Preparation and characterization of metal oxide/carbon nanotube nanocomposites for photocatalytic and photo-electrocatalytic hydrogen production: A review

Abstract

The study of metal oxide and carbon nanomaterials for hydrogen evolution reaction (HER) catalysis stands out among the innovative solutions required by the rapid advancement of renewable energy. The potential of metal oxide catalysts to produce hydrogen is promising. In parallel, a lot of attention is being given to carbon nanotubes because of their numerous applications in environmental remediation and renewable energy. Despite an extensive amount of research on these materials, their composites, metal oxide-carbon nanotubes (MO-CNTs), remain relatively underexplored for their photocatalytic and photoelectrocatalytic (PEC) capabilities. This comprehensive study fills this knowledge gap by concentrating on the techniques used in producing and characterizing MO-CNTs composites. Aiming ways of developing methods to produce green hydrogen energy that is cost-effective. The most economical method of producing hydrogen remains to be photocatalysis. One of the important properties in photocatalysis is the band gap of the material. Many metal oxides are reported to have a high band gap; for example, recent reports indicate that TiO₂ has a band gap of 3.11 eV. Upon forming the composite with CNTs, the band gap was reduced to 3.09 eV. The reduction in the band gap is responsible for improved hydrogen production. To further understand the composites of MO-CNTs, characterization techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), cyclic voltammetry (CV), and chronoamperometry (CA), were thoroughly discussed. These techniques assist in understanding and improving the MO-CNTs composites and the synergy between the MO and CNTs. The synergistic effects of the metal oxides and carbon nanotubes are responsible for the outstanding enhancement in hydrogen production under light irradiation, as revealed by photocatalytic tests.