Grantee Research Project Results
Final Report: In-Situ Measurement of Vehicle Exhaust Emissions Using Supramolecular Conducting Polymer Films
EPA Contract Number: 68D02074Title: In-Situ Measurement of Vehicle Exhaust Emissions Using Supramolecular Conducting Polymer Films
Investigators: Ram, Manoj
Small Business: Fractal Systems Inc.
EPA Contact: Richards, April
Phase: I
Project Period: October 1, 2002 through July 31, 2003
Project Amount: $99,975
RFA: Small Business Innovation Research (SBIR) - Phase I (2002) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , SBIR - Air Pollution , Small Business Innovation Research (SBIR)
Description:
The U.S. Environmental Protection Agency needs on-vehicle emissions data (NO, NO2, N2O, SO2, NH4, CO, CO2, diesel and gasoline hydrocarbons, particulate matter mass, etc.) from on-highway and non-road vehicles, while the vehicles are being operated. There is a need to develop rapid, cost-effective, low-power, non-intrusive, rugged sensors that can be easily installed. To be useful as an engine exhaust measurement system, a sensor, device, and/or technique has to be able to detect the above gases precisely at relevant low concentrations.
Fractal Systems, Inc.'s effort has focused on the use of highly organized, ultrathin conducting polymer and/or conducting polymer/SnO2 films with high conductivity and excellent selectivity for the above gases, generated from exhaust engines. The supramolecular approach has been utilized to fabricate films of conducting materials via the layer-by-layer (LBL) self-assembly technique. In addition, metal oxide-doped conducting polymers have been fabricated using the same technique with the purpose of establishing selectivity for the different gases. The different films were fabricated at various solid surfaces and electrodes. Using earlier experience and work on ultrathin ordered films, Fractal Systems, Inc., intended to develop highly sensitive, cost-effective, simple, and reliable sensors for in-situ measurement of engine exhaust gases, with high selectivity and low detection limit in real time. In Phase II, real-world measurements will be performed in which the gas will be collected from an exhaust engine and tested in the laboratory for sensor calibration in the temperature range of interest to yield meaningful data.
The extensive characterization effort has yielded some excellent results from several materials for sensing three major gases, namely CO, NO2 and SO2. Ultraviolet (UV) visible spectroscopy, fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), four-point probe conductivity, and AC impedance measurements were used to quantify the different characteristics of the various polymer systems and to ensure reproducibility, stability, and reliability. By achieving the same film thickness in each fabrication process, Fractal Systems, Inc., was able to control the parameters that are of importance for gas detection, primarily diffusion time of the gas into the electrode layer. Thus far, the company has achieved ppb sensitivity in seconds, and selectivity has been achieved by material design. For example, polyaniline or polyaniline-SnO2 nanocomposite are CO selective, while poly(hexylthiophene) or its SnO2 nanocomposite is NO2 selective. The different techniques indicate that the materials are stable, with no resistance change with time or temperature up to 500°C.
This approach is unique in that it allows for the fabrication of films with thicknesses that can be varied from monolayer to several hundred nm. One additional advantage is the possibility of fine tuning the structure and composition of the films to accommodate gas or analyte sensitivity and selectivity. Together with the simple fabrication technique (low cost), possible miniaturization, and multi-analyte sensor fabrication, this approach is likely to result in viable devices for the marketplace. It is planned to build these devices with several digits on one electrode array where each digit, which consists of a different material, can be separately addressable. Fractal Systems, Inc., will be collaborating with HRL Laboratories for such future efforts, and the company has interacted with Figaro for marketing the technology, which will provide a significant step forward in automotive exhaust detection, among several other industrial applications.
Summary/Accomplishments (Outputs/Outcomes):
This effort consisted of several tasks that revolved around the synthesis, processing, and characterization of a few selected stable, conducting polymers followed by gas testing for sensitivity and selectivity. The substrate preparation for ultrathin film deposition is an important task for obtaining the desired number of layers. The substrates used always had a hydrophilic surface coated with a first layer that consists of polystyrene sulfonate (PSS). The type of surface treatment affects the outcome with respect to material properties. Before deposition of the PSS layer, the substrates often are protonated by tetramethylsilane for better layer deposition. The various conducting polymers have been used to fabricate films of conducting polymer with the LBL technique chemically and electrochemically, using well-known procedures. Films with different thicknesses of one single polymer, layers of two different polymers, and copolymers have been fabricated. The polymers synthesized thus far are being characterized using UV-visible spectroscopy, electrochemical impedance spectroscopy, cyclic voltammetry, AFM, and electrical conductivity measurements. Deposition of supramolecular layered films was achieved on different substrates, such as glass and indium tin oxide (ITO) coated glass plates.
The polymers and composites were characterized using various techniques, including UV-visible spectroscopy, FTIR, AFM, and resistance and impedance measurements before, during, and after exposure to the different gases. The different materials also were evaluated after a thermal treatment at high temperatures using the same techniques to test their stability.
Optical spectroscopy of conducting polymers is a well-known technique for characterization of the conducting states and conjugated chain morphology. Each polymer has its characteristic optical absorption bands, which correspond to inter-and/or intra-gap states. Each polymer also has a characteristic color that can be determined from the position of the transition band. The intra-gap energy states typically are affected when one of the conducting polymers is exposed to oxidizing or reducing species. The gases being used in this effort have a doping effect on the polymers, and therefore, optical spectroscopy is a good tool for understanding the processes that are taking place as a result of exposure to those gases. Adsorption of and interaction with the different gases differs from one form of the same polymer to another. FTIR results indicated that the structure is better defined (narrower peaks) in Fractal Systems, Inc.'s materials, partially due to the preparation technique, which results in more ordered materials. This technique also is valuable for analysis of the membranes after exposure to gases. The company has found that the structure is basically unaffected by adsorption of the different gases.
The conductivity of the different films was measured using the two- or four-probe techniques. The treated (protonated) surface showed better conductivity than the hydrophilic surfaces in the absence of protonation. This means that protonation is a requirement for deposition of high-quality films. Conductivity measurements yielded the range needed for gas detection using simple measurement setups. Each polymer has its own range of conductivity that depends on its structure and whether the polymer is alone or in the form of a composite with metal oxides. The uniformity in the deposition process and surface topography of the films were observed by AFM. The AFM images reveal a surface topography of typical granular patterns, and the size of spheres ranges from a few nm to 250 nm, depending on the number of film layers and the nature of the material.
Cyclic voltammetry (CV) was used to determine redox potentials and the CV profiles of the polymers, parameters that are important for indicating whether certain gases are reactive with the polymer system. Most of the systems investigated exhibit CVs that change in the presence of a reactive gas and remain unchanged in the presence of non-reactive gases. Typically, this type of behavior is reversible when the gas is desorbed from the system. Thermal stability was studied in the range from room temperature to 800°C. Most polymers, and particularly their nanocomposites, remain intact when heated up to 400°C. Above 500°C, the material's morphology becomes more granular and less effective in gas detection.
Fractal Systems, Inc., monitored the change of resistance of various materials prepared with different numbers of monolayers (thicknesses) upon exposure to CO, NO2 and SO2. The company detected ppb levels with films that are in the range of few to 100 monolayers. Selectivity was achieved by using the correct material (e.g., a polypyrrole film is specific for one gas, while a polyaniline film is specific for a different gas; the same observation is valid for the nanocomposite films). A typical thickness that proved to be practical was one in which 20-25 layers was used in the deposition process.
Conclusions:
Fractal Systems, Inc., fabricated conducting polymers and their nanocomposites and nanohybrid structures on glass, ITO-coated glass, and interdigitated electrodes. The films were characterized using various techniques. Interdigitated electrodes were used for the detection of CO, NO2, and SO2 gases thus far. Poly(hexylthiophene)-SnO2 is very sensitive to CO gas, as is the case for other polymer systems, but it has a faster response time. Measurements indicate that the nanocomposite supramolecular films are excellent candidates for recognition and detection of such gases. The sulfonated polyaniline-SnO2 hybrid structure also is very sensitive to CO, but the recovery time, although fast (ms), is slower compared to other polymers, particularly poly(hexylthiophene)-based films. The layered structure of poly(hexylthiophene)-based composite shows a high sensitivity for NO2 gas as well, without having any effect in the presence of SO2 gas. This means that sensor poisoning from such a gas will not take place. The number of bilayers necessary for achieving fast gas sensing has been determined on the ms scale. Such a rate can be modified by changing the number of layers with precision. To achieve selectivity, it has been shown that the different polymer systems respond differently to different gases. Therefore, the likely scenario will involve the use of a multi-chip sensor in which each chip would be selective for a given gas. More experiments are needed to determine the type of polymer and the gas that it selectively recognizes. In Phase II, the effect of temperature on gas-sensing properties of the best candidates will be determined. Fractal Systems, Inc., has started to interact with HRL Laboratories and Figaro for commercializing the technology, and Foresight Science and Technology has had input in identifying the markets where the sensors could have an added value.
Supplemental Keywords:
vehicle exhaust emissions, automotive exhaust sensors, supramolecular conducting polymer films, ultrathin films, engine, layer-by-layer self-assembly, LBL, CO, NO2, SO2, ultraviolet-visible spectroscopy, UV, fourier transform infrared spectroscopy, FTIR, atomic force microscopy, AFM, cyclic voltammetry, CV, polystyrene sulfonate, PSS, nanocomposite, gas selectivity, small business, SBIR., Scientific Discipline, Air, air toxics, Environmental Monitoring, Engineering, Chemistry, & Physics, Environmental Engineering, particulate matter, particulates, atmospheric particles, air pollutants, vehicle emissions, automotive emissions, nanotechnology, emissions measurement, air sampling, automotive exhaust, emissions, supramolecular conducting polymer films, in-situ method, atmospheric aerosols, aromatic hydrocarbons, exhaust, nitrogen oxides (Nox)The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.