Investigation on LaF<SUB>3</SUB>-Impregnated Porous Silicon Heterostructur as Potentionmetric Sensor for Fluoride Ion in Aqueous Medium

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Figure 2. Experimental setup for C-V measurement

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The LaF₃ surface responses by interacting with the fluoride ion in solution (electrolyte) and an additional potential develop in the electrolyte, which varies the width of the depletion layer. Depending upon the concentration of fluoride ion (pF) in the electrolyte the resulting surface charge at the interface of the sensing structure affects the depletion layer which also affects the flat band voltage of the sensing system. The basic chemical reaction with the LaF₃sensing layer with the fluoride ion as follows:

However, from the shift of the C-V curves on the potential axis ^[32], concentration of fluoride ion was determined. The shift of the C-V curves can be explained by the surface potential variation of the sensor enabled by the surface ionization with F^-ion. Home made fluoride solution buffered at constant pH 5.24, with various concentrations, have been experimented with the developed sensor. The standard solution was prepared from Sodium stocks solution. The stock solution was prepared from NaF and buffered at pH 5.24 to avoid unwanted change of the pH of test electrolytes. Buffer solution made of acetic acid, sodium chloride and EDTA, where the EDTA acts as a chelating agent. Again this solution was diluted and made a series of standard solution of different pF. Then different pF (-log [F^-]) of the aqueous solution of sodium fluoride has been buffered with buffer solution.

3. Result and Discussion

The impregnation the pores of PS by LaF₃ has been confirmed by the SEM on the cross-section of the LaF₃/PS/Si system (Figure 3) that has been annealed at 400°C for 10 min. LaF₃ on PS pores is clearly visible as the white clusters. Several points on the cross-section were selected for EDX to know their corresponding compositions.

Figure 3. Cross-sectional SEM image of the LaF₃/PS heterostructure

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The EDX spectrum of a point on the cross-section is shown in Figure 4. The ratio of the La and F atom was 1:15 at that point. For a stoichiometric LaF₃deposition the ratio should be 1:3. In our case the fluorine source was HF and why the stoichiometry of LaF₃ was not good. If TiF4 or TaF5 type of electrolyte could be used we could have received better stoichiometry of LaF₃.

Figure 4. EDX spectra on the cross-section of the LaF3/PS heterostructure

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Sensors with two conditions, namely, as-deposited (without annealing) and, annealed at 400°C, were investigated. As the as-deposited sensor shows hardly any shift in potential axis with change in fluoride ion concentration ^[35], from now on only the annealed structure will be investigated as the sensor. The response for a particular solution is obtained by plotting the capacitance for the corresponding bias. The typical response of the sensor for a pF 2.28~4.28 solutions is shown in Figure 5. As shown in Figure 5, the capacitance for sensors start decreasing when the depletion region is created in the interface of LaF3 and porous-Si by the applied bias and the surface potential at the LaF3/electrolyte interface which corresponds to the Nernstian voltage of LaF3 for pF solutions. The minimum level of the capacitance was observed at highly depleted condition at a bias of -1 V. The change in the concentration (pF) of the fluoride solution is measured by determining the shift in the C-V curve on the potential axis (bias) from any reference pF value.

Figure 5. C-V response of the sensor for different pF solution

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The potential change for different pF found from the zero crossing point in the second derivative of C-V response signal. Figure 6 shows the second derivative of signal of 4.28 pF solution.

Figure 6. Second derivative of C-V response showing the inflexion point

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After calculating the inflexion potentials for all the fluoride concentrations (pFs) the calibration curve was obtained, which is shown in Figure 7. From the calibration curve the average sensitivity ^{[5, 14]} of sensor was found to be 400 mV/pF. This over Nernstian behavior of the sensor is believed to be due to the enlargement of the active area of PS and excellent adhesion between sensing fluorides and the surface of the porous silicon ^{[33, 34]}.

Figure 7. Sensitivity curve of the sensor for different pF solution

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