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Letter

Vertically Oriented Ti−Fe−O Nanotube Array Films: Toward a Useful Material Architecture for Solar Spectrum Water Photoelectrolysis

Gopal K. Mor, Haripriya E. Prakasam, Oomman K. Varghese, Karthik Shankar, and Craig A. Grimes*

Department of Electrical Engineering and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802

Nano Lett., 2007, 7 (8), pp 2356–2364

DOI: 10.1021/nl0710046

Publication Date (Web): July 3, 2007

Corresponding author. E-mail: cgrimes@engr.psu.edu.

Abstract

In an effort to obtain a material architecture suitable for high-efficiency visible spectrum water photoelectrolysis, herein we report on the fabrication and visible spectrum (380−650 nm) photoelectrochemical properties of self-aligned, vertically oriented Ti−Fe−O nanotube array films. Ti−Fe metal films of variable composition, iron content ranging from 69% to 3.5%, co-sputtered onto FTO-coated glass are anodized in an ethylene glycol + NH₄F electrolyte. The resulting amorphous samples are annealed in oxygen at 500 °C, resulting in nanotubes composed of a mixed Ti−Fe−O oxide. Some of the iron goes into the titanium lattice substituting titanium ions, and the rest either forms α-Fe₂O₃ crystallites or remains in the amorphous state. Depending upon the Fe content, the band gap of the resulting films ranges from about 380 to 570 nm. The Ti−Fe oxide nanotube array films are utilized in solar spectrum water photoelectrolysis, demonstrating 2 mA/cm² under AM 1.5 illumination with a sustained, time−energy normalized hydrogen evolution rate by water splitting of 7.1 mL/W·hr in a 1 M KOH solution with a platinum counter electrode under an applied bias of 0.7 V. The surface morphology, structure, elemental analysis, optical, and photoelectrochemical properties of the Ti−Fe oxide nanotube array films are considered.

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