Stop complaining about me being unsophisticated, it's because I'm so charming

Silicon, the chemical element with the second highest abundance in the earth's crust after oxygen, is the essence of the "earth" element in all major civilizations. Silicon builds the architectural foundation of human society and lays the material cornerstone of the information age. With the continuous advancement of the preparation technology of functional mesoporous materials, the application of silicon materials will bring unlimited possibilities to the world in the future.

01Ordinary protagonist element——Silicon

Silicon, with the chemical symbol Si, is the 14th member of the periodic table. It is an inconspicuous but ubiquitous "behind-the-scenes hero". From the universe to the earth, from sand to high-tech chips, silicon is irreplaceable in our daily lives and scientific explorations.

Silicon element and its structure

Silicon ranks eighth in abundance in the universe, accounting for only 0.07% of the total mass of the universe. In the solar system, silicon is one of the important elements that make up the lithosphere of terrestrial planets (such as Earth and Mars). Scientists have also found widespread silicon in meteorite samples, indicating that silicon has been involved in the chemical evolution of the Earth since the early formation of the solar system.

The abundance table of elements in the universe, silicon ranks 8th

In the earth's crust, silicon becomes the second most abundant element after oxygen, accounting for 27.7% of the mass of the earth's crust, and has leapt from a "supporting role" to one of the true "protagonists".

Abundance table of elements in the earth's crust

Most silicon exists in the form of silicates and silicon dioxide. Silicon combines with oxygen to form a rich variety of minerals. Quartz, feldspar, mica, these familiar minerals are all masterpieces of silicon. The deserts and beaches on the earth are the "showcases" of silicon dioxide in nature. These sand grains composed of silicon have become an important tool for shaping the earth's landforms through geological processes such as weathering and transportation.

Silicon commonly found in nature

The chemical nature of silicon was not known in ancient times, but it is inseparable from the progress of human civilization. In the Stone Age, silica (mainly composed of SiO₂) was used as the main material for making stone tools. Silica is hard and sharp, making it an ideal choice for making tools such as knives and axes. Clay and limestone (containing silicates) were used as early building materials, laying the foundation for human settlement. As an important component of "soil", silicon not only supported the rise of ancient civilizations, but also opened up the road for human exploration of natural materials.

Stone knives, stone axes and other tools left by prehistoric humans In ancient Greek philosophy, soil is considered to be one of the four elements (fire, air, water and earth) that make up the material world. In ancient China's "Five Elements Theory", soil is considered to be one of the five basic elements that make up the world. Soil is the mother of all things and supports the cycle of nature. In fact, the "soil" understood by the ancients was mostly rocks and minerals with silicates and silicon dioxide as the main components.

An illustration of the Five Elements Theory in Ancient China

Although ancient people did not understand the chemical nature of silicon, they inadvertently explored the basic properties of silicon compounds through simple labor practices and created early pottery, bricks and tiles and other iconic inventions of human civilization.

02The embodiment of civilization - ceramics

Ceramics is one of the earliest artificial synthetic materials mastered by humans. Its core lies in the ingenious combination of "earth" and "fire". With silicate minerals as the main raw materials, through high-temperature firing, the silicon-oxygen compounds in the minerals undergo chemical reactions, and finally form hard and beautiful ceramic products.

As early as the Neolithic Age, pottery had already appeared in the Yangshao Culture and Longshan Culture in China, which opened up mankind's exploration of silicon-based materials.

Yangshao culture painted pottery basin with geometric patterns Source: Palace Museum

The peak of Chinese ceramics technology appeared in the Tang and Song dynasties, especially the emergence of porcelain, which made China the center of world ceramic culture. Another meaning of the word "China" - porcelain, comes from the excellent quality and technological innovation of Chinese ceramics. Whether it is the elegance of blue and white porcelain, the delicacy of Ru kiln, or the brilliance of Jingdezhen kiln fire, porcelain has become an important symbol of Chinese culture.

Shanghai Science and Technology Museum "Blue and White Porcelain" Special Exhibition Source: iDaily Media

03Lens for understanding the world——glass

If ceramics are the concrete embodiment of Chinese civilization, then glass is the invisible promoter of Western scientific enlightenment. Through glass, humans can not only see the microscopic cellular world, but also observe the vast galaxy of the universe. This transparent silicon-based material has changed the way humans perceive the world.

The invention of glass can be traced back to the Middle East in 2000 BC. Ancient craftsmen discovered during high-temperature firing that sand (mainly composed of SiO₂) mixed with alkaline substances (such as plant ash) would form a transparent solid material. This accidental discovery made glass an important invention of early humans.

Perfume bottle Eastern Mediterranean region circa 500 BC Source: Hunan Provincial Museum

In the 1st century BC, people in the Middle East invented glass blowing technology, which made the shapes and uses of glass more diverse. From windows to wine glasses, from lampshades to jewelry, glass has gradually been integrated into every aspect of human life.

The technology of glass blowing has been used since ancient Roman times.

Image source: Quanzhizhi Although glass was popularized earlier in the Middle East and Europe, its application in China was relatively limited. It was not until the Han Dynasty that glass was introduced to China through the Silk Road as a rare luxury. Imagine that if Chinese craftsmen accidentally added alkaline substances when firing ceramics, it is possible to accidentally make glass at high temperatures. The core of this chemical reaction is that alkaline oxides lower the melting point of silicon oxide, allowing it to form a transparent amorphous structure at a lower temperature.

The transparency and processability of glass have made it play an irreplaceable role in the modern scientific revolution. In the 16th century, Galileo used glass lenses to make a telescope, presenting the details of the universe to human eyes for the first time.

Galileo showing his telescope

Image source: London Science Museum. During the same period, the invention of the microscope enabled humans to explore the microscopic world, from cells to microorganisms, and glass became an important tool for scientific enlightenment.

Microscope image source: London Science Museum

The invention of glass fiber has completely changed the way of communication. Optical fiber, which is mainly composed of silicon dioxide, can transmit optical signals with extremely low loss, becoming the central nervous system of modern information networks. On October 6, 2009, Chinese scientist Charles Kao, who was born in Jinshan, Shanghai, won the Nobel Prize in Physics for his "breakthrough achievements in the transmission of light in optical fibers in the field of optical communications". He was hailed as the "Father of Fiber Optic Communications", which can be said to be a great achievement that has been passed down through the ages!

Kao displays the Nobel Prize gold medal and award certificate

04 From small pieces of sand come a tower: the material world made up of silicon and oxygen elements

Silicon has four electrons in its outer layer, and it can form a stable covalent bond by sharing electrons with oxygen atoms. The oxides formed by silicon and oxygen, especially the silicon-oxygen tetrahedron (SiO₄) structure, can form an extremely stable crystal structure by sharing oxygen atoms.

The structure of silicon tetroxide

These structures are strong and heat-resistant, so they can survive for a long time in the high temperature and high pressure environment of the earth. For example, silica (SiO2) and various silicate minerals (such as feldspar and mica) are typical representatives of silicon oxides, which constitute the main components of the earth's crust. Silicon-oxygen tetrahedrons are not only stable, but also very flexible. They can form chain, layer or three-dimensional network structures by sharing oxygen atoms, thus creating a variety of different substances and forming a rich material world.

Silicate crystal structures based on silicon-oxygen tetrahedrons: (a) chain structure, (b) planar layer structure, and extension outside the plane can form a three-dimensional structure

Image source: Zhao Dongyuan

For example: Silica tetrahedrons form a three-dimensional network structure by sharing oxygen atoms, forming hard silica, which is widely found in sand, quartz and rocks; the pore structure formed by silica tetrahedrons gives silica gel good adsorption capacity, and is often used as a desiccant and catalyst; when silica tetrahedrons and aluminum tetrahedrons are intertwined to form a regular pore structure, such a pore only allows smaller molecules to enter, just like a sieve, and is called a molecular sieve. Zeolite molecular sieves have good ion exchange, adsorption and catalytic properties, and have become an indispensable catalyst in the petrochemical industry.

The important role of molecular sieve catalysts in petrochemicals

05Mesopores—More Than Just a Little Bigger

Fossil energy is a non-renewable primary energy source. Although we continue to develop green and clean energy, the world is still and will be dependent on primary energy for a long time. At the same time, the chemical industry not only provides energy for the world, but is also closely related to our food, clothing, housing and transportation.

Although zeolite molecular sieve is a kind of aluminosilicate crystalline mineral existing in nature, it has always been the frontier field of material science research due to its huge industrial application prospects. In the 1950s, researchers began to synthesize zeolite, mainly by adjusting the silicon-aluminum ratio and adjusting the pore structure to improve catalytic performance.

Milestones in the development of zeolite molecular sieve industrial catalysts in the second half of the 20th century Source: Zhao Dongyuan

The pores of several traditional molecular sieves are all micropores (<1nm), which limits the cracking of heavy oil residues and the catalysis of macromolecular fine chemicals. In fact, many years ago, people have discovered that zeolite molecular sieves can produce some mesopores (2-50nm) during the dealumination process, but their pore sizes are uneven and arranged in disorder. In the past 30 years, dozens of mesoporous silica molecular sieves have been synthesized one after another, and the FDU series developed by the team of Academician Zhao Dongyuan of Fudan University is the leader in this field.

Mesoporous silica: (a) powder sample, (b) microstructure transmission electron microscopy (TEM) image and (c) schematic diagram of catalytic mechanism. Image source: Zhao Dongyuan

One of the biggest advantages of FDU is that it can expand the composition of mesoporous materials from silicon oxide to organic polymers, mesoporous carbon and metal oxides, etc. It is a systematic synthesis methodology that realizes the directional synthesis of functional mesoporous materials and enriches the application fields of mesoporous materials.

Excellent properties of functional mesoporous materials and their application fields

Image source: Zhao Dongyuan

06The Discovery of Silicon: From Lavoisier to Berzelius

Although silicon is widely present in nature, it is hidden under the outer layer of compounds and is difficult for humans to identify and extract. Its discovery process is a history of scientific exploration spanning hundreds of years, and is a microcosm of mankind's gradual understanding of chemical elements and breakthroughs in the mysteries of nature.

At the end of the 18th century, Antoine Lavoisier, the "father of modern chemistry", conducted a systematic study of the basic composition of matter.

Antoine-Laurent de Lavoisier, 1743-1794, France He first proposed the concept of "element" in his book "The Fundamentals of Chemistry" published in 1789, and listed a table of elements known at the time. In this table, Lavoisier speculated that quartz (SiO₂) might contain an unknown basic element, but he did not succeed in extracting this element. He named this hypothetical element "silice", which means "substance in quartz", marking the first time that silicon entered the scientific field of vision.

Lavoisier's table of elements, with silice at the end

This genius chemist highly respected experiments. He once said: "There is no other way except to seek truth through the natural path of experiment and observation." Around 1775, Lavoisier used quantitative chemical experiments to overturn the "phlogiston" theory, expounded the oxidation theory of combustion, summarized the law of conservation of mass, and ushered in the era of quantitative chemistry.

Lavoisier was doing experiments, and his wife was helping him record them.

In the 19th century, British chemist Humphry Davy tried to electrolyze quartz in the hope of extracting its core components. He believed that the elements in quartz might be a metal, so he named the element "silicium" and classified it into the metal family. However, although Davy's experimental concept was correct, his technology and equipment were not enough to successfully extract silicon.

Humphry Davy (1778–1829)

Although no substantial progress was made, Davy laid the foundation for the naming of silicon. Later, the name was further modified to "silicon" and is still used today. In 1811, French chemist Joseph Louis Gay-Lussac first synthesized silicon tetrafluoride (SiF₄) in an experiment, which was the first time that humans prepared silicon compounds. He further tried to reduce silicon tetrafluoride with metallic potassium, but due to technical limitations, he only obtained some extremely impure amorphous silicon. The silicon impurity content was too high to be further analyzed, and it was impossible to confirm whether it was an independent element.

Joseph Louis Gay-Lussac (1778–1850)

The person who actually separated silicon from nature was Swedish chemist Jöns Jacob Berzelius, a prominent figure in the field of chemistry in the 19th century, known as the "father of analytical chemistry." In 1823, Berzelius successfully extracted silicon by improving the reduction technique.

Jöns Jacob Berzelius (1779–1848)

In the experiment, Berzelius found that under heating conditions, potassium metal could capture the fluorine element in silicon tetrafluoride, thereby releasing elemental silicon. This process produced an amorphous silicon, which, although still containing impurities, was basically confirmed to be a new element. Berzelius continued to improve the experimental method, and through multiple refining and purification, he finally produced a higher purity elemental silicon. This silicon has a gray-black, opaque crystal structure, high hardness, and metallic luster. His research not only established the independent element status of silicon, but also laid the foundation for the chemical research of silicon.

Gray-black, opaque crystalline silicon

07High-purity silicon——the cornerstone of the information age

Crystalline silicon has high hardness, high melting point, and a unique tetrahedral structure. Since all valence electrons of silicon participate in the formation of σ bonds, it is non-conductive at room temperature. This property lays a theoretical foundation for the application of silicon as a semiconductor material. With the continuous advancement of chemical technology, scientists have gradually mastered more efficient purification technologies, which have further improved the purity of silicon, gradually regularized the structure, and gradually made the preparation of high-purity silicon possible. At first, silica sand was chemically purified and smelted to remove impurities to obtain polycrystalline silicon. Then, through the "pulling method" or "zone melting method", polycrystalline silicon is further purified and pulled into single crystal silicon rods. The purity of these single crystal silicon usually reaches 99.9999% or even higher, which is enough to meet the strict requirements of the semiconductor industry for materials.

Schematic diagram of the Czochralski process and single crystal silicon rods and wafers. Subsequently, single crystal silicon is cut into wafers less than 1 mm thick. These wafers are the basis for chip manufacturing. After entering the factory, they go through a series of process steps such as lithography, doping, etching and coating, testing and packaging. Silicon wafers are endowed with powerful computing power and transformed into integrated circuits carrying billions of transistors - chips.

Conceptual image of modern silicon-based integrated circuits

Today, a chip with an area of ​​only a few square millimeters can accommodate more than 10 billion transistors, and the precision of these transistors reaches the nanometer level. Small chips support all these high-tech technologies from personal computers to smartphones, from artificial intelligence to cloud computing. The name "Silicon Valley" has directly engraved the technological status of silicon on the global technology map.

08Leading to infinite possibilities

In order to cope with the energy crisis, the world is exploring clean and efficient green energy. In the process of global energy transformation towards green and low-carbon, solar energy is undoubtedly one of the most ideal renewable energy sources. Silicon photovoltaic cells directly convert sunlight into electrical energy through the photoelectric effect, and their core is semiconductor silicon crystals.

Application of semiconductor silicon in photovoltaic cells It is worth mentioning that silicon plays a dual role in the energy field: on the one hand, it continuously optimizes the utilization efficiency of fossil energy as a molecular sieve catalyst; on the other hand, it promotes the green energy revolution through photovoltaic and energy storage technology. This "dual identity" makes silicon a link between traditional energy and future energy. In addition to the energy field, silicon is even closely related to life sciences, especially in supporting bone and skin health. Silicon is involved in the synthesis of collagen, but it is easily degraded. Therefore, as the silicon element degrades, our skin will wrinkle and age.

Silicon degradation causes skin aging

In addition, organosilicon compounds have formed a large-scale chemical industry, realizing the powerful combination of carbon and silicon. The performance of these materials is different from that of carbon plastics and polyethylene. They are heat-resistant and cold-resistant, and their mechanical properties and electrical insulation properties are very stable. In particular, they are suitable for use as medical materials due to their good air permeability and physiological inertness. It can be said that although silicon is already everywhere in the material world, it can still be developed into thousands of uses for the benefit of mankind and the world. With the continuous advancement of technology, perhaps in the near future, soft semiconductors, flexible glass, liquid silicon, nanoceramics, silicon-based batteries, silicon-based macroporous chiral molecular sieves and other materials that are still in scientific imagination will become a reality, leading the upcoming scientific and technological revolution. Even, will the world currently enjoyed exclusively by carbon-based life usher in the advent of silicon-based composite life?

Infinite Possibilities of Silicon Applications

Don't be surprised, don't shrink back, the world of chemistry is full of surprises. Roald Hoffmann, Nobel Prize winner in Chemistry, once said: "Chemical synthesis depends half on design and half on chance. A synthetic chemist is not just a logician and strategist, he is also an explorer who speculates, imagines and creates." The story of silicon is a history of the inheritance of the scientific spirit. It reflects scientists' curiosity and persistence in the unknown, and also witnesses the continuous advancement of experimental technology. The discovery of silicon reveals that our understanding of nature will never end, and it also inspires us to continue to explore more unknown possibilities. Image source: The unmarked images in the article are from the Internet

Written by: Huang Xunjie (Science Communication Center, Shanghai Science and Technology Museum)

Scientific reviewer: Li Wei (Professor of Chemistry Department, Fudan University)

Editor: rain