Spatially Resolved Imaging of Inhomogeneous Charge Transfer Behavior in Polymorphous Molybdenum Oxide. I. Correlation of Localized Structural, Electronic, and Chemical Properties Using Conductive Probe Atomic Force Microscopy and Raman Microprobe Spectroscopy

A detailed study of electrochemically deposited molybdenum oxide thin films has been carried out after they were sintered at 250 °C. Conductive probe atomic force microscopy (CP-AFM), Raman microscopy, and X-ray photoelectron spectroscopy (XPS) techniques were employed to assess the complex structural, electronic, and compositional properties of these films. Spatially resolved Raman microprobe spectroscopy studies reveal that sintered molybdenum oxide is polymorphous and phase segregated with three types of domains observed comprising orthorhombic α-MoO3, monoclinic β-MoO3, and intermixed α-/β-MoO3. CP-AFM studies conducted in concert with Raman microprobe spectroscopy allowed for correlation between specific compositional regions and localized electronic properties. Single point tunneling spectroscopy studies of chemically distinct regions show semiconducting current−voltage (I−V) behavior with the β-MoO3 polymorph exhibiting higher electronic conductivity than intermixed α-/β-MoO3 or microcrystalline α-MoO3 domains. XPS valence level spectra of β-MoO3 films display a small structured band near the Fermi level, indicative of an increased concentration of oxygen vacancies. This accounts for the greatly enhanced electronic conductivity of β-MoO3 as these positively charged cationic defects (anion vacancies) act to trap excess electrons. Connections between structural features, electronic properties, and chemical composition are established and discussed. Importantly, this work highlights the value of using spatially resolved techniques for correlating structural and compositional features with electrochemical behaviors of disordered, mixed-phase lithium insertion oxides

Spatially Resolved Imaging of Inhomogeneous Charge Transfer Behavior in Polymorphous Molybdenum Oxide. I. Correlation of Localized Structural, Electronic, and Chemical Properties Using Conductive Probe Atomic Force Microscopy and Raman Microprobe Spectroscopy

A detailed study of electrochemically deposited molybdenum oxide thin films has been carried out after they were sintered at 250 °C. Conductive probe atomic force microscopy (CP-AFM), Raman microscopy, and X-ray photoelectron spectroscopy (XPS) techniques were employed to assess the complex structural, electronic, and compositional properties of these films. Spatially resolved Raman microprobe spectroscopy studies reveal that sintered molybdenum oxide is polymorphous and phase segregated with three types of domains observed comprising orthorhombic α-MoO3, monoclinic β-MoO3, and intermixed α-/β-MoO3. CP-AFM studies conducted in concert with Raman microprobe spectroscopy allowed for correlation between specific compositional regions and localized electronic properties. Single point tunneling spectroscopy studies of chemically distinct regions show semiconducting current−voltage (I−V) behavior with the β-MoO3 polymorph exhibiting higher electronic conductivity than intermixed α-/β-MoO3 or microcrystalline α-MoO3 domains. XPS valence level spectra of β-MoO3 films display a small structured band near the Fermi level, indicative of an increased concentration of oxygen vacancies. This accounts for the greatly enhanced electronic conductivity of β-MoO3 as these positively charged cationic defects (anion vacancies) act to trap excess electrons. Connections between structural features, electronic properties, and chemical composition are established and discussed. Importantly, this work highlights the value of using spatially resolved techniques for correlating structural and compositional features with electrochemical behaviors of disordered, mixed-phase lithium insertion oxides

Spatially Resolved Imaging of Inhomogeneous Charge Transfer Behavior in Polymorphous Molybdenum Oxide. I. Correlation of Localized Structural, Electronic, and Chemical Properties Using Conductive Probe Atomic Force Microscopy and Raman Microprobe Spectroscopy

A detailed study of electrochemically deposited molybdenum oxide thin films has been carried out after they were sintered at 250 °C. Conductive probe atomic force microscopy (CP-AFM), Raman microscopy, and X-ray photoelectron spectroscopy (XPS) techniques were employed to assess the complex structural, electronic, and compositional properties of these films. Spatially resolved Raman microprobe spectroscopy studies reveal that sintered molybdenum oxide is polymorphous and phase segregated with three types of domains observed comprising orthorhombic α-MoO3, monoclinic β-MoO3, and intermixed α-/β-MoO3. CP-AFM studies conducted in concert with Raman microprobe spectroscopy allowed for correlation between specific compositional regions and localized electronic properties. Single point tunneling spectroscopy studies of chemically distinct regions show semiconducting current−voltage (I−V) behavior with the β-MoO3 polymorph exhibiting higher electronic conductivity than intermixed α-/β-MoO3 or microcrystalline α-MoO3 domains. XPS valence level spectra of β-MoO3 films display a small structured band near the Fermi level, indicative of an increased concentration of oxygen vacancies. This accounts for the greatly enhanced electronic conductivity of β-MoO3 as these positively charged cationic defects (anion vacancies) act to trap excess electrons. Connections between structural features, electronic properties, and chemical composition are established and discussed. Importantly, this work highlights the value of using spatially resolved techniques for correlating structural and compositional features with electrochemical behaviors of disordered, mixed-phase lithium insertion oxides

Spatially Resolved Imaging of Inhomogeneous Charge Transfer Behavior in Polymorphous Molybdenum Oxide. II. Correlation of Localized Coloration/Insertion Properties Using Spectroelectrochemical Microscopy

A newly developed spectroelectrochemical imaging approach for directly assessing lithium ion insertion energetics and kinetics in mixed-phase, polymorphous MoO3 is reported. Two variants of spectroelectrochemical microscopy were used to monitor insertion dynamics and to follow electrochemically induced phase transformations at specifically identified structural and compositional domains. Cyclovoltoabsorptometric (dOD/dE) measurements carried out in LiClO4/propylene carbonate solutions reveal that the lithium insertion is nonuniform and can be directly correlated with phase-segregated domains comprising α-MoO3, β-MoO3, and intermixed α-/β-MoO3. Lithium insertion is found to proceed by a staging process where each phase displays energetically distinct insertion behaviors. Chronoabsorptometric imaging measurements allow for the simultaneous estimation of lithium diffusion coefficients, ionic conductivities, and lithium capacities at isolated phases within the polymorphous material. The lithium diffusion coefficient and ionic conductivity is largest for domains comprising intermixed α-/β-MoO3, whereas it is smallest at domains consisting of β-MoO3. The higher diffusion coefficient observed for intermixed α-/β-MoO3 domains is most likely due to larger thermodynamic enhancement factors for the mixed phase domains than for domains consisting of either α-MoO3 or β-MoO3. Estimation of capacity values within each uniquely identified domain reveals that the lithium insertion capacity is about 4 times greater in α-MoO3 than in β-MoO3. The discrepancies between the lithium insertion capacities can be rationalized in terms of lattice oxygen defects, which effectively reduce the number of available lithium insertion sites in β-MoO3 as compared to α-MoO3

Electrochemical Behavior of Flavin Adenine Dinucleotide Adsorbed onto Carbon Nanotube and Nitrogen-Doped Carbon Nanotube Electrodes

Flavin adenine dinucleotide (FAD) is a cofactor for many enzymes, but also an informative redox active surface probe for electrode materials such as carbon nanotubes (CNTs) and nitrogen-doped CNTs (N-CNTs). FAD spontaneously adsorbs onto the surface of CNTs and N-CNTs, displaying Langmuir adsorption characteristics. The Langmuir adsorption model provides a means of calculating the electroactive surface area (ESA), the equilibrium constant for the adsorption and desorption processes (<i>K</i>), and the Gibbs free energy of adsorption (Δ<i>G</i>°). Traditional ESA measurements based on the diffusional flux of a redox active molecule to the electrode surface underestimate the ESA of porous materials because pores are not penetrated. Techniques such as gas adsortion (BET) overestimate the ESA because it includes both electroactive and inactive areas. The ESA determined by extrapolation of the Langmuir adsorption model with the electroactive surface probe FAD will penetrate pores and only include electroactive areas. The redox activity of adsorbed FAD also displays a strong dependency on pH, which provides a means of determining the p<i>K</i>_a of the surface confined species. The p<i>K</i>_a of FAD decreases as the nitrogen content in the CNTs increases, suggesting a decreased hydrophobicity of the N-CNT surface. FAD desorption at N-CNTs slowly transforms the main FAD surface redox reaction with <i>E</i>_1/2 at −0.84 V into two new, reversible, surface confined redox reactions with <i>E</i>_1/2 at −0.65 and −0.76 V (vs Hg/Hg₂SO₄), respectively (1.0 M sodium phosphate buffer pH = 6.75). This is the first time these redox reactions have been observed. The new surface confined redox reactions were not observed during FAD desorption from nondoped CNTs

Spatially Resolved Imaging of Inhomogeneous Charge Transfer Behavior in Polymorphous Molybdenum Oxide. II. Correlation of Localized Coloration/Insertion Properties Using Spectroelectrochemical Microscopy

A newly developed spectroelectrochemical imaging approach for directly assessing lithium ion insertion energetics and kinetics in mixed-phase, polymorphous MoO3 is reported. Two variants of spectroelectrochemical microscopy were used to monitor insertion dynamics and to follow electrochemically induced phase transformations at specifically identified structural and compositional domains. Cyclovoltoabsorptometric (dOD/dE) measurements carried out in LiClO4/propylene carbonate solutions reveal that the lithium insertion is nonuniform and can be directly correlated with phase-segregated domains comprising α-MoO3, β-MoO3, and intermixed α-/β-MoO3. Lithium insertion is found to proceed by a staging process where each phase displays energetically distinct insertion behaviors. Chronoabsorptometric imaging measurements allow for the simultaneous estimation of lithium diffusion coefficients, ionic conductivities, and lithium capacities at isolated phases within the polymorphous material. The lithium diffusion coefficient and ionic conductivity is largest for domains comprising intermixed α-/β-MoO3, whereas it is smallest at domains consisting of β-MoO3. The higher diffusion coefficient observed for intermixed α-/β-MoO3 domains is most likely due to larger thermodynamic enhancement factors for the mixed phase domains than for domains consisting of either α-MoO3 or β-MoO3. Estimation of capacity values within each uniquely identified domain reveals that the lithium insertion capacity is about 4 times greater in α-MoO3 than in β-MoO3. The discrepancies between the lithium insertion capacities can be rationalized in terms of lattice oxygen defects, which effectively reduce the number of available lithium insertion sites in β-MoO3 as compared to α-MoO3

Picomolar Peroxide Detection Using a Chemically Activated Redox Mediator and Square Wave Voltammetry

A method for low-level, low-potential electrochemical detection of hydrogen peroxide using a chemically activated redox mediator is presented. This method is unique in that it utilizes a mediator, Amplex Red, which is only redox-active when chemically oxidized by H2O2 in the presence of the enzyme horseradish peroxidase (HRP). When employed in concert with microelectrode square wave voltammetry to optimize sensing at ultralow concentrations (<1 μM), this method exhibits marked improvements in analytical sensitivity and detection limits (limit of detection as low as 8 pM) over existing protocols. Sensing schemes incorporating both freely diffusing and immobilized HRP are evaluated, and the resulting analytical sensitivities are 1.22 ± 0.04 and (2.1 ± 0.6) × 10-1 μA/(μM mm2), respectively, for peroxide concentrations in the high picomolar to low micromolar range. A second linear region exists for lower peroxide concentrations. Furthermore, quantitative enzyme kinetics analysis using Michaelis−Menten parameters is possible through interpretation of data collected in this scheme. Km values for soluble and immobilized HRP were 84 ± 13 and 504 ± 19 μM, respectively. This method is amenable to any biological detection scheme that generates hydrogen peroxide as a reactive product

Spatially Resolved Measurement of Inhomogeneous Electrocoloration/Insertion in Polycrystalline Molybdenum Oxide Thin Films via Chronoabsorptometric Imaging

A new integrated electrochemical and transmission optical microscopy approach is presented which allows for elucidation of inhomogeneous ion/charge-transfer behavior in polycrystalline electrochromic/insertion materials. Spatially resolved Li+ diffusion coefficients and ionic conductivities are determined from the time-lapsed optical density imaging response monitored during electrochemical potential-step perturbation. Non-uniform coloration changes and dispersed insertion kinetics are observed and associated with domain specific reactivity of polymorphous materials comprising α-MoO3 and β-MoO3

Anomalous Electrochemical Dissolution and Passivation of Iron Growth Catalysts in Carbon Nanotubes

Catalytically synthesized carbon nanotubes (CNTs) such as those prepared via chemical vapor deposition (CVD) contain metallic impurities including Fe, Ni, Co, and Mo. Transition metal contaminants such as Fe can participate in redox cycling reactions that catalyze the generation of reactive oxygen species and other products. Through the nature of the CVD growth process, metallic nanoparticles become encased within the CNT graphene lattice and may still be chemically accessible and participate in redox chemistry, especially when these materials are utilized as electrodes in electrochemical applications. We demonstrate that metallic impurities can be selectively dissolved and/or passivated during electrochemical potential cycling. Anomalous Fe dissolution and passivation behavior is observed in neutral (pH = 6.40 ± 0.03) aqueous solutions when using multiwalled CNTs prepared from CVD. Fe particles contained within these CNTs display intriguing, potential-dependent Fe redox activity that varies with supporting electrolyte composition. In neutral solutions containing dibasic sodium phosphate, sodium acetate, and sodium citrate, FeII dissolution and surface confined FeII/III redox activity are significant despite Fe being encapsulated within CNT graphene layers. However, no apparent Fe dissolution is observed in 1 M potassium nitrate solutions, suggesting that the electrolyte composition plays an important role in observing FeII dissolution, passivation, and surface confined FeII/III redox activity. Between potentials of 0 and −1.1 V versus Hg/Hg2SO4, the primary redox-active Fe species are surface FeII/III oxides/oxyhydroxides. This FeII/III surface oxide redox chemistry can be completely suppressed by passivating Fe through repeated cycling of the CNTs in supporting electrolyte. By increasing the potential to more negative values (> −1.3 V), FeII dissolution may be induced in  electrolyte solutions containing acetate and phosphate and inhibited by addition of sodium benzoate, which adsorbs on exposed Fe particles, effectively passivating them. Finally, we observe that the FeII/III redox chemistry or subsequent passivation does not affect the onset of oxygen reduction at nitrogen-doped CNTs, suggesting that the surface-bound FeII species is not the primary catalytically active site for oxygen reduction in these materials

Mechanistic Discussion of the Oxygen Reduction Reaction at Nitrogen-Doped Carbon Nanotubes

The oxygen reduction reaction (ORR) at undoped and nitrogen-doped carbon nanotubes (CNTs and N-CNTs, respectively) was studied by cyclic voltammety, rotating disk electrode voltammetry, and gasometric analysis in neutral and alkaline aqueous solutions. At undoped CNTs, the ORR proceeds by two successive two-electron processes with hydroperoxide (HO2–) as the intermediate. At N-CNTs, the ORR occurs through a “pseudo”-four-electron pathway involving a catalytic regenerative process in which hydroperoxide is chemically disproportionated to form hydroxide (OH–) and molecular oxygen (O2). The ORR mechanism at both undoped and N-doped varieties is supported by steady state polarization and gasometric measurements of hydroperoxide disproportionation rates. An enhancement of over 1000-fold for hydroperoxide disproportionation is observed for N-CNTs, with rates comparable to the best known peroxide decomposition catalysts. A positive correlation between nitrogen content and ORR activities is observed where the ORR potential shifts by up to 11.6 mV per at. % N incorporated into the N-CNTs and exhibits an oxygen reduction potential, Ep, of −0.23 V vs Hg/Hg2SO4 (+0.640 V vs NHE) in 1 M Na2HPO4 for N-CNTs containing 7.4 at. % N. A detailed mechanism is proposed that involves a dual site reduction in which O2 is initially reduced at a N–C type site in a 2-electron process to form HO2–, which then can undergo either further electrochemical reduction to form OH– species or chemical disproportionation to form OH– species and molecular O2 at a decorating FexOy/Fe surface phase