Ascorbate-Assisted Growth of Hierarchical ZnO Nanostructures: Sphere, Spindle, and Flower and Their Catalytic Properties

Abstract

A simple solution-based method to prepare mainly flowerlike zinc oxide (ZnO) nanostructures using the ascorbate ion as a shape-directing/capping agent at relatively low temperature (ca. 30 and 60 degrees C) was described. However, we observed that different shapes of hierarchical ZnO nanostructures such as flowerlike, spindlelike, and spherical could be obtained with an increase in the synthesis temperature from 60 to 90 degrees C. The effects of other organic capping agents on the shape of hierarchical ZnO nanostructures were also studied. FTIR, FESEM, and XRD characterization were performed on the formed ZnO nanostructures to understand the role of ascorbate in the growth of flowerlike morphology. The nucleation and growth process can regulate by changing the metal precursor and ascorbate ion concentrations. We were able to identify intermediate nanostructures such as spherical/quasi-spherical and spindle that are very much on the pathway of formation of large, flowerlike ZnO nanostructures. Electron microscopy results indicated that these spherical/quasi-spherical ZnO nanoparticles might aggregate through oriented attachment to produce spindlelike and flowerlike nanostructures. On the basis of these results, a possible growth mechanism for the formation of flowerlike ZnO nanostructures was described. The optical properties of these differently shaped ZnO nanostructures were also described. The catalytic activities of the as-synthesized spherical and flowerlike ZnO nanostructures were tested in the Friedel-Crafts acylation reaction of anthracene with benzoyl chloride. The catalysis results indicated that the catalytic activity of flowerlike ZnO nanostructures is slightly higher than the spherical counterpart.

Contribution of TEMPO-Oxidized Cellulose Gel in the Formation of Flower-Like Zinc Oxide Superstructures: Characterization of the TOCgel/ZnO Composite Films

“…In comparison with SEM images provided in Figures 2 and 3, it is evident that the flower-like ZnO superstructures are formed only in the presence of carboxylates, which indicate the crucial role of carboxylate moieties of TOCgel in nucleation and growth process of ZnO in a particular direction that ultimately results in the formation of such anisotropic flower-like morphology, which is in accordance with the results reported elsewhere by other researchers [41]. These results indicate that the flower-like ZnO architectures can be formed in the presence of ascorbate ions which are similar to carboxylate moieties of oxidized cellulose material (TOCgel).…”

“…The position of this peak appears to be slightly shifted in the former composite film (~1625 cm −1 ), which could be related to the adsorption of carboxylate onto ZnO structures [41,55]. To further prove the presence of ZnO superstructures incorporated in the TOCgel/ZnO composite film, X-ray photoelectron spectroscopic study has been used.…”

“…The synthesis of ZnO flower-like structures was performed directly onto TOCgel surfaces through hydrothermal routes with some modification from the previous literature [34,41]. Indeed, the growth of ZnO superstructures onto the TOCgel surface can be related to the treatment of TOCgel with both Zinc nitrate and HMT solutions, which led to the nucleation and the growth of discrete seeds at the cellulosic fiber surface.…”

“…Consequently, such adsorption could facilitate the secondary structure nucleation on the initial structure (through the intersection of nanosheets) in order to reduce the interfacial activation energy, and finally contribute to the construction of the well-shaped flower-like ZnO superstructures with porous surfaces within the former TOCgel/ZnO composite film as illustrated in Figure 7. Earlier, several researchers have also demonstrated the effect of citrate and ascorbate solutions on the formation of flower-like ZnO structures under hydrothermal route [34,41]. In our work, carboxylates moieties present along the oxidized cellulose fibers display similar effects in the tuning of the shape of ZnO and, fortunately contribute to the elaboration of a flexible composite material in which flower-ZnO superstructures are well-dispersed within the fibrous network of TOCgel as confirmed by the SEM images (Figure 2).…”

“…Yin et al also developed novel ZnO nanorod-assembled hollow superstructures for photoluminescence and catalysis applications using hydrothermal reaction via sodium carboxymethyl cellulose (CMC) assistance [40]. Flower-like ZnO nanostructures with controllable sizes and higher catalytic activity were fabricated through a solution-phase route in the presence of ascorbate as a shape directing/capping agent [41].…”