B. Synthesis of ZnSe by hydrothermal method; –
In this study synthesis of ZnSe nanoparticles were synthesized by using hydrothermal method. The synthesis was carried out by preparing two different solution. First 0.544gm Zinc acetate was added in 100ml ethanol. The solution was heated under water at 60° for 30min to achieve transparent solution. In second solution 0.3580gm sodium selenite was taken in 10ml deionized water. After completely dissolve add 40ml sodium hydroxide (1mol/lit) (or 20ml ammonia) and 10ml hydrazine hydrate then stirred using a magnetic stirrer for uniform dissolution. The solutions were obtained then mixed, stirred and kept into autoclave for hydrothermal at 180° for 24hrs. After centrifugation, the precipitate was collected. Finally, obtained precipitated dried in oven at 60°. The prepared sample further have been characterized by various physical techniques.

2.2 Sensor fabrication: –
A. Preparation of sensor solution: –
The pastes of thick film ZnSe were prepared by mixing appropriate amount of ZnSe (prepared as per above), the polyvinylpyrrolidone (PVP) as a surfactant to control the ZnSe sphere size and an organic vehicle as a temporary binder. The organic vehicle is a mixture of 2-2-butoxy ethoxy ethyl acetate, 2- butoxy ethanol, ?-terpineol and ethyl cellulose. The ZnSe powder to PVP ratio was optimized as 20:30. The formulated pastes using these percentages give good thixotropy and printability. Finally, the uniform pastes were formed by ultra-sonication for 5min. The two different sets of pastes were prepared with different stochiometric ratio of ZnSe (as prepared above).
B. Preparation of thick film: –
The humidity sensors used in this study were all prepared on insulated glass substrates with 1 cm × 1.5 cm size and a 500 µm thickness. Before the drop-coating of sensing materials, the insulated glass was well-cleaned with acetone and deionized water. Then, formulated paste of PVP-ZnSe were sequentially dropped onto the surface of the insulated glass substrates and dried under the IR lamp. To avoid rectifying features, ohmic contacts are essential. In our experiment, the ohmic contact of the samples were made by simple silver panting. The fabricated ZnSe thick films were further used as humidity sensors.

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