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The interaction between adjacent active sites is crucial to balance the efficiency and utilization of functional atoms in single-atom catalysts. Herein, the catalytic activity of hydrogen evolution reaction at different site (nitrogen coordinated transition metal centers embedded in graphene) distances was comprehensively investigated by density functional theory calculations. The results show that a proximity effect of reactivity and site spacing can be identified in the Co-series single-atom catalysts. Although the proximity effect is more linearly responded with the site spacing along x direction, an optimal distance of ∼0.8 and ∼2.8 nm are found for Co and Rh, Ir atoms, respectively. An in-depth analysis of the electronic property reveals that the proximity effect is caused by the distinct net charge of the active site, which is affected by the d z 2 position relative to EF. Subsequently, an excess electron nodal channel in x direction was found to serve as a communication pathway between the active sites. Through the finding in this work, an optimal Fe-N2C2 structure was deliberately designed and has shown prominent proximity effect as Co-series do. The results reported in this work provide a simple and effective tuning method for the reactivity of a single-atom catalyst. [ABSTRACT FROM AUTHOR]

Understanding how the electronic state of transition metal atoms can influence molecular adsorption on a substrate is of great importance for many applications. Choosing NH3 as a model molecule, its adsorption behavior on defected SnS2 monolayers is investigated. The number of valence electrons n is controlled by decorating the monolayer with different transition metal atoms, ranging from Sc to Zn. Density-Functional Theory based calculations show that the adsorption energy of NH3 molecules oscillates with n and shows a clear odd–even pattern. There is also a mirror symmetry of the adsorption energies for large and low electron numbers. This unique behavior is mainly governed by the oxidation state of the TM ions. We trace back the observed trends of the adsorption energy to the orbital symmetries and ligand effects which affect the interaction between the 3σ orbitals (NH3) and the 3d orbitals of the transition metals. This result unravels the role which the spin state of TM ions plays in different crystal fields for the adsorption behavior of molecules. This new understanding of the role of the electronic structure on molecular adsorption can be useful for the design of high efficiency nanodevices in areas such as sensing and photocatalysis. [ABSTRACT FROM AUTHOR]

Two-dimensional material-supported single metal atom catalysts have been extensively studied and proved effective in electrocatalytic reactions in recent years. In this work, we systematically investigate the OER catalytic properties of single metal atoms supported by the NiN2 monolayer. Several typical transition metals with high single atom catalytic activity, such as Fe, Co, Ru, Rh, Pd, Ir, and Pt, were selected as catalytic active sites. The energy calculations show that transition metal atoms (Fe, Co, Ru, Rh, Pd, Ir, and Pt) are easily embedded in the NiN2 monolayer with Ni vacancies due to the negative binding energy. The calculated OER overpotentials of Fe, Co, Ru, Rh, Pd, Ir and Pt embedded NiN2 monolayers are 0.92 V, 0.47 V, 1.13 V, 0.66 V, 1.25 V, 0.28 V, and 0.94 V, respectively. Compared to the 0.57 V OER overpotential of typical OER noble metal catalysts IrO2, Co@NiN2 and Ir@NiN2 exhibit high OER catalytic activity due to lower overpotential, especially for Ir@NiN2. The high catalytic activity of the Ir embedded NiN2 monolayer can be explained well by the d-band center model. It is found that the adsorption strength of the embedded TM atoms with intermediates follows a linear relationship with their d-band centers. Besides, the overpotential of the Ir embedded NiN2 monolayer can be further reduced to 0.24 V under −2% biaxial strain. Such findings are expected to be employed in more two-dimensional material-supported single metal atom catalyzed reactions. [ABSTRACT FROM AUTHOR]

Designing p–n junctions without introducing foreign atoms has attracted considerable attention, especially as far as high carrier concentrations are concerned. Here, we systematically investigate the structural and electronic properties of polar GaN/MoSSe by first-principles calculations. The diversity of GaN and MoSSe structures and their polarization leads to versatile heterostructures with different electronic behaviors. Particularly, the band alignment can be effectively modified from type-I or type-II to type-III, forming a self-doped p–n junction. Interestingly, the carrier concentration in self-doped p–n junctions is large (>3.48 × 1012 cm−2). This unique behavior is mainly attributed to the charge redistribution and intrinsic electric field induced by polarization, leading to a shift of the band edge positions and induction of the quantum Stark effect. This work provides a perspective for regulating vdW heterostructures and shows possibilities for designing self-doped p–n semiconductors for low-power and multi-functional device applications. [ABSTRACT FROM AUTHOR]

Electrochemical conversion of CO2 (CO2RR) into high-value fuel is identified as one of the promising approaches to achieve carbon neutrality. The synthesis of high-efficiency CO2 reduction electrocatalysts with high C2:C1, selectivity remains a field of intense interest. Previous studies have shown that the presence of Cu(I) is beneficial for the reduction of CO2 into C2 products. However, the stable presence of Cu(I) remains controversial, especially in the negative potential window. Here we report a simple and easily scalable catalyst precursor Cu2(OH)3Cl/C, which automatically forms in-situ chlorine-doped Cu/Cu2O heterointerface during electrocatalysis. The catalyst not only exhibits a Faradaic efficiency of 33.03% but also provides a long-term stability of Cu+, gaining a stable electrolysis of 11 h, with an ethylene/methane ratio over 50. The experimental results and mechanistic studies confirm that the presence of Cl− inhibits the reduction of Cu+, inducing the formation of Cu0/Cu+, and reduces the reaction energy of the intermediate ⋆CO dimerization, thereby facilitating the formation of C2 products. This work provides a feasible way to synthesize copper ions with long-term and stable positive charge in CO2RR and expands a new way to synthesize ethylene industrial products in the future. [ABSTRACT FROM AUTHOR]

Z-scheme van der Waals heterojunctions are very attractive photocatalysts attributed to their excellent reduction and oxidation abilities. In this paper, we designed InN/XS2 (X = Zr, Hf) heterojunctions and explored their electronic structure properties, photocatalytic performance, and light absorption systematically using first-principles calculations. We found that the valence-band maximum (VBM) and conduction-band minimum (CBM) of the InN/XS2 (X = Zr, Hf) heterojunctions are contributed by InN and XS2, respectively. Photo-generated carriers transferring along the Z-path can accelerate the recombination of interlayer electron–hole pairs. Therefore, the photogenerated electrons in the CBM of the InN layer can be maintained making the hydrogen evolution reaction occur continuously, while photogenerated holes in the VBM of the Ti2CO2 layer make the oxygen evolution reaction occur continuously. The band edge positions of heterojunctions can straddle the required water redox potentials, while pristine InN and XS2 (X = Zr, Hf) can only be used for photocatalytic hydrogen evolution or oxygen evolution, respectively. Furthermore, the HER barriers can be tuned by transition metal doping. With Cr doping, the hydrogen evolution reaction (HER) barriers decrease to −0.12 for InN/ZrS2 and −0.05 eV for InN/HfS2, very close to the optimal value (0 eV). In addition, the optical absorption coefficient is as high as 105 cm−1 in the visible and ultraviolet regions. Therefore, the InN/XS2 (X = Zr, Hf) heterojunctions are expected to be excellent photocatalysts for water splitting. [ABSTRACT FROM AUTHOR]

Low activity and poor product selectivity of CO2 reduction have seriously hampered its further practical application. Introducing p-block atoms to the catalyst is regarded as a promising strategy due to the versatility of p orbitals and diversity of p-block elements. Here, we systematically studied the influence of p-block atom X (X = C, N, O, S, and Se) on CO2 catalytic properties on a Sn(200) surface by first-principles calculation. Our work shows that all the p-block atoms are relative stable with Ef in the range of −5.11 to −3.59 eV. Further calculation demonstrates that the diversity of the p-block atoms results in unique CO2 electrocatalytic activity and product selectivity. Interestingly, the p-block C atom shows bi-functional activity to form two-electron products HCOOH and CO, with the corresponding energy barriers remarkably low at about 0.19 eV and 0.28 eV. In particular, the p-block S(Se) atom appears to have striking HCOOH selectivity, with the energy barrier to form HCOOH only a quarter of that to form the CO product. This unusual behavior is mainly attributed to the adsorption strength and frontier orbital interaction between the p-block atom and intermediates. These findings can effectively provide a valuable insight into the design of highly efficient CO2 electrocatalyst. [ABSTRACT FROM AUTHOR]

Understanding how the electronic state of transition metal atoms can influence molecular adsorption on a substrate is of great importance for many applications. Choosing NH 3 as a model molecule, its adsorption behavior on defected SnS 2 monolayers is investigated. The number of valence electrons n is controlled by decorating the monolayer with different transition metal atoms, ranging from Sc to Zn. Density-Functional Theory based calculations show that the adsorption energy of NH 3 molecules oscillates with n and shows a clear odd-even pattern. There is also a mirror symmetry of the adsorption energies for large and low electron numbers. This unique behavior is mainly governed by the oxidation state of the TM ions. We trace back the observed trends of the adsorption energy to the orbital symmetries and ligand effects which affect the interaction between the 3σ orbitals (NH 3 ) and the 3d orbitals of the transition metals. This result unravels the role which the spin state of TM ions plays in different crystal fields for the adsorption behavior of molecules. This new understanding of the role of the electronic structure on molecular adsorption can be useful for the design of high efficiency nanodevices in areas such as sensing and photocatalysis.

Two-dimensional material-supported single metal atom catalysts have been extensively studied and proved effective in electrocatalytic reactions in recent years. In this work, we systematically investigate the OER catalytic properties of single metal atoms supported by the NiN 2 monolayer. Several typical transition metals with high single atom catalytic activity, such as Fe, Co, Ru, Rh, Pd, Ir, and Pt, were selected as catalytic active sites. The energy calculations show that transition metal atoms (Fe, Co, Ru, Rh, Pd, Ir, and Pt) are easily embedded in the NiN 2 monolayer with Ni vacancies due to the negative binding energy. The calculated OER overpotentials of Fe, Co, Ru, Rh, Pd, Ir and Pt embedded NiN 2 monolayers are 0.92 V, 0.47 V, 1.13 V, 0.66 V, 1.25 V, 0.28 V, and 0.94 V, respectively. Compared to the 0.57 V OER overpotential of typical OER noble metal catalysts IrO 2 , Co@NiN 2 and Ir@NiN 2 exhibit high OER catalytic activity due to lower overpotential, especially for Ir@NiN 2 . The high catalytic activity of the Ir embedded NiN 2 monolayer can be explained well by the d-band center model. It is found that the adsorption strength of the embedded TM atoms with intermediates follows a linear relationship with their d-band centers. Besides, the overpotential of the Ir embedded NiN 2 monolayer can be further reduced to 0.24 V under -2% biaxial strain. Such findings are expected to be employed in more two-dimensional material-supported single metal atom catalyzed reactions.

Z-scheme van der Waals heterojunctions are very attractive photocatalysts attributed to their excellent reduction and oxidation abilities. In this paper, we designed InN/XS 2 (X = Zr, Hf) heterojunctions and explored their electronic structure properties, photocatalytic performance, and light absorption systematically using first-principles calculations. We found that the valence-band maximum (VBM) and conduction-band minimum (CBM) of the InN/XS 2 (X = Zr, Hf) heterojunctions are contributed by InN and XS 2 , respectively. Photo-generated carriers transferring along the Z-path can accelerate the recombination of interlayer electron-hole pairs. Therefore, the photogenerated electrons in the CBM of the InN layer can be maintained making the hydrogen evolution reaction occur continuously, while photogenerated holes in the VBM of the Ti 2 CO 2 layer make the oxygen evolution reaction occur continuously. The band edge positions of heterojunctions can straddle the required water redox potentials, while pristine InN and XS 2 (X = Zr, Hf) can only be used for photocatalytic hydrogen evolution or oxygen evolution, respectively. Furthermore, the HER barriers can be tuned by transition metal doping. With Cr doping, the hydrogen evolution reaction (HER) barriers decrease to -0.12 for InN/ZrS 2 and -0.05 eV for InN/HfS 2 , very close to the optimal value (0 eV). In addition, the optical absorption coefficient is as high as 10 5 cm -1 in the visible and ultraviolet regions. Therefore, the InN/XS 2 (X = Zr, Hf) heterojunctions are expected to be excellent photocatalysts for water splitting.

Defects play a crucial role in the surface reactivity and electronic engineering of titanium dioxide (TiO 2 ). In this work, we have used an active learning method to train deep neural network potentials from the ab initio data of a defective TiO 2 surface. Validations show a good consistency between the deep potentials (DPs) and density functional theory (DFT) results. Therefore, the DPs were further applied on the extended surface and executed for nanoseconds. The results show that the oxygen vacancy at various sites are very stable under 330 K. However, some unstable defect sites will convert to the most favorable ones after tens or hundreds of picoseconds, while the temperature was elevated to 500 K. The DP predicated barriers of oxygen vacancy diffusion were similar to those of DFT. These results show that machine-learning trained DPs could accelerate the molecular dynamics with a DFT-level accuracy and promote people's understanding of the microscopic mechanism of fundamental reactions.

Low activity and poor product selectivity of CO 2 reduction have seriously hampered its further practical application. Introducing p-block atoms to the catalyst is regarded as a promising strategy due to the versatility of p orbitals and diversity of p-block elements. Here, we systematically studied the influence of p-block atom X (X = C, N, O, S, and Se) on CO 2 catalytic properties on a Sn(200) surface by first-principles calculation. Our work shows that all the p-block atoms are relative stable with E f in the range of -5.11 to -3.59 eV. Further calculation demonstrates that the diversity of the p-block atoms results in unique CO 2 electrocatalytic activity and product selectivity. Interestingly, the p-block C atom shows bi-functional activity to form two-electron products HCOOH and CO, with the corresponding energy barriers remarkably low at about 0.19 eV and 0.28 eV. In particular, the p-block S(Se) atom appears to have striking HCOOH selectivity, with the energy barrier to form HCOOH only a quarter of that to form the CO product. This unusual behavior is mainly attributed to the adsorption strength and frontier orbital interaction between the p-block atom and intermediates. These findings can effectively provide a valuable insight into the design of highly efficient CO 2 electrocatalyst.