Polyaniline functionalized with sulfonate groups (SPANI) shows excellent solubility in water. Single-walled carbon nanotubes functionalized with carboxylic groups (f-SWCNTs) and then hybridized with freshly prepared zinc sulphide (ZnS) nanocrystals have been found to be good luminescent material with tuned emission properties. Nanocomposite of sulfonated polyaniline with embedded SWCNT/ZnS nanohybrid fibers has been prepared by a simple solution mixing process and characterized by using high resolution transmission electron microscopy (HRTEM), X-ray diffractometry, Raman spectroscopy, FTIR spectroscopy, and thermogravimetric analysis (TGA). The study of optical properties by UV-vis absorption and photoluminescence spectroscopy reveals that the composite is a luminescent material of enhanced emission intensity in the visible region of the spectrum.
Since their discovery by Iijima [
While there are published reports on CNT-based SPANI composites [
The SWCNTs used in our work were supplied by Chengdu Organic Chemicals Co. Ltd., China, with average diameter, length, and purity, as stated by the manufacturer, being 1-2 nm, 1–3
The as-received SWCNTs were purified by heating in a muffle furnace at 350°C in air for 6 h followed by soaking and stirring in 6 M HCl for 12 h. The acid-treated SWCNTs were filtered using vacuum filtration system (Millipore, pore size
We took 30 mL of zinc nitrate solution and 0.0995 gram of f-SWCNT was dispersed in it. The solution was stirred in a magnetic stirrer for 1 h at room temperature. 15 mL of methanol with sodium sulfide flakes added to it was sonicated to obtain a saturated solution which was added dropwise to the zinc nitrate solution containing SWCNTs till the pH became 7. The solution was stirred for 1 h and filtered using Whatman filter paper. The precipitate was washed with methanol followed by deionized water and filtered again. The sample was then dried under the IR lamp.
0.2 M aniline hydrochloride and 0.25 M ammonium persulfate solution were prepared and mixed in equal volumes and left overnight for polymerization to take place. The salt precipitate (PANI) was collected by filtration using a Whatman filter paper. It was then mixed with 1,2-DCE and heated to 80°C under stirring using a magnetic stirrer (REMI 2 MLH). Chlorosulfonic acid diluted with DCE was added dropwise to the reaction mixture and stirred at 80°C for 1 h. The semisolid precipitate of sulfonated polyaniline (SPANI) obtained by filtration was then mixed with 400 mL deionized water and heated to 60°C and stirred for 2 h to promote hydrolysis. The solution was further diluted with water and filtered through a cellulose membrane using a vacuum filtration system (Millipore). The final sample (SPANI) was collected over the filter membrane and dried in air at room temperature.
The SWCNT/ZnS hybrid was dispersed in deionized water and ultrasonicated. The prepared dispersion was mixed with SPANI dispersion in deionized water with constant stirring. The resulting solution was heated at 50–60°C for 2-3 hours, with stirring. Then, it was cooled and filtered using vacuum filtration system (Millipore). The sample collected by filtration was dried in air at room temperature to get SPANI/SWCNT/ZnS composite with SWCNT content of 6 wt.% and ZnS content of 7 wt.%.
The micrographs of the sample were obtained using high resolution transmission electron microscope (HRTEM model JEOL JEM-2010; operating acceleration voltage—200 kV). The XRD patterns were obtained using Philips PANalytical X-Pert Pro diffractometer. The molecular structures of the samples were characterized by a Perkin Elmer Spectrum RX I FTIR Spectrometer. The laser source used in the spectrometer was a He-Ne laser (633 nm). Raman spectroscopy was performed using EZ-Raman-M field portable Raman analyzer (Enwave Optronics, Inc.). A diode laser of wavelength 785 nm was used as excitation source. The thermogravimetric analysis (TGA) was carried out with Perkin-Elmer Pyris- 1 TGA thermogravimetric analyzer at a heating rate of 10°C/min in nitrogen atmosphere. The optical absorbance spectra were recorded using a HITACHI U-3010 UV-visible absorption spectrophotometer. Photoluminescence spectra of the samples were acquired using a Perkin-Elmer LS-55 fluorescence spectrophotometer.
The scheme of reactions is as in Scheme
The HRTEM micrograph of SPANI/SWCNT/ZnS nanocomposite is shown in Figure
HRTEM image of SPANI/SWCNT/ZnS nanocomposite.
The structural characteristics of SPANI, SWCNT, SPANI/SWCNT, and SPANI/SWCNT/ZnS have been analyzed by X-ray diffractograms shown in Figures
(a) XRD pattern of SWCNT, SPANI, SPANI/SWCNT, and SPANI/SWCNT/ZnS. (b) Smoothened XRD pattern of SPANI/SWCNT/ZnS structure.
The mean size (
In FTIR spectra as shown in Figure
FTIR spectra of SPANI, SWCNT, SWCNT/ZnS, and SPANI/SWCNT/ZnS.
Pure PANI shows the characteristic stretches at 1560, 1480, 1300, and 1140 cm−1. The stretches at 1560 and 1480 cm−1 arise due to the stretching vibrations of quinoid ring (–N=quinoid=N–) and the benzenoid ring (–N-benzenoid-N–), respectively. The stretch at 1300 cm−1 is due to C–N stretching and that at 1140 cm−1 is due to C=N stretching. In SPANI, the stretches at 700 and 615 cm−1 arise due to S–O and C–S stretching vibrations, respectively. The stretch at 815 cm−1 is assigned to the aromatic C–H out-of-plane bending vibration of the 1,2,4-trisubstituted aromatic rings. Similar results have been reported earlier [
The Raman spectrum of SWCNT in Figure
Raman spectra of SPANI, SWCNT, SWCNT/ZnS, and SPANI/SWCNT/ZnS.
The rogravimetric analysis (TGA) was done to investigate the thermal stability of the samples. Figure
TGA thermograms of SPANI, SPANI/SWCNT/ZnS, and SWCNT.
The UV-visible absorption spectra of SPANI, SPANI/SWCNT, and SPANI/SWCNT/ZnS are compared and shown in Figure
UV-vis absorbance spectra of SPANI, SPANI/SWCNT, and SPANI/SWCNT/ZnS.
The optical band gap
Plot of (
Figure
(a) Photoluminescence spectra for SPANI, SPANI/SWCNT, and SPANI/SWCNT/ZnS. (b) Photoluminescence spectra for SPANI/SWCNT/ZnS nanocomposite at different excitation wavelengths.
We reported the synthesis of a novel luminescent material containing sulfonated polyaniline, embedded with functionalized SWCNTs hybridized with ZnS nanocrystals, by a simple chemical process. Characterization established the desired nanostructure of the composite material. The energy band gap of 3.73 eV for pure SPANI was found to be increased to 4.49 eV for the composite. The PL emission from the composite showed a broad emission range in the visible region between 400 and 450 nm with the peak emission at 416 nm, with appreciable enhancement in the intensity of emission, leading to the formation of a novel luminescent polymer nanocomposite.