Jeiwan Tan

A new class of chiral semiconductors: chiral-organic-molecule-incorporating organic–inorganic hybrid perovskites

An organic–inorganic hybrid perovskite incorporating chiral organic molecules is demonstrated as a new class of chiral semiconductors. Chiral perovskites exhibit oppositely-signed circular dichroism (CD) according to the S- and R-configurations of chiral organics. The CD signals can be also varied by changing the crystalline orientation and thickness of the chiral perovskite films.

Photoelectrodes based on 2D opals assembled from Cu-delafossite double-shelled microspheres for an enhanced photoelectrochemical response

Although a unique light-harvesting property was recently demonstrated in a photocathode based on 2-dimensional (2D) opals of CuFeO2-shelled SiO2 microspheres, the performance of a monolayer of ultra-thin CuFeO2-shelled microspheres is limited by ineffective charge separation. Herein, we propose an innovative design rule, in which an inner CuFeO2/outer CuAlO2 double-shelled heterojunction is formed on each partially etched microsphere to obtain a hexagonally assembled 2D opal photoelectrode. Our Cu-delafossite double-shelled photocathode shows a dramatically improved charge separation capability, with a 9-fold increase in the photocurrent compared to that of the single-shelled counterpart. Electrochemical impedance spectroscopy clearly confirms the reduced charge transport/transfer resistance associated with the Cu-delafossite double-shelled photocathode, while surface photovoltage spectra reveal enhanced polarization of the photogenerated carrier, indicating improved charge separation capability with the aid of the heterojunction. Our finding sheds light on the importance of heterojunction interfaces in achieving optimal charge separation in opal architectures as well as the inner-shell/electrolyte interface to expedite charge separation/transport.

Controlled Electrodeposition of Photoelectrochemically Active Amorphous MoSx Cocatalyst on Sb2Se3 Photocathode

Amorphous molybdenum sulfide (a-MoS x) is a promising hydrogen evolution catalyst owing to its low cost and high activity. A simple electrodeposition method (cyclic voltammetry) allows uniform formation of a-MoS x films on conductive surfaces. However, the morphology of a-MoS x deposited on a TiO2/Sb2Se3 photocathode could be modulated by varying the starting potential. The cathodically initiated a-MoS x showed conformal filmlike morphology, whereas anodic initiation induced inhomogeneous particulate deposition. The filmlike morphology of a-MoS x was subjected to catalyst activation, which improved the photocurrent density and reduced the charge-transfer resistance at the semiconductor/electrolyte interface, as compared to that of its particulate counterpart. X-ray photoelectron spectroscopy confirmed that different chemical states of a-MoS x (photoelectrochemically active sites) were developed on the basis of the electrodeposited a-MoS x morphology. The research provides an effective approach for uniformly depositing cost-effective a-MoS x on nanostructured photoelectrodes, for photoelectrochemical water splitting.

Thermally driven in situ exsolution of Ni nanoparticles from (Ni, Gd)CeO2 for high-performance solid oxide fuel cells

In situ exsolution of metal nanoparticles affords a high content and uniform distribution of metal nanocatalysts without complex synthetic processes. To implement this strategy in practical electrodes for solid oxide fuel cells, understanding the exsolution process in terms of synthesis temperature and atmosphere is a prerequisite. Herein, we demonstrate that Ni:Gd co-doped ceria (GNDC) can be effectively used as an in situ exsolution system, from which substitutionally doped Ni is thermally exsolved as NiO nanoparticles strongly attached to the surface of GNDC, the host oxide, and subsequently reduced to a Ni nanocatalyst under anodic operation conditions. The exsolution procedures were characterized by X-ray diffraction, Raman spectroscopy, and transmission electron spectroscopy, which revealed that the evolution of Ni nanoparticles could be solely controlled by thermal treatment. The thermally exsolved Ni nanocatalyst from the 5 mol% Ni-doped GNDC electrode exhibits a polarization resistance comparable to that of the mechanically mixed Ni-GDC composite electrode, with a significant increase in the calculated triple phase boundary density despite having a low Ni volume fraction. By employing the GNDC layer as a functional layer of an anode-supported SOFC, we demonstrated the utilization of the thermally exsolved Ni nanocatalyst combined with fluorite-structured doped-ceria as an electrode of SOFCs at low temperature.

Time-Resolved Observations of Photo-Generated Charge Carrier Dynamics in Sb2Se3 Photocathodes for Photoelectrochemical Water Splitting

Solar-energy conversion by photoelectrochemical (PEC) devices is driven by the separation and transfer of photogenerated charge carriers. Thus, understanding carrier dynamics in a PEC device is essential to realizing efficient solar-energy conversion. Here, we investigate time-resolved carrier dynamics in emerging low-cost Sb2Se3 nanostructure photocathodes for PEC water splitting. Using terahertz spectroscopy, we observed an initial mobility loss within tens of picoseconds due to carrier localization and attributed the origin of carrier localization to the rich surface of Sb2Se3 nanostructures. In addition, a possible recombination at the interface between Sb2Se3 and the back contact is elucidated by time-resolved photoluminescence analysis. We also demonstrated the dual role of the RuO x co-catalyst in reducing surface recombination and enhancing charge transfer in full devices using intensity-modulated spectroscopy. The relatively low onset potential of the Sb2Se3 photocathode is attributed to the sluggish charge transfer at a low applied bias rather than to fast surface recombination. We believe that our insights on carrier dynamics would be an important step toward achieving highly efficient Sb2Se3 photocathodes.

Chemically Driven Enhancement of Oxygen Reduction Electrocatalysis in Supported Perovskite Oxides

Perovskite oxides have the capacity to efficiently catalyze the oxygen reduction reaction (ORR), which is of fundamental importance for electrochemical energy conversion. While the perovskite catalysts have been generally utilized with a support, the role of the supports, regarded as inert toward the ORR, has been emphasized mostly in terms of the thermal stability of the catalyst system and as an ancillary transport channel for oxygen ions during the ORR. We demonstrate a novel approach to improving the catalytic activity of perovskite oxides for solid oxide fuel cells by controlling the oxygen-ion conducting oxide supports. Catalytic activities of (La0.8Sr0.2)0.95MnO3 perovskite thin-film placed on different oxide supports are characterized by electrochemical impedance spectroscopy and X-ray absorption spectroscopy. These analyses confirm that the strong atomic orbital interactions between the support and the perovskite catalyst enhance the surface exchange kinetics by ∼2.4 times, in turn, improving the overall ORR activity.