A Comparative Density Functional Study of Hydrogen Peroxide

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A Comparative Density Functional Study of Hydrogen Peroxide Adsorption and Activation on the Graphene Surface Doped With N, B, S, Pd, Pt Au, Ag and Cu Atoms Derya Düzenli J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06131 • Publication Date (Web): 17 Aug 2016 Downloaded from http://pubs.acs.org on August 18, 2016

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The Journal of Physical Chemistry

A Comparative Density Functional Study of Hydrogen Peroxide Adsorption and Activation on the Graphene Surface Doped with N, B, S, Pd, Pt Au, Ag and Cu Atoms Derya Düzenli* Chemical Engineering Department, Middle East Technical University, 06800 Ankara, Turkey Mineral Analysis and Technology, General Directorate of Mineral Research and Exploration, 06800 Ankara, Turkey [email protected] ABSTRACT The adsorption of the hydrogen peroxide (H2O2) molecule which is known as the common name of reactive oxygen species in living cell is theoretically investigated over pure graphene and heteroatoms (nitrogen, boron, sulphur) and metal atoms (silver, gold, copper, palladium and platinum) doped graphene surface using Density Functional Theory method (DFT). This study consists of the optimization of the pure and doped graphene surfaces, adsorption of the gas molecule on the top of the doped atoms and neighboring carbon atoms and analyzes the behavior of the gas molecule over the various adsorption sites. First principle calculation results reveal that the copper-doped and silver-doped graphene surfaces are the most thermodynamically favorite surfaces for the formation of water molecule directly. Moreover, sulphur-doped surface shows a superior performance among the heteroatoms doped surface. Additionally, the gap between the orbital energies of the system has an effect on the surface behavior against to the H2O2 molecule. INTRODUCTION Graphene is a monolayer of sp2 hybridized carbon atoms arranged in a two-dimensional honeycomb lattice network.

1,2

Known as the thinnest and mechanically strongest material,

ACS Paragon Plus Environment


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graphene has some excellent properties such as high surface area, high electrical and thermal conductivity and low Johnson noise.3-6 Graphene has attracted many researchers to investigate its potential application in electrodes, hydrogen storage materials, solid-state gas sensor, biosensors, and catalysis due to the electronic, chemical, mechanical and optical properties.7,8 Pure graphene shows zero-band gap semiconductor properties because of the overlapping the valance and conduction bands at the Brillouin zone.9,10 This property of graphene limits its applications in various areas. The improvement of adsorption and catalytic properties of graphene by enhancing its chemical reactivity and electronic structure has been a main topic for several studies. Therefore, new versatile methods such as doping with different atoms, preparing graphene with defect11 and chemical functionalization.12-14 have been widely used together or separately to adjust its band-gap for various applications especially as a gassensor. Detection and adsorption of common and polluting gases in a short contact time (even in 1 ppb) are important environmental issues.4,6,15,16 Doping with heteroatoms (B, S or N) is an excellent method to open the band-gap and to provide efficient way to electron transfer. Graphene turns to p- and n-type semiconductor and becomes suitable for electrochemical biosensing, supercapacitor and fuel cell application.10,17-19 Furthermore, graphene (pure and modified) has been investigated experimentally and theoretically to adsorb several gas molecules such as NO5,6,20, NO24,6, NH34,6, CO4,6,20,21, hydrogen peroxide (H2O2)13, CO26 and formaldehyde (CH2O)22,23 due to being an excellent candidate as a gas sensor. Beside adsorption mechanism, dissociation and oxidation of molecules and catalytic reaction mechanism have been studied over metal modified graphene surface.20,24-27 One of the reaction mechanisms performed over graphene surface is hydrogen peroxide (H2O2) detection and reduction reaction. Hydrogen peroxide is an important molecule in nature and extensively used such as food manufacturing, chemical synthesis, fuel cell, pharmaceutical and clinical analysis. Detection of H2O2 molecule resulted from the important


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reactions in the living cells is crucial for diagnosing pathological and physiology conditions of the cells.28,29. According to the literature studies, carbon based biosensor could be used for H2O2 detection.13,30-32 Pure graphene and nitrogen-doped graphene surface have been used for reduction of H2O2 and reported that catalytic activity of doped graphene surface is much higher than that of pure graphene surface according to the results obtained from experimental and theoretical studies.30,33 In this study, adsorption and activation of H2O2 molecule are investigated by DFT method over pure and heteroatoms (N, B, S) and metal atoms (Au, Pt, Cu, Pd, Ag) modified graphene surfaces. The effect of doped atom over electronic and mechanical properties of the graphene surfaces and adsorption/reduction properties are examined by the first principle approach. COMPUTATIONAL DETAILS Examined graphene surface has been constructed with 27 benzene rings including 73 carbon and 21 hydrogen atoms. Carbon atoms are positioned around the central carbon atom as three lines. Hydrogen atoms are placed as a fourth line to terminate the dangling bonds of the terminal carbon atoms of the structure. Terminal H atoms are kept fixed to simulate continuity of the surface. All carbon and doped atoms of the surface, the adsorbent and product molecules are kept relaxed. The structural parameters of the model surface where carbon atoms having dangling bonds at the edge are saturated with H atoms is in good agreement with the similar graphene model reported in many theoretical works.6,7,30 Obtained structure is given in Figure 1a. In order to prepare doped surfaces, central carbon atom is replaced with a single N, B, S, Pt, Pd, Au, Ag and Cu atoms. Density functional theory (DFT) calculations are conducted by using Gaussian 0934 software. B3LYP method, which is known as a high-quality density functional method certainly for this type of organic chemistry reactions is used in the calculations.35-37 Los Alamos LANL2DZ effective core pseudo-potentials are used for metal atoms and 6-31G(d,p) basis set is used for



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the remaining atoms. The convergence criteria involving gradients are reported as 0.000450 hartree/bohr for maximum force, 0.000300 hartree/radian for root-mean-square (rms) force, 0.001800 bohr for maximum displacement, and 0.001200 radian for rms displacement in Gaussian software. Single point energy (SPE) calculations are conducted as the starting point of the study to find correct spin multiplicity (SM) values for the pure and doped-graphene surfaces and hydrogen peroxide molecule. SPE calculations are run for different SM values and the SM value that gives the lowest SPE is taken as the correct spin multiplicity for the system. This SM value is used for the following calculations on the corresponding systems. After determining SM values, H2O2 molecule and (pure and modified) graphene surfaces are fully optimized by equilibrium geometry (EG) calculations. The highest occupied molecular orbital energy (HOMO) and lowest unoccupied molecular orbital (LUMO) energy values are obtained by full population analysis. Vibrational frequency calculations are also added in the EG calculations to check the stability of the obtained geometries and to obtain vibrational frequencies of the system. The recommended scaling factor is 0.961338 for B3LYP method to reproduced experimental fundamentals. Finally, interaction of H2O2 molecule with pure or doped graphene surfaces has been studied by using the same EG calculation procedure. The examined various adsorption sites (a, b, c etc.) for H2O2 molecule located within first two layers are shown in Figure 1a&b. The geometry with the lowest energy belongs to each surfaces is determined and used to decide the most favored surface for adsorption of H2O2. The interaction energy of H2O2 over the graphene surfaces is calculated by using (energy data from zero point corrections calculations) the following equation: ∆‫ܧ‬ௌ௬௦௧௘௠ = ‫ܧ‬ௌ௨௥௙௔௖௘ିுమ ைమ − ൫‫ܧ‬ுమ ைమ − ‫ܧ‬ௌ௨௥௙௔௖௘ ൯


[1]

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‫ܧ‬ௌ௨௥௙௔௖௘ିுమ ைమ is the computed energy of the optimized system including geometries of the H2O2 and the graphene surface, ‫ܧ‬ுమைమ is the energy of the gas phase H2O2 molecule, and ESurface is the energy of the pure or doped graphene surface. The energy values obtained by equilibrium geometry calculation for the final and initial geometries are used to find the binding energy, Gibbs free energy and enthalpy values of the systems by using the same equation (Eqn.1). RESULTS AND DISCUSSION Structural and electronic effects of doping on the graphene surface: The optimized pure graphene structure with an average bond distance of 1.44 Å and various possible adsorption sites (site a to site p) investigated in the present study are shown in Figure 1a&b. Doped atoms are inserted into the optimized structure by removing of a carbon atom located at the center (shown with red circle in Figure 1a). In addition to the adsorption sites, H2O2 molecule is placed on top of a carbon and doped atoms. After determining the ideal gas molecule orientation over the pure graphene surface, the same orientation has been examined for all surfaces for each adsorption sites and the adsorption energy is calculated for all of them. Optimization calculation with full population analysis containing frequency analysis is performed for pure graphene and atom doped-graphene surfaces. Vibration analysis shows that the geometrical structures of all the surfaces are stable. Spin multiplicity (SM) numbers are found as 1 for boron-doped (BG), nitrogen-doped (NG) and silver-doped (AgG), 2 for pure graphene, sulphur-doped (SG), platinum-doped (PtG), and palladium-doped (PdG), 3 for gold-doped (AuG) and copper-doped (CuG) graphene surface with SPE calculation. SM number is found as 1 for H2O2 molecule. These determined SM numbers are used through all calculations. To understand the effect of the doped atoms on the graphene surface, geometric structures and electronic properties of the optimized surfaces are investigated. The change in the surface



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structure of graphene after doping with different atoms is shown in Figure 2a-g. The planar surface of the pure graphene is preserved as expected after an addition of the B and N atoms due to the close atomic mass of these atoms to C atom. Table 1 indicates the bond distance and bond angles between the central atom and adjacent carbon atoms (C8, C9 and C10) for the optimized pure and doped graphene surfaces. The distance between the substituted atom at the center and the neighboring carbon atoms (C8, C9, and C10 as shown in Figure 1a) of the graphene is 1.50 Å for BG and 1.43 Å for NG surface while the distance is 1.44 Å for undoped graphene surface. B-doping causes the elongation of the C-B bond distance due to the relatively higher atomic radius of B than that of carbon atom while there is no change in the angle between doped atom and adjacent carbon atoms for B and N-doped surfaces. However, S-doped graphene and especially metal (Au, Ag, Cu, Pt, Pd) doped graphene surfaces protrude out of plane and cause deformation of the hexagonal ring that substituted in as shown in Figures 2b-g. The carbon-doped atom distance is 1.76 Å for SG. The results given for NG, BG and SG surfaces are comparable with the values reported by Dai group.6 An increase in the bond distance and change in the angles are more significant for metal-doped graphene surfaces because of significantly larger radius of metal atoms. The bond-distances between the metal and neighboring C atoms are homogeneous for CuG, PtG and PdG systems. Furthermore, these bond distances are 1.91, 1.95 and 1.96 Å for CuG, PtG and PdG respectively. Ma et al. reported the distance between Pt-C atom as 1.92 Å.39 However, the bond lengths vary from 2.07 to 2.24 Å for AuG which is longer than reported value (dAuG=1.98

Å)40 and 2.19 to 2.64 Å for AgG surface. The difference in the calculated bond

distance for AuG surface can be caused by the method used in both studies. The larger atomic radius of Ag atom among the other metals causes the higher bond elongation in the structure. Additionally, the angles between the neighboring carbons and doped atom are getting closer for all surfaces.


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Doping with various atoms changes also the electronic structure of graphene surface. Mulliken charge of the doped-atoms and neighboring carbon atoms, HOMO-LUMO energy values of each surfaces and energy gap between the HOMO and LUMO are given in Table 2. The values given in Table 2 reveal that electrons are transferred from the carbon atoms of graphene to the B and N atom because of the higher electron affinity of these two atoms than carbon atom. Substantially negative charge density of N atom makes adjacent carbon atoms much higher positive than that of in the B-doped graphene surface. Both atoms lead to p-type doping of graphene.41, 42 On the other hand, the charge densities on the carbon atoms adjacent to the metal atoms and S atom have more negative values than that of the pure graphene as result of withdrawing electron from doped atom. This charge distribution over the adjacent carbon atoms is not homogenous for all surfaces. Hence, the behavior of the peroxide molecule over different adsorption sites including top of the doped atoms and neighboring carbon atoms is examined to find the configuration that gives the minimum binding energy of the gas molecule. The change in the energy band gap (∆Eg) is also calculated from the difference between HOMO and LUMO energy values to clarify the effect of substitution of the atoms in the graphene structure. The band gap of pure graphene decreases with the substitution of all doped-atoms especially in SG surface which facilitates the interaction between the H2O2 molecule and the surfaces. Furthermore, there is no difference in the HOMO representation of the systems while significant changes are observed for the LUMO images with respect to the type of the substituted atoms as depicted in Figure S1-S9. The charge concentrates on the some parts of the surface for heteroatom doped surfaces. However the localization of charge over doped atoms and neighboring carbon atoms are more significant for metal doped graphene surfaces except PtG. The charge distribution looks more homogenous over PtG surface than the other surfaces. Adsorption behavior of H2O2 on the pure and doped graphene surface: After optimizing all surfaces, adsorptive H2O2 molecule is optimized separately before placing it over different



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graphene surfaces. The calculated O-O and O-H bond lengths are 1.454 and 0.971 Å and OO-H and H-O-O-H bond angles are 99.72 and 118.53° which are in agreement with the values presented in the literature.30 Before adding the H2O2 molecule to the surface, different possible orientations of the molecule are tested to determine the most stable adsorption configuration. The molecule placed perpendicular to the central and neighboring carbon atoms via oxygen does not interact with the surface. In the other case, H2O2 is positioned parallel to the surface both via hydrogen and oxygen atom of the molecule and it binds to the surface via oxygen atoms. After determining the most appropriate H2O2 orientation, the molecule is placed at the different adsorption sites around the central atom (site a to p as shown in Figure 1b) and on top of the doped atoms (N, S, B, Pd, Pt, Au, Ag and Cu). The energies of different configurations over doped atoms and neighboring carbon atoms are calculated for all optimized surfaces (pure and heteroatoms/metal-doped graphene) and the configuration that gives the minimum energy is used to find the binding energy of H2O2 over the surfaces. The simulation is started with the pure graphene plane and H2O2 molecule is placed over the central carbon atom at a distance of 1.41 Å. The simulation results show that when molecule moves from initial point to the surface, it splits into two hydroxide (OH) species spontaneously instead of adsorption over the surface as shown in Figure 3a. The enlargement in the bond distance of O-O from 1.45 Å to 2.46 Å given in Table 3 supports the dissociation of H2O2 which is accepted as the rate determining step for the H2O2 reduction reaction.13 Two OH species bind with the central carbon atom (C1) and neighboring carbon atom (C9) with a distance of 1.46 Å and cause the protrusion of these surface atoms from the surface. The charge densities of carbon atoms increase from 0.015 to 0.054 for C1 and -0.005 to 0.051 for C9 (the values given in Table S1) while there is a slightly change for each atoms of the surface after OH binding. There is a small charge transfer from the molecule to the surface (0.04 as shown in Table S1). All these values are a sign of the formation of a C-O chemical


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bond. Dissociation of the H2O2 molecule into two OH species over the pure graphene surface is an endothermic process and the energy difference between final and initial geometries is 0.57 eV. The positive value of the binding energy, enthalpy and Gibbs free energy given in Table 3 reveals that PG surface is not active thermodynamically and needs energy to dissociate H2O2 molecule. Beside the central carbon atom and its adjacent sites (site a to p), H2O2 molecule is also placed over the carbon atoms which have more negative Mulliken charge values (-0.023 charge over C29 and -0.048 charge over C37) located between the first and second line as depicted in Figure 1a. The adsorption behavior over these sites is less favorable than other sites according to the binding energy value. However, all the doped atoms enhance adsorption ability of graphene to the H2O2 molecule and enable the exothermic dissociation and reduction reaction route compared with the pure graphene surface according to the adsorption energy values as shown in Table 3 and Figure 4a&b. Hydrogen peroxide is activated through different pathways and prefers one of the adsorption sites over different surfaces. It dissociates into two OH species over NG, BG, PtG and PdG like PG surface while it reduces directly into the water molecule and remains an adsorbed oxygen atom over SG, AgG, AuG and CuG surfaces. Therefore, the results can be demonstrated in two groups. Dissociation of the H2O2 molecule into two OH species over surfaces can be examined as a first group. The second group includes the surface that forms water molecule directly and remains an adsorbed oxygen atom over it. Figures 3a-i show the side views of the activation of H2O2 over the various surfaces. Among the first group, the lowest adsorption energy belongs to the NG surface. The most suitable adsorption site that gives the maximum binding energy (-0.37 eV) is determined as site m for NG surface (Figure 1b). Hydrogen peroxide molecule placed on top of the N atom goes away from the surface and prefers two carbon atoms located at site m (Figure 3b). Adsorbent has made a bond via oxygen with carbon atoms at a distance of 1.45 Å (dC5-O and



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dC9-O) as shown in Table 3. The carbon atoms adjacent to the N atom (C8, C9 and C10) have a higher positive charge (~0.233) compared with the other surface carbon atoms and the second highest charge (0.043) belongs to the carbon atom connected with C9 (C5 and C4). Relatively lower binding energy is obtained for other adsorption sites. Moreover, the H2O2 molecule placed over site f and l binds to the B atom and one of the C atoms of the surface located at this sites (C9 and C10, Figure 1a) over BG surface with a bond length of ~1.50 Å (dC9-O, dC10-O and dB-O) as shown in Figure 3c. An adsorption energy of -0.98 eV is obtained at site l (dC9-O and dB-O) and there is a slightly increase to -1.01 eV energy at site f (dC10-O and dBO).

Most probably, the small difference in Mulliken charge of the atoms (C9=0.013 and

C10=0.016) causes appearing of this difference in the binding energy. Meanwhile other adsorption sites performance including on top of the B atom is so pure when compared with the site f and l. The distance between two oxygen atoms of H2O2 molecule elongates to ~2.5 Å from 1.45 Å over NG and BG surfaces. A significant elongation in O-O bond and the distance between the C-O atoms support the chemical bond formation between OH species and surface atoms. Adsorption enthalpies and energies of the surfaces increase from N to B doped surfaces and all values are negative as depicted in Table 3. However, Gibbs free energy values of these surfaces indicate that only adsorption and dissociation of H2O2 molecule over the BG surface occurs spontaneously when molecule contacts with the doped surfaces. A distinct decrease in the negative charge value of B atom (Table 2) enhances the adsorption behavior of this surface. On the other hand, relatively large negative adsorption energy of OH species is obtained for PdG and PtG surfaces. H2O2 molecule is first placed over Pd atom because of having a high positive charge (0.298 eV) over it. Both oxygen atoms of the H2O2 molecule connect with a metal atom having -1.28 eV of bonding energy. The molecule placed at site f and l connects to the surface with -2.18 eV of energy. On the other hand, the binding energy increases to -2.29 eV for site d. One of the OH species connects with the Pd atom with a distance of 2.06 Å (dPd-


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O)

and the other OH species binds to graphene at C13 (-0.016 of Mulliken charge) with a

distance of 1.42 Å (dC13-O) . OH species binds to the C13 atom more strongly than Pd atom so it rises out of the surface as shown in Figure 3e. Therefore, the configuration over site d is taken account for the determination of the other properties of the surface and adsorbed species. The other sites that examined during the study also give the lower adsorption energy than site d. An adsorption energy of H2O2 over PtG surface increases to 3.12 eV for the gas molecule positioned at site l. Both OH species are adsorbed over Pt atom (Figure 3f) even though the H2O2 molecule is placed at site e, f, h and l and give the close energy value. This most probably takes place because of the high positive charge of Pt atom (0.728 eV). However, when molecule is directly positioned over Pd atom, O-O bond of molecule again breaks but OH groups are being held relatively with less binding energy (-1.85 eV). The bond distance between OH species and Pt (dC-O=2.09-1.99 Å) is larger than that of over PdG surface (dC/M-O=2.06-1.42 Å). Both an adsorption enthalpy and Gibbs free energy value are negative for PdG and PtG surfaces. In our study, dissociation of the H2O2 molecule takes place spontaneously on the BG, PdG and PtG surfaces. However, there is an energy need for PG and NG surfaces according to the Gibbs free energy of the system. Luque et al.13 reported that splitting of H2O2 molecule on the perfect layer of graphene is not thermodynamically favorable. The energy need for this reaction is 0.52 eV13 while it is 0.57 eV in the present study. On the other hand, the reaction takes place with a releasing energy of -0.26 eV on the defective surface and functionalization of this defect site with carboxyl groups improves the reaction. Wu et al.30 investigated the H2O2 reduction reaction mechanism over pristine and N-doped graphene surfaces via DFT method. Calculation results showed that the breakage of the O-O bond can be possible when H+ is added into the system during adsorption otherwise H2O2 molecule is physically adsorbed over the surface. One H2O molecule forms and then leaves from the surface and one chemically bound OH group remains on the surface with an addition of H+ into the system



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during the first step. According to the calculation results in the study, the direct reduction of H2O2 molecule into two OH species is not thermodynamically possible.30 As stated before, H2O2 molecule exhibits different behavior over the SG, AuG, AgG and CuG surfaces. When it is placed on top of S atom, one H2O molecule directly forms and then desorbs from the surface. The distance between the surface and water molecule is 3.37 Å and the remaining oxygen atom makes a bond with S atom at a distance of 1.49 Å as depicted in Figure 3d. Gibbs free energy and enthalpy values show that the reaction can take place as shown in Table 3. Generally, two OH molecules form over the different adsorption sites over SG surface and the binding energy less than -2.94 eV. Also, H2O molecule forms when molecule is placed at the other adsorption site like site e, but the energy of system is again so low. The adsorption of water molecule on graphene occurs physically because of hydrophobic nature of graphene.43 So it can be said that once H2O forms, it will leave from the surface directly. To understand the interaction of the formed H2O molecule with the surface, frequency values of corresponding bond are checked. The bonds at 3656, 3755 and 1594 cm-1 are assigned to the symmetric and asymmetric OH stretching vibration and the bending mode for H2O at gas phase.44 In our calculation, the corresponding frequency values are 3654, 3763 and 1601 cm-1 for H2O molecule at vacuum. All calculated frequency values for SG, AuG, AgG and CuG surfaces are given in Table 4. The symmetric and asymmetric OH stretching frequency at 3601 cm-1 and 3729 cm-1 and they shift 52 cm-1 and 34 cm-1 from the gas phase molecule as shown in Table 4 while a small shift is observed for H-O-H bending frequency as 21 cm-1 (from 1601 cm-1 to 1622 cm-1). Although these peaks are somewhat shifted, the close values of frequencies imply that formed H2O molecule in the system is similar to that of H2O at gas phase.


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On the other hand, partially large negative energy value of the system is obtained for metal doped surfaces. The highest adsorption energy and enthalpy are calculated for CuG surface as 4.01 eV and 3.94 eV. Moreover, AgG surface has very close energy values. The large negative value of ∆G (3.47 eV) also belongs to CuG surface as depicted in Table 3. The distance of the formed H2O molecule is 2.12 Å from the Cu atom and 3.42 Å from the CuG surface. The maximum energy is obtained around the metal atom not over it for three surfaces, at site f for CuG and AuG surface and site a for AgG as shown in Figure 3g-i. The H2O-metal distance is 2.30 Å for AuG and 2.41 Å for AgG surface. The band of H2O molecule over CuG shows similar frequencies with SG surface. However, the symmetric OH stretching band is around 3479 cm-1 and 3380 cm-1 which is shifts 175 cm-1 and 274 cm-1 from the gas molecules for AuG and AgG surfaces (Table 4). The H atoms of H2O point to the surface over AgG (Figure 3h), so an interaction between H atoms and surface atoms must be stronger than that of the other surfaces and this might explain this large frequency shift. On the other hand, H-O-H bending frequency is very close the H2O molecule at gas phase for AgG and AuG surfaces. This peak is also assignment of C=O stretching which is observed for CuG, AgG and AuG surfaces. Taking the relationship between the energy gap between HOMO-LUMO (∆Eg) and the adsorption energy values in Tables 2&3, it can be said that the adsorption energy inversely related with the energy gap for heteroatom doped surfaces (NG, BG and SG) and small band gap of SG surface causes a reduction of H2O2 instead of dissociation into OH species. While it directly related with energy gap for metal doped surfaces (PdG, PtG, AuG and AgG) expect for CuG surface. An increase in the band gap energy leads to again formation of H2O directly. Additionally, the charge of the atoms interacting with a gas molecule increase due to the electron transfer from surface atoms to H2O2 for all surfaces as seen in Table S1. CONCLUSIONS



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Adsorption and activation of the hydrogen peroxide molecule over the pure and doped graphene surfaces are studied by DFT method to examine the surface sensitivity to the gas molecule. The distance between the adsorbed molecule and surface atoms (carbon and doped atoms) and the binding energy of the molecule to the surfaces reveal that the chemical bond occurs between nitrogen, boron, palladium and platinum doped-graphene surfaces and hydroxide molecule. Reduction of the H2O2 molecule into the water and adsorbed oxygen occurs spontaneously over sulphur, gold, silver and copper doped surfaces. Among various doped surfaces, the most sensitive surface is determined as the Cu-doped graphene surface according to the thermodynamic properties such as binding energy, enthalpy and Gibbs free energy. An increase in the gap between HOMO-LUMO energies affects positively metaldoped surfaces adsorption energies and the formation of H2O while it conversely affects heteroatoms-doped surfaces. Supporting Information Supporting Information Available: Table of Mulliken charge of the atoms after and before adsorption and HOMO and LUMO representations of optimized geometries. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENTS The author thanks Prof. Dr. Isik Onal for help with the Gaussian 09 software package. The numerical calculations reported in this paper were fully performed at TUBITAK ULAKBIM, High

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REFERENCES [1] Pumera, M.; Ambrosi, A.; Bonanni, A.; Chng, E. L. K.; Poh, H. L. Graphene For Electrochemical Sensing and Biosensing. Trend. Anal. Chem. 2010, 29, 954-965.


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TABLE CAPTIONS Table 1. Bond distances and angles of the optimized pure and doped–graphene surfaces.

Table 2. Mulliken charge of doped atoms and neighboring carbon atoms, HOMO energies (eV), LUMO energies (eV) and energy gap between HOMO-LUMO.

Table 3. Bond distances (Å) between oxygen and surface atoms and oxygen-oxygen distance after activation of the H2O2 molecule, enthalpy (∆H, eV), Gibbs free energy (∆G, eV) and adsorption energy (∆EAds, eV).

Table 4. Vibrational frequencies of water molecule formed over the doped-graphene surfaces.

FIGURE CAPTIONS Figure 1. (a) Optimized structure of the pure graphene and (b) various adsorption sites over the surface.



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Page 20 of 25

Figure 2. The side view of atom doped-graphene surfaces a) NG and BG b) SG c) PdG d) PtG e) AuG f) AgG and g) CuG surface. Figure 3. Side view of adsorbed H2O2 over a) pure b) NG c) BG d) SG e) PdG f) PtG g) AuG h) AgG and i) CuG surfaces. Figure 4. a) Binding energy of two OH species over un-doped and N, B, Pd and Pt-doped surfaces and b) Energies of the SG, AuG, AgG and Cu-doped surfaces. TABLES Table 1. Bond distances and angles of the optimized pure and doped–graphene surfaces. Bond Distance (Å) Cluster C/M*-C8

Bond Angle (°)

C/M-C9

C/M-C10

C8-C/M-C10

C8-C/M-C9

C9-C/M-C10

PG

1.44

1.44

1.44

120.12

119.94

119.94

NG

1.43

1.43

1.43

120.11

119.95

119.94

BG

1.50

1.50

1.50

120.15

119.92

119.92

SG

1.76

1.76

1.76

104.21

104.42

104. 24

PdG

1.96

1.96

1.96

89.56

89.51

89.37

PtG

1.95

1.94

1.95

91.90

91.79

91.80

AuG

2.23

2.24

2.07

78.66

76.68

77.88

AgG

2.64

2.53

2.19

72.51

70.68

51.74

CuG

1.91

1.91

1.92

89.53

96.06

89.73

*M=N, B, S, Pd, Pt, Au, Ag, Cu

Table 2. Mulliken charge of doped atoms and neighboring carbon atoms, HOMO energies (eV), LUMO energies (eV) and energy gap between HOMO-LUMO. Mulliken Charge Cluster

C/M*

C8

C9

C10

HOMO

LUMO

∆Eg

PG

0.015

0.001

-0.005

0.001

-5.088

-3.156

1.932

NG

-0.928

0.238

0.233

0.238

-3.864

-2.694

1.170

BG

-0.208

0.016

0.013

0.016

-4.544

-3.401

1.143

SG

0.686

-0.465

-0.465

-0.468

-3.973

-3.592

0.381


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PdG

0.298

-0.301

-0.298

-0.303

-4.408

-3.184

1.224

PtG

0.728

-0.448

-0.442

-0.448

-4.435

-3.184

1.252

AuG

0.053

-0.220

-0.238

-0.267

-4.599

-3.184

1.415

AgG

0.208

-0.266

-0.119

-0.164

-4.898

-3.238

1.660

CuG

0.298

-0.297

-0.298

-0.323

-4.463

-3.102

1.361


Table 3. Bond distances (Å) between oxygen and surface atoms, and oxygen-oxygen distance after activation of the H2O2 molecule, enthalpy (∆H, eV), Gibbs free energy (∆G, eV) and adsorption energy (∆EAds, eV). Cluster

C/M*-O1

C/M-O2

O1-O2

∆G

∆H

∆EAds

(Å)

(Å)

(Å)

(eV)

(eV)

(eV)

PG

1.46

1.46

2.46

1.28

0.68

0.57

NG

1.46

1.44

2.46

0.31

-0.27

-0.37

BG

1.46

1.53

2.53

-0.39

-0.99

-1.01

SG

1.49

3.37

2.93

-2.48

-2.90

-2.94

PdG

2.06

1.42

2.57

-1.66

-2.26

-2.29

PtG

2.09

1.99

2.63

-2.52

-3.08

-3.12

AuG

2.30

1.24

2.75

-2.88

-3.42

-3.49

AgG

1.27

2.41

2.67

-3.46

-3.90

-3.94

CuG

2.12

1.26

2.77

-3.47

-3.94

-4.01


Table 4. Vibrational frequencies of water molecule formed over the doped-graphene surfaces. Bond

Type of frequency

Vacuum

SG

AuG

AgG

CuG

O-H

Symmetric stretching

3654

3602

3479

3380

3604

O-H

Bending

1601

1622

1587

1599

1574

H-O-H

Asymmetric stretching

3763

3729

3699

3708

3728



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FIGURES (a)

(b)

Figure 1. (a) Optimized structure of the pure graphene and (b) various adsorption sites over the surface.

(a)

(b)

(c) (d)

(e)

(f)

(g) Figure 2. The side view of atom doped-graphene surfaces a) NG and BG b) SG c) PdG d) PtG e) AuG f) AgG and g) CuG surface.


Page 22 of 25

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a

b

c d

e

f

g

h

i

Figure 3. Side view of adsorbed H2O2 over a) pure b) NG c) BG d) SG e) PdG f) PtG g) AuG h) AgG and i) CuG surfaces.



1 PG

0.5 0

a

NG

Adsorption energy, eV

-0.5 BG

-1

-1.5 -2

PdG

-2.5 PtG

-3

-3.5

Reaction step

0

b -0.5 -1

Adsorption energy, eV

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Page 24 of 25

-1.5 -2 -2.5

SG -3

AuG -3.5

AgG -4

CuG -4.5

Reaction step

Figure 4. a) Binding energy of two OH species over un-doped and N, B, Pd and Pt-doped surfaces and b) Energies of the SG, AuG, AgG and Cu-doped surfaces.


Page 25 of 25

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ABSTRACT GRAPHICS

Graphene

OH

B-Graphene

S-Graphene


A Comparative Density Functional Study of Hydrogen Peroxide

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