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ZnO-Templated Synthesis and MetalInsulator Transition of VO2 Nanostructures Xuefei Li, and Raymond E. Schaak Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b05231 • Publication Date (Web): 23 Feb 2019 Downloaded from http://pubs.acs.org on February 25, 2019
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Chemistry of Materials
ZnO-Templated Synthesis and Metal-Insulator Transition of VO2 Nanostructures Xuefei Li and Raymond E. Schaak* Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA ABSTRACT: Vanadium dioxide (VO2) exhibits a metal-insulator transition (MIT) that is accompanied by steep changes in electrical and optical properties, making it an important component of device architectures that require facile switching between metal and insulating states. VO2 nanostructures are particularly useful components of such devices, given their unique size-dependent properties and processing capabilities. Here, we show that VO2 nanostructures can be synthesized by chemical transformation of ZnO nanoparticles, which are readily available and serve as morphological templates. Commercially available and colloidally synthesized ZnO nanoparticles react with VOSO4 in water at room temperature to form amorphous VO2, which can be crystallized to the switchable M1 phase of VO2 upon thermal annealing. Experiments probing various particle dimensions, shapes, surface ligands, and reaction parameters suggest that the reaction occurs by depositing VO2 on the ZnO particles, which serve as a sacrificial template. The ZnO-derived VO2 nanostructures exhibit reversible structural transformations between the metallic (R-VO2) and insulating (M1-VO2) phases. Dopants such as Al3+, which modify both the VO2 phase and the MIT properties, can be incorporated. The transition temperature also varies with particle size and reaction parameters. Synthesizing VO2 in solution using ZnO as a sacrificial template provides a potentially scalable route to diverse VO2 nanostructures that exhibit metal-insulator transitions.
excellent using these fabrication methods, and therefore they are useful for fundamental studies and small-scale prototype devices. However, there are practical limits on the product scalability and uniformity, which limits applications that require large quantities of uniform and free-standing VO2 nanoparticle building blocks, such as plasmonic antennas and metasurfaces.17-19 Additionally, because the VO2 MIT is a first-order phase transition, phase co-existence and switching speed can be influenced and engineered by many factors, including defect types and concentrations,20-22 dimensions of the material,23,24 local strain and/or electronic effects arising from chemical dopants,25,26 and external strain conditions of the VO2 material.27 It is therefore important to develop synthetic methods that produce free-standing VO2 particles with nanoscale dimensions for ensemble studies of the transition properties and eventually for applications that require solution-processable materials integration and homogeneous metal-insulator transitions. Such free-standing VO2 nanostructures have been synthesized primarily by thermally converting metastable VO2(B) structures, which are obtained via hydro- or solvothermal methods from higher valent vanadium precursors such as V2O5.28,29 Topochemical thermal transformations have also been reported,30 and hydrothermal exfoliation of bulk VO2 powder was reported to form nanobelt structures.31 Colloidal VOx nanoparticles were also synthesized and solution processed into patterned films, followed by rapid thermal annealing to form VO2.37 However, in general, solution-synthesized VO2 nanostructures are notoriously difficult to synthesize because of challenges in targeting the correct stoichiometry and phase, given the complexity of the V-O phase diagram,32,33 while also controlling morphology. Here, we show that ZnO,
INTRODUCTION Vanadium dioxide (VO2) is a prototypical strongly correlated material, where electron-electron interactions lead to unique functions. In VO2, this enables abrupt switching properties that emerge from vastly different free electron densities spanning many orders of magnitude.1-3 The thermodynamically stable VO2 phase, typically denoted as the M1 phase (monoclinic, P21/c) for undoped and unstrained VO2 at room temperature, exhibits a metal-insulator transition (MIT) at 341K concomitant with a structural transformation to the higher symmetry rutile R phase (tetragonal, P42/mnm), which is stable at higher temperatures.4 The transition is typically accompanied by a change in electrical conductivity1 that spans several orders of magnitude, as well as an up to 80% change in optical reflectance in the near infrared (NIR) region.5 These steep changes in electrical and optical properties between the two states can be triggered thermally, electrically, and optically.6,7 Size-dependent properties emerge when the VO2 particle sizes are less than a few hundreds of nanometers, resulting in modified transition temperatures, hysteresis, and the on/off ratio.8 The incorporation of nanoscale VO2 building blocks is therefore important for switchable electronic and optical applications including Mott-type field effect transistors (FETs)9 and memory devices,10 steep slope transistors,11 synchronized oscillators,12 and thermally switchable metamaterials.13 Despite significant interest in the underlying physics and applications of the VO2 MIT in nanoscale materials, the most commonly studied VO2 nanostructures consist of single crystalline nanobeams14 and nanoplatelets15 that are solidified and reduced from V2O5 droplets on substrates, as well as nanoscale epitaxial thin films that strongly interact with the underlying substrates.16 The crystallinity and purity of such samples are
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reacted with VOSO4×xH2O. For transformation using ZnO particles annealed in air, commercial ZnO particles dried into powder form were annealed at 475 °C for 1 hour before cooling to room temperature. For control reactions that included ZnCl2, 300 mg of ZnCl2 were dissolved along with 20 mg of ZnO particles, and reaction with 50 mg and 100 mg of VOSO4×xH2O were carried out under otherwise identical conditions. For control reactions using alternative vanadium sources with nitrate anions, 122 mg of amorphous VO2 particles obtained from transformation of ZnO particles with VOSO4×xH2O were dissolved in 5 mL of DI water, and 5 mL of 2% nitric acid was added swiftly to the amorphous VO2 nanoparticle suspension under stirring. The amorphous VO2 particles were dissolved instantaneously upon addition of nitric acid, forming a clear blue-colored solution. The solution was then swiftly injected into 5 mL of a ZnO particle suspension containing 24 mg of ZnO, and the mixture was centrifuged at 13,500 rpm.
which is widely available as nanoparticles in various shapes and sizes,34 serves as a sacrificial template for the formation of amorphous VO2 upon reaction with VOSO4 in water at room temperature. The amorphous VO2 transforms to crystalline VO2 upon annealing. ZnO particles therefore pre-define the dimensions, morphologies, and uniformity of the product VO2 particles, circumventing some key challenges in VO2 nanoparticle synthesis. Using this approach, the composition and oxidation states in the VO2 nanostructures, which influence their MIT properties, can be chemically tuned.
EXPERIMENTAL SECTION Chemicals and Materials Vanadyl sulfate hydrate (VOSO4×xH2O, 3 < x < 5, 99.99%), oleic acid (OLAC), oleylamine (OLAM), octadecene (ODE), 1-octadecanol (ODA), sodium dodecyl sulfate (SDS), zinc acetate dihydrate (Zn(OAc)2×2H2O), zinc chloride (ZnCl2), ZnO nanoparticles (average size ~25nm), ZnO nanowires (diameter = 90 nm, length = 1µm), and Al(NO3)3×9H2O were purchased from Sigma-Aldrich. Isopropanol (IPA) was purchased from VWR. Nanopure distilled and deionized (DI) water (18 MΩ) was obtained from a Barnstead Nanopure Analytical Ultrapure water system. All chemicals except VOSO4×xH2O were used as received. VOSO4×xH2O was dispersed in DI water and stirred under ambient conditions in the control experiments for making smaller VO2 particles.
Transformation of Amorphous to Crystalline VO2 Wires The amorphous VO2 nanoparticles and nanowires suspended in 1 mL of IPA were dropcasted into an Al2O3 boat to dry over 30 min. The dried nanoparticles were then placed in a tube furnace at ambient pressure under Ar flow and purged for 30 min before annealing. The temperature of the tube furnace was increased to 475 °C over 20 min and then kept at that temperature for 30 min before cooling to room temperature over 12 hours. The powder after annealing was collected and dispersed in isopropanol for characterization. For V2O3 and V6O13 control reactions, amorphous VO2 particles in dried powder form were annealed in forming gas to 250 °C, held at that temperature for 1 hour, cooled down to room temperature over 4 hours, heated to 550 °C over one hour in forming gas for V2O3 and mixed Argon and air for V6O13, held at 550 °C for 2 hours, and then cooled to room temperature.
Synthesis of Amorphous VO2 Nanoparticles and Nanowires Amorphous VO2 nanoparticles and nanowires were obtained by reaction between ZnO nanoparticles and VOSO4×xH2O dissolved in DI water. For complete transformation reactions, 80 mg of commercially purchased ZnO nanoparticles or nanowires were dispersed in 5 mL of DI water under sonication, and 400 mg of VOSO4×xH2O dissolved in 5 mL of DI water was added to the dispersed ZnO solution swiftly (within 2-3 seconds) via 5 mL syringes. The color of the white ZnO suspension instantaneously turned to purple after the addition of the VOSO4 solution under sonication. The mixture was then diluted with a mixture of DI water and IPA having a ~9:1 DI:IPA ratio, and then centrifuged at 13,500 rpm for 3 min to wash. The particles were washed one more time by 9:1 DI:IPA, and then dispersed in 1 mL of isopropanol. The synthesis of Al3+ doped amorphous VO2 nanoparticles used the same procedure, except that 7 mg of Al(NO3)3×9H2O were added to the ZnO suspension. For partial transformation reactions, smaller amounts of VOSO4×xH2O (200mg, 240mg, 280mg, 320mg, and 360mg) were added under otherwise identical conditions. For the transformation reaction involving oxidized VOSO4×xH2O, 400mg VOSO4×xH2O dissolved in 5 mL DI was stirred at 850 rpm for 21 hours and 45 hours before reaction with ZnO particles under the same conditions. For reaction of ZnO particles with added surface ligands, ZnO particles were synthesized solvothermally or from established reactions. For example, to obtain OLAM-capped ZnO particles, 200 mg of Zn(OAc)2×2H2O in 8.6 mL of OLAM was transferred to a 18-mL autoclave and heated to 190 °C in 1 hour, held 8 hours before cooling to room temperature and washing with ethanol. Octadecanol (ODA) and OLAM capped ZnO nanoparticles were synthesized using an established method.35 The particles were dried in hexanes and then redispersed in 1% (by weight) of SDS in DI water before being
Solvent Reduction and Re-oxidation of Crystalline VO2 Particles The crystalline VO2 particles (35 mg) were dispersed in a mixture of 3 mL of OLAC and 5.6 mL of ODE, heated to 220 °C in a 18 mL autoclave over 1 hour, kept at 220 °C for 8-10 hours, and cooled to room temperature over 8 hours. The mixture after the reaction was washed with ethanol twice and redispersed in hexane for further characterization. Re-oxidation of the solvent annealed particles was performed in a tube furnace with mixed air and Ar carrier gas. The particles in dry powder form were heated to 550 °C over 30 min and held at 550 °C for one hour before cooling to room temperature. Characterization Transmission electron microscopy (TEM) images, highresolution TEM (HRTEM) images, selected area electron diffraction (SAED) patterns, high-angle annular dark-field scanning transmission electron microscopy (STEM-HAADF) images, energy dispersive X-ray spectroscopy (EDS) spectra and maps, and in-situ heating TEM images were obtained using a FEI Talos field emission TEM operating at 200kV. Esprit software was used for analysis of EDS mapping and spectra. Elemental quantification used the energy range of 0.5 keV to 10 keV, covering the vanadium Kα peak at 4.95 keV, the zinc Kα peak at 8.64 keV, and the sulfur Kα peak at 2.31 keV. The
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Chemistry of Materials
EDS element maps for Zn include signal from nearby Cu Kα signals, so the ensemble EDS spectra are used for quantification. The in-situ heating chips were purchased from Protochips and used without further treatment. ES Vision software (Emispec) and ImageJ software were used to measure reciprocal space distances in the SAED and FFT images. Powder Xray diffraction (XRD) patterns were collected on a Bruker Advance D8 and a PANalytical Empyrean X-ray diffractometer with Cu Kα radiation at room temperature or a PANalytical XPert Pro MPD X-ray diffractometer with Cu Kα radiation at elevated temperatures, with a HTK setup. XRD samples were dropcast in thick layers on a low background Si substrate; data acquired on the D8 instrument contained a noticeable broad, low-intensity background for all samples, including the blank zero-background holder. Simulated powder XRD patterns were made using the CrystalMaker and CrystalDiffract software suite. Crystal structure data for the metallic rutile phase VO2(R) is from reference 42, and reference patterns for the insulating monoclinic M1 and M2 phases are from JCPDS 440252 and 33-1441, respectively. Structural transformation data were obtained from a TA Q2000 differential scanning calorimeter under N2 atmosphere, with a heating/cooling rate of 10 °C/min in the range of –20 to 140 °C at a heating rate of 10 °C per minute. The DSC chamber was purged with N2 at 20 °C for 30 min and then 60 °C for 30 min before each measurement started, to prevent oxidation of VO2 by air at elevated temperature. Inductively Coupled Plasma Emission Spectrometry (ICP-AES) measurements were performed by Thermo iCAP 7400 instrument, with diluted (1600 times) DI solution of supernatants for the control experiments shown in Figure 2a-c and Figure S2, and 16 mL of 2% nitric acid was used to dissolve 5.7 mg of Al-doped VO2 particles shown in Figure 5. The detection limit is 0.01µg/ml for Al, 0.005 µg/ml for V, and 0.005 µg/ml for Zn. High purity standards (EPA certified standard 200.7) were used to calibrate the results.
RESULTS AND DISCUSSION Figure 1. (a-b) TEM image and powder XRD pattern of commercially purchased ZnO particles. The simulated XRD pattern is for 25 nm ZnO. (c-d) TEM image and powder XRD pattern of amorphous VO2 particles obtained from reacting 80 mg of commercial ZnO particles with 400 mg of VOSO4·xH2O in water at room temperature. (e) EDS spectrum, HAADF-STEM image, and EDS element maps of the amorphous VO2 particles. The EDS spectrum includes Cu from the Cu TEM grid, adventitious Si, sulfur from adsorbed sulfate, and vanadium but no zinc.
Various ZnO nanostructures serve as sacrificial templates for the formation of amorphous VO2 upon reaction with VOSO4 in water at room temperature. Subsequent thermal annealing produces crystalline VO2, which exhibits the characteristic metal-insulator transition.
Reaction of ZnO Particles with VOSO4 We began by studying commercially available ZnO nanoparticle templates, as they do not have strongly bound surface ligands that are present in solution-synthesized ZnO. A representative TEM image, shown in Figure 1a, reveals that the ZnO particles have smallest dimensions on the order of approximately 25 nm, with particle morphologies that include spherical, elongated, and multi-faceted, as well as some that appear more plate-like. The particles are somewhat aggregated, as expected because they do not have bound ligands that help enhance dispersibility. The XRD data in Figure 1b confirm that the sample consists of crystalline wurtzite ZnO without observable impurities. The peak broadening in the experimental XRD pattern is consistent with that of a simulated pattern for 25-nm grain sizes. While it is possible that amorphous impurities are present, the broad low-intensity background features match those of a blank sample holder, suggesting that amorphous components are not contributing to the XRD signal.
To transform the ZnO particles to amorphous VO2, VOSO4·xH2O dissolved in de-ionized water was added swiftly to ZnO particles dispersed in de-ionized water under sonication using syringe injection. The VOSO4·xH2O (400 mg) was used in significant excess relative to ZnO (80 mg). Figure 1c shows a TEM image of the product, which has larger particles with smallest dimensions of approximately 50 nm. The particles in Figure 1c retain the general morphology and interconnectedness of the ZnO seed particles in Figure 1a. The corresponding XRD pattern in Figure 1d is broad without any clearly-defined peaks, suggesting that the product is amorphous. The broad peaks are centered close to the primary peaks of ZnO. EDS and HAADF-STEM data for the amorphous VO2 particles are shown in Figure 1e. An EDS spectrum taken for an ensemble of particles shows predominantly vanadium with only
