Photocatalysis can realize the conversion of solar energy into chemical energy (such as photocatalytic decomposition of water to produce hydrogen), which is an important way to obtain new energy. The development of photocatalytic materials that can effectively absorb visible light (wavelengths of 400-700 nm) is a prerequisite for achieving efficient solar photocatalytic conversion. However, most stable photocatalytic materials have low visible light absorption. Doping can reduce the band gap of the photocatalytic material and is a basic means to increase the visible light absorption of the photocatalytic material. Anatase TiO2 is the most extensively studied photocatalytic material. Currently, the doping method has been used to increase the visible light absorption of the material to a certain extent, but it still cannot achieve full-spectrum strong absorption.
Since 2004, Shenyang National (Joint) Laboratory of Materials Science, Institute of Metal Research, Chinese Academy of Sciences has been working to solve the problem of full-spectrum strong absorption of wide-band gap photocatalytic materials. The previous series of studies revealed that the spatial distribution of doped atoms is the essential factor that determines whether doping can reduce the band gap. That is, surface doping can only introduce localized energy levels in the band gap, and bulk doping can reduce the band gap. . At the same time, the idea of ​​using a layered structure to achieve the homogeneous distribution of doped atoms in the bulk phase is proposed to increase the visible light absorption of the photocatalytic material. However, how to achieve bulk doping of doped atoms in non-layered materials such as TiO2 has not been a breakthrough.
Recently, the laboratory proposed the use of interstitial atoms to weaken the bonding between metal atoms and oxygen (MO) to achieve a new mechanism to replace lattice oxygen doped atoms into the bulk phase. Gradient-doped anatase TiO2 was obtained to achieve visible light Full-spectrum strong absorption expands the active light response range of TiO2 photo-electrolysis hydrogen production to 700nm.
The difficulty of doping anions into the metal oxide bulk phase is essentially caused by the high bond energy of the MO bond and the difference in charge between the doping ions and the substitutional lattice ions. The researchers adopted the preliminarily developed "dopant and precursor combined into one" characteristic preparation idea, using TiB2 crystal as a precursor, and obtained interstitial boron-doped anatase TiO2 micrometers through hydrothermal and subsequent heat treatment processes. Spheres, and boron exhibits a gradient distribution from the surface of the sphere to a bulk thickness of about 50 nm. Theoretical studies show that the gap Bσ + (σ ≤ 3) ions can effectively weaken the surrounding Ti-O bonds, so that the energy required for N to replace the lattice oxygen of the weakened Ti-O bonds is significantly reduced, and the gap Bσ + Improves the stability of N-doped TiO2. The experiment found that not only N3- can effectively replace lattice oxygen, but also the spatial distribution of N3- is consistent with the gap Bσ +, showing a similar gradient distribution. It shows that the gap Bσ + plays a key guiding role in the spatial distribution of N-doping. The root cause is that Bσ + weakens the surrounding Ti-O bond, so that N3-selectively replaces the oxygen in the weakened Ti-O bond in the bulk phase. At the same time, the extra electrons contributed by the gap Bσ + can effectively compensate for the charge difference between N3- and O2-.
The B / N gradient co-doped anatase TiO2 material obtained by the study showed a unique red color (Figure a), and had a high absorbance in the full spectrum of visible light (Figure b). The photocatalytic performance study shows that the response range of this material's photoelectrolytic hydrogen production activity is close to 700nm. This result indicates that it is possible to use TiO2-based photocatalytic materials to achieve efficient visible light decomposition of water to produce hydrogen.
This work provides a new way of thinking about how to achieve visible light absorption of wide-bandgap photocatalytic materials based on doping, which can be used to develop high-performance visible light photocatalytic materials. The research results have been published in Adv. Funct. Mater. (2012, 22, 3233-3238), Energy & Environmental Science (2012, DOI: 10.1039 / C2EE22930G).
This work was supported by major research projects of the National Natural Science Foundation of China, the 973 project of the Ministry of Science and Technology and the "Solar Energy Action Plan" of the Chinese Academy of Sciences.
Figure a: Photograph of red TiO2; Figure b: UV-Vis absorption spectrum of red and white TiO2
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