Facile Synthesis and Photocatalytic Property of Titania/Carbon Composite Hollow Microspheres with Bimodal Mesoporous Shells

Titania/carbon composite hollow microspheres with bimodal mesoporous shells are one-pot fabricated by hydrothermal treatment of the acidic (NH4)2TiF6 aqueous solution in the presence of glucose at 180◦C for 24 h and then calcined at 450◦C. The asprepared samples were characterized by XRD, SEM, TEM, HRTEM, UV-visible spectroscopy, and nitrogen adsorption-desorption isotherms. The photocatalytic activity of the as-prepared samples was evaluated by daylight-induced photocatalytic decolorization of methyl orange aqueous solution at ambient temperature. The effects of calcination time on the morphology, phase structure, crystallite size, specific surface area, pore structures, and photocatalytic activity of the microspheres were investigated. The results indicated that the as-obtained TiO2/C composite hollow spheres generally exhibit bimodal mesopore size distribution with their peak intra-aggregated mesopore size in the range of 2.3–4.5 nm and peak interaggregated mesopore size in the range of 5.7– 12.7 nm, depending on specific calcination time. The daylight-induced photoactivity of as-obtained hollow TiO2/C microspheres generally exceeds that of Degussa P25. The influences of calcination time on the photoactivity are discussed in terms of carbon content, phase structures, and pore structures.


Introduction
Photocatalytic degradation of organic compounds for the purpose of purifying water or wastewater from industries and households has attracted great attention in the past decade [1][2][3][4][5][6][7].Among various oxide semiconductor photocatalysts, titania is a very important photocatalyst due to its biological and chemical inertness, strong oxidizing power, nontoxicity, and long-term stability against photo and chemical corrosion.In order to commercialize this treatment technique, it is of great importance to improve the preparative methods of titania, because the morphology, microstructures, and photocatalytic activity of TiO 2 are significantly influenced by the preparative conditions and methods [8][9][10][11].
The fabrication of TiO 2 hollow structures attracts special attention due to their low density, high surface area, good surface permeability and larger light-harvesting capacities [9].Especially, micro-or nanometer scale TiO 2 hollow spheres with controllable structure, composition, and properties have shown a promising perspective in many fields such as catalysts, adsorbents, sensors, light fillers, and chemical reactors [12][13][14][15][16].While hollow TiO 2 structures have been synthesized without the assistance of templates by the spray-drying technique and via Ostwald ripening and chemically induced self-transformation [17][18][19][20], the templating method has been most frequently applied for the synthesis of hollow TiO 2 structures with tailored properties.For example, Hard templates (e.g., polymer latex, carbon, and anodic aluminium oxide templates) and soft International Journal of Photoenergy templates (e.g., supermolecules, ionic liquids, surfactant, and organogel) have been extensively employed to produce TiO 2 hollow structures either by controlled surface precipitation of inorganic molecule precursors or by direct surface reactions utilizing specific functional groups [21][22][23][24][25][26][27].Very recently, a general method for the synthesis of metal oxide hollow spheres has been developed using carbonaceous polysaccharide microspheres prepared from saccharide solution as templates by Sun and coworkers and Titirici and coworkers [28,29].
Carbon materials (e.g., amorphous carbon, activated carbon, and graphite) have attracted considerable attention due to their widespread applications as absorbents, catalyst supports, and nanocomposites [30].Carbon is chemically inert at low temperature and a suitable support for titanium dioxide photocatalysts.Various studies have demonstrated that titanium/carbon composite photocatalysts can result in a synergistic effect of both adsorption and photocatalysis and can dramatically improve the daylight-induced photocatalytic activity of titania for different modal organic components [30][31][32][33][34][35][36][37].
However, preparation of well-crystallized titanium/carbon composite hollow microspheres with high daylightinduced photocatalytic activity keeps still a great challenge.In this study, titanium/carbon composite hollow microspheres with bimodal mesoporous shell are one-pot fabricated according to the method reported by Titirici and coworkers [29] and their visible light photocatalytic activity is investigated.

Sample Preparation.
All chemicals used in this study were reagentgrade without further purification.Distilled water was used in all experiment.(NH 4 ) 2 TiF 6 was used as a titanium source.In a typical synthesis, 15 g (75.5 mmol) of glucose and 3.0 g (15.1 mmol) of (NH4) 2 TiF 6 were dissolved in 80 and 40 mL of distilled water under stirring, respectively.The above two solutions were mixed and the pH of the mixed solutions was adjusted to ca. 3 using 1 M HCl or NaOH aqueous solutions.Then the reaction solution was transferred into a 200 mL Teflon-lined stainless steel autoclave, followed by hydrothermal treatment of the mixture at 180 • C for 24 h.After hydrothermal reaction, the black or puce precipitates were centrifuged and then washed with distilled water and absolute alcohol for 5 times.The washed precipitates were dried in a vacuum oven at 60 • C for 8 h.Finally, the dried mixtures were calcined in air at 450 • C for 0.5-4.0h, the TiO 2 /C composites, with the color from black to grey when the calcined time was increased, were obtained.The abbreviations of TiO 2 /C composites with different calcined time are shown in Tables 1 and 2.

Characterization.
The carbon content of the composite catalysts was monitored using a DTA-TG instrument (Netzsch STA 449 C) in airflow of 100 mL min −1 at a heating rate of 10 • C min −1 from room temperature to 900 • C. The X-ray diffraction (XRD) patterns obtained on an X-ray diffractometer (type HZG41B-PC) using Cu Kα irradiation at a scan rate of 0.05 • 2θ s −1 were used to determine the identity of any phase present and their crystallite size.The accelerating voltage and the applied current were 15 kV and 20 mA, respectively.The phase composition of TiO 2 can be calculated from the integrated intensities of anatase (101), rutile (110), and brookite (121) peaks.If a sample contained anatase and rutile two phases, the mass fraction of rutile could be calculated according to the following equation (1) [38].
where A A and A R represent the integrated intensity of the anatase (101) and rutile (110) peaks, respectively.The average crystallite sizes of anatase and rutiles were determined according to the Scherrer equation using the FWHM data of each phase after correcting the instrumental broadening [38].Morphology observation was performed on a JSM-5610LV scanning electron microscope (SEM, JEOL, Japan).Crystallite sizes and shapes were observed using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) (JEOL-2010F at 200 kV).The samples for TEM observation were prepared by dispersing the TiO 2 powders in an absolute ethanol solution under ultrasonic irradiation; the dispersion was then dropped on carbon-copper grids.The Brunauer-Emmett-Teller (BET) surface area (S BET ) of the powders was analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA).All the samples were degassed at 180 • C prior to nitrogen adsorption measurements.The BET surface area was determined by multipoint BET method using the adsorption data in the relative pressure (P/P 0 ) range 0.05 ∼ 0.3.Desorption isotherm was used to determine the pore size distribution via the Barret-Joyner-Halender (BJH) method, assuming a cylindrical pore modal [39][40][41].The nitrogen adsorption volume at the relative pressure (P/P 0 ) of 0.994 was used to determine the pore volume and average pore size.UV-visible diffuse reflectance spectra of as-prepared TiO 2 powders were obtained for the dry-pressed disk samples using a UV-visible spectrophotometer (UV2550, Shimadzu, Japan).BaSO 4 was used as a reflectance standard in the UV-visible diffuse reflectance experiment.The morphologies of TiO 2 powders were observed using scanning electron microscopy (SEM) (type JSM-5610LV, Japan) with an acceleration voltage of 20 kV.

Measurement of Photocatalytic Activity.
The evaluation of photocatalytic activity of the prepared samples for the photocatalytic decolorization of methyl orange aqueous solution was performed at ambient temperature, as reported in our previous studies [42].Experiments were as follows: 0.04 g of the prepared powders were dispersed in a 20 mL of methyl orange aqueous solution with a concentration of 3.1 × 10 −5 mol L −1 in a dish (with a diameter of ca.7.0 cm).The solution was allowed to reach an adsorption-desorption equilibrium among the photocatalyst, methyl orange, and  a The BET surface area was determined by multipoint BET method using the adsorption data in P/P 0 range from 0.05 to 0.3.b Pore volume and average pore size were determined by nitrogen adsorption volume at P/P 0 = 0.994.c The porosity is estimated from the pore volume determined using the desorption data at P/P 0 = 0.994.
water before daylight irradiation.An 18-W daylight lamp (3 cm above the dish) was used as a light source.The integrated daylight intensity was 0.46 ± 0.01 mW/cm 2 , as measured by a UV radiometer (made in the photoelectric instrument factory of Beijing Normal University) with the peak intensity of 420 nm.After visible-light irradiation for 60 min, the reaction solution was filtrated, and the concentration of methyl orange aqueous solution was determined by a UV-visible spectrophotometer (UV-2550, SHIMADZU, Japan).As for the methyl orange aqueous solution with low concentration, its photocatalytic decolorization is a pseudo-first-order reaction and its kinetics may be expressed as ln(c 0 /c) = kt, where k is the apparent rate constant, and c 0 and c are the adsorption-desorption equilibrium and reaction concentrations of methyl orange, respectively.Each set of photocatalytic measurements were repeated for three times, and the experimental error was found to be within ±5%.

Results and Discussion
3.1.Crystal Structure.Figure 1 shows the XRD pattern of the composites catalysts prepared at 450  from 0.5 to 4 h, the carbon content of TiO 2 /C samples changes from 32.9 to 1.9%.The presence of carbon layer may suppress the phase transformation of anatase to rutile to a certain extent.Similar results were reported by Shanmugam et al. and Tsumurae al. [33,43].The carbon layers are acting as barriers for phase transformation of anatase to rutile.microspheres.Figure 2 displays the SEM images of hollow TiO 2 /C microspheres obtained after hydrothermal reaction and calcination in air at 450 • C for different time.It can be seen from Figure 2(a) that after calcination for 0.5 h, the microspheres with surface wrinkles have a uniform diameter of ca. 4 μm.It is interesting to note that with increasing calcination time, the diameter of microspheres decrease drastically probably due to a large amount of carbon removed during calcination, resulting in drastic shrinkage of corresponding hollow spheres.At 400 • C for above 2 h, the samples are composed of hollow spheres with a diameter range from 0.5 to 2.0 μm and cavities are occasionally found in some broken microspheres with a shell thickness ∼400 nm (Figure 2(c) and inset in Figure 2(d)).

SEM and TEM Studies.
The morphology and microstructures of TiO 2 /C hollow spheres are further investigated by TEM analysis.Figure 3(a) shows a typical TEM image of the samples calcined at 450 • C for 2 h.There is a strong contrast difference observed for all microspheres with dark edge and bright center, clearly confirming their hollow structures.Further observation indicates that the prepared hollow microspheres appear unique sphere-in-sphere superstructures.Li and coworkers reported the similar sphere-in-sphere superstructures prepared solvothermally in glycerol, alcohol, and ethyl ether, which allow multireflections of electromagnetic waves, such as ultraviolet and visible light, within their interior cavities, endowing these spheres with greatly enhanced properties [44].Figure 3(b) presents a typical HRTEM lattice image of the TiO 2 nanoparticles in the shell of TiO 2 /C hollow microspheres.The selected area electron diffraction (SAED) patterns (inset in Figure 3(b)) reveal the polycrystalline nature of the anatase and rutile phases for the TiO 2 /C hollow microspheres.By measuring the lattice fringes, the resolved interplanar distances are ca.0.35 and 0.33 nm, corresponding to the (101) planes of anatase and the (110) planes of rutile, respectively.This further confirms the mixed biphase structures of hollow microspheres.0.4 and 0.8, the hysteresis loops are of type H2, which can be observed in the pores with narrow necks and wider bodies (ink-bottle pores) [45,46].However, at high relative pressures between 0.8 and 1.0, the shape of the hysteresis loop is of a type H3, associated with plate-like particles giving rise to narrow slit-shaped pores [39,46].Further observation indicates that with increasing calcination time, the hysteresis loops shifted to a higher relative pressure region.The isotherms of the TiO 2 /C composites prepared at 450 • C for 0.4-4 h having two hysteresis loops indicate bimodal pore-size distributions in the mesoporous regions existed in the shells.

Pore Structure and BET Surface Areas.
Figure 5 shows the corresponding pore-size distributions of the TiO 2 /C composite hollow spheres calcined at 450 • C for 0.5-4 h.All samples show bimodal pore-size distributions, consisting of finer intra-aggregated pores and larger interaggregated pores.For example, the sample calcined at 450 • C for 0.5 h has small mesopores (peak pore: ca.2.3 nm) and larger mesopores (peak pore: ca.5.8 nm), which are related to finer aggregated pore formed between small anatase crystallites 11 nm in size, and larger aggregated pore produced by large rutile crystallites 36 nm in size (see Table 1), respectively.Further observation indicates that with increasing calcination time, the corresponding maximum peaks shift to the right, indicating the increase of pore size.There are two possible factors resulting in the increase of pore size.One is that the aggregation of greater crystallites forms bigger pores.The other is that the content of carbon in the shells decreases, which probably inserts into the pore of TiO 2 to cause the decrease in pore size.The effects of calcination time on physical properties of TiO 2 /C hollow microspheres are shown in Table 2.With increasing calcination time, the BET-specific surface areas, pore volumes, and porosity steadily decrease.However, the average pore size increases.This is ascribed to the fact that increase of calcination time resulted in decrease of the carbon content in the TiO 2 /C composites.

UV-Vis Spectrum.
Usually, the carbon content obviously influences light absorption characteristics of TiO 2 /C composites [32][33][34][35].Figure 6 shows the UV-visible absorption spectra of the TiO 2 /C composite hollow spheres calcined at 450 • C for 0.5-4 h.A significant increase in the absorption at wavelengths shorter than 400 nm can be assigned to the intrinsic band gap absorption of TiO 2 [3].It is noticeable that there is an obvious correlation between the calcination time and the UV-vis spectrum change.With increasing calcination time, the absorption in the near UV and visible-light region gradually decreases and the absorption edge of the samples shows an obvious blue shift.The differences in adsorption are attributed to the change of carbon content in the TiO 2 /C composites.This clearly indicates an increase in the band gap International Journal of Photoenergy energy of TiO 2 .Band gap energy could be estimated from Figure 6.The intercept of the tangent to the plot would give an approximation of the band gap energy for indirect band gap materials such as TiO 2 [47,48].The band gap energies were estimated to be about 3.05, 3.09, 3.15, and 3.20 eV for the TiO 2 /C hollow spheres calcined at 450 • C for 0.5, 1.0, 2.0, and 4 h, respectively.Obviously, the longer calcination time is, the bigger value of the band energies.This is due to the difference in carbon content of TiO 2 /C composites [34,49].

Photocatalytic Activity.
The photocatalytic activity of the as-prepared TiO 2 /C composite hollow spheres was evaluated by the daylight-induced photocatalytic decolorization of methyl orange aqueous solution at ambient temperature.For comparison, the daylight-induced photocatalytic activities of the commercial TiO 2 powder Degussa P25 (P25) were also tested.Figure 7 shows the comparisons of the apparent rate constants of P25 and the TiO 2 /C composite hollow spheres at various calcination time.It can be seen that the TiO 2 /C-0.5 sample has a high daylight-induced photocatalytic activity.This is due to the fact that the TiO 2 /C-0.5 sample has the large carbon content of about 32.9% and surface area of 210.5 m 2 /g (see Table 1).With increasing calcination time, the daylight-induced photocatalytic increases.At 450 • C for 2 h, the photocatalytic activity of the TiO 2 /C-2 sample reaches a maximum value, and its activity exceeds that of Degussa P25, which is recognized as an excellent photocatalyst [3,[43][44][45].On the one hand, this is attributed to the former having bimodal mesoporous structures, which is more beneficial in enhancing the adsorption and desorption of reactants and products, respectively [45].On the other hand, the high photocatalytic activity of the TiO 2 /C-2 sample is due to its large surface area and high adsorption ability in the near UV and visible-light region.With further increasing calcination time, the phototcatalytic activity of the powders decreases.The highest daylight-induced phototcatalytic activity of the TiO 2 -2 sample is due to the following factors.
Usually, TiO 2 /C composite photocatalysts can result in a synergistic effect of both TiO 2 and carbon on the photocatalytic degradation of different modal organic components.Carbon can dramatically improve the photocatalytic activity of titania as coadsorbent due to its very high surface area.Therefore, the carbon content in the TiO 2 /C composite catalysts plays an important role in its visible-light phototcatalytic activity.It can be seen from Figure 7 that the TiO 2 /C-2 sample with higher carbon content shows the superior photocatalytic activity compared with the TiO 2 /C-4 sample.This may be ascribed to the fact that higher carbon content results in a intense increase in absorption in the near UV and visible-light region and a red shift in the absorption edge of the TiO 2 -2 sample (as shown in Figure 6), implying that the TiO 2 -2 sample can be easier activated by visible International Journal of Photoenergy 7 light and more photo-generated electrons and holes can be generated and participate in the photocatalytic reactions.However, adsorption of carbon is not only factor for the enhancement of the photocatalytic activity of TiO 2 .For example, compared with the TiO 2 -2 sample, the two TiO 2 /C-0.5 and TiO 2 /C-1 samples with the larger carbon content have lower daylight-induced photocatalytic activity.This result can be interpreted in two aspects.On one hand, the surplus carbon of the two TiO 2 /C-0.5 and TiO 2 /C-1 samples can scatter the photons in the photoreaction system [35].On the other hand, the lower carbon content of the TiO 2 /C-2 sample enables the methyl orange aqueous solution to get the active site easier and faster [33].Therefore, the TiO 2 /C-2 sample with an optimum carbon content of about 5.0% has the highest daylight-induced phototcatalytic activity.
Apart from the above carbon content, another possible understanding is that the TiO 2 -1 sample possesses a relative large surface area and good anatase crystallinity (as shown in Tables 1 and 2).Usually, a large specific surface area may enhance the rate of photocatalytic degradation reactions, as a large amount of adsorbed organic molecules promote the photocatalytic reaction [50,51].However, the specific surface areas and crystallinity usually appear to be two conflicting intrinsic properties for TiO 2 nanoparticles [52].The powders with a large surface area are usually associated with large amounts of crystalline defects or weak crystallization, which favor the recombination of photo-generated electrons and holes, leading to a poor photoactivity [53][54][55].For example, with increasing calcination time from 0.5 to 2 h, the surface area decreases greatly, but the photocatalytic activity increases.This may be due to the increase of the relative anatase crystallinity (as shown in Table 1).So a balance between specific surface area and crystallinity is a very important factor in determining the photocatalytic activity of TiO 2 powders.According to the above results and discussion, the TiO 2 /C-2 sample possesses a relative large surface area and good anatase crystallinity (as shown in Tables 1 and 2), resulting in a highest daylight-induced phototcatalytic activity.

Conclusions
Titania/carbon composite hollow microspheres with bimodal mesoporous shells are one-pot fabricated by hydrothermal treatment of the acidic (NH 4 ) 2 TiF 6 aqueous solution in the presence of glucose at 180 • C for 24 h and then calcined at 450 • C for 2-4 h.Calcination time obviously influenced on the morphology, phase structure, crystallite size, specific surface area, pore structures, and daylight-induced photocatalytic activity of the as-obtained hollow microspheres.All TiO 2 /C composite hollow spheres generally exhibit bimodal mesopore size distribution with their peak intra-aggregated mesopore size in the range of 2.3-4.5 nm and peak interaggregated mesopore size in the range of 5.7-12.7 nm.The TiO 2 -2 sample showed the highest daylight-induced phototcatalytic activity and greatly exceeded that of Degussa P25.
Figure 4  shows the nitrogen adsorption-desorption isotherms of the TiO 2 /C composite hollow spheres calcined at 450 • C for 0.5-4 h.It can be seen that all the samples show a type IV isotherm with two hysteresis loops.The shapes of two hysteresis loops are different from each other.At low relative pressures between

− 1 )Figure 7 :
Figure 7: Effects of calcination temperatures on the apparent rate constants of the TiO 2 /C composite hollow microspheres.

Table 1 :
Effects of calcination time on phase structure, phase content, and average crystallite sizes of TiO 2 /C composite microspheres at 450 • C. Average crystalline size of TiO 2 was determined by XRD using Scherrer equation.Relative anatase crystallinity: the relative intensity of the diffraction peak from the anatase (101) plane (indicated in parentheses, reference = sample calcined at 450 • C for 0.5 h).c Determined by XRD method.
a Estimated from the data of TGA-DSC measurement.b

Table 2 :
Effects of calcination time on surface areas and pore parameters of TiO 2 /C composite microspheres at 450 • C.
• C for different calcined time.The phase content, crystal size, and relative anatase crystallinity of TiO 2 /C samples are shown in Table 1.At 450 • C for 0.5 h, anatase and rutile phases appear, and their mass percentages are 86.2% and 13.8%, respectively.With increasing calcined time, crystallite size, relative anatase crystallinity, and rutile content increases gradually.This may be ascribed to the fact that the longer calcined time enhance grain growth and favor the phase transformation of anatase to rutile.On the other hand, with increasing calcined time