Improved recovery time and sensitivity to H2 and NH3 at room temperature with SnOx vertical nanopillars on ITO

Abstract

Nanostructured SnO₂ is a promising material for the scalable production of portable gas sensors. To fully exploit their potential, these gas sensors need a faster recovery rate and higher sensitivity at room temperature than the current state of the art. Here we demonstrate a chemiresistive gas sensor based on vertical SnO_x nanopillars, capable of sensing < 5 ppm of H₂ at room temperature and 10 ppt at 230 °C. We test the sample both in vacuum and in air and observe an exceptional improvement in the performance compared to commercially available gas sensors. In particular, the recovery time for sensing NH₃ at room temperature is more than one order of magnitude faster than a commercial SnO₂ sensor. The sensor shows an unique combination of high sensitivity and fast recovery time, matching the requirements on materials expected to foster widespread use of portable and affordable gas sensors.

Figure 1

Scanning electron microscope images of vertical nanostructured SnO_x pillars on ~150 nm thick ITO Sn-reduced substrate. (a) 45° tilt angle, and (b) 0° tilt angle. The two figures share the same scale bar. The images are taken before starting the sensing tests and is representative of the post-sensing morphology, as no change can be noticed under the SEM. (c) Response of vertical nanostructured SnO_x pillars after exposure for 100 s to several gases at low temperatures (30 or 90 °C) in low vacuum (10⁻⁵ mbar).

Figure 2

Response of SnO_x nanopillars sensor to low pressures of H₂ (corresponding to ppt amount) at 230 °C in UHV.

Figure 3

(a) Resistivity variation of the vertical SnO_x nanopillars sensor (blue line) compared with the FIGARO sensor (dashed red line) after an exposure to 30 ppm of NH₃ in air at RT. The NH₃ exposure period has been marked with a green background. Both recovery times are fitted with double exponential functions, illustrated by dashed black lines. (b) Observed fluctuations can be compared with (c) the signal noise. (d) Shows the resistivity variation of our vertical SnO_x nanopillars sensors (blue line) compared with the FIGARO response (dashed red line) after an exposure to 3.6 and 2.2 ppm of NH₃ (pulse in the millisecond range).

Figure 4

Experimental response of the sensor as a function of NH₃ exposure at RT in air, with about 50% of relative humidity. The power law fit is meant as a guide for the eye.

Figure 5

Schematic diagrams of the nanopillar electron bands under different conditions (UHV and ambient air) after exposure to different gases. (a) O₂ in UHV, (b) H₂ or CH₄ in UHV, (d) O₂ in air, (e) NH₃ in air with pre-adsorbed O₂, (f) H₂O in air with pre-adsorbed O₂. (c) Reference band diagram for the n-doped SnO_x nanopillars. The dotted lines represent the donor level (E_d), the dashed line the intrinsic Fermi level (E_i).

Figure 6

Energy band alignment at the ITO/SnO₂ isotype heterojunction (adapted from ref.).

Figure 7

Schematic of the electron injection from the SnO₂ nanopillars to the substrate upon the exposure to a reducing gas. The only junctions are at the nanopillar/ITO interface. Therefore, the current from source to drain does not have to pass through any junction.