Jun Young Cheong

Nanoscale PdO Catalyst Functionalized Co3O4 Hollow Nanocages Using MOF Templates for Selective Detection of Acetone Molecules in Exhaled Breath

The increase of surface area and the functionalization of catalyst are crucial to development of high-performance semiconductor metal oxide (SMO) based chemiresistive gas sensors. Herein, nanoscale catalyst loaded Co3O4 hollow nanocages (HNCs) by using metal-organic framework (MOF) templates have been developed as a new sensing platform. Nanoscale Pd nanoparticles (NPs) were easily loaded on the cavity of Co based zeolite imidazole framework (ZIF-67). The porous structure of ZIF-67 can restrict the size of Pd NPs (2-3 nm) and separate Pd NPs from each other. Subsequently, the calcination of Pd loaded ZIF-67 produced the catalytic PdO NPs functionalized Co3O4 HNCs (PdO-Co3O4 HNCs). The ultrasmall PdO NPs (3-4 nm) are well-distributed in the wall of Co3O4 HNCs, the unique structure of which can provide high surface area and high catalytic activity. As a result, the PdO-Co3O4 HNCs exhibited improved acetone sensing response (Rgas/Rair = 2.51-5 ppm) compared to PdO-Co3O4 powders (Rgas/Rair = 1.98), Co3O4 HNCs (Rgas/Rair = 1.96), and Co3O4 powders (Rgas/Rair = 1.45). In addition, the PdO-Co3O4 HNCs showed high acetone selectivity against other interfering gases. Moreover, the sensor array clearly distinguished simulated exhaled breath of diabetics from healthy peoples breath. These results confirmed the novel synthesis of MOF templated nanoscale catalyst loaded SMO HNCs for high performance gas sensors.

Rational design of Sn-based multicomponent anodes for high performance lithium-ion batteries: SnO2@TiO2@reduced graphene oxide nanotubes

Ultra thin TiO2 layer (2 nm)-coated SnO2 nanotubes (NTs) wrapped by reduced graphene oxide (rGO) sheets (SnO2@TiO2@rGO) were synthesized as high capacity anode materials for lithium-ion batteries. The rationally designed anodes exhibited superior rate capability while maintaining a high discharge capacity of over 840 mA h g−1 at a current density of 500 mA g−1 after 50 cycles.

Formation of a Surficial Bifunctional Nanolayer on Nb2O5 for Ultrastable Electrodes for Lithium-Ion Battery

Safe and long cycle life electrode materials for lithium-ion batteries are significantly important to meet the increasing demands of rechargeable batteries. Niobium pentoxide (Nb2 O5 ) is one of the highly promising candidates for stable electrodes due to its safety and minimal volume expansion. Nevertheless, pulverization and low conductivity of Nb2 O5 have remained as inherent challenges for its practical use as viable electrodes. A highly facile method is proposed to improve the overall cycle retention of Nb2 O5 microparticles by ammonia (NH3 ) gas-driven nitridation. After nitridation, an ultrathin surficial layer (2 nm) is formed on the Nb2 O5 , acting as a bifunctional nanolayer that allows facile lithium (Li)-ion transport (10-100 times higher Li diffusivity compared with pristine Nb2 O5 microparticles) and further prevents the pulverization of Nb2 O5 . With the subsequent decoration of silver (Ag) nanoparticles (NPs), the low electric conductivity of nitridated Nb2 O5 is also significantly improved. Cycle retention is greatly improved for nitridated Nb2 O5 (96.7%) compared with Nb2 O5 (64.7%) for 500 cycles. Ag-decorated, nitridated Nb2 O5 microparticles and nitridated Nb2 O5 microparticles exhibit ultrastable cycling for 3000 cycles at high current density (3000 mA g-1 ), which highlights the importance of the surficial nanolayer in improving overall electrochemical performances, in addition to conductive NPs.

Brush-Like Cobalt Nitride Anchored Carbon Nanofiber Membrane: Current Collector-Catalyst Integrated Cathode for Long Cycle Li–O2 Batteries

To achieve a high reversibility and long cycle life for lithium-oxygen (Li-O2) batteries, the irreversible formation of Li2O2, inevitable side reactions, and poor charge transport at the cathode interfaces should be overcome. Here, we report a rational design of air cathode using a cobalt nitride (Co4N) functionalized carbon nanofiber (CNF) membrane as current collector-catalyst integrated air cathode. Brush-like Co4N nanorods are uniformly anchored on conductive electrospun CNF papers via hydrothermal growth of Co(OH)F nanorods followed by nitridation step. Co4N-decorated CNF (Co4N/CNF) cathode exhibited excellent electrochemical performance with outstanding stability for over 177 cycles in Li-O2 cells. During cycling, metallic Co4N nanorods provide sufficient accessible reaction sites as well as facile electron transport pathway throughout the continuously networked CNF. Furthermore, thin oxide layer (<10 nm) formed on the surface of Co4N nanorods promote reversible formation/decomposition of film-type Li2O2, leading to significant reduction in overpotential gap (∼1.23 V at 700 mAh g-1). Moreover, pouch-type Li-air cells using Co4N/CNF cathode stably operated in real air atmosphere even under 180° bending. The results demonstrate that the favorable formation/decomposition of reaction products and mediation of side reactions are hugely governed by the suitable surface chemistry and tailored structure of cathode materials, which are essential for real Li-air battery applications.

Expanding depletion region via doping: Zn-doped Cu2O buffer layer in Cu2O photocathodes for photoelectrochemical water splitting

We report photoelectrochemical hydrogen evolution reaction using a Cu2O-based photocathode with a layer doped with Zn ions. The doping results in the shift of the onset flat-band potential of the photocathode, likely a consequence of maximized band-bending in the Cu2O/Zn : Cu2O heterojunction. Systematic electrochemical analysis reveals that expansion of depletion region is responsible for the enhanced photoelectrochemical performance, e.g., the increase of photocurrent and reduced internal resistance.

MOF derived ZnCo2O4 porous hollow spheres functionalized with Ag nanoparticles for a long-cycle and high-capacity lithium ion battery anode

Huge volume expansion during discharge is always a critical problem of high capacity conversion anodes for next generation lithium-ion (Li-ion) batteries. Although extensive efforts have been devoted to controlling the volume expansion using nanostructuring and surface engineering till now, more simple and facile approaches have to be considered due to the complicated and inefficient synthetic methods. Here, we report a straightforward synthesis of Ag coated ZnCo2O4 porous hollow spheres (ZnCo2O4@Ag HSs): (i) immobilization of metal–organic frameworks (MOFs) including Zn and Co metal nodes onto polystyrene sphere templates, (ii) calcination (∼450 °C) for the removal of core polystyrene sphere templates and oxidation of MOFs to produce a mesoporous ZnCo2O4 HSs, and (iii) a subsequent Ag-mirror reaction for 10 min, resulting in the formation of ZnCo2O4@Ag HSs. This porous hollow morphology not only effectively relieves the strain stemming from the volume expansion of transition metals, but also facilitates the efficient electron transport for Li+ diffusion by shortening the Li-ion diffusion path during a lithiation/delithiation process. Moreover, uniformly decorated Ag nanoparticles are beneficial to the formation of a stable solid electrolyte interface (SEI) layer as well as an increased electrical conductivity of ZnCo2O4. The MOF derived porous ZnCo2O4@Ag HSs exhibited remarkably stable cycling performance (a capacity value of 616 mA h g−1 after 900 cycles at a current density of 1 A g−1) and an excellent capacity retention of 80% at a very high current density of 20.0 A g−1.

Elaborate Manipulation for Sub-10 nm Hollow Catalyst Sensitized Heterogeneous Oxide Nanofibers for Room Temperature Chemical Sensors

Room-temperature (RT) operation sensors are constantly in increasing demand because of their low power consumption, simple operation, and long lifetime. However, critical challenges such as low sensing performance, vulnerability under highly humid state, and poor recyclability hinder their commercialization. In this work, sub-10 nm hollow, bimetallic Pt-Ag nanoparticles (NPs) were successfully formed by galvanic replacement reaction in bioinspired hollow protein templates and sensitized on the multidimensional SnO2-WO3 heterojunction nanofibers (HNFs). Formation of hollow, bimetallic NPs resulted in the double-side catalytic effect, rendering both surface and inner side chemical reactions. Subsequently, SnO2-WO3 HNFs were synthesized by incorporating 2D WO3 nanosheets (NSs) with 0D SnO2 sphere by c-axis growth inhibition effect and fluid dynamics of liquid Sn during calcination. Hierarchically assembled HNFs effectively modulate surface depletion layer of 2D WO3 NSs by electron transfers from WO3 to SnO2 stemming from creation of heterojunction. Careful combination of bimetallic catalyst NPs with HNFs provided an extreme recyclability under exhaled breath (95 RH%) with outstanding H2S sensitivity. Such sensing platform clearly distinguished between the breath of healthy people and simulated halitosis patients.

Feasible Defect Engineering by Employing Metal Organic Framework Templates into One-Dimensional Metal Oxides for Battery Applications

Facile synthesis of rationally designed nanostructured electrode materials with high reversible capacity is highly critical to meet ever-increasing demands for lithium-ion batteries. In this work, we employed defect engineering by incorporating metal organic framework (MOF) templates into one-dimensional nanostructures by simple electrospinning and subsequent calcination. The introduction of Co-based zeolite imidazole frameworks (ZIF-67) resulted in abundant oxygen vacancies, which induce not only more active sites for Li storage but also enhanced electrical conductivity. Moreover, abundant mesoporous sites are formed by the decomposition of ZIF-67, which are present both inside and outside the resultant SnO2-Co3O4 nanofibers (NFs). Attributed to the creation of vacancy sites along with the synergistic effects of SnO2 and Co3O4, SnO2-Co3O4 NFs exhibit an excellent reversible capacity for 300 cycles (1287 mA h g-1 at a current density of 500 mA g-1) along with superior rate capabilities and improved initial Coulombic efficiency compared with pristine SnO2 NFs. This is an early report on utilizing MOF structures as the defect formation platform into one-dimensional nanostructures, which is expected to result in superior electrochemical performances required for advanced electrodes.

In Situ High-Resolution Transmission Electron Microscopy (TEM) Observation of Sn Nanoparticles on SnO2 Nanotubes Under Lithiation

We trace Sn nanoparticles (NPs) produced from SnO2 nanotubes (NTs) during lithiation initialized by high energy e-beam irradiation. The growth dynamics of Sn NPs is visualized in liquid electrolytes by graphene liquid cell transmission electron microscopy. The observation reveals that Sn NPs grow on the surface of SnO2 NTs via coalescence and the final shape of agglomerated NPs is governed by surface energy of the Sn NPs and the interfacial energy between Sn NPs and SnO2 NTs. Our result will likely benefit more rational material design of the ideal interface for facile ion insertion.

An Impedance-Transduced Chemiresistor with a Porous Carbon Channel for Rapid, Nonenzymatic, Glucose Sensing

A new type of chemiresistor, the impedance-transduced chemiresistor (ITCR), is described for the rapid analysis of glucose. The ITCR exploits porous, high surface area, fluorine-doped carbon nanofibers prepared by electrospinning of fluorinated polymer nanofibers followed by pyrolysis. These nanofibers are functionalized with a boronic acid receptor and stabilized by Nafion to form the ITCR channel for glucose detection. The recognition and binding of glucose by the ITCR is detected by measuring its electrical impedance at a single frequency. The analysis frequency is selected by measuring the signal-to-noise ( S/ N) for glucose detection across 5 orders of magnitude, evaluating both the imaginary and real components of the complex impedance. On the basis of this analysis, an optimal frequency of 13 kHz is selected for glucose detection, yielding an S/ N ratio of 60-100 for [glucose] = 5 mM using the change in the total impedance, Δ Z. The resulting ITCR glucose sensor shows a rapid analysis time (<8 s), low coefficient of variation for a series of sensors (<10%), an analysis range of 50 μM to 5 mM, and excellent specificity versus fructose, ascorbic acid, and uric acid. These metrics for the ITCR are obtained using a sample size as small as 5 μL.