MS Modeling's DMol3 has been used to investigate the nitrogen dioxide sensing mechanism of tin dioxide nanoribbons.
Understanding the sensing mechanism will enable the efficient design and manufacture of nanoscale chemical sensors - an important commercial application of nanotechnology.
Nanowires and Nanoribbons
Practical challenges with carbon nanotubes involving cost of synthesis, control of chirality and diameter, separation from bundles, and attachment of functional groups have prompted researchers to explore other types of one-dimensional nanostructures. Two of the most promising candidates in this regard are nanowires and nanoribbons. As the name suggests, nanoribbons are solid objects (unlike nanotubes, which are hollow) with a near-uniform rectangular cross-section.
Nanoribbons and Their Potential Applications
So far, nanoribbons have primarily been synthesized from the oxides of metals and semiconductors. In particular, SnO2 and ZnO nanoribbons have been materials systems of great current interest because of potential applications as catalysts, in optoelectronic devices, and as chemical sensors for pollutant gas species and biomolecules. Although they grow to tens of microns long, the nanoribbons are remarkably single-crystalline and essentially free of dislocations. Thus they provide an ideal model for the systematic study of electrical, thermal, optical, and transport processes in one-dimensional semiconducting nanostructures, and their response to various external process conditions.
Tin Dioxide Nanoribbons as Chemical Sensors
Recent experiments with SnO2 nanoribbons indicate that these are highly effective in detecting even very small amounts of harmful gases like NO2. Upon adsorption of these gases, the electrical conductance of the sample decreases by more than an order of magnitude. More interestingly, it is possible to get rid of the adsorbates by shining UV light, and the electrical conductance is completely restored to its original value. Such single-crystalline sensing elements have several advantages over conventional thin-film oxide sensors: low operating temperatures, no ill-defined coarse grain boundaries, and high active surface-to-volume ratio.
Investigating the NO2 Sensing Mechanism of SnO2
To be able to fully commercialize their potential, it is important to better understand the sensing mechanism of such systems.
With this goal in mind, researchers at the Brookhaven National Laboratory, Lawrence Berkeley National Laboratory, and Accelrys used MS Modeling's DMol3 to investigate the nitrogen dioxide sensing mechanism of SnO2 nanoribbons.
Reporting in Nano Letters , the researchers examined the NO2-sensing mechanism of SnO2 nanoribbons with exposed (1 0`1) and (0 1 0) surfaces.
Figure 1. Molecular model of a SnO2 nanoribbon, showing its exposed surfaces and edges. Periodic boundary condition was employed in the actual calculations.
The density functional theory (DFT) calculations revealed that:
• The most stable adsorbed species involved an unexpected NO3 group doubly bonded to Sn centers
• An orders-of-magnitude drop in electrical conductance can be explained by significant electron transfer to the adatoms
• Computed binding energies were consistent with adsorbate stability up to 700 K, with X-ray absorption spectroscopy indicating predominantly NO3 species on the nanoribbon surface.
Investigating the O2 and CO Sensing Mechanism of Nanoribbons
The ability of the nanoribbons to sense O2 and CO was also investigated.
In the case of O2, the response of the nanoribbon was highly sensitive to the concentration of O-vacancies on the surface. Thus, in the absence of any surface vacancies, the calculation predicted negligible charge transfer. However, when surface vacancies were present, an O2 molecule can adsorb as a peroxide bridge, and withdraw a significant amount of electronic charge from the nanoribbon surface, thereby decreasing its electrical conductance.
In the case of CO adsorption, there was a net electron transfer from the CO to the nanoribbon surface. Thus the calculation predicted an increase in nanoribbon electrical conductance upon CO-adsorption, in agreement with experimental results.