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Undoped and Cu doped ZnO films of two different molarities deposited by spray pyrolysis using zinc nitrate and cupric chloride as precursors show polycrystalline nature and hexagonal wurtzite structure of ZnO. The crystallite size varies between 10 and 21 nm. Doping increases the transmittance of the films whereas the optical band gap of ZnO is reduced from 3.28 to 3.18 eV. With increment in doping the surface morphology changes from irregular shaped grains to netted structure with holes and then to net making needle-like structures which lends gas sensing characteristics to the films. Undoped ZnO shows maximum sensitivity at 400 ∘ C for higher concentration of CO 2 . The sensitivity of Cu doped sample is maximum at 200 ∘ C for all CO 2 concentrations from 500 to 4000 ppm.

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Undoped and Fe doped ZnO films of different molarities deposited by spray pyrolysis method using zinc nitrate and ferric chloride as precursors show polycrystalline nature and hexagonal wurtzite structure. Crystallite size decreases with an increase in dopant concentration from 0 at.% to 3 at.%. Doping improves the transmission of the films whereas it reduces the optical band gap of ZnO from 3.28 eV to 3.17 eV. The morphology resembles flake-like structures which collapse when the dopant is introduced. The samples are found to be sensitive to CO 2 gas. Undoped ZnO shows maximum sensitivity at 350°C for higher concentration of CO 2 . Doped samples show maximum sensitivity at 200°C for all CO 2 concentrations i.e. from 500 ppm to 4000 ppm. Maximum sensitivity is achieved at temperatures 350°C, 250°C, 300°C and 450°C for the samples prepared using precursor solution of 0.1 M molarity.

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