Graphene-Semiconductor Nanocomposites

Graphene-Semiconductor Nanocomposites

(Parte 1 de 2)

DOI: 10.1021/la900905h 13869Langmuir 2009, 25(24), 13869–13873 Published on Web 05/19/2009 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 ofChemistry andBiochemistry, University ofNotre 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 transportthathasbeenshowntoproducestrongconductivity.1,2Given the economicalcost ofgraphene, 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, cannotbereadilyseparatedintoindividualsheetsordispersedina solventmedium.Several methodsofpreparationofgrapheneand graphene oxide include the oxidation of graphite combined with thermal exfoliation, the treatment of graphite fluorides with alkyl lithiumreagents,andtheoxidationofgraphitefollowedbystrong 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.1,12

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 probedtheZnOemissiontomonitortheelectrontransferbetween 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 †Part of the “Langmuir 25th Year: Nanoparticles synthesis, properties, and assemblies” special issue.

*Address Correspondence to this author. E-mail: pkamat@nd.edu (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, 9. (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.

(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. (1) 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, 1, 9012–9015. (17) Kongkanand, A.; Kamat, P. V. ACS Nano 2007, 1, 13–21. (18) Kongkanand, A.; Domınguez, 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.

13870 DOI: 10.1021/la900905h Langmuir 2009, 25(24), 13869–13873

Article Williams and Kamat 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 Ab ase, 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.

AFMsampleswerepreparedbydropcastingdilutemixturesof the semiconductor nanoparticles and GO sheets in ethanol onto heated, freshly cleaved mica substrates. Atomic force microscopy (AFM) was conducted using a Digital Nanoscope I in tapping mode. An etched silicon tip was used as a probe to image the samples.

Optical and Emission Measurements. Absorption spectra wererecordedusingaShimadzuUV-3101PCspectrophotometer. 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.Inethanol,they can bepreparedin the size-quantizedregime (2-5 nm) by controlling the hydrolysis temperature.2 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.2-24 In the present experiments, the emission quenching represents electron transferfromtheexcitedZnOnanoparticlestotheGOtoproduce reduced GO (RGO).

ZnO þhνfZnOðh þeÞ f C H OH

ZnOðeÞþ•C2H4OH ð1Þ

ZnOðeÞþ GOfZnO þRGO 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 byGO. For all ofthe samples examinedinFigure1,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

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

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

(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. (2) 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.

Williams and Kamat Article

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) fðtÞ¼ A1et=τ þ A2et=τ ð3Þ

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

Æτæ ¼ Xn

Aiτi 2

Aiτi 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 32.7ns. 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 averagelifetimeofZnOemissionfrom30to14nsisobservedwith 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 inthelong-livedcomponentisrelativelysmall.Onthebasisofthis 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).

Bysubstitutingthevaluesof0.67and2.98nsasthelifetimesofthe

ZnO-GO and ZnO systems, we obtain ket as 1.2 109 s-1.T his 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. Usingthis strategy, we were ableto 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 nanoparticlesuspension,respectively.Nosuchcolorchangescouldbeseen if we irradiate a graphene suspension in ethanol by excluding ZnO. The insetinFigure 4 (right)shows thegrowth ofabsorption at 460 nm. The shift in absorption occurs as a result of the darkeningofsolutionandismaximizedafterapproximately7min 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.

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