Coaxial Heterogeneous Structure of TiO2 Nanotube Arrays with CdS as a Superthin Coating Synthesized via Modified Electrochemical Atomic Layer Deposition

We report the fabrication and characterization of CdS/TiO2 nanotube-array coaxial heterogeneous structures. Such structures may potentially be applied in various photocatalytic fields, such as water photocatalytic decomposition and toxic pollutant photocatalytic degradation. Thin films of CdS are conformally deposited onto TiO2 nanotubes using a modified method of electrochemical atomic layer deposition. We propose that such nanostructured electrodes can overcome the poor absorption and high charge-carrier recombination observed with nanoparticulate films. The practical electrochemical deposition technique promotes the deposition of CdS onto the TiO2 tube walls while minimizing deposition at the tube entrances, thus preventing pore clogging. The coaxial heterogeneous structure prepared by the new electrochemical process significantly enhances CdS/TiO2 and CdS/electrolyte contact areas and reduces the distance that holes and electrons must travel to reach the electrolyte or underlying conducting substrate. This results in enhanced photon absorption and photocurrent generation. The detailed synthesis process and the surface morphology, structure, elemental analysis, and photoelectrochemical properties of the resulting films with the CdS/TiO2 nanotube-array coaxial heterogeneous structure are discussed. In comparison with a pure TiO2 nanotube array, a 5-fold enhancement in photoactivity was observed using the coaxial heterogeneous structure. This methodology may be useful in designing multijunction semiconductor materials for coating of highly structured substrates.