A superlattice interface and S-scheme heterojunction for ultrafast charge separation and transfer in photocatalytic H2 evolution

Abstract

The rapid recombination of photoinduced charge carriers in semiconductors fundamentally limits their application in photocatalysis. Herein, we report that a superlattice interface and S-scheme heterojunction based on Mn_0.5Cd_0.5S nanorods can significantly promote ultrafast charge separation and transfer. Specifically, the axially distributed zinc blende/wurtzite superlattice interfaces in Mn_0.5Cd_0.5S nanorods can redistribute photoinduced charge carriers more effectively when boosted by homogeneous internal electric fields and promotes bulk separation. Accordingly, S-scheme heterojunctions between the Mn_0.5Cd_0.5S nanorods and MnWO₄ nanoparticles can further accelerate the surface separation of charge carriers via a heterogeneous internal electric field. Subsequent capture of the photoelectrons by adsorbed H₂O is as fast as several picoseconds which results in a photocatalytic H₂ evolution rate of 54.4 mmol·g^-1·h^-1 without any cocatalyst under simulated solar irradiation. The yields are increased by a factor of ~5 times relative to control samples and an apparent quantum efficiency of 63.1% at 420 nm is measured. This work provides a protocol for designing synergistic interface structure for efficient photocatalysis.

Fig. 1. Schematic illustration of proposed layout.

According to the consecution of universal spatial charge separation, only synergy of superlattice interface and S-scheme heterojunction can achieve the ultrafast and universal spatial separation and transfer of charge carriers. The red and green marks represent unsatisfactory and satisfactory separation efficiency, respectively.

Fig. 2. Synthesis of synergistic interface structure in SL-MCS/MW NRs.

Synthetic process and possible formation mechanism of synergistic interface structure in SL-MCS/MW NRs.

Fig. 3. Atomic-scale synergistic interface structure in SL-MCS/MW NRs.

a Atomic-scale resolution HAADF-STEM image of SL-MCS region marked in Supplementary Fig. 5c with corresponding schematic diagrams of close-packing style and structural model, where A, B, C and a, b, c represent close-packing positions of Mn/Cd (gray sphere in structural model) and S (yellow sphere in structural model), respectively. b Magnified image of the area marked in (a). c Atom arrangement model of (b). d FFT pattern of (a). e Strain distribution of (a) simulated by GPA. f Atomic-scale resolution HAADF-STEM image of heterointerface in SL-MCS/MW NRs. g FFT pattern of (f). EDX mapping of (h) Mn, (i) Cd, (j) S, (k) W, and (l) O element, respectively.

Fig. 4. PHE performance of all as-prepared catalysts and comparison with reported interface engineering strategies.

a Average PHE rate, where N.D. represents no detection. b The optical absorbance-dependent AQE measurement of SL-MCS/MW NRs. Error bars in (a) and (b) represent standard deviation. c Cycling stability test of SL-MCS/MW NRs. d Comparison of PHE performance and AQE (λ = 420 nm) with different reported photocatalysts marked by numbers 1–14. 1: Mn_0.8Cd_0.2S (no interface), 2: Mn_0.25Cd_0.75S/MoS₂ (Schottky interface), 3: NiS/Mn_0.25Cd_0.75S (Schottky interface), 4: FeWO₄/Mn_0.5Cd_0.5S (p-n junction), 5: CdWO₄/Mn_0.5Cd_0.5S (Z-scheme heterojunction), 6: CdS/PT polymer (S-scheme heterojunction), 7: g-C₃N₄/CdS/TiO₂ DRSP (dual S-scheme heterojunctions), 8: HOCN-0.01 (homojunction & Ohmic contact), 9: CZS/NiS_x (twin homojunction & Schottky heterojunction), 10: CdZnS NCSSs (twin homojunction), 11: Ni-Cd_0.5Zn_0.5S (twin homojunction & Schottky heterojunction), 12: Cd_0.5Zn_0.5S/WO_3-x (twin homojunction & Z-scheme heterojunction), 13: (3CdS/Au)-4ZnS QDNWs (quasi-superlattice interface), and 14: Cu_1.8S-ZnS ASLNWs (quasi-superlattice interface).

Fig. 5. Mechanism of bulk charge separation via superlattice interface.

a Partial charge density and (b) macro-average electrostatic potential line profiles of WZ-MCS. c Partial charge density and (d) macro-average electrostatic potential line profiles of SL-MCS. Insets in (a, c) are partial charge density distribution images with cyan isosurface value of 0.001 e/Bohr³.

Fig. 6. Mechanism of surface charge separation via S-scheme heterojunction.

a Atomic force microscopy (AFM) image of SL-MCS/MW NRs. Corresponding surface potential distribution (b) under dark state and (c) light irradiation of 420 nm. d Line profiles of surface potential from point A to B, marked in (b) and (c). e Schematic illustration of charge separation and transfer mechanism of S-scheme heterojunction in SL-MCS/MW NRs.

Fig. 7. Kinetic behavior of photoinduced charge carriers near the band edge.

Two-dimensional fs-TA spectra plotted by color mapping of as-prepared photocatalysts suspended in (a–d) MeCN and (e–h) ultrapure water with the same delay timescale from femtosecond to nanosecond.

Fig. 8. Quantitative study of ultrafast behavior of charge carriers near the band edge.

Decay kinetics fitting of as-prepared photocatalysts suspended in (a–d) MeCN and (e–h) ultrapure water probed at 440, 475 nm and 450, 480 nm, respectively.