Date of Degree

9-2017

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor

Teresa J. Bandosz

Committee Members

Charles Michael Drain

Maria Tamargo

Stephen O’Brien

Subject Categories

Chemistry | Environmental Chemistry | Materials Chemistry | Physical Sciences and Mathematics

Keywords

gas sensing, toxic gases, ammonia sensing, nanoporous carbons, surface chemistry, adsorption

Abstract

Activated carbons, either synthetic, developed in our laboratory, or commercial, were prepared or further modified, in order to introduce specific heteroatoms such as oxygen, nitrogen and sulfur to their matrices. Chips coated with thin layers of the carbon samples were used for the sensing of gaseous ammonia. They were exposed to continuous cycles of various ammonia concentrations (10-500 ppm), and changes in normalized resistance were analyzed. In all cases linear responses were recorded and the chips reached sensitivities as high as 31%, which are comparable to those of modified graphene-based sensors. The applied specific surface chemical modifications were an effective means to control the type of the charge carriers (electrons or holes), and thus the electronic and transport properties. The mechanism of the reversible sensing was governed by several processes including specific interactions between the surface functional groups and the molecules of the target gas, pore-filling with ammonia (especially of pores smaller than 0.7 nm), electron–hole conductivity, and charge transport through ionic conductivity. Strongly acidic carboxylic and sulfonic groups played an important role in ammonia sensing by promoting charge transport via ionic conductivity, due to the formation of NH4+. Among all N-containing groups, nitrogen located in six-membered rings (pyridines and quaternary nitrogen), rather than nitrogen on the periphery (amines, amides) played the most important role in sensing. An important aspect was the conversion of the conduction type from predominantly p- to predominantly n- upon oxidation of the carbon surface due to introduction of electron withdrawing nitro groups to the matrix. Owing to the high porosity of the oxidized carbon and the polarity of the formed -NO2 groups present in the pore system, opposite signal changes compared to the initial counterpart were recorded (decrease instead of increase in the normalized resistance). Interesting changes in the electrical response were noticed for S- and N-dual-doped carbons. Their ability to activate oxygen and generate superoxide ions resulted in oxidation of ammonia to nitrogen dioxide. NO2 adsorbed in the pore system caused an increase in the population of holes (h+) as charge carriers in the matrix, which led to a conductivity increase upon ammonia exposure. The surface chemical and structural features of the carbons acted either synergistically or competitively. When the chips were exposed to H2S, they showed a very low sensitivity to this gas. A high surface acidity of the carbons enhanced their affinity towards NH3 adsorption, contributing to a selective ammonia detection. The role of the specific chemical arrangement of the heteroatoms on ammonia sensing is extensively examined and analyzed.

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