1. Introduction

Hexavalent chromium Cr(VI) compounds in drinking water have serious toxic or lethal effects on humans and animals, which are environmental contaminants from leather tanning, electroplating, pigments, and textile production, etc [1–4 ]. The increasing morbidities of bladder, liver, kidney cancers are highly related to the aqueous Cr(VI) induced into the environment without any pretreatment. Therefore, the development of new technologies to effectively reduce Cr(VI) is of great importance [5]. Many pioneering works such as magnetic polyaniline [6], photocatalytic reduction [7,8 ], microbial reduction [9], and adsorption technique [10] have been proved to be efficient approaches for the reduction of aqueous Cr(VI). For example, Zhang et al. reported the visible-light-driven photocatalytic reduction of Cr(VI) in the presence of g-C₃N₄ and SnS₂/SnO₂ nano-heterojunction [11,12 ]. Qiu et al. fabricated a magnetic carbon under the carbonization temperature of 800 °C, which has good activity in the reduction of aqueous Cr(VI) within acidic solutions [13]. Among all available methods, visible-light-driven photocatalytic reduction of Cr(VI) in the presence of nano-semiconductors is a promising strategy, since the photo-catalysts are efficient, cost-economically, environmental-friendly and do not use or discharge any perilous chemicals. At present, the visible-light-driven photo-catalytic activity should be significantly improved for the practical application in reduction of aqueous Cr(VI).

Recently, the construction of metal sulfides with porous nano-structures has been proved to be a successful way for developing more efficient photo-catalysts in the reduction of aqueous Cr(VI). Compared with various metal sulfides nano-materials with solid interiors, such as wires, belts, needles, and other complex solid counterparts, porous nano-particles are of fine structure with a high surface area, excellent carrier transport-mobility, better surface grafting properties. Increasing evidence has shown that the excellent photo-catalytic activity of metal sulfides nano-particles is strongly depends on the porous surfaces and hollow interiors. Therefore, the controlled synthesis of various metal sulfides porous nano-materials is important. In this letter, we design a simple, versatile, and rapid route to controllably synthesize ZnS/Zn porous nano-material directly from bulk Zn target based on laser ablation in liquid solution containing thioacetamide (TAA), hydrochloric acid (HCl) and hexadecyl trimethyl ammonium bromide (CTAB). Laser fabrication in liquid (LAL) with high non-equilibrium processing character is a novel green approach to the synthesis of meta-stable phases of nano-materials [14,15 ]. The formation of ZnS/Zn porous nano-structures is highly related to the laser induced-Zn plasma, and non-equlibrium nucleation process including ultra-rapid acid etching. Compared with previous reports by using SnS₂, SnS₂/SnO₂, and SnS₂/TiO₂ [3,5,8 ], the as-prepared ZnS/Zn porous nano-cages exhibit significantly enhanced photo-catalysis activity and excellent visible-light-driven photo-catalytic reduction of aqueous Cr(VI). The aim of this work is to extend a new method of synthesizing the high-performance photo-catalysts in reduction of aqueous Cr(VI) under visible light.

2. Experimental section

In a typical experiment, a well-polished Zn metal is used as a target placed on a cubic container, which is filled with 8 mL liquid solution containing 0.3 M TAA, 0.05 M CTAB and distilled water. The TAA can provide the sulfur sources, and CTAB is used as surfactant. The different amounts of HCl were added to the mixture solution, which provides an acidic condition to prevent the formation of ZnO and to modify the ZnS/Zn nano-materials. A 1064 nm laser beam with a pulse duration of 10 ns and 10 Hz repetition rate from a YAG laser (Quanta Ray, Spectra Physics) was focused on the target by a quartz lens with a 40 mm focal length. The incident power density was approximately 4 GW/cm² and the ablation lasted for 30 min. After the laser ablation, the precipitates of the products were centrifuged at 4200 rpm for 10 min by an ultracentrifuge. Then, the obtained materials were washed with deionized water two times to remove the TAA and CTAB from nano-particles. The as-prepared nano-materials weights were carefully measured by an electronic analytical balance with an accuracy of 0.1 mg. After laser ablation for 30 min, the mass of the ZnS/Zn nano-particles is measured about 7.4 mg, which is slightly higher than that (7.1 mg) of ZnS/Zn nano-cages. The main reason should be related to the hole-structures in the ZnS/Zn nano-cages. The sediments were dropped on a copper mesh and dried in an oven at room temperature for observation by transmission electron microscopy (JEOL-JEM-2100F). In addition, the crystallographic investigations of the porous nano-particles were analyzed by X-ray diffraction (XRD) patterns (Rigaku, RINT-2500VHF) using Cu Kα radiation (λ = 0.15406nm). Photocatalytic-reduction of Cr(VI) under visible-light irradiation using 18 W lamp with about 60 lm/w, and wavelength of 400~750 nm. In a typical process, 2.7 mg ZnS/Zn nano-particles was added into 10 mL of 1 × 10⁻² M aqueous K₂Cr₂O₇ solution. At the end of reduction, the photo-catalysts were removed from the solution by centrifuged at 4200 rpm for 10 min. The Cr(VI) concentrations were measured by the absorption spectra of Cr(VI) solution, which were carried by a UV-Vis-IR spectrometer (UV-1800, Shimadzu).

3. Result and discussion

After cumulative pulse laser ablation of pure Zn metal target in activated solution containing distilled water, 0.3 M TAA, 0.05 M CTAB, and 5 μL HCl, low magnification and high resolution images of the nano-particles are collected with transmission electron microscopy (TEM; Figs. 1(a)-1(d) , respectively). The morphology in Fig. 1(a) clearly shows that numerous core-shell-like nano-particles with diameters varying from about 40 to 60 nm are interconnected and accreted with each other. Closer inspections of the individual core-shell nano-particles in Fig. 1(b)-1(c) confirm that the spherical-shaped structures exhibit smooth surface and clear boundary between cores and shells. The HRTEM image in Fig. 1(d) provides a typical structural detail of the cross region between the core and shell. The core region marked by orange lines in Fig. 1(d) with a d-spacing of 0.23 nm is indexed as the (100) plane in the Zn structure. In addition to the well crystalline of the core, it is further noted that the shell should be amorphous material without any lattices fringes. The distribution histograms of the nano-particles sizes and corresponding shell thickness are illustrated in Fig. 1(e)-1(f), respectively. The statistical results are obtained by measuring the diameters and the shells of more than 200 particles in sight on the TEM images. The result in Fig. 1(e) reveals that the as-prepared nano-particles have narrow size dispersion. As shown in Fig. 1(f), the average thickness of the corresponding shell thickness is about 10 nm. Moreover, elemental mapping images of the typical core-shell structures (insets in Fig. 1(b)) show that the core-shell nano-particles are composed of Zinc and Sulfur elements. The ratio of Zn to S in the nano-particles is about 7:3, which is consistent with the Zn core-ZnS shell structure. We increased the amount of HCl to 10 μL in the activated solution, and found that the interconnected ZnS/Zn core-shell structures changed to porous nano-cages with numerous surface pores, as shown in Fig. 2(a) . The average diameter of the final nano-cages is approximately 30~50 nm, which is slightly smaller than the size of core-shell structure in Fig. 1(a). The high-magnification TEM image of representative nano-cage is shown in Fig. 2(b). It is confirmed that the products are characterized by an obvious interior cavity and porous surface. Moreover, the porous nano-cage is found to be well crystalline according to the clear lattice fringes. Correspondingly, the lattice fringes with spacing of 0.25 nm can be identified as Zn (002) plane. The pores in nano-cage are shown as contrasting lighter images with their walls as darker ones, due to different penetration depths of the incident electron beam. The crystallographic investigation of the as-prepared nano-cages is established by X-ray diffraction (XRD in Fig. 2(c)). The XRD pattern clearly reveals that a series of (002),(100), (101), (102), (103), (110), (004), (112) and (201) Zn diffraction peaks centered at 36.28°, 39.00°, 43.21°, 54.30°, 70.12°, 70.67°, 77.04°, 82.08° and 86.53° were indeed detected. Furthermore, the EDS pattern in Fig. 2(d) illustrates that signals from S, Zn can be clearly detected, and the measured ratio of them is about 6:4.

 

Fig. 1 (a) The typical low-magnification TEM image of the nano-particles by pulsed laser ablation of Zn target in activated solution containing 0.3 M TAA, 0.05 M CTAB, and 5 μL HCl. (b-c) The representative enlarge TEM images of the productions. (d) HRTEM image of the core-shell like structure. (e-f) The distribution histograms of the ZnS/Zn nano-particles size and the corresponding shell thicknesses, respectively

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Fig. 2 (a) The representative low-magnification TEM images of the nano-cages by laser ablation of Zn target in the aqueous solution of 0.3 M TAA, 0.05 M CTAB and 10 μL HCl. (b) The HRTEM image of the individual nano-cage. (c-d) XRD pattern of the nano-cages and the result of the EDS.

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It is necessary to note that the measured ratio of Zn element in ZnS/Zn nano-cage is some lower than the ZnS/Zn core-shell structure, implying the ablated Zn material can be dissolved and removed from the hybrid nano-composites.

The possible ZnS/Zn nano-cage growth process has been proposed in the following section. Fig. 3 provides a schematic growth diagram of ZnS/Zn core-shell nano-particle and nano-cage fabricated with low and enough HCl in activated solution, respectively. The few hydrogen chlorides in solution will significantly improve the TAA hydrolyzing degrees, providing the sulfur sources. In the laser ablation (0~10 ns) process, the superheating will play a critical role in the early stage, resulting in the formation of Zn plasma characterized by excited Zn ions with high temperature and pressure. The hot and dense Zn plasma initiated by the laser ablation expands rapidly in liquid, resulting in the formation of Zn crystalline. Meanwhile, during laser induced excited Zn ions in the activated liquid, the outsider region of the Zn plasma can easily conjunct with S element. The homogeneous nucleation of Zn and S will take place in the stage of rapid condensation of the plasma, and sharply terminate due to expiration of the pulse. It will result in the formation of ZnS shell with amorphous structure on the core Zn crystalline. The acid condition plays an important role for the formation of ZnS/Zn core-shell structure, which is also the main reason for the fabrication of ZnS/Zn nano-cages. As shown in Fig. 3, enough concentration of HCl will result in an ultra-rapid acid etching process within the hot Zn plasma. The ultra-rapid chemical reaction enables some part of Zn material to be dissolved/removed from Zn crystalline, and then provides the formation of abundant voids in the final particles. The acid etching process will also rapidly terminate because of the formation of ZnS shell structure, resulting in the fabrication of ZnS/Zn nano-cages.

 

Fig. 3 The schematic growth of ZnS/Zn core-shell nano-particle and nano-cage structures by multiple pulses laser ablation of Zn target in liquid solution containing low and enough HCl.

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Finally, the unique photo-catalytic performances of the as-prepared ZnS/Zn nano-materials are illustrated in Fig. 4 . The results show the photo-catalytic reductions of 10 mL of 1 × 10⁻² M aqueous K₂Cr₂O₇ solution under visible light irradiation in the presence of the ZnS/Zn core-shell nano-particles and nano-cages, respectively. The dosage of the photo-catalyst in each dichromate solution is 2.7 mg. As shown in Fig. 4(a), the as-prepared core-shell-like ZnS/Zn nano-particles exhibit good photo-catalytic activity. The main absorption of Cr(VI) at about 360 nm decreases from 1 to 0.242 with the exposure time of 0~15 min, which is also agreement with the gradual color change of the solution from yellow to light yellow (the inset in Fig. 4(a)). Then, the absorption peak gradually reduced to about 0.04 (the reduction of nearly 96% Cr(VI)) within 50 min of irradiation. Interestingly, some more enhanced photocatalytic activity by using ZnS/Zn nano-cages as photo-catalyst is illustrated in Fig. 4(b). Compared with the result of core-shell structures, the absorption of Cr(VI) at 360 nm sharply dropped from 1 to 0.03 (the reduction of nearly 97% Cr(VI)) within 15 min, which can be confirmed by the color change of the solution from yellow to colorless (the inset in Fig. 4(b)). Figure 4(c) shows the reduction time dependence of the relative concentration C/C₀ of the Cr(VI) by using 2.7 mg ZnS/Zn core-shell nano-particles and nano-cages, respectively. Where C and C₀ are the concentration of Cr(VI) after visible light irradiation and initial solution, respectively. As for ZnS/Zn nano-cages, the Cr(IV) content (99.9%) can be completely removed from solution under visible light irradiation for 20 min. The completely reduction time is about one third of the result by using core-shell nano-particles. Moreover, Fig. 4(d) shows the completely reduction times as functions of ZnS/Zn core-shell nano-particles and nano-cages concentrations. In brief, the required reduction time decreases linearly with increase of ZnS/Zn nano-materials concentration. The enhanced photo-catalytic activity of the ZnS/Zn nano-cages should be highly related to the porous surface structure. It has been proved that the visible-light derived photo-catalytic reduction of Cr(VI) is strongly affected by photo-excited electron and dispersed electron-hole structure [3]. The dispersed electron-hole is easily to be formed in the ZnS/Zn nano-cage structures with higher surface area than the core-shell structures. The ZnS/Zn nano-cage fabricated by laser ablation in solution is an excellent photo-catalyst with enhanced activity in reduction of Cr(VI) under visible light irradiation. It has significant implications for polluted waters treatments in the further.

 

Fig. 4 (a-b) Photocatalytic reduction of 10 mL of 1 × 10⁻² M aqueous K₂Cr₂O₇ solution under visible light irradiation in presence of the as-prepared ZnS/Zn core-shell nano-particles and nano-cages, respectively. The photo-catalyst in each solution is 2.7 mg. The insets show the direct photographs of gradual color change of the solution with irradiation time of 0~15 min. (c) Photo-catalytic reduction-time dependence of the relative concentration C/C₀ of the Cr(VI) in the solution. (d) The completely reduction times as functions of ZnS/Zn core-shell nano-particles and nano-cages concentrations.

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4. Conclusions

In conclusion, we have reported the synthesis of ZnS/Zn core-shell nano-particles and nano-cages by pulses laser ablation of Zn metal in activated liquid. The enough HCl in the solution plays an important role in the fabrication of nano-cage structures, owing to the ultra-rapid acid etching and removing of Zn in the early stage of the nucleation process. Benefiting from the unique porous surface structure, the as-prepared ZnS/Zn nano-cages show excellent photo-catalytic activity in reduction of aqueous Cr(VI) under visible-light irradiation. The results suggest that the ZnS/Zn nano-cage is a promising photo-catalyst in utilization of solar energy for reduction of aqueous Cr(VI) under visible-light irradiation. The developed route will offer a simple and versatile way of synthesizing highly efficient photo-catalysts for reduction of other toxic elements in the environment.