Open Access
Add to BibSonomyAbstract
Thinned silica fibers were fabricated by drawing conventional single mode silica fiber through flame heated method and well-arrayed ZnO nanorods were grown on the thinned silica fibers by a hydrothermal method. The structure enables efficient light coupling between the fiber and the nanorods. With the unique property of high surface to volume ratio of one-dimensional ZnO nanorods, light coupled to nanorods array enhances the optical interaction between the device and the ambient environment. Sensitive humidity sensor was demonstrated by launching laser into ZnO nanorod-covered fibers. Theoretical and experimental results are presented.
©2012 Optical Society of America
1. Introduction
ZnO nano-wires/rods have been recognized as a kind of extremely important materials in a broad range of high-technology applications, e.g., field-effect transistor [1], light-emitting diode [2], ultraviolet laser [3], biological detector [4], and solar cells [5]. Specially, ZnO nanorods in highly oriented and ordered arrays have been demonstrated to be important for sensing applications [6‒9]. Various fabrication techniques have been established for growing ZnO nanorods, such as vapor transport [10], physical vapor deposition [11], and chemical vapor deposition [12]. These growth methods require a relatively high temperature during the synthesis procedure. In 2003, Vayssieres reviewed a growth of arrayed nanorods and nanowires of ZnO from aqueous solutions through a hydrothermal method [13]. This method is energy-efficient and inexpensive to form massive high-quality ZnO nanorods, since it avoids using high temperature and complex vacuum environment. Growing ZnO nanowires and oriented ZnO nanorods on flat substrates has been extensively demonstrated in various platforms such as Si [14], glass [15], carbon cloth [16], and Al foil [17] etc. However, very limited literatures discuss the well-arrayed nanorods on curve surface, for example around optical fibers. It has been reported that this hybrid structure enhances the interaction of fiber-guided light with dye molecules on ZnO nanowires for solar cells [5]. Since ZnO is a high-refractive-index material to silica fiber, the structure allows fiber-guided light couple into ZnO nanorod waveguides [18]. The well-arrayed nanorods are very favorable for sensing applications as it has large surface to volume ratio. Thus optical coupling between the easy-to-access fiber and nanorods are potentially promising for various novel optical sensing applications. In this work, we report the growth of well-arrayed, large-scale ZnO nanorods on thinned silica fiber via the hydrothermal method and demonstrate optical transmission and humidity sensing properties of ZnO nanorod array covered silica fiber.
2. Fabrication and characterization
The thinned fibers used in our experiments are about 10 mm in length and 5, 10, and 20 µm in diameter, which were fabricated by drawing bare single mode silica fiber (core/cladding diameters: 8.2/125 µm) through flame heated method referred to [19]. At first, a commercial single-mode fiber was softened by an alcohol burner with nozzle of about 5-mm diameter. Then the softened fiber was drawn by two three-dimensional stages which control the drawing speed. At the beginning, the drawing speed is controlled at about 10 µm/s, and is increased gradually to about 100 µm/s when the diameter gets to the predetermined values (5 to 20 µm in this work). Well-arrayed ZnO nanorods were grown on the fibers using the following steps: (1) ZnO seed particles were synthesized in a colloidal solution [20]. A 4 mM zinc acetate [Zn(CH3COO)2⋅2H2O, 99% purity] solution was prepared in 40 ml of ethanol [CH3CH2OH, 99.7% purity] under vigorous stirring at 50 °C for 30 min. Another 4 mM sodium hydroxide [NaOH, 96% purity] solution was prepared in 40 ml of ethanol under vigorous stirring at 50 °C for 1 h. Both solutions were then cooled down to room temperature. During cooling, sodium hydroxide was added dropwise to the zinc acetate solution under continuous stirring. The mixture of the two solutions was then filled in a temperature controlled water bath at 70 °C for 2 h. It should be noted that the colloidal solution was stored at room temperature and found to be stable for months. (2) The ZnO seed particles were securely coated on the thinned silica fibers by dipping the fibers into the colloidal solution for 15 min and then annealing at 150 °C for 15 min for three times. (3) After uniformly coating the fibers with ZnO seed particles, hydrothermal ZnO nanorods growth was carried out by suspending the pretreated fibers in the growth solution at 100 °C under vigorous stirring for 12 hours. The solution was prepared by dissolving 0.1878 g of zinc nitrate hydrate (Zn(NO3)2⋅6H2O, 99.0% purity) and 0.0881 g of hexamethylenetetramine (HMTA, 99.0% purity) in 250 mL deionized water. (4) The fibers covered by high density ZnO nanorods were rinsed with deionized water followed by baking at 150 °C for 1 hour.
Figure 1(a) shows a representative scanning electron microscope (SEM) image of the bare fiber with diameter of 5 μm. Figure 1(b) shows the SEM image of the fiber covered by ZnO nanorods. The grown ZnO nanorods are pointing outwards from the fiber surface and forming in well arrayed pattern. The fiber/nanorods structure looks like a “test-tube brush”. By comparing Figs. 1(b) with 1(a), estimated maximum length of the nanorods is 2.5 µm. Higher magnification SEM image [Fig. 1(c)] shows that each nanorod exhibits a hexagonal end facet. The density of nanorods which stands for the average distribution of ZnO nanorods is approximately 8.5 rods per µm2 estimated by total number of 76 nanorods in this 3.47 × 2.58 µm2 area.
Fig. 1 (a) SEM images of a 5-µm bare fiber; (b) A “test-tube brush”-like fiber/nanorods structures comprising 5-µm fiber covered by ZnO nanorods with approximately 2.5-µm length; (c) Higher magnification SEM image of the ZnO nanorods in the area of 3.47 × 2.55 µm2.
Figure 2(a) shows the statistical analysis of the nanorod diameter distributions in Fig. 1(c), which shows the dominant diameter is in range of 200−300 nm. Figure 2(b) is the X-ray diffraction (XRD) pattern of the ZnO nanorods. The diffraction peaks corresponding to the (100), (002), (101), (102), (110), (103), (200), (112), (201), and (202) diffraction planes are observed. All the obtained peaks are well matched with the Joint Committee on Powder Diffraction Standards card no. 36-1451, which corresponds to wurtzite crystal structure. Figure 2(c) shows the optical transmission spectrum for the ZnO nanorods in the wavelength range of 350−780 nm using ultraviolet-visible (UV-VIS) spectrophotometer. This indicates that ZnO nanorods have flat transmittance band from 550 nm to 780 nm. Through this work, we used a diode-pumped solid state laser for later characterizations. The wavelength of the laser is 644 nm and the output power is continuously variable from 0 to 60 mW.
Fig. 2 (a) Diameter distribution histogram of the ZnO nanorods showed in Fig. 1(c); (b) XRD pattern and (c) transmission spectrum of the grown ZnO nanorod array.
Figure 3(a) shows optical microscope image of a 5-µm bare fiber with 100-µW light coupled from left port using 644-nm laser as lightsource. The light is guided and confined inside the fiber, except little light scattered out from some defects. Figure 3(b) shows the 5-µm ZnO nanorod-covered fiber with the identical laser power. An extensive part of light is guided into the nanorods and leaked out from the nanorod facets due to the refractive index of ZnO (n = 1.99 at 644-nm wavelength) is higher that that of silica fiber (n = 1.46 at 644-nm wavelength). The extra loss of the ZnO nanorod-cover fiber is 9.2 dB, estimated by comparing its output power (3.2 µW) from that of the bare fiber (26.8 µW). Figure 3(b) inset is the microscope image showing red speckles when coupled power is reduced to 10 µW. Light leaked out by each nanorod is clearly discernible. The image suggests that fiber-guided light couples to the majority of the nanorods, and in turn scatters out.
Fig. 3 Optical microscope images of (a) 5-µm bare fiber and (b) ZnO nanorod-covered fiber with 644-nm laser light input from the left port with 100 µW coupled power. Inset: Magnified image of nanorod-covered fiber with lower coupled power of 10 µW. Numerical simulations of the electric field distribution of (c) 5-µm bare fiber and (d) ZnO nanorod-covered fiber.
To investigate the optical properties of the fibers in this work, a two-dimensional finite element method-based software (COMSOL Multiphysics 3.5) is used to simulate the phenomena observed in the experiment. The results are shown in Figs. 3(c) and 3(d). The information about the structure used for the numerical simulation is as follows. The wavelength of input light is 644 nm. The diameter and length of the bare fiber (Fig. 3c) are 5 µm and 35 µm, respectively. The refractive indices of the fiber and surrounding medium are 1.46 and 1.0 (for air), respectively. The diameter of the ZnO nanorods covered fiber (Fig. 3d) is also 5 µm and the diameter and length of the ZnO nanorods is 200 nm and 2.5 µm, respectively. The number of ZnO nanorods covered on the fiber is 66 and the refractive index of ZnO nanorods is 1.99. In the simulation, the density of the nanorods is set to be a quarter of that in the experiment to clearly show the distribution of electric field near the fiber and to overcome the limitations of the data storage and processing capability of the personal computer. Compared with Fig. 3(c), Fig. 3(d) shows an extensive part of light coupled into the ZnO nanorods and leaked out from the nanorod facets. The output intensity decreased sharply compared to the bare fiber. The simulation result indicates that the existence of the ZnO nanorods causes a larger proportion of light penetrating into the surrounding medium. The increased proportion of the light outside the fiber enhances the interaction between light and the surrounding medium and consequently results in a stronger sensitivity to any changes in the surrounding medium, i.e. the relative humidity in this work.
3. Humidity sensing experiment
The mutual coupling between the fiber and the nanorods array enables using large surface area of the nanorods for sensing purpose. Here we present a proof-of-principle experiment to utilize this structure for humidity sensor. Experimental setup is schematically presented in Fig. 4 . Thinned ZnO nanorod-covered silica fiber was put inside the humidity chamber. The two ends of the fiber were connected to a 644-nm laser and an optical power meter. Relative humidity (RH) in the chamber was changed by adjusting mixing ratio of wet gas and dry gas. A commercially available humidity sensor (an impedance hygrometer with an accuracy of 2.5 RH%) was inserted into the chamber to monitor the RH. The inset of Fig. 4 shows the higher magnification image of fiber/nanorods structures fixed by a clamp for protection purposes.
To start the measurement, the initial humidity level was reduced to 10% RH by passing dry air into the chamber. The fiber output power was measured at this humidity level with an optical power meter. The humidity was then increased by passing wet gas in. The maximum achievable relative humidity level is 95%. All these measurements were recorded at a stable temperature of 25 °C. The sensitivity has been defined as |ΔP/Δn|, where ΔP is the fractional change of the output power and Δn is the change of relative humidity in the chamber. Figure 5 shows the measured normalized transmission of 5-µm-diameter bare fiber and the ZnO nanorod-covered fibers in diameters D of 5, 10, and 20 µm upon RH changed from 10% to 95%. The normalized transmission of all the structures decreases monotonously as the RH rises. The normalized transmission of the 5-µm bared fiber decreases 5% with a flat slop upon humidity change from 10% RH to 95% RH, and the maximum sensitivity is 0.0007 RH–1. In comparison, transmission of 5-µm nanorod-covered fiber shows steep intensity reduction of 76% upon the same humidity change. The maximum sensitivity can reach to 0.014 RH–1. The normalized transmission of 10-µm fiber decreases 30% from 10% RH to 95% RH. The maximum sensitivity is 0.0057 RH–1. The normalized transmission of 20-µm fiber shows even flatter slope. It decreases 4% from 10% RH to 95% RH. The maximum sensitivity is 0.0008 RH–1, comparable to that of 5-µm bare fiber. Note that the curves for the 5-µm-diameter bare fiber and the ZnO nanorod-covered fibers in diameters of 20 µm are very close to each other. This is because the ZnO nanorods covered on the 20-µm-diameter fiber lead to an increase of effective refractive index of the surrounding medium, which makes the guiding property of the 20-µm ZnO nanorod-covered fiber very similar to that of the 5-µm bare fiber. This issue will be discussed in details in later sections. Among all the structures, 5-µm ZnO nanorod-covered fiber shows the highest sensitivity. This is because thinner fibers have a larger proportion of light propagating outside as evanescent waves and have more high-order modes which are leaked out of the fibers [21], i.e. more light interacts with the surrounding mediums. The result suggests that thinner fiber core is favorable for nanorod-covered fiber based humidity sensor. Compared to 50-mm long nanoparticle-coated fiber sensor with 0.0012 RH–1 sensitivity reported by Khijwania et al. [22], our sensor attained a sensitivity more than 10 times larger yet with coated fiber length 5-times shorter. Thus per unit length sensitivity enhancement of nanorod-covered fiber over nanoparticle-coated fiber is more than 50-fold.
Fig. 5 Measured normalize transmission of the 5-µm-diameter bare fiber and the ZnO nanorod-covered fibers in diameters D of 5, 10, and 20 µm as a function of RH changed from 10% to 95%.
4. Discussion
The 5-µm ZnO nanorod-covered fiber exhibits sensitivity almost 20 times larger than that of the bare fiber. This phenomenon can be explained as following: to start with, the ZnO nanorods have dry air on their surface. When the nanorods are exposed to an environment of humidity, rapid surface adsorption of water molecules onto ZnO nanorods occurs. The optical properties of the ZnO nanorod surfaces are modulated by the surface adsorption of water molecules on the ZnO nanorods. With an increase of the relative humidity, more water molecules will be absorbed on the ZnO nanorod surfaces. The increasing water molecules cause an increase in both effective refractive index (neff) of the surrounding medium and absorption coefficient of the ZnO nanorod surfaces, which leads to a larger leakage and absorption of light. Here we provide a simulation to certify the impact of the neff variation, as shown in Fig. 6 . According to our calculations, the effective refractive index neff varies from 1.698 to 1.718 with the relative humidity changed from 10% to 95%. The electric field distributions in 5-µm ZnO nanorod-covered fiber are obtained with the effective refractive index neff varying from 1.698 to 1.718 corresponding to different cases of relative humidity. It can be seen that the magnitude of the scattered electric field increases while the light guided in the fiber core becomes weaker gradually with the increase of neff. The normalized transmission variation with neff is extracted from the simulations, as shown in Fig. 7 . It can be seen that the normalized transmission of 5-µm ZnO nanorod-covered fiber decreases from 1 (for neff = 1.698) to 0.51 (for neff = 1.718). Moreover, the increasing water molecules cause a stronger light absorption by the ZnO nanorod surfaces, which is attributed to the much higher absorption coefficient of water (0.336 m‒1) than that of air (10‒4 m‒1) at the wavelength of 644 nm. Due to the array of nanorods with high surface to volume ratio on the fiber, even minute variation of the relative humidity can accumulate to result in an extensive output intensity change of the fiber power.
Fig. 6 Numerical simulations of electric field distribution in a surrounding medium with effective refractive indices neff of (a) 1.698, (b) 1.706, (c) 1.713, and (d) 1.718.
It should be pointed out that the applied laser wavelength also affects the sensing property of the structure. To show the effect of the laser wavelength, further experiments of humidity sensing with 473- and 532-nm lasers (input power of 100 µW) have also been carried out. Figure 8 shows the humidity sensing responses of 5-μm ZnO nanorod-covered fiber with 473- (blue) and 532-nm (green) lasers. Both plots of normalized transmission exhibit decreasing tendencies as the relative humidity increases, which are consistent with that of 644-nm wavelength. For the 473- and 532-nm wavelengths, the average sensitivities reach about 0.012 RH‒1 and 0.013 RH‒1, respectively, both lower than that for 644-nm wavelength (0.014 RH‒1). The insets of Fig. 8 are optical microscope images of the 5-μm ZnO nanorod-covered fiber with 473- and 532-nm lasers coupled from the left port. It can be seen that lights are strongly scattered out by the ZnO nanorods. For 473- and 532-nm wavelengths, the output powers are 4.3 and 3.9 µW, respectively. The extra losses can then be calculated by referring output powers to that of the bare fiber (26.8 µW) and the results are 8.0 and 8.4 dB for 473- and 532-nm wavelengths, respectively, both lower than that for 644-nm wavelength (9.2 dB). In general, the longer wavelength induces more light leakage for the same diameter fiber. So the wavelength of 644 nm exhibits the highest sensitivity in the experiments. This phenomenon can further confirm the humidity sensing property of this structure.
Fig. 8 Measured normalized transmission of the 5-µm ZnO nanorod-covered fiber using 473- and 532-nm lasers (input power of 100 µW) as light sources. The insets are optical microscope images of the 5-µm ZnO nanorod-covered fiber with 473- and 532-nm lasers coupled from the left ports.
5. Conclusions
In summary, we have fabricated ZnO nanorods radially grown on the thinned silica fiber. A detailed experimental investigation is carried out to analyze the light transmission loss and the humidity response characteristics. At 644-nm wavelength, the optical transmission loss is 9.23 dB for a 5-µm fiber with 10-mm length section covered by ZnO nanorods. The device exhibits optical transmission decreases relatively linearly in the broad range of 10%−95% RH. The strong light coupling between the fiber and ZnO nanorods with large surface to volume ratio enables attaining high sensitivity to the humidity change. The maximum measured humidity sensitivity is 0.014 RH–1 for 5-µm ZnO nanorod-covered fiber, showing a more than 50-fold unit-length sensitivity increase compared to ZnO nanoparticle-covered fibers. This investigation paves the path towards the development of fiber optical sensors which can be deployed at remote and unmanned stations. The ZnO nanorod-covered fiber is attractive for other application such as chemical and biological sensing using optical methods, in which light coupling to high surface to volume ratio nano structure is of great benefit.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grants 61007038, 60625404 and 10974261).
References and links
1. H. T. Ng, J. Han, T. Yamada, P. Nguyen, Y. P. Chen, and M. Meyyappan, “Single crystal nanowire vertical surround-gate field-effect transistor,” Nano Lett. 4(7), 1247–1252 (2004). [CrossRef]
2. X. W. Sun, J. Z. Huang, J. X. Wang, and Z. Xu, “A ZnO nanorod inorganic/organic heterostructure light-emitting diode emitting at 342 nm,” Nano Lett. 8(4), 1219–1223 (2008). [CrossRef] [PubMed]
3. M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, and P. Yang, “Room-temperature ultraviolet nanowire nanolasers,” Science 292(5523), 1897–1899 (2001). [CrossRef] [PubMed]
4. T. Y. Liu, H. C. Liao, C. C. Lin, S. H. Hu, and S. Y. Chen, “Biofunctional ZnO nanorod arrays grown on flexible substrates,” Langmuir 22(13), 5804–5809 (2006). [CrossRef] [PubMed]
5. B. Weintraub, Y. Wei, and Z. L. Wang, “Optical fiber/nanowire hybrid structures for efficient three-dimensional dye-sensitized solar cells,” Angew. Chem. Int. Ed. Engl. 48(47), 8981–8985 (2009). [CrossRef] [PubMed]
6. Q. Wan, Q. H. Li, Y. J. Chen, T. H. Wang, X. L. He, J. P. Li, and C. L. Lin, “Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors,” Appl. Phys. Lett. 84(18), 3654–3656 (2004). [CrossRef]
7. H. T. Wang, B. S. Kang, F. Ren, L. C. Tien, P. W. Sadik, D. P. Norton, S. J. Pearton, and J. Lin, “Hydrogen-selective sensing at room temperature with ZnO Nanorods,” Appl. Phys. Lett. 86(24), 243503 (2005). [CrossRef]
8. L. Liao, H. B. Lu, J. C. Li, H. He, D. F. Wang, D. J. Fu, C. Liu, and W. F. Zhang, “Size dependence of gas sensitivity of ZnO nanorods,” J. Phys. Chem. C 111(5), 1900–1903 (2007). [CrossRef]
9. F. Fang, J. Futter, A. Markwitz, and J. Kennedy, “UV and humidity sensing properties of ZnO nanorods prepared by the arc discharge method,” Nanotechnology 20(24), 245502 (2009). [CrossRef] [PubMed]
10. M. H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, and P. Yang, “Catalytic growth of zinc oxide nanowires by vapor transport,” Adv. Mater. (Deerfield Beach Fla.) 13(2), 113–116 (2001). [CrossRef]
11. Y. C. Kong, D. P. Yu, B. Zhang, W. Fang, and S. Q. Feng, “Ultraviolet-emitting ZnO nanowires synthesized by a physical vapor deposition approach,” Appl. Phys. Lett. 78(4), 407–409 (2001). [CrossRef]
12. J. J. Wu and S. C. Liu, “Low-temperature growth of well-aligned ZnO nanorods by chemical vapor deposition,” Adv. Mater. (Deerfield Beach Fla.) 14(3), 215–218 (2002). [CrossRef]
13. L. Vayssieres, “Growth of arrayed nanorods and nanowires of ZnO from aqueous solutions,” Adv. Mater. (Deerfield Beach Fla.) 15(5), 464–466 (2003). [CrossRef]
14. J. H. Lee, I. C. Leu, Y. W. Chung, and M. H. Hon, “Fabrication of ordered ZnO hierarchical structures controlled via surface charge in the electrophoretic deposition process,” Nanotechnology 17(17), 4445–4450 (2006). [CrossRef]
15. M. Wei, D. Zhi, and J. L. MacManus-Driscoll, “Self-catalysed growth of zinc oxide nanowires,” Nanotechnology 16(8), 1364–1368 (2005). [CrossRef]
16. S. H. Jo, D. Banerjee, and Z. F. Ren, “Field emission of zinc oxide nanowires grown on carbon cloth,” Appl. Phys. Lett. 85(8), 1407–1409 (2004). [CrossRef]
17. A. Umar, B. K. Kim, J. J. Kim, and Y. B. Hahn, “Optical and electrical properties of ZnO nanowires grown on aluminium foil by non-catalytic thermal evaporation,” Nanotechnology 18(17), 175606 (2007). [CrossRef]
18. T. Voss, G. T. Svacha, E. Mazur, S. Müller, C. Ronning, D. Konjhodzic, and F. Marlow, “High-order waveguide modes in ZnO nanowires,” Nano Lett. 7(12), 3675–3680 (2007). [CrossRef] [PubMed]
19. J. Yu, R. Feng, and W. She, “Low-power all-optical switch based on the bend effect of a nm fiber taper driven by outgoing light,” Opt. Express 17(6), 4640–4645 (2009). [CrossRef] [PubMed]
20. Z. Hu, G. Oskam, and P. C. Searson, “Influence of solvent on the growth of ZnO nanoparticles,” J. Colloid Interface Sci. 263(2), 454–460 (2003). [CrossRef] [PubMed]
21. L. Tong, J. Lou, and E. Mazur, “Single-mode guiding properties of subwavelength-diameter silica and silicon wire waveguides,” Opt. Express 12(6), 1025–1035 (2004). [CrossRef] [PubMed]
22. R. Aneesh and S. K. Khijwania, “Zinc oxide nanoparticle based optical fiber humidity sensor having linear response throughout a large dynamic range,” Appl. Opt. 50(27), 5310–5314 (2011). [CrossRef] [PubMed]
