Patterns of assembled colloidal particles can form on substrates during solvent evaporation and here such  patterns were studied for carbon-rich colloids. Carbon-rich colloids are of fundamental and technological importance, and we prepared monodispersed colloidal hydrochar dispersions by hydrothermal carbonization of glucose. The dispersions were purified by dialysis, and under certain circumstances line patterns formed on the substrates on the evaporation of water and under other circumstances tile-like patterns formed instead. The formation of lines was studied as a function of the addition of sodium dodecylsulfate (SDS), the pH of the dispersion, the geometry of the glass container, and the colloidal particle concentration. The lines consisted of dense assemblies of hydrochar particles. Typically, the lines formed in parallel to the drying front; but on up-concentration, they also formed orthogonally as an effect of likely fingering instabilities. The line width increased with the successive evaporation of water. The horizontal cracks in the tile-like patterns that formed for example without the addition of SDS also formed in parallel to the drying from, and the thickness of the “tiles” increased on successive evaporation of water. The sharper lines that formed on the addition of SDS was tentatively ascribed to the effect of solubilization or moderated interactions. At higher concentrations we obsereved a continuous layer of colloidal particles at the interface also between the lines. The mechanism for the line pattern fromation was derived from the literature on other colloids and for the formation of the tile-like patterns a mechanism was proposed. With this study, we extended the literature on line formation on substrates to a new and important colloidal system – the colloidal hydrochars. The study could also have technological implications to in-situ line printing within microfluidic devices.

Self-aggregated colloids can be used for the preparation of materials, and we studied long rod-like aggregates formed on the evaporation of water from dispersed particles of colloidal hydrochar. The monodispersed hydrochar particles (100–200 nm) were synthesized by the hydrothermal carbonization ofglucose and purified through dialysis. During the synthesis they formed colloidal dispersions which wereelectrostatically stable at intermediate to high pH and at low ion strengths. On the evaporation of water,macroscopically large rods formed from the dispersions at intermediate pH conditions. The rods formedat the solid-water interface orthogonally oriented with respect to the drying direction. Pyrolysis renderedthe rods highly porous without qualitatively affecting their shape. A Cu-Si alloy was reactively infiltratedinto the in-situ pyrolyzed hydrochars and composites of tricopper silicide (Cu3Si)-silicon carbide(SiC)/carbon formed. During this process, the Si atoms reacted with the C atoms, which in turned causedthe alloy to wet and further react with the carbon. The shape of the underlying carbon template wasmaintained during the reactions, and the formed composite preparation was subsequently calcined intoa Cu3Si-SiC-based replica of the rod-like assemblies of carbon-based colloidal particles. Transmission andscanning electron microscopy, and X-ray diffraction were used to study the shape, composition, andstructure of the formed solids. Further studies of materials prepared with reactive infiltration of alloysinto self-aggregated and carbon-based solids can be justified from a perspective of colloidal science, aswell as the explorative use of hydrochar prepared from real biomass, exploration of the compositionalspace in relation to the reactive infiltration, and applications of the materials in catalysis. 

Small particles of chemically activated carbons could be of relevance for several applications as they can be dispersed and used or processed further into structured materials. We hypothesized that it would be beneficial to freeze-dry the dispersions of hydrochar particles prepared from hydrothermal carbonization of glucose before chemical activation when it comes to the redispersibility, which was confirmed. Such superactivated carbons prepared by chemical activation in KHCO3 could be redispersed if the hydrochar had been dried properly before the activation. The dispersed particles were aggregates with diameters of about 600 nm, which was about 5-times larger  than the underlying particles. On drying of the water dispersions of the hydrochar particles, it was observed that long bent macroscopic rods formed as a yarn. The bent rods consisted of compact assemblies of hydrochar particles formed by a directed assembly at the glass-water (air) interface. These rods could keep their macroscopic shape on chemical activation if they were pyrolyzed in N2 before the chemical activation. It was though noted that the highest porosity was observed for the preparation that had not been pretreated by pyrolysis in N2. The specific surface areas for the activated carbons were up to about 1800 m2/g and CO2 uptake of 7.1 mmol/g at 0 C and 1 atm. Even if the freeze-dried activated carbons displayed a sufficient redispersibility, we expect that it can be further enhanced. 

Activated carbon nanofibers were prepared with a high uptake of CO2 of 9.36 mmol/g at 101 kPa and 0 °C. Electrospun nanofibers of polyvinylpyrrolidone and colloidal hydrochar were oxidized and chemically activated with KHCO3 or K2CO3. The activated carbon nanofibers may be relevant for CO2 capture or capture of other small molecules.