Phonon Energy Transfer in GrapheneâPhotoacid Hybrids - The

Article pubs.acs.org/JPCC

Phonon Energy Transfer in Graphene−Photoacid Hybrids Xiaoyong Pan,† Hao Li,† Kim Truc Nguyen,† George Grüner,§,∥ and Yanli Zhao*,†,‡ †

Division of Chemistry and Biological Chemistry, School of Physical & Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371 ‡ School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 § Department of Physics and Astronomy, University of California Los Angeles, 405 Hilgard Avenue, Los Angeles, California 90095, United States S Supporting Information *

ABSTRACT: Three water-soluble pyrene derivatives, i.e., 1pyrenesulfonic acid sodium salt (PAS), 8-hydroxy-1,3,6pyrenetrisulfonic acid trisodium salt (HPTS), and 6,8dihydroxy-1,3-pyrenedisulfonic acid disodium salt (DHPDS), were employed in noncovalent functionalization of graphene. The phonon coupling interaction between the HPTS and DHPDS photoacids and graphene was demonstrated by UV− vis and photoluminescence spectroscopies, and the proposed mechanism of the phonon transfer was verified by temperature-dependent absorption spectroscopy. Graphene plays the role as a modulator in these graphene/photoacid hybrid systems, which switches the equilibrium between different species of the photoacids. Current work presents the pioneering investigation of phonon coupling (phonon energy transfer) in the graphene− photoacid systems.

1. INTRODUCTION Graphene has attracted much attention on account of its potential applications in the development of new materials and devices.1 Chemical functionalizations of graphene with various organic molecules have been investigated extensively in order to create functional composite materials that combine the advantageous characteristics and properties of both components, largely extending its application domains.2 However, rare research attention has been paid to the experimental investigation of the membrane aspect of graphene. The 2D structure of graphene processes the out-of-plane vibration mode that is known as phonon, providing graphene with the appealing properties capable of having wide applications in nanoelectronic devices. In particular, the experimental effort of employing the vibration energy of graphene has not been reported yet to our best knowledge.1b In current research, three pyrene derivatives (1-pyrenesulfonic acid sodium salt (PAS), 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt (HPTS), and 6,8-dihydroxy-1,3-pyrenedisulfonic acid disodium salt (DHPDS), whose chemical structures are shown in Scheme 1) were chosen in order to investigate their interactions with graphene. The phonon energy transfer1b between graphene and HPTS- or DHPDSbased photoacids was determined under certain conditions. In these graphene/photoacid hybrid systems, graphene plays the role as a modulator, which switches the equilibrium between different species of the photoacids. In contrast to tremendous research efforts on charge/energy transfers between different components of various graphene composite materials,3 our current work presents the pioneering investigation of phonon © 2012 American Chemical Society

Scheme 1. Chemical Structures of the Pyrene Derivatives Employed: PAS, HPTS, and DHPDS

coupling (phonon energy transfer) in the graphene−photoacid systems.

2. EXPERIMENTAL SECTION Materials. Extra pure grade of fine power graphite was supplied by Merck Chemicals International Inc. Sodium dodecyl sulfate (SDS), 1-pyrenesulfonic acid sodium salt (PAS), 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt (HPTS), and 6,8-dihydroxy-1,3-pyrenedisulfonic acid disodium salt (DHPDS) were purchased from Aldrich and were used as received. Potassium permanganate, hydrogen peroxide solution (30%), sulfuric acid (98%), and sodium borohydride (NaBH4) were supplied by Regent Chemicals Pte Ltd. and were used without further purification. Dimethyl sulfoxide (DMSO) was supplied by Sino Chemical Co. Pte Ltd. and was used as received. Deionized (DI) water with a resistance of 18.2 MΩ was obtained from a Millipore Simplicity 185 system. SingleReceived: November 12, 2011 Revised: January 16, 2012 Published: January 18, 2012 4175

dx.doi.org/10.1021/jp2109016 | J. Phys. Chem. C 2012, 116, 4175−4181

The Journal of Physical Chemistry C

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Figure 1. UV−vis spectra of HPTS and DHPDS in the absence/presence of graphene. (a) Absorbance spectra of HPTS in the absence/presence of graphene. (b) Absorbance spectra of DHPDS in the absence/presence of graphene. The concentration of the solutions is 1 × 10−5 mol L−1 in H2O.

eter. For the characterization of solutions with UV−vis and photoluminescence measurements, a dilute concentration (1 × 10−5 M) was used. Resonance Raman scattering (RRS) measurements were conducted on a Renishaw Ramanscope in the backscattering configuration under the same experimental setups. Stokes spectra of GO, graphene, and various hybrids were collected with a 457 nm (2.71 eV) laser. Atomic force microscopy (AFM) measurements were performed on a MFP3D microscope (Asylum Research, Santa Barbara, CA) with a cantilever (Arrow NC, Nanoworld) in tapping mode. Scan rate was set to be 1 Hz at various scan sizes. Measurements of pH values of all the involved solutions were conducted with a Jenco 6173 benchtop pH meter.

walled carbon nanotubes (SWNTs) were purchased from SouthWest Nanotechnologies, Inc., and were used as received. Graphene Oxide (GO). GO was prepared according to a modified Hummers method.4 Graphite (2 g) was introduced into a cold concentrated H2SO4 solution (100 mL), and the mixture was homogenized by stirring. To the mixture was gradually added KMnO4 (6 g) with stirring in an ice bath in order to avoid rapid rise of the mixture temperature. The resulted solution was diluted in DI water (250 mL) after reaction for 2 h. Subsequently, 30% H2O2 (20 mL) was added into the solution at room temperature. After reaction for 3 days, the resulting solution was subjected to dialysis until the pH value of the solution reached ∼7 in order to completely remove acid and metal ions. The produced GO solution was centrifuged and filtered followed by freeze-drying, yielding GO as dark brown powder. Successful preparation of GO was verified by Raman and UV−vis spectroscopies (Figure S1 in the Supporting Information). Reduced Graphene (RG). Graphene was prepared according to the previously reported method starting with GO solution.5,2b The pH value of GO aqueous solution (10 mL) was adjusted to be 10−11. Then, NaBH4 (5 mg) was added to the GO solution. The mixture was stirred at 90 °C for 1 day, and the solid product was collected by centrifugation or filtration. Effective reduction of GO to graphene was confirmed by Raman and UV−vis spectroscopies (Figure S1 in the Supporting Information). The AFM image (Figure S2 in the Supporting Information) indicates that the dimension of the graphene flakes is on the order of 1 μm. Noncovalent Functionalization of Graphene with Pyrene Derivatives. The homogenization process was briefly outlined below: RG (1 mg) was dropped into solution (10 mL) of pyrene derivatives. Sonication for 15 min at 60 W was followed by centrifugation at 1000g for 1 h. The supernatant suspension was collected and underwent dialysis in order to remove excess pyrene derivatives. Characterization Techniques. UV−vis absorbance measurements were conducted with a Shimadzu UV-3600 UV−vis− NIR spectrophotometer. DI water, 1% SDS solution, or DMSO was used for the background scan for GO/graphene suspension with/without pyrene derivatives in corresponding solvents. The photoluminescence spectrum measurements were conducted on a Perkin-Elmer Instrument LS 55 luminescence spectrom-

3. RESULTS AND DISCUSSION GO and graphene were prepared according to the reported methods in the literature.4,5 All the Raman spectra (Figure S1a in the Supporting Information) of graphite, GO, and graphene consist of three major peaks, so-called D band (∼1340 cm−1), G band (1590 cm−1), and 2D band (2680 cm−1).6 The occurrence of the D-band Raman feature is ascribed to the presence of disorder in sp2-hybridized carbon atoms. It is usually used to qualitatively characterize the extent of covalent functionalization of sp2-hybridized carbon frameworks. The substantial increases of the D-band feature on GO and graphene as contrast to that of graphite indicate clearly functionalization of the sp2-hybridized carbon frameworks. Since the reduction of GO to graphene will only remove oxygen-containing functional groups and the framework defect cannot be restored, there is no significant decrease of the Dband intensity on graphene as compared with that of GO. Successful reduction of GO to graphene was verified by the shift of absorption peak from 226 to 265 nm (Figure S1b in the Supporting Information) due to the partially restoration of the conjugated sp2-hybridized carbon frameworks. The pyrene derivatives associate with graphene surfaces by means of π−π stacking interactions.2b The synthetic procedure for noncovalent functionalization of graphene with these pyrene derivatives is presented in the Experimental Section. The influence of graphene on the optical properties of PAS, HPTS, and DHPDS was investigated by UV−vis and photoluminescence spectroscopies. Figure 1 shows the influence of graphene on the optical absorption of the 4176



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Figure 2. Optical behavior of HPTS and DHPDS at different pH values in the absence and presence of graphene. (a) UV−vis spectra of HPTS at different pH values in the absence and presence of graphene. (b) UV−vis spectra of DHPDS at different pH values in the absence and presence of graphene. (c) Photoluminescence spectra of HPTS at pH 2 in the absence and presence of graphene, with excitation wavelengths of 390 and 456 nm. (d) Photoluminescence spectra of HPTS at pH 12 in the absence and presence of graphene, with excitation wavelengths of 390 and 456 nm. (e) Photoluminescence spectra of DHPDS at pH 2 in the absence and presence of graphene, with excitation wavelengths of 408 and 461 nm. (f) Photoluminescence spectra of DHPDS at pH 12 in the absence and presence of graphene, with excitation wavelengths of 408 and 461 nm.

corresponding to ground and excited electronic states can be calculated from the energy difference of 0−0 electronic transition of acid and its conjugated base.7a The relationship can be expressed as pKa* = pKa − (hν1 − hν2)/2.3RT. In this equation, pKa and pKa* stand for the negative logarithm (base 10) of the equilibrium constants of electronic ground and excited states, respectively. ν1 and ν2 represent the frequencies of average absorption or fluorescence transitions of acid and its conjugated base, respectively. R is an ideal gas constant, and T stands for temperature. The above equation indicates that some of the photon energy absorbed by the acid form is used in the dissociation of the acid into its conjugated base, which increases

compounds HPTS and DHPDS. The spectra of PAS in the absence and presence of graphene are presented in Figure S3 of the Supporting Information. It can be seen that there is no remarkable impact of graphene on the absorption spectrum of PAS. On the contrary, significantly different absorption spectra were observed in the presence of graphene for HPTS and DHPDS. The most striking differences are the emergence of a new peak (for HPTS) and a substantial increase of the peak intensity (for DHPDS) around 456 nm. Generally, the conjugated acid/base species of photoacids in their equilibrated ground state have characteristic absorption bands centered at different wavelengths.7 The difference of the pKa values 4177



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Figure 3. Photoluminescence spectra of HPTS and DHPDS in the absence and presence of graphene. (a) Photoluminescence spectra and excitation spectra of HPTS in the absence and presence of graphene. (b) Photoluminescence spectra of DHPDS in the absence and presence of graphene. Note: the “ex” in the figure represents excitation spectra and “g” represents the spectra in the presence of graphene.

will shift the equilibrium toward favorable formation of the deprotonated species HPDS− and PDS2−, which is verified by the substantial increase of the absorbance features around 456 nm.7b It should be mentioned that the role of phonon energy transfer on the shift of acid/base equilibrium would be overwhelmed under strong acidic/alkaline conditions. pH influences on the shift of acid−base equilibrium of photoacids in the absence and presence of graphene were investigated by UV−vis and photoluminescence spectroscopies (Figure 2). Figure 2a,b shows the absorbance spectra of HPTS and DHPDS at different pH values. Under strong acidic and alkaline conditions, the absorbance spectra are insensitive to graphene. No obvious peak change can be observed for both photoacids. The observation indicates that the acid−base equilibrium will not be affected by graphene under strong acidic or alkaline conditions. This is qualitatively in good agreement with our proposed mechanism that the pH value and phonon energy transfer are somewhat concerted factors that will both influence the equilibrium. At strong acidic or alkaline pH value, the influence of the phonon energy transfer is overwhelmed by the pH effect. The photoluminescence spectra (Figure 2c−f) of HPTS and DHPDS under strong acidic or alkaline condition confirm this interpretation. The fluorescence quenching by graphene can be obviously observed for both HPTS and DHPDS, which is quite similar to the reported fluorescence quenching phenomena by graphene. The trivial influence of graphene on photoluminescence of the photoacids is also compatible with our proposed mechanism. The transfer of phonon energy is surmounted by the pH effect. In addition, spectroscopic investigations (Figure S4 in the Supporting Information) indicate that fine adjustments of the pH value will presumably lead to the effect of vibration energy transfer under mild acidic or alkaline conditions, which is confirmed by the optical behavior at nearly neutral pH values. Photoluminescence studies of HPTS and DHPDS were also investigated in order to further support our assumption that the interactions between graphene and photoacids affect the acidities of the photoacids. The obtained spectra are shown in Figure 3. We did not study PAS by photoluminescence spectroscopy because its absorbance spectrum has already indicated that it undergoes little interactions with graphene layers.

the leaving capability of the acid protons so that the acidity of the photoacid increases in its electronic excited states, compared to that in its ground state.7a In Figure 1a, the absorption band centered at 456 nm in the spectrum of HPTS/graphene hybrid is assigned to the conjugated base PTS−, which is not observed in the spectrum of HPTS in acid media in the absence of graphene.7 Upon the introduction of graphene, (a) the content ratio of the conjugated base increased substantially, judged from the obvious enhancement of its absorption at 456 nm, and (b) the pH values of HPTS solution decreased from 6.1 to 5.4. In a control experiment, the pH value of graphene solution is about 6.6 under the same conditions. To our surprise, in the presence of graphene, the acidity of HPTS solution increases without the photon absorption. This is mainly due to the absorption of phonon energy by the photoacid in its ground state. This assumption is consistent with the fact that the energy difference between the acidic and alkaline species exactly matches the phonon energy of the 2D vibration band (2680 cm−1). The phonon-activated molecule may be the true excited species, which shows stronger acidity, as observed for the photoacids in the electronic excited states. The increased content ratio of the conjugated base well verifies that the existence of graphene will affect the equilibrium, since the decrease of the pH value will suppress the shift of the equilibrium toward the formation of PTS− given no other variables. Fluorescence resonance energy transfer (FRET) between HPTS and graphene is unlikely to be responsible for the observations, since FRET, if it exists, will not effectively affect absorption spectra. Moreover, the trivial overlap integral between the fluorescence spectrum of HPTS and the absorbance spectrum of graphene indicates that FRET is ineffective. Charge transfer between HPTS and graphene is also not the main factor responsible for these observations, since it normally results in an obvious absorption peak shift due to the modification of the electronic band structures. Based on the reported pKa value for HPTS that is 7.3,7b the pKa* of the 2D phonon activated HPTS species can be calculated to be 1.9 (7.3 − 0.32 eV/2.3RT). This estimated pKa* value indicates that high extent of the equilibrium shift of the phonon activated species toward the formation of conjugated base at pH of 5.4 is highly likely, which is in good agreement with our observation.4a A similar discussion can be applied to DHPDS, and the introduction of graphene 4178



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Figure 4. Temperature effect of the phonon coupling interaction in the HPTS/graphene hybrid: (a) first round scan; (b) second round scan.

determines the phonon occupation number.1b UV−vis spectroscopy of HPTS was employed in the investigation. The reasons why we used HPTS instead of DHPDS for this study rely on the facts that the phonon coupling between HPTS and graphene is relatively simple and HPTS has only one phenol group, resulting in less conjugated basic species in the acid/base equilibriums than that for DHPDS. Figure 4 shows the UV−vis absorbance spectra of HPTS at different temperature in the presence of graphene. In the absence of graphene, impact of temperature on the absorbance spectra of HPTS is not obvious (Figure S5 in the Supporting Information). This observation is inconsistent with our anticipation that the UV−vis absorbance spectra of HPTS should exhibit strong temperature dependence. The temperature dependence is by far not monotonic. The absorption feature around 456 nm decreases at the beginning. Further increase of the temperature until 50 °C resulted in a reverted tendency, i.e., absorption increase around 456 nm with increasing temperature. The initial decrease of the absorption feature presumably resulted from the cleavage of the hydrogenbonding interactions7a,d between solvent molecules and the protons on the photoacid, which inevitably increases the activation energy for the deprotonation. The role of the hydrogen-bonding interaction in the deprotonation process was confirmed in the successive round scan of the temperaturedependent behavior. During the second round scans (Figure 4b), the UV−vis absorbance features of the photoacid around 456 nm exhibit almost purely monotonic characteristics. The absorption band centered at around 456 nm decreases at higher temperature, a similar observation as we observed in the first round scans except that it shows higher rate than that in the first round scans. This observation could be attributed to the fact that the role of phonon coupling was maximized at 80 °C for both scan rounds. With the contribution from phonon coupling maximized, the effect of the hydrogen-bonding interaction will be magnified. This is why the decreasing tendency of the absorption feature around 456 nm in the second round scans was much more obvious than that in the first round scans. Moreover, with the scan temperature kept lower than the highest temperature (80 °C) ever achieved in the first round scans, the contribution from the phonon coupling cannot increase any more compared with that in the first round scans. This observation can explain the monotonic characteristic of

We observed (Figure 3) that the fluorescence emissions of both HPTS and DHPDS are strongly quenched by graphene when the excitation is performed at around 400 nm, an observation that is in good agreement with the published results.8 Nevertheless, when the excitation is performed at 456 nm, the fluorescence intensities of both HPTS and DHPDS enhance strongly in the presence of graphene, which is contradictory to the well-known fluorescence quenching ability of graphene. This counterintuitive fluorescent behavior of HPTS and DHPDS can be well explained by the shift of acid− base equilibrium in the presence of graphene. The emission bands centered at around 510 nm are attributed to the conjugated basic species, HPDS−, and PDS2−.7a,b The increase of the emission intensity indicates that the ratio of these disassociated species increases in the presence of graphene. This is in consistent with the results observed with UV−vis spectra. The fluorescence intensity increase of HPTS around 510 nm with an excitation band centered at 456 nm was also verified by the excitation spectra of HPTS (black line in Figure 3a). The influence of graphene on the acid−base equilibrium results from the phonon energy transfer from graphene to the photoacids, the elaboration of which will be discussed in the following section. Phonon transfer from graphene to ground-state photoacids was observed for both HPTS and DHPDS (Figure 1). Transfer of phonon from excited photoacids to graphene was also observed for DHPDS. In Figure 3b, the higher energy portion (∼450 nm) of the fluorescent spectra was more dramatically suppressed than lower energy portion (∼500 nm), indicating that phonon was transferred from excited species to graphene so that the equilibrium was shifted toward the formation of lower energy excited species. Moreover, phonon transfer with reversed direction, i.e., from ground state DHPDS species to graphene and from graphene to excited DHPDS species, is also possible at a controlled pH value (Figure S4 in the Supporting Information). On the basis of the UV−vis absorbance and photoluminescence results discussed above, the following mechanism can be proposed: in the presence of graphene, the transfer of the 2D band (2680 cm−1) phonon energy between graphene and photoacid species occurs, which switches the equilibrium of conjugated acid/base. In order to confirm the assumption, we investigated the temperature influence on the phonon coupling interaction since temperature is a critical parameter, which 4179



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Figure 5. Proposed mechanism of the phonon coupling interaction in the HPTS/graphene hybrid. Attachment of HPTS on graphene provides the platform for the phonon energy transfer between graphene and HPTS. Phonon-excited HPTS shows enhanced deprotonation ability and is thus stronger acid than its ground state.

can also be observed with replacement of graphene by GO, which is consistent with our proposed mechanism since both graphene and GO exhibit strong 2D vibration band (Figure S1a). Moreover, a controlled experiment in which the SWNT− photoacid hybrids prepared through noncovalent functionalization showed quite marginal phonon coupling interaction between SWNTs and photoacids (Figure S7 in the Supporting Information). The fact that SWNTs that had also been known to have the 2D band (2680 cm−1)9 did not show the phonon energy transfer indicates that the two-dimensional framework of graphene plays a critical role for the facile attachment of photoacids, which is a prerequisite for the phonon coupling.

the decreasing tendency of the peak intensity around 456 nm, since the hydrogen-bonding interactions became weakened at higher increasing temperature. On the other hand, the abrupt increase of the absorption peak intensity from the first round scan at 80 °C to the second round scan at 20 °C resulted from the restoration of hydrogen-bonding interactions7a,d between solvent molecules and the proton of the phenol group on HPTS. Nevertheless, the more obvious absorption features around 456 nm for the second round scan at 20 °C as contrast to that in the first round at the same temperature may result from the molecular orientation alignment of HPTS on graphene. Thus, the phonon coupling will probably optimize the molecular orientation to facilitate further phonon energy transfer, which is similar to the case of resonance energy transfer (RET) where attractive intermolecular pairing force will also be generated.3a It can be concluded that the increase of the temperature will increase the phonon occupation density, which will facilitate phonon energy transfer. The phonon energy transfer will in turn improve the alignment of HPTS with respect to the graphene framework, further increasing the possibility of phonon energy transfer. The quantum mechanics essence of the phonon coupling interaction indicates that convertible species most susceptible to this interaction are those with the energy difference exactly matches integer unit of phonon energy.1b The emergence of the sharp peak in the UV− vis spectra for DHPDS in the presence of graphene results from this kind of “selective” interaction. With the assistance of the sonication for only 3 min, the initial absorption feature of the HPTS−graphene hybrids before temperature-dependent investigation was recovered, and the absorbance spectrum is included in Figure 4. This observation further confirms that the unique absorbance characteristics in the presence of graphene result from the cooperative contribution from the phonon coupling and hydrogen bond formation because the sonication will partially interrupt the phonon coupling by spoiling the optimized molecular orientation. The proposed mechanism based on the obtained results and the related analysis is schemed in Figure 5. In the presence of graphene, phonon transfer from graphene to HPTS will activate the molecule. The activated molecule exhibits stronger deprotonation ability and thus is stronger acid than its ground state. Further investigation indicates that the phonon mediated acid/base equilibrium is insensitive to certain solvents and can be clearly observed in other solvents such as DMSO (Figure S6 in the Supporting Information). Phonon-mediated equilibrium

4. CONCLUSION Phonon coupling interaction between graphene and photoacids has been well demonstrated, and temperature effects have verified the phonon energy transfer mechanism. The influence of the hydrogen-bonding interactions on the acid/base equilibrium shift has also been identified. The current work provides a facile technique to alter the photoacid properties and paves an alternative way for the fundamental investigation of the interaction between aromatic molecules and graphene. Since a phonon is a quantum mechanical description of a specific type of vibrational motion, the enhancement of the phonon energy transfer will increase the vibrational energy of the phonon acceptor, generating the localized heat at the phonon acceptor. Photothermal damage to cells is currently one of the most promising approaches in the treatment of cancer and infectious diseases.10 When the hybrids are located inside or around the target cells, the transferred phonon is anticipated to increase the temperature of the phonon acceptor, leading to the apoptosis. As a result, such phonon coupling interaction may find its potential application in photothermal therapy.

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ASSOCIATED CONTENT

S Supporting Information *

Additional optical spectra and AFM image. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +65-6316-8792; e-mail: [email protected]. 4180



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Present Address ∥

Visiting Professor, Chemistry Department, Faculty of Science, King Abdul-Aziz University Jeddah, Saudi Arabia, 21589. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Singapore National Research Foundation Fellowship (NRF2009NRF-RF001-015) and Nanyang Technological University.

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REFERENCES

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