Bio-template synthesis of spirulina/TiO₂ composite with enhanced photocatalytic performance

Abstract

A visible-light photocatalytic, mesoporous, hierarchical spirulina/TiO₂ composite with dye-sensitized surface was fabricated through a one-step hydrothermal process. The microstructure, mesoporous characteristics, surface morphology, as well as the visible-light photocatalytic activities are studied. The spirulina/TiO₂ had an anatase phase and an enhanced harvesting of visible light. It was found that the spirulina/TiO₂ composite exhibited higher specific surface area with narrow distributed mesopores. The photocatalytic activity of spirulina/TiO₂ was evaluated by decolorizing methyl orange aqueous solutions under visible light irradiation. As a bio-template, spirulina prevents TiO₂ from further aggregating and provides photosynthesis pigments as in situ dye-sensitizing source. The enlarged specific surface area and dye-sensitized surface improved the visible-light photocatalytic activity of spirulina/TiO₂.

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Introduction

Titania is a classic photocatalyst which can make use of the sunlight to produce hydrogen (total water splitting),¹ generate photocurrent (dye sensitized solar cells),² fabricate organics (photosynthesis)³ as well as degrade pollutants (water purification).⁴ It has been deeply investigated for its possible application in environment and energy fields. Unfortunately, titania is restrained from further utilization by recombination of photo-generated electron/hole pairs and inability to respond to visible light.⁵ To solve the listed problems, researchers have made revolutionary progress and discovered efficient ways. Traditional methods include nanosizing,^6–8 metal-ion doping,⁹ anion doping,^10,11 noble metal loading^12–14 and dye sensitization,^15,16etc., which are easy to be realized. Nowadays, self-assembly,^17,18 bio-templating^19,20 and surfactant templating²¹ have been developed and have harvested a lot of achievements to synthesize hierarchical titania micro/nano structures,^22,23etc.. The controllable synthesis of anatase TiO₂ with dominant {001} high-energy facets,^6,7,24,25 have also been a hot spot in synthesizing titania photocatalyst.

Recently, bio-templating has drawn more attention in the field of oxides material synthesis. The synthesized materials range from magnetic leaf²⁶ to artificial photocatalytic leaves.¹⁹ The sophisticated natural plant templates provide porous and hierarchical structures for better light-harvesting and more reaction centres. The sinuous vessels provide adequate access for matter transportation. Moreover, the abundant elements are able to be in situ doped into oxides. Our research group has developed a series of bio-templating methods and prepared LiFePO₄/C cathode materials with outstanding electrochemical performance in lithium-ion batteries^27,28 and different carbide (B₄C, TiC, TaC, etc.) nanostructures with remarkable mechanical properties.^29–32

Spirulina, a typical blue-green algae, is famous for its photosynthetic ability and nutrition. During the earth’s history, the development of blue-green algae has played an important role in harvesting solar energy directly and transferring considerable CO₂ into O₂, which was crucial for the further evolution of creatures. Attributed to the optimised helical structure (shown in Fig. 1a and b) and abundant photosynthesis pigments called phycobiliproteins (PBPs), spirulina obtains a high photosynthetic efficiency. Inspired by the photosynthetic ability of spirulina, the authors put forward an idea to combine spirulina and titania together. Spirulina is not only a promising bio-template to prepare hierarchical titania micro/nanostructures but also a potential natural dye sensitizer source to help titania attain the ability to utilize visible light. By duplicating spirulina structure and in situ dye sensitization, an improved photocatalytic performance has been realized.

Fig. 1 (a), (b) Optical microscopy images of spirulina.

Experimental

Materials

Titanium(IV) sulfide (Ti(SO₄)₂, 96%, CP) and glutaraldehyde (25%, aqueous solution) were purchased from Sinopharm Chemical Reagent Co., Ltd. Methyl orange (MO, AR) was purchased from Shanghai SSS Reagent Co., Ltd. Spirulina was obtained from College of Chemical Engineering and Materials Science, Zhejiang University of Technology. All reagents were used without any further purification.

Synthesis of spirulina/TiO₂ composite

A hydrothermal process was conducted to prepare spirulina/TiO₂ composite. 1.0 g of spirulina was added into 40.0 mL of distilled water. The mixture was heated and kept boiling for 10 min. After that, the mixture was cooled in air down to room temperature and 4.0 mL of glutaraldehyde was added into it, followed by magnetic stirring at room temperature for 1 h. Before hydrothermal process, 6.0 g of Ti(SO₄)₂ was dissolved into the mixture. The mixture was then transferred into a teflon-lined stainless steel reactor (50 mL) and heated at 180 °C in an oven for 12 h. After the reaction was completed, the reactor was cooled in air down to room temperature. The pH of the product was adjusted to 7 by distilled water. After vacuum filtration, the precipitate was dried at 80 °C to acquire spirulina/TiO₂ composite. The filtrate (Solution ST) was preserved for further tests.

Synthesis of control group

The pure TiO₂ was prepared through a hydrothermal process. 6.0 g of Ti(SO₄)₂ was added into 44.0 mL of distilled water. The solution was then transferred into a teflon-lined stainless steel reactor (50 mL) and heated at 180 °C in an oven for 12 h. The pH of the reaction product was adjusted to 7 by distilled water. After vacuum filtration, the precipitate was dried at 80 °C to acquire pure TiO₂.

To verify the adsorption of photosynthesis pigments on the composite, a hydrothermal treated pure spirulina product was prepared as follows. 1.0 g of spirulina was added into 40.0 mL of distilled water. The mixture was heated and kept boiling for 10 min. After that, the mixture was cooled in air down to room temperature and 4.0 mL of glutaraldehyde was added into it, followed by magnetic stirring at room temperature for 1 h. Then the mixture was transferred into teflon-lined stainless steel reactor (50 mL) and heated at 180 °C in an oven for 12 h. The product was vacuum filtrated and the filtrate (Solution S) was preserved for further testing.

Characterization

The specific surface areas and pore sizes of the photocatalysts were determined by Brunauer-Emmett-Teller (BET) method using an ASAP 2020 (Micromeritics Instruments) surface area and pore analysers via nitrogen adsorption at 77 K. Before nitrogen adsorption measurement, both the spirulina/TiO₂ and pure TiO₂ were degassed at 120 °C. A desorption isotherm was used to determine the pore size distribution by the Barret-Joyner-Halender (BJH) method. The morphologies of the photocatalysts were examined by scanning electron microscopy (SEM, Hitachi, SU-70) and transmission electron microscopy (TEM, FEI, Tecnai G2 F20) equipped with an energy dispersive X-Ray spectroscopy (EDX) detector. The X-ray diffraction (XRD) patterns of the photocatalysts were observed by an X’Pert Pro diffractometer using Cu-Kα radiation (λ = 0.15418 nm). The UV-vis diffuse reflection spectra (DRS) of photocatalysts (referenced to BaSO₄) and UV-vis absorption spectra of Solution S and Solution ST (referenced to distilled water) were recorded by an ultraviolet and visible spectrophotometer (Shimazu, UV-2550). The Fourier transform infrared spectroscopy (FTIR) of photocatalysts and Solution S were carried out with KBr pellet using a Nicolet-6700 FTIR spectrometer.

Photocatalytic performance

The visible-light photocatalytic activities of the photocatalysts were evaluated by the photocatalytic degradation of MO aqueous solution at 20 °C controlled by circulating water cooling. 1.5 g of each sample was dispersed into a 1500 mL 10 mg L⁻¹ MO aqueous solution in a beaker. The beaker was then kept in darkness for 24 h to avoid the physical adsorption affecting the degradation rate. A 35 W halogen lamp was used as the light source, which contain both ultra-violet and visible light. The UV-vis absorption patterns of photocatalytic degraded solution samples were recorded by ultraviolet and visible spectrophotometer (Shimazu, UV-2550).

Results and discussion

The specific surface area of the photocatalysts was determined by BET analysis (Table 1). The results are 239.23 m² g⁻¹ for spirulina/TiO₂ and 54.75 m² g⁻¹ for pure TiO₂, which definitely shows an enlarged specific surface area of spirulina/TiO₂ composite compared with pure TiO₂.

Table 1 BET test results of the samples

Sample	BET surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Average pore size by BET (nm)
Spirulina/TiO2	239.23	0.33	5.52
Pure TiO2	54.75	0.32	23.51

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Fig. 2a, b displays the nitrogen isotherms of spirulina/TiO₂ and pure TiO₂, respectively. The isotherms of the samples can be categorized in Type IV with two capillary condensation steps, implying bimodal pore size distributions in the mesoporous and macroporous regions.³³ According to the previous research, a bimodal pore size distribution results from two types of different aggregates in the powders.^18,33 The hysteresis loop in the lower relative pressure range (0.4 < P/P₀ < 0.8) is corresponding to finer intra-aggregated pores, and that in the higher relative pressure range (0.8 < P/P₀ < 1) is related to larger inter-aggregated pores.

Fig. 2 (a), (b) Nitrogen adsorption isotherms for spirulina/TiO₂ and pure TiO₂, respectively; (c), (d) Size distribution of mesopores for spirulina/TiO₂ and pure TiO₂, respectively.

Generally, the pattern of spirulina/TiO₂ (Fig. 2a) can be approximately categorized into Type IV with a type-H1 hysteresis loop. This indicates the presence of narrow distributed mesopores.^18,34,35 The higher adsorption at high relative pressure indicates the macropores are also present. These are consistent with the corresponding pore size distribution (Fig. 2c), which shows a clear bimodal pore size distribution. It shows a pore diameter peak at 38 nm in mesoporous region and a pore diameter peak at 71 nm in macroporous region. The pattern of pure TiO₂ (Fig. 2b) can be categorized into Type IV with Type-H3 hysteresis loop which reveals the presence of mesopores and macropores. This is confirmed by the pore size distribution (shown in Fig. 2d), which shows that the pure TiO₂ contains mesoporous peak at 36 nm and macroporous peak at ca. 300 nm. This indicates that pure TiO₂ is severely aggregated. Though spirulina/TiO₂ and pure TiO₂ have similar mesoporous peaks, the pore volume of spirulina/TiO₂ is larger than pure TiO₂. The results undoubtedly support the fact that the specific surface area of TiO₂ was enlarged by adding spirulina during hydrothermal process. This process formed a TiO₂ hierarchical structure and avoided TiO₂ from further aggregating to form larger inter-aggregated pores.

Fig. 3a and b display typical pure TiO₂ morphology. The sizes of the particles range widely from several micrometres to tens of micrometres and the particle surfaces are not regular. The inset in Fig. 3b is a magnification of the pure TiO₂ surface corresponding to the rectangular area, showing that the grain size of pure TiO₂ is ca. 30 nm. The typical morphology of spirulina/TiO₂ is shown in Fig. 3c and d. The composites are well dispersed with a length of 15–30 μm and a diameter of ca. 1–5 μm. As shown in the inset in Fig. 3d, the TiO₂ grains are homogenously distributed on the surface of spirulina. The grain size of spirulina/TiO₂ is similar to pure TiO₂. As known to us, the specific surface area is not only related to the grain size but also related to the surface structure, aggregate and morphology. As shown in Fig. 3a, the pure TiO₂ is severely aggregated compared with spirulina/TiO₂ (shown in Fig. 3c). Therefore, the specific surface area of spirulina/TiO₂ was larger than pure TiO₂, consistent with the BET results.

Fig. 3 (a), (b) SEM images of pure TiO₂. The inset is a magnification of pure TiO₂ surface corresponding to the rectangular area in (b); (c), (d) SEM images of spirulina/TiO₂. The inset is a magnification of spirulina/TiO₂ surface corresponding to the rectangular area in (d).

Fig. 4a shows a typical TEM image of pure TiO₂ particles with a diameter of 3 μm. As shown in Fig. 4b, the grains of pure TiO₂ maintain a diameter of ca. 30 nm. The selected area electron diffraction (SAED) displayed in Fig. 4c was performed over a group of particles. The pattern shows clearly the presence of concentric rings due to the polycrystalline structure. The corresponding interplanar spacings are carefully measured, calculated and analysed. The results perfectly match the anatase phase (JCPDS, No. 21-1272). Fig. 4d, e, f and g are high resolution transmission electron microscope (HRTEM) images of pure TiO₂ with crystallographic orientation of [13̄1], [010], [111̄] and [011̄], respectively. The corresponding fast Fourier transformation (FFT) patterns reveal that the areas shown in Fig. 4 d, e, f and g are single-crystalline domains. The interplanar spacing and the interfacing angle illustrated in Fig. 4d, e, f and g match the anatase phase precisely, which is consistent with the XRD test.

Fig. 4 (a), (b) TEM images of pure TiO₂; (c) SAED pattern of pure TiO₂; (d), (e), (f), (g) HRTEM images of pure TiO₂ with FFT patterns inset.

Fig. 5a is a typical TEM image of spirulina/TiO₂ composite. The composites are covered by titania nanoparticles. The grains have a size of ca. 20–30 nm as shown in Fig. 5b. Different from the morphology of pure TiO₂, spirulina/TiO₂ exhibits intriguing morphological characteristics and a porous hierarchical structure (Fig. 5a). The SAED pattern of spirulina/TiO₂ is displayed in Fig. 5c. Similar to pure TiO₂, the SAED pattern presents concentric rings which indicate a polycrystalline structure. The corresponding interplanar spacing matches the anatase phase well. Fig. 5d is a scanning transmission electron microscope (STEM) image corresponding to Fig. 5a. A magnification image obtained from the rectangular area in Fig. 5d is shown in Fig. 5e. Fig. 5f, g, h, and i correspond to Ti, O, C, and N maps, respectively, revealing that the elements mentioned above are well distributed. The EDX spectrum (Fig. 5j) is obtained from the area of Fig. 5e. The strong background signals of Cu come from the TEM grid. The HRTEM image of titania nanoparticles (Fig. 5k) shows three grains with different crystallographic orientations, indicating that the composite is polycrystalline. Fig. 5l is the HRTEM image magnified from the area 1 in Fig. 5k with crystallographic orientation of [010]. The corresponding FFT diffraction pattern proves that the area shown in Fig. 5l is a single-crystalline domain. The lattice planes (103) and (200) are illustrated in Fig. 5l. The interplanar spacing and the interfacing angle match the anatase phase (JCPDS, No. 21-1272). The magnified images and FFT patterns of area 2 with crystallographic orientation of [11̄0] and area 3 with crystallographic orientation of [010] are shown in Fig. 5m and n, respectively. The lattice planes illustrated in Fig. 5m and n also match the anatase phase. Though the diffraction direction of area 3 is the same as that of area 1, the lattice plane (200) direction in area 3 is different from area 1, indicating that area 1 and 3 are different domains. In addition, the TEM and HRTEM images (Fig. 5b, c and k) reveal distinct porous structures which can provide more adsorption sites for photo degradation. Meanwhile the morphology of pure TiO₂ (shown in Fig. 4a and e–g) are less of nano pores and smaller specific surface area than spirulina/TiO₂, which are consisting with BET test results.

Fig. 5 (a) TEM image of a Spirulina/TiO₂ with titania covering the spirulina; (b) TEM image of the TiO₂ particles on the spirulina surface; (c) SAED pattern of Spirulina/TiO₂; (d) STEM image of the same composite as in (b); (e) High magnification image obtained from the rectangular area in (d); (f), (g), (h) and (i) Ti, O, C, N maps, respectively, obtained from the area of (e); (j) The EDX spectrum obtained from area in (e); (k) HRTEM image of the Spirulina/TiO₂; (l), (m) and (n) high magnification images obtained from the rectangular area 1 to 3 in (k), respectively, with the corresponding FFT diffraction patterns inset.

The XRD patterns for the two samples are shown in Fig. 6c. The presence of anatase phase of TiO₂ can be obviously observed (JCPDS, No. 21-1272) in both photocatalysts. The spirulina/TiO₂ and pure TiO₂ exhibit similar intensive peaks and full width at half maximum, which means a similar grain size based on Scherrer’s equation.³⁶

Fig. 6 (a) Samples of pure TiO₂ and spirulina/TiO₂; (b) Filtrate of only spirulina (Solution S) and spirulina/TiO₂ (Solution ST) after hydrothermal reaction; (c) XRD patterns of spirulina/TiO₂ and pure TiO₂; (d) UV-vis DRS spectra of spirulina/TiO₂, pure TiO₂; (e) FTIR spectra of spirulina/TiO₂, pure TiO₂ and Solution S; (f) UV-vis absorption spectra of Solution S and Solution ST.

The DRS spectra of the photocatalysts are shown in Fig. 6d. Slight changes are observed for the fundamental absorption edges of titania, which is located in the UV region at ca. 380 nm.³⁷ However, the harvesting of visible light is significantly enhanced for spirulina/TiO₂. The yellow colour of spirulina/TiO₂ is caused by the adsorption of dyes from the solution. The colour is light orange for Solution S and ST as shown in Fig. 6b. The UV-vis absorption spectra of Solution S and ST are shown in Fig. 6f, which reveals that concentration of dyes decreases when the TiO₂ participate in the hydrothermal reaction. Based on the UV-vis test results shown in Fig. 6d and f, it can be demonstrated that the pigments were adsorbed on TiO₂ during the hydrothermal reaction. As mentioned in the Introduction, spirulina contains abundant photosynthesis pigments called PBPs. A typical kind of PBP is called C-phycocyanin (CPC) which is linked with fluorescent linear tetrapyrrole chromophore, phycocyanobilin (PCB),^38–40 as shown in Fig. 7b. CPC can be widely found in spirulina and accounts for approximately 15-20% of the dry weight of spirulina.³⁹ However, CPC finally decomposes during the hydrothermal process because it was a protein and unstable in the hydrothermal environment. The residual PCB was the dye which can be chemically adsorbed on TiO₂. The peaks in the UV-vis spectrum of solution ST is similar to that of pure PCB chromophore,⁴¹ suggesting the existence of PCB. The peaks at 258 nm and 307 nm respectively correspond to π–π* and n–π* transitions of pyrrole. The peaks at 400 nm can be assigned to n–π* transition of C=O.

Fig. 7 (a) Schematic diagram of formation mechanism; (b) schematic diagram of pigment-adsorption process.

Further evidences for the existence of PCB come from the FTIR results (Fig. 6e) of spirulina/TiO₂, pure TiO₂ and Solution S. In the spectrum of Solution S, the three peaks at 2960 cm⁻¹, 2921 cm⁻¹ and 2855 cm⁻¹ demonstrate the existence of nCH₃–, –CH₂– and –CH–. The other two peaks at 1452 cm⁻¹ and 1393 cm⁻¹ are assigned to –CH₂ bending vibration and to C–CH₃ bending vibration, respectively. The peak at ca. 1662 cm⁻¹ corresponds to both C=O stretching vibration and –NH– bending vibration. The peak at 1576 cm⁻¹ can be assigned to C=C stretching vibration. These groups basically compose the PCB structure (shown in Fig. 6b). In the FTIR spectra of spirulina/TiO₂, and pure TiO₂, three intense vibration bands at 3400 cm⁻¹, 1637 cm⁻¹, 520 cm⁻¹ correspond to O–H stretching vibrations, H–O–H bending vibrations and TiO₂ fundamental vibrations, respectively.^37,42 Particularly, the spectrum of spirulina/TiO₂ has some characteristic peaks which correspond to the FTIR spectrum of Solution S. These are strong evidences for the adsorption of PCB on TiO₂.

Based on the characterization results, the formation mechanism of spirulina/TiO₂ is illustrated in Fig. 7a and b. The phase transformation from amorphous TiO₂ to anatase is favoured by the acidic high-pressure aqueous reaction environment. As known to us, the phase transformation of TiO₂ is due to the rearrangement of TiO₆ octahedra. The crystallization process was supposed to initiate through face-sharing polycondensation of these octahedra.³³ Based on the previous research^43,44 and the isoelectric point of TiO₂ (pH 5.5–6),⁴⁵ the Ti–OH group in TiO₆ octahedra should be partially protonated to give TiOH₂⁺ in this acidic condition (pH = 1) when forming TiO₂. Hence, the surface of TiO₂ would have a positive charge density and tend to be attracted by the negative anions. The spirulina had a sheath composed of polysaccharides containing poly-hydroxyl, making the spirulina surface electronegative and easier to adsorb TiO₂ with positive surface.³⁹ The hydroxyl on both TiO₂ surface and spirulina surface then dehydrated to form C–O–Ti bonds, which made TiO₂ and spirulina integrated. After integration, the TiO₂ kept growing, nucleating, phase transforming and finally covering the surface of spirulina in anatase phase. This process ensured the growth of TiO₂ on spirulina surface and prevented TiO₂ from aggregating. Therefore, spirulina can acquire a narrow distribution of mesopores without macropores over 100 nm. The pigment-adsorption process is illustrated in Fig. 7b. At the initial stage of hydrothermal process, CPC released and was chemically adsorbed on the surface of TiO₂. However, CPC, as a protein, could not suffer the extreme hydrothermal acidic environment (pH = 1) at high temperature (180 °C) and finally decomposed into PCB and other fragments.^41,46,47 The PCB was more thermostable⁴¹ and the carboxyl in its structures^38–40 could react with the hydroxyl on TiO₂ surface and be chemically adsorbed on the surface of TiO₂. Both of the processes occurred simultaneously so that the hierarchical structure and dye sensitized surface could be acquired via one-step hydrothermal process.

The visible-light photocatalytic activities of the photocatalysts were performed by the photocatalytic degradation of MO aqueous solution at ambient temperature. The UV-vis patterns of MO degraded by the photocatalysts are illustrated in Fig. 8a and b. The peak at 280 nm corresponds to the π–π* transition of phenyl ring. After degradation, the peak at 280 nm blue shifts to 250 nm, which is caused by the disappearance of the conjugation between the phenyl ring and azo bonds. The maximum absorption peak of MO at 450 nm corresponds to the n–π* transition of azo bonds (–N=N–), which is taken as the symbol of the concentration based on Lambert-Beer Law. As shown in Fig. 8c, the degradation rate of spirulina/TiO₂ is 95.2% in 90 min while the one of pure TiO₂ is 79.3%. By fitting the plots, the straight lines of ln(c_MO, 0/c_MO)-t are drawn in Fig. 8d, which indicates a typical first-order reaction.⁴⁸ From the slopes, the reaction rate constants can be calculated, 0.03436 min⁻¹ for spirulina/TiO₂ and 0.01701 min⁻¹ for pure TiO₂. As known to us, the valid photocatalyst is TiO₂, which is only a part of spirulina/TiO₂. We tested the actual content of TiO₂ in spirulina/TiO₂ by burning spirulina/TiO₂ sample and the result was 67.8% (more details can be found in the ESI†). Although the actual content of TiO₂ in spirulina/TiO₂ is less than that of pure TiO₂, spirulina/TiO₂ degraded MO much faster than pure TiO₂. It is a general consequence of porous hierarchical structure and dye sensitization of spirulina/TiO₂, which provide more reaction centres for degrading MO and a more effective utilization of light.

Fig. 8 (a), (b) UV-vis spectra of MO solution degraded by spirulina/TiO₂ and pure TiO₂, respectively; (c) degradation line; (d) photo-degradation kinetics line.

To support the effect of spirulina-hierarchical structure, a TiO₂ sample prepared with only photosynthesis pigments was tested. The further synthesis process and characterization results can be found in the ESI.† The typical morphology of CPC/TiO₂ is shown in Fig. S1a and b, see ESI†. Without spirulina, TiO₂ maintain a pure-TiO₂-like morphology. The particle sizes range from 2–10 μm and are also severely aggregated. The grain size shown in Fig. S1b, see ESI† is ca. 30 nm, which is similar to spirulina/TiO₂. The presence of anatase (JCPDS, No. 21-1272) can be observed in Fig. S1c, see ESI†.The degradation line and degradation kinetics line of CPC/TiO₂ are shown in Fig. S1e and f, respectively, see ESI†. Compared with the reaction constant calculated in Fig. 8d, the value of CPC/TiO₂ (0.02517 min⁻¹) is between spirulina/TiO₂ (0.03436 min⁻¹) and pure TiO₂ (0.01701 min⁻¹). On one hand, this indicates that the photocatalytic performance of CPC/TiO₂ is enhanced by the dye-sensitization. On the other hand, it shows that the porous hierarchical structure of spirulina/TiO₂ does have effect the enhancement of photocatalytic performance.

Hoffmann et al.⁴⁹ reviewed the mechanism of titania photo degradation reaction in detail and pointed out a variety of factors effecting the performance of titania. The groups (·OH, TiOH⁺, H₂O₂, etc.) with oxidation ability are able to be produced by complex reaction of photo-generated holes and electrons with water (H₂O) and oxygen (O₂).⁴⁹ There are some intrinsic factors of the photocatalysts that need to be taken into consideration to enhance the photo-catalytic performance of titania. To increase the production of photo-generated carriers, utilization of the visible light has to be involved by doping and dye sensitization. To intensify the practical photo degradation reaction of the photo-generated carriers, it is suggested to delay the recombination of the electron/hole pairs, accelerate the transmission of charge carriers and enlarge the specific surface area of the photocatalysts to adsorb more contaminants and hence more reaction centres.

The schematic diagram of spirulina/TiO₂ degrading MO under visible light is illustrated in Fig. 9. A primary source of the electrons is the photo excited dye. The electrons from the highest occupied molecular orbital transfer to the lowest unoccupied molecular orbital and then inject into the conduction band of TiO₂.^50–53 This process makes it more effective to use visible light than pure TiO₂ which merely generates the electrons in the valance band in response to ultraviolet light. The porous hierarchical structure of spirulina/TiO₂ makes it possible to adsorb more MO and O₂ on the surface and create more reaction centres.⁴ Meanwhile, the transportation of oxidizing groups can be accelerated as well. The reaction rate is then improved.

Fig. 9 Schematic diagram of spirulina/TiO₂ degrading MO under visible light.

Conclusions

Through a simple hydrothermal process, we successfully fabricated the spirulina/TiO₂ composite. The MO degradation experiment under halogen lamp proved that the spirulina/TiO₂ composites were efficient photocatalysts in dealing with the azo-compound pollutants. The remarkable porous hierarchical structure and dye sensitized surface contributed to the enhanced photocatalytic performance compared with the pure TiO₂. The photosynthesis pigments realized an enhancement of visible light absorption to improve the efficiency of light energy utilization. The particular porous hierarchical structure of the composite and the biological characteristics of spirulina decreased the ratio of electron/hole pair recombination and increased the reaction centres, which resulted in the higher photocatalytic rate. This research presents a new method in mass-fabricating hierarchical oxides micro/nanostructures which may be utilized in lithium-ion batteries, photo-luminescence materials, etc..

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51172205 and 51002138), the Natural Science Foundation of Zhejiang Province (Y4110505, Y4090420 and Y4110523), the Qianjiang Project of Zhejiang Province (2010R10029), the Scientific Research Foundation for the Returned Overseas Chinese Scholars (2010609), the foundation of Science and Technology Department of Zhejiang Province (2011C21009) and the New Century Excellent Talents in University (NCET 111079).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ra21753h

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