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Developing highly active catalysts for hydrogen evolution reaction based on earth-abundant materials is challenging. Nitrogen doping has recently been reported to improve catalytic properties by modifying the electrochemical properties of titanium carbide MXene. However, systematic doping engineering, such as optimization of doping concentration, doping site, and thermodynamic phase stabilization have not been systematically controlled, which retards the reliable production of high-activity MXene catalysts. In this study, the optimum doping concentration of nitrogen and doping process conditions on O-functionalized Ti2C MXene for hydrogen evolution reaction were investigated using density functional theory with thermodynamics. To confirm the optimum nitrogen concentration, the catalytic properties are examined considering the Gibbs free energy of hydrogen adsorption and conductivity for 2.2-11.0 at % nitrogen concentration. It was confirmed that 8.8 at % nitrogen-doped Ti2CO2 had optimum catalytic properties under standard conditions. Moreover, when the doping concentration was higher, the decrease in the adsorption energies of hydrogen and the transition in the energy dispersion of the conduction band led to deterioration of the catalytic properties. Through theoretical results, the feasible process conditions for optimum nitrogen concentration while maintaining the structure of MXene are presented using a thermodynamics model taking into account chemical reactions with various nitrogen sources. This study provides further understanding of the nitrogen-doping mechanism of Ti2CO2 for hydrogen evolution reactions.

Borosulfates are classified as silicate analogue materials. The number of crystallographically characterized compounds is still limited, whereas the structural diversity is already impressive. The anionic substructures of borosulfates exhibit vertex-connected (BO4 )- and (SO4 )-tetrahedra, whereas bridging between two (SO4 )- or even between two (BO4 )-tetrahedra is scarce. The herein presented compound Sr[B3 O(SO4 )4 (SO4 H)] is the first borosulfate with a triple-vertex linkage of three (BO4 ) tetrahedra via one common oxygen atom. DFT calculations complement the experimental studies. Bader charges (calculated for all atoms) as well as charge-density calculations give hint to the electron distribution within the anionic substructure and density-of-states calculations support the interpretation of the bonding situation.

In this study, vertically aligned p–n junction TiO2 nanotubes can be formed by anodization of the Ti substrate following the addition of a high electric field in a Pd precursor-containing electrolyte. The bottom region of the TiO2 nanotubes with a high concentration of Pd because of the high electric field which was induced to be p-type. In contrast, the region with a low Pd concentration (top of the TiO2 nanotubes) was determined to be n-type, similar to the pristine TiO2. The concentration profile of the dopant in TiO2 nanotubes was investigated via TOF-SIMS and XPS. Defect formation energies in TiO2 nanotubes were estimated using density-functional theory calculation to understand the p–n junction formation. The p–n junction TiO2 nanotubes showed a high photocatalytic hydrogen production rate of 25.2 μL cm−2 h−1 under solar light irradiation as a result of the enhancement of visible light photoactivity.

Brookite, the least studied crystalline phase of TiO2, recently has been found to have excellent photocatalytic activities, comparable to that of anatase TiO2. However, its activity is highly depen...

Here we report the miscibility gap of CuSb1-xBixS2 (CABS), a promising photo energy conversion material for band gap engineered solar cells, and evaluate its applicability via a combination of theoretical predictions and experimental verifications. Our ab initio calculations and thermodynamic modeling revealed that the CABS random alloy system has optimal band gap values in the range of 1.1–1.5 eV when synthesized at room temperature. The CABS system, synthesized by mechanochemical methods, exhibited optical band gap values in very good agreement with theoretical predictions, as well as lowered kinetic energy barriers for enhanced nucleation.

The development of cost-effective and high-performance electrocatalysts for water oxidation has attracted intense research interest. It was reported recently that the interface between the amorphous and crystalline phases plays a significant role in the electrocatalytic activity of transition metal compounds. It was reckoned therefore that an increase in the density of the crystalline–amorphous phase boundary would enhance the electrochemical water oxidation on the catalyst. In this work we develop a new and facile strategy for inducing high density crystalline–amorphous phase boundaries via selective fluorination surface doping. This resulted in excellent characteristics of the engineered material for electrochemical water splitting. An initial computational simulation is carried out to design the crystalline–amorphous phase boundary material and an experimental verification follows for demonstration and optimization of the impact of surface doping. We conclude that the engineering of the interface using this facile and cost-effective strategy maximizes the crystalline and amorphous phases of metal–metalloids, which can be used to fabricate low-cost and efficient electrocatalysts for water oxidation.

Highlights Single-atom M-N4/N-doped carbons (M = Fe, Co) prepared as OER/ORR catalysts.Ultrasonication-assisted plasma engineering used for catalyst synthesis.Co-N4/NC outperformed benchmark commercial catalysts in practical Zn–air battery test.DFT calculations provided insights into the origin of superior ORR/OER performance. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-020-00581-4, As bifunctional oxygen evolution/reduction electrocatalysts, transition-metal-based single-atom-doped nitrogen–carbon (NC) matrices are promising successors of the corresponding noble-metal-based catalysts, offering the advantages of ultrahigh atom utilization efficiency and surface active energy. However, the fabrication of such matrices (e.g., well-dispersed single-atom-doped M-N4/NCs) often requires numerous steps and tedious processes. Herein, ultrasonic plasma engineering allows direct carbonization in a precursor solution containing metal phthalocyanine and aniline. When combining with the dispersion effect of ultrasonic waves, we successfully fabricated uniform single-atom M-N4 (M = Fe, Co) carbon catalysts with a production rate as high as 10 mg min−1. The Co-N4/NC presented a bifunctional potential drop of ΔE = 0.79 V, outperforming the benchmark Pt/C-Ru/C catalyst (ΔE = 0.88 V) at the same catalyst loading. Theoretical calculations revealed that Co-N4 was the major active site with superior O2 adsorption–desorption mechanisms. In a practical Zn–air battery test, the air electrode coated with Co-N4/NC exhibited a specific capacity (762.8 mAh g−1) and power density (101.62 mW cm−2), exceeding those of Pt/C-Ru/C (700.8 mAh g−1 and 89.16 mW cm−2, respectively) at the same catalyst loading. Moreover, for Co-N4/NC, the potential difference increased from 1.16 to 1.47 V after 100 charge–discharge cycles. The proposed innovative and scalable strategy was concluded to be well suited for the fabrication of single-atom-doped carbons as promising bifunctional oxygen evolution/reduction electrocatalysts for metal–air batteries. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-020-00581-4

Charge storage in supercapacitors is strongly related to the bond characteristics and electronic structure of electrode materials. Graphene-based materials are widely used in a supercapacitor due to the easily tunable properties and high surface/volume ratios. However, we claim that the typical covalent bond characteristics of 2D carbon materials originating from this 2pπ orbital is not very suitable to the application in supercapacitor. Here, we suggest an efficient way to improve the supercapacitor performance by tuning the covalency of bonding between the graphene-based electrode and potassium ion. We, for the first time, also introduce a simple solvent-free scale-up doping technique to prepare fluorine-doped graphene oxide (FGO) by direct plasma treatment on graphene oxide (GO) powder at ambient pressure. The FGO enabled fast electrochemical charge transfer and provided a large number of active sites for redox reactions during supercapacitor operation, and those mechanisms were thoroughly studied by various electrochemical analyses. As a result, the fabricated symmetric supercapacitor using FGO electrodes exhibited a maximum power density (~3200 W/kg) and energy density (~25.87 Wh/kg) with superior cycle stability (20000 cycles) without capacitance loss. Furthermore, the computational calculation results clarified the roles of semi-ionic C–F bonding of FGO: huge charge accumulation on the electrodes and superior electrical conductivity. Thus, our study demonstrates a facile strategy to develop promising functionalized materials, which can enhance the viability of supercapacitor for the next generation of energy storage systems.

The catalytic mechanism of the initial reaction of lithium oxides formation on buckled graphitic carbon nitrides (g-C3N4) as a catalyst for non-aqueous Li–O2 batteries was investigated using density function theory. The overpotential of heptazine were superior to triazine and precious metal. It was revealed that the difference in overpotential between heptazine and triazine was mainly a result of the gap of charge voltages. Using potential-dependent surface phase diagram, it was confirmed that the charge voltage was determined by the LiO2 formation. The hybridization of Li and bound N in the cavity revealed a clear distinction between substrates. Through the charge analysis, these results stemmed from the electron difference of bound N that was caused by the cavity structural property depending on the substrates. These findings may provide insight for designing of N-doped carbon materials as catalysts for Li–O2 batteries, which possess overpotential that are lower than existing catalyst materials.

A combined solvothermal method and post-annealing to synthesize highly active ZnS-ZnO nanosheets is presented. ZnO was produced on ZnS nanosheets by thermal annealing ZnS/ethylenediamine in air. From combined studies of experimental and computational works, we revealed that during annealing, carbon (C) element was a major dopant from the decomposition of ethylenediamine, which was unintentionally doped into the ZnS-ZnO, and gave major impacts on enhanced visible-light photocatalysis. Rhodium (Rh) was deposited by in situ photoreduction to form a ZnS-ZnO/Rh catalyst composite. This multijunctioned photocatalyst was outstanding for H2 generation from pure water under solar simulating light due to effective charge separation by Z-scheme heterojunction of ZnS and ZnO and Schottky junction of Rh-cocatalysts/semiconductors. This facile method realizes multijunctioned ZnS-ZnO/Rh photocatalysts with substantial defects that are very promising for solar-energy harvesting applications.

The feasibility of designing two-dimensional (2D) materials was investigated using monolayer Cr 2 C, a member of the MXene family, with F and OH functional groups on each side of the surface. Interestingly, this study shows that the magnetic and electronic properties of Cr 2 C with F and OH functional groups (i.e., Cr 2 CF(OH), with one functional group on each side) can be predicted using previously known properties of Cr 2 C with only one kind of functional group (i.e., either F or OH on both sides). Unlike typical functionalized 2D materials with different functional groups, each functional group of Cr 2 CF(OH) has a negligible effect on the magnetic and electronic properties of the other side. This unique phenomenon results from the small differences in geometry between the functional groups, such as the coordination number, bond length, and thickness; the properties themselves come from those of Cr 2 CF 2 and Cr 2 C(OH) 2 , despite having different functional groups. Due to the preserved magnetic moments of the Cr atoms bound with each functional group, Cr 2 CF(OH) exhibits ferrimagnetism. Likewise, semiconducting characters are present because the atomic contributions of each energy level from Cr 2 CF 2 and Cr 2 C(OH) 2 are maintained. Thus, the present work demonstrates that a combination of functional groups that shows certain desired properties can be selected when designing Cr 2 C-based spintronic and electronic devices.

As a strategy to improve the catalytic performances of two-dimensional transition metal dichalcogenide materials, doping and defect engineering are widely used. However, it is extremely challenging to find proper new materials and doping/defect engineering conditions relying only on experimental trial-and-error. In this study, use of vanadium disulfide (VS2) was suggested as an effective cathode catalyst for Li-air batteries (LABs), under the condition of proper doping engineering to optimize electrochemical performances. To investigate the dopant screening and doping processing optimization of VS2, a theoretical concept, which combines first-principles calculations and thermodynamic modeling, was first derived and suggested. Through our approach, it is worth noting that F- and N-doping on H-VS2 shows superior overpotential properties (0.68 V and 0.76 V) than the carbon-based cathode and NH3 and O2 as reactant for T-VS2, and N2 and H2 for H-VS2 are necessary to control the N doping on VS2. Our theoretical work provides the guideline for application of VS2 to cathode of LABs and leads to further insights for designing a new cathode materials based on two-dimensional materials.

Herein, holey, thin, conductive nickel substituted cobalt molybdenum boride (Ni-CMB) nanosheets have been designed to obtain superior electrochemical HER performance with small overpotential of 69 mV at 10 mA cm-2 current density and lower Tafel slope of 76.3 mV dec-1 in alkaline medium. Incorporation of Ni leads to improved conductivity and favorable hydrogen adsorption on Mo sites, which collectively yield efficient electrocatalytic H2 production from Ni-CMB catalyst. The ultrathin nature (thickness = 5.0 nm) of the designed material expectedly helps to attain high exposure of active sites and facile charge transportation through the nanosheets. Additionally, the decorated mesopores (average size = 3.86 nm) on nanosheets have benefitted towards faster electrolyte diffusion, easy gas escape from catalyst surface to support high electrocatalytic performance. Finally, well-maintained morphology of the sample and evolution of HER active sites in the material have guaranteed long-term, sustainable hydrogen production even at high current densities, which clearly demonstrate its superiority over an expensive electrolyzer (Pt-C) in alkaline water.

Perpendicular magnetic anisotropy (PMA) in thin film is the key factor to obtain good properties for high density storage devices. Although the importance of the properties of PMA is well-known for Co/Pd thin film, it is still unclear which surface structure and composition indicate that PMA is present. In this work, the surface structure and magnetic properties of L10-ordered Co/Pd (001) were analyzed using density functional theory calculations. It was confirmed that only Pd-rich A among the facile surface structures indicates PMA properties. However, according to the calculated surface energy, not only Pd-rich A but also Co-rich B is among the most energetically stable structures. The density of states showed a clear distinct electronic structure between Pd-rich A and Co-rich B derived from the structural difference. This result indicates that PMA is not always present in a sandwich structure such as Pd-rich A. The results provide a useful guide to magnetic devices created using L10-ordered Co/Pd (001) on substrate Pd.