comparable absorption coefficients,33 indicates that pure R-TiO2 mainly has L-acidity.34 The B- and L-acid sites on the R-TiO2 surface are tentatively assigned to terminated Tis-OH groups (and bridge OH groups) and coordinatively unsaturated surface Ti4+ ions, respectively. Also, the absorption peaks at 2962, 2929, and 2858 cm-1 are assignable to the C-H stretching vibrations of V2, V13, and V20 modes, respectively. At n ) 1 (spectrum b), the V19b(B) signal intensifies, which may result from the generation of new B-acid sites with the formation of Ti-O-Si bonds.35 On the other hand, the V19b(L) signal weakens, and the adsorption
SiOx Ultrathin Layer on TiO2
J. Phys. Chem. C, Vol. 112, No. 23, 2008 8705
Figure 5. Solid-state 29Si NMR spectra: a, TMCTS/R-TiO2; b, SiOx(n ) 1)/R-TiO2; c, SiOx(n ) 2)/R-TiO2; d, SiOx(n ) 4)/R-TiO2.
Figure 6. Difference DRIFT spectra before and after pyridine adsorption of R-TiO2 (a) and SiOx(n)/R-TiO2 (b-d): b, n ) 1; c, n ) 2; d, n ) 4.
amount of pyridine decreases as indicated by the νC-H signal intensities. At n g 2 (spectra c and d), the absorption of V19b(B) and V19b(L) disappears accompanied by inhibition of the adsorption. Liu et al. reported that pure SiO2 has neither B- nor L-acidity.36 Evidently, the acid sites are completely lost by the formation of the SiOx-UTL. Prior to the evaluation of photocatalytic activity, the influence of the SiOx-UTL coating on the optical property of R-TiO2 was studied. R-TiO2 with a band gap of 3.0 eV has a strong absorption band below 410 nm due to interband transition. No change in the absorption spectrum was observed with the formation of the SiOx-UTL at n e 4. The fact that the superior optical property of R-TiO2 as a white pigment is maintained is of great importance from the practical viewpoint. To study the effect of the SiOx-UTL coverage on the photocatalytic activity of R-TiO2, gas-phase decomposition of CH3CHO37 and liquidphase decomposition of 2-NAP38 were carried out as test reactions. Figure 7A shows time courses for the photocatalytic decomposition of CH3CHO in the presence of R-TiO2 and SiOx(n)/R-TiO2. UV-light irradiation after achievement of the
Figure 7. (A) Time courses for the photocatalyzed CH3CHO decomposition: a, R-TiO2; b, SiOx( n ) 1)/R-TiO2; c, SiOx(n ) 2)/R-TiO2; d, SiOx(n ) 4)/R-TiO2. (B) Plots of ln(C0/C) vs irradiation time: a, R-TiO2; b, SiOx(n ) 1)/R-TiO2; c, SiOx(n ) 2)/R-TiO2; d, SiOx(n ) 4)/R-TiO2. (C) n dependence of k.
adsorption equilibrium in the dark leads to the decrease in the concentration of CH3CHO in the R-TiO2 system (curve a). Noticeably, the decomposition rate is significantly reduced by the SiOx-UTL coating (curves b-d). As shown in Figure 7B, the linearity in the ln(C0/C) versus irradiation time plots indicates that this reaction follows the first-order rate law: C0 and C denote the initial CH3CHO concentration and the concentration at arbitrary irradiation time, respectively.8 The apparent rate
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Figure 8. (A) n dependence of the decomposition ratio of 2-NAP after 1 h of irradiation. (B) Photocurrent response of R-TiO2/SnO2 (a) and SiOx/R-TiO2/SnO2 (b-d): b, n ) 1; c, n ) 2; d, n ) 4. The inset is a field emission scanning electron microscope (FE-SEM) image of R-TiO2/SnO2.
constant (k) determined from the slope of the straight line was used as an indication of the photocatalytic activity. Figure 7C shows plots of k versus n, indicating that the photocatalytic activity of R-TiO2 drastically decreases with the SiOx-UTL coating to be almost completely suppressed at n g 2. Figure 8A shows the ratio of 2-NAP decomposed after 1 h of irradiation in the presence of R-TiO2 and SiOx/R-TiO2 as a function of n. A trend for the n dependence of the photocatalytic activity similar to that in the CH3CHO decomposition is apparent, and it lowers to a self-decomposition level (
