| s a typical semiconductor material, ZnO has attracted worldwide attention for its potential optoelectronic, electrical and catalytic applications. Among various developed synthetic techniques, hydrothermal synthesis by using of zinc salt precursor is one of the most favourite approaches, a process that can be used for the direct formation of ZnO or the mediate forming zinc hydroxide which could be further converted into ZnO by pyrolysis. The morphology transformation of ZnO is generally decided by the chemical processes of transition state and intermediates which are subsequently transformed into ZnO in hydrothermal synthesis. Thus, it’s important for giving more attention on study of the chemical processes of transition state. In traditional opinion, the crystalline process of either ZnO or zinc hydroxide is mainly referred to the crystal growth of clusters by aggregation or undergoing Ostwald ripening due to the fast formation of nuclei in hydrothermal reaction Observations from more deep studies suggest that crystalline process may follow more complex routes, where Ostwald ripening is always the final phenomena of crystal growth. How to design and control the crystallization proceeding through a multistep pathway has a decisive influence on the morphology of the final products. In this dissertation, a variety oft ZnO with novel morphotogies have been obtained in hydrothermal synthesis by introducing a series of soluble long-chain tetraalkylammonium carboxylate zinc salt to effectively regulate the reaction process. Additionally, ion-adjusting method has been developed to study the contribution of different ions to the morphological influence. Most importantly, some new rout and evidence of the crystalline transformation involved in reaction from zinc salt precursor to final products has been achieved successfully. The detailed work is as follows: 1. A series of uniformly unique three-dimensional （3D） structures of layered zinc hydroxides （LZHs） were synthesized via a hydrothermal reaction at 105 ℃ by introducing zinc N-Dodecyl-N,N-dimethylammonioacetic bromide （Zn（DDAB）2） as a soluble long-chain tetraalkylammonium carboxylate zinc salt precursor and urea as precipitation agent. These LZHs were then transformed into ZnO by pyrolysis at 600 ℃ for 2 h. During the reaction process, two kinds of LZHs with different crystal structures could be formed, depending on the ionic groups intercalated in zinc hydroxide layers:when carbonate anions were the only interlayer ions, the corresponding LZH compound was assigned as LZHC or zinc hydroxide carbonate; when the interlayer ions contained both carbonate anions and long-chain quaternary ammonium groups, the corresponding LZH compound was denominated as LZHDC. Interestingly, the LZHDC is transformed into LZHC though a structure and compositional transformation involving an exchange of long-chain quaternary ammonium groups by carbonate anions. Thus, we proposed a new route of stepwise growth and anion exchange mechanism involving conversion from zinc precursor to LZHDC to LZHC. This new route was distinguished from the classical crystalline evolution in a typical hydro thermal synthesis by using common zinc salt precursor. Depending on this new rote, we obtain a series of LZHs with unique 3D morphologies by carefully controllable time-dependent and concentration-dependent experiments. We also demonstrated the utility the LZHs as self-sacrificial templates for formation novel 3D ZnO structures by pyrolysis.2. ZnO microrod was prepared via a hydrothermal synthesis at 105 ℃ by using zinc N-Dodecyl-N,N-dimethylammonioacetic bromide （Zn（DDAB）2） as a soluble long-chain tetraalkylammonium carboxylate zinc salt precursor and hexamethyleneteramine （HMTA） as precipitation agent. We successfully synthesized a novel 3D binary structure consisting of two different crystal structures in one individual, with the head showing as layered basic zinc salt （LBZS） microsphere and the body as ZnO rod. This unique LBZS/ZnO binary structure indicates a topotactic relationship existed in the transformation from LBZS to ZnO, suggesting that the ZnO nuclei directly form from LBZS. In classical nucleation theory, the phase transformation usually takes place through dissolution-renucleation process referred to Ostwald’s Law. And the pyrolytic transformation of LBZS into ZnO occurs via a solid-phase transformation. During the synthesis process, careful concentration and time-controlled experiments are needed to achieve a key intermediate of stable self-assembled LBZS, which is subsequently converted into LBZS/ZnO binary structure by controlling transformation of LBZS to ZnO through the solid-phase transformation preceding the dissolution-renucleation process. The obtained structure demonstrate a possible growth mechanism involving stepwise evolution as:zinc salt→ LBZS→ LBZS/ZnO→ ZnO, giving an improvement of understanding of the growth of ZnO in solution. More importantly, this study provides new evidence that ZnO growth in solution synthesis could proceed via a solid-phase transformation. And a competitive procedure between solid-phase transformation and dissolution-renucleation process was demonstrated during this synthetic system. Additionally, uniform LBZSs with diversified 3D morphologies were expected successfully and pyrolyzed for forming unique 3D ZnO structures.3. A variety of ZnO structures were synthesized in hydrothermal synthesis at 105 ℃ by introduing zinc N-dodecyl-N,N-dimethylammonioacetic chloride （Zn（DDAC）2） as a soluble long-chain tetraalkylammonium carboxylate zinc salt precursor and hexamethyleneteramine （HMTA） as precipitation agent During the synthetic process, an intermediate, simonkolleite （Zn5（OH）8Cl2·H2O） with monodisperse multilayered 3D structure, was obtained successfully. Previous reports usually focused on preparation of two-dimensional （2D） simonkolleite microsheets with disordered stacking by using traditionally common zinc salt precursor. The anion DDAC- released from Zn（DDAC）2 is an amphiphilic ion mainly consisting of four parts such as acetate, chloride ion, long alkyl chain and quaternary ammonium groups. Considering the complex units consisted in the anion DDAC-, ion-adjusting experiments was designed and carried out to analyze the main factors influencing the forming multilayered 3D simonkolleite. The morphology proceeds a revolution from 2D microsheets to multilayered 3D structures, accompanied by addition sequence of chloride ion, acetate, long alkyl chain and quaternary ammonium groups. Therefore, the novel long-chain tetraalkylammonium carboxylate zinc salt has significant advantage in synthesis of multilayered 3D simonkolleite. Compared with 2D simonkolleite microsheets, the multilayered 3D simonkolleite exhibits much higher specific surface area (21.74→44.57 m2 g-1). Furthermore, the multilayered 3D simonkolleite acting as supercapacitor electrodematerial displays improved electro chemical performance in specific capacitance, rate performance and cycle stability. The multilayered 3D simonkolleite obtained a specific capacitance as high as 240 F g-1 at the current density of 5 A g-1 with a capacitance retention of over 97.1% after 2000 cycles, which isabout 40% higher than that of 2D simonkolleite microsheets. |
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