Fabrication of hollow PbS nanospheres and application in phenol release
© Ye et al.; licensee Springer. 2013
Received: 27 January 2013
Accepted: 25 April 2013
Published: 18 July 2013
This article demonstrates a versatile method to prepare the hollow PbS nanospheres via the template method. First, the latex poly (vinyl benzyl chloride) (PVBC) nanoparticles were synthesized by the radical polymerization, followed by the atom transfer reversible polymerization of lead (II) dimethacrylate (Pb (MA)2) on the surfaces of the latex nanoparticles. Then, the ethanethioamide was reacted with the nanoparticles to afford the PbS. By calcination at 600°C for 6 h, the template was removed to obtain the hollow PbS nanospheres. The structure, morphology and optical properties of the hollow PbS nanospheres were carefully investigated. The received hollow PbS nanospheres could be used for the controlled release of phenol after absorbing phenol solution.
Semiconductor materials have been the focus in material science, due to their prominent optical and electronic properties and their potential applications for the preparation of optical signal processors and switches (Ridley et al. 1999). As an important IV–VI group semiconductor, lead sulfide (PbS) has attracted considerable attention for many decades due to its specially small band gap (0.41 eV at 300 K) and a larger exciton Bohr radius of 18 nm (Machol et al. 1993). What is more, a blue shift from near infrared to visible region can occur. This kind of material presented fantastic optical and electric properties, which could be potentially applied widely in optoelectronic materials such as inductor, infrared detector, photoelectric converter and solar cells (McDonald et al. 2005; Deng et al. 2009; Gao et al. 2011; Lee et al. 2008; Noone et al. 2010; Asunskis et al. 2008; Jain et al. 2010). The design and fabrication of PbS nanostructure have attracted considerable attention in recent years. The synthesis of closed nanorods (Wang & Yang 2000), nanowires (Lau et al. 2009) and dendritic structures (Zhou et al. 2006) by different methods such as solvothermal, microwave irradiation and thermal decomposition have been reported by many groups. Additionally, PbS crystals with other morphologies have also been reported (Lee et al. 2002; Peng et al. 2008).
Recently, hollow micro- or nanospheres have attracted much attention because of their specific structures and potential applications. Owing to their low density, large surface area, and surface permeability, hollow spheres were widely used as artificial cell, catalysts, fillers, and capsules for controlled release of drugs and dyes (Huang et al. 1999; Caruso 2000). The hollow nanospheres could be often prepared via the template methods, such as the one-pot preparation of hollow silica spheres by using thermosensitive poly (N-isopropylacrylamide) as a reversible template (Du et al. 2009), and the fabrication of polymer nanocapsules with cross-linked organic–inorganic hybrid walls (Chen et al. 2012). The hollow PbS nanospheres have been obtained via the sonochemical synthesis method (Wang et al. 2006) and the block copolymer micro-emulsion based approach (Ding et al. 2007; Gao et al. 2005). However, few articles reported on the synthesis of hollow PbS nanospheres prepared with the assistance of any surfactant and atom transfer reversible polymerization (ATRP) process. Furthermore, the application of hollow PbS nanospheres in the field of controlled release needs to be explored.
In this paper, a simple chemical precipitation route was demonstrated for the fabrication of poly (vinyl benzyl chloride)@PbS (PVBC@PbS) core-shell and hollow PbS nanospheres. The structure, morphology and optical properties of the hollow nanospheres were investigated. The hollow PbS nanospheres embedded with phenol could be used for the controlled release of phenol. It is worth noting that the method demonstrated in this article may potentially find useful application in the field of controlled release.
The vinyl benzyl chloride (VBC), polyethylene-polypropylene glycol, sodium dodecylbenzenesulphonate, azobisisobutyronitrile (AIBN) and ethanethioamide were purchased from Sinopharm Chemical Reagent Corporation Ltd. The p-toluenesulfonylchloride and 2, 2'-dipyridyl (Bpy, >99%) were purchased from ACROS. The solvents were purchased from Tianjin Fuchen Chemical Reagent Factory. The chemicals were used as received without further purification. Copper (I) chloride (CuCl) was purified according to procedures described in the literature (Matyjaszewski et al. 1999). Lead dimethacrylate (LDMA) was synthesized according to the literature (Dave 1984) and re-crystallized from ethanol.
Preparation of poly (vinyl benzyl chloride) (PVBC) latex nanospheres
Vinyl benzyl chloride (21.4 g, 0.14 mol), polyethylene-polypropylene glycol (4.0 g), sodium dodecylbenzenesulphonate (0.5 g, 1.43 mmol) and deionized water (74.0 mL) were charged into a 250 mL flask. After vigorous stirring for 30 min, the AIBN (0.065 g, 0.39 mmol) was added and the reaction mixture was heated to 65°C and stirred for 10 h. Then, the mixture was poured into a 500 mL beaker containing ethanol. The precipitated polymer was filtered and extracted with ethanol and deionized H2O for 6 h. The PVBC nanospheres were dried in vacuum at 80°C for 12 h and isolated as white solid in a weight yield of 87%, and the result could be reproduced by the careful control of the reaction conditions.
Surface-initiated atom transfer radical polymerization
Into a Pyrex tube, the PVBC latex nanospheres (0.04 g) were dispersed in 40 mL of ethanol solution containing LDMA (3.2 g, 8.5 mmol) and p-toluenesulfonylchloride (64.8 mg, 0.34 mmol). The mixture was purged by nitrogen for 30 min, followed by the addition of CuCl (33.7 mg, 0.34 mmol) and Bpy (106.2 mg, 0.34 mmol). The CuCl and Bpy were used as the catalyst in the atom transfer radical polymerization. The Pyrex tube was sealed and kept at 90°C for several hours after purging with argon for another 10 min. The poly (vinyl benzyl chloride) grafted poly (lead dimethacrylate) (PVBC-g-PLDMA) nanospheres were collected by centrifuging at 20000 rpm. The collected nanospheres were re-dispersed in ethanol and centrifuged at 1000 rpm for 5 min to remove any Cu (II) precipitate formed in the ATRP process. The nanospheres were then collected by centrifuging at 20000 rpm and subjected to repeated cycles of washing with ethanol and centrifugation to remove the untreated monomer and homo-polymer before further characterization.
Reaction with ethanethioamide
The PVBC-g-PLDMA nanospheres were dispersed into 10 mL ethanol in a 100 mL flask. Ethanethioamide (2.25 g, 0.03 mol, 3 M solution in ethanol) was added in 3 h under vigorous stirring. The reaction mixture was stirred for additional 12 h, and the precipitation was isolated with centrifugation and washed with ethanol for 5 times. The received PVBC@PbS nanospheres were dried in vacuum at 40°C for 24 h.
Removing the PVBC templates by calcination
The PVBC@PbS core-shell hybrid nanospheres were added into quartz dish and then put into tubular-furnace, after vacuum pumping and circulated by argon. The temperature was raised to 600°C and stood for 6 h, followed by cooling to room temperature. The nanospheres were dispersed into 50 mL THF in a 100 mL flask and stirred for 12 h. The mixture was filtered and washed with deionized water for 3 times. The received hollow PbS nanospheres were dried in vacuum at 40°C for 24 h.
Release behavior of hollow PbS nanospheres loaded with phenol
Due to the sensitive channel pathway in hollow PbS nanospheres, the controlled release behavior of hollow PbS nanospheres loaded with phenol was investigated. The hollow PbS nanospheres embedded with phenol was prepared by three different ways. First, 100 mg of hollow PbS nanospheres was added into 20 mL phenol aqueous solution (0.01 M). The hollow PbS nanospheres were then collected by centrifugation after incubation at room temperature for 24 h and washing by deionized water for 5 times. Second, hollow PbS nanospheres (100 mg) were heated to 300°C under nitrogen and then added into 20 mL phenol aqueous solution (0.01 M) directly. The nanospheres were then collected after stirring for 30 min and washed by deionized water for 5 times. Third, hollow PbS nanospheres (100 mg) were added into 20 mL phenol aqueous solution (0.01 M) and stirred for 3 h under vacuum at room temperature. The nanospheres were collected after washing by deionized water for 5 times. The absorbance of the solution was measured by the UV spectrophotometer, and the phenol concentration was calculated by the use of the standard curves.
The hollow PbS nanospheres embedded with phenol were added into dialysis bags, followed by the soaking in 50 mL deionized water under stirring. The solution was measured by the UV spectrophotometer at wavelength of 268 nm. After 36 h, the data were collected and analyzed in comparison with the release behavior of pristine phenol.
The fourier transform infrared resonance (FTIR) spectra of the samples dispersed in KBr disks were recorded on a SHIMADZU IRprestige-21 spectrophotometer. Transmission electron microscope (TEM) analysis was used to characterize the morphology of the nanospheres. In a typical experiment, several drops of the colloidal dispersion were introduced onto a carbon film supported by a copper grid. The droplet was allowed to dry in air, and then observed under a JEM-2010 (HR) transmission electron microscope operating at an acceleration voltage of 100 kV. The X-ray diffraction (XRD) study of the samples was carried out on a Bruker D8 Focus X-ray diffractometer, operating at 40 kV and 40 mA with a copper target (λ = 1.54 Å) and at a scanning rate of 2°/min. The ultraviolet–visible spectra of the samples were obtained with a Hitachi UV-2300 spectrophotometer. The photoluminescence (PL) of the samples was measured on a Shimadzu RF-5301PC spectrofluorophotometer. Both experiments were performed at the ambient temperature.
Results and discussion
Release behavior of phenol adsorbed by hollow PbS nanospheres
The standard curve between concentration and absorbance (determined by UV spectra) for the phenol aqueous solution has been carefully prepared. It is revealed that a good linear relationship could be obtained at the phenol concentration below 30 mg/L. Based on the standard curve, the phenol concentration in solution could then be calculated after the measurement of absorbance in solution.
The hollow PbS nanospheres have been successfully fabricated via the convenient template method, including the preparation of PVBC latex nanoparticles by radical polymerization, grafting of PLDMA onto PVBC via ATRP, and removal of the polymer template by calcination. The hollow PbS nanospheres were highly crystalline and exhibited the quantum confinement due to the smaller average size of PbS nanospheres than that of the excitonic Bohr radius of the bulk PbS. The hollow PbS nanospheres could be used for the controlled release of phenol, which might be contributing to the existence of PbS nanosphere shell.
This Project Supported by Scientific Research Fund of Anhui Provincial Education Department (KJ2010B104).
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