CuS hierarchical hollow microcubes with improved visible-light photocatalytic performance

Abstract

In this report, a simple Kirkendall-based process using freshly prepared Cu₂O microcubes as sacrificial templates has been successfully demonstrated for the fabrication of CuS hierarchical and hollow micro/nanocubes (HHCs). Individual CuS HHCs were assembled with plenty of nanosheet-like building units. CuS HHCs showed improved photocatalytic activity in the degradation of methylene blue (MB) in the presence of H₂O₂. Under visible light irradiation, the photocatalytic activity of CuS HHCs was 3.7 and 5.8 times higher than those of Cu₂O/CuS and Cu₂O, respectively. Furthermore, they also showed good recyclability in the visible region. It appears that the enhanced photocatalytic performance of the hierarchical and hollow micro/nanostructures can be ascribed to the synergetic effect of the efficient light harvesting and high surface area of the CuS HHCs. The facilely prepared CuS HHC products are promising materials in the fields of photocatalytic and optoelectronic applications.

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1 Introduction

The complex/novel structures of inorganic materials from nanoscale to microscale size, including two dimensional, hierarchical and hollow structures, are of interest to chemists and materials scientists,^1–4 because of their unique structure-induced chemical and physical properties that may bring a series of opportunities for their potential application as sensors,^5–7 coatings, photoelectric devices^8,9 and chemical catalysis.^10–12 Specific structures may correspond to distinct performance,¹³ and a single material with complex structure, or increasing the structural complexity of the material, may realize the invention of some unique optical, electrical, and surface properties. As expected, various methods, such as hard templates, soft templates, as well as physical/chemical processes based on the Kirkendall effect, Ostwald ripening, chemically induced self-transformation, and so on,^14–16 have been devoted to constructing complex/hollow structures for different properties and applications. Among these methods, the sacrificial template-direct chemical transformation method based on the Kirkendall effect has been demonstrated to be an effective approach.^17,18

As an important transition metal semiconductor, copper sulfide (CuS) with complex phase structures and valence states has been the focus of intense interest due to not only its optical electrical properties but also its potential applications in photocatalysis,^19,20 as a cathode material,²¹ optical filter,²² and nonlinear optical material applications in thermoelectric cooling materials²³ etc. Up to now, a variety of CuS nanostructures such as nanoparticles, nanowires, nanotubes, hollow spheres, snowflake-like and hierarchical structures have been fabricated. Recently, many efforts have been developed to the synthesis of CuS hollow micro/nanostructures. For example, Xu et al.²⁴ reported the synthesis of hollow spherical copper sulfide nanoparticle assemblies using 2-hydroxypropyl-β-cyclodextrin as a template in the presence of sonication. Li et al. prepared porous hollow microspheres of CuS under hydrothermal conditions by treating a copper(I)–L-cysteine complex formed from CuCl₂·2H₂O and L-cysteine at 160 °C for 12 h,^25,26 and observed that the absorption ranges of the CuS hollow microspheres were narrower and more obvious than those of the CuS snowflake-like patterns and CuS flower-like microspheres. By a microwave-assisted Cu-complex transformation route, Liu et al.²⁷ fabricated uniform nanoporous CuS nanotubes consisting of nanoparticles. The CuS nanotube-modified electrode showed excellent electrocatalytic activity towards the oxidation of glucose. Tanveer et al.²⁸ synthesized CuS hollow microspheres with enhanced photocatalytic activity through a PVP derived solvothermal technique. The CuS microspheres had spherical architectures with various sizes and shapes. Jiao et al. obtained well-defined non-spherical Cu₂S and Cu₇S₄ mesocages with single-crystalline shells by using shape-controlled Cu₂O crystals as sacrificial templates based on the Kirkendall effect.²⁹ However, to the best of our knowledge, the simple and shape controllable synthesis of hierarchical hollow microcubes of CuS, which can exhibit excellent physical and chemical properties, have rarely been reported. Consequently, it is necessary to develop a facile method to fabricate CuS hierarchical hollow microcubes consisting of nanosheets as building blocks.

Herein, we present a facile chemical route for the synthesis of copper sulfide hierarchical hollow microcubes (HHCs) with nanosheet building blocks at room temperature, exploiting uniform cubic Cu₂O as both precursor and sacrificial templates. Our method not only offers a simple approach to synthesize copper sulfide HHCs but opens a low cost route for the synthesis of micro/nanostructures. Furthermore, the copper sulfide HHCs showed an effective catalytic property and stability by the degradation of methylene blue (MB) as the model contaminant under visible light irradiation. Finally, the possible mechanism and kinetics of H₂O₂ assisted visible light photocatalytic activity using copper sulfide HHCs as the catalyst were also discussed.

2 Experimental

2.1 Preparation of Cu₂O cube templates

All of the chemicals were of analytical purity and used as received without further purification. Cu₂O was fabricated by the typical approach reported previously.³⁰ Firstly, a reaction solution composed of copper sulfate (2.0 mL of 0.68 M), PVP (0.5 mM, K-30, M_w = 30 000) and 33.2 mL distilled water was prepared in a glass flask by constant strong stirring for 15 min, and then 2.0 mL of mixture solution (0.74 M sodium citrate and 1.2 M anhydrous sodium carbonate) was dropped into the above reaction solution, resulting in a dark blue color change. After about 10 min, 2.8 mL of a 1.0 M L/D-glucose solution was slowly injected into the reaction with further stirring for 15 min; lastly, the reaction solution was immersed in a water bath at 80 °C for 0.5 h to drive the reduction process. The as-synthesized dark orange material was collected, rinsed with distilled water and ethanol several times, and dried in a vacuum at 60 °C overnight. The as-synthesized sample (Cu₂O) was obtained. By comparison, the varied PVP concentrations were also investigated while maintaining other parameters, and the corresponding different Cu₂O crystals with various morphologies were prepared.

Synthesis of CuS hollow cubes. Typically, 0.1 g of as-prepared Cu₂O microcubes was ultrasonically dispersed in 40 mL of water, and then the mixture solution was added to Na₂S aqueous solution (100 mL, 0.015 M) under magnetic stirring at room temperature. A red solution firstly appeared, which then changed to gray, with further stirring for 20 min. The obtained intermediates (the intermediate sample, Cu₂O/CuS) were collected and rinsed with distilled water several times. Then, the intermediates were transferred into a beaker with sodium citrate solution (150 mL, 0.08 M), with further continuous stirring for 8 h. The precipitate (the final sample, CuS) was collected and washed repeatedly with distilled water and ethanol, and then dried under vacuum overnight. Moreover, CuS powders of the smashed CuS HHCs were obtained using a ball-milling machine (QM-3SP2) for 4 h (ball mill speed of 30 rpm, the ball-milling condition was very mild to minimize the ball-milling induced generation of different defects).

2.2 Characterizations

The structures and purity of the as-prepared products were characterized by X-ray powder diffraction (XRD; Rigaku D/Max-2550 PC) with Cu Kα radiation. The morphology and size of samples were observed using a scanning electron microscope (SEM, S-4800) with an X-ray energy dispersive spectrometer (EDS) and a JEM-2100F high-resolution transmission electron microscope. X-ray photoelectron spectroscopy (XPS, PHI-5000C ESCA system, Perkin Elmer) analysis was used to determine the chemical binding states of the constituent elements. UV-vis diffuse reflectance spectroscopy was performed on a Perkin Elmer Lambda 35 spectrophotometer, using BaSO₄ as the reference.

2.3 Photocatalytic activity experiments

Methylene Blue (MB) is a popular probe molecule in heterogeneous catalytic reactions because it is a typical dye resistant to biodegradation and direct photolysis. For the evaluation of catalytic activity, degradation experiments of the MB dye with/without the assistance of hydrogen peroxide (H₂O₂) were carried out under visible light at ambient temperature. The experimental procedures are as follows: 20 mg of sample was added into the solution composed of H₂O₂ (1.3 mL, 30%, w/w) and MB solution (50 mL, 10 mg L⁻¹) to form a dispersion. Afterwards, the dispersion was kept in the dark under magnetic stirring for 30 min to ensure adsorption/desorption equilibrium. Then, the dispersion under magnetic stirring was placed approximately 10 cm below a xenon lamp (500 W, Model PLS-SXE300) with a cut-off filter that only emits visible light (λ > 400 nm). At each sampling time (5 min), the irradiation was switched off and about 3.5 mL of the dispersion was taken and centrifuged. The absorbance spectrum of the MB solution was analyzed using a UV-1901 spectrophotometer (maximum absorption peak at 663 nm). After testing, the solution was returned and the irradiation was resumed. Also, the photocatalytic activity of the samples was measured under solar light irradiation simulated by a Xe lamp without a filter while keeping the other conditions constant.

3 Results and discussion

3.1 Structure and morphology

XRD analysis was used to investigate the composition and phase purity of the samples. Fig. 1 presents the XRD patterns of the as-synthesized sample, the intermediate sample, and the final sample. All the diffraction peaks in the XRD pattern of Fig. 1a can be indexed as the cubic symmetry of cuprite (Cu₂O, JCPDS no. 05-0667), confirming that the as-synthesized sample is phase-pure cubic Cu₂O. No other obvious peaks indicating impurities due to metallic Cu and CuO are observed in the as-synthesized sample. Fig. 1b shows the XRD pattern of the intermediate sample formed by reaction of the Cu₂O with the Na₂S solution. Compared with pure Cu₂O, the intensity of Cu₂O diffraction peaks obviously decreases in the intermediate sample, and we can also clearly see that there are five characteristic diffraction peaks at around 27.1°, 29.3°, 31.8°, 47.9° and 58.7°, corresponding to the (100), (102), (103), (110) and (116) crystal planes of the hexagonal phase of CuS, respectively. It shows that the intermediate sample is composed of Cu₂O and CuS. The XRD pattern in Fig. 1c illustrates that the final sample was prepared by reaction of Cu₂O/CuS with sodium citrate. The peak corresponding to Cu₂O completely disappeared, and all diffraction peaks match well with the standard data of pure CuS (JCPDS no. 06-0464). XRD spectra help us to conclude that there is a chemical reaction and transformation of Cu₂O to CuS, and the above XRD results demonstrate that the samples of Cu₂O, Cu₂O/CuS and CuS have been successfully prepared by our experimental method.

Fig. 1 XRD patterns of (a) the as-synthesized Cu₂O sample, (b) the intermediate sample (Cu₂O/CuS), and (c) the final sample (CuS).

The composition and purity of the final sample (CuS) were further investigated by XPS analysis. Fig. 2a shows the XPS spectrum of Cu2p and it obviously displays that the binding energies of Cu2p_3/2 and Cu2p_1/2 are centered at 933.1 and 953.2 eV, respectively, which are essentially identical binding energies for the Cu2p orbital in accord with Cu(II).^31,32 The XPS spectrum of S2p in Fig. 2b represents a peak at 161.6 eV, which is attributed to the S²⁺ species.³³ The Cu/S ratio calculated in this case is 0.98, which is very close to the stoichiometry of CuS.

Fig. 2 Cu2p (a) and S2p (b) XPS spectra of the final sample (CuS).

The morphology, size and micro/nanostructure of the products were observed in detail from SEM images. A low-magnification SEM image (Fig. 3a) shows that the as-synthesized sample of Cu₂O consists of well-defined and uniform cubes with an average edge length of ∼800–900 nm. The inset of Fig. 3a shows an individual microcube with smooth faces and regular shapes, having a high degree of symmetry in its structure. Fig. 3b shows the SEM images of the intermediate sample formed by the reaction of the Cu₂O microcubes with Na₂S solution. Uniform cubes similar to the Cu₂O cubes were prepared. The cubes with a rough surface have an edge length of ∼0.85–1 μm. From the broken microcubes in the inset of Fig. 3b, one can see that the microcubes of the intermediate sample are core–shell structures. The inner core is a cube with an edge length of ∼600 nm and the outer shell, with a thickness of ∼100–150 nm, is composed of many similar nanosheets as building blocks. Fig. 3c and d present SEM images of CuS cubes (the final sample), prepared by the reaction of Cu₂O/CuS with sodium citrate. The sizes of the cubes typically range from 0.9 to 1.1 μm, with an average edge length of ∼1 μm. Some partially broken microcubes in Fig. 3d clearly show a hollow interior, which indicates the formation of cubic mesocages. The shell of the CuS hollow microcubes is assembled from many closely packed nanosheets. The shell thickness of the CuS hierarchical hollow microcubes with a rough surface is about ∼100–250 nm. This information also suggests that there is a complete transformation of Cu₂O microcubes to CuS hierarchical hollow microcubes (HHCs).

Fig. 3 (a) SEM images of Cu₂O microcubes with symmetrical structure and smooth faces, (b) SEM image of Cu₂O/CuS core–shell microcubes, (c) and (d) SEM images of the final product of CuS HHCs. The scale bar in the insets is 500 nm.

In order to further characterize the morphology and crystalline structure of the as-prepared Cu₂O/CuS core–shell microcubes and CuS HHCs, TEM and HRTEM measurements were performed and are shown in Fig. 4. TEM images of Cu₂O/CuS core–shell microcubes and CuS HHCs are in good agreement with the SEM images. As shown, the TEM image of Cu₂O/CuS core–shell microcubes shows their solid cubic nature, having high symmetry in their structure, with extraordinarily rough faces (Fig. 4a). From the HRTEM image of the outer shell of Cu₂O/CuS core–shell microcubes in Fig. 4b, the lattice spacing has been calculated, which is 0.33 nm, corresponding to the (100) crystal plane of CuS. The TEM image of CuS clearly demonstrates the hollow nature of the synthesized microcubes in Fig. 4c. For every single CuS hollow cube, the contrast between the dark edge and the pale center clearly provides convincing evidence of the hollow structure. The HRTEM image of the CuS shell shows lattice spacing of 0.19 nm, which corresponds to the (110) crystal plane of CuS HHCs (Fig. 4d). Combined with the XRD and SEM results, the above observations demonstrate that the CuS HHCs were successfully fabricated by our method.

Fig. 4 (a and b) TEM images of Cu₂O/CuS core–shell microcubes, (c and d) TEM images of CuS HHCs.

This procedure has also been extended to other Cu₂O crystal templates. Cu₂O crystals with various well-defined morphologies, such as octahedral and polyhedral shapes, were chosen as sacrificial templates. Different morphologies of CuS hierarchical hollow microstructures were then obtained. CuS octahedral hollow structures were fabricated by using octahedral Cu₂O crystals as templates. Fig. 5a presents a typical SEM image of CuS hollow octahedrons with main edge lengths of ∼1.0–1.1 μm prepared by Cu₂O octahedrons (inset of Fig. 5a) with main edge lengths of ∼800–900 nm. The diffraction peaks in the XRD patterns of the CuS hollow octahedron sample can be indexed to a hexagonal CuS phase in Fig. 5c, and the result is well matched with EDS analysis in Fig. 5d. Using Cu₂O polyhedrons as the templates (inset of Fig. 5b), CuS hollow polyhedrons can be synthesized (Fig. 5b).

Fig. 5 (a and b) SEM images of CuS with different hierarchical hollow structures, the insets of (a) and (b) are SEM images of Cu₂O and the scale bars are 500 nm. (c) XRD patterns of CuS and Cu₂O, and (d) EDS spectrum of CuS octahedral hollow structures.

3.2 Formation mechanism

In order to understand the formation process of CuS HHCs, systematic investigations were carried out by means of XRD patterns and SEM images at different reaction stages in the synthetic process. Fig. 6 shows the XRD patterns of the products under different reaction stages and times. Fig. 6a shows the XRD pattern of Cu₂O microcubes as the precursor and template. After 20 min of the reaction of the Cu₂O microcubes with Na₂S solution (the intermediate sample), in the first stage (0–20 min), some diffraction peaks corresponded to the cuprite (Cu₂O, JCPDS no. 05-0667), while the other diffraction peaks were indexed to the hexagonal phase of CuS (JCPDS no. 06-0464), Fig. 6b. This indicated that the product consisted of Cu₂O and CuS. With the reaction time expanded to 1 h and 4 h (by reaction of the intermediate sample with sodium citrate), in the second stage (20 min to 8 h), the peaks belonging to the Cu₂O and CuS material were observed in Fig. 6c and d, and the relative intensities of the peaks for the CuS material became stronger. With the reaction time up to 8 h in the second stage, all diffraction peaks in Fig. 6e were assigned to hexagonal CuS crystals, and no diffraction peaks of the Cu₂O material were observed. The results indicate that the final product was pure CuS material. The results suggested that products with different composition were obtained at different stages in our experiments.

Fig. 6 XRD patterns of the product with different reaction times: (a) 0 min (Cu₂O), (b) 20 min, (c) 1 h, (d) 4 h, and (e) 8 h.

Fig. 7 shows a series of SEM images of as-obtained products collected at different stages. These SEM images clearly present the transformation process from microcubes with smooth surfaces to hierarchical hollow microcubes with many nanosheets as building blocks. As seen in Fig. 7a, only the smooth surface microcubes (Cu₂O reactant) could be observed when the reaction time was 0 min in the first stage. When the reaction time was increased to 20 min in the first stage, every Cu₂O/CuS core–shell microcube was composed of a Cu₂O microcube as the core and a microcube assembled from CuS nanosheets as the shell (Fig. 7b). When the reaction time was increased to 1 h in the second stage, Cu₂O/CuS core–shell microcubes composed of a smaller microcube as the core and a larger nanosheet assembly as the shell formed (Fig. 7c). When the reaction time was expanded to 4 h and 8 h in the second stage, hierarchical hollow microcubes with many nanosheets as building blocks were observed, as shown in Fig. 7d and 3d. These results suggested that the reaction stage/time had an important effect on the size, shape and composition of the products and showed the formation process of hierarchical hollow microcubes with many nanosheets.

Fig. 7 SEM images of the product with different reaction times: (a) 0 min (Cu₂O), (b) 20 min, (c) 1 h, and (d) 4 h.

Hence, the formation mechanism of CuS hierarchical hollow microcubes (HHCs) is proposed and illustrated in Fig. 8. A Kirkendall-based process was believed to be responsible for the transformation from Cu₂O to CuS HHCs.³⁴ In the first step of the reaction time, from 0 min to 20 min, as the solubility product constant K_sp of copper sulfide is very small (K_sp: 10⁻⁴⁸), Cu²⁺ with a low concentration can only exist on the surface of Cu₂O microcube templates, without diffusing into the body of the solution. S²⁻ reacts with the Cu²⁺ on the surface of the templates to produce CuS. The newly formed CuS exists as small flakes on the surfaces of the templates, and then forms a CuS shell with rough surfaces. The CuS shell prevents further direct chemical reaction between the inner Cu₂O microcube and the outer sulfur ion. Because the diffusion velocity of copper ions is faster than that of sulfur ions in the CuS shell,³⁵ which is named as the Kirkendall effect, the consumption rate of the Cu₂O core is larger than the production rate of CuS on the inside of the shell. As a result, the outer edge length of the formed CuS shell is larger than that of the precursor Cu₂O microcubes, while the inner edge length becomes smaller. At the same time, a gap between the Cu₂O core and the CuS shell is formed.

Fig. 8 Schematic illustration of the formation mechanism of a hierarchical hollow cube of CuS.

In the second step of the reaction time, from 20 min to 8 h, the citrate anion plays an important role in controlling the formation of hollow microcubes and copper sulfides. Citrate ligands will coordinate with copper ions to form copper–citrate complexes, which can promote the Cu₂O core solubility of the intermediate (Cu₂O/CuS).³⁶ When the concentration of sodium citrate solution is kept in a suitable range, the production of CuS on the outside of the shell is faster than that on the inside of the shell, and the Cu₂O core will dissolve completely after a period of time. As the reaction proceeds, the Cu₂O/CuS core/shell structure disappears and the CuS HHCs can be obtained. On the other hand, generally, the pH of sodium citrate solution is lower than that of Na₂S aqueous solution, which makes the oxidation ability of O₂ stronger; citrate may be a hard ligand, and complexing Cu(II) with a hard ligand will reduce the electrode potential of copper species, thus stabilizing the Cu(II) oxidation state rather than Cu(I), Cu(0) or other lower oxidation states. Therefore, Cu(I) is more easily oxidized into Cu(II) by O₂ in the solution.^36–38 The reaction process from Cu₂O to CuS can be expressed in the following equations:

2Cu₂O + O₂ + 4H₂O → 4Cu²⁺ + 8OH⁻ (1)

2Cu²⁺ + 2cit³⁻ → [Cu₂(cit)₂]²⁻ (2)

2Cu₂O + 4S⁻ + O₂ + 4H₂O → 4CuS + 8OH⁻ (3)

3.3 UV-vis spectra of the products

The UV-vis absorption spectra of the Cu₂O cubes, Cu₂O/CuS core–shell structure cubes, as well as CuS hollow cubes, are shown in Fig. 9. Cu₂O (Fig. 9a) exhibits absorption both in the UV light and visible light region, and the onset of the absorption edge for Cu₂O is 650 nm.³⁹ As observed in Fig. 9b, the Cu₂O/CuS core–shell microcubes show two broad absorption bands at 400 nm and 750 nm. In comparison with Cu₂O/CuS, the CuS HHCs clearly show a broad and strong absorption in the visible light range (λ = 550–750 nm, Fig. 9c). The band gap energy (E_g) of the CuS HHCs calculated on the basis of the corresponding absorption edge was 1.31 eV, showing a significant red-shift compared with the bulk value. It suggested that these HHCs with nanosheet building blocks might display good photocatalytic behaviors under visible light.

Fig. 9 UV-vis spectra of (a) Cu₂O microcubes, (b) Cu₂O/CuS core–shell microcubes, and (c) CuS HHCs.

3.4 Photocatalytic activity of the products

As indicated above, the result from UV-vis absorption of the CuS hierarchical hollow microcubes (HHCs) predicted their improved photocatalytic performance in visible light, and we have investigated their photocatalytic activities by choosing the photocatalytic degradation of MB in the presence of H₂O₂ under visible light. The corresponding photocatalytic properties have been demonstrated in Fig. 10. In the presence of H₂O₂, Fig. 10a shows the variation in the absorption spectra of MB solution using 20 mg of CuS HHCs as a catalyst after exposure to visible light for different durations. It could be seen that the main absorption peak is located at 663 nm, which corresponds to the MB molecules. The absorbance value of MB solution decreases rapidly with extension of exposure time, and disappears almost completely after about 30 min, suggesting that the MB solution in the presence of H₂O₂ is effectively photodegraded by the CuS HHCs.

Fig. 10 (a) Time-dependent UV-vis spectra of MB aqueous solution in the presence of CuS HHCs and H₂O₂ under visible light; (b) MB absorption in the presence of CuS HHCs and H₂O₂ under simulated solar light or visible light irradiation for 30 min; (c) line (a) is MB absorption of CuS HHCs in the dark, line (b–m) is the degradation efficiency of MB in the presence of visible light under different conditions. Line (b) without any catalyst, line (c) in the presence of commercial CuS nanoparticles, line (d) in the presence of H₂O₂, line (e) in the presence of commercial CuS nanoparticles and H₂O₂, line (f) in the presence of Cu₂O, line (g) in the presence of CuS powders of the smashed CuS HHCs, line (h) in the presence of Cu₂O/CuS, line (i) in the presence of CuS HHCs, line (j) in the presence of Cu₂O and H₂O₂, line (k) in the presence of CuS powders of the smashed CuS HHCs and H₂O₂, line (l) in the presence of Cu₂O/CuS and H₂O₂, and line (m) in the presence of CuS HHCs and H₂O₂; (d) five cycles of photocatalytic activities of CuS HHCs for the degradation of MB.

Considering the practical application of the photocatalysts, the photocatalytic performance test under simulated solar light irradiation was also carried out in the same way. Fig. 10b shows the absorption of MB solution with 20 mg of CuS HHCs in the presence of H₂O₂ after exposure to simulated solar light illumination for 30 min. The intensity of the characteristic absorption peak decreases rapidly and shifts down considerably. About 95.91% of MB was degraded after 30 min simulated solar light irradiation. Obviously, the CuS HHCs in the presence of H₂O₂ show higher photocatalytic performance under visible light than under simulated solar light.

For comparison, further photodecomposition experiments were carried out to assess the superiority of the catalyst under the same experimental conditions (Fig. 10c). As shown, the degradation ratio of MB is hardly observed for the CuS HHCs in the dark (line a) and in the absence of catalysts under visible light (line b), illustrating that the absorption and photoinduced self-decomposition of MB could be negligible under our experimental conditions. The degradation ratio = 100% × (C₀ − C)/C₀, where C₀ and C are the equilibrium concentration of MB before and after light irradiation. When commercial CuS nanoparticles, H₂O₂ and Cu₂O microcubes were used solely, the MB degradation ratio was only 7.01% (line c), 9.33% (line d) and 14.8% (line f), respectively. When CuS powders of the smashed CuS HHCs, Cu₂O/CuS core–shell microcubes and CuS HHCs were used, the degradation rate of MB increased to 15.51% (line g), 20.64% (line h) and 26.1% (line i), respectively. However, the co-presence of CuS HHCs and H₂O₂ is helpful for the degradation process. Fig. 10c, line (m) shows the absorption spectra of MB solution that was tested at different times when CuS HHCs and H₂O₂ were used. It is obvious to see that the intensity of the absorption of MB quickly decreased and the degradation ratio reached about 97.16% after 30 min. Furthermore, the activity of CuS HHCs was much higher than that of commercial CuS nanoparticles (13.13%, line e), Cu₂O microcube powders (41.38%, line j), CuS powders of the smashed CuS HHCs (46.43%, line k) and Cu₂O/CuS core–shell microcubes (59.45%, line l) under the same conditions, due to their hollow structure and high surface area.

The stability of photocatalysts is also an important factor for their practical applications. Therefore, cycling degradation experiments of CuS HHCs after the MB degradation experiments were also conducted. As shown in Fig. 10d, the second degradation of MB was 95.59% and 98.72% for the third time. After repeating the procedure 5 times, the degradation rate still remained at 95.16%. This result indicates that CuS HHCs can still remain stable and efficient during organic dye degradation. This shows that they demonstrate promising potential to become an excellent recyclable catalyst.

In order to better understand the photocatalytic efficiency of our products, the photocatalytic degradation kinetics of MB was also investigated. The photocatalytic degradation reaction can be modeled as a pseudo-first-order reaction with the kinetics expressed by the equation, ln(C/C₀) = −kt, where “k” is the reaction rate constant, and “t” is the irradiation time. The reaction rate constant k, which is equal to the corresponding slope of the fitting line, is shown in Fig. 11a. The curve for CuS HHCs is linear, the ratio of ln(C₀/C) increased at a constant rate with increase in time. The slopes are shown in Fig. 11b, obviously demonstrating that the photocatalytic efficiency (reaction rate constant) of the above photocatalysis followed the order of CuS + H₂O₂ (0.099 min⁻¹) > Cu₂O/CuS + H₂O₂ (0.027 min⁻¹) > Cu₂O + H₂O₂ (0.017 min⁻¹) > CuS (0.011 min⁻¹) > Cu₂O/CuS (0.008 min⁻¹) > Cu₂O (0.005 min⁻¹). It is clear that the k value of CuS HHCs is far higher than those of the others, almost 3.7 times and 5.8 times higher than Cu₂O/CuS core–shell microcubes and Cu₂O microcubes, respectively, suggesting that the activity of CuS HHCs shows a significant improvement in photodegradation.

Fig. 11 (a) Kinetic linear simulation curves of MB photocatalytic degradation over different samples under visible light irradiation; (b) the reaction rate constant k of each sample.

The underlying photodegradation mechanism might involve acceleration in the photodecomposition of H₂O₂ over CuS HHCs, giving a large number of oxidants. The amount of oxidant is usually determined by the surface active sites of the catalysts. The relevant chemical reactions include separation of electron–hole pairs due to irradiation and subsequent scavenging of these electrons and trapping of holes by H₂O₂ molecules. A brief description of the reaction mechanism is described below:^40,41

Cu²⁺ + hν → h_vb⁺ + e_cb⁻ (4)

H₂O₂ + h_vb⁺ → ˙OOH + H⁺ (5)

H₂O₂ + e_cb⁻ → ˙OH + OH⁻ (6)

˙OH radicals can attack an organic substrate RH such as MB:

RH + ˙OH → R˙ + H₂O (7)

When the as-prepared HHCs of CuS were irradiated with visible light in the presence of H₂O₂, the photo-generated electrons and holes could be captured by H₂O₂ molecules, and yielded oxidants, such as highly reactive hydroxyl radicals (˙OH). The ˙OH could oxidize MB into small molecules like CO₂, H₂O.⁴² It has been demonstrated that because of their high oxidative capacities, the photo-generated oxidant species are in favor of oxidizing organic contaminants.^43,44

The obtained CuS HHCs possess the highest photocatalytic activity among all the as-prepared products due to the following several factors. Firstly, the HHCs have the largest surface area due to the coexistence of hollow interior and hierarchical shell walls with nanosheet building blocks. The special hierarchical hollow architectures absorb more MB molecules, which allows for more efficient transport of the injected electrons from the excited MB dye, leading to enhancement of the photocatalytic performance. Secondly, the special nanosheet shell walls not only reduce reflection, thus harvesting the light more effectively (Fig. 12), but also promote transfer of light-generated charge carriers to the reactive surface and allow rapid diffusion of reactants and products during the reaction.²⁸

Fig. 12 Light reflection models in different microcubes.

4 Conclusions

In summary, CuS hierarchical hollow microcubes (HHCs) composed of many nanosheets were successfully fabricated using smooth Cu₂O microcubes as sacrificial templates. CuS HHCs could be controlled by adjusting the reaction procedure with excellent reproducibility. A possible mechanism for the formation of CuS HHCs has also been proposed, suggesting that a Kirkendall-based process played an important role in the transformation from Cu₂O microcubes to CuS HHCs. The CuS HHCs exhibit enhanced photocatalytic activity for the degradation of MB solution in the presence of H₂O₂ under visible light irradiation, and the photocatalytic activity of CuS HHCs was 3.7 and 5.8 times higher than those of Cu₂O/CuS and Cu₂O, respectively. Furthermore, they also show good recyclability in the visible region. It appears that the synergetic effect resulting from the efficient light harvesting and high surface area of the hierarchical and hollow micro/nanostructures is an important factor in the enhanced photocatalytic performance of the CuS HHCs. The facilely prepared CuS HHC products are promising materials in fields such as photocatalytic and optoelectronic applications.

Acknowledgements

This research was supported by the Foundation of Shanghai University of Engineering Science (Grant no. 2012gp13, E1-0501-15-0105), Innovation Program of Shanghai Municipal Education Commission (Grant no. 14ZZ160), Open Fund of State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University (Grant no. LK1209).

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