Leveraging Hot Electrons for Optical Detection of Hydrogen and Methane

Metal nanostructures such as Au, Ag and Cu can couple visible light efficiently through the creation of resonant localized surface plasmons (LSPs) which are collective oscillation of free electrons on the surface of the metal. The decay of localized surface plasmons leads to the formation of hot electrons, which are known to catalyze chemical reactions when irradiated with a flux of low intensity visible photons. One such reaction is the dissociation of hydrogen which has been shown to occur via the transfer of hot electrons from Au to adsorbed H2 under visible illumination creating a transient negative H2- ion and facilitates the dissociation. In the first part of the talk, I will show how I have leveraged hot electron induced dissociation to develop fast optical H2 sensors based on changes in optical transmission and spectral shift in the localized surface plasmon resonance. The mechanism of sensing is still not well understood. The hypothesis is that there is the formation of a metastable gold hydride following the dissociation event, which leads to a change in the dielectric constant of pure Au, thereby causing the optical transmission to change. I have investigated how the changes in the size of the nanoparticle, wavelength and power of the excitation source affect the change in optical transmission of Au in hydrogen. I am currently using Density Function Theory, Discrete Dipole Approximation and in-situ spectroscopic ellipsometry to elucidate the mechanism and understand the interaction of Au with atomic hydrogen. In the second half of the talk I will focus on how I have used metallic alloys such as Au-Ag and Au-Cu to detect Methane and H2 respectively. These results are important because of the following reasons: the nanostructures used in these studies being surfactant free open up new avenues towards the design of efficient plasmonic catalysts. It is important to note that the observation of hot electron generation and photocatalytic H2 dissociation capabilities for spherical, larger nanostructures (> 20 nm) was previously reported to be negligible. On the contrary, in the present study, a ∼1-2% change in optical transmission was achieved at room temperature using nanostructures between 10 and 50 nm in diameter upon incoherent excitation. These experiments also demonstrate an inexpensive and simple optical spectroscopic technique for detecting hydrogen and methane at room temperature.

For further information please contact Amit Agrawal, 301-975-4633, amit.agrawal [at] nist.gov (amit[dot]agrawal[at]nist[dot]gov) @nist.gov>