N-Heterocyclic Olefin Type Ionic Liquid with Innate Soft Lewis-Base Character as an Effective Additive for Hybrid Quasi-2D and 3D Perovskite Solar Cells.

In optimizing perovskites with ionic liquid (IL), the comparative study on Lewis acid-base (LAB) and hydrogen-bonding (HB) interactions between IL and perovskite is lacking. Herein, methyl is substituted for hydrogen on 2-position of imidazolium ring of N-heterocyclic carbene (NHC) type IL IdH to weaken HB interactions, and the resulting N-heterocyclic olefin (NHO) type IL IdMe with softer Lewis base character is studied in both hybrid quasi-2D (Q-2D) and 3D perovskites. It is revealed that IdMe participates in constructing high-quality Q-2D perovskite (n = 4) and provides stronger passivation for 3D perovskite compared with IdH. Power conversion efficiency (PCE) of Q-2D PEA2 MA3 Pb4 I13 perovskite solar cells (PVSCs) is boosted to 17.68% from 14.03%. PCE and device stability of 3D PVSCs enhances simultaneously. Both theoretical simulations and experimental results show that LAB interactions between NHO and Pb2+ take the primary optimization effects on perovskite. The success of engineering LAB interactions also offers inspiration to develop novel ILs for high-performance PVSCs.

Synergistic Effect of Boron Nitride and Carbon Domains in Boron Carbide Nitride Nanotube Supported Single-Atom Catalysts for Efficient Nitrogen Fixation.

Developing the low-cost and efficient single-atom catalysts (SACs) for nitrogen reduction reaction (NRR) is of great importance while remains as a great challenge. The catalytic activity, selectivity and durability are all fundamentally related to the elaborate coordination environment of SACs. Using first-principles calculations, we investigated the SACs with single transition metal (TM) atom supported on defective boron carbide nitride nanotubes (BCNTs) as NRR electrocatalysts. Our results suggest that boron-vacancy defects on BCNTs can strongly immobilize TM atoms with large enough binding energy and high thermal/structural stability. Importantly, the synergistic effect of boron nitride (BN) and carbon domains comes up with the modifications of the charge polarization of single-TM-atom active site and the electronic properties of material, which has been proven to be the essential key to promote N2 adsorption, activation, and reduction. Specifically, six SACs (namely V, Mn, Fe, Mo, Ru, and W atoms embedded into defective BCNTs) can be used as promising candidates for NRR electrocatalysts as their NRR activity is higher than the state-of-the art Ru(0001) catalyst. In particular, single Mo atom supported on defective BCNTs with large tube diameter possesses the highest NRR activity while suppressing the competitive hydrogen evolution reaction, with a low limiting potential of -0.62 V via associative distal path. This work suggests new opportunities for driving NH3 production by carbon-based single-atom electrocatalysts under ambient conditions.

Realizing a Not-Strong-Not-Weak Polarization Electric Field in Single-Atom Catalysts Sandwiched by Boron Nitride and Graphene Sheets for Efficient Nitrogen Fixation.

Developing efficient single-atom catalysts (SACs) for nitrogen fixation is of great importance while remaining a great challenge. The lack of an effective strategy to control the polarization electric field of SACs limits their activity and selectivity. Here, using first-principles calculations, we report that a single transition metal (TM) atom sandwiched between hexagonal boron nitride (h-BN) and graphene sheets (namely, BN/TM/G) acts as an efficient SAC for the electrochemical nitrogen reduction reaction (NRR). These sandwich structures realize stable and tunable interfacial polarization fields that enable the TM atom to donate electrons to a neighboring B atom as the active site. As a result, the partially occupied pz orbital of a B atom can form B-to-N π-back bonding with the antibonding state of N2, thus weakening the N≡N bond. The not-strong-not-weak electric field on the h-BN surface further promotes N2 adsorption and activation. The NRR catalytic activity of the BN/TM/G system is highly correlated with the degree of positively polarized charges on the TM atom. In particular, BN/Ti/G and BN/V/G are identified as promising NRR catalysts with high stability, offering excellent energy efficiency and suppression of the competing hydrogen evolution reaction.

Tuning the activity of the inert MoS2 surface via graphene oxide support doping towards chemical functionalization and hydrogen evolution: a density functional study.

The basal plane of MoS2 provides a promising platform for chemical functionalization and the hydrogen evolution reaction (HER); however, its practical utilization remains challenging due to the lack of active sites and its low conductivity. Herein, using first principles simulations, we first proposed a novel and effective strategy for significantly enhancing the activity of the inert MoS2 surface using a graphene oxide (GO) support (MoS2/GOs). The chemical bonding of the functional groups (CH3 and NH2) on the MoS2-GO hybrid is stronger than that in freestanding MoS2 or MoS2-graphene. Upon increasing the oxygen group concentration or introducing N heteroatoms into the GO support, the stability of the chemically functionalized MoS2 is improved. Furthermore, use of GOs to support pristine and defective MoS2 with a S vacancy (S-MoS2) can greatly promote the HER activity of the basal plane. The catalytic activity of S-MoS2 is further enhanced by doping N into GOs; this results in a hydrogen adsorption free energy of almost zero (ΔGH = ∼-0.014 eV). The coupling interaction with the GO substrate reduces the p-type Schottky barrier heights (SBH) of S-MoS2 and modifies its electronic properties, which facilitate charge transfer between them. Our calculated results are consistent with the experimental observations. Thus, the present results open new avenues for the chemical functionalization of MoS2-based nanosheets and HER catalysts.

Oxygen-Molecule Adsorption and Dissociation on BCN Graphene: A First-Principles Study.

Boron and nitrogen co-doped (BCN) graphene is an attractive material for use as a metal-free oxygen reduction reaction electrocatalyst and as other catalysts due to its unique structure and electronic properties. Reported here is the structure, determined by using density functional theory, of the active O2 -dissociation site of BCN graphene containing different types of BN cluster. The results show that the edge termination and shape of substitutional BN clusters are two important factors that determine the catalytic activity of BCN graphene for the dissociation of molecular oxygen. N-Terminated triangular BN (t-BN) cluster doping can reduce the energy barrier more effectively compared to a t-BN with a B edge or quadrangular BN cluster. Interestingly, the B atom neighboring the N edge, only in the case of N-terminated t-BN doping, is determined to be the most active site for O2 dissociation due to the barrier being as low as 0.08 eV. The electronic structure calculations reveal that in addition to the large positive charge densities, the catalytic activity of graphene enhanced by B,N doping is also attributed to the increased density of states of the π* states of the active site around the Fermi level.

Regioselective synthesis of 2,3'-biindoles mediated by an NBS-induced homo-coupling of indoles.

Mild conditions have been developed to achieve NBS-induced homodimerization of indole derivatives with excellent regioselectivity at 15 °C in high efficiency. This method provides a simple route to a 2,3'-linked biindolyl scaffold from the electron-rich to moderately electron-poor indoles. In addition, [3,2-a]carbazole derivatives can also be prepared through this method.

Diels-Alder reactions of graphene oxides: greatly enhanced chemical reactivity by oxygen-containing groups.

Graphene oxides (GOs) or reduced GOs (rGOs) may offer extraordinary potential for chemical functionalization of graphene due to their unique electronic and structural properties. By means of dispersion-corrected density functional theory computations, we systematically investigated the Diels-Alder (DA) chemistry of GOs. Our computations showed that the dual nature of GOs as both a diene and a dienophile is stronger than that of pristine graphene. Interestingly, the interior bonds of a graphene surface modified by oxygen-containing groups could be functionalized by maleic anhydride (MA) and 2,3-dimethoxybutadiene (DMBD) through cycloaddition reactions, and the cycloaddition products of MA and DMBD are more favorable than the non-covalent complexes between these reagents and the GO surface. The feasibility of covalent functionalization of GOs as a diene and a dienophile strongly depends on the local structural environment of the oxygen groups, including the atomic arrangement and the number of these groups surrounding the reaction site. The exothermicities for (4+2) adducts of DMBD with GO are far larger than those of MA, which indicates that the dienophile character of the GO surface is stronger than its behavior as a diene.

Interfacial interaction of Ag nanoparticles with graphene oxide supports for improving NH3 and NO adsorption: a first-principles study.

We investigated the structural and electronic properties of Ag13 nanoparticles (NPs) deposited on graphene oxides (GOs) and the effect of the interfacial interaction on NH3 and NO adsorption using density functional theory calculations. The epoxy functional group and its neighboring sp(2) carbon atoms of GOs, rather than the hydroxyl group, are used as active sites to enhance the binding of Ag13 to graphene through the C-O-Ag and C-Ag chemical bonds. The stability of deposited Ag NPs depends on the chemical environment of active sites in GOs, including the atomic arrangement of epoxides and their concentration. The deposited Ag13 NPs are likely to be further oxidized to form Ag13O by neighboring oxygen irrespective of the oxidation level of GOs. The strong interfacial interaction of Ag13/GOs, which effectively tunes the position of the d-band center of NPs due to large charge transfer from Ag13 to GOs, has a significant impact on the adsorption of NH3. The NH3 is strongly adsorbed on deposited Ag13 through the formation of an N-Ag bond and an N···HO hydrogen bond between NH3 and O from C-O-Ag. The electronic structure calculations show that the hybridization of the HOMO of NH3 with the conduction bands of Ag13-GOs results in the strong donor doping by an NH3 molecule, and gives rise to larger charge transfers from NH3 to the hybrid, compared to NH3 adsorption on isolated Ag13 and GOs. The adsorption of NO on oxidized Ag13 on GOs is obviously improved due to the oxidation of NO to NO2 by its neighboring oxygen atoms. In contrast to NH3, the adsorbed NO acts as an acceptor. The calculated results show good agreement with experimental observations.

Realizing semiconductor-half-metal transition in zigzag graphene nanoribbons supported on hybrid fluorographene-graphane nanoribbons.

Hydrogenation and fluorination provide promising applications for tuning the properties of graphene-based nanomaterials. Using first-principles calculations, we investigate the electronic and magnetic properties of zigzag graphene nanoribbons (ZGNRs) supported on hydrogenated and fluorinated ZGNRs. Our results indicate that the support of zigzag graphane nanoribbon with its full width has less impact on the electronic and magnetic properties of ZGNRs, whereas the ZGNRs supported on fluorographene nanoribbons can be tuned to metal with almost degenerated ferro- and anti-ferromagnetic states due to the intrinsic polarization of substrate. The ZGNRs supported on zigzag hybrid fluorographene-graphane nanoribbons are spin-polarized half-semiconductors with distinct band gaps for spin-up and spin-down channels. Interestingly, in the absence of an external electric field, the spin-polarized band gaps of supported ZGNRs can be well modulated in the opposite direction by changing the ratio of fluorination to hydrogenation concentration in hybrid substrates. Furthermore, the ZGNRs supported on hybrid nanoribbons exhibit the half-semiconducting to half-metallic behavior transition as the interlayer spacing is gradually reduced, which is realized more easily for the hybrid support with a relatively wide fluorographene moiety compared to its narrow counterpart. Present results provide a novel way for designing substrate-supported graphene spintronic devices.

Engineering Spin Polarization of the Surface-Adsorbed Fe Atom by Intercalating a Transition Metal Atom into the MoS2 Bilayer for Enhanced Nitrogen Reduction.

The precise control of spin states in transition metal (TM)-based single-atom catalysts (SACs) is crucial for advancing the functionality of electrocatalysts, yet it presents significant scientific challenges. Using density functional theory (DFT) calculations, we propose a novel mechanism to precisely modulate the spin state of the surface-adsorbed Fe atom on the MoS2 bilayer. This is achieved by strategically intercalating a TM atom into the interlayer space of the MoS2 bilayer. Our results show that these strategically intercalated TM atoms can induce a substantial interfacial charge polarization, thereby effectively controlling the charge transfer and spin polarization on the surface Fe site. In particular, by varying the identity of the intercalated TM atoms and their vacancy filling site, a continuous modulation of the spin states of the surface Fe site from low to medium to high can be achieved, which can be accurately described using descriptors composed of readily accessible intrinsic properties of materials. Using the electrochemical dinitrogen reduction reaction (eNRR) as a prototypical reaction, we discovered a universal volcano-like relation between the tuned spin and the catalytic activity of Fe-based SACs. This finding contrasts with the linear scaling relationships commonly seen in traditional studies and offers a robust new approach to modulating the activity of SACs through interfacial engineering.

Cu(II)-catalyzed highly regio- and stereoselective construction of C-C double bonds: an efficient method for the ketonization-olefination of indoles.

A general and efficient method for the cross-coupling of indoles with ß-keto esters by using TEMPO/CuSO4·5H2O in air as oxidant has been developed. This reaction features high functional-group compatibility and an excellent selectivity. This methodology provides an alternative approach for the ketonization-olefination of indoles in moderate to good yields.

Tunable doping and band gap of graphene on functionalized hexagonal boron nitride with hydrogen and fluorine.

First-principles calculations have been used to investigate the structural and electronic properties of graphene supported on functionalized hexagonal boron nitride (h-BN) with hydrogen and fluorine atoms. Our results show that the hydrogenation and fluorination of the h-BN substrate modify the electronic properties of graphene. Interactions of graphene with fully hydrogenated or fully fluorinated h-BN and half-hydrogenated and half-fluorinated h-BN with H at N sites and F at the B sites can lead to n- or p-type doping of graphene. The different doping effect may be attributed to the significant charge transfer from graphene to the substrate. Interestingly, when graphene is supported on the functionalized h-BN with H at B sites and F at N sites (G/HBNF), a finite band gap of 79 meV in graphene is opened due to the equivalence breaking of two sublattices of graphene, and can be effectively modulated by changing the interlayer spacing, increasing the number of functionalized BN layers, and applying an external electric field. More importantly, the modification of the band gap in G/HBNF with a functionalized BN bilayer by the electric field is more pronounced than that of the single-layer h-BN, which is increased to 408 meV with 0.8 V Å(-1). Thus, graphene on chemically modified h-BN with a tunable and sizeable band gap may provide a novel way for fabricating high-performance graphene-based nanodevices.

Site-dependent catalytic activity of graphene oxides towards oxidative dehydrogenation of propane.

Graphene oxides (GOs) may offer extraordinary potential in the design of novel catalytic systems due to the presence of various oxygen functional groups and their unique electronic and structural properties. Using first-principles calculations, we explore the plausible mechanisms for the oxidative dehydrogenation (ODH) of propane to propene by GOs and the diffusion of the surface oxygen-containing groups under an external electric field. The present results show that GOs with modified oxygen-containing groups may afford high catalytic activity for the ODH of propane to propene. The presence of hydroxyl groups around the active sites provided by epoxides can remarkably enhance the C-H bond activation of propane and the activity enhancement exhibits strong site dependence. The sites of oxygen functional groups on the GO surface can be easily tuned by the diffusion of these groups under an external electric field, which increases the reactivity of GOs towards ODH of propane. The chemically modified GOs are thus quite promising in the design of metal-free catalysis.

Dual-Atom Metal and Nonmetal Site Catalyst on a Single Nickel Atom Supported on a Hybridized BCN Nanosheet for Electrochemical CO2 Reduction to Methane: Combining High Activity and Selectivity.

Atomically dispersed nitrogen-coordinated transition-metal sites supported on graphene (TM-N4-C) offer promising potential for the electrochemical carbon dioxide reduction reaction (CO2RR). However, a few TM-Nx-C single-atom catalysts (SAC) are capable of reducing CO2 to multielectron products with high activity and selectivity. Herein, using density functional theory calculations, we investigated the electrocatalytic performance of a single TM atom embedded into a defective BCN nanosheet for CO2RR. The N and B atom co-coordinated TM center, namely, TM-B2N2, constructs a symmetry-breaking site, which strengthens the overlapping of atomic orbitals, and enables the linear CO2 to be curved and activated, compared to the weak coupling of CO2 with the symmetric TM-N4 site. Moreover, the TM-B2N2 sites play a role of dual-atom active sites, in which the TM atom serves as the carbon adsorption site and the B atom acts as the oxygen adsorption site, largely stabilizing the key intermediates, especially *COOH. The symmetry-breaking coordination structures shift the d-band center of the TM atom toward the Fermi level and thus facilitate CO2 reduction to hydrocarbons and oxygenates. As a result, different from the TM-N4-C structure that leads to CO as the major product, the Ni atom supported on BCN can selectively catalyze CO2 conversion into CH4, with an ultralow limiting potential of -0.07 V, while suppressing the hydrogen evolution reaction. Our finding suggests that introduction of a nonmetal active site adjacent to the metal site provides a new avenue for achieving efficient multi-intermediate electrocatalytic reactions.

Adsorption of nitrogen oxides on graphene and graphene oxides: insights from density functional calculations.

The interactions of nitrogen oxides NO(x) (x = 1,2,3) and N(2)O(4) with graphene and graphene oxides (GOs) were studied by the density functional theory. Optimized geometries, binding energies, and electronic structures of the gas molecule-adsorbed graphene and GO were determined on the basis of first-principles calculations. The adsorption of nitrogen oxides on GO is generally stronger than that on graphene due to the presence of the active defect sites, such as the hydroxyl and carbonyl functional groups and the carbon atom near these groups. These active defect sites increase the binding energies and enhance charge transfers from nitrogen oxides to GO, eventually leading to the chemisorption of gas molecules and the doping character transition from acceptor to donor for NO(2) and NO. The interaction of nitrogen oxides with GO with various functional groups can result in the formation of hydrogen bonds OH⋅⋅⋅O (N) between -OH and nitrogen oxides and new weak covalent bonds C⋅⋅⋅N and C⋅⋅⋅O, as well as the H abstraction to form nitrous acid- and nitric acidlike moieties. The spin-polarized density of states reveals a strong hybridization of frontier orbitals of NO(2) and NO(3) with the electronic states around the Fermi level of GO, and gives rise to the strong acceptor doping by these molecules and remarkable charge transfers from molecules to GO, compared to NO and N(2)O(4) adsorptions on GO. The calculated results show good agreement with experimental observations.

Solid solutions of flexible host-guest supramolecules for tuning molecular motion and phase transitions.

By utilizing a supramolecular complex rather than an individual molecule as a deformable and elastic substitutional component, we put forward a solid-solution strategy and demonstrate an example of how two related yet non-isostructural crystalline host-guest compounds can form molecular solid solutions. Interestingly, such a strategy can effectively and continuously modulate the molecular motion and phase transition in them, as revealed by the variable-temperature/frequency dielectric responses.

Regulating Electronic Spin Moments of Single-Atom Catalyst Sites via Single-Atom Promoter Tuning on S-Vacancy MoS2 for Efficient Nitrogen Fixation.

The electrocatalytic activity of transition-metal (TM)-based catalysts is correlated with the spin states of metal atoms. However, developing a way to manipulate spin remains a great challenge. Using first-principles calculations, we first report the crucial role of the spin of exposed Mo atoms around an S-vacancy in the electrocatalytic dinitrogen reduction reaction on defective MoS2 nanosheets and propose a novel strategy for regulating the electronic spin moments by tuning a single-atom promoter (SAP). Single TM atoms adsorbed on a defective MoS2 basal plane serve as SAPs via a noncontact interaction with an exposed Mo active site, inducing a significant spin polarization that promotes N2 adsorption and activation. Interestingly, by changing only the adsorption site of the TM atom, we are able to change the spin moments of the Mo atom, over a wide range of tunable values. The spin moments can be tuned to largely improve the catalytic activity of MoS2 toward the reduction of N2 to NH3.

Carbon-doped zigzag boron nitride nanoribbons with widely tunable electronic and magnetic properties: insight from density functional calculations.

First-principles calculations within the local spin-density approximation have been used to investigate the electronic and magnetic properties of carbon chain-doped zigzag born nitride nanoribbons (ZBNNRs). Our results indicate that doped half-bare ZBNNRs with an H-passivated B edge and a bare C edge generally have a spin-polarized ground state with the ferromagnetic spin ordering localized at the C edge, independent of the doping concentration and the ribbon width. In particular, doped half-bare ZBNNRs for all widths may produce half-semiconducting --> half-metallic --> metallic behavior transitions without an external electric field as the doping proceeds gradually from the N edge to the B edge. The breakage of the symmetric spin distribution in the bipartite lattice and the coexistence of the edge state and the border state arising from charge transfer in these doped ZBNNRs are responsible for their tunable electronic and magnetic properties.

Synergistic Effect of Surface-Terminated Oxygen Vacancy and Single-Atom Catalysts on Defective MXenes for Efficient Nitrogen Fixation.

The production of ammonia (NH3) from molecular dinitrogen (N2) under ambient conditions is of great significance but remains as a great challenge. Using first-principles calculations, we have investigated the potential of using a transition metal (TM) atom embedded on defective MXene nanosheets (Ti3-xC2Oy and Ti2-xCOy with a Ti vacancy) as a single-atom electrocatalyst (SAC) for the nitrogen reduction reaction (NRR). The Ti3-xC2Oy nanosheet with Mo and W embedded, and the Ti2-xC2Oy nanosheet with Cr, Mo, and W embedded, can significantly promote the NRR while suppressing the competitive hydrogen evolution reaction, with the low limiting potential of -0.11 V for W/Ti2-xC2Oy. The outstanding performance is attributed to the synergistic effect of the exposed Ti atom and the TM atom around an extra oxygen vacancy. The polarization charges of the active center are reasonably tuned by the embedded TM atoms, which can optimize the binding strength of key intermediate *N2H. The good feasibility of preparing such TM SACs on defective MXenes and the high NRR selectivity with regard to the competitive HER suggest new opportunities for driving NH3 production by MXene-based SAC electrocatalysts under ambient conditions.

Density functional characterization of adsorption and decomposition of 1-propanethiol on the Ga-rich GaAs (001) surface.

Density functional calculations have been used to investigate adsorption and decomposition of 1-propanethiol on the Ga-rich GaAs (001) surface. The dissociative adsorption of 1-propanethiol on GaAs (001) to the chemisorbed propanethiolate and hydrogen was predicted to be quite facile. Followed by the C-S bond scission of the propanethiolate species, the surface propyl species was formed with a barrier of 47.2 kcal/mol for the low-energy route. The propyl species is an important precursor to propane through the C-H bond coupling and to propene via the beta-H elimination. Predicted activation free energies for the surface processes from the propyl species to propane and propene are 45.2 and 37.0 kcal/mol at 298.1 K, respectively, while the corresponding overall Gibbs free energies of reaction DeltaG are -49.3 and -21.2 kcal/mol relative to free 1-propanethiol. Therefore, both reaction routes are competitive, resulting in a product mixture, although the beta-H elimination from the propyl species is initially remarkably favorable dynamically. On the basis of our calculations, detailed mechanisms for adsorption and thermal decomposition of 1-propanethiol on the GaAs (001) surface were proposed, and the calculated results show good agreement with experimental observations.