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Semiconductor nanomaterials are one of the most important materials for photocatalytic and photovoltaic devices. The appropriate bandgap size of this type of semiconductor material allows sufficient light absorption in the solar spectral range, and photogenerated electrons and holes generated to drive the photocatalytic reaction or form photovoltaic potential. On the other hand, the large specific surface of nanomaterials can adsorb a large amount of reactant molecules or dye molecules in dye-sensitized solar cells, greatly improving the photocatalytic and photoelectric conversion efficiency.
However, the disadvantages of nanocrystallization of materials are the introduction of a large number of surface defects and bulk phase defects. The energy levels of these defect states are distributed between the band gaps, forming a binding center for photogenerated carriers. The result is a decrease in the number of defects. The ability of photoelectron reduction, the oxidizing ability of holes, and the electromotive force of photovoltaic cells. Therefore, it is imperative to develop a method for measuring the band gap intermediate state energy level of photocatalytic semiconductor nanomaterials. Since the conduction band electrons and the bound state electrons have different infrared absorption spectra, the time-resolved infrared spectrum has the ability to distinguish the photoconductive band electrons from the bound state electrons.
In 2013, the Institute of Physics, Chinese Academy of Sciences/Key Laboratory of Soft Assemblies, Beijing National Laboratory for Condensed Matter Physics, Professor Yu Xiangxiang set up a photocatalytic method for the measurement of the band gap intermediate state energy level of semiconducting nanomaterials. The Transient Infrared Absorption Excitation Energy Scanning Spectra (TIRA-EESS) determines the relative position of the bound Fermi level of anatase TiO2 nanoparticles and determines the valence band top to bound state fee. More than a dozen deep bound energy levels between electron energy levels (electron filled states) and bounded Fermi energy levels to multiple shallow bound energy levels at the bottom of the conduction band (electron unfilled states) (J. Phys. Chem. C 2013, 117, 18863−18869). Subsequent experiments show that the deep bound energy level comes from the surface defect state, and the shallow bound energy level comes from the bulk phase defect state. This method applied for a national invention patent.
On July 14, 2015, the research group published a research paper entitled "Band Alignment and Controllable Electron Migration between Rutile and Anatase TiO2" in Scientific Reports. It is an independently developed photocatalytic semiconductor nanomaterial band gap intermediate state energy. The application of research methods for grade measurement, the first author is doctoral student Mi Yang. TiO2 has three different crystal forms, namely anatase, rutile and plate titanium.
As the photocatalytic materials, anatase and rutile are generally used, and the anatase type has a higher photocatalytic activity than the rutile type. However, it was found that when a certain proportion of a mixture of anatase and rutile TiO2 was used as a photocatalyst, the photocatalytic activity of the mixed phase was significantly higher than that of the anatase or rutile TiO2 alone, showing a clear synergistic effect. . The anatase band gap is 3.2 eV, and the rutile band gap is 3.0 eV. Analogous to the concept of heterojunction in semiconductors, the researchers represented by Academician Li Can of the Dalian Institute of Chemical Physics, Chinese Academy of Sciences proposed to disassociate The concept explains the synergistic effect described above, that is, staggered levels of the energy levels of the conduction band and the valence band of anatase and rutile TiO2. Internationally, a large number of theoretical and experimental studies have been conducted around the relative arrangement of the conduction bands and the valence band energy levels of these two TiO2 crystal forms. All five possible relative arrangements have been supported by varying degrees of experimental or theoretical calculations. As shown in Figure 1), the controversy surrounding these issues has continued.
The group used band gap excitation scanning-transient infrared spectroscopy to determine the relative arrangement of the band positions of rutile and anatase TiO2 (see Figure 2). Firstly, this method was applied to distinguish the characteristic absorption spectra of the defect states and bulk defects (Ti3+) on the surface of TiO2, and it was proved that the transition energy levels of bulk Ti3+ were consistent within the experimental error range (<0.02eV). From these intrinsic energy levels as a reference point for comparing the relative positions of both the conduction band and the valence band, the relative arrangement of the energy bands of rutile and anatase TiO2 was determined.
The results show that the top of the rutile valence band is 0.2 eV higher than that of the anatase, which means that the photogenerated holes are directional migration. That is to say, the anatase phase migrates to the rutile phase, which contributes to the long-range separation of photogenerated charge and improves the photocatalytic efficiency. However, the conduction bands of the two are aligned, indicating that there is no thermodynamic preference for photoelectron transfer between the two phases. The group further proposed theoretically the dynamic criteria for judging the direction of photoelectron migration in two phases, revealing that the direction of photoinduced charge transfer is compounded by the mobility of electrons, the dielectric constant (related to particle size) and the photogenerated charge. (Response) Rate and other factors are controlled to predict the migration direction of photoelectrons under different conditions, and the experimental results are supported under several typical conditions, thus solving the long-standing unsolved problems in this field.
Another successful application example of this method is to explain why rutile TiO2 can achieve complete photolysis reaction of water (simultaneously achieve hydrogen release and oxygen evolution), while anatase TiO2 is only observed under non-special treatment conditions. Hydrogen, but no oxygen evolution was observed. If the anatase TiO2 is irradiated with UV light for a long time, complete photolysis of water can be observed.
The research group compared the surface defect state distributions of anatase TiO2 and rutile TiO2 using band gap excitation-transient infrared spectroscopy and found that anatase TiO2 contains a large amount of energy from the top of the valence band to an energy range higher than 0.6 eV. Bound state electrons result in a greatly reduced oxidative capacity of photogenerated holes in the bound state. When exposed to ultraviolet light for a long time, these deep bound electrons almost completely disappeared. In comparison, rutile TiO2 can hardly detect similar deep surface bound electrons. This explains the difference in the activity of two crystalline TiO2 during photolysis of water. The above results were published on July 2, 2015 as an important part of the cooperation paper of the Dalian Institute of Chemical Physics Li Can Group and the Institute of Physics of the Chinese Academy of Sciences, Achieving overall water splitting using titanium dioxide-based photocatalysts of different phases. On the Environmental, the Dalian Institute of Chemical Physics was the first author and the author of the correspondence, and the Institute of Physics was the partner.
This work was supported by the major research projects of the National Natural Science Foundation, the 973 Program of the Ministry of Science and Technology and the important direction project of the Knowledge Innovation Project of the Chinese Academy of Sciences.
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