engleski

Design of semiconductor (photo)catalysts and investigation of their electronic structure

Significant efforts have been made to develop new semiconductor catalysts for energy conversion and photocatalytic degradation of organic pollutants, pigments and dyes. It is challenging to identify, and design new semiconductor materials that are efficient, stable and abundant. Therefore, the knowledge of fundamental semiconductor (photo)electrochemistry principles is of the great importance. The crucial step in photoelectrochemical processes connected with solar energy conversion, including (photo)catalytic organic pollutant degradation is electron transfer, which occurs between the same energy levels of semiconductor and redox electrolyte species at the semiconductor|electrolyte interface. The flat band potential, EFB (Fermi level) as experimentally determined parameter, correlates the electron transfer between the semiconductor and the electrolyte solution and is linked to the semiconductor’s band edge position. For EFB determination, the Mott-Schottky (MS) analysis or photocurrent measurements can be used. In this work special attention was given to the accuracy of EFB determination. The aim of this work was also to broaden the photocatalytic system forward from single component catalyst’s to multi-component configuration. Composite semiconductor- semiconductor junction structure of the n-type Bi2S3 (narrow-band gap semiconductor), and the n- type Bi2O3 (wide-band gap semiconductor) was proposed. Both semiconductors have recently attracted much attention due to the natural abundance, non-toxicity, high catalytic activity and excellent electrical conductivity. Bi-films, formed potentiostatically on bismuth in an aqueous sodium hydroxide solution with and without the addition of sulfide ions, were investigated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and diffuse reflectance spectroscopy (DRS). The energy-band diagram for the proposed composite structure, n-Bi2S3| n-Bi2O3, was constructed taking into account the semiconducting parameters of the both semiconductors (deduced from the Mott-Schottky analysis and Tauc plots) and the thermodynamic data for the redox reactions occurring at the semiconductor|electrolyte interface. Special attention was given to the reactions that include the formation of free radical species prerequisite for the solar-driven photocatalytic degradation processes of organic pollutants. Absolute energy values of the conduction band edge, ECB and the valence band edge, EVB positions of n-Bi2O3 and n-Bi2S3 were deduced from the EFB (Fermi level) and correlated with the semiconductor’s stability against decomposition. The n-Bi2S3|n-Bi2O3 catalyst satisfies simultaneously the light adsorption and charge separation criteria, acting as possible suitable visible light-driven catalyst in advanced oxidation processes for wastewater treatment application.

Semiconductor Electrochemistry ; Semiconductors ; Surface Science ; Electronic Structure

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