Fig 1

Structure of (a) triazine and (b) tri-s-triazine fragments in g-C3N4 9, 10. Copyright 2015 John Wiley & Sons - Journals."

Fig 2

Reduction potentials of photocatalytic CO2 reduction to different products (V vs. NHE, pH = 7)."

Fig 3

Various N-precursors utilized in the preparation of g-C3N4 via different synthetic routes."

Table 1

Methods of preparing g-C3N4 using various precursors."

Fig 4

Schematic diagram illustrating synthesis of g-C₃N₄ through thermal polymerization using various nitrogen-rich precursors such as melamine 22, cyanamide 23, dicyandiamide 24, urea 25, and thiourea 26."

Fig 5

(a) Molecular structure of various precursors under identical g-C3N4 synthesis conditions (pyrolysis at 7 ℃∙min−1 and 500 ℃ for 2 h); (b) Predicted band structures of g-C3N4 obtained from diverse precursors 35. Copyright 2023 Elsevier."

Fig 6

(a) Schematic of the electrochemical preparation of g-C3N4, SEM images of g-C3N4 nanosheets synthesized under different voltages: (b) 3 V; (c) 5 V; (d) 7 V; (e) 9 V. (f) UV absorbance dependence on solution at different voltages. (g) FTIR spectra of g-C₃N₄ synthesized at arious voltages 37. Copyright 2020 Elsevier."

Fig 7

Schematic demonstrating the preparation of HCNMS, H-HCNMS, and OH-HCNMS 38. Copyright 2020 Elsevier."

Table 2

Modification strategy of g-C3N4 and its photocatalytic CO2 reduction performance."

Fig 8

Different morphologies of g-C3N4 75–80. Copyright 2014, 2021 Royal Society of Chemistry, Copyright 2022, 2023 Elsevier."

Fig 9

(a) Schematic diagram of controlled synthesis of CNQDs 86; Copyright 2014 Royal Society of Chemistry. (b) schematic diagram of g-C3N4 nanosheets and QDs with controllable size synthesized by ethanol in the presence of KOH 87; (c) TEM images of CNQDs, (d) diameter distribution of CNQDs; Copyright 2017 Royal Society of Chemistry (e) HRTEM of a single CNQDs 86; (f) schematic illustrating the formation process of CNQDs 88. Copyright 2011 Royal Society of Chemistry."

Fig 10

Typical (a) SEM and (b) TEM images of g-C3N4 with 1D structure 96; Copyright 2012 John Wiley & Sons - Journals. (c) formation process of porous g-C3N4 nanotubes, (d) SEM and (e) TEM images of g-C3N4 nanotubes 97. Copyright 2019 Springer Nature."

Fig 11

Schematic illustrations of exfoliation strategies for preparing g-C3N4 nanosheets by (a) H2SO4 (98 wt%); (b) typical AFM images and (c) TEM images of the as-prepared monolayer-C3N4 nanosheets 106; Copyright 2013 Royal Society of Chemistry. (d) mechanical grinding 107. Copyright 2017 American Chemical Society."

Fig 12

(a) Synthesis of HCNS and metal-based HCNS composites; (b) TEM image of HCNS-3 (scale bar equals 1 μm) 108; Copyright 2012 Springer Nature. (c) Formation process of 3D porous g-C3N4 ultra-thin nanosheets 109; Copyright 2021 Elsevier. (d) Schematic of the synthetic strategy for 3DOM g-C3N4 110 (polyvinylpyrrolidone abbreviated as PVP). Copyright 2016 Elsevier."

Fig 13

(a) Synthesis and (b) TEM image of PCN-S (inset HRTEM and AFM images); (c) Top and side views of optimized g-C3N4 and P-doped g-C3N4; (d) Band structures of doped samples 117. Copyright 2015 Royal Society of Chemistry."

Fig 14

(a) Optimized structure and potential doping sites of monolayer g-C3N4; (b) Proposed CO2 reduction mechanism over Co-doped g-C3N4 (CoCN) 120; Copyright 2023 Elsevier. (c) Synthesis method for copper-modified g-C3N4 nanorod bundles (CCNBs); (d) Proposed CO2 reduction mechanism over CCNBs 121. Copyright 2022 Elsevier."

Fig 15

(a) Schematic representation of the synthesis of Pt@Def-CN; (b) CO2 reduction conversion and carbon-based selectivity of photocatalysts including P-CN, Def-CN, Pt@Def-CN, and PtNPS@Def-CN; (c) Free energy diagram for CO2 → CH4 on Pt@Def-CN and Def-CN catalysts 130; Copyright 2022 John Wiley & Sons - Journals. (d) Fabrication process of Cu-CCN samples 131. Copyright 2020 American Chemical Society."

Fig 16

(a) Schematic for the preparation of graphitic carbon nitride nanosheets (g-CN-X); (b) Band gap of various g-CN-X samples; (c) Proposed mechanism for reaction in g-CN-10 143; (d) Schematic for the synthesis of nitrogen-vacancy g-C3N4 microtubes; (e) Band structure alignments of different samples 60. Copyright 2022, 2023 Elsevier."

Fig 17

Schematic diagram of three types of heterojunctions: (a) Type-Ⅰ, (b) Type-Ⅱ and (c) Type-Ⅲ heterojunction."

Fig 18

(a) Transfer pathways of carriers in Type-Ⅱ system and associated issues; (b) Transfer pathways of photogenerated carriers in an S-scheme heterojunction."

Fig 19

Charge transfer schematic in S-scheme heterojunctions."

Fig 20

(a) Synthesis of ZIS/HCNT heterostructures; SEM images of (b) HCNT, (c) ZIS, and (d) ZIS/HCNT-2; (e) Proposed charge transfer mechanism for the ZIS/HCNT S-scheme heterojunction 159. Copyright 2022 Elsevier."

Table 3

Photocatalytic CO2 reduction over g-C3N4 composites based on S-scheme heterojunctions."

Fig 21

(a) Schematic illustration for the preparation of the CN/Fe-MOF composite, (b) Schematic of the formation of the S-scheme heterojunction between CN and Fe-MOF 181. Copyright 2022 Elsevier."

Fig 22

(a) Schematic for the preparation of the g-C3N4/Tp-Tta COF S-scheme heterojunction, (b) Calculated work functions for Tp-Tta COF and defective g-C3N4, (c) Illustration of the carrier transfer in gCN/Tp-Tta COF S-scheme heterojunction 184. Copyright 2022 Elsevier."