Electrochemical preparation of photoelectrochemically active CuI thin films from room temperature ionic liquid

Cuprous iodide (CuI) thin films with photoelectrochemical activity were prepared by anodizing copper wire or copper-electrodeposited tungsten wire in the room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMI-PF6 RTIL) containing N-butyl-N-methylpyrrolidinium iodide (BMP-I). A copper coating was formed on the tungsten wire by potentiostatic electrodeposition in BMP-dicyanamide (BMP-DCA) RTIL containing copper chloride (CuCl). The CuI films formed using this method were compact, fine-grained and exhibited good adhesion. The characteristic diffraction signals of CuI were observed by powder X-ray diffractometry (XRD). X-ray photoelectron spectroscopy (XPS) also confirmed the formation of a CuI compound semiconductor. The CuI films demonstrated an apparent and stable photocurrent under white light illumination in aqueous solutions and in a RTIL. This method has enabled the electrochemical formation of CuI from a RTIL for the first time, and the first observation of a photocurrent produced from CuI in a RTIL. The coordinating strength of the anions of the RTIL is the key to the successful formation of the CuI thin film. If the coordinating strength of the anions of the RTIL is too strong, no CuI formation is observed.

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► CuI film can be formed by anodization of Cu in ionic liquid containing iodide.
► Coordinating strength of anion in ionic liquid determine the formation of CuI.
► Photocurrent of the CuI film can be observed in aqueous solution and in ionic liquid.
► Cu layer coated on conductive substrates can be converted to CuI.