Doped Graphene Systems Research Articles

First-principles study of the structural and electronic properties of BN-ring doped graphene

Since advanced silicon-based device components are moderately chemically tunable, doped graphene has emerged as a promising candidate to replace this semiconducting material in flexible miniaturized electronic devices. Indeed, heteroatom codoping (i.e., with boron and/or nitrogen) is an appealing strategy to tune both its structural and electronic properties, possibly inducing a band gap in graphene. However, presently synthesized BN-doped carbon-based materials are randomly doped, leading their electronic properties not to be reproducible. Using first-principles techniques, the present study investigates the periodic doping of graphene with borazine-like rings in order to search for an entirely new class of BNC hybrid 2D materials exhibiting high stabilities and optimized band gaps for optoelectronic applications. Ab initio calculations show that BN-ring doped graphene displays cohesive energies comparable with benchmark ideal periodic BNC systems (such as ${\mathrm{BC}}_{3}$ and ${\mathrm{BC}}_{2}\mathrm{N}$) with a decreasing linear trend toward high concentrations of BN rings. Band gaps of BN-ring doped graphene systems are calculated using many-body perturbation techniques and are found to be sensitive to the doping pattern and to be considerably larger for high concentration of BN rings exhibiting the same orientation. These predictions suggest that BN-ring doped graphene materials could be interesting candidates for the next generation of optoelectronic devices and open new opportunities for their synthesis using chemical bottom-up approaches.

Identification of ORR activity of random graphene-based systems using the general descriptor and predictive model equation

Carbon based electrocatalysts are well known promising candidates for the oxygen reduction reaction (ORR), but the random approach to find the best catalyst using experimental method delayed the screening process and it leads to a huge cost with less proficiency. Using Quantum Mechanics followed by Machine Learning (QM/ML) approach, we can predict the best catalyst faster way and the origin of the cause can be identified for further development of carbon-based catalyst. Using the π electronic descriptor unveiled using density functional theory, we applied the analytical simple fit method and six different machine learning algorithms to develop a highly effective predictive model to estimate ΔGOH. Furthermore, structural relations of ZGNR and AGNR are demonstrated to estimate the Dπ(EF), R-Oπ, and ΔGOH of different widths of ribbon that reduces additional DFT calculations. By applying both SVR predictive model and structural relations, we predicted the ORR performance of 2500 sites of GNRs and listed a few most ideal active carbon sites with lower overpotential (η < 0.5V). To validate our study, we predicted the ORR performance of different sites in 0D, 1D, 2D doped graphene systems using SVR model and confirmed the values with the DFT computed results.

First-principle calculation of electronic structure and optical properties of (P, Ga, P–Ga) doped graphene

Abstract First-principle calculations are used to study the electronic structures, electronic and optical properties of pure, phosphorus-doped, aluminum-doped, and phosphorus and aluminum co-doped graphene. The gap between the conduction band and valence band of intrinsic graphene is zero. However, when graphene is doped with P, Ga, and P–Ga, the gap in energy will be opened and to a different extent. In the three different doping results, the gap values between the conduction and valence bands of P, Ga, and P–Ga graphene are 0.11, 0.35, and 0.39 eV, respectively. When graphene is doped with P–Ga, more electrons (1.40 e) will be gained by carbon atoms compared to P-doped graphene (0.61 e), while fewer electrons (1.49 e) will be transferred to carbon atoms compared to Ga-doped graphene (1.75 e). After doping with P, Ga, and P–Ga, the overall intensity of the graphene absorption peak is weakened and more pronounced relative to the low-frequency peaks. This result can fully demonstrate that the band gap of the doped graphene system can be better tuned by adding phosphorus and gallium atoms.

Understanding the sp magnetism in substitutional doped graphene

Defect-induced magnetism in graphene has been predicted theoretically and observed experimentally. However, there are open questions about the origin of the magnetic behavior when substitutional impurities with $sp$ electrons are considered. The aim of this work is to contribute to the understanding of impurity-induced spin magnetism in doped graphene systems. Thus, the electronic structure and spin magnetic moments for substitutional doped graphene with impurities from groups IIIA (B, Al, and Ga) and VA (N, P, As, Sb, and Bi) of the periodic table were obtained within the framework of density functional theory. The nature of the magnetic ground state was determined from calculations of the total energy as a function of the spin magnetic moment using the fixed spin moment method. We show that the spontaneous magnetization in the studied systems arises from an electronic instability by the presence of a narrow impurity band at the Fermi level. Furthermore, we found that the emergence of spin polarization requires the impurity to introduce an extra electron to the graphene lattice and that the impurity-carbon hybridization is close to the $sp^3$ geometry. These features reveal that the charge doping sign and the hybridization degree play a fundamental role in the origin of $sp$ magnetism in substitutional doped graphene.

Enhanced hydrogen gas adsorption properties of B, N, and Au co-doped graphene in o-, m-, and p-configurations: DFT study

The adsorption of hydrogen gas molecules on boron, nitrogen, gold co-doped graphene has been investigated for different configurations using Density Functional Theory. Co-doped graphene systems outperform single doped graphene systems due to combined influence of hetero-atoms. Different configurations studied in present study include gold doped graphene (Au-G); boron‐gold (BAu); nitrogen‐gold (NAu); and boron‐nitrogen‐gold (BN-Au) co-doped graphene. In case of BN-Au co-doped graphene, the aspect of relative sites of boron and nitrogen has also been determined. Despite the fact that all configurations were confirmed to have interaction with hydrogen molecule, relative sites of boron and nitrogen resulted in distinct responses to hydrogen adsorption. The sensing performance has been evaluated by calculating adsorption energies and changes observed in electronic properties following geometry optimization. The primary goal of this research is to better interpret structure, stability, and prospective capability of hydrogen molecule detection by diverse combinations of boron, nitrogen, gold co-doped graphene geometries.

Formaldehyde gas sensing properties of transition metal-doped graphene: a first-principles study

The formaldehyde gas sensing properties of transition metal-doped graphene have been systematically investigated by first-principles calculations. The optimized geometries of transition metal-doped graphene with and without HCHO adsorption, adsorption energies, charge transfers and magnetic moments are obtained for various doped graphene systems. The calculated results show that Co-, Ni-, Cu-, Zn-, Pd- and Ag-doped systems have moderate adsorption energies, implying their potential applications as HCHO sensors. The two-probe sensor devices consisting of the central scattering region and pristine graphene electrodes with the armchair or zigzag transport directions are built, and the spin-polarized current–voltage curves are calculated by the non-equilibrium Green’s function method within the framework of density functional theory. For the Cu-, Zn- and Ag-doped systems, the average sensor response is over 80%, 40% and 290%, respectively. In particular, the Cu- and Ag-doped graphene devices have the highest value of response over 200% and 900% at low voltages, indicating a short response time and high sensitivity of these devices. The present study provides a theoretical basis for exploring practical applications of the transition metal-functionalized graphene as superior HCHO gas sensors.

First-principle study on electronic and optical properties of (Al, P, Al-P) doped graphene

First-principle calculations are used to study the electronic structures, electronic and optical properties of pure, phosphorus-doped, aluminum-doped, and co-doped graphene with phosphorus and aluminum. The results show that the band gap of graphene can be effectively opened as the carbon atoms were replaced by P and Al atoms, and a large band gap of 0.45 eV could be opened when the P and Al atoms co-replaced carbon atoms. In addition, the band gap could be also reached 0.40 eV when an Al atom replaces a carbon atom. When P is doped into graphene, the band gap value is only 0.12 eV. For co-doped graphene with phosphorus and aluminum, the lost number of the electrons which are around the aluminum (2.10 e) is smaller than that of the Al-doping graphene (2.27 e). However, the phosphorus atom loses less electrons (0.43 e) than that in P-doping graphene (1.40 e). Furthermore, the optical parameters of pure graphene and several other doped graphene systems were studied, including refractive index, dielectric function and absorption coefficient have been also calculated. These crucial results confirm existence of P, Al and P–Al doping induced in graphene is an effective method in band gap engineering. Our research provides the theoretical foundation of using the chemical doping with heteroatoms approach to improve the electronic and optical performances of graphene, expected to benefit the practical application of graphene in tunable optoelectronic devices.

Catalyst-free solvothermal synthesis of ultrapure elemental N- and B-doped graphene for energy storage application

Even though atomically thin graphene has excellent electronic mobility, is flexible and promptly removes unwanted heat from device interfaces, the wonder material is not yet been able to be employed in energy sector due to unavailability of electrochemically active sites. Doping graphene not only is expected to generate electrochemically active sites, will enhance inter-layer distance as a consequence of doping (up to 6 Å). On-demand tunable high level of doping of graphene is thus desirable to enable it for energy sector applications. While Raman spectroscopy of doped graphene attained via solvothermal method, attests to chemical fingerprints of doped graphene systems, X-ray photoelectron spectroscopy establishes that doping of graphene (as high as ~19.8%) by nitrogen and boron has been attained. While pristine as well as B-doped graphene exhibited EDLC behaviour, N doped graphene exhibited excellent signatures of pseudo-capacitive energy storage behaviour with energy density of 170 Wh/kg and power density as high as 8.3 kW/kg. Solvothermal technique of doping graphene being single step, facile and economic; seems to have great potential for pseudo-capacitive energy storage applications where scalable production of doped graphene is needed.

Revisiting the anchoring behavior in lithium-sulfur batteries: many-body effect on the suppression of shuttle effect

We apply the state-of-the-art many-body dispersion (MBD) method to study the anchoring behavior in lithium-sulfur (Li–S) batteries, which is closely related to the notorious “shuttle effect”. Based on the experimental results of metal sulfides (FeS and SnS2), we find that the MBD method gives a more accurate prediction of anchoring mechanism compared with other van der Waals (vdW) inclusive methods. We systematically investigate the anchoring mechanism of two prototypal anchoring materials—Ti2CF2 and doped-graphene systems. The many-body effect is found to play an important role on the reduction of anchoring behaviors, especially when the systems have large polarization and the vdW interactions predominate the anchoring behavior. Our work deepens the fundamental understanding of the anchoring mechanism, and provides a more accurate criterion for screening anchoring materials for suppressing the shuttle effect.

First principles study on the stability and thermodynamic properties of N[sbnd]Au co-doped graphene

Based on first-principles density functional perturbation theory and first-principles molecular dynamics (MD) method, the effects of N and Au dopants as well as doping concentration on the structure stability, thermal stability and thermodynamic properties of graphene were systemically studied. It is found that the formation energy, phonon dispersion curve and MD simulation analysis indicate that N-doped graphene is more stable than Au and NAu co-doped graphene at the ground state or room temperature (300 K). The graphene doped with N, Au and NAu pair at different doping concentration can significantly affect its thermodynamic properties. Regardless of low or high concentrations, the specific heat and entropy of the doped graphene system increase as temperature increases, whereas free energy decreases. The results provide a theoretical basis for regulating the thermal conductivity of graphene and for relevant developing device applications.

Fundamental Properties of Transition-Metals-Adsorbed Graphene.

The revealing properties of transition metal (T)-doped graphene systems are investigated with the use of the first-principles method. The detailed calculations cover the bond length, position and height of adatoms, binding energy, atom-dominated band structure, adatom-induced free carrier density as well as energy gap, spin-density distributions, spatial charge distribution, and atom-, orbital- and spin-projected density-of-states (DOS). The magnetic configurations are clearly identified from the total magnetic moments, spin-split energy bands, spin-density distributions and spin-decomposed DOS. Moreover, the single- or multi-orbital hybridizations in T-C, T-T, and C-C bonds can be accurately deduced from the careful analyses of the above-mentioned physical quantities. They are responsible for the optimal geometric structure, the unusual electronic properties, as well as the diverse magnetic properties. All the doped systems are metals except for the low-concentration Ni-doped ones with semiconducting behavior. In contrast, ferromagnetism is exhibited in various Fe/Co-concentrations but only under high Ni-concentrations. Our theoretical predictions are compared with available experimental data, and potential applications are also discussed.

Adsorption of Bromonitromethane over Graphene‐Based Substrates: A Density Functional Theory Analysis

AbstractDisinfection by‐products (DBPs) are present ubiquitously in drinking water. These are the by‐products of the water disinfection process and are a matter of great concern, as they affect human health. In this work, the adsorption of a model DBP, bromonitromethane (BNM), on pristine graphene, graphene oxide and Ni/Pd/Pt‐doped graphene systems has been investigated using density functional theory (DFT) studies. Two adsorption modes have been considered, based on the interaction of BNM with graphene surfaces via the Br atom and the NO2 functional group. While adsorption of BNM on the pristine graphene is purely physisorption, it has been found that the adsorption capacity of pristine graphene and graphene oxide can be significantly enhanced by doping with Ni/Pd/Pt metals. Particularly, for Ni, the graphene oxide substrate gets involved in H‐bonding with the adsorbed BNM. As a result, effective and favorable adsorption occurs. Partial density of states (PDOS) plots reveal the effect of metal doping on the band gap and reactivity of different substrates. Electron density difference plots give insights into the adsorption behavior of BNM.

DFT study on the adsorption of p-nitrophenol over vacancy and Pt-doped graphene sheets

The frequent and widespread use of p-nitrophenol at commercial scale is giving rise to several health hazards among living beings and damage to the ecosystem. In this work, the adsorption of para-nitrophenol (p-NP) on pristine/vacancy-graphene and Pt4-cluster doped graphene systems are investigated using density functional theory (DFT). Two modes of adsorption have been considered: perpendicular and parallel. While adsorption of p-NP on the pristine graphene substrate is unfavorable, it is found that the adsorption capacity of graphene can be significantly increased by creating defects and by doping with Pt-clusters. Different electronic properties, like partial density of states (PDOS), reveal that on doping with a Pt-cluster, the band gap of the surface decreases and the substrates start showing magnetic character. Electron density difference plots give insights into the adsorption behavior of p-NP.

(Invited) Understanding ORR Reaction on Nitrogen Doped Carbon Materials: Insight from Experiments and Calculations

In this communication, we will present recent results on the oxygen reduction reaction (OOR) on nitrogen doped carbons. DFT calculations on nitrogen doped graphene systems will be used to understand the key elements that activate the reaction on these materials. The study is centered on the monodentate chemisorption of molecular oxygen as the first step of the process. It will be shown that carbon atoms are the active sites provided that several conditions are fulfilled. First, water should be included in the models, since it stabilizes the adsorption of the oxygen molecule with the characteristics of a superoxide species on the carbon atoms, which are the active sites. This carbon atom should be able to transition from a sp2 hybridization state to sp3 upon adsorption of the oxygen molecule to have a favorable interaction energy with the oxygen molecule. This transition is favored destabilized carbon atoms, that is, those neighboring a nitrogen dopant. Second, a significant charge transfer to this site should occur, so that charge can be concentrated on the adsorbed oxygen molecule. This charge transfer requires the presence of additional nitrogen dopants, which are charge donors, even when they are in distant positions. When the nitrogen dopant is located in an edge of the graphene structure, a clearly favorable chemisorbed state for molecular oxygen was found when nitrogen-dopant is hydrogenated and located at an armchair edge. The chemisorbed state is further favored by additional available charge from other nitrogen. By contrast, the chemisorbed state of oxygen is much less favorable when the hydrogenated pyridinic nitrogen-dopants are located at zigzag edges. To demonstrate the applicability of this approach, the ORR was experimentally studied on an aza-fused π-conjugated microporous polymer, which has a very well characterized structure. The experimental results using a rotating disk configuration indicate that hydrogen peroxide is almost exclusively produced at very low overpotentials on these materials. The DFT calculations indicate half of the nitrogen atoms of the material are hydrogenated at the potentials at which the reaction takes place. This hydrogenation process destabilizes some carbon atoms in the lattice and also provide a source of charge. On these carbon atoms, molecular oxygen could be chemisorbed with the aid of charge transferred from the hydrogenated nitrogen atoms and solvation effects, as in the previous cases. Due to the low destabilization of the carbon sites, the resulting molecular oxygen chemisorbed state is only slightly stable, promoting the formation of hydrogen peroxide.

Dopant-Modulating Mechanism of Lithium Adsorption and Diffusion at the Graphene/Li2S Interface

Modifying graphene is important for exploiting sulfides as excellent electrode materials in high-capacity Li-ion and Li-S batteries, but a physical, mechanistic picture is needed to interpret the enhancement of electrochemical properties induced by different dopants. The authors find Li storage with intercalation character in the Li${}_{2}$S---doped graphene system, and reveal two mechanisms to enhance interfacial lithium adsorption and diffusion, plus the synergistic effect of codoping. These calculations give a theoretical basis for materials engineering using doped graphene for advanced energy storage.

Dissociative Adsorption of Molecular Hydrogen on BN-Doped Graphene-Supported Aluminum Clusters

The present work, demonstrates dissociative adsorption of molecular hydrogen on supported and unsupported aluminium clusters (Aln, n=4 - 8, 13) using Density Functional Theory based calculations. The studies reveal that the presence of a BN doped graphene surface support reduces the dissociative adsorption barrier of the bond in molecular hydrogen on even atom clusters. In particular, supported Al6 demonstrates a barrier-less dissociative adsorption towards the H2 molecule. These results demonstrate the excellent potential of supported Al nanoparticles for hydrogen storage and also the potential of doped graphene systems are catalyzing supports.

First-principles identification of site dependent activity of graphene based electrocatalyst

Graphene based metal free electrocatalysts have been considered as potential candidate for efficient oxygen reduction reaction (ORR) in fuel cell systems. Using density functional theory we investigated the site dependent ORR activity of nitrogen doped and divacancy (DV_555-777) graphene structures considering free energy calculations and correlate with the occupancy of pz(π) electrons. The dioxygen adsorption strength differs for each carbon site in case of N doped and DV_555-777 graphene system. Range of overpotential and on-set potential has been estimated for same system. Among the sites and systems considered, the C2′ site of 2N doped graphene system is most active towards oxygen reduction with lowest overpotential. We have found that the DV_555-777 is more active than the pure system. In this case also we have observed a range of overpotential considering various C sites. We have been estimated the occupancy of pz(π) electrons of each C site of different systems. The occupancy of pz(π) electrons of C atoms increase near the dopant site due to back-donation mechanism in N doped graphene system. A strong correlation has been identified in-between activity of each C site with the occupancy of pz(π) electrons of corresponding site. Overall we concluded that each C site of N doped and defective graphene has different on-set potential and pz(π) electrons play a major role to define the activity.

Adsorption behavior of B‐doped/N‐doped graphene sheets toward NO2, NO and NH3 molecules: A first‐principles study

Based on the first‐principles of density‐functional theory (DFT), the effects of NO2, NO, and NH3 adsorption on the change in geometric stability, adsorption properties, and electronic structures of B‐ or N‐doped graphene are investigated. For NO/NO2‐B/N‐doped graphene systems, NO/NO2 have more stability on B‐doped graphenes than them on N‐doped graphenes. The introduction of B atom can make the adsorption of NO/NO2 on graphene much easier. The stable configuration of B‐doped graphene is more likely to be gas sensor for detecting toxic gases such as NO2 and NO. This makes it possible to use B‐doped graphene as sensing materials for designing novel gas sensors. The interaction between NH3 and B/N‐doped graphene is rather weak. The results will provide a new direction for the adsorbed NO2, NO, and NH3 on graphene by introducing B/N doping atoms into graphene.

Mono and dual doped monolayer graphene with aluminum, silicon, phosphorus and sulfur

Herein, we investigated the mono and dual doped graphene (DDG) systems that can be constructed when 2 carbon atoms are replaced by two 3p dopants: Al, Si, P or S. Although the latter elements have an atomic radius much larger than that of carbon, for most of the systems studied the dopants prefer to interact between themselves instead with the carbon atoms. The exception to this rule was graphene doped with two P atoms which favored a para disposition of the dopants. While in some cases like, SiSi, SS and SiP doped graphene the traditional ortho disposition of the dopants is preferred; for other systems like AlS or AlP DDG graphene, only one of the dopants fits in the graphene plane, while the other adopts a bridged structure. The most extreme case is PS DDG, for which the S atom is covalently bonded to P and no direct S-C bonding is observed. Analysis of the formation energies indicated that the use of two dopants is a useful procedure to reduce the formation energies of doped graphenes. HSE calculations showed that addition second dopant to the monodoped graphene systems does not increase the size of the band gaps. On the contrary, in all cases studied the gaps determined for the doped graphenes containing two dopants were smaller than those calculated when just one dopant is present. Moreover, in some cases the addition of a second dopant restored the semimetallic character of graphene. Thus, care must be taken when it is postulated that a gap is opened by substitutional doping, since the presence and magnitude of the gaps completely depends on the concentration as well as the position of the dopants.

Computational electrochemistry of doped graphene as electrocatalytic material in fuel cells

AbstractIn this work, we present an overview on how density functional theory calculations can be used to design novel electrocatalytic materials for fuel cells. In particular, we focus the attention on non‐metal doped graphene systems, which were reported to present excellent performances as electrocatalysts for the oxygen reduction reaction (ORR) at the cathodic electrode of fuel cells and are, thus, considered promising substitutes of platinum or platinum alloys electrodes. The methodology, originally proposed by Nørskov et al. (J. Phys. Chem. B 2004, 108, 17886) for electrochemical processes at metal electrodes, is revisited and applied specifically to doped graphene. Finite molecular models of graphene are found to perform as well as periodic models for localized properties or reactions. Therefore, the sophisticated molecular quantum mechanics methodologies can be safely used to compute reliable Gibbs free energies of reaction in an aqueous environment for the various steps of reduction (at the cathode) or of oxidation (at the anode). Details of the reaction mechanisms and accurate cell onset‐ or over‐potentials can be derived from the Gibbs free energy diagrams. The latter are computational quantities which can be directly compared to experimentally obtained cell overpotentials. Modeling electrocatalysis at fuel cells is, thus, an extremely powerful and convenient tool to improve our understanding of how fuel cells work and to design novel potentially active electrocatalytic materials. In this work, we present two specific applications of B‐doped graphene, as electrocatalysts for the ORR at a half‐cell cathode and for the methanol oxidation reaction (MOR) at a half‐cell anode.