Study of valence states of copper in copper-phosphate glasses

Phosphate glasses containing CuO with composition, [(CuO) _x (P₂O₅)_1–x ], x=0.10, 0.20, 0.25, 0.30, 0.40 and 0.50, were studied by magnetization, X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS). It was observed that compositional changes take place in going from batch to glass and these changes are more pronounced for low copper concentration. The ratio [Cu²⁺/Cu_total] as a function of x was determined from XPS and magnetization combined with RBS. The magnetization measurements suggest that more than 90% of the copper ions exist in the Cu²⁺ state in the glasses, while the XPS data show that less than 50% of the copper ions may be in the Cu²⁺ state. The low Cu²⁺ states detected by XPS may have resulted form reduction of copper ions upon exposure of the samples to X-ray radiation during measurement.Author to whom all correspondence should be addressed.     

Thermoelectric power in Gd3+-substituted Cu-Cd ferrites

Polycrystalline Cd _x Cu_1−x Fe_2−y Gd _y O₄ ferrites fory=0·0 and 0·1 were prepared by ceramic technique. X-ray diffractograms of powder samples show cubic symmetry withx⩾0·2 fory=0·0 and 0·1, while compositions withx=0·0 fory = 0·0 and 0·1 are tetragonal. The thermopower measurements for Gd³⁺-undoped ferrites in the temperature range 300 K to 788 K shown-type conductivity forx⩾0·2. The substitution of Gd³⁺ changedn-type conductivity of the compositions top-type. The mobilities calculated show decreasing trend on Gd³⁺ substitution. The values of activation energy ΔE and drift mobilityE _d suggest polaron formation in substituted samples. The conduction mechanism is explained on the basis of localized model and formation of Gd³⁺+Fe²⁺ stable pairs at B site and Cu¹⁺+Fe³⁺ at A site.     

Photo-Induced Construction and Recovery of Cu+ Sites in Metal–Organic Frameworks

The adjustment of the valence state of metal ions is crucial for various applications because peculiar activity originates from metal ions with specific valence. Cu⁺ can interact with molecules possessing unsaturated bonds like CO via π-complexation, while Cu²⁺ doesn't have such ability. Meanwhile, Cu⁺ sites are easily oxidized to Cu²⁺, leading to the loss of activity. Despite great efforts, the development of a facile method to construct and recover Cu⁺ sites remains a pronounced challenge. Here, for the first time a facile photo-induced strategy is reported to fabricate Cu⁺ sites in metal–organic frameworks (MOFs) and recover Cu⁺ after oxidation. The Cu²⁺ precursor was loaded on NH₂-MIL-125, a typical visible-light responsive Ti-based MOF. Visible light irradiation triggers the formation of Ti³⁺ from Ti⁴⁺ in framework, which reduces the supported Cu²⁺ in the absence of any additional reducing agent, thus simplifying the process for Cu⁺ generation significantly. Due to π-complexation interaction, the presence of Cu⁺ results in remarkably enhanced CO capture capacity (1.16 mmol g⁻¹) compared to NH₂-MIL-125 (0.49 mmol g⁻¹). More importantly, Cu⁺ can be recovered conveniently via re-irradiation when it is oxidized to Cu²⁺, and the oxidation-recovery process is reversible.     

Metal clusters and nanoparticles assembled in zeolites: an example of stable materials with controllable particle size

An ion-exchangeable zeolite (mordenite) is used to control the formation of nanoparticles and clusters within the solid matrix by the hydrogen reduction of metal ions (Ag⁺, Cu²⁺, and Ni²⁺). SiO₂/Al₂O₃ molar ratio in mordenite appears to be an efficient tool to manage the reducibility of the metal ions. Few-atomic silver clusters in line with the larger silver nanoparticles were observed with DRS for the reduced Ag⁺-exchanged mordenites. Cu²⁺-exchanged ones produce the copper nanoparticles with different optical appearance, and Ni²⁺-exchanged mordenites are reduced up to complicated species with no explicit assignment of metal particles under the conditions studied.     

Scanning Tunneling Spectroscopic Studies of the Pairing State of Cuprate Superconductors

Quasiparticle tunneling spectra of both hole-doped (p-type) and electron-doped (n-type) cuprates are studied using a low-temperature scanning tunneling microscope. The results reveal that neither the pairing symmetry nor the pseudogap phenomenon is universal among all cuprates, and that the response of n-type cuprates to quantum impurities is drastically different from that of the p-type cuprates. The only ubiquitous features among all cuprates appear to be the strong electronic correlation and the nearest-neighbor antiferromagnetic Cu²⁺-Cu²⁺ coupling in the CuO₂ planes.     

Facile fabrication of p–n heterojunctions for Cu2O submicroparticles deposited on anatase TiO2 nanobelts

In this paper, Cu₂O particle-deposited TiO₂ nanobelts with p–n semiconductor heterojunction structure were successfully prepared via a two-step preparation process to investigate electron-transfer performance between p-type Cu₂O and n-type TiO₂. Various measurement results confirm that the amount of pure Cu₂O submicroparticles, with diameters within the range of 200–400 nm and deposited on the surface of TiO₂ nanobelts, can be controlled, and that the purity of Cu₂O is heavily affected by reaction time. Visible-light photodegradation activities of Rhodamine B show that photocatalysts have little or no photocatalytic activities mainly due to their p–n heterojunction structure, indicating that there hardly appears any electron-transfer from Cu₂O to TiO₂.     

Control of electrical conductivity with the admixture of chlorine in copper tellurite glasses

Copper tellurite glasses containing. CuCI₂ with composition 65TeO₂-(35–x)CuO-xCuCl₂ (mol%) with x=0, 1, 2, 3, 4, 5 were prepared by quenching the melt. An increase in density with the addition of CuCl₂ and with a corresponding decrease in molar volume, has been observed. The d.c. conductivity of copper tellurite glasses is found to be very sensitive to the reduced valency rationC=[Cu⁺]/[Cu_total] and depends on the relative concentrations of Cu⁺ (reduced valency state) and Cu²⁺ (higher valency state) ions. It is found that by adding cupric chloride to the melt when the glass is formed, the chlorine in the salt which acts as an oxidizing agent alters the ratio of the concentrations of Cu⁺ and Cu²⁺ ions in the glass and hence the conductivity. It is found that more than 2 mol% of cupric chloride reduces the conductivity very sharply due to the formation of chlorine clusters in the form of local TeCl₂₊ whereas less than 2 mol% of CuCl₂ leads to an increase in conductivity due to the Cu⁺Cu²⁺ transition which is negligibly affected by the chlorine due to the formation of TeCl₂ which is amorphous in nature. The increase and decrease of electrical conductivity of glasses containing less or more than 2 mol% CuCl₂ is also interpreted in terms of the electronic transitions between the orbitals of tellurium 3d electrons, their binding energies and peak widths on the basis of XPS (X-ray Photoelectron Spectroscopy) study, and it was found that the increase in bandwidth supports the idea of clustering of chlorine above 2 mol% CuCl₂ and causing a decrease in the conductivity. Overall the conductivity is found to be somewhat uncontrollable in these glasses because it is not simply dependent on the concentration of Cu²⁺ ions.     

Investigation of structural transport and magnetic properties of the system Li0.25Cu0.5Fe2.25- x Al x O4

Experimental data on X-ray electrical conductivity, thermoelectric coefficient, magnetic hysteresis and infrared absorption spectra of the system Li_0.25Cu_0.5Fe_2.25–x Al _x O₄ are presented. All the compounds, 0x2.25, showed cubic symmetry. Lattice constant values progressively decreased on increasing the Al³⁺ content. X-ray intensity calculations, magnetic hysteresis and infrared spectroscopy studies indicated the presence of Li⁺, Al³⁺ and Fe³⁺ at tetrahedral and octahedral sites, while Cu²⁺ is present only at the octahedral site. The activation energy and threshold frequency increased with increasing values ofx. The compounds withx1.50 aren-type, and those withx2.0 arep-type semiconductors. Magnetic hysteresis indicated that compounds withx1.50 are ferrimagnetic, and those withx2.0 are antiferrimagnetic. High coercive force,H _c, values and remanence ratios (J _R/J_S) showed that all the compounds exceptx2.0 exhibit single-domain behaviour. The probable ionic configuration for the system is suggested as Li _0.15 ⁺ Al _0.5 ³⁺ Fe _0.35 ³⁺ [Li _0.1 ⁺ Cu _0.5 ²⁺ Fe _0.4 ³⁺ Al³⁺]O ₄ ^2– .     

Structural,magnetic and electrical study of MgCoMnO4

A new compound MgCoMnO₄ has been synthesised by the oxide method. It crystallizes in a tetragonal spinel structure witha=8.30 A andc=8.46 A. The observed crystal symmetry is associated with the existence of Mn⁺³ on theB sites. The compound isn type semi-conductor with activation energy ΔE=0.33 eV. The electrical properties show that it can be regarded as a properly substituted MgMn₂O₄ by Co⁺³ ions. It is paramagnetic withC _M=5.75 and ϑ _a =−160°K. All these results show the ionic configuration of the compound as Mg⁺² [Co⁺³ Mn⁺³] O₄.     

Performance characteristics of a plasma-type sputter ion source

A low-power, plasma-type sputter source for the production of positive ions of non-volatile elements has been constructed and tested. Argon and copper were employed as support gas and sample material, respectively. The source output was measured as a function of the discharge current, the axial magnetic field, the sputter voltage, the spacing between the sputter sample and the source exit, the diameter of the sputter sample, and the argon pressure. It was found that the Cu⁺ current, I_Cu, increases with increasing discharge current i_d as I_Cu ∝ i_dⁿ, where n 2. I_Cu increases with decreasing sample-to-exit spacing s. For not too small spacings, I_Cu ∝ s⁻². Cu⁺ currents of up to 6 μA were produced at a total power consumption of less than 60 W. The simultaneous prese nce of an intense Ar⁺ component in the beam causes space charge expansion which becomes significant at low energies and large ratios of the drift length to the beam-size defining apertures. Loss of target current may also be caused by neutralization of ions in charge-transfer collisions outside of the source.     

Is Tl2Ba2CuO6 a superconductor-metal array?

From structural analysis we have shown in a previous investigation that the series TlBa₂Ca _n Cu _n+1O_2n+5 and Tl₂Ba₂Ca _n Cu _n+1O_2n+6 are composed of superconductor-semiconductor arrays. In this paper, we demonstrate that the unit cell of the Tl₂Ba₂Ca _n Cu _n+1O_2n+6 series can further be viewed as composed of a composite block of superconductor-metal-semiconductor, where TlBa₂CuO₅ is the fundamental superconductor, TlO the metallic unit, and CaCuO₂ the semiconductor block. It is indicated that theT _c of Tl(2201) can be deduced from that of Tl(1201) based on Kresin's model of composite systems.     

Conduction mechanism in Sn4+ substituted copper ferrite

Thermoelectric power (α) and electrical resistivity (ρ) are reported for the system Cu_1+x Sn _x Fe_2−2x O₄ (wherex=0·05, 0·1, 0·15, 0·2 and 0·3) from room temperature to 800 K. The compositions withx=0·05 and 0·2 exhibitn-type conduction while the compositions withx=0·1 and 0·15 showp- ton-type conduction change after 423 K. The conduction at low temperature (i.e. < 400 K) is due to impurities, while at higher temperature (i.e. > 400 K), it is due to polaron. Hopping conduction phenomenon for the present system has been explained on the basis of localized model of electrons. Additional localization may arise due to Sn⁴⁺ + Fe²⁺ stable pairs at B-site and Cu¹⁺ + Fe³⁺ pair at A-site.     

On the stereochemistry of cations in the doping block of superconducting copper oxides

Metal-oxygen complexes containing Cu, Tl, Hg, Bi, and Pb cations are electronically active in superconducting copper oxides by stabilizing single phases with enhancedT _c, whereas other metal-oxygen complexes deteriorate copper oxide superconductivity. Cu. Tl, Hg, Bi, and Pb in their actual oxidation states are closed-shelld ¹⁰ or inerts ² pair ions. Their electronic configurations have a strong tendency to polarize the oxygen environment. The closed shelld ions with low-lyingnd ¹⁰nd ⁹(n+1)s excitations form linear complexes through hybridization polarizing the apical oxygens. Comparatively lownd ⁹(n+1)s excitation energies distinguish Cu^1+,3+, Tl³⁺, Hg²⁺, from other closed-shelld ¹⁰ ions deteriorating copper oxide superconductivity, e.g., Zn²⁺.     

On the role of transition metal elements as structure-stabilising agents in cuprate superconductors

We outline here several key aspects of the crystal chemistry of (high-valent) transition metal-stabilised cuprate superconductors. An examination of the chemistry of the mercurocuprate homologous series HgBa₂Ca_n−1Cu_nO_2n+2+δ leads us to propose that the inherent instability of the analogous strontium system (HgSr₂Ca_n−1Cu_nO_2n+2+δ) arises from bond-length mismatch at the interface between the rock-salt and perovskite layers. A simple set of principles can now be formalised for the stabilisation of strontium mercurocuprates via partial substitution of mercury by a high-valent transition metal within the crystal structure of interest. The chemistry of three different cuprate systems, in which high-valent transition metal ions adopt a pivotal role as structure-stabilising agents, are illustrated here. First, we demonstrate how a combination of structural stabilisation (by the substitution of rhenium onto the mercury site) combined with control of the copper oxidation state (via judicious control of the Nd³⁺:Ca²⁺ ratio), allows the successful synthesis of superconducting strontium analogues of HgBa₂CaCu₂O_6+δ in the (Hg_1−xRe_x)Sr₂(Nd_1−yCa_y)Cu₂O_6+δ system. Second, we report the successful preparation of superconducting Hg_1−xCr_xSr₂CuO_4+δ thin films, by laser ablation and ex situ mercuration. Finally, we describe a new family of layered cuprates with the general chemical formula (Cu_1−xTM_x)Sr₂(Y_1.2Ce_0.8)Cu₂O_10−δ, with TM=Ti, Cr, Mo, W, V and Re, in which these transition metal substituents, once again, fulfil a key role in structure stabilisation.     

A study of interaction between copper and manganese in a soda-borate glass by ESR

The ESR spectra of 30 Na₂O-70 B₂O₃ glasses, containing very low amounts of copper and manganese singly and in mixed proportions, have been studied. The copper glass shows that Cu²⁺ is present in a distorted octahedral environment. The manganese glass shows that Mn²⁺ is also present in an octahedral site possibly with some distortion. The mixed copper-manganese glasses show that both Cu²⁺ and Mn²⁺ are present with Cu⁺ and Mn³⁺, and that there is a considerable interaction between these two ions giving rise to the exchange coupled Cu²⁺-Mn²⁺ pairs. The covalency of Cu²⁺-O bonding decreases in the mixed glasses. The possibility of spin diffusion and nuclear relaxation in the Mn²⁺ site is proposed to account for the absence of the Mn²⁺ hyperfine lines in the spectra of mixed glasses. The possibility of nuclear relaxation of the Cu²⁺ site is not completely ruled out from the broadening of the hyperfine lines in the mixed glasses, as compared to that in the copper glass.     

Study of the colouring process in copper ruby glasses by optical and EPR spectroscopy

The mechanism responsible for the development of the ruby colour in silica glasses containing copper have been investigated by means of optical and EPR spectroscopy. During the striking treatment, the evolution of the concentration of Cu⁺ and Sn²⁺ has been followed through their luminescence bands, whereas the Cu²⁺ concentration has been monitored by the EPR spectra. To account for the data, a model for the colouring which involves two simultaneous redox reactions: (i)2Cu⁺Cu²⁺+Cu⁰ (ii)2Cu²⁺+Sn²⁺2Cu⁺+Sn⁴⁺ has been proposed. According to it, the role of tin is to act as a redox buffer to regenerate Cu⁺, therefore favouring the coalescence of Cu₂O colloids.     

Changes in valency state of ions in CuMn2O4 at high temperatures

Thin films of copper manganite with three different molar ratios of CuMn viz 11, 12 and 13 have been deposited on quartz plates. The electrical conductivity of these thin films has been studied as a function of temperature. The energy of activation for electrical conduction in these compounds is found to increase from 0.20 eV to about 0.62 eV above 600° K. Differential thermal analysis of bulk sample of copper manganite also shows a small endothermic change at 600° K, while thermogravimetric analysis does not show any change in weight. It has been concluded from these results that the stable form of the ionic configuration of copper manganite at low temperatures, i.e. Cu¹⁺[Mn³⁺Mn⁴⁺]O₄ changes at high temperatures to Cu²⁺[Mn₂ ³⁺]O₄     

A Robust n-n Heterojunction: Cu?N and B?N Boosting for Ambient Electrocatalytic Nitrogen Reduction to Ammonia

An n-n type heterojunction comprising with Cu N and B N dual active sites is synthesized via in situ growth of a conductive metal–organic framework (MOF) [Cu₃(HITP)₂] (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) on hexagonal boron nitride (h-BN) nanosheets (hereafter denoted as Cu₃(HITP)₂@h-BN) for the electrocatalytic nitrogen reduction reaction (eNRR). The optimized Cu₃(HITP)₂@h-BN shows the outstanding eNRR performance with the NH₃ production of 146.2 µg h⁻¹ mg_cat⁻¹ and the Faraday efficiency of 42.5% due to high porosity, abundant oxygen vacancies, and Cu N/B N dual active sites. The construction of the n-n heterojunction efficiently modulates the state density of active metal sites toward the Fermi level, facilitating the charge transfer at the interface between the catalyst and reactant intermediates. Additionally, the pathway of NH₃ production catalyzed by the Cu₃(HITP)₂@h-BN heterojunction is illustrated by in situ FT-IR spectroscopy and density functional theory calculation. This work presents an alternative approach to design advanced electrocatalysts based on conductive MOFs.     

Hybrid Cu0 and Cux+ as Atomic Interfaces Promote High‐Selectivity Conversion of CO2 to C2H5OH at Low Potential

The mixing of charge states of metal copper catalysts may lead to a much improved reactivity and selectivity toward multicarbon products for CO₂ reduction. Here, an electrocatalyst model composed of copper clusters supported on graphitic carbon nitride (g‐C₃N₄) is proposed; the connecting Cu atoms with g‐C₃N₄ can be oxidized to Cu^{x +} due to substantial charge transfer from Cu to N atoms, while others stay as Cu⁰. It is revealed that CO₂ can be captured and reduced into *CO on the Cu_t⁰ site, owing to its zero oxidation state. More importantly, C–C coupling reaction of two *CHO species on the Cu_t⁰–Cu_b^{x +} atomic interface can occur with a rather low kinetic barrier of 0.57 eV, leading to the formation of the final C₂ product, namely, C₂H₅OH. During the whole process, the limiting potential is just 0.68 V. These findings may open a new avenue for CO₂ reduction into high‐value fuels and chemicals.     

Controlled reduction of Cu to Cu with an N,O-type chelate under hydrothermal conditions to produce Cu2O nanoparticles

N,O-type organic chelates reduced coordinated Cu²⁺ ions under hydrothermal reaction conditions to produce Cu₂O/CuO nanoparticles. Chelates in which the N and O atoms are closely spaced produced smaller amounts of CuO nanoparticles, indicating their higher ability to reduce Cu²⁺ ions to Cu⁺ ions. [Cu(Gly)₂]² with the shortest ligand chain length produced only Cu₂O nanoparticles and, therefore, can be used as a single molecule precursor for the synthesis of Cu₂O nanoparticles.