Eliminating Halogen Vacancies Enables Efficient MACL-Assisted Formamidine Perovskite Solar Cells.

Methylammonium chloride (MACl) additive is almost irreplaceable in high-performance formamidine perovskite photovoltaics. Nevertheless, Some of the problems that can arise from adding MACl are rarely mentioned. Herein, it is proposed for the first time that the addition of MACl would cause the non-stoichiometric ratio in the perovskite film, resulting in the halogen vacancy. It is demonstrated that the non-synchronous volatilization of methylamine cations and chloride ions leads to the formation of halogen vacancy defects. To solve this problem, the NH4 HCOO is introduced into the perovskite precursor solution to passivate the halogen vacancy. The HCOO- ions have a strong force with lead ions and can fill the halogen vacancy defects. Consequently, the champion devices' power conversion efficiency (PCE) can be improved from 21.23% to 23.72% with negligible hysteresis. And the unencapsulated device can still retain >90% of the initial PCE even operating in N2 atmosphere for over 1200 h. This work illustrates another halogen defect source in the MACl-assisted formamidine perovskite photovoltaics and provides a new route to obtain high-performance perovskite solar cells.

Tunable electronic and optical properties of ferroelectric WS2/Ga2O3heterostructures.

To integrate two-dimensional (2D) materials into van der Waals heterostructures (vdWHs) is regarded as an effective strategy to achieve multifunctional devices. The vdWHs with strong intrinsic ferroelectricity is promising for applications in the design of new electronic devices. The polarization reversal transitions of 2D ferroelectric Ga2O3layers provide a new approach to explore the electronic structure and optical properties of modulated WS2/Ga2O3vdWHs. The WS2/Ga2O3↑ and WS2/Ga2O3↓ vdWHs are designed to explore possible characteristics through the electric field and biaxial strain. The biaxial strain can effectively modulate the mutual transition of two mode vdWHs in type II and type I band alignment. The strain engineering enhances the optical absorption properties of vdWHs, encompassing excellent optical absorption properties in the range from infrared to visible to ultraviolet, ensuring promising applications in flexible electronics and optical devices. Based on the highly modifiable physical properties of the WS2/Ga2O3vdWHs, we have further explored the potential applications for the field-controlled switching of the channel in MOSFET devices.

Reconfigurable band alignment of SWSe/h-BP heterostructures for photoelectric applications.

The integration of two-dimensional (2D) materials into van der Waals heterostructures (vdWHs) is regarded as an effective strategy for fabricating multifunctional devices. Herein, the effects of the vertical electric field and biaxial strain on the electronic, optical and transport properties of SeWS (SWSe)/h-BP vdWHs are systematically investigated using density functional theory calculations. The study shows that electric fields and biaxial strain can modulate not only the band gap but also the band alignment to produce multifunctional device applications. The SWSe/h-BP vdWHs can be used as highly efficient 2D exciton solar cells with a power conversion efficiency of up to 20.68%. In addition, the SWSe/h-BP vdWHs present a significant negative differential resistance (NDR) with a peak-to-valley ratio of 1.12 (1.18). The present work may provide a direction for tunable multiple-band alignments in SWSe/h-BP vdWHs and help achieve multifunctional device applications.

Single-Atom Anchored g-C3N4 Monolayer as Efficient Catalysts for Nitrogen Reduction Reaction.

Electrochemical N2 reduction reaction (NRR) is a promising approach for NH3 production under mild conditions. Herein, the catalytic performance of 3d transition metal (TM) atoms anchored on s-triazine-based g-C3N4 (TM@g-C3N4) in NRR is systematically investigated by density functional theory (DFT) calculations. Among these TM@g-C3N4 systems, the V@g-C3N4, Cr@g-C3N4, Mn@g-C3N4, Fe@g-C3N4, and Co@g-C3N4 monolayers have lower ΔG(*NNH) values, especially the V@g-C3N4 monolayer has the lowest limiting potential of -0.60 V and the corresponding limiting-potential steps are *N2+H++e-=*NNH for both alternating and distal mechanisms. For V@g-C3N4, the transferred charge and spin moment contributed by the anchored V atom activate N2 molecule. The metal conductivity of V@g-C3N4 provides an effective guarantee for charge transfer between adsorbates and V atom during N2 reduction reaction. After N2 adsorption, the p-d orbital hybridization of *N2 and V atoms can provide or receive electrons for the intermediate products, which makes the reduction process follow acceptance-donation mechanism. The results provide an important reference to design high efficiency single atom catalysts (SACs) for N2 reduction.

Theoretical exploration of the structural, electronic and optical properties of g-C3N4/C3N heterostructures.

Integration of graphene-like carbon nitride materials is essential for nanoelectronic applications. Using density-functional theory (DFT), we systematically investigate the structural, electronic and optical properties of a s-triazine-based g-C3N4/C3N heterostructure under different modified conditions. The g-C3N4/C3N van der Waals heterostructure (vdWH) formed has an indirect bandgap with type-II band alignment and the band structures can be tuned from type-II band alignment to type-I band alignment by applying biaxial strains and external electric fields (Efield). Compared to single transition metal (TM) atoms at g-C3N4/C3N surfaces, the TM atoms anchored in the interlayer region exhibit more stability, and the corresponding bandgaps are changed from 0.19 eV to 0.61 eV. In addition, the g-C3N4/C3N heterostructure has a strong absorption coefficient in the ultraviolet-visible light region along the x direction. It is found that compressive strain has a large influence on the absorption coefficient of the g-C3N4/C3N system. With the increased compressive strain, the absorption spectra in the visible light region disappeared. Tensile strain has a slight effect on the absorption range, but causes a red shift of the absorption spectrum. In comparison, the light absorption coefficient of the g-C3N4/C3N system remains almost unchanged under the Efield conditions. In summary, the formation of a s-triazine-based g-C3N4/C3N heterostructure has shown potential for applications in nanoelectronic and optoelectronic devices.

VS2nanosheet as a promising candidate of recycle and reuse NO2gas sensor and capturer: a DFT study.

We describe the utilization of VS2nanosheet as high sensing response, reuse, and thermodynamic stability at room temperature NO2and NO gas sensors by using the density functional theory method. We focus on the electronic structures and adsorption energy toward a variety of gaseous molecules (such as O2, CO, H2O, NH3, NO, and NO2) adsorbed on the VS2nanosheet. The results show that chemical interactions existed between NO/NO2molecules and VS2nanosheet due to sizable adsorption energy and strong covalent (S-N) bonds. In particular, the adsorption energies, charge transfer and electronic properties between NO2adsorbed system is significantly changed compared with the other gas molecules (CO, NO, H2O, NH3, and O2) adsorbed systems under biaxial strains, which is effective to achieve the capture or reversible release of NO2for cycling capability. Our analysis indicates that VS2nanosheet is promising as electrical devices candidate for NO2high-performance gas sensor or capturer.

Comparative Study of NO and CO Oxidation Reactions on Single-Atom Catalysts Anchored Graphene-like Monolayer.

Noble metal single-atom catalysts (NM-SACs) anchored at novel graphene-like supports has attracted enormous interests. Gas sensitivity, catalytic activity, and d-band centers of single NM (Pt and Pd) atoms at graphenylene (graphenylene-NM) are investigated using first-principle calculations. The adsorption geometries of gas reactants on graphenylene-NM sheets are analyzed. It is found that the adsorption energies of reactant species on graphenylene-Pt are larger than those on graphenylene-Pd, because the d-band center of the Pt atom is closeser to the Fermi level. The NO and CO oxidation reactions on graphenylene-NM are investigated via four catalytic mechanisms, including Langmuir-Hinshelwood (LH), Eley-Rideal (ER), New ER (NER), and termolecular ER (TER). The results show that the NO and CO oxidations via LH and TER mechanisms can occur owing to the relatively small energy barriers. Moreover, the interaction of 2NO+2CO via ER mechanism is the energetically more favorable reaction. Although the NO oxidation via the NER mechanism has rather low energy barriers, the reaction is unlikely to occur due to the low adsorption energy of O2 compared with CO and NO. This research may provide guidance for exploring the catalytic performance of SACs on graphene-like materials to remove toxic gas molecules.

BN cluster-doped graphdiyne as visible-light assisted metal-free catalysts for conversion CO2 to hydrocarbon fuels.

Carbon dioxide electrochemical reduction reaction (CO2RR) under ambient conditions provides an intriguing picture for conversion of CO2 to useful fuels and chemicals. Here by means of density functional theory (DFT) computations, the formation configuration and CO2RR catalytic activity of boron nitrogen cluster-doped graphdiyne (BN-doped GDY) were systematically investigated. The band structure and optical adsorption spectra reveal that BN-doped GDY exhibits semiconductor with the band gap of 0.902 eV and shows photothermal effect under visible and even infrared light irradiation. The BN-doped GDY could act as a hot spot to enhance CO2RR. The adsorption configurations of various reaction intermediates indicate that boron atoms are active sites, which can be further confirmed by charge analysis. Based on thermodynamic analysis, the reaction pathways and onset potentials were studied as compared with Cu(111) surface. For the production of CO, the onset potential for BN-doped GDY (-1.06 V) is higher than that for Cu(111) surface. While for the reduction of CO2 to HCOOH, CH4, CH3OH, and C2H4 on BN-doped GDY, the onset potentials are lower than that on Cu(111) surface, which are -0.57 V, -0.62 V, -0.57 V, and -0.82 V, respectively. Moreover, the onset potential of competitive hydrogen evolution reaction on BN-doped GDY is high to -0.82 V, which shows us a good selectivity towards to CO2RR rather than HER. Our results may pave a new avenue for the conversion of CO2 into high-value fuels and chemicals.

Charge transfer and strain tuned antiferromagnetism in the two-dimensional CrCl3/[Mo2C(-O)]2 heterojunction.

Magnetic ordering in two-dimensional materials with atomic level thickness has been one of the most important issues in condensed matter physics and material science. Most previous studies have focused on the two-dimensional ferromagnetic systems, while the antiferromagnetic systems have been much less touched. Here, by using first-principles calculations and Monte Carlo simulation, a two-dimensional antiferromagnetic heterojunction: CrCl3/[Mo2C(-O)]2, is predicted, by tuning the electronic distribution. The ferromagnetic coupling between the Cr-Cr atoms in the CrCl3/(Mo2C)2 heterostructure is enhanced by the transferred electrons from Mo2C, which will occupy the t2g orbits of Cr. With the O adsorbed on the Mo2C, the Cr-Cl bond length increases and the superexchange interaction is decreased. The magnetic ground state changes to antiferromagnetism. More interestingly, under a moderate compressive biaxial strain, its Néel temperature of CrCl3/(Mo2C-O)2 can be significantly increased for the enhanced direct exchange of Cr-Cr atom with a value of 146 K. The heterojunction is useful for two-dimensional spintronic logic, ultrafast magnetodynamic devices and information storage for new generation computer devices.

Single-atom metal-modified graphenylene as a high-activity catalyst for CO and NO oxidation.

Herein, the adsorption behaviors and interactions of different gas species on single-metal atom-anchored graphenylene (M-graphenylene, M = Mn, Co, Ni, and Cu) sheets were investigated by first-principles calculations. At first, the single metal atom tends to adsorb on the hollow or bridge site of graphenylene, and the formed M-graphenylene systems exhibit varied magnetic properties. The reactants (NO, CO, O2, O, CO2, and NO2) adsorbed on the Mn-, Co-, and Ni-graphenylene sheets exhibit higher stability than those adsorbed on the Cu-graphenylene sheet. Moreover, the co-adsorption configurations of NO-O2, CO-O2, 2NO, and 2CO on the M-graphenylene sheets were comparably studied, which are considered as the initial states for NO and CO oxidation. It was found that the energy barriers for the formation of OONO and OOCO complexes on Mn-graphenylene by the Langmuir-Hinshelwood (LH) mechanism are larger than those in the case of Co-graphenylene (<0.4 eV). The possible reactions for the oxidation of 2CO by the 2NO molecules on the M-graphenylene sheets were also considered, because of the adsorbed NO molecules are more stable than the CO and O2 molecules. Furthermore, the energy barrier for the oxidation of CO on Mn-graphenylene via the Eley-Rideal (ER) mechanism (2NO + 2CO → 2CO2 + N2) is smaller (<0.3 eV) than those in the cases of other substrates. These results illustrate that the single-metal atom-modified graphenylene can be used as a potential novel carbon-based catalyst with high activity.

Graphdiyne coordinated transition metals as single-atom catalysts for nitrogen fixation.

The reduction of N2 molecules to NH3 is a very challenging task in chemistry. The electrocatalytic nitrogen reduction reaction (NRR) is a promising technology for NH3 synthesis. By using first-principles calculation, a new class of single-atom catalysts (SACs), graphdiyne coordinated single transition metal atoms (TM@GDY, TM = Sc-Zn, Y-Cd, and La-Hg) were designed, and the NRR catalytic character of TM@GDY was systematically investigated. The results demonstrated that some TM@GDY (TM = Ti, V, Fe, Co, Zr, Rh, and Hf) monolayers exhibit better NRR activities than a Ru(0001) stepped surface. There is an obvious linear correlation between the limiting potential and the atomic N adsorption energy, which confirms that the N adsorption energy may be a descriptor for evaluation of the NRR catalytic performance. The V@GDY monolayer possesses the best NRR catalytic character with the lowest limiting potential of -0.67 V and the potential-limiting step (PLS) of *N2→ *NNH for both alternating and distal mechanisms. Our results highlight a new family of efficient and stable TM@GDY catalysts and provide useful guidelines for SAC development and practical applications.

Steady semiconducting properties of monolayer PtSe2 with non-metal atom and transition metal atom doping.

Based on density functional theory, the electronic structure and magnetic properties of monolayer PtSe2 doped with different atoms were studied. The Pt and Se atoms are replaced by a transition metal atom (Mn) and a non-metal atom X (X = N, P, As), respectively. The pristine monolayer PtSe2 is a semiconductor with an indirect band gap of 1.352 eV. For one non-metal atom doping, the doped system exhibits indirect band gap magnetic semiconducting properties and the magnetic moment is less than 1 µB and mainly comes from the hybridization of Pt-5d and X-p orbitals. The N-Doped system still retains the magnetic semiconducting properties under strain (from -10% to 13%) and the band gap varies from 0.059 eV to 1.308 eV. For two X doped systems, three different configurations are considered. The doped systems retain the indirect band gap semiconducting properties except for the third nearest neighbor N-doped system (direct band gap). But, for all N-doped and the second nearest neighbor P-doped systems, the magnetic moment increases to more than double. Meanwhile, all X-doped monolayer PtSe2 systems exhibit p-type semiconducting characteristics. For (Mn, X) co-doped systems, the magnetic moments are mainly localized in the Mn 3d orbital and there is strong p-d hybridization between Mn atoms and X atoms. The (Mn, N/P) co-doped system still exhibits magnetic semiconducting properties. These results are important for designing semiconductor devices and electronic spin devices based on monolayer PtSe2.

Charge-compensated co-doping of graphdiyne with boron and nitrogen to form metal-free electrocatalysts for the oxygen reduction reaction.

The development of metal-free catalysts for the oxygen reduction reaction (ORR) is critical for rechargeable metal-air batteries and full cells. Various light non-metallic-atom-doped graphdiyne (GDY) materials have been designed based on density functional theory and evaluated as efficient ORR electrocatalysts. Volcano curve correlations between the overpotential and Gibbs adsorption free energies of oxygenated intermediates have been derived for the ORR. Optimized compensated B,N co-doped GDY monolayers exhibit the lowest ORR overpotential of 0.57 V. In compensated B,N co-doped GDY, the B atoms are positioned in the benzene rings, and the N atoms are positioned in the acetylene linkages. Moreover, some B,N co-doped GDYs have two active sites with almost the same catalytic activity: the B atom and the C atom bearing an effective positive charge nearest to the doped N atom. Based on the theoretical analysis, ideal doped GDY monolayers are expected to exhibit comparable catalytic activity to Pt-based catalysts.

Multifarious Interfaces, Band Alignments, and Formation Asymmetry of WSe2-MoSe2 Heterojunction Grown by Molecular-Beam Epitaxy.

Monolayer (ML) transition-metal dichalcogenides (TMDs) continue to attract research attention, and the heterojunctions formed by vertically stacking or laterally stitching two different TMDs, e.g., MoSe2 and WSe2, may have many interesting electronic and optical properties and thus are at the center stage of current research. Experimentally realizing such heterojunctions with desired interface morphologies and electronic properties is of great demand. In this work, we report a diverse interface structure in molecular-beam epitaxial WSe2-MoSe2 heterojunction. The corresponding electronic bands show type-II band alignment for both monolayer ML-ML and ML-bilayer lateral junctions irrespective of the presence or absence of step states. Interestingly, a strong anisotropy in lateral heterojunction formation is observed, where sharp interfaces are obtained only when WSe2 deposition precedes MoSe2. Reversing the deposition order leads to alloying of the two materials without a notable boundary. This is explained by a step segregation process as suggested by the first-principles total energy calculations.

Modulation of the electronic properties and spin polarization of 2H VS2 nanoribbons by tuning ribbon widths and edge decoration.

The band structures and spin-polarization characteristics of armchair and zigzag VS2 nanoribbons with different terminated edges are investigated based on density functional theory (DFT) calculations with a spin polarized meta-GGA. The results reveal that zigzag 2H VS2 nanoribbons exhibit metal, half-metal, or semiconductor electrical characteristics with different edge decorations or ribbon widths. And the spin polarized ratio can achieve 100% self-polarization for the zigzag VS2 nanoribbons with V atom edges. The Curie temperatures (TC) estimated by mean field approximation simulations for the zigzag 2H VS2 nanoribbons with terminated edges of V systems are 276 K. These preliminary findings offer an effective treatment option for controllable and adjustable spintronic devices.

Formation Mechanism, Geometric Stability and Catalytic Activity of a Single Iron Atom Supported on N-Doped Graphene.

Based on density functional theory (DFT) calculations, the formation geometries, stability and catalytic properties of single-atom iron anchored on xN-doped graphene (xN-graphene-Fe, x=1, 2, 3) sheet are systemically investigated. It is found that the different kinds and numbers of gas reactants can effectively regulate the electronic structure and magnetic properties of the 3 N-graphene-Fe system. For NO and CO oxidation reactions, the coadsorption configurations of NO/O2 and CO/O2 molecules on a reactive substrate as the initial state are comparably analyzed. The NO oxidation reactions through the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms have relatively smaller energy barriers than those of the CO oxidation processes. In comparison, the preadsorbed 2NO reacting with 2CO molecules (2NO+2CO→2CO2 +N2 ) through ER reactions (<0.4 eV) are energetically more favorable processes. These results can provide beneficial references for theoretical studies on NO and CO oxidation and designing graphene-based catalyst for toxic gas removal.

Molecule-level graphdiyne coordinated transition metals as a new class of bifunctional electrocatalysts for oxygen reduction and oxygen evolution reactions.

Single-atom catalysts (SACs) are highly desired for maximizing the efficiency of metal atoms and can entail high selectivity and activity. Bifunctional catalysts enable higher performance and lower cost than two single-function catalysts. Supported single-atom bifunctional catalysts are therefore of great economic interest and scientific importance. Density functional theory calculations are used to design SACs, isolated transition metal anchored on graphdiyne (TM@GDY), for oxygen reduction (ORR) and oxygen evolution (OER) reactions in alkaline media. A dual-volcano plot is constructed to thoroughly describe and predict the catalytic activity toward ORR and OER processes on TM@GDY materials. The results demonstrate that GDY could provide a unique platform for synthesizing uniform SACs with high catalytic activity toward ORR and OER. The theoretical evaluations show that Ni@GDY and Pt@GDY catalysts possess comparable electrocatalytic activity for ORR and OER in alkaline media. The study not only provides deep insights into the catalytic activity of TM@GDY, but also guides the design of GDY based SACs.

Graphdiyne doped with sp-hybridized nitrogen atoms at acetylenic sites as potential metal-free electrocatalysts for oxygen reduction reaction.

Exploring metal-free electrocatalysts with high efficiency and lower cost for oxygen reduction reaction (ORR) is necessary to realize the commercialization of fuel cells. In this paper, the ORR mechanisms on nitrogen-doped graphdiyne (GDY) are investigated using the first principles calculations. It is found that the GDY doped with sp-hybridized N at acetylenic sites can activate molecular oxygen (O2). The kinetically most favorable reaction pathway is O2 → OOH → O + H2O → OH → H2O, which is an efficient four-electron ORR process. The first reaction step O2 → OOH is the rate determining step (RDS), and the energy barrier is 0.61 eV. The energy barrier of RDS is smaller than that of pure Pt (0.80 eV). Therefore, these results illustrate that sp-hybridized N-doped GDY is a promising carbon-based metal-free ORR catalyst.

Effect of toxic ligands on O2 binding to heme and their toxicity mechanism.

Heme, as the cofactor and active site of Hb, enables Hb to carry out the necessary function required for O2 management for life, that is, reversible O2 binding for transport. In this paper, the microscopic mechanism of heme-associated poisoning has been elucidated from the perspective of electronic interaction by performing first-principles calculations. The results show that the functional groups (-CHO, -COOH, -NO2, -NH2) and CN exhibit a stronger affinity for heme than O2 and are more likely to occupy the O2 binding site, which results in the loss of the ability of heme to carry O2. Moreover, the addition of functional groups, CO and CN to heme at the side site can cause a pronounced enhancement toward the O2 binding characteristics of heme, which prevents heme from releasing O2 to oxygen-consuming tissues as the blood circulates. The reversible O2 binding function of heme is disrupted by the presence of these toxic ligands in the heme binding pocket, which greatly affects O2 transport in the blood. The inability of tissues to obtain O2 leads to tissue hypoxia, which is the main cause of poisoning. Based on the energy, geometry and electronic properties, the hypoxia mechanism proposed by us coincides well with experiment, and the research has the potential to provide a theoretical reference for the relevant areas of bioscience.

In Situ Engineering of Intracellular Hemoglobin for Implantable High-Performance Biofuel Cells.

The key challenge for the broad application of implantable biofuel cells (BFCs) is to achieve inorganic-organic composite biocompatibility while improving the activity and selectivity of the catalysts. We have fabricated nanoengineered red blood cells (NERBCs) by an environmentally friendly method by using red blood cells as the raw material and hemoglobin (Hb) embedded with ultrasmall hydroxyapatite (HAP, Ca10 (PO4 )6 (OH)2 ) as the functional BFC cathode material. The NERBCs showed greatly enhanced cell performance with high electrocatalytic activity, stability, and selectivity. The NERBCs maintained the original biological properties of the natural cell, while enhancing the catalytic oxygen reduction reaction (ORR) through the interaction between -OH groups in HAP and the Hb in RBCs. They also enabled direct electron transportation, eliminating the need for an electron-transfer mediator, and showed catalytic inactivity for glucose oxidation, thus potentially enabling the development of separator-free BFCs.