This paper describes the design of a photoelectrochemical (PEC) cell using carbon-doped titanium dioxide (TiO2 - x C x ) nanotube arrays as the photoanode and platinum, Pt nanoparticles incorporated in TiO2 (titania) nanotube arrays, as the cathode. The PEC cell is found to be highly efficient (i.e., gives good photocurrent at a low external bias, j p = 2.5−2.8 mA/cm2 at −0.4 VAg/AgCl), inexpensive (only 0.4 wt % Pt on TiO2), and robust (continuously run for 80 h without affecting the photocurrent) for hydrogen generation by water splitting under the illumination of simulated one sun intensity. The synthesis of the photoanode is carried out by the sonoelectrochemical anodization technique using aqueous ethylene glycol and ammonium fluoride solution. This anodization process gives self-organized hexagonally ordered TiO2 nanotube arrays with a wide range of nanotube structure, which possess good uniformity and conformability. As-synthesized titania nanotubes are annealed under reducing atmosphere (H2), which converts the amorphous nanotube arrays to photoactive anatase phase as well as it helps in doping of the carbon (from the reduction of ethylene glycol) to give the TiO2- x C x type photoanode. The cathode material is prepared by synthesizing Pt nanoparticles (by reduction of a Pt salt to Pt(0)) into the titania nanotubular arrays by the incipient wetness method. Various characterization techniques, viz., field emission scanning electron microscopy, energy dispersive spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, electron diffraction, and glancing angle X-ray diffraction, etc., are used to study the morphology, phase, band gap, and doping of the nanotubes.

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Two different configurations of photoanodes based on anodic iron oxide were investigated for photoelectrochemical water oxidation. A self-ordered and vertically oriented array of iron oxide nanotubes was obtained by anodization of pure iron substrate in an ethylene glycol based electrolyte containing 0.1M NH4F + 3 vol% water (EGWF solution) at 50 V for 15 min. Annealing of the oxide nanotubes in a hydrogen environment at 500 °C for 1 h resulted in a predominantly hematite phase. The second type of photoanode was obtained by a two-step anodization procedure. This process resulted in a two-layered oxide structure, a top layer of nano-dendrite morphology and a bottom layer of nanoporous morphology. This electrode configuration combined the better photocatalytic properties of the nano-dendritic iron oxide and better electron transportation behaviour of vertically oriented nano-channels. Annealing of these double anodized samples in an acetylene environment at 550 °C for 10 min resulted in a mixture of maghemite and hematite phases. Photocurrent densities of 0.74 mA cm−2 at 0.2 VAg/AgCl and 1.8 mA cm−2 at 0.5 VAg/AgCl were obtained under AM 1.5 illumination in 1M KOH solution. The double anodized samples showed high photoconductivity and more negative flat band potential (−0.8 VAg/AgCl), which are the properties required for promising photoanode materials.

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