Formation of cerium oxide hollow spheres and investigation of hollowing mechanism
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Hollow structure materials have found important applications in many fields such as catalysis, energy storage and sensors. A facile and environmentally benign synthesis will favor their applications. Here we prepare a cerium oxide hollow structure with the size of micrometers through a simple one-step hydrothermal method. This process results in the formation of microspheres at 210 °C for only 2 h. Advantages of this method include no template, without surfactant-assistance or subsequent treatment during the synthesizing process. Explanation on the controlled crystal growth and hollowing mechanism via Ostwald Ripening is proposed elaborately based on the detailed experiment and observation. The FESEM images showed that the surface morphology of the sphere shell turned from initially smoothness into roughness with the increase of reaction time, relating to dissolution and re-deposition of nanoparticles onto inner and outside of the shell. The hollowing microstructure evolvement was vividly presented by observing samples taken out at different reaction stages, which makes visualization of Oswald ripening process. The prepared Cerium oxide microspheres were used to modify glassy carbon electrode, which acts as a sensor for sensitive determination of environmental hormone catechol (CC), demonstrating superior performance over the solid oxide. This work helps to deeply understand Ostwald ripening from a new perspective.
KeywordsHollow materials Hydrothermal method Morphology evolvement Electrochemical detection
Micro- and nanoscale hollow materials have attracted growing attention for decades, and various hollow structures [1, 2, 3, 4, 5] were prepared in recent years. Hollow materials have shown potential applications such as catalysis , energy storage [4, 7],sensor , and drug delivery ,which rely on intrinsic features of hollow structures, such as high surface area, hollow interior space, high stability, low density and good permeability. Therefore, it is necessary to study how to control the crystal structure and morphology in order to give full play to the potential application value of these hollow materials. Generally speaking, the synthesis of hollow materials can be classified into two cases: template and template-free. Template assisted material synthesis may utilize hard templates such as silica  ,carbon sphere  ,polystyrene  and metal oxide [13, 14], or soft templates including emulsion micelles  and gas bubbles [16, 17]. However, template synthesis can be time-consuming and expensive, and poor crystallinity materials are often produced during the synthesis process . For example, Titirici et al. reported synthesis of metal oxide hollow spheres by adopting template method during the hydrothermal treatment. Carbon spheres are formed with metal ions incorporated into the hydrophilic shell and the removal of carbon via calcination yields hollow metal oxide spheres. The process contains multi-steps and produces materials with unsatisfactory crystallinity . In contrast, template free approaches toward the synthesis of hollow structures such as galvanic erosion [19, 20], the Kirkendall effect [21, 22], and inside-out Ostwald ripening [5, 23, 24],come with extraordinary advantages, including time-saving and economical, good reproducibility and high yield. Ostwald ripening, a phenomenon which was first described by Wilhelm Ostwald in 1896,  involves the dissolution of small crystals and redeposition of the dissolved species on the surface of large particles, which results in growth of larger domain at the expense of the smaller ones. This phenomenon is thermodynamically controlled because small particles with higher surface energy are unstable. In other words, small partcles with higher surface energy appear to have a higher solubility than the large ones. Ostwald ripening has been widely used to explicate the formation of the hollow interior in a crystal. For instance, Zeng et al. reported for the first time the synthesis of hollow TiO2 nanospheres based on Ostwald ripening mechanism, including the hydrolysis of TiF4 under hydrothermal conditions in a high pressure vessel reactor . The experimental process can be described as follows: Firstly, TiF4 forms amorphous solid TiO2 microspheres by hydrolysis, which is composed of many small crystallites. Secondly, when the reaction time was prolonged, the ripening process occurs by mass transfer between the solid core and the external chemical solution. Finally, the nanospheres eventually become hollow. The Ostwald ripening mechanism also was utilized to synthesize various metal complexes, such as Cu2O, SnO2, ZnS, Sb2S2, NiS and SiO2 [3, 5, 27, 28, 29, 30]. The above-mentioned hollow particles can be used in catalysis, lithium and sodium storage.
However, the formation mechanism of these hollow structures was not fully understood. For example, how the matter disappears inside the particle lacks convincing explanation. Direct observation by in situ transmission electron microscopy (TEM) and in situ transmission electron microscopy (X-ray microscopy) is helpful to reveal the mechanism of hollowing, which provide real-time information on crystal structure at different reaction stages [19, 20, 31]. It is observed that Oswald ripening is a dynamic material transfer process. By using in situ TEM, Yu et al. . reported the formation of hollow iron oxide nanorods by decomposition of single crystal β-FeOOH. Unlike previously reported inside-out Ostwald ripening, they proposed a hollowing mechanism termed as “shell-induced Ostwald ripening”, which involves the dissolution and redepositioning of small nanoparticles on the inner surface of the shell to produce cavities. Although in situ TEM observation offers convenience to reveal hollowing mechanisms, the details of shell morphology could not be observed clearly in the event of particles size at micrometer. FESEM, which presents clear morphology of particles from nano to micro size, can reveal the evolvement of the whole surface morphology of the micrometer particles taken out at different synthesis stage to enrich Oswald ripening theory from another perspective.
Here, inspired by the Oswald ripening mechanism, we report a new one-step synthesis of CeO2 hollow microspheres to explore surface morphology evolvement without template, surfactant assistance or subsequent treatment. By focusing on the shell evolvement using FESEM to observe entire particles image, we have concisely depicted a hollowing mechanism that enhances previously reported inside-out Ostwald ripening. In our observations, amorphous cores dissolve and nanoparticles redeposit on the interior surface of the shell while small nanoparticles in solution deposit on the outer shell layer simultaneously, generating the hollow cavity and a dense rough shell. This work explains how matter disappears and where it moves. The direct observation of surface morphology using FESEM makes visualization of the Oswald ripening process. The strategy of synthesis and characterization in this study provides general implications for the microscopic morphological evolution of other hollow metal oxide micro- and nanomaterials.
2 Experimental section
The chemicals were obtained from the following sources and used without further purification, which include Cerium(III) nitrate hexahydrate (Ce(NO3)3·6H2O, 99%, Sigma aldrich); Citric acid (C6H8O7·H2O, 99.5%, Shanghai Lingfeng Chemical Co. China); Catechol (C6H6O2, 99%, Aladdin); Sodium dihydngen phoshate anhydrous (NaH2PO4, 99%, Aladdin); and Sodium phosphate dibasic (Na2HPO4, 99%, Aladdin).
2.2 Synthesis of CeO2 hollow microspheres
In a typical experiment, firstly, Ce(NO3)3·6H2O (0.4130 g) and C6H8O7·H2O (0.1000 g)(mole ratio = 2:1) were dissolved in 2 mL deionized water and sonicated for 20 min. The mixture solution was placed into a 20 ml Teflon-lined stainless steel autoclave and heated for 0.5 h, 0.75 h, 1 h, and 2 h at 210 °C respectively. After the autoclave was cooled to room temperature naturally, pale yellow precipitates were collected by centrifugation at 5000 rpm for 3 min and washed three times with distilled water and absolute alcohol respectively, and finally dried under vacuum at room temperature for 8 h.
2.3 Fabrication of the modified electrode
Before modification, the bare glassy carbon electrode (GCE) was polished repeatedly with 6, 1, and 0.05 μm alumina powders. After polishing, in order to clean the electrode surface, it was washed with doubly distilled water, and sonicated in ethanol and doubly distilled water for 5 min. Finally, it was dried under nitrogen atmosphere and ready for use. For comparison, an amount of hollow CeO2 microspheres and solid CeO2 was dispersed in 5 wt% of Nafion and methanol (1:2) respectively to obtain colloidal sol and the sol was dripped onto the treated GCE. Then the modified electrode was dried at room temperature and subjected for CV analysis.
2.4 Electrochemical measurements of catechol (CC)
The electrochemical property of the CeO2 microspheres was measured on a CHI 660E electrochemical workstation (Shanghai, China). A conventional three-electrode cell was used. The glassy carbon electrode (GCE) with 3.0-mm diameter served as the working electrode. A saturated calomel electrode (SCE) and a platinum wire electrode were used as the reference electrode and the counter electrode. N2 atmosphere was kept during the whole experiments.
2.5 Characterization of CeO2 microspheres
The compositions of the CeO2 microspheres were identified by X-ray diffraction (XRD, Bruker D8 advance, Germany) with Cu target radiation (λ = 0.15418 nm), and range of measurement angle is 20−80°. The microstructures were characterized by field emission scanning electron microscopy (FESEM; FEI Nova Nano SEM 450, America) with an accelerated voltage of 5 kV. The X-ray photoelectron spectroscopy (XPS) was analyzed by PHI 1800 XPS system (Al target, λ = 0.834 nm, Japan). The surface area and porosity of the CeO2 hollow microspheres were investigated by means of Brunauer–Emmett–Teller (BET) analysis and nitrogen adsorption and desorption isotherm at 77 K (ASAP-2020, America).
3 Results and discussion
According to the IUPAC definition, Ostwald ripening refers to the “dissolution of small crystals or sol particles and re-deposition of the dissolved species on the surfaces of larger crystals or sol particles” . During Ostwald ripening, relatively small nanoparticles have a greater migration tendency than the larger ones in solution, which increases the possibility of collision and agglomeration. Therefore, with the increase of reaction temperature, the growth of single crystal nanoparticles is at the expense of smaller ones or agglomerated species in the solution. The dissolution of small nanoparticles and aggregates is driven by their higher chemical potential than that of larger particles .
3.1 Microstructure evolution of the CeO2 microspheres
3.2 Hollowing formation theories of CeO2 hollow microspheres
3.3 XRD and XPS
3.4 Electrochemical detection of catechol
In summary, we developed a new facial hydrothermal method to prepare uniform CeO2 microspheres without a template. This process possesses outstanding advantages including time-saving, low cost, good reproducibility and high yield compared with traditional template-directed strategies. The mechanism of controlled growth and hollowing of the microspheres via Ostwald Ripening was proposed elaborately based on the detailed observation. The FESEM images vividly showed the evolution of the surface morphology of the sphere shell. The phase transformation occurred on a stable crystal region (shell layer) so that the uniformity of crystal structure was maintained, which explains clearly how matter disappeared and transferred, to which little attention has been paid in the previous reports. The direct observation of surface morphology evolvement of CeO2 from a solid core-shell structure to a hollow microsphere with FESEM makes vividly visualization of the Oswald ripening process. The material was electrochemically active and could be explored for more applications like catalysis owing to its prominent redox ability.
This work was financially supported by the National Natural Science Foundation of China (51777209), Guangdong Provincial Key Laboratory (2014B030301014), and Shenzhen Peacock Program (KQJSCX20170731163718639).
Compliance with ethical standards
Conflicts of interest
The authors declare no competing financial interest.
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