Rate-determining step crystal face

In this case it was found that 7 6 [35,36], i.e. the reaction occurs essentially via the valence band. In addition, an ideal slope of 60 mV/decade was found with p-GaAs at anodic polarization [37]. From this result it must be concluded that only one hole is involved in the rate-determining step. Investigations with the electrodes with, in one experiment the Ga (111) face and in a second experiment the As (1 iT) face contacting the electrolyte, yielded no difference in dissolution rate [35]. This result is an indication that the dissolution occurs mainly at steps as illustrated for GaP in Fig. 8.10. The influence of steps is discussed in detail by Morrison in his book [39], On the other hand, the flatband potential and the onset potential of the photocurrent do depend on the crystal faces. The difference in flatband potential is in the order of about 0.1 V [39]. Accordingly, the Helmholtz layer is somewhat different for the two surfaces. 

Now, it is generally accepted that the dissociated adsorption of N2 is the rate determining step, because it has been confirmed by many results from surface science experiments. The contradictions about the pathway of nitrogen dissociation adsorption still remains, as different conclusions were obtained from different experimental measures. Meanwhile, there are also different results for different metals and crystal faces under various experimental conditions. Some experiments showed that N2 was dissociated directly to nitrogen atoms on Fe sm-face, and others found that N2 were dissociated to nitrogen atoms by intermediates, based... 

Thus, the rate-determining step for crystal growth is not the diffusion process but the attachment of a molecule in the correct orientation at the correct site on the growth face. Only when a molecule becomes firmly bonded on at least two sides to similar molecules can it be considered a part of the crystal. 

In this theory it is assumed that a chain stem, one fold period long, is laid down on the lateral growth face of the crystal. This is the slowest step because the stem has only one surface face on which to sit. Once this stem is in place, however, an adjacent stem is more easily laid down (i.e., has a lower free energy barrier to cross), because it can now contact two surfaces, the crystal substrate and the side face of the first stem. The row therefore quickly fills up once the first stem is deposited. The growth rate of the crystal (primary crystallization) is thus largely determined by secondary nucleation (Figure 10-31). 

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