Research Article www.acsami.org
Synthesis of Hollow Nanotubes of Zn2SiO4 or SiO2: Mechanistic Understanding and Uranium Adsorption Behavior Shalini Tripathi,† Roopa Bose,§ Ahin Roy,† Sajitha Nair,§ and N. Ravishankar*,† †
Materials Research Centre, Indian Institute of Science, Bangalore 560012, India Department of Atomic Energy, Atomic Minerals Directorate for Exploration and Research, Nagrabhavi, Bangalore 560012, India
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S Supporting Information *
ABSTRACT: We report a facile synthesis of Zn2SiO4 nanotubes using a two-step process consisting of a wet-chemical synthesis of core−shell ZnO@SiO2 nanorods followed by thermal annealing. While annealing in air leads to the formation of hollow Zn2SiO4, annealing under reducing atmosphere leads to the formation of SiO2 nanotubes. We rationalize the formation of the silicate phase at temperatures much lower than the temperatures reported in the literature based on the porous nature of the silica shell on the ZnO nanorods. We present results from in situ transmission electron microscopy experiments to clearly show void nucleation at the interface between ZnO and the silica shell and the growth of the silicate phase by the Kirkendall effect. The porous nature of the silica shell is also responsible for the etching of the ZnO leading to the formation of silica nanotubes under reducing conditions. Both the hollow silica and silicate nanotubes exhibit good uranium sorption at different ranges of pH making them possible candidates for nuclear waste management. KEYWORDS: hollow, silicate nanotube, in situ TEM, Kirkendall diffusion, uranyl species, surface charge
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INTRODUCTION Hollow nanostructures have emerged as an exotic class of functional nanomaterials with applications in different fields1−8 including catalysis,1,4,7,9,10 biomedical applications,1,4 optoelectronics,1 and water purification.11 The Kirkendall process, based on the differences in the diffusivities of the diffusing species involved, is one of the most popular routes for the synthesis of hollow structures.6,12−19 In terms of materials, ZnO-based ternary compounds20 exhibit superior functionalities as compared to the pristine ZnO phase.21 Zn2SiO4 is an important compound having several interesting properties leading to applications involving luminescence,22 gas sensing,23 toxic metal-ion absorption,24,25 and catalysis.26 Several studies have shown that hollow nanostructures of Zn2SiO4 can be very useful for Li+-ion intercalation27 and adsorption of toxic metal ions like Pb2+ and Cd2+.11 The ion-absorption property originates from ion-exchange as well as the capability to accommodate foreign ions at vacant interstitial sites.11 Facile wet-chemical synthetic techniques to fabricate the onedimensional (1-D) nanowires/nanorods,28 two-dimensional (2D) sheets,11 and zero-dimensional (0-D) spheres29 are available in literature. However, a simple chemical route for the synthesis of hollow 1-D structured Zn2SiO4 is not yet reported. Common methods adopted to fabricate such hollow nanostructures encompass several physical processes such as solid-state reaction,20 solid−vapor process,30 and atomic layer deposition (ALD)31 wherein SiO2 is coated over ZnO nanostructures © XXXX American Chemical Society
followed by annealing. The barrier for Zn2SiO4 formation is the diffusion of Zn2+ cations and O2− anions across the interface and requires a very high temperature (typically >900 °C)32 to overcome the large activation barrier for defect nucleation.33 In this work, we demonstrate that the use of a wet-chemical route facilitates facile synthesis of Zn2SiO4 nanotubes. We synthesize ZnO nanorods using hydrothermal synthesis and use the conventional sol−gel technique to coat a porous shell of silica. We show that the high defect density in the form of voids at the interface between ZnO and the silica shell facilitates enhanced diffusion, while the presence of accessible pores facilitates transport of oxygen to the interface, thus significantly lowering the temperature of formation of Zn2SiO4. The mechanism of formation of Zn2SiO4 has been studied in situ by heating the ZnO@SiO2 inside the transmission electron microscope (TEM). We investigated the effect of various reaction parameters such as temperature and atmosphere on the formation of the Zn2SiO4 phase. We report the formation of hollow amorphous SiO2 nanotube in a reducing atmosphere through sacrificial templating of ZnO rods. Furthermore, the fabricated silicate and silica nanotubular structures show excellent uranium adsorption property. Thus, we present a conceptual platform of defect nucleation and propagationReceived: June 16, 2015 Accepted: November 16, 2015
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DOI: 10.1021/acsami.5b09805 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
ACS Applied Materials & Interfaces
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RESULT AND DISCUSSION The fabrication of Zn2SiO4 nanotubes was achieved by heating SiO2-coated hydrothermally prepared ZnO nanorods (NRs) under atmospheric conditions. The ZnO NRs@SiO2 nanostructure was obtained by Stöber method, by employing the hydrolysis and immediate condensation of Si-ester under alkaline conditions in the presence of aliphatic alcohol to form amorphous SiO2.34 Figure 1a shows bright-field TEM image of the ZnO nanorods. The wurtzite ZnO rods show preferred growth
controlled formation mechanism of hollow 1-D Zn2SiO4, based on extensive real-time monitoring of the phenomenon using in situ TEM.
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Research Article
EXPERIMENTAL SECTION
Synthesis of ZnO Nanorods. In a typical synthesis, 14.75 g of zinc acetate dihydrate (Zn(ac)2·2H2O) was uniformly dispersed in 62.5 mL of methanol by stirring for 30 min in an oil bath maintained at 65 °C. To this, 7.4 g of KOH dissolved in 32.5 mL of methanol was added under vigorous stirring. The whole mixture was heated to reduce the volume to half the original volume. Then, the solution was transferred in a Teflon bomb, sealed in a stainless steel autoclave, and heated at 120 °C for 6 h. The white precipitate obtained was washed several times with methanol and water and dried under vacuum at 80 °C for 2 h. Silica Coating over Nanostructures. The silica coating over the ZnO nanorods was performed by Stöber process. Desired nanostructure (0.2 g) was dispersed in a mixture of 9 mL of deionized water and 20 mL of ethanol by ultrasonication for 30 min. After that 0.5 mL of 25% ammonia solution and 0.5 mL of tetraethylorthosilicate (TEOS) was added to it, followed by stirring for 3 h. The final product was collected by centrifugation and washed with water and ethanol several times. The sample was dried under vacuum at 80 °C for 2 h. Fabrication of Zn2SiO4 Nanotubes. In a typical synthesis of silicate nanotubes, the ZnO@SiO2 nanorods were heated in atmospheric condition at 900 °C for 30 min to form the complete nanotube structure. However, to study the systematic formation of the silicate, the heating was performed at various temperatures ranging from 500 to 900 °C. Fabrication of SiO2 Nanotubes. The ZnO@SiO2 core−shell was heated under a reducing atmosphere of 95% Ar + 5% H2 gas mixture, at 800 °C for 2 h. Characterization Techniques. For the different phases, formed at different annealing conditions, in between ZnO@SiO2 and Zn2SiO4; the X-ray diffraction patterns were recorded using a Philips X’pert pro diffractometer equipped with a radiation source of Cu Kα. The in situ heating was performed in Tecnai T20 TEM operating at 200 kV, whereas the compositional informational studies using energy dispersive X-ray spectroscopy (EDS) were performed in a Tecnai F30 TEM operating at 300 kV. Sample preparation for TEM studies involving imaging, diffraction, and spectroscopy were done with Ccoated Cu grid, while for in situ thermal study of Kirkendall process, sample was drop cast over Protochips (E-chips from Audro). UV− visible spectroscopic studies were performed using PerkinElmer Lambda 750 UV−vis Spectrometer. The photoluminescence spectra were recorded using Horiba Lab Raman HR 800 spectrometer using a laser of 325 nm. Uranium Adsorption Study. Instrumentation. All inductively coupled plasma optical emission spectrometry (ICP-OES) measurements were performed using Horiba JobinYvon France model 2000(2). The instrument is equipped with rapid scanning monochromator (Focal Length 640 mm), ruled grating of 4320 grooves/ mm, and modified Czerny−Turner mounting system with 0.005 nm/ mm (first order) dispersion, 40.68 MHz crystal controlled radio frequency generator using forwarded power of 1000−1200 W. The pH of the solutions was measured using pH meter (Elico India make). Reagents. All reagents and chemicals used were of AnalaR (BHD pool, England) or GR quality. A stock solution 1000 mg/L of U(VI) was prepared by dissolving 0.5275 g of spectroscopically pure uranyl nitrate hexahydrate (UO2(NO3)2·6H2O) in 250 mL with 4 mL of 1 M HNO3. A Zn stock solution 1000 mg/L was prepared by weighing spectroscopically pure zinc granules in 5% HNO3. All solutions were diluted with high purity Millipore water having conductivity of 0.01 mhos; the pH of the solution was adjusted using dilute ammonia and doubly distilled HNO3. Uranium and zinc were estimated using inductively coupled plasma atomic emission spectroscopy (ICP-AES) at the most sensitive wavelengths.
Figure 1. Low magnification BF images of (a) ZnO nanorods and (b) ZnO@SiO2 core−shell nanostructures formed upon Stöber coating.
along the c-direction. The presence of ZnO nanorods in the medium during the sol−gel synthesis of silica leads to a preferred formation of silica on the ZnO nanorods by heterogeneous nucleation (Figure 1b). The X-ray diffraction (XRD) pattern of the core−shell structure shows peaks corresponding to wurtzite ZnO phase. Absence of any crystalline SiO2 peaks in the XRD indicates the amorphous nature of the SiO2 shell. The thickness of the silica shell can be controlled by varying the concentration of the Si-ester in the reaction. In a typical synthesis, the SiO2 shell thickness is ∼15− 25 nm. To form the ternary Zn2SiO4 nanotube, the ZnO@SiO2 was heated in air. Figure 2 shows the XRD patterns of the as-
Figure 2. XRD of ZnO@SiO2 rods heated at 800 °C and a completely formed Zn2SiO4 nanotube upon completion of reaction at 900 °C.
synthesized ZnO@SiO2 and the same sample heated in air at 900 °C. As can be seen from the figure, there is a complete conversion of ZnO@SiO2 to Zn2SiO4. The gradual phase evolution with temperature was also monitored. Figure S1 shows the XRD patterns of the as-synthesized ZnO@SiO2 samples and the samples heated in air at 700, 800, and 900 °C, showing the evolution of silicate phase at different temperatures. While the peaks seen in the as-synthesized sample B
DOI: 10.1021/acsami.5b09805 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
the SiO2-coated ZnO NRs show a quenching of the band-edge emission of ZnO upon heating to 900 °C that also supports the complete formation of willemite Zn2SiO4. Such void-mediated nanotube formation from a core−shell structure indicates migration of Zn and O atoms at the interface from the nanorod into the SiO2 shell. Annealing ZnO-SiO2 couple involves a one-way mass flux because of the almost stationary SiO2 phase.13 It is remarkable that we achieved the ternary nanotubes at 900 °C without any costly catalyst, whereas previous studies have employed either much higher temperature or a noble metal catalyst for the easier nucleation of Kirkendall voids.27,33 We attribute the reason for the lower temperature of reaction to the porous nature of wet-chemically prepared Stöber silica. Brunauer−Emmett−Teller measurements with N2 adsorption show that the surface area of the synthesized materials follows the following trend: SiO2 tubes (53.8 m2 g−1) > ZnO@ SiO2 (13.0 m2 g−1) > Zn2SiO4 tubes (8.8 m2 g−1) > ZnO NRs (11.26 m2 g−1). This explains the fact that formation of Zn2SiO4 from ZnO@SiO2 is driven by internal void formation, which remains inaccessible in the adsorption experiment. The inherent porosity of Stöber SiO2 drives the mechanism through surface diffusion of ZnO with a much lower kinetic energy barrier (158 kJ mol−1),35 rather than bulk diffusion (347−405 kJ mol−1).36 In addition, the pores could favor the supply of atmospheric O2 at the reaction interface, facilitating the silicate formation, as depicted in Scheme 1. However, for this mechanism to be operative, the pores must be interconnected and accessible. In case of a bulk system with planar interfaces (Scheme 1a), crossing the SiO2 barrier demands very high energy for atomic migration, resulting in high activation barrier. This barrier is again lowered for a 1-D nanotube because of the low radius of curvature leading to a higher outward diffusion from the core. To investigate the accessibility of the silica shell, we treated the ZnO@SiO2 with dilute hydrochloric acid solution (0.5 N HCl solution). This resulted in etching of ZnO leading to SiO2 tube formation, as shown in Figure 4. This observation unambiguously proves that Stöber SiO2 consists of interconnected pores that allows for exchange of gaseous or liquid species. To study the effect of atmosphere on the reaction, ZnO@SiO2 was heated at 900 °C for 2 h in a reducing atmosphere consisting of a mixture of 5% H2 and 95% Ar. A dramatic change in the morphology of the product is seen in this case.
correspond to phase-pure ZnO (JCPDS card No. 80−0075) phase, peaks corresponding to willemite Zn2SiO4 (JCPDS card No. 85−0453) can be seen from the samples heated in air. The solid-state reaction between ZnO and SiO2 is initiated at the interface by the diffusion of Zn2+ and O2− ions into the silica shell, while there is negligible diffusion into the ZnO side. This asymmetry in diffusion leads to the nucleation of voids at the interface that can be used as an indicator for the progress of the reaction. The bright-field TEM image in Figure 3a shows a
Figure 3. Formation of Zn2SiO4 nanotubes from ZnO@SiO2 nanorods at different temperature in atmospheric conditions. (a) Room temperature and (b) 700, (c) 800, and (d) 900 °C.
core−shell ZnO@SiO2 nanorod. The presence of small voids (
