Using light as a medium to promote a green future

  Apply photocatalytic materials on the wall, and the indoor organic pollutants can be decomposed under the light; add photocatalytic materials to the sewage, and the pollutants in the water will also be decomposed into non-toxic and harmless substances; spray on the curtain wall glass The photocatalytic materials on the surface can play a self-cleaning role, making urban buildings known as “cement forests” into real green buildings… The
  magic of photocatalytic materials does not stop there. It can also convert solar energy into chemical energy by absorbing sunlight and catalyzing redox reactions.
  The role of light
  In 1972, the British “Nature” magazine published a paper in which it proposed a method of using titanium dioxide electrodes to photolyze water to produce hydrogen and oxygen. It can be said that it is this paper that created a new research field of photocatalysis, making photocatalytic materials represented by titanium dioxide become the “darling” of the chemical community.
  Why can titanium dioxide catalyze the photolysis reaction of water? This starts with its structure.
  Titanium dioxide is a semiconductor whose energy level structure consists of a lower-energy valence band and a higher-energy conduction band. The energy difference between the valence band and the conduction band is called the bandgap energy. As for what is the valence band and what is the conduction band, we can use an analogy to help understand. The valence band is like the downstream of the river, the conduction band is like the upstream of the river, and the electrons are like the boats in the river. When there is no external energy, due to the effect of water flow, the boats are all gathered downstream, that is, when the semiconductor material is in the ground state, the electrons are all distributed on the valence band. When the boat is started with enough energy, it can also travel upstream of the river when it goes upstream. Similarly, when a semiconductor material is excited with enough energy, electrons can transition from the valence band to the conduction band, and this part of the energy required is the band gap energy.
  If light is irradiated on the titanium dioxide material, and the energy of the light is greater than or equal to the band gap energy, then a part of the electrons in the valence band will be excited, jump to the conduction band, and flow freely on the conduction band; and the electrons “jump” After reaching the conduction band, there are vacancies left on the valence band. If this process is described in a professional way, the photocatalytic material is excited by light to generate photogenerated electrons and holes.
  Next, the electrons and holes travel to different locations on the surface of the catalyst. The holes are eager to get electrons (that is, have strong oxidizing power), so they can oxidize water molecules to oxygen, and the electrons participate in the hydrogen evolution reaction. This is why titanium dioxide is able to split water under light conditions.
  In fact, the application of photocatalytic materials such as titanium dioxide is mainly concentrated in the fields of energy and environment, and both use their properties that can be excited by light to generate electrons and holes. The former refers to the conversion of solar energy into chemical energy by initiating a series of redox reactions through photocatalysts, such as the aforementioned splitting of water to produce hydrogen, as well as the catalytic reduction of carbon dioxide, etc.; the basic principle of the latter is to allow photocatalytic materials to produce Active oxygen species with strong oxidizing ability can oxidize and decompose pollutants in water or air.
  Promising photocatalytic materials
  The paper in the journal “Nature” allows people to see the unique potential of photocatalytic materials in the field of hydrolysis hydrogen production, and this result is very encouraging. Because if this technology can achieve large-scale industrial application, it means that solar energy can be converted into chemical energy, and the problem of energy scarcity will be solved. But unfortunately, the photolysis reaction of water is very difficult, and people have not achieved further breakthroughs for a while. Soon after, at the end of the 1970s, it was discovered that under the action of photocatalysts, pollutants in water bodies could be decomposed non-selectively, so some researches on the application of photocatalytic technology in the field of environment were carried out.
  The reason why photocatalytic materials can treat sewage also benefits from the electrons and holes generated after it is excited by light. After the holes migrate to the catalyst surface, the water molecules adsorbed on the catalyst surface will be oxidized to hydroxyl radicals. Hydroxyl radicals have a very strong oxidizing ability, and can oxidize most organic pollutant molecules and some inorganic pollutant molecules into non-toxic and harmless substances such as carbon dioxide and water.
  According to a similar mechanism, photocatalytic materials can also decompose pollutants in the air and play a role in purifying the air. For example, some air purifiers use photocatalytic technology, and the “mystery” lies in the photocatalytic material on the filter layer; coating the photocatalytic material on the soundproof wall of the highway can decompose automobile exhaust. Photocatalytic materials can also be used for sterilization and disinfection. After it is applied on the surface of utensils or other objects, it can not only kill bacteria, but also decompose bacteria. Compared with nano-silver, which is mainly used in sterilizing liquids, this is a unique advantage of photocatalytic materials. During the COVID-19 epidemic, sterilizing solutions based on photocatalytic materials also emerged as the times require, and have achieved certain applications.
  Make up for the shortcomings and break through the bottleneck
  Some people may ask: Since photocatalytic materials are so powerful, they can not only purify the air, treat sewage, but also convert solar energy into chemical energy, why does its scope of application not seem to be everywhere? This is because photocatalytic materials still have limitations that cannot be ignored in practical applications.
  The biggest limitation is that its bandgap energy doesn’t match the solar spectrum. Taking titanium dioxide as an example, its bandgap energy is 3.2 electron volts. Correspondingly, it can only absorb ultraviolet light with a wavelength less than 387 nanometers, but most of the energy of sunlight is concentrated in the visible light band of 400-600 nanometers. accounted for less than 6%. That is to say, the utilization of solar energy by photocatalytic materials is not efficient, and it is necessary to expand its absorption range of light, or to shift this range to the visible light band.
  Another limitation is that the efficiency of the photocatalytic reaction is not high enough. Why are the most commonly used methods for wastewater treatment still physical (such as adsorption, sedimentation) or biological methods? It is because the efficiency of photocatalytic technology is not high enough. If efficiency and cost are considered comprehensively, the “cost performance” of photocatalytic technology appears to be low, so it is only suitable for situations with high toxicity and low concentration of water pollutants, such as the treatment of pharmaceutical industrial wastewater.
  How to make photocatalytic materials absorb more visible light? We mainly adopted the method of element doping. Doping transition metal or non-metal ions into photocatalytic materials can change the bandgap structure, extend the absorption wavelength to the visible light band, or generate new doping energy levels, or narrow the bandgap, thereby using Lower energy light can excite photocatalytic materials to generate electrons and holes.
  As for how to improve catalytic efficiency, we must first figure out what causes the low efficiency. As mentioned earlier, photogenerated electrons and holes will migrate to different positions on the surface of the catalyst to undergo reduction and oxidation reactions, respectively. But in fact, there is another possibility for their whereabouts, which is to recombine on the surface and recombine together. Once recombination occurs, the catalyst is deactivated. This is an important reason for the low photocatalytic efficiency. Therefore, how to quickly separate and transfer electrons and holes while inhibiting their recombination becomes a key factor to improve catalytic efficiency.
  Based on such analysis, a series of highly active photocatalysts, such as photocatalytic materials loaded with space-separated dual co-catalysts, were designed by constructing heterophase interfaces. As the name implies, double co-catalysts are the introduction of two co-catalysts into the system, which respectively capture electrons or holes in the reaction and use themselves as active centers for reduction or oxidation reactions. As long as the two catalysts are separated from each other in terms of spatial structure, once the electrons and holes are generated, they will be trapped in different positions immediately, and will not contact each other, and naturally they will not recombine. We also proposed some other methods for constructing heterogeneous interfaces, including constructing “heterojunction” structures, preparing Z-shaped frameworks, and so on.
  At present, my country has proposed the goal of carbon peak and carbon neutrality, which brings new opportunities for the development of photocatalytic technology. To achieve the goal of carbon peaking and carbon neutrality, it is necessary to completely change the current chemical industry production method dominated by “thermocatalysis”. Compared with thermal catalysis, photocatalysis does not require high temperature and high pressure reaction conditions, and does not require complicated operating equipment. Most importantly, the energy source of photocatalysis is only solar energy, which is a truly green and low-carbon production method.
  Photocatalysts have great potential for application, and it is possible to use photocatalytic materials as catalysts in the future for extremely important reactions in the chemical industry such as olefins and ammonia synthesis. If solar energy can be converted into chemical energy on a large scale in this way, it will undoubtedly be a complete revolution.

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