Three-Dimensional ZnO Hierarchical Nanostructures: Solution Phase Synthesis and Applications

Abstract

Zinc oxide (ZnO) nanostructures have been studied extensively in the past 20 years due to their novel electronic, photonic, mechanical and electrochemical properties. Recently, more attention has been paid to assemble nanoscale building blocks into three-dimensional (3D) complex hierarchical structures, which not only inherit the excellent properties of the single building blocks but also provide potential applications in the bottom-up fabrication of functional devices. This review article focuses on 3D ZnO hierarchical nanostructures, and summarizes major advances in the solution phase synthesis, applications in environment, and electrical/electrochemical devices. We present the principles and growth mechanisms of ZnO nanostructures via different solution methods, with an emphasis on rational control of the morphology and assembly. We then discuss the applications of 3D ZnO hierarchical nanostructures in photocatalysis, field emission, electrochemical sensor, and lithium ion batteries. Throughout the discussion, the relationship between the device performance and the microstructures of 3D ZnO hierarchical nanostructures will be highlighted. This review concludes with a personal perspective on the current challenges and future research.

Keywords: field emission; hierarchical nanostructures; lithium ion batteries; photocatalysis; sensor; solution phase synthesis; zinc oxide.

Figure 1

Crystal structure model of wurtzite ZnO (reprinted from [5] with permission, Copyright—Elsevier B.V., 2004), and typical morphologies of 1D ZnO nanostructures with exposed facets (reprinted from [2] with permission, Copyright—Elsevier B.V., 2009).

Figure 2

Typical 3D ZnO hierarchical nanostructures and their applications as photocatalysts, field electron emitters, electrochemical sensors, and electrodes for batteries.

Figure 3

Typical solution phase methods for the synthesis of 3D ZnO hierarchical nanostructures. (a) Precipitation; (b) microemulsions; (c) hydrothermal/solvothermal; (d) sol-gel; (e) chemical bath deposition; and (f) electrochemical deposition.

Figure 4

Schematic illustration of the formation process of ZnO 3D hierarchical structures via the combination of (a) growth and in situ etching (reprinted from [8] with permission, Copyright—The Royal Society of Chemistry, 2012); (b) capping and etching (reprinted from [23] with permission, Copyright—The Royal Society of Chemistry, 2015).

Figure 5

Typical 3D ZnO hierarchical nanostructures synthesized by solution phase methods: (a–d) microemulsion process (reprinted from [25] with permission, Copyright—The Royal Society of Chemistry, 2012); (e) repetitive hydrothermal (reprinted from [35] with permission, open access, American Chemical Society, 2011); (f) ion-mediated hydrothermal (scale bars = 500 nm, reprinted from [36] with permission, Copyright—Macmillan Publishers Limited, 2011); (g) sol-gel method (reprinted from [39] with permission, Copyright—Elsevier Ltd and Techna Group S.r.l., 2013); (h,i) electrochemical deposition (reprinted from [49] with permission, Copyright—The Owner Societies, 2011); and (j) chemical bath deposition method (reprinted from [51] with permission, Copyright—American Chemical Society, 2015).

Figure 6

Schematic illustration on the photocatalytic processes in ZnO.

Figure 7

Photocatalytic degradation of RhB via (a,b) 3D ZnO hierarchical nanostructures with different morphologies (reprinted from [8] with permission, Copyright—The Royal Society of Chemistry, 2012); (c–e) ZnO needle flowers and Au nanoparticles/ZnO needle flowers composite (reprinted from [65] with permission, Copyright—The Royal Society of Chemistry, 2011).

Figure 8

(a) Potential energy of an electron near the cathode surface (reprinted from [67] with permission, Copyright American Vacuum Society, 2007); (b) illustration of field electron emission from a tip (reprinted from [68] with permission, Copyright—Elsevier B.V., 2004).

Figure 9

The comparison of field emission properties of ZnO nanowires with flat ends and nanoneedles with sharp ends. (a) Transmission electron microscopy (TEM) images; (b) Current density (J)-applied electric field (E) curves; (c) Fowler–Nordheim plots; (d) stability of the emission current density under a constant electric field of 6.0 V μm⁻¹ (reprinted from [74] with permission, Copyright—Elsevier B.V., 2017).

Figure 10

Schematic illustration of the electrochemical sensor testing.

Figure 11

(a) cyclic voltammograms of bare and modified GCE with pure ZnO, Au/ZnO, and Ag/ZnO in the absence of H₂O₂ and in the presence of H₂O₂; (b) amperometric response of three modified GCE at constant voltage of −0.45 V with successive addition of 1 μM H₂O₂ in 0.05 M PBS under stirring; (c) corresponding calibration curves of the three modified electrodes and (d) stability plot of the three modified GCE at constant potential of −0.45 V in the presence of 1 μM H₂O₂ (reprinted from [81] with permission, Copyright—Springer Science+Business Media Dordrecht, 2016).

Figure 12

(a,b) Electron microscopy images and (c–e) lithium storage properties of the ZnO needle flowers and the Au nanoparticles/ZnO needle flowers composite (reprinted from [65] with permission, Copyright—The Royal Society of Chemistry, 2011).

Figure 13

(a) Schematic illustrating the synthesis procedures of ZnO@ZnO QDs/C core-shell nanorod arrays on carbon cloth; (b,c) TEM and high-resolution transmission electron microscopy (HRTEM) images; (d,e) lithium storage properties of ZnO@ZnO QDs/C core-shell structures (reprinted from [100] with permission, Copyright—John Wiley and Sons, 2015).