Monitoring of the Plasmon Resonance of Gold Nanoparticles in Au/TiO₂ Catalyst under Oxidative and Reducing Atmospheres

Abstract

Diffuse optical reflectance of Au/TiO 2 powder catalysts, prepared by the deposition-precipitation method, is measured in the UV-visible range in controlled atmosphere. An intense optical absorption observed around 550 nm is interpreted by the excitation of plasmon resonances in the 4 nm gold nanoparticles (NPs). The location, intensity, and width of the absorption can be reproduced theoretically by using a distribution of shapes of the NPs. The changes of the reflectance upon exposure of the Au/TiO 2 catalyst to different atmospheres (O 2 /He, H 2 /He, CO/He) are measured in real-time by use of a homemade differential diffuse reflectance (DDR) spectrometer. The derivative-like DDR spectrum shows that the exposure to oxygen leads to the adsorption of oxygen species at the surface of the Au NPs. By modeling the DDR spectrum, we analyze different possible processes that could occur during the oxygen exposure and we find that the main effect is a charge transfer from gold to the adsorbed oxygen species, combined with a slight flattening of the Au NPs. The exposure to CO gives a similar but much smaller effect than the one of oxygen. Finally, real-time optical measurements performed during the exposures to oxygen indicate that two different sets of particles are probably present in the catalyst, and react with different kinetics, slightly flat three-dimensional NPs and very flat almost two-dimensional NPs.

Ultrasensitive and fast single wavelength plasmonic hydrogen sensing with anisotropic nanostructured Pd films

“…The carrying gas presented so far was chosen to be Ar due to its inertness to Pd, in order to investigate the intrinsic capabilities of the TAS method with comparison to previously investigated plasmonic Pd-based sensors. Nevertheless, we have verified the response of the sample to pure O 2 , to pure synthetic dry air and to 50% relative humidity in Ar, particles, leading to charge transfer between metallic Pd and the adsorbed molecules, in the same way as it has been shown previously for adsorption of oxygen and of hydrogen on gold [21,69,70].…”

Mechanism of hydrogen adsorption on gold nanoparticles and charge transfer probed by anisotropic surface plasmon resonance

“…Hence, the high sensitivity of the Au NPs LSPR upon gas adsorption makes it an efficient tool for gathering information on their chemical properties. In particular, the red or blue shift of the LSPR, and its amplitude, are directly related to the sign and to the amount of charge transfer between Au and adsorbed species, and should inform on the charge transfer between Au and adsorbed H. 24,25 However, the amount of adsorbed H atoms on the Au NPs, hence the transferred charge, is expected to be small, and thus the shift difficult to measure. Most of LSPR-based sensors rely on the shift in wavelength of the position of the LSPR, ∆λ, induced by the analyte adsorption.…”

Gas Sensing with Nanoplasmonic Thin Films Composed of Nanoparticles (Au, Ag) Dispersed in a CuO Matrix

“…Figure 8 presents the LSPR peak position (transmittance) of the three systems, during five cycles under vacuum, and O 2 at atmospheric pressure. As expected from this type of sensor, the transmittance shift due to a change in the refractive index is typically very short, in the order of tenths of percentage points [2,47]. Anyway, it is possible to observe that the films responded consistently to the presence of the gas.…”