Dynamic Interaction of NO₂with a Nanostructure Modified Porous Silicon Matrix: Acidity, Sensor Response, and the Competition for Donor Level Electrons

Abstract

We describe a reversible response matrix and the dynamic interplay, as a moderately acidic NO 2 analyte interacts at room temperature with a TiO 2 , SnO x , Cu x O (x = 1,2), and A x O (x 1) metal oxide nanostructure deposited n-type nanopore coated microporous porous silicon (PS) interface. A significant variable response matrix is measured and the dynamic nature of analyte-interface processes characterizing the semiconductor-analyte interaction as NO 2 couples to the decorated semiconductor majority charge carriers. The competition for electrons between NO 2 and the TiO 2 decorated PS interface leads to distinct variations in sensor response as a function of increased TiO 2 concentration and displays a time variance as the extraction of electrons by NO 2 reaches a limiting charge carrier depletion and the decorated PS surface becomes electron withdrawing. This dynamic reversal is also demonstrated by distinct time-dependent PS interface responses to NO 2 and NO where the degree of electron extraction reaches a maximum and is again countered by the depleted n-type PS. Electron depleted SnO x and Au x O treated PS interfaces display a significant remnant effect as the decorated surface can become electron withdrawing for extended periods, forming a stronger acid and extracting electrons from NO 2. Exposure to NH 3 also demonstrates the remnant effects of depletion.We have outlined 1 an approach to "nanostructure directed electron transduction vs. chemisorption" on a sensor/microreactor interface. The fractional deposition of nanostructured metal oxide islands is used to modify sensitive surface layers created utilizing a hybrid nanopore covered microporous matrix formed on "p-or n-type" silicon. The core of this approach is the Inverse Hard / Soft acid / base (Table I) interaction model (IHSAB). This model provides a general approach to optimally design sensors with improved and variable sensitivity and conversion efficiency for a variety of gases, in an array-based format, 5 by correlating the tenants of acid/base interaction and the properties of extrinsic semiconductors. [1][2][3][4] In the present study, we demonstrate not only the variable surface sensitivities that are introduced by the playoff between the untreated extrinsic semiconductor interface and the matrix of fractionally deposited nanostructured islands, but also the dynamic time-dependent response that exemplifies the manner in which these nanostructure decorated interfaces interact with the analyte NO 2 , striking at the heart of the competitive electron dynamics that characterizes these systems.The IHSAB principle correlates with a basis in directed electron transduction with minimized chemisorption,and is in complement to the HSAB principle for hard/soft acid/base interactions first put forth by Pearson et al. 6 and later correlated with density functional theory (DFT) 7,8 and Chemical Reaction Theory 9 by Pearson, Parr, 7,8 Cohen et al. 9 and others. 10 In contrast to the HSAB principle which outlines a means to form strong ionic or covalent b...

A Variable Response Phosphine Sensing Matrix Based on Nanostructure Treated p and n-Type Porous Silicon Interfaces

“…We suggest that this results as the decorated p + doped interface extracts electrons more effectively than does the p-type decorated interface. TiO 2 , as a strong acid, can compete with the p + interface [24], [32] during the electron transduction process. Cu x O as a moderate acid also is capable of competing with the p+ interface for electrons while Au x O, as a weak acid, does not compete nearly as effectively for these electrons.…”

Nanostructure directed interfaces created from nanoparticle island sites deposited to microporous arrays and forming sensor platforms

“…These results provide a comparative but by no means exhaustive list. For these analytes, concentrations at the sub-ppm level can be routinely measured at room temperature using the configuration of Figure 1 (LEL (Lower Exposure Limit) = 0.94ppm for H 2 S [44], 0.84 ppm for SO 2 , < 0.5 ppm for NO 2 , 1ppm for CO,< 0.5 ppm for NH 3 with the low ppb range readily accessible, and < 0.5 ppm for PH 3 ( amenable to extension to the ppb range)) [2,[44][45][46][47][48]. The LEL's that we quote here correspond to the lowest measured concentration directly accessible to our measurements, in the linear regime of response, with no extrapolation.…”

Sensitive and selective detection of H2S and application in the presence of toluene, benzene, and xylene

“…Observed sensitivities and the sensor system reversibility can be predicted from the recently developing inverse hard/soft acid/base (IHSAB) model [3][4][5][6][9][10][11][12][13][14], which provides a means of linking chemical selectivity and the mechanism of sensor response. This model combines the basic tenants of acid/base chemistry (the ability of bases to donate electrons and acids to seek electrons) and semiconductor physics.…”

“…The selection of these nanostructures and the variable and controllable reversible interaction they introduce for sensor applications is well predicted by the IHSAB model [3][4][5][6][9][10][11][12][13][14] as it dictates the coupling of analyte/interface acid-base interactions with the properties of the majority charge carriers in an extrinsic semiconductor. The inverse hard and soft acid and base (IHSAB) concept [3][4][5][6][9][10][11][12][13][14] complements the tenants of HSAB interactions [15]. Based on the reversible interaction of hard acids and bases with soft bases and acids, the IHSAB principle enables the selection of interacting materials that do not form strong covalent or ionic chemical bonds; thus it represents the inverse of the HSAB model [15] for significant bond formation based on strong ionic (hard acid/base) or covalent (soft acid/base) interactions and chemical bond formation.…”