17December2018

Nano-Micro Letters

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Review of Water-Assisted Crystallization for TiO2 Nanotubes

Xiaoyi Wang¹, Dainan Zhang¹, Quanjun Xiang¹, Zhiyong Zhong¹, Yulong Liao^1,2	Abstract Full Text Html PDF w/ Links Export Citation Figures
¹State Key Laboratory of Electronic Thin Film and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, People’s Republic of China ²Center for Applied Chemistry, University of Electronic Science and Technology of China, Chengdu, People’s Republic of China +Show more
Nano-Micro Lett. (2018) 10: 77
First Online: 15 November 2018 (Review)
DOI:10.1007/s40820-018-0230-4
*Corresponding author. E-mail: yulong.liao@uestc.edu.cn

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Brief description of the overall contents
Schematic illustration of the crystallization process in water
a Digital photographs showing the appearance change of a TiO2 NT/Ti film before and after the treatment. b XRD patterns demonstrating the phase transformation before (black line) and after (red line) the water soaking for the TiO2 NT/Ti film. c Schematic showing the nucleation of anatase from amorphous TiO2 induced by water [40].
XRD patterns of anodic TiO2 NT array films crystallized in a water, b ethanol, and c ethylene glycol at 90 °C for 20 h. d Digital photographs of the treatment system at 90 °C with treatment duration rising from 0 to 20 h. e Magnified digital photograph of the 20-h sample. f Selected area diffraction pattern of the precipitated TiO2 nanoparticles, indicating the anatase phase of the precipitated nanoparticles [52]. g Schematic diagram of the dissolution-recrystallization-precipitation mechanism.
a Flowchart for the low-temperature crystallization of amorphous TiO2 nanotubular arrays by solid–gas reaction. In a Teflon-lined stainless autoclave, the as-anodized amorphous TiO2 nanotubes fabricated by anodization reacted with water vapor to yield anatase phase. Morphology images of TiO2 nanotubular arrays prepared by anodizing Ti foil at 20 V for 20 min in 0.5 wt% HF solution, followed by b hydrothermal solid–gas crystallization and c hydrothermal solid–liquid crystallization at 160 °C for 4 h [54].
a XRD patterns of the as-hydrothermal samples after annealing at 450 °C for 3 h in air (lines 1, 2, 3 represent the 0.05 M, 0.2 M, and 0.5 M Zn (Ac)2 solutions). SEM images of the samples hydrothermally treated with Zn(Ac)2 solutions with different concentrations at 200 °C for 6 h: b 0.05 M and c 0.5 M. d SEM image of anatase TNTs after hydrothermal treatment in 0.2 M Zn (Ac)2 at 200 °C for 6 h [67]. e Schematic illustration of the reaction processes in the presence of anatase or amorphous TNTs.
a Schematic diagram showing the fabrication of crystalline TNTs by immersing the as-anodized sample in the solution of ammonia. b Low-resolution XPS spectra of as-anodized TNAs and ammonia-treated TNAs (10 h). The inset shows the corresponding N 1s core-level XPS spectra with high resolution. c The corresponding normalized Ti 2p core-level XPS spectra with high resolution together with their difference spectrum (‘ammonia-treated TNAs’ minus ‘as-anodized TNAs’). d I–V characteristics of the as-anodized TNAs and ammonia-treated TNTs annealed for 5, 8, 10, and 15 h, respectively. The inset: a simplified sketch of the two-point measurement arrangement [78].
a Schematic illustration of the crystallization process at room temperature: first step: electrochemical anodization; second step: pulsed crystallization. b The mechanism of the superfast room temperature crystallization of TNTs. c XRD patterns of the as-anodized and pulsed-crystallized TNTs [82].
SEM characterization of the morphological evolution as a function of water soaking time: a as-anodized TiO2 nanotube arrays, b intermediate double-walled TiO2 nanotube arrays obtained by water soaking for 15–25 h, c intermediate wire-in-tube TiO2 nanoarchitecture formed by water soaking for 25–72 h, and d mesoporous TiO2 nanowire arrays obtained by a long-term water soaking (> 72 h). e Scheme illustrating the morphology transformation of the as-anodized TiO2 nanotubes upon water soaking. The red arrows denote the direction along which the dissolution–precipitation of TiO6 octahedra proceeds [40].
SEM, HRTEM and SAED images of the a as-anodized TNTs, b after water soaking for 1 day, and c after water soaking for 3 days [83]
a Schematic illustration of the fabrication stages of the nanoparticle decorated TiO2 NT array photoanode: (A) untreated TiO2 NT array on FTO substrate (UT-NT), (B) ALD amorphous TiO2 thin layer coated on the TiO2 NT array (AT-NT), (C) cross-sectional view of (B), (D) AT-NT electrode post-water treated for 1 day (WT1-NT), and (E) AT-NT electrode water treated for 2 days (WT2-NT). b TEM image of the sample after water treatment. c The corresponding HRTEM lattice images of the circled portions in b. d Incident photoconversion efficiency spectra of DSSCs using untreated and water-treated TiO2 NT array photoanodes. SEM images of e as-anodized TNTs, f after being immersed in water at 90 °C for 8 h, and g after being immersed in ammonia solution (the concentration of Volammonia/Volwater = 1:1) at 90 °C for 8 h [45, 75].
a UV–Vis absorption spectra of the TiO2 NT-powders after different heat treatment duration times at 92 °C. The inset shows the corresponding estimated bandgap. b The absorption spectra of water immersed TNTs and ammonia solution immersed TNTs with different concentrations. The inset shows the corresponding estimated bandgap, A/DI = Volammonia/Volwater. c Schematic of the band structure of pure TNTs and nitrogen-doped TNTs prepared by immersing the as-anodized sample in ammonia solution [39, 55].
a Schematic illustration of the reaction process for degrading pollutants by TiO2 under irradiation. b Photocatalytic degradation kinetics of the MO (methyl orange) aqueous solution and c the degradation rate constant by using the TiO2 NT-powders obtained by water immersion with different treatment duration times at 92 °C (the inset of b shows the MO degradation processing with the TiO2 NT-powders after hot water treatment at 92 °C for 35 h as photocatalyst, and S-450 refers to the sample sintered at 450 °C for 3 h) [39].
Schematic illustration of the configuration of DSSCs based on TNTs [125].
Typical TEM images of the TNTs for a as-anodized and b after HSGM (hydrothermal solid–gas method) treatment with 120 μL water put into a 50 mL Teflon autoclave. c Cyclic voltammograms of the TNTAs without HSGM treatment and TNTAs treated by HSGM under different vapor pressures at a scan rate of 100 mV s−1. d Galvanostatic charge–discharge (GCD) curves of the HSGM-treated TNTAs at a current density of 0.5 mA cm−2. HSGM-120, 180, 240, 1000 represent the different water content in the Teflon autoclave, which leads to the different vapor pressures [53].
a Schematic illustration of the water-assisted crystallization process for preparing porous anatase TiO2 nanospheres from sol–gel derived amorphous particles. b TEM images of the original amorphous TiO2 spheres. c TEM images of the samples after immersing in water at 75 °C for 30 min. d XRD patterns of products obtained by immersing in water at 75 °C for different time periods. e The change of surface area as a function of heating time period for samples treated at different temperatures. f Schematic illustration of the water-assisted crystallization strategy for conversion of amorphous TiO2 layer to mesoporous crystalline shells. Typical TEM images of the samples at different preparation steps: g SiO2@TiO2 core–shell structures prepared by sol–gel coating and h mesoporous TiO2 hollow nanostructures after removing SiO2 cores [141, 142].
SEM images of the TiO2 nanofibers treated with water steam at different temperatures of a, b 150 °C; c, d 350 °C; and e, f 550 °C. g XRD patterns of the samples heat-treated at different temperatures in water steam. h, i HRTEM and SAED images of the water steam-treated TiO2 nanofibers prepared at 550 °C. j Nitrogen adsorption–desorption isotherms of the samples heat-treated at different temperatures in water steam [152].
SEM images of the as-anodized SnO2 samples a before and b after soaking in water for 2 h at room temperature and c after soaking in water for 2 h at 60 °C. The insets are the photographs showing the appearance of the corresponding SnO2 samples. d Schematic illustration showing the transformation from amorphous to rutile SnO2 assisted by water soaking. TEM images of the SnO2 samples e as-anodized and f after water soaking at 60 °C for 2 h; the insets show the corresponding SAED patterns. g Long-term stability profiles and Coulombic efficiency of the water-crystallized SnO2 at 0.3 C for 500 cycles [159].

Abstract

TiO₂ nanotubes (TNTs) have drawn tremendous attention owing to their unique architectural and physical properties. Anodizing of titanium foil has proven to be the most efficient method to fabricate well-aligned TNTs, which, however, usually produces amorphous TNTs and needs further thermal annealing. Recently, a water-assisted crystallization strategy has been proposed and investigated by both science and engineering communities. This method is very efficient and energy saving, and it circumvents the drawbacks of thermal sintering approach. In this paper, we review the recent research progress in this kind of low-temperature crystallization approach. Here, various synthetic methods are summarized, and the mechanisms of the amorphous–crystalline transformation are analyzed. The fundamental properties and applications of the low-temperature products are also discussed. Furthermore, it is proved that the water-assisted crystallization approach is not only applicable to TNTs but also to crystallizing other metal oxides.

Keywords

TiO₂ nanotube, Crystallization, Water-assisted, Low-temperature

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