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Title: Development of some heterostructured photocatalysts for hydrogen evolution from water
Authors: Hu, Liangsheng
Degree: Ph.D.
Issue Date: 2018
Abstract: Photochemical hydrogen generation from water is considered as one of the most promising solutions to tackle our present energy and environment problems. However, the solar-to-hydrogen energy conversion efficiency is still very low due to two major limiting factors encountered in most existing photocatalysts: the wide bandgap energy of photocatalysts and the fast recombination of photo-generated charge carriers. It is highly desirable to develop new approaches to improve the conversion efficiency. Hybrid photocatalysts, composed of more than one photocatalysts and/or combination of photocatalyst and/or co-catalyst, are beneficial in suppressing the recombination of charge carriers and widening the absorption range of solar light. In this thesis, several effective strategies to rationally design heterogeneous photocatalysts are presented in order to overcome those intrinsic limitations of photocatalysts. To narrow the bandgap of titanium dioxide (TiO₂) and prevent recombination of photo-generated holes and electrons, order/disorder Janus type TiO₂ nanoparticles consisting of ordered blue-anatase and disordered black-rutile phases were fabricated by a controlled magnesiothermic reduction process. In this process, the rutile and anatase phases of commercial P25-TiO₂ were reduced to different degrees at the same temperature due to their different surface enthalpies. The obtained order/disorder Janus architecture shows approximately 1,300% enhancement in photocatalytic hydrogen evolution compared to the untreated sample. The disordered black-rutile phase broadens the light absorption range as well as boosts the separation of charge carriers, while the ordered blue-anatase phase maintains the high catalytic activity.
To further enhance the efficiency of solar-to-hydrogen conversion, phosphorus was doped onto the order/disorder Janus heterojunction TiO₂ nanoparticles. Such introduction of an impurity energy level further narrows down the bandgap of ordered-anatase, thus allowing a better utilization of solar spectrum. The photocatalytic hydrogen generation rate of phosphorus doped Janus P25-TiO₂ has increased by 42 % from that of the undoped sample. In order to elevate the efficiency of photocatalytic hydrogen evolution on TiO₂ films, a layer of vertically aligned 3-dimensional (3D) superlattice gold nanorod (AuNR) array was introduced between the substrate (glass) and TiO₂ films. Significant enhancement in hydrogen generation rate was observed from the 3D AuNR array/TiO₂ heterogeneous nanostructures, which exceeds 58 times that of TiO₂ film. The direct electron transfer or near field enhancement from the inserted 3D AuNR arrays remarkably enhanced the generation of exciton carrier in TiO₂. The optimum conditions for maximized utilization of the surface plasmon resonance (SPR) effect on photocatalysis were also investigated. In order to determine the optimal distance for SPR effect without adverse interference from Forster resonance energy transfer (FRET), an interlayer of insulating polydopamine (PDA) or conducting polypyrrole (PPY) was introduced, with controlled thickness, between the gold core and the cuprous sulfide (Cu₉S₅) shell. The study shows that the photocatalytic efficiency of Cu₉S₅ is tuneable and largely affected by the size of Au nanoparticle and the thickness and conductivity of the interlayer.
Subjects: Hong Kong Polytechnic University -- Dissertations
Hydrogen as fuel
Pages: xv, 272 pages : color illustrations
Appears in Collections:Thesis

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