Water Oxidation Catalyst via Heterogenization of Iridium Oxides on Silica: A Polyamine-Mediated Route To Achieve Activity and Stability

Heterogenization of nanostructured iridium-based catalysts to simultaneously achieve activity and stability in the catalytic water oxidation with cerium ammonium nitrate (CAN) as the oxidant is reported herein. We demonstrate that a polyamine-mediated assembly process to disperse iridium species on mesoporous silica spheres facilitates the fabrication of nanosized iridium oxides under optimal thermal treatment. From comprehensive morphological and electronic structure studies including electron microscopy, UV–vis spectroscopy, XANES, and EXAFS, we show that the influence of polyamine is crucial in stabilizing catalytically active iridium oxides in the mesoporous silica matrix. While the functionalization of the silica surface with polyamine facilitates interaction with the negatively charged iridium precursor, the presence of polyamine further enables control of the dispersion and crystallization of the generated iridium oxides during the thermal treatment at 573 K. As a consequence, the catalyst exhibits enhanced activity with higher TON along with desirable stability to allow it to be recycled while keeping the activity intact. The activity and stability of the synthesized catalyst in comparison with those of IrCl₃ and IrO₂ reveal that balancing between the dispersion and crystallization of iridium oxides is crucial in heterogenization of the catalyst

Search for Origin of Room Temperature Ferromagnetism Properties in Ni-Doped ZnO Nanostructure

The origin of room temperature (RT) ferromagnetism (FM) in Zn_1–<i>x</i>Ni_<i>x</i>O (0< <i>x</i> < 0.125) samples are systematically investigated through physical, optical, and magnetic properties of nanostructure, prepared by simple low-temperature wet chemical method. Reitveld refinement of X-ray diffraction pattern displays an increase in lattice parameters with strain relaxation and contraction in Zn/O occupancy ratio by means of Ni-doping. Similarly, scanning electron microscope demonstrates modification in the morphology from nanorods to nanoflakes with Ni doping, suggests incorporation of Ni ions in ZnO. More interestingly, XANES (X-ray absorption near edge spectroscopy) measurements confirm that Ni is being incorporated in ZnO as Ni²⁺. EXAFS (extended X-ray absorption fine structure) analysis reveals that structural disorders near the Zn sites in the ZnO samples upsurges with increasing Ni concentration. Raman spectroscopy exhibits additional defect driven vibrational mode (at 275 cm^–1), appeared only in Ni-doped samples and the shift with broadening in 580 cm^–1 peak, which manifests the presence of the oxygen vacancy (V_O) related defects. Moreover, in photoluminescence (PL) spectra, we have observed a peak at 524 nm, indicating the presence of singly ionized V_O⁺, which may be activating bound magnetic polarons (BMPs) in dilute magnetic semiconductors (DMSs). Magnetization measurements indicate weak ferromagnetism at RT, which rises with increasing Ni concentration. It is therefore proposed that the effect of the Ni ions as well as the inherent exchange interactions arising from V_O⁺ assist to produce BMPs, which are accountable for the RT-FM in Zn_1–<i>x</i>Ni_<i>x</i>O (0< <i>x</i> < 0.125) system

Physiochemical Investigation of Shape-Designed MnO₂ Nanostructures and Their Influence on Oxygen Reduction Reaction Activity in Alkaline Solution

In this work, five types of MnO₂ nanostructres (nanowires, nanotubes, nanoparticles, nanorods, and nanoflowers) were synthesized with a fine control over their α-crystallographic form by hydrothermal method. The electrocatalytic activities of these materials were examined toward oxygen reduction reaction (ORR) in alkaline medium. Numerous characterizations were correlated with the observed activity by analyzing their crystal structure (TGA, XRD, TEM), material morphology (FE-SEM), porosity (BET), inherent structural nature (IR, Raman, ESR), surfaces (XPS), and electrochemical properties (Tafel, Koutecky–Levich plots and % of H₂O₂ produced). Moreover, X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) analysis were employed to study the structural information on the MnO₂ coordination number as well as interatomic distance. These combined results show that the electrocatalytic activities are significantly dependent on the nanoshapes and follow an order nanowire > nanorod > nanotube > nanoparticle > nanoflower. α-MnO₂ nanowires possess enhanced electrocatalytic activity compared to other shapes, even though the nanotubes possess a much higher BET surface area. In the ORR studies, α-MnO₂ nanowires displayed Tafel slope of 65 mV/decade, n-value of 3.5 and 3.6% of hydrogen peroxide production. The superior ORR activity was attributed to the fact that it possesses active sites composed with two shortened Mn–O bonds along with a Mn–Mn distance of 2.824 Å, which provides an optimum requirement for the adsorbed oxygen in a bridge mode favoring the direct 4 electron reduction. In accordance with the first principles based density functional theory (DFT), the enhancement in ORR activity is due to the less activation energy needed for the reaction by the (211) surface than all other surfaces

Hierarchical Polyoxometallate Confined in Woven Thin Films for Single-Cluster Catalysis: Simplified Electrodes for Far-Fetched O₂ Evolution from Seawater

The highly anticipated artificial conversion of water to oxygen for the imperishable growth of renewable energy requires efficient water oxidation catalysts (WOCs) to drive the exciting 4e– transformation at low driving potentials. Herein, we describe the freestanding thin film of P5Q7 (TFPQ), where Preyssler [P5W30O110]14– (P5) clusters are woven with [CH3(CH2)6]4N(Br) chains (Q7) to confine P5 clusters and maximize its catalytic exposure. The TFPQ-supported electrode shows OER at record-low overpotentials at 10 mAcm2 (η10 = 130 and 490 mV), rapid migration of electrons (Tafel, 35 and 56 mVdec–1), turnover frequency (TOF, 8.55 s–1), in alkaline water (1 M KOH), and natural seawater, respectively. Evenly dispersed and confined conducting P5 clusters with a delocalized charge cloud shows ∼3 times lower η10 and eventually high OER efficiency than nonconfined clusters. The TFPQ electrodes showed a prolonged stability of minimum 1000 cycles in alkaline water and seawater, without the leaching of true catalytic species P5

Quantitative Structure of an Acetate Dye Molecule Analogue at the TiO₂–Acetic Acid Interface

The positions of atoms in and around acetate molecules at the rutile TiO₂(110) interface with 0.1 M acetic acid have been determined with a precision of ±0.05 Å. Acetate is used as a surrogate for the carboxylate groups typically employed to anchor monocarboxylate dye molecules to TiO₂ in dye-sensitized solar cells (DSSC). Structural analysis reveals small domains of ordered (2 × 1) acetate molecules, with substrate atoms closer to their bulk terminated positions compared to the clean UHV surface. Acetate is found in a bidentate bridge position, binding through both oxygen atoms to two 5-fold titanium atoms such that the molecular plane is along the [001] azimuth. Density functional theory calculations provide adsorption geometries in excellent agreement with experiment. The availability of these structural data will improve the accuracy of charge transport models for DSSC

Hierarchical Polyoxometallate Confined in Woven Thin Films for Single-Cluster Catalysis: Simplified Electrodes for Far-Fetched O₂ Evolution from Seawater

The highly anticipated artificial conversion of water to oxygen for the imperishable growth of renewable energy requires efficient water oxidation catalysts (WOCs) to drive the exciting 4e– transformation at low driving potentials. Herein, we describe the freestanding thin film of P5Q7 (TFPQ), where Preyssler [P5W30O110]14– (P5) clusters are woven with [CH3(CH2)6]4N(Br) chains (Q7) to confine P5 clusters and maximize its catalytic exposure. The TFPQ-supported electrode shows OER at record-low overpotentials at 10 mAcm2 (η10 = 130 and 490 mV), rapid migration of electrons (Tafel, 35 and 56 mVdec–1), turnover frequency (TOF, 8.55 s–1), in alkaline water (1 M KOH), and natural seawater, respectively. Evenly dispersed and confined conducting P5 clusters with a delocalized charge cloud shows ∼3 times lower η10 and eventually high OER efficiency than nonconfined clusters. The TFPQ electrodes showed a prolonged stability of minimum 1000 cycles in alkaline water and seawater, without the leaching of true catalytic species P5

Hierarchical Polyoxometallate Confined in Woven Thin Films for Single-Cluster Catalysis: Simplified Electrodes for Far-Fetched O₂ Evolution from Seawater

The highly anticipated artificial conversion of water to oxygen for the imperishable growth of renewable energy requires efficient water oxidation catalysts (WOCs) to drive the exciting 4e– transformation at low driving potentials. Herein, we describe the freestanding thin film of P5Q7 (TFPQ), where Preyssler [P5W30O110]14– (P5) clusters are woven with [CH3(CH2)6]4N(Br) chains (Q7) to confine P5 clusters and maximize its catalytic exposure. The TFPQ-supported electrode shows OER at record-low overpotentials at 10 mAcm2 (η10 = 130 and 490 mV), rapid migration of electrons (Tafel, 35 and 56 mVdec–1), turnover frequency (TOF, 8.55 s–1), in alkaline water (1 M KOH), and natural seawater, respectively. Evenly dispersed and confined conducting P5 clusters with a delocalized charge cloud shows ∼3 times lower η10 and eventually high OER efficiency than nonconfined clusters. The TFPQ electrodes showed a prolonged stability of minimum 1000 cycles in alkaline water and seawater, without the leaching of true catalytic species P5

Room-Temperature Magneto-dielectric Effect in LaGa_0.7Fe_0.3O_3+γ; Origin and Impact of Excess Oxygen

We report an observation of room-temperature magneto-dielectric (RTMD) effect in LaGa_0.7Fe_0.3O_3+γ compound. The contribution of intrinsic/resistive sources in the presently observed RTMD effect was analyzed by measuring direct-current (dc) magnetoresistance (MR) in four-probe geometry and frequency-dependent MR via impedance spectroscopy (MRIS). Present MRIS analysis reveals that at frequencies corresponding to grain contribution (≥1 × 10⁶ Hz for present sample), the observed MD phenomenon is MR-free/intrinsic, whereas at lower probing frequencies (<1 × 10⁶ Hz), the observed MD coupling appears to be MR-dominated possibly due to oxygen excess, that is, due to coexistence of Fe³⁺ and Fe⁴⁺. The magnetostriction is anticipated as a mechanism responsible for MR-free/intrinsic MD coupling, whereas the MR-dominated part is attributed to hopping charge transport along with Maxwell–Wagner and space charge polarization. The multivalence of Fe ions in LaGa_0.7Fe_0.3O_3+γ was validated through iodometric titration and Fe K-edge X-ray absorption near-edge structure measurements. The excess of oxygen, that is, coexistence of Fe³⁺ and Fe⁴⁺, was understood in terms of stability of Fe⁴⁺ by means of “bond-valence-sum” analysis and density functional theory-based first-principles calculations. The cation vacancies at La/Ga site (or at La and Ga both) were proposed as the possible origin of excess oxygen in presently studied compound. Present investigation suggests that, to justify the intrinsic/resistive origin of MD phenomenon, frequency-dependent MR measurements are more useful than measuring only dc MR or comparing the trends of magnetic-field-dependent change in dielectric constant and tan δ. Presently studied Fe-doped LaGaO₃ can be a candidate for RTMD applications