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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 17 — Aug. 16, 2010
  • pp: 18401–18406
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Synthesis of ZnO nanoflowers and their wettabilities and photocatalytic properties

Xiaodong Guo, Quanzhong Zhao, Ruxin Li, Huaihai Pan, Xiaoyang Guo, Anyuan Yin, and Weilin Dai  »View Author Affiliations


Optics Express, Vol. 18, Issue 17, pp. 18401-18406 (2010)
http://dx.doi.org/10.1364/OE.18.018401


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Abstract

By combing laser direct writing and hydrothermal growth, we demonstrate the growth of three-dimensional flowerlike ZnO nanostructures from aqueous solution. Our approach offers synthetic flexibility in controlling film architecture, coating texture and crystallite size. The wettability is studied by measurement of time-dependent contact angles in the as-grown samples. In addition, superior photocatalytic activity of the flowerlike ZnO nanostructures in the degradation of Rhodamine B is investigated as well. The influence factors and formation mechanism of the flowerlike ZnO nanostructures are also analyzed and discussed.

© 2010 OSA

1. Introduction

As well known, the wetting behavior of a solid surface is one of the most important aspects of surface physical chemistry, which affects the degree of interaction a surface with the surrounding species. According to Cassie’s law [18

18. A. B. D. Cassie and S. Baxter, “Wettability of porous surfaces,” Trans. Faraday Soc. 40, 546–551 (1944). [CrossRef]

], wettability is determined by both the chemical composition and the geometrical structure of solid surfaces. It has been reported that the CF3 terminated surfaces possess the lowest free energy and thus the highest hydrophobicity. Furthermore, research has been focused on tunable wettability by controlling surface topographic structures in order to enhance long-life functional durability. Recent studies have revealed that micro- and nanostructures exhibiting hierarchical roughness cause an amplification of photoresponsive contact angle (CA) changing [19

19. R. Rosario, D. Gust, A. A. Garcia, M. Hayes, J. L. Taraci, J. W. Dailey, and S. T. Picraux, “Lotus effect amplifies light-induced contact angle switching,” J. Phys. Chem. B 108(34), 12640–12642 (2004). [CrossRef]

].

On the other hand, nanoscale materials are also believed to perform much better than their bulk counterparts in photocatalytic properties due to the higher surface-to-volume ratio and higher redox potentials. 3D micro/nanocomposite structures would possess high specific area on the surface of the particles, and likely be an ideal host material for catalysis, water treatments, and other highly functional and effective devices [20

20. L. S. Zhong, J. S. Hu, H. P. Liang, A. M. Cao, W. G. Song, and L. J. Wan, “Self-assembled 3D flowerlike iron oxide nanostructures and their application in water treatment,” Adv. Mater. 18(18), 2426–2431 (2006). [CrossRef]

22

22. Y. B. Li, Y. Bando, and D. Golberg, “MoS2 nanoflowers and their field-emission properties,” Appl. Phys. Lett. 82(12), 1962–1964 (2003). [CrossRef]

]. In the photocatalysis processes, the illuminated semiconductors catalyst absorb light and generate electronic species which lead to complete oxidation of organic components in wastewaters. Among the wide band gap semiconductors catalyst employed, ZnO and TiO2 are the most extensively used photocatalyst due to their high photocatalytic activity. Although TiO2 is considered as the most active photocatalyst, ZnO also can be a suitable alternative to TiO2 because its cost is lower and it has a similar band gap energy. In addition, the potential of electron derived from ZnO is more negative than that generated by TiO2, thus ZnO has a stronger electron affinity.

In this paper, we report the selective growth 3D flowerlike ZnO nanostructures by combining laser direct writing and hydrothermal method. The wettabilities and photocatalytic activities of the flowerlike ZnO nanostructures are studied. This work provides a new way to design desirable micro/nanoarchitectures. It is also worth to mention that our technique is a very potential route in the preparation of metal and semiconductor nanostructures on various material substrate. This controllable technique opens the possibility of creating novel nanostructures for future functional devices.

2. Experimental section

As shown in the left of Fig. 1
Fig. 1 Fabrication process of 3D flowerlike nanostructures: deposition of a PMMA layer on a GaN/LiALO2 substrate, laser processing and hydrothermal growth of ZnO nanostructure. The right side shows the schematic diagram of photoreaction apparatus.
, 3D
Fig. 3 CCD images of CA measurements of ZnO grid surfaces at different time: (a) 0 min, (b) 10 min, (c) 20 min, (d) 30 min, (e) 40 min, respectively (Media 1). (f) Water CA spectra of ZnO grid as a function of time, the time interval is 2 min.
flowerlike ZnO arrays were fabricated in three steps: substrate preparation, laser processing, and selective growth of nanostructures on the substrate. In a typical procedure, a 1 µm thick layer of PMMA was spun on the GaN/LiAlO2 substrate at a speed of 4000 rpm. After that, the substrate was baked on a hot plate at 100°C for 10 min. The second step was laser processing. A commercial regenerative amplified Ti:Sapphire laser (Spitfire, Spectra-Physics) with a pulse duration of 120 fs and a repetition rate of 1 kHz was used in this experiment. The sample was mounted on a computer-controlled x-y-z translation stage. The surface of the sample was positioned perpendicular to the propagation direction of the incident laser beam in the focal plane of a 100 × objective lens (NA = 0.8). The last step was selective growth ZnO nanostructures. The aqueous precursor was prepared by mixing zinc nitrate and hexamethyltetramine (HMT) at a molar ratio of 1:1 and a zinc concentration of 0.025 mol/L. After the pretreated substrates were immersed in the bottle filled with the above precursor solution, the whole system was heated from room temperature and maintained at the constant temperature of 90°C for 120 minutes with continuous stirring. After the system cooled down to room temperature, the substrate was washed with distilled water, and dried at room temperature.

The wettability of the ZnO nanostructure films was measured by means of the water contact angle using a contact angle system (SL200B) at ambient temperature of 10°C. After dropping a 2 μL water drops onto the film surface, images were recorded with a CCD camera after adjusting contrast and focus. Subsequently, the contact angles can be determined by the analysis system. The photocatalytic activity measurement was conducted by putting the substrate into 100 mL of Rhodamine B (2.5 × 10−7M) solution, as shown in the right of Fig. 1. The suspension was irradiated by using a 8 W fluorescent UV lamp (365 nm) under continuous stirring at room temperature. The lamp was positioned 10 cm away from the suspension, and parallel to the sample. Prior to the irradiation, the reaction mixture was kept in the dark for 10 min to ensure sufficient adsorption of the dye. Analytical samples for absorption measurement were taken out from the reaction suspension at different time intervals. A spectrophotometer (Jasco V570) was used to record the absorption spectra of the solutions.

3. Results and discussion

Figure 2
Fig. 2 FE-SEM images of ZnO flowerlike nanostructures (a) single flower, (b) flower with different size, (c) flower matrix, (d) continuous line-mode structures, (e) ZnO grid, (f) EDX analysis took from the flower petals, and the substrate (the inset), respectively.
shows the general morphologies of the product, the location at which the ZnO nanostructures is defined by the site of the irradiation dot. In Fig. 2a, a single flower has a diameter of 5 μm which is made up of many thin nanopetals. The nanopetals with smooth surfaces are about 100 nm in thickness; these edge-standing nanopetals provide necessary surface roughness as well as a highly porous structure to our later wettability engineering and film functionalization. In this dot, the substrate was irradiated by 20 femtosecond laser pulses with an energy of 1 μJ. Larger dots template the growth of larger flowers, and finally, the nanopetals were connected to each other to build the 3D flowerlike structures. Figure 2b clearly demonstrates that by tuning the laser irradiation conditions, the ablated dot can be size adjusted. The control of flower density is another important aspect in spatial organization. Figure 2c shows a matrix of flowers, the distance between the flowers can be mediated through the pre-determined dots. We further studied the relations between the laser parameters and structure. Interestingly, while the substrate was irradiated in line-scan mode at a speed of 10 μm/s and a pulse energy of 1 μJ, the flower grows out of the opening, and could form a correspondingly long range architecture along the irradiated lines. Figure 2d shows that the nanostructures extend along the laser scanning line, forming a continuous line of uniform thickness. Similarly, we successfully obtained a ZnO micro-grid by laser cross scanning (Fig. 2e). It is also worth to mention that no nanostructures are observed in the un-irradiated area. Clearly, judicious choice of pattern size offers control and flexibility in designing nanomaterial architectures. As shown in Fig. 1f, energy dispersive X-ray spectroscopy (EDX) analysis proves that only Zn and O exist in the product (the element C originates from the thin C layer sputtering on the sample in the test). During the hydrothermal growth process, the negative nature of the growth unit [Zn(OH)4 2-] will lead to different growth rates of planes. When there is no organic additive in solution, spherical ZnO particles will be easily developed because of the Ostwald ripening process [23

23. J. A. Marqusee and J. Ross, “Kinetics of phase transitions: Theory of Ostwald ripening,” J. Chem. Phys. 79(1), 373–378 (1983). [CrossRef]

]. In our experiments, hexamethyltetramine (HMT) is expected to serve as the organic template during the heating process, thus dynamically modifying the nucleation process. The substrate/crystal surface has a boundary layer of charged ions, with increasing concentration of Zn2+ and OH-, ZnO nuclei will be developed under the low precursor concentration and the action of HMT. The self-assembly of a number of active sites that trigger the nucleation at the interface, promoting the formation of petal crystals extending from the interface.

The study of photocatalytic activities is important for searching strategies to design functional nanostructures. The as-produced ZnO nanoflowers were investigated for the applicability in photodegrading organic dyes of Rhodamine B (RB). The use of this aromatic compound as a model dye is mainly due to their recurrent occurrence in the industrial field. The reaction time was scheduled to be 0, 1, 2, 3, 4 and 5 h, respectively. The degradation is monitored by studying the decrease in absorbance of RB and quantified by plotting a first order decay plot of the absorbance. As shown in Fig. 4a
Fig. 4 (a) UV-vis adsorption spectra of RB solution catalyzed by the ZnO nanostructure films under UV irradiation. (b) Degradation of the RB under UV irradiation as a function of time.
, the absorption spectra corresponding to the RB molecules at 552 nm decreases in intensity rapidly with the extension of exposure time. The decomposition rate is high within the first 2 h. With time evolution, the concentration of RB decreases, the probability of RB reacting with ZnO nanostructures decreases, and consequently, the decomposition rate decreases.

The photodegradation of RB solution accords with the exponential decay formula C(t) = C(0) exp (-kt) [28

28. J. Matos, J. Laine, and J. M. Hermann, “Synergy effect in the photocatalytic degradation of phenol on a suspended mixture of titania and activated carbon,” Appl. Catal. B 18(3–4), 281–291 (1998). [CrossRef]

], where C(t)is the relative concentration of RB at time t, C(0) is the initial relative concentration, and k is the photodegradation rate constant, the result is plotted and shown in Fig. 3b. The solid squares are the experimental data, and the solid line is the fitting curve. The rate constant of the degradation reaction for RB is 0.58 ± 0.03 h−1. It is worth to mention that, due to the very small thickness and low density of the nanostructures on the substrate, the total weight of the nanostructures that evolved in the photodegradation is less than 0.1 mg, which cannot be determined by our analytical balance. Further experiments are required to clarify the relationship between the weight of the nanostructures and the degradation rate, and to compare it with normal photocatalysts in degradation of organic dyes.

4. Conclusion

To summarize, a novel method was proposed for the selective growth of 3D ZnO flowerlike nanostructures. The flowerlike structure could be varied by adjusting hydrothermal reaction conditions and laser irradiated parameters. The wettability is studied by measurement of time-dependent contact angles. In addition, superior photocatalytic activity of the flowerlike ZnO nanostructures in the degradation of Rhodamine B has been demonstrated; the rate constant of the degradation reaction for RB is 0.58 ± 0.03 h−1. Our results will be expected to find vast applications in the synthesis of other similar oxide nanocrystals, and creating novel nanostructures for future functional devices.

Acknowledgement

References and links

1.

Y. S. Ding, X. F. Shen, S. Gomez, H. Luo, M. Aindow, and S. L. Suib, “Hydrothermal growth of manganese dioxide into three-dimensional hierarchical nanoarchitectures,” Adv. Funct. Mater. 16(4), 549–555 (2006). [CrossRef]

2.

A. Chen, X. Peng, K. Koczkur, and B. Miller, “Super-hydrophobic tin oxide nanoflowers,” Chem. Commun. (Camb.) (17): 1964–1965 (2004). [CrossRef]

3.

B. Liu and H. C. Zeng, “Mesoscale organization of CuO nanoribbons: formation of “dandelions”,” J. Am. Chem. Soc. 126(26), 8124–8125 (2004). [CrossRef] [PubMed]

4.

S. Park, J. H. Lim, S. W. Chung, and C. A. Mirkin, “Self-assembly of mesoscopic metal-polymer amphiphiles,” Science 303(5656), 348–351 (2004). [CrossRef] [PubMed]

5.

X. Wang, J. Zhuang, Q. Peng, and Y. D. Li, “A general strategy for nanocrystal synthesis,” Nature 437(7055), 121–124 (2005). [CrossRef] [PubMed]

6.

Y. P. Fang, A. W. Xu, A. M. Qin, and R. J. Yu, “Selective synthesis of hexagonal and tetragonal dysprosium orthophosphate nanorods by a hydrothermal method,” Cryst. Growth Des. 5(3), 1221–1225 (2005). [CrossRef]

7.

J. A. van Kan, A. A. Bettiol, and F. Watt, “Proton beam writing of three-dimensional nanostructures in hydrogen silsesquioxane,” Nano Lett. 6(3), 579–582 (2006). [CrossRef] [PubMed]

8.

J. W. Hsu, Z. R. Tian, N. C. Simmons, C. M. Matzke, J. A. Voigt, and J. A. Liu, “Directed spatial organization of zinc oxide nanorods,” Nano Lett. 5(1), 83–86 (2005). [CrossRef] [PubMed]

9.

S. Xu, Y. Wei, M. Kirkham, J. Liu, W. Mai, D. Davidovic, R. L. Snyder, and Z. L. Wang, “Patterned growth of vertically aligned ZnO nanowire arrays on inorganic substrates at low temperature without catalyst,” J. Am. Chem. Soc. 130(45), 14958–14959 (2008). [CrossRef] [PubMed]

10.

Y. J. Kim, C. H. Lee, Y. J. Hong, and G. C. Yi, “Controlled selective growth of ZnO nanorod and microrod arrays on Si substrates by a wet chemical method,” Appl. Phys. Lett. 89, 163128–163130 (2006.). [CrossRef]

11.

C. A. Mirkin, “Programming the assembly of two- and three-dimensional architectures with DNA and nanoscale inorganic building blocks,” Inorg. Chem. 39(11), 2258–2272 (2000). [CrossRef]

12.

A. Ivanisevic and C. A. Mirkin, ““Dip-Pen” nanolithography on semiconductor surfaces,” J. Am. Chem. Soc. 123(32), 7887–7889 (2001). [CrossRef] [PubMed]

13.

A. P. Joglekar, H. H. Liu, E. Meyhöfer, G. Mourou, and A. J. Hunt, “Optics at critical intensity: applications to nanomorphing,” Proc. Natl. Acad. Sci. U.S.A. 101(16), 5856–5861 (2004). [CrossRef] [PubMed]

14.

Y. F. Guan and A. J. Pedraza, “Synthesis and alignment of ZnO and ZnO nanoparticles by laser-assisted chemical vapor deposition,” Nanotechnology 19(4), 045609 (2008). [CrossRef] [PubMed]

15.

Q. Z. Zhao, S. Malzer, and L. J. Wang, “Formation of subwavelength periodic structures on tungsten induced by ultrashort laser pulses,” Opt. Lett. 32(13), 1932–1934 (2007). [CrossRef] [PubMed]

16.

X. D. Guo, R. X. Li, Y. Hang, Z. Z. Xu, B. K. Yu, Y. Dai, B. Lu, and X. W. Sun, “Coherent linking of periodic nano-ripples on a ZnO crystal surface induced by femtosecond laser pulses,” Appl. Phys., A Mater. Sci. Process. 94(2), 423–426 (2009). [CrossRef]

17.

F. F. Luo, B. Qian, G. Lin, J. Xu, Y. Liao, J. Song, H. Y. Sun, B. Zhu, J. R. Qiu, Q. Z. Zhao, and Z. Z. Xu, “Redistribution of elements in glass induced by a high-repetition-rate femtosecond laser,” Opt. Express 18(6), 6262–6269 (2010). [CrossRef] [PubMed]

18.

A. B. D. Cassie and S. Baxter, “Wettability of porous surfaces,” Trans. Faraday Soc. 40, 546–551 (1944). [CrossRef]

19.

R. Rosario, D. Gust, A. A. Garcia, M. Hayes, J. L. Taraci, J. W. Dailey, and S. T. Picraux, “Lotus effect amplifies light-induced contact angle switching,” J. Phys. Chem. B 108(34), 12640–12642 (2004). [CrossRef]

20.

L. S. Zhong, J. S. Hu, H. P. Liang, A. M. Cao, W. G. Song, and L. J. Wan, “Self-assembled 3D flowerlike iron oxide nanostructures and their application in water treatment,” Adv. Mater. 18(18), 2426–2431 (2006). [CrossRef]

21.

W. N. Li, J. K. Yuan, X. F. Shen, S. Gomez-Mower, L. P. Xu, S. Sithambaram, M. Aindow, and S. L. Suib, “Hydrothermal synthesis of structure- and shape-controlled manganese oxide octahedral molecular sieve nanomaterials,” Adv. Funct. Mater. 16(9), 1247–1253 (2006). [CrossRef]

22.

Y. B. Li, Y. Bando, and D. Golberg, “MoS2 nanoflowers and their field-emission properties,” Appl. Phys. Lett. 82(12), 1962–1964 (2003). [CrossRef]

23.

J. A. Marqusee and J. Ross, “Kinetics of phase transitions: Theory of Ostwald ripening,” J. Chem. Phys. 79(1), 373–378 (1983). [CrossRef]

24.

C. W. Extrand, S. I. Moon, P. Hall, and D. Schmidt, “Superwetting of structured surfaces,” Langmuir 23(17), 8882–8890 (2007). [CrossRef] [PubMed]

25.

R. N. Wenzel, “Resistance of solid surfaces to wetting by water,” Ind. Eng. Chem. 28(8), 988–994 (1936). [CrossRef]

26.

H. K. Sun, M. Luo, W. J. Weng, K. Cheng, P. Y. Du, G. Shen, and G. R. Han, “Room-temperature preparation of ZnO nanosheets grown on Si substrates by a seed-layer assisted solution route,” Nanotechnology 19(12), 125603 (2008). [CrossRef] [PubMed]

27.

Y. O. Popov, “Evaporative deposition patterns: spatial dimensions of the deposit,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 71(33 Pt 2B), 036313 (2005). [CrossRef] [PubMed]

28.

J. Matos, J. Laine, and J. M. Hermann, “Synergy effect in the photocatalytic degradation of phenol on a suspended mixture of titania and activated carbon,” Appl. Catal. B 18(3–4), 281–291 (1998). [CrossRef]

OCIS Codes
(140.3390) Lasers and laser optics : Laser materials processing
(240.6670) Optics at surfaces : Surface photochemistry
(160.4236) Materials : Nanomaterials

ToC Category:
Laser Microfabrication

History
Original Manuscript: June 1, 2010
Revised Manuscript: July 20, 2010
Manuscript Accepted: July 25, 2010
Published: August 12, 2010

Citation
Xiaodong Guo, Quanzhong Zhao, Ruxin Li, Huaihai Pan, Xiaoyang Guo, Anyuan Yin, and Weilin Dai, "Synthesis of ZnO nanoflowers and their wettabilities and photocatalytic properties," Opt. Express 18, 18401-18406 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-17-18401


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References

  1. Y. S. Ding, X. F. Shen, S. Gomez, H. Luo, M. Aindow, and S. L. Suib, “Hydrothermal growth of manganese dioxide into three-dimensional hierarchical nanoarchitectures,” Adv. Funct. Mater. 16(4), 549–555 (2006). [CrossRef]
  2. A. Chen, X. Peng, K. Koczkur, and B. Miller, “Super-hydrophobic tin oxide nanoflowers,” Chem. Commun. (Camb.) (17): 1964–1965 (2004). [CrossRef]
  3. B. Liu and H. C. Zeng, “Mesoscale organization of CuO nanoribbons: formation of “dandelions”,” J. Am. Chem. Soc. 126(26), 8124–8125 (2004). [CrossRef] [PubMed]
  4. S. Park, J. H. Lim, S. W. Chung, and C. A. Mirkin, “Self-assembly of mesoscopic metal-polymer amphiphiles,” Science 303(5656), 348–351 (2004). [CrossRef] [PubMed]
  5. X. Wang, J. Zhuang, Q. Peng, and Y. D. Li, “A general strategy for nanocrystal synthesis,” Nature 437(7055), 121–124 (2005). [CrossRef] [PubMed]
  6. Y. P. Fang, A. W. Xu, A. M. Qin, and R. J. Yu, “Selective synthesis of hexagonal and tetragonal dysprosium orthophosphate nanorods by a hydrothermal method,” Cryst. Growth Des. 5(3), 1221–1225 (2005). [CrossRef]
  7. J. A. van Kan, A. A. Bettiol, and F. Watt, “Proton beam writing of three-dimensional nanostructures in hydrogen silsesquioxane,” Nano Lett. 6(3), 579–582 (2006). [CrossRef] [PubMed]
  8. J. W. Hsu, Z. R. Tian, N. C. Simmons, C. M. Matzke, J. A. Voigt, and J. A. Liu, “Directed spatial organization of zinc oxide nanorods,” Nano Lett. 5(1), 83–86 (2005). [CrossRef] [PubMed]
  9. S. Xu, Y. Wei, M. Kirkham, J. Liu, W. Mai, D. Davidovic, R. L. Snyder, and Z. L. Wang, “Patterned growth of vertically aligned ZnO nanowire arrays on inorganic substrates at low temperature without catalyst,” J. Am. Chem. Soc. 130(45), 14958–14959 (2008). [CrossRef] [PubMed]
  10. Y. J. Kim, C. H. Lee, Y. J. Hong, and G. C. Yi, “Controlled selective growth of ZnO nanorod and microrod arrays on Si substrates by a wet chemical method,” Appl. Phys. Lett. 89, 163128–163130 (2006.). [CrossRef]
  11. C. A. Mirkin, “Programming the assembly of two- and three-dimensional architectures with DNA and nanoscale inorganic building blocks,” Inorg. Chem. 39(11), 2258–2272 (2000). [CrossRef]
  12. A. Ivanisevic and C. A. Mirkin, ““Dip-Pen” nanolithography on semiconductor surfaces,” J. Am. Chem. Soc. 123(32), 7887–7889 (2001). [CrossRef] [PubMed]
  13. A. P. Joglekar, H. H. Liu, E. Meyhöfer, G. Mourou, and A. J. Hunt, “Optics at critical intensity: applications to nanomorphing,” Proc. Natl. Acad. Sci. U.S.A. 101(16), 5856–5861 (2004). [CrossRef] [PubMed]
  14. Y. F. Guan and A. J. Pedraza, “Synthesis and alignment of ZnO and ZnO nanoparticles by laser-assisted chemical vapor deposition,” Nanotechnology 19(4), 045609 (2008). [CrossRef] [PubMed]
  15. Q. Z. Zhao, S. Malzer, and L. J. Wang, “Formation of subwavelength periodic structures on tungsten induced by ultrashort laser pulses,” Opt. Lett. 32(13), 1932–1934 (2007). [CrossRef] [PubMed]
  16. X. D. Guo, R. X. Li, Y. Hang, Z. Z. Xu, B. K. Yu, Y. Dai, B. Lu, and X. W. Sun, “Coherent linking of periodic nano-ripples on a ZnO crystal surface induced by femtosecond laser pulses,” Appl. Phys., A Mater. Sci. Process. 94(2), 423–426 (2009). [CrossRef]
  17. F. F. Luo, B. Qian, G. Lin, J. Xu, Y. Liao, J. Song, H. Y. Sun, B. Zhu, J. R. Qiu, Q. Z. Zhao, and Z. Z. Xu, “Redistribution of elements in glass induced by a high-repetition-rate femtosecond laser,” Opt. Express 18(6), 6262–6269 (2010). [CrossRef] [PubMed]
  18. A. B. D. Cassie and S. Baxter, “Wettability of porous surfaces,” Trans. Faraday Soc. 40, 546–551 (1944). [CrossRef]
  19. R. Rosario, D. Gust, A. A. Garcia, M. Hayes, J. L. Taraci, J. W. Dailey, and S. T. Picraux, “Lotus effect amplifies light-induced contact angle switching,” J. Phys. Chem. B 108(34), 12640–12642 (2004). [CrossRef]
  20. L. S. Zhong, J. S. Hu, H. P. Liang, A. M. Cao, W. G. Song, and L. J. Wan, “Self-assembled 3D flowerlike iron oxide nanostructures and their application in water treatment,” Adv. Mater. 18(18), 2426–2431 (2006). [CrossRef]
  21. W. N. Li, J. K. Yuan, X. F. Shen, S. Gomez-Mower, L. P. Xu, S. Sithambaram, M. Aindow, and S. L. Suib, “Hydrothermal synthesis of structure- and shape-controlled manganese oxide octahedral molecular sieve nanomaterials,” Adv. Funct. Mater. 16(9), 1247–1253 (2006). [CrossRef]
  22. Y. B. Li, Y. Bando, and D. Golberg, “MoS2 nanoflowers and their field-emission properties,” Appl. Phys. Lett. 82(12), 1962–1964 (2003). [CrossRef]
  23. J. A. Marqusee and J. Ross, “Kinetics of phase transitions: Theory of Ostwald ripening,” J. Chem. Phys. 79(1), 373–378 (1983). [CrossRef]
  24. C. W. Extrand, S. I. Moon, P. Hall, and D. Schmidt, “Superwetting of structured surfaces,” Langmuir 23(17), 8882–8890 (2007). [CrossRef] [PubMed]
  25. R. N. Wenzel, “Resistance of solid surfaces to wetting by water,” Ind. Eng. Chem. 28(8), 988–994 (1936). [CrossRef]
  26. H. K. Sun, M. Luo, W. J. Weng, K. Cheng, P. Y. Du, G. Shen, and G. R. Han, “Room-temperature preparation of ZnO nanosheets grown on Si substrates by a seed-layer assisted solution route,” Nanotechnology 19(12), 125603 (2008). [CrossRef] [PubMed]
  27. Y. O. Popov, “Evaporative deposition patterns: spatial dimensions of the deposit,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 71(33 Pt 2B), 036313 (2005). [CrossRef] [PubMed]
  28. J. Matos, J. Laine, and J. M. Hermann, “Synergy effect in the photocatalytic degradation of phenol on a suspended mixture of titania and activated carbon,” Appl. Catal. B 18(3–4), 281–291 (1998). [CrossRef]

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