Investigation of Copper-containing photocathode for enhanced solar hydrogen production via photoelectrochemical water splitting

Nguyen Hoang Lam 영남대학교 대학원 2023 국내박사

최근 몇 년간, 환경 친화적이고 오염물질이 없는 수소 생산 방법으로써 광촉매 물분해에 대한 관심이 증가하고 있습니다. 이 분야에서의 주요 과제 중 하나는 태양광을 이용하여 효율적으로 물을 분해할 수 있는 시스템을 저렴하고 비독성인 재료를 사용하여 개발하는 것입니다. PEC(광전기화학) 수분해 시스템에 사용되는 광전극에서는 상업 생산에 사용될 수 있는 특정 속성을 갖는 반도체가 이상적입니다. 그러나 이러한 물질들의 보고된 효율은 때때로 이론적 최대값보다 낮습니다. 따라서 광촉매 활성을 향상시키기 위해서는 광전극의 효율을 제한하는 요인들에 대한 명확한 이해가 필요합니다. 본 논문의 4, 5, 6 장에는 실험에 대한 구체적인 내용이 수록되어 있습니다. 4장에는 XRD, SEM 및 TEM 기술을 사용하여 분석된 구리 기반 분말 NPs (Cu/Cu2O 나노복합상)의 결정 구조에 초점을 맞추었으며, W-H, N-R, Rietveld 정제 등의 몇 가지 방법을 사용하여 PNCs의 결정 구조를 심도있게 분석하기 위해 노력했습니다. 용액 합성 방법 중 다양한 합성 조건에서 PNC의 결정 구조 변화를 관찰하기 위해 두 가지 실험을 진행하였습니다. 해당 실험을 통해 결정 구조의 철저한 분석과 비교를 하였습니다. 5장에서는 CuO 박막의 구조적 및 광학적 특성 및 PEC 성능에 초점을 맞추었습니다. 브로콜리 모양을 가진 이 박막은 물분해 응용을 위한 광전극으로 사용하기 위해 수열법을 사용하여 제조하였습니다. XRD, SEM, XPS, PL 및 UV-Vis와 같은 다양한 기술을 사용하여 박막의 형태학적, 구조적 및 광학적 특성을 측정하였습니다. 또한 이러한 박막의 PEC 성능은 삼전극 시스템에서 평가되었습니다. 6장에서는 CuO 박막의 성능 향상 방법에 대해 설명합니다. PEC 물분해 응용에서 CuO의 단점을 해결하기 위해 수열법, 스핀 코팅 및 열처리를 통해 FTO 기판 상에 CuO와 CuBi2O4를 합성하여 이종접합 광전극을 제작했습니다 (FTO/CuO/CuBi2O4). 각 샘플의 특성(합성 방법, 구조, 구성, 형태 등)은 다양한 합성 조건에서 철저히 논의하였습니다. CuBi2O4를 CuO와 접합함으로써 두 개의 반도체가 밀접하게 결합되어 광전극의 PEC 효율이 향상되었습니다. 또한 CuO 층은 전자 생성 층으로 작용하고, CuBi2O4 층은 광부식을 방지하고 전자를 전극/전해질 접합부로 운반하여 HER을 용이하게 합니다 In recent years, there has been a growing interest in using photocatalytic water splitting as a way to produce hydrogen that is both environmentally friendly and free of pollutants. One of the primary challenges in this field is developing systems that can efficiently dissociate water using sunlight, while also relying on inexpensive and non-toxic materials. For photocathodes used in PEC water splitting systems, it is ideal to have semiconductors with specific properties that can be used in commercial production. However, the reported efficiencies for these materials are often below their theoretical maximum values. Therefore, it is important to have a clear understanding of the factors limiting the efficiency of these photocathodes if the photocatalytic activity is to be improved. The experiments in this dissertation are structured into three sections. The fourth chapter of this dissertation focused on the crystalline structure of copper-based powder NPs (Cu/Cu2O nanocomposite phase), which was analyzed using XRD, SEM, and TEM techniques. There are some methods such as W-H, N-R, and Rietveld refinement were used to deeply analyze the crystalline structure of PNCs. Two experiments were conducted to observe the changes in the crystalline structure of PNC under different synthesis conditions in the colloidal solution method. A thorough analysis and comparison of the crystalline structure were performed. In the fifth chapter, the focus is on the structural, and optical properties of CuO thin films, as well as their PEC performance. These thin films, which exhibit a broccoli-like structure, were prepared using a hydrothermal method to serve as a photocathode for water splitting applications. Various techniques, including XRD, SEM, XPS, PL, and UV-Vis, were used to measure and characterize the morphological, structural, and optical properties of the thin films. Additionally, the PEC performance of these thin films was evaluated in a three-electrode system. In the sixth chapter, a method for enhancing the performance of CuO thin films is described. To address the drawbacks of CuO in PEC water splitting applications, a heterojunction photocathode was produced by synthesizing CuO and CuBi2O4 on an FTO substrate (FTO/CuO/CuBi2O4) through hydrothermal methods, spin coating, and annealing. The characteristics of each sample under various synthesis conditions, including synthesis method, structure, composition, and morphology are thoroughly discussed. The integration of CuBi2O4 with CuO resulted in the two semiconductors being closely bonded, enhancing the PEC efficiency of the photocathode. Additionally, the layer of CuO acted as the electron-generating layer, while the CuBi2O4 layer prevented photo-corrosion and aided in the transportation of carriers to the electrode/electrolyte interface to facilitate HER

Environmental pollution and bioconcentration by perfluoroalkyl substances in Korea and Vietnam

Nguyen, Hoang Lam Chonnam National University 2016 국내박사

Environmental contamination monitoring is the preliminary work to investigate ecotoxicology and potential risk on human health of a pollutant. Data on the occurrence and spatial distribution characteristics of a target pollutant is necessary for further understanding of the exposure level of the chemical in an ecosystem, which can be an important factor that directly or indirectly induces adverse effects on biotic and abiotic media in ecosystems, ecological food chains or human health. Perfluoroalkyl substances (PFASs) are a group of fully fluorinated chemicals which has the unique properties including resistance to hydrolysis, photolysis, bio-degradation and thermal stability and have been used in diverse commercial application since the 1950s. Although the production and use of the major concerns PFASs in environmental contamination, perfluorooctane sulfonic acid (PFOS) and its related chemicals had been widely banned, their homologues, neutral precursor and other new PFASs continue to be produced. Due to their persistence and bioaccumulation, some PFASs can elicit harmful effects in terrestrial and aquatic organisms. The toxicity and persistence of PFASs in human have raised considerable concerns and several biomonitoring studies in environment throughout the world. Despite this, available studies on PFASs in Korean and Vietnamese freshwater ecosystems are limited. The overall aim and objective of this study are to determine and support the baseline contamination extent and bioconcentration of PFASs in both abiotic and biotic matrices involved in freshwater aquatic environment of Korea and Vietnam. The ecological and human health potential risk due to expose of PFASs in the environmental matrices were evaluated. The target chemicals analyzed in this study are 13 non-polymer and ionic PFASs including nine perfluorocarboxylic acid (PFCAs; C4 and C6-C14) and four perfluoroalkyl sulfonic acid (PFSAs; C4, C6, C8 and C10). Water (n = 55), sediment (n = 55) and tissues (n = 466) of blood, liver and whole body of various freshwater fishes including crucian carp (Carassius auratus), catfish (Silurus asotus), common carp (Cyprinus carpio), northern snake head (Channa argus), skygager (Erythroculter erythropterus), Korean piscivourous chub (Opsariichthys uncirostris), barbell steed (Hemibarbus labeo), blue gill (Lepomis macrochirus), bass (Micropterus salmoides), mandarin fish (Siniperca scherzeri), and medaka fish (Oryzias latipes) were collected from thirty sampling sites located in six major rivers and lakes (Namhan River, Bukhan River, Nakdong River, Nam River, Yeongsan River and Sangsa Lake) and canals in nine rice field regions (Yeosu, Gwangyang, Hadong, Sacheon, Naju, Gimhae, Andong, Kunsan, Dangjin) in Korea from 2012 to 2014 were analyzed for the occurrence of PFASs. Furthermore, the spatial distribution of PFAS concentration in coastal water from forty two sampling sites in the southeastern coastal areas of Korea including Ulsan, Busan and Gwangyang Bays was investigated. PFOS and perfluorooctanoic acid (PFOA) were consistently found as predominant PFASs in waters and sediments. The greatest PFASs concentration of these compounds were 15.1 ng PFOS/L and 42.2 ng PFOA/L for waters and 1.09 ng PFOS/ g wet weight (ww) and 1.82 ng PFOA/g ww for sediments. The greatest concentrations of PFOS and PFOA in water samples were found in sites located downstream of sewage water treatment plants (WWTPs). PFOS (up to 365 ng/ml) and long chain PFCAs (LC-PFCAs) including perfluorodecanoic acid (PFDA; up to 261 ng/g wet weight), perfluoroundecanoic acid (PFUnDA; up to 141 ng/ml) and perfluorododecanoic acid (PFDoDA; up to 158 ng/ml) were predominant PFASs which accounted for 81.2% ± 16.4% of the total PFASs concentration in fish tissues analyzed. The mean ratios of PFAS concentration between crucian carp blood and liver were above 2 suggesting higher levels in blood than in liver of this fish. Significant positive correlations (p < 0.001) were observed between PFOS, PFDA, PFUnDA concentrations in blood and liver tissues of crucian carp. This result suggests that blood can be used for nonlethal monitoring of these compounds in this fish. A general ascending order of mean total PFASs concentration of water (0.013 ng/ml) < sediment (0.44 ng/g ww) < bass liver (0.64 ng/g ww) < northern snake head liver (9.71 ng/g ww) < common carp liver (10.1 ng/g ww) < blue gill liver (14.7 ng/g ww) < catfish liver (16.4 ng/g ww) < medaka fish whole body (22.7 ng/g ww) < crucian carp liver (24.5 ng/g ww) < barbel steed liver (50.9 ng/g ww) < mandarin fish liver (51.5 ng/ g ww) < crucian carp blood (83.7 ng/ml) < skygager liver (169 ng/g ww) < Korean piscivourous chub liver (351 ng/g ww). Except for the Nakdong River and Yeongsan River, the total PFAS concentrations in water samples collected in rivers and lakes from Korea were below 10 ng/L. No potential chronic and/or acute effects on aquatic organisms due to PFOS and PFOA levels measured in waters were expected. Potential risk of PFOS and PFOA exposure in sediment to benthic organisms and the immediate risk of PFOS exposure to fish were not likely. The interim Korea water quality criteria for PFOA (23.3 ng/L) and PFOS (34.0 ng/L) were calculated. Among individual PFASs, only PFOS was found to bioaccumulate to all of twelve fish tissues analyzed in this study. The mean bioconcentration factor (BCFs) of PFOS (concentration in biota/concentration in water) was as follows: bass liver (63 L/kg) < common carp liver (7,664 L/kg) < northern snake head liver (7,952 L/kg) < catfish liver (8,511 L/kg) < medada whole body (8,874 L/kg) < blue gill liver (9,245 L/kg) < crucian carp liver (11,577 L/kg) < Korean piscivorous chub liver (16,948 L/kg) < crucian carp blood (31,938 L/kg) < skygager liver (31,949 L/kg) < mandarin fish liver (34,788 L/kg) < barbel steed (58,086 L/kg). Although PFOS concentrations in water samples were comparable with those of PFOA, the BCFs of PFOA in fish tissues were hundred-fold less than those of PFOS. Similar to PFOS, LC-PFCAs including PFDA, PFUnDA, PFDoDA and PFTrDA were very bioaccumulative in most of fish tissues. Different PFAS composition patterns in fish tissues suggest species-specific and tissue-specific bioaccumulation. Only PFDA and PFOS were consistently found to be very bioaccumulative in medaka fish whole body tissues in 2012 – 2014. The overall trend of PFOS contamination in water is comparable to the trend of that found in whole body of medaka fish and significant positive correlation between PFOS concentration in water and medaka fish whole body (p = 0.014) was found. Medaka fish was suggested as a good bioindicator for monitoring PFOS in water. Despite the relatively consistent of PFASs concentration in environmental matrices, there is a continuous accumulating trend of PFASs in tissues of aquatic organisms in Korea whether they exposure to point sources (crucian carp) or non point sources (medaka fish) of PFASs discharging into their habitat. The profiles of occurrence and spatial distribution of PFASs in various environmental media suggest the existence of several sources of PFASs and the continuing input PFASs in Korean aquatic environment. Water (n = 36), sediment (n = 10) and biota tissues (n = 162) of liver, whole body, muscle and soft tissues from five fish species including stripped snake head (Chana striata), dusky sleeper (Eleotris fusca), shark catfish (Pangasius elongatus), flying barb (Esomus danricus), tilapia (Oreochrommic niloticus), two crustacean species including giant prawn (Macrobranchium rosengbergii) and freshwater paddle crab (Varuna litterata), one gastropod species of golden applesnail (Pomacea canaliculata) and one bivalve species of golden freshwater clam (Corbicula fluminea) were collected from 36 sampling sites in major river basins of Vietnam including the Sai Gon, Red, Ma, Da Rang, Da Nong, Cai, Quan Truong, Dong Nai and Mekong Rivers from 2013 to 2015 and analyzed for the presence of PFASs. Occurrence of PFASs in canal water in rice field areas (n = 3) coastal water (n = 2), tap water (n = 12) and well water (n = 8) collected in eight different regions in Vietnam was investigated. Water (n = 8) and sediment (n = 6) samples collected in the discharge canal of wastewater treatment plants (WWTPs) and landfills were analyzed to identify sources of pollution. In Vietnam, the PFASs concentration determined in waters, sediment and biota collected in major rivers were relatively low in a comparison with those previously reported in other countries including Korea. Overall, the results indicate a general ascending order of mean PFASs concentration in river ecosystem consisting water (1.79 ng/L), sediment (0.21 ng/g ww), gastropod tissues (0.33 ng/g ww), crustacean tissues (0.44 ng/g ww), bivalve tissues (0.73 ng/g ww) and fish tissues (2.81 ng/g ww). PFOS and PFOA were predominant PFASs in surface water with the greatest concentrations of 53.5 ng/L and 40.2 ng/L found in a surface water sample collected from a channel that receives wastewater treatment plant discharge, respectively. Except for the Sai Gon River, the total PFAS concentrations in water samples collected from the Red, Ma, Da Rang, Da Nong, Cai, Quan Truong, Dong Nai and Mekong Rivers from were less than 5 ng/L. PFOS and perfluorohexane sulfonic acid (PFHxS) were found as predominant PFASs in sediment. The greatest PFAS level in biota tissues was found as 16.9 ng PFUnDA/ g ww for a fish liver. Some long-chain PFCAs including perfluorononanoic acid (PFNA), PFUnDA and perfluorotridecanoic acid (PFTrDA) as well as PFHxS were more abundant than other compounds in biota tissues. Similar to profile of PFASs distribution in Korea aquatic ecosystems, different PFAS composition patterns in biota tissues suggest compound-, species-, and tissues-specific bioaccumulation. No potential chronic and/or acute effects on aquatic organisms due to exposure to PFOS and PFOA levels measured in surface water, and human health risk due to consuming tap and well water as drinking water were expected. The interim Vietnam water quality criteria for PFOA (9.32 ng/L) and PFOS (2.96 ng/L) were calculated. The rank order of mean BCF of PFOS in biological groups was; crustacean (115 L/kg), gastropod (1,117 L/kg), fish (1,120 L/kg) and bivalve (2,110 L/kg). WWTP discharge is probably the “point source” of aqueous discharges of PFASs into the aquatic environment. The profiles of occurrence and spatial distribution of PFASs in various environmental media in this study provide baseline information for a better understanding of the PFAS contamination characteristic in Vietnam. Although the relatively greater contamination levels of PFASs in Korea than those in Vietnam were found in the present study, it is difficult to explain this. Industrialization degree and economic growth gap may be attributed to this difference but the relationship between these factors and environmental pollution is complex and multi-dimensional. The relatively greater concentration of PFASs in environmental matrices from Korea than those from Vietnam in the present studies is consistent with previous studies and represents the historical and ongoing deposition of PFASs in Korean and Vietnamese aquatic environment. Further studies should focus on identifying the existence and status of PFASs sources in Korean freshwater ecosystems and more studies with greater numbers of water and aquatic organism samples are needed for accurate the sources and pathways of exposures, and bioaccumulation of PFASs in aquatic environment in Vietnam. It’s necessary to evaluate the combined potential adverse effect of multiple PFASs on aquatic organisms, especially long chain PFASs.