Atomic Charge Transfer Research Articles

Transition Metal and N Doping on AlP Monolayers for Bifunctional Oxygen Electrocatalysts: Density Functional Theory Study Assisted by Machine Learning Description.

It is vital to search for highly efficient bifunctional oxygen evolution/reduction reaction (OER/ORR) electrocatalysts for sustainable and renewable clean energy. Herein, we propose a single transition-metal (TM)-based defective AlP system to validate bifunctional oxygen electrocatalysis by using the density functional theory (DFT) method. We found that the catalytic activity is enhanced by substituting two P atoms with two N atoms in the Al vacancy of the TM-anchored AlP monolayer. Specifically, the overpotential of OER(ORR) in Co- and Ni-based defective AlP systems is found to be 0.38 (0.25 V) and 0.23 V (0.39 V), respectively, showing excellent bifunctional catalytic performance. The results are further presented by establishing the volcano plots and contour maps according to the scaling relation of the Gibbs free-energy change of *OH, *O, and *OOH intermediates. The d-band center and the product of the number of d-orbital electrons and electronegativity of the TM atom are the ideal descriptors for this system. To investigate the activity origin of the OER/ORR process, we performed the machine learning (ML) algorithm. The result indicates that the number of TM-d electrons (Ne), the radius of TM atoms (rd), and the charge transfer of TM atoms (Qe) are the three primary descriptors characterizing the adsorption behavior. Our results can provide a theoretical guidance for designing highly efficient bifunctional electrocatalysts and pave a way for the DFT-ML hybrid method in catalysis research.

Local Lattice Distortion in High-Entropy Carbide Ceramics

Using the ab initio calculations, we study the lattice distortion of HfNbTaTiVC5, HfNbTaTiZrC5 and MoNbTaTiVC5 high-entropy carbide (HEC) ceramics. Results indicate that the Bader atomic radius and charge transfer in HECs is very close to those from binary carbide. The degree of lattice distortion strongly depends on the alloying element. The Bader atomic radius can excellently describe the lattice distortion in HEC. Further, the corresponding atomic radius and formation enthalpy of binary carbides may be indicators to predict the single-phase HECs.

A computational study of a reduced dye and its O2 reduction: Implication on H2O2 production with dye-sensitized photocathodes

Anion radicals of dyes (dye−•), including four coumarin 343 conformers and a perylenemonoimide derivative, as well as their reactions with the O2 molecule, were investigated by density functional theory. All the dye−• species reacted with O2 to form H∙∙∙O hydrogen bonds via C–H groups, followed by rigorous analysis by the natural bond orbital method. A complete atomic charge and spin transfer occurred upon the reaction, verifying that the reduction of O2 by dye−• occurred via a one-electron-transfer process. The exergonic reaction free energies revealed that an intermediate [dyeO2]−• complex can be easily formed and spontaneously dissociated into the neutral dye and O2−• subsequently. Most of the [dyeO2]−• intermediates did not react further with H+ and HO2•, and although some [dyeO2]−• species reacted, no atomic charge or spin transfer occurred. These results suggest that the dyes did not participate in the subsequent reactions of O2−• with H+ and HO2•, such as disproportionation, leading to the efficient production of H2O2 as the final product through HO2• and HO2− species.

Atomic interfacial structure and charge transfer mechanism on in-situ formed BiOI/Bi2O2SO4 p–n heterojunctions with highly promoted photocatalysis

Developing novel and highly efficient Bi-based photocatalysts induced by visible light, and determining their interfacial mechanism represent two significant challenges in the environmental catalysis field. Herein, the preparation of bismuth oxysulfate, Bi2O2SO4, and in-situ couping with BiOI to form a novel BiOI/Bi2O2SO4 p–n heterojunction are described. The optimized BiOI/Bi2O2SO4 heterojunction exhibits remarkable photocatalytic NO oxidation under visible-light irradiation, with multifold higher activities than pristine BiOI or Bi2O2SO4. The BiOI/Bi2O2SO4 heterojunction enforces a directional charge transfer induced by the internal electric field, and substantially boosts the charge separation of photogenerated electron-hole pairs at the Bi2O2SO4 and BiOI interface. The electron-rich interfacial environment promotes reactant activation and reactive oxygen species generation, and prevents the formation of toxic NO-derived by-products, resulting in superior photocatalytic activity. Considered in its entirety, the current work introduces a promising Bi-based photocatalyst and provides guidance for the rational design of highly efficient heterojunction photocatalysts.

Twinning enhanced electrical conductivity and surface activity of nanostructured CuCrO2 gas sensor

Nanotwins stand at an essential frontier of condensed matter physics research, due to not only the optimized properties, but also the theoretic value from their unique coherent microstructure. They have been ever used to strengthen the metals or alloys, enhance the electrical conductivity, and boost the photocatalytic performance previously. However, the understanding of correlation between the twinned nanostructure and the charge transport in the functional metal oxide semiconductors, especially the gas sensitive nanomaterials, is still insufficient. In this paper, twinned CuCrO2 nanoparticles in gaseous chemiresistive property are comparatively examined to explore the attribution of twin boundaries to the charge transfer and activity of Cu atoms. This clarification of the structure-function relationship may pave a new way to the insightful acquirement of nanotwins in the application of gas detection.

Interface atom mobility and charge transfer effects on CuO and Cu2O formation on Cu3Pd(111) and Cu3Pt(111)

We bombarded mbox{text{Cu}_{3}text{Pd}(111)} and mbox{text{Cu}_{3}text{Pt}(111)} with a 2.3 eV hyperthermal oxygen molecular beam (HOMB) source, and characterized the corresponding (oxide) surfaces with synchrotron-radiation X-ray photoemission spectroscopy (SR-XPS). At 300,text{K}, CuO forms on both mbox{text{Cu}_{3}text{Pd}(111)} and mbox{text{Cu}_{3}text{Pt}(111)}. When we increase the surface temperature to 500,text{K}, mbox{text{Cu}_{2}text{O}} also forms on mbox{text{Cu}_{3}text{Pd}(111)}, but not on mbox{text{Cu}_{3}text{Pt}(111)}. For comparison, mbox{text{Cu}_{2}text{O}} forms even at 300,text{K} on Cu(111). On mbox{text{Cu}_{3}text{Au}(111)}, mbox{text{Cu}_{2}text{O}} forms only after 500,text{K}, and no oxides can be found at 300,text{K}. We ascribe this difference in Cu oxide formation to the mobility of the interfacial species (Cu/Pd/Pt) and charge transfer between the surface Cu oxides and subsurface species (Cu/Pd/Pt).

Effects of atomic substitutional doping on electronic structure of monolayer Janus WSeTe

Based on the first principles calculations, the effects of substitutional doping of nitrogen, halogen and 3d transition metal elements on the electronic structure of monolayer Janus transition metal dichalcogenides WSeTe are studied in this paper, where the VASP software package is used based on density functional theory to perform calculations through using both the projector augmented wave method and the GGA-PBE functional method. A monolayer WSeTe hexagonal crystal system with 4 × 4 supercells is established, which contains 48 atoms. When VA (VIIA) element substitutes for monolayer WSeTe, one of the Se atoms is replaced with a nitrogen (halogen) atom; when the 3d transition metal element substitutes for monolayer WSeTe, one of the W atoms is replaced with a transition metal atom. Through the analysis of band structure, charge transfer and magnetism, it is found that VA (VIIA) nonmetallic elements doped monolayer WSeTe due to the introduction of the hole (electronic) doped, makes the Fermi level shift downward (upward), thus transforming into a p(n) type semiconductor. The Ti and V element substitutional doped monolayer WSeTe will present semiconductor-metal transformation. A doping for each of Cr, Co, Mn, Fe element doesn’t lead semiconductor material properties to change, but the each of Co, Mn, Fe element doped monolayer WSeTe can create a band gap of less than 20 meV. The VIIA (VA) non-metallic element and 3d transition metal element doped monolayer WSeTe will not have a huge influence on the original geometric structure of the material. Due to the charge transfer and doped atoms on the top of the valence band hybridization phenomenon, the Rashba spin splitting intensity near the <i>Γ</i> point of the top valence band increases with the increase of the atomic number of the doped atoms in the same main group when VIIA and VA non-metallic elements are doped. Moreover, the increase in atomic number and charge transfer have a greater influence on the strength of Rashba spin-orbit coupling than the change in electronegativity. The 3d transition metal element substitution doped single-layer WSeTe has obvious spin polarization phenomenon, which produces valley polarization near the Fermi level and introduces magnetism. In particular, since Cr-doped WSeTe retains the original semiconductor properties of WSeTe and has a large energy valley polarization, it may have a wide range of applications, such as in the field of spintronic devices. The monolayer WSeTe doped separately with Cr, Mn and Fe element produces an impurity band with fully polarized spin electrons in the band gap. The results are of great significance in systematically understanding the properties of monolayer WSeTe doping model and can provide theoretical reference for designing the monolayer WSeTe based electronic devices.

Removal of azo dye from aqueous solution by a low-cost activated carbon prepared from coal: adsorption kinetics, isotherms study, and DFT simulation.

The high-risk organic pollutants produced by industries are of growing concern. The highly porous coal-based activated carbon (AC) having a specific surface area of 3452.8m2/g is used for the adsorption of azo dye from synthetic solution. The sorbent is characterized through BET, SEM, TEM, XRD, FT-IR, TGA, and zeta potential. The sorbent exhibits -18.7mV surface charge, which is high enough for making suspension. The maximum dye uptake of 333mg/g is observed in sorbent under acidic medium. The thermodynamics parameters like ∆G, ∆H, and ΔS were found to be -12.40kJmol-1, 39.66kJmol-1, and 174.55Jmol-1K-1 at 293K, respectively, revealing that the adsorption mechanism is spontaneous, endothermic, and feasible. The experimental data follows the Langmuir and D-R models. The adsorption follows pseudo 2nd-order kinetics. DFT investigation shows that the dye sorption onto AC in configuration No. 4 (CFG-4) is more effective, as this configuration has high ∆H (enthalpy change) and adsorption energy (Eads). This is confirmed by Mullikan atomic charge transfer phenomenon.

Regulating Charge Transfer of Lattice Oxygen in Single-Atom-Doped Titania for Hydrogen Evolution.

Single-atom catalysts have attracted much attention. Reported herein is that regulating charge transfer of lattice oxygen atoms in serial single-atom-doped titania enables tunable hydrogen evolution reaction (HER) activity. First-principles calculations disclose that the activity of lattice oxygen for the HER can be regularly promoted by substituting its nearest metal atom, and doping-induced charge transfer plays an essential role. Besides, the realm of the charge transfer of the active site can be enlarged to the second nearest atom by creating oxygen vacancies, resulting in further optimization for the HER. Various single-atom-doped titania nanosheets were fabricated to validate the proposed model. Taking advantage of the localized charge transfer to the lattice atom is demonstrated to be feasible for realizing precise regulation of the electronic structures and thus catalytic activity of the nanosheets.

Toward the design of efficient adsorbents for Hg2+ removal: Molecular and thermodynamic insights

AbstractA systematic DFT study was performed to evaluate the effect of oxygenated functional groups for Hg2+ adsorption in aqueous systems. This work includes several aspects usually neglected in many current works, namely, ground‐state multiplicity, solvation effects, establishment of thermodynamic parameters, atomic charge transfer, and modeling of infrared spectra. In addition, two carbonaceous models were studied to account for both the effect of the carbonaceous matrix and the oxygenated functional groups on the Hg2+ binding. Adsorption energies indicated that Hg2+ adsorption on the unsaturated model is favored in the following order: phenol > lactone > semiquinone > carboxyl, whereas for the saturated model, the Hg2+ adsorption energy decrease order is: carboxyl > semiquinone > lactone. Thermodynamic parameters confirmed that the adsorption process is spontaneous (unsaturated model), while the infrared spectra provided an insight at the atomic level about the experimentally reported bands. Our results contributed to a deeper understanding of the current experimental information on the effect of the surface functional groups on the Hg2+ adsorption over carbonaceous materials as different active sites can be present on oxygenated carbonaceous materials for metal adsorption. The results also create new ways to improve the performance of adsorption capability of mercury and other pollutants.

Alloying effect in silver-based dilute nanoalloy catalysts for oxygen reduction reactions

To address the sluggish kinetics of the electrochemical oxygen reduction reaction (ORR), we consider a family of silver-based dilute nanoalloys by alloying the Ag(1 1 1) with one 3d transition metal atom M (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) as ORR catalysts in alkaline media. The M1Ag(1 1 1) dilute nanoalloys exhibit higher stability when M is on the subsurface (2L-M1Ag(1 1 1)) than when M is on the surface (1L-M1Ag(1 1 1)) of the Ag(1 1 1) facet as observed from surface segregation and mixing energies, and this subsurface stability is attributed to the relative positive charge transfer of surface Ag atoms as revealed from atomic charge analysis. Thus, 2L-M1Ag(1 1 1) catalysts are further investigated for their electronic structure and ORR activities. Particularly, 2L-Cu1Ag(1 1 1), 2L-Ni1Ag(1 1 1) and 2L-Zn1Ag(1 1 1) dilute nanoalloys exhibit a free-atom-like d state, and 2L-Mn1Ag(1 1 1) and 2L-Cu1Ag(1 1 1) dilute nanoalloys exhibit an ORR overpotential of 0.459 and 0.468 V, respectively. For the first time, the ternary 2L-Cu1Mn1Ag(1 1 1) is theoretically predicted with an overpotential of 0.450 V. Motivated by this prediction, we prepare a ternary CuMnAg nanoalloy by using the pulse laser deposition method. These ternary CuMnAg and dealloyed DE-CuMnAg catalysts exhibit an ORR overpotential of 0.50 and 0.47 V that is close to the predicted overpotential and that of a commercial Pt/C catalyst.

The charge transfer of intercalated Li atoms around islands on Li-halide (F, Br, Cl) surface of SEIs: A first principles calculation

The solid electrolyte interface (SEI) not only play the role of surface protective film for lithium metal anodes in lithium-sulfur batteries but also use to suppress the growth of lithium dendrites during the charging process. The relationship between the morphology of the SEI and the deposition behavior of lithium atoms determines the effect of homogenization of the dendrites. The Li-halide (LiF, LiCl, and LiBr) solid electrolyte interface is comparatively investigates by comprehensive first-principles calculations on Li metal anodes. Surface adsorption, surface diffusion, and charge transfer of Li atoms during the deposition process on the Li-halide SEIs with aggregated Li–Ha2Li2 island configurations is systematically calculates to achieve the orientation of Li atoms from deposition. However, the Li atom has high migration energy regardless of the uniformity of the surface of LiF-SEI, which has high stability making the charge transfer of Li atoms less affected by the subneighbor and subsurface Ha atoms. The Li atom deposition behavior makes the charge transfer and lithium halide (LiHa) SEI potential tend to be evenly distributed, which in turn leads to uniform deposition of Li atoms.

Structural order and charge transfer in highly strained carbon nanobelts

We present a computational study of the atomic morphology, structural order and charge transfer properties of radially π-conjugated, closed-loop, and highly strained chiral carbon nanobelts (CNBs).

Electronic properties of graphene-ZnO interface: a density functional theory investigation

Electronic properties of graphene/ZnO interface have been theoretically investigated by applying first principles density functional theory calculations. This interface is demonstrated to have interesting electrical, optical and chemical properties and therefore, is employed in different applications. In our investigation the interface between graphene and different ZnO surfaces such as polar Zn-terminated and O-terminated surfaces as well as nonpolar surface are considered. Different interface properties such as equilibrium atomic structure, binding energy, charge transfer and band alignment are calculated for these interfaces. The calculated binding energies between graphene and different ZnO surfaces are within the range of van der Waals or physical adsorption. The results clearly reveal the essential role of oxygen density at the interface. The O- and Zn-terminated ZnO surfaces show the lowest and highest binding energies, respectively. The amount of charge transfer and the direction of interfacial dipole are also dominated by the number of oxygen atoms at the graphene/ZnO interface. Calculations for the interfacial band alignment reveal that a high/low density of oxygen atoms at the interface results in a Schottky/Ohmic contact. It is also shown that inducing oxygen vacancies at an oxygen rich interface leads to the lowering of the Schottky barrier.

Polarization properties of AlN (101̅0) and (112̅0) non-polar surfaces: maximally localized Wannier functions study

AlN (101̅0) and (112̅0) surfaces are usually considered to be non-polar facets. However, polarization may be induced in the surface due to the surface dangling bonds. In this work, using the average local dipole evaluated by maximally localized Wannier functions, we investigated polarization properties of these two surfaces layer-by-layer. It is found that a remarkable polarization which is more than eight times that of the bulk, is induced on these two surfaces. Moreover, we found that the polarization oscillates around the bulk value in the inner surface layers, and quickly approaches to the bulk value at a depth of four layers. Such polarization variation was further analyzed from the viewpoints of atomic rearrangement and charge transfer. Thus (101̅0) and (112̅0) surfaces are actually not completely non-polar facets, but rather non-polar bulk core combined with polar surface layers which would potentially affect the surface properties. The present study may provide a deeper understanding of the non-polar AlN surfaces.

Comparison of the oxygen reduction mechanisms in a GBCO–SDC-impregnated cathode and a GBCO cathode

In this paper, a GdBaCo2O5+δ–Ce0.8Sm0.2O1.9 (GBCO–SDC) composite cathode was prepared by the impregnation method with GdBaCo2O5+δ (GBCO) as the impregnating phase and Ce0.8Sm0.2O1.9 (SDC) as the cathode skeleton. X-ray diffraction (XRD) analysis showed a good chemical compatibility between GBCO and SDC, and there were no obvious impurities at high temperature. The particle size of GBCO was approximately 80 nm. The continuous GBCO phase was deposited on the surface of porous SDC backbones, which greatly improved the electrochemical activity of the cathode. The polarization resistance of GBCO–SDC-impregnated cathode was 0.50 Ω cm2 at 600 °C, which was only ~ 28.6% of that of pure GBCO cathode. The oxygen reduction reaction (ORR) at GBCO cathode mainly involved the adsorption–dissociation of oxygen molecules and charge-transfer process of oxygen atoms. The two ORR rate-controlling steps were the adsorption–dissociation process at 500–600 °C and the charge-transfer processes at 600–700 °C, respectively. The oxygen reduction reaction at GBCO–SDC-impregnated cathode mainly involved the diffusion of oxygen ions through the three phase boundaries and the charge transfer of oxygen atoms, in which the charge-transfer process was always the ORR rate-controlling step.

Approaches on the understanding of nanoporous carbon reactivity with polyatomic ions

Correlations among atomic structure, chemical reactivity, charge transfer and electronic structure properties were investigated in nanoporous carbon at different mass densities. The model structures have been generated by a heat-quench procedure using Density Functional Theory calculations and first-principles quantum molecular dynamics simulations. Reactive points seem to be highly dependent of the local structure of the material. The results showed that sponge-like nanoporous carbons are good candidates to graft polyoxometalates since they presented a stronger interaction with the carbon substrate. The charge transfer is from the nanoporous carbon to the polyoxometalate and a covalent interaction was observed for a density of 0.75 g/cm3. Our materials developed an electronic band around the Fermi level.

Interaction between dyes and SeCN−–(SeCN)2 redox mediator in dye-sensitized solar cells

The intermolecular interactions of selenocyanate anion (SeCN−) and selenocyanogen molecule ((SeCN)2) as a SeCN−–(SeCN)2 redox couple with a state-of-the-art TPA-TTAR-T-A organic dye (1) and a typical Ru-complex N3 dye (2) were investigated by means of density functional theory. The N atom of SeCN− mainly interacts with the oxidized 1 via the CH bond of the TPA donor part; however, there is no atomic charge or spin transfer. The Se atom of SeCN− interacts with the oxidized 2 via the terminal S atom of the isothiocyanate ligand with atomic charge and spin transfer of 60%. If a second SeCN− is considered for both dyes, the atomic charge and spin are transferred completely, forming the Se–Se bond equivalent of the isolated (SeCN)2 radical, NCSeSeCN−. The 1–(SeCN)2 complexes via the N atom of the dye cyano group at the anchor moiety have far more configurations compared to 1–I2. The 2–(SeCN)2 complexes through S⋯SeSe bonding are also much more numerous than 2–I2 with S⋯ II bonds. Based on the computational results, the regeneration mechanism and recombination reaction in dye-sensitized solar cells with SeCN−–(SeCN)2 electrolyte are discussed.

C, Si, Ge AND Sn SURFACE DOPING OF WURTZITE AlN NANOCRYSTALS: Ab INITIO STUDY USING WURTZOIDS STRUCTURES

Wurtzite nanostructures have been recently described using molecular building blocks called wurtzoids. These wurtzoids are utilized in the present work to describe aluminum mononitride (AlN) nanostructures including its surface doping with group IV elements i.e. C Si, Ge and Sn. Calculations are performed for bare, and hydrogen surface passivated wurtzoids. Results show that hydrogen-passivated (HP) AlN-wurtzoids have energy gaps that are very near to the experimental bulk AlN. Longitudinal optical (LO) vibrational frequencies are also very near to bulk experimental value with blue and red frequency shifting for bare, and hydrogen surface passivated wurtzoids. Doped AlN-wurtzoid2c with group IV elements show a reduction of the energy gap. The gap generally decreases as the atomic number of the dopant increases. The electronic and vibrational properties can be interpreted using the charge transfer between atoms. Minimum atomic charge transfer is for carbon atom doping that leads to a maximum reduction of the energy gap of bare and entirely hydrogen surface passivated wurtzoids. The doped carbon atom charge transfer also induces the highest increase in LO vibrational mode.

A first-principles study of the effects of Au dopants in HfO2-based RRAM

Metal dopants play an important role in the resistive switching mechanism of HfO2-based resistive random access memory (RRAM), which can improve the performance of the RRAM device. Although Cu and Gd ions as metal dopants were widely explored, Au dopant is little focused so far. So the effects of Au doping in defect-free m-HfO2 using the first-principles calculation were investigated in this paper, with the purpose of revealing the contribution of dopants in the forming process of conducting filaments (CFs). Based on the first principle, the statistical models are firstly built to simulate the system of the Au-doped m-HfO2 with and without an oxygen vacancy (Vo). Then the electronic properties, the charge transfer of atoms, and the interaction between Au dopants and Vos are calculated and analyzed. The results show that the formation energy of the Au-dopant at the interstitial site is 5.673eV less than that of the Au-dopant at the substitutional site, which means the interstitial site is found to be the more stable position. Moreover, the interstitial Au dopant induces gap states of Hf and O atoms into the band gap of m-HfO2. As a result, the conductivity of the Au-HfO2 and the Au-HfO2-Vo systems is enhanced substantially. The results of the DOS, the band structure and the partial charge density show that the intermediate energy level crosses over the Fermi level and the electrons could connect the Au-dopant with nearby Hf atoms. According to these results, it is found that the Au CFs can be generated in the process of doping. In addition, the Vos at different positions near the Au-dopant have diverse formation energies, which highly indicate that these Vos are involved in the forming of the CFs. It may be concluded that the Vos CFs and the Au-dopants can be formed and play a role in the process of the resistive switching.