Bandgap-shifted semiconductor surface and method for making same, and apparatus for using same

Abstract

Titania is a semiconductor and photocatalyst that is also chemically inert. With its bandgap of 3.2 and greater, to activate the photocatalytic property of titania requires light of about 390 nm wavelength, which is in the ultra-violet, where sunlight is very low in intensity. A method and devices are disclosed wherein stress is induced and managed in a thin film of titania in order to shift and lower the bandgap energy into the longer wavelengths that are more abundant in sunlight. Applications of this stress-induced bandgap-shifted titania photocatalytic surface include photoelectrolysis for production of hydrogen gas from water, photovoltaics for production of electricity, and photocatalysis for detoxification and disinfection.

1 . A photoelectrolytic cell for production of first and second gases from a liquid, the cell comprising:
 a container capable of holding the liquid;   a photoelectrode disposed within the container and capable of generating the first gas upon exposure to radiation;   a counterelectrode disposed within the container electrically connected to the photoelectrode and capable of generating a second gas when the photoelectrode is exposed to radiation; and   a septum arranged between the photoelectrode and the counterelectrode to separate the first and second gases.   
     
     
         2 . A photoelectrolytic cell according to  claim 1  wherein the photoelectrode has a plurality of apertures extending therethrough, said apertures serving to improve migration of ions between the photoelectrode and the counterelectrode. 
     
     
         3 . A photoelectrolytic cell according to  claim 1  wherein the photoelectrode is a photoanode, the counterelectrode is a cathode, and the photoelectrolytic cell further comprises a second anode disposed within the container, the second anode not being photolytically active but being electrically connected to the cathode. 
     
     
         4 . A photoelectrolytic cell according to  claim 3  further comprising an auxiliary septum arranged between the second anode and the cathode. 
     
     
         5 . A photoelectrolytic cell according to  claim 1  wherein the septum is formed of an open cell material, an open cell foam, a microporous material such as fritted glass or ceramic, or an ion exchange membrane such as a fluoropolymer. 
     
     
         6 . A photoelectrolytic cell according to  claim 1  wherein the photoelectrode comprises a semiconductor film on a substrate, the semiconductor film having a bandgap not supporting spontaneous photoelectrolysis of water in visible light wavelengths present in sunlight, the substrate having surface undulations with a spatial period smaller than the wavelength of visible light that cause stress in the semiconductor film and thereby shift the bandgap therein to support spontaneous photoelectrolysis of water in visible light. 
     
     
         7 . A photoelectrolytic cell for production of at least one gas from a liquid, the cell comprising:
 a container capable of holding the liquid;   a photoanode disposed within the container;   a cathode disposed within the container and electrically connected to the photoanode, such that when the photoanode is exposed to radiation, at least one gas will be generated by the photoanode and the cathode; and   a second anode disposed within the container, the second anode not being photolytically active but being electrically connected to the cathode.   
     
     
         8 . A photoelectrolytic cell according to  claim 7  wherein the photoanode has a plurality of apertures extending therethrough, said apertures serving to improve migration of ions between the photoelectrode and the counterelectrode. 
     
     
         9 . A photoelectrolytic cell according to  claim 7  wherein the photoelectrode comprises a semiconductor film on a substrate, the semiconductor film having a bandgap not supporting spontaneous photoelectrolysis of water in visible light wavelengths present in sunlight, the substrate having surface undulations with a spatial period smaller than the wavelength of visible light that cause stress in the semiconductor film and thereby shift the bandgap therein to support spontaneous photoelectrolysis of water in visible light. 
     
     
         10 . Apparatus for generating electricity and for carrying out photo-induced reactions, the apparatus comprising:
 a primary reflector arranged to concentrate radiation incident thereon to a primary focus;   a dichroic mirror disposed at or adjacent the primary focus and arranged to pass a first band of radiation therethrough and to reflect a second band of radiation having wavelengths differing from those of the first band;   photovoltaic means for converting radiation to electricity; and   photo-reactor means for carrying out at least one photo-induced reaction, the photo-reactor means comprising at least one photoactive electrode,   wherein one of the photovoltaic means and the photo-reactor means is arranged to receive the first band of radiation passing through the dichroic mirror, and the other of the photovoltaic means and the photo-reactor means is arranged to receive the second band of radiation reflected from the dichroic mirror.   
     
     
         11 . Apparatus according to  claim 10  wherein the dichroic mirror comprises a secondary reflector arranged to direct radiation incident thereon to a secondary focus. 
     
     
         12 . Apparatus according to  claim 11  the apparatus having a Dall-Kirkham reflective design, with an elliptical primary reflector and a cylindrical secondary reflector, or a Cassegrain design with a parabolic primary reflector and a hyperbolic secondary reflector. 
     
     
         13 . Apparatus according to  claim 10  wherein the primary reflector comprises:
 a support member; 
 two end caps mounted on the support member and spaced apart from one another, each end cap having a mounting surface facing the other end cap, and a slotted guide into its mounting surface; and; 
 a flexible reflector material having a reflective surface inserted into the slotted guides on the two end caps so that the reflective surface of the flexible substrate concentrates incident radiation on the primary focus. 
 
     
     
         14 . Apparatus according to  claim 10  wherein the photo-reactor means comprises a container capable of holding liquid; a photoanode; and a cathode electrically connected to the photoanode, wherein the container is substantially cylindrical and at least part of the container is light transmissive such that the light transmissive part of the container concentrates light on the photoanode. 
     
     
         15 . Apparatus according to  claim 10  wherein the photo-reactor means comprises a container capable of holding liquid, the container having first and second apertures extending therethrough, and a liquid circulation tube disposed outside the container extending from the first aperture to the second aperture thereof such that liquid contain from the interior of the container through the first aperture, through the liquid circulation tube and back into the container through the second aperture; and heat extraction means arranged to extract heat from the liquid in the liquid circulation tube. 
     
     
         16 . Apparatus according to  claim 10  wherein the photo-reactor means comprises a photoelectrode comprising a semiconductor film on a substrate, the semiconductor film having a bandgap not supporting spontaneous photoelectrolysis of water in visible light wavelengths present in sunlight, the substrate having surface undulations with a spatial period smaller than the wavelength of visible light that cause stress in the semiconductor film and thereby shift the bandgap therein to support spontaneous photoelectrolysis of water in visible light. 
     
     
         17 . Apparatus according to  claim 10  wherein the photo-reactor means comprises a photoelectrolytic cell for production of first and second gases from a liquid, the cell comprising:
 a container capable of holding the liquid; 
 a photoelectrode disposed within the container and capable of generating the first gas upon exposure to radiation; 
 a counterelectrode disposed within the container electrically connected to the photoelectrode and capable of generating a second gas when the photoelectrode is exposed to radiation; and 
 a septum arranged between the photoelectrode and the counterelectrode to separate the first and second gases. 
 
     
     
         18 . Apparatus according to  claim 17  wherein the photoelectrode is a photoanode, the counterelectrode is a cathode, and the photoelectrolytic cell further comprises a second anode disposed within the container, the second anode not being photolytically active but being electrically connected to the cathode. 
     
     
         19 . Apparatus according to  claim 18  further comprising an auxiliary septum arranged between the second anode and the cathode. 
     
     
         20 . Apparatus according to  claim 17  wherein the septum is formed of an open cell material, an open cell foam, a microporous material such as fritted glass or ceramic, or an ion exchange membrane such as a fluoropolymer.