Surface plasmon resonance enhanced hydrogen evolution from water with graphitic carbon nitride photocatalyst

Abstract

Energy crisis becomes an important issue during the development of modern industry. Until now, greenhouse effect and air pollution are the most serious problems. Therefore, development of alternative sustainable clean energies is becoming more and more urgent for researchers and industries. The use of solar energy to generate hydrogen via water splitting with photocatalyst is considered a potential solution for the global energy problem. Titanium dioxide (TiO₂) was studied as photocatalyst for photocatalytic hydrogen generation under ultraviolet (UV) light by Japanese scientists in 1972. Due to drawbacks such as high bandgap energy in TiO₂ as photocatalyst, it is important for researchers to develop new promising photocatalysts with desirable properties. Graphitic carbon nitride (g-C₃N₄) is a new kind of conjugated polymer semiconductor which could be used for visible-light-responsive photocatalyst. In fact, from the perspective of two-dimensional (2D) layered system, g-C₃N₄ is a graphite-like layered material. The g-C₃N₄ structure is a 2D frameworks of tri-s-triazine units connected with planar amino groups in each layer, which are stacked layer by layer via weak Van der Waals force. In this work, g-C3N4 as a fascinating material was synthesized successfully by thermal condensation of melamine, which was then used for photocatalytic hydrogen generation under simulated sunlight irradiation. Noble metals, such as gold (Au), silver (Ag) and copper (Cu), are good candidates for localized surface plasmon resonance (LSPR). They display intense peaks in visible and near-infrared spectral region due to LSPR. Importantly, the LSPR effect is dependent not only on the size and shape of metallic nanostructure but also on the material's nature and surroundings. Gold nanorods (Au NRs) were prepared via the colloidal seed-mediated approach using cetyltrimethyl ammonium bromide (CTAB) as surfactant in this work. Under the optimal condition, by means of adding different concentrations of L-Cysteine (L-Cys), Au NRs formed different assembled structures that are random, aligned and aggregated. In order to further increase the hydrogen generation efficiency of g-C₃N₄, a series of composites as photocatalysts were prepared through combining different Au NRs assembled structures with g-C₃N₄. Interestingly, the hydrogen generation rates of the composites are proportional to the SPR enhancements of different plasmonic-Au NRs assembled structures. The random and aggregated structures with some vertical Au NRs in contact with g-C₃N₄, can provide a better circuit for continuous flow of hot charge carriers compared to the horizontally aligned Au NRs. In addition, horizontally aligned Au NRs can reduce the Schottky barrier between Au NRs and g-C₃N₄, which enhances the back electrons from g-C₃N₄ to Au NRs and thus slow down the generation of H2 from g-C₃N₄ surface. In chapter 1, a brief introduction of background, fundamental of photocatalytic hydrogen evolution and photocatalysts for hydrogen evolution from water are given. The mechanism of assembling different orientations of Au NRs motifs is described. The mechanism of LSPR enhancement effects and the formation of Schottky barrier are explained. In addition, the mechanism of semiconductor-metal composites as photocatalysts is discussed. Chapter 2 describes the detailed information of experiments, including chemicals used, synthetic procedure, characterization and equipment used. Chapter 3 presents the result of semiconductor-metal composites as photocatalysts for photocatalytic hydrogen generation. Different Au NRs assembled structures were characterized by UV-vis absorbance spectroscopy as well as TEM and SEM. Subsequently, different Au NRs assembled structures were combined with g-C₃N₄, and the photocatalytic hydrogen generation reactions were studied and the rates of reaction were compared. The effect of SPR enhancement is discussed in this chapter. Finally, chapter 4 gives a summary and conclusion of this thesis work.