Recyclable MFe2O4 (M = Mn,Zn, Cu,Ni, Co) coupled micro–nano bubbles for simultaneous catalytic oxidation to remove NOx and SO2 in flue gas

NO_x can be efficiently removed by micro–nano bubbles coupling with Fe³⁺ and Mn²⁺, but the catalyst cannot be reused and the adsorption wastewater should be treated. This work developed a new technology that uses micro–nano bubbles and recyclable MFe₂O₄ to simultaneously remove NO_x and SO₂ from flue gas, and clarified the effectiveness and reaction mechanism. MFe₂O₄ (M = Mn, Zn, Cu, Ni and Co) prepared by a hydrothermal method was characterized. The results show that MFe₂O₄ can be activated to produce ˙OH which can accelerate the oxidation absorption of NO_x. Compared with no catalyst, the NO_x conversion rate increased from 32.85% to 83.88% in the NO_x–SO₂–MFe₂O₄-micro–nano bubble system, while the removal rate of SO₂ can reach 100% at room temperature. The catalytic activities of MFe₂O₄ showed the following trend: CuFe₂O₄ > ZnFe₂O₄ > MnFe₂O₄ > CoFe₂O₄ > NiFe₂O₄. The results provide a new idea for the application of advanced oxidation processes in flue gas treatment.

NO_x-SO₂-MFe₂O₄-micro–nano bubbles system for NO_x removal.     

Rapid and selective electrochemical transformation of ammonia to N2 by substoichiometric TiO2-based electrochemical system

In this study, we have developed a continuous-flow electrochemical system towards the rapid and selective conversion of ammonia to N₂, based on a tubular substoichiometric titanium dioxide (Ti₄O₇) anode and a Pd–Cu co-modified Ni foam (Pd–Cu/NF) cathode, both of which are indispensable. Under the action of a suitable anode potential, the Ti₄O₇ anode enables the conversion of Cl⁻ to chloride radicals (Cl˙), which could selectively react with ammonia to produce N₂. The anodic byproducts, e.g. NO₃⁻, were further reduced to N₂ at the Pd–Cu/NF cathode. EPR and scavenger experiments confirmed the dominant role of Cl˙ in ammonia conversion. Complete transformation of 30 mg L⁻¹ ammonia could be obtained over 40 min of continuous operation under optimal conditions. The proposed electrochemical system also exhibits enhanced oxidation kinetics compared to conventional batch systems. This study provides new insights into the rational design of a high-performance electrochemical system to address the challenging issue of ammonia pollution.

A continuous-flow electrochemical system for rapid and selective conversion of ammonia to N₂ was proposed. The system consists of a tubular substoichiometric titanium dioxide (Ti₄O₇) anode and a Pd–Cu co-modified Ni foam (Pd–Cu/NF) cathode.     

High performance GZO/p-Si heterojunction diodes fabricated by reactive co-sputtering of Zn and GaAs through the control of GZO layer thickness

The effect of thickness of Ga doped ZnO (GZO) layer on the performance of GZO/p-Si heterojunctions fabricated by reactive co-sputtering of Zn–GaAs target is investigated. GZO films were deposited at 375 °C with 0.5% GaAs area coverage of Zn target and 5% O₂ in sputtering atmosphere. X-ray diffraction and X-ray photoelectron spectroscopy show that c-axis orientation of crystallites, Ga/Zn ratio and oxygen related defects depend substantially on the thickness of films. The 200–350 nm thick GZO films display low carrier concentration ∼10¹⁷ cm⁻³, which increases to >10²⁰ cm⁻³ for thicker films. The diodes fabricated with >500 nm thick GZO layers display non-rectifying behaviour, while those fabricated with 200–350 nm thick GZO layers display nearly ideal rectification with diode factors of 1.5–2.5, along with, turn-on voltage ∼1 V, reverse saturation current ∼10⁻⁵ A, barrier height ∼0.4 eV and series resistance ∼200 Ω. The drastically improved diode performance is attributed to small Ga/Zn ratio (∼0.01) and extremely low dopant activation (∼0.3%), owing to diffusion and non-substitutional incorporation of Ga in thin GZO layers, which cause self-adjustment of doping concentration. These factors, together with c-axis orientation and chemisorbed oxygen at grain boundaries, facilitate ideal diode characteristics, not reported earlier for GZO/p-Si heterojunctions.

The effect of thickness of Ga doped ZnO (GZO) layer on the performance of GZO/p-Si heterojunctions fabricated by reactive co-sputtering of Zn–GaAs target is investigated.     

Co1−xBaxFe2O4 (x = 0, 0.25, 0.5, 0.75 and 1) nanoferrites as gas sensor towards NO2 and NH3 gases

Co_1−xBa_xFe₂O₄ (x = 0, 0.25, 0.5, 0.75 and 1) nanoferrites were synthesized using a controlled chemical co-precipitation technique. Their structural, optical, dielectric and gas sensing properties were characterized by X-ray diffractometry, UV-Vis spectroscopy and an LCR meter with a gas sensing unit. The crystalline sizes were estimated using the Scherrer formula and were found to be 7.8 nm, 14.4 nm, 21.8 nm, 16.5 nm and 30.3 nm for x = 0, 0.25, 0.5, 0.75 and 1, respectively. The fundamental optical band gaps were calculated by extrapolating the linear part of (αhυ)²vs. hυ of the synthesized nanoferrites. The SEM and EDX spectra also confirmed the formation of nanoferrites. Dramatic behavior was observed in the dielectric constant and dissipation factor with varying temperature, which provides a substantial amount of information about electric polarization. The synthesized nanoferrites were tested towards NO₂ and NH₃ gases. The order of sensitivity (%) towards NH₃ was analyzed as x = 0.75 > x = 0.5 > x = 0.25 > x = 0 > x = 1, while the order was x = 0 > 0.75 > 1 > 0.5 > 0.25 for NO₂ gas.

XRD pattern and sensitivity (%) as a function of flow rate (ppm) of Co_1−xBa_xFe₂O₄ (x = 0, 0.25, 0.5, 0.75 and 1.0) nanoferrites towards NO₂ and NH₃ gases.     

Compatibility and thermal decomposition mechanism of nitrocellulose/Cr2O3 nanoparticles studied using DSC and TG-FTIR

Nano metal oxides are common combustion catalysts for enhancing the burning rate of solid propellants. Cr₂O₃ nanoparticles (NPs) are efficient combustion catalysts for the pyrolysis of energetic components. In this study, Cr₂O₃ NPs were synthesized via a modified sol–gel method and further used for studying the thermal decomposition of nitrocellulose (NC). Differential scanning calorimetry (DSC) and thermogravimetry-Fourier-transform infrared spectroscopy (TG-FTIR) analyses indicate that the Cr₂O₃ NPs can be safely used with NC and the mechanism of the reaction between Cr₂O₃/NC and pure NC follows the Avrami–Erofeev equation: f(α) = 3(1 − α)[−ln(1 − α)]^1/3/2. The peak temperature and activation energy (E_a) for the thermal decomposition of Cr₂O₃/NC are lower than those of pure NC. NO₂ was detected at a lower temperature after NC was mixed with Cr₂O₃ NPs; this indicated the catalytically accelerated bond cleavage of NC by Cr₂O₃ NPs.

Cr₂O₃ have good compatibility with nitrocellulose and catalyze decomposition of NC through decrease the activation energy. The mechanism showed Cr₂O₃ can accelerate the O–NO₂ bond cleavage and speed up the secondary reaction.     

Hollow ZnO nanorices prepared by a simple hydrothermal method for NO2 and SO2 gas sensors

Chemoresistive gas sensors play an important role in detecting toxic gases for air pollution monitoring. However, the demand for suitable nanostructures that could process high sensing performance remains high. In this study, hollow ZnO nanorices were synthesized by a simple hydrothermal method to detect NO₂ and SO₂ toxic gases efficiently. Material characterization by some advanced techniques, such as scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy, demonstrated that the hollow ZnO nanorices had a length and diameter size of less than 500 and 160 nm, respectively. In addition, they had a thin shell thickness of less than 30 nm, formed by an assembly of tiny nanoparticles. The sensor based on the hollow ZnO nanorices could detect low concentration of NO₂ and SO₂ gasses at sub-ppm level. At an optimum operating temperature of 200 °C, the sensor had response values of approximately 15.3 and 4.8 for 1 ppm NO₂ and 1 ppm SO₂, respectively. The sensor also exhibited good stability and selectivity, suggesting that the sensor can be applied to NO₂ and SO₂ toxic gas detection in ambient air.

Hollow ZnO nanorices with an ultrathin shell show excellent response to NO₂ and SO₂ gases.     

Synthesis and performance evaluation of nanostructured NaFexCr1−X(SO4)2 cathode materials in sodium ion batteries (SIBs)

This research work focuses on the synthesis and performance evaluation of NaFe_xCr_1−X(SO₄)₂ (X = 0, 0.8 and 1.0) cathode materials in sodium ion batteries (SIBs). The novel materials having a primary particle size of around 100–200 nm were synthesized through a sol–gel process by reacting stoichiometric amounts of the precursor materials. The structural analysis confirms the formation of crystalline, phase pure materials that adopt a monoclinic crystal structure. Thermal analysis indicates the superior thermal stability of NaFe0_.8Cr_0.2(SO₄)₂ when compared to NaFe(SO₄)₂ and NaCr(SO₄)₂. Galvanostatic charge/discharge analysis indicates that the intercalation/de-intercalation of a sodium ion (Na⁺) into/from NaFe(SO₄)₂ ensues at about 3.2 V due to the Fe²⁺/Fe³⁺ active redox couple. Moreover, ex situ XRD analysis confirms that the insertion/de-insertion of sodium into/from the host structure during charging/discharging is accompanied by a reversible single-phase reaction rather than a biphasic reaction. A similar sodium intercalation/de-intercalation mechanism has been noticed in NaFe_0.8Cr_0.2(SO₄)₂which has not been reported earlier. The galvanostatic measurements and X-ray photoelectron spectroscopy (XPS) analysis confirm that the Cr²⁺/Cr³⁺ redox couple is inactive in NaFe_xCr_1−X(SO₄)₂ (X = 0, 0.8) and thus does not contribute to capacity augmentation. However, suitable carbon coating may lead to activation of the Cr²⁺/Cr³⁺ redox couple in these inactive materials.

This research work focuses on the synthesis and performance evaluation of NaFe_xCr_1−X(SO₄)₂ (X = 0, 0.8 and 1.0) cathode materials in sodium ion batteries (SIBs).     

Covalent organic framework with sulfonic acid functional groups for visible light-driven CO2 reduction

In this study, a covalent organic framework (TpPa–SO₃H) photocatalyst with sulfonic acid function groups was synthesized using a solvothermal method. The morphologies and structural properties of the as-prepared composites were characterized by X-ray diffraction, infrared spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, N₂ adsorption–desorption measurements, and field emission scanning electron microscopy. An electrochemical workstation was used to test the photoelectric performance of the materials. The results show that TpPa–SO₃H has –SO₃H functional groups and high photocatalytic performance for CO₂ reduction. After 4 h of visible-light irradiation, the amount of CO produced is 416.61 μmol g⁻¹. In addition, the TpPa–SO₃H photocatalyst exhibited chemical stability and reusability. After two testing cycles under visible light irradiation, the amount of CO produced decreased slightly to 415.23 and 409.15 μmol g⁻¹. The XRD spectra of TpPa–SO₃H were consistent before and after the cycles. Therefore, TpPa–SO₃H exhibited good photocatalytic activity. This is because the introduction of –SO₃H narrows the bandgap of TpPa–SO₃H, which enhances the visible light response range and greatly promotes the separation of photogenerated electrons.

In this study, covalent organic framework (TpPa–SO₃H) photocatalyst with sulfonic acid function groups was synthesized using a solvothermal method.     

Intriguing electronic,optical and photocatalytic performance of BSe,M2CO2 monolayers and BSe–M2CO2 (M = Ti,Zr, Hf) van der Waals heterostructures

Using density functional (DFT) theory calculations, we have investigated the electronic band structure, optical and photocatalytic response of BSe, M₂CO₂ (M = Ti, Zr, Hf) monolayers and their corresponding BSe–M₂CO₂ (M = Ti, Zr, Hf) van der Waals (vdW) heterostructures. Optimized lattice constant, bond length, band structure and bandgap values, effective mass of electrons and holes, work function and conduction and valence band edge potentials of BSe and M₂CO₂ (M = Ti, Zr, Hf) monolayers are in agreement with previously available data. Binding energies, interlayer distance and Ab initio molecular dynamic simulations (AIMD) calculations show that BSe–M₂CO₂ (M = Ti, Zr, Hf) vdW heterostructures are stable with specific stacking and demonstrate that these heterostructures might be synthesized in the laboratory. The electronic band structure shows that all the studied vdW heterostructures have indirect bandgap nature – with the CBM and VBM at the Γ–K and Γ-point of BZ for BSe–Ti₂CO₂, respectively; while for BSe–Zr₂CO₂ and BSe–Hf₂CO₂ vdW heterostructures the CBM and VBM lie at the K-point and Γ-point of BZ, respectively. Type-II band alignment in BSe–M₂CO₂ (M = Ti, Zr, Hf) vdW heterostructures prevent the recombination of electron–hole pairs, and hence are crucial for light harvesting and detection. Absorption spectra are investigated to understand the optical behavior of BSe–M₂CO₂ (M = Ti, Zr, Hf) vdW heterostructures, where the lowest energy transitions are dominated by excitons. Furthermore, BSe–M₂CO₂ (M = Ti, Zr, Hf) vdW heterostructures are found to be potential photocatalysts for water splitting at pH = 0, and exhibit enhanced optical properties in the visible light zones.

Using density functional theory calculations, we have investigated the electronic band structure, optical and photocatalytic response of BSe, M₂CO₂ (M = Ti, Zr, Hf) monolayers and their corresponding BSe–M₂CO₂ (M = Ti, Zr, Hf) van der Waals heterostructures.     

Mesoporous MnOx–CeO2 composites for NH3-SCR: the effect of preparation methods and a third dopant

In this study, different preparation methods including an oxalate route, a nano-casting strategy and a traditional co-precipitation route were applied to obtain MnOx–CeO₂ mixed oxides for selective catalytic reduction (SCR) of NO with NH₃. The catalyst prepared from the oxalate route showed improved performance for NOx conversion and SO₂ + H₂O durability. To further improve the SO₂ and H₂O resistance of catalysts, ternary oxides were prepared from the oxalate route. The catalysts were studied by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, X-ray photoelectron spectroscopy (XPS), H₂ temperature-programmed reduction (H₂-TPR), NH₃ temperature-programmed desorption (NH₃-TPD), SO₂ temperature-programmed desorption (SO₂-TPD), and in situ diffuse reflectance infrared fourier transform spectroscopy (in situ DRIFTS). The nickel–manganese–cerium ternary oxide showed the best SO₂ and H₂O durability. The reason can be ascribed to its smaller pores, amorphous structure, and moderate amount of surface Mn³⁺/oxygen species, which could decrease chemical adsorption of SO₂.

In this study, an optimal oxalate route was used to obtain nickel/cobalt doped MnOx–CeO₂ mixed oxides. Nickel doped MnOx–CeO₂ showed excellent NH₃-SCR activity and H₂O + SO₂ resistance.     

Single crystal growth and structure analysis of type-I (Na/Sr)–(Ga/Si) quaternary clathrates

Single crystals of (Na/Sr)–(Ga/Si) quaternary type-I clathrates, Na_8−ySr_yGa_xSi_46−x, were synthesized by evaporating Na from a mixture of Na–Sr–Ga–Si–Sn in a 6 : 0.5 : 1 : 2 : 1 molar ratio at 773 K for 12 h in an Ar atmosphere. Electron-probe microanalysis and single-crystal X-ray diffraction revealed that three crystals from the same product were Na_8−ySr_yGa_xSi_46−x with x and y values of 7.6, 2.96; 8.4, 3.80; and 9.1, 4.08. It was also shown that increasing the Sr and Ga contents increased the electrical resistivity of the crystal from 0.34 to 1.05 mΩ cm at 300 K.

Single crystals of (Na/Sr)–(Ga/Si) quaternary type-I clathrates, Na_8−ySr_yGa_xSi_46−x, were synthesized by evaporating Na from a mixture of Na–Sr–Ga–Si–Sn in a 6 : 0.5 : 1 : 2 : 1 molar ratio at 773 K for 12 h in an Ar atmosphere.     

Effects of amino acid ionic liquids with different cations ([N2Py], [N2222], [P2222], and [C2mim]) on wheat seedlings

The ecotoxicity of four ionic liquids with different cations (N-ethyl-pyridine alanine [N₂Py][Ala], tetraethyl phosphine l-α-amino propionic acid salt [P₂₂₂₂][Ala], 1-ethyl-3-methyl-imidazolium alanine [C₂mim][Ala], and tetraethyl ammonium l-α-amino propionic acid salt [N₂₂₂₂][Ala]) was assessed in hydroponically-grown wheat seedlings at concentrations from 200–1200 mg L⁻¹. The results showed that type of cation has a significant influence on the growth, chlorophyll and nutrient uptake of wheat seedlings (P < 0.05). We observed decreased dry weight and shorter roots and shoots in the treated seedlings with increasing IL concentrations. The contents of Chl a and Chl b in wheat seedlings exposed to ILs showed the trend of firstly increasing followed by a decrease with increasing IL concentrations, but they peaked at different concentrations of ILs. In addition, the exposure of wheat seedling to ILs containing different cations (200–1200 mg L⁻¹) led to first an increase and then a decrease of nitrogen content, and reduced the content of phosphorus and potassium. Moreover, the cellular structures, including nuclei, mitochondria, chloroplasts, cell membranes, and the cell walls of wheat leaf and root were affected to varying degrees by 600 mg L⁻¹ ILs. The negative impacts of ILs on wheat seedlings ranked from high to low were: [N₂Py][Ala] > [N₂₂₂₂][Ala] > [P₂₂₂₂][Ala] > [C₂mim][Ala]. In this work, the relatively stronger toxicity of [N₂Py][Ala] was likely contributed by ethanol, which was used to dissolve [N₂Py][Ala]. Therefore, it is not recommended to use N-ethyl-pyridine alanine ([N₂Py][Ala]) widely in practical applications.

The ecotoxicity of four ionic liquids with different cations was assessed in hydroponically-grown wheat seedlings at concentrations from 200–1200 mg L⁻¹.     

Hydrogen production from CO2 reforming of methane using zirconia supported nickel catalyst

The use of hydrogen as an alternative fuel is an attractive and promising technology as it contributes to the reduction of environmentally harmful gases. Finding environmentally friendly cheap active metal-based catalysts for H₂ rich syngas via dry reforming of methane (DRM) for industrial applications has posed a challenge. In this paper, H₂ production via CO₂ reforming of methane was investigated over 5Ni/ZrO₂ catalysts. The catalytic performance of all prepared catalysts was evaluated in a microtubular fixed bed reactor under similar reaction conditions (i.e., activation temperature at 700 °C, feed flow rate of 70 ml min⁻¹, reaction temperature 700 °C for 440 min reaction time) of CO₂ reforming of methane. Different characterization techniques such as; BET, CO₂-TPD, TGA, XRPD, Raman, and TEM, were used. The study of the textural properties of catalysts established that the BET of pristine catalyst (5NiZr) was enhanced by the addition of modifiers and promoters. A bimodal TPR distribution in the reduction temperature range of 250–550 °C was recorded. In the CO₂-TPD analysis, the strength of basicity came in this order: 5Ni15YZr > 5Ni10YZr > 5Ni5YZr > 5NiZr > 5Ni20YZr. The investigation of catalyst modifiers (MgO and Y₂O₃) resulted in the Y₂O₃ modifier improving the activity and catalytic performance better than MgO, which generated a hydrogen yield of 22%. 15% Y₂O₃ modifier loading gave the highest H₂ yield 53% in the phase of different loadings of yttria. The study of the influence of promoters (Cs, Ga, and Sr) revealed that the catalytic performance of 5Ni15YZr catalysts promoted with Sr towards the H₂ yield enhanced the activity to 62%. The promoted catalysts displayed lower carbon deposition compared to the unpromoted catalyst, which provided 25.6 wt% weight loss.

The use of hydrogen as an alternative fuel is an attractive and promising technology as it contributes to the reduction of environmentally harmful gases.     

Improving the optical and thermoelectric properties of Cs2InAgCl6 with heavy substitutional doping: a DFT insight

The next-generation indium-based lead-free halide material Cs₂InAgCl₆ is promising for photovoltaic applications due to its good air stability and non-toxic behavior. However, its wide bandgap (>3 eV) is not suitable for the solar spectrum and hence reduces its photoelectronic efficiency for device applications. Here we report a significant bandgap reduction from 2.85 eV to 0.65 eV via substitutional doping and its effects on the optoelectronic and opto-thermoelectric properties from a first-principles study. The results predict that Sn/Pb and Ga and Cu co-doping will enhance the density of states significantly near the valence band maximum (VBM) and thus reduce the bandgap via shifting the VBM upward, while alkali metals (K/Rb) slightly increase the bandgap. A strong absorption peak near the Shockley–Queisser limit is observed in the co-doped case, while in the Sn/Pb-doped case, we notice a peak in the middle of the visible region of the solar spectrum. The nature of the bandgap is indirect with Cu–Ga/Pb/Sn doping, and a significant reduction in the bandgap, from 2.85 eV to 0.65 eV, is observed in the case of Ga–Cu co-doping. We observe a significant increase in the power factor (PF) (2.03 mW m⁻¹ K⁻²) for the n-type carrier after Pb-doping, which is ∼3.5 times higher than in the pristine case (0.6 mW m ⁻¹ K⁻²) at 500 K.

The next-generation indium-based lead-free halide material Cs₂InAgCl₆ is promising for photovoltaic applications due to its good air stability and non-toxic behavior while it shows good thermoelectric properties when doped with Pb.     

Natural phosphate-supported Cu(ii), an efficient and recyclable catalyst for the synthesis of xanthene and 1,4-disubstituted-1,2,3-triazole derivatives

Cu(NO₃)₂ supported on natural phosphate, Cu(ii)/NP, was prepared by co-precipitation and applied as a heterogeneous catalyst for synthesizing xanthenes (2–3 h, 85–97%) through Knoevenagel–Michael cascade reaction of aromatic aldehydes with 1,3-cyclic diketones in ethanol under refluxing conditions. It was further used for regioselective synthesis of 1,4-disubstituted-1,2,3-triazoles (1–25 min, 95–99%) via a three-component reaction between organic halides, aromatic alkynes and sodium azide in methanol at room temperature. The proposed catalyst, Cu(ii)/NP, was characterized using X-ray fluorescence, X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, Brunauer–Emmett–Teller, Barrett–Joyner–Halenda and inductively coupled plasma analyses. Compared to other reports in literature, the reactions took place through a simple co-precipitation, having short reaction time (<3 hours), high reaction yield (>85%), and high recyclability of catalyst (>5 times) without significant decrease in the inherent property and selectivity of catalyst. The proposed protocols provided significant economic and environmental advantages.

Cu(NO₃)₂ supported on natural phosphate, Cu(ii)/NP, was prepared by co-precipitation and characterized. The Cu(ii)/NP catalyzed the synthesis of xanthenes and triazoles. The proposed protocols provided significant economic and environmental advantages.     

SnO–Sn3O4 heterostructural gas sensor with high response and selectivity to parts-per-billion-level NO2 at low operating temperature

Considering the harmfulness of nitrogen dioxide (NO₂), it is important to develop NO₂ sensors with high responses and low limits of detection. In this study, we synthesize a novel SnO–Sn₃O₄ heterostructure through a one-step solvothermal method, which is used for the first time as an NO₂ sensor. The material exhibits three-dimensional flower-like microparticles assembled by two-dimensional nanosheets, in situ-formed SnO–Sn₃O₄ heterostructures, and large specific surface area. Gas sensing measurements show that the responses of the SnO–Sn₃O₄ heterostructure to 500 ppb NO₂ are as high as 657.4 and 63.4 while its limits of detection are as low as 2.5 and 10 parts per billion at 75 °C and ambient temperature, respectively. In addition, the SnO–Sn₃O₄ heterostructure has an excellent selectivity to NO₂, even if exposed to mixture gases containing interferential part with high concentration. The superior sensing properties can be attributed to the in situ formation of SnO–Sn₃O₄ p–n heterojunctions and large specific surface area. Therefore, the SnO–Sn₃O₄ heterostructure having excellent NO₂ sensing performances is very promising for applications as an NO₂ sensor or alarm operated at a low operating temperature.

A novel SnO–Sn₃O₄ heterostructural gas sensor with high response and selectivity to ppb-level NO₂ at 75 °C and room temperature.     

Improved microstructure and significantly enhanced dielectric properties of Al3+/Cr3+/Ta5+ triple-doped TiO2 ceramics by Re-balancing charge compensation

The charge compensation mechanism and dielectric properties of the (Al_xCr_0.05−x)Ta_0.05Ti_0.9O₂ ceramics were studied. The mean grain size slightly changed with the increase in the Al³⁺/Cr³⁺ ratio, while the porosity was significantly reduced. The dielectric permittivity of the co-doped Cr_0.05Ta_0.05Ti_0.9O₂ ceramic was as low as ε′∼ 10³, which was described by self-charge compensation between Cr³⁺–Ta⁵⁺, suppressing the formation of Ti³⁺. Interestingly, ε′ can be significantly increased (6.68 × 10⁴) by re-balancing the charge compensation via triple doping with Al³⁺ in the Al³⁺/Cr³⁺ ratio of 1.0, while a low loss tangent (∼0.07) was obtained. The insulating grains of [Cr_0.05³⁺Ta_0.05⁵⁺]Ti_0.9⁴⁺O₁₂ has become the semiconducting grains for the triple-doped Al_x³⁺[Cr_0.05−x³⁺Ta_0.05−x⁵⁺][Ta_x⁵⁺Ti_x³⁺Ti_0.9+x⁴⁺]O_12+3x/2. Considering an insulating grain with low ε′ of the Cr_0.05Ta_0.05Ti_0.9O₂ ceramic, the electron-pinned defect-dipoles and interfacial polarization were unlikely to exist supported by the first principles calculations. The significantly enhanced ε′ value of the triple-doped ceramic was primarily contributed by the interfacial polarization at the interface between the semiconducting and insulating parts, which was supported by impedance spectroscopy. This research gives an underlying mechanism on the charge compensation in the Al³⁺/Cr³⁺/Ta⁵⁺-doped TiO₂ system for further designing the dielectric and electrical properties of TiO₂-based ceramics for capacitor applications.

The dielectric properties of Cr³⁺/Ta³⁺ co-doped TiO₂ can be significantly improved by triple doping with Al³⁺ due to the re-balance of charge compensation.     

Stability of 2H- and 1T-MoS2 in the presence of aqueous oxidants and its protection by a carbon shell

Two-dimensional molybdenum disulfide (MoS₂) is emerging as a catalyst for energy and environmental applications. Recent studies have suggested the stability of MoS₂ is questionable when exposed to oxidizing conditions found in water and air. In this study, the aqueous stability of 2H- and 1T-MoS₂ and 2H-MoS₂ protected with a carbon shell was evaluated in the presence of model oxidants (O₂, NO₂⁻, BrO₃⁻). The MoS₂ electrocatalytic performance and stability was characterized using linear sweep voltammetry and chronoamperometry. In the presence of dissolved oxygen (DO) only, 2H- and 1T-MoS₂ were relatively stable, with SO₄²⁻ formation of only 2.5% and 3.1%, respectively. The presence of NO₂⁻ resulted in drastically different results, with SO₄²⁻ formations of 11% and 14% for 2H- and 1T-MoS₂, respectively. When NO₂⁻ was present without DO, the 2H- and 1T-MoS₂ remained relatively stable with SO₄²⁻ formations of only 4.2% and 3.3%, respectively. Similar results were observed when BrO₃⁻ was used as an oxidant. Collectively, these results indicate that the oxidation of 2H- and 1T-MoS₂ can be severe in the presence of these aqueous oxidants but that DO is also required. To investigate the ability of a capping agent to protect the MoS₂ from oxidation, a carbon shell was added to 2H–MoS₂. In a batch suspension in the presence of DO and NO₂⁻, the 2H–MoS₂ with the carbon shell exhibited good stability with no oxidation observed. The activity of 2H–MoS₂ electrodes was then evaluated for the hydrogen evolution reaction by a Tafel analysis. The carbon shell improved the activity of 2H–MoS₂ with a decrease in the Tafel slope from 451 to 371 mV dec⁻¹. The electrode stability, characterized by chronopotentiometry, was also enhanced for the 2H–MoS₂ coated with a carbon shell, with no marked degradation in current density observed over the reaction period. Because of the instability exhibited by unprotected MoS₂, it will only be a useful catalyst if measures are taken to protect the surface from oxidation. Further, given the propensity of MoS₂ to undergo oxidation in aqueous solutions, caution should be used when describing it as a true catalyst for reduction reactions (e.g., H₂ evolution), unless proven otherwise.

Two-dimensional molybdenum disulfide (MoS₂) is emerging as a catalyst for energy and environmental applications.     

Sulfonic acid-functionalized PCP(Cr) catalysts with Cr3+ and –SO3H sites for 5-ethoxymethylfurfural production from glucose

5-Ethoxymethylfurfural (EMF) has been identified as a potential biofuel and fuel additive, for which the production from glucose (the most abundant and inexpensive monosaccharide) in a one-step process would be highly desirable. Here, the synthesis of sulfonic acid-functionalized porous coordination polymers (PCPs) and their application as catalysts for EMF synthesis are reported. PCP(Cr)-BA (PCP material with Cr³⁺ ions and H₂BDC-SO₃H linkers) and PCP(Cr)-NA (PCP material with Cr³⁺ ions and H₂NDC(SO₃H)₂ linkers) materials containing both Cr³⁺ sites and Brønsted-acidic –SO₃H sites were prepared. The morphology, pore structure, acidity, chemical composition, and thermal stability of the two functionalized PCP(Cr) catalysts were analyzed by systematic characterization. The catalysts featured a porous morphology and dual Cr³⁺ and –SO₃H sites, which enabled the cascade conversion of glucose to EMF. PCP(Cr)-BA exhibited higher performance than PCP(Cr)-NA with an EMF yield of 23.1% in the conversion of glucose at 140 °C after 22 h in an ethanol/water system. In addition, the as-prepared catalyst exhibited a high stability in the current catalytic system for EMF production from glucose with a constant catalytic activity in a four-run recycling test without an intermediate regeneration step.

The PCP(Cr)-BA catalysts featured porous morphology and dual Cr³⁺ and –SO₃H sites, which enabled the cascade conversion of glucose to EMF. In addition, the as-prepared catalyst exhibited a high stability in the current catalytic system.     

Intrinsic structural distortion and exchange interactions in SmFexCr1−xO3 compounds

The effect of substituting different amounts of magnetic metal Fe on the magnetic properties of SmFe_xCr_1−xO₃ (0 < x < 0.5) is reported here in order to probe the relation between the structural distortion and magnetism in these materials. The structural properties of the samples were characterized using X-ray diffraction with Rietveld refinements, and Raman spectroscopy carried out at ambient temperature. Magnetization data reveals the Neel temperature (T_N, where the Cr(Fe) ions order) increases with an increase in the average B-site ionic radius, and average Cr(Fe)–O–Cr(Fe) bond angle. By fitting the temperature dependence of the magnetic susceptibility to the Curie–Weiss law modified by the Dzyaloshinskii–Moriya (DM) interaction, the strengths of the symmetric and antisymmetric Cr(Fe)–Cr(Fe) exchange interactions (J and D) were determined. It was found that the strength of the symmetric interaction J (reflected in the changes in the Neel temperature) increases with the replacement of Cr³⁺ with Fe³⁺, which is ascribed to the changes in the average Cr(Fe)–O–Cr(Fe) bond angle and bond lengths. Meanwhile, the antisymmetric interaction D a slightly decreases, which may be ascribed to the displacement of oxygen ions (dO) away from their “original” middle point.

The relationship between intrinsic structural distortions and exchange interactions in SmFe_xCr_1−xO₃ compounds was studied.