Page 126 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Chapter 6
Ks =[Y3+][OH-]3 Kw =[H+][OH-]
[H + ] = Kw ([Y 3+ ] / Ks )1/3 pHst =-log10[H+]
(6.2)
(6.3)
(6.4)
(6.5)
It is expected that the yttrium hydroxide will deposit at pH of 7.1 from the 0.05 mol/L Y(NO3)3•6H2O aqueous solution. Figure 6.11 shows the pH value of the solution during the plasma treatment. One can observe that the pH increases drastically within the first 20 min, from 5.53 to 6.56. This can be explained by the occurrence of an electrochemical reaction in which OH- radicals are generated from water molecules, leading to an initial apparent increase of pH. However, as reaction proceeds, pH only undergoes a mild further increase from 6.56 to 6.76 during the rest of the process. A reasonable explanation is due to the limited penetration ability of electrons, relatively high concentration OH- is formed locally at the solution surface. The generated OH- is rapidly consumed by chemical reactions to form yttrium hydroxide, leading to a pH value below 7.1 of bulk solution and remains approximately constant. Direct evidence is reflected in Supplementary Material Figure S4, where white flocs are firstly observed at the electrolyte surface near the plasma-liquid interaction area. To conclude, the above experimental phenomena are in good agreement with the precipitation equilibrium analysis, suggesting the release of hydroxyl ions from water can also be achieved by this technique.
Figure 6.11 Variation of pH with time during plasma treatment
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