Photocatalytic CO₂ reduction of Ag/Ag₂S/Ti₃C₂T_X heterojunctions with enhanced interfacial charge transfer

Abstract

Photocatalytic reduction of CO₂ to produce organic fuels is a promising strategy for addressing carbon reduction and energy scarcity. Transition metal carbides (Ti₃C₂T_X) are of particular interest due to their unique layered structures and excellent electrical conductivity. However, the practical application of Ti₃C₂T_X is limited by the poor separation efficiency of photogenerated charge carriers and the low migration ability of photogenerated electrons. Herein, Ag/Ag₂S/Ti₃C₂T_X heterojunctions were synthesized by depositing Ag/Ag₂S nanoparticles onto layered Ti₃C₂T_X substrates using a combination of co-precipitation and photoreduction methods. Fluorescence spectra, UV diffuse reflection, and photoelectric chemical characterizations demonstrated that Ag/Ag₂S/Ti₃C₂T_X heterojunctions provided effective channels for the reverse and synergistic migration of electrons and holes, leading to improved spatial separation. Notably, the Ag component in the composite acts as an electron acceptor and reactive center, significantly enhancing the migration ability of photogenerated electrons. The total alcohol yield over Ag/Ag₂S/Ti₃C₂T_X (125.3 μmol g_catal.⁻¹) was 5.1 times higher than that on Ag₂S (24.7 μmol g_catal.⁻¹) and 2.1 times higher than on Ti₃C₂T_X (60.7 μmol g_catal.⁻¹). This study offers valuable insights into designing efficient photocatalytic CO₂ reduction catalysts.

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1 Introduction

The escalating consumption of fossil fuels exacerbates energy scarcity. Concurrently, the persistent combustion of fossil fuels contributes to a heightened concentration of CO₂ within the atmosphere, with the consequent greenhouse effect precipitating global climate alterations.^1–5 Strategies to diminish atmospheric greenhouse gases, particularly CO₂, whilst addressing energy deficiencies, have emerged as focal points of research.^6–9 Notably, CO₂, recognized as an economical and non-toxic C1 resource, possesses the potential to be transformed into an array of organic compounds, including alcohols, low carbon olefins, aldehydes, ethers, and esters, via diverse methodologies.^6,10 Its utilization as a raw material can simultaneously alleviate the greenhouse effect and provide solutions to energy crises. It is crucial to acknowledge that CO₂ exhibits considerable thermodynamic stability, boasting the highest oxidation state of carbon within its molecular structure. The dissociation energy of the C=O bond (∼430 kJ mol⁻¹) surpasses that of C–H (∼336 kJ mol⁻¹) and C–C (∼750 kJ mol⁻¹) bonds. Consequently, the conversion of CO₂ necessitates prior activation, demanding an input of energy surpassing the dissociation threshold of the C=O bond.¹¹ Photocatalytic CO₂ reduction represents an emergent technique for decrementing CO₂ levels. Leveraging renewable solar energy, photocatalysts trigger the transformation of inert CO₂ molecules into valuable fuels or chemical entities. This approach, harnessing renewable and non-pollutive solar energy, facilitates carbon reclamation and is characterized by numerous benefits: elimination of supplementary reducing gases, assurance of a non-toxic procedure, absence of pollutant generation, facilitation of material reuse, simplification of reaction apparatus, and streamlined operational condition controls.¹²

Ti₃C₂T_X, a novel graphene-analogous material, is characterized by its superior electrical conductivity and unique layered configuration, rendering it particularly promising for exploration within the realm of photocatalytic CO₂ reduction.^2,13–17 This distinctive layered architecture facilitates the swift transit and segregation of photoinduced electron–hole pairs while offering numerous active sites conducive to photocatalysis. Nonetheless, pristine Ti₃C₂T_X exhibits certain limitations, including a propensity for the facile recombination of photoinduced electron–hole pairs. Peng et al. have demonstrated that (001) TiO₂/Ti₃C₂T_X nanocomposites, synthesized via a straightforward hydrothermal approach, can significantly enhance photocatalytic efficacy.¹⁸ In these composites, the integration of (001) TiO₂ nanoflakes within the stratified Ti₃C₂T_X culminates in the formation of an interfacial heterostructure. Leveraging the elevated conductivity inherent to two-dimensional Ti₃C₂T_X facilitates the segregation of photoinduced charge carriers at the Ti₃C₂T_X–TiO₂ interface, thereby mitigating electron–hole recombination. Empirical results underscore that Ti₃C₂T_X can be adeptly engineered to construct heterojunction composite photocatalysts paired with semiconductors of corresponding energy bands. Such judicious pairing not only curtails the recombination of electron–hole pairs but also amplifies the CO₂ photoreduction efficiency.

Metal sulfides, characterized by their unique structures, exceptional properties, and narrow band gaps, readily absorb light and demonstrate catalytic activity. Owing to these attributes, they hold significant promise for applications spanning environmental remediation and energy generation among others.^19–22 Notably, silver sulfide (Ag₂S) possesses a minimal band gap of 0.92 eV, ensuring a high absorption rate of solar illumination coupled with robust photoelectric characteristics and chemical resilience.^23–26 The conduction band of Ag₂S is positioned at approximately −0.45 eV, while its valence band is around 0.38 V. Importantly, the conduction band's potential is more negative than the reduction thresholds required for converting CO₂ into methanol and ethanol, underscoring its potential in photocatalytic CO₂ reduction endeavors. Presently, Ag₂S is predominantly employed in realms such as photocatalytic degradation of contaminants, detoxification of heavy metals, and hydrogen evolution.^24,27 Nevertheless, standalone Ag₂S suffers from elevated electron–hole recombination rates and a marked propensity for aggregation during synthesis, which compromises its photocatalytic efficacy. Researchers have ascertained that the incorporation of suitable substrates during synthesis effectively circumvents Ag₂S aggregation.^28–30 Moreover, these substrates aid in the efficient segregation of photogenerated electrons and holes. Silver, endowed with a low Fermi energy level, frequently serves as an electron sink. Photogenerated electrons derived from silver-based composite systems can be conveyed to the silver matrix via the interfacial region.^31,32 Jiang et al. have conducted an exhaustive examination of the electronic configurations in four distinct chalcogenide-based silver catalysts.³³ Their findings elucidate that the positions of the conduction and valence bands, the migratory velocity of photoinduced charge carriers, and the overall photostability and catalytic proficiency of these materials are governed by the band gap dimensions and the alignment of their respective bands. Consequently, the selection of catalysts for specific photocatalytic transformations varies accordingly.

To inhibit the recombination of photogenerated carriers in Ti₃C₂T_X layered materials and enhance the number of photogenerated electrons with strong reducing capabilities for CO₂ photoreduction, the co-precipitation and photoreduction methods were employed to deposit Ag/Ag₂S onto Ti₃C₂T_X in this study. The resulting compact heterojunction, formed by the amalgamation of Ag/Ag₂S with Ti₃C₂T_X, effectively prevented the recombination of electron–hole pairs. Upon photoexcitation, electrons with high reductive capabilities from the Ag₂S were efficiently transferred to the Ti₃C₂T_X material via the heterojunction. Concurrently, photogenerated electrons, both from the Ti₃C₂T_X material and Ag₂S transferred to the layered substrate, were directed to the metallic Ag with a low Fermi energy level. The synthesized Ag/Ag₂S/Ti₃C₂T_X composites were then tested for photocatalytic CO₂ reduction, and their catalytic activity was evaluated. Additionally, the study explored how varying Ag₂S loadings affect the photocatalytic performance of the Ag/Ag₂S/Ti₃C₂T_X composites.

2 Experimental sections

2.1 Preparation of Ti₃C₂T_X

Ti₃C₂T_X materials were prepared using a procedure which is a modified recipe of the original version reported previously.^13,34 1 g of Ti₃AlC₂ powders was added slowly into 10 mL of 40 wt% HF and etched at 25 °C for 36 h. The resulting precipitate was washed with deionized water and centrifuged at 8000 rpm for 5 min, that the pH of the supernatant was around 6. The samples after centrifuging were vacuum drying at 60 °C for 12 h.

2.2 Preparation of Ag/Ag₂S/Ti₃C₂T_X composites

First, 0.1 g of layered Ti₃C₂T_X was added into 40 mL of deionized water and stirred for 30 min. Then, 0.1 M AgNO₃ was injected into the solution and stirred for 1 h, sonicated for 15 min. Subsequently, 0.05 M Na₂S·9H₂O was slowly added and stirred for 2 h. Finally, the black precipitate obtained by exposure of xenon light for 30 min was washed several times with deionized water to remove the impurities on the surface and then dried at 80 °C for 12 h. The Ag/Ag₂S/Ti₃C₂T_X composites with different mass ratio of Ag₂S and Ti₃C₂T_X (6 wt%, 6.5 wt%, 7 wt%, 7.5 wt%, 8 wt%) was denoted as 6AAT, 6.5AAT, 7AAT, 7.5AAT, and 8AAT, respectively. In addition, pristine Ag₂S was synthesized via the above recipe in the absence of layered Ti₃C₂T_X and photoreduction. And the Ag₂S/Ti₃C₂T_X (mass ratio of Ag₂S and Ti₃C₂T_X is 7.5 wt%, denoted as 7.5AT) was synthesized via the above recipe in the absence of photoreduction.

2.3 Characterizations

X-ray diffraction (XRD) measurements of the prepared catalytic materials were carried out on a powder diffractometer with instrument model D8 Advance X. Scanning electron microscopy (SEM) images were acquired on a Nova Nano SEM450 (FEI, USA) to observe the morphology and topography of the samples. Transmission electron microscopy (TEM) images were obtained on a JEM-2010F field emission electron microscope to characterize the morphological structure and high-resolution lattice fringes of the prepared photocatalysts, as well as the electron diffraction of the selected regions. X-ray photoelectron spectroscopy (XPS) analysis was performed on an Axis Ultra DLD to obtain the data of the species and valence states of the elements, elemental content and chemical bonding in the prepared materials. UV-vis diffuse reflectance spectra (DRS) were recorded in a fluorescence spectrometer (LAMBDA950) equipped with an integrating sphere using BaSO₄ as the reference. Fourier transform infrared (FTIR) spectra were recorded on a Bruker Vertex 70 spectrometer to obtain characteristic bonding peaks between specific functional groups or elements. Photoluminescence spectroscopy (PL) was recorded on a F7000 fluorescence spectrometer (Hitachi, Japan). Specific surface areas and pore properties were measured using model TriStar-II to analyzed the specific surface area, pore size, and adsorption properties of the materials. Electrochemistry was used to study the conductive properties of photocatalytic materials and the transfer of electrons across material interfaces. Mott–Schottky and electrochemical impedance spectroscopy (EIS) measurements were performed on the electrochemical workstation (CHI660D) using graphite as the counter electrode, saturated calomel electrode as a reference electrode and the modified fluorine-doped tin oxide (FTO) glass as the working electrode. The electrolyte solution for the Mott–Schottky with transient photocurrent response was 0.5 M Na₂SO₄ solution.

2.4Photocatalytic CO₂ reduction tests

The efficiency of photocatalytic reduction of CO₂ was evaluated by the rate of catalytic reduction of CO₂ to organic alcohols under catalyst illuminated from a xenon lamp (λ ≥ 420 nm). The catalyst (30 mg) was dispersed in 30 mL of deionized water at 4 °C. The reactor system was ensured that the reactants were only H₂O and CO₂ using the vacuum pump to remove gases such as air and dissolved O₂ from the device and aqueous solution. High-purity CO₂ (99.995%) was then introduced into the reactor without illumination for 0.5 h to obtain the saturated aqueous solution of CO₂ and followed by full-spectrum illumination for 4 h, where the sampling time interval was 1 h. The photoreduction generated products were detected through gas chromatograph (GC), and the yields of methanol and ethanol were calculated by standard curves of methanol and ethanol (Section S1, Table S1, and Fig. S1†).

3 Results and discussion

3.1 Structural and morphological characterizations

The crystalline phases of Ti₃C₂T_X, Ag₂S, and the Ag/Ag₂S/Ti₃C₂T_X composites were characterized by analyzing their XRD patterns (Fig. 1a). All synthesized samples exhibited pronounced XRD diffraction peaks, suggesting a commendable degree of crystallinity.^13,18,35 Notably, several distinctive diffraction peaks at 25.9°, 28.9°, 31.5°, 34.3°, 36.8° for the singular Ag₂S material correspond to the planes of (−111), (111), (−112), (−121), and (121), respectively. These characteristic peaks matched well with JCPDS card no. 14-0072 for Ag₂S. Concurrently, the Ti₃C₂T_X material manifested its characteristic diffraction peaks at 8.8° and 18.3°, indexing to the (002) and (004) crystal planes respectively, which matched with JCPDS card no. 52-0875. Ag/Ag₂S/Ti₃C₂T_X composites were discerned that upon integrating Ag and Ag₂S with Ti₃C₂T_X, which displayed the main structure Ti₃C₂T_X with some signature peaks of Ag₂S materialized at 25.9°, 28.9°, 34.3° and 36.8°. Concurrently, the emergent peaks at 38.1°, 44.3°, and 64.4° were identified as the characteristic peaks of elemental Ag, correlating with the JCPDS card no. 04-0783. The overlaid XRD patterns elucidated that the Ag/Ag₂S/Ti₃C₂T_X composites, irrespective of varying loadings retain analogous diffraction peaks to those of Ag, Ag₂S, and Ti₃C₂T_X. This observation was indicative of the fact that the amalgamation of Ag/Ag₂S with Ti₃C₂T_X has not induced alterations in the crystalline configurations of either Ag₂S or Ti₃C₂T_X. Two augmentations in peak intensities corresponding to the (−112) and (−121) crystal planes of Ag₂S were observed with the Ag₂S loading progressively increased. Similarly, an enhancement in peak intensities at 38.1° related to the (111) crystal planes of Ag was noted. Cumulatively, these findings substantiate that both Ag and Ag₂S were adeptly integrated into the layered Ti₃C₂T_X matrix without any resultant modifications to their crystal structures. N₂ adsorption–desorption analysis was conducted on the synthesized Ti₃C₂T_X, Ag₂S, and Ag/Ag₂S/Ti₃C₂T_X composites, and the obtained curves were presented in Fig. 1b. The isotherms for Ti₃C₂T_X, Ag₂S, and Ag/Ag₂S/Ti₃C₂T_X composites were characterized by Type IV isotherms with H3-type hysteresis loops, suggesting these materials exhibit mesoporous structures with slit-shaped pores.^13,26Table 1 revealed that the specific surface areas for Ti₃C₂T_X, Ag₂S, 6AAT, 6.5AAT, 7AAT, 7.5AAT, and 8AAT measured 3.52, 4.48, 7.22, 7.50, 7.54, 8.06, and 8.2 m² g⁻¹ respectively. Their average pore diameters stood at 15.62, 7.92, 16.12, 16.95, 16.62, 15.68, and 16.96 nm, while the pore volumes were documented as 0.014, 0.009, 0.029, 0.032, 0.031, 0.32, and 0.034 cm³ g⁻¹. Notably, upon compounding Ti₃C₂T_X with Ag/Ag₂S, there was a marked enhancement in the specific surface area, pore volume, and pore size. The pore size (Fig. S2†) of Ag/Ag₂S/Ti₃C₂T_X composites was increased in the range of 5–50 nm with increasing Ag₂S content. This phenomenon suggested that Ag/Ag2S particles were effectively anchored onto the Ti₃C₂T_X substrate, leading to an augmented specific surface area and pore size. Such modification was beneficial for enhancing the interaction between the catalyst and the reactant, subsequently exposing a greater number of active sites.

Fig. 1 XRD patterns (a) and N₂ adsorption–desorption curve (b) of Ag₂S, Ti₃C₂T_X, and Ag/Ag₂S/Ti₃C₂T_X composites. SEM images of (c) Ti₃C₂T_X, (d) 7.5AAT composites. TEM (e) and HRTEM (f) images of 7.5AAT composites.

Table 1 BET surface area and porosity of samples

Sample	BET surface area (m^2 g^−1)	Pore size (nm)	Pore volume (cm^3 g^−1)
Ti3C2TX	3.52	15.62	0.014
Ag2S	4.48	7.92	0.009
6AAT	7.22	16.12	0.029
6.5AAT	7.50	16.95	0.032
7AAT	7.54	16.62	0.031
7.5AAT	8.06	15.68	0.032
8AAT	8.20	16.96	0.034

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SEM image of layered Ti₃C₂T_X synthesized via the acid etching technique was shown in Fig. 1c, revealing the derived Ti₃C₂T_X possesses a uniformly layered structure characterized by smooth and planar surfaces. The thickness of the Ti₃C₂T_X layers varies between 50–100 nm. Notably, these layers predominantly consisted of two-dimensional nano-sheets separated by substantial pore spaces. This particular layered configuration enhanced the capacity for adsorbing gaseous CO₂ while concurrently offering an increased number of photoactive sites and shorter paths for photogenerated carrier transport, as referenced in literature.^36,37 As shown in Fig. 1d, the SEM image of the 7.5AAT composites revealed that the fundamental laminar architecture remained unaltered post-incorporation of Ag/Ag₂S with the comparison with pristine Ti₃C₂T_X, suggesting that the Ag/Ag₂S integration did not compromise the inherent lamellar structure of Ti₃C₂T_X. Moreover, Ag/Ag₂S/Ti₃C₂T_X surface hosted numerous particles with an approximate diameter of 25 nm, dispersed uniformly across its smooth plane. This observation confirmed that following the co-precipitation and photo-reduction procedures, Ag/Ag₂S particles were securely anchored onto the Ti₃C₂T_X layers without any noticeable aggregation. TEM and High-Resolution Transmission Electron Microscopy (HRTEM) images of the 7.5AAT composite were conducted. TEM image in Fig. 1e underscored the layered disposition adorned with a copious loading of particle substances on its exterior, aligning with SEM findings. HRTEM image in Fig. 1f allows for the precise measurement of the lattice fringes with a spacing of 0.203 nm, 0.250 nm, and 0.357 nm, indexing to the (200) plane of metallic Ag, the (006) plane of Ti₃C₂T_X, and the (110) plane of Ag₂S, respectively.^18,29,38 These measurements affirmed the establishment of a densely packed heterojunction among Ti₃C₂T_X, Ag, and Ag₂S.

High-resolution XPS spectra with a Gaussian fitting process focusing on Ti 2p, C 1s, Ag 3d, and S 2p peaks were conducted. Specifically, Fig. 2a offers an in-depth view of the Ti 2p spectrum. Both Ti₃C₂T_X and the 7.5AAT composite exhibited four distinct doublet peaks for Ti 2p. This spectrum was characterized by two primary energy levels, 2p_3/2 and 2p_1/2, maintaining an approximate area ratio of 2 : 1 and demonstrating an electron binding energy differential of approximately 5.7 eV. It is noted that the peaks located at 454.97, 455.68, 457.11, and 459.80 eV correspond to the Ti 2p_3/2 energy level in the Ti₃C₂T_X material, which are attributed to the titanium–carbon (Ti–C) bond, with the titanium–halide (Ti–X) bond associated with substoichiometric titanium carbide (TiC_x) or titanium-based oxycarbide, alongside the reduced states of titanium oxygen (Ti_xO_y) and the titanium–fluoride (Ti–F) bond. In comparison to the Ti₃C₂T_X material, the peak positions associated with the reduced state Ti_xO_y of titanium (Ti) ions in the 7.5AAT composited remain consistent with the peak linked to the Ti 2p_3/2 energy level in both the Ti–X bond and titanium oxycarbide. Notably, there was a discernible shift in the binding energy for the peak related to the Ti 2p_3/2 energy level of the Ti–C bond in the 7.5AAT composite by 0.22 eV toward a lower value. Furthermore, an emergent Ti 2p_3/2 energy level peak at 459.05 eV, corresponding to the titanium–oxygen (Ti–O) ionic bond, suggested alterations in the chemical environment surrounding the Ti–C bond subsequent to the amalgamation of Ti₃C₂T_X with Ag/Ag₂S, resulting in partial oxidation of titanium-to-titanium dioxide (TiO₂).^18,34,35,39 The significant change in the relative intensity of the peaks in the Ti 2p spectra of the two samples also observed, which is attributed to partial oxidation of the Ti₃C₂T_X surface during the loading of the catalytic materials. This process introduces Ti–O bonds, leading to variations in the Ti 2p peak intensities and shifts. It was imperative to note that TiO₂ exhibits photoresponse exclusively to ultraviolet light, demonstrating no responsiveness to illumination within the visible and near-infrared spectrum.⁴⁰ XPS analysis of C 1s depicted in Fig. 2b to determine the binding energies of C 1s in its various forms. Within the Ti₃C₂T_X material, the binding energies were identified as 281.77 eV, 284.80 eV, 286.55 eV, and 288.50 eV, corresponding to the C in Ti–C bond, C–C bond, C–O bond, and C–F bond, respectively. In the 7.5AAT material, there was a notable shift in the binding energy for C in the Ti–C bond and C–O bond, with decrease of 0.27 eV and 0.36 eV, respectively. In contrast, the binding energy associated with the C–F bond remained unchanged.¹³Fig. 2c presented a high-resolution scan of Ag 3d. The distinct peaks observed at 368.34 eV and 374.34 eV in Ag₂S align with the characteristic peaks of Ag 3d_5/2 and Ag 3d_3/2, respectively. This observation suggested that Ag within the Ag₂S material exhibited a valency of +1. For the 7.5AAT composite, the characteristic peaks for Ag 3d_5/2 and Ag 3d_3/2 of Ag⁺ were positioned at 367.74 eV and 373.77 eV, respectively, revealing a significant offset of approximately 0.6 eV compared to the Ag₂S material. Furthermore, the characteristic peaks for 7.5AAT at 367.60 eV and 373.52 eV correspond to the Ag 3d_5/2 and Ag 3d_3/2 peaks of Ag, confirming the incorporation of metallic Ag within the composite material.⁴¹Fig. 2d presented a high-resolution scan of S 2p. The peaks located at 161.55 and 162.73 eV correspond to S 2p_3/2 and S 2p_1/2 of the S element in unadulterated Ag₂S,³⁰ respectively. These peaks denoted a valence state of −2 for the S element. Compared with pure Ag₂S, a notable shift to lower binding energies was observed for S 2p_3/2 and S 2p_1/2 in the 7.5AAT composite. Such findings suggested that upon the compounding of Ag/Ag₂S with Ti₃C₂T_X, the Ag/Ag₂S material became deposited atop the Ti₃C₂T_X layered substrate, thereby forming a heterojunction. This recombination induced alterations in the chemical milieu of the constituent elements and precipitates an electron flux between the materials, which subsequently modified the binding energy of each element within the composite matrix. To quantitatively assess the proportions of Ag⁰ and Ag⁺ within the Ag/Ag₂S/Ti₃C₂T_X composites, an analysis of the peak areas associated with Ag⁺ and Ag⁰ in the Ag 3d high-resolution XPS spectra for 6AAT, 6.5AAT, 7AAT, 7.5AAT, and 8AAT composites was conducted. The results were compiled in Table 2. The peak area ratios of Ag⁰ to Ag⁺ for these composites were as follows: 1 : 3.22 (6AAT), 1 : 3.07 (6.5AAT), 1 : 2.98 (7AAT), 1 : 2.88 (7.5AAT), and 1 : 2.70 (8AAT). This data elucidated that with increasing Ag₂S loading, there was a concomitant rise in the peak areas corresponding to both Ag⁺ and Ag⁰. The Ag⁰ content, relative to the Ag/Ag₂S/Ti₃C₂T_X composites, escalated in tandem with heightened Ag₂S loading. This phenomenon underscored that throughout the synthesis of the Ag/Ag₂S/Ti₃C₂T_X composites, a subset of Ag ions became attached to the Ti₃C₂T_X surface. Upon illumination, these surface-adsorbed Ag ions underwent reduction to yield Ag⁰, and a greater generation of Ag⁰via photoreduction when an increased number of Ag ions were adsorbed onto the Ti₃C₂T_X substrate with Ag₂S loading amplified.

Fig. 2 High-resolution XPS spectra of (a) Ti 2p, (b) C 1s, (c) Ag 3d and (d) S 2p in Ag₂S, Ti₃C₂T_X, and Ag/Ag₂S/Ti₃C₂T_X composites.

Table 2 Peak area and peak area ratio of Ag⁺ and Ag⁰ in Ag/Ag₂S/Ti₃C₂T_X composites from Ag 3d high-resolution XPS

Samples	Ag^0 peak area	Ag^+ peak area	Ag^0/Ag^+ peak area ratio
6AAT	6477.5	20910.6	1 : 3.22
6.5AAT	7530.8	23183.7	1 : 3.07
7AAT	8414.5	25127.4	1 : 2.98
7.5AAT	9516.4	27434.4	1 : 2.88
8AAT	10 726	28984.7	1 : 2.70

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3.2 Photoelectric property of Ti₃C₂T_X and Ag/Ag₂S/Ti₃C₂T_X composites

To investigate the optical absorption characteristics of the synthesized catalyst materials, an ultraviolet-visible infrared diffuse reflection analysis was conducted on Ag₂S, Ti₃C₂T_X, and Ag/Ag₂S/Ti₃C₂T_X composites. As depicted in Fig. 3a, the primary light absorption spectrum of the fabricated semiconductor photocatalyst material predominantly resided within the ultraviolet-visible light domain. Moreover, a partial absorption in the near-infrared region was also observed, suggesting an elevated sunlight utilization efficiency for the material. Both singular materials, Ag₂S and Ti₃C₂T_X, exhibiting black coloration, manifest robust light absorption capabilities. Marginal variances in light absorption among the composite materials-specifically 6AAT, 6.5AAT, 7AAT, 7.5AAT, and 8AAT-were discerned, which can be ascribed to the pronounced light absorption intensities inherent to Ag₂S and Ti₃C₂T_X materials. As illustrated in Fig. S3,† by plotting hν against (Ahν)² and subsequently determining the intersection point of its external tangent with the abscissa, one can ascertain the band gaps of pristine Ag₂S and Ti₃C₂T_X materials. Notably, the band gaps for pure Ag₂S and Ti₃C₂T_X materials were empirically determined to be 0.86 eV and 1.26 eV, respectively. To ascertain the positions of the conduction bands for the Ag₂S and Ti₃C₂T_X semiconductor materials, Mott–Schottky measurements were executed on both substances. As depicted in Fig. 3b, the flat band potentials (V_fb) for Ag₂S and Ti₃C₂T_X were observed to be −0.71 V and −0.57 V relative to the SCE, respectively. To convert the SCE reference potential to the NHE reference potential, you can use the following formula:

E_{fb (vs. NHE)} = E_{fb (vs. SCE)} + E⁰ + 0.059 pH (1)

where E⁰ (the potential difference between the SCE and NHE reference electrodes at standard conditions) is 0.2415 V and pH is 0. Pertinent literature indicated that both Ag₂S and Ti₃C₂T_X materials were characterized as n-type semiconductors.^25,42–44 For n-type semiconductors, the conduction band potential is more negative compared to the flat band potential, typically by around 0.1 eV.^45,46 Subsequent computations revealed that the conduction band potentials for Ag₂S and Ti₃C₂T_X materials were equivalent to −0.57 and −0.43 V relative to NHE, respectively. Utilizing ultraviolet-visible infrared diffuse reflection analysis, the inherent band gaps of pristine Ag₂S and Ti₃C₂T_X materials were determined to be 0.86 eV and 1.26 eV, respectively. Using the following equation:

E_VB = E_CB + E_g (2)

where E_VB is the valence band, E_CB is conduction band bottom, and E_g is the band gap. The valence bands of Ag₂S and Ti₃C₂T_X were deduced to be 0.29 and 0.83 V, respectively.

Fig. 3 UV-vis absorbance spectra (a) of Ag₂S, Ti₃C₂T_X, and Ag/Ag₂S/Ti₃C₂T_X composites. Mott–Schottky curves (b) obtained on Ag₂S and Ti₃C₂T_X. (c) PL spectra and (d) EIS Nyquist plots of Ag₂S, Ti₃C₂T_X, and Ag/Ag₂S/Ti₃C₂T_X composites.

Fig. 3c presented the fluorescence emission spectra for Ti₃C₂T_X and various Ag/Ag₂S/Ti₃C₂T_X composites, all measured at an emission wavelength of 370 nm. Pristine Ti₃C₂T_X exhibited the highest fluorescence intensity. However, upon modification with Ag/Ag₂S, there was a diminishment in the fluorescence intensity of the resultant composites. This reduction suggested that the Ag/Ag₂S modification effectively hinders the recombination of electron–hole pairs in Ti₃C₂T_X. Furthermore, the variation in fluorescence intensity among the composites, each modified with differing amounts of Ag/Ag₂S, implied disparities in the separation velocities of photoinduced charges, contingent upon the Ag₂S loading. Upon the introduction of a minimal quantity of Ag/Ag₂S, there was a notable diminution in fluorescence intensity. This outcome was attributed to the presence of heterojunctions that inhibit the recombination of photogenerated carriers. Conversely, when the Ag/Ag₂S loading surpassed an optimal threshold, superfluous Ag/Ag₂S became polyvalent on the Ti₃C₂T_X substrate. This accumulation engendered sited that facilitate the recombination of electron–hole pairs, leading to an elevation in fluorescence intensity.^29,31,34 Notably, the composite designated as 7.5AAT manifested the most diminished fluorescence intensity, underscoring its superior efficacy in separating electron–hole pairs. In parallel, the 7.5AT composite, devoid of metallic Ag, exhibited fluorescence intensity inferior to that of pristine Ti₃C₂T_X yet exceeding that of 7.5AAT. This juxtaposition substantiated that photoinduced electrons within the composite were conveyed towards the metal Ag, characterized by its low Fermi level. Such migration impeded the reunification of photoinduced electron–hole pairs, wherein the metallic Ag functions as an electron sink within the composite matrix.

Fig. 3d illustrated the alternating current (AC) impedance curve for the synthesized material. Compared with pristine Ag₂S, a notable reduction in the AC impedance arc radius was observed for the 6AAT, 6.5AAT, 7AAT, 7.5AAT, and 8AAT composites. This diminution suggested that the modification of Ti₃C₂T_X with Ag/Ag₂S enhanced the velocity of photogenerated electron migration, thereby facilitating efficient transport of photoinduced carriers. Intriguingly, the Ti₃C₂T_X substrate, inherently characterized by superior electrical conductivity, manifested the smallest AC impedance arc radius, correlating with the most rapid migration of photo-induced carriers. A subsequent analytical perspective revealed that the AC impedance arc radii for the 6AAT, 6.5AAT, 7AAT, 7.5AAT, and 8AAT composites initially diminish and then subsequently augment. This trend delineated that at a lower Ag/Ag₂S loading, the carriers, upon photoexcitation, could be expeditiously conveyed and partitioned via the heterojunction. In instances where the Ag/Ag₂S loading surpasses an optimal threshold, consequently forming a recombination center for Ag/Ag₂S tended to polymerize on the sample surface, photogenerated carriers, thereby impeding carrier migration. As depicted in Fig. S4,† it presented a transient photocurrent test plot for the synthesized materials. Notably, the current intensity exhibited by 7.5AAT is higher than that of pristine Ag₂S and Ti₃C₂T_X. Cumulatively, these findings substantiated that the Ag/Ag₂S/Ti₃C₂T_X composite adeptly facilitated the separation of photo-induced electron–hole pairs. Moreover, the incorporation of metallic Ag augmented the propagation speed of photogenerated electrons, concomitantly elevating the photocatalytic efficacy of the Ag/Ag₂S/Ti₃C₂T_X composite.

3.3 Photocatalytic reduction of CO₂ over Ag/Ag₂S/Ti₃C₂T_X composites

To evaluate the photoreduction efficacy of various photocatalytic materials towards CO₂, an assessment of the photocatalytic activities of the synthesized materials was conducted. As illustrated in Fig. 4a and b, the primary products derived from the photocatalytic reduction of CO₂ were identified as methanol and ethanol. Subsequent to a 4 hours photocatalytic reduction reaction employing substrates such as Ti₃C₂T_X, Ag₂S, 7.5AT, 6AAT, 6.5AAT, 7AAT, 7.5AAT, and 8AAT composites, the resultant methanol yields manifested an initial ascending trend, followed by a descending pattern. Specifically, the methanol yields were recorded as 37.6, 14.7, 61.3, 73.5, 87.1, 90.8, 107.4, and 68.7 μmol g_catal.⁻¹, respectively. Conversely, ethanol production exhibited an initial rise, followed by a decline, and subsequently stabilized at consistent yields, with values measured at 23.1, 10.0, 27.9, 28.2, 17.8, 19.3, 17.9, and 19.7 μmol g_catal.⁻¹, correspondingly. Notably, the 7.5AAT substrate demonstrated superior performance, achieving the highest cumulative yield of 125.3 μmol g_catal.⁻¹, which was approximately 5.1-fold greater than the aggregate yields of Ag₂S (24.7 μmol g_catal.⁻¹). Additionally, an evaluation of the photocatalytic reduction capability of the 7.5AT substrate devoid of metallic Ag was undertaken, revealing a methanol yield of 61.3 μmol g_catal.⁻¹, an ethanol yield of 27.9 μmol g_catal.⁻¹, and a cumulative yield of 89.2 μmol g_catal.⁻¹. This data indicated that the overall production of the 7.5AAT composite was 1.4 times greater than that of the 7.5AT composite. The cycling stability of CO₂ reduction over 7.5AAT was inspected in Fig. S5.† The total yield of methanol and ethanol obtained using 7.5AAT in five cycles are 125.30, 120.17, 115.87, 111.46 and 109.48 μmol g_catal.⁻¹, respectively. The performance over 7.5AAT represents little decrease after five cycling runs, but the 7.5AAT exhibited unchanged morphology and crystal structure after tests (Fig. S6†). Such findings suggested that Ag functions as both an electron acceptor and an active site within the Ag/Ag₂S/Ti₃C₂T_X composite, thereby enhancing the photocatalytic reduction of CO₂. Given the aforementioned experimental outcomes, coupled with comprehensive material characterization and analytical assessment, several factors contributing to the elevated photocatalytic efficacy of the Ag/Ag₂S/Ti₃C₂T_X composite can be identified. Primarily, the inherent conductivity and rapid electron transit characteristics of the Ti₃C₂T_X layered substrate are noteworthy. Secondly, the elevated energy at the sulfide conduction band edge amplifies, and the reduction potential of electrons residing on the Ag₂S conduction band is particularly favorable for CO₂ reduction. Moreover, the incorporation of metallic Ag, characterized by its low Fermi level within the composite matrix, facilitates its role as an electron acceptor and reaction locus. Photoexcited electrons, originating from both Ag₂S and Ti₃C₂T_X, are shuttled to metallic Ag via the heterojunction, subsequently engaging with adsorbed CO₂ molecules. This mechanism not only amplifies the number of electrons participating in the photocatalytic reduction but also enhances the overall photocatalytic efficiency. However, it is crucial to modulate the quantity of Ag/Ag₂S judiciously because an excess may result in the aggregation of Ag and Ag₂S on the Ti₃C₂T_X layered surface. Such aggregation can impede the efficient separation of photoinduced electron–hole pairs, adversely impacting the photocatalyst performance of the material to a certain extent.

Fig. 4 Yields of (a) methanol and (b) ethanol from photocatalytic CO₂ reduction over Ag₂S, Ti₃C₂T_X, and Ag/Ag₂S/Ti₃C₂T_X composites with increasing irradiation time under visible light.

3.4 Proposed mechanism involved in CO₂ photoreduction over Ag/Ag₂S/Ti₃C₂T_X composites

The examination of experimental outcomes revealed a notable enhancement in the catalytic efficacy of the Ag/Ag₂S/Ti₃C₂T_X composite towards the photoreduction of CO₂. The reduction mechanism is depicted in Fig. 5. The improvement of photocatalytic efficiency can be attributed to three primary factors. First, the layered Ti₃C₂T_X possesses superior electrical conductivity, thereby augmenting the transit of photoinduced electrons, which in turn facilitates the segregation of electrons and holes to a discernible extent. Second, the Ag₂S conduction band is particularly amenable to CO₂ reduction to give the pronounced reducing capability of photoinduced electrons on the sulfide conduction band. Concurrently, the formation of a heterojunction between Ag₂S and Ti₃C₂T_X further encourages the segregation of electron–hole pairs. Moreover, the presence of metallic Ag, characterized by its low Fermi level, enables it to function as an electron sink and active reaction site. Photoinduced electrons generated by the composite are shunted to Ag, where they engage with the surface-adsorbed CO₂, leading to a commensurate increase in photoreduction efficacy. The energy levels associated with the valence band, conduction band and forbidden band of Ag₂S and Ti₃C₂T_X can be determined utilizing ultraviolet-visible infrared diffuse reflection spectroscopy and Mott–Schottky analysis. By semiconductor photocatalysis principles, upon exposure to light irradiation, both Ag₂S and Ti₃C₂T_X produce photo-induced charge carriers. These include photo-induced electrons, which exhibit potent reducing capabilities, and photo-induced holes, known for their robust oxidizing effects. The establishment of heterojunctions facilitates the transfer of photo-induced electrons from semiconductor materials possessing a more electronegative conduction band to those with a less electronegative conduction band. Simultaneously, photo-induced holes transition from semiconductors with a higher valence band to those with a lower valence band, thereby inhibiting the recombination of photo-induced charge carriers. In essence, metallic Ag functions as an electron sink and provides an active reaction site. Photo-induced electrons, originating from Ag₂S and Ti₃C₂T_X upon photoexcitation, not only engage in the photoreduction process but are also channeled to metallic Ag via the heterojunction, facilitating the reduction of CO₂ adsorbed onto the material's surface. The resultant Ag/Ag₂S/Ti₃C₂T_X composite effectively curtails the recombination of photo-induced electron–hole pairs, prolonging the lifespan of the photo-induced carriers.

Fig. 5 Schematic diagram of bandgap structure and photocatalytic CO₂ reduction over Ag/Ag₂S/Ti₃C₂T_X composites.

4 Conclusions

In this study, Ag/Ag₂S/Ti₃C₂T_X heterojunction composites were synthesized by integrating coprecipitation and photoreduction techniques, wherein Ag/Ag₂S was loaded onto the layered Ti₃C₂T_X. Upon photoexcitation, both Ag₂S and Ti₃C₂T_X materials generated photo-induced carriers. The conduction band of Ag₂S was more electronegative than that of Ti₃C₂T_X, leading to the transfer of photo-induced carriers from Ag₂S to the Ti₃C₂T_X substrate via the heterojunctions. Owing to the substantial presence of photoproduced Ag on Ti₃C₂T_X and the low Fermi level of Ag, the photogenerated electrons on Ti₃C₂T_X were shuttled to Ag particles. This mechanism effectively inhibited the recombination of electron–hole pairs and enhanced CO₂ photocatalytic reduction efficiency. Additionally, metallic Ag improved photogenerated electrons migration efficiency and prolonged carrier lifespan, acting as an active site in the photocatalytic reaction. Ag/Ag₂S/Ti₃C₂T_X heterojunction thus significantly improved CO₂ photoreduction performance, as evidenced by the increased yields of methanol and ethanol. This work offers valuable insights for designing intimate heterostructures that enable effective photogenerated carrier separation and full spectral absorption, thereby enhancing photocatalytic CO₂ reduction.

Data availability

The data that support the findings of this study are available from the corresponding author upon request.

Author contributions

F. L., W. X., and B. Z. conceived the idea and supervised the entire project. W. X., W. D., and Y. Z. designed the methodology and conducted the experiments. X. L., B. Z., and W. D. analyzed the data and visualized the data. F. L. and Y. C. wrote the manuscript. B. Z. contributed to funding support. F. L., W. X., and B. Z. commented on or edited the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (22406208), the Zhaoqing City Science and Technology Innovation Guidance Project (No. 2023040304001), the Zhaoqing University High-level Project Cultivation Program (GCCZK202416), the Zhaoqing University Doctor Start-up Fund (240016) and Zhaoqing University Teaching Reform Project (zlgc2024060).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4na00969j

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