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TiO(2) is one of the most studied compounds in materials science. Owing to some outstanding properties it is used for instance in photocatalysis, dye-sensitized solar cells, and biomedical devices. In 1999, first reports showed the feasibility to grow highly ordered arrays of TiO(2) nanotubes by a simple but optimized electrochemical anodization of a titanium metal sheet. This finding stimulated intense research activities that focused on growth, modification, properties, and applications of these one-dimensional nanostructures. This review attempts to cover all these aspects, including underlying principles and key functional features of TiO(2), in a comprehensive way and also indicates potential future directions of the field.

TiO2 nanotubes: synthesis and applications.

Photoelectrochemical (PEC) cells offer the ability to convert electromagnetic energy from our largest renewable source, the Sun, to stored chemical energy through the splitting of water into molecular oxygen and hydrogen. Hematite (α-Fe(2)O(3)) has emerged as a promising photo-electrode material due to its significant light absorption, chemical stability in aqueous environments, and ample abundance. However, its performance as a water-oxidizing photoanode has been crucially limited by poor optoelectronic properties that lead to both low light harvesting efficiencies and a large requisite overpotential for photoassisted water oxidation. Recently, the application of nanostructuring techniques and advanced interfacial engineering has afforded landmark improvements in the performance of hematite photoanodes. In this review, new insights into the basic material properties, the attractive aspects, and the challenges in using hematite for photoelectrochemical (PEC) water splitting are first examined. Next, recent progress enhancing the photocurrent by precise morphology control and reducing the overpotential with surface treatments are critically detailed and compared. The latest efforts using advanced characterization techniques, particularly electrochemical impedance spectroscopy, are finally presented. These methods help to define the obstacles that remain to be surmounted in order to fully exploit the potential of this promising material for solar energy conversion.

Solar Water Splitting: Progress Using Hematite (α‐Fe2O3) Photoelectrodes

Lignocellulosic biomass: a sustainable platform for the production of bio-based chemicals and polymers

The demand for petroleum dependent chemicals and materials has been increasing despite the dwindling of their fossil resources. As the dead-end of petroleum based industry has started to appear, today's modern society has to implement alternative energy and valuable chemical resources immediately. Owing to the importance of lignocellulosic biomass being the most abundant and bio-renewable biomass on earth, this critical review provides insights into the potential of lignocellulosic biomass as an alternative platform to fossil resources. In this context, over 200 value-added compounds, which can be derived from lignocellulosic biomass by various treatment methods, are presented with their references. Lignocellulosic biomass based polymers and their commercial importance are also reported mainly in the frame of these compounds. This review article aims to draw the map of lignocellulosic biomass derived chemicals and their synthetic polymers, and to reveal the scope of this map in today's modern chemical and polymer industry.

One-dimensional nanostructures exhibit quantum confinement that leads to unique electronic properties, making them attractive as the active elements for various applications. Iron oxide (α-Fe2O3 or hematite) nanotubes are of particular interest in catalysis, sensor devices, Li-ion battery, environmental remediation and photocatalysis. Here, we report a simple sonoelectrochemical anodization method to grow smooth and ultrathin (5−7 nm thick) Fe2O3 nanotube arrays (3−4 μm long) on Fe foil in as little as 13 min. The prepared catalyst has shown tremendous potential to split water to generate hydrogen under solar light illumination. A photocurrent density of 1.41 mA/cm2 is observed for hematite nanotube arrays with more than 50% being contributed by the visible light components of the solar spectrum.

Water Photooxidation by Smooth and Ultrathin α-Fe2O3 Nanotube Arrays

A coupled semiconductor material is prepared by filling one-dimensional (1D) titania (TiO2) nanotubes (NTs) with cadmium sulfide (CdS) nanoparticles (NPs). Self-assembled TiO2 NTs (length = 550 nm, diameter = 80 nm) are prepared by the sonoelectrochemical anodization method. These NTs are functionalized with CdS NPs by a single-step electrodeposition method. This material harvests solar light in UV as well as visible light (up to 510 nm) region. An eight to 9-fold enhancement in photoactivity is observed using CdS functionalized TiO2 NTs compared to pure TiO2 NTs and commercial P25 NPs. This methodology will be useful in designing multijunction semiconductor materials confined inside 1D nanochannels.

Synthesis of Coupled Semiconductor by Filling 1D TiO2 Nanotubes with CdS

Self-organized, vertically oriented TiO2 nanotube arrays prepared by the sonoelectrochemical anodization method are functionalized with palladium (Pd) nanoparticles of approximately 10 nm size. A simple incipient wetness method is adopted to distribute the Pd nanoparticles uniformly throughout the TiO2 nanotubular surface. This functionalized material is found to be an excellent heterogeneous photocatalyst that can decompose nonbiodegradable azo dyes (e.g., methyl red and methyl orange) rapidly (150-270 min) and efficiently (100%) under ambient conditions using simulated solar light in the absence of any external oxidative radicals such as hydrogen peroxide.

Functionalization of Self-Organized TiO2 Nanotubes with Pd Nanoparticles for Photocatalytic Decomposition of Dyes under Solar Light Illumination

Synthesis of hematite (α-Fe(2)O(3)) nanostructures on a titania (TiO(2)) nanotubular template is carried out using a pulsed electrodeposition technique. The TiO(2) nanotubes are prepared by the sonoelectrochemical anodization method and are filled with iron (Fe) by pulsed electrodeposition. The Fe/TiO(2) composite is then annealed in an O(2) atmosphere to convert it to Fe(2)O(3)/TiO(2) nanorod-nanotube arrays. The length of the Fe(2)O(3) inside the TiO(2) nanotubes can be tuned from 50 to 550 nm by changing the deposition time. The composite material is characterized by scanning electron microscopy, transmission electron microscopy and diffuse reflectance ultraviolet-visible studies to confirm the formation of one-dimensional Fe(2)O(3)/TiO(2) nanorod-nanotube arrays. The present approach can be used for designing variable one-dimensional metal oxide heterostructures.

https://iopscience.iop.org/article/10.1088/0957-4484/19/31/315601/pdf

Synthesis of Fe2O3/TiO2 nanorod–nanotube arrays by filling TiO2 nanotubes with Fe

Multiferroic materials such as bismuth iron oxide (BiFeO3/BFO) have strong potential for solar hydrogen generation. In this paper, we have described a sol-gel method using urea and polyvinyl alcohol at low temperature to synthesize high purity BFO nanoparticles of size 50- 60 nm. A complicated and high temperature process is usually employed to synthesize BFO. It is observed that the material has an energy band gap of 2.1 eV which makes it a potential material towards solar applications. Photoelectrochemical (PEC) and photocatalytic tests are conducted to evaluate BFO's performance towards hydrogen generation by splitting of water using simulated solar light. PEC generation of hydrogen using fabricated electrodes (BFO nanoparticles coated on titanium foil) shows that the material is photoactive and can evolve hydrogen from water using a visible light. Photocatalytic tests for hydrogen generation are also performed using the nanoparticles in 1(M) KOH solution under simulated solar light. Upon analysis of the evolved gases by gas chromatography (GC), hydrogen and oxygen are detected. Furthermore, it is also observed that BFO generates three times more hydrogen than commercial titania P25 catalyst under similar experimental conditions.

Subarna Banerjee

Papers

Water Photooxidation by Smooth and Ultrathin α-Fe2O3 Nanotube Arrays

Synthesis of Coupled Semiconductor by Filling 1D TiO2 Nanotubes with CdS

Functionalization of Self-Organized TiO2 Nanotubes with Pd Nanoparticles for Photocatalytic Decomposition of Dyes under Solar Light Illumination

Synthesis of Fe2O3/TiO2 nanorod–nanotube arrays by filling TiO2 nanotubes with Fe

Bismuth Iron Oxide Nanoparticles as Photocatalyst for Solar Hydrogen Generation from Water