Chinese Journal of Chemical Physics  2019, Vol. 32 Issue (6): 708-714

#### The article information

Hao Chen, Na Dong, Kai Wang, Yi Yao, Faqiang Xu

Dark Color ZnO Quasi-One-Dimensional Nanostructures Grown by Hydrothermal Method and Modulation of their Optical Properties

Chinese Journal of Chemical Physics, 2019, 32(6): 708-714

http://dx.doi.org/10.1063/1674-0068/cjcp1903045

### Article history

Received on: March 12, 2019
Accepted on: April 20, 2019
Dark Color ZnO Quasi-One-Dimensional Nanostructures Grown by Hydrothermal Method and Modulation of their Optical Properties
Hao Chen , Na Dong , Kai Wang , Yi Yao , Faqiang Xu
Dated: Received on March 12, 2019; Accepted on April 20, 2019
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China
Abstract: Zinc oxide has a large energy gap and thus it has potential application in the field of solar cells by tuning the absorption of sunlight. In order to enhance its absorption of sunlight, dark color zinc oxides have been prepared by traditional hydrothermal method directly using a zinc foil as both source and substrate. We found that we could tune the optical properties of ZnO samples by changing the temperature. In particular, increasing temperature could significantly reduce the reflectivity of solar energy in the visible range. We speculate that the phenomenon is relevant to the sharp cone morphology of the ZnO nanorods grown on the surface of Zn foils, which furthermore enhance refraction and reflection of light in the nanorods. The capacity to improve the light absorption of ZnO may have a bright application in raising the efficiency of solar cells.
Key words: Hydrothermal method    Dark color ZnO samples    Enhanced optical properties
Ⅰ. INTRODUCTION

Energy is of great significance to today's production needs and household use [1, 2]. In recent decades, however, energy shortages have appeared in lots of countries and regions [3]. Researchers have been struggling to find new clean energy to substitute non-renewable energy in order to achieve sustainable development [4]. Among the known recyclable energy, solar energy is favored by scientific researchers for its universality, harmlessness, and durability [5, 6]. But solar energy has a few weaknesses which have limited its applications [7, 8]. For example, only a small part of solar energy exposed to the Earth's surface can be effectively used, although the total radiation of solar energy per year is equivalent to 130 trillion tons of coal [9-11]. Consequently, how to maximize the usage of solar energy is the primary issue in realizing the practical application of solar energy.

ZnO, as a wide bandgap semiconductor material, has attracted abundant interests in the last few decades for its applications in piezotronics [12], electrochromics [13], solar cells [14], photocatalysis [15], etc. The main downside of ZnO, as a kind of light-harvesting material, is lack of absorption in visible light, due to the high exciton binding energy of 60 meV at room temperature, corresponding to its direct bandgap of 3.37 eV [16-18]. Some scientists are devoted to finding ways to adjust its bandgap for increasing the solar adsorption among the whole spectral range. Fred et al. created strong absorptive ZnO by designing its hemi-urchin shape [23]. Chen et al. have achieved controlling the geometries of ZnO nanorods to decrease the reflectivity [24]. ZnO nanostructures could be synthesized by many methods, like chemical vapor deposition [19], hydrothermal method [20], pulsed laser deposition [21], and electrochemical deposition [22]. And hydrothermal method provides a promising way for monitoring properties of nanostructured materials because of its low cost, high efficiency, and possibility for large-scale production [25-28].

Herein, we performed a facile hydrothermal procedure to fabricate quasi-one-dimensional ZnO nanorods samples showing alterable colors, from gray, brown to dark black. The changeable colors correspond to the variation of reflectivity or absorption of zinc oxides. The effect of the reaction conditions on the morphology and the optical properties is systematically investigated. Furthermore, the possible mechanism of changeable colors of ZnO samples is also proposed.

Ⅱ. EXPERIMENTS

Zn foils (99.9% purity, Sinopharm Chemical Reagent C, Ltd.), NaOH, H$_2$O$_2$ (30.0% purity, Sinopharm Chemical Reagent Co. Ltd.) and deionized water are used in the whole preparation process. And all reagents are analytically pure.

The detailed processing steps applied in this study are illustrated as follows. Firstly, 30 mL of the prepared solution (NaOH aqua and H$_2$O$_2$) was added into a Teflon-lined stainless steel autoclave with a capacity of 50 mL. Secondly, the Zn foil was ultrasonically rinsed by ethanol and deionized water for several times. After cleaning, the Zn foil was put into the autoclave with the solution. Finally, the prepared autoclave was sealed and put into an oven which was heated to a certain temperature and kept for 3 h. Afterward, the autoclave was cooled to room temperature naturally. The resulting products over the Zn foil were collected by washing with distilled water for several times and then dried in air at 60 ℃ for 3 h. During the whole experiments, the temperature (100-200 ℃), the concentration of NaOH aqueous solution (0.02-0.08 mol/L) and H$_2$O$_2$ (2-8 mL of volume) were adjusted to find relationships between external conditions and optical properties.

The microstructure analysis was carried out on field emission scanning electron microscope (FESEM, Hitachi SU8200), transmission electron microscope (TEM, JEOL JEM-2011), and high-resolution transmission electron microscope (HRTEM, JEOL JEM-2100F). The growth direction and crystallinity were also analyzed by X-ray diffraction measurement (XRD, Rigaku TTR-Ⅲ Ⅹ-ray diffractometer with Cu K$\alpha$ radiation). The photoluminescence spectra were recorded on a JY Fluorolog-3-Tou fluorescence spectrophotometer using the 325 nm exciton line of the xenon lamp at room temperature. The UV-Vis absorption spectra were obtained on Shimadzu SOLID3700 spectrophotometer.

Ⅲ. RESULTS AND DISCUSSION

In the beginning, we selected two specific reaction temperatures, 110 and 190 ℃. Just as shown in FIG. 1 inserts, it is surprisingly found that the two samples display different colors and as the temperature increases, the color gets deeper and deeper from gray to dark black. In order to further investigate the reason that the ZnO samples' colors changed regularly, SEM and TEM were applied for the microstructure analysis. FIG. 1 shows the SEM images of the ZnO nanorods on Zn foils with growth temperature of 110 ℃ and 190 ℃, respectively. At lower temperature, uniformly thin ZnO nanorods with flat top grow on the Zn foils. But with the temperature increasing, the tapered nanorods of larger diameter and some nanoparticles fail to grow into nanorods, as depicted in FIG. 1(b). Evidently, the colors of the samples are closely related to the structure of ZnO.

 FIG. 1 SEM images of the ZnO samples growing on Zn foils at (a) 110 ℃ and (b) 190 ℃ respectively. The inserts are optical images showing the colors of gray and dark black.

FIG. 2 exhibits the TEM images of the as-prepared ZnO nanorods which show the morphologies and microstructure details of the ZnO nanorods growing at 110 and 190 ℃. It is obvious that the ZnO nanorods have different morphologies at diverse temperature conditions, just as shown in FIG. 1. FIG. 2(a) is the TEM image of the ZnO nanorod obtained at 110 ℃ which has a flat end and the diameter of approximately 121.8 nm. However, as can be seen from FIG. 2(c), the nanorod with a diameter of 335.0 nm produced at 190 ℃ has a very sharp tip. The details of crystal characteristics are further determined by the HRTEM images in FIG. 2 (b) and (d). The HRTEM image of the circled part of the nanorod displayed in FIG. 2(a) is shown in FIG. 2(b), exhibiting well-defined lattice fringes, implying that the ZnO nanorod has high-quality crystallization. The interplanar spacing is measured to be about 0.26 nm, which corresponds to (0001) crystal planes of the wurtzite phase, being well consistent with the results analyzed above. In FIG. 2(d), however, apart from the (0001) crystal plane, the interplanar spacings of 0.24 and 0.28 nm are also found at the tip end of the nanorod synthesized at 190 ℃, which corresponds to the (101) and (110) faces. There is no doubt that all the results demonstrated above affirm that the temperature changes can arouse the alteration of the morphology and structure of ZnO, which in turn causes the changes in the colors of ZnO samples.

 FIG. 2 TEM images of the ZnO samples growing on Zn foils at (a, b) 110 ℃ and (c, d) 190 ℃ respectively. (a, c) Low-magnification and (b, d) high-resolution.

In order to explore the reason for ZnO color changes, series experiments were conducted by tuning the temperature trying to modulate the morphology and structure of ZnO, and the results are shown in FIG. 3. FIG. 3 shows the optical images of the products prepared at the temperature from 100 ℃ to 200 ℃ for 3 h. As the temperature continues to rise, the color of the ZnO sample gradually varies from gray, brown to dark black. At the same time, the micro shape of the top of ZnO nanorod changes from flat and the transition morphology to cusp can also be observed in the process of modulating temperature. Moreover, the growth direction and crystallinity were also analyzed by X-ray diffraction measurement with the purpose of studying the peculiar phenomenon. FIG. 4 shows that there are only two phases, hexagonal ZnO and Zn in all the samples, meaning that no impurities exist in the products. In contrast to the standard XRD patterns for ZnO, the crystal planes (100), (002), (102), (110), (103), and (112) of ZnO can be determined. The strong and sharp diffraction peaks suggest that the obtained ZnO structures are well crystalline in nature. In addition, the ZnO lattice parameters of all products can be calculated to the average $a$=3.224 Å and $c$=5.183 Å, which is well matched with the standard values. Apart from the fundamental information obtained above, it can be seen that the diffraction peak intensities of (100), (102), (110), (103), and (112) rise with the increasing temperature. Just as described in FIG. 3, the results of XRD indicate that the appearance of sharp top of ZnO nanorods is in line with the change of six peak intensities, further proving that temperature can control the structure of ZnO nanorods.

 FIG. 3 The optical photos with SEM images of ZnO nanorod prepared at 100-200 ℃ from upper left to lower right with a temperature interval of 10 ℃.
 FIG. 4 XRD patterns of ZnO nanorod produced by tuning the temperature.

As we all know, the color of an object is related to the nature of light absorption on its surface, meaning that objects will appear black if they absorb all light in the visible light range. Thus, diffuse reflectance UV-Vis spectra were carried out to investigate the mechanisms of the interesting color changing of ZnO samples from gray to dark black, the results are shown in FIG. 5. The inset images show the average reflectivity of ZnO samples at changeable temperatures. For all samples, the reflectance is low in the UV region ($<$370 nm) and increases significantly at around 380 nm, which corresponds to the ZnO band-gap of 3.37 eV [31]. With respect to the influence of temperature on the reflectance of ZnO samples, it is obviously seen that as the temperature changes from 100 ℃ to 200 ℃, the average reflectance steps down and the maximum value (31.247%) is about nine times larger than the minimum (3.114%), which is a huge reduction. Thus, the absorption of ZnO samples in the visible light can be significantly improved by increasing temperature, though the absorption band of zinc oxide is usually in ultraviolet light.

 FIG. 5 UV-Vis reflectance of ZnO samples prepared by tuning temperature (100-200 ℃). The insert image shows the average reflectivity of ZnO samples.

The luminescent properties of as-prepared ZnO nanorods were carried out by the room-temperature photoluminescence (PL) spectroscopy using the Xe lamp with an excitation wavelength of 325 nm, and the results are displayed in FIG. 6. There is a common view that the intensity of the peak in UV region not only reflects the recombination rate of carriers, but also plays an important role in determining the crystal quality of ZnO [29, 30]. As shown in FIG. 6, all the samples grown at 100-200 ℃ have a sharp and strong UV emission at 377-379 nm, which corresponds to ZnO band gap about 3.37 eV. It is clearly seen that the peak intensity has a significant enhancement (the peak intensity at 200 ℃ is about twenty times than that at 100 ℃) with increasing temperature. Therefore, it can be inferred that the rising temperature is able to promote the recombination efficiency of excitons and the crystallization of ZnO structures. As for visible light emission, there are three kinds of leading point defects, that is electron transition from Zn$_\textrm{i}$ to O$_\textrm{i}$, Zn$_\textrm{i}$ to V$_\textrm{O}$, and conduction band to O$_\textrm{i}$ [39-41]. In FIG. 6, the green emissions are suppressed along with the increase of temperature, which demonstrates that deep levels gradually disappeared when the temperature is increasing and further affirms that structural defects, especially interstitial oxygen atoms and interstitial zinc atoms, have been reduced through increasing temperature. Thus, we may well conjecture that the enhancement of temperature gives ZnO nanorods more opportunities to grow into perfect crystals and thus reduce the defects.

 FIG. 6 Normalized photoluminescence spectra of ZnO nanorods prepared at the regulated temperature.

Having concluded that the producing temperature is the main term towards all phenomena of ZnO samples, it is now necessary to discuss the mechanism in it. There were several explanations in the literature explaining the formation of black zinc oxides. The simplest interpretation is that a certain amount of carbon on the surface of ZnO is the vital factor for the formation of black ZnO samples via MOCVD method, in which reactors usually contain carbon sources [32]. Tian et al. have reported that it was the pyramid ZnO arrays that caused the novel functionality in antireflection [33]. They found that in the black arrays all the ZnO structures had pyramid tips, but after scratching and ion etching of the ZnO surface, the pyramid tops were got rid of. Meanwhile, the black color disappeared. Therefore, they believed that the pyramid shape of ZnO nanorods gave rise to the black color. Regretfully the transformation of colors from gray to black was not studied systematically and moreover, no mechanism was proposed. In addition, several researchers have devoted to studying the effect of hydrogenation on the formation of black ZnO samples [31, 34, 35]. The authors generally considered that hydrogenation could introduce additional V$_\textrm{o}$ and interstitial hydrogen (H$_\textrm{i}$) defects because of the presence of Zn-O-H bonds.

As for our experiments, no any carbon and hydrogen atoms were introduced in the whole process, therefore, we suppose that the change of morphology and structure of ZnO nanorods may be the intrinsic reason leading to the color change of the ZnO samples. It is widely acknowledged that the roughness of an object's surface can affect the light absorption or reflectance [33, 36-38]. As demonstrated above, the ZnO nanorods with hexagonal pyramid tops appear in our samples and each cusp of nanorod has six faces, which immensely increase the light reflection frequency among the nanorods, like the light well, capturing light for broadband and leading to the drop of the reflectivity of ZnO samples. Thus, the black ZnO samples can be observed. Besides, the formation of the hexagonal pyramid can be affected by the growth temperature, so it is convenient to obtain the dark color ZnO samples using this method.

Aiming at verifying our conjecture that higher temperature was the main factor about the formation of black ZnO samples and the ZnO nanorods with hexagonal pyramid tops, experiments via tuning H$_2$O$_2$ additive amount and NaOH solution concentration were designed and conducted. In these experiments, it was ensured that only one variable was adjusted and the growth temperature was set at 160 ℃ and the preparation time was 3 h. As shown in FIG. 7, the colors of different ZnO samples are approximately the same brown as the ZnO sample in FIG. 3, that is, the color of the ZnO samples obtained at 160 ℃ is always brown, regardless that the amount of H$_2$O$_2$ additive is 2 mL or 8 mL and NaOH concentration is 0.02 mol/L or 0.08 mol/L. FIGs. 7 and 8 illustrate that there is no evident difference among the SEM, XRD, UV-Vis, and PL results of ZnO samples prepared by changing NaOH solution concentration and H$_2$O$_2$ additive amount. In other words, NaOH and H$_2$O$_2$ are not able to make ZnO samples dark black or improve ZnO optical properties substantially. As a consequence, these two experiments directly confirm that high temperature is the pivotal element towards producing dark black ZnO samples.

 FIG. 7 The optical photos with SEM images of ZnO nanorods prepared by regulating H$_2$O$_2$ additive volume (2-8 mL) and NaOH solution concentration (0.02-0.08 mol/L), respectively.
 FIG. 8 (a) XRD patterns, (b) UV-Vis reflectance, and (c) normalized photoluminescence spectra of ZnO nanorods produced by regulating H$_2$O$_2$ additive volume (2-8 mL) and NaOH solution concentration (0.02-0.08 mol/L), respectively. It was ensured that only one variable was adjusted throughout the experiments.
Ⅳ. CONCLUSION

In conclusion, a practical hydrothermal method was utilized to prepare black ZnO nanorod samples with tunable optical properties on a large scale. At the same time, we authenticate that the elevated temperature greatly affects the formation of dark black ZnO samples. The implied mechanism is that temperature is able to impact the morphology of ZnO nanorods, forming hexagonal pyramid structures, further acting on the colors and optical properties of ZnO samples. We optimistically believe that the black ZnO has great potential in solar energy harvesting.

Ⅴ. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.11575187) and the National Key Research and Development Program (No.2016YFB0700205).

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