Determination of the Mechanism for the Decrease in Zinc Oxide Surface Area upon High-Temperature Drying

□ High-temperature drying is required to remove chemisorbed water from the zinc oxide surface. High-temperature drying of a very small particle size zinc oxide powder (median particle size ~23 nm) resulted in a substantial decrease in the surface area. The surface areas (BET analysis of 77 K nitrogen vapor adsorption data) of ZnO samples dried at 500 °C decreased continually as the drying time was increased. Although the surface area decrease was fastest during the first 5 h, a 64% decrease in surface area was found after 20 h. The decrease in surface area was not due to a collapse of pore structure. Comparison of nitrogen vapor adsorption and desorption isotherms as well as geometric calculations of surface area indicated that both the original and final particles were nonporous. X-ray diffractograms of the original powder and of powders dried at two temperatures were all identical. Thus, no change in crystal structure occurred as a result of drying at 500 °C. Atomic force microscopy provided substantial evidence that the surface area decrease was due to a shift in the particle size distribution to a larger mean size. It was verified using two different experiments that ZnO exhibited significant sublimation at 500 °C. It was concluded that the increase in particle size was due to a sublimation/condensation process that obeyed the Kelvin equation. The effect of ZnO particle size on the vapor pressure ratio in the Kelvin equation was modeled at 500 °C for several different assumed solid surface tensions. Drying conditions for ZnO were then selected which balanced maximum removal of chemisorbed water and minimum surface area decrease. Water vapor adsorption isotherms for ZnO at 25 °C were subsequently obtained. Differences in the isotherms resulting from the presence or absence of a chemisorption contribution could clearly be demonstrated.