Stabilizing Ti(III) Species in Black TiO via a Phosphate Capping Layer for Sub-Parts-per-Billion-Level NO Detection at Room Temperature.

The requirement of a high operating temperature to achieve sufficient sensitivity is a common challenge for metal oxide semiconductor (MOS)-based chemiresistive gas sensors because of their intrinsic poor conductivity and scarce active sites. In this study, utilizing the phosphate group as a surface capping layer, we show that the electrochemical reduction (ECR) technique is a simple and effective method to endow MOS nanoarrays with improved conductivity for a long time, even after they undergo high-temperature treatment in air. Using TiO nanotube arrays (Ti NTs) grown on a Ti chip as a proof of principle, a large number of Ti(III) and oxygen vacancy (O) species were created by the ECR technique in a phosphate ion-containing electrolyte. The affinity between phosphate groups and TiO enables the phosphates to act as a capping layer blocking oxygen penetration, thus stabilizing most of the Ti(III) and O species after a double annealing treatment at 450 °C and storage for 3 months at room temperature (RT). Using NO as a model target, the formation of an S-scheme TiO/BiVO heterojunction on the sensing chip resulted in a remarkable NO sensing performance at RT, with a response of 16.4 toward 100 ppb NO (the response is defined as the ratio of the sensor's resistance in the target gas to that in air) and rapid response/recovery rates (27/55 s). Moreover, the hydrogen bond formed between HO and phosphate groups endowed the sensor with good humidity resistance. Further loading the sensing chip onto an unmanned aerial vehicle demonstrated its high applicability, enabling on-site environmental detection and providing an alternative to traditional gas sensing devices for high-sensitivity, real-time monitoring of trace target gases.