Weekly Science and Technology Award｜Promising Photocatalytic Materials

In 1972, "Nature" published a paper proposing a method of using titanium dioxide electrodes to photolyze water to produce hydrogen and oxygen. Photocatalytic materials represented by titanium dioxide became the "darling" of the scientific community.

Why can titanium dioxide be driven by light? This starts with its structure.

Titanium dioxide is a semiconductor, and its energy level structure consists of a valence band with lower energy and a conduction band with higher energy. The energy difference between the valence band and the conduction band is called the band gap energy. What is the valence band and what is the conduction band? For example, the valence band is like the downstream of a river, the conduction band is like the upstream of a river, and the electrons are like a small boat in the river. When there is no external energy, the boat will stay downstream due to the effect of the water flow, that is, when the semiconductor material is in the ground state, the electrons are all distributed in the valence band. When the boat gets enough energy to start, it can go upstream and come to the upstream of the river. That is, when the semiconductor material gets enough energy, the electrons can be excited and jump from the valence band to the conduction band, and the required energy is the band gap energy.

If light is irradiated onto the titanium dioxide material, and the energy of the light is greater than or equal to the band gap energy, then some of the electrons in the valence band will be excited and jump to the conduction band, where they flow freely. After the electrons "jump" to the conduction band, vacancies are left in the valence band. To describe this process in professional terms, the photocatalytic material is excited by light, generating photogenerated electrons and holes.

Next, the photogenerated electrons and holes are distributed at different locations on the surface of the material. Holes are eager to get electrons, so they have strong oxidizing ability, while electrons show strong reducing properties. This is why titanium dioxide can decompose water under light conditions. In fact, the light absorption of materials, bulk phase separation of photogenerated charges, and surface transfer are the three basic processes of photocatalysis.

According to this principle, photocatalytic materials can use the inexhaustible solar energy to electrolyze the extremely abundant seawater on the earth, thereby "continuously" producing hydrogen and oxygen. The prospect is very attractive. Unfortunately, photocatalytic materials have inherent defects in practical applications.

One is that its band gap energy does not match the solar spectrum. The light absorption range of photocatalytic materials is mostly concentrated in the ultraviolet band, but the energy of sunlight is mostly concentrated in the visible light band of 400-600 nanometers, and ultraviolet light accounts for less than 6%. This means that the use of solar energy by photocatalytic materials is not efficient.

The second is that the efficiency of the photocatalytic reaction is not high enough. As mentioned earlier, the photogenerated electrons and holes will migrate to different locations on the catalyst surface, undergoing reduction and oxidation reactions respectively. But this is only the most ideal situation. In fact, they may also recombine on the surface and recombine together, which will cause the catalyst to deactivate and ultimately seriously reduce the photocatalytic efficiency. Therefore, in this process, we need to find a way to quickly separate the photogenerated electrons and holes to different places.

Through a series of basic research, Liu Gang's team from the Institute of Metal Research, Chinese Academy of Sciences, has found some solutions to these two problems. A research project based on "Energy Band and Microstructure Regulation of Photocatalytic Materials" was launched. They found that the spatial distribution of band structure modifiers within the grains of photocatalytic materials is the essential factor in regulating the band gap and thus changing the overall light absorption range. Therefore, two ideas were proposed: using atomic structure channels to promote diffusion and using interstitial heteroatoms to weaken strong bonds to reduce bond breaking energy, which greatly widened the spectral range that photocatalytic materials can absorb. In order to achieve the spatial separation of photogenerated electrons and holes, they developed a two-dimensional photocatalytic material with short-range charge migration characteristics, and designed a core/shell structured photocatalytic material containing unsaturated/saturated valence cations, breaking through the charge separation limitations of photogenerated electrons and holes due to the inherent mismatch in mobility. The project was awarded the second prize of the National Natural Science Award in 2020 in November 2021.

The team also achieved selective exposure of the crystal face of photocatalytic materials, clarified the association mechanism between crystal face characteristics and band edge positions and charge surface transfer, and laid the foundation for the realization of controllable surface charge transfer. The relevant research results not only strongly promoted the development of efficient solar-driven photocatalytic materials, but also radiated to multiple inorganic non-metallic functional materials research fields.

(Text: Gu Miaofei, deputy editor of Science Pictorial, Shanghai Science and Technology Press; review expert: Li Cunpu, professor of the School of Chemistry and Chemical Engineering, Chongqing University)

China Association for Science and Technology Department of Science Popularization

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