Understanding the electrocatalytic mechanism of self-template formation of hierarchical Co₉S₈/Ni₃S₂ heterojunctions for highly selective electroreduction of nitrobenzene

Abstract

Aqueous electrochemical nitroarene reduction reaction using H₂O as the sustainable hydrogen source is an emerging technology to produce functionalized anilines. However, the development of low-cost electrocatalysts and the fundamental mechanistic understanding of the selective NO-RR still remain challenging. Herein, self-supporting hierarchical nanosheets consisting of high-density Co₉S₈/Ni₃S₂ heterojunctions on Ni foam (Co₉S₈/Ni₃S₂-NF) are constructed via an in situ self-template strategy. With combined advantages of high-loading, high surface exposure, efficient conductivity and unique electronic structure of the Co₉S₈/Ni₃S₂ interface, the as-prepared Co₉S₈/Ni₃S₂-NF exhibits efficient electrocatalytic NO-RR performance, including up to 99.0% conversion and 96.0% selectivity towards aniline, and outstanding functional group tolerance. Mechanistic investigations and theoretical calculations reveal that electron transfer from Ni₃S₂ to Co₉S₈ is beneficial for the co-adsorption of H₂O and nitrobenzene molecules at the interfacial sites, promoting the formation of active hydrogen and subsequent reduction of nitrobenzene. Additionally, the interfacial charge transfer breaks the symmetry of two active Co sites at the Co₉S₈/Ni₃S₂ interface, which markedly reduces the energy barrier for reduction of nitrobenzene to aniline. This work offers a successful example for the interfacial engineering of metal sulfide-based heterojunctions with excellent electrocatalytic nitroarene reduction performance, and also paves the way for the in-depth understanding of the corresponding mechanism.

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Introduction

Functionalized aniline (PhNH₂) is a group of vital chemical stocks in many industries, including dye, drug, conductive polymer, transparent electrode and pseudocapacitor.^1–3 Traditional PhNH₂ production via thermocatalytic or direct-amination endures the issues of high energy consumption, high carbon emission, high cost and unsafe proton donors (Table S1†).^4–9 In addition, current nitroarene reduction reactions (NO-RR) typically utilize rare noble metals (Pt, Pd, Ru, Rh, etc.) as catalysts, which however face a significant challenge in achieving satisfactory chemoselectivity and are vulnerable to functional group poisoning.^10–12 Moreover, the harsh reaction conditions lead to low compatibility with fragile groups (–OCH₃, –OH, –Cl, –F, –Br, or/and –COCH₃, etc.), which severely limits the scope of substrates for NO-RR.^2,13–16 Therefore, it is crucial to develop noble-metal-free catalysts that enable selective reduction of functionalized nitroarenes with fragile side groups under mild conditions. In recent years, electrochemical organic synthesis has inspired the field with multiple advantages including ambient reaction conditions, utilization of sustainable electrical energy, and controllable selectivity.^17–20 With continuous efforts, electrocatalytic nitroarene reduction has been greatly promoted, especially via the development of high-performance electrocatalysts (Table S2†) into the most promising technology for PhNH₂ production.^{2,13,15,16,21,22} However, few studies uncover the active origins and fundamental mechanism of the selective NO-RR process under aqueous conditions.

In view that reactive hydrogen (H*) participates and plays a crucial role in the aqueous electrochemical NO-RR, H₂O activation performance of NO-RR electrocatalysts should be carefully considered for the modulation of their activities.^13,15 Previous studies have confirmed that highly conductive transition metal sulfides (TMSs) with favorable H₂O adsorption show promising potential in H₂O splitting to replace precious metals.^23–31 Nevertheless, pure TMSs exhibit weak adsorption on nitrobenzene (PhNO₂), which leads to premature desorption of PhNO₂ on the catalyst surface, leading to insufficient and incomplete reduction to PhNH₂.¹⁵ To this end, interface engineering is an efficient strategy towards electronic structure regulation of interfacial active sites for designing efficient electrocatalysts.^{23,27,32–35} For example, Fu et al. successfully fabricated a MoO₂–FeP heterojunction for efficient electrochemical hydrogen evolution and 5-HMF reaction, and interfacial electron redistribution between MoO₂–FeP was confirmed as the origin of enhanced activity.²³ In addition to the electronic structure, reasonable geometry regulations including the microstructure of electrocatalysts and selection of substrates are of vital importance.^36–38 Construction of anisotropic nanosheets on conductive substrates can also increase active site exposure, accelerate mass transfer, and improve the electrical conductivity of electrocatalysts.^39,40

Herein, self-supporting hierarchical nanosheets composed of Co₉S₈/Ni₃S₂ heterojunctions were constructed on Ni foam (Co₉S₈/Ni₃S₂-NF), with unique and abundant active interfaces that promote hydrogenation and conversion of nitroarenes into functionalized PhNH₂. The as-prepared Co₉S₈/Ni₃S₂-NF shows efficient electrocatalytic reduction of PhNO₂ to PhNH₂ with up to 99.0% conversion and 96.0% selectivity under alkaline conditions, and demonstrates NO-RR favorable functional group tolerance. Both theoretical calculations and experimental characterization identify the critical role of electron transfer from Ni₃S₂ to Co₉S₈, which favors H₂O activation for the formation of H* and adsorption of PhNO₂ onto Co sites, thus improving the NO-RR activity. This work offers a design principle for NO-RR electrocatalysts via low-cost TMS-based heterojunction engineering, and also provides a systematic understanding of the electrocatalytic reduction mechanism during NO-RR.

Results and discussion

The construction of Co₉S₈/Ni₃S₂-NF was achieved by in situ sulfuration of Co–Ni layered-double-hydroxide (LDH) precursors pre-formed on the Ni foam (CoNi-LDH/NF) (Fig. 1a). First, CoNi-LDH/NF was prepared by a hydrothermal method with the Ni foam substrate placed in the Co salt and urea precursor solution at 120 °C. Subsequently, in situ sulfuration of self-sacrificing CoNi-LDH/NF was conducted by a second hydrothermal process with Na₂S as the S source under 160 °C, and the final Co₉S₈/Ni₃S₂-NF was obtained. For better control of the morphology and catalytic performance of CoNi-LDH/NF precursors, the optimized hydrothermal time and the ratio of Co salts to urea is 8 hours and 1 : 1, respectively. For comparison, catalysts obtained by in situ growth of cobalt sulfide on a titanium mesh (Co₉S₈/Ti) and nickel sulfide on Ni foam (Ni₃S₂/NF) were also successfully synthesized (see “Experimental section” for more details).

Fig. 1 (a) Schematic illustration of the formation of Co₉S₈/Ni₃S₂-NF. (b) SEM image of the CoNi-LDH/NF precursor. (c) and (d) SEM images, (e)–(g) TEM image, (h) SAED pattern, and (i) HAADF-STEM image and corresponding EDS elemental mapping of Co₉S₈/Ni₃S₂-NF.

To reveal the morphology and microstructure features of the catalytic materials, field-emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used. CoNi-LDH shows a smooth surface and uniform growth on Ni-foam (see Fig. 1b and S1†). The Co₉S₈/Ni₃S₂-NF exhibits an obvious hierarchical nanosheet structure (i.e., nanosheets on the top of nanosheets), which implies that the sulfuration process takes place in situ on the CoNi-LDH/NF templates (Fig. 1c and d). Similarly, as shown in Fig. 1e and f, TEM images with different magnifications can more intuitively demonstrate the hierarchical nanosheets structure of Co₉S₈/Ni₃S₂-NF. High-resolution TEM (HRTEM) further reveals clear lattice fringes and intimate contact between Co₉S₈ (440) and Ni₃S₂ (110), confirming the formation of the heterojunction (see Fig. 1g). In addition, the selected area electron diffraction (SAED) pattern of Co₉S₈/Ni₃S₂-NF displays diffraction rings which can be satisfactorily ascribed to the (311) and (440) crystal planes of Co₉S₈, and the (110) crystal plane of Ni₃S₂ (Fig. 1h). EDS elemental maps (Fig. 1i and S2†) show the homogeneous distribution of Co, Ni, S and O elements, further supporting the successful synthesis of Co₉S₈/Ni₃S₂-NF, and the cobalt/nickel atomic ratio is about 3.48 (Table S3†). SEM images of Co₉S₈/Ti and Ni₃S₂/NF samples indicate that the monometallic TMS is spherical, and therefore it cannot grow evenly on the substrate (Fig. S3 and S4†). Further control of hydrothermal conditions can change the structure of the CoNi-LDH/NF templates (see SEM images in Fig. S5 and S6†) and hence the morphology of the catalysts. All these results confirm that the Co₉S₈/Ni₃S₂-NF with hierarchical nanosheets was successfully prepared, presumably with exposure of more active sites that benefits the mass diffusion during NO-RR.

X-ray diffraction (XRD) patterns were obtained to verify the phase structure of the catalysts. In Fig. 2a, the XRD pattern of Co₉S₈/Ni₃S₂-NF exhibits characteristic diffraction signals ascribed to the (311) and (440) planes of Co₉S₈, as well as the (110) plane of Ni₃S₂ with (110) arising from the Ni foam. The XRD patterns of other samples (Co₉S₈ powder, Co₉S₈/Ti and Ni₃S₂/NF) are shown in Fig. S7–S9,† indicating that all samples were successfully synthesized. Additionally, CoNi-LDH/NF shows distinct diffraction characteristic signals for the hydrotalcite-like LDH phase, indicating that CoNi-LDH was successfully constructed on Ni foam.^41–43 X-ray photoelectron spectroscopy (XPS) was performed to obtain further insight into the chemical states of each element and the electron effect between Co₉S₈ and Ni₃S₂ in Co₉S₈/Ni₃S₂-NF. The XPS survey spectra in Fig. S10a† reveal the coexistence of Ni, Co, and S signals in Co₉S₈/Ni₃S₂-NF, while no signal for S is observed in CoNi-LDH/NF, and the corresponding elemental quantifications are shown in Table S4.† Fig. S10b† shows that the proportions of Co and Ni in various Co₉S₈/Ni₃S₂-NF change with the addition of Co salts for CoNi-LDH/NF production, and the optimal Co₉S₈/Ni₃S₂-NF has the highest cobalt content among the control samples. To further figure out electron transfer between Co₉S₈ and Ni₃S₂, the high-resolution Co 2p, Ni 2p, and S 2p spectra were analyzed in detail.⁴⁴ The Co 2p spectrum of Co₉S₈/Ni₃S₂-NF was deconvoluted into six peaks including two satellite peaks, and the others observed at 780.5 and 796.0 eV for Co²⁺ as well as 779.1 and 794.6 eV for Co³⁺ are ascribed to the spin–orbit doublets Co 2p_1/2 and 2p_3/2, respectively (Fig. 2b).⁴⁵ Compared to Co₉S₈/Ti, the ratio of Co³⁺ to Co²⁺ in Co₉S₈/Ni₃S₂-NF is decreased, suggesting that the average valence state of Co is decreased.^33,46 Moreover, the Ni 2p spectrum of Co₉S₈/Ni₃S₂-NF (Fig. 2c) can also be deconvoluted into six subpeaks, corresponding to Ni³⁺ (854.9/872.3 eV), Ni²⁺ (853.8/871.2 eV), and two satellite peaks of Ni 2p_1/2 and Ni 2p_3/2, severally.⁴⁷ Interestingly, compared to Ni₃S₂/NF, the ratio of the peak area of Ni³⁺ to Ni²⁺ for Co₉S₈/Ni₃S₂-NF is higher than that for Ni₃S₂/NF, showing that the average valence state of Ni is raised.^25,48 Therefore, it can be concluded that electrons transfer from Ni₃S₂ to Co₉S₈ through their interfaces. Moreover, the S 2p spectra in Fig. S11† demonstrate the presence of metal–sulfur bonds in Co₉S₈/Ni₃S₂-NF.^40,49 Besides, the XPS survey spectra of the other control samples during optimization are detailed in Fig. S12–S14.† Notably, elemental oxygen is detected in almost all materials due to the large amount of hydroxyl or H₂O molecules adsorbed on the catalyst surface during the hydrothermal reaction (Table S4†).

Fig. 2 (a) XRD patterns of the CoNi-LDH/NF precursor and Co₉S₈/Ni₃S₂-NF. (b) High-resolution XPS Co 2p spectra of Co₉S₈/Ni₃S₂-NF and Co₉S₈/Ti. (c) High-resolution XPS Ni 2p spectra of Co₉S₈/Ni₃S₂-NF and Ni₃S₂-NF. (d) FTIR spectra and (e) Raman spectra of Co₉S₈/Ni₃S₂-NF, Co₉S₈/Ti and Ni₃S₂-NF. (f) Nitrogen adsorption–desorption isotherms of CoNi-LDH/NF, Ni₃S₂/NF and Co₉S₈/Ni₃S₂-NF.

The Fourier transform infrared (FTIR) spectra were recorded to analyze the surface groups. Fig. 2d exhibits the FTIR spectra of Co₉S₈/Ni₃S₂-NF, Co₉S₈/Ti and Ni₃S₂/NF samples, and the two strong typical peaks at 3436 and 1631 cm⁻¹ are ascribed to the stretching and bending vibration of O–H of surface-adsorbed H₂O molecules.^50,51 Besides, the obvious peaks at 1116 and 1020 cm⁻¹ are attributed to the asymmetrical stretch of metal–sulfur, whereas the peak at 621 cm⁻¹ corresponds to symmetrical vibrations. The presence of an additional band at 1381 cm⁻¹ is due to the vibration of sulfide ions in the crystal in the TMS microstructures.⁵² Furthermore, compared with single metal analogues, the characteristic peaks at 194, 459, 507, 602 and 660 cm⁻¹ are assigned to the vibrational modes of the Co₉S₈ phase in Co₉S₈/Ni₃S₂-NF, and the peaks in the 150–400 cm⁻¹ region represent the phonon modes of heazlewoodite-phase Ni₃S₂ (Fig. 2e).^53–56 In addition, the FTIR and Raman spectra of CoNi-LDH can be seen in Fig. S15 and S16,† illustrating the successful synthesis of the template.^57,58 The Brunauer–Emmett–Teller (BET) surface area and pore texture of Co₉S₈/Ni₃S₂-NF were analyzed using N₂ adsorption–desorption measurements. Compared with Ni₃S₂/NF, Co₉S₈/Ni₃S₂-NF results in a higher BET surface area of 7.37 m² g⁻¹ (Fig. 2f), and a higher average pore size of about 14.32 nm (Fig. S17†) due to the hierarchical nanosheet structure (Fig. 2f and Table S5†).³³

To explore the electrochemical NO-RR properties of the catalysts under ambient aqueous conditions, an H-type cell was constructed with a Pt plate counter electrode, an Hg/HgO reference electrode, and the self-supporting Co₉S₈/Ni₃S₂-NF as the working electrode (Fig. S18†). First, linear sweep voltammetry (LSV) curves of nitroarenes reduction and hydrogen evolution reaction (HER) were recorded in 1.0 M KOH solution using 1,4-dioxane as the co-solvent. In the presence of PhNO₂ (0.8 mmol), Co₉S₈/Ni₃S₂-NF exhibits the lowest onset potential (E_onset) of 0.371 V versus the reversible hydrogen electrode (RHE) compared with Co₉S₈/Ti and Ni₃S₂/NF catalysts (Fig. 3a), indicating the best NO-RR performance. Besides, Co₉S₈/Ni₃S₂-NF also has the most positive E_onset for the HER (Fig. 3a), which suggests that Co₉S₈/Ni₃S₂-NF is favorable for H₂O dissociation and H* generation. Previous studies have shown that such in situ generated H* is beneficial for subsequent protonation reactions of PhNO₂.¹⁵ In addition, the Nyquist plots from electrochemical impedance spectroscopy (EIS) show that Co₉S₈/Ni₃S₂-NF shows a much lower internal resistance and smaller charge-transfer resistance compared with the control catalysts (Fig. 3b), implying better charge transfer capability of Co₉S₈/Ni₃S₂-NF. The solution resistance (R_s), electrode resistance (R_a), charge transfer resistance (R_ct) and constant phase element (CPE) in the equivalent circuit model (Fig. 3b, inset) were extracted by fitting the experimental results and are listed in Table S6.† The smaller R_ct of Co₉S₈/Ni₃S₂-NF than Ni₃S₂/NF and Co₉S₈/Ti reveals a faster charge transfer process in NO-RR.⁵⁹ During the catalyst optimization, LSV curves (Fig. S19†) and Nyquist plots (Fig. S20†) of all samples were also recorded. In order to confirm the intrinsic activity of NO-RR of the obtained metal sulfides, electrochemical active surface areas (ECSA) and normalized current density by ECSA were also investigated.^60–62 According to CV curves without faradaic processes (Fig. S21a–c†), the calculated capacitance and the ECSA are summarized in Table S7.† The ECSA of Co₉S₈/Ni₃S₂-NF is 3.0 times and 1.7 times larger than that of Ni₃S₂/NF and Co₉S₈/Ti respectively, indicating that the active sites on Co₉S₈/Ni₃S₂-NF are more accessible. Moreover, the normalized LSV curve of Co₉S₈/Ni₃S₂-NF exhibits a higher current density than Ni₃S₂/NF and Co₉S₈/Ti, showing the best intrinsic activity for NO-RR of Co₉S₈/Ni₃S₂-NF (Fig. S21e†).

Fig. 3 (a) LSV curves of the cathode at a scan rate of 5 mV s⁻¹ in 1.0 M KOH solution (Diox/H₂O, 2 : 5 v/v) with and without 0.8 mmol of PhNO₂. (b) Nyquist plots of Co₉S₈/Ni₃S₂-NF, Co₉S₈/Ti and Ni₃S₂-NF catalysts. (c) Potential-dependent conversion (Con.) of PhNO₂, selectivity (Sel.) and FE of PhNH₂ over Co₉S₈/Ni₃S₂-NF within 8 h at 0.121 V (vs. RHE). (d) Time-dependent evolution of PhNO₂ and PhNH₂ products over Co₉S₈/Ni₃S₂-NF (inset: selectivity (Sel.) of all products). (e) Conversion (Con.) of PhNO₂, selectivity (Sel.) and FE of PhNH₂ of Co₉S₈/Ni₃S₂-NF, Co₉S₈/Ti and Ni₃S₂/NF within 8 h at 0.121 V (vs. RHE). (f) Cycle-dependent performance of transforming PhNO₂ into PhNH₂ over Co₉S₈/Ni₃S₂-NF within 8 cycles (8 h per cycle) at 0.121 V (vs. RHE).

To explore the potential-dependence of the selectivity of PhNH₂, the electrocatalytic reduction of PhNO₂ over Co₉S₈/Ni₃S₂-NF was carried out at various potentials from 0.421 to −0.279 V (vs. RHE) (Fig. 3c) by gas chromatography (GC) (Fig. S22–S23†). The wide E_onset gap between NO-RR and HER indicates efficient and highly selective reduction of PhNO₂ to PhNH₂ on the Co₉S₈/Ni₃S₂-NF electrode at different potentials from 0.321 to −0.079 V (vs. RHE). Specifically, the optimal catalytic performances for PhNO₂ to PhNH₂ were achieved at 0.121 V (vs. RHE) with up to 99.0% conversion, 96.0% selectivity and 95.3% faradaic efficiency (FE) (Table S8†). Nevertheless, PhNO₂ cannot be reduced at potentials higher than 0.421 V (vs. RHE) and demonstrates low conversion and FE at potentials more negative than −0.079 V (vs. RHE) due to the dominant HER. In Fig. 3d, time-dependent transformations reveal that PhNO₂ is almost completely converted to PhNH₂ within ∼8 h at 0.121 V (vs. RHE), and the corresponding selectivity of NO-RR is depicted in the inset of Fig. 3d. By contrast, lower conversion and selectivity are obtained over Co₉S₈/Ti and Ni₃S₂/NF cathodes at 0.121 V (vs. RHE), showing the promotion effect of the Co₉S₈/Ni₃S₂ heterojunction in NO-RR (Fig. 3e). The other control catalysts were also evaluated to verify the more significant electrochemical performance of Co₉S₈/Ni₃S₂-NF (Fig. S24†).

Furthermore, the electrocatalytic stability of PhNO₂ reduction is assessed over Co₉S₈/Ni₃S₂-NF. After 8 cycles (8 h per cycle) of electrolysis at 0.121 V vs. RHE, Co₉S₈/Ni₃S₂-NF maintained high selectivity and FE for PhNO₂ to PhNH₂ conversion (Fig. 3f and Table S9†). Furthermore, the current efficiency and Nyquist plots are almost consistent with the starting cycle (Fig. S25†). Negligible changes were discovered by further SEM, TEM, XRD and XPS characterization, implying the excellent durability of the Co₉S₈/Ni₃S₂-NF cathode for NO-RR (Fig. S26–S28 and Tables S10, S11†). All these electrochemical results demonstrate that Co₉S₈/Ni₃S₂-NF shows excellent catalytic activity and promising cycle stability for application in the electrocatalytic conversion of PhNO₂ for PhNH₂ production.

To understand the underlying origin of the excellent activity of the Co₉S₈/Ni₃S₂-NF heterostructure, density functional theory (DFT) calculations were further employed.^2,16,63,64 According to the HRTEM and XRD results, Co₉S₈ (440) and Ni₃S₂ (110) lattice planes are preferentially exposed in Co₉S₈/Ni₃S₂-NF, and thus the models of the Co₉S₈ (440), Ni₃S₂ (110), and Co₉S₈ (440)/Ni₃S₂ (110) heterojunction surfaces were optimized (Fig. S29†). To evaluate the electronic interaction between Co₉S₈ and Ni₃S₂, the calculated charge density difference within the Co₉S₈/Ni₃S₂ heterojunction was analyzed. As displayed in Fig. 4a, charge density difference for the Co₉S₈/Ni₃S₂ model (front and left views) clearly shows a significant charge rearrangement occurring at the interface of Co₉S₈/Ni₃S₂.³⁵ Additionally, electron accumulation is also observed on the Co atoms close the Co₉S₈/Ni₃S₂ interface (marked as a blue cycle) due to electron transfer (2.01 e⁻) from Ni₃S₂ to Co₉S₈, which was consistent with the XPS and Raman results. Such electron transfer at the Co₉S₈/Ni₃S₂ interface is more directly manifested in the corresponding radial distribution function (Fig. 4b). Furthermore, the calculated projected density of states reveals that the Co₉S₈/Ni₃S₂ heterostructure has a higher electron density of 1.96 at the Fermi level than single Co₉S₈ (1.60) and Ni₃S₂ (0.64) (Fig. 4c), which can facilitate electron transfer.⁶⁵ Notably, all three models present no band gap crossing the Fermi level, indicating that the Co₉S₈/Ni₃S₂ heterostructure preserves the metallic nature of Ni₃S₂ and Co₉S₈ with high conductivity.

Fig. 4 (a) The front and left views of calculated charge density difference of the Co₉S₈/Ni₃S₂-NF heterostructure. (b) The corresponding radial distribution function over Co₉S₈/Ni₃S₂-NF. (c) The calculated projected density of states on Co₉S₈/Ni₃S₂-NF, Co₉S₈/Ti and Ni₃S₂/NF systems. (d) Calculated H₂O adsorption energy. (e) The catalytic pathway on the Co₉S₈/Ni₃S₂ surface. (f) The computational energy profile of the optimized intermediates for Co₉S₈ (440), Ni₃S₂ (110), and the Co₉S₈ (440)/Ni₃S₂ (110) heterojunction. The fuchsia, caeseous, yellow, gray, blue, red, and white spheres represent the Co, Ni, S, C, N, O and H atoms, respectively. The yellow and green regions represent electron accumulation and depletion, respectively.

The adsorption energies of H₂O (ΔE_H2O) and the energy profile of the PhNO₂ reduction were further studied. As shown in Fig. 4d, the Co₉S₈/Ni₃S₂ heterojunction has a more negative ΔE_H2O value of −2.14 eV than Co₉S₈ (−1.89 eV) and Ni₃S₂ (−1.59 eV), which implies effective H₂O activation for H* generation on the Co₉S₈/Ni₃S₂ interface.²³ This calculation result is in line with the HER experimental measurements in alkaline medium (Fig. S19†), and the in situ formed active H* at the interface will participate in the transfer hydrogenation of PhNO₂.

According to the structural optimization of intermediates in the aqueous electrochemical reduction of PhNO₂, the corresponding energy profile of PhNO₂ hydrogenation on the Co₉S₈/Ni₃S₂ surface along the pathway was calculated. The catalytic cycle is displayed in Fig. 4e, and specific elementary steps are as follows: (i) ; (ii) ; (iii) PhNOOH* + H + e⁻ → PhNHO*OH; (iv) PhNHO*OH + H + e⁻ → PhNHO* + H₂O; (v) PhNHO* + H + e⁻ → PhNHOH*; (vi) PhNHOH* + H + e⁻ → PhNH₂ + OH*; (vii) OH* + H + e⁻ → H₂O, in which * represents the adsorption site. Notably, the PhNO₂ molecule will spontaneously adsorb on two Co atoms close to the Co₉S₈/Ni₃S₂ interface, such a diatomic metal-site adsorption mechanism has also been reported in thermal catalysis under high pressure.⁶³ For comparison, both Co₉S₈ and Ni₃S₂ were also considered, and the corresponding mechanism process is shown in Fig. S30.†

Fig. 4f exhibits the computational energy profiles of the optimized intermediates for Co₉S₈ (440), Ni₃S₂ (110), and the Co₉S₈ (440)/Ni₃S₂ (110) heterojunction at U = 1.23 V vs. the reversible hydrogen electrode.⁶⁶ Logically, Co₉S₈/Ni₃S₂ exhibits a lower energy absorption of 0.15 eV on the rate determining step (RDS) compared to Co₉S₈ of 1.82 eV and Ni₃S₂ of 0.73 eV, conforming to the unique catalytic activities of Co₉S₈/Ni₃S₂-NF again. Compared with pure Co₉S₈, the Co₉S₈/Ni₃S₂ model is more beneficial to the desorption of OH (S6 → S7), which is because the Co₉S₈/Ni₃S₂ interface breaks the charge symmetry of two Co atom sites (Fig. 4a).

Hence, the OH intermediate is prone to adhere to one Co atom at the Co₉S₈/Ni₃S₂ surface, but it exhibits strong bridging adsorption on symmetric two cobalt atoms at the Co₉S₈ surface (Fig. 4e and S29†). Overall, calculations demonstrate that the Co₉S₈/Ni₃S₂ heterojunction is beneficial for activation of H₂O, reducing the energy barrier of subsequent PhNO₂ hydrogenation and facilitating electron transfer at interfaces.

On the basis of DFT results, ultraviolet photoelectron spectroscopy (UPS) measurements were performed to gain further insights into interfacial electron transfer capacity. Fig. S31† shows that the work functions of Co₉S₈/Ni₃S₂-NF, Co₉S₈/Ti and Ni₃S₂/NF catalysts were calculated to be 6.77, 6.28 and 5.89 eV, severally. The low work function of Co₉S₈/Ni₃S₂-NF indicates a rapid electron donation capacity from the catalyst surface to the adsorbed nitro groups.^67,68 Therefore, theoretical calculations and experiments together show that the construction of the Co₉S₈/Ni₃S₂ heterojunction facilitates the efficient reduction of PhNO₂ to PhNH₂.

The general applicability of the electrochemical selective hydrogenation of nitroarenes towards aminoarenes was extended with excellent performance over the Co₉S₈/Ni₃S₂-NF cathode (Table 1).^2,69 Obviously, a series of functionalized aminoarenes with electro-donating groups of –OCH₃ and –OH, and electron withdrawing groups of –Cl and –F on the ortho, meta or para positions can be obtained in excellent yields (entries 1–6). Nitroarenes with other fragile –Br and –COCH₃ groups were also successfully converted with excellent yields (entries 7–10). Due to the two hexatomic rings of the 5-nitroquinoline substrate, the difficult adsorption of 5-nitroquinoline on the catalyst leads to a low conversion of NO-RR compared to other functionalized aminoarenes (entry 11). Overall, these results signify that the potential applicability of Co₉S₈/Ni₃S₂-NF provides a good opportunity for subsequent generation of more complex molecules, which are challenging in traditional synthesis methods.

Table 1 Selective electroreduction of various substituted nitroarenes over the Co₉S₈/Ni₃S₂-NF cathode^a

Entry	Reactant	Product	Con. (%)	Sel. (%)	FE (%)
a Reaction conditions: nitro substrates (0.8 mmol), Co9S8/Ni3S2-NF (working area: 1 cm^2), 1.0 M KOH solution (Diox/H2O, 2 : 5 v/v), room temperature, −0.421 ∼ −0.279 V (vs. RHE), 8 h. The conversion (Con.), selectivity (Sel.) and faradaic efficiency (FE) were determined by gas chromatography.
1			100.0	97.5	96.9
2			96.3	100.0	96.2
3			93.6	95.4	96.0
4			100.0	97.1	98.1
5			100.0	99.3	97.0
6			100.0	97.6	96.3
7			97.7	97.9	95.0
8			100.0	99.5	90.2
9			99.5	99.9	98.1
10			97.1	98.5	96.4
11			79.4	98.3	84.0

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Conclusions

In summary, self-supporting hierarchical nanosheets composed of Co₉S₈/Ni₃S₂ heterojunctions were fabricated via an in situ sulphuration strategy using layered double hydroxide as a self-sacrificing template. Because of the integration of plentiful active Co₉S₈/Ni₃S₂ interfaces, open nanostructure, and high conductivity, the as-prepared Co₉S₈/Ni₃S₂-NF shows outstanding NO-RR performance in alkaline aqueous medium. Both the experimental results and theoretical calculations reveal electron transfer from Ni₃S₂ to Co₉S₈, which is identified by electron redistribution at interfaces. For this reason, the Co₉S₈/Ni₃S₂ heterojunction is more conducive to the adsorption of H₂O and PhNO₂ for the formation of H* and further protonation of PhNO₂. Furthermore, interfacial charge transfer can break the symmetry of two active Co sites at the Co₉S₈/Ni₃S₂ interface to change the strong bridging adsorption of OH on Co sites, which markedly reduces the energy barrier. In addition, Co₉S₈/Ni₃S₂-NF also exhibits good functional group tolerance and excellent cycling stability for electrocatalytic NO-RR. This work reveals the electrocatalytic mechanism for NO-RR on metal sulfide-based heterojunctions, and reflects the advantage and great potential of interfacial engineering for green synthesis.

Data availability

All data associated with this article have been included in the main text and ESI.†

Author contributions

Y. C., M. D., and P. Z. conceived the concept and supervised the project. K. Y. contributed to valuable guidance and comments. X. W. and L. L. searched the literature and performed the synthesis, characterization and electrochemical experiments. G. X. and M. S. assisted with the HR-TEM testing and data analysis. Y. W. assisted with the XRD testing and data analysis. The manuscript was written through contributions of all authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge financial support from the National Natural Science Foundation of China (52073137, 22172075, and 92156024), the Education Department Science and Technology Research Foundation of Jiangxi Province (GJJ210318), the Natural Science Foundation of Jiangxi Province (20212BAB213018, 20203BDH80W011, and 20212BAB214028), and the Thousand Talents Plan of Jiangxi Province (jxsq2019102002).

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Footnotes

† Electronic supplementary information (ESI) available. See https://doi.org/10.1039/d2sc03585e

‡ These authors contributed equally.

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