Carbon-incorporated TiO2 photoelectrodes prepared via rapid-anodic oxidation for efficient visible-light hydrogen generation (original) (raw)
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Photo-electrochemical generation of hydrogen using hybrid titanium dioxide nanotubular arrays
2006
The effects of heat-treatment, Pt modification of the surface and surface oxide layer on the efficiency for the photoelectrochemical generation of hydrogen were investigated at p-CdTe films deposited electrochemically at-0.35V (vs. Ag/AgC1) from an aqueous sulfuric acid solution of pH = 1.4 containing 1M CdSO4 and 1 mM TeOz. By heat-treatment in a He atmosphere, the crystalline diameter increased and, thus, the number of grain boundaries at which effective electron-hole recombination takes place decreased. Free metallic Te existing near the surface, which acts as a recombination center, was removed by the treatment at the temperatures above 350~ The cathodic photocurrent was increased by the heat-treatment due to these two effects. The cathodic photocurrent was decreased and even the anodic photocurrent was observed at the films treated at too high a temperature and for too long a time. The films were converted to n-type by these treatments because of the removal of the lattice Te from CdTe crystal. Pt modification, either vacuum deposition or codeposition, improved the efficiency, but the photocurrent is small, suggesting that the major reason for the low efficiency is still the bulk recombination. The oxide layer was easily formed on the CdTe films by exposure to air but dissolved by contacting with solution and had no effect on the photoelectrochemical properties of the film.
Carbon and potassium-embedded TiO 2 nanotube arrays were rapidly formed via anodic oxidation of the Ti metal in ethylene glycol (EG) containing potassium hydroxide (KOH). The incorporation of KOH allowed the simultaneous control of electrochemical oxidation and chemical dissolution, resulting in the equilibrium growth of nanotube arrays with a maximum growth rate of ∼353 nm min −1 . The anodic growth of nanotube arrays in the hydroxyl (OH)-rich environment induced the formation of anatase crystallites by bridging between the dissociated H 2 O molecules and OH group of octahedra in TiO 2 . High aspect ratio nanotube arrays with a large pore size formed in EG electrolyte containing KOH could efficiently harvest the light energy, thereby enhancing the photocatalytic efficiency. High reaction sites of nanotube arrays with high surface area promoted the diffusion of charge carriers to the electrolyte. Furthermore, the strong e − donation nature of adsorbed-potassium species on nanotubes facilitated the photoelectrochemical properties. Nanotube arrays formed in EG electrolyte containing 1 wt% of 1.0 M KOH exhibited a remarkable capability to generate hydrogen at an evolution rate up to ∼658.3 L min −1 cm −2 and the photoconversion efficiency of ∼2.5%.
2009 Photoeletrochemical generation of hydrogen over carbon-doped TiO2 photoanode (Zhou et al).pdf
Nanostructured carbon-doped TiO 2 (C-TiO 2 ) thin film was prepared by pulsed laser deposition (PLD) on ITO substrate. Results of X-ray diffraction (XRD) analysis of the thin film showed that the crystal structure of the C-TiO 2 was a hybrid of rutile and anatase. Data of X-ray photoelectron spectroscopic (XPS) analysis confirmed the successful incorporation of carbon into TiO 2 molecules, which formed defects. These defects narrowed the band gap of the C-TiO 2 from 3.25 eV of pure TiO 2 to 3.15 eV. The photoelectrochemical property of the C-TiO 2 film was examined by evaluating the efficiency of hydrogen generation by water splitting in a two-compartment electrochemical system. The fastest rate of gas production was obtained when the thin film was irradiated by light at a wavelength of 325 nm; the photoresponse under visible radiation was limited, however. Applying a bias potential across the photoanode and its counter electrode can increase hydrogen generation effectively. Organic substance added to the photoanodic chamber was photocatalytically degraded which increased the photocurrent and subsequently enhanced hydrogen generation. ß
International Journal of Hydrogen Energy, 2008
Carbon modified n-type titanium oxide ðCM-n-TiO 2 Þ photoelectrodes were synthesized by flame oxidation of 0.2 mm thick Ti metal sheets for hydrogen production by photoelectrochemical splitting of water. The photocurrent density at optimized CM-n-TiO 2 photoelectrodes (synthesized using flame temperature of 825 C and oxidation time of 15 min) was found to be 6:38 mA cm À2 at a minimal external applied potential of 0.24 V under illumination with light intensity of 70 mW cm À2 from a 150 Watt Xenon Lamp. The same value of photocurrent density was found when the wavelength dependent photocurrent densities at the same applied potential of 0.24 V were integrated. This optimized CM-n-TiO 2 photoelectrode was found to split water with maximum photoconversion efficiency of 9.02% under white light illumination. Also, a very close value of photoconversion efficiency of 9.01% was obtained from the wavelength dependent photocurrent density, j p ðlÞ under monochromatic light illumination. Carbon modification lowered the original bandgap energy from 3.0 to 2.65 eV and generated a mid-gap band at 1.6 eV above the valence band of the optimized CM-n-TiO 2. Photocurrent measurements, UV-Vis spectra, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to characterize CM-n-TiO 2 photoelectrodes.
Photoeletrochemical generation of hydrogen over carbon-doped TiO2 photoanode
Applied Catalysis B-environmental, 2009
Nanostructured carbon-doped TiO 2 (C-TiO 2) thin film was prepared by pulsed laser deposition (PLD) on ITO substrate. Results of X-ray diffraction (XRD) analysis of the thin film showed that the crystal structure of the C-TiO 2 was a hybrid of rutile and anatase. Data of X-ray photoelectron spectroscopic (XPS) analysis confirmed the successful incorporation of carbon into TiO 2 molecules, which formed defects. These defects narrowed the band gap of the C-TiO 2 from 3.25 eV of pure TiO 2 to 3.15 eV. The photoelectrochemical property of the C-TiO 2 film was examined by evaluating the efficiency of hydrogen generation by water splitting in a two-compartment electrochemical system. The fastest rate of gas production was obtained when the thin film was irradiated by light at a wavelength of 325 nm; the photoresponse under visible radiation was limited, however. Applying a bias potential across the photoanode and its counter electrode can increase hydrogen generation effectively. Organic substance added to the photoanodic chamber was photocatalytically degraded which increased the photocurrent and subsequently enhanced hydrogen generation.
Chemistry: A European Journal, 2017
In the present work we report on the key factors dictating the photoelectrochemical (PEC) performance of suboxide titania (TiO x)n anotubes.T iO x nanotubes were produced by asystematic variation of reduction heat treatments of TiO 2 in Ar/H 2 .T he properties of the TiO x tubes were investigated by electron paramagnetic resonance (EPR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), solid-state conductivity,r eflectivitym easurements, photocurrents pectroscopy,a nd photoelectrochemi-cal hydrogen evolution. In line with earlier literature, these suboxide tubes show ad rastically improved photoelectrochemicalw ater-splitting performance compared to non-reduced anatase TiO 2 tubes. In this work we show that the key improvement in water-splitting performance is due to the strongly improved conductivity of TiO x semimetalic tubes, reaching1 3.5 KW per tube compared to 70 MW (for non-reduced anatase), and is not due to the enhanced visible-light absorbance.
2009
Two different configurations of photo anodes based on anodic iron oxide were investigated for photo electrochemical water oxidation. Self ordered and vertically oriented array of iron oxide nanotubes was obtained by anodization of pure iron substrate in ethylene glycol based electrolyte containing 0.1 M NH4F + 3 vol% water (EGWF solution) at 50 V for 15 minutes. Annealing of the oxide nanotubes in hydrogen environment at 500 °C for 1 h resulted in predominantly hematite phase. The second type of photo anode 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 photo catalytic properties of the nano-dendritic iron oxide and better electron transportation behavior of vertically oriented nano-channels. Annealing of these double anodized samples in acetylene environment at 550 °C for 10 minutes resulted in a mixture of maghemite and hematite phases. Photo current densities of 0.74 mA/cm2 at 0.2 VAg/AgCl and 1.8 mA/cm 2 at 0.5 VAg/AgCl were obtained under AM 1.5 illumination in 1 M KOH solution. The double anodized samples showed high photo conductivity and more negative flat band potential (-0.8 VAg/AgCl), which are the properties required for promising photo anode materials. Apart from the above work, mild steel which is 10 times less the cost of Ti is also being tested for its photoelectrochemical properties. TiO2 nanotubes synthesized and annealed in different conditions are compared for their quantum efficiency is also carried out in this work. Quantum efficiency measurements gives more reliable and photocurrent data towards photoelectrochemical applications.
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].
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