Development of Titanium Dioxide Based Visible Light Photoelectrode for Photoelectrochemical Applications

基於二氧化鈦的可見光光電極研發及其在光電化學中的應用

Student thesis: Doctoral Thesis

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Award date10 Aug 2017

Abstract

Hydrogen is one of the candidates to function as a primary energy source in solving energy crisis as it is not only a clean energy but also contains high energy capacity. As of now, natural gas steam reformation and electrolysis have become the major technologies for the production of commercial hydrogen that suffer from large energy consumptions together with high costs. Photoelectrochemical (PEC) water splitting is believed to be one of the most promising technologies employed for the generation of hydrogen as it uses solar energy to split water into oxygen and hydrogen. The photoelectrodes used in a PEC system are usually semiconductors, which should have a band structure that the conduction band edge must be higher than the hydrogen redox potential and the valence band edge must conversely be lower than the oxygen redox potential. Taking into account the existence of over-potential in both hydrogen and oxygen evolution reactions, the minimum band gap of photoelectrodes is required to be 1.8 eV. Moreover, an appropriate photoelectrode should possess good electric conductivity and sufficient potential diference between band edge and half reaction potential to ensure the efficient charge transfer, long-term stability against corrosion and an adequate band gap for the efficient usage of solar spectrum. One dimensional TiO2 nanorod arrays (TiO2 NRAs) is chemically stable, cost-effective and non-toxic in addition to being capable of offering direct electrical pathways for the transportation of photo-generated electrons. However, the industrial application of TiO2 has been restricted by the large band gap (3.0 eV of rutile TiO2). In this dissertation, we primarily throw emphasis on tuning the band gaps of TiO2, engineering the surface and bulk defects to enhance the photoresphonse and opening its photoresponse to longer wavelength region.

In the first chapter, detailed information regarding the PEC system will be delivered, including the components of a PEC device, working principle and materials for the fabrication of both photoanodes and photocathodes.

The second chapter presents a brief overview of TiO2 semiconductor. It primarily focuses on the properties as well as catalytic applications. Their merits and demerits together with recent developments in curbing the shortcomings are also included.

In the third chapter, one-dimensional TiO2 NRAs are successfully synthesized on conducting fluorine doped tin oxide (FTO) glass. Nevertheless, the pristine TiO2 NRAs that are fabricated by a hydrothermal method typically possess high concentration of defects in addition to adsorbing the impurities on its surface. Hence, post-annealing is put to application in nitrogen (N2) and oxygen (O2) for the improvement of the performance of TiO2 NRAs in a PEC cell. It is discovered that the annealing treatment is capable of desorbing the chlorine (Cl) atoms from TiO2 NRAs in addition to augmenting the grain size. Removal of Cl ions after annealing downshifts of the energy bands of TiO2NRAs increases the resistance for charge carrier transport and transfer. Annealing in O2 can fill the surface oxygen vacancy (Vo) together with expanding the depletion layer. Consequently, a maximum photocurrent density of 1.38 mA/cm2 at 1.3 V vs. RHE is detected from TiO2 NRAs annealed in O2, which is approximately 28 times higher as compared with that of the pristine TiO2 NRAs.

In the fourth chapter, more efforts have been applied for the improvement of photoresponse of oxygen annealed TiO2 NRAs (O2-TiO2) to visible light region. Black TiO2 has gathered a good amount of research interest recently because of its narrowed band gap that can be excited by visible light. Through the implementation of a simple one-step vacuum annealing approach, black TiO2 NRAs with intentionally doped Ti3+/Vo are attained. The thickness of the amorphous layer (in which Ti3+/Vo located) can be controlled by varying the annealing temperature and time duration. The amorphous layer covered TiO2 NRAs brought to light the enhanced light absorption, reduced charge transport resistant and increased donor density. Resultantly, the TiO2 NRAs annealed in vacuum at 500 ℃ for 30 minutes exhibited the highest photocurrent density of 1.68 mA/cm2 at 1.23 V vs. RHE that gets enhanced 68% in comparison with that of pristine TiO2 NRAs.

In the fifth chapter, the black TiO2 NRAs prepared by a thermal vacuum de-oxygen (TVDO) approach, was surface engineered by illuminating with AM 1.5G light for 7000s in 0.2 M Na2SO4 solution for the induction of in-situ oxidation of Ti3+. The decrease of Ti3+ was confirmed by both XPS and EPR data, but the Ti3+ was not fully vanished. O 1s XPS spectra demonstrated the formation of Ti-OH on the surface of in-situ oxidized black TiO2 that facilitates the water oxidation reaction. As a consequence, the photocurrent density of black TiO2 got increased from 1.60 mA/cm2 to that of 1.89 mA/cm2 of surface engineered black TiO2 NRAs at 1.23 V versus RHE.

In the final chapter, a brief conclusion of the whole thesis together with outlook for the further development will be provided with.

    Research areas

  • Photoelectrochemical water splitting, annealed TiO2, Cl desorption, oxygen vacancies, hydroxyl groups