서강대학교

초록/요약

Subject I. Studies on Photocatalytic Semiconductors for Solar Water Oxidation Chapter 1. Photochemical Properties of Cu Doped Hematite. The photochemical properties of Cu doped hematite nanocrystal were in-vestigated intensively. The Cu doped hematite nanocrystals were prepared by hy-drothermal method, changing the molar ratio of Cu precursors. The XRD and XPS techniques are used for revealing crystal and chemical state of Cu doped hematite nanocrystal. Raman spectroscopy was also used for confirming Cu atoms replacing Fe position in Cu doped hematite crystal. The UV-vis and UPS were used for as-signing electronic band position for photocatalytic properties. Cu doped hematite showed the enhanced photocatalytic properties with photodegradation of methyl orange. Finally, by checking magnetic hysteresis loops of Cu doped hematites with VSM, it was revealed that the magnetic property of Cu doped hematite nanocrystal was increased after doping Cu into hematite nanocrystal, get the dis-tortion of magnetic sub-lattices. Chapter 2. Enhanced Photoelectrochemical Water Splitting Activity of Hematite by Inserting Ultrathin Insulating Under-layer. To improve the photoelectric properties of hematite as a photoanode for solar water splitting, a SiO2 layer has been inserted at the junction between the FTO substrate and Ti-doped hematite film. Hematite films with junction treatment showed promising photoelectrochemical properties with maximum photocurrent with 0.75mA/cm2 at 1.23V vs. RHE under one sun condition. This is due to in-serted SiO2 layer inducing diffusion of Si atom from SiO2 layer to Ti-hematite thin film. Thus, this work reports a new direction for the junction control between hematite film and FTO substrate. Subject II. Studies on Materials for High Efficient CO2 Reduction with Product Selectivity. Chapter 3. Role of the Nafion Functional Layer on Metal-Free Photoelec-trochemical CO2 Reduction Reaction System with High Liquid Product Selectivity In this study, the simple and metal-free photoelectrochemical CO2 reduction sys-tem of (040) oriented BiVO4 of photoanode with Nafion functional layer coated TiO2 cathode (Nafion/TiO2) produced methanol as exclusive product. The func-tional layer of Nafion coated on the TiO2 electrode for the efficient proton transport could suppress the dimerization of mono-carbon radical intermediates for high selective CO2 reduction into methanol through the one-pot reaction. The pho-toelectrochemical CO2 reduction system of (040) oriented BiVO4 of photoanode and TiO2 cathode produced an ethanol as the primary product with solar to fuel efficiency including methanol as by-product with solar to liquid fuel efficiency of 0.82 %. However, high selective liquid product production through CO2 reduction into methanol with 57.3 % of faradaic efficiency was achieved by deposition of Nafion functional layer on TiO2, with hydrogen as the by-product. The liquid product selectivity for methanol was increased after the functional Nafion layer deposition onto TiO2 cathode surface. The specified mechanism of CO2 reduction was investigated by in-situ XAFS measurement on the adsorption and reduction process of CO2 molecule on the cathode surface and low temperature EPR meas-urement on the intermediate radical detection. It has been proved that the Nafion functional layer contributed on enhanced proton transport, which has been checked with the increased higher current value of Nafion/TiO2 cathode than that of pristine TiO2 cathode. This resulted in the suppression of dimerization of mono carbon radical intermediates to have the exclusive methanol production on photoe-lectrochemical CO2 reduction. The present results represent the strategy on high product selectivity on solar fuel production via metal-free artificial photosynthesis system of CO2 reduction reaction by introducing novel approach of reduction po-tential tuning with external bias potential and conduction band of cathode, in ad-dition to the functional Nafion layer coated on the cathode. Chapter 4. Reduction Potential Tuning on Liquid Product Selectivity for CO2 Reduction of Artificial Photosynthesis Reaction Here, Simple metal-free photoelectrochemical CO2 reduction system with thermodynamic reduction potential tuning and reaction kinetics control during CO2 reduction reaction were introduced for the selective reduction products with high efficiency. A thermodynamic reduction potential control was achieved by controlling band energy level of reduced graphene oxide (rGO) by difference reduction degree of graphene oxide (GO) on TiO2 film. Different reaction kinetics during solar fuel production was investigated by time ressolved photoluminescence spectroscopy (TR-PL) based on the different reduction dgree of rGO. The (040) crystal facet engineered BiVO4 (040-BVO)│KHCO3│rGO/TiO2 system was illuminated with solar light with applied external bias potential tuning from -0.4 V to -1.2 V (vs. NHE) for CO2 reduction reaction. The applied external bias potential integrated with fermi energy level of rGO on the cathode surface of TiO2 film showed the noticable production of formaldehyde by CO2 reduction reaction. The difference on reaction kinetics by different degree of rGO reduction showed an enhanced reaction rate during CO2 reduction reaction. Based on the results, TiO2 cathode coated with rGO obtained by 0.5 hr reduction reaction showed the optimum thermodynamic potential for formaldehyde production and the highest electron transfer kinetics for ethanol formation during CO2 reduction reaction. The electron paramagnetic resonance spectroscopy (EPR) showed the intermediate CO2 anion radical (CO2-.) during CO2 reduction and 13C labeled CO2 gas chromatography with mass spectrometer (GC-MS) study proved that produced solar fuels were not originated from rGO and other carbon sources except purged CO2 gas. The correlation between solar fuel production and CO2 reduction potential tuning was studied with Ti-O bond distance change during photoelectrochemical CO2 reduction reaction by in-situ synchrotron X-ray absorption study. We suggest that the presented results represent the viable strategy to get high product selectivity and high efficiency of solar fuel production by controlling thermodynamical reduction potential and reaction kinetics during solar fuel production reaction.   Subject III. Constructing Artificial Photosynthesis System for Green Utilization of Solar Energy. Chapter 5. Facile Approaches on Photochemical CO2 reduction with Flexi-ble Multi-Layered Membrane Catalysts. Simple and facile approach for artificial photosynthesis with flexible multi lay-ered membrane catalysts is suggested. The g-C3N4 and BiVO4 particle were syn-thesized by self-condensation and hydrothermal method. g-C3N4 membrane cata-lyst and g-C3N4/BiVO4 layered membrane catalyst were fabricated by casting and shaping of Nafion polymer mixture. XRD, FT-IR and XPS analysis proved that the intrinsic properties of g-C3N4 and BiVO4 were maintained after fabricating flexible membrane catalyst. The internal connection of g-C3N4 and BiVO4 parti-cles in flexible membrane catalyst for smooth transport of photogenerated electron was revealed by TEM and photoelectrochemical analysis. Finally, photochemical CO2 reduction reaction was performed with flexible membrane catalysts. The g-C3N4 membrane catalysts produced 147 μM of ethanol during 12 hrs. of CO2 re-duction reaction while the g-C3N4/BiVO4 layered membrane catalysts produced 256 μM of ethanol during 12 hrs. of CO2 reduction reaction. This is due to the higher solar light harvesting and fast hole-charge separation from layered BiVO4 membrane catalysts leaded higher electron transport rate to g-C3N4 membrane cat-alysts, promoting the CO2 reduction reaction on the surface of g-C3N4 membrane catalyst.