Effect of Synthesis Parameters for Synthesizing Mesoporous ZnO Microparticles

Zinc oxide is regarded as non-toxic to living organisms. It is a widely researched transparent semiconductor with a wide band gap (Eg = 3.37 eV) and a high exciton binding energy (∼60 meV), offering promising applications in solar cells, antibacterial techniques, sensing, photocatalysis, and optical applications [1-5]. There are several methods to achieve this type of material. However, in many cases, they are complicated and expensive. On the other hand, mesoporous materials are highly versatile, and they have several applications in our lives due to their crystalline structure and physicochemical properties. They have high surface area which makes it an ideal material for energy absorption, catalysis and storage. Also, the mesopores can be structurally ordered, especially in materials such as mesoporous silica (MCM-41, SBA-15) and their permeability facilitates the transport of larger molecules, making it very useful for chemical and biological applications [1, 6]. Actually, there is few information about preparation of ZnO mesoporous microparticles. In this research work, both the synthesis of zinc oxide mesopores and the synthesis conditions that affect their pore size and surface area are presented. This research presents a simple and easy method for synthesizing mesoporous zinc oxide without using an organic template. The preparation of mesoporous zinc oxide samples was following, first, a solution (S1) was prepared by dissolving 0.02 mol of zinc acetate dihydrate (Sigma-Aldrich) in 21.6 g of distilled water. A second solution (S2) was prepared by dissolving 0.5 mol of NaOH (J.T. Baker) in 21.6 g of distilled water. Then, solution S2 was added to solution S1 slowly, drop by drop, and left stirring at 500 rpm for 30 min. The resulting gel was placed in a stainless-steel autoclave at 120 °C for one day in an oven to give hydrothermal treatment to the sample. Subsequently, the resulting suspension was washed with 100 ml of distilled water and centrifuged at 12,000 rpm for 10 min. Then, the material obtained was dried in an oven at 80 °C for one day. Finally, the samples were calcined at 560 °C (with a ramp of 10°C/min) for 1 hour. The pore size and surface area were measured through BET equipment (Nova touch LX1, Quantachrome Instruments) and images related to the morphology of the microparticles were obtained by using a high-resolution scanning electron microscope (HR-SEM, JEOL JSM-7600F). Table 1 shows the results obtained for surface area and pore size analysis. This result showed that surface area and pore size were influenced by the hydrothermal treatment temperature used. The samples 2 and 3 showed the lowest surface area and the highest pore size compared with the other samples. This can be due that the samples contain microparticles (Fig. 1 and 2). The use of different alkaline material and content water shows opposite behavior. This result shows that this synthesis procedure has low cost, easily and highly reproducible to obtain zinc oxide mesopores and could be reproduced for other types of materials [7]. Surface area and pore size of zinc oxide samples after calcination. *Sample synthesized without using autoclave during hydrothermal treatment. Surface area and pore size of zinc oxide samples after calcination. *Sample synthesized without using autoclave during hydrothermal treatment. SEM images. a) sample 4 and b) sample 8. SEM images. a) sample 10 and b) sample 12.