Nanostructure Modified Gas Sensor Detection Matrix for NO Transient Conversion of NO to NO2

Abstract

Porous silicon (PS) conductometric gas sensors are used to create a sensitivity matrix for the room temperature detection of NOx (NO, NO2). “P-type” nanopore coated microporous silicon is treated with tin, nickel, copper, and gold, electrolessly deposited onto the PS surface to form SnOx, NiO, CuxO, and AuxO nanostructured centers as confirmed by XPS measurements. The relative sensitivities of these modified PS gas sensor surface sites have been measured under 1–5 ppm NO exposure. An improved sensitivity of up to 10 times that of untreated PS is observed for a 1 ppm exposure. The choice of deposits is based on the hard to soft acid character of the nanostructured metal oxide islands that are fractionally deposited on the semiconductor interface and their effect on the physisorption of NO, a weak base, dictated by an inverse pattern (IHSAB) to the hard-soft acid base concept. NO, a free radical, can interact with oxygen sites on the modified PS sensor interfaces, to effect a transient NO2 signal unique to PS-based NO sensors, which is not observed as other basic analytes including NH3, PH3, H2S, SO2, and CO interact with “p-type” PS.A comparison is made between the current PS sensor systems which operate at room temperature and electrochemical and traditional metal oxide sensors.

Nanostructure‐Driven Analyte–Interface Electron Transduction: A General Approach to Sensor and Microreactor Design

“…The recorded signal for NO on n-type PS corresponds to 100 W ppm À1 which considerably exceeds the 2 W ppm À1 signal recorded for p-type PS. [16] For both n-and p-type PS, the observed change in response corresponds to an increased resistance. Thus, with ntype PS, the amphoteric NO radical acts like an acid withdrawing electrons whereas with p-type PS, the NO radical acts like a weak base, contributing electrons but at a much lower rate.…”

“…An N 2 flow onto the sensor is kept constant at 200 sccm at all times during the experiment and diluted NO 2 , NO, and NH 3 are mixed (separately) with the N 2 flow as we test the sensor response to these gases. [15,16] The positive resistance changes for NO 2 and NO vary linearly and correspond to an exposure to 1, 2, 3, 4, or 5 ppm (10 ppm for NO 2 ) of test gas. The recorded signal for NO 2 on n-type PS is that of an acid that has a significant electron affinity [12] and withdraws electrons from the PS interface.…”

“…The observed trends, while striking, correlate well with the relative responses observed as NO interacts with p-type PS (Table 2). [16] These data for ptype PS lead to the positioning of NO (weak base) below Cu Table 3, based on the relative response matrix for SnO x , NiO, Cu x O, and Au x O.…”

“…Schematic diagrams of the complete working sensor platform have already been presented. [2,15,16] The PS interface was generated by electrochemical anodization of 7-13 W cm…”

“…The anodized sample was cleaned in MeCN for 10 min to purge any residue in the pores due to the etch solution. [2,15,16] Subsequently, it was immersed for several minutes in HF and then methanol. The PS had a porosity of 50-80 % with the micropore diameters varying from 0.8 to 1.5 mm and pore depths varying from 10 to 30 mm.…”