Titania Nanotubes by Electrochemical Anodization for Solar Energy Conversion

Tsui, Lok‐kun; Zangari, Giovanni

doi:10.1149/2.010407jes

Cited by 33 publications

(26 citation statements)

References 222 publications

(283 reference statements)

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“…Anodic TiO 2 nanotubes (ATNTs) and other porous anodic oxides have attracted considerable scientific interests due to their various applications [1][2][3] (e.g., solar energy materials, magnetic semiconductors) and mysterious formation mechanisms. [4][5][6] Several interesting mechanisms of ATNTs have been reported in many famous journals in recent years.…”

mentioning

confidence: 99%

Formation Mechanism of Gaps and Ribs Around Anodic TiO₂Nanotubes and Method to Avoid Formation of Ribs

Chong

Gao

et al. 2015

J. Electrochem. Soc.

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Anodic TiO 2 nanotubes (ATNTs) have been studied extensively for many years. However, their mysterious formation mechanism still remains unclear. The formation of gaps and ribs around the nanotubes has not been elucidated. Here, various surface and cross-section morphologies of ATNTs obtained under different anodizing conditions and their evolution process have been investigated in detail. Based on many experimental facts, new explanations for the gaps and ribs are presented. An entire surface layer covered on the nanotubes plays a primary role on the formation of gaps and ribs. The gaps result from the radial distribution of the electric field at the pore bottom. No newly-formed oxide will exist along the gap direction, because the electric filed along the gap is the minimum. The ribs result from the electrolyte entering into the wider gaps among the ATNTs due to the rupture of the entire surface layer. The rings or ribs on the outer wall of ATNTs are formed at the electrolyte/Ti interface due to the discontinuous existence of a small amount of electrolyte within the gap base. The present viewpoint was demonstrated by an original micro-dam, which can block the electrolyte entering into the gaps and avoid the formation of ribs.

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confidence: 99%

Formation Mechanism of Gaps and Ribs Around Anodic TiO₂Nanotubes and Method to Avoid Formation of Ribs

Chong

Gao

et al. 2015

J. Electrochem. Soc.

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“…3,10 The superior electrode among the air-annealed samples, A650, was composed of 68% anatase and 32% rutile. A850 exhibited rutile crystallites in near absence of anatase crystallite, indicating the almost complete phase transition of anatase crystallites in NT walls.…”

Section: Pec Performances Of Tiomentioning

confidence: 99%

“…[2][3][4][5][6][7][8][9][10] Importantly, TiO 2 can be a platform for developing visible-light-responsive photoelectrodes for water splitting under sunlight because it is easily sensitized by dye molecules, quantum dots, and narrow bandgap semiconductors. 2,11 Impurity doping also can provide visible light absorption to TiO 2 by forming an impurity level and a sub-band in the bandgap.…”

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confidence: 99%

Rutile Titania Particulate Photoelectrodes Fabricated by Two-Step Annealing of Titania Nanotube Arrays

Amano

Mukohara

Shintani

2018

J. Electrochem. Soc.

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TiO 2 nanotube arrays fabricated by anodization and post annealing are extensively studied as an n-type semiconductor electrode for photoelectrochemical (PEC) water splitting owing to their large surface area and efficient electron transport property. The rutile phase is believed to be an inactive component of the TiO 2 nanotube's electrode because annealing at high temperatures decreases the PEC efficiency with the transformation of anatase into rutile crystallites. In contrast, herein, we found that photoelectrodes prepared by two-step annealing, in which TiO 2 nanotubes annealed in air at 650 • C were then treated in a nitrogen atmosphere at a higher temperature, exhibited higher PEC efficiency despite the anatase nanotube structure changes to rutile particles. Sheet resistance measurement and Mott-Schottky analysis showed that the enhanced efficiency is attributed to a significant increase in donor density by partial reduction of rutile TiO 2 . The second annealing in the reductive atmosphere is essential to provide a columnar arrangement of TiO 2 crystallites with high donor density resulting in high PEC properties for the oxidation of water to O 2 . This suggests that improving the electron transport property by the enhanced electrical conductivity and the interconnected nanocrystalline structure is important to enhance the PEC property of rutile TiO 2 particulate photoanodes. Photoelectrochemical (PEC) water splitting is a promising technology for producing H 2 using solar energy. A key component necessary to achieve a PEC system is the development of highly efficient photoelectrodes. Since the discovery of PEC water oxidation on TiO 2 electrodes, n-type oxide electrodes have been extensively studied as photoanodes for four-electron oxidation of water to evolve O 2 .1 Among them, highly ordered vertically oriented TiO 2 nanotube (NT) electrodes have attracted much research attention owing to their high efficiency in the PEC reaction after crystallization to anatase by annealing.2-10 Importantly, TiO 2 can be a platform for developing visible-light-responsive photoelectrodes for water splitting under sunlight because it is easily sensitized by dye molecules, quantum dots, and narrow bandgap semiconductors.2,11 Impurity doping also can provide visible light absorption to TiO 2 by forming an impurity level and a sub-band in the bandgap. 12,13The nanotubular crystalline structure exhibits a large surface area, which increases the interface of a semiconductor/liquid and decreases the traveling length of photogenerated holes, which results in improved charge separation.2 The large pore volume inside the hollow structure provides fast diffusion of electrolytes and evolved O 2 gas. Furthermore, the one-dimensional interconnected nanocrystalline structure provides excellent electron transport pathways for the photoexcited electrons to the back contact substrate.2 The fast electron transport, which means long diffusion lengths of photoexcited electrons, enables the optical path length, i.e. the NT length, to increa...

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“…It is used as an ultraviolet radiation absorber in sunscreen [303] as well as a pigment for white paints, plastics, paper, and cosmetics [304][305][306]. TiO 2 also has some very interesting photocatalytic behavior, leading to its use as photo-induced catalyst for chemical application as well as in photovoltaic applications [47,[307][308][309]. Nano-structured TiO 2 particles, nanotubes, and nanoporous membranes are a key component to the operation of dye-sensitized solar cells, where the photoinduced electron-hole splitting occurs at the nano-structured TiO 2 surface [310][311][312].…”

Section: Applications and Background Of Tiomentioning

confidence: 99%

“…These properties can include semiconductivity [28,29], ferroelectricity [30][31][32], ferromagnetism [29,[33][34][35][36], or superconductivity [37][38][39]. Functional oxides are used in applications including microelectronics [5,8,[40][41][42][43][44][45], photovoltaics [12,13,46,47], piezoelectrics [9][10][11], and thermoelectrics [16,18,19,[48][49][50][51]. I will discuss much more about the applications, and specifically the thermally driven aspects of the applications utilizing functional oxide materials below and in the chapters to come.…”

Section: Introductionmentioning

confidence: 99%

Understanding Phonon Interactions with Defects in Functional Oxide Materials

Donovan¹

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Understanding of nano-scale thermal transport has enabled significant advances in a wide variety of fields including microelectronics, alternative energy, optics, and many more [1][2][3][4]. Utilized in many of these applications are a class of materials known as functional oxides. These materials are oxygen compounds with specific functional properties of interest (e.g., dielectrics [5][6][7][8], piezoelectrics [9][10][11], photovoltaics [12][13][14], thermoelectrics [15][16][17][18][19]). Often it is the defects in these materials that enable functionality and, depending on the processing and operating conditions, the concentration and types of defects that are present in these materials can vary widely [20]. In this dissertation, I aim to identify the role that defects play in functional oxide materials with respect to thermal transport.The major thermal carriers in most functional oxide crystals are collective lattice vibrations, or phonons [21]. Phonon-dominated thermal conductivity is dictated by the scattering of these thermal carriers with a variety of other components or the material system, including defects in the crystal [22,23]. I concentrate on three types of defects that are common in functional oxides: boundaries, extrinsic point defects, and intrinsic point defects.Boundaries such as film boundaries and grain boundaries with nano-scale dimensions can be detrimental to the thermal conductivity of a material. As the length of the uninterrupted crystal reduces to the phonon mean free paths in the crystal, phonons will scatter readily off of nano-scale boundaries and will not be able to propagate as they would in a bulk environment. This becomes particularly relevant in technologies such as the microelectronics industry, where device size scales are well below common phonon mean free paths in the constituent materials. To study this, I turn to one of the more common functional oxides used in the microelectronics industry, BaTiO 3 .I study the effects of nano-scale grains on thin films of BaTiO 3 grown via chemical solution deposition. The films studied are 150 nm and range in grain size from 36 nm -63 nm. I show that the thermal conductivity of these nano-grained films scales with the grain size and film thickness and demonstrate agreement of this trend with analytical models. This result demonstrates that despite a complex crystal structure, there is a mean free path spectrum of phonons in BaTiO 3 that significantly exceeds the dimensions of the grains and film (contrary to the common "gray" mean 2 free path assumption seen in literature).To address the role of extrinsic point defects, or dopants, on the thermal conductivity of functional oxides, I study the effects of dysprosium doping in thin films of cadmium oxide deposited by molecular beam epitaxy. In this experiment, the addition of Dy dopants in CdO actually increases the thermal conductivity initially, owing to a reduction in the equilibrium concentration of oxygen vacancies (a type of defect that is intrinsic to all oxides). The de...

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Titania Nanotubes by Electrochemical Anodization for Solar Energy Conversion

Cited by 33 publications

References 222 publications

Formation Mechanism of Gaps and Ribs Around Anodic TiO₂Nanotubes and Method to Avoid Formation of Ribs

Formation Mechanism of Gaps and Ribs Around Anodic TiO₂Nanotubes and Method to Avoid Formation of Ribs

Rutile Titania Particulate Photoelectrodes Fabricated by Two-Step Annealing of Titania Nanotube Arrays

Understanding Phonon Interactions with Defects in Functional Oxide Materials

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Titania Nanotubes by Electrochemical Anodization for Solar Energy Conversion

Cited by 33 publications

References 222 publications

Formation Mechanism of Gaps and Ribs Around Anodic TiO2Nanotubes and Method to Avoid Formation of Ribs

Formation Mechanism of Gaps and Ribs Around Anodic TiO2Nanotubes and Method to Avoid Formation of Ribs

Rutile Titania Particulate Photoelectrodes Fabricated by Two-Step Annealing of Titania Nanotube Arrays

Understanding Phonon Interactions with Defects in Functional Oxide Materials

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Formation Mechanism of Gaps and Ribs Around Anodic TiO₂Nanotubes and Method to Avoid Formation of Ribs

Formation Mechanism of Gaps and Ribs Around Anodic TiO₂Nanotubes and Method to Avoid Formation of Ribs