Nanocrystalline ZnO thin films - influence of sol-gel conditions on the underlying chemistry and film microstructure and transparency

Zinc oxide (ZnO) thin films were spin-coated onto glass substrates by a low-cost sol-gel process. The influence of the zinc acetate dihydrate concentration (0.35 M and 0.75 M) and two different solvents (methanol and 2-methoxyethanol) on the underlying chemical mechanism, surface morphology, microstructure and optical properties of the films was investigated. FTIR results support the formation of [CH3COOZn](+), Zn(OH)(2) and Zn-O-CH-CH2-NH2 as intermediate products/by-products, particularly when methanol and higher precursor concentration were used. XRD patterns show a preferred orientation of crystallization of wurtzite along the (002) plane, more evident when 2-methoxyethanol was used, due to the longer available time for the reactions completion and crystal growth. SEM analysis shows a wrinkled surface in the films that is denser when methanol and the highest precursor concentration were used in the synthesis. The ZnO films synthesized with 2-methoxyethanol exhibited higher optical transmittance (>= 90%) than the films made with methanol (60-70%). The transparency in the visible range is enhanced in the case of 2-methoxyethanol and higher Zn(II) concentration (>= 90%). The optical band gap values of the films were in the range 3.10-3.50 eV. (C) 2015 Published by Elsevier Ltd. Selection and peer-review under responsibility of TEMA - Centre for Mechanical Technology and Automation.