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Notes Dependence of TiO2 Photocatalytic Activity upon Its Film Thickness
Scheme 1. Procedure for Preparing SG-TiO2 Films
Hiroaki Tada* and Makiko Tanaka Nippon Sheet Glass Techno-Research Co. Ltd., 1, Kaidoshita, Konoike, Itami, Hyogo 664, Japan Received May 2, 1996. In Final Form: October 22, 1996
I. Introduction If the semiconductor absorbs light with energy greater than that of the bandgap, a photon will excite an electron from the valence band to the conduction band, thereby generating a hole in the valence band. After these charges diffuse to the surface of the semiconductors, the reaction of either the photoexcited electron with reducible adsorbates and/or the hole with oxidizable adsorbates can take place. Among a wide range of applications based on these principles, those related to environmental chemistry and pollution chemistry have been of interest recently.1-6 Fujishima and Hashimoto et al. proposed a system using a film of TiO2, a representative n-type semiconductor, coated on the inside of window glass for air purification of buildings.7 Also, it has been demonstrated that a photocatalytic device consisting of TiO2 films formed on internal lightguides is quite effective in decomposing model organic pollutants in water.8 Since the light from the backside of the film can be used as an energy source in these uses, the film thickness of semiconductors is thought to be an important factor affecting the performance of the photocatalytic devices. However, to the best of our knowledge, there are few papers concerned with this subject. This paper describes experimental and theoretical studies of the relationship between the photocatalytic activity and the TiO2 film thickness, particularly when TiO2 is exposed to back face illumination. II. Experimental Section Preparation of the Sol-Gel (SG)-TiO2 Film. Quartz plates (20 × 50 mm2) were used as substrates. Prior to the TiO2 coating, they were soaked in an alkaline solution (pH ) 13.7) for 30 s and subsequently rinsed using pure H2O (conductivity < 0.3 µS cm-1) with sonification for 10 min. SG-TiO2 films were coated on the substrates according to the procedure shown in Scheme 1. The solution of Ti(OiPr)4 (Tokyo Kasei Co., reagent grade) was greatly stabilized by adding acetylacetone (Kishida Chemicals Co., >99%). The stabilization is a result of the formation of the chelate (Ti(OiPr)2(acac)2), which is manifested by a color change of the solution from pale yellow to red.9 The standard concentration of the total Ti complexes (Cs) was fixed at 0.67 mmol dm-3. The concentration of the coating solution (C) was (1) Bard, A. J. J. Photochem. 1979, 10, 59. (2) Matthews, R. W. J. Phys. Chem. 1987, 91, 3328. (3) Tunesi, S.; Anderson, M. A. Chemosphere 1987, 16, 1447. (4) Jackson, N. B.; Wang, C. M.; Luo, Z.; Schwitzebel, J.; Ekerdt, J. G.; Brockand. J. R.; Heller, A. J. Electrochem. Soc. 1991, 138, 3660. (5) Fox, M. A. CHEMTECH 1992, 22, 680. (6) Photocatalytic Purification and Treatment of Water and Air; Ollis, F. D., Al-Ekabi, H., Eds.; Elsevier Science Publishers: Amsterdam, 1993. (7) Fujishima, A.; Hashimoto, K.; Kubota, Y. J. Surf. Sci. Soc. Jpn. 1995, 16, 188. (8) Tada, H.; Honda, H. J. Electrochem. Soc. 1995, 142, 3438. (9) Shinmou, K.; Tohge N.; Minami, T. Jpn. J. Appl. Phys. 1994, 33, L1181.
reduced to Cs/2 for preparing thinner films, while the procedure was repeated for thicker films (cycle number ) n). In order to express the coating conditions, a parameter, n′, defined as n × (C/Cs), was introduced. Characterization of the SG-TiO2 Film. The TiO2 films obtained after annealing were confirmed to be anatase crystallites by X-ray diffraction (XRD) analysis. Diffraction peaks (2θ) were observed at 25.3, 37.9, and 48.1° and could be assigned to diffraction from (101), (004) and (200) planes of anatase, respectively.10 Depth profile analyses were conducted using a PHI 5600Ci Physical Electronics X-ray photoelectron spectrometer. Photoelectrons were generated by Al (KR1,2) X-ray radiation (hν ) 1486.6 eV) at 150 W of power. The unpolarized X-ray photons struck the sample surface with their k-vector (kx) at an angle of 45° with respect to the surface. The electrons were always collected at an angle of 90° relative to kx. The positions of the following photoelectron lines were considered in the depth profile evaluation: Ti(2p), Si(2p), O(1s), and Na(1s). Each sample was depth profiled with Ar+ ions rastered over a 4 × 4 mm2 area by alternately acquiring data and then sputtering at a rate of ca. 3 nm min-1. Each sputtering interval lasted 3 min. Evaluation of the Photocatalytic Activity. The TiO2/ quartz samples were washed in the same manner as the quartz substrates. A fully wetting surface against H2O was obtained by this treatment. A 200 µL portion of 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS, Shin-Etsu Chem., >98%) was allowed to react with the TiO2/quartz samples placed in a vacuum chamber under ca. 10 Torr at 80 °C for 0.5 h. Then the temperature was raised to 100 °C, evacuating for an additional 0.5 h to remove physisorbed TMCTS. It was confirmed by Fourier-transformed infrared spectroscopy that a methylsiloxane monolayer is formed on the surface of TiO2 via Si-O-Ti covalent bonds with this treatment.11,12 The samples were irradiated in (10) JCPDS No. 21-1272, the International Centre for Diffraction Data, Philadelphia, PA, 1988. (11) Tada, H. Langmuir 1995, 11, 3281. (12) Tada, H.; Nakamura, K.; Nagayama, H. J. Phys. Chem. 1994, 98, 12452.
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Figure 1. (A) Plots of the TiO2 film thickness vs n′ ()n (C/Cs). (B) SEM image of the TiO2 film (n′ ) 0.5). air with a 500 W high-pressure mercury arc (wavelength > 300 nm) whose light intensity at 365 nm was 2.1 mW cm-2. Static contact angles were measured by using a contact angle meter (Model CA-D, Kyowa Interface Science Co.) at room temperature (20 ( 1 °C). Water droplets with a diameter of approximately 2 mm were placed at six positions for one sample and the average was adopted as the contact angle (reproducibility within (3.5%). The photocatalytic activity of the TiO2 films having the methylsiloxane overlayer were assessed by the decreasing rate of the H2O contact angle with photoirradiation.13
III. Results and Discussion Figure 1A shows TiO2 film thicknesses determined by scanning electron microscopy (SEM) as a function of n′. The thickness increases linearly ca. 100 nm per n′ in the region of n′ < 1 (a, n ) 1, C ) Cs/2; b, n ) 1, C ) Cs). The slope decreases to ca. 40 nm per cycle for n′ above 2 (c, n ) 2, C ) Cs; d, n ) 3, C ) Cs; e, n ) 4, C ) Cs). SEM (Figure 1B) also indicated that a porous film with a number of surface asperities is formed on the substrate by the SG method. The surface roughness of the underlaid TiO2 film seems to be responsible for the significant decrease of the slope in the region of n′ > 1. Figure 2A shows variations of H2O contact angles of methylsiloxane overcoated TiO2/quartz samples with back face illumination time (t). The initial contact angles for (13) Tada, H.; Tanaka, M. Thin Solid Films 1996, 281/282, 404.
Figure 2. (A) Time dependence of H2O contact angles (back face illumination). (B) Plots of ln(f0/ft) vs t (back face illumination). (C) Plots of k as a function of the film thickness (datum c is the plot for the TiO2 film on the glass substrate).
all samples are almost equal (90 ( 3°). A conclusion deduced previously from molecular orbital calculations13 that the conformation of TMCTS is changed from all-trans type in the gas phase to all-cis type on the TiO2 surface, where all the CH3 groups are oriented outward from the surface, explains the hydrophobic character of the meth-
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Notes
ylsiloxane overcoated TiO2. Photoirradiation leads to a monotonous decrease in the contact angle for all the samples. However, the contact angle of the methylsiloxane overcoated quartz substrate (without any TiO2 film) was invariant with illumination up to 35 min. The increase in the surface hydrophilicity in TiO2-coated quartz is ascribable to the TiO2-photoinduced oxidation of the outermost Si-CH3 groups to Si-OH groups.14 Figure 2B shows plots of ln(f0/ft) vs t, where f0 and ft denote the coverages of the Si-CH3 groups at t ) 0 and t ) t, respectively. The f0 and ft values were obtained by analyzing the contact angles using eq 1
f ) (1 - cos θt)/(1 - cosθ0)
(1)
where θ0 and θt are the H2O contact angles at t ) 0 and t ) t, respectively. Because of the constancy of the initial contact angle, the f value at t ) 0 is defined as unity for every sample in the present analysis. In each case, the data follow a straight line (R > 0.97 ( 0.02), whose slope provides an apparent rate constant for the oxidation of the Si-CH3 groups (k/min-1). The values of k for back face illumination (kb) were determined for the TiO2/quartz samples by the least-squares method to be 0.018 (n′ ) 0.5), 0.082 (n′ ) 1), 0.035 (n′ ) 2), and 0.029 (n′ ) 3), respectively. Also, the half-lives of the Si-CH3 groups given by (ln 2)/k are 38.5 min (n′ ) 0.5), 8.5 min (n′ ) 1), 19.8 min (n′ ) 2), and 23.9 min (n′ ) 3), respectively. This reaction was previously shown to follow the Langmuir-Hinshelwood mechanism. Since the oxidation is induced by the photocarriers (h+‚‚‚e-), the apparent rate constant can be assumed to be proportional to their concentration at the surface.14 Figure 2C shows the dependence of k on the TiO2 film thickness. When the light is irradiated from the direction of the TiO2 surface (a, substrate ) quartz), the value of k (kf) increases monotonically with increasing film thickness, reaching a plateau of 0.36 ( 0.04 min-1 above 140 nm (ds). This can be simply accounted for by the increase in the amount of light absorbed by TiO2. Then the absorption coefficient of the SG-TiO2 film is estimated from the reciprocal of ds to be ∼7 × 104 cm-1, which is in agreement with the literature values of 104 to 105 cm-1.15 On the other hand, in the case of photoirradiation from backside of the TiO2 film (b, substrate ) quartz), the photocatalytic activity is significantly reduced compared to the case of the front face illumination and a maximum of k (kb) is present at about 100 nm. The reason is discussed below. The diffusion process of photocarriers (h+‚‚‚e-) generated in the TiO2 film to the surface is analyzed on the basis of a diffusion model shown in Figure 3A. J expresses the flux of the photocarriers from the TiO2/substrate interface to the surface. In the photostationary state, J2 is equal to the sum of J1 and J3. J3 is the flux of the photocarriers generated in the slab, ∆x. Neglecting the surface recombination of the photocarriers and the excess charge on the particle, eq 2 can be written
d/dx(Dp(dp/dx)) + C0p0 exp(-Rx) ) 0
(2)
where Dp is the diffusion coefficient of the photocarriers (cm2 s-1), τp is the lifetime of the photocarriers, p0 and p are the concentrations of the photocarriers at x ) 0 and x ) x, respectively, R is the absorption coefficient of TiO2, and C0 is a proportional constant. (14) Tada, H. Langmuir 1996, 12, 966. (15) Gerischer, H.; Heller, A. J. Electrochem. Soc. 1992, 139, 113.
Figure 3. (A) A diffusion model of the photocarriers in the TiO2 film. (B) Values of ps’ calculated as functions of x and Lp.
Equation 1 can be solved using Kirchhoff’s transformation to give
ps ) constant {-exp(-Rx) - x(Dpτp)1/2 + 1}
(3)
where ps is a function of x proportional to the concentration of the photocarriers (see Appendix). Figure 3B shows ps′ ()ps/constant) calculated as functions of x and Lp ()(Dpτp)1/2) employing an R value of 7 × 104 cm-1. Lp denotes the diffusion length of the photocarriers with the dimension of nanometers. This indicates the presence of an optimum film thickness (xmax), which is observed in the experiment (Figure 2C). In the region of x < xmax, ps′ increases with increasing film thickness due to the increase in the amount of light absorbed by TiO2. On the other hand, in the region of x > xmax, most of the photocarriers are thought to recombine, disappearing before arrival at the surface. The other observations that xmax decreases with decreasing Lp and with increasing R (data not shown) are also consistent with this consideration. According to Graetzel et al., τp is on the order of 100
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ns.16 In the case of Lp ) 300 nm, xmax is ca. 100 nm, close to the experimental value (Figure 2C). Then, Dp, which is regarded as the average diffusion constant of h+ and e-, can be calculated to be 9 × 10-3 cm2 s-1. It is reported that the mobility of the electrons in TiO2 rutile monocrystal (µe) is approximately 0.5 cm2 V-1 s-1.16 Einstein’s equation of De ) µekBT/e, where De is the diffusion coefficient of the electron, kB is the Boltzmann constant, and e is the electricity of electron, yields a value of 2 × 10-2 cm2 s-1 as De. Since the movement of the photocarriers requires crossing grain boundaries in these polycrystalline TiO2 films, the diffusion coefficient should be reduced significantly.17 Considering this, the fairly good agreement of these two values also supports the validity of the present analyses. The final important result is that the maximum of ps′ (ps′(xmax)) is strongly dependent on Lp. When Lp increases by a factor of 1.5, the maximum of ps′ increases by a factor of 3.8. This suggests that crystallinity, defects, and impurities of the TiO2 film greatly affect its photocatalytic activity. Datum c in Figure 2C shows the kf for the SGTiO2 film coated on a soda-lime glass substrate. In spite of the same film thickness, the kf is ca. 0.56 times decreased compared to that of the film on quartz substrate. Fujishima et al. reported the similar result and attributed that to the generation of Na2TiO3 crystalline phase near the interface (crystallographic data are not shown in the literature).18 However, such a phase could not be detected in our experiments. This inconsistency may be due to the difference in the method for preparing the TiO2 film. Figure 4 shows XPS depth profiles of the compositions in the SG-TiO2 films coated on quartz (A) and a sodalime-glass (B) substrates. In sample A, sharp changes of Ti and Si concentrations are observed at the TiO2/substrate interface (15 < sputter time < 30 min). The TiO2 film contains a significant quantity of C, which results from the organic residuals after annealing, and a small amount of Na+ ions as impurities. In sample B, it is evident that a large amount of Na+ ions diffuse into the TiO2 film from the glass substrate. The amount of Na is ca. 3.2 times in the film and further as much as 8.4 times on the surface greater than amounts in sample A. In other respects, the two profiles are similar. These findings suggest that Na+ ions act as the recombination center of the h+‚‚‚e- pairs or disorder the crystallinity of TiO2, leading to the significant reduction of the photocatalytic activity in sample B. IV. Conclusions The photocatalytic activity of the TiO2 films was assessed by the rate of the photoinduced oxidation of the methylsiloxane monolayer chemisorbed. The apparent rate constant was obtained by analyzing the variation of the H2O contact angle with photoirradiation time. It was revealed that an optimal thickness is present for back face illumination, while the rate increases monotonously with increasing thickness for front face illumination. The distribution of the photocarrier concentration in the film was calculated by solving the diffusion equation for a diffusion model. This reproduced the existence of an optimal thickness decreasing with an increase in R. It (16) Energy Resources through Photochemistry and Catalysis; Graetzel, M., Ed.; Academic Press: New York, 1983. (17) O’Regan, B.; Moser, J.; Anderson M.; Graetzel, M. J. Phys. Chem. 1990, 94, 8720. (18) (a) Negishi, N.; Iyoda, T.; Hashimoto, K.; Fujishima, A.Chem. Lett. 1995, 841. (b) Kikuchi, Y.; Hashimoto, K.; Fujishima, A. Chem. Ind. 1995, 12, 937.
Figure 4. XPS depth profiles of the compositions in the SGTiO2 films coated on quartz (A) and a soda-lime-glass (B).
was further indicated that the surface concentration of the photocarriers increases with increasing their diffusion length. A marked difference of the photocatalytic activity between the TiO2 films coated on quartz and glass substrates was confirmed, which could be interpreted in terms of the difference in the photocarrier’s diffusion length induced by impurity Na+ ions. These results above lead to a conclusion that the crystallinity and the defects of TiO2 as well as the film thickness and the surface area have a great influence on the photocatalytic activity, providing fundamental information upon the design of the photocatalytic devices for air and water purification. Acknowledgment. The authors express their sincere gratitude to M. Kikuta of Nippon Sheet Glass Co., Ltd., for the helpful discussion on the model calculation. Also, thanks are due to A. Hattori, M. Yamada, and K. Maenaka of Nippon Sheet Glass Techno-Research Co., Ltd., for experimental support. Appendix
d/dx(Dp(dp/dx)) + C0p0 exp(-Rx) ) 0 using Kirchhoff’s transformation of u ) Dp(0) dp
(2)
∫p0Dp(p)/
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d2u/dx2 ) -(C0p0/Dp(0)) exp(-Rx)
Notes
Then
u(x) ) -(c0p0/Dp(0)R2) exp(-Rx) + c1x + c2
(5)
Assuming that Dp is independent of the photocarriers’ concentration (Dp(0) ∼ Dp(p) ) Dp), u(x) becomes equal to p(x)
p(x) ) -(c0p0/DpR2) exp(-Rx) + c1x + c2
(6)
c1 and c2 can be determined from the next boundary conditions
p(0) ) p0
p((Dpτp)1/2) ∼ 0
(4)
(7)
(8)
c1 ) (p0/(Dpτp)1/2DpR2)(C0 exp(-R(Dpτp)1/2) - C0DpR2) (9) c2 ) p0(1 + C0/DpR2)
(10)
Substituting eqs 9 and 10 into eq 6, one can obtain the final equation.
ps(x) ) constant (-exp(-Rx) - x/(Dptp)1/2 + 1) where constant ) C0p0/DpR2. LA960437D
(3)
