Copper sulfide cages wholly exposed with nanotwinned building blocks

Abstract

We have demonstrated a facile protocol for the synthesis of CuS cages wholly exposed with nanotwinned building blocks via a sacrificial Cu₂O template solution route. The as-prepared CuS cages exhibit higher photocatalytic activity for enhancing degradation of methylene blue.

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Morphological and structural tailoring of crystals has been an exciting challenge for current material synthesis due to the special shape and structure-dependent effect, which would develop their significant scientific values and widespread potential applications.¹ As one type of prospective structures with low density, large surface area, good conductivity and permeability, for charge and mass (gas) transport, hollow three-dimensional (3D) superstructures have attracted tremendous interest because of their potential applications in sensors, catalysts, lithium ion batteries, drug-delivery carriers, etc.^2–10 To date, hollow 3D superstructures with different building blocks can be successfully synthesized by precisely controlling the nucleation and growthvia a “bottom–up” self-assembly route. Many recent efforts have been employed to the synthesis of hollow architectures; however, most of the synthesized hollow architectures are spherical shaped and have polycrystalline shells.² Therefore, it is still a challenge for material scientists to develop new facile methods to fabricate well-defined non-spherical hollow superstructures with unique building blocks.

As a non-stoichiometric p-type semiconductor (direct band gap 1.2–2.0 eV) with unique optical, electric and thermal properties,^2–4copper sulfide is a promising material with applications in solar cells, optical filters, nanoswitches, thermoelectric and photoelectric transformers, superconductors, gas sensors and lithium-ion batteries.^2–4,11 For the above application, a microstructure tailoring which determines the surface atomic arrangement and surface energy is of great importance. So far, non-spherical copper sulfide micro- and nanocages with single-crystalline shells (such as cubes, octahedra and 18-facet polyhedra) have been artificially synthesized by using cuprous oxide (Cu₂O) crystals as sacrificial templates.^2–4 To the best of our knowledge, although much attention has been recently developed towards Cu₂O-templated growth of various copper sulfide cages, there are no reports on the synthesis of hollow CuS polyhedral architectures exposed with nanotwinned building blocks.

Nanotwinned structure, an attractive building block with unique mechanical properties and high chemical activity,^12–15 might be widely used in electronics, optics and catalysis.^11–15 Recently, Guo and co-workers have demonstrated that the introduction of nanotwins into photocatalytic semiconductors (Cd_1−xZn_xS) could importantly enhance their photocatalytic activity, because the twins inside a photocatalyst can not only keep the transport property of free charges as in perfect crystals (because of their highly ordered structures), but also prevent the re-combination of holes and electrons (due to the possibility of forming an electrostatic field).¹⁵ The special effects of nanotwins for enhancing the photocatalytic activity for H₂ evolution from water under visible light irradiation have been published.¹⁵ In addition, Chowdhury and co-workers have also reported that TiO₂ with nanotwinned structures can play a crucial role in improving their catalytic activity.¹⁴ Based on the above excellent literature examples, it can be believed that the employment of nanotwinned building blocks into copper sulfide might bring exciting structure-dependent properties.

In a previous report,¹⁶ unique polyhedral 26-facet CuS hollow architectures selectively decorated with nanotwinned, mesostructural and single crystalline shells have been successfully synthesized. Consequently, the shape-controlled synthesis of polyhedral hollow CuS cages wholly exposed with nanotwinned building blocks is an interesting and challenging topic for both fundamental study and potential applications. To date, the relevant experimental investigation is still unavailable. Herein, we present the first evidence on the synthesis of cubic CuS cages exposed with high-activity nanotwinned building blocks. The formation of nanotwinned CuS polyhedral hollow cages not only enriches the family of copper sulfide nanostructures, but also makes the purposive crystal design of spatial arrangement possible. Additionally, it offers a good opportunity to understand the significance of copper sulfide nanotwinned structures for enhancing the catalytic degradation of methylene blue (MB).

The synthetic strategy to prepare well-defined CuS cages with nanotwinned building blocks is based on an inward replacement/etching route.^2–4 The polyhedral CuS cages with nanotwinned building blocks can be synthesized by a three-step process, i.e. the formation of uniform and monodisperse cubic Cu₂O crystals, then the synthesis of Cu₂O/CuS core/shell architectures in a mixed anhydrous ethanol solution composed of sodium sulfide and sodium hydroxide, finally the inner Cu₂O cores are dissolved completely with an ammonia solution.^2–4 The pure cubic phase of 6-facet Cu₂O templates with smooth surfaces and uniform sizes, as shown in Fig. S1 and S2†, can be prepared through the reduction of a copper hydroxide complex with D-(+)-glucose powder based on a template-free complex precursor solution synthesis route.^17,18 After addition of Cu₂O templates into the mixed anhydrous ethanol solution composed of sodium sulfide and sodium hydroxide, a gradual conversion of Cu₂O cores to CuS shells occurred.² The pure primitive hexagonal phase of CuS hollow architectures, as shown in Fig. S3† (XRD pattern, see ESI†), can be obtained after the inner Cu₂O cores are removed completely with ammonia solution for 72 hours. All the diffraction peaks are indexed according to the standard hexagonal structure of CuS (JPCDS file no. 06-0464). No peaks of impurities such as copper oxide or other copper sulfides were detected, indicating the high purity of the as-prepared CuS hollow cages.

Fig. 1 displays the typical FESEM images of the as-prepared cubic products at different magnifications. From the low-magnification FESEM result (Fig. 1a), it can be found that CuS cages have the excellent cubic architectures in our synthesis. Some partially broken particles can also be observed in Fig. 1a, indicating that cubic cages with hollow interiors are formed. A typical individual cubic CuS cage is shown in Fig. 1b, which obviously indicates that the formation of rough surfaces of the shells with well-assembled nanoplate building blocks can be achieved.

Fig. 1 Typical FE-SEM images of the as-prepared cubic CuS cages. (a) Low-magnification; (b) individual particle.

The morphology and structural characterization of the as-prepared cubic CuS cages are also obtained using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) investigations. The results of the cubic CuS cages are shown in Fig. 2. Fig. 2a is the low-magnification TEM image of an individual CuS cage, and it can be seen that the particle has three pairs of well-defined square surfaces, leading to the formation of cubic shapes. Especially, an extremely strong contrast difference between their edges (dark) and facets (bright) provides the obvious evidence of the hollow nature. The SAED pattern corresponding to the square shell is displayed in Fig. 2b, the diffraction pattern with somewhat dispersed and elongated spots implies that the square shell has a relatively imperfect single crystalline characteristic, and it can be indexed to the hexagonal phase of CuS viewed along the [0001̄] zone axis. This observation indicates that the square shell has nearly the same crystallographic orientations among nanoplate building blocks as single crystals, therefore, the formation of CuS mesostructures occurred on the square shells, which can be attributed to an oriented attachment mechanism determined by the self-assembly of these nanoplate building blocks, involving aggregation, rotation and realignment.^19,20Fig. 2c displays a typical TEM image of the partial area of the cage, and it can be observed that the particles are made of rough shells and are void inside. Fig. 2d shows the high-magnification TEM image of the square shell, and it can be closely found that the shell is composed of abundant nanoplate building blocks with irregular alignment. Some of these nanoplates have joined together to form clear nanograins of the shell. Other nanoplates disorderly attach and form cavities or mesopores inside the cage shell (marked by a blue circle). The detailed microstructure of a nanoplate on the square shell is further investigated by HRTEM, as shown in Fig. 2e. A high density of planar defects is clearly observed, which forms a domain structure with nanotwinned planes.¹¹ The corresponding fast Fourier transform (FFT) of the HRTEM images taken from the area marked with a red square of the planar defects (Fig. 2e) is shown in Fig. 2f. It is expressed as heavy streaking of the reflections, which is similar to that occurring in the previous report,¹¹ indicating the formation of nanotwinned planes in the nanoplates.

Fig. 2 (a) Low-magnification TEM image of the as-prepared cubic CuS cages; (b) SAED pattern of the square shell; (c) and (d) high-magnification TEM image; (e) HRTEM image of a typical nanotwinned nanoplate building block; (f) FFT image of the HRTEM image taken from the area marked with a red square in Fig. 2e.

The formation of CuS hollow architectures via in situ sacrificial Cu₂O templates is believed to be the result of the Kirkendall effect, which has been discussed in previous reports.^2–4 To the best of our knowledge, these novel hollow CuS architectures wholly exposed with nanotwinned building blocks were not reported previously. After addition of Cu₂O templates into the reaction system, a thin CuS layer composed of many nanoplates was formed immediately on the surface of Cu₂O, as evidenced by a fast color change of the Cu₂O templates from dark red to black. Further reaction depends on the diffusion of copper or sulfur ions through this CuS interface, finally leading to the formation of voids in the particles. The strain energy might be stored in the CuS interfaces during the mass exchange process, and this stored strain energy can be released by forming a twinning when the growth of nanoplates is completed.¹¹Reduction of the surface free energy by fusing and eliminating the high-activity nanotwinned planes can drive the spontaneous ordering assembly to form these hierarchical superstructures.¹¹ The exact formation mechanism of nanotwinned CuS hollow cages is yet uncertain, further studies are still needed to uncover the underlying principles.

Previous references have reported that the introduction of nanotwins into semiconductors can effectively improve their photocatalytic property.^14,15 To demonstrate the potential application of these as-synthesized cubic CuS hollow cages with nanotwinned building blocks in the degradation of organic contaminants, we have investigated their photocatalytic activities by choosing the photocatalytic degradation of MB dye in the presence of hydrogen peroxide (H₂O₂). The catalytic reaction was conducted under natural light. UV–vis spectra were used to demonstrate the photocatalytic degradation activity of MB dye (Fig. 3). The characteristic absorption peak at 664 nm of MB was monitored to follow the catalytic degradation process. Fig. 3a is the optical absorption spectra of MB tested at different durations without any catalyst, only about 2.9% of the MB (Fig. 3f, Panel A) was degraded after 40 min. In addition, the assistance of H₂O₂ can be in favor of the bleaching of MB, which is similar to the experiments described in previous reports.^21,22 Herein, we also conducted photocatalytic tests under the same conditions as aforementioned, but without feeding H₂O₂ into the reaction system for degradation of MB (Fig. 3b). However, no noticeable bleaching was observed after 40 min of visible light irradiation (Fig. 3f, Panel B), which indicates that the H₂O₂ is crucial under such conditions. Fig. 3d shows the optical absorption spectra of MB tested at different intervals in the presence of the common cubic CuS cages without nanotwinned building blocks, which were synthesized based on the previous report (Fig. S4, see ESI†).² The intensity of the absorption peak at 664 nm of MB decreased rapidly with the extension of exposure time (Fig. 3d), indicating the noticeable photocatalytic degradation of MB, and about 75.1% of the MB (Fig. 3f, Panel D) was degraded after 40 min. The above results indicate that the presence of CuS catalysts can accelerate the degradation of MB, which confirmed the significant role CuS played in the photocatalytic degradation of MB. Moreover, it was found that the photocatalytic activity was higher than that of commercial CuS powders (Fig. 3f, Panel C). Further experiment was carried out to compare the catalytic activity of the as-prepared cubic CuS cages with nanotwinned building blocks. It can be found that the intensity of the absorption peak at 664 nm of MB decreased even further than that of the above hollow CuS cages (Fig. 3e), and about 92.3% of the MB (Fig. 3f, Panel E) was degraded after 40 min, which proved that nanotwinned building blocks played a vital role in enhancing the degradation of MB dye. The special effect of the external introduction of nanotwinned building blocks into crystals for enhancing the photocatalytic property is similar to the process that occurs in TiO₂ and Cd_1−xZn_xS.^14,15 Hence, the decomposition of the MB aqueous solution at 40 min in the presence of the above samples is as follows: cubic CuS cages with nanotwinned building blocks (92.3%) > cubic CuS cages without nanotwinned building blocks (75.1%) > commercial CuS powders (57.9%).

Fig. 3 Absorption spectra of photodegradation of MB by different catalysts under natural light: (a) without any catalyst + H₂O₂. (b) Cubic CuS cages with nanotwinned building blocks + no H₂O₂. (c) Commercial CuS+ H₂O₂; (d) Cubic CuS cages without nanotwinned building blocks + H₂O₂. (e) Cubic CuS cages with nanotwinned building blocks + H₂O₂. (f) A plot of the extent of photodegradation of MB by different catalysts under natural light. Panel A: without any catalyst; Panel B: cubic CuS cages with nanotwinned building blocks + no H₂O₂; Panel C: commercial CuS powders; Panel D: cubic CuS cages without nanotwinned building blocks; Panel E: cubic CuS cages with nanotwinned building blocks.

The underlying photodegradation mechanism might involve the acceleration in photodecomposition of H₂O₂ over CuS cages to give a large number of oxidants. The amount of oxidant is determined by the surface active sites of catalysts.²³ The relevant chemical reactions include the electron–hole pair separation by UV irradiation and subsequent scavenging of the electrons and trapping of holes by H₂O₂ molecules, as shown below:

CuS + hν → h_vb+ + e_cb⁻ (1)

H₂O₂ + h_vb+ → ˙OOH + H⁺ (2)

H₂O₂ + e_cb⁻ → ˙OH + OH⁻ (3)

˙OOH → ˙O₂⁻ + H⁺ (4)

When the CuS cages were irradiated with UV light in the presence of H₂O₂, holes in the conduction band (CB) would be excited to the valence band (VB), with simultaneous formation of electrons in the CB.²⁴ The electrons and holes can be captured by H₂O₂ molecules, leading to the formation of the oxidants (eqn (1)–(4)). It has been demonstrated that the photogenerated oxidant species are in favor of oxidizing organic contaminants because of their high oxidative capacities.²¹ In our experiment, the formation of the nanotwinned building blocks is the reason for improving the photocatalytic activity, because the nanotwins inside the CuS cages might keep the transport property of free charges as in perfect crystals, and could decrease the recombination of photoinduced electrons and holes, and increase the lifetime of the electron–hole pairs.¹⁵ Thus the photocatalytic capacity of the CuS cages with larger amount of nanotwinned building blocks would be improved. Based on the above discussion, it is believed that the photocatalytic superiority of the as-prepared CuS hollow cages is attributed to their wholly exposed nanotwinned building blocks. Note that the photodegradation experiment of MB dye here was carried out under illumination by natural light. This suggests that the potential application might be quite feasible in practice.

In summary, facile synthesis of CuS hollow cages wholly exposed with nanotwinned building blocks was successfully achieved via an in situ sacrificial Cu₂O template solution route. The photocatalytic superiority of the as-prepared CuS cages can be attributed to their active components of nanotwinned building blocks, which exhibited higher catalytic activity for enhancing photocatalytic degradation of MB dye. The present study not only opens a new route to the synthesis of nanotwinned CuS cages, but also is of fundamental importance for the investigation of their potential applications in other fields, including optics, sensors and so forth.

Acknowledgements

We thank the support from the National Science Foundation of China (NSFC no. 51071116), National Basic Research Program of China (no. 2010CB635101) and National High Technology Research and Development Program of China (no. 2009AA03Z320).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, FESEM images, XRD patterns, TEM images and UV-vis absorption spectra. See DOI: 10.1039/c1ce06135f

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