Pristine Compound Research Articles

INFLUENCE OF Ba SUBSTITUTION IN SrTiO3 PEROVSKITE: STUDIES OF PHOTOCATALYSIS AND CATALYTIC BIGINELLI REACTION

Crystalline Barium substituted Sr1-xBaxTiO3 (x =0.0, 0.04, and 0.08) system nanomaterials were successfully prepared via modified sol-gel citrate precursor technique at 800˚C temperatures. A comparative study of the structural, morphological, optical properties and catalytic activity is reported. Synthesized nanomaterials were analyzed by X-ray diffraction and FTIR. Pristine SrTiO3 compound found to be cubic structure. On substitution of Ba in Sr-site, the slight distortion in XRD diffraction peaks were observed. Photo-catalytic efficiency using UV-Visible Spectroscopy was investigated by photo-degradation of synthetic dye in Laundry wastewater employing solar light, and found to be promising photo-catalysts. The catalytic potentiality was also explored for the synthesis of dihydropyrimidinone using Biginelli reaction and the product was found to increase upto 20% for Sr0.2Ba0.8TiO3 with the highest concentration of Barium.

Feature‐rich structural, electronic, magnetic and optical properties of the fluorine‐ and nitrogen‐incorporated CaF2 compound

AbstractIn this work, two new compounds, namely CaF3 and CaF2N, are designed by incorporating fluorine and nitrogen into the CaF2 fluorite structure. The newly created materials show good thermodynamic and structural stability. CaF2 is an insulator with a band gap of 10.92 eV, exhibiting non‐magnetic feature. Under the incorporation effects, CaF3 becomes a ferromagnetic half‐metallic material with insulator spin‐up state with an energy gap of 11 eV and metallic spin‐down state. While CaF2N presents the magnetic insulator characteristic with an energy gap of 9.21 and 5.74 eV in the spin‐up and spin‐down channels, respectively. Consequently, diverse magnetic properties with total spin magnetic moments of 1 μB of CaF3 and 3 μB of CaF2N are obtained, being produced mainly by spin‐up state of the incorporated atoms. Interestingly, the population analysis indicates that Ca2+ ion remains its oxidation state in newly created systems. Unusual properties of CaF3 and CaF2N materials provide the possibility of harvesting the visible and far‐ultraviolet light with the absorption coefficient reaching values of 30 (104/cm) and 65.6 (104/cm), respectively. Theoretical predictions in this study suggest that the designed materials may be prospective candidates for use in spintronic and optoelectronic devices, applications that have not been done accounting for the pristine CaF2 compound yet.

Scattering Mechanisms and Suppression of Bipolar Diffusion Effect in Bi2Te2.85Se0.15Ix Compounds.

We investigated the anisotropic thermoelectric properties of the Bi2Te2.85Se0.15Ix (x = 0.0, 0.1, 0.3, 0.5 mol.%) compounds, synthesized by ball-milling and hot-press sintering. The electrical conductivities of the Bi2Te2.85Se0.15Ix were significantly improved by the increase of carrier concentration. The dominant electronic scattering mechanism was changed from the mixed (T ≤ 400 K) and ionization scattering (T ≥ 420 K) for pristine compound (x = 0.0) to the acoustic phonon scattering by the iodine doping. The Hall mobility was also enhanced with the increasing carrier concentration. The enhancement of Hall mobility was caused by the increase of the mean free path of the carrier from 10.8 to 17.7 nm by iodine doping, which was attributed to the reduction of point defects without the meaningful change of bandgap energy. From the electron diffraction patterns, a lattice distortion was observed in the iodine doped compounds. The modulation vector due to lattice distortion increased with increasing iodine concentration, indicating the shorter range lattice distortion in real space for the higher iodine concentration. The bipolar thermal conductivity was suppressed, and the effective masses were increased by iodine doping. It suggests that the iodine doping minimizes the ionization scattering giving rise to the suppression of the bipolar diffusion effect, due to the prohibition of the BiTe1 antisite defect, and induces the lattice distortion which decreases lattice thermal conductivity, resulting in the enhancement of thermoelectric performance.

Impact of UV Radiation on the Raman Signal of Cystine: Implications for the Detection of S-rich Organics on Mars.

Traces of life may have been preserved in ancient martian rocks in the form of molecular fossils. Yet the surface of Mars is continuously exposed to intense UV radiation detrimental to the preservation of organics. Because the payload of the next rovers going to Mars to seek traces of life will comprise Raman spectroscopy tools, laboratory simulations that document the effect of UV radiation on the Raman signal of organics appear critically needed. The experiments conducted here evidence that UV radiation is directly responsible for the increase of disorder and for the creation of electronic defects and radicals within the molecular structure of S-rich organics such as cystine, enhancing the contribution of light diffusion processes to the Raman signal. The present results suggest that long exposure to UV radiation would ultimately be responsible for the total degradation of the Raman signal of cystine. Yet because the degradation induced by UV is not instantaneous, it should be possible to detect freshly excavated S-rich organics with the Raman instruments on board the rovers. Alternatively, given the very short lifetime of organic fluorescence (nanoseconds) compared to most mineral luminescence (micro- to milliseconds), exploiting fluorescence signals might allow the detection of S-rich organics on Mars. In any case, as illustrated here, we should not expect to detect pristine S-rich organic compounds on Mars, but rather by-products of their degradation.

Single-Molecule Conductance of 1,4-Azaborine Derivatives as Models of BN-doped PAHs.

The single-molecule conductance of a series of BN-acene-like derivatives has been measured by using scanning tunneling break-junction techniques. A strategic design of the target molecules has allowed us to include azaborine units in positions that unambiguously ensure electron transport through both heteroatoms, which is relevant for the development of customized BN-doped nanographenes. We show that the conductance of the anthracene azaborine derivative is comparable to that of the pristine all-carbon anthracene compound. Notably, this heteroatom substitution has also allowed us to perform similar measurements on the corresponding pentacene-like compound, which is found to have a similar conductance, thus evidencing that B-N doping could also be used to stabilize and characterize larger acenes for molecular electronics applications. Our conclusions are supported by state-of-the-art transport calculations.

Rhenium Doping of Layered Transition-Metal Diselenides Triggers Enhancement of Photoelectrochemical Activity.

The ever decreasing sources of fossil fuels have launched extensive research of alternative materials that might play a key role in their replacement. Therefore, the scientific community continuously investigates the possibilities of maximizing the working capacity of such materials in order to fulfill energy challenges in the near future. In this context, doping of the semiconducting materials is a versatile strategy to trigger their physicochemical properties as well their electrochemical performance. Herein, the impact of rhenium doping toward photoelectrochemical activity of MoSe2 and WSe2 was studied. Our results indicate that rhenium as a dopant contributes to better overall electrochemical performance, that is, an easier electron transfer of these materials compared to pristine compounds. Additionally, the photoelectrochemical measurements revealed that the doping with rhenium generated an enhancement of the photocurrent response of MoSe2 as well as WSe2 under UV light illumination.

Hydrogen isotope exchange behavior of protonated lithium metal compounds

The exchange behaviors of hydrogen isotopes between protonated lithium metal compounds and deuterated water or tritiated water were investigated. The various protonated lithium metal compounds were prepared by acid treatment of lithium metal compounds with different crystal structures and metal compositions. The protonated lithium metal compounds could more effectively reduce the deuterium concentration in water compared with the corresponding pristine lithium metal compounds. The H+ in the protonated lithium metal compounds was speculated to be more readily exchangeable with hydrons in the aqueous solution compared with Li+ in the pristine lithium metal compounds, and the exchanged heavier isotopes were speculated to be more stably retained in the crystal structure compared with the light protons. When the tritiated water (157.7 kBq/kg) was reacted with the protonated lithium metal compounds, the protonated lithium manganese nickel cobalt oxide was found to adsorb and retain twice as much tritium (163.9 Bq/g) as the protonated lithium manganese oxide (69.9 Bq/g) and the protonated lithium cobalt oxide (75.1 Bq/g) in the equilibrium state.

Local disorder and structure relation induced by magnetic exchange interactions in A2(Mo1−yMny)2O7 pyrochlores

We present an extended X-ray absorption fine structure study at the Mo K-edge of A2(Mo1−yMny)2O7 (A = Gd, Ho; y = 0.05 and 0.10) pyrochlores, as a function of temperature and composition, coupled to diffractometric and magnetic characterizations. Extending the study reported in our previous paper on the pristine A2Mo2O7 compounds to these Mo/Mn partially substituted samples, where we hypothesize a competition between double-exchange and superexchange couplings, we aim to check which structure parameters are related to the nature of the nearest-neighbor magnetic interactions, looking for the presence and evolution of order and structure anomalies. Two Ho samples Ho2(Mo1−yMny)2O7 (y = 0.05 and 0.10) keep the spin-glass nature of the parent compositions with a strong distortion of the MoO(1)6 octahedron and of the Mo‒Ho and Mo‒Mo second shell. On the other hand, two Gd2(Mo1−yMny)2O7 samples, ferromagnetic at low temperature in the undoped case, show the appearance on a local scale of a spin-glass transition mirrored by a quite high structure disorder and by a distortion coherent with a lattice frustration after Mn-doping. Therefore, the Gd and Ho Mn-doped samples display a similar frustrated behavior, differently from the corresponding undoped ones.

Ultralow Thermal Conductivity and Enhanced Figure of Merit for CuSbSe2 via Cd-Doping

Thermoelectric properties of CuSb1–xCdxSe2 (x = 0–0.08) compounds, prepared by vacuum melting, were studied at temperatures of 300–675 K. The results indicate that Cd doping causes both remarkable increase in the Seebeck coefficient and drastic drop of lattice thermal conductivity. The enhancement of thermopower originates mainly from increase of electronic density of states, while the drop of lattice thermal conductivity can be ascribed to enhanced phonon scattering by introduced impurity (dopant) atoms. As a consequence, thermoelectric figure of merit ZT is improved with a maximum ZT = 0.55 (at 675 K) being reached for CuSb0.98Cd0.02Se2, which is around 2.2-fold higher than that of the CuSbSe2 pristine compound. Our results indicate that Cd substitution is a feasible way to improve thermoelectric performance of the CuSbSe2-based system.

Lattice effects on the multiferroic characteristics of (La, Ho) co-substituted BiFeO3

The influence of La3+ and Ho3+ co-substitution on the structural, electrical and magnetic properties of rhombohedrally distorted polycrystalline BiFeO3 is reported. The presence of a tiny residual secondary phase of Bi2Fe4O9, Bi25FeO40 is evident in all the samples, while La doping suppresses the impurity phase and favors the grain growth (size of 1–2 µm) of the pristine compound. A fivefold increase in the dielectric constant (230 at 1 MHz) is achieved for the samples with the highest La content with a remarkable reduction in the dielectric loss. LaxHoyBi1-(x+y)FeO3 ceramics exhibit lossy ferroelectric loops and the magnitude of polarization reduces upon the co-substitution. The antiferromagnetic BiFeO3 transforms to ferromagnetic at low temperatures due to the annihilation of the cycloid-spin structure. The co-substitution effectively improved the magnetic properties. ‘Ho’ substitution helps in achieving the enhanced magnetization values while ‘La’ substitution helps in increasing the magnetic coercivity, HC. The variation of the HC with the temperature is interpreted by means of the two sublattice model.

Nitronyl Nitroxide Bifunctionalized Electron-Poor Chromophores: Synthesis of Stable Dye Biradicals by Lewis Acid Promoted Desilylation.

Open shell organic molecules bearing π-cores are of great interest for optical, electronic, and magnetic applications but frequently suffer fast decomposition or lack synthetic accessibility. In this regard, nitronyl nitroxides are promising candidates for stable (bi-)radicals due to their high degree of spin delocalization along the O-N-C-N-O pentad unit. Unfortunately, they are limited to electron-rich systems so far. To overcome this limitation, we developed a synthetic procedure for the twofold spin decoration of electron-poor chromophores (Ered = -1158 mV) with nitronyl nitroxide radical moieties via selective deprotection/oxidation of the respective silylated precursors with boron fluoride and subsequent quenching with tetraethyl orthosilicate. Nitronyl nitroxide biradicals PBI-NN, IIn-NN, PhDPP-NN, ThDPP-NN, and FuDPP-NN bridged by perylene bisimide (PBI), isoindigo (IIn), and diketopyrrolopyrrole (DPP) pigment colorants were finally obtained as bench stable compounds after periodate oxidation with yields of 60-81%. The absorption spectral signatures of the chromophores remain preserved in the open shell state and match the ones of the pristine parent compounds, which allowed an a priori prediction of their optical properties. Consequently, we achieved twofold spin labeling while keeping the intrinsic properties of the electron deficient chromophores intact.

Radiolytic Degradation of Soil Carbon from the Mojave Desert by 60Co Gamma Rays: Implications for the Survival of Martian Organic Compounds Due to Cosmic Radiation.

The martian surface has been continuously exposed to galactic cosmic radiation. Since organic compounds are degraded by ionizing radiation, knowledge of their decay constants is fundamental to predicting their stability on the martian surface. In this study, we report the radiolysis constant for the destruction of soil organic compounds at a starting concentration of ∼2011 μg C/gsoil from the Mojave Desert. The soils were exposed to gamma irradiation with absorbed doses of up to 19 MGy at room temperature, representing ∼250 million years of exposure to galactic cosmic rays. The destruction of total soil organic carbon and the formation of gases were investigated by a sequential on-line analytical array coupled to gas chromatography-mass spectrometry. Soil inorganic and organic carbon were degraded exponentially with a radiolysis constant 0.3 MGy-1(30%) producing mostly carbon dioxide (93.2%), carbon monoxide (6.2%), and methane (0.6%). Using the dose rate measured by the Radiation Assessment Detector on board the Curiosity rover, we make predictions on the survival of organic compounds in the cold martian subsurface. It is estimated that soil organic compounds with initial concentrations as those found today at the Mojave Desert would have been destroyed to levels <1 ppb at 0.1 m in depth in ∼2000 Myr. Pristine organic compounds are expected to be present at a depth of ∼1.5 m. These results are relevant for the search of organic compounds in past, present, and future missions to Mars. In particular, we predict that the upcoming ExoMars will encounter pristine organic compounds at this depth.

The pulmonary toxicity of carboxylated or aminated multi-walled carbon nanotubes in mice is determined by the prior purification method

BackgroundInhalation of multi-walled carbon nanotubes (MWCNTs) poses a potential risk to human health. In order to safeguard workers and consumers, the toxic properties of MWCNTs need to be identified. Functionalization has been shown to either decrease or increase MWCNT-related pulmonary injury, depending on the type of modification. We, therefore, investigated both acute and chronic pulmonary toxicity of a library of MWCNTs derived from a common pristine parent compound (NC7000).MethodsMWCNTs were thermally or chemically purified and subsequently surface functionalized by carboxylation or amination. To evaluate pulmonary toxicity, male C57BL6 mice were dosed via oropharyngeal aspiration with either 1.6 or 4 mg/kg of each MWCNT type. Mitsui-7 MWCNT was used as a positive control. Necropsy was performed at days 3 and 60 post-exposure to collect bronchoalveolar lavage fluid (BALF) and lungs.ResultsAt day 3 all MWCNTs increased the number of neutrophils in BALF. Chemical purification had a greater effect on pro-inflammatory cytokines (IL-1β, IL-6, CXCL1) in BALF, while thermal purification had a greater effect on pro-fibrotic cytokines (CCL2, OPN, TGF-β1). At day 60, thermally purified, carboxylated MWCNTs had the strongest effect on lymphocyte numbers in BALF. Thermally purified MWCNTs caused the greatest increase in LDH and total protein in BALF. Furthermore, the thermally purified and carboxyl- or amine-functionalized MWCNTs caused the greatest number of granulomatous lesions in the lungs. The physicochemical characteristics mainly associated with increased toxicity of the thermally purified derivatives were decreased surface defects and decreased amorphous content as indicated by Raman spectroscopy.ConclusionsThese data demonstrate that the purification method is an important determinant of lung toxicity induced by carboxyl- and amine-functionalized MWCNTs.

A comparative study of the structural, optical, magnetic and magnetocaloric properties of HoCrO3 and HoCr0.85Mn0.15O3 orthochromites

In the present study, the physical properties including structural, optical and magnetic of HoCr1-xMnxO3 (x = 0, 0.15) have been thoroughly investigated and compared. A detailed structural analysis of the compounds provided a way to correlate the optical and magnetic properties with the change in Cr-site ionic radius, tilt angles and distortion in CrO6 octahedra due to Mn substitution. Further, the shift in the Raman phonon modes was associated with the increased structural distortion owing to Mn substitution in HoCrO3 compound. The variation in the Urbach energy values, measured from the diffuse reflectance spectroscopy, was found to be closely related to the octahedral distortion. The optical band gap reduced from 3.22 eV to 2.01 eV after Mn substitution. Other optical parameters such as skin depth, refractive index, extinction coefficient and optical conductivity were also perceived to vary noticeably in the substituted compound. The overall structural distortion with Mn substitution resulted in the decrease of antiferromagnetic transition temperature of the pristine compound. The strong magnetic entropy change was observed at low temperatures in pure HoCrO3 compound and its value was enhanced on Mn substitution. Therefore, the present work provides a convenient method to tune the multifunctional properties of HoCrO3 compound, which has practical applications such as photocatalyst and low-temperature magnetic refrigerant.

Polyoxometalate-Based Compounds for Photo- and Electrocatalytic Applications.

Photo/electrocatalysis of water (H2 O) splitting and CO2 reduction reactions is a promising strategy to alleviate the energy crisis and excessive CO2 emissions. For the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and CO2 reduction reaction (CO2 RR) involved, the development of effective photo/electrocatalysts is critical to reduce the activation energy and accelerate the sluggish dynamics. Polyoxometalate (POM)-based compounds with tunable compositions and diverse structures are emerging as unique photo/electrocatalysts for these reactions as they offer unparalleled advantages such as outstanding solution and redox stability, quasi-semiconductor behaviour, etc. This Minireview provides a basic introduction related to photo/electrocatalytic HER, OER and CO2 RR, followed by the classification of pristine POM-based compounds toward different catalytic reactions. Recent breakthroughs in engineering POM-based compounds as efficient photo/electrocatalysts are highlighted. Finally, the advantages, challenges, strategies and outlooks of POM-based compounds on improving photo/electrocatalytic performance are discussed.

Effect of alkali metal deintercalation on superconductivity in the KxFe2–ySe2 system

In this work, the intercalation and subsequent deintercalation of alkali metal (K) were performed on new tetragonal β-FeSe compounds, and the effect on their superconductivity was studied. The FeSe precursor was intercalated with potassium by solid-state reaction, and then the potassium was selectively etched by topochemical reaction with polar chemical solvents. The K-intercalated FeSe showed phase separation, into a major insulating phase with Fe vacancy and a minor superconducting phase without vacancy. After the topotactic deintercalation reaction, the deintercalated FeSe exhibited increased lattice parameters compared to the pristine FeSe compound, and antiferromagnetic behavior instead of superconductivity. The magnetic property can be attributed to the phase separation of the K-intercalated phases. The deintercalated phase retained the β-FeSe structure with Fe vacancies, which changed the superconductor to an antiferromagnetic metal.

A DFT+U study on the thermodynamic properties of defective Gd2Zr2O7 pyrochlore

With many technologies and applications downscaling to nanometer dimensions, single point defects can modify the physical properties of compounds significantly, and advancing the fundamental understanding is critical to defect engineering and control of material properties. In the present study, first-principles calculations based on density functional theory (DFT) are carried out to study how the presence of point defects (vacancy, interstitial and antisite) affects the mechanical and thermal properties of Gd2Zr2O7 pyrochlore. As compared with the pure Gd2Zr2O7, the defective states exhibit smaller elastic moduli and better ductility. The Debye temperatures for the defective Gd2Zr2O7 are generally lower than those for pristine compound, indicating that the defective states have larger thermal expansion coefficient. This study suggests that under irradiation the thermomechanical properties of pyrochlores are influenced significantly, and intentional controlling of the point defects may benefit to improve their mechanical stability.

Cu-doped layered P2-type Na0.67Ni0.33-xCuxMn0.67O2 cathode electrode material with enhanced electrochemical performance for sodium-ion batteries

A series of Cu-doped layered P2-type Na0.67Ni0.33-xCuxMn0.67O2 (x = 0, 0.05, 0.10, 0.15, 0.20, 0.33) were fabricated using a convenient solid-state method and studied as cathode materials for sodium-ion batteries. The microstructure and morphology of the cathode materials were examined by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) techniques. The electrochemical characteristics of Na0.67Ni0.33-xCuxMn0.67O2 samples have been investigated systematically and it shows good capacity retention, cycling stability and rate performance by introducing electrochemically active Cu2+ ions as substituents. When x= 0.15, the sample delivers an initial discharge capacity of 120 mAh g−1 at 0.1 C in the voltage range 2–4.3 V with a capacity retention of 78% after 200 cycles, and a reversible capacity of 62 mAh g-1 can be obtained at a high current rate of 20 C. Compared with the pristine compound, the enhanced electrochemical performance can be attributed to the Cu2+ inserted into the transition metal (TM) layer, which stabilizes the P2-phase structure against P2-O2 phase transition when charging to high voltage. Meanwhile, the presence of copper also contributes to the reversible capacity based on the Cu2+/Cu3+ redox reaction. This strategy can improve the cyclability and rate performance by enhancing the stability between TM layers.

Enhanced Functional Properties of Ti3C2Tx MXenes as Negative Electrodes in Sodium‐Ion Batteries by Chemical Tuning

AbstractThe large implementation of electrochemical energy storage devices requires the development of new chemistries tailored for specific uses. Sodium‐ion batteries (SIBs) can cover different application fields, however the state‐of‐the‐art negative material, hard carbon, suffers from poor cyclability and rate capability. MXenes are a vast class of 2D‐materials of the general formula Mn+1XnTx (M = transition metal, X = C or N, and T = M‐terminating group) with peculiar structural features able to reversible intercalate chemical species, such as alkaline cations. The MXene compound Ti3C2Tx is one of the most investigated, thanks to the easy preparation route by etching of the pristine compound Ti3AlC2. In this work, the effect of the etching conditions and of the postsynthesis thermal treatments on the chemical, morphological, and structural properties of Ti3C2Tx are investigated, and in turn the correlation between its features and the functional properties as negative materials in SIBs are studied. The Ti3C2Tx obtained in high hydrofluoric concentration and after a 300 °C thermal treatment shows 110 mAh g−1 at 30 mA g−1 with an average potential of 1.33 V versus Na+/Na, 100% and good rate capability, since it is still able to deliver 73 mAh g−1 at 1500 mA g−1.

Consequences of long-term water exposure for bulk crystal structure and surface composition/chemistry of nickel-rich layered oxide materials for Li-ion batteries

Water exposure of layered nickel-rich transition metal oxide electrodes, widely used in high-energy lithium-ion batteries, has detrimental effects on the electrochemical performance, which complicates electrode handling and prevents implementation of environmentally benign aqueous processing procedures. Elucidating the degradation mechanisms in play may help rationally mitigate/circumvent key challenges. Here, the bulk structural consequences of long-term (>2.5 years) deuterated water (D2O) exposure of intercalation materials with compositions LixNi0.5Co0.2Mn0.3O2 (NCM523) and LixNi0.8Co0.1Mn0.1O2 (NCM811) are studied by neutron powder diffraction (NPD). Detailed inspection of the NPD data reveals gradual formation of a secondary crystalline phase in all exposed samples, not previously reported for this system. This unknown phase forms faster in liquid- compared to vapor-exposed compounds. Structural modelling of the NPD data shows a stable level of Li/Ni anti-site defects and does not indicate any significant changes in lattice parameters or hydrogen-lithium (D+/Li+) exchange in the structure. Consequently, the secondary phase formation must take place via transformation rather than modification of the parent material. X-ray photoelectron spectroscopy data indicate formation of LiHCO3/Li2CO3 at the surface and a Li-deficient oxide in the sub-surface region of the pristine compounds, and the presence of adsorbed water and transition metal hydroxides at the exposed sample surfaces.