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2.1. Materials

Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Loba Chemie, Mumbai, India 98%) and zinc acetate dihydrate (Zn(CH3COO)2·2H2O, Daejung Chemicals, Gyeonggi-do, Korea 98%) were chosen as zinc precursors of ZnO powders. Hexamethylenetetramine ((CH2)6N4, Ajax Finechem, New South Wales, Australia), monoethanolamine (HOC2H4NH2, Sigma Aldrich, St. Louis, MO, USA, 99%), and 2-methoxyethanol (HOC2H4OCH3, Alfa Aesar, Ward Hill, MA, USA 99%) were used as received without further purification. Microscope slides were used as substrates representing window glass.

2.2. Synthesis of ZnO with Different Ratios of ZA/ZN

The synthesis of ZnO particles with different molar ratios of zinc acetate (ZA) and zinc nitrate (ZN) was performed using a simple precipitation method []. The synthesis flow chart in summarizes the method used to produce ZnO powders. Typically, the synthesis of ZnO particles began with the preparation of ZA and ZN precursor solutions. The ZA solution was prepared by dissolving 0.6 M ZA and 0.4 M monoethanolamine in 2-methoxyethanol at room temperature with continuous stirring for 30 min before heating at 90 °C for 1 h. The ZN solution was produced by dissolving 0.015 M ZN and 0.015 M hexamethylenetetramine in deionized (DI) water. The ZN solution was subsequently stirred until the solution became clear. Then, ZA and ZN solutions were mixed with ZA/ZN at mole ratios of 80/1, 40/1, and 20/1. During the mixing procedure, the precipitates formed instantly in the mixed solutions. The mixed solutions were subsequently incubated in a temperature-controlled oven at 90 °C for 2 h. The precipitates were washed through several rounds of rinsing with ethanol and then DI water and with centrifugation, then dried in the oven for 12 h. Finally, the samples were calcined at 550 °C for 6 h in a muffle furnace (CWF 1200, Carbolite Gero, Neuhausen, Germany). Calcination is required for the oxidative transformation of Zn(OH)42- into ZnO []. The ZnO particles prepared with the various mole ratios of ZA/ZN at 80/1, 40/1, and 20/1 are referred to as ZnO-80/1, ZnO-40/1, and ZnO-20/1, respectively.