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

Abstract

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 S2- 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