| Energy shortage and environmental pollution have become the two major problems in the development process of the world. Photocatalytic technology is expected to be one of the most promising ways to solve these two problems. Because the photocatalytic technology can be used to directly change the most abundant, cheap and sustainable solar energy into the clean hydrogen energy, can also be used for solar degradation of organic pollutants and conversion of carbon dioxide into carbon monoxide and methane fuel to clean up the environment. The research and development of anew and high efficient semiconductor photocatalytic materials is the key to the application of photocatalytic technology.Polymer photocatalytic materials have been attracting wide attention due to their low cost, turnable structural design, and visible light response. Our research group developed a new type of polyimide (PI) semiconductor photocatalytic materials. As well as other polymer semiconductor photocatalytic materials, PI also has the advantages of wide source, low price, adjustable structure and so on. Morever, the synthesis process is more energy efficient, green and environmental friendly than g-C3N4, thus resulting in a more broad prospect for development. However, as with other polymer semiconductor photocatalytic materials, PI also has low carrier mobility, weak absorption of visible light and so on, and its photocatalytic activity is still low. Therefore, it is urgent and practical to improve the carrier mobility and broaden the range of visible light absorption for improving the photocatalytic activity of PI.This paper mainly aimed at the shortcomings of PI as photocatalytic materials, and carried out the following major work:First of all, one step solid state thermal polymerization method was used to develop the Mo doped polymer photocatalytic material BMO/PI. The existence form of Mo component in the polymer structure and the interaction between PI and Mo were studied systematically. The effects of Mo on the crystallinity, band structure and photocatalytic properties of PI were investigated. Characterization results showed that N atoms of the triazine ring provide Lone pair electrons to unsaturated Mo atoms of black molybdenum oxide forming Mo-N coordination bond, resulting in a large number of Mo5+ ion in BMO/PI and as electronic pond to promote the transmission of electrons. In the process of the introduction of Mo, MoO3 as a solid acid catalyst from thermal decomposition of ammonium molybdate as Mo precursor, promoted some oligomers further polymerization in the reaction system, resulting in a significantly enhancement of the crystallinity of BMO/PI compaired to the PI, which is advantageous to the improvement of the carrier mobility. After the introduction of Mo, the position of the conduction band (CB) and the valence band (VB) were decreased, at the same time, the band gap narrowing, which is beneficial to improve the stability and enhance visible absorption of photocatalytic material systems. The photoelectric properties and photocatalytic activity of BMO/PI were tested, and the mechanism was discussed. Compared with pure BMO/PI, the visible light absorption of PI was significantly enhanced, and the activity of photocatalytic degradation of organic pollutants was significantly increased. The 3.0BMO/PI shows the highest photocatalytic activity, which is about 4 times higher than PI.Secondly, MoO3/PI composite semiconductor photocatalytic materials with different content of molybdenum trioxide were synthesized using the method of dipping solid state heat, thus molybdenum oxide single crystal nanosheets in situ crystallization grow on the surface of the PI. Interface structure, surface profile and carrier transport properties of the composites are studied. The effects of different contents of MoO3 on the crystallinity, photoelectric properties and photocatalytic activity of PI were investigated. On the PI surface, the α-MoO3 followed the order of growth rate{001}>{100}>{010} along the crystallographic axis, exposing the dominant (010) crystal plane. Morever, the induced effect of PI lamellar morphology and the special interaction between Mo and PI resulted in the crystallization epitaxial growth of MoO3 single-crystal nanosheets and the formation of dense interfacial structure between MoO3 and PI, contributing greatly to the transfer of photogenerated charge carriers. The experimental results show that the visible light absorption range of MoO3/PI is obviously widened, and the radiative recombination of the photo-induced electron and hole is effectively suppressed. In particular, the photocatalytic hydrogen production of 3.0MoO3/PI is higher than that of 3.0Pt/PI under the same conditions, which shows that the morphology of the catalyst and the interface between catalyst and cocatalyst are very important to improve the photocatalytic activity.Finally, molybdenum sulfide quantum dots (MQDs) were in situ decorated on the surface of PI, which aims to improve the thermodynamic driving force of photocatalytic hydrogen evolution from water splitting over PI using the quantum confinement effect of MQDs, and the excellent in-plane carrier transport ability of MQDs can greatly reduce the recombination rate of the photogenerated holes and electrons. Research results showed that most sizes of the MQDs are about 3 nm, which showed a single layer and more uniformly embedded in the surface of PI. Compared to bulk MoS2, such small nano structure of MQDs shortened photoinduced carrier transmission distance, increased the interfacial contact area between MQDs and PI, while the number of surrounding exposed unsaturated S atom increased significantly, which can provide more active sites for hydrogen production, resulting in a significant improvement in the photocatalytic hydrogen evolution activity and stability. In particular, the photocatalytic hydrogen evolution activity of 1.0MQDs/PI sample was 400% higher than that of 1.0Pt/PI under the same conditions. Our study shows that the monolayer molybdenum sulfide quantum dots can instead of noble metal Pt as cocatalyst of PI, which provides new ideas and methods for the synthesis of other metal compounds quantum dots as cocatalysts to develop efficient polymer semiconductor photocatalytic materials. |
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