Inverted Photonic Glasses Made of Carbon Nitride as Functional Gradient Materials for Photocatalysis

01/06/2025 to 31/05/2028

Photocatalysis is one of the most important functionalities of semiconductor materials, especially when converting readily available starting compounds such as water or carbon dioxide into more valuable products, which in turn can be used in the chemical value chain. Carbon nitride is considered one of the most promising semiconductors, as it consists of readily available elements and can be produced from simple and inexpensive precursors. To use the sun as an inexhaustible source of energy for photocatalysis, it is advantageous to work with semiconductors that have different electronic band gaps. Depending on the composition, carbon nitride not only offers this option, but the position of the conduction band of graphitic carbon nitride enables the photoreduction of CO2. Another way to increase the yield of photons for photocatalysis is by “slowing down photons”. This refers to two effects: 1. the maximization of scattering in a material and 2. the reduction of the group velocity of the wave packets in certain zones of the photonic band structure.
The project aims to synthesize novel carbon nitride materials that can utilize such “slowed photon” effects for photocatalysis. We plan to use an ultracentrifugation-based process to produce photonic glasses with an inverted structure for the first time. Pores of uniform size in a carbon nitride matrix have a short-range order but no long-range order. If the energy of the photonic pseudogap matches the electronic bandgap, an improvement in photocatalytic efficiency is expected, which is being investigated as part of the project. The synthesis method we have developed enables us to produce gradient materials in which the electronic and photonic band gaps vary along a spatial coordinate. The change in the electronic band gap is controlled by a change in the composition of the carbon nitride (doping), whereby we will precisely characterize the optoelectronic properties and the dynamics of the photogenerated charge carriers using various methods, including transient absorption spectroscopy. We envision CO2 can be photoreduced several times in the new gradient metamaterials, as they have a corresponding multi-junction architecture.
The project is a collaborative effort between the materials chemistry group led by Dr. Polarz and the group led by Dr. Lauth, which specializes in the physicochemical characterization of optoelectronic properties of semiconductor nanostructures.

In comparison to photonic crystals, there are no phase boundaries in a glass, which makes the latter an essential characteristic of the materials. Furthermore, the isotropic character of the photonic glasses is expected to allow angle-independent light absorption due to the multiple scattering and slow photon effects.