### Recent Submissions

• #### 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.
• #### Illuminating Initial Carbon-Carbon Bond Formation during the Early Stages of Methane Dehydroaromatization.

(Angewandte Chemie (International ed. in English), Wiley, 2020-06-11) [Article]
Still in 2020, methane dehydroaromatization (MDA) is among the most challenging processes in catalysis science due to the inherent harsh reaction conditions and fast catalyst deactivation. To improve it further, understanding the initial C-C bond formation mechanism is sine qua non. 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 two independent C-H activation pathways. Moreover, this study emphasizes the significance of mobility-dependent host-guest chemistry between inorganic zeolite and its organic trapped species during heterogeneous catalysis.
• #### Lattice deformation in epitaxial Fe3O4 films on MgO substrate studied by polarized Raman spectroscopy∗

(Chinese Physics B, IOP Publishing, 2020-05-25) [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.
• #### Impact of small promoter amounts on coke structure in dry reforming of methane over Ni/ZrO2

(Catalysis Science & Technology, Royal Society of Chemistry (RSC), 2020-05-22) [Article]
Coke deposition is one of the main challenges in the commercialisation of dry reforming of methane over supported Ni catalysts. Besides the coke quantity, the structure of the deposits is also essential for the catalyst lifetime. Accordingly, in this study, we analysed the effect of Na, K, and Cs promoters on both these variables over Ni/ZrO2 catalysts. Besides blocking the most active coke-forming sites already at low loading, the promoting effect of the alkali metals is also contributed to by their coke gasification activity. To evaluate the additional impact of the latter, the behaviour of alkali-doped catalysts was compared to that for Mn-doped catalysts, exclusively featuring the site-blocking promotion mechanism. While the conversion is barely affected by the type of promoter, it has a profound effect on the amount and the composition of carbon deposits formed during the reaction. Promoting with K or Mn reduces the coke content to a similar degree but with less carbon fibres observed in the case of K. The promotion by Cs and Na results in the lowest coke content. The superior performance of Cs and Na-doped Ni/ZrO2 catalysts is attributed to the enhanced coke gasification via carbonate species on top of the site blocking effects.
• #### 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.
• #### Impact of Small Promoter Amounts on Coke Structure in Dry Reforming over Ni/ ZrO2

(American Chemical Society (ACS), 2020-04-23) [Preprint]
<jats:p>Coke deposition is one of the main challenges in the commercialization of dry reforming of methane over supported Ni catalysts. Besides the coke quantity, the structure of the deposits is also essential for the catalyst lifetime. Accordingly, in this study, we analysed the effect of different metal promoters on both these variables over Ni/ ZrO<sub>2</sub> catalysts. Alkali metals are known to block the most active coke forming sites already at low loading, leading to an investigation of Na, K and Cs. To analyse the possible contributions of coke gasification activity of the alkali metals, Mn was additionally used as a comparison. While the conversion is barely affected by the type of promoter, it has profound effect on the amount and the composition of carbon deposits formed during reaction: Addition of K or Mn reduces the coke content to a similar degree but with less carbon fibres observed in the case of K. Promotion by Cs and Na results in the lowest coke content, which is attributed to enhanced coke gasification via carbonate species</jats:p>
• #### [Cu81(PhS)46(tBuNH2)10(H)32]3+ Reveals the Coexistence of Large Planar Cores and Hemispherical Shells in High-Nuclearity Copper Nanoclusters

(Journal of the American Chemical Society, American Chemical Society (ACS), 2020-04-21) [Article]
Copper-based nanomaterials have attracted tremendous interest due to their unique properties in the fields of photoluminescence and catalysis. As a result, studies on the correlation between their molecular structure and their properties are of great importance. Copper nanoclusters are a new class of nanomaterials that can provide an atomic-level view of the crystal structure of copper nanoparticles. Herein, a high-nuclearity copper nanocluster with 81 copper atoms, formulated as [Cu81(PhS)46(tBuNH2)10(H)32]3+ (Cu81), was successfully synthesized and fully studied by X-ray crystallography, X-ray photoelectron spectroscopy, hydrogen evolution experiments, electrospray ionization mass spectrometry, nuclear magnetic resonance spectroscopy, and density functional theory calculations. Cu81 exhibits extraordinary structural characteristics, including (i) three types of novel epitaxial surface-protecting motifs; (ii) an unusual planar Cu17 core; (iii) a hemispherical shell, comprised of a curved surface layer and a planar surface layer; and (iv) two distinct, self-organized arrangements of protective ligands on the curved and planar surfaces. The present study sheds light on structurally unexplored copper nanomaterials and paves the way for the synthesis of high-nuclearity copper nanoclusters.
• #### Effect of surface roughness on the anomalous Hall effect in Fe thin films

(Physical Review B, American Physical Societyrevtex@aps.org, 2020-04-01) [Article]
Surface roughness plays an important role on the magnetotransport properties of thin films, especially in ultrathin films. In this work, we prepared Fe thin films with various surface roughness by using different seed layers and studied the electrical transport and anomalous Hall effect. By tuning surface roughness scattering, the longitudinal resistivity (ρxx) measured at 5 K increases by one order of magnitude and the corresponding anomalous Hall resistivity (ρAHE) increases by three times with increasing roughness. The intrinsic, skew-scattering, and side-jump contributions to ρAHE were separated from our data. The anomalous Hall angle depends on the surface roughness, which may be of importance to the material engineering for achieving large spin Hall angle.
• #### Ligand-free gold nanoclusters confined in mesoporous silica nanoparticles for styrene epoxidation

(Nanoscale Advances, Royal Society of Chemistry (RSC), 2020-03-18) [Article]
We present a novel approach to produce gold nanoclusters (Au NCs) in the pores of mesoporous silica nanoparticles (MSNs) by sequential and controlled addition of metal ions and reducing agents. This impregnation technique was followed to confine Au NCs inside the pores of MSNs without adding external ligands or stabilizing agents. TEM images show a uniform distribution of monodisperse NCs with an average size of 1.37 ± 0.4 nm. Since the NCs are grown in situ in MSN pores, additional support and high temperature calcination are not required to use them as catalysts. The use of Au NC/MSNs as a catalyst for the epoxidation of styrene in the presence of tert-butyl hydroperoxide (TBHP) as a terminal oxidant resulted in an 88% conversion of styrene in 12 h with a 74% selectivity towards styrene epoxide. Our observations suggest that this remarkable catalytic performance is due to the small size of Au NCs and the strong interaction between gold and the MSNs. This catalytic conversion is environmentally friendly as it is solvent free. We believe our synthetic approach can be extended to other metal NCs offering a wide range of applications.
• #### Spray-coated graphene oxide hollow fibers for nanofiltration

(Journal of Membrane Science, Elsevier BV, 2020-03-09) [Article]
Advances in process intensification in the chemical and pharmaceutical industry depends on the availability of more sustainable separation methods. For this progress, membranes with high stability in organic solvents are needed. GO-coated membranes could be advantageous in these applications, being chemically, thermally and mechanically stable. However, simple, scalable, low-cost fabrication methods, particularly for GO deposition on hollow fibers are still in an early phase. We propose here a simple spray-coating method for deposition of GO sheets on crosslinked hollow fiber supports. We first fabricated polyetherimide hollow fiber membranes and crosslinked them with hexamethylene diamine. These supports have strong tolerance to various organic solvents. The amide reaction arising between the imide groups of polyetherimide and the amine groups of hexamethylene diamine provide even higher chemical and mechanical stability. Thereafter, the spray-coating of GO dispersions led to the formation of a stable selective layer on the hollow fibers. An excellent adhesion between GO and the substrate was achieved. The chemical reasons for that were investigated by solid state NMR. Overall, this simple method enables the application of GO hollow fiber membranes in organic solvent nanofiltration, with high performance demonstrated in water and acetone.
• #### Highly Stable Phosphonate-Based MOFs with Engineered Bandgaps for Efficient Photocatalytic Hydrogen Production

Photoactive metal-organic frameworks (MOFs) represent one of the most promising materials for photocatalytic hydrogen production, but phosphonate-based MOFs have remained largely underdeveloped compared to other conventional MOFs. Herein, a photocatalyst of 1D titanium phosphonate MOF is designed through an easy and scalable stirring hydrothermal method. Homogeneous incorporation of organophosphonic linkers can narrow the bandgap, which is due to the strong electron-donating ability of the OH functional group that can efficiently shift the top of the valence band, moving the light absorption to the visible portion of the spectrum. In addition, the unique 1D nanowire topology enhances the photoinduced charge carrier transport and separation. Accordingly, the titanium phosphonate nanowires deliver remarkably enhanced photocatalytic hydrogen evolution activity under irradiation of both visible light and a full-spectrum simulator. Such concepts of engineering both nanostructures and electronic states herald a new paradigm for designing MOF-based photocatalysts.
• #### Constructing Polymorphic Nanodomains in BaTiO 3 Films via Epitaxial Symmetry Engineering

(Advanced Functional Materials, Wiley, 2020-02-24) [Article]
Ferroelectric materials owning a polymorphic nanodomain structure usually exhibit colossal susceptibilities to external mechanical, electrical, and thermal stimuli, thus holding huge potential for relevant applications. Despite the success of traditional strategies by means of complex composition design, alternative simple methods such as strain engineering have been intensively sought to achieve a polymorphic nanodomain state in lead-free, simple-composition ferroelectric oxides in recent years. Here, a nanodomain configuration with morphed structural phases is realized in an epitaxial BaTiO3 film grown on a (111)-oriented SrTiO3 substrate. Using a combination of experimental and theoretical approaches, it is revealed that a threefold rotational symmetry element enforced by the epitaxial constraint along the [111] direction of BaTiO3 introduces considerable instability among intrinsic tetragonal, orthorhombic, and rhombohedral phases. Such phase degeneracy induces ultrafine ferroelectric nanodomains (1–10 nm) with low-angle domain walls, which exhibit significantly enhanced dielectric and piezoelectric responses compared to the (001)-oriented BaTiO3 film with uniaxial ferroelectricity. Therefore, the finding highlights the important role of epitaxial symmetry in domain engineering of oxide ferroelectrics and facilitates the development of dielectric capacitors and piezoelectric devices.
• #### Effect of Strontium Doping Into CaBi2Nb2O9 Aurivillius Oxide Nanoceramics: Structural and Electrical Properties

(Journal of Nanoelectronics and Optoelectronics, American Scientific Publishers, 2020-02-14) [Article]
<jats:p>Herein, we report the synthesis and characterization of Ca<jats:sub>1-<jats:italic>x</jats:italic></jats:sub>Sr<jats:sub><jats:italic>x</jats:italic></jats:sub>Bi<jats:sub>2</jats:sub>Nb<jats:sub>2</jats:sub>O<jats:sub>9</jats:sub> (CSBNO) (0 ≤ <jats:italic>x</jats:italic> ≤ 1) nanoceramics prepared using sucrose-assisted sol–gel combustion methods. The synthesized nanoceramics were characterized by different tools like differential thermal and thermogravimetric analyzer (DTA-TG), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) spectrometer, X-ray powder diffraction (XRD), and impedance analyzer. DTA-TG reveals that the optimum temperature of calcination of CBN is higher than 1000 °C. The FTIR revealed the formation of CaBi<jats:sub>2</jats:sub>Nb<jats:sub>2</jats:sub>O<jats:sub>9</jats:sub> (CBNO) at 614 cm$^{-1}$. The XRD confirmed that all samples exhibited orthorhombic crystal structure. Increased orthorhombic distortion was spotted for doped CBNO and the structure acquires extra orthorhombicity through Sr doping. The TEM measurement inspected the increase of the grain size due to the inclusion of strontium into the orthorhombic crystal structure of CBNO from 56 nm to 76 nm. The dielectric constant measurement demonstrated that the increase in Sr content is associated with steady decrease in Curie temperature from 1207 K up to 720 K. The dielectric loss exhibited a minimum value at <jats:italic>x</jats:italic> = 0.5 and high stability along the temperature range of 300–850 K. Such property may enable this nanocomposite to be used for the application of FeRAM.</jats:p>
• #### LongQC: A Quality Control Tool for Third Generation Sequencing Long Read Data.

(G3 (Bethesda, Md.), Genetics Society of America, 2020-02-12) [Article]
We propose LongQC as an easy and automated quality control tool for genomic datasets generated by third generation sequencing (TGS) technologies such as Oxford Nanopore technologies (ONT) and SMRT sequencing from Pacific Bioscience (PacBio). Key statistics were optimized for long read data, and LongQC covers all major TGS platforms. LongQC processes and visualizes those statistics automatically and quickly.
• #### Ultra miniaturized InterDigitated electrodes platform for sensing applications

(Microelectronic Engineering, Elsevier BV, 2020-02-08) [Article]
InterDigitated Electrodes (IDEs) is a generic platform for a wide range of diverse applications with their implementation in sensing modules being a major one. We propose the use of IDCs with deep sub-micron critical dimension; equally spaced electrodes of 200 nm width for enhanced sensing performance and also the method of fabrication thereof. The transducer configuration was studied theoretically with a finite element method simulation by using COMSOL Multiphysics. The miniaturization of the IDEs up to 200 nm critical dimension with an adequate sensing area for the deposition of the polymeric materials is considered beneficial in terms of sensitivity gain. The IDCs were designed to deliver capacitance values of few pF in order to be compatible with already developed miniaturized low-power readout electronics. The transducers fabrication is performed with conventional microelectronic/micromachining processing and then coated with several semi-selective polymeric films. Besides the fabrication of multiple sensor arrays (chips) on the same silicon wafer, the miniaturization offers the integration with the readout electronics on the same chip. The evaluation of the sensing performance of the semi-selective polymer coated sensors is performed upon exposure to vapours of pure and binary mixtures of VOCs and humidity in various concentrations. The sensors demonstrate high sensitivity to the examined analytes as a result of the miniaturization, while their semi-selectivity is a key for applications in complex vapour environment discrimination.
• #### Direct imaging of an inhomogeneous electric current distribution using the trajectory of magnetic half-skyrmions

(Science Advances, American Association for the Advancement of Science (AAAS), 2020-02-08) [Article]
The direct imaging of current density vector distributions in thin films has remained a daring challenge. Here, we report that an inhomogeneous current distribution can be mapped directly by the trajectories of magnetic half-skyrmions driven by an electrical current in Pt/Co/Ta trilayer, using polar magneto-optical Kerr microscopy. The half-skyrmion carries a topological charge of 0.5 due to the presence of Dzyaloshinskii-Moriya interaction, which leads to the half-skyrmion Hall effect. The Hall angle of half-skyrmions is independent of current density and can be reduced to as small as 4° by tuning the thickness of the Co layer. The Hall angle is so small that the elongation path of half-skyrmion approximately delineates the invisible current flow as demonstrated in both a continuous film and a curved track. Our work provides a practical technique to directly map inhomogeneous current distribution even in complex geometries for both fundamental research and industrial applications.
• #### A landscape of genomic alterations at the root of a near-untreatable tuberculosis epidemic.

(BMC medicine, Springer Science and Business Media LLC, 2020-02-05) [Article]
BACKGROUND:Atypical Beijing genotype Mycobacterium tuberculosis strains are widespread in South Africa and have acquired resistance to up to 13 drugs on multiple occasions. It is puzzling that these strains have retained fitness and transmissibility despite the potential fitness cost associated with drug resistance mutations. METHODS:We conducted Illumina sequencing of 211 Beijing genotype M. tuberculosis isolates to facilitate the detection of genomic features that may promote acquisition of drug resistance and restore fitness in highly resistant atypical Beijing forms. Phylogenetic and comparative genomic analysis was done to determine changes that are unique to the resistant strains that also transmit well. Minimum inhibitory concentration (MIC) determination for streptomycin and bedaquiline was done for a limited number of isolates to demonstrate a difference in MIC between isolates with and without certain variants. RESULTS:Phylogenetic analysis confirmed that two clades of atypical Beijing strains have independently developed resistance to virtually all the potent drugs included in standard (pre-bedaquiline) drug-resistant TB treatment regimens. We show that undetected drug resistance in a progenitor strain was likely instrumental in this resistance acquisition. In this cohort, ethionamide (ethA A381P) resistance would be missed in first-line drug-susceptible isolates, and streptomycin (gidB L79S) resistance may be missed due to an MIC close to the critical concentration. Subsequent inadequate treatment historically led to amplification of resistance and facilitated spread of the strains. Bedaquiline resistance was found in a small number of isolates, despite lack of exposure to the drug. The highly resistant clades also carry inhA promoter mutations, which arose after ethA and katG mutations. In these isolates, inhA promoter mutations do not alter drug resistance, suggesting a possible alternative role. CONCLUSION:The presence of the ethA mutation in otherwise susceptible isolates from ethionamide-naïve patients demonstrates that known exposure is not an adequate indicator of drug susceptibility. Similarly, it is demonstrated that bedaquiline resistance can occur without exposure to the drug. Inappropriate treatment regimens, due to missed resistance, leads to amplification of resistance, and transmission. We put these results into the context of current WHO treatment regimens, underscoring the risks of treatment without knowledge of the full drug resistance profile.
• #### Titanium Carbide MXene Nucleation Layer for Epitaxial Growth of High-Quality GaN Nanowires on Amorphous Substrates

(ACS Nano, American Chemical Society (ACS), 2020-02-03) [Article]
Growing III-nitride nanowires on 2D materials is advantageous, as it effectively decouples the underlying growthsubstrate from the properties of the nanowires. As a relatively new family of 2D materials, MXenes are promising candidates as III-nitride nanowire nucleation layers capable of providing simultaneous transparency and conductivity. In this work, we demonstrate the direct epitaxial growth of GaN nanowires on Ti3C2 MXene films. The MXene films consist of nanoflakes spray coated onto an amorphous silica substrate. We observed an epitaxial relationship between the GaN nanowires and the MXene nanoflakes due to the compatibility between the triangular lattice of Ti3C2 MXene and the hexagonal structure of wurtzite GaN. The GaN nanowires on MXene show good material quality and partial transparency at visible wavelengths. Nanoscale electrical characterization using conductive atomic force microscopy reveals a Schottky barrier height of ∼330 meV between the GaN nanowire and the Ti3C2 MXene film. Our work highlights the potential of using MXene as a transparent and conductive preorienting nucleation layer for high-quality GaN growth on amorphous substrates.
• #### Enhanced photocatalytic hydrogen evolution from organic semiconductor heterojunction nanoparticles

(Nature Materials, Springer Science and Business Media LLC, 2020-02-03) [Article]
Photocatalysts formed from a single organic semiconductor typically suffer from inefficient intrinsic charge generation, which leads to low photocatalytic activities. We demonstrate that incorporating a heterojunction between a donor polymer (PTB7-Th) and non-fullerene acceptor (EH-IDTBR) in organic nanoparticles (NPs) can result in hydrogen evolution photocatalysts with greatly enhanced photocatalytic activity. Control of the nanomorphology of these NPs was achieved by varying the stabilizing surfactant employed during NP fabrication, converting it from a core-shell structure to an intermixed donor/acceptor blend and increasing H2 evolution by an order of magnitude. The resulting photocatalysts display an unprecedentedly high H2 evolution rate of over 60,000 µmol h-1 g-1 under 350 to 800 nm illumination, and external quantum efficiencies over 6% in the region of maximum solar photon flux.
• #### Coated sulfated zirconia/SAPO-34 for the direct conversion of CO2 to light olefins

(Catalysis Science & Technology, Royal Society of Chemistry (RSC), 2020-01-30) [Article]
The conversion of CO<sub>2</sub> to light olefins <italic>via</italic> bifunctional catalysts (<italic>i.e.</italic> metal oxides/zeolites) is a promising approach to tackle CO<sub>2</sub> emissions and, at the same time, reduce fossil-fuel dependence by closing the carbon cycle.