Comparative Photocatalytic Ability of Nanocrystal-Carbon Nanotube

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Comparative Photocatalytic Ability of Nanocrystal-Carbon Nanotube and -TiO2 Nanocrystal Hybrid Nanostructures Kyung Hwan Ji, Dong Myung Jang, Yong Jae Cho, Yoon Myung, Han Sung Kim, Yunhee Kim, and Jeunghee Park* Department of Chemistry, Korea UniVersity, Jochiwon 339-700, Korea ReceiVed: July 9, 2009; ReVised Manuscript ReceiVed: October 5, 2009

We synthesized various nanocrystal (e.g., CdS, CdSe, and Cu2S)-carbon nanotube (NC-CNT) and NC-TiO2 hybrid nanostructures using the solvothermal method and compared their photocatalytic ability toward the visible-light-driven degradation of methylene blue (MB) dye. The free CdS NCs exhibited higher degradation efficiency than the CdSe and Cu2S NCs. The photocatalytic abilities of the NCs were found to determine the relative degradation efficiency of their CNT and TiO2 hybrid nanostructures. These results suggest that the oxidative N-demethylation degradation involves the transfer of holes from the NCs to MB. The hybridization of the NCs with the TiO2 NCs and CNTs enhances the oxidative degradation rate to the same extent, suggesting that the interfacial electron transfer process from the NCs to the attached CNTs (or TiO2), which retards the recombination of the electrons and holes, is comparable for both hybrid nanostructures. 1. Introduction Photocatalysis has attractive potential applications in the conversion of solar energy into chemical energy (e.g., the production of H2 by water splitting) as well as the degradation of toxic water pollutants.1-3 Most photocatalytic degradation studies have focused on the use of nanocrystalline titania (TiO2, band gap (Eg) ) 3.2 eV for anatase and 3.0 eV for rutile). However, the efficiency of these UV-irradiated systems is poor, because the most intense region of the solar spectrum is approximately ∼2.6 eV. Finding photocatalysts with high visible light activity remains a challenge. Recent research demonstrated the improved photocatalytic activity of semiconductor (e.g., CdS, CdSe)-TiO2 nanocrystal (NC) hybrid nanostructures toward the degradation of water and organic molecules.4-17 The electron transfer from the conduction band (CB) of the NC sensitizer to that of TiO2 can lead to efficient and longer charge separation by minimizing the electron-hole recombination. However, the photocatalytic abilities of these NC sensitizers have not been compared yet, although this information would provide valuable information on the mechanism of the photocatalytic redox reaction. NC-carbon nanotube (CNT) hybrid nanostructures have been considered as another good photocatalyst candidate, because of their excellent adsorption and charge transfer abilities.18-26 The excited electrons in the CB of the wide-band gap TiO2 (or ZnO, Ta3N5, CdS, ZnS) NCs migrate into the CNTs, thereby decreasing the possibility of the recombination of the electron-hole pairs, resulting in the quicker degradation of the target molecules. Nevertheless, there have been few reports on visiblelight-driven photocatalytic degradation using semiconductor NCCNT hybrid nanostructures. This paper reports the synthesis of colloidal semiconductor CdS (Eg (bulk) ) 2.4 eV), CdSe (Eg (bulk) ) 1.7 eV), and β-Cu2S (Eg (bulk) ) 1.2 eV) NCs and their multiwalled CNT (MWCNT) and TiO2 hybrid nanostructures, which were prepared by the in situ solvothermal method. The electronic * To whom correspondence should be addressed. E-mail: parkjh@ korea.ac.kr.

structures of the hybrid nanostructures were investigated using X-ray photoelectron spectroscopy (XPS). Detailed analytical investigations of their band structures were performed using UV-visible absorption, photoluminescence spectra, and cyclic voltammogram (CV) measurements. We investigated the photocatalytic ability of the NCs and hybrid nanostructures toward the degradation of aqueous methylene blue (hereafter referred to simply as MB) solution. MB, a brightly colored and blue cationic thiazine dye (ε ) 105 L mol-1 cm-1 at λmax ) 660 nm), was selected because it is frequently used as a reactant to test the photocatalytic power of various hybrid nanostructures.4,7,8,13,15,17,27-29 Its degradation mechanism has been extensively investigated using TiO2 NCs and CdS bulk powders.30-36 Herein, we focused on the relative photocatalytic abilities of CdS, CdSe, and Cu2S NCs (in the free and hybrid forms) toward the degradation of MB under visible light irradiation. Furthermore, we first compared the hybridization effect of those NCs on the photodegration of MB, for two different electron acceptors, TiO2 and CNTs. We found notably that the photocatalytic ability of the NCs determines the degradation activity of the CNT and TiO2 hybrid nanostructures, and that the interfacial electron-transfer processes from the NCs to the CNTs and TiO2 NCs are comparable to each other. 2. Experimental Section 2.1. Synthesis of Free CdS, CdSe and Cu2S NCs, and their CNT (or TiO2) Hybrid Structures. The synthesis procedures of the water-soluble colloidal NCs are summarized in the Supporting Information. The CdS and CdSe (with ZnSe/ZnS double-shell) NCs were synthesized using the solvothermal method reported by other research groups.37,38 The Cu2S NCs were synthesized using the solvothermal method, as described elsewhere.39 The hybrid nanostructures were synthesized using the in situ growth of NCs on the dispersed MWCNTs (Aldrich) and TiO2 NCs (Degussa P25, Anatase:Rutile)3:1). 2.2. Characterization. The products were characterized by field-emission transmission electron microscopy (FE TEM, FEI TECNAI G2 200 kV, and Jeol JEM 2100F) and high-voltage transmission electron microscopy (HVEM, Jeol JEM ARM

10.1021/jp906476m CCC: $40.75  2009 American Chemical Society Published on Web 10/22/2009

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Figure 1. HRTEM images of the (a) CdS, (b) CdSe, (c) Cu2S, (d) CdS-CNTs, (e) CdSe-CNTs, (f) Cu2S-CNTs, (g) TiO2 (avg. diameter ) 20 nm), (h) CdS-TiO2 NCs, and (i) Cu2S-TiO2 hybrid nanostructures. The mean sizes of the CdS, CdSe, and Cu2S NCs were 3.5 ( 0.3, 4 ( 0.4, and 7 ( 1 nm, respectively. The insets in (e,f) show the lattice-resolved TEM images and corresponding FFT ED patterns of the CdS and Cu2S NCs in the CNT hybrid nanostructures, respectively.

1300S, 1.25 MV). The high-resolution X-ray diffraction (XRD) patterns were obtained using the 8C2 or 3C2 beamline of the Pohang Light Source (PLS) with monochromatic radiation (λ ) 1.54520 Å). X-ray photoelectron spectroscopy (XPS) was carried out using the 8A1 beamline of the PLS and a laboratorybased spectrometer (VG Scientifics ESCALAB 250) using a photon energy of 1486.6 eV (Al KR). Thermal gravimetric analysis (TGA, TA Instrument, SDT 2960) was used to measure the wt % of the NCs in the hybrid nanostructures. The Raman spectroscopy measurements (Horiba Jobin-Yvon HR-800 UV) were recorded using an Ar ion laser (λ ) 514.5 nm). A UV-visible-NIR absorption spectrometer (Varian, Cary 1000) was used to identify the band edge position of the NCs. The photoluminescence (PL) measurements were carried out using an He-Cd laser (λ ) 325 nm) as the excitation source. The laser power was 400 nm. Five milligrams of the NCs or NC-TiO2, NC-CNT hybrid nanostructures was suspended in an aqueous solution (50 mL) containing 0.1-1 µM MB by sonication for 30 min. The solution was placed in a reaction vessel equipped with a quartz window (exposed to light under dry air flow) and stirred with a magnetic stirrer. Aliquots (1 mL) were sampled at various irradiation times and filtered through a cellulose filter (0.25 µm) to remove the suspended catalysts. The UV-visible absorption spectrum of the filtered solution was measured using a spectrophotometer and the concentration was estimated by the integration of the absorption peak. 3. Results 3.1. Morphology and Composition. Figure 1a-c shows the high-resolution TEM (HRTEM) images of the CdS, CdSe, and Cu2S NCs, respectively. Their mean sizes were estimated to 3.5 ( 0.3, 4 ( 0.4, and 7 ( 1 nm, respectively, by counting at least 30 NCs. Figure 1d-f shows the TEM images of the MWCNTs, CdS NC-, and Cu2S NC-MWCNT hybrid nanostructures, respectively. The mean diameter of the MWCNTs was 20 ( 5 nm. All of the MWCNTs were decorated homogeneously with single-crystalline NCs, having the same size as that of the free forms. The lattice-resolved TEM images of the CdS and Cu2S NCs in the hybrid nanostructures are shown in the insets of Figure 1e,f, respectively, together with the corresponding fast-Fourier transformed electron diffraction (FFT ED) patterns. The (111) planes of the CdS NCs are separated by a distance of 3.4 Å (zone axis ) [011]), which is consistent with the reference value of zinc blende CdS (JCPDS No. 800019; a ) 5.811 Å). The (102) plane of the wurtzite Cu2S NCs

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Figure 2. XRD patterns of free CdS, CdSe, and Cu2S NCs. The peaks corresponding to the zinc blende CdS, wurtzite CdSe, and wurtzite Cu2S are identified.

with d ) 2.4 Å (JCPDS No. 26-1116; a ) 3.961 Å, c ) 6.722 Å) is identified at the zone axis of [22j01]. Figure 1g-i corresponds to the TEM images of the TiO2, CdS-TiO2, and Cu2S-TiO2 hybrid nanostructures, respectively. The mean diameter of the TiO2 NCs was 20 ( 2 nm. The TiO2 NCs were decorated with a number of NCs, whose size is about the same as that of the free or CNT hybrid forms. XPS measurements were performed to determine the content of NCs in the hybrid nanostructures, which was found to be 40 wt % on average for both the CNT TiO2 (Supporting Information, Figure S1). The EDX data of the free NCs and their TiO2 hybrid nanostructures are shown in the Supporting Information, Figure S2. 3.2. XRD Pattern and XPS. The XRD patterns confirm the composition and crystal structure of the free NCs, as shown in Figure 2. The peak positions are consistent with those of the reference values of zinc blende CdS (JCPDS No. 80-0019; a ) 5.811 Å), wurtzite CdSe (JCPDS No. 77-230; a ) 4.299 Å, c ) 7.010 Å), and wurtzite Cu2S (JCPDS No. 26-1116; a ) 3.961 Å, c ) 6.722 Å). The average sizes of the NCs were estimated using the Debye-Scherrer equation and found to be 4, 4, and 8 nm for the CdS, CdSe, and Cu2S NCs, respectively, which are consistent with the results obtained from the TEM images. The XRD patterns of the NC-CNT and NC-TiO2 hybrid nanostructures are shown in the Supporting Information, Figure S3. Figure 3a shows the fine-scanned XPS C 1s spectra for the MWCNTs and their CdS and Cu2S NC hybrid nanostructures. As the NCs are deposited, the full-width at half-maximum (fwhm) of the asymmetric band, centered at 284.5 eV, increases significantly from 1.0 to 2.0 (CdS) or 1.5 eV (Cu2S). The binding energy of the C atoms bonded to the defects through dangling bonds (-OH or -COOH) appears at a higher energy relative to that of the graphite C atoms, which causes an asymmetry at the higher energy region. The peak broadening may provide evidence for the binding interaction with the NCs that were created on the surface of CNTs during the solvothermal growth of the NCs. The Raman spectra show that the intensity ratio (ID/IG) of the defect peak (D band) to the graphite peak (G band) is higher for the hybrid nanostructures (Supporting Information, Figure S4). It suggests that the binding interaction with the NCs would be through the defect sites of graphitic layers. The fine-scanned Ti 2p3/2 peaks of the TiO2 NCs, CdS-TiO2, and Cu2S-TiO2 hybrid nanostructures are displayed in Figure

Ji et al. 3b. As the CdS and Cu2S NCs are deposited on the TiO2 NCs, the peaks become broader; their fwhm values increase from 1.0 to 1.1 eV (CdS) and 1.4 eV (Cu2S), respectively. The Cd 3d5/2 and Cu 2p3/2 peaks also show a tendency to increase in width upon the deposition of the NCs on the TiO2 NCs or CNTs (Figure 3c,d). Their fwhm values are marked in the figures. All these peak broadenings would indicate the existence of binding interaction between the NCs and CNTs (or TiO2 NCs). It is noteworthy that a similar level of broadening was observed for both hybrid nanostructures. This suggests that there is a comparable binding interaction of NCs with the CNTs and TiO2 NCs. 3.3. UV Absorption, PL, and CV Data. The UV-visible absorption and photoluminescence (PL) spectra of the CdS, CdSe, and Cu2S NCs were measured, as shown in Figure 4. The absorption bands appear at 3.04, 2.05, and 1.34 eV, respectively. The CdS, CdSe, and Cu2S NCs exhibited bandedge emission bands at 2.92, 2.03, and 1.30 eV, respectively. We measured the CV data of the NCs in order to obtain the potential energy of their valence band (VB)/conduction band (CB) (or HOMO/LUMO), as shown in Figure 5a. The CV ′ ) and reduction curves show that the onset oxidation (Eox ′ ) are 1.8 and -1.3 V for CdS, respectively, and potentials (Ered ′ ) 1.0 and Ered ′ ) -1.1 V for CdSe, and Eox ′ ) 0.55 and Ered ′ Eox ) -0.85 V for Cu2S. Then the VB/CB band energy was ′ and Ered ′ values, assuming the energy calculated from these Eox level of ferrocene/ferrocenium (Fc/Fc+) to be -4.8 eV below the vacuum level. The formal potential of Fc/Fc+ was measured to be 0.075 V against an Ag/Ag+ reference electrode. Therefore, EVB(EHOMO) ) -(E′ox+4.725) eV; ECB(ELUMO) ) -(E′red + 4.725) eV, where the onset potential values are relative to the Ag/Ag+ reference electrode.40,41 The band gaps were calculated to be 1.4, 2.1, and 3.1 eV for the Cu2S, CdSe, and CdS NCs, which are nearly the same as those estimated from the absorption edges, viz. 1.34, 2.05, and 3.04 eV, respectively. As the NCs form a hybrid nanostructure with the CNTs, the onset oxidation and reduction potentials remain nearly unchanged. A close look reveals that the oxidation of the graphitic layers occurs at the onset potential of 0.02 V. The energy diagram of the VB/CB (or HOMO/LUMO) levels of the NCs, calculated from their E′ox and E′red values, are shown in Figure 5b. The HOMO and LUMO levels of MB are known to be -6.11 and -4.25 eV, respectively.34 The VB/CB levels of TiO2 are assumed to be the same as that of anatase bulk phase. 3.4. Photodegradation of MB. The photocatalytic degradation measurements of MB using the various NCs and NC-CNT hybrid nanostructures were carried out under visible light irradiation from a 100 W halogen lamp (intensity ) 0.25 W/cm2) using a UV filter (>400 nm), by monitoring the change in concentration of MB. Figure 6a displays the degradation of the UV-visible absorption spectrum of 0.5 µM MB (in 50 mL solution) upon visible light irradiation when 5 mg (0.1 g/L) of the CdS NCs is used. The maximum absorption of MB occurs at λmax ) 660 nm but shifts toward a shorter wavelength (615 nm) during the degradation. This absorption band was integrated to monitor the temporal concentration change of MB. Figure 6b displays the degradation of the concentration ([MB]t/[MB]0) versus irradiation time (t, min) when 5 mg of the CdS, CdSe, and Cu2S NCs were used. The concentration of MB remained unchanged for 3 h in the absence of light irradiation or catalysts. The degradation occurs following a single-exponential decay function (shown as a fitted line). The greatest degradation yield (∼100% at 3 h) was observed when the CdS NCs were used.

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Figure 3. (a) Fine scanned C1s spectra of CNTs, CdS-CNT, and Cu2S-CNT hybrid nanostructures. (b) Fine scanned Ti 2p3/2 spectra of TiO2, CdS-TiO2, and Cu2S-TiO2 hybrid nanostructures. (c) Fine-scanned Cd 3d5/2 peaks of the free CdS NCs, CdS-CNT, and CdS-TiO2 hybrid nanostructures. (d) Fine-scanned Cu 2p3/2 peaks of the free Cu2S NCs, Cu2S-CNTs, and Cu2S-TiO2 hybrid nanostructures.

Figure 4. UV-visible absorption and photoluminescence spectra of the CdS, CdSe, and Cu2S NCs in ethanol. Their absorption bands appear at 1.34, 2.05, and 3.04 eV, respectively. The Cu2S, CdSe, and CdS NCs exhibited band-edge emission bands at 1.30, 2.03, and 2.92 eV, respectively.

The concentration of MB was reduced by 40% (at 3 h) when the CdSe and Cu2S NCs were used. The degradation yield was reduced linearly when a lesser amount of the photocatalysts was employed. The degradation of MB was also monitored using 5 mg of CNTs and NC-CNT hybrid nanostructures (Figure 6c). We make sure that there is no change in the concentration of MB for 3 h in the absence of catalysts. The degradation yield of MB at 3 h is 40% when the CNTs were used. However, as they formed a hybrid nanostructure with the CdS NCs (40 wt %, 2 mg), the degradation yield (at 3 h) increased from 40 to ∼100%. There was a negligible increase in the degradation efficiency when using the hybrid with either the CdSe or Cu2S NCs, thus

highlighting the higher catalytic ability of the CdS NCs, which is consistent with that of the free NCs. The degradation of MB in the absence of light irradiation was monitored using the CNTs and NC-CNT hybrid nanostructures, showing a 10% decrease of the MB concentration. This indicates that the adsorption ability of the CNTs contributes to increasing the degradation efficiency. The degradation data was measured for 5 mg of TiO2 and its CdS and Cu2S NC hybrid nanostructures, as shown in Figure 6d. The concentration of MB remained unchanged for 3 h in the absence of catalysts or light irradiation. The degradation yield of the TiO2 NCs was 30% at 3 h, whereas that of their hybrid with the CdS NCs showed an increased value of 100%. This means that the efficiency of the TiO2 NCs is multiplied by higher than three times when they form a hybrid structure. The contribution of the NCs to the degradation efficiency is comparable to that in the CNT hybrid nanostructures. There was a negligible increase in the degradation efficiency when using the CdSe or Cu2S NC hybrid nanostructures, which is consistent with the result obtained for the CNT hybrid nanostructures. The degradation data was measured for various initial concentrations of MB in the range of 0.1-1 µM, as shown in the Supporting Information, Figure S5. The results indicate that as the initial concentration of MB decreases, the degradation efficiency increases, which is consistent with the results reported by other groups.31,33 The presumed reason for this is that at higher concentrations some of the active sites of the photocatalysts are covered by MB molecules. Over this concentration range, the same sequence (CdS > CdSe ≈ Cu2S) is maintained. Assuming the Langmuir-Hinshelwood (L-H) mechanism, the initial rate (V0) ([MB] ≈ [MB]0) can be written as follows: ([MB]0/V0) ) (1/kK) + ([MB]0/k), where k is the degradation

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Figure 5. (a) CV curves of Cu2S, CdSe, and CdS NCs and their CNT hybrid nanostructures. (b) VB/CB levels of Cu2S, CdSe, and CdS NCs determined from the CV data and those of TiO2, CNTs, and MB.

Figure 6. (a) Evolution of UV-visible absorption spectrum of 0.5 µM MB upon visible light irradiation when 5 mg (0.1 µg/L) of the CdS NCs is used. (b) Photocatalytic degradation ([MB]t/[MB]0) of 0.5 µM MB upon visible-light irradiation using 5 mg of the free CdS, CdSe, and Cu2S NCs. The decay data points were fitted with an exponential decay function (solid lines). (c) [MB]t/[MB]0 vs irradiation time for 5 mg of the CNTs (with and without light), CdS-, CdSe-, and Cu2S-CNT hybrid nanostructures. (d) [MB]t/[MB]0 vs irradiation time for 5 mg of TiO2, CdS-, and Cu2S-TiO2 hybrid nanostructures., Langmuir-Hinshelwood plots (([MB]0)/(V0) vs [MB]0) of the (e) free NCs and (f) NC-TiO2 and NC-CNT hybrid nanostructures used to obtain the values of the degradation rate constant (k) and adsorption equilibrium constant (K) of MB on the surface of the photocatalysts.

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TABLE 1: Oxidation Rate Constant (k) of MB and Adsorption Equilibrium Constant (K) of MB, Assuming Langmuir-Hinshelwood Mechanism photocatalysts

k (µM · h-1)

K (µM-1)

CdS CdSe Cu2S CNTs CdS-CNT Cu2S-CNTs TiO2 CdS-TiO2 Cu2S-TiO2

0.52 0.17 0.16 0.10 0.50 0.15 0.12 0.52 0.18

14 1.2 1.7 ∞ 15 35 14 10 25

rate constant and K is the adsorption equilibrium constant of MB on the surface of the photocataysts.31,33 The initial reaction rate at various [MB]0 values was obtained during the first 60 min of illumination. Figure 6e,f shows the L-H plots of ([MB]0)/ (V0) versus [MB]0 for the free NCs and hybrid nanostructures, respectively. From the intercept and slope, we obtained the values of k and K (Table 1), respectively. All of these data demonstrate that the photocatalytic activity of the CdS NCs is higher than that of the CdSe and Cu2S NCs. The free CdS NCs exhibit a 3 times higher degradation rate constant than the CdSe and Cu2S NCs. As the CNTs and TiO2 NCs form hybrids with the CdS NCs, the degradation rate increases significantly by about 5 times in both cases. The K values of the CNTs and hybrid nanostructures are larger than those of the TiO2 series, as expected. To find the steadiness of the photocatalytic ability, we carried further experiments to test the three times recycling of the photocatalysts by adding fresh MB solution after each run (to make same initial concentration), as shown in the Supporting Information, Figure S6. The results show that the photocatalytic activity of the CdS (and CdSe) NCs and TiO2 hybrid nanostructure reduces by 20-30% after first run, but negligibly after second run. The Cu2S NCs exhibit an excellent stability in the photocatalytic activity. Nevertheless, the relative efficiency of the photocatalysts remains as the same order. 4. Discussion The free CdS NCs exhibit a 3 times higher degradation rate than the CdSe and Cu2S ones. This higher degradation rate might be directly related to their superior photocatalytic activity toward the degradation of MB. The blue shift of the absorption band during the degradation was ascribed to the oxidation of MB to various N-demethylated forms (e.g., thionine, λ ) 600 nm), which is consistent with the results of previous works using CdS bulk powder.34,36 The color of the MB solution fades when all or part of the auxochromic methyl groups degrade. The photoexcitation of MB takes place to form appropriate singlet or triplet states. According to the potential energy diagram (as shown in Figure 5b), the electron injection from the excited MB (at LUMO level) into the CB of the NCs is forbidden. On the other hand, the electron transfer from the CB of the NCs to the LUMO level of MB is allowed for all three types of NCs. MB (or its degradation intermediates) can react with the VB holes (h+) by acting as a sacrificial reagent and interfere with the electron/hole (e-/h+) recombination process. It is important that the VB holes of the NCs were able to extract the electrons from the MB only in the case of the CdS NCs, whose VB potential energy is below that of the HOMO level of MB. The N-methyl group can facilitate the attack of the VB holes on MB, which is likely to be the major step in the photocatalytic oxidative degradation, as suggested in previous works.34,36

The CV data of the CNT hybrid nanostructures indicated that the band gap and VB/CB levels of the NCs were not much different from those of the NCs. The relative photocatalytic abilities of the NCs determine the degradation efficiency of the hybrid nanostructures; the degradation of MB was accelerated by the hybridization of the CdS NCs, but not by that of the CdSe or Cu2S NCs. Therefore, it is evident that the photogenerated VB holes of the NCs are still effective in the oxidative degradation of MB in the case of the hybrid nanostructures. We observed that the addition of 2 mg of NCs (corresponding to 40 wt % in the hybrid nanostructures) to the 5 mg CNTs or TiO2 colloidal solution (i.e., not in the hybrid form) did not increase the degradation rate. It is also noted that the use of 2 mg of the CdS NCs alone induces a photodegradation yield of 60% at 3 h, whereas the addition of the CNTs (or TiO2 NCs) decreases it to that obtained with the CNTs (or TiO2 NCs) alone. The dispersion of the larger size CNTs and TiO2 NCs would block the absorption of the free NCs by the scattering the visible light, unless they form hybrid nanostructures. Therefore, the interfacial electron-transfer process would actually enhance the degradation efficiency of the hybrid nanostructures. The CNTs are relatively good electron acceptors, while the semiconductor NCs can act as good electron donors under visible light illumination. Since O2 is adsorbed on the largearea surface of the CNTs (excluding the NC-deposited area), the electrons transferred from the excited NCs to the CNTs will have more opportunity to come into contact with the O2 molecules, yielding O2•-. As the photogenerated electrons undergo such an interfacial electron transfer process, the recombination of e-/h+ is retarded, which improves significantly the photocatalytic activity of the NCs. The larger adsorption ability (K) of the CNTs than that of TiO2 would contribute to increasing the degradation efficiency, as shown in Figure 6c,d. In the NC-TiO2 hybrid nanostructures, the photoexcited electrons of the NCs are also transferred to the CB of the TiO2 NCs, and are scavenged mainly by O2 to yield O2•-.4-17 The hybridization of CdS NCs also enhances the oxidation rate constant (k) by a factor of 5, suggesting that the interfacial electron transfer process may take place to an extent which is comparable to that in the case of the CNTs. This could be correlated with the comparable binding interaction of the NCs with the CNTs and TiO2 NCs (as discussed above using the XPS data), because the hybrid formation procedure is identical. It should be mentioned that the photocalatytic activity and binding interaction may be dependent on the synthesis method. Nevertheless, the present work provides valuable information on the advantage of the hybrid nanostructures as photocatalysts. 5. Conclusion Water-dispersed CdS, CdSe, and Cu2S NCs and their CNTs and TiO2 hybrid nanostructures were synthesized using the solvothermal method. The average sizes of the CdS, CdSe, and Cu2S NCs were 3.5, 4, and 7 nm, respectively. The analysis of the fine-scanned XPS electronic structures of the free NCs and hybrid nanostructures revealed that the binding interaction between the NCs and CNTs (or TiO2 NCs) was comparable in each case. The band gap and VB/CB levels were similar for the free and hybridized NCs, based on the UV-visible absorption and PL spectra and CV data. The photocatalytic degradation of MB was performed under visible light irradiation. The CdS NCs exhibited the highest degradation efficiency among the three types of NCs, suggesting that the degradation of MB occurs mainly via the N-demethylation oxidation process. The relative photocatalytic abilities

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of the NCs determine the photodegradation efficiency of the hybrid nanostructures. The higher adsorption ability of the CNTs contributes to increasing the degradation efficiency. The degradation kinetics were analyzed using the LindemannHinshelwood model. The hybridization of the CdS NCs increases significantly the oxidative degradation rate of the TiO2 NCs and CNTs by the same factor, indicating that the interfacial electron transfer process may take place comparably in both cases. Acknowledgment. This study was supported by KOSEF (R01-2008-000-10825-0; 2008-02364), KRF (2008-314-C00175), and MKE under the ITRC support program supervised by the IITA (2008-C1090-0804-0013). This research was also supported by the WCU (World Class University) program through the NRF funded by the Ministry of Education, Science, and Technology (R31-10035). The HVEM (Daejeon), XRD (Taegu), and XPS (Pusan) measurements were performed at the KBSI. The experiments at the PLS were partially supported by MOST and POSTECH. Supporting Information Available: The XPS, EDX, XRD, Raman spectra, degradation data of MB depending on the initial concentration of MB (0.1-1 µM), and recycle test of photocatalysts. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (2) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625. (3) Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Nature 2006, 440, 295. (4) Yu, J. C.; Wu, L.; Lin, J.; Li, P.; Li, Q. Chem. Comm. 2003, 1552. (5) Tada, H.; Mitsui, T.; Kiyonaga, T.; Akita, T.; Tanaka, K. Nat. Mater. 2006, 5, 782. (6) Bessekhouad, Y.; Chaoui, N.; Trzpit, M.; Ghazzal, N.; Robert, D.; Weber, J. V. J. Photochem. Photobiol., A 2006, 183, 218. (7) Wu, L.; Yu, J. C.; Fu, X. J. Mol. Catal A: Chem. 2006, 244, 25. (8) Wang, J.; Liu, Z.; Zheng, Q.; He, Z.; Cai, R. Nanotechnology. 2006, 17, 4561. (9) Kim, J. C.; Choi, J.; Lee, Y. B.; Hong, J. H.; Lee, J. I.; Yang, J. W.; Lee, W. I.; Hur, N. H. Chem. Commun. 2006, 5024. (10) Li, H.; Zhu, B.; Feng, Y.; Wang, S.; Zhang, S.; Huang, W. J. Solid State Chem. 2007, 180, 2136. (11) Park, H.; Choi, W.; Hoffmann, M. R. J. Mater. Chem. 2008, 18, 2379. (12) Jang, J. S.; Choi, S. H.; Kim, H. G.; Lee, J. S. J. Phys. Chem. C 2008, 112, 17200.

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