Boron and Nitrogen co-Doped Graphene used as Counter Electrode

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Boron and Nitrogen Co-doped Graphene Used As Counter Electrode for Iodine Reduction in Dye-Sensitized Solar Cells Kuan-Yu Lin,† Minh Tho Nguyen,*,†,‡,§ Keiko Waki,∥ and Jyh-Chiang Jiang*,† †

Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan Institute for Computational Science and Technology (ICST), Ho Chi Minh City, 700000 Vietnam § Department of Chemistry, KU Leuven, B-3001 Leuven, Belgium ∥ Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama-shi 226-8502, Japan Downloaded via KAOHSIUNG MEDICAL UNIV on November 15, 2018 at 17:52:07 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: A counter electrode (CE), which acts as an efficient catalyst, plays a pivotal role in dye-sensitized solar cells (DSSCs). Graphene doped by heteroatoms has been regarded as a material having a high reduction catalytic activity to develop efficient Pt-free alternative CE. However, the doping effects accounting for such a catalytic activity remain unknown. In this study, by means of density functional theory calculations, we determined the band gaps, formation energies, and regions of chargeinduced impurities to examine the possibilities of B-doped, N-doped, and B−N co-doped graphene (BNG) sheets to replace platinum as CE. Our results demonstrated that the B−N co-doped graphene (BNG) is suitable to be used for CE owing to its small band gap, small formation energy, and having appropriate region of charge-induced impurities. In addition, we considered the iodide reduction reaction on the negatively charged BNG sheet. After injecting two extra electrons, the I2 molecule can strongly adsorb on the BNG surface and the I2 decomposition can be achieved with a small activation energy. A theoretical analysis of the obtained results provides us with some useful guidelines for identifying low-cost and effective CEs in the DSSC devices.

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INTRODUCTION There has been increasing interest in the dye-sensitized solar cells (DSSCs) in part due to their sustainable and eco-friendly advantages.1 In fact, the DSSCs can be used not only in environmentally friendly photovoltaic devices but also in cleanenergy applications.2−4 In addition, DSSCs are attractive approaches owing to their low cost, easy fabrication, flexibility, and relatively high efficiency. In general, a DSSC is composed of three components including a dye-sensitized photoanode, an electrolyte with a I3−/I− reduction−oxidation (redox) couple, and a counter electrode (CE).5 Platinum (Pt) deposited on a conductive substrate is generally used as a CE. Platinum has frequently been prescribed for the collection of electrons from an external circuit and also to function as a catalyst in the reduction reaction of the redox electrolyte occurring at the CE.6−11 The platinum electrode thus plays a crucial role in the transformation of the oxidation state of the electrolyte into a reduction state. However, Pt is well known as a very expensive metal, and limited worldwide reserves of Pt-containing catalysts tend to restrict their large-scale commercial applications.5 In search for a solution to this practical problem, significant efforts have been devoted to the development of Ptfree alternative materials with the aim to completely or partially replace the Pt-based CEs. © XXXX American Chemical Society

Many Pt-free alternative materials for CEs have in fact been proposed and studied in detail. For instance, conductive organic polymers,12 transition-metal compounds,13 composite materials,14 and carbon materials15 have been suggested to replace Pt-based CEs. Of the Pt-free materials proposed, carbon materials have recently emerged as the most commonly used ones, owing to their high electrical conductivity, low cost, and chemical stability. The abundance of different types of carbon nanomaterials, including carbon black,16 carbon nanotubes,17,18 graphene sheets,19−21 graphite nanoplatelets,22 porous carbon,23,24 carbon nitride,25,26 and active carbon27 are offering several new opportunities for the development of advanced Pt-free CE catalysts with promising performance. Owing to its high electrical conductivity, high thermal stability, and corrosion resistance, graphene has been considered as a potential candidate for this purpose.15 However, a pure graphene sheet consistently lacks a high catalytic activity for the reduction of I3− to I−, which is a key reaction in the functioning of a DSSC.28 It was reported that the heteroatom-doped graphene has high electrocatalytic Received: July 19, 2018 Revised: October 10, 2018 Published: November 5, 2018 A

DOI: 10.1021/acs.jpcc.8b06956 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

electrocatalytic activity on the iodide reduction with the aim to highlight the impacts of the B−N co-doping on the catalytic activities of graphene.

activity for fast reduction and regeneration of triiodide/iodide (I3−/I−) redox couple.29,30 As reported in other previous works, the overall I−3 reduction reaction on the CE can be written as

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COMPUTATIONAL DETAILS In the present study, structural, energetic, and electronic parameters are determined using density functional theory calculations. All the calculations are carried out using the Vienna ab initio simulation package42 with the Perdew− Burke−Ernzerhof functional.43 Calculations using plane-wave basis sets and a projector augmented wave method44 describe electron−ion interactions. Long-range dispersion correction incorporated in the optB88-vdW functional45 is adopted to describe van der Waals (vdW) interactions between the I2 molecule and the graphene sheet. We use the climbing image nudged elastic band46 method to locate the transition structures (TS) and thereby to calculate the energy barriers for I2 decomposition. A vibrational frequency analysis is subsequently carried out to verify the uniqueness of an imaginary vibrational mode, confirming the true nature of the saddle point. Graphene surface is optimized with a kinetic cutoff energy of 400 eV and a 3 × 3 × 1 mesh points in the k-space based on Monkhorst−Pack scheme for p(3 × 3) and p(6 × 6) supercells. The p(6 × 6) graphene supercell is generated to avoid the interactions between periodic structures along the reaction pathways. All the geometrical parameters are relaxed until the total energy is converged to 10−4 eV and the Hellmann−Feynman forces are smaller than 0.01 eV/Å. To simulate the different doped graphene nanosheets, we consider three periodic model structures, which contain the boron-doped, nitrogen-doped, and co-doped graphene. The p(6 × 6) doped graphene sheets are used to study the interactions between the doped graphene surfaces considered and the I2 molecule with periodic boundary conditions in the x−y plane. The vacuum space is set to 12 Å in the z-direction to avoid the interactions of adjacent slabs. To validate the stability of various doped graphene, including boron, nitrogen, and the pair of B−N co-doped atoms, we calculate their formation energies (ΔEf), which are evaluated by the following equation

I−3 (sol) + 2e− → 3I−(sol)

The reaction mechanism of the reduction reaction can be described by the steps31 I−3 ↔ I 2 + I−

(1)

I 2 + 2* ↔ 2I*

(2)

I* + e− ↔ I−

(3)

where the asterisk (*) indicates the adsorbed atom. The step (1) has been verified to be very fast and considered to be in equilibrium. Therefore, the iodide reduction reaction is determined by the steps (2) and (3). It is well known that a feasible way of fundamentally modifying the structural and electronic properties of an elemental cluster is through suitable doping. When doped by heteroatoms such as B or N atoms, or by combined B−N atoms, the electronic and chemical properties of the graphene sheet have indeed been found to be much changed.32−35 Due to the relatively large differences in the electronegativities of the dopants with respect to that of carbon atom, these heteroatomic dopants are expected to cause substantial changes in the electronic configuration, charge distribution, and thereby electronic properties of carbon skeletons.36,37 In this regard, although the atomic size of both boron and nitrogen dopants is similar to that of carbon, their electronic configurations and polarity are much different from each other. Such a difference in the electronic configuration could improve the less active site on which the reduction reaction occurs, and the similar atomic sizes tend to cause only small distortions in the carbon lattice. To confirm the performance of doped graphene used for Ptfree CE, electric conductivity, which has been considered as an important factor in DSSCs, needs to be evaluated. In fact, studies on the electrical conductivity of graphene materials have been reported.38,39 Mizuta et al.40 revealed that the resistance is much changed with the increasing or decreasing carrier density in heteroatom-doped graphenes. They also suggested that even a small charge transfer after molecule adsorption can strongly affect the electron transport in doped graphene surface by inducing charged impurity. Overall, recent studies on co-doped carbon nanotubes and graphene with boron and nitrogen atoms have clearly revealed that these nanomaterials can be used for metal-free electrodes owing to their higher catalytic activity, long-term operation stability, and larger tolerance to corrosive effect.41 These results showed that the enhanced electrocatalytic performance of the N-doped nanomaterials can be attributed to the electron-accepting ability of nitrogen atoms. In particular, a co-doping with two different elements would create a unique electronic structure with a synergistic coupling effect. This effect can in turn enhance their electrocatalytic activity in the reduction reaction. However, the question on how such a doping effect influences the electrocatalytic activity remains unanswered. In an attempt to address this important issue, we consider the use of negatively charged boron and nitrogen codoped nanosheets and study a synergistic coupling effect to the adsorption energies of I2 molecule. We also investigate the

ΔEf = Edoped ‐ graphene − Epristine − n[μ(B/N) − μ(C)]

where Edoped‑graphene and Epristine are the energies of the optimized structure of doped graphene and pure graphene sheet (pristine), respectively, and n is the number of dopant atoms. μ(B/N) and μ(C) are the chemical potentials of boron and nitrogen atoms and obtained by calculating the αrhombohedral boron, the total energy of the N2 molecule, and pristine graphene structure. The adsorption energy (Eads) of I2 molecule and desorption energy (Edes) of I atom on the doped graphene surface are calculated as follows Eads = (Etotal − Esurface − E I2)

Edes = (E I + E I *) − E2I *

where Etotal, Esurface, and EI2 are the total energies of I2 molecule adsorbed on the doped graphene nanosheets, the isolated adsorbent, and the isolated I2 molecule, respectively. EI and EI* are the total energies of the isolated I atom in gas phase and I atom on the doped graphene nanosheets, respectively. To understand the interactions in more detail, we also plot the B


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in electronic configurations and reducing the band gap. Figure 1 shows that pristine graphene has indeed a zero band gap, but the values of band gap of the doped graphene by substitutional boron and nitrogen atoms increase sharply. The size of band gap is highly related to dopant concentration, and this increasing band gap is going to limit the amount of electrons populated in the conduction band, eventually leading to a poorer conductivity. Nonetheless, the value of band gap increases slowly in the BNG surface. It is significant to note that the value of band bap still ranges below 0.3 eV when the B−N dopant concentration increases up to 33.3%, indicating that the BNG surface is a better candidate for the CE owing to its inherent small band gap. To validate the stability of doped graphene sheets, we compare the formation energy (ΔEf) of each doped graphene sheet with different dopant concentration. The calculated ΔEf values are depicted in Figure 2. According to the above

electron density difference (EDD) contour maps and projected density of states (PDOS) for I2 molecule adsorbed on the B− N co-doped graphene (BNG) surface. The electron transfer between partner molecules is determined using the Bader charge analysis.47

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RESULTS AND DISCUSSION Electronic Properties of Doped Graphene. Starting from the basal plane graphene, which comprises one monolayer with a hexagonal configuration, a two-atom unit cell is first fully optimized for the two-dimensional graphene structure. The calculated C−C bond length matches well with the experimental value of 1.42 Å.48 The unit cell of graphene structure is expanded to a p(6 × 6) supercell that contains 72 carbon atoms to study the interaction of the graphene surface with the iodine molecule. Previous experimental studies showed that the concentrations of the B-doped, N-doped, and B−N co-doped graphenes reach their maximal values at 13.0, 15.6, and 27.0%, respectively.49,50 We consider the dopant concentrations of boron and nitrogen atoms on the basis of the previous results of Tan et al.36,51 while designing the B and N co-doped graphene sheets. Graphene is known to be metallic with a zero band gap, but the band gap of B and N co-doped graphene would become open upon doping.52,53 A variable band gap, which is increased in doped graphene, leads to a change in the conductivity in going from a metallic property to a semiconductor. As shown in Figure 1, we

Figure 2. Calculated formation energies with B, N, and B−N doping concentration.

equation of ΔEf, a positive value of ΔEf denotes that formation of these doped graphene sheets is energetically endothermic, and the large value of ΔEf represents a thermodynamically unfavorable formation. As shown in Figure 2, as the dopant concentration increases from 0 to 33.3%, all of the formation energy values are increased. In particular, as the dopant concentration increases, the formation energy of the B-doped graphene sheet increases significantly, making it difficult to form B-doped graphene sheet with a high concentration of B atoms. Compared to the B- and N-doped graphene sheets, the BNG sheet is characterized by a small formation energy, indicating that the BNG should be easier to be synthesized than B-doped and N-doped graphene sheets. Due to the favorable formation and good conductivity, we expect that BNG sheet is a more suitable candidate to replace Pt as the CE material. Figure 3a−c displays the electron density difference (EDD) of I2 molecule adsorbed on B-doped, N-doped, and B−N codoped graphene surfaces, respectively. Yellow and blue lines refer to the electron-rich and electron-deficient areas, respectively. The I2 molecule causes a strong localization of electron density depletion around the adsorption site, namely, the positively charged impurity. The wide region of positively charged impurity in boron-doped graphene surface, as shown in Figure 3a, produces a Coulomb potential envelope in the boron-doped graphene, which is strong enough to scatter an electron and completely block the electron transport. In

Figure 1. Variation in band gap with B, N, and B−N doping concentration.

calculate the band gaps of 72 atomic graphic supercells of graphene, which correspond to different dopant concentrations by B, N, and pair of B−N doping. According to the limitation of experimental synthesis,25 the B and N doping concentrations vary from 0 to 11.1% by doping 2 (2.8%), 4 (5.6%), 6 (8.3%), and 8 (11.1%) atoms (Supporting Information Figures S1 and S2). However, the doping concentration of B−N is considered from 0 to 33.3% (Supporting Information Figure S3). The energies of single-atom-doped and B−N co-doped graphene sheets are listed in Tables S1 and S2. It was pointed out that the dopants of nitrogen or boron atoms will break the electroneutrality of sp2 carbon and change the electronic configurations, causing the band gap to increase significantly.54 However, co-doping with boron (lower electronegativity) and nitrogen (higher electronegativity) atoms can create unique electronic structure with a synergistic coupling effect between heteroatoms, contributing to the small change C


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requirement to be considered for the injection of extra electrons, we could reaffirm that the BNG sheet can be regarded as a better alternative material for CEs. We subsequently consider the adsorption of the I2 molecule on the BNG sheet. We consider all the possible adsorption sites for the I2 molecule, directly on top of a C, B, or N atom, above the center of a hollow site, and a bridge site of the C−B, C−N, and B−N atoms. Figure 5 shows the two lowest-energy

Figure 3. Electron density difference (EDD) of boron-doped, nitrogen-doped, and boron and nitrogen co-doped graphene sheets by the adsorption of I2 molecule. (a−c) The electron density difference (EDD) distribution along the I2 molecule on boron-doped, nitrogen-doped, and boron and nitrogen co-doped graphene sheet, respectively. The isosurface value is 0.001 e/au.

addition, a comparison of the charged-induced impurity on different doped graphene surfaces points out that the impurity region on the boron-doped graphene surface is the largest, and that on the BNG surface is the smallest. Accordingly, the electron transport in the BNG surface is more conductive and thereby it is a more suitable CE material. I2 Adsorption on B−N Co-doped Graphene Sheet. Graphene with B and N co-doping concentration up to 33.3% still has good conductivity and small formation energy (Figures 1 and 2). Besides, a Coulomb scattering of charged impurities insignificantly affects the electronic transport in the BNG surface (Figure 3). Therefore, in this study, we chose BNG with a doping concentration of 33% as a counter electrode to investigate its catalytic performance. Figure 4a shows the stable structure of BNG with a doping concentration of 33%. Because the role of a CE in a DSSC scheme is to accept electrons from external circuitry, we explore this further in carrying out calculations for negatively charged systems. For this purpose, we introduce two excess electrons into the p(6 × 6) supercell. The charge density of two electrons negatively charged BNG surface is 9.18 × 1013 e−/cm2. By analyzing the DOS of neutral and negatively charged BNG surfaces, Figure 4b reveals that the neutral BNG nanosheet (gray area) shows a small band gap. On the contrary, the negatively charged BNG nanosheet (yellow area) is metallic, and the value of Fermi level shifts upward to −1.2 eV. As a good electrical conductivity is an important

Figure 5. Top view and side view of the optimized configuration for I2 molecule adsorption on the B−N co-doped graphene surface with (a) end-on and (b) side-on mode.

configurations of I2 molecule on the neutral BNG surface, and the calculated adsorption energies and structural parameters are summarized in Table 1. Other adsorption energies and Table 1. Calculated Adsorption Energy, I−I Bond Length, and Distance from I2 to BNG Surface under Neutral and Negatively Charged Systems neutral system end-on side-on

negatively charged system

Eads (eV)

dI−I (Å)

hI−S (Å)

Eads (eV)

dI−I (Å)

hI−S (Å)

−0.45 −0.48

2.79 2.81

2.99 3.59

−1.55 −1.67

3.36 3.69

3.16 3.65

structural parameters are listed in Table S1 of the Supporting Information. For the end-on adsorption mode (Figure 5a), the linear I2 molecule adsorbs on top of B site. The distance between the I atom and the B atom is 2.99 Å, and the bond length of the I2 molecule is elongated from 2.68 to 2.79 Å. On the other hand, for the side-on adsorption mode, the linear I2 molecule is adsorbed on a bridge site between boron and nitrogen atoms (Figure 5b). The distance between the surface

Figure 4. (a) Top view of the optimized structure of p(3 × 3) supercell of B and N co-doped graphene (BNG). (b) The calculated density of states (DOS) of the BNG sheet. The gray and yellow areas denote the neutral and negatively charged BNG sheets, respectively. The black dashed line is the Fermi level. D


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The Journal of Physical Chemistry C and I2 molecule is ∼3.59 Å, and the bond length of I2 molecule 2.81 Å. The calculated adsorption energies of I2 with end-on and side-on amount to −0.45 and −0.48 eV, respectively. According to the calculated adsorption energies listed in Table S1 (Supporting Information), the corresponding adsorption energies remain almost unchanged, indicating that I2 molecule can favorably adsorb on the BNG sheet in both end-on and side-on modes. To simulate the electrocatalytic property, we now consider the injection of two extra electrons into the BNG supercell. The effects of the negatively charged system can be evaluated (Table 1). The distances between the I atom and the charged BNG surface are 3.16 and 3.65 Å for the end-on and the sideon adsorption modes, respectively. In addition, both adsorption modes clearly show significant structural change with the I−I bond elongation to 3.36 and 3.69 Å. In this case, the strength of I2 adsorption becomes remarkably stronger than that on the neutral BNG surface, and the adsorption energies for the end-on and the side-on modes are −1.55 and −1.82 eV, respectively. Accordingly, the side-on adsorption mode on the negatively charged BNG surface turns out to be more favorable, and it is expected to exert more influence on the following decomposition of the I2 molecule. Electronic Properties of I2 Adsorbed on BNG Sheet. To understand the underlying cause of the strong adsorption of the I2 molecule on the negatively charged BNG sheet, we now calculate the Bader charges, the electron density differences (EDD), and the projected density of states (PDOS) of the related structures. The MOs of the gaseous I2 can be designated as (σ5s)2(σ5s * )2(σ5p)2(π5p)4(π5p * )4(σ5p * )0, where (π5p * ) is the antibonding LUMO. For the purpose of the BNG sheet to serve as an electrocatalytic CE substrate, an electron transfer should occur from the co-doped graphene surface to the I2 molecule. Addition of electron to the antibonding (σ5p * ) orbital tends to reduce the bond order and thereby increase the I−I bond length. The longer bond length of the I2 molecule indicates that the electron transfer from the BNG sheet in a negatively charged system is more than that in the neutral system. The ability of electron transfer is an important factor for a CE material because it is expected to donate electrons to form an iodide anion. Results of a charge analysis for both neutral and negatively charged systems are compared in Table 2, where a positive

density (the red lines) along the axis of the I−I bond, which belongs to the antibonding (σ5p * ) orbital. An increasing electron density in that region emphasizes an electron transfer from the BNG surface to an I2 (σ*5p) orbital, which would reduce the bond order and increase the I−I bond length. Not only the amount of electron density accumulation around I2 but also the depletion of charge (the blue lines) above the BNG surface in the negatively charged systems is more than that in the neutral counterparts, which is in good agreement with the amounts of charge transfer given in Table 2. To further understand the interaction between surface and I2, partial density of states (PDOS) for the I2 molecule before and after adsorption are plotted in Figure 6. As given above,

Figure 6. Projected density of states (PDOS) for the I2 molecule after the I2 adsorption (a) in the neutral system and (b) in the negatively charged system.

* )2 the MOs of the gas-phase I2 are designated as (σ5s)2 (σ5s (σ5p)2 (π5p)4 (π*5p)4 (σ*5p)0. As shown in Figure 6a,b, after the I2 adsorption, the σ5p* area below the Fermi level increases, indicating that the σ5p* orbital of I2 gains electrons from the BNG surface. Moreover, the area of σ5p* below the Fermi level in the charged system is much larger than the area in the neutral system, indicating that more electrons have been transferred from the charged BNG sheet to the I2 molecule. Electron transfer from the BNG surface to σ5p* will reduce the bond order and increase the I−I bond length. The result is consistent with the longer bond length of the I2 molecule in the negatively charged system given in Table 1. Furthermore, after I2 adsorption, the I2 peaks below the Fermi level are broadened, especially in negatively charged system, showing that the I2 molecule has a stronger interaction with the BNG sheet in the charged system. Energetics of the I2 Reduction Reaction. Figure 7 shows a profile that is a portion of the potential energy surface (PES) describing the adsorption and dissociation of I2 on the neutral BNG surface. The potential energy profile is referred to as the energy of the end-on adsorbed I2 (I2*(end-on)) mode. Relative energies and structural parameters of intermediates and final product are also illustrated in Figure 7. The end-on adsorbed I2 can undergo an isomerization through transition structure TS1, with a tiny barrier 0.08 eV, to side-on adsorbed I2 (I2*(side-on)), which exhibits a favorable pre-reactive configuration for the following dissociation. Through the surface catalytic effect, then the I−I bond can be broken by transferring an adsorbed I atom to another B−N pair with an energy barrier of 1.07 eV.

Table 2. Change of Bader Charges (ΔQ in e) after I2 Molecule on B−N Co-doped Graphene Sheet Obtained from the Bader Charge Analysis in the Neutral and Negatively Charged Systems ΔQ (neutral system)

end-on side-on

BNG surface

I(1)

0.13 0.15

−0.04 −0.07

ΔQ (charged system)

I(2)

BNG surface

I(1)

I(2)

−0.09 −0.08

0.98 1.42

−0.44 −0.71

−0.54 −0.71

value represents an electron donation and a negative value stands for an electron acceptance. A small amount of electron is actually transferred from the BNG surface to I2 in the case of neutral systems. However, the electron transfer is substantially increased in going from the negatively charged BNG sheet. The two-dimensional contour maps of EDD corresponding to the adsorption of the I2 molecule on the BNG surface are displayed in Figure S4. There is an increment of electron E


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The reaction on the negatively charged BNG surface is slightly endothermic by 0.25 eV, as compared to the larger endothermic reaction by 0.9 eV on the neutral BNG surface. Furthermore, we also considered the decomposition of I2 molecule on the singly charged BNG sheet. The relative energies and structural parameters of intermediates and final product are illustrated in Figure S5. The result demonstrated that the tendency of the I−I bond dissociative process on the singly charged BNG sheet is similar to that on the doubly charged system. The calculated reaction energetics point out that the activation for I−I bond breaking on the negatively charged BNG surface is both thermodynamically and kinetically more favorable. Following the dissociation, the iodide anion is able to desorb with desorption energies of 0.41 and 0.22 eV for the neutral and negatively charged systems, respectively (Figures 7 and 8). Related to the desorption energy (Edes), desorption of adsorbed I anion should be fast with a small Edes, and decomposition of I2 molecule is the ratedetermining step, indicating that the adsorbed I anion is easy to desorb from the BNG surface. For the sake of comparison, we calculate the Edes of adsorbed I atom on Pt(100) surface (see details in Supporting Information Table S4). Besides, we also calculated Edes, including the solvent (acetonitrile). The optimized configurations are shown in Figure S6, and the values of Edes are listed in Table S5. As can be seen in Table S5, the influence of solvent effect on the Edes is not so significant. Hence, we considered Edes without the solvent effect in this study. Table S4 shows that Edes of an adsorbed I anion on two electrons negatively charged Pt(100) surface reaches a value as large as 0.87 eV. Compared to the Edes of I anion on the BNG surface, it can thus be expected that the desorption of adsorbed I anion on the negatively charged BNG sheet is thermodynamically more favorable than that on a negatively charged Pt(100) surface. This result lends an additional support to the B−N codoped graphene to be a candidate material for use as a Pt-free counter electrode.

Figure 7. Energy profile of the I2 molecule decomposition on the BNG (black line) at the neutral system. All the energies are informed in eV relative to the reactants, namely, I2*(end-on) and distances are in angstrom units.

By examining the transition structure TS2, it can be noticed that the activated I−I bond is stretched to 4.48 Å and the transition vector of the imaginary frequency is indeed directed toward a pair of B−N atoms. The I−I distance is stretched up to 5.52 Å after complete decomposition. The large energy barrier suggests that the decomposition of the I2 molecule is a rather difficult process to occur under these conditions. Furthermore, the overall decomposition is found to be endothermic, with a reaction energy of 0.90 eV, indicating a thermodynamically unfavorable reaction. The result shows that the decomposition of the I2 molecule is overall not favored on the neutral BNG surface. We now examine the decomposition of I2 molecule on the negatively charged BNG sheet. According to the potential energy profile illustrated in Figure 8, the reduction reaction

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CONCLUSIONS In the present theoretical study, we carried out first-principle calculations to study the electrical conductivity and stability of B-doped, N-doped, and B−N co-doped graphene sheets. The B−N co-doped graphene (BNG) exhibits greater advantages in both conductivity and stability, which is an important aspect for the counter electrode (CE) in DSSCs. Our results show that the negatively charged BNG nanosheet offers more stabilized adsorption sites and enhances the ability of electron transfer from the surface, thereby increasing the electrocatalytic activities. Such a behavior in electron transfer implies a key difference in the catalytic effects of the negatively charged BNG surface. Moreover, we found that the more facile I−I bond dissociative process occurs on the negatively charged BNG surface, with a lower energy barrier of ∼0.3 eV. The BNG surface acts as a stronger electron donor along the dissociative reaction pathway. The larger electronegativity of nitrogen atoms makes them to more readily interact with the I2 molecule. The charged B−N co-doped graphene nanosheets can thus act as electrocatalytic materials that reduce the reaction barrier for the I−I bond cleavage. The present findings suggest that the BNG surface emerges as a promising candidate for the replacement of the Pt-based CE catalyst in a DSSC device. It is hoped that these results would stimulate the experimental search and implementation

Figure 8. Energy profile of the I2 molecule decomposition on the BNG (black line) in the negatively charged system.

pattern on the negatively charged BNG surface is similar to that on the neutral BNG surface. It is also initiated by a rotation of I2 from end-on to side-on through the transition structure TS1-a, with a small barrier of 0.03 eV, being in fact a free rotation process. The I−I bond length is at this stage stretched from 3.36 to 3.69 Å. Then, the I−I bond can easily be dissociated with a lower energy barrier of 0.31 eV. In the final state seen in Figure 8, both dissociated I atoms are connected to the bridge site of the B−N pair and completely separated from each other, as indicated by the I−I distance of 5.61 Å. The bridge sites thus offer stabilized adsorption positions for the formation of I atoms on the BNG surface. F


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of Pt-free CEs using carbon-based nanomaterials with advantages of low cost and high efficiency.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b06956. Optimized structures (B-doped, N-doped, and B−N codoped graphene sheets varied the doping concentrations from 2.8 to 33.3%) are summarized; two-dimensional contour of electron density difference (EDD) is presented; energy profile of the I2 molecule decomposition on the BNG at the single charged system is shown to compare the double charged system; desorption configurations of adsorbed I atom with and without solvent effect on Pt(100) surface and the BNG surface are also summarized (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected] (M.T.N.). *Email: [email protected] (J.-C.J.). ORCID

Minh Tho Nguyen: 0000-0002-3803-0569 Jyh-Chiang Jiang: 0000-0002-4225-5250 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.-C.J. thanks the financial support from the Ministry of Science and Technology, Taiwan (MOST 106-2113-M-011-001). The authors are also thankful to the National Center of HighPerformance Computing (NCHC) for donating computer time and facilities. Work at ICST (MTN) was supported by the Department of Science and Technology of Ho Chi Minh City, Vietnam, under a Grant 2018/HĐ-SKHCN entitled: Theoretical Design of Metal-free Organic Materials and Counter Electrodes for Solar Cells. Work at the School of Materials Science (Mizuta Group) was supported by Advanced Institute of Science and Technology (JAIST) in Japan.

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Boron and Nitrogen co-Doped Graphene used as Counter Electrode

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