High Electronegativity Research Articles

Adsorption mechanism of Zn2+, Ni2+, Cd2+, and Cu2+ ions by carbon-based adsorbents: interpretation of the adsorption isotherms via physical modelling.

A theoretical physicochemical and thermodynamic investigation of the adsorption of heavy metals Zn2+, Cd2+, Ni2+, and Cu2+on carbon-based adsorbents was performed with statistical physics fundaments. Particularly, the experimental adsorption isotherms of heavy metal removal, at 30°C and pH 5, using adsorbents obtained from the pyrolysis of three biomasses (cauliflower cores, broccoli stalks, and coconut shell) were modelled and interpreted with a homogeneous statistical physics adsorption model. Calculations indicated that the heavy metal adsorption with these carbon-based materials was a multi-ionic process where several ions interact simultaneously with the same carboxylic functional group on the adsorbent surface. Adsorption capacities for these metal ions and adsorbents were correlated with electronegativity theory, which established that the adsorbate with the highest electronegativity was more readily adsorbed by the carboxylic functional groups available on the adsorbent surfaces. Also, the chemical compositions of biomass precursors explained achieved adsorption capacities for these metallic ions. The best adsorbent for heavy metal removal was obtained from CC biomass pyrolysis. Calculated adsorption energies for heavy metal removal could be associated with physisorption-type forces. Finally, the adsorption mechanism analysis was complemented with the determination of adsorption thermodynamic functions using the statistical physics.

Understanding the promoter effect of bifunctional (Pt, Ni, Cu)-MoO3-x/TiO2 catalysts for the hydrodeoxygenation of p-cresol: A combined DFT and experimental study

The effect of Pt, Ni, and Cu promoters on the partial reduced MoO3-x/TiO2 catalysts for the hydrodeoxygenation (HDO) reaction of p-cresol was investigated through experiments and density functional theory (DFT) calculations. The MoO3-x/TiO2 was firstly prepared and then Pt, Ni, or Cu precursor was introduced. The mixed Mo4+, Mo5+, and Mo6+ states were identified by XPS. The HR-TEM and TEM-EDX images suggest that the MoO3-x becomes amorphous and is well-dispersed on TiO2 supports leading to direct contact of the metal on MoO3-x/TiO2. The HDO of p-cresol under a low hydrogen pressure (5 bar) at 300 °C showed the %conversion as the following order: Pt-MoO3-x > Ni-MoO3-x > Cu-MoO3-x > MoO3-x. DFT results explained the differences of catalytic performance of the catalysts and the role of the promoters in enhancing the H2 activation. The high electronegativity and density of state near the Fermi level are responsible for the high catalytic performance of Pt-MoO3-x/TiO2, which shows barrierless for H2 dissociation and allows this catalyst to be active under a milder reaction temperature and pressure. These results deepen the fundamental understanding of the role of the promoters toward the key reaction step in HDO, which is beneficial for guiding the development of catalysts for biomass conversion.

Influence of Fluorinated Components on Perovskite Solar Cells Performance and Stability.

Several valuable scientific investigations have been conducted these last few years in materials design and device engineering for perovskite solar cells (PSCs) to make them competitive compared to traditional silicon-based photovoltaic technologies. Consequently, high power conversion efficiency beyond 25% is nowadays reported. However, their long-term stability remains a significant challenge to overcome. Herein, the influence of fluorinated compounds on each layer of PSCs devices and their impact on the resulted device performances and stability is spotlighted. The fluorinated compounds exhibit attractive properties due to their very high electronegativity attributed to the fluorine atom, and their strong hydrophobicity. Thus, the introduction of these compounds is found to be a successful strategy to positively suppress the surface trap states, enhancing charge collection and reducing interfacial charge recombination. Besides, a better film quality and better energy level alignment is obtained, resulting in the improvement of device photovoltaic parameters such as the open-circuit voltage (Voc ), short-circuit current (Jsc ), and fill factor (FF), and then, the device's overall power conversion efficiency (PCE). Their long-term stability is also found to further be improved.

Synthesis of functional conjugated microporous polymer/TiO2 nanocomposites and the mechanism of the photocatalytic degradation of organic pollutants

We successfully synthesize six kinds of functional conjugated microporous polymer (CMP) containing amino, hydroxyl, carboxyl and ester groups by a Sonogashira–Hagihara coupling reaction. These CMPs exhibit type-I nitrogen gas sorption isotherms, and their pore widths are approximately 1.28–1.86 nm with a relatively narrow pore size distribution that is attributed to microporous structures. By blending CMPs with TiO2 under solvothermal conditions, TiO2 is uniformly coated onto the surfaces of the CMPs, and CMP/TiO2 nanocomposites are synthesized successfully. The novel nanocomposites show excellent photocatalytic properties for the degradation of organic pollutants, such as methylene blue, ciprofloxacin and tetracycline, under visible light. The degradation rate of organic pollutants is higher than 96.8%, and after 5 recycling cycles, the degradation efficiency is still more than 93%. The effects of the different functional groups of CMPs on the photocatalytic properties are discussed for the first time. CMP/TiO2 with carboxyl groups show the smallest band gap, highest photocurrent intensity and lowest resistance compared to those of nanocomposites with amino, hydroxyl and ester groups. This result may be caused by the high electronegativity of carboxyl groups, which enhances the charge transfer rate. The photocatalytic mechanism of the CMP/TiO2 nanocomposites is investigated, and the superoxide anion (·O2−) is the main active species in the photocatalytic degradation process.

Effects of N-, P-, or O-containing ligands on gold-based complex catalysts for acetylene hydrochlorination

In line with the fulfillment of international Minamata Convention and the urgent need for developing non-mercury catalysts, Au-Lx/AC catalysts were synthesized and evaluated for the hydrochlorination of acetylene. The results showed that the interaction between ligands and Au species contributed to the enhanced catalytic activity with important and different roles of each ligand. Depending on the high electronegativity of heteroatoms, all the prepared catalysts had effectively improved their adsorption and activation capacity of hydrogen chloride. Specifically, the N-containing ligands in catalysts increased the dispersion of Au species and an enhanced adsorption of acetylene was observed. Whilst, the coordination of P-containing ligands with Au can increase the relative content of active Aun+ species and the O-containing ligands rather decreased the adsorption of acetylene. Furthermore, all the prepared Au-Lx/AC catalysts showed good stabilities based on the inhibition of the agglomeration of Au components or of coking deposition.

Gold nanocrystal decorated trimetallic metal organic frameworks as high performance electrocatalysts for oxygen evolution reaction

Electronic states of active sites of catalysts play a key role in determining electrocatalytic performances. Engineering electronic states through decoration of functional entities is an effective way to advance catalyst design. In this work, Au nanocrystal decorated Fe, Co, and Ni based trimetallic metal organic frameworks (MOFs) (termed Au/(FCN)MOFs), grown in-situ on nickel plates (NP), are developed as high performance oxygen evolution reaction (OER) electrocatalysts. At optimal Au nanocrystal decoration, the Au/(FCN)MOF/NP electrode achieves significantly improved OER performances over the (FCN)MOF/NP electrode, lowering η10 and η500 from 242 and 340 mV to 216 and 290 mV, respectively, Tafel slope from 42.1 to 31.7 mV dec−1, and current density decay from 6 to 5% after 50 h operations at an initial current density of 160 mA cm-2. The enhancement is attributed to the significantly elevated oxidation states of Fe, Co, and Ni, induced by decoration of the Au nanocrystals of high electronegativity, as evidenced with high-resolution X-ray photoelectron spectroscopy and in-situ Raman spectroscopy studies. Furthermore, the main active metal centers are identified with in-situ X-ray absorption spectroscopy study to be Ni and Co, with Fe playing a supporting role to help Ni acquire high valence states to realize enhanced OER activities.

QSAR studies on indole-azole Analogues using DTC tools; imidazole ring is more favorable for aromatase inhibition

Quantitative structure activity relationship (QSAR) studies have been performed on indole-azole based compounds to correlate molecular descriptors and aromatase inhibitory activity using Drug Theoretics and Cheminformatics (DTC) tools. For 2- and 3- [(aryl) (azolyl)methyl]indole derivatives (set-I), MATS4s, AATS4e, and ATSC7i showed good correlation with significant R2 and Q2 values as 0.93 and 0.83, while for 1-aryl-2-[(1H-imidazol-1-yl)methyl]-6-substituted-1H-indole derivatives (set-II), the derived QSAR model showed significant R2 and Q2 values as 0.93 and 0.82 respectively, with the descriptors AVP-0, MATS7s, minHaaCH. Electronic parameters such as high electronegativity and ionization potential have unfavourable influence on anti-aromatase activity of both series of compounds. Among the imidazole and 1, 2, 4-triazole rings, imidazole nucleus is more favourable due to its low electrostatic potential. The findings of the study might guide in future designing and development of more potent indole-azole based aromatase inhibitors.

Cross-dehydrogenative N-N couplings.

The relatively high electronegativity of nitrogen makes N–N bond forming cross-coupling reactions particularly difficult, especially in an intermolecular fashion. The challenge increases even further when considering the case of dehydrogenative N–N coupling reactions, which are advantageous in terms of step and atom economy, but introduce the problem of the oxidant in order to become thermodynamically feasible. Indeed, the oxidizing system must be designed to activate the target N–H bonds, while at the same time avoid undesired N–N homocoupling as well as C–N and C–C coupled side products. Thus, preciously few intermolecular hetero N–N cross-dehydrogenative couplings exist, in spite of the central importance of N–N bonds in organic chemistry. This review aims at analyzing these few rare cases and provides a perspective for future developments.

Robust Artificial Solid‐Electrolyte Interfaces with Biomimetic Ionic Channels for Dendrite‐Free Li Metal Anodes

AbstractA robust artificial solid electrolyte interphase (SEI) film with biomimetic ionic channels and high stability is rationally designed and fabricated by combining the ClO4−‐decorated metal‐organic framework (UiO‐66‐ClO4) and flexible lithiated Nafion binder (Li‐Nafion). The high electronegativity and lithiophilicity of ClO4− groups chemically anchored into the UiO‐66 channels endow the SEI film with an excellent single‐ion conducting pathway, high Li+ transference number, and outstanding ionic conductivity, which can effectively prohibit undesirable reactions of the Li metal with the electrolyte and regulate fast and uniform Li+ flux. With further assistance of the flexible Li‐Nafion binder, the resulting UiO‐66‐ClO4/Li‐Nafion (UCLN) composite film exhibits an excellent mechanical strength to suppress the growth of Li dendrites and maintain the integral stability of the Li metal anodes during cycling. Consequently, the UCLN coated Li metal anodes (Li@UCLN) deliver remarkable cycling stabilities even under a high current density of up to 20 mA cm−2 and large areal capacity of up to 30 mAh cm−2, as well as enhanced rate capacities and cycle lifespans in the full cells even under the harsh conditions for high‐energy density applications.

The many faces of fluorine: Some noncovalent interactions of fluorine compounds

Following a brief survey of some anomalous properties of fluorine, both as an atom and as a constituent of molecules, we computed electrostatic potentials on molecular surfaces to examine some noncovalent interactions of fluorinated compounds. We demonstrate that the relative rarity and weakness of organic fluorine acting as a hydrogen bond acceptor can be attributed to the low charge capacity of atomic fluorine, which is associated with its low polarizability. This prevents covalently-bonded fluorine from acquiring as much negative character as would be anticipated from its high intrinsic electronegativity. Then we show that the differing interactive behaviors of hydrocarbons and perfluorocarbons can also be interpreted on the basis of their molecular surface electrostatic potentials. The aqueous solubilities of linear alkanes and linear perfluoroalkanes can be expressed quite well as functions of their intrinsic polarities, defined in terms of their electrostatic potentials, and their molecular volumes. The molecular electrostatic potentials of the linear perfluoroalkanes explain their inertness, and their ability to repel water, i.e. the “Teflon effect”.

2D square nanosheets of Anatase TiO2: A surfactant free nanofiller for transformer oil nanofluids

Rutile TiO2 nanoparticle with high dielectric constant has already become an automatic choice as a favorable nanofiller for efficacious transformer oil (TO) nanofluids (NFs) development, thanks to their large surface area and splendid electron trapping capacity. In this work, the role of phase and morphology of TiO2 on the properties of TO has been deciphered for the first time. For this, solvothermally prepared ultra-thin, square nanosheets of purely anatase phase TiO2 (a-TiO2) with 50–70 nm dimensions are used for NF preparation in two types of commercial mineral oil. The dielectric breakdown strength and thermo-physical properties of the resultant TO NFs have been investigated thereafter. A significant enhancement (27–30%) in breakdown voltage (BDV) at very low filler fraction (0.005 wt%) due to accelerated charge scavenging capability by virtue of the inherent surface oxygen vacancies, high electronegativity and slightly higher band-gap than rutile phase is observed. Additionally, the thermal conductivity of TO is seen to rise by 13–23% (±5%) at 50C which is facilitated by long phonon lifetime mediated inter-sheet transfer of heat. Further, the electronegativity induced strong repulsive forces among the dispersed nanofillers at ultra-low concentrations leads to a surfactant-free stable liquid insulation over a prolonged time. Besides, the acidity and viscosity, the increment of which with filler fraction causes a serious drawback for NFs, are below par the alarming level of TO due to low filler fraction of a-TiO2. All the above beneficial features along with improved flash and pour points makes the synthesized 2D a-TiO2 a budding candidate for efficient NF development at very low filler fraction without any usage of surfactant.

Theoretical understanding for anchoring effect of MOFs for lithium-sulfur batteries

Metal organic framework materials (MOFs) ZIF-8, ZIF-67 and MOF-5 have been widely used to solve the shuttle effect of lithium-sulfur batteries based on the great anchoring effect. Elucidating the anchoring effect has important implications for the MOFs design. The first-principles calculation shows that the adsorption of polysulfides intermediates occurs through the Li-heteroatom (N/O) bonding with the capacity order of MOF-5 > ZIF-8 > ZIF-67. The electronegativity of Zn in ZIF-8 is lower than that of Co in ZIF-67, leading to a higher charge density of N atoms in the ligand, which results in greater adsorption. The electronegativity of O in the ligand of MOF-5 is higher than that of N in ZIF-8, which contributes to closer Li-O interaction. Besides, MOF-5 provides enough space for both Li atoms to bond with O atoms of the ligand. Accordingly, MOFs with low electronegativity metal center, high electronegativity ligand heteroatom, and sufficient ligand framework space would have great anchoring effect.

Application of coal fly ash for treatment of wastewater containing a binary mixture of copper and nickel

The adsorptive properties of coal fly ash in removing copper and nickel from waste water was investigated using batch and fixed bed column studies. Various process parameters such as solution pH, solid loading, residence time, temperature, bed height and flow rate were tested to determine the effectiveness of fly ash. The study revealed that fly ash is an effective adsorbent in the selective separation of copper and nickel from binary mixtures. Copper showed a higher competitive strength and greater affinity to active sites as it was easily adsorbed than nickel due to its smaller atomic radius and higher electronegativity. Analysis of the batch studies results indicated that the removal process followed the Pseudo Second Order kinetic model and the modified Langmuir isotherm (qm of 12.18 and 1.99 mg/g for copper and nickel respectively). Values of ΔGº, ΔHº and ΔSº showed a more energetically favourable sorption process which is endothermic in nature (ΔHo of 50 and 62 kJ/mol for copper and nickel respectively), a random solid – liquid interface and a low adsorbate – adsorbent interaction. The Thomas model better described the mechanism for adsorption of copper and nickel in the fixed bed column study and it agreed with the models in batch studies because the model predicts that the sorption process follows the Langmuir isotherm and was derived based on the second order kinetics. Generally, the study found that the predominant mechanism of adsorption was chemisorption which is defined by higher heating values, a unimolecular layer and a homogeneous approach.

Transition Metal-Based Ternary Catalysts Prepared By Electrodeposition for Hydrogen Evolution Reaction

To solve the global warming problem caused by indiscriminate use of fossil fuel, hydrogen production technology via water electrolysis is drawing attention. It is known that the Pt catalyst has been reported to be excellent for hydrogen evolution reaction (HER), which inhibits commercialization of water electrolysis due to its high price. For this reason, non-noble metal-based catalysts have been investigated to achieve cost-effectiveness with acceptable catalytic performance. It has recently been studied focusing on transition metals (Ni, Co, Mo, etc.) combined with non-metals (B, N, O, S, P, etc.) and reported indicating improved HER performance of transition metal borides and transition metal phosphides. In this study, nickel based complex containing phosphides and borides catalysts (Ni-P-B) were prepared on the carbon paper substrate using electrodeposition method. Phosphorous is reported to act as an active site due to its abundance of electrons and high electronegativity, and thus the catalytic performance of transition metal phosphides to HER increased with an increase in P ratio. On the other hand, boron has a different aspect. In transition metal borides, boron was expected to act as an active site due to its abundance and high electronegativity, but in practice the boron's electrons increase catalyst activity by donating electrons to the empty d-orbital. Nickel-based compounds containing phosphorus and boron (Ni–P–B) may be catalysts for HER, given the advantages of nickel phosphides and borides, but little research has been done on this. An experiment was conducted to increase the performance of the transition metal by adjusting the ratio of P and B in combination with the different properties of the transition metal phosphides and borides. Deposition parameters such as deposition potential/time and bath configuration were changed to control their morphology and composition. Their catalytic activity for HER was tested in half-cell measurement with the acidic electrolyte. Then the activity orders based on geometric current, specific activity, and mass activity were connected with the results from analysis on material properties. Furthermore, employing the optimized catalyst in cathode, the performance of proton exchange membrane water electrolyzer was tested.

Electrodeposited Transitional Metal-Based Electrocatalysts for Hydrogen Evolution Reaction

As the fossil fuel is continuously declining, the necessity of a sustainable and clean source of energy is increasing. Hydrogen has been considered as a potential alternative due to its “zero emission” when reacting with oxygen in electrochemical cells. In addition, hydrogen is also a promising energy carrier/ storage to compensate the intermittent nature of other renewable energy sources (e.g. sunlight, wind, and tides). However, majority of hydrogen production comes from the combustion of fossil fuel which doesn’t make hydrogen a practical sustainable energy. Electrocatalytic hydrogen evolution reaction (HER) in water electrolysis therefore has drawn considerable attention owing to water and oxygen as the “green” reactant and by-product respectively.Early works with electrocatalytic HER involved noble metals (e.g. Pt) which is not cost-effective for industrial scale production. Therefore, transitional metal-based electrocatalysts (TMEs) have been intensively studied owing to the affinity of unpaired d-band electrons for chemisorption of hydrogen atoms. Among different strategies to improve the intrinsic properties of TMEs, coupling with other materials synergistically promotes the kinetics of HER. Nickel with minimum energy for hydrogen adsorption among various nonprecious metals exhibits the most efficient catalytic activities as a binary alloy with molybdenum in alkaline HER. Hydrogen spillover and averaging effect between the hydrogen adsorption energies of nickel and molybdenum facilitate the catalytic activity of nickel molybdenum alloy. On the other hand, transitional metal phosphides, especially cobalt with similar energy for hydrogen adsorption as nickel, have been considered as promising TMEs for acidic HER. With high electronegativity, phosphorus atoms can trap the positively charged protons and consequently enhance the dissociation of hydrogen. Moreover, dissolution of phosphorus-doped cobalt is less thermodynamically favored than that of cobalt, resulting in better electrocatalytic stability. To further enhance the HER catalytic performance, both types of electrocatalysts are synthesized at non-equilibrium by electrodeposition techniques to introduce chemical disorders and metaphases as well as to widen the range of composition. Different properties of deposits (e.g. composition, morphology, and grain size) can be tuned by controlling electrolyte composition and electrodeposition parameters.

(Invited) Surface Properties of Perovskite Oxynitrides Influence Solar Water Splitting Activity

Photoelectrochemical (PEC) water splitting using semiconductors has been a promising means to directly produce hydrogen gas from water. Semiconductors capable of absorbing a wide wavelength range of visible light are in the limelight at the moment, because the favourable optical property causes thermodynamically high solar-to-hydrogen (STH) conversion efficiency. The n-type perovskite oxynitrides AB(O,N)3 (A=Ca, Sr, Ba and La, B=Ti, Ta and Nb) have intensive absorption bands in visible light above 600 nm so that they are one of the leading candidates for the solar water splitting.1 For instance, LaNbON2 has an Eg of 1.7 eV (wavelength λ = 750 nm) and thus potential for the STH conversion efficiency of 29.5% in reference to an incident photon-to-current efficiency of unity. However, the water splitting activity using the oxynitrides is relatively low and unstable under long-term illumination.1-2 The synthesis of oxynitrides, namely, the nitridation of starting oxides to the corresponding oxynitrides, is performed mostly in harsh conditions including a high temperature, long nitridation time, and a high flow rate of NH3. Such the severe nitridation obviously increases a bulk crystallinity of oxynitrides, which it permits efficient light absorption and charge separation under irradiation. In particular, it is necessary to prepare high-crystalline BaBO2N because the oxynitrides have the absence of stoichiometric, crystalline oxide precursors (with Ba/B ratio of unity) for nitridation.3-4 Unfortunately, the severe nitridation enhances reductions of B-site cations to a lower oxidation state (e.g., Nb5+ to Nb4+ or Nb3+) leading to anion vacancy and/or surface impurity phases such as NbO x N y , owing to the high electronegativity of the B-site cations. The surface defect sites, where the photoreaction takes place, promotes recombination of photoexcited holes and electrons during the PEC water oxidation, thus resulting in low photocurrent density. There were several reports on how to decrease defect density of oxynitrides during the nitridation, including flux-assisted nitridation and etching treatment by an aqua regia.4-5 However, these methods should be positively considered the suitable flux media and unintended doping depending on each oxynitride, and the partial loss of oxynitride particles. There was also a limitation in improving the photocurrent density.Herein we present the active and stable solar water oxidation over oxynitride photoanodes, which was remarkably improved by surface annealing in an inert Ar flow to increase the degree of crystallinity and as well to suppress the generation of surface defects.6 Also, we report the quantitative relationship between the surface defect density of the oxynitride and its photocurrent density, demonstrating that surface properties of oxynitrides greatly influence the solar water splitting performance.7 Oxides (or carbonates) contains A-site cations and B2O5 (B=Ta, Nb) were blended and calcined in air at different temperatures from 1273 to 1573 K for 30 h to obtain crystalline oxides such as Ba5B4O15. The starting oxides were nitrided at different temperatures and durations under a NH3 flow. The products were washed by distilled water and dried naturally. Subsequently, the as-prepared oxynitrides were annealed under an Ar flow at an appropriate temperature depending on oxynitride. For PEC measurements, particulate oxynitride photoanodes were prepared by a particle transfer method or an electrophoretic deposition.The nitridation of Ba5B4O15 caused highly-defective BaBO2N surface due to Lewis base and Ba-rich conditions of the starting oxide. The annealing in Ar both enhances the crystallinity of amorphous surface and effectively suppresses the formation of reduced defects. As a result, the annealed BaNbO2N photoanode exhibited greatly enhanced photocurrent of 5.2 mA cm-2 at 1.23 VRHE during sunlight-driven water oxidation, which is approximately twenty times higher than that of as-prepared BaNbO2N. BaTaO2N is thermally more stable in an Ar flow as compared with BaNbO2N, so that the high-temperature annealing at 1073 K was found to improve both the bulk and surface properties of inactive as-prepared BaTaO2N. Consequently, the particulate BaTaO2N photoanode followed by the Ar annealing produced a half-cell STH energy conversion efficiency of 1.4% at 0.88 VRHE. In addition, the solar water oxidation retained 79% of the initial photocurrent over 24 h. Surface and bulk characterizations of perovskite oxynitrides annealed in Ar and the corresponding photoresponse will be discussed in detail in the presentation.References J. Seo et al., Angew. Chem. Int. Ed., 2018, 57, 2.T. Hisatomi et al., Energy Environ. Sci., 2013, 6, 3595.J. Seo et al., Adv. Energy Mater., 2018, 1800094.S. Jadhav et al., J. Mater. Chem. A, 2020, 8, 1127.M. Matsukawa et al., Nano Lett., 2014, 14, 1038.J. Seo et al., ACS Appl. Energy Mater., 2019, 2, 5777.J. Seo et al., J. Mater. Chem. A, 2019, 7, 493.

Enantioselective Michael Addition Reactions to Construct SCF3‐containing Stereocenter Catalyzed by Chiral Quaternary Phosphonium Salts

AbstractThe trifluoromethylthio group is significant in drug design for its high stability, electronegativity, and lipophilicity. Herein, a series of indenone structures containing the SCF3 group and an adjacent chiral quaternary stereocenter were synthesized by an asymmetric Michael addition reaction using an ion pair catalyst derived from amino acids. The desired products were obtained in excellent yields (up to 98%) and good to high stereoselectivities (up to 97% ee and >20:1 dr).magnified image

Precursor selection in hybrid molecular beam epitaxy of alkaline-earth stannates

One of the challenges of oxide molecular beam epitaxy (MBE) is the synthesis of oxides containing metals with high electronegativity (metals that are hard to oxidize). The use of reactive organometallic precursors can potentially address this issue. To investigate the formation of radicals in MBE, we explored three carefully chosen metal-organic precursors of tin for SnO2 and BaSnO3 growth: tetramethyltin (TMT), tetraethyltin (TET), and hexamethylditin (HMDT). All three precursors produced single-crystalline, atomically smooth, and epitaxial SnO2 (101) films on r-Al2O3 (101¯2) in the presence of oxygen plasma. The study of growth kinetics revealed reaction-limited and flux-limited regimes except for TET, which also exhibited a decrease in the deposition rate with increasing temperature above ∼800 °C. Contrary to these similarities, the performance of these precursors was dramatically different for BaSnO3 growth. TMT and TET were ineffective in supplying adequate tin, whereas HMDT yielded phase-pure, stoichiometric BaSnO3 films. Significantly, HMDT resulted in phase-pure and stoichiometric BaSnO3 films even without the use of an oxygen plasma (i.e., with molecular oxygen alone). These results are discussed using the ability of HMDT to form tin radicals and therefore assisting with Sn → Sn4+ oxidation reaction. Structural and electronic transport properties of films grown using HMDT with and without oxygen plasma are compared. This study provides guideline for the choice of precursors that will enable the synthesis of metal oxides containing hard-to-oxidize metals using reactive radicals in MBE.

Unusual substituent effects in the Tr···Te triel bond

AbstractThe triel bond between the π‐hole on the triel atom of TrR3 (Tr = B, Al, Ga; R = H, F, Cl, Br) and a lone pair on the Te atom of H2Te is examined using ab initio methods. For Tr = B, the triel bond is weakened as the R substituent becomes more electronegative, while the opposite pattern is noted for Al and Ga. The weakest triel bond of all occurs in H2Te‐BF3 (2.9 kcal/mol) but is much stronger for all the other complexes (>11 kcal/mol). The placement of electron‐releasing OH, NH2, and CH3 R′ substituents on the R′2Te base strengthens the triel bond, whereas the opposite occurs for the electron‐withdrawing CN. Despite its high electronegativity, the F substituent causes a strengthening of the interaction, which is due in large part to the formation of secondary chalcogen bonds that involve the σ‐holes on Te in F2Te. However, the F atom on TrF3 is unable to act as an electron donor in these chalcogen bonds, leading to weakened F2Te··TrF3 interactions.

Ba(MoO2F)2(XO3)2 (X = Se and Te): First Cases of Noncentrosymmetric Fluorinated Molybdenum Oxide Selenite/Tellurite Through Unary Substitution for Enlarging Band Gaps and Second Harmonic Generation.

Nonlinear optical (NLO) materials have critically important applications in advanced laser technologies. However, achieving a good balance between the mutual competing NLO properties and band gap within one molecular structure remains a great challenge. In this study, two alkaline earth metal fluorinated molybdenum oxide selenite/tellurite, Ba(MoO2F)2(XO3)2 [X = Se (BMFS) and Te (BMFT)], were synthesized through a facile unary substitution: BMFS was obtained by partial substitution of oxygen atoms with highly electronegative fluorine in the parent compound BaMo2O5(SeO3)2 (BMS), while BMFT was achieved by further replacing lone-pair Se4+ cations in BMFS with heavier Te4+ cations in the same main group. By partially replacing oxygen with fluorine, BMFS shows a broadened band gap and enhanced second harmonic generation (SHG) response compared to BMS owing to the high electronegativity of fluorine anions and the favorable orientation and alignment of NLO-active [MoO5F]5- and [SeO3]2- groups, which is relatively rare for unary anion substitution. BMFS and BMFT are isostructural and both belong to the polar space group Aba2, featuring a three-dimensional (3D) double-layered framework composed of 2D [MoO4F(XO3)]∞ anionic layers interconnected by divalent barium cations. Both BMFS and BMFT exhibit good optical performance, including large SHG responses (3× and 4× KH2PO4), wide band gaps (3.30 and 3.27 eV) and optical transparency window, and high laser damage thresholds (60× and 53× AgGaS2), demonstrating their potential applications as promising second-order NLO crystals. DFT calculations have elucidated the crucial role of the [MoO5F]5- groups in the enlarged band gaps and enhanced SHG responses in BMFS and BMFT. This work proposes a feasible unary substitution strategy for synthesizing the first polar fluorinated molybdenum oxide selenite/tellurite with synchronously enlarged band gaps and SHG efficiency.