XPS Technique Reveals Surface Chemistry of Bio-Based Hybrid Materials

Researchers at the Fraunhofer Institute for Applied Polymer Research (IAP) are using a technique called X-ray photoelectron spectroscopy, or XPS, to study the surface chemistry of bio-based materials being developed for uses such as water treatment, catalysis and functional coatings. The work, done with the Materials Chemistry research group of Professor Andreas Taubert at the University of Potsdam, focused on materials made by combining natural raw materials with inorganic components, and the latest results were published in the New Journal of Chemistry.

The researchers produced biochar — a carbon-rich material made by heating organic matter with little oxygen — from spent coffee grounds and orange peels, then combined it with clay minerals and metal oxides to create hybrid materials for water purification. Dr. Jiyong Kim, a surface analysis expert at Fraunhofer IAP, used XPS to study how thermal treatment and hybrid formation changed the chemistry at the materials' surfaces and interfaces. In XPS, a material's surface is irradiated with X-rays and the energy of the emitted particles is measured. "XPS is like an extremely fine surface scanner. It allows us to read the top 1 to 10 nanometers of a material like a chemical map," Kim said, adding that the analysis needs only minimal sample preparation.

Materials made from natural raw materials are chemically more complex than conventional laboratory chemicals, because heating biomass such as coffee grounds or orange peels triggers many reactions at once, producing carbon with uneven and sometimes unpredictable surface chemistry. In the coffee-ground biochar, XPS showed that thermal treatment reorganized the surface carbon into a more carbon-rich, graphite-like structure that remained chemically varied and rich in defect sites — properties that can help the material bind pharmaceutical compounds, dyes and other organic pollutants. "A detailed understanding of the exact chemistry in a material helps us accurately understand not only the material composition but also the individual changes that occur in a material once it is used in an application like water remediation or in an electrochemical device," said Taubert.

In bio-based hybrid materials, the organic and inorganic components are joined at very small scales, and what happens at their interface can strongly influence the material's later properties. "With hybrid materials, it is not enough to know which elements are present. What matters is how the organic and inorganic components interact and whether new chemical environments form at their interfaces. XPS allows us to make these interactions visible," Kim said. In hybrid systems made of orange-peel biochar, clay and titanium dioxide, the analysis showed that the organic and inorganic components form chemical bonds at their interfaces during thermal processing, involving titanium, oxygen, carbon and aluminum.

In nitrogen-modified photocatalysts — materials that use light to break down pollutants — XPS also let the researchers distinguish nitrogen built into the titanium-containing inorganic structure from residual nitrogen in organic material. Fraunhofer IAP said the technique helps companies and research partners identify the causes of batch-to-batch variation, aging effects and surface changes in bio-based materials, guiding the systematic optimization of new material formulations.