Synthesis of Bismuth Vanadate Thin Films for Highly Efficient Photoelectrochemical Water Splitting and Triboelectric Nano Generators

釩酸鉍薄膜在高效光電化學水分解和摩擦納米發電機中的應用

Student thesis: Doctoral Thesis

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Award date14 Sep 2021

Abstract

Photoelectrochemical (PEC) water splitting is the process of hydrogen production that employs sunlight, water, and specially designed photoactive materials, (i.e., semiconductors or photosensitizers) to ramify back the water molecules into their building blocks (i.e., hydrogen and oxygen). This is a renewable fuel generation technology with trivial or no emission of greenhouse gases. PEC cells which are derived from semiconducting materials (fusion of semiconductors, doping, and heterojunctions) play pivotal roles in numerous significant applications such as storage and conversion of solar energy to renewable green energy and they are also applicable for nitrogen fixation. In 1998, Akihiko Kudo and fellows came up with some shreds of evidence in favor of BiVO4 activity for oxygen evolution under visible light irradiation. Since then, BiVO4 has grabbed the attention owing to its favorable bandgap (i.e., 2.4-2.5 eV), low production cost, high theoretical current density (i.e., 7.5 mA cm-2), non-toxic nature, and good stability under visible light irradiation in almost neutral media. However, some limitations on the PEC performance of BiVO4 include poor intrinsic stability in the acidic or basic media ranging ~ 3> pH >11, charge carrier recombination, and low charge transportation. 

In the present work, we deposited pristine BiVO4 thin films using facile spin coating synthetic procedure and achieved the remarkable PEC performance with current density up to 3.17 mA cm-2 under SE (substrate-to-electrolyte) illumination that exhibited quite good stability at 1.23 V vs. RHE for water splitting. By adjusting a few parameters for the annealing process and optimizing the heating rate of the furnace to the targeted annealing temperature (i.e., 475 °C), we tuned the morphological and electronic properties of the BiVO4 films mainly by introducing a quite good population of oxygen vacancies which act as shallow donors of electrons and can create shallow energy levels inside the bandgap of BiVO4 by introduction of V4+ species in the lattice sites of the crystal, thereby increasing photogenerated charge density and reducing electron and hole pair recombination. We recorded the data for a series of 7 samples from 10 °C rise per minute to 70 °C rise per minute to reach 475 °C, and named them as BV-10 °C, BV-20 °C, BV-30 °C, BV-40 °C, BV-50 °C, BV-60 °C, and BV-70 °C. Out of those, BV-60 °C outperformed to achieve 3.17 mA cm-2 photocurrent density. The morphology of the films changed from densely stacked particles to the porous structure which effectively boosted the photoactivity of the material, while the presence of oxygen vacancies was confirmed by careful analysis of XPS data. The bandgap of as-prepared BiVO4 thin film was reduced from 2.55 eV to 2.33 eV, as compared to that of literature. 

The literature demonstrated that the output of triboelectric nanogenerators (TENG) can be tuned by modulating the triboelectric polarity using efficient photoactive semiconductor materials by incorporating sunlight. Hence, we came up with an idea of employing our highly photoactive material (BiVO4) for this application which is not reported yet. A 4-fold increase (3.1 µA – 6.1 µA) in the short-circuit current was recorded. The oxygen vacancies introduced V4+ ions as charge traps and hence the charge retention of the device increased significantly. The photocurrent plateau appeared because of the formation of BiVO4 Schottky contact inside TENG. This work opened a new avenue to enhance the PEC cell output performance by designing a controlled heating rate protocol for BiVO4 thin film synthesis. Moreover, BiVO4 thin films are applied successfully as a triboelectric frictional layer in TENG device, and their output performance prevails over commonly employed triboelectric materials (e.g., PDMS and PTFE).