In plain words
Making crystals in a sealed steel pot, an autoclave, in which water – or another solvent – is heated well above its normal boiling point. The pressure keeps it liquid, and hot, pressurised water dissolves and moves substances that barely dissolve at all under normal conditions, so crystals can grow from it at a few hundred degrees instead of the far higher temperatures a furnace would need. Nature grows quartz and many minerals this way; in the laboratory it gives of BiOCl, LDHs and tellurene, and the framework materials known as COFs and MOFs.
Going deeper
Water above its boiling point
In a closed vessel, water heated beyond 100 °C cannot boil away; its pressure rises instead, to about 15 bar at 200 °C. Under these conditions water dissolves silicates, oxides and sulfides that are practically insoluble at room temperature, and carries them through the solution far faster. Seeds and a small temperature difference between two parts of the vessel then let crystals grow – the way most quartz for watches and electronics is made, in large autoclaves at over 300 °C.
For the vessels are small Teflon-lined steel autoclaves at 100 to 250 °C, filled with metal salts, a reducing or oxidising agent and often a surfactant or polymer. When the solvent is not water – ethylene glycol, an alcohol, a mixture of organic solvents – the method is called solvothermal; the principle is the same.
What it makes
Hydrothermal routes give bismuth oxyhalides such as BiOCl and BiOBr as thin plates, layered double hydroxides with better-ordered layers than simple precipitation, SnS2 nanosheets, and flakes of two-dimensional tellurium, grown at about 180 °C from a tellurium salt with a polymer that makes them flatten into sheets.
Solvothermal synthesis is the standard route to crystalline framework materials: organic frameworks, first made this way in 2005, and many layered metal–organic frameworks. There the bonds form reversibly in solution, so wrongly placed building blocks come off and re-attach until they fit, which is what makes ordered crystals possible at all.
Strengths and limits
The method is cheap, scales easily, runs at low temperature and can be steered: the solvent, the pH, the temperature and additives that cling to particular crystal faces decide whether the product grows as cubes, rods or sheets. Its products, though, are usually small crystals or powders with a spread of sizes and thicknesses, and their surfaces carry the surfactants, polymers and solvent they grew in.
Those residues change contacts, and optical properties, so careful washing and a check of what remains – by X-ray photoelectron or infrared spectroscopy, for example – belong in any report. For electronic devices, flakes grown in solution rarely match the quality of crystals grown from the vapour or exfoliated from bulk.
For specialists
Crystallisation from solution in a sealed vessel above the solvent’s boiling point at autogenous pressure – water for hydrothermal, organic solvents such as ethylene glycol, DMF or mesitylene–dioxane for solvothermal synthesis – usually at 100 to 250 °C in Teflon-lined steel autoclaves, and at higher temperatures and pressures for minerals and quartz. Raised solubility and transport allow crystalline products at low temperature, and pH, solvent, surfactants and capping agents steer the shape towards thin plates. It yields BiOX and LDH nanosheets, SnS2 and tellurene flakes and crystalline COFs and MOFs, whose reversible bond formation in solution corrects errors as they grow. The products are typically small, polydisperse crystals or powders whose surfaces carry ligands and solvent.