Stabilizing Ti(III) Species in Black TiO2 via a Phosphate Capping Layer for Sub-Parts-per-Billion-Level NO2 Detection at Room Temperature

Abstract

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 TiO₂ nanotube arrays (Ti NTs) grown on a Ti chip as a proof of principle, a large number of Ti(III) and oxygen vacancy (O_V) species were created by the ECR technique in a phosphate ion-containing electrolyte. The affinity between phosphate groups and TiO_2-x enables the phosphates to act as a capping layer blocking oxygen penetration, thus stabilizing most of the Ti(III) and O_V species after a double annealing treatment at 450 °C and storage for 3 months at room temperature (RT). Using NO₂ as a model target, the formation of an S-scheme TiO_2-x/BiVO₄ heterojunction on the sensing chip resulted in a remarkable NO₂ sensing performance at RT, with a response of 16.4 toward 100 ppb NO₂ (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 H₂O 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.

Keywords: ECR TiO2 nanotubes; NO2 detection; S-scheme heterojunction; drone integration; oxygen vacancies; room temperature.