Cobalt Phthalocyanine–Graphene Oxide Nanocomposite - American

Article pubs.acs.org/JPCC

Cobalt Phthalocyanine−Graphene Oxide Nanocomposite: Complicated Mutual Electronic Interaction Jing-He Yang,†,‡ Yongjun Gao,† Wei Zhang,† Pei Tang,† Juan Tan,‡ An-Hui Lu,‡ and Ding Ma†,* †

College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China Department of Catalysis Chemistry and Engineering, Dalian University of Technology, China 116024

‡

S Supporting Information *

ABSTRACT: Through the adsorption/intercalation of cobalt phthalocyanine (CoPc) onto/ into graphene oxide (GO) layers, CoPc−GO nanocomposite was prepared via a simple solvent evaporation method driven by the electronic interaction between CoPc and GO. The interaction between GO and CoPc has been studied in detail by various methods. The result suggests that the interaction does not follow a simple donor−acceptor mode, but, instead, it is complicated two-way process including the transfer of electron from the graphitic domain to the adsorbed/intercalated CoPc, and a feedback from the Co ions through the ligand-like attacking of oxygen functional groups of GO to the central cobalt ions. The obtained structural hybrid materials have potential in the electrochemical detection of the compounded medicine.

graphene oxide through the π-stacking system of two components. The nanocomposites have a complicated mutual electronic interaction between the two components, which was evaluated in detail by multiple-characterization methods. Graphene oxide (GO) was prepared from graphite powder by a modified Hummers method.13 For preparing CoPc−GO, designated amount of CoPc was put into the mixed solvent containing 1.5 g AlCl3 and 200 mL acetone and then the mixture was stirred for 2 h after being ultrasonically dispersed for 1 h following the adding of 1 g GO. The stirring mixture was then put into the oil bath at 333 K until the solvent was evaporated. The powder obtained was washed first with a mixture containing ethanol, water, and ammonia (volume ratio: 90% CH3CH2OH, 5% H2O, 5% NH3·H2O) and then with pure ethanol until filtrate was colorless. The sample was dried at 333 K for 24 h to get composite containing 2.6 w .t. % CoPc (2.6% CoPc−GO) and 1 wt % CoPc (1% CoPc−GO). (Supporting Information) The control samples were processed using the same method and there were no significant difference between the control samples and the primary ones (Figure S1 of the Supporting Information). TEM images reveal that the 2.6%CoPc−GO material consists of more randomly aggregated, crumpled sheets with the size smaller than that of the flat GO (parts A and B of Figure 1). The 2.6%CoPc−GO prepared was first characterized by Raman spectroscopy (part C of Figure 1) with CoPc, GO, and graphite as control. The Raman spectrum of graphite displays a strong G peak at 1580 cm−1 and no visible D band. Instead, obvious D band centered at 1340 cm−1 was observed on both GO and CoPc−GO sample, with that of the latter

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n guest−host systems, electronic donor−acceptor interaction is important issue, which influences the construction of the composite functional materials that can be used in catalysis, energy conversion, or the sensing fields.1 Significantly, graphene or graphene oxide with a combination of armchair and zigzag edges that are analogous to cis- and transpolyacetylenes, and particularly a giant 2D π-system has attracted lots of attention due to its promising potential in serving as the host for active guest materials. Indeed, besides tetrathiafulvalenes, amines, porphyrins-fullerenes, and prussian blue-carbon nanotubes via the covalent interaction and π−π interaction,2,3 it has been reported that Pt, Pd, Au, Ni, TiO2, and even porphyrin can be composited with graphene to form various of functional materials with designated applications.4−7 Of these guests, materials with π-domain were especially interesting as the π−π stacking may strongly promote the guest−graphene interaction and pointedly enhance their performance. We recently reported that organometallic compounds with a π-electron system like metallocene/metal phthalocyanine can be adsorbed/intercalated onto/into graphene/layered carbon materials.8,9 This process was driven by a strong π−π interaction between the guest molecules and hosts and in some cases by an additional electrostatic attraction between the two components.10 For guest molecules with much more complicated modules, or host with much more heterogeneous surface (such as graphene oxide), the mode for the interaction was much more complicated and in debates.11,12 However, it is desired to know the detailed information of how the host−guest interact with each other so as to shed light on the structure−property relationship. Herein, we developed a facile wet-chemical route to synthesize cobalt phthalocyanine-graphene oxide (CoPc−GO) nanostructures, in which it involves the intercalation/ adsorption of cobalt phthalocyanine into/onto the layers of © 2013 American Chemical Society

Received: November 8, 2012 Revised: January 18, 2013 Published: January 31, 2013 3785

dx.doi.org/10.1021/jp311051g | J. Phys. Chem. C 2013, 117, 3785−3788

The Journal of Physical Chemistry C

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sheets. After introduction of 2.6wt.% CoPc, the (001) peak further shifted down to 2θ = 10.3° indicating an enlarged inter planar space of 0.85 nm.15 This 0.13 nm expansion of the dspacing by CoPc molecular suggests that CoPc can be easily intercalated in between the hosting GO sheets through the π−π interaction between residual sp 2 carbons of GO and phthalocyanine. Part E of Figure 1 shows the UV−vis spectra of CoPc, GO, and CoPc−GO. The absorption maxima of pristine CoPc are at 661 and 598 nm. The Q-bands shift a little bit for 1% CoPc− GO and were further red-shifted to 667 and 604 nm respectively for 2.6%CoPc−GO. This indicates the adsorption/intercalation of CoPc onto/into the carbon sheets, by which the strong π−π interaction between CoPc and GO leading to relocation of the electrons from graphene sheets to the phthalocyanine group of CoPc,10 and thereby reducing the highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbitals (LUMO) gap of the CoPc. The donation of electron and thereby the change of the charge density at the Pc was also revealed by the N 1s XPS spectra, where a down shift of 0.5 eV was observed on 2.6%CoPc−GO as compared to the pristine CoPc (part A of Figure 2). Significantly, we also noticed a 0.7 eV shift on Co 2p3/2 signal after intercalation of 2.6 wt % CoPc into GO (from 780.9 to 780.2 eV, part B of Figure 2). The intercalation-induced shift indicates that the electron transfer from the GO to the Co ion resulting in a reduction of the ion’s effective oxidation state and therefore its XPS signal has shifted toward Co(0) position (e.g., 778.1 eV for Co metal). The result of 1%CoPc−GO follows the same trend. Those results demonstrated that after the adsorption/ intercalation of CoPc in the GO, electron donation from GO to CoPc happened with both the metal center and phthalocyanine ring gained the charge. However, when we check the C 1s XPS of GO and CoPc−GO, it was amazing to find that after the introduction of CoPc, the signals of oxygenfunctional groups of GO have shifted in reference to that of ring carbon at 284.8 eV (part C of Figure 2). In particular, the signal corresponding to the carbonyl carbon has shifted from 287.3 to 286.8 eV indicating the gain of the electron density on those groups. The electron transfer was also verified by FTIR spectra (part F of Figure 1), where the red-shift of the CO carbonyl stretching at 1733 cm−1 of GO to 1728 cm−1 at CoPc−GO was observed (the band at 1601 cm−1 is due to the water adsorption or the vibration of the skeletal of unoxidized graphitic domains).16 With the unexpected enhanced electron density of the oxygen-functional groups on the GO sheets, the electron transfer mechanism between the host, GO, and the guest, CoPc became more confused. Apparently, both the components gained the charge, but where the electron comes from? As for graphene oxide, it is actually consists of two parts, that is, graphitic domain and

Figure 1. TEM images of GO (A); and 2.6% CoPc−GO (B); the Raman spectra of 2.6%CoPc−GO, GO, CoPc and graphite (C); XRD patterns (D); UV−vis spectra (E); and FTIR spectra (F) of GO, CoPc, and CoPc−GO.

being ever broader. Meanwhile, the G band position of the two samples shifted slightly to 1587 cm−1. This can be attributed to the presence of the oxygen functional group induced defects during the preparation of graphene oxide14 as well as the adsorption/intercalation of CoPc on/between the carbon sheets. No obvious peaks corresponding to CoPc has been observed on CoPc−GO, which is due to the relatively low concentration of CoPc (2.6 wt %) and the insensitivity of Raman method. To identify that whether CoPc has diffused onto/into the graphene sheets to form the hybrid materials, XRD, UV−vis, and FTIR were employed to characterize the samples. As shown by XRD in part D of Figure 1, GO reveals an intense peak at 2θ = 12.3° corresponding to d-spacing of 0.72 nm, which is roughly double the value of graphite with the (002) reflection centered at 25.2°. The swollen basal distance is due to the formation of oxygen-functional groups such as hydroxyl, epoxy, and carboxyl on the surface of graphene

Figure 2. N 1s (A), Co 2p (B), and C 1s (C) XPS of GO and CoPc−GO. 3786

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Figure 3. CVs on GCE (black line), CoPc/GCE (blue line), GO/GCE (green line), and 2.6%CoPc−GO/GCE (red line) electrodes in presence of PBS solution (0.2 mol L −1) containing 5.0 × 10 −4 mol L−1 ACOP and 5.0 × 10 −4 mol L−1 ASA at pH = 7.2 and 50 mV s−1. Inset: CVs on 2.6% CoPc−GO/GCE in absence (dotted line) and presence (solid line) of mixture of 5.0 × 10 −4 mol L−1 ACOP and 5.0 × 10 −4 mol L−1 ASA (A); CVs on CoPc/GO electrodes in a solution containing mixture of 5.0 × 10 −4 mol L−1 ACOP and 5.0 × 10 −4 mol L−1 ASA in 0.2 mol L−1 PBS at pH 7.2 and different sweep rates. Insert: linear relationship between peak current versus the square root of the scan rate (scan rate< 30 mV/S) and the scan rate (scan rate> 30 mV/S) (B); SWV for 2.6%CoPc−GO electrodes in 0.2 mol L−1 PBS solution containing the ACOP and ASA (same concentrations from 3 to 90 μmol L−1 at pH 7.2) (C); linear calibration curve for the determination of ACOP and ASA (D).

The obtained structural hybrids were used in the electrochemical detection of compounded medicine. Steady-state cyclic voltammograms (CVs) by using 2.6%CoPc−GO/GCE electrode in absence and presence of a mixture of ACOP (0.5 mM) and ASA (0.5 mM) were shown in the insert in part A of Figure 3. The result indicated that CoPc−GO/GCE had a very good response to ACOP and ASA. CVs responses on GCE, CoPc/GCE, GO/GCE, and 2.6%CoPc−GO/GCE electrodes in presence 0.5 mM ACOP and ASA mixture (part A of Figure 3). With the bare GC electrode, only ACOP showed a quasireversible redox behavior with relative weak redox peaks but ASA almost did not respond. Although there are peaks corresponding to ACOP and ASA at CoPc/GCE and GO/ GCE, the peaks were weak. Instead, 2.6%CoPc−GO/GCE presented well-defined and quasi-reversible redox peaks corresponding to the electrochemical reaction of ACOP and ASA, which implied its good electrocatalytic activity. CVs of ACOP and ASA over 2.6%CoPc−GO/GCE were depicted in part B of Figure 3 with scan rate ranging from 5 to 100 mV/s. The relation between redox peak currents and the scan rates implied a hybrid kinetic mechanism. At lower scan rates (from 5 to 50 mV/s), the electrochemical redox behavior of ACOP and ASA over the 2.6%CoPc−GO/GCE surface was a diffusion-controlled process, whereas at higher scan rates (>50 mV/s) it was a surface absorption-controlled process with the current peak being proportional to the scan rate. It meant that the reaction occurred not only at reactive sites within the adsorbed assembly but also at the outer surface of the electrode.17 The phenomena suggest that CoPc insertion does not change the nature of GO, and its own, they constitute a stable composite and 2.6%CoPc−GO is a good and sensitive sensor for the compounded medicine such as ACOP/ASA complex. Square wave voltammogram (SWV) of ACOP and

oxygen-functional groups. The graphitic domain is mainly composed of sp2 carbon, which is apt to interact with the Pc ring of introduced CoPc through the π-stacking mechanism. However, the oxygen-functional groups especially the carbonyl species tend to act as a ligand that coordinated on the central metal ions of CoPc, and this accounts for the enhanced electron density of those groups after the introduction of CoPc. On the basis of the results above and those clues, we proposed that the charge transfer between GO and CoPc cannot be explained as a simple donator−acceptor mode; actually, it is much more complicated. Through the π−π interaction between the graphitic domain of GO and phthalocyanine, the relocation of the electron from the GO to Pc happened. The relocation of electron from the graphitic domain of GO to Pc was further verified by the blue shift of the adsorption of graphitic domain after the adsorption/intercalation of CoPc (part E of Figure 1, the native GO has an adsorption at around 240 cm−1). Meanwhile, the oxygen-functional groups on GO have ligandlike functions through which the electron has been attracted from the central metal ions of CoPc. Those explained well the observed shift of Q-band and C 1s XPS peaks on UV− vis and XPS experiments, respectively. In addition, the small reduction of the effective oxidation state of Co after insertion into the GO is due to the compensation effect between the electron donation from the Pc ring/graphitic domain, and attraction of electron from the oxygen-functional groups in the direction perpendicular to the Pc ring plane. Taking all, the interaction between GO and CoPc is a two-way process including a donation of electron from the graphitic domain to the adsorbed/intercalated CoPc, and a feed-back from the Co ions through the ligandlike attacking of oxygen-functional groups of GO to the central cobalt ions. 3787

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ASA mixtures was carried out on 2.6%CoPc−GO/GCE (part C of Figure 3). The SWV amperometric currents showed a prefect linear relationship with the concentration of ACOP in the range of 3 to 50 μM, 50 to 90 μM and ASA in the range of 3 to 90 μM with a detection limit of 1 μM at a signal-to-noise ratio of 3 (part D of Figure 3), and meanwhile a wide detection window single standard curve indicates a very good stability and sensitivity of the electrode especially to ASA. In conclusion, we synthesized CoPc−GO nanocomposites through a simple solvent evaporation method. The electronic interaction between the adsorbed/intercalated CoPc and the GO host was studied in detail by UV−vis, XPS, IR, and XRD methods. It was concluded that a complicated mutual interaction mode was responsible for the observed electron transfer between the guest and host. The nanocomposite showed a good response for the detection of compounded medicine such as ACOP/ASA in the electrochemical oxidation reactions.

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ASSOCIATED CONTENT

S Supporting Information *

Experiment details including XRD, UV−vis, IR, and XPS. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], fax: +86 10 62758603, tel: +86 10 62758603. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work received financial support from the Natural Science Foundation of China (21173009, 21222306) and 973 Project (2011CB201402, 2013CB933100).

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REFERENCES

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