Article pubs.acs.org/JPCC
Nanocomposites of CdS Nanocrystals with Montmorillonite Functionalized with Thiourea Derivatives and Their Use in Photocatalysis Cristiane C. Nascimento,† George R. S. Andrade,† Erick C. Neves,‡ Cintya D’Angeles Espirito Santo Barbosa,§ Luiz P. Costa,⊥ Ledjane S. Barreto,†,‡ and Iara F. Gimenez*,† †
Programa da Pós-graduaçaõ em Ciência e Engenharia de Materiais, Universidade Federal de Sergipe, Av. Marechal Rondon s/n, Cidade Universitária “Prof. José Aloísio de Campos”, CEP 491000-000, São Cristóvão - SE, Brazil ‡ Núcleo de Ciência e Engenharia de Materiais, Universidade Federal de Sergipe, Av. Marechal Rondon s/n, Cidade Universitária “Prof. José Aloísio de Campos”, CEP 491000-000, São Cristóvão - SE, Brazil § Núcleo de Pós-graduaçaõ em Química, Universidade Federal de Sergipe, Av. Marechal Rondon s/n, Cidade Universitária “Prof. José Aloísio de Campos”, CEP 491000-000, São Cristóvão - SE, Brazil ⊥ ITP − Instituto de Tecnologia e Pesquisa, Universidade Tiradentes, Av. Murilo Dantas, 300 - Prédio do ITP Bairro Farolândia, CEP: 49032-490, Aracaju, Sergipe S Supporting Information *
ABSTRACT: Nanocomposites of CdS were prepared in refluxing DMF using a montmorillonite organofunctionalized with thiourea derivatives, with different proportions of the matrix relative to Cd2+ ions. The matrix provided sulfur atoms for CdS formation as well as specific binding sites for systematic anchoring of the nanocrystals. However, addition of thiourea to the reaction medium was required in order to yield cubic CdS nanocrystals, and increasing the thiourea/Cd2+ molar ratio led to the formation of mixtures of cubic and hexagonal CdS. The samples obtained using a thiourea/Cd2+ ratio of 2, with cubic CdS, exhibited photoluminescence due to both direct recombination and surface trap states. The distribution of CdS throughout the matrix was characterized by the presence of aggregates (with diameters in the region of 50 nm) of nanocrystals (with diameters less than 7 nm) on the outer surface as well as small pillars in the interlayer region. The photocatalytic performance of the nanocomposites was tested in the degradation of Rhodamine 6G under sunlight irradiation, with promising results in terms of degradation time and degree of mineralization.
1. INTRODUCTION
provide a range of possible uses. Ion exchange, organofunctionalization, and pillarization have been exploited in the preparation of semiconductors. Furthermore, these materials are abundant, high surface area solids, and possess unique properties that enable them to be combined with metal sulfides to produce functionally useful materials.6 Their application as catalysts and photocatalysts7 has attracted attention, especially in the case of clay/CdS systems, whose photocatalytic activity can be higher than that of CdS alone.8 A variety of different clays have been studied, including montmorillonite, laponite,9 rectorite,8 and vermiculite,10 among others. In most cases, the preparation approach involves the exchange of the metal ions into the interlayer region, followed by exposure to H2S or Na2S. An alternative method involves the precipitation of CdS in cetyltrimethylammonium (CTA) micelles, followed by deposition of the preformed particles.11
Semiconductor nanocrystals, also known as quantum dots (QDs), have been extensively studied from both theoretical and practical perspectives. These are especially interesting nanomaterials that exhibit size-dependent optical and photophysical properties that can be controlled by surface modifications. These properties arise from the characteristic electronic structure of nanosized semiconductors, for which the magnitude of the band gap energy separating the valence and conduction bands is critically dependent on size for particles with sizes below the Bohr radius.1 CdS is one of the II−VI semiconductors that has been most studied. Various methods have been proposed for its preparation,2−4 most of which have aimed at stabilizing the particle size in the nanometric range as well as adapting the materials for use in specific applications.5 The growth of QDs over solid supports can help to accomplish both objectives, and several classes of supports have been used for this purpose. Layered solids such as clays and clay minerals are included among the supports of choice because different preparation approaches © 2012 American Chemical Society
Received: February 28, 2012 Revised: September 18, 2012 Published: September 22, 2012 21992
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Another approach is based on the formation of a Cd[NH2−CS− NH2]2Cl2 complex, where NH2−CS−NH2 is thiourea. The complex is added to a suspension of the clay, followed by hydrothermal treatment, yielding hexagonal CdS pillars in the intergallery spaces of the clay as well as nanoparticles in the outer space.12 The chemical modification of clays using silane coupling agents enables the introduction of functional groups that make the material suitable for interaction with different species.13 An example is organofunctionalization with sulfur-containing groups, which can improve the affinity of the supports for adsorption of heavy metal ions.14−16 Modification of the solid support with thiol and other sulfur-containing groups has been explored in the case of mesoporous matrices for the synthesis of metallic nanoparticles17 and is a novel technique for clays and semiconductor nanoparticles. Functionalization of silica and carbon surfaces with thiourea-analogue groups has been described in the contexts of catalysis18 and metal ion adsorption.19 This modification strategy may potentially influence the formation of the CdS particles by providing a sulfur source while improving certain characteristics of the supported nanoparticles, such as the homogeneity of the distribution of the nanocrystals. Here we propose the use of montmorillonite clay modified by thiourea-analogue groups for the preparation of CdS nanoparticles. The samples were characterized by UV/vis reflectance spectroscopy, photoluminescence (PL), XRD, and TEM imaging. The possible application of the material as a photocatalyst for dye degradation was tested using Rhodamine 6G.
Figure 1. Representation of the thiourea-derivative group anchored onto the clay surface.
radiation (λ = 1.5418 Å), operated at 40 kV/40 mA in continuous mode with a scanning rate of 2°/min (see Supporting Information). Thermogravimetric analyses employed a TA Instruments model SDT 2960 analyzer fitted with Pt pans and operated using a flow of nitrogen (100 mL min−1) and a heating rate of 10 °C min−1. 2.3. Preparation of Nanocomposites. 2.3.1. Influence of Different Proportions of the Matrix. The nanocomposites of MT-TU with CdS were prepared by dispersing different masses of MT-TU (100, 150, 200, and 300 mg) in 25 mL of a DMF solution containing 0.04 mol L−1 of cadmium acetate and 0.08 mmol L−1 of thiourea (thiourea/Cd2+ molar ratio = 2). The mixture was heated under reflux for 10 min with constant stirring (boiling point of DMF = 153 °C). After cooling to room temperature, the solid was filtered, washed with acetone, and dried at 45 °C for 12 h. The samples were denoted MT100, MT150, MT200, and MT300 according to the mass of MT-TU used. Characterization was accomplished by UV/vis reflectance spectroscopy, using an Ocean Optics HR2000 spectrophotometer coupled to an integrating sphere. PL spectra were acquired using an ISS/PC1 photon counting spectrofluorimeter equipped with a 30 W Xe lamp and 2 mm slits. XRD powder patterns were measured using the Rigaku RINT 2000/PC instrument. Transmission electron microscopy (TEM) images were obtained using a JEOL 2100 TEM-MSC microscope operated at an acceleration voltage of 200 kV. Samples were prepared for TEM observation by dropping a suspension of the nanocomposite in isopropanol onto a holey carbon-coated copper grid and allowing the solvent to evaporate at 80 °C in an oven. 2.3.2. Dependence of the CdS Crystal Phase on the Thiourea/Cd2+ Molar Ratio. The effect on the crystal phase of different thiourea/Cd2+ molar ratios (2, 5, 10, and 20) was studied while maintaining a constant MT-TU/Cd2+ ratio (equivalent to that of the MT200 sample) and a constant reaction medium volume. The samples were characterized by XRD under the same conditions described above (section 2.2.2). 2.4. Photocatalysis Tests. The sample with the smallest CdS nanocrystals (MT200), as characterized by TEM, was used for the photocatalysis tests. 30 mg portions of the photocatalyst were added to 10 mL aliquots of a 3 × 10−4 mol L−1 Rhodamine 6G solution placed in conical flasks fitted with stopcock adapters. The suspensions were stirred under sunlight using a multipoint magnetic stirrer. The experiments were conducted between the hours of 13:00 and 15:00, when fluctuations in sunlight intensity were negligible. Meteorological data indicated that in the region where the experiments were carried out, the average radiative flux
2. EXPERIMENTAL SECTION 2.1. Materials. Natural montmorillonite from Brazil was used in this work. The reagents (3-aminopropyl)trimethoxysilane and phenyl isothiocyanate were purchased from Sigma. Cadmium acetate and thiourea were purchased from ACROS Organics and used as received. N,N-Dimethylformamide (DMF) (Vetec), acetone (Cromoline), and Rhodamine 6G (R6G) (Merck) were used without purification. All solutions were prepared with ultrapure water generated by a Milli-Q purification system. 2.2. Modifications of the Clay. The sulfur-modified montmorillonite was prepared in a two-step procedure involving an initial grafting with (3-aminopropyl)trimethoxysilane, followed by conversion of the amino group to a thiourea derivative (Figure 1) by reaction with phenyl isothiocyanate. Experimental details are given below. 2.2.1. Grafting with (3-Aminopropyl)trimethoxysilane (Formation of MT-APTS). 3 g of montmorillonite (previously stirred in 100 mL of 1 mol L−1 NH4Cl for 24 h, centrifuged, and dried at 60 °C for 12 h) was suspended in 40 mL of a toluene solution containing 1.3 mL (7.5 mmol) of (3-aminopropyl)trimethoxysilane. The mixture was refluxed under N2 for 48 h. The resulting solid was filtered and then washed with anhydrous toluene and subsequently with anhydrous dichloromethane. 2.2.2. Preparation of Montmorillonite Functionalized with Thiourea Derivatives (MT-TU). 2.0 g of MT-APTS was treated with 0.72 mL (0.81 g, 6 mmol) of phenyl isothiocyanate dissolved in 50 mL of anhydrous ethanol, stirring the mixture under reflux for 24 h. The product was isolated by filtering the resulting solid and washing with anhydrous ethanol and used in the subsequent steps. During all preparation steps, the samples were characterized by XRD using a Rigaku RINT 2000/PC instrument with Cu Kα 21993
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Figure 2. UV/vis reflectance (a) and photoluminescence (b) spectra for CdS-containing samples.
was around 2900 kJ m−2 at the time of the experiments. During irradiation, a 1 mL aliquot was removed every 20 min for measurement of the UV/vis spectrum, until the absorption bands disappeared. All experiments were carried out in duplicate. An analogous series of experiments were carried out under dark conditions in order to evaluate the possible adsorption of the dye onto the clay support. All UV/vis measurements in solution were carried out using a PerkinElmer Lambda 45 UV/ vis spectrophotometer. Measurements of COD (chemical oxygen demand) were performed with the initial dye solution and also with the final liquid phase after the photocatalysis tests, using standard methods.20 Cd2+ concentrations were also determined (in duplicate) in the solutions after the photocatalysis experiments, using a Varian FS 220 spectrophotometer.
3. RESULTS AND DISCUSSION The samples containing CdS nanocrystals were characterized by UV/vis reflectance and photoluminescence spectroscopy, XRD, and TEM. Figure 2 shows the reflectance spectra obtained for the MT100, MT150, and MT200 samples, with maximum absorption wavelengths varying from 469 to 456 nm. It is worth noting that sample MT300 exhibited reflectance and PL spectra that were identical to those for MT150 and MT200. The absorption band for bulk CdS occurs at 515 nm, and here the shift of the absorption band to lower wavelengths may be evidence of quantum confinement.21 Changes in band position with increasing relative matrix proportion could suggest variations in the particle diameter.22 However, estimation of particle sizes by the effective mass approximation23 gave values of 4.0 nm (MT100) and 4.5 nm (all other samples), indicating that the UV/vis spectroscopy results 21994
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Figure 3. XRD pattern for the modified montmorillonite (MT-TU) and for the CdS-containing samples: (a) view of the 3−60° region in 2θ; (b) zoom of the 3−8° region.
The XRD data, on the other hand, revealed a clear effect of the relative matrix proportion. It is evident from Figure 3 that the relative intensities of the CdS peaks increased as the matrix proportion decreased. As the basal peak of the clay could still be observed, it appears that the presence of CdS did not induce a complete exfoliation of the stacked layers. As the matrix proportion increased, the basal peak broadened and shifted progressively to lower angles (see zoom in Figure 3b), suggesting that there was a progressive inclusion of CdS particles into the interlayer region of the clays. The basal peak shift (estimated by comparison with the original montmorillonite, MT) was subnanometric (MT: 1.64 nm; MT300: 1.74 nm; MT200: 1.82 nm; MT150: 1.84 nm; MT100: 1.90 nm), which in principle could be considered very small. However, Han and co-workers compared the basal peak shifts observed for a series of clay/CdS nanocomposites with a theoretical estimation of the maximum expected value.6 It was concluded that the observed values were larger than the expected values because the CdS pillars did not form a uniform layer within the galleries.6 In the case of CdS, broad peaks at around 2θ = 26.7°, 44.2°, and 51.8° could be assigned to planes 111, 220, and 311 of the cubic phase, respectively. The broadness of the peaks is consistent with the nanometric size of the particles. There was a predominance of the cubic phase for thiourea/Cd2+ = 2, while mixtures of cubic and hexagonal CdS were observed as the proportion of thiourea was increased. The relative intensities of the hexagonal phase peaks increased progressively, although no pure hexagonal CdS was obtained for thiourea/Cd2+ ratios of up to 20 (see Supporting Information). This observation suggests the participation of thiourea-analogue groups on the matrix as a source of sulfur atoms, as discussed below. The hexagonal wurtzite phase of CdS is thermodynamically the most stable and can be found in both the bulk material and nanoparticles, while cubic zinc blende CdS is only found in
Figure 4. Thermogravimetric (TG) curves: (a) original montmorillonite; (b) MT-TU; (c) MT100; (d) MT150; (e) MT200.
did not reflect any clear influence of relative matrix proportion on particle size. The photoluminescence spectra (Figure 2b,c) revealed two distinct regions for all samples. The bands in the region 475−550 nm are related to the intrinsic conduction band−valence band electron−hole recombination. The presence of emission at lower energy regions was evidenced by a broad and weak band at 550− 650 nm due to electron−hole recombination in surface trap states.24 Analogously to the absorption data, the PL spectra were not particularly sensitive to changes in the matrix proportion. A curve fit was performed in order to measure the band position more accurately (Figure 2c), and it was observed that in all cases the surface trap band occurred at almost the same position. 21995
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Figure 5. TEM images: (a) MT-TU clay; (b−f) CdS-containing samples (MT200 was chosen as illustrative). In (b) and (c), the scale bars represent 100 and 10 nm, respectively.
nanoparticles.25 The phase behavior of CdS nanoparticles prepared from Cd−thiourea complexes depends on the molar ratio between precursors,26 the annealing temperature,26 the thermolysis conditions (reflux,27,28 hydrothermal,6,9,12 and microwave29,30), and the solvent used.27 Grimes and coworkers26 discussed the effect of the thiourea/Cd ratio on the phase behavior in terms of the way in which the symmetry of the precursor complex favored each crystal phase. It was pointed out that interpenetrating tetrahedra are in an eclipsed conformation in hexagonal CdS and in a staggered conformation in cubic CdS. It was also noted that low thiourea ratios led to precursor
tetrahedra with lower symmetry, which would cause steric hindrance in an eclipsed conformation. In this case, the cubic phase would therefore be favored. On the other hand, the hexagonal phase would be favored by the more symmetrical precursor tetrahedra that are typical at higher thiourea proportions. We believe that this was a significant factor here. In addition, the coordination of Cd2+ ions to thiourea groups on the matrix surface may have contributed to steric hindrance during the formation of the CdS nanoparticles, inducing adoption of the staggered conformation by the precursor complex. This would 21996
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Figure 6. TEM images of CdS-containing samples: (a, b) MT200; (c, d) MT150; (e, f) MT100.
Other factors that may influence the phase behavior of CdS nanocrystals include the hydrothermal conditions under which the cubic nanocrystals can readily dissolve and then reprecipitate in the hexagonal form.32 Another significant parameter is the nanocrystal size, which can be influenced by the solvent used.27 Although less stable thermodynamically, cubic CdS is nucleated faster since it has a lower interfacial energy than the hexagonal phase and thus a lower nucleation barrier.33 If the nanocrystals are allowed to grow, the phase volume related to larger nanocrystals will favor the more stable hexagonal phase.
favor cubic structure and cause the organic groups to be consumed during CdS formation, which was corroborated by the thermal analyses. The TG curves obtained for all samples are shown in Figure 4, where it can be seen that MT-TU exhibited a mass loss at 237 °C, which was absent for unmodified MT. This was attributed to decomposition of organic moieties in the temperature range observed for free thiourea derivatives.27 Such mass loss was not observed for samples containing CdS, for which the curve resembled that for unmodified MT in this temperature range. 21997
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Figure 7. Results of the photocatalysis tests: (a) evolution of the solution absorption spectra with time during the photocatalysis; (b) comparison of the concentration relative to the initial value at each time in the photocatalysis and adsorption experiments; (c) decolorization efficiency according to time; (d) Langmuir−Hinshelwood kinetic model plots.
some filling of the interlayer space, although further analyses will be needed to confirm this possibility. TEM images for the samples prepared using different proportions of the matrix are shown in Figure 6, at magnifications favoring the observation of separate nanocrystals. Although there was a substantial variability in nanocrystal size, in general larger isolated particles were favored as the matrix proportion decreased. An accurate evaluation of size distribution was not possible since the particle boundaries were not well-defined, and there was superposition of particles located at different depths in the matrix. However, a crude estimate gave values of 5, 6.5, and 10 nm for samples MT200, MT150, and MT100, respectively. The possible use of the material as a photocatalyst was tested using Rhodamine 6G (R6G), a xanthene dye employed in lasers and as a fluorescent probe in biological applications. Figure 7a shows the variation in the absorption spectrum of R6G with time of exposure to sunlight, with a progressive decrease in intensity across the entire spectrum. Between 0 and 140 min of irradiation, there was a shift in the maximum absorption wavelength, from 527 to 503 nm, which could be attributed to N-deethylation of R6G. This has been suggested to occur mainly on the surface of the photocatalyst, while the degradation itself is described as a solution process.34,35 The decrease in intensity can be explained by the decolorization that accompanies photocatalysis, which is likely to be caused both by modifications in the chromophore group and by the mineralization of the dye molecule. Here we observed that COD decreased by one-third following irradiation, which gives a measure of the degree of mineralization. Although the dye
The TEM images (Figure 5) showed that prior to the deposition of CdS (Figure 5a) the surface of the clay was uniform, without the presence of grains with higher contrast. The image of the sample containing CdS (Figure 5b) shows aggregates (larger than 50 nm) distributed across the outer surface of the clay, each composed of CdS nanocrystals of smaller size (Figure 5c). The spatial distribution of the aggregates suggested that the organic groups on the clay acted as specific interaction sites, ensuring a good anchoring of the inorganic particles. Separate smaller spherical particles of low contrast can be seen around the aggregates (Figure 5d), which were probably formed in the interlayer regions of the clay, as will be discussed. Since the formation of CdS occurs in a single step, it is impossible to avoid the presence of particles outside the interlayer region. Peng and co-workers reported the formation of CdS nanocrystals within the galleries of rectorite and observed that washing the samples after adsorption of Cd2+ ions, prior to sulfurization, was crucial to avoid formation of aggregates.8 However, here the properties of the material did not suffer from any detrimental effect of the presence of the aggregates, since quantum confinement was clearly observed. This provides an explanation for the observation that the spectroscopic properties were not sensitive to different preparation conditions. An image obtained at higher magnification (Figure 5e) shows separate particles with sizes close to 5 nm. Further insight into the morphology of the material is provided by Figure 5f, which shows a view of stacked layers, corroborating conservation of the layered structure. The presence of dark points between the layers could be indicative of 21998
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was not completely degraded, the decrease after 140 min was comparable to that described for other photocatalysts after similar exposure times.36 Analyses showed that there was leaching of Cd2+ during the process (final concentration of 4 ppm), but at a lower rate than observed previously.28 Another point to be considered in the present case is that adsorption of R6G onto the photocatalyst could also contribute to the decrease in intensity. It is important to clarify that although adsorption onto the clay surface could lead to an erroneous attribution of decolorization to photocatalysis, the adsorption of dye molecules over the CdS particles is not undesirable, since it is considered to be the initial step in the photocatalysis process. To evaluate this possibility, an analogous series of experiments were carried out under dark conditions. The results (Figure 7b) showed that adsorption did, in fact, occur during approximately the first 20 min, although it was not possible to quantify the relative contributions of the clay and semiconductor surfaces to the overall adsorption. When the entire process was considered in terms of photocatalytic efficiency (eq 1), an efficiency of 99% was achieved after 140 min (Figure 7c). Peng and co-workers8 obtained a photodegradation efficiency of 94.6% for CdS/ rectorite nanocomposite, using a two-step procedure that allowed the removal of Cd2+ ions adsorbed on the outer surface. Our results indicate that despite the impossibility of doing the same using the one-step method chosen here, the properties of the material were not prejudiced. efficiency (%) =
C0 − C × 100 C0
AUTHOR INFORMATION
Corresponding Author
*Tel 55-79-21056652; Fax 55-79-21056651; e-mail gimenez@ ufs.br. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to CNPq, Capes, and Fapitec (Brazilian funding agencies) for financial support. The participation of LNNano-LME (Campinas, SP, Brazil) in the TEM analyses is gratefully acknowledged. We are also indebted to Professors F. S. Dias (UFC) and E. F. S. Vieira (UFS).
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REFERENCES
(1) Brus, L. E. J. Chem. Phys. 1984, 80, 4403−4409. (2) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706−8715. (3) Pileni, M. P. Langmuir 1997, 13, 3266−3276. (4) Yuwen, L. H.; Bao, B. Q.; Liu, G.; Tian, J.; Lu, H. T.; Luo, Z. M.; Zhu, X. R.; Boey, F.; Zhang, H.; Wang, L. H. Small 2011, 2, 1456−1463. (5) El Hamzaoui, H.; Bernard, R.; Chahadih, A.; Chassagneux, F.; Bois, L.; Jegouso, D.; Hay, L.; Capoen, B.; Bouazaoui, M. Nanotechnology 2010, 21, 134002−134010. (6) Han, Z.; Zhu, H.; Ratinac, K. R.; Ringer, S. P.; Shi, J.; Liu, J. Microporous Mesoporous Mater. 2008, 108, 168−182. (7) Rajeshwar, K.; Osugi, M. E.; Chanmanee, W.; Chenthamarakshan, C. R.; Zanoni, M. V. B.; Kajitvichyanukul, P.; Krishnan-Ayer, R. J. Photochem. Photobiol. C 2008, 9, 171−192. (8) Xiao, J.; Peng, T.; Ke, D.; Zan, L.; Peng, Z. Phys. Chem. Miner. 2007, 34, 275−285. (9) Han, Z.; Yang, Q.; Shi, J.; Lu, G. Q.; Lewis, S. W. Solid State Sci. 2008, 10, 563−568. (10) Matejka, V.; Supova, M.; Klemm, V.; Rafaja, D.; Valaskova, M.; Tokarsky, J.; Leskova, J.; Plevova, E. Microporous Mesoporous Mater. 2010, 129, 118−125. (11) Praus, P.; Kozák, O.; Koci, K.; Panacek, A.; Dvorsky, R. J. Colloid Interface Sci. 2011, 360, 574−579. (12) Han, Z.; Zhu, H.; Bulcock, S. R.; Ringer, S. P. J. Phys. Chem. B 2005, 109, 2673−2678. (13) He, H.; Duchet, J.; Galy, J.; Gerard, J. F. J. Colloid Interface Sci. 2005, 288, 171−176. (14) Guimarães, A. M. F.; Ciminelli, V. S. T.; Vasconcelos, W. L. Appl. Clay Sci. 2009, 42, 410−414. (15) Tonle, I. K.; Ngamenia, E.; Tcheumi, H. L.; Tchieda, V.; Carteret, C.; Walcarius, A. Talanta 2008, 74, 489−497. (16) Tchinda, A. J.; Ngameni, E.; Kenfack, I. T.; Walcarius, A. Chem. Mater. 2009, 21, 4111−4121. (17) Wu, P.; Bai, P.; Lei, Z. B.; Loh, K. P.; Zhao, X. S. Microporous Mesoporous Mater. 2011, 141, 222−230. (18) Chen, H.-T.; Trewin, B. G.; Wiench, J. W.; Pruski, M.; Lin, V. S.-Y. Top. Catal. 2010, 53, 187−191. (19) An, X.; Zheng, H. Carbon 2003, 41, 2889−2896. (20) APHA Standard Methods for the Examination of Water and Wastewater, 20th ed.; American Public Health Association: Washington, DC, 1998. (21) Rajeshwar, K.; Tacconi, N. R.; Chenthamarakshan, C. R. Chem. Mater. 2001, 13, 2765−2782. (22) Smith, A. M.; Nie, S. Acc. Chem. Res. 2010, 43, 190−200. (23) Brus, L. J. Phys. Chem. 1986, 90, 2555−2560. (24) Liqiang, J.; Yichun, Q.; Baiqi, W.; Shudan, L.; Baojiang, J.; Libin, Y.; Wei, F.; Honggang, F.; Jiazhong, S. Sol. Energy Mater. Sol. Cells 2006, 90, 1773−1787. (25) Bandaranayake, R. J.; Wen, G. W.; Lin, J. Y.; Jiang, H. X.; Sorensen, C. M. Appl. Phys. Lett. 1995, 67, 831−833. (26) Bao, N.; Shen, L.; Takata, T.; Domen, K.; Gupta, A.; Yanagisawa, K.; Grimes, C. A. J. Phys. Chem C 2007, 111, 17527−17534.
(1)
Finally, fitting of the data to the Langmuir−Hinshelwood kinetic model7 gave a straight line (Figure 7d), confirming that the process followed this model, which is widely employed to describe the kinetics of reactions occurring at solid surfaces as pseudo-first-order processes. The good fit to this model may indicate that although the surface of the photocatalyst was covered by both dye molecules and reactive species such as hydroxyl radicals,37 the coverage only varied significantly with time in the case of the dye, since reactive species were constantly generated in the reaction medium by the interaction between water molecules and holes.
4. CONCLUSIONS Modification of montmorillonite with thiourea-analogue groups had a significant effect on the stabilization of CdS nanocrystals in the pure cubic phase during a single step preparation procedure, due to the participation of organic moieties as a source of sulfur. The distribution of CdS was controlled both on the outer surface, in the form of aggregates composed of nanocrystals smaller than 10 nm, and in the interlayer space, as small pillars whose size was dependent on the relative proportion of the matrix. The nanocomposites were active as photocatalysts for the degradation of Rhodamine 6G by sunlight.
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ASSOCIATED CONTENT
S Supporting Information *
XRD characterization of modifications in diffraction patterns after functionalization. This material is available free of charge via the Internet at http://pubs.acs.org. 21999
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(27) Orimi, R. L.; Shahtahmasebi, N.; Tajabor, N.; Kompany, A. Physica E 2008, 40, 2894−2898. (28) Andrade, G. R. S.; Nascimento, C. C.; Neves, E. C.; Barbosa, C. D. E. S.; Costa, L. P.; Barreto, L. S.; Gimenez, I. F. J. Hazard. Mater. 2012, 203−20, 151−157. (29) Wada, Y.; Kuramoto, H.; Anand, J.; Kitamura, T.; Sakata, T.; Mori, H.; Yanagida, S. J. Mater. Chem. 2001, 11, 1936−1940. (30) Majumder, M.; Karan, S.; Chakraborty, A. K.; Mallik, B. Spectrochim. Acta, Part A 2010, 76, 115−121. (31) Naumov, A. V.; Semenov, V. N.; Goncharov, E. G. Inorg. Mater. 2001, 37, 539−543. (32) He, X.; Gao, L. J. Phys. Chem. C 2009, 113, 10981−10989. (33) Ricolleau, C.; Audinet, L.; Gandais, M.; Gacoin, T.; Boilot, J.-P.; Chamarro, M. J. Cryst. Growth 1996, 159, 861−866. (34) Watanabe, T.; Takirawa, T.; Honda, K. J. Phys. Chem. 1977, 81, 1845−1851. (35) Seery, M. K.; George, R.; Floris, P.; Pillai, S. C. J. Photochem. Photobiol. A 2007, 189, 258−263. (36) Kansal, K.; Singh, M.; Sud, D. J. Hazard. Mater. 2007, 141, 581− 590. (37) Fox, M. A.; Dulay, M. A. Chem. Rev. 1993, 93, 341−357.
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