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Title: Synthesis of nanoscale quaternary chalcogenide and their heterostructures for photocatalytic hydrogen evolution
Authors: Ha, Enna
Degree: Ph.D.
Issue Date: 2015
Abstract: The advent in copper-based quaternary chalcogenide semiconductor nanomaterials development has brought forward a new age in the efficient conversion of solar energy to electricity or chemical energy for storage. Solution-based synthesis of nanoparticles is an ideal technique that offers the ease of synthesis and the possibility of scale-up to meet future industrial needs. This thesis concentrates on the delicate tuning of the morphology and composition of nanoscale quaternary chalcogenides and their metal/semiconductor nanohybrids, which will give a fundamental understanding in the reaction dynamics for both photovoltaic and photocatalytic processes. Mercaptobenzoic acid (MBA) was used in the novel mediated synthesis of Cu2ZnSnS4 (CZTS) nanoparticles, and was demonstrated to have a decisive effect on the control of CZTS morphology. By tuning the molar ratio between MBA and elemental sulphur, either kesterite CZTS nanoparticles or wurtzite CZTS nanorods were synthesized with high purity and monodispersity. Tracing the growth of CZTS nanorods revealed a seeded-growth mechanism via Cu2S, whereby tin would first substitute copper in Cu2S in the superionic state, followed by further substitution with zinc at a higher temperature, yielding thermodynamically stable CZTS. As the chemical composition has a great effect on the optical, electrical, and catalytic properties on semiconductors, a general synthetic technique for quaternary metal chalcogenide nanocrystals with a tuneable composition (CuxMySnzS4, where M = Mn, Fe, or Co) was developed. Zinc were successfully substituted by other transition metals to form new Cu2MSnS4 chalcogenide nanocrystals. Band gap tunable Cu2ZnxFe1-xSnS4 nanocrystals have also been prepared and studied. This synthetic method was further applied to the composition-controlled synthesis of CuxZnySnzS4 nanocrystals. Amongst all the derivatives, Zn-rich CZTS nanoparticles showed the best performance in photocatalytic hydrogen evolution.
In order to improve the photocatalytic activity of CZTS nanoparticles, a plasmonic metal was incorporated into CZTS to form a core/shell nanostructure. Au/CZTS core/shell was synthesized with Au nanoparticle core of average diameter 15 nm being fully coated by a CZTS nano-shell. This core/shell structure was found to enhance the photocurrent and photocatalytic activity of the catalyst compared to CZTS. Surface plasmon resonance(SPR)-induced direct electron transfer or near field enhancement from gold to CZTS can be provided as the prospective photocatalytic mechanism of this heterostructured nanomaterial. The structure-related enhancement in photocatalytic activities were further investigated systematically by incorporating gold nanomaterials of different morphology into Cu2FeSnS4 (CITS) nanoparticles. Delicate tuning of different structures was achieved by adjusting the experimental procedure. Auspherical/CITS core/shell NPs were synthesized via a seed-mediated method by injecting freshly prepared spherical Au nanoparticles into a solution of Cu-Fe-Sn-oleylamine precursors, whereas Aumulti-rod/CITS core/shell NPs were synthesized by one-pot synthesis in which Au-oleylamine precursors were nucleated and grown to seeds in the presence of Cu-Fe-Sn-oleylamine precursors at a lower temperature. The results of photocatalytic hydrogen generation test indicated that Aumulti-rod/CITS core/shell nanoparticles performed better than pure CITS nanoparticles or Auspherical/CITS core/shell nanoparticles. This enhancement in catalytic activity could be attributed to the size and shape effects of the gold nanomaterial, which have strong influence on the SPR property. Multi-rod gold cores exhibit stronger SPR effect with wider path for exciton transportation, which in turn leads to a higher catalytic activity.
Subjects: Nanoparticles.
Nanostructured materials.
Hong Kong Polytechnic University -- Dissertations
Pages: xvi, 239 leaves : illustrations (some color) ; 30 cm
Appears in Collections:Thesis

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