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Project Number19-72-00136

Project titleDevelopment of approaches to design of high-performance gas sensors based on the system "titanium wire - array of titanium dioxide nanotubes"

Project LeadFedorov Fedor

AffiliationAutonomous Non-Profit Organization for Higher Education "Skolkovo Institute of Science and Technology",

Implementation period 07.2019 - 06.2021 

Research area 02 - PHYSICS AND SPACE SCIENCES, 02-202 - Semiconductors

Keywordsgas sensor, transition metal oxide, electrochemical anodization, arrays of titanium dioxide nanotubes, catalytic bead sensor, selectivity



The need to control the state of the environment leads to the development of sensor devices, characterized by a simple principle of operation and low cost. However, at present, the analysis of gases and gas mixtures is carried out mainly with the help of gas chromatographs or spectrometers of various types, the use of which is limited by the requirements for the time of obtaining the result, mass-dimensional characteristics and energy consumption. Therefore, more and more attention is paid to the use of fast-response and miniaturized gas sensors. Since gas sensitive material is a key element of the sensor, it is important to search primarily for new sensor materials and technologies for their fabrication for sensor applications, as well as to consider re-engineering of traditional sensors based on accumulated knowledge / progress, including in nanotechnology. In this project, we propose to develop approaches to the design of new single-electrode sensors of catalytic type in a single process by anodizing titanium wire. The oxide layers, arrays of titanium dioxide nanotubes obtained by anodizing a metal titanium wire, would represent the active layer, since they show high catalytic activity, and the titanium wire, on which the oxide layer is synthesized, is characterized by high melting point, which makes it possible to use resistive (Joule) heating to activate this sensor, which, thus, allows to implement a sensor in the architecture of "titanium wire - matrix of titanium dioxide nanotubes". In the project, we will try to solve the problem of clarifying the mechanism of sensitivity of titanium dioxide nanotubes formed during the anodization of titanium filaments, and develop approaches to the creation and controlled change of the properties of this material. The problem under consideration is connected, first of all, with the identification of fundamentals interplays of thermocatalytic and chemoresistive effects that determine the way of operation of such gas sensors. Thus, this project is directly related to the combination of the technology of preparation/synthesis of active material with the technology of manufacturing of sensors based on it. In the project we will explore and develop methods to increase sensitivity by modifying the oxide layer with a noble metal, as well as the possibility of using these sensors for selective detection of gas impurities – by combining several sensors in one array. The scientific novelty of this study is predetermined by the progress in the field of anodic fabrication of such nanostructures, which allows us to investigate the sensor effects associated with the catalytic and semiconductor properties of this material. The study of these systems will be carried out for the first time and will be aimed at achieving maximum efficiency of these sensors, which will allow us to develop a new direction in the design of such systems.

Expected results
The implementation of the project will help to deepen our understanding of the processes of synthesis of nanotubular structures at surfaces with the initial curvature, as well as an understanding of the connection of chemoresistive / thermocatalytic effects with the sensor architecture; will allow to develop technological approaches to the synthesis (design) of single-electrode sensors within a single technological process by the method of anodization of titanium wire. In particular, the project: (1) we will develop and optimize approaches to the fabrication of sensors based on arrays of oxide nanotubes with different geometric parameters on a titanium wire, as well as approaches to nanotubular layer modification to improve sensor properties; (2) we obtain systematic data on the dependence of the sensor response on the characteristics of the nanotubular oxide layer, its relationship with the thickness of the heating wire and heating power when exposed to reducing gases (alcohols, acetone, toluene); (3) we obtain data on the implementation of the sensor array (based on the developed sensors) for selective recognition of gas mixtures; (4) based on the results of the project implementation we provide a reasonable assessment of the prospects of the proposed approach for practical application. The obtained data will allow to develop an understanding of the sensitivity mechanisms and details of design of these sensors. Taking into account the use of modern approaches for the synthesis of sensor material (synthesis of anodic titanium oxide), and progress in their use to fabricate sensor systems – we assume that the obtained results will correspond to the world level of research. This project sets not only an important fundamental basis, but also has a real practical application in the form of a possible realization of new types of highly sensitive and selective gas sensors suitable for mass production. These sensors may find wide application in households, industry, transport, trade, and also in security systems.



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.



1. Fedorov F.S., Vasilkov M.Yu., Shurigina L.Yu., Nasibulin A.G. Электрохимическое формирование массивов нанотрубок TiO2 на поверхности титановой нити XI Международная научная конференция «Современные методы в теоретической и экспериментальной электрохимии», г. Плес, Ивановская обл., 7-11 сентября 2020 г. Тезисы докладов. Иваново: Институт химии растворов им. Г.А. Крестова РАН, 2020. - 147 с., - (year - 2020).

2. Vasilkov M.Yu., Lashkov A.V., Fedorov F.S., Sysoev V.V. Исследование сенсорных свойств титановой нити, декорированной нанотрубками диоксида титана «Наноэлектроника, нанофотоника и нелинейная физика»: тез. докл. XV Всерос. конф. молодых ученых. –Саратов: Изд-во “Техно-Декор”, 2020. – 351 с.: илл., - (year - 2020).

3. Fedorov F.S., Vasilkov M.Yu., Goldt A., Shurygina L.I., Nasibulin A.G. Synthesis of Pt nano-microspheres at TiO2 –decorated Ti wire Proceedings of First virtual Bilateral Conference on Functional Materials (BiC-FM), 8-9 October, 2020, Moscow: Skolkovo Institute of Science and Technology, 2020., - (year - 2020).

4. Fedorov F.S., Yaqin A., Krasnikov D.V., Kondrashov V.A., Ovchinnikov G., Kostyukevich Yu., Osipenko S., Nasibulin A.G. Detecting Cooking State of Grilled Chicken by Electronic Nose and Computer Vision Techniques Food Chemistry, Volume 345, 30 May 2021, 128747 (year - 2021).

5. Fedorov F.S., Simonenko N.P., Trouillet V., Volkov I.A., Plugin I.A., et al. Microplotter-Printed On-Chip Combinatorial Library of Ink-Derived Multiple Metal Oxides as an “Electronic Olfaction” Unit ACS Applied Materials and Interfaces, - (year - 2020).

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.



1. Andrey V.Lashkov, Fedor S.Fedorov, Mikhail Yu.Vasilkov, Alexey V.Kochetkov, Ilia V.Belyaev, Ilia A.Plugin, Alexey S.Varezhnikov, Anastasia N.Filipenko, Stepan A.Romanov, Albert G.Nasibulin, Ghenadii Korotcenkov, Victor V.Sysoev The Ti wire functionalized with inherent TiO2 nanotubes by anodization as one-electrode gas sensor: A proof-of-concept study Elsevier B.V., Volume 306, P. 127615 (year - 2020).

2. Fedor S. Fedorov, Maksim A. Solomatin, Margitta Uhlemann, Steffen Oswald, Dmitry A. Kolosov, Anatolii Morozov, Alexey S. Varezhnikov, Maksim A. Ivanov, Artem K. Grebenko, Martin Sommer, Olga E. Glukhova, Albert G. Nasibulin and Victor V. Sysoev Quasi-2D Co3O4 nanoflakes as an efficient gas sensor versus alcohol VOCs Royal Society of Chemistry, Volume 8, Issue 15, P. 7214 (year - 2020).