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The effect on the electrocatalytic oxygen evolution reaction (OER) of cobalt incorporation into the metal oxide sheets of the layered manganese oxide birnessite was investigated. Birnessite and cobalt-doped birnessite were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and conductivity measurements. A cobalt:manganese ratio of 1:2 resulted in the most active catalyst for the OER. In particular, the overpotential (η) for the OER was 420 mV, significantly lower than the η = 780 mV associated with birnessite in the absence of Co. Furthermore, the Tafel slope for Co/birnessite was 81 mV/dec, in comparison to a Tafel slope of greater than 200 mV/dec for birnessite. For chemical water oxidation catalysis, an 8-fold turnover number (TON) was achieved (h = 70 mmol of O 2 /mol of metal). Density functional theory (DFT) calculations predict that cobalt modification of birnessite resulted in a raising of the valence band edge and occupation of that edge by holes with enhanced mobility during catalysis. Inclusion of extra cobalt beyond the ideal 1:2 ratio was detrimental to catalysis due to disruption of the layered structure of the birnessite phase.

Carbonization of nature-inspired polydopamine can yield thin films with high electrical conductivity. Understanding of the structure of carbonized PDA (cPDA) is therefore highly desired. In this study, neutron diffraction, Raman spectroscopy, and other techniques indicate that cPDA samples are mainly amorphous with some short-range ordering and graphite-like structure that emerges with increasing heat treatment temperature. The electrical conductivity and the Seebeck coefficient show different trends with heat treatment temperature, while the thermal conductivity remains insensitive. The largest room-temperature ZT of 2 × 10 -4 was obtained on samples heat-treated at 800 °C, which is higher than that of reduced graphene oxide.

The properties of water molecules located close to an interface deviate significantly from those observed in the homogeneous bulk liquid. The length scale over which this structural perturbation persists (the so-called interfacial depth) is the object of extensive investigations. The situation is particularly complicated in the presence of surface charges that can induce long-range orientational ordering of water molecules, which in turn dictate diverse processes, such as mineral dissolution, heterogeneous catalysis, and membrane chemistry. To characterize the fundamental properties of interfacial water, we performed molecular dynamics (MD) simulations on alkali chloride solutions in the presence of two types of idealized charged surfaces: one with the charge density localized at discrete sites and the other with a homogeneously distributed charge density. We find that, in addition to a diffuse region where water orientation shows no layering, the interface region consists of a "compact layer" of solvent next to the surface that is not described in classical electric double layer theories. The depth of the diffuse solvent layer is sensitive to the type of charge distributions on the surface and the ionic strength. Simulations of the aqueous interface of a realistic model of negatively charged amorphous silica show that the water orientation and the distribution of ions strongly depend on the identity of the cations (Na(+) vs Cs(+)) and are not well represented by a simplistic homogeneous charge distribution model. While the compact layer shows different solvent net orientation and depth for Na(+) vs Cs(+), the depth (~1 nm) of the diffuse layer of oriented waters is independent of the identity of the cation screening the charge. The details of interfacial water orientation revealed here go beyond the traditionally used double and triple layer models and provide a microscopic picture of the aqueous/mineral interface that complements recent surface specific experimental studies.

Charge transport in a porphyrin with four identical pyridyl substituents, 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (TPyP), was investigated using the scanning tunneling microscopy break junction method. To determine the dominant pathway, we studied two structurally similar porphyrins, o-DPyP and p-DPyP. Our experiments reveal that charge transport through TPyP in a break junction configuration does not follow the traditional assumption, i.e., the shortest path between the neighboring side groups. Instead, the charge transport pathway was dominated by the farthest anchoring groups. Furthermore, these single molecule experiments can distinguish between the two structural isomers, which is important in molecular discrimination, porphyrin chemistry, and molecular electronics., (© 2011 American Chemical Society)

We describe an ultrabroadband IR-visible sum-frequency (SF) setup that allows simultaneous acquisition of the entire vibrational spectrum of water molecules at mineral surfaces in the OH stretching region without ever tuning the IR laser pulses. Our newly developed 800-nm pumped noncollinear optical parametric amplifier (NOPA) generates broadband mid-IR pulses (~1800-3500 nm, or ~2900 - 6000 cm-1) with bandwidths >600 cm-1 at half-maximum at near 3500 cm-1. Using the ultra-broadband IR NOPA, we constructed a sum-frequency vibrational spectrometer that allowed the acquisition of spectra of the OH stretches of water at hydrophilic and hydrophobic silica surfaces, over the frequency range ~2900 - 3800 cm-1, within 60 s, much shorter than with scanning SFG spectrometers. The ultra-broadband SFG spectrometer reported here can be potentially applied to time-resolved measurements of kinetics at interfaces.

Studying electron transfer reactions of porphyrin molecules is important for a wide range of applications including biology, molecular devices, artificial photosynthesis, information storage, and fuel cells. It is known that porphyrins adsorbed in a self-assembled monolayer at an electrochemical interface may lose their electrochemical activity. However, the mechanism of the suppressed electrochemical activity is not clear. In this article, the electrochemical behavior of the two-dimensional network structures of 5,10,15,20-tetrakis(4-carboxylphenyl)-21H,23H-porphyrin (TCPP) molecules, formed via intermolecular hydrogen bonding on Au(111), was investigated by electrochemical scanning tunneling microscopy (EC-STM). Three types of domains, including a square network with molecules trapped inside, square packing, and hexagonal close-packing structures have been observed under various pH conditions. The difference in STM contrast between oxidized and reduced TCPP allows the slow electrochemical reduction of adsorbed TCPP to be visualized by STM. For the first time, the pH dependent reduction of porphyrins was imaged by EC-STM, revealing the mechanism of porphyrin slow reduction at electrochemical interfaces. TCPP reduction can be accelerated either by tuning the working electrode potential to a more negative value or by lowering the H(+) concentration. A redox reaction model was proposed based on the pH dependent reduction of TCPP to elucidate the fundamental aspects of porphyrin redox reactions.

Time-resolved sum frequency generation (tr-SFG) reveals that the vibrational energy relaxation rate of O-H stretching of dilute HDO in D(2)O at the silica interface is markedly different from that of bulk water. As compared to the bulk liquid, the vibrational lifetime (T(1)) of HDO is shorter at the charged surface than in the bulk, but longer at the neutral surface. The vibrational decoupling of the O-H of the HDO species leads to the observation of a frequency-dependent T(1) of the O-H stretch, which is shorter at the red than the blue side of the hydrogen-bonded OH spectral region. This correlates with the red-shift of the SFG spectra with increasing surface charge and is consistent with a theoretical model that relates the vibrational lifetime to the strength of the hydrogen-bond network.

FLOSS (fluorescence labeling of surface species) enables one to identify and quantify very low concentrations of surface functional groups. Unlike most surface analytical techniques, FLOSS can provide absolute, as well as relative, surface coverage determination. However, as with any other surface derivatization technique, FLOSS provides a lower limit to surface coverage. The specificity of FLOSS for a particular functional group is the key to this application. In one FLOSS protocol, amine-modified dyes are used to label surface aldehyde groups. However, amine-modified dyes, in principle, can bind to both aldehyde and carboxyl groups, limiting specificity. In this paper, we report that the FLOSS protocol devised results in less than 0.5 % of the carboxyl-modified dyes binding to the surface amine groups. Therefore, the presence of carboxyl groups on the surface should have a limited effect on the detection of aldehyde groups by amine-modified dye. Quenching of fluorescence can potentially affect quantitative measurements. To address this issue, the densities of surface functional groups of CHO-, NH2-, and epoxy-coated glass surfaces were quantified using FLOSS and compared to surface densities estimated by other methods. The FLOSS technique was extended to glass surfaces by using visible absorbing and emitting dyes. The lower detection limit is on the order of 10(9) groups/cm2.

Time-resolved second-harmonic generation (SHG) was used to study the hot-carrier dynamics and nonlinear optical properties of S-terminated and Cl-terminated Ge(111) interfaces on the femtosecond time scale. The hot-carrier second-order nonlinear optical susceptibilities were determined to be 720 +/- 50 times greater than the valence-band second-order nonlinear optical susceptibilities for the Ge(111)-S system and 880 +/- 100 times greater in the Ge(111)-Cl system. Furthermore, the ground- and excited-state second-order nonlinear optical susceptibilities are suggested to be out of phase for Ge(111)-S and Ge(111)-Cl systems, leading to a pump-induced decrease in the SHG signal as opposed to the increase in the SHG signal observed in the Ge(111)-GeO2 system. Although the SHG response reaches a steady state in 415 +/- 90 fs in the Ge(111)-GeO2 system, a faster response is observed in the Ge(111)-S system, 220 +/- 85 fs, and in the Ge(111)-Cl system, 172 +/- 50 fs. This suggests significantly faster carrier cooling at the Ge(111)-Cl and Ge(111)-S interfaces, with significant implications for hot-carrier mediated device degradation, and migration to high-K dielectrics.

The effect of adsorption on molecular properties and reactivity is a central topic in interfacial physical chemistry. At electrochemical interfaces, adsorbed molecules may lose their electrochemical activity. The absence of in situ probes has hindered our understanding of this phenomenon and electrode reactions in general. In this work, classical electrochemistry and electrochemical scanning tunneling microscopy (EC-STM) were combined to provide molecular level insight into electrochemical reactions and the molecular adsorption state at the electrolyte-electrode interface. The metal-free porphyrin 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (TPyP) adsorbed on Au(111) in 0.1 M H(2)SO(4) solution was chosen as a model system. TPyP is found to irreversibly adsorb on Au(111) over a wide range of potentials, from -0.25 to 0.6 V(SCE). The adsorption state of TPyP has a dramatic effect on its electrochemistry. Preadsorbed, oxidized TPyP displays no well-defined cathodic peaks in cyclic voltammograms in sharp contrast to solution-phase TPyP. Our present work provides direct, molecular level evidence of the electrochemically "invisible" species. Electrochemical activity of absorbed species is recovered by allowing the oxidized molecule sufficient time (tens of minutes) to reduce. The redox state of adsorbed TPyP also affects the nature of the adsorption. Oxidized species can apparently only form monolayers. However, multilayers, stable enough to be imaged by STM, can form when the adsorbed TPyP is in the reduced state. This suggests that by controlling the electrochemistry one can either promote or suppress the formation of multilayers.

It is demonstrated that surface vibrational spectroscopy can be used to probe local ordering of adsorbates and the nature of inter-adsorbate interactions through dynamical dipole coupling among neighboring adsorbates. Our studies show that for CO adsorbed on Cu(100), first and second nearest neighbor interactions are repulsive. This repulsion prevents the formation of long range ordered structures at low and medium coverages and can be best understood as an adsorbate induced modification in the local electron density, i.e., Friedel oscillation.

Single-molecule electrical conduction studies are used to evaluate how the molecular linking unit influences the tunneling efficiency in metal-molecule-metal (m-M-m) junctions. This work uses conducting-probe atomic force microscopy (CP-AFM) to compare the molecular conduction of two pi-bonded molecules: one with a single thiol linker, and another with a conjugated double thiol linker at both ends of the molecules. The results demonstrate that the molecule with conjugated double thiol linkers displays higher conduction in gold-molecule-gold junctions than nonconjugated single thiol-gold contacts.