Facile synthesis of hierarchical ZnO microstructures with enhanced photocatalytic activity

The ZnO microflowers were synthesized by a simple hydrothermal process. In a typical synthesis procedure, 0.744 g Zn(NO₃)₂·6H₂O and 0.6 g NaOH were dissolved in the reaction solution containing 15 mL ethanol and 10 mL water under vigorous stirring for 10 min. Then, the mixture was transferred into a 30 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 24 h. The autoclave was taken out and allowed to cool naturally to room temperature. The as-prepared product was separated by centrifugation, washed several times with distilled water and ethanol, and finally dried at 60 °C in air for 5 h.

The XRD patterns of the products were examined using a Rigaku D/max 2500V/PC X-ray diffractometer with CuKα radiation (λ = 1.5406 Å), employing a scanning rate of 0.017 s^-1. The morphology of the as-synthesized products was examined by field emission scanning electron microscope (FE-SEM, JEOL JSM-6700F), transmission electron microscopic (TEM)/high-resolution TEM (HRTEM) (Philips Tecnai G2 F20).

All photoreaction experiments were carried out in a photocatalytic reactor system, which consisted of a cylindrical borosilicate glass reactor vessel with an effective volume of 500 mL, a cooling water jacket, and a UV-A lamp (8 W medium-pressure mercury lamp (Institute of Electric Light Source, Beijing) positioned axially at the center as a visible light. The reaction temperature was kept at 20 °C by circulating the cooling water. A special glass frit as an air diffuser was fixed in the reactor to uniformly disperse air into the solution. Photocatalytic activities of the prepared samples were examined by the degradation of MB under UV-A light irradiation. For each run the reaction suspension was freshly prepared by adding 0.10 g of catalyst into 250 mL of aqueous MB solution, with an initial concentration of 5 mg·L^-1. The aqueous suspension containing MB and the catalyst was then irradiated under UV-A light with constant aeration. At the given time intervals, the sample was withdrawn from the suspension and centrifuged at 4000 revolutions per minute (rpm) for 15 min to remove the catalyst. The absorption spectra were recorded using an UV-Vis spectrophotometer (Perkin-Elmer Lambda 45). The maximum characteristic absorption wavelength of MB was positioned at 664 nm.

Fig. 1a shows the XRD pattern of as-prepared ZnO hierarchical microstructures. All diffraction peaks can be indexed to the hexagonal wurtzite structure of ZnO (JCPDS Card No. 36-1451). No other noticeable peaks of impurities have been detected, which indicates high purity of the as-prepared ZnO. The morphologies of the product were examined with SEM. Fig. 1b shows a typical SEM image of the ZnO microflowers, composed of interleaving thin plates or nanosheets. The microflowers are about 2 µm in diameter.

In this section, we report our results of the pH effect on the morphology of ZnO crystals. When the pH of the solution is 13.1, the product is composed of nanoparticles of about 200 nm to 400 nm in diameter (Fig. 2a). When the pH of the solution is increased to 13.6, the nanoplates with the size of 150 nm to 350 nm are synthesized (Fig. 2b). When the pH of the solution is further increased to 13.7, the formed nanosheets are getting thinner and they gather together to form the flower-like microstructures (Fig. 2c). The complex structures are made of nanosheets which grow continuously from one layer to another in a helical fashion. The above experimental results show that solution pH has a strong influence on the morphology of ZnO crystals.

As it is well known, the morphology of nanocrystals in morphology-controlled synthesis method can be controlled by intrinsic crystal structure [22]. Fig. 1a demonstrates that the as-prepared ZnO crystal is a close-packed hexagonal structure. Thus, the ZnO crystal growth habit demonstrates the full crystalline 2D nanoplates. The possible growth mechanism of the ZnO microflowers has been illustrated in Fig. 3. First, Zn²⁺ and OH^-react easily and lead to fast nucleation and congregation, resulting in sphere-like core, which could serve as heterogeneous nucleation sites, providing high-energy sites for the growth of primary crystalline particles. Then, a secondary nucleation is favorable, and cubic angular protuberances grow into nanopetals combining with the remaining primary particles, finally forming the 3D flower-like hierarchitecture, which is similar to what has been described as the terrace-step-kink model [23]. This is similar to the formation process of the flowerlike nanocrystals, such as CoS [24], CuO [25], TiS₂ [26], In₂O₃ [27], AlOOH [28], Bi₂MoO₆ [29], Ni(OH)₂ [30] reported previously.

The photocatalytic activity of the as-prepared ZnO samples was evaluated by degradation of MB dye under UV-A light irradiation. Fig. 4 shows the degradation of these samples by monitoring the concentration of MB solution at the maximum absorption of 664 nm, in which C/C₀ stands for the MB concentration ratio after and before a certain reaction time. After addition of ZnO photocatalyst into the MB solution, the photocatalytic activity of ZnO crystals can be well tuned by the morphology. According to the photocatalysis data, after irradiation for 3 h, the MB remaining in the solution is about 30 %, 10 %, 5 % and 1 % for the ZnO samples, respectively (Fig. 4). Especially, the ZnO nanoplates exhibit the most excellent photocatalytic performance, and they could almost completely degrade MB dye in 3 h. Therefore, these experiments indicate that the photocatalytic activity of ZnO nanocrystals could be significantly improved by tailoring the shape and surface structure. As to the photocatalytic mechanism of ZnO nanoflowers in MB, we assume that the electrons injection from the photo-excited MB molecules to ZnO nanoflowers leads to oxidative decomposition of the electron-deficient MB. The ZnO nanoflowers show a higher photocatalytic activity comparing to the nanoplates, due to the higher surface energy of the (0 0 0 1) surface exposure and hierarchical structure formation in the nanoflowers.