I've made the process of annealing at a temperature of 400 C for a period of two hours, the film became a brown and adhesion is weak, and can be removed easily., What is the reason for that?
Nickel may have partly reduced to metallic nickel; therefore I would use a more oxidizing environments / oxidizing ligands, such as nitrates or citrates
NiO thin films can easily prepared via spin coating
Nickel acetate + 2-methoxy ethanol can be used for preparation of sol-gel.
Each layer ie. after add the drops while spinning you have to anneal the film at 300 to 400 deg for 2 to 5 min. You have to coat at least 10 layers to obtain good film.
This article can help you.
"Optical, structural and electrochromic properties of nickel oxide films produced by sol–gel technique" Solar Energy
NiO thin films can prepared by spin coating as following:
Nickel ion solution was prepared by dissolving 0.59 g of NiCl2..6H2O (0.05M) in 100 ml of ethanol as a solvent. The solution was stirred slowly for 1 hr at 20 oC by using magnate stirrer device, then acetic acid was added to be 10% in ethanol, to obtain clear green homogeneous solution and then mixed slowly for 12 hour at (20-22) oC, until get the green solution.The deposition of the films were made by spin-coating technique under the conditions of speed 3000 rpm and time 15 s. Taking 20µl from the solution and dropped it on the substrate by using spin coater device. After that the sample was dried by using electric oven at 100 oC for 2 min, to remove the effect of the solvent (ethanol). This process was repeated four times, to get sample with 4 layers and the same way to get on sample with 30 layers. Finally the sample annealed to 5000C by using jinyu-700 device to reduce the crystal defects and increasing the conductivity of the sample.