A New Visible-Light Photocatalyst: CdS Quantum Dots Embedded

The resulting sol solution was gelled in an open Petri dish at 40 °C in air for .... This confirms that CdS QDs are well embodied into the TiO2 mesop...
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Environ. Sci. Technol. 2009, 43, 7079–7085

A New Visible-Light Photocatalyst: CdS Quantum Dots Embedded Mesoporous TiO2 G U I - S H E N G L I , †,‡ D I E - Q I N G Z H A N G , † A N D J I M M Y C . Y U * ,† Department of Chemistry, Environmental Science Programme and Centre of Novel Functional Molecules, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong (China), and Department of Chemistry, Shanghai Normal University, Shanghai 200234, China

Received April 24, 2009. Revised manuscript received July 20, 2009. Accepted July 23, 2009.

Cadmium sulfide quantum dots (QDs) sensitized mesoporous TiO2 photocatalysts were prepared by preplanting cadmium oxide as crystal seeds into the framework of ordered mesoporous titanium dioxide and then converting CdO to CdS QDs through ionexchange. The presence of CdS QDs in the TiO2 framework extended its photoresponse to the visible-light region by accelerating the photogenerated electron transfer from the inorganic sensitizer to TiO2. The new photocatalyst showed excellent photocatalytic efficiency for both oxidation of NO gas in air and degradation of organic compounds in aqueous solution under visible light irradiation. The photocatalysts were characterized by XRD, N2 adsorption-desorption, TEM, XPS, UV/ vis, and PL spectroscopy. The relationship between the physicochemical properties and the photocatalytic performance of the sample is discussed.

1. Introduction The depletion of energy resources and the degradation of the natural environment are two of the most urgent issues facing modern society. Visible light-driven photocatalytic technology can help to alleviate both problems by splitting water for green energy hydrogen production and degrading toxic pollutants (1, 2). The widely used photocatalyst TiO2 is only active in the UV range (3). Although dye-sensitized and transition metal-doped or nonmetal-doped TiO2 makes the utilization of visible light possible (4-9), many researchers focus their efforts on the design and development of coupling TiO2 with a low band gap semiconductor material (10-14). This interest is due to the fact that stable and efficient dyes are rare, whereas dopants are inevitably a recombination centers for the photogenerated electrons and holes. Cadmium sulfide is widely used to sensitize TiO2 for visible-light-driven applications (15, 16). TiO2/CdS composites can be prepared by sol-gel, chemical bath deposition and electro-deposition techniques (17-19). To obtain significantly improved photoelectrochemical properties, the architecture of the composite material is critical. Ordered mesoporous TiO2 offers a robust template for the incorporation of quantum-confined inorganic semiconductor sensitizers. Due to their larger surface area and multiple scattering, * Corresponding author phone: +(852)2609-6268; fax: +(852)26035057; e-mail: [email protected]. † The Chinese University of Hong Kong. ‡ Shanghai Normal University. 10.1021/es9011993 CCC: $40.75

Published on Web 08/12/2009

 2009 American Chemical Society

mesoporous TiO2 allows more light to be harvested. It also possesses continuous pore channels that facilitate the transfer of reactant molecules (20, 21). Our group has reported a number of ordered mesoporous metal ion or metal oxide sensitized TiO2 visible light photocatalysts and their superior catalytic performance (22, 23). Herein, we reported a new efficient and stable visiblelight driven photocatalyst, in which highly dispersed semiconductor quantum dots are incorporated within the framework of ordered mesoporous TiO2. To the best of our knowledge, ordered mesoporous TiO2 with CdS quantum dots (QDs) embedded in its framework has never been used as a visible-light driven photocatalyst for air and water purification. This is realized by planting CdO as a seed into the framework of ordered mesoporous TiO2 and then converting it to CdS by ion-exchange at room temperature. The introduction of CdS QDs into the TiO2 pore wall framework can effectively extend the photoresponse of TiO2 to the visible-light region and accelerate the electron transfer between CdS and TiO2 without destroying the ordered mesoporous structures. Enhanced visible-light photocatalytic performance is therefore expected.

2. Experimental Section 2.1. Sample Preparation. CdO/TiO2: 1.5 g of poly(alkyleneoxide) block copolymer (Pluronic F-127, BASF) was dissolved in 19 mL of ethanol (EtOH) containing 0.14 g Cd(NO3)2 · 4H2O. To this solution was added 0.015 mol of titanium tetrachloride (Aldrich) with vigorous stirring for 0.5 h. The resulting sol solution was gelled in an open Petri dish at 40 °C in air for 4 days. The as-prepared transparent sample was then calcined at 400 °C for 4 h in air. CdS/TiO2: 0.3 g CdO/TiO2 was dispersed in 50 mL of Na2S aqueous solution (0.2 M) with stirring for 6.0 h at room temperature. The products were subsequently filtered, washed with water and collected. A graphical illustration for the synthesis route is shown in Supporting Information (SI) Figure S1. For comparison, a pure ordered mesoporous TiO2 was also prepared by the same method. 2.2. Characterization. Low and wide-angle X-ray diffraction measurements were carried out in a parallel mode (ω ) 0.5°, 2θ varied from 0.5° to 5°, and 20° to 80°) using a Bruker D8 Advance X-ray diffractometer (Cu KR radiation, λ ) 1.5406 Å). The crystal size of anatase was calculated from Scherrer’s equation: D ) 0.9λ/βcosθ, where D is the crystal size, λ is the wavelength of X-ray radiation (0.15406 for Cu KR radiation), β is the full width at half-maximum of the (101) peak of anatase TiO2, and θ is the diffraction angle (24, 25). High-resolution transmission electron microscopy (HRTEM) was recorded in JEOL-2010F at 200 kV. The electron microscopy samples were recorded prepared by grinding and dispersing the powder in acetone with ultrasonication for 20 s. Carbon-coated copper grids were used as sample holders. The N2-sorption isotherms were recorded at 77 K in a Micromeritics ASAP 2010 instrument. All the samples were degassed at 150 °C and 10-6 Torr for 24 h prior to the measurement. The Brunauer-Emmett-Teller approach was used to determine the surface area. X-ray photoelectron spectroscopy (XPS) measurement was done with a PHI Quantum 2000 XPS system with a monochromatic Al KR source and a charge neutralizer. All the binding energies were referenced to the C1s peak at 284.8 eV of the surface adventitious carbon. The diffuse reflectance spectra of the samples over a range of 200-800 nm were recorded by a Varian Cary 100 Scan UV-vis system equipped with a VOL. 43, NO. 18, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. XRD patterns for CdS/TiO2 in wide-angle (a) CdO/TiO2, CdS/TiO2 in small-angle (b).

Labsphere diffuse reflectance accessory. The photoluminescent spectra (PLS) were recorded on a Varian Cary-Eclipse 500. 2.3. Photocatalytic Testing. 2.3.1. Gas Phase Oxidation of Nitric Oxide. The oxidation of NO gas in air at ambient temperature in a continuous flow reactor was chosen as a probe gas photocatalytic reaction. The volume of the rectangular reactor, made of stainless steel and covered with Saint-Glass, was 4.5 L (10 H × 30 L × 15 W cm). A 300 W commercial tungsten halogen lamp (General Electric) was used as the simulated solar light source. A piece of Pyrex glass was utilized to cut off the UV light below 400 nm. The lamp was vertically placed outside the reactor above the sample dish. Four minifans were fixed around the lamp to avoid the temperature rise of the flow system. The samples were prepared by coating an aqueous suspension of the mesoporous materials onto a dish with a diameter of 12.0 cm. The weight of the photocatalysts used for each experiment was kept at 0.1 g. The samples were dried at 70 °C and then cooled to room temperature. The stock NO gas had a concentration of 48 ppm (N2 balance, BOC gas) that was traceable to the National Institute of Standards and Technology (NIST) standard. The initial concentration of NO was diluted to about 400 ppb by the air stream supplied by a zero air generator (Thermo Environmental Inc. model 111). The humidity level of the NO flow was maintained at 70% (2100 ppmv) by passing the zero air streams through a humidification chamber. The gas streams were premixed thoroughly by a gas blender and the flow rate was controlled at 4 L/min byamassflowcontroller.Afterreachingadsorption-desorption equilibrium, the lamp was turned on. The concentration of NO was continuously monitored by a chemiluminescence NO analyzer (Thermo Environmental Instruments Inc. model 42c), which monitors NO, NO2, and NOx (NOx represents NO + NO2) at a sampling rate of 0.7 L/min (26). The removal rate (%) of NO was calculated based on the following equation: NO removal rate(%) ) ([NO]in - [NO]out)/[NO]in × 100% 2.3.2. Aqueous Phase Degradation of Organic Compounds. The photocatalytic degradation of methylene blue was carried out in an aqueous solution at ambient temperature. Briefly, in a 100 mL beaker, 0.08 g of ordered mesoporous CdS/TiO2 photocatalyst was suspended in 60 mL aqueous solution containing 10 ppm methylene blue. After 1 h of adsorption/ desorption equilibrium, the photocatalytic degradation of methylene blue was initiated by irradiating the reaction mixture with a commercial 300 W tungsten halogen spotlight surrounded with a filter that restricted the illumination to the 400-660 nm range (27). The light source was located at 8 cm from the reaction solution. Oxygen under atmospheric 7080

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pressure was bubbled through the reaction continuously. Photodegradation was monitored by measuring the absorbance of the solution at 664 nm. Only less than 2.0% methylene blue decomposed after 3 h in the absence of either the photocatalyst or the light irradiation and, thus, could be neglected in comparison with the methylene blue degraded via photocatalysis. The photocatalytic activities of the mesoporous CdS/TiO2 and pure TiO2 samples were also measured by the degradation of 4-chlorophenol in an aqueous solution. A 300W tungsten halogen lamp with a 400 nm cutoff filter was used as visible light source. O2 was bubbled into the reaction solution, containing 0.08 g of catalyst and 60 mL of 1.0 × 10-4 M 4-chlorophenol, throughout the experiment. After reacting for 4 h, the absorbance of the solution was measured at the characteristic wavelength of 4-chlorophenol (λ ) 224 nm).

3. Results and Discussion Wide-angle X-ray diffraction (WXRD) and small-angle X-ray diffraction (SXRD) were used to identify the crystalline phase and mesostructural ordering of the CdS/TiO2 sample. Figure 1a shows the WXRD. The diffraction peaks at 2θ of 25.3, 36.9, 38.2, 38.6, 48.1, 53.5, 55.6, 62.7, and 75.0° are attributed to anatase-TiO2 (JCPDF 21-1272). Meanwhile, there were two additional diffraction peaks at 2θ of 28.2 and 43.6, which could be attributed to (101) and (110) crystal planes of hexagonal CdS phase (JCPDF 41-1049) with a space group of P63mc(186). It should be pointed out that these peaks were rather weak due to the ultrasmall diameter of the CdS QDs. The other diffraction peaks of CdS were not clearly resolved because they overlapped with that of anatase TiO2. We also investigated the WXRD pattern of CdO/TiO2, as shown in SI Figure S2. No diffraction patterns ascribed to CdO phase can be found, perhaps owing to its ultrasmall size and high dispersity in TiO2. Nevertheless, those broaden peaks of CdS indicate that ultrafine CdS QDs can be produced through in situ converting CdO seeds by ion-exchange with S2- though obvious WXRD diffraction peaks of CdO can not be resolved in CdO/TiO2. The particle size of anatase for the CdS/TiO2 composites is found to be 10.3 nm, estimated from the fwhm of the TiO2 (101) peak by using the Scherrer’s formula. This indicates that the CdS/TiO2 sample is composed of crystallized anatase. The crystallization of the TiO2 mesoporous framework is a key factor for applying the sample in devices that utilize its semiconductor properties (28). From the lower 2θ region (Figure 1b), a strong peak at 0.93° (2θ) can be seen clearly in the XRD pattern of the CdO/ TiO2 composites, together with relatively weak peaks at 1.63, 1.85, and 2.45°. These diffraction peaks are the (100), (110), (200), and (210) reflections of the 2D hexagonal space group (p6 mm). The cell parameter of the CdO/TiO2 sample is 5.9

FIGURE 2. (a) N2-sorption isotherms (inset) and corresponding pore size distribution curves for CdO/TiO2 (a) and CdS/TiO2 (b). nm, calculated based on the equation of a0 ) 2d100/3j , in which, d ) nλ/2Sinθ, n ) 1, λ ) 1.5406 Å, θ is the diffraction angle of (100) peak (29). The sharp and strong (100) peak, together with the presence of the (110) and (200) peaks further demonstrates that the CdO/TiO2 sample is well organized in the meso scale. Upon ion-exchange, these diffraction peaks of (100), (110), (200), and (210) were red-shifted to about 1.00, 1.71, 1.97, and 2.51°, respectively. The cell parameter of the CdS/TiO2 sample is 5.1 nm. These results show that the in situ converting CdO seeds to CdS via ion-exchange has not destroyed the ordered structures of the mesoporous framework except for a slight decrease of cell parameter. Figure 2 shows the nitrogen adsorption-desorption isotherms (inset) and pore size distribution plots for the mesoporous CdO/TiO2 and CdS/TiO2. Both samples exhibit a type-IV isotherm being representative of mesoporous solids. The specific surface area of the CdS/TiO2 sample is 145 m2g-1 using the Brunauer-Emmett-Teller (BET) method. The pore diameter of the CdS/TiO2 is 4.1 nm (estimated using the desorption branch of the isotherm) with very narrow pore size distribution. The CdO/TiO2 samples possesses virtually identical average pore diameter (4.0 nm) and specific surface area (176 m2g-1), considering a typical uncertainty of 5% for BET surface area measurements (30). The ion-exchange with S2- results in a slight decrease, from 0.170 to 0.136 cm3/g, in the pore volume of CdO/TiO2. These results illustrate that the ion-exchange with S2- does not significantly change the textural properties of CdO/TiO2. The pore wall thicknesses of CdO/TiO2 and CdS/TiO2 are calculated to be about 1.9 and 1.0 nm, respectively, according to the equation of W ) a0 - P, in which, a0 is the cell parameter, and P is the pore diameter. The thin pore wall of CdO/TiO2 owing to the ionexchange further suggests that the CdS QDs are planted in the TiO2 network of the mesoporous pore walls by substituting the CdO seeds. The BET results also show that the mesoporous channels remain open. Such open mesoporous architecture with large surface area and connected poresystem plays an important role in catalyst design for its being able to improve the molecular transport of reactants and products (22). The transmission electron microscopy (TEM) images further confirm the long-range order structure of CdO/TiO2 (Figure 3a). The image also shows no trace of cadmium oxides in the pore channels, indicating that all of the cadmium oxide species are embodied into the mesoporous framework of TiO2. This result is consistent with that calculated from the BET data. Figure 3b shows that such ordered structure can be well maintained even after ion-exchange with S2-, though there are some observed distortions of the pore channels owing to the in situ transformation of CdO to CdS. Meanwhile, an elemental map is shown in Figure 3c. The red areas represent the S distribution, and the black areas correspond to the pores of the mesoporous CdS/TiO2. As illustrated in

the map, virtually all CdS quantum dots are highly dispersed in the pore walls of the mesoporous TiO2. This confirms that CdS QDs are well embodied into the TiO2 mesoporous network. The nanocrystalline nature of hexagonal CdS (solid ellipses) and anatase TiO2 (dot ellipses) can be well-defined in the HRTEM image of CdS/TiO2 as shown in Figure 3d. The particle size of the CdS QDs is about 4.0 nm, and the anatase TiO2 owns a particle size of 10.0 nm, which is consistent with that calculated from the XRD data. The particle size of CdS and TiO2 (relative standard deviation σ below 4.0%) were obtained from a statistical analysis of over 20 particles from several different HRTEM images. It is also obvious that heterogeneous junction between CdS and TiO2 has been derived by the present means owing to the intimate contact between CdS and TiO2. As known, the junction will lead to a more efficient interelectron transfer between the two components and improve the charge separation and therefore the photocatalytic activity (31). The XPS spectra of the as prepared mesoporous TiO2 and CdS/TiO2 (see SI Figure S3) show the characteristic spin-orbit split of Ti2p3/2 (SI Figure S3a), O1s (SI Figure S3b), Cd3d5/2 and Cd3d3/2 signals (SI Figure S3c), and S2p peak (SI Figure S3d). The ion-exchange of O2- with S2- at room temperature has no significant influence on the position of the Ti2p and O1s peaks. However, negative shifts of the binding energy of Cd3d are observed at about 0.4 and 0.3 eV for Cd3d5/2 and Cd3d3/2, respectively. The S2p peak is found at 161.4 eV (SI Figure S3d), corresponding to S2- of CdS nanoparticles (32, 33). The XPS results further suggest the formation of CdS QDs in the mesoporous TiO2 framework. The conversion ratio of CdO to CdS is estimated to be 96% based on the S to Cd molar ratio from XPS measurements. UV-visible diffuse reflectance spectroscopy (DRS) was used to characterize the electronic states of the as prepared samples. Figure 4 shows the UV-visible absorption spectra of mesoporous TiO2 and CdS/TiO2 samples. For large energy gap of anatase (3.2 eV), the former has no significant absorbance for visible-light. The samples of ordered mesoporous CdS/TiO2 exhibits a broad absorption bands from 200 to 500 nm, indicating the effective photoabsorption property for this ordered structure composite photocatalyst system. The ion exchange reaction leads to the growth of new visible absorption extending beyond the TiO2 band gap absorption and with an onset of approximately 500 nm. This new band provides strong evidence for the formation of CdS QDs indeed embedded in the pore wall of mesoporous TiO2, thus altering its electronic and optical properties. The enhanced ability to absorb visible-light of this type of ordered mesoporous CdS/TiO2 makes it a promising photocatalyst for solar-driven applications. SI Figure S4 shows that both TiO2 and CdS/TiO2 samples display a PLS peak at around 382 nm. It is an emission peak from band edge free excitation, mainly corresponding to the VOL. 43, NO. 18, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. (a) Standard TEM of CdO/TiO2, (b) TEM image of CdS/TiO2, (c) The chemical map of CdS/TiO2 (red areas correspond to the S distribution) and (d) HRTEM image of CdS/TiO2.

FIGURE 4. UV-visible absorption spectra of TiO2 and CdS/TiO2.

number of surface oxygen vacancies and/or surface defects (34-36). From TiO2 to CdS/TiO2, the intensity of the peak around 382 nm is increased. These results demonstrate that the introduction of CdS QDs into the ordered mesoporous TiO2 framework results in the increase of both the number of the oxygen vacancies and/or defects in the TiO2 crystal and the ability for light absorption. 7082

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Figure 5 shows the relative variations of NO removal rate against irradiation time for the pure TiO2 and CdS/TiO2 samples upon simulated solar-light irradiation at a single pass flow through the reactor under a humidity level of 2100 ppmv. Before turning on the light, the adsorption/desorption equilibrium had been reached. When the light was switched on, the photocatalytic oxidation of NO was initiated. The control experiment exhibited negligible activity without the photocatalyst. However, the removal rate of NO over the CdS QDs sensitized mesoporous TiO2 photocatalyst reached 45% after 15 min irradiation (Figure 5a). Comparing to that of C-doped TiO2, the activity was enhanced by about 84% (26). For the pure TiO2 sample, only about 9% NO gas was removed, indicating its low photoresponse to simulated solar light irradiation. This is because the pure mesoporous TiO2 can only be activated by the UV light, which only accounts for 4%, from solar light. The CdS QDs sensitized mesoporous TiO2 not only showed a good activity but also exhibited a high stability. The high photocatalytic activity could be maintained for the duration of the experiment. It is not surprising that the pure mesoporous TiO2 and the control experiment show no activity for NO degradation under visible light irradiation (Figure 5b). To investigate the recyclability of CdS/TiO2, a sample after one trial was washed and dried for the subsequent photoreaction cycles. As shown in SI

FIGURE 5. NO removal rate of TiO2 and CdS/TiO2 in a single pass flow of air under simulated solar light (a) and visible light (b) irradiation. Initial concentration of NO ) 400 ppb.

FIGURE 6. Photodegradation of methylene blue (a) and 4-chlorophenol (b) for mesoporous TiO2 and CdS/TiO2 under visible light irradiation (λ > 400 nm). A ) the concentration of pollutants, A0 ) initial concentration of pollutants. Figure S5, the NO removal rate for CdS/TiO2 was wellmaintained after five cycles under visible light irradiation. We also investigated the effect of the content of CdS on the photocatalytic performance of CdS/TiO2 in the treatment of NO. As shown in SI Figure S6, the sample with 3% molar ratio of CdS/TiO2 exhibits the highest NO removal rate under both solar light and visible light irradiation (SI Figure S6). The oxidation reaction of NO in this paper is believed to be initiated by · OH and O2· - radicals. The OH radicals are formed in air in the following reactions. + CdS/TiO2 + visible light f +eCB +hVB

(1)

·eCB + O2ads f O2ads

(2)

2eCB + O2 + 2H+ f H2O2

(3)

·· H2O2 + O2ads f HOads + OH- + O2

(4)

· NO + HOads f HNO2

(5)

· f NO2 + H2O HNO2 + HOads

(6)

· NO2 + HOads f HNO3

(7)

·f NONO + O2ads 3

(8)

According to the above mechanism, avoiding the undesirable electron-hole pair recombination is important for the continuous formation of · OH and O2· - radicals. The photocatalytic oxidation of NO over the CdS QDs sensitized mesoporous TiO2 photocatalyst involves a series of oxidation steps by the · OH

radical: NO f HNO2 f NO2 f HNO3. The NO removal rate slowly decreased with the irradiation time, which can be attributed to the accumulation of HNO3 on the catalyst surface resulting in deactivation of TiO2 photocatalysts (37, 38). To further evaluate and compare the photocatalytic performance of the mesoporous TiO2 and CdS/TiO2 in aqueous contaminants, the decomposition of methylene blue and 4-chlorophenol was used as probe photoreactions. As shown in Figure 6a, the pure mesoporous sample is ineffective but the mesoporous CdS/TiO2 composites show a very high decomposition rate of methylene blue under the irradiation of visible-light (400 nm < λ < 660 nm). Since there is a concern of methylene blue photosensitization under visible-light irradiation (39), a nonlight absorbing substrate, 4-chlorophenol, was also used to confirm the photocatalytic performance of the as-prepared catalysts. As shown in Figure 6b, similar results were obtained. Meanwhile, the dependence of the photocatalytic degradation efficiency of methylene blue and 4-chlorophenol on the CdS content in TiO2 was also studied (SI Figure S7). The sample containing 3% CdS exhibited the highest photocatalytic performance. The excellent photocatalytic performance in both gas phase and aqueous reactions for the CdS QDs sensitized mesoporous TiO2 composites can be attributed to its low band gap energy, fast electron transfer velocity, large surface area and ordered mesoporous structures. Low band gap energy makes it a promising photocatalyst for solar-driven applications. Fast electron transfer between CdS and TiO2 may lead to higher quantum efficiency, supplying more photogenerated electrons to be used in photocatalytic reactions. A large surface area will not only supply more active sites for the degradation reaction of organic compounds, but also effectively promote the separation efficiency VOL. 43, NO. 18, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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of the electron-hole pairs (40). Meanwhile, the light harvesting is also enhanced due to the large surface area and multiple scattering (20, 41). The high catalytic activity is also related to the open ordered mesoporous architecture. Chemical reactions are most effective when the transport paths through which molecules move into or out of the nanostructured materials are included as an integral part of the architectural design (42). In summary, framework embedded CdS quantum dots sensitized ordered mesoporous TiO2 was fabricated by planting CdO as a seed into the TiO2 network and then converting it to CdS by ion-exchange at room temperature. The resulting ordered mesoporous CdS/TiO2 composites possess a well crystallized anatase phase, large specific surface area, low band gap energy, and a tight contact between CdS and TiO2, resulting in an excellent visible-light photocatalytic activity.

(14) (15) (16) (17) (18) (19)

Acknowledgments This research was supported by a Strategic Investments Scheme administrated by The Chinese University of Hong Kong. We thank Prof. S. C. Lee’s group (Department of Civil and Structural Engineering of The Hong Kong Polytechnic University) for their assistance on the gas-phase measurements.

Supporting Information Available Schematic synthesis route; WXRD of CdO/TiO2, Highresolution XPS spectra of CdO/TiO2 and CdS/TiO2; PLS spectra of TiO2 and CdS/TiO2; Cyclability of CdS/TiO2 for NO removal, Dependence of NO, methylene blue and 4-chlorophenol removal rates on the molar ratio of CdS/ TiO2. This material is available free of charge via the Internet at http://pubs.acs.org.

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