Titanium Dioxide-Coated Silica Waveguides for the Photocatalytic

LAWRENCE W. MILLER,. M. ISABEL ... In such systems, UV light is propagated through the ... light is incident from an optically rarer medium (in this c...
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Environ. Sci. Technol. 1999, 33, 2070-2075

Titanium Dioxide-Coated Silica Waveguides for the Photocatalytic Oxidation of Formic Acid in Water LAWRENCE W. MILLER, M. ISABEL TEJEDOR-TEJEDOR, AND MARC A. ANDERSON* University of WisconsinsMadison, Water Chemistry Program, 660 North Park Street, Madison, Wisconsin 53706

Photooxidation of organic compounds on the surface of titanium dioxide (TiO2) is a potential method of removing organic pollutants from water. By coating TiO2 on transparent substrates and illuminating the catalyst with internally reflected light, it may be possible to increase the amount of illuminated photocatalyst in a given reactor volume. Planar, silica internal reflection elements (IREs) were coated with thin, porous, nanoparticulate films of TiO2. UVvisible internal reflection spectroscopy was performed in order to determine that visible and near-UV light propagated through the modified IREs in an attenuated total reflection (ATR) mode. The TiO2-coated IREs were employed in a photocatalytic reactor, and their ability to oxidize formic acid was assessed. Apparent quantum yields and quantum efficiencies of formic acid oxidation as a function of catalyst film thickness and incident angles of internally propagating UV light (310-380 nm) were determined. Quantum efficiency was enhanced when UV light propagated through the TiO2-coated waveguide in an ATR mode. Photocatalytic reactors based on waveguide-supported TiO2 films operating in an ATR mode may utilize light more effectively than reactors based on direct irradiance of TiO2 and could facilitate the scale-up of photocatalytic oxidation processes for commercial remediation applications.

Introduction Titanium dioxide (TiO2) has been extensively studied as a photocatalyst for the oxidation of organic compounds in water and air. Several reviews and books provide an overview of this research and an explanation of TiO2-mediated photocatalytic processes (1-6). Organic compounds are completely mineralized to CO2 and H2O on the surface of TiO2. This process occurs in the presence of UV light of ca. 380 nm or less and a suitable electron acceptor such as O2 (7). Because TiO2-mediated photocatalytic processes occur at ambient temperatures, the technology is attractive as a possible method for the remediation of liquid and gas-phase waste streams. Various studies over the past two decades have shown that optical fibers coated with TiO2 can be employed in the photocatalytic oxidation of organic compounds in water (814). In such systems, UV light is propagated through the optical fibers to activate the TiO2 photocatalyst coating. This concept has been explored in an effort to overcome some of the common barriers to practicability associated with TiO2* Corresponding author phone: (608)262-2674; fax: (608)262-0454; E-mail: [email protected]. 2070

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based photocatalytic processes: namely, separation of photocatalyst from reactant, poor catalyst surface area to reactor volume ratio, and poor utilization of activating ultraviolet radiation (7). While TiO2-coated optical fibers can be employed to overcome the above-mentioned problems, there is an inherent difficulty in developing TiO2-coated waveguides that transmit UV light. Peill and Hoffmann (12, 13) found that the propagation of UV light through a TiO2-coated optical fiber was limited to a maximum fiber length of ca. 10-15 cm. The authors attributed the relatively short propagation distance to UV light propagating through the TiO2-coated fibers in a frustrated total reflection (FTR) mode. This means that at each reflection at the fiber/TiO2 interface, a portion of the UV light was refracted out of the fiber, and the TiO2 absorbed a portion of the refracted light. Upon successive reflections, the UV light intensity in the fiber was quickly diminished. An FTR mode of light propagation is to be expected when light is incident from an optically rarer medium (in this case, silica) to an optically denser medium (TiO2) (15). However, we hypothesized that it may be possible to propagate UV light through a TiO2-coated silica waveguide in an attenuated total reflection (ATR) mode by limiting the thickness of the TiO2 film. In such a system, the optical nature of the interface is determined by the refractive indices of the silica waveguide and the exterior medium (in this case, water). In an ATR mode of propagation, light incident upon the silica/TiO2/ water interface is totally reflected back into the silica. At each reflection, the TiO2 film absorbs a portion of the light that has greater energy than its band gap (λ e 380 nm). Given a sufficient number of reflections, the TiO2 film eventually absorbs all UV light with no refractive losses out of the waveguide. We have demonstrated that this is indeed the case, and that fused silica internal reflection elements (IREs) coated with thin films of TiO2 synthesized via the sol-gel process will propagate light in an ATR mode. Thus, TiO2-coated silica waveguides will propagate light with no apparent refractive losses. We employed TiO2-coated silica IREs in a photoreactor to oxidize formic acid in water. We explored the relationship between film thickness and light propagation through the IREs. We also explored the relationship between both TiO2 film thickness and light propagation angle and the apparent quantum yield of formic acid oxidation. We suggest that photocatalytic reactors employing an array of TiO2-coated, silica ATR waveguides could be used as the basis for an energetically efficient, commercial remediation system.

Experimental Section Preparation and Characterization of TiO2 Films. TiO2 films were deposited onto silica IREs (50 mm × 20 mm × 2 mm parallelepipeds, Harrick Scientific Corp.) by dip-coating from a colloidal sol at a rate of ca. 1 cm/min. This step was followed by controlled heating in a furnace (Thermolyne, model 30400). A water-based TiO2 sol was used to deposit a porous layer of TiO2 onto one side of the IRE. The coated IRE was then heated to 350 °C at a rate of 3 °C/min and held for 2 h in the furnace. By repeating the coating step the desired number of times, we could vary film thicknesses. The synthetic procedure used to prepare water-based, porous TiO2 films is described in an earlier published study by Aguado and Anderson (16) (Colloid C in the cited study). Thin films of TiO2 prepared by the method described therein proved to be effective photocatalysts. It should be noted that Colloid C as prepared by Aguado and Anderson was 10.1021/es980817g CCC: $18.00

 1999 American Chemical Society Published on Web 05/04/1999

TABLE 1. TiO2 Film Thickness No. TiO2 layers

TiO2 film thickness (nm)

1 2 3 4 5

35 ( 4 55 ( 5 94 ( 8 128 ( 8 150 ( 12

FIGURE 1. Schematic representation of TiO2-coated silica internal reflection element photoreactor. concentrated after dialysis by evaporating water from the suspension. This concentration step is not employed for this project. Thus, the TiO2 sol employed is approximately 2 wt % TiO2 at a pH of ca. 3.4. Precursor chemicals were obtained from Aldrich Chemical (Milwaukee, WI), and laboratory water (18 Mohms-cm) was generated by a Barnstead system. Film thicknesses were determined by performing profilometry measurements (Tencor Instruments, model 200) on optically flat quartz plates that were coated under the same conditions as the IREs. Profilometry measurements were corroborated with SEM measurements (LEO, Model 982). The refractive index of the TiO2 films was determined by ellipsometry (Gaertner Scientific Corp., model L116C) measurements made on films deposited on a silicon wafer. The silicon wafer was first heated to 350 °C in air to form a layer of SiO2. Three layers of TiO2 were deposited, and the film was then heated to 350 °C for 2 h. Internal Reflection Photoreactor. The TiO2-coated IREs were employed in a continuous-flow, recirculating reactor to determine their effectiveness for the photooxidation of formic acid (see Figure 1). Formic acid was chosen as the target compound because it oxidizes to CO2 and H2O with no stable intermediates or byproducts. The reactor system was illuminated by a light source (Oriel Instruments Corp., model 66024 lamp housing with a 450 W Xe-Hg arc lamp). Light was filtered through a 10 cm IR water filter (Oriel, 6123) and a 310-380 nm UV band-pass filter (Oriel, 59810). The IRE was held in an internal reflection spectroscopy liquid-cell holder (Harrick Scientific Corp., model MEC-1S). This assembly was mounted on a variable angle pedestal (Harrick Scientific Corp., model TRMP-VAM) that was positioned in front of the light source. Collimated light from the source shone on the edge of the IRE and was propagated internally to the TiO2 coating. A 10 mL solution of aqueous formic acid (concentration ) 10 mg/L as carbon, 833 µmol/ L) was contained in a separate vessel with a gas inlet and medium glass frit. O2 was introduced into the reaction vessel at a rate of 5 mL/min. O2 introduction ensured that the reactant solution was well mixed and saturated with dissolved O2. The formic acid was recirculated over the coated face of the IRE at a flow rate of 6 mL/min with a peristaltic pump (SciLog Corp., model 1041). Formic acid concentration was monitored with a carbon analyzer (Shimadzu, model TOC5000). Light flux to the IRE was measured using a radiometer (International Light Corporation). The spectral distribution was assumed to be that of the UV band-pass filter (Oriel, model 59810). UV-Visible Internal Reflection Spectroscopy. Absorbance of UV light in the region of 310-380 nm by TiO2 films

FIGURE 2. SEM micrograph of TiO2 film on quartz substrate. Three layers of TiO2 heated at 350 °C for 2 h. coated onto silica IREs was measured using a UV-Visible spectrophotometer (Hewlett-Packard, model HP8452) and a variable angle internal reflection spectroscopy cell holder (Harrick Scientific Corp., model TRMP-VAM). Transmittance and internal reflection spectra of phenolphthalein (1:1 mixture of saturated phenolphthalein in methanol and 1 M NaOH in water) were obtained using the same instrument.

Results Characterization of TiO2 Thin Films. Table 1 shows the relationship of the number of TiO2 layers to the film thickness. It has been shown that multiple layers yield a uniform film upon heating (17). In fact, this can be observed in Figure 2, an SEM micrograph of a TiO2 film on quartz. The film appears to be homogeneous throughout its cross-section. The thickness, 90 nm, is the same as that measured by profilometry. The refractive index of the TiO2 film was determined to be 1.9 ( 0.1. The refractive index of the film can be assumed to be a composite of the refractive index of TiO2 particles and air-filled pores:

nd,film ) nd,airxχair + nd,TiO2xχTiO2

(1)

where χair and χTiO2 are the compositional fractions of the pores and solid TiO2 in the film. Given a refractive index of 2.6 for solid anatase TiO2, the porosity of the TiO2 is calculated to be 45 ( 7%. Thus, the refractive index of the film when the pores are filled with water (nd ) 1.33) is ca. 2.0. Internal Reflection Spectroscopy with TiO2-Coated IREs. To determine whether light propagates through TiO2-coated waveguides in an ATR mode, we measured the internal reflection UV-visible spectrum of phenolphthalein using both uncoated and TiO2-coated silica waveguides (see Figure 3). Because the spectra are qualitatively similar in both cases, and because they resemble spectra obtained by transmission indicates that the light propagates through both the coated and uncoated crystals in an ATR mode (15). If the light propagated through the TiO2-coated silica in an FTR mode, the spectrum obtained would be distinctly different. The VOL. 33, NO. 12, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Phenolphthalein spectra. (A) Transmission spectrum of 1 mmol/L phenolphthalein in water. (B) Internal reflection spectrum of 1:1 mixture of saturated phenolphthalein in methanol/1 M NaOH in water. Spectrum obtained with uncoated silica IRE. (C) Internal reflection spectrum of saturated phenolphthalein solution obtained with silica IRE coated with 35 nm TiO2. (D) Internal reflection spectrum of saturated phenolphthalein solution obtained with silica IRE coated with 500 nm TiO2. the propagating light was less than the critical angle for the silica/water interface (θc ) ∼65° ) sin-1 (nd,water/nd,silica)). The critical angle is referenced to the normal to the waveguide/ film interface. When the internal angle of incidence is below the critical angle, light propagates in an FTR mode. Thus, most of the light is lost out of the crystal through refraction at the interface.

FIGURE 4. Qualitative difference between internal reflection spectrum and external reflection spectrum. spectrum would resemble that obtained from external reflection (see Figure 4) (15). The character of external reflection spectra is determined by the dispersion in the refractive index of the sample rather than by absorption by the sample. The fact that it is possible to generate internal reflection spectra with TiO2-coated silica IREs provides further evidence that light propagates through the waveguides in an ATR mode. We observed that it was not possible to measure internal reflection spectra when the internal angle of incidence of 2072

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If placing a TiO2 film onto the silica caused the light to propagate in an FTR mode, a similar drop-off in transmittance at all wavelengths of light should be observed. However, only significant losses in the absorbing region of TiO2 (380 nm) spectral region. Light Flux Measurements. The flux of light incident on the catalyst-coated IRE was measured with a radiometer. The measurement yielded total irradiance in mW/cm2. These measurements were accurate to within ( 0.5 mW/cm2. The energy of light that propagated within the IRE was assumed to be the measured flux multiplied by the surface area of the IRE crystal edge (0.4 cm2). This number was multiplied by

FIGURE 5. Percent transmittance vs wavelength obtained with silica IRE coated with varying thicknesses of TiO2 at incident angle of 68°.

TABLE 2. Percent Light Absorbed by TiO2 Film TiO2 film thickness (nm)

% input light absorbed by TiO2 at 68°

35 90 150 500

50 63 70 91

0.96 to account for a ca. 4% reflection loss from the crystal edge. This quantity was converted to units of µeins/h by a factor of 1.08 × 10-5 eins/h per 1 mW. This conversion was based on a maximum throughput of the band-pass filter at 360 nm (i.e., E (einsteins) ) Nhc/λ, λ ) 360 nm, 1 mW ) 10-3 J/s). The light absorbed by the catalyst at different coating thicknesses for an input angle of 68° is shown in Table 2. This quantity was determined by comparing the percent transmittance curves (Figure 5) to the spectral distribution of the band-pass filter. The spectral distribution of the bandpass filter ranged from 310 to 380 nm with a maximum at 360 nm. The fraction of light absorbed by the various TiO2 films at each wavelength was calculated as:

FIGURE 6. Quantum efficiency vs incident angle for photooxidation of formic acid in IRE photoreactor. TiO2 film thickness ) 90 nm. Flux of light to reactor ) 14.5 mW/cm2. from 3 mL/min to 10 mL/min with no change in reaction rate observed. Changes in formic acid concentration were determined relative to light input to the catalyst, and a quantity, the quantum efficiency for the system, was determined. This quantity is defined as the quantity of formic acid oxidized per unit time (µmoles/h.) divided by the total irradiance (µeinsteins/h.) entering the reactor (12,19). This quantity can be represented as

Φ′ )

(3)

d[hν]incident/dt

where [HCO2H]0 is the initial formic acid concentration, V is the reactant solution volume (10 mL), and [hv]incident is the total irradiance of UV light incident on the IRE. The apparent quantum yield is defined as the amount of formic acid degraded per time (µmoles/h) divided by the light absorbed by the catalyst (µeinsteins/h) (19). This is represented as

[(I0 - I)/I0]λ,film ) (1 - %T/100)λ,film × (Iλ/Iλ,max)filter (2) The first term on the right side of eq 2 represents the fraction of light absorbed by the film at a given wavelength, and the second term is the fractional intensity of the light. The integral with respect to wavelength of the filter spectral distribution from 310 to 380 nm was assumed to be equal to the measured light intensity. The numerically evaluated integral over the same interval of the fractional absorbance (eq 2) was the fraction of the input intensity absorbed by the catalyst. Photooxidation of Formic Acid with TiO2-Coated IREs. Rates of formic acid oxidation were based on the difference between initial formic acid concentration and the concentration remaining after 4 h of operation. Concentration was measured as total organic carbon in the solution. The pH of the initial formic acid solution was 3.8. In a study by Kim, et al. (18), photocatalytic oxidation rates of formic acid were independent of initial concentration above 17 mg/L (4.4 mg/L as carbon, 370 µmol/L). Thus, the rate of reaction is assumed to be constant throughout the 4 h duration of each experiment. A blank was run with no illumination, and no decrease in formic acid concentration was detected. The O2 flow rate was varied from 5 mL/min to 15 mL/min with no difference in rate. Hence, the reactant was assumed to be well mixed and saturated with O2. The reactant flow rate was varied

- (d[HCO2H]V)/dt|0

Φ)

-(d[HCO2H]V)/dt|0

(4)

d[hν]absorbed/dt

This equation is similar to that for quantum efficiency; however, here, [hv]absorbed represents the total irradiance of UV light apparently absorbed by the catalyst. The term apparent quantum yield reflects the fact that the extent of scattering of the evanescent wave by the particulate TiO2 film is unknown. Thus, the apparent quantum yield is likely a lower bound to the true quantum yield. Figure 6 shows the variation of quantum efficiency of formic acid oxidation with angle of incidence of the input light. Figure 7 shows the variation in apparent quantum yield and quantum efficiency with variation in catalyst film thickness (θ ) 68°). The error bars in the figures depicting variations in apparent quantum yield and quantum efficiency represent a 95% confidence limit based on the standard deviation of six measurements of formic acid concentration change made at an incident light angle of 60° and a film thickness of 90 nm.

Discussion Fused silica is a common waveguide material for transmitting UV light. TiO2 has a high refractive index relative to silica (recall that our porous TiO2 films have a composite refractive VOL. 33, NO. 12, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 7. Apparent quantum yield and quantum efficiency vs TiO2 film thickness for photooxidation of formic acid in IRE photoreactor. Incident angle ) 68°. Flux of light to reactor ) 7 mW/cm2.

FIGURE 8. Schematic representation of internal reflection at thinfilm interface. index of ca. 1.9 vs a refractive index of 1.5 for silica). Thus, it would seem that one of the preconditions for ATR at the interface between a silica waveguide and a TiO2 film is violated, and light must propagate through the coated waveguide in an FTR mode. This is the situation described by Peill and Hoffmann (12, 13). It is known, however, that light can propagate through a medium, n1, in an ATR mode despite the presence of a thin film of refractive index n2 > n1 at the interface with a third medium of refractive index, n3 < n1 (see Figure 8). Harrick (15, p. 51) describes a situation where ATR occurs because the character of the interface is determined largely by the two semi-infinite media (n1 and n3). ATR at a thin-film interface has three experimental consequences: (1) internal reflection spectra measured at these interfaces resemble spectra obtained via transmission; (2) the critical angle for ATR is independent of the refractive index of the thin film and is dependent on the refractive indices of the two semiinfinite media determined by

φc ) sin-1 (n3/n1)

(5)

and (3) if medium n2 absorbs, the absorption of the propagating light by n2 occurs via coupling to an evanescent wave at the interface, and the extent of this coupling to the evanescent wave is affected by the film thickness. For a silica/ TiO2/water interface, θc ) ca. 65°. The similarity of the observed internal reflection spectra of phenolphthalein to transmission spectra (Figure 3) and the fact that the critical angle is unchanged despite the deposition of a TiO2 film onto the silica IRE indicates that the TiO2-coated IREs propagate light in an ATR mode. 2074

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TiO2-coated silica waveguides may have important implications for the design of photocatalytic reactors. Because light is propagating through the waveguides in an ATR mode, there are no losses due to refraction. Thus, given sufficient propagation distance, the TiO2 catalyst will absorb almost all light (