A one-step thermal decomposition method to prepare anatase TiO2 nanosheets with improved adsorption capacities and enhanced photocatalytic activities

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A one-step thermal decomposition method to prepare anatase TiO2 nanosheets with improved adsorption capacities and enhanced photocatalytic activities Wenting Li, Chunli Shang, Xue Li ∗ Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, School of Chemistry and Chemical Engineering, University of Jinan, 336 West Road of Nan Xinzhuang, Jinan 250022, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 21 July 2015 Received in revised form 25 September 2015 Accepted 26 September 2015 Available online xxx Keywords: TiO2 nanosheets Thermal decomposition Adsorption Photocatalytic activity

a b s t r a c t Anatase TiO2 nanosheets (NSs) with high surface area have been prepared via a one-step thermal decomposition of titanium tetraisopropoxide (TTIP) in oleylamine (OM), and their adsorption capacities and photocatalytic activities are investigated by using methylene blue (MB) and methyl orange (MO) as model pollutants. During the synthesis procedure, only one type of surfactant, oleylamine (OM), is used as capping agents and no other solvents are added. Structure and properties of the TiO2 NSs were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption analysis, UV–vis spectrum, X-ray photoelectron spectroscopy (XPS) and Photoluminescence (PL) methods. The results indicate that the TiO2 NSs possess high surface area up to 378 m2 g−1 . The concentration of capping agents is found to be a key factor controlling the morphology and crystalline structure of the product. Adsorption and photodegradation experiments reveal that the prepared TiO2 NSs possess high adsorption capacities of model pollutants MB and high photocatalytic activity, showing that TiO2 NSs can be used as efficient pollutant adsorbents and photocatalytic degradation catalysts of MB in wastewater treatment. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Recently, two-dimensional (2D) anisotropic nanostructures of TiO2 , especially ultrathin nanosheets (NSs), have attracted tremendous research attention because of their unique physiochemical properties derived from larger surface area and structural flexibility and wide range of applications, such as in solar cells, catalysts, lithium ion batteries, adsorbents, and so on [1–14]. Compared to TiO2 nanocrystals and other anatase TiO2 nanostructures, the ultrathin TiO2 NSs with high-energy exposed (0 0 1) facets exhibit superior performance serving as photocatalysts [5,9,15–21] and electrode materials for lithium-ion batteries [9,22,23]. For example, it has been proved that 2D NSs can improve photoelectric conversion efficiency in dye-sensitized solar cells due to good crystallinity, high pore volume, large lateral size, and enhanced light scattering of TiO2 NSs [12–14]. Layered protonated titanate NSs exhibit excellent adsorption capacities for methylene blue and Pb2+ because of their

∗ Corresponding author. E-mail address: chm [email protected] (X. Li).

high specific surface areas and excellent ion-exchange capability [24]. Varieties of synthetic methods such as chemical exfoliation [3,25], hydrothermal/solvothermal methods [1,6,8,13–24,26–43], chemical vapor deposition [44], etc. have been developed to prepare TiO2 NSs. Chemical exfoliation is a common method used to fabricate ultrathin 2D anatase TiO2 NSs [24,25]. Although the swelling–exfoliation behavior of layered titanates has been studied since the mid-1990s [25], the exfoliation procedure of TiO2 nanosheets is still a considerably complicated and timeconsuming process [3]. A hydrothermal method with hydrofluoric acid (HF) as a morphology controlling agent and titanium alkoxides as precursor is usually exploited to prepare TiO2 NSs with high-energy exposed (0 0 1) facets [14]. After successful synthesis of anatase TiO2 single crystals with a large percentage of reactive (0 0 1) facets by Yang et al. [1], various systems and capping agents have been used to generate TiO2 with exposed high-energy facets [1,9,16,18,20–23,26–43]. However, the HF or its compound is corrosive. Moreover, the hydrothermal/solvothermal methods need high-pressure equipment such as Teflon-lined stainless steel autoclave which limits large-scale production of TiO2 NSs [14,44]. Recently, HF-free synthetic strategies, such as solvothermal

http://dx.doi.org/10.1016/j.apsusc.2015.09.214 0169-4332/© 2015 Elsevier B.V. All rights reserved.

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Fig. 1. (a) Low-magnification and (b) high-magnification TEM images of TiO2 NSs synthesized at 270 ◦ C without aging, the molar ratio of TTIP/OM was 1:1. (c) HRTEM image of TiO2 NSs, insert is its SAED pattern. (d) Typical XRD pattern of TiO2 NSs.

Fig. 2. (a, a’) TEM and HRTEM images of TiO2 NSs synthesized at 270 ◦ C for 2 h. (b, b’) TEM and HRTEM images of TiO2 NSs synthesized at 270 ◦ C for 6 h, insert is its SAED pattern. The molar ratio of TIP/OM was 1:1.

alcoholysis of TiF4 , gas-phase thermal oxidation of TiCl4 or using tetrafluoroborate-based ionic liquid as stabilizing agent, toluenewater biphasic interfacial reaction method, have been developed to synthesize (0 0 1) facets exposed anatase TiO2 single crystals [27,30,32,45,46]. In addition, a low concentration of alkoxide

precursors is used in aqueous systems to prevent nanocrystals from aggregation and/or further growth, which makes it difficult to achieve the large-quantity preparation of TiO2 nanocrystals [21]. To overcome the limitations mentioned above, nonaqueous system has been developed and nanostructured anatase with high-energy

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Fig. 3. XPS spectra of TiO2 NSs synthesized at 270 ◦ C for 6 h, the molar ratio of TTIP/OM was 1:1. (a) Survey, (b) Ti2p, (c) O1s, (d) C1s and (e) N1s. In (d), black trace shows the experimental data, blue, red, cyan and green traces show the curves fitted to the experimental data, and the magenta traces show the envelope, i.e. the sum of the blue, red, cyan and green traces. (f) N1s XPS spectrum of TiO2 NSs sample after calcination at 450 ◦ C for 4 h in air (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

facets are prepared under solvothermal conditions by using titanium (IV) isopropoxide as the titania precursor, a primary amine as the capping agent, and benzyl alcohol as the reaction medium [20]. Although various strategies have been exploited to prepare anatase TiO2 NSs with high proportion of (0 0 1) facets, a more simple and facile synthetic technique to anatase NSs on a large scale is still needed. In this work, we report a one-step route to produce anatase TiO2 NSs with high specific surface areas through thermal decomposition of titanium tetraisopropoxide (TTIP) precursors in oleylamine

(OM). Only one type of surfactant, was chosen as capping agents due to its advantages, such as its liquid state at room temperature, easy removal via centrifugation, high boiling point, and low cost [47]. By simply adjusting the reaction conditions such as the molar ratio of TTIP/OM the morphology of TiO2 nanoparticles can be tailored. The resultant TiO2 NSs possess porous structure and large surface area up to 378 m2 g−1 . Adsorption and photocatalytic degradation experiments reveal that the prepared TiO2 NSs are efficient adsorbents and catalysts for the photocatalytic degradation of pollutant MB.

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Fig. 4. (a) and (b) Low-magnification and high-magnification TEM images of TiO2 NSs synthesized at 270 ◦ C for 2 h, the molar ratio of TTIP/OM was 1:8. (c, d) Low-magnification and high-magnification TEM images of TiO2 nanoparticles synthesized at 270 ◦ C for 2 h, the molar ratio of TTIP/OM was 1:32.

2. Experimental 2.1. Materials Titanium tetraisopropoxide (TTIP, 97%) was purchased from Alfa Aesar. Oleylamine (OM) was purchased from J&K Scientific Ltd., China. Methylene blue (MB) and methyl orange (MO) purchased from Sigma–Aldrich were used as a standard material to estimate adsorption and photocatalytic degradation. 2.2. Synthesis of TiO2 NSs TiO2 NSs were prepared by mixing TTIP and OM and heating to 270 ◦ C using thermal decomposition method. In a typical synthesis, TTIP (10.48 mL) was added to degassed OM (10.0 g, the molar ratio of TTIP/OM was 1:1) in a 100 mL three-necked round bottom under stirring. Then the mixture was slowly heated to 270 ◦ C and aged at this temperature for 0 h, 2 h and 6 h under nitrogen flow. After the reaction vessel was cooled to room temperature, an excess amount of acetone was added to the gel-like product to precipitate TiO2 NSs. The sample was centrifuged at 8000 rpm for 15 min with acetone to separate the formed TiO2 NSs. The purification was repeated at least 2 times to remove the excess amount of OM and byproducts. The products were dispersed in nonpolar solvents such as hexane and toluene for further characterization. 2.3. Characterization UV–Vis spectra of the samples were recorded on a TU1810 spectrometer (Beijing Purkinje General Instrument Co., China). Dilute dispersions of the TiO2 NSs were measured in quartz cuvettes, using pure solvent as a reference. X-ray Diffraction (XRD) was performed using a D8 FOCUS diffractometer (Bruker-AXS, Germany) with ˚ Transmission electron microscopy Cu K␣ radiation ( = 1.5418 A). (TEM) and high resolution TEM (HRTEM) were performed on a JEM-2100 electron microscope (JEOL Ltd., Japan) operating at

200 kV. The samples were prepared by mounting a drop of the dispersion on a carbon-coated Cu grid and allowing it to dry in air. Proton nuclear magnetic resonance (1 H NMR) measurements were carried out on a Bruker BioSpin (400 MHz) instrument. Fourier transform infrared (FTIR) spectra were recorded with a FTIR-8400S spectrophotometer in the range of 4000–400 cm−1 with KBr plates. Nitrogen adsorption/desorption experiments were performed on a Micromeritics ASAP 2020 surface area analyzer after outgassing at 300 ◦ C for 5 h prior to analysis. The standard multipoint Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface area. The pore size distributions of the materials were derived from the adsorption branches of the isotherms on the basis of the Barett–Joyner–Halenda (BJH) model. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250 with Al Ka excitation. Photoluminescence spectra were measured using FLS920 instrument (Edinburgh Instruments, UK) with an excitation wavelength of 290 nm. Thermogravimetric analyses were performed with a Diamond TG/DTA instrument (Perkin-Elmer, USA). The samples were heated in nitrogen from 100 to 800 ◦ C at a heating rate of 10 ◦ C min−1 . 2.4. Adsorption experiments 20.0 mg TiO2 NSs were dispersed in 40 mL of MB aqueous solution (C0 = 10.0, 30.0, 50.0, 70.0, 100.0, 150.0, 200.0, 250.0 mg/L). The adsorption process was carried out in dark condition to avoid the photocatalytic influence. At fixed contact time, the samples were taken, centrifuged, and the optical absorption spectra of the samples were obtained using a UV–Vis spectrophotometer. To obtain the maximum adsorption capacity (Qmax ) of NSs, the suspensions were stirred for 20 h in the dark at room temperature to achieve the adsorption and desorption equilibration. The Qmax = V(C − Ce )/W, where C0 and Ce are the initial and equilibrium concentrations of MB, respectively. V is the volume of MB solution, and W is the total addition amount of the TiO2 NSs.

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Fig. 5.

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H NMR spectra of TTIP, OM and the mixture of TTIP and OM after reaction at 140 ◦ C and 250 ◦ C in CDCl3 .

2.5. Photocatalytic activity measurements The photodegradation of MB dyes was observed on the basis of the absorption spectroscopic technique. In a typical process, 40.0 mL of MB aqueous solution with a concentration of 10.0 mg/L

and 20.0 mg of TiO2 NSs were placed in a quartz conical flask. Under stirring, the conical flask was irradiated with UV light at 254 nm produced by 2 × 15 W UV lamps (Spectronics Co., USA). At given intervals, 3 mL of suspension was extracted and then centrifuged at 10,000 rpm for 5 min to get rid of the catalysts from the

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Fig. 6. FTIR spectra of (a) OM and (b) a mixture of TIP and OM (1:1) after reaction at 140 ◦ C.

supernatant. Then, the solution was analyzed by recording the UV–Vis spectrum of MB at the maximum absorbance of 665 nm. 3. Results and discussion 3.1. Synthesis of TiO2 NSs TiO2 NSs was synthesized simply via thermal decomposition process using TTIP as a source and OM as capping agents. No expensive and toxic organic solvents, such as 1-octadecene, phenyl ether, and benzyl ether, are required in this synthesis. TEM images shown in Fig. 1a demonstrated that the product obtained at 270 ◦ C without aging consisted of sheet-like structures though the outline was not rectangular. The thickness of NSs is ∼3.5–9 nm, which is a stack of about 9–21 layers (measured thickness of a single layer from TEM image is ∼0.42 nm) (Fig. 1b). The HRTEM image (Fig. 1c) and selected-area electron diffraction (SAED) pattern indicated the incomplete crystallization of TiO2 NSs. Fig. 1d showed a typical XRD pattern of TiO2 NSs. The diffraction peaks could be indexed to anatase-phase TiO2 (JCPDS No. 21-1272), indicating that the product was anatase TiO2 . It should be noted that the diffraction peaks are relatively broad, implying that the crystallinity of the TiO2 NSs is relatively small. This is consistent with the SAED result. To observe the stability, the change of the crystallinity or crystal size of the TiO2 NSs formed during synthesis, the reaction time aged at 270 ◦ C was increased to 2 h and 6 h. Fig. 2 shows the TEM images of the samples with aging time of 2 h and 6 h. The shape of the TiO2 NSs was almost maintained, indicating that the TiO2 NSs are stable at this temperature. The lattice spacings are clearly observed in Fig. 2a´ı and b´ı and diffraction spots appear in SAED pattern. These results indicate that the crystallinity of the TiO2 NSs aged for 2 h or 6 h is higher than that of the sample synthesized without aging. After decomposition of OM molecules completely, the final weights measured by TGA (Fig. S1) are about 63.7%, 64.3% and 68.6% for the samples aged for 0 h, 2 h and 6 h, respectively, illustrating that the surface of TiO2 is covered by OM molecules. The surface information of the prepared TiO2 NSs was detected by XPS. Characteristic peaks of Ti, O, C and N were observed (Fig. 3). The Ti 2p spectrum shows peaks at 458.1 and 463.9 eV corresponding to the 2p3/2 and 2p1/2 spin–orbital components, respectively [1,48]. The binding energy of O1s at 529.5 eV is attributed to the O2− in TiO2 . A broadening on the higher bonding energy side at 531.5 eV (indicated by an arrow) indicates the presence of another type of oxygen. This might be due to the presence of oxygen and nitrogen from the same lattice units in TiO2 [49]. The binding energy signal of

C1s at 284.8 eV was recorded. The N1s spectrum (Fig. 3e) consists of two strong peaks and two weak peaks. One strong peak at around 399.5 eV corresponding to amino groups [50,51], another strong peak at 401.2 eV corresponding to N Ti O, Ti O N or Ti N O [52]. Burda et al. observed a N1s core level at 401.3 eV from the detailed XPS investigations of nano N TiO2 and suggested that there is N Ti O bond formation due to nitrogen doping and no oxidized nitrogen is present [52b]. It was reported that the presence of oxidized nitrogen such as Ti O N and/or Ti N O linkages should appear above 400 eV [53,54]. Two weak peaks at 400.4 eV and 402.3 eV may correspond to O = C N and O N, respectively [53a,54]. N1s XPS spectra suggest that a part of OM molecules undergo chemical change and some N-doped TiO2 structures may form during the thermal decomposition process. The atom percents of C1s, N1s, O1s and Ti2p are 52.37, 1.93, 33.06 and 12.63, respectively. These results indicate that the surfaces of the TiO2 NSs are covered by OM molecules. It has been observed that calcination at high temperature can decrease the amount of heteroatom doping in TiO2 [49]. To further prove the existence of the nitrogen doping, the TiO2 NSs sample was calcinated at 450 ◦ C for 4 h in air to remove OM molecules. The N1s spectrum around 400.1 eV was observed (Fig. 3f). The atom percents of C1s, N1s, O1s and Ti2p are also changed into 13.68, 0.95, 52.16 and 33.21, respectively. Asahi et al. described that the peak around 400 eV corresponds to N2 molecules incorporated into the TiO2 lattice [55a]. Nosaka et al. ascribed the peak at 400 eV to the formation of doping of the other type [55b], where nitrogen doping was performed with 1 M organic compound and calcination at 450 ◦ C for 1 h. Sato et al. observed only one peak at 400 eV in the N1s XPS spectra of doped samples prepared from TTIP and calcined at 300 and 350 ◦ C [55c]. Therefore, it is reasonable to deduce from the above results that some N-doped TiO2 structures may exist in the TiO2 NSs. 3.2. Possible formation mechanism of TiO2 NSs To understand the growth mechanism of TiO2 NSs, a series of experiments was carried out by changing the molar ratio of TTIP/OM. The XRD patterns of the synthesized samples (Fig. S2) show the diffraction peaks corresponding to crystalline phases. However, the diffraction peaks are hardly detected when the molar ratio of TTIP/OM was decreased to 1:32, indicating that the small size of TiO2 nanoparticles and lack of orientation in the crystalline network [56]. Fig. 4 shows the TiO2 nanostructures obtained with TTIP/OM molar ratios of 1:8 and 1:32. TiO2 NSs are also formed when the molar ratio of TTIP/OM is 1:8. By careful observation, one can see from Fig. 4b that small TiO2 nanoparticles (labeled with white circles) also exist. Further decreasing the molar ratio of TTIP/OM to 1:32 results in the formation of TiO2 nanocrystals with sizes of 2–3 nm, as revealed by the HRTEM image in Fig. 4d. One can conclude that the OM plays a key role in the formation of the TiO2 NSs. The amount of OM can affect the formation of the TiO2 nanostructures, which can be used to control the morphology. Although there have been several reports on the synthesis of metal oxide nanocrystals by the thermal decomposition of metal alkoxide or carboxylate precursors, a detailed mechanism up to present was not given. It has been reported that TTIP molecules are complexed with OM to form TTIP–OM complex monomers [57]. To prove the formation of TTIP–OM complexes, the structure of the complex monomer synthesized by the reaction of equimolar TTIP and OM at 140 ◦ C was characterized by 1 H NMR. The change of the peak shape of the TTIP from multiplet to singlet and the shift of the peak value from 4.49 to 4.42 ppm (Fig. 5) indicate that TTIP and OM can form TTIP–OM complex monomers. FTIR spectra of OM and the mixture of TTIP and OM after reaction at 140 ◦ C were shown in Fig. 6. Appearing of a new characteristic N H stretching band at 1516 cm−1 and the shift of the N H band of NH2 from

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3335 cm−1 to 3246 cm−1 also support the formation of TTIP–OM complex monomers. When the temperature is increase to 250 ◦ C, the mixture gradually turned from a pale yellow to a pale green. 1 H NMR result (Fig. 5) of the mixture of TTIP and OM after reaction at 250 ◦ C shows that the peak shape of the TTIP at 4.49 was changed into multiplet again, the signal of isopropyl alcohol at 3.97 (peak l) and a new peak (m) appeared. These results revealed that the OM dissociates from the TTIP–OM complex monomers and decomposition of TTIP begins. When the mixture was increased to 270 ◦ C, the TTIP molecules were decomposed by cleavage of the C O bonds and then discrete nanocrystals formed by nucleating and growing process [58]. OM as an efficient activation reagent could also accelerate the formation of nanoparticles [47,59]. The amount of OM affects the formation of TiO2 NSs, which can be used to control the morphology of TiO2 nanoparticles. When the molar ratio of TTIP/OM is small (a large amount of OM), all facets of TiO2 nanocrystals are covered with excess OM molecules, further crystal growth and assembly cannot take place. Therefore only TiO2 nanocrystals are obtained. When the molar ratio of TTIP/OM is increased, TiO2 nanocrystals not covered completely with OM molecules are unstable and tend to assemble with each other to decrease the surface energy, which results in the formation of 2D network of TiO2 nanocrystals first and then transition into TiO2 NSs [60]. 3.3. Nitrogen adsorption/desorption experiments To determine the specific surface area and the pore volumes of the TiO2 NSs, nitrogen gas adsorption–desorption isotherms at 77 K were measured, and the typical isotherm and the pore size distribution are shown in Fig. 7. According to the IUPAC classification, the nitrogen adsorption–desorption isotherm is attributed to the Langmuir IV type. The formation of the hysteresis loops in the plot of Fig. 7A reveal the presence of mesopores. One can see from Fig. 7B that the TiO2 NSs samples have bimodal pore size distribution in the mesoporous area. The average pore sizes and pore volumes are 4.8 nm and 0.50 cm3 g−1 for the sample without aging, 4.3 nm and 0.46 cm3 g−1 for the sample with aging time of 2 h, and 6.6 nm and 0.62 cm3 g−1 for the sample with aging time of 6 h, respectively. The values of the BET specific surface area are 378, 360 and 347 m2 g−1 for the samples with aging time of 0 h, 2 h and 6 h, respectively, which are comparable to or larger than the values of typical layered protonated titanate NSs (379 m2 g−1 ) [24], surface-fluorinated anatase TiO2 NSs (253 m2 g−1 ) [61], graphenebased mesoporous titania NSs (297 m2 g−1 ) [62], and hollow TiO2 spheres (233.8 m2 g−1 ) [63]. It can also be see that when the aging time is increased, the surface areas of the samples decrease slightly due to the increase of the crystallinity.

Fig. 7. Nitrogen adsorption/desorption isotherms (A) and corresponding BJH pore size distribution curves (B) of TiO2 NSs. TiO2 NSs synthesized at 270 ◦ C for 0 h (a), 2 h (b) and 6 h (c), the molar ratio of TIP/OM was 1:1.

nanoparticles of dimensions >2 nm [65a]. Monticone et al. found no quantum size effect in anatase TiO2 nanoparticles for sizes 2Rg 1.5 nm [65b]. As discussed above, some N-doped TiO2 structures may exist in the sample (Fig. 3e). It has been reported that N doping can make the original band gap of TiO2 smaller [66]. Considering the formation of TiO2 NSs, which is formed by 2D network of TiO2 nanocrystals, and the existence of some N-doped TiO2 structures in TiO2 NSs, it is deduced that the obtained value of the band gap energy (3.10 eV) is reasonable.

3.4. UV absorption analysis and bandgap energy

3.5. Photoluminescence (PL) of TiO2 NSs

Fig. 8a shows a diffuse reflectance spectrum for TiO2 NSs. A characteristic absorption in the UV region due to the charge transfer was observed. The absorption edge was around 400 nm for the TiO2 NSs, which is larger than the edge for anatase TiO2 bulk and nanoparticles. The band gap energy of this change transfer was calculated based on UV–Vis spectrum. The relationship between (˛h)1/2 and the photo energy calculated based on Fig. 8a is shown in Fig. 8b. The band gap energy of TiO2 NSs is about 3.10 eV, which is slightly smaller than that of TiO2 nanoparticles. The bandgap energy shift is predominantly governed by the nanoparticle size and thickness of the sheet-like structures. It has been reported that smaller nanoparticles and TiO2 NSs often exhibit a larger band gap energy [3,64]. However, the value obtained in this experiment is slightly smaller than that in other cases. Serpone et al. have concluded that no size effects were observed for TiO2

The optical properties of TiO2 NSs, nanodots and the P25 were investigated by photoluminescence (PL) spectroscopy. The PL spectra of TiO2 NSs, nanodots and the P25 are compared in Fig. 9. Compared to P25, TiO2 NSs and nanodots exhibit stronger PL and the obtained PL spectrum of TiO2 NSs (Fig. 9a) is quite different from the spectra of TiO2 nanodots and P25. The peak positions and the band energies are shown in Fig. 9. For the TiO2 NSs, the PL peaks corresponding to direct transitions X1b → X2b (3.59 eV) and X1b → X1a (3.45 eV) were observed [67a,b]. The peak at 455 nm (2.72 eV) is assigned to the band edge luminescence of the anatase TiO2 particles. The broad band centered at 544 nm, which is similar to peak in the luminescence spectrum of the colloidal solution of exfoliated TiO2 NSs [64a], is assigned to a high density of surface defect sites. Many such defect sites exist at mid-band gap energies [67c,d]. For the TiO2 nanocrystals, in addition to the peaks at

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Fig. 8. UV–Visible spectrum of (a) and (b) (˛h)1/2 as a function of photo energy for TiO2 NSs, TiO2 NSs synthesized at 270 ◦ C for 2 h, the molar ratio of TIP/OM was 1:1.

Fig. 9. PL spectra of the TiO2 NSs (a), TiO2 nanocrystals (b) and P25 (c). (a) TiO2 NSs synthesized at 270 ◦ C for 2 h, the molar ratio of TIP/OM was 1:1; (b) TiO2 nanocrystals prepared at 270 ◦ C for 2 h, the molar ratio of TIP/OM was 1:32.

about 400 nm, 455 nm and 566 nm, a small peak at 330 nm (3.75 eV) was observed, which may be due to the size effect. For P25, there are five small peaks observed in the wavelength range from 350 to 470 nm. These PL signals are due to the emission of bandgap transition, xcitonic PL from surface oxygen vacancies and defects of the P25 sample. 3.6. Adsorption capacity Motivated by the high specific surface areas and mesoporous structures, the performance of TiO2 NSs as a potential adsorbent for removal of MB was measured. The adsorption isotherms of dye molecules as a function of MB concentration on three kinds of TiO2 NSs prepared by changing the aging time (0 h, 2 h and 6 h) are shown in Fig. 10. One can see from Fig. 10 that the adsorption capacity of the samples decreases with the increase of aging time due to the decrease of specific surface area of TiO2 NSs. The maximum adsorption capacities (Qmax ) of the three kinds of TiO2 NSs are about 186, 128 and 118 mg g−1 for the samples prepared with aging time of 0 h, 2 h and 6 h, respectively, illustrating the potential of TiO2 NSs as a superior adsorbent for practical applications in waste pollutant treatment. The values compare very favorably to adsorption capacities of several common adsorbents reported in the literature, such as zeolite (16.37 mg g−1 , MB), activated sewage char (120 mg g−1 , MB), titania (6 mg g−1 , MB), H3 PO4 -activated various micro-mesoporous carbons (138–190 mg g−1 , MB) [68,69]. The value (186 mg g−1 ) is also comparable to data reported for other

Fig. 10. Dye adsorption isotherms of MB (40 mL, C0 = 10–250 mg/L) on three kinds of TiO2 NSs (20 mg). The solid lines are the fitted curves. TiO2 NSs synthesized at 270 ◦ C for 0 h (a), 2 h (b) and 6 h (c), the molar ratio of TIP/OM was 1:1.

layered titanate products, for example, hollow spheres (154 mg g−1 , MB) and microspherulites (236 mg g−1 , MB) [70–72]. The high adsorption capacity of TiO2 NSs are assigned to the high surface area [73], and mesopore structures of the materials. In addition, oxygen vacancies on the surfaces of TiO2 NSs due to incomplete crystallization, the existence of surface functional groups, such as NH groups, can interact with MB molecules via hydrogen bonds and/or electrostatic forces can also promote the adsorption of dye molecules [60]. The Qmax values of NSs synthesized at different molar ratios of TTIP/OM measured at the same conditions are 184 mg g−1 (for TTIP/OM = 1/2), 114 mg g−1 (for TTIP/OM = 1/8) and 88 mg g−1 (for TTIP/OM = 1/32), respectively. Considering the influence of the molar ratio of TTIP/OM on the TiO2 nanostructures, one can conclude that the decrease of the absorption capacity is due to the shape change of TiO2 nanoparticles from sheet-like to spherical morphology. To understand the mechanism of adsorption, pseudo-first- and pseudo-second-order models were used to simulate the adsorption data for various contact times from Fig. 11a. The absorption evolution curve can be well fitted to the following pseudo second-order kinetic model (Fig. 11b). The linear form of the pseudo-secondorder model is expressed as follows: t 1 t = + qt qe k2 q2e

(1)

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Fig. 11. (a) UV–Vis spectrum of MB (40 mL, 10 mg/L) in the presence of TiO2 NSs (10 mg). (b) Pseudo-second-order kinetics of MB onto TiO2 NSs. C0 = 10 mg/L; mass of TiO2 NSs = 10 mg; solution volume = 80 mL. TiO2 NSs synthesized at 270 ◦ C for 2 h, the molar ratio of TIP/OM was 1:1.

where qe and qt refer to the amount of adsorbed dye at equilibrium and at time t, respectively, and k2 is the equilibrium rate constant of the pseudo-second-order kinetic model. The values of qe and k2 were calculated from the slope and intercept of Eq. (1), are 68 mg g−1 and 1.5 × 10−3 g mg−1 min−1 , respectively. The correlation coefficient has large values (R2 = 0.99888). It can be concluded that the adsorption of MB on TiO2 NSs follows the pseudo-second-order kinetic model which relies on the assumption that chemisorption and effective electrostatic interactions play a major role in the adsorption process [74]. Comparing the adsorption capacities of the adsorbents mentioned above (references [68–72]), we can conclude that TiO2 NSs shows efficient adsorption properties and is a promising candidate for environmentally friendly adsorbents in water treatment. 3.7. Enhanced photocatalytic activity of TiO2 NSs The as-prepared TiO2 NSs with high surface areas are also expected to exhibit high photocatalytic efficiency. In this experiment, MB (cationic dyes) was first used as pollute molecules to investigate the photocatalytic performance. For comparison, Degussa P25 TiO2 was used as a reference. Because adsorption has significantly influence on the photocatalytic process, preadsorption is done for P25 in the dark prior to UV irradiation. To illustrate that the degradation efficiency has big difference between P25 catalyst and TiO2 NSs, the pre-adsorption time is set 60 min for P25 catalyst and 0 min for TiO2 NSs. Fig. 12a shows the evolution of MB absorption spectra in the presence of TiO2 NSs catalyst, from which it can be see that the absorbance peaks of MB decreased quickly with irradiation time. One can see from Fig. 12b that the degradation efficiency of the TiO2 NSs was much higher than that of P25 TiO2 . For P25 catalyst system, the MB was removed 99.3% after 50 min. For TiO2 NSs catalyst system, the MB was removed 99.7% within 15, 5 and 5 min for the TiO2 NSs samples prepared by changing the age time 0 h, 2 h and 6 h. The enhanced photocatalytic activity is ascribed to the high adsorption capacity due to the high surface area, a high density of surface defect sites and the mesostructure of TiO2 NSs. In addition, the existence of some N-doped TiO2 structures with a narrow band gap is also responsible for the significant increase in photocatalytic activity [75]. It has been reported that stronger adsorption, resulted from the higher surface area, to the substrate is one of the important reasons for the higher photoreactivity of the TiO2 hollow spheres when compared with TiO2 nanoparticles [73]. Although the crystallinity of the sample prepared without aging is low, the photocatalytic activity is much higher than that of P25. We observed previously that

Fig. 12. (a) UV–Vis spectra of MB solution under UV irradiation for different times using TiO2 NSs as photocatalyst; (b) the change in the concentration of MB as a function of irradiation time using different photocatalysts. Curves: (1) in the presence of P25, the solution was equilibrated in the dark for 60 min prior to UV irradiation; (2–4) in the presence of TiO2 NSs, the solutions were not equilibrated prior to UV irradiation. TiO2 NSs synthesized at 270 ◦ C for 0 h (curve 2), 2 h (curve 3) and 6 h (curve 4), the molar ratio of TIP/OM was 1:1.

amorphous titania catalysts with larger surface areas exhibited enhanced photocatalytic activity for the decomposition of MB [76]. Compared to the sample prepared without aging, the higher photoactivity in the samples prepared with aging time for 2 and 6 h is attributed to the increase of crystallinity. The samples prepared

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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2015.09. 214.

References

Fig. 13. The change in the concentration of MO as a function of irradiation time using different photocatalysts, C0 = 10.0 mg/L. Curves: (1) in the presence of P25. (2–4) in the presence of TiO2 NSs. TiO2 NSs synthesized at 270 ◦ C for 0 h (curve 2), 2 h (curve 3) and 6 h (curve 4), the molar ratio of TIP/OM was 1:1. The solution was equilibrated in the dark for 30 min prior to UV irradiation.

with aging time for 2 and 6 h have almost the same photocatalytic activities due to the smaller crystallinity difference and surface areas difference between the two samples. To further prove that adsorption play an important role in the enhanced photocatalytic activity, the degradation of MO (anionic dyes) was performed. Because the ligand on the surface of TiO2 NSs is oleylamine molecules, the interaction between amine groups with negative electric field effect and anionic MO is repulsive. Therefore the adsorptive capacity for MO is very poor. Prior to UV irradiation, no obvious color changes were observed for the MO solutions, and C/C0 of the MO solution after equilibrium in the dark equaled almost 1.0 (Fig. 13), indicating that the adsorptive capacity of TiO2 NSs for MO is negligible. The change in the concentration of MO as a function of UV irradiation time using different photocatalysts is shown in Fig. 13. The degradation efficiency of the TiO2 NSs was much lower than that of P25 TiO2 . Compared to Fig. 12b, one can see that stronger adsorption is one of the important reasons for improving photocatalytic activity of TiO2 [73]. In addition, the TiO2 NSs prepared without aging shows the higher photoactivity when the irradiation time was less than 210 min, which is attributed to the higher surface area. However, when the irradiation time was longer than 210 min, the photoactivities of the samples prepared with aging time for 2 and 6 h exceed that of the sample prepared without aging due to the increase of crystallinity. 4. Conclusions Anatase TiO2 NSs with high surface area were synthesized via a reproducible one-step thermal decomposition method. During the synthesis procedure, only one type of amine surfactant, oleyamine, was used as capping agents and no other organic solvents were added. The results indicate that the TiO2 NSs possess high surface area up to 378 m2 g−1 . The concentration of capping agents is found to be a key factor controlling the morphology of the product. Adsorption and photodegradation experiments reveal that the prepared TiO2 NSs possess high adsorption capacities of model pollutants MB and high photocatalytic activity, showing that TiO2 NSs are efficient pollutant adsorbents and photocatalytic degradation catalysts of pollutant MB in wastewater treatment. Acknowledgements This work was funded by the National Natural Science Foundation of China (51173069, 51473068).

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