I prepared ZnO via thermal decomposition of zinc acetate dihydrate. I evaluated its performance for dye degradation under UV light. It exhibited 42% dye degradation in slurry system. After keeping it in the Lab (temperature of around 30 deg.) for about 2 yrs, it showed 15% performance enhancement under identical experimental conditions. In fact, I unintentionally did this and therefore have no expectation for the result. I speculated that its properties are altered and some characterizations are thus required for elucidation. But, why does its performance change ?
*I assume the dye properties are unchanged