Influence of crystallographic orientation

A distinct answer has been found, however, with respect to the influence of crystallographic orientation of pyrolytic graphite on the rates of various reactions. It could be shown that the rate of cathodic oxygen reduction at the basal plane of graphite is much lower than at surfaces with edge orientation (Morcos and Yeager, 1970). To the contrary, the rates of simple redox reactions hardly depend on face orientation. 

Formation of 2D Me surface alloy and 3D Me bulk alloy have to be taken into account besides 2D Meads overlayer formation for Me UPD systems with nonvanishing Me solubility in S. Influences of crystallographic orientation and crystal imperfection density of S on the rate of 2D and 3D Me-S alloy formation are observed. 2D Me-S surface alloy formation processes are pronounced on S(lll). The mechanisms of 2D Me surface alloy and 3D Me bulk alloy formation processes are still not well understood. More realistic models are necessary to describe these processes... 

Hamelin A (1984) Underpotential deposition of lead tm single crystal faces of gold. Part I. The influence of crystallographic orientation of the substrate. J Electroanal Chem 165(1-2) 167-180... 

Almost all that is known about the crystal face specificity of double-layer parameters has been obtained from studies with metal single-crystal faces in aqueous solutions. Studies in nonaqueous solvents would be welcome to obtain a better understanding of the influence of the crystallographic structure of metal surfaces on the orientation of solvent molecules at the interface in relation to their molecular properties. 

It is very difficult in view of the vast amount of experimental data to draw general conclusions that would hold for different, let alone all electrocatalytic systems. The crystallographic orientation of the surface undoubtedly has some specific influence on adsorption processes and on the electrochemical reaction rates, but this influence is rather small. It can merely be asserted that the presence of a particular surface orientation is not the decisive factor for high catalytic activity of a given electrode surface. 

The crystalline structure of adsorbents directly influences their gas sensitivity. Depending on the type of the problem to be addressed ad-sorption-sensitive semiconductors are monocrystals or monocrystal films with a predetermined crystallographic orientation of of>erational surface vacuum baked polycrystalline films, which, from their electrical standpoint, are similar to monocrystals but differ from the latter by ulti-... 

In acidic solutions, the corrosion rate is relatively high. Studies on cadmium monocrystals and polycrystals in acidic chloride solutions revealed anodic dissolution independent of the crystallographic orientation the dissolution rate was controlled by the mass transport of CdCl" ions [331]. The inhibitive influence of adsorbed organic substances, for example, alcohols [332], phenotiazine [333], and some polymers (e.g. poly (vinyl alcohol), poly(acrylic acid), sodium polyacrylate. 

Also, Kolb and coworkers [468] have studied initial stages of Co deposition on Au(lll) and Au(lOO) using CV and STM. Depending on the crystallographic orientation of the substrate and electrochemical properties, either three- or two-dimensional growth has been observed. The influence of surface reconstruction on the nucleation and growth behavior has also been discussed. 

Studies on single crystal faces have indicated that, on the whole, the crystallographic orientation has a minor influence on the electrocatalytic activity. Of the possible explanations which can be offered [292], a reasonable one is that the high coverage with adsorbed hydrogen far from equilibrium smoothes down the differences in A//ad from face to face. Actually, quantum chemical calculations reproduce more closely the situation at 0H-O, which does not usually correspond to the real situation under cathodic load. Another possibility is that the hydridic... 

Toraya s WPPD approach is quite similar to the Rietveld method it requires knowledge of the chemical composition of the individual phases (mass absorption coefficients of phases of the sample), and their unit cell parameters from indexing. The benefit of this method is that it does not require the structural model required by the Rietveld method. Furthermore, if the quality of the crystallographic structure is poor and contains disordered pharmaceutical or poorly refined solvent molecules, quantification by the WPPD approach will be unbiased by an inadequate structural model, in contrast to the Rietveld method. If an appropriate internal standard of known quantity is introduced to the sample, the method can be applied to determine the amorphous phase composition as well as the crystalline components.9 The Rietveld method uses structural-based parameters such as atomic coordinates and atomic site occupancies are required for the calculation of the structure factor, in addition to the parameters refined by the WPPD method of Toraya. The additional complexity of the Rietveld method affords a greater amount of information to be extracted from the data set, due to the increased number of refinable parameters. Furthermore, the method is commonly referred to as a standardless method, since the structural model serves the role of a standard crystalline phase. It is generally best to minimize the effect of preferred orientation through sample preparation. In certain instances models of its influence on the powder pattern can be used to improve the refinement.12... 

In conclusion, it seems that for both the AP-CVD and the LP-CVD processes, deposition parameters have been found, for which the introduction of a dopant component does not significantly perturb the crystallographic orientation of the ZnO films. However, it is, at this stage, not possible to identify the mechanisms and critical parameters governing the influence of dopants on film structure. Nevertheless, some speculative statements can be made in the case of the AP-CVD process, the deposition temperature seems to be the critical factor, i.e., above 400°C the growth orientation of ZnO films is no more perturbed by the introduction of a dopant. For the LP-CVD... 

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