Construction of 3 D Arrays of Cylindrically Hierarchical Structures with ZnO Nanorods Hydrothermally Synthesized on Optical Fiber Cores

With ZnO nanorods hydrothermally synthesized on manually assembled arrays of optical fiber cores, 3D arrays of ZnO nanorodbased cylindrically hierarchical structures with nominal pitch 250 μm or 375 μm were constructed. Based on micrographs of scanning electron microscopy and image processing operators of MATLAB software, the 3D arrays of cylindrically hierarchical structures were quantitatively characterized.The values of the actual diameters, the actual pitches, and the parallelism errors suggest that the process capability of the manual assembling is sufficient and the quality of the 3D arrays of cylindrically hierarchical structures is acceptable. The values of the characteristic parameters such as roughness, skewness, kurtosis, correlation length, and power spectrum density show that the surface morphologies of the cylindrically hierarchical structures not only were affected significantly by Zn concentration of the growth solution but also were anisotropic due to different curvature radii of the optical fiber core at side and front view.


Introduction
ZnO nanorod has larger surface-to-volume ratio, wider band-gap, better electron transfer ability, higher isoelectrical point, and nontoxicity [1][2][3][4] and has been frequently employed to construct arrays of ZnO nanorod-based cylindrically hierarchical structures with larger specific surface area, expectant surface morphology, desirable wettability, and photocatalytic activity [5,6].As an enzymatic electrode or a photocatalytic reactor, the array of cylindrically hierarchical structures was widely used to detect the concentration of glucose, cholesterol, urea, uric acid, hydroperoxide, and phenol [7] as well as to photodegrade methylene blue and methylene orange in waste water [8].Synthesizing parameters of ZnO nanorods determine the surface morphology and the wettability of ZnO nanorod-based cylindrically hierarchical structure and eventually affect the enzyme immobilization of the aforementioned biosensors [9,10] as well as the photodegradation efficiency of the target agents [11,12].Therefore, construction and quantitative characterization of various 3D arrays of ZnO nanorod-based cylindrically hierarchical structures with desirable surface morphology and larger surface area are prerequisite to the above mentioned biosensors and waste water treatment domains.
Construction of an array of ZnO nanorod-based cylindrically hierarchical structures includes the production of a cylinder array as the microscale smooth substrate and the synthesis of ZnO nanorods as the nanoscale rough surface.A stainless steel wire mesh with specific pitch and diameter has been frequently used as an array of microscale cylinders [13].Of all approaches including hydrothermal synthesis [14], chemical vapor deposition (CVD) [15], and thermal evaporation method [16] to synthesize ZnO nanorods on a cylindrical substrate, simple hydrothermal synthesis not only is of low cost but also leads to ZnO nanorods of high quality.For instance, Li et al. [5] synthesized ZnO nanorods with CVD and hydrothermal routes, respectively, on stainless steel wire meshes not only to study the wettability switching under the alternation of UV irradiation and dark storage but also to separate water from oil.Vu et al. [17] hydrothermally synthesized ZnO nanostructures on a stainless steel wire mesh and investigated the effect of different synthesis parameters on the yield and the geometric dimensions of the samples.Besides the stainless steel mesh, a Cu wire mesh is also used as an array of microscale cylinders.For example, Lin et al. [18] attached CdSe nanoparticles to the surface of the radially orientated ZnO nanorods which were hydrothermally synthesized on a Cu wire mesh substrate as a wire-shaped electrode to improve the electrolyte mass transport and light harvesting.Nevertheless, the disadvantages of stainless steel and Cu meshes are that they consist of only two layers of microscale cylinders and have finite effective surface areas.As an ideal microscale cylinder and being of low cost, optical fiber cores have potential to be used to construct an array of more than 2 layers of microscale cylindrical substrates [19].To our knowledge, synthesizing ZnO nanorods on an array of optical fiber cores with specific pitch has not been reported in other literatures.
In situ measurement route comparing the features of a scanning electron microscopy (SEM) micrograph with related scale bar is widely employed to coarsely estimate the limited geometric parameters of a ZnO nanorod-based cylindrically hierarchical structure.For instance, Vu et al. [8] measured the thicknesses of the ZnO nanorod film on a stainless steel mesh.Usman Ali et al. [20] approximately estimated the diameters and the lengths of ZnO nanorods synthesized on an Ag wire as a working electrode.Zhang and Jiao [21] roughly figured out the diameters of ZnO nanowires synthesized on carbon fibers.Similarly, Ehlert and Sodano [22] roughly estimated the diameters and the lengths of ZnO nanorods grown on polymer fibers, and Huang et al. [23] approximately derived the diameters and the lengths of ZnO nanorods synthesized on carboxylic acid functionalized aramid fibers.Since the line edges of the features are not extracted from the above mentioned SEM micrographs, the aforementioned measurement data are insufficient and the characterization parameters are deficient.This led to incomplete geometric characterization of these ZnO nanorod-based cylindrically hierarchical structures.Moreover with the in situ measurement route mentioned above, it is difficult to quantitatively characterize the surface morphology of any cylindrically hierarchical structure.
In this paper, 3D arrays of 2 and 3 layers of ZnO nanorodbased cylindrically hierarchical structures were constructed.More geometric parameters were introduced and the surface morphologies were quantitatively characterized.The process capability of the manual assembling was evaluated, and the influencing factors of the surface morphologies of the cylindrically hierarchical structures were identified.(Φ125 m Quanzhou Anpon Company, China), and polydimethylsiloxane (PDMS) solvent and hardener (Sylgard184, Dow Corning Company, USA) were employed.

Construction of the 3D Arrays of Cylindrically
Hierarchical Structures

Manually Assembling the 3D Arrays of Optical Fiber
Cores.Dual-mode optical fibers with core diameter Φ125 m and length 30 mm were immersed in acetone solution for 15 min.The optical fiber cores were pulled out from the plastic sheaths, subjected to ultrasonic cleaning in absolute ethyl alcohol and DI water for 5 min, respectively, and dried in air.These optical fiber cores were manually placed next to each other on a glass substrate and fixed securely at one end by PDMS with the ratio of solvent to hardener 10 : 1.The assembly of the optical fiber cores and the glass substrate were dried in the drying oven at 80 ∘ C for 30 min.Every other one optical fiber core regarded as a positioning fixture was removed from the glass substrate; thus the 1st layer of optical fiber cores with nominal pitch 250 m was formed.Similarly, the 2nd layer of optical fiber cores was vertically placed upon the 1st layer, and a 3D array of 2 layers of optical fiber cores with nominal pitch 250 m was produced; see Figure 1.Once more, the 3rd layer of optical fiber cores was vertically placed upon the 2nd layer, and a 3D array of 3 layers of optical fiber cores with nominal pitch 250 m was generated.
The aforementioned construction process was repeated and every other two neighboring optical fiber cores regarded as positioning fixtures were removed; likewise, the 3D arrays of 2 and 3 layers of optical fiber cores with nominal pitch 375 m were formed, respectively.The ZnO seed solution of 2 mL was dropped on 6 optical fiber cores, and the optical fiber cores were annealed at 120 ∘ C for 10 min in the drying oven.These dropping and annealing processes were repeated twice more.The optical fiber cores were put into 6 beakers, respectively, and the growth solutions of Zn 2+ concentrations 25, 40, 55, 70, 85, and 100 mM were poured into these beakers quickly.The beakers were sealed with preservative films and then put into the water bath at 90 ∘ C for 2.5 h.The optical fiber cores were taken out, cleaned with DI water for 5 min in the ultrasonic washer, and dried in air.Therefore, 6 cylindrically hierarchical structures with different surface morphologies were produced.

Synthesizing ZnO Nanorods on the 3D Arrays of Optical Fiber
Similarly, the ZnO seed solution was deposited on the aforementioned 3D arrays of 2 and 3 layers of optical fiber cores with nominal pitches 250 m and 375 m.The arrays were put into the growth solution of Zn 2+ concentration 100 mM at 90 ∘ C for 2.5 h.Hence, the 3D arrays of 2 and 3 layers of cylindrically hierarchical structures with nominal pitches 250 m and 375 m were formed.

Quantitatively Characterizing the 3D Arrays of Cylindrically Hierarchical Structures.
Based on SEM micrographs and filtration, line edge detection, and fitting operators of MATLAB software, not only the profiles of the 3D arrays of cylindrically hierarchical structures but also those of the cylindrically hierarchical structures at side and front view were extracted.In terms of these extracted profiles, the actual diameters, the actual pitches, and the parallelism errors of the 3D arrays of cylindrically hierarchical structures as well as the characteristic parameters of the surface morphologies of the cylindrically hierarchical structures were determined.The characteristic parameters include surface roughness (Ra), skewness (Sk), kurtosis (Ku), correlation length (), and power spectrum density (PSD).In light of these profiles and the datum lines, the actual diameters, the actual pitches, and the parallelism errors of the 3D arrays of 2 and 3 layers of cylindrically hierarchical structures were determined; see Table 1.

Characterization of the 3D Arrays of Cylindrically Hierarchical Structures.
The actual diameters of cylindrically hierarchical structures in Figures 2(a) and 2(d) increased gradually from the bottom to the top layer.It is attributed to the square blindholes formed by the 3D array of the optical fiber cores along with the glass substrates; see regions in grey in Figure 3.These square blind-holes resulted in different Zn 2+ diffusion rates of the growth solution to the surfaces of the optical fiber cores among the 1st, the 2nd, and the 3rd layers.That is, Zn 2+ concentration of the growth solution around the surfaces of optical fiber cores at the 1st layer was lower, that at the 2nd layer was low, and that at the 3rd layer was high.Hence, the growth rates of ZnO nanorods on the optical fiber cores increased from the 1st through the 2nd and to the 3rd layer and resulted in increasing actual diameters of the cylindrically hierarchical structures from the bottom to the top layer.
The averages of the actual pitches of the 3D array of the cylindrically hierarchical structures in Figure 2(a) deviated less from nominal pitch 250 m than that in Figure 2(d) from nominal pitch 375 m.The positioning fixture removing every other one optical fiber core resulted in less deformation of PDMS than taking away every other two neighboring optical fiber cores.Meanwhile, the more the deformation of PDMS is, the larger the actual pitch deviation is.This suggests that the process capability of manually assembling the 3D array of optical fiber cores with nominal pitch 250 m is better than that with nominal pitch 375 m.Similar conclusion can be drawn from the parallelism errors of the cylindrically hierarchical structures relative to data A and B in Figure 2(a) as well as that with regard to data C, D, and E in Figure 2(d).The top left and the bottom left insets in Figure 4 demonstrate the cylindrically hierarchical structures at side and front view, respectively.Obviously ZnO nanorods were uniformly synthesized all around the smooth optical fiber cores and radially orientated in normal direction.This suggests that desirable rough surfaces of high spatial frequencies were superimposed on the smooth cylindrical surfaces of low spatial frequency and the cylindrically hierarchical structures with expectant surface morphologies were produced.

Qualitative Characterization of the Surface Morphologies of the
Figure 5 shows the XRD spectrum of ZnO nanorods in Figure 4(a).All ten peaks are indexed in 2 range of 20 ∘ to 80 ∘ (JCPDS Card number 36-1451).This indicates that the orientation of ZnO nanorods is random to some extent.The strength of (002) peak is stronger than that of other peaks.This suggests that ZnO nanorods grew along -axis and the crystallinity is high.The XRD analysis is in agreement with the SEM micrograph of ZnO nanorods in Figure 4(a).Similar conclusion between SEM micrographs of ZnO nanorods in Figures 4(b)-4(f) and related XRD analysis can also be drawn.
Curves in red and in blue in both insets of Figures 4(a  From the profiles at front view, the median curves were fitted.With the median curves subtracted from the profiles at front view, the surface morphologies of the cylindrically hierarchical structures at front view were obtained; see curves in blue in Figures 6(a Comparing the profiles in Figure 6(a) with corresponding ones in Figure 6(b), obviously, the fluctuation ranges of the surface heights of the former profiles are larger than those of the latter ones.This concludes that the larger the Zn 2+ concentration of the growth solution, the smoother the profiles at side and front view.For both hierarchical structures, the profiles in blue almost envelop that in red.This suggests that the profiles at front view are rougher than those at side view.That is, anisotropic surface morphologies of the cylindrically hierarchical structures appeared.

Quantitative Characterization of the Surface Morphologies of the Cylindrically Hierarchical Structures.
Similarly, the profiles of the surface morphologies of the cylindrically hierarchical structures in Figures 4(b)∼4(e) were extracted too.According to these profiles, the characteristic parameters Ra, , Sk, Ku, and PSD of corresponding surface morphologies were determined.

Characteristic Parameters
Ra, , Sk, and Ku. Figure 7(a) exhibits that the increase of Zn 2+ concentration of the growth solution resulted in the decrease of both Ra front and Ra side values.It suggests that the profiles of the cylindrically hierarchical structures at front and side view gradually became smoother.With Zn 2+ concentration of the growth solution increasing, the diameters of ZnO nanorods gradually became larger.This led to the decreasing sizes of the pits among ZnO nanorods, gave rise to better aligned ZnO nanorods or even ZnO nanostructures, and produced smoother surfaces.Moreover, Ra front values are slightly larger than corresponding Ra side .This suggests that the profiles at front view are a bit rougher than the ones at side view and anisotropic surface morphologies were formed.This is attributed to the difference of the curvature radii of the optical fiber core at front and side view.The curvature radius of the optical fiber core at front view is significantly less than that at side view.This resulted in radially orientated ZnO nanorods from the surfaces of the optical fiber cores at cross section and further led to rougher profiles of the cylindrically hierarchical structures at front view.
The changes of both  front and  side values also demonstrate significant anisotropic surface morphologies of the cylindrically hierarchical structures.This is ascribed to the different curvature radii of the optical fiber cores at side and front view.As Zn 2+ concentration of the growth solution increased,  side values became larger steadily.This suggests that the profiles at side view turned from more random into rather correlated.It is ascribed to more uniformly aligned ZnO nanorods of larger diameters as well as less sizes of the pits among these ZnO nanorods.With Zn 2+ concentration of the growth solution increasing,  front values first decreased, then kept approximately stable, and eventually increased.As Zn 2+ concentration of the growth solution was 25 mM, ZnO nanorods of smaller diameters radially were orientated from the surface of the optical fiber core.This resulted in larger sizes of the pits among the bundles of ZnO nanorods and further gave rise to larger values of  front .With Zn 2+ concentration of the growth solution increasing continually, the diameters of ZnO nanorods increased whilst the sizes of the pits decreased, but the sum of both kept stable to some extent, so  front values changed insignificantly.When Zn 2+ concentration of the growth solution reached and was larger than 85 mM, ZnO nanostructures with even larger diameters appeared.This led to larger  front values which approximated to  side ones and isotropic surface morphologies of the cylindrically hierarchical structures emerged.
Most Sk front values in Figure 7(b) are less than Sk side ones whilst the latter distributed symmetrically around zero (corresponding to a Gaussian rough profile).This suggests that the profiles at front view have more pits than those at side view whilst, on the profiles at side view, the number of pits is approximately equal to that of bumps.More radiation of ZnO nanorods on the optical fiber core at front view contributed much to this anisotropic behavior of the surface morphologies.Some Ku front values in Figure 7(b) were larger than 3 (corresponding to a Gaussian rough profile) whilst most Ku side values were less than 3.This means that the fluctuation ranges of the surface heights of the profiles at front view are larger than those at side view.These anisotropic surface morphologies of the cylindrically hierarchical structures are attributed to the radially orientated ZnO nanorods from the optical fiber core at front view.As Zn 2+ concentration of the growth solution reached 100 mM, both Ku front and Ku side values are less than 3 and nearly equal to each other.This suggests that the surface morphologies of the cylindrically hierarchical structure at front and side view became isotropic.It is ascribed to the appearance of ZnO nanostructures instead of ZnO nanorods.8(a) demonstrates the PSD curves of both profiles of the cylindrically hierarchical structures at front and side view produced at Zn 2+ concentration 25 mM of the growth solution.Apparently, PSD front curve in blue has two major spatial frequencies whilst PSD side curve in red has one.Also, the PSD front values corresponding to the major spatial frequencies are quite larger than related PSD side ones.These suggest that the profile at front view is rougher than that at side view and the cylindrically hierarchical structure presented anisotropic surface morphology.Similar conclusion can also be drawn from the PSD curves of the profiles of the cylindrically hierarchical structure at front and side view formed at Zn 2+ concentration 100 mM of the growth solution in Figure 8(b).PSD curves in blue and in red in Figure 8(a) have longer tails than corresponding ones in Figure 8(b).This suggests that both profiles of the former cylindrically hierarchical structure at front and side view have wider spatial frequency ranges than those of the latter one.Furthermore, the PSD values corresponding to major spatial frequencies of the former cylindrically hierarchical structure are significantly larger than those of the latter one.These mean that the profiles of the former cylindrically hierarchical structure at front and side view are rougher than those of the latter one.The appearance of ZnO nanostructures instead of ZnO nanorods is attributed to this evolution of the surface morphologies of the cylindrically hierarchical structures.

Conclusion
With ZnO nanorods hydrothermally synthesized on manually assembled arrays of 2 and 3 layers of optical fiber cores with nominal pitches 250 m and 375 m, the 3D arrays of ZnO nanorod-based cylindrically hierarchical structures were constructed.More geometric parameters were introduced and the 3D arrays of cylindrically hierarchical structures were quantitatively characterized in detail.The characterization results suggest that the process capability of the manual assembling is sufficient and the quality of the 3D arrays of cylindrically hierarchical structures is acceptable.The anisotropic surface morphology of the cylindrically hierarchical structure was caused by the different curvature radii of the optical fiber core at front and side view and was affected significantly by Zn 2+ concentration of the growth solution.The 3D arrays of ZnO nanorod-based cylindrically

Figure 1 :
Figure 1: Schematic of a 3D array of 2 layers of optical fiber cores with nominal pitch 250 m.
Figure 2 demonstrates the 3D arrays of 2 and 3 layers of cylindrically hierarchical structures with nominal pitches 250 m and 375 m.Curves in red (the 1st layer) and in green (the 2nd layer) in Figure 2(a) are the extracted profiles of the 3D array of 2 layers of cylindrically hierarchical structures with nominal pitch 250 m, whilst curves in red (the 1st layer), in green (the 2nd layer), and in yellow (the 3rd layer) in Figure 2(d) are those of the 3D array of 3 layers of cylindrically hierarchical structures with nominal pitch 375 m.Circled letters A and B in Figure 2(a) and C, D, and E in Figure 2(d) are datum lines to evaluate the parallelism errors of corresponding line edges.
Cylindrically Hierarchical Structures.For the cylindrically hierarchical structure produced at low Zn 2+ concentration of the growth solution, hexagonal ZnO nanorods are evident and are randomly aligned, and the diameters are small (Figure4(a)).As Zn 2+ concentration of the growth solution gradually increased, the diameters of ZnO nanorods became larger, and the direction became less random (Figures 4(b) and 4(c)), even recrystallization among ZnO nanorods occurred (Figures 4(d) and 4(e)), and eventually ZnO nanostructures instead of ZnO nanorods appeared (Figure 4(f)).
) and 4(f) are the extracted profiles of corresponding cylindrically hierarchical structures at side and front view.
) and 6(b).Curves in red in Figures6(a) and 6(b) are the extracted profiles in red in top left insets of Figures 4(a) and 4(f) which represent the surface morphologies of the cylindrically hierarchical structures at side view.

Figure 6 :
Figure 6: The profiles of the cylindrically hierarchical structures at front and side view produced at Zn 2+ concentrations (a) 25 mM and (b) 100 mM of the growth solution.

ZnZnFigure 7 :
Figure 7: Effect of Zn 2+ concentration of the growth solution on the characteristic parameters Ra,  (a), and Sk, Ku (b) of the surface morphologies of the cylindrically hierarchical structures.

Figure 8 :
Figure 8: PSD curves of the profiles of the cylindrically hierarchical structures at front and side view produced at Zn 2+ concentrations (a) 25 mM and (b) 100 mM of the growth solution.

Table 1 :
Quantitative characterization of the 3D arrays of cylindrically hierarchical structures.