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

• Published in: Nano letters 2007-08, Vol.7 (8), p.2356-2364
• Main Authors: Mor, Gopal K, Prakasam, Haripriya E, Varghese, Oomman K, Shankar, Karthik, Grimes, Craig A
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Cross-disciplinary physics: materials science; rheology; Crystallization - methods; Electrolysis - methods; Electron states and collective excitations in thin films, multilayers, quantum wells, mesoscopic and nanoscale systems; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Exact sciences and technology; Fullerenes and related materials; Hydrogen - chemistry; Iron - chemistry; Macromolecular Substances - chemistry; Materials science; Materials Testing; Membranes, Artificial; Molecular Conformation; Nanoscale materials and structures: fabrication and characterization; Nanotechnology - methods; Nanotubes; Nanotubes - chemistry; Nanotubes - ultrastructure; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures; Oxygen - chemistry; Particle Size; Photochemistry - methods; Physics; Solar Energy; Surface Properties; Titanium - chemistry; Visible and ultraviolet spectra; Water - chemistry
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/nl0710046

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.