Nickel oxide films by chemical spray

Nickel oxide (NiO) has potential applications in numerous fields such as a semiconductor, in capacitor-inductor devices, tuned circuits, transparent heat mirrors, thermistors, batteries, micro-supercapacitors, electrochromic and chemical sensing devices owing to its remarkable optical and electrical properties and its wide bandgap. Nickel oxide has also been investigated for potential photocatalytic activity, and it has been identified as a promising alternative photocatalyst for/in addition to zinc oxide and titanium oxide due to its low cost, low toxicity, high stability, and ability to degrade pollutants. However, most studies on the photocatalytic activity (of) NiO use its powdered form, which has several drawbacks ….

Among the techniques developed to synthesize NiO thin films, spray pyrolysis is a facile technology for the robust synthesis of thin films. Moreover, it is a low-cost approach that can be easily commercialized for large-scale industrial production. Notably, efficient deposition of NiO films via spray pyrolysis requires a suitable precursorthat/which determines the homogeneity and crystallinity of the resulting films. For instance, those produced using nickel chloride precursors display more roughness and crystallinity than those produced by nickel nitrate and nickel acetate precursors.

Although the film properties depend on different preparation parameters, the deposition temperature (Ts) is often considered critical. An increase in Ts increases the crystallite size regardless of the precursor used and may also affectthe bandgap of NiO films.. However, the effects of Ts on NiO film properties, especially the surface chemical composition of the sprayed films, remains largely underexplored in the literature..

On this account, a team of researchers from Tallinn University of Technology: Mr. Zengjun Chen, Dr. Tatjana Dedova, Professor Ilona Oja Acik, Dr. Mati Danilson and Professor Malle Krunks investigated the effects of Ts and solvent type (aqueous- and alcohol-based) on the properties of NiO films grown via spray pyrolysis method using Nickel acetate as precursor material. The Ts ranged between 300 to 420 °C. The research team also investigated the correlation between surface chemical composition wettability and photocatalytic activity towards methyl orange degradation by NiO films. The research is published in the journal, Applied Surface Science.

The authors showed that Ts was the core parameter that influenced wettability, surface chemical composition and photocatalytic activity of the NiO films. In contrast, the solvent type controlled the structural and optical properties. XPS spectra revealed that the sprayed film surfaces contained the NiO phase, Ni(OH)₂ and hydroxyl groups. The quantity of the hydroxyl groups decreased with an increase in the Ts. Additionally, films produced at lower deposition temperature (Ts = 300 °C) had displayed surfaces richer in hydroxyl groups and exhibited improved photocatalytic activity and superhydrophilic nature. However, films produced by both alcoholic and aqueous solutions displayed similar trends regarding wettability and surface chemical composition properties. The notable difference between the two solutions was observed in the films crystallinity and optical bandgap value. Thje films produced by aqueous solution were polycrystalline, with the mean crystallite size increasing from 4nm to 10nm when Ts was increased from 320 to 420 °C and with optical bandgasp of 3,4 eV. Films from alcoholic solution were amorphous and showed a bandgap of 4.0 eV.

In a nutshell, Tallinn University of Technology scientists conducted a comprehensive systematic study of the different properties of NiO films grown by spray pyrolysis regarding solvent type and deposition temperature. An important finding is that deposition temperature was the dominant parameter affecting the surface chemical composition and the photocatalytic degradation performance of NiO films. The difference between the films produced by aqueous and alcoholic solutions was also established. In a statement to Advances in Engineering, Professor Malle Krunks, the corresponding author said their study will expand NiO films preparation technologies and their perspective industrial applications.