Charge neutralization process of mobile species at any distance from the electrode/solution interface. 2. Concentration gradients during potential pulse experiments

The theoretical model presented in part 1 of this work is employed to simulate and fit experimental probe beam deflection (PBD) data of Fe(CN)6(3-)/Fe(CN)6(4-) and Fe3+/Fe2+ couples. Current and beam deviation dependency on time at constant potential (chronoamperometry and chronodeflectometry) is analyzed via a new treatment based on the migration and diffusion properties of all the species involved. The diffusion coefficients of electroactive species are obtained by fitting chronoamperometric curves. Those coefficients are then employed to simulate the respective chronodeflectometric profiles. The experimental data and the theoretical function are fitted by the minimum squares Simplex algorithm. The effect of working with systems in which both electroactive species are charged is discussed in detail. Specifically, the possibility of quantitative analysis of nonspecific techniques data is analyzed when a relative high concentration of supporting electrolyte is used. Such analysis widens the scope of techniques as PBD since in many cases the effect of supporting electrolyte species could be negligible as compared to the response of electroactive species. The variation of the refraction index with the concentration gradient of each soluble species is also discussed.     

Synthesis of boron-doped diamond films by DC plasma CVD using a CH4+CO2+H2 gas mixture at lower substrate temperature and formation of an n-Si/p-diamond heterojunction

Diamond films were synthesized by direct current plasma chemical vapour deposition using a CH₄+CO₂+H₂ gas mixture on Si substrates. The optimum deposition conditions were determined. It was found that 0.4 A/cm² current density, at applied voltage of 1 kV, resulted in good-quality diamond films. The substrate temperature was 750 K which is considerably lower than the conventional requirement of ∼1100 K. Boron doping was achieved by passing a portion of the gas mixture through boric acid dissolved in methanol. The boron-doped p-type diamond films were deposited on an n-type single crystalline Si substrate and an n-Si/p-diamond heterojunction was fabricated. The p-n junction was characterized in terms of current-voltage (I-V) and capacitance-voltage (C-V) measurements.     

Charge neutralization process of mobile species at any distance from the electrode/solution interface. 1. Theory and simulation of concentration and concentration gradients developed during potentiostatic conditions

The theoretical framework of a general model for the simulation of concentration profiles of electroactive and nonelectroactive species, driven by an electrochemical process under potentiostatic conditions, is presented. Based on this analysis, finite differences simulations are performed to calculate the actual profiles under different experimental conditions. Furthermore, the effect of experimental parameters (diffusion coefficients of the ions of the redox couple or the supporting electrolyte, charge of the different species, etc.) on the concentration profiles is also examined. The results obtained when low and high concentrations of supporting electrolyte are compared aid understanding of the effect of the electrolyte on the measurements. The presented model also underlines the role of supporting electrolyte species when nonspecific techniques are employed to measure the concentration changes produced by electroactive species. On the other hand, if a highly specific technique were used to detect changes in the concentration or concentration gradient of a given species, then it would be possible to estimate the respective profiles of the other species. The simulations suggest that techniques measuring concentration gradients are more sensitive to determining concentration changes than those involving a measurable linearly related to concentration.