Diffusion-assisted synthesis of crystalline materials in rigid gels

Abstract

This study explores diffusion-assisted synthesis in rigid gel matrices, utilizing reaction–diffusion processes to fabricate crystalline materials with controlled size and morphology. The presented techniques focus on synthesizing various classes of materials, such as inorganic precipitates, metal–organic frameworks, and gold nanoparticles, using gel column and flow-through gel reactors, as well as reactive wet stamping. In these setups, the reagents are initially spatially separated, and one of the reagents diffuses into gels containing the other reagent, producing crystals with sizes that increase linearly from the gel interface. The gel matrix prevents sedimentation and aggregation, allowing the undisturbed growth of larger crystals. Additionally, the experimental setup provides a spatiotemporal control over the mass flux of the reagents, thus controlling the rates of nucleation and crystal growth. Theoretical models can explain the linear dependence of the crystal size and attribute larger crystal sizes to regions of lower supersaturation, which favor growth over nucleation. We also discuss advanced methods, including orthogonal diffusion and electric field-assisted synthesis, that can enhance spatial control and crystal morphology. Compared to traditional bulk wet synthesis, diffusion-assisted methods offer exceptional control over crystal size, shape, and dispersity. Prospects include scaling up macroscopic crystal synthesis, refining reactor designs for 2D and 3D configurations, and exploring applications in catalysis, biomedicine, and environmental remediation.