Evidence for a strong spin-phonon interaction in cupric oxide

Tuning C1 /C2 Selectivity of CO2 Electrochemical Reduction over in-Situ Evolved CuO/SnO2 Heterostructure

Heterostructured oxides with versatile active sites, as a class of efficient catalysts for CO2 electrochemical reduction (CO2 ER), are prone to undergo structure reconstruction under working conditions, thus bringing challenges to understanding the reaction mechanism and rationally designing catalysts. Herein, we for the first time elucidate the structural reconstruction of CuO/SnO2 under electrochemical potentials and reveal the intrinsic relationship between CO2 ER product selectivity and the in situ evolved heterostructures. At -0.85 VRHE , the CuO/SnO2 evolves to Cu2 O/SnO2 with high selectivity to HCOOH (Faradaic efficiency of 54.81 %). Mostly interestingly, it is reconstructed to Cu/SnO2-x at -1.05 VRHE with significantly improved Faradaic efficiency to ethanol of 39.8 %. In situ Raman spectra and density functional theory (DFT) calculations reveal that the synergetic absorption of *COOH and *CHOCO intermediates at the interface of Cu/SnO2-x favors the formation of *CO and decreases the energy barrier of C-C coupling, leading to high selectivity to ethanol.

Water Oxidation by a Copper(II) Complex with 6,6'-Dihydroxy-2,2'-Bipyridine Ligand: Challenges and an Alternative Mechanism

Recently, copper(II) complexes have been extensively investigated as oxygen-evolution reaction (OER) catalysts through a water-oxidation reaction. Herein, new findings regarding OER in the presence of a Cu(II) complex with 6,6'-dihydroxy-2,2'-bipyridine ligand are reported. Using scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, Raman spectroscopy, in situ visible microscopy, in situ visible spectroelectrochemistry, X-ray absorption spectroscopy, and electrochemistry, it is hypothesized that the film formed on the electrode's surface in the presence of this complex causes an appropriated matrix to produce Cu (hydr)oxide. The resulting Cu (hydr)oxide could be a candidate for OER catalysis. The formed film could form Cu (hydr)oxide and stabilize it. Thus, OER activity increases in the presence of this complex.

Cu/CuOx@C Composite as a High-Efficiency Electrocatalyst for Oxygen Reduction Reactions

Among clean energy transformation devices, fuel cells have gained special attention over the past years; however, advancing appropriate non-valuable metal impetuses to halfway supplant the customary Pt/C impetus is still in progress. In this paper, we propose a specific electrocatalyst in the formula of highly-active Cu species, associated with coated carbon (Cu@C-800), for oxygen reduction reaction (ORR) through post-treatment of a self-assembled precursor. The optimized catalyst Cu@C-800 showed excellent ORR performance (i.e., the onset potential was 1.00 V vs. RHE, and half-wave potential of 0.81 V vs. RHE), high stability, resistance to methanol, and high four-electron selectivity. The enhancement is attributed to the synergy between the carbon matrix and a high explicit surface region and rich Cu nano-species.

Ultrasensitive surface-enhanced Raman spectroscopy detection of gaseous sulfur-mustard simulant based on thin oxide-coated gold nanocone arrays

Surface Enhanced Raman Spectroscopy (SERS) could be a powerful technique for detecting trace gaseous sulfur-mustard, but it is still challenging due to the difficulty in efficiently capturing sulfur-mustard molecules by normal SERS substrates. Here, a chemically trapping strategy is presented for such detection via coating an ultrathin metal-oxide sensing layer on a SERS substrate. In the strategy, a SERS substrate Au-wrapped Si nanocone array is designed and fabricated by Si wafer-based organic template-etching and appropriate Au deposition, and coated with an ultrathin CuO for chemically capturing sulfur-mustard molecules. The validity of such strategy has been demonstrated via taking the gaseous 2-chloroethyl ethyl sulfide (a simulant of sulfur-mustard, or 2-CEES for short) as the target molecules. The response of the CuO-coated SERS substrate to the gaseous 2-CEES is detectable within 10 min, and the lowest detectable concentration is 10 ppb or less. Further experiments have shown that there exists an optimal CuO coating thickness which is about 6 nm. The CuO coating-based capture of 2-CEES molecules is attributed to the surface hydroxyl-induced specific adsorption, which is subject to the pseudo-second-order kinetics and Freundlich-typed model. This study presents the practical SERS chips and new route for the trace detection of gaseous sulfur-mustard.

Femtometer atomic displacement, the root cause for multiferroic behavior of CuO unearthed through polarized Raman spectroscopy

Recently, CuO has been proposed as a potential multiferroic material with high transition temperature. Competing models based on spin current and ionic displacements are invoked to explain ferroelectricity in CuO. The theoretical model based on ionic displacement predicted very small displacement (∼10^-5Å) along thebaxis. Experimentally detecting displacements of such a small amplitude in a particular direction is extremely challenging. Through our detailed angle resolved polarized Raman spectroscopy study on single crystal of CuO, we have validated the theoretical study and provided direct evidence of displacement along thebaxis. Our study provides important contribution in the high temperature multiferroic compounds and showed for the first time, the use of the polarized Raman scattering in detecting ionic displacements at the femtometer scale.

Studies on Sensing Properties and Mechanism of CuO Nanoparticles to H2S Gas

In this work, the high crystalline copper oxide (CuO) nanoparticles were fabricated by a hydrothermal method, and their structural properties were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The sensing results show that CuO nanoparticles exhibit enhanced sensitivity and good selectivity for hydrogen sulfide (H2S) gas at a low temperature. There are two working mechanisms involved in the H2S sensing based on CuO nanoparticle sensors. They are the H2S oxidation mechanism and the copper sulphide (CuS) formation mechanism, respectively. The two sensing mechanisms collectively enhance the sensor's response in the H2S sensing process. The Cu-S bonding is stable and cannot break spontaneously at a low temperature. Therefore, the CuS formation inhibits the sensor's recovery process. Such inhibition gradually enhances as the gas concentration increases from 0.2 ppm to 5 ppm, and it becomes weaker as the operating temperature rises from 40 °C to 250 °C. The XPS results confirmed the CuS formation phenomenon, and the micro Raman spectra demonstrated that the formation of CuS bonding and its decomposition can be effectively triggered by a thermal effect. Gas-sensing mechanism analysis supplied abundant cognition for the H2S sensing phenomena based on CuO materials.

Strain modulated magnetocaloric effect in (111) oriented La0.7Sr0.3MnO3-SrRuO3 superlattices

Magnetocaloric effect in (111)-oriented La0.7Sr0.3MnO3-SrRuO3 (LSMO-SRO) superlattices grown with both the stacking orders by reversing the individual layer thickness on (111)-oriented SrTiO3(STO) substrates using the pulsed laser deposition technique has been studied. Pseudomorphic growth with 0.64% in-plane tensile strain in [11 unit cell (u.c.)SRO/3u.c.LSMO]×15 superlattice is favourable for a larger change in entropy (ΔSM) as compared to relaxed growth with in-plane compressive strain in [11u.c.LSMO/3u.c.SRO]×15 superlattice. The reduction of ΔSM in [11u.c.LSMO/3u.c.SRO]×15 could be due to the orientation-dependent in-phase and out-of-phase tilt of the unit cell between ±1° along the 103pc of the {103}pc, which softens the exchange coupling and leads to the faster alignment of the magnetization near the Curie temperature (TC). Stabilization of the orthorhombic phase of LSMO in the superlattices with both stacking orders is evidenced from the existence of anomaly around the TC of LSMO and SRO in the temperature-dependent phonon frequency shifts. Reduction in symmetry of LSMO from the rhombohedral to orthorhombic structure modulates the Mn-O-Mn bond length and angles, which induces the spin reorientations and hence, modifies the electronic and magnetic properties in these LSMO-SRO superlattices. The ΔSM of these superlattices suggest that the strain, magnitude of the magnetic field, volume and magnetization of the ferromagnet can control the magnetocaloric effect. These results will be useful for designing the magnetic entropy based devices to improve renewable energy systems.

The effects of currents and potentials on the selectivities of copper toward carbon dioxide electroreduction

Copper electrodes have been shown to be selective toward the electroreduction of carbon dioxide to ethylene, carbon monoxide, or formate. However, the underlying causes of their activities, which have been attributed to a rise in local pH near the surface of the electrode, presence of atomic-scale defects, and/or residual oxygen atoms in the catalysts, etc., have not been generally agreed on. Here, we perform a study of carbon dioxide reduction on four copper catalysts from -0.45 to -1.30 V vs. reversible hydrogen electrode. The selectivities exhibited by 20 previously reported copper catalysts are also analyzed. We demonstrate that the selectivity of carbon dioxide reduction is greatly affected by the applied potentials and currents, regardless of the starting condition of copper catalysts. This study shows that optimization of the current densities at the appropriate potential windows is critical for designing highly selective copper catalysts.

Corrosion inhibition of copper in aqueous chloride solution by 1H-1,2,3-triazole and 1,2,4-triazole and their combinations: electrochemical, Raman and theoretical studies

Triazoles are well-known organic corrosion inhibitors of copper. 1H-1,2,3-Triazole and 1,2,4-triazole, two very simple molecules with the only difference being the positions of the nitrogen atoms in the triazole ring, were studied in this work as corrosion inhibitors of copper in 50 mM NaCl solution using a set of electrochemical and analytical techniques. The results of electrochemical tests indicate that 1H-1,2,3-triazole exhibited superior inhibitor properties but could not suppress anodic copper dissolution at moderate anodic potentials (>+300 mV SCE), while 1,2,4-triazole, although it exhibited higher anodic currents, suppressed anodic copper dissolution at very anodic potentials. Density functional theory calculations were also performed to interpret the measured data and trends observed in the electrochemical studies. The computational studies considered either the inhibitors isolated in the gaseous phase or adsorbed onto Cu(111) surface models. From the calculations, the mechanisms of the inhibitive effects of both triazoles were established and plausible mechanisms of formation of the protective films on the Cu surface were proposed. The results of this study hold positive implications for research in the areas of catalysis, and copper content control in water purification systems.

Cu1-x Fe x O: hopping transport and ferromagnetism

Single phase, sol-gel prepared Cu_1-x Fe _x O (0 ≤ x ≤ 0.125) powders are characterized in terms of structural, electronic and magnetic properties. Using dielectric and magnetic studies we investigate the coupling of electron and spin. The electrical conductivities and activation energies are studied with increasing Fe content. Modelling of experimental conductivity data emphasizes a single hopping mechanism for all samples except x = 0.125, which have two activation energies. Hole doping is confirmed by confirming a majority Fe³⁺ substitution of Cu²⁺ in CuO from X-ray photoelectron spectroscopy studies (XPS). Such a substitution results in stabilized ferromagnetism. Fe substitution introduces variation in coercivity as an intrinsic magnetic property in Fe-doped CuO, and not as a secondary impurity phase.

Investigation of optical absorption and photothermal conversion characteristics of binary CuO/ZnO nanofluids

This work aimed at studying the optical absorption and photothermal conversion of binary CuO/ZnO nanofluids that can be used as catalysts in methanol reforming hydrogen production.

High Pressure Experimental Studies on CuO: Indication of Re-entrant Multiferroicity at Room Temperature

We have carried out detailed experimental investigations on polycrystalline CuO using dielectric constant, dc resistance, Raman spectroscopy and X-ray diffraction measurements at high pressures. Observation of anomalous changes both in dielectric constant and dielectric loss in the pressure range 3.7–4.4 GPa and reversal of piezoelectric current with reversal of poling field direction indicate to a change in ferroelectric order in CuO at high pressures. A sudden jump in Raman integrated intensity of A_g mode at 3.4 GPa and observation of Curie-Weiss type behaviour in dielectric constant below 3.7 GPa lends credibility to above ferroelectric transition. A slope change in the linear behaviour of the A_g mode and a minimum in the FWHM of the same indicate indirectly to a change in magnetic ordering. Since all the previous studies show a strong spin-lattice interaction in CuO, observed change in ferroic behaviour at high pressures can be related to a reentrant multiferroic ordering in the range 3.4 to 4.4 GPa, much earlier than predicted by theoretical studies. We argue that enhancement of spin frustration due to anisotropic compression that leads to change in internal lattice strain brings the multiferroic ordering to room temperature at high pressures.

Spin phonon interactions and magnetodielectric effects in multiferroic BiFeO3-PbTiO3

Here, we report the observation of magneto-dielectric and magneto-structural coupling in (1  -  x)BiFeO3-xPbTiO3 i.e.(1  -  x)BF-xPT) solid solutions with compositions in the vicinity of morphotropic phase boundary, as manifested by a combination of temperature dependent magnetic, Raman and dielectric measurements. Whilst x-ray diffraction and Raman spectroscopy suggest absence of any structural phase transition between 90-300 K, temperature dependent magnetic studies reveal magnetic anomalies in the solid solutions. These results are complemented by identical observations in the dielectric measurements at similar temperatures indicating a coupling between magnetic and electric order parameters. Further, Raman studies on rhombohedral i.e. x  =  0.20 samples reveal a coupling between the magnetic structure and the lattice, causing spin-phonon interactions that are possibly responsible for observed magneto-dielectric effects. Our results illustrate that the phase transitions in BiFeO3-PbTiO3 system are fewer than expected and are attributed to a spatial averaging in an inhomogeneous albeit single-phase material due to clustering of Fe- or Ti-ions on different length scales.

Infrared probe of spin-phonon coupling in antiferromagnetic honeycomb lattice compound Li₂MnO₃

We investigated temperature-dependent infrared-active phonon modes of honeycomb Li2MnO3 which shows an antiferromagnetic transition at T(N)  =  36 K. In the far-infrared frequency region, we observed fourteen phonon modes. We obtained the temperature dependence of each phonon mode from the analysis of optical conductivity spectra by using the Lorentz and the Fano-type oscillator models. We found that the resonance frequencies of nine phonon modes showed an anomalous behavior near T(N) that should be attributed to the spin-phonon coupling. We calculated the magnitude of the spin-phonon coupling constant from the shift in the resonance frequencies of the phonon modes below T(N). Our results suggest that Li2MnO3 is weakly frustrated and that spin-phonon coupling plays a role in antiferromagnetic ordering.

A new look at oxide formation at the copper/electrolyte interface by in situ spectroscopies

The oxide layer passivating copper consists mainly of a complex, defect-rich oxide on the basis of copper mixed oxide, Cu₄O₃.

Temperature dependence of crystal field excitations in CuO

We report a study on the temperature dependence of charge-neutral crystal field (dd) excitations in cupric oxide, using nonresonant inelastic x-ray scattering spectroscopy. Thanks to a very high-energy resolution (ΔE = 60 meV), we observe thermal effects on the dd excitation spectrum fine structure between temperatures of 10-320 K. The spectra broaden considerably with increasing temperature, consistently with an enhancement of the coupling between crystal field excitations and the temperature-dependent continuum of states above the band gap. We discuss this and other mechanisms that may explain this temperature dependence.

Significant enhancement of optical absorption through nano-structuring of copper based oxide semiconductors: possible future materials for solar energy applications

An excellent optical absorption coefficient twice that of Si is successfully achieved in a nanostructured phase mixture of copper based oxide semiconductors.

Short-range spin-phonon coupling in in-plane CuO nanowires: a low-temperature Raman investigation

We report an application of low-temperature Raman scattering on in-plane CuO nanowires, in which an overview of the characteristic parameter of spin-phonon coefficient, the interaction of incident light with the spin degrees of freedom, and size effects will be given. The appearance of spin-phonon coefficient decrease reflects the existence of finite size effect.

Second-order Raman scattering in CuO

Polarized second-order Raman scattering spectra of CuO single crystals are reported. It is shown that for some scattering geometries the second-order processes dominate the inelastic light scattering spectra. Group-theoretical symmetry analysis of the selection rules for the first- and second-order scattering processes is performed and phonon dispersion relations are calculated within density functional theory. The main spectral features of the two-phonon spectra are assigned to overtones of the vibrational branches at various special points across the Brillouin zone.

Field- and pressure-induced phases in Sr4Ru3O10: a spectroscopic investigation

We have investigated the magnetic-field- and pressure-induced structural and magnetic phases of the triple-layer ruthenate Sr4Ru3O10. Magnetic-field-induced changes in the phonon spectra reveal dramatic spin-reorientation transitions and strong magnetoelastic coupling in this material. Further, we are able to deduce key magnetoelastic coupling parameters, and evidence that the magnetic moments are localized on the Ru sites. Additionally, pressure-dependent Raman measurements at different temperatures reveal an anomalous negative Gruneisen parameter associated with the B(1g) mode (approximately 380 cm(-1)) at low temperatures (T < 75 K), which can be explained consistently with the field-dependent Raman data.

Magnetism in dense hexagonal iron

The magnetic state of hexagonal close-packed iron has been the subject of debate for more than three decades. Although Mössbauer measurements find no evidence of the hyperfine splitting that can signal the presence of magnetic moments, density functional theory predicts an antiferromagnetic (afm) ground state. This discrepancy between theory and experiment is now particularly important because of recent experimental findings of anomalous splitting in the Raman spectra and the presence of superconductivity in hexagonal close-packed iron, which may be caused by magnetic correlations. Here, we report results from first principles calculations on the previously predicted theoretical collinear afm ground state that strongly support the presence of afm correlations in hexagonal close-packed iron. We show that anomalous splitting of the Raman mode can be explained by spin-phonon interactions. Moreover, we find that the calculated hyperfine field is very weak and would lead to hyperfine splitting below the resolution of Mössbauer experiments.

Coupling between molecular vibrations and liquid crystalline order parameters

Specific Raman active modes in two prototype cyanobiphenyl liquid crystals are shown to display a temperature dependent softening proportional to either the nematic or smectic order parameters, while other vibrations (like the C identical withN stretch mode) remain unaltered. This selective coupling between intramolecular vibrations and the liquid crystalline order is related to the intrinsic symmetry of the modes. The method provides a simple, microscopic, noninvasive optical technique with which the liquid crystalline order parameters can be qualitatively mapped out.