Fabrication of TiO 2 @ Yeast-Carbon Hybrid Composites with the Raspberry-Like Structure and Their Synergistic Adsorption-Photocatalysis Performance

In the present work, we report the preparation and photocatalytic properties of TiO 2 @yeast-carbon with raspberry-like structure using a pyrolysis method. The products are characterized by field emission scanning electron microscopy (FE-SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), thermal gravimetric anddifferential thermal analysis (TGA-DTA), Fourier transformed infrared spectroscopy (FT-IR), and ultraviolet visible spectroscopy (UV-VIS), respectively. The results show that the hybrid TiO 2 @yeast-carbon microspheres have ordered elliptic shapes of uniform size (length = 3.5 ± 0.3 μm; width = 2.5 ± 0.5 μm). UV-VIS ascertains that the as-prepared microspheres possess an obvious light response in a wide range of 250–400 nm. In the decomposition of typical model pollutants including methylene blue and congo red, the hybrid composites exhibited excellent photocatalytic activity for the methylene blue due to the enhanced adsorption ability. Further investigation reveals that the combined effect of adsorption from the yeast-carbon core and photocatalytic degradation from the attached TiO 2 nanoparticles were responsible for the improvement of the photocatalytic activities. Hereby, the raspberry-like TiO 2 @yeast-carbon has promising applications in water purification.


Introduction
In the past few years, raspberry-like composite particles with well-defined structures have become the subject of rapidly growing interest due to their high surface roughness and potential applications [1,2].The unique raspberrylike composites have combined excellent characteristics of host particles and guest particles, such as higher surface area, increased chemical and biological stability, rich surface chemical component, and different magnetic or optical properties [3].
The high surface area, large pores (macroporosity), and the presence of surface hydroxyl groups make carbon substance an ideal catalyst support [4][5][6].Like the titania photocatalysts supported on the carbon matrix by means of several strategies [7][8][9] appear to have various benefits and advantages for providing a cheap and effective wastewater treatment and remediation options [10,11].For example, Hu et al. [12] obtained TiO 2 nanoparticles/carbon nanotubes particles with raspberry-like structure by combining sol-gel and electrospinning methods.Liu et al. [13] demonstrated the formation of TiO 2 /carbon fibers (ACFs) composite photocatalyst via sol-gel method and the TiO 2 /ACFs is especially helpful for the removal of low molecular weight organic pollutants in the contaminated water.Areerachakul et al. [14] prepared well-structured TiO 2 /granular-activated carbon (GAC) hybrid particles.The TiO 2 /GAC showed excellent capabilities in decomposing the herbicide of metsulfuronmethyl (MM) from waste water.In brief, using carbon materials above-mentioned as catalyst supports has increased the photodegradation rate by progressively allowing an increased quantity of substrate to come in contact with the TiO 2 by means of adsorption.In this respect, carbon matrix has been proven to be an invaluable support in promoting the photocatalytic process [15][16][17] through providing a synergistic effect by creating a common interface between both the carbon phase and the TiO 2 nanoparticle phase.Such synergistic effect can be explained as an enhanced adsorption of the target pollutant onto the carbon phase followed closely by a transfer through an interphase to the TiO 2 phase, giving a complete photodegradation process [18].
The yeast-carbon is a porous and amorphous solid carbon material, which is derived mainly from baker's yeast.For instance, Nacoo and Aquarone [19] reported the fabrication of yeast-carbon by carbonizing in a gas-heated muffle.The obtained yeast-carbon possessed higher surface area.Similarly, Guan et al. [20] synthesized amphiphilic porous hollow carbonaceous materials via mild hydrothermal treatment of yeast cells.The resultant carbon material displayed effective adsorption of organic chemicals in wastewater treatment.Thus, in the present study, a novel TiO 2 @yeastcarbon microsphere with raspberry-like morphology was fabricated firstly by a single-step strategy based on the pyrolysis method.The obtained hybrid microspheres were characterized by FE-SEM, EDS, XRD, TGA-DTA, FT-IR, and UV-VIS, respectively.A possible mechanism for the formation of the composite microspheres was proposed.In addition, the synergistic effect of adsorption-photocatalysis performance in the TiO 2 @yeast-carbon microspheres was evaluated by examining the decolonization of methylene blue and congo red.

Materials.
The powdered yeast was purchased from Angel Yeast Company.Photocatalyst was TiO 2 from Degussa and was used without further purification.In all preparations, absolute ethanol and double-distilled water were used.Methylene blue (MB) and congo red (CR) were analytic grades and were used as the model pollutants in present work.@Yeast-Carbon Hybrid Microspheres.In a typical synthesis procedure, 125.0 mg yeast powder was washed with distilled water and absolute ethanol for three times, respectively.The yeast was dissolved in 20.0 mL of distilled water and stirred vigorously for 30 minutes.The pH was adjusted to approximately 3 by adding drop wise sulfuric acid.Further, 10.0 mg TiO 2 was dispersed in 20.0 mL of distilled water, using ultrasonic vibration for 1.0 minute.The pH was adjusted to approximately 9-10 with sodium hydroxide and stirred for 30.0 min.Then the suspended TiO 2 and yeast were gathered by centrifugation from their own suspensions and redistributed in 20.0 mL of distilled water.Afterwards, TiO 2 and yeast suspensions were mixed and magnetically stirred for 1.5 h at room temperature and left for 3.0 h without further stirring or shaking to ensure the formation of TiO 2 @yeast particles.Thus, the mixture was centrifuged in three or more cycles to remove the undesired components and finally desiccated at 353 K for 1.0 h.Subsequently the TiO 2 @yeast particles were calcined at 573 K for 1.0 h in a nitrogen pipe furnace and cooled to room temperature.After that, the TiO 2 @yeast-carbon composite microspheres were obtained.

Sample Analysis.
Philips XL-30 field emission scanning electron microscope (FE-SEM) was used to observe the morphology of samples.X-ray diffraction (XRD) patterns were collected on X. Pert Pro diffractometer using Cu K radiation ( = 0.15418 nm) at a scanning rate of 10 ∘ /min.Thermal gravimetric analysis and differential thermal analysis (TGA-DTA) were carried out on an EXSTAR apparatus at a heating rate of 40 ∘ /min in flowing high purity N 2 .Fourier-transform infrared spectroscopy measurements (FT-IR) were recorded with a Bio-Rad FTS135 spectrometer in the rage of 370-4000 cm −1 .UV-VIS diffuse reflectance spectra (UV-VIS) were measured on a HITACHI 340 UV-VIS spectrophotometer.

Photocatalytic Activity.
The methylene blue (MB) dye and congo red (CR) are widely used in dye industry.Therefore, the elimination of these compounds is becoming an increasingly important environmental problem.Photocatalytic degradation of MB and CR was evaluated under UV irradiation in an aqueous media.The initial concentration of MB and CR was set as 2∼3 mg/L, respectively.The amount of photocatalysts including yeast-carbon and TiO 2 @yeastcarbon were kept at 0.25 g/L.Before UV irradiation, the suspension containing photocatalyst, MB, and CR was deposited within 50.0 min to establish adsorption-desorption equilibrium.Then the suspension was irradiated under a UV lamp (the intensity of irradiation is 60 W).The concentration of MB and CR was traced by UV-VIS spectroscopy.The absorbance characteristic at bands 666.4 nm and 499.0 nm was taken to determine MB and CR concentration by using a calibration curve, respectively.

Results and Discussion
3.1.Materials Characterization.The shape and structure of the samples are shown in Figure 1.The SEM image in Figure 1(a) shows that the prime yeast cells have a regular ellipsoidal shape with a diameter varying between 2.6 and 3.7 m. Figure 1(b) shows an image of the precursor of TiO 2 @yeast-carbon.Compared with the bared yeast, the size of the TiO 2 @yeast (length = 3.7 ± 0.4 m; width = 2.6 ± 0.5 m) was gently increased due to the attachment of TiO 2 nanoparticles.The picture in Figure 1(d) shows an overall image of the TiO 2 @yeast-carbon microspheres.The particles maintained the shape of the original precursor and had the average diameter (length = 3.7 ± 0.3 m; width = 2.5 ± 0.5 m).Further, higher resolution image in Figure 1(f) shows that the nano-sized TiO 2 particles randomly decorated the surface of yeast spheres, which have the morphology of raspberry-like composites.Similar morphology has been reported for the synthesis of the raspberry-like PMMA/SiO 2 hybrid microspheres [21].Moreover, the nanoparticles TiO 2 were relatively dispersed on the yeast-carbon in comparison with the agglomerated TiO 2 in the TiO 2 /spherical activated carbons composed by Oh et al. [22].In addition, the raspberry-like morphology of the TiO 2 @yeast-carbon microspheres can be confirmed through the EDS analysis.In the insert image in Figure 1(f), the sample contained Ti, C, O, S, and P; no other impurity element is detected, confirming that the TiO 2 particles were coated on the surface of the yeastcarbon.By comparing the TiO 2 @yeast precursor with the TiO 2 @yeast-carbon product, it can be seen that the color of particles changed from pure white to ash black.Thus, it can be inferred that the carbonization of the yeast occurred in the pyrolysis process.In this respect, the control experiments of the yeast-carbon without the attachment of nanoparticles TiO 2 have been conducted under the same conditions.The image of yeast-carbon in Figure 1(c) indicates that the yeastcarbon with an average diameter (length = 3.5 ± 0.4 m; width = 2.3 ± 0.50 m) has smooth surface morphology and rich pore structure, which contrasts with the morphology of yeast-carbon synthesized by Shen et al. [23] in which a relatively high distribution of broken shells was observed.XRD patterns of yeast, yeast-carbon, TiO 2 @yeast-carbon, and TiO 2 are displayed in Figure 2. The broad peak around 2 = 20 ∘ indicates that the yeast carbon (in Figure 2(a)) and the yeast (in Figure 2(b)) can be assigned to amorphous species.In Figures 2(c TiO 2 (JCPDS.No: 21-1276) [25].The relative intensity of diffraction peaks of TiO 2 @yeast carbon is approximately identical with the original components of the TiO 2 as guest particles (P 25 TiO 2 : 78% anatase-type TiO 2 and 22% rutiletype TiO 2 ).The broad peaks at 2 = 20 ∘ in the Figures 2(c) and 2(d) are mainly caused by the amorphous structure of yeast-carbon.
In Figure 3, the distinct decrease in weight of the bare yeast is shown during a wide temperature range of 280-600 ∘ C. The rapid weight loss of approximately 50.0% at the temperature range of 280-370 ∘ C may be associated with the split of the polysaccharide chains in the yeast [26,27], which is accompanied by a broad exothermal peak at about 370 ∘ C in the DTA curve.The ultimate weight loss from 400 to 600 ∘ C can be resulted from the cross-linking of oligosaccharides [28] in the yeast, which is accompanied by the endothermic peak at 500 ∘ C in the DTA curve.The total weight loss reaches to about 30.0% and the last weight of the residual ashes is approximately 70.0% of the original yeast biomass.In comparison with the naked yeast, the carbonization reaction of the TiO 2 @yeast precursor was started at temperatures about 250 ∘ C. The broad exothermal peak at around 200-300 ∘ C may be related to the carbonized decomposition of organic substrate in the yeast along with the rapid weight loss of 50.0%.It is worth noting that the broader exothermal peak appeared at around 300-450 ∘ C.This may be caused by the release of constitution water and further crystallization of TiO 2 [29].The wider exothermal peak at 450-600 ∘ C can be ascribed to the crystal shift from anatase to rutile phase.
After pyrolysis entirely, the raspberry-like TiO 2 @yeastcarbon microspheres can be attained, which has been proved by SEM analysis.The connection between the guest particles TiO 2 and the host particles of the yeast-carbon can be elaborated further by FT-IR analysis.FT-IR spectra of the yeast, yeast-carbon, TiO 2 @yeast precursor, TiO 2 and TiO 2 @yeast-carbon composites are presented in Figure 4, respectively.For the yeast-carbon, the band around 1570 cm −1 can be ascribed to -C=C-stretching bond originated from the inherent structure of yeast, and the bands at 1704 and 1210 cm −1 correspond to C=O, C-O stretching of carboxyl groups, respectively [30].As for TiO 2 @yeast-carbon composites, the broad and intense peak at 3500-3200 cm −1 can be assigned to the stretching of -OH group due to the bound water in the TiO 2 @yeast-carbon composites [31,32].The stretching bond corresponding to skeletal Ti-O-Ti is clearly represented in the region of 509 cm −1 .The band at 1171 cm −1 suggests the presence of C-O bond [33].The band at around 620 cm −1 can be attributed to the Ti-O-C vibration, which indicates that TiO 2 nanoparticles were chemically bonded with yeast-carbon in hybrid particles [34].Moreover, the band at 3350 cm −1 is more prominent in TiO 2 @yeast-carbon composites than that of TiO 2 , indicating that there were more hydroxyl groups in the TiO 2 @yeast-carbon hybrid particles.Generally, more hydroxyl groups lead to generation of more • OH radicals in photocatalysis, which can enhance the photocatalytic activity of TiO 2 [35].
Based on the characterization discussed above, the following mechanism that appeared in Scheme 1 is proposed to account for the formation procedure of the raspberrylike TiO 2 @yeast-carbon hybrid microspheres.In Scheme 1, the opposite zeta potentials of yeast cells with a positive charge (H + ) and the nanoparticles TiO 2 with a negative charge (OH − ) were adjusted previously by tuning the pH of their own aqueous suspensions.Then the TiO 2 @yeast hybrid precursor with raspberry-like morphology was obtained once the aforementioned aqueous suspensions were mixed [36].In the obtained raspberry-like TiO 2 @yeast precursor, the yeast acted as the host cores and TiO 2 as the guest particles.Afterwards, the carbonization of yeast core in the precursors of raspberry-like TiO 2 @yeast can be fulfilled by a gradual temperature-rise procedure, which can obtain the hybrid TiO 2 @yeast-carbon composites.TGA-DTA results showed that the final weight loss is approximately 50.0% in the process  of carbonization.In the obtained raspberry-like TiO 2 @yeastcarbon, TiO 2 nanoparticles were chemically bonded with yeast-carbon through FT-IR analysis.
Figure 5 shows the UV-VIS diffuse reflectance spectra of the yeast-carbon, TiO 2 nanoparticles, and the TiO 2 @yeastcarbon hybrid particles.In Figure 5(a), the yeast-carbon showed a strong absorption in the range from the UV to the visible light, which was similar to the commercial activated carbon [37].In Figure 5(b), the band gap energy (  ) of nanoparticles TiO 2 is estimated about 3.2 ev [38], corresponding to a threshold wavelength of 376 nm.In comparison with the nanoparticles TiO 2 , in Figure 5(c), TiO 2 @yeastcarbon composites remain optical response in a wide range of 250-400 nm.Besides, in the insert images of Figure 5, the color of the TiO 2 @yeast-carbon sample was ash black in comparison with the pure white TiO 2 @yeast precursor, which was consistent with its adsorption spectrum.

Combined Adsorption and Photocatalytic Degradation of Methylene Blue and Congo
Red.The raspberrylike TiO 2 @yeast-carbon microspheres containing an incom- pletely covered surface of yeast-carbon may possess unique properties for getting rid of water pollutants.The rich pore structure of yeast-carbon might promote adsorption of organic dyes, while the outer TiO 2 nanoparticles can be in charge of the photocatalytic degradation of the dyes [39].That is to say that immobilizing TiO 2 nanoparticles on adsorbentlike yeast-carbon can result in a synergistic effect on the efficient degradation of dye in the photocatalytic process.Specially, yeast-carbon core has the capability to extend the separation lifetime of photogenerated e − /h + from outer TiO 2 nanoparticles [40] and thus increasing the quantum efficiency of TiO 2 .In turn, TiO 2 nanoparticles can destroy dyes by photocatalytic oxidation, thus regenerating the yeastcarbon in situ.In order to test the activity of the composite TiO 2 @yeast-carbon catalysts, the photocatalytic experiments  were carried out for the degradation of MB and CR aqueous solution widely used as model compound for photocatalytic nanomaterial standardization.
From the results shown in Figure 6, the MB and CR aqueous solution is barely photolyzed by UV light irradiation in Figure 6(a).In Figure 6(b) the adsorption-desorption equilibrium was set up within 50.0 min dark environment.The adsorption capacity of the TiO 2 @yeast-carbon microspheres for MB was higher than that for CR, which can be assigned to the negatively charged surface of the catalyst.Hereafter, the experiments of the removal of dyes in aqueous solution were preceded for about 120.0 min under UV light irradiation.During 50.0∼140.0minutes, only the adsorption procedures of MB and CR dyes onto the yeastcarbon continuously occurred in the yeast-carbon suspension in Figure 6(b).In the presence of the TiO 2 @yeastcarbon catalysts, the adsorption and photocatalysis occurred simultaneously in Figure 6(c).Thus, the degradation rate of the dye in the TiO 2 @yeast-carbon catalyst suspension was significantly higher than that in the yeast-carbon suspension.After 140.0 minutes, the adsorption of dyes onto the yeast-carbon tended to reach the adsorption equilibrium.However, the photocatalytic reactions under UV irradiation still proceeded constantly by TiO 2 @yeast-carbon catalysts.Finally, approximately 87.0% degradation rate of MB was achieved, whereas only around 30.0% CR was removed from the suspension.The distinct disparity of degradation rate between MB and CR dye aqueous solution can be attributed to their own adsorption performance onto the TiO 2 @yeastcarbon catalyst.
The adsorption constant for MB and CRwas listed in Table 1.From the results in Table 1, the maximum adsorption amount of the hybrid TiO 2 @yeast-carbon catalysts for cationic MB was significantly higher than that for anionic CR.In addition, the Langmuir model for MB and CR yields a somewhat better fit than the Freundlich model.The value of 1/n is equal to 0.94 (MB) and (0.13), respectively, which indicates that the adsorption may belong to the favorable adsorption [41].This prior adsorption of MB than CR dyes on the surface of the TiO 2 @yeast-carbon microspheres may be assigned to the negatively charged properties of composite catalyst.The discriminatory adsorption for the MB and CR dyes by the TiO 2 @yeast-carbon composite catalyst inevitably leads to the disparity of above-mentioned photocatalytic degradation rate.Therefore, the adsorption behavior on the surface of the TiO 2 @yeast-carbon microspheres has a direct impact on the photocatalytic degradation of the MB and CR dyes.

Conclusions
In summary, we prepared hybrid raspberry-like TiO 2 @yeastcarbon utilizing pyrolysis method.The as-synthesized hybrid TiO 2 @yeast-carbon had ordered elliptic shapes of uniform size (length = 3.7 ± 0.3 m; width = 2.5 ± 0.5 m).The potential applications of these novel composites for the removal of contaminants were ascertained for the removal of MB and CR.The results showed that the hybrid composites exhibited excellent photocatalytic activity for the MB due to its enhanced adsorption ability.The novel TiO 2 @yeastcarbon hybrid microspheres have potential applications not only in polluted water treatment but also in other areas such as sensors devices and dye sensitized solar cells.

Figure 1 :
Figure1: SEM images of (a) the naked yeast, (b) general observation of the raspberry-like TiO 2 @yeast precursor, (c) the yeast-carbon, (d) the overall view of the raspberry-like TiO 2 @yeast-carbon microspheres, (e) the selected raspberry-like TiO 2 @yeast-carbon microspheres, and (f) typical raspberry-like TiO 2 @yeast-carbon microspheres observed under high magnification.