Synthesis and Characterization of Semiconductor Tantalum Nitride

Dec 13, 2010 - Department of Chemical Engineering, UniVersity of Illinois at Chicago, 810 South Clinton Street, Chicago,. Illinois 60607-4431, United ...
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J. Phys. Chem. C 2011, 115, 647–652

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Synthesis and Characterization of Semiconductor Tantalum Nitride Nanoparticles Chiun-Teh Ho,§ Ke-Bin Low,‡ Robert F. Klie,| Kazuhiko Maeda,⊥,≈ Kazunari Domen,⊥ Randall J. Meyer,§,* and Preston T. Snee†,* Department of Chemical Engineering, UniVersity of Illinois at Chicago, 810 South Clinton Street, Chicago, Illinois 60607-4431, United States, Research Resource Center, 845 West Taylor Street, Chicago, Illinois 60607-7058, United States, Department of Physics, UniVersity of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607-7056, United States, Department of Chemical System Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan, Department of Chemistry, UniVersity of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607-7056, United States ReceiVed: October 21, 2010; ReVised Manuscript ReceiVed: NoVember 29, 2010

We have developed colloidal synthesis methods to create nanoparticles (NPs) of tantalum nitride. The particle sizes and crystallinity can be controlled through the use of different organic solvents and reaction times; we demonstrate here NPs ranging in size from 2 to 23 nm. While electron microscopy and selected area diffraction demonstrate the synthesis of NPs of tantalum nitride (which may be partially oxidized), results from X-ray photoelectron spectroscopy reveal that the majority of the tantalum in our sample is present as an unidentified molecular-scale oxide species. Introduction The generation of alternative and renewable energy sources is a significant challenge essential toward sustaining long-term economic stability. Solar energy collection and H2 production using water splitting photocatalysts are important processes toward achieving our energy goals. Considerable research has focused on overall water splitting using TiO2 based photocatalysts as a method of hydrogen generation.1-4 However, TiO2 based photocalysts possess band gaps that lie within the UV region (3.0 eV for the rutile and 3.2 eV for the anatase); as a result, TiO2 cannot efficiently harvest solar energy.5 In contrast, low band gap semiconductors may absorb solar light but have low reduction potentials that prevent photogeneration of H2.6 Recently, Domen et al. have shown that several metal nitride and oxynitride materials have the appropriate redox potentials coupled with low band gaps to allow for solar energy harvesting.6,7 While high band gap oxides are heavily investigated for this purpose, the fact that nitrogen has an atomic valence bondforming orbital (2p) with a higher oxidation potential than oxygen raises the valence bands of (oxy)nitride materials without affecting their conduction (i.e., H+ reducing) potentials. Shown in Scheme 1 are the band diagrams of tantalum oxide and (oxy)nitrides that illustrate this concept. Thus, Ta3N5 (Eg ) 2.1 eV) and TaON (Eg ) 2.5 eV) are potentially stable and environmentally benign photocatalysts.8 A recent study by Ishikawa et al. examined the use of Ta3N5 as a photoelectrode;9 however, the photocatalytic activity for H2 evolution was found to be 1 order of magnitude lower than that for O2 evolution.10 * Email: [email protected], [email protected]. § Department of Chemical Engineering, University of Illinois at Chicago. ‡ Research Resource Center, University of Illinois at Chicago. | Department of Physics, University of Illinois at Chicago. ⊥ Department of Chemical System Engineering, The University of Tokyo. ≈ Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan. † Department of Chemistry, University of Illinois at Chicago.

SCHEME 1: Illustration of the Band Gaps of Ta2O5, TaON, and Ta3N5a

a

Reproduced from ref 6.

As such, the use of Ta3N5 for water splitting required sacrificial reagents. A possible solution to resolve this issue is to raise the conduction potential though the effects of quantum confinement.11,12 Quantum confinement in semiconductor nanoparticles (NPs) can be used to tune the redox properties of the material if the physical dimensions of the NPs become smaller than the exciton radius of an electon-hole pair; the resulting localization of the charge carriers causes an increase in their kinetic energies which in turn alters the band gap. Thus, nanoparticles of tantalum nitride may have enhanced reduction efficiencies.

10.1021/jp110105u  2011 American Chemical Society Published on Web 12/13/2010

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Presently, metal nitrides are synthesized through ammonolysis of metal oxide powders at high temperature.10 For example, Lu et al. used crystalline Ta2O5 as a starting material to synthesize particles of Ta3N5 on the submicrometer scale with surface areas on the order of 10 m2/g and pore sizes ranging from 10 to 20 nm.13 Henderson and Hector also synthesized a porous Ta3N5 material with 20-30 nm crystal facets by nitriding amorphous Ta2O5 with NH3 gas between 680 and 900 °C.14 Choi and Kumta15 made tantalum nitride nanoparticles by reacting TaCl5 with ammonia for several hours in choloroform; after drying, the materials were exposed to NH3 at high temperature. In their study, agglomerates of ∼20 nm crystallites of Ta3N5 were made at 600 °C while larger crystal faceting was observed when the temperature was raised to 800 °C.15 Particle size may also affect the ammonolysis as Zhang and Gao have shown the temperature required to transform the oxide to Ta3N5 is reduced with a ∼20 nm Ta2O5 NP template.16 Quantum confinement effects were observed in 26 nm NPs as UV/vis diffuse reflectance spectroscopy demonstrated these Ta3N5 nanoparticles exhibited a slight blue-shifted absorption onset compared to 75 nm Ta3N5 NPs synthesized from a larger template. Hector and co-workers also used a solvothermal synthesis to make Ta3N5 in an autoclave at 500 °C.17,18 However, the Ta3N5 products were amorphous and needed to be annealed at 600-700 °C to form crystals. All the studies discussed above showed that even under high temperature nitriding conditions without air exposure, some oxygen and/or carbon content was observed. The current synthesis methods for Ta3N5 production using high temperature gaseous NH3 exposure generate materials with a large degree of surface heterogeneity. This engenders difficulty when we try to characterize the materials and occludes studies of active sites for photocatalytic reactions. A nanoscale homogeneity in the crystallinity and uniformity in the as-produced materials is required to develop a detailed understanding of photocatalytic behavior. We have made progress on the development of colloidal synthesis procedures for Ta3N5 nanoparticles based on similar methods to create emissive semiconductor NPs. Motivating this research is the fact that highly crystalline and homogeneous samples of materials such as CdSe19,20 and PbSe21 nanoparticles can be produced routinely due in part to the intense development of colloidal synthetic procedures to create such emissive NPs. Preparing monodisperse nanoparticles requires a short nucleation event followed by a slow growth of the nuclei that form.22 Rapid nucleation can be achieved by injecting semiconductor precursors into a vessel containing a hot, coordinating solvent provided that the temperature of the solvent in the vessel is sufficient to decompose the reagents to form reactive, supersaturated species (see Scheme 2). The formation of nuclei relieves this supersaturation; these small clusters slowly grow into NPs. Particle sizes can be controlled by adjusting the temperature, reagent concentrations, choice of surfactants and solvents, and the reaction time. Experimental Section All manipulations were carried out using standard air free procedures. Tri-n-octylphosphine oxide (TOPO) was purchased from Aldrich and purified by vacuum distillation, retaining the fraction transferred between 260 and 280 °C. Tris(trimethylsilyl)amine, lithium nitride, 1-octadecene (ODE), methanol, 1-butanol, and hexane were purchased from Aldrich and oleylamine (OA) from Acros. Tri-n-octylphosphine (TOP, 97%), tantalum(V) chloride, and pentakis(dimethylamino)tantalum(V) were purchased from Strem.

Ho et al. SCHEME 2: Rapid Injection Procedure for Synthesizing Ta3N5 NPs

Purification of Nanoparticles. During growth, surfactants adsorb to the surface of the nanoparticles and provide an organic surface passivation that stabilizes the NPs and mediates their growth.22 This organic layer dictates the NPs solubility and how they are processed; for example, Ta3N5 nanoparticles made in ODE and TOPO are soluble in hexane. Introducing a polar nonsolvent such as methanol reduces the barrier to aggregation and induces flocculation. The nanoparticles may then be isolated by centrifuging to remove the excess high molecular weight solvents; this is necessary for many types of characterization. The procedure can be repeated for further purification. In our studies, we have found that each method of preparation requires a different workup procedure that alters our ability to characterize the materials; the specific methods of purification are discussed below. Method 1. Approximately 5 g of ODE was dried and degassed in a 50 mL three-neck reaction vessel while stirring at 120 °C under vacuum for 30 min. Next, 0.1 g of TaCl5 stored in an inert atmosphere glovebox was transferred into the reaction vessel. The solution was degassed again at 120 °C for another 30 min. The temperature of the reaction vessel was then increased to at 310 °C under a dry N2 atmosphere. The injection solution was prepared in an inert atmosphere by adding 0.109 g of tris(trimethylsilyl)amine to 2 mL of TOP. The amine solution was quickly injected to the vigorously stirring reaction vessel though a rubber septum, producing a deep red solution. The temperature decreased suddenly to 280 °C after injection, which was restored to 300 °C for an additional 20 min. In an effort to purify the NPs, it is necessary to remove unreacted precursors and extra long alkyl chained surfactant as much as possible. However, it was difficult to redisperse precipitated NPs prepared in this method into any solvent. As a result, samples synthesized by method 1 could not be precipitated and were instead simply diluted before analysis. Two drops of a dilute solution containing Ta3N5 NPs were placed on a holey carbon grid, which is then dried at room temperature for characterization with electron microscopy. Method 2. A second route to the production of Ta3N5 nanoparticles was developed by replacing tris(trimethylsilyl)amine used in method 1 with 0.162 g of lithium nitride, which

Semiconductor Tantalum Nitride Nanoparticles was injected into a reaction vessel containing 3.0 g of TOPO, 1.0 g of oleylamine, and 0.1 g of TaCl5 at 300 °C. The vessel had been degassed under vacuum at 120 °C for 1 h before injection. In method 2, the reaction time is used to control the particle size after Li3N is injected. After half an hour, the reaction solution is cooled to 60 °C, slightly above the melting point of TOPO. A 5 mL aliquot of the reaction solution was removed, to which 25 mL of anhydrous methanol is added. The addition of anhydrous methanol resulted in the reversible flocculation of the Ta3N5 NPs, which helps remove excess solvent. The flocculant was isolated from the supernatant by centrifugation. The flocculant was redisperesed into a solvent mixture consisting of 1 mL of hexane and 1 mL of butanol to which 10 mL of anhydrous methanol was added. A dark brown precipitate is observed and isolated. A final rinse of the precipitate with 5 mL of anhydrous methanol is performed; the precipitate was then dried under vacuum. The dried material was redispersed in hexane for further characterization. Method 3. In this procedure, 0.194 g of tris(trimethylsilyl)amine and 0.20 g of pentakis(dimethylamino)tantalum(V) were dissolved in 2 mL of ODE in a glovebox; this solution was injected into a vessel containing 3.4 g of ODE at 300 °C for 30 min. The solvent had been degassed under vacuum at 120 °C for 1 h before injection. Purification steps were performed in a glovebox where 1 mL of the reaction solution is mixed with 1 mL of anhydrous hexane. Addition of 10 mL of anhydrous ethanol into the vial caused the material to flocculate. The flocculant was isolated from the supernatant by centrifugation and was redispersed into 2 mL of hexane for further characterizations. Transmission Electron Microscopy. A JEOL JEM-3010 operating at 300 kV was used for transmission electron microscopy (TEM) measurements. The JEM-3010 is an ultrahigh resolution analytical electron microscope with a point resolution of 0.17 nm. Imaging was performed in a bright field mode with an objective aperture selected to permit lattice imaging. TEM samples were prepared by adding one or two drops of a dilute hexane dispersion of NPs on the surface of a copper grid with holey carbon films. Adjusting the concentration of the dilute solution can change the coverage of nanoparticles on the grid. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) data were collected on a Kratos AXIS-165 operating in a monochromatic Al X-ray (15 kV, 10 mA) in an ultrahigh vacuum (10-9 to 10-10 Torr) chamber. Difficulties were encountered when calibrating the XPS spectra of various tantalum (oxy)nitride species prepared by different methods; as such, the Ta 4p3/2 peaks were normalized to the same binding energy. However, the Ta 4p3/2 binding energy is known to be a function of the oxygen content of tantalum (oxy)nitride materials.23 Absorption Spectroscopy. Absorption spectra were collected using a Varian Cary300. Results and Discussion We have examined the formation of colloidal crystalline Ta3N5 by injecting a variety of reactive tantalum and nitrogen precursors24-27 into hot coordinating solvents under an inert atmosphere. Characterization by TEM (Figure 1) has demonstrated the formation of crystalline Ta3N5 particles by method 1, where tris(trimethylsilyl)amine is injected into a hot coordinating solvent containing TaCl5. Ta3N5 NPs prepared by method 1 have a 23 ( 4 nm diameter with

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Figure 1. (a) Materials synthesized by method 1 at 300 °C. (b) Particles made by method 1 at 200 °C. These micrographs demonstrate that injection at lower temperatures results in the formation of smaller particles.

Figure 2. (a) High resolution TEM of a 7.5 nm NP synthesized via method 1 and (b) Fourier transformation of electron micrograph reveals that the lattice spacings (Å) of this nanoparticle made by method 1 is most consistent with the Ta3N5 material system. See Table 1.

TABLE 1: Lattice Spacings from Fourier Transformation of HRTEM Micrographs and X-ray Diffraction of Tantalum Nitride measured lattice spacing (Å)a

Ta3N5 lattice spacing (Å)b

orientation

% difference

3.39 2.57 2.24 1.69 1.61 1.43

3.4122 2.5534 2.2827 1.7053 1.6009 1.4189

(111) (040) (132) (006) (135) (046)

0.65 0.65 1.87 0.90 0.57 0.78

a

From HRTEM. b From XRD.

injection at 300 °C; lowering the temperature to 200 °C led to smaller 10 ( 3 nm Ta3N5 nanoparticles, as shown in Figure 1b. The synthesis is sensitive to the purity levels of the tantalum precursor as well; no Ta3N5 NPs were observed using 99.9% TaCl5 (the use of a 99.99% purity precursor is necessary). The structures of these materials were examined on a particle-to-particle basis by Fourier transforming high resolution TEM images and quantifying the diffraction patterns; a singular example is shown in Figure 2. Comparing these results to the XRD data of bulk Ta3N5 as tabulated in Table 1 demonstrates that the Ta3N5 crystal structure is a good match to the experimental data. After purification, the isolated materials are dark brown in color in contrast to pure bulk Ta3N5 powders, which are red. This dark color indicates the formation of products other than Ta3N5 NPs; while energy dispersive X-ray (EDX) analysis confirmed the presence of tantalum, a nitrogen signal

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Figure 3. (a) 2-8 nm Ta3N5 NPs created after 20 min heating in method 2. (b) Selected area diffraction pattern from Ta3N5 samples.

Figure 4. (a) 10 nm Ta3N5 NPs were made by method 2 after 1 h of heating. (b) Absorption spectrum. Inset shows two limiting regimes of indirect band gap behavior.

was not observed, possibly due to overlap with large signals from oxygen and carbon. The reaction yield is very low and difficult to quantify. Heating the reaction vessel for a longer period of time, or slowly injecting more tantalum and nitride precursors into the original reaction vessel, does not appear to augment the reaction yield. This fact, as well as our inability to process the samples effectively, makes further characterization by methods such as absorption or emission spectroscopy and XPS difficult. When developing method 1, we noticed that TaCl5 has poor solubility in ODE, which is consistent with the fact that TaCl5 is very soluble in aromatic hydrocarbons like benzene and

Ho et al. mesitylene. We attribute the low reaction yield to the poor solubility of the tantalum precursor. The addition of a nonsolvent to induce NP precipitation was not very successful when the nanoparticles are prepared in ODE as in method 1. However, when we used TOPO/TOP/oleylamine as solvents, processing Ta3N5 NPs is more facile. While method 1 affords the formation of 10-23 nm Ta3N5 NPs, a further decrease in the size of our nanoparticles is desirable to examine the quantum confinement of Ta3N5 NPs for band gap engineering. Method 2 made small spherical Ta3N5 NPs varying in diameter from 2 to 10 nm as shown in Figures 3a and 4a. A TOPO/oleylamine mix appeared to be a better solvent for TaCl5 as compared to ODE; further, TOPO can be heated to a higher injection temperature (360 °C). The processing via precipitation with nonsolvents was facile, which allowed for more characterization of the materials compared to samples made via method 1. This is likely why we were able to measure the selected area diffraction (SAD, Figure 3b) pattern for materials prepared by method 2; these data are tabulated in the Supporting Information (Table S1). The results demonstrate that Ta3N5 is the best match for the structural parameters determined from SAD results. When examining the SAD data, we found that it was difficult to identify diffraction rings of highly deflected electrons scattering off of short lattice spacings. To resolve this issue, we subtracted out the Gaussian-shaped background intensity from the SAD data and calculated the radial distribution of intensity from the center of the pattern; the results are shown in Figure 5. Compared to XRD data from authentic samples of Ta2O5, TaON, and Ta3N5, the short d-spacings are a clear match for the (oxy)nitrides compared to Ta2O5. We examined the optical properties to see if quantum confinement effects can be observed in these materials given previous results that demonstrated a slight blue-shifted absorption16 when Ta3N5 particles decreased in size from 75 to 26 nm. As shown in the inset of Figure 4b, there appear to be two possible absorption onsets. The weak and flatfeatured absorption profiles are indicative of indirect band gap behavior; one absorption onset can be extrapolated to 467 nm (2.66 eV) and the other to 515 nm (2.41 eV). We believe that the lower energy feature can be explained by

Figure 5. (a) Selected area diffraction of NPs prepared by method 2 compared to XRD data of Ta2O5 powder. (b) Selected area diffraction of NPs prepared by method 2 compared to XRD data of Ta3N5 and TaON powders. The SAD and XRD intensities should not necessarily correlate as X-ray and electron diffraction are sensitive to different scattering factors.

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Figure 6. XPS spectra of authentic nitride and oxynitride materials in comparison to colloidal nanoparticles made by method 1 illustrating the differences in the relative Ta to N content. The XPS spectra were calibrated to the same Ta 4p3/2 binding energy (404 eV) to demonstrate the differences in the N content, although the absolute Ta 4p3/2 binding energy is a function of the level of oxygen content.23

Figure 7. (a) and (b) TEM micrographs of materials synthesized by method 3.

quantum confinement effects in Ta3N5 NPs. The more blueshifted onset may be indicative of an oxide shell of a higher band gap material;28 specifically, TaON as the presence of the oxynitride is supported by the SAD data in Figure 5 and XPS results. Unfortunately, we were not able to quantify the emission most likely due to the low yield of NPs, loss of surface passivation during purification, and the indirect band gap nature of Ta3N5. Bulk Ta3N5 synthesized by nitriding Ta2O5 powders is very stable in ambient conditions.29 However, it is not clear if the same is true of Ta3N5 nanoparticles created by our colloidal synthetic method. We have measured the XPS spectra of samples made by method 2 and compared the results to data for Ta3N5 and TaON powders from Domen’s group. It was unfortunately difficult to calibrate all the XPS spectra with a common standard; as such, the Ta 4p3/2 features were fitted

to have the same 404 eV binding energy although the oxidation level of tantalum (oxy)nitride is known to change the absolute Ta 4p3/2 binding energy.23 As can be seen in Figure 6, our samples have a weak nitride peak compared to the bulk powder samples. The oxygen peak in the same XPS spectrum (Supporting Information, Figure S1) is very strong. These results suggest that we have a very low yield of Ta3N5 and that our sample is highly contaminated by a molecularscale tantalum oxide species that cannot be observed via electron microscopy or diffraction. While we encountered difficulties in consistently calibrating the absolute binding energies in these XPS spectra, the relative binding energies (BE) between the Ta 4f7/2 (∼25 eV, data not shown) and the N 1s (∼396 eV) transitions are known to be indicative of the oxidation state of the material.23 In our measurements, this difference ∆BE ) BE(N 1s) - BE(Ta 4f7/2) in Ta3N5 (∆BE ) 371.5 eV) is greater than that for TaON (∆BE ) 370.8 eV). The difference observed in the nanoparticle sample prepared by method 2 (∆BE ) 371.0 eV) is between the pure tantalum nitride and oxynitride; this is consistent with our conclusion that we are forming partially oxidized Ta3N5 NPs. To address the low yield of NPs prepared by methods 1 and 2, a highly reactive pentakis(dimethylamino)tantalum(V) precursor was used in method 3. After coinjection with tris(trimethylsilyl)amine into ODE at 300 °C, an emissive red-colored solution was formed after cooling to room temperature. While the red color is indicative of formation of large Ta3N5 NPs, the as-prepared materials are highly air sensitive. Once the septum from the reaction vessel is removed and the solution is exposed to the atmosphere, the red color and emission immediately disappears. This suggests the formation of small, air-sensitive clusters of tantalum24,25 vs large Ta3N5 nanoparticles. All purifications of materials made by this method were preformed in an inert-atmosphere glovebox via precipitation with anhydrous ethanol after dilution with anhydrous hexane. Even under these conditions, the red color was perturbed by processing and the resulting solutions in hexane were cloudy; this prevented characterization by absorption or emission spectroscopy. The NPs created by method 3 were imaged with electron microscopy. As shown in Figure 7a,b, pentakis(dimethylamino)tantalum(V) created poorly crystalline Ta5+ polydisperse and spherical nanoparticles from 3 to 10 nm in diameter. As shown in the Supporting Information (Figure S2), XPS reveals a Ta 4p3/2 feature at 401.5 eV; this confirms that tantalum is in the +5 oxidation state. A small shoulder at 397.4 eV is also seen, which is likely due to a N 1s transition and compares favorably to the nitride peak at 396.5 eV of Ta3N5 thin films synthesized by sputtering.30 Oxides are nonetheless likely present as analysis with EDX confirmed the presence of a significant amount of oxygen. Table 2 summarizes the processes and results of all three methods of NP synthesis.

TABLE 2: Summary of the Three Methods Used to Produce Tantalum (Oxy)nitride Nanoparticles Ta precursor N precursor solvent result

method 1

method 2

method 3

TaCl5 tris(trimethylsilyl)amine ODE/TOP crystalline 10f23 nm NPs

TaCl5 Li3N TOPO/TOP/OA crystalline 2f10 nm NPs

pentakis(dimethylamino)tantalum(V) tris(trimethylsilyl)amine ODE amorphous 3f10 nm NPs

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Conclusions Our group has attempted several preparations for the synthesis of Ta3N5 NPs using the methods developed for creating emissive semiconductor nanoparticles. The best crystalline ∼2-10 nm diameter NPs were made by TaCl5 and lithium nitride precursors in long alkyl chain phosphine solvents, such as trioctylphosphine oxide/oleylamine, which seem to be better for solubilizing the TaCl5 precursor. There is also evidence for band gap engineering as colloidal NPs have absorption onsets greater than the bulk powders by ∼0.3 eV. While TEM, electron diffraction, XPS, and absorption spectroscopic analyses support the formation of Ta3N5 that may be partially oxidized, the low reaction yields and low relative N to Ta ratio from XPS suggest much of the tantalum precursor oxidizes into a molecular species during the synthesis and processing of the NPs. Such a molecular oxidized species would not be observed with diffraction or electron microscopy. This is also consistent with the known air sensitivity of organometallic tantalum clusters.24 The O2 reactivity and poor solubility in solvents like octadecene contribute to a low yield of Ta3N5 NPs. While we have demonstrated success in Ta3N5 nanoparticle synthesis, the very low yields coupled to problems with oxidation suggest our methods are not likely to be effective for large scale production of metal nitride nanoparticles. Acknowledgment. This work was supported by the UIC Chancellor’s discovery fund and by the University of Illinois at Chicago. We thank Stephen Guggenheim, Jamin Krinsky, and Michael Trenary for helpful discussions and assistance with several experiments. Supporting Information Available: Selected area diffraction data as well as X-ray photoelectron spectra and a deconvolution graph. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ni, M.; Leung, M. K. H.; Leung, D. Y. C.; Sumathy, K. Renewable Sustainable Energy ReV. 2007, 11, 401–425. (2) Hameed, A.; Gondal, M. A. J. Mol. Catal. A: Chem. 2004, 219, 109–119.

Ho et al. (3) Yamakata, A.; Ishibashi, T.; Onishi, H. J. Mol. Catal. A: Chem. 2003, 199, 85–94. (4) Yamakata, A.; Ishibashi, T.; Onishi, H. Int. J. Photoenergy 2003, 5, 7–9. (5) Chen, X.; Mao, S. S. Chem. ReV. 2007, 107, 2891–2959. (6) Maeda, K.; Domen, K. J. Phys. Chem. C 2007, 111, 7851–7861. (7) Takata, T.; Hitoki, G.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. Res. Chem. Intermediat. 2007, 33, 13–25. (8) Abe, R.; Higashi, M.; Domen, K. J. Am. Chem. Soc. 2010, 132, 11828–11829. (9) Ishikawa, A.; Takata, T.; Kondo, J. N.; Hara, M.; Domen, K. J. Phys. Chem. B 2004, 108, 11049–11053. (10) Hara, M.; Hitoki, G.; Takata, T.; Kondo, J. N.; Kobayashi, H.; Domen, K. Catal. Today 2003, 78, 555–560. (11) Brus, L. E. J. Chem. Phys. 1983, 79, 5566–5571. (12) Ekimov, A. I.; Onushchenko, A. A. JETP Lett. 1984, 40, 1136– 1139. (13) Lu, D. L.; Hitoki, G.; Katou, E.; Kondo, J. N.; Hara, M.; Domen, K. Chem. Mater. 2004, 16, 1603–1605. (14) Henderson, S. J.; Hector, A. L. J. Solid State Chem. 2006, 179, 3518–3524. (15) Choi, D.; Kumta, P. N. J. Am. Chem. Soc. 2007, 90, 3113–3120. (16) Zhang, Q. H.; Gao, L. Langmuir 2004, 20, 9821–9827. (17) Mazurrider, B.; Chirico, P.; Hector, A. L. Inorg. Chem. 2008, 47, 9684–9690. (18) Mazumder, B.; Hector, A. L. J. Mater. Chem. 2008, 18, 1392– 1398. (19) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706–8715. (20) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 183–184. (21) Steckel, J. S.; Coe-Sullivan, S.; Bulovic, V.; Bawendi, M. G. AdV. Mater. 2003, 15, 1862–1866. (22) Murray, C. B.; Sun, S. H.; Gaschler, W.; Doyle, H.; Betley, T. A.; Kagan, C. R. IBM J. Res. DeV. 2001, 45, 47–56. (23) Lamour, P.; Fioux, P.; Ponche, A.; Nardin, M.; Vallat, M. F.; Dugay, P.; Brun, J. P.; Moreaud, N.; Pinvidic, J. M. Surf. Interface Anal. 2008, 40, 1430–1437. (24) Krinsky, J. L.; Anderson, L. L.; Arnold, J.; Bergman, R. G. Angew. Chem., Int. Ed. 2007, 46, 369–372. (25) Krinsky, J. L.; Anderson, L. L.; Arnold, J.; Bergman, R. G. Inorg. Chem. 2008, 47, 1053–1066. (26) Wang, J. J.; Grocholl, L.; Gillan, E. G. Nano Lett. 2002, 2, 899– 902. (27) Grocholl, L.; Wang, J. J.; Gillan, E. G. Chem. Mater. 2001, 13, 4290–4296. (28) Dabbousi, B. O.; RodriguezViejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463–9475. (29) Hitoki, G.; Ishikawa, A.; Takata, T.; Kondo, J. N.; Hara, M.; Domen, K. Chem. Lett. 2002, 736–737. (30) Chun, W. J.; Ishikawa, A.; Fujisawa, H.; Takata, T.; Kondo, J. N.; Hara, M.; Kawai, M.; Matsumoto, Y.; Domen, K. J. Phys. Chem. B 2003, 107, 1798–1803.

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