Ultrasonication-Assisted Hydrothermal Synthesis of Ultralong TiO2 Nanotubes

Rare Metal Materials and Engineering Volume 43, Issue 10, October 2014 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2014, 43(10): 2326-2329.

Ultrasonication-Assisted Hydrothermal Ultralong TiO2 Nanotubes Teng Honghui1, 2, 1

Xu Shukun 1,

ARTICLE

Synthesis

of

Wang Jiku2 2

Northeastern University, Shenyang 110004, China; Jilin Normal University, Siping 136000, China

Abstract: Ultralong titanate nanotubes were synthesized by treating general anatase TiO2 in alkaline solution using a facile sonication-assisted hydrothermal method. The as-synthesized products via sonication and sonication-hydrothermal treatment were characterized by SEM, TEM, OM and XRD. The effects of sonication power, sonication frequency and sonication temperature on the morphology and the crystal structure of the products were investigated. The results show that in the range of ultrasonic power (200, 300, 400, 500 W) and frequency (20, 40, 80 kHz), titanium dioxide particles were swelled in alkali aqueous solution to get the hedgehog flocculent intermediate products by ultrasonic irradiation when the ultrasonic temperature was equal or higher than 60 °C; several tens of microns long titanate nanotubes were achieved using the intermediate products as raw materials under the mild hydrothermal conditions of 90 °C and 5 h. The possible formation mechanism for ultralong titanate nanotubes was also discussed. The hedgehog floccules synthesized by sonication play an important role to synthesize the several tens of micrometers long nanotubes. Key words: ultralong titanate nanotubes; floccule; sonication; hydrothermal synthesis

One-dimensional TiO2 nanostructure materials with various morphologies including nanowires and nanotubes have attracted a great deal of interest due to the uniquely optical and electrical properties and the potential applications in photocatalysis [1,2], photovoltaic cells [3], Li-ion battery [4], and sensors [5,6]. The approaches in developing titanate nanotubes mainly include template method [1], anodic oxidation [7], and hydrothermal method [8]. Among them, the hydrothermal technique is widely employed to prepare titanate nanotubes by treating the TiO2 powder precursors in alkali aqueous solution, because of its many advantages like cost- effectiveness, mild reaction condition and simple equipment requirement [9]. Recently, a sonication assisted hydrothermal method for the synthesis of titanate nanotubes has received great interest due to the fact that sonication pretreatment plays an important role in fabricating titanate nanotubes[10-13]. The sonication pretreatment has a strong influence on hydrothermal temperature and time, the length distribution and the length of the synthesized product. Ma et al [11] prepared nanotubes with

the lengths up to 600 nm by hydrothermal treatment at 110 °C for 4 h after the sonication of 380 W for 1 h. The sonication with sufficient power plays an important role to promote intercalating Na+ into titania lattices and breaking the Ti-O-Ti bonds. However, Seo et al stated that titanate nanotubes of 1.5 μm in length were obtained by hydrothermal treatment at 70 °C for 48 h after the sonication of 150 W for 16 h [12]. Nawin and his colleagues[10,13] reported that titanate nanotubes were achieved by hydrothermal treatment at 120 °C for 3 d with sonication of 38 w for 8 min. They proposed that sonication speeds up the dispersion of nanoparticles by breaking the intermolecular interactions between titanium dioxide particles and concentrated sodium hydroxide solution. This contradiction in the experimental finding most likely resulted from the difference in sonication treatment, i.e., the difference in sonication temperature and the duration employed. To the best of our knowledge, the roles of sonication temperature and ultrasonic frequency in sonication assisted hydrothermal synthesis have not been reported so far.

Received date: October 25, 2013 Foundation item: National Natural Science Foundation of China (21077041); Science Foundation of Jilin Province of China (201205067) Corresponding author: Xu Shukun, Ph. D., Professor, Department of Chemistry, Northeastern University, Shenyang 110004, P. R. China, Tel: 0086-24-83681343, E-mail: [email protected] Copyright © 2014, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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In this work, a systematic study was carried out to understand the effects of sonication temperature, sonication frequency and sonication power on morphology and crystalline structure of titanate product while keeping the hydrothermal temperature/time constant. We have found that the TiO2 suspension was transformed to a sort of floccule under a suitable sonication conditions before hydrothermal treatment. The floccule has a remarkable effect on the following hydrothermal temperature and time. The morphology and crystalline structure of this floccule was also investigated.

1

Experiment

Ultralong titanate nanotubes were synthesized by sonication-hydrothermal treatment using commercial TiO2 particles (the average particle size of 40 nm, anatase) as starting material. In a typical procedure, 2 g of TiO2 was mixed in 50 mL of 10 mol/L NaOH aqueous solution in a Teflon vessel. This solution was sonicated at different temperatures, different sonication powers and different sonication frequencies. The sonication was not performed until the floccule was achieved. The parameter of the experiment was varied according to Table 1. Four per five of the floccules was transferred into a 100 mL Teflon-lined autoclave by thermal treatment at 90 °C for 5 h. The autoclave was allowed to cool to room temperature. This sample and one per five of the floccule were discharged into plastic beaker, diluted with 1 L of distilled water, and then filtrated under vacuum. The filter cake was washed with distilled water repeatedly until the pH value of the washing solution was less than 7. The washed precipitates were dried in a vacuum oven at –45 °C for 12 h. The as-synthesized products by sonication and sonication-hydrothermal treatment will be henceforth represented as Si and Shi, respectively; i mean that sample number in Table 1. All the chemicals were of analytical grade and used without further purification as

received from Sinopharm Chemical Reagent Co., Ltd. The morphologies of the products were characterized by scanning electron microscopy (SEM, Rigaku S-570, Japan) and transmission electron microscopy (TEM, JEOL JEM-2010HR, Japan). The morphology of the products was also characterized by optics microscopy (OM, Motic DM-1802, German). The crystalline phases of the samples were characterized by a Japan Riguku D/max 2500 X-ray powder diffractometer (XRD) with Cu Kα irradiation (λ= 0.1542 nm ).

2

Results and Discussion

Fig.1 shows the images of the products synthesized via hydrothermal treatment with and without the sonication pretreatments. The TiO2 suspension solution (Fig.1a) can be changed to a sort of floccule (Fig.1b) via sonication

S1 S2 S3 S4 S5 S6 S7 S8 S9

Sh1 Sh2 Sh3 Sh4 Sh5 Sh6 Sh7 Sh8 Sh9

Frequency/kHz

Power/W

T/°C

Time/h

20 40 80 40 40 40 40 40 40

500 500 500 200 300 400 500 500 500

70 70 70 70 70 70 60 40 20

3 3 3 6 5 3 4 -

b

c

d

3 μm

3 μm e

e1

f

f1

3 μm

3 μm

Table 1 Sonication experimental condition for synthesis of TiO2 nanotubes from TiO2 nanoparticles Samples No.

a

g

h

50 nm

20 nm

Fig.1 Images of TiO2 suspensions without (a-OM image, c-SEM image) and with (b-OM image, d-SEM image) sonication at 500 W and 70 °C for 3 h before hydrothermal treatment, and

“-” the floccule was not achieved after 24 h via sonication

images of titanate products without (e-SEM image, e1-OM

treatment; under different sonication conditions without (samples

image, g-TEM image) and with (f-SEM image, f1-OM image,

S1 to S9) and with followed hydrothermal treatment (samples Sh1

h-TEM image) sonication after hydrothermal treatment at 90

to Sh9)

°C for 5 h

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Fig.2 OM images of products synthesized at 90 °C for 2 h under different sonication conditions without (samples S1 to S9) and with (samples Sh1 to Sh9) followed hydrothermal treatment

Na2Ti3O7 Anatanse-TiO2 TiO2 S9 S8 S7 S6 S5 S4 S3 S2 S1 Sh9 Sh8 Sh7 Sh6 Sh5 Sh4 Sh3 Sh2

Intensity/a.u.

pretreatment. The typical SEM image of the floccule (Fig.1d) shows that the TiO2 particles are swollen and transformed from the spherical shape into the hedgehog configuration, and Fig.1c shows that the TiO2 precursors have spherical particles configuration. The typical SEM (Fig.1f), TEM (Fig.1h), and OM (Fig.1f1) images of the products synthesized via hydrothermal process with sonication pretreatment show tubular morphology. The nanotubes possess about 20 nm of diameter and 1 up to 20 μm of length. The SEM (Fig.1e), TEM (Fig.1g), and OM (Fig.1e1) images of products without sonication pretreatment show the swollen spherical particles (the average particle size increases from 40 nm to 60 nm), and platelet morphology. These observations lead to the following conclusion: the sonication treatment plays an important role to promote intercalating Na+ into titania lattices and breaking the Ti-O-Ti bonds[6]. As a result, the TiO2 particles are swollen and changed to floccule; the resulting floccules imply that the ultralong nanotubes can be synthesized via followed hydrothermal treatment. So the floccules can or not be produced under different sonication conditions as listed in Table 1. The titanate floccules can be synthesized via 3 h of sonication treatment time at different sonication frequencies. The titanate floccules are produced via the longer sonication time at low sonication power (200 and 400 W). When sonication temperature is lower than 60 °C, the titanate floccules are not synthesized even via 24 h of sonication time, and no tubular structures (Sh8 and Sh9 in Fig.2) are synthesized by followed hydrothermal process. The resulting floccules take 4 h of longer sonication time at 60 °C than 3 h at 70 °C, and a combination of nanotubes and remaining platelet structures can be produced by followed hydrothermal process (Sh7 in Fig.2). The OM images of the products synthesized under different sonication conditions without (samples S1 to S9) and with followed hydrothermal treatment (samples Sh1 to Sh9) at 90 °C for 2 h are shown in Fig.2. Remarkable differences in the morphologies of the products can be observed. Many aggregations of particles and large or small platelet structures are noticed, and no tubular structure is formed under different sonication conditions without hydrothermal treatment (samples S1 to S9). The ultralong tubular structures can be produced under different sonication conditions except for 20 and 40 °C of sonication temperatures (no floccule) via hydrothermal process (samples Sh1 to Sh7). These nanotubes are randomly oriented and entwined, forming an intertexture-like hierarchical structured film. Fig.3 shows XRD patterns of the products synthesized under different sonication conditions without (samples S1 to S9) and with hydrothermal treatment (samples Sh1 to Sh9), including a typical XRD pattern for the raw TiO2 material. In the XRD patterns of the floccules (samples S1 to S7 in Fig.3) there appears the peak at 2θ of 10° indicating the

Sh1

10

20

30

40

50

60

70

2θ/(°) Fig.3 XRD patterns of products synthesized at different sonication conditions without (samples S1 to S9) and with (samples Sh1 to Sh9) followed hydrothermal treatment

layered photonic behavior, and anatase peak (around 2θ=25°) is weakened. Samples S8 and S9 in Fig.3 keep anatase phase along with the raw TiO2 materials. It suggests that anatase-typed TiO2 precursors can react quickly with NaOH

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and result in the formation of titanate under suitable sonication conditions, and increasing of sonication power or temperature is beneficial to this reaction. It is found that titanate nanostructure with anatase phase could be obtained under the different sonication conditions by followed hydrothermal process. In XRD patterns of the products (samples Sh1 to Sh7 in Fig.3) appear the peaks at 2θ= 10.44°, 15.65°, 29.61°, and 34.13° indicating that the as-synthesized nanotubes have the chemical composition Na2Ti3O7 (PDF# 31-1239), and the peaks are enhanced comparing with the floccules, while a number of weak anatase peaks are also presented in the XRD patterns indicating that a small amount of anatase-typed nanocrystals still remain in the products.

3

Conclusions

1) The ultralong nanotubes can be prepared via a low temperature of 90 °C and short duration of 5 h hydrothermal process with sonication pretreatment. 2) The titanate floccule synthesized via sonication treatment plays an important role to prepare ultralong titanate nanotubes by followed hydrothermal process. 3) Sonication temperature and power have a remarkable effect on the transformation from TiO2 particles to the titanate floccules. 4) The sonication-hydrothermal combination technique is expected to be used as a facile controlled, low-cost, quick

method for large scale synthesis of titanate nanotubes.

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