Regenerated Cellulose Nanocomposite Films for Highly Efficient

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J. Phys. Chem. C 2009, 113, 16021–16026

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CdS/Regenerated Cellulose Nanocomposite Films for Highly Efficient Photocatalytic H2 Production under Visible Light Irradiation Dingning Ke, Shilin Liu, Ke Dai, Jinping Zhou, Lina Zhang, and Tianyou Peng* College of Chemistry and Molecular Science, Wuhan UniVersity, Wuhan 430072, China ReceiVed: April 12, 2009; ReVised Manuscript ReceiVed: July 15, 2009

The photocatalytic activities of CdS nanoparticles immobilized in porous regenerated cellulose (RC) films with different pore sizes were investigated. The resulting CdS/RC nanocomposite films were characterized by using transmission electron microscopy, scanning electron microscopy, X-ray diffraction, thermal gravimetric analysis, and UV-vis diffuse reflectance spectroscopy. The mean pore sizes of the porous RC films can be modulated from about 20 to 57 nm by adjusting the concentration of the cellulose solution, and the porous structures within RC film act as reacting sites to lead to the embedment of CdS nanoparticles with a mean particle diameter of about 8 nm. The photocatalytic H2 evolution efficiencies over the obtained CdS/RC nanocomposite films were investigated by using Na2S-Na2SO3 mixed solution as a sacrificial reagent under visible-light (λ g 420 nm) irradiation. Experimental results indicate that the present nanocomposite films revealed obvious predominance, such as high visible-light photoactivity for H2 production, enduring photostability, and convenient regeneration in comparison with CdS nanoparticles suspension system. This new pathway for the fabrication of portable visible-light-driven photocatalyst is important for the H2 production via “green” processes. 1. Introduction Hydrogen, as an attractive sustainable clean energy source, has the potential to take the place of fossil energy resources, particularly coal and oil.1-3 Since the photoelelctrochemical splitting of water into H2 and O2 on a titanium dioxide (TiO2) electrode was first reported in 1972,4 photocatalytic H2 production over semiconductors has been attracting extensive attention.5,6 However, most of semiconductors, such as TiO2, mainly absorb the ultraviolet due to their wide bandgap (∼3.2 eV for anatase TiO2), which only contains ∼4% radiant energy of the sunlight. Therefore, to develop novel photocatalysts with visible-light response is indispensable for the photocatalytic H2 production techniques.7-9 Among the reported photocatalysts with visible-light response, cadmium sulphide (CdS) has been extensively studied because of its excellent water photosplitting property, in that its bandgap (∼2.3 eV) corresponds well with the spectrum of sunlight and its conduction band edge is more negative than the H2O/H2 redox potential.10 Nevertheless, CdS is prone to photocorrosion during the photochemical reaction because CdS itself is oxidized by the photogenerated holes, a fact which obstructs the largescale application of the photocatalytic H2 production over CdS nanoparticles in a suspension system.11 Therefore, some reducing agents (such as, S2-, SO32-, or S2O32-) have also been applied for stabilizing CdS nanoparticles in an aqueous suspension system.2,12 Moreover, syntheses of CdS nanoparticles in some heteromatrices have also attracted greater research interest, as the relative matrix can effectively suppress the photocorrosion of CdS during the photochemical reaction.13-16 For example, coupling CdS nanoparticles with another wide bandgap semiconductor and incorporating CdS nanoparticles into some inorganic matrix with layer or porous structures has been * To whom correspondence should be addressed. Tel: +86-27-87218474; Fax: +86-27-6875-4067. E-mail: [email protected].

reported for the stabilization and immobilization of the CdS nanoparticles.17-21 It has been reported that mesoporous silica18,19 and alumina20,21 can improve the stability and enhance the photocatalytic activity of the loaded CdS nanoparticles, but the above inorganic compounds used as the matrix may keep the light from reaching the CdS.20 Therefore, the use of organic porous films in the synthesis of heteromatrices has been the subject of immense interest due to their distinctive optical and convenient reactivating properties.22 Furthermore, organic porous films as matrices of CdS nanoparticles also provide an interface for the charge transfer, and then for the improvement of photocatalytic efficiency.23-26 In past decades, cellulose films have been extensively used as commercial materials in the field of membrane separation processes due to their relatively low cost, porous structure, high thermal stability, remarkable hydrophilic property, and good compatibility with biological compounds.27 Zhang’s group have reported that the cellulose can be dissolved in LiOH/urea (4.2/ 12.0 wt %) aqueous solution precooled to -12 °C,28-30 and cellulose films with good mechanical properties were prepared from the cellulose solutions.29 The hydrophilic pores within the cellulose film make it quite suitable as a host matrix for the embedment of some semiconductor (e.g., CdS) nanoparticles. Moreover, the porous structures within the cellulose film can also prevent the agglomeration and growth of CdS nanoparticles into excessively large particles. Therefore, CdS nanoparticles with smaller particle sizes and larger surface areas can be expediently immobilized in the cellulose film to obtain CdS/ cellulose nanocomposite. The photogenerated electrons and holes inside those CdS nanoparticles are able to quickly migrate to the particles’ surfaces, while the large surface areas of CdS within the nanocomposite films are also inclined to quicken the interfacial charge carrier transfer and separation, which can greatly decrease the bulk recombination of the photogenerated carriers during the photochemical reaction. Moreover, CdS/

10.1021/jp903378q CCC: $40.75  2009 American Chemical Society Published on Web 08/06/2009

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cellulose film also possesses obvious advantage, such as convenient reactivation from an industrial viewpoint. Herein, the porous RC films with different pore sizes were selectively prepared by adjusting the concentration of cellulose solution, and then CdS nanoparticles were homogeneously embedded in its porous structures. The photocatalytic H2 production efficiency of the obtained CdS/RC nanocomposite films under visible-light irradiation was also investigated in detail. 2. Experimental Section 2.1. Materials. The cellulose (cotton linter pulp) used was provided by Hubei Chemical Fiber Group Ltd. (Xiangfan, China). The cellulose sample was washed with distilled water 5× and dried in a vacuum oven at 70 °C prior to use. Its viscosity-average molecular weight (Mη) was determined by using an Ubbelohde viscometer in LiOH/urea aqueous solution at 25 ( 0.05 °C and, calculated from the equation [η] ) 3.72 × 10-2 Mw0.77 to be 13.6 × 104.28 Other chemical reagents were commercially available with analytical grade and used without further purification. 2.2. Preparation of Regenerated Cellulose Films. CdS/ regenerated cellulose (CdS/RC) nanocomposite films were prepared according to Zhang’s patent.30 Cellulose was added immediately into 200 g LiOH/urea aqueous solution (5.0/12.0 wt %), which was precooled to -12 °C in a refrigerator, and then stirred vigorously for 5 min at ambient temperature to dissolve it. The resultant cellulose solution was subjected to centrifugation at 8000 rpm for 10 min at 10 °C to eliminate some bubbles in the viscous solution. The resultant viscous bubble-free solution was cast on a glass plate and the thickness of the solution was controlled about 0.25 mm by using a glass rod with two metal wires at each end, then immersed it into 5 wt % H2SO4 solution to regenerate for 5 min under a coagulation bath (30 °C). The regenerated cellulose (RC) films were washed with running water and deionized water, and then air-dried at ambient temperature. The RC films prepared with cellulose concentration of 4, 4.5, and 5 wt % were coded as RC-4, RC4.5, and RC-5, respectively. The obtained RC films were immersed into 0.1 M CdCl2 solution for 12 h, and then picked out and rinsed with deionized water 3×. Subsequently, the resulting Cd2+-loaded films were put into Na2S solution (0.05 M) for 20 min, and then rinsed with deionized water until the color of the films became uniformity. Finally, the resulted CdS/RC nanocomposite films (CdS/RC-4, CdS/RC-4.5, and CdS/RC-5) were pasted on PMMA plate and dried at ambient conditions. 2.3. Characterization of Materials. Scanning electron microscope (SEM) observations were carried out on a FEI Quanta 200 electron microscope. High-resolution transmission electron microscopy (HRTEM) image was performed on a JEOL JEM 2010 FEF (UHR) at 200 kV. The nanocomposite films were embedded in an epoxy resin. Ultrathin slices for HRTEM measurements were obtained via sectioning on a LKB-8800 ultraome. X-ray diffraction (XRD) patterns were obtained on a XRD-6000 diffractometer using Cu KR as radiation (λ ) 0.15418 nm). The diffuse reflectance absorption spectra (DRS) were recorded on a Cary 5000 UV-vis-NIR spectrophotometer equipped an integrating sphere by using BaSO4 as a reference. Thermal gravimetric analysis (TGA) was carried out by using a thermogravimetric analyzer (Netzsch, Germany). The films were ground into powder and about 5 mg of the powder was placed in a platinum pan and heated from 20 to 700 °C at a rate of 10 K · min-1 under air atmosphere. The tensile strength

Figure 1. SEM images of the pristine RC films and CdS/RC nanocomposite films: (a) RC-4; (b) RC-4.5; (c) RC-5; (d) CdS/RC-4; (e) CdS/RC-4.5; (f) CdS/RC-5.

(σb) and elongation at break (εb) of the films in the dry state were measured on a universal tensile tester (CMT 6503, Shenzhen SANS Test machine Co. Ltd., China) according to ISO 527-2, 1993 (E) at a speed of 5 mm/min, The σb and εb values were the average results of five measurements. 2.4. Photocatalytic Activity Test. The photocatalytic H2 evolution experiments were carried out in a closed gascirculation system. A 250-W Xe-illuminator (CHF-XM-250W, Beijing Trusttech Co., China) was used as light source, which was collimated and focalized into 5 cm2 parallel faculae, then translated into uprightness light by viewfinder. A cutoff filter (Kenko L-42, λ g 420 nm) was employed for visible-light irradiation. Pt as a cocatalyst for the promotion of H2 evolution was photochemically deposited on the surface of the nanopposite films by using H2PtCl6 as precursor solution. The photocatalytic reaction was performed in a reaction cell (Pyrex glass) containing Pt-loaded nanocomposite films (0.050 g) and Na2S (0.25 M)-Na2SO3 (0.35 M) mixed solution (100 mL), then the suspension was thoroughly degassed to remove air completely, and the reactor was irradiated from the top. The amount of evolved H2 was determined by gas chromatography (GC, SP6800A, thermal conductivity detector, 5 Å molecular sieve columns and Ar carrier). For evaluating the photostability of the nanocomposite films, 0.050 g of CdS/RC-4.5 was immersed into 100 mL Na2S (0.25 M)-Na2SO3 (0.35 M) mixed solution under visible-light irradiation at different time intervals, and then the Cd2+ contents in the remnant solutions were detected by a 7500a ICP-MS system (Agilent, United States). 3. Results and Discussion 3.1. Microstructure Analyses of CdS/RC Films. The microstructures and morphologies of the pristine RC films and CdS/RC nanocomposite films are shown in Figure 1, and the SEM images for the pristine RC-4, RC-4.5, and RC-5 films were observed at the freeze-dried state of the swollen film. As can be seen from Figure 1, parts a, b and c, the pristine RC films display porous structures, which are a result of the phase separation of the cellulose solution during the regenerating process, and the solvent-rich regions contribute to the pore formation.29 The pore size distribution in the films becomes more uniform and the pore size is getting smaller with enhancing the cellulose’s concentration. On the basis of Hagen-Poiseuille calculating method,27 the mean pore size of RC-4, RC-4.5, and RC-5 film is 57, 43, and 20 nm, respectively. Namely, the pore size within the RC films can be modulated by adjusting the

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Figure 3. X-ray diffraction patterns of the pristine RC film and CdS/ RC nanocomposite films: (a) RC-4.5; (b) CdS/RC-4; (c) CdS/RC-4.5; and (d) CdS/RC-5.

Figure 2. HRTEM images of the CdS/RC nanocomposite films: (a), (d) CdS/RC-4; (b) CdS/RC-4.5, and (c) CdS/RC-5.

cellulose concentration during the regenerating process. As can be seen from the SEM images of CdS/RC (dry at air) in Figure 1, parts d, e, and f, the porous structures in the pristine RC films disappear and some scattered particles are observed over the flat surfaces of the composite films, indicating that CdS nanoparticles can fill into the porous structures within RC film, Cd2+ ions could be readily impregnated into the porous structures of RC film and attached onto the cellulose macromolecules via electrostatic interactions when the RC film was immersed into CdCl2 solution, then react with Na2S aqueous solution in the present reaction conditions. In fact, the colorless RC film was transformed into yellow immediately after the present embedment condition of CdS, which is a sign of the formation of CdS nanoparticles. In comparison with other organic films,22-25 the porous structures within RC film could effectively suppress the growth and agglomeration of CdS nanoparticles. Moreover, the porous RC film with remarkable hydrophilic property possesses stronger electrostatic interactions with Cd2+, which results in a larger Cd2+-loading amount and further stabilize the formed CdS nanoparticles.31 Namely, the porous structures of the cellulose could serve as excellent supports to hold and stabilize these inorganic nanoparticles.32 Therefore, it is rational to speculate that the decreased pore size within the RC film may result in a higher CdS-loading amount in a certain range. TEM images of the CdS/RC-4, CdS/RC-4.5, and CdS/RC-5 nanocomposite films are shown in Figure 2. As can be seen, the CdS nanoparticles within CdS/RC-4, CdS/RC-4.5, and CdS/ RC-5 nanocomposite films possess a particle size distribution in the range of 5-12 nm with a mean particle size of 8.4, 8.2, and 8.6 nm, respectively. The relative smaller particle sizes of CdS are beneficial for the fast electron transfer and the suppression of the bulk recombination of the photogenerated carriers, and then for the improvement of the photocatalytic activity. The similar particle sizes of the incorporated CdS nanoparticles among three kinds of nanocomposite films with different pore sizes may be due to the same concentration of CdCl2 and Na2S solutions during the present precipitation process. Moreover, more detailed mirostructures of CdS nanoparticles within CdS/RC-4.5 can be observed from the HRTEM image (Figure 2d). The fringe spacings of 3.36 Å can be indexed

Figure 4. TGA curves of the pristine RC film and CdS/RC nanocomposite films: (a) RC-4.5; (b) CdS/RC-4; (c) CdS/RC-4.5; and (d) CdS/ RC-5.

to (111) crystal plane of CdS, suggesting high crystallinity of CdS nanoparticles. 3.2. XRD Analyses of the CdS/RC Films. The X-ray diffraction (XRD) patterns for the pristine RC-4.5 film and CdS/ RC nanocomposite films are depicted in Figure 3. As can be seen, there are three distinctive peaks at 2θ ) 12.4°, 20.2°, and 22.2°, which corresponds to the (1ıj0), (110), and (200) crystal plane of cellulose II crystalline, respectively.33 A peak at 2θ )26.6° assigned to the (111) crystal plane of CdS is also clearly observed, implying that cubic-phase CdS (JPDS10-0454) existed in the CdS/RC films.34 Apart from this low-intensity peak, other peaks of cubic CdS are inconspicuous, suggesting that the CdS-loading amount in the RC film is limited. The peak intensity of cubic CdS slightly intensifies with enhancement of the cellulose’s concentration, as observed from Figure 2, indicating the increase of the CdS-loading amount in the nanocomposite films. Though the relatively low signal-to-noise ratio of the XRD pattern did not allow a reliable quantification of CdS-loading amount in the nanocomposite films, the exact quantification of CdS and cellulose in the nanocomposite films can be obtained through the following TGA technique. 3.3. Thermal Gravimetric Analyses of the CdS/RC Films. Figure 4 shows the results of TGA experiments for pristine RC4.5 films and CdS/RC nanocomposite films. As can be seen, the decomposition temperature of the pristine RC-4.5 film is about 353.5 °C, and the pristine RC-4.5 film completely decomposes (with 100% weight loss) at about 560 °C. Whereas the decomposition temperatures of the nanocomposite films slightly shift to lower temperatures in comparison with the pristine one, suggesting that CdS-loading can reduce the thermal stability of an RC film to a certain extent. This is reasonable, considering that CdS nanoparticles would partially break the intermolecular hydrogen bonds of the cellulose and incorporate into the films through physical interactions instead of chemical reactions between CdS and RC films. However, the above limited decrease in the thermal stability would not influence the applicability of the nanocomposites films in photocatalytic reaction.32

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Figure 5. The stress-strain curves for the pristine RC and nanocomposite films in the dry state: (a) RC-4.5; (b) CdS/RC-4; (c) CdS/RC4.5 and, (d) CdS/RC-5.

Once the temperature was enhanced to 450 °C, the composite film was transformed into CdS and trace CdO (as shown in Supporting InformationSI1), and then transformed into pure CdO at 600 °C. Therefore, the weight percentage of CdS in the nanocomposite films can be calculated according to the following equation:

WCdS(%) ) WCdO ·

MCdS · 100% MCdO

Here, WCdS is the weight percentage of CdS nanoparticles within the nanocomposite film, and WCdO is the weight percentage of CdO derived from the TGA curve at 600 °C. MCdS and MCdO are the molar mass of corresponding CdS and CdO, respectively. The calculated WCdS values for CdS/RC-4, CdS/RC-4.5, and CdS/RC-5 are 1.96, 2.91, and 3.27 wt %, respectively. Obviously, the weight percentage of the embedded inorganic component increased with enhancing the cellulose’s concentration, which is in agreement with the above-mentioned discussion on the observation of SEM images and XRD analyses. 3.4. Mechanical Properties of the CdS/RC Films. Figure 5 shows the mechanical properties of pristine RC-4.5 film and CdS/RC nanocomposite films. The composite films exhibit excellent mechanical properties and the tensile-stress values range from 97 to 125 MPa, which are obviously higher than that of the RC film (86 MPa). Usually, intramolecular and intermolecular hydrogen bondings in the cellulose chains would prevent close contact between inorganic fillers and the cellulose film. The internal cracks of the aggregate particles make them stress concentrators, and this results in a reduction of the mechanical properties of the materials.35 However, in this work, CdS nanoparticles were in situ synthesized in the porous structured cellulose films, leading to an increase in the mechanical properties. This implies that OH groups on the cellulose are tightly bound to the surface of CdS, and CdS cross-linking results in the relatively strong adhesion between the cellulose and the CdS particles, which would make up the decrease of mechanical properties resulting from the destruction of the hydrogen bondings of cellulose itself.36 It is noteworthy that the CdS/RC-4.5 nanocomposite film shows the best mechanical properties among the nanocomposite films, suggesting the strongest adhesion between the cellulose and the CdS particles in CdS/RC-4.5 nanocomposite film. 3.5. UV-vis Diffuse Reflectance Absorption Spectra of the CdS/RC Films. Figure 6 shows the diffuse reflectance absorption spectra of the pristine RC-4.5 film and CdS/RC nanocomposite films. The pristine RC film does not exhibit absorption in the visible light region, although it shows less absorption in the UV light region, while a considerable

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Figure 6. UV-vis diffuse reflectance absorption spectra (DRS) of the pristine RC film and CdS/RC nanocomposite films: (a) RC-4.5; (b) CdS/RC-4; (c) CdS/RC-4.5, and (d) CdS/RC-5.

Figure 7. (a) Effect of Pt-loading amount on the photocatalytic H2 evolution rate over the CdS/RC-4.5 films. (b) Time course of the photocatalytic H2 evolution efficiency over the nanocomposite films: (a) RC-4.5; (b) CdS/RC-4; (c) CdS/RC-4.5; and (d) CdS/RC-5. Reaction conditions: 8 wt % Pt-loaded film (0.050 g) containing Na2S (0.25 M)-Na2SO3 (0.35 M) mixed solution (100 mL), visible-light irradiation (λ > 420 nm) from 250 W Xe-lamp.

absorbance in the visible light region can be observed from the nanocomposite films in spite of the limited CdS-loading amounts (reference the above TGA results), which can be attributed to the loading of CdS nanoparticles. The absorption edges of CdS/ RC nanocomposite films are close to each other due to the similar CdS particle sizes and CdS contents. Moreover, the absorption spectra of the nanocomposite films are very similar to that of the pure cubic phase CdS,37 implying that the CdS nanoparticles embedded in the present porous RC film basically maintain their original structural characteristics.36 The inhabitation of CdS nanoparticles within the present nanocomposite films is beneficial for maintaining more photocatalytic active sites in comparison with the nanoparticle suspension system because agglomeration usually occurred in the latter.38 Therefore, it can be concluded that the present RC film is a suitable host matrix for the CdS photocatalyst. 3.6. Photocatalytic Activity of the CdS/RC Films. The photocatalytic H2 production efficiency of CdS/RC films was too low to detect by GC without Pt-loading and sacrificial reagents, but can be greatly improved after Pt-loading and addition of sacrificial reagent.2 Figure 7a shows the dependence of the photocatalytic H2 evolution rate over the CdS/RC-4.5 nanocomposite film upon the Pt-loading amount. This arises from the catalytic performance of the loaded Pt nanoparticles and the efficient charge separation at the interfaces due to the Schottky potential barrier between the cocatalyst and photocatalyst.39 The Pt-loading on the nanocomposite film can be verified from the energy dispersive X-ray spectroscopy in Figure SI2 of the Supporting Information. As can be seen, the H2 evolution rate rapidly increases from 0.652 mmol g-1 h-1 to the maximum of 1.323 mmol g-1 h-1 when the Pt-loading amount was enhanced from 0.5 wt % to 8 wt %, at which point it quickly decreases to 0.341 mmol g-1 h-1 once the Pt-loading amount was further enhanced from 8 wt % to 13 wt %. The excessive Pt nanoparticles may occupy the active sites and cover

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Figure 8. Time course of the photocatalytic H2 yield efficiency over the same CdS/RC-4.5, three continuous runs. Reaction conditions: 8 wt % Pt-loaded film (0.050 g) containing Na2S (0.25 M)-Na2SO3 (0.35 M) mixed solution (100 mL), visible-light irradiation (λ > 420 nm) from 250 W Xe-lamp.

the surfaces of the photocatalyst to shield the incident light after a long-term light irradiation, and then lead to a decrease of the photocatalytic efficiency.40 Thus, it can be concluded that 8 wt % is the optimal Pt-loading amount for the photocatalytic H2 production over the present nanocomposite films. Figure 7b shows the photocatalytic H2 evolution efficiency over the pristine RC-4.5 and CdS/RC nanocomposite films with 8 wt % Pt-loading from Na2SO4-Na2S mixed solution under visible-light irradiation for 5 h. As can be seen, no obvious H2 evolution can be observed over the pristine RC-4.5 film, and the nanocomposite films show obvious photoactivity for H2 production. Among those, CdS/RC-4.5 film exhibits the best photocatalytic H2 evolution efficiency (6.721 mmol g-1) during the 5 h light irradiation, whereas the H2 evolution efficiency over CdS/RC-4 film is lower than that of the CdS/RC-5 film. The increasing concentration of cellulose solutions from 4.0% to 4.5% resulted in an enhancement of the CdS-loading amount according to aforementioned discussions on the TGA experiments. Namely, CdS/RC-4.5 film seems to possess a suitable CdS-loading amount for the photocatalytic H2 production in the present nanocomposite film system. The photons can be efficiently absorbed by the CdS nanoparticles embedded in CdS/ RC-4.5 nanocomposite film, and the H2 production efficiency can stably increase, though the utilization factor of the irradiation during the photocatalytic reaction is still low.2 Once the CdSloading amount exceeds an optimal value (e.g., CdS/RC-5 films), the photons absorbed by the nanocomposite film reached the stable maximum; however, the photons cannot be continuously injected into CdS nanoparticles, which increases the recombination probability of photogenerated carriers,2 and therefore, the photocatalytic efficiency of the CdS/RC-5.0 nanocomposite films decreases instead of increases. For evaluating the photostability of the nanocomposite films, the Pt-loaded CdS/RC-4.5 film after the first run of 5 h photochemical reaction was picked out from the sacrificial reagent solution, washed with distilled water, and then dried at 65 °C. The same film was used for the second and third run of the photochemical reaction under the same conditions. Figure 8 shows the photocatalytic activity of the CdS/RC-4.5 film decreased slightly after two runs of the photochemical reaction. The photocatalytic H2 evolution efficiency at the third run of the photoreaction retains ∼93% of the first run H2 evolution efficiency (6.721 mmol g-1). Moreover, it is worth noting that there is no Cd2+ in the remnant solution after the photochemical reaction with 15 h visible-light irradiation was detected through ICP-MS, implying that the photocorrosion of CdS in the present nanocomposite film system is very limited. Moreover, the fairly strong adhesion of CdS crystallites in the nanocomposite film

J. Phys. Chem. C, Vol. 113, No. 36, 2009 16025 could be attributed to the strong interaction between the CdS nanoparticles and RC films due to their rough and porous texture as seen in the SEM observations from Figure 1. On the bases of aforementioned discussions, three factors can be considered in order to explain the high photoactivity of the present CdS/RC nanocomposite film. First, CdS in the nanocomposite film can maintain a relatively large surface area due to the immobilization of the porous structures in comparison with CdS nanoparticles suspension system, in which CdS nanoparticles usually tend toward agglomeration, and leads to a lower light absorption efficiency. Namely, the CdS nanoparticles dispersed in the nanocomposite film can efficiently restrain the agglomeration among CdS nanoparticles, and then improve the light absorption efficiency. Second, the remarkable hydrophilic properties of RC film are indispensable for the improvement of the photoactivity of CdS due to its efficient interaction with the sacrificial reagent. When the photogenerated electrons and holes migrate to the surfaces of CdS nanoparticles, the photogenerated holes expediently react with the adsorbed sacrificial reagent, which can greatly restrain the bulk recombination of the photogenerated charges. Third, the porous structures within the RC film would efficiently restrict the growth of CdS nanoparticles, and a smaller particle is beneficial for the photogenerated carrier transfer to CdS surfaces, and then for a lower bulk recombination probability. Furthermore, the CdS nanoparticles embedded in the cellulose film can be conveniently recycled with little loss in their photocatalytic activity after recycling. The photoactivity of the used CdS/RC film can be simply restored by washing with water. There is no evident difference in the photoactivity for the second and third repetitive uses of the same CdS/RC nanocomposite films (reference Figure 8), suggesting that these CdS nanoparticles in the nanocomposite film indeed possess a favorable photochemical stability. 4. Conclusions The photocatalytic H2 production of CdS nanoparticles immobilized in porous regenerated cellulose (RC) films with different pore sizes were investigated. The mean pore sizes in the porous RC films can be modulated from about 57 to 20 nm by increasing the concentration of the cellulose solution, which also induced an increase of the CdS-loading amount. The experimental facts indicated that the loaded CdS nanoparticles were cubic phase with an average particle size of about 8 nm, and the porous RC film corresponded to cellulose II crystalline. Moreover, the composite films exhibit better mechanical properties in comparison with the RC film. Although the CdS-loading amount is low (1.96-3.27 wt %), the CdS/RC nanocomposite films possess an obvious optical absorption in the region of 250-800 nm, with a main absorption edge (λab) at about 510 nm. A high photocatalytic H2 production efficiency of about 1.323 mmol g-1 h-1 under visible-light irradiation (λ g 420 nm) has been attained over the Pt-loaded CdS/RC nanocomposite film synthesized by 4.5% cellulose solution due to the efficient light absorption, fast carrier transfer, and photochemical reaction between the loaded CdS nanoparticles and electrolyte interfaces. In a long-period visible-light irradiation, the present CdS/RC nanocomposite film shows excellent fixity and photostability in comparison with a nanoparticle suspension system, indicating its promise in practical applications. The present synthesis strategy could be a general method for other semiconductor photocatalyst/porous cellulose films.

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Acknowledgment. This work is supported by the National “863” Foundation (2006AA03Z344), Natural Science Foundation of China (20573078, 20871096), and Program for New Century Excellent Talents in University (NCET-07-0637), China. The authors acknowledge the assistance of Center for Electron Microscopy, Wuhan University, for HRTEM. Note Added after ASAP Publication. This article was published ASAP on August 6, 2009. Figures 3 and 8 have been modified. The correct version was published on August 17, 2009. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Meyer, T. J. Nature 2008, 14, 778. (2) Bao, N. Z.; Shen, L. M.; Takata, T.; Domen, K. Chem. Mater. 2008, 20, 110. (3) Maeda, K.; Teramura, K.; Lu, D.; Saito, N.; Inoue, Y.; Domen, K. Angew. Chem., Int. Ed. 2006, 45, 7806. (4) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (5) Khan, S.; Al-Shahry, M.; Ingler, W. B. Science 2002, 297, 2243. (6) Park, J. H.; Kim, S. W.; Bard, A. J. Nano. Lett. 2006, 6, 24. (7) Zou, Z. G.; Ye, J. H.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625. (8) Ishikawa, A.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. J. Am. Chem. Soc. 2002, 124, 13547. (9) Zhang, L. Z.; Djerdj, I.; Cao, M. H.; Antonietti, M.; Niederberger, M. AdV. Mater. 2007, 19, 2083. (10) Jing, D. W.; Guo, L. J. J. Phys. Chem. B 2006, 110, 11139. (11) Meissner, D.; Memming, R.; Kastening, B. J. Phys. Chem. 1988, 92, 3476. (12) Kiwi, J.; Gratzel, M. Nature 1979, 281, 657. (13) So, W. W.; Kim, K. J.; Moon, S. J. Int. J. Hydrogen Energy 2003, 29, 229. (14) Kida, T.; Guan, G.; Yoshida, A. Chem. Phys. Lett. 2003, 371, 563. (15) Serpone, N.; Borgarello, E.; Gra¨tzel, M. J. Chem. Soc., Chem. Commun. 1984, 342. (16) Fujii, H.; Ohtaki, M.; Eguchi, K.; Arai, H. J. Mol. Catal. A 1998, 129, 61.

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