Conformal Nanoscale Zirconium Hydroxide Films for Decomposing


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Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Conformal Nanoscale Zirconium Hydroxide Films for Decomposing Chemical Warfare Agents Seokmin Jeon,† Robert B. Balow,‡ Grant C. Daniels,‡ Jesse S. Ko,† and Pehr E. Pehrsson*,‡ †

National Research Council (NRC) Research Associateship Program and ‡Chemistry Division, US Naval Research Laboratory, Washington, D.C. 20375, United States

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S Supporting Information *

ABSTRACT: Amorphous zirconium hydroxide (ZH) has attracted recent attention for its high decomposition reactivity with chemical warfare agents (CWA). Conformal, 50 nm thick amorphous ZH films were produced on arbitrarily shaped metallic substrates by cathodic electrodeposition from ZrOCl2 in aqueous solution, with nanoscale control of the film thickness. The films had a root-mean-square (rms) roughness of ∼6−8 nm, ∼2.4 nm nanoscale pores, and a high specific surface area of 132 m2 g−1. These rapidly grown, cost-effective, and scalable films may be more suitable for some in situ decontamination needs than post-event application of powders prepared by conventional wet-synthesis methods. The morphology and chemical properties of the electrodeposited films were characterized with scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis mass spectrometry (TGA-MS), and nitrogen-sorption porosimetry. The decomposition activity of ZH toward dimethyl methylphosphonate (DMMP), a CWA simulant, was probed with gas chromatography−mass spectrometry (GC-MS) and in situ attenuated total reflection infrared spectroscopy (ATR) by monitoring the evolution of gas-phase methanol and the coverage of surface-bound methoxy and phosphonate species during and after DMMP dosing. We compared the chemical activity of electrochemically synthesized ZH (EZ), commercial ZH (CZ) and ZrO2 nanopowders, and calcined EZ at 100−500 °C. The ATR, XPS, and XRD results indicate that calcination to 500 °C decreased DMMP decomposition due to the loss of hydroxyls and conversion to crystalline ZrO2 and that EZ and CZ had virtually identical surface reactions. This study provides a framework for characterizing the evolution of complex reactions during the transition from amorphous-to-crystalline material. KEYWORDS: zirconium hydroxide (Zr(OH)4), electrodeposition, film, chemical warfare agent simulant, dimethyl methylphosphonate (DMMP)



INTRODUCTION Zirconium hydroxide (ZH) is a promising material for sequestering and decomposing toxic chemicals, especially chemical warfare agents (CWA).1−7 It has the highest known decomposition activity for the toxic nerve agent VX (O-ethyl S[2-(diisopropylamino)ethyl]methylphosphonothiolate). 1 Other sophisticated Zr-based materials such as Zr-based metal−organic frameworks (MOFs)7,8 and hybrid ZrOx(OH)y with high-surface-area carbon supports9,10 may outperform amorphous ZH in decomposing some chemical warfare agents; however, they often require more complex preparation and may not be easily scalable. On the other hand, electrodeposition produces nanoscale, conformal ZH films on relatively large substrates, in an ambient one-pot solution, within a few seconds to minutes. Such easy preparation and scalability could be critical for using these materials in practical applications. The rapid hydrolysis of toxic adsorbates on ZH is attributed to a defective, amorphous structure with high surface area and © XXXX American Chemical Society

a high density of undercoordinated Zr atoms (Lewis acid) and hydroxyl groups (Lewis base). However, these attributes also make it difficult to understand its structure and surface chemistry. Structural models, including the cyclic tetramer unit cell, [Zr4(OH)8·16H2O]8+, and polymerized 2D layers11−15 have been proposed, but the degree of in- and out-of-plane polymerization and the concentrations of terminal and bridge hydroxides and undercoordinated Zr atoms, key contributors to ZH’s high reactivity in contrast to crystalline or amorphous ZrO2, are still poorly understood. Amorphous ZH is usually prepared by wet synthesis. Common methods include precipitation of Zr salts such as ZrCl4 and ZrOCl2 with various bases including urea, NH4OH, or NaOH9,16,17 and other methods such as sol−gel hydrolysis of Zr(OBu)4 with ethanol6 and zirconium carbonate with Received: February 4, 2019 Accepted: April 3, 2019

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DOI: 10.1021/acsanm.9b00194 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Nano Materials Scheme 1. Illustration of ZH Sample Preparation in This Work

HNO3.14 The resultant powdery ZH is useful for many applications but has limitations for preparing uniform, largescale thin films. By contrast, electrodeposition provides scalable and easy-toprepare nanoscale conformal films on arbitrarily shaped metallic substrates and is thus potentially useful for lightweight coatings, membranes, or filter applications. There are multiple reported electrochemical synthesis methods for ZH, including electroplating with halide ions and a Zr anode,18 and cathodic electrodeposition of ZrOCl2,19−21 ZrO(NO3)2,22−24 or ZrCl425 electrolyte. In such methods, ZH is the typical precursor in producing zirconia (ZrO2), but because ZH is not the end product for many of these studies, its material and chemical properties are often not determined thoroughly. Because of the lack of a well-defined crystal structure, it is difficult to compare the stoichiometry and crystallographic purity of materials prepared with different synthesis methods, which obscures whether a ZH sample has the same physical and chemical properties as comparatively well-studied commercial ZH nanopowders (CZ). Because most reactivity studies use CZ,1−4,26−29 evaluating the physicochemical properties of the electrodeposited material with a well-studied control material, CZ, is essential. Variation of synthesis parameters such as pH, concentration of counterions, temperature, and aging time could produce different levels of polymerization/condensation and alter the chemical activity.13,17 For instance, Zr(OH)4 powder prepared by a conventional precipitation method had a surface area of 31.82 m2 g −1 .10 On the other hand, well-engineered commercial Zr(OH)4 nanopowder showed a surface area of 432 m2 g−1 (measured in this work). Accordingly, chemical activity should be very different for these nominally identical materials, so thorough characterization and comparison to a well-studied control material is critical for this research. Here, we characterize electrochemically synthesized zirconium hydroxide (EZ) using SEM, AFM, Raman spectroscopy, XRD, XPS, TGA-MS, and nitrogen sorption porosimetry. We then evaluate the decomposition performance of EZ with DMMP, a Sarin simulant, using GC-MS and in situ ATR, and

compare its material properties and DMMP decomposition performance to better characterized CZ.



EXPERIMENTAL METHODS

Reagents. The following reagents were used as received and without further purification, unless otherwise noted: zirconium(IV) oxychloride (ZrOCl2·xH2O, 99.9%, Alfa Aesar), zirconium(IV) oxide (ZrO2, 5 μm, 99%, Sigma-Aldrich), and dimethyl methylphosphonate (DMMP, 97%, Sigma-Aldrich). Deionized (DI) water was obtained from a Thermo Fisher Scientific Barnstead GenPure ultrapure water system (18.2 Mohm·cm). CZ powder was purchased from Magnesium Electron Ltd. (manufacturer’s part number 1501/03). Ultrahigh purity (UHP) N2 (99.999%) was obtained from Airgas. Electrochemical Synthesis. Scheme 1 shows an overview of the sample preparation in this work. EZ was synthesized by cathodic electrochemical deposition using a three-electrode cell described in previous reports.19−21 The working electrode was either a piece of SiO2/Si(100) (1 μm thermal SiO2 on Si(100), n-type, 10 × 20 mm2) coated by electron-beam-assisted physical vapor deposition with 20 nm Au (base pressure 100 °C.1 In addition, EZ has lower porosity, which makes it harder for water molecules to diffuse out of the inner particle region and elevates the apparent water desorption temperature in the TGA thermogram. In contrast to CZ, washed EZ loses 3.3% in weight between 350 and 450 °C. For EZ film particulates, the loss percentage is up to 15% at 300−400 °C. TGA-MS data show the weight loss in washed EZ is mostly CO2 and water, not Cl-related products, suggesting contamination by adventitious hydrocarbons. This TGA data differs from that in Nakajima’s report, where a 50% weight loss was observed at around 350 °C in similarly prepared and washed EZ, which was identified by TGA-MS as desorption of HCl and water.19 In addition to the hydrocarbon contamination in the washed EZ, weight loss at 350−450 °C could also result from the amorphous-to-crystal transition of ZrO2 at around 400 °C.36 Elemental analysis (see XPS survey spectrum in section 4 of the Supporting Information) of a typical EZ film particulate sample shows that EZ consists of Zr (19.75%), O (60.02%), C (16.43%), and Cl (3.80%). The average O/Zr ratios calculated from several EZ and CZ samples are 3.08 ± 0.21 and 2.93 ± 0.12, which are similar to each other but higher than 2.63 ± 0.02 in commercial ZrO2. The Cl contamination dropped from 3.5−4.8% to 0.2% after washing in DI water, consistent with the TGA-MS data. The atomic percentage of carbon varied from 16.4 to 28.5%. In general, the carbon increased as a function of time, since ambient organic molecules and CO2 reacted with the ZH surface,29 even though the material was stored in a sealed vial. The washes performed using DI water to remove residual Cl ions did not reduce the adventitious carbon. Figure 3 shows high-resolution XPS data of CZ and EZ film particulates, a 50 nm EZ film on Au/SiO2/Si, and ZrO2. The Zr 3d5/2 peaks are fitted with a single component (blue plots in Figure 3a), with 182.2−182.3 eV in binding energy and 1.24− 1.59 eV in fwhm, respectively. This component is associated with the Zr4+ oxidation state, including Zr−OH, Zr−OH−Zr, and Zr−O−Zr species; an assignment supported by a Zr foil control with a native oxide surface (not shown here). The Zr 3d5/2 peaks are fitted with components at 182.4 eV (Zr4+) and 178.3 eV (metallic Zr0) in binding energy. The O 1s region is fitted with three components (blue, green, and magenta in Figure 3b), attributed to bridging O (i.e., Zr−O(H)−Zr), terminal O (i.e., Zr−OH), and O in physisorbed water, respectively, based on literature assignments.26,37 The assignment of the second component is more complicated since the binding energies of sources such as terminal OH bound to Zr, chemisorbed water on a coordinatively unsaturated Zr (cus-Zr) atom through a

Figure 3. High-resolution XPS in (a) Zr 3d and (b) O 1s regions for (i) CZ, (ii) washed EZ particulates, (iii) 50 nm EZ film on Au/SiO2/ Si, and (iv) ZrO2. Gray circles and red line represent raw data and fitted envelope function, respectively. Dashed vertical lines guide the eyes to compare with peak binding energies.

metal−ligand bond,38 and O in carbonate contaminants could be in this binding energy range.39 Assuming that the densities of the cus-Zr defects and carbonate contaminants are lower than the stoichiometric terminal OH, we assign the second O 1s component to terminal OH, as reported previously in other studies.26 ZrO2 also has three components, but the first, Zr−O−Zr, is dominant since ideal stoichiometric ZrO2 consists of an O bridging to two Zr atoms. However, the terminal O component (green in plot iv, Figure 3b) could originate with surface water chemisorbed and dissociated on surface Zr to produce Zr−OH. The area ratio of terminal to bridging O in ZrO2 is ∼0.25. The ratios of the terminal to bridging O in EZ and CZ are 0.74 and 0.51, respectively. The higher value in EZ suggests more terminal-OH, again consistent with TGA data showing more water loss from EZ by polymerization/ condensation at T > 100 °C. On the other hand, in CZ, higher water loss occurred by desorption of physisorbed water at