Converting light into chemical and electrical energy offers the opportunity to effectively harvest incoming solar radiation. To develop these concepts into real-world technologies, new materials systems need to be developed. This research program focusses on the synthesis and characterization of a new class of nanocomposites involving chemically reactive alkaline earth oxides (barium oxide and strontium oxide) and titanium dioxide, a well-established photocatalyst. Upon thermal annealing, such composites can transform at least partially into ferroelectric perovskites that are expected to promote charge separation in the presence of light.
Nanoparticles & Film Architectures
Scheme (left) and Scanning Electron Microscope (SEM) images (right) of a hybrid nanoparticle film supported on a sputtered thin oxide film.
The influence of spontaneous polarization on the surface chemistry and separation of photogenerated charge carriers will be investigated in the metal oxide grains and on the compositionally graded interface layers. We will explore size effects on structure, strain and ferroelectric properties and use microscopy, X- ray diffraction and spectroscopic techniques.
The knowledge acquired during this project will be used to improve the CO2 conversion into added-value chemicals, which is a particularly timely endeavor that could provide a new path to mitigate global warming. This project will contribute to the rational development of photoactive materials for energy conversion and photocatalysis. Moreover, we believe that this work will be highly influential for materials science activities that focus on sensors, piezoelectric energy harvesters and for light induced processes in functional electroceramics.
One part of the project aims at the hybridization of complex high-surface-area nanoparticles ensembles with model 2D structures as they can be produced by sputter-based techniques (Figure 1). The objective is to create metal oxide-graded nanostructures with enhanced charge separation properties. The selection of materials for this project is based on their intrinsic properties. These include titanium dioxide (TiO2, a photocatalytic material capable of harnessing solar radiation, and barium titanate (BaTiO3), a ferroelectric metal oxide that can extend the lifetime of photoexcited electron-hole pairs thanks to its spontaneous lattice polarization. The samples are characterized using Scanning Electron Microscopy (SEM) imaging, X-ray Diffraction (XRD), Electron Paramagnetic Resonance (EPR) spectroscopy, among other techniques.
Different nanoparticle architectures will be prepared by depositing aqueous slurries (containing pre-synthesized oxide particles together with organic additives) onto the sputter-coated substrates (Figure 1). Interconnected particle networks will then be generated by thermal annealing. As an integral part of the project, we will have to consider the possibility that extrinsic point defects get incorporated into nanoparticle films during the fabrication process. The impact of such defects on the materials’ performance, which is often ignored or at least overlooked by most research groups, needs to be considered when discussing the functional properties of different nanoparticle architectures. Importantly, we observed for TiO2 nanoparticle films that a characteristic point defect associated with the presence of carbon impurities inside the lattice accumulates in the nanoparticle necks and solid-solid interfaces during material processing (Figure 2). These carboneck centers act as electron traps and form within the nanoparticle network as well as in the region between the nanoparticles and the supporting substrate.
Carbonecks enter the Particle Network
Scheme illustrating the formation of carboneck centers in nanoparticle-based mesoporous films.
Nanoparticle powders with high specific surface areas and tunable densities correspond to a real-life system, quite representative of what the industry could mass-produce. High surface area nanofibers or sintered and coarsened BaTiO3 nanoparticle structures are analyzed with regard to their ferroelectricity-assisted charge separation properties.
Grain Configurations
From Barium Titanate Nanoparticles that were grown by Gasphase Synthesis to sintered Ceramic Surfaces.
We discovered that barium titanate is unstable in aqueous media, which leads to the incongruent dissolution of barium and subsequent barium carbonate formation (Figure 3). This behavior can have a negative influence on the ferroelectric materials properties of the BaTiO3, hence it can change the charge separation behavior.
The insights gained from this research program should pave the way for creating more efficient photocatalytic systems that drive sustainable solutions in the global effort to combat climate change.
Researchers
Guillem Vives Ollé, MSc.
Kerstin Neuhauser, MSc.
Gregor Zickler, Dr. MSc.
Thomas Berger, Assoc. Prof. Dr.
Gilles Bourret, Assoc. Prof. Dr.
Oliver Diwald, Prof. Dr.
Publications
[1] On the Importance of Nanoparticle Necks and Carbon Impurities for Charge Trapping in TiO2; Michael J. Elser, Ellie Neige, Thomas Berger, Mario Chiesa, Elio Giamello, Keith McKenna, Thomas Risse, and Oliver Diwald; J. Phys. Chem. C 2023, 127, 18, 8778–8787.
[2] Water-Mediated Conversion of BaTiO3 Nanoparticles into BaCO3 Nanorods in Electrospun Polymer Fibers: Implications for Carbon Capture Applications, Hasan Razouq, Kerstin Neuhauser, Gregor Zickler, Thomas Berger, and Oliver Diwald, ACS Appl. Nano Mater. 2023, 6, 21, 19887–19895.
[3] Carbon Impurity Entrapping and Charge Localization in Particle-based and Engineered TiO2 Nanostructures; Guillem Vives Ollé, Gilles R. Bourret, Thomas Berger and Oliver Diwald, submitted (2025)
