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High sensitivity plasmonic sensing of hydrogen over a broad dynamic range using catalytic Au-CeO2 thin film nanocomposites Nora Houlihan, Nicholas Karker, Radislav A. Potyrailo, and Michael Andrew Carpenter ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b01193 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018
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ACS Sensors
High sensitivity plasmonic sensing of hydrogen over a broad dynamic range using catalytic Au-CeO2 thin film nanocomposites Nora M. Houlihan 1, Nicholas Karker1, Radislav A. Potyrailo2, Michael A. Carpenter1* 1 SUNY
Polytechnic Institute, College of Nanoscale Engineering and Technology Innovation, 257 Fuller Road, Albany, New York 12203, United States 2 General Electric Global Research Center, Niskayuna, NY 12309, USA Keywords: hydrogen sensor, LSPR, plasmonic, SOFC, gold, ceria Abstract Next-generation gas-sensor technologies are needed for diverse applications including environmental surveillance, occupational safety, and industrial process control. However, the dynamic range using existing sensors is often too narrow to meet demands. In this work plasmonic films of Au-CeO2 that detect hydrogen with 0.38% and 60% lower and upper detection limits in an oxygen-free atmosphere experiment are demonstrated. The observed 15 nm peak shift was 4x stronger versus other plasmonic H2 sensors. The proposed sensing mechanism that involves H2 dissociation by Auδ+ nanoparticles, was validated using XPS, kinetics, and Arrhenius studies. Our understanding of this remarkable sensing behavior in oxygen-free conditions opens new horizons for packaging, art conservation, industrial process control and other applications where conventional oxygen-dependent sensors lack broad dynamic range.
Gas sensor technologies continue to be challenged with regards to signal quality, stability and dynamic range.1 The need for high quality gas sensors are numerous and include industrial safety and process controls, environmental monitoring and homeland security among others.1,2 In critical applications analytical grade instruments are preferred due to their detection resolution and selectivity. However, even with cost constraints, the inconvenience of instrumentation size, lack of distributed detection capability, and power requirements these instruments can be an inopportune necessity for a wide range of sensing applications given the lack of alternative options.3–6 Chemical sensors have long been sought to ease these constraints, however typical devices have insufficient dynamic range and stability.1,7–10 The work presented here examines an optical based sensor for the detection of a broad range of H2 concentrations in a zero oxygen background. While there has been significant research on electrochemical metal oxide, field-effect transistors, and Pdbased room temperature sensors for H2 detection, in most cases oxygen must be present to create a depletion region in a metal oxide semiconducting film to sense a change in conductance or resistance.11,12 Optical sensing measurements are much better suited for the high concentrations of hydrogen since no electrical components are used, eliminating any ohmic contacts as well as any potential electrical discharge hazards within H2 rich environments.13 Earlier studies have shown that the plasmon band of a Au-SiO2 film on a fiber optic cable can be used for H2 detection at percent levels,14 however, this work relied on a return to an oxygen-containing baseline condition for efficient recovery of the films. Other work showed detection up to 100% H2 with a SrTiLaO3 coated optical fiber13 and later
work showed a similar fiber could detect CO, CO2, and CH3 in a water containing baseline,15 though both works exhibited significant drift over 20 hour time frames. Palladium based localized plasmon resonance sensors have also shown promise for sensing in high hydrogen concentration environments, although long term stability is still lacking.16 While lower H2 concentrations were targeted for O2 containing applications, previous work has demonstrated sensing of H2 with Au-ceria nanocomposites.17 Additional research on Au-CeO2 catalysts mostly focuses on preferential oxidation of carbon monoxide in H2 rich streams.18–20 Other studies have examined the plasmon band of Au hemispheres after exposure to multiple repeats of 10% H2 in an N2 background and observed up to a 2% signal change at room temperature.21 While responsive to high levels of H2, no other concentrations were reported and the measurements were done at room temperature. Furthermore, the response varied significantly for each cycle and was a shortterm test lasting less than 10 minutes, thus long-term stability was not demonstrated. Sensors that can operate in oxygen-free conditions are needed for numerous applications where conventional oxygendependent sensors lose their gas-response ability. Examples include food and medical packaging22,23, art conservation,24 industrial process control,25 and many others.26,27 One specific application is the monitoring of gases for reliable operation of solid oxide fuel cells (SOFCs). Sensors are needed to accurately monitor concentrations of hydrogen, and other gases that come from the steam reforming of methane or natural gas. These gases need to be carefully monitored as the inlet stream composition directly affects SOFC operation characteristics.28,29 Specifically, by monitoring the CO/H2
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Figure 1. A representative schematic of the proposed mechanism for hydrogen dissociation on the Au/Ceria interface and the impact on the free electron density and dielectric constant of the ceria. (a) dissociative H2 adsorption mechanism in zero O2 conditions. (b) reduced signal change when O2 is present (teal line) compared to signal change in zero O2 conditions (black line) with the Drude equation. (c) mechanism when O2 is present where the H2 and O2 react with the Au-ceria nanocomposite leading to a reduced signal change.
ratio in real time at the inlet of the SOFC, efficiency can be increased.30 Given these needs and current technology limitations it is clear that new methods of in situ gas monitoring continue to require a low cost sensor that can monitor ppm to percent levels of H2 at high temperatures. The present work has revisited the sensing characteristics of Au-CeO2 thin films in the absence of O2 and have shown blue shift signal changes of 6.6% of the plasmon band, which is approximately 4x the amount observed in previous studies the highest hydrogen concentration and almost 2x the amount shown by earlier findings and furthermore the sample does not saturate its response until H2 concentrations as high as 60%.17,21 Such a unique response in zero oxygen conditions, as will be shown for the present work, is a result of a dominant H2 dissociative adsorption on oxidized Au species at the ceria interface as schematically shown in Figure 1a. This leads to an increase in charge density, No and a blue shift in the plasmon band as noted from the Drude equation in Figure 1b.31,32 Hydrogen spillover is also possible which would increase and cause a red shift in the plasmon band as noted from the
Drude equation. However, given the dominant blue shift, spillover can only be a minor reaction channel if it is occurring in the present study.32 In previous work, it was determined that with an increase in the O2 concentration there is a decrease in the signal change for the same concentration of H2 and Figure 1b shows a representative schematic of their reported signal change at 500C for 1% H2 in dry air (teal line) as well as the representative schematic signal change seen in this work for 5% H2 in N2 (black line).17 Reduced signal changes in the presence of O2 are due to reactions that increase , Figure 1c, which given the competing effects noted in the Drude equation leads to an overall reduced plasmonic response. Such competing mechanisms were recently highlighted in the Au-YSZ system where with increasing YSZ thickness a null response could be observed as both channels cancelled out their respective changes to the plasmon peak position.33 In addition, the present work demonstrates excellent sensitivity to concentrations ranging from 5-20% over the course of a 16 day stability experiment. Therefore, for the first time it is presented that AuCeO2 thin films show great promise for plasmonic sensing
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Figure 2. (a) SEM micrograph of an uncapped Au-CeO2 thin film. (b) baseline 300 °C plasmon curve of the sample shown in (a) in black, and the plasmon curve when the sample is exposed to 20% H2 in red at 300C. (c) The change in plasmon peak position as a function of time as the sample is exposed to 5, 8, 12, 15, and 20% H2 over 16 days. (d) An overlay of the first and last set of exposures from the 16day stability test. (e) Calibration curve for the 16-day stability test.
at 300oC in zero oxygen conditions and percent levels of H2. These results are highlighted along with kinetics analysis and a discussion of the potential reaction mechanisms that are shown in Figure 1. The Au-CeO2 thin films were fabricated in a layer-by-layer approach where gold was deposited between two layers of CeO2 on a quartz substrate. A ceria base layer of 75 nm was deposited with physical vapor deposition followed by a high temperature anneal at 1000°C for 1 hour in argon. This was followed by a 3 nm gold evaporation and a subsequent anneal process at 900°C for 5 minutes in argon for dewetting and formation of hemispherical gold nanoparticles of the size shown in Figure 2a. The anneal process after the gold evaporation step forms AuNPs with an average diameter of 15.0 5.0 nm (Supplementary Figure S1). Previous studies have shown that the catalytic activity of AuNPs increases when the diameter is
