Formation of nanoneedle Cu(0)/CuS nanohybrid thin film by the disproportionation of a copper(I) complex at an oil–water interface and its application for dye degradation

Abstract

A nanoneedle Cu(0)/CuS nanohybrid thin film was synthesized by reaction of the [Cu(PPh₃)₃I] complex with Na₂S at the interface between toluene and water at room temperature. The catalytic activity of Cu(0)/CuS nanoneedles was investigated in the degradation of dyes in the presence or absence of H₂O₂. On the basis of comparative kinetic studies, the following trend was obtained for the degradation of different dyes in the presence of H₂O₂ and CuS catalyst: neutral dyes > cationic dyes > anionic dyes. Although the most popular degradation of methylene blue dye was studied using TiO₂ semiconductor particles in the presence of UV light, the present procedure involves only ambient conditions and sunlight in the room.

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1. Introduction

Industrial wastewater is one of the most critical environmental pollutants and is predicted to become a major threat to human beings, biodiversity and hydrosphere ecosystems.¹ For example, poisonous groups, such as benzene and azo, present in industrial wastewater may contribute to an increasing risk of bladder cancer incidence among humans.² Dye wastewater contains a large number of various toxic pollutants, which accounts for ten percent of all industrial wastewater.³ To degrade toxic pollutants in dye wastewater, many approaches such as ozonation, adsorption, photochemical and electrochemical degradation have been developed.⁴ Unfortunately, most of these methods lack the high efficiency required to degrade diverse toxic substances in dye wastewater and are limited by their high cost of operation. The method of H₂O₂ catalytic oxidation is feasible and economical. This method with transition metal sulfides and oxides as catalysts releasing highly reactive hydroxyl radicals has been proven to be effective in treating dye-containing wastewater to convert some organic macromolecules into small pollution-free molecules, H₂O and CO₂.⁵

These days, a great interest has been focused to copper sulfides owing to their special properties arising from its varied nonstoichiometric composition, complex structures, nanocrystal morphologies, valence states, etc. The copper sulfides stoichiometry vary starting from CuS₂ at the deficient of copper side to Cu₂S at the rich of copper side.⁶ CuS, one of the important semiconductor transition metal chalcogenides, transforms into a superconductor at low-temperature⁷ and has potential application as sensor, p-type semiconductor, cathode material, solar energy converter, optical filter, nonlinear optical material and catalyst.⁸ Using the following strategies, efficiency of the semiconductor has been increased through prompted charge separation, but the limitation regarding the e⁻−h⁺ pair recombination is not solved. To overcome this obscurity, a new strategy has been developed where noble metal (Pd, Pt, Au, Ag and Cu) platforms are incorporated to inhibit e⁻−h⁺ pair recombination owing to a suitable work function. They serve this purpose in the following ways.

(a) Increasing the charge separation between the valance band (VB) and conduction band (CB) in the semiconductor material⁹

(b) Scavenging photogenerated electrons across the interface¹⁰

(c) Providing a low over potential redox pathway¹¹

The copper metal has a high work function value, which is very much suitable for up taking electrons from the CB of CuS nanoparticles (NPs).

In recent years, CuS with various morphologies have been fabricated.¹² In fact, many methodologies have been explored which include solution thermolysis and solventless, solution phase reactions, sacrificial templating, solvothermal or hydrothermal method, ultrasonic and microwave irradiation, microemulsion, template-assisted methods, chemical vapor deposition, electrodeposition and so on.¹³

The “liquid–liquid interfacial assembly” is a novel strategy provides a powerful bottom-up approach for nanofilm and nanodevice fabrication due to low cost and low environmental impact.¹⁴ Recently, we have reported that various types of Pt, Pd, Pt-based and Pd-based nanostructures thin films can be easily obtained at the liquid–liquid (organic–aqueous) interface (LLI).¹⁵ To the best of our knowledge, there is no report concerning fabrication of Cu(0)/CuS nanohybrid thin film at liquid–liquid interface by the disproportionation of a copper(I) complex. In this paper, we report the synthesis of nanoneedle Cu(0)/CuS nanohybrid thin film at toluene–water interface via an easy method at ambient temperature without using any stabilizing agent. This strategy is an inexpensive and easy method for preparation of Cu(0)/CuS nanoneedles and only a syringe and beaker are required for a high-quality thin film to be easily produced in a short time. Furthermore, the Cu(0)/CuS nanoneedles exhibit a high catalytic activity in the reaction of dye degradation.

2. Experimental

All of the chemical compounds were purchased from Merck and Sigma-Aldrich companies. X-ray diffraction (XRD) patterns of the as-prepared electrocatalyst were recorded using a Bruker AXS (D8, Avance) instrument equipped with Cu-Kα radiation. Transmission electron microscopy (TEM) images of the electrocatalyst were recorded using a Philips CM-10 TEM microscope operated at 100 kV. The surface atomic concentration and chemical composition of the samples were investigated by X-ray photoelectron spectroscopy (XPS) equipped with an Al Kα X-ray source at energy of 1486.6 eV in an ultrahigh vacuum (UHV) system with a base pressure lower than 2 × 10⁻⁹ Torr. UV-Vis studies were carried out by using Perkin-Elmer Lambda 25 spectrophotometer. [Cu(PPh₃)₃I] complex was synthesized using reported procedure.¹⁶

2.1. Preparation of nanoneedle Cu(0)/CuS nanohybrid thin film at the toluene–water interface

A solution of [Cu(PPh₃)₃I] (2 mM, 25 ml) in toluene was sonicated for 5 min to prepare a white color solution. This solution was stand in contact with double distilled water (25 ml) in a 100 ml beaker. Once the two layers were stabilized, an appropriate volume of aqueous Na₂S·9H₂O (5 ml, 0.33 M) was injected into the aqueous layer using a syringe with minimal disturbance to the toluene layer. The onset of reaction was marked by a coloration of the oil–water interface. With the passage of time, the color became more vivid, finally resulting in a film at the oil–water interface.

2.2. Experimental of catalytic activity evaluation

The catalytic properties of the Cu(0)/CuS nanoneedles was investigated in the presence and absence of H₂O₂ by the UV-Vis absorption spectra in a UV-Vis spectroelectrochemical cell. Considering strong absorption in the visible region and excellent skeletal stability, cationic dyes like methylene blue (MB), malachite green (MG) and brilliant green (BG), neutral dye like methyl red (MR) and anionic dye like methyl orange (MO) were selected to examine their degradation (see ESI Fig. S1†). In a typical experiment, hydrogen peroxide (0.5 ml) was mixed with the dye solution (2.5 ml) in a quartz cell (3 ml) and the reaction initiated by adding Cu(0)/CuS nanoneedles (3 mg ml⁻¹, 0.2 ml). The UV-Vis absorption spectra were recorded by applying a UV-Vis spectrometer. Also, similar procedure was applied for the experiments in the absence of H₂O₂. Furthermore, the degradation reaction of MB was examined by KBrO₃ and K₂S₂O₈ as oxidant. The results showed that H₂O₂ is the better oxidant.

3. Results and discussion

3.1. Physicochemical characterization of the catalyst

In this study, the Cu(0)/CuS nanoneedles was synthesized by the reaction of [Cu(PPh₃)₃I] complex with Na₂S at toluene–water interface, as shown in Fig. 1. [Cu(PPh₃)₃I] complex was dissolved in toluene at room temperature and then contacted with water. The aqueous solution of Na₂S was injected into interface with minimal disturbance to initiate the reaction and thin film formation was indicated by the interface color change from white to greenish black.

Fig. 1 Schematic illustration of the nanoneedle Cu(0)/CuS nanohybrid thin film formation at toluene–water interface, (a) stabilized solution of [Cu(PPh₃)₃I] in toluene and water, (b) dropwise addition of Na₂S to the stabilized solution, (c) Cu(0)/CuS nanoneedles thin film appeared at the toluene–water interface after adding Na₂S, (d) remove the toluene (top) phase by a syringe and (e) deposited thin film on a glass.

The solid Cu(0)/CuS nanoneedles well dispersed in H₂O when the mixture is sonicated for 5 min. The optical property of the solution is ascertained using UV-Vis spectroscopy. Fig. 2a presents the absorption spectrum bearing a λ_max = 400 nm as the reminiscent of the absorption for Cu(0)/CuS nanoneedles in H₂O. According to the equation αE_p = K(E_p − E_g)^1/2, a plot of (αE_p)² vs. E_p based on the direct transition is shown in Fig. 2b, where α is the absorption coefficient, K is a constant, E_p is the discrete photoenergy and E_g is the band gap energy. The extrapolated value of E_p at α = 0 gives absorption edge energies correspondence to E_g = 1.45 eV.

Fig. 2 (a) UV-Vis spectrum, and (b) band gap energy of as-prepared Cu(0)/CuS nanoneedles.

The XRD analysis (Fig. 3) shows a complex pattern obtained for the film after 24 h reaction at the toluene/water interface. The strong diffraction peaks of (101), (102), (103), (006), (110), (108), (202), (116), and (208) are attributable to CuS.¹⁷ The other patterns corresponding to the diffraction of (111), (200), and (220) are attributed to Cu(0).¹⁸

Fig. 3 XRD pattern of the Cu(0)/CuS nanoneedles.

The coexistence of Cu(0) and CuS after the reaction of interfacial between the copper(I) complex in toluene and the Na₂S solution can be understood as disproportionation, due to the following relevant standard potentials, relative to the hydrogen electrode:¹⁹

which finally leads to Cu(0) and CuS in a basic solution, and the proposed interfacial reactions are described as follows:

Cu(PPh₃)₃I_(org) + Na₂S_(aq) → Cu₂S_(in) + 3PPh_3(org) + NaI_(aq) (2)

2Cu₂S_(in) → 2CuS_(in) + 2Cu_(in) (3)

The subscripts “in”, “org” and “aq”, in the equations represent interfacial, organic and aqueous phases, respectively. The Cu₂S intermediate is formed from ligand exchange between PPh₃ and S. The copper sulfide is in turn transformed into Cu₂S,²⁰ which disproportionates to Cu(0) and CuS, as shown in eqn (3).

Fig. 4 shows TEM image of the as-prepared Cu(0)/CuS nanohybrid thin film, which includes nanoneedle structures with an average diameter of about 11.25 nm.

Fig. 4 (a) TEM image of the as-prepared standard film obtained after 24 h from a liquid–liquid interface containing a 2 mM toluene solution of [Cu(PPh₃)₃I] and 5 ml of 0.33 M Na₂S solution, (b) histogram of particle size distribution.

The nature of the emerging film was examined by systematically varying factors such as the contact time at the interface and the relative amounts of concentration of precursors. In Fig. 5, we show the TEM images of the films sampled after contact times of 6 and 12 h. These images may be compared with that of the standard film shown in Fig. 4 corresponding to a contact time of 24 h. Clearly, an increase in contact time increases the coverage of the film on the substrate, with no observable change in the average diameter of the nanoneedles.

Fig. 5 TEM images of the Cu(0)/CuS nanoneedles collected from a liquid–liquid interface containing a 2 mM toluene solution of [Cu(PPh₃)₃I] and 5 ml of 0.33 M Na₂S solution with different contact times: (a) 6, and (b) 12 h.

The use of high concentrations of the Na₂S results in less-uniform films with altered distributions in the particle diameter, as can be seen from a comparison of Fig. 6 with Fig. 4. When the concentration of Na₂S was doubled, the distribution was somewhat narrower with diameter in the 13–14 nm range (Fig. 6a). A further increase in the concentration of the Na₂S results in broader distributions in diameters, as shown in Fig. 6b. We notice that the close packing of the particles seen in Fig. 4 is lost at higher concentrations of the Na₂S. The films formed at higher concentrations of the Na₂S also exhibit a distinct tendency to form multilayers. However, when the concentrations of the [Cu(PPh₃)₃I] complex and Na₂S were increased simultaneously, the film primarily consisted of a monolayer of nanoneedles with a broader size distribution and a higher coverage (Fig. 7). The maximum catalytic activity was achieved for the sample with the higher concentrations of the [Cu(PPh₃)₃I] complex and Na₂S.

Fig. 6 TEM images of films collected after 24 h from a liquid–liquid interface containing a 2 mM toluene solution of [Cu(PPh₃)₃I] with (a) 0.66 M and (c) 1.32 M of Na₂S solution. (b and d) Histograms of particle size distribution.

Fig. 7 TEM image of the film collected after 24 h from a liquid–liquid interface containing 8 mM toluene solution of [Cu(PPh₃)₃I] and 5 ml of 1.32 M Na₂S solution.

XPS was used to determine the composition of the as-prepared Cu(0)/CuS nanoneedles. The XPS spectrum indicates the purity and the presence of sulfur, copper and carbon from the reference. For the Cu 2p_3/2 peaks of Cu(0)/CuS nanoneedles, Cu(0) and Cu(II) were observed at 932.41 and 934.70 eV, respectively (Fig. 8). The contents of Cu(II) and Cu(0) are 74.6% and 25.4%, respectively.

Fig. 8 Cu 2p XPS spectrum of Cu(0)/CuS nanoneedles.

3.2. Catalytic evaluation

The changes in the spectrum during a typical experiment and a typical set of absorbance–time curves for the dye degradation reaction are shown in Fig. 9 and S2–S5,† respectively, for MB, MG, BG, MR and MO. For example, as shown in Fig. 9a, the absorbance (A) decreases with time for MB that the characteristic absorption peak at 663 nm is used to monitor the reaction of dye degradation. The catalytic activity of Cu(0)/CuS nanoneedles is estimated by monitoring the successive decrease in absorption intensity of MB at 663 nm in the absence and presence of H₂O₂, as shown in Fig. 9c and d, respectively.

Fig. 9 Degradation of MB dye (a) plot of absorbance (A) vs. wavelength and the inset image indicates the decolorisation reaction in the absence of H₂O₂, (b) plot of absorbance (A) vs. wavelength and the inset image indicates the decolorisation reaction in the presence of H₂O₂, (c) plot of absorbance (A) vs. time (t) in the absence of H₂O₂ and (d) plot of absorbance (A) vs. time (t) in the presence of H₂O₂.

The spectrum of the initial solution of MR (in the absence of H₂O₂) shows maximum absorption peak located at 427 nm, which originates from the strong molecular absorption of MR (Fig. S4b, ESI†). The MR band at 427 nm was shifted in the 522 nm after the addition of H₂O₂ to the MR solution (Fig. S4b, ESI†). Hydrogen peroxide is a weak acid with strong oxidizing properties. The MR dye converts to the structure 1 in the presence of H₂O₂ (Fig. 10).²¹

Fig. 10 Structure of (a) MR and (b) MR + H₂O₂.

Similarly, the spectrum of the initial solution of MO (in the absence of H₂O₂) shows maximum absorption peak which is located at 493 nm and after the addition of H₂O₂ to the MO solution the band at 473 nm was shifted (Fig. S5b, ESI†).

All of the catalytic decomposition reactions of dyes studied was pseudo-first order kinetics. The pseudo-first-order rate constants (k_obs) were evaluated by nonlinear least-squares fitting of the absorbance (A)–time (t) profiles to the first-order equation A_t = A_∞ + (A₀ − A_∞)exp(−k_obst). The values of apparent rate constants (k_app) (s⁻¹) for these reactions are summarized in Table 1.

Table 1 The rate constants k_app (s⁻¹) for the reaction of dye degradation by the Cu(0)/CuS nanoneedles at 25 °C

Dye	Peroxide	kapp (s^−1)
MB	—	1.97 × 10^−3
MB	H2O2	8.40 × 10^−3
MG	—	6.67 × 10^−4
MG	H2O2	6.82 × 10^−3
BG	H2O2	5.48 × 10^−3
MR	H2O2	1.36 × 10^−2

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The absorbencies at λ = 663 nm were collected with time and the apparent rate constant was found to be 1.97 × 10⁻³ s⁻¹ (Fig. 4c) in the absence of H₂O₂ and 8.40 × 10⁻³ s⁻¹ in the presence of H₂O₂ (Fig. 9d) for MB. The absorbencies at λ = 616 nm were collected with time and the apparent rate constant was found to be 6.67 × 10⁻⁴ s⁻¹ in the absence of H₂O₂ and 6.82 × 10⁻³ s⁻¹ in the presence of H₂O₂ for MG as shown in Fig. S2c and d (ESI),† respectively. Also, a similar procedure was employed to investigate the catalytic decomposition reaction of BG and MR as shown in Fig. S3c and S4c (ESI),† respectively. The rate constants for these reactions were found to be 5.48 × 10⁻³ s⁻¹ and 1.36 × 10⁻² s⁻¹ for the decomposition reactions of BG and MR, respectively in the presence of H₂O₂. The catalytic decomposition reactions of MR and BG (in the absence of H₂O₂) and MO (in the presence and absence of H₂O₂) were very slow and their plots of absorbance (A) vs. time (t) were not investigated.

The Cu(0)/CuS nanoneedles were employed as the catalyst for adsorption and degradation of dye molecules and the related experiments were carried out with the addition of H₂O₂. In our experiment, H₂O₂ produced highly reactive hydroxyl radicals that could oxidize dye into smaller molecules (H₂O, CO₂, etc.). The catalytic property of Cu(0)/CuS nanoneedles was related closely to the amount of hydroxyl radicals. Without the assistance of the catalyst, H₂O₂ was unable to degrade dye solutions alone. This observation suggests that the presence of the catalyst can significantly increase the dye degradation. The catalytic properties of the CuS were investigated by the UV-Vis absorption spectra, as shown in Fig. 9 and S2–S5 (ESI†). It shows the efficient catalysis of H₂O₂ to release ˙OH and ˙O₂⁻ species and degrade dyes in a short time. The reaction mechanism is described in Fig. 11. The effective band gap (1.85 eV) differs from the actual band gap (2.2 eV) for CuS.²² As a result the effective band gap of Cu(0)/CuS decreases (1.45 eV). Now, when CuS NPs get excited by the incoming photon during the photocatalysis, the excited electron (e⁻) goes to the conduction band creating hole (h⁺) in the valence band. As the band gap of CuS is low, it is expected that the e⁻−h⁺ pair recombination will be facile. Here, Cu plays a vital role to inhibit the charge recombination by withdrawing the photogenerated electron toward itself and thus can scavenge the photogenerated electrons from the CuS NPs. As the electron in the CB of CuS is transferred to Cu, the process of charge recombination is inhibited, and Cu provokes the reaction to create hydroxyl radicals. Cu(0)/CuS nanoneedles absorb light energy under visible light illumination, resulting holes (h_VB⁺) in the VB and in electrons (e_CB⁻) in the CB (Fig. 11) (eqn (4)).²³ The h_VB⁺ can react with surface adsorbed hydroxyl groups to form hydroxyl radicals (˙OHs) on the surface of catalyst (eqn (7)).²⁴ The e_CB⁻ can react with dissolved O₂ to form ˙O₂⁻ (eqn (8)). The e_CB⁻ and h_VB⁺ react with H₂O₂ to form ˙OH and ˙O₂⁻ species (eqn (9)–(11)).²⁵ When these double bonds are exposed to free radicals, dye molecules react with free radicals and the electrons are transferred via free radicals to double bonds of the dye molecules and dissociation of dye molecules occurs, which are converted into byproducts (eqn (12)).²⁶

Cu(0)/CuS + hν → Cu(0)/CuS(h⁺–e⁻) (4)

Cu(0)/CuS(h⁺–e⁻) → Cu(0)(e⁻)/CuS(h⁺) (5)

H₂O → OH⁻ + H⁺ (6)

CuS(h⁺) + OH⁻ → ˙OH (7)

Cu(0)(e⁻) + O₂ → ˙O₂⁻ (8)

CuS(h⁺) + H₂O₂ → ˙OOH + H⁺ (9)

Cu(0)(e⁻) + H₂O₂ → ˙OH + OH⁻ (10)

˙OOH → ˙O₂⁻ + H⁺ (11)

Dye + ˙OH + ˙O₂⁻ → degradation products (12)

Fig. 11 Mechanism of the dye degradation on Cu(0)/CuS nanoneedles.

There is also a Fenton process in operation whereby Cu²⁺ reacts with H₂O₂ to also generate hydroxyl radicals and is another source of reactive species. A brief description of the reaction mechanism is described below:²⁷

Cu²⁺ + H₂O₂ → Cu⁺ + HO₂˙ + H⁺ (13)

Cu⁺ + H₂O₂ → Cu²⁺ + HO⁻ + HO˙ (14)

Hydroxyl radicals play a significant role in the dye degradation process. To investigate the role of hydroxyl radicals, the degradation process was carried out in the presence of hydroxyl radical scavengers like thiourea and ethanol. Thiourea and ethanol are capable of deactivating the hydroxyl radicals and its derivatives.²⁸ When the thiourea (0.2 ml, 221 mg ml⁻¹) was used as a scavenger, the rate decreased from 8.40 × 10⁻³ s⁻¹ to 1.38 × 10⁻³ s⁻¹ for degradation of MB dye. The rate of the degradation reaction of MB dye decreased from 8.40 × 10⁻³ s⁻¹ to 8.96 × 10⁻⁴ s⁻¹ in the presence of ethanol (1 ml) as a scavenger. The effect of a variety of ⁻OH scavengers was determined. These compounds are listed in Table 2. The rate constants decrease in the presence of more concentration of hydroxyl radical scavengers.

Table 2 Effect of ˙OH scavengers on the rate constants k_app (s⁻¹) for the reaction of degradation of MB in the presence of 0.5 ml H₂O₂

˙OH scavenger	[Scavenger]	kapp (s^−1)
Thiourea	0.1 ml, 221 mg ml^−1	7.36 × 10 ^−3
Thiourea	0.2 ml, 221 mg ml^−1	1.38 × 10 ^−3
Ethanol	470 μl	6.54 × 10 ^−3
Ethanol	1000 μl	8.96 × 10 ^−4

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As depicted in Fig. 12, the removal of MB during the first catalytic run could be achieved above 90% after 630 s. After six recycles for the catalytic degradation of MB, the catalytic activity of Cu(0)/CuS nanoneedles just slightly decreased. As can be seen in Fig. 12, the degradation efficiency of Cu(0)/CuS nanoneedles can reach up to 84% after 630 s even if the catalysts had been utilized for several times, which indicates that this catalyst can maintain good stability.

Fig. 12 The cyclic utilization of the as-prepared Cu(0)/CuS nanoneedles for the degradation of MB with the addition of H₂O₂ for 630 s.

4. Conclusion

In summary, the interfacial disproportionation of copper(I) complex allows the low-cost and facile preparation of Cu(0)/CuS nanoneedles at the liquid–liquid interface. The Cu(0)/CuS nanoneedles show a high catalytic activity in the reaction of dye degradation. In the presence of H₂O₂, it is observed that the kinetic of degradation reaction is quite faster for the molecules of neutral dye (k_app = 1.36 × 10⁻² s⁻¹ for MR) than cationic dyes (k_app = 8.40 × 10⁻³ s⁻¹ for MB, k_app = 6.82 × 10⁻³ s⁻¹ for MG and k_app = 5.48 × 10⁻³ s⁻¹ for BG) or anionic dye (very slow for MO) (see Fig. 9 and ESI Fig. S2–S5†). In the absence of H₂O₂, it was found that the degradation process is quite faster for cationic dyes like (k_app = 1.97 × 10⁻³ s⁻¹ for MB and k_app = 6.67 × 10⁻⁴ s⁻¹ for MG) but quite slow for anionic dyes like MO (see Fig. 9 and ESI Fig. S2–S5†). The cationic dye molecules embrace the catalyst surface by the electrostatic field force and electron transfer and formation of free radicals on the surface of the catalyst is facilitated. For neutral and anionic dye molecules this effect is not operative as such. The structure of 1 include positive charge on nitrogen, that make easier the dye degradation by Cu(0)/CuS nanoneedles.

Acknowledgements

We thank the Yasouj University Research Council and the Iranian Nanotechnology Initiative Council for their support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14995b

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