The photocatalytic activities of Au nanoparticle-loaded anatase (Au/anatase) and rutile (Au/rutile) for green organic synthesis are compared under illumination of UV and visible light. Whereas Au/anatase shows a higher UV-light activity for the reduction of nitrobenzene than Au/rutile, the replacement of anatase by rutile greatly increases the visible-light activity of Au/TiO2 for the oxidation of alcohols to carbonyl compounds. The quantum efficiencies (molecules produced/incident photons) for the Au/rutile and Au/anatase systems for the selective oxidation of cinnamyl alcohol to cinnamaldehyde were calculated to be 1.4 × 10–3 at λ = 585 ± 15 nm and 0.33 × 10–3 at λ = 555 ± 15 nm, respectively. This superiority of rutile over anatase as the support of Au nanoparticle (NP) plasmon photocatalyst is also confirmed in the heterosupramolecular system consisting of Au/TiO2 and a cationic surfactant. In the system using Au/rutile, a quantum efficiency of 6.8 × 10–3 at λ = 585 ± 15 nm has been achieved for the cinnamyl alcohol oxidation. Also, the plot of the visible-light activity versus Au particle size (d) for the Au/rutile system shows a volcano-shaped curve with a maximum at d ≈ 5 nm, while the activity of the Au/anatase system weakly depends on d. Photoelectrochemical measurements indicate that the Au/rutile system favors the localized surface plasmon resonance (LSPR) induced interfacial electron transfer from Au to TiO2. Further, intrinsic Fano analysis for the absorption spectra of Au/TiO2 suggests that the elongation of the LSPR lifetime with the Au NP loading on rutile is primarily responsible for the enhancement of the alcohol oxidation. We concluded that the optimum d value is determined by the factors of the LSPR absorption intensity, the interfacial electron transfer efficiency, and the surface area.

Hydrogen production from water using sunlight as an energy source is the key to constructing sustainable and clean energy systems. For this purpose, various visible light photocatalysts including metal oxides and non-metal oxides for water splitting have been developed. [1,2] However, the total conversion efficiency from solar energy to hydrogen energy remains low at present. As the alternative, photoelectrochemical cells for water splitting with TiO 2 photoanodes have continuously been pursued since the discovery of the Honda and Fujishima effect.[3] Also, the recent rapid progress in the techniques for preparing nanostructured TiO 2 films gives a great impetus to the research.[4] The conversion efficiency under UV light irradiation (320 < l < 400 nm) attains to 6.8 % by the use of a TiO 2 nanotube array photoanode.[5] The upcoming major challenge is increasing the conversion efficiency under visible light irradiation to utilize sunlight more effectively. Khan et al. has chemically prepared porous carbon-doped TiO 2 , achieving a photoconversion efficiency of 1 % by using it as a photonoade under illumination from Xe lamp.[6] Also, Ogisu et al. have reported an incident photon-to-current efficiency (IPCE) at l = 420 nm and À0.2 V (vs Ag/AgCl) of 12.9 % for hydrogen production from water containing hole scavengers (S 2À and SO 3 2À ions) in a photoelectrochemical cell with CdS quantum dot (QD)-loaded TiO 2 photoanodes.[7] Further, Gao et al. have shown under AM 1.5 simulated sunlight (138.4 mW cm À2 ) that the photocurrent at 0 V (vs Ag/AgCl) of TiO 2 nanotube array (NTA) photoanode in 0.1 mol dm À3 Na 2 S increases from 0.17 mA cm À2 up to 1.68 mA cm À2 with loading CdTe QDs, although the conversion efficiency is not given. [8] In the QD-sensitized photoelectrochemical (QD-SPEC) cells for hydrogen production, the QD-photosensitizer has so far been only limited to CdS and CdTe. However, these photoanodes only absorb the light with wavelengths below 550 nm, which should restrict their overall conversion efficiencies. On the other hand, PbS with light absorption in the whole visible range in the bulk state operates as an efficient photosensitizer for photocatalysts [9,10] and QDsensitized solar cells.[11] The narrow gap PbS photoanode needs the sacrificial electron donors to suppress its photocorrosion; however, sulfur electron donors present abundantly in nature should not devaluate the QD-SPEC cell. Thus, it can be highly expected also as the photosensitizer for the QD-SPEC cell for the hydrogen production. We have recently developed a mercaptoacetic acid (MAA)-surface modified photodeposition (PD) technique for directly coupling PbS QDs with mesoporous TiO 2 nanocrystalline films (PbS/mp-TiO 2 ), [12] while the successive ionic layer adsorption and reaction (SILAR) method has mainly been used. [13][14][15][16] The PD technique, taking advantage of the TiO 2 photocatalysis, has an important characteristic that the electron transfer from QDs to TiO 2 is inherently guaranteed.Here we report hydrogen production from...

CdS quantum dots (QDs) have been incorporated into mesoporous TiO 2 nanocrystalline films by a photodeposition (PD) technique we have recently developed [CdS(PD)/mp-TiO 2 ], and for comparison, the conventional successive ionic layer adsorption and reaction (SILAR) and self-assembled monolayer (SAM) methods have also been used for preparing the coupling system. The most important characterstic of the PD technique is that the efficicent interfacial charge transfer between the semiconductors is guaranteed because the photocatalytic redox property of TiO 2 is taken advatage of to form the heteronanojunction. The N 2 adsorption-desorption data analysis by the Barret-Joyner-Halenda method and the elemental depth profile by electron probe microanalysis showed that CdS QDs are distributed in the mesopores of the film without pore-blocking in the PD sample and with partial pore-blocking in the SILAR sample, whereas only the upper part of the film is covered with CdS QDs in the SAM sample. The PD technique enables one to control the loading amount and particle size of CdS QDs by UV-light irradiation time (λ > 320 nm) with excellent reproducibility. Owing to these unique features, sandwich-type solar cells using the CdS(PD)/mp-TiO 2 (photoanode showed a power conversion efficiency (η) under simulated sunlight (AM 1.5, 100 mW cm -2 ) of up to 2.51% more than those for the cells employing CdS(SILAR)/mp-TiO 2 (η ) 1.21%) and CdS(SAM)/mp-TiO 2 (η ) 0.14%).

At present, narrow gap semiconductor quantum dot (QD)-loaded mesoporous TiO 2 nanocrystalline films (mp-TiO 2 ) are the key material in the photoelectrochemical (PEC) devices for the conversion of solar energy to electric and chemical energy. [1,2] To prepare such coupling systems, the self-assembled monolayer (SAM) method for immobilizing QDs on the TiO 2 surface through bifunctional coupling agents (BCAs) has conveniently been used.[3] However, the loading capacity of QDs is small because their diffusion into the interior of the mesopores is difficult, while a strict size control is possible. Also, the insulating BCA molecules intervening between QD and TiO 2 could be a barrier for the interfacial electron transfer.[4] On the other hand, the photodeposition (PD) technique which takes advantage of the photoinduced properties-that is photocatalysis and surface superhydrophilicity-of TiO 2 , is applicable to synthesize the coupling systems on the nanoscale. [5][6][7][8][9][10] We have recently shown that the PD technique has the following unique and important features in the application to the mp-TiO 2 :[11] i) the electron transfer from QDs to TiO 2 is inherently guaranteed, ii) a large amount of QDs can be deposited on-not only-the external surfaces, but also the inner surfaces of mp-TiO 2 without pore-blocking, iii) the band energy of QDs are tunable by irradiation time due to the size quantization. When QD/mp-TiO 2 is applied to the PEC cells, these features should increase the light-harvesting efficiency and the electron-collection efficiency. An additional merit of the PD technique is that it affords a simple one-pot process with excellent reproducibility and this heightens its practical value. Consequently, a power conversion efficiency as high as 2.5 % has been achieved for a QD-sensitized solar cell (QD-SSC) using the CdS QD/mp-TiO 2 photoanode prepared by the PD technique.[11] A subject to be solved in the PD technique is the control of the QD size, which is of great importance for increasing the interfacial electron-transfer efficiency.[12] Additionally, while the light absorbed by CdS having a band gap (E g ) of 2.4 eV is limited to the wavelength region at l < 520 nm, PbS (E g = 0.41 eV) can absorb the light in the whole visible region, and thus, is very attractive from the viewpoint of efficient solar energy utilization. Very recently, the coupling of PbS QDs with TiO 2 nanotubes has been shown to increase the visible light photocatalytic activity for dye decomposition, [13] and the incident photon-to-current conversion efficiency (IPCE) over 35 % have been attained for PbS QD-SSC in a wide wavelength region.[14] Also, the PD of PbS on TiO 2 has been reported in the particulate system by Zhukovskiy et al.; however, the particle size distribution is fairly wide, and the detailed reaction mechanism remains unknown.[15]Here we report the ultrafast size-controlled PD of PbS QDs on TiO 2 using mercaptoacetic acid (MAA) as a surface modifier. To our knowledge, this is the first report on the surface modifi...

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