Stress-induced bandgap-shifted semiconductor photoelectrolytic/photocatalytic/photovoltaic surface and method for making same

Abstract

Titania is a semiconductor and photocatalyst that is also chemically inert. With its bandgap of 3.0, 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 apparatus for production of hydrogen gas from an aqueous medium, the apparatus comprising:
 a housing capable of holding an aqueous medium, the housing permitting ultraviolet and visible light to enter the interior of the housing;   a first electrode, the first electrode comprising a substrate having an undulating surface and a semiconductor film on the undulating surface, the semiconductor film conforming to the undulating surface so as to induce a stress of at least about 100 mPa in at least part of the semiconductor film, thereby altering the bandgap of the stressed part of the semiconductor film;   a second electrode; and   means for collecting hydrogen gas generated at the second electrode substantially free from oxygen generated at the semiconductor film.   
     
     
         2 . A photoelectrolytic apparatus according to  claim 1  wherein the undulations on the substrate are substantially cylindrical, hemispherical, or sinusoidal in profile and have a pitch not exceeding about 370 nm. 
     
     
         3 . A photoelectrolytic apparatus according to  claim 1  wherein the semiconductor comprises titania, doped titania of the formula nTi x O y , or a metal titanate. 
     
     
         4 . A photoelectrolytic apparatus according to  claim 1  wherein the housing is substantially cylindrical. 
     
     
         5 . A photoelectrolytic apparatus according to  claim 1  wherein the housing is comprised of UV transmitting acrylic, borosilicate glass, quartz, or fused silica. 
     
     
         6 . A photoelectrolytic apparatus according to  claim 1  wherein at least part of the semiconductor film is stressed so as to reduce its bandgap to not greater than about 3.0 eV. 
     
     
         7 . A photoelectrolytic apparatus according to  claim 1  wherein the substrate comprises polycarbonate or titanium. 
     
     
         8 . A photoelectrolytic apparatus according to  claim 7  wherein the titanium is tubular 
     
     
         9 . A photoelectrolytic apparatus according to  claim 1  further comprising means for concentrating incident radiation and substantially focusing it on to the titania electrode within the housing. 
     
     
         10 . A photoelectrolytic apparatus according to  claim 9  wherein the housing is substantially cylindrical. 
     
     
         11 . A photoelectrolytic apparatus according to  claim 9  wherein the housing is comprised of one or more of UV transmitting acrylic, borosilicate glass, quartz, and fused silica. 
     
     
         12 . A photoelectrolytic apparatus according to  claim 9  further comprising a photovoltaic strip having a length approximately equal to the housing length and a width substantially equal to or less than the width of the housing and a spectral filter of substantially the same dimensions as the photovoltaic strip, the spectral filter being placed between the titania electrode and the concentrating means, such that a first part of the incident radiation passes through the filter and on to the titania electrode, but a second part of the incident radiation is reflected by the filter on to the photovoltaic strip, the negative terminal of the photovoltaic strip being connected to the second electrode, and the positive terminal being connected to the titania electrode, such that electricity produced by the photovoltaic strip upon illumination provides a bias voltage to the photoelectrolytic cell for increased hydrogen production.