âClickableâ Metal-Oxide Nanomaterials Surface-Engineered by Gas

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‘Clickable’ Metal Oxide Nanomaterials Surface-Engineered by Gas-Phase Covalent Functionalization with Prop-2-ynoic Acid Chuan He, Ryan Janzen, Shi Bai, and Andrew V. Teplyakov Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b05124 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 24, 2019

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Chemistry of Materials

‘Clickable’ Metal Oxide Nanomaterials Surface-Engineered by GasPhase Covalent Functionalization with Prop-2-ynoic Acid Chuan He, Ryan Janzen, Shi Bai, Andrew V. Teplyakov University of Delaware, Department of Chemistry and Biochemistry, Newark, DE, USA ABSTRACT The surface properties of metal oxide nanomaterials (MONMs) can be tuned by reacting with a variety of organic compounds. Reactions of the oxides with functionalized carboxylic (R-COOH) or phosphonic acids (R-PO(OH)2) and various approaches utilizing silylation (such as with R-Si(X)3, where the X could be Cl or -OCH3) are often used to deliver the target modifier R to the oxide surface. However, the liquid phase reactions between metal oxides and these agents often cause agglomeration or multilayer growth, morphology change, or surface etching. In this paper, we report a novel approach that circumvents all of these problems. The proposed two-step functionalization approach utilizes exposure of the oxide materials to prop-2-ynoic acid (HCC-COOH, propiolic acid) in the gas phase as a first step. The second step can then utilize the created CC for postmodification that introduces any pre-designed functionality to the surface via “click” chemistry with azides (R-N3). Gas-phase exposure of prop-2-ynoic acid was carried out onto surfaces of different metal oxide nanomaterials (ZnO nanorods, CuO nanowires, TiO2 nanoparticles, CeO2 nanoparticles) under medium vacuum (×10-2 Torr), and this step was demonstrated to preserve the nanostructure of all the materials studied. The surface modification with “click” chemistry was tested via Cu(I)-catalyzed reaction of benzyl azide, with an added bonus of the CuO materials not requiring the presence of the copper catalyst. The combination of microscopic and spectroscopic investigations including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and solid-state nuclear magnetic resonance spectroscopy (ss-NMR) was used to follow the process and to compare with the traditional liquid-phase modification schemes.

__________________________________________ * Corresponding author: Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716. Tel.: (302) 831-1969; Fax: (302) 831-6335; e-mail: [email protected] 1

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INTRODUCTION Metal oxide nanomaterials (MONMs) are attracting substantial interest because of their versatile physical and chemical properties, wide range of compositions, and possibility to prepare a diverse set of nanostructures.1–3 These properties can be expanded even further by surface sensitization of these materials with organic or organometallic compounds that would introduce additional functionality to the materials leading to their multiple applications as pigments, sensors, catalysts and extend their use in optics, electronics, and energy conversion.4–8 Often, the nanostructured forms of these metal oxides possess remarkably different properties compared to the bulk materials because of the distinct surface properties or other effects that become dominant with the decrease of the crystallite size to the level of atomic or molecular scales.9–11 A number of advanced synthetic methods have been developed to tailor the features of the metal oxide nanomaterials such as size, shape, phase, and crystallinity.12–14 Among those techniques, the chemical synthesis of MONMs shows great potential in designing unique nanostructures. The highly developed and broadly used methods are largely based sol–gel processing,15 electrochemical methods,16 hydrothermal and solvothermal techniques,17,18 chemical vapor deposition19 and a number of other approaches.20 For example, sol–gel processing has been used to synthesize the TiO2 nanoparticles of uniform morphology and size, with a great efficiency to solar energy conversion.21 Electrochemical methods have been utilized to grow CuO nanorods that can be used as a hydrogen peroxide sensor.22 These methods show a high level of chemical control and selectivity, while retaining a high surface area, a high microporosity and an ordered 3-D structure. Solvo/hydrothermal methods have been applied to synthesize CeO2-supported nano-particles for the application

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of CO2 methanation at low temperature.23 Chemical vapor deposition (CVD) has been used to control the growth of ZnO nanorods and other nanostructures, which can be sensitized with dye molecules to exhibit a set of very attractive properties for sensing and energy conversion.24–26 One of the advantages of this approach is the fact that it allows chemical methods to be used for growing nanostructures on a variety of substrate materials. Overall, preparation of various oxide nanostrutures has been widely studied, and multiple reviews recount the approaches and methods to target production of specific nanomaterials. However, two major issues often have to be addressed following the nanostructure preparation. First, the surfaces of the MONMs produced are often very reactive and need to be stabilized to avoid agglomeration. Second, the surfaces of such materials are often desired to be chemically functionalized for a specific application. The problem with the first of these issues is that desensitization of reactive surfaces may decrease chemical reactivity towards a target process (sensing, catalysis, etc.).27–29 The problem with the latter is the fact that oxide materials can only react selectively with a handful of functional groups, and many of them can cause etching or other morphological changes in the material itself. The most common chemical approaches to oxide surface sensitization include the use of dilute solutions of carboxylic or phosphonic acids, or silylation.20,30 In other words, a target functionality (such as a dye molecule, for example) needs to be modified with a reactive carboxylic (-COOH) or phosphonic (–PO(OH)2) groups or with appropriately substituted reactive center involving silicon, such as R-Si(X)3, where the X could be Cl or -OCH3.20,30 The inherent reactivity requirement for such functional groups suggests the possibility of surface etching of many target materials. While some oxides, such as TiO2, are sufficiently stable during chemical sensitization

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process, making them very attractive candidates for practical applications; others are very sensitive to this processing step. In those cases, the potential for aggregation, polymerization of organic reactants, and surface etching prevents the use of surface sensitization for realistic applications and devices.29,31,32 Thus, new methods for MONMs surface modification are always in demand. Recently, a novel approach utilizing a two-step chemical procedure to sensitize the surface of ZnO nanostructures has been developed.33–35 The major advantage of this approach compared to the conventional surface functionalization processes is the use of a gas-phase exposure of the surface of ZnO nanostructures to prop-2-ynoic acid as a first step of chemical modification. It was shown that the morphology of the nanostructured ZnO material is fully preserved during this modification step and that the modification itself actually stabilizes the surface of the nanomaterial, so that it can undergo further postmodification in liquid phase. The second step involved a copper-catalyzed alkyne-azide cycloaddition (commonly referred to as an example of “click” chemistry),36 and this step did not affect the morphology of the material either. The use of carboxylic acid derivative to achieve the same end result showed that the morphology of the nanomaterial is affected greatly even if a dilute acid solution is used. Moreover, the time-resolved measurements suggested that the new procedure is perfectly adequate for measuring the behavior of the dye molecule strongly coupled to the oxide surface, while the surface chemically modified in a liquid phase contained a number of different species that were not coupled to the surface. The idea of a two-step sensitization of oxide materials based on a first step of saturating the oxide surface with a gas-phase pro-2-ynoic acid, as illustrated in Scheme 1, is extended in this work to become a general oxide modification approach and to illustrate

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the potential of this method to be adopted for a wide variety of other materials. The chemistry depicted in Scheme 1 is oversimplified and may involve surface hydroxyl groups present on surfaces of oxide materials. Nevertheless, previous work on surface reactions of ZnO nanopowder suggests that in gas-phase modification, the displacement processes37 and surface preparation by brief annealing lead to the formation of the same end-products in a reaction with carboxylic acids as shown in Scheme 1.33–35,37

Scheme 1. Proposed surface binding models of the prop-2-ynoic acid on metal oxide nanomaterials, including CuO, TiO2, CeO2 and previously studied ZnO surfaces.

In this work, surfaces of several metal oxide nanomaterials (ZnO nanorods, CuO nanowires, TiO2 nanoparticles and CeO2 nanoparticles) were exposed to the gas-phase prop-2-ynoic acid under medium vacuum conditions following a simple pre-treatment by preheating in vacuum to remove impurities weakly adsorbed on the surface. This step produced fully morphologically preserved surfaces with reactive alkyne functional groups for further modification with azido-compounds by click reaction. Selected results described previously for ZnO materials33–35 are reinforced here with additional spectroscopic characterization and serve as a reliable reference for comparison with chemistries of CuO, TiO2, and CeO2. The combination of microscopic and spectroscopic

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investigations including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and solid-state nuclear magnetic resonance spectroscopy (ss-NMR) has been used to confirm the “clickable” functionality and the morphology preservation. In case of TiO2, the use of the two-step modification approach with the first step being a reaction with a gas-phase species is not required, as the morphology of this material is preserved regardless of whether the first step is performed in liquid or gas phase. However, for all other metal oxides studied, there was a profound difference between gas phase and liquid phase modification schemes. RESULTS AND DISCUSSION Morphology-Preserving Preparation of Prop-2-ynoic Acid-Modified MONMs In order to demonstrate that the sensitization of the oxide nanomaterials with gasphase prop-2-ynoic acid preserves their morphology a number of oxides were investigated. The materials included ZnO nanorods grown via chemical vapor deposition method, commercially available TiO2 nanoparticles (

âClickableâ Metal-Oxide Nanomaterials Surface-Engineered by Gas

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