Graphene−Semiconductor Nanocomposites: Excited-State

pubs.acs.org/Langmuir © 2009 American Chemical Society

Graphene-Semiconductor Nanocomposites: Excited-State Interactions between ZnO Nanoparticles and Graphene Oxide† Graeme Williams‡ and Prashant V. Kamat* Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556. ‡ University of Waterloo cooperative education student Received March 14, 2009. Revised Manuscript Received April 21, 2009 Graphene oxide sheets suspended in ethanol interact with excited ZnO nanoparticles and undergo photocatalytic reduction. The luminescence quenching of the green emission of ZnO serves as a probe to monitor the electron transfer from excited ZnO to graphene oxide. Anchoring of ZnO nanoparticles on 2-D carbon nanostructures provides a new way to design semiconductor-carbon nanocomposites for catalytic applications.

Introduction Graphene sheets are analogous to unrolled 2-D carbon nanotubes. They are individual sheets separated from the large, stacked-sheet structure of graphite. The carbon sp2 network of single and bilayer graphene exhibits unique 2-D electronic transport that has been shown to produce strong conductivity.1,2 Given the economical cost of graphene, there is a significant drive within the scientific community to gain a greater understanding of its properties and explore its possible applications. For example, the potential for conductive graphene-based films and graphenesheet transparent conductive films has been explored.3-6 However, photochemistry and photovoltaic aspects of 2-D carbon nanostructures, graphene and graphene oxide (GO) sheets, are relatively unstudied and provide an exciting new array of ideas and applications. Although graphite and graphite oxide have been known to be in existence since the last century, it is only recently that graphene and graphene oxide sheets have been prepared and characterized in a systematic way.7 The most significant challenge in the preparation of graphene is overcoming the strong exfoliation energy of the π-stacked layers in graphite.8 The recent focus on graphene sheets involves the simple “micromechanical cleavage” of graphite, where a piece of Scotch tape is used to remove individual sheets of graphene. Micrographitic powder, however, cannot be readily separated into individual sheets or dispersed in a solvent medium. Several methods of preparation of graphene and graphene oxide include the oxidation of graphite combined with thermal exfoliation, the treatment of graphite fluorides with alkyl † Part of the “Langmuir 25th Year: Nanoparticles synthesis, properties, and assemblies” special issue. *Address Correspondence to this author. E-mail: [email protected].

(1) Gomez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Nano Lett. 2007, 7, 3499 –3503. (2) Castro, E. V.; Novoselov, K. S.; Morozov, S. V.; Peres, N. M. R.; Dos Santos, J.; Nilsson, J.; Guinea, F.; Geim, A. K.; Neto, A. H. C. Phys. Rev. Lett. 2007, 99. (3) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Nature 2007, 448, 457–460. (4) Wang, X.; Zhi, L. J.; Mullen, K. Nano Lett. 2008, 8, 323–327. (5) Watcharotone, S. Nano Lett. 2007, 7, 1888–1892. (6) Kim, K.; Park, H. J.; Woo, B. C.; Kim, K. J.; Kim, G. T.; Yun, W. S. Nano Lett. 2008, 8, 3092–3096. (7) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339–1339. (8) Schniepp, H. C.; Li, J. L.; McAllister, M. J.; Sai, H.; Herrera-Alonso, M.; Adamson, D. H.; Prud’homme, R. K.; Car, R.; Saville, D. A.; Aksay, I. A. J. Phys. Chem. B 2006, 110, 8535–8539.

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lithium reagents, and the oxidation of graphite followed by strong sonication.8-10 The factors dictating stable dispersions of graphene in various organic solvents and its interactions with substituted organic compounds have also been discussed.11,12 One obvious challenge is to utilize these 2-D carbon nanostructures as conductive carbon mats so that one can anchor semiconductor or metal nanoparticles and facilitate tailored catalytic reactions (Scheme 1). Initial efforts to utilize graphene as a 2-D carbon support to anchor metal nanoparticles have already been reported.13,14 ZnO and TiO2 nanoparticles have recently been examined in combination with carbon nanotubes, showing an electron accepting and storing capacity of the nanotubes.15-18 Hence, it is reasonable to expect that GO sheets may play a similar role of providing unique 2-D architecture to support semiconductor catalyst nanoparticles. The ability of semiconductor nanoparticles to partially reduce GO samples when excited with UV light was demonstrated recently with TiO2.19 In our quest to further explore the interaction between graphene oxide and semiconductor nanoparticles, we have now probed the ZnO emission to monitor the electron transfer between the semiconductor nanoparticles and GO sheets (Scheme 1). The photochemical processes that illustrate the partial reduction of GO in a ZnO nanoparticle suspension are presented here.

Experimental Section Materials. Graphene oxide was prepared by the Hummers’ method, where micrographitic powder was mixed with strong oxidizing agents, filtered, and dried.7 The oxidation process functionalizes the graphene sheets with various hydroxyl and epoxy groups, in addition to carbonyl and carboxyl groups along (9) Worsley, K. A.; Ramesh, P.; Mandal, S. K.; Niyogi, S.; Itkis, M. E.; Haddon, R. C. Chem. Phys. Lett. 2007, 445, 51–56. (10) Niyogi, S.; Bekyarova, E.; Itkis, M. E.; McWilliams, J. L.; Hamon, M. A.; Haddon, R. C. J. Am. Chem. Soc. 2006, 128, 7720–7721. (11) Paredes, J. I.; Villar-Rodil, S.; Martinez-Alonso, A.; Tascon, J. M. D. Langmuir 2008, 24, 10560–10564. (12) Rochefort, A.; Wuest, J. D. Langmuir 2009, 25, 210–215. (13) Muszynski, R.; Seger, B.; Kamat, P. J. Phys. Chem. C 2008, 112, 5263– 5266. (b) Seger, B.; Kamat, P. V.; J. Phys. Chem. C 2009, 113, 7990–7995. (14) Xu, C.; Wang, X.; Zhu, J. W. J. Phys. Chem. C 2008, 112, 19841–19845. (15) Vietmeyer, F.; Seger, B.; Kamat, P. V. Adv. Mater. 2007, 19, 2935–2940. (16) Kongkanand, A.; Kamat, P. V. J. Phys. Chem. C 2007, 111, 9012–9015. (17) Kongkanand, A.; Kamat, P. V. ACS Nano 2007, 1, 13–21. (18) Kongkanand, A.; Domı´ nguez, R. M.; Kamat, P. V. Nano Lett. 2007, 7, 676–680. (19) Williams, G.; Seger, B.; Kamat, P. V. ACS Nano 2008, 2, 1487–1491.

Published on Web 05/19/2009

DOI: 10.1021/la900905h

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Scheme 1. Excited-State Interaction between ZnO and Graphene Oxide

the edges of the sheets. The dried product was suspended in ethanol and sonicated in order to disperse GO sheets. The details of the procedure are described elsewhere.13 The method of synthesis for zinc oxide nanoparticles, adopted from the literature,20,21 involves the addition of zinc acetate to an ethanol solution, followed by sonication in an ice bath.15 A base, lithium hydroxide, was then added to the solution, and the reaction vessel was further sonicated at room temperature, allowing the particles to complete the growth process. AFM samples were prepared by drop casting dilute mixtures of the semiconductor nanoparticles and GO sheets in ethanol onto heated, freshly cleaved mica substrates. Atomic force microscopy (AFM) was conducted using a Digital Nanoscope III in tapping mode. An etched silicon tip was used as a probe to image the samples. Optical and Emission Measurements. Absorption spectra were recorded using a Shimadzu UV-3101 PC spectrophotometer. Emission spectra were recorded using an SLM-S 8000 spectrofluorometer. Emission lifetimes were measured using a Horiba Jobin Yvon single-photon counting system with a diode (277 nm, 1 MHz repetition, 1.1 ns pulse width) excitation source. Filtered light (λ > 300 nm) from an Oriel 150 W xenon arc lamp was used to carry out steady-state photolysis. All experiments were conducted under ambient conditions.

Results and Discussion Fluorescence Studies. ZnO nanoparticles, with a bulk bandgap of 3.37 eV, are photocatalytically active under UV irradiation. In ethanol, they can be prepared in the size-quantized regime (2-5 nm) by controlling the hydrolysis temperature.22 These particles exhibit green emission under bandgap excitation.23 This green emission (λmax ≈ 530 nm), arising from oxygen vacancies, serves as a probe to monitor the interfacial electron-transfer processes. The ability of ZnO nanoparticles to transfer photogenerated electrons to carbon nanotubes has been demonstrated from the quenching of ZnO emission.15 Figure 1 shows the fluorescence of a 1 mM solution of ZnO nanoparticles with varying amounts of graphene oxide added to the solution. The decrease in fluorescence yield suggests that an additional pathway for the disappearance of the charge carriers dominates because of the interactions between the excited ZnO particles and the GO sheets. As demonstrated earlier, such emission quenching represents interfacial charge-transfer processes.22-24 In the present experiments, the emission quenching represents electron (20) Spanhel, L.; Anderson, M. A. J. Am. Chem. Soc. 1991, 113, 2826–2833. (21) Kamat, P. V.; Patrick, B. Photochemistry and Photophysics of ZnO Colloids. In Symp. Electron. Ionic Prop. Silver Halides; The Society for Imaging Science and Technology: Springfield, Va, 1991. (22) Kamat, P. V.; Patrick, B. J. Phys. Chem. 1992, 96, 6829–34. (23) Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Phys. Chem. B 2003, 107, 7479–7485. (24) Kamat, P. V.; Huehn, R.; Nicolaescu, R. J. Phys. Chem. B 2002, 106, 788– 794.

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Figure 1. Emission spectra of a 1 mM ZnO suspension at different GO concentrations: (a) 0, (b) 0.035, (c) 0.09, (d) 0.14, (e) 0.20, and (f ) 0.24 mg/mL

Figure 2. Absorption of a 1 mM ZnO nanoparticle suspension in ethanol containing different amounts of GO.

transfer from the excited ZnO nanoparticles to the GO to produce reduced GO (RGO). ZnO þ hν f ZnOðh þ eÞ

C2 H5 OH

f

ZnOðeÞ þ• C2 H4 OH

ZnOðeÞ þ GO f ZnO þ RGO

ð1Þ ð2Þ

The ethoxy radicals produced during the hole oxidation step are also reductive in nature and thus contribute to the reduction process. No such reductive radicals are produced in the absence of ZnO. The absorption spectra of ZnO suspensions containing different amounts of GO are shown in Figure 2. Upon close examination of these absorption spectra, it is evident that GO absorbs strongly in the UV region and may interfere with the excitation at 315 nm of ZnO nanoparticles in emission studies. The absorbance difference between ZnO-GO and ZnO at 315 nm can be used to estimate the fraction of excited light absorbed by GO. For all of the samples examined in Figure 1, the decreased absorption at 315 nm due to GO is significantly lower than the fluorescence quenching observed for the same sample. For example, at the highest GO concentration (0.24 mg/mL) the contribution of GO absorption at 315 nm amounts to 25%, but the extent of ZnO quenching was more than 95% of the pristine ZnO emission yield. Thus, we consider the contribution of GO Langmuir 2009, 25(24), 13869–13873

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in the long-lived component is relatively small. On the basis of this analysis, we can conclude that the static interaction between ZnO particles and GO is dominated by the fast component. If electron transfer between ZnO and GO is the only process dominating the fast decay component in the ZnO-GO system, then we can estimate the electron-transfer rate from the emission lifetimes (eq 5). 1 1 k et ¼ ðZnO  GOÞ - ðZnOÞ τ τ

Figure 3. Fluorescence decay traces of 1 mM ZnO with varying GO concentrations. An incident laser pulse wavelength of 279 nm was used for this experiment. Emission was monitored at 530 nm. Inset shows the dependence of lifetime on GO concentration. Table 1. Kinetic Analysis of ZnO Emission Decay at Varying GO Concentrations GO concentration (mg/mL)

A1

τ1 (ns)

A2

τ2 (ns)

Æτæ (ns)

χ2

0 0.05 0.10 0.15 0.20 0.25

0.420 0.434 0.579 0.809 2.09 2.29

2.98 2.84 2.14 1.44 0.67 0.67

0.412 0.423 0.394 0.364 0.235 0.186

32.7 28.4 26.7 26.7 23.7 19.3

30.2 ( 1.0 26.0 ( 0.8 25.5 ( 0.6 24.0 ( 0.6 19.1 ( 0.4 13.7 ( 0.3

2.40 2.81 2.35 2.70 2.20 2.61

absorption of excitation light contributing to the decreased fluorescence yield to be relatively small. If the interaction between the excited ZnO and GO is indeed responsible for emission quenching, then it should be possible to time resolve the process from the emission decay. Figure 3 shows the emission decay of a 1 mM solution of ZnO nanoparticles at varying concentrations of GO. The resulting fluorescence can be fit to exponential curves in order to derive decay time constants, which can be used to calculate average lifetimes of the fluorescence. The inset shows the decrease in average lifetime as a function of GO concentration. The ZnO emission decay is multiexponential, which is typical for semiconductor nanocrystals. The decay was analyzed using biexponential kinetics (eq 3) f ðtÞ ¼ A1 et=τ1 þ A2 et=τ2

ð3Þ

The lifetimes (τ1 and τ2), preexponential factors (A1 and A2), and average lifetime Æτæ of ZnO emission at different GO concentrations are summarized in Table 1. The average lifetime, which was determined using eq 4, provides an overall comparison of the quenching behavior. Æτæ ¼

n X Ai τ i 2 i ¼1

Ai τ i

ð4Þ

In the absence of GO, the fast component of the emission decay has a lifetime of 3.0 ns whereas the slower component has a lifetime of 32.7 ns. Both of these lifetimes decrease with increasing GO concentration. To assess the influence of decay on ZnO emission, we compared the average lifetimes. A decrease in the average lifetime of ZnO emission from 30 to 14 ns is observed with increasing concentration of GO. The short component shows a nearly 20-fold decrease in lifetime (2.98 to 0.67 ns) when the GO concentration is increased to 0.25 mg/mL. However, the decrease Langmuir 2009, 25(24), 13869–13873

ð5Þ

By substituting the values of 0.67 and 2.98 ns as the lifetimes of the ZnO-GO and ZnO systems, we obtain ket as 1.2  109 s-1. This estimate of the electron-transfer rate constant sets the upper limit based on the analysis of the fast component. Photocatalytic Reduction of GO. An interesting aspect of photoinduced electron transfer between excited ZnO and GO is the convenience of on-demand reduction with UV-light irradiation. Using this strategy, we were able to achieve partial reduction of GO in a UV-irradiated TiO2 suspension.19 This photocatalytic reduction method is similar to other reduction methods that are employed to obtain conductive 2-D layers of graphene. Chemical reduction methods have been shown to be effective in attaining good conductivities that are only an order of magnitude lower than that of bulk graphite1,4 Recent endeavors have even allowed for the reduction of graphene oxide in solution by chemical reduction with hydrazine monohydrate, maintaining the disperse sheets through electrostatic repulsions by the precise removal of electrolytes.25 Ethanol suspensions of GO and ZnO nanoparticles were exposed to UV light (λ > 300 nm) using a 150 W Oriel xenon arc lamp and a copper sulfate filter. The solutions were deaerated by purging with nitrogen for approximately 15 min. The removal of air from the system is crucial because dissolved oxygen competes for the photoinduced electron, thus making the reduction of GO ineffective. Because the ZnO-GO suspension in ethanol was irradiated with UV light, it changed color from light brown to dark black. This change in color is very similar to the color change observed during the chemical reduction of graphene oxide. As suggested earlier, this color change is indicative of “partial restoration of the [conjugated] π network” in the graphene sheets.26 Figure 4 shows the color and absorption changes seen during the UV irradiation of a ZnO-GO nanoparticle suspension, respectively. No such color changes could be seen if we irradiate a graphene suspension in ethanol by excluding ZnO. The inset in Figure 4 (right) shows the growth of absorption at 460 nm. The shift in absorption occurs as a result of the darkening of solution and is maximized after approximately 7 min of irradiation. It should be noted that a maximized color change does not necessarily correspond to a complete reduction. Additional exposure of UV light is necessary to maximize the reduction of GO. AFM Characterization. ZnO-GO composite (3:1 ZnO/ GO by mass) systems were examined by tapping mode atomic force microscopy (AFM). A typical AFM image of ZnO nanoparticles anchored to GO sheets is given in Figure 5. The corresponding section-depth profile of the image is also provided. The thickness of an individual sheet of graphene is 0.34 nm, whereas graphene oxide sheet thicknesses have a dry value of 0.65 nm (as determined by X-ray diffraction (XRD)) (25) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101–105. (26) Becerril, H. A.; Mao, J.; Liu, Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. ACS Nano 2008, 2, 463–470.

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Figure 4. (Left) Suspensions of ZnO, ZnO-GO, and ZnO-RGO. The color changes from brown (middle vial) to dark brown (right vial) as we irradiate the ZnO-GO sample with UV light. (Right) Absorption spectra of 0.5 mg/mL GO and 1 mM ZnO in deaerated ethanol at different UV irradiation times of (a) 0, (b) 60, (c) 180, and (d) 420 s. The inset shows the growth of absorption with UV irradiation time.

Figure 5. AFM analysis of ZnO nanoparticles deposited onto graphene oxide sheets. Table 2. Resistances of Unreduced and Photocatalytically Reduced GO

lateral resistance (kΩ) sheet resistance (kΩ/sq) a

ZnO-GO

ZnO-RGO

TiO2-GOa

TiO2-RGOa

583 ( 107 2920 ( 535

16.3 ( 5.16 81.7 ( 25.8

233 ( 105 1170 ( 525

30.5 ( 8.33 152 ( 41.6

From ref 19.

and more typical values of 1-1.3 nm for a hydrated sample examined by AFM.8,27 The composite system studied in this and shown in Figure 5 therefore exhibits mono- and bilayers of graphene, as indicated by 1 to 2 nm sheet heights. Although we were initially interested in individual graphene sheets, it should be noted that the presence of bilayer and few-layer graphene is not necessarily indicative of a bad sample. Studies have shown that bilayer graphene can exhibit conductivities significantly greater than those for single-layer graphene.1 The ZnO-GO system consists of nanoparticles on the order of 2-7 nm, and the nanoparticles appear to be completely covering the graphene sheets. Film Resistance. Upon photocatalytic reduction, it is expected that the reduced GO (RGO) sheets exhibit an improvement in conductivity. Solid-film resistivity tests were performed to evaluate the effectiveness of the photocatalytic reduction of GO in improving the conductivity. Borosilicate glass slides were sputtered with a 40 nm layer of gold except for a 2 mm gap in the center. Large resistances between the gold terminations were confirmed before further manipulations to the slides were made. The GO and RGO solutions were then drop cast on the gap between the gold terminations at slightly elevated temperatures (27) Stankovich, S.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Carbon 2006, 44, 3342–3347.

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(50-60 C) in order to ensure immediate evaporation of the ethanol solvent. This was done to ensure a fairly uniform coverage of the GO film. However, a variation in the film thickness is evident from the large standard deviations witnessed for these samples. The gold terminations were then connected to a digital multimeter where resistance measurements of the GO and RGO samples were made. Sheet resistance values were calculated by dividing measured lateral resistances by the square of the sample (equal to the ratio of the length to the width of the gap). A comparison was also made between the photocatalytic reduction using ZnO and TiO2 nanoparticles. A summary of the results of the resistance measurements averaged for five different samples is given in Table 2. A similar method of determining resistivity changes was previously used for measurements of chemically reduced graphene oxide.28 The lateral resistance of a reduced graphite oxide sample across a 2-mm-long, 3-mm-wide gap reported in this study was 12 kΩ. Lateral resistance values of 16 and 31 kΩ found for the ZnO-RGO and TiO2-RGO samples observed in the present experiments are of the same order as those in previous reports. The unreduced lateral resistance of 32 MΩ28 is lower than the several hundred MΩ resistance seen for GO films. The presence of semiconductor particles (ZnO or TiO2) in the GO film is expected (28) Kotov, N. A.; Dekany, I.; Fendler, J. H. Adv. Mater. 1996, 8, 637–641.

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to contribute to this enhanced photoconductivity of the unreduced GO film. Nevertheless, the lower resistance observed with photocatalytically reduced GO films confirms the effectiveness of semiconductor nanoparticles in facilitating reduction under bandgap excitation.

on-demand reduction of GO using UV irradiation is potentially useful in fine tuning conductivity over a single graphene sheet. In addition, graphene-semiconductor nanoparticle composites can also provide new ways to tailor catalytic and photocatalytic reactions.

Conclusions The electron transfer from excited ZnO nanoparticles to GO is effective in carrying out reduction and decreasing the resistivity of chemically functionalized graphene films. On the basis of the luminescence decay measurements, we estimate the apparent electron-transfer rate constant to be 1.2  109 s-1. The

Acknowledgment. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. We also thank Brian Seger for his help in synthesizing GO and helpful discussions. This is contribution number NDRL 4798 from the Notre Dame Radiation Laboratory.

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