CdS-TiO2 photoelectrodes Formed through Pulsed Electrodeposition of Cadmium on TiO2 Nanotube Films and Treatment with H2s for Application in Solar Cells

TiO₂ nanotubes films (TNAs), compared with classic nanocrystalline photoanodes, can accelerate electron transport and diminish the recombination probability, making it suitable for use as a photoelectrode in solar cells [1]. However, one of the critical drawbacks of TiO₂ is its wide band gap (3.0 and 3.2 eV for the rutile and anatase phases, respectively). That means, only ultraviolet region of the solar spectrum (about 7%) can be utilized by pure TiO₂. Therefore, different methods were developed to extend the light harvesting region of TiO₂, including dye sensitization and coupling with low band gap semiconductors. Coupled CdS/TNAs photoelectrode has been considered to be an important technique because CdS has a narrow band gap (2.4 eV) and its conduction band level is higher than that of TiO₂. Upon illumination, the excited electrons from CdS can be rapidly transferred to TNAs, arriving at the electron collectors through the nanotube microstructure [1]. Different methods have been reported to couple CdS with TNAs, being the most commonly used: sequential chemical bath deposition (SCBD) and electrochemical deposition. However, with these methods the CdS is obtained for precipitation reactions between ions Cd²⁺ and S^2- located in the interface electrode-electrolyte, therefore, no direct contact is ensured among CdS and TNAs. In this way, to guarantee direct contact between CdS and TNAs, an electrochemical/chemical (E/C) process was proposed in this work. This process is similar as that developed by Penner and his group to deposit CdS nanocrystals on HOPG substrates. TiO₂ nanotube films were grown by potentiostatic anodization of titanium foils at 30 V during 2 hours in 0.05 M NH₄F in ethylene glycol (10% water) electrolyte [2]. As-prepared films were rinsed with ethanol and water, left to air dry, and heat treated in ambient air at 450º C for 30 min (slope rate 10 ºCmin^-1). E/C method consists of three steps [3]: (1) Cd nanoparticles were deposited in heat treated TNAs by pulsed electrodeposition to different reduction potentials [4] and varying on and off times; (2) the modified films were annealing at 400ºC for 30 min (slope rate 10 ºCmin^-1) to form CdO/TNAs; (3) finally, the CdO/TNAs were heated in H₂S atmosphere at 300º C for 30 min (slope rate 10 ºCmin^-1) to convert CdO into CdS. The CdS/TNAs photoelectrodes were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV-Vis diffuse reflectance absorption spectra (DRS). We also investigated the photoelectrochemical behavior of the CdS/TNAs photoelectrodes under visible illumination.

Acknowledgements.The authors are indebted to the CONACyT (Mexico) for their financial support to carry out this work (Projects CB-2008/105655 and INFR-2011-1-163250). J.E. Carrera‑Crespo is grateful to CONACyT for the PhD fellowship granted. The authors thank to LDRX (T 128) UAM-I for XRD measurements.

References.

[1] X. Ma, Y. Shen, G. Wu, Q. Wu, B. Pei, M. Cao, F. Gu, “Sonication-assisted sequential chemical bath deposition of CdS nanoparticles into TiO2 nanotube arrays for application in solar cells”, J Alloy Compd (2012) 538: 61-65

[2] P. Acevedo-Peña, L. Lartundo-Rojas, I. González, “Effect of water and fluoride content on morphology and barrier layer properties of TiO2 nanotubes grown in ethylene glycol-based electrolytes”, J Solid State Electr (2013) 17: 2939-2947

[3] R. M. Penner, “Hybrid electrochemical/chemical synthesis of quantum dots”, Acc Chem Res (2000) 33: 78-86

[4] J. E. Carrera-Crespo, P. Acevedo-Peña, M. Miranda-Hernández, I. González, “Electrocrystallization of cadmium on anodically formed titanium oxide”, J Solid State Electr (2013) 17: 445-457