Citrate–nitrate synthesis of nano-structured titanium dioxide ceramics for gas sensors

Nano-structured TiO2-based ceramics were synthesized using a citrate–nitrate auto combustion method. Phase purity, particle size, and surface area of TiO2 nano-powder were studied as a function of citrate-to-nitrate ratio (C/N = 0.1–0.5), water addition during synthesis (0 and 10 vol%), and yttrium as a metal ion dopant (0 and 10 wt.%). The powder X-ray diffraction data showed that all synthesized TiO2-based powder was primarily anatase phase. BET surface area analysis showed an increase in specific average surface area up to 48 m2/g with increasing C/N ratio and the addition of water during synthesis. Powder XRD, BET, and TEM measurements confirmed particle size ranged from 10 to 50 nm depending on synthesis parameters. C/N ratio of 0.5 and addition of water during synthesis resulted in the smallest particle size. Sintered compacts made from this nano-powder showed an increase in bulk conductivity with increasing temperature inside a tube furnace open to the air. Addition of yttrium increased conductivity by 3 times over that of pure TiO2. Higher conductivity was attributed to higher energy of electrons to overcome conduction barriers and higher carrier concentrations caused by both temperature effects and yttrium addition. Thick film sensors made from the synthesized TiO2 nano-powder with higher surface area showed an improved response to carbon monoxide at 600 °C. Yttrium doping in TiO2 also increased response to CO compared with pure TiO2.