An ionic liquid route to prepare copper sulphide nanocrystals aiming at photocatalytic applications

Abstract

Copper sulphide crystallizes in a variety of phases that exhibit distinct optical properties including different optical band gaps in the visible region. The crystalline phases are associated with unique properties shown by semiconductor nanocrystals that could be exploited to produce new photocatalysts with tuned optical behavior. This research reports on the use of ionic liquids as a reaction medium to produce copper sulphide nanocrystals whose crystalline phase depends on the synthesis conditions. Noteworthy, these solvents allow the production of water dispersible copper sulphides following the thermal treatment of Cu(II) alkyldithiocarbamates complexes, used here as single-molecule precursors, thus improving their potential for heterogeneous photocatalysis in the aqueous phase. Finally, the ensuing copper sulphides have been preliminarily assessed for their photocatalytic activity in the degradation of rhodamine B (RhB). The results indicate that the ionic liquid route described here has an impact on the copper sulphides produced and thereby on their performance as photocatalysts.

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Introduction

Semiconducting transition metal chalcogenides have attracted great attention owing to their size-dependent properties and their potential applications which include sensors, solar cells, catalysts and optoelectronic devices.¹ In particular, copper sulphide, a well known p-type semiconductor, have attracted great attention because of its stoichiometry-dependent band-gap.² These solids crystallize in a variety of stable and metastable phases at room temperature, from copper-rich chalcocite (Cu₂S) to the highest oxidized form covellite (CuS), with several non-stoichiometric phases (Cu_2−xS) in between those two compositions. Motivated by the need to produce copper sulphide nanocrystals (NCs) with well-controlled stoichiometry, several methods have been reported including the use of colloidal methods,³ microwave irradiation,⁴ solvothermal treatment,^4c,5 solid-state reactions,⁶ sonochemical synthesis,⁷ microemulsion,^5d,8 and surfactant-assisted synthesis.^5e,9

Recent efforts have explored chemical routes for nanocrystals synthesis using ionic liquids (ILs) as the reaction medium.¹⁰ Compared to conventional solvents, ILs are adequate to dissolve a variety of precursor compounds ranging from ionic inorganic salts to molecular metal–organic precursors. ILs present several advantages as solvents such as negligible vapour pressure, non-flammability, high thermal and chemical stability, and a wide temperature range for the liquid state. Additionally, they can be used when anhydrous or oxygen poor conditions are required and, due to their low solubility in polar solvents, reaction products can be easily extracted with effective recycles, allowing reuse procedures, which in certain cases is economically crucial due to their cost. These features can be advantageous for nanomaterials synthesis but just a few studies have been reported, such as the synthesis of CdS by a single-source route¹¹ and of Pd colloids in phosphonium ILs.¹² To the best of our knowledge, the use of ILs for the synthesis of crystalline semiconductor polytypes has not been reported and therefore this study was undertaken to explore the ILs route for copper sulphide nanomaterials.

Semiconductor photocatalysis has been of great interest for multiple environmental applications, including the photocatalytic reduction of CO₂,¹³ and the photo-oxidation of organic pollutants.¹⁴ Polycrystalline titanium dioxide (TiO₂) has been the top-choice as semiconductor photocatalyst, namely due to its photostability and relatively low toxicity. TiO₂ is a wide band gap semiconductor (anatase: 3.2 eV; rutile: 3.0 eV), therefore solar radiation harvesting is limited because the E_g is located in the UV (∼4% solar radiation). A number of strategies have been reported aiming at the development of more efficient photocatalysts activated by sunlight by coupling TiO₂ to visible sensitizers.¹⁵ In this respect, several metal chalcogenides have been investigated by exploiting band gap tuning via adjustment of particle size distribution.¹⁶ Among these inorganic compounds, copper sulphide has been reported to harvest photons efficiently in the visible spectral region, exhibiting narrow band-gap whole range is 1.2–2.2 eV, depending on the crystalline phase.^2,17 Here, we demonstrate that copper sulphide NCs can be prepared through a single-source approach by thermolysis of Cu(II) dialkyldithiocarbamate complexes in ILs. Furthermore, the photocatalytic activity of the ensuing copper sulphide nanomaterials have been evaluated using rhodamine B (RhB) solutions under visible-light irradiation with the assistance of hydrogen peroxide.

Experimental

Reagents and methods

Copper nitrate tri-hydrate (Carlo Erba, 99.5%), dibutylamine (Sigma-Aldrich, 99%), carbon disulphide (Panreac), sodium hydroxide (Acros Organics, 98.5%), sodium diethyldithiocarbamate trihydrate (Sigma-Aldrich), oleylamine (Sigma-Aldrich) and 30% (w/w) aqueous hydrogen peroxide (Riedel-de-Häen) were used as received. The ionic liquids trihexyl(tetradecyl) phosphonium dicyanamide, [TDTHP][N(CN)₂] (Cytec, 96.5%), and trihexyl(tetradecyl) phosphonium bis(trifluoromethylsulfonyl)imide, [THTDP][NTf₂] (Cytec, 93.7%) were dried under moderate vacuum at room temperature, for a minimum of 72 h. All other solvents were supplied by commercial sources and used as received.

Copper diethyldithiocarbamate, {Cu[S₂CN(C₂H₅)₂]₂}, and copper dibutyldithiocarbamate, {Cu[S₂CN(C₄H₉)₂]₂}, were investigated as single-molecule precursors for copper sulphides and were prepared by adaptation of literature methods.^18,19 Hence, the compound {Cu[S₂CN(C₂H₅)₂]₂} was prepared by the addition of Cu(II) salt to a solution containing the diethyldithiocarbamate anion. In a typical procedure an aqueous solution of Cu(NO₃)₂ (10 mmol, 2.4 g) was treated, under stirring, with an excess of aqueous Na(S₂CN(C₂H₅)₂). The brown solid obtained, {Cu[S₂CN(C₂H₅)₂]₂}, was collected by filtration and the final product recrystallized from hot acetone.¹⁸

{Cu[S₂CN(C₂H₅)₂]₂}. Microanalysis (wt%): found: C, 33.5; H, 4.98; N, 7.6; S, 35.4. Calc. for Cu[S₂CN(C₂H₅)₂]₂: C, 33.4; H, 5.6; N, 7.8; S, 35.6. ¹H NMR (CDCl₃) δ (ppm): 1.22 (t, 3H, CH₂CH₃); 3.73 (q, 2H, CH₂CH₃). {Cu[S₂CN(C₄H₉)₂]₂} was obtained by insertion of CS₂ into a secondary amine. Typically, butylamine (20 mmol) and then CS₂ (13.5 mmol) were added dropwise to 50 mL of Cu(NO₃)₂ (10 mmol) ethanolic solution at 60 °C. To this mixture, 20 mmol of NaOH have been added while stirring for 3 h at 60 °C. The brown solid formed was filtered and the product was recrystallized from hot acetone.¹⁹

{Cu[S₂CN(C₄H₉)₂]₂}. Microanalysis (wt%): found: C, 44.8; H, 6.9; N, 5.5; S, 28.0. Calc. for Cu[S₂CN(C₄H₉)]₂: C, 45.8; H, 7.7; N, 5.93; S, 27.1. ¹H NMR (CDCl₃) δ (ppm): 0.94 (t, 3H, CH₂CH₂CH₂CH₃); 1.25 (m, 2H, CH₂CH₂CH₂CH₃); 1.32 (m, 2H, CH₂CH₂CH₂CH₃); 3.73 (t, 2H, CH₂CH₂CH₂CH₃).

Synthesis of copper sulphide NCs

These syntheses have been performed under a N₂ stream, using a round flask equipped with a Liebig condenser, placed in a heating mantle set at the required temperature. The experimental apparatus was placed inside a fume cupboard equipped with local exhaust ventilation. The thermolysis reactions were performed using as solvents the ionic liquids [THTDP][N(CN)₂] and [THTDP][NTf₂]. Two methodologies were employed for the synthesis of copper sulphides; hot-injection of the precursor in ILs and the heating of a batch of the IL containing previously dissolved precursor. A typical synthesis by the injection method was as follows: 0.233 mmol of precursor was dissolved in 4 mL of ILs. This solution was swiftly injected into 16 mL of hot ILs through a syringe, under stirring and N₂ atmosphere. The reaction was kept at the required temperature over 3 h, and then the brown mixture was cooled to 45 °C and methanol was added. The suspended solid was isolated by centrifugation (3461 g over 20 min by using a Hettich EBA 20 centrifuge) and thoroughly washed with methanol. For the batch synthesis, a typical procedure was as follows: 0.233 mmol of precursor was dissolved in 20 mL of ILs under N₂ flow and stirring. The mixture was heated at the required temperature (e.g. 180 °C) and kept for 3 h. After cooling the copper sulphide nanopowders were obtained similarly as described above. En route to the synthesis of copper sulphide nanocrystals by the above described methods, and in order to compare the crystalline phase obtained without a solvent, the single-molecule precursors decomposition was also performed in the solid state using a tubular furnace at 500 °C for 2 h, under N₂ atmosphere, placed inside a fume cupboard with efficient exhaust ventilation.

Photocatalytic studies

The photocatalytic activity of as-prepared copper sulphides was preliminary evaluated for the degradation of rhodamine B aqueous solutions under visible-light irradiation, at room temperature. In a typical run, the photocatalyst (10 mg) was dispersed into 150 mL of RhB aqueous solution (20 mg mL⁻¹) by sonication during 15 minutes and stirred for 1 h in the dark, in order to establish adsorption/desorption equilibrium of RhB molecules. The photoreactor was filled with this suspension and then was irradiated by a 400 W halogen lamp (Haloline Eco 400 W, Osram). At regular intervals, aliquots were withdrawn from the reaction mixture and, after centrifugation, the concentration of RhB was estimated by measuring the absorbance and by applying the Beer–Lambert law (λ_max = 554 nm). For comparative purposes, the photocatalytic degradation of RhB was also carried out in the presence of Degussa P25 TiO₂ and H₂O₂ using the same apparatus. Also, in order to clarify the photocatalytic reaction mechanism, the degradation of RhB was performed using a hydroxyl (˙OH) radical scavenger, tert-butanol. The photocatalytic degradation rate was calculated according to the formula:
where C₀ and C_t are the RhB initial and final concentrations, respectively.

Instrumentation

X-ray power diffraction (XRD) data were collected using a PAN analytical Empyrean X-ray diffractometer equipped with Cu-K_α radiation source (λ = 1.54178 Å) at 45 kV/40 mA. The UV-Vis measurement spectra were performed using a Jasco V-560 spectrophotometer. FT-Raman spectra were recorded using a Horiba Jobin-Yvon LabRam HR800 equipped with a multichannel air cooled (−70 °C) CCD (Nd:YAG laser with excitation wavelength of 532 nm, laser power set at 25 mW). X-ray photoelectron spectroscopy (XPS) analysis was performed using a Kratos AXIS Ultra HSA, with VISION software for data acquisition and CASAXPS software for data analysis. The analysis was carried out with a monochromatic Al K_α X-ray source (1486.7 eV), operating at 15 kV (90 W), in fixed analyser transmission mode, with a pass energy of 40 eV for regions ROI and 80 eV for survey. Data acquisition was performed with a pressure lower than 1 × 10⁻⁶ Pa, and it was used a charge neutralization system. The effect of the electric charge was corrected by the reference of the carbon peak (284.6 eV). Spectra analysis was performed by peak fitting with Gaussian–Lorentzian peak shape and Shirley type background subtraction (or linear, taking in account the data). Transmission electron microscopy (TEM) was performed using a JEOL JEM-2010 microscope operating at 200 kV. The samples were prepared by placing an aliquot of the ethanolic suspension on a copper grid coated with amorphous carbon film and left to dry in air. Scanning electron microscopy (SEM) analysis was performed using a FEG-SEM Hitachi SU70 microscope, operating at 15 kV, coupled to a Bruker EDX analyzer. The ¹H NMR spectra of the precursors in CDCl₃ were recorded using a Bruker Avance 300 spectrometer at 300.13 MHz. The chemical shifts were express as δ (ppm) relatively to tetramethylsilane (TMS) as internal reference.

Results and discussion

Phosphonium ILs with two distinct counter anions were used here as solvents (Fig. S1†). As compared to other ILs, these salts show good thermal and chemical stability.²⁰ This aspect is of particular relevance in this study due to the potential effect of the reaction temperature on the type of polymorph obtained for copper sulphide. Fig. 1 and 2 show the powder XRD patterns for copper sulphide samples obtained by thermal decomposition of the precursor {Cu[S₂CN(C₄H₉)₂]₂} in [TDTHP][N(CN)₂] and [THTDP][NTf₂]. For comparative purposes, samples prepared by thermolysis of the precursors in oleylamine were also analyzed; oleylamine is a coordinating solvent commonly used in the synthesis of nanocrystals by the hot-injection method.²¹

Fig. 1 Powder XRD patterns of copper sulphide NCs prepared by thermolysis of {Cu[S₂CN(C₄H₉)₂]₂}, at 180 °C, in: (A) [THTDP][N(CN)₂], (B) [THTDP][NTf₂] and (C) oleylamine. The vertical lines correspond to the standard diffractions peaks attributed to copper sulphide: (A) and (C) rhombohedral-digenite type (ICDDPDF file no. 00-047-1748); (B) monoclinic-djurleite type (ICDDPDF file no. 00-023-0959).

Fig. 2 Powder XRD patterns of copper sulphide samples prepared by thermolysis of {Cu[S₂CN(C₄H₉)₂]₂} in [TDTHP][N(CN)₂] at: (A) 120 °C, (B) 180 °C and (C) 240 °C. The vertical lines correspond to the standard diffractions peaks attributed to copper sulphide: (A) cubic phase-digenite type (ICDDPDF file no. 00-024-0061), (B) rhombohedral phase-digenite type (ICDDPDF file no. 00-047-1748) and (C) monoclinic phase with djurleite type (ICDDPDF file no. 00-023-0959).

The XRD identification of copper sulphide phases in polycrystalline samples can be a difficult task due to the diversity of possible phases, similar diffraction patterns and occurrence of mixtures.²² Indeed, a number of reports have described the synthesis of covellite (CuS) or chalcocite (Cu₂S) but a less number of studies report the formation of non stoichiometric phases (Cu_2−xS) despite the marked influence of the synthesis conditions on the polymorph obtained. For instance, Fig. 1 shows the diffraction patterns for copper sulphide NCs prepared at 180 °C, by the hot-injection method, using the ILs or oleylamine as solvents. While metastable rhombohedral Cu_1.8S (digenite) formed when using [TDTHP][N(CN)₂] and oleylamine, in the presence of [THTDP][NTf₂] the less oxidized phase Cu_1.94S (djurleite) was also obtained. This is a first evidence for a solvent effect on the synthesis of copper sulphides polymorphs using the hot-injection method.

As expected, variations in the temperature of synthesis also led to distinct copper sulphide crystalline phases. This is illustrated for the case of using [TDTHP][N(CN)₂] as the solvent, at distinct reaction temperatures, leading to nanocrystalline powders characterized by the powder XRD shown in Fig. 2. Table 1 summarizes the crystalline phases identified for each set of synthesis conditions using the method described here.

Table 1 Copper sulphide nanophases obtained at different reaction conditions

Solvent	Temp (°C)	Average size^a (nm)	Phase	JCPDS file no.
a Calculated from XRD using the Debye–Scherrer formula.
[THTDP][N(CN)2]	120	43	Cu1.8S cubic	00-024-0061
	180	30	Cu1.8S rhombohedral	00-047-1748
	240	22	Cu1.94S monoclinic	00-023-0959
[THTDP][NTf2]	120	—	—	—
	180	11	Cu1.94S monoclinic	00-023-0959
	240	42	Cu1.8S rhombohedral	00-047-1748
Oleylamine	120	41	Cu1.8S rhombohedral	00-047-1748
	180	18	Cu1.8S rhombohedral	00-047-1748
	240	16	CuS hexagonal	00-006-0464

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There is not an obvious dependency of the phase obtained on the synthesis conditions. Instead, the crystalline phase seems dependent on the conjugation of the type of solvent and reaction temperature that assist the thermolysis of the precursor.

It is instructive to compare the identity of the powders obtained by dry pyrolysis of the precursor {Cu[S₂CN(C₄H₉)₂]₂} with that one produced by the corresponding process of liquid phase thermolysis. The solid state pyrolysis of such precursor led to the formation of cubic copper sulphide-digenite structure (Fig. S2†), which similarly to most of the liquid phase thermolysis products (Table 1) also resulted from a partial reduction of Cu(II) ions, in the complex, to Cu(I) ions in the final metal sulphide. However, a distinct phase was obtained in this case, thus suggesting that there is an influence of the solvent on the type of crystalline phase obtained. Previously, Sceney et al.²³ have reported the chalcocite phase (Cu₂S) as the product from the thermal decomposition of {Cu[S₂CN(C₂H₅)₂]₂}, with concomitant reduction of Cu(II) to Cu(I). Here, the copper sulphide obtained from dry pyrolysis corresponds to Cu_1.8S,²² thus our results are in line with other reports,² in the sense that the most reduced form Cu₂S (chalcocite) was not observed for any of the samples obtained by the thermolysis of the Cu(II) dithiocarbamate precursor. Fig. 3 shows in situ XRD patterns for a rhombohedral digenite sample submitted to variable temperature treatment (30–400 °C).

Fig. 3 In situ powder XRD patterns for nanocrystalline copper sulphide submitted to thermal treatment: ★ rhombohedral Cu_1.8S; ♦ monoclinic Cu_1.94S.

The structural changes associated to the temperature dependent XRD patterns can be divided in two main stages: (i) up to 280 °C, there were no significant changes in the crystalline phase; (ii) above this temperature, up to 400 °C, the diffraction peaks are in agreement with the presence of Cu_1.94S.

In order to clarify the valence of Cu and S in the several copper sulfide nanocrystals prepared, XPS analysis was performed by using the C 1s peak as reference. The XPS survey spectra indicate the presence of Cu and S as well as C and O adsorbed due to exposure to air (Fig. S3†). The XPS spectra of the Cu 2p and S 2p core levels for the rhombohedral (digenite type) and covellite samples are shown in Fig. 4. The Cu 2p core level XPS spectra for the three types of copper sulphide show the binding energies corresponding to Cu 2p_3/2 and 2p_1/2. The Cu 2p_3/2 spectrum of covellite nanocrystals shows two peaks at 932.4 eV and 933.6 eV, which are in agreement with published values for Cu⁺ and Cu²⁺.^24b,27 For digenite (Cu_1.8S) two peaks are observed, at 932.2 eV and 933.3 eV, showing the presence of Cu⁺ and Cu²⁺ sites. The Cu 2p_3/2 region of covellite was characterized by an asymmetric tail, which has been reported for this crystalline phase.²⁸ The asymmetric shape of the peaks were slightly reduced from covellite to digenite nanocrystals. In addition, for both structures, a weak peak corresponding to a satellite (indicated by an arrow) was observed around 943 eV, indicating the paramagnetic chemical state of Cu²⁺.^27a

Fig. 4 Fitted XPS spectra of copper sulphide (Cu 2p, S 2p) phases: CuS (A and C) and Cu_1.8S (B and D).

The S 2p XPS core level for covellite is characterized by three peak profiles typical of covellite.²⁹ Peak fitting indicates that there are distinct sulphur sites in the sample. The peaks at 161.2 and 162.1 eV correspond to the S 2p_3/2 core levels for S²⁻ and S₂²⁻, respectively. In the case of digenite, the peaks corresponding to sulphides and disulphide are also observed and appear at 161.6 eV and 162.6 eV, respectively. The peaks at 168.6 eV and 169.4 eV, in S 2p core level XPS spectrum for digenite and covellite, respectively, are attributed to sulphate due to adventitious surface oxidation.^27c,30c

Depending on the stoichiometry, copper sulphide (Cu_2−xS) materials show different values for the energy band gap (E_g) and this is has been of great interest for a number of photoapplications.^2,17c,24 The energy band gap varies from 1.3 eV in djurleite (x = 0.06), 1.75 eV in digenite (x = 0.2) and 2.2 eV in covellite (x = 1).²⁵ Additionally, band gap tuning can also be accessed by varying particle size of the semiconductor (quantum size effects) leading to new possibilities for the application of these materials. Table 1 shows the crystallite size for the several copper sulphides prepared in this work, as determined by the Debye–Scherrer equation.²⁶ Although these values give a crude estimate on particle size, overall the observed crystallite size ranges between 11 and 43 nm. These dimensions are above 10 nm, which has been reported as the size threshold above which quantum size effects are expected to occur for this material.² However, this assumption needs further confirmation because the dielectric constants for Cu_2−xS materials also change with chemical composition that might lead into larger exciton Bohr radius.² For clarifying this point the experimental visible spectra of the Cu_2−xS samples are of great utility in the current study. For example, Fig. 5 indicates that the optical band gap for samples prepared in the IL [TDTHP][N(CN)₂] is close to the values for the respective E_g of the bulk materials. Thus the optical band gap of covellite (CuS) and rhombohedral digenite nanocrystalline samples as prepared by liquid phase thermolysis is 2.2 eV and 1.7 eV, respectively. This indicates that the E_g in both cases are basically consistent with that of bulk CuS (2.2 eV) and bulk Cu_1.8S (1.75 eV). Also for the djurleite sample, the optical data in Fig. 5 gives a band gap of 1.4 eV of the bulk sample, which is also in agreement with relatively large particles, as illustrated for this case in Fig. 6.

Fig. 5 UV-Vis absorption spectra of nanocrystalline copper sulphide samples prepared in [TDTHP][N(CN)₂] at: (a) 120 °C, (b) 180 °C and (c) 240 °C.

Fig. 6 TEM images of djurleite nanocrystals obtained in [TDTHP][N(CN)₂] ionic liquid at 240 °C (injection temperature).

Studies of copper sulphide NCs as heterogeneous photocatalysts for the degradation of organic dyes are seldom reported. Recently, Ding et al.³¹ have reported the photocatalytic properties of CuS nanoflowers using rhodamine B and, after 2 hours under halogen lamp irradiation, a degradation rate close to 90% was found. Other studies have shown that self-assembled structures and nanodisks of CuS can degrade 85% and 93% of RhB, respectively, over 2 h under visible-light irradiation.³² Li et al. have described the photocatalytic activity of CuS structures for the decomposition of methylene blue under solar light irradiation.^1f,32 Nanocrystals of CuS were also evaluated on the photodegradation of RhB under visible light irradiation, showing that less than 20% of RhB is degraded. In this study, the authors have shown that the photodegradation efficiency of the CuS could be enhanced by its encapsulation with ZnS shells, achieving ∼50% degradation after irradiation by visible light during 2 h.^1h More recently, various types of copper sulphide have been used in the photodegradation of rhodamine B; in this case the presence of H₂O₂ as co-catalyst improved the photodegradation rate to 98%, at room temperature.^1g Also, other organic pollutants such as nitrobenzene and 4-nitrophenol have been efficiently degraded under visible light irradiation by using CuS photocatalysts.³³ An important feature for the nanomaterials obtained by the ionic liquid route as described here is their easy dispersion in water, hence resulting in colloidal stable suspensions. Together with the possibility of crystalline phase and optical band gap tuning, these materials can be of great interest for heterogeneous semiconductor photocatalysis. In order to perform a preliminary assessment of their potential in visible photocatalysis, selected copper sulphides nanophases were tested in the presence of H₂O₂. In these tests, RhB was selected as an organic dye model because it shows strong absorption in the visible region (554 nm) and is photostable in the absence of photocatalyst. In fact, negligible values of RhB degradation under visible-light irradiation were observed in the absence of catalyst.

Fig. 7(I) shows the percentage of photodegradation of RhB as function of time of light irradiation in the presence and absence of Cu_1.8S, at different reaction conditions. It is clear that the photodegradation of RhB in the presence of either Cu_1.8S or H₂O₂ alone was very low; about 10% and 3.5% of RhB were degraded after 120 minutes, respectively (Fig. 7c and d). Furthermore, in presence of both Cu_1.8S and H₂O₂ but in dark conditions, only 25% of RhB was decomposed after 2 h irradiation. Still, for the conditions investigated, the maximum RhB photodegradation (close to 100%) after 2 h irradiation, was observed in presence of djurleite and by adding to the system 1.5 mL of H₂O₂ (30% w/w). Fig. 7(II) compares the photodegradation rate of RhB, as a function of time, in presence of three copper sulphide NCs prepared by the IL route: rhombohedral digenite, djurleite and covellite. Both djurleite and digenite nanophases show high visible-light photocatalytic activity, with fast RhB degradation rate at the early stage of the photocatalytic process, with about 80% of RhB degraded after 15 minutes. On the other hand, covellite exhibited weaker photocatalytic activity than djurleite and digenite, circa 22% degradation after 120 minutes light irradiation. In fact, both digenite and djurleite polytypes exhibit strong absorption in the visible region, while covellite absorbs strongly in the near-IR region (Fig. S4†). For sake of comparison, the photocatalytic activity of commercial Degussa P25 TiO₂ was also evaluated in these conditions. The photodegradation of RhB after 15 min and 120 min was 12% and 84%, respectively, which are values lower than those achieved in the presence of djurleite and digenite, under visible light irradiation; it is well known that P25 TiO₂ performs more efficiently under UV irraditation.³⁴

Fig. 7 Percentage rate for the degradation of RhB aqueous solutions (20 mg L⁻¹). (I) Cu_1.8S as photocatalyst: (a) H₂O₂ 30% (w/w); 400 W halogen lamp irradiation; (b) H₂O₂ 30% (w/w); dark conditions; (c) no co-catalyst; 400 W halogen lamp irradiation; (d) no catalyst, H₂O₂ 30% (w/w); 400 W halogen lamp irradiation. (II) Comparison of the photocatalytic activity of Cu_1.8S, Cu_1.94S and CuS nanomaterials, assisted by H₂O₂ 30% (w/w) as co-catalyst and under 400 W halogen lamp irradiation. Lines aim to guide the eye.

In the presence of djurleite, for which maximum RhB photodegradation was achieved, the wavelength for maximum absorption (λ_max) of the dye was slightly shifted from 554 nm to 530 nm, along with a decrease in the absorbance during the photocatalytic reaction. A similar blue shift was reported for the RhB oxidation using metal oxides as photocatalysts, which was associated to stepwise removal of the N-ethyl group in the degradation of RhB (N,N,N′,N′-tetra-ethylated rhodamine).³⁵

The characteristic λ_max for RhB and for the de-ethylated rhodamine species are 554 nm (RhB), 535 nm (N,N,N′-tri-ethylated rhodamine), 522 nm (N,N-di-ethylated rhodamine) and 498 nm (rhodamine), respectively.³⁶ Thus, these results suggest that the RhB photodegradation occurs via de-ethylation of a single N-ethyl group followed then by oxidation of the dye.

Several research groups have investigated the photocatalytic mechanism applied to the degradation of water dissolved organic dyes, using copper sulfide as the catalyst.^{32,33,37–44} Based on the literature, it is possible to suggest a mechanism that also takes into account our own data. Hence, when energy superior to the E_g of copper sulfide irradiates the semiconductor, photogenerated electrons (e⁻) and holes (h⁺) are formed, respectively in the conduction band (CB) and valence band (VB). The photogenerated charge carriers (e⁻ and h⁺) have the ability to promote chemical processes with adsorbed dye molecules at the particles' surfaces, though electron–hole recombination is a competitive process.³⁸ Photogenerated electrons will be trapped by dissolved O₂, generating superoxide radicals (O₂⁻˙), which can further combine with holes to yield ˙OH radicals. Photogenerated electrons can also be captured by H₂O₂, which is an electron acceptor, increasing the amount of reactive hydroxyl radicals (˙OH) and limiting electron–hole recombination. The hydroxyl radicals can also to be obtained by the interaction of –OH and photogenerated holes. In fact, when the photodegradation of RhB with H₂O₂ was performed in the presence of tert-butanol, a hydroxyl (˙OH) radical scavenger, significant decrease on the RhB degradation was observed, suggesting that RhB degradation proceed mainly via radical mechanism. However, chemical generated hydroxyl radicals due to the hemolytic cleavage of H₂O₂ (Fenton-type reaction) have also been considered to be involved in the photocatalytic behavior of CuS, which can thus explain its catalytic activity in dark conditions.^43,44

Conclusions

Copper sulphide nanocrystals were synthesized using the decomposition of Cu(II) alkyldithiocarbamate complexes in ILs. This method yields water dispersible nanocrystals of diverse Cu_2−xS polytypes, which were further exploited as heterogeneous photocatalysts on RhB degradation under visible-light irradiation, in presence of H₂O₂ as co-catalyst. Preliminary assessment of the copper sulphide nanophases obtained by this IL route, shows that the degradation rate of RhB seem to depend on the type of copper sulphide obtained and their ability to disperse in water. So, the best degradation values of RhB were obtained in the presence of djurleite and digenite, both prepared in ILs, showing almost 100% of degradation after 120 minutes. The copper sulphide obtained in oleylamine are less dispersed in water, giving the lowest values of RhB degradation. Therefore, the possibility to obtain a particular polymorph in controlled synthesis conditions is of great relevance due to distinct photocatalytic behavior. Future work on these systems will attempt to investigate the reuse of these catalysts namely as supported materials.

Acknowledgements

This research was financed in the scope of the FCT Project UTAP-ICDT/CTMNAN/0025/2014 and through the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. The authors are very grateful to Dr Carlos Sá (CEMUP), Dr Marta Ferro and MSc M. C. Azevedo for technical support. A. C. E. also thanks FCT for a post-doctoral grant (SFRH/BPD/86780/2012).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Chemical structure of ILs used; powder XRD pattern of copper sulphide samples obtained at solid state; XPS data of copper sulphide samples with rhombohedral and hexagonal structures and UV-Vis absorption spectrum of hexagonal copper sulphide phase. See DOI: 10.1039/c5ra27569e

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