Ex situ measurements

While characterization of the electrode prior to use is a prerequisite for a reliable correlation between electrochemical behaviour and material properties, the understanding of electrochemical reaction mechanisms requires the analysis of the electrode surface during or after a controlled electrochemical experiment. Due to the ex situ character of photoelectron spectroscopy, this technique can only be applied to the emersed electrode, after the electrochemical experiment. The fact that ex situ measurements after emersion of the electrode are meaningful and still reflect the situation at the solid liquid interface has been discussed in Section 2.7. 

More than a decade ago, Hamond and Winograd used XPS for the study of UPD Ag and Cu on polycrystalline platinum electrodes [11,12]. This study revealed a clear correlation between the amount of UPD metal on the electrode surface after emersion and in the electrolyte under controlled potential before emersion. Thereby, it was demonstrated that ex situ measurements on electrode surfaces provide relevant information about the electrochemical interface, (see Section 2.7). In view of the importance of UPD for electrocatalysis and metal deposition [132,133], knowledge of the oxidation state of the adatom in terms of chemical shifts, of the influence of the adatom on local work functions and knowledge of the distribution of electronic states in the valence band is highly desirable. The results of XPS and UPS studies on UPD metal layers will be discussed in the following chapter. Finally the poisoning effect of UPD on the H2 evolution reaction will be briefly mentioned. 

Most recently, these same authors have employed an in situ cell (Fig. 14) for carrying out these experiments. Again they studied nitrite- and chromate-passivated films. The results obtained in this case are quite different from the ex situ measurements. In addition,... 

The Fourier transforms for both films are again quite similar, but as for the ex situ measurements, the chromate-passivated films appear to have a more glassy structure. It should be mentioned that these studies employed a rather limited data range which makes spectral differentiation difficult. 

The results are shown in Figures 3.76(a) and (b). Figure 3.76(a) shows the conductivity as a function of the potential of the polymer-coated electrode and Figure 3.76(b) shows the conductivity as a function of the charge injected per monomer ring. The conductivity of the fully oxidised polymer was found to be significantly lower than that obtained by other workers via ex situ measurements, 10 2 1 cm 1 compared with I02 fi 1 cm The difference... 

Ex situ measurements in the presence of dissolved oxygen have proved that the mixed monolayer was stable in the solution free of 6TG and guanine. Madueno etal. [Ill] have also studied adsorption and phase formation of 6TG on mercury electrode. At high potentials, the molecules were chemisorbed and were able to form a self-assembled monolayer. When the potential was scanned to more negative values, reductive desorption of the monolayer was observed. Cathodic voltam-metric peaks, which are typical of a 2D condensed phase transition, divided the potential window into two regions one, in which self-assembled monolayer was stable, and the second, in which a physisorbed state existed. 

In ex-situ measurements the catalyst is outside a proper reactor and it is in a state which is more or less different from the active state. The advantage is that we have a wide choice of temperature, pressure and gas composition. The disadvantage is that we don t really know how far the state is from the active state and we have no easy way of finding out. 

The big advantage of making ex situ measurements is that they allow the application of methods used in surface chemistry when no solution is present. Some of these ex situ methods (LEED or XPS) are described in Chapter 6. In electrochemical situations in which the critical questions concern, for example, passivation of metals involving oxides or sulfide films, there is no accompanying disadvantage in the use of these well-developed and accurate methods. 

However, in many electrochemical situations (e.g., methanol oxidation), the question is one of what entities and how many of them are present on the surface when a certain potential is applied to the electrode, while the solvent layer is still adsorbed on the electrode. One has to ponder the effect of removing the electric potential that orients the reactants on the electrode and the solvent (this happens when one applies a vacuum) and ask Is the information that can now be extracted from the surface (i.e., in ex situ measurements) still relevant to what is wanted—information on entities present during the electrochemical reaction, which must necessarily involve a certain potential applied across the reactants, in the presence of the solvent ... 

Particle size measurement is one of the essential requirements in almost all uses of colloids. However, our discussion in Section 1.5 makes it clear that this is no easy task, especially since even the definition of particle size is difficult in many cases. A number of techniques have been developed for measuring particle size and are well documented in specialized monographs (e.g., Allen 1990). Optical and electron microscopy described in the previous section can be used when ex situ measurements are possible or can be acceptable, but we also touch on a few nonintrusive methods such as static and dynamic light scattering (Chapter 5) and field-flow fractionation (see Vignette II Chapter 2) in other chapters. 

Field turbidity meters may be part of the multiple field parameter meters or they may be available as standalone units. A typical meter has a range of 0-1000 NTUs and an accuracy of + 2-3 percent. Some meters have a submersible turbidity probe that allows in situ measurements, while others require sample collection for ex situ measurements. To perform an ex situ measurement, we pour the sampled water into a glass measuring cuvette (usually a vial with a cap), seal it, and insert into a measuring chamber of a portable nephelometer. The read-out device will give us a turbidity value in NTUs. 

However, when the adsorption process is reversible, one cannot use the traditional approach to measuring the effect of the deposition potential on the coverage, i.e. immersing a clean electrode in a deposition solution under potential control, followed by ex situ measurement of the surface coverage in a blank electrolyte solution. 

During the investigation of CO poisoning in PEM fuel cell anodes, Ciureanu and Wang [14] compared in situ and ex situ measurements. They summarized three primaiy electrochemical methods that could be used for investigating the mechanism by which H2 and H2/CO mixtures are oxidized ... 

Figure 9.7 Raman spectra of samples that were heat treated corresponding to A-C (ex situ measurements). Raman spectrum at 132°Cis in good agreementwiththatofa-AIH, reported in the literature. Ref [20].

In the Grand Accelerateur National dTons Lourds (GANIL, Caen, France) facility, ion beams are delivered in order to understand the specific characteristics of SHI-matter interactions. In particular, polymers are investigated. Infrared spectroscopy is a very powerful tool to characterize organic species in general and polymers in particular FT-IR spectra have been recorded in GANIL since 1994 in order to understand the specificities of SHI-polymers behavior. The first studies were ex situ measurements. Since 2003, SHI irradiations have been analyzed by means of in situ FT-IR spectroscopy. 

While this sensor employed only a model nucleic acid probe (poly(A)), it demonstrates the principle of detection of the complementary sequence, the analyte poly(U). Figure 7.9(a) shows that a frequency decrease of 600 Hz was observed following the hybridization step, Step 3, and that this decrease was not observed with a control sensor that did not possess surface-bound poly(A). The sensor exhibits a relatively small frequency change that is superimposed on a large initial resonance frequency (9 MHz), and so S/N is low in addition, difficulties associated with ex situ measurements at constant humidity preclude the use of this device for practical DNA or RNA detection. 

A different interaction mechanism is expected, and indeed observed, when the cluster-assembled films are exposed to dry air, since oxygen is known to readily react chemically with carbon chains [7]. Figure 2.9 shows the evolution of the Raman spectrum during exposure to dry air. As reported in Table 2.1, the C peak decay is faster for dry air than for the other gas (Zair — 0-6 h, with a partial pressure of oxygen of 100 mbar), and the corresponding asymptotic value is significantly smaller (Rg — 3-4%) and comparable to what observed in ex situ measurements. 

Correlations of in situ and ex situ observations. The characterization methods of surface science have already been established within an electrochemical context, because they can provide structural definition of fine distance scales as well as atomic composition of a surface and, sometimes, vibrational spectroscopy of adsorbates. These ex situ methods normally involve transfer of an electrode from the electrochemical environment to ultrahigh vacuum, and the degree to which they provide accurate information about structure and composition in situ is continuously debated. Additional work is needed to clarify the effect of emersion of samples and their transfer to ex situ measurement environments. The most appropriate experimental course requires observations by techniques that can be employed in both environments. Vibrational spectroscopy, ellipsometry, radiochemical measurements, and x-ray methods seem appropriate to the task. Once techniques suited to this problem are established, emphasis should be placed on the refinement of transfer methods so that the possibilities for surface reconstruction and other alterations in interfacial character are minimized. 

The interaction of iodine with palladium and platinum electrodes was studied in different electrolyte solutions and on single crystals with electrochemical techniques and UHV spectroscopic ex situ measurements [111]. The chemisorption of atomic iodine on palladium researched extensively because of its ability to protect the surface from air and water interactions. Moreover, it is able to induce a surface reconstruction from a stepped surface to a (1 x 1) unreconstructed one. 

The first attempt to use ex-situ measurements of the oxidation state of Pt and O in these thin films was made by Kim et al. [3] who used XPS. They were able to study oxide films from the beginning of oxidation about +1.0V (RHE) up to + 2.5 V using simple insertion into the UHV system after washing the electrode. They interpreted their results in terms of three Pt... 

Local probe techniques are carried out ex-situ , non-situ or in-situ with respect to applied environmental conditions. Ex-situ local probe investigations are performed under UHV conditions on well-defined substrates, e.g., single-crystal surfaces. Such ex-situ measurements are often made in far fiom real conditions, which are characterized by adsorption and film formation. Therefore, ex-situ UHV techniques are usually combined with appropriate transfer devices to switch substrates fi om the real environment to UHV and vice versa. Non-situ local probe measurements are also started under UHV conditions to characterize the bare substrate surface, but they are continued under a finite vapor pressure in order to form adsorbates or mono- or multi-atomic (-molecular) films modeling real environmental conditions. In-situ local probe measurements are carried out at solid/liquid or solid/gas interfaces under defined real conditions involving adsorption and film formation. 

The STM imaging resolution is mainly determined by the tunneling mechanism, which may be identified by distance tunneling spectroscopy (DTS). DTS is characterized by measuring the tunneling current, U, as a function of the tip-substrate distance, d, at constant tunnel voltage Et. For ex-situ measurements is the applied... 

Complex Index of Refraction of Soot. Soot refractive index has been measured by several researchers. The experimental techniques used can be broadly categorized as in situ and ex situ techniques. In the former, the measurements are performed nonintrusively in a flame environment. The necessary information is retrieved either from spectral transmission data or both the transmission and scattering information, as in Refs. 215-224. The ex situ measurements involve the reflection/transmission of incident spectral radiation on planar pellets of soot, and the optical properties are determined using the Fresnel relations [225]. An alternative ex situ technique was used by Janzen [226], who dispersed the soot particles in a KBr matrix and used transmission measurements to extract the required optical properties. 

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