Preparation of porous zinc oxide microspheres guided by HEPES

HEPES is abbreviated of 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, which is a Good's buffer with pH buffer range of 6.8~8.2, and widely used in cell culture because of its good ability at maintaining physiological pH. It can also be used to prepare porous zinc oxide microspheres.

As a II-VI semiconductor compound, zinc oxide has a band gap of 3.37 eV and an exciton binding energy of 60 meV. Due to its special piezoelectric and optical properties, zinc oxide is widely used in many fields such as solar cells, sensors, and voltage sensitive resistors, piezoelectric materials, antibacterial agents, and photocatalysis fields. The morphology and size of zinc oxide have an important influence on its properties and applications, especially porous zinc oxide microspheres, which exhibit low density and high specific surface properties in catalysts, gas sensors, drug delivery and other fields.

At present, different methods have been reported to synthesize porous zinc oxide microspheres, such as high temperature calcination, chemical vapor deposition, and chemical bath deposition. However, these methods have high reaction temperatures, are complicated to operate, and are difficult to control. The solvothermal method is widely used for the synthesis of zinc oxide micro/nano materials due to its simple equipment and mild reaction conditions. However, the synthesis of porous zinc oxide microspheres by solvothermal method often requires the addition of a templating agent or a porogen. After the completion of the reaction, further post-treatment is required to remove the templating agent, which increases the complexity of the process and environmental pollution.

In order to solve the above problems, the researchers [1] developed a preparation method for porous zinc oxide microspheres by using HEPES molecules, which has low cost and simple operation, and the obtained zinc oxide has uniform morphology, high specific surface area and multi-stage pore structure. The specific operations are as follows:

(1) 2~6 mmol of organic zinc salt (zinc acetate or zinc acetylacetonate) are sonicated in 30~70mL organic solvent (TEG, DEG, EG, DMF) for 10 minutes, then add 2~8 mmol HEPES;

(2) The mixed solution is placed in a stainless steel high pressure autoclave lined with 100 mL of polytetrafluoroethylene, and reacted at 150°C for 6 to 18 hours;

(3) The obtained product was washed by centrifugation at 10000 rpm/min for 10 minutes, and the supernatant was removed, repeat 5 times to remove the residual solvent and HEPES. The product was dried in a 60°C oven for 24 hours and then naturally cooled.

In this method, HEPES molecules play an important role in the formation of porous zinc oxide microspheres. After the addition of the HEPES molecule, it is adsorbed on the surface of the nucleus by electrostatic interaction with the zinc oxide seed crystal in the sol. The presence of the HEPES molecule blocks further clustering and aggregation of the nucleus. Porous zinc oxide microspheres are formed by the continued growth of the HEPES molecules under solvothermal conditions. The HEPES molecules and solvent in the surface and voids of the microspheres can be completely removed after multiple washings with deionized water.

HEPES (CAS 7365-45-9) molecules have the advantages of non-toxicity and environmental friendliness. The method is easy to operate, simple in equipment, low in synthesis temperature, low price in raw materials, good in repeatability, and suitable for industrial production. The prepared porous zinc oxide microsphere has the advantages of uniform size, multi-stage pore structure (pore size 4~30 nm) and large specific surface area (43.4~69.6m2/g), and can be used as photocatalyst and gas sensor.

Reference

[1]CHEN Rong, LI Qin, YANG Hao, Lv Zhong. A preparation method of porous zinc oxide microspheres guided by HEPES molecularly. 2014, CN103482682A.

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