Fig. 1. Various species of CNMs.

Fig. 2. Recent works of CNMs in photocatalytic H2 production. (a) Quantity of related literature in the recent decade. (b) Origin country and quantity of related literature. (c) Categories and proportion of related literature. (Data from Web of Science, index keywords are “carbon”, “photocatalytic” and “hydrogen”).

Fig. 3. Schematic diagram of photocatalytic H2 production.

Fig. 4. Comparison of advantages and disadvantages of CNMs in photocatalytic H2 evolution.

Fig. 5. (a) DFT-optimized cell structures of ideal graphene (i), graphene with hexavacancy (ii), and corresponding quaternary (iii), pyridinic (iv), and pyrrolic (v) N-doped graphenes. (b) Calculated spin-polarized band structure and N 2p PDOSs of quaternary N-doped graphene. Reprinted with permission from Ref. [123]. Copyright 2015, ACS. (c) Schematic of the X-doped graphene nanoribbons, showing the possible positions of dopants. Reprinted with permission from Ref. [124]. Copyright 2015, Wiley.

Fig. 6. Pattern of CNMs in photocatalytic H2 production.

Fig. 7. Scheme of the CNMs as the supporting material under typical situations, (a) CNMs as electron acceptor, (b) CNMs as electron donor; (c) CNMs as electron acceptor and donor.

Fig. 8. (a) Schematic diagram of photocatalyst preparation process. (b) Schematic illustration of the photocatalytic hydrogen evolution mechanism in ex-TA@C catalysts. (c) HRTEM images pattern of ex-TA@C hybrids. (d) Energy band schematic illustration for TA@C and ex-TA@C hybrids. (e) UV-vis diffuse reflectance absorption spectra of TA NPs, TA@C and ex-TA@C catalysts. Reprinted with permission from Ref. [146]. Copyright 2020, Elsevier.

Fig. 9. (a) The Fourier transform of EXAFS spectra derived from EXAFS. (b) FT-EXAFS curves between the experimental data and fit. (c) The schematic multi-scale design of Pt-NCDs/TiO2 photocatalyst. Reprinted with permission from Ref. [147]. Copyright 2020, RSC.

Fig. 10. (a) Low magnification TEM images of CdS/CNF mats. (b) Time profiled photocurrent generations on CdS-loaded CNF mats under visible light. (c) Diffused reflectance UV-visible spectra of CdS-loaded CNF mats. (d) Schematic diagram for visible-light-induced H2 production of CdS loaded CNF system. Reprinted with permission from Ref. [155]. Copyright 2015, Elsevier.

Fig. 11. (a) Experimental (symbols) and fitted (solid lines) Nyquist plots of CSCPE and GPE. (b) Transient photocurrent response of bare CdS QDs and CdS/CSC. (c) A photocatalytic mechanism proposed for visible-light-driven H2 production on Pt/CdS/CSC from water containing lactic acid as hole scavenger. LA, CB and VB label the lactic acid, conduction band and valence band, respectively. Reprinted with permission from Ref. [156]. Copyright 2016, Elsevier.

Fig. 12. (a) Preparation of CdS/GDY composite and its photocatalytic process. (b) TEM image of GDY2.5. (c) Band structures of GDY and CdS. Reprinted with permission from Ref. [121]. Copyright 2019 ACS.

Fig. 13. (a) TEM image of Co-NG/g-C3N4, scale bar 500 nm. (b) LSV curves of a cathodic scan from 0 to -2.3 V (vs. Ag/AgCl) in TEOA solution. (c) Photoluminescence spectrums of an anodic scan from 0 to 1.8 V (vs. Ag/AgCl) in TEOA solution for Co-NG/g-C3N4 and g-C3N4 samples. (d) Photo-current measurement of catalysts in 0.5 mol/L Na2SO4 solution under 0.5V (vs. Ag/AgCl). Reprinted with permission from Ref. [166]. Copyright 2018, Elsevier.

Fig. 14. (a) A comparison of jH2 of CNT and CNT/Ag NWs at various potentials. (b) Raman spectra of AB, CB, GF, and CNT. (a,b) Reprinted with permission from Ref. [167]. Copyright 2019, Elsevier. (c) TEM Micrographs of Cu1/TiO2-60. (d) Schematic diagram of the g-C layer encapsulating Cu0 nanoparticles to enhance the performance of materials. Reprinted with permission from Ref. [168]. Copyright 2020, ACS.

Fig. 15. (a) Illustration of the PEC water oxidation at the NFCB photoanode. (b) Schematic diagram illustrating the separation of photo-generated electrons and holes in the water splitting system. (a,b) Reprinted with permission from Ref. [170]. Copyright 2017 RSC. (c) Schematic representation of excited-state processes for doped C-dots excited at core state (at 320 nm) and surface state (at 420 nm). Reprinted with permission from Ref. [171]. Copyright 2019, ACS.

Fig. 16. (a) UV/Vis diffuse reflectance spectra of BCN, P-TCN, and a series of P-TCN/GQDs photocatalysts with different ratios of GQDs. (b) Transient photoelectrochemical responses for BCN, P-TCN, and P-TCN/GQDs-0.15 under visible-light irradiation. (c) Possible photocatalytic mechanism for hydrogen evolution over P-TCN/GQDs-0.15 under visible light irradiation. Reprinted with permission from Ref. [172]. Copyright 2018, Wiley.

Fig. 17. Influence of different factors of CNMs on photocatalytic HER performance and its own problems.

Fig. 18. (a) Schematic illustration of the role of typical CNMs as electron acceptor and transport channel in photocatalytic H2 production. (b) Comparison of work function of typical CNMs with the CB of several typical semiconductor photocatalysts.

Fig. 19. Partial and total density of states of free H2O (i), H2O@(101) (ii), H2O@Cu/(101) (iii). The blue areas show where the electron density has been enriched with respect to the fragments, and the yellow areas show where the density has been depleted. Reprinted with permission from Ref. [263]. Copyright 2018, ACS.

Fig. 20. (a) Potential-dependent evolution in Nyquist plots of FeP/CN. (b) Cyclic voltammograms vs overpotential of FeP/XC, FeP/CNT and FeP/CN. (a,b) Reprinted with permission from Ref. [139]. Copyright 2019, RSC. (c) Proposed mechanism of enhanced visible light photocatalysis in DGRAu and GOAu hybrid photocatalysts. Reprinted with permission from Ref. [265]. Copyright 2016, RSC.

Fig. 21. (a) RhCrOx/STO:Al/N-G photocatalysts preparation method. (b) Raman spectra of G (black), N-G (red) and comG (blue). (c) High resolution XPS for C 1s recorded for N-G sample and the best deconvolution for their individual components. Reprinted with permission from Ref. [266]. Copyright 2019, Elsevier.

Fig. 22. (a) Schematic illustration of the synthesis of AA-functionalized CNTs, Pt nanoparticle deposition, and CdS nanohybrids. Digital pictures show the dispersion of CNTs, Af-CNT, Pt-Af-CNTs before and after sonication. FTIR spectrum of pristine MWCNT compared with functional groups presented in (b) Cf-CNT, Nf-CNT, Sf-CNT materials and, (c) Af-CNT, Pt-Af-CNT compared with pure ascorbic acid. Reprinted with permission from Ref. [97]. Copyright 2018, Elsevier.

Fig. 23. (a) Schematic illustration for the synthetic process of hierarchical Co/NGC@ZIS cages. (b) TEM image of p-Co/NGC nanocages; Steady-state PL spectra (c) and EIS spectra (d) of Co/NGC@ZIS and ZnIn2S4. Reprinted with permission from Ref. [270]. Copyright 2019, Wiley.

Fig. 24. (a) Schematic of the formation of the TiO2-x/CNT composites. (b) UV-vis spectra with different samples. (c) Nyquist plots for TiO2, TiO2-x, TiO2/CNT and TiO2-x/CNT. Reprinted with permission from Ref. [284]. Copyright 2017, RSC.

Fig. 25. (a) Schematic diagram of synthesis process for Zn0.5Cd0.5S-MWCNT-TiO2 ternary nanocomposites. (b) Proposed electron/hole transfer mechanism for MWCNT-mediated Z-scheme system. (c) Estimated relative band positions of Zn0.5Cd0.5S-MWCNT-TiO2 nanocomposite. Reprinted with permission from Ref. [285]. Copyright 2017, Elsevier.

Fig. 26. (a) HRTEM images of TiO2/GY-5. (b) Photoluminescence spectra of TiO2/γ-graphyne nanocomposites at an excitation wavelength of 330 nm. (c) UV-vis diffuse reflectance spectra of TiO2/γ-graphyne nanocomposites. (d) Schematic illustration of possible band structures of TiO2/γ-GY nanocomposites. Reprinted with permission from Ref. [119]. Copyright 2018, RSC.

Fig. 27. Schematic illustrating the photocatalytic mechanism for the Ag-TiO2 (a) and Ag-rGO-TiO2 (b) samples under visible light irradiation. Reprinted with permission from Ref. [289]. Copyright 2018, Elsevier.

Fig. 28. (a) Enlarged spectra of the GO, TG, and WTG samples in the range 1100-1750 cm-1. (b) EIS Nyquist plots of the T, W, TG, WT, and WTG samples. (c) Schematic illustration of the S-scheme heterojunction-based charge transfer mechanism in WO3/TiO2/rGO composite. Reprinted with permission from Ref. [290]. Copyright 2020, Elsevier.

Fig. 29. (a) Valence band spectra, inset: the enlarged view of bend edge region and the schematic representation of energy levels. (b) Steady-state photoluminescence spectra of NiPO and RGO modified NiPO. (c) Plausible mechanism for proton reduction reaction by RGO-NiPO photocatalyst. Reprinted with permission from Ref. [291]. Copyright 2019, ACS.

Fig. 30. (a) Schematic diagram for fabrication of Co-graphene composites. Reprinted with permission from Ref. [294]. Copyright 2018, ACS. (b) EXAFS fitting curve of Pt-SAs/C R-space. (c) Calculated Gibbs free energy diagram of HER for Pt/C, Pt-SAsC4 and defect-C at the equilibrium potential. The charge density difference of defect C (d) and Pt-SAs-C4 (e). Yellow and light blue isosurfaces denote an increase and decrease of 0.002 eÅ-3 for electronic density, respectively. Reprinted with permission from Ref. [296]. Copyright 2020, RSC.

Fig. 31. Schematic illustrations of the nanostructures and the calculated DOSs based on the UV-visible diffuse reflectance spectra and the VB-XPS results, for the reference TiO2, C/TiO2 and reduced TiO2. Reprinted with permission from Ref. [303]. Copyright 2014, RSC.

Fig. 32. (a) Schematic illustration for the fabrication of Ag@C@TiO2 NTs core-shell nanocomposite. (b) Schematic illustration of the hydrogen generation and dye degradation process by using the Ag@C@TiO2 NTs core-shell nanocomposite catalyst under simulated visible light irradiation. Reprinted with permission from Ref. [305]. Copyright 2018, ACS.