Photoelectrochemical hydrogen production using CdS nanoparticles photodeposited onto Li-ion-inserted titania nanotube arrays

This study reports the synthesis of cadmium sulfide (CdS) nanoparticles on Li⁺-inserted TiO₂ nanotube array (Li-TNA) to fabricate Li-TNA/CdS heterojunction electrodes for photoelectrochemical (PEC) hydrogen production under air mass (AM) 1.5 light and solar visible light (λ > 420 nm). For fabrication of the heterojunction, Li⁺ is rapidly inserted into TNA pre-grown on Ti foil, and CdS is then photodeposited onto the Li-TNA electrodes for varying deposition times. Surface analyses reveal that sub-100-nm polycrystalline CdS particles partly cover the Li-TNA (length: ∼800 nm, pore diameter: ∼100 nm), enabling the heterojunction to utilize AM 1.5 light as well as visible light. In aqueous solutions of sulfide and sulfite, the Li-TNA/CdS exhibits an incident photon-to-current efficiency (IPCE) of ∼20% (λ = 420 nm) while generating H₂ at a Faradaic efficiency of ∼100%. This PEC performance is superior to that of TNA/CdS, which is attributed to the Li⁺-enhanced charge transfer at the TNA/CdS interface. Electrochemical impedance analysis shows that the charge-transfer resistance of the TNA is reduced by ∼60% by Li⁺ insertion. Time-resolved photoluminescence decay profiles further reveal that the charge transfer in Li-TNA is completed within 0.8 ns, which is ∼33% faster than that in TNA. The sample surface is analyzed using scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible spectroscopy, and the PEC behavior of the samples is discussed in detail.