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Electrode Integration of Nanostructured Metal and Metal Oxide Materials Based on in-situ Growth Methods for Environmental Sensors
Title
Electrode Integration of Nanostructured Metal and Metal Oxide Materials Based on in-situ Growth Methods for Environmental Sensors.
Creator
Wang, Xiaochen, Cho, Hyoung Jin, Fang, Jiyu, Chen, Quanfang, Lee, Woo Hyoung, University of Central Florida
Abstract / Description
In the past decades, increased human population and activities have introduced a large amount of pollutants into the environment. Various types of conventional analytical instruments were used for monitoring the emitted chemicals with low detection limit, high accuracy, and discrimination power. However, many of these methods are laboratory-based owing to sample collection, transportation, extraction, and purification steps. To make real-time on-site monitoring possible, miniaturized sensors...
Show moreIn the past decades, increased human population and activities have introduced a large amount of pollutants into the environment. Various types of conventional analytical instruments were used for monitoring the emitted chemicals with low detection limit, high accuracy, and discrimination power. However, many of these methods are laboratory-based owing to sample collection, transportation, extraction, and purification steps. To make real-time on-site monitoring possible, miniaturized sensors with various integrated elements were developed. One of the most well-known strategies is to utilize nanostructured materials with enhanced sensing properties for those devices. For a majority of the current state of art devices, the synthesis of nanostructured materials and device integration are done separately, that is, (")synthesis first and then integration(") approach which involves two separate process steps. However, this approach comes with some disadvantages such as misalignment, contamination, as well as disconnection between nanomaterials and electrodes.To overcome the aforementioned technical challenge, several synthesis methods were developed and validated for in-situ integration of nanostructured metal and metal oxide materials for environmental sensors in this work. The electroplating technique combined with photolithography was used to make the predefined metal electrodes. Then, with subsequent post-treatments, nanostructured metals and metal oxides could be produced in-situ and directly integrated in the electrodes without any extra transfer process steps.In the process of developing a phosphate sensor, nanofibrous Co electrodes were fabricated by pulsed electroplating of Co-Cu alloy and dealloying the Cu component. A linear potentiometric response to phosphate in the 10-5 to 10-2 M concentration range was obtained which validated the sensor's function. A mechanism based on mixed potential response was proposed to elucidate the Co electrode behavior in aqueous solutions with varying pH conditions and optimum pH ranges for working devices were proposed.In addition to the alloying and dealloying method, the template assisted electroplating method was also investigated. A nanoporous Co-Cu electrode fabricated by electroplating through a sacrificial glass fiber template was obtained. A linear amperometric response to phosphate with suppressed oxygen interference was achieved in a 10-5 to 10-2 M concentration range of phosphate. An analysis of the cyclic voltammetry characterization results provided a direction for further exploring an optimized electrode polarization potential range for suppressing oxygen interference while maintaining a good sensitivity to phosphate. Based on this result, we improved the fabrication process with another template: in-situ hydrothermally grown ZnO nanoflakes on the electrode surface, as a template for uniform nanostructured Co electroplating. The cyclic voltammetry characterization of the fabricated electrode showed an amperometric response in the range of 10-6 to 10-2 M of phosphate where the limit of detection (LOD) was enhanced compared with the previous work.For the flammable gas sensor development, the in-situ oxidation of Cu was utilized to form nanowires for sensing electrode fabrication. Multiple CuO nanowires were synthesized in-situ on the electroplated interdigitated Cu electrodes on a hotplate at 500 ? in air. The nanowires were successfully integrated as a sensing element into the device, forming bridges between two electrodes. The sensor's behavior was characterized by a current-voltage measurement. Simple processing parameters could be utilized for controlling the electrode morphologies and determining the characteristics of contacts - Schottky or Ohmic - at the electrode interface. A hypothesis was proposed to explain the transition phenomenon between Schottky and Ohmic contact modes, providing an important baseline for future device design and fabrication. Finally, the fabricated sensor was tested for a flammable gas detection using saturated ethanol vapor at room temperature, which implicates a low power consumption gas sensor without elevating the sensor temperature unlike traditional gas sensors.
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Date Issued
2017
Identifier
CFE0007139, ucf:52312
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0007139
Electroplated micro- and nanoscale structures for emitters and sensors
Title
Electroplated micro- and nanoscale structures for emitters and sensors.
Creator
Wang, Xiaochen, Cho, Hyoung, Fang, Jiyu, Chen, Quanfang, Lee, Woo Hyoung, University of Central Florida
Abstract / Description
In the electroplating process, dissolved metal cations are reduced by electrical current to a form a coherent metal coating on an electrode. Therefore, electroplating is primarily applied to modify the surface properties of an object (e.g. abrasion and wear resistance, corrosion protection, lubricity, aesthetic qualities, etc.), but also be applied to build up high aspect ratio structures on undersized parts or to form devices by electroforming.Compared with other common MEMS ...
Show moreIn the electroplating process, dissolved metal cations are reduced by electrical current to a form a coherent metal coating on an electrode. Therefore, electroplating is primarily applied to modify the surface properties of an object (e.g. abrasion and wear resistance, corrosion protection, lubricity, aesthetic qualities, etc.), but also be applied to build up high aspect ratio structures on undersized parts or to form devices by electroforming.Compared with other common MEMS (microelectromechanical systems) metal device fabrication techniques, such as vapor depositions, electroplating has several outstanding advantages. First, the fabrication process is cost-efficient because electroplating process can be set up easily without complex and expensive facilities. Second, the fabrication condition of electroplating is less demanding and does not require high temperature or low pressure. Furthermore, the process is applicable to making various features consisting of nanometer to millimeter scale particles, wires, and films. Thus, in this thesis, based on the design requirements of electrospray emitters and environmental sensors, the electroplating method was chosen to fabricate micro- and nanoscale structures for such applications.Electrospray is an atomization technique by which an electrically conductive liquid through a small capillary is charged with high voltage (kV) and ejected to a ground electrode. To minimize the electric field edge effect of the emitter nozzles to get even electro-hydrodynamic pulling force on the liquid among the nozzles and minimize variation from one emitter to another, the device needs to have the viscous pressure drop across each nozzle dominant over the electro-hydrodynamic pulling force. Therefore, embedded structures that can create high flow impedance are desirable to achieve uniform feeding of low flow rate of liquid to each emitter.We designed and fabricated in-plane metallic electrospray devices with an embedded array of micropillars within a microchannel by photolithography and electroplating. The novelty of the proposed research lies in its embedded flow restriction structure, scalability, and ease of fabrication. The formation of jets as well as the flexing capability of the emitter was achieved. The other application of electroplating was demonstrated in the fabrication of environmental sensors. Utilizing a pulsed electroplating method, Co-Cu metal alloy films were prepared and Cu was selectively etched to fabricate nanoporous electrodes which could be used to measure both absolute levels and changes of phosphate concentration in aqueous environments. The formation of cobalt phosphate compound could be used for the detection. The increased surface area and relatively simple fabrication protocols make the proposed method attractive and promising for many environmental sensing applications.
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Date Issued
2014
Identifier
CFE0005274, ucf:50548
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0005274