Nickel-zinc mixed metal oxide : coprecipitation synthesis and application as gas sensors

As the atmospheric pollution has considerably increased in the recent years, the detection of harmful and flammable gases is a subject of growing importance in both domestic and industrial environments. Gas sensors using metal oxide semiconductors have been the subject of extensive investigations because they have several advantageous, features such as simplicity in device structure, low cost for fabrication, robustness in practical applications and adaptability to a wide variety of reducing or oxidizing gases. One of the main challenges in the development of metal-oxide gas sensors is the enhancement of selectivity to a particular gas. For this purpose, the sensor optimal temperature, the use of doping elements, the formation of composite materials are investigated. To achieve better selectivity in the case of composites, the synthesis method has a great influence on the sensor performance. This thesis reports the synthesis, characterization and gas sensor investigation of nickel-zinc mixed metal oxide prepared by thermal decomposition of the corresponding precursors (nickel-zinc malonate), presynthesized by coprecipitation. The precursors, nickel zinc malonate blends, with various Ni/Zn ratio, were synthesized by coprecipitation in an aqueous solution by finding the optimal condition (pH =7, T=90 °C and t=3H)and characterized by ICP-AES, FTIR and TG. The obtained results showed that the precursor is a homogeneous mixture of nickel malonate and zinc malonate. TG indicates that the decomposition take place at around 360 °C. Alternatively the nickel malonate precursor was modified by a surfactant (oleylamine) and the characterization of this modified precursor showed clearly the presence of the surfactant in the precursor. The thermal decomposition products were characterized by FTIR, XRD, SEM, TEM, XPS and ToF-SIMS. XRD indicates the formation of one phase (Ni1-xZnxO), identified as the cubic NiO structure when the Zn percentage is lower than 20 %, and two phases (Ni1-xOZnxO/ZnO) when the Zn percentage is equal or greater than 20 %. In the case of Zn doped NiO the variation of the NiO lattice parameter indicates that the zinc has substituted nickel in its crystal structure. ToF-SIMS confirms the presence of the Zn and Ni in the same structure while XPS indicates that they are both under Zn2+ and Ni2+ form. The morphology of the particles was determined by SEM and TEM and the results indicate that the pure NiO and ZnO are spherical while the composite material has undefined morphology. For all samples, the particle size was less than 80 nm. It was found that the oleylamine reduces considerably the particle size of NiO (56 % of reduction). The sensing performance of all the samples was investigated. The sensitivity of films of mixed nickel zinc oxide to CO, H2, NO2 was tested for different concentrations and temperatures. For the doped material, it is found that the conductivity of NiO increases with increasing the amount of Zn up to 4 %. The optimal sensitivity and the selectivity to CO of NiO increase with 2 % of Zn doping. For the composite material, it is found that the Ni1-xZnxO/ZnO (1:1)composite material is more sensitive than the pure Ni1-xZnxO and ZnO at 300 °C to CO. A good selectivity to NO2 at 225 °C was also observed for the composite material. The performance of the composite material was explained by the formation of a p-n heterojunction.