Surface Modification of a MXene by an Aminosilane Coupling Agent

MXenes, two-dimensional (2D) transition metal carbides and/or nitrides, possess surface termination groups such as hydroxyl, oxygen, and fluorine, which are available for surface functionalization. Their surface chemistry is critical in many applications. This article reports amine functionalization of Ti3C2Tx MXene surface with [3-(2-aminoethylamino)-propyl]trimethoxysilane (AEAPTMS). Characterization techniques such as X-ray photoelectron spectroscopy verify the success of the surface functionalization and confirm that the silane coupling agent bonds to Ti3C2Tx surface both physically and chemically. The functionalization changes the MXene surface charge from −35 to +25 mV at neutral pH, which allows for in situ preparation of self-assembled films. Further, surface charge measurements of the functionalized MXene at different pH values show that the functionalized MXene has an isoelectric point at a pH around 10.7, and the highest reported positive surface charge of +62 mV at a pH of 2.58. Furthermore, the existence of a mixture of different orientations of AEAPTMS and the simultaneous presence of protonated and free amine groups on the surface of Ti3C2Tx are demonstrated. The availability of free amine groups on the surface potentially permits the fabrication of crosslinked electrically conductive MXene/epoxy composites, dye adsorbents, high-performance membranes, and drug carriers. Surface modifications of this type are applicable to many other MXenes

Bulk and Surface Chemistry of the Niobium MAX and MXene Phases from Multinuclear Solid-State NMR Spectroscopy

MXenes, derived from layered MAX phases, are a class of two-dimensional materials with emerging applications in energy storage, electronics, catalysis, and other fields due to their high surface areas, metallic conductivity, biocompatibility and attractive optoelectronic properties. MXene properties are heavily influenced by their surface chemistry, but a detailed understanding of the surface functionalization is still lacking. Solid-state nuclear magnetic resonance (NMR) spectroscopy is sensitive to the interfacial chemistry, the phase purity including the presence of amorphous/nanocrystalline phases, and the electronic properties of the MXene and MAX phases. In this work, we systematically study the chemistry of Nb MAX and MXene phases, Nb2CTx and Nb4C3Tx, with their unique electronic and mechanical properties, using solid-state NMR spectroscopy and examine a variety of nuclei ( 1 H, 13C, 19F, 27Al and 93Nb) with a range of one- and two-dimensional correlation, wideline, high-sensitivity, high-resolution, and/or relaxation-filtered experiments. Hydroxide and fluoride terminations are identified, found to be intimately mixed, and their chemical shifts are compared with other MXenes. This multinuclear NMR study demonstrates that diffraction alone is insufficient to characterize the phase composition of MAX and MXene samples as numerous amorphous or nanocrystalline phases are identified including NbC, AlO6 species, aluminum nitride or oxycarbide, AlF3×nH2O, Nb metal, and unreacted MAX phase. To the best of our knowledge, this is the first study to examine the transition-metal resonances directly in MXene samples, and the first 93Nb NMR of any MAX phase. The insights from this work are employed to enable the previously-elusive assignment of the complex overlapping 47/49Ti NMR spectrum of Ti3AlC2. The results and methodology presented here provide fundamental insights on MAX and MXene phases and can be used to obtain a more complete picture of MAX and MXene chemistry, to prepare realistic structure models for computational screening, and to guide the analysis of property measurements.<br /

Bulk and Surface Chemistry of the Niobium MAX and MXene Phases from Multinuclear Solid-State NMR Spectroscopy.

MXenes, derived from layered MAX phases, are a class of two-dimensional materials with emerging applications in energy storage, electronics, catalysis, and other fields due to their high surface areas, metallic conductivity, biocompatibility, and attractive optoelectronic properties. MXene properties are heavily influenced by their surface chemistry, but a detailed understanding of the surface functionalization is still lacking. Solid-state nuclear magnetic resonance (NMR) spectroscopy is sensitive to the interfacial chemistry, the phase purity including the presence of amorphous/nanocrystalline phases, and the electronic properties of the MXene and MAX phases. In this work, we systematically study the chemistry of Nb MAX and MXene phases, Nb2AlC, Nb4AlC3, Nb2CTx, and Nb4C3Tx, with their unique electronic and mechanical properties, using solid-state NMR spectroscopy to examine a variety of nuclei (1H, 13C, 19F, 27Al, and 93Nb) with a range of one- and two-dimensional correlation, wide-line, high-sensitivity, high-resolution, and/or relaxation-filtered experiments. Hydroxide and fluoride terminations are identified, found to be intimately mixed, and their chemical shifts are compared with other MXenes. This multinuclear NMR study demonstrates that diffraction alone is insufficient to characterize the phase composition of MAX and MXene samples as numerous amorphous or nanocrystalline phases are identified including NbC, AlO6 species, aluminum nitride or oxycarbide, AlF3·nH2O, Nb metal, and unreacted MAX phase. To the best of our knowledge, this is the first study to examine the transition-metal resonances directly in MXene samples, and the first 93Nb NMR of any MAX phase. The insights from this work are employed to enable the previously elusive assignment of the complex overlapping 47/49Ti NMR spectrum of Ti3AlC2. The results and methodology presented here provide fundamental insights on MAX and MXene phases and can be used to obtain a more complete picture of MAX and MXene chemistry, to prepare realistic structure models for computational screening, and to guide the analysis of property measurements

Modified MAX Phase Synthesis for Environmentally Stable and Highly Conductive Ti3C2 MXene

One of the primary factors limiting further research and the commercial use of the two-dimensional (2D) MXene titanium carbide (Ti3C2), as well as MXenes in general, is the rate at which freshly made samples oxidize and degrade when stored as aqueous suspensions. Here, we show that including excess aluminum during synthesis of the Ti3AlC2 MAX phase precursor leads to the creation of Ti3AlC2 grains with improved stoichiometry and crystallinity. Ti3C2 nanosheets produced from the improved Ti3AlC2 are of higher quality, as evidenced by their increased resistance to oxidation and an increase in their electrical conductivity to 20,000 S/cm. Our results indicate that defects created during the synthesis of Ti3C2 (and by inference, other MXenes) lead to the previously observed instability. We show that by eliminating those defects results in Ti3C2 that is highly stable in aqueous solutions and in air. Aqueous suspensions of single- to few-layer Ti3C2 flakes produced from the modified Ti3AlC2 have a shelf life of over ten months, compared to one to two weeks for Ti3C2 produced from conventional Ti3AlC2, even when stored in ambient conditions. Freestanding films made from Ti3C2 suspensions stored for ten months show minimal decreases in electrical conductivity and negligible oxidation. Oxidation of the improved Ti3C2 in air initiates at temperatures that are 100-150°C higher than conventional Ti3C2. The observed improvements in both the shelf life and properties of Ti3C2 will facilitate the widespread use of this material. <br /

Overcoming the Limitations of MXene Electrodes for Solution-Processed Optoelectronic Devices

MXenes constitute a rapidly growing family of 2D materials that are promising for optoelectronic applications because of numerous attractive properties, including high electrical conductivity. However, the most widely used titanium carbide (Ti3C2Tx) MXene transparent conductive electrode exhibits insufficient environmental stability and work function (WF), which impede practical applications Ti3C2Tx electrodes in solution-processed optoelectronics. Herein, Ti3C2Tx MXene film with a compact structure and a perfluorosulfonic acid (PFSA) barrier layer is presented as a promising electrode for organic light-emitting diodes (OLEDs). The electrode shows excellent environmental stability, high WF of 5.84 eV, and low sheet resistance R-S of 97.4 omega sq(-1). The compact Ti3C2Tx structure after thermal annealing resists intercalation of moisture and environmental contaminants. In addition, the PFSA surface modification passivates interflake defects and modulates the WF. Thus, changes in the WF and R-S are negligible even after 22 days of exposure to ambient air. The Ti3C2Tx MXene is applied for large-area, 10 x 10 passive matrix flexible OLEDs on substrates measuring 6 x 6 cm. This work provides a simple but efficient strategy to overcome both the limited environmental stability and low WF of MXene electrodes for solution-processable optoelectronics.N

Highly Conductive and Transparent Reduced Graphene Oxide Nanoscale Films via Thermal Conversion of Polymer-Encapsulated Graphene Oxide Sheets

Despite noteworthy progress in the fabrication of large-area graphene sheetlike nanomaterials, the vapor-based processing still requires sophisticated equipment and a multistage handling of the material. An alternative approach to manufacturing functional graphene-based films includes the employment of graphene oxide (GO) micrometer-scale sheets as precursors. However, search for a scalable manufacturing technique for the production of high-quality GO nanoscale films with high uniformity and high electrical conductivity is still continuing. Here we show that conventional dip-coating technique can offer fabrication of high quality mono- and bilayered films made of GO sheets. The method is based on our recent discovery that encapsulating individual GO sheets in a nanometer thick molecular brush copolymer layer allows for the nearly perfect formation of the GO layers via dip coating from water. By thermal reduction the bilayers (cemented by a carbon-forming polymer linker) are converted into highly conductive and transparent reduced GO films with a high conductivity up to 10⁴ S/cm and optical transparency on the level of 90%. The value is the highest electrical conductivity reported for thermally reduced nanoscale GO films and is close to the conductivity of indium tin oxide currently in use for transparent electronic devices, thus making these layers intriguing candidates for replacement of ITO films