Flame synthesis of TiO₂ nanoparticles for ultrasensitive gas sensing

A new method is proposed to fabricate nanocrystalline titania (TiO₂) films of controlled crystalline size and film thickness. The method uses the laminar, premixed, stagnation flame approach, combining particle synthesis and film deposition in a single step. A rotating disc serves as a combination of substrate holder and stagnation-surface flame stabilizer. Disc rotation repetitively passes the substrates over a thin-sheet, fuellean ethylene-oxygen-argon flame doped with titanium tetraisopropoxide (TTIP). Convective cooling of the stagnation surface keeps the substrate well below the flame temperature, allowing thermophoretic forces to deposit a uniform film of particles that are nucleated and grown via the flame stabilized adjascent to the surface. The particle film grows typically at a net rate of ~1 µm/sec. The film is made of narrowly distributed, crystalline TiO₂ particles several nanometers in diameter and estimated to be initially ~90% porous. Analysis shows that the rotation of the stagnation surface does not reduce the stability of a stagnation flame, nor does it affect the fundamental chemistry of particle nucleation and growth that occurs between the flame and the stagnation surface. ❧ Titania particles produced using this method are phase-pure single-crystal anatase with a narrow and controllable size distribution with median diameter ranging from a few nanometers to around 20 nm, characteristics desirable in the production of highly sensitive and selective gas sensors. Conductometric gas sensors were fabricated using this new technique by repeatedly translating interdigitated electrodes over a TTIP doped flame. The conductometric response of TiO₂ to surface reactions is a result of the gain and loss of oxygen ions and the corresponding change in electrical resistance arising from the change in titania stoichiometry. For example, a reducing gas reacts with surface oxygen, removing it and enriching the free electron concentration, thereby changing the resistance of the nanoparticle network. Networks made of smaller particles—with a greater surface area to volume ratio—offer a twofold advantage: first, they offer a greater number of surface sites for surface reactions and second, the electrical effects taking place at the surface of the semiconductor are more pronounced. For these reasons, the sensitivity and selectivity of the gas sensing films prepared by the flame technique is expected to improve notably compared to existing film preparation methods. In this dissertation work, the conductometric gas sensing limits of stagnation flame synthesized TiO₂ nanoparticle films to CO were examined and compared to commercial titania. Flame deposited electrodes show good sensitivity to carbon monoxide (CO) at concentrations as low as 5 PPM as well as long term stability with reproducible signals.