A One‐Step Approach to the Synthesis of ZrO₂‐Modified TiO₂ Nanotubes in Supercritical Carbon Dioxide

Abstract

Recently, considerable effort has been devoted to synthesizing materials with one-dimensional (1D) nanostructures due to their unique properties and potential applications in a diversity of applications including catalysis, high-efficiency solar cells, coatings, and sensors.[1] Particularly, titania (TiO 2 ) nanotubes are receiving considerable attention due to titania's high activity, strong oxidation capability, and chemical stability. In addition, being a low-cost material, TiO 2 is favorable for potential large-scale commercial applications. The performance of titania nanomaterials strongly relies on their crystallinity, crystallite size, crystal structure, specific surface area, thermal stability and quantum efficiency. [2,3] These properties highly depend on both the synthesis method, and the subsequent thermal-treatment technique -calcination. In some cases, doping with a second metal has been found very effective to improve the properties of TiO 2 . Zirconia has been reported as one of the most suitable dopants to enhance the thermal stability and activity of TiO 2 nanomaterials. [4][5][6] Conventionally, anodization, [7] template techniques, [8,9] hydrothermal processes, [10,11] and soft chemical processes [12] have been used to prepare TiO 2 nanotubes. However, each of these methods has limitations. The template technique requires high calcination temperatures to remove the template, resulting in a collapse of the tubular structure in the product.[12]Anodizing processes produce nanotubes with relatively-large diameters.[13] The multistep hydrothermal process requires a large amount of inorganic solvents, which may lead to environmental pollution. Due to these limitations, alternative processes using a green solvent such as supercritical carbon dioxide (scCO 2 ) are attractive to synthesize TiO 2 nanotubes. Previously, our group developed a direct sol-gel process to synthesize TiO 2 nanofibers, [14] where scCO 2 was used in place of an organic solvent. There are several favorable properties that make CO 2 an attractive solvent for the synthesis and processing of high-quality porous materials. Firstly, CO 2 is inexpensive, environmentally benign and non-flammable, with mild critical conditions (P c ¼ 73.8 bar; T c ¼ 31.1 8C), which makes this a green solvent that is suitable for both laboratory and commercial scale application. Furthermore, its physical properties such as density and solubility can be tuned by adjusting the operating temperature and pressure. Low viscosity, zero surface tension and high diffusivity of scCO 2 are also favorable in synthesizing superior ultrafine and uniform nanomaterials. In addition, scCO 2 can be easily and completely removed from products by venting; hence, no drying process is required and the porous structure can be maintained without collapsing the nanostructure. In addition, scCO 2 can be easily recycled after the pressure is diminished for potential scale-up applications. While exploring the synthesis of ZrO 2 -modified TiO 2 nanostructures by sol-gel methods in sc...

Facile synthesis of Ag3VO4/β-AgVO3 nanowires with efficient visible-light photocatalytic activity

“…The apparent rate constant k was shown in inset. It can be seen that the composite with mole ratios of 30% Ag 3 VO 4 /b-AgVO 3 shows the highest value of apparent rate constant k, which was 9 and 2.4 times higher than pure b-AgVO 3 and Ag 3 VO 4 . This indicates that the composite The stability of photocatalysts is an important factor for the further application.…”

“…[1][2][3] Over the past several decades, more and more photocatalysts have been developed, including TiO 2 , ZnO, Ta 2 O 5 and CuO. [4][5][6][7][8][9][10] However, most of them had a high rate of charge carrier recombination and no response in the visible light range. It is very important to effectively utilize solar energy to overcome the above two limitations.…”

Co-doped MoS2 NPs with matched energy band and low overpotential high efficiently convert CO2 to methanol

“…The main reasons are manifested from two aspects: firstly, transition metal-doping could change the original crystal structure of catalyst, form new active sites and then increase reductive activity [21,22], for example, Rahima [23] reported Zr doped TiO 2 nanotubes prepared by sol-gel methods, in which the crystal structure http://dx.doi.org/10.1016/j.apsusc.2015.06.178 0169-4332/© 2015 Elsevier B.V. All rights reserved. was changed and resulted in the production of a hexamer with a high surface area and greater thermal, made the catalytic reaction more intense; secondly, transition metal owned multi-valence, the doping catalyst could form capture trap of electron-hole, decrease the rate of electron-hole recombination and increase the catalytic activity, Kiriakidou [24] found that W-doped TiO 2 for photocatalytic degradation obtained upward flat band potential and Fermi level upward, surface barrier of which becomes higher and electron-hole separate efficiently, finally, photocatalytic effect is enhanced.…”

Green solvent-based approaches for synthesis of nanomaterials

“…Ion solubility, which is high under ambient conditions, decreases with the increased density, whereas the solubility of organic molecules increases significantly. Taking advantage of SC-CO 2 solutions and SC water, many SCF-based approaches have been developed for the synthesis of nanomaterials [4][5][6][7][8][9]. In our laboratory, we have have paid particular attention to the synthesis of nanocomposites and replication of biotemplates and the results obtained are reviewed here.…”