Dual CdS Nanoparticle-Deposited Vertically Aligned Titanate Nanotube Heterostructure Photoanode

Dual CdS nanoparticles improve the photoelectrochemical (PEC) performance of vertically aligned titanate nanotubes (i.e., VA-TNTs). This is achieved by first depositing a CdS layer via an ion-exchange method (i.e., CdS/VA-TNT-10), followed by nanoconfined CdS nanoparticles over CdS/VA-TNT-10 (i.e., CdS/VA-TNT-2S) via a vertically successive chemical bath deposition reaction. Field emission scanning electron microscopy and transmission electron microscopy results confirm that the morphology of monolayer CdS resembles large protrude spherical size "dew drops" due to the accumulation of low-concentration CdS nanoparticles at the inner pores of the titanate nanotubes over a long reaction period. The time-restricted high-concentration ultrafine nanoparticle monolayer (CdS/VA-TNT-SiL) shows 2-fold enhancement in the photocurrent density as compared to the CdS/VA-TNT-10 in the mixed sulfide Na₂S/Na₂SO₃ electrolyte. Also, the photocurrent density for the double-layered CdS/VA-TNT-2S heterostructure photoanode is 3 times higher than that for the CdS/VA-TNT-1S photoanode. This excellent PEC performance is ascribed to the manner in which the calcination process establishes an appropriate heterojunction at the interface of CdS nanoparticles after each deposition, which allows the maximum solar visible light-to-energy conversion and efficient charge separation. Meanwhile, the base VA-TNT substrate provides a facile photogenerated electron-hole transport, which reduces the recombination losses in the CdS/VA-TNT-2S photoanode.