### Recent Submissions

• #### Charge-Storage Mechanism of Aluminum-Sulfur Batteries

(ECS Meeting Abstracts, The Electrochemical Society, 2020-11-23) [Article]
The electrochemical performance of aluminum-sulfur batteries is beset by poor stability and sluggish charge-storage properties. To address these issues, carbon allotropes have been used as electrode fillers, but successful outcomes remain inexplicably elusive. Here, a composite of sulfur and small diameter single-walled carbon nanotubes is synthesized and used as a cathode for reversible aluminum-sulfur batteries. The assembled electrode delivers a high capacity of 1024 mAh/g and effectively reduces the cell polarization by 37%. Moreover, the use of small-diameter SWCNT helps in lowering the electrolyte-to-sulfur ratio down to 17 μg/ml, an important step toward lean electrolyte conditions. Despite that, the capacity fade of the Al-S battery cannot be fully reversed. As we show, the consequence of the interaction between the electrolyte and S is the buildup of insoluble and poorly conductive discharge products, which inhibit charge diffusion and progressively deactivate the electrode, ultimately causing capacity decay. Overall, this works clarifies the carbon–sulfur–electrolyte interactions and their role in the underlying charge-storage mechanism of Al-S batteries.
• #### Cyclized polyacrylonitrile anode for alkali metal ion batteries

(Angewandte Chemie International Edition, Wiley, 2020-11-16) [Article]
Alkali metal (Li, Na, and K) ion batteries are vital in portable and large-scale stationary energy storage. Recently, organic anodes have attracted increasing attention for alkali metal ion batteries due to their chemical diversity and potential high capacity. In this work, we discovered that cyclized polyacrylonitrile (cPAN) can serve as a superior anode for alkali metal ion batteries. Remarkably, upon activation cycling, as an anode of lithium-ion battery, cPAN exhibits a reversible capacity as high as 1238 mAh g-1 under a current density of 50 mA g-1. Based on electrochemical experiments and first-principles calculations, it is demonstrated that the hexagonal carbon ring, piperidine ring, and pyridine nitrogen in ladder cPAN are the main active sites for lithium-ion storage. In addition, we show that cPAN displays a unique potential-dependent solid electrolyte interphase formation from 0.1 to 0.01 V vs. Li/Li+. Furthermore, cPAN displays decent performance as an anode in SIBs and PIBs. The discovery of cPAN anode could pave the way for the future development of organic anodes for alkali metal ion batteries.
• #### Active and stable Fe-based catalyst, mechanism, and key role of alkali promoters in ammonia synthesis

(Journal of Catalysis, Elsevier BV, 2020-11-13) [Article]
Worldwide NH3 production reached 0.18 Gton in 2019, and 1-2 % of the global CO2 emissions are due to large-scale NH3 synthesis (1 billion tons of CO2 / year). A catalyst for ammonia synthesis has been obtained by pyrolysis of iron phthalocyanine (FePc) precursor under N2, followed by impregnation with alkali metals (Na, Li, K, and Cs) and H2 treatment. Characterization (XPS, XRD, HR-TEM, ICP-OES, TGA, CHNS analysis, and BET) revealed nano-sized core-shell structures formed during H2 treatment, with Fe in the core and promoters (“Cs2O” and “K2O”) with carbon on the shell. The alkali metals partially inhibit the methanation process of carbon. These Fe NPs were found to be very active and stable catalysts, as compared to the commercial iron-based catalyst KM1 (Haldor-Topsoe). Activities of promoted catalysts follow the order: K>Cs>Na∼Li, with more than 6% of NH3 at 400 °C and 7 MPa, and contact time (WHSV) of 12000 ml.g-1.h-1 with K. The apparent activation energy was found to be 31 kJ.mol-1 and 34 kJ.mol-1 for 3-K-FePc700 and 10-Cs-FePc700 suggesting the facile activation of N2 on the catalysts surface. DFT-based predicted atomic and electronic structures reveal a similarity in the partial charge distribution on surface Fe species with K or Cs. Surprisingly the main effect of alkali is related to the geometrical repartition of alkali, leading to a larger number of exposed iron atoms, active sites, in the case of K than Cs. The alkali (present as metal oxide) leaves at medium coverage of the surface some exposed Fe(0) for N2 non-dissociative chemisorption (end-on type). The free energy profile demonstrates that the thermodynamic stability of the reaction intermediates for nitrogen reduction reaction (NRR) increases with pressure indicating better feasibility of the reaction at higher pressures.
• #### Intrinsic efficiency limits in low-bandgap non-fullerene acceptor organic solar cells

(Nature Materials, Springer Science and Business Media LLC, 2020-10-23) [Article]
In bulk heterojunction (BHJ) organic solar cells (OSCs) both the electron affinity (EA) and ionization energy (IE) offsets at the donor–acceptor interface should equally control exciton dissociation. Here, we demonstrate that in low-bandgap non-fullerene acceptor (NFA) BHJs ultrafast donor-to-acceptor energy transfer precedes hole transfer from the acceptor to the donor and thus renders the EA offset virtually unimportant. Moreover, sizeable bulk IE offsets of about 0.5 eV are needed for efficient charge transfer and high internal quantum efficiencies, since energy level bending at the donor–NFA interface caused by the acceptors’ quadrupole moments prevents efficient exciton-to-charge-transfer state conversion at low IE offsets. The same bending, however, is the origin of the barrier-less charge transfer state to free charge conversion. Our results provide a comprehensive picture of the photophysics of NFA-based blends, and show that sizeable bulk IE offsets are essential to design efficient BHJ OSCs based on low-bandgap NFAs.
• #### Bending strain tailored exchange bias in epitaxial NiMn/γ′-Fe4N bilayers

(Applied Physics Letters, AIP Publishing, 2020-09-28) [Article]
The strain tunable exchange bias has attracted much attention due to its practical applications in flexible and wearable spintronic devices. Here, the flexible epitaxial NiMn/c0-Fe4N bilayers are deposited by facing-target reactive sputtering. The maximum strain-induced change ratios of exchange bias field HEB and coercivity HC (jDHEB/HEBj and jDHC/HCj) are 51% and 22%, respectively. A large strain-induced jDHEB/HEBj appears in a thicker ferromagnetic layer, but a large jDHC/HCj) appears in a thinner ferromagnetic layer. At a compressive strain, the antiferromagnetic anisotropy of the tetragonal NiMn layer increases, resulting in an increased HC of NiMn/c0-Fe4N bilayers. The bending-strain induced changes of anisotropy magnetoresistance and planar Hall resistance are also observed at low magnetic fields. The bending-strain tailored magnetic properties can be ascribed to the distributions of ferromagnetic and antiferromagnetic anisotropies.
• #### Hydrogen Selective Catalytic Reduction of Nitrogen Oxide on Pt-and Pd-Based Catalysts for Lean-Burn Automobile Applications

(SAE International, 2020-09-15) [Conference Paper]
The utilization of H2 to catalytically treat NO emissions under lean-burn engine exhaust conditions was studied on Pt-and Pd-containing catalysts supported on CeO2 and MgO. The catalytic performance was examined using a fixed-bed reactor whose dry effluent gas stream was analyzed by an online FTIR analyzer. The catalysts NO conversion and N2 selectivity were measured in the range of 125-3000C with a feed gas composition of 0.05%NO/1%H2/10%O2/N2. The CeO2-based catalysts exhibited higher NO conversion, and the most effective catalyst was Pd/CeO2, with a conversion of 67% and selectivity of 70% near 2300C. The prepared solids were characterized using different techniques (BET, ICP-OES, CO pulse chemisorption, STEM, EELS and EDS) to correlate the structural and morphological properties of the metallic phase and the support with the catalytic activity. CeO2 is a more effective support as it yields higher metal dispersion and better facilitates the reduction of the Pt and Pd catalysts.
• #### Extension of the Surface Organometallic Chemistry to Metal-Organic Framework: development of well-defined single site [(≡Zr-O-)W(=O)(CH2tBu)3] olefin metathesis catalyst.

(Journal of the American Chemical Society, American Chemical Society (ACS), 2020-09-09) [Article]
We report here the first step by step anchoring of a W(≡CtBu)(CH2 tBu)3 complex on a highly crystalline and mesoporous MOF, namely Zr-NU-1000, using Surface organometallic Chemistry (SOMC) concept and methodology. SOMC allowed us to selectively graft the complex on the Zr6 clusters and characterize the obtained single site material by using state of the art experimental methods including extensive solid-state NMR techniques and HAADF-STEM imaging. Further FT-IR spectroscopy revealed the presence of a W=O moiety arising from the in situ reaction of the W≡CtBu functionality with the coordinated water coming from the 8-connected hexanuclear Zr6 clusters. All the steps leading to the final grafted molecular complex have been identified by DFT. The obtained material was tested for gas phase and liquid phase olefin metathesis and exhibited higher catalytic activity than the corresponding catalysts synthesized by different grafting methods. This contribution establishes the importance of applying SOMC to MOF chemistry to get well defined single site catalyst on MOF inorganic secondary building units, in particular the in situ synthesis of W=O alkyl complexes from their W carbyne analogues.
• #### A Multilayered Electron Extracting System for Efficient Perovskite Solar Cells

(Advanced Functional Materials, Wiley, 2020-09-04) [Article]
Power conversion efficiencies of perovskite solar cells (PSCs) have rapidly increased from 3.8% to a certified 25.2% within only a decade. Eliminating possible recombination losses at the interfaces is essential to further improve both efficiency and stability of this class of emerging photovoltaic technology. Herein, a simple approach for improving the electron extraction of the PC60BM electron transport layer (ETL) is presented by sequentially depositing Al:ZnO (AZO) and triphenyl-phosphine oxide (TPPO) on top of it, in a p–i–n device configuration. The efficiency of the resulting CH3NH3PbI3-based solar cell is shown to improve from 14.6%, measured for the control PC60BM-only cell, to 17.9% for double-ETL (PC60BM/AZO) and 19.2% for triple-ETL (PC60BM/AZO/TPPO)-based devices, respectively. Optimized triple-ETL-based cells exhibit high fill factor of 82%. The combination of electrical and quantum mechanical calculations shows that efficiency improvement is attributed to reduced trap-assisted recombination at the interface and better energy level alignment due to chemical interactions between PC60BM, AZO, and TPPO. Moreover, it is shown that the use of multilayer ETL results in better device stability (T80 ≈ 800 h) under constant illumination. This work opens new possibilities for inexpensive highly efficient and stable multilayered contacts for PSCs by combining organic small molecules and metal oxides.
• #### Covalent Organic Frameworks as Negative Electrodes for High-Performance Asymmetric Supercapacitors

(Advanced Energy Materials, Wiley, 2020-09-02) [Article]
New covalent organic frameworks (COFs), encompassing redox-functionalized moieties and an aza-fused π-conjugated system, are designed, synthesized, and deployed as negative electrodes in asymmetric supercapacitors (ASC), for the first time. The Hex-Aza-COFs are synthesized based on the solvothermal condensation reaction of cyclohexanehexone and redox-functionalized aromatic tetramines with benzoquinone (Hex-Aza-COF-2) or phenazine (Hex-Aza-COF-3). The redox-functionalized Hex-Aza-COFs show a specific capacitance of 585 F g−1 for Hex-Aza-COF-2 and 663 F g−1 for Hex-Aza-COF-3 in a three-electrode configuration. These values are the highest among reported COF materials and are comparable with state-of-the-art pseudocapacitive electrodes. The Hex-Aza-COFs exhibit a wide voltage window (0 to −1.0 V), which allow the construction of a two-electrode ASC device by combining them with RuO2. The complementary potential windows of Hex-Aza-COF-3 and RuO2 enable an asymmetric device with a high voltage window of 1.7 V. The RuO2//Hex-Aza-COF-3 ASC device achieves an energy density value of 23.3 W h kg−1 at a power density of 661.2 W kg−1. The newly developed negative COF materials open new prospects for the development of high-performance ASCs.
• #### Quantifying the Transverse-Electric-Dominant 260 nm Emission from Molecular Beam Epitaxy-Grown GaN-Quantum-Disks Embedded in AlN Nanowires: A Comprehensive Optical and Morphological Characterization

(ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2020-09-01) [Article]
There has been a relentless pursuit of transverse electric (TE)-dominant deep ultraviolet (UV) optoelectronic devices for efficient surface emitters to replace the environmentally unfriendly mercury lamps. To date, the use of the ternary AlGaN alloy inevitably has led to transverse magnetic (TM)-dominant emission, an approach that is facing a roadblock. Here, we take an entirely different approach of utilizing a binary GaN compound semiconductor in conjunction with ultrathin quantum disks (QDisks) embedded in AlN nanowires (NWs). The growth of GaN QDisks is realized on a scalable and low-cost Si substrate using plasma-assisted molecular beam epitaxy as a highly controllable monolayer growth platform. We estimated an internal quantum efficiency of ∼81% in a wavelength regime of ∼260 nm for these nanostructures. Additionally, strain mapping obtained by high-angle annular dark-field scanning transmission electron microscopy is studied in conjunction with the TE and TM modes of the carrier recombination. Moreover, for the first time, we quantify the TE and TM modes of the PL emitted by GaN QDisks for deep-UV emitters. We observed nearly pure TE-polarized photoluminescence emission at a polarization angle of ∼5°. This work proposes highly quantum-confined ultrathin GaN QDisks as a promising candidate for deep-UV vertical emitters.
• #### Smart covalent organic networks (CONs) with “on-off-on” light-switchable pores for molecular separation

(Science Advances, American Association for the Advancement of Science (AAAS), 2020-08-19) [Article]
Development of the new-generation membranes for tunable molecular separation requires materials with abilities beyond strict separation. Stimuli response could remotely adjust the membrane selectivity. Azobenzene derivatives can be photo-switched between trans and cis isomers under ultraviolet or visible light. Here, the azobenzenes were implanted as light switches to bridge the flexible cyclen building blocks. The smart covalent organic network membranes fold and unfold as origami that can be photo-switched between on-state (large) and off-state (small) pores. The cis membranes with off state under ultraviolet (UV) light have higher dye rejection than trans membranes with on-state channels. By controlling the trans-to-cis azobenzene isomerization via UV/Vis light, the pore size can be remotely controlled at the molecular level and the solvent permeance and dye rejection can be dynamically tuned.
• #### Lattice deformation in epitaxial Fe3O4 films on MgO substrate studied by polarized Raman spectroscopy∗

(Chinese Physics B, IOP Publishing, 2020-08-02) [Article]
The lattice structures of epitaxial Fe3O4 films deposited on MgO were studied systematically using polarized Raman spectroscopy as a function of film thickness, where interesting phenomena were observed. Firstly, the spectral conflict to the Raman selection rules (RSRs) was observed under cross sectional configuration, which can be attributed to the tetragonal deformation in the growth direction due to the lattice mismatch between Fe3O4 and MgO. Secondly, the blue-shift and broadening of Raman peaks evidenced the decrease of the tensile strain in Fe3O4 film with decreased thickness. Thirdly, distinct from the other Raman modes, the lowest T 2g mode exhibited asymmetric lineshape, which can be interpreted using the spatial correlation model. The increased correlation length introduced in the model can well explain the enhanced peak asymmetry feature with decreasing thickness. These results provide useful information for understanding the lattice structure of epitaxial Fe3O4 film.
• #### CCDC 2019617: Experimental Crystal Structure Determination

(Cambridge Crystallographic Data Centre, 2020-07-30) [Dataset]
• #### Initial Carbon−Carbon Bond Formation during the Early Stages of Methane Dehydroaromatization

(Angewandte Chemie, Wiley, 2020-07-20) [Article]
Methane dehydroaromatization (MDA) is among the most challenging processes in catalysis science owing to the inherent harsh reaction conditions and fast catalyst deactivation. To improve this process, understanding the mechanism of the initial C−C bond formation is essential. However, consensus about the actual reaction mechanism is still to be achieved. In this work, using advanced magic-angle spinning (MAS) solid-state NMR spectroscopy, we study in detail the early stages of the reaction over a well-dispersed Mo/H-ZSM-5 catalyst. Simultaneous detection of acetylene (i.e., presumably the direct C−C bond-forming product from methane), methylidene, allenes, acetal, and surface-formate species, along with the typical olefinic/aromatic species, allow us to conclude the existence of at least two independent C−H activation pathways. Moreover, this study emphasizes the significance of mobility-dependent host–guest chemistry between an inorganic zeolite and its trapped organic species during heterogeneous catalysis.
• #### Evidence for Silica Surface Three- and Five-Membered Metallacycle Intermediates in the Catalytic Cycle of Hydroaminoalkylation of Olefins Using Single-Ti-Metal Catalysts

(Organometallics, American Chemical Society (ACS), 2020-06-30) [Article]
The single-site silica-supported group IV metal amido complex [Ti(NMe2)4] gives the tris(amido)-supported fragment [(=Si−O−)Ti(−NMe2)3], which transforms into a three-membered metallacycle (called a metallaaziridine) by an αH transfer between two amido ligands. When the three-membered metallacycle reacts with 1-octene, it gives a five-membered metallacycle by insertion of the double bond into the M−C bond of the metallaziridine. These two metallacycles, key intermediates in the catalytic cycle of the hydroaminoalkylation of terminal olefins, were isolated and fully characterized following the surface organometallic chemistry (SOMC) concept and procedures. This paper shows that surface organometallic chemistry can be used to identify and fully characterize three- and five-membered metallacycles of Ti in the hydroaminoalkylation of olefins.
• #### Capacity Retention Analysis in Aluminum-Sulfur Batteries

(ACS Applied Energy Materials, American Chemical Society (ACS), 2020-06-15) [Article]
The electrochemical performance of aluminum-sulfur batteries is beset by poor stability and sluggish charge-storage properties. To address these issues, carbon allotropes have been used as electrode fillers, but successful outcomes remain inexplicably elusive. Here, a composite of sulfur and small-diameter single-walled carbon nanotubes was studied as a cathode for AlCl3:[EMIM]-based aluminum batteries. The presence of carbon nanotubes, while enabling a high capacity (1024 mAh g-1) with slower decay and reducing the electrolyte-to-sulfur ratio, is insufficient to fully stabilize the cell's performance. In fact, the main obstacle is in the interaction between sulfur and chloroaluminate ions. As we show, there is a gradual buildup of insoluble and poorly conductive discharge products that inhibit the diffusion of electroactive ions and, ultimately, cause capacity decay. Overall, this work sheds light on the carbon-sulfur-electrolyte interactions and their role on the underlying charge-storage mechanism of aluminum-sulfur batteries.
• #### Phenanthroline Covalent Organic Framework Electrodes for High-Performance Zinc-Ion Supercapattery

(ACS Energy Letters, American Chemical Society (ACS), 2020-06-08) [Article]
Aqueous zinc-ion batteries and capacitors are potentially competitive grid-scale energy storage devices because of their great features such as safety, environmental friendliness, and low cost. Herein, a completely new phenanthroline covalent organic framework (PA-COF) was synthesized and introduced in zinc-ion supercapatteries (ZISs) for the first time. Our as-synthesized PA-COF shows a high capacity of 247 mAh g-1 at a current density of 0.1 A g-1, with only 0.38% capacity decay per cycle during 10â€»000 cycles at a current density of 1.0 A g-1. Although covalent organic frameworks (COFs) are attracting great attention in many fields, our PA-COF has been synthesized using a new strategy involving the condensation reaction of hexaketocyclohexanone and 2,3,7,8-phenazinetetramine. Detailed mechanistic investigations, through experimental and theoretical methods, reveal that the phenanthroline functional groups in PA-COF are the active zinc ion storage sites. Furthermore, we provide evidence for the cointercalation of Zn2+ (60%) and H+ (40%) into PA-COF using inductively coupled plasma atomic emission spectroscopy and deuterium solid-state nuclear magnetic resonance (NMR). We believe that this study opens a new avenue for COF material design for zinc-ion storage in aqueous ZISs.
• #### Topological electronic state and anisotropic Fermi surface in half-Heusler GdPtBi

(Journal of Physics: Condensed Matter, IOP Publishing, 2020-06-05) [Article]
Half-Heusler alloys possess unique and desirable physical properties due to their thermoelectricity, magnetism, superconductivity, and weak antilocalization effects. These properties have become of particular interest since the recent discovery of topological Weyl semimetal state for which the electronic bands are dispersed linearly around one pair of Weyl nodes, with opposite chirality (i.e., chiral anomaly). Here, we report the transport signatures of topological electronic state in a half-Heusler GdPtBi single crystal. We show that the non-trivial  Berry phase, negative magnetoresistance and giant planner Hall effect arise from the chiral anomaly and that the Shubnikov-de Haas (SdH) oscillation frequency in GdPtBi is angle-dependent with an anisotropic Fermi surface (FS). All transport signatures not only demonstrate the topological electronic state in half-Heusler GdPtBi crystals, but also describe the shape of the anisotropy FS.
• #### Piezotronic AlGaN nanowire Schottky junctions grown on a metal substrate

(AIP Advances, AIP Publishing, 2020-05-11) [Article]
The non-centrosymmetric crystal structures of polar-semiconductors comprising GaN, InN, AlN, and ZnO intrigued the scientific community in investigating their potential for a strain-induced nano-energy generation. The coupled semiconducting and piezoelectric properties produce a piezo-potential that modulates the charge transport across their heterostructure interfaces. By using conductive-atomic force microscopy, we investigate the mechanism that gives rise to the piezotronic effect in AlGaN nanowires (NWs) grown on a molybdenum (Mo) substrate. By applying external bias and force on the NWs/Mo structure using a Pt–Ir probe, the charge transport across the two adjoining Schottky junctions is modulated due to the change in the apparent Schottky barrier heights (SBHs) that result from the strain-induced piezo-potential. We measured an increase in the SBH of 98.12 meV with respect to the background force, which corresponds to an SBH variation $\textstyle\frac{\partial\phi}{\partial F}$ of 6.24 meV/nN for the semiconductor/Ti/Mo interface. The SBH modulation, which is responsible for the piezotronic effect, is further studied by measuring the temperature-dependent I–V curves from room temperature to 398 K. The insights gained from the unique structure of AlGaN NWs/Mo shed light on the electronic properties of the metal-semiconductor interfaces, as well as on the potential application of AlGaN NW piezoelectric nanomaterials in optoelectronics, sensors, and energy generation applications.
• #### CCDC 1944717: Experimental Crystal Structure Determination : hexatetracontakis(mu-benzenethiolato)-ddotriacontakis(mu-hydrido)-decakis(2-methylpropan-2-amine)-henoctaconta-copper tris(tetrafluoroborate) unknown solvate

(Cambridge Crystallographic Data Centre, 2020-05-05) [Dataset]