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Fabrication of titanium dioxide nanotubes with good morphology at high calcination temperature and their photocatalytic activity
Accepted Manuscript Fabrication of titanium dioxide nanotubes with good morphology at high calcination temperature and their photocatalytic activity Liangpeng Wu, Xu Yang, Juan Li, Yanqin Huang, Xinjun Li PII:
S0254-0584(17)30725-3
DOI:
10.1016/j.matchemphys.2017.09.022
Reference:
MAC 19989
To appear in:
Materials Chemistry and Physics
Received Date: 3 March 2017 Revised Date:
25 July 2017
Accepted Date: 12 September 2017
Please cite this article as: L. Wu, X. Yang, J. Li, Y. Huang, X. Li, Fabrication of titanium dioxide nanotubes with good morphology at high calcination temperature and their photocatalytic activity, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.09.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Fabrication of titanium dioxide nanotubes with good morphology at high calcination temperature and their photocatalytic activity
Lia,b,* a
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Liangpeng Wua,b, Xu Yanga,b, Juan Lia,b, Yanqin Huanga,b, Xinjun
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CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, P. R. China b Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, P. R. China
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Fax: +86 20 87057677; Tel: +86 20 87057781 E-mail: [email protected];
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ACCEPTED MANUSCRIPT Abstract: The hydrothermally synthesized titanate nanotubes were first modified by silane coupling agent as a pillar support, and then calcined at 450 oC. Finally, the surface coated silica was etched by concentrated NaOH to obtain pure titanium dioxide nanotube. The nanotubes were characterized by X-ray diffraction, high-resolution scanning and transmission electron microscopy,
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N2 adsorption-desorption isotherm measurements and photoluminescence spectra. The results revealed that titanium dioxide nanotubes have good tubular morphology, high anatase crystallinity and large surface area. Due to the better 1D nano-tubular morphology and higher separation efficiency of the photogenerated electron-hole pairs, titanium dioxide nanotube exhibits improved
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photocatalytic activity for the degradation of methyl orange.
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Keywords: Titanium oxide nanotubes, Good morphology, Photocatalytic activity, Methyl orange
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1. Introduction Since the photocatalytic splitting water over the TiO2 electrodes was discovered by Fujishima and Honda in 1972 [1], TiO2 becomes a hot topic and has been extensively applied for photocatalytic water splitting and CO2 reduction [2-4], photocatalytic degradation of pollutants
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and bacteria [5,6], dye-sensitized solar cells [7], sensors [8] and drug release plat form [9] and so on. It is well known that the photocatalytic performance depends not only on the crystallinity of TiO2 but also on the structure and morphologies in these applications [10,11].
Typically, TiO2 exhibits three major crystalline structures in nature: anatase (tetragonal), rutile
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(tetragonal), and brookite (orthorhombic) [12]. Other synthetic phases, TiO2(B), TiO2(H), and TiO2(R) as well as several high pressure polymorphs have also been reported [13]. Among these
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phases, anatase and rutile are the most important crystal structures for photocatalytic applications. The difference is that anatase-TiO2 with its lower electron mobility usually exhibits higher activity than rutile [14]. However, Rutile-TiO2 has the high scattering effect which leads to protection from the ultraviolet light [15]., Besides the crystalinity and the morphology of TiO2 can also influence the photocatalytic activity strongly. TiO2 nanostructural materials with different
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dimensionalities, such as nanoparticles, nanosheets, nanocables, nanofibers, nanowires, nanobelts, nanorods, nanotubes, and interconnected architectures, have been fabricated and enabled us to take full advantage of the unique properties [16-20]. Among these morphologies, TiO2 nanotubes
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have been received more and more attention due to their easily fabrication, high specific surface area, ion changeable ability, less charge recombination, and good chemical stability. As an idea
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photocatalytic candidate, it can also provide a fast transfer pathway, which facilitates electron transfer and subsequently improves the electron transfer efficiency, and thus benefits the photocatalytic activity [21]. The synthesis techniques of TiO2 nanotubes may be achieved by various routes including template-assisted fabrication, electrochemical anodization and alkaline hydrothermal treatment [22]. Among these preparation methods, alkaline hydrothermal method has merited more attention due to the simple procedure and cost-effective method for large scale production of nanotubes. In the past decade, Much previous efforts were mainly focused on the effects of hydrothermal parameters on the structure and the formation mechanism of nanotubes [23]. The pristine
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ACCEPTED MANUSCRIPT nanotubes (titanate nanotubes) with high specific surface area are often used as supports and adsorption materials, but the photocatalytic activity of titanate nanotubes was found to be markedly lower than that of the standard TiO2 or P25 in the photocatalytic reaction, such as of NH3 oxidation, organic degradation, as well as in the reaction of dye oxidation in aqueous
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suspensions [24]. This can be mainly attributed to the low crystallinity of titanate nanotubes. To improve the crystallization degree of titanate nanotubes, titanate nanotubes are transformed to the anatase titanium dioxide nanotubes by heat-treatment at elevated temperatures (usually below 400 o
C) [25]. In recent years, the “pillar-effect” method was used to protect the nanotubular structure
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at 400 oC. Zong et al used lanthanum nitrate to dope the titanate nanotubes, the obtained product was dried and calcined at 400 oC, and then removed La2O3 nanoparticles by washing with the
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diluted acid to obtain anatase TiO2 nanotubes [26]. Li et al. selected glucose to act as the pillar effect particles to maintain the structure of TiO2 nanotubes. Titanate nanotubes impregnated glucose was calcined in air at 400 °C, glucose would be removed completely during the calcination process and the anatase TiO2 nanotubes were obtained [27]. However, when the calcination temperature was above 400 oC, the one-dimensional multilayer nanotubular structure
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of titanate nanotubes would collapse during the dehydration process, and the BET surface area would decrease sharply [28]. So how to control the crystal phase transition and avoid the collapse of TiO2 nanotubes at more than 400 °C are the key points.
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Herein, we present a novel method to obtain titanium dioxide nanotubes with good morphology as well as high anatase crystalline. Firstly, titanate nanotubes were synthesized by hydrothermal
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method using TiO2 nanopowers as the precursors in alkali solution. Secondly, titanate nanotubes were modified by silane coupling agent, the obtained products was dried and calcinated at 450 °C. Finally, the surface coated silica was easily removed with the sodium hydroxide solution by the etching method to obtain anatase TiO2 nanotubes (Scheme S1). The morphology and structure of TiO2 nanotubes were characterized, and the photocatalytic performance was investigated under UV light irradiation. The relationship between the structure and the photocatalytic activity was discussed. This study may provide useful information and an effective approach for the preparation of TiO2 nanotubes at high calcinated temperature. Scheme S1
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ACCEPTED MANUSCRIPT 2. Experimantal 2.1 Materials Sodium hydroxide (NaOH, >96%), hydrochloric acid (HCl, 36%-38%), glacial acetic acid (CH3COOH, >99.5%) and ethanol (C2H5OH, >99.7%) were purchased from Guangzhou Chemical Reagent Factory. 3-methacryloxpropyltrimethoxysilane (KH-570, 99%) was purchased from
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Sinopharm Chemical Reagent Co., Ltd. TiO2 powders was purchased from Pangang Group Research Insitute Co., Ltd. P25 was purchased from Hualisen Trading Co., Ltd. Deionized water was used for all experiments.
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2.2 Catalysts preparation
All reagents are of analytical grade and used without any further purification. According to a
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typical procedure [29], 100 ml of 10 M NaOH solution and 2 g of TiO2 powders were put into a 250 ml Teflon-lined flask. The flask was maintained at 110 ºC under atmospheric pressure and stirred for 36 h, and cooled to room temperature naturally. The product was washed with deionized water until the pH value of the filtrate was about 7. Then the precipitate was bathed with 0.1 M HCl solution for 5 h and subsequently washed by deionized water until the pH value of the
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filtrate was about 7. Finally, the washed samples were dried at 60 ºC in air (named as titanate nanotubes), annealed at 450 ºC in the air for 1 h and marked as TiO2NTs. 1 g of as-prepared titanate nanotubes was ultrasonically dispersed in 50 ml deionized water for 10 min, and then the pH was adjusted to 3-4 using 5 wt% acetic acid. After stirring at room
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temperature for 1 h, 50 ml of ethanol dispersed with 0.2 mL 3-methacryloxypropyl -trimethoxysilane (KH570) was added into the solution and the mixture was stirred at 80 ºC for 5
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h. The precipitate was washed with deionized water followed by ethanol, dried at 60 ºC for 24 h in air and annealed at 450 ºC in the air for 1 h and marked as TiO2NTs-KH570. After the etching process to remove the SiO2 shell, the as-obtained TiO2NTs-KH570 were dispersed in 30 mL of 1.0 M sodium hydroxide solution and stirred for 3 h at room temperature. The precipitate was washed with deionized water followed by ethanol, dried at 60 ºC for 24 h in air and marked as TiO2NTs-1. 2.3 Catalysts Characterization A JEOL JSM-6700 field-emission scanning electron microscope (FESEM) was used to characterize the morphologies of the samples. High-resolution transmission electron microscopy (HRTEM) with selected area electron diffraction (SAED) was carried out using a JEM-2100F 5
ACCEPTED MANUSCRIPT microscope at 200 kV (JEOL Co., Ltd). Elemental mapping by energy-dispersive X-ray spectroscopy (EDX) was employed to gain further insight into the structure of the samples. Crystal-line phases of the samples were investigated with X-ray diffraction (XRD, X’Pert-PRO, PANalytical, Holland) equipped with Cu Ka radiation (l = 0.154056 nm) at an accelerating voltage
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of 40 kV and a current of 40 mA. The patterns were recorded in the 2θ range from 10o to 80o at a scan rate of 1.51 min-1. X-ray photoelectron spectroscopy (XPS) was performed with an AXIS Ultra DLD (Kratos, Britain) to examine the catalysts’ electronic properties. Fourier transform infrared spectra (FT-IR) were characterized in the range of 400-4000 cm-1 using a TENSOR27
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FT-IR spectrophotometer. The samples were pressed to conventional pellets at ambient conditions and measured in the transmission mode after blending with KBr powder. N2 adsorption-desorption
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isotherms were measured with a Tristar 3010 isothermal nitrogen sorption analyzer (Micromeritics Instruments, USA) using a continuous adsorption procedure. The BET method was used for surface area calculation, the pore size distribution (pore diameter and pore volume of the samples) wasdetermined by the BJH method. Photoluminescence (PL) spectra of the samples were recorded using a PerkinElmer LS550 spectrofluorimeter with a 150 W xenon lamp as the excitation source.
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2.4 Photocatalytic measurement
The photocatalytic activity of these samples was investigated using a self-made photocatalytic reactor at ambient temperature. A high pressure mercury lamp (125 W, λ= 365 nm)
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was used as the light source and preheated for 30 min in order to reach a stable irradiation during photocatalytic degradation. In each experiment, the reaction suspension was prepared by adding
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0.1 g of the catalysts into 350 mL of methyl orange aqueous solution with an initial concentration of 20 mg/L. Before the irradiation, the suspension was magnetically stirred in the dark for 30 min in order to reach an adsorption-desorption equilibrium. The cooling water was recycled throughout the reaction period to maintain the room temperature. The absorbance peak of methyl orange was recorded every 10 min to enable calculation of the degradation rate. A UV-vis (U-3010, Hitachi) spectrophotometer was used to determine the concentration of the methyl orange solution during the photocatalytic degradation reaction.
3. Results and discussion As is well known, TiO2NTs are obtained due to the slow dissolution-recrystallization process 6
ACCEPTED MANUSCRIPT and low growth kinetic of nanotubes at static condition [30]. In the first stage, the Ti-O-Ti bonds of the TiO2 powder are broken during the primary stage of hydrothermal treatments. Ti-O-Na and Ti-OH layered nanosheets on the surface of TiO2 are formed under the action of 10 M NaOH. In the second stage, the layered nanosheets are exfoliated with the increase of curling tendency,
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leading to the formation of layered sodium titanate nanotubes [31]. In the third stage, the as-prepared sodium titanate nanotubes are rinsed by diluted HCl solution and deionized water. Na+ ions are exchanged and H+ ions are distributed loosely in the interlayer space. The Na-O-Ti bonds are thought to react with acid and water to form the new H-O-Ti and Ti-O-Ti bonds [32]. Finally,
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titanate nanotubes are transformed into the multilayers of anatase titanium dioxide nanotubes under the certain temperature sintering.
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The morphology and structure of TiO2NTs and TiO2NTs-1 were assessed using FE-SEM and HRTEM measurements. In our previous experiments, titanate nanotubes have several hundreds of nanometers in length and a strong tendency to agglomerate. Also they are multilayer, a hollow cavity and open at both ends with diameters of about 10 nm. As shown in Fig. 1(a) and (b). After calcinating at 450 oC, most of the titanate nanotubes become short with a relative rough surface
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(see inset Fig. 1(b)), indicating the nanotubular structure partially collapsed to the nanoparticles. This can be ascribed to the dehydration of interlayered OH groups during calcination [33]. For the KH570 modified titanate nanotubes calcinated at 450 oC, the nanotubular morphology of the
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modified titanate nanotubes is similar with that of primary titanate nanotubes (see Fig. S1), indicating that the surface coated silica plays a key role in keeping the nanotubular structure. The
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introduction of KH570 onto the titanate nanotubes surfaces was confirmed by FT-IR (see Fig. S2). When the silica was removed by etching of 0.1 M NaOH, the nanotubular structure was almost completely preserved. The results also well agree with that observed by Kim et al [34]. Figure 1
The crystal structure of TiO2NTs, TiO2NTs-KH570 and TiO2NTs-1 was measured by XRD analysis. As is shown in Fig. 2, Titanate nanotubes transformed to anatase TiO2NTs when calcined at 450 °C, and the diffraction peaks at about 2θ = 25.4°, 37.8°, 48.2°, 53.9°, 55.190° and 62.7° are corresponding to the (101), (004), (200), (105), (211) and (204) crystal faces of anatase (JCPDS 21-1272), respectively [35]. The TiO2NTs-KH570 shows similar XRD pattern as that of TiO2NTs, 7
ACCEPTED MANUSCRIPT but broadened and weakened intensity for each diffraction peaks, indicating that the presence of silica can sluggish its crystallization degree [36]. The EDX and EDS obtained from TEM analysis in Fig. S3 and S4 showed that the composition of TiO2NTs-KH570 consisted of O, Si and Ti elements, which confirmed the presence of silica on the surface of TiO2NTs. Compared to the
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XRD pattern of TiO2NTs-KH570, the intensity of diffraction peaks of TiO2NTs-1 is strengthened, and the XRD pattern of TiO2NTs-1 was indexed solely to the crystalline anatase phase of TiO2 without any discernible silica characteristic diffraction peaks. This may be due to the small amount of silica or the silica atoms serve as an interstitial atom well-inserted into the crystal lattice
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of TiO2. In addition, the corresponding SAED pattern in Fig. S4 also indicates that these samples of TiO2NTs, TiO2NTs-KH570 and TiO2NTs-1 show the well-hold crystalline nature of anatase
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TiO2. Figure 2
To further confirm the existence of silica, XPS analysis was performed. It can be seen clearly from Fig. 3a, the wide-scan XPS spectra testify that Ti, O and C elements exist in TiO2NTs,
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TiO2NTs-KH570 and TiO2NTs-1, and the photoelectron peak for C1s at 284 eV is probably attributed to the contamination caused by the residual carbon from the precursor solution and specimen handing or pumping oil from the XPS instrument itself [37]. The binding energy peaks of 458.9 and 464.5 eV correspond to Ti 2p1/2 and Ti 2p3/2, indicating that titanium is entirely at
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presence of Ti4+. As shown in Fig. 3b, the Si 2p and Si 2s binding energies in TiO2NTs-KH570 are detected at 102.2 eV and 153.1 eV, which is consistent with the literature reported [38]. Compared
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with the TiO2NTs, the O 1s and Ti 2p peaks in TiO2NTs-KH570 exhibit a blue shift. It is because pauling values of Si (1.8) are larger than that of Ti (1.5), the combination of Ti atoms and Si atoms with larger electronegative will lead to the decrease of coordination number of Ti [39]. Therefore, O 1s and Ti 2p binding energy of TiO2NTs are smaller than that of TiO2NTs-KH570. These results confirmed the presence of KH570 onto the TiO2NTs surfaces in TiO2NTs-KH570. In addition, we found that compared to TiO2NTs-KH570, O 1s and Ti 2p peaks in TiO2NTs-1 exhibit a red shift compared to that in TiO2NTs-KH570, the binding energy peaks of Si in TiO2NTs-1 are negligible, indicating that most of silica was removed from TiO2 nanotubes. However, O 1s and Ti 2p peaks in TiO2NTs-1 exhibit a blue shift compared to that in TiO2NTs, indicating the good 8
ACCEPTED MANUSCRIPT nanotubular morphology and efficient electron transfer property. Figure 3 N2 adsorption-desorption isotherms and the corresponding pore diameter distribution of the TiO2NTs and TiO2NTs-1 sample are presented in Fig. 4. The isotherms for both of the catalysts are
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of type III (BDDT classification) with a large hysteresis hoop of type H3, the presence of mesopores (2-50 nm). Moreover, the observed hysteresis loop approach P/P0 = 1, suggesting the presence of macropores (>50 nm) [40]. It is known from Fig. 4(b), the corresponding pore size
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distributions of TiO2NTs-1 confirmed the two types of pores: the smaller pores (<8 nm) can be attributed to the pores inside the nanotubes, while the larger pores (8-100 nm) can be attributed to
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the aggregation of the nanotubes. It can be seen that pore size distributions of TiO2NTs shifted from the direction of the large pore size, it may be attributed to collapse of the pore channels inside the nanotubes structure. The physical properties, including BET surface area, average pore size and total pore volume are summarized in Table S1. The BET specific surface area of TiO2NTs-1 is 192 m2/g, which increased apparently compared to that of TiO2NTs (SBET=132 m2/g).
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The increased BET surface area should be associated with the good nanotubular morphology of TiO2NTs-1, as verified by the results of TEM images. Figure 4
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The photocatalytic activity of P25, the prepared TiO2NTs and TiO2NTs-1 was evaluated by probing the photocatalytic decomposition of MO under UV-vis light irradiation as a function of
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irradiation time and the experiment error was within 5%. The strong absorption peak at 464.5 nm of MO aqueous solution was monitored at certain time intervals to realize the degradation kinetics. Fig. 5 shows the photocatalytic degradation rate of MO under UV irradiation. The value of degradation rate for TiO2NTs is about 60% after 60 min of irradiation, while for TiO2NTs-1, the ratio of degraded MO is markedly increased to 90% after only 40 min of irradiation. To compare the reaction kinetics of TiO2NTs and TiO2NTs-1, we assume that MO follows the first-order rate law, ln (C/C0) = kt, where k is the apparent rate constant for degradation. It can be seen the linear relationship of ln (C/C0) with irradiation time in Fig. 4(b). According to the above reaction kinetic equation, the calculated apparent reaction rate constants k for P25, TiO2NTs and TiO2NTs-1 were
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ACCEPTED MANUSCRIPT 0.028, 0.016 and 0.071, respectively. It means that the k values of TiO2NTs-1was the largest. It is noteworthy that the photocatalytic activity of TiO2NTs-1 was higher than P25and TiO2NTs. Figure 5 In general, when TiO2 is irradiated by UV light, conduction band electrons (eCB-) and valence
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band holes (hVB+) are generated and separated over the surface of TiO2. It is also possible that the charge carriers recombine with the release of heat on the surface of TiO2 [32]. The valence band holes can oxidize surface adsorbed water or hydroxide to produce hydroxyl radicals, while the
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conduction bands electrons can absorbed by dissolved oxygen to generate superoxide (O2-). In the degradation process, the hydroxyl radicals or superoxide play a key role to attacks organic
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compounds. It is well known that the photocatalytic properties of TiO2 strongly depend on the morphology, crystallinity, and specific surface area, the effective integration of good crystalline with a large surface area is expected to effectively promote the separation and transfer of photo-induced charge carriers and thus enhancing photocatalytic efficiencies [41]. On the basis of the above characterizations, the difference in the photocatalytic degradation activity of MO over
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TiO2NTs and TiO2NTs-1 was mainly ascribed to the specific surface area and the separation efficiency of the photogenerated electron-hole pairs. As discussed with the SEM and TEM results, the length of the TiO2NTs decreased, and the nanotubular shape of TiO2NTs has partially collapsed to small nanoparticles with larger particle sizes. Whereas the nanotubular structure of TiO2NTs-1
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still retains under the same calcination temperature. As is well known that the catalyst with high specific surface area has larger number of active sites per catalyst mass-volume, and the active
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sites are better dispersed. In our experimental result, The BET specific surface areas of TiO2NTs-1and TiO2NTs were 192 and 132 m2/g, respectively. In addition, the sample of TiO2NTs-1 possesses the perfect 1D nanotubular structure, which is propitious to the separation and transmission for the photogenerated electron-hole pairs. As shown in Fig. S5, the PL intensity of TiO2NTs-1 is much lower than that of TiO2NTs, revealing a higher separation efficiency of photoinduced carriers over TiO2NTs-1. Hence TiO2NTs-1 exhibits the enhanced photocatalytic activity. We think that TiO2 nanotubes with good nanotubular morphology at high calcinations temperature should have great applications in the field of catalysis, solar cells, gas sensing, photocatalytic H2O splitting and CO2 reduction, the separation and purification processes of water 10
ACCEPTED MANUSCRIPT or air pollution, as well as several biomedical engineering.
4. Conclusion TiO2 nanotubes with good nanotubular morphology at high calcinations temperature had been successfully fabricated using titanate nanotubes as the precursor and silane coupling agent as a
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pillar support. The BET specific surface area of TiO2NTs-1 was 192 m2/g, which was much larger than that of the TiO2NTs obtained by direct calcination of titanate nanotubes at 450 oC. And the TiO2NTs-1sample exhibits the higher photocatalytic degradation activity for MO, which should be
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attributed to the better 1D nanotubular morphology with large BET specific surface area and higher separation efficiency of the photogenerated electron-hole pairs.
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Acknowledgments
The work was supported by Guangdong Natural Science Foundation (No. 2015A030313715), the National Natural Science Foundation of China (No. 51661145022).
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ACCEPTED MANUSCRIPT propylene on Pd-loaded anatase TiO2 nanotubes under visible light irradiation, Nanoscale Res. Lett. 11 (2016) 271. [28] J. Yang, Z. Jin, X. Wang, W. Li, J. Zhang, S. Zhang, X. Guo, Z. Zhang, Study on composition, structure and formation process of nanotube Na2Ti2O4(OH)2, Dalton. Trans. 20 (2003)
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ACCEPTED MANUSCRIPT photocatalytic activity of the neodymium-doped TiO2 nanotubes, Appl. Surf. Sci. 255 (2009) 8624-8628. [38] H.B. Cao, P.F. Du, L.X. Song, J. Xiong, J.J. Yang, T.H. Xing, X. Liu, R.R. Wu, M.C. Wang, X.L. Shao, Co-electrospinning fabrication and photocatalytic performance of TiO2/SiO2
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ACCEPTED MANUSCRIPT Figures Captions Fig. 1 SEM images of TiO2NTs (a) and TiO2NTMs-1 (c), HRTEM images of TiO2NTs (b) and TiO2NTs-1 (d).
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Fig. 2 XRD patterns of TiO2NTs, TiO2NTs-KH570 and TiO2NTs-1 Fig. 3 XPS spectra of TiO2NTs, TiO2NTs- KH570 and TiO2NTs-1: a wide-scan XPS spectra, b Si 2p narrow scan, c O 1s narrow scan, d Ti 2p narrow scan.
distribution of TiO2NTs and TiO2NTs-1.
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Fig. 4 (a) Nitrogen adsorption-desorption isotherm. (b) The corresponding BJH pore-size
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Fig. 5 (a) Photocatalytic degradation of MO (20 mg/L) over P25, TiO2NTs and TiO2NTs-1 under
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UV-vis light irradiation, (b) Degradation kinetic analysis of MO.
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ACCEPTED MANUSCRIPT Highlights • Titanium dioxide nanotube was synthesized by hydrothermal, modification and etching process. • Titanium dioxide nanotube has good morphology at high calcination temperature.
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• Titanium dioxide nanotube exhibits improved photocatalytic activity.
S0254-0584(17)30725-3
DOI:
10.1016/j.matchemphys.2017.09.022
Reference:
MAC 19989
To appear in:
Materials Chemistry and Physics
Received Date: 3 March 2017 Revised Date:
25 July 2017
Accepted Date: 12 September 2017
Please cite this article as: L. Wu, X. Yang, J. Li, Y. Huang, X. Li, Fabrication of titanium dioxide nanotubes with good morphology at high calcination temperature and their photocatalytic activity, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.09.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Fabrication of titanium dioxide nanotubes with good morphology at high calcination temperature and their photocatalytic activity
Lia,b,* a
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Liangpeng Wua,b, Xu Yanga,b, Juan Lia,b, Yanqin Huanga,b, Xinjun
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CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, P. R. China b Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, P. R. China
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Fax: +86 20 87057677; Tel: +86 20 87057781 E-mail: [email protected];
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ACCEPTED MANUSCRIPT Abstract: The hydrothermally synthesized titanate nanotubes were first modified by silane coupling agent as a pillar support, and then calcined at 450 oC. Finally, the surface coated silica was etched by concentrated NaOH to obtain pure titanium dioxide nanotube. The nanotubes were characterized by X-ray diffraction, high-resolution scanning and transmission electron microscopy,
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N2 adsorption-desorption isotherm measurements and photoluminescence spectra. The results revealed that titanium dioxide nanotubes have good tubular morphology, high anatase crystallinity and large surface area. Due to the better 1D nano-tubular morphology and higher separation efficiency of the photogenerated electron-hole pairs, titanium dioxide nanotube exhibits improved
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photocatalytic activity for the degradation of methyl orange.
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Keywords: Titanium oxide nanotubes, Good morphology, Photocatalytic activity, Methyl orange
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1. Introduction Since the photocatalytic splitting water over the TiO2 electrodes was discovered by Fujishima and Honda in 1972 [1], TiO2 becomes a hot topic and has been extensively applied for photocatalytic water splitting and CO2 reduction [2-4], photocatalytic degradation of pollutants
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and bacteria [5,6], dye-sensitized solar cells [7], sensors [8] and drug release plat form [9] and so on. It is well known that the photocatalytic performance depends not only on the crystallinity of TiO2 but also on the structure and morphologies in these applications [10,11].
Typically, TiO2 exhibits three major crystalline structures in nature: anatase (tetragonal), rutile
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(tetragonal), and brookite (orthorhombic) [12]. Other synthetic phases, TiO2(B), TiO2(H), and TiO2(R) as well as several high pressure polymorphs have also been reported [13]. Among these
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phases, anatase and rutile are the most important crystal structures for photocatalytic applications. The difference is that anatase-TiO2 with its lower electron mobility usually exhibits higher activity than rutile [14]. However, Rutile-TiO2 has the high scattering effect which leads to protection from the ultraviolet light [15]., Besides the crystalinity and the morphology of TiO2 can also influence the photocatalytic activity strongly. TiO2 nanostructural materials with different
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dimensionalities, such as nanoparticles, nanosheets, nanocables, nanofibers, nanowires, nanobelts, nanorods, nanotubes, and interconnected architectures, have been fabricated and enabled us to take full advantage of the unique properties [16-20]. Among these morphologies, TiO2 nanotubes
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have been received more and more attention due to their easily fabrication, high specific surface area, ion changeable ability, less charge recombination, and good chemical stability. As an idea
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photocatalytic candidate, it can also provide a fast transfer pathway, which facilitates electron transfer and subsequently improves the electron transfer efficiency, and thus benefits the photocatalytic activity [21]. The synthesis techniques of TiO2 nanotubes may be achieved by various routes including template-assisted fabrication, electrochemical anodization and alkaline hydrothermal treatment [22]. Among these preparation methods, alkaline hydrothermal method has merited more attention due to the simple procedure and cost-effective method for large scale production of nanotubes. In the past decade, Much previous efforts were mainly focused on the effects of hydrothermal parameters on the structure and the formation mechanism of nanotubes [23]. The pristine
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ACCEPTED MANUSCRIPT nanotubes (titanate nanotubes) with high specific surface area are often used as supports and adsorption materials, but the photocatalytic activity of titanate nanotubes was found to be markedly lower than that of the standard TiO2 or P25 in the photocatalytic reaction, such as of NH3 oxidation, organic degradation, as well as in the reaction of dye oxidation in aqueous
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suspensions [24]. This can be mainly attributed to the low crystallinity of titanate nanotubes. To improve the crystallization degree of titanate nanotubes, titanate nanotubes are transformed to the anatase titanium dioxide nanotubes by heat-treatment at elevated temperatures (usually below 400 o
C) [25]. In recent years, the “pillar-effect” method was used to protect the nanotubular structure
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at 400 oC. Zong et al used lanthanum nitrate to dope the titanate nanotubes, the obtained product was dried and calcined at 400 oC, and then removed La2O3 nanoparticles by washing with the
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diluted acid to obtain anatase TiO2 nanotubes [26]. Li et al. selected glucose to act as the pillar effect particles to maintain the structure of TiO2 nanotubes. Titanate nanotubes impregnated glucose was calcined in air at 400 °C, glucose would be removed completely during the calcination process and the anatase TiO2 nanotubes were obtained [27]. However, when the calcination temperature was above 400 oC, the one-dimensional multilayer nanotubular structure
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of titanate nanotubes would collapse during the dehydration process, and the BET surface area would decrease sharply [28]. So how to control the crystal phase transition and avoid the collapse of TiO2 nanotubes at more than 400 °C are the key points.
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Herein, we present a novel method to obtain titanium dioxide nanotubes with good morphology as well as high anatase crystalline. Firstly, titanate nanotubes were synthesized by hydrothermal
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method using TiO2 nanopowers as the precursors in alkali solution. Secondly, titanate nanotubes were modified by silane coupling agent, the obtained products was dried and calcinated at 450 °C. Finally, the surface coated silica was easily removed with the sodium hydroxide solution by the etching method to obtain anatase TiO2 nanotubes (Scheme S1). The morphology and structure of TiO2 nanotubes were characterized, and the photocatalytic performance was investigated under UV light irradiation. The relationship between the structure and the photocatalytic activity was discussed. This study may provide useful information and an effective approach for the preparation of TiO2 nanotubes at high calcinated temperature. Scheme S1
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ACCEPTED MANUSCRIPT 2. Experimantal 2.1 Materials Sodium hydroxide (NaOH, >96%), hydrochloric acid (HCl, 36%-38%), glacial acetic acid (CH3COOH, >99.5%) and ethanol (C2H5OH, >99.7%) were purchased from Guangzhou Chemical Reagent Factory. 3-methacryloxpropyltrimethoxysilane (KH-570, 99%) was purchased from
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Sinopharm Chemical Reagent Co., Ltd. TiO2 powders was purchased from Pangang Group Research Insitute Co., Ltd. P25 was purchased from Hualisen Trading Co., Ltd. Deionized water was used for all experiments.
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2.2 Catalysts preparation
All reagents are of analytical grade and used without any further purification. According to a
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typical procedure [29], 100 ml of 10 M NaOH solution and 2 g of TiO2 powders were put into a 250 ml Teflon-lined flask. The flask was maintained at 110 ºC under atmospheric pressure and stirred for 36 h, and cooled to room temperature naturally. The product was washed with deionized water until the pH value of the filtrate was about 7. Then the precipitate was bathed with 0.1 M HCl solution for 5 h and subsequently washed by deionized water until the pH value of the
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filtrate was about 7. Finally, the washed samples were dried at 60 ºC in air (named as titanate nanotubes), annealed at 450 ºC in the air for 1 h and marked as TiO2NTs. 1 g of as-prepared titanate nanotubes was ultrasonically dispersed in 50 ml deionized water for 10 min, and then the pH was adjusted to 3-4 using 5 wt% acetic acid. After stirring at room
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temperature for 1 h, 50 ml of ethanol dispersed with 0.2 mL 3-methacryloxypropyl -trimethoxysilane (KH570) was added into the solution and the mixture was stirred at 80 ºC for 5
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h. The precipitate was washed with deionized water followed by ethanol, dried at 60 ºC for 24 h in air and annealed at 450 ºC in the air for 1 h and marked as TiO2NTs-KH570. After the etching process to remove the SiO2 shell, the as-obtained TiO2NTs-KH570 were dispersed in 30 mL of 1.0 M sodium hydroxide solution and stirred for 3 h at room temperature. The precipitate was washed with deionized water followed by ethanol, dried at 60 ºC for 24 h in air and marked as TiO2NTs-1. 2.3 Catalysts Characterization A JEOL JSM-6700 field-emission scanning electron microscope (FESEM) was used to characterize the morphologies of the samples. High-resolution transmission electron microscopy (HRTEM) with selected area electron diffraction (SAED) was carried out using a JEM-2100F 5
ACCEPTED MANUSCRIPT microscope at 200 kV (JEOL Co., Ltd). Elemental mapping by energy-dispersive X-ray spectroscopy (EDX) was employed to gain further insight into the structure of the samples. Crystal-line phases of the samples were investigated with X-ray diffraction (XRD, X’Pert-PRO, PANalytical, Holland) equipped with Cu Ka radiation (l = 0.154056 nm) at an accelerating voltage
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of 40 kV and a current of 40 mA. The patterns were recorded in the 2θ range from 10o to 80o at a scan rate of 1.51 min-1. X-ray photoelectron spectroscopy (XPS) was performed with an AXIS Ultra DLD (Kratos, Britain) to examine the catalysts’ electronic properties. Fourier transform infrared spectra (FT-IR) were characterized in the range of 400-4000 cm-1 using a TENSOR27
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FT-IR spectrophotometer. The samples were pressed to conventional pellets at ambient conditions and measured in the transmission mode after blending with KBr powder. N2 adsorption-desorption
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isotherms were measured with a Tristar 3010 isothermal nitrogen sorption analyzer (Micromeritics Instruments, USA) using a continuous adsorption procedure. The BET method was used for surface area calculation, the pore size distribution (pore diameter and pore volume of the samples) wasdetermined by the BJH method. Photoluminescence (PL) spectra of the samples were recorded using a PerkinElmer LS550 spectrofluorimeter with a 150 W xenon lamp as the excitation source.
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2.4 Photocatalytic measurement
The photocatalytic activity of these samples was investigated using a self-made photocatalytic reactor at ambient temperature. A high pressure mercury lamp (125 W, λ= 365 nm)
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was used as the light source and preheated for 30 min in order to reach a stable irradiation during photocatalytic degradation. In each experiment, the reaction suspension was prepared by adding
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0.1 g of the catalysts into 350 mL of methyl orange aqueous solution with an initial concentration of 20 mg/L. Before the irradiation, the suspension was magnetically stirred in the dark for 30 min in order to reach an adsorption-desorption equilibrium. The cooling water was recycled throughout the reaction period to maintain the room temperature. The absorbance peak of methyl orange was recorded every 10 min to enable calculation of the degradation rate. A UV-vis (U-3010, Hitachi) spectrophotometer was used to determine the concentration of the methyl orange solution during the photocatalytic degradation reaction.
3. Results and discussion As is well known, TiO2NTs are obtained due to the slow dissolution-recrystallization process 6
ACCEPTED MANUSCRIPT and low growth kinetic of nanotubes at static condition [30]. In the first stage, the Ti-O-Ti bonds of the TiO2 powder are broken during the primary stage of hydrothermal treatments. Ti-O-Na and Ti-OH layered nanosheets on the surface of TiO2 are formed under the action of 10 M NaOH. In the second stage, the layered nanosheets are exfoliated with the increase of curling tendency,
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leading to the formation of layered sodium titanate nanotubes [31]. In the third stage, the as-prepared sodium titanate nanotubes are rinsed by diluted HCl solution and deionized water. Na+ ions are exchanged and H+ ions are distributed loosely in the interlayer space. The Na-O-Ti bonds are thought to react with acid and water to form the new H-O-Ti and Ti-O-Ti bonds [32]. Finally,
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titanate nanotubes are transformed into the multilayers of anatase titanium dioxide nanotubes under the certain temperature sintering.
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The morphology and structure of TiO2NTs and TiO2NTs-1 were assessed using FE-SEM and HRTEM measurements. In our previous experiments, titanate nanotubes have several hundreds of nanometers in length and a strong tendency to agglomerate. Also they are multilayer, a hollow cavity and open at both ends with diameters of about 10 nm. As shown in Fig. 1(a) and (b). After calcinating at 450 oC, most of the titanate nanotubes become short with a relative rough surface
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(see inset Fig. 1(b)), indicating the nanotubular structure partially collapsed to the nanoparticles. This can be ascribed to the dehydration of interlayered OH groups during calcination [33]. For the KH570 modified titanate nanotubes calcinated at 450 oC, the nanotubular morphology of the
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modified titanate nanotubes is similar with that of primary titanate nanotubes (see Fig. S1), indicating that the surface coated silica plays a key role in keeping the nanotubular structure. The
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introduction of KH570 onto the titanate nanotubes surfaces was confirmed by FT-IR (see Fig. S2). When the silica was removed by etching of 0.1 M NaOH, the nanotubular structure was almost completely preserved. The results also well agree with that observed by Kim et al [34]. Figure 1
The crystal structure of TiO2NTs, TiO2NTs-KH570 and TiO2NTs-1 was measured by XRD analysis. As is shown in Fig. 2, Titanate nanotubes transformed to anatase TiO2NTs when calcined at 450 °C, and the diffraction peaks at about 2θ = 25.4°, 37.8°, 48.2°, 53.9°, 55.190° and 62.7° are corresponding to the (101), (004), (200), (105), (211) and (204) crystal faces of anatase (JCPDS 21-1272), respectively [35]. The TiO2NTs-KH570 shows similar XRD pattern as that of TiO2NTs, 7
ACCEPTED MANUSCRIPT but broadened and weakened intensity for each diffraction peaks, indicating that the presence of silica can sluggish its crystallization degree [36]. The EDX and EDS obtained from TEM analysis in Fig. S3 and S4 showed that the composition of TiO2NTs-KH570 consisted of O, Si and Ti elements, which confirmed the presence of silica on the surface of TiO2NTs. Compared to the
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XRD pattern of TiO2NTs-KH570, the intensity of diffraction peaks of TiO2NTs-1 is strengthened, and the XRD pattern of TiO2NTs-1 was indexed solely to the crystalline anatase phase of TiO2 without any discernible silica characteristic diffraction peaks. This may be due to the small amount of silica or the silica atoms serve as an interstitial atom well-inserted into the crystal lattice
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of TiO2. In addition, the corresponding SAED pattern in Fig. S4 also indicates that these samples of TiO2NTs, TiO2NTs-KH570 and TiO2NTs-1 show the well-hold crystalline nature of anatase
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TiO2. Figure 2
To further confirm the existence of silica, XPS analysis was performed. It can be seen clearly from Fig. 3a, the wide-scan XPS spectra testify that Ti, O and C elements exist in TiO2NTs,
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TiO2NTs-KH570 and TiO2NTs-1, and the photoelectron peak for C1s at 284 eV is probably attributed to the contamination caused by the residual carbon from the precursor solution and specimen handing or pumping oil from the XPS instrument itself [37]. The binding energy peaks of 458.9 and 464.5 eV correspond to Ti 2p1/2 and Ti 2p3/2, indicating that titanium is entirely at
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presence of Ti4+. As shown in Fig. 3b, the Si 2p and Si 2s binding energies in TiO2NTs-KH570 are detected at 102.2 eV and 153.1 eV, which is consistent with the literature reported [38]. Compared
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with the TiO2NTs, the O 1s and Ti 2p peaks in TiO2NTs-KH570 exhibit a blue shift. It is because pauling values of Si (1.8) are larger than that of Ti (1.5), the combination of Ti atoms and Si atoms with larger electronegative will lead to the decrease of coordination number of Ti [39]. Therefore, O 1s and Ti 2p binding energy of TiO2NTs are smaller than that of TiO2NTs-KH570. These results confirmed the presence of KH570 onto the TiO2NTs surfaces in TiO2NTs-KH570. In addition, we found that compared to TiO2NTs-KH570, O 1s and Ti 2p peaks in TiO2NTs-1 exhibit a red shift compared to that in TiO2NTs-KH570, the binding energy peaks of Si in TiO2NTs-1 are negligible, indicating that most of silica was removed from TiO2 nanotubes. However, O 1s and Ti 2p peaks in TiO2NTs-1 exhibit a blue shift compared to that in TiO2NTs, indicating the good 8
ACCEPTED MANUSCRIPT nanotubular morphology and efficient electron transfer property. Figure 3 N2 adsorption-desorption isotherms and the corresponding pore diameter distribution of the TiO2NTs and TiO2NTs-1 sample are presented in Fig. 4. The isotherms for both of the catalysts are
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of type III (BDDT classification) with a large hysteresis hoop of type H3, the presence of mesopores (2-50 nm). Moreover, the observed hysteresis loop approach P/P0 = 1, suggesting the presence of macropores (>50 nm) [40]. It is known from Fig. 4(b), the corresponding pore size
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distributions of TiO2NTs-1 confirmed the two types of pores: the smaller pores (<8 nm) can be attributed to the pores inside the nanotubes, while the larger pores (8-100 nm) can be attributed to
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the aggregation of the nanotubes. It can be seen that pore size distributions of TiO2NTs shifted from the direction of the large pore size, it may be attributed to collapse of the pore channels inside the nanotubes structure. The physical properties, including BET surface area, average pore size and total pore volume are summarized in Table S1. The BET specific surface area of TiO2NTs-1 is 192 m2/g, which increased apparently compared to that of TiO2NTs (SBET=132 m2/g).
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The increased BET surface area should be associated with the good nanotubular morphology of TiO2NTs-1, as verified by the results of TEM images. Figure 4
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The photocatalytic activity of P25, the prepared TiO2NTs and TiO2NTs-1 was evaluated by probing the photocatalytic decomposition of MO under UV-vis light irradiation as a function of
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irradiation time and the experiment error was within 5%. The strong absorption peak at 464.5 nm of MO aqueous solution was monitored at certain time intervals to realize the degradation kinetics. Fig. 5 shows the photocatalytic degradation rate of MO under UV irradiation. The value of degradation rate for TiO2NTs is about 60% after 60 min of irradiation, while for TiO2NTs-1, the ratio of degraded MO is markedly increased to 90% after only 40 min of irradiation. To compare the reaction kinetics of TiO2NTs and TiO2NTs-1, we assume that MO follows the first-order rate law, ln (C/C0) = kt, where k is the apparent rate constant for degradation. It can be seen the linear relationship of ln (C/C0) with irradiation time in Fig. 4(b). According to the above reaction kinetic equation, the calculated apparent reaction rate constants k for P25, TiO2NTs and TiO2NTs-1 were
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band holes (hVB+) are generated and separated over the surface of TiO2. It is also possible that the charge carriers recombine with the release of heat on the surface of TiO2 [32]. The valence band holes can oxidize surface adsorbed water or hydroxide to produce hydroxyl radicals, while the
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conduction bands electrons can absorbed by dissolved oxygen to generate superoxide (O2-). In the degradation process, the hydroxyl radicals or superoxide play a key role to attacks organic
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compounds. It is well known that the photocatalytic properties of TiO2 strongly depend on the morphology, crystallinity, and specific surface area, the effective integration of good crystalline with a large surface area is expected to effectively promote the separation and transfer of photo-induced charge carriers and thus enhancing photocatalytic efficiencies [41]. On the basis of the above characterizations, the difference in the photocatalytic degradation activity of MO over
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TiO2NTs and TiO2NTs-1 was mainly ascribed to the specific surface area and the separation efficiency of the photogenerated electron-hole pairs. As discussed with the SEM and TEM results, the length of the TiO2NTs decreased, and the nanotubular shape of TiO2NTs has partially collapsed to small nanoparticles with larger particle sizes. Whereas the nanotubular structure of TiO2NTs-1
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still retains under the same calcination temperature. As is well known that the catalyst with high specific surface area has larger number of active sites per catalyst mass-volume, and the active
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sites are better dispersed. In our experimental result, The BET specific surface areas of TiO2NTs-1and TiO2NTs were 192 and 132 m2/g, respectively. In addition, the sample of TiO2NTs-1 possesses the perfect 1D nanotubular structure, which is propitious to the separation and transmission for the photogenerated electron-hole pairs. As shown in Fig. S5, the PL intensity of TiO2NTs-1 is much lower than that of TiO2NTs, revealing a higher separation efficiency of photoinduced carriers over TiO2NTs-1. Hence TiO2NTs-1 exhibits the enhanced photocatalytic activity. We think that TiO2 nanotubes with good nanotubular morphology at high calcinations temperature should have great applications in the field of catalysis, solar cells, gas sensing, photocatalytic H2O splitting and CO2 reduction, the separation and purification processes of water 10
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4. Conclusion TiO2 nanotubes with good nanotubular morphology at high calcinations temperature had been successfully fabricated using titanate nanotubes as the precursor and silane coupling agent as a
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pillar support. The BET specific surface area of TiO2NTs-1 was 192 m2/g, which was much larger than that of the TiO2NTs obtained by direct calcination of titanate nanotubes at 450 oC. And the TiO2NTs-1sample exhibits the higher photocatalytic degradation activity for MO, which should be
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attributed to the better 1D nanotubular morphology with large BET specific surface area and higher separation efficiency of the photogenerated electron-hole pairs.
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Acknowledgments
The work was supported by Guangdong Natural Science Foundation (No. 2015A030313715), the National Natural Science Foundation of China (No. 51661145022).
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Fig. 2 XRD patterns of TiO2NTs, TiO2NTs-KH570 and TiO2NTs-1 Fig. 3 XPS spectra of TiO2NTs, TiO2NTs- KH570 and TiO2NTs-1: a wide-scan XPS spectra, b Si 2p narrow scan, c O 1s narrow scan, d Ti 2p narrow scan.
distribution of TiO2NTs and TiO2NTs-1.
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ACCEPTED MANUSCRIPT Highlights • Titanium dioxide nanotube was synthesized by hydrothermal, modification and etching process. • Titanium dioxide nanotube has good morphology at high calcination temperature.
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• Titanium dioxide nanotube exhibits improved photocatalytic activity.