The formation of mesoporous silicon is confirmed by surface SEM observations in our previous work , showing homogeneously distributed pores over the entire sample surface. The average pore size is in the range of 10-15 nm. The same morphologies were observed after thermal oxidized of porous silicon sample to stabilize its surface. We note that the mesopores are still discernible
FTIR spectroscopy has been proved to be a very powerful technique to detect the intermolecular interaction between the polymer and the interface. Figure (1) shows the infrared spectrum of porous silicon before and after oxidation. Before the oxidation, the mesoporous silicon exhibits three bands due to υSi-Hx stretching modes (x=1,2,3) localized at 2090, 2110 and 2150 cm-1, a band at 910 cm-1 ascribed to δSi-H2 scissor mode and two bands at 630 and 680 cm-1 corresponding to δSiHx. The homogeneity of the porous layer is confirmed by the presence of periodic oscillations (figure (1)) .
After the surface oxidation, the infrared spectra confirm the significant oxidation of the PSi surface as it is indicated by the decrease of υSi-Hx and δSiHx bands . An important vibration band of the Si-O bond containing the symmetric and antisymmetric mode stretching vibration of the Si-O-Si bond at 1107cm-1 and 1163 cm-1, respectively, and the SiO2 stretching vibration at 1147 cm-1. We note again the appearance of a broad absorption between 3480 and 2900 cm−1 due to terminal hydroxyl groups (Si OH) and likely to adsorbed water molecules.
Contact angle measurements provide an indication of the surface reactions efficiency. The contact angle of the mesoporous silicon determined by this method is 122° indicating the hydrophobic behavior of this surface (Figure (2.a)). After the oxidation of PSi surface, we observe a decrease in contact angle to 13° (Figure (2b)).
Functionalization of PSi surface by PPy
Figure (3) shows the cyclic voltammogram of 0.01M pyrrole in anhydrous acetonitrile solution containing 0.1 M C16H36BF4, on mesoporous silicon electrode, between -2 V and 3 V with a scan rate of 50 mV s−1. Pyrrole oxidation has two irreversible features; we observe the appearance of a shoulder located at approximately 1.14 V and a peak at 2 V corresponding to the formation of a radical cation and a dication during pyrrole monomer oxidation. The formation of PPy was visually observed. Since the number of electropolymerization cycles determines the thickness of polypyrrole layer, we preferred to stop the scanning after two cycles to prevent the polymer detachment. Indeed, in previous manipulations we found that PPy layer is not stable on the porous silicon beyond three scans.
The surface of PSi/PPy structure is characterized by SEM. The side view shows the PPy film forming a thick layer (Figure (4)). However, the PPy film does not adhere to the surface substrate suggesting that the structure obtained is a deposit and not a covalent grafting.
Researchers have faced the same problem of adherence when studying polypyrrole deposition on various substrates and several methods have been proposed to overcome this problem . In this work, we have proceeded to a thermal oxidation of the PSi hydride surface to compensate for the low adherence of the polymer.
Functionalization of oxidized PSi surface by PPy
Figure (5) shows the cyclic voltammogram of 0.01M pyrrole in anhydrous acetonitrile solution containing 0.1 M C16H36BF4, on oxidized mesoporous silicon electrode, between -2 V and 2V. Scan rate = 50 mV s−1. We observed in the first scan a shoulder at 0.5 V, corresponding to the monomer oxidation to form cation radical leading to the formation of the polypyrrole layer.
In the reverse scan, we observed a cathodic peak located at 0.1 V characteristic of the reduction of the polymer deposit formed during the polymerization reaction. During the recording of further cyclovoltammograms as depicted in Figure (5), the oxidation peak current shifts to positive values and the reduction peak shifts to negative values. The displacement of these potentials is accompanied by an increase of the current intensity of the oxidation and reduction peaks confirming the deposition of the polypyrrole film on the oxide mesoporous silicon surface. With repeated cycles, an increasingly thick dark film was deposited on the oxidized porous silicon surface.
Figure (6) shows SEM micrographs of PPy films deposited with cyclic voltammetry (CV) on oxidized mesoporous silicon surface. The obtained film is thin and appears homogeneous over the whole surface. The deposit is characterized by a cauliflower-like structure consisting of microspherical grains of approximately 1µm diameter. A. El Jouahri and al obtain the same morphologies on the study of the electrochemical behavior of PPy coated carbon steel in aqueous NaCl solution by scanning electrochemical microscopy [20-21].
This modification was confirmed by another technique. The FTIR spectra depicted in the figure (7) in the range 450–4000 cm−1 show the presence of the major expected peaks of the PPy on the oxidized PSi surface.
The absorption centered at 1541 cm-1 and 1462 cm-1 correspond to the c-c and c-n vibration pyrrole rings. Also the peaks at 1298, 1180 and 1039 cm-1 can be attributed to the C-H plane stretch vibration. The peak at 963 ascribed to the C-C out of plane deformation .
Electrochemical Behavior of 4-NP:
The electrochemical behavior of 4-NP at the PSi/PPy hybrid structure was investigated using cyclic voltammetry (CV).
After addition of para-nitrophenol, the voltammetric behavior as shown in Figure (8) is strongly affected and the shape of the curve is modified. The different structures were tested in a concentration range of 10-2 to 10-8M. A large peak appears around 1.7 V. We note a shift of this peak with the variation of the concentration. The oxidation peak may be attributed to the oxidation of the 4-(hydroxyamino)phenol which is the reduction reaction product of the paranitrophenol .
The calibration curve for p-NPh in tampon PBS solution was investigated in terms of the relationship between p-NPh concentration and the current oxidation peak.
The calibration curve (Figure (9)) shows that the intensity of oxidation peaks increases linearly with the concentration of p-Nph. The linearity is obtained over a concentration range of 10-2 to 10-8M with regression coefficients of 0.959, which implies a good calibration of this detector and it can be used in a large concentration range. We can conclude from these results that the hybrid systems PSi Oxide/PPy are sensitive to para-nitrophenol.