Theoretical Designs of Photoresponsive Energy-Storage Materials

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Theoretical Designs of Photoresponsive Energy-Storage Materials Based on Attachment of #-Conjugated Molecules onto Sulfur-Doped Graphene Jun Zhao, and Jing Ma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09247 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 22, 2016

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The Journal of Physical Chemistry

Theoretical Designs of Photoresponsive Energy-Storage Materials Based on Attachment of π-Conjugated Molecules onto Sulfur-Doped Graphene Jun Zhao,a,b Jing Maa,* a

School of Chemistry and Chemical Engineering, Institute of Theoretical and Computational Chemistry,

Key Laboratory of Mesoscopic Chemistry of MOE, Nanjing University, Nanjing 210093, P. R. China b

School of Physics and Optoelectronic Engineering, Yangtze University, Jingzhou, Hubei 434023, P. R.

China

ABSTRACT: Introducing heteroatom active sites and functional units is essential for achieving high-performance graphene in potential applications of optoelectronic devices and sustainable solar-heat energy conversion/storage. Density functional theory (DFT) calculations with long-range van der Waals effects (vdW-DF2) were performed to study the electronic structures and energy storage/release of graphene through sulfur (S)-doping and physisorption of π-conjugated photoresponsive molecules, trans/cis-azobenzene (AB) derivatives with electron-donating substituent group and trans/cis-stilbene (ST), respectively. With the increase of the S doping concentration from 0.4 to 0.8 atom/nm2, the bandgap of graphene exhibits the enhanced metallic characteristics with a direct-to-indirect transition. Although AB and ST molecules have different unsaturated bridge bonds, -N=N- versus -CH=CH-, physisorption of these two photoresponsive molecules onto the graphene can both 1 ACS Paragon Plus Environment


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broaden band gap to about 0.02 eV, as a result of the π-π interfacial interactions. Under the exposure to the solar light, the facile trans-to-cis isomerization of AB (ST) molecule adsorbed onto graphene renders the energy storage of about 1.04 eV (0.49 eV) in each molecule. The noncovalent physisorption of trans/cis-AB molecules onto graphene is unexpectedly more favorable to energy storage than that of covalent binding. In addition, a multifunctional graphene model, with the combination of both S-doping and physisorption of photoresponsive moelcules, could not only open a bandgap of about 0.27eV, but also induce energy storage of 0.84 eV per molecule via the conformational change from trans to cis AB isomer. Strong charge localization at S dopant may become the active sites for catalysis and energy storage, and meanwhile, photoactive adsorbates could further promote the energy conservation and release.

1. INTRODUCTION Due to the unique physicochemical properties,1-6 graphene7-9 has attracted tremendous research attention in many fields including gas sensing10-13 and electronic device.14-17 Despite these important developments, the lack of a bandgap and the resultant poor switching behavior in graphene transistors imposed limits on graphene-based semi-conductor device performance. Thus, to achieve high-performance graphene for future applications, it is essential to open up a significant band gap.18-23 A number of approaches have been developed to tailor the electronic structures of graphene,24-27 in which heteroatom doping and surface functionalization are the two most promising strategies. 2 ACS Paragon Plus Environment

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Figure 1. Schematic illustration of the multifunctional model of graphene through S-doping, physisorption of trans/cis-AB

and trans/cis-ST on the pristine-graphene and the combination of S doping and surface functionalization. The cyan, white,

red, blue and yellow colored atoms represent the carbon, hydrogen, oxygen, nitrogen and sulfur atoms, respectively.

Among various heteroatom dopings on graphene, sulfur (S) doping is of particular interest as the resulting materials are expected to have a band gap opening28-31 due to the electron-withdrawing character of S atom.32,33 Up to now, different experimental strategies,34-36 including chemical vapor deposition technique, thermal reaction and so forth, have been employed to synthesize S-doped graphene. S-doped graphene was demonstrated to possess great potential for broad applications in fuel cells,37 metal-free electrocatalyst,38 supercapacitors,39 lithium ion batteries,40 storage H2 and capturing CO2.41 However, studies aimed to build multifunctional graphene through diversified modulations are still lacking. In the present work, we will focus on the combination of heteroatom doping and surface

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functionalization by π-conjugated photoresponsive molecules to realize the electronic structure engineering and energy storage/release of graphene, as shown in Figure 1. Upon absorption of solar light, the π-conjugated photoactive molecules, such as azobenzene (AB) and stilbene (ST), could undergo a conformational change from trans isomer to a higher energy metastable cis-configurations, thus storing energy42-60. Such stored solar energy can be further utilized as heat release by the external trigger, such as light, voltage and so on. In this work, donor-π-acceptor-type (D-π-A) molecules, trans/cis-AB derivatives and trans/cis-ST are introduced to deposite onto the substrate to tailor the electronic structures of graphene. Although physisorption may not be as effective as S doping for bandgap opening, an alternative probability of energy conversion/storage could be anticipated with the utilization of these photoresponsive molecules. Interestingly, both of these two kinds of photoresponsive molecules have two π-conjugated benzene rings but different unsaturated bridge bonds, -N=N- vs. -CH=CH-. Will these different bridge bonds make different contributions on the band gap opening? How does the adsorption orientation of π-conjugated molecules impact the electronic structures? How much energy can be stored? Will the band gap of graphene exhibit the additive effects when heteroatom doping and surface adsorption routes are simultaneously applied? We will answer these questions through systematic calculations of the electronic structures and energy storage/release of graphene through S-doping and physisorption of π-conjugated photoresponsive molecules using DFT method with vdw-DF2 corrections. Multifunctional graphene model, with the combination of S-doping and physisorption of photoresponsive molecules, will be built for catalysis and energy storage. It will be shown that S doping approach can effectively tune the 4 ACS Paragon Plus Environment

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electronic structures of graphene. When exposed to light, photoactive molecules can store solar energy in molecular bonds and convert it to thermal energy with the external trigger. We will also display that the S-doping effect and the photoresponsive energy storage/release function are orthogonal to each other. The present multifunctional graphene model may be applied to the fabrication of graphene-based optoelectronics and the solar energy utilization and storage.

2. COMPUTATIONAL METHODS

All the geometry optimizations and electronic structure calculations are performed by using the density functional theory (DFT) in Vienna ab initio simulation package (VASP),61,62 which utilizes a plane-wave basis set to solve the Kohn-Sham equations. For the graphene monolayer, a 7×7 unit cell consisting of 98 carbon atoms is chosen as supercell. We make use of generalized approximation (GGA) formulated by Perdew and Burke and Ernzerhof (PBE) as the exchange-correlation functional.63 S doping effect on the pristine graphene is investigated with the doping concentration increasing from 0.4 to 0.8 atom/nm2. Here, the doping concentration is defined as the ratio of the number of S atoms and the area of the substrate. The sulphur doping effects on the single-walled carbon nanotubes and graphene with sulphur concentration of 1.7-4 atom% were also studied at GGA-PBE level, and the electronic structures were found to be modified depending on the sulphur content.29 The π-conjugated molecules, trans/cis-AB derivatives (with R=-C2H5C(O)NH-) and trans/cis-ST, C6H5-CH=CH-C6H5, are physisorbed onto the pristine or S-doped graphene layer, respectively. The periodic boundary condition (PBC) is applied and a 40 Å vacuum layer is 5 ACS Paragon Plus Environment


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used to avoid possible interactions between periodic images. A 7×7×1 Monkhorst-Pack64 k-point mesh is used for the Brillouin Zone (BZ) integration in geometries relaxations and 400 eV is used as kinetic cutoff energy. All geometric optimizations were done until the electronic self-consistent energy was less than 10-4 eV and the force on each atom was less than 1meV/Å by using the conjugate-gradient method. To obtain accurate density of states (DOS) result, the k-point mesh is increased to 15×15×1. The ab initio vdW-DF2 functional65,66 implemented in VASP is employed to describe the vdW interactions between graphene substrate and adsorbed molecule, since previous theoretical calculations67 have shown that the van der Waals (vdW) interactions play a critical role in properly predicting the properties of graphene based systems using first-principles studies. The binding energy (Eb) of the physisorbed nanostructure is calculated according to the definition of

Eb = Ecomplex - Esub - Ead

(1),

where Ecomplex is the ground state energy of the complex system with adsorbed nanostructure,

Esub and Ead are the individual energies of the substrate and the adsorbate, respectively. The storing energy (∆Ec-t) is estimated by the energy difference between the cis and trans isomers, i.e.,

∆Ec-t = Ecis - Etrans

(2),

where Ecis and Etrans are the energies of the nanostructures when the cis or trans isomer is adsorbed onto the substrate, respectively. In order to understand the doping effects at different size scales and the influence of boundary effects, some cluster models of graphene fragment (without PBC) were studied for 6 ACS Paragon Plus Environment

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comparison with the PBC nanosheet. The optimized structures and frontier molecular orbitals of fragment clusters are obtained at B3LYP/6-311+G(d) level by using the GAUSSIAN 09 program.68 For the selected cluster models: (p-graphene: C32H14; S-graphene: C31H14S; 2S/2S’-graphene: C30H14S2), the ground state spin multiplicity is singlet. The energy gap is defined as the energy difference between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the cluster models.

3. RESULTS AND DISCUSSION

The modifications of graphene and their influence on the functions will be presented in the following three steps, as shown in Figure 1. Firstly, S atom, with different doping concentration ns, is doped into pristine-graphene to tailor its electronic structures. Then, with π-conjugated photoresponsive molecules (trans/cis-AB and trans/cis-ST) physisorbed onto the pristine-graphene, electronic structures, energy storage and charge redistribution of two systems are investigated. Lastly, by combining above two methods, electronic structures and energy conversion/storage of a multifunctional material will also be addressed.

3.1. Electronic Structures Modification by S-Substitution. On the basis of the PBC model, the optimized geometries of pristine-graphene (p-graphene), S-doped-graphene (S-graphene), 2S-doped-graphene (2S-grahene or 2S'-graphene) are shown in Figure 2a, with the S-doping concentration increasing from 0 to 0.8 atom/nm2. Unlike the planar form of p-graphene, in S-graphene, due to the sp3-like hybridization, the dopant S atom protrudes out of the plane. The buckling distance, δ, of S atom is 1.57 Å under the consideration of vdW interaction, a little longer than that obtained from the exclusion of vdW interaction (1.43Å). 7 ACS Paragon Plus Environment


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The local geometric deformation caused by S-substitution somewhat induces the strain in plane. The C-C bond distance in p-graphene is 1.42 Å, while the corresponding C-S bond length a1 in S-graphene expands to 1.75 Å, which is close to the previous calculation results28 at GGA-PBE level (1.74 Å) with a smaller 4×4 supercell. The three C atoms bonded to the S dopant are lifted up by about 0.78 Å. The bond angle θ is reduced from 120° with a typical sp2 hybridization to 101° of sp3 frame.

Figure 2. Optimized structures of p-, S-, 2S- and 2S'-grahpene for (a) PBC model and (b) cluster model with HOMO/LUMO

molecular orbitals of the four clusters obtained at B3LYP/6-311+G(d) level. The doping concentration (ns) of S atoms for each system in PBC model is 0, 0.4, 0.8 and 0.8 atom/nm2, respectively. The definitions of geometry parameters are depicted


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on the top of each Figure, and the buckling distance δ and bond lengths of a1(a1'), a2 and a3 are in units of Angstrom, bond angel θ is in unit of degree.

It is interesting to notice that there exist two kinds of dual S-doped models, 2S- and 2S'-graphene systems, with two S atoms buckling on p-graphene surface at the same side and the opposite side, respectively. The total energy of 2S-graphene is just 0.14 eV higher than that of 2S'-graphene model, implying that these two kinds of dual S-doped models may coexist in the experiment. More interestingly, 2S'-graphene configuration has better planarity than 2S-graphene model. The average buckling distance δ is 1.66 Å for the same side one but 1.35 Å for the opposite one. The smaller buckling distortion for 2S'-graphene may be attributed to the strain release of the two S atoms doping at the opposite side. How to understand the doping effects on local electronic structures with the increasing doping level? To answer this question, we resort to a cluster model, which is cut from the graphene sheet and the strain in the fragments can be released at free boundary, as illustrated in Figure 2b. In order to ensure the consistency with respect to the concentration of sulphur doping, the size of the p-graphene and S-graphene clusters are required to be the same as that of 2S-graphene. The buckling distance δ for S-graphene cluster is decreased from 1.57 Å (in PBC model) to 0.99 Å due to the release of strain at the free boundary of cluster model. Correspondingly, the C-S bond distance a1 stretches to 1.80 Å and a3 decreases to 1.38 Å. The smaller clusters of the p- and S-graphene (C13H9 and C12H9S) are also investigated to estimate the size effect, as shown in Figure S1 in the Supporting Information. For these two smaller clusters, the ground state spin multiplicity is doublet. It is noted that the geometric 9 ACS Paragon Plus Environment


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structure is not influenced significantly by the size of the cluster. The buckling distance δ is increased to 1.32 Å with the decrease of the S-graphene cluster size, indicating a larger geometric distortion for the smaller cluster. In the case of 2S- and 2S'-graphene fragment clusters, the buckling distance δ is decreased to 1.56 and 1.22 Å, respectively, smaller than those in PBC model. This is also ascribed to the strain release at free boundary. The HOMO and LUMO of p-graphene in Figure 2b are assigned to bonding π and antibonding π* orbitals, respectively. For the cluster model of p-graphene, the HOMO-LUMO gap is 2.92 eV. In contrast, when one S atom substitutes a C atom, lone pair electrons break the π electrons delocalization, and accordingly, the molecular orbital transition between HOMO and LUMO is mixed with n-π* transition. Electron-withdrawing atom S induces the LUMO and HOMO energy levels of p-graphene fragment to shift down and up by about 0.08 and 0.66 eV, respectively. As depicted in Figure S1 in the Supporting Information, for the smaller p- and S-graphene clusters (C13H9 and C12H9S), the energy gaps between singlet occupied molecular orbital (SOMO) and LUMO are of 3.99 and 3.26 eV, respectively. This suggests that the sizes of the clusters have obvious impacts on the energy gaps. For two dual S-doped clusters, the energy gaps (2S-graphene: 1.90 eV; 2S'-graphene: 1.63 eV) are both smaller than that in single S-doped cluster model (Figure 2b). This indicates that S doping with higher concentration decreases the HOMO-LUMO energy gaps, thereby improving the chemical reactivity of graphene. It will be shown that with the increase of the S doping concentration, S-doped graphene in PBC model will experience a direct-to-indirect band-gap transition.


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Figure 3. Electronic band structures, total density of states (DOS) structures and the isosurface plots of the band

decomposed charge density distribution corresponding to CBM and VBM for (a) p-graphene, (b) S-graphene, (c) 2S-graphene and (d) 2S'- graphene. The isovalue is 0.0008 e/Bohr3. The Fermi level is set to zero and indicated by the

dashed line.



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S-substitution significantly changes the band structures of the p-graphene, which can be observed in Figure 3. The p-graphene is undoubtedly the gapless semiconductor with the linear dispersion, E = ℏυk , in the vicinity of K point (Figure 3a). Moreover, the total projected density of states (PDOS) of the p-graphene is mainly contributed by the p-orbital due to sp2 planar structure. In order to show the orbital contributions at the Fermi-level, the band decomposed charge density distributions at valence band maximum (VBM) and conduction band minimum (CBM) are also depicted. The CBM and VBM for p-graphene indicate characteristics of delocalized π-bonding and π*-antibonding states, respectively. The most important consequence of introducing a single S atom is the appearance of a direct band gap of 0.25 eV and a sharp peak near the Fermi level, indicating its semiconductive behavior (Figure 3b). The prominent charge transfer between the dopant and the nearest C atoms is induced by the single dopant S atom, thus the total PDOS is intensively modified around the Fermi level. Inspecting from the VBM and CBM of S-graphene, the band gap is induced by the n-π* transition, in good accordance with that obtained from fragment clusters. The charge localization around the dopant S atom unveils that the C-S bond could act as an important catalytic active site for attractive functional materials in energy conversion/storage and adsorption as metal-free, electrocatalysts for fuel cells,69 anode materials in Li-ion batteries70-71 and so on. It was addressed that the introduction of dopant S atom could improve the selectivity of oxygen reduction reaction (ORR).72-81 Theoretical calculations76 predicted that S-doping would induce more strain and defect sites in the graphene panel due to the larger atom radius and the change of spin density, contributing to the enhanced ORR activity of S-doped graphene. In experiments, some S-doped-graphene-based systems, e.g., 12 ACS Paragon Plus Environment

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S-doped graphene ‘Idli’,78 edge activated S doped Fe-N-graphene,79 and Pt nanowires on S-doped graphene,80 have been demonstrated to exhibit excellent catalytic activity and some of them even outperform commercial Pt/C in terms of durability and selectivity (in addition to the considerably lower materials cost), indicating that S-doped graphene templates hold great potential in replacing conventional Pt/C catalysts in alkaline media.77 In spite of these achievements, more intensive efforts are required to provide the relationship between the catalytic activity and the microstructure for S-doped graphene systems. The dual S doped graphene exhibits a very different electronic band structure compared with single doped case. With the increase of the S doping concentration, the band gap decreases from 0.25 to 0.07 eV, undergoing a direct-indirect transition, as illustrated in Figure 3c and 3d. For 2S- and 2S'-graphene, the DOS is both no longer zero at the Fermi level and an equal indirect gap of 0.07 eV is induced for both two systems, indicating that the metallic properties of graphene are enhanced by introducing two S atoms. These is consistent with the previous results of doubly S doped graphene,29 in which, however, only doping at the same side was calculated. The 2S'-graphene model has not yet been theoretical studied to the best of our knowledge. The similarity in the electronic structures for both 2S- and 2S’-graphene models demonstrates that the S doping at same side or opposite side onto graphene may coexist in experiments. Moreover, the band decomposed charge density distributions at VBM/CBM (Figure 3c-3d) show that the delocalization feature of the π electrons are destroyed by the lone pair electrons of two S dopants. Charges tend to accumulate at the buckled S atoms and the lifted C atoms. It is possible to vary the electronic properties of the graphene sheet by adjusting the doping level of sulfur into the sheet lattice. 13 ACS Paragon Plus Environment


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3.2. Energy Conversion Achieved by Physisorption of Photoresponsive Molecules. We then study the physisorption of the photoresponsive π-conjugated molecules (trans/cis-AB derivative and trans/cis-ST) onto the p-graphene, starting from identifying the most energetically favorable adsorption site. As shown in Figure 4, three different sites including the hollow and bridge site (HB), the bridge and bridge site (BB), and the top and bridge site (TB) have been considered in the investigation. Such a definition of adsorption sites was borrowed from other works about the adsorption of aromatic molecules onto graphene.82 It is found that the most stable adsorption mode for trans-AB derivative deposited onto graphene is at the HB site (see Figure 4a). We should recall that the same adsorption site of the unsubstituted AB molecule deposited onto graphene42 was reported to be the stable configuration at GGA-PBE level, indicating the substitution of AB with -C2H5C(O)NHgroup will not lead to much difference about the adsorption site. Accordingly, hereafter we construct all nanostructures at the HB site of the graphene substrate.

Figure 4. Three binding sites of trans-AB molecule adsorbed onto p-graphene, where H, B and T represents the hollow,

bridge and top positions, respectively. The grey, white, red, blue and yellow colored atoms represent the carbon, hydrogen,

oxygen, nitrogen and sulfur atoms, respectively.


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Figure 5 displays the optimized structures and charge difference isosurfaces of the trans/cis-AB and trans/cis-ST molecules physisorbed onto the p-graphene. Since the orientation of the adsorbate onto the substrate is practically difficult to be controlled in experimental synthesis, two different orientations of the trans- and cis-AB adsorbed onto the p-graphene are investigated, as shown in Figure 5a. Here, the substituent group (R=-C2H5C(O)NH-) of AB molecule is below or above the trans/cis-AB surface, denoting as 'down' or 'up' orientation. To evaluate the stability of the physisorbed nanostructure, we calculate the binding energy (Eb). All of the energies of studied systems with and without adsorbates are summarized in Table S1 in the Supporting Information. From equation (1), it can be expected that the relative values of Eb are insensitive to choice of the surface models, PBC vs. cluster models. For simplicity, only the PBC calculation results are presented in the following sections.



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Figure 5. Top view and side view of the optimized atomic structures and the charge difference isosurfaces of (a)

trans/cis-AB with down and up orientations and (b) trans/cis-ST adsorbed onto adsorbed onto p-graphene. Cyan and light

yellow isosurfaces represent regions of hole and electron accumulations, respectively. The isovalue is 0.0005 e/Bohr3. The

binding energies (Eb) of each nanostructure and the distances between the graphene layers and the adsorbates are presented in the side views. The grey, white, red, blue and yellow colored atoms represent the carbon, hydrogen, oxygen, nitrogen and

sulfur atoms, respectively.

All the physisorptions are exothermic processes, indicating that the trans/cis-AB molecules can be stably attached onto p-graphene. The up oriented trans/cis-isomers (Eb=-0.97/-0.75 eV) are energetically favorable than those (Eb=-0.82/-0.38 eV) of down oriented cases. For trans-AB in 'down' orientation, the adsorption distance is 4.59 Å, suggesting an obvious 16 ACS Paragon Plus Environment

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physisorption process. The adsorption distance of cis-AB in 'down' orientation is increased by 0.44 Å, which may be ascribed to the weakening of the π-π interaction in cis isomer. In the case of up orientation, the steric hindrance effects are reduced due to the rotation of the terminal group, resulting in a shorter adsorption distance (trans: 3.29 Å; cis: 3.32 Å). For the trans isomer in down orientation, the main charge transfer comes from the π-π stacking interaction between the trans-AB surface and the pristine substrate. The -N=Nunsaturated bridge bond creates a localized region of hole accumulation. In the case of cis-AB, due to the departure of one of the benzene rings from the substrate, the original π-π stacking interaction is damaged and thus the amount of localized charge density is decreased. Interestingly, the change of the adsorption orientation seems to affect the distribution of the charge density. Unlike the down oriented model, the main charge transfer of up oriented system occurs around the -NH- group region. The electron-donating C(O)-C2H5 substituent group rotates and makes the -NH- group exposed to the graphene substrate. One can find a localized region of hole accumulation around the -NH- group and the corresponding electron accumulation region on the graphene sheet. In order to examine the impacts of the different bridge linking units on the adsorption properties and electronic structures, another π-conjugated molecule, trans/cis-ST, is also introduced to be physisorbed onto p-graphene (Figure 5b). It is noted that the bridge bond in trans/cis-ST is -CH=CH-, different from -N=N- in trans/cis-AB. For trans- and cis-ST, due to the strong π-conjugation without any substituent group, the respective binding energies become larger and the adsorption distances get shorter. Although the binding (cis: Eb=-2.04 eV; trans: Eb=-1.78 eV) is stronger than that of trans/cis-AB, fewer charge transfer is found between ST molecule and graphene sheet. The 17 ACS Paragon Plus Environment


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different charge transfer feature may result in different energy storage property, which will be discussed in the following part. The above trans/cis-AB and trans/cis-ST molecules are expected to function as photoresponsive energy storage units. The energy can be stored in the chemical bonds of photoresponsive molecules through the trans-to-cis isomerization. In terms of ∆Ec-t, the capacity of energy storage can be estimated by using equation (2), with all the computational results of cis and trans isomers listed in Table S2 in the Supporting Information. As listed in Figure 5a, for down and up oriented AB systems, the storing energy, ∆Ec-t, is of 1.04 and 0.82 eV per photoresponsive molecule, respectively. To obtain such stored energy, an external trigger, like heat or light, is needed to overcome the thermal barrier so as to release a net energy. Grossman et al.52 predicted that the energy of 1.55 eV can be stored with 2,2',5'-trihydroxy diazobenzene molecules upon a photoinduced trans-to-cis isomerization based on the GGA-PBE calculations. This net result was a 260% increase in ∆E relative to gas phase AB molecule (0.59 eV). Another model of solar thermal fuels53 of high energy density was fabricated experimentally by templating photoswitchable AB derivatives on rigid low-mass carbon nanotubes, in which the energy stored per AB molecule was 1.25 eV. Recently, Feng et al.83 has designed a nano-template for solar thermal fuels with a high functionalization density composed of AB molecules with methosyl and/or carboxyl groups covalently attached on the surface of graphene nanosheets. This nano-template has been demonstrated to achieve a high experimental storage capacity of 112 W h kg-1 and a high theoretical storage energy of 2.16 eV based on PBE calculations.83 More recently, a chromophore/hybrid template with the bis-azobenzene molecules covalently bound to 18 ACS Paragon Plus Environment

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reduced graphene oxide has been demonstrated experimentally to have a high energy density of 80 W h kg-1 for solar thermal fuel, with a calculated dramatic enthalpy increase of 2.56 eV.84 The order of magnitude of our predicted energy storage is comparable to those obtained in the above theoretical and experimental works, suggesting an attractive energy storage capability of the multifunctional graphene model.

Figure 6. Energy storage of trans/cis-AB isomer noncovalently and covalently bound to p-graphene, respectively.

A question comes up that whether it is more effective for energy storage and conversion when the trans/cis-AB isomer are noncovalently adsorbed onto p-graphene. As shown in Figure 6, for the selected AB derivative, the energy release is of 1.04 and 0.62 eV per molecule when trans/cis-AB isomer is noncovalently and covalently bound to p-graphene, respectively. This unexpectedly indicates that the noncovalent adsorption of trans/cis-AB molecules onto p-graphene is more favorable to energy storage than that of covalently bound. Notably, from the point view of the experimental synthesis and fabrication, noncovalent adsorption of trans/cis-AB isomer onto the substrate may be more feasible than that of covalently bound for solar thermal fuel, suggesting a promising platform in designing solar energy storage devices. It must be mentioned that the reported energy storage of 1.04 eV per 19 ACS Paragon Plus Environment


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AB molecule during conformational change may be difficult to be fully achieved in experiment, since the calculated values were obtained for an idea nonconvalent adsorption with the orderly packed AB isormers lying flat onto the substrate. The present theoretical model has a low coverage of about 0.4 molecule/nm2, and the intermolecular interactions between two adjacent molecules are negligible in such a sparse molecular array. But when the coverage is increased, the molecules are forced to come closer to each other, some other factors, such as the transition in packing style and molecular orientations as well as the collective effect of the neighboring arrays, may add the complicity and difficulty in experimental control of energy storage in the non-convalently bound materials.

Figure 7. Electronic band structures and density of states (DOS) structures of (a) trans/cis-AB and (b) trans/cis-ST adsorbed

onto adsorbed onto p-graphene. The zoomed band structures (denoted by the orange circled area) are shown at both sides.

The Fermi level is set to zero and indicated by the black dashed line.


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To further understand the influence of the different adsorbate on the adsorption properties and energy storage, the electronic band structures and DOS of two adsorbed nanostructures are illustrated in Figure 7. Both trans- and cis-AB molecule adsorbed onto p-graphene can open a band gap of about 20 meV, as depicted Figure 7a. It is interesting to compare our results with other theoretical results,82 in which, by DFT calculations, the most stable configuration of benzene adsorbed onto graphene was reported to be the 'AB' site (hollow site in our work) without band gap opening and the bridge site configuration could induce a 19.7 meV band gap. Interestingly, as illustrated in Figure 7b, the band gap opening of trans/cis-ST attached onto graphene is also about 20 meV, indicating that the unsaturated bridge bond, -CH=CH- or -N=N- makes little contribution to band gap tailoring, while the π-π interfacial interactions between two conjugated benzene rings and the graphene surface play the main role in this band gap opening. Both trans/cis-AB and trans/cis-ST molecules adsorbed onto graphene modify the DOS weakly around the Fermi level. For the adsorptions of trans/cis-AB, there emerges a striking PDOS peak at about -1.4 eV and a substantial PDOS peak at about 0.9 eV (Figure 7a), owing to the electron accumulation as a result of the interfacial vdW interactions. Moreover, the adsorption of the cis-isomer molecule induces a remarkable PDOS peak at about -1.68 eV caused by charge transfer between the adsorbate and the pristine-graphene substrate. The same phenomenon also appeared in the cis SO3-AB-2CH3 molecule adsorbed onto pristine graphene at GGA-PBE level.42 The DOS distribution in Figure 7b shows that the total DOS is almost identical with the local PDOS of the substrate graphene. The trans-ST molecule doped the substrate graphene indistinctively, except two peaks at -1.2 and 1.5 eV. The physisorption 21 ACS Paragon Plus Environment


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discussed above seems not as efficient as S-doping on the electronic structure modulation for graphene, but it provides a promising approach to store energy for solar thermal fuels via attaching photoresponsive molecules. 3.3. Multifunctional Graphene with S-Doping and Physisorption. Finally, by combining S-substitution and physisorption of π-conjugated photoresponsive molecules, we design a multifunctional graphene model, as shown in Figure 8. It is very important to find that the band gap of such hybrid model exhibits an additive effect of each individual unit. Moreover, a certain amount of energy can be stored via the isomerization of the photoresponsive molecules. When the S doping concentration is of 0.4 atom/nm2, as depicted in Figure 8a, the values of Eb for trans- and cis-AB adsorped onto S-graphene are -0.58 and -0.34 eV, respectively. The adsorption distance for trans-AB is 4.57 Å, which is comparable to that on p-graphene (4.59 Å in Figure 5a). However, the adsorption of the cis-AB on the S-graphene presents great differences. The buckling distance δ of the S-graphene sheet is substantially increased to 1.76 Å (1.57 Å in Figure 2a), indicating the enhancement of sp3 hybridization for the S atom. Thus, the distance between the S and the N atoms is substantially decreased to 4.16 Å (5.03 Å in Figure 5a), leading to prominent charge transfer between the dopant S atom and the –N=N- group. Inspecting from the charge difference isosurfaces of trans/cis-AB adsorbed onto S-graphene, we can find that the electrons accumulate around the dopant S atom, which are transferred from the above trans/cis-AB molecules. The charge density is delocalized across the entire trans isomer conjugated surface. Interestingly, under the exposure to the solar light, about 0.84 eV energy per


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molecule can be stored in the chemical bonds of system via the conformational change from trans to cis AB isomer.

Figure 8. (a) Geometric structures, including top view and side view of the optimized atomic structures and the charge

difference isosurfaces; (b) Electronic band structures and density of states (DOS) structures of trans/cis-AB adsorbed onto

S-graphene. Cyan and light yellow isosurfaces represent regions of hole and electron accumulations, respectively. The isovalue is 0.0005 e/Bohr3. The binding energies (Eb) of each nanostructure and the distances between the graphene layers and the adsorbates are presented in the side views. The grey, white, red, blue and yellow colored atoms represent the carbon,

hydrogen, oxygen, nitrogen and sulfur atoms, respectively. The zoomed band structures (denoted by the orange circled area)

are shown at both sides. The Fermi level is set to zero and indicated by the black dashed line.

As illustrated in Figure 8b, a band gap of 0.27 eV is introduced by attaching trans/cis-AB molecules onto S-graphene, which reveals that bandgap opening of S-doping (0.25 eV) and 23 ACS Paragon Plus Environment


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physisorption (0.02 eV) follows the adductive rules. For the trans-isomer adsorbed nanostructure, except for the widening in the PDOS gap, the total PDOS near the Fermi level does not experience much change compared with the corresponding result of the S-graphene (Figure 8b). Two PDOS peaks observed at the 0.5 eV (0.65 eV) and -1.5 eV (-1.32 eV) are mainly contributed by the trans (cis) isomers. The charge transfer between the trans-AB and the substrate is more striking compared to the cis-isomer case. This trend was also shown in the previous study of SO3-AB-SO3 deposited onto pristine graphene at GGA-PBE level.42 As is well known that PBE functional always underestimates the band gap. To alleviate such problem, some exchange potential functionals, such as Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional85 and meta-GGA calculations of modified Becke-Johnson exchange potential (MBJ86,87), have been implemented for several systems. Unfortunately, for our multifunctional graphene models, HSE06 functional calculations are computationally too expensive. Generally, HSE06 functional could further increase the band gap by about 30%～ 50%. For trans/cis-AB molecules deposited onto S-graphene, the band gap may be corrected to be 0.29～0.41 eV. We also tested the MBJ calculations for the selected models. However, we failed to get the converged results. In general, the presence of the trans- and cis-isomers physisorbed on the graphene monolayer only influences the PDOS far away from the Fermi level. These results give a hint of designing a multifunctional material based on the orthogonality in electron feature of each function unit.

4. CONCLUSIONS


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We have investigated the electronic structures and energy storage/release of graphene through S-doping and physisorption of π-conjugated photoresponsive molecules by DFT calculations with vdW corrections. With the increase of the S doping concentration from 0.4 to 0.8 atom/nm2, the bandgap of graphene was opened to different degree, exhibiting the enhanced metallic characteristics with a direct-to-indirect transition. The physisorption of two kinds of π-conjugated photoresponsive molecules onto p-graphene could both broaden the band gap of about 20 meV, attributed to the π-π stacking interactions. Neither the bridge bond -N=N- nor -CH=CH- made significant contribution on band gap opening. More importantly, the energy of up to 1.04 eV per molecule can be stored during the conformational changes under the solar light. On the basis of the orthogonality of S-doping effect and the photoresponsive energy storage/release function, we designed the multifunctional graphene model for energy storage (∆E=0.84 eV/molecule) and bandgap opening (270 meV). Our designs of photoresponsive energy-storage materials may provide a new route for experimental fabrication of new layered materials with promising application in optoelectronics and sustainable energy storage in energy technologies.

ASSOCIATED CONTENT

Supporting Information

Figure S1 shows the optimized structures of smaller cluster models of p-graphene and S-graphene with SOMO/LUMO molecular orbitals obtained at B3LYP/6-311+G(d) level. The buckling distance δ and bond lengths of a1, a2 and a3 are in units of Angstrom, bond angel θ is in unit of degree. Table S1 shows the binding energies Eb (eV) of various adsorbed nanostructures. 25 ACS Paragon Plus Environment


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Table S2 shows the storing energies ∆Ec-t (eV) of various adsorbed nanostructures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21273102, 21290192, 21673111, 11347010, 11404037 and 11304022). We are grateful to the High Performance Computing Centre of Nanjing University for providing the IBM Blade cluster system. REFERENCES (1) Morozov, S. V.; Novoselov, K. S.; Katsnelson, M. I.; Schedin, F.; Elias, D. C.; Jaszczak, J. A.; Geim, A. K. Giant Intrinsic Carrier Mobilities in Graphene and Its Bilayer. Phys. Rev. Lett. 2008, 100, 016602. (2) Geim, A. K. Graphene: Status and Prospects. Science 2009, 324, 1530-1534. (3) Chen, J. H.; Jang, C.; Xiao, S.; Ishigami, S. M.; Fuhrer, M. S. Intrinsic and Extrinsic Performance Limits of Graphene Devices on SiO2. Nat. Nanotechnol. 2008, 3, 206-209. (4) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, P.; Choi, J. Y.; Hong, B. H. Large-Scale Pattern Growth of Graphene Films for Stretchable Transparent Electrodes. Nature 2009, 457, 706-710. 26 ACS Paragon Plus Environment

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TOC Graphic


Theoretical Designs of Photoresponsive Energy-Storage Materials

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