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Nature's Porous Materials: From Beautiful to Practical

Porous materials are materials that contain voids, channels, holes, or basically pores. This type of material has always attracted a lot of attention as the presence of pores means that the material possesses an internal surface area of interest for all type of applications (see Chapter 2). Nowadays, many porous materials are made in the laboratory and can even be produced on a large industrial scale (see Chapters 3 and 4). However, many porous materials are naturally occurring and were first produced in "Nature's laboratory" without any human influence. In fact, mankind has often based the preparation procedures of synthetically porous materials on processes that occur in nature.

Nature has found a way to produce beautiful and practical porous materials and they can be very diverse: tissue or bones in the human body and animals, rocks, fruit, and so on. A general overview with some examples is presented in Figure 1.1. Besides that, mankind has found its own way to introduce porosity in many materials as some examples clearly demonstrate (Figure 1.2). Ceramics, bricks, and clothing are a few items that were developed very early.

Figure 1.1 Examples of naturally occurring porous materials: lemons, snowflakes, sea sponges, coral reef, egg shells, butterfly wings (European peacock butterfly), soil, and sandstones.

Source: All photographs are public domain.

Figure 1.2 Synthetic porous materials, all made by mankind: Concrete road, paper, fabric of clothes, chalk, ceramics, cake, bread, pottery, bricks, and artificial sponges for cleaning.

Source: All photographs are public domain.

This chapter describes a few carefully selected naturally occurring porous materials. It aims to give the reader a taste of what is available in nature. These materials are also the foundation for development of synthetic porous materials that are more elaborately described in Chapters 3-9 of this book. Silicas and zeolites are also materials that were originally found in nature before a synthetic procedure was discovered to produce them. They will not be covered in this chapter, as they are described in depth in Chapters 3 and 4.

1.1 Living Porosity

1.1.1 Butterflies

Porous materials can be found in animal and human bodies. The bones and lungs of humans are famous examples of ingenious porous structures. In particular, the bones of a human skeleton are very robust, despite their high porosity, as they must support and protect our body and vital organs, respectively. Animals can also create porous structures of very diverse and beautiful shapes. For example, sponges are multicellular organisms that have an entire body containing pores. The wings of butterflies are not only colorful and useful to fly, but they are also porous (Figure 1.3). The cuticle on the scales of these butterflies' wings is composed of nano- and microscale, transparent, chitin-and-air layered structures. Rather than absorb and reflect certain light wavelengths as pigments and dyes do, these multiscale structures cause light that hits the surface of the wing to diffract and interfere. Cross ribs that protrude from the sides of ridges on the wing scale diffract incoming light waves, causing the waves to spread as they travel through spaces between the structures. The diffracted light waves then interfere with each other so that certain color wavelengths cancel out (destructive interference) while others are intensified and reflected (constructive interference). The varying heights of the wing scale ridges appear to affect the interference such that the reflected colors are uniform when viewed from a wide range of angles.

Figure 1.3 (a) Optical image of M. menelaus; (b,c) Scanning Electron Microscope (SEM) image of the nanostructure of the wing under different magnification. (d) Optical image of P. u. telegonus; (e) SEM image of the nanostructure of the blue region; the insert in (e) is the high magnification of SEM image; (f) SEM image of the nanostructure of the fiber region; and, the insert in (f) is the high magnification of SEM image. (g) Optical image of O. c. lydius; (h,i) the SEM image of the nanostructure of the wing according to different magnification.

Source: Reproduced with permission. Taken from Ref. [1], open access: https://creativecommons.org/licenses/by/4.0/.

1.1.2 Algae

Single-celled diatoms can also produce porous structures, however, on a very different scale. Diatoms are microalgae that can be abundantly found in, for example, oceans all around the world. They are part of the phytoplankton family and contribute a staggering 20% of total oxygen produced on our planet every year. They are very unique and useful small creatures and, moreover, they produce a porous cell wall or protective shell called a frustule [2]. The frustule consists of two overlapping structures with identical shapes but slightly different in size. They are called the thecae or valve, and a girdle band or expansion joint holds the two thecae together.

The frustule is entirely made from silica, with a very well-defined structure and unique for every diatom species. It is estimated that approximately 200 000 separate species exist with very different frustules [3]. The dimensions of the frustules can be very different depending on the species. Pore sizes range from 3 nm up to a few hundred nm [4].

A few examples of different species are presented in Figures 1.4-1.6. These figures clearly show the different morphologies, but also diverse types of porosity. These frustules do not only have beautiful porous structures, they can also be used practically.

Figure 1.4 SEM images of purified diatom frustules of Coscinodiscus sp. (a), Melosira sp. (b) and Navicula sp. (c). Scale bar = 5 µm.

Source: Reproduced with permission of John Wiley & Sons, Ltd. Taken from Ref. [4c]

Figure 1.5 SEM images of diatom frustules after 1% HF treatment: (a) and (b) Melosira after 2 and 3 h, respectively; (c) and (d) Navicula after 1 and 2 h, respectively.

Source: Reproduced with permission of Springer Nature. Taken from Ref. [4c].

Figure 1.6 Electron micrographs of the pore structures of different diatom species: (a) Lauderia borealis; (b) Odontella sinensis; (c) Thalassiosira weissflogii; (d) Coscinodiscus granii; (e) Navicula salinarum; (f) Nitzschia sigma; (g) Stauroneis constricta. Scale bar = 5 µm (a) and 0.1 µm (b-g).

Source: Reproduced with permission of the RSC. Taken from Ref. [4b].

These algae can be produced on a large industrial scale as they possess a very fast growth rate and only need a limited amount of space. Moreover, they use carbon for photosynthesis, which also makes them very interesting. It is believed that diatoms for these reasons are a very promising alternative biomass resource to produce biofuels. Additionally, they present a new source of porous silica with very defined pore sizes and distinct morphologies. The silica source can be further used as support for all kinds of applications (Chapter 5).

As an example, here we show how we extracted the silica from algae and used it as a photocatalyst for air purification [5]. Diatom frustules were extracted from a sample containing a cultivation of Thalassiosira pseudonana in its salt water medium. After an initial washing procedure to remove the majority of the salts, an acid treatment was used to remove any remaining carbonates and partially digest the organic matter. After washing away the acid, calcination in air at 550 °C was used to completely free the frustules of organic components. The resulting pure silica sheets are shown in Figure 1.7.

Figure 1.7 Silica extracted as diatom frustules from the algae species Thalassiosira pseudonana.

Source: Reproduced with permission of Elsevier. Taken from Ref. [5].

It can be clearly seen that these silica sheets contain very uniform pores. We then deposited titania nanoparticles onto these frustule sheets. The results are shown in Figure 1.8.

Figure 1.8 TEM images of the optimized titanium functionalized frustules, showing an overview of the nanoparticles (a) and a detail of the nanoparticles contained inside the pores.

Source: Reproduced with permission of Elsevier. Taken from Ref. [5] with permission.

It is remarkable how all the titania nanoparticles are situated in the pores of the silica nanosheets. These materials were shown to be very active photocatalysts for ambient air purification, outperforming the current commercial benchmarks.

1.1.3 Bamboo

Another example of an organic source that has a high silica content are bamboo leaves. An amount of 1 g of bamboo leaves contains 0.03 g of silica. A careful extraction is again key to extract the beautiful and fluffy silica flakes as presented in Figure 1.9.

Figure 1.9 Extraction of silica out of bamboo.

As we zoom in closer on the silica...