Semiconductor Thin Films with Tunable Electronic Structures for Photoelectrochemical Applications


Student thesis: Doctoral Thesis

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Award date29 Nov 2018


Photoelectrochemical (PEC) cells that based on semiconductor materials play crucial roles in many important applications such as solar energy storage, conversion and industrial production reactions (e.g. hydrogen evolution, carbon dioxide reduction and nitrogen fixation). The multi-electron transfer chemistry and multiple redox equivalent accumulation in photo-induced redox reactions make PEC process kinetically slow and retarded the progress in this field. To date, various semiconductor, such as, TiO2, BiVO4, g-CN, CdS, MoS2 and phosphorene have been demonstrated to be promising photoanodes/cathodes.

Since 2009, when Prof. Xinchen Wang introduced g-CN to the photocatalysis field, it has attracted much attention due to its moderate band gap, high stability under light irradiation and non-toxicity. In this thesis, we employed a simple thermal vapor condensation (TVC) method to deposit uniform g-CN films under ambient pressure. By making an appropriate adjustment to the TVC method, we textured the surface morphology of the g-CN film in-situ effectively. Aiming at seeking further improvement of g-CN films, we tuned the electronic structures and regulated the physicochemical properties of g-CN-based photoanodes via doping. The influence of different heteroatoms on the PEC performance was investigated experimentally and theoretically. Based on the density functional theory (DFT) and time-dependent density functional based tight-binding (TD-DFTB) calculations, we confirmed that the heteroatom with weak electronegativity like boron would result in a spatially complementary orbital distribution over the modified g-CN films. While for heteroatom with strong electronegativity like oxygen would contribute to the band-gap states and reduce the band gap of g-CN films effectively by introducing the bandgap states. Then, we systematically investigated photoelectrochemical properties of the doped g-CN films. We found that the photocurrent density of the modified g-CN films was enhanced with the introduction of heteroatoms, which was due to enhanced light absorption, decreased charge transport and charge transfer resistance. The g-CN films that were modified with B atoms exhibit long-term photostability (i.e. maintains 90% of initial photocurrent over 15h) due to relatively stable electronic structure in the hole accumulated situation. Our results highlight the significance of component tailored design in g-CN-based photoanodes which can further advance the use of g-CN films’ photocatalytic properties for a numbers of PEC applications.

Apart from the g-CN, BiVO4 is considered as another promising photoanode for water oxidation reaction. The IPCE of BiVO4-based photoanode upon 420 nm illumination at 1.6 V vs RHE is estimated to be 20%, which makes it very suitable to drive visible-light water splitting. However, the energy lost and poor charge transfer in BiVO4-based photoanodes hinder their PEC applications. To solve this problem, we prepared vertically oriented Mo-doped BiVO4 nanoworm array films by pulsed laser deposition (PLD) technique. Photoanodes doped with 1% molybdenum at vanadium site provided superior photocurrent density, up to 2.1 mA/cm2 and 1.7 mA/cm2 at 1.23 V vs. reversible hydrogen electrode (RHE) with Na2SO3 as the hole scavenger for substrate to electrode illumination and electrolyte to electrode illumination, respectively. The improved photoelectrochemical (PEC) performance of 1 % Mo:BiVO4 photoanodes primarily originated from an appropriate regulated composition and a desirably controlled porous morphology. The outstanding PEC performance highlights the significance of vertically oriented nanoworm array nanoarchitecture with a large surface area as a promising photoanode that ensures enhanced light absorption without sacrificing effective charge collection. This study is expected to enable BiVO4 to be applied as efficient photoanodes in PEC water splitting process and to open the door to the large-scale manufacture of metal oxides photoanodes using the PLD technique for highly efficient photochemical performance.

In summary, the g-CN films and BiVO4 films were rationally designed and studied in-depth in the present thesis, targeting at revealing the relationship between the component and their PEC performance. It is believed that the strategies and results in this thesis can shed new light on the compositional design of semiconductor materials for efficient PEC energy storage/conversion devices.