Synthesis and characterizations of aluminium/gallium co-doped zinc-oxide nanostructures for hydrogen sensors

Abstract

Transparent electrode materials with the transparency of up to 80% in the visible wavelength region and resistivity as low as 10-3 Ω·cm are in high demand for various optoelectronic applications. It is believed that nanostructured Zinc oxide (ZnO) with modified electronic structure properties may be a promising alternative to Indium tin oxide (ITO). This is due to its similarity in band gap energy. This research attempted to improve the structural, morphological, optical and electrical properties of ZnO nanostructures by co-doping it with aluminium (Al) and gallium (Ga) (AGZO) in order to use the material as a hydrogen gas sensor. To achieve this goal, a series of Al/Ga co-doped ZnO nanostructure (NS) films with new morphologies was synthesised on the p-type Si (100) substrate using combined sol-gel and spin coating methods. Samples were annealed at different temperatures, time durations and laser energies to examine their effects on overall properties. The influence of varying Ga content was determined. FESEM images showed two new different morphologies for two different samples with increasing Ga content nanoleaves Ga (4 at. %) and nanopeanuts-like (5 at. %). The optical bandgap energy (3.26 - 3.20 eV) of AGZO nanofilms was discerned to be lower than pure ZnO films (3.37 eV). The resistivity of the prepared AGZO nanofilms at 1 at. % of Ga was found to be 4.571 x 10-3 Ω·cm which was lower than Ga free (AZO) thin films (6.4 x 10-2 Ω·cm). The sensing performance of the AGZO film was evaluated at (100 and 150) °C under varying H2 gas concentrations (250 – 1750 ppm). The best sensitivity was achieved at a H2 gas concentration of 1750 ppm. The improvement of overall properties was attributed to Ga mediated enhanced polycrystalline growth of the films and the production of new two different nanostructures. It was affirmed that the present method of sample preparation is simple and economical. The systematic characterisations might constitute a basis for producing high-quality ZnO nanofilms suitable for assorted applications. The laser annealing is a fast, cheap and better method than the thermal annealing method, but the annealing by the thermal method is the best to make a good hydrogen gas sensor in this study.