Photoelectrochemical Water Oxidation Characteristics of Anodically Fabricated TiO2 Nanotube Arrays: Structural and Optical Properties (original) (raw)
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The present work demonstrates for the first time the facile fabrication of TiO2 nanotube arrays (TNTAs) by a fluoride-free solid-state anodization process using LiClO4 containing solid polymeric electrolyte. The resulting nanotubes were tested for photoelectrochemical water splitting. The elimination of liquid electrolytes in electrochemical anodization constitutes a paradigm shift for the formation of nanoporous and nanotubular metal oxides. Our results open a new area of research that uses the distinctive properties of solid polymer electrolytes to achieve targeted doping and nano-morphologies. Characterization of the grown TNTAs indicated solid state anodized TNTAs to consist purely of the anatase phase of titania. The solid-state anodization process provides several advantages over conventional liquid electrolytes such as easy handling and processing, better charge transport, environmentally benign chemicals and methodology. Photoelectrochemical water splitting experiments were performed which confirmed the viability of TNTAs grown by the new solid-state process for photocatalytic applications.
Journal of Solid State Electrochemistry, 2017
Efficient photoanodes are designed of vertically aligned anatase TiO 2 nanotube arrays (anatase TNTAs) decorated with different shaped rutile TiO 2 structures (particles, 1D n a n o r o d s , 3 D m i c r o f l o w e r s) t o i m p r o v e t h e photoelectrochemical water oxidation performance of pristine TNTAs. Anatase TNTAs were prepared by anodic oxidation of Ti substrate in NH 4 F electrolyte, and the rutile percent and shape are controlled by tuning the TiCl 4 treatment time (20-120 min). The effects of treatment time on the morphology, c r y s t a l s t r u c t u r e , b a n d g a p , p h o t o c u r r e n t , a n d photoconversion efficiency are characterized by FESEM, HRTEM, XRD, UV-Vis diffuse reflectance spectroscopy, and photoelectrochemical measurements. XRD data confirmed that TiCl 4 treatment induced the formation of the rutile phase of TiO 2 over anatase TNTAs and the rutile amount in the TNTAs was increased upon increasing the treatment time. Additionally, the band gap of TNTAs was gradually decreased by increasing the rutile percent. The optimum treatment time is 80 min, which produces TiO 2 photoanode array that possess the following characteristics: rutile/anatase mixed phase, nanorode/nanotube mixed morphology, 3.09 eV band gap, two times increase in photocurrent, and almost more than twofold enhancement in the photoconversion efficiency relative to pure TNTA.
The Journal of Physical Chemistry C, 2009
A significant enhancement in the photoconversion efficiency of anodically grown titania nanotube array photoanodes is observed when crystallized using an infrared annealing process. This infrared (IR) annealing treatment is performed over relatively short periods, 5-15 min, over the temperature range 300-600°C with the anatase crystallite size increasing with annealing temperature and duration. Used as photoanodes to photoelectrochemically split water, the 15 min, 600°C IR crystallized nanotube arrays show a three-electrode photoconversion efficiency of 13.13% under UV illumination (100 mW/cm 2 , 320-400 nm, 1 M KOH). A reduction in the carrier trap states and an increased charge carrier transport due to reduction of the barrier layer thickness are believed responsible for the significant conversion efficiency seen with the IR annealed samples.
A significant enhancement in the photoconversion efficiency of anodically grown titania nanotube array photoanodes is observed when crystallized using an infrared annealing process. This infrared (IR) annealing treatment is performed over relatively short periods, 5-15 min, over the temperature range 300-600 degrees C with the anatase crystallite size increasing with annealing temperature and duration. Used as photoanodes to photoelectrochemically split water, the 15 min, 600 degrees C IR crystallized nanotube arrays show a three-electrode photoconversion efficiency of 13.13% under UV illumination (100 mW/cm(2) 320-400 nm, 1 M KOH). A reduction in the carrier trap states and an increased charge carrier transport due to reduction of the barrier layer thickness are believed responsible for the significant conversion efficiency seen with the IR annealed samples
Nature Publishing Group, 2018
Nowadays, increasing awareness of environment and fossil fuels protection stimulates intensive research on clean and renewable sources of energy. Production of hydrogen from water through solar-driven splitting reactions is one of the most promising approaches in the field of photoelectrochemistry (PEC). In this work we have fabricated well-aligned, highly-ordered, smooth-mouth TiO 2 nanotube arrays (TNAs) in a two-step anodization process of titanium foil, which were then used as photoelectrodes for PEC water splitting. It demonstrates for the first time correspondence between non-linear component characteristics of multiscale rough surface and crystalline structure of annealed TNAs measured at various fabrication stages and their photoelectrochemical response. The as-anodized TNAs with isotropic surface (deduced from AFM and SEM images) and largest figure of merit (according to their PEC performance) were annealed at 450 °C in air. Scale-invariant descriptors of the surface structure of the deposits involved: fractal dimension, corner frequency, roughness, size of nanostructures and their dominant habits. Moreover, X-ray diffraction data processed using the Rietveld method confirmed coexistence of various oxides, for example: TiO 2 in the form of anatase, TiO and Ti 3 O 5 phases in the TNAs under study pointing that previous well-established mechanisms of the TNA growth were to certain degree incomplete. Photoelectrochemical (PEC) water splitting is one of the most favorable approaches for H 2 production as a clean energy vector of the future. Since the work by Fujishima and Honda in 1972 1 , increasing research has been carried out towards this issue using electrodes made of various materials, e.g. semiconductors. Unfortunately, their practical application has encountered a number of technical complications. Among the metal oxides that has been taken into consideration, titanium dioxide (TiO 2) is found to be promising in PEC water splitting 2-9 due to its appropriate band-gap structure, superior chemical and optical stability and low cost. In particular, TiO 2 nanotube arrays prepared in anodization processes have numerous advantages over TiO 2 nanoparticle films resulting from facile preparation procedure, high surface-to-volume ratio for contact with the electrolyte, large light harvesting efficiency improved by light scattering into tubular morphology, and high electron mobility induced by their unidirectional channel 10,11. Some strategies such as doping or semiconductor heterocoupling were used for modification of TiO 2 nanotube arrays to be activated under visible light 12-14 .
Described is the synthesis of TiO2 nanotube array films by anodization of Ti foil in HCl electrolytes containing different H2O2 concentrations. Highly ordered nanotube arrays up to 860 nm in length, 15 nm inner pore diameter, and 10 nm wall thickness were obtained for one hour anodizations using a 0.5 M HCl aqueous electrolyte containing 0.1-0.5 M H2O2 concentrations for anodization potentials between 10-23 V. The use of ethylene glycol as the electrolyte medium significantly alters the anodization kinetics and resulting film morphologies; nanotube bundles several microns in length achieved for anodization potentials between 8 V and 18 V in only a few minutes. The nanotube arrays obtained from the ethylene glycol electrolytes show relatively higher photocurrents, approximate to 0.8 mA cm(-2) under AM 1.5. Under 100 mW cm(-2) AM 1.5 illumination a 500 degrees C annealed 1 cm(2) nanotube array sample, obtained by anodization of a Ti foil sample in ethylene glycol + 0.5 M HCl + 0.4 M H2O2 electrolyte, demonstrates a hydrogen evolution rate of approximately 391 mu L h(-1) by water photoelectrolysis, time-power normalized evolution rate of 3.9 mL W-1 h(-1), with water splitting confirmed by the 2 : 1 ratio of evolved hydrogen to oxygen
MRS Proceedings, 2011
The present work reports the synthesis of self-organized strontium-doped titania nanotubes arrays as a potential material for photocatalytic water splitting. Electrochemical anodization process was used to grow such material under various electrochemical conditions. The effect of dopant concentration on the morphology and photoelectrochemical properties of the material was investigated. The microstructure, morphology and composition of as-prepared and heat treated nanotubes were characterized by field emission scanning electron microscopy (FESEM), x-ray diffraction (XRD), transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS). The results showed that increasing the dopant concentration up to its solubility limit results in higher photoelectrochemical activity. A preliminary proof of concept of the photocatalytic activity of the fabricated material was estimated in terms of the use of such material as a photoanode for photoelectrochemical water splitting.
International Journal of Hydrogen Energy, 2012
Sonication assisted anodization of titanium in a fluorinated ethylene glycol and water electrolyte using Ti itself as a cathode is investigated. The prepared anodic film has a highly ordered nanotube-array surface architecture. The resulting TiO 2 nanotubes at potential 20e40 V have various diameters (30e100 nm), tube length (3e12 mm) and wall thicknesses (6e15 nm). The tube diameter and wall thickness are increased with the anodization time while the overall length of the nanotube arrays is controlled by the duration of the anodization time. In addition, apart from the anodization time, formation of nanotubes is governed by the distance and supplied voltages between the two electrodes, for a given electrolyte. The crystal structure and surface morphology of the annealed anodic films are investigated by XRD and SEM, respectively. The corresponding photoelectrochemical water splitting efficiency (PCE) was calculated under UV light. Our results show a very high PCE under UV (315e400 nm, 100 mW/cm 2) irradiation. The maximum value of PCE for hydrogen generation obtained was 29% which is one of the best results reported in literature [1].
Aminated TiO2 nanotubes as a photoelectrochemical water splitting photoanode
Catalysis Today, 2016
The present work reports on the enhancement of TiO 2 nanotubes photoelectrochemical water splitting rate by decorating the nanostructure with an amine layer in a hydrothermal process using diethylenetriamine (DETA). The aminate coated TiO 2 tubes show a stable improvement of the photoresponse in both UV and visible light spectrum and under hydrothermal conditions, 4-fold increase of the photoelectrochemical water splitting rate is observed. From intensity modulated photocurrent spectroscopy (IMPS) measurements significantly faster electron transport times are observed indicating a surface passivating effect of the Ndecoration.
In recent years, considerable efforts have been made to design and discover photoactive nanostructured materials that can be used as anodes in water photoelectrolysis cells. Herein, we report on the growth of a novel photoanode material composed of self-ordered, vertically oriented nanotube arrays of titanium–palladium mixed oxynitride films via anodization of Ti–Pd alloy in an electrolyte solution of formamide containing NH4F at room temperature, followed by annealing in an ammonia atmosphere. The nanostructure topology was found to depend on both the anodization time and the applied voltage. Our results demonstrate the ability to grow mixed oxynitride nanotube array films that are several micrometers thick. The Ti–Pd oxynitride nanotube array films were utilized in solar-spectrum water photoelectrolysis, demonstrating a photocurrent density of 1.9 mA/cm2 and a 5-fold increase in the photoconversion efficiency under AM 1.5 illumination (100 mW/cm2, 1.0 M KOH) compared to pure TiO2 nanotubes fabricated and tested under the same conditions. The obtained efficiency is among the highest reported values for a TiO2 nanotube-based photoelectrochemical cell. This enhancement in the photoconversion efficiency is related to the synergistic effects of Pd alloying, nitrogen doping, and the unique structural properties of the fabricated nanotube arrays.