Highlights
- Switching the precursor solvent between water, water-ethanol, and ethanol modulated ZnO film thickness from 875 nm down to 150 nm.
- The water-ethanol (1:1) precursor produced a distinctive water-hyacinth-shaped nanostructure with the largest crystallite size.
- The all-water precursor gave the most uniform grain structure and the strongest room-temperature ammonia response (36 toward 100 ppm).
- Solvent choice also shifted the optical band gap of the films, including a twin band gap observed for the ethanol-only sample.
Abstract
This study examined how the solvent used to prepare the precursor solution affects spray-pyrolysis-deposited zinc oxide (ZnO) thin films. Water, a 1:1 water-ethanol mixture, and pure ethanol were used to dissolve zinc nitrate hexahydrate before spraying onto heated glass substrates. Changing the solvent altered the precursor's viscosity, boiling point, and pH, which in turn changed the deposited film's thickness from 875 nm down to 150 nm as the ethanol content increased. The water-ethanol mixture produced an unusual water-hyacinth-shaped nanostructure and the largest crystallite size, while the all-water film gave the most uniform grain structure and the best response to ammonia gas at room temperature.
Research summary
Zinc oxide (ZnO) thin films deposited by spray pyrolysis are widely used for gas sensors, and their properties depend heavily on the deposition conditions. This study investigated how the choice of solvent used to dissolve the zinc precursor — water, a water-ethanol mixture, or pure ethanol — affects the thickness, structure, and gas-sensing behavior of the resulting ZnO film, all deposited at the same substrate temperature.
What the study examined
- ZnO thin films spray-deposited on glass using zinc nitrate hexahydrate dissolved in water, a 1:1 water-ethanol mixture, or pure ethanol
- Film thickness, crystal structure, and grain morphology across the three solvent conditions
- Optical band gap and electrical conductivity as a function of solvent medium
- Room-temperature sensing response of each film toward ammonia and other gases
Main findings
Changing the solvent medium substantially changed the deposited film. Thickness dropped from 875 nm with pure water, to 450 nm with the water-ethanol mixture, to 150 nm with pure ethanol, consistent with ethanol’s lower boiling point limiting how much material reached the substrate before evaporating. The water-ethanol mixture also produced the largest crystallite size and a distinctive, densely packed nanostructure that the authors likened to water hyacinth, while the pure ethanol film showed more random grain shapes and weaker crystallinity.
The films’ optical band gaps also shifted with solvent choice, including a twin band gap observed for the ethanol-deposited film. For gas sensing, the water-based film — with its uniform, loosely packed spherical grains — gave the strongest ammonia response among the three, reaching a response of 36 toward 100 ppm at room temperature, more than the water-ethanol or ethanol-based films.
Why it matters
The results show that a single, simple change — the solvent used to prepare the spray precursor — can be used as a practical knob for tuning ZnO film thickness, crystal structure, and gas-sensing performance without altering the deposition equipment or the zinc source itself. The unexpected water-hyacinth nanostructure and twin optical band gap also point to further degrees of freedom worth exploring in future ZnO thin-film work.
Citation
G. Ezhilarasan, Prabakaran Shankar, G. K. Mani, J. B. B. Rayappan. Modulation of ZnO Film Thickness and Formation of Water-Hyacinth Nanostructure. European Physical Journal: Applied Physics 67 (2014) 20301.
