•γ-CuCl is an intrinsic p-type semiconductor owing to the low formation energy of Cu vacancy.•The low diffusion barrier of Cu vacancy make the long-standing n-type conductivity cannot be realized in γ-CuCl.•The microscopic origin of doping asymmetry in γ-CuCl is discussed.Doping asymmetry is a pervasive issue in wide band gap semiconductors. We demonstrated that γ-CuCl is one of them with an intrinsic p-type semiconductor by first-principles calculations. The valence band maximum of γ-CuCl is dominated by the antibonding state of Cu-3d and Cl-3p, resulting in a high energy position. We further find that Cu vacancy has a relatively low diffusion barrier in addition to its low formation energy, implying that the long-standing n-type conductivity is hard to realize in γ-CuCl even with non-equilibrium approaches.

Tin (Sn) halide perovskite absorbers have attracted much interest because of their nontoxicity as compared to their lead (Pb) halide perovskite counterparts. Recent progress shows that the power conversion efficiency of FASnI3 (FA = HC(NH2)2) solar cells prevails over that of MASnI3 (MA = CH3NH3). In this paper, we show that the organic cations, i.e., FA and MA, play a vital role in the defect properties of Sn halide perovskites. The antibonding coupling between Sn-5s and I-5p is clearly weaker in FASnI3 than in MASnI3 due to the larger ionic size of FA, leading to higher formation energies of Sn vacancies in FASnI3. Subsequently, the conductivity of FASnI3 can be tuned from p-type to intrinsic by varying the growth conditions of the perovskite semiconductor; in contrast, MASnI3 shows unipolar high p-type conductivity independent of the growth conditions. This provides a reasonable explanation for the better performance of FASnI3-based solar cells in experiments with respect to the MASnI3-based solar cells.

•Indium is miscible with Mg, while Al, Ti, and Nb prefer to the inner layers of Mg.•The existence of Ti or Nb atoms enhances the diffusion of H atoms into bulk Mg.•Ti/Nb is beneficial to the formation of H–Mg–H trilayer, as well as Mg hydride.We have systematically studied the role of four typical metal impurities (Ti, Nb, Al, and In) in the diffusion of hydrogen atoms into magnesium bulk by first-principles calculations. We find that Ti, Nb, and Al energetically prefer to substitute Mg atoms in the inner layers rather than the outmost layer, which In favors, with the consideration of H adsorption. The existence of the subsurface Ti or Nb atoms enhances hydrogen atom diffusion with respect to the pure Mg system, beneficial to the formation of H–Mg–H trilayer structure and its subsequent transition to Mg hydride. The doped Al or In atoms, however, provide no obvious help to the formation of MgH2.

•H-bonding and dative bonding show a synergistic effect on Si/Ge(100).•H2O and HF are found to be energetically favored to cluster with the pre-adsorbed H2O.•Dissociation barriers of H2O on Ge(100) decrease significantly when assisted by H-bonding, especially with HF.•H2O molecule dissociates spontaneously on Si(100) when catalyzed by HF.The adsorption of various combinations of three polar molecules (NH3, H2O, and HF) on Si/Ge(100) surface is studied by first-principles calculations. On a given pre-adsorbed substrate, the H-bonding and dative bonding always show a synergistic effect, together with the electrostatic attraction, dominating the adsorption and dissociation of polar molecules on Si/Ge(100). Both H2O and HF are found to be energetically favored to cluster with the pre-adsorbed H2O molecule on Si/Ge(100) surface. Catalyzed by H2O, the dissociation barriers of H2O on Ge(100) decrease from ~ 0.7 to ~ 0.4 eV, in comparison with the nearly an order of magnitude reduction on Si(100). Furthermore, H2O molecule dissociates spontaneously on Si(100) and the barriers are lowered to ~ 0.2 eV on Ge(100) when catalyzed by HF, providing an efficient approach for dissociation of H2O on Si(100) and Ge(100) surfaces.

We demonstrate that the competition between Pauli exclusion and H-bonding dominates the adsorption of H2O on silicene through first-principles calculations. It explains the bewildering problem that isolated H2O is inert on silicene while isolated NH3 tends to chemisorption. Moreover, Pauli exclusion can be overcome by the synergetic effect of Si···O dative bonding and intermolecular H-bonding. As a result, H2O molecules are readily to chemisorb in clusters. It is expected that the competition is in general polar molecule adsorption on silicene and, thus, crucial for the adsorption mechanism.

Highlights•Rich phase transitions were predicted in SrMnO3 under negative pressures.•The required negative pressure could be reduced by doping with ions of larger radii.•The G-AFM configuration of cubic SrMnO3 could be destabilized, in line with experiments.A magnetic configuration phase transition from antiferromagnetic (AFM) to ferromagnetic (FM) ordering is observed in cubic SrMnO3 under negative pressure of −9 GPa by density functional calculations, while the ground state of G-type AFM ordering is maintained under the positive pressure. To realize the negative pressure, SrMnO3 with chemical pressure by Sr site doping of Ba and La ions is further investigated, respectively. It is found that the required negative pressure is reduced by ∼1 GPa for the cubic SrMnO3 favoring FM configuration when doped with Ba, though it keeps C- or G-AFM under positive pressures. In addition, the stability of G-AFM configuration is destroyed in cubic SrMnO3 when doped with La under various pressures.

The experimental observation that H2O molecules could be dissociated with a unity sticking coefficient on an Si(100)-2×1 surface in low temperatures (especially as low as 80 K) was not well understood because earlier theoretical attempts showed a relative high dissociation barrier. Herein, using density-functional theory calculations, we find that the additional H2O molecule tends to cluster with the preadsorbed one, with corresponding adsorption energy as high as 0.88 eV. In particular, the calculated barriers of two possible dissociation paths are lowered by nearly an order of magnitude, with the second H2O molecule acting as a catalyst. This leads to the feasibility of the H2O molecule dissociating at low temperatures.

The interactions of a hydrogen atom with clean, vacancied, and transition metal-doped (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Au, Pt) Mg(0001) surfaces are investigated using first-principles calculations. The H adsorption on Mg(0001) with TMs doped within the second layer is generally more stable than that on clean Mg but clearly weaker than that on Mg surfaces with TM in the first layer. We find, however, that all these TM atoms prefer to substitute for the Mg atoms in the second layer rather than for those in the outermost layer of the Mg surface. To enhance the catalytic effect of the TM dopants, we investigated various co-doping conditions of TMs, and we found that i) Ti is a good “assistant” that stabilizes co-doped Co, Ni, Pd, Ag, Pt, and Au within the first layers and that ii) Ni and Co are more easily incorporated into the first layer of a Mg surface when co-doped with Ti, V, and Nb. These observations may lead to a possible approach to stabilize the TM dopants within the first layer and thus promote the hydrogenation of Mg accordingly.Highlights► H adsorption is enhanced with transition metals (TMs) in the outermost layer of Mg. ► However, most TMs favor incorporation into the second layer of Mg surfaces. ► Ti can stabilize the co-doped Co, Ni, Pd, Ag, Pt, and Au within the first layers. ► Ni and Co are easily incorporated into the first layer with co-doped Ti, V, and Nb.

The interaction of H2 with clean, Ni and Nb doped Mg(0001) surface are investigated by first-principles calculations. Individual Ni and Nb atoms within the outermost surface can reduce the dissociation barrier of the hydrogen molecule. They, however, prefers to substitute for the Mg atoms within the second layer, leading to a weaker catalytic effect for the dissociation of H2, a bottleneck for the hydriding of MgH2. Interestingly, co-doping of Ni and Nb stabilizes Ni at the first layer, and results in a significant reduction of the dissociation barrier of H2 on the Mg surface, coupled with an increase of the diffusion barrier of H. Although codoped Ni and Nb shows no remarkable advantage over single Nb here, it implies that the catalytic effect could be optimized by co-doping of “modest” transition metals with balanced barriers for dissociation of H2 and diffusion of H on Mg surfaces.

Density functional theory (DFT) calculations are employed to study H2O and CO dissociations on a set of CuNi bimetallic surfaces aiming at exploring the optimal Ni ensemble on Cu(111) for an efficient water–gas shift (WGS) process, i.e., splitting H2O with high reactivity and avoiding CO activation. We found that Ni additives in the Cu(111) surface layer including a Ni monomer can remarkably enhance water splitting. Meanwhile, H2O dissociation barriers (Eact) are strongly correlated with the H adsorption energies (Ead): the larger Ead, the smaller Eact. Moreover, H2O dissociation may be more practical via the dissociated H* rather than OH closing to Ni atoms. For the scission of the C–O bond, the process is unfavorable on Ni monomers, though it is obviously promoted on Ni dimers, trimers, and other ensembles with higher Ni content. It is deduced that the selectivity of the Cu–Ni bimetallic catalysts toward WGS would decrease with increasing Ni concentration. These findings suggest that the bimetallic CuNi catalysts with highly dispersed Ni ensembles containing lower Ni concentration should exhibit high performance for the WGS process.

Ni additives into Cu catalyst can enhance the activity to the water–gas shift (WGS) reaction. However, an undesirable side reaction (methanation) would arise synchronously, consequently sharply degrading the selectivity to WGS. Herein, we propose an improved CuNi model system with potential excellent performance (both activity and selectivity) toward WGS, i.e., the inverse NiO1–x/Cu(111) (x < 1). The unsaturated Niδ+ species are expected to facilitate the rate-limiting step of WGS remarkably, H2O dissociation, and subsequently, a rather smooth potential energy surface is found in the rest of the steps of WGS over the interface of NiO1–x/Cu(111), indicating a high reactivity. Meanwhile, a weak interaction between CO and NiO1–x and a low activity of NiO1–x/Cu(111) toward CO dissociation imply that the oxidized Niδ+ species can effectively suppress the undesirable methanation found in CuNi catalysts, expecting to improve its selectivity toward WGS. The model system may be also applied to catalyze CO oxidation at proper conditions.

We have studied hydrogen adsorption on the Mg(0001) surface under biaxial strain, using density-functional theory calculations. A phase diagram is obtained for an intuitive sense of how the strain and hydrogen chemical potential affect the structural stabilities of Mg–H system. It is found that the compressive (negative) strains facilitate the formation of the H–Mg–H trilayers, a precursor of the transition to magnesium hydride, due to the fact that the lattice constant of H–Mg–H trilayer is shorter than that of pure Mg. However, the magnesium hydride is more energetically favored with greater lattice constant caused by tensile (positive) strains which exceed +6%. During the hydrogenation, the H–Mg–H trilayer and MgH2 bulk-like structures could be coexisting, where the strain is able to modulate their relative stabilities. These findings are helpful for the understanding of hydrogenation/dehydrogenation of the Mg–H system and could ultimately improve the design of Mg-based hydrogen storage materials.

NO co-adsorption with X (X = Na, O, S, and Cl) on Au and Pd(111) surfaces is studied using density functional theory (DFT) calculations to get a deeper insight into the extraordinary sulfur enhanced adsorption on the Au surface. It is found that both electronegative and electropositive adatoms can enhance NO adsorption on Au(111). In Na + NO/Au(111), the strong electrostatic attraction between Na and NO dominates and stabilizes NO adsorption, though Na-induced surface negative charging weakens NO adsorption. In (O, S, Cl) + NO/Au, the electronegative atoms would induce a slight surface distortion and enhance NO adsorption accordingly. NO adsorption on Pd(111) is enhanced by Na, but weakened by electronegative species. We suggest that the unique features of noble metals, i.e., the narrow DOS at the Fermi level (EF) and the deep buried d-band center, should play an important role in the promotion of NO adsorption on their surface as the CO case.

Density functional theory (DFT) calculations are performed to investigate CO and O2 adsorption as well as CO oxidation on the AumPdn (m + n = 2–6) bimetallic clusters. It is found that the adsorption energies of both CO and O2 on AumPdn (m + n = 2–6) are greater than those on the pure gold clusters of corresponding sizes, and unexpectedly greater than those on Pd clusters in some cases. At the same time, the calculated reaction barrier of CO oxidation on Au2Pd is lower than those on Au3 and Pd3, indicating that Au/Pd bimetallic cluster could potentially have a better catalytic activity for CO oxidation potentially.Graphical abstractHighlights► CO and O2 could be more stable on Au/Pd than Au or Pd cluster of same size. ► CO oxidation barrier on Au2Pd is found to be lower than that on Au3 and Pd3. ► Au/Pd bimetallic clusters could have higher activity for CO oxidation.

S and CO adsorption and coadsorption on Pd(111), Au(111), and PdAu(111) surfaces were studied, with the aim of providing insight into the poisoning effects of S and searching for novel sulfur-tolerant catalyst materials by first-principles calculations. It was found that both neighboring and next-nearest-neighboring adsorption sites are poisoned by a single preadsorbed S atom for CO adsorption on Pd(111). On Au(111), Ead of CO increases because of preadsorbed S, indicating that preadsorbed sulfur does not always act as a poison. The bimetallic PdAu(111) surface was found to have better S poison resistance than the Pd(111) and better activity than the Au(111) monometallic surfaces for CO adsorption. Meanwhile, the CO stretching frequencies on each surface with and without S were calculated to depict the effects of S preadsorption on the C−O bond strength.

Zr mono-doped and (Zr, N) co-doped ZnO are investigated by the first-principles calculations. It is found that Zr prefers to substitute Zn site under most growth conditions. The passive (N–Zr–N) complexes create a fully occupied impurity band above the valence-band maximum (VBM) of ZnO, which helps p-type conductivity by reducing the ionization energy, consistent with a new approach to overcome the doping asymmetry [Y.F. Yan, J.B. Li, S.H. Wei, and M.M. Al-Jassim, Phys. Rev. Lett. 98 (2007) 135506]. In comparison with (Ga, N) co-doping, (Zr, N) is found to be probably better dopants to push p-type conductivity in ZnO through the new approach with easier formation of the passive impurity band.

NO co-adsorption with X (X = Na, O, S, and Cl) on Au and Pd(111) surfaces is studied using density functional theory (DFT) calculations to get a deeper insight into the extraordinary sulfur enhanced adsorption on the Au surface. It is found that both electronegative and electropositive adatoms can enhance NO adsorption on Au(111). In Na + NO/Au(111), the strong electrostatic attraction between Na and NO dominates and stabilizes NO adsorption, though Na-induced surface negative charging weakens NO adsorption. In (O, S, Cl) + NO/Au, the electronegative atoms would induce a slight surface distortion and enhance NO adsorption accordingly. NO adsorption on Pd(111) is enhanced by Na, but weakened by electronegative species. We suggest that the unique features of noble metals, i.e., the narrow DOS at the Fermi level (EF) and the deep buried d-band center, should play an important role in the promotion of NO adsorption on their surface as the CO case.

Tin (Sn) halide perovskite absorbers have attracted much interest because of their nontoxicity as compared to their lead (Pb) halide perovskite counterparts. Recent progress shows that the power conversion efficiency of FASnI3 (FA = HC(NH2)2) solar cells prevails over that of MASnI3 (MA = CH3NH3). In this paper, we show that the organic cations, i.e., FA and MA, play a vital role in the defect properties of Sn halide perovskites. The antibonding coupling between Sn-5s and I-5p is clearly weaker in FASnI3 than in MASnI3 due to the larger ionic size of FA, leading to higher formation energies of Sn vacancies in FASnI3. Subsequently, the conductivity of FASnI3 can be tuned from p-type to intrinsic by varying the growth conditions of the perovskite semiconductor; in contrast, MASnI3 shows unipolar high p-type conductivity independent of the growth conditions. This provides a reasonable explanation for the better performance of FASnI3-based solar cells in experiments with respect to the MASnI3-based solar cells.