Category Archives: Surface Characterization

Detection of the Surface of Crystalline Y2O3 Using Direct 89Y Dynamic Nuclear Polarization #DNPNMR

Brownbill, Nick J., Daniel Lee, Gaël De Paëpe, and Frédéric Blanc. “Detection of the Surface of Crystalline Y2O3 Using Direct 89Y Dynamic Nuclear Polarization.” The Journal of Physical Chemistry Letters 10, no. 12 (June 20, 2019): 3501–8.

https://doi.org/10.1021/acs.jpclett.9b01185

Nuclei with low gyromagnetic ratio (γ) present a serious sensitivity challenge for nulear magnetic resonance (NMR) spectroscopy. Recently, dynamic nuclear polarization (DNP) has shown great promise in overcoming this hurdle by indirect hyperpolarization (via 1H) of these low-γ nuclei. Here we show that at a magnetic field of 9.4 T and cryogenic temperature of about 110 K direct DNP of 89Y in a frozen solution of Y(NO3)3 can offer signal enhancements greater than 80 times using exogeneous trityl OX063 monoradical by satisfying the cross effect magic angle spinning (MAS) DNP mechanism. The large signal enhancement achieved permits 89Y NMR spectra of Y2O3 and Gd2O3-added Y2O3 materials to be obtained quickly (∼30 min), revealing a range of surface yttrium hydroxyl groups in addition to the two octahedral yttrium signals of the core. The results open up promises for the observation of low gyromagnetic ratio nuclei and the detection of corresponding surface and (sub-)surface sites.

Mapping the dynamics of methanol and xenon co-adsorption in SWNTs by in-situ continuous-flow hyperpolarized 129Xe NMR

Xu, Shutao, Xin Li, Cheng Sun, Anmin Zheng, Weiping Zhang, Xiuwen Han, Xianchun Liu, and Xinhe Bao. “Mapping the Dynamics of Methanol and Xenon Co-Adsorption in SWNTs by in-Situ Continuous-Flow Hyperpolarized 129Xe NMR.” Physical Chemistry Chemical Physics 21, no. 6 (2019): 3287–93.

https://doi.org/10.1039/C8CP07238H.

A comparative study of the adsorption and desorption processes of methanol in two kinds of nanochannels (i.e. MCM-41 and SWNTs) is performed by in situ continuous-flow laser-hyperpolarized 129Xe NMR. The high sensitivity and short acquisition time of hyperpolarized 129Xe allow for probing the molecular dynamics in a confined geometry under real working conditions. Hyperpolarized 129Xe NMR spectra indicate that the methanol adsorption behavior in nanochannels is determined by the characters of adsorption sites and that the methanol adsorption rate in the nanochannels of SWNTs is faster than in MCM-41. The experimental data shown in this work also indicate that there is a change in gas phase 129Xe NMR signal intensity during the adsorption and desorption of methanol in SWNTs. This may be because there is a strong depolarization of hyperpolarized 129Xe in SWNTs.

Surface chemical heterogeneity modulates silica surface hydration #DNPNMR #ODNP

Schrader, Alex M., Jacob I. Monroe, Ryan Sheil, Howard A. Dobbs, Timothy J. Keller, Yuanxin Li, Sheetal Jain, M. Scott Shell, Jacob N. Israelachvili, and Songi Han. “Surface Chemical Heterogeneity Modulates Silica Surface Hydration.” Proceedings of the National Academy of Sciences 115, no. 12 (March 20, 2018): 2890–95.

https://doi.org/10.1073/pnas.1722263115.

An in-depth knowledge of the interaction of water with amorphous silica is critical to fundamental studies of interfacial hydration water, as well as to industrial processes such as catalysis, nanofabrication, and chromatography. Silica has a tunable surface comprising hydrophilic silanol groups and moderately hydrophobic siloxane groups that can be interchanged through thermal and chemical treatments. Despite extensive studies of silica surfaces, the influence of surface hydrophilicity and chemical topology on the molecular properties of interfacial water is not well understood. In this work, we controllably altered the surface silanol density, and measured surface water diffusivity using Overhauser dynamic nuclear polarization (ODNP) and complementary silica–silica interaction forces acrosswater using a surface forces apparatus (SFA). The results show that increased silanol density generally leads to slower water diffusivity and stronger silica– silica repulsion at short aqueous separations (less than ∼4 nm). Both techniques show sharp changes in hydration properties at intermediate silanol densities (2.0–2.9 nm−2). Molecular dynamics simulations of model silica–water interfaces corroborate the increase in water diffusivity with silanol density, and furthermore show that even on a smooth and crystalline surface at a fixed silanol density, adjusting the spatial distribution of silanols results in a range of surface water diffusivities spanning ∼10%. We speculate that a critical silanol cluster size or connectivity parameter could explain the sharp transition in our results, and can modulate wettability, colloidal interactions, and surface reactions, and thus is a phenomenon worth further investigation on silica and chemically heterogeneous surfaces.

Resolving the Core and the Surface of CdSe Quantum Dots and Nanoplatelets Using Dynamic Nuclear Polarization Enhanced PASS–PIETA NMR Spectroscopy #DNPNMR

Piveteau, Laura, Ta-Chung Ong, Brennan J. Walder, Dmitry N. Dirin, Daniele Moscheni, Barbara Schneider, Janine Bär, et al. “Resolving the Core and the Surface of CdSe Quantum Dots and Nanoplatelets Using Dynamic Nuclear Polarization Enhanced PASS–PIETA NMR Spectroscopy.” ACS Central Science 4, no. 9 (September 26, 2018): 1113–25.

https://doi.org/10.1021/acscentsci.8b00196

Understanding the surface of semiconductor nanocrystals (NCs) prepared using colloidal methods is a longstanding goal of paramount importance for all their potential optoelectronic applications, which remains unsolved largely because of the lack of site-specific physical techniques. Here, we show that multidimensional 113Cd dynamic nuclear polarization (DNP) enhanced NMR spectroscopy allows the resolution of signals originating from different atomic and magnetic surroundings in the NC cores and at the surfaces. This enables the determination of the structural perfection, and differentiation between the surface and core atoms in all major forms of size- and shape-engineered CdSe NCs: irregularly faceted quantum dots (QDs) and atomically flat nanoplatelets, including both dominant polymorphs (zinc-blende and wurtzite) and their epitaxial nanoheterostructures (CdSe/CdS core/shell quantum dots and CdSe/CdS core/crown nanoplatelets), as well as magic-sized CdSe clusters. Assignments of the NMR signals to specific crystal facets of oleate-terminated ZB structured CdSe NCs are proposed. Significantly, we discover far greater atomistic complexity of the surface structure and the species distribution in wurtzite as compared to zinc-blende CdSe QDs, despite an apparently identical optical quality of both QD polymorphs.

Probing the surface of γ-Al2O3 by oxygen-17 dynamic nuclear polarization enhanced solid-state NMR spectroscopy #DNPNMR

Li, Wenzheng, Qiang Wang, Jun Xu, Fabien Aussenac, Guodong Qi, Xingling Zhao, Pan Gao, Chao Wang, and Feng Deng. “Probing the Surface of γ-Al2O3 by Oxygen-17 Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy.” Physical Chemistry Chemical Physics 20, no. 25 (June 27, 2018): 17218–25.

https://doi.org/10.1039/C8CP03132K.

γ-Al2O3 is an important catalyst and catalyst support of industrial interest. Its acid/base characteristics are correlated to the surface structure, which has always been an issue of concern. In this work, the complex (sub-)surface oxygen species on surface-selectively labelled γ-Al2O3 were probed by 17O dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP-SENS). Direct 17O MAS and indirect 1H–17O cross-polarization (CP)/MAS DNP experiments enable observation of the (sub-)surface bare oxygen species and hydroxyl groups. In particular, a two-dimensional (2D) 17O 3QMAS DNP spectrum was for the first time achieved for γ-Al2O3, in which two O(Al)4 and one O(Al)3 bare oxygen species were identified. The 17O isotropic chemical shifts (δcs) vary from 56.7 to 81.0 ppm and the quadrupolar coupling constants (CQ) range from 0.6 to 2.5 MHz for the three oxygen species. The coordinatively unsaturated O(Al)3 species is characterized by a higher field chemical shift (56.7 ppm) and the largest CQ value (2.5 MHz) among these oxygen sites. 2D 1H → 17O HETCOR DNP experiments allow us to discriminate three bridging (Aln)-μ2-OH and two terminal (Aln)-μ1-OH hydroxyl groups. The structural features of the bare oxygen species and hydroxyl groups are similar for the γ-Al2O3 samples isotopically labelled by 17O2 gas or H217O. The results presented here show that the combination of surface-selective labelling and DNP-SENS is an effective approach for characterizing oxides with complex surface species.

Bulk Nuclear Hyperpolarization of Inorganic Solids by Relay from the Surface #DNPNMR

Björgvinsdóttir, Snædís, Brennan J. Walder, Arthur C. Pinon, and Lyndon Emsley. “Bulk Nuclear Hyperpolarization of Inorganic Solids by Relay from the Surface.” Journal of the American Chemical Society 140, no. 25 (June 27, 2018): 7946–51.

https://doi.org/10.1021/jacs.8b03883

NMR is a method of choice to determine structural and electronic features in inorganic materials, and has been widely used in the past, but its application is severely limited by its low relative sensitivity. We show how the bulk of proton-free inorganic solids can be hyperpolarized in a general strategy using impregnation dynamic nuclear polarization through homonuclear spin diffusion between low-g nuclei. This is achieved either through direct hyperpolarization or with a pulse-cooling cross-polarization method, transferring hyperpolarization from protons to heteronuclei at particle surfaces. We demonstrate a factor 50 gain in overall sensitivity for the 119Sn spectrum of powdered SnO2, corresponding to an acceleration of a factor >2500 in acquisition times. The method is also shown for 31P spectra of GaP, for 113Cd spectra of CdTe, and 29Si spectra of α-quartz.

Water Dynamics from the Surface to the Interior of a Supramolecular Nanostructure #DNPNMR #ODNP

Ortony, Julia H., Baofu Qiao, Christina J. Newcomb, Timothy J. Keller, Liam C. Palmer, Elad Deiss-Yehiely, Monica Olvera de la Cruz, Songi Han, and Samuel I. Stupp. “Water Dynamics from the Surface to the Interior of a Supramolecular Nanostructure.” Journal of the American Chemical Society 139, no. 26 (July 5, 2017): 8915–21.

https://doi.org/10.1021/jacs.7b02969

Water within and surrounding the structure of a biological system adopts context-specific dynamics that mediate virtually all of the events involved in the inner workings of a cell. These events range from protein folding and molecular recognition to the formation of hierarchical structures. Water dynamics are mediated by the chemistry and geometry of interfaces where water and biomolecules meet. Here we investigate experimentally and computationally the translational dynamics of vicinal water molecules within the volume of a supramolecular peptide nanofiber measuring 6.7 nm in diameter. Using Overhauser dynamic nuclear polarization relaxometry, we show that drastic differences exist in water motion within a distance of about one nanometer from the surface, with rapid diffusion in the hydrophobic interior and immobilized water on the nanofiber surface. These results demonstrate that water associated with materials designed at the nanoscale is not simply a solvent, but rather an integral part of their structure and potential functions.

Characterizing Substrate-Surface Interactions on Alumina-Supported Metal Catalysts by Dynamic Nuclear Polarization-Enhanced Double-Resonance NMR Spectroscopy #DNPNMR

Perras, F.A., et al., Characterizing Substrate-Surface Interactions on Alumina-Supported Metal Catalysts by Dynamic Nuclear Polarization-Enhanced Double-Resonance NMR Spectroscopy. J Am Chem Soc, 2017. 139(7): p. 2702-2709.

https://www.ncbi.nlm.nih.gov/pubmed/28112506

The characterization of nanometer-scale interactions between carbon-containing substrates and alumina surfaces is of paramount importance to industrial and academic catalysis applications, but it is also very challenging. Here, we demonstrate that dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP SENS) allows the unambiguous description of the coordination geometries and conformations of the substrates at the alumina surface through high-resolution measurements of 13C-27Al distances. We apply this new technique to elucidate the molecular-level geometry of 13C-enriched methionine and natural abundance poly(vinyl alcohol) adsorbed on gamma-Al2O3-supported Pd catalysts, and we support these results with element-specific X-ray absorption near-edge measurements. This work clearly demonstrates a surprising bimodal coordination of methionine at the Pd-Al2O3 interface.

Surface-selective direct 17O DNP NMR of CeO2 nanoparticles #DNPNMR

Hope, M.A., et al., Surface-selective direct 17O DNP NMR of CeO2 nanoparticles. Chem Commun (Camb), 2017. 53(13): p. 2142-2145.

https://www.ncbi.nlm.nih.gov/pubmed/28134945

Surface-selective direct 17O DNP has been demonstrated for the first time on CeO2 nanoparticles, for which the first three layers can be distinguished with high selectivity. Polarisation build-up curves show that the polarisation of the (sub-)surface sites builds up faster than the bulk, accounting for the remarkable surface selectivity.

Surface-selective direct 17O DNP NMR of CeO2 nanoparticles #DNPNMR

Michael A. Hope et. al, Chem. Commun., 2017,53, 2142-2145 

http://pubs.rsc.org/en/Content/ArticleLanding/2017/CC/C6CC10145C

Surface-selective direct 17O DNP has been demonstrated for the first time on CeO2nanoparticles, for which the first three layers can be distinguished with high selectivity. Polarisation build-up curves show that the polarisation of the (sub-)surface sites builds up faster than the bulk, accounting for the remarkable surface selectivity.

Have a question?

If you have questions about our instrumentation or how we can help you, please contact us.