The convenient fabrication of metal-based nanocages with obvious interior hollow spaces and porous-shell structures has stimulated extensive investigations in recent years.^[1–5] The unique architectures with tunable optical and electronic properties play an important role in ultra-fast responsive sensor, super-active catalyst, and biomedical material, etc. For example, the polymer poly (N-isopropylacrylamide) loaded on a gold nanocage can be released in a controllable way by using a near-infrared laser, which is very suited for biomedical application.^[3] Moreover, because of unique hollow spaces and porous wall structures, the metal-sulfide/oxide nanocages can also be used for efficiently decontaminating liquid organic/heavy metal ions pollutants.^[6,7] The organic pollutants that originate from printing and dyeing industries, especially some complex aromatic molecular such as methyl blue (MB) are generally persistent and stable to heat, light or oxidization agents.^[8–10] These stable pollutants have been regarded as serious environmental issues. Most recently, our group has confirmed that the obtained Cu₂O@Cu nanocomposites exhibit excellent adsorption performance for the removal of MB molecules from waste water.^[11] The possibility of adopting laser beam as a versatile tool for constructing complex-nanostructures has received increasing attention in the nanomaterial systems.^{[2,4,10–16]} Generally, most of metal-sulfide/oxide nanocages have been synthesized by chemical fabrication via complicated soft directing agents or stabilizers.^[12–16] However, these residual reagents on the nanomaterials will bring some uncontrollable/unpredictable secondary chemical pollution into the waste water treatment. Therein, the laser-induced fabrication of pure nanocomposites is of great significance for the practical adsorption of organic pollution in water purification. Many of nanostructures such as amorphous metal–oxide (Fe, Co, and Ni oxides) nanoparticles^[10] and porous Cu₂O@CuO nanocomposites^[11] have been developed by laser ablation in different medium liquids. The controlled synthesis of well-defined metal-based nanocages with adsorption performances for the removal of organic contaminants has not been extensively explored so far.

Here in this work, for the first time, we report on the one-step synthesis of well-defined CdS nanocages with an overall size of ∼ 20 nm by laser ablation of bulk Cd target in thioacetamide (TAA) solution. The 75-mg TAA in 10-mL distilled water will provide sulfur sources for the fabrication of metal-sulfide nanostructure. During laser ablation in liquid condition, the CdS nanocages should be highly related to the initial generation of Cd vapor bubbles through a model of micro-explosive boiling. Moreover, we find that the as-prepared CdS nanocages with obvious interior void spaces and distinctive porous-shell structures exhibit excellent adsorption performance for removing the MB molecules from liquid solution. The maximum adsorption capacity can reach up to 11813.3 mg/g. It is even slightly higher than that of NiO amorphous nanostructures in previous work,^[10] and much greater than those in many other reports.^[9,11,14] In the present work we provide a convenient and versatile method of constructing the well-defined metal-based nanocages. The obtained CdS nanocages can serve as superior adsorbent materials for removing the organic compound from solution.

The experimental apparatus based on laser-induced fabrication in liquid has been widely illustrated in detail elsewhere.^{[2,4,10–16]} Briefly, a well-polished pure (99.99%) Cd wafer with a diameter of 1.5 cm and thickness of 2 mm was placed on the bottom of a rotating glass dish (∼ 500 rpm). It was filled with 10-mL distilled water solution containing 75-mg thioacetamide (TAA). A 1064-nm laser beam with 10-ns pulse duration, 10-Hz repetition rate and ∼ 5.4 GW/cm² power density originating from a Q-switched Nd-YAG (Yttrium Aluminum Garnet) laser (Quanta Ray, Spectra Physics) was used for laser ablation. After 15-min laser ablation, the dark–brown solution was formed, implying the generation of colloidal suspensions. Immediately, the products were carefully washed with distilled water three times and centrifuged at rpm for 15 min in an ultracentrifuge. Compared with polymers, the TAA can be removed from the products after rinsing it in distilled water. The transmission electron microscopy (JEOL-JEM-2100F) equipped with energy dispersive x-ray spectroscopy, field emission scanning electron microscope (SEM, Hitachi, S-4800), x-ray diffraction pattern (XRD, Rigaku, RINT-2500 VHF with Cu Kα radiation: λ = 0.15406 nm), Fourier transform infrared (FTIR) spectra (ALPHA-T, Bruker), and x-ray photoelectron spectra (XPS, PHI Quantera SXM with an Al K α = 280.00-eV excitation source) were used to reveal the CdS nano-cage structures. In a typical adsorption experiment, the decontamination of MB molecules was carried out by simply adding the as-prepared CdS nanocages into MB water solution (pH∼6.8) under constant stirring (350 rpm) at room temperature. After 0 min–10 min reaction, the MB concentrations (centrifuged at rpm for 15 min) were separately determined by the absorbance spectra (UV-Vis-IR spectrometer, UV-1800, Shimadzu).

After laser ablation of Cd target in TAA solution, the typical transmission electron microscopy (TEM) and high-resolution scanning electron microscope (HRSEM) images of the as-prepared products are illustrated in Figs. 1(a) and 1(b), respectively.

Fig. 1.

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	Fig. 1. (a) and (b): Typical TEM and HRSEM images of the as-prepared products. (c) and (d): XRD pattern and EDS results of the obtained nanomaterials.

The images clearly reveal that numerous quasi-spherical nanomaterials are indeed of nano-cage structure, which are shown as contrasting lighter images with their walls as darker ones due to different penetration depths of the incident electron beam. The obtained products are characterized by obvious interior hollow spaces and distinctive shell structures. The average overall size of these nanocages is about 20 nm obtained by measuring the diameters of more than 260 nano-structures in sight on the TEM and SEM images. Without using any dispersing agent in this paper, the generated nanocages tend to interconnect with each other, forming a necklace-like short curvilinear structure. In addition, the crystallographic investigation of the products is established by XRD spectrum in Fig. 1(c). The XRD pattern illustrates that a series of (111), (200), (220), (311), (222), and (400) CdS (JCPDS, no. 21-0829) diffraction peaks is clearly detected at 28.22°, 32.78°, 47.05°, 56.03°, 58.76°, and 68.98°, respectively. Because of the relatively high peak at 28.22° in XRD pattern, the preferential alignment of the (200) orientation should be formed in CdS nanocages. Moreover, the chemical compositions of nanocages are determined by energy-dispersive x-ray spectroscopy (EDS) in Fig. 1(d). The EDS result shows that the product is only composed of Cd and S elements, and their relative ratio is about 51.2:48.8, which is consistent with the CdS chemical composition.

In order to obtain more detailed information about the obtained products, the enlarged TEM image in Fig. 2(a) provides a representative structural detail of the CdS nanocages. In addition to the obvious hollow interior spaces, a closer view of these nancages confirms that the outside shell has a porous structure with numerous surface nano-pores. It is very beneficial for the adsorption of organic molecules in water. Moreover, the high resolution TEM image (top-right inset in Fig. 2(a)) illustrates that the lattice fringes with a periodicity corresponding to a d-spacing of 0.316 nm could be indexed to the (111) plane in the CdS nanocage (JCPDS, no. 21-0829). On the other hand, the left-bottom inset in Fig. 2(a) shows the elemental mapping image of the individual nanocage. The result clearly confirms that the uniform distributions of Cd (red dots) and S (green dots) throughout the outside shell structure. Moreover, the shell thickness-distribution histograms are obtained by measuring the shells of more than 260 nanocages in sight on the TEM images The result in Fig. 2(b) reveals that the CdS nanocages have narrow shell thickness dispersion in a range of 4 nm–6.5 nm and the average shell thickness is about 5.4 nm.

Fig. 2.

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	Fig. 2. (color online) (a) Representative HRTEM image of the CdS nanocages. The top-right inset shows the corresponding lattice fringe. The left-bottom inset shows the elemental mapping image of the individual nanocage. (b) Mean thickness distribution histogram of the shells in CdS nanocages.

The x-ray photoelectron spectroscopy (XPS) patterns of the obtained CdS nanocages are illustrated in Fig. 3, in order to further verify the element valence states of Cd and S species in the products. Firstly, the binding energies are calibrated by referencing the C 1s peak at 284.8 eV to reduce the sample charge effect, which has been illustrated in many previous researches.^[11,14] The O 1s spectral line with very weak peak intensity at 530.8 eV is attributed to the surface oxide after keeping them in an oven. The relatively high peaks of Cd 4d, Cd 3d, and Cd 3p, as well as S 2p and S 2s can be clearly detected in Fig. 3(a). Figure 3(b) shows that the S 2p peak at 162.1 eV and S 2p at 163.2 eV reveal the formation of element valence state of S²⁻ in the obtained product. Moreover, the peaks at 405 eV (Cd 3d ) and 412 eV (Cd 3d ) in Fig. 3(c) illustrate that the oxidation of Cd²⁺ formed in the nanocages.

Fig. 3.

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	Fig. 3. XPS spectra of the CdS nanocages obtained by laser ablation of Cd target in solution. (a) Survey structure, (b) S 2p and S 2p of the products, (c) Cd 3d and Cd 3d of the products.

In summary, the above results are the best evidence for the formation of CdS nanostructures by laser ablation of Cd target in TAA solution. In the following section, based on laser-induced Cd vapor bubbles in TAA solution, we will describe the possible growth of CdS nanocages. Briefly, at the moment of 1064-nm laser beam arriving at Cd target in liquid condition, the Cd surface layers absorb well the laser energy. The absorbed photon energy of the laser beam will trigger the rapid boiling and vaporization of explosive Cd plasma on the irradiated surface. Immediately, the micro-explosive boiling with high temperature (thousands of kelvins^[12]) will then lead to the formation Cd²⁺ ion vapor bubbles at solid-liquid interface. The superheated explosive Cd vapor bubbles in the solution can drastically promote the liquid temperature and then significantly improve the surrounded TAA hydrolyzing degrees, resulting in the formation of S²⁻ ions in liquid The nucleation of Cd and S ions (TAA hydrolyzing reaction) will take place at Cd vapor bubble-liquid interface. At the end of the explosive Cd species, the nucleation process will sharply terminate due to the rapid collapse of vapor bubbles. During the pulse (10 ns) laser ablation, the unique rapid CdS nucleation process at bubble–liquid interface gives rise to the formation of porous shell structure and obvious interior hollow space. Without using any complicated stabilizers or soft directing agents, the CdS nanocages can be formed by the laser-induced Cd vapor bubbles in liquid condition. The Cd vapor bubble plays an important role in forming the CdS nanocages. It is highly related to the laser power density. To verify this hypothesis, a lower laser power (∼ 3 GW/cm²) is adopted in experiment. The morphologies of the obtained nanomaterials are illustrated in Fig. 4. The low- magnification and enlarged TEM images in Figs. 4(a)–4(b) show that the products are obvious core-shell structures instead of the hollow-shell architectures. As shown in Fig. 4(b), the core region with a d-spacing of 0.234 nm is indexed as the ((101) plane in Cd structure (JCPDS, no. 05-0674). The shell region with a periodicity corresponding to a d-spacing of 0.316 nm should be indexed with reference to the CdS(111) plane structure (JCPDS, no. 21-0829). It can be deduced that the obtained nanomaterial has Cd core and a CdS shell structure. On the other hand, too high a laser power ( GW/cm²) will also cause the frames to completely collapse on the nanocages because of too hot a Cd plasma formed on the irradiated spot. Therefore, the moderate laser beam is suitable for fabricating the CdS nanocages with stable hollow-shell structure.

Fig. 4.

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	Fig. 4. (color online) (a) and (b) Typical low- magnification and enlarged TEM images of the nanomaterials by 1064-nm laser ablation with a lower power density of 3 GW/cm2 in TAA solution.

Finally, as for the removal of MB molecules from the water liquid, the excellent adsorption performances of the obtained CdS nanomaterials are demonstrated in Fig. 5. In the presence of 0.36-mg CdS core–shell nanomaterials and 0.36-mg CdS nanocages, respectively, the corresponding adsorption behaviors of MB solution (80 mg/L, 2 mL) are illustrated by the absorbance spectra (Fig. 5(a)). After 1-min adsorption process, the main absorption band of MB molecules at about 587 nm decreases from about 2.731 a.u. to 1.911 a.u., and 0.004 a.u. for CdS core–shell nanomaterials and CdS nanocages, respectively. The adsorptions of 30.6% and 99.8% MB molecules can be achieved in the presence of CdS core–shell nanomaterials and CdS nanocages, respectively. The results clearly reveal that the obtained CdS nanocages exhibit enhanced adsorption performance for removing MB molecules from the solution. As for the adsorption performance of as-prepared CdS nanocages, in order to provide the quantitative result, the amount of MB absorbed at time t should be calculated according to following equations:^[7,10] 1 2 where (mg/L), (mg/L), and (mg/L) are the MB concentration at initial, any time t, and equilibrium state, respectively; is the volume of the solution (in unit mL) and m is the mass of the CdS nanocages. If the value of could be maintained/sustained for further prolonging time, then the equilibrium adsorption condition can be determined. Therefore, in the presence of 0.36-mg CdS nanocages, the reduction performance of MB solution (90 mg/L, 80 mL) versus reaction time is illustrated in Fig. 5(b). The corresponding peak intensity of MB molecules at 587 nm (Fig. 5(c)) drops from about 2.76 a.u. to 1.132 a.u. with the adsorption time increasing from 0 min to 7 min. Then, it holds the same level with a nearly constant of ∼ 1.129 a.u. for further prolonging time to 9 min. Therefore, according to Eq. (2), the maximum MB adsorption capacity can be calculated to be about 11813.3 mg/g in this paper. It is even slightly higher than that of NiO amorphous nanostructure^[10] and much higher than those in many previous reports.^[9,11,14] Compared with the nanomaterials with solid interiors, the obtained CdS nano-cage architectures with obvious interior hollow spaces and porous shell structures exhibit superior adsorption performances for removing the MB molecules from the solution. The main mechanism is related to the positive charge regions on CdS nanocages surfaces. In this paper, the unique sharp and rapid CdS nucleation at bubble-liquid interface should originate from laser-induced superheating of Cd species. After pulsed (10 ns) laser fabrication, the rapid quenching process will enhance and improve the disorder degree of Cd metal, making them stay at higher excited states. It will result in the formation of excited Cd metal with positive charges on the surfaces of CdS nanocages. The excellent absorption performance can be achieved by the strong chemical bonds between positive sites of the CdS nanocages and the negative charges of – SO₃ functional groups in MB molecules. On the other hand, the –OH groups formed on the surface will also be suitable for the absorption process, which has been verified in previous researches.^{[10,11,14,17]} As shown in Fig. 5(d), the FTIR spectrum shows that the obtained CdS nanocages are covered by OH-enriched groups with spectral line located at about 3445 cm⁻¹. After the adsorption process, the hydrogen bonding interaction will be generated between –OH groups and the oxygen groups in MB molecules, which also has been widely illustrated in previous researches.^{[10,11,14,17]} After adsorbing MB molecules, the FTIR spectra of the CdS absorbents show that the –OH groups at about 3445 cm⁻¹ significantly decrease, implying the formation of hydrogen bonding. Moreover, several enhanced FTIR spectral lines originating from MB structures can be detected in the absorbents. For example, the C–N stretching vibration at 1389 cm⁻¹ and the –SO₃Na functional groups at 1165, 1120, and 1003 cm⁻¹ are clearly observed in FTIR spectra. The obtained result is the best evidence for the chemical bonds between the MB molecule and CdS nanocage, which is consistent with the results in previous reports.^{[10,11,14,17]} Without using any complicated stabilizers or soft directing reagents, the obtained pure CdS nanocages with super adsorption performances have promising high applicability in wastewater treatment.

Fig. 5.

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Fig. 5. (a) Reduction performance of the removal of MB molecules from the water solution(80 mg/L, 2 mL) in the presence of 0.36-mg CdS core–shell nanomaterials and 0.36-mg CdS nanocages, respectively. (b) and (c) The adsorption behaviors of MB solution (90 mg/L, 80 mL) via different reaction times. The required CdS nanocages is 0.36 mg. (d) The FTIR spectra of the as-prepared CdS nanocages before and after adsorbing MB molecules.

In this work, the well-defined CdS nanocages with obvious interior hollow architectures and distinctive porous-shell structures are fabricated by laser ablation of Cd target in TAA solution. To reveal the mechanism, the initial generation of Cd vapor bubbles through laser-induced superheating of Cd species is proposed. Moreover, the as-prepared CdS nanocages exhibit super adsorption performance for removing the MB molecules from liquid at room temperature. The maximum adsorption capacity can reach up to 11813.3 mg/g. Without using any potentially toxic chemical agent, the pure CdS nancages with excellent adsorption performance should be established as advanced nanostructures for removing the organic pollutants from waste water. The laser-induced fabrication opens up possibilities for using laser beam as a convenient approach to sculpting novel complex nanostructures.