Crystal growth and structural analysis of perovskite chalcogenide BaZrS3_3 and Ruddlesden-Popper phase Ba3_3Zr2_2S7_7

Perovskite chalcogenides are gaining substantial interest as an emerging class of semiconductors for optoelectronic applications. High quality samples are of vital importance to examine their inherent physical properties. We report the successful crystal growth of the model system, BaZrS$_3$ and its Ruddlesden-Popper phase Ba$_3$Zr$_2$S$_7$ by flux method. X-ray diffraction analyses showed space group of $Pnma$ with lattice constants of $a$ = 7.056(3) \AA\/, $b$ = 9.962(4) \AA\/, $c$ = 6.996(3) \AA\/ for BaZrS$_3$ and $P4_2/mnm$ with $a$ = 7.071(2) \AA\/, $b$ = 7.071(2) \AA\/, $c$ = 25.418(5) \AA\/ for Ba$_3$Zr$_2$S$_7$. Rocking curves with full-width-at-half-maximum of 0.011$^\circ$ for BaZrS$_3$ and 0.027$^\circ$ for Ba$_3$Zr$_2$S$_7$ were observed. Pole figure analysis, scanning transmission electron microscopy images and electron diffraction patterns also establish high quality of grown crystals. The octahedra tilting in the corner-sharing octahedra network are analyzed by extracting the torsion angles.Comment: 4 Figures, 2 Table

Pendant Hydrogen-Bond Donors in Cobalt Catalysts Independently Enhance CO_2 Reduction

The bioinspired incorporation of pendant proton donors into transition metal catalysts is a promising strategy for converting environmentally deleterious CO_2 to higher energy products. However, the mechanism of proton transfer in these systems is poorly understood. Herein, we present a series of cobalt complexes with varying pendant secondary and tertiary amines in the ligand framework with the aim of disentangling the roles of the first and second coordination spheres in CO_2 reduction catalysis. Electrochemical and kinetic studies indicate that the rate of catalysis shows a first-order dependence on acid, CO_2, and the number of pendant secondary amines, respectively. Density functional theory studies explain the experimentally observed trends and indicate that pendant secondary amines do not directly transfer protons to CO_2, but instead bind acid molecules from solution. Taken together, these results suggest a mechanism in which noncooperative pendant amines facilitate a hydrogen-bonding network that enables direct proton transfer from acid to the activated CO_2 substrate

Ideal Bandgap in a 2D Ruddlesden-Popper Perovskite Chalcogenide for Single-junction Solar Cells

Transition metal perovskite chalcogenides (TMPCs) are explored as stable, environmentally friendly semiconductors for solar energy conversion. They can be viewed as the inorganic alternatives to hybrid halide perovskites, and chalcogenide counterparts of perovskite oxides with desirable optoelectronic properties in the visible and infrared part of the electromagnetic spectrum. Past theoretical studies have predicted large absorption coefficient, desirable defect characteristics, and bulk photovoltaic effect in TMPCs. Despite recent progresses in polycrystalline synthesis and measurements of their optical properties, it is necessary to grow these materials in high crystalline quality to develop a fundamental understanding of their optical properties and evaluate their suitability for photovoltaic application. Here, we report the growth of single crystals of a two-dimensional (2D) perovskite chalcogenide, Ba3Zr2S7, with a natural superlattice-like structure of alternating double-layer perovskite blocks and single-layer rock salt structure. The material demonstrated a bright photoluminescence peak at 1.28 eV with a large external luminescence efficiency of up to 0.15%. We performed time-resolved photoluminescence spectroscopy on these crystals and obtained an effective recombination time of ~65 ns. These results clearly show that 2D Ruddlesden-Popper phases of perovskite chalcogenides are promising materials to achieve single-junction solar cells.Comment: 4 Figure

An investigation and optimisation of electrical power generation scenarios for a sustainable Malaysia

PhD ThesisThe Malaysian Government has been introducing fuel diversification policies over the past decade by considering other sources of fuel such as alternative and renewable energy into the electricity mix as a measure to lengthen the oil and gas reserves against premature depletion. Since electricity consumption forms about a fifth of the total energy consumption, and directly impacts the country’s economy and people’s well-being, it is necessary to pay emphasis on Malaysia’s intermediate to long-term power sector planning by identifying sustainable options which will enhance Malaysia’s energy security and simultaneously mitigate climate change in line with the commitments set in the Paris Agreement. This study attempts to provide a comprehensive foresight analysis in relation to the electricity generation portfolios by exploring different energy resources and technologies to meet the electricity demand through 2015 to 2050 by a modelling approach known as Malaysia TIMES Electricity Model (MYTEM). The multiple scenarios which collectively forms MYTEM were developed by deploying ‘The Integrated Market Allocation-Energy Flow Optimisation Model System’ or in brief known as the TIMES model generator. The examined scenarios are business as usual (BAU), the two nuclear scenarios where one of them simulates the inclusion of the 2.0 GW nuclear power (NUC2) and the other demonstrates the nuclear expansion plan to reach cumulative nuclear power to 4.0 GW (NUC4), as well as the four renewable plus storage scenarios which were specified based on the application of 6 and 7 types of renewable technologies plus the integration of 7 and 14 days storage generation capacity respectively (RNW6S7, RNW6S14, RNW7S7, and RNW7S14). The results indicated that by 2050, the electricity demand for Malaysia is expected to grow to 892.30 PJ from base year levels of 475.92 PJ. One of the significant findings from the renewable energy assessment revealed that based on the International Electro-technical Commission (IEC) standards, class II offshore wind turbines have great potential for grid-connected utility-scale power generation in the South China Sea since the wind speed falls within the class II velocity range from 7.5 ms-1 to 8.5 ms-1 at altitudes between 50 to 200 m. Apart from this, Malaysia has great potential to gain electricity yield from other renewable resources such as hydro, solar, geothermal, biomass, and biogas. Out of all the MYTEM scenarios, the RNW7S14 scenario would be the most feasible model for implementation from an investment perspective and the most effective model for CO2 abatement, followed by RNW7S7, RNW6S14, and RNW6S7. The intermittency issue caused by renewables can be resolved with the integration of pumped hydro storage (PHS) system into the grid. ii To conclude, MYTEM substantiated that Malaysia does not need to embrace nuclear power as other renewable-based technologies such as hydropower could generate the equivalent baseload and peak load electricity, while solar photovoltaics combined with PHS system could cater to the rise in electricity demand which occurs in the afternoon due to the increase in air-condition usage and industrial sector demand. Furthermore, MYTEM demonstrated that by 2050, 98.37% of the electricity generation portfolio could be sourced from renewable energy which simultaneously enhances Malaysia’s energy security and decarbonises the environment. Ultimately, this study contributed to knowledge by providing a novel consolidated research methodological framework in modelling the reference energy system specially customised for electrical power that could be applied to other long term energy resource optimisation studies at country levelPublic Service Department (JPA) of Malaysia and to all Malaysians who funded my doctoral studies

Taming Tin(IV) Polyazides

The first charge-neutral Lewis base adducts of tin(IV) tetraazide, [Sn(N3)4(bpy)], [Sn(N3)4(phen)] and [Sn(N3)4(py)2], and the salt bis{bis(triphenylphosphine)iminium} hexa(azido)stannate [(PPN)2Sn(N3)6] (bpy = 2,2′-bipyridine; phen = 1,10-phenanthroline; py = pyridine; PPN = N(PPh3)2) have been prepared using covalent or ionic azide-transfer reagents and ligand-exchange reactions. The azides were isolated on the 0.3 to 1 g scale and characterized by IR and NMR spectroscopies, microanalytical and thermal methods and their molecular structures determined by single-crystal XRD. All complexes have a distorted octahedral Sn[N]6 coordination geometry and possess greater thermal stability than their Si and Ge homologues. The nitrogen content of the adducts of up to 44 % exceed any SnIV compound known hitherto

Electronically Modified Cobalt Aminopyridine Complexes Reveal an Orthogonal Axis for Catalytic Optimization for CO₂ Reduction

The design of effective electrocatalysts for carbon dioxide reduction requires understanding the mechanistic underpinnings governing the binding, reduction, and protonation of CO₂. A critical aspect to understanding and tuning these factors for optimal catalysis revolves around controlling the electronic environments of the primary and secondary coordination sphere. Herein we report a series of para-substituted cobalt aminopyridine macrocyclic catalysts 2–4 capable of carrying out the electrochemical reduction of CO₂ to CO. Under catalytic conditions, complexes 2–4, as well as the unsubstituted cobalt aminopyridine complex 1, exhibit i_(cat)/i_p values ranging from 144 to 781. Complexes 2 and 4 exhibit a pronounced precatalytic wave suggestive of an ECEC mechanism. A Hammett analysis reveals that ligand modifications with electron-donating groups enhance catalysis (ρ < 0), indicative of positive charge buildup in the transition state. This trend also extends to the Co^(I/0) potential, where complexes possessing more negative E(CoI/0) reductions exhibit greater i_(cat)/i_p values. The reported modifications offer a synthetic lever to tune catalytic activity, orthogonal to our previous study of the role of pendant hydrogen bond donors

Proton-Assisted Reduction of CO_2 by Cobalt Aminopyridine Macrocycles

We report here the efficient reduction of CO_2 to CO by cobalt aminopyridine macrocycles. The effect of the pendant amines on catalysis was investigated. Several cobalt complexes based on the azacalix[4](2,6)pyridine framework with different substitutions on the pendant amine groups have been synthesized (R = H (1), Me (2), and allyl (3)), and their electrocatalytic properties were explored. Under an atmosphere of CO_2 and in the presence of weak Brønsted acids, large catalytic currents are observed for 1, corresponding to the reduction of CO_2 to CO with excellent Faradaic efficiency (98 ± 2%). In comparison, complexes 2 and 3 generate CO with TONs at least 300 times lower than 1, suggesting that the presence of the pendant NH moiety of the secondary amine is crucial for catalysis. Moreover, the presence of NH groups leads to a positive shift in the reduction potential of the Co^(I/0) couple, therefore decreasing the overpotential for CO_2 reduction

Understanding the role of crystallographic shear on the electrochemical behavior of niobium oxyfluorides

The effects of shear planes in perovskite materials have been studied in order to identify their role in the electrochemical behavior of Li⁺ intercalation hosts. These planes modulate the structural stability and ionic transport pathways and therefore play an intimate role in the characteristics and performance of shear compounds. Herein, two Nb-based compounds, NbO₂F and Nb₃O₇F, were chosen as representative perovskite and shear derivatives respectively to investigate the role of crystallographic shear. A series of operando measurements, including X-ray diffraction and X-ray absorption spectroscopy, in conjunction with structural analysis, Raman spectroscopy, and detailed electrochemical studies identified the effect of shear planes. It was found that shear planes led to increased structural stability during Li⁺ (de)intercalation with shear layers being maintained, while perovskite layers were seen to degrade rapidly. However, disordering in the shear plane stacking introduced during delithiation ultimately led to poor capacity retention despite structural maintenance as Li⁺ diffusion channels are disrupted