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J. Phys. Chem. B 1999, 103, 8490-8492
Mesoporous Metal Oxide Semiconductor-Clad Waveguides Lawrence W. Miller, M. Isabel Tejedor, Bryce P. Nelson, and Marc A. Anderson* Water Chemistry Program, UniVersity of WisconsinsMadison, 660 N. Park St., Madison, WI 53706 ReceiVed: April 26, 1999; In Final Form: July 6, 1999
Optical waveguides were prepared by depositing a sol gel-derived titania film onto a silica substrate. The titania film is mesoporous, with pore sizes ranging from 3 to 8 nm. Deposition of the titania does not change the critical angle of total internal reflection. Thus, the titania-coated waveguides propagate light in an attenuated total reflection mode, despite the relatively high refractive index (n ) 1.8 in air) of the titania film relative to the silica substrate (n ) 1.5). The optical and structural properties of these films suggest the possibility of developing efficient photocatalytic reactors or improved optical chemical sensors.
Introduction An optical waveguide consisting of a transparent core and a porous metal oxide semiconductor cladding could be scientifically and technologically useful. For example, optical fibers have been suggested as a means of light distribution to supported photocatalysts.1,2 Multiple oxide-clad fibers or planar waveguides could capture light and distribute it to the photocatalyst via successive internal reflections at the waveguide surface. Thus, a single light source could be used to illuminate a large surface area of photocatalytic semiconductor material. In this way, the light utilization efficiency of heterogeneous photocatalytic systems, such as photooxidation reactors or solar cells, could be enhanced. Metal oxide-clad waveguides can also be used as optical chemical sensors.3,4 In this configuration, chemical species (analytes) are concentrated within the path of interrogating radiation by adsorption onto the abundant surface of the porous metal oxide film. Changes in the throughput of radiation yield spectroscopic data. Concentration of analytes within the light path should yield lower detection limits. The surface of the metal oxide film could also be functionalized with particular analyte-sensitive species. The development of metal oxide-clad waveguides has been experimentally difficult due to the high refractive index of many metal oxides and the difficulties in producing transparent, homogeneous metal oxide films. For example, titania-coated silica fibers developed for photocatalytic oxidation of organic compounds did not successfully guide light.5,6 Light was refracted out of these systems due to the high refractive index of titania relative to silica and the optical roughness of the titania films. Attempts to develop porous metal oxide-clad optical sensors were limited to systems where the waveguide core material had a higher refractive index than the metal oxide cladding.3,4 We have developed titania-clad silica waveguides that successfully propagate light despite a high refractive index of the titania cladding relative to the silica core. These structures successfully guide light because the titania films are largely defect-free, composed of particles that are small relative to the wavelength of the propagating light, and the film and substrate boundaries are parallel. Waveguides propagate light via successive total internal reflections at the waveguide boundary. Total internal reflection * To whom correspondence should be addressed.
Figure 1. Schematic of light reflection at the boundary of two semiinfinite phases (refractive indices n1 and n3) separated by a thin film of refractive index n2 (n2 > n1 > n3). The thickness of n2 is considered to be on the order of the wavelength of the reflected light. When θ1 is greater than the critical angle θc ) sin-1(n3/n1), no light is transmitted into phase 3.
at a three-phase interface is shown schematically in Figure 1. This figure shows a three-phase system consisting of two semiinfinite dielectric mediums of refractive indices n1 and n3 (n3 < n1) separated by a dielectric medium of finite thickness with refractive index n2 (n2 > n1 > n3). Phases 1 and 3 are semiinfinite and transparent. Phase 2 is finite with a thickness on the order of the wavelength of the incident light. Coherent light incident from phase 1 onto the phase 2 will be reflected, transmitted, or absorbed. The extent of reflection, transmission, or absorption can be completely predicted by complex Fresnel equations if the following parameters are known: the angle of incidence of the light striking the interface, the complex refractive indices of the three phases, the thickness of phase 2, and the polarization of the incident light relative to the plane of incidence.7 In the system we describe here, phase 1 is a glass or silica substrate, phase 2 is a mesoporous titania film, and phase 3 is air. Light incident from phase 1 onto the interface will be totally internally reflected (reflection ) 100%) provided the following conditions are met: (1) the incident angle of the light exceeds the critical angle θc ) sin-1(n3/n1); (2) phase 2 is planar and parallel to phases 1 and 3; (3) phase 2 is nonabsorbing and free of scattering defects. At incident angles greater than the critical angle, the angle of refraction into phase 3 is imaginary. The
10.1021/jp991340d CCC: $18.00 © 1999 American Chemical Society Published on Web 09/16/1999
MOS-Clad Waveguides presence of a plane, parallel film (phase 2, titania) at the glass/ air boundary does not affect the critical angle for total internal reflection (see Figure 1). If θ2 is the critical angle for total reflection at the titania/air interface, then sin θ2 ) n3/n2, and light incident on the interface at all angles greater than θ2 will be totally reflected. From the law of refraction, sin θ2 ) (n1/ n2) sin θ1, so the critical angle for total internal reflection is θc ) θ1 ) sin-1(n3/n1). At incident angles greater than θc, the reflection is total and independent of the wavelength of the light and the thickness of phase 2 in the region where phase 2 is nonabsorbing. If phase 2 is absorbing, the reflection is attenuated, and a portion of the light is absorbed or scattered by phase 2 at each reflection. This attenuation can be predicted from complex Fresnel equations that include a complex refractive index term, nˆ ) n(1 + iκ), where κ is the extinction coefficient. If the surface of phase 2 is rough relative to the wavelength of the incident light, a portion of the light is transmitted into phase 3, and the reflection is said to be frustrated.7 While total reflection is independent of the thickness of phase 2, it will be seen that this thickness is practically limited by scattering of the light within the titania film.
J. Phys. Chem. B, Vol. 103, No. 40, 1999 8491
Figure 2. Percent reflectivity vs angle of incidence for a single reflection of 632.8 nm laser light incident on a 270 nm thick, porous TiO2 film (composite refractive index in air ) 1.8) from a glass prism (refractive index ) 1.515). The solid line represents the theoretically predicted reflectivity, and the open markers represent the measured reflectivity. The percent reflectivity was normalized to a value of 100% for reflectivity measured at a fixed incident angle of 45°.
Experimental Section We coated glass substrates (nd ) 1.515) with mesoporous titania films by dip-coating from a colloidal sol.8 The colloidal titania sol was prepared by hydrolyzing titanium isopropoxide in a nitric acid solution. After hydrolysis, the sol was dialyzed in water until the pH measured ca. 3.5. The glass substrates were cleaned with detergent, rinsed in water, and air-dried. The substrates were immersed in the colloidal sol and withdrawn at a steady rate of ca. 3 cm/min. By repeating the dip-coating process, the thickness of the titania films was increased. The resulting films consist of aggregates of ca. 5 nm primary particles of anatase phase titania. The films are characterized by a high porosity (ca. 50%), high surface area (>150 m2/g), and a large pore radius (3-8 nm).8 We measured the relative intensity of a single reflection of parallel-polarized, 632.8 nm HeNe laser light at the glass/titania/air interface over a range of incident angles with a hemispherical Kretchmann prism.9 The titania-coated glass substrates were attached to the prism with a refractive index matching fluid (ethylene glycol). Titania film thickness and refractive index were determined independently by profilometry and ellipsometry, respectively. Results and Discussion Figure 2 shows the percent reflectivity as a function of incident angle for a 270 nm thick, porous titania film (nd ) 1.8 in air). The open markers represent the experimental data, and the solid line is the theoretically predicted result. The theoretical values were obtained from a numerical solution of reflectance equations for parallel-polarized light at a three-phase boundary as described in Figure 1.7 Phase 2 (titania film) was assumed nonabsorbing and was assigned a refractive index of 1.8 (determined by ellipsometry measurements). The experimental and theoretical results match closely, showing the critical angle matches that expected by the relative refractive indices of the air and glass (θc ) sin-1(n3(1.00)/n1(1.515) ) 41.3°). Because the deposition of a mesoporous titania film onto a glass substrate does not change the critical angle for total internal reflection, we conclude that reflection at the glass/titania/air interface is not frustrated. Thus, a silica or glass substrate coated with titania will act as a waveguide, propagating light in an ATR mode via successive reflections at the silica/titania/air boundary.
Figure 3. UV-visible internal reflection spectra of various thicknesses of mesoporous TiO2 films. The spectra were recorded using 50 mm × 20 mm × 2 mm, silica internal reflection elements coated on one side with TiO2. The spectra were obtained at an internal incident angle of 60°, representing six reflections at the TiO2/silica interface. An uncoated silica crystal was used as a blank.
Light incident on the glass/titania/air interface is totally reflected but the reflection is attenuated through absorption or scattering by the titania film. Figure 3 shows this attenuation over a range of wavelengths for different film thicknesses. The interaction is a combination of scattering and/or absorption of a transverse wave and a standing wave. At the glass/titania interface, ATR cannot occur because of the relatively high refractive index of the titania. Therefore, a portion of the light is refracted into the TiO2 film. However, ATR does occur at the titania/air interface. Thus, no light is lost from the waveguide through refraction at the surface. The superposition of the incident and reflected waves yields a standing wave at the interface.7 As can be seen in Figure 3, the attenuation in the nonabsorbing region increases with increasing film thickness. This indicates that the scattering of the light by the titania particles increases with the increase in path length through the film. Mesoporous metal oxide-clad waveguides need not be limited to silica/titania structures. Any porous metal oxide can be coated onto any transparent substrate. Total internal reflection will occur at the substrate/film interface, regardless of their relative refractive indices, provided that the metal oxide films are composed of particles that are small relative to the wavelength of the incident light, and that the film and substrate boundaries
8492 J. Phys. Chem. B, Vol. 103, No. 40, 1999 are parallel. This property allows for significant freedom in the design of optical sensors because the substrate material can be chosen for optimal transmission of a given wavelength of light while the film can be tailored to the particular analyte or matrix to be examined. Likewise, photocatalytic metal oxides can be supported on any support that is transparent to the wavelength of the activating radiation. This feature could enhance the economical scale-up of photocatalytic reactors by increasing the amount of illuminated catalyst surface per reactor volume.
Miller et al. References and Notes (1) (2) (3) (4) (5) (6) (7) (8) (9)
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