Photoredox catalysis enabled by atomically precise metal nanoclusters

Abstract

Atomically precise metal nanoclusters (NCs), distinguished by their unique electronic structures, quantum confinement effects, and enriched active sites, have been considered highly promising photosensitizers for light harvesting and conversion. However, the ultra-short carrier lifetime and poor stability of metal NCs remarkably retard their widespread applications in photocatalysis. In this study, we achieved the modulation of carrier separation over metal NCs via heterostructure engineering by smartly integrating atomically precise silver NCs [Ag₁₆(GSH)₉] with transition metal chalcogenides (TMCs). The favorable energy level alignment between metal NCs and TMCs facilitates the electron transfer from the metal NCs to the TMCs, leading to a significantly prolonged charge lifetime and considerably enhanced photoactivity toward the selective reduction of nitro compounds to amino derivatives under visible light. The photocatalytic mechanism of these composite photosystems is elucidated herein. This work advances our fundamental understanding of charge transfer mechanisms over atomically precise metal NCs for solar energy conversion.

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1. Introduction

Atomically precise metal nanoclusters (NCs) usually consist of a few to several hundreds of metal atoms with diameters comparable to the Fermi wavelength of electrons (<2 nm) and thus exhibit unique physicochemical properties and energy band structures that are quite distinct from those of conventional metal nanocrystals.^1–4 These metal NCs feature discrete energy band structures characterized by a transition gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).^5–8 In previous studies,⁹ metal NCs have been used as photosensitizers for constructing heterostructured photocatalysts towards versatile visible-light-driven heterogeneous photocatalysis, including photocatalytic hydrogen generation, CO₂ reduction, environmental remediation, and selective organic transformation.^10–12 Intriguingly, the peculiar atom-stacking mode and abundant active sites of metal NCs endow the heterostructure with significantly enhanced photoactivities. However, to date, few metal NCs have been exploited for photocatalysis, with charge transfer characteristics and photocatalytic mechanism remaining elusive, which stems mainly from a scarcity of metal NCs with photosensitization capacity, the ultra-fast charge recombination rate of metal NCs, and the poor stability of metal NCs, markedly retarding the exploration of high-performance metal NC-based photocatalytic systems.¹³

Transition metal chalcogenides (TMCs) consist of anionic sulfur atoms bonded to transition metals and have been widely recognized as promising photocatalysts due to their suitable band gap, favorable energy band position, and abundance of active sites.^14–16 Nevertheless, the exploration of robust and stable TMC photocatalysts is often hindered by rapid photoinduced electron–hole recombination, photo-corrosion, and short carrier lifetimes. The introduction of an elegant interface,^17,18 modulation of the nanoarchitecture^19,20 and construction of heterostructures^21–23 are effective methods to solve the problems of TMCs. Co-catalyst engineering has also been proven to be an effective strategy to reinforce the charge separation of TMCs. For example, Jin et al. introduced graphdiyne into the NiCo₂O₄ photosystem through reasonable design, which is innovative and has the potential to attract the interest of readers.²⁴ Intriguingly, the construction of heterostructured photocatalysts provides a convenient, easily accessible, and efficacious strategy to finely tune the interfacial charge transport pathway by fully harnessing the synergy of building blocks for improved photocatalytic efficiency.²⁵ Hence, we speculate that the construction of heterostructures by rationally integrating atomically precise metal NCs with TMCs can optimally harness the photosensitization effect of metal NCs and physicochemical merits of TMCs. The TMCs feature suitable energy level positions and abundant catalytically active sites, which enable them to form favorable energy level alignments with metal NCs to foster interfacial charge transfer, as well as their synergistic cooperation, ultimately giving rise to the significantly enhanced photocatalytic activities of the composite photosystems.

Herein, atomically precise metal NCs [Ag₁₆/(GSH)₉] are self-assembled on the TMCs (CdIn₂S₄, CIS) to produce heterostructured photocatalysts in which metal NCs serve as light-harvesting antennas for photosensitizing TMCs. The favorable energy level alignment and intimate interfacial contact enable effective charge transfer between metal NCs and TMCs. We have found that the self-assembled CIS/Ag₁₆(GSH)₉ heterostructure shows significantly boosted photoactivities toward the reduction of aromatic nitro compounds to amino derivatives under visible light irradiation. The results imply that electrons photogenerated over Ag₁₆/(GSH)₉ can rapidly transfer from the LUMO level to the conduction band (CB) of CIS, thereby enhancing the charge separation of Ag₁₆/(GSH)₉ and prolonging the charge lifetime. Our work is anticipated to offer a new idea for precisely mediating the charge transport behaviors of atomically precise metal NCs for solar energy conversion.

2. Experimental section

2.1. Materials

Indium chloride tetrahydrate (InCl₃·4H₂O), cadmium chloride (CdCl₂·2.5H₂O), thiourea (Tu), ethylenediamine, ethanol (C₂H₅OH), sodium borohydride (NaBH₄), reduced L-glutathione (GSH), sodium hydroxide (NaOH), silver nitrate (AgNO₃), sodium sulfite anhydrous (Na₂SO₃), 4-nitroaniline (4-NA), 3-nitroaniline (3-NA), 2-nitroaniline (2-NA), 1-bromo-4-nitrobenzene, 1-chloro-4-nitrobenzene, 4-nitroanisole, 4-nitrotoluene (4-NT), nitrobenzene (NB), O-nitroacetophenone, and deionized water (DI H₂O, Millipore, 18.2 MΩ cm resistivity) were used. All glassware was washed with aqua regia and rinsed with ethanol and ultrapure water. All the materials were of analytical grade and used as received without further purification.

2.2. Preparation of CdIn₂S₄ (CIS)²⁶

Appropriate amounts of analytical grade CdCl₂·2.5H₂O and thiourea (Tu) were added to a Teflon-lined stainless-steel autoclave of 50 mL capacity, which was filled with ethylenediamine up to 90% of the capacity. The autoclave was maintained at 160 °C for 12 h and then air-cooled to room temperature. The precipitate was filtered off and washed with distilled water and absolute ethanol to remove residual impurities. After being dried in a vacuum at 70 °C for 3 h, the collected products were characterized as CdS. The prepared CdS and InCl₃·4H₂O in a molar ratio of 1 : 2 and excess Tu were placed in an autoclave that was then filled with distilled water up to 90% of the total volume. The autoclave was sealed and heated under autogenerated pressure at 180 °C for 10 h, and other procedures similar to those used to prepare CdS were applied. A final orange-yellow powder was obtained for characterization.

2.3. Preparation of Ag₁₆(GSH)₉ NCs²⁷

Aqueous solutions of AgNO₃ (20 mM) and GSH (50 mM) were prepared with DI H₂O. A fresh aqueous solution of NaBH₄ (112 mM) was prepared by dissolving 43 mg of NaBH₄ in 2 mL of 1 M NaOH solution, followed by the addition of 8 mL of DI H₂O. NaOH was added to the NaBH₄ solution to improve the stability of borohydride ions against hydrolysis. In a typical experiment to synthesize Ag₁₆(GSH)₉ NCs, aqueous solutions of AgNO₃ (125 μL, 20 mM) and GSH (150 μL, 50 mM) were first mixed in DI H₂O (4.85 mL) under vigorous stirring to form thiolate–AgI complexes, followed by the addition of an aqueous solution of NaBH₄ (50 μL, 112 mM). A deep-red solution of Ag₁₆(GSH)₉ NCs (ca. 5 mL) was collected after 5 min. This Ag₁₆(GSH)₉ solution was then incubated at room temperature for 3 h and the deep-red solution gradually decomposed into a colorless solution, forming thiolate–AgI complexes. Subsequently, a certain amount of 112 mM NaBH₄ (50 μL) was introduced into this colorless solution under vigorous stirring, giving rise to the formation of a light-brown Ag₁₆(GSH)₉ solution after 15 min. Without stirring, this light-brown Ag₁₆(GSH)₉ solution was incubated at room temperature for 8 h. A strong red or green emission was then observed in the aqueous phase. The Ag₁₆(GSH)₉ NCs were collected without purification and stored at 4 °C for further use.

2.4. Preparation of CIS/Ag₁₆(GSH)₉ heterostructures

CIS/Ag₁₆(GSH)₉ heterostructures were fabricated via a self-assembly method under ambient conditions. The Ag₁₆(GSH)₉ NCs aqueous solution was slowly added dropwise into the CIS colloid aqueous solution under vigorous stirring for 2 h. The liquid volume percentage of Ag₁₆(GSH)₉ NCs in the nanocomposites was controlled to be 5%, 10%, 20%, 30% and 40% with the samples denoted as CIS/xAg₁₆(GSH)₉ (x = 0.05, 0.1, 0.2, 0.3, 0.4). Finally, the mixture was centrifuged and dried in an oven at 60 °C.

2.5. Characterization

Fourier transform infrared (FTIR) spectroscopy measurements were performed on a Nicolet IS50 spectrometer. The powder X-ray diffraction (XRD) patterns were recorded using a MiniFlex 600 X-ray powder diffractometer. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Fisher Scientific ESCALAB 250 X-ray instrument equipped with a standard and monochromatic source (Al Kα, 1486.6 eV), and the binding energies were normalized to the signal for C 1s at 284.8 eV. HRTEM investigations were performed on a TECNAI G2F20 (FEI). Elemental mapping analysis was conducted using EDX. The ultraviolet-visible (UV-vis) absorption spectra of the Ag₁₆(GSH)₉ were acquired on a Cary 5000 instrument (Agilent, USA). UV–Vis diffuse (UV-Vis DRS) reflectance spectra were recorded on a UV-VIS-NIR spectrometer (Cary 5000). Raman spectra were recorded using an InVia Reflex. Photoluminescence (PL) spectra were acquired on a Varian Cary Eclipse spectrometer with an excitation wavelength of 350 nm.

2.6. Photocatalytic reduction performances

For the photoreduction reaction, a 300 W Xe lamp (PLS-SXE300D, Beijing Perfect Light Co. Ltd, China) equipped with a 420 nm cut-off filter (λ > 420 nm) was used as the irradiation source. Here, 10 mg of catalyst and 40 mg of sodium sulfite anhydrous (Na₂SO₃) were added to 30 mL of 4-NA aqueous solution (10 ppm) and saturated with bubbling N₂ under ambient conditions. Before visible light illumination, the above suspension was stirred in the dark for 30 min to ensure the establishment of adsorption–desorption equilibrium between the sample and reactant. During the process of the reaction, 2 mL of sample solution was collected at a certain time interval and centrifuged to completely remove the catalyst (8000 rpm, 15 min), and the supernatant was analyzed using a UV-vis absorption spectrophotometer. The procedure for the photoreduction of other aromatic nitro compounds is similar to that of 4-NA. The photoactivities of the samples were defined as follows.

2.7. Photoredox performances^28,29

(a) Photocatalytic reduction of aromatic nitro compounds. PEC water splitting measurements were performed using a conventional three-electrode configuration on an electrochemical workstation (CHI660E, CHI Shanghai, Inc.). The samples (1 cm × 1 cm) were directly employed as the working electrodes, and Pt foil, as well as the Ag/AgCl electrode, served as the counter and reference electrodes, respectively. The electrolyte was composed of 0.5 M Na₂SO₄ (pH = 6.69) aqueous solution. PEC water splitting performances were evaluated under simulated sunlight irradiation from a 300 W Xe lamp (PLS FX300HU, Beijing Perfect Light Co. Ltd, China) equipped with an AM 1.5 filter or visible light (λ > 420 nm). The potentials of the electrodes were calibrated against the normalized hydrogen electrode (NHE) based on the following formula:

(b) Photocatalytic degradation of organic pollutants. Typically, 20 mg of catalyst powder was suspended in 50 mL methyl orange (MO) aqueous solution (20 mg L⁻¹). Before light irradiation, the mixtures were magnetically stirred for 1 h in the dark to reach the adsorption–desorption equilibrium. Then, a 300 W Xe lamp (λ > 420 nm) was used as the visible light source to irradiate the suspensions under vigorous stirring. At a certain interval (20 min), 1 mL of the suspension was collected, followed by the addition of 2 mL of water, and centrifuged to get rid of the catalysts. The supernatant was monitored by measuring the maximum absorbance at 464 nm for MO using a UV-Vis spectrophotometer. The removal efficiency is expressed by the ratio of C_t/C₀, where C_t is the reaction concentration and C₀ is the initial concentration.

3. Results and discussion

3.1. Characterization of the CIS/Ag₁₆(GSH)₉ heterostructure

As displayed in Scheme 1, the CIS/Ag₁₆(GSH)₉ heterostructure was fabricated by self-assembling Ag₁₆(GSH)₉ NCs on the CIS matrix under ambient conditions. It should be emphasized that the preparation of Ag₁₆(GSH)₉ NCs is based on the strong binding between the GSH ligand and the Ag core, which forms the monolayer of GSH molecules on the Ag₁₆(GSH)₉ NCs surface.²⁷ Note that the various polar functional groups such as carbonyl and amide bonds on the GSH ligand cause Ag₁₆(GSH)₉ NCs to interact with CIS via intermolecular forces by which Ag₁₆(GSH)₉ can be spontaneously and uniformly anchored to the surface of CIS to form the heterostructure.

Scheme 1 A schematic illustration depicting the self-assembly of the CIS/Ag₁₆(GSH)₉ heterostructure.

The morphology and size of Ag₁₆(GSH)₉ NCs were analyzed by transmission electron microscopy (TEM). Fig. S1a† shows Ag₁₆(GSH)₉ NCs with an ultra-small size of ca. 1.9 nm (Fig. S1b†). In the UV-visible absorption spectrum (Fig. S1c†), Ag₁₆(GSH)₉ NCs showed an absorption peak at 480 nm, which is different from the surface plasmon resonance absorption of conventional silver nanoparticles, indicative of the distinct electronic structure of Ag₁₆(GSH)₉ NCs.^30–33 As shown in the SEM image (Fig. 1a), pristine CIS exhibits a nanorod-like structure. The TEM image of CIS (Fig. 1c) confirmed the one-dimensional structure and revealed a lattice spacing of about 0.330 nm (Fig. 1d) corresponding to the (311) crystalline facet of CIS. Note that no Ag₁₆(GSH)₉ NCs were observed in the SEM image of the CIS/Ag₁₆(GSH)₉ heterostructure (Fig. 1b), which is because the size of Ag₁₆(GSH)₉ NCs (ca. 1.9 nm) is too small to be discerned. However, the TEM image indicates that Ag₁₆(GSH)₉ NCs are uniformly distributed on the surface of CIS (Fig. 1e). This can be strongly verified by the SEM mapping (Fig. S2†) and EDS results (Fig. S3†), which confirmed the deposition of Ag₁₆(GSH)₉ NCs on the CIS surface. Elemental mapping results from TEM measurements indicated the uniform deposition of Ag₁₆(GSH)₉ NCs on the CIS substrate, as displayed in Fig. 1(f–j), where the Ag signal from NCs can be observed. The Fourier transform infrared spectroscopy (FTIR) results for CIS and CIS/Ag₁₆(GSH)₉ are provided in Fig. S4 and Table S1,† in which the characteristic peaks at 1038, 1142 and 2951 cm⁻¹ correspond to the stretching vibration modes of C–N and C–H bonds on the CIS, and a distinct stretching vibration mode is observed at 2358 cm⁻¹, which corresponds to the N–H bond of Ag₁₆(GSH)₉ NCs.^34–36

Fig. 1 SEM images of (a) CIS and (b) the CIS/Ag₁₆(GSH)₉ heterostructure. TEM image of (c) CIS. (d) TEM and HRTEM (inset) images of CIS. (e) HRTEM image of the CIS/Ag₁₆(GSH)₉ heterostructure. (f) The elemental mapping result of the CIS/Ag₁₆(GSH)₉ heterostructure for (g) S, (h) In, (i) Cd, and (j) Ag signals.

Crystal structures of the samples were analyzed by X-ray diffraction (XRD) as displayed in Fig. 2a.²⁶ XRD peaks at 14.13°, 23.18°, 27.24°, 28.48°, 33.00°, 40.73°, 43.31°, and 47.41°, correspond to the (111), (220), (311), (222), (400), (422), (511) and (440) crystal facets of CIS (JCPDS no. 27-0060). The sharp diffraction peaks of CIS were observed, which indicated the good crystallization of the samples. It should be noted that no peaks attributable to Ag₁₆(GSH)₉ NCs were observed in the XRD results of the CIS/Ag₁₆(GSH)₉ heterostructure, which may be due to the relatively low loading amount of Ag₁₆(GSH)₉ NCs or its generic amorphous properties. As shown in Fig. 2b, the light absorption band edges of CIS and CIS/Ag₁₆(GSH)₉ in the UV diffuse reflectance spectra (DRS) results are all located at ca. 650 nm, which is due to the band gap photoexcitation of the CIS matrix. Notably, the absorption of the CIS/Ag₁₆(GSH)₉ heterostructure is slightly enhanced when Ag₁₆(GSH)₉ NCs are loaded onto the CIS matrix, which is due to the photosensitization effect of Ag₁₆(GSH)₉ NCs.^37,38 This can also be demonstrated by the color change of the CIS and CIS/Ag₁₆(GSH)₉ heterostructure (Fig. 2b, inset). According to the Kubelka–Munk function versus photon energy, the bandgap (E_g) values of CIS and the CIS/Ag₁₆(GSH)₉ heterostructure (Fig. 2c) were roughly determined as ca. 2.26 and 2.27 eV, respectively. Ag₁₆(GSH)₉ NCs deposition does not substantially change the bandgap of the CIS substrate, which is due to the shielding absorption of the NCs and CIS substrate. Fig. 2e shows three bands at ca. 250, 309, and 367 cm⁻ in the Raman spectra of CIS and the CIS/Ag₁₆(GSH)₉ heterostructure, which correspond to the E_g, A_1g, and F_1u modes of the CIS substrate, respectively.^39,40 Similarly, no peaks of Ag₁₆(GSH)₉ NCs were observed in the Raman results, probably due to the low deposition amount or amorphous properties of NCs. The specific surface areas of CIS and the CIS/Ag₁₆(GSH)₉ heterostructure were determined as 15.6227 m² g⁻¹ and 15.0465 m² g⁻¹, respectively, as shown in Fig. 2d and Table S1.† The results indicate that the loading of Ag₁₆(GSH)₉ NCs on the surface of the CIS substrate does not drastically change the specific surface area of the catalyst, which suggests that the specific surface area is not a key factor affecting the photocatalytic performances of the samples.

Fig. 2 (a) XRD patterns and (b) DRS results of CIS and the CIS/Ag₁₆(GSH)₉ heterostructure, with (c) transformed plots calculated by the Kubelka–Munk function vs. the energy of light along with sample color in the inset [left: CIS; right: CIS/Ag₁₆(GSH)₉]. (d) N₂ adsorption–desorption isotherms of CIS and the CIS/Ag₁₆(GSH)₉ heterostructure, with pore size distribution curves in the inset. (e) Raman spectra of CIS and the CIS/Ag₁₆(GSH)₉ heterostructure. High-resolution (f) S 2p, (g) In 3d, (h) Cd 3d, and (i) Ag 3d spectra of (I) CIS and (II) CIS/Ag₁₆(GSH)₉.

Fig. S5† shows the survey X-ray photoelectron spectra (XPS) of CIS and CIS/Ag₁₆(GSH)₉ with the signals of C, Cd, In, and S. Specifically, as displayed in Fig. S5a,† a clear Ag signal was detected in the survey XPS spectrum of the CIS/Ag₁₆(GSH)₉ heterostructure, which suggests that Ag₁₆(GSH)₉ NCs were successfully loaded on the CIS surface. The high-resolution N 1s and C 1s spectra (Fig. S5b & c†) also indicate the loading of Ag₁₆(GSH)₉ NCs on the CIS substrate. As shown in Table S2† and Fig. 2f (I), the binding energies (BEs) of the high-resolution S 2p spectrum of CIS are about 161.57 eV (S 2p_3/2) and 162.76 eV (S 2p_1/2), and these peaks correspond to the S²⁻ species.⁴¹ As shown in Fig. 2g (I), the BEs of the high-resolution In 3d spectrum of CIS are about 444.95 eV (In 3d_5/2) and 452.53 eV (In 3d_3/2), and these peaks mainly originate from the In³⁺ species.⁴² As shown in Fig. 2h (I), the main peaks in the high-resolution Cd 3d spectrum of CIS are located at 405.33 eV (Cd 3d_5/2) and 412.06 eV (Cd 3d_3/2), which correspond to the Cd²⁺ species.⁴² Compared with the CIS substrate, as shown in Fig. 2f (II), g (II) and h (II), the peak positions of the CIS/Ag₁₆(GSH)₉ heterostructure were not shifted, indicating that the surface valence states of Ag₁₆(GSH)₉ NCs and CIS were not altered by NCs deposition. Notably, double-peak bands were observed in the high-resolution Ag 3d spectrum (Fig. 2i) of the CIS/Ag₁₆(GSH)₉ heterostructure, wherein the peaks at 367.82 eV (Ag 3d_5/2) &373.79 eV (Ag 3d_5/2) and 368.29e V (Ag 3d_5/2) & 374.34 eV (Ag 3d_5/2) correspond to the Ag⁺ and Ag⁰ species, respectively. This confirmed the successful loading of Ag₁₆(GSH)₉ NCs on the CIS surface. Table S3† summarizes the relationship between the chemical bonding species and BEs for the CIS substrate and CIS/Ag₁₆(GSH)₉ heterostructure.

3.2. Photocatalytic activities

To investigate the photocatalytic activities of the CIS/Ag₁₆(GSH)₉ heterostructure, the selected probe reaction was the anaerobic selective photoreduction of nitroaromatic compounds to amino compounds under visible light irradiation (>420 nm) and ambient conditions. Here, 4-nitroaniline (4-NA) was used as the representative nitro compound in the photocatalytic reaction; according to the optical absorption spectrum of 4-NA, its absorbance at 380 nm gradually decreased with the addition of a hole trapping agent (Na₂SO₃) and the continuous bubbling of nitrogen. This confirmed the successful reduction of 4-NA to 4-phenylenediammonium (4-PDA).⁴³ We ascertained that it was indeed a photocatalytic reaction based on the control experiments without catalyst or light irradiation (Fig. S6†). The influence of Ag₁₆(GSH)₉ NCs loading percentage on the photocatalytic activities was then probed (Fig. S7†). In our work, the loading percentage of Ag₁₆(GSH)₉ NCs on the CIS was tuned by varying the volume of Ag₁₆(GSH)₉ NCs. The results show that the photoactivity of CIS/Ag₁₆(GSH)₉ gradually increased with increasing the Ag₁₆(GSH)₉ NCs loading amount and reached the maximum value when x = 0.3 (x refers to the volume ratio of NCs), and then it decreased when the Ag₁₆(GSH)₉ NCs loading amount was further increased. This may be because active sites on the CIS surface and light absorption of CIS are shielded by the metal NCs, resulting in decreased carrier density and reduced photoactivity; therefore, the optimal loading ratio of the CIS/Ag₁₆(GSH)₉ heterostructure can be determined.

The photocatalytic activity of the CIS/Ag₁₆(GSH)₉ heterostructure toward 4-NA reduction increased by ca. 65% in conversion as compared with the blank CIS substrate, confirming that Ag₁₆(GSH)₉ NCs plays a great role in boosting the photocatalytic reaction. In addition to 4-NA (Fig. 3a), the photocatalytic activities of the CIS/Ag₁₆(GSH)₉ heterostructure for the selective reduction of other nitro compounds were also explored and interestingly, similar results were observed. As shown in Fig. 3b–i, the CIS/Ag₁₆(GSH)₉ heterostructure exhibited a more enhanced photocatalytic activity relative to the blank CIS for the selective photoreduction of 3-NA (Fig. 3b), 2-NA (Fig. 3c), 1-chloro-4-nitrobenzene(Fig. 3d), 1-bromo-4-nitrobenzene (Fig. 3e), o-nitroacetophenone (Fig. 3f), 4-nitroanisole (Fig. 3g), NB (Fig. 3h), and 4-NT (Fig. 3i) under visible light irradiation. Notably, the photocatalytic performances of the CIS/Ag₁₆(GSH)₉ heterostructure were generally enhanced by a factor of 2–3 relative to the pristine CIS substrate, which strongly implies the synergistic effect of CIS and Ag₁₆(GSH)₉ NCs in boosting the photocatalytic activity. Besides the photoreduction reaction, the photocatalytic oxidative capacity of the CIS/Ag₁₆(GSH)₉ heterostructure was also probed by the oxidative degradation of methyl orange (MO) under visible light irradiation. Fig. S8† indicates that the CIS/Ag₁₆(GSH)₉ heterostructure demonstrated enhanced degradation efficiency in comparison with the blank CIS.

Fig. 3 Photoactivities of CIS and the CIS/Ag₁₆(GSH)₉ heterostructure toward the selective photoreduction of nitroaromatics under visible irradiation (λ > 420 nm) with the addition of hole scavengers and N₂ purging, including (a) 4-NA, (b) 3-NA, (c) 2-NA, (d) 1-chloro-4-nitrobenzene, (e) 1-bromo-4-nitrobenzene, (f) 2-nitroacetophenone, (g) 4-nitroanisole, (h) NB, and (i) 4-nitrotoluene together with the reaction model.

As shown in Fig. S9,† the photocatalytic performances of the CIS/Ag₁₆(GSH)₉ heterostructure with and without adding an electron trapping agent (K₂S₂O₈) were probed. The results show that the photocatalytic activity of the CIS/Ag₁₆(GSH)₉ heterostructure was remarkably reduced with adding K₂S₂O₈ to the reaction system, thus suggesting that photogenerated electrons play a decisive role in the photocatalytic reduction reaction of aromatic nitro compounds. Of particular note, we found that the photocatalytic activity of the CIS/Ag₁₆(GSH)₉ heterostructure was markedly reduced without adding the hole-trapping agent (Na₂SO₃), thus proving that the quenching of holes by Na₂SO₃ is essential in the photocatalytic reduction reaction. The addition of hole-trapping agents ensures that the electrons function as the sole active species and can efficiently photoreduce nitroaromatic compounds to amino derivatives, thus fulfilling the photoreduction catalysis. The stability of the CIS/Ag₁₆(GSH)₉ heterostructure was explored by a cyclic reaction, which is of paramount importance for practical applications. It was found that the stability of the CIS/Ag₁₆(GSH)₉ heterostructure was not substantially decreased after the cyclic test, which may be due to the rapid transfer of holes from CdIn₂S₄ to the metal NCs, which reduced the charge separation and retarded the photo-corrosion of CdIn₂S₄. The relatively good photocatalytic activity of the CIS/Ag₁₆(GSH)₉ heterostructure was observed after the cyclic reaction (Fig. S10†), which proved the favorable stability of the CIS/Ag₁₆(GSH)₉ heterostructure. Alternatively, XRD and XPS analyses of the CIS/Ag₁₆(GSH)₉ heterostructure after the cyclic reaction were also performed to further evaluate its photostability. As shown in Fig. S11 & S12,† the crystal structure and surface elemental valence states of the CIS/Ag₁₆(GSH)₉ heterostructure were not changed after the cyclic reaction, and the analytical results were the same as those of the newly prepared catalysts, which once again proved its good stability.

3.3. PEC performances

Photoelectrochemical (PEC) measurements were carried out to probe the charge separation efficiency of the CIS/Ag₁₆(GSH)₉ heterostructure. As shown in Fig. 4a, the transient photocurrent of the CIS/Ag₁₆(GSH)₉ heterostructure is significantly higher than that of the CIS substrate, indicating that the CIS/Ag₁₆(GSH)₉ heterostructure demonstrates a more efficient charge separation efficiency relative to the blank CIS. Mott–Schottky (M–S) results were probed to reveal the carrier density (Fig. 4b). The carrier density of the CIS/Ag₁₆(GSH)₉ heterostructure was calculated based on the (M–S) results (Fig. 4b), which far exceeded that of the CIS substrate (Fig. 4c). Electrochemical impedance spectroscopy (EIS) results suggest that the CIS/Ag₁₆(GSH)₉ heterostructure demonstrated a smaller semicircular arc radius relative to CIS, which implies its lower interfacial charge transfer resistance (Fig. 4d). As shown in Fig. 4e, the CIS/Ag₁₆(GSH)₉ heterostructure exhibited a larger photovoltage than CIS, which also confirmed the more efficient charge separation over the CIS/Ag₁₆(GSH)₉ heterostructure compared with CIS. This is consistent with other PEC results. As shown in Fig. 4f, the photoluminescence (PL) intensity of the CIS/Ag₁₆(GSH)₉ heterostructure is lower than that of CIS, confirming that the CIS/Ag₁₆(GSH)₉ heterostructure shows more enhanced charge migration efficiency compared with the CIS substrate. The charge carrier density was calculated using the following equation:where ε is the dielectric constant (ε_CIS = 6.60), ε₀ is the vacuum permittivity (8.86 × 10⁻¹² F m⁻¹), e⁰ is the unit of charge (1.6 × 10⁻¹⁹ C), and V is the potential.

Fig. 4 (a) On–off transient photocurrents, (b) M–S results, (c) carrier density, (d) EIS results, (e) open-circuit potential decay plots, and (f) PL spectra of CIS and the CIS/Ag₁₆(GSH)₉ heterostructure (excitation wavelength: 350 nm).

3.4. Photocatalytic mechanism

Scheme 2 presents the photocatalytic mechanism of the CIS/Ag₁₆(GSH)₉ heterostructure according to the above analysis. First, the energy levels of the CIS substrate were determined by M–S and DRS results. As shown in Fig. S13,† the flat-band potential of the CIS was determined as −0.3 V vs. NHE. Considering its bandgap value of E_g = 2.27 eV, the VB of CIS was calculated as 1.97 V vs. NHE based on the formula E_g = E_VB − E_CB. The HOMO–LUMO levels of Ag₁₆(GSH)₉ NCs were determined from the CV and UV-vis absorption spectrum (Fig. S13†). As shown in Fig. S14a,† carbon paper without loading Ag₁₆(GSH)₉ NCs demonstrated no obvious peak, whereas an apparent peak at 0.78 V vs. NHE appeared after depositing Ag₁₆(GSH)₉NCs, and the peak intensity was boosted with increasing the amount of Ag₁₆(GSH)₉ NCs. Therefore, the HOMO and LUMO levels of Ag₁₆(GSH)₉ NCs were determined as −1.8 V vs. NHE and 0.78 V vs. NHE, respectively. Ag₁₆(GSH)₉ NCs and CIS form the type-II energy level alignment, which is beneficial for interfacial charge transfer.⁴⁴ Thus, when CIS and Ag₁₆(GSH)₉ NCs are simultaneously photoexcited under visible light irradiation to produce electron–hole charge carriers, the electrons in the LUMO level of Ag₁₆(GSH)₉ NCs migrate to the CB of CIS, while the holes in the VB of CIS are transferred to the HOMO level of Ag₁₆(GSH)₉ NCs,⁴⁵ promoting the carrier separation and boosting the photocatalytic activity. Consequently, the electrons participate in the photocatalytic reduction of nitroaromatics to amino compounds. It should be noted that the holes accumulating in the LUMO level of Ag₁₆(GSH)₉ NCs are effectively captured by hole scavengers or participate in the dye oxidative degradation reaction.

Scheme 2 Schematic illustration of the photocatalytic mechanism of the CIS/Ag₁₆(GSH)₉ heterostructure.

4. Conclusions

To conclude, the CIS/Ag₁₆(GSH)₉ heterostructure was constructed by the self-assembly of atomically precise Ag₁₆(GSH)₉ NCs on the CIS substrate under ambient conditions. The self-assembled CIS/Ag₁₆(GSH)₉ heterostructure demonstrated remarkably enhanced and stable photocatalytic performances in the selective reduction of nitroaromatic compounds to amino derivatives under visible light irradiation in an anaerobic environment, far surpassing CIS under the same experimental conditions. This is mainly ascribed to the efficient charge transport between Ag₁₆(GSH)₉ NCs and the CIS substrate, thereby effectively preventing the recombination of photogenerated electron–hole pairs. The favorable charge transfer between Ag₁₆(GSH)₉ NCs and CIS was determined. Our work provides a typical paradigm for fabricating semiconductor–metal NC heterostructures for solar energy conversion.

Author contributions

Junyi Zhang: Writing the original draft, editing, formal analysis. Linjian Zhan: Methodology. Boyuan Ning: Investigation, validation. Yunhui He: Instrument testing, Zhixin Chen: Instrumental analysis, Guangcan Xiao and Fang-Xing Xiao: Supervision.

Data availability

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

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21703038, 22072025). The financial support from the State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science, is acknowledged (No. 20240018).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi01826e

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