UHasselt, imo-imomec, Energyville, Materials Chemistry, DESINe group, Diepenbeek, Belgium

Solution-based synthesis methods have been investigated at the DESINe group for the past 25 years.   Over the years, many different materials such as oxides, metals, phosphates, sulfides, thiophosphates and composites as well as hybrids have been investigated.  These materials have interesting functional properties and can serve many different applications for example in microelectronics, solar cells, touch screens, (micro)batteries, catalysts for various reactions and photoelectrodes, the latter being the most recent application DESINe has focused on.

The liquid solution phase is the common point in all these syntheses, but a wide variety of “method families” can be distinguished and within each family a large diversity of routes exists.  Though reaction equations are often presented in a simple manner, the true mechanisms of inorganic syntheses are much less understood than e.g. those of organic synthesis. This imposes challenges to predict which synthesis route, and which exact conditions among a large number of parameters, will successfully lead to the desired material.  DESINe has developed the “aqueous solution-gel route” over the past decades.   As a result of the fundamental research that was done, by now, almost all relevant metal ions of the periodic system can be solubilized into precursors. These precursors are now a box of building blocks, with which almost any metal oxide can be formed.   However, challenges are still great, and will be touched upon in the lecture

• Temperature limitations impose limitations to the substrates that can be used, where glass in some cases needs replacement by quartz
• Large amounts of CO₂ are emitted typically in chemical solution deposition routes, imposing a risk of negative LCA, whereas the use of bio-based citric acid as the coordinating ligand can provide counterweight
• Careful analysis of the phase (or phase mixture) which is obtained is required, since secondary phase formation can occur depending on the thermodynamics governing the specific materials besides the reaction kinetics
• Sufficient attention must be given to ensure C content removal from the precursor   
• Morphology control is possible from highly porous to highly dense thin films ranging from few nanometers to some hundreds of nanometers.

Hydrothermal precipitation is an interesting alternative to provide even stronger morphological variations, and is particularly useful when simple or binary materials are envisaged.  It also is useful for a wider range of inorganics, beyond oxides and phosphates. Besides, its low reaction temperature is promising both for ecological and economic reasons. However, it has its own challenges as well, for example the growth of films is much less common than the formation of powders.

These points will be illustrated by case studies of CuBi₂O₄ and CuFeO₂, as photocathodes, as well as BiWO₆, as a photoanode, for H₂ production and CO₂ conversion respectively.  The first results obtained in the young collaboration between prof. Shin’s lab and DESINe will be touched upon as well, as an outlook towards further common work.

The authors acknowledge support by the Flemish Research Foundation (FWO, G053519N, mobility grant), by SYNCAT, a Flemish cSBO Catalisti MOT3 Moonshot, Vlaamse Veerkracht green H2 and UHasselt cleanH2 BOF21GP04 projects.