(Invited) Atomic Layer Deposition of MOF from Dissolved Precursors ― ‘Solution ALD’ (sALD)

Abstract

Metal organic frameworks (MOFs) are crystalline three-dimensional porous solids featuring a high surface area, which have attracted considerable attention in a wide range of applications e.g. in catalysis, hydrogen storage, gas separation, thermoelectric applications or as chemical sensors. Beyond solvothermal synthesis of bulk MOF crystals, growing MOFs thin films has been limited so far to rather conventional solution-based techniques. However, traditional growth methods from solution such as spin-coating, spray-coating or dip-coating generate MOFs thin films with a rather low film quality and an alarming lack of a precise film thickness control under 1 nm. This renders them unsuitable for most industrial applications and severely limits their future application potential.Atomic layer deposition (ALD) provides this capability, but is limited by the necessity to use gaseous precursors in vacuum conditions at temperatures typically above 100°C. This is incompatible with the typically high temperature sensitivity of many hybrid metal-organic compounds such as the MOFs.In this perspective, we have transferred the principles of ALD to precursors dissolved in liquid solvents. The ‘solution ALD’ (sALD) method can be implemented in a variety of microfluidic or slot-die processing devices. We have demonstrated that the principles of classical, gas-based ALD —self-limiting surface reactions— are reproduced in sALD. sALD shares the fundamental properties of standard ‘gas ALD’ (gALD), in particular the self-limiting surface chemistry and the ability to coat deep pores in a conformal manner Furthermore, sALD allows the experimentalist to design new, advantageous, reactions of known semiconductors (oxides, heavier chalcogenides), or to deposit materials otherwise inaccessible to ALD altogether, such as polymers, ionic solids, and metal-organic frameworks. They may be obtained in highly pure, crystalline form at room temperature, either as planar films or as conformal coatings of porous substrates. Based on these advances, we have been able to make the first sALD-derived films of a MOF, Cu-BDC. At any chosen thickness, sALD MOF films feature significantly lower roughness and better adhesion than those generated by the best spray-coating and dip-coating methods available currently.