Smilyk V. Photoelectrochemical and electrochromic properties of bismuth and copper tungstates and vanadates

The dissertation work is devoted to establishing the influence of the conditions of synthesis of thin coatings based on copper vanadates and tungstates by electrochemical and chemical methods, composition and structure on their photoelectrochemical and electrochromic properties. The dissertation examines original approaches to the synthesis and investigation of the properties of thin coatings, composites and heterostructures of bismuth and copper vanadates and tungstates, which can be used as photoanodes (bismuth vanadate) and photocathodes (copper vanadate) in photoelectrochemical cells for the decomposition of organic pollutants and photoelectrochemical separation solar hydrogen, solar cells, smart glasses, optical sensors (copper tungstate). For the synthesis of thin coatings of bismuth vanadates obtained by electrochemical deposition, acidic solutions of pH = 4.8 - 5 based on vanadyl sulfate and bismuth nitrate were used. The formation of bismuth vanadate occurs due to the anodic formation of vanadic acid ions and bismuthyl ions existing at this pH. Based on the analysis of polarization curves and Purbet diagrams, the reaction potentials were established and the stages of the electrodeposition process of thin BiVO4 coatings were determined. It was concluded that the deposition of thin coatings takes place according to a mixed mechanism, which includes the electrochemical stage of oxidation of the vanadyl ion to H2VO4- and the interaction of H2VO4- with the bismuthyl ion. Thermogravimetric and differential thermal analyzes showed that the formation of crystalline thin coatings occurs at temperatures above 200°C. This corresponds to the temperature of decomposition of hydroxides into oxides and water, after which they crystallize. Heterostructures and BiVO4(WO3) composites were deposited separately. Analysis of X-ray patterns of powders from thin coatings of BiVO4 and BiVO4(WO3) after heat treatment at 500°C showed that they consist mainly of monoclinic BiVO4 and WO3. To measure the thickness of such thin coatings during their growth, the method of two-beam interferometry was used. The optical registration scheme included two silicon photodetectors that record the intensity of two rays reflected at different angles of incidence φ1 and φ2 from a sample with a growing film. Studies of the influence of the thickness of thin BiVO4 coatings on the photocurrent quantum yield have shown that thick thin coatings (1-2 μm) have a lower quantum yield compared to films with a thickness of 200-400 nm. This fact is explained by the physical properties of polycrystalline thin BiVO4 coatings, namely low electrical conductivity. An increase in thickness leads to an increase in energy expenditure for recombination losses and an increase in their electrical resistance, and as a result, the efficiency of charge transfer decreases. At the same time, on thin films, visible light makes a greater contribution to the photocurrent compared to films with a thickness of more than 500 nm, where the region of the maximum quantum yield of the photocurrent falls on near-ultraviolet radiation. To increase the efficiency of these thin coatings, a heterostructure of BiVO4 with tungsten oxide was obtained. It was established that due to tungsten trioxide, the overall quantum yield index increases, which is related to the electrocatalytic properties of WO3. It was established that in the films of BiVO4 composites with WO3, in contrast to heterostructures and thin coatings of pure BiVO4, the spectral characteristics show a higher value of the photocurrent quantum yield in the region of light absorption of BiVO4. Thin Cu2O-Cu3VO4 coatings were obtained by electrochemical synthesis. The analysis of polarization curves and Purbet diagrams for copper and vanadium ions made it possible to analyze the possible reactions occurring during the deposition of thin coatings. At potentials from 0 to -0.2 V, mainly monovalent copper oxide Cu2O with impurities Cu3VO4 precipitated in the working solution. Due to the contribution of copper oxide, which largely dominates Cu3VO4 in the synthesized films, the thin coatings are photosensitive at (320-750 nm) with the participation of (Cu+-Cu2+ phototransitions) and show p-type conductivity, which makes them promising for use together with BiVO4 in photoelectrochemical cells for converting solar energy into electrical energy. Copper tungstate was obtained in two stages by electrochemical deposition on a Cu2O cathode. Analysis of the electrochromic properties of of copper tungstate thin coatings showed that they can be a promising electrochromic material, the color spectrum of which is wider due to the participation of copper oxide compounds compared to the well-researched electrochromic material based on tungsten trioxide.