•Y-N4 doped graphene is stable thermodynamically.•ORR is a four-electron process.•The most favorable pathway is the OOH hydrogenation to form OH + OH with energy barrier of 1.09 eV.The pyrolyzed transition metal and nitrogen-derived carbon are recently proposed as promising candidates in substituting Pt catalyst for oxygen reduction reaction (ORR) in the fuel cells. In this study, the active sites and reaction pathways for ORR on Y-N4 doped graphene are investigated theoretically. The ORR elementary reactions could take place within a small region around the Y-N4 moiety and its adjacent eight carbon atoms. ORR is a four electron process. The kinetically most favorable pathway is the OOH hydrogenation into OH + OH species, in which the formation of the second H2O is the rate-determining step with an energy barrier of 1.09 eV. The free energy change is discussed on different electrode potentials.

•The geometrical structures are hardly affected by substitution effect.•The absorption intensity is enhanced for mer-Ir(R-pmb)3 compared to fac-Ir(R-pmb)3.•The CN and Ph substitutions cause significant red shift on emissions.•The emissive nature is dominated by ligand with large structural change upon S0 → T1.A DFT/TDDFT investigation was carried out to understand the electronic structures and photophysical properties of a series of facial and meridional Ir(III) complexes fac-Ir(R-pmb)3 and mer-Ir(R-pmb)3 (R = H, CF3, CN, OCH3, tBu, Ph, respectively) bearing phenylbenzimidazolinato ligand. Substitution effects and fac-mer isomer factors were both investigated. The results herein show that the variation of substituent on the phenyl ring does not cause obvious effects on the metal-related bond lengths and absorption spectra of these complexes. However, the HOMO and LUMO energy levels are affected significantly by the substitution effects. Meanwhile, the most intense absorptions of mer-Ir(R-pmb)3 are found to be blue shifted (∼10 nm) compared with their fac isomers, and the absorption intensities of mer Ir(III) complexes are l enhanced. In addition, we also find that both the electron-donating and electron-withdrawing substituents lead to a red shift on the emissions of these fac and mer Ir(III) carbene complexes and the emissive natures are controlled by the chelate ligand with significant structural changes upon S0 → T1 excitation.

As one of the potential candidates for electrocatalysis, non-precious transition metal and nitrogen co-doped graphene has attracted extensive attention in recent years. A deep understanding of the oxygen reduction reaction (ORR) mechanism including the specific active sites and reaction pathways will contribute to the further enhancement of the catalytic activity. In this study, the reaction mechanism for ORR on Fe–N3 doped graphene (Fe–N3-Gra) is investigated theoretically. Our results show that Fe–N3-Gra is thermodynamically stable. The ORR elementary reactions take place within a small region around the Fe–N3 moiety and its adjacent six C atoms. HOOH does not exist on the catalyst surface, indicating a direct four-electron process for Fe–N3-Gra. The kinetically most favorable pathway is O2 hydrogenation, in which the formation of the second H2O is the rate-determining step with an energy barrier of 0.87 eV. This value is close to 0.80 eV for pure Pt, suggesting that Fe–N3-Gra could be a potential electrocatalyst. Free energy changes at different electrode potentials are also discussed.

Heteroatom doped graphene has caused particular interest in recent years due to its promising ORR (oxygen reduction reaction) activity in fuel cells. Sn doped divacancy graphene (Sn–Gra) was predicted to be a good candidate as a cathode catalyst in the previous study. In this work, the detailed ORR mechanism has been studied for Sn–Gra. The calculated charge transfer indicates that Sn and its adjacent four C atoms are the catalytic reaction sites. The unstable intermediate HOOH suggests that Sn–Gra experiences a four-electron ORR process. The most favorite pathway is the hydrogenation of the O2 molecule. The rate determining step is the hydrogenation of OOH to form H2O + O with the energy barrier of 0.75 eV. This value is slightly smaller than 0.80 eV for Pt, implying that Sn–Gra is a potential cathode catalyst for ORR. The predicted working potential is 0.16 V for the most favorite pathway. We expect that this study could provide new insights for the design of low-cost and highly efficient electrocatalysts in fuel cells.

Non-precious metal catalysts have attracted particular interest in recent years due to their promising ORR (oxygen reduction reaction) activity in fuel cells. In this work, the structural stability and ORR mechanism of CoN3 embedded graphene have been studied theoretically in acid media. The results indicate that CoN3 embedded graphene is stable thermodynamically. The kinetically most favorable reaction pathway for the ORR is a four-electron process. The process of OOH hydrogenation to generate O + H2O is the most favorable pathway. In the rate determining step, the energy barrier is 0.38 eV, much smaller than the theoretical value of ∼0.80 eV for pure Pt. The predicted working potential is 0.4 V for the most favorite pathway. Besides the lower energy barrier, the smaller Tafel slope compared with pure Pt in both low and high overpotential regions also suggests that CoN3 embedded graphene is a promising electrocatalyst for the ORR.

Heterojunction interfaces in perovskite solar cells play an important role in enhancing their photoelectric properties and stability. Till date, the precise lattice arrangement at TiO2/CH3NH3PbI3 heterojunction interfaces has not been investigated clearly. Here, we examined a TiO2/CH3NH3PbI3 interface and found that a heavy atomic layer exists in such interfaces, which is attributed to the vacancies of methylammonium (MA) cation groups. Further, first-principles calculation results suggested that an MA cation-deficient surface structure is beneficial for a strong heterogeneous binding between TiO2 and CH3NH3PbI3 to enhance the interface stability. Our research is helpful for further understanding the detailed interface atom arrangements and provides references for interfacial modification in perovskite solar cells.

The ethanol dissociation process on an iron cluster was estimated by nonequilibrium quantum chemical molecular dynamics simulations and static potential energy surface calculations based on the density-functional tight-binding potential. The competition among reaction pathways related to C–H, O–H, C–C, C–O cleavage, and H2 formation were studied. The schematic of ethanol as a carbon source to grow single-walled carbon nanotubes on iron clusters was also predicted. The simulations highlighted the C–O and O–H bond cleavage were more favorable on iron cluster than the other pathways due to the lower barrier and higher exothermicity and C2Hx (x = 4–6) were the major intermediates. The ethanol dissociation pathway on iron catalysts promised the two carbon atoms in ethanol were nearly equivalent and had similar contribution for the further single-walled carbon nanotube growth, consistent with the observation in previous experimental work.

Quantum chemical molecular dynamics simulations of graphene growth from small island precursors in different carbon nucleation densities on the Ni(111) surface at high temperatures have been conducted. The results indicate that small islands are not static, i.e. lateral diffusion and vertical fluctuation are frequently observed. In the case of low carbon nucleation density, carbon atoms or small carbon patches diffuse and attach to the edge of the nuclei to expand the size of the growing carbon network. The growth of graphene precursors is accompanied by the corresponding changes in the bonding of nickel atoms with the precipitation of subsurface carbon atoms. This is because the carbon–carbon interaction is stronger than the nickel–carbon interaction. In the case of high carbon nucleation densities, the dominant ripening mechanism depends on different growth stages. In the initial stage, the coalescence of carbon islands takes place via the Smoluchowski ripening mechanism. In the later stage the Smoluchowski ripening process is damped owing to the higher diffusion barrier of larger clusters and the restriction of movement by self-assembled nickel step edges. The cross-linking mechanism eventually takes over by the coalescence of extended polyyne chains between graphene islands. In either case, the Ostwald ripening process is not found in our molecular dynamics simulations due to the stability of carbon–carbon bonds within the islands. These investigations should be instructive to the control of graphene growth in experiments.

The reaction mechanisms for oxygen reduction reaction (ORR) on phosphorus doped divacancy graphene (P-GDV) are investigated by using the density functional theory method. Our results showed that all of the possible ORR elementary reactions could take place within a small region around the P atom and its adjacent four carbon atoms. The hydrogenation of O2 molecule which forms OOH and hydrogenation of OOH which forms H2O + O have negligible energy barrier. This reaction pathway is also the kinetically most favorable. The rate-determining step is the final step in the pathway, i.e., the hydrogenation of OH into H2O with an energy barrier of 0.85 eV. Therefore, ORR mechanism on P-GDV would be a four electron process. The free energy diagram of the ORR predicted that for the most favorable pathway, the working potential is 0.27 V. Consequently, our theoretical study suggests that P doped graphene with intrinsic carbon defects could possess good catalytic activity for ORR.

As one of the potential candidates of electrocatalysts, non-precious transition metal and nitrogen embedded graphene has attracted extensive attention in recent years. Deep understanding of the oxygen reduction reaction (ORR) mechanism including the specific active sites and reaction pathways will contribute to the further enhancement of their catalytic activity. In this work, density functional study is conducted on the ORR process of CoN2 and CoN4 embedded graphene in acid medium. The calculated formation energy shows that both CoN2 and CoN4 embedded graphenes are thermodynamically stable configurations. On the CoN4 site, the reaction pathway to form HOOH is the most favorable pathway. However, CoN4 site does not promote complete ORR and HOOH is the final product. Meanwhile, for CoN2 embedded graphene, the reaction pathway of HOOH dissociation is also the most favorable pathway and the energy barrier is 0.58 eV in the rate-determining step. This implies that CoN2 site serves as the second site for the complete ORR on the CoNx embedded graphene. Therefore, the HOOH formed on the CoN4 site can be dissociated on the CoN2 site, leading to a dual-site 2 × 2e− ORR mechanism. Finally, the effect of different electrode potentials on the ORR is discussed.

•The adsorption of poisoning species CO is weakened in Cu1@Pd3(111).•Cu1@Pd3(111) promote the catalytic activity for formic acid dissociation.•H accumulation, electron accumulation and potential accelerate the deactivation.•Cu1@Pd3(111) has better durability than pure Pd(111) under low anode potential.The bimetallic Cu1@Pd3(111) catalyst has been synthesized recently and exhibits better catalytic activity and durability compared with pure Pd(111) as anode catalyst in direct formic acid fuel cells (DFAFCs). In this work, we studied the reaction mechanism of formic acid dissociation on both Pd(111) and Cu1@Pd3(111) by using the density functional method. Our calculations showed that the surface adsorption of the poisoning species CO on Cu1@Pd3(111) is weakened mainly by the strain effect rather than the CuPd ligand effect. The Cu1@Pd3(111) can effectively promote the catalytic activity for formic acid dissociation by decreasing the barrier of CO2 formation from the preferential trans-COOH intermediate and increasing the barrier of CO formation from the reduction of CO2. We found that the H atom accumulation, electron accumulation and low electrode potential could accelerate the catalyst deactivation due to the contamination of the poisoning species CO. Furthermore, under low anode potential, the Cu1@Pd3(111) has better durability than pure Pd(111), which can be attributed to the unfavorable CO formation and the favorable CO desorption.

The electronic structures and photophysical properties of the recently reported Ir(III) cyclometallated complexes containing the 2-(1-phenoxy-4-phenyl)-5-methylpyridine ligand (1) were studied via density functional theory and time-dependent density functional theory calculations. To improve the performance and establish the structural–photophysical property relationships, a series of Ir(III) complexes 2–6 with –CH3/H and t-Bu substituents have been designed according to the experimental complex 1. The calculated results demonstrate that the different substituted ligands not only modify the absorption and emission bands, but also enhance the phosphorescent quantum efficiency. The t-Bu substituent increases the energy levels of the LUMO for complexes 4, 5 and 6 and broadens the HOMO–LUMO energy gaps. When compared with 1, the emission wavelengths for 3 and 6 are red-shifted considerably, while they are slightly blue-shifted for 4 and 5. In particular, the –CH3 and t-Bu substituents in 2, 4 and 5 lead to a relatively high quantum yield. Therefore, the designed complexes 2, 4 and 5 with –CH3/t-Bu substituents are expected to be promising phosphorescence emitters with high quantum efficiency.

The non-precious metal graphene catalyst doped with Fe–Px are recently proposed as a promising candidate in substituting Pt for catalyzing oxygen reduction reaction (ORR) in fuel cells. Systematic DFT calculations are performed to investigate the catalytic activity and the ORR mechanism on the Fe–Px (x = 1–4) system in acid medium in this work. Our results indicated that the configuration with one Fe and two P atoms codoped at zigzag edge site (Fe–P2–zig-G) is the most stable, in excellent agreement with the experimental observation that the ratio of Fe and P is nearly 1:2. The four-electron reduction mechanism for ORR on the Fe–P2–zig-G is via the competing OOH hydrogenation pathways (to form either OH + OH or O + H2O). The rate determining step is the O2 hydrogenation with an energy barrier of 0.43 eV, much smaller that of calculated 0.80 eV for pure Pt. In addition, the highest energy barrier of the studied ORR mechanism is the O2 dissociation with an energy barrier of 0.70 eV, a value also smaller than that of pure Pt. This demonstrated that the zigzag edge site of the Fe–P2 codoped graphene should be active for the ORR.

The particular physicochemical properties of nanomaterials are able to elicit unique biological responses. The property activity relationship is usually established for in-depth understanding of toxicity mechanisms and designing safer nanomaterials. In this study, the toxic role of specific crystallographic facets of a series of polyhedral lead sulfide (PbS) nanocrystals, including truncated octahedrons, cuboctahedrons, truncated cubes, and cubes, was investigated in human bronchial epithelial cells (BEAS-2B) and murine alveolar macrophages (RAW 264.7) cells. {100} facets were found capable of triggering facet-dependent cellular oxidative stress and heavy metal stress responses, such as glutathione depletion, lipid peroxidation, reactive oxygen species (ROS) production, heme oxygenase-1 (HO-1) and metallothionein (MT) expression, and mitochondrial dysfunction, while {111} facets remained inert under biological conditions. The {100}-facet-dependent toxicity was ascribed to {100}-facet-dependent lead dissolution, while the low lead dissolution of {111} facets was due to the strong protection afforded by poly(vinyl pyrrolidone) during synthesis. Based on this facet-toxicity relationship, a “safe-by-design” strategy was designed to prevent lead dissolution from {100} facets through the formation of atomically thin lead-chloride adlayers, resulting in safer polyhedral PbS nanocrystals.

The methanation of syngas (CO and H2) on the Ce-doped Ni(111) surface (Ce–Ni(111)) has been investigated by using the density functional method. The doped Ce enhances the adsorption energy of the intermediates on the catalytic surface, except for H2, particularly for O-containing species. On the Ce–Ni(111) surface, the reaction pathway CO + 3H2 → CHO + 5H → CH + O + 5H → CH4 + H2O is the most favorite, in which the energy barrier is 1.18 eV for the rate-determining step. Compared with the pure Ni(111) surface, the doping of Ce improves the catalytic activity both thermodynamically and kinetically. The microkinetic analysis also supports that the methanation of syngas has high reaction rate on the Ce–Ni(111) compared with the pure Ni(111). The temperature has great influence on the reaction rate, while H2/CO ratio shows only slightly impact. Our study also explains the experimental observation that the doped Ce can reduce the reaction temperature from ∼500 °C on the pure Ni(111) to ∼340 °C on the Ce–Ni(111) surface. The coverage of CHO is the largest on the Ce–Ni(111) surface. We expect that the obtained results could be useful for the future experimental study in searching the high efficient catalysts.

Toxicological responses of nanomaterials have been closely correlated to their physicochemical properties, and establishment of a property–activity relationship of nanomaterials is favorable for a deep understanding of the nanomaterials’ toxicity mechanism, prospectively predicting nanomaterials’ potential hazards and rationally designing safer nanomaterials. Faceted nanomaterials usually exhibit more versatile and effective performance than spherical nanomaterials due to their selectively exposed crystallographic facets with high densities of unsaturated atoms. These facets have high surface reactivity, capable of eliciting strong interactions with biological systems. Few studies paid attention to the toxic behaviors of faceted nanomaterials in terms of their distinctive facets. In the present study, the toxicological role of the crystallographic facets of TiO2 nanomaterials was investigated, and the precise property–activity relationship was exploited to clearly understand the toxicity of faceted nanomaterials. A series of faceted TiO2 nanocrystals with the morphology of truncated octahedral bipyramids were prepared to expose different percentages of {101} and {001} facets on the surface. Density functional theory calculation revealed that {101} facets could only molecularly absorb water molecules while {001} facets due to their surface-unsaturated Ti atoms could dissociate the absorbed water molecules to generate hydroxyl radicals. Biophysical assessments corroborated the increased production of hydroxyl radicals on the {001} facets compared to {101} facets, which endowed {001} facets with strong hemolytic activity and elicited severe toxicities. A series of increased oxidative stress toxicological responses, including cellular ROS production, heme oxygenase-1 expression, cellular GSH depletion, and mitochondrial dysfunctions, were triggered by faceted TiO2 nanocrystals with progressively increased {001} percentages, demonstrating the toxicological roles of {001} facets.Keywords: facets; oxidative stress; surface reactivity; TiO2 nanocrystals; toxicity

The electrocatalyst, nitrobenzene molecular doped graphene, for the oxygen reduction reaction (ORR) is investigated by the first-principles calculations. We find that zigzag edge (Z), doped armchair edge (NBA), and the opposite-side edge of doped zigzag nanoribbon (NBZ-2) are three active centers that contribute to the efficient catalytic performance. Our calculations suggest that such excellent electrocatalytic properties originate from the induced high asymmetry spin density and charge redistribution. The calculated onsite potentials are −0.13, −0.43, and −0.11 V for Z, NBA, and NBZ-2, which are close to the experimental values of −0.20 V on NBG and −0.24 V on graphene. We also find that the electrocatalytic activity and the tolerance of methanol depend on the doped configurations. Therefore, the carefully controllable synthesis is highly expected to further improve the ORR activity of nitrobenzene-doped graphene.

The mechanisms of the oxygen reduction reaction (ORR) on Pt/Cu(111) and Pt/Cu(100) have been investigated by using density functional theory. Compared with pure Pt(111) and Pt(100), the adsorptions of ORR intermediates are weakened on both Pt/Cu(111) and Pt/Cu(100) surfaces. The ORR follows the oxygen dissociation mechanism on Pt/Cu(100) which is the same as that on pure Pt(100). However, the ORR mechanism is the peroxyl dissociation mechanism on pure Pt(111), hydrogen peroxide dissociation on Pt/Cu(111). The rate determining step is OH + H+ + e− → H2O on Pt/Cu(100) and pure Pt(100), O + H+ + e− → OH on pure Pt (111) and OOH + H+ + e− → H2O2 on Pt/Cu(111). Compared with the energy barrier of the rate determining step on pure Pt(111) (0.86 eV) and Pt(100) (0.76 eV), the ORR reaction activity is improved on Pt/Cu(111) and hindered on Pt/Cu(100), with the barriers of 0.40 and 0.85 eV, respectively. For the effects of electric potential, OH protonation is favorable thermodynamically at a broad electrode potential (from 0 to 1.23 V) on the Pt/Cu(111) surface, in agreement with the high durability of Pt/Cu observed in experiments. The working potentials of Pt/Cu(111) and Pt/Cu(100) are predicted to be 0.39 and 0.73 V, respectively.

A DFT/TDDFT method was applied to explore the geometrical, electronic and photophysical properties of the recently reported oxazoline- and thiazoline-containing iridium(III) complexes [(ppy)2Ir(oz)] (1) and [(ppy)2Ir(thoz)] (2). The calculated absorption and emission wavelengths are in agreement with experimental data. Based on complexes 1 and 2, a series of Ir(III) complexes 3, 4, 5 and 6 with different N^O ligands have been designed. The calculated results reveal that the different ancillary ligands not only affect the absorption spectra properties but also tune the emission colour. Compared with 2, the higher quantum yield of the experimental observation for 1 was explained by its larger MLCT contribution and smaller singlet–triplet splitting energy (ΔES1–T1). From this point of view, the designed complexes 3, 5 and 6 are expected to be potential phosphorescence emitters in OLEDs with high quantum efficiency.

•Crystal structure of TaB3 is predicted using the evolutionary algorithm USPEX code.•The structural and mechanical properties of tantalum borides are investigated by DFT.•The stable phases are found by enthalpy-pressure relationship and convex hull.•oC16–TaB3 has a estimated hardness (41.2 GPa) and indentation strength (22.8 GPa).•High pressure is advantageous to syntheses of ruthenium Triborides oC16–TaB3.The phase stability, elastic, mechanical, dynamical and electronic properties of tantalum borides, i.e., Ta2B, TaB, Ta3B4, Ta5B6, TaB2 and TaB3, have been investigated by first-principles. The calculated convex hull indicates that at ambient conditions, the ground state phases are tI12–Ta2B, oC8–TaB, oC22–Ta5B6, oI14–Ta3B4, and hP3–TaB2; while at 75 GPa, they are tI12–Ta2B, oC8–TaB, oC22–Ta5B6, oI14–Ta3B4, hP3–TaB2 and oC16–TaB3; oC8–TaB, oC22–Ta5B6, oI14–Ta3B4, oC16–TaB3 are the most stable phases at 120 GPa. The enthalpy-pressure relationship reveals that the hP3–TaB2 is the most stable below 75 GPa, while the predicted oC16–TaB3 becomes the most stable above 75 GPa. Combining the estimated hardness (41.2 GPa) and indentation strength (22.8 GPa) for oC16–TaB3, it is suggested that oC16–TaB3 is hard or potential superhard. Since it is not available experimentally, further experimental synthesis could be rewarding.

•Na-doped Sr2CrOsO6 is investigated by using the density functional theory.•Both NaSr5Cr3Os3O18 and NaSrCrOsO6 are half metals.•The Curie temperature of NaSr5Cr3Os3O18 and NaSrCrOsO6 is higher than room temperature.The insulating Sr2CrOsO6 has the highest Curie temperature of 725 K among the double perovskites so far. In this study, by doping with Na, NaSr5Cr3Os3O18 and NaSrCrOsO6 are investigated by using the density functional theory. The calculated results indicated that the hole generated by Na goes to Os 5d t2g orbitals. This makes one of the insulating spin channels in Sr2CrOsO6 to be metallic in the Na-doped compounds. Thus, they become half metals. The estimated magnetic ordering temperature is 579 K for NaSr5Cr3Os3O18 and 615 K for NaSrCrOsO6, which are higher than the room temperature. Therefore, we expect that the Na-doped Sr2CrOsO6 would be promising candidates as spintronic material.

Due to their useful physical and chemical characteristics, transitional metal borides have attracted much attention. In this study, the structural, thermodynamical, mechanical, dynamical and electronic properties of 5d transitional metal triborides TMB3 (TM = Hf–Au) are investigated by density functional theory. For each triboride, five structures are considered, i.e., m-AlB2 (modified AlB2), OsB3, FeB3, WB3 and TcP3 structures. The calculated lattice parameters are in good agreement with previously theoretical results. Thermodynamic stability of compounds is predicted and the formation enthalpy increases from TaB3 to AuB3. The calculated phonon dispersion curves demonstrate that each TMB3 in the most stable structure is dynamically stable. The calculated density of states shows that they are all metallic. Among the studied compounds, OsB3-ReB3 (OsB3-ReB3 represents ReB3 in OsB3 structure, the same hereinafter) has the largest shear modulus (261 GPa), bulk modulus (355 GPa) and Young modulus (630 GPa). WB3-WB3 has the second largest shear modulus (257 GPa). This suggests that OsB3-ReB3 and WB3-WB3 might be potential ultra-incompressible and superhard materials.

The influence of hydrogen for CH4 dissociation on Cu(1 1 1) and Ni(1 1 1) surfaces has been investigated by using the density functional theory. The two possible reactions, i.e. H-abstraction reaction (CHx + H → CHx−1 + H2) and direct dehydrogenation reaction (CHx + H → CHx−1 + 2H), are studied. Our results show that H-abstraction reaction has higher energy barrier than direct dehydrogenation reaction on Cu(1 1 1), while for Ni(1 1 1), only the direct dehydrogenation reaction is observed. The microkinetic analysis supports that H-abstraction reaction is less competitive than the direct dehydrogenation reaction at broad coverage of H atom on Cu(1 1 1) surface. The major intermediate changes from CH to CH3 on Cu(1 1 1) and Ni(1 1 1) with the increase of H2 partial pressure. Furthermore, the behavior of free C atoms on both clean and H pre-adsorbed metal surfaces is discussed. The adsorbed H atom hinders the polymerization of the C atoms on Cu(1 1 1), resulting in sufficient time for C relaxed to the most stable site and further lead to a prefect graphene pattern formation, while H atom has little effect on such process for Ni(1 1 1).

La2VMnO6 is an insulating ferrimagnet experimentally. By substituting La with Sr, La2−xSrxVMnO6 (x = 0.5, 1.0, 1.5, 2.0) was investigated using the density functional theory. Our results indicated that half metallic properties are obtained for x = 0.5, 1.5, 2.0. For x = 1.0, it is insulating. With the increase of hole doping, the holes initially go to V 3d orbitals for x = 0.5, 1.0, and after that, the holes go to the Mn 3d orbitals for x = 1.5, 2.0. Ferrimagnetic coupling between V and Mn is found to be the ground state for x = 0.5, 1.5, while for x = 1.0 and 2.0, ferromagnetic and antiferromagnetic couplings between Mn and Mn are competitive for the ground state.

A DFT/TDDFT investigation was performed on the electronic structures, absorption and emission spectra, as well as the phosphorescence efficiency of [(ppy)2Ir(P^SiO)] (1) and the derivatives (1a, 1b, 1c and 1d) with CN-substitution at different positions in ppy ligands, as well as [(dfppy)2Ir(P^SiO)] (2) [where ppy = 2-phenylpyridine, dfppy = 2-(2,4-difluorophenyl)pyridine and (P^SiO) is an organosilanolate ancillary chelate]. The calculated results reveal that the introduction of CN leads to a significant red shift for 1a–1d in absorption spectra compared with that of 1. Moreover, the CN substitution at different positions on C^N ligands may be an efficient approach towards tuning emitting color. 1b, 1c, and 1d lead to a blue shift of emission spectra compared with 1, while an obvious red shift is observed for 1a. The high quantum yield of 1 (0.90) compared to 2 (0.59) is explained based on the S1–Tn splitting energies and energy gap between 3MLCT/π–π* and 3MC d–d states, and the evaluation of the radiative and nonradiative rate constants for all the complexes is also studied. The designed complexes 1a, 1c and 1d are expected to be potential phosphorescence emitters in OLEDs with high quantum efficiency.

A DFT/TDDFT investigation was carried out on a series of homoleptic triphenylamine-featured Ir(III) complexes 1a–1c [1a: (fac-tris[2-phenyl-4-(2-(N,N-diphenylamino)phenyl)pyridine]iridium(III)); 1b: (fac-tris[2-phenyl-4-(3-(N,N-diphenylamino)phenyl)pyridine]iridium(III)); 1c: (fac-tris[2-phenyl-4-(4-(N,N-diphenylamino)phenyl)pyridine]iridium(III))] with one triphenylamine unit in the 2-phenylpyridine (ppy) ligand and 2a–2c [2a: (fac-tris[2,4-bis(2-(N,N-diphenylamino)phenyl)pyridine]iridium(III)); 2b: (fac-tris[2,4-bis(3-(N,N-diphenylamino)phenyl)pyridine]iridium(III)); 2c: (fac-tris[2,4-bis(4-(N,N-diphenylamino)phenyl)pyridine]iridium(III))] with two triphenylamine units in the ppy ligand, respectively, aiming to gain insight into the influence of number and ligation position of triphenylamine units on the photophysical and electronic properties of the studied complexes. Complexes 2a–2c have been synthesized recently. For comparison, the parent complex Ir(ppy)3 was also investigated. The calculated results reveal that the introduction of the triphenylamine unit leads to enhanced charge-injection abilities and a balanced charge-transfer process compared with Ir(ppy)3. The different ligation positions of triphenylamine unit have an obvious effect on the absorption intensities for these complexes. The emissions of 1a–1c and 2a–2c undergo significant red shift with the introduced triphenylamine unit in ppy ligands compared with that of Ir(ppy)3, while the extent of red shift shows an apparent dependence on the number of triphenylamine units. The factors that might affect the quantum yield have been discussed.

Redox-switchable second-order nonlinear optical (NLO) responses of a series of ferrocene-tetrathiafulvalene (Fc–TTF) hybrids have been studied based on density functional theory calculations. The hyper-Rayleigh scattering (HRS) responses as well as the dynamic (λ = 1064 nm) HRS hyperpolarizabilities have been calculated in the gas phase within the T convention. The electron-correlation effects have been investigated. The long-range corrected LC-BLYP and wB97X-D functionals provide satisfactory results. The electron donor strength of the Fc–TTF in a donor-π-conjugated-acceptor structure has been assessed. The results indicate that the Fc unit does not play the role of the electron donor in the Fc–TTF unit. Because the Fc–TTF hybrid unit is a multistep redox center, the one- and two-electron-oxidized processes have been considered to control the second-order NLO responses. For a known Fc–TTF hybrid, one-electron-oxidization leads to a significant increase of the HRS hyperpolarizability, while the calculated HRS hyperpolarizabilities are not affected by the two-electron-oxidization according to our DFT calculations. Interestingly, in another system the two-electron-oxidization significantly enhances the HRS hyperpolarizability, and the one-electron-oxidization does not largely affect the HRS hyperpolarizability.

The electronic structure, absorption and emission spectra, as well as phosphorescence efficiency of (ppy)2Ir(PPh2^SiO) (1), (ppy)2Ir(P(CH3)2^SiO) (2), (ppy)2Ir(PH2^SiO) (3), and (dfppy)2Ir(PPh2^SiO) (4) [where ppy = 2-phenylpyridne, dfppy = 2-(2,4-difluorophenyl)pyridine and (PR2^SiO) is an organosilanolate ancillary chelate] were investigated by using density functional theory (DFT) and time-dependent DFT (TDDF) methods. The results revealed that the subtle differences in geometries and electronic structures result in different spectral properties and the quantum yields. Compared with 1, the substituent H in 3 leads to an obvious red shift in absorption spectra, while the substituent CH3 leads to a blue shift for 2 in the emission spectra. Moreover, the S1–T1 splitting energy (ΔES1–T1), the transition dipole moment (μS1, transition from S0 → S1) and the energy gap between the metal-to-ligand charge transfer 3MLCT/π–π* and metal-centered 3MC/d–d states (ΔEMC–MLCT) were also calculated. It was found that the designed complexes 2 and 3 have smaller ΔES1–T1, larger μS1 and ΔEMC–MLCT, which make them have higher quantum yield compared with the experimentally synthesized complexes. Therefore, they are expected to be the potential candidates as emitting materials with high quantum yield.

The electronic structures and photophysical properties of a series of heteroleptic Ir(III) carbene complexes (fpmi)3−xIr(N^N′)x (x = 1, 2) [fpmi = 1-(4-fluorophenyl)-3-methylimidazolin-2-ylidene-C,C2′, N^N′ = 2-(1H-pyrrol-2-yl)pyridinato (1a(x = 1)/1a′(x = 2)); 2-(1H-pyrazol-5-yl)pyridinato (2a/2a′); 2-(1H-1,2,4-triazol-5-yl)pyridinato (3a/3a′); 2-(1H-tetrazol-5-yl)pyridinato (4a/4a′)] with different azole-pyridine-based N^N′ ligands as ancillary and main chelate, respectively, are investigated using the density functional method. It is found that, with the systematic addition of N substitution in N^N′ ligands, the HOMO–LUMO energy gap follows a decreasing order from 1a to 4a with ancillary N^N′ ligands, but increasing order for 1a′–4a′ with main N^N′ ligands. Besides, the emission spectra are blue shifted with the addition of N substitution and the emissive state of all of the studied complexes is predominantly controlled by the N^N′ ligand, regardless of being ancillary or the main chelate. Furthermore, the evaluation of the radiative (kr) and nonradiative (knr) rate constants for all of the complexes are also investigated based on the calculated results. It's found that the quantum yield (ΦPL) shows an apparent dependence on the selection of different N^N′ ligands and the N^N′ ligands as main chelate present a small kr and large knr, which is not beneficial for efficient materials. While the pyrazole- and 1,2,4-triazole-pyridine-based N^N′ ligands as ancillary chelate is believed to be more favorable for highly efficient phosphorescence emitters in OLEDs.

•The structural and mechanical properties of ruthenium borides are investigated by DFT.•The stable phases are found by enthalpy–pressure relationship and convex hull.•The driving force of the good elastic constant is covalent bond between Ru and B.•High pressure is advantageous to syntheses of ruthenium borides.The phase stability and mechanical properties of ruthenium borides Ru7B3, RuB, Ru2B3, RuB2, RuB3 and RuB4 have been investigated systemically by first-principles calculations within density functional theory. The results show that ReB2–RuB2 (ReB2–RuB2 represents RuB2 in ReB2-type structure, the same hereinafter), TcP3–RuB3 and MoB4–RuB4 are more thermodynamically and mechanically stable than other structures at ambient conditions. The large bulk modulus, large shear modulus, large Young’s modulus, small Poisson’s ratio and large Debye temperature show that they should be ultra-incompressible hard material. Further analyses on density of states unravel the covalent bonding (B–B and Ru–B) as the force of the good mechanical properties. Combing enthalpy–pressure relationship with convex hull, it is interesting to note that the synthesized Ru7B3–Ru7B3, Ru2B3–Ru2B3 and hypothetical WB–RuB, ReB2–RuB2 should be the ground state phases at zero pressure, while around 60 GPa the predicted MoB4–RuB4 becomes a stable phase; Ru7B3–Ru7B3, WB–RuB, ReB2–RuB2 and MoB4–RuB4 are the most stable phases at about 100 GPa. High pressure is advantageous to synthesis of ruthenium borides.

•The diphenylamine unit is beneficial for hole injection and charge-transfer balance.•A remarkable color tuning is realized by altering the position of diphenylamine unit.•The diphenylamine unit has different influences on the emissions of 1a–1c and 2a–2c.The electronic structures and photophysical properties of a series of homoleptic Ir(III) complexes with triphenylamine-featured thiazole (1: fac-tris(2-phenylthiazole)iridium(III)) and benzothiazole (2: fac-tris[6-(1-naphthalenylphenylamino)-2-phenylbenzothiazole]iridium(III)) based ligands are investigated using the density functional method. By introducing the diphenylamine unit at 2-, 3- and 4-position of the phenyl ring in (C^N) ligands, 1a–1c (1a: fac-tris[2-(2-(N,N-diphenylamino)phenyl)thiazole]iridium(III); 1b: fac-tris[2-(3-(N,N-diphenylamino)phenyl)thiazole]iridium(III); 1c: fac-tris[2-(4-(N,N-diphenylamino)phenyl)thiazole]iridium(III)) and 2a–2c (2a: fac-tris[6-(1-naphthalenylphenylamino)-2-(2-(N,N-diphenylamino)phenyl)benzothiazole]iridium(III); 2b: fac-tris[6-(1-naphthalenylphenylamino)-2-(3-(N,N-diphenylamino)phenyl)benzothiazole]iridium(III); 2c: fac-tris[6-(1-naphthalenylphenylamino)-2-(4-(N,N-diphenylamino)phenyl)benzothiazole]iridium(III)) are studied to get insight into the influence of different ligation positions of diphenylamine unit on the photophysical properties. The calculated results showed that the incorporation of diphenylamine unit into (C^N) ligands is beneficial to raise the HOMO levels, reducing the barrier height for hole injection and enhancing the balance of charge-transfer process. In addition, a remarkable color tuning could also be realized by altering the ligation positions of the diphenylamine units. A 82–132 nm blue shift is found for the thiazole-based 1a–1c compared with 1, while a relatively small red shift (7–35 nm) is observed on the emissions of benzothiazole-based 2a and 2c compared with 2. Meanwhile, a qualitative analysis on the parameters that would affect the quantum yields (ΦPL) of the studied complexes has also been presented.

•The CN substituent is favorable for enhancing absorption intensity.•Changing position of CN substituent is an efficient way to tuning emitting color.•The ΦPL is increased with the introduction of CN substituent in carbene ligand.•The one-CN-substituented 2 and 3 might be efficient blue-emitting materials.The electronic structures and photophysical properties of four heteroleptic Ir(III) complexes 1 [(Mepmi)2Ir(pypz)], 2 [(CNpmi)(Mepmi)Ir(pypz)], 3 [(Mepmi)(CNpmi)Ir(pypz)] and 4 [(CNpmi)2Ir(pypz)] (where Mepmi = 1-(4-methyl-phenyl)-3-methylimdazolin-2-ylidene-C,C2′; CNpmi = 1-(4-cyano-phenyl)-3-methylimdazolin-2-ylidene-C,C2′; pypz = 2-(1H-pyrazol-5-yl)pyridinato) are investigated using the density functional method. The influence of different positions and numbers of CN-substitution has been explored. The calculated results reveal that the introduction of CN substituent leads to lowered HOMO and LUMO energy levels, increased HOMO–LUMO energy gaps, and the enhanced intensity of the absorption spectra. Meanwhile, the CN-substitution in carbene ligand is found to be an efficient approach of tuning the emitting color. The one CN substituent in 2 leads to an obvious blue shift of emission spectra, while a slight red shift is observed for the 3 and 4 in comparison with 1. The qualitative analysis on the quantum yields of four complexes has been carried out with the assistance of ΔES1−T1 (singlet–triplet energy splitting), MLCT% (metal-based charge transfer contribution in T1 state) and μS1μS1 (S0 → S1 transition electric dipole moment). It showed that the designed complex 2 and 3 with one CN substituent in carbene ligand may be the potential blue-emitting materials and possess high quantum efficiency.

The mechanism and kinetics of graphene formation from amorphous nickel carbides have been investigated employing quantum chemical molecular dynamics (QM/MD) simulations. Amorphous Ni3C, Ni2C, and NiC were employed to elucidate the role of the subsurface carbon density (ρC) on graphene formation. In each case, the nickel carbide phase underwent rapid carbon precipitation, resulting in a segregated nickel–carbon structure. The kinetics of graphene formation was most favorable for high carbon densities. At low ρC, i.e., Ni3C and Ni2C, there was a tendency for the formation of a number of small carbon fragments that failed to coalesce due to their inability to diffuse over the nickel surface. Graphene formation was only observed in the presence of high carbon densities that were relatively localized. These simulations, therefore, suggest that graphene nucleation is not immediately related to the presence of catalyst carbide phases.

The possible C2Hy (y = 2–6) formation reactions (CHx + CHz → C2Hy (y = x + z)) and activated second-order CHx+1 + CHz–1 reactions (CHx + CHz → CHx+1 + CHz–1) during CH4 dissociation on Cu(100) surface have been investigated by using the density functional theory. Our results show that C2Hy (y = 2, 4) formation reactions are favorable both kinetically and thermodynamically, compared with the direct dehydrogenation of CH4 (CHx → CHx–1 + H) and second-order CHx+1 + CHz–1 reactions. The second-order CHx+1 + CHz–1 reactions are less competitive compared with the direct dehydrogenation of CHx. Both DFT calculations and microkinetic model demonstrate that the reaction CH + CH → C2H2 is a major channel to produce C2Hy at a temperature of 860 °C, followed by CH3 + CH → C2H4. When the H2 influence is introduced, the major intermediate changes from CH to CH3 on Cu(100) surface with the increase of H2 partial pressure, while the coverage difference between CH and CH3 is not significant. This means that both species will have a large influence on the graphene growth mechanism.

The heteroatom-doped fullerene is systematically studied as an electrocatalyst for oxygen reduction reaction (ORR) on the cathode of fuel cells under the alkaline and acid conditions. Nitrogen-doped C60, i.e. C59N, can facilitate the ORR process and it is a promising candidate for efficient ORR electrocatalysts. The intrinsic C60 also possesses a high catalytic activity, which is different from graphene and is probably attributed to the curvature or pentagon defect of fullerene. We deduce that the curvature and pentagon defect are important factors to tune the catalytic ability. The fitted activity volcano diagram shows that the best achievable ORR activity of the heteroatom-doped fullerene, in the thermodynamic viewpoint, appears at ΔG*OH = 0.25 eV, which could be realized by dual or three-element-doping of C60 cage. The estimated standard equilibrium potential values are −0.52 and 0.25 V in alkaline and acid medium, respectively. Since the catalytic activity of the doped fullerenes depends on their IP and Egap, here we provide a simple and easy method, by measuring the IP and Egap, to roughly evaluate the catalytic ability of designed catalysts. This is vital to guide the design of new ORR catalysts with high catalytic activity and durability.

Designing the low cost, long durability and high efficient substitutes for platinum (Pt) electrocatalyst to facilitate ORR is significant for the large-scale commercial application of fuel cells. In this work, single Pd atoms supported on graphitic carbon nitride (i.e., Pd/g-C3N4) with different Pd coverages acting as electrocatalyst for ORR is investigated by using the density functional theory calculations. Our study shows that the doping of Pd atoms can effectively decompose H2O2, leading to a four-electron mechanism via the sequential hydrogenation of *O2 giving *O+*H2O on Pdx/g-C3N4 (x = 1–4). With the increase of Pd coverages, the energy barriers increase in the rate determining step, which are 0.39 eV, 0.63 eV, 0.68 eV and 2.61 eV for Pdx/g-C3N4 (x = 1–4), respectively. This implies that Pdx/g-C3N4 (x = 1–3) have lower energy barrier than Pt (calculated value 0.80 eV), showing high ORR activity compared with Pt. The oxidized Pd2/g-C3N4 (i.e., Pd2/g-C3N4−O) shows similar ORR activity to Pd2/g-C3N4, but with different rate determining step. The working potentials are also discussed for the studied catalysts. Especially, the working potential for Pd2/g-C3N4 is up to 0.60 V, comparable to calculated value 0.73 V for Pt/Cu(100).

The electronic structures and photophysical properties of the recently reported Ir(III) cyclometallated complexes containing the 2-(1-phenoxy-4-phenyl)-5-methylpyridine ligand (1) were studied via density functional theory and time-dependent density functional theory calculations. To improve the performance and establish the structural–photophysical property relationships, a series of Ir(III) complexes 2–6 with –CH3/H and t-Bu substituents have been designed according to the experimental complex 1. The calculated results demonstrate that the different substituted ligands not only modify the absorption and emission bands, but also enhance the phosphorescent quantum efficiency. The t-Bu substituent increases the energy levels of the LUMO for complexes 4, 5 and 6 and broadens the HOMO–LUMO energy gaps. When compared with 1, the emission wavelengths for 3 and 6 are red-shifted considerably, while they are slightly blue-shifted for 4 and 5. In particular, the –CH3 and t-Bu substituents in 2, 4 and 5 lead to a relatively high quantum yield. Therefore, the designed complexes 2, 4 and 5 with –CH3/t-Bu substituents are expected to be promising phosphorescence emitters with high quantum efficiency.

A DFT/TDDFT investigation was carried out on a series of homoleptic triphenylamine-featured Ir(III) complexes 1a–1c [1a: (fac-tris[2-phenyl-4-(2-(N,N-diphenylamino)phenyl)pyridine]iridium(III)); 1b: (fac-tris[2-phenyl-4-(3-(N,N-diphenylamino)phenyl)pyridine]iridium(III)); 1c: (fac-tris[2-phenyl-4-(4-(N,N-diphenylamino)phenyl)pyridine]iridium(III))] with one triphenylamine unit in the 2-phenylpyridine (ppy) ligand and 2a–2c [2a: (fac-tris[2,4-bis(2-(N,N-diphenylamino)phenyl)pyridine]iridium(III)); 2b: (fac-tris[2,4-bis(3-(N,N-diphenylamino)phenyl)pyridine]iridium(III)); 2c: (fac-tris[2,4-bis(4-(N,N-diphenylamino)phenyl)pyridine]iridium(III))] with two triphenylamine units in the ppy ligand, respectively, aiming to gain insight into the influence of number and ligation position of triphenylamine units on the photophysical and electronic properties of the studied complexes. Complexes 2a–2c have been synthesized recently. For comparison, the parent complex Ir(ppy)3 was also investigated. The calculated results reveal that the introduction of the triphenylamine unit leads to enhanced charge-injection abilities and a balanced charge-transfer process compared with Ir(ppy)3. The different ligation positions of triphenylamine unit have an obvious effect on the absorption intensities for these complexes. The emissions of 1a–1c and 2a–2c undergo significant red shift with the introduced triphenylamine unit in ppy ligands compared with that of Ir(ppy)3, while the extent of red shift shows an apparent dependence on the number of triphenylamine units. The factors that might affect the quantum yield have been discussed.

The mechanisms of the oxygen reduction reaction (ORR) on Pt/Cu(111) and Pt/Cu(100) have been investigated by using density functional theory. Compared with pure Pt(111) and Pt(100), the adsorptions of ORR intermediates are weakened on both Pt/Cu(111) and Pt/Cu(100) surfaces. The ORR follows the oxygen dissociation mechanism on Pt/Cu(100) which is the same as that on pure Pt(100). However, the ORR mechanism is the peroxyl dissociation mechanism on pure Pt(111), hydrogen peroxide dissociation on Pt/Cu(111). The rate determining step is OH + H+ + e− → H2O on Pt/Cu(100) and pure Pt(100), O + H+ + e− → OH on pure Pt (111) and OOH + H+ + e− → H2O2 on Pt/Cu(111). Compared with the energy barrier of the rate determining step on pure Pt(111) (0.86 eV) and Pt(100) (0.76 eV), the ORR reaction activity is improved on Pt/Cu(111) and hindered on Pt/Cu(100), with the barriers of 0.40 and 0.85 eV, respectively. For the effects of electric potential, OH protonation is favorable thermodynamically at a broad electrode potential (from 0 to 1.23 V) on the Pt/Cu(111) surface, in agreement with the high durability of Pt/Cu observed in experiments. The working potentials of Pt/Cu(111) and Pt/Cu(100) are predicted to be 0.39 and 0.73 V, respectively.

La2VMnO6 is an insulating ferrimagnet experimentally. By substituting La with Sr, La2−xSrxVMnO6 (x = 0.5, 1.0, 1.5, 2.0) was investigated using the density functional theory. Our results indicated that half metallic properties are obtained for x = 0.5, 1.5, 2.0. For x = 1.0, it is insulating. With the increase of hole doping, the holes initially go to V 3d orbitals for x = 0.5, 1.0, and after that, the holes go to the Mn 3d orbitals for x = 1.5, 2.0. Ferrimagnetic coupling between V and Mn is found to be the ground state for x = 0.5, 1.5, while for x = 1.0 and 2.0, ferromagnetic and antiferromagnetic couplings between Mn and Mn are competitive for the ground state.

A DFT/TDDFT investigation was performed on the electronic structures, absorption and emission spectra, as well as the phosphorescence efficiency of [(ppy)2Ir(P^SiO)] (1) and the derivatives (1a, 1b, 1c and 1d) with CN-substitution at different positions in ppy ligands, as well as [(dfppy)2Ir(P^SiO)] (2) [where ppy = 2-phenylpyridine, dfppy = 2-(2,4-difluorophenyl)pyridine and (P^SiO) is an organosilanolate ancillary chelate]. The calculated results reveal that the introduction of CN leads to a significant red shift for 1a–1d in absorption spectra compared with that of 1. Moreover, the CN substitution at different positions on C^N ligands may be an efficient approach towards tuning emitting color. 1b, 1c, and 1d lead to a blue shift of emission spectra compared with 1, while an obvious red shift is observed for 1a. The high quantum yield of 1 (0.90) compared to 2 (0.59) is explained based on the S1–Tn splitting energies and energy gap between 3MLCT/π–π* and 3MC d–d states, and the evaluation of the radiative and nonradiative rate constants for all the complexes is also studied. The designed complexes 1a, 1c and 1d are expected to be potential phosphorescence emitters in OLEDs with high quantum efficiency.

The non-precious metal graphene catalyst doped with Fe–Px are recently proposed as a promising candidate in substituting Pt for catalyzing oxygen reduction reaction (ORR) in fuel cells. Systematic DFT calculations are performed to investigate the catalytic activity and the ORR mechanism on the Fe–Px (x = 1–4) system in acid medium in this work. Our results indicated that the configuration with one Fe and two P atoms codoped at zigzag edge site (Fe–P2–zig-G) is the most stable, in excellent agreement with the experimental observation that the ratio of Fe and P is nearly 1:2. The four-electron reduction mechanism for ORR on the Fe–P2–zig-G is via the competing OOH hydrogenation pathways (to form either OH + OH or O + H2O). The rate determining step is the O2 hydrogenation with an energy barrier of 0.43 eV, much smaller that of calculated 0.80 eV for pure Pt. In addition, the highest energy barrier of the studied ORR mechanism is the O2 dissociation with an energy barrier of 0.70 eV, a value also smaller than that of pure Pt. This demonstrated that the zigzag edge site of the Fe–P2 codoped graphene should be active for the ORR.

Non-precious metal catalysts have attracted particular interest in recent years due to their promising ORR (oxygen reduction reaction) activity in fuel cells. In this work, the structural stability and ORR mechanism of CoN3 embedded graphene have been studied theoretically in acid media. The results indicate that CoN3 embedded graphene is stable thermodynamically. The kinetically most favorable reaction pathway for the ORR is a four-electron process. The process of OOH hydrogenation to generate O + H2O is the most favorable pathway. In the rate determining step, the energy barrier is 0.38 eV, much smaller than the theoretical value of ∼0.80 eV for pure Pt. The predicted working potential is 0.4 V for the most favorite pathway. Besides the lower energy barrier, the smaller Tafel slope compared with pure Pt in both low and high overpotential regions also suggests that CoN3 embedded graphene is a promising electrocatalyst for the ORR.