Dealloying-driven synthesis of sea-urchin like titanate nanowires and hierarchically porous anatase TiO2 nanospindles with enhanced photocatalytic performance

Corrosion Science 98 (2015) 651–660

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Dealloying-driven synthesis of sea-urchin like titanate nanowires and hierarchically porous anatase TiO2 nanospindles with enhanced photocatalytic performance Zhengfeng Zhao a,b , Jing Xu a , Caiyun Shang a , Ran Ye c , Yan Wang a,b,∗ a

School of Materials Science & Engineering, University of Jinan, No. 336, West Road of Nan Xinzhuang, Jinan 250022, PR China Shandong Provincial Key Laboratory of Preparation & Measurement of Building Materials, University of Jinan, No. 336, West Road of Nan Xinzhuang, Jinan 250022, PR China c Department of Biosystems & Soil Science, University of Tennessee, 2506 E. J. Chapman Drive, Knoxville, TN, USA b

a r t i c l e

i n f o

Article history: Received 12 January 2015 Received in revised form 25 May 2015 Accepted 9 June 2015 Available online 20 June 2015 Keywords: Alloy SEM TEM De-alloying Alkaline corrosion

a b s t r a c t Hydrogen titanate (H-titanate) is an important material to obtain the high performance TiO2 . Here we present a facile strategy to synthesize H-titanate nanowires through dealloying method and subsequent acid treatment. By calcination or hydrothermal treatment, the H-titanate could transform into nanostructured anatase TiO2 with diverse morphologies. The hydrothermal processing gives rise to the formation of hierarchically porous anatase TiO2 nanospindles. Importantly, the spindle-like anatase TiO2 with high specific surface area (63.8 m2 /g) shows enhanced photocatalytic performance for degradation of methyl orange. Our strategy is promising for synthesis of other nanostructured metal hydroxides/oxides with various morphologies/properties. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Hydrogen titanate (H-titanate) discovered by Kasuga et al. [1,2] in 1998 has attracted much attention because of its potential applications in photocatalysis, wastewater treatment, ion exchange, lithium ion storage, gas sensors, and solar energy conversion [3–8]. Titanate possesses an open and layered structure, and common structural features similar to anatase [9–12]. Owing to its low cost, low toxicity, and relative stability both in acidic and basic conditions, titanate is regarded as a promising precursor to fabricate TiO2 nanocrystals. TiO2 with diverse microstructures of nanoshuttle [9], nanospindle [10], and nanobelt [11] have been developed through treating titanate using different fabrication methods. The mechanisms of phase and shape transformation between titanate and TiO2 have been studied under different reaction conditions. Wang et al. [9] have presented that titanate wires were sacrificed to form anatase TiO2 shuttles through a dissolution/recrystallization process. Zhu et al. [12] have reported that a composite structure

∗ Corresponding author at: School of Materials Science & Engineering, University of Jinan, No. 336, West Road of Nan Xinzhuang, Jinan 250022, PR China. Tel.: +86 531 82765473; fax: +86 531 87974453. E-mail address: mse [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.corsci.2015.06.007 0010-938X/© 2015 Elsevier Ltd. All rights reserved.

of titanate fibers covered with anatase TiO2 nanoparticles was attributed to the dehydration of titanate nanofibers due to the reaction with acid, accompanying by an in situ rearrangement of structural units. Moreover, titanate has been used as an important precursor to prepare functional nanomaterials, such as SrTiO3 and BaTiO3 [13–15]. TiO2 as one of the most advanced industrial materials, plays a significant role in many applications such as photocatalysis [16], dye-sensitized solar cells [17], gas sensors [18], biological coatings [19], and Li ion battery materials [20]. Recently, the application of TiO2 photocatalysts has mainly been focused on decomposition of toxic and hazardous organic pollutants in contaminated air and water, which is of great importance for environmental protection [21–23]. Now, the synthesis of anatase TiO2 with promising properties on a large scale is still a challenge, especially via controllable methods. Generally, the alkali titanate is derived from the alkaline treatment of TiO2 particles using a hydrothermal method under highly basic conditions. And the morphologies of alkali titanate can be strongly affected by the synthesis conditions, including temperature, duration of reaction time, and concentration of the alkaline solution [24–27]. The H-titanate could be obtained via protonating the alkali titanate, which involves stirring the alkali titanate in dilute acid for hours in order to remove the residual alkali ions

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completely [28,29]. Moreover, very high concentrations of aqueous NaOH solution are usually used to synthesize these alkali titanate products, and the reaction time is typically in the range of 12–48 h [30,31]. So this traditional method to fabricate H-titanate becomes time-consuming and energy-wasting. A simple and energy-saving route is urgent for fabrication of such materials. Comparatively, dealloying as a simple and efficient method has received growing interests in the past decade. Dealloying (i.e., selectively removing one or more active metals from alloys) has been proven to be effective in producing free-standing three-dimensional (3D) nanoporous metals in large amount [32–40]. In this work, we have obtained the H-titanate nanowires using a facile dealloying method. The anatase TiO2 with nanospindle and nanorod structures has been prepared by hydrothermal and calcination methods through employing H-titanate nanowires as the sacrificing precursors. The microstructure and formation mechanism of nanostructured TiO2 have been further investigated. Additionally, the photocatalytic performance of the as-obtained TiO2 has been evaluated via degrading methyl orange (MO) under the UV-light condition.

2. Experimental 2.1. Preparation of Al90 Ti10 precursor ribbons Al90 Ti10 (nominal composition, at%) alloy was prepared from Al (purity, 99.99 wt%) and Ti (purity, 99.9 wt%) in a quartz crucible under argon atmosphere using a high-frequency induction furnace. Using a single roller melt spinning instrument, the prealloyed ingot was remelted by high-frequency induction heating and then melt-spun onto a copper roller with a diameter of 0.25 m at a speed of 2000 revolutions per minute (rpm) in a controlled argon atmosphere. The ribbons obtained were typically 10–30 ␮m in thickness, 3–5 mm in width, and several centimeters in length. 2.2. Preparation of hydrogen titanate (H-titanate) samples The as-spun Al90 Ti10 ribbons were dealloyed in a 4 mol/L NaOH aqueous solution under free corrosion conditions. After dealloying for 12 h at ambient temperature, sodium titanate (Na-titanate) [5,31,41,42] was obtained. Then the as-dealloyed samples were washed thoroughly with deionized water for several times, in order to remove the residual chemical substances. After drying in air, the as-dealloyed samples were immersed in a 0.1 mol/L HCl aqueous solution for 12 h and washed thoroughly with distilled water. Finally, the H-titanate was acquired and could be used as the precursor to fabricate anatase TiO2 . 2.3. Fabrication of anatase TiO2 In the present study, hydrothermal and calcination methods were used to prepare anatase TiO2 . The hydrothermal procedure was described as follows. Firstly, the H-titanate precursors were dispersed in the mixture of water and alcohol with equal volumes and stirred vigorously for 30 min using ultrasonic cleaner. After adding 2 mL of dimethylamine into the mixture, the stirring treatment was continued for another 30 min. Then the resulting mixture was sealed in a Teflon-lined stainless steel autoclave with a capacity of 50 mL and maintained at 453 K for 12 h. Finally, the autoclave was cooled to room temperature naturally, the turbid suspensions inside the autoclave were centrifugally separated and the precipitates were collected and washed. Alternatively, the H-titanate precursors were calcined at 873 K for 2 h in an argon atmosphere and cooled to room temperature. Then the calcined products were

obtained. It can be seen from the following that the anatase TiO2 samples were successfully fabricated by the above two methods. 2.4. Characterization All products obtained (Na-titanate, H-titanate, and anatase TiO2 nanomaterials) were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). Thermal analysis of the as-obtained samples was performed on a thermo gravimetric analyzer (TGA) and differential scanning calorimetry (DSC) at a heating rate of 20 K/min. In addition, N2 adsorption–desorption experiments of the samples were carried out at 77 K by a Gold APP V-Sorb X800 surface area and porosity analyzer. Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface area. The pore size distribution was measured from the desorption branch of isotherm using the corrected form of Kelvin equation by the Barrett–Joyner–Halenda (BJH) method. 2.5. Photocatalytic activity test In photodegradation experiments, MO was used as the model pollutant because it is a water-soluble azo colorant and is widely used in textile, printing, paper manufacturing, pharmaceutical and food industries. The photocatalytic experiments were carried out by adding 5 mg TiO2 photocatalysts into 50 mL MO solution (20 mg/L). The 250 W high-voltage mercury lamp with UV-light was used as the light source. The suspension was stirred in the dark for 60 min to obtain the saturated adsorption of MO before illumination. After different irradiation time, the concentration of the MO solution was measured by monitoring the absorbance at 464 nm (the maximum absorbance wavelength of MO) on a UV–vis spectrophotometer. The photodegradation efficiency, given as a percentage, refers to the difference in the concentration of MO solution before irradiation (C0 ) and after light irradiation for M (M = 1, 2, 3, 4, and 5 h) (CM ) divided by C0 (i.e., 100[C0 − CM ]/C0 ). 3. Results and discussion 3.1. Phase analysis and microstructure of the Na-titanate and H-titanate Fig. 1 presents the XRD pattern of the as-spun Al90 Ti10 ribbons. According to the standard PDF files (# 65-2869 and # 65-4505), the precursor alloy is composed of two phases: Al and Al3 Ti. It is obvious that the contents of Al3 Ti and Al are almost the same in the as-spun alloy. A large number of Al in the precursor alloy ensures that the dealloying can be carried out facilely. Fig. 2(a) shows the XRD pattern of the samples synthesized by dealloying the Al90 Ti10 ribbons in the 4 mol/L NaOH solution. The Al and Al3 Ti phases could not be detected in the XRD pattern of the as-dealloyed samples, suggesting that both phases in the rapidly solidified Al–Ti alloys can be fully dealloyed after immersing in the NaOH solution. The removal of Al from Al–Ti does not result in the formation of nanoporous Ti/titanium oxide, just like nanoporous core–shell Cu@Cu2 O nanocomposite by dealloying the Al–Cu alloys [39,40]. The XRD profile of the as-dealloyed samples is similar to that of Na-titanate reported in the literature [42,43]. Thus the Natitanate samples were obtained through dealloying the Al90 Ti10 precursor in the NaOH solution. Fig. 2(b) presents the XRD pattern of the H-titanate [12,15], which was prepared by acid treatment of the Na-titanate samples in the 0.1 mol/L HCl solution. After acid treatment, the diffraction peaks of XRD pattern have changed a little compared with that of the Na-titanate in Fig. 2(a). The main peak

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Al: PDF#65-2869 Al3Ti: PDF#65-4505

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Fig. 2. XRD patterns of the (a) Na-titanate and (b) H-titanate samples.

(2 = 28◦ ) of the Na-titanate shifts toward a lower angle (2 = 25◦ ) for the H-titanate, as marked by arrows in Fig. 2. This indicates the lattice expansion due to intercalation of water molecules into the interlayer space of the H-titanate [44]. The formation mechanism of the H-titanate could be elucidated by the cation-exchange mechanism [31,45]. The Na-titanate samples could be protonated while preserving their pristine layered structures when being immersed in the acid solution. In addition, two weak peaks appear at about 25◦ and 48◦ for the H-titanate samples, which is attributed to the transformation from H-titanate to TiO2 through a little dehydration [43]. Fig. 3 shows the FESEM microstructure of the Na-titanate samples. The samples are composed of connecting particles with large-sized channels (hundreds of nanometers) in between (Fig. 3(a)). The SEM image at a higher magnification shows that the particles exhibit a sea-urchin like structure, which is composed of a large number of nanowires (Fig. 3(b)). In addition, the EDS analysis shows that only Na, Ti, and O can be identified and all of Al atoms are removed during dealloying of the Al90 Ti10 ribbons in the NaOH solution as shown in Fig. 3(c). In combination with the XRD result of Fig. 2(a), the EDS results further confirm that the as-dealloyed samples are the Na-titanate phase. During dealloying process, Al phase in the Al–Ti alloys selectively dissolved into the NaOH solution, which can remain the large-sized channels. And the Al in Al3 Ti phase dissolved continuously due to its high content and chemical

Fig. 3. (a and b) FESEM images of the resulted Na-titanate samples after dealloying in the 4 mol/L NaOH solution at room temperature for 12 h. (c) Typical EDS spectrum corresponding to the area in (a).

activity in the NaOH solution [37]. Besides, the fast removal of Al would cause the diffusion of Ti atoms [38], which tend to react with the Na+ and OH− ions. In order to get rid of Na+ , the Na-titanate samples were further treated by the dilute HCl solution. Fig. 4 presents the FESEM images of the acid-treated samples. The porous microstructure of the acid-treated samples is similar to that of the as-dealloyed ones (Fig. 4(a)). The particles in the acidtreated samples are also comprised of sea-urchin like nanowires

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Fig. 4. (a–c) FESEM images of the resulted H-titanate samples obtained by the acid treatment of Na-titanate samples in the 0.1 mol/L HCl solution at room temperature for 12 h. (d) Typical EDS spectrum corresponding to the area in (a).

(Fig. 4(b)). Furthermore, it can be seen that the nanowires are several nanometers in diameter and about 200 nm in length, and are distributed radially like blooming fireworks (Fig. 4(c)). Fig. 4(d) presents a typical EDS spectrum of the acid-treated samples. It is clear that Na is completely removed and only Ti and O could be detected. According to the XRD and EDS results (Fig. 2(b)), the acidtreated samples are the H-titanate. Na+ ions could be corroded by the H+ ions in the HCl solution, which tends to form H-titanate during the acid corrosion. This is called cation-exchange, which does not significantly change the morphology of the samples.

TGA and DSC were further carried out to evaluate the water content and investigate the thermodynamic behavior of the H-titanate. Fig. 5 shows the TGA and DSC curves of the H-titanate samples. The TGA curve indicates a total weight loss of 29.57% over the temperature range of 323–873 K, which is higher than the previous report [46]. This weight loss of H-titanate during the dehydration reaction can be attributed to the loss of water molecules physically

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Fig. 5. (a) TGA and (b) DSC scans for the H-titanate samples at a heating rate of 20 K/min in an argon atmosphere.

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Fig. 6. XRD patterns of the anatase TiO2 powders by the treatment of H-titanate with two methods: (a) calcination at 873 K for 2 h and (b) hydrothermal treatment at 453 K for 12 h.

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Fig. 8. Nitrogen adsorption–desorption isotherms and BJH pore size distribution curve (inset) of the anatase TiO2 fabricated by the hydrothermal method.

Fig. 7. FESEM images of the anatase TiO2 samples after hydrothermal treatment of H-titanate at 453 K for 12 h.

Fig. 9. FESEM images of the anatase TiO2 samples after calcining the H-titanate samples at 873 K for 2 h.

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Fig. 10. (a) TEM and (c and d) HRTEM images showing the microstructure of the anatase TiO2 fabricated by the hydrothermal method. (b) SAED pattern corresponding to (a). (e) FFT pattern corresponding the HRTEM image in (d).

adsorbed on the surface or trapped in the interparticle space. It has been reported that the layers in H-titanate were collapsed to produce anatase and rutile phases after complete dehydration, indicating that the layered H-titanate became thermally unstable [46,47]. The sea-urchin like structure and tiny nanowires greatly

improve the specific surface area of the H-titanate prepared by the dealloying method. Obviously, the large specific surface area of the H-titanate is more beneficial to the physical adsorption of water molecules. However, it has also been assumed that free interlayer water of H-titanate could be evaporated up to 373 K and the

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H-titanate reached the stable state [48]. In such a case, the weight loss decreases to 23.26% in the range of 373–873 K (Fig. 5(a)). There exists an obvious endothermic peak (300–500 K) on the DSC curve of the H-titanate (Fig. 5(b)), corresponding to the evaporation of all free interlayer water and most of the crystal water. With increasing temperature up to 873 K, the residual crystal water continues to evaporate. But it does not exhibit the second endothermic peak because of the minimal residual of the crystal water. Above 873 K, a new endothermic peak appears owing to the phase transformation from anatase to rutile [49]. It is worth noting that only the dehydration reaction occurs when the H-titanate is calcined at 873 K in an argon atmosphere. 3.2. Microstructure of the anatase TiO2 prepared by hydrothermal and calcination methods According to the TGA and DSC results (Fig. 5), the H-titanate samples were calcined at 873 K in the vacuum tube furnace. Moreover, the H-titanate was used as the precursor to prepare anatase TiO2 by the hydrothermal method. Fig. 6 shows the XRD patterns of the as-obtained samples prepared by the above two methods. It is clear that the anatase TiO2 (PDF # 21-1272) was successfully obtained by both the calcination and hydrothermal methods. The sharp and strong diffraction peaks suggest the good crystallinity and high crystalline purity of the as-prepared anatase TiO2 samples. Fig. 7 shows the FESEM images of the anatase TiO2 obtained by the hydrothermal method. The TiO2 samples exhibit a hierarchically porous structure. The large pores are open, bicontinuous, and their sizes range from several hundred nanometers to two microns (Fig. 7(a) and (b)). It can be seen at higher magnification that the islands between these large pores are highly rough and nanoporous (Fig. 7(b) and (c)). Moreover, these islands are composed of quantities of spindle-like TiO2 nanostructures with the diameter of 10–30 nm and the length of 50–100 nm (Fig. 7(c)). The hierarchically porous structure of the spindle-like anatase TiO2 samples has also been probed by nitrogen adsorption/desorption experiments. Fig. 8 presents the nitrogen adsorption–desorption isotherms and Barret–Joyner–Halenda (BJH) pore size distribution curves (inset) of the spindle-like anatase TiO2 samples. Based upon the Brunauer classification [50], Fig. 8 displays type IV adsorption–desorption isotherms, which are typical characteristics of mesoporous materials. This result suggests the inter-connected mesoporosity and high pore connectivity in the framework. According to the adsorption–desorption isotherms, the Brunauer–Emmett–Teller (BET) surface area of the anatase TiO2 samples was determined to be as high as 63.8 m2 /g. The pore size distribution of the samples was determined using the Barrett–Joyner–Halenda (BJH) model analysis (inserted in Fig. 8). The BJH pore size distribution indicates that the anatase TiO2 samples prepared by the hydrothermal method have two kinds of pore diameters of 2–4 and 30–200 nm, which further confirms the hierarchically porous structure of the TiO2 samples and is reasonably in agreement with the SEM results. Fig. 9 presents the FESEM images of the anatase TiO2 samples obtained by calcination of H-titanate at 873 K. The anatase TiO2 samples display the porous structure (Fig. 9(a)), which inherits the characteristic of the H-titanate precursor (Fig. 4(a)). However, the surface morphology seems to be coral-like and abundant short nanorods appear on the island surface (Fig. 9(b)), rather than retention of the pristine nanowires of the H-titanate precursor (Fig. 4(b)). The sea-urchin like H-titanate nanowires have transformed to massive anatase TiO2 nanorods during the calcination process. TEM and SAED were used to further prove the microstructure of the anatase TiO2 samples. Fig. 10 shows the TEM/HRTEM images and corresponding SAED pattern of the TiO2 samples obtained by the hydrothermal method. The spindle-like morphology of TiO2

Fig. 11. (a) TEM image of the anatase TiO2 obtained by the calcination method, (b) the corresponding SAED pattern, and (c) a HRTEM image of one nanorod.

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Fig. 12. Schematic representation of crystal structure for the layered H-titanate and its reconstruction to form spindle-like anatase TiO2 during the hydrothermal process.

nanostructures can be clearly observed (Fig. 10(a)), and one spindle is highlighted in Fig. 9(c). The sizes of these TiO2 nanospindles are 10–30 nm in diameter and 30–80 nm in length, which is reasonably consistent with the SEM observation. The corresponding SAED pattern in Fig. 10(b) shows regular diffraction rings, which can be indexed as (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 0 4), (1 1 6), and (2 1 5) planes of the anatase TiO2 (PDF # 21-1272). The electron diffraction results further verify the formation of the anatase TiO2 phase during the hydrothermal process. The ordered lattice fringes of the nanospindle are clearly observed (Fig. 10(c) and (d)), indicating that the nanospindle is a single nanocrystal with good crystallinity. From the distance between the adjacent lattice fringes, the interplanar spacing of 0.353 nm can be obtained, which matches well with the (1 0 1) plane of anatase TiO2 (Fig. 10(d)). Fig. 10(e) shows the fast Fourier transformation (FFT) pattern of the HRTEM image in Fig. 10(d). The FFT pattern corresponds to the [0 1 0] zone axis of the anatase TiO2 and the related reflections have also been indexed in Fig. 10(e). From the HRTEM image and FFT pattern, we can conclude that the spindle-like TiO2 crystals preferentially grow along the [0 0 1] direction during the hydrothermal process, as highlighted in Fig. 10(d). It has been reported that the dimethylamine plays an important role in the hydrothermal process, which is known to be easily adsorbed on the (1 0 1) surface of TiO2 [9,10,51,52]. The nanospindle is confined by a set of (1 0 1) planes, which is attributed to the lower surface energy of the (1 0 1) plane than that of other planes [51]. Therefore, both the addition of dimethylamine and the lower surface energy of (1 0 1) plane contribute to the formation of the unique spindle-like morphology of the anatase TiO2 nanocrystals in the present work. Fig. 11 shows the TEM image and corresponding SAED pattern of the TiO2 samples obtained by the calcination method. The TEM results clearly reveal the short nanorod-like structure in the calcined samples (Fig. 11(a)). This morphology confirms the in situ rearrangement of structural units in the H-titanate to form anatase TiO2 [12], when the sea-urchin like H-titanate samples were calcined at 873 K for 2 h. The corresponding SAED pattern verifies the as-calcined samples are the anatase TiO2 (Fig. 11(b)). The related reflections are also marked in Fig. 11(b). Fig. 11(c) shows the HRTEM image of one TiO2 nanorod. Regular lattice fringes run across the whole nanorod, and the interplanar spacing is 0.355 nm corresponding to the (1 0 1) crystal plane of the anatase TiO2 . 3.3. Formation mechanism of the anatase TiO2 nanomaterials 2Al + 2NaOH + 2H2 O = 2NaAlO2 + 3H2 ↑

(1)

Ti + NaOH + H2 O → Na-titanate

(2)

+

Na-titanate + H → H-titanate + Na H-titanate → TiO2 → H2 O

+

(3) (4)

Generally, the active elements are priority corrosion in the certain solution during the dealloying process [53]. At present, Al atoms in the Al90 Ti10 precursor firstly react with OH− to form AlO2 − dissolving into the NaOH solution as Al3+ (Eq. (1)). At the same time, Ti atoms in Al3 Ti start to be oxidated by OH− and become Ti4+ . It is known that Ti4+ can easily form the precipitate (here, Na-titanate) in the alkaline solution (Eq. (2)). During the cation exchange process in the dilute HCl solution, the Na+ is replaced by H+ forming the Htitanate precursor (Eq. (3)). Moreover, the H-titanate can maintain the structural feature of the Na-titanate [42]. During the calcination or hydrothermal process, the H-titanate samples dehydrate to form the anatase TiO2 (Eq. (4)). Both H-titanate and anatase TiO2 contain TiO6 octahedra with the zigzag layer shape that shares four edges with others. The common feature of crystal lattice indicates that the phase transition is a topochemical reaction [47,48,54]. Fig. 12 illustrates an idealized structural representation of phase transformation between H-titanate and anatase TiO2 during the hydrothermal process. The layered titanate is easily delaminated, exfoliated, and split into discrete TiO6 monomer or oligomer units in the presence of dimethylamine. The TiO6 monomer or oligomer units would then be assembled into the anatase TiO2 [10,15]. In the hydrothermal environment, dimethylamine is adsorbed on the surface of TiO6 octahedra, which leads to the preferential grow along the [0 0 1] direction and further the formation of spindle-like TiO2 nanocrystals. During the calcination process, the H-titanate nanowires dehydrate due to the high temperature, which is accompanied by an in situ rearrangement of structural units. The layered titanate nanowires transform into anatase TiO2 nanorods through the in situ dehydration reaction. In spite of different length scales, the as-obtained anatase TiO2 (nanorod) resembles the morphology (nanowire) of the H-titanate precursor. 3.4. Photocatalytic properties of the anatase TiO2 nanomaterials Fig. 13(a) and (b) shows the UV–vis absorption spectra for the degradation of MO by the anatase TiO2 fabricated by the hydrothermal and calcination method. Fig. 13(c) shows the corresponding degradation efficiency versus the radiation time. The characteristic absorption peak of MO at 464 nm gradually decreases with increasing irradiation time (Fig. 13(a) and (b)). But two kinds of catalysts prepared by the two methods exhibit quite different photocatalytic activity. Obviously, the activity of the spindle-like anatase TiO2 obtained by the hydrothermal method is much better than that of the TiO2 obtained by the calcination method (Fig. 13(c)). The MO degradation efficiency of the spindle-like anatase TiO2 can approach more than 95% but that of the rod-like anatase TiO2 is less than 45% after UV light irradiation of 5 h. The difference in photocatalytic activity of TiO2 obtained through different methods could be rationalized as follows. The

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Fig. 13. UV–vis absorption spectra of the degradation of methyl orange by (a) the spindle-like anatase TiO2 fabricated by the hydrothermal method, (b) rod-like anatase TiO2 fabricated by the calcination method, (c) The plot of photodegradation ratio versus the irradiation time.

photocatalytic properties of TiO2 are derived from the formation of photogenerated charge carriers (hole and electron), which occurs upon the absorption of ultraviolet (UV) light corresponding to the band gap [55]. UV irradiation induces the formation of electron-hole pairs, whose charge carriers react with chemical species such as water molecule and molecular oxygen in the air to produce hydroxyl radicals (• OH) and superoxide radical anions (O2 •− ), respectively, which contributes to the decomposition of organic molecules at the TiO2 surface [56]. Thus active sites on the TiO2 surface play a dominant role in the photocatalytic degradation process [57,58]. The TiO2 samples prepared by the hydrothermal method have a unique spindle-like morphology and expose much more active sites than those obtained by the calcination method. Partial step sites are highlighted by short red arrows in Fig. 10(d). In addition, the hierarchically porous structure of the hydrothermally synthesized TiO2 is also beneficial to its photocatalytic activity. The interconnected large pores could enhance the transfer of involved reaction species, and the nanopores could greatly increase the specific surface areas providing more active sites.

4. Conclusions In summary, sea-urchin like Na-titanate nanowires can be fabricated through the facile dealloying strategy using Al–Ti alloy as the precursor. Further acid treatment could induce the transition from Na-titanate to H-titanate while preserving the morphology of sea-urchin like nanowires. Besides, anatase TiO2 nanostructures can be prepared through calcination or hydrothermal treatment of H-titanate, and the morphology of TiO2 depends upon the processing method. The hydrothermal treatment results in the formation of hierarchically porous anatase TiO2 nanospindles with the preferential growth direction of [0 0 1]. The nanospindle-like anatase TiO2 exhibits the enhanced photocatalytic performance, owing to the

unique hierarchically porous structure, high specific surface area (63.8 m2 /g) and active sites on the surface. Our findings can provide valuable information for synthesis of nanostructured titanate and TiO2 with diverse morphologies.

Acknowledgements The authors acknowledge financial support from the National Natural Science Foundation of China (No. 51171072), and Excellent Middle-age and Young Scientists Research Award Foundation of Shandong Province (No. BS2012CL002).

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