Annotation of the results obtained in 2020
Within the framework of the 2nd year of the project, we carried out the synthesis and microscopic measurements of the morphological characteristics of the obtained porous platinum structures on the surface of an array of titanium dioxide nanotubes on a titanium wire varying the concentrations of the platinum precursor and the reducing agent (formic acid). We optimized the thermal cycling parameters (the sweep speed, the maximum current value) for the sensor made on the basis of the obtained material towards volatile organic compounds in a mixture with air; selective recognition of analytes in a mixture with air was carried out using machine learning methods. A simple approach to the synthesis of porous platinum microspheres on an array of TiO2 nanotubes grown on a titanium wire was developed. The synthesis of Pt microspheres is facilitated by a high molar excess of formic acid compared to the platinum precursor. As a result of the study, it was found that the optimal ratio of precursor and reducing agent for obtaining porous structures of the TiO2@Pt type is about 1:1250. The resulting Pt microspheres possess bi-hierarchical porous structures madeof Pt nanorods up to 5-10 nm in size with channels that are replicas of titanium dioxide nanotubes. The growth of the spheres is regulated by two-dimensional diffusion with a constant or decreasing rate of nucleation of Pt nanorods, including their aggregation. Platinum microspheres synthesized directly on an array of TiO2 nanotubes on a Ti wire are used as a single-electrode selective sensor for alcohols in the framework of thermal cycling protocols. This sensor demonstrates a response to alcohol vapors (for example, methanol, ethanol, isopropanol, and butanol-1) mixed with air. It is assumed that the sensor response is related to the catalytic processes occurring on Pt microspheres. With an increase in the molecular weight of the analyte, the values of the sensor responses become indistinguishable from each other, which may be due to steric factors and limits the use of this sensor for CnH2n+1OH alcohols at n>3. Selective recognition of these alcohol homologs is successfully implemented even when using a single sensor under machine learning protocols such as Random Forest. It was found that the method used also allows us to determine the concentration of the analyte in a mixture with air.

 

Publications

---

Annotation of the results obtained in 2019
In the frame of this project, we studied and developed the concept of a single-electrode sensor based on an anodized titanium wire. In the proposed design, the wire is applied both as a heater and as a measuring resistive element. For this purpose, we carried out a search and optimization of anodization conditions of metal titanium wire to obtain an array of oxide nanotubes with the required geometric characteristics, and we studied variation of the thickness of the titanium wire during the anodizing process. As a result, protocols for creating single-electrode sensors based on Ti wire coated with a layer of titania nanotubes with a high aspect ratio were developed. During the electrochemical fabrication of a sensor based on the system "titanium wire-matrix of nanotubes of titanium dioxide" in the chosen electrolyte, we observed a linear reduction of the diameter of titanium wire to ~67 µm up to its breakage and stabilization of the thickness of the oxide layer at ~10 µm with the formation of rough surfaces due to the domain structure of the original metallic Ti. It was found that for a long time of anodization, the etching process should be the main factor determining the thickness of the nanotube layer. The resulting arrays of titanium dioxide nanotubes can be described by the following formula, TiO1,89. The nanotubes crystallize in the anatase phase under Joule heating of an anodized titanium wire. We evaluated dependence of electrical and sensor properties on the characteristics of the nanotubular layer and the heating power of the titanium wire. It was shown that the thermocatalytic effect appears only at high concentrations of test vapors at a small heating power. At the increasing heating power, the thermocatalytic effect in the form of a "positive" response changes to a chemoresistive effect in the form of a "negative" response, determined by a decrease in resistance when the analyte appears. Thus, with a decrease in the diameter of the metal thread and an increase in the heating temperature, the chemoresistive effect begins to dominate. The effect value is affected by the ratio between the oxide layer thickness and the metal thickness. We found the optimal stability and sensor response for the wires with a diameter of 110 and 135 micrometers, showing a response to 0.1%-1.0% of test vapors. For these samples, the optimal response to isopropanol vapors, 0.1%, mixed with air, was observed at a power in the range of 0.6-0.7 W. The observed sensory response is reversible and is characterized by repeatability, and the response/recovery time is no more than 10 s. The obtained sensors are characterized by a stable sensor response and linear concentration dependence. Measurements using an IR camera and numerical simulations in the Comsol ® show that the temperature of sensor depends almost linearly on the heating power and at the optimal power P=0.62 W belongs to the temperature range of 300-380 °C (IR camera), which is confirmed by modeling (320 °C). Heat dissipation in the wire is associated with three main processes, and mainly occurs through convection, which is 88%, 7% - radiation, and 5% - thermal conductivity, in the case of a wire with a diameter of 135 micrometers under optimal operating conditions of the sensor. Wires of smaller diameter have a higher temperature at the same power, which is due to the smaller contribution of convection to heat dissipation. Thus, it was shown that the anodizing of Ti wire is an appropriate approach for implementing a cost-effective protocol for the production of discrete single-electrode gas sensors without the use of complex technological operations. However, it is possible to realize a catalytic sensor by modifying the wire surface using known catalysts that promote the catalytic oxidation of flammable gases. In this case, the effect of temperature modulation in the working area will play a significant role.

 

Publications

---