Metal-Free Carbon Nitride (CN)-Based Materials
Photoelectrochemical cells (PECs), which directly convert sunlight into chemical fuels, are considered one of the most promising and economically feasible future technologies for producing alternative energy sources. At the heart of a PEC lies a light-harvesting semiconductor, which, under solar illumination, transforms the absorbed photons into excited electronic states (holes and electrons). Depending on the pH conditions, the excited charged states can then be used for electrochemical production of fuels such as hydrogen, by reducing protons from water to H2(g). PECs that are based on a semiconductor/liquid electrolyte junction have the potential to commercially compete with traditional fossil fuel-based production methods because they are relatively simple and convert solar energy to fuels. Production of solar fuel utilizing PECs requires low-cost, robust, and highly efficient semiconductors, which should have good conductivity and be able to transfer charge rapidly at the semiconductor/liquid electrolyte interface, display long-term stability, possess good light-harvesting properties, and have a suitable energy band position for the desired reaction. Up to now, the PEC field has been dominated mostly by metal-based materials (oxides, sulfides, (oxy)nitrides, and organo(metallic) dyes) and despite the significant progress in this field, semiconductors that fulfill all the aforementioned requirements as PEC semiconductors do not exist today, and novel materials are still much sought after. Thus, the development of suitable semiconductor materials is an immense challenge for allowing PECs to fulfill their role in the energy-devices landscape.
“Metal-free”, light element materials based only on carbon, nitrogen, phosphorus, sulfur, and boron – referred to here as CNs and CNXs (X = P, B, or S) – exhibit electronic, chemical, and photophysical properties that range from insulators through semiconductors to semi-metals. As such, they have the potential to dramatically accelerate the development of numerous fields, such as electronic devices, energy-related applications, lubricants, and fire-retardant materials. The electronic, photophysical, and catalytic properties of these CNX materials largely depend on the ratio between the different elements and on their spatial organization. Simple carbon nitride (CN) materials have been explored as promising photo- and electrocatalysts for various reactions, such as water splitting and carbon dioxide reduction, with potential use in fuel cells, pollutant degradation, and other applications. The insertion of heteroatoms into their structure results in a remarkable combination of low price, tunable electronic and catalytic properties, environmentally friendly nature, and stability under harsh conditions, which positions CNXs as ideal materials for photoelectrochemical applications. However, to date, progress on the utilization of CN and CNX materials for PECs has been limited by the fact that no method is currently available for the preparation of high-quality, large-scale, and homogenous CN layers on a conductive substrate with simultaneous control over the chemical, electronic, and catalytic properties and our lack of basic knowledge of the layer’s photophysical and photoelectrochemical properties. Moreover, the understanding of the physical and catalytic properties of these materials through structure-activity relationships remains poor.
In my group, we develop new strategies that allow the controlled synthesis and growth of metal-free materials on various substrates for photoelectrochemical cells, ranging from carbon nitride to nitrogen-doped carbon and new carbon-nitrogen-phosphorus/boron/sulfur materials. We aim to gain a fundamental understanding of their growth mechanisms and the resulting structural, chemical, photophysical, and photoelectrochemical properties. Our approach allows us to overcome the current limitations of the traditional synthetic and growth methods for CNXs layers with controlled properties by designing and encoding the elemental composition of the final material at the molecular level. Our new synthetic path, based on the rational selection of the reaction monomers, taking into account their intermolecular interactions, prior their calcination at high temperature allows us to target specific required properties for PEC: suitable optical band gap, crystal structure, porosity, layer thickness, and catalytic activity, as well as the design of a beneficial electronic structure for efficient charge separation and collection. This research is highly interdisciplinary, as it combines materials science, photoelectrochemistry, and supramolecular chemistry. It will open up new avenues in these fields, in particular in the synthesis and deposition of metal-free materials, which hopefully will result in the integration of lightweight materials into energy–conversion and other devices.