Full length articleRedistribution of π and σ electrons in boron-doped graphene from DFT investigation
Introduction
Graphene, consisting of a single atomic layer of sp2-hybridized carbon atoms, has been widely explored due to its unique 2-D structure [1]. Novel physical and electronic properties associated with its 2-D structure make graphene one of the most promising materials to use in electronic devices, supercapacitors, batteries, solar cells, sensors, and catalysis [[2], [3], [4]]. In order to enhance the performance of graphene in these applications, many efforts have been focused on the modification of pristine graphene, one of the methods is boron or nitrogen doping since both elements are carbon's neighbors in the periodic table [[5], [6], [7], [8], [9], [10]].
As a counter electrode (CE) for dye-sensitized solar cells, boron doped graphene (BG) exhibits higher conversion efficiency (6.73%) than Pt CE (6.34%) due to stronger bonding strength between BG/substrate and the lower electric resistance between the BG electrode/electrolyte interface [11]. For adsorption of small molecules, graphene has an excellent catalytic performance for the removing of phenol, and the catalytic activity in phenol oxidation is improved after nitrogen and boron doping [12]. Density functional theory (DFT) study also shows that BG is a good candidate for photosynthesis by the investigation of the adsorption mechanism of CO2 and SO2 molecules on BG [13]. In the application of oxygen reduction reaction (ORR), BG displays outstanding electrocatalytic activity in alkaline electrolytes, which is similar to that of Pt catalysts. Compared with pristine graphene layer, the ORR in BG exhibits a one-step four-electron process rather than a two-step two-electron pathway, which leads to lower onset potential and larger reduction current [14]. The electron-deficient boron atom acts as the adsorption point for O2 molecular and promotes the splitting of OO bond. In recent years, graphene has also been widely used in the field of supercapacitors [15,16]. In these applications, the improvement of graphene conductivity is one of the goals pursued by researchers. It has been reported that a top-gated single-layer graphene transistor is able to reach electron- or hole-doping levels of up to 5 × 1013 cm−2 [17]. The electrochemical impedance measurement indicates that BG has a lower charge transfer resistance compared with the pristine graphene [18,19], a higher electrical conductivity of BG is favorable for enhancing ion and electron transport in supercapacitors. As an anode material in lithium-ion battery, boron doped graphene also performs well [[20], [21], [22], [23]]. The exceptional results are attributed to manifold effects of the boron doping on electrical conductivities, electrode/electrolyte wetting properties, defected sites, interlayer distances, thermal stability, thus making the doped graphene superior to the pristine graphene concerning its Li-ion absorption and diffusion properties [24,25].
Modification in physical properties can be traced back to the changing of electronic structures [[26], [27], [28]]. Experimental and theoretical studies on chemically doped graphene showed that the introduction of heteroatoms into graphene can modify the electronic band structure of graphene [[29], [30], [31], [32]], and consequently tune the catalytic or electrocatalytic activity, adsorption property and so on [[32], [33], [34]]. The improvement of adsorption ability by B、N doping is attributed to the freer flowing of electrons transferred from the graphene basal layer to the oxidant [35]. Especially for the system with BB bonds which exhibits extraordinary bandage to oxygen atom [36]. For the enhancement of ORR performance, theoretical calculations performed by Yang et al. indicate that there are two reasons [37]. First, boron has a lower electronegativity than carbon, and the positively polarized boron atoms attract the negatively polarized oxygen atoms, leading to chemisorption. Second, boron sites function as electron donors for oxygen reduction reaction, the graphitic π-electron density can be transferred to the free pz orbital of boron. Meanwhile, a similar opinion is also proposed in the explanation of the advancement of supercapacitors. Boron doping in carbon networks can facilitate charge transfer between neighboring carbon atoms and thus enhance the electrochemical performance of carbon-based materials. Therefore, BG based all-solid-state supercapacitors exhibit a high specific capacitance, better rate capability, and enhanced energy density or power density with respect to undoped graphene [15,16]. For lithium ion battery, DFT calculations revealed that Li atom shows much stronger binding on B-doped graphene which greatly enhances the adsorption and storage performance of Li [38,39]. Bonding character and binding energies between Li and graphene are tuned with the help of doped boron atom [40,41].
In brief, boron doped graphene shows good performance in electronic devices, energy storage, and catalysis. Since all these physical properties are closely associated with the changes in electronic structures, adequate attention should be paid to the redistribution of electrons by dopant-incorporation. Most investigations tend to attribute the changes of electronic properties to the redistribution of π electrons while neglecting the changes in σ electrons [5,29,30]. However, that is insufficient for understanding some micromechanisms in B-doped graphene. For instance, if we only consider the redistribution of π electrons, the direction of charge transfer (from B to C) in substituted B-doped graphene could not be explained reasonably because the direction should have been from C to B due to the one less electron of B [42]. Furthermore, how the components (π and σ states) of electronic structure are influenced by boron incorporation has not been fully illuminated. As far as we know, only a handful of researches have considered the rearrangement of σ electrons by chemical doping. Radovic et al. have pointed out that σ electron localization occurred on carbon atoms because of the higher electronegativity of carbon than boron [43]. The transfer of π electrons is restricted by the polarization of σ bonds [44]. Boldyrev et al. have found that only delocalized σ-interactions are existed in an all-boron graphene α-sheet [45,46]. In addition to this previous work, a more in-depth study should be performed to figure out the changes in electronic structure which help to establish a theoretical foundation for the promotion of physical properties. Based on these factors, we employ first principles theory to investigate the redistribution of π and σ electrons in B-doped graphene over a wide range of doping concentration.
Section snippets
Simulation method
All calculations are performed within the framework of density functional theory (DFT) formalism by using the Vienna ab initio simulation package (VASP) code [47]. Projected augmented wave (PAW) method is applied with Perdew-Burke-Ernzerhof (PBE) gradient corrected function to describe the exchange-correlation term [48,49]. The valence electrons are described by plane wave basis sets with energy cutoff of 680 eV to a more accurate calculation [50]. The conjugate gradient method is applied to
Redistribution of π states
The first step toward understanding the electronic variation is to understand the redistribution of π electrons under boron incorporation. A redistribution of π electrons is revealed from the perspective of both quantity and energy. Compared with the symmetrically distributed partial density of states (PDOS) of pristine graphene (Fig. 2), the direct reflection of the changes in electron number is the shift of Fermi energy to lower level, as shown in Fig. 3(a)–(c) which depicts the PDOS for
Conclusions
The effect of boron doping to the electronic properties of graphene has been studied by first-principles DFT investigation. It is found that the introduction of boron in graphene not only affects the redistribution of π electrons but also σ electrons. For π states, boron substitution changes the energy distribution of π electrons which homogeneously distributed on pristine graphene evolving into a distribution with energies gradually decreased from boron atom to further carbon atoms. The
Acknowledgments
This work is supported by the National Natural Science Foundation of China (Grant No. 21271114 and No. 91326203).
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