2D Molybdenum Carbide MXenes for Enhanced Selective Detection of Humidity in Air

Precise control of ambient humidity is key to secure safe environmental conditions in many practical applications, including biomedical processing, microelectronics and health monitoring. Still, many commercial sensors are not able to detect very low H2O concentrations. In this respect, new devices with high sensitivity are needed to detect< 50 part per million (ppm) H2O concentrations.
With this goal in mind, we developed a novel sensor for enhanced selective detection of humidity based on the use of transition metal carbides and nitrides, the so called MXenes. These materials exhibit a 2D layered morphology which is uniquely beneficial for gas-sensing applications; furthermore, they offer tailorable surface chemistry, low-noise metallic conductivity, mechanical flexibility, and low cost processability.
Using selective chemical etching we successfully synthesized transparent single and few-layer thick Mo2CTMXene flakes with average size of 500–800 nm (Figure 1). After chemical etching multilayer MXenes nanosheets were delaminated from the Mo2Ga2C precursor with tetrabutylammonium hydroxide (TBAOH), resulting in the formation of a stable suspension of Mo2CTMXenes sheets, where Txrepresents an O, OH or F termination. The aqueous Mo2CTsuspension was then drop-casted over the chip to cover the electrode area and subsequently dried at 60 °C in air. Finally, the gas-sensing properties of the Mo2CTx layer were studied in a flow-mode setup towards water, alcohols, ammonia, and acetone.
Using this approach, we designed and realized a prototypical chemiresistive-type sensor, which demonstrates superior sensitivity among other MXenes towards H2O vapors. We found that the humidity vapors enhance the resistance of the Mo2CTx poly-flake layer with a high signal-to-noise value, which allows for the reversible detection down to 10 ppm of H2O. To the best of our knowledge, this is the lowest value reported to date. Importantly, the sensing properties are stable and preserved even when the sensor chip is stored for 6 months in ambient conditions.

 figure 1

Figure 1.  Illustration of the fabrication procedure of Mo2CTx MXene-based multielectrode sensor chips. a) crystal structure of Mo2Ga2C phase; b) multilayer Mo2CTx MXenes structure terminated by various Tx groups (black) inherited from chemical etchants; c) delaminated MXene suspension containing Mo2CTx flakes; d) drop-casting of Mo2CTx suspension over the sensor chip; e) optical image of the final sensor after drying

figure 2

Figure 2.  High-resolution photoemission spectra of (a) Mo3d, (b) C1s and (c) O1s core level lines for the as-prepared Mo2CTxMXenes and corresponding spectra of (d) Mo3d, (e) C1s and (f) O1s lines after exposure in air. 

The origin of the long-term stability of the designed sensor was investigated by high-resolution photoemission spectroscopy at the VUV-Photoemission beamline of Elettra. Figure 2 shows the photoemission spectra of Mo 3d, C 1s, and O 1s core level lines of the drop cast Mo2CTMXenes as prepared (top row) and after their exposure in air (bottom row). In both cases, the Mo 3d spectra show similar features, exhibiting two distinct components which we attributed to Mo-C-Tand MoOspecies (Fig. 1(a,d)). The increase of the oxide component after air exposure, which is seen in the O 1s spectrum (Fig. 2(c,f)), determines a  decrease of the work function from 4.22 ±0.15eV to 3.57 ±0.05 eV. Spectroscopy also showed that air exposure results in a higher carbon contamination. Notably, our analysis revealed strong similarities between aged and as-prepared samples, demonstrating the stability of the sensors over months.
In conclusion, our work shows that the low-noise resistance signal of developed Mo2CTxbased sensor allows the detection of H2O down to 10 ppm, which is the lowest value obtained to date. Our study opens up novel opportunities for using 2D Mxenes in gas sensing with high sensitivity at room temperature.


This research was conducted by the following research team:

Hanna Pazniak1, Alexey S. Varezhnikov2, Dmitry A. Kolosov3, Ilya A. Plugin2, Alessia Di Vito4, Olga E. Glukhova3,5, Polina M. Sheverdyaeva6, Marina Spasova1, Igor Kaikov7, Evgeny A. Kolesnikov8, Paolo Moras6, Alexey M. Bainyashev2, Maksim A. Solomatin2, Ilia Kiselev7, Ulf Wiedwald1, Victor V. Sysoev2


1  Faculty of Physics and Center for Nanointegration Duisburg-Essen, University of Duisburg-Essen, Duisburg, Germany 
2  Yuri Gagarin State Technical University of Saratov, Saratov, Russia
Department of Physics, Saratov State University, Saratov, Russia
University of Rome Tor Vergata, Roma, Italy
Laboratory of Biomedical Nanotechnology, I.M. Sechenov First Moscow State Medical University, Moscow, Russia
Institute of Structure of Matter (ISM-CNR), Trieste, Italy
Breitmeier Messtechnik GmbH, Ettlingen, Germany
8  National University of Science & Technology (NUST) MISIS, Moscow, Russia

Contact persons:

Hanna Pazniak, email:



H. Pazniak, A. S. Varezhnikov, D. A. Kolosov, I. A. Plugin, A. Di Vito, O. E. Glukhova, P. M. Sheverdyaeva, M. Spasova, I. Kaikov, E. A. Kolesnikov, P. Moras, A. M. Bainyashev, M. A. Solomatin, I. Kiselev, U. Wiedwald, and V. V. Sysoev, 2D Molybdenum Carbide MXenes for Enhanced Selective Detection of Humidity in Air. Advanced Materials 2104878 (2021). https://doi.org/10.1002/adma.202104878

Last Updated on Tuesday, 04 January 2022 14:04