Our research program aims to expand our understanding of materials, surfaces, and interfaces and to apply this knowledge to make advances in a number of technologically emerging and societally critical areas. Currently, at the fundamental level, we are interested in low-dimensional materials, hybrid organic/inorganic materials, metal oxides and silicates. In these materials systems, we aim to establish synthesis-structure-property relationships, and to understand and control their surfaces and interfaces. These insights are helping to guide our interest in impacting areas such as environment, health, sustainability and energy as detailed below.
Chemical Sensing
Starting from the days of the canary in the coal mine, technologies have been developed to assist with the detection of chemicals in the environment, whether they be toxic, combustible, or simply contain an unpleasant odor. At the core of gas sensor development is the exploitation of some chemical or physical property of the target chemical to detect its presence. There are several broad classes of techniques, each with its own advantages and limitations, including electrochemical sensors, conductometric sensors, calorimetric sensors, optical sensors, acoustic sensors, and more. Below some of our current activities in this area are detailed.
Fundamental understanding of MOX-based sensors:
Chemiresistive sensors, which transduce target gas concentrations based on the change in resistance of a sensing material, provide sensitive, low-cost detection of gaseous analytes in applications such as environmental monitoring, quality control, and clinical diagnostics. Semiconducting metal oxides (MOX) such as SnO2 are an industry-standard material system for chemiresistive sensing. We aim to develop MOX-based sensors with stable long-term characteristics by investigating how the structural and electronic properties of MOX respond to different operating temperatures, durations, and environments. To enhance the stability, sensitivity, and selectivity of our sensors, we are also investigating catalytically active noble metals, such as Pt, Au, and Pd, loaded onto MOX materials to form noble metal-loaded MOX nanocomposites. These nanocomposites are integrated with interdigited electrodes to form fully functioning sensors. To understand the sensing mechanisms of pristine MOX and metal-loaded MOX nanocomposites, we are correlating charge transport processes elucidated by impedance spectroscopy with structural and chemical information conveyed by X-ray photoelectron spectroscopy. Mature sensors will leverage energy-efficient (~15 mW to reach 500 °C) poly-Si and SiC microheater platforms previously developed by our group to enable robust, low-power gas microsensors for a variety of applications.Materials innovations:
A classic challenge in gas sensing is the tunability of the sensing material for the selective absorption of target gases without interference from unwanted species. Metal-organic frameworks (MOFs), made up of metal-cluster nodes connected by organic linkers, can achieve selective adsorption owing to their high chemical and structural tunability. Their selectivity and flexibility make MOFs attractive for gas sensing, as realized in novel low-power, low-footprint, on-chip devices such as the chemical-sensitive field-effect transistor, previously demonstrated by our group. In this project, we aim to explore the large library of MOFs (consisting of different metal nodes and organic linkers) towards novel electronic sensor arrays. This effort includes investigating the underlying electronic transduction mechanisms of MOFs through resistance measurement, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and infrared and Raman spectroscopies, targeting important polluting and toxic gases such as CO2, CH4, CO, CH2O.Device innovations:
Developing cheap and efficient sensors for monitoring different gases (such as CO2) is of great importance in many industries, and to environmental and human health, including food storage, microbial investigation, air-quality assessment, and capnography. The most common CO2 gas sensor is nondispersive infrared (NDIR) sensors which, while very effective, have proven difficult to miniaturize and to reduce cost. Among reported methods for gas detections, colorimetric sensors stand out for their simplicity, passive nature, and capability of exhibiting color changes detectable to human eyes, providing a user-friendly and convenient platform in practical applications. We aim to develop sensing materials based on MOFs with specific chemical entities to generate strong and reversible gas adsorptions. For example, the porous, robust ZIF-8, which is constructed by Zn2+ ions and 2-methylimidazole linkers can be modified with ethylenediamine (ED) post-synthesis. ED reacts with CO2 (in methanol) to form a zwitterion intermediate, which further protonates the incorporated pH indicator (phenol red, PSP) to generate a color change. With the rise of interest in the Internet of Things (IoT), the need for low-power sensors for monitoring the working environment has been in spotlight. Considering the number of sensors required to provide real time monitoring, creating sustainable and self-powered sensors is essential. Triboelectric nanogenerator (TENG), which converts mechanical motion to electrical energy, is one of the most promising candidates for realizing self-powered sensors due to its sensitivity to surface material properties and ability to generate consistent signals depending on mechanical input. We aim to develop functionalized TENG devices whose characteristics change in response to a target gas, enabling self-powered chemical sensing.Sustainable Agriculture
One-third of all food produced globally (1.3 billion tons) is wasted annually, amounting to an estimated loss of $935 billion per year. In developing regions, 29% of food loss occurs during production and storage, for instance due to spoilage. Accurately monitoring crops in the field, in storage, and in transit thus plays a critical role in reducing food loss. The concentrations of gaseous volatile organic compounds (VOCs) are correlated with the status of fruits and vegetables, and provide chemical means for agricultural monitoring. For example, avocados emit phenols when under-ripened and tomatoes emit large alcohols when infected by fungi. Some VOCs, such as ethylene, control the rate at which fruits ripen—a vital concern when produce is shipped internationally and post-harvest ripening may lead to spoilage. Looking at tree nuts, Pistachio is a 3.6-billion-dollar industry in California. Although pistachios are not perishable foods, they are susceptible to fungal contamination and insect infestation. Infected nuts can rapidly accumulate aflatoxins. Aflatoxins are considered a serious food safety issue. The early detection of aflatoxin can lower the incidence of infected pistachio exports to the European Union. This will facilitate the export of high value products, with lower financial risk for U.S. processors and growers in both the short and long term. It is our plan to accomplish early, and more accurate, detection of aflatoxin using our advanced sensor technology to sense the specific volatile organic compounds (VOCs) it releases as a result of its metabolic reactions. Earlier and more sensitive detection may prevent the manifestation of aflatoxin in pistachios during the transport process. If we are successful, our proposed technology will improve the sensitivity and specificity of aflatoxin detection for all pistachios produced within the U.S. This will result in an improved industry food safety standard.
Green Construction
Global usage of concrete has tripled in the last 40 years, and continues to grow rapidly, placing immense pressure on the environment while requiring its use for safe and effective infrastructure. Concrete accounts for roughly 10% of worldwide CO2 emissions annually. A promising method for directly reducing the CO2 emissions associated with concrete is through replacement of cement, the primary binding material in concrete, with a percentage of carbon, creating so called carbon-incorporated cement composites (CCC). Carbon may be sourced from the waste product of methane pyrolysis, a process that is being explored to produce hydrogen fuel at large with lower CO2 footprint, making CCCs a method for carbon sequestration. Along with the environmental benefits, CCCs have displayed beneficial mechanical properties in the form of tensile strength and allow for opportunities with in-situ structural health monitoring arising from the electrical conductivity differences of solid carbon in concrete. Previous work has demonstrated the capability for CCCs to monitor compressive, tensile, and flexural stresses in concrete members at carbon replacements of 0.6% (wt.). This work looks to increase the carbon replacement to levels up to 10% while maintaining (or improving) the sensing, mechanical, and workability properties of concrete. To achieve this, surface modification of carbon materials, namely carbon fibers, via various methods will be used to increase dispersion of fibers in concrete necessary for mechanical and electrical effects and as an enabler towards low-carbon intensity hydrogen fuel.
RECENT NEWS
-
- April 2023 – Anthony receives the SURF Rose Hill Fellowship!
- April 2023 – Stuart receives the NSF Graduate Fellowship!
- February 2023 – Xiaohong Zhu receives the EBI-Shell Postdoctoral Fellowship!
- May 2022 – Veronica receives the SURF Rose Hill Fellowship!
- May 2022 – Jeffrey graduates with the highest honors and heads to Stanford for graduate studies!
- March 2022 – Zhou successfully completes his PhD and joins Stanford as a postdoc!