Wain A.J.,National Physical Laboratory United Kingdom
Electrochemistry Communications | Year: 2014
Scanning electrochemical microscopy (SECM) provides a unique approach to combinatorial screening, and this technique has found a wide variety of applications in recent years. This mini-review provides a brief summary of progress in this field, highlighting some of the most significant developments in SECM screening for electrocatalysis, photoelectrochemistry and biosensing applications. © 2014 Published by Elsevier B.V. All rights reserved.
Shard A.G.,National Physical Laboratory United Kingdom
Surface and Interface Analysis | Year: 2014
A simple approach to estimating the detection limits of X-ray photoelectron spectroscopy (XPS) for any element in any elemental matrix is presented, using the intensity of the background at the expected position for the photoelectron peak to be detected. The approach has been extended to estimate the detection limit for all elements from lithium to bismuth in a similar range of elemental matrices. Using a number of assumptions, it is possible to obtain a reasonable estimate of the background intensity at any electron kinetic energy in the XPS spectrum of an element. Therefore, a detection limit for an arbitrary element homogeneously distributed in that matrix can be estimated. The results show that, although most elements are detectable at about the 1 at.% to 0.1 at.% level, for heavy elements in a light element matrix, the detection limit can be better than 0.01 at.%, whereas for light elements in a heavy element matrix, detection limits above 10 at.% are not uncommon. Two charts detailing the detection limits for all combinations of trace and matrix elements from lithium (Z = 3) to bismuth (Z = 83) are provided for Al Kα and Mg Kα X-ray sources using a typical hemispherical analyser instrument which provides 10 6 counts eV for the Ag 3d5/2 peak from pure silver. These detection limits can be scaled to estimate the detection limits for any given instrument and operating conditions if the intensity of the Ag 3d5/2 peak from pure silver under those conditions is known. © 2014 John Wiley & Sons Ltd.
Wain A.J.,National Physical Laboratory United Kingdom
Electrochimica Acta | Year: 2013
Understanding the effects of particle size and surface structure on the electrochemical behavior of nanomaterials is of critical importance to the optimization of electrocatalysts. In this work the electrocatalytic activity of arrays of spherical gold nanoparticles (AuNPs) of varying diameter was imaged using scanning electrochemical microscopy (SECM), focusing on the oxygen reduction reaction (ORR) and the electrooxidation of hydrogen peroxide. Unlike most previous reports of catalyst array screening wherein tip generation-substrate collection (TG-SC) is the favored approach, redox competition and tip collection modes were instead employed. For both reactions studied the electrocatalytic activity, normalized to surface area, was observed to increase with decreasing particle diameter in the range 5-50 nm. Characterization of the AuNP surface structure using lead underpotential deposition revealed that this trend can be correlated to the ratio of Au(1 1 0) to Au(1 1 1) sites. For the ORR this is consistent with the established view that Au(1 1 0) is more active than Au(1 1 1). Conversely, for hydrogen peroxide electrooxidation on AuNPs this is in contrast to Au single crystal data, suggesting the subtle influence of higher index sites on this reaction.
Margolis H.S.,National Physical Laboratory United Kingdom
Chemical Society Reviews | Year: 2012
Over the past twelve years, notable advances have occurred in a diverse range of scientific areas following the development of femtosecond optical frequency combs. Compared to a conventional laser source, the distinguishing feature of a femtosecond comb is that it provides a broadband source with well-defined phase coherence across the optical spectrum. This makes it a unique tool for spectroscopic applications, simultaneously providing high spectral resolution and broad spectral coverage. This tutorial review provides an introduction to femtosecond optical frequency combs, covering their principles of operation and examples of how they can be applied to spectroscopy. In this way it aims to demonstrate their potential as a versatile spectroscopic tool that could play a very significant role in future advances in the chemical sciences. © 2012 The Royal Society of Chemistry.
Seah M.P.,National Physical Laboratory United Kingdom
Journal of Physical Chemistry C | Year: 2013
An analysis is made of the sputtering yields of materials for argon gas cluster ion beams used in SIMS and XPS as a function of the beam energy, E, and the cluster size, n. The analysis is based on the yield data for the elements Si and Au, the inorganic compound SiO2, and the organic materials Irganox 1010, the OLED HTM-1, poly(styrene), poly(carbonate), and poly(methyl methacrylate). The argon primary ions have cluster sizes, n, in the range 100-16 000 and beam energies, E, from 2.5 to 80 keV. It is found that the elemental and compound data expressed as the yields, Y, of atoms sputtered per primary ion may all be described by a simple universal equation: Y/n = (E/An) q/[1 + (E/An)q-1] where the parameters A and q are established by fitting. The sputtering yields of the three organic materials are given as yield volumes expressed in nm3. For these, an extra parameter B is included multiplying the right-hand side of the equation where B is found by fitting to be of the order (0.18 nm)3 to (0.26 nm) 3. This universal equation exhibits no threshold energy, and no deviation was observed from the equation, indicating that any threshold energy would have to be significantly below E/n = 1 eV per atom. The equation also shows that doubling the cluster size at the same energy per atom simply doubles the sputtering yield so that in this sense, and probably this sense alone, the sputtering effects are linearly additive. The parameter A is related, inversely, to the mean sputtered fragment size, and the low A values for organic materials are indicative of high yield volumes. For materials with low A values, the universal equation is close to a linear dependence on energy, and if that linear dependence is assumed, an apparent threshold energy is predicted and observed experimentally. © Published 2013 by the American Chemical Society.