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Photoluminescent ZnO nanoparticles have wide applications in biolabeling. A dual phase hydrothermal method has been developed in this paper to synthesize nanoflower-shaped ZnO nanoparticles. Hydrogen peroxide was identified as a unique oxygenic source to promote the formation of ZnO nanoflowers from the organic zinc precursor. The reaction mechanism for the formation of ZnO nanoflowers was proposed and studied by Fourier transform infrared (FTIR). The as-prepared hydrophobic colloidal ZnO nanoparticles could be subsequently modified to water-soluble ZnO nanoflowers via a ligand exchange process with aminethanethiol HCl. The structure and optical properties of the ZnO nanoparticles were studied by transmission electron microscopy, X-ray diffraction, and photoluminescence measurement (PL). Both types of ZnO nanoflowers demonstrated good photoluminescent properties which could have wide applications.
© 2014 Optical Society of America
1. Introduction
Nanomaterials, differing from those of the corresponding bulk materials, always have novel physical, chemical and optical properties, which make them potential candidates for a wide range of applications [1, 2]. ZnO nanoparticles have a wide band gap (3.37 eV at room temperature) and large exciton bonding energy (~60 meV). They have been widely used for solar cells, light-emitting diodes, photonic crystals, photodetectors and biological sensors [3–6]. Since pioneer reports from Xiong et al. [7], the application of photoluminescent ZnO nanoparticles in biolabeling have emerged to replace Cd-based fluorescent labels, due to their non-toxicity, less expensive and chemically stable properties [8–13].
To date, a few methods have been developed to synthesize photoluminescent ZnO nanoparticle. The sol–gel route is a traditional method to prepare ZnO nanoparticles, however, the as-prepared ZnO nanoparticles are usually not very stable and tend to aggregate due to their high surface energy [14]. In photoluminescence (PL), the visible emission properties of ZnO are strongly dependent on their defects. In order to obtain stable photoluminescent ZnO nanoparticles, ligands and core-shell structures have been explored to protect the ZnO from high concentration of defects. For instance, ligand capping technique to bind a protective ligand on ZnO nanoparticle surface has been used to stabilize the ZnO nanoparticles. Various surfactants/polymers, such as oleic acid, 1-octadecanol, diethanolamine (DEA), and polymethyl methacrylate (MMA) [15–17], were used. The core-shell architecture of the nanoparticle structure is another technique to stabilize photoluminescent ZnO [7, 8, 11]. Meanwhile, the doping technique has also been shown by Liu et al. to stabilize the particles as well as to control the particle sizes [12].
In some of the theoretical and experimental works, ZnO nanoflowers have been reported to have high performance in various applications, such as photocatalysts, sensors, solar cells etc., due to their unique morphologies and surface states, including surface defects (surface oxygen vacancies), bound excitons, and surface adsorption [18–21]. However, the photoluminescent ZnO nanoflowers are rarely reported [22, 23]. The synthesis of well-defined photoluminescent ZnO nanoflowers remain a challenge. Herein, we report a dual-phase hydrothermal method to produce highly monodispersed photoluminescent flower-shaped ZnO nanoparticles. The as-prepared ZnO nanopartilces with blue emission wavelength at around 420 nm could be readily dispersed in nonpolar solvents, such as hexane and toluene. For the application of biolabeling, water soluble ZnO nanoparticles are always required. We have also applied a ligand exchange method to obtain the water-soluble ZnO nanoparticles while retaining its optical property [24–26].
2. Experimental
2.1 Synthesis of ZnO nanoparticles
Zinc stearate, and amineothanethiol·HCl (AET) were purchased from Alfa Aesar. 1-Octadence (ODE, tech 90%) was purchased from Aldrich. Hydrogen peroxide (H2O2, tech 30%) was purchased from Shanghai Yuanda Co., Ltd. Toluene and methanol were purchased from Sinopharm Chemical Reagent Co., Ltd. All of the chemicals were used without further purification. Ultrapure water used in all experiments was produced by a Milli-Q apparatus (Millipore) with resistivity over 18.2 MΩ cm.
For a typical synthesis process, a mixture of 0.2 mmol zinc stearate (126.5 mg) and 0.2 mmol ODE (5.64 mL) were added into a 25 mL three-neck round bottom flask. The reaction mixture was heated to 260 °C using a heating mantle at a rate of 2 °C/min with the reaction flask back-filled with N2.At this temperature, 2 mL H2O2 was added dropwise into the flask at a rate of 1 drop/min. The reaction begins at the point when the solution turned from colorless to dark orange. The reaction was then refluxed at 260 °C for an additional 2 hours. The pH of the reaction mixture in the flask was kept between 6 to 7. At the end of the reaction, the nanoparticles were separated and washed using a 1:1 (v/v) mixture of toluene and methanol. The final products were dispersed in 1 mL toluene or chloroform for storage.
2.2 Preparation of water-soluble nanocrystals
The AET-coated water-soluble nanocrystals were synthesized by a phase transfer method modified from the literature [24–26]. A 100μL crude solution of as-prepared ZnO nanoparticles were firstly dissolved in 1 mL chloroform, followed by the addition of 0.5 mL of methanol with AET. Subsequently, 1 mL of ultrapure water was added to the suspension, causing the solution to flocculated, which resulted in a dual-phase mixture (water was on the upper layer). After a vigorous shaking, the ZnO nanoparticles were finally exchanged into the water phase.
Electron microscope specimens were prepared by dispersing the suspension of nanoparticles in toluene (1 mg/mL) and drop-casting onto transmission electron microscopy grids. Transmission electron microscopy (TEM) images were collected using the JEOL TEM 2010 working at 200 kV, equipped with energy dispersive X-ray analyzer (Phoenix). X-ray diffractions (XRD) were carried out with a Rigaku D/max-2500 using filtered Cu Kαradiation. UV-vis absorption (Abs) spectra and photoluminescence (PL) of various samples were recorded by Hitachi UV-4100 and Fluorolog-3-Tau Steady-State/Lifetime Spectrofluorometer, respectively. Fourier transform infrared (FTIR) spectroscopic study was carried out with a Magna-IR 750 (Nicolet Instrument Co.) FTIR spectrometer.
3. Results and discussion
Figure 1(a) shows a TEM image of typical ZnO flower-like nanocrystals with the mean size of about 30 nm. Detailed structure of a nanoparticle can be observed under the high-resolution transmission electron microscopy (HRTEM) image (Fig. 1(b)). As shown in Fig. 1(b), the ZnO nanoparticle is composed of one core and several leaves with average size about 5 nm and 10 nm respectively. HRTEM images also revealed that the nanoparticle is a single crystal with the lattice space of 2.81Å, which can be assigned to the (211) planes of ZnO.
Fig. 1 (a) TEM image of flower-shaped ZnO nanoparticles; (b) High-resolution TEM image of a single ZnO nanoparticle and (c) the corresponding SAED pattern of nanoparticles.
XRD result of the ZnO nanoparticles is shown in Fig. 3. Nearly all the characteristic peaks of synthetic ZnO nanoparticles (top curve) could be observed and were very consistent with the standard values of bulk ZnO (bottom vertical lines) (JCPDS card 36-1451), indicating that the ZnO nanoparticles are of the wurtzite structure. The average size of the nanoparticles was calculated through the Debye-Scherrer equation to be 31.5 nm, and the corresponding size distribution is recorded in the Fig. 2(a).These results are also consistent with the observation in Fig. 1(a). Selected area electron diffraction (SAED) pattern of ZnO nanoparticles was also performed and the result is shown in Fig. 1(c). Seven diffraction rings were observed and further assigned to the (100), (002), (101), (102), (110), (103) and (200) planes of wurtzite ZnO, which is consistent with the XRD pattern in Fig. 3.
Fig. 2 Size distribution of ZnO nanoparticles before (a) and after (b) ligand exchange. There was no significant change in the size distribution, which shows that the flower-shaped ZnO nanoparticles remained as a stable colloid after ligand exchange.
The possible reaction processes of hydroxyester elimination reaction between zinc stearate complex and H2O2 are shown in Fig. 5, which could be further confirmed by FTIR spectroscopy (Fig. 4(a)). Before the addition of H2O2, the peaks of -CH2- vibration (1462 cm−1) and –COO- asymmetric bending (1536 cm−1) of zinc stearate could be observed (dotted line in Fig. 4(a) [27, 29]. However, an ester -C = O vibration (1739 cm−1) appeared in the FTIR spectrum after the ZnO nanocrystals were formed (solid line in Fig. 4(a)) [28]. Furthermore, the as-prepared ZnO nanoparticles were found to be readily dispersed in nonpolar solvents, such as hexane and toluene. This behavior suggests that the surface of nanoparticles were capped by the acid stearate [28] to form a hydrophobic corona. Consequently, we can conclude that the stearate was conjugated to the surface of ZnO nanoparticles [27, 28].The key difference between the synthesis process described here and other reported synthesis methods of ZnO nanocrystals is that hydrophilic regent H2O2 as the oxygenic source was used instead of hydrophobic ones, such as alkyl alcohol [27]. Thus, in the presence of hydrophobic reaction environment (zinc stearate in ODE), the reaction between zinc stearate and H2O2 only occurs at the interface of the two phases.
Fig. 4 (a) FTIR data for zinc stearate (dashed line) and ZnO nanoparticles (solid line); (b) FTIR data for AET-ZnO (dashed line) and pure AET (solid line).
Fig. 5 Possible processes of hydroxy ester elimination reaction between zinc stearate complex and hydrogen peroxide.
Figure 6 and Fig. 7 show the probable mechanism for the formation of the flowery nanostructure. One typical HRTEM image of a flower-shaped nanoparticle is shown in Fig. 6(a). The visible lattice fringes of all nanoparticles were found to run across the entire structure, indicating oriented attachment of several particles to form the flower-shape. Interfaces between primary particles were visible (Fig. 6(a)), but they did not affect the lattice orientation. Figure 6(b) shows that the primary particles were also single crystals. The complicated structure of flower-shaped particle was further confirmed by rotating the sample as shown in Fig. 7. When the sample was rotated (−19.5°, 0°, + 19.5°), the two dimensional projection of each particle was kept the same, but the structure of each particle exhibited observable changes (inset in red circle). The rotation experiments (Fig. 7) and HRTEM observations (Fig. 6) further imply that each flower-shaped nanoparticle was formed by geometrically random but lattice-oriented attachment of multiple primary nanocrystals [27].
Figure 8 shows the Abs and PL properties of ZnO nanoparticles. The Abs spectrum exhibits a band-edge absorption feature at λ = 360 nm (3.44 eV), which is slightly greater than that of the bulk (~3.36 eV). As discussed previously, the flower-shaped ZnO nanoparticles were composed of several small single crystalline particles. Figure 1(b) shows that each petal of this particle was ~6nm. Therefore, we attributed the enhanced band-edge absorption to the quantum-confinement effect. Quantum-confinement effect was also observed in the ZnO nanotetrapod with a radius of ~6 ± 0.5 nm, where the absorption onset was observed at ~362 nm (3.42 eV) [1]. The PL spectrum obtained from the as-prepared nanoparticles shows a wide blue emission centered at λ = 420 nm (2.9 eV) with a full width half maximum (FWHM) of 0.5 eV. The very weak green emission of ZnO nanoparticles indicated that the nanostructures contained few defects such as oxygen vacancies. The reason for such behavior may be due to the fact that, when H2O2 decomposed as a strong oxidant in the reaction, a large amount of oxygen was produced and filled up the oxygen vacancies in the nanostructures.
Fig. 8 PL and absorption spectra of as-prepared hydrophobic ZnO nanoparticles(black solid lines) and aqueous AET-ZnO nanoparticles (red dashed line). Inset: the corresponding photographs of aqueous AET-ZnO nanoparticles under daylight (1) and 365 nm UV excitation (2), and the aqueous AET under 365 nm UV excitation (3).
With regards to biolabeling, the hydrophobic surface of nanoparticles has to be converted into a hydrophilic surface. In this paper, we have achieved water-soluble ZnO nanoparticles by modifying the surface ligands of as-prepared nanoparticles with AET with a phase transfer technique [24–26]. The surface characteristic of AET capped ZnO nanoparticles (AET-ZnO) was investigated by FTIR spectroscopy. From the spectrum, the ester -C = O vibration (1739 cm−1) (solid line in Fig. 4(a)) disappeared after exchanging the ligands of ZnO nanocrystals while the sharp peak appeared near 1620 cm−1 (dashed line in Fig. 4(b)) [27], which was assigned to the N-H deformation vibration [30, 31]. Furthermore, it was also observed that the elements of sulfur and nitrogen were detected on the AET-ZnO nanoparticles by X-ray photoelectron spectroscopy measurement. Therefore, we concluded that AET were attached to the surface of nanoparticles. Additionally, there was little change on the PL spectrum of AET-ZnO nanoparticles in comparison to that of the as-prepared ZnO nanoflowers (Fig. 8). It had been demonstrated that the optical properties of ZnO nanoparticles were preserved even after the ligand exchange process. The particle sizes of the as-prepared ZnO nanoflowers and AET-ZnO nanoparticles were also analyzed by dynamic light scattering (DLS) (Fig. 2(b)). There was no obvious change between the two types of ZnO nanoparticles, demonstrating that the ligand exchange method was a good method to prepare water soluble ZnO nanoflowers.
Meanwhile, the photoluminescence properties of the AET-ZnO nanoparticles could be observed by naked-eyes. As shown in Fig. 8, photographs (1) and (2) displayed the pictures of the water soluble AET-ZnO nanoparticles under the irradiation of sunlight and UV light (365 nm), respectively. The AET-ZnO nanoparticles solution was transparent under sunlight but emitted blue light under UV excitation. As a comparison, no blue emission was observed on the pure AET water solution under UV irradiation (365 nm) as shown in photograph (3). It can be concluded that the blue emission of the solution in photograph (2) is attributed to the hydrophilic ZnO nanoflowers, which is consistent with the PL spectrum of AET-ZnO nanoparticles in Fig. 8.
3. Conclusion
We, herein, reported a first dual-phase synthetic method to prepare photoluminescent ZnO nanoflowers with an ester-elimination reaction with an oxygen resource of H2O2 and a zinc stearate precursor. Various techniques were used to characterize the nanoflower structures of the as-prepared ZnO nanoparticles. A detailed mechanism study was performed to understand the oxygenic role of H2O2 in the reaction. Meanwhile, the as-prepared hydrophobic colloidal ZnO nanoflowers could be efficiently converted into water-soluble ZnO nanoparticles via a ligand exchange process. The physical and optical properties of the ZnO nanoflowers, such as photoluminescence and particle sizes, were perfectly retained. The photoluminescent ZnO nanoflowers have wide applications in biolabeling, diagnosis. The water soluble ones are extremely useful in biological fluorescent imaging.
Acknowledgments
We are grateful for financial support from AG plus Technologies and the NSFC (U1204207) of China and initial funding of Hundred Young Talents Plan at Chongqing University (0210001104430 and 0210001104431)
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