Microwave-Assisted Synthesis of Porous Aggregates of CuS Nanoparticles for Sunlight Photocatalysis.

Solvated two-dimensional nanosheets of copper hydroxy dodecylsulfate in 1-butanol react with thiourea under microwave irradiation to yield surfactant-free porous aggregates of CuS nanoparticles. These aggregates exhibit excellent photocatalytic activity toward degradation of methylene blue, methyl orange, and 4-chlorophenol in natural sunlight. While the high surface area (14.74 m2 g-1) and porosity increase the active reaction centers for adsorption and degradation of organic molecules, quantum confinement results in a low recombination of photogenerated electrons and holes. Chemical and photogenerated hydroxyl radicals cause the oxidation of the dyes and 4-chlorophenol.

Introduction

Copper sulfide (hexagonal covellite, CuS) is a p-type semiconductor with a band gap of ∼2 eV that enables it to absorb a large fraction of the solar spectrum.[1,2] In addition, a high concentration of charge carriers due to copper vacancies leads to plasmonic absorption in the infrared region.[3−8] Thus, CuS nanostructures are of interest for photovoltaic[1,2,9−13] and photocatalytic applications.[1,2,14−22] The morphology, size, and surface area of CuS nanostructures control its photocatalytic efficiency.[1,2] Porous nanostructures with a large active surface area are highly desired for catalytic applications.[23−25] Coprecipitation, solvothermal reactions, or hot injection synthesis routes yield monodispersed or hierarchical self-assembled CuS nanostructures.[1,2] In general, porous nanostructures are usually synthesized with the aid of templates such as block copolymers, surfactant mesophases, and lyotropic liquid crystals.[26,27] The templates or pore-directing agents are eventually removed by chemical or thermal methods.[23−27] Sacrificial template-directed chemical transformation based on the Kirkendall effect is known to yield porous hollow CuS nanostructures.[14,28−33] Xu et al.[34] reported the synthesis of nanoporous CuS through dealloying of Ti–Cu alloy using 15 M H2SO4. Facile and efficient synthesis of porous CuS is still a challenge.

Exfoliation of layered transition metal hydroxides yields colloidal dispersions of solvated 2-D metal hydroxide layers.[35,36] These nanosheets can be chemically reacted at room temperature or under solvothermal conditions to yield nanostructures of metals and metal oxides.[36−40] In contrast to multistep reactions involving metal salts/complexes and surfactants, a single-step reaction employing solvated hydroxide nanosheets as a precursor results in unusual shapes and interesting properties.[36] Although we have employed this method to synthesize nanostructures of metal oxides and metals (through solvothermal reduction), we have not explored the possibilities of reacting the hydroxide nanosheets with suitable reagents to form compounds other than metals and metal oxides. Would it be possible to treat the metal hydroxide nanosheets with a chalcogenide source to form metal chalcogenide nanostructures? Because the decomposition of hydroxide sheets[41] prior to sulfidation is expected to result in a porous structure, metal sulfide formed could also be porous. Keeping in mind the need for developing simpler routes to porous CuS, and to explore the possibilities of extending the use of solvated metal hydroxide nanosheets in the synthesis of chalcogenide nanostructures, we have studied the synthesis of CuS nanostructures using solvated nanosheets of copper hydroxy dodecylsulfate.

Facilitation of recycling and reuse of water demands a sustainable approach for waste water treatment. Advanced oxidation processes (AOP) are efficiently able to destroy the persistent organic pollutants through Fenton reactions, wherein strongly oxidizing hydroxyl radicals (·OH) generated in situ with the aid of primary oxidants (ozone, hydrogen peroxide, oxygen) and/or energy sources (ultraviolet light) or catalysts (Fe2+, titanium dioxide) nonselectively oxidize the organic compound to carbon dioxide and water.[42] AOP also encompass techniques that involve other reactive oxidants, such as SO4·– and Cl· and also those that enable direct electron transfer. Homogeneous AOP that involve activation of H2O2 to produce ·OH through dissolved Fe2+/Fe3+ redox pair suffer from fundamental drawbacks such as retarded reduction kinetics of the redox cycle and pH-dependent efficiency. Nanomaterials (2-D layered solids, semiconductor metal oxides/sulfides) with inherent properties such as rapid charge transfer, large surface area, optimal charge diffusion, and path length have been explored as heterogeneous catalysts[43−46] in AOP to overcome these drawbacks. In this context, porous aggregates of CuS nanoparticles are explored as a photocatalyst in photochemical degradation of organic dyes (methylene blue, methyl orange) and carcinogenic 4-chlorophenol under natural sunlight.

Results and Discussion

The XRD pattern of layered copper hydroxy dodecylsulfate (Figure A(a)) shows a series of 00l reflections, indicating that the hydroxy salt is well ordered along the stacking direction. The basal spacing of 2.9 nm calculated from the 00l reflections confirms the interlayer incorporation of dodecylsulfate ions.[37] The absence of in-plane reflections (Figure A(a)) indicates an oriented growth along the c direction. The SEM image of the as-prepared copper hydroxy dodecylsulfate (Figure B) exhibits a layered morphology with platelets of varying lateral sizes.

Figure 1

(A) XRD patterns of (a) copper hydroxy dodecylsulfate and (b) porous aggregates of CuS nanoparticles obtained upon microwave irradiation of the colloidal dispersion of copper hydroxy dodecylsulfate in the presence of thiourea. (B) SEM image of the as-prepared layered copper hydroxy dodecylsulfate.

Copper hydroxy dodecylsulfate exfoliates in 1-butanol, resulting in a colloidal dispersion of mono- to few-layer-thick nanosheets of varying lateral sizes.[37] Upon microwave heating, the mixture of copper hydroxy salt nanosheets and thiourea yields a black product. The XRD pattern of the product (Figure A(b)) could be indexed to an hexagonal covellite CuS phase with cell parameters a = 3.79 Å and c = 16.34 Å (JCPDS 78-2121). Peaks due to impurities are not observed, indicating the formation of a pure CuS phase. The average crystallite size calculated from XRD peak broadening is ∼10 nm. The infrared spectrum of CuS (Figure S1b, Supporting Information) indicates that it is free of the surfactant dodecylsulfate.

The chemical composition of porous aggregates of CuS was further probed by X-ray photoelectron spectroscopy (XPS). Core-level Cu 2p spectra (Figure a) exhibit peaks at 932.35 and 953.16 eV, indicating the presence of Cu2+ in CuS.[48] Satellite peaks at 938.43 and 959.37 eV correspond to the paramagnetic chemical state of Cu2+.[48] Binding energies of 161.69 and 162.89 eV in the case of S 2p core-level spectra (Figure b) indicate the presence of S2–.

Figure 2

XPS spectra showing Cu 2p (a) and S 2p (b) core-level peak regions of porous aggregates of CuS.

The morphology of CuS was investigated by scanning and transmission electron microscopy. The SEM image (Figure S2, Supporting Information) indicates that CuS largely consists of aggregates of roughly spherical nanoparticles. Low-magnification (Figure a,b) and high-magnification (Figure c) bright-field TEM images of CuS reveal that nanoparticles of ∼10 nm form a porous network. The HRTEM image of porous CuS nanostructures (Figure d) clearly shows that the nanoparticles are single crystals exhibiting fringes due to 103 planes.

Figure 3

(a–c) Bright-field TEM images and (d) HRTEM image of CuS nanostructures obtained upon microwave irradiation of the colloidal dispersion of copper hydroxy dodecylsulfate in the presence of thiourea.

The mechanism of formation of porous aggregates of CuS nanoparticles is schematically depicted in Figure . Upon microwave heating, the thermally unstable copper hydroxide nanosheets undergo dehydroxylation and simultaneous sulfidation in the presence of an excess of S2– released upon decomposition of thiourea. The rapid dehydroxylation of the hydroxide layers results in a porous network of nanoparticles of uniform sizes. Atomic force microscopy images of copper hydroxy dodecylsulfate layers and porous CuS (Figure S3, Supporting Information) corroborate the mechanism.

Figure 4

Schematic representation of formation of porous aggregates of CuS nanoparticles.

The surface area and pore size of CuS nanostructures were obtained from nitrogen adsorption–desorption isotherms (Figure a) using the multiple-point BET method. While the specific surface area was measured using the lower part of the adsorption isotherm, the desorption curve of the isotherm was used for pore size analysis. While the surface area of 14.74 m2 g–1 of our nanoporous CuS is lower than that reported for porous CuS by Xu et al.,[34] the pore volume of 11 × 10–2 cm3 g–1 and pore radius of 2.12 nm are comparable with that reported in the literature for nanoporous CuS.[48]

Figure 5

(a) Nitrogen adsorption–desorption curves, (b) UV–visible (diffuse reflectance) spectrum, and (c) Tauc plot of porous aggregates of CuS obtained upon microwave irradiation of the colloidal dispersion of copper hydroxy dodecylsulfate in the presence of thiourea.

Porous aggregates of CuS nanoparticles exhibit strong absorption in the entire visible region (Figure b). Absorption beyond 800 nm (near-IR region) is attributed to the interband transitions of free carriers from valence to unoccupied states.[3−8,49] Intense absorption in the entire visible range implies that porous CuS would be a good visible light harvester. The optical energy band gap (Eg) is determined from the optical spectra by extrapolating the linear region of the plot of (αhν)[2] versus hν (Figure c), where hν is the photon energy, α is the absorption, h is the Planck’s constant, and ν is the frequency. Eg is estimated to be 2.0 eV, which is comparable to the band gap of CuS reported in the literature.[48]

The photocatalytic activity of porous aggregates of CuS nanoparticles in the degradation of MB, MO, and 4-chlorophenol was evaluated under natural sunlight. In all cases, prior to photocatalytic degradation, adsorption–desorption equilibrium in the dark indicates negligible adsorption. Figure a–c represents time-resolved UV–visible absorption spectra of photocatalytic degradation of MB, MO, and 4-chlorophenol, respectively. In each of the cases, the intensity of the prominent absorption decreases with time. The log (absorbance) versus time plots (Figure d,e) of MB and 4-chlorophenol degradation indicate a pseudo-first-order kinetics with rate constants of 4.6 × 10–1 and 4.5 × 10–2 min–1, respectively. Over several degradation cycles (Figure f), porous CuS exhibits consistent catalytic activity with no significant change in morphology, composition, or surface area (Figure S4, Supporting Information).

Figure 6

Degradations of dyes (10 mg L–1) and 4-chlorophenol (100 mg L–1) were traced through UV–visible absorption spectra of reaction mixtures containing 15 mg of porous CuS aggregate as photocatalyst, 1 mL of 30% H2O2, and dye/4-chlorophenol. Evolution of absorption spectra with time in the case of (a) MB, (b) MO, and (c) 4-chlorophenol. Log (absorbance) versus time plots for (d) MB and (e) 4-chlorophenol degradations. (f) Catalytic efficiency over repeated cycles in the case of MB degradation.

Table compares the photocatalytic degradation of methylene blue or 4-chlorophenol in the presence and absence of the oxidizer H2O2. Since water and adsorbed oxygen on the surface of CuS may produce[34,42,48] reactive ·OH and ·O2–, one would expect the degradation to occur even in the absence of H2O2. Although this is the case, the conversion percentages are quite low (71% in 60 min for MB and a poor 30% in 60 min for 4-chlorophenol). When H2O2 is added, the degradation kinetics improves dramatically (∼100% for MB in 10 min and 93% for 4-chlorophenol in 60 min). This is due to facile formation of ·OH radicals from H2O2,[34,42,48] which increases the net oxidizing species concentration in solution. In addition, H2O2 accepts the photogenerated electrons from CuS, thus preventing electron–hole recombination.[34,42,48] In the absence of a catalyst, H2O2 is inefficient in photochemical degradation of the pollutants (36% degradation of MB in 60 min). Even in the dark, CuS catalyzes the degradation of the pollutants. However, the conversion percentages are quite low.

Table 1

Effect of H2O2 on the Sunlight Photocatalytic Degradation of Dye/4-Chlorophenol in the Presence of Porous Aggregates of CuS Nanoparticles

		degradation
		no catalyst		catalyst
		sunlight		dark		sunlight
		with H2O2		with H2O2		without H2O2		with H2O2
pollutant	conc (mg L–1)	time (min)	%	time (min)	%	time (min)	%	time (min)	%
methylene blue	10	60	36	60	62	60	71	10	100
4-chlorophenol	100	60	∼0	60	26	60	30	60	93

The time taken for complete MB degradation decreases with the mass of the catalyst (Figure a). Nonlinear dependency appears to be exponential with saturation beginning at 15 mg of the catalyst. One of the requisites of an ideal catalyst is its ability to catalyze degradation of organics under all pH conditions. The effect of pH on the degradation of MB (Figure b) clearly indicates that porous aggregates of CuS nanoparticles effectively catalyze dye degradation in a wide pH range of 3–8. Spontaneous decomposition of H2O2 at pH > 8 leads to poor generation of ·OH, thereby lowering the rate of dye degradation.

Figure 7

Variation of the photocatalytic MB (100 mL, 10 mg L–1) degradation efficiency with (a) mass of porous aggregates of CuS nanoparticles as catalyst and (b) pH.

In comparison to what has so far been reported in the literature (Table ), the enhanced photocatalytic activity under ambient conditions in natural sunlight renders porous aggregates of CuS nanoparticles not only a superior catalyst but also a desired material for environmental amelioration. Porous aggregates of CuS nanoparticles as a catalyst outperform most of CuS-based nanocatalysts in MB degradation and are far superior in MO degradation compared to CuS nanoflowers. More interestingly, their performance in 4-chlorophenol degradation is over 30 times better than that of hierarchical CuS.

Table 2

Comparison of Photocatalytic Activities of Porous Aggregates of CuS Nanoparticles with Other CuS Nanostructures Reported in Literature

				degradation
catalyst	BET surface area (m2/g)	mass of catalyst (mg)	pollutant conc in ppm (volume in mL)	irradiation source	time (min)	%	reference
methylene blue
CuS porous aggregate	14.74	15	10 (100)	sunlight	10	98	present
CuS (commercial)		15	10 (100)	sunlight	60	90	present
CuS nanocaved	13.60	10	10 (10)	visible	5	95	(50)
Cu1.8Se-CuS		50	10 (30)	UV	12	96	(51)
CuS nanoporous	28.70	10	1000 (6)	Xe (500 W)	16	98	(34)
CuS-ZnO		100	5 (50)	Xe (500 W)	20	63	(52)
CuS flowers	9.27	5	5 (30)	visible	25	98	(53)
CuS nanotube		10	6 (40)	visible	25	98	(54)
CuS hierarchical	14.77	25	6 (40)	sunlight	30	99	(55)
CuS nanosheet	36.00	20	12 (100)	sunlight	48	93	(56)
CuS-rGO		30	20 (50)	W (150 W)	60	99	(57)
TiO2-CuS		30	6 (100)	Xe (500 W)	60	90	(58)
CuS nanoplates	30.43	30	6 (40)	visible	120	53	(59)
CuS-rGO		1	4 (10)	Xe (500 W)	120	81	(49)
methyl orange
CuS porous aggregate	14.74	15	10 (100)	sunlight	15	98	present
CuS (commercial)		15	10 (100)	sunlight	60	79	present
CuS flowers	9.27	5	5 (30)	visible	25	8	(34)
Cu1.94S-rGO	110.00	10	10 (400)	LED (18 W)	60	79	(60)
CdS-CuS	31.7	30	10 (50)	Xe (500 W)	150	93	(15)
4-chlorophenol
CuS porous aggregate	14.74	15	100 (100)	sunlight	60	93	present
CuS (commercial)		15	100 (100)	sunlight	60	38	present
CuS hierarchical	15.30	50	100 (50)	metal halide	300	83	(48)
carbon nitride	176.0	40	15 (80)	Xe (300 W)	60	100	(61)

The efficient degradation of the different dyes and 4-chlorophenol indicates that porous aggregates of CuS nanoparticles are an effective photocatalyst with the ability to utilize a large fraction of the solar spectrum. In addition, the porous network of fine CuS particles induces a large density of low coordinated atoms on the surface and edges.[48] Defective high-energy CuS surfaces favor fast ionic transfer between the surface and interior, facilitating high reactivity between the reactive species and pollutants.[48]

Conclusions

Microwave irradiation of solvated 2-D copper hydroxy salt nanosheets in the presence of thiourea yields porous aggregates of CuS nanoparticles. The successful formation of CuS nanostructures of interesting morphology suggests that exfoliated metal hydroxide monolayers can be utilized as precursors in the synthesis of metal chalcogenide nanostructures. Metal hydroxide 2-D nanosheets act as self-templates yielding a porous network with a large surface area. Porous aggregates of CuS nanoparticles exhibit superior visible light catalytic activity and excellent recyclability in the degradation of dyes and 4-chlorophenol.

Experimental Section

Synthesis of Copper Hydroxy Dodecylsulfate

Layered copper hydroxy acetate, Cu2(OH)3(OOCCH3)·H2O, was prepared according to the method reported by Yamanaka et al.[47] Fifty milliliters of 0.1 M NaOH was added dropwise to 50 mL of 0.094 M cupric acetate solution with continuous vigorous stirring. The blue precipitate obtained was washed with copious quantities of water followed by acetone and dried at 65 °C. This solid was subjected to an anion exchange reaction to obtain copper hydroxy dodecylsulfate, as reported in our earlier work.[37] Copper hydroxy acetate (0.8 mmol, 0.2040 g) was added to a 30 mL solution containing 9.76 mmol (2.8145 g) sodium dodecylsulfate, and the mixture was stirred at room temperature for 6 days. The resultant solid was separated by centrifugation, washed with water followed by acetone, and dried at 65 °C.

Synthesis of Copper Sulfide Nanoparticle Aggregates

Copper hydroxydodecylsulfate (0.2 g) was exfoliated in 100 mL of 1-butanol by sonication for 1 h at room temperature to form a colloidal dispersion.[37] Thiourea (0.1 g) was added to this dispersion, and the mixture was sonicated for 5 min. The resulting mixture was subjected to microwave (MW) heating for 5 min in a domestic microwave oven (2.45 GHz) operated at 800 W. At the end of heating, the solution was cooled to room temperature. The black product was isolated by centrifugation, washed with a mixture of ethanol and water (1:1, v/v) followed by acetone, and dried in vacuum at room temperature.

Photocatalytic Degradation of Organic Pollutants

Porous aggregates of CuS nanoparticles were used as a catalyst in photochemical degradation of organic dyes, methylene blue (MB) and methyl orange (MO). Fifteen milligrams of CuS was dispersed in 100 mL of MB/MO solution (10 mg L–1), and the mixture was stirred in the dark for 1 h to establish adsorption–desorption equilibrium. One milliliter of 30% H2O2 solution was added, and the mixture was allowed to react under irradiation by natural sunlight and constant stirring. Degradations of MB and MO were monitored by measuring the absorbance of MB at 664 nm and that of MO at 464 nm. Porous CuS (15 mg) was also used in natural sunlight photocatalytic degradation of 4-chlorophenol (100 mL, 100 mg L–1) in the presence of 1 mL of 30% H2O2.

Characterization

All samples were characterized by recording powder X-ray diffraction (XRD) patterns using a PANalytical X’Pert Pro diffractometer (Cu Kα radiation, secondary graphite monochromator, scanning rate of 1° 2θ/min). Scanning electron microscopy (SEM) analysis was carried out using a Zeiss Ultra 55 field-emission scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (EDS). Transmission electron microscopy (TEM) images were acquired with a Tecnai T20 operated at 200 kV. UV–visible spectra of the reaction mixtures were recorded on a Perkin Elmer (LS 35) UV–Visible spectrometer. The nitrogen physisorption isotherms were recorded at 77 K using a BELSORP system after degassing of samples at 60 °C for 3 h.