Tuning surface electronic structure of (CuGa)xZn1‒2xGa2S4 photocatalyst for efficient nitrate-to-ammonia conversion

doi:10.1016/S1872-2067(25)64895-9

Abstract

Abstract:

The photocatalytic conversion of nitrate (NO₃^-) into ammonia (NH₄⁺) under mild conditions offers a promising approach for mitigating environmental nitrate contamination. The efficiency of this process is fundamentally governed by the adsorption and activation of NO₃^- and its intermediates, which are significantly influenced by the surface electronic properties of the catalyst, particularly the position of the d-band center. However, conventional approaches to tune the surface electronic structure such as doping with extraneous elements or forming heterojunctions often alter the overall band structure seriously, typically leading to reduced photocatalytic activity. In this study, the d-band state of (CuGa)_xZn_1‒2_xGa₂S₄ semiconductor is engineered through Al³⁺ surface decoration without affecting the conduction band or the bandgap to enhance NO₃^- adsorption and activation. X-ray photoelectron spectroscopy and X-ray absorption fine structure analyses reveal that the surface doping of Al³⁺ do not induce obviously energy band structure change but the d-band center, which shift more closer to Fermi level in comparison with pristine material. Electronic energy band analyses indicate that Al³⁺ decoration does not significantly alter the conduction band or bandgap. Moreover, the Al³⁺-modified material demonstrates a substantial improvement in photocatalytic conversion of NO₃^- into NH₄⁺, increasing the NH₄⁺ production rate from 0.18 to 0.93 mmol h^-1 g^-1. Density functional theory calculations further revealed that the d-band center of Al³⁺/(CuGa)_xZn_1-2_xGa₂S₄ shifted closer to the Fermi level, moving from -4.75 to -4.54 eV compared to the pristine (CuGa)_xZn_1-2_xGa₂S₄. This shift lowered the Gibbs free energy for the adsorption of NO₃^- reduction intermediates, thereby enhancing the conversion efficiency of NO₃^- into NH₄⁺. This work introduces an effective strategy for surface d-band states modulation without altering the intrinsic band structure to improve nitrate reduction performance, offering deep insights into the future design of materials for environmental remediation applications.

Key words: Photocatalysis, Electronic structure, d-band center, Sulfide, Nitrate reduction

Peng Liu, Lian Duan, Baopeng Yang, Mingwei Sun, Gen Chen, Xiaohe Liu, Min Liu, Ning Zhang. Tuning surface electronic structure of (CuGa)_xZn_1‒2_xGa₂S₄ photocatalyst for efficient nitrate-to-ammonia conversion[J]. Chinese Journal of Catalysis, 2026, 83: 172-182.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(25)64895-9

https://www.cjcatal.com/EN/Y2026/V83/I4/172

Figures/Tables 5

Fig. 1. (a) XRD patterns of (CuGa)xZn1-2xGa2S4 and different proportions of Al3+/(CuGa)xZn1?2xGa2S4. SEM images of (CuGa)xZn1?2xGa2S4 (b) and Al3+/(CuGa)xZn1?2xGa2S4-0.01% (c). (d) TEM images of (CuGa)xZn1?2xGa2S4. TEM images (e), HRTEM image (f), and HAADF image with EDS elemental line scan image (g-l) of Al3+/(CuGa)xZn1-2xGa2S4-0.01%.

Fig. 2. High-resolution XPS spectra of Zn 2p (a) and S 2p (b) of the (CuGa)xZn1-2xGa2S4 (bottom) and Al3+/(CuGa)xZn1?2xGa2S4-0.01% (upper). (c) XANES spectra of Zn K-edge over the (CuGa)xZn1?2xGa2S4 (green line) and Al3+/(CuGa)xZn1?2xGa2S4-0.01% (orange line). (d) Zn K-edge k3-weighted FT-EXAFS spectra of the (CuGa)xZn1?2xGa2S4 (green line) and Al3+/(CuGa)xZn1?2xGa2S4-0.01% (orange line). Continuous Cauchy wavelet transforms (CCWT) of (CuGa)xZn1?2xGa2S4 (e) and Al3+/(CuGa)xZn1?2xGa2S4-0.01% (f).

Fig. 3. UV-vis adsorption spectra and Eg calculation diagram (a), XPS-VB spectra (b), the d-band centers were measured by high-resolution XPS (c), scheme of relatively energy band positions (d), PL spectra (e) and TRPL spectrum (f) of (CuGa)xZn1?2xGa2S4 (green line) and Al3+/(CuGa)xZn1-2xGa2S4 ?0.01% (orange line).

Fig. 4. NH4+ yield vs. irradiation time (a), H2 evolution vs. irradiation time (b), NH4+/H2 unit yield and NH4+ selectivity (c). (d) 1H NMR spectrum and the spectrum of ion chromatography for the reaction solution of Al3+/(CuGa)xZn1?2xGa2S4-0.01%. (e) Long-period photocatalytic performance of Al3+/(CuGa)xZn1?2xGa2S4-0.01%. (f) FT-IR spectra of Al3+/(CuGa)xZn1?2xGa2S4-0.01% (after 0, 10-, 15-, 20-, 25-, and 30- min light irradiation).

Fig. 5. Atomic structure over (112) surface (a), calculated charge density distribution over (112) surface for (CuGa)xZn1?2xGa2S4 (upper) and Al3+/(CuGa)xZn1?2xGa2S4 (bottom) (b). The average length of Zn-S bonds (c) and Ga-S bonds (d) around the Zn active site on the (112) surface. d-band centers for (CuGa)xZn1?2xGa2S4 (e) and Al3+/(CuGa)xZn1?2xGa2S4 (f). (g) The schematic changes of d-band center positions. (h) The adsorption energies of *NO3 on the Zn, Ga, and Al atoms exposed on the (112) surface of Al3+/(CuGa)xZn1?2xGa2S4. (i) The adsorption process of NO3- reduction intermediates over Al3+/(CuGa)xZn1?2xGa2S4 on Zn atom. (j) Gibbs free energy diagram for the pathways of NO3- conversion into NH3 over (CuGa)xZn1?2xGa2S4 on Zn atom and Al3+/(CuGa)xZn1?2xGa2S4 on Zn/Al atoms based on DFT calculations.

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Tuning surface electronic structure of (CuGa)_xZn_1‒2_xGa₂S₄ photocatalyst for efficient nitrate-to-ammonia conversion

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