Graphene Nanocomposites with Highly

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Controlled Synthesis of CeO2/Graphene Nanocomposites with Highly Enhanced Optical and Catalytic Properties Linhai Jiang, Mingguang Yao, Bo Liu, Quanjun Li, Ran Liu, Hang Lv, Shuangchen Lu, Chen Gong, Bo Zou, Tian Cui, and Bingbing Liu* State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China

Guangzhi Hu and Thomas Wågberg Department of Physics, Umeå University, 901 87 Umeå, Sweden ABSTRACT: In this paper, CeO2 nanocubes with the (200)terminated surface/graphene sheet composites have been prepared successfully by a simple hydrothermal method. It is found that the CeO2 nanocubes with high crystallinity and specific exposed surface are well dispersed on well-exfoliated graphene surface. The (200)-terminated surface/graphene sheet composites modified electrode showed much higher sensitivity and excellent selectivity in its catalytic performance compared to a CeO2 nanoparticle-modified electrode. The photoluminescence intensity of the CeO2 anchored on graphene is about 30 times higher than that of pristine CeO2 crystals in air. The higher oxygen vacancy concentration in CeO2 is supposed to be an important cause for the higher photoluminescence and better electrochemical catalytic performance observed in the (200)-terminated surface/graphene sheet composites. Such ingenious design of supported well-dispersed catalysts in nanostructured ceria catalysts, synthesized in one step with an exposed high-activity surface, is important for technical applications and theoretical investigations.

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surface, so-called “type III” surface, is more reactive and catalytically more important than (111) and (110) surfaces.16,18,19 The surface energy of the (100) plane diverges, and thus its structure is considered less stable, due to the existence of a dipole moment perpendicular to the (100) surface.20,21 Strikingly, the CeO2 nanocubes with (200)-terminated surface have been successfully synthesized in experiment very recently,20 which enables us to study its catalytic activity, and indeed a very high activity and catalytic efficiency has been found.22 Thus, the controllable synthesis of CeO2 nanoparticles (NPs) with various morphologies is very important in both nanocrystal synthesis and catalytic applications. However, despite the high catalytic activity of nanocrystals, as-prepared nanocrystals usually tend to form aggregates leading to a lower catalytic activity. An efficient solution, widely used in industry, is to utilize supporting substrates to anchor the nanocatalyst and enhance its dispersion.23,24 Rational design of catalysts and catalyst supports is considered as a prominent route to achieve better reaction activities and selectivities.22,25 The dispersion of CeO2 nanocrystals on graphene layer as a CeO2 nanoparticle/graphene hybrid composite could be an important way both in the engineering

INTRODUCTION Graphene-based metal oxides nanocomposites have received large attention for their excellent properties and potential applications in many technological fields.1−3 In the composites, graphene can serve as high-performance support because of its ideal two-dimensional (2D) structure, good chemical stability, excellent electrical conductivity, and huge specific surface area.4−7 Metal oxides can be uniformly dispersed on the plane of graphene, and the charge transfer at the interface of these hybrid materials can show a synergistic effect to induce properties that are different from those of each individual component. Several metal oxides, such as RuO2, ZnO, TiO2, Co3O4, and MnO2/graphene nanocomposites etc., have been investigated and shown to exhibit various extraordinary enhanced properties.2,8−11 CeO2 is one of the most important rare earth materials, which has been extensively studied for many technological applications including catalytic, luminescent, fuel cells, and solar cells due to its chemical stability, high oxygen storage capacity, etc.12−15 Especially, CeO2 nanocrystals have been the focus of intense research because the catalytic reactivity of nano-CeO2 is strongly dependent on the surface structure.16 Sayle et al. predicated that (110) and (310) surfaces are more reactive in the oxidation of CO than (111) due to the ready formation of oxygen vacancies on them.17 In particular, both experimental and theoretical studies have shown that the (100)-terminated © 2012 American Chemical Society

Received: February 15, 2012 Revised: April 27, 2012 Published: May 2, 2012 11741

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RESULTS AND DISCUSSION Figure 1a−d shows the transmission electron microscopy (TEM) images of chemically exfoliated reduced graphene oxide

of a high-performance CeO2 catalyst and for obtaining a material with other unique properties. So far, two studies on the synthesis of CeO2 nanoparticle/graphene hybrid composites have been reported, either by depositing 1.4 nm CeO2 nanoparticles onto three-dimensional graphene sheets or by a hydrothermal method.26,27 However, those composites contain poor crystalline CeO2 nanoparticles and the CeO2 nanoparticles have no well-shaped exposed surface. Up until now, the synthesis of CeO2 well-dispersed nanocrystals with specific exposed surface anchored on well-exfoliated graphene has not been reported and has still been a challenge. In this work, we report the synthesis of highly crystalline CeO2 nanocubes with the (200)-terminated surface/graphene hybrid compositions (COGSCs) by using a simple and efficient hydrothermal method. The catalytic activity of the composites toward a model organic substance, uric acid (UA), has been evaluated. The obtained hybrid composite exhibits much higher sensitivity and excellent catalytic performance on UA compared to either a CeO2 nanoparticle or a CeO2 nanocube modified electrode. In addition, the CeO2 in COGSCs shows a striking enhancement of photoluminescence with the emission intensity up to 30 times higher than that of the pristine unsupported CeO2 nanocubes.

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Article

Figure 1. TEM images of (a) GSs and (b and c) COGSCs with low magnification (b) and high magnification (c). (d) HRTEM image of an individual CeO2 cube, and the inset shows the corresponding FFT pattern.

EXPERIMENTAL SECTION

Preparation of COGSCs graphite oxide (GO) was obtained from natural flake graphite powder by a modified Hummers method.28,29 The COGSCs were prepared by a facile hydrothermal synthesis method. An amount of 20 mg of GO was dissolved in 40 mL of distilled water with ultrasonic treatment for 2 h to form a light yellow-brown suspension. Subsequently, 1 mmol of Ce(NO3)3·6H2O was added into the suspension under magnetic stirring and 1 mL of NH3·H2O was injected into the mixture. Then the mixture was transferred into a Teflon-lined stainless steel autoclave (50 mL) and reacted at 220 °C for 24 h in an oven. The resulting product was separated by centrifuging and washed several times with distilled water and ethanol, respectively. Finally, the obtained composites were dried in air at 50 °C for 48 h. For comparison, a CeO2 cube sample has also been synthesized by a hydrothermal synthesis method reported previously.27 The electrochemical measurement was carried on an Autolab PGSTAT30 with a three-electrode cell at room temperature. The counter and the reference electrodes are a Pt wire and a saturated calomel electrode, respectively. A glassy carbon electrode coated with the as-prepared materials was used as the working electrode. The working electrode was prepared as follows: (i) a glassy carbon electrode was first polished with 0.05 m aluminum oxide paste on a chamois and then rinsed successively in acetone, ethanol, and water under a ultrasonication for 5 min; (ii) 2.5 L of CeO2 nanoparticle (or COGSCs) suspension mixed with dimethylformamide (DMF) (2 mg/mL) was carefully dropped onto the electrode surface and dried at room temperature without any heating process. Phosphate buffer solution (concentration, 0.1 M; pH 6.8) was used as supporting electrolyte during the electrochemical measurement. The electrolyte solution was bubbled with argon for at least 30 min to remove dissolved oxygen before electrochemical measurement.

sheet and the hybrid product after the hydrothermal synthesis, respectively. From Figure 1a, it is observed that the synthesized solvent-dispersed reduced graphene oxide sheets (GSs) are transparent with some clearly visible wrinkles, suggesting that the obtained GSs are mainly composed of single or few layers of graphene.30 From Figure 1, parts b and c, it is clear that a large number of CeO2 nanocubes with average size of 10 nm are homogeneously anchored onto the surface of the GSs. It is suggested that the well-dispersed CeO2 nanoparticles on the GSs surface could also act as spacers and thus prevent the GSs to restack, which increases the stability of the single- or fewlayers exfoliated reduced graphene oxide sheets.2 The highresolution TEM (HRTEM) image in Figure 1d combined with fast Fourier transform (FFT) analysis (inset) displays the clear (200) and (220) lattice fringes with the interplanar spacing of 0.27 nm, respectively, implying that the CeO2 nanocubes are only enclosed by the (200) planes.23 Such cube shapes have rarely been observed for CeO2 in previous studies.20 Raman spectroscopy is considered to be a very convenient and nonvolatile technique for the characterization of graphitic materials. Figure 2 shows the recorded Raman spectra of GO, CeO2 nanocubes, and COGSCs. From the spectra we can see that the D/G intensity ratio of COGSCs (∼1.21) is higher than that of the GO (∼0.83). This change suggests a decrease in the average size of the sp2 domains upon reduction of the exfoliated GO.31 Furthermore, we observed that the 2D/G and S3/2D (Figure 2b) intensity ratios (where the S3 peak is a secondorder peak due to the D−G combination, and 2D refers to the graphene 2D peak) of the GSs also decreased significantly after hydrothermal reduction, suggesting that the GO was much reduced.32 Besides the Raman peaks from the reduced GO, a typical Raman peak at 457 cm−1 can be assigned to the F2g vibration mode of the anchored CeO2 particles, which agrees with our TEM observations that the CeO2 nanocubes are well anchored on the GSs surface.33 In comparison with graphene and CeO2, remarkable shifts are observed in the Raman peaks 11742

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peaks are the same as those of as-prepared CeO2 nanocubes. The sharp diffraction peaks from both samples suggest a high crystallinity of our synthesized CeO2 nanocrystals. In addition, only a very weak and broad diffraction peak from graphene oxide can be observed in the pattern, which suggests that the graphene oxide was well reduced, and the produced sample mainly contains single- or few-layer graphene product.2 This result is consistent with our TEM observations and Raman measurements. As mentioned above, one of the most important potential applications of nanostructured rare earth oxides is as luminescent devices.36 However, CeO2 usually exhibits very weak broad-band emission.35 So far only a few researchers reported that doping CeO2 with lanthanide ions could be one way to enhance its luminescent properties. Nevertheless, such doping processes often require relatively high reaction temperatures (100−1000 °C), special equipment, or multiple reaction steps and also an introduction of impurities into the crystal lattice.13,37 Here, we examined the photoluminescent (PL) properties of our synthesized COGSCs at room temperature (RT) and compared with that of CeO2 nanocubes under the identical instrumental conditions. The obtained PL spectra are shown in Figure 4, and the excitation wavelength is

Figure 2. Raman spectra of (a) GO, (b) CeO2 nanocubes, and (c) COGSCs. The F2g vibration mode of CeO2 is also marked.

of COGSCs composites. The D and G bands of graphene are blue-shifted from 1360 and 1604 cm−1 to 1352 and 1599 cm−1, respectively. These significant frequency shifts of the graphene in COGSCs suggest charge transfer between CeO2 nanocubes and graphene sheets.34 For the CeO2 component in COGSCs, the F2g band is shifted from 464 to 457 cm−1, and the blue shift can be understood by charge transfer from graphene sheets to CeO2 nanocubes, which results in the increase of Ce3+ concentration and thus the oxygen vacancies concentration of CeO2 in COGSCs. This is further supported by our following X-ray photoelectron spectroscopy (XPS) results (see below). To know the crystalline structure of the obtained samples, Xray diffraction (XRD) measurements are performed on the assynthesized GO, CeO2, and COGSCs, and the resulting XRD patterns are shown in Figure 3. The most evident diffraction

Figure 4. PL spectra at 514 nm of (a) CeO2 (its intensity was enlarged by 10) and (b) COGSCs. The laser power is 0.15 mW.

325 nm. In agreement with previous reports, CeO2 nanocubes exhibit a very weak broad-band emission with maxima at 400− 440 nm at RT (its PL spectrum was enlarged by 10 times in the figure). These emission bands (400−500 nm) have been suggested to be relative to the oxygen vacancy in CeO2 nanocrystals.13,35,37 Remarkably, the PL efficiency of CeO2 in the hybrid composite is significantly enhanced, reaching a 30fold enhancement in the intensity compared to the same condition produced CeO2 nanocubes (without graphene hybridization). Furthermore, the spectrum of COGSCs exhibits similar features to that of CeO2 nanocubes, a broad-band character from 330 to 650 nm with the most intensive peak at ∼410 nm (3.03 eV), indicating a similar luminescence mechanism for both samples. It is also interesting to note that such high enhancement of the PL in COGSCs has not been observed in the other previous graphene-based hybrids. Nanostructured CeO2 has been reported to be effective for the determination of UA and ascorbic acid (AA) simultaneously and individually when it is used to modify the glassy carbon electrode by the electrochemical technique (in Figure 5).38 The electrocatalytic ability of the as-prepared materials was also investigated with the cyclic voltammetry (CV) method in 0.1 M phosphate buffer solution bubbled with argon for at least 30

Figure 3. XRD patterns of (a) GO, (b) CeO2, and (c) COGSCs.

peak is found at 2θ = 10.4° in the pattern of GO (Figure 3a), corresponding to a d-spacing of 0.76 nm, and is from the (001) reflection of graphite oxide.35 This d-spacing is significantly expanded compared to that of 0.34 nm for pristine graphite, suggesting that the natural graphite was oxidized into GO by the formation of oxygen-containing functional groups located on both sides of graphene sheet which leads to a roughness on the atomic scale by the structural defects (sp3 bonding) generated on the originally flat graphene sheet.36 The XRD pattern of the as-prepared CeO2 nanocubes (Figure 3b) shows a typical cubic fluorite-type structure (cell parameter a = 5.37 Å, O5h (Fm3m) space group) (JCPDS no. 81-0792). In the pattern of COGSCs (Figure 3c), we can see that the major diffracted 11743

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Figure 5. Cyclic voltammograms of 100 μM uric acid (UA) in 0.1 M phosphate buffer solution at pH 6.8: CeO2 nanoparticles (b), CeO2 nanocubes (c), COGSCs (d), and glassy carbon (a) electrodes. Scanning rate: 50 mV/s.

Figure 6. XPS spectra of (a) CeO2 nanocubes and (b) COGSCs.

that the concentration of Ce3+ in CeO2 increased from 9.3% in CeO2 nanocubes to 18.5% in COGSCs.40 Due to the presence of Ce3+ ions in CeO2 nanocrystals, oxygen vacancies are introduced in the crystal to compensate the effective negative charge associated with the trivalent ions.35 The XPS results thus give very strong support that the oxygen vacancy concentration in COGSCs (equals to Ce3+ concentration) is higher than that in the CeO2 nanocubes, and consequently, the emission band is 30 times higher than that of CeO2 nanocubes. Such high oxygen vacancy concentration is from the special six (200) exposed planes in nanocubes, as well as from the chargetransfer effect from graphene sheets to CeO2 nanocubes (see above discussion in our Raman results). The so highly enhanced PL in such composites has not been reported in this hybrid system, in sharp contrast to other metal oxides nanocrystal/graphene hybrid composites, for example, ZnO and TiO2/graphene composites, for which the PL of these nanocrystals is usually suppressed after the hybridization.41,42 In addition, the increased concentration of Ce3+ could also significantly enhance the electrochemical catalytic ability of COGSCs on UA. Furthermore, some other factors should also be taken into account for the enhanced catalytic ability of CeO2 on UA after hybridization. The loading of CeO2 leads to the separation of neighboring GSs and, consequently, results in a rich porous texture and an enhanced available surface area. The well-dispersed CeO2 on the graphene surface consequently ensures more exposed surfaces for a better electrochemical oxidation of the reagent. And also, due to the introduction of GSs as support in the composite, the conductivity of the nanocrystals should be greatly increased, which results in a faster charge transfer during the reaction and thus an improved catalytic efficiency. In summary, the great improvement in electrochemical catalytic ability of COGSC electrodes is due to increased oxygen vacancy concentration in CeO2 and the unique particle−sheet structure, which allows the full utilization of the advantages of both GSs and CeO2.

min. As is well-known, bimolecular UA shows a slow electrontransfer process on most traditional bulk electrodes such as gold, Pt, and carbon-based electrodes.38 As shown in Figure 5, the oxidation potential of UA for the bare glassy carbon electrode is observed at the potential of 0.482 V, with a current density of 13.5 μA/cm2 (curve a). The oxidation potential of UA significantly decreases (to 0.391 V) at the cerium oxide nanoparticle-modified electrode (curve b), indicating that CeO2 nanoparticles can effectively accelerate the electrochemical oxidation of UA, with the current intensity of 16.67 mA/cm2. For the cerium oxide nanocubes with the (100)-terminated surface (curve c) the catalytic activity toward UA oxidation is slightly more efficient with a current intensity of 16.91 mA/cm2 and a decrease of the oxidation potential to 0.377 V. However, as shown in Figure 5, curve d, cerium oxide nanocubes supported on the graphene are by far the most efficient with a lowering of the oxidation peak of UA to 0.348 V representing a decrease of about 0.144 V compared to that of a bare glassy carbon electrode. Meanwhile, the current intensity of UA obviously increases to 20.12 mA/cm2, which is about 1.5-fold higher than that of the unmodified glassy carbon electrode. The above results show that COGSCs show much better electrocatalytic activity toward UA oxidation than that of the CeO2 nanoparticles and nanocubes. This also implies that graphene composites can have potential as electrochemical sensors for the determination of some other biomolecules. To reveal a possible mechanism to explain the significantly enhanced luminescence and the excellent electrochemical catalytic performance of our COGSCs, we have performed XPS. From the PL results, we can conclude that the PL mechanism for our COGSCs is similar as to that of CeO2. For CeO2, it is suggested that the concentration of oxygen vacancies strongly affects the PL and that a higher PL intensity is expected for higher vacancy concentration.13,35,37 Also, the concentration of oxygen vacancy (corresponding to Ce3+ concentration in CeO2) is related to the electrochemical catalytic ability of CeO2 on UA. The oxygen vacancy concentration is closely linked to the ease with which the cerium can change oxidation states. This is partially due to the similar energy of the 4f and 5d electronic states and the low potential energy barrier to electron density distribution between them.39 Figure 6 shows the XPS spectra of CeO2 nanocubes and COGSCs, which shows the presence of Ce4+ and Ce3+ ions in the nanostructured CeO2. By fitting the curves and then calculating the area of the fitted peaks, we conclude

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CONCLUSIONS CeO2 nanocubes with the (200)-terminated surface/graphene sheet composites (COGSCs) have been prepared successfully by a simple hydrothermal method. The COGSCs-modified electrode exhibits very high catalytic performance, showing much better reaction activities and selectivity compared to a CeO2 nanocube-modified electrode. The PL efficiency of the CeO2 cubes hybridized with graphene is about 30 times higher than that for pristine CeO2 nanocrystals in air. Higher PL and better catalytic performance are explained by a higher oxygen 11744

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vacancy concentration in graphene-supported CeO2 cubes. Such ingenious design of supported well-dispersed catalysts in nanostructured ceria catalysts, synthesized in one step with an exposed high-activity surface, is expected to have important technical applications such as solar cells and as a good electrochemical sensing material for the determination of some biomolecules. The successful synthesis of well-shaped nanocrystals on graphene surfaces is also of importance for further theoretical investigations.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: 86-431-85168256. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the National Basic Research Program of China (2011CB808200), the NSFC (10979001, 51025206, 51032001, 21073071, 11004075, 11004072, 11104105), and the Cheung Kong Scholars Programme of China.

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