Constructing Ternary CdS–Graphene–TiO2 Hybrids on the Flatland of

Article pubs.acs.org/JPCC

Constructing Ternary CdS−Graphene−TiO2 Hybrids on the Flatland of Graphene Oxide with Enhanced Visible-Light Photoactivity for Selective Transformation Nan Zhang, Yanhui Zhang, Xiaoyang Pan, Min-Quan Yang, and Yi-Jun Xu* State Key Laboratory Breeding Base of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou, 350002, P.R. China S Supporting Information *

ABSTRACT: The ternary CdS−graphene−TiO2 hybrids (CdS−GR− TiO2) have been prepared through an in situ strategy on the flatland of graphene oxide (GO). The structure and properties have been characterized by a series of techniques, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission scanning electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), UV−vis diffuse reflectance spectra (DRS), electrochemical analysis, photoluminescence spectra (PL), nitrogen adsorption− desorption, and electron spin resonance spectra (ESR). Combined with our previous results, it is found that the introduction of the thirdcomponent TiO2 can maintain the morphology and porosity of the samples, whereas it is able to tune the energy band, increase the surface area, and facilitate the electron transfer, thus prolonging the lifetime of photogenerated carriers. Taking photocatalytic selective oxidation of various alcohols to their corresponding aldehydes as model reactions, the ternary CdS−GR−TiO2 hybrid exhibits enhanced photocatalytic activity compared with its foundation matrix binary CdS−GR. The improved photocatalytic performance can be attributed to the combined interaction of the longer lifetime of photogenerated electron−hole pairs, faster interfacial charge transfer rate, and larger surface area. In addition, a possible reaction mechanism has been proposed. This work indicates that the careful design of graphene−based composites by coupling graphene to suitable, multiple semiconductors allows the achievement of more efficient photocatalysts, which may have the great potential to improve the capacity for photocatalytic processes significantly. As a proof-of-concept, it is expected that this work could offer new inroads into exploration and utilization of graphene−based nanocomposites as a fertile ground for energy conversion.

1. INTRODUCTION Graphene (GR), a 2D single layer graphite with close-packed conjugated hexagonal lattices, has become a sparkling rising star on the horizon of material science in the past few years.1−3 GR is recognized as the basic building block of all-dimensional carbon materials, including 0D fullerene (C60), 1D carbon nanotubes (CNTs), and 3D graphite, considering that it can be wrapped into 0D C60, rolled into 1D CNTs, or stacked into 3D graphite.2,4,5 The planar structure and long-range π-conjugation of GR endow it with superior and unique properties, such as high values of Young’s modulus (∼1.0 TPa), large spring constants (1−5 N m1−), high mobility of charge carriers (>200 000 cm2 V−1s−1 at electron densities of 2 × 1011 cm−2), large theoretical specific surface area (2630 m2·g−1), excellent thermal conductivity (∼5000 W m−1 K−1), and optical transmittance (∼97.7%).5−15 Because of its remarkable nature, GR has drawn intense interests and been applied in various areas, such as nanoelectronics, optoelectronics, chemical and biochemical sensing, polymer composites, H2 production and storage, intercalation materials, drug delivery, supercapacitors, catalysis, and photovoltaics.10−49 © 2012 American Chemical Society

In view of its lamellar 2D structure, extremely high specific surface area, excellent transparency, superior electron mobility, and high chemical stability, GR can be used as an ideal highperformance candidate for photocatalyst carrier or promoter.13 Thus far, GR-based photocatalysts have been enjoying great research interests for their potential in photocatalysis.4,5,10−13,19,50−58 However, the processes in which they have been utilized often involve photocatalytic degradation of pollutants and water splitting.16,53,59−65 Notably, photocatalytic selective transformation has attracted ever-increasing attention because it holds great promise for providing an alternative to the conventional synthetic pathways.66−81 The required mild conditions and the possibility to decrease the generation of undesired substances highlight its potential as a promising route for organic synthesis. Through a literature survey, it can be found that the study of GR-based nanocomposites for photocatalytic selective transformation is quite insufficient. Received: April 12, 2012 Revised: May 29, 2012 Published: June 26, 2012 18023

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Scheme 1. Schematic Chart for Fabrication of Ternary CdS−GR−TiO2 Hybrids

demonstrates that the careful design of graphene−based composites by coupling graphene with multiple semiconductors enables the synthesis of more efficient photocatalysts with improved photocatalytic performance toward specific applications. As a proof-of-concept, it is expected that this current research could provide a new thread of thought to prepare GRbased multicomponent hybrid photocatalysts and widen their applications in heterogeneous photocatalysis, particularly in photocatalytic selective transformation under ambient conditions.

The primary results obtained by our group demonstrate that GR-based nanocomposites can serve as a type of efficient photocatalysts for selective transformations driven by visiblelight irradiation.66,82 We have prepared GR−TiO2 nanocomposites by an in situ process in a solution phase that led to an intimate interfacial contact between GR and TiO2.66 Taking photocatalytic selective oxidation of alcohols as probing reactions, the GR−TiO2 nanocomposites exhibited enhanced performance compared with CNT−TiO2 counterpart, which disclosed the prominent advantage of GR over CNT on both controlling the morphology of GR−TiO2 nanocomposite and enhancing the photocatalytic activity of TiO2. In a subsequent work,82 we reported that CdS−GR nanocomposites obtained via a facile one-step solvothermal treatment displayed higher photocatalytic activity toward selective oxidation of alcohols to their corresponding aldehydes under mild conditions than bare CdS. In this nanocomposite, the CdS nanoparticles overspread on the graphene scaffold evenly. The improved photoactivity of CdS−GR can be ascribed to the integrative effect of enhanced light absorption intensity, high electron conductivity of GR, and its significant influence on the morphology and structure of the samples. In addition, it should be noted that the studies on GR-based photocatalysts often focus on binary nanocomposites; that is, GR is coupled to only another single component. However, beyond dual-ingredient hybrid systems, multicomponent hybrid nanomaterials are expected to provide improved photoactivity and new insight into the development of novel 3D nanoarchitectures with versatile and extraordinary properties.50,83−88 For example, Kamat and coworkers83 have constructed organic/inorganic ternary composites of porphyrin, ZnO nanoparticles, and reduced graphene oxide (GO) by a bottom-up strategy. This hierarchical electron transfer (ET) cascade system exhibited remarkably high photocurrent generation due to the occurrence of multistep ET on the electrode. Amal’s group84 has prepared photoreduced graphene oxide (PRGO)/BiVO4−Ru/SrTiO3:Rh for Z-scheme photocatalytic water splitting under visible-light irradiation. In the light of these pioneering researches, more efforts should be devoted to preparing multicomponent GR-based nanocomposites for better functional performance and wider applications instead of getting stuck in binary composites. On the basis of the above issues and our previous results,66,82 in the present work, we fabricate ternary hybrids of CdS−GR− TiO2 by an in situ strategy on the flatland of GO and utilize them in the selective oxidation of alcohols to aldehydes, a fundamental and important transformation in the industries.69,71,75−83 It is found that CdS and TiO2 nanoparticles carpet the graphene nanosheets and the photoactivity of these ternary nanocomposites surpasses the binary one. This work

2. EXPERIMENTAL SECTION 2.1. Preparation. Materials. Cadmium acetate (Cd(CH3COO)2·2H2O), dimethyl sulfoxide (C2H6OS2, DMSO), graphite powder, sulfuric acid (H2SO4), nitric acid (HNO3), hydrochloric acid (HCl), potassium persulfate (K2S2O8), phosphorus pentoxide (P2O5), potassium permanganate (KMnO4), hydrogen peroxide 30% (H2O2), and ethanol (C2H6O) were supplied by Sinopharm chemical reagent Shanghai, China). Titanium(IV) fluoride (TiF4) was purchased from Alfa Aesar China (Tianjin, China). All materials were used as received without further purification. Deionized (DI) water used in the preparation was from local sources. Synthesis. The preparation of ternary CdS−GR−TiO2 hybrids is illustrated in Scheme 1. a. Preparation of Graphene Oxide. GO was synthesized from natural graphite powder by a modified Hummers method.89 The detail of the typical process can be referenced in the previous report.16,82 b. Synthesis of CdS−GR Nanocomposite. The given amount of the as-prepared GO was dispersed in 40 mL of DMSO by ultrasonication to obtain the homogeneous GO− DMSO dispersion. According to the obtained photocatalytic activities,82 5% weight ratio of GR was chosen. Then, 0.106 g Cd(CH3COO)2·2H2O was added to the above solution. The mixture was stirred vigorously for 30 min and then transferred to 50 mL Teflon-lined stainless-steel autoclave. The solvothermal treatment was performed at 453 K for 12 h. After that, the products were cooled to room temperature, separated by centrifugation and washed with acetone three times and absolute ethanol one time. Followed by a dry process, CdS− 5% GR nanocomposite was obtained. c. Fabrication of CdS−GR−TiO2 Nanocomposites. The asprepared CdS−5% GR nanocomposite was dispersed in 60 mL of DI water completely by ultrasonication. A certain volume of 8 mM TiF4 was added to the above aqueous suspension. This mixture was mixed under stirring for 2 h and then heated to 353 K in an oil-bath kept stirring for 12 h. Then, the resulting solution was centrifugated and washed until the ion concentration of the supernatant was 420 nm) for 3 h over CdS−GR−TiO2 and CdS−GR nanocomposites. Note: CG is short for CdS−5% GR. 18026

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Figure 5. Time-online photocatalytic selective oxidation of alcohols to aldehydes over the CdS−5% GR−10% TiO2 and CdS−5% GR photocatalysts under visible-light irradiation (λ > 420 nm) under ambient conditions: (a) benzyl alcohol, (b) p-methyl benzyl alcohol, (c) p-methoxyl benzyl alcohol, (d) p-nitro benzyl alcohol, (e) p-fluoro benzyl alcohol, (f) p-chloro benzyl alcohol, (g) cinnamyl alcohol, (h) 3-methyl-2-buten-1-ol, and (i) 2-buten-1-ol.

performance toward the selective oxidation of benzyl alcohol to benzaldehyde compared with binary CdS−5% GR nano-

composite based on both of the conversion of benzyl alcohol and the yield of benzaldehyde, although the selectivity for 18027

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introduction of the third component TiO2. To ascertain the detail of energy band, Mott−Schottky plots have been analyzed for CdS−5% GR and CdS−5% GR−10% TiO2 nanocomposites. As shown in Figure 7, the plots with the positive

benzaldehyde obtained over the ternary hybrids decreases slightly. This is reasonable considering the fact that with the evolution of the reaction, part of the target product is further oxidized to produce benzyl acid or carbon dioxide. Among the CdS−GR−TiO2 nanocomposites with different weight ratio of TiO2, CdS−5% GR−10% TiO2 nanocomposite shows the highest photocatalytic performance. Therefore, it is chosen as the testing candidate to verify if such a photoactivity improvement is general for other substituted alcohols. As shown in Figure 5, in comparison with CdS−5% GR, the CdS− 5% GR−10% TiO2 nanocomposite possesses higher activities for all testing reactions, including the selective oxidation of benzylic alcohols and allylic alcohols to the corresponding aldehydes. Clearly, the ternary CdS−5% GR−10% TiO2 nanocomposite possesses its advantages over its foundation base, binary CdS−5% GR, for photocatalytic selective oxidation of alcohols in the light of both conversion and yield. Considering their compositions, the improvement in the photocatalytic activity of the ternary CdS−5% GR−10% TiO2 hybrid should be ascribed to the introduction of the third component TiO2. To understand the causes for enhancing the photocatalytic performance of the ternary hybrids, we have performed some other characterizations, including UV−vis DRS, electrochemical analysis, PL, and nitrogen adsorption−desorption. The DRS spectra as shown in Figure 6A suggest that the introduction of

Figure 7. Mott−Schottky plots for CdS−5% GR and CdS−5% GR− 10% TiO2 nanocomposites in 0.2 M Na2SO4 aqueous solution (pH 6.8).

slope are observed, which is consistent with the typical feature for n-type semiconductors.82,90 The flat band potential (EFB) of CdS−5% GR−10% TiO2 and CdS−5% GR nanocomposites, as calculated from the X intercepts of the linear region, are found to be −0.63 and −0.72 V versus Ag/AgCl (equivalent to −0.43 and −0.52 V versus normal hydrogen electrode, NHE), respectively. Obviously, EFB of the ternary hybrid is lowerlying than that of the binary nanocomposite. It is known that the EFB measured according to the Mott−Schottky relationship equals Fermi level (EF) for n-type semiconductor.91,92 Namely, EF of CdS−5% GR−10% TiO2 nanocomposite shifts positively in comparison with CdS−5% GR, which may result from the Fermi-level equilibration. Because the calculated EF of GR and the conduction band (CB) edge (ECB) of TiO2 are −0.08 and −0.29 V versus NHE, respectively,38,93 both of which are less negative than ECB of CdS (−0.52 V vs NHE), electrons will transfer from CdS to graphene nanosheets or TiO2 when they are in contact. The accumulation of electrons inevitably causes shifting of the apparent Fermi level (EF*) to achieve equilibrium, as illustrated in the upper right inset of Figure 8.

Figure 6. UV−vis diffuse reflectance spectra (DRS) of CdS−5% GR and CdS−5% GR−10% TiO2 nanocomposites (A) and the plot of transformed Kubelka−Munk function versus the energy of light (B).

TiO2 has insignificant impact on the absorption intensity of the samples. However, the characteristic absorption sharp edge of CdS−5% GR−10% TiO2 nanocomposite undergoes a red shift compared with CdS−5% GR. The band gap of the ternary nanocomposite is narrowed as reflected in Figure 6B. From our previous reports,82 we know that the introduction of GR into the matrix of CdS could lead to a band-gap narrowing of the semiconductor CdS because of the chemical bonding between semiconductor and graphene. In this case, the same phenomenon is also observed, which is caused by the

Figure 8. Illustration of the proposed reaction mechanism for selective oxidation of alcohols to corresponding aldehydes over the ternary CdS−GR−TiO2 hybrids under visible-light irradiation; the upper right inset demonstrates the energy band of ternary CdS−GR−TiO2 nanocomposites. 18028

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known that the high-frequency arc in the Nyquist plots is related to the charge-transfer limiting process and ascribed to the double-layer capacitance (Cdl) in parallel with the charge transfer resistance (Rct) at the contact interface between electrode and electrolyte solution.82,98 Notably, the introduction of the third component causes significant decrease in the arc compared with CdS−5% GR, indicating that the ternary system has much smaller Rct than the binary CdS−5% GR,97 which suggests the much more efficient transfer of charge carriers over CdS−5% GR−10% TiO2 than that over CdS−5% GR. This in turn should contribute to the enhanced photoactivity of the CdS−5% GR−10% TiO2 nanocomposite. Besides, the surface area and porosity of the binary CdS−5% GR and ternary CdS−5% GR−10% TiO2 samples have been characterized. It is easy to see from Figure S2 of the Supporting Information that both of the nitrogen adsorption−desorption isotherms are ascribed to type IV isotherm with typical H3 hysteresis loop characteristic of mesoporous solids according to the IUPAC classification.99 The pore volume of CdS−5% GR and CdS−5% GR−10% TiO2 nanocomposites is 0.12 and 0.16 cm3·g−1, respectively, and their average pore diameter is 8.2 and 7.3 nm corresponding to the binary and ternary nanocomposites, severally. The BET surface area of the ternary hybrid (89 cm2·g−1) is larger than its foundation matrix CdS− 5% GR (57 cm2·g−1), suggesting that the ternary system could provide more active sites for photocatalytic reactions to proceed. On the basis of the above discussion, it can be concluded that the enhanced photocatalytic performance of the ternary CdS− GR−TiO2 hybrids can be attributed to the combined interaction of the longer lifetime of photogenerated electron− hole pairs, faster interfacial charge transfer rate, and larger surface area. Accordingly, a probable reaction mechanism has been proposed, as illustrated in Figure 8. Under the visible-light irradiation (λ > 420 nm), the electrons are excited from the valence band (VB) of CdS to its CB. The photogenerated electrons transfer to graphene nanosheets and TiO2 owing to their intimate interfacial contact. According to the values of energy band, this process is thermodynamically permissible; then, the electrons react with the adsorbed oxygen molecule to give superoxide radicals, whose existence in the system has been confirmed by the results of ESR spectra. As displayed in Figure 11, the superoxide radicals are detectable and stable in the reaction system. At the same time, the hole perching in the VB of CdS can oxidize the absorbed alcohols to form alcohol radicals;82 then, the alcohol radicals are oxidized by oxygen or superoxide radicals to give the targeted product. To

The same phenomena of Fermi-level equilibration have also been observed in noble metal−semiconductor systems.94−96 Therefore, the results of DRS and Mott−Schottky measurement are supportive of each other. On the basis of the above analysis, it can be inferred that the photogenerated electrons could transfer to graphene nanosheets and TiO2 from the CB of CdS. Therefore, the separation of photoinduced electron−hole pairs is improved, and the lifetime of the carriers is prolonged, which can be confirmed by the transient photocurrent response, as shown in Figure 9. It is

Figure 9. Transient photocurrent response of CdS−5% GR and CdS− 5% GR−10% TiO2 nanocomposites in 0.2 M Na2SO4 (pH 6.8) aqueous solution without bias versus Ag/AgCl.

easy to observe that the photocurrent of CdS−5% GR−10% TiO2 is enhanced significantly, demonstrating the longer life span obtained on the ternary hybrid than CdS−5% GR. This is strongly evidenced by the results of PL spectra. It can be seen from Figure S1 in the Supporting Information that the PL intensity of CdS−5% GR−10% TiO2 is much weaker than that of binary CdS−5% GR nanocomposite, indicating that the recombination of photogenerated electron−hole pairs is hampered in the ternary system. The photoinduced carriers with prolonged lifetime of CdS−5% GR−10% TiO2 hybrid is beneficial for photocatalytic process and should contribute to its enhanced photocatalytic activity. To determine further the advantage of the ternary CdS−5% GR−10% TiO2 hybrid over CdS−5% GR nanocomposite, we have also performed EIS Nyquist plots. As shown in Figure 10,

Figure 10. Nyquist impedance plots of CdS−5% GR and CdS−5% GR−10% TiO2 nanocomposites under visible-light irradiation in 0.2 M Na2SO4 aqueous solution (pH 6.8).

the Nyquist plots of the CdS−5% GR and CdS−5% GR−10% TiO2 electrode materials under visible-light irradiation obtained in 0.2 M Na2SO4 electrolyte solution give rise to semicycles at high frequencies. Considering the same preparation of the electrodes and electrolyte, the high-frequency semicircle should be associated with the resistance of the electrodes.82,97 It is

Figure 11. ESR spectra of superoxide radical species trapped by DMPO in CdS−5% GR−10% TiO2 dispersion in the solvent of BTF under visible-light irradiation. 18029

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(2012J06003), Program for Changjiang Scholars and Innovative Research Team in Universities (PCSIRT0818), Program for Returned High-Level Overseas Chinese Scholars of Fujian province, and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, is gratefully acknowledged.

demonstrate the advantage of two routes for ET in the ternary system, the activity of CdS−10% TiO2, which is prepared using the same procedures to that for CdS-GR-TiO2 nanocomposites except the addition of GO, has also been evaluated. It is obvious to see from Figure S3 of the Supporting Information that compared with the binary CdS−5% GR and CdS−10% TiO2, the ternary CdS−5% GR−10% TiO2 hybrid exhibits the best photocatalytic performance, suggesting that the two routes for ET in the ternary system favor more efficient separation of the photoinduced electron−hole pairs, thus further prolonging the lifetime of the carriers and enhancing the photocatalytic activity. What’s more, the stability tests for CdS−5% GR−10% TiO2 nanocomposite (Figure S4 of the Supporting Information) indicate that this ternary hybrid possesses a stable durability of photoactivity. The XRD patterns of the fresh and used samples, as shown in Figure S5 of the Supporting Information, are almost the same, signifying the stability of the photocatalysts, which is consistent with results of stability tests.

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4. CONCLUDING REMARKS In summary, ternary CdS−GR−TiO2 hybrid has been fabricated through an in situ strategy on the flatland of GO. In this ternary system, the CdS nanoparticles overspread on the surface of graphene nanosheets evenly and the TiO2 nanoparticles decorate the CdS−GR foundation base uniformly. It is found that the introduction of TiO2 can further enhance the photocatalytic performance toward the selective oxidation of alcohols to corresponding aldehydes without changing the morphology and porosity properties of the samples. The improved photocatalytic activity can be ascribed to the combined interaction of the longer lifetime of photogenerated electron−hole pairs, faster interfacial charge-transfer rate, and larger surface area. As a proof-of-concept, this work demonstrates that the careful design of graphene-based composites by coupling graphene with multiple semiconductor compounds is favorable to the development of next-generation photocatalyst systems, which would have the potential to improve their capacity for photocatalytic processes significantly. It is expected that this work could provide new inroads into exploration and utilization of graphene-based nanocomposites as photocatalyst for artificial photosynthesis.

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ASSOCIATED CONTENT

S Supporting Information *

PL spectra, nitrogen adsorption−desorption isotherms, photoactivity comparison of the binary CdS−5% GR, CdS−10% TiO2, and ternary CdS−5% GR−10% TiO2 nanocomposites, stability tests, and XRD patterns of the fresh and used CdS−5% GR−10% TiO2 nanocomposites. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support by the National Natural Science Foundation of China (20903023, 21173045), the Award Program for Minjiang Scholar Professorship, the Natural Science Foundation of Fujian Province for Distinguished Young Investigator Grant 18030

dx.doi.org/10.1021/jp303503c | J. Phys. Chem. C 2012, 116, 18023−18031

The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp303503c | J. Phys. Chem. C 2012, 116, 18023−18031