Urban roadside greenery as a carbon sink: Systematic assessment considering understory shrubs and soil respiration.

Roadside greenery is an efficient strategy for maximizing ecosystem services, including carbon sequestration in urban settings. However, the quantification of carbon sequestration is not comprehensive because understory shrubs and soil respiration have not been thoroughly considered. We developed an integrated methodology that combined field measurements and greenhouse incubation to comprehensively assess carbon sequestration in roadside greenery systems. The system was defined as an 8 m long section comprising a single tree (Zelkova serrata), 79 shrubs (Euonymus japonicus), and soil. Annual carbon uptake by a tree was estimated using an allometric equation derived from an official government report. For shrubs, carbon uptake was measured in the field by monitoring CO2 concentration change in the chamber enclosing the leaves and stems. Annual carbon uptake by shrubs was estimated by using the regression equation among carbon uptake, air temperature, and photosynthetically active radiation. We also estimated shrub root respiration by combining net primary production (NPP) from the greenhouse incubation and measured pruning effect in the field. This enabled us to differentiate heterotrophic respiration from the total soil respiration. The overall methodology accurately assessed net ecosystem production (NEP) from the roadside greenery system, which is 0.528 kg C m-2 yr-1. If this figure is extended to all roads in the target city, it can offset daily carbon emitted from the total registered passenger vehicles in the target city. Considering that shrubs sequester an amount equivalent to 29.3 % of the carbon sequestered by tree species, the current greenhouse gas inventory should include shrubs as an important carbon sink. As we also revealed that roadside soil has high carbon vulnerability, proper soil management is needed to enhance NEP. Our systematic approach evaluating the carbon balance within the roadside greenery system can be applied to other cities, contributing to enhance global understanding of urban carbon cycle.

MoO3@MoS2 Core-Shell Structured Hybrid Anode Materials for Lithium-Ion Batteries.

We explore a phase engineering strategy to improve the electrochemical performance of transition metal sulfides (TMSs) in anode materials for lithium-ion batteries (LIBs). A one-pot hydrothermal approach has been employed to synthesize MoS2 nanostructures. MoS2 and MoO3 phases can be readily controlled by straightforward calcination in the (200-300) °C temperature range. An optimized temperature of 250 °C yields a phase-engineered MoO3@MoS2 hybrid, while 200 and 300 °C produce single MoS2 and MoO3 phases. When tested in LIBs anode, the optimized MoO3@MoS2 hybrid outperforms the pristine MoS2 and MoO3 counterparts. With above 99% Coulombic efficiency (CE), the hybrid anode retains its capacity of 564 mAh g-1 after 100 cycles, and maintains a capacity of 278 mAh g-1 at 700 mA g-1 current density. These favorable characteristics are attributed to the formation of MoO3 passivation surface layer on MoS2 and reactive interfaces between the two phases, which facilitate the Li-ion insertion/extraction, successively improving MoO3@MoS2 anode performance.

Metastable hexagonal close-packed palladium hydride in liquid cell TEM.

Metastable phases-kinetically favoured structures-are ubiquitous in nature1,2. Rather than forming thermodynamically stable ground-state structures, crystals grown from high-energy precursors often initially adopt metastable structures depending on the initial conditions, such as temperature, pressure or crystal size1,3,4. As the crystals grow further, they typically undergo a series of transformations from metastable phases to lower-energy and ultimately energetically stable phases1,3,4. Metastable phases sometimes exhibit superior physicochemical properties and, hence, the discovery and synthesis of new metastable phases are promising avenues for innovations in materials science1,5. However, the search for metastable materials has mainly been heuristic, performed on the basis of experiences, intuition or even speculative predictions, namely 'rules of thumb'. This limitation necessitates the advent of a new paradigm to discover new metastable phases based on rational design. Such a design rule is embodied in the discovery of a metastable hexagonal close-packed (hcp) palladium hydride (PdHx) synthesized in a liquid cell transmission electron microscope. The metastable hcp structure is stabilized through a unique interplay between the precursor concentrations in the solution: a sufficient supply of hydrogen (H) favours the hcp structure on the subnanometre scale, and an insufficient supply of Pd inhibits further growth and subsequent transition towards the thermodynamically stable face-centred cubic structure. These findings provide thermodynamic insights into metastability engineering strategies that can be deployed to discover new metastable phases.

Development of a highly active FeNC catalyst with the preferential formation of atomic iron sites for oxygen reduction in alkaline and acidic electrolytes.

Nitrogen-doped porous carbons containing atomically dispersed iron are prime candidates for substituting platinum-based catalysts for oxygen reduction reaction (ORR) in fuel cells. These carbon catalysts are classically synthesizedviacomplicated routes involving multiple heat-treatment steps to form the desired Fe-Nx sites. We herein developed a highly active FeNC catalyst comprising of exclusive Fe-Nx sites by a simplified solid-state synthesis protocol involving only a single heat-treatment. Imidazole is pyrolyzed in the presence of an inorganic salt-melt resulting in highly porous carbon sheets decorated with abundant Fe-Nx centers, which yielded a high density of electrochemically accessible active sites (1.36 × 1019 sites g-1) as determined by the in situ nitrite stripping technique. The optimized catalyst delivered a remarkable ORR activity with a half-wave potential (E1/2) of 0.905 VRHE in alkaline electrolyte surpassing the benchmark Pt catalyst by 55 mV. In acidic electrolyte, an E1/2 of 0.760 VRHE is achieved at a low loading level (0.29 mg cm-2). In PEMFC tests, a current density of 2.3 mA cm-2 is achieved at 0.90 ViR-free under H2-O2 conditions, reflecting high kinetic activity of the optimized catalyst.

Nanoscale light element identification using machine learning aided STEM-EDS.

Light element identification is necessary in materials research to obtain detailed insight into various material properties. However, reported techniques, such as scanning transmission electron microscopy (STEM)-energy dispersive X-ray spectroscopy (EDS) have inadequate detection limits, which impairs identification. In this study, we achieved light element identification with nanoscale spatial resolution in a multi-component metal alloy through unsupervised machine learning algorithms of singular value decomposition (SVD) and independent component analysis (ICA). Improvement of the signal-to-noise ratio (SNR) in the STEM-EDS spectrum images was achieved by combining SVD and ICA, leading to the identification of a nanoscale N-depleted region that was not observed in as-measured STEM-EDS. Additionally, the formation of the nanoscale N-depleted region was validated using STEM-electron energy loss spectroscopy and multicomponent diffusional transformation simulation. The enhancement of SNR in STEM-EDS spectrum images by machine learning algorithms can provide an efficient, economical chemical analysis method to identify light elements at the nanoscale.

Curved Structure of Si by Improving Etching Direction Controllability in Magnetically Guided Metal-Assisted Chemical Etching.

Metal-assisted chemical etching (MACE) is widely used to fabricate micro-/nano-structured Si owing to its simplicity and cost-effectiveness. The technique of magnetically guided MACE, involving MACE with a tri-layer metal catalyst, was developed to improve etching speed as well as to adjust the etching direction using an external magnetic field. However, the controllability of the etching direction diminishes with an increase in the etching dimension, owing to the corrosion of Fe due to the etching solution; this impedes the wider application of this approach for the fabrication of complex micro Si structures. In this study, we modified a tri-layer metal catalyst (Au/Fe/Au), wherein the Fe layer was encapsulated to improve direction controllability; this improved controllability was achieved by protecting Fe against the corrosion caused by the etching solution. We demonstrated curved Si microgroove arrays via magnetically guided MACE with Fe encapsulated in the tri-layer catalyst. Furthermore, the curvature in the curved Si microarrays could be modulated via an external magnetic field, indicating that direction controllability could be maintained even for the magnetically guided MACE of bulk Si. The proposed fabrication method developed for producing curved Si microgroove arrays can be applied to electronic devices and micro-electromechanical systems.

Characterization of Cr (VI) - Containing solid phase particles in dry dust deposition in Daejeon, South Korea.

Solid phase speciation of chromium in dry dust deposition and road paint was determined using transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS). Spherical black carbon aggregates in dry dust deposition contained discrete nano-sized lead chromate (PbCrO4) and zinc chromate (ZnCrO4), which likely originated from yellow traffic paint and zinc chromate primer (corrosion resistant pigment), respectively based on their main applications. Road marking paint samples from auto roads included lead chromate particles whose size, shape, composition and crystal structure were similar to those in dry dust deposition. A slight difference was found at the particle boundary. Namely, lead chromate in traffic paint was encapsulated by silica-bearing matrix, while discrete lead chromate in the black carbon of dry deposition was rarely enclosed within silica-bearing matrix. The Cr K edge X-ray absorption near edge structure (XANES) spectrum for dry deposition showed the characteristic pre-edge resonance peak similar to the spectra for the Cr(VI) standard and road paint, but the lower intensity. The amount of Cr(VI) accounted for approximately 45% of the total Cr in the dust sample. The solid phase speciation of Cr observed using TEM and their valence states determined using XANES were consistent with the chemical speciation determined using the sequential extraction. The contribution of this study is that XANES was applied to identify Cr valance states in urban dust deposition and zinc chromate was found as a Cr(VI) phase in dry dust deposition using TEM. These study results provide novel data on Cr speciation and Cr(VI)-containing mineral phases in dry dust deposition and their potential sources. Based on the wide use of lead and zinc chromate, atmospheric contamination with these solid phase speciation of Cr(VI) is expected in other cities in the world.

Electrically Driven Reversible Phase Changes in Layered In2 Se3 Crystalline Film.

An unconventional phase-change memory (PCM) made of In2 Se3 , which utilizes reversible phase changes between a low-resistance crystalline ß phase and a high-resistance crystalline γ phase is reported for the first time. Using a PCM with a layered crystalline film exfoliated from In2 Se3 crystals on a graphene bottom electrode, it is shown that SET/RESET programmed states form via the formation/annihilation of periodic van der Waals' (vdW) gaps (i.e., virtual vacancy layers) in the stack of atomic layers and the concurrent reconfiguration of In and Se atoms across the layers. From density functional theory calculations, ß and γ phases, characterized by octahedral bonding with vdW gaps and tetrahedral bonding without vdW gaps, respectively, are shown to have energy bandgap value of 0.78 and 1.86 eV, consistent with a metal-to-insulator transition accompanying the ß-to-γ phase change. The monolithic In2 Se3 layered film reported here provides a novel means to achieving a PCM based on melting-free, low-entropy phase changes in contrast with the GeTe-Sb2 Te3 superlattice film adopted in interfacial phase-change memory.

Density control and wettability enhancement by functionalizing carbon nanotubes with nickel oxide in aluminum-carbon nanotube system.

Excellent mechanical properties of carbon nanotubes (CNTs) make them ideal reinforcements for synthesizing light weight, high strength metal matrix composite. Aluminum is attractive matrix due to its light weight and Al/CNT composites are promising materials for various industrial applications. Powder metallurgy and casting techniques are normally used for bulk fabrications of composites. Casting process which can mass-produce delicate product is more suitable than existing powder metallurgy in view point of application in industries. In CNT-metal matrix composites, however, composite bulk fabrication has been limited because of the large density gap and poor wettability between the metal and CNTs. This study suggests a method for alleviating such problems. It was found that the wettability between aluminum and CNT could be enhanced by functionalizing the CNTs with nickel oxide. This functionalization of CNTs with heavier element also reduces the density gap between the matrix and reinforcements. It is suggested that this method could possibly be used in a casting process to enable mass fabrication of CNT-metal matrix composites.

Enhanced morphological and thermal stabilities of nickel germanide with an ultrathin tantalum layer studied by ex situ and in situ transmission electron microscopy.

The formation and morphological evolution of germanides formed in a ternary Ni/Ta-interlayer/Ge system were examined by ex situ and in situ annealing experiments. The Ni germanide film formed in the Ni/Ta-interlayer/Ge system maintained continuity up to 550°C, whereas agglomeration of the Ni germanide occurred in the Ni/Ge system without Ta-interlayer. Through microstructural and chemical analysis of the Ni/Ta-interlayer/Ge system during and after in situ annealing in a transmission electron microscope, it was confirmed that the Ta atoms remained uniformly on the top of the newly formed Ni germanide layer during the diffusion reaction. Consequently, the agglomeration of the Ni germanide film was retarded and the thermal stability was improved by the Ta incorporation.

Mechanism of Pt loading on multi-walled carbon nanotubes.

Microwave treatment of multi-walled carbon nanotubes (MWCNTs) with nitric acid (HNO3) and 0.2 M sodium chlorate (NaClO3) can generate and enhance defects on the surfaces of MWCNTs. These defects are the important sites to load Pt nanoparticles (NPs). We investigated the defect induced Raman spectra and observed a decrease in the R-values (D-band/G-band peak ratio) and a slight up-shift of the both peaks as the amount of loaded Pt NPs increased. Using the Brunauer-Emmett-Teller (BET) method, we observed that the pore size distribution and the pore volume changed according to the amount of Pt NPs loaded. Fewer micropores and mesopores were observed on MWCNTs loaded with Pt NPs. Based on the pore size distribution calculated from the BET results, Pt NPs loaded mainly on pores/defects with a size of 2-8 nm. Transmission electron microscopy and Raman spectroscopy results confirmed that most well-crystallized Pt NPs loaded on the surface defect sites and pores spontaneously through the exchange of electrons between Pt and C atoms.

Fabrication of CdTe/Te hetero-nanostructures by vapor-solid process.

CdTe/Te core-shell heterostructures were fabricated by the vapor phase synthesis at low temperatures using a quartz tube furnace. Two step vapor-solid processes were employed. First, various tellurium structures such as nanowires, nanorods, nanoneedles, microtubes and microrods were synthesized under different deposition conditions. These tellurium nanostructures were then used as substrates in the second step to synthesize the CdTe/Te core-shell heterostructures. Using this method, various sizes, shapes and types of CdTe/Te core-shell structures were fabricated under a range of conditions. The temperature, pressure and gas flow rate were very important parameters for synthesizing these nanostructures in the vapor phase process. The morphology, crystal structure and orientation were characterized by field-emission scanning microscopy, field-emission transmission electron microscopy and X-ray diffraction. The vapor phase process at low temperatures appears to be an efficient method for producing a variety of Cd/Te hetero-nanostructures. In addition, the hetero-nanostructures can be tailored to the needs of specific applications by deliberately controlling the synthetic parameters.