English

• CO₂ reduction into CH₄ or other hydrocarbons together with H₂O oxidation into O₂ by solar energy is a promising strategy to alleviate the global warming effect and generate value-added chemicals [1–5]. However, it is great challenge to convert CO₂ under mild conditions using water caused by the stable-bond of C═O in CO₂ molecule, multi-electrons coupling process and competing water splitting reactions. Recently, various semiconductors have been constructed for photocatalytic CO₂ reduction reaction (CO₂RR), such as metal oxides [6,7], metal sulfides [8], carbon-based materials [9], metal organic frameworks [10]. Especially, layered double hydroxides (LDHs) with two-dimensional layered structures, tunability of metal cations, and excellent photoelectric property have been widely used in CO₂RR [11]. However, most of LDHs also suffer from severe agglomerations, and rapidly charge recombination, leading to unsatisfactory CO₂RR activity. Therefore, decorating LDHs with other porous and stable components can greatly increase exposed active sites and inhibit recombination of electron-hole pairs, thus promoting their CO₂RR performance with H₂O [12].

  In₂O₃ is considered as an ideal semiconductor for activization of CO₂ [13,14], due to the its high stability, oxygen defect as well as rich hydroxyl group, and its appropriate band gap is ~2.7 eV, so that the redox potentials can meet simultaneously the requirement for CO₂ reduction and water oxidation. Besides, except the above advantages, MOFs-derived In₂O₃ nanotube after pyrolysis can inherit porous structure and unique morphology from MIL-68(In) [15]. In our previous work, MIL-68(In)-derived In₂O₃ nanotube coupled with Co@C—N exhibited an excellent photocatalytic CO₂ conversion [16]. Besides, carbon dots have attracted wide attention toward photocatalytic CO₂RR, attributed to the unique geometric structure (diameter less than 10 nm, thickness between 0.5 and 3 nm), excellent electron transport efficiency and adjustable light response range [17–19]. Li et al. reported that CDs improved the CH₄ selectivity of g-C₃N₄ during photocatalytic CO₂RR due to proton-coupled ability of CDs [20]. Considering the above advantages, the fabrication of multicomponent catalysts, based on integration of hierarchical heterostructure (LDHs/In₂O₃) and quantum effect (CDs), is particularly important in promoting the CO₂ conversion efficiency and CH₄ selectivity.

  Herein, the ternary heterostructures of CDs/NiAl-LDH@In₂O₃ (C-DH@IN) nanorods were fabricated using in situ growth strategy of CDs and NiAl-LDH nanosheets on MIL-68(In)-derived In₂O₃ after calcination, in which the C-DH@IN catalyst indicated high photocatalytic CO₂RR activity and CH₄ selectivity in pure water under simulated solar irradiation. The optimal CO and CH₄ yields over 3% C-DH@IN are up to 7.12 and 10.67 µmol h⁻¹ g⁻¹, and the CH₄ selectivity is 85.70%, which is 1.81 and 1.15 times higher than that of NiAl-LDH and DH@IN-50, respectively. The photocatalytic mechanism and intermediate HCOO⁻ and CH₃O⁻ products are investigated systematically using in situ Fourier transform infrared (FT-IR) and electron spin resonance (ESR) spectra.

  The diffraction peaks of pure Ni-Al LDH (Fig. 1a) at 11.5°, 23.1°, 34.9°, 39.3°, 46.8°, 60.9° and 62.1° are consistent to (003), (006), (012), (015), (018), (110) and (113) facets respectively, corresponding to the typical LDHs (JCPDS No. 22-0452) [21]. The peaks of In₂O₃ at 21.5°, 30.5°, 35.4°, 37.6°, 41.8°, 45.7°, 51.0°, 55.9° and 60.6° belong to the (211), (222), (400), (411), (332), (431), (440), (611) and (622) diffraction planes of cubic In₂O₃ (JCPDS No. 06-0416) [22], respectively. All the as-prepared DH@IN displays the diffraction peaks of both In₂O₃ and NiAl-LDH, and no obvious impurity peaks are observed. Additionally, as In₂O₃ loading continues to increase, the peaks intensity of In₂O₃ are obviously improved, and meanwhile that of NiAl-LDH is weakened, indicating the successful formation of DH@IN. Besides, the pristine CDs present two broad diffraction peaks at 25° and 43°, consistent with (002) and (100) planes of graphitic carbon, respectively (Fig. S1 in Supporting information) [23]. The C-DH@IN composites show the similar diffraction peaks as DH@IN, suggesting that the crystalline structure of DH@IN is maintained after the addition of the CDs. Moreover, there is no obvious peak belonging to CDs in the C-DH@IN due to their low content, ultra-small size as well as uniform dispersion [24].

  Figure 1

  

  Figure 1.  (a) X-ray diffractometer (XRD) patterns of In₂O₃, NiAl-LDH, DH@IN-50 and C-DH@IN composites. (b) Raman spectra of In₂O₃, NiAl-LDH, DH@IN-50 and 3% C-DH@IN composites. (c) XPS survey spectra of NiAl-LDH, DH@IN and 3% C-DH@IN. High resolution XPS spectra of O 1s. (d–i) Corresponding elemental mapping images of C-DH@IN.

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  The presence of CDs and surface defects of C-DH@IN were verified by Raman spectra (Fig. 1b). The DH@IN heterostructure shows the peaks of both In₂O₃ and NiAl-LDH at 130 and 149 cm⁻¹. In addition, the 3% C-DH@IN has typical D band (1360 cm⁻¹) and G band (1580 cm⁻¹) caused by CDs [25], suggesting that CDs have been successfully embedded into the DH@IN composites. The BET surface area values of In₂O₃ and NiAl-LDH are 32 and 52 m²/g, respectively, yet the DH@IN-50 exhibits the highest surface area (82 m²/g), meaning that the heterostructure possesses more porous structure after in-situ synthesis process. With addition of CDs, 3% C-DH@IN (61 m²/g) is decreased compared with DH@IN-50, which reveals that the implanted CDs partially occupy the pores of DH@IN-50.

  From scanning electron microscope (SEM) and transmission electron microscope (TEM) images (Fig. S2 in Supporting information), after construction of C-DH@IN, the nanoflower morphology of NiAl-LDH disappears and In₂O₃ nanotube is well defined wrapped in multilayer NiAl-LDH nanosheets to fabricate the multi-layer heterostructure, which possesses numerous catalyst active sites and intimate contact between NiAl-LDH and In₂O₃ resulting from the spatial effect. Besides, the addition of CDs has no effect on the morphology of DH@IN and the NiAl-LDH nanosheets are uniformly dispersed on the In₂O₃ surface, which exhibits that the strong interaction can facilitates the migration of photogenerated carriers and light absorption, thus promoting the photocatalytic performance. As observed in the high-resolution transmission electron microscope (HRTEM) image (Fig. S2 h), the interplanar distances of 0.29, 0.26 and 0.21 nm correspond to the (222), (012) and (100) planes of In₂O₃, NiAl-LDH and CDs [26], respectively. As depicted in Fig. S14 (Supporting information), the TEM image of the used 3% C-DH@IN shows no deviation from its initial state, maintaining In₂O₃ nanotube with multi-layer structure. In addition, the elemental composition of C-DH@IN was investigated by energy dispersive spectrometer (EDS) spectra in Fig. S2i, and from the results, the C, O, In, Ni and Al elements are present in the C-DH@IN. The elemental mapping of C-DH@IN exhibits that the C, Ni and Al elements are well-dispersed on the In and O elements, which suggests that the NiAl-LDH and CDs are uniformly coated on In₂O₃ without agglomeration, furthering confirming that the multi-layer nanotube is constructed successfully.

  The elemental compositions and chemical states of NiAl-LDH, DH@IN-50 and 3% C-DH@IN were detected using X-ray photoelectron spectroscopy (XPS) spectra (Fig. S3a in Supporting information). The In 3d spectrum (Fig. S3b in Supporting information) of DH@IN consists of two primary peaks at 444.0 and 451.5 eV, which are assigned to In 3d_5/2 and In 3d_3/2 of In³⁺, respectively [27]. The binding energies of C-DH@IN slightly move to the lower binding energy in comparation with binary DH@IN, probably caused by the strong interaction between CDs and DH@IN. In the O 1s spectra (Fig. 1c), for NiAl-LDH, two peaks situated at 532.7 and 531.5 eV are assigned to adsorbed oxygen (O_abs) and hydroxide groups (O—H), while the DH@IN exhibits that three characteristic peaks at 529.3, 531.4 and 532.9 eV are consistent with lattice oxygen (O_L), oxygen vacancies (O—H) [28], and adsorbed oxygen (O_abs), respectively, indicating that the as-prepared catalysts possess the abundant O—H. The Ni 2p peaks of NiAl-LDH (Fig. S3c in Supporting information) at 856.3 and 873.9 eV result from Ni 2p_3/2 and Ni 2p_1/2 [29], respectively, in accordance with the Ni²⁺ oxidation state. Obviously, after formation of C-DH@IN, the peaks of Ni 2p move toward a higher binding energy in comparation with bare NiAl-LDH, illustrating that the photoexcited electrons are migrated from NiAl-LDH to In₂O₃ upon hybridization. Similar, the Al 2p spectrum of C-DH@IN (Fig. S3d in Supporting information) is composed of two peaks at 69.3 and 74.8 eV, due to Al 2p_3/2 and Al 2p_1/2 of Al³⁺ [30], which also move to higher binding energy compared with NiAl-LDH. For C-DH@IN (Fig. S3e in Supporting information), the peaks of C 1s spectrum at 284.6, 286.2 and 288.7 eV are consistent with C—C, C—O and C═O bonds, respectively, which belong to CDs [31]. From the elemental mapping images of C-DH@IN (Figs. 1d–i), it can be observed that the C, Ni and Al elements are uniformly coated on the In₂O₃ nanotubes, forming a distinctive multi-shell structure.

  The photocatalytic CO₂RR performances of C-DH@IN composites were performed in H₂O solution without any photosensitizer and sacrificial agent under simulated solar irradiation. The CO and CH₄ yields of In₂O₃ are 3.27 and 2.16 µmol g⁻¹ h⁻¹, respectively, while the CO and CH₄ yields of NiAl-LDH are very low, only 2.45 and 0.55 µmol g⁻¹ h⁻¹, respectively. Besides, the photocatalytic performances of binary DH@IN catalysts are much higher than those of In₂O₃ and NiAl-LDH, exhibiting that the formed heterojunction can facilitate the separation of photoexcited electron-hole pairs. Notably, the (CO + CH₄) yields in C-DH@IN are improved significantly after introduction of CDs, and the optimal CO and CH₄ yields of 3% C-DH@IN are up to 7.12 and 10.67 µmol g⁻¹ h⁻¹ (Fig. 2a), respectively, exceeding the performances of many other similar catalysts (Table S2 in Supporting information). In addition, the CO and CH₄ yields of the C-DH@IN are affected by adjusting the loading of CDs.

  Figure 2

  

  Figure 2.  (a) Photoreduction performances, (b) R_electron and (c) carbon products selectivity of In₂O₃, NiAl-LDH, DH@IN-50 and C-DH@IN composites. (d) Stability test of 3% C-DH@IN with four 5 h cycles. (e) Production rates of CO and CH₄ under various reaction conditions. (f) XRD patterns of 3% C-DH@IN before and after CO₂ photoreduction experiment.

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  On account of the formula R_electron = 2R(CO) + 8R(CH₄), Fig. 2b presents the average electron consumption rate (R_electron), in which R(CO) and R(CH₄) mean the yields of CO and CH₄ respectively. The R_electron of 3% C-DH@IN is up to 99.59 µmol g⁻¹ h⁻¹, which is 4.19 and 10.74-fold higher than those of In₂O₃ and DH@IN, respectively. It is noted that, the selectivity of CH₄ over 3% C-DH@IN is as high as 85.7% (Fig. 2c), whereas that for NiAl-LDH and DH@IN is only 47.3% and 74.3%, respectively, demonstrating that more generated CH₄ is due to the introduction of CDs that could absorb more H⁺. The control experiment was carried out to further verify the products (CH₄ + CO) source (Fig. 2e). When the Ar atmosphere is used to replace the CO₂ atmosphere, trace amounts of CO are detected by chromatography, indicating that the CH₄ is indeed from CO₂ instead of other carbon source. Furthermore, no gaseous products are observed in the absence of catalyst or in a dark environment, suggesting that catalyst and light source are necessary for CO₂RR. The 3% C-DH@IN catalyst maintains excellent photocatalytic performance after four 5-h cycles, which indicates that the constructed heterojunctions possess good stability (Fig. 2d). Fig. 2f shows that the main crystalline phase of C-DH@IN remains unchanged after the photocatalytic reaction, suggesting that C-DH@IN has excellent stability and reusability.

  With the increase of CDs loading, the absorption edge of C-DH@IN moves to the larger wavelengths, indicating that the ternary heterostructure significantly promotes the visible-light absorption. The band gap values of In₂O₃ and NiAl-LDH can be obtained from the Tauc curves (Fig. S4b in Supporting information), where the band-gap values of In₂O₃ and NiAl-LDH are 2.68 and 2.25 eV, respectively. The pure In₂O₃ exhibits an intense steady-state photoluminescence (PL) signal at 470 nm (Fig. 3a), indicating a severe recombination of photoinduced charge [32]. Compared to pristine In₂O₃, the DH@IN-50 displays a significant PL quenching, indicating that the charge carrier recombination is sufficiently suppressed. After incorporation of CDs, the PL intensity of C-DH@IN decreases significantly, resulting from the further improvement of charge transfer efficiency. Furthermore, the average PL lifetimes (τ_ave) of In₂O₃, DH@IN-50 and 3% C-DH@IN were estimated by time-resolved photoluminescence decay (TR-PL) spectra (Fig. 3b), with values of 2.56, 1.34 and 0.64 ns for In₂O₃, DH@IN-50 and 3% C-DH@IN [33], respectively. In addition, the 3% C-DH@IN shows faster PL decay and shorter average lifetime compared to DH@IN-50, indicating that CDs suppress the charge recombination and promote charge transfer.

  Figure 3

  

  Figure 3.  (a) PL and (b) TRPL decay of In₂O₃, DH@IN-50 and 3% C-DH@IN. (c) Transient photocurrent responses and (d) EIS Nyquist plots of In₂O₃, NiAl-LDH, DH@IN-50 and 3% C-DH@IN. Mott–Schottky plots of (e) In₂O₃ and (f) NiAl-LDH.

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  Photoelectrochemical tests were used to explore the photoexcited charge separation efficiency of In₂O₃, NiAl-LDH, DH@IN-50, and 3% C-DH@IN. The incorporation of CDs into DH@IN can dramatically enhance the current density of C-DH@IN and promote carrier separation (Fig. 3c). Fig. 3d displays that the arc radius of 3% C-DH@IN is the smallest, indicating that 3% C-DH@IN photocatalyst has lower resistance as well as good charge transfer capability compared with pure In₂O₃ and NiAl-LDH [34]. In order to study the energy band structure, the Mott–Schottky (M–S) curves of In₂O₃, and NiAl-LDH were measured at different frequencies as shown in Figs. 3e and f. The flat-band potentials (E_FB) values for In₂O₃ and NiAl-LDH can be determined from the intercepts of M–S plots, which are −0.76 and −0.97 V, respectively (vs. Ag/AgCl). E_FB values of In₂O₃ and NiAl-LDH are calculated to be −0.56 and −0.77 V (vs. NHE), respectively, according to the equation E_NHE = E_Ag/AgCl + 0.197. It is well known that the conduction band (E_CB) of the n-type semiconductor is about 0.10 V below E_FB. As a result, E_CB of In₂O₃ and NiAl-LDH are determined to be −0.66 and −0.87 V vs. NHE, respectively. Based on the equation E_CB = E_VB - E_g, the E_VB values of In₂O₃ and NiAl-LDH are 2.02 and 1.38 V (vs. NHE), respectively.

  Electron spin resonance (ESR) spectra of In₂O₃, DH@IN-50 and 3% C-DH@IN were used to investigate the existence of ^·O₂⁻ as well as ^·OH (Fig. 4a and Fig. S15 in Supporting information). The DMPO-^·O₂⁻ signals indicate that the photoexcited electrons-enriched E_CB on C-DH@IN is more negative than the potential of O₂/^·O₂⁻ (−0.33 V vs. NHE), thus leading to the production of ^·O₂⁻ active species. More importantly, the clear DMPO-^·OH signals can be observed (Fig. 4b), which means that the holes-accumulated E_VB is more positive than the potential of OH⁻/^·OH (1.99 V vs. NHE) [35]. These findings verify that the C-DH@IN constructs the S-scheme heterojunction and thus possesses a strong redox ability.

  Figure 4

  

  Figure 4.  (a) DMPO-^·O₂⁻ and (b) DMPO-^·OH spin-trapping ESR spectra of 3% C-DH@IN. (c) In situ FT-IR of 3% C-DH@IN. (d) The photocatalytic reaction pathways of C-DH@IN. (e) Schematic illustration of charge transfer over C-DH@IN under simulated solar light.

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  Furthermore, the in-situ FT-IR spectra of C-DH@IN were carried out in the range of 1200–1800 cm⁻¹ to study the reaction pathway of CO₂RR (Fig. 4c). The multiple intermediate products are detected and corresponding peak intensity is enhanced with prolonged irradiation time. The peaks of m-CO₃²⁻ (1419, 1488, 1522 and 1541 cm⁻¹), b-CO₃²⁻ (1372, 1559 and 1627 cm⁻¹) and HCO₃⁻ (1219, 1396, 1440, 1461 and 1478 cm⁻¹) groups are detected during photocatalytic CO₂RR [36], due to the interaction of absorbed CO₂ and H₂O on C-DH@IN catalyst. Meanwhile, H₂O as only solvent is reacted with photoexcited holes to produce O₂, and notably, the peaks of HCOO⁻ (1637 and 1648 cm⁻¹), COO⁻ (1361 cm⁻¹) and CH₃O⁻ (1698 and 1732 cm⁻¹) groups emerge with illumination time, revealing that HCOO⁻ and CH₃O⁻ are considered as the active species and key intermediate for CO₂RR [37]. Besides, the primary CO₂⁻ (1684 cm⁻¹) peaks are observed caused by the conversion of CO₂ to CO, and the peaks at 1448 and 1387 cm⁻¹ result from the methyl groups of CH₄, which verify the generation of CO and CH₄, implying that multi-electron transfer pathway exists in the CO₂RR process of C-DH@IN. Atomic force microscopy together with Kelvin-probe force microscopy (AFM-KPFM) is used to explore the potential surface distribution of C-DH@IN (Fig. S16 in Supporting information). The surface potential under solar irradiation is higher than that in the darkness, revealing C-DH@IN S-scheme heterojunction with a strong built-in electric field.

  Accordingly, the proposed S-scheme for photocatalytic CO₂RR on C-DH@IN is illustrated in Fig. 4e. The S-scheme heterojunction with the intimate contact from the multi-shell nanotube facilitates that the electrons are transferred from In₂O₃ to NiAl-LDH and generates an internal electric field (IEF) caused by the different Fermi levels between the above two semiconductors, thus resulting in the band edge bending. Driven from IEF as well as band bending, the elections in the E_CB of In₂O₃ recombine with holes in the E_VB of NiAl-LDH, and thus the electrons on the E_CB of NiAl-LDH and holes on the E_VB of In₂O₃ are well preserved, which exhibits that the reserved electrons and holes possess the stronger redox capability for CO₂RR. Notably, CDs in C-DH@IN as an electron mediator and charge transport highway at the interface between In₂O₃ and NiAl-LDH can effectively boost the charge transfer and enhance the redox reaction.

  In summary, we report a novel S-scheme C-DH@IN heterojunction for photocatalytic CO₂RR, which was fabricated through a facile in situ hydrothermal method. In the pure water without sacrificial agent, the 3% C-DH@IN catalyst shows the highest CO₂RR activity (CO: 7.12 µmol h⁻¹ g⁻¹; CH₄: 10.67 µmol h⁻¹ g⁻¹) under simulated solar irradiation, and CH₄ selectively of optimal catalyst is up to 85.7%. The ESR, in-situ FT-IR and photoelectric measurements verify the S-scheme mode in C-DH@IN heterostructure with strong redox capability, and meanwhile exhibit that HCOO⁻ and CH₃O⁻ as crucial intermediates are existed in conversion of CO₂ into CH₄. The CDs in C-DH@IN catalyst can acts as an "electron bridge" between NiAl-LDH and In₂O₃, to facilitate the establishment of the IEF and band bending, and what is more, CDs can absorb more H⁺ to promote the CH₄ selectively. This study provides a new strategy for the design and development of robust and efficient photocatalysts for the selective conversion of CO₂ to hydrocarbon fuels.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 21876015, 21703019), Qinglan Project Foundation of Jiangsu Province.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.108942.

  
  
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  Figure 1  (a) X-ray diffractometer (XRD) patterns of In₂O₃, NiAl-LDH, DH@IN-50 and C-DH@IN composites. (b) Raman spectra of In₂O₃, NiAl-LDH, DH@IN-50 and 3% C-DH@IN composites. (c) XPS survey spectra of NiAl-LDH, DH@IN and 3% C-DH@IN. High resolution XPS spectra of O 1s. (d–i) Corresponding elemental mapping images of C-DH@IN.

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  Figure 2  (a) Photoreduction performances, (b) R_electron and (c) carbon products selectivity of In₂O₃, NiAl-LDH, DH@IN-50 and C-DH@IN composites. (d) Stability test of 3% C-DH@IN with four 5 h cycles. (e) Production rates of CO and CH₄ under various reaction conditions. (f) XRD patterns of 3% C-DH@IN before and after CO₂ photoreduction experiment.

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  Figure 3  (a) PL and (b) TRPL decay of In₂O₃, DH@IN-50 and 3% C-DH@IN. (c) Transient photocurrent responses and (d) EIS Nyquist plots of In₂O₃, NiAl-LDH, DH@IN-50 and 3% C-DH@IN. Mott–Schottky plots of (e) In₂O₃ and (f) NiAl-LDH.

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  Figure 4  (a) DMPO-^·O₂⁻ and (b) DMPO-^·OH spin-trapping ESR spectra of 3% C-DH@IN. (c) In situ FT-IR of 3% C-DH@IN. (d) The photocatalytic reaction pathways of C-DH@IN. (e) Schematic illustration of charge transfer over C-DH@IN under simulated solar light.

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