Controlled Growth and Photocatalytic Properties of CdS Nanocrystals

Oct 21, 2005 - Cd2(OH)3(DS)‚2.5H2O (CdDS), where DS stands for dodecyl sulfate sandwiched between two adjacent inorganic layers, have been ...
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J. Phys. Chem. B 2005, 109, 21602-21607

Controlled Growth and Photocatalytic Properties of CdS Nanocrystals Implanted in Layered Metal Hydroxide Matrixes Ying Guo, He Zhang, Yao Wang, Zuo-Lei Liao, Guo-Dong Li, and Jie-Sheng Chen* State Key Laboratory of Inorganic Synthesis and PreparatiVe Chemistry, College of Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: August 7, 2005; In Final Form: September 15, 2005

Layered double hydroxide Cd1-xAlx(OH)2(DS)x‚3.0H2O (CdAlDS) and a related hydroxide salt compound Cd2(OH)3(DS)‚2.5H2O (CdDS), where DS stands for dodecyl sulfate sandwiched between two adjacent inorganic layers, have been synthesized and used as precursors for CdS nanoparticle growth. Through a gas/ solid reaction, CdS nanocrystals implanted in the layer matrixes of the layered double hydroxides are grown, and the sizes of the nanocrystals vary in the range of 3-6 nm in diameter. The presence of trivalent Al cations in the layered double hydroxide can be taken advantage of to control the size of the CdS nanocrystals, and it also helps to prevent the formed nanocrystals from extraction from the solid matrixes. The nano-CdS implanted composite exhibits high photocatalytic activity for degradation of the nonbiodegradable rhodamine B under both UV and visible irradiations.

Introduction Under conventional preparation conditions, crystallites with a nanometer size tend to aggregate or to undergo Ostwald ripening, resulting in particles exceeding the quantum effect domains.1,2 To prevent the particle size from increasing, the synthesis of nanoparticles usually involves the use of terminating and/or stabilizing reagents.2 One of the approaches to terminate and to stabilize quantum-sized nanoparticles is encapsulation of the particles in inorganic and organic hosts.3 As an important II-VI semiconductor and photocatalytic material, nanosized CdS has received extensive interest in control of morphology and size.4 There have been considerable advances in the stabilization of CdS nanoparticles through the use of solid materials including zeolites,3a mesoporous silica,5 clays,6 layered titanates7a,b or niobate,7c polymer films,8 Nafion,3b,9 and Langmuir-Blodgett films.10 These solid matrixes provide media more suitable than colloidal solutions for controlling the CdS particle aggregation, and among them the layered compounds have been exploited as a host for the formation of CdS nanoparticles in the interlayer region. Interlayer CdS nanoparticles are likely to be highly anisotropic2a,11 and have been found to show promising photocatalytic properties.6,7a,12 However, as is known, the interlayer region of a layered host compound has only one strained dimension, and therefore, the size of the nanoparticles formed in the interlayer region cannot be controlled in all three dimensions. In this paper, we describe an alternative approach to grow CdS nanoparticles in a layered host compound. In our case, the semiconductor nanoparticles are not sandwiched between adjacent inorganic layers but are implanted in the layer matrixes of the host compound instead. The layered host compound we used is a particular layered double hydroxide (LDH) containing divalent Cd and trivalent Al atoms. Layered double hydroxides are a family of lamellar materials which find wide applications as catalysts or catalyst supports, * Corresponding author. Phone: [email protected].

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adsorbents, and anion exchangers.13 These compounds can be represented by the general formula [MII1-xMIIIx(OH)2]x+(Am-)x/m‚ nH2O,14 where MII is a divalent metal ion, MIII a trivalent metal ion, and Am- an anion located in the interlayer gallery. Another type of layered compound related to LDHs are the so-called layered hydroxide salts in which there are no trivalent metal atoms and a part of the hydroxide groups are replaced by anions, and generally the corresponding formula for these compounds is [MII2(OH)3]+(A-)‚nH2O.15 We chose a particular layered double hydroxide Cd1-xAlx(OH)2(DS)x‚3.0H2O (CdAlDS) and a related hydroxide salt compound Cd2(OH)3(DS)‚2.5H2O (CdDS), where DS stands for dodecyl sulfate sandwiched between two adjacent inorganic layers, as precursors for the CdS nanocrystal preparation. Through a gas/solid reaction route, CdS nanocrystals are implanted and stabilized in the inorganic sheets of the lamellar metal hydroxides. It is interesting to note that depending on the Cd/Al ratio in the LDH precursor, the size of the CdS nanoparticles can be varied from 3 to 6 nm. Furthermore, the photocatalyic performance of the nano-CdS implanted composite for the degradation of rhodamine B (RhB) has been investigated and found to be distinctly superior to that of bulk CdS and even to that of nanosized TiO2 in terms of activity. Experimental Section Materials Preparation. Synthesis of Cd1-xAlx(C12H25SO4)x‚ 3.0H2O (CdAlDS) Precursors. The LDH precursor compounds were synthesized through direct solution reaction of Cd(NO3)2‚ 4H2O, Al(NO3)3‚6H2O, and the surfactant dodecyl sulfate (C12H25SO4Na). Typically, an aqueous solution (15.8 mL) of 0.02 mol of Cd(NO3)2‚4H2O and 0.01 mol of Al(NO3)3‚6H2O was mixed with a solution of the dodecyl sulfate surfactant (0.02 mol of sodium dodecyl sulfate in 85 mL of H2O). An aqueous solution of 2 mol‚L-1 NaOH was added to the above solution until the pH reached 8.0. The resulting mixture was aged at room temperature for 6 h, and a white precipitate was formed. The precipitate was separated through centrifugation, washed with distilled water, and dried under vacuum. Other CdAl-LDH

10.1021/jp054400q CCC: $30.25 © 2005 American Chemical Society Published on Web 10/21/2005

CdS Nanocrystals in Layered Metal Hydroxides precursors with different Cd/Al ratios were also successfully obtained through varying the Cd/Al molar ratio in the reaction mixture. Synthesis of Cd2(OH)3(C12H25SO4)‚2.5H2O (CdDS) Precursor. The synthesis procedure for CdDS was similar to that for the CdAlDS compounds. A 2 mol‚L-1 NaOH aqueous solution was added dropwise to the mixture of 0.02 mol of Cd(NO3)2‚ 4H2O and an equal molar dodecyl sulfate in 100 mL of H2O until a particular pH (8.0) was reached. Consequently, a white precipitate was formed and separated through centrifugation. Gas/Solid Reaction. The layered precursor material in its powdered form was sealed in a glass vessel, and the vessel was evacuated. Afterward, excessive H2S gas was injected into the vessel. Upon introduction of the H2S gas, the color of the precursor in the vessel turned from white to light yellow. The reaction was carried out for 2 h at 100 °C in order to ensure that the precursor was sulfurized properly. All the precursor materials were subject to the gas/solid sulfurization reaction separately to form differently sized CdS nanoparticles implanted in the metal hydroxide layer matrixes. Characterization. General Characterization. The powder X-ray diffraction (XRD) patterns were recorded on a Siemens D5005 diffractometer with Cu KR (λ ) 1.5418 Å) radiation. The UV-vis diffuse reflectance spectra were measured on a Perkin-Elmer Lambda 20 spectrometer. The transmission electron microscope (TEM) images were taken on a JEOL JEM3010 electron microscope, whereas the scanning electron microscope (SEM) images were obtained on a JEOL JSM-6700F electron microscope. Solid-state 27Al MAS NMR spectroscopy was performed on a Varian Infinity Plus 400 NMR spectrometer operated at 104.20 MHz. The spinning speed was 10 kHz, and the chemical shift reference was 1.0 M Al(NO3)3 aqueous solution. The FT-IR spectra of the samples dispersed in KBr pellets were obtained within the 4000-500 cm-1 wavenumber region on a Bruker IFS 66V/S FT-IR spectrometer. The thermogravimetric analyses (TGA) were conducted on a Netzsch STA 449C thermal analyzer under a flow of dry air at a heating rate of 20 K min-1. The C, H, N, and S elemental analysis was performed on a Perkin-Elmer 2400 elemental analyzer, whereas the metal contents were determined by inductively coupled plasma (ICP) analysis on a Perkin-Elmer Optima 3300DV ICP spectrometer. It was revealed from the ICP, CHNS elemental, and TG/DTA analyses that the composition formulas of the products were Cd2Al(OH)6(C12H25SO4)‚3.0H2O (found: Cd, 33.4%; Al, 4.0%; C, 21.61%; H, 5.53%; S, 4.75%) for CdAlDS (Cd/Al ) 2/1) and Cd2(OH)3(C12H25SO4)‚2.5H2O (found: Cd, 34.1%; C, 23.91%; H, 5.38%; S, 5.33%) for CdDS, respectively. Photocatalytic Testing. Photocatalytic experiments in aqueous solution were performed in a water-cooled quartz vessel. The UV light was generated from a 400 W high-pressure mercury lamp, and the visible light was generated from a 500 W xenon lamp. A suspension containing a powdered catalyst (720 mg) and fresh aqueous solution of RhB (720 mL; 50 ppm) was ultrasonicated for 5 min and magnetically stirred in the dark for at least 30 min to establish an adsorption/desorption equilibrium of the RhB sepcies. At given irradiation time intervals, a series of aqueous solutions in a certain volume were collected and filtered through a Millipore filter for analysis. The photocatalytic performance of the catalyst was estimated by monitoring the visible absorbance (at λ ) 555 nm) characteristic of the target RhB through UV-vis spectroscopy using a PerkinElmer Lambda 20 UV-vis spectrometer. Since the molar absorptivity of the dye was very high, the sample after filtration was diluted twice to accurately quantify the dye concentration.

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Figure 1. Powder X-ray diffraction patterns of (a) CdAlDS with a Cd/Al ratio of 2/1 and (b) CdDS within the 2θ ranges of 2-10° (left) and 4-40° (right).

Figure 2. Schematic models for (a) CdAlDS and (b) CdDS intercalated with dodecyl sulfate anions.

Results and Discussion From the powder X-ray diffraction (XRD) patterns (Figure 1), it is seen that both types of precursor compounds have a well-ordered layer structure, although the reflection intensities for CdAlDS (Cd/Al ) 2/1) are, to a certain degree, lower than those for CdDS. As the sum of the length of the DS molecule (21.3 Å) and the layer thickness (about 4.8 Å) almost coincides with the d-spacing value (27.1 Å) of CdAlDS on the basis of the XRD data,16 a monolayer of surfactant chains oriented perpendicularly and interdigitated between the inorganic brucitelike layers of CdAlDS is envisioned (Figure 2a). The basal spacing of CdDS (36.3 Å) is larger than that of CdAlDS, and therefore, the surfactant molecules in CdDS should be arranged in a way different from that for the DS molecules in CdAlDS. From the composition analysis results (Experimental Section) we know that the content of surfactants in CdDS is higher than that in CdAlDS. The high density of surfactant molecules in the interlayer region of CdDS prevents the molecules from being interdigitated because there is not enough free space around each surfactant chain to accommodate molecules from the opposite direction. Taking into account the basal spacing value (36.3 Å) for CdDS, it is more likely that the surfactant molecules in CdDS are arranged in a bilayer mode and tilted at an angle of R ≈ 48° (Figure 2b). On the other hand, the XRD patterns of the CdAlDS samples (not shown) with Cd/Al ratios of 1/1,

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Figure 3. Powder X-ray diffraction patterns for (a) CdS-1a, (b) CdS1b, (c) CdS-1c, (d) CdS-1d, and (e) CdS-2.

4/1, and 6/1 (the Cd/Al ratios were confirmed by the ICP analysis) are more or less the same as the one for the CdAlDS sample with a Cd/Al ratio of 2/1, except that the relative diffraction intensities vary from sample to sample. It is believed that the surfactant molecules in all the CdAlDS samples are similarly oriented because the basal spacings for these compounds are almost identical. The color of the dehydrated CdAlDS and CdDS precursors turns from white to yellow rapidly when the precursors were exposed to H2S gas at about 100 °C. This color change suggests that CdS particles are formed in the products (CdS-1 and CdS2) through the gas/solid reaction. Effects of the Cd/Al ratio in the CdAlDS precursor on the H2S-treated material have been examined, and the corresponding products are designated CdS1a through CdS-1d for Cd/Al ratios of 1/1, 2/1, 4/1, and 6/1, respectively. The XRD peaks typical of ordered layer stacking for both CdS-1 and CdS-2 almost disappear, whereas three broad peaks corresponding to the (111), (220), and (311) reflections of cubic CdS4c,17 show up for CdS-1 (Figure 3). The average size of the cubic CdS nanocrystals estimated from the Scherrer formula18 increases with the increase of the Cd/Al ratio as follows: 2.0 nm (CdS-1a), 3.0 nm (CdS-1b), 4.0 nm (CdS-1c), and 6.0 nm (CdS-1d). In contrast, the CdS nanoparticles in CdS-2 appear to be a hexagonal phase, and their average size is about 9.0 nm on the basis of the XRD data (Figure 3e). It should be pointed out that in the XRD pattern of CdS-2, a weak (200) diffraction peak of cubic CdS is also present besides the wurtzite peaks, suggesting that the resulting product contains a small amount of cubic CdS phase as well as the predominant hexagonal phase. It has been reported previously that larger nanoparticles tend to crystallize as the hexagonal phase, whereas smaller CdS nanocrystals are likely to exist as the cubic phase.18 Therefore, it is not unusual that the CdS nanocrystals in CdS-1 belong to the cubic phase, whereas those in CdS-2 are mainly hexagonal in phase. Figure 4 shows the corresponding transmission electron microscope (TEM) images for CdS-1b and CdS-2. The particle sizes in these images are in agreement with those estimated from the XRD data. Inspection of the TEM images also reveals that the CdS nanoparticles in both CdS-1b and CdS-2 are separated from one another by noncrystalline regions. The TEM image with a higher magnification for the CdS nanocrystal (Figure 4b) in CdS-1b shows a lattice d-spacing of about 0.33 nm corresponding to the (111) reflection.17,19 A spacing of about 0.36 nm has also been observed for the higher magnification TEM image (Figure 4d) of the CdS particle in CdS-2, and this spacing value matches the plane separation of the (100) reflection of a hexagonal CdS crystal.20 For CdS-1, the treatment of the CdAlDS precursor with H2S gas at 100 °C leads to the nucleation of CdS nanocrystals which grow further. Since the

Figure 4. Transmission electron microscope (TEM) images of (a) CdS1b, (b) a single CdS particle in CdS-1b with a higher magnification, (c) CdS-2, and (d) a single CdS particle in CdS-2 with a higher magnification.

Figure 5. UV-vis diffuse reflectance spectra of (a) CdS-1b, (b) CdS2, and (c) bulk CdS.

layer matrixes contain trivalent AlIII species around the CdII cations in the CdAlDS precursor, the further growth of the formed CdS nuclei is effectively hindered and terminated at an early stage. As a result, the particle size of the CdS nanocrystals in CdS-1 remains smaller, especially when a CdAlDS precursor with a small Cd/Al ratio is used. The situation for CdS-2 is somewhat different. Upon nucleation of CdS nanoparticles, the cadmium hydroxide species in the layer matrix surrounding the CdS nuclei in the CdDS precursor cannot prevent the nanoparticles from growth in an effective way, and therefore the particle size of the CdS nanocrystals in CdS-2 appears to be larger. The particle size difference of CdS nanocrystals in CdS-1b and CdS-2 is further demonstrated by the UV-vis diffuse reflectance spectra (Figure 5) of the two materials. In comparison with that of the bulk CdS, the absorption edge of CdS-2 formed from the sulfurization of CdDS blue-shifts by about 70 nm, whereas the absorption edge of CdS-1b formed from CdAlDS blue-shifts by 120 nm. The blue shift in the UV-vis spectrum due to an increase in the band gap is associated with a decrease in the particle size, characteristic of the so-called quantum size effects.17 The 27Al MAS NMR spectra of CdAl-LDH before and after sulfurization are shown in Figure 6. We can monitor the chemical environments of the Al atoms and elucidate the effect of sulfurization process on the surrounding of Al atoms through inspection of the 27Al MAS NMR spectra. From the structure point of view, octahedral [Cd(OH)6] and octahedral [Al(OH)6] in CdAl-LDH share edges to form layers with the brucite (Mg(OH)2) structure, and the positive charges in the inorganic layers

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Figure 6. 27Al MAS NMR spectra of CdAlDS before (a) and after (b) sulfurization treatment. The / stands for a spinning sideband.

Figure 8. Scanning electron microscope (SEM) images of (a) CdAlDS, (b) CdS-1b, (c) CdDS, and (d) CdS-2.

Figure 7. The IR spectra of CdAlDS (left) and CdDS (right) before (a) and after (b) sulfurization treatment.

are balanced by dodecyl sulfate anions sandwiched between the layers. From Figure 6, it is seen that the signal at around 13 ppm (Figure 6a) arising from octahedrally coordinated Al21 in the inorganic layers of the precursor material also exists after sulfurization (Figure 6b). However, after sulfurization, besides the 13 ppm signal, a new broad peak centered at around -1.9 ppm appears. Obviously, the hydroxyls around the Cd atoms in the precursor compound have been consumed and replaced by sulfur anion during the sulfurization treatment, causing the distortion of the [Al(OH)6] octahedra to some extent. The replacement of the hydroxyls by sulfur anions also increases the shielding ability of the hydroxyl groups surrounding the Al atoms adjacent (through the OH bridges) to the Cd atoms, and as a result, the chemical shift of these Al atoms moves toward higher field whereas that of the remaining Al atoms which are not immediately adjacent to the Cd-S species is unchanged, as observed from the 27Al MAS NMR spectra. The IR spectra (Figure 7) of CdAlDS and CdDS show that besides a broad absorption band at around 3500 cm-1 due to the presence of hydroxyl groups of LDH, there exist strong C-H stretching bands at 2919 and 2844 cm-1 and a C-H bending band at around 1469 cm-1, confirming the presence of DS molecules in the two precursor samples. As expected, the absorption bands of the sulfate ion at 1255, 1226, 1083, 998, and 593 cm-1 also appear, whereas absorptions of nitrate (1380 cm-1) and carbonate (1450 cm-1) ions are not observed for CdAlDS and CdDS, indicating that these compounds are highly pure in terms of intercalating anions. However, after sulfurization, the broad absorption band at around 3500 cm-1 decreases distinctly in intensity, reflecting the partial replacement of the hydroxyls by sulfur anions during the transformation of the CdAlDS and CdDS precursors to the CdS-1 and CdS-2 materials. Although the orderliness of layer stacking for the sulfurized samples is lost as indicated by the XRD patterns, the morphologies of the samples undergo no distinct variation after the sulfurization reaction (Figure 8). It is seen that the compounds before and after sulfurization all appear as irregular platelike particles characteristic of a sheet structure, and the particle

diameter varies from 2 to 80 µm. Therefore, the sulfurization treatment for both CdAlDS and CdDS does not affect the macroscopic integrity of the samples. As is known, TiO2 has been extensively used as an efficient photocatalyst in the chemical decomposition of many organic substrates.22 In principle, ultrafine particles of TiO2 should be superior to large-sized TiO2 crystals because the former possesses larger external surface areas than the latter. However, in practice when tested as photocatalyst, TiO2 ultrafine particles are easy to disperse in the reaction system and it is difficult to separate them from the final reacted mixture.23 Therefore, although with a high surface area, TiO2 ultrafine particles may not be suitable for use as a photocatalyst. Another disadvantage of TiO2 as a photocatalyst is that it responds only to UV light, and this retards the utilization of light in the visible region.7a On the other hand, CdS is a photocatalyst with a relatively narrow band gap (2.4 eV) and it has been shown that CdS is photocatalytically active under irradiation with visible light.24,25 We anticipate that the combination of the photocatalytic activity of CdS and the property of LDH along with the expansible interlayer space will be advantageous for photocatalysis. In this regard, we explored the photocatalytic properties of CdS-1 and CdS-2 using rhodamine B, which is relatively difficult to degrade, as a model organic substrate in aqueous media for degradation testing. The photocatalytic performance is estimated by monitoring26 the absorbance at λ ) 555 nm characteristic of the RhB molecule. As shown in Figure 8, CdS-1b is very active for the degradation of the dye molecules under UV irradiation ([RhB] ) 50 ppm, 720 mL dispersion, and 720 mg of CdS-1b loading) (Figure 9a). Furthermore, CdS-1b is recyclable for the photocatalytic reaction, and no CdS nanoparticles are extracted from the solid matrixes during the reaction process. The photocatalytic behavior of other CdS-1 samples is similar to that of CdS-1b. CdS-2 is as active as CdS-1b, implying that the difference in CdS particle size for these two materials is not large enough to influence the photocatalytic activity (Figure 9b). However, after the photocatalytic reaction process, a considerable amount of small particles are extracted from the solid matrixes of CdS-2 and they enter into the liquid phase to form a colloid. These small particles in the colloid can get through the Millipore filter, and UV-vis spectroscopy and XRD patterns confirm that the small particles extracted from the CdS-2 solid material correspond to CdS nanocrystals. Therefore, it is believed that the CdS nanoparticles in CdS-2 are less stabilized than those in CdS-1 and the matrixes with

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Figure 11. Schematic representation of the growth of CdS nanocrystals implanted in the metal hydroxide layer matrixes. Figure 9. Concentration change of RhB irradiated with UV light as a function of irradiation time (tirr) in the presence of (a) CdS-1b, (b) CdS2, (c) TiO2 (Degussa P-25), (d) bulk CdS, and (e) no catalyst. Ct and C0 stand for the RhB concentrations after and before irradiation.

Figure 10. Concentration change of RhB irradiated with a xenon lamp light as a function of irradiation time (tirr) in the presence of (a) CdS1b, (b) bulk CdS, (c) TiO2 (Degussa P-25), and (d) no catalyst. Ct and C0 stand for the RhB concentrations after and before irradiation.

AlIII in CdS-1 samples play an important role in holding the CdS nanoparticles during photocatalytic processes. For comparison, the photocatalytic performance of the commercial nanosized TiO2 (Degussa P-25) (Figure 9c) and the bulk CdS (Figure 9d) ([RhB] ) 50 ppm, 720 mL dispersion, and 720 mg of TiO2 or bulk CdS loading) was also assessed under the same experimental conditions. It is seen from Figure 8 that, as a photocatalyst, CdS-1 is distinctly superior to the bulk CdS and to the nanosized TiO2 (we also encountered separation difficulty for the nanosized TiO2-containing system after photocatalytic testing) for the degradation reaction in terms of activity. In the absence of a catalyst, RhB degrades very slowly under UV irradiation (Figure 9e). Most importantly, we have observed that CdS-1b shows a considerable photocatalytic activity for RhB degradation when a xenon lamp is used as the irradiation source for solar light simulation (the main part of the xenon lamp radiation is in the visible region, i.e., λ > 400 nm) (Figure 10a). For comparison, the photocatalytic activities of P-25 and bulk CdS under xenon lamp irradiation have also been tested. It is found that P-25 catalyzes the dye degradation very slowly, whereas the activity of bulk CdS (Figure 10b) is marginally superior to that of P-25 (Figure 10c). The poor photocatalytic performance of P-25 under xenon lamp irradiation is reasonable since TiO2 absorbs no radiation in the visible region. Bulk CdS does absorb visible light, but the photogenerated holes and electrons are less effective in the catalytic process, as addressed later on in this paper. Xenon light illumination on RhB in the absence of a catalyst leads to no degradation of the dye at all (Figure 10d), indicating that the RhB degradation needs an electron-transfer mediator under visible light. In general, the superior photocatalytic property of CdS-1 is rationalized by the presence of small CdS nanocrystals implanted

and stabilized in the layer matrixes of the composite material. The photoinduced holes and electrons in the CdS nanocrystals migrate to the nanoparticle surface easily, and they are effectively trapped at the interface between the nanoparticle and the layer matrix, where they are captured by the reactants in the solution. Furthermore, despite the replacement of hydroxyls by sulfur anions, there are still many hydroxyls surrounding the CdS nanocrystals in CdS-1, as confirmed by the IR spectra. The hydroxyls can capture the photoproduced h+, preventing the recombination of the h+ and e-, and consequently improve the photocatalytic activity.23 In contrast, the bulk CdS surface is less efficient in trapping the photoinduced holes and electrons, and as a result, it is less active photocatalytically. On the other hand, although the host layers of the CdAlDS precursor are markedly altered in terms of long-range ordering after H2S treatment, the dodecyl sulfate molecules still exist, rendering the adjacent layer matrixes containing the CdS nanocrystals apart from each other, as schematized in Figure 11. The pillaring of surfactants in combination with the swelling capability of the layered materials in solution leads CdS-1 to provide sufficient space for H2O and the dye molecules to penetrate from the solution to the active centers of the catalyst. As a comparison, we also tested the photocatalytic properties of an H2S-treated (Cd,Al)-containing LDH intercalated with nitrate ions instead of the dodecyl sulfate ions, but only insignificant photocatalytic activity was found for this material. Therefore, the intercalation of bulky anions such as dodecyl sulfate is crucial for the CdS implanted LDH to exhibit high photocatalytic activities. In the case of CdS-2, although it shows an initial photocatalytic activity similar to that of CdS-1, the CdS nanocrystals implanted in the metal hydroxide matrixes are less stabilized, and therefore CdS-2 is not favorable for photocatalytic application. To further assess the stability of the nanoparticles implanted in the different catalysts in aqueous solution, CdS-1 (with various Cd/Al ratios) and CdS-2 (without trivalent Al) were immersed into water, respectively. The corresponding aqueous mixtures were sonicated and stirred for 48 h and then left static overnight. It was found that the CdS-1 sample precipitated from the treated aqueous system and no CdS nanoparticles were extracted from the CdS-1 solid, whereas CdS-2 became a suspension solution. A part of each treated system was collected and filtered through a Millipore filter for analysis. The amount of CdS nanoparticles extracted out from the composites was estimated by UV-vis spectroscopy of the filtrates. No CdS nanoparticles were detected for the CdS-1 filtrate, whereas the filtrate of CdS-2 contained a considerable amount of CdS nanoparticles. These results clearly demonstrate that the presence of AlIII species in the inorganic layers plays a critical role in stabilizing the CdS nanoparticles implanted in the layer matrixes. Conclusions We demonstrate a new approach for the controlled growth of CdS nanocrystals which are implanted and stabilized in the

CdS Nanocrystals in Layered Metal Hydroxides inorganic layer matrixes of an LDH host. Such semiconductor/ inorganic layered composites are attractive because the presence of the trivalent AlIII ions in the metal hydroxide precursor is favorable for size controlling and tuning of the implanted CdS nanocrystals. Moreover, with AlIII, the implanted nanocrystals are more tightly bound in the matrixes than without AlIII, and as a result, the nano-CdS implanted composite exhibits high stability against nanoparticle extraction from the solid host when used as a photocatalyst in an aqueous system, permitting catalytic reactions to take place at the active sites. The photocatalytic activity of the nanocomposite is distinctly superior to that of bulk CdS and TiO2 for the photodegradation of rhodamine B under irradiation of both UV and visible light. It is highly possible for the approach described in the current paper to be extended to the preparation of other composite materials with implanted functional nanocrystals. Acknowledgment. We gratefully acknowledge the National Natural Science Foundation of China and the Education Ministry of China for financial support. References and Notes (1) (a) Guo, S.; Popovitz-Biro, R.; Weissbuch, I.; Cohen, H.; Hodes, G.; Lahav, M. AdV. Mater. 1998, 10, 121. (b) Dabbousi, B. O.; RodriguezViejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. 1997, 101, 9463. (2) (a) Cao, G.; Rabenberg, L. K.; Nunn, C. M.; Mallouk, T. E. Chem. Mater. 1991, 3, 149. (b) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmuller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665. (c) Pradhan, N.; Efrima, S. J. Am. Chem. Soc. 2003, 125, 2050. (d) Kwon, K. W.; Shim, M. J. Am. Chem. Soc. 2005, 127, 10269. (e) Sone, E. D.; Stupp, S. I. J. Am. Chem. Soc. 2004, 126, 12756. (3) (a) Wang, Y.; Herron, N. J. Phys. Chem. 1987, 91, 257. (b) Kakuta, N.; White, J. M.; Campion, A.; Bard, A. J.; Fox, M. A.; Webber, S. E. J. Phys. Chem. 1985, 89, 48. (c) Wankhede, M. E.; Haram, S. K. Chem. Mater. 2003, 15, 1296. (d) Zhang, Z. T.; Dai, S.; Fan, X. D.; Blom, D. A.; Pennycook, S. J.; Wei, Y. J. Phys. Chem. B 2001, 105, 6755. (e) Bunker, C. E.; Harruff, B. A.; Pathak, P.; Payzant, A.; Allard, L. F.; Sun, Y. P. Langmuir 2004, 20, 5642. (f) Lemon, B. I.; Crooks, R. M. J. Am. Chem. Soc. 2000, 122, 12886. (g) Du, H.; Xu, G. Q.; Chin, W. S.; Huang, L.; Ji, W. Chem. Mater. 2002, 14, 4473. (4) (a) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 183. (b) Holman, J.; Ye, S.; Nevandt, D. J.; Davies, P. B. J. Am. Chem. Soc. 2004, 126, 14332. (c) Cao, Y. C.; Wang, J. H. J. Am. Chem. Soc. 2004, 126, 14336. (d) Shenton, W.; Pum, D.; Sleytr, U. B.; Mann, S. Nature 1997, 389, 585. (e) Shen, G. Z.; Cho, J. H.; Yoo, J. K.; Yi, G. C.; Lee, C. J. J. Phys. Chem. B 2005, 109, 9294. (f) Sherman, R. L.; Ford, W. T. Langmiur 2005, 21, 5218. (g) Gao, T.; Wang, T. J. Phys. Chem. B 2004, 108, 20045. (h) Zhang, P.; Cao, L. Langmuir 2003, 19, 208. (5) (a) Xu, W.; Liao, Y. T.; Akins, D. L. J. Phys. Chem. B 2002, 106, 11127. (b) Besson, S.; Gacoin, T.; Ricolleau, C.; Jacquiod, C.; Boilot, J. P. Nano Lett. 2002, 2, 409. (c) Wang, S. Z.; Choi, D. G.; Yang, S. M. AdV. Mater. 2002, 14, 1311. (d) Hirai, T.; Okubo, H.; Komasawa, I. J. Phys. Chem. B 1999, 103, 4228. (e) Tura, C.; Coombs, N.; Dag, O. Chem. Mater. 2005, 17, 573. (6) (a) Enea, O.; Bard, A. J. J. Phys. Chem. 1986, 90, 301. (b) Stramel, R. D.; Nakamura, T.; Thomas, J. K. Chem. Phys. Lett. 1986, 130, 423. (c)

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