Enhanced Photoelectrochemical Activity of the TiO2/ITO Nanocomposites Grown onto Single-Walled Carbon Nanotubes at a Low Temperature by Nanocluster Deposition (original) (raw)
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ACS Sustainable Chemistry & Engineering, 2019
The effect of calcination temperature on the photoelectrochemical properties of TiO 2 nanotube arrays (TNTAs) has been investigated in many studies. Most work focused on improving the photoelectrochemical properties through optimization of the microstructure. In this paper, however, an anatase/rutile phase junction formed in TiO 2 nanotubes has been demonstrated to account for the enhancement of the photoelectrochemical performance. Observations by the UVvisible diffuse reflectance spectra, glancing incidence angle X-ray diffraction (GIA-XRD) and electrochemical impedance spectroscopy indicate that the rutile fraction is at the bottom of the nanotubes while the anatase fraction at the body of the nanotubes. The TNTAs with the coexistence of about 60% anatase and 40% rutile exhibit the optimal performance and show the 1.4 times improved photocurrent density compared with the pure anatase TNTAs. Detailed synchrotron radiation photoemission spectroscopy further confirms the existence and effect of the phase junction. The results suggest photogenerated electrons transfer from the rutile phase to anatase phase in the nanotubes due to the band edge alignment, which facilitates the photogenerated carriers separation and transport along the nanotubes and leads to apparent enhancement of the photoelectrochemical behavior.
Solar Energy Materials and Solar Cells, 2015
In this study, we demonstrate the evolution of morphology and band gap tuning of TiO 2 nanotube (TNT) arrays by an electrochemical anodization method. Both XRD and Raman spectra confirm the crystallinity of TNT arrays in a phase pure anatase form. The crystallinity of TNT increases up to the anodization time of 60 min and with further increase in time, the crystallinity decreases due to the destruction of tubular structures. By increasing the anodization time, the optical absorption of TiO 2 is extended up to the visible region. The well defined visible light photoluminescence emissions reveal the structural defects, oxygen vacancies (F or F 2 þ , F þ) and radiative recombination sites created within the band gap. The nanotubes fabricated at an anodization time of 60 min show the highest photoconversion efficiency of 11.86% in the potassium hydroxide (KOH) electrolyte containing ethylene glycol.
Enhancement and limits of the photoelectrochemical response from anodic TiO[sub 2] nanotubes
Applied Physics Letters, 2005
TiO 2 nanotube layers were grown on titanium by a self-organized anodic oxidation. The layers consist of arrays of individual tubes with a length of ϳ2 m, a diameter of ϳ100 nm, and a wall thickness of ϳ10 nm. These layers can be annealed to an anatase structure which strongly increases the photocurrent efficiency. Moreover, the nanotube layers can-under certain conditions-exhibit a drastically enhanced photocurrent compared to compact anatase layers. These strong changes in the photoresponse are attributed to the characteristics of the space charge layer within the tube wall.
Applied Surface Science, 2014
We report the significance of crystallinity on photoelectrochemical and the photocatalytic degradation of methyl orange of titanium dioxide (TiO 2) nanotube arrays. The TiO 2 nanotube arrays are fabricated by electrochemical anodization of titanium substrates in fluoride based aqueous electrolyte for various anodization time. The degree of crystallinity and phase purity (anatase) is confirmed from X-ray diffraction and Raman spectra. High resolution scanning electron microscope is used to analyze the surface morphology of forming nanotubes. The UV-visible absorption spectrum shows the enhanced absorption in the visible region which is further confirmed using photoluminescence spectra. The photoelectrochemical properties of the prepared samples are studied from linear sweep photovoltammetry measurements and a maximum photocurrent density of 1.32 mA/cm 2 is observed. The enhanced photoelectrochemical activity is attributed to the higher crystallinity which increases the charge carrier separation and extends its light absorption from ultraviolet to visible region owing to lower band gap of 2.751(7) eV.
Anatase-type TiO 2 nanotube arrays (TiO 2-NTAs) were grown on Ti foil by anodic oxidation in CH 3 COOH/NH 4 F solutions followed by thermal treatment. The surface of TiO 2-NTAs was further decorated by palladium and silver metal clusters through a chemical-reduction method and its photocatalytic activity was tested by investigating the degradation of p-nitrophenol (PNP) in aqueous solution under visible-light irradiation and electrical polarization. The effects of preparation variables both on microstructural properties of samples and photocatalytic activity were examined by using the 3D response surface and the 2D contour plots. The experimental investigations carried out by using XRD, SEM, HRTEM, EDS, XRF, ICP-AES, XPS, DRS, and PL, demonstrated a strong relation between the phase structure and the photocatalytic activity of TiO 2-NTAs. Titania nanotubes grown in acetic acid solution and thermally post-treated have stable anatase crystal structure, to a point that by performing annealing at 800 • C for 3 h, only the 35% of anatase transforms into rutile. Finally, it was shown that the TiO 2-NTAs decorated with Pd(0.72 wt%) and Ag(1.26 wt%) particles show higher photocatalytic activity compared with nanotubes modified with single metal particles. It is believed that the high photoactivity of TiO 2 nanotubes decorated with Pd–Ag heterostructures is due to the prolonged lifetimes of photogenerated electron–hole pairs. The possible mechanism for the enhanced photocatalytic activity is discussed in detail.
In this article, we present recent advances that we have achieved toward improving the properties of anodically formed semiconducting TiO2 nanotubes as well as nanowire arrays as electrodes for oxidative photoelectrochemistry. The morphology, crystallinity, composition, and illumination geometry of nanotube or nanowire arrays are critical factors in their performance as photoelectrodes. We discuss the key aspects relating to each factor and the advances achieved in improving each. With respect to the more fully investigated nanotube arrays, the ability to control the morphological parameters such as pore size, tube length, and wall thickness of the nanotube architecture has enabled high performance in applications such as water photoelectrolysis, photocatalysis, dye-sensitized solar cells, and heterojunction TiO2-polymer hybrid solar cells. We begin by reviewing the photoelectrochemical performance of state-of-the-art nanotube arrays fabricated on planar substrates. We then present more recent results related to the growth of TiO2 nanotube arrays on nonplanar substrates designed in such a way that reflected light normally lost to free space is instead directed to a different point on the device, in turn improving overall photoconversion efficiency. Insofar as the crystallinity of the nanotubes is concerned, the use of a high-temperature oxygen or air-ambient anneal to crystallize the nanotube arrays is disadvantageous, since it results in a thick barrier layer where recombination losses occur and also because it precludes compatibility with polymeric substrates. In this regard, we discovered a two-step fabrication process for synthesis of crystallized nanotube arrays at low-temperatures. The photoelectrochemical applications of TiO2 are limited by its large electronic band gap. We briefly review the cationic and anionic doping approaches popularly used to modify the TiO2 band gap. We consider the use of ternary oxide systems containing titania as both a structural support and corrosion-inhibitor, in particular fabrication and performance of n-type Ti-Fe-O nanotubes and p-type copper-rich Cu-Ti-O nanotubes, with a note on our recent synthesis of iron oxide nanotube arrays by anodic oxidation of iron. Fabrication and photoelectrochernical properties of CdS-TiO2 and CdTe-TiO2 nanotube array heterojunction photoelectrodes are discussed. The article concludes by examining low temperature synthesis, and resulting properties, of single crystal vertically oriented TiO2 nanowire arrays on transparent conductive glass substrates; preliminary investigation of these nanowire array photoelectrodes for water photolysis reveals them to have low series resistance and provide excellent separation of photogenerated charges.
Recent Advances in the Use of TiO 2 Nanotube and Nanowire Arrays for Oxidative Photoelectrochemistry
Journal of Physical Chemistry C, 2009
In this article, we present recent advances that we have achieved toward improving the properties of anodically formed semiconducting TiO 2 nanotubes as well as nanowire arrays as electrodes for oxidative photoelectrochemistry. The morphology, crystallinity, composition, and illumination geometry of nanotube or nanowire arrays are critical factors in their performance as photoelectrodes. We discuss the key aspects relating to each factor and the advances achieved in improving each. With respect to the more fully investigated nanotube arrays, the ability to control the morphological parameters such as pore size, tube length, and wall thickness of the nanotube architecture has enabled high performance in applications such as water photoelectrolysis, photocatalysis, dye-sensitized solar cells, and heterojunction TiO 2 -polymer hybrid solar cells. We begin by reviewing the photoelectrochemical performance of state-of-the-art nanotube arrays fabricated on planar substrates. We then present more recent results related to the growth of TiO 2 nanotube arrays on nonplanar substrates designed in such a way that reflected light normally lost to free space is instead directed to a different point on the device, in turn improving overall photoconversion efficiency. Insofar as the crystallinity of the nanotubes is concerned, the use of a high-temperature oxygen or air-ambient anneal to crystallize the nanotube arrays is disadvantageous, since it results in a thick barrier layer where recombination losses occur and also because it precludes compatibility with polymeric substrates. In this regard, we discovered a twostep fabrication process for synthesis of crystallized nanotube arrays at low-temperatures. The photoelectrochemical applications of TiO 2 are limited by its large electronic band gap. We briefly review the cationic and anionic doping approaches popularly used to modify the TiO 2 band gap. We consider the use of ternary oxide systems containing titania as both a structural support and corrosion-inhibitor, in particular fabrication and performance of n-type Ti-Fe-O nanotubes and p-type copper-rich Cu-Ti-O nanotubes, with a note on our recent synthesis of iron oxide nanotube arrays by anodic oxidation of iron. Fabrication and photoelectrochemical properties of CdS-TiO 2 and CdTe-TiO 2 nanotube array heterojunction photoelectrodes are discussed. The article concludes by examining low temperature synthesis, and resulting properties, of single crystal vertically oriented TiO 2 nanowire arrays on transparent conductive glass substrates; preliminary investigation of these nanowire array photoelectrodes for water photolysis reveals them to have low series resistance and provide excellent separation of photogenerated charges.
Solar Energy Materials and Solar Cells, 2008
Photoinduced electron transfer between anatase and rutile in nanosized TiO 2 , prepared by a sol-gel method, was revealed by means of the surface photovoltage technique, and its effects on the photocatalytic performance in the degradation of a phenol solution were investigated. Also, the role of the surface states during the processes of photo-physics and photochemical reactions was discussed. In the as-prepared TiO 2 sample consisting of anatase and rutile, the photoinduced electrons can easily transfer from anatase surface states to rutile, as well as from anatase conduction band to rutile. These factors are responsible for the strong surface photovoltage response and high photocatalytic activity. Moreover, the surface states related to oxygen vacancies can induce photocatalytic reactions under visible irradiation, especially in the resulting biphase TiO 2 , due to the electron transfer from anatase surface states to rutile.