Atomic-Scale Insight into Exsolution of CoFe Alloy Nanoparticles in La0.4Sr0.6Co0.2Fe0.7Mo0.1O3−δ with Efficient CO2 Electrolysis

In situ exsolution of metal nanoparticles in perovskite under reducing atmosphere is employed to generate a highly active metal–oxide interface for CO₂ electrolysis in a solid oxide electrolysis cell. Atomic-scale insight is provided into the exsolution of CoFe alloy nanoparticles in La_0.4Sr_0.6Co_0.2Fe_0.7Mo_0.1O_3−δ (LSCFM) by in situ scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy and DFT calculations. The doped Mo atoms occupy B sites of LSCFM, which increases the segregation energy of Co and Fe ions at B sites and improves the structural stability of LSCFM under a reducing atmosphere. In situ STEM measurements visualized sequential exsolution of Co and Fe ions, formation of CoFe alloy nanoparticles, and reversible exsolution and dissolution of CoFe alloy nanoparticles in LSCFM. The metal–oxide interface improves CO₂ adsorption and activation, showing a higher CO₂ electrolysis performance than the LSCFM counterparts.     

Atomic‐Scale Insight into Exsolution of CoFe Alloy Nanoparticles in La0.4Sr0.6Co0.2Fe0.7Mo0.1O3−δ with Efficient CO2 Electrolysis

In situ exsolution of metal nanoparticles in perovskite under reducing atmosphere is employed to generate a highly active metal–oxide interface for CO₂ electrolysis in a solid oxide electrolysis cell. Atomic‐scale insight is provided into the exsolution of CoFe alloy nanoparticles in La_0.4Sr_0.6Co_0.2Fe_0.7Mo_0.1O_3?δ (LSCFM) by in situ scanning transmission electron microscopy (STEM) with energy‐dispersive X‐ray spectroscopy and DFT calculations. The doped Mo atoms occupy B sites of LSCFM, which increases the segregation energy of Co and Fe ions at B sites and improves the structural stability of LSCFM under a reducing atmosphere. In situ STEM measurements visualized sequential exsolution of Co and Fe ions, formation of CoFe alloy nanoparticles, and reversible exsolution and dissolution of CoFe alloy nanoparticles in LSCFM. The metal–oxide interface improves CO₂ adsorption and activation, showing a higher CO₂ electrolysis performance than the LSCFM counterparts.     

Understanding the dynamics of signal transduction for adsorption of gases and vapors on carbon nanotube sensors

Adsorption dynamics and their influence on signal transduction for carbon nanotube-based chemical sensors are explored using continuum site balance equations and a mass action model. These sensors are shown to possess both reversible and irreversible binding sites that can be modeled independently. For the case of irreversible adsorption, it is shown that the characteristic response time scales inversely with analyte concentration. It is inappropriate to report a detection limit for this type of sensor since any nonzero analyte concentration can be detected in theory but at a cost of increasing transduction time with decreasing concentration. The response curve should examine the initial rate of signal change as a function of analyte concentration. Conversely, a reversible sensor has a predefined detection limit, independent of the detector geometry with a characteristic time scaling that becomes constant in the zero analyte concentration limit. A simple analytical test is presented to distinguish between these two mechanisms from the transient response of a nanotube sensor array. Two systems appearing in the literature are shown to have an irreversible component, and regressed surface rate constants for this component are similar across different sensor geometries and analytes.     

Nanostructure-driven analyte-interface electron transduction: a general approach to sensor and microreactor design

A concept describing the nanostructure-directed dynamics of acid/base interaction and the balance between physisorption and chemisorption on an extrinsic semiconductor interface is evaluated and compared for n- and p-type semiconductors. The inverse hard/soft acid/base (IHSAB) concept, as it complements the HSAB concept, defines the nature of a dominant physisorption behavior and enables the creation of a matrix of controllable interactions. The technology results in the coupling of Lewis acid/base chemistry with the extrinsic semiconductor majority carriers. Nanoporous silicon layers facilitate the application of nanostructured metal/metal oxides, which provide sensitivity and selectivity for the modified interface. Applied fractional depositions can produce a dominant reversible physisorptive (sensors) or chemisorptive (microreactors) interaction at the semiconductor interface as the nanostructures act as antennas to focus the interaction. The dynamic natures of n- and p-type silicon are evaluated and compared, by focusing on the controlled manipulation of these semiconductors as they are modified with nanostructures and interact with the gas-phase analytes. The observed semiconductor responses correlate well with the temperature dependence of the extrinsic semiconductor, the population of the donor or acceptor levels, and the inherent mobilities of electrons. The response of the modified n-type semiconductors is found to exceed that of comparable p-type systems. The IHSAB concept can be extended to assess the properties of several additional semiconductor interfaces including nanowires. The results obtained not only pertain to sensor and microreactor array design, but also suggest the importance of the dynamic changes created, as the majority charge-carrier concentrations are manipulated and the Fermi energies are modified through chemical interaction.     

Bottom‐Up Fabrication of a Sandwich‐Like Carbon/Graphene Heterostructure with Built‐In FeNC Dopants as Non‐Noble Electrocatalyst for Oxygen Reduction Reaction

High‐performance non‐noble electrocatalysts for oxygen reduction reaction (ORR) are the prerequisite for large‐scale utilization of fuel cells. Herein, a type of sandwiched‐like non‐noble electrocatalyst with highly dispersed FeN_x active sites embedded in a hierarchically porous carbon/graphene heterostructure was fabricated using a bottom‐up strategy. The in situ ion substitution of Fe³⁺ in a nitrogen‐containing MOF (ZIF‐8) allows the Fe‐heteroatoms to be uniformly distributed in the MOF precursor, and the assembly of Fe‐doped ZIF‐8 nano‐crystals with graphene‐oxide and in situ reduction of graphene‐oxide afford a sandwiched‐like Fe‐doped ZIF‐8/graphene heterostructure. This type of heterostructure enables simultaneous optimization of FeN_x active sites, architecture and interface properties for obtaining an electron‐catalyst after a one‐step carbonization. The synergistic effect of these factors render the resulting catalysts with excellent ORR activities. The half‐wave potential of 0.88 V vs. RHE outperforms most of the none‐noble metal catalyst and is comparable with the commercial Pt/C (20 wt %) catalyst. Apart from the high activity, this catalyst exhibits excellent durability and good methanol‐tolerance. Detailed investigations demonstrate that a moderate content of Fe dopants can effectively increase the intrinsic activities, and the hybridization of graphene can enhance the reaction kinetics of ORR. The strategy proposed in this work gives an inspiration towards developing efficient noble‐metal‐free electrocatalysts for ORR.     

Paramagnetic complexes of 9, 10-anthraquinone and 9-fluorenone on the metal oxide surface

Paramagnetic complexes of 9, 10-anthraquinone and 9-fluorenone adsorbed on the surface of calcium, magnesium, zinc, zirconium, and aluminum oxides and modified Al₂O₃ as well as on mixed oxides were studied by ESR and electron-nuclear double resonance. Radical anions that do not interact with Lewis acid sites are generated on the surfaces of oxides with electrondonating properties (CaO, MgO). Paramagnetic complexes of the anthraquinone or fluorenone radical anion with Lewis acid sites (coordinatively unsaturated metal cations) are formed in other cases. Several types of similar complexes can be formed. Mechanisms of interaction of the probe molecules with the metal oxide surface were proposed.     

Synthesis of nickel nanoparticles supported on metal oxides using electroless plating: Controlling the dispersion and size of nickel nanoparticles

Nickel nanoparticles supported on metal oxides were prepared by a modified electroless nickel-plating method. The process and mechanism of electroless plating were studied by changing the active metal (Ag) loading, acidity, and surface area of metal oxides and were characterized by UV–vis spectroscopy, transmission electron microscopy, scanning electron microscopy, and H₂ chemisorption. The results showed that the dispersion of nickel nanoparticles was dependent on the interface reaction between the metal oxide and the plating solution or the active metal and the plating solution. The Ag loading and acidity of the metal oxide mainly affected the interface reaction to change the dispersion of nickel nanoparticles. The use of ultrasonic waves and microwaves and the change of solvents from water to ethylene glycol in the electroless plating could affect the dispersion and size of nickel nanoparticles.     

Lewis Acid Coupled Electron Transfer of Metal–Oxygen Intermediates

Redox‐inactive metal ions and Brønsted acids that function as Lewis acids play pivotal roles in modulating the redox reactivity of metal–oxygen intermediates, such as metal–oxo and metal–peroxo complexes. The mechanisms of the oxidative C?H bond cleavage of toluene derivatives, sulfoxidation of thioanisole derivatives, and epoxidation of styrene derivatives by mononuclear nonheme iron(IV)–oxo complexes in the presence of triflic acid (HOTf) and Sc(OTf)₃ have been unified as rate‐determining electron transfer coupled with binding of Lewis acids (HOTf and Sc(OTf)₃) by iron(III)–oxo complexes. All logarithms of the observed second‐order rate constants of Lewis acid‐promoted oxidative C?H bond cleavage, sulfoxidation, and epoxidation reactions of iron(IV)–oxo complexes exhibit remarkably unified correlations with the driving forces of proton‐coupled electron transfer (PCET) and metal ion‐coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes were taken into account. The binding of HOTf and Sc(OTf)₃ to the metal–oxo moiety has been confirmed for Mn^IV–oxo complexes. The enhancement of the electron‐transfer reactivity of metal–oxo complexes by binding of Lewis acids increases with increasing the Lewis acidity of redox‐inactive metal ions. Metal ions can also bind to mononuclear nonheme iron(III)–peroxo complexes, resulting in acceleration of the electron‐transfer reduction but deceleration of the electron‐transfer oxidation. Such a control on the reactivity of metal–oxygen intermediates by binding of Lewis acids provides valuable insight into the role of Ca²⁺ in the oxidation of water to dioxygen by the oxygen‐evolving complex in photosystem II.     

Nanostructure‐Directed Physisorption vs Chemisorption at Semiconductor Interfaces: The Inverse of the HSAB Concept

A concept, complementary to that of hard and soft acid–base interactions (HSAB‐dominant chemisorption) and consistent with dominant physisorption to a semiconductor interface, is presented. We create a matrix of sensitivities and interactions with several basic gases. The concept, based on the reversible interaction of hard‐acid surfaces with soft bases, hard‐base surfaces with soft acids, or vice versa, corresponds 1) to the inverse of the HSAB concept and 2) to the selection of a combination of semiconductor interface and analyte materials, which can be used to direct a physisorbed vs chemisorbed interaction. The technology, implemented on nanopore coated porous silicon micropores, results in the coupling of acid–base chemistry with the depletion or enhancement of majority carriers in an extrinsic semiconductor. Using the inverse‐HSAB (IHSAB) concept, significant and predictable changes in interface sensitivity for a variety of gases can be implemented. Nanostructured metal oxide particle depositions provide selectivity and complement a highly efficient electrical contact to a porous silicon nanopore covered microporous interface. The application of small quantities (much less than a monolayer) of nanostructured metals, metal oxides, and catalysts which focus the physisorbtive and chemisorbtive interactions of the interface, can be made to create a range of notably higher sensitivities for reversible physisorption. This is exemplified by an approach to reversible, sensitive, and selective interface responses. Nanostructured metal oxides developed from electroless gold (Au_xO), tin (SnO₂), copper (Cu_xO), and nickel (NiO) depositions, nanoalumina, and nanotitania are used to demonstrate the IHSAB concept and provide for the detection of gases, including NH₃, PH₃, CO, NO, and H₂S, in an array‐based format to the sub‐ppm level.     

Construction and Electrochemical Property Studies of DNA Duplexes Tethered to Gold Electrode via Au−C Bond

Effective linkage of DNA onto metal surfaces plays a crucial role in the applications of DNA as electrochemical recognition, signal output and amplification devices for gene and protein detections, specific analyte recognitions, catalysis, and so on. Here we report a promising and operationally simple approach for the construction of double‐stranded DNA‐linked Au interface via Au?C bond (RdsDNA‐C?Au), upon efficient in situ cleavage of trimethylsilyl end group of 4‐[(trimethylsilyl) ethynyl] benzoic acid and subsequent dehydration condensation between NH₂‐modified DNA and benzoic acid. Due to the introduction of large conjugated π group (4‐carboxyphenylethynyl) as the “bridge bond”, the conductivity of the RdsDNA‐C?Au interface is greatly improved. As a result, under commonly used DNA packing density (>0.5 pmol cm^?2), the surface‐confined electron transfer at the interface is simply mediated by the stacked‐bases of dsDNA, independent of the orientation of dsDNA (tethered to the electrode at 5′‐ or 3′‐end). Also, compared to the traditional RdsDNA‐S?Au interface via alkanethiol linker, the RdsDNA‐C?Au interface displays more sensitive electrochemical response and excellent stability. Due to the better stability, conductivity and simple electron transfer mechanism, the RdsDNA‐C?Au interface is anticipated to be widely used in electrochemistry‐involved molecular recognitions, gene and protein detections with higher sensitivity and accuracy.     

Mixed metal fluorides as doped Lewis acidic catalyst systems: a comparative study involving novel high surface area metal fluorides

MF₃-doped/MgF₂ systems with enhanced Lewis acidity are reported, which are obtained either by the conventional aqueous route of co-precipitation or, by a novel non-aqueous soft chemistry route. The latter gives outstanding high surface areas and exhibits potent Lewis acid catalyst behaviour. The doped solid metal fluorides with dopant metals such as Ga, In, Fe, V are discussed in terms of the modified Tanabe model, which is adopted for metal fluoride systems. The two doped but differently prepared systems are analysed according to their surface characteristics by BET surface area, pore-size distribution and XPS/XAES as well as for the solid state structure by scanning electron microscopy (SEM), XRD and -MAS-NMR. The surface properties were evaluated by photoacoustic IR-spectroscopy of pyridine adsorbates and selected catalytic reactions.The exemplarily investigated GaF₃-doped/MgF₂ system reveals modified intrinsic properties of the solid mixture culminating in very high surface areas of a structurally distorted mesoporous solid and electrostatic charge rearrangements causing increased Lewis acid sites.     

Electrochemical Properties of Ag@iron Oxide Nanocomposite for Application as Nitrate Sensor

Ag@iron oxide nanocomposite powders were synthesized via a two‐step chemical method. Characterization by UV‐Vis, XRD, SEM‐EDX and TEM revealed they are composed of nanosized crystalline silver particles in strict contact with amorphous iron oxide(s). The electrochemical behavior of the synthesized Ag@iron oxide composite was investigated by cyclic voltammetry. Compared with the single phase‐modified electrodes, the Ag@iron oxide/SPCE electrode exhibits an enhanced cathodic current in response to the target analyte, due to a synergistic effect between Ag crystallites and amorphous iron oxide nanoparticles. An amperometric sensor for detection of nitrate based on Ag@iron oxide modified screen‐printed electrode (Ag@iron oxide/SPCE) has been fabricated, showing a good sensitivity (663 µA mM^?1 cm^?2) and a detection limit of 30 µM.     

Investigation of new chelation ion chromatography procedure to determine the surface composition of powdered metal oxide samples in the solid state

Numerous tests have been conducted on the feasibility of characterizing the surfaces of metal oxide powders using HPLC. An in-line filter housing was modified to serve as a sample chamber to replace the sample loop. A gradient pump was used to gradually increase eluent acidity to find the conditions at which the surface of a metal oxide powder began to dissolve. The theoretical masses of surface monolayers of metal oxide powders were compared with the experimentally determined masses of dissolved material thought to be from the surface to test whether surface and bulk dissolution phenomena in acidic conditions are separable and quantifiable. A set of methods was tested that could first dissolve a metal oxide sample's surface, then separate and detect analyte species by chelation ion chromatography. Surface characterization by ion chromatography could be more cost-effective than existing methods, and reveal chemical properties of the sample where existing methods only give physical composition and properties.     

On‐Chip Tailorability of Capacitive Gas Sensors Integrated with Metal–Organic Framework Films

Gas sensing technologies for smart cities require miniaturization, cost‐effectiveness, low power consumption, and outstanding sensitivity and selectivity. On‐chip, tailorable capacitive sensors integrated with metal–organic framework (MOF) films are presented, in which abundant coordinatively unsaturated metal sites are available for gas detection. The in situ growth of homogeneous Mg‐MOF‐74 films is realized with an appropriate metal‐to‐ligand ratio. The resultant sensors exhibit selective detection for benzene vapor and carbon dioxide (CO₂) at room temperature. Postsynthetic modification of Mg‐MOF‐74 films with ethylenediamine decreases sensitivity toward benzene but increases selectivity to CO₂. The reduced porosity and blocked open metal sites caused by amine coordination account for a deterioration in the sensing performance for benzene (by ca. 60 %). The enhanced sensitivity for CO₂ (by ca. 25 %) stems from a tailored amine–CO₂ interaction. This study demonstrates the feasibility of tuning gas sensing properties by adjusting MOF–analyte interactions, thereby offering new perspectives for the development of MOF‐based sensors.     

Investigation of Methanol Behavior at the Designed Electrochemical Sensor based on Ni(II) Complex and Graphene Nanosheets

In this work, the electrochemical oxidation of methanol was investigated by different electrochemical methods at a carbon paste electrode (CPE) modified with (N‐5‐methoxysalicylaldehyde, N´‐2‐hydroxyacetophenon‐1, 2 phenylenediimino nickel(II) complex (Ni(II)–MHP) and reduced graphene oxide (RGO), which is named Ni(II)‐MHP/RGO/CPE, in an alkaline solution. This modified electrode was found to be efficient for the oxidation of methanol. It was found that methanol was oxidized by the NiOOH groups generated by further electrochemical oxidation of nickel(II) hydroxide on the surface of the modified electrode. Under optimum conditions, some parameters of the analyte (MeOH), such as the electron transfer coefficient (α), the electron transfer rate constant) k_s), and the diffusion coefficient of species in a 0.1 M solution (pH = 13), were determined. The designed sensor showed a linear dynamic range of 2.0–100.0 and 100.0–1000.0 μM and a detection limit of 0.68 μM for MeOH determination. The Ni(II)‐MHP/RGO/CPE sensor was used in the determination of MeOH in a real sample.     

Gas sensing mechanism in chemiresistive cobalt and metal-free phthalocyanine thin films

The gas sensing behaviors of cobalt phthalocyanine (CoPc) and metal-free phthalocyanine (H2Pc) thin films were investigated with respect to analyte basicity. Chemiresistive sensors were fabricated by deposition of 50 nm thick films on interdigitated gold electrodes via organic molecular beam epitaxy (OMBE). Time-dependent current responses of the films were measured at constant voltage during exposure to analyte vapor doses. The analytes spanned a range of electron donor and hydrogen-bonding strengths. It was found that, when the analyte exceeded a critical base strength, the device responses for CoPc correlated with Lewis basicity, and device responses for H2Pc correlated with hydrogen-bond basicity. This suggests that the analyte-phthalocyanine interaction is dominated by binding to the central cavity of the phthalocyanine with analyte coordination strength governing CoPc sensor responses and analyte hydrogen-bonding ability governing H2Pc sensor responses. The interactions between the phthalocyanine films and analytes were found to follow first-order kinetics. The influence of O2 on the film response was found to significantly affect sensor response and recovery. The increase of resistance generally observed for analyte binding can be attributed to hole destruction in the semiconductor film by oxygen displacement, as well as hole trapping by electron donor ligands.     

Adsorption of Carbon Dioxide on Unsaturated Metal Sites in M2(dobpdc) Frameworks with Exceptional Structural Stability and Relation between Lewis Acidity and Adsorption Enthalpy

A series of metal–organic frameworks (MOFs) M₂(dobpdc) (M=Mn, Co, Ni, Zn; H₄dobpdc=4,4′‐dihydroxy‐1,1′‐biphenyl‐3,3′‐dicarboxylic acid), with a highly dense arrangement of open metal sites along hexagonal channels were prepared by microwave‐assisted or simple solvothermal reactions. The activated materials were structurally expanded when guest molecules including CO₂ were introduced into the pores. The Lewis acidity of the open metal sites varied in the order Mn<Co<Ni>Zn, as confirmed by C=O stretching bands in the IR spectra, which are related to the CO₂ adsorption enthalpy. DFT calculations revealed that the high CO₂ binding affinity of transition‐metal‐based M₂(dobpdc) is primarily attributable to the favorable charge transfer from CO₂ (oxygen lone pair acting as a Lewis base) to the open metal sites (Lewis acid), while electrostatic effects, the underlying factor responsible for the particular order of binding strength observed across different transition metals, also play a role. The framework stability against water coincides with the order of Lewis acidity. In this series of MOFs, the structural stability of Ni₂(dobpdc) is exceptional; it endured in water vapor, liquid water, and in refluxing water for one month, and the solid remained intact on exposure to solutions of pH 2–13. The DFT calculations also support the experimental finding that Ni₂(dobpdc) has higher chemical stability than the other frameworks.     

Surface plasmon resonance within ion implanted silver clusters

Surface plasmon resonance (SPR) belongs to the most sensitive indicators for changes in analyte concentrations or other sample properties, which depend on the refractive index in the medium. Surface plasmons represent collective electron oscillations in metal cluster or metal layers of diameter or thickness in the nanometer range. Such layers or clusters are used in many optical sensors in order to enhance the interaction between electromagnetic radiation and analyte. Clusters are preferred to enhance Raman scattering and IR absorption, whereas layers are used for SPR in the visible range. We tested the applicability of ion implanted clusters in order to enhance the stability of the metal coatings of the SPR sensor elements. A model based on the effective media theory was developed in order to enhance the sensor capabilities. The potential of the SPR with ion implanted metal clusters consists in durable resonance layers for biochemical sensors. Received: 21 December 1997 / Revised: 6 March 1998 / Accepted: 12 March 1998     

Surface plasmon resonance within ion implanted silver clusters

Surface plasmon resonance (SPR) belongs to the most sensitive indicators for changes in analyte concentrations or other sample properties, which depend on the refractive index in the medium. Surface plasmons represent collective electron oscillations in metal cluster or metal layers of diameter or thickness in the nanometer range. Such layers or clusters are used in many optical sensors in order to enhance the interaction between electromagnetic radiation and analyte. Clusters are preferred to enhance Raman scattering and IR absorption, whereas layers are used for SPR in the visible range. We tested the applicability of ion implanted clusters in order to enhance the stability of the metal coatings of the SPR sensor elements. A model based on the effective media theory was developed in order to enhance the sensor capabilities. The potential of the SPR with ion implanted metal clusters consists in durable resonance layers for biochemical sensors. Received: 21 December 1997 / Revised: 6 March 1998 / Accepted: 12 March 1998     

Myoglobin‐Clay Electrode for Nitric Oxide (NO) Detection in Solution

Sodium montmorillonite was prepared via a colloidal chemical approach and deposited onto glassy carbon electrodes (GCE). Myoglobin was immobilized on the clay membrane modified electrode by spontaneous adsorption. Characterization of the myoglobin/clay/glassy carbon electrode (Mb/clay/GCE) showed a quasi‐reversible, electrochemical redox behavior of the adsorbed protein with a formal potential of ?0.380±0.010 V (vs. Ag/AgCl). The heterogeneous electron transfer rate constant was found to be strongly influenced by the buffer concentration. The Mb/clay/GCE was stable for several days in solution. The interaction of the immobilized Mb with nitric oxide (NO) is characterized by coordination chemistry. The reaction was found to be reversible and could be applied for NO detection in the nanomolar concentration range by a voltammetric analysis. In addition a mixed protein electrode with co‐immmobilized myoglobin (Mb) and cytochrome c (Cyt.c) was developed. By choice of the electrode potential both proteins can be addressed independently.