Laser-Induced Surface Modification at Anatase TiO₂ Nanotube Array Photoanodes for Photoelectrochemical Water Oxidation

Abstract

Nanostructured titanium dioxide (TiO 2 ) presents considerable potential as a photoanode in low cost, sustainable photoelectrochemical systems for solar water splitting. The wide band gap combined with the presence of trap states and reduced water oxidation kinetics limit, however, the photocurrent performance to only ∼1 mA cm −2 . Increasing the disorder of the crystal structure at the surface, on the other hand, has been proven to increase light absorption via band gap narrowing, and conversion efficiency. In this work, anodized TiO 2 nanotubes have been irradiated with a pulsed UV laser in deionized water environment to introduce lattice disorder. As a result, the photocurrent improved by 1.6-fold under simulated sunlight compared with pristine TiO 2 nanotube arrays at 1.23 V RHE . For all samples the water oxidation reaction kinetics is determined to be the limiting step for the solar-to-current conversion at low bias (0.5−0.7 V RHE ), while modified nanotube arrays display a 78% water oxidation selectivity at 1.23 V RHE , compared to 65% for the pristine TiO 2 nanotubes. The electronic density of states of the modified nanotubes is evaluated using electrochemical impedance spectroscopy, revealing that selective laser irradiation improved the number density of shallow donors while reducing the density of deep trap states.

The In-Depth Studies of Pulsed UV Laser-Modified TiO2 Nanotubes: The Influence of Geometry, Crystallinity, and Processing Parameters

“…The thickness of the re-solidified layer is ca. 300 nm for all samples, and the same observation was reported by Xu et al [20]. Raman spectra recorded for laser annealed titania samples, as well as for reference ones, are shown in Figure 3.…”

“…In the case of laser annealed crystalline TiO 2 nanotubes, the narrowing of the energy band gap can be easily observed. This phenomenon can be related to the introduction of additional states, so-called "shallow donors", resulting from the degradation of the crystal structure [20]. However, on the contrary to the as-anodized samples, the values of bandgaps for calcined TiO 2 NTs modified by laser rise with the length of the nanotubes.…”

“…Moreover, since Raman spectra recorded from laser annealed samples were not compared with calcined ones the results did not provide full information about the process. Xu et al [20] modified calcined titania NTs by means of pulsed 248 nm laser, and the irradiation under optimum conditions resulted in enhanced photoelectrochemical response. However, that approach is limited to the small samples due to the requirement of their immersion in deionized water and their rotation with a motorized actuator to homogenize area of modification.…”

The Effect of Laser Re-Solidification on Microstructure and Photo-Electrochemical Properties of Fe-Decorated TiO2 Nanotubes

“…Since the recombination rate of photo-generated minority carriers depends on the diffusion path length to the electrode surface, where they can take part in a water molecule oxidation [13], the structural engineering is especially focused in a way to produce highly porous structures. As it has been already shown [14][15][16][17][18][19][20], the preparation of TiO 2 in a form of various nanostructures, like nanobelts, nanorods or nanotubes drastically improves photo-electrodes performance in a reference to the plane material. However, its tubular structure synthesized by anodization is most frequently examined due to perpendicular orientation of hollow cylinders regarding the metal substrate reducing the charge recombination and the scalability of the fabrication process [15,21].…”

“…That technique is extremely fast, is easy-scalable for large areas, and due to a wide range of processing parameters can be applied to various materials [39]. The modification of anatase TiO 2 -NTs electrode using Nd:YAG laser was shown to enhance charge carrier donor density over one order of magnitude and a shift of the flat-band potential up to +0.7 V vs. Ag|AgCl|0.1 M KCl [16,39]. However, the modification leads to the photo-response improvement by no more than a few tens of percent.…”