Gas sensors made from electrospun nanofibers doped by functionalized carbon nanotubes

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Gas sensors made from electrospun nanofibers doped by functionalized carbon nanotubes

 

Author: Yang, An
Title: Gas sensors made from electrospun nanofibers doped by functionalized carbon nanotubes
Degree: Ph.D.
Year: 2009
Subject: Hong Kong Polytechnic University -- Dissertations.
Gas-detectors.
Nanotubes.
Nanofibers.
Department: Institute of Textiles and Clothing
Pages: xxi, 185 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2306720
URI: http://theses.lib.polyu.edu.hk/handle/200/3729
Abstract: Wearable sensors are much desired devices for industrial safety and personal protection. They require light weight and low electricity consumption in addition to sensing performance. Conventional metal oxide gas sensors are effective at only temperatures above 200 oC, which requires high electric energy and is not safe in combustion gases. Nanostructures of metal oxide materials are of great interest due to the improved sensitivity and potentially low operation temperature of nanostructures compared to the bulk materials. On the other hand, carbon nanotubes have been demonstrated as promising sensors for detecting gas molecules with fast response and high sensitivity at room temperature. Hence, the objective of this thesis is to investigate various nanostructures doped by functionalized carbon nanotubes and explore their application as wearable room temperature gas sensors. A preliminary study of electrospinning process of nanofibers was made. Effects of applied electrical field and solution flow rate to diameter and morphology of electrospun fibers were studied. Pure tin dioxide nanofibers were fabricated by calcining electrospun nanofibers of PVA/stannic hydroxide sol composite as precursor. FESEM was utilized to investigate the structure and morphology of tin dioxide nanofibers before and after calcination. A simple method for dispersing MWCNTs into tin oxide precursor solutions has been developed and the hybrid SnO2/MWCNTs nanofibers were synthesized by electrospinning followed by calcination in air at 500 oC. A simple method has been developed for the preparation of porous SnO2 nanobelts by calcining electrospun nanofibers of polyethylene oxide/stannic hydroxide sol composite in an open atmosphere. Microstructural analysis shows that the prepared nanobelts consist of a continuous network of interconnected SnO2 grains. The synthesized SnO2 nanobelts possess a high surface area and continuous porosity. Porous SnO2/MWCNTs composites were successfully fabricated by a PVA fiber-template method. The effects of applied voltage, collection distance between the tip to target and the flow rate of the solution on the morphological appearance and average diameter of the as-spun PVA fibers were investigated. It is found that the solution flow rate has the most significant effect on fiber formation, followed by the applied voltage and tip-target distance. Uniform ultrafine PVA fibers (203 +-25 nm) were obtained by electrospinning of 8wt% aqueous PVA solution at 18.4kV when the flow rate was 0.025mm/min and tip-target distance is 12.5cm. Electrospun PVA fibers were then used as sacrificial templates for coating with SnO2/MWCNTs precursor solution using a sol-gel deposition technique. Porous structures were formed after the removal of the PVA fiber templates through heat treatment. FESEM showed that the resulting composite materials exhibited an extended network of features separated by large pores with a diameter of l-2um approximately. TEM and Raman spectroscopy have been employed to characterize the composite structure. Topological defects including open-ended structures and stepped surface with open edges of graphic sheet were created on the MWCNTs after the calcination process. A gas measurement system has been used to perform electrical and gas sensing characterization. Two types of gas sensor devices were made based on flexible PET substrates and evaluated with CO gas of a wide range of concentrations. The measurements were carried out by using the sensors fabricated from SnO2/MWCNTs composite fiber mats at steady state. The results show that the n-type SnO2/MWCNTs nanofibers were able to detect carbon monoxide at 50 ppm at room temperature, while the pure SnO2 nanofibers were insensitive up to 500 ppm. This shows that the doping MWCNTs contribute to the improved sensor sensitivity. Sensors fabricated from porous SnO2/MWCNTs composites exhibit a reversible and reproducible response, at a bias voltage of 0.5V, to CO in the range of 45-400 ppm at room temperature. The effects of humidity, working voltage and doping concentration on the gas sensing properties were investigated and established. It is found that humidity does play an important role in the gas sensitivity. The mechanistic study by TEM shows that after being pretreated in acidic environment under sonication and calcination in air, abundant defect sites are created on the surface of MWCNTs walls. The defect would serve as a binding site for the CO molecule leading to changes in the electronic structure and subsequently in the adsorbate binding energy and charge transfer between gas molecules and sensing materials, consistent with the observation of sensitive CO detection at room temperature. The role of tin dioxide was explored. Our experimental results show that pure SnO2 nanostructures are insensitive to CO gases at room temperature. The SnO2 may act as a carrier for electrons or the dopant (MWCNTs) in its oxidized state acts a strong acceptor for electrons of the host semiconductor (SnO2). This induces an electron-depleted space-charge layer near the interface. Another possible effect of a carrier for electrons can be described as: The deposited clusters of dopants can provide more preferred adsorption and activation sites for the target gas CO from which activated fragments are spilled over onto SnO2 to react with the adsorbed oxygen. As a result, the surface coverage with oxygen and therefore surface potential barrier is reduced and accompanied by a change in conductance, while the cluster itself remains unchanged. Future work is needed to obtain a throughout understanding of the interactions between the adsorbed gas molecules and the SnO2/MWCNTs composites.

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