Cesium lead halide perovskite quantum dots (QDs) have gained significant attention as next-generation optoelectronic materials; however, their properties are highly dependent on their surface ...chemistry. The surfaces of cuboidal CsPbBr3 QDs have been intensively studied by both theoretical and experimental techniques, but fundamental questions still remain about the atomic termination of the QDs. The binding sites and modes of ligands at the surface remain unproven. Herein, we demonstrate that solid-state NMR spectroscopy allows the unambiguous assignment of organic surface ligands via 1H, 13C, and 31P NMR. Surface-selective 133Cs solid-state NMR spectra show the presence of an additional 133Cs NMR signal with a unique chemical shift that is attributed to Cs atoms terminating the surface of the particle and which are likely coordinated by carboxylate ligands. Dipolar dephasing curves that report on the distance between the surface ammonium ligands and Cs and Pb were recorded using double resonance 1H{133Cs} RESPDOR and 1H{207Pb} S-REDOR experiments. Model QD surface slabs with different possible surface terminations were generated from the CsPbBr3 crystal structure, and theoretical dipolar dephasing curves considering all possible 1H–133Cs/207Pb spin pairs were then calculated. Comparison of the calculated and experimental dephasing curves indicates the particles are CsBr terminated (not PbBr2 terminated) with alkylammonium ligands substituting into some surface Cs sites, consistent with the surface-selective 133Cs NMR experiments. These results highlight the utility of high-resolution solid-state NMR spectroscopy for studying ligand binding and the surface structure of nanomaterials.
1H solid-state NMR spectra are plagued by low resolution, necessitating the use of complex pulse sequences or specialized equipment. We introduce a new resolution enhancement method, inspired by ...super-resolution microscopy, that uses a 2D Hahn-echo experiment to constrain deconvolution. The result is an effective doubling of the MAS frequency.
Our civilization relies on synthetic polymers for all aspects of modern life; yet, inefficient recycling and extremely slow environmental degradation of plastics are causing increasing concern about ...their widespread use. After a single use, many of these materials are currently treated as waste, underutilizing their inherent chemical and energy value. In this study, energy-rich polyethylene (PE) macromolecules are catalytically transformed into value-added products by hydrogenolysis using well-dispersed Pt nanoparticles (NPs) supported on SrTiO3 perovskite nanocuboids by atomic layer deposition. Pt/SrTiO3 completely converts PE (M n = 8000–158,000 Da) or a single-use plastic bag (M n = 31,000 Da) into high-quality liquid products, such as lubricants and waxes, characterized by a narrow distribution of oligomeric chains, at 170 psi H2 and 300 °C under solvent-free conditions for reaction durations up to 96 h. The binding of PE onto the catalyst surface contributes to the number averaged molecular weight (M n) and the narrow polydispersity (Đ) of the final liquid product. Solid-state nuclear magnetic resonance of 13C-enriched PE adsorption studies and density functional theory computations suggest that PE adsorption is more favorable on Pt sites than that on the SrTiO3 support. Smaller Pt NPs with higher concentrations of undercoordinated Pt sites over-hydrogenolyzed PE to undesired light hydrocarbons.
Due to its extremely low natural abundance and quadrupolar nature, the (17)O nuclide is very rarely used for spectroscopic investigation of solids by NMR without isotope enrichment. Additionally, the ...applicability of dynamic nuclear polarization (DNP), which leads to sensitivity enhancements of 2 orders of magnitude, to (17)O is wrought with challenges due to the lack of spin diffusion and low polarization transfer efficiency from (1)H. Here, we demonstrate new DNP-based measurements that extend (17)O solid-state NMR beyond its current capabilities. The use of the PRESTO technique instead of conventional (1)H-(17)O cross-polarization greatly improves the sensitivity and enables the facile measurement of undistorted line shapes and two-dimensional (1)H-(17)O HETCOR NMR spectra as well as accurate internuclear distance measurements at natural abundance. This was applied for distinguishing hydrogen-bonded and lone (17)O sites on the surface of silica gel; the one-dimensional spectrum of which could not be used to extract such detail. Lastly, this greatly enhanced sensitivity has enabled, for the first time, the detection of surface hydroxyl sites on mesoporous silica at natural abundance, thereby extending the concept of DNP surface-enhanced NMR spectroscopy to the (17)O nuclide.
In this study, a revolution in solid-state nuclear magnetic resonance (SSNMR) spectroscopy is taking place, attributable to the rapid development of high-field dynamic nuclear polarization (DNP), a ...technique yielding sensitivity improvements of 2–3 orders of magnitude. This higher sensitivity in SSNMR has already impacted materials research, and the implications of new methods on catalytic sciences are expected to be profound.
Ultrawideline dynamic nuclear polarization (DNP)-enhanced 195Pt solid-state NMR (SSNMR) spectroscopy and theoretical calculations are used to determine the coordination of atomic Pt species supported ...within the pores of metal–organic frameworks (MOFs). The 195Pt SSNMR spectra, with breadths reaching 10 000 ppm, were obtained by combining DNP with broadbanded cross-polarization and CPMG acquisition. Although the DNP enhancements in static samples are lower than those typically observed under magic-angle spinning conditions, the presented measurements would be very challenging using the conventional SSNMR methods. The DNP-enhanced ultrawideline NMR spectra served to separate signals from cis- and trans-coordinated atomic Pt2+ species supported on the UiO-66-NH2 MOF. Additionally, the data revealed a dominance of kinetic effects in the formation of Pt2+ complexes and the thermodynamic effects in their reduction to nanoparticles. A single cis-coordinated Pt2+ complex was confirmed in MOF-253.
The interactions between polymers and metal oxide surfaces affect the physical properties of polymer composites and the selectivity of catalysts used for plastic conversion. Gauging the strengths of ...these interactions and how they depend on adsorption conformations and polymer structure is nevertheless quite challenging. Herein, we evaluate whether a recently introduced dynamic nuclear polarization-based surface-to-atom distance measurement method can be used to gauge the specific interactions between five vinyl polymers and an oxide support (alumina). The adsorption conformations of polymer monolayers were studied using site-specific 13C{27Al} rotational-echo saturation-pulse double-resonance (RESPDOR) experiments. The method is effective in revealing the average distance between the adsorbed polymer and the surface, as well as the fraction of the monomers that are adsorbed. Polymers that possess larger side chains are more weakly adsorbed, as evidenced by their formation of more numerous loop and tail structures. The measured RESPDOR dephasing data further reveals that polymer adsorption conformations are largely disordered and involve few hydrogen bonding interactions.
High-ordered multiple-quantum coherences have been used to gauge the size of coupled networks of nuclear spins but are seldom used in modern high-resolution solid-state NMR spectroscopy due to the ...low resolution of earlier techniques. Herein, we extend this work to the third dimension by designing an MQ spin counting experiment with full chemical resolution of the nuclei involved in the coherences. The technique uses multiplex processing to simultaneously acquire MQ/1Q correlation 2D spectra, with all even-ordered values of M, in the same amount of time it would take to acquire the 2Q/1Q spectrum alone. We apply this multiplexed MQ/1Q correlation method to probe site clustering on functionalized silica surfaces and find the first evidence of clusters involving three or more functionalities. When applied to a supported zirconium metallocene, the observation of a 6Q coherence between six cyclopentadienyl proton nuclei confirms the colocation of two cyclopentadienyl ligands on a single metal center.
This is the first report of a multifunctional separator for potassium‐metal batteries (KMBs). Double‐coated tape‐cast microscale AlF3 on polypropylene (AlF3@PP) yields state‐of‐the‐art ...electrochemical performance: symmetric cells are stable after 1000 cycles (2000 h) at 0.5 mA cm−2 and 0.5 mAh cm−2, with 0.042 V overpotential. Stability is maintained at 5.0 mA cm−2 for 600 cycles (240 h), with 0.138 V overpotential. Postcycled plated surface is dendrite‐free, while stripped surface contains smooth solid electrolyte interphase (SEI). Conventional PP cells fail rapidly, with dendrites at plating, and “dead metal” and SEI clumps at stripping. Potassium hexacyanoferrate(III) cathode KMBs with AlF3@PP display enhanced capacity retention (91% at 100 cycles vs 58%). AlF3 partially reacts with K to form an artificial SEI containing KF, AlF3, and Al2O3 phases. The AlF3@PP promotes complete electrolyte wetting and enhances uptake, improves ion conductivity, and increases ion transference number. The higher of K+ transference number is ascribed to the strong interaction between AlF3 and FSI− anions, as revealed through 19F NMR. The enhancement in wetting and performance is general, being demonstrated with ester‐ and ether‐based solvents, with K‐, Na‐, or Li‐ salts, and with different commercial separators. In full batteries, AlF3 prevents Fe crossover and cycling‐induced cathode pulverization.
Dendrite‐free potassium metal anode and potassium metal battery are achieved by a multifunctional separator of double‐coated reactive AlF3 on polypropylene. It possesses improved electrolyte wetting and uptake, ion conductivity, and ion transference numbers, facilitating uniform K‐ion flux, and plays as an artificial solid electrolyte interphase (SEI), facilitating stable KF, Al2O3, and AlF3 SEI.
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