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Park, Jin Woo,Mahadik, Mahadeo A.,An, Gil Woo,Lee, Su Yong,Piao, Guangxia,Choi, Sun Hee,Chae, Weon-Sik,Chung, Hee-Suk,Park, Hyunwoong,Jang, Jum Suk Elsevier 2018 Solar energy materials and solar cells Vol.187 No.-
<P><B>Abstract</B></P> <P>In present work, we synthesized ZnFe<SUB>2</SUB>O<SUB>4</SUB> nanorods on a fluorine-doped tin oxide substrate using spray coating method followed by two-step high-temperature quenching (HTQ). X-ray photoelectron spectroscopy (XPS) results indicate that Sn<SUP>4+</SUP> is diffused from the FTO substrate after the second quenching, which could help in minimizing the recombination of photogenerated carriers. Photoelectrochemical measurements of the ZnFe<SUB>2</SUB>O<SUB>4</SUB> nanorod photoelectrodes quenched at 780 °C, 800 °C, and 820 °C indicate that among the studied samples (ZFO1, ZFO2 and ZFO3), the highest photocurrent density was observed for nanotextured ZFO3 photoelectrodes (130 µA cm<SUP>−2</SUP> at 1.23 V vs RHE). The photoelectrochemical performances of the ZnFe<SUB>2</SUB>O<SUB>4</SUB> nanorods after the second quenching were compared with those of the firstly quenched ZnFe<SUB>2</SUB>O<SUB>4</SUB> nanorod samples; water-oxidation photocurrent density of the former (ZFO3) was increased by 6.9 times compared with that of the first quenching (PZFO). Intensity modulated photocurrent spectroscopy (IMPS) and photoluminescence (PL) results confirm the faster charge extraction was achieved for the ZFO3 photoelectrode. Thus, the overall photocurrent density during the second quenching process results from the effectively improved crystallinity, the reduced strain and suppressed charge–carrier recombination's on both the surface as well as in the bulk of the ZnFe<SUB>2</SUB>O<SUB>4</SUB> nanorods. In terms of solar water splitting, these research findings provide an effective route for the synthesis of other nanostructures.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Highly efficient ZnFe<SUB>2</SUB>O<SUB>4</SUB> photoanode was synthesized by spray pyrolysis and two step quenching. </LI> <LI> First quenching process transforms the Zn-sprayed β-FeOOH nanorods to ZnFe<SUB>2</SUB>O<SUB>4</SUB>/ZnO. </LI> <LI> Second quenching improves the Sn<SUP>4+</SUP> diffusion and reduces the charge recombination's. </LI> <LI> The charge-transfer mechanism is well proposed. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Park, Hyunwoong,Ou, Hsin-Hung,Colussi, Agustí,n J.,Hoffmann, Michael R. American Chemical Society 2015 The journal of physical chemistry. A, Molecules, s Vol.119 No.19
<P>The conversion of CO2 and water into value-added fuels with visible light is difficult to achieve in inorganic photocatalytic systems. However, we synthesized a ternary catalyst, CdS/(Cu-TNTs), which is assembled on a core of sodium trititanate nanotubes (TNTs; NaxH2xTi3O7) decorated with elemental copper deposits followed by an overcoat of CdS quantum dot deposits. This ternary photocatalyst is capable of catalyzing the conversion of CO2 and water into C1C3 hydrocarbons (e.g., CH4, C2H6, C3H8, C2H4, C3H6) upon irradiation with visible light above 420 nm. With this composite photocatalyst, sacrificial electron donors are not required for the photoreduction of CO2. We have shown that water is the principal photoexcited-state electron donor, while CO2 bound to the composite surface serves as the corresponding electron acceptor. If the photochemical reaction is carried out under an atmosphere of 99.9% (CO2)-C-13, then the product hydrocarbons are built upon a C-13 backbone. However, free molecular H2 is not observed over 5 h of visible light irradiation even though proton reduction in aqueous solution is thermodynamically favored over CO2 reduction. In terms of photocatalytic efficiency, the stoichiometric fraction of Na+ in TNTs appears to be an important factor that influences the formation of the observed hydrocarbons. The coordination of CO2 to surface exchange sites on the ternary catalyst leads to the formation of surface-bound CO2 and related carbonate species. It appears that the bidentate binding of O=C=O to certain reactive surface sites reduces the energy barrier for conduction band electron transfer to CO2. The methyl radical (CH3), an observed intermediate in the reaction, was positively identified using an ESR spin trapping probe molecule. The copper deposits on the surface of TNTs appear to play a major role in the transient trapping of methyl radical, which in turn self-reacts to produce ethane.</P>
Photoinduced charge transfer processes in solar photocatalysis based on modified TiO<sub>2</sub>
Park, Hyunwoong,Kim, Hyoung-il,Moon, Gun-hee,Choi, Wonyong The Royal Society of Chemistry 2016 ENERGY AND ENVIRONMENTAL SCIENCE Vol.9 No.2
<P>High efficiency solar photocatalysis requires an effective separation of photogenerated charge carriers and their rapid transport to the semiconductor interface. The mechanisms and kinetics of charge separation and interfacial/ interparticle charge transfers (CT) are significantly influenced by both the bulk and surface properties of the semiconductor. The surface properties are particularly important because the photocatalysis should be driven by the interfacial CT. The most popular and the most investigated semiconductor photocatalyst is based on bare and modified TiO2. This article highlights the interfacial and interparticle CTs under the bandgap excitation of TiO2 particles, visible light-induced photochemical processes via either dye-sensitization or ligand-to-metal CTs at surface modified TiO2 particles, and the applications of the photo-processes to pollutant degradation and simultaneous hydrogen production. While a variety of surface modification techniques using various nanomaterials and chemical reagents have been developed and tested so far, their effects are very diverse depending on the characteristics of the applied photocatalytic systems and even contradictory in some cases. Better understanding of how the modification influences the photoinduced CT events in semiconductors is required, particularly for designing hybrid photocatalysts with controlled CTs, which is sought-after for practical applications of photocatalysis.</P>