We report the successful synthesis of ZnO porous nano-cages with controllable hollow spaces by simply using laser ablation of Zn target in liquid medium containing deionized water and ammonia (Vwater:Vammonia = 7:1~5:1). In addition to the porous surface, the created interior space of the ZnO nano-cage substantially increases with the ammonia concentration. The related growth mechanism has been illustrated based on the ultra-rapid alkaline etching process. Moreover, numerous Zn(NH3)42+ clusters generated by the selective etching route readily penetrate into the ZnO porous structures and can be embedded in these unique nano-cages. It is envisaged that these composite ions/ ZnO porous nano-cages have significant implications for gas sensing and catalytic applications. The synthetic scheme used here should also be applicable to other semiconductors.
© 2014 Optical Society of America
As one of many challenges in the design and synthesis of hollow nano-objects, ZnO porous nano-cages with a controllable interior space and a distinctive porous shell have received increasing attention in recent years owing to their importance in the fabrication of super-active catalyst, faster responsive sensor, biomedical material, etc [1–8]. Compared with various ZnO nano-materials possess solid interiors, such as wires, belts, needles, and other complex solid counterparts, ZnO porous nano-cages are characterized as fine nano-particles with lower densities, larger specific surface area, and higher permeability [4–7]. Moreover, the large fraction of void space in ZnO nano-cage can be used to encapsulate and control release of sensitive materials, such as drugs, cosmetics, and DNA, etc, since the unique porous nano-structure exhibits a well adsorptive capacity of several chemical molecules and ions. Because the properties of the ZnO porous nano-cages are strongly dependent on their porous surfaces and hollow interiors, the controlled synthesis of various ZnO nano-cages is of great significance for the specific applications [1–8].
Among all available synthesis methods, such as thermal evaporation , ultrasonic irradiation , template-based method , laser ablation in liquid medium (LAL) represents an attractive technique with its highly nonequilibrium processing character, which allows synthesis of novel metastable phases of material by the addition ionic surfactants during the ablation process [2,4,9–12]. Recently, many interesting works such as generation of hollow MgO particles , hollow Al2O3 nanostructure , unique Pt hollow structure , and fullerene–like Fe19Ni81 nanoparticle , have been reported using laser ablation in liquid condition. It has been believed that the formation of these hollow particles directly from bulk material should be related to laser generated bubbles via bubble surface pinning by cluster or particles [9,11–13]. Compared with these reactive metals, ZnO nano-cages with hollow interior space were hard to produce by laser ablation of bulk Zn metal in water [11–13]. This technique should be improved for the fabrication of ZnO porous nano-cages.
As for the laser ablation of Zn target in liquid, it is inevitable that the zinc hydroxide Zn(OH)2 will be mixed into the ZnO structures, resulting in the formation of ZnO/Zn(OH)2 hybrid nano-composites [2,4]. On the other hand, the Zn(OH)2 byproduct is amphoteric hydroxide, which will dissolve in weak alkaline condition by the decomposition reaction. The liquid solution can be adjusted to obtain desirable nano-materials. If the ablation of Zn metal is carried out in a liquid phase containing some ammonia hydroxide, the ablated Zn(OH)2 material can be dissolved and removed from the ZnO/Zn(OH)2 hybrid nano-composites. Then ZnO nano-cages will be obtained due to the decomposition reaction occurred between zinc hydroxide and the activated liquid. In this work, we report the successful synthesis of ZnO porous nano-cages with hollow spaces by simply using laser ablation of Zn target in liquid medium containing deionized water and ammonia (Vwater:Vammonia = 7:1~5:1). The ammonia concentration played a critical role for the final nano-particles. If the ammonia concentration below and high than that used in this paper, only some incomplete hollows-like and numerous collapses structures, instead of ZnO nano-cages, were formed after laser ablation. Moreover, the ZnO nano-cages obtained by this simple and versatile strategy trend to interconnect with each other via magnetic-dipole interaction, and then forming short curvilinear groups with necklace-like structure. In addition, the interconnected ZnO porous nano-cages exhibits a well adsorptive capacity of meta-stable Zn(NH3)42+ clusters. On the other hand, ZnO/Zn(OH)2 hybrid nano-composites produced in deionized water were compared with ZnO porous nano-cages, in order to get an insight into the translation from hybrid composites to the porous nano-cages. Meanwhile, a detailed discussion of the related mechanism is addressed. The aim of this work is to extend the scope of porous nano-cages that pulsed laser ablation in liquid can fabricate, and inspire deeper investigations for generating more complex structures by this strategy.
2. Experimental setup
The experimental apparatus herein used pulsed laser ablation of solid target in liquid have been described in details elsewhere [2,4,7,9,11–13]. Briefly, a well polished pure (99.99%) zinc metal target was placed on the bottom of a rotating glass dish filled with 6mm depth of liquid solution containing deionized water and ammonia (Vwater:Vammonia = 7:1~5:1). A Q-switched Nd-YAG(Yttrium Aluminum Garnet) laser (Quanta Ray, Spectra Physics) operating at wavelengths of 1064 nm with a pulse duration of 10ns and 10Hz repetition rate was focused onto the zinc target surface. The power density of the laser beam was about 8 GW/cm2, and the ablation lasted for 1 hour. Moreover, ZnO/Zn(OH)2 hybrid nano-composites produced in deionized water were also obtained at the same condition. After laser ablation, the products were carefully washed in ethanol three times. The colloidal suspensions were dropped on a copper mesh and dried in oven at 60°C for observation by transmission electron microscopy (JEOL-JEM-2100F). Morphological investigations and chemical compositions were also carried out with field emission scanning electron microscope (Hitachi S-4800) equipped with energy-dispersive x-ray spectroscopy (EDS). The specimens for scanning electron microscope (SEM) imaging were prepared by small drops of colloidal suspensions placed on silicon wafers and dried in oven at 60oC. The crystallographic investigations of the nano-particles were established by X-ray diffraction (XRD) patterns (Rigaku RINT-2500VHF) using Cu Kα radiation (λ = 0.15406nm). The absorption spectra of the nano-particles were measured with a UV–Vis–IR spectrometer (Cary 50).
3. Results and discussion
After laser ablation of Zn metal in deionized water and activated liquid, the typical morphologies of the nano-particles were analyzed by transmission electron microscopy (TEM), as shown in Figs. 1(a) and 1(b), respectively. Figures 1(c) and 1(d) represent the high resolution images of representative nano-particles in Figs. 1(a) and 1(b), respectively. The low-magnification image in Fig. 1(a) clearly shows that numerous quasi -spherical nano-materials with solid interiors are accreted with some nano-sheets on the out surfaces. The HRTEM image in Fig. 1(c) provides the structural detail of the nano-particle by laser ablation of Zn target in deionized water. The areas marked by blues lines in Fig. 1(c) with a periodicity corresponding to a d-spacing of 0.271nm could be indexed with reference to a β-Zn(OH)2 structure . Similarly, the regions marked by red lines in Fig. 1(c) with a d-spacing of 0.262nm are thus indexed as (002) plane in the wurtzite ZnO structure. It is confirmed that the deionized water used as the ablation media in this paper, results in final particles characterized by ZnO/Zn(OH)2 hybrid nano-composites. By the addition of ammonia (Vwater:Vammonia = 7:1), the nano-particles indeed have different morphologies, as shown in Fig. 1(b). Closer view of these nano-particles in Fig. 1(b) indicates that the products are hollow nano-particles with numerous surface pores. According to the statistics, the pores observed in Fig. 1(b) are in the size of 3~5nm. The average diameter of the final particles is approximately 40nm with a narrow size distribution, which is noticeably smaller than that in Fig. 1(a). Moreover, the porous nano-cages trend to interconnect with each other via magnetic-dipole interaction, and then forming short curvilinear groups with necklace-like structure. The HRTEM image in Fig. 1(d) illustrates that the nano-cage structure is found to be well crystalline according to the clear lattice fringes, but obviously porous. Correspondingly, the lattice fringes with spacing of 0.262 nm can be indentified for ZnO(002) plane. The pores in ZnO 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 above results in Fig. 1 clearly confirm that the ablated Zn(OH)2 material can be dissolved and removed from the ZnO/Zn(OH)2 hybrid nano-composites, due to the reaction occur between zinc hydroxide and activated liquid. Following this mechanism, a higher concentration of ammonia in the activated liquid should result in higher void space in the ZnO nano-cages. To verify it, we increased the concentration of ammonia via deionzed water to 1:5, and found that the interconnected ZnO nano-cages with an obvious interior cavity were obtained, as illustrated in Fig. 2. More clearly, the created interior space and specific surface areas of the ZnO porous nano-cages substantially increase with the ammonia concentration. Some typical regions referred as ZnO nano-cages and hollow structures are marked by blue and yellow lines in Fig. 2, respectively. The shell thickness of the nano-cages is measured about 8 nm. In addition to the unique hollow structure, it is further noted that the numerous amorphous materials without any lattice fringes were absorbed/trapped in the ZnO porous nano-cages, as the representative regions marked by red lines in Fig. 2. These unknown materials will be investigated in following section.
Figures 3(a) and 3(b) depict XRD patterns of the nano-particles prepared in deionized water and activated liquid(Vwater:Vammonia = 5:1), respectively. As shown in Fig. 3(a), besides the ZnO diffraction peaks, a series of (00l) β-Zn(OH)2 diffraction peaks were also indeed found in the low angle range(10°~25°) from the products. While, the XRD pattern of nano-cages formed in the activated liquid only contains diffraction peaks from wurtzite ZnO in Fig. 3(b), which is the best confirmation of the removing Zn(OH)2 byproduct through selective alkaline etching process. Moreover, no other peaks were found in Fig. 3(b), indicating that the trapped materials in ZnO nano-cages should be amorphous structures. Then, the exterior morphologies and chemical compositions of these nano-materials are further examined with scanning electron microscopy (SEM) equipped with energy-dispersive x-ray spectroscopy (EDS).
The SEM images of the samples from deionized water and activated liquid (Vwater:Vammonia = 5:1) are shown in Figs. 4(a) and 4(c), respectively. Figures 4(a) and 4(b) are the low and high magnification images of ZnO/Zn(OH)2 hybrid nano-composites, which reveal that the quasi-spherical nano-particles with average diameter of 200 nm are likely fabricated one by one separately, since there are almost not hinge joints. In addition, the result of EDS in Fig. 4(e) clearly demonstrates that the nano-particles are composed of Zinc and Oxygen elements. The ratio of Zn to O in the nano-particles is about 2/3, which is different from the stoichiometry of ZnO due to some Zn(OH)2 formed in the final products. As for the SEM image of ZnO nano-cages in Fig. 4(c), a very different morphology presents that numerous flower-like ZnO micro-structures aggregate with multi-leaves, and almost all of them show same morphology. From the enlarged image of a ZnO micro-flower in Fig. 4(d), each flower is make up of many nano-sheets, and then each sheet is consist of numerous nano-particles. The average diameter of these nano-paritcles in Fig. 4(d) is about 23nm, which is consistent with the size of ZnO nano-cages in Fig. 1(b). Furthermore, the EDS pattern in Fig. 4(e) shows that the ZnO micro-flowers are not only made of zinc and oxygen, but also composed of nitrogen element. The ratio of Zn, O to N in the nano-particles is about 5:3:7. Since nitrogen element found in EDS, the unknown materials trapped in ZnO nano-structures in Fig. 2 should be some compounds with ratio of Zn to N elements being about 2:7, this is obviously different from a common Zn3N2 material. In principle, some light elements, such as hydrogen, cannot be found in EDS pattern. The problem of what other compositions in these compounds will be partly solved by the absorption spectra of the nano-particles using a UV–Vis–IR spectrometer.
Some remarkable differences in UV-visible optical absorption spectrums of ZnO/Zn(OH)2 hybrid nano-composites and ZnO nano-cages were observed in Fig. 5(a). When the continuous light passed through the hybrid nano-composites, an absorption edge at ~375nm due to the interband or exciton absorption of ZnO nanoparticles was observed in Fig. 5(a)(blue curvilinear). In the case of ZnO nano-cages, only the absorption spectrum at around 210 nm can be clearly detected, as the purple curvilinear shown in Fig. 5(a). The experimental result exhibits fairly narrow absorption ultra-spectrum with the maximum peak centered at 212 nm and its full width half maximum (FWHM) of 13nm, which is completely consistent with the absorbance spectra and evaluation region of NH3 [15,16]. Since the metastable Zn(NH3)42+ clusters has been discussed in previous work [17,18], it is reasonable to deduce that the unknown material absorbed in ZnO nano-cages should be Zn(NH3)42+ clusters, considering the compounds with the ratio of Zn and N being about 2:7 in Fig. 4(e). In the following section, we will describe the possible growth processes for the formation of ZnO porous nano-cages. Figure 5(b) provides a schematic growth diagram of ZnO porous nano-cages by laser ablation of Zn metal in ammonia water.
In brief, the growth mechanism of final particle involves the formation of Zn(OH)2 nano-particles firstly, according to the following reaction formulas :2,4,7,9–13] will result in a ultra-rapid alkaline etching process within a single laser pulse(10ns). The ultra-rapid chemical reaction enable the Zn(OH)2 to be dissolved/removed from ZnO/Zn(OH)2 hybrid composites, as the following formulas [17,18]:
The ultra-rapid reaction in a confined space enables the formation of abundant voids in the final particles that greatly deviates from the equilibrium process. Because of the rapid cooling due to expiration of the pulse (>10ns) and exhaustive expansion of the vapor, the metastable Zn(NH3)42+ clusters cannot further hydrolyzed to ZnO without the additional energy, which is different from the completely reaction discussed in previous report [17,18]. The hollow like ZnO nano-cages should be formed via this complicated reaction. Moreover, the nonoequilibrium condition during the fabrication of ZnO nano-cages can also lead to various defects, resulting in the present of abundant disorderly arranged Zn and O species, which has been verified in previous works [2,7]. After laser ablation, the rapid quenching process enables the disorder degrees of Zn and O species become more and more intensive, which makes these species at a highly excited state. As a result, these Zn(NH3)42+ clusters readily penetrate into the porous structures of the ZnO nano-cages by the defect formation energy, which will be finally trapped and adsorbed via the interaction with the electron-rich excited oxygen of ZnO nano-cages. In this paper, a higher concentration of ammonia in the activated liquid (Vwater:Vammonia = 5:1) would elevate the reaction rate and site, which leads to more completely dissolution of Zn(OH)2 material during laser ablation process, and then the fabrication of higher void space in the ZnO nano-cages. It is believed that the adsorptive capacity of Zn(NH3)42+ clusters will be further improved by the enlarged specific surface area and active sites for reaction. Moreover, the embedding state of ion-clusters, rather than usual surface attachment, would make ZnO porous nano-cages as specific UV-light absorbing device in the further.
In summary, the ZnO porous nano-cages with hollow interiors have been devised through laser ablation of Zn metal in liquid medium containing deionized water and ammonia (Vwater:Vammonia = 7:1~5:1). To reveal the related mechanism, the weak alkaline etching process involving ultra-rapid dissolution of Zn(OH)2 from ZnO/Zn(OH)2 hybrid nano-composites has been proposed. The ZnO nano-cages trended to interconnect with each other via magnetic-dipole interaction. Moreover, the interconnected ZnO porous nano-cages exhibit a single absorption spectrum at around 210nm with its FWHM of 13nm, which was explained by the adsorption of the metastable Zn(NH3)42+ clusters during formation of ZnO nano-cages. On the basis of the present findings, this work provides a new paradigm to obtain hollow-like nano-cage directly from bulk materials and inspires deeper investigations for generation more complex structures by this strategy.
This work was supported by the Natural Science Foundation of China under Grant Nos.11105085, 11275116 and 11375108, the Excellent Youth and Middle Age Scientists Fund of Shandong Province under grant No. BS2012CL024, and the Project of Shandong Province Higher Educational Science and Technology Program under grant No.J12LA51.
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