Making sensors from 2-D materials

Sheets of materials composed of just a single layer of bonded atoms could act as highly selective and sensitive detectors of key polluting gases. Rajeev Ahuja and colleagues at Uppsala University, Sweden, report on computational simulations exploring the possibilities of the materials, called MXenes, in the journal Applied Materials Today.

MXenes contain transition metals together with carbon or nitrogen atoms and another non-metal atom or chemical group. These three components are combined according to the general formula M_n+1X_nT_x where M represents the transition metal, X is carbon or nitrogen, and T is the “terminal” atom or group. The Uppsala University team investigated the predicted properties of MXenes composed of titanium, nitrogen and sulphur (Ti₂NS₂) and of vanadium, nitrogen and sulphur (V₂NS₂). These are some of the lightest and thinnest MXenes.

Sheets of chemicals that are just one layer of bonded atoms thick are referred to as “two-dimensional” (2-D) materials as the layer extends only in two dimensions. They are attracting great interest from both theorists and experimentalists due to the versatile and often unique properties that become apparent without the bulk of a material extending into the third dimension.

“2-D materials are regarded as promising as gas sensors due to their high surface to volume ratio, outstanding surface tunability and e?cient operation at room temperature,” Ahuja explains. He points out that there is increasing demand for more sensitive and selective gas sensors due to the many toxic gases that are contributing to rising air pollution levels.

Ahuja and his colleagues used understanding of the quantum mechanical electronic properties of chemical arrangements to predict the interaction of some MXenes with gases. They focused on the interaction of their MXenes with eight different gases. Their most significant prediction is that 2-D sheets of either Ti₂NS₂ or V₂NS₂ have significant potential for detecting nitrogen monoxide, nitrogen dioxide, hydrogen sulphide and sulphur dioxide. These are some of the most troublesome air pollutants released by vehicles and industry. “They are major contributors towards health problems, climate change, and global warming,” says Ahuja.

The researchers also discuss some ways in which the MXene sheets might be incorporated into operational sensing devices, using electrical effects created when the detected molecules are adsorbed. Their calculations suggest that MXene sensors could offer sensitivity down to the level of a few parts per billion, with record-breaking signal-to-noise ratios. The key next step, however, is for the work of this theoretical research group to be tested in real experiments.

“We believe our findings should catch immediate attention from experimentalists,” Ahuja points out, expressing hope that practical applications might follow in the near future.

He also believes that the research could have wider implications due the general insights it offers into the interaction of specific gas molecules with MXenes. This might benefit other work exploring the potential of MXenes for selectively absorbing gases or separating specific gas molecules from more complex mixtures.

Article details:

Ahuja, R. et al: “Exploring two-dimensional M₂NS₂ (M?=?Ti, V) MXenes based gas sensors for air pollutants,” Applied Materials Today (2020).