1. Introduction

Over the several past years, multifunctional zinc oxide (ZnO) nanostructures [1] which yielded large improvements of device efficiency in various fields such as photovoltaic [2, 3], light emitting [4, 5], ultraviolet photodetecting [6, 7], chemical gas sensing [8, 9], and piezoelectric [10, 11] applications, have attracted great interest due to their unique optical and optoelectronic properties. Particularly, two- or three-dimensional (2D or 3D) ZnO nanostructures as building blocks have provided great potential for dye-sensitized semiconductors and hybrid solar cells, since they offer excellent surface morphologies with a high density and large surface area [3, 12–15]. Among them, the flower or sea-urchin shaped ZnO nanostructures, wet-chemically grown on periodic 2D or 3D templates, are favorable for the self-assembly of artificial nanostructures because of good periodic arrangement with controllable surface morphology through a relatively low temperature fabrication process. Recently, the well-ordered hollow-urchin or flower shaped ZnO nanostructures using the polystyrene microspheres or subwavelength gratings (SWGs) were demonstrated by the electrochemical deposition and hydrothermal method [12, 16]. For 2D or 3D ZnO nanostructures, the densely integrated ZnO nanostructures on transparent conductive oxide (TCO) coated glass promoted the light conversion efficiency in dye-sensitized solar cells due to efficient light scattering by increasing the diffuse transmitted light on the surface of ZnO nanostructures [12]. Furthermore, for the silicon (Si) solar cells, the hemi-urchin shaped ZnO nanostructures on Si SWG were able to improve the cell efficiency by enhancing the light absorption owing to their broadband and wide-angle antireflection geometry [17].

Meanwhile, sunlight (solar radiation) is becoming more important energy resource, so that many research efforts are focusing on the study of photon managements, i.e. collecting, trapping, and concentrating the sunlight in photovoltaic devices [18–21]. Recently, for collecting the sunlight, the absorption was maximized by the antireflection coating layer with tuning the shape and geometry at the surface of device [22–25] and the light trapping or concentrating could be efficiently achieved by coating or embedding the metal nanoparticles which induce the localized surface plasmon resonance modifying the light path of photons or enhancing the light intensity near the metal nanoparticles [26–29]. In utilizing the ZnO nanostructures, there have been also intensive efforts to manage the photons of sunlight by reducing the surface reflection and enhancing the light scattering [30–33]. However, it is difficult to generate sufficient light scattering using only a vertically-aligned 1D ZnO nanostructure because of its high verticality and limited surface area [34]. In this paper, we demonstrated considerable improvements of the light scattering in urchin-aggregation shaped closely-packed ZnO nanostructures with a periodic alignment as 3D building blocks on indium tin oxide (ITO) coated glass by a simple procedure based on the dip-coated monolayer of silica microspheres and a subsequent wet-chemical growth of ZnO nanorods with a thin sputtered aluminum-doped ZnO (AZO) seed layer by hydrothermal method, together with theoretical analysis. For a deep understanding of urchin-aggregation shaped ZnO nanostructures on their optical characteristics, the finite difference time domain (FDTD) simulation was also performed [35].

2. Simulation of urchin-aggregation shaped ZnO nanostructures

The structure as integrated building blocks should be properly optimized to modify the behavior of light because it is closely correlated to the light diffusion. Figure 1 shows the calculated electric fields of light passing through the (a) ITO/glass, (b) silica microspheres (300 nm)/ITO/glass, (c) silica microspheres (1 μm)/ITO/glass, and (d) ZnO nanorods (0.8 μm height, 50 nm size)/silica microspheres (1 μm)/ITO/glass. The thickness of ITO films is fixed at 200 nm, which has been widely used as a TCO coating layer in photovoltaic and optoelectronic devices. In FDTD simulations, the amplitude of y-polarized electric field was calculated for the incident plane wave with a normalized Gaussian beam profile propagating in the z direction at λ ~600 nm. The refractive indices of glass, ITO, silica microspheres, and ZnO nanorods were 1.46, 1.88, 1.54, and 1.99, respectively. For the bare ITO coated glass in Fig. 1(a), the incident light propagated smoothly without any change of its propagation direction. This behavior confirms that the light interference rarely occurs. When the monolayer of silica microspheres was employed on ITO coated glass, however, the light interference patterns could be observed as shown in Fig. 1(b) and Fig. 1(c). For 300 nm silica microspheres on ITO/glass, the light interfered within nearly 1 μm distance from the surface, but it returned to be z-propagating plane wave at the longer distance, as can be seen in Fig. 1(b). It is found that the subwavelength (i.e. smaller than the wavelength of light) microspheres monolayer could not efficiently diffuse the light. This reason is that the subwavelength structure suppresses all but zeroth-order diffraction. As shown in Fig. 1(c), in contrast, the light passing through the 1 μm silica microspheres monolayer on ITO/glass evidently caused strong interference patterns at both near and far distances from the surface, which allows for efficient diffuse light scattering. This means that the structure with a longer period than the light wavelength is more beneficial to the light diffusion. The use of the ZnO nanorod arrays (NRAs) on the 1 μm silica microspheres monolayer (i.e. urchin-aggregation shaped ZnO nanostructures) further enhances the light interference and diffusing properties as shown in Fig. 1(d). Thus, the urchin-aggregation shaped ZnO nanostructures may act as light diffusion centers to highly improve the light scattering.

 

Fig. 1 Calculated electric fields of light passing through the (a) ITO/glass, (b) silica microspheres (300 nm)/ITO/glass, (c) silica microspheres (1 μm)/ITO/glass, and (d) ZnO nanorods (0.8 μm height, 50 nm size)/silica microspheres (1 μm)/ITO/glass. The thickness of ITO film is fixed at 200 nm.

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3. Results and discussion

3.1. Fabrication of urchin-aggregation shaped ZnO nanostructures

Figure 2 shows the schematic diagram of the fabrication processes of urchin-aggregation shaped ZnO nanostructures on ITO/glass by the hydrothermal method using a thin sputtered AZO seed layer. The corresponding scanning electron microscope (SEM) images are also shown. The SEM images show (i) the monolayer of silica microspheres of 970 nm on ITO/glass, (ii) the deposited AZO layer on upper middle part of silica microspheres of 970 nm, and (iii) the ZnO NRAs on AZO/silica microspheres of 970 nm/ITO/glass. After cleaning the ITO/glass, the monolayer of silica microspheres was deposited by a dip coating technique [36]. For an efficient deposition, the hydrophilic surface of ITO/glass was prepared. To form the monolayer of silica microspheres, the samples were vertically dipped into the silica microsphere colloidal solutions and then pulled up very slowly. The silica microspheres with different diameters of 320 nm, 540 nm, and 970 nm were used. As shown in Fig. 2, the silica microspheres monolayers were formed in a periodic two-dimensional hexagonal closely-packed arrangement. For growing the ZnO NRAs on silica microspheres by the hydrothermal growth, a thin AZO seed layer was deposited on the monolayer of silica microspheres by radio-frequency (RF) magnetron sputtering method. The middle upper part of silica microspheres could be finely covered by the AZO films due to the excellent step coverage ability in sputtering process as shown in the magnified SEM image of Fig. 2(i). Then, the AZO coated samples were dipped into the equimolar growth solution (10 mM zinc nitrate hydrate, 10 mM hexamethylenetetramine, 1 liter deionized water) at 85-88 °C for 10 hour. After drying the samples on a hot plate, the urchin-aggregation shaped ZnO nanostructures on the monolayer of silica microspheres on ITO/glass were fabricated.

 

Fig. 2 Schematic diagram and SEM images of the fabrication process of urchin-aggregation shaped ZnO nanostructures on ITO/glass by the hydrothermal method using a thin sputtered AZO seed layer. The SEM images show (i) the monolayer of silica microspheres of 970 nm on ITO/glass, (ii) the deposited AZO on upper middle part of silica microspheres of 970 nm, and (iii) the ZnO NRAs on AZO/silica microspheres of 970 nm/ITO/glass.

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Figure 3(a) shows the top-view and cross-sectional SEM images of the ZnO NRAs on (i) AZO/ITO/glass and AZO/silica microspheres of (ii) 320 nm, (iii) 540 nm, and (iv) 970 nm/ITO/glass. The thickness of AZO seed layer was approximately 20 nm, which was estimated from the calibrated thickness monitored by a quartz crystal oscillator. The photoluminescence (PL) spectra of the corresponding samples are shown in Fig. 3(b). The 2θ scan X-ray diffraction (XRD) patterns are also shown in the inset of Fig. 3(b). When the ZnO NRAs were grown on the flat AZO seed layer, they were vertically aligned with a dominant growing direction of c-axis in the wurzite structure as can be seen in the inset of Fig. 3(b) [32]. While, the ZnO NRAs grown on silica microspheres were formed as the urchin-aggregation shaped architectures along the hexagonal closely-packed arranged monolayers. As the diameter of silica microspheres was increased, the ZnO nanorods were more densely integrated on the AZO surface covering the silica microspheres because of their larger surface area for growing ZnO NRAs. As can be seen in Fig. 3(b), the PL peak intensity was enhanced with increasing the diameter of silica microspheres due to the increased density of ZnO NRAs. From XRD patterns, the (100), (101), and (102) XRD peaks of ZnO were increased with increasing the diameter of silica microspheres. It is noticeable that the ZnO NRAs were well aligned with a wide range of bending angles because the ZnO nanorods were more radically extended over a large surface area during the growth. This may provide a superior morphology for efficient light scattering due to the wider bending angle of the aligned ZnO NRAs [33].

 

Fig. 3 (a) Top-view and cross-sectional SEM images of the ZnO NRAs on (i) AZO/ITO/glass and AZO/silica microspheres of (ii) 320 nm, (iii) 540 nm, and (iv) 970 nm/ITO/glass, and (b) PL spectra of the corresponding samples. The 2θ scan XRD patterns are also shown in the inset of (b).

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3.2. Characterization of optical property for urchin-aggregation shaped ZnO nanostructures

In order to estimate the light scattering efficiency, the total (diffuse + specular) and diffuse transmittances (T_T and T_D) were characterized, which were measured by using UV-vis-NIR spectrophotometer (Cary 5000, Varian, USA) equipped with an integrating sphere in the wavelength range of 300-1200 nm. Clearly, the specular component of optical transmission corresponds to the zeorth-order diffraction while the diffuse component is related to the higher-order diffractions. Figure 4 shows the measured T_T as a function of wavelength for the ZnO NRAs on AZO/silica microspheres of 320 nm, 540 nm, and 970 nm. For a comparison, the T_T spectra of the ITO/glass, ZnO NRAs on AZO/ITO/glass are also shown. The insetshows the corresponding T_D as a function of wavelength. The ITO coated glass exhibited high T_T values over 80% in the wavelength range of 400-910 nm. When increasing the diameter of silica microspheres for urchin-aggregation shaped ZnO nanostructures, the T_T was slightly decreased with increased oscillations. This can be explained from the Fresnel reflection by a change in the effective refractive index via the densely integrated ZnO nanostructures and silica microspheres. As shown in the inset of Fig. 4. Meanwhile, the T_D was considerably increased with increasing the diameter of silica microspheres. For silica microspheres of 320 nm, 540 nm, and 970 nm, the T_D values of samples were 30.9%, 42.8%, and 64.8%, respectively, at the wavelength of 600 nm. It is clear that the light scattering is more efficiently induced in the larger silica microsphere at incident wavelengths, which can be understood from the FDTD simulation results in Fig. 1. In comparison of ITO/glass without and with ZnO NRAs, the bare ITO coated glass produced very low T_D values of < 3% in the wavelength range of 300-1200 nm, but the T_D was increased by employing the vertically-aligned ZnO NRAs with an average value of ~20% at wavelengths of 400-700 nm. However, the wide bending angle and high density of ZnO NRAs were required for efficient light scattering in vertically-aligned 1D ZnO nanostructures on the flat substrate [33, 34]. For the urchin-aggregation shaped ZnO nanostructure (i.e. ZnO NRAs on AZO/silica microspheres of 970 nm/ITO/glass), the T_D value was higher than 50% in a wide wavelength range of 400-965 nm.

 

Fig. 4 Measured T_T as a function of wavelength for the ITO/glass, ZnO NRAs on AZO/ITO/glass, and ZnO NRAs on AZO/silica microspheres of 320 nm, 540 nm, and 970 nm/ITO/glass. The inset shows the corresponding T_D as function of wavelength.

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To provide a better insight into the improvement of light scattering in urchin-aggregation shaped ZnO nanostructures, the calculated transmittance haze ratios (H_T = T_D/T_T) for the ITO/glass and ZnO NRAs on ITO/glass without and with silica microspheres were compared in Fig. 5 . The insets show the oblique-view SEM images of ZnO NRAs on ITO/glass without and with silica microspheres of 970 nm. The photographs of the water droplets on the samples are also shown in the inset. For ITO coated glass, as expected, the H_T was very low. The ZnO NRAs on AZO/ITO/glass also exhibited low H_T values below 27%. Instead, the incorporation of silica microspheres into the structures leads to a significant increase in H_T. The H_T was increased with increasing the diameter of microspheres due to the enhanced T_D. The urchin-aggregation shaped ZnO nanostructure with 970 nm silica microspheres yielded a high H_T value of > 70% in the wavelength range of 400-900 nm, indicating the fairly high improvement in light trapping and scattering abilities. Moreover, the urchin-aggregation shaped ZnO nanostructures give rise to a high density of ZnO NRAs, keeping a high total transmittance over a wide wavelength region. On the other hand, the surface wettability is an important property of solid surfaces. The nanostructures can be also used to modify the surface property. In order to explore the surface property of urchin shaped ZnO nanostructures compared to the conventional ZnO NRAs, the contact angle of water droplets on the sample surface was determined by using a contact angle measurement system. The urchin-aggregation shaped ZnO nanostructures provided an improved surface hydrophobicity, which may be useful for photovoltaic applications [37]. In view of the surface macroscopic properties on wettability, for ZnO NRAs on AZO/ITO/glass, the water droplet spread rapidly out, corresponding to a contact angle of 36.65° which is close to the hydrophilic, whereas forthe urchin-aggregation shaped ZnO nanostructure the contact angle was increased to 90.51°, as shown in the inset of Fig. 5. For silica microspheres of 320 nm and 540 nm, the urchin-aggregation shaped ZnO nanostructures also exhibited the increased contact angles of 88.37° and 90.45°, respectively (not shown here). The contact angle was almost not changed when the size of silica microspheres in urchin-aggregation shaped ZnO nanostructures was varied from 320 nm to 970 nm. This indicates that the modified surface of integrated ZnO NRAs on silica microspheres improves the hydrophobicity by preventing the penetration of water into the surface, which is desirable for certain applications.

 

Fig. 5 Measured H_T as a function of wavelength for the ITO/glass, ZnO NRAs on AZO/ITO/glass, and ZnO NRAs on AZO/silica microspheres of 320 nm, 540 nm, and 970 nm/ITO/glass, The insets show oblique-view SEM images and photographs of the water droplets for ZnO NRAs on ITO/glass with and without silica microspheres of 970 nm.

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5. Conclusion

We fabricated the urchin-aggregation shaped closely-packed hierarchical ZnO nanostructures by simply growing the ZnO nanorods with a thin AZO seed layer onto the monolayer of dip-coated silica microspheres on ITO/glass using the hydrothermal method. The effect of silica microspheres and ZnO NRAs on the light scattering of ITO/glass samples was investigated, with theoretical simulations. The urchin-aggregation shaped ZnO nanostructure would result in a significant enhancement in T_D over a wide range of wavelengths. This interpretation was supported by the numerical simulations. It was found that the diffuse light scattering was affected by the size of silica microspheres. The T_D was increased with increasing the diameter of silica microspheres, indicating a similar tendency with the theoretical results. For 970 nm silica microspheres, a H_T value as high as > 70% at wavelengths of 400-900 nm was achieved, keeping a high total transmittance. Additionally, the hydrophobicity of the sample was improved compared to the ZnO NRAs on AZO/ITO/glass. These results suggest that the urchin-aggregation shaped hierarchical ZnO nanostructures with large light scattering efficiency are a very promising candidate for various photovoltaic and optoelectronic device applications.