Electron Transport Properties through Graphene OxideâCobalt

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Electron Transport Properties through Graphene Oxide—Cobalt Phthalocyanine Complexes Gloria Ines Cardenas-Jiron, Paola Leon, Diego Cortes, and Jorge M Seminario J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp405951p • Publication Date (Web): 15 Oct 2013 Downloaded from http://pubs.acs.org on October 30, 2013

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The Journal of Physical Chemistry Cárdenas-Jirón, León-Plata, Cortes-Arriagada and Seminario

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Graphene oxide and cobalt phthalocyanine

Electron Transport Properties through Graphene Oxide—Cobalt Phthalocyanine Complexes Gloria I. Cárdenas-Jirón,1* Paola León-Plata,2 Diego Cortes-Arriagada,1 and Jorge M. Seminario2,3,4* 1

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 2

3

Department of Chemical Engineering, Department of Electrical and Computer Engineering 4 Materials Science and Engineering Program Texas A&M University College Station, Texas, USA

ABSTRACT

We present a theoretical study using density functional theory at the M05-2X/631G(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.

* authors whom correspondence should be addressed E-mail: [email protected]; [email protected] Keywords: cobalt phthalocyanine, graphene oxide, current-voltage, molecular orbitals

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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 that allow extensive nanotechnology applications.3 Between the most remarkable G properties are the high electronic mobility at room temperature3 (larger than 15,000 cm2V-1s-1), the resistance to pressure which is 200 times stronger than steel,4 and the high thermal conductivity of 5.3 x 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 and organic molecule sensors,9 development of field-effect superconductors,10 and the manufacturing of devices for the storage of hydrogen in fuel cells.11 Trough 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 mono-vacancies, and multi-vacancies,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-electrons15. However, due to 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 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 NMR19-20, AFM21-22 and STM23 suggest that the basal plane of GO possess mainly hydroxyl and epoxide groups, while along to 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,28 29 suggesting that these sites cause the retention of functional groups in the G obtained by GO reduction28 so-called RGO (reduced graphene oxide). The GO and G emerge as promising materials for the development of gaseous sensors such as those for CO2, NO2, an H2,30-33 molecular (HCN, DDT),34 and biological (DNA, H2O2),35-36. Also, optoelectronics has been benefitted by the optical and conductive properties of G and GO,

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being used for the design of transparent electrodes,37 LEDs,38-39 and solar cells28, 30; in this case the π-π interactions are an advantage between the organics systems and G40-42. Otherwise, the metallophthalocyanines (MPc`s) 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 MPc`s act as mediators in electrochemical processes of charge transfer being useful for the detection of a diversity of organic molecules

46-53

. MPc`s

have also shown an efficient conversion light-to-electron, useful to be used as dyes in photovoltaic cells

54-58

. Because to the optic and electrochemical properties of MPc's, 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 synthesized by covalent and non-covalent functionalization, being used as complex central metal: zinc59-61, copper62-63, iron64 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 photosynthesis59, 61-62. It has also been observed an increase in the solubility in aqueous solution when regard G64, an electrocatalytic activity improved for potential application in electrochemical sensors64 and an extinction coefficient of non-linear optics (NLO) larger to G and MPc65. 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 oxygencontaining 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 approximation8, 66(LDA) 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 arrangements type supermolecule. For example, a recent study using the PBE functional corrected for dispersion forces shows that the long-range interactions contribute until a 50% to the binding energy of an arrangement formed by NO or O2 adsorbed on

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doped G.70 Due to 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

non-covalent 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 GVac-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 starting point the functional groups are added at the center of the graphene model, and then the functionalized graphene is re-optimized.

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 one, with the exception of 1, 2 and 3 that are doublet. The fragments with spin multiplicity one are graphene and epoxide functionalized graphene. Those fragments with spin

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multiplicity doublet are cobalt phthalocyanine, and functionalized graphene with hydroxyl and carboxyl. The complexes have a total of ~1435 basis functions. 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 ~7µA 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 M05-2X,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 Gaussian 09.75 We denominate this level of theory as M05-2X/6-31G(d)/LANL2DZ. Although the functional M05-2X is parameterized only for nonmetals, at difference of M05 that is parameterized 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

G

G-Def

G-Vac

CoPc

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

1: G-O-CoPc

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2: G-Def-O-CoPc


3: G-Vac-O-CoPc

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4: G-OH-CoPc

7: G-COOH-CoPc

5: G-Def-OH-CoPc

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6: G-Vac-OH-CoPc

8: G-Def-COOH-CoPc

Figure 2. M05-2X/6-31G(d)/LANL2DZ optimized molecular complexes 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 include 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 interface to the electrodes; thus a voltage difference can be applied to the complexes. As shown in Figure 3, the model 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 Gaussian09 program.

The density of states (DOS) of the bulk contacts (gold) is calculated with the program Crystal06,78-79 where DFT with 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 Gaussian 09 program at each bias voltage are inputed into the Green’s functions

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approach to caclulate 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

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).

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 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 experiment a conduction of electrical current. The characteristics of the complexes as semiconductor materials will be determined by these properties.

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Molecular Structure. The optimized molecular structures for all the complexes (1-8) are showed 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, 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 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 non-bonding between C1 and C2 atoms, allowing that the O atom is closer to the G plane, forming an angle C1OC2 of ∼117º. It means that in 3 would not exist an epoxide ring 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 is 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 slightly increase of rCo-O in 2 by ∼0.03 Ǻ could be explained for 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. On the other hand, the value of rCo-O in 3 (∼2.63 Ǻ) indicates that a non covalent bond exist 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 ∼3.35Å, which constitutes a reasonable distance for π-π interactions.

Table 1. Relevant optimized geometrical parameters for the complexes 1-9 and their fragments. Atom labels are shown in Scheme 1.

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System G-O G-Def-O G-Vac-O

rC1-O (rC2-O) 1.432 (1.432) 1.427 (1.430) 1.380 (1.380)

rCo-O -

Electron Transport Properties through Graphene OxideâCobalt

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