Researchers at the University of Hamburg have developed a 3D-printed nanoparticle aerogel process. The process is based on direct ink writing (DIW) 3D printing technology using titanium dioxide (TiO2) nanoparticles aerogels for printing. It can be used to produce solar steam power generation or thermochemical heat storage devices.

The researchers say they are currently using titanium dioxide as the basis for their study, but their technique could be extended to other materials as well, and nanoparticle inks could be designed with specific functions.

△ Modular 3D printing method based on TiO2 nanoparticle aerogel. A) Schematic diagram of the three steps of the 3D printing process: I) inks, either pure TiO 2 or loaded with AuNPs or AuNRs, are formulated by gelation of the respective nanoparticle dispersions. Ii) 3D printing ink by mechanically extruding nanoparticle gels in a liquid bath. Iii) The obtained macromicrostructure solvent gels were processed into aerogels by CO2 supercritical drying. B-d) 3D-printed photographs of aerogel and E-G) corresponding 3D models

Transform direct ink writing 3D printing technology

Aerogel is a porous solid that can maintain the characteristics of a single nanomaterial at a macroscopic scale. Although nanomaterials of various sizes, shapes, compositions, and surface chemistry can be manufactured, mass manufacturing of devices based on macroscopic nanomaterials remains a challenge.

One of the major difficulties in nanomaterial processing is maintaining nanoscale properties at multiple length scales. Gel pouring is a method for processing nanomaterials into aerogels, but the limited range of available mold shapes and the limited tunability of aerogel shapes manufactured in this way hinder the fabrication of complex microstructures.

The researchers decided to base their 3D printing on extruded direct ink writing, a technology developed from ceramic processing where materials and equipment can be made from prefabricated particles. However, ceramics and nanoparticle based aerogels have different properties due to the difference in particle size and porosity.

Therefore, researchers need to prepare additive free 3D-printing inks that are compatible with direct-ink writing processes, similar to traditional casting aerogels, in order to obtain nanomaterial properties in 3D-printed aerogels.

Layered structure of TiO 2 aerogel 3D printed with 250 μm conical nozzle. Optical micrographs of centimeter level 3D printed TiO 2 aerogel in a) side view and b) top view. C) SEM images of multiple independent filaments, and TiO 2 aerogel with microstructures highlighted on the surface of filaments at d) low and e) high magnification consists of a fine nanopore network. F) TEM images of aerogel fragments indicate that each branch of TiO 2 aerogel consists of cross-linked TiO 2 nanoparticles.

3D printing nanoparticle based aerogel

The researchers developed a DIW 3D printing process capable of printing TIO2 aerogel. Instead of using rheological additives to ensure printability, which can be detrimental to the properties of nanomaterials, the printing process takes place in an alkaline liquid bath. This allowed the researchers to maintain the nanoscale properties of the ink while still creating macroscopic translucent aerogel geometry.

Ink is first formed by nanoparticle gels, then 3D printed in a liquid bath, and then post-processed by supercritical drying. The researchers found that inks with a particle concentration of 4% were best for 3D printing. Using Hyrel 3D's Engine HR 3D printer, the 3D printing process takes place in a liquid bath to overcome gel damage caused by evaporation when printing aerogel in air.

In addition, the researchers found that they could easily process multicomponent inks by mixing TIO2 with other nanoparticles, such as gold nanoparticles (AuNP) or gold nanorods (AuNR), prior to the gelation process.

Obstacles in the experiment of aerogel DIW based on TiO 2 nanoparticles. A) Optical micrograph time series of TiO 2 gel filaments to be processed after extrusion in air through a nozzle with an inner diameter of 410 μm. Optical micrograph time series of the microgrid at 0, 12, and 24 h after printing in NH 3-filled heptane at B-D) and F-H), respectively. E) Schematic diagram of the microgrid printed using a nozzle with diameter D = 2 r and wire center distance h. Although (a) all inks are fitted with pH indicators to account for PH-induced TiO 2 condensing gels. The micromesh in (B,f) is printed with a 250 μm nozzle and consists of 23 layers, where each layer is an array of parallel filaments with a center-to-center distance H of 500 μm.

The research team printed geometry with high shape fidelity and precision, including void-free cubes, 3D meshes, boats with large overhangs, and other multimaterial geometry. The 3D-printed aerogel contains a randomly organized interconnected mesoporous network with a pore size in the 20 nm range, and has the typical relative surface area and low density of conventional cast metal oxide aerogel.

The researchers also successfully combined the excellent thermal insulation of metal oxide aerogels with the photothermal properties of plasma gold nanorods. Their DIW 3D printing process not only defines the size of the printed material, but also the composition and photothermal properties at any desired point.

The most important breakthrough, however, was being able to design the microstructure of the photothermal gel they made to provide better light penetration and more uniform heating. According to the team, this could enable a new generation of photothermal devices for solar steam generation or thermochemical heat storage.

To achieve this, the team used two print heads loaded with different nanomaterials. Tio 2 inks absorb UV radiation and appear translucent, but when loaded with gold nanorods, intense extinction occurs in the visible and near infrared (NIR) ranges due to plasma excitation. The free-form capabilities of DIW technology have been used to locally define the photothermal properties of aerogels and improve their heat production and photodistribution properties.

△ Multi-material printing with pure TiO 2 and AuNR/TiO 2 mixed inks. Aunr-loaded inks appear a) reddish in the form of hydrogel and b) green in the form of aerogel. C) UV/Vis absorption spectra indicate that the color change is caused by the blue shift of the longitudinal plasmon formants while changing the dielectric environment from liquid to air by supercritical drying. D,e) Thermal infrared camera images of structured aerogel illuminated with 300 W Xe light source turned off and on, respectively. F,g) Schematic diagram of photothermal heating of TiO 2 and AuNR/TiO 2 aerogel under visible light irradiation. The pure titanium dioxide aerogel was kept low temperature, while the AunR-loaded aerogel was heated up due to strong light absorption by plasma AuNR.

In addition to TiO2, this method can also be applied to SiO2, Al2O3 or ZrO2 nanoparticle based aerogels commonly used in photothermal devices. Photothermal heating of plasmonic nanoparticles has been used in the past in prototype devices for clean water regeneration, energy generation, and photothermal catalysis, but has until now been limited to thin films due to the inability to construct nanoscale properties on a 3D macroscopic scale.

By achieving more uniform heat generation in macroscopic objects, the researchers believe their DIW 3D printing technology provides an entirely new way to fabricate large 3D structured photothermal devices.

For more information about the study, Please refer to the Additive Free, published in Advanced Functional Materials Gelled nanoinks as a 3D printing Toolbox for hierarchically structured bulk aerogels, "in the paper. The article was co-authored by M. Rebber, M. Trommler, I. Lokteva, S. Ehteram, A. Schropp, S. Konig, M. Froba, and D. Koziej.

Effect of △ microstructure on temperature and light distribution of 3D-printed AuNR/TiO 2 aerogel. A) Schematic diagram of typical photothermal measurements. The sample is illuminated from one side while an infrared camera records the temperature of the front or one of the sides. Exemplary temperature data for volume geometry are shown as superposition of cubic unstructured samples. B) Temperature time trajectory of body surface temperature under repeated illumination. The inset shows the infrared camera images at t 1, t 2 and t 3 as shown in the time trajectory diagram. C) Schematic representation of light penetrating three types of microstructure (unstructured block, alignment and shift). 3D models from left to right show cross sections and expected light absorption for each geometry. Adjacent layers of the geometry are colored for clarity. D) Thickness dependence of the temperature profiles of the three microstructures measured. An exponential function is used to fit the block temperature distribution to extract the block extinction coefficient σ block. The temperature profiles of the aligned and shifted geometry follow an exponential trend but have either 1/2 or 1/4 of the volume decay constant σ block, respectively.

Source: Antarctic Bear 3D Printing network