A team of scientists from Moscow State University. MV Lomonosov Moscow State University and Research Center "Kurchatov Institute" has developed a gas sensor based on nanocrystalline indium oxide — a material that has long been used as a sensor that can determine the presence of nitrogen dioxide in the air. Electrical properties of these materials are directly dependent on the structure of the surface. If the surface of the indium oxide align molecules than oxygen molecules, it immediately changes the conductivity. In this study, researchers examined the effect of indium oxide nanocrystal size on its sensitivity to nitrogen dioxide, and determined the optimal particle size of indium oxide to create a sensor with the greatest sensitivity.
Nitrogen dioxide (NO2) — one of the most toxic gases contained in the atmosphere, therefore it is necessary to monitor its concentration in the air. This can be done by means of semiconductor sensors that are sensitive to the increased content of various gases in the environment. The principle of operation of such devices is that they are able to change their electrical conductivity depending on the amount of adsorbed gas molecules on the surface.
The sensitivity of indium oxide to nitrogen oxides also long known: it increases with the increase of the specific surface of indium oxide. It is logical to assume that the most sensitive to the presence of molecules in the atmosphere of nitrogen oxide is a material with the smallest particle size. So whether it’s actually — decided to test the authors of the study. Scientists have studied the effect of the size of indium oxide nanocrystals on its sensitivity to nitrogen dioxide. They published their findings in the latest issue of the journal "Russian Nanotechnology."
Researchers synthesized nanocrystalline indium oxide sol-gel method (method based on — the technology to obtain materials with specific chemical, physical and mechanical properties, the final stage in which the material is obtained as a gel). Then, during the day it was subjected to high temperature treatment, and then applied to a glass substrate and determining the phase composition, the particle size of the oxide and the specific surface area was evaluated. In the next step scientists measured sample conductivity depending on the amount of adsorbed nitrogen dioxide.
As a result of all this pain and it turned out that the size of the nanocrystals of indium oxide is greater, the higher the annealing temperature of the sample. It was observed a significant change in conductivity of the samples at a concentration of nitric oxide 0.00001 percent or higher. Conductivity indium oxide decreases rapidly in the presence of nitric oxide and the initial value taken in air in a few minutes at the same temperature.
During the experiments, the authors determined that the sensor signal is made on the basis of nanocrystalline indium oxide increases monotonically with increasing concentration of nitrogen dioxide in the gas mixture. Interestingly, the highest sensitivity was observed for the sample of a metal oxide with average size of nanocrystals and surface area.
Thus, the authors refute the initial theoretical predictions that the particle size reduction sensors provide maximum sensitivity. In fact, the following occurs: a decrease of nanocrystals at first an increase and then decrease sensitivity. The authors attributed this phenomenon is that, on the one hand, the touch signal is determined by the specific surface of the nanocrystals, which may be increased by reducing the size of the nanocrystals, and the other — reduction of the nanocrystals reduces the so-called height of the potential barrier and thus the sensor signal . Therefore, in this case, there is an optimum particle size of indium oxide, which is characterized by the maximum sensitivity to the presence of nitric oxide in the environment. Scientists estimate that in such an ideal gas sensor particles should be of slightly more than 8 nm.
This work was supported by the Ministry of Education and Science of the Russian Federation (state contract № 02.527.11.0008) under the Federal Program "Research and development on priority directions of scientific-technological complex of Russia for 2007-2013".