The effects of thermal treatment on the optical and structural properties of cobalt oxide thin films synthesized in the pores of PVP by chemical bath deposition technique were investigated. Films deposited were crystalline. The optical properties of the films were got from absorbance, transmittance reflectance, refractive index, absorption coefficient, and extinction coefficient measurements. The synthesized films turned out to be cobalt oxyhydroxide , CoO(OH), nanocrystals. The crystals obtained were of size 41.84？nm; however, as annealing temperature increased, the size decreased to 16.28？nm. The absorption coefficient, refractive index, and extinction coefficient were found to increase with increase in annealing temperature though not sequentially. For the same energy ranges of the incident photons, the absorption coefficient and refractive index ranged from 0.2 to 1.8 and from 1.4 to 2.3, respectively. The energy band-gap of the films ranged from 1.96？eV to 2.22？eV. 1. Introduction Five species of cobalt oxide (CoO2, Co2O3, CoO(OH), Co3O4, and CoO) have been reported [1, 2]. Both Co3O4 and CoO(OH) are easily obtained by thermally decomposing cobalt salts under oxidizing reactions [3]. Cobalt oxides have applications in superconductivity in electronics, electrochemical properties in microbatteries and high density batteries [4]. Cobalt oxyhydroxide, CoO(OH), has a hexagonal structure in which a divalent metal cation is located at an octahedral site which is coordinated by six hydroxyl oxygen [2]. CoO(OH) has been proposed as an alternative material for CO detection at low temperatures, for improving Co3O4-based gas sensor [5]. Well-spread CoO(OH) can be used as the conductive network in rechargeable alkaline batteries [6]. CoO(OH) is a promising material for fuel cells [7] and capacitors [8]. Generally, cobalt oxide materials can be deposited using several techniques, such as precipitation, sputtering, pulsed laser deposition, spray pyrolysis, sol-gel, hydrothermal synthesis, and electrochemical deposition [9]. But chemical bath deposition (CBD) technique is one of the simplest, cheapest, cost saving, convenient, and highly reproducible techniques that can be used for this deposition. It has been well known as a prevalent low temperature aqueous technique for directly depositing large area thin films of semiconductors [10]. It can be used for deposition of both conducting and nonconducting layers from solutions by electrochemical processes, without application of external fields [11]. CBD has the following advantages: it allows films to be deposited
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