Modification of Carbon Nanotubes with Cadmium Sulfide Quantum Dots to Obtain Electrode Materials for Current Sources

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Zinc was used as the auxiliary electrode. For all investigations, the reference electrode was a silver-chloride electrode connected through a salt bridge. All potentials are given versus the silver-chloride reference electrode. The electrochemical curves were recorded under galvanostatic conditions. The oxygen source was a U-shaped electrolyzer with alkaline electrolyte. Oxygen was supplied to the gas electrodes under an excess pressure of 0.01 mPa. Before operation, the oxygen electrode was blown down with oxygen during an hour. The deposited cadmium sulfide was identified by X-ray fluorescence analysis by means of an ELVAX SBR-01 X-ray energy spectrometer. The electron micrographs have been obtained by means of a JEM-100 CXII electron microscope.

3. Results and Discussion

The dependence of optical density on wavelength is shown in Figure 2. Since CdS is a semiconductor in which direct band-to-band optical transitions are observed ^[7], the energy gap width E_g was determined by commonly used procedures as a tangent in the absorption spectrum at the inflection points of the curve from the relation (Dh ν)² ≈ h ν up to the value Dh ν = 0. The particle size was determined from the optical energy gap width E_g. semiconductor particles in the region of steep rise of the plot of D(λ) by quantum-chemical calculations described in ^{[8, 9]}. For CdS particles, the values of E_g were determined in two spectral regions, and the values for particles of two sorts have been determined: E_g1=2.49-2.53eV, which values correspond to large particles (D > 10nm), and E_g2=2.70- 2.95eV at a particle diameter of 2nm. Samples of MWCNTs were dipped into a solution after the necessary optical density for the fabrication of 2nm particles had been reached and held for 30 minutes; then they were dried at 90°C and washed. This procedure was repeated several times to obtain different amounts of deposited cadmium sulfide. Cadmium and sulfur peaks have been found, which corroborates deposition of cadmium sulfide on nanotubes. The deposited cadmium sulfide was identified by X-ray fluorescence analysis. Figure 3 shows the spectrum of a nanocomposite deposited onto carbon nanotubes.

Figure 2. Spectral dependence of the optical density D of a colloidal solution on wavelength for CdS particles. Thickness of quartz cell: 10nm

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Figure 3. Spectrum of a nanocomposite deposited onto carbon nanotubes

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The elemental cadmium content has been determined by spectral analysis to be 1, 2, 4 and 7wt%. Figure 4 shows micrographs of a nanocomposite with a cadmium content of 4 wt% and Figure 5 4-7wt% of cadmium content. The size of particles deposited onto carbon nanotubes allows one to assert that they were modified with cadmium sulfide quantum dots. Oxygen electrodes have been fabricated from the samples obtained of MWCNTs with deposited cadmium sulfide. Figure 6 shows electrical characteristics of oxygen electrodes based on carbon nanotubes modified with cadmium sulfide quantum dots. For comparison, this figure shows current density - potential curves for electrodes based on carbon nanotubes treated with sodium sulfide (Figure 6, curve 1) and electrodes based on carbon nanotubes with deposited metallic platinum (Figure 6, curve 6).

Figure 4. Micrograph of a nanocomposite based on MWCNTs mofdified with cadmium sulfide quantum dots with a catalyst particle size of up to 10nm

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Figure 5. Micrograph of a nanocomposite based on MWCNTs mofdified with cadmium sulfide, containing 7wt% elemental cadmium, with a particle size of about 60nm

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As is seen from Figure 6, the characteristics of oxygen electrodes based on carbon nanotubes modified with cadmium sulfide quantum dots (Figure 6, curve 5) approach those of platinum-based oxygen electrodes. When the amount of cadmium sulfide deposited onto nanotubes is increased, an increase in characteristics is observed at first (Figure 6, curves 3-5); then a decrease in characteristics with increase in the amount of cadmium to 7wt% is observed, which may be due to the formation of large agglomerates of deposited catalyst, as is seen from the micrograph in Figure 5. Comparison with electrodes based on MWCNTs with deposited 1wt% metallic platinum (Figure 6, curve 6) shows that carbon nanotubes modified with cadmium sulfide quantum dots are a promising material for oxygen electrodes. A test of the oxygen electrodes obtained for the stability of characteristics in time showed the electrical characteristics to be stable during three months long operation. Calculated electrochemical characteristics of the oxygen electrode of power cell based on multiwalled carbon nanotubes modified with cadmium sulfide quantum dots are 280mW cm^-2 with the polarization of 250 mV at room temperature.

Figure 6. Dependence of potential on current density for oxygen electrodes based on nanocomposites of MWCNTs with deposited 0.02 g/cm²cadmium sulfide containing elemental cadmium (wt%): (1) pure MWCNTs treated with sodium sulfide, (2) 7, (3) 1, (4) 2, (5) 4, (6) MWCNTs with 1 wt% metallic Pt

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