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Visible Light-induced Charge Transfers and Photoconversions in Photocatalyst-polymer Composites

Title
Visible Light-induced Charge Transfers and Photoconversions in Photocatalyst-polymer Composites
Authors
김수정
Date Issued
2017
Publisher
포항공과대학교
Abstract
Solar energy utilization is considered as one of the most powerful ways to solve the facing environmental and energy problems of global society since solar energy is clean and infinite energy source. Photocatalytic reactions using homogeneous and heterogeneous photocatalysts have been widely studied for direct conversion of solar light to chemical energy or degradation of environmental pollutants. Various photocatalysts have been suggested for this purpose, however most of them are functioned only under UV irradiation which accounts for only 5% of whole incident solar spectrum. Therefore visible light utilization by economical materials is highly required for practical photocatalytic processes. In this study, various types of photocatalyst-polymer composites are developed to enhance the visible light absorption and the following charge transfer to promote the photoconversion efficiencies. In the first topic, I investigated the photosynthetic CO2 conversion in homogeneous system by immobilizing RuL dye and Re(I) cocatalyst in Nf resin for efficient charge transfer under visible light irradiation. In the second topic, I studied dopamine-derived polymers decorated on TiO2 to sensitize visible light for photocatalytic degradation of water pollutants. In the third topic, I synthesized the metal-free polymeric semiconductor, KPF6-modified C3N4, for visible light-induced photocatalytic H2O2 production through selective O2 reduction. 1. Photosynthetic CO2 reduction to CO has been widely investigated based on homogeneous Re(I) complex spreading its branches to formation of supramolecular complex and alteration of ligands for visible light utilization. As practical point of view, it will be pleased to make combinations with heterogeneous materials in simple way to easily enhance the photoactivity without the use of highly technical skills. To accomplish this objective, the immobilization of photosensitizer, RuL and cocatalyst, Re(I) within Nf structure has been proposed here. Although Re(I) could only make weak physical bonding with Nf, RuL could firmly bind to Nf by electrostatic and hydrophobic attraction. The presence of Nf enhanced the photocatalytic conversion efficiency (QY and TON) upto 1.5 times compared to homogeneous Re(I)-RuL mixture under visible light irradiation (λ > 420 nm, > 495 nm, and monochromatic 495±10 nm). The immobilization of RuL on Nf accelerates radiative decay of RuL* to prevent the energy to be wasted as heat and Re(I) on Nf inhibits the formation of unreactive Re(I) dimer at the same time. This is a unique feature of Nf whereas other polymers such as PDDA, PEG, and PSS did not show enhancing effect in photocatalytic activity. The several factors like charges, polymer structure, and proton coupling may have comprehensively affected the photocatalytic reaction assigning the special characteristic for Nf which maintains its durability upto 5 cycles. 2. Visible-light induced chemical transformation using inexpensive photocatalytic materials has been proposed as an eco-friendly method for energy and environmental applications. In this work, I employed polymers of environmentally benign dopamine derivatives as a low cost sensitizer of titania and systematically investigated their properties for the visible light photocatalytic transformation in aquatic environment. Dopamine (DA) and its derivatives (norepinephrine and nitrodopamine) were chosen as monomers and their polymers (pDA, pNE, and pNDA) were synthesized and subsequently complexed with TiO2. The visible light-induced catalytic transformations were successfully demonstrated for the reduction of Cr(VI) to Cr(III), dechlorination of CCl4, oxidation of As(III) to As(V), and H2O2 production via dioxygen reduction using the polymer-complexed TiO2. pDA-TiO2 exhibited the highest activities and the activity of pDA-TiO2 was much higher than DA-TiO2 in all tested cases, which indicates that the polymerized form of DA forms a stronger and more efficient surface complex on TiO2 surface for visible light sensitization. The dopamine-derived polymers could efficiently transfer electrons to TiO2 conduction band under visible light to initiate the reductive transformations whereas the oxidative transformation of organic substrates were largely inhibited because the organic polymer layer on TiO2 should scavenge any oxidizing radical species. pDA and pNE exhibited far better activities than pNDA due to the extensive π electron delocalization induced by 5,6-dihydroxyindole structure. This was also supported by the higher photon-to-current conversion and lower charge transfer resistance obtained with pDA-TiO2 and pNE-TiO2 (compared with pNDA-TiO2), which was observed with photoelectrochemical measurements. pDA should be an attractive visible light sensitizer for aquatic transformations. 3. Photochemical production of H2O2 through O2 reduction has been proposed as an alternative method of solar energy storage and carbon nitride (CN) photocatalyst was selected for this purpose in this study. The incorporation of KPF6 into the CN structure highly enhanced the apparent quantum yield (AQY) of H2O2 production in the UV and visible light region. The AQY of KPF6-modified CN was measured to be 35.9 % and 24.9 % under the monochromatic irradiation of 370 and 420 nm, respectively, which is 8.3 and 26.7 times higher than that of bare CN. The KPF6-enhanced activity is ascribed to several factors including (i) the enhanced absorption of UV and visible light, (ii) higher charge carrier density, (iii) retarded radiative recombination of charge pairs, (iv) highly enhanced selective 2-electron transfer to O2, and (v) hindered photo-decomposition of in-situ generated H2O2. Due to a high selectivity to O2, the enhancement factors on other control tests such as reduction of POM and chromate, reductive conversion of CCl4, H2 prodcution were negligible. This study demonstrates a simple way to produce a large amount of H2O2, which could be potentially utilized for a variety of applications including solar energy conversion to useful chemicals and environmental remediation.
URI
http://postech.dcollection.net/jsp/common/DcLoOrgPer.jsp?sItemId=000002378136
https://oasis.postech.ac.kr/handle/2014.oak/93683
Article Type
Thesis
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