Flame-annealed single crystal

Over the past 10 years it has been demonstrated by a variety of in situ and ex situ techniques187,188 485 487 488 534 that flame-annealed Au faces are reconstructed in the same way as the surfaces of samples prepared in UHV,526-534 and that the reconstructed surfaces are stable even in contact with an aqueous solution if certain precautions are taken with respect to the potential applied and the electrolyte composition 485,487,488 A comprehensive review of reconstruction phenomena at single-crystal faces of various metals has been given by Kolb534 and Gao etal.511,513... 

Some measurements with Pt single-crystal faces have been published recently.140,210,773 Iwasita and Xia210 prepared platinum single crystals according to the method of Clavilier et a/.186 773 After flame annealing and cooling in an H2 + Ar mixture, the electrode was protected... 

Figure J. Preparation of a welJ-defined single-crystal electrode by four steps, (a) Mirror-rinished crystal is annealed under gas flame, and (b) quenched in air and then pure water, (c) The crystal is covered with a droplet of pure water to protect it from pollution by impurities in air and (d) is introduced into the cell, where the crystal contacts the meniscus of the electrolyte solution. (From Ref. 25.)...

It is noteworthy that prior to the advent of scanning probe microscopy electrochemically driven reconstruction phenomena had been identified and studied using traditional macroscopic electrochemical measurements [210,211], However, STM studies have provided insight as to the various atomistic processes involved in the phase transition between the reconstructed and unreconstructed state and promise to provide an understanding of the macroscopically observed kinetics. An excellent example is provided by the structural evolution of the Au(lOO) surface as a function of potential and sample history [210,211,216-223], Flame annealing of a freshly elec-tropolished surface results in the thermally induced formation of a dense hexagonal close-packed reconstructed phase referred to as Au(100)-(hex). For carefully annealed crystals a single domain of the reconstructed phase... 

Figure 2. (A) The arrangement of single-crystal basal planes in a flame-annealed Clavilier bead. (B) Single-crystal facets on a Clavilier bead of Pt. (C). Hanging-meniscus configuration for voltammetric experiments with a single-crystal facet of a Clavilier bead.

In addition to flame annealing methods for the preparation of single crystal faces of metals with extended atomically smooth terraces [5.10-5.13], the already traditional technique of electrolytic growth of single crystals [5.6-S.9] has remained unrivaled for the preparation of perfect, screw dislocation-free faces or faces with single, isolated screw dislocations. ... 

Before each measurement, single crystals were first elec-trochemically polished35 and then flame annealed for several minutes, cooled down, and either immersed into an external electrochemical cell for cyclic voltammetry (CV) characterization or mounted into an electrochemical cell of an STM. For both CV and in situ STM results, sample potentials were measured and presented vs. Ag/AgCl reference electrode. 

Au(lll) facets formed on a single crystal bead were produced by annealing Au wire in a H2-O2 flame to form a droplet, which was quenched in pure water saturated with H. The bead was transferred into a STM cell... 

The corresponding set of spectra obtained by ATR-SEIRAS with a vacuum-evaporated Au(lll) thin-film electrode and 1 mM pyridine-i-0.1 M NaClO. solution in H2O is shown in Fig. 8.8b [55]. The vacuum-evaporated film electrode was subjected to flame annealing before use to give a (111) quasi-single crystal surface. The spectra were recorded with unpolarized radiation and by a simple potential difference tactic with the reference potential of -0.8 V. These spectra are represented in the absorbance units [A=-log(R/i o)]. and thus the up-going bands correspond to pyridine adsorbed on the electrode. Largely different from... 

This means that one needs tips free of adsorbates and contamination, and very smooth surfaces, which cannot support SERS. Well-annealed single-crystalline metal surfaces are suitable substrates in this respect. An easy way to prepare such crystals is to use the flame-annealing technique developed by Clavilier [147]. This simple procedure not only removes contamination, but also produces very smooth surfaces. These smooth surfaces are not SERS active in the usual sense. 

The surface electrochemistry of Pt single-crystal electrodes has been exhaustively studied using cyclic voltammetry [5, 8-12, 61-67]. Phenomena of a step reconstruction and step coalescence have been observed [61]. For Pt(lll)-H20 interface, prepared by the flame annealing method, a double-layer charging has been observed only in a very narrow potential region (0.1 < E < 0.35 V (SCE) in 0.05 M H2SO4), which depends on the chemical... 

For some active metals, the flame annealing method carmot be used, and the electrochemical poKshing method comes to the fore. For example, a good single crystal copper surface can be obtained by electrochemical polishing for 20 s in a solution of 60% H3PO4 in water, at 2.0 V. For most crystal surfaces, in order to help preserve the perfectness of the surface, the electrode is normally immersed into a solution with potential control. 

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