Effect of Nanotube Diameter on Photo-Electro-Chemical Properties of Carbon Quantum Dot Functionalized TiO2 Nanotubes (original) (raw)
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Nanotechnology, 2016
Hydrogen fuels generated by water splitting using a photocatalyst and solar irradiation are currently gaining the strength to diversify the world energy matrix in a green way. CdS quantum dots have revealed a hydrogen generation improvement when added to TiO2 materials under visible-light irradiation. In the present paper, we investigated the performance of TiO2 nanotubes coupled with CdS quantum dots, by a molecular bifunctional linker, on photocatalytic hydrogen generation. TiO2 nanotubes were obtained by anodization of Ti foil, followed by annealing to crystallize the nanotubes into the anatase phase. Afterwards, the samples were sensitized with CdS quantum dots via an in situ hydrothermal route using 3-mercaptopropionic acid as the capping agent. This sensitization technique permits high loading and uniform distribution of CdS quantum dots onto TiO2 nanotubes. The XPS depth profile showed that CdS concentration remains almost unchanged (homogeneous), while the concentration rela...
Materials Today: Proceedings, 2018
The films of TiO 2 nanotube arrays were fabricated by the anodization of Ti metal sheets for photocatalytic hydrogen production. The anodization was carried out in electrolytes prepared by mixing ethylene glycol (EG), ammonium fluoride (0.3 wt % NH 4 F) and deionized water (2 Vol % H 2 O). DC power supply was used at a constant voltage of 50 Volts. The nanotube arrays were aged in different amounts of ammonia (NH 3) as nitrogen dopants. The morphology of the nanotube arrays was characterized by scanning electron microscopy (SEM). The phase and structure of the TiO 2 nanotube arrays were determined by X-ray diffraction (XRD). The phase of the nanotubes is transformed from anatase to rutile when annealing temperature is changed from 450 °C to 500 °C or higher. The phase of the nanotubes became completely rutile when the nanotube arrays were 700 °C or higher, and the nanotubes were transformed to nanacrystalline. X-ray Photoelectron spectroscopy (XPS) was used to determine the dopant presenting in TiO 2 structure. UV-vis spectroscopy was used to study the optical property of the nanotubes. The diameters of TiO 2 nanotubes were about 200 nm. The highest density of TiO 2 nanotubes was obtained when the nanotubes were doped with 6% by volume of ammonia. The photocatalytic activity was examined without an external applied potential. The maximum photocurrent density was 2.7 mA/cm 2 under illumination of 100 mW/cm 2 corresponding with photoconversion efficiency of 3.3%.
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.
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%.
Nano Energy, 2015
To make the best and highest use of solar light is the main direction and object of photocatalysis and water-splitting. Although UV and visible active photocatalysts have been extensively investigated, the use of near-infrared (NIR) wave band of solar light remains a nearly blank area. Here we report the UV-visible-NIR broad spectrum active photocatalytic property of CQDs/hydrogenated TiO 2 (H-TiO 2 ) nanobelt heterostructures. The improved UV and visible photocatalytic property can be attributed to improved optical absorption, charge carrier trapping, and hindering of the photogenerated electronhole recombination of oxygen vacancies and Ti 3 + ions in TiO 2 nanobelts created by hydrogenation. The NIR photocatalytic activity is from photo-induced electron transfer, electron reservoir, and upconverted PL properties of CQDs, which can absorb NIR light and convert into visible light and transfer to visible photocatalytic active H-TiO 2 nanobelts. This work offers a simple strategy for the fabrication of a wide spectrum of active heterostructured photocatalysts by assembling CQDs on the surface of UV-visible photocatalysts, which opens a door for photodegradation, photocatalytic water splitting, and enhanced solar cells using sunlight as light source.
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.
Novel multiwalled carbon nanotubes-graphene-TiO 2 (CNT-GR-TiO 2) composite materials without noble metal co-catalysts are designed for photocatalytic decomposition of water using solar light. The CNT-GR-TiO 2 nano-composite shows the highest H 2 production rate of 29 mmol h −1 g −1 under the full spectrum of solar light irradiation. The rate of H 2 production is 8-fold higher than the commercial TiO 2 (Degussa P25) and the estimated solar energy conversion efficiency is 14.6%. Spectroscopic and photocatalytic studies reveal that graphene acts as an electron reservoir through which interfacial charge transfer occurs for water splitting. The UV-vis-DRS study shows that the absorption peak maximum for anatase TiO 2 occurs at ∼315 nm, which is shifted to ∼355 nm and 380 nm for GR-TiO 2 and CNT-GR-TiO 2 composites, respectively. The EPR spectra of GR-TiO 2 and CNT-GR-TiO 2 composites indicate that graphene and multiwalled carbon nanotubes in the composites promote the generation of Ti 3+ and oxygen vacancies and in turn reduce the band gap of anatase TiO 2 from 3.32 eV to 2.79 eV. This is corroborated by XPS and photoluminescence analyses of the samples. The role of CNTs is to prevent the restacking of graphene nanosheets and provide additional electron transport channels thereby suppressing the recombination rate of electron-hole pairs in the CNT-GR-TiO 2 composite. The combination of all these factors results in increasing the hydrogen production rate from 19 mmol h −1 g −1 (anatase TiO 2) to 22 mmol h −1 g −1 (GR-TiO 2) to 29 mmol h −1 g −1 (CNT-GR-TiO 2).
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.