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  • Editor: Bernard Kippelen
  • Vol. 20, Iss. S2 — Mar. 12, 2012
  • pp: A340–A350
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Bacteria-directed construction of hollow TiO2 micro/nanostructures with enhanced photocatalytic hydrogen evolution activity

Han Zhou, Tongxiang Fan, Jian Ding, Di Zhang, and Qixin Guo  »View Author Affiliations


Optics Express, Vol. 20, Issue S2, pp. A340-A350 (2012)
http://dx.doi.org/10.1364/OE.20.00A340


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Abstract

A general method has been developed for the synthesis of various hollow TiO2 micro/nanostructures with bacteria as templates to further study the structural effect on photocatalytic hydrogen evolution properties. TiO2 hollow spheres and hollow tubes, served as prototypes, are obtained via a surface sol-gel process using cocci and bacillus as biotemplates, respectively. The formation mechanisms are based on absorption of metal-alkoxide molecules from solution onto functional cell wall surfaces and subsequent hydrolysis to give nanometer-thick oxide layers. The UV-Vis absorption spectrum shows that the porous TiO2 hollow spheres have enhanced light harvesting property compared with the corresponding solid counterpart. This could be attributed to their unique hollow porous micro/nanostructures with microsized hollow cavities and nanovoids which could bring about multiple scattering and rayleigh scattering of light, respectively. The hollow TiO2 structures exhibit superior photocatalytic hydrogen evolution activities under UV and visible light irradiation in the presence of sacrificial reagents. The hydrogen evolution rate of hollow structures is about 3.6 times higher than the solid counterpart and 1.5 times higher than P25-TiO2. This work demonstrates the structural effect on enhancing the photocatalytic hydrogen evolution performance which would pave a new pathway to tailor and improve catalytic properties over a broad range.

© 2012 OSA

1. Introduction

Solar energy incident on the earth’s surface by far exceeds all human energy needs [1

1. N. S. Lewis and D. G. Nocera, “Powering the planet: chemical challenges in solar energy utilization,” Proc. Natl. Acad. Sci. U.S.A. 103(43), 15729–15735 (2006). [CrossRef] [PubMed]

]. Converting solar energy into renewable and clean energy resources is required in order to solve global energy and environmental issues [2

2. M. Hambourger, G. F. Moore, D. M. Kramer, D. Gust, A. L. Moore, and T. A. Moore, “Biology and technology for photochemical fuel production,” Chem. Soc. Rev. 38(1), 25–35 (2009). [CrossRef] [PubMed]

, 3

3. M. Woodhouse and B. A. Parkinson, “Combinatorial approaches for the identification and optimization of oxide semiconductors for efficient solar photoelectrolysis,” Chem. Soc. Rev. 38(1), 197–210 (2008). [CrossRef] [PubMed]

]. The photocatalytic splitting of water is one of the most promising techniques for converting light energy into hydrogen fuel [4

4. F. E. Osterloh, “Inorganic materials as catalysts for photochemical splitting of water,” Chem. Mater. 20(1), 35–54 (2008). [CrossRef]

, 5

5. H. Zhou, T. Fan, and D. Zhang, “An insight into artificial leaves for sustainable energy inspired by natural photosynthesis,” ChemCatChem 3(3), 513–528 (2011). [CrossRef]

]. Since Honda and Fujishima discovered photocatalytic H2 evolution over TiO2 in 1972 [6

6. A. Fujishima and K. Honda, “Electrochemical photolysis of water at a semiconductor electrode,” Nature 238(5358), 37–38 (1972). [CrossRef] [PubMed]

], many efforts have been made to promote its photocatalytic efficiency. One approach is to change the components by doping non-metal elements, metals, metal oxides and so forth [7

7. M. Ni, M. K. H. Leung, D. Y. C. Leung, and K. Sumathy, “A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production,” Renew. Sustain. Energy Rev. 11(3), 401–425 (2007). [CrossRef]

]. The other one is to develop various micro/nanostructures such as nanoparticles, nanosheets, nanotubes, nanowires [8

8. X. Chen and S. S. Mao, “Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications,” Chem. Rev. 107(7), 2891–2959 (2007). [CrossRef] [PubMed]

10

10. H. Zhou, E. M. Sabio, T. K. Townsend, T. Fan, D. Zhang, and F. E. Osterloh, “Assembly of core-shell structures for photocatalytic hydrogen evolution from aqueous methanol,” Chem. Mater. 22(11), 3362–3368 (2010). [CrossRef]

], among which, hollow micro/nanostructures are emerging as a promising area because of their high surface area and high surface permeability with superior photocatalytic activities relative to conventional bulk materials [11

11. Z. Liu, D. D. Sun, P. Guo, and J. O. Leckie, “One-step fabrication and high photocatalytic activity of porous TiO2 hollow aggregates by using a low-temperature hydrothermal method without templates,” Chemistry 13(6), 1851–1855 (2007). [CrossRef] [PubMed]

]. Many researches have been devoted on the synthesis of hollow micro/nanostructures with various morphologies including hollow spheres, tubes, fibers, etc [12

12. Z. Wu, F. Dong, W. Zhao, H. Wang, Y. Liu, and B. Guan, “The fabrication and characterization of novel carbon doped TiO2 nanotubes, nanowires and nanorods with high visible light photocatalytic activity,” Nanotechnology 20(23), 235701 (2009). [CrossRef] [PubMed]

, 13

13. S. Xuan, W. Jiang, X. Gong, Y. Hu, and Z. Chen, “Magnetically separable Fe3O4/TiO2 hollow spheres: fabrication and photocatalytic activity,” J. Phys. Chem. C 113(2), 553–558 (2009). [CrossRef]

] by using silica/PS spheres, carbonaceous polysaccharide spheres, carbon nanotubes, etc. as templates. However, it is still difficult to find a general method to obtain various morphologies of hollow micro/nanostructured TiO2, which also blocks the further study on the structural effect on their photocatalytic performance.

Nature provides a multiplicity of materials, architectures and functions resulting from stringent selection processes for the past millions of years. Biological systems found in nature combine many inspiring properties such as sophistication, miniaturization and hierarchical organizations which may be hardly attainable through any chemical methods. Bio-inspired fabrication by templating has been demonstrated to be a versatile route to hollow micro/nanostructures [14

14. T. Fan, S. K. Chow, and D. Zhang, “Biomorphic mineralization: from biology to materials,” Prog. Mater. Sci. 54(5), 542–659 (2009). [CrossRef]

, 15

15. H. Zhou, T. Fan, and D. Zhang, “Biotemplated materials for sustainable energy and environment: Current status and challenges,” ChemSusChem 4(10), 1344–1387 (2011). [CrossRef] [PubMed]

]. A rich variety of biological structures with complex morphologies have been used as templates to generate TiO2 hollow micro/nanostructures for photocatalysis [16

16. J. He and T. Kunitake, “Preparation and thermal stability of gold nanoparticles in silk-templated porous filaments of titania and zirconia,” Chem. Mater. 16(13), 2656–2661 (2004). [CrossRef]

20

20. X. Li, T. Fan, H. Zhou, S. K. Chow, W. Zhang, D. Zhang, Q. Guo, and H. Ogawa, “Enhanced light-harvesting and photocatalytic properties in morph-TiO2 from green leaf biotemplates,” Adv. Funct. Mater. 19(1), 45–56 (2009). [CrossRef]

]. However, many natural templates are commonly lack of structural multiformity and speciality. To conquer this problem, we turn to a very old branch-microorganism for help.

When entering this mysterious meso/micro-scopic world, we will pause to admire the surprising and bewildering riot of shapes, sizes, and aggregates of various kinds of microorganism. A typical example is bacteria. There are cells that look like lemons, teardrops, or oblong spheroids; some are bent, curved, flat sided, triangular, bean shaped, or helical; others are rounded, squared, pointed, curved, or tapered. Other organisms grow as branched or unbranched filaments, live in sheathed or unsheathed chains, or aggregate in primitive or highly organized multicellular composites. Inspired by their structural multiformity, we’d like to template their sophisticated structures to form the corresponding biomorphic hollow structures (e.g. hollow spheres, hollow tubes, hollow nanohelixs, hollow twin spheres, hollow squares and other kinds of 3D structures) with tailored morphologies and dimensions. The obtained various hollow micro/nanostructures can provide a wide space for the further fundamental study of the structural effect on the photocatalytic performance. Additionally, bacteria are abundant in nature and can be used for large scale production because of their exponential growth and relatively short cell cycle.

Herein, we put forward a general approach to generate biomorphic TiO2 hollow spheres and hollow tubes using two species of bacterium cocci and bacillus as biotemplates, respectively. The possible formation mechanisms involving the surface sol-gel process are proposed. The UV-Vis absorption spectra shows that the hollow spheres have enhanced light-harvesting property compared with the corresponding solid counterpart which are probably attributed to the multiple scattering and rayleigh scattering processes of light within hollow structures. Moreover, compared with the solid counterpart and commercial P25 TiO2, the biomorphic TiO2 hollow spheres and hollow tubes have superior photocatalytic hydrogen evolution activities under UV and visible light irradiation, which mainly attribute to their unique porous hollow structures. This bacteria-templated method represents a versatile means for various hollow micro/nanostructures. This work also builds up a working prototype to design and fabrication of photocatalysts with enhanced catalytic activities from the point of structure.

2. Experimental section

2.1 Chemicals and materials

Lactobacillus powder of Streptococcus thermophilus (Str. theromophilus) and Lactobacillus bulgaricus (L. bulgaricus) were provided by Beijing Zhuanger Company. Tetrabutyltitanate (Ti(OBu)4) and ethyl alcohol (EtOH, 100%) were used without any treatment.

2.2 Synthesis of TiO2 hollow structures

In a typical procedure, the bacterial templates were placed in a suction filtering unit (0.45μm), and a precursor diluted with 3 ml Ti(OBu)4 in 30 ml ethanol was dropped into the bacterial templates. Immediately, precursor solution infiltrated into the space of the structure. Excess precursor solution was removed by filtration. The precursor adsorbed on the templates was immediately hydrolyzed by the water vapor in air. Consequently, the bacterial templates were covered with a thin oxide and hydroxide. This coating process was repeated three to five times to control the coating thickness. The bacterial templates were removed by calcination in air at 700 °C for 1h. Obtained samples were cooled to room temperature in air naturally. The corresponding TiO2 nanoparticles were prepared under the same conditions without using the bacteria as biotemplates.

2.3 Characterization

X-ray diffraction (XRD) measurements were carried out on a Bruker-AXS D8 Advance instrument operating at a voltage of 40 kV and a current of 40 mA with Cu Kα radiation (λ = 1.5406 Å). Field-emission scanning electron microscopy (FESEM, FEI sirion 200) was operated under 5.0Kv (Acc.V), and the spot size (SE) was 3.0; the work distance (WD) was 13.5 mm. Transmission electron microscopy (TEM) measurements were performed on a JEM-2010 transmission electron microscope operated at an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA) was conducted on a TA 2050 thermo-analyzer with the heating rate of 15°C/min from ambient temperature to 700°C. Nitrogen adsorption–desorption isotherms were measured with a Micromeritics ASAP 2010 adsorption analyzer (Micromeritics Instrument Corp., Norcross, GA). UV-vis absorption spectroscopy was recorded using a Varian Cary UV-vis-NIR spectrophotometer in the spectral range 200–800 nm. 0.25 g of each sample was pressed between two pieces of quartz glass within the 3 × 3 cm area to cover the aperture through which the excitation light passed. A BaSiO4 plate was used as the basic line for the spectra. The same quality for the all of the samples was taken during the measurement to delineate the structural effects and to diminish the data error at the maximum.

2.4 Photocatalytic hydrogen and oxygen evolution experiments

Irradiation was performed at 25°C using a Xe arc lamp (400 mW/cm2). Quantum flux was measured in the flask by ferrioxalate actinometry. Photoreduction of H+ to H2 was performed by dispersing 100 mg of the catalyst powder in an aqueous methanol solution (MeOH, 20 vol %, 50ml) as the sacrificial reagent without pH control, while photooxidation of H2O to O2 was conducted by dispersing 100 mg of the catalyst powder in an aqueous silver nitrate solution (AgNO3, 0.05 M, 50 ml). The temperature of the reactant solution was maintained at room temperature by a flow of cooling water during the reaction. The flask filled with 50 ml reaction solutions and 100 mg of the respective catalysts was connected to a gas chromatography system. The flask was evacuated and purged four times with argon gas, and the stirred mixture was then irradiated for 5 h with periodic removal of gas samples. Gas samples were analyzed with a Varian gas chromatograph employing a Supelco molecular 60/80 sieve 5A column with Ar as the carrier gas and a thermal conductivity detector (TCD).

3. Results and discussions

3.1 Formation mechanisms

We choose a very familiar bacteria lactobacillus as biotemplates first to demonstrate the biotemplating synthesis of biomorphic replicas. Here, two species of lactobacillus Streptococcus thermophilus (Str. theromophilus) and Lactobacillus bulgaricus (L. bulgaricus) were used as templates to direct the formation of biomorphic TiO2 hollow spheres and tubes via the surface sol-gel process, respectively. As illustrated in Fig. 1
Fig. 1 Schematic illustration of the biotemplating synthesis of biomorphic hollow structures via the surface sol-gel process.
, this process is based on chemical adsorption of metal alkoxide from solution onto functionalized cell wall surfaces to form a covalently bound layer, followed by hydrolysis to give a gel layer. TiO2 gel layers are deposited to coat the individual bacteria by repeating the surface sol-gel deposition cycle. Layer thickness can be controlled by the concentration of the sol-gel precursor solution and the repeating times. Finally, biomorphic replicas are obtained after calcinations, retaining the original morphologies of the bacteria templates.

The possible coordination interactions between bacteria and precursors are shown in the green pane. Bacteria possess abundant functional groups on the cell wall surfaces, and provide a suitable substrate for metal oxide deposition via the surface sol-gel process. The cell wall of Str. theromophilus, L. bulgaricus and many gram-positive bacteria is primarily made up of peptidoglycan (PG), which is a polymer of N-acelglucosamine and N-acetylmuramic acid. The two other important constituents are teichoic acid and teichuronic acid. Additionally, S-layer proteins present as the outermost component of the cell wall are reported to function as templates in the natural mineralization process and are known to bind nanoparticles [21

21. S. Schultze-Lam, G. Harauz, and T. J. Beveridge, “Participation of a cyanobacterial S layer in fine-grain mineral formation,” J. Bacteriol. 174(24), 7971–7981 (1992). [PubMed]

]. These biological components are covered predominantly with carboxyls (R-COOH), phosphomonoesters (R-OPO3H2), phosphodiesters ((RO)2-P(OH)2), amines (R-NH3+), and hydroxyls (R-OH) [22

22. W. Jiang, A. Saxena, B. Song, B. B. Ward, T. J. Beveridge, and S. C. B. Myneni, “Elucidation of functional groups on gram-positive and gram-negative bacterial surfaces using infrared spectroscopy,” Langmuir 20(26), 11433–11442 (2004). [CrossRef] [PubMed]

] which are able to bind metal anions through coordination interactions. Our results also represent an extension of the biotemplating synthesis of various hollow structures templated with other shapes of bacterium such as vibrios, spirillum, square bacteria, fusiform bacilli, and so forth, leading to the formation of corresponding hollow structures with controllable morphologies and dimensions. We believe that a similar approach is applicable to other materials like SnO2 [23

23. J. Huang, N. Matsunaga, K. Shimanoe, N. Yamazoe, and T. Kunitake, “Nanotubular SnO2 templated by cellulose fibers: synthesis and gas sensing,” Chem. Mater. 17(13), 3513–3518 (2005). [CrossRef]

], ZrO2 [24

24. H. Ogihara, M. Sadakane, Y. Nodasaka, and W. Ueda, “Shape-controlled synthesis of ZrO2, A2O3, and SiO2 nanotubes using carbon nanofibers as templates,” Chem. Mater. 18(21), 4981–4983 (2006). [CrossRef]

], Ga2O3 [25

25. X. Sun, J. Liu, and Y. Li, “Use of carbonaceous polysaccharide microspheres as templates for fabricating metal oxide hollow spheres,” Chemistry 12(7), 2039–2047 (2006). [CrossRef] [PubMed]

], Cr2O3 [25

25. X. Sun, J. Liu, and Y. Li, “Use of carbonaceous polysaccharide microspheres as templates for fabricating metal oxide hollow spheres,” Chemistry 12(7), 2039–2047 (2006). [CrossRef] [PubMed]

, 26

26. C. Lin, Y. Li, M. Yu, P. Yang, and J. Lin, “A facile synthesis and characterization of monodisperse spherical pigment particles with a core/shell structure,” Adv. Funct. Mater. 17(9), 1459–1465 (2007). [CrossRef]

], CeO2 [25

25. X. Sun, J. Liu, and Y. Li, “Use of carbonaceous polysaccharide microspheres as templates for fabricating metal oxide hollow spheres,” Chemistry 12(7), 2039–2047 (2006). [CrossRef] [PubMed]

], Fe2O3 [26

26. C. Lin, Y. Li, M. Yu, P. Yang, and J. Lin, “A facile synthesis and characterization of monodisperse spherical pigment particles with a core/shell structure,” Adv. Funct. Mater. 17(9), 1459–1465 (2007). [CrossRef]

], ZnCo2O4 [26

26. C. Lin, Y. Li, M. Yu, P. Yang, and J. Lin, “A facile synthesis and characterization of monodisperse spherical pigment particles with a core/shell structure,” Adv. Funct. Mater. 17(9), 1459–1465 (2007). [CrossRef]

], etc. via the surface sol-gel process.

3.2 Characterization of the TiO2 hollow structures

The calcination process was traced to suggest the formation of TiO2 hollow structures by conducting TGA and DTA surveys in Fig. 2(a)
Fig. 2 (a) TGA and DTA curves of the bacterial template. (b) XRD patterns of TiO2 hollow spheres after calcination at 700þC, with the inset of the hybrids without calcination.
. As shown in the TGA curve, the major removal of water occurs between 60 and 100 °C (as observed from the endothermic peak in the DTA curve). DTA curve shows two exothermic peaks at 365.3 °C and 549.2 °C, which should represent the decompositions of the precursors and the bacterial template, respectively. It is explicit that the bacteria templates absolutely pyrolyzed by 700°C. So the calcination temperature is fixed at 700 °C in order to remove the organic template completely. Crystallographic characterization was performed with XRD in Fig. 2(b). The results indicate that the hybrids without calcination are amorphous. The samples calcined at 700 °C were mainly of anatase forms with a little of rutile forms.

Figure 3(a)
Fig. 3 FESEM images of (a) original templates of Str. theromophilus. (b) bacteria/TiO2 gel hybrid spheres using Str. Theromophilus as templates, with the inset of a magnified image. The surface sol-gel deposition was repeated five times. (c) TEM image of an individual bacteria/TiO2 gel hybrid diplo-spheres by templating of a duplicating Str. Theromophilus cell. The surface sol-gel deposition was repeated five times. (d) FESEM image of TiO2 hollow spheres with five repeating cycles. (e) TEM image of TiO2 hollow spheres with five repeating cycles, (f) an individual TiO2 hollow sphere with three repeating cycles, with the inset of the SAED pattern.
indicates that the original morphology of Str. theromophilus is approximately spherical with the diameter of about 500 nm. The size can be controlled by choosing different growth period. After contacting with the precursors, the cell walls were covered with a thin layer of amorphous titania, as shown in Fig. 3(b). The diameter of the bacteria/TiO2 gel hybrid spheres is about 500nm estimated by the magnified image in the inset of Fig. 3(b). Figure 3(c) is a TEM image of an individual bacteria/TiO2 gel hybrid diplo-sphere by templating of a duplicating cell. As indicated by the arrowheads, the titania gel wall can be identified with a thickness of about 50nm. After calcination at 700°C, TiO2 hollow spheres are obtained with conservation of the spherical geometry of the original template as shown in Fig. 3(d). Some broken spheres with apparent cavity demonstrate the hollow nature of the products. The surface sol-gel deposition was repeated five times for this sample. The hollow structure of the as-prepared TiO2 spheres is further confirmed by TEM. The pale center together with the dark edge is the evidence for the hollow structure (Fig. 3(e)). Figure 3(f) shows an individual TiO2 hollow sphere with three repeating cycles with the shell thickness of about 15 nm.

Similarly, L. bulgaricus, a rod-shaped bacterium was also used as templates to synthesize hollow tubes. The size of L. bulgaricus is approximately 200-500 nm wide and 1µm long as shown in the inset of Fig. 4(a)
Fig. 4 FESEM image of (a) bacteria/TiO2 gel hybrid tubes using L. bulgaricus as the templates, with the inset of a TEM image of the template L. bulgaricus (b) TEM image of an individual TiO2 nanotube by calcination of the bacteria/ TiO2 gel hybrid tubes at 700°C. The surface sol-gel deposition was repeated five times for these samples.
. After contacting with the precursors, bacteria/TiO2 gel hybrid tubes were obtained as shown in Fig. 4(a). Figure 4(b) shows an individual biomorphic TiO2 hollow nanotube yielded by calcination with a size of about 200-300 nm wide and 800nm long, which retains the original morphology of the template. The surface sol-gel deposition was repeated five times for this sample. The shell thickness is about 30-40 nm.

N2 adsorption-desorption isotherms were employed to further examine the porous nature of the biomorphic TiO2 hollow structures as shown in Fig. 5(a)
Fig. 5 (a) Nitrogen adsorption-desorption isotherm and BJH pore size distribution plot (inset) of TiO2 hollow spheres. (b) UV-Vis absorption spectra of biomorphic TiO2 hollow spheres and TiO2 nanoparticles. (c) schematic illustration of light pathway within hollow structures. (1) multiple scattering within hollow cavity. (2) light incident on smaller radii voids and is uniformly scattered and (3) forward scattering may be greater than backward scattering in the case of higher radii voids.
. The isotherm exhibit type III characteristics with a type H3 loop, which is usually indicative of broad size-distribution of pore sizes extending into the large mesopore and/or macropore range [27

27. W. C. Li, A. H. Lu, C. Weidenthaler, and F. Schuth, “Hard-templating pathway to create mesoporous magnesium oxide,” Chem. Mater. 16(26), 5676–5681 (2004). [CrossRef]

]. The Barrett-Joyner-Halenda (BJH) pore size distribution (the inset in Fig. 5(a)) shows that the porous biomorphic TiO2 hollow spheres are basically mesoporous with a pore size distribution peak around 14 nm while macropores are also observed in PSDs with a small contribution.

3.3 Light-harvesting property

Light-harvesting properties were characterized by UV-vis spectroscopy. Figure 5(b) compares UV-Vis absorption spectra of biomorphic TiO2 hollow spheres and TiO2 nanoparticles synthesized under the same experimental condition but without using biotemplates. 0.25 g of each sample was pressed between two pieces of quartz glass for measurements. It is obviously that TiO2 hollow spheres have strongly enhanced light-harvesting in the UV and visible region than the solid counterpart, which is probably attributed to their unique porous hollow structures. Hendricks and Howell [28

28. T. J. Hendricks and J. R. Howell, “New radiative analysis approach for reticulated porous ceramics using discrete ordinates method,” J. Heat Transfer 118(4), 911–917 (1996). [CrossRef]

, 29

29. T. J. Hendricks and J. R. Howell, “Adsorption/scattering coefficient and scattering phase functions in reticulated porous ceramics,” J. Heat Transfer 118(1), 79–87 (1996). [CrossRef]

] indicate that the porous (hollow) structure of ceramics creates complex electromagnetic scattering and interference patterns within the structure. The nature of the scattering process depends on the void size and the wavelength of light. As the wavelength approaches the pore size, multiple scattering processes arises [30

30. C. Dodson, J. Spicer, M. Fitch, P. Schuster, and R. Osiander, “Propagation of terahertz radiation in porous polymer and ceramic materials,” AIP Conf. Proc. 760, 562–569 (2005). [CrossRef]

, 31

31. W. M. Robertson, G. Arjavalingam, and S. L. Shinde, “Microwave dielectric measurements of zirconia-alumina ceramic composites: a test of the Clausius–Mossotti mixture equations,” J. Appl. Phys. 70(12), 7648–7650 (1991). [CrossRef]

]. In our study, the synthesized hollow spheres have a cavity diameter of about 500nm~1µm, which is comparable with the wavelength of UV and visible light (200-800nm). So the enhanced optical absorption of biomorphic TiO2 hollow spheres partially accounts for multiple-and back-scattering effects at the large voids (hollow cavity) as illustrated in Fig. 5(c)-(1). Simultaneously, when voids of sizes are extremely small compared to the wavelength of light, the Rayleigh scattering phenomenon takes place [32

32. M. Banerjee, S. K. Datta, and H. Saha, “Enhanced optical absorption in a thin silicon layer with nanovoids,” Nanotechnology 16(9), 1542–1548 (2005). [CrossRef]

, 33

33. H. Seel and R. Brendel, “Optical absorption in crystalline Si films containing spherical voids for internal light scattering,” Thin Solid Films 451–452, 608–611 (2004). [CrossRef]

]. Rayleigh scattering is characterized by the fact that the fraction of incident light energy which is scattered is distributed equally in the forward and backward hemispheres. The enhanced optical absorption in a thin silicon layer with nanovoids has also been reported [32

32. M. Banerjee, S. K. Datta, and H. Saha, “Enhanced optical absorption in a thin silicon layer with nanovoids,” Nanotechnology 16(9), 1542–1548 (2005). [CrossRef]

, 33

33. H. Seel and R. Brendel, “Optical absorption in crystalline Si films containing spherical voids for internal light scattering,” Thin Solid Films 451–452, 608–611 (2004). [CrossRef]

]. So the nanovoids of the exterior shells with sizes of several nanometers as demonstrated by N2 adsorption-desorption isotherms also provide an ideal pathway through which the electromagnetic wave can diffuse and scatter (Fig. 5c-(2)). What’s more, as shown in the inset of Fig. 5(a), there are still some voids with larger radius ranging from tens of nanometers to hundreds of nanometers. As for such voids, forward scattering may be greater than backward scattering (Fig. 5c-(3)). As a result, complicated scattering processes take place in hollow micro/nanostructures, thus, the structure and morphology of the materials can be correlated with the light-harvesting performance.

3.4 Photocatalytic hydrogen evolution

Photocatalytic studies for H2 evolution of our samples were carried out as described in the experimental section. Time-resolved plots for H2 evolution are presented in Fig. 6(a)
Fig. 6 (a). Hydrogen evolution from the samples in 20% aqueous methanol under UV and visible light irradiation, with the inset of the hydrogen evolution rates of four typical samples. (b). Schematic illustration of the photocatalytic hydrogen evolution processes.
, while numerical data are shown in Table 1

Table 1. Photochemical hydrogen evolution in 20% methanol (aq) under UV and visible light irradiation

table-icon
View This Table
. Apparent quantum efficiencies (QE = 2·[H2]/I) were calculated from the mean H2 evolution rate (mol/s) and the quantum flux I (mol/s) of the irradiation system. The sample of TiO2 hollow spheres evolves H2 at a rate of 0.57 µmol/h in pure water. The activity of it in pure water is low which might be attributed to the high recombination of the photogenerated electrons and holes as well as the back reaction between the produced H2 and O2. When the reaction was performed in 20% aqueous methanol, a known sacrificial electron donor [34

34. K. Domen, A. Kudo, M. Shibata, A. Tanaka, K. Maruya, and T. Onishi, “Novel photocatalysts, ion-exchanged K4Nb6O17, with a layer structure,” J. Chem. Soc. Chem. Commun. (23), 1706–1707 (1986). [CrossRef]

], the hydrogen evolution rate is about 110 times higher than in pure water, producing a total H2 amount of 312.75 µmol after 5 h, with the evolution rate of 62.55 µmol/h (QE = 0.69%). The activity is about 3.6 times higher than that of TiO2 nanoparticles synthesized without templates (QE = 0.193% in 20% aqueous methanol). The hydrogen evolution rate of TiO2 hollow tubes is 57.77 µmol/h, which is comparable with that of hollow spheres but a little lower. We also compare our samples with commercial Degussa P25 TiO2, one commercial photocatalyst with high activity and has been widely used as a standard TiO2 reference material. Our TiO2 hollow structured samples show higher catalytic activities than P25. The hydrogen evolution rate is about 1.5 times higher than P25. The hydrogen evolution rates of the four typical samples are compared in the inset of Fig. 6(a). The mechanism of the photochemical hydrogen evolution process is depicted in Fig. 6(b). Furthermore, O2 could also be detected as shown in Fig. 7
Fig. 7 Oxygen evolution from TiO2 hollow spheres in 0.05M aqueous silver nitrate solution under UV and visible light irradiation.
when the reaction was performed in 0.05 M silver nitrate aqueous solution, a known sacrificial hole donor. The decrease in activity with reaction time is primarily attributable to the deposition of metallic silver on the catalyst surface, which blocks light absorption and obstructs active sites [35

35. A. Ishikawa, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi, and K. Domen, “Oxysulfide Sm2Ti2S2O5 as a stable photocatalyst for water oxidation and reduction under visible light irradiation (λ ≤ 650 nm),” J. Am. Chem. Soc. 124(45), 13547–13553 (2002). [CrossRef] [PubMed]

].

Compared with TiO2 nanoparticles, the as-obtained TiO2 hollow spheres and hollow tubes are endowed with superior photocatalytic hydrogen evolution activities. This is mainly attributable to their unique structures. We have demonstrated the enhanced light harvesting in porous hollow spheres taking into account the Rayleigh scattering between small nanovoids and the gradual transition from Rayleigh scattering to diffraction phenomena in the case of large voids including multiple-scattering and back-scattering effects. So the enhanced light-harvesting abilities of hollow structures could capture more photons for the photocatalytic reactions. Meanwhile, the porous hollow structures could offer more absorption and reaction sites for the photocatalytic reactions and allow rapid diffusion of various reactants and products during the reaction. The differences of photocatalytic performances between hollow spheres and hollow tubes may result from their structures involving morphologies, specific surface areas, pore volumes, pore sizes and so forth. So that it’s possible to tune their photocatalytic performance by changing these structure parameters. Furthermore, the as-obtained TiO2 hollow structures have better photocatalytic hydrogen evolution activities than that of P25 TiO2, which shows that the TiO2 hollow structures may have potential commercial applications as catalysts.

4. Conclusions

To summarize, we have presented a general strategy for the facile synthesis of hollow TiO2 micro/nanostructures with bacteria as templates based on surface sol-gel process. We used hollow spheres and tubes as the prototypes to demonstrate the structural effect on improving the photocatalytic hydrogen evolution properties. The hollow TiO2 structures exhibit superior photocatalytic hydrogen evolution activities in the presence of sacrificial reagents under UV and visible light irradiation. The hydrogen evolution rates on average is 3.6 times higher than that of the corresponding solid counterpart and 1.5 times higher than P25-TiO2. This should attribute to their unique hollow structures with microsized hollow cavities and nanovoids which can bring about multiple scattering and rayleigh scattering of light, respectively. This bacteria-templated approach could provide a versatile pathway to various hollow micro/nanostructures including hollow cables, hollow twin spheres, hollow chain spheres, hollow squares and so forth. Thus, the structure of hollow micro/nanostructures may be varied to tailor and improve their catalytic properties over a broad range. The surface sol-gel process can also be extended to other materials such as SnO2, ZrO2, Ga2O3, Cr2O3 and so forth. The research may represent an easy way to design and synthesis of a new group of micro/nanostructured photocatalysts to harness light energy efficiently for renewable and clean energy resources.

Acknowledgments

This work is supported by the National Natural Science Foundation of China (50972090, 51172141 and 51102163), the Dawn Program of Shanghai Education Commission (08SG15), the Shanghai Rising star Program (10Qh1401300) and the Research Fund for the Doctoral Program of Higher Education (20100073110065 and 20110073120036).

References and links

1.

N. S. Lewis and D. G. Nocera, “Powering the planet: chemical challenges in solar energy utilization,” Proc. Natl. Acad. Sci. U.S.A. 103(43), 15729–15735 (2006). [CrossRef] [PubMed]

2.

M. Hambourger, G. F. Moore, D. M. Kramer, D. Gust, A. L. Moore, and T. A. Moore, “Biology and technology for photochemical fuel production,” Chem. Soc. Rev. 38(1), 25–35 (2009). [CrossRef] [PubMed]

3.

M. Woodhouse and B. A. Parkinson, “Combinatorial approaches for the identification and optimization of oxide semiconductors for efficient solar photoelectrolysis,” Chem. Soc. Rev. 38(1), 197–210 (2008). [CrossRef] [PubMed]

4.

F. E. Osterloh, “Inorganic materials as catalysts for photochemical splitting of water,” Chem. Mater. 20(1), 35–54 (2008). [CrossRef]

5.

H. Zhou, T. Fan, and D. Zhang, “An insight into artificial leaves for sustainable energy inspired by natural photosynthesis,” ChemCatChem 3(3), 513–528 (2011). [CrossRef]

6.

A. Fujishima and K. Honda, “Electrochemical photolysis of water at a semiconductor electrode,” Nature 238(5358), 37–38 (1972). [CrossRef] [PubMed]

7.

M. Ni, M. K. H. Leung, D. Y. C. Leung, and K. Sumathy, “A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production,” Renew. Sustain. Energy Rev. 11(3), 401–425 (2007). [CrossRef]

8.

X. Chen and S. S. Mao, “Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications,” Chem. Rev. 107(7), 2891–2959 (2007). [CrossRef] [PubMed]

9.

I. Kontos, V. Likodimos, T. Stergiopoulos, D. S. Tsoukleris, P. Falaras, I. Rabias, G. Papavassiliou, D. Kim, J. Kunze, and P. Schmuki, “Self-organized anodic TiO2 nanotube arrays functionalized by iron oxide nanoparticles,” Chem. Mater. 21(4), 662–672 (2009). [CrossRef]

10.

H. Zhou, E. M. Sabio, T. K. Townsend, T. Fan, D. Zhang, and F. E. Osterloh, “Assembly of core-shell structures for photocatalytic hydrogen evolution from aqueous methanol,” Chem. Mater. 22(11), 3362–3368 (2010). [CrossRef]

11.

Z. Liu, D. D. Sun, P. Guo, and J. O. Leckie, “One-step fabrication and high photocatalytic activity of porous TiO2 hollow aggregates by using a low-temperature hydrothermal method without templates,” Chemistry 13(6), 1851–1855 (2007). [CrossRef] [PubMed]

12.

Z. Wu, F. Dong, W. Zhao, H. Wang, Y. Liu, and B. Guan, “The fabrication and characterization of novel carbon doped TiO2 nanotubes, nanowires and nanorods with high visible light photocatalytic activity,” Nanotechnology 20(23), 235701 (2009). [CrossRef] [PubMed]

13.

S. Xuan, W. Jiang, X. Gong, Y. Hu, and Z. Chen, “Magnetically separable Fe3O4/TiO2 hollow spheres: fabrication and photocatalytic activity,” J. Phys. Chem. C 113(2), 553–558 (2009). [CrossRef]

14.

T. Fan, S. K. Chow, and D. Zhang, “Biomorphic mineralization: from biology to materials,” Prog. Mater. Sci. 54(5), 542–659 (2009). [CrossRef]

15.

H. Zhou, T. Fan, and D. Zhang, “Biotemplated materials for sustainable energy and environment: Current status and challenges,” ChemSusChem 4(10), 1344–1387 (2011). [CrossRef] [PubMed]

16.

J. He and T. Kunitake, “Preparation and thermal stability of gold nanoparticles in silk-templated porous filaments of titania and zirconia,” Chem. Mater. 16(13), 2656–2661 (2004). [CrossRef]

17.

J. Huang and T. Kunitake, “Nano-precision replication of natural cellulosic substances by metal oxides,” J. Am. Chem. Soc. 125(39), 11834–11835 (2003). [CrossRef] [PubMed]

18.

S. R. Hall, H. Bolger, and S. Mann, “Morphosynthesis of complex inorganic forms using pollen grain templates,” Chem. Commun. (Camb.) (22), 2784–2785 (2003). [CrossRef] [PubMed]

19.

H. Zhou, X. Li, T. Fan, F. E. Osterloh, J. Ding, E. M. Sabio, D. Zhang, and Q. Guo, “Artificial inorganic leafs for efficient photochemical hydrogen production inspired by natural photosynthesis,” Adv. Mater. (Deerfield Beach Fla.) 22(9), 951–956 (2010). [CrossRef] [PubMed]

20.

X. Li, T. Fan, H. Zhou, S. K. Chow, W. Zhang, D. Zhang, Q. Guo, and H. Ogawa, “Enhanced light-harvesting and photocatalytic properties in morph-TiO2 from green leaf biotemplates,” Adv. Funct. Mater. 19(1), 45–56 (2009). [CrossRef]

21.

S. Schultze-Lam, G. Harauz, and T. J. Beveridge, “Participation of a cyanobacterial S layer in fine-grain mineral formation,” J. Bacteriol. 174(24), 7971–7981 (1992). [PubMed]

22.

W. Jiang, A. Saxena, B. Song, B. B. Ward, T. J. Beveridge, and S. C. B. Myneni, “Elucidation of functional groups on gram-positive and gram-negative bacterial surfaces using infrared spectroscopy,” Langmuir 20(26), 11433–11442 (2004). [CrossRef] [PubMed]

23.

J. Huang, N. Matsunaga, K. Shimanoe, N. Yamazoe, and T. Kunitake, “Nanotubular SnO2 templated by cellulose fibers: synthesis and gas sensing,” Chem. Mater. 17(13), 3513–3518 (2005). [CrossRef]

24.

H. Ogihara, M. Sadakane, Y. Nodasaka, and W. Ueda, “Shape-controlled synthesis of ZrO2, A2O3, and SiO2 nanotubes using carbon nanofibers as templates,” Chem. Mater. 18(21), 4981–4983 (2006). [CrossRef]

25.

X. Sun, J. Liu, and Y. Li, “Use of carbonaceous polysaccharide microspheres as templates for fabricating metal oxide hollow spheres,” Chemistry 12(7), 2039–2047 (2006). [CrossRef] [PubMed]

26.

C. Lin, Y. Li, M. Yu, P. Yang, and J. Lin, “A facile synthesis and characterization of monodisperse spherical pigment particles with a core/shell structure,” Adv. Funct. Mater. 17(9), 1459–1465 (2007). [CrossRef]

27.

W. C. Li, A. H. Lu, C. Weidenthaler, and F. Schuth, “Hard-templating pathway to create mesoporous magnesium oxide,” Chem. Mater. 16(26), 5676–5681 (2004). [CrossRef]

28.

T. J. Hendricks and J. R. Howell, “New radiative analysis approach for reticulated porous ceramics using discrete ordinates method,” J. Heat Transfer 118(4), 911–917 (1996). [CrossRef]

29.

T. J. Hendricks and J. R. Howell, “Adsorption/scattering coefficient and scattering phase functions in reticulated porous ceramics,” J. Heat Transfer 118(1), 79–87 (1996). [CrossRef]

30.

C. Dodson, J. Spicer, M. Fitch, P. Schuster, and R. Osiander, “Propagation of terahertz radiation in porous polymer and ceramic materials,” AIP Conf. Proc. 760, 562–569 (2005). [CrossRef]

31.

W. M. Robertson, G. Arjavalingam, and S. L. Shinde, “Microwave dielectric measurements of zirconia-alumina ceramic composites: a test of the Clausius–Mossotti mixture equations,” J. Appl. Phys. 70(12), 7648–7650 (1991). [CrossRef]

32.

M. Banerjee, S. K. Datta, and H. Saha, “Enhanced optical absorption in a thin silicon layer with nanovoids,” Nanotechnology 16(9), 1542–1548 (2005). [CrossRef]

33.

H. Seel and R. Brendel, “Optical absorption in crystalline Si films containing spherical voids for internal light scattering,” Thin Solid Films 451–452, 608–611 (2004). [CrossRef]

34.

K. Domen, A. Kudo, M. Shibata, A. Tanaka, K. Maruya, and T. Onishi, “Novel photocatalysts, ion-exchanged K4Nb6O17, with a layer structure,” J. Chem. Soc. Chem. Commun. (23), 1706–1707 (1986). [CrossRef]

35.

A. Ishikawa, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi, and K. Domen, “Oxysulfide Sm2Ti2S2O5 as a stable photocatalyst for water oxidation and reduction under visible light irradiation (λ ≤ 650 nm),” J. Am. Chem. Soc. 124(45), 13547–13553 (2002). [CrossRef] [PubMed]

OCIS Codes
(160.6000) Materials : Semiconductor materials
(350.5130) Other areas of optics : Photochemistry

ToC Category:
Photocatalysis

History
Original Manuscript: September 8, 2011
Revised Manuscript: December 11, 2011
Manuscript Accepted: December 11, 2011
Published: March 9, 2012

Virtual Issues
Vol. 7, Iss. 5 Virtual Journal for Biomedical Optics

Citation
Han Zhou, Tongxiang Fan, Jian Ding, Di Zhang, and Qixin Guo, "Bacteria-directed construction of hollow TiO2 micro/nanostructures with enhanced photocatalytic hydrogen evolution activity," Opt. Express 20, A340-A350 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-S2-A340


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References

  1. N. S. Lewis and D. G. Nocera, “Powering the planet: chemical challenges in solar energy utilization,” Proc. Natl. Acad. Sci. U.S.A.103(43), 15729–15735 (2006). [CrossRef] [PubMed]
  2. M. Hambourger, G. F. Moore, D. M. Kramer, D. Gust, A. L. Moore, and T. A. Moore, “Biology and technology for photochemical fuel production,” Chem. Soc. Rev.38(1), 25–35 (2009). [CrossRef] [PubMed]
  3. M. Woodhouse and B. A. Parkinson, “Combinatorial approaches for the identification and optimization of oxide semiconductors for efficient solar photoelectrolysis,” Chem. Soc. Rev.38(1), 197–210 (2008). [CrossRef] [PubMed]
  4. F. E. Osterloh, “Inorganic materials as catalysts for photochemical splitting of water,” Chem. Mater.20(1), 35–54 (2008). [CrossRef]
  5. H. Zhou, T. Fan, and D. Zhang, “An insight into artificial leaves for sustainable energy inspired by natural photosynthesis,” ChemCatChem3(3), 513–528 (2011). [CrossRef]
  6. A. Fujishima and K. Honda, “Electrochemical photolysis of water at a semiconductor electrode,” Nature238(5358), 37–38 (1972). [CrossRef] [PubMed]
  7. M. Ni, M. K. H. Leung, D. Y. C. Leung, and K. Sumathy, “A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production,” Renew. Sustain. Energy Rev.11(3), 401–425 (2007). [CrossRef]
  8. X. Chen and S. S. Mao, “Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications,” Chem. Rev.107(7), 2891–2959 (2007). [CrossRef] [PubMed]
  9. I. Kontos, V. Likodimos, T. Stergiopoulos, D. S. Tsoukleris, P. Falaras, I. Rabias, G. Papavassiliou, D. Kim, J. Kunze, and P. Schmuki, “Self-organized anodic TiO2 nanotube arrays functionalized by iron oxide nanoparticles,” Chem. Mater.21(4), 662–672 (2009). [CrossRef]
  10. H. Zhou, E. M. Sabio, T. K. Townsend, T. Fan, D. Zhang, and F. E. Osterloh, “Assembly of core-shell structures for photocatalytic hydrogen evolution from aqueous methanol,” Chem. Mater.22(11), 3362–3368 (2010). [CrossRef]
  11. Z. Liu, D. D. Sun, P. Guo, and J. O. Leckie, “One-step fabrication and high photocatalytic activity of porous TiO2 hollow aggregates by using a low-temperature hydrothermal method without templates,” Chemistry13(6), 1851–1855 (2007). [CrossRef] [PubMed]
  12. Z. Wu, F. Dong, W. Zhao, H. Wang, Y. Liu, and B. Guan, “The fabrication and characterization of novel carbon doped TiO2 nanotubes, nanowires and nanorods with high visible light photocatalytic activity,” Nanotechnology20(23), 235701 (2009). [CrossRef] [PubMed]
  13. S. Xuan, W. Jiang, X. Gong, Y. Hu, and Z. Chen, “Magnetically separable Fe3O4/TiO2 hollow spheres: fabrication and photocatalytic activity,” J. Phys. Chem. C113(2), 553–558 (2009). [CrossRef]
  14. T. Fan, S. K. Chow, and D. Zhang, “Biomorphic mineralization: from biology to materials,” Prog. Mater. Sci.54(5), 542–659 (2009). [CrossRef]
  15. H. Zhou, T. Fan, and D. Zhang, “Biotemplated materials for sustainable energy and environment: Current status and challenges,” ChemSusChem4(10), 1344–1387 (2011). [CrossRef] [PubMed]
  16. J. He and T. Kunitake, “Preparation and thermal stability of gold nanoparticles in silk-templated porous filaments of titania and zirconia,” Chem. Mater.16(13), 2656–2661 (2004). [CrossRef]
  17. J. Huang and T. Kunitake, “Nano-precision replication of natural cellulosic substances by metal oxides,” J. Am. Chem. Soc.125(39), 11834–11835 (2003). [CrossRef] [PubMed]
  18. S. R. Hall, H. Bolger, and S. Mann, “Morphosynthesis of complex inorganic forms using pollen grain templates,” Chem. Commun. (Camb.) (22), 2784–2785 (2003). [CrossRef] [PubMed]
  19. H. Zhou, X. Li, T. Fan, F. E. Osterloh, J. Ding, E. M. Sabio, D. Zhang, and Q. Guo, “Artificial inorganic leafs for efficient photochemical hydrogen production inspired by natural photosynthesis,” Adv. Mater. (Deerfield Beach Fla.)22(9), 951–956 (2010). [CrossRef] [PubMed]
  20. X. Li, T. Fan, H. Zhou, S. K. Chow, W. Zhang, D. Zhang, Q. Guo, and H. Ogawa, “Enhanced light-harvesting and photocatalytic properties in morph-TiO2 from green leaf biotemplates,” Adv. Funct. Mater.19(1), 45–56 (2009). [CrossRef]
  21. S. Schultze-Lam, G. Harauz, and T. J. Beveridge, “Participation of a cyanobacterial S layer in fine-grain mineral formation,” J. Bacteriol.174(24), 7971–7981 (1992). [PubMed]
  22. W. Jiang, A. Saxena, B. Song, B. B. Ward, T. J. Beveridge, and S. C. B. Myneni, “Elucidation of functional groups on gram-positive and gram-negative bacterial surfaces using infrared spectroscopy,” Langmuir20(26), 11433–11442 (2004). [CrossRef] [PubMed]
  23. J. Huang, N. Matsunaga, K. Shimanoe, N. Yamazoe, and T. Kunitake, “Nanotubular SnO2 templated by cellulose fibers: synthesis and gas sensing,” Chem. Mater.17(13), 3513–3518 (2005). [CrossRef]
  24. H. Ogihara, M. Sadakane, Y. Nodasaka, and W. Ueda, “Shape-controlled synthesis of ZrO2, A2O3, and SiO2 nanotubes using carbon nanofibers as templates,” Chem. Mater.18(21), 4981–4983 (2006). [CrossRef]
  25. X. Sun, J. Liu, and Y. Li, “Use of carbonaceous polysaccharide microspheres as templates for fabricating metal oxide hollow spheres,” Chemistry12(7), 2039–2047 (2006). [CrossRef] [PubMed]
  26. C. Lin, Y. Li, M. Yu, P. Yang, and J. Lin, “A facile synthesis and characterization of monodisperse spherical pigment particles with a core/shell structure,” Adv. Funct. Mater.17(9), 1459–1465 (2007). [CrossRef]
  27. W. C. Li, A. H. Lu, C. Weidenthaler, and F. Schuth, “Hard-templating pathway to create mesoporous magnesium oxide,” Chem. Mater.16(26), 5676–5681 (2004). [CrossRef]
  28. T. J. Hendricks and J. R. Howell, “New radiative analysis approach for reticulated porous ceramics using discrete ordinates method,” J. Heat Transfer118(4), 911–917 (1996). [CrossRef]
  29. T. J. Hendricks and J. R. Howell, “Adsorption/scattering coefficient and scattering phase functions in reticulated porous ceramics,” J. Heat Transfer118(1), 79–87 (1996). [CrossRef]
  30. C. Dodson, J. Spicer, M. Fitch, P. Schuster, and R. Osiander, “Propagation of terahertz radiation in porous polymer and ceramic materials,” AIP Conf. Proc.760, 562–569 (2005). [CrossRef]
  31. W. M. Robertson, G. Arjavalingam, and S. L. Shinde, “Microwave dielectric measurements of zirconia-alumina ceramic composites: a test of the Clausius–Mossotti mixture equations,” J. Appl. Phys.70(12), 7648–7650 (1991). [CrossRef]
  32. M. Banerjee, S. K. Datta, and H. Saha, “Enhanced optical absorption in a thin silicon layer with nanovoids,” Nanotechnology16(9), 1542–1548 (2005). [CrossRef]
  33. H. Seel and R. Brendel, “Optical absorption in crystalline Si films containing spherical voids for internal light scattering,” Thin Solid Films451–452, 608–611 (2004). [CrossRef]
  34. K. Domen, A. Kudo, M. Shibata, A. Tanaka, K. Maruya, and T. Onishi, “Novel photocatalysts, ion-exchanged K4Nb6O17, with a layer structure,” J. Chem. Soc. Chem. Commun. (23), 1706–1707 (1986). [CrossRef]
  35. A. Ishikawa, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi, and K. Domen, “Oxysulfide Sm2Ti2S2O5 as a stable photocatalyst for water oxidation and reduction under visible light irradiation (λ ≤ 650 nm),” J. Am. Chem. Soc.124(45), 13547–13553 (2002). [CrossRef] [PubMed]

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