Light-harvesting multi-walled carbon nanotubes and CdS hybrids: Application to photocatalytic hydrogen production from water

Abstract

This study reports the synthesis and surface characterization of multi-walled carbon nanotubes (CNT), CdS, and metal catalyst (M) hybrids (CdS/CNT/M), and their novel application to photocatalytic hydrogen production under visible light (λ > 400 nm) in the presence of electron donor (Na₂S and Na₂SO₃). In the binary hybrids between CNT and CdS (CdS/CNT) the CNT annealed at 500 °C (h-CNT) has the larger amount of hydrogen production than crude (c-CNT) or acid-treated CNT (a-CNT) due to highly improved purity and suitable work function. When hybridized with CdS and M, however, a-CNT has the largest amount of hydrogen production (a-CNT > h-CNT > c-CNT) even though all the CNTs have similar functional groups for binding metal catalyst on their surfaces. Photocurrent measurements also indicated that CdS/a-CNT/Pt ternary generates a higher photocurrent than that of CdS/a-CNT binary (ternary > binary > CdS alone). In such ternary hybrids, Pt, Ni, and Ru are found to be effective in catalyzing proton/water but other metals (Pd, Au, Ag, Cu) showed very low activities with the following order: Pt > Ni > Ru > Pd > Au > Ag > Cu. The enhanced hydrogen production in the binary and ternary hybrids is ascribed partially to suitably positioned work functions among the hybrid components and thereby vectorial charge transfer through the work function energy gradient. Detailed surface studies were also described using Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).