Fig. 1. Structural polytypes of MoS2: 2H, 3R, and 1T.

Fig. 2. Structure of single-layer graphene.

Fig. 3. (a) Schematic illustration of the synthesis of Ni QD@NC@rGO composites. Reproduced with permission from Ref. [119], Copyright 2019 American Chemical Society. (b) Schematic of the structure of a MoS2-based biodegradable sensor. Reproduced with permission from Ref. [124], Copyright 2018 Springer Nature. (c) Schematic overview of the preparation of layered PAA-MoS2 nanosheets and the concept of the ionic-strength-induced electrodeposition of MoS2 nanosheets from PAA-MoS2 aqueous dispersion. Reproduced with permission from Ref. [137], Copyright 2017 Elsevier Ltd. (d) Schematic illustration of the exfoliation process. Reproduced with permission from Ref. [146], Copyright 2019 Elsevier Ltd.

Fig. 4. (a) Illustration of the synthesis of MoS2 in different solutions. Reproduced with permission from Ref. [153], Copyright 2015 Elsevier Ltd. Transmission electronic microscopy (TEM) images of MoS2/CN materials prepared at different temperatures. (b) 120 °C; (c) 160 °C; (d) 200 °C; (e) 240 °C. Reproduced with permission from Ref. [152], Copyright 2016 Elsevier Ltd.

Fig. 5. (a) Procedures for the hetero-assembly of Ni-Fe LDH nanosheets and graphene for water splitting. Reproduced with permission from Ref. [75], Copyright 2015 American Chemical Society. (b) Schematic of the configurations of N-C@P-MoS2 in the HER. Reproduced with permission from Ref. [154], Copyright 2018 Elsevier Ltd. (c) Schematic illustration for the preparation of a Co3S4@MoS2 heterostructure via a two-step temperature-raising hydrothermal procedure. Reproduced with permission from Ref. [155], Copyright 2018 Elsevier Ltd.

Fig. 6. (a) Scanning tunneling microscopy (STM) topography of as-treated MoS2 showing a mixture of 2H (bright) and 1T (dark) domains. (b) Magnified image of a domain boundary, denoted by the dashed square. (c) dI/dV spectra of the 2H and 1T phases. (d) STM image of MoS2 after hydrogenation. (e) Magnified image of the depression features, denoted by the dashed square. (f) Projected density of states (PDOS) of S and H atoms at the phase boundary before and after hydrogenation. (g) Top views of atomic models of the zigzag type 2H-1T-phase boundaries and the four kinds of 2H-2H boundaries. (h) Comparison of the Gibbs free energies of H adsorbed on the different boundaries in the HER. Reproduced with permission from Ref. [54], Copyright 2019 Springer Nature.

Fig. 7. (a) Surface energy per unit cell for 2H-MoS2 as a function of the applied potential for the basal plane of 2H-MoS2 with different adsorbate species at a fixed sulfur vacancy. (b) Surface energy per unit cell for a range of S-vacancy concentrations. (c) When several S-vacancy sites are generated in succession, new S vacancies are most stable when formed next to an existing S-vacancy. (d) Optical microscopy (left panel; scale bar: 2 mm) and SEM images (right panel; scale bar: 20 mm) of a monolayer MoS2 film before (P-MoS2) and after (V-MoS2) desulfurization. (e) X-ray photoelectron spectroscopy (XPS) of the Mo 3d and (f) S 2p peaks of pristine MoS2 (P-MoS2) and MoS2 with S vacancies (V-MoS2). Reproduced with permission from Ref. [161], Copyright 2016 Springer Nature.

Fig. 8. (a) Evolution of the structures of MoS2 and TEM image of as-synthesized MoS2. (b) TEM images of the as-synthesized MoS2. (c,d) HRTEM images of the stacked individual layers of as-synthesized MoS2. (e) EPR spectra generated by the dangling Mo-S bonds for MoS2 with various defects compared to that of 2H MoS2. (f) High-resolution XPS of the Mo 3d and S 2s regions for the 2H MoS2 electrodes. (g) Evolution of the S:Mo and Mo(UC):Mo(IV) ratios as a function of the annealing temperature of the electrodes. Reproduced with permission from Ref. [165], Copyright 2019 American Chemical Society.

Fig. 9. (a) TERS line image across the edge of bilayer MoS2. (b) Typical TERS spectra of the basal plane and the edge of bilayer MoS2. (c) Plots of the normalized intensities of the two TERS peaks (396 and 406 cm-1) with the tip position. (d) Schematic diagram of the band reconstruction and electronic transition region (ETR) of MoS2 near the edge. De is the electron density. (e) Typical line-trace TERS spectra of the zigzag edge (left panel) and armchair edge (right panel) in the spectral range of the A1g mode. (f) Plots of peak position with the tip position. (g) Calculated Raman spectra and lattice vibration of the basal plane. (h) AFM image of mechanically exfoliated single-layer MoS2 with different edge angles on an Au substrate. (i) Illustration of the relationship between the angles and edge structures. (j) TERS spectra of four edges in the spectral range of the A1g mode. Reproduced with permission from Ref. [174], Copyright 2019 Springer Nature.

Fig. 10. (a) High-angle annular dark-field imaging scanning transmission electron microscopy (HADDF-STEM) image of A-Ni@DG. (b) Magnified image of the defective area. (c) Magnified image of the defective area. Illustrations of three different types of catalytic active sites corresponding to a single Ni atom supported on (d) perfect hexagons, (e) D5775, and (f) a Di-vacancy. PDOS of the three configurations: (g) perfect hexagons, (h) D5775, and (i) a Di-vacancy. (j) Energy profiles of the three configurations for the HER. (k) HER polarization curves of A-Ni@DG, A-Ni@G, and Pt/C measured in 0.5 M H2SO4. Reproduced with permission from Ref. [177], Copyright 2018 Elsevier Ltd.

Fig. 11. (a) Comparison between the Raman spectra of graphite and SLG/FLG-DE. (b) I(D)/I(G) vs fwhm(G) (full width at half-maximum of the G band) plot for SLG/FLG-DE. (c) Statistical analysis of I(D)/I(D′) for SLG/FLG-DE. (d) Theoretical calculations of the Gibbs free energy profiles for the HER at pH = 0 for graphene. Reproduced with permission from Ref. [178], Copyright 2019 American Chemical Society. (e) Definition of the "activated" A-region (green) and "structurally-disordered" S-region (red). (f) I(D)/I(G) data points from three different monolayer graphene samples as a function of the average distance LD between defects induced by the ion bombardment procedure. Reproduced with permission from Ref. [179], Copyright 2010 Elsevier Ltd.

Fig. 12. (a,b) SEM and TEM images of a 20 wt% DRM-C sample. (c) HRTEM image of the selected area marked in (b). (d) Schematic illustration of the band structure of CdS, DFM, and DRM. (e) Schematic illustration of the photogenerated charge transfer in the DRM-C heterostructure under visible-light irradiation (>420 nm). Reproduced with permission from Ref. [184], Copyright 2018 Elsevier Ltd. (f) Schematic of the preparation processes of C-MoS2-X @CdS and MoS2@CdS nanocomposites. (g) Valence band XPS spectra of CdS. (h) Schematic illustrations of the reaction mechanism of C2-10%MoS2-X@CdS. (h) Reproduced with permission from Ref. [186], Copyright 2020 Elsevier Ltd.

Fig. 13. (a) Schematic of M?2M?Xenes. Free energy diagram of the HER process for (b) Cr2TiC2O2 and (c) Cr2VC2O2. (d) Exchange current i0 of M?2M?C2O2 at different H coverages. (e) Exchange current i0 of Cr2VC2 at an H coverage of θ=3/8, while the other three M?2M?C2 have a coverage of θ=1/4. Reproduced with permission from Ref. [187], Copyright 2018 American Chemical Society.

Fig. 14. SEM images of (a) NiFe-LDH and (b) (Ni, Fe)S2@MoS2 Mo:S-0.25:5. (c,d) Magnified SEM images corresponding to (a)-(b). (e) XPS spectra of the Mo 3d level of (Ni, Fe)S2@MoS2 and MoS2; S 2p level of (Ni, Fe)S2@MoS2, (Ni, Fe)S2, and MoS2; and Ni 2p and Fe 2p levels of (Ni, Fe)S2@MoS2 and (Ni, Fe)S2. Reproduced with permission from Ref. [197], Copyright 2019 Elsevier Ltd. (f) Schematic illustration of the synthesis and growth of hierarchical CoMoNiS-NF-xy composites. (g) XRD patterns of Ni3S2/NF, Co9S8/Ni3S2/NF, MoS2/Ni3S2/NF, and CoMoNiS-NF-31. (h,i) Theoretically optimized structures of MoS2/Co9S8 interfaces. Reproduced with permission from Ref. [202], Copyright 2019 American Chemical Society.

Fig. 15. (a) Raman spectra of monolayer MoS2 films on Au, glassy carbon (GC), Ti, Ni, and Pt substrates. (b) XPS spectra of the MoS2 films on the different substrates. (c) Tafel slope vs. the A1g peak of the MoS2 films on different substrates. (d) Calculated absorption energy of hydrogen atoms and the measured Tafel slope of the monolayer MoS2 films on different substrates. (e) Schematic illustration of the transport of electrons from the substrate to the outermost layer of the film. (f) Exchange current density of the monolayer MoS2 films on different substrates. (g) Polarization curves of the monolayer MoS2 thin films on different substrates in acidic media. Reproduced with permission from Ref. [158], Copyright 2020 American Chemical Society.

Fig. 16. (a) Proposed mechanism for the photocatalytic hydrogen production process of the CMo/CdS photocatalyst. Reproduced with permission from Ref. [213], Copyright 2018 Elsevier Ltd. (b) Photocatalytic hydrogen evolution mechanism for the CdS-MoS2 composite. Reproduced with permission from Ref. [217], Copyright 2020 Elsevier Ltd. (c) Transient photocurrent response and (d) room-temperature PL spectra of all the as-prepared samples (λex = 434 nm). (e) Schematic illustration of the proposed mechanism for the enhanced photocatalytic H2 evolution. Reproduced with permission from Ref. [216], Copyright 2019 Elsevier Ltd.

Fig. 17. (a) Schematic illustration of the synthesis procedure of MoS2/G inlaid nanosheets. Reproduced with permission from Ref. [235], Copyright 2016 American Chemical Society. (b) Illustration of the synthesis of 3D porous Pt/G-MoS2 hybrids. Reproduced with permission from Ref. [236], Copyright 2018 Elsevier Ltd. (c-e) Raman spectra, XPS spectra, and nitrogen adsorption-desorption isotherm and pore size distribution of NiS/G-3. (f,g) ΔGH* values of different catalysts determined using theoretical calculations of the proposed overall water splitting mechanism on NiS/G. Reproduced with permission from Ref. [231], Copyright 2019 Elsevier Ltd.

Fig. 18. (a) Schematic illustration of the synthesis of NG/CCN. XPS spectra and high-resolution N 1s peaks of NG/CCN (b) before calcination and (c) after calcination. Reproduced with permission from Ref. [245], Copyright 2019 Elsevier Ltd. (d,e) DOS for the CNQDs and CNQDs@G. (f) Free energy diagrams for hydrogen evolution at the equilibrium potential of the CNQDs@G and QDs@G. (g) Volcano plots of j0 as a function of the TGH* for CNQDs@G and other catalysts. Reproduced with permission from Ref. [248], Copyright 2018 American Chemical Society.

Fig. 19. Top (upper) and side (lower) views of the optimized (a) g-C3N4@BG, (b) g-C3N4@NG, (c) g-C3N4@OG, (d) g-C3N4@FG, (e) g-C3N4@PG, and (f) g-C3N4@SG hybrids. (g) Top (upper) and side (lower) views of the optimized g-C3N4@G hybrid. Blue and gray balls represent N and C atoms, respectively, and the unit cell of g-C3N4@G is indicated by solid gray lines. (h) Calculated band structure (left) and PDOS (right) of g-C3N4@G. (i) Calculated free energy diagram of the HER at the equilibrium potential for g-C3N4@G and Pt for different H* coverages. Reproduced with permission from Ref. [249], Copyright 2016 American Chemical Society.

Fig. 20. (a) Scheme of the fabrication of TiOF2@Ti3C2Tx. (b) Cross-section of the monolayer TiOF2@Ti3C2Tx sheet. (c) OH terminal bilayer structural model and adsorption configuration. (d) O terminal bilayer structural model and adsorption configuration. (e) Stable adsorption structures at the top site on O, OH, and F surface terminations. Reproduced with permission from Ref. [252], Copyright 2019 American Chemical Society.

Fig. 21. (a) Schematic illustration of the preparation of g-C3N4@Ti3C2 QD composites. Reproduced with permission from Ref. [253], Copyright 2019 American Chemical Society. (b) Calculated free-energy diagram of the HER at the equilibrium potential (U=0 V) on the surface of a 2-2-1 O-terminated Ti3C2 supercell at different H* coverage values. (c) Calculated free-energy diagram of the HER at the equilibrium potential (U = 0 V). (d) Charge separation and transfer in the CdS/Ti3C2 system under visible-light irradiation. Reproduced with permission from Ref. [255], Copyright 2017 Springer Nature.

Fig. 22. (a) HRTEM image of EBP. (b) Bright-field TEM image and (c) dark-field TEM image and corresponding element distribution image of EBP@NG(1:8). Scale bar: (a) 1 nm; (b, c) 500 nm. (d) Illustration of the interfacial charge redistribution between NG and EBP. (e) Illustration of the differential charge density of NG and EBP (blue: electron-rich areas; yellow: hole-rich areas). Calculated free energy diagrams of (f) the HER on BP and BP-NG and (g) the OER on NG and BP-NG at a potential (U) of 1.23 V. Reproduced with permission from Ref. [242], Copyright 2019 American Chemical Society.

Fig. 23. (a) Schematic diagram of the fabrication process for SA Co-D 1T MoS2. (b) 2H and (c) 1T atomic structures of MoS2 assembled with a Co atomic layer as calculated by first principles. (d) Energies of 2H MoS2 and 1T MoS2 assembled with a Co atomic layer as a function of the Co-Co distance as calculated by the first-principles method. (e) Calculated Co projected d-density of states for different coverages. (f) Electron charges of Co and S atoms adjacent to Co as a function of Co coverage. (g) Polarization curves of different catalysts tested in Ar-saturated 0.5 M H2SO4. Reproduced with permission from Ref. [262], Copyright 2019 Springer Nature.

Fig. 24. (a) Two-step synthesis of Pd,Zn-MoS2. High-resolution spectra of the (b) Mo 3d region for the Zn-MoS2 and Pd,Zn-MoS2 samples and (c) S 2p region for the Zn-MoS2 and Pd,Zn-MoS2 samples. (d) Ultraviolet photoelectron spectroscopy (UPS) of Zn-MoS2. (e) Stripe model with the Mo edge and S-edge sites. Reproduced with permission from Ref. [268], Copyright 2020 Elsevier Ltd. (f) Hydrogen adsorption free energy diagram of doped and undoped MoS2. (g) Total density of state (TDOS) for Mn-doped and undoped MoS2, respectively. Reproduced with permission from Ref. [269], Copyright 2019 American Chemical Society.

Fig. 25. (a) Schematic illustration of the preparation of N-doped graphene with Ru nanoparticles and Co atoms. TEM images of (b) NG/Ru and (c) CoNG/Ru. (d) TEM image of CoNG. Reproduced with permission from Ref. [273], Copyright 2019 American Chemical Society. (e) Variation of the calculated adhesion energies of Ir among the different models. Color code: C: gray; N: blue; cyan: Ir. (f) Top and side views of hydrogen-adsorbed models. Color code: C: gray; N: blue; cyan: Ir: yellow: H. Reproduced with permission from Ref. [275], Copyright 2019 Elsevier Ltd.

Fig. 26. Charge densities of structures with (a) one or (b) two H adsorbed on SV+3N, and with one H adsorbed on the N1-site of SV+3N+Ni; top view and 45°-tilted view. In (c), the charge density interaction between H and N2/N3 is weakened because the N2/N3 electrons are more localized along the N-Ni bonds. Reproduced with permission from Ref. [180], Copyright 2018 American Chemical Society. (d) EIS Nyquist plots for Au/STO, Cu/STO and Cu@C/STO. (e) Photoluminescence (PL) spectra of Cu/STO, Cu@C/STO (λex = 460 nm). (f) Time-resolved PL spectra of Cu/STO, Cu@C/STO. Simulated differential charge density illustrates the alterations of electron distributions with different G thickness: (g) 0 layer, (h) monolayer, (i) bilayer. Reproduced with permission from Ref. [293], Copyright 2020 Elsevier Ltd.

Fig. 27. (a) SEM image of Mo2CTx:Co, and (b) HAADF-STEM image and elemental mapping for delaminated Mo2CTx:Co sheets. (c) Schematic representation of the structure of Mo2CTx:Co. (d) Background-corrected polarization curves for Mo2CTx:Co and Mo2CTx, recorded using a scan rate of 10 mV s-1. (e) Geometric electrode area and (f) electrochemically active surface area. (g) Mo2CO2:Co model structure used for DFT calculations. (h) Computed values of ΔGH on the Mo2CO2 and Mo2CO2:Co surfaces. (i) Reaction coordinate for hydrogen evolution on Mo2CO2 and Mo2CO2. Reproduced with permission from Ref. [294], Copyright 2019 American Chemical Society.

Fig. 28. (a) Atomic structures of the chemisorption of H2O, H-OH, HO, and H on freestanding Pt55 and Pt22Ru33 NCs and Pt55 and Pt22Ru33 NCs supported on oxidized BP monolayers. (b) Reaction energy diagram of the dissociation of water on the freestanding and BP-supported Pt55 and Pt22Ru33 NCs. (c) Free energy diagrams of hydrogen evolution at zero potential on the freestanding and BP-supported NCs. Reproduced with permission from Ref. [298], Copyright 2019 American Chemical Society. (d) Schematic illustration of the synthesis of BP NSs/Pt. (e) Time courses of ErB-sensitized photocatalytic H2 evolution catalyzed by BP NSs, free Pt nanoparticles, B-BP/Pt, and BP NSs/Pt. Reproduced with permission from Ref. [299], Copyright 2019 Elsevier Ltd.

Fig. 29. (a) Schematic illustration of the synthesis of C-MoS2 and MoS2. (b) HRTEM image of C-MoS2. (c) TEM image and the corresponding element mapping images of C, Mo, and S in C-MoS2. Top and side views of the sp2-hybridized orbitals (red dashed circles) at the top of the valence band (d) and the empty 2p orbitals (red dashed circles) perpendicular to the basal plane at the bottom of the conduction band (e) of C-MoS2. Top view of the electrostatic potential of water adsorbed on the basal plane of C-MoS2 (f) and MoS2 (g), and corresponding side view of the bonding and non-bonding orbitals. (h) Relative energy diagram along the reaction coordinate, including the first (left panel) and second (right panel) water splitting processes on the basal planes of MoS2 and C-MoS2, respectively. Reproduced with permission from Ref. [302], Copyright 2019 Springer Nature.

Fig. 30. (a) Schematic illustration of the self-templated synthesis of NMoS2/CN. (b) N2 adsorption/desorption isotherms of N-MoS2/CN and MoS2/g-C3N4. (c) XPS spectra of the N 1s region with fitting curves for NMoS2/CN and NCNS. (d) Normalized Mo K-edge XANES spectra of Mo, MoN, MoS2, MoS2/CS, and N-MoS2/CN. (e) Corresponding Fourier transform (FT) of the Mo K-edge EXAFS spectra (lines) and fitting curves (circles). Reproduced with permission from Ref. [64], Copyright 2019 American Chemical Society.

Fig. 31. (a) Schematic illustration of the active sites in MoS2. (b) HER free energy diagrams for different sites in the basal plane. (c) Partial charge density distribution of a single N-MoS2 monolayer with adsorbed H. (d) Partial charge density distribution of a single N,F-MoS2 monolayer with adsorbed H. (e) Band structures of pristine MoS2. (f) Band structures of N,F-doped MoS2. Reproduced with permission from Ref. [309], Copyright 2017 American Chemical Society.

Fig. 32. (a) Schematic illustration of the synthesis of N/S co-doped graphene. Reproduced with permission from Ref. [319], Copyright 2015 Elsevier Ltd. (b) Structural models and charge density of H adsorbed on the surface of graphene, N-doped graphene, and S-doped graphene. Reproduced with permission from Ref. [320], Copyright 2010 American Chemical Society. (c) H atom combined on the C atom; H atom combined on the N (d) or S (f) dopant atoms; H atom combined on the C atom near the N (e) or S (g) dopant atoms. Several possible sulfur-doped graphene clusters. Reproduced with permission from Ref. [322], Copyright 2015 Elsevier Ltd. (h) Sulfur atoms adsorbed on the surface of a graphene cluster; sulfur atoms substituted at the (i) zigzag and (j) armchair edges; SO2 substituted at the (k) zigzag and (l) armchair edges; and (m) sulfur ring clusters connecting two pieces of graphene. Reproduced with permission from Ref. [321], Copyright 2014 American Chemical Society.

Fig. 33. (a) Comparison of photocatalytic H2 production activities of samples with different weight percentages of the cocatalysts for 1 h. (b) Different molar contents of MoS2/G with 2.0 wt% cocatalyst. (c) Photocatalytic H2 production activities of MoS2/G-CdS with 2.0 wt% cocatalyst and a MoS2 to graphene molar ratio of 1:2 that were annealed at different temperatures, and (d) their cycling behavior over 5 h. (e) Photocatalytic H2 production activities of the MoS2/G-CdS composite and Pt/CdS (0.5 wt%) with the highest H2 generation for 1 h and (f) their cycling behavior over 5 h. (g) Schematic illustration of the microstructure of MoS2 and (h) its cocatalytic mechanism of H2 generation in lactic acid solution. Reproduced with permission from Ref. [363], Copyright 2014 American Chemical Society.

Fig. 34. TEM images of graphene oxide (a) and (b) the GC1.0 sample. (c) Comparison of the photocatalytic activity of the GC0, GC0.25, GC0.5, GC1.0, GC2.0, and GC5.0 samples and N-doped TiO2 for photocatalytic H2 production from methanol aqueous solution under visible-light irradiation. (d) UV-vis diffuse reflection spectra of the GC0, GC0.25, GC0.5, GC1.0, GC2.0, and GC5.0 samples. (e) Proposed mechanism for the enhanced electron transfer in the graphene/g-C3N4 composites. Reproduced with permission from Ref. [371], Copyright 2011 American Chemical Society.

Fig. 35. (a) Calculated DOS of a single-layered Ti2CFx slab. The orange shading highlights the DOS contribution near the Fermi level. (b) Temperature-dependent resistance of Ti2CTx nanosheets. Reproduced with permission from Ref. [373], Copyright 2018 Elsevier Ltd. (c) Illustration of the synthesis of N-doped MXene from the Ti3AlC2 MAX phase. (d) XPS images of Ti3C2Tx and N-doped Ti3C2Tx MXene annealed at various temperatures. (e) Core-level XPS of the F 1s level of Ti3C2Tx and of N-doped Ti3C2Tx annealed at various temperatures. (f) ΔGH* diagram of different H adsorption states. Reproduced with permission from Ref. [376], Copyright 2019 American Chemical Society.

Fig. 36. (a) Green and yellow regions represent the HOMO and LUMO of all systems. The HOMO and LUMO of undoped BP overlap on all the P atoms, while those of the B-, C-, Si-, O-, F-, and Te-doped BP are largely separated. (b) Top view and (c) side view of the crystal structure of a 4×4×1 doped supercell of monolayer BP. Reproduced with permission from Ref. [91], Copyright 2020 Elsevier Ltd.

Fig. 37. (a) H adsorption energy ΔEH-S on a S site as a function of the S adsorption energy ΔES on the metal sites closest to the edge. (b) ΔES and ΔEH-S as a function of εd. (c) Linear fitting of ΔEH-S, ΔES, and ΔESH as a function of εd. (d) Adsorption energy ΔE of various key reaction intermediates as a function of the d-band center εd. H, O, OH, CHO, and COOH are adsorbed onto S sites with stable S and H coverage. Reproduced with permission from Ref. [377], Copyright 2014 American Chemical Society. Band structure and DOS of a bilayer graphene/MoS2 heterostructure when hydrogen is adsorbed (e) on the graphene side and (f) on the MoS2 side. Reproduced with permission from Ref. [378], Copyright 2017 American Chemical Society. Different charge densities of (g) H/Ni and (h) Gr/H/Ni. Top and side views of the H-adsorption configurations on graphene-covered metal surfaces: (i) and (j) for Ni and Cu; (k) and (l) for Pd, Rh, Pt, Au, and Ag substrates. Reproduced with permission from Ref. [379], Copyright 2016 American Chemical Society.

Fig. 38. (a) Adsorption sites for a hydrogen molecule on a graphene layer. (b) DFT t DF adsorption energy versus the lateral distance of H2 on the graphene-N. Reproduced with permission from Ref. [380], Copyright 2016 Elsevier Ltd. (c) Geometry-optimized paired groups (CONH2) with mono-H adsorption and H2 stabilized between them. (d) Orbital charge densities for efficient s-p band overlap for proton-electron exchange. (e) Comparison of the PDOS of the groups. (f) Variation in the PDOS of the paired groups during the HER process. (g) Group (CONH2)-catalyzed HER formation energy pathway under alkaline conditions. (h) Potential H-bond influence on the structural variation of a pair of functional groups. Reproduced with permission from Ref. [383], Copyright 2019 Springer Nature.

Fig. 39. (a) Top: Top view of the atomic structure of bare MXene with different adsorption sites indicated. (b) HER volcano plot for the investigated MXenes. (c) Expansion of the region at the top of the volcano. (d) Atomic structure of M2′M″C2 and M2′M2″C3 viewed from the top. 3D scatterplot of the M-O bond ICOHP, number of electrons gained by the oxygen atom (Bader charge), and ΔGH of 24 different double transition metal carbides in (e) octahedral and (f) trigonal prismatic configurations. Reproduced with permission from Ref. [385], Copyright 2016 American Chemical Society. (g) Free energy diagram of the HER process on Ti2CO2, V2CO2, Nb2CO2, Ti3C2O2, and Nb4C3O2 under standard conditions. Reproduced with permission from Ref. [384], Copyright 2017 American Chemical Society.