Tuning the photonic band gap (PBG) to the electronic band gap (EBG) of Au/TiO2 catalysts resulted in considerable enhancement of the photocatalytic water splitting to hydrogen under direct sunlight. Au/TiO2 (PBG-357 nm) photocatalyst exhibited superior photocatalytic performance under both UV and sunlight compared to the Au/TiO2 (PBG-585 nm) photocatalyst and both are higher than Au/TiO2 without the 3 dimensionally ordered macro-porous structure materials. The very high photocatalytic activity is attributed to suppression of a fraction of electron-hole recombination route due to the co-incidence of the PBG with the EBG of TiO2 These materials that maintain their activity with very small amount of sacrificial agents (down to 0.5 vol.% of ethanol) are poised to find direct applications because of their high activity, low cost of the process, simplicity and stability.

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Probing into the relationship between charge carrier dynamics and photocatalytic reactions is central to the understanding and therefore the design of photocatalysts. We have studied the charge carrier lifetime of Pt/C3N4 and compared it to that of C3N4 alone using femtosecond pump–probe transient absorption spectroscopy in the 500–900 nm signal range. Parallel photocatalytic reactions were also conducted to link the extracted lifetime to the corresponding rates of hydrogen production from water. The presence of Pt (with a mean particle size of ca. 2.5 nm) on C3N4 decreased the lifetime of excited electrons in the conduction band (CB). This has occurred in pure water as well as in the presence of the organic sacrificial agent used. These results suggest that Pt particles on this n-type semiconductor act as electron trap centers (either by pumping away CB electrons or by creating trap centers at the interface). The corresponding increase in reaction rates can be linked to this electron transfer.

The main characteristics of noble metals on semiconductor photocatalysts within the context of hydrogen ions reduction are particles’ size, electronic structure, and dispersion. In this work, we have systematically studied Au, Pd, and Pt particles on TiO2 with mean diameters of 5.2–5.8, ca. 2, and 1.2–1.5 nm, respectively, at different coverages. Ethanol photo-reforming with water was used as the example from which extraction of reaction rates per mass, per particle, and per atom has been analyzed. Irrespective of the metal nature, a narrow range for maximum catalytic performance was observed when the rate is normalized per unit mass or unit mole. Starting from a very low metal content (0.25 wt % for each metal), the H2 production rates decreased with increasing number of Pd, Pt, or Au particles. However, the highest rate per particle is that of gold at any metal coverage. This rate exceeded by 2 orders of magnitude that of Pt and by 1 order of magnitude that of Pd. These results indicate that, unlike the case of thermal catalytic reaction, large particles perform better than small particles. Extraction of reaction rates from this study and from previous studies on Ni and Ag deposited on TiO2 indicated a direct relationship with the work function of the metals and a volcano-shape relationship with their d-band center position.

Despite many observations that plasmonics can enhance photocatalytic reactions, their relative role in the overall reaction rate is not thoroughly investigated. Here we report that silver nanoparticles contribution in the reaction rate by its plasmonic effect is negligible when compared to that of Pd (Schottky effect). To conduct the study a series of AgÀPd/TiO 2 catalysts have been prepared, characterized and tested for H 2 production from water in the presence of an organic sacrificial agent. Pd was chosen as a standard high work function metal needed for the Schottky junction to pump away electrons from the conduction band of the semiconductor and Ag (whose work function is ca. 1 eV lower than that of Pd) for its high plasmonic resonance response at the edge of the bandgap of TiO 2 . While H 2 production rates showed linear dependency on plasmonic response of Ag in the PdÀAg series, the system performed less than that of pure Pd. In other words, the plasmonic contribution of Ag in the AgÀPd/TiO 2 catalyst for hydrogen production, while confirmed using different excitation energies, is small. Therefore, the "possible" synergistic effect of plasmonic (in the case of Ag) and Schottky-mechanism (in the case of Pd) is minor when compared to that of Schottkyeffect alone.

The effect of electrode area, electrolyte concentration, temperature, and light intensity (up to 218 sun) on PV electrolysis of water is studied using a high concentrated triple-junction (3-J) photovoltaic cell (PV) connected directly to an alkaline membrane electrolyzer (EC). For a given current, the voltage requirement to run an electrolyzer increases with a decrease in electrode sizes (4.5, 2.0, 0.5, and 0.25 cm2) due to high current densities. The high current density operation leads to high Ohmic losses, most probably due to the concentration gradient and bubble formation. The EC operating parameters including the electrolyte concentration and temperature reduce the voltage requirement by improving the thermodynamics, kinetics, and transport properties of the overall electrolysis process. For a direct PV–EC coupling, the maximum power point of PV (P max) is matched using EC I–V (current–voltage) curves measured for different electrode sizes. A shift in the EC I–V curves toward open-circuit voltage (V oc) reduces the P op (operating power) to hydrogen efficiencies due to the increased voltage losses above the equilibrium water-splitting potential. The solar-to-hydrogen (STH) efficiencies remained comparable (∼16%) for all electrode sizes when the operating current (I op) was similar to the short-circuit current (I sc) irrespective of the operating voltage (V op), electrolyzer temperature, and electrolyte concentration.

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The photocatalytic water splitting activity of nanocomposite photocatalysts of TiO 2 with CoO x was studied under UV and visible light, and the catalysts were characterized by XRD, XPS, and UV-vis techniques. The presence of CoO x enhances the hydrogen production activity of TiO 2 by five times at an optimal loading of. 2 wt. %. To investigate the role of CoO x , the photocatalytic activity was also studied under visible light and with different amounts of sacrificial agent. Our results indicate that the increasing activity was not due to increasing absorption of the visible light but most likely due to the role of CoO x nanoparticles as hole scavengers at the interface with TiO 2. XPS Co2p analyses of CoO/TiO 2 showed a considerable decrease in their signal after prolonged reaction time (44 h) when compared to that of the fresh catalyst. Because part of Co 2+ cations is dissolved in solution, in neutral or acidic pH, the possible increase in the reaction rate upon their addition to TiO 2 under UV excitation was investigated. No change in the reaction rate was observed upon, on purpose, addition Co 2+ cations to TiO 2 under UV excitation. Thus, one may rule out the reduction of Co 2+ to Co 0 with excited electrons within TiO 2. In order to further increase the reaction rate, we have synthesized and tested a hybrid system composed of CoO and Pd nanoparticles (Pd wt. % = 0.1, 0.3, 0.5, and 1 wt. %) where 0.3 wt. % Pd-2 wt. % CoO/TiO 2 showed the highest rate.