TiO2 as a gas sensor: The novel carbon structures and noble metals as new elements for enhancing sensitivity – A review

The development of highly selective, stable, and sensitive materials for the various gas emissions is a challenging task concerning the numerous applications: environmental control, security systems, energy, and, more recently, biological monitoring. One of the major challenges to reach this activity's requirements is developing nanostructured functional and smart materials that can selectively detect and monitor the gases at high temperatures. The semiconductor materials based on metallic oxides have gained increasing attention within this scenario, especially titanium dioxide (TiO2), due to their excellent properties. The literature presents multiple sensors based on TiO2 nanoparticles to detect various gases: CO, ethanol, benzene, toluene, NO2, O2, CO, SO2, NOx, NH3, Cl2, H2S, alkanes, among others. This review initially explores some aspects of TiO2 and its properties regarding the sensor sensitivity's influence. Then, the insertion of noble metals (Ag, Pt, Au, and Ru) and carbon structures (graphene, carbon nanotube, and other carbon forms) was also explored in the search to improve the TiO2 capacity as a gas sensor. This review focuses on the most significant achievements related to TiO2 and TiO2 combined with carbon structures or noble metals for gas sensor applications. Several studies were presented, highlighting the synthesis methods employed, the sensor response/recovery times, and some detection mechanisms after the carbon and noble metal insertion. Finally, the influence of a triple combination between TiO2-carbon-metal was also explored.