Phase purification of Cu–S system towards Cu_1.8S and its catalytic properties for a clock reaction

Abstract

Controlling the composition and crystal phase is an important issue to tune material physical/chemical properties. Herein, it was found that triphenyl phosphine (TPP) can be used as a phase transfer agent to transform CuS, Cu₃₉S₂₈ phases into pure low-sulfur Cu_1.8S phase. When mixed phase copper sulfides were reacted with triphenyl phosphine under suitable temperature, sulfur was extracted to produce the low-sulfur Cu_1.8S phase. It was also demonstrated that the Cu–S product can effectively catalyze a clock reaction between methylene blue and hydrazine in aqueous medium. In addition, the photothermal conversion properties of the Cu–S based products were studied. The results show that the purified Cu_1.8S materials show enhanced or similar properties than the original mixed-phase Cu–S products.

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Introduction

Metal chalcogenide semiconductor materials have been intensively studied in recent years out of scientific interest and because of their wide technological application.^1–3 Their physical and chemical properties can be highly modified by minute changes in their microscale morphology and size. Besides morphology and size, chemical composition and crystal phase are another two basic parameters for tuning their physical or chemical properties through influencing the bonding characteristic, vacancies, and so on.^4,5 For examples, careful tuning the oxygen stoichiometry in copper oxide materials can induce the transferring of various superconducting, electronic, and magnetic states.⁶ FeSe at a very precise composition in window of 50.6–51.0% Fe is superconducting, while NiAs-type FeSe with a nominally 1 : 1 of Fe to Se (42.0–50.5%) is not superconducting.⁷ Hence, precise controlling and modulating the composition or crystal phase of metal chalcogenide materials is of large scientific and technological importance.

The electrical conductivity breadth from metallic to semiconducting to superconducting and the defect chemistry owing to nonstoichiometry for copper sulfides render them attractive for various applications.^8–22 Copper sulfides with various Cu/S ratios have demonstrated great potential for wide applications in fields such as solar cells, photoelectric transformers, nano-switches, thermoelectric materials, electrocatalysts and photocatalysts.^23,24 The Cu–S system has at least nine kinds of different crystal phase with the varied x value in Cu_2−xS (x = 0–1) including Cu₂S (γ- and β-chalcocite),²⁵ Cu_1.98S (djurleite),²⁶ Cu_1.8S (digenite),^27,28 Cu_1.75S (anilite),^29,30 and CuS (covellite),^31,32 as shown in the X-ray diffraction pattern database. The energy band structure and physical/chemical properties of Cu_2−xS are highly dependent on the stoichiometric factor, 2 − x. However, it is not always straightforward for synthesizing a copper sulfide product with targeting phase; usually, mixed phase copper sulfide products are often obtained through usual synthesis strategies. Up to date, some methods including phase transformation route have been devoted for the preparation of copper sulfide products with pure specific phase.^26,33–35 For example, it was demonstrated that Cu₇S₄ and CuS can be obtained from the freshly formed Cu₉S₈ nanocrystals.³⁶ With a solid-state annealing route, CuS also undergoes a reduction to tetragonal cuprous sulfide Cu₂S.³⁷ Recently, Fang et al. reported facile synthesis of Cu₃₉S₂₈ microcrystals via a solvothermal route.³⁸ Alivisatos et al. has demonstrated the temperature-induced structural transformations of Cu₂S nanorods from a low to a high chalcocite structure and investigated their size dependent phase transformation behaviour.^39,40 Zhong et al. reported a phase transformation process from rhombohedral Cu_1.8S nanocrystals to hexagonal CuS clusters.⁴¹ In spite of these valuable investigations, relative to highly studied PbS and CdS micro-/nanoscale materials, copper sulfides have not been extensively studied. In addition, as copper sulfide easily forms different but close stoichiometric phases, controlling of copper sulfide crystal phase with simple method is still a challenge.^42–44

Among the various Cu_2−xS materials, Cu_1.8S, which is a useful p-type semiconductor with indirect bandgap of 1.5–1.6 eV, has got much attention due to its rich properties and potential applications.^14,40,45,46 Cu_1.8S is known to be used as thermo- or photoelectric transformers and high-temperature thermistors. The higher copper deficiency in Cu_1.8S material makes it show unique localized surface plasmon resonance effect (LSPR).⁴⁰ Due to the LSPR effect, Cu_1.8S has shown excellent photothermal conversion effect and would be used as new type of photothermal agent.^45,46 Especially, it was also demonstrated that Cu_1.8S with higher ion deficiency can be used as catalyst inducing the formation of heterostructures.¹⁴

In this study, a simple and low cost hydrothermal method was developed for the synthesis of copper sulfide product. The obtained product contains two kinds of copper sulfides (CuS and Cu_1.8S). Based on this, we described a new chemical extraction route with triphenyl phosphine (TPP) as extraction agent. This extraction process can transform CuS, Cu₃₉S₂₈ phases into pure low-sulfur metastable rhombohedral phase Cu_1.8S. This chemistry has at least three important implications. First, sulfur-rich copper sulfides can be chemically transformed into Cu_1.8S phase via TPP extraction of sulfur. This highlights the possibility of precise Cu_1.8S phase targeting with this route. Secondly, impure samples contain mixtures of multiple copper sulfides including CuS, Cu_1.8S, Cu₃₉S₂₈ can be purified by TPP extraction of sulfur, with the final product exclusively being metastable rhombohedral Cu_1.8S phase. Third, it seems that the Cu_1.8S product obtained with purification route shows similar properties to that of Cu_1.8S sample obtained with direct preparation route.

Experimental

Materials

CuCl₂·2H₂O, thiourea, polyvinylpyrrolidone (PVP, K30, M_w = 58 000), ethylene glycol, triphenylphosphine, N,N-dimethyl-formamide (DMF), chloroform, and ethanol employed in this research were analytical grade and used without further purification. Deionized water was used in the experiments.

Synthesis of mixed-phase copper sulfide products

In a typical synthesis, 1.36 mmol of CuCl₂·2H₂O, 1.85 mmol of thiourea, and 0.2 g of PVP (K-30) were dissolved in 18 mL of ethylene glycol to obtain a clear solution. The solution was then transferred into a 30 mL of Teflon-lined autoclave, sealed and heated at 180 °C for 12 h and then cooled down to room temperature naturally. The final products were collected by centrifugation and washed with deionized water and absolute ethanol for several times. The as-obtained products were then dried in a 45 °C vacuum oven for further characterization. The sample contains mixed phase copper sulfides, CuS and Cu_1.8S.

Synthesis of Cu₃₉S₂₈ phase copper sulfide

Cu₃₉S₂₈ phase sample was prepared according to a reported method.³⁸ In a typical synthesis, 2 mmol of Cu(NO₃)₂·3H₂O, 2 mmol of thiourea, and 2.0 g of PVP were absolutely dissolved in 20 mL of DMF. The obtained system was then stirred for about 30 min to obtain a clear green solution, which was then transferred into a 30 mL Teflon-lined stainless steel autoclave, sealed and heated at 120 °C for 12 h. After cooling down to room temperature naturally, the obtained Cu₃₉S₂₈ product was collected by centrifugation, washed with distilled water and absolute ethanol, and dried at 45 °C under vacuum.

Phase transformation of copper sulfides to Cu_1.8S phase

Typically, 5 g of TPP was heated to 100 °C in a 50 mL of three-necked flask and was kept at this temperature for 15 min with stirring. 30 mg of copper sulfide sample (mixture of CuS and Cu_1.8S, or Cu₃₉S₂₈) was then added. After reaction at 100 °C for 2 h, the product was collected by centrifugation, washed with chloroform and ethanol for several times, and dried at 45 °C under vacuum.

Characterization

XRD patterns were recorded on an X-ray diffractometer (Bruker D8 Advance diffractometer) with Cu Kα radiation (λ = 1.5406 Å) at a scanning rate of 5° min⁻¹. Ultraviolet-visible (UV-vis) spectroscopy measurements were performed on a UV-2450 ultraviolet-visible spectrophotometer. The morphology and microstructure analyses were conducted on Hitachi S-4800 field emission scanning electron microscope (SEM) and a JEM-2010 transmission electron microscope (TEM).

Catalytic properties

In a typical reaction, 100 μL (∼1 × 10⁻⁵ mol L⁻¹, about 17 μg mL⁻¹) of copper sulfide aqueous dispersion was mixed with an 200 μL of methylene blue (MB) aqueous solution (with concentration of 5 × 10⁻⁴ mol L⁻¹) in a 1 cm quartz cuvette, and the volume of the reaction system was added up to 3 mL with deionized water. 100 μL of 2.5 M aqueous hydrazine hydrate solution was then added to the reaction mixture to induce the reaction. After addition of hydrazine hydrate solution, the blue color of MB gradually disappeared. The reaction mixture regained its original blue shade just shaking the mixture in air for several seconds. For detecting the color change of the reaction system, time-dependent absorption spectra were recorded with a UV-visible spectrophotometer at room temperature.

Measurement of photothermal conversion effect of the copper sulfide materials

For measuring temperature change due to the photothermal conversion effect of copper sulfide materials, 980 nm near infrared (NIR) laser was delivered through a quartz glass cuvette containing aqueous dispersion (1 mL with concentration of 0.03 mg mL⁻¹) of copper sulfide samples with magnetic stirring. The light source is 0.5 W with a laser beam diameter of 6 mm (Ningbo Ruanming Laser Technology Co., Ltd China). A thermocouple with an accuracy of 0.5 °C was inserted into the aqueous dispersion of the tested samples for recording the change of real-time temperature. The temperature was recorded one time per 20 s.

Results and discussion

Synthesis and characterization

In our study, TPP was used as the crystal-phase transform agent. TPP is usually used as nanoparticle capping agent owing to its strong coordination ability of phosphorus atom, phosphorous source for chemical synthesis. Especially, it is also used as a chemical agent in the form of TPP–S complexes. The phase transform process is simple. Direct heating of the copper sulfide dispersion in TPP will induce the phase transform of copper sulfides. Fig. 1 shows the powder X-ray diffraction (XRD) data for the purification process. It is clear that the original Cu–S products contain two kinds of copper sulfide phase, hexagonal covellite CuS (JCPDS no. 65-3561) and rhombohedral digenite Cu_1.8S (JCPDS no. 26-0476). After treated with TPP at 100 °C for 0.5 h, the XRD diffraction peaks corresponding to hexagonal CuS decrease, suggesting the transform process of CuS to Cu_1.8S is conducted. With 2 h of treatment, the obvious enhancement of the peaks at about 2θ = 46.2°, 54.8° and the absolute disappearing of the peak at about 2θ = 47.9° indicate that pure phase Cu_1.8S is obtained after this treatment. This result suggests that TPP can successfully extract sulfur from sulfur-rich copper sulfide, CuS, forming the corresponding low-sulfur compound, Cu_1.8S phase.

Fig. 1 XRD patterns of (a) the synthesized mixed phase sample containing CuS and Cu_1.8S and the products after treated by TPP for (b) 0.5 h, (c) 1 h, and (d) 2 h at 100 °C. The standard JCPDS XRD patterns for CuS (no. 65-3561) and Cu_1.8S (no. 26-0476) are also shown in the bottom.

We also investigate the morphology change of the copper sulfide products during the phase transformation process. Fig. 2 shows the SEM and TEM images of the mixture phase copper sulfides. It can be seen that the original mixture phase copper sulfide product is composed of flower-like micro-spheres with size of 3–5 μm (Fig. 2a and b). The flower-like microspheres are assembled by sheet-like units. Some irregular particles are loaded on the flower-like microstructure, suggesting the mixture phase. TEM image shows similar results (Fig. 2c). Inset of Fig. 2c shows the selected area electron diffraction pattern (SAED). The pattern can be indexed into CuS and Cu_1.8S, although they have similar crystal plane spacing. Two kinds of clear lattice fringes with different spacing in the high-resolution TEM indicate the high crystallinity and two different crystal phases involved in it (Fig. 2d). The lattice fringe with spacing of 0.278 nm would be indexed to (10 10) plane of Cu_1.8S, while that of 0.198 nm can be attributed to (008) plane of CuS. After phase purification with TPP, the morphology of the copper sulfide changed. The assembled flower-like microspheres are broken, leaving disorder sheet-like units (Fig. 3a and b). The corresponding SAED pattern and HRTEM shows pure Cu_1.8S phase with high crystallinity (Fig. 3c and d). The morphology change indicates the chemical reaction between copper sulfide and TPP.

Fig. 2 (a and b) SEM, (c) TEM, and (d) HRTEM images of the mixed phase copper sulfides. Inset of (c) shows the SAED pattern of the product.

Fig. 3 (a) SEM, (b) TEM, (c) SAED pattern, and (d) HRTEM images of the purified Cu_1.8S product.

It was found that the formation of Cu_1.8S phase by TPP treatment is highly phase-targeting. Increasing the treatment temperature to 200 °C, the obtained product is still Cu_1.8S phase. Interestingly, other copper sulfide phase, for example Cu₃₉S₂₈, can also be transferred into the rhombohedral Cu_1.8S phase by this phase transform route. The corresponding results are shown in Fig. 4. Cu₃₉S₂₈ sample was synthesized according to a reported method. Although the XRD patterns are difficult to distinguish owing to the relatively weak crystalline and the close XRD patterns between Cu₃₉S₂₈ and Cu_1.8S phase, after TPP treatment, an obvious shift of the strongest peak is clearly observed, suggesting the crystal phase transformation.

Fig. 4 XRD patterns of (a) the synthesized Cu₃₉S₂₈ product and (b) that after treated by TPP for 2 h at 100 °C. The standard JCPDS XRD patterns for Cu₃₉S₂₈ (no. 36-0380) and Cu_1.8S (no. 26-0476) are shown in the bottom.

These results suggest that TPP can extract sulfur from sulfur-rich copper sulfide, forming metal-rich Cu_1.8S phase.

This extract process is quite different from the previous reported Cu(I) induced phase transformation route.⁴³ In that case, Cu(I) specie was found to induce Cu_1.1S phase to Cu_1.1–1.5S phase. It was proposed that a fraction of Cu⁺ ions from the solution enters the Cu_1.1S lattice, matched by a transfer of electrons from the solution. In our route, it seems that TPP extract sulfur from sulfur-rich covellite CuS product, causing the phase transformation. There are reports about crystal structure of hexagonal covellite CuS.^47–50 Studies have shown that the valence of copper in covellite CuS is monovalent with formalism of (Cu⁺)₃(S₂²⁻)(S⁻) or (Cu⁺)₃(S₂⁻)(S²⁻).^43,47 Recent experiments and calculations have put the valence of Cu between 1 and 1.5 (1.33 for CuS from calculations).⁵⁰ While, all studies indicate that sulfur in covellite phase exists with various valence or forms. Fig. 5 shows the crystal structure illustration of covellite CuS phase and digenite Cu_1.8S. From the structural point of view, it is important to note the similarities of the Cu or S sub-lattice between covellite and digenite in the corresponding close-packed planes, i.e. (001) and (10−1), respectively. It would make it easily to transfer covellite CuS to digenite Cu_1.8S under suitable reaction conditions.

Fig. 5 Crystal structure illustration of covellite CuS and digenite Cu_1.8S.

In our case, the extract route is possibly driven by the large formation constant of the TPP–S complexes. The bonding of sulfur and TPP would be stronger, which cause the following reaction proceed.

CuS + TPP → Cu_1.8S + TPP–S

In the phase transformation process, solid copper sulfide reacts with liquid TPP causing the phase transformation. Thus the reaction rate would be controlled by the solid–liquid interface reaction.

Catalytic properties

It is known that nanoscale copper-based compounds have very good catalytic performance.^51,52 To investigate the catalytic properties of Cu_1.8S products obtained from the purified procedure, the reaction between methylene blue (MB) and hydrazine in aqueous medium was conducted. The reaction shows quite slow reaction rate if no any catalysts were involved. With suitable catalysts, chemical oscillation phenomenon between blue color MB and colorless leucomethylene blue (LMB) can be observed with periodic shaking. Although it was reported that some copper-based materials can effectively catalyze this reaction, CuO or Cu nanoparticles did not show any catalytic effect for it.^51,53

In the presence of copper sulfide catalyst, hydrazine will gradually reduce MB at room temperature, forming colorless LMO (as shown in Scheme 1). While, upon further shaking reaction system in air, the colorless LMO will be oxidized again by air forming blue MB. The redox process demonstrates a simple “clock reaction”, which provides an engaging illustration of redox phenomena and reaction kinetics. As shown in Fig. 6, the original blue reaction solution gradually becomes weak with the increasing of duration time. With duration time of about 20 min, only the liquid–air interface shows blue color, while the solution body becomes colorless. This is reasonable since oxygen in air will oxidize LMO. Further prolonging the duration time to 3 h, the air in the sealed system is possibly exhausted, so the whole solution is colorless. Once shaking the reaction system in air, blue color quickly reappears.

Scheme 1 The chemical oscillation reaction based on methylene blue ions.

Fig. 6 Schematic representation of the “clock reaction” catalyzed by Cu_1.8S, that is, blue color fading and regeneration of MB.

Based on the fact that MB exhibits an intense absorption band in the region of 500–700 nm and LMB shows no absorption at this region, we monitored the process of the clock reaction by a UV-visible spectrophotometer. With the MB blue color bleaching, a steady decrease of the absorbance of MB was measured at same intervals, as shown in Fig. 7a. It should be noted that in the absence of copper sulfide catalyst, no such a decrease in the absorbance of MB was observed in the same experimental condition. Thus, the important role of the synthesized copper sulfide product in the “clock reaction” is no doubt concluded.

Fig. 7 (a) Absorption spectra for successive decolorization of MB and (b) colour regeneration catalyzed by Cu_1.8S.

It seems that the synthesized Cu_1.8S product has a strong catalytic ability for the reduction of MB by hydrazine at room temperature. We also investigated the catalytic activities of mixed phase copper sulfides and Cu₃₉S₂₈ product. In contrast, the catalytic activities of them are relatively low (Fig. SI-1 and SI-2, see ESI†). After complete reduction of MB, the solution containing colourless LMB can again regenerate the blue color in the presence of small amount aerial O₂ on shaking of the reaction mixture openly and so the characteristic absorption band appears (shown in Fig. 7b).

We then investigated the influence of various experimental parameters on the reaction kinetics. It was found that without hydrazine, the reaction cannot proceed. More hydrazine is involved, the reaction rate is quickly. While, it seems that the catalyst dosage don't have obvious influence on the reaction rate in our investigated range. In the presence of copper sulfide catalysts, the plot of absorption factor as a function of time (Fig. 8a) shows a profile of exponential equation, A_t = A₀e^−kt, which is consistent with a pseudo-first-order reaction. Thus, pseudo-first-order reaction kinetics was applied for the evaluation of catalytic activity in our case. The relation of ln(A_t/A₀) (at peak of 664 nm) versus time is shown in Fig. 8b. The slope of the straight line gives the rate constant of the catalytic reaction, 0.0323 min⁻¹ for pure Cu_1.8S. While, mixed phase copper sulfide sample shows similar and relatively lower rate constant, 0.0297 min⁻¹. This indicates that the purified procedure don't change the catalytic activity of the copper sulfide product. In addition, the Cu₃₉S₂₈ product shows the poorest catalytic activity among the tested three samples. It is proposed that the tiny composition difference and the different crystal phase cause their different properties. Copper sulfides with different phases have different copper and sulfur ions arrangement in the crystal, which will cause the different chemical surroundings for the copper or sulfur ions. This finally induces the different catalytic or related properties.

Fig. 8 Plots of (a) A_t and (b) ln(A_t/A₀) of MB vs. reaction time in the presence of mixed copper sulfides, Cu₃₉S₂₈, and pure Cu_1.8S.

We then proposed a possible catalytic principle for the reaction. It is proposed that copper sulfide would promote the electron transfer process from hydrazine to MB.⁵¹ During the reaction, hydrazine and MB may be absorbed onto the copper sulfide surface due to the reasonable affinity. It is believed that hydrazine supplies electrons to MB via copper sulfide, causing the generation of colorless LMB. Hydrazine transfers electrons to Cu(I) to reduce it to Cu(0). Then, MB and dissolved oxygen in the system oxidizes Cu(0) back to Cu(I).⁵¹ During this process, MB was reduced forming colorless LMB. The electron-transfer processes can be supported by the redox potential values of φ(N₂/N₂H₄) = −1.16 V, φ(Cu⁺/Cu) = 0.52 V, φ(MB/LMB) = 1.08 V.^51,54 In the reaction system, excess hydrazine in turn decreases the dissolved oxygen concentration in water, which facilitates LMB formation. While, indeed, detailed catalytic mechanism needs further study.

In addition, it is recently reported that Cu–S system is a new promising semiconductor photothermal conversion platforms with relatively high photothermal conversion efficiency, good photostability, synthetic simplicity, low toxicity and low cost.⁴² It can effectively induce temperature elevation under NIR irradiation, which would provide a possible route for cancer treatment.

The NIR photothermal conversion property of the obtained Cu–S samples in the aqueous dispersions was then examined at a fixed concentration of 0.03 mg mL⁻¹. The results are shown in Fig. 9. The pure water system was also tested for comparison. It is obvious that, for the pure water, the NIR irradiation (980 nm) caused a temperature increase of only about 4 °C after 8 min. For the aqueous dispersion of Cu–S products, the NIR irradiation induced temperature elevation is much higher than the pure water. For the mixtured sample with CuS and Cu_1.8S phase, the temperature increase is 6 °C, while the pure phase Cu_1.8S sample gives a higher temperature increase of 7 °C, suggesting the better photothermal conversion property of Cu_1.8S than that of CuS. It should be noted that all the aqueous dispersions reach a temperature platform with irradiation for 8 min, suggesting a thermal equilibrium between laser energy input and thermal diffusion towards environment are obtained at this stage. After that, the laser is closed, the temperature then decreases to room temperature.

Fig. 9 Temperature variations for pure water and aqueous dispersions of the obtained copper sulfide samples with NIR irradiation time for 0–8 min, and then the laser was shut off. The concentration of the Cu–S products is 0.03 mg mL⁻¹.

Conclusions

We have demonstrated that TPP can be used for the phase-selective extraction of sulfur from sulfur-rich copper sulfides. This provides a new phase transformation strategy for manipulating the compositions and structures of Cu–S system, which would also have applicability to purification of multiphase samples. In addition, the clock reaction catalyzed by Cu–S based product has been demonstrated. Our study on the catalytic properties and the photothermal conversion properties of the Cu–S based products reveals that the obtained Cu_1.8S materials show enhanced or similar properties than the original mixed-phase Cu–S products.

Acknowledgements

The authors are grateful for financial support from National Natural Science Foundation of China (No. 51203069, 51102117, 21201010, 21203079), Jiangsu Natural Science Foundation (No. BK2012276), and Cultivating Project of Young Academic Leader from Jiangsu University.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17652b

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