- Email: [email protected]
Controllable synthesis of uniformly distributed hollow rutile TiO2 hierarchical microspheres and their improved photocatalysis
Materials Chemistry and Physics 143 (2013) 446e454
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Controllable synthesis of uniformly distributed hollow rutile TiO2 hierarchical microspheres and their improved photocatalysis Yanming Xue, Jing Lin*, Ying Fan, Ammar Elsanousi, Xuewen Xu, Jiao Mi, Jie Li, Xinghua Zhang, Yang Lu, Tingting Zhang, Chengchun Tang* School of Material Science and Engineering, Hebei University of Technology, Tianjin 300130, PR China
h i g h l i g h t s Controllable conditions of the hollow TiO2 microspheres were investigated detailedly. A condensation process has been proposed as the formation mechanism of the hollow TiO2 microspheres. The specific surface area of the hierarchical TiO2 microspheres was measured as high as 37 m2 g1. Optimum photocatalytic conditions and kinetic study were investigated detail. Hollow hierarchical TiO2 structure displayed improved photocatalytic activity compared to bulk one.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 March 2013 Received in revised form 12 September 2013 Accepted 20 September 2013
We reported the controllable synthesis of uniformly distributed hierarchical hollow microspheres composed of rutile TiO2 nanorods as building blocks, prepared by a hydrothermal method without employing any templates/substrates or surfactants. The homogenous hollow microspheres were obtained by optimizing the experimental conditions including hydrothermal temperatures and tetrabutyl titanate (TBOT) concentrations. A detailed formation mechanism was also proposed. The samples were analyzed by BrunauereEmmetteTeller (BET) specific surface area analysis and ultravioletevisible (UV eVis) diffuse reflectance spectra (DRS). The photocatalytic results for methylene orange (MO) degradation showed that the hollow hierarchical microspheres exhibited the best photocatalytic activity among the as-synthesized products. Further optimum photocatalytic conditions were determined to study the degradation rate, decolorization and TOC (total organic carbon) removal efficiencies, and reaction kinetics in detail. Under optimum conditions, the contrastive photocatalytic experiments indicated that the photocatalytic activity was enhanced markedly when assembling the single-crystal rutile TiO2 nanorods into hollow hierarchical microstructures. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Oxides Semiconductors Microstructure
1. Introduction Numerous studies on water reuse have been focused primarily on effectively removing organic pollutants in wastewater by inexpensive, environment-friendly and easy-operating approaches [1,2]. Advanced oxidation processes using titanium dioxide (TiO2) nanomaterials have attracted considerable attention for decades due to their excellent physical and chemical properties, such as actual feasibility, biocompatibility, photostability, chemical inertness and favorable band position [3e8]. However, TiO2, as a wide band gap semiconductor (w3.0 or 3.2 eV), can utilize only w5% of
* Corresponding authors. Fax: þ86 22 60202660. E-mail addresses: [email protected] (J. Lin), [email protected] (C. Tang). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.09.026
the full sunlight spectrum for the degradation of organic compounds, presenting a poor efficiency [9]. Therefore, further enhancement of the photodegradation rate of TiO2 by various strategies is one of the new challenges in the related field. As resulted in several recent studies, diverse morphologies of TiO2 micro-/nano-materials can present distinctly different photocatalytic activities [10e19]. It is well known that rutile TiO2 is the most common crystal form and more thermodynamically stable than anatase or brookite phase at all temperatures, allowing it to be more applicable in many fields [20]. Therefore, fabrication and investigation of different morphologies of rutile TiO2 micro-/nanomaterials are of great significance and worth devoting efforts. Hollow structures have attracted much attention recently, because of their specific features for many important applications, e.g., catalysis, drug delivery, lithium-ion batteries and solar cells
Y. Xue et al. / Materials Chemistry and Physics 143 (2013) 446e454
[21e24]. These hollow structures can be usually fabricated by using either removable hard/soft-template as direct reagents or convenient template-free methods depending on Kirkendall diffusion and Ostwald ripening [25e28]. As to TiO2 hollow structures, they can effectively improve the photocatalytic properties due to their high surface areas and mesoporous structures [29e31]. Thanks to the contributions and efforts from many researchers, a variety of hollow TiO2 structures have been successfully fabricated in elaborately controllable technologies [32e37]. Actually, exploring a facile and controllable approach for general fabrication of hollow TiO2 micro-/nano-structures is of relative difficulty and is still a challenge. As for the recent research on fabrication of hollow rutile TiO2 microspheres, various strategies, such as mixed-solvent method [33], hydrothermal method [38,39] and laser irradiation method [32] have been developed. However, these methods may have some disadvantages, such as cumbersome routes, undetermined optimum conditions and potential dangerousness. In addition, the well-crystallized and well-dispersed properties of the as-reported hollow TiO2 quasi-micro/microspheres still need to improve. Therefore, our aim is to fabricate well-crystallized hollow rutile TiO2 microspheres with good dispersibility by using a simple, controllable and safe method. Herein, we report a one-step hydrothermal process for the synthesis of hierarchical hollow rutile TiO2 microspheres. Through varying the experimental conditions, the hollow microspheres could be controllably synthesized. We also proposed their growth mechanism based on experiment results. In addition, a series of photocatalytic experiments were preformed to compare the photocatalytic activities of the as-prepared products with different morphologies. The optimum photocatalytic conditions, degradation rates and reaction kinetics were found. 2. Experimental 2.1. Synthesis In a typical experiment, the hierarchical hollow microspheres were prepared as follows: 34 g tetrabutyl titanate (TBOT, (C4H9O)4Ti, 98%, Tianjin Fine Chemical Co. Ltd.) (0.1 mol) and 35 g concentrated hydrochloric acid (HCl, 38%, Tianjin Fine Chemical Co. Ltd.) were mixed in a beaker, and stirred for 10 min (herein, the TBOT concentration in the mixture was 49 wt%, simplified as 49%). 50 g of the as-stirred solution was placed in a stainless steel autoclave, and heated at 170 C for 24 h. Then, the slight-yellow precipitate was collected in the reaction solution. Finally, the targeted products were obtained after further filtering, washing and drying. To investigate the effect of TBOT concentration and the hydrothermal temperature on the hierarchical structure formation, further experiments were performed by (i) changing the TBOT concentration from 38 wt% to 63 wt% (simplified as 38%e63%) at a constant temperature of 170 C and (ii) changing the hydrothermal reaction temperature in the range of 120e220 C at a fixed TBOT concentration of 49 wt% (49%). 2.2. Characterizations The crystallographic properties of the products were characterized by X-ray Powder Diffractometry (XRD, Bruker D8 ADVANCE, Germany; measured at room temperature with a scan rate of 10 /min). The morphology and identity of the products were analyzed by FieldEmission Scanning Electron Microscope (FESEM, Hitachi S-4800, Japan; measured at room temperature with a voltage of 3 kV), Transmission Electron Microscope (TEM, JEM-2010, Japan; at room temperature with a voltage of 200 kV). The specific surface areas were analyzed by BrunauereEmmetteTeller (BET) using an automated gas
447
sorption analyzer (Quantachrome Instruments 1900, USA; using nitrogen adsorptionedesorption isotherm measured at 77 K). The diffuse reflectance ultravioletevisible (UV/Vis) spectrum of each product was measured by a spectrophotometer (HITACHI U-3900, Japan; at room temperature under the air atmosphere with a spectral resolution of 0.1 nm, wavelength accuracy 0.1 nm (adjusting at 656.1 nm)). The amount of total organic carbon (TOC) in the solution was detected by a TOC analyzer (SHIMADZU-TOC-VCPH/VCPN SERIES, Japan). 2.3. Methylene orange degradation experiment The photocatalytic degradation of bleaching methylene orange (MO) was obtained at room temperature through UV/Vis light (using a fluorescence lamp with a wavelength up to 430 nm). Appropriate amount of the as-synthesized powder was ultrasonically dispersed in different-concentration MO solution. The mixture was then placed in the dark under continuous stirring. At regular 20 min intervals, w3 ml of the turbid liquid was removed. Each sample was centrifugated to separate the catalyst particles from the solution, and the relative MO concentration of the liquid supernatant was determined using an UV/Vis spectrophotometer by monitoring the highest MO adsorption peaked at 463 nm. The pH (1 pH 11) of the solution was adjusted with an appropriate amount of NaOH and HCl solution. 3. Results and discussion 3.1. The morphology and crystal structure of the hierarchical hollow microspheres The morphological overview and crystallographic properties of the hollow-microsphere products are shown in Fig. 1. Welldispersed microspheres are on full display throughout the SEM image of Fig. 1a. Their average diameter is estimated to be w4 mm (Fig. 1b and c). From the broken hemisphere shown in Fig. 1b, one can see the hollow structure of the microspheres. The average diameter distribution of the hollow inner core is estimated to be 1.5 w 2 mm. The external shell structure is composed of regularly outward-radiating nanorods (Fig. 1d). The diameter and length of each nanorod subunit is w50 nm and w1 mm, respectively. By indexing the XRD data and comparing with the JCPDS card No. 650190, the phase composition of the dandelion-like hierarchical microspheres is identified as rutile TiO2 with lattice constants of a ¼ b ¼ 0.459 nm, c ¼ 0.297 nm and space group of P42/mnm. The XRD analysis also indicates that the product possesses high-purity and well-crystallized properties (see Fig. 1e). TEM images and the corresponding selected area electron diffraction (SAED) pattern of the nanorod subunits are presented in Fig. 2. A cluster of the nanorods is shown in Fig. 2a where each crystal rod directs toward the center. The nanorod is a symmetric square prism structure with a pyramid-like apex at the end (Fig. 2b). SAED pattern (Fig. 2c) clearly indicates that the hollow dandelion-like microspheres are composed of monocrystal nanorods. The SAED pattern also reveals the presence of (111), (110) and (001) planes directly correspond to the lattice spacing of 0.22, 0.32 and 0.25 nm of the tetragonal rutile TiO2 nanorods. The elongated growth of nanorod here is along the [001] axis. 3.2. Morphology dependence on experimental conditions Investigations on the dependence of the product morphology on various preparation conditions reveal that the fabrication of hollow TiO2 microspheres relies mainly on: (i) the TBOT
448
Y. Xue et al. / Materials Chemistry and Physics 143 (2013) 446e454
Fig. 1. SEM images of the hierarchical microspheres synthesized at 170 C and TBOT concentration of 49 wt% in the concentrated hydrochloric acid solution: (a) uniform-distribution panorama of the microspheres; (b) broken semi-microsphere showing the hollow structure; (c) single-distributed microsphere; (d) outward-radiating nanorods as the subunits for building the hierarchical structure; (e) XRD pattern of the microspheres in reference to that of standard PDF card of rutile TiO2.
concentration in the initial solution; and (ii) the controlled hydrothermal temperatures. By varying the TBOT concentration and fixing the hydrothermal temperature at 170 C, the SEM images show absolutely distinctive morphologies, while the XRD results illustrate almost identical crystalline characteristics (Fig. 3). The morphology exhibits random aggregated particles at a TBOT concentration of 63% (Fig. 3a), while at a concentration of 58%, nanorod clustering is initiated (Fig. 3b). Solid microspheres with regularly outward-radiating nanorods (diameters of sub-100 nm) start to appear at a concentration of 53% (Fig. 3c). Uniform hollow microspheres are formed only at concentrations ranging from 46 to 49%, with an average nanorod
subunit diameter less than 50 nm (Fig. 3d and e). Observations indicate that at TBOT concentrations between 38 w 43%, the hollow structure of hierarchical microspheres disappears, and random aggregations of solid TiO2 microspheres can be obtained, as shown in Fig. 3f. From the XRD results shown in Fig. 3g, we can conclude that the crystallographic parameters of the products are nearly independent on the TBOT concentration in the solutions, and all the as-prepared samples show the same crystal structure (rutile TiO2). The influence of hydrothermal-treatment temperature on the morphological and crystallographic structure of the products has also been studied by varying the hydrothermal temperatures
Fig. 2. TEM images of the hollow hierarchical microspheres synthesized at 170 C and TBOT concentration of 49 wt%: (a) a cluster of nanorod subunits from broken microsphere; (b) single-nanorod subunit; (c) SAED pattern of the single nanorod.
Y. Xue et al. / Materials Chemistry and Physics 143 (2013) 446e454
449
Fig. 3. SEM images of the products obtained at the temperature of 170 C and different TBOT concentrations of: (a) 63%; (b) 58%; (c) 53%; (d) 49%; (e) 46%; (f) 38e43%. (g) The corresponding XRD patterns of the products as-obtained with different TBOT concentrations.
between 120 and 220 C, while keeping the TBOT concentration of 49%, as illustrated in Fig. 4. When the temperature is in the range between 120 and 140 C, microspheres with smooth surfaces and large cracks can be observed (Fig. 4a and b). Within this stage, the XRD pattern shows wide diffraction peaks, indicating that small TiO2 crystals in nanoscale size or a solid TiO2 structure with relatively poor crystallinity formed (Fig. 4g). A hollow morphology can be found at temperatures ranging from 160 to 180 C (Fig. 4c and d). Here, the average diameter of the corresponding subunit is estimated to be w50 nm, which possesses much higher crystallinity than the solid spheres. When elevating the temperature over 180 C, the hollow structure disappears. In the temperature range of 200e220 C, solid microspheres with distributing dense nanorods as subunits on the surface are observed. The average diameter of these subunits is w70 nm (Fig. 4e and f). Meanwhile, the corresponding crystallinity is further improved compared to that between 160 and 180 C. These results clearly indicate that an increase in the hydrothermal temperature leads to an enhancement in crystallinity of the products (Fig. 4g). 3.3. Proposed growth mechanism The growth mechanism for the hierarchical hollow structures is schematically proposed as shown in Fig. 5. Some researchers have reported a similar structure and formation mechanism [38e41]. On the basis of the above-mentioned results, TBOT concentrations and hydrothermal temperatures are crucial for the formation of hollow rutile TiO2 microspheres. Actually, the amount and quality of TiO2 crystal nucleus in the mixed solution are just a real cause for the formation of the distinct morphologies, where the TBOT hydrolysis
rate plays a vital role in the nucleation process. The introduced 38% HCl provides enough Hþ to reduce the self-ionization of water in the solution efficiently and further lowering the hydrolysis rate of TBOT. Hence, the existence of 38% HCl is important for the nucleation process during the hydrothermal synthesis. Due to the same content of HCl in each solution, the dependence of different morphologies on the TBOT concentration is our first discussion focus. The growth mechanism of the products prepared at different TBOT concentrations at 170 C is demonstrated in Fig. 5a. It is predicted that when the TBOT concentration is very high (at 63%), the hydrolysis rate is extremely low and the rutile TiO2 crystal nucleus forms slowly due to a low concentration of Ti4þ in the surrounding solution. During the limited reaction time, only some small particles form as the final product. When the TBOT concentration is set between 53%and 58%, a single-crystal nucleus of rutile TiO2 is rapidly formed by homogeneous nucleation. At the same time, some small crystal nuclei are attached to its surface via a heteronucleation process. Due to existence of Ti4þ in the solution, the crystal growth on the surface of multimers occurs along the low energy face of the crystal nuclei, gradually leading to the emergence of ordered nanorod-based 3D solid clusters or microspheres. When the concentration is in the range of 46%e49%, the amount of TiO2 crystal nuclei increases. In order to understand the formation mechanism of hollow microspheres, further experiments with different hydrothermal reaction times were carried out. The SEM images and XRD patterns of the samples are shown in Fig. 6. At the early stage of TBOT hydrolysis, metastable anatase TiO2 and amorphous TiO2 nuclei are aggregated (the as-marked peaks in the bottom of Fig. 6e (1 h) and (3 h)). Subsequently, the TiO2 changes from anatase and amorphous to the dense rutile crystalline state via a condensation process
Fig. 4. SEM images of the products with identical TBOT concentration of 49% in the original acid solution, with varying hydrothermal temperatures as: (a) 120 C; (b) 140 C; (c) 160 C; (d) 180 C; (e) 200 C; (f) 220 C. (g) The corresponding XRD patterns of the products at different temperatures.
450
Y. Xue et al. / Materials Chemistry and Physics 143 (2013) 446e454
(Fig. 6e (6 h) and (15 h)). This process is often accompanied by volume change and inner-stress generation in the materials. Thus the initial spheres can be condensed to hollow structures. Simultaneously, due to a saturated Ti4þ solution surrounding the sphere surface, the nanorods growth begins to occur by Ostwald ripening, finally leading to the formation of hollow dandelion-like hierarchical microspheres. At concentrations lower than 46%, the hydrolysis rate is very fast and a mass of polycrystalline clusters can be formed. The supersaturated rutile TiO2 nuclei are jointed to their coordination sites and assembled into the irregular aggregations. Due to the existence of Ti4þ ions in the solution, the nanorods grow on the aggregation surfaces and, finally, the aggregation-like microspheres are generated. The formation mechanism of the diverse samples prepared at different reaction temperatures ranging from 120 to 220 C with TBOT concentration of 49% is proposed in Fig. 6b. At temperatures lower than 160 C, most of the solution inside the autoclave is in liquid phase, which makes the TBOT hydrolysis easily and fast. The relatively low reaction temperature can maintain the poor crystallinity and low density of the product. As a result, a condensation process could not occur at this hydrothermal reaction stage. The formation mechanism of the products prepared at 160e180 C is similar to those of the above-mentioned cases with the TBOT concentration of 46e49%. When the temperature is in the range of 180e220 C, most of the solution in the autoclave exists as vapor, and the TBOT hydrolyzes slowly. At the temperature, the relativelylarge rutile TiO2 nuclei directly form, and they aggregate with each other. Because of plenty of Ti4þ in solution and Ostwald ripening, nanorods grow along the aggregated nuclei. Finally, the ordered nanorod-based solid 3D microsphere forms. 3.4. BET analysis
Fig. 5. The proposed growth mechanism of the hierarchical products obtained by: (a) adjusting the TBOT concentration in the original acid solution at the fixed hydrothermal temperature of 170 C; (b) varying the hydrothermal temperature with fixing the TBOT concentration at 49%.
The BET parameters of the as-prepared product were obtained by using a nitrogen sorption analysis (Fig. 7). According to the IUPAC (International Union of Pure and Applied Chemistry) classifications of hysteresis loops, the isotherm is attributed to type H3 hysteresis, which does not exhibit any limited adsorption P/P0 varying from 0 to 1. This indicates a non-rigid aggregation of the TiO2 nanorods, which gives rise to numerous slit-shaped pores in the microspheres, as shown in Fig. 7a. The hollow microsphere structures have specific surface areas (Fig. 7b) up to 37 m2 g1 for the sample obtained at 180 C with TBOT concentration of 49%, which is the highest value among the as-prepared samples. This
Fig. 6. SEM images of the samples obtained after the hydrothermal treatment at 170 C by different reaction times: (a) 1 h; (b) 3 h; (c) 6 h; (d) 15 h. (e) The corresponding XRD patterns for different reaction times, the as-marked peaks representing the anatase TiO2 phase.
Y. Xue et al. / Materials Chemistry and Physics 143 (2013) 446e454
451
Fig. 7. Measurement of BET parameters: (a) nitrogen adsorptionedesorption isotherm of a hollow microsphere product, under 77 K; (b) histograms showing the different BET specific surface area distributions of the as-fabricated product with changing TBOT concentration at 170 C (the top one) and with varying temperatures for the same TBOT concentration of 49% (the bottom one).
implies that the hollow microspheres have the potential to enhance the photocatalytic activities superior to the other samples. 3.5. UVevis diffuse reflectance spectra analyses The UVevis diffuse reflectance spectra (DRS) of the corresponding products (with the specific surface area of 27 and 37 m2 g1, respectively), similar-sized bulk rutile TiO2 and P25 are comparatively presented in Fig. 8. Fig. 8a displays that all absorptions are confined within the near ultraviolet (NUV) region. For the hollow rutile TiO2 structures, their absorption edge located at w413 nm basically coincides with the similar-sized bulk rutile TiO2. This implies that the single-crystal rutile nanorod, as an elementary unit of the hollow structure, has an invariable electronic structure and light-adsorption property compared to the bulk product. The rutile TiO2 product has a broader absorption region than that of P25, which reveals different essential attributes between anatase and rutile type TiO2 due to P25 containing the component of anatase TiO2. On the basis of their adsorption spectra, the energy band gap (Eg) for the as-measured TiO2 samples can be predicted by the following equation [42]:
ahn ¼ A hn Eg
n=2
(1)
where a, n, Eg and A are the absorption coefficient, the light frequency, the band gap energy and a constant, respectively [43]. The n is determined as 4 due to the indirect transition type of rutile TiO2 [43]. The Eg of the hollow hierarchical microspheres and the bulk rutile TiO2 are obtained from the plots of (ahn)1/2 versus (hn) and determined to be w3.0 eV, which is lower than that of P25. 3.6. MO degradation The performance of the hollow microspheres in degrading MO under ultraviolet irradiation has been investigated (Figs. 9 and 10). The results shown in Fig. 9a and b reveal that the hollow dandelionlike hierarchical TiO2 microspheres exhibit higher photocatalytic activities than that of other samples with distinct morphologies. Obviously, after 160 min, the degradation efficiency of hollow TiO2 structures is at least 5% higher than that of the solid structures. The hollow hierarchical TiO2 microspheres synthesized at 180 C with TBOT concentration of 49% show the highest photocatalytic activity. This remarkable photocatalytic capacity may be attributed to its special hollow morphology and high crystallinity (as illustrated in
Fig. 8. (a) UVevis diffuse reflectance spectra of the similar-sized bulk TiO2, P25 and the samples (with the specific surface areas of 27 and 37 m2 g1, respectively). (b) Corresponding band gaps (Eg) of different samples.
452
Y. Xue et al. / Materials Chemistry and Physics 143 (2013) 446e454
Fig. 9. MO photodegradation of the different samples (100 ml of 20 ppm MO solution, catalyst dosage of 100 mg): (a) obtained at 170 C with different TBOT concentration; (b) obtained by fixing the TBOT concentration at 49% with different hydrothermal reaction temperatures.
Figs. 5 and 6). The hollow structure with outward-radiating monodispersed nanorods can offer more active sites during the photocatalytic reaction process, and the well-crystallized single-crystalline TiO2 nanorods can effectively hinder the recombination of electronehole pairs under ultraviolet irradiation. These two features make the as-synthesized hollow microspheres highly active for photocatalysis. In the photocatalytic experiments, we also
determined the optimum conditions of this prominent sample (synthesized at 180 C with TBOT concentration of 49%), including initial dye concentration, catalyst dosage and pH value (see Fig. 10aec). By fixing the catalyst dosage at 30 mg and varying the MO concentration from 5 to 30 ppm at pH of 7, the optimum initial dye concentration can be found as 5 ppm (Fig. 10a). With changing the amount of TiO2 microspheres from 5 to 35 mg, further optimum
Fig. 10. Optimum conditions of photocatalytic experiments using as-obtained hollow microspheres (at 180 C with TBOT concentration of 49%): (a) optimum initial dye concentration; (b) optimum catalyst dosage; (c) optimum PH and contrast decomposition MO of the hollow microspheres with the similar-sized bulk rutile TiO2 and the P25 under the optimum conditions.
Y. Xue et al. / Materials Chemistry and Physics 143 (2013) 446e454
453
Fig. 11. Graphics for kinetic study and degradation efficiencies under the optimum conditions: (a) plot of ln(C0/Ct) vs irradiating time with linear fitting; (b) decolorization and TOC removal efficiencies in 20, 60 and 120 min.
catalyst dosage can be determined as 30 mg by photocatalysis degradation of 50 ml 5 ppm MO solution (pH ¼ 7) within 120 min (Fig. 10b). By fixing the catalyst dosage and initial MO concentration at 30 mg and 5 ppm respectively, the optimum pH for a highly effective degradation is 5 after 120 min (Fig. 10c). As a result, for 5 ppm MO solution, the optimum photocatalytic conditions are 30 mg catalyst dosage and pH ¼ 5. In addition, the comparative degradation experiments were performed among the TiO2 microspheres (synthesized at 180 C with TBOT concentration of 49%), similar-sized bulk rutile TiO2 and P25, under the optimum conditions (Fig. 10d). The blank test (MO solution without any catalyst) exhibits almost no catalysis. Noticeably, although the hollow dandelion-like rutile TiO2 demonstrates lower dye decomposition than that of P25 within 120 min, it displays enhanced photocatalytic activity comparable to the similar-sized bulk rutile TiO2. The degradation efficiency for the as-synthesized hollow microspheric structure can reach 99.03% after 120 min, which is lower than that of P25 (after only 60 min, reaching up to 99.56%), but higher than that of the bulk TiO2 (with only 95.19% after 120 min). This enhancement may be resulted from the increase in active sites as compared to the bulk one, due to the existence of special outward-radiating nanorod structures. The result indicates that the rutile TiO2 could exhibit remarkably enhanced photocatalytic capacity when fabricated into hollow hierarchical microsphere with radial nanorods. In order to demonstrate better the reaction kinetics of the MO degradation under the optimum conditions, we utilized the apparent pseudo-first-order model in the present experiments expressed as [44]:
ln
C0 ¼ Kapp t Ct
(2)
where the Kapp is the apparent pseudo-first-order rate constant (with unit of min1); Ct is the MO solution concentration at time t (mg L1 ¼ ppm); and C0 is the initial MO concentration (mg L1 ¼ ppm). By means of first-order linear fitting, the graphics was plotted in Fig. 11a. The slope of the line represents Kapp, which can be calculated to be 0.03769 (9.9819 104). The linear fitting implies that this photocatalytic reaction is a first-scale kinetic reaction. Under the optimized photocatalytic conditions, the decolorization and TOC (total organic carbon) removal efficiencies can be obtained after 20, 60 and 120 min (Fig. 11b). The photocatalytic reaction can efficiently destroy the conjugated color systems in MO dye molecules, leading to an obvious decolorization process. The decolorization efficiency reaches 99.61% by ultraviolet irradiating
in 120 min. However, all TOC removal efficiencies are retarded by w30% in different time period (after 120 min, only 71.73%). This indicates that within 120 min of ultraviolet irradiation, the photocatalytic oxidation reaction fails to fully destroy the whole structures of MO dye molecules, and the apparent degradation rate is not equal to actual mineralization level of MO organics. 4. Conclusion In summary, the controllable hydrothermal process was proposed to fabricate hierarchical hollow rutile TiO2 microspheres. The mean diameter distribution of the hollow inner core was over the range of w1.5e2 mm. The single-crystal nanorod subunit was estimated to be w50 nm in average diameter and w1 mm in length. The optimum experimental conditions for fabricating hierarchical hollow structures were at the TBOT concentration of 46e49% and hydrothermal temperature range of 160e180 C for 24 h. The formation of the hollow structure has been proposed as a condensation process from anatase and amorphous to the dense rutile crystalline state. The specific surface area of hollow microspheres reached 37 m2 g1, and their absorption edge (413 nm) and energy gap (3.0 eV) were in good accordance with the bulk ones. The hollow microspheres synthesized at 180 C and TBOT concentration of 49% display the highest photocatalytic activity among all the asprepared products with different morphologies. The optimum degradation conditions for the hollow microsphere were at catalyst dosage of 30 mg, with 50 ml of 5 ppm MO solution and pH of 5, within 120 min. Under the optimum conditions, the photocatalytic reaction was a first-scale kinetic reaction, and showed a superior photocatalytic performance compared to the similar-sized bulk counterpart. Further exploration showed that the apparent degradation efficiency is not equal to the actual TOC removal efficiency. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 10974041, 51172060, 51202055, 21103056), Natural Science Foundation of Hebei Province (Grant No. E2012202040, E2012202114), the Key Basic Research Program of Hebei Province of China (No. 12965135D), the National Basic Research Program of China (973 Programs, No. 2011CB612301) and the Innovation Fund for Excellent Youth of Hebei University of Technology (No. 2012001). References [1] O. Legrini, E. Oliveros, A.M. Braun, Chem. Rev. 93 (1993) 671.
454
Y. Xue et al. / Materials Chemistry and Physics 143 (2013) 446e454
[2] M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [3] J. Zhou, Y. Cheng, J. Yu, J. Photochem. Photobiol. A 223 (2011) 82. [4] U.M. Gaya, A.H. Abdullah, J. Photochem. Photobiol. C 9 (2007) 1. [5] J.G. Yu, Y.R. Su, B. Cheng, Adv. Funct. Mater. 17 (2007) 1984. [6] D.G. Shchukin, R.A. Caruso, Chem. Mater. 16 (2004) 2287. [7] J.H. Park, S.Y. Jung, R. Kim, N.G. Park, J. Kim, S.S. Lee, J. Power Sources 194 (2009) 574. [8] Y. Li, T. Sasaki, Y. Shimizu, N. Koshizaki, J. Am. Chem. Soc. 130 (2008) 14755. [9] S.J. Ding, X. Yin, X.J. Lü, Y.M. Wang, F.Q. Huang, D.Y. Wan, ACS Appl. Mater. Interfaces 4 (2012) 306. [10] J.A. Byrne, B.R. Eggins, J. Electroanal. Chem. 457 (1998) 61. [11] L. Liu, H.J. Liu, Y.P. Zhao, Y.Q. Wang, Y.Q. Duan, G.D. Gao, M. Ge, W. Chen, Environ. Sci. Technol. 42 (2008) 2342. [12] Y.K. Du, J. Rabani, J. Phys. Chem. B 107 (2003) 11970. [13] M. Adachi, Y. Murata, M. Harada, S. Yoshikawa, Chem. Lett. 8 (2000) 942. [14] J.E.G.J. Wijnhoven, W.L. Vos, Science 281 (1998) 802. [15] N.L. Wu, S.Y. Wang, I.A. Rusakova, Science 285 (1999) 1375. [16] S. Madhugiri, B. Sun, P.G. Smirniotis, J.P. Ferraris, K.J. B Jr., Microporous Mesoporous Mater. 69 (2004) 77. [17] M. Qin, W. Zhao, J.L. Chu, J.K. Qu, L. Wang, S.H. Li, H. Zhang, H.X. Zhao, Z.H. Yu, T.Y. Xue, J. Li, Q. Guo, T. Qi, Mater. Res. Bull. 48 (2013) 1076. [18] S. Chen, W. Chu, Y.Y. Huang, X. Liu, D.G. Tong, Mater. Res. Bull. 47 (2012) 4514. [19] I.B. Troitskaia, T.A. Gavrilova, V.V. Atuchin, Phys. Procedia 23 (2012) 65. [20] D.A.H. Hanaor, M.H.N. Assadi, S. Li, A. Yu, C.C. Sorrell, Comput. Mech. 50 (2012) 185. [21] X.L. Fang, Z.H. Liu, M.F. Hsieh, M. Chen, P.X. Liu, C. Chen, N.F. Zheng, ACS Nano 6 (2012) 4434. [22] C. Rungnim, U. Arsawang, T. Rungrotmongkol, S. Hannongbuab, Chem. Phys. Lett. 550 (2012) 99. [23] L. Zhou, D.Y. Zhao, X.W. Lou, Angew. Chem. Int. Ed. 51 (2012) 239.
[24] Z.H. Dong, X.Y. Lai, J.E. Halpert, N.L. Yang, L.X. Yi, J. Zhai, D. Wang, Z.Y. Tang, L. Jiang, Adv. Mater. 24 (2012) 1046. [25] Y. Zeng, X. Wang, H. Wang, Y. Dong, Y. Ma, J.N. Yao, Chem. Commun. 46 (2010) 4312. [26] Z.Y. Wang, D.Y. Luan, C.M. Li, F.B. Su, S. Madhavi, F.Y.C. Boey, X.W. Lou, J. Am. Chem. Soc. 132 (2010) 16271. [27] Z.Y. Fei, S.C. He, L. Li, W.J. Ji, C.T. Au, Chem. Commun. 48 (2012) 853. [28] C.P. Zhao, X.W. Zhang, Y.P. Zhang, Y.L. Xing, X.J. Zhang, X.H. Zhang, J.S. Jie, CrystEngComm 14 (2012) 819. [29] H.J. Zhang, G.D. Du, W.Q. Lu, L.L. Cheng, X.D. Zhu, Z. Jiao, CrystEngComm 14 (2012) 3793. [30] S.H. Yao, X.Y. Jia, L.L. Jiao, C. Zhu, Z.L. Shi, Indian J. Chem. Sect. A 51A (2012) 1049. [31] T.Y. Zhao, Z.Y. Liu, K. Nakata, S. Nishimoto, T. Murakami, Y. Zhao, L. Jiang, A. Fujishima, J. Mater. Chem. 20 (2010) 5095. [32] H.Q. Wang, M. Miyauchi, Y. Ishikawa, A. Pyatenko, N. Koshizaki, Y. Li, L. Liang, X.Y. Li, Y. Bando, D. Golberg, J. Am. Chem. Soc. 133 (2011) 19102. [33] B. Peng, L.F. Tan, D. Chen, X.W. Meng, F.Q. Tang, ACS Appl. Mater. Interfaces 4 (2012) 96. [34] Y. Wang, X.W. Su, S. Lu, J. Mater. Chem. 22 (2012) 1969. [35] J.Q. Li, D.F. Wang, Z.L. He, Z.F. Zhu, J. Am. Ceram. Soc. 94 (2011) 1639. [36] H.G. Yang, H.C. Zeng, J. Phys. Chem. B 108 (2004) 3492. [37] Y.J. Kim, S.Y. Chai, W.I. Lee, Langmuir 23 (2007) 9567. [38] X.X. Li, Y.J. Xiong, Z.Q. Li, Y. Xie, Inorg. Chem. 45 (2006) 3493. [39] S.J. Liu, J.Y. Gong, B. Hu, S.H. Yu, Cryst. Growth Des. 9 (2009) 203. [40] S.S. Mali, C.A. Betty, P.N. Bhosale, P.S. Patil, CrystEngComm 13 (2011) 6349. [41] H.M. Zhang, H. Yu, Y.H. Han, P.R. Liu, S.Q. Zhang, P. Wang, Y.B. Cheng, H.J. Zhao, Nano Res. 4 (2011) 938. [42] X. Zhang, Z.H. Ai, F.L. Jia, L.Z. Zhang, J. Phys. Chem. C 112 (2008) 747. [43] J. Cao, B.Y. Xu, H.L. Lin, B.D. Luo, S.F. Chen, Chem. Eng. J. 185-186 (2012) 91. [44] X.P. Lin, T. Huang, F.Q. Huang, W.D. Wang, J.L. Shi, J. Phys. Chem. B 110 (2006) 24629.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Controllable synthesis of uniformly distributed hollow rutile TiO2 hierarchical microspheres and their improved photocatalysis Yanming Xue, Jing Lin*, Ying Fan, Ammar Elsanousi, Xuewen Xu, Jiao Mi, Jie Li, Xinghua Zhang, Yang Lu, Tingting Zhang, Chengchun Tang* School of Material Science and Engineering, Hebei University of Technology, Tianjin 300130, PR China
h i g h l i g h t s Controllable conditions of the hollow TiO2 microspheres were investigated detailedly. A condensation process has been proposed as the formation mechanism of the hollow TiO2 microspheres. The specific surface area of the hierarchical TiO2 microspheres was measured as high as 37 m2 g1. Optimum photocatalytic conditions and kinetic study were investigated detail. Hollow hierarchical TiO2 structure displayed improved photocatalytic activity compared to bulk one.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 March 2013 Received in revised form 12 September 2013 Accepted 20 September 2013
We reported the controllable synthesis of uniformly distributed hierarchical hollow microspheres composed of rutile TiO2 nanorods as building blocks, prepared by a hydrothermal method without employing any templates/substrates or surfactants. The homogenous hollow microspheres were obtained by optimizing the experimental conditions including hydrothermal temperatures and tetrabutyl titanate (TBOT) concentrations. A detailed formation mechanism was also proposed. The samples were analyzed by BrunauereEmmetteTeller (BET) specific surface area analysis and ultravioletevisible (UV eVis) diffuse reflectance spectra (DRS). The photocatalytic results for methylene orange (MO) degradation showed that the hollow hierarchical microspheres exhibited the best photocatalytic activity among the as-synthesized products. Further optimum photocatalytic conditions were determined to study the degradation rate, decolorization and TOC (total organic carbon) removal efficiencies, and reaction kinetics in detail. Under optimum conditions, the contrastive photocatalytic experiments indicated that the photocatalytic activity was enhanced markedly when assembling the single-crystal rutile TiO2 nanorods into hollow hierarchical microstructures. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Oxides Semiconductors Microstructure
1. Introduction Numerous studies on water reuse have been focused primarily on effectively removing organic pollutants in wastewater by inexpensive, environment-friendly and easy-operating approaches [1,2]. Advanced oxidation processes using titanium dioxide (TiO2) nanomaterials have attracted considerable attention for decades due to their excellent physical and chemical properties, such as actual feasibility, biocompatibility, photostability, chemical inertness and favorable band position [3e8]. However, TiO2, as a wide band gap semiconductor (w3.0 or 3.2 eV), can utilize only w5% of
* Corresponding authors. Fax: þ86 22 60202660. E-mail addresses: [email protected] (J. Lin), [email protected] (C. Tang). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.09.026
the full sunlight spectrum for the degradation of organic compounds, presenting a poor efficiency [9]. Therefore, further enhancement of the photodegradation rate of TiO2 by various strategies is one of the new challenges in the related field. As resulted in several recent studies, diverse morphologies of TiO2 micro-/nano-materials can present distinctly different photocatalytic activities [10e19]. It is well known that rutile TiO2 is the most common crystal form and more thermodynamically stable than anatase or brookite phase at all temperatures, allowing it to be more applicable in many fields [20]. Therefore, fabrication and investigation of different morphologies of rutile TiO2 micro-/nanomaterials are of great significance and worth devoting efforts. Hollow structures have attracted much attention recently, because of their specific features for many important applications, e.g., catalysis, drug delivery, lithium-ion batteries and solar cells
Y. Xue et al. / Materials Chemistry and Physics 143 (2013) 446e454
[21e24]. These hollow structures can be usually fabricated by using either removable hard/soft-template as direct reagents or convenient template-free methods depending on Kirkendall diffusion and Ostwald ripening [25e28]. As to TiO2 hollow structures, they can effectively improve the photocatalytic properties due to their high surface areas and mesoporous structures [29e31]. Thanks to the contributions and efforts from many researchers, a variety of hollow TiO2 structures have been successfully fabricated in elaborately controllable technologies [32e37]. Actually, exploring a facile and controllable approach for general fabrication of hollow TiO2 micro-/nano-structures is of relative difficulty and is still a challenge. As for the recent research on fabrication of hollow rutile TiO2 microspheres, various strategies, such as mixed-solvent method [33], hydrothermal method [38,39] and laser irradiation method [32] have been developed. However, these methods may have some disadvantages, such as cumbersome routes, undetermined optimum conditions and potential dangerousness. In addition, the well-crystallized and well-dispersed properties of the as-reported hollow TiO2 quasi-micro/microspheres still need to improve. Therefore, our aim is to fabricate well-crystallized hollow rutile TiO2 microspheres with good dispersibility by using a simple, controllable and safe method. Herein, we report a one-step hydrothermal process for the synthesis of hierarchical hollow rutile TiO2 microspheres. Through varying the experimental conditions, the hollow microspheres could be controllably synthesized. We also proposed their growth mechanism based on experiment results. In addition, a series of photocatalytic experiments were preformed to compare the photocatalytic activities of the as-prepared products with different morphologies. The optimum photocatalytic conditions, degradation rates and reaction kinetics were found. 2. Experimental 2.1. Synthesis In a typical experiment, the hierarchical hollow microspheres were prepared as follows: 34 g tetrabutyl titanate (TBOT, (C4H9O)4Ti, 98%, Tianjin Fine Chemical Co. Ltd.) (0.1 mol) and 35 g concentrated hydrochloric acid (HCl, 38%, Tianjin Fine Chemical Co. Ltd.) were mixed in a beaker, and stirred for 10 min (herein, the TBOT concentration in the mixture was 49 wt%, simplified as 49%). 50 g of the as-stirred solution was placed in a stainless steel autoclave, and heated at 170 C for 24 h. Then, the slight-yellow precipitate was collected in the reaction solution. Finally, the targeted products were obtained after further filtering, washing and drying. To investigate the effect of TBOT concentration and the hydrothermal temperature on the hierarchical structure formation, further experiments were performed by (i) changing the TBOT concentration from 38 wt% to 63 wt% (simplified as 38%e63%) at a constant temperature of 170 C and (ii) changing the hydrothermal reaction temperature in the range of 120e220 C at a fixed TBOT concentration of 49 wt% (49%). 2.2. Characterizations The crystallographic properties of the products were characterized by X-ray Powder Diffractometry (XRD, Bruker D8 ADVANCE, Germany; measured at room temperature with a scan rate of 10 /min). The morphology and identity of the products were analyzed by FieldEmission Scanning Electron Microscope (FESEM, Hitachi S-4800, Japan; measured at room temperature with a voltage of 3 kV), Transmission Electron Microscope (TEM, JEM-2010, Japan; at room temperature with a voltage of 200 kV). The specific surface areas were analyzed by BrunauereEmmetteTeller (BET) using an automated gas
447
sorption analyzer (Quantachrome Instruments 1900, USA; using nitrogen adsorptionedesorption isotherm measured at 77 K). The diffuse reflectance ultravioletevisible (UV/Vis) spectrum of each product was measured by a spectrophotometer (HITACHI U-3900, Japan; at room temperature under the air atmosphere with a spectral resolution of 0.1 nm, wavelength accuracy 0.1 nm (adjusting at 656.1 nm)). The amount of total organic carbon (TOC) in the solution was detected by a TOC analyzer (SHIMADZU-TOC-VCPH/VCPN SERIES, Japan). 2.3. Methylene orange degradation experiment The photocatalytic degradation of bleaching methylene orange (MO) was obtained at room temperature through UV/Vis light (using a fluorescence lamp with a wavelength up to 430 nm). Appropriate amount of the as-synthesized powder was ultrasonically dispersed in different-concentration MO solution. The mixture was then placed in the dark under continuous stirring. At regular 20 min intervals, w3 ml of the turbid liquid was removed. Each sample was centrifugated to separate the catalyst particles from the solution, and the relative MO concentration of the liquid supernatant was determined using an UV/Vis spectrophotometer by monitoring the highest MO adsorption peaked at 463 nm. The pH (1 pH 11) of the solution was adjusted with an appropriate amount of NaOH and HCl solution. 3. Results and discussion 3.1. The morphology and crystal structure of the hierarchical hollow microspheres The morphological overview and crystallographic properties of the hollow-microsphere products are shown in Fig. 1. Welldispersed microspheres are on full display throughout the SEM image of Fig. 1a. Their average diameter is estimated to be w4 mm (Fig. 1b and c). From the broken hemisphere shown in Fig. 1b, one can see the hollow structure of the microspheres. The average diameter distribution of the hollow inner core is estimated to be 1.5 w 2 mm. The external shell structure is composed of regularly outward-radiating nanorods (Fig. 1d). The diameter and length of each nanorod subunit is w50 nm and w1 mm, respectively. By indexing the XRD data and comparing with the JCPDS card No. 650190, the phase composition of the dandelion-like hierarchical microspheres is identified as rutile TiO2 with lattice constants of a ¼ b ¼ 0.459 nm, c ¼ 0.297 nm and space group of P42/mnm. The XRD analysis also indicates that the product possesses high-purity and well-crystallized properties (see Fig. 1e). TEM images and the corresponding selected area electron diffraction (SAED) pattern of the nanorod subunits are presented in Fig. 2. A cluster of the nanorods is shown in Fig. 2a where each crystal rod directs toward the center. The nanorod is a symmetric square prism structure with a pyramid-like apex at the end (Fig. 2b). SAED pattern (Fig. 2c) clearly indicates that the hollow dandelion-like microspheres are composed of monocrystal nanorods. The SAED pattern also reveals the presence of (111), (110) and (001) planes directly correspond to the lattice spacing of 0.22, 0.32 and 0.25 nm of the tetragonal rutile TiO2 nanorods. The elongated growth of nanorod here is along the [001] axis. 3.2. Morphology dependence on experimental conditions Investigations on the dependence of the product morphology on various preparation conditions reveal that the fabrication of hollow TiO2 microspheres relies mainly on: (i) the TBOT
448
Y. Xue et al. / Materials Chemistry and Physics 143 (2013) 446e454
Fig. 1. SEM images of the hierarchical microspheres synthesized at 170 C and TBOT concentration of 49 wt% in the concentrated hydrochloric acid solution: (a) uniform-distribution panorama of the microspheres; (b) broken semi-microsphere showing the hollow structure; (c) single-distributed microsphere; (d) outward-radiating nanorods as the subunits for building the hierarchical structure; (e) XRD pattern of the microspheres in reference to that of standard PDF card of rutile TiO2.
concentration in the initial solution; and (ii) the controlled hydrothermal temperatures. By varying the TBOT concentration and fixing the hydrothermal temperature at 170 C, the SEM images show absolutely distinctive morphologies, while the XRD results illustrate almost identical crystalline characteristics (Fig. 3). The morphology exhibits random aggregated particles at a TBOT concentration of 63% (Fig. 3a), while at a concentration of 58%, nanorod clustering is initiated (Fig. 3b). Solid microspheres with regularly outward-radiating nanorods (diameters of sub-100 nm) start to appear at a concentration of 53% (Fig. 3c). Uniform hollow microspheres are formed only at concentrations ranging from 46 to 49%, with an average nanorod
subunit diameter less than 50 nm (Fig. 3d and e). Observations indicate that at TBOT concentrations between 38 w 43%, the hollow structure of hierarchical microspheres disappears, and random aggregations of solid TiO2 microspheres can be obtained, as shown in Fig. 3f. From the XRD results shown in Fig. 3g, we can conclude that the crystallographic parameters of the products are nearly independent on the TBOT concentration in the solutions, and all the as-prepared samples show the same crystal structure (rutile TiO2). The influence of hydrothermal-treatment temperature on the morphological and crystallographic structure of the products has also been studied by varying the hydrothermal temperatures
Fig. 2. TEM images of the hollow hierarchical microspheres synthesized at 170 C and TBOT concentration of 49 wt%: (a) a cluster of nanorod subunits from broken microsphere; (b) single-nanorod subunit; (c) SAED pattern of the single nanorod.
Y. Xue et al. / Materials Chemistry and Physics 143 (2013) 446e454
449
Fig. 3. SEM images of the products obtained at the temperature of 170 C and different TBOT concentrations of: (a) 63%; (b) 58%; (c) 53%; (d) 49%; (e) 46%; (f) 38e43%. (g) The corresponding XRD patterns of the products as-obtained with different TBOT concentrations.
between 120 and 220 C, while keeping the TBOT concentration of 49%, as illustrated in Fig. 4. When the temperature is in the range between 120 and 140 C, microspheres with smooth surfaces and large cracks can be observed (Fig. 4a and b). Within this stage, the XRD pattern shows wide diffraction peaks, indicating that small TiO2 crystals in nanoscale size or a solid TiO2 structure with relatively poor crystallinity formed (Fig. 4g). A hollow morphology can be found at temperatures ranging from 160 to 180 C (Fig. 4c and d). Here, the average diameter of the corresponding subunit is estimated to be w50 nm, which possesses much higher crystallinity than the solid spheres. When elevating the temperature over 180 C, the hollow structure disappears. In the temperature range of 200e220 C, solid microspheres with distributing dense nanorods as subunits on the surface are observed. The average diameter of these subunits is w70 nm (Fig. 4e and f). Meanwhile, the corresponding crystallinity is further improved compared to that between 160 and 180 C. These results clearly indicate that an increase in the hydrothermal temperature leads to an enhancement in crystallinity of the products (Fig. 4g). 3.3. Proposed growth mechanism The growth mechanism for the hierarchical hollow structures is schematically proposed as shown in Fig. 5. Some researchers have reported a similar structure and formation mechanism [38e41]. On the basis of the above-mentioned results, TBOT concentrations and hydrothermal temperatures are crucial for the formation of hollow rutile TiO2 microspheres. Actually, the amount and quality of TiO2 crystal nucleus in the mixed solution are just a real cause for the formation of the distinct morphologies, where the TBOT hydrolysis
rate plays a vital role in the nucleation process. The introduced 38% HCl provides enough Hþ to reduce the self-ionization of water in the solution efficiently and further lowering the hydrolysis rate of TBOT. Hence, the existence of 38% HCl is important for the nucleation process during the hydrothermal synthesis. Due to the same content of HCl in each solution, the dependence of different morphologies on the TBOT concentration is our first discussion focus. The growth mechanism of the products prepared at different TBOT concentrations at 170 C is demonstrated in Fig. 5a. It is predicted that when the TBOT concentration is very high (at 63%), the hydrolysis rate is extremely low and the rutile TiO2 crystal nucleus forms slowly due to a low concentration of Ti4þ in the surrounding solution. During the limited reaction time, only some small particles form as the final product. When the TBOT concentration is set between 53%and 58%, a single-crystal nucleus of rutile TiO2 is rapidly formed by homogeneous nucleation. At the same time, some small crystal nuclei are attached to its surface via a heteronucleation process. Due to existence of Ti4þ in the solution, the crystal growth on the surface of multimers occurs along the low energy face of the crystal nuclei, gradually leading to the emergence of ordered nanorod-based 3D solid clusters or microspheres. When the concentration is in the range of 46%e49%, the amount of TiO2 crystal nuclei increases. In order to understand the formation mechanism of hollow microspheres, further experiments with different hydrothermal reaction times were carried out. The SEM images and XRD patterns of the samples are shown in Fig. 6. At the early stage of TBOT hydrolysis, metastable anatase TiO2 and amorphous TiO2 nuclei are aggregated (the as-marked peaks in the bottom of Fig. 6e (1 h) and (3 h)). Subsequently, the TiO2 changes from anatase and amorphous to the dense rutile crystalline state via a condensation process
Fig. 4. SEM images of the products with identical TBOT concentration of 49% in the original acid solution, with varying hydrothermal temperatures as: (a) 120 C; (b) 140 C; (c) 160 C; (d) 180 C; (e) 200 C; (f) 220 C. (g) The corresponding XRD patterns of the products at different temperatures.
450
Y. Xue et al. / Materials Chemistry and Physics 143 (2013) 446e454
(Fig. 6e (6 h) and (15 h)). This process is often accompanied by volume change and inner-stress generation in the materials. Thus the initial spheres can be condensed to hollow structures. Simultaneously, due to a saturated Ti4þ solution surrounding the sphere surface, the nanorods growth begins to occur by Ostwald ripening, finally leading to the formation of hollow dandelion-like hierarchical microspheres. At concentrations lower than 46%, the hydrolysis rate is very fast and a mass of polycrystalline clusters can be formed. The supersaturated rutile TiO2 nuclei are jointed to their coordination sites and assembled into the irregular aggregations. Due to the existence of Ti4þ ions in the solution, the nanorods grow on the aggregation surfaces and, finally, the aggregation-like microspheres are generated. The formation mechanism of the diverse samples prepared at different reaction temperatures ranging from 120 to 220 C with TBOT concentration of 49% is proposed in Fig. 6b. At temperatures lower than 160 C, most of the solution inside the autoclave is in liquid phase, which makes the TBOT hydrolysis easily and fast. The relatively low reaction temperature can maintain the poor crystallinity and low density of the product. As a result, a condensation process could not occur at this hydrothermal reaction stage. The formation mechanism of the products prepared at 160e180 C is similar to those of the above-mentioned cases with the TBOT concentration of 46e49%. When the temperature is in the range of 180e220 C, most of the solution in the autoclave exists as vapor, and the TBOT hydrolyzes slowly. At the temperature, the relativelylarge rutile TiO2 nuclei directly form, and they aggregate with each other. Because of plenty of Ti4þ in solution and Ostwald ripening, nanorods grow along the aggregated nuclei. Finally, the ordered nanorod-based solid 3D microsphere forms. 3.4. BET analysis
Fig. 5. The proposed growth mechanism of the hierarchical products obtained by: (a) adjusting the TBOT concentration in the original acid solution at the fixed hydrothermal temperature of 170 C; (b) varying the hydrothermal temperature with fixing the TBOT concentration at 49%.
The BET parameters of the as-prepared product were obtained by using a nitrogen sorption analysis (Fig. 7). According to the IUPAC (International Union of Pure and Applied Chemistry) classifications of hysteresis loops, the isotherm is attributed to type H3 hysteresis, which does not exhibit any limited adsorption P/P0 varying from 0 to 1. This indicates a non-rigid aggregation of the TiO2 nanorods, which gives rise to numerous slit-shaped pores in the microspheres, as shown in Fig. 7a. The hollow microsphere structures have specific surface areas (Fig. 7b) up to 37 m2 g1 for the sample obtained at 180 C with TBOT concentration of 49%, which is the highest value among the as-prepared samples. This
Fig. 6. SEM images of the samples obtained after the hydrothermal treatment at 170 C by different reaction times: (a) 1 h; (b) 3 h; (c) 6 h; (d) 15 h. (e) The corresponding XRD patterns for different reaction times, the as-marked peaks representing the anatase TiO2 phase.
Y. Xue et al. / Materials Chemistry and Physics 143 (2013) 446e454
451
Fig. 7. Measurement of BET parameters: (a) nitrogen adsorptionedesorption isotherm of a hollow microsphere product, under 77 K; (b) histograms showing the different BET specific surface area distributions of the as-fabricated product with changing TBOT concentration at 170 C (the top one) and with varying temperatures for the same TBOT concentration of 49% (the bottom one).
implies that the hollow microspheres have the potential to enhance the photocatalytic activities superior to the other samples. 3.5. UVevis diffuse reflectance spectra analyses The UVevis diffuse reflectance spectra (DRS) of the corresponding products (with the specific surface area of 27 and 37 m2 g1, respectively), similar-sized bulk rutile TiO2 and P25 are comparatively presented in Fig. 8. Fig. 8a displays that all absorptions are confined within the near ultraviolet (NUV) region. For the hollow rutile TiO2 structures, their absorption edge located at w413 nm basically coincides with the similar-sized bulk rutile TiO2. This implies that the single-crystal rutile nanorod, as an elementary unit of the hollow structure, has an invariable electronic structure and light-adsorption property compared to the bulk product. The rutile TiO2 product has a broader absorption region than that of P25, which reveals different essential attributes between anatase and rutile type TiO2 due to P25 containing the component of anatase TiO2. On the basis of their adsorption spectra, the energy band gap (Eg) for the as-measured TiO2 samples can be predicted by the following equation [42]:
ahn ¼ A hn Eg
n=2
(1)
where a, n, Eg and A are the absorption coefficient, the light frequency, the band gap energy and a constant, respectively [43]. The n is determined as 4 due to the indirect transition type of rutile TiO2 [43]. The Eg of the hollow hierarchical microspheres and the bulk rutile TiO2 are obtained from the plots of (ahn)1/2 versus (hn) and determined to be w3.0 eV, which is lower than that of P25. 3.6. MO degradation The performance of the hollow microspheres in degrading MO under ultraviolet irradiation has been investigated (Figs. 9 and 10). The results shown in Fig. 9a and b reveal that the hollow dandelionlike hierarchical TiO2 microspheres exhibit higher photocatalytic activities than that of other samples with distinct morphologies. Obviously, after 160 min, the degradation efficiency of hollow TiO2 structures is at least 5% higher than that of the solid structures. The hollow hierarchical TiO2 microspheres synthesized at 180 C with TBOT concentration of 49% show the highest photocatalytic activity. This remarkable photocatalytic capacity may be attributed to its special hollow morphology and high crystallinity (as illustrated in
Fig. 8. (a) UVevis diffuse reflectance spectra of the similar-sized bulk TiO2, P25 and the samples (with the specific surface areas of 27 and 37 m2 g1, respectively). (b) Corresponding band gaps (Eg) of different samples.
452
Y. Xue et al. / Materials Chemistry and Physics 143 (2013) 446e454
Fig. 9. MO photodegradation of the different samples (100 ml of 20 ppm MO solution, catalyst dosage of 100 mg): (a) obtained at 170 C with different TBOT concentration; (b) obtained by fixing the TBOT concentration at 49% with different hydrothermal reaction temperatures.
Figs. 5 and 6). The hollow structure with outward-radiating monodispersed nanorods can offer more active sites during the photocatalytic reaction process, and the well-crystallized single-crystalline TiO2 nanorods can effectively hinder the recombination of electronehole pairs under ultraviolet irradiation. These two features make the as-synthesized hollow microspheres highly active for photocatalysis. In the photocatalytic experiments, we also
determined the optimum conditions of this prominent sample (synthesized at 180 C with TBOT concentration of 49%), including initial dye concentration, catalyst dosage and pH value (see Fig. 10aec). By fixing the catalyst dosage at 30 mg and varying the MO concentration from 5 to 30 ppm at pH of 7, the optimum initial dye concentration can be found as 5 ppm (Fig. 10a). With changing the amount of TiO2 microspheres from 5 to 35 mg, further optimum
Fig. 10. Optimum conditions of photocatalytic experiments using as-obtained hollow microspheres (at 180 C with TBOT concentration of 49%): (a) optimum initial dye concentration; (b) optimum catalyst dosage; (c) optimum PH and contrast decomposition MO of the hollow microspheres with the similar-sized bulk rutile TiO2 and the P25 under the optimum conditions.
Y. Xue et al. / Materials Chemistry and Physics 143 (2013) 446e454
453
Fig. 11. Graphics for kinetic study and degradation efficiencies under the optimum conditions: (a) plot of ln(C0/Ct) vs irradiating time with linear fitting; (b) decolorization and TOC removal efficiencies in 20, 60 and 120 min.
catalyst dosage can be determined as 30 mg by photocatalysis degradation of 50 ml 5 ppm MO solution (pH ¼ 7) within 120 min (Fig. 10b). By fixing the catalyst dosage and initial MO concentration at 30 mg and 5 ppm respectively, the optimum pH for a highly effective degradation is 5 after 120 min (Fig. 10c). As a result, for 5 ppm MO solution, the optimum photocatalytic conditions are 30 mg catalyst dosage and pH ¼ 5. In addition, the comparative degradation experiments were performed among the TiO2 microspheres (synthesized at 180 C with TBOT concentration of 49%), similar-sized bulk rutile TiO2 and P25, under the optimum conditions (Fig. 10d). The blank test (MO solution without any catalyst) exhibits almost no catalysis. Noticeably, although the hollow dandelion-like rutile TiO2 demonstrates lower dye decomposition than that of P25 within 120 min, it displays enhanced photocatalytic activity comparable to the similar-sized bulk rutile TiO2. The degradation efficiency for the as-synthesized hollow microspheric structure can reach 99.03% after 120 min, which is lower than that of P25 (after only 60 min, reaching up to 99.56%), but higher than that of the bulk TiO2 (with only 95.19% after 120 min). This enhancement may be resulted from the increase in active sites as compared to the bulk one, due to the existence of special outward-radiating nanorod structures. The result indicates that the rutile TiO2 could exhibit remarkably enhanced photocatalytic capacity when fabricated into hollow hierarchical microsphere with radial nanorods. In order to demonstrate better the reaction kinetics of the MO degradation under the optimum conditions, we utilized the apparent pseudo-first-order model in the present experiments expressed as [44]:
ln
C0 ¼ Kapp t Ct
(2)
where the Kapp is the apparent pseudo-first-order rate constant (with unit of min1); Ct is the MO solution concentration at time t (mg L1 ¼ ppm); and C0 is the initial MO concentration (mg L1 ¼ ppm). By means of first-order linear fitting, the graphics was plotted in Fig. 11a. The slope of the line represents Kapp, which can be calculated to be 0.03769 (9.9819 104). The linear fitting implies that this photocatalytic reaction is a first-scale kinetic reaction. Under the optimized photocatalytic conditions, the decolorization and TOC (total organic carbon) removal efficiencies can be obtained after 20, 60 and 120 min (Fig. 11b). The photocatalytic reaction can efficiently destroy the conjugated color systems in MO dye molecules, leading to an obvious decolorization process. The decolorization efficiency reaches 99.61% by ultraviolet irradiating
in 120 min. However, all TOC removal efficiencies are retarded by w30% in different time period (after 120 min, only 71.73%). This indicates that within 120 min of ultraviolet irradiation, the photocatalytic oxidation reaction fails to fully destroy the whole structures of MO dye molecules, and the apparent degradation rate is not equal to actual mineralization level of MO organics. 4. Conclusion In summary, the controllable hydrothermal process was proposed to fabricate hierarchical hollow rutile TiO2 microspheres. The mean diameter distribution of the hollow inner core was over the range of w1.5e2 mm. The single-crystal nanorod subunit was estimated to be w50 nm in average diameter and w1 mm in length. The optimum experimental conditions for fabricating hierarchical hollow structures were at the TBOT concentration of 46e49% and hydrothermal temperature range of 160e180 C for 24 h. The formation of the hollow structure has been proposed as a condensation process from anatase and amorphous to the dense rutile crystalline state. The specific surface area of hollow microspheres reached 37 m2 g1, and their absorption edge (413 nm) and energy gap (3.0 eV) were in good accordance with the bulk ones. The hollow microspheres synthesized at 180 C and TBOT concentration of 49% display the highest photocatalytic activity among all the asprepared products with different morphologies. The optimum degradation conditions for the hollow microsphere were at catalyst dosage of 30 mg, with 50 ml of 5 ppm MO solution and pH of 5, within 120 min. Under the optimum conditions, the photocatalytic reaction was a first-scale kinetic reaction, and showed a superior photocatalytic performance compared to the similar-sized bulk counterpart. Further exploration showed that the apparent degradation efficiency is not equal to the actual TOC removal efficiency. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 10974041, 51172060, 51202055, 21103056), Natural Science Foundation of Hebei Province (Grant No. E2012202040, E2012202114), the Key Basic Research Program of Hebei Province of China (No. 12965135D), the National Basic Research Program of China (973 Programs, No. 2011CB612301) and the Innovation Fund for Excellent Youth of Hebei University of Technology (No. 2012001). References [1] O. Legrini, E. Oliveros, A.M. Braun, Chem. Rev. 93 (1993) 671.
454
Y. Xue et al. / Materials Chemistry and Physics 143 (2013) 446e454
[2] M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [3] J. Zhou, Y. Cheng, J. Yu, J. Photochem. Photobiol. A 223 (2011) 82. [4] U.M. Gaya, A.H. Abdullah, J. Photochem. Photobiol. C 9 (2007) 1. [5] J.G. Yu, Y.R. Su, B. Cheng, Adv. Funct. Mater. 17 (2007) 1984. [6] D.G. Shchukin, R.A. Caruso, Chem. Mater. 16 (2004) 2287. [7] J.H. Park, S.Y. Jung, R. Kim, N.G. Park, J. Kim, S.S. Lee, J. Power Sources 194 (2009) 574. [8] Y. Li, T. Sasaki, Y. Shimizu, N. Koshizaki, J. Am. Chem. Soc. 130 (2008) 14755. [9] S.J. Ding, X. Yin, X.J. Lü, Y.M. Wang, F.Q. Huang, D.Y. Wan, ACS Appl. Mater. Interfaces 4 (2012) 306. [10] J.A. Byrne, B.R. Eggins, J. Electroanal. Chem. 457 (1998) 61. [11] L. Liu, H.J. Liu, Y.P. Zhao, Y.Q. Wang, Y.Q. Duan, G.D. Gao, M. Ge, W. Chen, Environ. Sci. Technol. 42 (2008) 2342. [12] Y.K. Du, J. Rabani, J. Phys. Chem. B 107 (2003) 11970. [13] M. Adachi, Y. Murata, M. Harada, S. Yoshikawa, Chem. Lett. 8 (2000) 942. [14] J.E.G.J. Wijnhoven, W.L. Vos, Science 281 (1998) 802. [15] N.L. Wu, S.Y. Wang, I.A. Rusakova, Science 285 (1999) 1375. [16] S. Madhugiri, B. Sun, P.G. Smirniotis, J.P. Ferraris, K.J. B Jr., Microporous Mesoporous Mater. 69 (2004) 77. [17] M. Qin, W. Zhao, J.L. Chu, J.K. Qu, L. Wang, S.H. Li, H. Zhang, H.X. Zhao, Z.H. Yu, T.Y. Xue, J. Li, Q. Guo, T. Qi, Mater. Res. Bull. 48 (2013) 1076. [18] S. Chen, W. Chu, Y.Y. Huang, X. Liu, D.G. Tong, Mater. Res. Bull. 47 (2012) 4514. [19] I.B. Troitskaia, T.A. Gavrilova, V.V. Atuchin, Phys. Procedia 23 (2012) 65. [20] D.A.H. Hanaor, M.H.N. Assadi, S. Li, A. Yu, C.C. Sorrell, Comput. Mech. 50 (2012) 185. [21] X.L. Fang, Z.H. Liu, M.F. Hsieh, M. Chen, P.X. Liu, C. Chen, N.F. Zheng, ACS Nano 6 (2012) 4434. [22] C. Rungnim, U. Arsawang, T. Rungrotmongkol, S. Hannongbuab, Chem. Phys. Lett. 550 (2012) 99. [23] L. Zhou, D.Y. Zhao, X.W. Lou, Angew. Chem. Int. Ed. 51 (2012) 239.
[24] Z.H. Dong, X.Y. Lai, J.E. Halpert, N.L. Yang, L.X. Yi, J. Zhai, D. Wang, Z.Y. Tang, L. Jiang, Adv. Mater. 24 (2012) 1046. [25] Y. Zeng, X. Wang, H. Wang, Y. Dong, Y. Ma, J.N. Yao, Chem. Commun. 46 (2010) 4312. [26] Z.Y. Wang, D.Y. Luan, C.M. Li, F.B. Su, S. Madhavi, F.Y.C. Boey, X.W. Lou, J. Am. Chem. Soc. 132 (2010) 16271. [27] Z.Y. Fei, S.C. He, L. Li, W.J. Ji, C.T. Au, Chem. Commun. 48 (2012) 853. [28] C.P. Zhao, X.W. Zhang, Y.P. Zhang, Y.L. Xing, X.J. Zhang, X.H. Zhang, J.S. Jie, CrystEngComm 14 (2012) 819. [29] H.J. Zhang, G.D. Du, W.Q. Lu, L.L. Cheng, X.D. Zhu, Z. Jiao, CrystEngComm 14 (2012) 3793. [30] S.H. Yao, X.Y. Jia, L.L. Jiao, C. Zhu, Z.L. Shi, Indian J. Chem. Sect. A 51A (2012) 1049. [31] T.Y. Zhao, Z.Y. Liu, K. Nakata, S. Nishimoto, T. Murakami, Y. Zhao, L. Jiang, A. Fujishima, J. Mater. Chem. 20 (2010) 5095. [32] H.Q. Wang, M. Miyauchi, Y. Ishikawa, A. Pyatenko, N. Koshizaki, Y. Li, L. Liang, X.Y. Li, Y. Bando, D. Golberg, J. Am. Chem. Soc. 133 (2011) 19102. [33] B. Peng, L.F. Tan, D. Chen, X.W. Meng, F.Q. Tang, ACS Appl. Mater. Interfaces 4 (2012) 96. [34] Y. Wang, X.W. Su, S. Lu, J. Mater. Chem. 22 (2012) 1969. [35] J.Q. Li, D.F. Wang, Z.L. He, Z.F. Zhu, J. Am. Ceram. Soc. 94 (2011) 1639. [36] H.G. Yang, H.C. Zeng, J. Phys. Chem. B 108 (2004) 3492. [37] Y.J. Kim, S.Y. Chai, W.I. Lee, Langmuir 23 (2007) 9567. [38] X.X. Li, Y.J. Xiong, Z.Q. Li, Y. Xie, Inorg. Chem. 45 (2006) 3493. [39] S.J. Liu, J.Y. Gong, B. Hu, S.H. Yu, Cryst. Growth Des. 9 (2009) 203. [40] S.S. Mali, C.A. Betty, P.N. Bhosale, P.S. Patil, CrystEngComm 13 (2011) 6349. [41] H.M. Zhang, H. Yu, Y.H. Han, P.R. Liu, S.Q. Zhang, P. Wang, Y.B. Cheng, H.J. Zhao, Nano Res. 4 (2011) 938. [42] X. Zhang, Z.H. Ai, F.L. Jia, L.Z. Zhang, J. Phys. Chem. C 112 (2008) 747. [43] J. Cao, B.Y. Xu, H.L. Lin, B.D. Luo, S.F. Chen, Chem. Eng. J. 185-186 (2012) 91. [44] X.P. Lin, T. Huang, F.Q. Huang, W.D. Wang, J.L. Shi, J. Phys. Chem. B 110 (2006) 24629.