Fabrication of hierarchical core–shell Au@ZnO heteroarchitectures initiated by heteroseed assembly for photocatalytic applications
Introduction
Semiconductor–metal heterostructured nanomaterials have attracted great interest in recent years because they can not only combine the unique properties of metals and semiconductors, but also generate novel electrical, optical, and catalytic properties due to the synergetic interaction between the metal and the semiconductor components [1], [2], [3], [4], [5], [6]. As a representative class of semiconductor–metal nanostructured materials, Au–ZnO nanocomposites have received special attention due to their promising applications in solar energy conversion [7], biological detection [8], [9], sensing fields [10], [11], [12] as well as photocatalysis [13], [14], [15], [16]. Moreover, from the perspective of fundamental science, it is of great significance to explore novel photoelectric mechanisms tied to the morphologies and structures of Au–ZnO heterojunction materials, and their relevant catalytic, optical and electric properties [2], [17], [18], [19], [20], [21], [22]. Therefore, great efforts have been devoted to fabricating Au–ZnO nanocomposites with different shapes and structures recently. For example, a variety of ZnO–Au heterostructures including ZnO NRs/Au NPs [17], [18], [22], [23], [24], flower-like ZnO/Au NPs [7], [10], [13], [14], [25], [26], ZnO nanocones/Au NPs [15], [27], and ZnO NPs/Au NPs [9], [28], [29], [30], [31], [32] have been synthesized. However, in most of these work, gold NPs were merely dispersed or deposited on the surface of presynthesized ZnO nanostructures. In contrast, inverse structure form of the two components such as Au@ZnO core–shell structures where ZnO nanostructures grown on Au NPs prepared via reverse synthetic approaches was seldom reported [33], [34], [35], due probably to the synthesis difficulty of growing ZnO nanostructures on gold nuclei. Nonetheless, as reported by Xu’s group, core–shell structured M@TiO2 (M = Au, Pd, Pt) nanocomposites encapsulating metal NPs within the semiconductor shell in fact exhibited much higher stabilities against aggregation and undesirable metal corrosion in the practical photocatalysis applications than their oxide supported counterparts [36], [37]. Thus, more subtle structure design for Au–ZnO core–shell nanocomposites is highly desired to fully explore their potential advantages as promising heterostructured photocatalysts. It is generally recognized that effective separation and transportation of photogenerated charges as well as high light harvesting of photocatalysts play decisive roles in determining their photocatalytic performance [14], [19], [38]. Therefore, for Au@ZnO heterostructures to acquire high photocatalytic activity, particular structure designs are required: First, complete separation of Au cores and ZnO shells with defined interfaces. This is beneficial for not only effective electron–hole separation and transfer at the interfaces of the cores and shells, but also the subsequent reduction and oxidation half reactions proceeded separately in the core and shell regions [39]. Second, highly epitaxial ZnO nanostructure with high carrier mobility. It can facilitate charge transportation and thus reduce charge recombination. Third, open hierarchical architectures of the Au@ZnO nanocomposites with large specific surface area (SSA). These features will guarantee high light trapping capacity and provide abundant accessible sites for effective molecule adsorption/desorption [38].
Through a rational design, here we demonstrate the fabrication of 3D hierarchical core–shell Au@ZnO dandelions through a simple solution approach involving heteroseed induced nucleation and epitaxial growth of ZnO nanorods. The synthesized product is composed of Au NP cores and ZnO shells of nanorod arrays grown radially on Au cores. The as-prepared Au@ZnO dandelions are attractive for photocatalysis according to the structure advantages discussed above: separate Au and ZnO components confined in respective core and shell regions with good interfacial contact for effective charge separation and transfer at the interface of Au cores and ZnO shells, 1D ZnO nanorods with high carrier mobility for fast and direct electron transportation over all shells, dandelion-like hierarchical architectures with relatively high SSA for efficient light trapping and molecule adsorption/desorption. Besides, radial ZnO nanorod arrays anchored tightly on Au NP cores can avoid serious aggregation relative to common catalysts in the form of dispersed nanoparticles, guaranteeing high catalytic stability and long lifetime of the catalyst. To the best of our knowledge, the novel Au@ZnO dandelions are reported here for the first time. As expected, the typical Au@ZnO dandelions display higher photocatalytic activity and stability when evaluated through RhB photodegradation, compared with other pure ZnO nanorods (ZnO NRs), ZnO nanorod-Au NP hybrid (ZnO NRs–Au NPs, prepared by depositing Au NPs on the surface of ZnO nanorods).
Section snippets
Materials
Chloroauric acid (HAuCl4⋅4H2O), Rhodamine B (RhB) and mercaptopropionic acid (MA) were purchased from Alfa Aesar. Anhydrous zinc acetate (Zn(CH3COO)2) and sodium citrate were purchased from Sigma–Aldrich Company. Hexamethylene tetramine(HMT), potassium hydroxide(KOH) and anhydrous ethanol were purchased from Shanghai Chemical Reagent Co., Ltd. All the above reagents are of analytical grade and used as purchased.
Synthesis of the Au@ZnO dandelions
Briefly, Au nanoparticles with the size of ∼20 nm were synthesized by reduction of
Results
The representative SEM images of the product obtained under standard conditions are shown in Fig. 1a and b. According to Fig. 1a, the product consists of high-yield dandelion-like spherical assemblies with about 1–1.5 μm in size. Fig. 1b is an enlarged SEM image of these dandelion-like assemblies, which suggests that each “dandelion” is comprised of radially arrayed nanorods with 50–100 nm in diameter and 500–700 nm in length growing around a core. The typical TEM image in Fig. 1c further reveals
Discussions
To investigate the formation mechanism of the Au@ZnO dandelions, the morphology evolution of Au NPs at different synthesis stages before and after ZnO seed coating was traced (Fig. 2). Fig. 2a presents the typical Au NPs synthesized according to Frens’s method [40]. It can be seen that the gold NPs without ZnO seed coating are monodisperse with a uniform diameter of about 20 nm. The HRTEM image (inset in Fig. 1a) shows several discrete lattice fringes with equal lattice distance of 0.23 nm
Conclusions
In summary, we have prepared novel Au@ZnO core–shell dandelions with 3D hierarchical architectures through a solution growth approach initiated by heteroseed assembly and nucleation. Compared with pure ZnO NRs and heterostructured ZnO NRs–Au NPs, the typical Au@ZnO dandelions display higher photocatalytic activity and stability in photodegradation of RhB. The enhanced performance is ascribed to its special core–shell hierarchical heterostructures that are beneficial for photogenerated charge
Acknowledgments
This work was financially supported by National Natural Science Foundation (21001082, 21273161 and 21101117), Shanghai Innovation program (13ZZ026), Scientific Research Foundation for the Returned Overseas Chinese Scholars of SEM, The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (2013-13), Visiting scholar fund of the Key Laboratory for Ultrafine Materials of Ministry of Education, East China University of Science and Technology, and
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These authors contributed equally to this work.