My research focuses in particle science and technology. Following is a list of projects I am currently or was previously involved in:
1. Rapid Measurements of Particle Hygroscopic Growth with a Humidity-Controlled Fast Integrated Mobility Spectrometer (HFIMS)
2017-present, Environmental and Climate Sciences Department, Brookhaven National Laboratory
PI: Dr. Jian Wang
Hygroscopicity is a key parameter in determining the impact of atmospheric aerosols on global radiation and climate change. Tandem differential mobility analyzer (TDMA) system is the most widely used instrument for determining the aerosol hygroscopicity. However, TDMA measurements are relatively slow because the time needed for a full scan of DMA voltage is typically in the range of minutes. The slow measurement speed becomes a significant issue when unsteady sources of aerosols are being studied, for example, direct measurement of combustion aerosols and measurement of aerosols onboard mobile platforms.
The recently developed water-based fast integrated mobility spectrometry (WFIMS) allows rapid mobility-based measurement of particle size distribution. It uses a parallel plate mobility system to separate charged particles with different mobilities. Upon exiting the mobility separator, the spatially separated particles are condensationally grown in a three-stage water-based growth channel and imaged onto a CCD array. The size distribution is obtained by counting particles located in each mobility bin shown on the image, providing a near instantaneous measurement (1 Hz) of mobility size distributions. The use of water vapor enables independent control of RH in the mobility separation region, which cannot be achieved with alcohol vapor due to their lower diffusivity. A humidity-controlled FIMS (HFIMS), consisting of a DMA, a relative humidity control unit, and a WFIMS coupled in series, was tested for measuring the hygroscopic growth of particles. In this study, a data inversion algorithm is developed to derive the growth factor distribution of the DMA-classified particles. The inversion algorithm uses the known transfer functions of the upstream DMA and the WFIMS, and calculates the growth factor distribution that reproduces the position distribution of particles measured by the WFIMS. The growth factor distributions of ambient particles at various RHs are analyzed with the inversion algorithm. Further optimization of the HFIMS system will be discussed.
2. Sub 2 nm Particle Characterization in Systems with Aerosol Formation and Growth (Ph. D. Dissertation)
2014 - 2017, Aerosol and Air Quality Research Laboratory, Washington University in St. Louis
PI: Professor Pratim Biswas
Aerosol science and technology enable continual advances in material synthesis and atmospheric pollutant control. Among these advances, one important frontier is characterizing the initial stages of particle formation by real time measurement of particles below 2 nm in size. Sub 2 nm particles play important roles by acting as seeds for particle growth, ultimately determining the final properties of the generated particles. Tailoring nanoparticle properties requires a thorough understanding and precise control of the particle formation processes, which in turn requires characterizing nanoparticle formation from the initial stages. The knowledge on particle formation in early stages can also be applied in quantum dot synthesis and material doping. This dissertation pursued two approaches in investigating incipient particle characterization in systems with aerosol formation and growth: (1) using a high-resolution differential mobility analyzer (DMA) to measure the size distributions of sub 2 nm particles generated from high-temperature aerosol reactors, and (2) analyzing the physical and chemical pathways of aerosol formation during combustion.
Part. 1. Particle size distributions reveal important information about particle formation dynamics. DMAs are widely utilized to measure particle size distributions. However, our knowledge of the initial stages of particle formation is incomplete, due to the Brownian broadening effects in conventional DMAs. The first part of this dissertation studied the applicability of high-resolution DMAs in characterizing sub 2 nm particles generated from high-temperature aerosol reactors, including a flame aerosol reactor (FLAR) and a furnace aerosol reactor (FUAR). Comparison against a conventional DMA (Nano DMA, Model 3085, TSI Inc.) demonstrated that the increased sheath flow rates and shortened residence time indeed greatly suppressed the diffusion broadening effect in a high-resolution DMA (half mini type). The incipient particle size distributions were discrete, suggesting the formation of stable clusters that may be intermediate phases between initial chemical reactions and downstream particle growth. The evolution of incipient cluster size distributions further provided information on the gaseous precursor reaction kinetics, which matched well with the data obtained through other techniques.
Part 2. The size distributions and their evolution measured by the DMAs help explain the physical pathways of aerosol formation. The chemical analysis of the incipient particles is an important counterpart to the existing characterization method. The chemical compositions of charged species were measured online with an atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF). The tandem arrangement of the high-resolution DMA and the APi-TOF realized the simultaneous measurement of the mobility and the mass of combustion-generated natively charged particles, which enabled their chemical and physical formation pathways to be derived. The results showed that the initial stages of particle formation were strongly influenced by chemically ionized species during combustion, and that incipient particles composed of pure oxides did not exist. The effective densities of the incipient particles were much lower than those of bulk materials, due to their amorphous structures and different chemical compositions. Measuring incipient particles with high-resolution DMAs is limited because a DMA classifies charged particles only, while the charging characteristics of sub 2 nm particles are not well understood. The charge fraction of combustion-generated incipient particles was measured by coupling a charged particle remover and a diethylene-based condensation particle counter (Airmodus A10). A high charge fraction was observed, confirming the strong interaction among chemically ionized species and formed particles. The combustion system was modeled by using a unimodal aerosol dynamics model combined with Fuchs’ charging theory, and showed that the charging process indeed affected particle formation dynamics during combustion.
Particulate matter (PM) is a crucial factor of air quality affecting visibility, human health, and global climate.PM is quantified using PM10, PM2.5, or PM1, according to inhalation and deposition properties in the human respiratory system, representing the mass concentration of particles below 10 μm, 2.5 μm, and 1 μm in aerodynamic size, respectively. The measurement of PM indices can rely on various instruments, among which impactors, cyclones, tapered element oscillating microbalances (TEOM), and beta attenuation monitors (BAM) are commonly used. Other instruments, such as Dusttrak and SidePak (TSI Inc.) use light-scattering to obtain particle mass concentrations, while scanning mobility particle sizers (SMPS) and aerodynamic particle sizer (APS) derive particle mass concentrations from measured particle size distributions. Temporal and spatial PM index may differ from each other significantly due to the limited transport coefficient of particles. In order to obtain accurate local PM indices down to streets or blocks, a higher density of measurement sites mapping out the entire area is required, while the expense of the complicated instruments mentioned above frustrates the plan. With prototypes first assembled in smoke detectors and air purifiers decades ago, particle sensors become popular in recent years with the usage of portable PM monitors, due to the remarkably low price of around $ 10 USD and the merging need for “big data”. Although cannot generate data for enacting air pollution regulations or for health studies due to the less accuracy compared to the advanced instruments, these low-cost sensors can be used in locating pollution hotspots or generating coarse 3-D map of PM concentrations for individuals, industries, and environmental agencies. In a broad sense, the usage of the low-cost particle sensors can also raise the awareness of air quality among the society. Hence, it is worthwhile to study and evaluate the performance of the particle sensors and further provide instructions to customers and manufacturers.
The real-time and local measurement of PM concentration can be achieved by networking the particle sensors wirelessly. We are using Arduino and XBee boards to communicate among the sensors and the computers, weaving a network reporting indoor and outdoor PM concentraitons.
Tandem Differential Mobility Analyzer (TDMA) systems are utilized to investigate the size change of sub-micron particles under certain environments, including heating, humidifying, and reaction conditions. In a TDMA, monodisperse particles generated by the first DMA is introduced into the environment for investigation. Then the size of the grown or shrinked particles is further measured with the second DMA. By determining the extent of growth or shrinkage, the effect of environment on particle shape could be evaluated. The study on TDMA system is mainly focused on measurement of hygroscopicity and volatility of sub-micron particles. The existence of sub-micron particles can greatly influence global climate forcing due to their optical properties. On the other hand, the formation of these particles is largely dependent on the humidity in atmosphere, since particle growth is mainly a result of condensation of water vapor on newly formed nanometer range particles. Therefore, by knowing how size of particles with different chemical composition will react in environment with certain humidity, the growth or shrinkage of particles can be determined. Further evaluation of optical property and climate forcing can be estimated and modeled. A Labview program was designed to obtain particle size distribution information in three modes, including SMPS mode, heating mode for volatility study, and humidifying mode for hygroscopicity study.
A uniform, high-quality nanostructured TiO2 film used in Dye Sensitized Solar Cells (DSSCs) is synthesized by a novel premixed stagnation swirl flame synthesis system, followed by a short-time sintering and densification process. By simply tuning synthesizing and annealing parameters, we could prepare films with different morphology and characteristics, i.e. particle size, surface area and packing density. The whole process for film preparation costs around 1 hour in time, which is significantly less than the time needed (~12 hours) using traditional sol-gel method. The cell reaches an efficiency of 5.7% under AM 1.5G incident irradiance of 22mW/cm2 without using a back scattering layer or an anti-reflective layer. The role of TiO2 film annealing time in cell performance is investigated. It was found that there exists an optimum value of annealing time. As annealing time increases, cell efficiency increases at first which is caused by a decrease of crystal deficiency; then, cell efficiency drops down because annealing decreases film surface area, which results in a decrease of dye loading. In our research, we also found that the procedure of densification determines the performance of DSSCs in shrinking film thickness and adding electron path ways.