1.   Introduction

Zinc oxide (ZnO), which is an important semiconductor, possesses excellent physical and chemical properties [1–2]; these properties promote wide applications, such as light-emitting diodes [3–4], ultraviolet (UV) detectors [5–6], gas sensors [7], and solar cells [8–10]. In addition, ZnO is a nontoxic green material, and its synthesis does not introduce toxic materials during the growth process. Importantly, ZnO exhibits direct bandgap of approximately 3.37 eV and low exciton binding energy of 60 meV [11]. Evidently, these unique properties enable ZnO crystals easily absorb UV light and generate electron–hole pairs, which make them suitable for optoelectronic applications, especially UV detectors [5].

ZnO micro/nanostructures vary in terms of physical properties, which results in their improved properties compared with those of bulk ZnO. To date, two primary methods are used for the synthesis of ZnO micro/nanostructures: hydrothermal reaction and chemical vapor deposition [12–15]. The synthesis of ZnO crystals with high-quality and low-density lattice defects can be achieved through chemical vapor deposition; however, such a process requires the use of expensive vacuum equipment [16]. The solution in the hydrothermal reaction of ZnO is usually aqueous, and the growth mechanism shows a close relation to external conditions, such as solution pH [17], temperature, reaction concentration, and additive [12]. This method is conventionally used for the synthesis of various ZnO micro/nanostructures. However, this synthesis procedure often requires the use of an autoclave, which implies high energy consumption and load-bearing requirements. ZnO morphology engineering can also influence the performance of specific applications. Therefore, the synthesis of ZnO micro/nanostructures with morphology engineering under mild conditions, including at room temperature, remains a challenge.

In this work, a facile solution method was developed to prepare ZnO micro/nanostructures at room temperature using precursors containing zinc nitrate (Zn(NO₃)₂·6H₂O), sodium hydroxide (NaOH), and ammonia (NH₃·H₂O) solution. In particular, the concentration of NH₃·H₂O plays a crucial role in controlling the reaction rate for morphology engineering [18–19]. Adjustment of the morphology of ZnO micro/nanostructures, from random nanoparticles to regular nanosheets, microflowers, and single crystals, can be achieved. This approach paves a novel means to fabricate ZnO micro/nanostructures under low-cost, mild, and environment-friendly conditions. ZnO microflowers and nanosheets were used in interdigitated-electrode UV detectors. The microflower-based device exhibited high performance and operation stability based on the band structure and three-dimensional light capture.

2.   Experimental

The raw materials included Zn(NO₃)₂·6H₂O, NaOH, and NH₃·H₂O solution (25%–28%). Zn(NO₃)₂·6H₂O and NaOH served as sources of Zn²⁺ and OH^–, respectively. NH₃·H₂O can be combined with Zn²⁺ to tune the reaction rate on the effect of ZnO product structures. As shown in Fig. 1, 1.16 g Zn(NO₃)₂·6H₂O was dissolved in 130 mL distilled water with stirring typically, and then NH₃·H₂O solution (1–6 mL) was slowly added to the solution and mixed well to achieve full complexation. During this period, the solution immediately turned turbid and clarified. Next, 0.288 g NaOH was dissolved in 6 mL distilled water completely by ultrasound, which was slowly added dropwise to the above solution under stirring. Finally, the chemical reaction was kept at room temperature 50, 70, and 90°C, respectively. After the reaction, the white precipitates were washed with deionized water and dried at 70°C in an oven overnight.

Figure  1.  Schematic diagram of the synthesis process for ZnO micro/nanostructures.

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Flexible polyethylene terephthalate (PET) substrates were washed with deionized water and ethanol and dried via N₂ flow. Then, the Au electrodes were deposited on the flexible PET substrates through thermal evaporation, and the interdigitated 20 fingers (width 100 µm, interfinger spacing 100 µm, and length 10 mm) were patterned by UV lithography. The ZnO powders exhibited dispersion in ethanol, deposition on the interdigitated Au electrodes, and drying at 70°C to form flexible UV detectors.

The phase of the obtained white powders was checked via X-ray diffraction (XRD) pattern with Cu-K_α irradiation (λ = 1.5418 Å, Bruker D8 Advance X-ray diffractometer). The morphology was characterized via scanning electron microscopy (SEM, FEI NOVA Nano SEM 450) and transmission electron microscopy (TEM, JEOL/JEM-F200). The UV–visible light absorption was measured using a LAMBDA 1050 spectrometer (PerkinElmer). Regarding the UV detector, the current–voltage (I–V) and current–time (I–t) curves were measured on electrochemical workstation (Model 600E Series). The UV light was originated from a light source (365 nm, ZF-1, Shanghai ChiTang Industrial Co. Ltd).

3.   Results and discussion

3.1   Structural and morphological characterization

Fig. 2 shows the XRD patterns of the obtained white precipitates. The diffraction peaks at 31.5°, 34.1°, 36.0°, 47.2°, 56.3°, 62.6°, 67.6°, 68.8°, and 77.0° can be indexed to the (100), (002), (101), (102), (110), (103), (112), (201), and (202) planes of hexagonal wurtzite ZnO phase (JCPDS No. 36-1451), respectively [18–19]. Evidently, when the NaOH solution was directly added to the Zn(NO₃)₂·6H₂O solution, the white ZnO precipitates formed immediately. In the product XRD pattern, an additional peak was located at 25.83°. If the NH₃·H₂O solution participated in the reaction, the resulting products also comprised the ZnO phase. The more NH₃·H₂O added, the lower the recreation rate. Thus, low chemical reaction rate can improve the crystallinity, and thus, with the increase in NH₃·H₂O amount, the diffraction peak intensities of ZnO products showed a gradual increase.

Figure  2.  XRD patterns of ZnO micro/nanostructures synthesized using various amounts of NH₃·H₂O solution.

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The chemical reaction can be expressed as follows [18–19]:

The synthesis of ZnO precipitates can be divided into three stages. First, Zn²⁺ ions were combined with NH₃ from the NH₃·H₂O solution to form ion complexes of Zn(NH₃)${}_x^{2+}$ (x = 1, 2, 3, 4). When the NH₃·H₂O solution was added to the Zn(NO₃)₂ solution, the solution immediately turned turbid and clarified under rocking. The Zn(NH₃)${}_x^{2+}$ complexes slowly released Zn²⁺ after the addition of OH⁻ to form Zn(OH)${}_{4}^{2-}$ [20]. Finally, the Zn(OH)${}_{4}^{2-}$ was transferred to the stable ZnO phase. Therefore, the amount of NH₃·H₂O added can be used to manage the reaction rate of the synthesized ZnO [21]. The increase in NH₃·H₂O amount led to high degree of complexation, which slowed down the reaction rate of ZnO. In contrast, heating speeds up this release process. Thus, the higher the temperature, the faster the reaction rate.

Room-temperature chemical reactions (about 20°C) offer evident advantages. Typically, the process saves energy and facilitates product generation without requiring high-temperature condition. This provides an idea for mass production in industrialization. In this work, room-temperature reaction was first carried out. The dose of the added NH₃·H₂O solution was increased from 0 to 6 mL, with a step of 1 mL. The samples synthesized under the NH₃·H₂O doses from 0 to 4 mL comprised white precipitates. If the NH₃·H₂O solution increased to 5 mL, the products cannot be obtained immediately, and the reaction consumed a considerable amount of time. After 5 d, millimeter-sized crystals can be evidently observed. In particular, when 6 mL or more NH₃·H₂O was added, no product can be observed at room temperature. Heating can increase the reaction activity [22–23]. The increase in the temperature may enable the synthesis of ZnO with NH₃·H₂O addition of 6 mL or more. Dried white precipitates were obtained after the reaction under heat.

SEM and TEM were applied in the investigation of the morphology of ZnO products. Fig. 3 shows the morphology of the ZnO synthesized at room temperature using various amounts of NH₃·H₂O. In the absence of NH₃·H₂O, the reaction occurred quickly, which led to the random and aggregated morphology of ZnO nanoparticles (Fig. 3(a)). After the addition of 1 mL NH₃·H₂O in the reaction, the products also aggregated but assumed sheet shape with microscale surface (Fig. 3(b)). Evident ZnO nanosheets were observed when 2 mL NH₃·H₂O was used (Fig. 3(c)). With the increase in NH₃·H₂O to 3 mL (Fig. 3(d)), the nanosheets become more evident and regular with sizes of 3–5 µm. Moreover, the sheets gradually became thinner. On the one hand, the increase in NH₃·H₂O concentration reduced the reaction rate. On the other hand, stirring the reaction solution provided tangential shear force. Therefore, two-dimensional (2D) ZnO nanosheets can be formed at an appropriate NH₃·H₂O concentration. When the NH₃·H₂O added was increased to 4 mL, the nanosheets thickened due to the slow reaction rate (Fig. 3(e)), and the size of the nanosheets decreased to approximately 1 µm. When the NH₃·H₂O concentration was increased to 5 mL, the reaction rate further slowed down, and the product transformed into macroscopic and millimeter-scale single crystals after 5 d (Fig. 3(f)).

Figure  3.  SEM images of ZnO micro/nanostructures synthesized at room temperature using various amount of NH₃·H₂O: (a) 0; (b) 1 mL; (c) 2 mL; (d) 3 mL; (e) 4 mL; (f) 5 mL.

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The 2D ZnO nanosheets were also characterized via TEM technology. Fig. 4(a) shows a typical 2D ZnO nanosheet having the polycrystalline characteristics observed from the selected area electron diffraction (SAED) (Fig. 4(b)). The spotted rings corresponded to the (100), (002), (101), (102), (110), and (103) lattice planes of hexagonal wurtzite ZnO. The high-resolution TEM image in Fig. 4(c) reveals that the lattice fringes with distances of 0.284 (0.287) and 0.267 nm (0.268 nm) were indexed to the ZnO (100) and (002) planes, respectively. The random domains further conformed to the polycrystalline nature of 2D ZnO nanosheets. To the best of our knowledge, this phenomenon is the first documentation of polycrystalline 2D ZnO nanosheets [24].

Figure  4.  (a) TEM image, (b) selected area electron diffraction, and (c) high-resolution TEM image of the typical ZnO nanosheet.

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Given the presence of the reaction with 6 mL NH₃·H₂O addition at room temperature, the temperature was set to 50, 70, and 90°C. The required reaction times periods varied with temperature. The reaction times under 50, 70, and 90°C lasted for 48, 24, and 12 h, respectively, with the corresponding SEM images displayed in Fig. 5(a)–(c). All products had microflower morphology, which was composed of nanorods. As the temperature rose to 90°C, some microflowers collapsed as independent nanorods due to the rapid reaction rate. Consequently, moderate temperature aids in attaining more regular morphology.

Figure  5.  SEM images of ZnO structures synthesized at various temperatures and NH₃·H₂O: (a) 50°C, 6 mL; (b) 70°C, 6 mL; (c) 90°C, 6 mL; (d) 70°C, 4 mL; (e) 70°C, 5 mL; (f) 70°C, 7 mL.

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The reaction rate slowed down with the increased amount of NH₃·H₂O addition. Moreover, when the NH₃·H₂O reached 4 mL, the ZnO nanosheets thickened due to the slower reaction rate, and the output of immediately collected ZnO precipitates decreased at room temperature. Thus, when the temperature was set at 70°C, the NH₃·H₂O amount begin from 4 mL. Fig. 5(d)–(f) shows the SEM images of the ZnO synthesized at 70°C with NH₃·H₂O addition of 4, 5, and 7 mL, respectively. Fig. 5(d) shows the presence of microflowers and microspheres comprising nanoparticles and nanorods. Regular ZnO microflowers were obtained when the NH₃·H₂O addition increased to 5 and 6 mL, as shown in Fig. 5(e) and Fig. 5(b), respectively. When it further increased to 7 mL, excessive NH₃·H₂O led to high-alkaline environment, which destroyed the ZnO products to random aggregation of nanoparticles (Fig. 5(f)).

3.2   Optoelectronic application

The ZnO microflowers synthesized using 5 mL NH₃·H₂O addition at 70°C can accomplish 3D light capture, which enables the efficient absorption of UV light (Fig. S1, see the supplementary material) [25]. The ZnO microflowers were further applied to the UV detector (Fig. 6). Fig. 6(a) shows the I–V curves of the UV detectors in the dark and under 365 nm UV, and Fig. 6(b) displays the amplified dark current. An evident photocurrent of the ZnO UV detectors was observed. The calculated light on/off ratio at 1 V was approximately 50. In addition, the Schottky contacts formed between ZnO microflowers and Au electrodes. Fig. 6(c) shows the switching curve of current in the dark and under UV illumination with a constant bias voltage of 1.0 V. The rising and decaying photocurrents obey the exponential function, which can be fitted with the corresponding time. The two exponential function Eqs. (4) and (5) used for respective rising and decaying photocurrents (I_t) with the time (t) are expressed as follows [26–27]: The two exponential function Eqs. (4) and (5) used for respective rising and decaying photocurrents (I_t) with the time (t) are expressed as follows

Figure  6.  (a) I–V characteristics of ZnO-microflower-based UV detectors with and without 365 nm UV illumination, (b) I–V characteristics of ZnO UV detectors without light irradiance and (c) reversible switching of electrical current for ZnO detector at 1 V biasing voltage in the dark state and under 365 nm UV illumination.

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where I₀ denotes the dark current, A₁, A₂, and A₃ indicate positive constants, τ_r1 and τ_d (τ_d1 and τ_d2) are time constants for rising and decaying photocurrents, respectively. On the basis of curve fittings, the rise time constant for ZnO was τ_r1 = 21.3 s, and the decay time constants were τ_d1 = 13.7 s and τ_d2 = 36.3 s, which are similar to the previous results on ZnO nanowire arrays [26–27]. In addition, the UV sensitivity of 2D ZnO nanosheets was measured (Fig. S2). The photocurrent is lower than that of the microflower-based device. Therefore, the obtained ZnO microflowers can be applied as excellent UV detectors.

4.   Conclusion

ZnO micro/nanostructures were prepared through simple and low-cost hydrothermal method, nearly at room temperature, through the introduction of NH₃·H₂O solution. The NH₃ in NH₃·H₂O solution combined with the Zn²⁺ precursor to form complexes, which can be used to tune the reaction rate and influence of the resulting ZnO structures. At a precise amount of NH₃·H₂O addition, the polycrystalline 2D ZnO nanosheets can be obtained as a result of the stirring-induced tangential shear force at room temperature. Meanwhile, during relatively slow reaction, ZnO single crystals formed after 5 d at room temperature. Heating (such as 70°C) can increase the reaction rate, which leads to 3D microflowers. The ZnO microflowers can effectively capture UV light as detectors. Therefore, this facile and low-cost synthesis method can be applied in tuning ZnO micro/nanostructures and be easily scaled-up for industrial production, which will promote their optoelectronic applications, typically in UV detectors.