Chemical bath deposition ͑CBD͒ is an advantageous thin film deposition technique for depositing compound semiconductors at low temperature. In this paper, nickel oxide thin films were prepared by CBD from an aqueous solution composed of nickel sulfate, potassium persulfate, and ammonia at room temperature. Thin film growth mechanisms were studied by using quartz crystal microbalance, UV-vis absorption, and photon correlation spectroscopy. The data indicate that film growth is strongly dependent upon mixing conditions and competes with homogeneous particle formation. No film formation was observed without the addition of persulfate. A growth mechanism based on the combination of particle sticking and molecule level heterogeneous growth is proposed. The as-deposited film contained ␣-Ni͑OH͒ 2 and 4Ni͑OH͒ 2 ·NiOOH·xH 2 O and was converted to nickel oxide ͑NiO͒ by thermal annealing according to thermogravimetric, X-ray diffraction and X-ray photoelectron spectroscopy measurements.NiO is a transition metal oxide that has several potential applications, such as smart window, 1 solar thermal absorber, 2 electrodes for batteries, 3 and photoelectrocatalysts. NiO thin films have been prepared by various techniques, including thermal evaporation, 4 sputtering, 5 spray pyrolysis, 6 sol-gel, 7 chemical vapor deposition, 8 electrochemical deposition, 9 and chemical solution deposition. 10 Chemical solution deposition, also called chemical bath deposition ͑CBD͒, is an advantageous technique because of its low cost and low-temperature operating conditions. There are a few reports on the deposition of NiO thin films by chemical bath deposition. Pramanik and Bhattacharya 11 deposited NiO thin films using nickel sulfate, ammonia, and persulfate aqueous solution at room temperature. The obtained films were polycrystalline NiO with a black appearance. Varkey and Fort 12 deposited nickel oxide thin films using nickel sulfate and ammonia solution over the temperature range 60-80°C. Pejova et al. 13 used a bath containing nickel nitrate and urea at a temperature of 100°C. The as-deposited films were Ni͑OH͒ 2 ·H 2 O with a green appearance and were transformed into nickel oxide films after thermal annealing.In this study, the growth mechanism of NiO thin film by the chemical bath deposition following Pramanik's chemistry was studied in real time by using quartz crystal microbalance, UV-vis absorption, and photon correlation spectroscopy ͑PCS͒. The resulting thin films and particles were characterized by scanning electron microscopy ͑SEM͒, X-ray diffraction ͑XRD͒, UV-vis absorption, X-ray photoelectron spectroscopy ͑XPS͒, and thermogravimetric analysis ͑TGA͒.
ExperimentalSolution for CBD was prepared by mixing 40 mL of 1 M nickel sulfate ͑Alfa Aesar͒, 30 mL of 0.25 M potassium persulfate ͑Ald-rich͒, 10 mL of aqueous ammonia ͑28-30% NH 3 , EM Science͒, and 20 mL of deionized water in a 250 mL Pyrex beaker at room temperature ͑recipe A͒. The mixing was carried out by a 5 cm magnetic stir bar with 200 rpm. Substrates used for depositions inc...
