LaCoO₃ has been extensively studied for their outstanding physical and chemical properties, which have applications in fields like heterogeneous catalysis, solid oxide fuel cells (SOFC’s), gas permeation membranes and gas sensors, to name a few [1,2]. LaCoO₃ is a p-type semiconductor material, possessing the perovskite-type structure and has a band gap of 2.2 eV [3]. In this work, the detection of ultraviolet light (UV), as well as other light sources of different wave lengths (λ), was investigated for this oxide. For this purpose, polycrystalline LaCoO₃ was synthesized by the solution-polymerization method, using an aqueous solution containing La and Co nitrates and polyvinyl alcohol. The identification of single-phase LaCoO₃ was made by X-ray powder diffraction on samples calcined at 700ºC, in air. The surface microstructure was observed by field emission scanning electron microscopy, showing abundant porous sheets formed by networks of interconnected nanoparticles. Images obtained by transmission electron microscopy indicate that LaCoO₃ crystallites have an average size of 50 nm. Measurements for detecting UV and light in the visible region were done at room temperature, using light emitting diodes (LEDs) of different λ and optical irradiances (E_e). The latter were done on LaCoO₃ thick films prepared by depositing a suspension of the as-prepared powder on alumina substrates, with silver wires as electrodes. A typical graph showing the variation of the electrical resistance (R) with time, caused by the exposure of LaCoO₃ to UV radiation (λ = 365 nm, E_e = 217 mW/cm²) and darkness, can be observed in Fig. 1. The results show a uniform response pattern, characterized by a decrease of resistance under UV light. The quantitative detection of UV radiation was also evaluated by varying the optical irradiance from 22 to 217 mW/cm². The results indicate a non linear relationship between R and E_e, tending to a constant value while increasing E_e. I–V curves were also performed on LaCoO₃ films confirming the latter results. For other light sources such as blue (λ = 449 nm), red (642 nm) and infrared (843 nm) reliable responses were also observed, noting that the change of R decreased by increasing λ. The results indicate that nanostructured LaCoO₃ can be used as a wearable detector for UV radiation and a as sensitive material for photocells, among others. Even though this work was focused on UV detection, a good performance using light of larger λ was observed.

References

[1] Y. Wang, J. Ren, Y. Wang, F. Zhang, X. Liu, Y. Guo, G. Lu J. Phys. Chem. C 112 (2008) 15293.

[2] M. Ghasdi, H. Alamdari, Sens. Actuat. B 148 (2010) 478.

[3] S. Mukhopadhyay, M.W. Finnis, N.M. Harrison, Phys. Rev. B 87 (2013) 125132.