In Situ Monitoring of Non-Thermal Plasma Cleaning of Surfactant Encapsulated Nanoparticles

Abstract

Surfactants are widely used in the synthesis of nanoparticles, as they have a remarkable ability to direct their growth to obtain well-defined shapes and sizes. However, their post-synthesis removal is a challenge, and the methods used often result in morphological changes that defeat the purpose of the initial controlled growth. Moreover, after the removal of surfactants, the highly active surfaces of nanomaterials may undergo structural reconstruction by exposure to a different environment. Thus, ex situ characterization after air exposure may not reflect the effect of the cleaning methods. Here, combining X-ray photoelectron spectroscopy, in situ infrared reflection absorption spectroscopy, and environmental transmission electron microscopy measurements with CO probe experiments, we investigated different surfactant-removal methods to produce clean metallic Pt nanoparticles from surfactant-encapsulated ones. It was demonstrated that both ultraviolet-ozone (UV-ozone) treatment and room temperature O2 plasma treatment led to the formation of Pt oxides on the surface after the removal of the surfactant. On the other hand, when H2 was used for plasma treatment, both the Pt0 oxidation state and nanoparticle size distribution were preserved. In addition, H2 plasma treatment can reduce Pt oxides after O2-based treatments, resulting in metallic nanoparticles with clean surfaces. These findings provide a better understanding of the various options for surfactant removal from metal nanoparticles and point toward non-thermal plasmas as the best route if the integrity of the nanoparticle needs to be preserved.

In Situ Ambient Pressure Photoelectron Spectroscopy Study of the Plasma–Surface Interaction on Metal Foils

“…These findings are in agreement with previous ex situ XPS studies examining hydrogen plasma interaction with transition metals. 11 , 12 Notably, the exposure to hydrogen plasma made it possible to reduce the metal oxides at room temperature due to the plasma’s high reactivity. As a result of this reduction, H 2 O is formed and thus observed in the O 1s spectrum, which aligns with reported post-plasma XPS measurements.…”

“…Although in situ techniques such as Raman spectroscopy , and IR spectroscopy have been applied, one powerful chemical analysis technique for surface characterization currently not implemented in situ is X-ray photoelectron spectroscopy (XPS), as it is traditionally operated in a high vacuum. As such, previous investigations on the plasma–surface interface perform ex situ measurements by exposing the sample to the plasma environment before returning to vacuum conditions for XPS measurements. − While these ex situ XPS measurements provide vital information regarding the post-plasma surface chemistry, it cannot fully describe the intermediate chemical reactions or temporary chemical states that occur during the plasma environment. Ambient pressure XPS, however, enables measurements in gas environments with pressures typically in the order of mbars.…”