Electron Hopping Through Single-to-Few-Layer Graphene Oxide

May 7, 2012 - confirming the ability of GO to transport electrons through its plane. Illumination- ... the deposition of Ag NPs on the side of GO oppo...
0 downloads 0 Views 2MB Size
Download PDF
Letter pubs.acs.org/JPCL

Electron Hopping Through Single-to-Few-Layer Graphene Oxide Films. Side-Selective Photocatalytic Deposition of Metal Nanoparticles Ian V. Lightcap, Sean Murphy, Timothy Schumer, and Prashant V. Kamat* Radiation Laboratory and Department of Chemistry and Biochemistry, Chemical & Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Single- to few-layer graphene oxide (GO) sheets have been successfully anchored onto TiO2 films using electrophoretic deposition. Upon UV illumination of TiO2−GO films, photogenerated electrons from TiO2 are captured by GO. These electrons are initially used in GO’s reduction, while additional electron transfer results in storage across its sp2 network. In the presence of silver ions, deposition of silver nanoparticles (NPs) is accomplished on the GO surface opposite the TiO2, thus confirming the ability of GO to transport electrons through its plane. Illuminationcontrolled reduction of silver ions allows for simple selection of particle size and loading, making these semiconductor−graphene−metal (SGM) films ideal for custom catalysis and sensor applications. Initial testing of SGM films as surface-enhanced resonance Raman (SERRS) sensors produced significant target molecule signal enhancements, enabling detection of nanomolar concentrations. SECTION: Energy Conversion and Storage; Energy and Charge Transport

T

Scheme 1. Photogenerated Electrons in TiO2 Injected into a Single-Layer RGO Sheet under UV Illuminationa

he preeminence of graphene’s electronic properties is well-described.1−8 Recent work has begun to leverage those excellent properties, specifically graphene oxide’s (GO's) ability to function as an electron shuttle, into enhanced catalysis, battery, and photovoltaic performance.9−14 A method has already been developed to extend the versatility of graphene by pairing the 2-D material with titanium dioxide nanoparticles (NPs).15 The coupling of GO with semiconductor NPs creates a photoactive graphene substrate that can be utilized for the selective deposition of metal NPs.16 UV illumination of a colloidal TiO2 suspension results in the photogeneration of electron−hole pairs. As shown in our earlier study,17 by scavenging holes with ethanol, it is possible to transfer electrons into GO. These electrons not only reduce GO to reduced graphene oxide, RGO, but also become stored across its sp2 network. These stored electrons can be used on-demand to reduce metal ions to metal NPs. When TiO2-assisted reduction of Ag+ is carried out in GO suspensions, Ag deposition occurs without spatial or side selectivity with respect to GO. If one could deposit GO on a photocatalytically active film substrate such as TiO2, it should be possible to transport electrons across its sp2 network. The obvious question is whether electrons in GO/RGO are able to hop from one side to the other and mediate side-selective reduction of metal ions. An ideal semiconductor−graphene− metal (SGM) architecture is presented in Scheme 1. Such controlled deposition of Ag NPs offers an excellent opportunity to develop surface-enhanced Raman spectroscopy (SERS) substrates. © XXXX American Chemical Society

a

The electrons are then used to reduce Ag+ ions on the opposing side of the RGO sheet. The process results in Ag nanoparticles on the side of RGO opposite the TiO2.

In addition to serving as an electron-mediating substrate, graphene itself has been shown to be SERS-active.18,19 Graphene can also function as a concentrator of target Received: April 5, 2012 Accepted: May 7, 2012

1453

dx.doi.org/10.1021/jz3004206 | J. Phys. Chem. Lett. 2012, 3, 1453−1458

The Journal of Physical Chemistry Letters

Letter

molecules. It has been previously shown that organic species interact with graphene through van der Waals forces, increasing local concentrations of the molecule near the active surface.20,21 This behavior is analogous to the concentrating effect observed between organic material and activated carbon.22 Additionally, the epoxy, hydroxy, and carboxylic acid functional groups found on GO enhance interactions with target molecules due to dipole/dipole attractions.23 We present here the unique property of RGO to shuttle electrons in a direction orthogonal to its basal plane and transfer them to Ag+ ions and Ag NPs. This property enables the deposition of Ag NPs on the side of GO opposite TiO2. The ability of these composite films to serve as surfaceenhanced resonance Raman spectroscopy (SERRS) sensors for the detection of organic compounds in nanomolar concentrations is demonstrated. The SGM architectures discussed here show excellent potential for creating custom metal NP arrays for use in next-generation sensing and catalyst systems. Deposition of Single-Layer GO on TiO2. Several methods exist for creating mono-to-few-layer films of graphene. Most of these methods involve the use of finely tuned temperature and pressure conditions for chemical vapor deposition of graphene sheets on carefully prepared substrates.24−27 However, chemical exfoliation of graphite offers a convenient alternative to produce GO sheets in suspension.28,29 These sheets can be dispersed on any given substrate by spin coating or electrophoretic deposition. In the present work, single-layer GO was deposited onto TiO2 films via controlled electrophoretic deposition (EPD). Scanning electron microscopy (SEM) was used to examine surface features along each step of the process. First, a thin TiO2 film is prepared by spreading a paste on an optically transparent electrode (OTE) via the doctor blade method, followed by annealing at 500 °C. The SEM image of the neat TiO2 film is shown in Figure 1A. In order to deposit

Scheme 2. Electrophoretic Deposition Used to Deposit Single Sheets of GO on TiO2

the following discussion, we will refer to the reduced GO in post-illumination SGM films as RGO. Deposition of single-to-few layers of GO can be controlled by using the appropriate concentration of GO, voltage, and deposition time. The optimum conditions for achieving singlelayer GO on TiO2 films were found to be 0.5 mg/mL GO in ethanol, 30 V dc, and 1 min deposition time. The SEM image of single-layer GO stretched over a nanostructured TiO2 surface is shown in Figure 1B. The characteristic wrinkles of graphene are clearly seen, and it is also possible to see the details of the TiO2 layer just beneath the graphene sheet. While incoming SEM beam electrons penetrate through the graphene, secondary electrons with lower energies are screened when passing back through the graphene, producing the shadowy appearance of TiO2. When SEM is utilized with lower accelerating voltages, incident electrons are unable to penetrate the graphene sufficiently to image the TiO2 sublayer. Using this strategy, the finer surface details of GO are revealed (Supporting Information, Figure S2). Photocatalytic Activation of Graphene for Metal NP Nucleation and Growth. TiO2−GO films are activated for metal nanoparticle deposition by UV illumination. The films are placed in a N2-purged, 2.0 × 10−5 M AgNO3 solution (2.0 × 10−2 M AgNO3 for nanomolar sensitive films) in ethanol prior to backside illumination. Equations 1−4 describe the electron-transfer process in SGM films. Upon UV illumination, electrons are photogenerated in TiO2 and transferred to GO, while holes are scavenged by ethanol. The electrons are then shuttled to the opposite side of GO, where they are readily available for the reduction of Ag+ ions to Ag NPs.

Figure 1. (A) SEM image of a TiO2 NP film. (B) A single layer of GO electrophoretically deposited over a TiO2 NP film. Wrinkles in the GO sheet and the shadowing of TiO2 NPs by GO are observed.

single sheets of GO, the TiO2 films were submersed in a dispersion of GO in ethanol. A graphic representing the EPD process is shown in Scheme 2. The OTE/TiO2 film was held ∼4 mm apart from another OTE, and a 30 V dc voltage was applied between the two electrodes. Under application of an electric field, the negatively charged GO sheets are driven to the positive electrode where the TiO2 film is positioned. As shown earlier, the EPD process also enables the reduction of GO to RGO.30 While this deposition process may result in partial reduction, further reduction of GO is achieved when the TiO2 film is UV-illuminated (see Supporting Information, Figure S1). The reduction of GO under these conditions confirms photogenerated electron transfer from TiO2 to the GO. In

TiO2 + hν(UV) → TiO2 (e + h)

(1)

TiO2 (e) + GO → TiO2 + (R)GO(e)

(2)

TiO2 (h) + C2H5OH → TiO2 + •C2H4OH

(3)

(R)GOn(e) + n Ag + → (R)GO + Ag n

(4)

This sequential electron-transfer process allows for the tuning of metal NP size by regulation of illumination exposure time in TiO2 films. Additionally, the RGO substrate should serve as a platform to disperse metal NPs due to delocalization of electrons within its extended sp2 network. The binding of metal ions at defects such as epoxide sites further aids nucleation of Ag nanoparticles.31 1454

dx.doi.org/10.1021/jz3004206 | J. Phys. Chem. Lett. 2012, 3, 1453−1458

The Journal of Physical Chemistry Letters

Letter

Morphologies of Ag NPs deposited on TiO2-only and TiO2− RGO films were evaluated with SEM. Both substrates were subjected to identical conditions and illuminated for 10 min to induce Ag NP growth. As seen in the Figure 2A SEM image, Ag

Figure 2. SEM images show the surface morphology of Ag NPs created via irradiation of TiO2 or TiO2−RGO films for 10 min in the presence of AgNO3. (A) SEM image of Ag NPs reduced directly on TiO2. (B) Ag NP deposition on RGO produces smaller particles with high surface loading. (C) A higher-magnification image of the SGM film from (B) shows significant coverage of small (∼2−5 nm diameters) Ag NPs on RGO.

particles formed directly on the TiO2 layer are larger (100−500 nm across) and less spherically symmetric than those found on RGO (Figure 2B and C). Larger Ag NPs on TiO2 most likely result from the uneven growth and coalescence of smaller NPs. The Ag NPs formed on the RGO shown in Figure 2B are welldispersed, the majority of which are considerably smaller (5−20 nm) than Ag NPs on TiO2. The high-magnification image provided in Figure 2C depicts the smallest Ag NPs (∼2−5 nm diameter) found covering the surface of RGO between larger particles (∼50 nm diameters). This effect provides evidence that RGO’s sp2 network, in addition to its wrinkles and surface defects, make it an excellent substrate for metal NP nucleation. The question remains whether particle sizing in SGM films can be fine-tuned. The growth of Ag NPs in SGM films was examined using a systematic study of Ag NP morphology as a function of irradiation time. Figure 3 displays SEM images taken following illumination times of 1, 2, and 3 min. During UV illumination of the TiO2−RGO film, electrons originating in the TiO2 layer are transferred to and through the RGO sheet and then used to reduce Ag+ ions. After only 1 min of illumination, Ag NPs nucleate evenly onto the RGO surface away from the TiO2 with an average diameter of ∼25 nm (Figure 3A). Closer examination of the image reveals an affinity of Ag NP deposition at RGO wrinkle sites. As illumination continues, the average NP diameter grows to ∼50 nm at 2 min (Figure 3B) and then to a maximum diameter of ∼100 nm at 3 min (Figure 3C). Further irradiation results in increased loading of Ag NPs on RGO, where smaller metal NPs (5−20 nm diameters) coat the remaining available surfaces (Figure 2C). Loading of smaller metal NPs on the RGO surface is important for increased activity in selected catalysis and sensing applications.32−34 Unlike the previous TiO2-assisted, solution-based Ag NP deposition method,15 the present approach allows for sideselective deposition of Ag NPs on the side of RGO opposite the TiO2. Other reported methods for interfacing Ag NPs with GO necessitate the use of coupling agents that inhibit direct contact between GO and Ag and add steps to the fabrication of composite films.35,36 The method presented here allows for simple control of Ag NP growth by illumination time for tunable sensing and catalysis applications. Growth of Ag NPs in SGM films was further elucidated through the use of UV−visible absorption spectrophotometry.

Figure 3. SEM images show the Ag NP size distribution as a function of irradiation time. (A) After 1 min, Ag NP diameters are 10−42 nm. (B) After 2 min, Ag NP diameters are 30−110 nm. (C) After 3 min, Ag NP diameters are 15−150 nm.

Figure 4 presents the absorption profiles for neat TiO2 on OTE, TiO2−GO, and SGM films following 1, 2, 3, and 10 min of UV irradiation. Because single layers of graphene absorb ∼2% incident light across the visible spectrum, the slight increase in absorption of TiO2−GO over TiO2-only films is expected (traces a and b).37 A sharp rise in absorption across the visible spectrum is seen after only 1 min of irradiation. This increase in absorption is due to initial reduction of GO to RGO followed by the formation of Ag NPs. The absorption changes resulting from the reduction of GO by photogenerated electrons from TiO2 were isolated and can be seen in Supporting Information (Figure S1). Traces taken following 2 and 3 min of irradiation of SGM films resulted in further broadband absorption increases across the visible spectrum. After 10 min of irradiation, increased loading of Ag NPs on the RGO surface resulted in a soft peak, emerging at 400−550 nm. This peak is attributed to the surface plasmon resonance signal of Ag NPs. 1455

dx.doi.org/10.1021/jz3004206 | J. Phys. Chem. Lett. 2012, 3, 1453−1458

The Journal of Physical Chemistry Letters

Letter

for TAPP in the presence of SGM films was compared to the response of the target molecule in the presence of a blank OTE (Supporting Information, Figure S6). As seen in Figure 2C and SI Figure S3, a high coverage of Ag NPs exists on the RGO sheet for 10 and 20 min illumination times (2.0 × 10−5 M AgNO3), which results in increased interactions between TAPP and the LSPR of Ag NPs. SERRS spectra shown in Figure 5A displays a strongly enhanced TAPP response with the SGM film. Enhancement of TAPP Raman bands is confirmed by the position of peaks in SERRS spectra.43,44 SGM films exhibiting high sensitivity to low concentrations of TAPP (5−20 nM) were achieved in these experiments (Figures 5B). Of additional benefit to the efficacy of these composites as SERRS sensors is graphene’s ability to interact with target molecules. Improved interaction should result in augmented concentrations of TAPP near the RGO surface, which is covered with Ag NPs. If this scenario is accurate, it should be possible to observe improved SERRS enhancements as a function of interaction time between SGM films and TAPP. The SERRS enhancement of TAPP in the SGM film was monitored as a function of the soaking time. Results shown in Figure 5A confirm the role that RGO plays as a concentrator of target species from solution. The TAPP SERRS signal is enhanced by a factor of ∼1.6 for the SGM film allowed to soak in the presence of TAPP solution for an extended period of time (Figure 5A). Previous studies have shown the usefulness of metal−GO nanostructured films for SERS sensing, demonstrating that GO with additional functionalization can interact with various target molecules with a signal-to-noise ratio of ∼3.35 In the present work, we were able to detect a nanomolar range TAPP solution with a signal-to-noise ratio of ∼15. SGM films thus combine both the desirable characteristics of controlled metal NP deposition and increased target molecule interaction with its RGO−metal NP substrate. Because the SGM films are simple to fabricate and require no extra functionalization of GO in order to interface the material with Ag NPs, the detection method should provide greater versatility. In summary, photocatalytic activation of single-to-few-layer GO films provides a convenient method to deposit desired size silver nanoparticles with side selectivity. The metal nanoparticle growth and density on the GO film can be controlled by varying the illumination time and metal ion concentration during deposition. With its unique procedure of illuminationcontrolled electron transfer and metal deposition, SGM films have the potential for adaptation into a wide range of custom catalyst and sensor applications.

Figure 4. Absorption spectra of TiO2, TiO2−GO, and SGM films under different irradiation times. Ag nanoparticle deposition and growth cause increased surface plasmon resonance absorption in SGM films.

Figures S4 and S5 are included in the Supporting Information to show the versatility of SGM films for different or additional metal NP deposition as well as the potential in SGM films for multilayer graphene−metal NP stacking. Utilization of SGM NP Films as SERRS Sensors. SERRS is a valuable tool for the detection of trace molecules.38−40 SERRS devices utilize the interaction between the target molecule and the localized surface plasmon resonance (LSPR) of noble metal NPs as means to amplify the target Raman signal. The SGM films developed in this study offer the opportunity to serve as a SERRS substrates for detecting low-level organic molecule concentrations from solution. Additionally, the graphene in SGM films serves as an ideal SERRS substrate due to fluorescence quenching along with its Raman-enhancing properties.41,42 To explore the potential of these films as chemical sensors, we utilized a porphyrin derivative as a model target compound. SGM films were placed in nanomolar solutions of 5,10,15,20tetrakis(4-aminophenyl)-21H,23H-porphyrin (TAPP) in ethanol. Scheme 3 illustrates the interaction between the LSPR of metal NPs and the target molecule resulting in amplification of the target molecule Raman signals. The SERRS enhancement Scheme 3. Detection of TAPP through SERRS Enhancement When the Molecule Interacts with the LSPR of Ag NPsa

■

EXPERIMENTAL SECTION Preparation of TiO2−GO Films. TiO2 paste (Dyesol 90T) was applied via the doctor blade method to the conducting side of a premasked OTE. The TiO2 film area was 2 cm2. Films were annealed at 500 °C for 1 h. Graphene oxide (GO) was prepared using a modified Hummers technique.28 Before deposition of GO on TiO2, the GO dispersion in ethanol was gently sonicated for 10 min and allowed to settle for 50 min. Electrophoretic deposition of single-layer GO onto TiO2 involved submersion of the TiO2 film into ∼3 mL of 0.5 mg/mL GO dispersion (taken from the top of the dispersion after settling). Another OTE was held parallel to the TiO2 film at a distance of 4 mm (conducting side in). A voltage of 30 V was applied between the two OTEs and held constant for the 1 min deposition.

a

Additionally, the RGO substrate serves to concentrate TAPP near the Ag NPs, allowing for more intimate Ag NP−TAPP interactions, which lead to Raman signal enhancements. 1456

dx.doi.org/10.1021/jz3004206 | J. Phys. Chem. Lett. 2012, 3, 1453−1458

The Journal of Physical Chemistry Letters

Letter

Figure 5. SERRS spectra for the target molecule, TAPP. (A) Strong SERRS signal is obtained using SGM films to detect submicromolar concentrations of TAPP. Soaking of SGM films in TAPP produces enhanced SERRS response, demonstrating the concentrating effects of RGO. (B) Nanomolar TAPP SERRS spectra. (a) Blank OTE film in TAPP, (b−d) SGM film in (b) 5 nM, (c) 10 nM, and (d) 20 nM TAPP solutions. The illumination time for Ag nanoparticle deposition was 2 min. All spectra have undergone baseline subtraction.

Deposition of Metal NPs on a TiO2−GO Film. The TiO2−GO film from the previous step was placed in a N2-purged, 20 μM AgNO3 solution in ethanol. The film was placed ∼10 cm from a 300 W Xe lamp (Oriel) and was back-side-illuminated (∼0.6 W/cm2). Optical and Structural Characterization of SGM Films. Diffuse reflectance absorption spectra were recorded using a Shimadzu UV-3101PC spectrophotometer. Surface morphologies of SGM films were examined using a FEI Magellan-400 field emission scanning electron microscope (FESEM) and a Hitachi S-4500 FESEM. SERRS Evaluation. The Raman signal enhancement for TAPP in the presence of SGM films was analyzed using a Renishaw Raman microscope (RM1000) equipped with a 514 nm argon ion laser excitation source. Integrated intensities of Raman bands were analyzed with Renishaw WiRE software. SGM films were immersed in an ethanol TAPP solution and placed in the front side (with respect to incident laser) of a sealed 5 mm optical cuvette.

■

Basic Energy Sciences of the U.S. Department of Energy through Award DE-FC02-04ER15533. This is contribution number NDRL No. 4916 from the Notre Dame Radiation Laboratory.

■

(1) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10451−10453. (2) Zhang, Y. B.; Tan, Y. W.; Stormer, H. L.; Kim, P. Experimental Observation of the Quantum Hall Effect and Berry’s Phase in Graphene. Nature 2005, 438, 201−204. (3) Gomez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Electronic Transport Properties of Individual Chemically Reduced Graphene Oxide Sheets. Nano Lett. 2007, 7, 3499−3503. (4) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. TwoDimensional Gas of Massless Dirac Fermions in Graphene. Nature 2005, 438, 197−200. (5) Du, X.; Skachko, I.; Barker, A.; Andrei, E. Y. Approaching Ballistic Transport in Suspended Graphene. Nat. Nanotechnol. 2008, 3, 491− 495. (6) Bolotin, K. I.; Sikes, K. J.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. L. Ultrahigh Electron Mobility in Suspended Graphene. Solid State Commun. 2008, 146, 351−355. (7) Yu, D.; Dai, L. Self-Assembled Graphene/Carbon Nanotube Hybrid Films for Supercapacitors. J. Phys. Chem. Lett. 2010, 1, 467− 470. (8) Niyogi, S.; Bekyarova, E.; Hong, J.; Khizroev, S.; Berger, C.; de Heer, W.; Haddon, R. C. Covalent Chemistry for Graphene Electronics. J. Phys. Chem. Lett. 2011, 2, 2487−2498. (9) Seger, B.; Kamat, P. V. Electrocatalytically Active Graphene− Platinum Nanocomposites. Role of 2-D Carbon Support in PEM Fuel Cells. J. Phys. Chem. C 2009, 113, 7990−7995. (10) Jafri, R. I.; Rajalakshmi, N.; Ramaprabhu, S. Nitrogen Doped Graphene Nanoplatelets as Catalyst Support for Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cell. J. Mater. Chem. 2010, 20, 7114−7117. (11) Liang, M. H.; Zhi, L. J. Graphene-Based Electrode Materials for Rechargeable Lithium Batteries. J. Mater. Chem. 2009, 19, 5871−5878. (12) Murray, I. P.; Lou, S. J.; Cote, L. J.; Loser, S.; Kadleck, C. J.; Xu, T.; Szarko, J. M.; Rolczynski, B. S.; Johns, J. E.; Huang, J.; et al.

ASSOCIATED CONTENT

S Supporting Information *

Absorption spectra showing the TiO2-assisted reduction of GO in SGM films under illumination are presented in Figure S1. Figures S2−S5 show surface features of GO, SGM films with high Ag NP loading, SGM films with Au NPs, and SGM films with Ag NPs between stacked GO sheets. Also shown are Raman spectra of SGM films compared with neat OTE and various other blanks. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The research described herein was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of 1457

dx.doi.org/10.1021/jz3004206 | J. Phys. Chem. Lett. 2012, 3, 1453−1458

The Journal of Physical Chemistry Letters

Letter

Graphene Oxide Interlayers for Robust, High-Efficiency Organic Photovoltaics. J. Phys. Chem. Lett. 2011, 2, 3006−3012. (13) Ng, Y. H.; Lightcap, I. V.; Goodwin, K.; Matsumura, M.; Kamat, P. V. To What Extent Do Graphene Scaffolds Improve the Photovoltaic and Photocatalytic Response of TiO2 Nanostructured Films? J. Phys. Chem. Lett. 2010, 1, 2222−2227. (14) Kamat, P. V. Graphene-Based Nanoassemblies for Energy Conversion. J. Phys. Chem. Lett. 2011, 2, 242−251. (15) Lightcap, I. V.; Kosel, T. H.; Kamat, P. V. Anchoring Semiconductor and Metal Nanoparticles on a 2-Dimensional Catalyst Mat. Storing and Shuttling Electrons with Reduced Graphene Oxide. Nano Lett. 2010, 10, 577−583. (16) Kamat, P. V. Graphene-Based Nanoarchitectures. Anchoring Semiconductor and Metal Nanoparticles on a Two-Dimensional Carbon Support. J. Phys. Chem. Lett. 2010, 1, 520−527. (17) Williams, G.; Seger, B.; Kamat, P. V. TiO2−Graphene Nanocomposites. UV-Assisted Photocatalytic Reduction of Graphene Oxide. ACS Nano 2008, 2, 1487−1491. (18) Xu, H.; Chen, Y. B.; Xu, W. G.; Zhang, H. L.; Kong, J.; Dresselhaus, M. S.; Zhang, J. Modulating the Charge-Transfer Enhancement in Gers Using an Electrical Field under Vacuum and an N/P-Doping Atmosphere. Small 2011, 7, 2945−2952. (19) Vinodgopal, K.; Neppolian, B.; Lightcap, I. V.; Grieser, F.; Ashokkumar, M.; Kamat, P. V. Sonolytic Design of Graphene Au Nanocomposites. Simultaneous and Sequential Reduction of Graphene Oxide and Au(III). J. Phys. Chem. Lett. 2010, 1, 1987−1993. (20) Jung, N.; Crowther, A. C.; Kim, N.; Kim, P.; Brus, L. Raman Enhancement on Graphene: Adsorbed and Intercalated Molecular Species. ACS Nano 2010, 4, 7005−7013. (21) Wojcik, A.; Kamat, P. V. Reduced Graphene Oxide and Porphyrin. An Interactive Affair in 2-D. ACS Nano 2010, 4, 6697− 6706. (22) Torimoto, T.; Ito, S.; Kuwabata, S.; Yoneyama, H. Effects of Adsorbents Used as Supports for Titanium Dioxide Loading on Photocatalytic Degradation of Propyzamide. Environ. Sci. Technol. 1996, 30, 1275−1281. (23) He, H. Y.; Klinowski, J.; Forster, M.; Lerf, A. A New Structural Model for Graphite Oxide. Chem. Phys. Lett. 1998, 287, 53−56. (24) Juang, Z.-Y.; Wu, C.-Y.; Lu, A.-Y.; Su, C.-Y.; Leou, K.-C.; Chen, F.-R.; Tsai, C.-H. Graphene Synthesis by Chemical Vapor Deposition and Transfer by a Roll-to-Roll Process. Carbon 2010, 48, 3169−3174. (25) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, P.; Choi, J. Y.; Hong, B. H. Large-Scale Pattern Growth of Graphene Films for Stretchable Transparent Electrodes. Nature 2009, 457, 706−710. (26) May, J. W. Platinum Surface Leed Rings. Surf. Sci. 1969, 17, 267. (27) Li, X. S.; Cai, W. W.; An, J. H.; Kim, S.; Nah, J.; Yang, D. X.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; et al. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312−1314. (28) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339−1339. (29) Konkena, B.; Vasudevan, S. Understanding Aqueous Dispersibility of Graphene Oxide and Reduced Graphene Oxide through pKa Measurements. J. Phys. Chem. Lett. 2012, 3, 867−872. (30) An, S. J.; Zhu, Y.; Lee, S. H.; Stoller, M. D.; Emilsson, T.; Park, S.; Velamakanni, A.; An, J.; Ruoff, R. S. Thin Film Fabrication and Simultaneous Anodic Reduction of Deposited Graphene Oxide Platelets by Electrophoretic Deposition. J. Phys. Chem. Lett. 2010, 1, 1259−1263. (31) Goncalves, G.; Marques, P. A. A. P.; Granadeiro, C. M.; Nogueira, H. I. S.; Singh, M. K.; Grácio, J. Surface Modification of Graphene Nanosheets with Gold Nanoparticles: The Role of Oxygen Moieties at Graphene Surface on Gold Nucleation and Growth. Chem. Mater. 2009, 21, 4796−4802. (32) Thiele, E. W. Relation between Catalytic Activity and Size of Particle. Ind Eng Chem 1939, 31, 916−920.

(33) Campbell, C. T.; Parker, S. C.; Starr, D. E. The Effect of SizeDependent Nanoparticle Energetics on Catalyst Sintering. Science 2002, 298, 811−814. (34) Tsao, C. S.; Tzeng, Y. R.; Yu, M. S.; Wang, C. Y.; Tseng, H. H.; Chung, T. Y.; Wu, H. C.; Yamamoto, T.; Kaneko, K.; Chen, S. H. Effect of Catalyst Size on Hydrogen Storage Capacity of PtImpregnated Active Carbon via Spillover. J. Phys. Chem. Lett. 2010, 1, 1060−1063. (35) Liu, X. J.; Cao, L. Y.; Song, W.; Ai, K. L.; Lu, L. H. Functionalizing Metal Nanostructured Film with Graphene Oxide for Ultrasensitive Detection of Aromatic Molecules by Surface-Enhanced Raman Spectroscopy. ACS Appl. Mater. Interfaces 2011, 3, 2944−2952. (36) Fu, X. Q.; Bei, F. L.; Wang, X.; Yang, X. J.; Lu, L. D. SurfaceEnhanced Raman Scattering of Silylated Graphite Oxide Sheets Sandwiched between Colloidal Silver Nanoparticles and Silver Piece. J. Raman Spectrosc. 2010, 41, 370−373. (37) Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M. R.; Geim, A. K. Fine Structure Constant Defines Visual Transparency of Graphene. Science 2008, 320, 1308−1308. (38) Campion, A.; Kambhampati, P. Surface-Enhanced Raman Scattering. Chem. Soc. Rev. 1998, 27, 241−250. (39) Haynes, C. L.; McFarland, A. D.; Van Duyne, R. P. SurfaceEnhanced Raman Spectroscopy. Anal. Chem. 2005, 77, 338 A−346 A. (40) Halvorson, R. A.; Vikesland, P. J. Surface-Enhanced Raman Spectroscopy (SERS) for Environmental Analyses. Environ. Sci. Technol. 2010, 44, 7749−7755. (41) Xie, L.; Ling, X.; Fang, Y.; Zhang, J.; Liu, Z. Graphene as a Substrate to Suppress Fluorescence in Resonance Raman Spectroscopy. J. Am. Chem. Soc. 2009, 131, 9890−9891. (42) Schedin, F.; Lidorikis, E.; Lombardo, A.; Kravets, V. G.; Geim, A. K.; Grigorenko, A. N.; Novoselov, K. S.; Ferrari, A. C. SurfaceEnhanced Raman Spectroscopy of Graphene. ACS Nano 2010, 4, 5617−5626. (43) Murphy, S.; Huang, L.; Kamat, P. V. Charge-Transfer Complexation and Excited-State Interactions in Porphyrin−Silver Nanoparticle Hybrid Structures. J. Phys. Chem. C 2011, 115, 22761− 22769. (44) Wang, H.; Xu, J.; Wan, J.; Zhao, Y.; Zheng, X. Excited State Structural Dynamics of Tetra(4-aminophenyl)Porphine in the Condensed Phase: Resonance Raman Spectroscopy and Density Functional Theory Calculation Study. J. Phys. Chem. B 2010, 114, 3623−3632.

1458

dx.doi.org/10.1021/jz3004206 | J. Phys. Chem. Lett. 2012, 3, 1453−1458