Colloidal Synthesis of Carbon Dot-ZnSe Nanoplatelet Van der Waals Heterostructures for Boosting Photocatalytic Generation of Methanol-Storable Hydrogen.

Methanol is not only a promising liquid hydrogen carrier but also an important feedstock chemical for chemical synthesis. Catalyst design is vital for enabling the reactions to occur under ambient conditions. This study reports a new class of van der Waals heterojunction photocatalyst, which is synthesized by hot-injection method, whereby carbon dots (CDs) are grown in situ on ZnSe nanoplatelets (NPLs), i.e., metal chalcogenide quantum wells. The resultant organic-inorganic hybrid nanoparticles, CD-NPLs, are able to perform methanol dehydrogenation through CH splitting. The heterostructure has enabled light-induced charge transfer from the CDs into the NPLs occurring on a sub-nanosecond timescale, with charges remaining separated across the CD-NPLs heterostructure for longer than 500 ns. This resulted in significantly heightened H2 production rate of 107 µmole·g-1·h-1 and enhanced photocurrent density up to 34 µA cm-2 at 1 V bias potential. EPR and NMR analyses confirmed the occurrence of α-CH splitting and CC coupling. The novel CD-based organic-inorganic semiconductor heterojunction is poised to enable the discovery of a host of new nano-hybrid photocatalysts with full tunability in the band structure, charge transfer, and divergent surface chemistry for guiding photoredox pathways and accelerating reaction rates.

Evaluation of the X-ray/EUV Nanolithography Facility at AS through wavefront propagation simulations.

Synchrotron light sources can provide the required spatial coherence, stability and control to support the development of advanced lithography at the extreme ultraviolet and soft X-ray wavelengths that are relevant to current and future fabricating technologies. Here an evaluation of the optical performance of the soft X-ray (SXR) beamline of the Australian Synchrotron (AS) and its suitability for developing interference lithography using radiation in the 91.8 eV (13.5 nm) to 300 eV (4.13 nm) range are presented. A comprehensive physical optics model of the APPLE-II undulator source and SXR beamline was constructed to simulate the properties of the illumination at the proposed location of a photomask, as a function of photon energy, collimation and monochromator parameters. The model is validated using a combination of experimental measurements of the photon intensity distribution of the undulator harmonics. It is shown that the undulator harmonics intensity ratio can be accurately measured using an imaging detector and controlled using beamline optics. Finally, the photomask geometric constraints and achievable performance for the limiting case of fully spatially coherent illumination are evaluated.

Stabilizing Cobalt-free Li-rich Layered Oxide Cathodes through Oxygen Lattice Regulation by Two-phase Ru Doping.

The application of Li-rich layered oxides is hindered by their dramatic capacity and voltage decay on cycling. This work comprehensively studies the mechanistic behaviour of cobalt-free Li1.2 Ni0.2 Mn0.6 O2 and demonstrates the positive impact of two-phase Ru doping. A mechanistic transition from the monoclinic to the hexagonal behaviour is found for the structural evolution of Li1.2 Ni0.2 Mn0.6 O2, and the improvement mechanism of Ru doping is understood using the combination of in operando and post-mortem synchrotron analyses. The two-phase Ru doping improves the structural reversibility in the first cycle and restrains structural degradation during cycling by stabilizing oxygen (O2- ) redox and reducing Mn reduction, thus enabling high structural stability, an extraordinarily stable voltage (decay rate <0.45 mV per cycle), and a high capacity-retention rate during long-term cycling. The understanding of the structure-function relationship of Li1.2 Ni0.2 Mn0.6 O2 sheds light on the selective doping strategy and rational materials design for better-performance Li-rich layered oxides.

Introducing 4s-2p Orbital Hybridization to Stabilize Spinel Oxide Cathodes for Lithium-Ion Batteries.

Oxides composed of an oxygen framework and interstitial cations are promising cathode materials for lithium-ion batteries. However, the instability of the oxygen framework under harsh operating conditions results in fast battery capacity decay, due to the weak orbital interactions between cations and oxygen (mainly 3d-2p interaction). Here, a robust and endurable oxygen framework is created by introducing strong 4s-2p orbital hybridization into the structure using LiNi0.5 Mn1.5 O4 oxide as an example. The modified oxide delivers extraordinarily stable battery performance, achieving 71.4 % capacity retention after 2000 cycles at 1 C. This work shows that an orbital-level understanding can be leveraged to engineer high structural stability of the anion oxygen framework of oxides. Moreover, the similarity of the oxygen lattice between oxide electrodes makes this approach extendable to other electrodes, with orbital-focused engineering a new avenue for the fundamental modification of battery materials.

Effect of Long- and Short-Range Disorder on the Oxygen Ionic Conductivity of Tm2(Ti2-xTmx)O7-x/2 "Stuffed" Pyrochlores.

The long-range average and short-range local structures in the Tm2(Ti2-xTmx)O7-x/2 (x = 0.00-0.67) series were studied using a combination of diffraction and spectroscopic techniques. The long-range average structure, established from synchrotron X-ray and neutron powder diffraction data, shows the development of multiphase regions from x = 0.134 and the formation of antisite cation disorder from x = 0.402. The crystal field splitting of the Ti4+ ions, as derived from the Ti L3-edge X-ray absorption near-edge structure (XANES) spectroscopy, decreases gradually from 2.17 to 1.92 eV with increasing Tm3+ content (x), reflecting the increase in coordination number from 6 to predominantly 7. This is consistent with a gradual evolution of the short-range local disorder from x = 0.00 to 0.67. These results suggest that local disorder develops gradually throughout the entire composition range, whereas changes in the long-range disorder occur more suddenly. Electrochemical impedance spectroscopic results show an increase in oxygen ionic conductivity at 1000 °C, by a factor of 4 upon doping at x = 0.268. This suggests that inducing small amounts of disorder into the pyrochlore structure, by stuffing, may lead to applications of this material as a solid electrolyte in solid-oxide fuel cells.

Mechanistic implications of Li-S cell function through modification of organo-sulfur cathode architectures.

Copolymeric organo-sulfur based electrodes provide a unique framework to explore and subsequently improve lithium-sulfur (Li-S) cells. There is a general difference in the way copolymers trap lithium during cell function compared to inorganic carbon-sulfur composites. Using a chain-like polyterpene copolymeric architecture involving the copolymerization of squalene monomer with sulfur (poly(S-r-squalene)), the first evidence for distinguishable differences in the entrapment of lithiated species, when using different copolymeric architectures, is provided. Investigation of poly(S-r-squalene) as an active cathode material via X-ray Absorption Near-Edge Structure (XANES) spectroscopy and high-resolution solid-state Nuclear Magnetic Resonance (NMR) reveal notable differences compared to previously studied poly(S-r-DIB) (proposed to have a less branched architecture) between the lithium environments present during electrochemistry that can be directly linked to the copolymeric structural features. Subtle but pertinent effects based on the copolymeric architecture related to the solid-electrolyte interphase (SEI) formed from the electrolytic components are also uncovered through these techniques. This work offers a comprehensive study on poly(S-r-squalene) and reveals that foundational inverse vulcanisation conditions such as choice of crosslinking monomer can dramatically impact lithium transport and SEI formation for the copolymeric electrode.

Outstanding Thermal Conductivity of Single Atomic Layer Isotope-Modified Boron Nitride.

Materials with high thermal conductivities (κ) are valuable to solve the challenge of waste heat dissipation in highly integrated and miniaturized modern devices. Herein, we report the first synthesis of atomically thin isotopically pure hexagonal boron nitride (BN) and its one of the highest κ among all semiconductors and electric insulators. Single atomic layer (1L) BN enriched with ^{11}B has a κ up to 1009 W/mK at room temperature. We find that the isotope engineering mainly suppresses the out-of-plane optical (ZO) phonon scatterings in BN, which subsequently reduces acoustic-optical scatterings between ZO and transverse acoustic (TA) and longitudinal acoustic phonons. On the other hand, reducing the thickness to a single atomic layer diminishes the interlayer interactions and hence umklapp scatterings of the out-of-plane acoustic (ZA) phonons, though this thickness-induced κ enhancement is not as dramatic as that in naturally occurring BN. With many of its unique properties, atomically thin monoisotopic BN is promising on heat management in van der Waals devices and future flexible electronics. The isotope engineering of atomically thin BN may also open up other appealing applications and opportunities in 2D materials yet to be explored.

Elucidation of structures and lithium environments for an organo-sulfur cathode.

Composite organo-sulfur cathodes provide a unique platform for the realization of lithium-sulfur (Li-S) cells. However, difficulties arise in the interpretation of the function of these electrodes in Li-S cells and the role they play in suppressing the so-called 'shuttle effect'. This work focuses on monitoring in detail the structural evolution and lithium environments during charge-discharge cycles in a lithium half-cell of an organo-sulfur cathode, which was synthesised by inverse vulcanisation with 1,3-diisopropenylbenzene. For the first-time in organo-sulfur materials, high resolution solid state 7Li-1H and 13C-1H double resonance NMR spectroscopy coupled with X-ray absorption near-edge structure (XANES) and X-ray diffraction (XRD) are used to develop a detailed structural model of the cathode material and its lithium environments as a function of cycle number. This work provides the first experimental evidence via 2D NMR spectroscopy of distinct molecular proximities of the lithium species with respect to the sulfur, the organic skeleton and the electrolyte in the cathode material. This approach enables us to develop unparalleled understanding of the mechanisms of the high charge capacity of 607 mA h g-1, rationalising initial capacity drop and suppression of capacity fade with cycling. These results also show new possibilities on how to better understand electrode function to further increase the lithium capacities of organo-sulfur cathode materials, which can in turn lead to performance-enhanced Li-S cells.

Quick AS NEXAFS Tool (QANT): a program for NEXAFS loading and analysis developed at the Australian Synchrotron.

An analysis program for near-edge X-ray absorption fine-structure (NEXAFS) spectra has been developed and implemented at the soft X-ray beamline of the Australian Synchrotron. The program allows for instant viewing of corrected data channels including normalizations to a standard, double normalizations when the standard itself has an undesired spectral response, and background subtraction. The program performs simple compositional analysis and peak fitting and includes rapid common calculations such as the average tilt angle of molecules with respect to the surface, and the determination of the complex index of refraction, which previously required intensive manual analysis. These functionalities make common manipulations carried out with NEXAFS data quick and straightforward as spectra are collected, greatly increasing the efficiency and overall throughput of NEXAFS experiments.

An unconventional method for measuring the Tc L3-edge of technetium compounds.

Tc L3-edge XANES spectra have been collected on powder samples of SrTcO3 (octahedral Tc(4+)) and NH4TcO4 (tetrahedral Tc(7+)) immobilized in an epoxy resin. Features in the Tc L3-edge XANES spectra are compared with the pre-edge feature of the Tc K-edge as well as other 4d transition metal L3-edges. Evidence of crystal field splitting is obvious in the Tc L3-edge, which is sensitive to the coordination number and oxidation state of the Tc cation. The Tc L3 absorption edge energy difference between SrTcO3 (Tc(4+)) and NH4TcO4 (Tc(7+)) shows that the energy shift at the Tc L3-edge is an effective tool for studying changes in the oxidation states of technetium compounds. The Tc L3-edge spectra are compared with those obtained from Mo and Ru oxide standards with various oxidation states and coordination environments. Most importantly, fitting the Tc L3-edge to component peaks can provide direct evidence of crystal field splitting that cannot be obtained from the Tc K-edge.

Key role of bismuth in the magnetoelastic transitions of Ba3BiIr2O9 and Ba3BiRu2O9 as revealed by chemical doping.

The key role played by bismuth in an average intermediate oxidation state in the magnetoelastic spin-gap compounds Ba3BiRu2O9 and Ba3BiIr2O9 has been confirmed by systematically replacing bismuth with La(3+) and Ce(4+). Through a combination of powder diffraction (neutron and synchrotron), X-ray absorption spectroscopy, and magnetic properties measurements, we show that Ru/Ir cations in Ba3BiRu2O9 and Ba3BiIr2O9 have oxidation states between +4 and +4.5, suggesting that Bi cations exist in an unusual average oxidation state intermediate between the conventional +3 and +5 states (which is confirmed by the Bi L3-edge spectrum of Ba3BiRu2O9). Precise measurements of lattice parameters from synchrotron diffraction are consistent with the presence of intermediate oxidation state bismuth cations throughout the doping ranges. We find that relatively small amounts of doping (∼10 at%) on the bismuth site suppress and then completely eliminate the sharp structural and magnetic transitions observed in pure Ba3BiRu2O9 and Ba3BiIr2O9, strongly suggesting that the unstable electronic state of bismuth plays a critical role in the behavior of these materials.

2D Ferromagnetic M3GeTe2 (M = Ni/Fe) for Boosting Intermediates Adsorption toward Faster Water Oxidation.

In this work, 2D ferromagnetic M3GeTe2 (MGT, M = Ni/Fe) nanosheets with rich atomic Te vacancies (2D-MGTv) are demonstrated as efficient OER electrocatalyst via a general mechanical exfoliation strategy. X-ray absorption spectra (XAS) and scanning transmission electron microscope (STEM) results validate the dominant presence of metal-O moieties and rich Te vacancies, respectively. The formed Te vacancies are active for the adsorption of OH* and O* species while the metal-O moieties promote the O* and OOH* adsorption, contributing synergistically to the faster oxygen evolution kinetics. Consequently, 2D-Ni3GeTe2v exhibits superior OER activity with only 370 mV overpotential to reach the current density of 100 mA cm-2 and turnover frequency (TOF) value of 101.6 s-1 at the overpotential of 200 mV in alkaline media. Furthermore, a 2D-Ni3GeTe2v-based anion-exchange membrane (AEM) water electrolysis cell (1 cm2) delivers a current density of 1.02 and 1.32 A cm-2 at the voltage of 3 V feeding with 0.1 and 1 m KOH solution, respectively. The demonstrated metal-O coordination with abundant atomic vacancies for ferromagnetic M3GeTe2 and the easily extended preparation strategy would enlighten the rational design and fabrication of other ferromagnetic materials for wider electrocatalytic applications.

Influence of cationic surfactants on the formation and surface oxidation states of gold nanoparticles produced via laser ablation.

We report on the time evolution of gold nanoparticles produced by laser ablation in the presence of the cationic surfactants cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC) in aqueous solution. The broader applicability of a laser-induced nanoparticle formation kinetic model previously developed by us for the case of anionic surfactants in aqueous solution [ J. Phys. Chem. C 2010 , 114 , 15931 - 15940 ] is shown to also apply in the presence of cationic surfactants. We explore the surface properties of the nanoparticles produced in the presence of the cationic surfactants via synchrotron X-ray photoelectron spectroscopy (XPS). The XPS data indicate that at CTA(+) concentrations approximating the aqueous critical micelle concentration Au(III) is present on the nanoparticle surface. Such oxidation is not observed at (i) lower CTA(+) concentrations, (ii) in the presence of an anionic surfactant, or (iii) in the case of pure water as a solvent.

Chemically synthesised atomically precise gold clusters deposited and activated on titania. Part II.

Synchrotron XPS was used to investigate a series of chemically synthesised, atomically precise gold clusters Au(n)(PPh3)y (n = 8, 9 and 101, y depending on the cluster size) immobilized on anatase (titania) nanoparticles. Effects of post-deposition treatments were investigated by comparison of untreated samples with analogues that have been heat treated at 200 °C in O2, or in O2 followed by H2 atmosphere. XPS data shows that the phosphine ligands are oxidised upon heat treatment in O2. From the position of the Au 4f(7/2) peak it can be concluded that the clusters partially agglomerate immediately upon deposition. Heating in oxygen, and subsequently in hydrogen, leads to further agglomeration of the gold clusters. It is found that the pre-treatment plays a crucial role in the removal of ligands and agglomeration of the clusters.

The influence of surface structure on H4SiO4 oligomerization on rutile and amorphous TiO2 surfaces: an ATR-IR and synchrotron XPS study.

Silicic acid (H(4)SiO(4)) is ubiquitous in natural aquatic systems. Applications of TiO(2) in these systems will be influenced by H(4)SiO(4) sorption and oligomerization reactions on the TiO(2) surface, and this can affect many aspects of TiO(2) reactivity. The spatial arrangement of sorption sites on a metal oxide surface can promote specific lateral interactions, such as oligomerization, between sorbed species. In this work we explore the relationship between surface structure and interfacial H(4)SiO(4) oligomerization by quantifying the extent of H(4)SiO(4) sorption and oligomerization on three TiO(2) phases; a rutile phase having well-developed (110) faces (R180), a rutile phase with poorly developed (110) faces (R60), and an amorphous TiO(2) (TiO(2(am))). The in situ ATR-IR spectra measured over time as 0.2 mM H(4)SiO(4) reacted with TiO(2) were quite different on the three TiO(2) phases. The percentage of the surface H(4)SiO(4) that was present as oligomers increased over time on all phases, but after 20 h almost all H(4)SiO(4) on the R180 surface was oligomeric, while the H(4)SiO(4) on TiO(2(am)) was predominantly monomeric. The extent of H(4)SiO(4) oligomerization on R60 was intermediate. When the TiO(2) phases reacted with 1.5 mM H(4)SiO(4) the ATR-IR spectra showed oligomeric silicates dominating the surface of all three TiO(2) phases; however, after 20 h the percentage of the surface H(4)SiO(4) present as three-dimensional polymers was ∼30, 10, and 0% on R180, R60, and TiO(2(am)) respectively. The Si 2s photoelectron peak binding energy (BE) and the H(4)SiO(4) surface coverage (Γ(Si)) were measured by XPS over a range of Γ(Si). For any given Γ(Si) the Si 2s BE's were in the order R180 > R60 > TiO(2(am)). A higher Si 2s BE indicates a greater degree of silicate polymerization. The ATR-IR and XPS results support the existing model for interfacial H(4)SiO(4) oligomerization where linear trimeric silicates are formed by insertion of a solution H(4)SiO(4) between suitably orientated adjacent bidentate sorbed monomers. The TiO(2(am)) has previously been shown to consist of ∼2 nm diameter particles with a highly disordered surface. When compared to the TiO(2(am)) surface, the regular arrangement of TiO(6) octahedra on the rutile (110) face means that sorbed H(4)SiO(4) monomers on adjacent rows of singly coordinated oxygen atoms are oriented so as to favor linear trimer formation. Higher silicate polymers can form between adjacent trimers, and this is favored on the rutile (110) surfaces compared to the TiO(2(am)). This is also expected on the basis of the arrangement of surface sites on the rutile (110) surface and because the high surface curvature inherent in a ∼2 nm spherical TiO(2(am)) particle would increase the spatial separation of adjacent trimers.

Epitaxial growth of an atom-thin layer on a LiNi0.5Mn1.5O4 cathode for stable Li-ion battery cycling.

Transition metal dissolution in cathode active material for Li-based batteries is a critical aspect that limits the cycle life of these devices. Although several approaches have been proposed to tackle this issue, this detrimental process is not yet overcome. Here, benefitting from the knowledge developed in the semiconductor research field, we apply an epitaxial method to construct an atomic wetting layer of LaTMO3 (TM = Ni, Mn) on a LiNi0.5Mn1.5O4 cathode material. Experimental measurements and theoretical analyses confirm a Stranski-Krastanov growth, where the strained wetting layer forms under thermodynamic equilibrium, and it is self-limited to monoatomic thickness due to the competition between the surface energy and the elastic energy. Being atomically thin and crystallographically connected to the spinel host lattices, the LaTMO3 wetting layer offers long-term suppression of the transition metal dissolution from the cathode without impacting its dynamics. As a result, the epitaxially-engineered cathode material enables improved cycling stability (a capacity retention of about 77% after 1000 cycles at 290 mA g-1) when tested in combination with a graphitic carbon anode and a LiPF6-based non-aqueous electrolyte solution.

Tailoring the microenvironment in Fe-N-C electrocatalysts for optimal oxygen reduction reaction performance.

Fe-N-C electrocatalysts, comprising FeN4 single atom sites immobilized on N-doped carbon supports, offer excellent activity in the oxygen reduction reaction (ORR), especially in alkaline solution. Herein, we report a simple synthetic strategy for improving the accessibility of FeN4 sites during ORR and simultaneously fine-tuning the microenvironment of FeN4 sites, thus enhancing the ORR activity. Our approach involved a simple one-step pyrolysis of a Fe-containing zeolitic imidazolate framework in the presence of NaCl, yielding a hierarchically porous Fe-N-C electrocatalyst containing tailored FeN4 sites with slightly elongated Fe-N bond distances and reduced Fe charge. The porous carbon structure improved mass transport during ORR, whilst the microenvironment optimized FeN4 sites benefitted the adsorption/desorption of ORR intermediates. Accordingly, the developed electrocatalyst, possessing a high FeN4 site density (9.9 × 1019 sites g-1) and turnover frequency (2.26 s-1), delivered remarkable ORR performance with a low overpotential (a half-wave potential of 0.90 V vs. reversible hydrogen electrode) in 0.1 mol L-1 KOH.

Photoreduction kinetics of sodium tetrachloroaurate under synchrotron soft X-ray exposure.

We report on the time evolution of the sodium tetrachloroaurate (NaAuCl(4)) chemical properties as a function of soft X-ray exposure in a dried sample on a silicon surface using X-ray photoelectron spectroscopy (XPS). Our investigations provide mechanistic insight into the photoreduction kinetics from Au(III) to Au(I) and then Au(I) to Au(0). We unambiguously show that XPS photoreduction occurs in stepwise fashion via the Au(I) state. Both photoreduction steps undergo first-order kinetics.

Validating the Electronic Structure of Vanadium Phosphate Cathode Materials.

Investigation of the electronic structure of contending battery electrode materials is an essential step for developing a detailed mechanistic understanding of charge-discharge properties. Herein, we use synchrotron soft X-ray absorption spectroscopy (XAS) in combination with complementary experiments and density functional theory calculations to map the electronic structure, band positioning, and band gap of prototype vanadium(III) phosphate cathode materials, Na3V2(PO4)3, Li3V2(PO4)3, and K3V3(PO4)4·H2O, for alkali-ion rechargeable batteries. XAS fluorescence yield and electron yield measurements reveal substantial variation in surface-to-bulk atomic structure, vanadium oxidation states, and density of oxygen hole states across all samples. We attribute this variation to an intrinsic alkali metal surface depletion identified across these alkali metal vanadium(III) phosphates. We propose that an alkali-depleted surface provides a beneficial interface with the bulk structure(s) that raises the Fermi level and improves surface charge transfer kinetics. Furthermore, we discuss how this effect can play a significant role in reducing the electronic and ionic diffusion limitations of alkali vanadium phosphates in alkali-ion rechargeable batteries. These findings clarify the electronic structure and properties of alkali metal vanadium phosphates and offer guidance on future strategies to improve vanadium phosphate battery performance.

Bias-Polarity-Dependent Direct and Inverted Marcus Charge Transport Affecting Rectification in a Redox-Active Molecular Junction.

This paper describes the transition from the normal to inverted Marcus region in solid-state tunnel junctions consisting of self-assembled monolayers of benzotetrathiafulvalene (BTTF), and how this transition determines the performance of a molecular diode. Temperature-dependent normalized differential conductance analyses indicate the participation of the HOMO (highest occupied molecular orbital) at large negative bias, which follows typical thermally activated hopping behavior associated with the normal Marcus regime. In contrast, hopping involving the HOMO dominates the mechanism of charge transport at positive bias, yet it is nearly activationless indicating the junction operates in the inverted Marcus region. Thus, within the same junction it is possible to switch between Marcus and inverted Marcus regimes by changing the bias polarity. Consequently, the current only decreases with decreasing temperature at negative bias when hopping is "frozen out," but not at positive bias resulting in a 30-fold increase in the molecular rectification efficiency. These results indicate that the charge transport in the inverted Marcus region is readily accessible in junctions with redox molecules in the weak coupling regime and control over different hopping regimes can be used to improve junction performance.