The Room-Temperature Chemiresistive Properties of Potassium Titanate Whiskers versus Organic Vapors

The development of portable gas-sensing units implies a special care of their power efficiency, which is often approached by operation at room temperature. This issue primarily appeals to a choice of suitable materials whose functional properties are sensitive toward gas vapors at these conditions. While the gas sensitivity is nowadays advanced by employing the materials at nano-dimensional domain, the room temperature operation might be targeted via the application of layered solid-state electrolytes, like titanates. Here, we report gas-sensitive properties of potassium titanate whiskers, which are placed over a multielectrode chip by drop casting from suspension to yield a matrix mono-layer of varied density. The material synthesis conditions are straightforward both to get stable single-crystalline quasi-one-dimensional whiskers with a great extent of potassium replacement and to favor the increase of specific surface area of the structures. The whisker layer is found to be sensitive towards volatile organic compounds (ethanol, isopropanol, acetone) in the mixture with air at room temperature. The vapor identification is obtained via processing the vector signal generated by sensor array of the multielectrode chip with the help of pattern recognition algorithms.

The precursor, potassium polytitanate, has been sintered and further used to prepare potassium titanate whiskers [S1]. The as-sintered precursor has been inspected by scanning electron microscopy, SEM (Tescan VEGA 3, Brno, Czech Republic), under an accelerating voltage of 15 kV. The SEM images are shown in the Figure A1. The potassium polytitanate is presented by irregularly shaped nanoparticles which are agglomerated to macro-assemblies of few micrometers. The calcination of the potassium polytitanate at 1050 ± 10 °С for 1 hour results in appearance of whiskers imbedded into glass phase; the SEM images are presented in the Figure A2. The images are recorded following the grinding of the obtained product.
The X-ray diffraction (XRD) of the potassium polytitanate has been carried out using ARL X'TRA diffractometer (Ecublens, Switzerland), with CuKα source of 0.15412 nm wavelength. The data are given in Figure A3. XRD results of the potassium polytitanate studies possess no evident characteristic reflections from crystalline planes that is caused by irregular agglomerates to appear in quasi-amorphous state. The calcination leads to formation of two phases, H2Ti8O17 and K2Ti4O9. Figure A2. SEM images of sintered potassium titanate whiskers embedded into glass phase. Figure A3. X-ray diffraction (XRD) patterns of as-prepared potassium polytitanate (red) and of the sintered potassium titanates after calcination at 1050 °C (black).
The obtained material is represented by quasi-1D whiskers, whose width is in sub-micrometer range, while the length reaches up to hundreds of micrometers. The glass phase is related to unreacted residuals.
Further ultrasonic treatment of the grinded product in acidic media supports the removal of residual base and an increase of specific surface area. The final product is presented in the Figure A4. There are whiskers of different widths and thicknesses, which are mainly in the range of tens of nanometers while the length is in micrometer range. The sonification in acidic media results in replacement of potassium ions by hydrogen ions [S2,S3] which facilitates the increase of the specific surface of the material, as measured by BET method (Quantachrome Nova2200, Boynton Beach, FL, 3 USA). The increase of the surface is the most drastic for the slurry treated to get the lowest pH (6-7) value. The dependence of specific surface on the pH treatment is summarized in the Table A1. Figure A4. SEM images of potassium titanate whiskers after sonification in acidic media. Table A1. Specific surface of the potassium titanates obtained after calcination and further milling depending on number of sonification cycles in acidic media.
No Cycle, a.u. рН of slurry Specific surface, Ssp, m 2 /g 1 -12-14 5-9 2 3 8-9 15-35 3 5 6-7 170-250 The final whiskers are characterized by layered structure, which is supported by HR-TEM studies ( Figure A5). We have performed the elemental analysis of the final titanate structures by Secondary Neutral Mass Spectrometry (SNMS) in INA 3 system (Leybold-Heraeus, Germany). In that technique, the sample is located in vacuum chamber, where its surface is bombarded by positive ions that are extracted from RF-excited argon plasma [S4]. Being induced by the ion bombardment, material is removed from the sample mainly as neutral atoms, which are ionized by electron impact when passing through the plasma. After suppression of thermal ions, a quadrupole mass spectrometer is finally used for mass analysis. Since the atoms could be excited not only from the sample, but also from the mask and a whole setup, first the spectrum was measured for the elemental composition of 4 the carrier. The data received after these measurements were used to correct the number of atoms that were extracted from the sample in the received spectra. The details are given in [S5-S7]. The spectrum has been recorded to analyze the elemental composition at the surface of the polytitatanes. The obtained data are summarized in the Table A2. The additional TEM/SAED measurements of obtained whiskers been taken using FEI Tecnai G2F20 S-Twin TMP (Eindhoven, The Netherlands) instrument applying accelerating voltage of 200 kV. Line resolution is 0.14 nm. The SAED results ( Figure A6) that were taken in a repeated set of characterization measurements, reveal d-values of 3.65, 3.49, 11.79 and 5.89 Å, correspondingly for (010), (110), and (100)  Our FFT results that were obtained at HR-TEM image ( Figure A7) additionally confirm the d-values calculated to be 3. 74, 3.48, 9.47, 4.73 Å, for the planes (010) and (110), (100), (200) planes. Moreover, the d-values for higher-order planes are 2.41, 1.74, 1.87, 2.37, 2.93, and 2.00 Å in case of (310), (220), (020), (400), (210), (410) planes, respectively.

B. Sensor preparation
We have optimized the suspensions based on the final product to get almost a single-whisker layer, depending on the concentration of the whiskers in solutions. A set of dispersions has been prepared where whisker concentration is varied from 5.0% mass to 0.0002% mass. Dispersion has been drop-casted on the Si/SiO2 wafer and let dry at room temperature. SEM inspection suggests that employing suspensions with a concentration of whiskers lower than 0.002% does not lead to the formation of contacts to support the current flow between the electrodes ( Figure B1). Utilization of 5% suspension results in appearance of layer with thickness of several whiskers. At this morphology some whiskers are limited to be exposed to gas environment. So, we have utilized the suspension of 0.1% mass for the chip preparation.
C. Gas-sensing characteristics Figure C1. Typical change of the median conductance of the titanate whisker layer under appearance of the isopropanol vapors, 5 kppm, in the mixture with air, (1), (2) denote pure lab air and mixture of air with isopropanol vapors, respectively.  Figure C2. The gas response of the multisensor chip based on titanate whiskers towards organic vapors, ~ 5 kppm concentration, in the mixture with lab air at room temperature distributed over the chip array and corresponding conductance values at chip exposure to air and the vapors, acetone (a), ethanol (b), isopropanol (c) in the mixture with air.