Novel plasmonic metal-semiconductor hybrid photocatalysts for enhanced organic decomposition

Abstract

Electron-hole pair separation is a crucial process to realize solar energy harvesting which can drives most of the photocatalytic reactions on the semi-conductor surface. Plasmon-induced hot carrier generation unleash optical band gap limitation, extending the solar energy conversion spectrum to visible and near-infrared range. This is regarded as a promising alternative for the electron-hole separation process. However, the plasmonic-induced hot carrier generation and extraction efficiency remain low, hindering the large-scale application. In this thesis, a wide range of plasmonic metal/semiconductor hybrid nanostructures are synthesized by wet chemistry method which aims to join semiconducting titanium dioxide (Ti02) with metallic plasmonic nanostructure into many different configurations. More importantly, this thesis demonstrates a highly efficient strategy to extract the hot carries by assembling plasmonic metal/semiconductor hybrid nanostructures on Au film to construct a plasmonic film-coupled nanocavity system. The main results are summarized as follow: 1. Uniform Gold nanorods (GNR) are synthesized via seed­mediated method, realizing a flexible tuning on the plasmonic response over the visible to infrared range. Besides, by the precise control of the titanium chloride (TiCl₃) hydrolysis process, metallic plasmonic nanostructures such as GNR and Au nanosphere are coated with a semiconducting Ti0₂ shell or be further decorated with metallic co-catalyst. This results in a monodispersed plasmonic/semiconductor hybrid structure which makes the plasmonic particle promising for the plasmonic-induced photocatalytic reactions. The preliminary photocatalysis results suggest that the morphology design of plasmonic/semiconductor hybrid structure is of high importance for photocatalytic reaction, since the spatial separation of electron/hole pair can supress the recombination. 2. A film-coupled plasmonic photocatalysis system with an extraordinary hot carrier generation and extraction efficiency 1s constructed by assembling Au/Ti0₂ dumbbell nanostructures on a thin gold (Au) film. Benefited from the Au thin film, the film-coupled system has an enhanced photon absorption efficiency accompanied with an amplified plasmon resonance near field strength on the Au nanostructure. This resulted in a remarkable enhancement on both photocurrent (> 10 times) and photocatalysis reaction rate (> 3 times) compared with the counterpart's dumbbell nanostructures assembled on an ITO film. Under the excitation of the localized surface plasmons in this particle-on-film nanocavities, the on-top GNRs serves as an efficient hot electron generator, consequently injecting hot electrons over the Au/Ti02 Schottky barrier to participate in the photocatalytic reaction occurred on the Ti0₂ surface. Apart from the plasmonic hot carrier generation, an alternative hot carrier generation mechanism is jointly confirmed by the incident-wavelength­dependent photocurrent and spectra dependent reaction rate measurements where hot electrons are generated through the Au film optical d-band transition. This mechanism is a secondary hot carrier source which promotes additional hot electron to the film-coupled nanocavity system and spontaneously contributes to the photocatalytic reactions. 3. The strong coupling plasmonic film-coupled system is constructed by depositing with an ultra-thin Ti02 semiconducting layer on an Au film. This thin Ti0₂ layer acts as a hot electrons collector and a optical spacer to sustain a gap hybridized resonance mode of the atop plasmonic nanostructures. The plasmonic resonance is characterized by a single­particle darkfield optical measurement, revealing three plasmon resonance modes supported in this system: Dipolar resonance mode, gap hybridized plasmonic resonance and magnetic resonance mode. More importantly, the wavelength-dependent photocurrent measurement suggests the hybridized magnetic resonance mode has an outstanding photocurrent enhancement at the near-infrared region. This result explored a new photocurrent generation mechanism, also provided a basis for the future theoretical study.