Electron-hole recombination limits the efficiency of TiO2 photocatalysis. We have investigated the efficacy with which anatase/carbon nanotube (CNT) composite materials reduce charge recombination and enhance reactivity. We synthesized nanostructured assemblies composed of different proportions of anatase (5 or 100 nm) and either single-or multi-walled CNTs. The composites were prepared using a simple low temperature process in which CNTs and anatase nanoparticles were dispersed in water, dehydrated at 80 degrees C, and dried at 104 degrees C. The structures of the various TiO2/CNT composites were characterized by scanning electron microscopy (SEM) and their function was tested by phenol oxidation. Charge recombination was compared by measuring the photoluminescence spectra of select composites. We found that a nanostructured composite assembled from the 100 nm anatase and single-walled CNTs (SWCNTs) exhibited enhanced and selective photocatalytic oxidation of phenol in comparison to both pure anatase and Degussa P25. A mechanism for the enhanced reactivity is proposed in which electrons are shuttled from TiO2 particles to the SWCNTs as a result of an optimal TiO2/ CNT arrangement that stabilizes charge separation and reduces charge recombination. In addition, the SWCNT assembly provides better catalyst-support (dispersal and connection) than multi-walled CNTs.

Summary Modern civilization is dependent upon fossil fuels, a nonrenewable energy source originally provided by the storage of solar energy. Fossil fuel dependence has severe consequences including energy security issues and greenhouse gas emissions. The consequences of fossil fuel dependence could be avoided by fuel-producing artificial systems that mimic natural photosynthesis, directly converting solar energy to fuel. This review describes the three key components of solar energy conversion in photosynthesis: light harvesting, charge separation, and catalysis. These processes are compared in natural and artificial systems. Such a comparison can assist in understanding the general principles of photosynthesis and in developing working devices including photoelectrochemical cells for solar energy conversion.

Coupled CuO−TiO2 nanocomposite photocatalysts were prepared by a deposition precipitation method and were characterized with a variety of techniques. Electron paramagnetic resonance (EPR) spectroscopy was employed to study the local structures of surface/interfacial Cu2+ sites using Cu2+ as a sensitive paramagnetic probe. The addition of bulk CuO to TiO2 led to decreased photocatalytic efficiency in the degradation of methylene blue. However, doping with a very small amount of CuO (0.1 wt % copper loading) significantly enhanced the photocatalytic activity of TiO2. EPR study of the TiO2 surface revealed the presence of both highly dispersed CuO clusters and substitutional Cu2+ sites (Ti−O−Cu linkages) at 0.1 wt % copper loading. The data suggest that the Ti−O−Cu linkages contributed to the improved photooxidative activity of the 0.1% CuO−TiO2 nanocomposite. In contrast, at higher loadings the bulk form of CuO created charge recombination centers lowering the photoactivity of the composites.

Framework nitrogen atoms of carbon nitride (C3N4) can coordinate with and activate metal sites for catalysis. In this study, C3N4 was employed to harvest visible light and activate Co2+ sites, without the use of additional ligands, in photochemical CO2 reduction. Photocatalysts containing single Co2+ sites on C3N4 were prepared by a simple deposition method and demonstrated excellent activity and product selectivity toward CO formation. A turnover number of more than 200 was obtained for CO production using the synthesized photocatalyst under visible-light irradiation. Inactive cobalt oxides formed at relatively high cobalt loadings but did not alter product selectivity. Further studies with X-ray absorption spectroscopy confirmed the presence of single Co2+ sites on C3N4 and their important role in achieving selective CO2 reduction.

A novel class of derivatized acetylacetonate (acac) linkers for robust functionalization of TiO2 nanoparticles (NPs) under aqueous and oxidative conditions is reported. The resulting surface adsorbate anchors are particularly relevant to engineering photocatalytic and photovoltaic devices since they can be applied to attach a broad range of photosensitizers and photocatalytic complexes and are not affected by humidity. Acac is easily modified by CuI-mediated coupling reactions to provide a variety of scaffolds, including substituted terpy complexes (terpy = 2,2':6,2''-terpyridine), assembled with ligands coordinated to transition-metal ions. Since Mn-terpy complexes are known to be effective catalysts for oxidation chemistry, functionalization with Mn(II) is examined. This permits visible-light sensitization of TiO2 nanoparticles due to interfacial electron transfer, as evidenced by UV-vis spectroscopy of colloidal thin films and aqueous suspensions. The underlying ultrafast interfacial electron injection is complete on a subpicosecond time scale, as monitored by optical pump-terahertz probe transient measurements and computer simulations. Time-resolved measurements of the Mn(II) EPR signal at 6 K show that interfacial electron injection induces Mn(II) --> Mn(III) photooxidation, with a half-time for regeneration of the Mn(II) complex of ca. 23 s.

A synergistic effect between anatase and rutile TiO2 is known, in which the addition of rutile can remarkably enhance the photocatalytic activity of anatase in the degradation of organic contaminants. In this study, mixed-phase TiO2 nanocomposites consisting of anatase and rutile nanoparticles (NPs) were prepared for use as photoanodes in dye-sensitized solar cells (DSSCs) and were characterized by using UV-vis spectroscopy, powder X-ray diffraction and scanning electron microscopy. The addition of 10-15% rutile significantly improved light harvesting and the overall solar conversion efficiency of anatase NPs in DSSCs. The underlying mechanism for the synergistic effect in DSSCs is now explored by using time-resolved terahertz spectroscopy. It is clearly demonstrated that photo-excited electrons injected into the rutile NPs can migrate to the conduction band of anatase NPs, enhancing the photocurrent and efficiency. Interfacial electron transfer from rutile to anatase, similar to that in heterogeneous photocatalysis, is proposed to account for the synergistic effect in DSSCs. Our results further suggest that the synergistic effect can be used to explain the beneficial effect of TiCl4 treatment on DSSC efficiency.

Highly photoactive, tetrahedral Ti4+ sites can be created, other than in zeolite cavities and on silica substrate, in mixed-phase TiO2 nanocomposites. The tetrahedral Ti4+ species was shown to be an intermediate formed during the thermally driven phase transformation from anatase to rutile.

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