Nano-Asterisks of Vanadium Dioxide As Electrodes for TNT Detection

Vanadium dioxide (VO₂) is a unique transition metal oxide that undergoes a first-order phase transition at approximately 340K from the insulating monoclinic phase (M) to the conducting rutile phase (R). This metal-to-insulator transition (MIT) results in a change in the resistivity and optical absorption of the material, which has made it useful in several electronic and electro-optic applications. Common synthetic techniques such as pulsed laser deposition (PLD), chemical vapor deposition (CVD), and magnetron sputtering allow for the synthesis of high quality films of VO₂ for these applications. Recently, however, hydrothermal methods have been used for the synthesis of relatively large quantities of VO₂(M) from inexpensive starting materials without the need for expensive instrumentation. While these hydrothermally synthesized VO₂(M) particles are being developed for electronic and electro-optic applications, their applicability in other areas, such as catalysis and sensing, is the focus of research in our group. 

To make nano-asterisks of VO2, the amount of vanadium precursor present during a hydrothermal treatment was varied to produce several different morphologies of VO₂(M) including plates, snowflake asterisks, and truncated asterisks.  Films of the particles were drop cast onto glass slides and four-point Van der Pauw resistivity measurements were used to determine the phase transition behavior of the particle films.  We observe resistivity changes of more than 2 orders of magnitude for drop cast films of VO₂(M) nano-asterisks.  Additionally, BET gas adsorption theory was used to determine the surface area of the particles in order to evaluate their potential as electrocatalytic materials. 

The electrochemical reduction of 2, 4, 6-Trinitrotoluene (TNT) was investigated using films of vanadium dioxide, from both thin layer films formed by PLD and films composed of nano-asterisks particles. Three distinct reduction peaks were observed in the potential range -0.50 V to -0.90 V (vs. an Ag/AgCl reference electrode), corresponding to the electrochemical reduction of the three nitro-groups on the TNT molecule. Adsorptive stripping voltammetry was performed to achieve detection down to 1 µg/L (4.4 nM), revealing a linear response to TNT concentration. These results are the first describing the use of VO₂ films as an electrochemical sensor for TNT, and open new avenues for further electrochemical research using this unique material.