Optimization of the Mechanical Properties and the Cytocompatibility for the PMMA Nanocomposites Reinforced with the Hydroxyapatite Nanofibers and the Magnesium Phosphate Nanosheets.

Commercial poly methyl methacrylate (PMMA)-based cement is currently used in the field of orthopedics. However, it suffers from lack of bioactivity, mechanical weakness, and monomer toxicity. In this study, a PMMA-based cement nanocomposite reinforced with hydroxyapatite (HA) nanofibers and two-dimensional (2D) magnesium phosphate MgP nanosheets was synthesized and optimized in terms of mechanical property and cytocompatibility. The HA nanofibers and the MgP nanosheets were synthesized using a hydrothermal homogeneous precipitation method and tuning the crystallization of the sodium-magnesium-phosphate ternary system, respectively. Compressive strength and MTT assay tests were conducted to evaluate the mechanical property and the cytocompatibility of the PMMA-HA-MgP nanocomposites prepared at different ratios of HA and MgP. To optimize the developed nanocomposites, the standard response surface methodology (RSM) design known as the central composite design (CCD) was employed. Two regression models generated by CCD were analyzed and compared with the experimental results, and good agreement was observed. Statistical analysis revealed the significance of both factors, namely, the HA nanofibers and the MgP nanosheets, in improving the compressive strength and cell viability of the PMMA-MgP-HA nanocomposite. Finally, it was demonstrated that the HA nanofibers of 7.5% wt and the MgP nanosheets of 6.12% wt result in the PMMA-HA-MgP nanocomposite with the optimum compressive strength and cell viability.

Direct observation of the formation and stabilization of metallic nanoparticles on carbon supports.

Direct formation of ultra-small nanoparticles on carbon supports by rapid high temperature synthesis method offers new opportunities for scalable nanomanufacturing and the synthesis of stable multi-elemental nanoparticles. However, the underlying mechanisms affecting the dispersion and stability of nanoparticles on the supports during high temperature processing remain enigmatic. In this work, we report the observation of metallic nanoparticles formation and stabilization on carbon supports through in situ Joule heating method. We find that the formation of metallic nanoparticles is associated with the simultaneous phase transition of amorphous carbon to a highly defective turbostratic graphite (T-graphite). Molecular dynamic (MD) simulations suggest that the defective T-graphite provide numerous nucleation sites for the nanoparticles to form. Furthermore, the nanoparticles partially intercalate and take root on edge planes, leading to high binding energy on support. This interaction between nanoparticles and T-graphite substrate strengthens the anchoring and provides excellent thermal stability to the nanoparticles. These findings provide mechanistic understanding of rapid high temperature synthesis of metal nanoparticles on carbon supports and the origin of their stability.

Revealing Sintering Kinetics of MoS2-Supported Metal Nanocatalysts in Atmospheric Gas Environments via Operando Transmission Electron Microscopy.

The decoration of two-dimensional (2D) substrates with nanoparticles (NPs) serve as heterostructures for various catalysis applications. Deep understanding of catalyst degradation mechanisms during service conditions is crucial to improve the catalyst durability. Herein, we studied the sintering behavior of Pt and bimetallic Au-core Pt-shell (Au@Pt core-shell) NPs on MoS2 supports at high temperatures under vacuum, nitrogen (N2), hydrogen (H2), and air environments by in situ gas-cell transmission electron microscopy (TEM). The key observations are summarized as effect of environment: while particle migration and coalescence (PMC) was the main mechanism that led to Pt and Au@Pt NPs degradation under vacuum, N2, and H2 environments, the degradation of MoS2 substrate was prominent under exposure to air at high temperatures. Pt NPs were less stable in H2 environment when compared with the Pt NPs under vacuum or N2, due to Pt-H interactions that weakened the adhesion of Pt on MoS2. Effect of NP composition: under H2, the stability of Au@Pt NPs was higher in comparison to Pt NPs. This is because H2 promotes the alloying of Pt-Au, thus reducing the number of Pt at the surface (reducing H2 interactions) and increasing Pt atoms in contact with MoS2. Effect of NP size: The alloying effect promoted by H2 was more pronounced in small size Au@Pt NPs resulting in their higher sintering resistance in comparison to large size Au@Pt NPs and similar size Pt NPs. The present work provides key insights into the parameters affecting the catalyst degradation mechanisms on 2D supports.

Novel PMMA bone cement nanocomposites containing magnesium phosphate nanosheets and hydroxyapatite nanofibers.

Lack of bioactivity and monomer toxicity are limiting factors of polymethyl methacrylate (PMMA) bone cement in orthopedic applications. Herein, we address these shortcomings by proposing two-dimensional magnesium phosphate (MgP) nanosheets and hydroxyapatite (HA) nanofibers as novel fillers in PMMA bone cement nanocomposites. Two-dimensional MgP nanosheets and one-dimensional HA nanofibers were synthesized by tuning the crystallization of the sodium-magnesium-phosphate ternary system and hydrothermal homogeneous precipitation, respectively. We show that MgP nanosheets exhibit antibacterial properties against Escherichia coli (E. coli). In addition, HA nanofibers with high level of bioactivity are the proper choice to induce cell viability in the nanocomposite. Results indicate that the combination of both fillers can act as deformation locks enhancing the compressive strength of the nanocomposites. The synthesized nanocomposite possesses excellent bioactivity, mechanical properties, and cytocompatibility potentially opening new paradigm in the design of next generation bone cement composites.

Revealing Grain-Boundary-Induced Degradation Mechanisms in Li-Rich Cathode Materials.

Despite their high energy densities, Li- and Mn-rich, layered-layered, xLi2MnO3·(1 - x)LiTMO2 (TM = Ni, Mn, Co) (LMR-NMC) cathodes require further development in order to overcome issues related to bulk and surface instabilities such as Mn dissolution, impedance rise, and voltage fade. One promising strategy to modify LMR-NMC properties has been the incorporation of spinel-type, local domains to create "layered-layered-spinel" cathodes. However, precise control of local structure and composition, as well as subsequent characterization of such materials, is challenging and elucidating structure-property relationships is not trivial. Therefore, detailed studies of atomic structures within these materials are still critical to their development. Herein, aberration corrected-scanning transmission electron microscopy (AC-STEM) is utilized to study atomic structures, prior to and subsequent to electrochemical cycling, of LMR-NMC materials having integrated spinel-type components. The results demonstrate that strained grain boundaries with various atomic configurations, including spinel-type structures, can exist. These high energy boundaries appear to induce cracking and promote dissolution of Mn by increasing the contact surface area to electrolyte as well as migration of Ni during cycling, thereby accelerating performance degradation. These results present insights into the important role that local structures can play in the macroscopic degradation of the cathode structures and reiterate the complexity of how synthesis and composition affect structure-electrochemical property relationships of advanced cathode designs.

Non-Dendritic Zn Electrodeposition Enabled by Zincophilic Graphene Substrates.

Rechargeable zinc (Zn) batteries suffer from poor cycling performance that can be attributed to dendrite growth and surface-originated side reactions. Herein, we report that cycling performance of Zn metal anode can be improved significantly by utilizing monolayer graphene (Gr) as the electrodeposition substrate. Utilizing microscopy and X-ray diffraction techniques, we demonstrate that electrodeposited Zn on Gr substrate has a compact, uniform, and nondendritic character. The Gr layer, due to its high lattice compatibility with Zn, provides low nucleation overpotential sites for Zn electrodeposition. Atomistic calculations indicate that Gr has strong affinity to Zn (binding energy of 4.41 eV for Gr with four defect sites), leading to uniform distribution of Zinc adatoms all over the Gr surface. This synergistic compatibility between Gr and Zn promotes subsequent homogeneous and planar Zn deposits with low interfacial energy (0.212 J/m2) conformal with the current collector surface.

In situ study of nucleation and growth dynamics of Au nanoparticles on MoS2 nanoflakes.

Two-dimensional (2D) substrates decorated with metal nanoparticles offer new opportunities to achieve high-performance catalytic behavior. However, little is known on how the substrates control the nucleation and growth processes of the nanoparticles. This paper presents the visualization of dynamic nucleation and growth processes of gold nanoparticles on ultrathin MoS2 nanoflakes by in situ liquid-cell transmission electron microscopy (TEM). The galvanic displacement resulting in Au nuclei formation on MoS2 was observed in real time inside the liquid cell. We found that the growth mechanism of Au particles on pristine MoS2 is in between diffusion-limited and reaction-limited, possibly due to the presence of electrochemical Ostwald ripening. A larger size distribution and more orientation variation is observed for the Au particles along the MoS2 edge than on the interior. Differing from pristine MoS2, sulfur vacancies on MoS2 induce Au particle diffusion and coalescence during the growth process. Density functional theory (DFT) calculations show that the size difference is because the exposed molybdenum atoms at the edge with dangling bonds can strongly interact with Au atoms, whereas sulfur atoms on the MoS2 interior have no dangling bonds and weakly interact with gold atoms. In addition, S vacancies on MoS2 generate strong nucleation centers that can promote diffusion and coalescence of Au nanoparticles. The present work provides key insights into the role of 2D materials in controlling the size and orientation of noble metal nanoparticles vital to the design of next generation catalysts.

Facet-Dependent Thermal Instability in LiCoO2.

Thermal runaways triggered by the oxygen release from oxide cathode materials pose a major safety concern for widespread application of lithium ion batteries. Utilizing in situ aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) at high temperatures, we show that oxygen release from LixCoO2 cathode crystals is occurring at the surface of particles. We correlated this local oxygen evolution from the LixCoO2 structure with local phase transitions spanning from layered to spinel and then to rock salt structure upon exposure to elevated temperatures. Ab initio molecular dynamics simulations (AIMD) results show that oxygen release is highly dependent on LixCoO2 facet orientation. While the [001] facets are stable at 300 °C, oxygen release is observed from the [012] and [104] facets, where under-coordinated oxygen atoms from the delithiated structures can combine and eventually evolve as O2. The novel understanding that emerges from the present study provides in-depth insights into the thermal runaway mechanism of Li-ion batteries and can assist the design and fabrication of cathode crystals with the most thermally stable facets.

Tailoring the Edge Structure of Molybdenum Disulfide toward Electrocatalytic Reduction of Carbon Dioxide.

Electrocatalytic conversion of carbon dioxide (CO2) into energy-rich fuels is considered to be the most efficient approach to achieve a carbon neutral cycle. Transition-metal dichalcogenides (TMDCs) have recently shown a very promising catalytic performance for CO2 reduction reaction in an ionic liquid electrolyte. Here, we report that the catalytic performance of molybdenum disulfide (MoS2), a member of TMDCs, can be significantly improved by using an appropriate dopant. Our electrochemical results indicate that 5% niobium (Nb)-doped vertically aligned MoS2 in ionic liquid exhibits 1 order of magnitude higher CO formation turnover frequency (TOF) than pristine MoS2 at an overpotential range of 50-150 mV. The TOF of this catalyst is also 2 orders of magnitude higher than that of Ag nanoparticles over the entire range of studied overpotentials (100-650 mV). Moreover, the in situ differential electrochemical mass spectrometry experiment shows the onset overpotential of 31 mV for this catalyst, which is the lowest onset potential for CO2 reduction reaction reported so far. Our density functional theory calculations reveal that low concentrations of Nb near the Mo edge atoms can enhance the TOF of CO formation by modifying the binding energies of intermediates to MoS2 edge atoms.

The influence of large cations on the electrochemical properties of tunnel-structured metal oxides.

Metal oxides with a tunnelled structure are attractive as charge storage materials for rechargeable batteries and supercapacitors, since the tunnels enable fast reversible insertion/extraction of charge carriers (for example, lithium ions). Common synthesis methods can introduce large cations such as potassium, barium and ammonium ions into the tunnels, but how these cations affect charge storage performance is not fully understood. Here, we report the role of tunnel cations in governing the electrochemical properties of electrode materials by focusing on potassium ions in α-MnO2. We show that the presence of cations inside 2 × 2 tunnels of manganese dioxide increases the electronic conductivity, and improves lithium ion diffusivity. In addition, transmission electron microscopy analysis indicates that the tunnels remain intact whether cations are present in the tunnels or not. Our systematic study shows that cation addition to α-MnO2 has a strong beneficial effect on the electrochemical performance of this material.