Fig. 1. Versatile applications of photocatalysis.

Fig. 2. Schematic illustration of a photocatalytic redox reaction.

Fig. 3. Schematic illustration of the band structure of typical conventional heterojunctions and S-scheme heterojunction.

Fig. 4. Schematic illustration of the internal field of a PN heterojunction.

Fig. 5. (a) Band structure of Schottky heterojunction; (b) Illustration of the oscillation of electrons in a metal particle; (c) Harmonic model to describe the plasmon resonance.

Fig. 6. (a) HRTEM image of g-C3N4/ZnTe. (b) Products of g-C3N4/ZnTe-x-y (x and y represented the mass ratio of g-C3N4 and ZnTe) at -1.1 V vs. Ag/AgCl. (c) Products of g-C3N4/ZnTe-1-2 at various applied potentials. (d,e) The optimized geometry structures and binding energies for CO2 (d) and CO (e) adsorption on g-C3N4, ZnTe, and g-C3N4/ZnTe. (f) Schematic diagram of band configuration and the charge separation at the interface of g-C3N4/ZnTe heterojunction. (reproduced with permission [68], copyright 2019, Elsevier).

Fig. 7. (a) Band structure of type-II I-BiOCl/I-BiOBr composite. Photocatalytic activities (b,d) and corresponding kapp constant (c,e) of various samples for MO and phenol degradation. (reproduced with permission [69], copyright 2017, Elsevier).

Fig. 8. (a) Band alignment of the QD@LDH@BiVO4 (QD: CdTe quantum dots, LDH: Co-based layered double hydroxide) photoanode. (b-d) SEM images of BiVO4, LDH@BiVO4 and QD@LDH@BiVO4, respectively. (e) HRTEM images of QD@LDH@BiVO4. (f) Oxidation efficiency of the surface-reaching holes injected into the solution species. (reproduced with permission [87], copyright 2016, American Chemistry Society).

Fig. 10. (a) TEM image of the La2Ti2O7/In2S3 heterojunction. (b) HRTEM image of the La2Ti2O7/In2S3 heterojunction. (c) Schematic illustration of the band edge positions for both La2Ti2O7 and In2S3. (reproduced with permission [70], copyright 2019, Elsevier).

Fig. 9. (a,b) Top and side view of the C2N/WS2 heterojunction. Grey, green, purple and orange colors denote C, N, W and S atoms, respectively. (c) Band structure of the C2N/WS2 heterojunction. (d) Absolute energy positions of VBM (valence band maxima) and CBM (conduction band minima) for all three structures with respect to vacuum level. (reproduced with permission [88], copyright 2018, Elsevier).

Fig. 11. (a) The charge transfer pathway between Cu2-xSe and CdS under illumination. (b,c) SEM and TEM images of 10?wt% Cu2-xSe/CdS composite. (d) Photocatalytic performance of CdS and different ratios of Cu2-xSe/CdS composites. (reproduced with permission [118], copyright 2019, Elsevier).

Fig. 12. (a) The schematic transformation from α-Bi2O3 to the β-Bi2O3. (b,c) TEM and HRTEM images of Bi2O3/Bi2SiO5 heterojunction. (d) Photocurrent responses of α-Bi2O3 and Bi2O3/Bi2SiO5 heterojunctions. The products with the SiO2 mass ratio of 1%, 5%, 10%, 15% and 20% were denoted as BiSi-1, BiSi-2, BiSi-3, BiSi-4 and BiSi-5, respectively. (reproduced with permission [119], copyright 2018, Elsevier).

Fig. 13. (a,b) TEM and HRTEM images of the BiOBr/La2Ti2O7 heterojunction. (c) Photocatalytic activity of various samples for RhB degradation. (d) Schematic diagram of the possible photocatalytic mechanism over BiOBr/La2Ti2O7 under both UV and visible light irradiation. (reproduced with permission [120], copyright 2016, Elsevier).

Fig. 14. (a) A schematic of the generation of CdS/Cd Schottky junction. (b) Mechanism of the improved photocatalytic activity for CdS/Cd. (c) STEM image of CdS/Cd viewed from the [100] direction. (d,e) Enlarged view of the corresponding areas in (c). (f) Comparison of the photocatalytic hydrogen evolution over CdS and CdS/Cd. (g) Cycling hydrogen evolution measurements for CdS/Cd. (reproduced with permission [135], copyright 2019, Elsevier).

Fig. 15. (A) Illustration of the structure of Ru/TiO2/RuO2 heterojunctions. (B) Band structure of the Ru/TiO2/RuO2 heterojunctions. (C,D,E,F) TEM and HRTEM images of the Ru/TiO2/RuO2 heterojunctions. (G,H) Photocatalytic H2 and O2 evolution over Ru8.0/TiO2 NBs-T, where T indicates the annealing temperatures. (reproduced with permission [125], copyright 2016, Wiley).

Fig. 16. (a) Schematic illustration showing the synthesis of Pt@MIL-125/Au and the corresponding Pt/MIL-125/Au and MIL-125/Au analogues. (b) Photocatalytic H2 production rates of different catalysts. (c) Schematic illustration showing the electron migration at the two metal/MOF interfaces. (reproduced with permission [153], copyright 2018, Wiley).

Fig. 17. (a) TEM image of PCN nanosheets. (b) TEM image of the CeO2/PCN heterojunction photocatalyst. Inset: corresponding HRTEM image of (111) plane of CeO2. (c) HAADF-TEM image of the CeO2/PCN heterojunction photocatalyst. (d) Band structure of pure CeO2 and PCN. (e) The statistic counts of bacteria colonies treated with different samples, respectively. (f) The bacteria removal efficiency over the prepared samples. (reproduced with permission [177], copyright 2020, Wiley).

Fig. 18. (a,b) TEM and HRTEM images of the TiO2/CdS nanofiber. (c) Comparison of photocatalytic H2-production activity of T, TC5, TC10, TC20, and C samples. TCx: T and C represent TiO2 and CdS, respectively, x indicates the weight percentage of CdS with respect to TiO2. (d) Cycling H2-production stability for the TC10 sample. (e) S-scheme charge separation and pathway in TiO2/CdS nanofiber. (reproduced with permission [178], copyright 2019, Wiley).

Fig. 19. (a,b,c) SEM, TEM and HRTEM images of BP/BiOBr photocatalyst. (d) Photocatalytic degradation efficiency of tetracycline (TC) over various samples. (e) Photocatalytic oxygen evolution rate of various samples. (f) Schematic illustration for the charge transfer and reaction mechanism. BP: Black phosphorus. xBP/BiOBr: x indicates the weight ratio of BP respect to BiOBr. (reproduced with permission [179], copyright 2020, Elsevier).

Fig. 20. (a) Schematic illustration of the preparation of COF/SC photocatalysts. (b,c) Photocatalytic performance of the prepared COF/SC samples. (d) Schematic charge separation in COF/SC with COF-138/TiO2 as an example. (reproduced with permission [180], copyright 2020, Wiley).

Fig. 21. (a) Schematic illustration of the (101)/(001) surface heterojunction of TiO2. (b) Schematic illustration of the adjustment of the facet ratio through morphology control. (c) SEM images of samples with different facet ratios. The (101)/(001) facet ratios of T1, T2, T3, and T4 were 49, 11.5, 1.78, and 0.087, respectively. (d,e) Bacterial viability of E. coli and S. aureus exposed to different concentrations of TiO2 nanocrystals, respectively. (reproduced with permission [188], copyright 2017, American Chemical Society).

Fig. 22. (a,b) Schematic illustration of the formation of TiO2 surface heterojunction. (c) Enlarged view showing the exposed facets of TiO2. (d-f) SEM images of TiO2 nanorods and TiO2 nanorods loaded with nanosheets. (g) Photocatalytic performance of various samples. (h) Band alignment of the (111) and (101) facets of rutile TiO2. FH-TiO2: Facet Heterojunction TiO2, TiO2 nanorods loaded with nanosheets; P25: commercial TiO2; NRs-anatase: nanorods with anatase phase; NRs-TiO2: nanorods with rutile phase. (reproduced with permission [190], copyright 2019, Wiley).

Fig. 23. (a) SEM image of BiOI. (b,c) HRTEM images of BiOI viewed from the perpendicular and parallel directions of the BiOI plate, respectively. (d) Schematic illustration of the growth of BiOI nanoplate at different PH values. (e) Band alignment of (110) and (001) facet of BiOI. (f) Degradation curves of phenol on BiOI samples. BiOI-X: X means X mL distilled water was added in the hydrolysis process. (reproduced with permission [205], copyright 2016, Elsevier).

Fig. 24. (a) The schematical formation of CNNA/rGO composite in the molten salt medium. CNNA, rGO represents crystalline carbon nitride nanorods and reduced graphene oxide, respectively. (b) FESEM image of CNNA/rGO. (c) Photocatalytic CO2-reduction property of bulk CN (bCN), CNNA and CNNA/rGO at various amount of rGO in water vapor-saturated wet CO2 gas. (reproduced with permission [236], copyright 2019, Elsevier).

Fig. 25. (a) Schematic illustration of NG/CdS HS preparation process. (b-e) Corresponding SEM images of materials sequentially appear from left to right in (a). (f) Photocatalytic CO2 reduction performance of CdS, CdG1, CdG2, CdG3, and CdG5. (g) Schematic illustration of the charge carrier migration mechanism. CdGx: x donates that the dosage of pyridine during fabrication was 10x μL. (reproduced with permission [237], copyright 2019, Wiley).

Fig. 26. TEM (a) and HRTEM (b) images of the WO3/TiO2/rGO (WTG) composite. (c) Photocatalytic activities of various samples. (d) Schematic illustration of the S-scheme heterojunction-based charge transfer mechanism in WO3/TiO2/rGO composite (reproduced with permission [238], copyright 2020, Elsevier).