Electron Transport Properties through Graphene OxideâCobalt

Article pubs.acs.org/JPCC

Electron Transport Properties through Graphene Oxide−Cobalt Phthalocyanine Complexes Gloria I. Cárdenas-Jirón,*,† Paola León-Plata,‡ Diego Cortes-Arriagada,† and Jorge M. Seminario*,‡,§,∥ †

Laboratorio de Química Teórica Departamento de Ciencias del Ambiente, Facultad de Química y Biología, Universidad de Santiago de Chile (USACH), Casilla 40, Correo 33, Santiago, Chile ‡ Department of Chemical Engineering, §Department of Electrical and Computer Engineering, and ∥Materials Science and Engineering Program, Texas A&M University, College Station, Texas 77843, United States ABSTRACT: We present a theoretical study using density functional theory at the M05-2X/6-31G(d)/LANL2DZ level of theory of the structural, stability, reactivity, and electrical properties of cobalt phthalocyanine (CoPc) adsorbed on functionalized graphene (G). The functionalization, localized at the center of graphene, is based on epoxide (−O), hydroxyl (−OH), and carboxyl (−COOH) complexes. Three types of graphene molecules are used: pristine, defect (Def), and vacancy (Vac). Binding energies show large stabilities for G−O−CoPc, from ∼−43 to −65 kcal/mol, and for G−OH−CoPc, from ∼−19 to −32 kcal/mol. No adsorption of CoPc occurs on G−COOH. The HOMO−LUMO gap is shorter by ∼0.5 eV for the complexes containing epoxide and hydroxyl (except for G−Vac) than for the functionalized G, thus implying a higher reactivity of the former. All these results together with the nature of the frontier molecular orbitals, which make functionalized G electron acceptor and CoPc electron donor species, explain the charge transfer properties of the complexes. Complexes containing epoxide functionalization present a better conduction of ∼18 μA (at ∼1 V) than those complexes containing hydroxyl functionalization (∼7 μA). These results show that the adsorption of cobalt phthalocyanine on functionalized graphene is feasible; yielding a tunable hybrid material that allows sensing because of the intrinsic electrical properties provided by functionalized G and CoPc.

1. INTRODUCTION The first decade of the 21st century has been tagged with the graphene (G) discovery,1,2 a material that attracts the attention of the scientific community because its unique properties that allow extensive nanotechnology applications.3 Between the most remarkable G properties are the high electronic mobility at room temperature3 (larger than 15, 000 cm2 V−1 s−1), the resistance to pressure, which is 200 times stronger than steel,4 and the high thermal conductivity of 5.3 × 103 W/mK surpassing the one of carbon nanotubes.5 The immediate applications of G are its use in the development of composite materials,6 electric batteries,7 design of gas sensors in solid state,8 organic molecule sensors,9 development of field-effect superconductors,10 and the manufacturing of devices for the storage of hydrogen in fuel cells.11 Through high-resolution transmission electronic microscopy (HR-TEM),12 it could be observed on the surface of G the existence of topological defects such as pentagon−heptagon pairs or Stone−Wales defects,13 monovacancies, and multivacancies,14 which at the same time can modulate electronic and chemical properties of G for changes of hybridization in the carbon atoms and modification of the density of π-electrons.15 However, because of the high cost of G produced by exfoliation16 and its low solubility, the researchers have used as an interesting alternative, the graphene oxide (GO), a structure © 2013 American Chemical Society

of G with several functional groups containing oxygen and hydrogen by treatment of graphite with strong oxidizers.17 Later, the exfoliation of the graphite oxide allowed getting plates of GO.18 The analysis by experimental techniques such as nuclear magnetic resonance (NMR),19,20 atomic force microscopy (AFM),21,22 and scanning tunneling microscopy (STM)23 suggest that the basal plane of GO possesses mainly hydroxyl and epoxide groups, while along the edges exist carbonyl and carboxyl groups. These functional groups modify the electronic structure of G24−26 and of its hydrophilic character,27 turning into a potential tool for development of tunable electronic materials.24,25 Besides, it has been shown theoretically that the adsorption of functional groups on sites of G with defects and vacancies is stronger than on pristine sites,2829 suggesting that these sites cause the retention of functional groups in the G obtained by GO reduction28 socalled RGO (reduced graphene oxide). The GO and G emerge as promising materials for the development of gaseous sensors such as those for CO2, NO2, H2,30−33 molecules (HCN, DDT),34 and biology (DNA, H2O2).35,36 Also, optoelectronics has been benefitted by the Received: June 17, 2013 Revised: October 13, 2013 Published: October 15, 2013 23664

dx.doi.org/10.1021/jp405951p | J. Phys. Chem. C 2013, 117, 23664−23675

The Journal of Physical Chemistry C

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Figure 1. Graphene models and cobalt(II) phthalocyanine (CoPc).

Figure 2. M05-2X/6-31G(d)/LANL2DZ optimized molecular complexes.

sized by covalent and noncovalent functionalization, being used as complex central metal: zinc,59−61 copper,62,63 iron,64 and cobalt.45 In the composite materials, MPc-G highlights the deactivation processes that appear in the fluorescence spectra that have been associated to charge transfer processes from the MPc toward G or GO. It is the reason for these hybrid systems be considered with potential application in optoelectronics, design of transparent electrodes for devices of solar energy conversion, and artificial photosynthesis.59,61,62 It has also been observed an increase in the solubility in aqueous solution when in regards to G,64 an electrocatalytic activity improved for potential application in electrochemical sensors64 and an extinction coefficient of nonlinear optics (NLO) larger to G and MPc.65 On the basis of the above-mentioned, in this work we investigated by density functional theory (DFT) the adsorption of cobalt(II) phthalocyanine (CoPc) on functionalized G with

optical and conductive properties of G and GO, being used for the design of transparent electrodes,37 LEDs,38,39 and solar cells;28,30 in this case, the π−π interactions are an advantage between the organics systems and G.40−42 Otherwise, the metallophthalocyanines (MPcs) are complexes of macrocyclic nature with an extended system of π density, which are adsorbed on graphite and carbon43−46 for the development of electrodes in where the MPcs act as mediators in electrochemical processes of charge transfer being useful for the detection of a diversity of organic molecules.46−53 MPcs have also shown an efficient conversion of light-toelectron, useful to be used as dyes in photovoltaic cells.54−58 Because of the optic and electrochemical properties of MPcs, the investigation and synthesis of novel hybrid materials with donor−acceptor properties as from G has centered the attention on the MPc. It is just as hybrid materials G (or GO) with metallophthalocyanines already have been synthe23665



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It is worth to mention that in a previous work,45 we showed that a localization of CoPc in a parallel form on the functionalized graphene produces a current flux of ∼7 μA between these molecular species. This is the reason why we adopt the same parallel orientation in the present work. All the calculations for the optimization and the analysis of the electronic structure are carried out using density functional theory (DFT) with the hybrid meta-GGA functional M052X,71−74 the 6-31G(d) basis set for C, H, N, and O, and the basis set and quasi-relativistic pseudopotential LANL2DZ for the cobalt atom as coded in the package Gaussian09.75 We denominate this level of theory as M05-2X/6-31G(d)/ LANL2DZ. Although the functional M05-2X is parametrized only for nonmetals, at difference of M05 that is parametrized for both metals and nonmetals, the former has the advantage that is a high-nonlocal functional with double amount of nonlocal exchange 2X (56%) that is required for the kind of systems studied in this work. Other works published in the literature also combine the main characteristics of M05-2X, the dispersion correction, with the presence of a transition metal, yielding good descriptions of the bonding interactions.76,77 In order to evaluate the electric properties of the complexes, we model an electric circuit placing two nanozised electrodes terminated in one gold atom, which is included in the complexes. These contacts are similar for all complexes to be tested; thus, their currents and other electrical properties can be compared. The ending gold atom of the tips are located at each side of the conjugated structures at distance of ∼3 Å from graphene and ∼3 Å from CoPc; these gold atoms act as an interface to the electrodes; thus, a voltage difference can be applied to the complexes. As shown in Figure 3, the model

oxygen-containing functional groups. We used different models of G, and the effects of the functionalization, defects in G surface, and the CoPc adsorption in the electronic properties of G are evaluated. It is worth to mention that the DFT has been widely used to study the adsorption of gas molecules and others ones on G, by using the local density approximation (LDA)8,66 and the generalized gradient approximation (GGA),41,67,68 being used for many authors the PBE functional.69 However, these methods do not include a correct description of the van der Waals interactions that is important in arrangement-type supermolecules. For example, a recent study using the PBE functional corrected for dispersion forces shows that the longrange interactions contribute until a 50% to the binding energy of an arrangement formed by NO or O2 adsorbed on doped G.70 Because of the possible existence of π−π interactions between CoPc and functionalized G, we have used in this study the exchange correlation functional M05-2X.71−74 It is defined for noncovalent interactions especially weak interaction, hydrogen bonding, and π−π stacking.

2. COMPUTATIONAL DETAILS We have used three graphene models: pristine graphene (G) with 19 benzene rings, graphene containing one Stone−Wales defect (G−Def), and graphene with one vacancy (G−Vac) (all are shown in Figure 1). The detail of the graphene model with defect and vacancy is the same as that published previously.45 Note that we used a graphene with terminal hydrogen with the aim to avoid the non planarity at the edges. However, the main interaction that we study corresponding to the functionalization and the cobalt atom are located at the center of the graphene and phthalocyanine, respectively; we guess that they would be not affected by the terminal hydrogens at the edges of the graphene. The functionalization of one oxygen-containing functional group is done on each graphene model that includes the epoxide (−O−), hydroxyl (−OH), and carboxyl (−COOH) groups. Later, a cobalt(II) phthalocyanine (CoPc) was adsorbed on the functionalized graphene generating the complex. The optimized molecular complexes are shown in Figure 2. Note that the complex G−Vac−COOH−CoPc is not included because convergence problems obtained for it by the vacancy region; thus, we only studied eight complexes. To obtain the equilibrium geometries for complexes 1−8, first we optimize the isolated graphene model; then using this geometry as the starting point, the functional groups are added at the center of the graphene model, and then the functionalized graphene is reoptimized. Finally, the adsorption of CoPc on functionalized graphene is carried out optimizing the whole complex. After the full optimization, a calculation of the vibrational frequencies is performed for each complex (1− 8), verifying that all frequencies have positive values and thus to be sure that all the optimized complexes correspond to minimum energy structures. This optimization procedure was adopted because the large molecular size and complexity of the molecules for which is needed to achieve the convergence. All the complexes show a total charge Q = 0 and a spin multiplicity of one, with the exception of 1, 2, and 3 that are doublet. The fragments with spin multiplicity of one are graphene and epoxide-functionalized graphene. Those fragments with spin multiplicity doublet are cobalt phthalocyanine and functionalized graphene with hydroxyl and carboxyl. The complexes have a total of ∼1435 basis functions.

Figure 3. Molecular model of the complex 1 attached to Au terminals containing graphene pristine and functionalization epoxide. Color code: carbon (gray), hydrogen (white), nitrogen (blue), oxygen (red), cobalt (steel blue), and gold (brown).

corresponding to CoPc is adsorbed on graphene pristine. Calculations to the augmented complexes, including the effect of an external electric dipole field that yields a bias voltage (V) between the contacts, are carried out at the same level of theory used for the geometry optimization, i.e., M05-2X/6-31G(d)/ LANL2DZ as encoded in the Gaussian09 program. The density of states (DOS) of the bulk contacts (gold) is calculated with the program Crystal06,78,79 where DFT with 23666



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Table 1. Relevant Optimized Geometrical Parameters for the Complexes 1−9 and Their Fragments; Atom Labels Are Shown in Scheme 1 system

rC1−O (rC2−O)

G−O G−Def−O G−Vac−O 1: G−O−CoPc 2: G−Def−O−CoPc 3: G−Vac−O−CoPc

1.432 (1.432) 1.427 (1.430) 1.380 (1.380) 1.429 (1.429) 1.427 (1.430) 1.391 (1.391) rC1−O

G−OH G−Def−OH G−Vac−OH 4: G−OH−CoPc 5: G−Def−OH−CoPc 6: G−Vac−OH−CoPc G−COOH G−Def−COOH 7: G−COOH−CoPc 8: G−Def−COOH−CoPc

rCo−O

2.173 2.203 2.633 rCo−O

∠C1OC2 63.677 60.394 117.123 63.604 60.246 119.804

∠C3N1N2C4

172.668 172.676 178.421 ∠C3N1N2C4

1.459 1.438 1.334 2.600 3.289 1.288 rC1−C(COOH)

1.765 1.768 1.992 rCo−O

rN1−H(COOH)

169.709 174.237 176.043 ∠C3N1N2C4

1.573 1.547 1.673 1.548

2.171 2.004

1.897 2.250

−173.709 −172.880

Scheme 1. Molecular Structure of (a) Graphene Molecule; (b) CoPc (Red Bonds Correspond to the Dihedral Angle C3−N1− N2−C4); R Is the Functionalization (−O−, −OH, and COOH)

between functionalized graphene and the cobalt phthalocyanine generate a charge transfer between them, as well as how the electronic reactivity is described by means of DFT reactivity descriptors. Finally, using a model for representing an electrode through a gold atom, we will investigate if the complexes experience a conduction of electrical current. The characteristics of the complexes as semiconductor materials will be determined by these properties. 3.1. Molecular Structure. The optimized molecular structures for all the complexes (1−8) are shown in Figure 2, and some relevant geometric parameters are presented in Table 1 and illustrated in Scheme 1. In the epoxide-functionalized G (G−O) and complexes 1−3, we found that the oxygen atom is covalently bonded to two carbon atoms in the G surface, forming an epoxide ring (Table 1 and Scheme 1). In the case of pristine G and G−Def, the optimized distance rC1−O (rC2−O) is ∼1.43 Ǻ , which is comparable to the experimental distance of ∼1.45 Ǻ found in epoxide groups on chrysenes88 and fullerenes,89 estimated by

periodic boundary conditions is used. The continuum of the bulk contacts DOS and the Hamiltonian and overlap matrices of the augmented complexes calculated with the Gaussian09 program at each bias voltage are inputed into the Green’s functions approach to calculate the electron transport using the program GENIP, which calculates current through single molecules.80−84 Besides the complexes current−voltage characteristics, GENIP also calculates the DOS and transfer functions of the junctions (complexes plus electrodes). More details about the procedure is included in previous works.85−87

3. RESULTS AND DISCUSSION We will start by analyzing if the cobalt phthalocyanine is able to be adsorbed onto functionalized graphene. With this aim, we will revise the molecular structure of these complexes from the relevant geometric parameters of the optimized molecular structures. Then from the energetic point of view, we will show if the adsorption leads to a stable complex. For the stable structures, we will be interested to know if the interaction 23667



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calculations.29,93,94 The COOH functionalization on a Stone− Wales defect leads to a decreasing of 0.2 Ǻ in rC1−C, which is a similar result to that published by Al-Aqtash and Vasiliev using a GGA functional.29 In 7, it is observed that CoPc causes an increase of the C1−C distance of ∼0.1 Ǻ , indicating that the functional group tends to get-out of the G surface; while no change in the distance C1−C is produced for 8. In complexes 7 and 8, the CoPc is located with the cobalt atom toward the oxygen of the carbonyl moiety in the functional group; the rCo−O parameter is ∼2.2 and ∼2.0 Ǻ for 7 and 8, respectively. In addition, the average distance between CoPc and G is larger than 7 Ǻ on the COOH group, while it is smaller (3.3 Ǻ ) in the G edges. Finally, the distance between the hydrogen of the COOH and the nitrogen atom of CoPc is ∼1.9 and ∼2.3 Ǻ for 7 and 8, respectively, suggesting the existence of a hydrogen bonding in these complexes. 3.2. Binding Energy. In order to study the interaction of the cobalt phthalocyanine (CoPc) on functionalized graphene (G), the binding energy (Ebind) is analyzed between them and calculated using Ebind = Ecomplex − (ECoPc + Efunctionalized‑G). We also use this equation in order to analyze the binding of the functional groups in the G lattices (pristine, defect, and vacancy). The results for Ebind calculated by the M05-2X/631G(d)/LAN2DZ level of theory are shown in Table 2.

means of X-ray diffraction. In addition, our results are in agreement with those ones obtained by Tang and Zhang90 who determined a distance of 1.42 Ǻ for the C−O bond by means of DFT calculations using the LSDA functional. For the complexes that contain a vacancy such as G−Vac−O and complex 3, the C−O distance is ∼1.39 Ǻ , which is shorter than in the other complexes. This can be explained by the nonbonding between C1 and C2 atoms, allowing that the O atom is closer to the G plane, forming an angle C1OC2 of ∼117°. It means that an epoxide ring would not exist in 3 because the bond C1−C2 does not exist. The rCo−O parameter in the complexes 1−3 accounts for the adsorption degree of CoPc on the functionalized graphene; in the case of 1 and 2, they are 2.17 and 2.20 Ǻ , respectively, which are slightly larger than the experimental values of perovskite compounds91 and dimeric complexes of cobalt92 (in the range of 1.86−2.14 Ǻ ). This suggests that a covalent interaction occurs for Co−O in 1 and 2. A slight increase of rCo−O in 2 by ∼0.03 Ǻ could be explained by a decrease in the electronic delocalization of graphene due to the defect. In addition, the slight deviation out of planarity of CoPc in 1 and 2, given by the dihedral C3N1N2C4 of ∼172.7° (deformation of the phthalocyanine core), suggests that an interaction between the π electronic density of Pc and G could occur. However, the value of rCo−O in 3 (∼2.63 Ǻ ) indicates that a noncovalent bond exists between Co and O and that the interaction between both atoms is weak. It is important to highlight that the average distance between CoPc and the G−O model is in the range of ∼3.35 Å, which constitutes a reasonable distance for π−π interactions. For the hydroxyl-functionalized G, a similar trend to epoxidefunctionalized G is found for rC1−O showing a decrease in the direction G > G−Def > G−Vac. The values obtained give account of a covalent functionalization (rC1−O) where the value of ∼1.46 Å for G−OH is in agreement with theoretical results.24,28,65,90 In the case of G−Def−OH and G−Vac−OH, the results for rC1−O (∼1.44 and 1.33 Å, respectively) are explained for the presence of the defect or vacancy site that provokes a decrease of the π-electron density that causes defects and vacancies in the G surface, and/or the hybridization changes of the carbon atom linked to OH group. CoPc adsorption on G−OH and G−Def−OH (4 and 5) causes the removal of the OH group from G; in these cases, the −OH group is linked to cobalt with a distance rCo−O ≈ 1.77 Å. In 4 and 5, the formed fragments G (G−Def) and HO− CoPc are separated by a distance of ∼4.4 Å. It is interesting to note that CoPc in the complex 6 does not produce the removal of the −OH group from the G−Vac surface. CoPc has a deviation in its planarity (176°) that is smaller to that found in 4 and 5. It is difficult to estimate an average distance between the G−Vac−OH and CoPc since this varies abruptly between 3.4 and 5.0 Å, being smaller in the G edges and larger in the functionalization site. This indicates that CoPc does not show affinity to interact on an OHfunctionalized site in G−Vac; this fact is also confirmed with the binding energy calculation. The Co−O distance in 6 is 2.8 Å, which is larger than the experimental value for the Co−O covalent bond (∼1.8−1.9 Å).91,92 In relation to the carboxylated G and G−Def, we found that the COOH group is covalently functionalized to the G surface with distances rC1−C of 1.57 and 1.55 Å, respectively; these distances are comparable with the distance for a simple bond C−C (1.54 Å), and those obtained as from DFT

Table 2. Binding (Ebind), HOMO, LUMO, and HOMO− LUMO Gap (ΔHL) Energies, for All the Complexes and the Isolated Fragments Calculated at the M05-2X/6-31G(d)/ LANL2DZ Level of Theory system G G−Def G−Vac CoPc G−O G−Def−O G−Vac−O G−OH G−Def−OH G−Vac−OH G−COOH G−Def−COOH G−Vac−COOH 1: G−O−CoPc 2: G−Def−O−CoPc 3: G−Vac−O−CoPc 4: G−OH−CoPc 5: G−Def−OH−CoPc 6: G−Vac−OH−CoPc 7: G−COOH−CoPc 8: G−Def−COOH−CoPc

Ebind (kcal/mol)

EHOMO (eV)

ELUMO (eV)

ΔHL (eV)

−36.91 −61.73 −112.67 −10.64 −28.76 −89.45 −4.91 −26.73 −78.46 −43.26 −43.57 −65.52 −32.05 −19.10 15.94 24.02 27.04

−6.00 −5.80 −5.89 −5.76 −5.97 −5.89 −5.32 −5.47 −5.76 −5.38 −5.26 −5.80 −5.71 −5.29 −5.34 −5.42 −5.56 −5.56 −4.99 −4.98 −5.16

−1.55 −2.01 −1.68 −1.69 −1.78 −1.81 −2.40 −1.50 −1.65 −1.44 −1.47 −1.64 −1.80 −1.83 −1.84 −2.22 −2.05 −2.09 −3.38 −2.94 −3.10

4.46 3.80 4.21 4.08 4.19 4.08 2.92 3.98 4.11 3.95 3.79 4.15 3.91 3.46 3.50 3.19 3.53 3.47 1.61 2.04 2.06

On one hand, we found that all the functionalized graphenes having epoxide and hydroxyl as the functionalization group show negative values for the binding energy, indicating that they are stable structures, in agreement with experimental results such as NMR,19,20 AFM,21,22 and STM,23 where it is suggested that epoxide and hydroxyl groups are localized in the basal plane of G. Besides, our DFT calculations for G−OH are consistent with those reported using DFT pseudopotential plane-wave methods,28 in the sense that the adsorption of 23668



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Figure 4. Structure of the π-bonds in (a) G−O, (b) G−Def−O, and (c) G−Vac−O obtained from the NBO Analysis.

Figure 5. Energy profile showing the energy changes associated with the formation of molecular complexes 4−6 and the removal of the −OH group from G due to the CoPc binding.

than in pristine G and G−Def, and the stability of 3 is favored by the opening of the epoxide ring decreasing the structural tension. (b) Natural Bond Orbital (NBO) analysis shows that in G−Vac the carbon atoms keep its sp2 character allowing it to not lose its aromaticity for oxygen binding; functionalized graphenes G−O and G−Def−O lose aromaticity at the functionalization due to the hybridization change in C1 and C2 from sp2 to sp3. Figure 4 shows the π-bonding in G−O, G− Def−O, and G−Vac−O built from the NBO analysis. (c) Another relevant point is that practically a charge transfer does not exist from G−Vac toward other regions of 3 keeping the electronic delocalization and stability (see charge transfer analysis). On the other hand, complexes 4 and 5 show stable bonding Co−O(OH) with Ebind of roughly −32 and −19 kcal/mol, respectively. In these cases, CoPc produces the removal of the −OH groups from G, but would it be possible for total breakup of the fragments after removal? To answer this question, an energy profile (Figure 5) that contains the total energy of the

hydroxyl groups on sites of G with defects and vacancies is stronger than on pristine sites,28 suggesting that these sites cause the retention of the functional group in the G. All of this confirm that the method and functional used are suitable for this kind of work. For complexes 1−3, negative values for Ebind were found indicating that the complexes gain stability in comparison with the isolated fragments and suggesting that these molecular structures would be stable in experimental conditions. The stability is favored by the formation of the Co−O bond and the presence of π−π interactions. Compounds 1 and 2 have similar Ebind values of ∼−43 kcal/mol suggesting that the change in the topology of graphene (pristine and defect) does not affect the binding of CoPc. This fact is probably due to the strong interaction that is produced between Co and O (epoxide) and not directly with the graphene. Moreover, 3 is ∼23 kcal/mol more stable (∼−66 kcal/mol) than 1 and 2, which could be explained on the following basis: (a) the epoxide group formation in G−Vac is more favorable (Ebind ≈ −113 kcal/mol) 23669



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isolated fragments (left), the energy of complexes 4−6 (center), and the fragments of complexes 4−6 in isolated form was built. Figure 5 shows that complexes 4 and 5 gain stability with respect to the isolated fragments and present a lower total energy (negative Ebind). After the removal caused by CoPc, the following step is the separation of the G and HO− CoPc fragments; this step contributes to increase the stability in 10 and 5 kcal/mol for 4 and 5, respectively. From the experimental point of view, this indicates that the CoPc adsorption would be useful for the selective removal of OH groups from the surface of graphene oxide. From the theoretical point of view, the arrangements 4 and 5 correspond to local minima in the path of the removal process. Some examples recently published have shown a pathway to reduce the graphene oxide and to remove the oxygen based groups. A microbial reduction of graphene oxide mediated by Shewanella95 and by Escherichia coli96 have demonstrated that compared to conventional methods, the microbial reduction approach is simple, scalable, cost-effective, and environmentally friendly. The mechanism suggested for Shewanella cells consists in an extracellular electron transfer for graphene oxide reduction mediated by an outer membrane of c-type cytochromes (iron porphyrin derivatives). These examples suggest that the reduction of graphene−OH as obtained for 4 and 5 are possible processes to occur and therefore kinetically viable. The complex 6 is unstable, and the −OH group is not removed as in 4 and 5 possibly because this is strongly adhered to G−Vac (Ebind ≈ −89 kcal/mol). In addition, a covalent bond between the Co and the −OH cannot be established. These two aspects can explain the instability of 6. We noticed that the complexes 7 and 8 have positive Ebind values of ∼24 and ∼27 kcal/mol, respectively, indicating that the CoPc binding on carboxylated graphene is not thermodynamically favored. The main reason is attributed to the spatial disposition of CoPc (Figure 2) that prevents a good interaction between the fragments. A different behavior is found for the graphene functionalized with carboxyl group; G− COOH, G−Def−COOH, and G−Vac−COOH, where Ebind values vary from −5 to −78 kcal/mol (Table 2) denoting an adsorption of the carboxyl group. A previous study carried out with the level of theory B3PW91/6-31G(d)/LANL2DZ for the energy but optimized at the B3LYP/6-31G(d)/LANL2DZ found that the CoPc binding on G−COO− leads to stable arrangements.45 Thus, our results suggest that CoPc adsorption on G−COOH would be possible in alkaline medium. We conclude until here that CoPc presents a binding on functionalized graphene (epoxide, hydroxyl) leading to stable arrangements, but only those containing pristine graphene (1 and 4) form more stable arrangements than the corresponding fragments (G−O and G−OH). In these cases, it means that the Co−O (Co−OH) interactions are stronger than the G−O (G− OH) ones. 3.3. Frontier Molecular Orbitals (FMO). The energy of the HOMO and LUMO molecular orbitals corresponding to 1−8 complexes, and the HOMO−LUMO gap (ΔHL) are shown in Table 2. Figure 6 shows the energies of the 30 highest occupied and the 30 lowest unoccupied MOs for the 1−8 complexes and the FMO surfaces. The pristine graphene (G) is also included as a reference. Overall, we observe a larger degeneration in the energy levels of the complexes than in graphene that could be attributed to the extended conjugation of the π electrons. The lower

Figure 6. Surfaces of the molecular orbitals (HOMO and LUMO) of complexes 1−8 calculated at the M05-2X/6-31G(d)/LANL2DZ level.

degeneration in graphene provokes a higher stabilization of the occupied molecular orbitals and destabilization of the unoccupied MOs indicating that a higher energy for the excitation from occupied to unoccupied MOs would be required. A similar behavior is observed for all the complexes in relation to the corresponding functionalized graphene, except for G−Vac−O, the complexes present a lower value for ΔHL giving account of the higher reactivity and their potential in the charge transport. In complexes 1−3, we found that ΔHL decreases with respect to their fragments by ∼1.3 eV, regarding the pristine G by ∼0.9 eV as compared with CoPc isolated by ∼0.7 eV (except for 3) considering the epoxide functionalized graphene. The 23670



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Figure 7. Current−voltage characteristics for the G−functionalization−CoPc complexes. According to the conjugate positions in Figure 2, at positive (direct) bias the current is from CoPc to G. Color curves are from the graphene models in the complexes: G (blue), G−Def (brown), and G−Vac (green); points are interconnected to help the eye.

larger decreasing in ΔHL for 3 is due to the vacancy in G, which can be seen to contribute in an important way to the LUMO molecular orbital. Molecular orbital surfaces show that for all complexes 1−3 the HOMO is located in the phthalocyanine ligand of CoPc, while the LUMO is distributed on G and the epoxide group with a minor contribution in the central ring of the CoPc. This indicates that the CoPc is the donor fragment and the G−O is the acceptor species. The electronic delocalization from CoPc to G−O suggests that 1−3 would be an interesting charge transfer sensor, as was found previously for the complex G− COOCoPc.45 The functionalization with hydroxyl groups causes a decreasing in the ΔHL value when the complex is formed (4−6), indicating that they have a larger electronic reactivity. The minor ΔHL found for 6 (1.61 eV) could be explained by its instability (Ebind ≈ 16 kcal/mol) and the larger electrophilic character. Besides, 4 and 5 possess similar ΔHL values (∼3.5 eV) suggesting that the electronic reactivity is similar for both.

In 4 and 5, HOMO and LUMO are located mainly in the Pc ligand, indicating that the phthalocyanine has a strong electrophilic character. In 5 appears a contribution of the G atoms to the HOMO, mainly in the defect site that is due to the sp3 carbons in the defect. However, in the complex 6, HOMO and LUMO are located at the ligand and G, respectively, suggesting the possibility of a charge transfer sensor. However, the instability of 6 given by a positive binding energy (15.94 kcal/mol) would prevent its possible use in electronic devices. However, we found that the functionalization of G and G− Vac with −COOH group decreases the ΔHL by ∼0.7 and ∼0.3 eV, respectively, indicating an increment in the reactivity; but in G−Def the −COOH functionalization increases ΔHL by 4 eV leading to a strong decrease in the reactivity. The interaction with CoPc forming 7 and 8 produces an important increase in the electronic reactivity (ΔHL ≈ 2 eV) with respect to the isolated fragments. However, this increase in the electronic reactivity is associated to the complex instability and the larger electrophilic character. HOMO and LUMO surfaces of 7 and 8 23671



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in both directions (CoPc → G and G → CoPc) could be attributed to the distortion of the phthalocyanine that provokes a larger approaching of the fragments in some regions. In summary, the adsorption of CoPc on functionalized graphene oxide (epoxide, hydroxyl, and carboxyl) generates structures able to yield current conduction in stable complexes. Using a hydroxyl functionalization, the conduction is improved with the adsorption of CoPc (∼7 μA) and significant with the epoxide functionalization (∼13−18 μA). Considering the stable complexes (negative binding energy), we found that defect and vacancy in graphene produce a lower current conduction than pristine. All currents were calculated at 1 V bias. The magnitude of current obtained (∼7 to 18 μA) could be useful for instance in the cases of interfaces for catalysis where currents in the order of hundreds of nanoamperes approaching to microamperes are desired.

show that cobalt atom has a non-negligible contribution in its strong donor and acceptor character. 3.4. Current−Voltage Analysis. Results obtained with the DFT-Green’s function procedure GENIP for complexes 1−8 are shown in Figure 7; a similar figure is shown to compare the effect on the graphene model (G, G−Def, and G−Vac). Current values for ∼1 V are also presented in Table 3. Table 3. Current−Voltage Values Obtained at the M05-2X/ 6-31G(d)/LANL2DZ Level of Theory complex Au−G−O−Au Au−G−OH−Au Au−G−COOH−Au Au−G−O−CoPc−Au Au−G−Def−O−CoPc−Au Au−G−Vac−O−CoPc−Au Au−G−OH−CoPc−Au Au−G−Def−OH−CoPc−Au Au−G−Vac−OH−CoPc−Au Au−G−COOH−CoPc−Au Au−G−Def−COOH−CoPc−Au

voltage (V)

current (μA)

0.95 −0.95 1.03 −1.03 1.06 −1.06 1.04 −1.04 1.08 −1.08 1.08 −1.08 1.11 −1.11 1.14 −1.14 1.08 −1.08 1.16 −1.16 1.28 −1.28

29.62 −29.32 0.12 −0.10 20.30 −26.00 18.05 −14.96 13.48 −13.16 15.13 −15.01 6.92 −6.80 5.29 −5.50 17.25 −11.25 46.84 −51.65 8.13 −7.75

4. SUMMARY AND CONCLUSIONS We investigated the adsorption of cobalt phthalocyanine on functionalized graphene and characterized the structural, energetic, reactivity, and electric properties with a theoretical study using density functional theory with the M05-2X functional and the combined 6-31G(d)/LANL2DZ basis sets. We use three graphene models (pristine, defect, and vacancy) and three functionalizations (epoxide, hydroxyl, and carboxyl) generating a total of eight complexes. Adsorptions in the range from −19 to −66 kcal/mol are achieved for the epoxide and hydroxyl functionalization; no adsorption was found for the carboxyl group. Overall, we found that HOMO−LUMO gap values confirm larger reactivity for complexes with epoxide and hydroxyl functionalizations than those without. In all cases, CoPc is the electron donor and functionalized graphene is the electron acceptor. We also found that epoxide complexes conduct more electric current (∼18 μA) than hydroxyl complexes (∼7 μA) at a bias voltage of 1 V. These two types of complexes would be potential candidates for charge sensing applications.

For all complexes, we define positive bias when the current runs from the phthalocyanine to the graphene, CoPc → G; the electric current is calculated for a few points around −1 V (negative bias) and +1 V (positive bias). The two gold contacts simulating the electrodes are ∼9.5 Å apart for the complexes and ∼6.5 Å for the graphene fragments. For the sake of comparison, we also calculated the current of the graphene pristine functionalized with epoxide, hydroxyl, and carboxyl (Figure 7a−c). The graphenes yield nearly symmetric currents for positive and negative bias. Epoxide and carboxyl favor the current transmission in the graphene with values from 30 and 20 μA at 1 V bias, respectively (Figure 7d−f). However, graphene− hydroxyl yields a small current of 0.12 μA (at 1 V) that could be explained by the presence of H in the OH group, which behaves as an insulator; this electroacceptor atom prevents the electron transport as verified by its Mulliken atomic charge of 0.43. The fragment comparison with each corresponding complex shows that in the case of G−O (∼30 μA), the adsorption of CoPc (1) decreases the current at positive bias (∼18 μA). However, in G−OH (∼0.12 μA) current increases to ∼7 μA for the complex 4, and in G−COOH (∼ 20 μA) it is increased to ∼47 μA for the complex 7. For the complexes shown to be stable by the calculation of the binding energy (1−5), we found similar current for different graphene models (pristine, defect, and vacancy), but this is slightly higher for those with graphene pristine (1 and 4). The high value found for G−COOH−CoPc

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AUTHOR INFORMATION

Corresponding Authors

*(G.O.C.-J.) E-mail: [email protected]. *(J.M.S.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.I.C.-J. thanks the financial support from CONICYT-CHILE through Project FONDECYT N° 1090700 and from DICYTUSACH through Project Apoyo Complementario, which provides computational resources. D.C.-A. thanks CONICYT for a Doctorate fellowship. J.M.S. acknowledges the highperformance computing support provided by the Texas A&M Supercomputer Facility and the Texas Advanced Computing Center as well as the financial support from the U.S. Defense Threat Reduction Agency (DTRA) through the U.S. Army Research Office (ARO), project no. W91NF-06-1-0231; and ARO/MURI project no. W911NF-11-1-0024.

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