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1.
Anal Chem ; 92(2): 2237-2243, 2020 01 21.
Artigo em Inglês | MEDLINE | ID: mdl-31874560

RESUMO

Probing pH gradients during electrochemical reactions is important to better understand reaction mechanisms and to separate the influence of pH and pH gradients from intrinsic electrolyte effects. Here, we develop a pH sensor to measure pH changes in the diffusion layer during hydrogen evolution. The probe was synthesized by functionalizing a gold ultramicroelectrode with a self-assembled monolayer of 4-nitrothiophenol (4-NTP) and further converting it to form a hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) redox couple. The pH sensing is realized by recording the tip cyclic voltammetry and monitoring the Nernstian shift of the midpeak potential. We employ a capacitive approach technique in our home-built Scanning Electrochemical Microscope (SECM) setup in which an AC potential is applied to the sample and the capacitive current generated at the tip is recorded as a function of distance. This method allows for an approach of the tip to the electrode that is electrolyte-free and consequently also mediator-free. Hydrogen evolution on gold in a neutral electrolyte was studied as a model system. The pH was measured with the probe at a constant distance from the electrode (ca. 75 µm), while the electrode potential was varied in time. In the nonbuffered electrolyte used (0.1 M Li2SO4), even at relatively low current densities, a pH difference of three units is measured between the location of the probe and the bulk electrolyte. The time scale of the diffusion layer transient is captured, due to the high time resolution that can be achieved with this probe. The sensor has high sensitivity, measuring differences of more than 8 pH units with a resolution better than 0.1 pH unit.

2.
Nat Commun ; 10(1): 5233, 2019 11 20.
Artigo em Inglês | MEDLINE | ID: mdl-31748552

RESUMO

In homoepitaxial crystal growth, four basic growth morphologies (idealized growth modes) have been established that describe the deposition of atoms on single crystal surfaces: step-flow, layer-by-layer, mound formation, and random/self-affine growth. Mound formation leads to nano-scale surface patterning. However, the formation of (nano)-islands, patterns, and roughness occurs also during ion bombardment, electrochemical etching and oxidation/reduction cycling. Here we show, in analogy to many particle/anti-particle formalisms in physics, the existence of the dualism between individual adatom and single vacancy growth modes. We predict that all standard adatom growth modes do exist also in their counter, vacancy version. For the particular case of mound formation, we derive the theoretical equations and show the inverse similarity of the solution. We furthermore treat simultaneous growth by adatoms and vacancies, and derive the analytical solution of the growth shape evolution of the mounds. Finally, we present an experimental verification, in which both adatom and vacancy mound formation are active. The theoretically predicted mound shape nicely fits the experimental observation.

3.
Nat Mater ; 17(3): 277-282, 2018 03.
Artigo em Inglês | MEDLINE | ID: mdl-29434306

RESUMO

Platinum plays a central role in a variety of electrochemical devices and its practical use depends on the prevention of electrode degradation. However, understanding the underlying atomic processes under conditions of repeated oxidation and reduction inducing irreversible surface structure changes has proved challenging. Here, we examine the correlation between the evolution of the electrochemical signal of Pt(111) and its surface roughening by simultaneously performing cyclic voltammetry and in situ electrochemical scanning tunnelling microscopy (EC-STM). We identify a 'nucleation and early growth' regime of nanoisland formation, and a 'late growth' regime after island coalescence, which continues up to at least 170 cycles. The correlation analysis shows that each step site that is created in the 'late growth' regime contributes equally strongly to both the electrochemical and the roughness evolution. In contrast, in the 'nucleation and early growth' regime, created step sites contribute to the roughness, but not to the electrochemical signal.

4.
J Chem Educ ; 94(9): 1209-1216, 2017 Sep 12.
Artigo em Inglês | MEDLINE | ID: mdl-28919643

RESUMO

As it connects to a large set of important fundamental ideas in chemistry and analytical techniques discussed in high school chemistry curricula, we review the exploding flask demonstration. In this demonstration, methanol vapor is catalytically oxidized by a Pt wire catalyst in an open container. The exothermicity of reactions occurring at the catalytic surface heats the metal to the extent that it glows. When restricting reactant and product gas flow, conditions may favor repetitive occurrence of a small explosion. We show how mass spectrometry and infrared spectroscopy allow for unravelling the chemical background of this demonstration and discuss various ideas on how to use it in a classroom setting to engage students' critical thinking about chemical research. Along the way, we show that two commonly published ideas about the chemical background of this demonstration are incorrect, and we suggest simple tests that may be performed in a high school setting either as an addition to the demonstration or as a student research project.

5.
J Chem Educ ; 94(9): 1285-1287, 2017 Sep 12.
Artigo em Inglês | MEDLINE | ID: mdl-28919644

RESUMO

The "exploding" flask demonstration presents a well-known illustration of heterogeneous catalyzed methanol oxidation. We find that for the same vapor pressure, the demonstration also works for all primary and secondary alcohols up to butanol but not for a tertiary alcohol. Also, we show that the demonstration works for a large range of transition metal catalysts. Hence, this demonstration, which is often applied for the repetitive explosions when methanol is used, may also be used to argue the requirement of initial dehydrogenation of the alcohol to an aldehyde in the catalytic reaction mechanism to support the general insensitivity to reactant molecules in heterogeneous catalysis in contrast to biological catalysis and to provide proof for activity trends as often depicted by volcano plots.

6.
Ultramicroscopy ; 182: 233-242, 2017 11.
Artigo em Inglês | MEDLINE | ID: mdl-28734230

RESUMO

A combined X-ray and scanning tunneling microscopy (STM) instrument is presented that enables the local detection of X-ray absorption on surfaces in a gas environment. To suppress the collection of ion currents generated in the gas phase, coaxially shielded STM tips were used. The conductive outer shield of the coaxial tips can be biased to deflect ions away from the tip core. When tunneling, the X-ray-induced current is separated from the regular, 'topographic' tunneling current using a novel high-speed separation scheme. We demonstrate the capabilities of the instrument by measuring the local X-ray-induced current on Au(1 1 1) in 800 mbar Ar.

8.
Phys Chem Chem Phys ; 18(41): 28451-28457, 2016 Oct 19.
Artigo em Inglês | MEDLINE | ID: mdl-27713936

RESUMO

Despite the widespread application of ultramicroelectrodes (UMEs), the customary method of their electrochemical characterization via outer sphere redox probing has serious limitations. In this study we provide additional insights into this subject by measuring not only outer sphere redox couples, but also blank voltammetries and the reactivity towards various catalytic reactions of Pt UMEs. The data for the UMEs are compared to those for macroscale Pt electrodes that can be flame-annealed. Although the reactivity is similar for reactions that are rather insensitive to the surface structure (and/or composition), UMEs perform much worse for more surface sensitive catalytic reactions. This effect can be explained by the UMEs being contaminated much faster, though it remains to be established if the origin of this contamination lies in the preparation method or in the impurities in the (high-purity grade) chemicals used. Our study recommends that catalytic reactivity measurements on ultramicroelectrodes should always be accompanied by blank voltammetries and the results have to be interpreted extremely carefully.

9.
J Chem Phys ; 144(24): 244706, 2016 Jun 28.
Artigo em Inglês | MEDLINE | ID: mdl-27369532

RESUMO

We have studied the adsorption and desorption of O2 on Pd(100) by supersonic molecular beam techniques and thermal desorption spectroscopy. Adsorption measurements on the bare surface confirm that O2 initially dissociates for all kinetic energies between 56 and 380 meV and surface temperatures between 100 and 600 K via a direct mechanism. At and below 150 K, continued adsorption leads to a combined O/O2 overlayer. Dissociation of molecularly bound O2 during a subsequent temperature ramp leads to unexpected high atomic oxygen coverages, which are also obtained at high incident energy and high surface temperature. At intermediate temperatures and energies, these high final coverages are not obtained. Our results show that kinetic energy of the gas phase reactant and reaction energy dissipated during O2 dissociation on the cold surface both enable activated nucleation of high-coverage surface structures. We suggest that excitation of local substrate phonons may play a crucial role in oxygen dissociation at any coverage.

10.
J Chem Phys ; 143(1): 014703, 2015 Jul 07.
Artigo em Inglês | MEDLINE | ID: mdl-26156488

RESUMO

Molecular adsorption and dissociation of O2 on the stepped Pt(553) surface have been investigated using supersonic molecular beam techniques and temperature programmed desorption. The initial and coverage-dependent sticking probability was determined with the King and Wells technique for various combinations of incident kinetic energy, surface temperature, incident angle, and surface coverage. A comparison with similar data for Pt(533) and Pt{110}(1 × 2) shows quantitatively the same high step-induced sticking at low incident energies compared to Pt(111). The enhancement is therefore insensitive to the exact arrangement of atoms forming surface corrugation. We consider energy transfer and electronic effects to explain the enhanced sticking. On the other hand, dissociation dynamics at higher incident kinetic energies are strongly dependent on step type. The Pt(553) and Pt(533) surfaces are more reactive than Pt(111), but the (100) step shows higher sticking than the (110) step. We relate this difference to a variation in the effective lowering of the barrier to dissociation from molecularly adsorbed states into atomic states. Our findings are in line with results from experimental desorption studies and theoretical studies of atomic binding energies. We discuss the influence of the different step types on sticking and dissociation dynamics with a one-dimensional potential energy surface.

11.
J Chem Phys ; 142(21): 214708, 2015 Jun 07.
Artigo em Inglês | MEDLINE | ID: mdl-26049517

RESUMO

We have determined the initial sticking probability of O2 on Pd(100) using the King and Wells method for various kinetic energies, surface temperatures, and incident angles. The data suggest two different mechanisms to sticking and dissociation. Dissociation proceeds mostly through a direct process with indirect dissociation contributing only at low kinetic energies. We suggest a dynamical precursor state to account for the indirect dissociation channel, while steering causes the high absolute reactivity. A comparison of our results to those previously obtained for Pd(111) and Pd(110) highlights how similar results for different surfaces are interpreted to suggest widely varying dynamics.

12.
Anal Chem ; 87(11): 5782-9, 2015 Jun 02.
Artigo em Inglês | MEDLINE | ID: mdl-25942527

RESUMO

Voltammetric scanning electrochemical cell microscopy (SECCM) incorporates cyclic voltammetry measurements in the SECCM imaging protocol, by recording electrochemical currents in a wide potential window at each pixel in a map. This provides much more information compared to traditional fixed potential imaging. Data can be represented as movies (hundreds of frames) of current (over a surface region) at a series of potentials and are highly revealing of subtle variations in electrode activity. Furthermore, by combining SECCM data with other forms of microscopy, e.g. scanning electron microscopy and electron backscatter diffraction data, it is possible to directly relate the current-voltage characteristics to spatial position and surface structure. In this work we use a "hopping mode", where the SECCM pipet probe is translated toward the surface at a series of positions until meniscus contact. Small amounts of residue left on the surface, upon probe retraction, demark the precise area of each measurement. We use these techniques to study hydrazine oxidation on a polycrystalline platinum substrate both in air and in a deaerated environment. In both cases, the detected faradaic current shows a structural dependence on the surface crystallographic orientation. Significantly, in the presence of oxygen (aerated solution) the electrochemical current decreases strongly for almost all grains (crystallographic orientations). The results highlight the flexibility of voltammetric SECCM for electrochemical imaging and present important implications for hydrazine electroanalysis.

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