Synergistic effect of Mo and Sn in a quaternary metal sulphide to activate N₂ adsorption for selective solar-driven ammonia production

Abstract

Photocatalytic nitrogen fixation struggles with low efficiency, owing to the challenges involved in breaking the nonpolar N≡N bond. Designing photocatalysts that convert N₂ to NH₃ under ambient conditions is key, as is improving adsorption sites and active centers for N₂ reduction to boost overall performance. Although CdS photocatalysts show promise, their efficiency is limited due to poor visible light absorption, slow carrier migration, and few active sites. This study presents a quaternary metal sulfide (Cd_1−2xMo_xSn_xS) synthesized via a hydrothermal method, resulting in exceptional durability and remarkable selectivity for N₂ reduction. The optimal % of Cd, Mo and Sn in Cd_1−2xMo_xSn_xS overcomes CdS's photo-corrosion issues, achieving NH₃ production rates up to 8 times higher (521.29 μmol g⁻¹ h⁻¹) than that of pure CdS (67.18 μmol g⁻¹ h⁻¹) under simulated solar light. Fourier transform infrared spectroscopy suggests that the fabricated robust photocatalyst follows a symmetric alternating pathway as the operation mechanism. DFT simulations illustrate the relationship between the d-band center and adsorption properties of Cd, Mo, and Sn, demonstrating that Mo and Sn synergistically enhance N₂ activation and NH₃ production. Experimental and theoretical results confirm that Mo and Sn synergistically boost photocatalytic N₂ reduction efficiency in Cd_0.60Mo_0.20Sn_0.20S.

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1. Introduction

Ammonia (NH₃) is an important raw material in agriculture which is a key component of fertilizers, supplying nitrogen for plant growth and boosting food production. However, its significance extends beyond traditional applications, as it has prominently emerged as a clean and secure energy vector, facilitating the utilisation of high-energy-density H₂ fuel.¹ The process of synthesizing NH₃ is underlined by the strong stability of the N₂ molecule having a substantial bond dissociation energy of 941 kJ mol⁻¹.² To break down the strong bonds of N≡N, high temperatures (300–600 °C) and high pressures (150–300 atmospheres) are essential along with pure forms of N₂ and H₂.^3,4 Industrially, the Haber Bosch process is the primary source of large-scale NH₃ production but has detrimental effects on the environment. Each year this process consumes 1–2% of the world’s energy, 3–5% of natural gas and 50% of H₂ which is mainly produced from steam reforming of fossil fuels,⁵ with the simultaneous emission of 2 tons of CO₂ for each ton of NH₃ produced which accounts for 3% of global greenhouse gas emissions.^6,7

We need a sustainable and CO₂-free approach for NH₃ production. Nitrogenase produced by Rhizobium bacteria is the motivation nature provides to develop efficient catalysts for conducting the N₂ fixation process under mild reaction conditions which are highly sustainable. The photocatalytic nitrogen reduction reaction (NRR) for artificial NH₃ production is a promising approach, allowing for direct synthesis of NH₃ from N₂ and water using sustainable solar energy under ambient conditions. In 1977, Schrauzer and Guth's study on an Fe-doped TiO₂ photocatalyst is the inspiration behind the extensive work done on photocatalytic NH₃ production from the reduction of the largely available N₂ gas.⁸ In most of the photocatalytic ammonia synthesis methods, the designed photocatalysts struggle with various obstacles, namely inefficient N₂ adsorption and activation and the simultaneous occurrence of the H₂ evolution reaction resulting in poor NRR efficiency with poor selectivity.⁹ Consequently, the prime challenge in photocatalytic NH₃ production is designing photocatalysts capable of mitigating H₂ evolution while enabling selective NRR.

Many sulphides, oxides, layered double hydroxides and carbon-based photocatalysts have been explored in the last few years in areas of photocatalysis such as H₂ evolution, ammonia synthesis, etc.^10–14 Recent advancements in medium and high entropy metal chalcogenide systems have demonstrated considerable potential in the area of catalysis.¹⁵ Among various photocatalysts, CdS is the one of the most important sulphide photocatalysts which is intensively studied due to its appropriate bandgap (∼2.4 eV), and absorption in the visible light region.¹⁶ The stumbling blocks in this path are mainly photo-corrosion associated with CdS (photo-oxidation properties of sulfides) and electron–hole recombination associated with CdS as well as with photocatalysis. Various strategies such as doping, heterojunction formation, cocatalyst loading, vacancy creation, etc. have been applied to overcome these two drawbacks.^17,18 Studies conducted by Zhao et al. depicted Mo-embedded boron nitride as having superior catalytic performance for N₂ fixation among other single transition metal atoms.¹⁹ Zhang et al. and his group showed that Mo acted as an active site for the adsorption of N₂ along with the successful creation of defect states near the Fermi level of W₁₈O₄₉, further facilitating the reduction of N₂.²⁰ Additionally, Kim and his group discovered Sn to be a donor of valence electrons for conduction in Sn–TiO₂, which enhanced the photocatalytic activity.²¹ N₂ fixation was also studied using different quaternary metal sulfide systems such as Zn_0.1Sn_0.1Cd_0.8S²² and g-C₃N₄/ZnSnCdS.²³

The synthesis of homogeneously distributed quaternary metal sulphide systems with pure phases presents a significant challenge, due to controlling their stoichiometry.^24,25 However, recent reports have showcased advancements in this domain, particularly in synthesising quaternary nanocrystals to improve photocatalytic efficiency.^26,27 Herein, a phase pure quaternary metal sulfide (Cd_1−2xMo_xSn_xS, where x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30 are considered) with a controlled composition was synthesized using the hydrothermal method for photocatalytic NH₃ production. By regulating the percentage of Cd : Mo : Sn in the Cd_1−2xMo_xSn_xS system, the NH₃ production rate can be significantly varied. The highest photocatalytic NH₃ production of 521.29 μmol g⁻¹ h⁻¹ is achieved for Cd_0.60Mo_0.20Sn_0.20S, showing an increase of ∼8 times compared to pure CdS (67.18 μmol g⁻¹ h⁻¹). Photostability was also tested for a total of 60 hours in 5 consecutive cycles of 12 h each, and high reproducibility along with high recyclability was also observed, which was further confirmed using the X-ray diffraction (XRD) and the X-ray photoelectron spectroscopy (XPS) data after photocatalysis. The combined influence of Cd, Mo and Sn enhances the efficiency of photocatalytic NH₃ synthesis through a synergistic effect. The main highlight of this work is the remarkable selectivity of the as-prepared photocatalyst in the production of NH₃, without the interference of the mostly observed predominant competing reaction of H₂ production. With the aid of extensive theoretical calculations, we intend to explain the remarkable selectivity as well as the mechanism involved here, by finding out the specific roles of different metal centres.

2. Experimental section

2.1. Synthesis of Cd_1−2xMo_xSn_xS nanoparticles

Quaternary sulfides (Cd_1−2xMo_xSn_xS) were prepared using the hydrothermal method, starting with dispersing stoichiometric amounts of Na₂MoO₄·2H₂O, SnCl₂·2H₂O, and Cd(CH₃COO)₂·2H₂O in 25 mL DI water. The resultant solution mixture was sonicated for 5 min to prepare homogeneously distributed metal salt solution. An excess amount of aqueous NH₂CSNH₂ solution was added dropwise to the metal salt dispersion followed by magnetically stirring for 4.5 h at room temperature. The mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave and maintained at 160 °C for 16 h. After the reactor was naturally cooled, the final product was obtained through centrifugation, followed by multiple washes with deionized water and ethanol. The obtained product was dried overnight at 60 °C. By varying the proportion of different metal salts taken, and keeping the amount of thiourea constant, different Cd-based quaternary systems (Cd_1−2xMo_xSn_xS, x = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30) were prepared under the same reaction conditions. For comparison, Cd-based ternary systems and pure CdS were also fabricated following the same method, but without the inclusion of Na₂MoO₄·2H₂O for Cd_0.80Sn_0.20S, SnCl₂·2H₂O for Cd_0.80Mo_0.20S, and both Na₂MoO₄·2H₂O and SnCl₂·2H₂O for pure CdS.

2.2. Characterization techniques

Various instruments were used to analyze the as synthesized photocatalysts. To identify the crystal phases of the samples a Bruker D8 Advance X-ray diffractometer (Cu Kα, λ = 1.5406 Å) was employed to record XRD. Morphological analysis of the samples was performed using transmission electron microscopy (TEM). High-resolution transmission electron microscopy (HR-TEM) was conducted and selected area electron diffraction (SAED) patterns were recorded to determine crystal planes using a UHR FEG-TEM, JEOL JEM 2100F field emission transmission electron microscope at 200 kV. Energy dispersive X-ray (EDX) mapping was performed to verify the elemental composition. XPS was conducted using a PHI 5000 VersaProbe III instrument and Al Kα radiation to analyse the chemical states of the elements in the samples. Ultraviolet photoelectron spectroscopy (UPS) was carried out with the same instrument using He I radiation to determine the band edge positions of the samples. Atomic absorption spectroscopy (AAS) was performed to calculate the molecular composition of the as-synthesized samples using an Analytik Jena contrAA 800D. The bandgaps of the samples were calculated using UV-vis diffuse reflectance spectroscopy (DRS). To quantify the NH₄⁺ ions produced after visible light illumination, ion chromatography (IC) was performed using an Eco IC Metrohm. An G3545A Agilent 8890A Custom GC Analyzer was utilized for the quantification of evolved H₂ gas. Nitrogen Temperature-Programmed Desorption (N₂-TPD) studies were conducted using a BELCAT II instrument (MicrotracBEL Corp.). To study the mechanism of N₂ fixation FT-IR was performed using a Thermo Fisher Scientific India (THERMO-NICOLET IS50) FT-IR instrument. Using a CHI 760E electrochemical workstation, photocurrent response was tested and electrochemical impedance spectroscopy (EIS) was performed. To study charge carrier kinetics, photoluminescence (PL) spectroscopy was conducted using a Shimadzu RF-6000 fluorescence spectrophotometer at 370 nm excitation.

2.3. Photocatalytic experiments

Photocatalytic N₂ fixation performance was evaluated using a specially designed quartz reactor to ensure maximum irradiation and minimum thermal energy loss. 20 mg of photocatalyst was added to a mixture of 40 mL DI water and 10 mL CH₃OH (as the hole scavenger) in the quartz reactor. This suspension was sonicated in the dark for 10 min to ensure complete dispersion of the photocatalyst and as a result, an increase in the exposure of the active surface. Consequently, the suspension was constantly stirred in the dark with N₂ flowing continuously for 45 min and then irradiated with a 300 W Xe lamp (100 mW cm⁻²) from the horizontal direction with continuous stirring. and a 5 mL aliquot was collected by using a syringe after every one-hour interval for 3 h. The obtained solution was filtered using a 0.45 μm PES syringe filter to remove the photocatalyst particles present. The final product was assessed and quantified with the Nessler's reagent spectrophotometry method and using ion chromatography.

2.3.1. Quantification of NH₃ using Nessler's reagent. To quantify NH₃, 100 μL sodium potassium tartrate solution and 300 μL Nessler's reagent were added to the 5 mL filtrate aliquot solution. The mixture was kept undisturbed for 15–20 min, and then the absorbance of the solution was measured using a UV-vis spectrophotometer (Analytik Jena SPECORD 250 PLUS UV-vis spectrophotometer). The colour of the solution becomes yellow or brown depending on the concentration of NH₄⁺ ions present in the solution. Initially, the UV-vis absorbance was measured for a series of standard NH₄Cl solutions to establish a calibration curve, followed by recording the data for the samples. The absorbance (Y-axis) vs. concentration (X-axis) curve was drawn to get the calibration curve. After further calculations using the equation derived from the graph, the concentrations of NH₃ were calculated from their corresponding absorbance values.

2.3.2. Quantification of NH₃ using ion chromatography. For the IC calibration method, a cationic exchange column (Metrosep C 6 - 150/4.0) was used, having a flow rate of the eluent (1.7 mmol L⁻¹ HNO₃) of 0.9 mL min⁻¹. At room temperature and 7.55 MPa system pressure, the peak for NH₄⁺ appears at ∼6.4 min and the intensity of the peak increases with an increase in NH₄⁺ ions present in the solution. The area (Y axis) vs. concentration (X axis) curve was drawn to get the calibration curve. After performing additional calculations, all the concentration values of NH₃ were calculated using the calibration curve (Fig. S9†).

2.3.3. Quantification of the byproduct H₂. The amount of H₂ evolved in the N₂ fixation experiment is quantified using a gas chromatograph (Agilent 8890 GC system, N₂ as carrier gas, and TCD detector). H₂ production is also checked in an Ar atmosphere instead of purging N₂.

2.3.4. Isotopic ¹⁵N₂ experiments. To identify the source of nitrogen in the photogenerated ammonia, ¹⁵N₂ gas (feeding gas; Sigma-Aldrich) was used as an alternative isotopic source of N₂ at 20 sccm flow rate. After 3 h of light illumination the resulting solution was collected for further analysis. The reaction solution was analysed through a nuclear magnetic resonance (NMR) spectrometer [Bruker 400 MHz].

2.4. Photoelectrochemical measurements

First, 12 mg of the catalysts were dispersed in a solution of 300 μL C₂H₅OH and 30 μL Nafion. After ultrasonication for 5 h, the resultant solution was drop-cast on indium-tin-oxide (ITO) glass covering 1 cm² area, and then dried overnight at 60 °C to finally obtain the working electrode. All the photoelectrochemical measurements (photocurrent and EIS) were performed using a three-electrode configuration in 0.5 M Na₂SO₄ solution purged with N₂ as the electrolyte comprising the fabricated electrode as the working electrode, Pt as the counter electrode and Ag/AgCl as the reference electrode. The photocurrent responses were recorded under 300 W Xe lamp irradiation (100 mW cm⁻²) at a bias voltage of 0.5 V vs. Ag/AgCl. EIS measurements were conducted without any illumination at a bias voltage of 1 V vs. Ag/AgCl in the frequency range of 100 kHz to 0.1 Hz.

2.5. Theoretical calculation

We conducted simulations using the first-principles-based density functional theory (DFT) tool, the Vienna Ab initio Simulation Package (VASP).²⁸ The calculations were performed with the revised Perdew–Burke–Ernzerhof approximation (RPBE)²⁹ for the exchange–correlation potential, employing a plane-wave basis set with an energy cutoff of 500 eV and the projector augmented wave (PAW) method. Corrections from the ideal gas limit approximation and the harmonic limit/hindered harmonic limit approximation model³⁰ implemented in the atomistic simulation environment (ASE)³¹ for free molecules and absorbates on the catalyst surface were utilized to derive the free energies reported in this work. Our study centered on a five-layered CdS slab with a (102) surface, hosting four Cd sites, and a 20 Å vacuum layer above the top slab to prevent interactions between periodic images, while the ions in the bottom three layers were constrained throughout the simulations. In the ternary and quaternary systems, Cd ions from the surface of the slab were substituted by Mo and/or Sn.

3. Results and discussion

3.1. Structural and morphological analyses

The Cd-based quaternary sulfides were fabricated using a simple hydrothermal method, as illustrated in Fig. 1a. XRD analysis was employed to explore the crystal phases of the as-prepared nanocrystals. The XRD patterns shown in Fig. 1b and S1† reveal evident peaks at 2θ values of 24.95°, 26.57°, 28.19°, 36.60°, 43.76°, 47.96° and 52.02°, corresponding to the crystal lattice planes (100), (002), (101), (102), (110), (103), and (112) of hexagonal CdS (JCPDS no. 41-1049).³² In addition, no identifiable diffraction peaks are observed aligning with MoS₂ and SnS₂, which confirms the successful incorporation of Mo and Sn into the CdS lattice forming a quaternary metal sulfide system instead of a physical mixture of MoS₂, SnS₂, and CdS. The smaller ionic radii of Mo⁶⁺ (59 pm)³³ and Sn⁴⁺ (69 pm)³⁴ as compared to Cd²⁺ (96 pm),³⁵ facilitate their occupancy in the crystal lattice. As a result of the substantial metal content, a noticeable expansion of the crystal lattice occurs,³⁶ which can be further confirmed by the shift in peak positions to lower 2θ values (Fig. 1c). The peak shifts further in the case of a quaternary sulphide system. Moreover, peak broadening is observed due to the slightly differing interplanar spacings arising from non-uniform strain.³⁷ Morphological analysis was conducted using TEM (Fig. S2a†), demonstrating the spherical structures of CdS. No significant change was observed in the morphology of Cd_1−2xMo_xSn_xS as compared to pure CdS (Fig. S3 and S4†). Further analysis of Cd_0.60Mo_0.20Sn_0.20S using SEM (Fig. 1d) and TEM (Fig. 1e) reveals a similar micromorphology. The HR-TEM images of the as-prepared multi-metal sulfides (inset of Fig. 1e, S3a and S4a†) reveal that the particle size is ∼12–15 nm, indicating that the size of the fabricated photocatalysts remains unchanged after metal incorporation. HR-TEM depicts the interplanar distance as 0.25 nm for Cd_0.60Mo_0.20Sn_0.20S, assigned to the presence of the active (102) facet of hexagonal CdS (Fig. 1f), which can be further confirmed from the SAED pattern (Fig. 1g). The existence of multiple crystalline planes proves the polycrystalline nature of the synthesised samples (Fig. 1g, S3b and S4b†).

Fig. 1 (a) Synthesis scheme of Cd-based quaternary samples; (b) XRD patterns of CdS, Cd_0.80Sn_0.20S, Cd_0.80Mo_0.20S and Cd_0.60Mo_0.20Sn_0.20S; (c) zoomed-in view of the highest intensity (002) peak; (d and e) SEM and TEM images of Cd_0.60Mo_0.20Sn_0.20S, and the inset shows a zoomed-in TEM image; (f and g) HR-TEM image and SAED pattern of Cd_0.60Mo_0.20Sn_0.20S; (h–l) TEM EDX elemental mappings of Cd_0.60Mo_0.20Sn_0.20S.

HR-TEM images of the ternary systems reveal the (102) plane as the active facet (Figures S3c,† S4c). The elemental images from EDX spectroscopy prove the homogeneous distribution of Cd, Mo, Sn, and S in the corresponding structures (Fig. 1h–l, S2b–d, S3d–g and S4d–g†). The composition of the multi-component sulfides is further identified using AAS (Table S1†). Considering Cd_0.60Mo_0.20Sn_0.20S, the mol% (molar ratio percentage) of Cd, Mo, and Sn as obtained from AAS is 59.87%, 21.02%, and 19.10% respectively, which is analogous to the expected values, revealing the true molar ratio composition as Cd_0.60Mo_0.21Sn_0.19S. The mol% values are well matched to the expected values for other synthesised photocatalysts.

XPS is a non-destructive surface analysis tool utilized to investigate the active chemical species at the surface of the catalysts and their respective degrees of oxidation. The survey scans (Fig. S5†) show the presence of Cd, Mo, Sn and S in the corresponding structures. The high-resolution narrow scan for Cd 3d (Fig. 2a) depicts two characteristic peaks for Cd²⁺, assigned to 3d_3/2 (411.84 eV) and 3d_5/2 (405.08 eV).³⁸ Starting with pure CdS while advancing towards quaternary metal sulphide, incorporation of high valence species (Mo⁶⁺ and Sn⁴⁺) renders Cd²⁺ electron-deficient, and thus a shift to higher binding energy (∼0.35 eV) is noticed. The Mo 3d spectrum (Fig. 2b) is fitted with two peaks attributed to 3d_3/2 (235.46 eV) and 3d_5/2 (232.35 eV), further confirming the presence of Mo⁶⁺.³⁹ On advancing from ternary to quaternary systems a slight shift to higher binding energy (∼0.22 eV) is observed, responsible for lowering the electron density on Mo⁶⁺. As shown in Fig. 2c the Sn 3d spectrum depicts the existence of Sn⁴⁺, as the spectrum is fitted with two peaks 3d_3/2 (495.40 eV) and 3d_5/2 (487.01 eV),⁴⁰ and no shift in binding energy is noticed. The S 2p spectrum (Fig. 2d) demonstrates two characteristic peaks for S²⁻, attributable to 2p_1/2 (162.61 eV) and 2p_3/2 (161.45 eV),³⁸ accompanied by a slight shift to higher binding energy (∼0.23 eV) in the presence of Sn.

Fig. 2 The high-resolution XPS narrow scans for (a) Cd 3d; (b) Mo 3d; (c) Sn 3d; (d) S 2p for CdS, Cd_0.80Sn_0.20S, Cd_0.80Mo_0.20S and Cd_0.60Mo_0.20Sn_0.20S, respectively.

3.2. Effect of metal incorporation on optical properties and the energy band structure

The optical characteristics of the photocatalysts are examined by recording the absorption spectra. Fig. 3a and S6a† show that the as synthesized photocatalysts exhibit characteristic semiconductor absorption in the visible light range having a band edge at around ∼600 nm. Incorporating Mo and Sn in the case of quaternary sulphide systems results in a slight red shift, demonstrated by a slight reduction in bandgaps, which significantly influences the tuning of photocatalytic behaviours.

Fig. 3 (a and b) UV-vis DRS spectra and Tauc plots of CdS, Cd_0.80Sn_0.20S, Cd_0.80Mo_0.20S, and Cd_0.60Mo_0.20Sn_0.20S, respectively; UPS spectra showing E_edge and E_cut-off (inset: complete UPS spectrum) of (c) CdS, (d) Cd_0.80Sn_0.20S, (e) Cd_0.80Mo_0.20S, and (f) Cd_0.60Mo_0.20Sn_0.20S, respectively; (g) band diagram of CdS, Cd_0.80Sn_0.20S, Cd_0.80Mo_0.20S, and Cd_0.60Mo_0.20Sn_0.20S, respectively (left to right).

The optical bandgaps as obtained from Tauc plots (Fig. 3b) are 2.25 eV, 2.20 eV, 2.13 eV and 2.15 eV for CdS, Cd_0.80Mo_0.20S, Cd_0.80Sn_0.20S and Cd_0.60Mo_0.20Sn_0.20S respectively, using the following equation:

(αhν)^1/γ = B(hν − E_g),

where α = absorption coefficient, h = Planck's constant, ν = frequency of light provided by the source, E_g = bandgap of the semiconductor, B = constant, and γ = 1/2 considering a direct bandgap.⁴¹

The insertion of Mo and Sn into a sulphide system is accompanied by a lowering in the bandgap proceeding from Mo to Sn as shown in Fig. 3b. Fig. S6b† shows the initial consistent reduction in bandgap with the growth in the percentage of incorporated multiple metallic sites to reach 2.15 eV for Cd_0.60Mo_0.20Sn_0.20S; afterward the bandgap widens with a further increase in addition. Furthermore, to understand the effect of Mo and Sn incorporation on the sulphide system, UPS spectra were recorded. Initially, the work function was determined using UPS (φ = hν − E_cut-off, where hν = energy of the photon for He I radiation = 21.2 eV and E_cut−off = secondary electron cut-off), followed by the determination of the valence band maximum level (VB_max = φ + E_edge – 4.44). As shown in Fig. 3c–f using a linear extrapolation, E_cut-off and E_edge are obtained. In addition, the conduction band minimum level (CB_min) is obtained by combining bandgaps from UV-vis DRS (Fig. 3b) with VB_max.⁴² The corresponding calculated band edge positions are depicted in the band diagram shown in Fig. 3g. Incorporating multiple metallic sites elevates both VB_max and CB_min levels proceeding from Mo to Sn, achieving the highest CB_min level for Cd_0.60Mo_0.20Sn_0.20S (−0.66 eV vs. 0.20 eV for Cd_0.80Mo_0.20S, −0.36 eV for Cd_0.80Sn_0.20S and 1.05 eV for CdS), again justifying its highest N₂ reduction capability (N₂/NH₄⁺ = 0.27 eV vs. NHE and N₂/NH₃ = −0.05 V vs. NHE).^14,43

3.3. Photocatalytic activities and photostability of the fabricated photocatalysts

The photocatalytic NH₃ production experiments are carried out at room temperature and under ambient pressure conditions, using simulated solar light (100 mW cm⁻²) as the excitation source, pictorially illustrated in Fig. S7.† The reaction medium comprises a photocatalyst thoroughly dispersed in 20% (v/v) methanol (CH₃OH) solution, saturated with N₂-gas where CH₃OH was used as a hole scavenger.⁴⁴ To determine the concentration of NH₃ produced during photocatalysis, calibration curves were employed which are plotted using the spectrophotometric method (Fig. S8†) and ion chromatography (Fig. S9†). Fig. 4a reveals that NH₃ production for pure CdS is substantially low (111.872 μmol g⁻¹, after 3 h), but increases ∼10.7 times in the case of a quaternary system (Cd_0.60Mo_0.20Sn_0.20S, 1202.679 μmol g⁻¹, after 3h). To check the reproducibility of the best-performing catalyst (Cd_0.60Mo_0.20Sn_0.20S), Cd_0.60Mo_0.20Sn_0.20S was synthesised in five distinct batches and its photocatalytic ammonia production was evaluated (Fig. S10a†), indicating a negligible variation in catalytic performance. These findings confirm the reproducibility and reliability of our synthesis process and catalyst design. The ammonia production of Cd_0.60Mo_0.20Sn_0.20S was assessed using methanol, ethanol, and isopropanol as hole scavengers to evaluate the role of hole scavengers (Fig. S10b†). These results indicate that methanol is the most effective hole scavenger for this system, likely due to the pH change in the solution. This observation aligns with the findings by Simon et al., who demonstrated that the pH of the solution plays a critical role in photocatalysis.⁴⁵ Furthermore, to get an insight into the major role played by Mo and Sn in enhancing the activities significantly, the photocatalytic activities of ternary systems (Cd_0.80Mo_0.20S and Cd_0.80Sn_0.20S) have been investigated.

Fig. 4 (a) Photocatalytic NH₃ production with time over CdS, Cd_0.80Mo_0.20S, Cd_0.80Sn_0.20S, and Cd_0.60Mo_0.20Sn_0.20S, respectively (left to right), (b) comparison of photocatalytic NH₃ generation by the Nessler's reagent method and ion chromatography, (c) comparative analysis of Cd_0.60Mo_0.20Sn_0.20S with various other highly efficient reported photocatalysts showing selectivity, (d) comparative analysis of Cd_0.60Mo_0.20Sn_0.20S with various other highly efficient reported photocatalysts for NH₃ production, (e) photostability of Cd_0.60Mo_0.20Sn_0.20S, (f) XRD patterns of Cd_0.60Mo_0.20Sn_0.20S: fresh (solid line) and after catalysis (dotted line).

Under 3 hours of visible light illumination, the ternary sulfides Cd_0.80Mo_0.20S and Cd_0.80Sn_0.20S achieved NH₃ production rates of 268.07 μmol g⁻¹ and 294.69 μmol g⁻¹, respectively, representing a 2.4 to 2.6-fold increase compared to pure CdS. Among the ternary systems, Cd_0.80Sn_0.20S demonstrates superior NH₃ production compared to Cd_0.80Mo_0.20S, due to its more favourable band edge position (Fig. 3g) and lower electron-hole recombination (discussed in Section 3.4). However, further modifications to the composition, as seen in Cd_0.60Mo_0.40S and Cd_0.60Sn_0.40S, resulted in reduced NH₃ production (Fig. S11†), suggesting that excessive addition of Mo and Sn diminishes the photocatalytic efficiency of the ternary systems. All these findings indicate that a synergistic effect of Mo and Sn plays a very important role in boosting photocatalytic activities. To investigate the influence of incorporated metal concentration on photocatalytic activities, various quaternary sulfides (Cd_1−2xMo_xSn_xS, where x, y = 0.05 to 0.30) were examined. It was found that photocatalytic activities increased with increased bimetallic (Mo and Sn) concentration, reaching a maximum efficiency for Cd_0.60Mo_0.20Sn_0.20S, after which further addition led to decreased activity (Fig. S12a†).

Furthermore, the NH₃ production is confirmed using ion chromatography, and very similar results are obtained, for instance, 1138.994 μmol g⁻¹ (after 3 h) for Cd_0.60Mo_0.20Sn_0.20S (Fig. 4b and S12b†). A striking feature of this work is the selectivity of Cd_0.60Mo_0.20Sn_0.20S towards N₂ reduction instead of H₂ production (H⁺ reduction) (Fig. 4c and Table S2†), without compromising the photocatalytic N₂ fixation performance. In the absence of light, photocatalysts, and N₂, the control experiment resulted in negligible NH₃ production. With CH₃OH serving as a hole scavenger, no H₂ is detected in the case of the Cd_0.60Mo_0.20Sn_0.20S catalyst when N₂ gas is introduced into the reactor as a feeding gas, showcasing its remarkable selectivity (Fig. S13a†). Moreover, excellent specificity is also observed in this case using Ar as a feeding gas as H₂ remains undetected. In the case of pure CdS, H₂ production is rare in the presence of N₂, though a significant amount of H₂ is detected in an Ar atmosphere (Fig. S13a†). To investigate the effectiveness of CH₃OH as a hole scavenger in N₂ fixation for designed photocatalysts, H₂ production is investigated using sodium sulfide and sodium sulfite (Na₂S/Na₂SO₃) in the presence of Ar. A minimal H₂ production is observed in the case of Cd_0.60Mo_0.20Sn_0.20S, whereas pure CdS shows a significant increase (Fig. S13b†). The results demonstrate that the incorporation of Mo and Sn, along with the use of CH₃OH as a hole scavenger, plays a crucial role in achieving remarkable selectivity for green NH₃ production. Moreover, to verify the source of the N atom in NH₃, ion chromatography is utilized for detecting NH₄⁺ ions in the presence of Ar instead of N₂ (Fig. S14†). There is no peak for NH₄⁺ in the case of Ar, indicating that the N₂ purged into the reaction medium is the sole source for NH₃ production. Additionally, our catalysts detected no NH₃ when dimethylformamide was used as the solvent. Furthermore, an isotopic labelling experiment with ¹⁵N₂ feeding gas confirmed the origin of NH₃ produced during the NRR, as shown in Fig. S15.† The ¹H NMR spectrum of ¹⁴NH₄⁺ exhibited a triplet coupling with a J_N−H of 52 Hz when ¹⁴N₂ was used as the feeding gas. In contrast, the ¹⁵NH₄⁺ spectrum showed a doublet coupling with distinct peak positions and a J_N−H of 74 Hz. These results align closely with published literature, confirming that NH₃ was entirely derived from the feeding gas, with no contribution from other sources.⁴⁶ These observations conclusively establish that NH₃ originates exclusively from nitrogen and water. It is evident that the as-prepared Cd_0.60Mo_0.20Sn_0.20S displays significantly superior N₂ reduction capability to other previously reported photocatalysts for N₂ fixation, clearly noticeable from the comparative analysis plot (Fig. 4d and Table S3†).

In addition to superior activity, the durability of a photocatalyst is a critical requirement for industrial applications. Therefore, we examined the photostability of the top-performing catalyst, as depicted in Fig. 4e. Fig. 4e illustrates the robustness of Cd_0.60Mo_0.20Sn_0.20S, as it exhibits consistent performance during five consecutive runs, consisting of 12 h each. To validate the structural and morphological integrity of Cd_0.60Mo_0.20Sn_0.20S, XRD and TEM data were collected after the fifth cycle; the results are depicted in Fig. 4f and S16.† The XRD pattern revealed no notable changes, confirming the phase stability of Cd_0.60Mo_0.20Sn_0.20S (Fig. 4f). Furthermore, morphological analysis through TEM reveals spherical particles with a diameter of ∼12–15 nm (Fig. S16a†) and showed a lattice fringe spacing of 0.25 nm aligning with the active (102) facet (Fig. S16c†). These findings are consistent with the SAED pattern observed (Fig. S16b†). Furthermore, elemental mappings (Fig. S16d–h†) confirm the homogeneous distribution of consistent amounts of Cd, Mo, Sn, and S, emphasizing that there is no leaching out of the elements, further confirming the robustness of Cd_0.60Mo_0.20Sn_0.20S. In addition, Fig. S17† illustrates the XPS spectral analysis of the recycled sample. Mo 3d and S 2p spectra (Fig. S17†) reveal a slight shift towards higher binding energy for the used sample, demonstrating that Mo⁶⁺ and S²⁻ act as the active centres for N₂ fixation, as further confirmed by DFT.

3.4. Reaction mechanism of photocatalytic N₂ fixation

To elucidate the mechanism, the FT-IR spectroscopy technique is utilized. After supplying N₂ gas into the reaction medium various absorption bands are detected in the 1000–4000 cm⁻¹ range under visible light irradiation. Fig. S18† depicts the FT-IR bands observed during photocatalysis conducted using Cd_0.60Mo_0.20Sn_0.20S. The series of peaks identified in order are: (i) 1112 cm⁻¹ for hydrazine (H₂N–NH₂), (ii) 1444 cm⁻¹ and 2953 cm⁻¹ for NH₄⁺ absorption, (iii) 1689 cm⁻¹ and 3253 cm⁻¹ for adsorbed NH₃, and (iv) 2159 cm⁻¹ is attributed to adsorbed N₂ (ref. 47 and 48) and (v) 3611 cm⁻¹ and 3725 cm⁻¹ to adsorbed H₂O.⁴⁹ Upon exposure to visible light, the absorption peaks for NH₃ and NH₄⁺ amplify with time. A noteworthy point here is the existence of hydrazine in the reaction medium in the N₂ reduction process confirming the symmetric alternating pathway.⁴⁷

Nitrogen Temperature-Programmed Desorption (N₂-TPD) was used to study N₂ chemisorption on catalysts (Fig. S19†). Nitrogen Temperature-Programmed Desorption (N₂-TPD) studies were conducted to investigate N₂ chemisorption on catalyst surfaces. The TPD curve's desorption peak temperature and area represent the strength and effectiveness of N₂ gas adsorption. Chemisorption is essential for photocatalytic N₂ fixation as it activates reaction centers. The results reveal that chemisorption predominates in the case of Cd_0.80Mo_0.20S, whereas Cd_0.80Sn_0.20S exhibits significantly higher physisorption. Interestingly, Cd_0.60Mo_0.20Sn_0.20S demonstrates an optimal balance of chemisorption, enhancing the rate of ammonia production.

To get an insight into the role of Mo and Sn in the quaternary sulphide system various photoelectrochemical measurements are conducted. Linear sweep voltammetry (LSV) being an efficient method for detecting the efficiency of water splitting (source of H⁺ for N₂ reduction) is employed.² The LSV curves (Fig. 5a) demonstrate the highest current density, and the lowest overpotential for water splitting using Cd_0.60Mo_0.20Sn_0.20S. Transient photocurrent measurements (Fig. 5b) conducted for Cd_0.60Mo_0.20Sn_0.20S (∼60 μA cm⁻²) exhibit ∼30 times more photocurrent response as compared to that of pure CdS, Cd_0.80Mo_0.20S, and Cd_0.80Sn_0.20S (∼1–2 μA cm⁻²). These results indicate a rapid electron–hole recombination rate for pure CdS, followed by Cd_0.80Mo_0.20S and then Cd_0.80Sn_0.20S, and the sluggish electron–hole recombination kinetics of Cd_0.60Mo_0.20Sn_0.20S. The efficient electron–hole segregation and high sensitivity towards visible light elucidate the higher availability of charge carriers, consistent with the photocatalytic N₂ fixation performance.¹⁶

Fig. 5 (a) LSV curves of CdS, Cd_0.80Mo_0.20S, Cd_0.80Sn_0.20S and Cd_0.60Mo_0.20Sn_0.20S under light conditions; (b) transient photocurrent response; (c) EIS-Nyquist plot of CdS, Cd_0.80Mo_0.20S, Cd_0.80Sn_0.20S and Cd_0.60Mo_0.20Sn_0.20S, respectively; (d) room-temperature PL spectra of photocatalysts excited at 370 nm.

To further recognize the role of incorporated high valence species (Mo⁶⁺ and Sn⁴⁺) in enhancing electron mobility, the electrochemical impedance spectroscopy (EIS) technique is used. The Nyquist plot (Fig. 5c) reveals the smallest semi-circle for Cd_0.60Mo_0.20Sn_0.20S, reflecting its lowest charge transfer resistance, further promoting electron–hole separation, consistent with the photocurrent responses. The above results imply that Cd_0.60Mo_0.20Sn_0.20S has rapid electron migration and enhanced charge carrier lifetime, which is further validated using photoluminescence (PL) spectroscopy (Fig. 5d). As can be observed from Fig. 5d, Cd_0.60Mo_0.20Sn_0.20S shows a lower intensity PL signal than pure CdS at ∼416 nm (excitation wavelength = 370 nm), verifying an increase in the supply of electrons after incorporation of Mo and Sn into CdS, resulting from sluggish charge-carrier recombination kinetics and enhanced charge carrier lifetimes.³⁸ The prominent availability of charge carriers is one of the contributing factors in enhancing the N₂ fixation properties of Cd_0.60Mo_0.20Sn_0.20S.

3.5. Theoretical viewpoint: the role of Sn and Mo

The d-band center model,⁵⁰ initially proposed by Hammer and Nørskov, correlates the electronic structure of metal sites with their catalytic properties. According to this model the position of the d-band center relative to the Fermi level influences the strength of the metal-adsorbate bond, thereby affecting the adsorption energy of molecules on the surface. The d-band center model predicts a uniform decrease (or increase) in the adsorption energy of a given molecule as the number of d-electrons increases (or decreases) across different transition metal surfaces. This relationship is crucial for understanding how different metals will interact with N₂, aiding in the understanding and designing of novel catalysts for nitrogen activation. Fig. 6a shows the d-band center of Cd, Mo, and Sn sites calculated from DFT simulations of the surfaces of pure, ternary and quaternary metal sulphide.

Fig. 6 (a) d-Band center; (b) visualization depicting N₂ adsorption and (c) free energy of H and N₂ adsorption on Cd, Sn and Mo sites on the catalyst surface at 300 K; (d) schematic diagram showing selective adsorption of N₂ on the Mo-site and the effect of both Mo and Sn on the catalyst.

The d-band center of the Cd site on quaternary, ternary and pure CdS surfaces lies deep below the Fermi level, resulting in weak physisorption of N₂ on the Cd site. Sn, being a post-transition and p-block element, has empty d-bands, and thus, the Sn site on the quaternary surface does not adsorb N₂. Conversely, the d-band center of the Mo site on the quaternary surface lies close to the Fermi level, leading to the chemisorption of N₂. Fig. 6b visualizes the optimized structure of N₂ adsorption on Cd, Sn, and Mo sites from the simulation results. Compared to the unabsorbed or weakly physisorbed N₂, the N–N bond length increases by 2.6% in the chemisorbed N₂ on the Mo site. Fig. 6c compares the free energy of adsorption of H and chemisorption of N₂ on Cd, Sn, and Mo sites. The comparison shows that Mo sites on both ternary (CdMoS) and quaternary (CdMoSnS) catalysts energetically favour N₂ adsorption over H. However, the strength of N₂ binding on the Mo site decreases, resulting in optimal binding of N₂ in the presence of Sn in the quaternary sulphide catalyst relative to the Mo site in the ternary (CdMoS) catalyst, which closely corresponds to the experimental results (Fig. S19†). This correlates with a decrease in the d-band center of the Mo site by 0.25 eV due to the presence of Sn, as shown in Fig. 6a. The simulation results indicate that a combination of Mo and Sn can be used to break the scaling law by selectively activating N₂ to avoid parasitic HER and can improve the NH₃ production rate, as observed experimentally (Fig. 6d).

4. Conclusion

In summary, we conducted a promising one-step hydrothermal fabrication of a quaternary metal sulfide (Cd_0.60Mo_0.20Sn_0.20S) exhibiting a remarkable NH₃ generation rate (521.294 μmol g⁻¹ h⁻¹) which is ∼4–4.5 times higher as compared to that of Cd_0.80Mo_0.20S and Cd_0.80Sn_0.20S, along with exceptional photo-stability. To elucidate such a noticeable performance, experiments were carried out to reveal the synergistic effect of Mo and Sn on boosting the reduction potential of the CB coupled with the suppressed charge-carrier recombination kinetics. FT-IR demonstrates a symmetric alternating pathway as the operating mechanism. Moreover, theoretical calculations recognize Mo as the active site for N₂ fixation. Further DFT calculations reveal the significance of Sn in attaining an outstanding selectivity towards N₂ fixation, according to the Sabatier principle. These results suggest that the combined effect of Mo and Sn can effectively break the scaling law, selectively activate N₂, and enhance NH₃ production rates, as observed experimentally. This research provides valuable insights for the design and development of efficient catalysts for N₂ activation and NH₃ production, with potential implications for sustainable energy and chemical synthesis applications.

Data availability

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

Author contributions

The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

We would like to thank Professor Satishchandra Ogale (Director, RISE, TCG CREST) for his insightful discussions. The authors would like to thank Sukanta Mandal and Ripan Kumar Biswas for their support in recording XPS and XRD data. J. P. would like to acknowledge the research grant (VIL58867) from VILLUM FONDEN for supporting this work.

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Footnote

† Electronic supplementary information (ESI) available: XRD; TEM and TEM-EDX elemental mapping; XPS survey scans; UV-vis DRS spectra and corresponding Tauc plots; N₂ fixation setup; calibration curve using the Nessler's reagent method and data obtained using ion chromatography; reproducibility of synthesis and photocatalytic NH₃ production; photocatalytic NH₃ production under various conditions using different photocatalysts; ¹H NMR; TEM and XPS after catalysis; FT-IR spectra; N₂-TPD; AAS table and comparative tables. See DOI: https://doi.org/10.1039/d5ta00645g

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