Abstract:
Apparatus for generating electricity and for carrying out photo-induced reactions comprises: a primary reflector ( 610 ) or other optic which concentrates radiation to a primary focus; a secondary reflector at the primary focus to direct radiation to a secondary focus; a photovoltaic device ( 602 ) to convert radiation to electricity; and a photo-reactor ( 116 ) having a photoactive electrode, one of the photovoltaic device ( 602 ) and the photoactive electrode ( 116 ) lies at the primary focus, and the other at the secondary focus. Electric potential generated by the photovoltaic device ( 602 ) may be used to provide a bias or over-voltage between the photoactive electrode and a counter electrode. The apparatus may be used to photolyze water or to carry out other photochemical reactions.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of copending application Ser. No. 10/424,259, filed Apr. 26, 2003 (Publication No. 2003/0228727), which claims benefit of Provisional Application Ser. No. 60/380,169, filed May 7, 2002. This application is also related to copending application Ser. No. 12/136,716, filed Jun. 10, 2008. The entire disclosures of all three of these applications are herein incorporated by reference. 
     
    
     BACKGROUND OF INVENTION 
       [0002]    This invention relates to a bandgap-shifted semiconductor surface, and a method for making same. This invention also relates to photocatalytic surfaces used in the process of photoelectrolysis, photovoltaics, and photocatalysis, and more specifically to induction and management of stress in a thin titania film photocatalytic surface to match the band gap of the titania more efficiently with the solar spectrum at the earth&#39;s surface for photoelectrolysis, photovoltaics, and photocatalysis. 
         [0003]    For general background information relating to this invention see: 
         [0004]    1. www.colorado.edu/˜bart/book/solar.htm: Bart J. Van Zeghbroeck, 1997, Chapter 4.8 (Photodiodes and Solar Cells) and Chapter Section 2.2.5 (Temperature and stress dependence of the energy bandgap). 
         [0005]    2. J. G. Mavroides, J. A. Kafalas, and D. F. Kolesar, “Photoelectrolysis of water in cells with SrTiO 3  anodes,” Applied Physics Letters, Vol. 28, No. 5, 1 Mar. 1976, and references therein. 
         [0006]    3. A. Fujishima and K. Honda, Nature, 238, 37 (1972) 
         [0007]    4. O. Khaselev and J. Turner, “A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production via Water Splitting,” Science, Vol. 280, 17 Apr., 1998. 
         [0008]    5. P. J. Sebastian, M. E. Calixto, and R. N. Bhattacharya, Electrodeposited CIS and CIGS thin film photocatalysts for hydrogen production by photoelectrolysis. 
         [0009]    6. T. Gerfin, M. Graetzel and L. Walder, Progr. Inorg. Chem., 44, 345-393 (1997), Molecular and Supramolecular Surface Modification of Nanocrystalline TiO 2  films: Charge-Separating and Charge-Injecting Devices. 
         [0010]    7. Guerra, J. M.,  Storage Medium Having a Layer of Micro - Optical Lenses Each Lens Generating an Evanescent Field , U.S. Pat. No. 5,910,940, Jun. 8, 1999. 
         [0011]    8. Guerra, J. M.,  Adsorption Solar Heating and Storage System , U.S. Pat. No. 4,269,170, May 26, 1981. 
         [0012]    9. Guerra, J. M.,  Photon tunneling microscopy applications , MRS Proceedings Volume 332, Determining Nanoscale Physical Properties of Materials by Microscopy and Spectroscopy, M. Sarikaya, H. K. Wickramasinghe and M. Isaacson, editors. Page 457,  FIG. 8   b  shows tensile stress fissures in diamond-like carbon coating on a silicon substrate.  FIG. 9   a  shows adhesion failure due to compressive stresses in a magnesium fluoride thin film coating on an acrylic substrate. 
         [0013]    10. Guerra, J. M.,  Storage Medium Having a Layer of Micro - Optical Lenses Each Lens Generating an Evanescent Field  ( application title: Optical Recording Systems and Media with Integral Near - Field Optics ), U.S. Pat. No. 5,910,940, Jun. 8, 1999. Assigned to Polaroid Corp. 
         [0014]    11. Guerra, J. M. and D. Vezenov,  Method of fabrication of sub - micron spherical micro - lenses . Patent Applied For Apr. 12, 2001. 
         [0015]    12. Guerra, J. M. et al, “Embedded nano-optic media for near-field high density optical data storage: modeling, fabrication, and performance,” Proceedings, Optical Data Storage Conference, SPIE, April, 2001. 
         [0016]    13. Guerra, J. M. et al, “Near-field optical recording without low-flying heads,” ISOM Technical Digest, Taipei, 2001. 
         [0017]    14. Guerra, J. M. et al, “Near-field optical recording without low-flying heads: Integral Near-Field Optical (INFO) Media,”  Japanese Journal of Applied Physics , scheduled publication March 2002 
         [0018]    15. J. M. Bennett et al, “Comparison of the properties of titanium dioxide films prepared by various techniques,” Appl. Opt. 28, 3303-3317 (1989) 
         [0019]    16. H. T. Tien and A. L. Ottova, “Hydrogen generation from water using semiconductor septum electrochemical photovoltaic (SC-SEP) cells,” Current Topics in Biophysics 2000, 25(1), 39-60. Modeled on nature&#39;s photosynthetic thylakoid membrane. 
         [0020]    The ills of our carbon-based energy are well-known: pollution of land and oceans, air pollution, and the global warming that is likely caused by the latter. In addition, there is the growing dependence on foreign oil (presently at 46%, up from 27% during the Oil Embargo during the Carter administration) with the economic, political, and human costs that result from that dependence. Hydrogen has been gradually emerging as the fuel of choice for the future and perhaps even the very near future. Fuel cell technology has recently advanced exponentially, with plans for miniature fuel cells to replace batteries in the ever power-hungry personal digital devices, and for combustion engines for automobiles in which hydrogen is the fuel. This last important application has made great progress in that the hydrogen can now be safely and efficiently stored in a host of metal hydride based materials, with the hydrogen being piped to or stored at local filling stations, with the associated cost and danger. In another approach, the hydrogen is split at the engine from toxic hydrogen-bearing liquids such as gasoline and alcohols. 
         [0021]    Ultimately, for a hydrogen-based energy to be completely beneficial, one would like to be able to split our most abundant resource, water, with a renewable energy source. Many have turned to solar cells to provide the electricity for electrolysis of water as a way to provide a stable and efficient storage for solar energy, with the stored hydrogen (adsorbed in a metal hydride, Ovshinsky et al) later used to create electric power in a fuel cell. However, the losses of the solar cell in converting sunlight to electricity, combined with the losses in the electrolytic splitting of water into hydrogen and oxygen, make for low efficiency overall. Further, the cost of the apparatus and lifetime of the components make the economic viability dim at this time. 
         [0022]    A promising path and highly sought-after goal is to use sunlight directly to split water. The free energy required for decomposing water into gaseous H 2  and O 2  is just 1.23 eV, so this seems possible given that the peak of the solar spectrum is about 2.4 eV (ref. Mavroides). However, the threshold energy for this reaction is 6.5 eV, so direct photodissociation is not possible. However, Honda and Fujishima (Nature 238, 37 (1972)) showed that the threshold energy required can be greatly reduced by introducing a photocatalytic semiconductor surface, such as titania. Immersing single crystal titania (n-type) and Pt electrodes in an aqueous electrolyte, connected externally to form an electrolytic cell, they observed development of gaseous oxygen at the titania electrode and gaseous hydrogen at the Pt electrode when the cell was illuminated. (In other photoelectrolytic cells, hydrogen collects at the semiconductor cathode and oxygen collects at the conducting anode, with a membrane preventing their recombining.) However, while they succeeded in activating titania as a photocatalyst, they required artificial light, such as a xenon lamp, with a photon energy of greater than 3.2 eV, the lowest energy gap of titania. Even so, their energy conversion efficiencies were low. Further, such light is in the ultraviolet part of the spectrum, and very little is present in sunlight at the surface of the earth (sunlight integrated over the 3 eV to 4 eV range is only 4 mW per square cm, compared to the 100 mW per square cm total in visible sunlight), so that titania photoelectrolysis with sunlight has less than 1% efficiency, and the photoelectrolysis quantum efficiency, independent of the solar spectrum, is only 1-2% unless a bias voltage is applied. For photoelectrolysis, as it is known, to spontaneously occur in sunlight, and with a practical efficiency, therefore requires the semiconductor to have a bandgap of about 1.7 electron volts (eV) in order to (1) have the energy required to split the water into hydrogen and oxygen gases, and (2) absorb at the peak of the solar spectrum for highest efficiency. 
         [0023]    Following this work, others (Turner and Warren) have investigated semiconductor alloys or compounds with lower bandgaps. For example, p-type GaInP 2  has a bandgap of 1.8 to 1.9 eV, which would work adequately in sunlight to produce a photocurrent that can be used to break down water into hydrogen and oxygen. However, they found that surface treatments in the form of metallated porphyrins and transition metals, such as compounds of ruthenium, were necessary to suppress the bandedge migration and allow bandedge overlap to occur. Without this treatment, hydrogen and oxygen cannot be produced because the conduction band and the Fermi level of the semiconductor do not overlap the redox potentials of water, i.e. when light shines on the semiconductor, electrons build up on the surface, shifting the bandedges and Fermi level further away from the overlap of the water redox potentials. The long term surface stability of these surface treatments are not known. 
         [0024]    Other attempts at photoelectrolytic cells with lower bandgap semiconductors typically (1) are corrosive in water, and (2) require a bias voltage, supplied by a conventional power source or by a photovoltaic cell or photodiode. The corrosion problem has been reduced by using platinum as the anode, and/or by combining different semiconductors. This again reduces economic viability. 
         [0025]    The titania electrode in the Honda/Fujishima cell has the important advantage that it does not undergo anodic dissociation in water, and titania is much less expensive than other semiconductors. Mavroides, Kafalas, and Kolesar demonstrated somewhat higher efficiency titania cells using n-type SrTiO 3  for its smaller electron affinity, after having confirmed the Honda/Fujishima results with titania in earlier work. They achieved 10% maximum quantum efficiency, an order of magnitude higher than for titania, but with light with energy hv (where h is Planck&#39;s constant and v is the light frequency) at 3.8 eV, compared to 3.2 eV required for the anatase form of titania. They believed this increase in efficiency was the result of band bending at the anode surface that is about 0.2 eV larger than for titania, resulting from the smaller electron affinity of SrTiO 3 . In their energy-level model for photoelectrolysis, the semiconductor serves as only the means for generating the necessary holes and electrons, without itself reacting chemically. In their model, the low quantum efficiency of titania is not due to inefficient carrier transfer, as others had shown that this was close to 100% with platinized—Pt cathodes and illuminated titania anodes, but rather to insufficient band-bending at the titania surface to cause efficient separation of the electron-hole pairs. The complete process, according to their model as in Ref. 2, (which is in substantial agreement with models of other researchers), is that photoelectrolysis occurs because electron-hole pairs generated at the semiconductor surface upon absorption of illumination with the required photon energy are separated by the electric field of the barrier, in the form of the energy-band bending at the surface, preventing recombination. The electrons move into the bulk of the anode and then through the external circuit to the cathode. There, they are transferred to the H 2 O/H 2  level of the electrolyte and hydrogen gas is released: 
         [0000]      2 e   − +2H 2 O→H 2 +2OH −   (1) 
         [0026]    Oxygen is produced at the same time as holes are transferred from the anode surface to the OH − /O 2  level of the electrolyte, as: 
         [0000]      2 p++ 2OH − →½O 2 H 2 O  (2) 
         [0027]    In other work that is farther a-field from this application, Graetzel invented a titania solar photovoltaic cell in which the functions of absorption of light and the separation of the electric charges (“electrons” and “holes”) are not both performed by the semiconductor (titania in this case). Instead, the light absorption is performed by a dye monolayer that is adsorbed onto titania particles, in one case, and onto titania nano-crystals, in another case. In this way he avoids the problem of titania&#39;s 3.2 eV bandgap. This technology is now being commercialized by, for example, Sustainable Technologies International. Others have followed his lead and replaced the dye absorber with quantum dot particles attached to the titania particles, where the quantum dots perform the light absorption (QD Photovoltaics, The University of Queensland). In all of this work, however, there is no attempt to alter the bandgap of the titania. Also, the titania layer is required to be microns thick, and is applied as a sol-gel. Such a process requires solvents and temperatures incompatible with polymer substrates. Further, an electrolyte is required to fill the porous gaps in the titania matrix and complete the cell. This electrolyte is non-aqueous and somewhat volatile, so packaging, cell lifetime, and effect on the environment remain problematic. Efficiencies are reported to be around only 5% at this point. Most importantly, such a device provides no direct access to the titania photocatalytic surface, and so cannot be used for hydrogen production, detoxification, or disinfection. 
         [0028]    Still further a-field is work by researchers at Oxford&#39;s Physics and Chemistry Departments, who are devising “inverted” photonic bandgap (PBG) crystals comprising polycrystalline titanium dioxides (titania), while earlier researchers achieved the same with self-assembled titania nano-spheres. Here, the bandgap is determined by the relative indices of refraction of the titania spheres and the empty or lower index media around and in between the spheres, the size of the spheres, and their geometrical arrangement. Again, there is no attempt to alter the bandgap of the titania spheres themselves, and the application is for directing, absorbing, and otherwise controlling light of a certain wavelength. The titania is used for its high refractive index of 2.4 to 2.6, which provides the desired index ratio of greater than 2 to if the immersion medium is air with in index of unity. 
         [0029]    So, titania has also been shown to have use in photovoltaic devices. And in addition to photoelectrolysis for hydrogen production, titania&#39;s photocatalytic properties have been shown to have beneficial application to disinfection by killing biological organisms, and detoxification by breaking down toxic chemicals. It will be seen that the invention disclosed herein, by enabling titania to function well in visible light, such as sunlight, also applies to photovoltaics, disinfection, and detoxification. 
         [0030]    In all of the above work, titania is either in the form of a slab cut from a crystal, and can be either of the most common polymorphs rutile or anatase, or is a thick film resulting from a sol gel process, or else are small particles of crystalline titania either in suspension or hot-pressed into a solid. No one is using, to our knowledge, titania in the form of a thin film deposited in a vacuum coating process. 
       SUMMARY OF THE INVENTION 
       [0031]    One would like a semiconductor photocatalyst with a bandgap that is better matched to the solar spectrum and/or artificial illumination for higher efficiency or even to work at all. In this invention, the bandgap of the known chemically-inert photocatalyst titania (TiO 2 ) is shifted and broadened to be active at wavelengths more prevalent in sunlight and artificial light by inducing and managing sufficiently high stress in titania by vacuum coating a thin film of titania onto a substrate, preferably of a different Young&#39;s modulus, with bending undulations on the surface of a spatial radius similar to the film thickness. The undulated coating also serves to self-focus and concentrate the incident light required for the process, increase photocatalytic surface area, and prevent delamination of the film from the substrate. The electrical activity so induced in the band-shifted titania subsequently by visible light is applied to photoelectrolysis (hydrogen production from water and light), photovoltaics (electrical power from sunlight), photocatalytic disinfection and detoxification, point-of-use photoelectrolysis for use in internal combustion engines, for example, and stress-induced tunable bandgap components for communications. In addition, the same stress-induced thin film bandgap shifting works with other semiconductors such as amorphous silicon, and with similar benefits. 
         [0032]    Accordingly, this invention provides for shifting, lowering, or reducing the size of, the optical bandgap of a semiconductor into optical wavelengths predominant in the illuminant by stressing (specifically straining) the semiconductor, where the semiconductor is a thin film, and/or where the stress is caused by conditions under which the thin film is formed, and/or where the stress is caused by the shape of the substrate on a nano scale, and/or where the stress is caused by the mechanical, chemical, and thermal properties of the substrate. 
         [0033]    In such a semiconductor, the bandgap may be shifted into longer wavelengths by heating. The semiconductor may be titania. The bandgap may be shifted into wavelengths that are abundant in the solar spectrum. The semiconductor may be a photocatalyst. The stress-inducing template profiles may also provide a mechanical lock to the coating so that the stress can exist without causing delamination of the coating from the substrate. The stress-inducing template profiles may create additional surface area without increasing the width or length of the surface, for additional efficiency in photocatalytic action. 
         [0034]    The photocatalyst may be used to split an aqueous solution into hydrogen gas and oxygen gas when irradiated. The illumination may be from the sun, or from artificial light. The stress-inducing profiles in the substrate may be one-dimensional, such as cylinders, or two-dimensional, such as spheres. The thickness of the titania layer may be chosen to be ¼ of the wavelength of the desired illumination, thereby acting as an anti-reflection filter and increasing absorption and decreasing reflection. 
         [0035]    The additional effective surface created by the substrate stress-inducing profiles facilitates and improves heat dissipation. The semiconductor may be formed by heat oxidation, or by anodizing. The semiconductor may be a contiguous film. The semiconductor may be a matrix of particles such as spheres. The substrate can be polymer, glass, silicon, stainless steel, copper, aluminum, or substrate material. 
         [0036]    The photocatalyst may be used to detoxify a medium in contact with it. The photocatalyst may also be used to disinfect a medium or biological agent in contact or proximal with it. 
         [0037]    The substrate may be transparent or reflective, and can be flexible. The substrate and titania formation are compatible with a roll-to-roll web manufacturing process. The substrate profiles may be embossed into the substrate with a stamper from a master, or molded into the substrate with a stamper from a master, or caused by reticulation in the substrate or in a layer applied to the substrate. 
         [0038]    The semiconductor used in the present invention can be titania, silicon, or other semiconductor. 
         [0039]    The titania-coated substrate(s) of the present invention can function as the anode in a photoelectrolytic cell, which further comprises some or all of the following: a housing, an aqueous electrolyte, a gas separation septum, a cathode, and a bias source. 
         [0040]    The present invention may be used in photovoltaic applications, for which the stress is enabling (titania) or improving (amorphous silicon), in photoelectrolysis, detoxification, disinfection, and point-of-use photoelectrolysis. The present invention may also be used for continual tuning of stress and bandgap properties for telecommunication applications, to alter and improve magnetic properties of thin films applied to hard drive disks for data storage, and to provide a corrugated substrate to which a desired titania or other thin film will adhere under stress but will not cause scatter or diffraction due to its sub-wavelength spatial period. 
         [0041]    This invention provides apparatus for utilizing different parts of the solar spectrum simultaneously to carry out photo-induced reactions and to generate electricity, the apparatus comprising:
       a primary reflector arranged to concentrate radiation incident thereon to a primary focus;   a secondary reflector disposed at or adjacent the primary focus and arranged to direct radiation incident thereon to a secondary focus;   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 disposed at or adjacent the primary focus, and the other of the photovoltaic means and the photo-reactor means is disposed at or adjacent the secondary focus.       
 
         [0047]    In this apparatus of the present invention, the photovoltaic means may use a first wavelength range for converting radiation to electricity and the photo-reactor means may use a second wavelength range at least part of which differs from the first wavelength range, and the secondary reflector may comprises a wavelength selective reflector arranged to reflect one of the first and second wavelength ranges to the secondary focus. The photo-reactor means may comprise a counter-electrode in addition to the photoactive electrode, and the apparatus may further comprising conductors connecting the photovoltaic means to the counter-electrode and photoactive electrode so that the voltage generated by the photovoltaic means is applied as a bias voltage across the counter-electrode and photoactive electrode. The photoactive electrode may comprise titania, desirably titania which is stressed such that at least part of the titania has its bandgap shifted to longer wavelengths in any of the ways taught herein. For example, the titania may have been produced by acid etching of titanium metal, followed by at least one of anodizing and heat oxidation of the acid etched titanium to convert at least part of the titanium to anatase. 
         [0048]    Also, in the apparatus of the present invention, the photo-reactor means may comprise a counter-electrode and a liquid-tight container surrounding the counter-electrode and the photoactive electrode, the container containing an aqueous medium capable of being electrolyzed to produce hydrogen and oxygen. The apparatus may further comprise a substantially tubular inner vessel disposed within the container and having apertures extending therethrough through which the aqueous medium can pass through the tubular inner vessel, the counter-electrode being disposed within the inner vessel, and the photoactive electrode having the form of a sheet outside and extending partially around the tubular inner vessel. 
         [0049]    In one form of the present apparatus, the photo-reactor means is disposed at or adjacent the secondary focus, and the photoactive electrode has substantially the form of a hollow tube having an aperture through which radiation can enter the tube, the inside surface of the photoactive electrode being photoactive. 
         [0050]    As an alternative to the use of tubular inner vessel, the apparatus may comprise a septum disposed within the container and essentially dividing the interior of the container into two chambers, with the photoactive electrode disposed in one chamber and the counter electrode in the other chamber. At least one portion of the septum adjacent the container may be provided with grooves which extend between, and provide ionic conduction pathways between, the two chambers. Alternatively, the septum may be formed of an open cell material, the open cells providing ionic conduction pathways between the two chambers. 
         [0051]    The present apparatus may comprise drive means for rotating the primary reflector around an axis to enable the primary reflector to track the sun. 
         [0052]    This invention also provides a method for bringing about a photoinduced chemical reaction in a liquid, the method comprising:
       providing an apparatus comprising:   a primary reflector arranged to concentrate radiation incident thereon to a primary focus;   a secondary reflector disposed at or adjacent the primary focus and arranged to direct radiation incident thereon to a secondary focus;   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 in contact with the liquid,   wherein one of the photovoltaic means and the photo-reactor means is disposed at or adjacent the primary focus, and the other of the photovoltaic means and the photo-reactor means is disposed at or adjacent the secondary focus   allowing electromagnetic radiation to fall on the primary reflector, to be reflected therefrom to the secondary reflector, and to be again reflected to the secondary focus, whereby at least part of the radiation falls on the photoactive electrode, thereby causing the photoactive electrode to bring about the reaction in the liquid, and a second part of the radiation falls on the photovoltaic means, thereby causing the photovoltaic means to generate an electric potential.       
 
         [0060]    In this method, the photovoltaic means may be electrically connected to the photoactive electrode so that the electric potential generated by the photovoltaic means is applied between the photoactive electrode and a counter electrode. The liquid may be an aqueous solution such that the reaction effected is the generation of hydrogen and oxygen gases from the liquid. 
         [0061]    The apparatus of the present invention may make use of a titania electrode produced by the following process, namely a process for producing a titania electrode comprising primarily anatase (with possibly a minor proportion of rutile) having a bandgap lower than that of unstressed anatase, the process comprising:
       (a) subjecting titanium metal to an etchant (which may be an acid etchant); and   (b) oxidizing at least part of etched titanium to anatase by at least one of (i) anodizing the titanium in an acid or other anodizing solution, and (ii) heating the titanium in an oxygen-containing atmosphere.       
 
         [0064]    In this process, the titanium metal used may be an impure form containing not more than about 99.6 percent titanium by weight, for example Grade 1 titanium having the following specification by weight:
       C 0.1% maximum   Fe 0.2% maximum   H 0.015% maximum   N 0.03% maximum   O 0.18% maximum   Ti 99.5% minimum, up to about 99.6%.
 
or Grade 2 titanium having the following specification by weight:
   C 0.1% maximum   Fe 0.3% maximum   H 0.015% maximum   N 0.03% maximum   O 0.25% maximum   Ti 99.2% minimum, up to about 99.6%.
 
The titanium metal used may be in the form of a foil, sheet or film from about 0.1 to about 1 mm thick.
       
 
         [0077]    Step (a) of the process, in which the nano-structures are formed, may be effected using sulfuric acid having a concentration of at least about 93 percent by weight at a temperature of about 60 to about 100° C. In a preferred from of the process, the sulfuric acid has a concentration in the range of about 93 to about 98 percent by weight and the acid etching is effected at a temperature of about 75 to about 85° C. The acid etching may be carried out for a period of from about 60 to about 600 seconds from the onset of visible bubbling. 
         [0078]    Step (b) of the process, in which the titania of substantially anatase morphology is formed, may be effected by anodizing in an aqueous medium having a pH in the range of about 1.5 to about 2.5 and at a temperature of about 60 to about 100° C. The anodizing may be effected at a maximum voltage of from about 70 to about 100 Volts. The anodizing may also be effected at a voltage which increases with time, for example the voltage may increase with time substantially according to the equation: 
         [0000]        V=V   Final (1 −e   −at ) 
         [0000]    where a is an arbitrary constant. 
         [0079]    Alternatively, the titania formation in step (b) may be effected by heat oxidizing the titanium at a temperature of at least about 630° C. for a period of not more than about 300 minutes, and preferably at a temperature of about 635 to 735° C. for a period of about 300 to about 10 minutes. The variation of the photoactivity of the resultant titania electrode with the time and temperature used in the heat oxidation step is somewhat complex and is discussed in the aforementioned copending application Ser. No. 12/136,716, to which the reader is referred for further information. The heat oxidation may be effected in air to which additional oxygen has been added. 
         [0080]    Other features of the invention will be readily apparent when the following detailed description is read in connection with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0081]    The structure and operation of the invention, together with objects and advantages thereof, may best be understood by reading the detailed description to follow in connection with the drawings in which unique reference numerals have been used throughout for each part and wherein: 
           [0082]      FIG. 1  is a perspective view of a preferred apparatus of the invention for generating hydrogen by photolysis of water. 
           [0083]      FIG. 2A  is a schematic cross-section through the cylindrical core of the apparatus shown in  FIG. 1 , the cross-section being taken in a plane including the axis of the cylindrical core. 
           [0084]      FIG. 2B  is a schematic cross-section through the cylindrical core shown in  FIG. 2A , the cross-section being taken in a plane perpendicular to the axis of the cylindrical core. 
           [0085]      FIG. 2C  is an enlarged cross-section taken in the same plane as  FIG. 2A  and illustrates the electrodes of the core, and the apertured tube lying between these electrodes, with this tube having louvered apertures. 
           [0086]      FIG. 3A  is a schematic cross-section similar to that of  FIG. 2A  through an alternative cylindrical core which can be substituted for the core shown in  FIG. 2A . 
           [0087]      FIG. 3B  is an enlarged cross-section, similar to that of  FIG. 2B , through the alternative core shown in  FIG. 3A  showing the forms of the electrodes, which are substantially planar but are oppositely curved and are separated by a flat septum. 
           [0088]      FIG. 3C  is a detailed view of one side faced of the flat septum shown in  FIG. 3B  showing the grooves provided in the side faces of the septum. 
           [0089]      FIG. 4  is a schematic cross-section, similar to that of  FIGS. 2B and 3B  through a third core assembly, in which the electrodes are substantially planar but are together curved the same way with the septum so as to better form a seal with the inside of the tube. 
           [0090]      FIG. 5  is a schematic side elevation of a modified cylindrical core and associated apparatus for making use of excess heat generated within the core during operation and for reducing temperature gradients along the core axis. 
           [0091]      FIG. 6  is a schematic cross-section, taken perpendicular to the axis of the cylindrical core, through the reflector and core of the apparatus shown in  FIGS. 1-4  to show the location of a photovoltaic strip. 
           [0092]      FIG. 7  is a schematic cross-section along the line  7 - 7  in  FIG. 6 , with part of the reflector omitted for clarity. 
           [0093]      FIG. 8  is a schematic cross-section, similar to that of  FIG. 6 , through a second modified form of the apparatus shown in  FIG. 1 ; in this second modified form, the core is disposed within the reflector. 
           [0094]      FIG. 9  is an enlarged cross-section through the photoactive electrode of the apparatus shown in  FIG. 8 , in which the optical integrating cylinder feature is seen in more detail. 
           [0095]      FIG. 10  is a perspective view of a multiple core photolysis apparatus of the present invention, optimized for vertical installation against a building wall or for use with heliostats. 
           [0096]      FIG. 11  is a perspective view of a second multiple core photolysis apparatus of the present invention suitable for mounting on the roof of a commercial or residential building. 
           [0097]      FIG. 12  is a block diagram showing various auxiliary apparatus used in conjunction with the apparatus of the present invention shown in  FIG. 1 . 
           [0098]      FIGS. 13A to 13D  are graphs showing the rate of hydrogen generation of solar conversion efficiency of an apparatus as shown in  FIGS. 1 ,  2  and  7  under varying conditions of temperature, illumination and bias voltage. 
           [0099]      FIG. 14  is a graph showing the conversion efficiency of an apparatus as shown in  FIGS. 1 ,  2  and  7  under varying bias voltage supplied by the photovoltaic strip therein, as a function of electrolyte recipe. 
       
    
    
     DETAILED DESCRIPTION 
       [0100]      FIG. 1  of the accompanying drawings illustrates a first photolysis apparatus (generally designated  100 ) of the present invention for the production of hydrogen and oxygen from water. The apparatus  100  comprises a squat cylindrical base  102 ; when the apparatus is installed in a fixed location, this base  102  may be installed directly on the ground or, for example, on a concrete pad, which may be equipped with power lines for driving the apparatus  100  as described below. Alternatively, if it is desired to make the apparatus  100  mobile, the base  102  may be mounted on a wheeled platform (not shown) which may be equipped with jacks or chocks (also not shown) for holding the wheeled platform stable at any desired location. 
         [0101]    A cylindrical support member  104  extends vertically upwardly from the base  102 , and a polar housing  106  runs across the upper end of support member  104 , the housing  106  being inclined so that its axis is parallel to that of the earth at the location where the apparatus  100  is being used. For simplicity, the housing  106  is shown as fixed relative to the support member  104 . However, since the optimum angle of inclination of the housing  106  to the support member  104  will vary with the latitude at which the apparatus  100  is to be used, in the case of a mobile apparatus  100  it may be desirable to provide means for varying the inclination of the housing  106  to the support member  104 . 
         [0102]    The polar housing  106  has the form of a hollow cylinder, and a polar shaft  108  is rotatably mounted with the housing  106  by means of radial bearings (not shown), so that the shaft  108  can rotate about the axis of the housing  106 , as indicated by the arrow in  FIG. 1 . Rotation of the shaft  108  relative to the housing  106  is effected by a electric motor (not shown) located within the support member  104 . A reflector assembly  110  provided with end caps  112 ,  114  and core assembly  116  are mounted via thrust bearings (not shown) on the shaft  108 , so that by rotating the shaft  108 , which is pointed at the North star and thus extends parallel to the axis of the earth, the reflector can follow the motion of the sun during the day. The reflector assembly  110  concentrates solar radiation on the core assembly  116  in a manner well known to those skilled in solar technology. 
         [0103]    The single-axis mount shown in  FIG. 1  is the presently preferred mount; adjustments for the seasonal variation in solar elevation can be made by using an oversized photocatalyst, which can accommodate changes in focus with the seasons, within the core assembly  116 , as described in detail below. A two-axis mount can alternatively be used to allow direct adjustment of the position of the reflector assembly  110  to allow for seasonal variation in solar elevation. 
         [0104]    The core assembly  116  will now be described in more detail with reference to  FIGS. 2A and 2B . As most easily seen in  FIG. 2A , the core assembly  116  comprises inner and outer hollow concentric tubes  202  and  204  respectively, which are formed of borosilicate glass, and polycarbonate respectively, although quartz or ultraviolet-transmissive acrylic polymer could alternatively be used for the outer tube  204 , and other materials can be used for the inner tube  202  depending on the desired operating temperature and pressure. The lower ends of the tubes  202  and  204  are received within a cylindrical recess in a lower endcap  206 , formed of poly(vinyl chloride); an annular seal  208 , formed by injecting silicone rubber around the tube  204 , extends around the outer tube  204  within the recess in the endcap  206  to provide a liquid-tight seal around the tube  204 . The upper end of outer tube  204  is received within a cylindrical central aperture in a flange member  210 , and an annular seal  212 , similar to the seal  208 , provides a liquid-tight seal around the tube  204 . 
         [0105]    The upper end of inner tube  202  extends beyond the upper end of outer tube  204  and is solvent welded within shallow cylindrical recess in a top plate  214 , which lies parallel to and adjacent flange member  210 . The top plate  214  is secured to flange member  210  by multiple bolts  216 , only two of which are visible in  FIG. 2A . An O-ring  218  is disposed between top plate  214  and flange member  210  radially outwardly of the upper end of outer tube  204 . 
         [0106]    The core assembly  116  further comprises a counter electrode  220  provided with a contact  222 , in the form of a titanium screw, which extends through the center of the top plate  214  to provide a negative terminal for the core assembly  116 . The counter electrode  220  is cylindrical and extends along the axis of the inner tube  202 . The counter electrode  220  is formed of carbon or graphite, although any material which is inert to the hydrogen generated at this electrode when the apparatus  100  is operating may be used, for example reticulated glassy carbon, or stainless steel, such as stainless steel  316 ; obviously, it is desirable to keep the counter electrode  220  as simple and inexpensive as possible. However, it may be desirable to roughen or otherwise increase the surface area of this electrode to improve the efficiency of the apparatus  100 . A hydrogen tube  224  extends from the upper end of inner tube  202  through the top plate  214  to provide a route for the hydrogen generated at the counter electrode  220  to leave the core assembly  116 . 
         [0107]    The second electrode of the core assembly  116  is a bandgap-shifted titania coated electrode  226  of the present invention. As best seen in  FIG. 2B , the titania coated electrode  226  has the form of a thin titanium sheet curved into substantially the form of an arc of a hollow cylinder, the curved sheet extending the full length of, and being wrapped around a portion of, the inner tube  202  so as to leave a small gap between the electrode and the inner tube (this gap is somewhat exaggerated in  FIG. 2B  for ease of illustration). The electrode  226  is in electrical contact with a titanium screw  228  ( FIG. 2A ), which is generally similar to the screw  222  previously described, extends through the top plate  214 , and acts as the positive terminal for the core assembly  116 . When the core assembly  116  is operating, oxygen is evolved at the electrode  226 , and this oxygen passes through the chamber formed by the facing surfaces of the flange member  210  and the top plate  214  and the O-ring  218 , and leaves the core assembly via an oxygen tube  230  similar to and extending parallel to the hydrogen tube  224  already described. The top plate  214  is also provided with an electrolyte supply tube (not shown) used for filling and refilling the core assembly  116  with electrolyte (described below). For reasons discussed below, the hydrogen and oxygen tubes  224  and  230  are provided with pressure relief valves (not shown) at locations further from the top plate  214  than is capable of being illustrated in  FIG. 2A . 
         [0108]      FIG. 2B  shows a cross-section perpendicular to the central axis of the core assembly  116  shown in  FIG. 2A , with the arrow indicating the face of the assembly which is intended to face the concentrated sunlight. The carbon counter electrode  220  has an outside diameter of 1 inch (25 mm). One provider is NAC Carbon Products, Inc., Elk Run Ave., Punxsutawney, Pa. The inner tube  202  has an internal diameter of 1.5 inch (38 mm) and an external diameter of 1.76 inch (45 mm). The outer tube  204  has an internal diameter of 2.18 inch (56 mm) and an external diameter of 2.365 inch (59 mm). The photoactive electrode  226  has a thickness of 0.010 inch (0.254 mm) and in practice fits somewhat more tightly around the inner tube  202  than is illustrated in  FIG. 2B . Apertures  402  (discussed in more detail below with reference to  FIG. 2C ) passing through the inner tube  202  are 0.575 inch (15 mm) in diameter and disposed on 2 inch (51 mm) centers. 
         [0109]    In operation, the tubes  202  and  204  are completely filled with an electrolyte solution capable of being photolyzed to hydrogen and oxygen. As shown in  FIG. 2C , to enable ions to flow between the electrodes  220  and  226 , while keeping the hydrogen and oxygen evolved at these electrodes separate, the inner tube  202  is provided with a series of apertures  402  lying beneath the electrode  226 , these apertures extending downwardly and radially outwardly, while the electrode  226  itself is provided with a series of vent louvers  404 . As indicated by the diagonal arrows in  FIG. 2C , the combination of the louvers  404  and apertures  402  provides a short, large effective cross-section path for ion flow between the electrodes  226  and  220  through the inner tube  202 , while keeping the evolved hydrogen and oxygen flowing separately upwardly, as indicated by the vertical arrows in  FIG. 2C , separated by the inner tube  202 . For additional insurance that the hydrogen and oxygen are kept separate, the apertures  402  may be covered by a fluoropolymer membrane, such as that sold commercially by E. I. du Pont de Nemours &amp; Co. under the Registered Trade Mark “NAFION”. Such a membrane may be wrapped around the inner tube  202  and sealing, especially if hydrogen and oxygen pressures are equalized. Microporous materials such as ceramic or glass frits or a methacrylate (contact lens plastic) permeable to oxygen may alternatively be used, and such microporous materials can sustain substantial pressure differences between the two tubes. However, it has been found that satisfactory separation of gas can be achieved without providing such a membrane over the apertures. 
         [0110]      FIG. 3A  shows as modified version (generally designated  116 ′) of the core assembly  116  shown in  FIG. 2A . In this modified version, the outer glass tube  204 ′ has a rounded, sealed lower end and is provided at its upper end with an outwardly-extending flange  240 , which is ground flat to accept an O-ring seal  218 ′. An annular backplate  210 ′ with threaded holes is positioned below the flange  240 , and an upper plate  214 ′, having a groove which receives the O-ring seal  218 ′ is bolted to the backplate  210 ′ to form a seal. The upper plate  214 ′ provided with an entrance port  242  for electrolyte and exit ports (only one exit port  230 ′ is shown in  FIG. 3A ) for gases generated. 
         [0111]    In the core assembly  116 ′ of  FIG. 3A , the inner tube  202  present in the core assembly  116  shown in  FIG. 2A  is eliminated, and instead a planar central septum  350 , extends diametrically across the tube  204 ′, effectively dividing this tube into two substantially hemi-cylindrical chambers, as most easily seen in  FIG. 3B . Note that the septum  350  does not make sealing contact with the lower end of the tube  204 ′. A titania/titanium electrode  226 ′ and a counter-electrode  220 ′ are mounted on opposed sides of the septum  350 . (For ease of illustration,  FIG. 3A  does not accurately represent the forms of these electrodes, which will be explained below with reference to  FIG. 3B .) 
         [0112]    Each of the electrodes  220 ′ and  226 ′ is provided at its upper end with a tab (designated  220 T and  226 T respectively), each tab extending horizontally and thus perpendicular to the main part of the electrode. Titanium screws  2205  and  226 S respectively pass through the tables  220 T and  226 T respectively and the secure the electrodes  220 ′ and  226 ′ respectively to the upper plate  214 ′. To ensure proper sealing around the screws  2205  and  226 S, O-rings or other sealing means may be provided where the screws pass through the upper plate  214 ′, but such conventional sealing means are omitted from  FIG. 3A  for clarity. The upper ends of the screws  2205  and  226 S protrude above the upper surface of the plate  214 ′ and are shaped and spaced to form a standard male plug size. This male plug may be connected via a female plug and cable to a photovoltaic strip (described below with reference to  FIG. 7 ) in order that the photovoltaic strip can provide a bias or over-voltage to the electrodes  220 ′,  226 ′. 
         [0113]    The planar septum  350  isolates the electrodes  220 ′,  226 ′ from each other to prevent a short circuit and also, as already noted, effectively divides the interior of the tube  204 ′ into two substantially hemi-cylindrical chambers, with one electrode being present in each chamber. The upper end of the septum plate is received within a groove in, and sealed to the upper plate  214 ′. The necessary seal to the polycarbonate upper plate  214 ′ may be formed by solvent welding, ultrasonic welding, heat welding, or a mechanical seal with or without silicone rubber adhesive sealant. The septum can be made of polycarbonate, polytetrafluoroethylene, silicone rubber, silicone rubber foam (closed or open cell) or other high temperature and inert material or plastic or combinations thereof, such as a polycarbonate septum with a sealing edge comprising silicone rubber tube that is split along its length and applied to the polycarbonate edges, or silicone rubber foam. 
         [0114]    As shown in  FIG. 3B , the electrodes  220 ′,  226 ′ have substantially the form of parts of thin, hollow cylinders, with the vertical edges of the electrodes in contact with the septum  350  but with the central portions of the electrodes spaced from the septum. 
         [0115]    The counter electrode  220 ′ shown in  FIGS. 3A and 3B  (and the similar counter electrode shown in  FIG. 4 ) will typically not be formed of the carbon, as is the counter electrode shown in  FIGS. 2A-2C ; instead the counter electrode  220 ′ will typically be formed of a planar electrode material such as stainless steel mesh, titanium mesh (platinized or not), TDA carbon strip or reticulated glass carbon. The TDA carbon strip is made from sheets that are reinforced with conducting carbon fiber and were obtained from TDA Research, 12345 W. 52 nd  Ave., Wheat Ridge, Colo. The edges of the septum  350  make contact with the inside wall of the tube  204 ′ by press-fit. The photoactive electrode  226 ′ and the counter-electrode  220 ′ are formed into the illustrated arch shape by making the electrodes with a width slightly larger than the inside diameter of the tube  204 ′. However the electrodes can also lie adjacent to the septum surface and even be bonded to the septum for ease of replacement of the entire assembly comprising septum, both electrodes, and top plate  214 ′. The arches of the electrodes can be concave or convex as it faces the concentrating reflector. 
         [0116]    Obviously, it is necessary to provide for ionic conduction pathways between the electrodes  220 ′ and  226 ′. However, unlike the inner tube  202  shown in  FIG. 2C , the main part of the septum is not provided with apertures. The ionic conduction pathway provided by the gap between the lower end of the septum  350  and the lower end of the tube  204 ′ is not, by itself, adequate for this purpose. Accordingly, to provide additional ionic conduction pathways, the side faces (designated  350 A in  FIG. 3C ) of the septum  350  in contact with the inner wall of the tube  204 ′, are interrupted periodically by slots or grooves  352  that are cut into the side faces  350 A. The grooves  352  may be substantially semi-circular, as illustrated in  FIG. 3C , “V”-shaped or linear and cut at an angle, preferably of 450 or more. The grooves  352  create a short ionic conduction pathway, similar to that provided by the apertures  402  shown in  FIG. 2C , while preventing the oxygen and hydrogen gases (indicated schematically at  354  and  356  respectively in  FIG. 3C ) from mixing due to buoyancy. Further, the grooves  352  can be alternating such that they impart a helical or screw effect to the flow of the electrolyte for enhanced convective flow. 
         [0117]    Alternatively, if the septum  350  is formed of (for example) a silicone rubber open cell foam strip 0.25-0.5 inch (6 to 13 mm) thick; the grooves  352  are not needed to create ionic conduction pathways since the open cell structure of the foam allows ionic communication to occur without allowing mixing of the gaseous products. In all cases the septum material must be capable of surviving temperatures of at least 100° C. and electrolytic solutions containing salts, acids, or bases. All of the materials discussed herein meet these requirements. Where adhesive is used to bond the titanium to the septum, acrylic adhesive is used. The titanium/titania electrode  220 ′ can also itself act as a septum, if edge guides are provided that effect a seal to the tube  202 ′ except in the area of the grooves  352 . 
         [0118]      FIG. 4  illustrates a radical cross-section, similar to that of  FIG. 3B , through a modified version of the apparatus of  FIGS. 3A-3C  in which the septum (designated  352 ′) is a flexible strip that seals to the inside wall of the borosilicate glass tube  204 ′ by intimate contact. The titania/titanium electrode  226 ″ and the counter electrode  220 ″ are of substantially the same arcuate form as the septum  352 ′ and are laminated thereto. Angled grooves (not shown in  FIG. 4 ) providing ionic conduction pathways are again formed into the edges of the septum  352 ′. This embodiment allows for a lighter core assembly that can have higher aspect ratios (i.e., the length to diameter ratio of the core assembly can be much higher) which is preferred for roof-top mountings, where the reflectors can be smaller in width for overall lower profile height. 
         [0119]    When any of the apparatus shown in  FIGS. 1-4  is operating, there is a natural convective flow of electrolyte parallel to the axis of the core assembly. This convective flow can be used to cool the electrolyte in order to maintain a desired operating temperature and/or to remove from the core assembly heat which can usefully be employed elsewhere, for example in space heating, thus improving the overall efficiency of use of the radiation incident upon the apparatus  100 .  FIG. 5  illustrates schematically a modified core assembly (generally designated  516 ) having the same central carbon anode  220  as previously described. However, the inner tube  502  of the modified core assembly  516  has a U-shaped external tube  504  joining its upper and lower ends. As indicated by the arrows in  FIG. 5 , electrolyte circulates upwardly through the inner tube  502  and downwardly through the external tube  504 , being cooled within the external tube  504  by a heat withdrawing apparatus indicated schematically at  506 . The external tube  504  can be formed of Grade 2 titanium tubing for resistance to corrosion, for increased tolerance to heat, and for increased hydrogen pressure. By a further modification of the apparatus shown in  FIG. 5 , the external tube  504  could be continuous with the titania electrode  226  ( FIG. 2 ) with the relevant portion of the titanium tube being treated to form a titanium photocatalyst of the present invention. This arrangement allows for reducing the temperature gradient along the photoactive portion of the titanium tube for more even operation along the length of this photoactive portion. Obviously, the core assemblies shown in  FIGS. 3A-3C  and  4  can also be modified as illustrated in  FIG. 5 . 
         [0120]    A closed convective loop is also present within the tube containing the carbon electrode and where the hydrogen is produced; this convection circulates the electrolyte in the “carbon” chamber past the interface with the “titania” chamber. As already noted, the interface between the two chambers can be a series of open holes, grooves, or a microporous material such as ceramic, fritted glass, or an ion exchange membrane such as fluoropolymer. This architecture enables higher differential pressures between the hydrogen and oxygen, in addition to increasing the rate of production and the production efficiency. The output is restricted by a ceramic frit to the pressure required, but keeps the aqueous electrolyte contained and circulating. Gas separation is achieved by common ports between the two electrolyte chambers that may be open holes, or the aforementioned microporous materials. The hydraulic pressure at the ports is substantially equal between the chambers so as not to damage the separation membrane, or if open holes are present, to avoid liquid flow across the holes. 
         [0121]    Although not shown in  FIG. 1 , the apparatus  100  also comprises a photovoltaic strip disposed along the axis of symmetry of the reflector assembly  110 , and the apparatus is designed so that each of the core assembly  116  and the photovoltaic strip can make maximum use of the wavelengths of incident radiation which they are best equipped to use; as discussed in more detail below, the photolysis reaction carried out by the electrode  226  ( FIG. 2A ) makes use of near ultraviolet and shorter wave visible (blue-green) wavelengths of radiation, whereas the photovoltaic strip makes use of wavelengths from about green in the visible range through red. For reasons explained below, in the preferred apparatus  100 , it is important that the outer tube  204  ( FIG. 2A ) transmit radiation of all the wavelengths used by both the electrode  226  and the photovoltaic strip, and this is one reason for forming the outer tube  204  of Type 3 borosilicate glass, which transmits radiation of all frequencies from infra-red to ultra-violet. 
         [0122]    As may be seen from  FIGS. 6 and 7 , the photovoltaic strip  602  is disposed along the axis of symmetry of the reflector assembly  110  and is mounted on a mirrored elliptical reflector member  610  which reflects solar radiation. Solar radiation from the reflector member  610  travels, as indicated at  612 , to the core assembly  116 , where the ultraviolet and short wave visible radiation is absorbed, while the remaining radiation is again reflected and travels, as indicated at  614 , to a secondary focus at the photovoltaic strip  602 . 
         [0123]    The reflection of the “unused” radiation (i.e., radiation not used by the titania electrode  226 ) from the core assembly  116  can be achieved in various ways, and one such way is illustrated schematically in  FIG. 7 , where the radiation  612  from the reflector member  610  passes through the outer tube  204  (hence the need for this tube to transmit all the wavelengths used by both the electrode  226  and the photovoltaic strip  602 ) and strikes the electrode  226 . The longer wavelength (green through red) radiation is reflected from the part-cylindrical electrode  602 , passes back through the outer tube  204  is travels, as indicated at  614 , to a secondary focus at the photovoltaic strip  602 . Although, as described in more detail below, the electrode  226  has minor undulation or other surface roughness needed for its photovoltaic efficiency, its surface is still sufficiently smooth to reflect and focus most of the unabsorbed longer wavelength radiation used by the photovoltaic strip  602 . For greater efficiency, the photovoltaic strip  602  may be covered by a coating which reflects the shorter wavelength radiation as well as the near-infra red used by the core assembly  116  to that core assembly. 
         [0124]    Placing the photovoltaic strip  602  on an “outrigger” (the reflector member  610 ) to the core assembly  116 , as shown in  FIG. 7 , ensures that the strip  602  does not occlude reception by the titania photoelectrode  226  of any of the concentrated light from the collector, so that the strip  602  is in the shadow of the core, while being close enough to receive substantially all of the diffuse reflection of un-absorbed solar radiation from the core. In this way, the cost of a dichroic spectral separator is eliminated, and the titania electrode itself reflects unused parts of the spectrum to the photovoltaic strip, which is optimized for the unused solar spectrum parts. The core assembly  116  shown in  FIG. 7  may of course be the core assemblies shown in  FIGS. 3A ,  3 B and  4 . 
         [0125]    As already indicated, it is not essential that the back reflection of the radiation to the photovoltaic strip be effected by the electrode  226 . Reflection may, for example, occur at the outer surface of the outer tube  204 , by coating this surface with either a dichroic mirror or filter comprising a thin film optical stack (typically alternating layers of high refractive index and low refractive index metal oxide layers, such as titania and magnesium fluoride or silicon dioxide) coated directly onto the outer tube, or a holographic mirror. Obviously, whatever reflective coating is used on the outer tube  204  should be wavelength selective such that the wavelengths required by the electrode  226  pass into the core assembly  116  through the outer tube  204  and only the longer wavelengths needed by the photovoltaic strip  206  are back reflected on to this strip. Alternatively, the necessary wavelength selective reflector can be coated on to an additional tube surrounding the core assembly  116 ; the provision of such an additional tube may also be useful for preventing mechanical damage to the core assembly and/or protecting persons or apparatus near the core assembly  116  from injury should the pressurized core fail during operation. 
         [0126]    As already indicated, the apparatus  100  uses a Dall-Kirkham reflective design, with an elliptical primary reflector, the reflector member  610 , and a cylindrical secondary reflector, the electrode  226 . In practice, this type of reflective design allows radiation concentration of about 30 suns without the need for precise optics, thus allowing a low cost, robust, light weight apparatus. Similar results can be achieved with a Cassegrain reflective design, with a parabolic primary reflector and a hyperbolic secondary reflector. The apparatus of the present invention may also may use of Newtonian (spherical primary and flat secondary reflectors, or a refractive concentrator, for example a Fresnel lens in preferably lenticular form that is manufactured from a UV-transparent material such as UVT (ultra violet transmitting) acrylic or borosilicate 3.3 glass. 
         [0127]    As shown in  FIG. 7 , the photovoltaic strip  602  is used to apply a bias voltage as well as over voltage across the electrodes  220  and  226 . Bias voltage is required for the hydrogen production to proceed vigorously, while the over-voltage overcomes the various electrochemical resistances in an electrolysis cell. For this purpose, opposed sides of the photovoltaic strip  602  are connected via conductors  620  and  622  to contacts  228  and  222  respectively and thence to the electrodes  226  and  220  respectively, with the positive conductor going to the electrode  226  and the negative conductor to the electrode  220 . Under typical practical conditions, the photovoltaic strip  602  will apply a bias voltage plus over-voltage varying from about 0.5 V to 8 V (direct current) across the electrodes  220  and  226 ; as demonstrated below, it has been found that a bias voltage plus over-voltage of about 5.5 V provides optimum efficiency for solar assisted photolysis of water. 
         [0128]    Obviously, a bias voltage and over-voltage can be applied to the electrodes  220  and  226  from a source other than a photovoltaic strip. Mains electricity can be used for non-solar electrolysis, or for solar-assisted electrolysis, after conversion of the high voltage AC mains electricity to low voltage direct current, which can then be supplied to the core assembly using the same conductors as for the photovoltaic strip. In this way, low cost over-capacity nighttime mains power can be used produce hydrogen; to generate hydrogen in the absence of illumination, the core assembly must be supplied with electricity at a voltage greater than the diode breakdown voltage of the core assembly, which is typically about 12 V. The mains electricity could, for example, be supplied by a wind farm (most wind farms produce the majority of their electricity at night), tidal generator or other generating apparatus the output of which varies with environmental conditions, thus providing a way to store the intermittent output from such a generating apparatus in the convenient form of hydrogen (with optional output of oxygen). 
         [0129]    In the apparatus  100  described above, the core assembly  116  is disposed at the primary focus of the reflector assembly  110  and the photovoltaic strip  602  at the secondary focus. It will readily be apparent to those skilled in radiation collection systems that the locations of the core assembly and photovoltaic strip could be reversed. Moreover, placing the core assembly at the secondary focus allows (optional) mounting of the core assembly within the reflector assembly. An apparatus (generally designated  800 ) of this type will now be described with reference to  FIGS. 8 and 9 . 
         [0130]    As shown in  FIG. 8 , the apparatus  800  is of the Cassegrain type, with a parabolic main reflector assembly  802  and a hyperbolic secondary reflector  806 , which is wavelength selective to reflect only shorter wavelengths and which overlies a photovoltaic strip  804 . As indicated by the broken lines in  FIG. 8 , incoming solar radiation is reflected from the main reflector assembly  802  as indicated at  812  and (if of appropriate wavelength) is further reflected from the secondary reflector  806  as indicated at  814  to a core assembly (generally designated  816 ) mounted within the reflector assembly  802 . This arrangement allows the core assembly  816  to be placed close to or within the main reflector assembly for easier access and interconnection, and enables easier, lower energy solar tracking with very little need for counter-balancing; the core assembly can be co-axial with the rotation axis of the main reflector assembly. 
         [0131]    Placing the core assembly at the secondary focus also allows for an advantageous modification of the form of this assembly. As schematically indicated in  FIG. 8 , the core assembly  816  comprises a transparent outer tube  818  similar to outer tube  204  ( FIG. 2 ) of apparatus  100 , but the internal arrangement of the core assembly  816  differs from that of the core assembly  116  of apparatus  100 ; in core assembly  816 , there is no inner tube and the electrodes  820  and  826  extend parallel to but spaced from each other. As most easily seen in the enlarged view of  FIG. 9 , the photoactive electrode  826  is formed as an “integrating cylinder”, that is to say the photoactive electrode  826  is substantially cylindrical with the photoactive surface on the inside, and having a slit  828  running axially along to cylinder such that the tightly focused light  814  from the secondary reflector enters the cylinder and is able to undergo multiple reflections within the cylinder multiple times until substantially completely absorbed by the photoactive surface. This increases the efficiency of photon conversion by the electrode  826 . 
         [0132]    The apparatus  800  is well adapted for construction as an extruded or molded plastic ribbed reflector design, with all feature for mounting the core assembly  816  and other components molded in. The main reflector assembly can, for example, use a rear-surface silver ultra-violet reflecting layer on ultra-violet transmissive acrylic polymer, and be epoxy-overcoated. Acrylic polymers can be flexed into the parabolic or hyperbolic main reflector shape, as required, and provide a smooth optical surface which is durable against hail and other weather. 
         [0133]    The main reflector assembly is typically one of two main types. In the first type, the end caps or end wings determine the shape of the mirror. The two end caps are connected to each other by a series of tubes, with one tube at each tip of the end caps, and one or more tubes in between. The tubes are solvent-cemented or otherwise secured connected to the end caps, and a tension rod can run down the center of the tube for added strength. Cross bracing between the tubes can be added for additional stiffness under wind loading. A slotted guide is attached to the facing inside surfaces of the end caps to define the shape of the main reflector. The main reflector material is inserted into the slotted guides. The main reflector material is made sufficiently flexible that it follows the shape determined by the guides faithfully, while also smoothing out any irregularities in the manufacture of the end caps or guides. The end caps are typically blow-molded of recycled plastic, and the molding process allows for many features to be easily integrated into the end caps, including the guides, mounting flanges, stiffening ribs, product identification, and safety and other information. The main reflector mounting material may, as already indicated, be ultra-violet resistant plastic, or may be powder coated or painted to resist ultra-violet degradation. The main reflector material can be an ultra-violet transmitting acrylic polymer such as poly(methyl methacrylate), known commercially as Plexiglas, with a mirror coating applied to its rear surface, this coating being optimized for reflection of ultra-violet as well as the visible and infra-red radiation. Alternately, the main reflector material can be an acrylic or polycarbonate sheet about ⅛ inch (about 3.2 mm) thick with a layer of sheet metal reflector adhered to its front surface facing the sun. The metal reflector in this case can be an anodized polished aluminum product, for example MIRO produced by Alanod GmbH. This product is overcoated with silicon dioxide and then titanium dioxide for improved ultra-violet reflection, the titanium dioxide also provides self-cleaning properties and ruggedness, since the hydrophilic nature of titanium dioxide causes rain to remove accumulated dirt from the surface, thereby reducing maintenance and improving lifetime. The metal dents easily, so in case of hail, the main reflector assembly is inverted, so that the acrylic polymer absorbs and deflects shocks caused by impact of hail. 
         [0134]    In the second type of reflector assembly, the assembly is an form or molded form having the cross section seen in  FIG. 8 , typically formed of a recycled plastic, and has edge receivers built in to receive either the mirror or the metal mirror sheet. For reflectors about 2 meters wide and 3 meters long, the acrylic backing mirror material should be about ⅛ inch to 3/16 inch (about 3.2 to 4.7 mm) thick, providing the optimum ratio of flexibility to stiffness for a smooth continuous optical curve. 
         [0135]    The apparatus  100  and  800  previously described are freestanding units provided with their own supporting members and intended to be disposed in open areas away from other structures. However, the apparatus of the present invention can also be designed to be mounted on a building wall or roof, and  FIGS. 10 and 11  illustrate two different embodiments of this type. 
         [0136]    The apparatus (generally designated  1000 ) shown in  FIG. 10  is of a “tower” type intended to be supported on a building wall. The apparatus  1000  comprises a plurality of elongate parallel core assemblies  1016 , each similar to the core assembly  116  shown in  FIGS. 2 and 7 ; for simplicity,  FIGS. 10 and 11  do not show the internal components of their core assemblies. Each core assembly  1016  extends along the axis of an outer cylinder  1018 , which is formed of an acrylic polymer capable of transmitting ultra-violet and visible radiation. Each outer cylinder  1018  is sealed at its lower end and its upper end is covered, and the cylinder is filled with water, so that is acts as a focusing lens concentrating sunlight on to the core assembly  1016  running along its axis. Advantageously, the water within the outer cylinders  1018  is mixed with sufficient glycerol to raise its refractive index from the 1.33 of pure water to match the 1.45 refractive index typical of acrylic polymers, thus improving the performance of the cylinder in concentrating solar radiation on the core assembly  1016 . The glycerol also acts as an antifreeze to prevent damage to the apparatus  1000  if the apparatus is exposed to freezing temperatures. Alternatively, a Fresnel lens formed of an ultra-violet transmitting polymer can replace the fluid-filled cylinder  1018 , and the use of such a Fresnel lens may be advantageous when the apparatus is to be mounted in a location (for example, on a roof which is not capable of supporting large loads per unit area) where the weight of the fluid-filled cylinders may be a problem. The outer cylinders  1018  also act as containment vessels should a core assembly fail during use, and thus allow operation of the core assemblies  1016  at higher pressures than would be safe if the outer cylinders were not present. Oxygen and hydrogen are removed from the apparatus  1000  via tubes  1020  and  1022  respectively, these tubes being housed within a protective manifold  1024 . 
         [0137]    In the apparatus  1000 , it is advantageous for the photoactive electrode to occupy a greater proportion of the hollow cylinder than the less than hemicylindrical electrode  226  shown in  FIG. 2 ; using a photoactive electrode which occupies more than a hemicylinder allows good use of solar radiation without the need for solar tracking such as that carried out by the apparatus  100  described above. For the same reason, the apparatus  100  typically does not incorporate a photovoltaic strip, since if such a strip is included the apparatus  1000  needs to be modified to allow the photovoltaic strip to remain at the secondary focus of the optical system. 
         [0138]    So far as possible consistent with the mounting location being used, the axes of the cylinders  1018  shown in  FIG. 10  should be tilted so as to parallel to the earth&#39;s axis, in the same way as the axis of the polar housing  106  of the apparatus  100  shown in  FIG. 1 . The cylinders  1018  should also be spaced apart so that they do not shadow each other. 
         [0139]    In a variant of the apparatus  1000  shown in  FIG. 10 , the fluid-filled cylinders  1018  are modified by inserting a second sheet of ultra-violet transmitting polymer within each cylinder, this second sheet being solvent welded to the inside surface of the main cylinder  1018  so as to form a meniscus focusing lens, which is filled with an optical oil or glycerol having a refractive index close to that of the ultra-violet transmitting polymer. The external form of the cylinder  1018  is unchanged but the core assembly  1016  is moved from the axis of the cylinder  1018  to adjacent the back surface thereof (i.e., adjacent the surface on which the apparatus  1000  is mounted) where the new primary focus is located. A photovoltaic strip may be mounted in the center of the rearward surface of the meniscus focusing lens. This form of the apparatus does require solar tracking, but such tracking is readily achieved by mounting the cylinders  1018  on rollers which can be rotated by an appropriate motor. 
         [0140]    The cylindrical tower apparatus  1000  shown in  FIG. 10  is more resistant to high winds than the apparatus  100  shown in  FIG. 1 , and is more easily integrated into a building design. The apparatus  1000  may also be considered more architecturally attractive than the freestanding apparatus  100 . 
         [0141]      FIG. 11  illustrates a further multi-core apparatus (generally designated  1100 ) of the present invention which is generally similar to the apparatus  1000  shown in  FIG. 10  but is adapted for roof mounting. The apparatus  1100  comprises a plurality of elongate parallel cores  1116  joined by a common manifold  1120 ; obviously, if desired, a second common manifold could be provided at the opposed ends of the cores  1116  from the manifold  1120 . However, in the apparatus  1100 , concentration of radiation is provided by a plurality of hemicylindrical, mirrored reflectors  1110 ; alternatively, a multiple Fresnel top sheet overlying the cores  1116  could be substituted for the reflectors  1110 . 
         [0142]      FIG. 12  illustrates schematically the various auxiliary apparatus which is used in conjunction with the apparatus  100  described above to collect and store the hydrogen and oxygen gases produced and to refill the apparatus with water to replace that electrolyzed. Although not shown in  FIG. 2 , the apparatus  100  is in fact provided with an additional tube extending through the top plate  214  through which additional way can be introduced into the apparatus, as schematically illustrated by “H 2 O” in  FIG. 12 . As already described, the apparatus  100  is also supplied with solar (or other) radiation, as schematically illustrated by “hv” in  FIG. 12 . The additional water (“feedstock”) can be ocean water. While the electrochemical potential for forming chlorine gas is very close to that of oxygen formation, it is still higher, so oxygen is formed preferentially over chlorine gas at low brine concentrations. Ocean water is only 3.5% sodium chloride by weight, which is a low concentration, and so chlorine is not formed. However, if ocean water is the sole replacement water, the salt concentration will grow within the apparatus  100  until it reaches saturation, or about 21% by weight, after which salt would precipitate out within the apparatus as sediment which have to be removed. However, because typically the formation of chlorine gas is not desired, the core assembly  116  ( FIG. 2 ) is flushed with ocean water at intervals (at least annually) to keep the salt concentration low. As indicated schematically at  1202  in  FIG. 12 , the apparatus includes provisions for filtering incoming water to remove algae, rust, iron, chlorine, fluoride, and other contaminants. After filtration, the water passes through a float valve system  1204  during cool, low pressure condition of the core assembly  116  at night or at other times of low pressure within the core assembly. 
         [0143]    As schematically indicated in  FIG. 12 , the hydrogen and oxygen gases leaving the apparatus  100  pass through pressure relief valves,  1206  and  1208  respectively, that provide both backpressure with the core assembly  116  and safety pressure release. The hydrogen then passes through compression apparatus  1210  and is stored under pressure in a tank  1212 ; the oxygen may be similarly compressed as indicated at  1214  and stored (storage tank not shown), or, depending upon the location of the apparatus  100  and commercial demand, may simply be vented to atmosphere. Those skilled in the art of gas collection will understand that a variety of additional apparatus may be included which is not shown in  FIG. 12 , for example thermal probes to monitor gas temperatures, and condensers and desiccants to conserve electrolyte and remove water vapor from the output gases. 
         [0144]    In this connection, it should be noted that, in the multiple core apparatus of  FIGS. 10 and 11 , the pipeline connections to individual core assemblies are arranged in parallel rather than serially, so that a leak in one core assembly only affects the leaking core assembly and not the entire apparatus. 
         [0145]    Although the apparatus of the present invention has been described above principally as used to generate hydrogen and oxygen from water, the chemistry of the electrolyte can be altered to produce different electrolysis products that may be useful for manufacturing processes. For example, if the electrolyte is a brine with high salt concentration, the products will be hydrogen gas, chlorine gas, chlorine water (water containing dissolved chlorine gas) and sodium hypochlorite, a bleach. If sodium carbonate or bicarbonate are used, the gaseous products are hydrogen and carbon dioxide. Additionally, if carbon dioxide bearing electrolytes are used, such as carbolic acid and/or carbonated electrolyte, the product can be carbon monoxide. In this way, carbon dioxide from fossil fuel plants or other production can be sequestered in an electrolyte and then converted with sunlight to carbon monoxide. The carbon monoxide is then used as the feedstock to produce methane or even gasoline-like liquid fuels by adding hydrogen, as is well known. Such processes are Fischer Tropsch or variants of them. But in this case the carbon monoxide and the hydrogen are produced with sunlight and a photolytic reaction. A liquid fuel produced in this way is carbon neutral, in that it sequesters as much carbon as it releases upon being combusted. Further, the infrastructure for handling, transporting, and using liquid fuels already exists. As already noted, for further explanations of the preferred types of titania electrodes for use in the preferred apparatus described above, the reader is referred to the aforementioned copending application Ser. No. 12/136,716, of even date herewith. 
       EXAMPLE 1 
       [0146]    This Example illustrates the effects of illumination conditions, bias voltage and temperature on the efficiency of hydrogen production and conversion efficiency of an apparatus as shown in  FIGS. 1-2  and  7 . 
         [0147]    An apparatus as described above with reference to  FIGS. 1-2  and  7  was subjected to artificial solar illumination conditions of AM 1.5 and AM 0.0, where AM means Air or Atmospheric Mass, the amount of atmosphere through which the sunlight must travel to reach the ground. So, AM 1.5 is typical for sea level conditions, while AM 0.0 is for a “space” application such as orbiting satellites. with the core assembly  116  maintained at either 25 or 80° C. and with varying bias voltages applied between the photoactive anode  226  and the cathode  220 . In these tests, the bias voltage was not supplied by the photovoltaic strip  602  ( FIG. 7 ) in order to permit the illumination of the photovoltaic strip and the bias voltage to be controlled independently.  FIG. 13A  shows the rate of hydrogen production (measured as the current passing between the electrodes) as a function of bias voltage at 25° C., while  FIG. 13B  shows the conversion efficiency as a function of bias voltage.  FIGS. 13C and 13D  parallel  FIGS. 13A and 13B  respectively but relate to measurements at 80° C. 
         [0148]    From  FIGS. 13A-13D , it will be seen that the rates of hydrogen production and conversion efficiency at 80° C. are more than double those at 25° C., and this improved performance with temperature is one major reason for constructing the apparatus so that it is able to operate at elevated temperatures. The improved performance at higher temperatures can be attributed to both bandgap shift and easier electrolysis. In all cases, hydrogen production increases with increasing bias voltage although under low illumination conditions the increase is small. Conversion efficiency tends to increase with bias voltage but eventually reaches a maximum value and thereafter declines. 
       EXAMPLE 2 
       [0149]    This Example illustrates (see  FIG. 14 ) the effects of bias voltage on conversion efficiency of an apparatus as shown in  FIGS. 1 ,  2  and  7 , as a function of the electrolyte composition. The best results, i.e. highest hydrogen production efficiency, are obtained with either an acidic electrolyte or a salt water electrolyte. The basic potassium hydroxide electrolyte performs best at zero voltage bias, but underperforms at higher voltage bias. 
         [0150]    The photoactive titania electrodes of the present invention can be used in any application in which photoactive titania electrodes have hitherto been used, as discussed in detail in the aforementioned parent application. 
         [0151]    In summary, this invention provides for shifting the optical bandgap of a semiconductor into longer optical wavelengths by stressing the semiconductor, where the semiconductor is a thin film, and where the stress is strain caused by some or all of the following: conditions under which the thin film is formed, the shape of the substrate on a nano and micro scale, and the mechanical, chemical, and thermal properties of the substrate. Titania is the preferred semiconductor photocatalytic embodiment, but the invention applies to any semiconductor that is photo-active, such as silicon, germanium, and their alloys, and compounds that include, in addition, gallium. The stress-inducing template profiles also provide a mechanical lock to the coating so that the stress can exist without causing delamination of the coating from the substrate. 
         [0152]    The aqueous source of hydrogen for the photoelectrolysis to act on can be water, sea water, an aqueous solution with electrolytes, or non-aqueous hydrogen-bearing liquids such as methanol or gasoline. 
         [0153]    While the invention has been described with reference to particular embodiments, it will be understood that the present invention is by no means limited to the particular constructions, and methods herein disclosed and/or shown in the drawings, but also comprises any modifications or equivalents within the scope of the claims. For example, the apparatus of the present invention shown in  FIGS. 1 ,  2  and  7  has a core assembly with cylindrical geometry, and tracks the sun by rotation about one axis. It will readily be apparent that this apparatus could use a core assembly with substantially spherical geometry (alternatively a planar core assembly could be used) which tracks the sun by rotation about two perpendicular axes. While a spherical core requires additional tracking of the sun, it allows for higher solar concentration than a cylindrical core so that smaller reflectors and/or core assemblies can be used. Other modifications of the specific apparatus described above will readily be apparent to those skilled in the art of light-powered photolysis and similar technologies.