This PDF file contains the front matter associated with SPIE Proceedings Volume 6650, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

The 20th Century was the age of the Petroleum Economy while the 21st Century is certainly the age of the Solar-Hydrogen Economy. The global Solar-Hydrogen Economy that is now emerging follows a different logic. Under this new economic paradigm, new machines and methods are once again being developed while companies are restructuring. The Petroleum Economy will be briefly explored in relation to oil consumption, Hubbert's curve, and oil reserves with emphasis on the "oil crash". Concerns and criticisms about the Hydrogen Economy will be addressed by debunking some of the "hydrogen myths". There are three major driving factors for the establishment of the Solar-Hydrogen Economy, i.e. the environment, the economy with the coming "oil crash", and national security. The New Energy decentralization pathway has developed many progressive features, e.g., reducing the dependence on oil, reducing the air pollution and CO₂. The technical and economic aspects of the various Solar-Hydrogen energy options and combinations will be analyzed. A proposed 24-hour/day 200 MWe solar-hydrogen power plant for the U.S. with selected energy options will be discussed. There are fast emerging Solar Hydrogen energy infrastructures in the U.S., Europe, Japan and China. Some of the major infrastructure projects in the transportation and energy sectors will be discussed. The current and projected growth in the Solar-Hydrogen Economy through 2045 will be given.

Photocatalytic and photoelectrochemical approaches to solar hydrogen production in our group were introduced. In photocatalytic water splitting system using NiO_x/ TiO₂ powder photocatalyst with concentrated Na₂CO₃ aqueous solution, solar energy conversion efficiency to H₂ and O₂ production (STH efficiency) was 0.016%. In addition, STH efficiency of visible light responding photocatalyst, NiOx/ promoted In_0.9Ni_0.1TaO₄, was estimated at 0.03%. In photoelectrochemical system using an oxide semiconductor film phptoelectrode, STH efficiencies of meosporous TiO₂ (Anatase) , mesoporous visible light responding S-doped TiO₂ (Anatase) and WO₃ film were 0.32-0.44% at applied potential of 0.35 V vs NHE, 0.14% at 0.55 V and 0.44% at 0.9 V, respectively. Finally, solar hydrogen production by tandem cell system composed of an oxide semiconductor photoelectrode, a Pt wire counter electrode and a dye-sensitized solar cell (DSC) was investigated. As photoelectrodes, meosporous TiO₂ (Anatase), mesoporous S-doped TiO₂ (Anatase), WO₃, BiVO₄ and Fe₂O₃ film were tested. STH efficiency of tandem cell system composed of a WO₃ film photoelectrode, and a two-series-connected DSC (Voc = 1.4 V) was 2.5-2.8%. In conclusion, it is speculated that more than 5% STH efficiency will be obtained by tandem cell system composed of an oxide semiconductor photoelectrode and a two-series-connected DSC in near future. This suggests a cost-effective and practical application of this system for solar hydrogen production.

Copper chalcopyrite films exhibit properties suitable for solar energy conversion processes such as direct bandgap, and excellent carrier transport. To explore the possibilities of solar-powered hydrogen production by photoelectrolysis using these materials, we have synthesized p-type polycrystalline CuGaSe₂ films by vacuum co-evaporation of the elemental constituents, and performed physical and electrochemical characterizations of the resulting films and electrodes. Based on CuGaSe₂ material with 1.65 eV bandgap, a 2.2 micron thick electrode exhibited an outdoor 1-sun photocurrent of 16 mA/cm², while a 0.9 micron thin device still produced 12.6 mA/cm² in conjunction with vigorous gas evolution. Flatband potential measurements and bias voltage requirements for saturation photocurrents indicate a valence band position to high for practical device implementation. Future photoelectrolysis devices may be based on copper chalcopyrites with lower valence band maximum in conjunction with a suitable auxiliary junction.

The effects of metal-ion doping or replacement on the photocatalytic performance for water splitting of d¹⁰ and d⁰ metal oxides and d¹⁰ metal nitride were studied. The photocatalysts examined were (1) α-Ga_2-2xIn_2xO₃ and ZnGa_2-2xIn_2xO₄ in which In³⁺ was added to Ga₂O₃ and ZnGa₂O₄, respectively, (2) Y_xIn_2-xO₃ being a solid solution of In₂O₃ and Y₂O₃, (3) metal ion doped CeO₂, and (4) metal ion doped GaN. The photocatalytic activity of 1 wt % RuO₂-loaded α-Ga_2-2xIn_2xO₃ increased sharply with increasing x, reached a maximum at around x=0.02, and considerably decreased with further increase in x. The DFT calculation showed that the band structures of α-Ga_2-2xIn_2xO₃ had the contribution of In 4d orbital to the valence band and of In5s orbital to the conduction band. Similar effects were observed for ZnGa_2-2xIn_2xO₄. RuO₂-dispersed Y_xIn_2-xO₃ had a capability of producing H₂ and O₂ in the range x=1.0-1.5 in which the highest activity was obtained at x=1.3. The structures of both InO₆ and YO₆ octahedra were deformed in the solid solution,, and the hybridization of In5s5p and Y4d orbitals in the conduction band was enhanced. Undoped CeO₂ was photocatalytically inactive, but metal ion-doped CeO₂ showed a considerable photocatalytic activity. The activation occurred in the case that metal ions doped had larger ion sizes than that of Ce⁴⁺. The small amount doping of divalent metal ions (Zn²⁺ and Mg²⁺) converted photocatalytically inactive GaN to an efficient photocatalyst. The doping was shown to produce p-type GaN which had the large concentration and high mobility of holes. The roles of metal ion doping and replacement in the photocatalytic properties are discussed.

ZnO has been cathodically electrodeposited from oxygen-saturated ZnCl₂ solution in the presence of monosaccharides as additives. While glucose had no influence on the morphology of the deposited ZnO films, a significant influence could be seen in the presence of glucuronic acid, showing that acidic groups are essential to obtain an influence on the film growth by enabling a strong interaction between the additive and the surface of the growing ZnO. The resulting films had a porous structure probably caused by the integration of aggregated additives as confirmed by TEM measurements. In addition, the films were found to have a strong crystallographic orientation as seen in the formation of stacking disc-like particles in SEM micrographs and confirmed by a strong 100 orientation observed in XRD measurements. A comparison with earlier results obtained with phthalocyanine dyes as additives in the electrodeposition of ZnO shows that additional OH groups in the additive molecules play an important role for the formation of this crystallographic orientation.

ZnO nanowire arrays appear as one of the most promising building blocks for photoelectrochemical devices. ZnO can be efficiently sensitized to solar light absorption by lining its surface with a solar light absorber or by doping with metal transition impurities. The particular morphology of ZnO nanowires may induce light scattering, increasing solar light absorption in the sensitizer. The electrodeposition of ZnO nanowire arrays from oxygen reduction was investigated in this work using Zn²⁺ precursor salts such as ZnSO₄ and Zn(CH₃COO)₂ instead of the most frequently used ZnCl₂. Important differences in the dimensions of the obtained nanowires were observed. The influence of the adsorbing behavior of Cl^-, SO₄^- and CH₃COO^- anions on the growth mechanism was discussed depending of the Zn²⁺ precursor. The anion concentration in solution was determined not only by the zinc precursor, but also by the supporting electrolyte (NaCl, Na₂SO₄ and CH₃COONa) concentrations. By using anions that exhibit different adsorbing properties on the different ZnO crystalline faces, a new strategy was developed to tailor the dimensions of the ZnO nanowires. The effects of nanowire length on the light scattering were investigated by optical spectroscopy. An overview of the influence of these effects on the sensitization of ZnO nanowires to solar light was presented by using ZnO/CdSe core-shell nanowires as an example.

We report here two approaches that we have developed recently to help solve the problem of energy crisis and global warming facing us today. One approach is to use nanoparticles attached to the end of single-walled carbon nanotubes to catalytically convert CO₂ and CH₄ into hydrogen and carbon fibers, which can then be used in hydrogen fuel cells and as the building material in transportation vehicles and many other structures. The second approach is to use silica nanowires as templates to make nanoscale electrodes to be used in solar cells. The main advantage of this type of solar cells is that it would be easy to incorporate them directly into glass windows on all the buildings.

With hydrogen being accepted as fuel for the future, the world is looking forward to development of clean and sustainable methods of its production from renewable energy. In this context, area of research in the PEC splitting of water assumes great significance and the challenge is to develop corrosion resistant, chemically stable semiconductor that absorbs sunlight in the visible region and also has the band edges matching to the redox level of water. The advent of nanotechnology has opened new vistas in the production of semiconductor with large surface area for solar energy absorption and other favourable properties, which has lead to restudy the old workhorses, viz α-Fe₂O₃ and TiO₂ in the PEC splitting of water. This communication reports the study on metal oxides, towards the photoelectrochemical splitting of water as function of material properties and characteristics of semiconductor- electrolyte junction, viz; particle size, suitable dopants, crystalline phase, surface morphology, resistivity, bandgap, donor density and flatband potential. Effect of sensitizers and surface modification has also been investigated. Both the techniques of surface modification: (i) depositing metal dots and (ii) swift heavy ion irradiation in α-Fe₂O₃ were observed to be much effective in improving the photoresponse of the material. α-Fe₂O₃ thin films prepared using spray pyrolysis having Zn dots (dot height: 260 Å) on its surface exhibited the best of photocurrent density (1.82 mA/cm²), at 0.6 V applied bias. Nitrogen doped nanostructured TiO₂ prepared by sol gel method exhibited much better photoresponse as compared to any other dopant.

Hydrogen production using water splitting by photoelectrochemical solar cells equipped with a TiO₂ photoelectrode has been attracting much attention. However, TiO₂ encounters serious difficulty in achieving hydrogen evolution. One solution to this difficulty is using a hydrogen-producing semiconductor, such as silicon, and an oxidation reaction other than oxygen evolution, such as oxidation of iodide ions into iodine (triiodide ion). In this study, microcrystalline silicon (μc-Si:H) thin films are used as photoelectrodes in the photodecomposition of HI for low-cost and efficient production of solar hydrogen. An n-μc-3C-SiC:H and an i-μc-Si:H layer are deposited on glassy carbon substrates using the hot-wire cat-CVD method. The μc-Si:H electrodes are modified with platinum nanoparticles through electroless displacement deposition. The platinum nanoparticles improve the electrode's stability and catalytic activity. The electrodes produce hydrogen gas and iodine via photoelectrochemical decomposition of HI with no external bias under simulated solar illumination. We also attempt solar water splitting using a multi-photon system equipped with the μc-Si:H thin film and TiO₂ photoelectrodes in series.

Bandgap, band edge positions as well as the overall band structure of semiconductors are of crucial importance in photoelectrochemical and photocatalytic applications. The energy position of the band edge level can be controlled by the electronegativity of the dopants, the pH of the solution (flatband potential variation of 60 mV per pH unit), as well as by quantum confinement effects. Accordingly, band edges and bandgap can be tailored to achieve specific electronic, optical or photocatalytic properties. Synchrotron radiation with photon energy at or below 1 keV is giving new insight into such areas as condensed matter physics and extreme ultraviolet optics technology. In the soft x-ray region, the question tends to be, what are the electrons doing as they migrated between the atoms. In this paper, I will present a number of soft x-ray spectroscopic study of nanostructured 3d metal compounds Fe₂O₃ and ZnO.

Titanium dioxide films are a critical component of many next-generation low cost solar cells. Film morphology has been identified as an efficiency-limiting property. A gas phase, single-step, rapid, atmospheric-pressure process to synthesize TiO₂ films with controlled morphology is reported. The process is based on a flame aerosol reactor (FLAR). Two different morphologies were synthesized for this report, granular and columnar. The granular morphology consists of nanoparticles aggregated into fractal structures on the substrate, and is characterized by high surface area and poor electronic properties. The columnar morphology is highly crystalline; composed of 1D structures oriented normal to the substrate, characterized by lower surface area and superior electronic properties. Films with both morphologies are applied to a hydrogen-producing photo-watersplitting cell and a photovoltaic dye-sensitized solar cell. For watersplitting, the columnar morphology outperforms the granular by almost 2 orders of magnitude, achieving a uv-light to hydrogen conversion efficiency of about 11%. In contrast, for the dye-sensitized solar cell, the granular morphology outperforms the columnar, due to enhanced dye absorption arising from the larger TiO₂ surface area.

We combine first-principles density functional theory, material synthesis and characterization, and photoelectrochemical (PEC) measurements to explore methods to effectively reduce the band gap of ZnO for the application of PEC water splitting. We find that the band gap reduction of ZnO can be achieved by N and Cu incorporation into ZnO. We have successfully synthesized ZnO:N thin films with various reduced band gaps by reactive RF magnetron sputtering. We further demonstrate that heavy Cu-incorporation lead to both p-type doping and band gap significantly reduced ZnO thin films. The p-type conductivity in our ZnO:Cu films is clearly revealed by Mott-Schottky plots. The band gap reduction and photoresponse with visible light for N- and Cu-incorporated ZnO thin films are demonstrated.

Photoexcited states of metal complexes are precursors for many important photochemical processes in solution phase which lead to solar hydrogen generation. Therefore, knowing their structures with atomic resolution and sufficient time resolution is crucial in correlating structures with molecular properties. Using x-ray transient absorption (XTA) spectroscopy, transient metal oxidation states, coordination geometry, and atomic rearrangements during photochemical processes can be probed. Such an approach complements with ultrafast optical laser spectroscopy in obtaining kinetics and coherence information among different excited states as well as intra- and intermolecular energy/charge transfer processes associated with solar energy conversion. Excited state structures of transition metal complexes, such as metalloporphyrins in solution, created by photoexcitation have been studied by XTA combined with optical transient absorption spectroscopy. Direct evidences of photoinduced redox reactions and coordination geometry changes as well as electronic configurations of the metals can be observed. These experimental studies are combined with quantum mechanical calculations to rationalize the evolution of the ultrafast excited state pathways with electronic configuration changes that may be responsible for the reactivity of the molecules in solar hydrogen generation. Preliminary time-resolved X-ray absorption near edge structure (XANES) studies on Pt coated TiO₂ nanoparticles during photocatalysis show a significant potential impact of XTA in understanding solar hydrogen production.

The catalysts commonly used for the H₂ producing reaction in artificial solar systems are typically platinum or particulate platinum composites. Biological catalysts, the hydrogenases, exist in a wide-variety of microbes and are biosynthesized from abundant, non-precious metals. By virtue of a unique catalytic metallo-cluster that is composed of iron and sulfur, [FeFe]-hydrogenases are capable of catalyzing H₂ production at turnover rates of millimoles-per-second. In addition, these biological catalysts possess some of the characteristics that are desired for cost-effective solar H₂ production systems, high solubilities in aqueous solutions and low activation energies, but are sensitive to CO and O₂. We are investigating ways to merge [FeFe]-hydrogenases with a variety of organic materials and nanomaterials for the fabrication of electrodes and biohybrids as catalysts for use in artificial solar H₂ production systems. These efforts include designs that allow for the integration of [FeFe]-hydrogenase in dye-solar cells as models to measure solar conversion and H₂ production efficiencies. In support of a more fundamental understanding of [FeFe]-hydrogenase for these and other applications the role of protein structure in catalysis is being investigated. Currently there is little known about the mechanism of how these and other enzymes couple multi-electron transfer to proton reduction. To further the mechanistic understanding of [FeFe]-hydrogenases, structural models for substrate transfer are being used to create enzyme variants for biochemical analysis. Here results are presented on investigations of proton-transfer pathways in [FeFe]-hydrogenase and their interaction with single-walled carbon nanotubes.

The sonoelectrochemical method is a highly efficient technique for the synthesis of well ordered and robust titanium dioxide nanotube arrays. Self ordered arrays of TiO₂ nanotubes of various diameters and length can be rapidly synthesized under an applied potential of 5-20 V in the presence of organic electrolyte solvents like ethylene glycol. The TiO₂ nanotubes prepared in the organic electrolytes and annealed under N2 atmospheres give a TiO₂-_xC_x type of semiconductor materials having a band gap of 2.0 eV. The hybride nanotubes demonstrated promising efficiency in splitting water in the presence of solar light. In addition, the modeling of titania nanotubes using the first principles of the Density Functional Theory (DFT) approach is underway for calculating electronic properties of the TiO₂ nanotubular structure. It is envisioned that the DFT modeling will yield valuable information in developing improved titania photoanodes for high efficiency photoelectrochemical splitting of water.

Titanium dioxide, the most popular photocatalyst, is inactive under visible light, which limits the practical application of TiO₂ as a solar energy harvesting catalyst. This study investigated the photocatalytic hydrogen production on TiO₂ nanoparticles whose surface was modified in different ways. Dye-sensitized TiO₂, nafion-coated TiO₂, CdS/TiO₂ nanocomposites, and surface fluorinated/platinized TiO₂ were prepared and tested for the hydrogen generation under visible or UV irradiation. We synthesized six ruthenium sensitizers having different numbers of carboxylic (c-RuL₃) or phosphonic (p-RuL₃) linkage groups, anchored them onto TiO₂ surface, and tested their visible light reactivity for hydrogen production. p-RuL₃ with two phosphonate groups was the most efficient for hydrogen production. On the other hand, Ru(bpy)₃²⁺ (as a cationic form) whose bipyridyl ligands were not functionalized with carboxylic acid groups was bound within the nafion layer on TiO₂ through electrostatic attraction. The visible light-sensitized H₂ production on Nf/TiO₂ using Ru(bpy)₃ ²⁺ was far more efficient than that on c-RuL₃-TiO₂. The roles of nafion layer on TiO₂ in the sensitized H₂ production are proposed to be two fold: to provide binding sites for cationic sensitizers and to enhance the local activity of protons in the surface region. TiO₂ nanoparticles sensitized with CdS quantum dots were also investigated for H₂ production. Finally, the simultaneously platinized and fluorinated TiO₂ (F-Pt-TiO₂) was tested for the generation of hydrogen under UV illumination. The production of hydrogen was negligible with F-TiO₂ and Pt-TiO₂ but was significant with F-Pt-TiO₂ even in the absence of organic electron donors. The hydrogen production was highly enhanced in the presence of 4-chlorophenol, which realized the simultaneous degradation of organic substrates and the production of hydrogen.

A temperature dependent Raman scattering study was conducted for the first time on LiAlH₄ and Li₃AlH₆ in an effort to monitor the reaction kinetics of these hydrides toward its application as a hydrogen storage/delivery system for fuel cell and other applications. LiAlH₄ demonstrated a gradual softening of its Raman modes as the temperature was increased from 25 to 145 °C while Li3AlH6 demonstrated a loss in its Raman signal as the temperature increased from 30 to 90 °C. The more sensitive Raman response of Li₃AlH₆ is believed to be the result of the weaker attraction between the octahedral AlH₆^3- units in the anionic network of the Li₃AlH₆ compared to the tetrahedral AlH₄^- units of LiAlH₄.

In this paper we describe the fabrication of amorphous SiC:H materials and using them as photoelectrodes in photoelectrochemical cells (PEC). With the increase of CH₄ flow (in SiH4 gas mixture) during growth, the bandgap, Eg, increases from ~ 1.8eV to ~2.0eV, while the photoconductivity decreases from ~10^-5 S/cm to ~10^-8 S/cm. These high-quality a-SiC:H materials with Eg of 2.0eV included into a solar cell configuration led to a conversion efficiency,η~7% on textured Asahi U type SnO₂ coated substrates, with the i-layer thickness of ~300nm. For a reduced i-layer thickness of ~100 nm, a current density, J_sc ~8.45mA/cm² has been achieved, Immersing the a-SiC:H(p)/a-SiC:H(i) structure in 0.33M H₃PO₄ electrolytes, produced a photocurrent of ~7mA/cm². With a further optimization we expect that the photocurrent could exceed 9mA/cm². With the use of this configuration substrate/silicon tandem device (a-Si/a-Si or a- Si/nc-Si)/a-SiC:H(p)/a-SiC:H(i), it may therefore be possible to increase the solar-to-hydrogen (STH) efficiencies to beyond 10%.

The electrochemical and photocatalytic properties of a TiO₂ film deposited on transparent electrodes were investigated. Its electrochemical behavior was typical of an n-type semiconductor electrode. Its photocatalytic activity, investigated for phenol degradation on an optical bench (area of 1 cm², 5 mL of solution), revealed small currents (3 μA) and poor total organic carbon (TOC) removal (5 %) when the electrode was biased at + 1.1 V in the dark for 3 h. Under polychromatic irradiation, the electrode presented 25 μA of current and 12 % of phenol degradation. A better performance was achieved for photoelectrocatalytic configuration, when the electrode was irradiated and biased with + 0.6 V. Experiments done under irradiation with a metallic vapor lamp using 9 cm² electrodes and 10 mL of solution revealed that heterogeneous photocatalysis configuration (HPC) resulted in 50 % of TOC removal, while 85 % was achieved by the electro-assisted process (EHPC). Both the configurations exhibited pseudo-first order kinetics for phenol degradation, but the rate constant was two times that of EHPC. The application of a potential bias to the TiO₂ porous electrode must enhance the photogenerated electron/hole separation, which minimize the charge recombination and increases its photocatalytic activity towards organic pollutant degradation.

Nanocrystalline TiO₂ (nc-TiO₂) in the anatase phase is widely used for photo-degrading organic pollutants in a variety of environmental applications. Herein we show that slow photons in photonic crystals fashioned from nc-TiO₂ can optically amplify the photocatalytic efficiency as a result of the longer path length of light and increased probability in anatase absorption, and that the optical amplification is tolerant to some degree of disorder. We investigated the photodegradation of adsorbed methylene blue on inverse TiO₂ opals (i-nc-TiO₂-o) with different stop-band energies under monochromatic and white light irradiation. By using template spheres with different diameters, the energy of the slow photons was tuned in-and-out of the anatase electronic absorption, thereby allowing the systematic study of the effects of photonic structure on the photo-degradation efficiency of TiO₂. Under monochromatic irradiation at 370 nm, a remarkable twofold enhancement was observed for i-nc-TiO₂-o with stop-band at 345 nm, as a result of slow photon coupling at 370 nm. Under white light (>300 nm) irradiation, an increase in the photo-degradation efficiency was observed when the stop-band moves from 370 to 300 nm, as a result of slow photon coupling and the suppression of stop-band reflection by the anatase absorption. By optimizing the energy of the photonic stop-band with respect to the semiconductor electronic band gap, we effectively harvested slow photons in the dielectric part of the material to give optically amplified photochemistry. Furthermore, we studied the effect of structural disorder on the photocatalytic efficiency of the inverse opals by introducing different fractions and sizes of guest spheres into the opal template. We found that half of the enhancement originally achieved by the inverse opal made from monodispersed spheres is conserved when the domain size of the host spheres remains above a critical threshold. Such a high tolerance to structural disorder provides strong support for the potential use of inverse TiO₂ opals in environmental cleanup and water treatment applications.

Titanium dioxide nanoparticles have been prepared by solution-phase methods in the three phases that occur naturally, anatase, rutile, and brookite. The amorphous titania starting material was prepared from titanium(IV) iso-propoxide using iso-propanol as solvent and a small quantity of water. The resulting material was treated hydrothermally in an acid digestion vessel at temperatures between 175 °C and 230 °C with different reactants to obtain the three phases or controlled mixtures of two phases. The nanomaterials were characterized by a variety of techniques, including X-ray diffraction, Raman spectroscopy, electron microscopy, dynamic light scattering, and UV-Vis absorbance spectrophotometry. The results illustrate the relation between the properties of the nanoparticles in the colloid, in the powder, and in nanostructured thin films prepared with the materials. A thorough understanding of synthesis methods is essential for the preparation of nanomaterials with tailored structural, morphological, and ultimately, physical properties.

Titania nanotubes (TiNTs) with high surface area were synthesized by hydrothermal reaction under strongly basic condition and their photocatalytic application was explored. After preparing a series of CdS-TiNT composite films with variation of the mole ratio (r) of TiNT/(CdS + TiNT), their photocatalytic activities for hydrogen production and the photocurrent generation under the visible light irradiation were examined. In the aspect of light absorption for the photocatalytic reaction, the CdS-TiNT composite films revealed similar amounts of absorption to their counterparts, i.e., CdS-TiO₂ particulate series. However, the former showed less significant synergistic effect in the photocurrent generation and lower photocatalytic activities compared to the latter. Consequently, it appears that TiNTs are not so effective photocatalyic material in spite of their larger surface areas rather than TiO₂ nano-particles, because they indicate a poor crystallinity and a less intimate interaction or contact with CdS particles owing to the tubular morphology and a readily agglomeration among themselves.

The green microalga Chlamydomonas reinhardtii is proposed to produce hydrogen in a low-cost system using the solar radiation in Yucatan, Mexico. A two-step process is necessary with a closed photobioreactor, in which the algae are firstly growth and then induced for hydrogen generation. Preliminary results are presented in this work with some planning for the future. Different culture broths, temperatures and light intensities were tested for biomass and hydrogen production in laboratory conditions. The first experiments in external conditions with solar radiation and without temperature control have been performed, showing the potential of this technique at larger scales. However, some additional work must be done in order to optimize the culture maintenance, particularly in relation with the temperature control, the light radiation and the carbon dioxide supply, with the idea of keeping an economic production.

Production of hydrogen by water splitting using solar energy is one of the long sought goals of hydrogen economy. Approximately 33% of solar radiation is emitted as high energy photons while the remaining 67% consists of primarily thermal energy. Utilization of both thermal and photonic energies within the solar spectrum is essential for achieving water splitting at high efficiency. At FSEC, we have developed a solar-thermochemical water splitting cycle for the production of hydrogen. In this cycle, the photonic portion of solar irradiance is diverted and used to drive the hydrogen production step, while solar thermal portion drives the oxygen generation step of the cycle. The photocatalytic hydrogen production step of the cycle employs aqueous ammonium sulfite solution that is oxidized to ammonium sulfate in the presence of nanosized photocatalysts. We have developed a technique for the preparation of polymer encapsulated nanosize photocatalysts that show high activity toward oxidation of ammonium sulfite aqueous solution. The use of nano-scale and defect free photocatalysts hinder the recombination of photo-generated electron-hole pairs, thereby increasing solar to hydrogen energy conversion efficiency.

Single-walled carbon nanotubes (SWNT) are promising candidates for use in energy conversion devices as an active photo-collecting elements, for dissociation of bound excitons and charge-transfer from photo-excited chromophores, or as molecular wires to transport charge. Hydrogenases are enzymes that efficiently catalyze the reduction of protons from a variety of electron donors to produce molecular hydrogen. Hydrogenases together with SWNT suggest a novel biohybrid material for direct conversion of sunlight into H₂. Here, we report changes in SWNT optical properties upon addition of recombinant [FeFe] hydrogenases from Clostridium acetobutylicum and Chlamydomonas reinhardtii. We find evidence that novel and stable charge-transfer complexes are formed under conditions of the hydrogenase catalytic turnover, providing spectroscopic handles for further study and application of this hybrid system.