Stable photoluminescent ZnO@Cd(OH)2 core–shell nanoparticles synthesized via ultrasonication-assisted sol–gel method
Highlights
► ZnO@Cd(OH)2 core–shell quantum dots are prepared via a sonochemical sol–gel method. ► These QDs are stable during storage, at high temperature and in alkaline solutions. ► Our method is rapid, simple, and effective for synthesizing highly luminescent QDs.
Introduction
Semiconductor nanoparticles (NPs), as a new class of luminescent materials, have shown potential applications in biological fluorescent labels [1], [2], gas sensors [3], [4], solar cells, and electronic transistors [5], [6], [7], [8]. Among the well-known NPs, ZnO has the advantages of nontoxicity and cheapness, with a wide band gap (3.37 eV) and a large exciton binding energy (60 meV) at room temperature, and thus, ZnO NPs can be used in ultraviolet laser devices and biomedical labels [1], [8]. However, ZnO NPs derived from the conventional sol–gel methods have relatively low quantum yield (QY) of visible emission due to the insufficient protection, and such prototypical ZnO NPs are usually not stable during storage [1], [9], [10]. Furthermore, their absorption spectra and fluorescence spectra exhibit gradual red-shift [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], because ZnO NPs tend to aggregate and undergo the Ostwald ripening process in colloids. ZnO visible emission originates from its defects which mainly locate on the surface of nanoparticles. Water is able to quench ZnO visible emission through surface passivation on oxygen vacancies [2]. Besides, many ions such as S2−, Fe2+/3+[34], Mn2+[35], [36], [37], [38], Co2+[37] and Ni2+[38], [39], can also quench ZnO luminescence, which may be ascribed to the surface passivation effects or the trap of the photogenerated electrons on the ZnO surface. Shi et al. [33] reported that the unprotected ZnO colloids became turbid suspensions after only 2 or 3 days at room temperature. Therefore, sufficient surface protection is regarded as the key to obtain stable ZnO NPs with strong visible emission.
To improve ZnO stability various methods have been tried, including organic ligands modification, polymer capping, and inorganic shell coating. However, choosing a proper modification route faces challenges, especially that how to protect ZnO visible emission during chemical modification. Norberg and Gamelin [40] synthesized colloidal ZnO NPs capped by dodecylamine and trioctylphosphine oxide with uniform size and shape, but the ZnO green emission was very weak. Guo et al. [41] and Yang et al. [42] reported that ZnO NPs capped by poly(viny pyrrolidone) showed enhanced ultraviolet emission and dramatically reduced defect-related green emission. Inorganic shells, such as SiO2[9], TiO2[43], Al2O3[44], [45], MgO [46] and Zn(OH)2[47], were also employed for coating ZnO NPs. For instance, our group synthesized ZnO@SiO2 core–shell NPs and such NPs were very stable in water, phosphate buffer saline, and cell culture medium [48]. But this inorganic-coating processes required multi-step complicated reactions which cost a lot of time [49], [50]. Therefore, it is necessary to develop facile methods to synthesized ZnO nanoparticles with stable fluorescence in aqueous solutions.
Recently, significant progress in nanomaterial syntheses has been achieved by developing sonochemical techniques [51], [52], [53], [54]. Upon irradiating liquids with ultrasound, the overgrowed bubbles will collapse and release the concentrated energy stored inside within a very short time (with a heating and cooling rate of over 1010 K s−1). This cavitational implosion is very localized and transient with a temperature of 5000 K and a pressure of 1000 bar [46], [55], [56], [57], [58]. Cavitation-induced sonochemistry provides a unique interaction between energy and matter and permits access to the synthesis of a wide variety of unusual nanostructured materials. For example, we have successfully doped Mg2+ ions into ZnO NPs and the products exhibited strong and stable visible emission [59].
In this report, ZnO NPs were synthesized using a facile ultrasonication-assisted sol–gel route. The morphology and structure of the ZnO NPs were analyzed in detail using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and infrared spectra (IR). From these results, we believe the products were ZnO NPs coated by Cd(OH)2, designated as ZnO@Cd(OH)2 core–shell NPs. The UV–Vis absorption and photoluminescence measurements demonstrated that the QY of such ZnO@Cd(OH)2 increased significantly as the Cd concentration increased. In the meantime, the stability of the ZnO@Cd(OH)2 NPs was enhanced remarkably both at room temperature (for a month) and at elevated temperatures up to 90 °C, even in alkaline aqueous solutions.
Section snippets
Materials
Dihydrated zinc acetate (Zn(Ac)2⋅2H2O), monohydrated lithium hydroxide (LiOH·H2O), dihydrated cadmium acetate (Cd(Ac)2⋅2H2O), and triethylene glycol (TEG) were all of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. Rhodamine 6G of high purity (Fluka) was used as a reference.
Synthesis of ZnO@Cd(OH)2 NPs
0.878 g of Zn(Ac)2⋅2H2O and 0.252 g of LiOH·H2O were mixed together in 80 ml of TEG at room temperature by stirring. After the solution became luminescent, different amount of Cd(Ac)2⋅2H2O was dissolved
Results and discussion
In XRD patterns of the ZnO@Cd(OH)2 NPs and the control (Fig. 1), each peak corresponds to ZnO hexagonal phase of the wurtzite structure, and there are no signals of any other compounds. These ZnO diffraction peaks do not show any shift in comparison with the standard (JCPDS No: 79-0228), indicating that Cd2+ ions did not incorporate into the ZnO lattice. This result is in accord with the report by Mishra et al. [50] who proved their products were ZnO@Cd(OH)2 core–shell NPs. The diffraction
Conclusions
In summary, the core–shell ZnO@Cd(OH)2 NPs with good stability and enhanced photoluminescence were successfully prepared via a facile ultrasonication-assisted sol–gel route. In the ultrasonic circumstance, acoustic cavitation creates extreme conditions inside the collapsing bubble and supplies a microenvironment for sonochemical reactions. Cd(Ac)2 was hydrolyzed rapidly to form a thin protective shell of Cd(OH)2 evenly deposited around the ZnO cores. Meanwhile, ultrasonic radiation lowers the
Acknowledgments
This work was supported by the National Basic Research Program of China (2013CB934102), National Natural Science Foundation of China (Grant Nos. 20873029, and 21271045) and NCET-11-0115.
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