Synthesis of ZnGa2O4 Hierarchical Nanostructure by Au Catalysts Induced Thermal Evaporation
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In this paper, ZnGa2O4 hierarchical nanostructures with comb-like morphology are fabricated by a simple two-step chemical vapor deposition (CVD) method: first, the Ga2O3 nanowires were synthesized and employed as templates for the growth of ZnGa2O4 nanocombs; then, the as-prepared Ga2O3 nanowires were reacted with ZnO vapor to form ZnGa2O4 nanocombs. Before the reaction, the Au nanoparticles were deposited on the surfaces of Ga2O3 nanowires and used as catalysts to control the teeth growth of ZnGa2O4 nanocombs. The as-prepared ZnGa2O4 nanocombs were highly crystallized with cubic spinel structure. From the photoluminescence (PL) spectrum, a broad band emission in the visible light region was observed of as-prepared ZnGa2O4 nanocombs, which make it promising application as an optical material.
KeywordsZnGa2O4 Hierarchical nanostructure Chemical vapor deposition Au catalyst
With the development of nanotechnology, low dimensional nanostructures are desired nanobuilding blocks for the assembly of various electronic and optical nanodevices to realize their potential applications [1–4]. So far, many kinds of low dimensional nanostructures, such as 1-D nanowires, nanorods, nanobelts, or 2-D nanosheets have been synthesized and studied. For low dimensional nanostructures, the morphology, structure, and size may sensitively affect the properties of nanostructures, so it is of high importance to fabricate nanostructures with designed morphology and size in a controlled way.
ZnGa2O4 is an important semiconducting material for applications in flat-panel displays as a blue phosphor, for its good cathode luminescence characteristics at low driving voltage and with more stability in high vacuum than sulfide-based phosphors [5–12]. Moreover, since ZnGa2O4 has a low resistivity at room temperature , it is also a promising transparent conducting oxide (TCO) when transparency through the violet to near UV region is desired. ZnGa2O4 are recently proven to be a promising photocatalyst for environmental purification of air and water polluted by organic compounds due to its photo-electrochemical properties [13, 14], it may have potential use in the environmental purification field.
In the past few years, ZnGa2O4 nanowires and thin films have been synthesized using various methods such as solid-state reaction [7, 15, 16], sputtering , sol–gel processing , electrophoresis , pulsed laser deposition , thermal evaporation [20, 21], and chemical vapor deposition [22–26]. However, the synthesis of hierarchical ZnGa2O4 nanostructures has not been investigated yet. As is known, the hierarchical nanostructures will improve the performance of materials in the field of optics, electronics, and catalysis [27–29]. In this paper, we present a novel route for the synthesis of ZnGa2O4 nanocombs in a controlled way by a simple CVD method. The optical properties of ZnGa2O4 nanocombs have been studied by the room-temperature PL, a broad band emission with the full wavelength at half maximum of about 175 nm in visible light region can be observed.
The synthesis of the ZnGa2O4 nanocombs was carried out in a conventional horizontal furnace in two steps, and the Ga2O3 nanowires were first synthesized as the templates for the following growth of ZnGa2O4 nanocombs. In brief, an alumina tube (outer diameter: 25 mm; length: 80 cm) was mounted horizontally inside a single-zone high temperature resistance furnace. For the synthesis of Ga2O3 nanowires, a mixture of Ga2O3 and active carbon powders (molar ratio 1:2) was put in an alumina boat that was located at the center of the furnace tube, and a silicon wafer coated with ~3-nm Au film was placed downstream at a distance of 4 cm. Before heating, the system was purged with 100-sccm (standard cubic centimeter per minute) high-purity argon (Ar, 99.999%) for 1 h. The furnace was heated up to 1,000°C and kept at this temperature for 30 min. After the furnace cooled down to room temperature, a layer of white products was deposited on the Si wafer.
The as-prepared Ga2O3 nanowires on Si substrate were coated with ~2-nm Au film through an ion coater Eiko-IB-3 (Vacuum: 0.2 Torr, electricity current: 6 mA for 10 s), and then annealed at 1,000°C for 30 min under the high-purity Ar gas surrounding. After annealing, Au particles formed from the congregation of Au film were well arranged on the side surface of the Ga2O3 nanowires, and they act as the secondary catalysts guiding the teeth growth.
Then, one gram of ZnO and active carbon powders (molar ratio 1:2) was put in an alumina boat placed at the center of an alumina tube. The Ga2O3 nanowires with the Au nanoparticles on its side were placed downstream at a distance of 4 cm. Before heating, a carrying gas (100 sccm Ar) was introduced into the tube for about 30 min. Under the constant flow of Ar, the furnace was rapidly raised to 850°C in 10 min and kept at this temperature for 10 min. After reaction, white products on the Si substrate were obtained.
The as-prepared samples were characterized using an X-ray diffraction (XRD, Philips X’pert PRO) with Cu K α radiation, field-emission scanning electron microscopy (FE-SEM, Sirion 200), high-resolution transmission electron microscopy (HRTEM, JEOL-2010), and photoluminescence (PL) spectrometer (JY Fluogolog-3-TAU, Xe lamp) at room temperature.
Results and Discussion
In recent reports, Ga2O3 nanowires are employed to synthesis ZnGa2O4 nanowires through high temperature reaction with ZnO vapor [16, 21]. Inspired by this, here in this paper, Ga2O3 nanowires were used as templates for the growth of ZnGa2O4 nanocombs. Catalyst induced growth is well known as a powerful method to control the growth of 1-D nanostructures. In order to guide the growth of the teeth of nanocombs, Au nanoparticles are introduced in our experiment. A thin layer of Au was deposited on the surface of as-grown Ga2O3 nanowires. After annealed at 1,000°C for 30 min, Au layer congregate into nanoparticles. As shown in Fig. 1c, Au nanoparticles arrange regularly on the side surface of Ga2O3 nanowires, which may derive from the difference of surface energy of Ga2O3 crystal planes. Orderly arrange of Au nanoparticles on the specific plane of Ga2O3 nanowire may have low energy and remain stable. This phenomenon is used to obtain the controlled growth of the teeth of ZnGa2O4 nanocombs. In addition, the annealing process is very important to get comb-like Ga2O3 nanostructures. We will discuss in the following.
In summary, we present an easy route to synthesize comb-like ZnGa2O4 nanostructures in a controllable way. The Ga2O3 nanowires were used as templates for the following growth of comb-like ZnGa2O4 nanostructures through the reaction with Zn and/or ZnO x vapor at high temperature. By annealing Ga2O3 nanowires coated with a thin layer of Au film at high temperature, the congregation of Au particles from Au film is the key to the formation of ZnGa2O4 nanoteeth via VLS mechanism. PL spectra for ZnGa2O4 nanocombs show a broad band emission in the visible light region from 400 to 575 nm at room temperature. This method can be easily applied to hierarchical nanostructure growth of other materials to enrich the family of low dimensional nanobuilding blocks and may find potential applications in nanotechnology.
This work was supported by the National Natural Science Foundation of China (No.50671099, 50172048, 10374090 and 10274085), the Ministry of Science and Technology of China (No.2005CB623603), and the Hundred Talent Program of Chinese Academy of Sciences.
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