Microwave-assisted hydrothermal synthesis of ZnO and Zn-terephthalate hybrid nanoparticles employing benzene dicarboxylic acids

Microwave-assisted hydrothermal synthesis of ZnO nanoparticles employing three isomers of benzene dicarboxylic acids, phthalic acid (PA), isophthalic acid (IPA) and terephthalic acid (TPA) as structure directing agents (SDAs) revealed their strength in the order of TPA > PA > TPA, for the synthesis with solution pH > 10, causing anisotropic crystal growth to result in a variety of specific nanostructures and reduction of particle size. TPA, however, behaved differently from the others when pH was reduced, resulting in a Zn-TPA metal–organic framework (MOF), composed of layered double hydroxide of Zn²⁺ (Zn LDH) stabilized by the intercalated TPA dianion. Synthesis under controlled pH revealed four different types of Zn-TPA MOFs. Three of them could be isolated to identify their structure and composition by means of XRD, TG–DTA and FT-IR, as Zn₃(OH)₄(TPA)·6H₂O, Zn₄(OH)₆(TPA) and Zn₂(OH)₂(TPA)·H₂O, with their interlayer distances of 14.4, 10.8 and 9.03 Å, as resulted from the syntheses when the initial pH of the precursor solution were adjusted to 7.0, 5.9 and 5.3, respectively. These Zn-TPA MOFs are hitherto unknown, especially for the presence of hydration water and significant change of the interlayer distance, which might have been achieved by the rapid crystallization under microwave radiation.