Volatile organic compounds (VOCs) are air pollutants that can have harmful health effects. Detecting the VOC levels in indoor air in a fast and accurate manner is, thus, important. Several metal oxides can be used for the detection of VOCs, but these sensors often need high working temperatures or have issues with response time and selectivity. MXenes, i.e., two-dimensional (2D) transition metal carbides, carbonitrides, or nitrides, can be used for the conductometric sensing of VOCs at room temperature. However, their small interlayer spacing limits the performance of such devices.

Lia A. Stanciu, Purdue University and Birck Nanotechnology Center, West Lafayette, IN, USA, and colleagues have developed sulfur-doped titanium carbide MXenes, Ti₃C₂T_x, which have an improved VOC-sensing performance compared with their undoped counterparts. The team started from Ti₃AlC₂, which was etched using LiF/HCl to remove the Al layers. The resulting MXene nanosheets were mixed with thiourea as a sulfur source and heated to 500 °C under an argon atmosphere to obtain the desired sulfur-doped Ti₃C₂T_x MXenes.

MXene solutions were then drop-cast onto flexible silver electrodes on a polyethylene terephthalate (PET) substrate to prepare the VOC sensors. The team evaluated the change in resistance upon exposure to four different VOCs (ethanol, hexane, hexyl acetate, and toluene) for both undoped and S-doped MXenes. They found that both types of MXenes can selectively detect toluene. Sulfur doping enhances the response significantly. The researchers attribute this to an increased interlayer spacing in the S-doped MXene, which facilitates the binding of the analyte.

• Sulfur-Doped Titanium Carbide MXenes for Room-Temperature Gas Sensing,
  Shoumya Nandy Shuvo, Ana Maria Ulloa Gomez, Avanish Mishra, Winston Yenyu Chen, Avinash M. Dongare, Lia A. Stanciu,
  ACS Sens. 2020.
  https://doi.org/10.1021/acssensors.0c01287