Vanadium dioxide (VO2) is a strongly associated transition metal oxide material with a wide range of application prospects. The most notable feature is that it has an insulation-metal phase transition property of four to five orders of magnitude at 68°C. The various photoelectric functional properties of VO2 are closely related to its phase transition, but its relatively high phase transition temperature has become a major bottleneck in practical applications. Through in-depth study of its phase change microscopic mechanism and exploration of effective phase change control methods to reduce the phase transition temperature is of great significance to promote its practical application.
Due to its small atomic radius, hydrogen atoms can effectively enter the VO2 lattice to achieve electron doping and achieve the purpose of controlling the phase transition. Using traditional high-temperature noble metal catalytic hydrogenation methods, the Zou Chongwen Research Group of the National Synchrotron Radiation Laboratory and the Jiangjun Research Group of the National Research Center of the Microscale Materials Science and Technology of China University of Science and Technology have implemented three stages of insulation-metal-insulation in VO2 thin films. In order of phase transitions, the mechanism of hydrogen-induced electron doping to fill the conduction band level of VO2 is revealed (Phys. Rev. B 96, 2017, 125130). However, conventional hydrogenation doping techniques rely on conditions such as high energy consumption of temperature and pressure, expensive precious metal catalysts, and the catalytic metal deposited on the surface of the hydrogenated material is also difficult to remove, and these unfavorable factors become a constraint on the hydrogenation phase of the VO2 material. Change regulation and application barriers.
Recently, researchers have broken through the traditional method of catalytic hydrogenation of high-temperature noble metals to regulate VO2 phase transitions, and realized the use of metal protons to drive protons of acid solutions into VO2 materials to achieve hydrogenation of materials at very low temperatures under mild conditions, inventing “pointsâ€. Iron into hydrogen technology.
The acid solution can easily corrode most of the oxides including VO2, so that the acid cannot be used as a hydrogen source for the hydrotreatment of the oxide material under normal temperature and pressure conditions. In the experiment, the researchers found that the VO2 film was not only not corroded by the acid solution but also quickly induced hydrogenation and induced phase transition. This phase change process has an extremely rapid diffusion effect, so that a two-inch diameter VO2 epitaxial film can be corrosion-resistant and metallized using only very small metal particles (1 mm in diameter), thereby achieving a similarity to the "gold to stone" "Point iron into hydrogen" effect. Theoretical predictions revealed the electron-proton co-doping mechanism behind this phenomenon. When a low work function metal contacts a high work function VO2, electrons are spontaneously injected into VO2. Due to the electrostatic induction effect, the protons in the acid solution are pulled into VO2, which causes the VO2 metalization and makes the formation of oxygen vacancies much easier. Raise to prevent acid corrosion. On the basis of already metallized VO2, if lower-work-function metals such as Al, Zn, etc. are used, more electrons and protons can continue to be injected, so that the electrons are filled to the top of the new valence band to form a new insulating state. Realize the three-phase sequential phase change from insulation-metal-insulation.
The electron-proton co-doping strategy realizes a three-state adjustment of “intrinsic-state-metal state-new insulation state†by simply contacting the acid liquid, metal particles, and VO2, and can not only develop into a compatible conventional environment. The doping method can also have a positive effect on the study of electronic synergy. The modification of materials has always been the focus of physical, chemical, and material science research. Doping is one of the most effective methods. Based on the principle of electron-proton co-doping, the researchers replaced the acid solution with a lithium ion solution and expanded it into an electron-ion co-doping strategy. It also achieved lithium ion doping and controlled the phase transition behavior of VO2. It has also been found that this strategy can achieve more oxide materials, such as titanium dioxide (TiO2) doping hydrogenation, verifying the universality of this doping technology. Relatively traditional doping techniques often use high temperature, high pressure, and precious metal catalysis. The research institute has developed a doping method that is more compatible with conventional mild environments, is easy to operate, and is extremely inexpensive, for the development of new functional materials. Both the device and the promotion of basic theory are of great significance.
The relevant research results were published in Nature-Communications. Ph.D. candidate Chen Yuliang and visiting scholar Wang Zhaowu were the co-first authors, associate researcher Zou Chongwen and professor Jiang Jun were correspondents. This study was supported by the National Key Basic Research and Development Program Young Scientist Special Project, the National Natural Science Foundation of China, the Central University Fundamental Research Funds Special Fund and the Chinese Academy of Sciences Youth Innovation Promotion Association.
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