Plasmon-Enhanced Photocatalytic Activity of Cadmium Sulfide

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Plasmon-Enhanced Photocatalytic Activity of Cadmium Sulfide Nanoparticle Immobilized on Silica-Coated Gold Particles Tsukasa Torimoto,*,†,‡ Hiroki Horibe,† Tatsuya Kameyama,† Ken-ichi Okazaki,† Shigeru Ikeda,§ Michio Matsumura,§ Akira Ishikawa,|| and Hajime Ishihara*,^ †

Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan § Research Center for Solar Energy Chemistry, Osaka University, Toyonaka, Osaka 560-8531, Japan Interdisciplinary Graduate School of Medicine and Engineering, The University of Yamanashi, Takeda 4-3-11, Kofu 400-8511, Japan ^ Graduate School of Engineering, Osaka Prefecture University, Gakuencho 1-1, Nakaku 599-8531, Sakai, Osaka, Japan

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bS Supporting Information ABSTRACT: Nanocomposite photocatalysts of CdS nanoparticles immobilized on Au core SiO2 shell particles, in which the SiO2 layer acts as an insulator layer to prevent direct electron transfer from photoexcited CdS to Au particles, were prepared. The photocatalytic activity of CdS particles for H2 evolution was greatly dependent on the distance between CdS and Au particles, due to the locally enhanced electric field produced by photoexcitation of the localized surface plasmon resonance (LSPR) peak of Au particles. Increase in Au core size enlarged the optimal distance between Au and CdS for the enhancement of photocatalysis. This behavior was theoretically reproduced by solving a self-consistent equation system, in which the range of energy dissipation became wider for larger diameter of the Au sphere, and then the balance with the enhancement of photoexcitation of CdS particles by the LSPR-induced electric field was changed. SECTION: Nanoparticles and Nanostructures

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unable physicochemical properties of size-quantized semiconductor nanoparticles (less than ca. 10 nm in size) have attracted much attention for applications to solar-light energy conversion systems or the development of new organic synthetic routes.1 8 Photogenerated electrons and holes in these nanoparticles exhibited higher reducing and oxidizing powers, respectively, than those in bulk particles, the degree being enhanced with a decrease in particle size. However, the obtained quantum yield was not high enough for practical applications such as application to efficient light energy conversion systems. One possible strategy for further improving quantum yield is to increase the efficiency of light absorption. Utilization of the electric field produced by the photoexcitation of localized surface plasmons in metal nanoparticles is one of the promising techniques. It is well-known that a metal particle of Au or Ag having an intense localized surface plasmon resonance (LSPR) peak can act as a nanoantenna for light trapping, resulting in photoexcitation of the LSPR peak to form a locally enhanced electric field in the proximity of metal nanoparticles.9 12 The LSPR-induced electric field can enhance the photoexcitation of chromophores near metal particles. For example, the photoluminescence (PL) intensity of semiconductor nanoparticles or organic dyes in the proximity of Au or Ag particles was greatly enhanced due to the LSPR-induced electric field.13 16 Furthermore, irradiation of dye molecules immobilized near Au nanoparticles produced a much larger photocurrent than that produced by irradiation of dye r 2011 American Chemical Society

molecules without Au particles.17 On the other hand, nonradiative energy transfer predominantly occurred from photoexcited semiconductor nanoparticles to metal when semiconductor nanoparticles were located close to metal particles, resulting in quenching of photoexcited semiconductor nanoparticles.14,18,19 Therefore, considering that the intensity of the LSPR-induced electric field decays with increase in distance from the metal surface, photocatalysts of semiconductor nanoparticles are expected to be effectively photoexcited if the distance between metal and semiconductor nanoparticles is appropriately adjusted. Watanabe et al.20 have recently reported that photodegradation of organic dyes in the presence of O2 molecules was enhanced by using TiO2 photocatalyst films deposited on SiO2-coated Ag nanoparticles due to LSPR-induced electric fields (plasmonic photocatalyst). However, the details, especially details regarding the influence of the semiconductor metal particle distance on activities of photocatalysts, are not known. Furthermore, although the intensity of LSPR-induced electric field is well-known to be dependent on the size of metal particles,9 13 to the best of our knowledge, little is known about the effect of the size of metal particles used as a nanoantenna on the photoactivities of semiconductor nanoparticles. Received: July 5, 2011 Accepted: July 26, 2011 Published: July 26, 2011 2057

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The Journal of Physical Chemistry Letters In this study, we prepared core shell-structured photocatalysts for efficient photoexcitation of semiconductor nanoparticles with an LSPR-induced electric field, with CdS nanoparticles being deposited on SiO2-coated Au particles to avoid direct electron transfer between Au and CdS particles. The photocatalytic activity of the CdS particles for H2 evolution varied depending on both the distance between CdS and Au particles and the size of Au cores. We, for the first time, found that the optimal distance between CdS and Au was remarkably enlarged with an increase in the Au particle size, the degree being quantitatively reproduced by the theoretical calculation. Direct deposition of Au on CdS photocatalysts is not suitable for the present study because photoinduced electron transfer can occur from CdS to Au particles on which photocatalytic reactions proceeded, and then the effect of the LSPR-induced electric field on the photocatalytic reactions may be modified. Silica-coated gold nanoparticles (Au@SiO2) were prepared by our previously reported method with a slight modification.21 Au nanoparticles having different average diameters, 19 and 73 nm, were prepared by the citrate reduction method22 and the seedmediated growth method,23 respectively, and used as core particles. After surface modification of the Au particles with 3-aminopropyltrimethoxy silane, the particles were uniformly covered with an amorphous SiO2 layer by hydrolysis of tetramethoxy silane (TMOS). The SiO2 shell thickness in the resulting Au@SiO2 particles could be controlled from 0.3 to 73 nm by varying the amount of TMOS added. Nanocomposite photocatalysts composed of CdS and Au particles with various Au CdS distances were prepared by deposition of CdS on Au@SiO2 particles (Figure 1a). Starting CdS particles having an average diameter of 5.0 nm were prepared by an Aerosol OT (AOT)-reversed-micelle method with a concentration ratio w (= [H2O]/[AOT]) of 12, and their surface was modified with 3-mercaptopropyltrimethoxysilane (MPTS) in a toluene solution as reported in our previous papers.24 The thusobtained MPTS-modified CdS particles (MPTS-CdS) were accumulated on Au@SiO2 particles. The calculated amount of Au@SiO2 particles suspended in 2-propanol was added to a 100 cm3 portion of MPTS-CdS toluene solution ([CdS] = 3.3 mmol dm 3), followed by refluxing the solution for 30 min, in which the atomic ratio of Cd to Au was fixed to 90. Hydrolysis of the remaining trimethoxysilyl groups was performed by dropwise addition of 100 cm3 of water to the refluxed solution, resulting in the formation of a thin SiO2 layer on the surface of CdS particles (CdS@SiO2) that were immobilized on Au@SiO2 particles. Thus-obtained CdS@SiO2-deposited Au@SiO2 particles had a semiconductor/insulator/metal structure and are denoted here as CdS@SiO2//Au@SiO2. After the suspension had been subjected to centrifugation, the obtained precipitates were washed several times with 2-propanol and then suspended in a 2-propanol-water (1:1) mixture solution. For comparison, SiO2-coated CdS particles (CdS@SiO2) were prepared without addition of Au@SiO2, the thickness of the SiO2 layer on the CdS particle being ca. 0.3 nm as reported previously.6,24 Photocatalytic hydrogen (H2) evolution was performed by irradiation light from a Xe lamp (λ > 350 nm) with light intensity of ca. 100 mW/cm2. Each photocatalyst containing 2.3 mg of CdS was suspended in an aqueous solution (5 cm3) containing 2-propanol (50 vol%) and irradiated under an argon atmosphere at a room temperature with vigorous magnetic stirring. Rhodium (Rh) as a cocatalyst was directly deposited onto CdS particles through photocatalytic reduction of Rh3+ ions by CdS particles.

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Figure 1. (a) Schematic illustration of the immobilization of CdS@SiO2 on Au@SiO2 particles. (b,c) TEM images of Au@SiO2 prepared with Au particles of 19 nm in diameter as a core (b) and CdS@SiO2-deposited Au@SiO2 particles (CdS@SiO2//Au@SiO2) (c).

Figure 2. Time courses of hydrogen evolution by photocatalysts of CdS@SiO2 (solid circles) and CdS@SiO2//Au@SiO2 particles with SiO2 layer thicknesses of 17 nm (open circles) and 2.8 nm (open squares) on Au cores. The experiments were performed by irradiation light from a 300-W Xe lamp (λ > 350 nm).

During photocatalytic H2 evolution, electrons photogenerated in CdS particles were mainly transferred to Rh particles and then consumed by the reduction of H+ ions.6 The amount of Rh loading was 0.37 atm % based on that of Cd atoms in the composites. Figure 1b shows a typical transmission electron microscopy (TEM) image of as-prepared Au@SiO2 particles with an Au core size of 19 nm and SiO2 shell thickness of 17 nm. The surface of Au particles was uniformly coated with an amorphous SiO2 layer. Hydrolyzing trimethoxysilyl groups of MPTS-CdS in the presence of Au@SiO2 gave nanocomposites, as shown in Figure 1c, in which CdS@SiO2 particles having a CdS core of ca. 5 nm diameter were densely immobilized on the SiO2 shell surface of Au@SiO2 particles, although some of the CdS@SiO2 particles formed aggregated secondary particles in the solution without accumulating on the Au@SiO2 particles, as observed in TEM images in a large area (Figure S1, Supporting Information). The size of the Au core and the SiO2 shell thickness seemed to be unchanged after immobilization of CdS@SiO2 particles. Figure 2 shows the time course of H2 evolution by irradiation to suspensions of CdS@SiO2//Au@SiO2 having SiO2 shell layers of various thicknesses on the Au core. For comparison, results for CdS@SiO2 particles are also shown. An almost linear increase in the amount of H2 was observed with irradiation, regardless of the 2058

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Figure 3. (a) Action spectra of H2 evolution of CdS@SiO2 (solid circles) and CdS@SiO2//Au@SiO2 particles with Au core size of 19 nm and SiO2 thickness of 17 nm (open circles). (inset) Absorption spectrum of CdS nanoparticles before SiO2 coating. (b) Wavelength dependence of fenhance obtained from the results shown in panel a. The extinction spectrum of Au@SiO2 used is shown as a solid line.

kind of photocatalyst, indicating that CdS particles worked as a stable photocatalyst under the experimental conditions used in this study. Although the simple particle mixture of CdS@SiO2 and Au@SiO2 did not cause any enhancement or retardation of H2 evolution rate (not shown), immobilization of CdS@SiO2 particles on Au@SiO2 caused the change in the photocatalytic activity of CdS particles, being dependent on the shell thickness of Au@SiO2: the H2 evolution rate (R(H2)), which was calculated from the slope of the linear time-course curves in Figure 2, became larger with immobilization of CdS particles on Au@SiO2 having an SiO2 layer thickness of 17 nm, while deposition of CdS particles on a thin SiO2 layer of 2.8 nm thickness on the Au core caused a decrease in photocatalytic activity in comparison with CdS@SiO2 particles only. Action spectra of photocatalytic reactions were obtained for CdS@SiO2 and CdS@SiO2//Au@SiO2 particles with an SiO2 layer thickness of 17 nm (Figure 3a). The expansion of wavelength region between 450 and 530 nm in Figure 3a was shown in Figure S2. The apparent quantum yield (Φapp) was calculated as the ratio of the rate of photogenerated electron consumption for H2 generation to the flux of incident photons. In both cases, H2 evolution was observed with irradiation of monochromatic light with a wavelength of less than 510 nm, and Φapp increased with a decrease in the wavelength of irradiation light. Since the absorption onset of CdS particles used (ca. 520 nm) was in good agreement with those of the action spectra, these facts indicated that the photoexcitation of CdS was prerequisite for photocatalytic H2 evolution in the presence or absence of Au particles; thus the photoexcitation of CdS particles via a multiphoton process scarcely occurred in the present reaction conditions, even if the LSPR peak of Au particles was photoexcited by the photons having an energy lower than the energy gap of CdS, that is, a light of wavelength longer than 530 nm. Ohtani et al. reported25 that photoexcitation of the LSPR peak of Au particles in Au-deposited

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TiO2 photocatalysts caused injection of electrons in Au particles to the conduction band of TiO2 particles, that is, photosensitization of TiO2 particles with Au particles, resulting in photocatalytic oxidation of organic compounds in the presence of O2 molecules. In these cases, the action spectra of the photocatalytic reactions agreed well with profiles of LSPR peaks in extinction spectra of Au particles loaded on TiO2. However, this was not true in the present case; the action spectra of H2 evolution for CdS@SiO2// Au@SiO2 corresponded to the absorption spectra of CdS but not the extinction spectra of Au particles, suggesting that direct electron transfer from Au to CdS could not occur even if LSPR peaks of Au particles were photoexcited, due to the presence of the insulator layer of an SiO2 shell on the Au particle surface. It should be noted that Φapp in Figure 3a was enhanced by the deposition of CdS@SiO2 on Au@SiO2 at any wavelength of monochromatic light irradiation. Since the diffuse reflectance spectra (Figure S3) showed that the absorption spectrum of CdS@SiO2 with wavelength less than ca. 500 nm was very similar to that of CdS@SiO2//Au@SiO2, the photocatalyst particles were assumed to have almost the same properties about light scattering and absorption in this wavelength region, regardless of the presence of Au particles, suggesting that the observed increase in Φapp in the presence of Au particles (Figure 3a) could not be attributed to the changes in the optical properties of CdS particles before and after the immobilization on SiO2-coated Au particles. For quantitative analyses, we calculated the enhancement factor (fenhance), which was defined in this study as the ratio of R(H2) of CdS@SiO2//Au@SiO2 nanocomposites to that of CdS@SiO2 particles. Figure 3b shows the dependence of fenhance on the wavelength of irradiation light. The spectral profile of fenhance corresponded to the LSPR peak observed in the extinction spectrum of Au@SiO2 used, indicating that photoexcitation of the LSPR peak of Au core particles played an important role in the enhancement of R(H2) for CdS particle photocatalysts. It would be interesting to clarify the dependence of CdS photocatalytic activity on interparticle spacing. The distance between the surfaces of CdS particles and Au core (dCdS-Au) was reasonably assumed to be the sum of SiO2 shell thickness of CdS@SiO2 (0.3 nm) and that of Au@SiO2. For composite photocatalysts having an Au core of 19 nm diameter, the dependence of fenhance on dCdS-Au is shown in Figure 4a, in which fenhance was calculated from R(H2) with irradiation of light from a Xe lamp (λ > 350 nm). The value of fenhance increased with an increase in dCdS-Au and then became larger than unity around dCdS-Au of ca. 10 nm. The optimum value of fenhance (1.5) was obtained at dCdS-Au of 17 nm. Further increase in dCdS-Au resulted in a gradual decrease in fenhance to ca. 1 beyond dCdS-Au of ca. 20 nm. The observed change in photocatalytic activity of CdS particles was probably due to energetic interactions between CdS and Au particles (but not electronic interactions). It has been reported that F€orster-type energy transfer from semiconductor nanoparticles to Au particles easily occurred when semiconductor nanoparticles were located in the close vicinity of metal particles, resulting in the quenching of semiconductor particles.14,18,19 Therefore, in the case where CdS particles were immobilized on the SiO2 surface with dCdS-Au smaller than ca. 10 nm, energy transfer from photoexcited CdS to Au particles predominantly occurred, resulting in attenuation of photocatalytic activity of the CdS particles, that is, fenhance < 1. On the other hand, it is wellknown that a metal particle of Au or Ag having an intense LSPR peak can act as a nanoantenna for light trapping, resulting in photoexcitation of the LSPR peak to form a locally enhanced 2059

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Figure 4. (a) Experimentally obtained dependence of fenhance for H2 evolution rate on dCdS-Au. Photocatalysts used were CdS@SiO2-deposited Au@SiO2 particles with Au cores of 19 nm (solid circles) or 73 nm (open squares). (b) Theoretically predicted (solid circles) and experimentally obtained optimal dCdS-Au (open triangles) for the most effective fenhance as a function of Au core size.

electric field in the proximity of metal nanoparticles.9 13 When the LSPR peak was photoexcited by photons having larger energy than the energy gap of CdS, the resulting LSPR-induced electric field could excite CdS particles. This was supported by the wavelength-dependence of fenhance (Figure 3b). Since the probability of energy transfer drastically declined with an increase in dCdS-Au, the photoexcitation of CdS caused by the LSPR-induced local electric field became remarkable, resulting in fenhance larger than 1. Furthermore, since the LSPR-induced local electric field in the vicinity of Au particles is well-known to decay with increase in distance from the metal surface, it is reasonable that further increase in dCdS-Au resulted in a decline in excitation probability of CdS and fenhance becoming unity. Furthermore, we investigated the influence of Au core size on the photocatalytic activity of CdS@SiO2//Au@SiO2 nanocomposites. R(H2) was similarly enhanced even if the size of the Au core used in Au@SiO2 was increased from 19 to 73 nm. However, the observed dependence of fenhance on dCdS-Au for nanocomposites having an Au core size of 73 nm became much wider than that for an Au core size of 19 nm (Figure 4a); an Au core of 73 nm gave the optimum fenhance of 1.5 at dCdS-Au= 36 nm, which decreased with further increase in dCdS-Au until fenhance was ca. 1 at dCdS-Au larger than ca. 50 nm. The wavelength dependence of fenhance at dCdS-Au= 36 nm also agreed well with the extinction spectrum of Au@SiO2 used (Au size of 73 nm) (Figure S4). In order to elucidate the different dCdS-Au dependence of fenhance for different Au core sizes, we performed theoretical calculation, based on the aforementioned mechanism. Using Green’s function into which the Drude-type dielectric function of an Au sphere is renormalized, we solved the self-consistent equation system comprising Maxwell’s equations and the equation of motion of the excited state in the CdS particle, which provides information on the selfconsistent response field of the system, the population of excited CdS particles, and the dissipation rate of the excitation (details are given in the Supporting Information). In the dielectric function

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of the Au sphere, we assume the plasma frequency of 8.8 eV, the nonradiative bulk damping constant of γP = 18 meV,26 and the standard values of other parameters of Au including those in the correction term of the damping constant for a nanoscale sphere.27 29 From Figure 3a, it is considered that absorption around λ = 350 nm is the most dominant contribution to photoexcitation of the CdS particle, and, therefore, the essential physics can be discussed by modeling the CdS particle as a representative single two-level system with a point dipole moment whose resonance energy is 3.54 eV (corresponding to λ = 350 nm). In this calculation, the dipole moment and the dephasing constant of the model CdS particle are deduced to 25 D and 0.1 meV, respectively, considering the correspondence of the theoretical estimation by our simple model to the experimental values of absorption coefficient of 5.26 cm 1 of CdS particles with a density of 1.49  1018 CdS-particles dm 3 at a photon energy of 3.54 eV. The configuration (dCdS-Au, θ, ϕ) of the CdS particle on the surface of Au@SiO2 is set to the optimal angle (θ0, ϕ0), wherein the electric field is enhanced most strongly. Figure S5 shows the relationship between the population of excited CdS particles and dCdS-Au for the several diameters of the Au sphere. A volcano-type dependence is obtained regardless of the Au particle size. The population increases for CdS particles approaching the Au surface. On the other hand, the population decreases for CdS particles that approach too closely, where quenching is due to energy transfer to the Au sphere and dissipation therein by the plasmon damping. As evidence of this, the quenching vanished when we set the whole nonradiative damping constant to zero. Figure 4b shows the theoretical relationship between optimal dCdS-Au for the photoexcitation of CdS particles and diameter of Au particles. The figure also shows experimental results obtained from Figure 4a. It is clearly shown that an increase in the diameter of Au particles increases the optimal dCdS-Au. The reason why the peak position shifts in a positive direction of the distance dCdS-Au for a larger Au sphere is that the range of energy dissipation becomes wider for a larger diameter of the Au sphere, due to a more extensive electric field of induced dipole in Au particles, and the balance with the enhancement of photoexcitation is changed, as confirmed by our calculation. This change of the balance is represented by Figure S6 and is qualitatively interpreted as follows: The excitation of Au sphere can be roughly regarded as the production of a dipole. Generally, the larger dipole provides higher electric field at a certain distance from the dipole. Therefore, CdS particles are more effectively excited with LSPRinduced electric field at a certain dCdS-Au for a larger Au sphere (Figure S6a) because it provides the larger dipole. On the other hand, the energy dissipation, that is, quenching of the photoexcited CdS particles occurs through the interaction with the electric field formed by this dipole, and hence, the spatial range of energy dissipation is also enlarged with an increase in the size of the Au sphere (Figure S6b). Although the present simulation by the simplified model leaves discussion of quantitative details as a future subject, our self-consistent calculation explains the essential profile of different dCdS-Au dependences for different diameters of the Au core. Furthermore, there is good agreement in Figure 4b between theoretical and experimental results. This fact confirms that the observed enhancement of photocatalytic H2 evolution was induced by enlargement of the population of excited CdS particles, that is, by photoexcitation of CdS nanoparticles by the LSPR-induced electric fields formed near Au particles. 2060

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The Journal of Physical Chemistry Letters In conclusion, we have successfully demonstrated that nanocomposite photocatalysts composed of Au particles and CdS particles separated by SiO2 insulating layers exhibited enhanced photocatalytic activity compared to that of CdS nanoparticles alone, due to effective photoexcitation of CdS particles with the LSPR-induced electric field in proximity to Au particles. The optimal distance between chromophores and metal particles for the enhancement of excitation was enlarged with an increase in the Au core size. Our findings of these facts and underlying mechanisms will be essentially important for designing and fabricating the novel plasmonic light-trapping systems. Furthermore, the wavelength of the LSPR peak is tunable in a wide range of visible and IR wavelength regions, depending on the shape and size of metal nanoparticles. Therefore, preparation of nanocomposites consisting of semiconductor and metal nanoparticles, both of which have an optical response in the IR region, will be a promising strategy for the fabrication of solar light energy conversion systems, such as photocatalysts and quantum dot solar cells. A study along this line is currently in progress.

’ ASSOCIATED CONTENT

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Supporting Information. Figures S1 S4 and theoretical calculation details associated with Figures S5 and S6. This information is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (T.T.); ishi@pe. osakafu-u.ac.jp (H.I.).

’ ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas “Strong Photon-Molecule Coupling Fields (No. 470)”, a Funding Program for Next Generation World-Leading Researchers from the Japan Society for the Promotion of Science, and Special Coordination Funds for Promoting Science and Technology from the Ministry of Education, Culture, Sports, Science and Technology of Japan. ’ REFERENCES (1) Kamat, P. V. Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion. J. Phys. Chem. C 2007, 111, 2834–2860. (2) Nozik, A. J.; Beard, M. C.; Luther, J. M.; Law, M.; Ellingson, R. J.; Johnson, J. C. Semiconductor Quantum Dots and Quantum Dot Arrays and Applications of Multiple Exciton Generation to Third-Generation Photovoltaic Solar Cells. Chem. Rev. 2010, 110, 6873–6890. (3) Sambur, J. B.; Novet, T.; Parkinson, B. A. Multiple Exciton Collection in a Sensitized Photovoltaic System. Science 2010, 330, 63–66. (4) Hoffman, A. J.; Carraway, E. R.; Hoffmann, M. R. Photocatalytic Production of H2O2 and Organic Peroxides on Quantum-Sized Semiconductor Colloids. Environ. Sci. Technol. 1994, 28, 776–785. (5) Silva, L. A.; Ryu, S. Y.; Choi, J.; Choi, W.; Hoffmann, M. R. Photocatalytic Hydrogen Production with Visible Light over Pt-Interlinked Hybrid Composites of Cubic-Phase and Hexagonal-Phase CdS. J. Phys. Chem. C 2008, 112, 12069–12073. (6) Pal, B.; Torimoto, T.; Iwasaki, K.; Shibayama, T.; Takahashi, H.; Ohtani, B. Size and Structure-Dependent Photocatalytic Activity of Jingle-Bell-Shaped Silica-Coated Cadmium Sulfide Nanoparticles for Methanol Dehydrogenation. J. Phys. Chem. B 2004, 108, 18670–18674.

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