Stevens Professors Discover a New Oxygen-Gold Structure

Interactions between oxygen and gold are fundamentally important in multiple and diverse areas of science and technology. Professors Fei Tian and Simon Podkolzin at the Department of Chemical Engineering and Materials Science discovered that in contrast with regular gold surfaces, oxygen interacts with gold nanoparticles differently and forms a special structure. They discovered a new oxygen-gold structure where two oxygen atoms are separated by a single gold atom as Au-O-Au-O-Au. This discovery will advance the development of improved catalysts with gold nanoparticles in chemical and petroleum-refining industries, improve techniques for bio imaging of living cells, lead to better understanding of spin-flip scattering processes of itinerant electrons in solid-state physics, advance optical detection methods at the single-molecule level, advance the development of better lithium batteries and improve understanding of oxygen-gold interactions in other areas of science and technology.

Professors Simon Podkolzin (left) and Fei Tian at Stevens Institute of Technology.

Professors Tian and Podkolzin reported their discovery in the publication entitled “Observation and Identification of an Atomic Oxygen Structure on Catalytic Gold Nanoparticles” DOI: 10.1002/anie.201706647 in Angewandte Chemie International Edition in September 2017, journal announcement on Twitter. The study was co-authored by graduate students Kai Liu, Tao Chen, Shuyue He and Jason Robbins. Angewandte Chemie is the journal of the German Chemical Society. The journal has been published since 1888, and it is one of the oldest and most respected scientific publications. The work in Prof. Podkolzin’s group was partially funded by the National Science Foundation under Grant CBET-1264453. The work in Prof. Tian’s group was funded by the American Chemical Society Petroleum Research Fund under Grant 55094-DNI5.

The new oxygen-gold structure, Au-O-Au-O-Au, was observed and identified on gold nanoparticles in catalytic decomposition of hydrogen peroxide to oxygen and water. This structure with an oxygen dimer separated by a gold atom is different from the known isolated atomic oxygen structures, which have only one oxygen atom, and different from the known molecular oxygen structures, which have two directly bonded oxygen atoms. The new structure was observed with in situ surface-enhanced Raman spectroscopic measurements and identified with density functional theory calculations. The experimental measurements were performed using monodisperse 5, 50 and 400 nanometer gold particles supported on silica with liquid-phase hydrogen and deuterium peroxides at multiple pH values. The calculations show that on surfaces with coordinatively unsaturated gold atoms, two oxygen atoms preferentially share a gold atom with a bond distance of 0.194-0.196 nanometers and additionally bind to two other surface gold atoms with a larger bond distance of 0.203-0.213 nanometers, forming the Au-O-Au-O-Au structure.

By studying catalytic decomposition of hydrogen peroxide to oxygen and water over gold nanoparticles, Professors Tian and Podkolzin discovered a new oxygen-gold structure where two oxygen atoms are separated by a single gold atom.

It is challenging to characterize oxygen structures on gold surfaces because the density of adsorption and reaction sites is typically extremely low. Flat gold surfaces are usually chemically inert, and only a small fraction of gold surface sites (coordinatively unsaturated sites, such as defects, steps and kinks) serves as catalytically active sites. Professors Tian and Podkolzin were able to address this challenge by utilizing ultra-high sensitive surface-enhanced Raman spectroscopic measurements collected with a custom-built system. An additional challenge is that properties of gold nanoparticles are typically very dependent on their size. This second challenge was addressed by evaluating monodisperse gold nanoparticles supported on silica. Monodisperse Au nanoparticles are usually synthesized in colloidal solutions with stabilizing organic ligands, which interfere with catalytic activity and spectroscopic measurements. This third challenge was addressed by developing a simple thermal treatment that removed stabilizing ligands, produced spectroscopically clean surfaces, and yet allowed gold nanoparticles to remain mostly monodisperse on the silica support.

Reaction kinetic measurements showed that the newly discovered oxygen-gold structure is not just a byproduct or a spectator species but an actual catalytic reaction intermediate because it forms on the same gold atoms that serve as active sites in catalytic decomposition of hydrogen peroxide. Therefore, this discovery will advance the development of improved catalysts for selective oxidation reactions that are urgently needed for sustainable and environmentally-friendly production of chemicals and will be helpful in energy research, nanotechnology and in numerous other fields of science and technology that rely on oxygen-gold interactions.

3-D visualization of the discovered oxygen-gold structure on a gold nanoparticle.