Date of Award

August 2020

Degree Type


Degree Name

Doctor of Philosophy



First Advisor

Junhong JC Chen

Committee Members

Deyang DQ Qu, Benjamin BC Church, Yongjin YS Sung, James JH Hill


gas sensor, heterostructures, nanomaterials, sensing mechanism


Chemiresistive sensors are the most widely investigated gas sensors due to their ease in fabrication, cost-effectiveness, simplicity of operation, and offer advances in miniaturization. Up to date, typical and well-researched resistive-type sensing materials include semiconductor metal oxides, noble metals, carbon-based nanomaterials (e.g., graphene and carbon nanotubes), and conducting polymers. Gas sensors based on a single material were found difficult to meet the practical requirements for multi-sensing properties, including sensitivity, selectivity, speed of response/recovery, stability, limit of detection, and room temperature operation. Rational design through a combination of chemically or electronically dissimilar nanomaterials is an effective route to enhancing gas sensing performance. Because the chemical composition varies with position, especially at the interface between two dissimilar materials, the newly hybridized structure is defined as a heterostructure. During the past decades, there has been significant research effort in exploring the nanocomposite heterostructures for chemiresistive room-temperature gas sensors. However, sensing mechanisms for such heterostructures are still elusive without solid analysis or direct characterization results. The objective of this dissertation study is to understand the sensing mechanisms of heterostructure-based chemiresistive gas sensors through in situ investigation and analysis under real operating conditions.

Various novel heterostructures have been developed for specific types of gas sensing, with a variety of in situ/operando techniques applied to investigate the sensing mechanisms toward different gases. Firstly, nickel oxide-tungsten oxide (NiO-WO3) nanowire-based heterostructures with various component ratios were fabricated via a facile, sonication-based solution mixing method. The exhibited heterojunction effect is maximally observed for W3N1 (75 mol% WO3-25 mol% NiO) and confirmed by observation of the increase in resistance due to the formation of a diode-like p-n junction at the NiO-WO3 interface. The excellent hydrogen sulfide (H2S) sensing performance for W3N1 is attributed to the p-n junction effect, sulfurization by H2S (formation of tungsten sulfides (WS2-x), and nickel sulfides (NiS1-x)), and the ideal ratio of the NiO component in the composite. The formation of reactive semi-metallic products due to sulfurization on the sensor surface was confirmed by in situ X-ray diffraction (XRD) analyses. Operando impedance measurements and resistor-capacitor (RC) equivalent circuit analyses during gas sensing experiments were performed to evaluate the effect of grain-grain boundary or the p-n junction on the sensing performance. It was found that for pure WO3 and W3N1 samples, these contributing effects are in the same direction, resulting in a cooperative and highly sensitive performance, whereas, for other compositions, the samples exhibited competing influences, resulting in low sensitivity.

Secondly, the gold doped tin oxide/reduced graphene oxide (Au-SnO2/rGO) ternary nanohybrid heterostructure was designed with improved room temperature hydrogen (H2) sensing performance. The sputtered Au nanoparticles enhanced both sensitivity and recovery of the SnO2-rGO platform. Such an enhancement was attributed to the increased surface area and the oxygen ions spillover effect of loaded Au nanoparticles. The catalytic effect of Au nanoparticles for hydrogen adsorption and desorption was then revealed through the temperature-dependent gas sensing test and the Arrhenius analysis. A better balance between sensitivity and recovery can be further achieved in the future by tuning the deposition conditions of Au nanoparticles. A prototype handheld device based on the Au-SnO2/rGO composites was finally developed for hydrogen detection. The prototype device demonstrates the potential for real-time hydrogen monitoring. The availability of such sensors will contribute to promoting a sustainable hydrogen economy, protecting public safety, and enhancing lead-acid battery safety in a wide range of applications.

Thirdly, the nickel-doped tin oxide-reduced graphene oxide (Ni/SnO2-rGO) ternary nanohybrid heterostructure was prepared with enhanced room temperature sulfur dioxide (SO2) sensing performance. The Ni additives significantly improved the lower detection limit (ppb level) of the SnO2-rGO platform. The SO2 concentration calibration curve is well fitted by the Langmuir isotherm. The humidity effect on the sensing performance was also investigated. The results suggested that current nanohybrid materials still suffer from the humidity effect. Metal oxide nanocomposite doping enhanced the SO2 sensing and activated the adsorption of water molecules, which diminished the sensor response to sulfur dioxide gas.

Finally, the Poly[3-(3carboxypropyl)thiophene-2,5-diyl]regioregular (PT-COOH)-GO binary nanocomposite heterostructure was prepared. The gas sensing properties were investigated toward NO2, NH3, SO2, and CO. The PT-COOH based sensors exhibited tunable sensing performance through the drain voltage modulation. PT-COOH-GO sensors indicated enhanced NO2 sensing performance with good sensitivity, recovery, and stable responses. The statistical signal analysis was conducted to obtain proof-of-concept results for gas discrimination through signal processing.

This study reveals the electronic conduction gas sensing model of multi-metal oxide -nanowires-based chemiresistive gas sensors through the combination of direct current (DC) and alternating current (AC) impedance measurements. The research also suggests that two-dimensional (2D) rGO with proper modifications can be efficient gas sensing materials toward various gaseous analytes. Combining in situ characterization and critical sensing factor analyses, results from the study will offer valuable and comprehensive insights for the rational design of superior heterostructure-based chemiresistive gas sensors.

Available for download on Wednesday, September 07, 2022