Field of Art

The present invention relates to S ² coated copper iron sulfide nanocrystals, and a method of preparation thereof, which affords non-toxic nanocrystals, composed from earth abundant elements, soluble in polar solvents with high phase and chemical purity. The resulting material acts as a highly efficient, chemoselective, recyclable and cost effective photocatalyst for the reduction of aromatic nitro compounds.

Background Art

Selective oxidation and reduction of organics for the synthesis of high added value compounds have attracted significant attention in recent years. Particularly regarding aromatic nitro-compounds, selective transformation to their aniline counterparts is considered as the key intermediate stage in the synthesis of pigments, polymers, pharmaceuticals, anti-oxidants and agrochemicals. These aniline derivatives are synthesized industrially by direct hydrogenation of nitro-compounds using noble metal-based catalysts (such as Pt, Pd or Au) and ¾ pressurized gas as reducing agent. The use of noble-metal catalysts and of extremely flammable high-pressure gases renders these processes costly, energy demanding and potentially hazardous. Therefore, identifying effective and sustainable catalysts based on earth-abundant elements, with high activity for the reduction of nitro-arenes under safer and eco-friendly conditions is a great challenge.

Developments in reduction technologies of nitro-arenes have demonstrated promising noble metal-free catalysts and alternative hydrogen sources, which do not require highly pressurized ¾ and elaborate experimental setups, such as H ₃ NBH ₃ , NaBTU, and NH ₂ NH ₂ .H ₂ O. The latter (hydrazine hydrate) is an attractive choice because of the high hydrogen content (8.0 wt %), simply separable by-products (only hydrogen and nitrogen) and scalable synthesis from ammonia, through the Haber-Bosch process. Furthermore, several low-cost catalytic systems, such as metal oxides (C0 ₃ O ₄ , Fe ₂ 0 ₃ , C O, T1O ₂ ), metal sulfides (C0S ₂ , CU ₂ S, CdS), metallic nanocomposites (Cu, Fe, Ni, Co etc) and coordination complexes (Co, Fe) have been investigated, improving the reduction reaction of nitro compounds. However, significant limitations remain, pertaining either to low selectivity and reaction rates, need for high reaction temperatures and use of highly pressurized ¾, long reaction times, use or production of toxic heavy metals or byproducts, respectively and low recyclability. Insights in plasmon-enhanced nanocatalysis for organic transformations aroused significant attention due to improved selectivities, enhanced reaction rates, and milder reaction conditions. However, most effective plasmonic catalysts are based on expensive noble metals, such as Au, Pt, Pd and their alloys, with limited alternatives. Recently, ternary chalcogenide (I-III-IV2) nanocrystals stimulated research due to their low toxicity, earth abundance and thus low-cost, high absorption coefficient, and tunable band gap. Among them, chalcopyrite (C ^’ uFcSi). with a distinctive golden luster, is a naturally occurring mineral, having a bulk band gap of 0.5 eV and tetragonal crystal structure, with tetrahedrally coordinated Cu and Fe ions with sulfur. In nano-form, CuFeS2 NCs have been synthesized, and exhibit localized surface plasmon resonance at 2.4 eV due to the presence of free carriers, resembling gold (Sugathan, A. et al. J. Phys. Chem. Lett. 2018, 9, 696-701). Importantly, CuFeS2 nanocrystals are non-emissive, whereby the excited surface plasmons relax through non-radiative damping -due to the intermediate energy bands- generating hot-electrons and heat (Ghosh, S. et al. Chem. Mater. 2016, 28, 4848-4858; Girma, W. M. et al. Acs Appl Mater Inter 2018, 10, 4590-4602; Chen, Q. et al. ACS Appl. Mater. Interfaces 2019, 11, 18133-18144.). In these previous publications, the CuFeS2 nanocrystals were coated with bulky organic ligands, which were then exchanged with hydrophilic macromolecules (such as polyethylene glycol, hyaluronic acid or bovine serum albumin). Furthermore, such plasmonic CuFeS2 nanocrystals have not been studied for the catalytic reduction of organic molecules before. In the present case, the exchange of the organic ligands of the CuFeS2 nanocrystals with the small and charged moieties of sulfide ions (S ² ) was found very effective pathway to turn the nanocrystals hydrophilic, and thus appropriate for the catalytic reaction in aqueous environments, and small enough in size to allow the interaction of the reactants with the surface of the nanocrystals.

Disclosure of the Invention

The present invention provides sulfide coated copper iron sulfide nanocrystals.

The sulfide anions are bound to the copper iron sulfide nanocrystals by interactions similar to coordination bonds, as confirmed by Raman spectroscopy. The surface of copper iron sulfide nanocrystals contains numerous electrophilic sites such as uncoordinated Cu7Fe ³⁺ sites which favour adsorption / coordination- like binding of sulfide anions. Counterions for establishing electroneutrality of the system are preferably cations from the initial sulfides used for ligand exchange reaction. Counterions do not bind to the nanocrystal surface and they were found to form a diffuse ionic layer surrounding the nanocrystal and maintain overall charge neutrality

The present invention provides a method for preparation of sulfide coated copper iron sulfide nanocrystals which contains the following steps: a) synthesizing oleylamine coated CuFeS2 nanocrystals; b) optionally subjecting the oleylamine CuFeS2 nanocrystals to purification using repeated centrifugation/precipitation by polar-non-polar solvent mixture and final dispersion in non-polar solvent; c) preparing an S ² ligand exchange solution by dissolving a sulfide in a mixture of water and a polar solvent,; d) contacting the product from step a) (or optionally b)) with the product of step c) under continuous stirring; e) separating the solid product formed in step d) from the solution.

The terms “copper iron sulfide” and “CuFeS2” are used herein interchangeably. The terms “S ² coated” and “sulfide coated” are used herein interchangeably.

The term “CuFeS2 nanocrystals” includes chalcopyrite, copper iron sulfide and iron copper sulfide with particle size ranges from 5 to 20 nm as measured by transmission electron microscopy (TEM). The nanocrystal may have various shapes including tetrahedral, trigonal, spherical, hexagonal or other polyhedral crystal shapes.

The term “S ² coated CuFeS2” is synonymous to terms: sulfide coated CuFeS2, S ² coated chalcopyrite, sulfide coated chalcopyrite, S ² coated copper iron sulfide, sulfide coated copper iron sulfide, S ² coated iron copper sulfide and sulfide coated iron copper sulfide. The nanocrystals preferably form aggregates having a particle size within the range from 5 to 50 nm. The nanocrystals may have various shapes including tetrahedral, trigonal, spherical, hexagonal or other polyhedral crystal shapes. The nanocrystals may contain cations as counter-ions (such as Na ⁺ , K ⁺ , Mg ²⁺ or NH ₄ ⁺ ).

The term “nanocrystals” refers herein to nanocrystals having the size from 5 to 20 nm. Step (a) preferably involves a hot injection method. The hot injection method involves dissolution of copper salts and iron salts in pure oleylamine at a temperature within the range of 40-160 °C under inert atmosphere, and a subsequent injection of a solution of a sulfur precursor into this solution.

The metal salt dissolution temperature varies from 40 °C to 160 °C, preferably this temperature is maintained for 1 h to 4 h.

The starting copper salt may preferably be selected from copper iodide, copper chloride, copper nitrate, copper acetate.

The starting iron salt may preferably be selected from iron chloride, iron sulfate, iron nitrate, iron acetate.

Quick injection of sulfur precursor solution to the metal solution at a temperature within the range of 150- 220 °C leads to the formation of supersaturated solution of CuFeS2 units, which immediately start the nucleation and growth of the nanocrystals.

The reaction with sulfur precursor solution takes place at 150 °C to 220 °C under inert atmosphere, preferably for the period of 1 min to 1 h.

The sulfur precursor solution is typically prepared by dissolving sulfur source in high boiling (boiling point range 200-350 °C at 1013 hPa) organic solvent under inert atmosphere at temperature within the range from 25 to 150 °C. The sulfur precursor may preferably be selected from hexamethyldisilathiane, sulfur powder and alkali metal sulfide. The organic solvent may preferably be selected from octadecene and oleylamine.

The step of purification of the CuFeS2 nanocrystals preferably involves addition of a mixture of polar and non-polar solvents for the precipitation under centrifugation and re-dispersion in non-polar solvent. The purification is preferably repeated several times to remove the excess ligands and metal salts.

The nanocrystal dispersion prepared in step (a) and optionally purified in step (b) is typically a dispersion of CuFeS2 nanocrystals in a non-polar solvent. The solvent may preferably be selected from chloroform, hexane, toluene, cyclohexane and mixtures thereof. The polar solvent, which is used for the purification step may preferably be selected from ethanol, methanol, acetone and mixtures thereof. The S ² ligand exchange solution is prepared by mixing metal sulfide with water and a polar solvent. The water : polar solvent ratio is preferably in the range of 1:10 to 1: 2. The sulfide in step c) is preferably selected from an alkali metal sulfide, an alkaline earth metal sulfide and ammonium sulfide. More preferably, the cation is selected from sodium, potassium, ammonium and magnesium; most preferably, the cation is sodium.

After contacting the oleylamine coated CuFeS2 nanocrystals (in non-polar solvent) with the S ² ligand exchange solution, the mixture is subjected to vigorous mixing for at least 15 minutes, for example for 30 minutes.

The products of steps (a) or (b), and (c) are preferably contacted in weight ratio of CuFeS2 nanocrystals to the sulfide being in the range from 4: 1 to 1: 1, more preferably about 2.5: 1 to 1.8:1.

The polar solvents used for the ligand exchange may preferably be selected from dimethylformamide (DMF), formamide (FA), dimethylsulfoxide (DMSO), A-methyl-2-pyrrolidone (NMP), N.N- dimethylacetamide (DMA) and mixture thereof.

The step of the separation of the product (S ² coated CuFeS2 nanocrystals) may be performed by known techniques such as centrifugation or sedimentation and repeated washings with non-polar solvents.

The invention also encompasses embodiments in which a different solvent is used for mixing and/or thermal treatment than the solvent used for the reaction with the metal sulfide reagent.

The method of invention allows to prepare chalcopyrite CuFeS2 nanocrystals coated with S ² ligands, which may form aggreggates with particle size ranges from 5 nm to 50 nm. S ² coated CuFeS2 nanocrystals exhibit localized surface plasmon resonance under light illumination, are soluble in polar solvents and stable at room temperature for several days. The process allowing to achieve these properties is simple and effective and uses economically effective starting compounds.

The S ² coating of CuFeS2 nanocrystal has thus desirable features enabling its use as a photothermal catalyst without the drawbacks for materials known in the art. In particular, the S ² coating replaces the insulating nature of the initially organic molecule-capped nanocrystals (such oleylamine or trioctylphosphine) and makes it more efficient in photocatalytic aromatic nitro-reduction reaction. Its excellent turnover frequency ( i.e. rate of production of the desired product), the lower reaction temperature, the shorter reaction time (4 h) and high chemo selectivity (higher than 90 %) in the photo reduction reaction are superior to the current state of the art. The highest achieved value of molar-based turnover frequency as described in the example was 22.9 h ¹ .

The present invention thus further involves the use of sulfide coated CuFeS2 nanocrystals as a photocatalyst for the catalytic reduction of nitroaromatic compounds.

FIG. 1. TEM images and SAED of the product of Example 1.

FIG. 2. Size distribution of the product of Example 1.

FIG. 3. UV-Vis absorption spectra of the products of Examples 1&2.

FIG. 4. XRD of the products of Examples 1&2.

FIG. 5. XPS of the product of Example 1.

FIG. 6. TEM images and SAED of the product of Example 2.

FIG. 7. Raman spectra of the products from Examples 1&2.

FIG. 8. FTIR spectra of the products of Examples 1&2.

FIG. 9. XPS survey spectra of the products of Examples 1&2.

FIG. 10. XPS analysis of the product of Example 2.

FIG. 11. FTIR spectra of the products of Example 3.

FIG. 12. Photo-catalytic reaction performance of the product of Example 2.

FIG. 13. XPS analysis of the product of Example 2 after the catalytic reaction.

FIG. 14. Catalytic comparisons of the product of Example 1&2 with basic metal sources.

FIG. 15. Catalytic recyclability of the product of Example 2.

FIG. 16. Photocatalytic reduction of various nitroarenes with the product of Example 2. The percentiles shown below each molecule correspond to the reaction yields for these products as determined by gas chromatography. The yields refer to the desired product and thus also represent the selectivities.

Examples of carrying out the Invention

Materials and methods: Copper iodide, Iron chloride, hexamethyldisilathiane, Na S^OFFO. oleylamine and octadecene were purchased from Sigma-Aldrich. Cyclohexane (pure), dimethylformamide (pure) and ethanol (absolute) were purchased from Penta, Czech Republic. All chemicals were used without further purification. Transmission electron microscopy (TEM) images were recorded on JEOL JEM-2100 TEM equipped with a LaBe type emission gun operating at 200 kV.

X-ray diffraction (XRD) patterns were recorded with a PANalytical X'Pert PRO MPD (PANalytical, The Netherlands) diffractometer in the Bragg-Brentano geometry, Co-Ka radiation (40 kV, 30 mA, l = 0.1789 nm) equipped with an X'Celerator detector and programmable divergence and diffracted beam antiscatter slits. The measurement range was 20:0 5°-105°, with a step size of 0.033°. The identification of the crystalline phases was performed using the High Score Plus software (PANalytical) that includes the PDF- 4+ database.

X-ray photoelectron spectroscopy (XPS) was carried out with a PHI VersaProbe II (Physical Electronics) spectrometer using an A1 Ka source (15 kV, 50 W). The obtained data were evaluated with the MultiPak (Ulvac - PHI, Inc.) software package.

UV-Vis absorption spectra were collected on a Cary 50 UV-Vis spectrophotometer (Varian).

Fourier transform infra-red (FT-IR) spectra were recorded on an iS5 FTIR spectrometer (Thermo Nicolet) using the Smart Orbit ZnSe ATR accessory. Briefly, a droplet of chloroform/ethanol dispersion of the relevant material was placed on a ZnSe crystal and left to dry and form a film. Spectra were acquired by summing 64 scans recorded under a nitrogen gas flow through the ATR accessory. ATR and baseline correction were applied to the collected spectra.

Raman spectra were recorded on a Raman microscope using the 613 nm excitation line of a diode laser After the reaction, the catalyst was separated by centrifugation and the product was obtained in ethanol or ethyl acetate and analyzed by GC employing chromatograph Agilent 6820 (Agilent, United States), equipped with flame ionization detector (FID). Products were identified by comparison of the retention time with standard chemicals and the quantitative analysis of the each content was analyzed by GC via interpolation from calibration curves.

Equations used to calculate the conversion, yield and selectivity are given below.

Moles all products ^ ^QQ

Conversion (%) Moles of reactant

Moles of desired product ^ -^QQ

Yield (%) Moles of reactant

Moles of desired product ^ -^ QQ

Selectivity (%) Moles of all products

Turnover frequency (TOF) was calculated according to the following equation: The moles of the catalyst were calculated by considering the dry weight and molecular formula (CuFcSi) of the catalyst. For example, 10 mg of the CuFeS2 catalyst was equal to 54.49 pmol. considering 183.52 as molecular weight.

Example 1: Synthesis of oleylamine coated CuFeS2 nanocrystals (CuFeS2-OLA)

1 mmol (0.192 g) of Cul and 1 mmol (0.162 g) of FeCL were dissolved in 27 mL of oleylamine and stirred in a three-necked round bottom flask. The mixture was heated to 100 °C under vacuum and maintained for

2 h to remove oxygen and moisture. The reaction atmosphere was then switched to nitrogen and then raised to 180 °C and stirred until the solution obtained a clear orange color. 2 mmol of hexamethyldisilathiane dissolved into 3 mL of 1-octadecene as swiftly injected into the above mentioned mixture under vigorous stirring, and the temperature was kept for lO min at 180 °C for the growth of the CuFeS2 nanocrystals. Then, the solution was cooled to room temperature under ambient conditions. Toluene and ethanol were added to the reaction mixture for centrifugation at 9418 ref for 5 min. The precipitate was then washed twice with an 1: 1 ethanoktoluene mixture for two times. Finally, the oleylamine-capped nanocrystals were collected and dried at 60 °C under reduced pressure.

The as synthesized oleylamine-capped CuFeS2 NCs displayed an average size of 8-10 nm with a large population having a prismatic crystal habit, as indicated by TEM (Figure la,b & Figure 2). The selected area electron diffraction (SAED, Figure lc) showed the characteristic diffraction rings of the (112), (204) and (312) lattice planes of the tetragonal CuFeS2 phase. UV-Vis absorption spectra of the NCs (Figure 3) showed a broad absorption in the visible region with maximum at 520 nm, attributed to the plasmon resonance of the CuFeS2 NCs. In the XRD pattern (Figure 4) all peaks were indexed to the tetragonal chalcopyrite CuFeS2 lattice, also confirming the purity of the bulk product.

The surface chemical states of the CuFeS2 NCs were also probed with XPS (Figure 5). The survey spectrum (Figure 5a) shows the presence of Cu, Fe, S, C, N and O elements. The Cu 2p high-resolution XPS spectra (Figure 5b) reflecting the typical spin-orbit splitting of the Cu atoms, resulting in Cu 2p _3/2 and Cu 2pia peaks with a separation of 19.8 eV, and no satellite peak, confirming the Cu (I) oxidation state of Cu and absence of Cu (II) species. The Fe 2p spectrum (Figure 5c) also showed the characteristic doublet due to spin-orbit splitting. The Fe 2p _3/2 envelope centered at 710.9 eV, corresponding to Fe (III) oxidation state, in accordance with previous results. The S 2p core level spectrum (Figure 5d) showed a spin orbit splitting (two main doublets), S 2p _3/2 and S 2r _ΐb , corresponding to the metal-sulfide (sulfide and disulfide) bonding states of sulfur. Example 2: Ligand exchange of oleylamine coated CuFeS ₂ nanocrystals with S ² by Na ₂ S (to form sulfide coated copper iron sulfide (CuFeS ₂ -S ² ))

0.1 g ofNa ₂ S.9H ₂ 0 was dissolved in 5 mL of ultrapure water and then 15 mL of dimethylformamide was added, to form the sulfide ligand-exchange solution. 0.2 g of oleylamine-coated CuFeS ₂ NCs were dispersed in 10 mL cyclohexane and then mixed with ligand-exchange solution and stirred for 30 min at room temperature. The mixture was centrifuged at 9500 ref for 5 min and the supernatant was discarded. The precipitate was washed 3 times with ethanol and the ligand-exchanged nanocrystals CuFeS ₂ -S ² were dried under vacuum at 60 °C. The final solid catalyst was grinded well in a mortar with a pestle before use. UV-Vis absorption spectra of the NCs before and after the ligand exchange (Figure 3) indicated that the main photophysical features of the material remained practically intact. TEM and SAED confirms the preservation of crystal size, shape and structure (Figure 6). XRD (Figure 4) also confirmed the preservation of the crystal structure before and after the ligand exchange reaction. Applying the Scherrer equation on the 35-degree diffractions, a crystallite size of 3 and 4.5 nm was calculated before and after ligand exchange, respectively. The smaller size obtained in comparison to that obtained from TEM is attributed to the fact that TEM is based on optical inspection and the smaller crystallites are not as visible as the bigger ones, leading to a size overestimation. The Raman spectra from both CuFeS ₂ -OLA and CuFeS ₂ -S ² nanocrystals (Figure 7) exhibit the three characteristic bands of CuFeS ₂ crystals, at 287, 351 and 470 cm ¹ , assigned to the Ai, B2 and E phonon modes of the CuFeS ₂ lattice. The E phonon mode at 470 cm ¹ is attributed to the Cu-S bonds. However, after the ligand exchange with S ² , the E band was enhanced, indicating the formation of new Cu-S bonds due to the S ² coordination with the Cu ⁺ atoms on the surface of the nanocrystals. The successful ligand exchange was unequivocally confirmed with FT-IR (Figure 8), showing the complete elimination of the oleylamine spectral features at 2987 and 2900 cm ¹ (assigned to the asymmetric and symmetric C-H stretching vibrations of the -CH ₂ - groups). Similarly, XPS (Figure 9) showed a dramatic reduction -or complete elimination- of the nitrogen peak (circled) in the CuFeS ₂ -S ² , due to the successful removal of the oleylamine coating. The appearance of the Na peak in the XPS spectrum of the CuFeS ₂ -S ² product (highlighted) is also attributed to the successful binding of the S ² with the free metal sites on the nanocrystal surface. S ² ligands bind (through a coordination bond) to the nanocrystal surface by displacing the original organic ligands based on the hard and soft acids and bases principle, producing an electrostatically stabilized colloid. Counter ions, like Na ⁺ in Na ₂ S, does not bind to the nanocrystal surface and they form a diffuse ionic layer surrounding the nanocrystal and maintain overall charge neutrality.

The survey spectrum of the CuFeS ₂ -S ² verified the presence of the Cu, Fe, S, C, Na and O elements (Figure 10a). The Cu 2p high-resolution XPS spectra (Figure 10b) reflecting the typical spin-orbit splitting of the Cu atoms, resulting in Cu 2pm and Cu 2p _\ n peaks with a separation of 19.8 eV, and no satellite peak, confirming the Cu (I) oxidation state of Cu and absence of Cu (II) species. The Fe 2p spectrum (Figure 10c) also showed the characteristic doublet due to spin-orbit splitting. The Fe 2p _3/2 envelope centered at 710.9 eV, corresponding to Fe (III) oxidation state, in accordance with previous results. The S 2p core level spectrum (Figure lOd) showed a spin orbit splitting (two main doublets), S 2p3 _/2 and S 2pm, corresponding to the metal-sulfide (sulfide and disulfide) bonding states of sulfur.

Example 3: Ligand exchange of oleylamine capped CuFeS2 nanocrystals with S ² by K2S or (NEL^S (to form CuFeS2-S ² )

0.1 g of K2S was dissolved in 5 mL of ultrapure water and then 15 mL of dimethylformamide was added, to form the sulfide ligand-exchange solution. 0.2 g of oleylamine-coated CuFeS2 NCs were dispersed in 10 mL cyclohexane and then mixed with the ligand-exchange solution and stirred for 30 min at room temperature. The mixture was centrifuged at 9418 ref for 5 min and the supernatant was discarded. The precipitate was washed 3 times with ethanol and the ligand-exchanged nanocrystals were dried under vacuum at 60 °C. The final solid catalyst CuFeS2-S ² was grinded well in a mortar with a pestle before use. The ligand exchange by using (NFD2S follows the same procedure except the usage of (NFD2S instead of K ₂ S.

The FTIR spectrum (Figure 11) confirms the successful ligand exchange, showing the complete elimination of the oleylamine spectral features at 2987 and 2900 cm ¹ (assigned to the asymmetric and symmetric C-H stretching vibrations of the -CFb- groups).

Example 4: Photocatalytic reaction using the product from Example 2

The CuFeS2-S ² catalyst (2 to 10 mg) was added in a common glass vial with a teflon-coated cap (5 mL), adding 0.1 and up to 5 mmol of nitrobenzene, 0.05 mL to 1 mL of hydrazine hydrate (50 % solution) and 3 to 0 mL of ethanol. After mixing by stirring (60 s) and sonication (30 s), the closed vial was irradiated in a reaction chamber equipped with a light source (EvoluChem PhotoRedOx box, attached with 34 W Kessil 150N LED having total irradiance of 22 mW/cm ² from 400-500 nm region, peaking at 450 nm) for 0 to 4 h under stirring at room temperature. In cases of solar simulator 1 Sun (100 mWcm ² ) intensity (Sciencetech Light Line A4-C250 equipped with an AM 1.5G filter, class CAA) was used.

The photocatalytic activity of the CuFeS2-S ² NCs for the hydrogenation of nitrobenzene (Figure 12a) was evaluated using hydrazine hydrate as a hydrogen and electron donor. Hydrazine is an attractive choice because of the high hydrogen content (8.0 wt %), simply separable by-products (only hydrogen and nitrogen) and scalable synthesis from ammonia, through the Haber-Bosch process. The reaction was performed under 400-500 nm of light, at a very low flux of 22 mW cm ² and maximum intensity at 450 nm. Although this is lower than the maximum of the plasmon band of the CuFeS2-S ² NCs (Figure 3), it has been observed that the direct generation of electron-hole pairs through interband transitions is strengthened using a shorter wavelength of excitation. Preliminary experiments for reaction optimization using 10 mg of the CuFeS2-S ² NCs catalyst showed that at 2 h of reaction and using 0.8 mmol of hydrazine afforded the product (aniline) at 100 % yield and selectivity using 0.1 mmol of the nitrobenzene substrate (Figure 12b, left part). To push further the limits of the catalyst we increased the amount of the substrate ten times (1 mmol), achieving similarly excellent results at 4 h of reaction and by increasing the amount of hydrazine to 16 mmol in 1 mL of ¾0 (Figure 12b, middle part). At these conditions the TOF reached a value of 4.6 h ¹ , being already among the highest reported. By challenging the catalyst even further by increasing the substrate to 5 mmol under the exact same conditions, aniline was also obtained at 100 % conversion and selectivity, affording the highest TOF of 22.8 h ¹ (Figure 12b, right part). Reactions without using the catalyst or without hydrazine did not yield any aniline, while a control reaction in the dark at 25 °C delivered a very low yield of 19 % (Figure 12b). The reaction yield and rate clearly depended on the amount of the catalyst, as shown in Figure 12c, dictating a maximum yield of 99.4 % and a ground-breaking TOF of 22.8 h ¹ with an optimum catalyst: substrate ratio of 10 mg per 5 mmol of nitrobenzene.

To draw further insights on the activity of the catalyst, control reactions were performed in the dark in an oil bath at 25 or 40 °C, affording aniline with a yield of 19.7 and 44.1 %, respectively (Figure 12d), indicating that CuFeS2-S ² NCs are intrinsically active towards the reduction, which is an important prerequisite for the ideal photocatalyst.

CuFeS2 is also a well-known photothermal agent for biomedical applications due to the intense absorption of light in the visible-near infrared region and its high photothermal conversion efficiency by thermalization of the photoexcited carriers within their broad intermediate bands. Therefore, light irradiation during the catalytic reaction indeed caused a spontaneous and significant temperature increase in presence of the catalyst. The maximum temperature of the reaction during light irradiation was 58 °C (Figure 12e, “light without fan”). When the same reaction was performed using the cooling fan of the photoreactor, the temperature stabilized at 33 °C giving a slightly lower yield of 89.7 % with 100 % nitrobenzene conversion (Figure 12e, “light with fan”). However, control reactions in the dark at 40 or 33 °C delivered significantly lower yields of 44.1 and 32 %, respectively (Figure 12d & Figure 12e, “dark”), than the reaction at 33 °C but under light. These results clearly indicated that the CuFeS2-S ² NCs must act not only through photo-thermal activation (passing the reaction energy barrier easier due to the higher temperature), but also through intermediate photo-excited species. The limits of the photo-thermal nanocatalyst were finally challenged under the optimized conditions but using a 1-sun solar-light simulator. The TOF of the reaction in this case was retained at very high levels ( ca . 84.2 % yield, or 20 h ¹ ), displaying complete conversion within 4 h (Figure 12e, “1 sun”).

The surface chemical states of the CuFeS2-S ² nanocrystals after the photocatalytic reaction were also probed with XPS (Figure 13). XPS analysis confirmed the full preservation of its structural features after photocatalytic reaction. The Cu 2p high-resolution XPS spectra (Figure 13b) reflecting the typical spin-orbit splitting of the Cu atoms, resulting in Cu 2p _3/2 and Cu 2pi _/2 peaks with a separation of 19.8 eV, and no satellite peak, confirming the Cu (I) oxidation state of Cu and absence of Cu (II) species. The Fe 2p spectrum (Figure 13c) also showed the characteristic doublet due to spin-orbit splitting. The Fe 2p _3/2 envelope centered at 710.9 eV, corresponding to Fe (III) oxidation state, in accordance with previous results. The S 2p core level spectrum (Figure 13d) showed a spin orbit splitting (two main doublets), S 2p _3/2 and S 2pi _/2 , corresponding to the metal-sulfide (sulfide and disulfide) bonding states of sulfur.

Example 5: Comparison of photocatalytic reaction using the product from Example 2, Example 1 and other metal salts or oxides

CuFeS2-S ² was added in a common glass vial with a teflon-coated cap (5 mL), adding 0.1 and up to 5 mmol of nitrobenzene, 0.05 mL to 1 mL of hydrazine hydrate (50 % solution) and 3 to 0 mL of ethanol. After mixing by stirring (60 s) and sonication (30 s), the closed vial was irradiated in a reaction chamber equipped with a light source (EvoluChem PhotoRedOx box, attached with 34 W Kessil 150N LED having total irradiance of 22 mW/cm ² from 400-500 nm region, peaking at 450 nm) for 0 to 4 h under stirring at room temperature.

In order to highlight further the role of the chalcopyrite nanocrystals, control experiments were performed using as catalysts Cu, Fe or S elements, as well as Fe203, FeCL, Cul, and mixtures thereof (same moles of metals as for the CuFeS2-S ² catalyst), which did not show significant catalytic activity (Figure 14).

CuFeS2 nanocrystals coated with the oleylamine molecules were also evaluated (Figure 14), showing significantly lower aniline yield than the S ² passivated nanocrystals, emphasizing the benefits of the ligand exchange.

Example 6: Photocatalytic reaction recyclability study using the product from Example 2

The recyclability of the catalyst CuFeS2-S ² was tested in the reaction of 1 mmol of nitrobenzene, in 1 mL of hydrazine hydrate, using 2 mg of the catalyst. The above regents were mixed well and sonicated for 30 s in a closed reaction vessel and then irradiated with for 4 hours under stirring at room temperature. After the reaction, the products were extracted by ethyl acetate and the catalyst separated by centrifugation. The ethyl acetate solution of the product was further diluted and analyzed in GC. The recovered catalyst was washed several times with ethanol and dichloromethane to remove any adsorbed molecules and finally dried under vacuum oven overnight at 60 °C before using it for the next cycle.

The recyclability of the CuFeS2-S ² nano-catalyst was investigated for five consecutive reactions under light irradiation with 1 mmol of nitrobenzene and 2 mg of catalyst (i.e. at its maximum performance, Figure 15). The results indicated that there was no significant loss in catalytic activity even after the 5 ^th cycle (100% conversion), and without the need of increasing the reaction time, or increasing the pressure and temperature, proving its excellent stability and reusability. It is also important to note that the recyclability was evaluated at the maximum activity limits of the catalyst, while other advanced catalysts can keep their activity only at lower conversions.

Example 7: Photocatalytic reaction of broad substrates study using the product from Example 2

The broad substrate scope of the catalyst CuFeS2-S ² was tested in the reaction of 1 mmol of aromatic nitrocompound, in 1 mL of hydrazine hydrate, using 2 mg of the catalyst. The above regents were mixed well and sonicated for 30 s in a closed reaction vessel and then irradiated with for 4 hours under stirring at room temperature. After the reaction, the products were extracted by ethyl acetate and the catalyst separated by centrifugation. The ethyl acetate solution of the product was further diluted and analyzed in GC. Besides the high activity of the CuFeS2-S ² nanocrystal catalyst, its ability to reduce effectively a wide variety of substrates (Figure 16) has additional and pivotal importance. Irrespectively of the presence and type of various functionalities in the nitro-arenes, the catalyst showed excellent yields of the desired product (indicated by the percentiles in Figure 16) and thus high selectivities towards the corresponding amines.