The physicochemical properties of carbon nanotube-titania nanocomposite for light harvesting.

Abstract

Nanocomposites for dye-sensitised solar cells (DSSCs) were synthesised from ultrasonic acid-treated multiwalled carbon nanotubes (MWCNTs) and titania precursors by means of sol-gel and chemical vapour deposition (CVD) processes. The wt.% of MWCNTs in nanocomposites were varied from 2 to 98. Physicochemical properties investigation forms the core of the study. Hence, nanocomposites were thoroughly characterised by means of thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), electron dispersive X-ray spectroscopy (EDX), Raman spectroscopy, Fourier transformation infra-red spectroscopy (FTIR), textural characteristics, inductively coupled plasma optical emission spectroscopy (ICP-OES), photoluminescence (PL) and powder X-ray diffraction (XRD) techniques. Nanocomposites were used as photoanode in assembled DSSCs. The gel electrolyte was polyvinyl acetate (PVAc) doped with LiI. Aluminium was the photocathode. The DSSCs were tested for their performance by illuminating them in a solar simulator device. Characteristic hollow MWCNTs morphology with titania particulates was obtained. Both synthetic methods coated small diameter MWCNTs well. EDX spectra showed titania and carbon peaks. The ICP-OES data correlated with TGA in that residual wt.% values were within the expected ranges. Defects from acid treatment lowered thermal stability of pristine MWCNTs from 640 °C to 625 °C. Ti-O-C bond (ca. 1110 cmˉ¹) and anatase form of titania (ca. 669, 577 and 411 cmˉ¹) were observed. Raman spectroscopy showed Eg, A₁g + B₁g₍₂₎ and B₁g₍₂₎ modes of anatase titania at ca. 630, 514 and 396 cmˉ¹ respectively. ID/IG trends indicate that titania reduced defects in MWCNTs. MWCNTs in nanocomposites from the CVD method had fewer defects, highly thermally stable and more uniformly coated, more crystalline, more porous and had smaller surface areas than sol-gel prepared nanocomposites. Nanocomposites had lower eˉ/h⁺ recombination and band gap energy than titania. The optimum MWCNTs wt.% in DSSCs was 15% and CVD nanocomposites were 900% more efficient. From this work, ideal nanocomposites physicochemical properties for DSSCs application include uniform morphology, a defect-free nature, crystallinity, large pore size and volume, and existence of chemical bonds between components. Other factors rather than band gap engineering such as absorption properties of DSSC components also affect DSSC capabilities. Also, the high eˉ conductivity nature of MWCNTs interferes with eˉ transport from the nanocomposites to the counter electrode at high MWCNTs wt.%.