Preparation and Application of a Novel S-Scheme Nanoheterojunction Photocatalyst (LaNi0.6Fe0.4O3/g-C3N4)

Abstract

Rapid recombination of photogenerated electrons and holes affects the performance of a semiconductor device and limits the efficiency of photocatalytic water splitting for hydrogen production. The use of an S-scheme nanoscale heterojunction catalyst for the separation of photogenerated charge carriers is a feasible approach to achieve high-efficiency photocatalytic hydrogen evolution. Therefore, we synthesized a three-dimensional S-scheme nanoscale heterojunction catalyst (LaNi_0.6Fe_0.4O₃/g-C₃N₄) and investigated its activity in photocatalytic water splitting. An analysis of the band structure (XPS, UPS, and Mott-Schottky) indicated effective interfacial charge transfer in an S-scheme nanoscale heterojunction composed of two n-type semiconductors. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) spectroscopy confirmed that the light-induced charge transfer followed the S-scheme mechanism. Based on the capture test (EPR) of •OH free radicals, it can be seen that the enhanced activity is attributed to the S-scheme carrier migration mechanism in heterojunction, which promotes the rapid adsorption of H⁺ by the abundant amino sites in g-C₃N₄, thus effectively generating H₂. The 2D/2D LaNi_0.6Fe_0.4O₃/g-C₃N₄ heterojunction has a good interface and produces a built-in electric field, improving the separation of e^- and h⁺ while increasing the oxygen vacancy. The synergistic effect of the heterostructure and oxygen vacancy makes the photocatalyst significantly better than LaNi_0.6Fe_0.4O₃ and g-C₃N₄ in visible light. The hydrogen evolution rate of the composite catalyst (LaNi_0.6Fe_0.4O₃/g-C₃N₄-70 wt %) was 34.50 mmol·h^-1·g^-1, which was 40.6 times and 9.2 times higher than that of the catalysts (LaNiO₃ and g-C₃N₄), respectively. After 25 h of cyclic testing, the catalyst (LaNi_0.6Fe_0.4O₃/g-C₃N₄-70 wt %) composite material still exhibited excellent hydrogen evolution performance and photostability. It was confirmed that the synergistic effect between abundant active sites, enriched oxygen vacancies, and 2D/2D heterojunctions improved the photoinduced carrier separation and the light absorption efficiency of visible light. This study opens up new possibilities for the logical design of efficient photodecomposition using 2D/2D heterojunctions combined with oxygen vacancies.

Figure 1

XRD patterns of the as-prepared LaNi_0.6Fe_0.4O₃ and LaNi_0.6Fe_0.4O₃/g-C₃N₄ photocatalysts.

Figure 2

SEM images of (a) LaNi_0.6Fe_0.4O₃, (b) g-C₃N₄, and (c, d) LaNi_0.6Fe_0.4O₃/g-C₃N₄-70 wt % composites.

Figure 3

(a) TEM image and (b) HRTEM image of LaNi_0.6Fe_0.4O₃; (c) TEM image, (d,e) high-magnification TEM images, and (f) selected area electron diffraction (SAED) pattern of LaNi_0.6Fe_0.4O₃/g-C₃N₄-70 wt %; (g) SEM image of LaNi_0.6Fe_0.4O₃/g-C₃N₄-70 wt % and energy-dispersive X-ray (EDS) images showing the C, N, O, Fe, Ni, and elemental distribution.

Figure 4

(a–c) AFM image and (d, e) profile image of the LaNi_0.6Fe_0.4O₃/g-C₃N₄-70 wt %.

Figure 5

X-ray photoelectron spectroscopy (XPS) full spectrum. (a) and (b) N 1s; (c) C 1s; (d) O 1s; (e) La 3d and Ni 2p, and (f) Fe 2p magnified high-resolution spectra for LaNi_0.6Fe_0.4O₃, g-C₃N₄, and LaNi_0.6Fe_0.4O₃/g-C₃N₄-70 wt %.

Figure 6

Fourier transform infrared (FT-IR) spectra of g-C₃N₄, LaNi_0.6Fe_0.4O₃, and LaNi_0.6Fe_0.4O₃/g-C₃N₄-70 wt %.

Figure 7

(a) UV–vis diffuse reflectance spectra (DRS) of LaNi_0.6Fe_0.4O₃, g-C₃N₄, and LaNi_0.6Fe_0.4O₃/g-C₃N₄; (b) the bandgap energy (E_g) diagram of LaNi_0.6Fe_0.4O₃ and LaNi_0.6Fe_0.4O₃/g-C₃N₄; (c, d) the Mott–Schottky (M-S) plots collected at 1000 Hz; and (e) the valence band (VB) X-ray photoelectron spectra (XPS) for LaNi_0.6Fe_0.4O₃ and g-C₃N₄.

Figure 8

(a) Electrochemical impedance spectra (EIS) plot; (b) photoluminescence (PL) spectra; (c) photocurrent response spectra; (d) time-resolved fluorescence spectra of LaNi_0.6Fe_0.4O₃, g-C₃N₄, and LaNi_0.6Fe_0.4O₃/g-C₃N₄-70 wt %.

Figure 9

(a) Photocatalytic hydrogen evolution rates of LaNi_0.6Fe_0.4O₃, g-C₃N₄, and LaNi_0.6Fe_0.4O₃/g-C₃N₄-65, 70, and 75 wt % as a function of irradiation time under full spectrum illumination; (b) long-term stability test of LaNi_0.6Fe_0.4O₃ under continuous illumination; (c) average hydrogen evolution rates during water splitting by LaNi_0.6Fe_0.4O₃, g-C₃N₄, and LaNi_0.6Fe_0.4O₃/g-C₃N₄-65, 70, and 75 wt % under full illumination; (d) long-term stability test of LaNi_0.6Fe_0.4O₃/g-C₃N₄-70 wt % under continuous illumination for 25 h.

Figure 10

Apparent quantum yields of the composite materials at different wavelengths.

Figure 11

(a) Water dispersion of BMPO-•O₂^– and (b) water dispersion of DMPO-•OH under full spectrum xenon lamp irradiation; (c) EPR spectra and (d) g-values of LaNiO₃, LaNi_0.6Fe_0.4O₃, and LaNi_0.6Fe_0.4O₃/g-C₃N₄-70 wt % composites from the bottom up.

Figure 12

Ultraviolet photoelectron spectra (UPS) of (a,b) LaNi_0.6Fe_0.4O₃ and (c,d) g-C₃N₄.

Figure 13

Schematic band diagrams for LaNi_0.6Fe_0.4O₃ and g-C₃N₄ (a) before contact, (b) after contact, and (c) charge transfer across the S-scheme heterojunction of LaNi_0.6Fe_0.4O₃/g-C₃N₄ during the reaction.