Engineering of Lewis acid–base interfaces in Cu₂S/ZnIn₂S₄ hollow hetero-nanocages for enhanced photocatalytic CO₂ reduction

Abstract

Selective photocatalytic CO₂ reduction (PCR) to CH₄ remains challenging due to the sluggish charge transfer kinetics and the involved complicated C₁ intermediates. Herein, deliberate engineering of Lewis acid–base interfaces in Cu₂S/ZnIn₂S₄ hollow hetero-nanocages (HHNCs) was carried out, and enhanced PCR activity and selectivity were achieved due to accelerated electron transfer and stabilized intermediates. Both experimental and theoretical results have demonstrated the construction of a Lewis base interface with Cu₂S and a Lewis acid interface with ZnIn₂S₄, which exhibited strong CO₂ adsorption and reduction of the Gibbs free energy in the hydrogenation step (*CO to *CHO). As a consequence, a CH₄ yield of 23.3 μmol g⁻¹ h⁻¹ under visible light irradiation (λ > 400 nm) was obtained with the Cu₂S/ZnIn₂S₄ HHNCs, approximately 13.7, 10.1 and 6.3 times higher than those of bare Cu₂S, ZnIn₂S₄ and a physically mixed sample (Cu₂S/ZnIn₂S₄-mix), respectively. The product selectivity of CH₄ was as high as 93.2%, in sharp contrast with 59.5% for the Cu₂S/ZnIn₂S₄-mix, 53.1% for Cu₂S and 35.4% for ZnIn₂S₄. This work demonstrates a rational strategy to engineer heterogenous Lewis acid–base interfaces for improving PCR activity and selectivity.

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New concepts

Selective photocatalytic CO₂ reduction (PCR) to CH₄ is highly coveted yet remains challenging due to sluggish charge transfer kinetics and the involvement of various C₁ intermediates. Constructing frustrated Lewis pair catalysts by assembling surface defect states, such as oxygen vacancies, is regarded as vital for CO₂ adsorption and activation. However, the thermodynamically unstable defect states inhibit the sustainability of PCR, which is averse to applications in industry. In this work, we have proposed a new strategy involving the deliberate construction of Lewis acid–base interfaces, in which nucleophilic O atoms and electrophilic C in CO₂ can be activated by combinations of electron acceptor (Lewis acid) interfaces and electron donor (Lewis base) interfaces to accelerate multiple proton coupled electron transfer and stabilize intermediates, aiming to enhance PCR activity and selectivity. These significant breakthroughs in this current forefront fundamental research make us believe that both the science and technique of this work are interesting and convincing enough for the broad readership.

1. Introduction

Photocatalytic CO₂ reduction (PCR) toward high value-added chemical products (such as CO, CH₄, C₂H₄, and CH₃OH) is considered to be an ingenious strategy for addressing global energy demand and environmental problems.^1,2 Among these hydrocarbons, methane (CH₄) is very important hydrocarbon feedstock for the storage of hydrogen and generation of multiple-carbon hydrocarbon molecules (such as C₂H₄ and C₂H₆) by non-oxidative coupling of methane and other commodity chemicals.^3,4 However, due to the dissociation energy of the C=O bond in CO₂ being as high as 750 kJ mol⁻¹, the PCR performance has remained limited up to now.⁵ Furthermore, special attention is paid to the conversion of CO₂ to CH₄, which requires the most electrons (8 electrons) of all the C₁ chemicals and is therefore one of the most challenging processes.^6,7 Recently, the design of dual-metal sites has been recognized as an excellent concept to intensify CO₂ adsorption and stabilize C₁ intermediates, enhancing PCR activity and selectivity.^8,9 Constructing frustrated Lewis pair catalysts by assembling surface defect states, such as oxygen vacancies, is regarded as vital for CO₂ adsorption and activation.^10,11 However, the thermodynamically unstable defect states inhibit the sustainability of PCR, which is averse to applications in industry.^12,13

Metal sulfide semiconductors with a broad photo-responsive range and advantageous band structures are suitable photocatalysts for PCR.^14,15 Among these materials, ZnIn₂S₄ is a commonly employed photocatalyst due to its chemical stability, non-toxic nature, and a suitable band gap for PCR. Nevertheless, the pristine ZnIn₂S₄ exhibits low PCR activity because of its low carrier mobility and limited CO₂ activation ability.^15,16 Conversely, Cu₂S features a narrow band gap and rapid carrier mobility.^17,18 Owing to the distinct photogenerated carrier migration rates of ZnIn₂S₄ and Cu₂S, in the presence of light, photogenerated electrons in the heterojunction photocatalyst will undergo migration from one catalyst to another, resulting in the formation of an electron-rich (Lewis base) side and an electron-poor (Lewis acid) side at the heterojunction interfaces, where Lewis acid–base interfaces will be established (Scheme 1).^19,20 Considering the coexistence of electrophilic C and nucleophilic O atoms in CO₂, when CO₂ is adsorbed on Lewis acid–base interfaces, the CO₂ molecule can be activated by combinations of electron acceptor (Lewis acid) interfaces and electron donor (Lewis base) interfaces to form a stable I_A–O–C–I_B configuration (where I_A and I_B represent Lewis acid interfaces and Lewis base interfaces, respectively).^21,22 The Lewis acid–base interfaces, formed by the accumulation of photogenerated electrons at a heterogeneous interface under light irradiation, exhibit enhanced stability compared to traditional frustrated Lewis pairs generated through the construction of defect sites.

Scheme 1 Illustration of the CO₂ adsorption configuration on Lewis acid–base interfaces. The cyan and the yellow regions agglomerated at the heterogeneous interface represent charge depletion and charge accumulation, respectively, where the Lewis acid–base interfaces will be established. The white, pink and black spheres represent hydrogen, oxygen and carbon atoms, respectively.

Inspired by the above considerations, the Lewis acid–base interfaces in Cu₂S/ZnIn₂S₄ hollow hetero-nanocages (HHNCs) were deliberately designed and fabricated to accelerate electron transfer and hydrogenate *CO to *CHO for enhancing the selective photocatalytic reduction of CO₂ to CH₄. The experimental results show that the electron aggregation region around Cu₂S interfaces and the electron depletion region around ZnIn₂S₄ interfaces upon irradiation lead to the generation of Lewis base sites at the Cu sites and Lewis acid sites at the In sites, which can dramatically enhance CO₂ adsorption and activation and accelerate the hydrogenation of *CO rather than desorption. Density functional theory (DFT) calculations further verified that the hydrogenation of *CO to *CHO becomes significantly favorable in contrast to its desorption to gaseous CO, thus contributing to the selectivity of CH₄. As a result, the as-prepared Cu₂S/ZnIn₂S₄ HHNCs achieved a CH₄ yield of 23.3 μmol g⁻¹ h⁻¹ under visible light irradiation (λ > 400 nm), which is about 6.3, 13.7 and 10.1 times that over a physically mixed sample (Cu₂S/ZnIn₂S₄-mix, 3.7 μmol g⁻¹ h⁻¹), Cu₂S (1.7 μmol g⁻¹ h⁻¹) and ZnIn₂S₄ (2.3 μmol g⁻¹ h⁻¹), respectively. In addition, the selectivity achieved is as high as 93.2% than the Cu₂S/ZnIn₂S₄-mix (59.5%), Cu₂S (53.2%) and ZnIn₂S₄ (35.4%).

2. Results and discussion

2.1 Composition and microstructure of Cu₂S/ZnIn₂S₄ HHNCs

Cu₂S/ZnIn₂S₄ HHNCs were obtained through a facile multistep templating strategy. Initially, the CuS NBs were synthesized by partial sulfidation and core etching of the Cu₂O NCs.^23,24 Subsequently, the CuS NBs were coated with ZnIn₂S₄ NSs and then annealed to yield the final Cu₂S/ZnIn₂S₄ HHNCs (Fig. 1a). The cubic phase Cu₂S (JCPDS no. 03-1071) and the hexagonal phase (JCPDS no. 60-2023) ZnIn₂S₄ were verified by powder X-ray diffraction (XRD) patterns (Fig. 1b).^24,25 The field-emission scanning electron microscopy (FESEM) image in Fig. S2 (ESI†) and the transmission electron microscopy (TEM) image in Fig. 1c show uniform morphology of Cu₂S/ZnIn₂S₄ HHNCs with abundant intersected ZnIn₂S₄ NSs on the surface of hollow Cu₂S NBs. The uniform distribution of Cu, Zn, In, and S elements within the whole Cu₂S/ZnIn₂S₄ HHNCs (Fig. S3, ESI†) was observed in elemental mapping images. The high-resolution TEM (HRTEM) image in Fig. 1d and that in Fig. S4 (ESI†) show lattice spacings of 0.320 nm and 0.333 nm, belonging to the (002) plane of cubic Cu₂S and the (100) plane of hexagonal ZnIn₂S₄, respectively, in agreement with the XRD results. Moreover, the heterointerface structure that existed on the Cu₂S/ZnIn₂S₄ HHNCs marked with purple imaginal lines is clearly observed. Additionally, geometric phase analysis (GPA) simulation was carried out to determine the interface environment of the Cu₂S/ZnIn₂S₄ HHNCs.^26,27 As shown in Fig. S5 (ESI†), the GPA images demonstrated obvious internal strain at Cu₂S and ZnIn₂S₄ of Cu₂S/ZnIn₂S₄ HHNCs. The distinct structural distortion stems from the lattice mismatch at the interface between Cu₂S and ZnIn₂S₄.^28,29 This lattice mismatch intensifies the built-in electric field, thus accelerating the migration of carriers and further promoting the formation of a Lewis acid–base interface at the interfaces of Cu₂S and ZnIn₂S₄.^26,29,30 The reciprocal spacings of the selected area electron diffraction (SAED) pattern and their relative intensity along with an integrated intensity-spacing profile were obtained, as shown in Fig. 1e. A set of composite diffraction patterns containing Cu₂S and ZnIn₂S₄ were observed with no impurity signal detected, demonstrating the successful fabrication of Cu₂S/ZnIn₂S₄ HHNCs.

Fig. 1 (a) Schematic illustration of the synthetic process of the Cu₂S/ZnIn₂S₄ HHNCs. (b) XRD patterns of the as-prepared Cu₂S NBs, ZnIn₂S₄ NSs, Cu₂S/ZnIn₂S₄ HHNCs and the Cu₂S/ZnIn₂S₄-mix. (c) TEM and (d) HRTEM images of the Cu₂S/ZnIn₂S₄ HHNCs. (e) Averaged position and intensity of diffraction rings in the SAED pattern (inset) of the Cu₂S/ZnIn₂S₄ HHNCs. (f) Cu 2p and (g) In 3d XPS spectra of the Cu₂S NBs, ZnIn₂S₄ NSs, Cu₂S/ZnIn₂S₄ HHNCs and the Cu₂S/ZnIn₂S₄-mix. (h) Raman spectra of the Cu₂S NBs, ZnIn₂S₄ NSs, Cu₂S/ZnIn₂S₄ HHNCs and the Cu₂S/ZnIn₂S₄-mix. (i) The structure models of the Cu₂S/ZnIn₂S₄ heterostructure.

X-ray photoelectron spectroscopy (XPS) was employed to analyze the surface chemical composition and valence states. The survey spectra reveal that the as-prepared Cu₂S/ZnIn₂S₄ HHNCs consist of Cu, S, Zn, and In elements, confirming the successful synthesis of the heterojunction structure (Fig. S6a, ESI†). The Cu 2p XPS spectra of Cu₂S NBs at 932.72 and 952.49 eV correspond to Cu 2p_3/2 and Cu 2p_1/2 of the Cu⁺ chemical state (Fig. 1f), consistent with the Auger spectra of Cu⁺ (Fig. S6b, ESI†).³¹ The S 2p spectrum indicates that the characteristic peaks centered at 161.11 and 162.27 eV can be assigned to S 2p_3/2 and S 2p_1/2 orbitals of S²⁻ (Fig. S6c, ESI†). Furthermore, two peaks in the Zn 2p XPS spectra for ZnIn₂S₄ NSs at 1021.88 and 1044.89 eV could be attributed to Zn 2p_3/2 and Zn 2p_1/2 of Zn²⁺, respectively (Fig. S6d, ESI†).^32,33 The In 3d XPS spectra of ZnIn₂S₄ NSs (Fig. 1g) contain two symmetry peaks at 444.35 and 451.87 eV, corresponding to In 3d_5/2 and In 3d_3/2 of In³⁺, respectively.^34,35 Significantly, the observed lower binding energy shift in the Cu 2p spectra and higher binding energy shift in the Zn 2p and In 3d spectra for Cu₂S/ZnIn₂S₄ HHNCs and the Cu₂S/ZnIn₂S₄-mix compared to their pristine counterparts suggest the formation of a strong interaction at the heterogeneous interfaces.³⁶ Furthermore, the more pronounced chemical shift for Cu₂S/ZnIn₂S₄ HHNCs than the physically mixed counterpart (Cu₂S/ZnIn₂S₄-mix) indicates that the in situ growth strategy can create stronger heterogeneous interfaces, which is advantageous for constructing Lewis acid–base interfaces. The existence of Lewis acid–base interfaces on Cu₂S/ZnIn₂S₄ was further confirmed through NH₃-temperature-programmed desorption (NH₃-TPD) and CO₂-temperature-programmed desorption (CO₂-TPD). As shown in Fig. S7a (ESI†), the desorption peaks of Cu₂S and ZnIn₂S₄ at 303.6 and 321.5 °C can be attributed to the chemical adsorption of CO₂ on the catalyst, respectively.^37,38 Compared to Cu₂S and ZnIn₂S₄, Cu₂S/ZnIn₂S₄ exhibited a higher desorption temperature (324.2 °C) and a larger desorption curve integral area, indicating that Cu₂S/ZnIn₂S₄ has more Lewis base sites.³⁷ In the meantime, two desorption peaks for Cu₂S/ZnIn₂S₄ HHNCs at 176.4 and 410.5 °C in NH₃-TPD measurements are observed (Fig. S7b, ESI†). Interestingly, there is no obvious NH₃-TPD signal for the single Cu₂S or ZnIn₂S₄ sample, which further indicates that the Lewis acid sites are only constructed at the Cu₂S/ZnIn₂S₄ heterojunction interface.³⁸

The chemical environment of the heterointerface for the Cu₂S/ZnIn₂S₄ HHNCs and control samples was investigated by Fourier-transform infrared spectra (Fig. S8, ESI†). In the case of ZnIn₂S₄ NSs, the obvious absorption band at 1375 cm⁻¹ was assigned to the vibrational mode of In–S bonds.^39,40 The absorption peak at 617 cm⁻¹ for Cu₂S NBs corresponded to the Cu–S vibration mode.^41–43 Importantly, it is observed that the In–S stretching vibrational peak of Cu₂S/ZnIn₂S₄ HHNCs shifts to a lower wave number when compared with that of ZnIn₂S₄ NSs. Moreover, the Cu–S bonds of Cu₂S/ZnIn₂S₄ HHNCs shift to a higher wave number compared to those of Cu₂S NBs. This suggests potential bonding modes between the In atom of ZnIn₂S₄ NSs and the Cu atom of Cu₂S NBs in Cu₂S/ZnIn₂S₄ HHNCs.⁴⁴ The results were further confirmed by Raman spectroscopy. As shown in Fig. 1h, the peak at 268 cm⁻¹ for Cu₂S NBs corresponded to the vibrational mode of the Cu–S bonds.⁴⁵ For ZnIn₂S₄ NSs, the Raman peaks at 187 cm⁻¹ and 332 cm⁻¹ were attributed to the In–S and Zn–S bonds, respectively.^46–49 Observingly, the remarkable Raman shift of the In–S and Cu–S bonds, along with the steady Raman vibration modes of the Zn–S bonds, indicates a potential connection through Cu–S–In bonds in the Cu₂S/ZnIn₂S₄-mix and Cu₂S/ZnIn₂S₄ HHNCs (Fig. 1i).^50,51 The more significant Raman peak shift observed in Cu₂S/ZnIn₂S₄ HHNCs, compared to the Cu₂S/ZnIn₂S₄-mix, suggests a robust interface interaction between Cu₂S NBs and ZnIn₂S₄ NSs facilitated by an in situ growth strategy.

2.2 Band structures and electron transfer pathways

The band structure of Cu₂S/ZnIn₂S₄ HHNCs and control samples was investigated by UV-vis diffuse reflectance spectroscopy (UV-vis DRS) and ultraviolet photoemission spectroscopy (UPS). Bare ZnIn₂S₄ exhibits an absorption edge at approximately 500 nm (Fig. 2a), corresponding to a band gap of about 2.62 eV estimated by the Tauc plot, while Cu₂S demonstrates a narrow band gap of 1.87 eV (Fig. 2b). UPS analysis was conducted to determine the band structures of Cu₂S NBs and ZnIn₂S₄ NSs from the E_cutoff (secondary electron cutoff) and E_edge (Fermi edge) recorded in Fig. 2c and d, respectively. The Φ values were calculated by subtracting the secondary electron cutoff (E_cutoff) from the excitation energy (21.22 eV), resulting in values of 5.09 and 4.70 eV for Cu₂S and ZnIn₂S₄ (vs. vacuum), respectively. The E_VB of Cu₂S and ZnIn₂S₄ was calculated to be −5.70 and −6.57 eV vs. vacuum (0.85 and 1.72 eV vs. NHE, at pH = 7), respectively.⁵² The E_CB was thus estimated to be −3.83 and −3.95 eV vs. vacuum (−1.02 and −0.90 eV vs. NHE, at pH = 7) for Cu₂S and ZnIn₂S₄, respectively.

Fig. 2 (a) UV-vis DRS spectra of the as-prepared Cu₂S NBs, ZnIn₂S₄ NSs, Cu₂S/ZnIn₂S₄ HHNCs and the Cu₂S/ZnIn₂S₄-mix. (b) Tauc plots, (c) secondary edge regions of UPS spectra and (d) low energy onset region of the Cu₂S NBs and ZnIn₂S₄ NSs. (e) Cu 2p, and (f) In 3d XPS spectra of Cu₂S/ZnIn₂S₄ HHNCs with and without irradiation. (g) Schematic illustration of the construction of Lewis acid–base interfaces on the Cu₂S/ZnIn₂S₄ heterojunction. (h) Calculated charge density difference and planar-averaged electron density difference of the Cu₂S/ZnIn₂S₄ heterojunction, where the cyan and the yellow regions agminated at the heterogeneous interface represent charge depletion and charge accumulation, respectively. The Cu, S, Zn and In atoms are represented by cyan, pink, reseda and fuchsia balls, respectively.

In situ irradiated XPS of Cu₂S/ZnIn₂S₄ HHNCs was employed to further investigate the actual mechanism of electron transfer. The Cu 2p spectra (Fig. 2e) of Cu₂S/ZnIn₂S₄ HHNCs at 932.73 and 932.50 eV demonstrate a significant shift of 0.35 eV toward a higher binding energy under irradiation than those under dark conditions, while the In 3d and Zn 2p spectra moved towards moderately lower values of 0.44 and 0.39 eV, respectively (Fig. 2f and Fig. S9, ESI†), attributed to the injection of electrons from Cu₂S to ZnIn₂S₄ across the heterogeneous interfaces. Typically, the band alignment structure of bare Cu₂S and ZnIn₂S₄ heterojunctions is depicted in Fig. 2g. Under dark conditions, electron transfer occurs from ZnIn₂S₄ to Cu₂S in Cu₂S/ZnIn₂S₄ HHNCs until equilibrium is achieved due to the disparity in E_f, which induces energy band bending in the space charge layer and the establishment of a built-in electric field pointing from ZnIn₂S₄ to Cu₂S. Upon irradiation, the synergistic effect of the built-in electric field and energy band bending leads to the spontaneous transfer of photoexcited electrons from the E_CB of Cu₂S to that of ZnIn₂S₄. Simultaneously, the holes at the E_VB of ZnIn₂S₄ are transferred to that of Cu₂S, establishing a type-II pathway. These results confirm that the electron transfer occurs from Cu₂S to ZnIn₂S₄ in the Cu₂S/ZnIn₂S₄ heterostructure under irradiation, resulting in the generation of an electron depletion layer at the side of ZnIn₂S₄ and an electron accumulation layer around Cu₂S, as depicted in Fig. 2h, and thereby forming Lewis acid–base interfaces in Cu₂S/ZnIn₂S₄ HHNCs.

2.3 PCR performances

The formation of Lewis acid–base interfaces has a significant impact on the activity and selectivity of PCR to CH₄. As illustrated in Fig. 3a and Fig. S10 and Table S1 (ESI†), ZnIn₂S₄ NSs predominantly produce CO (4.2 μmol g⁻¹ h⁻¹) with low CH₄ species (2.3 μmol g⁻¹ h⁻¹) during PCR under visible light irradiation (λ > 400 nm), consistent with previous ZnIn₂S₄ species.^53,54 H₂ was not detected due to the gas–solid reactor system with CO₂ and water vapor, where sustained CO₂ exposure on catalytic surfaces effectively suppressed hydrogen generation through water reduction.^55,56 The CH₄ selectivity of ZnIn₂S₄ NSs is approximately 35.4%. Although the selectivity of Cu₂S (53.1%) has indeed shown improvement compared to ZnIn₂S₄, the limited PCR activity (1.7 μmol g⁻¹ h⁻¹) hinders broader applications. Surprisingly, the in situ assembled heterojunction (Cu₂S/ZnIn₂S₄ HHNCs) demonstrates the highest CH₄ evolution rate (23.3 μmol g⁻¹ h⁻¹) with trace CO gas (1.3 μmol g⁻¹ h⁻¹), surpassing previously reported Cu-based, Zn-based and In-based photocatalysts (Table S2, ESI†). Liquid products (such as HCOOH) in the photocatalytic CO₂ reduction system were also not detected by ¹H nuclear magnetic resonance spectroscopy, indicating high specificity of Cu₂S/ZnIn₂S₄ in photocatalytic CO₂ reduction (Fig. S11, ESI†).^24,57 The CH₄ selectivity achieved is as high as 93.2% (Fig. 3b). Additionally, the photogenerated holes participate in H₂O oxidation during PCR, while the O₂ product rate for the Cu₂S/ZnIn₂S₄ HHNCs is about 50.3 μmol g⁻¹ h⁻¹ (Fig. S12, ESI†). However, the high PCR activity and selectivity of Cu₂S/ZnIn₂S₄ HHNCs are not reprinted by the Cu₂S/ZnIn₂S₄-mix (3.7 μmol g⁻¹ h⁻¹ for CH₄ and 2.5 μmol g⁻¹ h⁻¹ for CO), suggesting that the unique and compact interfacial arrangement plays a significant role in shaping the reactivity and specificity towards PCR. The activity and selectivity remained stable even after ten cycles (Fig. 3c). In addition, the consistent phase constitution (Fig. S13a, ESI†), morphology of the hollow cubic structure (Fig. S13b, ESI†) and chemical state of the element (Fig. S14, ESI†) provide inherent evidence that the Cu₂S/ZnIn₂S₄ HHNCs exhibit excellent photostability for PCR to CH₄. Control experiments (Fig. S15, ESI†) demonstrated that no products were detected in the absence of photocatalysts, H₂O, light irradiation, or under an Ar atmosphere, confirming that the production of CH₄ resulted from the reduction of CO₂ triggered using the Cu₂S/ZnIn₂S₄ HHNCs under light irradiation. Furthermore, the PCR activity of Cu₂S/ZnIn₂S₄ HHNCs is significantly weakened when a trace amount of pyridine or pyrrole is introduced into the reaction system (Fig. S16, ESI†), which is due to the adsorption of pyridine and pyrrole on Lewis acid and basic sites of the Cu₂S/ZnIn₂S₄ heterojunction, respectively.⁵⁸ This result further suggests that the Lewis acid–base interfaces of Cu₂S/ZnIn₂S₄ HHNCs play a crucial role in the PCR process. The isotopic labeling experiment was further performed to investigate the origin of CH₄ and CO through the replacement of ¹²CO₂ with ¹³CO₂. Ethane (C₂H₆) was employed as an internal standard substance (ISS). As shown in Fig. S17–S19 (ESI†), two peaks at m/z = 29 and 17 were identified and attributed to ¹³CO and ¹³CH₄, respectively. The peaks of ¹³CO and ¹³CH₄ gradually increased with the extension of the light exposure time (Fig. 3d), verifying that the products were exclusively derived from the reduction of CO₂.^19,59

Fig. 3 (a) CH₄ and CO evolution rates over different samples. (b) Product selectivity of CO and CH₄ evolution over different samples. (c) Cyclic tests of the Cu₂S/ZnIn₂S₄ HHNCs for photocatalytic CH₄ and CO evolution and CH₄ product selectivity. (d) Isotope-labeled mass spectra of gaseous products for photocatalytic reduction of ¹³CO₂ over Cu₂S/ZnIn₂S₄ HHNCs under visible-light irradiation.

2.4 Insights into the PCR mechanism

In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) measurements were conducted to investigate the proposed mechanism of high CH₄ selectivity for Cu₂S/ZnIn₂S₄ HHNCs by examining the intermediates and reaction pathways during the PCR. As presented in Fig. 4a and Fig. S20a (ESI†), the absorption peak of Cu₂S/ZnIn₂S₄ HHNCs at 1218 cm⁻¹ was attributed to the b-HCO₃⁻ species. Monodentate carbonate groups (m-CO₃²⁻) at around 1394, 1419, 1518 and 1540 cm⁻¹ and bidentate carbonate groups (b-CO₃²⁻) at 1243 and 1364 cm⁻¹ were also detected well.⁶⁰ All of the b-CO₃²⁻, b-HCO₃⁻ and m-CO₃²⁻ are generated from CO₂ and H₂O groups, which are considered as possible intermediates for the subsequent generation of C1 fuels. The adsorption bands at 1558 and 1653 cm⁻¹ were assigned to the typical *COOH intermediate during CO₂ reduction to CO and CH₄.^61–63 In addition, the observed *CHO and *CH₂O groups at 1468 and 1506 cm⁻¹, respectively, are regarded as crucial intermediates for the conversion of CO₂ to CH₄.^4,34,64 The negative in situ DRIFTS signals of Cu₂S/ZnIn₂S₄ HHNCs might be attributed to the fast consumption of reactants under light illumination due to the excellent photocatalytic CO₂ reduction capacity.^65,66 More importantly, the inconspicuous absorption peaks of *CHO and *CH₂O for ZnIn₂S₄ NSs (Fig. 4b and S20b, ESI†) indicate that the photocatalytic CO₂ methanation is insensitive, highlighting the pivotal role of interfacial reaction in this process, which is attributed to the construction of a Lewis acid–base interface.

Fig. 4 In situ DRIFTS spectra of (a) Cu₂S/ZnIn₂S₄ HHNCs and (b) ZnIn₂S₄ NSs. (c) The electron local function of Cu₂S/ZnIn₂S₄, where the value of function closer to 1 means that the electrons are more localized. (d) CO₂ adsorption energy of Cu₂S, ZnIn₂S₄ and Cu₂S/ZnIn₂S₄. Reaction pathways for CO₂ photocatalysis and calculated Gibbs free energies of reaction intermediates on (e) bare Cu₂S, (f) bare ZnIn₂S₄ and (g) Cu₂S/ZnIn₂S₄. The Cu, S, Zn, In, C, O and H atoms are represented by cyan, pink, reseda, fuchsia, brown, red and white balls, respectively.

DFT calculations were employed to investigate the effects of the electronic structure and the CO₂ reduction mechanism by constructing Lewis acid–base interfaces on the Cu₂S/ZnIn₂S₄ HHNCs. Fig. 4c illustrates that the charge redistribution of Cu₂S/ZnIn₂S₄ HHNCs is primarily confined to the heterostructure interfaces, where there is an accumulation of charge at the Cu sites of Cu₂S and a depletion of charge at the In sites of ZnIn₂S₄. This observation aligns with our hypothesis that Lewis acid–base interfaces are formed on the heterostructure interfaces of Cu₂S/ZnIn₂S₄ HHNCs. The asymmetric charge redistribution helps to perturb the spatial electronic distribution and increase the adsorption capacity of nonpolar CO₂ molecules. The adsorption of CO₂ represents a crucial aspect of PCR, directly influencing the photocatalytic activity. The N₂ adsorption–desorption measurements (Fig. S21, ESI†) reveal that the Cu₂S/ZnIn₂S₄ HHNCs (69.7 m² g⁻¹) demonstrate an intermediate specific surface area between ZnIn₂S₄ NSs (188.1 m² g⁻¹) and Cu₂S NBs (4.4 m² g⁻¹). However, the CO₂ uptake of Cu₂S/ZnIn₂S₄ HHNCs was 4.6 cm³ g⁻¹ (Fig. S22, ESI†), which was superior to that of bare Cu₂S (3.3 cm³ g⁻¹) and ZnIn₂S₄ (4.1 cm³ g⁻¹). The enhanced CO₂ adsorption capacity of Cu₂S/ZnIn₂S₄ is attributed to the formation of a Lewis acid–base interface. Based on the polarized electronic structure characteristics of the CO₂ molecule, the C atom in the CO₂ molecule is electrophilic, while the O atoms with lone pairs are nucleophilic. According to Lewis acid–base theory, the C atom and the O atom of the activated CO₂ molecule are regarded as Lewis acid and base sites, respectively. Therefore, the C atom tends to anchor at the basic site, while the O atom preferentially binds with the acidic site on the catalyst surface, forming an I_A–O–C–I_B configuration. The I_A–O–C–I_B intermediate facilitates the formation of the CHO* intermediate, which is further protonated to generate hydrocarbons,²¹ resulting in the formation of the Lewis acid–base interface facilitating the adsorption and activation of CO₂. The strong CO₂ adsorption property was further confirmed through theoretical calculation of CO₂ adsorption energy (Fig. 4d). The Cu₂S/ZnIn₂S₄ HHNCs with Cu–In dual sites adsorbing C and O, respectively, exhibit more negative adsorption energy (−1.68 eV) than the control sample compared to isolated Cu (0.16 eV) and In active sites (0.18 eV) of Cu₂S/ZnIn₂S₄ HHNCs demonstrating that interfacial dual sites are favorable for CO₂ adsorption capacity.⁶⁶ Furthermore, the lower adsorption energy of CO₂ in the case of Cu₂S/ZnIn₂S₄ compared to Cu₂S and ZnIn₂S₄ demonstrates that the Lewis acid–base interface structure with Cu–In dual sites favors CO₂ adsorption and facilitates subsequent activation.⁶⁷

The electron transfer between the intermediate states and Cu₂S/ZnIn₂S₄, Cu₂S, and ZnIn₂S₄ was visualized and quantified using charge density difference and Bader charge analysis. The Bader charge of the adsorbed *COOH changes from 0.365 |e| on Cu₂S (Fig. S23, ESI†) and 0.322 |e| on the ZnIn₂S₄ surface (Fig. S24, ESI†) to 0.391 |e| on the Cu₂S/ZnIn₂S₄ surface (Fig. S25, ESI†), indicating a more pronounced interaction between the *COOH intermediate and Cu₂S/ZnIn₂S₄ than that of Cu₂S or ZnIn₂S₄.⁶⁸ Theoretical calculations were further performed to analyze the reaction pathway and energy barriers during PCR. The Gibbs free energy barrier (ΔG) of the rate-determining step (RDS) on Cu₂S (Fig. 4e and Fig. S26, ESI†) and ZnIn₂S₄ (Fig. 4f and Fig. S27, ESI†) for the conversion of *CO₂ into *COOH was 1.46 and 2.22 eV, respectively. By contrast, the Gibbs free energy barrier for *COOH formation in Cu₂S/ZnIn₂S₄ (Fig. 4g and Fig. S28, ESI†) is significantly lower (0.37 eV). Furthermore, the RDS shifts from *COOH formation in Cu₂S and ZnIn₂S₄ to *CHO formation in Cu₂S/ZnIn₂S₄. While CO desorption is more favorable relative to subsequent hydrogenation in bare Cu₂S (Fig. 4e) and ZnIn₂S₄ (Fig. 4f), the desorption of *CO presents a challenge for Cu₂S/ZnIn₂S₄ HHNCs (Fig. 4g) due to a high energy requirement (ΔG = 2.59 eV), whereas the process of *CO hydrogenation leading to CHO* demonstrates a low energy barrier (ΔG = 1.48 eV), facilitating the photoreduction of CO₂ to CH₄ and illustrating remarkable CO-to-CH₄ selectivity.⁶⁹ The spontaneous desorption of *CO with an exothermic nature, rather than hydrogenation to *CHO intermediates for Cu₂S/ZnIn₂S₄ at bare Cu and In sites (Fig. S29, ESI†), suggests that the formation of Lewis acid–base interfaces is advantageous for CO₂-to-CH₄ conversion. In addition, the temperature-programmed desorption of CO (CO-TPD) was used to further illustrate the preference for CH₄ evolution on the Cu₂S/ZnIn₂S₄ surface. Compared to bare Cu₂S and ZnIn₂S₄, Cu₂S/ZnIn₂S₄ has a higher CO desorption temperature and the greatest CO adsorption capacity, which suggests that the surface of Cu₂S/ZnIn₂S₄ is more inclined to the CO hydrogenation process (Fig. S30, ESI†). Based on the aforementioned findings, it can be inferred that the construction of Lewis acid–base interfaces among Cu₂S/ZnIn₂S₄ HHNCs effectively facilitates the activation of CO₂, enhances the adsorption of *CO intermediates, and improves the selectivity for CH₄ production.

3. Conclusions

In conclusion, Cu₂S/ZnIn₂S₄ HHNCs featuring Lewis acid–base interfaces have been successfully synthesized using a straightforward multistep templating strategy. The experimental results show that the formation of an electron aggregation region around the Cu₂S interface and an electron depletion region around the ZnIn₂S₄ interface upon irradiation leads to the creation of Lewis base sites on the Cu sites and Lewis acid sites on the In sites. DFT calculations further reveal that Cu₂S/ZnIn₂S₄ with Lewis acid–base interfaces exhibits a more negative CO₂ adsorption energy and favorable hydrogenation of *CO to form *CHO compared to Cu₂S and ZnIn₂S₄, suggesting that CH₄ is the primary product in PCR progress. Under visible light irradiation (λ > 400 nm), the prepared Cu₂S/ZnIn₂S₄ HHNCs achieved a CH₄ yield of approximately 23.3 μmol g⁻¹ h⁻¹, which is approximately 6.3, 13.7, and 10.1 times higher than those over the Cu₂S/ZnIn₂S₄-mix, Cu₂S, and ZnIn₂S₄ respectively. The achieved product selectivity of CH₄ is 93.2%, surpassing that of the Cu₂S/ZnIn₂S₄-mix (59.5%), Cu₂S (53.1%), and ZnIn₂S₄ (35.4%). This work extends the concept of Lewis pairs from 0D (Lewis acid–base sites) to 2D (Lewis acid–base interfaces).

Author contributions

Yuanyuan Zhao: investigation, data curation, writing – original draft, and visualization. Kangjie Gao: investigation and validation. Jiaxin Li: visualization and formal analysis. Huanhuan Liu: conceptualization, methodology, and writing – review & editing. Fang Chen: methodology, validation, and data curation. Wentao Wang: software and validation. Yijun Zhong: methodology and formal analysis. Yong Hu: conceptualization, methodology, writing – review & editing, and supervision.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.†

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (22272150 and 22402181), the Zhejiang Provincial Ten Thousand Talent Program (2021R51009), a Project Supported by the Scientific Research Fund of Zhejiang Provincial Education Department (Y202457290), and the Jinhua Science and Technology Plan Project (2024-4-021). The work is carried out at the Shanxi Supercomputing Center of China, and the calculations are performed on TianHe-2. This research is also supported by the advanced computing resources provided by the Supercomputing Center of the USTC.

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Footnote

† Electronic supplementary information (ESI) available: Experimental and characterization section; DFT calculation details and structure models; SEM, TEM, EDS, FTIR, XPS, TPD, and BET results of Cu₂S/ZnIn₂S₄ HHNCs; and photocatalytic performance and comparison tables of Cu₂S/ZnIn₂S₄ HHNCs. See DOI: https://doi.org/10.1039/d5nh00355e

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