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Lithium-ion batteries suffer from reduced capacities and stabilities at low temperature due to poor Li intercalation to the graphite anode. Graphite has a high activation energy (∼0.6 eV) to accommodate Li ions, resulting in a substantial capacity drop at low temperatures. Additionally, it can induce the formation of Li dendrites on the surface of graphite. To address this issue, we designed and synthesized a redox-active fluorothianthrene-based MOF (SKIER-5). SKIER-5, which undergoes three-electron redox reactions resulting from the fluorothianthrene-based organic ligand and Ni, exhibited excellent electrochemical performance at various temperatures when used as an anode. In particular, the discharge capacities of SKIER-5 were significantly higher than those of commercial graphite at low temperatures (<−10 °C) because of the lower activation energy (∼0.23 eV) for charge transfer. Moreover, it maintained stability when cycled at −20 °C, highlighting its potential as a promising anode material in low-temperature environments. [ABSTRACT FROM AUTHOR]

Solar fuel production by photosynthetic systems strongly relies on developing efficient and stable oxygen-evolution catalysts (OECs). Cerium(IV) ammonium nitrate (CAN) has been the most commonly used sacrificial oxidant to investigate OECs. Although many metal oxides have been extensively investigated as OECs in the presence of CAN, mechanistic studies were rarely reported. Herein, first, Fe(III) (hydr)oxide (FeOxHy) was prepared by the reaction of Fe(ClO4)3 and KOH solution and characterized by some methods. Then, changes in Fe oxide in the presence of CAN during the OER were tracked using in situ Raman spectroscopy, in situ X-ray absorption spectroscopy, in situ visible spectroscopy, and in situ electron paramagnetic resonance spectroscopy. FeOxHy in the presence of CAN and during the OER converted to γ-Fe2O3 and [Fe(H2O)6]3+, and a small amount of oxygen was formed. A maximum turnover frequency and turnover number of 10−6 s−1 and 1.3 × 10−3 mol(O2)/mol(Fe) (for half an hour) in the presence of CAN (0.20 M) and FeOxHy were observed. [ABSTRACT FROM AUTHOR]

The electrochemical nitrate reduction reaction (eNO3RRs) can produce value-added chemicals such as ammonia (NH3) via multiple steps. Here, we demonstrate that bimetallic Cu–Pd–Ox alloy electrocatalysts can promote selective NH3 production from the eNO3RR, especially at low overpotential, by accelerating the rate-determining hydrogenation of NO2−, a critical intermediate, on the Cu catalyst. The synergistic effect of Cu and Pd affords 14 times higher NH3 yield (1.41 mg h−1 cm−2) at −0.2 VRHE with an optimum composition (Cu0.65Pd0.35Ox) than that obtained using bare CuOx. The operando near edge X-ray absorption fine structure (NEXAFS) and simulation results support that the high hydrogen affinity of Pd facilitates sequential hydrogenation on metallic Cu active sites, consequently increasing the NH3 selectivity and production rate. Our study sheds light on the mechanism of the catalytic eNO3RR and presents a design strategy for more advanced eNO3RR electrocatalysts. [ABSTRACT FROM AUTHOR]

In the context of energy storage, the oxygen-evolution reaction (OER, 2H2O → O2 + 4H+ + 4e−) through the water-oxidation reaction is a thermodynamically uphill reaction in overall water splitting. In recent years, copper(II) coordination compounds have been extensively used for the OER. However, challenges remain in finding the mechanism of the OER in the presence of these metal coordination compounds. Herein, the electrochemical OER activity is investigated in the presence of a copper(II) coordination compound at pH ≈ 7. While the investigations on finding true catalysts for the OER are focused on the working electrode, herein, for the first time, the focus is on the decomposition of copper(II) coordination compound (CuL3, L: 2,2′-bipyridine N,N′-dioxide) during the OER on the counter electrode toward the precipitation of copper(I) oxide and metallic Cu. [ABSTRACT FROM AUTHOR]

A cobalt coordination compound with azo-ligand linkers combined with linked bisulfonate moieties has been argued to be an efficient catalyst for the oxygen-evolution reaction (OER) (H.-T. Shi, X.-X. Li, F.-H. Wu and W.-B. Yu, Dalton Trans., 2017, 46, 16321.). In the previously published report, this cobalt compound (compound 1) was believed to display a high turnover frequency (5 s−1) at η = 720 mV at pH 9. Herein, the OER in the presence of compound 1 is reinvestigated. The nanosized oxide-based particles formed after the OER in the presence of compound 1 were tracked by electrochemical methods, scanning electron microscopy (SEM), energy dispersive spectrometry (EDX), X-ray diffraction studies (XRD), (High-resolution) transmission electron microscopy ((HR)TEM), Raman spectroscopy, X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS). Based on these experiments, it is proposed that a candidate for the true catalyst of the OER in the presence of compound 1 is cobalt oxide. During the OER and using chronoamperometry, the oxidation state of Co ions for the formed Co oxide is (III), but after consecutive CVs the oxidation states of Co ions for the formed Co oxide are (II) and (III). The results shed new light on the role of Co oxide nanoparticles formed in the presence of this Co coordination compound during the OER. Our experimental data also show that for the OER in the presence of a homogeneous (pre)catalyst, careful analyses to find the role of metal oxides are necessary for informed progress. The present findings also might help to find the mechanism of the OER in the presence of coordination compounds. [ABSTRACT FROM AUTHOR]

Development of an efficient and stable electrocatalyst for the oxygen evolution reaction (OER) is crucial to generate hydrogen via water splitting as a sustainable fuel. Nickel iron layered double hydroxides (NF-LDHs) are considered the most promising electrocatalysts for alkaline water oxidation among various low-cost transition metal-based electrocatalysts although exact mechanisms are still on debate. Herein, we disclose that quaternary multimetallic Ni–Fe–Al–Co LDHs (NFAC-MELDHs) function as one of the best catalysts for alkaline as well as seawater oxidation due to the synergetic effects among the four different redox-flexible metals. The multimetallic Ni–Fe–Al–Co LDHs are prepared via the metal–organic framework (MOF)-derived electrochemical incorporation of fourth transition metal (Co) into ternary Ni–Fe–Al LDHs grown by a hydrothermal reaction. Moreover, we reveal an exact electrocatalytic mechanism for the OER in NFAC-MELDHs via ex situ spectroscopies in combination with density functional theory (DFT) calculations. Redox-flexible Fe is identified with a real active site in synergy with the neighboring metals stabilizing adsorption of oxygen intermediates and simultaneously facilitating charge transfer. In consequence, NFAC-MELDHs exhibit one of the lowest overpotentials of 220 and 280 mV for affording a current density of 100 mA cm−2 in alkaline and simulating seawater solutions, respectively. More importantly, activity and stability merits in electrocatalysis for the OER are improved in the sequence of unary, binary, ternary, and quaternary LDHs, implying that catalyst design using multimetals for LDHs is a highly promising strategy. [ABSTRACT FROM AUTHOR]

Complex cation distribution in spinel solid solution, fosters defect generation and permutates the optical properties. To scrutinize the effect on structural properties, viz. the cation distribution and defect states upon substitution of Zn with Mg; and to tune the emission properties, Zn1−xMgxAl2O4 (0.01 ≤ x ≤ 0.30) nanocrystals are synthesized. The nanocrystals show increased inversion and generation of multiple defects, namely zinc vacancies, zinc interstitials, oxygen vacancies and antisite defects with increasing Mg content, which thereby impacts the optical band gap. Pentahedral coordination in addition to tetra- and octahedral coordination of Al has been observed, which infers the presence of oxygen vacancies and dangling bonds. Moire fringes formation has intimated the presence of two or more crystal lattices with higher Mg substitution. Band-to-band and defect-assisted photoluminescence shows the role of multiple defects, especially defect clusters, in deciphering the properties of the resulting crystals. Color change from bluish-white to pink has been achieved depending upon the excitation wavelength and emission mechanism, as proposed through a band model schematic. The presented study may be beneficial for designing the Zn1−xMgxAl2O4 nanocrystals with optimized emission properties. [ABSTRACT FROM AUTHOR]

Hydrogen peroxide production by enhanced electrocatalysts is an attractive alternative to the present commercial process. While the subnano/atomic dispersion in noble metal nanocatalysts is known to strongly enhance their catalytic efficiency and chemoselectivity, their excessive surface energy and consequent coarsening seriously compromise their physical/chemical stability. Here, we report a subnano/atomically dispersed Pt–Ag alloy (by a simply modified polyol process) that is resistant to agglomeration/Ostwald ripening. This catalyst does not follow a conventional four-electron oxygen reduction reaction (ORR) but selectively produces H2O2 without excessive degradation of its activity. We clarified the role of the alloying element, Ag, as follows: (1) selective activation of two-electron ORR by inhibiting O2 dissociation and (2) suppression of H2O2 decomposition by preventing the H2O2 adsorption. The present approach provides a convenient route for the direct generation of H2O2 as a simple byproduct of electricity generation by fuel-cell systems. [ABSTRACT FROM AUTHOR]

Phthalocyanines are a promising class of ligands for manganese because of their high binding affinity. This effect is suggested to be an important factor because phthalocyanines tightly bind manganese and stabilize it under moderate conditions. The strong donor power of phthalocyanine is also suggested as a critical factor to stabilize high-valent manganese phthalocyanine. Herein, a manganese(II) phthalocyanine, which is stable under moderate conditions, was investigated under harsh electrochemical water oxidation. By scanning electron microscopy, transmission electron microscopy, energy dispersive spectrometry, X-ray diffraction, extended X-ray absorption fine structure analysis, X-ray absorption near edge structure analysis, chronoamperometry, magnetic measurements, Fourier-transform infrared spectroscopy, and electrochemical methods, it is shown that manganese phthalocyanine, a known molecular complex showing good stability under moderate conditions, could not withstand water oxidation catalysis and ultimately is altered to form catalytic oxide particles. Such nanosized Mn oxides are the true catalyst for water oxidation. Besides, we try to go a step forward to find an answer as to how Mn oxides form on the surface of the electrode. [ABSTRACT FROM AUTHOR]

In the present study, the water-oxidizing activity of nickel(ii) phthalocyanine-tetrasulfonate tetrasodium (1), which is a stable Ni(ii) complex under moderate conditions, was investigated. The role of Ni oxide in water oxidation as a true catalyst was investigated. The electrodes after water oxidation by both the complex and Ni salt were analyzed and a relation was proposed between the decomposition of the Ni complex and water oxidation. On the surface of the electrode, there are some areas without any detectable nanoparticles; thus, the detection of such Ni oxides on the surface of the electrode is not easy in the first seconds of the reaction, even using some of the usual methods such as Scanning Electron Microscopy or electrochemical analysis. Such experiments indicated that a precise analysis is necessary to reject the role of nanoparticles in the presence of Ni phthalocyanine under water oxidation. The findings also showed that under water-oxidation conditions and in the presence of the complex, Ni oxide is a good candidate for a true catalyst. [ABSTRACT FROM AUTHOR]

Herein we report on a poly-peptide/Mn oxide nanocomposite as a model for the water-oxidizing catalyst in Photosystem II. The material is obtained by the reaction of poly-l-tyrosine, Mn(OAc)2·4H2O and KMnO4. HRTEM images of the nanocomposite indicate the presence of nanoparticles dispersed in a poly-l-tyrosine matrix with no crystalline phases. A broad peak at ∼400–550 nm in the UV-visible spectrum is related to Mn oxide formation. X-ray absorption studies suggest that the Mn atoms are in the oxidation state of +IV and a coordination number of around three was obtained for Mn in the structure. The water-oxidizing activity of the material under electrochemical conditions was also investigated. Electrochemical experiments proved the presence of Mn(iv) oxide. Determination of oxygen coupled to electrochemistry revealed the role of poly-l-tyrosine in decreasing the overpotential for water oxidation. In the next step, the interaction of the nanocomposite and imidazole as a model for the tyrosine 161 and histidine 190 residues in Photosystem II was studied with fluorescence spectroscopy using the multivariate curve resolution-alternating least squares method. [ABSTRACT FROM AUTHOR]

The interactions of energetic ions with multi-cation compounds and their consequences in terms of changes in the local electronic structure, which may facilitate intriguing hybridization between O 2p and metal d orbitals and magnetic ordering, are the subject of debate and require a deep understanding of energy transfer processes and magnetic exchange mechanisms. In this study, nanocrystals of ZnFe2O4 were exposed to O7+ ions with an energy of 100 MeV to understand, qualitatively and quantitatively, the metal–ligand field interactions, cation migration and magnetic exchange interactions by employing X-ray absorption fine structure measurements and X-ray magnetic circular dichroism to get deeper mechanistic insights. Nanosized zinc ferrite nanoparticles (NPs) with a size of ∼16 nm synthesized in the cubic spinel phase exhibited deterioration of the crystalline phase when 100 MeV O7+ ions passed through them. However, the size of these NPs remained almost the same. The behaviour of crystal deterioration is associated with the confinement of heat in this interaction. The energy confined inside the nanoparticles promotes cation redistribution as well as the modification of the local electronic structure. Prior to this interaction, almost 42% of Zn2+ ions occupied AO4 tetrahedra; however, this value increased to 63% after the interaction. An inverse effect was observed for metal ion occupancies in BO6 octahedra. The L-edge spectra of Fe and Zn reveal that the spin and valence states of the metal ions were not affected by this interaction. This effect is also supported by K-edge measurements for Fe and Zn. The t2g/eg intensity ratio in the O K-edge spectra decreased after this interaction, which is associated with detachment of Zn2+ ions from the lattice. The extent of hybridization, as estimated from the ratio of the post-edge to the pre-edge region of the O K-edge spectra, decreased after this interaction. The metal–oxygen and metal–metal bond lengths were modified as a result of this interaction, as determined from extended X-ray absorption fine structure measurements. These measurements further support the observation of cation migration from AO4 tetrahedra to AO6 octahedra and vice versa. The Fe L-edge magnetic circular dichroism spectra indicate that Fe3+ ions occupying sites in AO4 tetrahedra and BO6 octahedra exhibited antiferromagnetic-like ordering prior to this interaction. The NPs that interacted with energetic O ions displayed a different kind of magnetic ordering. [ABSTRACT FROM AUTHOR]

For the first time, using scanning electron microscopy, transmission electron microscopy, X-ray absorption near edge structure and extended X-ray absorption fine structure X-ray diffraction, it is showed that MnCO3, MnWO4, Mn3(PO4)2·3H2O, MnS and Mn(VO3)2·xH2O under the water-oxidation conditions and in the presence of cerium(iv) ammonium nitrate, are converted to Mn oxide without a high-range order. A mechanism is proposed for such conversion and as Mn oxide is an efficient water-oxidizing catalyst, it is thus a candidate as a contributor to the observed catalytic activity. [ABSTRACT FROM AUTHOR]

Herein, holmium oxide as a support for nanosized Mn oxide was used toward the synthesis of a new catalyst for water oxidation. A composite of Mn and Ho oxide was synthesized by a simple hydrothermal procedure and characterized by scanning electron microscopy, extended edge X-ray absorption fine structure, transmission electron microscopy, powder X-ray diffraction and electrochemical methods. The effect of calcination temperature on the composite performance for the water-oxidation process was examined in the range of 100–600 °C. The turnover frequencies for the catalyst toward water oxidation in the presence of cerium(iv) ammonium nitrate and photo-produced tris(2,2′-bipyridyl)ruthenium(iii), [Ru(bpy)3]3+ are 0.12 and 0.04 (mmol O2 per mol Mn s), respectively. In addition, the catalyst shows self-healing in the presence of cerium(iv) ammonium nitrate and under the water-oxidation conditions. [ABSTRACT FROM AUTHOR]

We report a simple methodology for the stabilization of Cu2O nanoparticles of size 10–30 nm on the polar pore surface of the polyoxomolybdate (NH4)12[Mo36(NO)4O108(H2O)16] ({Mo36}). For the first time, a Cu2O@{Mo36} composite has been utilized as a recyclable catalyst for the Huisgen 1,3-dipolar cycloaddition reaction of terminal alkynes, aryl/alkyl halides and NaN3 for the synthesis of 1,2,3-triazoles. [ABSTRACT FROM AUTHOR]

In the present work, Fe valence state, covalency effects, and metal–oxygen hybridization are discussed for ZnFe2O4 using X-ray absorption spectroscopy. A few sets of nano-sized and micro-sized zinc ferrite were synthesized using the nitrate method. Nanoparticles of ZnFe2O4 were synthesized by heating precursor at 300, 400, 500, 800, 1000, and 1200 °C for 1 h. To synthesize micro-sized ZnFe2O4, the obtained nanoparticles were annealed at 1200 °C for 12 h (bulk treatment). X-ray diffraction shows the presence of cubic spinel phase in nano-sized as well as micro-sized ZnFe2O4. Scanning electron microscopy measurements show that particle size ranges are 40–80 nm and 1–2 μm for nano-sized and micro-sized ZnFe2O4, respectively. Fe L-edge spectra of these materials envisage the presence of spectral features corresponding to t2g and eg symmetry states created due to Fe(2p3/2)-Fe(3d) and Fe(2p1/2)-Fe(3d) in octahedral crystal field. This reflects the presence of Fe3+ states in nano-sized and micro-sized ZnFe2O4. eg states dominate in micro-sized ZnFe2O4. O K-edge spectra for these materials can be distinguished by pre-edge and post-edge regions. Pre-edge and post-edge regions are associated with O(2p)–Fe(3d) and O(2p)–Fe(4s,4p) hybridized states. The extent of hybridization estimated from the intensity ratio of O(2p)–Fe(3d) and O(2p)–Fe(4s,4p) hybridized states is higher in nano-sized ZnFe2O4. [ABSTRACT FROM AUTHOR]

Ce1−xEuxO2 (0 ≤x≤ 0.3) nanoparticles (NPs) were synthesized by the chemical precipitation method. The microstructures and morphology were characterized by synchrotron X-ray diffraction and high resolution transmission electron microscopy. X-ray absorption near edge structure (XANES) spectra at the Eu M5,4-edge and atomic-multiplet calculations revealed that Eu3+ was predominantly present in the CeO2 lattice and Eu2+ was negligibly present within the entire doping range. The detailed analysis of the Ce M5,4-edge and the O K-edge has shown strong dependence of the Ce3+/Ce4+ ratio and oxygen vacancy with Eu content. Extended X-ray absorption fine structure (EXAFS) spectra at the Ce K-edge, along with theoretical fitting, have shown systematic variation in the coordination number, bond length and Debye–Waller factor with Eu doping. A blue shift in the absorption edge was observed which implies a net increase in the charge transfer gap between the O 2p and Ce 4f bands due to the increased number of Ce3+ ions in the Eu doped samples. The excitation and emission spectra of pure CeO2 NPs did not show any photoluminescence (PL) characteristic; however, Ce1−xEuxO2 (x = 0.1–0.3) NPs showed significant improvements in the 4f–4f, 5D0–7F2 and 5D0–7F1 transitions induced luminescence properties. Eu doping has two major effects on the electronic structure and optical properties of CeO2 NPs: the first, at an Eu content of 10 mol%, is the formation of Ce4+–O–Eu3+ networks, i.e., Eu3+ ions substitute the Ce4+ ions and introduce oxygen vacancies and Ce3+ ions in the host lattice, which favors the 5D0–7F2 induced PL properties. The other, at an Eu doping over 10 mol%, is the formation of both Ce4+–O–Eu3+ and Ce3+–O–Eu3+, i.e., Eu3+ ions not only take substitutional sites of Ce4+ ions but also replace a fraction of Ce3+ ions in the CeO2 lattice which favors 5D0–7F1 induced PL properties. As an application of CeO2 NPs towards the degradation of water pollutants, we demonstrated that the Ce1−xEuxO2 (0 ≤x≤ 0.3) NPs can serve as effective photocatalyst materials towards the degradation of the methyl-orange aqueous pollutant dye under UV light irradiation. [ABSTRACT FROM AUTHOR]

π-spacers and alkyl side chains play a key role in the optical, electrochemical, and photovoltaic properties of π-conjugated polymers. To investigate the collective effects of π-spacers, alkyl side chains, and various processing conditions on the photovoltaic properties, an array of four new low bandgap (LBG) π-conjugated polymers (P1–P4) was designed and synthesized for their application as donor materials in bulk heterojunction polymer solar cells (BHJ PSCs). These π-conjugated polymers contain a benzodithiophene (BDT) donor unit substituted with 2-ethylhexylselenyl or 2-hexyldecylselenyl as π-conjugated side chains and a dialkoxybenzothiadiazole (dialkoxyBT) electron deficient unit connected with thiophene or selenophene as π-spacers. Among the four polymers, the absorption spectra of P3 with the thiophene π-spacer showed a well enhanced vibronic shoulder peak between 620 and 650 nm, indicating that P3 possesses a strong interchain aggregation tendency and attains a planar backbone structure due to the non-covalent interactions arising between the sulfur atom in thiophene and the oxygen atom in dialkoxyBT. Under suitable device processing conditions optimized pristine PSCs of P3 showed a maximum power conversion efficiency (PCE) of 4.09%. After employing 1,8-diiodooctane as an additive, one of the PSC devices based on P2 displayed a PCE of 5.34%. The active layers of P1–P4 showed a positive response towards methanol treatment, especially the P3-based devices delivered an improved PCE of 5.63%, which was further assessed by electrical impedance spectroscopy. These findings in the current article provide a good specimen for efficiently fine tuning the optical and photovoltaic properties of π-conjugated polymers via varying the size of alkyl chains, π-spacer groups and device processing conditions for the imminent growth of LBG π-conjugated polymers. [ABSTRACT FROM AUTHOR]

Single phase nanoparticles (NPs) of CeO2, Ce0.5Zr0.5O2, Ce0.5Hf0.5O2 and Ce0.5Hf0.25Zr0.25O2 were successfully synthesized by co-precipitation method at constant pH and temperature. The X-ray diffraction results revealed that the additive atoms did not segregate to form secondary phases but led to grain size variation in the NPs. The 10 Dq values in the near edge X-ray absorption fine structure (NEXAFS) spectra at the O K-edge did not vary in the same way as the average grain size was changed for the doped CeO2 NPs. The deconvolution of Ce M5-edge and detailed analysis of O K pre-edge peak have shown the higher Ce+3/(Ce+3 + Ce+4) ratio in the Zr- and Hf-doped samples. The local atomic structure around the Ce, Zr and Hf atoms was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy at Ce K-edge, Zr K-edge and Hf L3-edge, respectively, and the EXAFS data were fitted with the theoretical calculations. The 4f occupancy, Ce+3/(Ce+3 + Ce+4) ratio of Ce ions, coordination number of Ce and Ce–Ce/Ce–O bond distances were sensitive to the additive atoms but not explicitly changed according to the grain size variation in the NPs. [ABSTRACT FROM AUTHOR]

Reversible, carbon dioxide mediated chemical hydrogen storage was first demonstrated using a heterogeneous Pd catalyst supported on mesoporous graphitic carbon nitride (Pd/mpg-C[sub 3]N[sub 4]). The Pd nanoparticles were found to be uniformly dispersed onto mpg-C[sub 3]N[sub 4] with an average size of 1.7 nm without any agglomeration and further exhibit superior activity for the dehydrogenation of formic acid with a turnover frequency of 144 h[sup -1] even in the absence of external bases at room temperature. Initial DFT studies suggest that basic sites located at the mpg-C3N4 support play synergetic roles in stabilizing reduced Pd nanoparticles without any surfactant as well as in initiating H[sub 2]-release by deprotonation of formic acid, and these potential interactions were further confirmed by X-ray absorption near edge structure (XANES). Along with dehydrogenation, Pd/mpg-C3N4 also proves to catalyze the regeneration of formic acid via CO[sub 2] hydrogenation. The governing factors of CO[sub 2] hydrogenation are further elucidated to increase the quantity of the desired formic acid with high selectivity. [ABSTRACT FROM AUTHOR]

In the present work, Fe valence state, covalency effects, and metal–oxygen hybridization are discussed for ZnFe2O4 using X-ray absorption spectroscopy. A few sets of nano-sized and micro-sized zinc ferrite were synthesized using the nitrate method. Nanoparticles of ZnFe2O4 were synthesized by heating precursor at 300, 400, 500, 800, 1000, and 1200 °C for 1 h. To synthesize micro-sized ZnFe2O4, the obtained nanoparticles were annealed at 1200 °C for 12 h (bulk treatment). X-ray diffraction shows the presence of cubic spinel phase in nano-sized as well as micro-sized ZnFe2O4. Scanning electron microscopy measurements show that particle size ranges are 40–80 nm and 1–2 μm for nano-sized and micro-sized ZnFe2O4, respectively. Fe L-edge spectra of these materials envisage the presence of spectral features corresponding to t2g and eg symmetry states created due to Fe(2p3/2)-Fe(3d) and Fe(2p1/2)-Fe(3d) in octahedral crystal field. This reflects the presence of Fe3+ states in nano-sized and micro-sized ZnFe2O4. eg states dominate in micro-sized ZnFe2O4. O K-edge spectra for these materials can be distinguished by pre-edge and post-edge regions. Pre-edge and post-edge regions are associated with O(2p)–Fe(3d) and O(2p)–Fe(4s,4p) hybridized states. The extent of hybridization estimated from the intensity ratio of O(2p)–Fe(3d) and O(2p)–Fe(4s,4p) hybridized states is higher in nano-sized ZnFe2O4. [ABSTRACT FROM AUTHOR]

Solar fuel production by photosynthetic systems strongly relies on developing efficient and stable oxygen-evolution catalysts (OECs). Cerium(IV) ammonium nitrate (CAN) has been the most commonly used sacrificial oxidant to investigate OECs. Although many metal oxides have been extensively investigated as OECs in the presence of CAN, mechanistic studies were rarely reported. Herein, first, Fe(III) (hydr)oxide (FeO x H y ) was prepared by the reaction of Fe(ClO 4 ) 3 and KOH solution and characterized by some methods. Then, changes in Fe oxide in the presence of CAN during the OER were tracked using in situ Raman spectroscopy, in situ X-ray absorption spectroscopy, in situ visible spectroscopy, and in situ electron paramagnetic resonance spectroscopy. FeO x H y in the presence of CAN and during the OER converted to γ-Fe 2 O 3 and [Fe(H 2 O) 6 ] 3+ , and a small amount of oxygen was formed. A maximum turnover frequency and turnover number of 10 -6 s -1 and 1.3 × 10 -3 mol(O 2 )/mol(Fe) (for half an hour) in the presence of CAN (0.20 M) and FeO x H y were observed.

Herein, we systematically studied the effect of various excitation processes on the structural, optical, and magnetic properties of ZnO films implanted with 80 keV Ar + , 110 keV Mn + , and 190 keV Ag + ions. Four different doses of 1 × 10 13 , 1 × 10 14 , 1 × 10 15 , and 2 × 10 16 ions per cm 2 were used for implantation. It was observed that the structural, optical, and magnetic properties of the implanted samples were dominantly affected at the highest dose of 2 × 10 16 ions per cm 2 . For lower doses, insignificant changes in these properties were observed. A comparison of various processes involved in the implantation process shows that both the electronic excitation and nuclear excitation processes are responsible for the changes in the structural, optical, and magnetic properties of the implanted ZnO films., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

In the context of energy storage, the oxygen-evolution reaction (OER, 2H 2 O → O 2 + 4H + + 4e - ) through the water-oxidation reaction is a thermodynamically uphill reaction in overall water splitting. In recent years, copper(II) coordination compounds have been extensively used for the OER. However, challenges remain in finding the mechanism of the OER in the presence of these metal coordination compounds. Herein, the electrochemical OER activity is investigated in the presence of a copper(II) coordination compound at pH ≈ 7. While the investigations on finding true catalysts for the OER are focused on the working electrode, herein, for the first time, the focus is on the decomposition of copper(II) coordination compound (CuL 3 , L: 2,2'-bipyridine N , N '-dioxide) during the OER on the counter electrode toward the precipitation of copper(I) oxide and metallic Cu.

A cobalt coordination compound with azo-ligand linkers combined with linked bisulfonate moieties has been argued to be an efficient catalyst for the oxygen-evolution reaction (OER) (H.-T. Shi, X.-X. Li, F.-H. Wu and W.-B. Yu, Dalton Trans. , 2017, 46 , 16321.). In the previously published report, this cobalt compound (compound 1) was believed to display a high turnover frequency (5 s -1 ) at η = 720 mV at pH 9. Herein, the OER in the presence of compound 1 is reinvestigated. The nanosized oxide-based particles formed after the OER in the presence of compound 1 were tracked by electrochemical methods, scanning electron microscopy (SEM), energy dispersive spectrometry (EDX), X-ray diffraction studies (XRD), (High-resolution) transmission electron microscopy ((HR)TEM), Raman spectroscopy, X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS). Based on these experiments, it is proposed that a candidate for the true catalyst of the OER in the presence of compound 1 is cobalt oxide. During the OER and using chronoamperometry, the oxidation state of Co ions for the formed Co oxide is (III), but after consecutive CVs the oxidation states of Co ions for the formed Co oxide are (II) and (III). The results shed new light on the role of Co oxide nanoparticles formed in the presence of this Co coordination compound during the OER. Our experimental data also show that for the OER in the presence of a homogeneous (pre)catalyst, careful analyses to find the role of metal oxides are necessary for informed progress. The present findings also might help to find the mechanism of the OER in the presence of coordination compounds.

Complex cation distribution in spinel solid solution, fosters defect generation and permutates the optical properties. To scrutinize the effect on structural properties, viz. the cation distribution and defect states upon substitution of Zn with Mg; and to tune the emission properties, Zn1-xMgxAl2O4 (0.01 ≤ x ≤ 0.30) nanocrystals are synthesized. The nanocrystals show increased inversion and generation of multiple defects, namely zinc vacancies, zinc interstitials, oxygen vacancies and antisite defects with increasing Mg content, which thereby impacts the optical band gap. Pentahedral coordination in addition to tetra- and octahedral coordination of Al has been observed, which infers the presence of oxygen vacancies and dangling bonds. Moire fringes formation has intimated the presence of two or more crystal lattices with higher Mg substitution. Band-to-band and defect-assisted photoluminescence shows the role of multiple defects, especially defect clusters, in deciphering the properties of the resulting crystals. Color change from bluish-white to pink has been achieved depending upon the excitation wavelength and emission mechanism, as proposed through a band model schematic. The presented study may be beneficial for designing the Zn1-xMgxAl2O4 nanocrystals with optimized emission properties.

Herein, the size dependent behavior of cobalt ferrite nanoparticles was investigated using synchrotron radiation based techniques. Scanning electron micrographs revealed the enhancement of particle/crystallite size with increase of annealing temperature. Moreover, the shape of these particles also changed with increase of crystallite size. Saturation magnetization increased with increase of crystallite size. The higher saturation magnetization for larger crystallite size nanoparticles was attributed to a cation distribution similar to that of bulk CoFe 2 O 4 . The optical band-gap of these nanoparticles decreased from 1.9 eV to 1.7 eV with increase of crystallite size. The enhancement of the optical band-gap for smaller crystallites was due to phenomena of optical confinement occurring in the nanoparticles. Fe L Co L-edge near edge extended X-ray absorption fine structure (NEXAFS) measurements showed that Fe and Co ions remain in the 3+ and 2+ state in these nanoparticles. The results obtained from Fe & Co K-edge X-ray absorption near edge structure (XANES)-imaging experiments further revealed that this oxidation state was possessed by even the crystallites. Extended X-ray absorption fine structure (EXAFS) measurements revealed distribution of Fe and Co ions among tetrahedral (A) and octahedral (B) sites of the spinel structure which corroborates the results obtained from Rietveld refinement of X-ray diffraction patterns (XRD). X-ray magnetic circular di-chroism (XMCD) measurements revealed negative exchange interaction among the ions situated in tetrahedral (A) and octahedral (B) sites. Theoretical and experimental calculated magnetic moments revealed the dominancy of size effects rather than the cation redistribution in the spinel lattice of CoFe 2 O 4 nanoparticles., Competing Interests: Authors declare no conflicts of interests., (This journal is © The Royal Society of Chemistry.)

Electrochemical hydrogen peroxide (H 2 O 2 ) production by the direct two-electron (2e - ) oxygen reduction reaction (ORR) has received much attention as a promising alternative to the industrially developed anthraquinone fabrication process. Transition metal (M) and nitrogen doped carbon (M-N-C, M = Fe or Co) catalysts are known to be active for four electron ORR pathways via two + two electron transfer, where the former is for the ORR and the latter for the peroxide reduction reaction (PRR). Here, we report mesoporous N-doped carbon/manganese hybrid electrocatalysts composed of MnO and Mn-N x coupled with N-doped carbons (Mn-O/N@NCs), which have led to the development of electrocatalysis towards the 2e - ORR route. Based on the structural and electrochemical characterization, the number of transferred electrons during the ORR on the Mn-O/N@NCs was found to be close to the theoretical value of the 2e - process, indicating their high activity toward H 2 O 2 . The favored ORR process arose due to the increased number of Mn-N x sites within the mesoporous N-doped carbon materials. Furthermore, there was a strong indication that the PRR is significantly suppressed by adjacent MnO species, demonstrating its highly selective production of H 2 O 2 (>80%) from the oxygen electrochemical process. The results of a real fuel cell device test demonstrated that an Mn-O/N@NC catalyst sustains a very stable current, and we attributed its outstanding activity to a combination of site-dependent facilitation of 2e - transfer and a favorable porosity for mass transport.

Phthalocyanines are a promising class of ligands for manganese because of their high binding affinity. This effect is suggested to be an important factor because phthalocyanines tightly bind manganese and stabilize it under moderate conditions. The strong donor power of phthalocyanine is also suggested as a critical factor to stabilize high-valent manganese phthalocyanine. Herein, a manganese(ii) phthalocyanine, which is stable under moderate conditions, was investigated under harsh electrochemical water oxidation. By scanning electron microscopy, transmission electron microscopy, energy dispersive spectrometry, X-ray diffraction, extended X-ray absorption fine structure analysis, X-ray absorption near edge structure analysis, chronoamperometry, magnetic measurements, Fourier-transform infrared spectroscopy, and electrochemical methods, it is shown that manganese phthalocyanine, a known molecular complex showing good stability under moderate conditions, could not withstand water oxidation catalysis and ultimately is altered to form catalytic oxide particles. Such nanosized Mn oxides are the true catalyst for water oxidation. Besides, we try to go a step forward to find an answer as to how Mn oxides form on the surface of the electrode.

In the present study, the water-oxidizing activity of nickel(ii) phthalocyanine-tetrasulfonate tetrasodium (1), which is a stable Ni(ii) complex under moderate conditions, was investigated. The role of Ni oxide in water oxidation as a true catalyst was investigated. The electrodes after water oxidation by both the complex and Ni salt were analyzed and a relation was proposed between the decomposition of the Ni complex and water oxidation. On the surface of the electrode, there are some areas without any detectable nanoparticles; thus, the detection of such Ni oxides on the surface of the electrode is not easy in the first seconds of the reaction, even using some of the usual methods such as Scanning Electron Microscopy or electrochemical analysis. Such experiments indicated that a precise analysis is necessary to reject the role of nanoparticles in the presence of Ni phthalocyanine under water oxidation. The findings also showed that under water-oxidation conditions and in the presence of the complex, Ni oxide is a good candidate for a true catalyst.

Performing water splitting for H 2 production is an interesting method to store different energies. For water splitting, an efficient and stable water-oxidizing catalyst is important. Ni-Fe (hydr)oxides are among the best catalysts for water oxidation in alkaline electrolytes. An Fe amount higher than 50% in Ni-Fe (hydr)oxides increases the overpotential for water oxidation. Thus, Ni-Fe (hydr)oxides with a high ratio of Fe to Ni have rarely been focused on for water oxidation. Herein, we report water oxidation using nanosized (Ni 1- x Zn x )Fe 2 O 4 . The catalyst was characterized via some methods and tested at pH values of 3, 7 and 11 in phosphate buffer. Nanosized (Ni 1- x Zn x )Fe 2 O 4 is a good catalyst for water oxidation only under alkaline conditions. In the next step, amperometry studies showed current densities of 3.50 mA cm -2 and 11.50 mA cm -2 at 1.25 V in 0.10 M and 1.0 M KOH solution, respectively. The amperometric measurements indicated high catalyst stability in both 0.10 M and 1.0 M KOH. Tafel plots were obtained in KOH solution at concentrations of both 0.10 M and 1.0 M. At pH = 13 in KOH solution (0.10 M), linearity of lg(j) vs. potential was shown, with two slopes relating to both relatively low (170.9 mV per decade) and high overpotentials (484.2 mV per decade). In 1.0 M KOH solution, the Tafel plot showed linearity of lg(j) vs. potential, with two slopes relating to both relatively low (192.5 mV per decade) and high overpotentials (545.7 mV per decade). After water oxidation, no significant change was observed in the catalyst., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Ca 2 V 2 O 7 , Sr 2 V 2 O 7 , and Ba 2 V 2 O 7 pyro-vanadates were synthesized using a modified chemical precipitation method and annealing. Detailed crystal structure, morphology, electronic structure and optical properties were investigated by XRD, UV-visible absorption, FTIR, Raman, FE-SEM, XANES, and photoluminescence spectroscopy. Rietveld refinement on the XRD patterns of Ca 2 V 2 O 7 , Sr 2 V 2 O 7 , and Ba 2 V 2 O 7 has confirmed the triclinic structure (space group; P 1̄(2)) of the pyro-vanadates. The band gap energy of Ca 2 V 2 O 7 , Sr 2 V 2 O 7 , and Ba 2 V 2 O 7 is estimated to be ∼2.67 eV, ∼2.97 eV and ∼3.09 eV, respectively. XANES spectra at the Ca L-edge, Sr K-edge and Ba L-edge have confirmed the Ca 2+ , Sr 2+ and Ba 2+ ions in the Ca 2 V 2 O 7 , Sr 2 V 2 O 7 and Ba 2 V 2 O 7 compounds, respectively. V K-edge XANES spectra have strengthened the presence of sub-pentavalent V ions in all of the pyro-vanadates. O K-edge XANES spectra of Ca 2 V 2 O 7 , Sr 2 V 2 O 7 and Ba 2 V 2 O 7 have shown dominating tetrahedral symmetry of the V ions which is also corroborated with the V K-edge XANES. Broad-band emission spectra, ranging from 400 nm to 700 nm, have been observed from the charge-transfer transitions of VO 4 tetrahedra. 3 T 1 → 1 A 1 and 3 T 2 → 1 A 1 transitions, from the VO 4 tetrahedra, have provided two distinct emission peaks from the compounds which exhibit a red-shift with the decreasing ionic-radii of alkali-earth metal ions. The mixed compounds, with equal weight proportions, have shown remarkable emission characteristics towards the realization of rare-earth element free white-light-emitting devices., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The interactions of energetic ions with multi-cation compounds and their consequences in terms of changes in the local electronic structure, which may facilitate intriguing hybridization between O 2p and metal d orbitals and magnetic ordering, are the subject of debate and require a deep understanding of energy transfer processes and magnetic exchange mechanisms. In this study, nanocrystals of ZnFe2O4 were exposed to O7+ ions with an energy of 100 MeV to understand, qualitatively and quantitatively, the metal-ligand field interactions, cation migration and magnetic exchange interactions by employing X-ray absorption fine structure measurements and X-ray magnetic circular dichroism to get deeper mechanistic insights. Nanosized zinc ferrite nanoparticles (NPs) with a size of ∼16 nm synthesized in the cubic spinel phase exhibited deterioration of the crystalline phase when 100 MeV O7+ ions passed through them. However, the size of these NPs remained almost the same. The behaviour of crystal deterioration is associated with the confinement of heat in this interaction. The energy confined inside the nanoparticles promotes cation redistribution as well as the modification of the local electronic structure. Prior to this interaction, almost 42% of Zn2+ ions occupied AO4 tetrahedra; however, this value increased to 63% after the interaction. An inverse effect was observed for metal ion occupancies in BO6 octahedra. The L-edge spectra of Fe and Zn reveal that the spin and valence states of the metal ions were not affected by this interaction. This effect is also supported by K-edge measurements for Fe and Zn. The t2g/eg intensity ratio in the O K-edge spectra decreased after this interaction, which is associated with detachment of Zn2+ ions from the lattice. The extent of hybridization, as estimated from the ratio of the post-edge to the pre-edge region of the O K-edge spectra, decreased after this interaction. The metal-oxygen and metal-metal bond lengths were modified as a result of this interaction, as determined from extended X-ray absorption fine structure measurements. These measurements further support the observation of cation migration from AO4 tetrahedra to AO6 octahedra and vice versa. The Fe L-edge magnetic circular dichroism spectra indicate that Fe3+ ions occupying sites in AO4 tetrahedra and BO6 octahedra exhibited antiferromagnetic-like ordering prior to this interaction. The NPs that interacted with energetic O ions displayed a different kind of magnetic ordering.

For the first time, using scanning electron microscopy, transmission electron microscopy, X-ray absorption near edge structure and extended X-ray absorption fine structure X-ray diffraction, it is showed that MnCO 3 , MnWO 4 , Mn 3 (PO 4 ) 2 ·3H 2 O, MnS and Mn(VO 3 ) 2 ·xH 2 O under the water-oxidation conditions and in the presence of cerium(iv) ammonium nitrate, are converted to Mn oxide without a high-range order. A mechanism is proposed for such conversion and as Mn oxide is an efficient water-oxidizing catalyst, it is thus a candidate as a contributor to the observed catalytic activity.

Ce1-xEuxO2 (0 ≤x≤ 0.3) nanoparticles (NPs) were synthesized by the chemical precipitation method. The microstructures and morphology were characterized by synchrotron X-ray diffraction and high resolution transmission electron microscopy. X-ray absorption near edge structure (XANES) spectra at the Eu M5,4-edge and atomic-multiplet calculations revealed that Eu(3+) was predominantly present in the CeO2 lattice and Eu(2+) was negligibly present within the entire doping range. The detailed analysis of the Ce M5,4-edge and the O K-edge has shown strong dependence of the Ce(3+)/Ce(4+) ratio and oxygen vacancy with Eu content. Extended X-ray absorption fine structure (EXAFS) spectra at the Ce K-edge, along with theoretical fitting, have shown systematic variation in the coordination number, bond length and Debye-Waller factor with Eu doping. A blue shift in the absorption edge was observed which implies a net increase in the charge transfer gap between the O 2p and Ce 4f bands due to the increased number of Ce(3+) ions in the Eu doped samples. The excitation and emission spectra of pure CeO2 NPs did not show any photoluminescence (PL) characteristic; however, Ce1-xEuxO2 (x = 0.1-0.3) NPs showed significant improvements in the 4f-4f, (5)D0-(7)F2 and (5)D0-(7)F1 transitions induced luminescence properties. Eu doping has two major effects on the electronic structure and optical properties of CeO2 NPs: the first, at an Eu content of 10 mol%, is the formation of Ce(4+)-O-Eu(3+) networks, i.e., Eu(3+) ions substitute the Ce(4+) ions and introduce oxygen vacancies and Ce(3+) ions in the host lattice, which favors the (5)D0-(7)F2 induced PL properties. The other, at an Eu doping over 10 mol%, is the formation of both Ce(4+)-O-Eu(3+) and Ce(3+)-O-Eu(3+), i.e., Eu(3+) ions not only take substitutional sites of Ce(4+) ions but also replace a fraction of Ce(3+) ions in the CeO2 lattice which favors (5)D0-(7)F1 induced PL properties. As an application of CeO2 NPs towards the degradation of water pollutants, we demonstrated that the Ce1-xEuxO2 (0 ≤x≤ 0.3) NPs can serve as effective photocatalyst materials towards the degradation of the methyl-orange aqueous pollutant dye under UV light irradiation.