Large Crystallites Research Articles

P-type conduction mechanism in continuously varied non-stoichimetric SnOx thin films deposited by reactive sputtering with the impedance control

We successfully fabricated a series of SnOx films varying from SnO2 to SnO using reactive sputtering. By precisely tailoring the transition region in reactive sputtering, a continuous structural evolution from SnO2 to SnO was observed with SnO2 films showing a typical columnar structure and SnO films having a dense film structure with larger crystallites. X-ray diffraction measurement confirmed that the fabricated SnO films coexist with the minor SnO2 and Sn3O4 phases. SnO films exhibit an unintentional p-type conductivity, and the interstitial oxygen possibly acts as the acceptor. The maximum hole mobility is 3.38cm2/Vs at a hole concentration of 1.12×1018cm−3. We propose a p-type conduction mechanism for those SnOx films with the major SnO phase coexisting with the minor SnO2 and Sn3O4 phases, in which the possible optimum for the hole transport can be achieved by tailoring the balance between the amounts of the SnO2/Sn3O4 phases and interstitial oxygen.

Selective Ammonia Detection at Ppb-Level with Si-Doped MoO3 Sensors for Food and Environmental Applications

Introduction Ammonia (NH3) is an important marker for food spoilage[1] as well as a toxic environmental pollutant emitted from concrete walls, fertilizers or refrigeration systems[2]. Therefore, low-cost compact sensors for the detection of hazardous NH3 are urgently needed. Metal-oxide (chemo-resistive) gas sensors are attractive for NH3 detection as they offer simple operation, low power consumption and can be integrated into portable devices.[3] In specific, the α-phase of MoO3 is promising for detection of NH3 down to 50 parts per billion (ppb) at dry conditions. However, due to the high operating temperatures of MoO3 gas sensors (250 to 500 °C), the material must be stabilized by doping or the addition of foreign oxides. Here, we show how tailoring material morphology, phase composition and particle size enables high NH3 sensing performance[4]. Method Pure and Si-doped MoO3 (0 – 20 wt%) nanoparticles were made by flame spray pyrolysis (FSP) and directly deposited onto Al2O3 sensor substrates with interdigitated Pt electrodes[4]. Subsequently, the sensor films were annealed at 450 °C for 5 h in an oven for thermal stabilization. The nanoparticles were analyzed using X-ray diffraction (XRD) patterns, nitrogen adsorption (Brunauer-Emmett-Teller, BET) as well as transmission electron microscopy (TEM) and electron diffraction (ED). The sensing performance was characterized as a function of SiO2 content with synthetic gas mixtures at a sensor temperature of 400 °C on a dynamic mixing setup. Furthermore, the selectivity towards possible interfering molecules of different chemical classes (e.g. acetone, CO and NO) was evaluated. Results and Conclusions The as-prepared, pure MoO3 powder shows fine crystallinity confirmed by transmission electron microscopy (TEM) (Figure 1a) and ED patterns (Figure 1b). By annealing the sensor substrates at 450 °C for 5 h, pure MoO x particles grow to ribbon-like structures (Figure 1d) with large crystallites, as indicated by XRD (triangles) and BET (open circles) and confirmed by bright spots in ED patterns (Figure 1c). Doping with Si reduces particle and crystal growth significantly, decreasing the crystal size from 147 to 65 nm and the particle size from 272 to 83 nm resulting in superior thermal stability. Excess Si (above 3 wt%) is not incorporated into the MoO3 lattice, but forms amorphous SiO2 domains, inhibiting further crystal growth, as subjected by the constant dXRD (Figure 1) and visible by TEM. Due to those structural changes, the sensor response and selectivity depend on the SiO2 doping content.Figure 2 shows the response of pure and Si-doped MoO3 sensors to 1000 ppb NH3 (triangles), acetone (circles), NO (squares) and CO (diamonds) at 400 °C. Increasing the SiO2 content from 0 to 3 wt% almost triples the NH3 response from 0.22 to 0.53 with only minor changes for the other analytes. This results from the reduced sinter neck size of MoO3 due to segregated SiO2 domains which locally narrow the conduction channel leading to an increased electron depletion and change the morphology resulting in a higher surface-to-volume ratio[4]. Therefore, charge carrier mobility and thus film resistance are dominated by surface phenomena increasing the sensor’s sensitivity. Moreover, Si-doping (3 wt%) improved the NH3 selectivity toward other interfering gases achieving a response ratio to acetone (S NH3 / SA ) of 3.6 and to NO of 7.9 and no sensitivity for CO (Figure 2). Above 3 wt% SiO2 content, the film resistance continuously increases (inset in Figure 2) due to the formation of large and inert SiO2 domains. In addition to the enhanced response and selectivity towards NH3, the sensor could clearly detect NH3 down to 400 ppb even in the presence realistic relative humidity (up to 90%)[4]. As a result, this sensor is promising for integration into a portable device for food spoilage detection and environmental monitoring.

SiO2 nanoparticle-regulated crystallization of lead halide perovskite and improved efficiency of carbon-electrode-based low-temperature planar perovskite solar cells**Project supported by the Fundamental Research Funds for the Central South University, China (Grant No. 2019zzts426), the National Natural Science Foundationof China (Grant Nos. 61172047, 61774170, and 51673218), the Scientific and Technological Project of Hunan Provincial Development and Reform Commission, China, the National Science

SiO2 nanoparticles were used to regulate the crystallizing process of lead halide perovskite films prepared by the sequential deposition method, which was used in the low-temperature-processed, carbon-electrode-basing, hole-conductor-free planar perovskite solar cells. It was observed that, after adding small amount of SiO2 precursor (1 vol%) into the lead iodide solution, performance parameters of open-circuit voltage, short-circuit current and fill factor were all upgraded, which helped to increase the power conversion efficiency (reverse scan) from 11.44(± 1.83)% (optimized at 12.42%) to 14.01(±2.14)% (optimized at 15.28%, AM 1.5G, 100 mW/cm2). Transient photocurrent decay curve measurements showed that, after the incorporation of SiO2 nanoparticles, charge extraction was accelerated, while transient photovoltage decay and dark current curve tests both showed that recombination was retarded. The improvement is due to the improved crystallinity of the perovskite film. X-ray diffraction and scanning electron microscopy studies observed that, with incorporation of amorphous SiO2 nanoparticles, smaller crystallites were obtained in lead iodide films, while larger crystallites were achieved in the final perovskite film. This study implies that amorphous SiO2 nanoparticles could regulate the coarsening process of the perovskite film, which provides an effective method in obtaining high quality perovskite film.

Morphology and size of Pt on Al2O3: The role of specific metal-support interactions between Pt and Al2O3

The catalytic performance of supported heterogeneous catalysts is significantly affected by the particle size and morphologies of the active phase. In this work, we studied how metal-support interaction affected the morphology and size of Pt clusters on Al2O3. Under oxidizing atmosphere, when the specific interaction between oxidized Pt and Al2O3 is dominant, highly dispersed 2D PtO2 rafts (1 nm) and atomically dispersed Pt atoms formed. At high Pt loadings, when only a fraction of the metal atoms can interact with the alumina support, or on weakly interacting supports (e.g., SiO2), 3D PtO2 particles (>2 nm) and large Pt crystallites (>10 nm) formed. Due to the weak interaction with the support, these 3D PtO2 particles can be reduced at a lower temperature (−40 °C) than 2D PtO2 rafts (~110 °C) and atomically dispersed Pt (>300 °C). The catalytic behavior for CO oxidation is also affected by the reduction properties of supported Pt catalysts. The activity of Pt/Al2O3 stepwise increases with the reduction temperature, which is closely related with the reduction of these three types of oxidized Pt species. In contrast, Pt/SiO2 didn’t show drastic activity change with the subsequent reduction treatments up to 500 °C because it contains mainly 3D PtO2 particles. All these results demonstrate that specific metal-support interactions guide the geometric and chemical properties and the corresponding catalytic performances of oxidized Pt in Pt/Al2O3 catalysts, underlining the importance of careful catalyst activation for the efficient utilization of metallic Pt in real-world catalytic applications.

Antifungal Efficacy of Chitosan-Modified Zinc Oxide Nanoparticles on Tube Sedge Products

The antifungal properties of ZnO were implemented in the real handicraft and showed promising results for the value addition of local products by sun-screen and fungi protections. The inhibition of Aspergillus sp. growth on tube sedge basketry by zinc oxide (ZnO) was demonstrated. ZnO nanoparticles synthesized with chitosan capping agents were analyzed by X-ray diffractometry (XRD), Fourier transform infrared (FTIR) spectrophotometry and thermogravimetric analysis (TGA). The crystallite size consistent with electron microscope images and surface area of ZnO were dependent on the amounts of chitosan. ZnO exhibited a large ultraviolet (UV) absorbance in an entire 200-400 nm range when large crystallites agglomerated into bulky aggregates. In the case of small amounts of chitosan used, small crystallites tending to agglomerate in close contacts enhanced antifungal activity on pieces of tube sedge basketry. The fungi inhibition by this chitosan-modified ZnO was attributed to the stress response in fungal hyphae and generation of hydrogen peroxide.

Effect of Ti content on microstructure and performance of carbon foam derived from mesophase pitch

A novel carbon foam was prepared from mixtures of mesophase pitch and titanium powder by the traditional process including foaming, carbonization and graphitization. Investigation was carried out to determine the effect on the structure and properties of the carbon foams through the addition of titanium to carbon foams. Results have shown that Ti can promote more perfect and larger crystallites and enhance the conductive/mechanical properties of carbon foams. The high thermal conductivity of 61.89 W/m K was obtained with an addition of 12 wt% Ti at heat treatment temperature of 2573 K. At the same time, the correlations between microstructure and properties are also discussed.

A prenucleation strategy for ambient fabrication of perovskite solar cells with high device performance uniformity

Humidity is known to be inimical to the halide perovskites and thus typically avoided during fabrication. The poor fundamental understanding of chemical interactions between water and the precursors hampers the further development of perovskite fabrication in ambient atmosphere. Here, we disclose a key finding that the ambient water could promote the formation of lead complexes, which when uncontrolled would make their way into large intermediate fibrillar crystallites and thus discontinuous perovskite films unfavorable for photovoltaics among others. To counter this effect, a prenucleation strategy is proposed, which embodies the controlled burst of profuse intermediate nuclei. Consequently, we are able to obtain a compact and uniform perovskite layer, which affords high efficiency perovskite solar cells. More excitingly, the solar cells show high performance uniformity, demonstrating the distinctive advantages of our prenucleation strategy. This work sheds light on developing reliable and cost-effective fabrication methods for industrial production of perovskite solar cells.

Localizing Binding Sites on Bioconjugated Hydrogen-Bonded Organic Semiconductors at the Nanoscale.

Hydrogen‐bonded organic semiconductors are extraordinarily stable organic solids forming stable, large crystallites with the ability to preserve favorable electrical properties upon bioconjugation. Lately, tremendous efforts have been made to use these bioconjugated semiconductors as platforms for stable multifunctional bioelectronics devices, yet the detailed characterization of bio‐active binding sites (orientation, density, etc.) at the nanoscale has not been achieved yet. The presented work investigates the bioconjugation of epindolidione and quinacridone, two representative semiconductors, with respect to their exposed amine‐functionalities. Relying on the biotin‐avidin lock‐and‐key system and applying the atomic force microscopy (AFM) derivative topography and recognition (TREC) imaging, we used activated biotin to flag crystal‐faces with exposed amine functional groups. Contrary to previous studies, biotin bonds were found to be stable towards removal by autolysis. The resolution strength and clear recognition capability makes TREC‐AFM a valuable tool in the investigation of bio‐conjugated, hydrogen‐bonded semiconductors.

Activated Carbon as a Support of Catalysts for the Removal of Nitrogen Oxides

Abstract Activated carbon was oxidised with concentrated nitric acid and impregnated with urea to form nitrogen-containing groups. Such a support was impregnated with cobalt, copper or silver nitrates to obtain catalysts for the selective catalytic reduction of nitrogen oxides with ammonia. Infrared spectra confirmed the formation of carboxylic and other organic oxygen-containing groups during oxidation. Nitrogen-containing species resulted from urea thermal decomposition. The metal-containing samples were hydrophilic. Cobalt and copper were present in the samples as small Co3O4 and CuO crystallites, while silver occurred in the form of large metallic crystallites, as seen from the X-ray diffraction patterns. Low temperature N2 sorption revealed that all samples were microporous solids, and the chemical and thermal treatment did not change their textural properties. The copper admixture caused the highest NO conversion, but worsened the selectivity and thermal stability of functionalised carbon support.

Corrosion of titanium alloys in pressurized water at 300 °C and 15 MPa

Commercially pure titanium (Ti CP) and two titanium alloys (Ti 64 and Ti10-2-3) representative of different metallurgical classes were exposed to pressurized water at 300 °C, 15 MPa in a dedicated stainless steel corrosion loop up to 1632 h. Mass uptake measurements as well as extensive characterization of the oxides formed on the polished samples using scanning and transmission electron microscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, photoelectrochemistry and glow discharge optical emission spectroscopy led to the following main conclusions: • The oxide scale was made of a thin and continuous TiO2 layer topped by small TiO2 anatase crystallites and large FeTiO3 ilmenite crystallites. • The interface tortuosity increased with exposure duration, as well as the amount of crystallites. From 217 h exposure, the thickness of the thin continuous TiO2 layer remained constant with exposure duration. • The mass uptake increased approximately linearly with exposure duration. These observations were consistent with a corrosion mechanism involving simultaneous oxide growth, dissolution and precipitation. The corrosion rate of the three studied materials was estimated to be around 3 µm/year.

In Ukrainian

There were highly-filled composites, based on (polypropylene) PP and containing 5 – 50 % mass. of calcite concentrate (CC), consisting in highly-dispersed calite powder, modified with organic compounds. It is established, that value for organic part content in CC was 24 % mass. CC and highly-flowing PP was used for process of concentrate granulation process. It is shown, that, when at increase of CC’s contents values, those ones of viscosity for compositions melts decreases simultaneously, dissonating with traditional laws for polymer melt flows, when those are intrisically and typically increasing. It is established, that melt flow index values, when at increase of CC contents for those ones, increasing also – from 3.1 g/10 min for pure PP, to 6,6 g/10 min for composition of 50 % mass. CC. It is discovered, that CC is having influence on melt and crystallization processes of PP. When at low (5 % mass.) and middle (10 – 20 % mass.) of CC contents values, there are meling and crystallization temperatures increasing – from 168 °C to 169 – 170 °C and from 115 °C to 117 – 118 °C, accordingly, and melting temperature range is narrowing – from 42 °C pure PP) to 38 – 34 °C, for composites of CC contents values of 30 – 50 % mass., accordingly, and crystallization temperature range – from 18 °C (pure PP, 0 % CC), to 16 – 15 °C, for composites of high CC contents values, accordingly also. There are increasing of phase transitions temperatures exists, pointing on formation of more large and perfect crystallites, however, narrowing in those ones range points on creation of more dimensionally uniform such of supermolecular structures. Such influence for CC on rheological behavior and temperature characteristics of phase transitons phenomena links with plastyfying effect of modifier‘s chains on CC surfaces of particles, and, simultaneously, at presence of highly-flowing PP as base polymer in. CC injection making some decrease here, namely for activation energy values for oxidative thermodestruction, this phenomen is expaining with more decrease of bounding degree in system, causing with presence of modifier additions in system, and one have influence on.

In Ukrainian

The prevalence of Cerium compounds, economic affordability, and unique redox properties determine the wide range of cerium oxide applications. The transition to nanosized particles of cerium oxide allows these materials to reversibly absorb and release oxygen, which determined the use of nano-CeO2 in biomedical technologies in the treatment of diseases associated with lesions due to oxidative stress. The review analyzed the state of modern scientific literature regarding the properties, methods for producing and using cerium nanooxide. The main characteristics of cerium oxide and the features of its structure are described, the mechanisms of action of the enzyme-like properties of nanoceria are given. It was shown that the coexistence of Ce3+ and Ce4+ ions on the surface is compensated by the formation of oxygen vacancies, which are essentially surface defects. Their number depends on the particle size, morphology and crystallinity of the material. These surface defects are catalytic centers that are capable of exhibiting the enzyme-like properties of nanoceria and effectively absorbing free radicals, which include reactive oxygen species. Physicochemical properties of nanoparticles depend on the synthesis parameters. These include the nature and type of precursor, stabilizing agent and solvent, the duration and temperature of the reaction, and the pH of the reactive mixture. So, microgravity affects morphology, average particle size, crystallinity; an increase in the aging time leads to the synthesis of large particles and crystallites, increases the degree of crystallinity. The synthesis temperature affects the size and structure of crystallites, agglomeration, surface defects and oxidation state. Varying the synthesis parameters and their control during the reaction allows one to obtain particles with desired physicochemical properties. The main methods are analyzed in detail: deposition, hydrothermal, microemulsion, combustion, sol-gel and green. It has been established that, due to its unique properties, nanosized cerium oxide has a wide range of applications. It is used as an abrasive powder for polishing glass and protecting it from corrosion; commercial sunscreens are produced on its basis, biosensors and solid oxide fuel cells are constructed, and catalysts are synthesized. Cerium oxide nanoparticles and materials based on it are widely used in environmental, industrial, bioanalytical and biomedical fields. Recently, the biomedical use of nanosized cerium oxide has been rapidly developing. The non-stoichiometric composition of cerium oxide nanoparticles allows it to effectively neutralize reactive oxygen species while protecting the body from oxidative stress. Nano-CeO2 is used in the treatment of inflammatory, cardiovascular and neurodegenerative diseases, it increases the activity of antimicrobials, is an agent for the delivery of therapeutic drugs to cancer cells.

Enhancing the electrical conductivity of PP/CNT nanocomposites through crystal-induced volume exclusion effect with a slow cooling rate

Polymer crystallization on CNT surfaces has been considered to be a critical reason for the relatively high percolation thresholds obtained in melt-blended polymer/CNT nanocomposites (e.g., generally > 1 wt% in most polyolefin/CNT systems). In this work, the crystal-nucleating capability of CNTs in polypropylene (PP) was controlled through addition of a sorbitol-based external nucleating agent (NA) and via controlling the cooling rate. Despite the competition between CNT and NA for crystal nucleation, little increase in conductivity was obtained as compared to PP/CNT with fast cooling (~150 °C/min). However, with a slow cooling process (~1.5 °C/min), large PP crystallites were induced with only a small number of nuclei. Therefore, the majority of the CNT particles were prevented from participating in the nucleation of PP crystals so that they could be concentrated to form conductive networks through an enhanced volume exclusion effect. This significantly increased the conductivity of the PP/CNT nanocomposites, and the percolation threshold was greatly reduced from 0.75 wt% to 0.36 wt%. The work highlights the crucial influence of controlling CNT nucleation capability and the polymer crystallite size on the conductivity using slow cooling in addition to annealing treatment.

Unveiling the vortex glass phase in the surface and volume of a type-II superconductor

Order-disorder transitions between glassy phases are common in nature and yet a comprehensive survey on the entailed structural changes is challenging since the constituents are in the micro-scale. Vortex matter in type-II superconductors is a model system where some of these experimental challenges can be tackled. Samples with point disorder present a glassy transition on increasing the density of vortices. A glassy yet quasi-crystalline phase, the Bragg glass, nucleates at low densities. The vortex glass stable at high densities is expected to be disordered, however its detailed structural properties remained experimentally elusive. Here we show that the vortex glass has large crystallites with in-plane positional displacements growing algebraically and short-range orientational order. Furthermore, the vortex glass has a finite and almost constant correlation length along the direction of vortices, in sharp contrast with strong entanglement. These results are important for the understanding of disorder-driven phase transitions in glassy condensed matter.

New Insights into the Ultrastructure of Bioapatite After Partial Dissolution: Based on Whale Rostrum, the Densest Bone.

Mineral particles in bone are interlaced with collagen fibrils, hindering the investigation of bioapatite crystallites (BAp). This study utilized a special whale rostrum (the most highly mineralized bone ever recorded) to measure the crystallites of bone BAp via long-term dissolution in water. The BAp in the rostrum has a low solubility (6.7 ppm Ca and 3.8 ppm P after 150 days dissolution) as well as in normal bones, which leads to its Ksp value of ~10-53. Atomic force microscopy results show tightly compacted mineral crystallites and confirm the low amount of collagen in the rostrum. Additionally, the mineral crystallites demonstrate irregular plate-like shapes with variable sizes. The small crystallites (~11 × 24 nm) are easily detached from BAp prisms, compared with the large crystallites (~50 nm). Moreover, various orientations of crystallites are observed on the edge of the prisms, which suggest a random direction of mineral growth. Furthermore, these plate-like crystallites prefer to be stacked layer by layer under weak regulation from collagen. The morphology of rostrum after dissolution provides new insights into the actual morphology of BAp crystallites.

Crystal texture-dependent magnetic and magnetotransport properties of half-metallic Fe3O4 films grown on oxidized Si substrates by reactive deposition

Nanocrystalline magnetite (Fe3O4) films with various preferred crystallite orientations were grown on oxidized silicon surface by reactive iron deposition in an oxygen atmosphere. Depending on the partial pressure of oxygen (PO2), the evolution of the structural, magnetic, and magnetotransport properties of the grown films was investigated. We found that the growth of films containing only the Fe3O4 phase occurs in a certain PO2 range, and the magnetite crystallites may have (311) or (110) preferred orientation. It was revealed that films with (311) and (110) textures have a column structure. An increase in the PO2 leads to a structure transformation from (311) to (110) texture with larger crystallites. A film with (110) texture showed higher values of saturation magnetization, magnetoresistance and spin polarization. The analysis of approach to magnetic saturation revealed that the local magnetic anisotropy of crystallites in textured films is much higher than the anisotropy of bulk magnetite due to the large surface contribution. The FORC diagram method in combination with the approach to magnetic saturation proved the existence of the exchange coupling between large and small grains. Our results open the reliable route for crystal texture-depending manipulation of functional properties of thin magnetite films for advanced spintronic applications.

Maximizing Active Site Concentrations at Ni-Substituted WS2 Edges for Hydrogenation of Aromatic Molecules.

High concentrations of Ni in bimetallic sulfide catalysts lead to the formation of segregated Ni sulfides (NiSx), which are rather inactive alone as large crystallites and even impede the accessibility of active sites at the sulfide slab edges that catalyze a multitude of hydrogenation reactions and H2 and CO2 activation processes. Treatment of Ni-WS2/γ-Al2O3 catalysts in aqueous acids, particularly concentrated HCl, results in a significant reduction of NiSx and ≤5-fold enhancement of the phenanthrene hydrogenation rate. Using infrared (IR) spectroscopy of probe molecules, we show that the acid-treated catalysts have a high concentration of accessible metal edge sites, a high degree of Ni substitution, and consequently a high sulfhydryl (SH) concentration at the slab edges in the presence of H2. The site-specific "turnover frequency" (based on SH concentrations determined by IR measurements) is identical for all parent and acid-treated sulfide catalysts studied, showing that excess NiSx does not influence electronic properties.

Perpendicular Weak Permanent Magnetic Field Effect on the Electrodeposited Nanostructured ZnO Film and its Kinetic Corrosion Behavior

In this work the effect of the permanent weak magnetic field on the electrodeposited ZnO nanostructured film on the ITO substrate elaborated under –0.5 and –0.8 V was studied. The deposits have been characterized by SEM, EDX, and electrochemical tests. The obtained results showed the increase of crystallites dimension in the presence of magnetic field under –0.5 V, and the decrease of the number of large crystallites of ZnO deposits under potential deposition equal to –0.8 V. The deposition under weak magnetic field change the current deposition, the open circuit potential (OCP), the polarization curve parameters after one week of corrosion and crystallographic parameters.

Study of phase transitional behavior and electrical properties of relaxor Ba0.85Ca0.15Zr0.05Ti0.95O3 lead free ceramic

Lead-free relaxor ferroelectric, Ba0.85Ca0.15Zr0.05Ti0.95O3 have been fabricated by a hydrothermal method. Single-phase X-ray diffraction patterns conforms very closely to perovskites structure and the samples have a phase with a tetragonal structure at room temperature. The FESEM image of Ba0.85Ca0.15Zr0.05Ti0.95O3 powder shows cylindrical like crystals. Small grains with average grain sizes of ∼10 ± 5 nm uniformly distributed all over the powder. The most intense diffraction rings from TEM correspond to large crystallites. A broad dielectric anomaly coupled with the dielectric maxima with increasing frequency indicates the second order diffuse phase transition in the system.

High Active Ultra-Low Pt Content Oxygen Reduction Reaction Catalyst Derived from Metal Organic Framework for PEMFC

Proton exchange membrane fuel cell (PEMFC) has been demonstrated as a highly efficient energy conversion technology. The sluggish oxygen reduction reaction (ORR) at the fuel cell cathode requires 3 to 5 times more catalysts than the hydrogen oxidation reaction at the anode.(1) Currently, platinum is still the catalyst material of choice. However, the heavy use of scarce Pt in the electrodes represents a major cost barrier to the commercialization of PEMFCs. Tremendous efforts have been dedicated to reduce or remove Pt usage through the development of Pt-transition metal catalysts, as highlighted recently by dealloyed PtNi (2), Mo-doped PtNi(3), and ordered PtCo(4); or through the development of transition metals-nitrogen-carbon based PGM-free catalysts (5, 6). In the fuel cell, catalysts should be highly dispersed over the electrode surface to be easily accessible by the reactants, especially at the high fuel cell current density where a large influx of O2 must be converted. For the Pt alloy catalyst of large crystallites, there won’t be enough crystallites available to spread over the electrode surface to encounter the incoming O2 if the total loading of platinum has to be maintained ultralow. Furthermore, unprotected Pt nano-alloys may lose their nanostructure and crystallinity from the dissolution and agglomeration during the ORR process. The PGM-free catalysts, on the other hand, have high catalytic site density with uniform distribution when prepared by homogenous precursors such as metal-organic framework or porous organic polymer, rendering them highly efficient in interacting with O2 flux. Their key drawback, however, is the poor stability operated under PEMFC condition. In this presentation, we will describe a method of preparing highly active yet stable electrocatalysts containing ultralow Pt content using Co or Co/Zn zeolitic imidazolate frameworks as precursors. The new catalysts contain Pt-Co core-shell nanoparticles (NPs) situated over the PGM-free catalytically active support (7). The ORR activities of the new catalysts were first tested in the O2 saturated acidic electrolyte by rotating ring disk electrode (RRDE) before fabricated into the membrane electrode assembly (MEA) and evaluated under fuel cell operating condition. The synergistic catalysis between Pt-Co NPs and PGM-free active substrate led to an unprecedented ORR performance in both RRDE and fuel cell. For example, the fuel cell test demonstrated a mass activity of 1.77 A/mgPt for the new catalysts and > 60% retention of the initial mass activity after 30,000 continuous voltage cycles from 0.6V – 1.0V. The characterizations of the fresh and the post-electrochemical test samples were investigated by x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), x-ray absorption near-edge structure (XANES), extended x-ray absorption fine structure (EXAFS), transmission electron microscopy (TEM), Raman spectroscopy and BET surface analysis. We also performed DFT calculation on the ORR thermodynamic barriers over both Pt-Co NPs and PGM-free site as the descriptors for reaction pathway. The results reveal that the synergistic interaction between Pt-Co and PGM active substrate contribute significantly to both activity and durability enhancements. Acknowledgments: This work was supported by U.S. Department of Energy, Fuel Cell Technologies Office through Office of Energy Efficiency and Renewable Energy. LC wishes to thank Argonne National Laboratory for Maria Goeppert Mayer Fellowship. The works performed at Argonne National Laboratory’s Center for Nanoscale Materials and Advanced Photo Source, U.S. Department of Energy Office of Science User Facilities, is supported by Office of Science, U.S. Department of Energy under Contract DE-AC02-06CH11357. H. A. Gasteiger, N. M. Markovic, Just a Dream-or Future Reality? Science 324, 48-49 (2009).B. Han et al., Record activity and stability of dealloyed bimetallic catalysts for proton exchange membrane fuel cells. Energy & Environmental Science 8, 258-266 (2015).X. Huang et al., High-performance transition metal–doped Pt3Ni octahedra for oxygen reduction reaction. Science 348, 1230-1234 (2015 ).D. Wang et al., Structurally ordered intermetallic platinum-cobalt core-shell nanoparticles with enhanced activity and stability as oxygen reduction electrocatalysts. Nat Mater 12, 81-87 (2013).L. Chong et al., Investigation of Oxygen Reduction Activity of Catalysts Derived from Co and Co/Zn Methyl-Imidazolate Frameworks in Proton Exchange Membrane Fuel Cells. ChemElectroChem 3, 1541-1545 (2016).J. Shui, C. Chen, L. Grabstanowicz, D. Zhao, D. J. Liu, Highly efficient nonprecious metal catalyst prepared with metal-organic framework in a continuous carbon nanofibrous network. Proc Natl Acad Sci U S A 112, 10629-10634 (2015).L. Chong et al., Ultralow-loading platinum-cobalt fuel cell catalysts derived from imidazolate frameworks. Science, (2018).