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J. Phys. Chem. B 2000, 104, 4440-4447
Real Time in Situ Spectroscopic Ellipsometry Studies of the Photocatalytic Oxidation of Stearic Acid on Titania Films J. T. Remillard,* J. R. McBride, K. E. Nietering, A. R. Drews, and X. Zhang Department of Physics, MD 3028, Scientific Research Laboratory, Ford Motor Company, P.O. Box 2053, Dearborn, Michigan 48121-2053 ReceiVed: October 12, 1999; In Final Form: January 10, 2000
We used in situ spectroscopic ellipsometry to measure the photocatalytic activity of titania films on fused silica and glass substrates. Amorphous and anatase TiO2 films with a variety of microstructures were prepared by reactive sputtering and pyrolytic deposition. The titania films were coated with thin, spin-cast films of stearic acid [CH3(CH2)16COOH] to represent an organic contaminant. Photooxidation rates were determined from ellipsometric measurements of the reduction in stearic acid film thickness during exposure to UV irradiation at 313 or 365 nm. The photooxidation rate was found to be proportional to IR, where I is the irradiance. The exponent R correlated with the TiO2 crystallinity, having values of approximately 0.7 and 0.8 for amorphous and anatase films, respectively. The largest photooxidation rate was observed for the pyrolytically deposited anatase sample on which X-ray reflectometry and spectroscopic ellipsometry measurements detected the presence of a low-density TiO2 surface layer. To assess the performance of these films in practical applications, the specimens were exposed to wavelength and irradiance conditions that simulated a solar UV spectrum. The most photocatalytically active sample had a stearic acid film removal rate of 22 nm/h, which would be suitable for self-cleaning window applications.
1. Introduction Photocatalytic titania coatings have generated considerable technological interest because of their ability to mineralize organic contaminants.1-10 When photons with energies greater than the TiO2 band gap (∼3.2 eV for anatase titania) are absorbed, electron-hole (e- - h+) pairs are created which can migrate to the titania surface where they either recombine or participate in redox reactions with adsorbed species. It is generally believed that the holes oxidize water to hydroxyl (h+ + H2O f •OH + H+) and the electrons reduce oxygen to superoxide (e- + O2 f O2•-). These radicals subsequently initiate a chain of reactions that oxidize the organic species to completion.11 Proposed commercial products that exploit the photooxidative properties of titania films include self-cleaning glass for automotive and architectural applications, selfdisinfecting tiles for use in hospitals, and microspheres for the remediation of oil spills. In addition, recent studies have shown that UV irradiation of the (110) surface of single-crystal rutile titania results in the formation of hydrophilic and oleophilic domains.12,13 Such highly amphiphilic surfaces are easily cleaned by rinsing with water and can be used as antifogging coatings for windows and mirrors. Photocatalytic activity is characterized by bringing a test organic contaminant (solid, liquid, or gas) into contact with a UVirradiated TiO2 surface and measuring its concentration (or that of its decomposition products) as a function of time. Stearic acid [CH3(CH2)16COOH] is frequently chosen for these tests because of its similarity to common solid organic contaminants (such as fingerprints) and because of its stability and low vapor pressure. Coatings of stearic acid are usually deposited on supported titania films using solution casting or Langmuir-Blodgett techniques. Paz et al. observed the photooxidation of stearic acid by measuring the decrease in integrated IR absorbance associated with C-H stretching vibrations.15,16 Sawunyama et al. used
atomic force microscopy (AFM) to monitor the surface topography of Langmuir-Blodgett stearic acid films as a function of irradiation time.17 Sitkiewitz et al. have studied the photocatalytic decomposition of stearic acid by measuring the CO2 produced as a result of mineralization.11 A variety of other analytical techniques (gas chromatography, mass spectroscopy, water contact-angle measurements, conductivity measurements, and friction force microscopy) and test compounds (benzene, n-octane, 3-octanol, 3-octonone, n-octanoic acid, acetaldehyde, 2-propanol) have been used to study the photoactivity and photoinduced amphiphilicity of titania.12,13,18-24 In this paper we discuss a real time in situ ellipsometric technique for determining the photocatalytic activity of titania films prepared using pyrolytic deposition and reactive sputtering on float glass and fused silica substrates. Photoactivity was assessed by spin-casting a thin layer of stearic acid onto the titania surface and ellipsometrically observing its reduction in thickness during exposure to UV radiation at 313 and 365 nm. The photooxidation rate was measured as a function of irradiance, and for all samples was found to follow a power law dependence. These measurements demonstrate that real time ellipsometry is a sensitive method for studying the photooxidative properties of planar titania films. Finally, to assess the performance of these films in practical applications, photooxidation experiments were performed by irradiating samples in an apparatus that simulated a solar UV spectrum. 2. Experimental Details Titania Film Fabrication. Sputtered titania films approximately 50 nm thick were deposited on 2.5 × 2.5 cm2 fused silica substrates (0.2 cm thick, polished to a 80/50 finish) using an Airco Temescal ILS-1600 vacuum coater equipped with an Advanced Energy MDX Spark-le accessory, operating in the self-triggered mode at 2 kHz. A 12.7 × 43.2 cm2 titanium target (99.9% pure) was sputtered in oxygen (99.994%) at a pressure of 5.0 × 10-3 Torr at 5.0 kW. Some of the samples were furnace
10.1021/jp9936300 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/08/2000
Oxidation of Stearic Acid on Titania Films annealed in air by ramping the temperature at 5 °C/min to 500 °C, soaking for 4 h, and slowly cooling to room temperature. Two sets of pyrolytically coated TiO2 samples were prepared by spraying a tetraisopropyl titanate (TIPT) solution onto glass emerging from a float furnace.25 The ∼4 m wide glass ribbon exited the furnace at ∼630 cm/min at a temperature of ∼620 °C. Nozzles were positioned ∼9 cm above the glass and aimed at an angle of 4° so that the spray covered an area of ∼1 × 4 m2. Additional oxygen was provided by a concentric air amplifier. One set of samples was made by spraying a 1:1 mixture by volume of TIPT and kerosene at a rate of 50 L/h onto a blue-colored glass, and another was made from a 2:1 mixture by volume of TIPT and 2-propanol that was sprayed at a rate of 37 L/h onto a brown-colored glass. We note that although two different soda-lime glasses were used in this work, their compositions were essentially the same, differing only in their concentrations of Co and Se, which are used as colorants. (For both glasses the concentrations of these elements are less than 60 ppm.) Diffusion of sodium ions from the glass into the titania film is believed to result in a reduction in photoactivity due to the creation of recombination centers for the photogenerated charges.15,16 However, since the sodium oxide concentrations for both float glasses are the same (13.6 wt %), any degradation in the photocatalytic performance due to this mechanism will be the same for both samples. Stearic Acid Film Application. The titania coated substrates were cleaned by ultrasonic agitation in a commercial detergent followed by rinses in deionized water and methanol. Films were spin-cast by applying 5 drops of a 4 × 10-3 M solution of stearic acid in n-decane to the substrate which was rotating at 3000 rpm. Samples were allowed to spin for 3 min, producing stearic acid films typically 30 Å in thickness. AFM analysis indicated the films produced using this technique were continuous and conformal with the titania surface. We also attempted to create thicker stearic acid films by either applying more drops or building up successive layers; however, these thicker films often appeared hazy and were judged to be of inferior quality for ellipsometric analysis. XRD Measurements. Wide-angle X-ray diffraction (XRD) and low-angle X-ray reflectivity (XRR) scans were collected on a Scintag PAD-V powder diffractometer using Cu KR radiation, slit collimation, and an energy dispersive Si(Li) detector. XRD scans were collected using a beam divergence of 1.4° in a Bragg-Bretano geometry for scattering angles 15° < 2θ < 90° with a step size of 0.03° and an integration time of 15 s/point. Data were background-subtracted and smoothed (three-point). Peaks in the XRD scans were compared to reference data of TiO2 phases.26 XRR data were collected using a beam divergence of 0.02° for scattering angles 0.3° < 2θ < 4° in steps of 0.01° and an integration time of 10 s/point. At scattering angles below 2θ ) 0.68° data were linearly scaled to correct for the specimen footprint (the sample did not intercept the full X-ray beam). The purely specular part of the reflectivity was extracted by subtracting the diffuse background near the specular peak position. Background-subtracted, footprint-corrected data were fit using the software program M-Layer to determine the layer thickness and surface vertical roughness.27 Interfaces were modeled as discrete “microlayers”, with their densities following a complementary error function dependence. Fits were performed with 201 microlayers for each interface. AFM Measurements. Surface topographic images were obtained using a commercial atomic force microscope (Digital Instrument Nanoscope III) with a silicon nitride tip operating
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Figure 1. Experimental configuration for in situ ellipsometric photooxidation measurements.
in the contact mode. The force between the tip and the sample was typically 2 nN. The images were taken in air with no additional surface preparations. A 1 × 1 µm2 scan was found to encompass the appropriate range of length scales: i.e., larger than the grain size but with sufficient resolution to see smaller features. The AFM roughness data were analyzed by averaging the scans taken from five randomly chosen locations. Ellipsometric Measurements. A schematic diagram illustrating the key features of our experiment is presented in Figure 1. The spectroscopic ellipsometer is a computer-controlled, rotating analyzer instrument manufactured by the J.A. Woollam Co. It measures the ellipsometric parameters ∆ and Ψ defined by rp/ rs ) (tan Ψ)ei∆, where rp and rs are the amplitude reflection coefficients for p- and s-polarized light, respectively. The ellipsometric parameters can be measured as a function of angle and wavelength, or dynamically as a function of time for a given angle at a small set of discrete wavelengths. For dynamic measurements, the practical limit on the number of wavelengths is determined by the time required to move the monochromator and the rate at which the sample film parameters (i.e., thickness and optical constants) evolve with time. Software provided with the instrument can calculate the ∆ and Ψ values for a model multilayer structure and determine fitting parameters (i.e., film thickness, refractive index, and extinction coefficient) from experimental data through regression analysis. This instrument is also capable of measuring sample transmittance and reflectance. The UV light source used to initiate photooxidation was a 200 W mercury arc lamp which was collimated, passed through an interference filter (fwhm of 10 nm) to obtain light at 313 or 365 nm, attenuated using neutral density filters, and coupled into a liquid-filled light guide 1 m in length. The guide exit was positioned ∼11 mm from the stearic acid coated titania sample. Power measurements were made using a Coherent LabMaster power meter equipped with a LM-2UV detector head. Experiments were conducted over an irradiance range of ∼0.01-40 mW/cm2. For the in situ photocatalysis studies, ellipsometric measurements of the stearic acid coated TiO2 samples were performed at two or three wavelengths as a function of time. Prior to exposing the sample to UV light, data were collected for ∼5 min to establish baseline values of ∆ and Ψ. As photooxidation proceeded, the values of ∆ and Ψ changed in real time due to the reduction in thickness of the stearic acid film. Data were acquired until no further changes in ∆ and Ψ were observed. Photooxidation measurements were also performed using a QUV-accelerated weathering tester (manufactured by Q-Panel Lab Products) which simulates a UV spectrum corresponding to the solar maximum conditions (global, noon sunlight, on the summer solstice, at normal incidence).28 Samples were placed in the weathering apparatus, removed for ellipsometric measure-
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Figure 3. Wide-angle XRD scans of two crystalline titania film specimens shown with a “stick figure” for a randomly oriented powder of anatase (PDF 21-1272). Data are offset for clarity.
Figure 2. AFM images of (a) pyrolytically deposited titania film on brown-colored float glass and (b) as-sputtered titania film on fused silica.
ments, and then replaced. This procedure was repeated until enough data were acquired to obtain an accurate determination of the photocatalytic activity. 3. Titania Film Characterization Figure 2a shows an AFM image of the pyrolytically deposited TiO2 film on brown float glass, and Figure 2b shows a similar measurement of the as-sputtered film on fused silica. The surface features of the titania films on the glass substrates are similar in size (50 nm), but the values of rms roughness are slightly different: 1.3 and 1.0 nm for the films on the brown and blue glass, respectively. The films deposited on the fused silica substrates have smaller features than the pyrolytically deposited samples, although they are comparable in roughness. For the as-sputtered film, values of 20 and 1.3 nm were obtained for the feature size and rms surface roughness, respectively. Annealing resulted in a more regular grain structure, but coalescence or grain aggregation was not observed. After annealing, values of 15 and 0.8 nm were obtained for the grain size and rms surface roughness, respectively. Figure 3 shows wide-angle XRD scans of the pyrolytically deposited film on brown float glass and the annealed sputtered film. Both samples revealed crystalline peaks which correspond closely to the anatase form of TiO2.29 The as-sputtered film on fused silica and the pyrolytically deposited film on blue float glass did not show any crystalline peaks. XRR scans all revealed Keissig fringes from the interference of X-ray waves scattered from the top and bottom interfaces, but the envelope of the fringes and the overall decay of intensity showed marked differences among the specimens. The pyrolytically deposited sample on brown float glass was distinct from the other specimens in that there was a rapid decay in the fringe amplitude and the intensity envelope with angle. XRR scans of this film and the as-sputtered film on fused silica are shown in Figure 4a with the corresponding fitting curves superimposed. Good agreement is observed between the data and fits over six decades of intensity. Initial fits to XRR data using a singlelayer model did not accurately reproduce the envelope of the
Figure 4. (a) Specular XRR scans for specimens of titania deposited on fused silica (as-sputtered) and brown float glass. Data shown are offset for clarity. Solid lines are fitting curves. (b) Density profiles determined by fitting.
Keissig fringes or the overall decay in intensities. A second model incorporating a thin surface layer was used to produce the fits shown. Fitted model parameters are shown in Table 1, and the refined density profiles are shown in Figure 4b. For both films, the surface layer does not have distinct boundaries but only provides an extended tail to the density profile. This is particularly true for the case of the pyrolytically deposited anatase film (brown float glass).
Oxidation of Stearic Acid on Titania Films
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TABLE 1: Parameters Used in the XRR Fits of Figure 4a
substrate roughness (Å) substrate density (g/cm3) TiO2 thickness (Å) TiO2 top roughness (Å) TiO2 density (g/cm3) surface thickness (Å) surface roughness (Å) surface density (g/cm3)
sputtered TiO2, amorphous
pyrolytic TiO2, anatase
13.5 2.70 489 26 3.82 19.1 23.6 0.44
17 3.00 530 55.0 3.99 73.8 57.0 0.55
Figure 6. Index of refraction profile for samples of titania deposited on fused silica (as-sputtered) and brown float glass determined from the graded-dielectric model.
Figure 5. Spectroscopic ellipsometry scan of as-sputtered titania on fused silica (data points). Solid lines are fit to the three-medium model.
From the preceding discussion it is apparent that the rms surface roughness as measured by XRR is larger than that measured by AFM, with the greatest discrepancy observed for the anatase film on brown float glass (5.7 versus 1.3 nm for XRR and AFM measurements, respectively). The origin of this disparity is unclear; however, it is possibly related to the fact that while AFM provides direct information about the sample surface height variations, the roughness measured by XRR also includes contributions due to film density. Spectroscopic ellipsometry (SE) measurements were carried out over the wavelength range 280-1000 nm at five angles of incidence (60°, 65°, 70°, 75°, and 80°) and initially analyzed using a three-medium model (air/TiO2/substrate). Fused silica and float-glass optical constants were first established by performing spectroscopic scans of bare substrates and fitting the resulting data to a Cauchy model. (Bare float-glass samples were obtained by polishing off the TiO2 films.) Titania optical constants were fit using a Kramers-Kronig-consistent parametric dispersion model developed by J.A. Woollam Co., which is appropriate for describing the optical constants of a broad range of dielectric and semiconductor materials.
The dotted curves in Figure 5 show SE scans of the as-sputtered sample on fused silica. A theoretical fit of these data (shown by the solid lines) using the three-medium model yields a TiO2 thickness of 493 Å. This fit also included a parameter to account for small thickness variations across the sample surface. Similar measurements and analysis result in thicknesses of 486, 521, and 418 Å for the annealed sputtered film on fused silica, and the pyrolytically deposited films on brown and blue float glass, respectively. Although use of the three-medium model produced satisfactory fits, the SE data were also analyzed using a graded-dielectric model in which the titania film was divided into two sublayers. Each sublayer consisted of a mixture of voids and TiO2 with optical constants determined using the Bruggemen effective medium approximation (EMA). The void density was linearly graded within each sublayer, and four parameters were determined from the fit: the total film thickness, the location of the interface between the two sublayers, and the void concentrations at the sublayer and film/air interfaces. Titania optical constants for the EMA calculation were those determined from fits using the three-medium model. Similar models have been used to describe titania films prepared on a variety of substrates.30 Fits of the SE measurements using the two models differed most for the anatase samples, with the largest improvement occurring for the titania film on brown float glass. For that specimen the mean standard error obtained using the gradeddielectric model was a factor of 2 lower than that obtained using the three-medium model. Use of the graded-dielectric model only slightly improved fits of the amorphous samples. Figure 6 compares the index of refraction (n) profiles obtained from the graded-dielectric model for the as-sputtered film on fused silica (amorphous) and pyrolytically deposited film on brown float
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Figure 8. Fraction of incident light absorbed by titania films. Arrows indicate wavelengths used in photocatalysis experiments.
TABLE 2: Parameters Characterizing the Optical and Photocatalytic Properties of the Titania Samplesa 313 nm
Figure 7. Index of refraction (top) and extinction coefficient (bottom) for titania samples.
glass (anatase). As expected from the excellent fit obtained using the three-medium model, n of the as-sputtered sample changes only slightly as a function of position within the film. In contrast, n of the titania film on brown float glass is constant over most of its thickness but drops from ∼2.5 to ∼1.2 within a ∼50 Å thick layer near the TiO2/air interface. Note that the film density profile as characterized by SE is qualitatively similar to that deduced from the XRR measurements (see Figure 4). Both techniques suggest all samples have a reduced density surface layer. For the amorphous samples, the density of this layer is only slightly less than that of the bulk film. For the anatase samples, however, the density of this layer is significantly lower than that of the bulk, with the greatest reduction in density observed for the TiO2 film on brown float glass. Although the graded-dielectric model provides a better characterization of the titania films, the in situ ellipsometric measurements are analyzed using the TiO2 optical constants derived from the simpler threemedium model since the photooxidation rates derived from fits using either model are nearly identical. Figure 7 shows plots of n and k as a function of wavelength for all specimens determined from analysis of the SE data using the three-medium model. The plots of k indicate the absorption edge of the sputtered samples is slightly red-shifted relative to that of the pyrolytically deposited samples. Recall that for applications in which photocatalysis is initiated by solar radiation it is desirable that the absorption edge extend as far as possible into the visible region of the spectrum to maximize the number of photogenerated electron-hole pairs. Figure 8 shows the fraction of light absorbed by each film, F, with the arrows indicating the UV wavelengths used to initiate photocatalysis. For each sample this quantity is calculated from the expression F ) 1 R - T, where R and T are the reflectance and transmittance
365 nm
sample
K
R
F
K
R
F
ηrel
sputtered, amorphous sputtered, anatase pyrolytic, amorphous pyrolytic, anatase
0.18 0.19 0.24 2.22
0.68 0.79 0.70 0.80
0.60 0.62 0.40 0.55
0.019 0.009 0.016 0.044
0.67 0.83 0.65 0.79
0.051 0.046 0.021 0.03
0.94 1.82 0.92 3.21
a The stearic acid film removal rate (R, Å/min) is given by R ) KIR, where I is the UV irradiance (mW/cm2). F is the fraction of incident light absorbed by the titania film. ηrel is the ratio of the photocatalytic efficiency at 313 nm to that at 365 nm. See the text for details.
Figure 9. Comparison of measured (dots) and calculated (lines) absorbance for titania samples deposited on fused silica.
calculated using the ellipsometrically determined optical constants (this analysis ignores the effects of scattering). These values (given in Table 2) are used when computing the samples’ relative photooxidative efficiency at the two wavelengths. To enable an independent check of the three-medium model’s accuracy, normal incidence transmittance (T) measurements of the TiO2 samples on fused silica substrates were performed using the ellipsometer. Figure 9 compares the measured absorbance (dots), defined as -log (T), with that calculated using the optical constants obtained from the three-medium model (solid lines).31 A similar comparison could not be made with the pyrolytically deposited samples because of large UV absorption by the floatglass substrates. As expected from the above discussion the optical properties of the films are well described by the threemedium model.
Oxidation of Stearic Acid on Titania Films
Figure 10. Spectroscopic ellipsometry scans of as-sputtered titania film on fused silica with stearic acid (data points). Solid lines are fit to the four-medium model.
4. Spectroscopic Ellipsometric Measurements of the Photooxidation Rate To model the results of the in situ experiments, we needed to determine the optical constants of our spin-cast stearic acid films. The as-sputtered sample on fused silica was well characterized by the three-medium model, so we used it as the test substrate and described the coated titania film using a fourmedium model (air/stearic acid/TiO2/substrate) in which a Cauchy expression was used for the stearic acid optical constants. A typical SE scan is shown by the dotted curves in Figure 10, with the fit shown by the solid lines. The spin-casting technique generally yielded films with an index of refraction close to that of liquid stearic acid (i.e., ∼1.44 at 500 nm). Figure 11 shows typical results from an in situ SE measurement in which a stearic acid film on sputtered amorphous titania is exposed to 313 nm light at an irradiance of 48 mW/cm2. Data were sequentially measured at 600, 700, and 750 nm at a 70° angle of incidence at a data sampling rate of ∼2 points/min. This combination of angle and wavelengths was chosen to ensure a large change in ∆ and Ψ would be observed as the stearic acid film was removed, thus improving our ability to accurately determine the photooxidation rate. The dotted curves in the top two panels show the changes in ∆ and Ψ as a function of time. Fits of these data using the four-medium model are shown by the solid lines, and the bottom panel shows the resulting film thickness as a function of time. We define the photooxidation rate, R, as the initial film removal rate, which for the data shown, is ∼2.0 Å/min. For all measurements the film removal rate was constant while the stearic acid layers were greater than approximately one monolayer in thickness (∼21 Å). Below this thickness the film removal rate was observed to
J. Phys. Chem. B, Vol. 104, No. 18, 2000 4445
Figure 11. (top) Ψ and (middle) ∆ measured at 70° as a function of time for exposure of the stearic acid film of Figure 10 to 48 mW/cm2 of 313 nm radiation (data points). Solid lines are fits using the fourmedium model, with the resulting stearic acid film thickness as a function of time shown in (bottom).
steadily decrease. This analysis assumes that the contaminant film density (and index of refraction) remains constant during the photooxidation process. While this may not be true for all contaminant materials and morphologies, it is most likely true for the stearic acid films used in these experiments, which were at most a few monolayers thick. Furthermore, any change in contaminant density at the onset of photooxidation would likely be small and should not appreciably alter the calculated initial film removal rate, which was used as the basis of comparison for the various titania samples. Figures 12 and 13 show log-log plots of the photooxidation rates as a function of irradiance for exposure to 313 and 365 nm light, respectively. The photocatalytic activities of the films on fused silica (anatase and amorphous phases) and blue float glass (amorphous phase) were comparable, while that of the pyrolytically deposited sample on brown float glass (anatase phase) was larger (especially for exposure to 313 nm light). It is unlikely that sodium contamination from the substrate plays a significant role in determining the photooxidation rates of our titania samples. As mentioned previously, the sodium oxide concentrations of the float glasses are the same; both samples should therefore experience a similar degradation in photocatalytic activity due to sodium ion diffusion into the titiania. Furthermore, there should be negligible sodium contamination from the fused silica substrates. To understand the relative magnitudes of the photooxidative rates, we turn to the results of the XRR and SE measurements. Recall that the titania film on brown glass was found to have a surface density significantly lower than that of the bulk. This is in contrast to the other TiO2 films, which are essentially uniform in density. It is likely, therefore, that the morphology of this surface region plays a
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Figure 14. Stearic acid film thickness as a function of time for exposure to the simulated solar UV spectrum. Solid lines are to guide the eye.
Figure 12. Initial stearic acid photooxidation rate, R, as a function of irradiance for exposure to 313 nm radiation (data points). Solid lines are fits using R ) KIR.
For all specimens, R was well fit using an expression of the form R ) ΚIR, where I is the irradiance. The values of R and K measured for our samples are summarized in Table 2. For both of the UV wavelengths, R appeared to be correlated with sample crystallinity: i.e., values of ∼0.8 and ∼0.7 were measured for the anatase and amorphous samples, respectively. Similar values of R have been observed by other workers.20 The value of R depends on the photocarrier recombination mechanism. For bimolecular photocarrier recombination, R ) 0.5, a condition commonly referred to as light-limited. By contrast, a light-rich condition is indicated when R ) 1 and implies the photoefficiency (see eq 1 below) is independent of the irradiance.19-21 Our measurements appear to have been performed in a regime intermediate between these extremes. Similar to other groups investigating the photocatalytic oxidation of stearic acid on titania films, we observed R to be independent of the UV flux (for comparable values of irradiance).11 However, some workers studying the decomposition of gas-phase contaminants (such as 2-propanol) observed a change in R as a function of irradiance.19,20,24 The photocatalytic efficiency is defined as the film removal rate divided by the number of photons absorbed by the film:
η≡ Figure 13. Initial stearic acid photooxidation rate, R, as a function of irradiance for exposure to 365 nm radiation (data points). Solid lines are fits using R ) KIR.
dominant role in determining the photooxidation rate. It is possible that the microstructure of this lower density layer could enhance the photooxidation rate of the brown glass sample relative to that of the others by increasing the stearic acid/titania contact area or the surface photocarrier density. Further support for this conclusion is given by the observation that the film removal rate of the titania film on brown glass exceeds those of the other samples by factors of ∼10 and ∼3 for exposure to 313 and 365 nm light, respectively. We note that the optical penetration depth (given by λ/4πk) for each titania sample is less than the film thickness at 313 nm, but larger than 10 times the film thickness at 365 nm. It is reasonable to assume that differences in surface morphology should have a greater impact on the photooxidation rate when the penetration depth is such that the majority of photocarriers are created close to the surface, as is the case for illumination at 313 nm.
KI R F(Iλ/hc)
(1)
where F is the fraction of light absorbed by the film, I is the irradiance, and λ is the wavelength. For a given sample, the ratio of the efficiency at 313 nm to that at 365 nm is then given by
ηrel )
( )( )( )
365 K313 F365 R313-R365 I 313 K365 F313
(2)
Because the value of R is approximately the same for both exposure wavelengths, the quantity ηrel is approximately independent of irradiance and is given in Table 2 for each sample. Note that for the amorphous samples the photocatalytic efficiency is nearly the same for both exposure wavelengths (ηrel ≈ 1). By contrast, the efficiencies of the two anatase samples increase by factors of ∼3.2 and 1.8, respectively, as the exposure wavelength is changed from 365 to 313 nm. Figure 14 is a plot of the stearic acid film thickness as a function of time for exposure to radiation simulating the solar maximum conditions. For these measurements the stearic acid
Oxidation of Stearic Acid on Titania Films coated titania coated surface faced the light source. Consistent with our in situ ellipsometric measurements, the photooxidation rate of the TiO2 film on brown float glass (∼22 nm/h) was ∼10 times larger than those of the other samples. Paz et al. have suggested that rates on the order of 20 nm/h are suitable for self-cleaning window applications.15 It is interesting to note that the photooxidation rates of the sputtered titania films are smaller than that of the sample on brown float glass, despite the fact that they absorb a larger fraction of the incident solar radiation (see Figure 8). As discussed above, the larger photoactivity of the titania film on the brown float glass is most likely due to the presence of a low-density surface TiO2 layer, which increases the surface area available for photooxidation. For exposure through the substrate (i.e., the titania film facing away from the light) the photooxidation rates for the films on fused silica are unchanged (because the UV light is transmitted through the substrate) while those for the pyrolytically deposited samples are reduced by a factor of ∼10 due to the large UV absorption of the float glass. 5. Conclusions Our work has demonstrated that in situ spectroscopic ellipsometry is a suitable technique for characterizing the photocatalytic activity of titania films on glass and fused silica substrates. Using this method, we measured the photooxidation rate of stearic acid on sputtered and pyrolytically deposited anatase and amorphous TiO2 as a function of UV wavelength and irradiance. From these measurements, we conclude (1) the photooxidation rate follows a power law dependence on the irradiance, with the value of the exponent and the photooxidative efficiency correlated with the sample crystallinity, (2) the largest photooxidation rate was observed for the pyrolytically deposited anatase sample (prepared on brown float glass) on which X-ray reflectometry and spectroscopic ellipsometry measurements detected the presence of a low-density surface TiO2 layer, and (3) under illumination conditions that simulate the solar UV spectrum, this sample’s photooxidation rate is suitable for use in self-cleaning window applications. Acknowledgment. We thank R. L. Crawley for bringing this problem to our attention and preparing the float glass samples. References and Notes (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69.
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