Functionalization is the process of adding new functions, features, capabilities, or properties to a material by changing the surface chemistry of the material. It is a fundamental technique used throughout chemistry, materials science, biological engineering, textile engineering, and nanotechnology. Functionalization is performed by attaching molecules or nanoparticles to the surface of a material, sometimes with a chemical bond but sometimes just through adsorption (that is, the thing you’re trying to attach sticks to the surface without forming a covalent or ionic bond).
To paraphrase aerogel scientist Dr. Debra Rolison, here’s a way to think about functionalization:
If making an aerogel is like building a house, then functionalization is like painting, decorating, and putting your stuff inside the house so that you can actually live, work, and have fun in it!
- Make a water-absorbing material waterproof
- Change the color of a material
- Render a surface antibiotic
- Make chemical sensors (“artificial noses”)
- Make non-magnetic materials magnetic
- Make sophisticated batteries
- …and more!
Flat surfaces such as silicon wafers or glass are commonly functionalized to make useful and interesting materials and devices. But aerogels, with ultrahigh surface areas wrapped up inside their porous skeletal frameworks, can also be functionalized to make materials with even more amazing properties than ordinary aerogels!
Aerogels are Three-Dimensional Surfaces
While aerogels are generally three-dimensional materials, they contain a tremendous amount of surface area. The surface of each and every nanosized strut that makes up the open-porous skeleton of the aerogel can potentially serve as a surface for molecules or particles to be attached to. And there’s a lot of surface area wrapped inside an aerogel–a typical piece of silica aerogel, say with a density of 0.1 g/cm3, easily has a surface area of 750 m2/g, which means a ice-cube sized piece having 2.2-cm sides (a little less than a cubic inch) has 750 m2, about 10% of a soccer (football) field.
Here’s an analogy to understand how aerogels can contain so much surface area in such a small volume. Imagine taking a piece of thick construction paper and crumpling up into a ball (and imagine further it doesn’t uncrumple itself when you let go). Now imagine taking a piece of newspaper and crumpling it into a ball. Certainly, you would be able to crumple the newspaper up into a smaller ball than the construction paper since it’s thinner, which means you would need to crumple up a much larger piece of newspaper to get the same size ball as your crumpled construction paper. Now imagine your paper was only a few hundred atoms thin. It would take a very large sheet of this hypothetical, nanoscopically thin paper to get the same size ball as your original ball of construction paper. Similarly, aerogels, made up of nanoscopically thin struts link together in a three-dimensional network, pack a lot of surface area into a small volume! The zillions of nooks and crannies carved out by these tiny struts add up to a lot of surface area in a small volume.
Why Aerogels Are Great Things to Functionalize
Usually when you functionalize something, the molecules you’re functionalizing with do something you care about. They soak stuff up you want to soak up, the light up when something you want to know about touches them, they prevent things from sticking you don’t want to stick, etc. And generally, the more of those molecules you can cram into a small space, the more powerful, efficient, and effective your material will be at doing what you want it to do.
Aerogels offer lots of advantages in this regard:
- Aerogels pack tremendous surface into a small volume (that is, they have a high surface-to-volume ratio)
- Aerogel backbones can be made of many different substances, providing tremendous chemical flexibility
- Aerogels can frequently made through ambient-temperature bench-top processes
- The open pore network of aerogels enables movement (mass transport) of stuff through the aerogel, providing avenues for getting things you might want to detect or adsorb to the functional groups that will sense or adsorb them
Waterproof Aerogels: An Example of Functionalization
One classic example of functionalization of aerogels is the making of hydrophobic (waterproof) silica aerogels. Silica aerogels like the kind described in the base-catalyzed recipe using TMOS under the Make section are normally hydrophilic, that is, they readily absorb moisture from the air (which causes them to shrink and become cloudy over time) and wick up liquid water on contact (which causes them to shrivel up and densify). This is because of surface hydroxyl (-OH) groups that cover the struts of the aerogel’s skeleton–sticky, polar groups to which water can readily stick by hydrogen bonding. To make a waterproof silica aerogel, we can replace the hydrogen on these sticky -OH groups with a much less sticky, non-polar group called trimethylsilyl (-Si(CH3)3). Transforming just 30% of these -OH groups into -OSi(CH3)3 groups is enough to make the struts of the aerogel repel water, allowing the aerogel to float in water indefinitely without wicking water into its pore network (which would cause the aerogel to shrivel up).
You can read more about how silica aerogels are made hydrophobic through functionalization under Learn>Flavors of Aerogel>Silica Aerogel and even how to make hydrophobic aerogels yourself under Make>Aerogel Recipes>Silica Aerogel>Hydrophobic and Subcritically-Dried Silica Aerogel.
How to Functionalize an Aerogel
There are three points in the process of making an aerogel where you can functionalize its surface: during gelation of the precursor gel, after gelation of the precursor gel, and after supercritical drying.
Functionalizing During Gelation
Special reactive monomers are included in the sol-gel process used to produce the gel precursor, resulting in a gel network with special chemical groups poking out of its struts. Examples include using methyltrimethoxysilane along with TMOS to put hydrophobic methyl (-CH3) groups onto a silica gel’s backbone so that the resulting aerogel will be hydrophobic.
- Advantage: Simplifies processing, reduces number of steps
- Disadvantage: Can be very tricky to develop a gel recipe that incorporates special reactive molecules as they can change pH, gel time, and chemical pathways needed to form the gel, and themselves may react with chemicals and solvents needed to form the gel.
Functionalizing After Gelation
Once a gel has formed, special reactive molecules can be introduced into the gel’s pore network by diffusion. Once the reactive molecules find their way into the pore network, a chemical reaction between them and the gel backbone can be initiated. The result is the bonding of the reactive molecule to the gel backbone, covering the backbone with new chemical groups! Examples include the reaction of trimethylchlorosilane with -OH groups to form -OSi(CH3)3 groups on the backbone to render the resulting aerogel hydrophobic.
- Advantage: Ensures proper functionalization of the gel backbone, allows greatest flexibility and control over how functionalization happens
- Disadvantages: Involves diffusion-limited infilitration which is slow and ill-suited for large, low-aspect-ratio monoliths such as cubes and spheres. May require exchange of pore fluid for a solvent that will not react with functionalization agent.
Functionalizing After Supercritical Drying
Chemically similar to functionalization of a wet gel after gelation as described for above, except for that the reactive molecules are introduced as a vapor into the pore network of an aerogel (not a wet gel). The vaporous reactive molecule diffuses its way through the dry pore network and finds its way to a surface group on a strut in the pore network, where a chemical reaction can be initiated result in the formation of special functional groups on the surface of the strut.
- Advantage: Eliminates tedious, slow solvent exchanges needed to functionalize after gelation, does not interfere with gel chemistry
- Disadvantages: Diffusion-limited process ill-suited for thick low-aspect-ratio monoliths, can be tricky to get reactive molecules into the aerogel pore network without invoking capillary collapse (shrinking/shrivelling), may be difficult to get reactive molecule for functionalization into vaporous form, often times more hazardous than liquid-based processing
Cool Stuff You Can Do With Functionalization
We mentioned waterproof silica aerogels as an example of functionalization already, but there’s lots of other things you can do through functionalization, like:
- Make brittle silica and metal oxide aerogels superstrong and tough by functionalizing the surface with polymers
- Make precursors for metallic aerogels by functionalizing metal oxide aerogels with carbon-rich polymers
- Make carbon aerogels into battery-like materials by functionalizing with electrochemical coatings such as manganese dioxide
- Make silica aerogels into oxygen sensors by functionalizing with oxygen-sensitive fluorophores
- Make carbon aerogels into efficient catalysts by functionalizing with precious metals
- Make silica or carbon aerogels into hydrogen storage materials by functionalizing with metals that form hydrides
- Make aerogels that absorb 20x their weight in oil by functionalizing with oleophilic groups
- Make aerogels that filter biomolecules like DNA by functionalizing with the complementary DNA sequence
And lots of other things too!
Hopefully you now have a perspective on why functionalization of aerogels is a powerful way to make new materials and one of the primary reasons why aerogels have such great technological potential. The fun starts when you think about aerogels as palettes for creating functional, active materials, not only as material destinations themselves.