Klaus Schmidt-Rohr

Physical Chemistry of Polymers & Nanocomposites, Analysis of Complex Organic Matter, Solid-state NMR Methods

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Research Interests

Our group studies the composition and nanoscale structure as well as dynamics of polymer materials, nanocomposites, natural organic matter, and thermoelectric tellurides, using a variety of selective or two-dimensional nuclear magnetic resonance (NMR) experiments, many of which were developed by us (http://www.public.iastate.edu/~nmrksr/). We have also introduced new methods for quantitative analysis of scattering data of nanostructured materials. This dual approach has enabled us to "solve" important aspects of the structure of the Nafion fuel-cell membrane and of the nanocomposite in bone.

1) Nanocomposites

We have established NMR as a tool for characterizing nanocomposites, typically consisting of inorganic nanoparticles dispersed in an organic polymer, not only in terms of composition, but also in terms of the location of various components relative to the organic-inorganic interface. The composition of the surface layers of both the nanoparticles and the matrix, which are particularly important since they are responsible for the interactions between the components, can be determined by selective NMR techniques, often based on heteronuclear dipolar couplings across the interface.

We have introduced several NMR methods for determining whether a material is a nanocomposite even in cases where standard electron microscopy fails to provide a specific signature. Spin diffusion from matrix 1H to 1H in the nanoparticles is one option to detect particles in the 1- 10 nm range, measurement of long-range 1H-X nucleus couplings using X{1H} HARDSHIP NMR another. The shape, size, and size distribution of nanoparticles can be determined from HARDSHIP NMR fits combined with quantitative analysis of small- and/or wide-angle scattering data. Nanocomposites and related systems that we have studied with these methods include:

  • Polymer/clay nanocomposites [Macromolecules, 2004]
  • Phosphate-glass / nylon nanocomposites [Chem. Mat., 2006]
  • The nanocomposite in bone [J. Chem. Phys., 2007]
  • Bone-mimetic nanocomposites [Chem. Mat., 2008]
  • Silicate nanospheres in tricalcium phosphate, a bone replacement material [Chem. Mat., 2008]
  • Nanodiamond with protonated surfaces [JACS, 2009]
  • Carbon-modified TiO2 [Chem. Mat., 2009]

As an example, Figure 1 shows long-range 29Si{31P} REDOR dephasing in 29Si-doped tricalcium phosphate. The analysis shows that silicate particles of 10 nm diameter or larger can be probed by this approach.

Figure 1. 29Si{31P} REDOR data of 5% and 10% 29Si,Zn-doped tricalcium phosphate (TCP) at 3 and 6 kHz MAS. Simulated REDOR curves for spherical silicate particles of 4 - 10 nm diameter are shown for reference. 1H{31P} REDOR dephasing of OH protons in NIST hydroxyapatite (triangles), recorded with identical 31P pulses, shows the decay expected for isolated ions in a phosphate matrix.

Our techniques and results on a few of these systems are briefly outlined in the following. Funding has mostly been provided by the Department of Energy, Basic Energy Sciences via the Ames Laboratory.

HARDSHIP NMR thickness / depth measurements. NMR can probe the thickness of phosphate, silicate, carbonate, and other nanoparticles in organic-inorganic nanocomposites using the strongly distant-dependent dipolar couplings between the abundant protons in the organic phase and X-nuclei (31P, 29Si, 13C, 27Al, 23Na, etc.) in the inorganic phase. This approach requires pulse sequences with heteronuclear dephasing only by the polymer or surface protons that experience strong homonuclear interactions, but not by dispersed OH or water protons in the inorganic phase, which have long transverse relaxation times T2,H. This goal is achieved by HeteronucleAr Recoupling with Dephasing by Strong Homonuclear Interactions of Protons (HARDSHIP). The pulse sequence alternates heteronuclear recoupling for ~0.15 ms with periods of homonuclear dipolar dephasing that are flanked by canceling 90o pulses. The heteronuclear evolution of the long-T2,H protons is refocused within two recoupling periods, so that 1H spin diffusion cannot significantly dephase these coherences. For the short-T2,H protons of a relatively immobile organic matrix, the heteronuclear dephasing rate depends simply on the heteronuclear second moment. A detailed analysis based on interaction representations shows that homonuclear interactions do not affect the dephasing, even though no homonuclear decoupling is applied, because long-range 1H-X dipolar couplings approximately commute with short-range 1H-1H couplings, and heteronuclear recoupling periods are relatively short. Experiments and simulations show that spherical particles of up to 10-nm diameter can be characterized quite easily. [J. Chem. Phys. 126, 054701-(1-16) (2007)]

Simulation of small-angle scattering from nanostructures. NMR can provide many unique insights into the composition and thickness of domains in nanocomposites and other nanostructured materials, but it is not suitable for determining the shape of the nanostructures. This important complementary information, together with independent information on size, can be obtained from small-angle X-ray or neutron scattering (SAXS/SANS) of these materials. We have become interested in simulating the scattered intensity quantitatively for two reasons: (i) For several nanomaterials of interest to us, in particular bone and Nafion, high-quality scattering data have been recorded by other groups but their interpretation has been only qualitative. (ii) The scattered intensity is the Fourier transform of the scattering density. We use numerical multidimensional Fourier transformation routinely in multidimensional NMR and know how to convert between the actual continuous scattering density and the discrete data required by a computer. Therefore, we have developed a simple numerical approach for calculating the q-dependence of the scattered intensity in SAXS/SANS. For a user-defined scattering density on a lattice, the scattered intensity I(q) is calculated by 3D (or 2D) numerical Fourier transformation and spherical summation in q-space, with a simple smoothing algorithm and an exact, simple correction for continuous rather than discrete (lattice-point) scattering density. The algorithm can be applied to particles of any shapes that can be defined on the chosen cubic lattice and with any size distribution, while those features pose difficulties to a traditional treatment in terms of form and structure factors. [J. Appl. Cryst. 40, 16-25 (2007)]

The nanocomposite in bone. The load-bearing material in bone is a nanocomposite of ~ 3-nm thick apatite (calcium phosphate) platelets imbedded in a collagen matrix, with a volume ratio typically near 40:50 and the remaining 10% accounted for by water. The nanocrystalline calcium phosphate is often referred to as hydroxyapatite, but hydroxide has been undetectable in vibrational spectra. Selective 1H{31P} nuclear magnetic resonance (NMR) spectra have enabled quantification of the 5- to 8-fold reduced hydroxide concentration. Carbonate, which is not a component of ideal hydroxyapatite, has been conclusively proven to be fully incorporated in the bioapatite nanocrystals of native bone. A carbonate ion substitutes for about one out of every eight phosphates. Our 13C{1H} and 31P{1H} HARDSHIP NMR distance measurements show that carbonate is neither a surface deposit nor a central nucleation layer, but has a reduced concentration in the crystallite center. Strongly bound water is proven for the first time and is distinguished from monohydrogen phosphate at the nanocrystal surface. Combining quantitative NMR of PO43-, OH-, CO32-, HPO42-, and bound H2O with information on cation concentrations from energy-dispersive analysis, we are able to provide a comprehensive analysis of the average composition of bone apatite.

Structure of nanodiamonds. Nanodiamonds, crystalline balls of a few thousand carbon atoms often with ~ 2 % nitrogen, are a fascinating material that is present in carbonaceous meteorites and has been synthesized by detonation or shock from carbon-containing precursors. Their surface and interior structure is still a matter of debate: Do they have an buckyball or onion-shell surface ? Or are they fully protonated ? Are there dangling bonds at the surface ? Is the interior hollow ? Why is the diameter of ~4 nm quite independent of synthesis conditions ? Is the lattice distorted, and if so, why ? How common are defects ? Where is the nitrogen ? Does slow growth in the interstellar medium result in the same structure as shock synthesis ? What is the origin of the broad downfield "foot" in the 13C spectrum ?

In order to answer many of these questions, we have studied synthetic nanodiamond with a crystalline core of 4.1-nm diameter by NMR spectroscopy, dipolar-coupling, relaxation and chemical shift anisotropy plus paramagnetic shift anisotropy measurements. The surface layer of these ~4.8-nm diameter carbon crystals is mostly protonated or bonded to OH or NH groups, while sp2-hybridized carbons make up < 1 % of the material. 13C{1H} HARDSHIP experiments, based on H-C dipolar dephasing by surface protons, show that seven carbon layers, in a shell of 0.63 nm thickness that contains 60 % of all carbons, mostly resonate more than 6 ppm downfield (within the 40 - 80 ppm range) from bulk diamond. The location and concentration of unpaired electrons has been studied in detail, based on their strongly distance-dependent effects on T1,C relaxation as well as the paramagnetic-shift anisotropy probed with chemical-shift anisotropy recoupling. Quantitative analysis of these effects shows that the unpaired electrons are at a 0.5-nm distance from the surface. There are ~ 30 unpaired electrons per nanodiamond particle. Based on the information from NMR and WAXD, we have developed a detailed model of the structure of the nanodiamond particles. [J. Am. Chem. Soc.131, 1426-1435 (2009)].

Supramolecular structure of diatom cell walls. The cell walls of diatoms consist mostly of amorphous hydrated silica patterned on the submicron scale by the organic biomolecular machinery of these unicellular organisms. They have been shown to provide mechanical protection and possibly act as photonic crystals directing sunlight to chlorophyll. We are investigating the supramolecular structure of T. pseudonana diatoms isotopically labeled with 13C, 29Si, and 15N at UC Santa Barbara (by Drs. Mark Brzezinski, Bradley F. Chmelka, et al.). More than 8 distinct components have been identified and characterized by multinuclear NMR: Silica (29Si), protein (13C, 15N), amines (13C, 15N), casing polysaccharide (13C), disordered polysaccharide (13C, 1H), two types of lipids (13C, 1H), and inorganic hydrated phosphate (31P, 1H). Their proximities on the 10-nm scale can be assessed most easily by 1H-spin-diffusion mediated T1H relaxation, detected after cross polarization to various X-nuclei.

2) The Structure of the Nafion Fuel Cell Membrane

The proton-exchange membrane (PEM) is a central, and often performance-limiting, component of all-solid H2/O2 fuel cells. Nafion®, the most widely used PEM, consists of a perfluorinated polymer that combines a hydrophobic Teflon-like backbone with hydrophilic ionic side groups. It stands out among polymer materials for its high, selective permeability to water and small cations. Generous funding of our research on Nafion has been provided by the Department of Energy, Basic Energy Sciences via the Ames Laboratory.

High-resolution 13C NMR of fluoropolymers. In spite of the technological significance of perfluorinated polymers such as Nafion, or poly(tetrafluoroethylene), PTFE/Teflon®, no 13C solid-state nuclear magnetic resonance (NMR) spectra of these temperature- and solvent-resistant materials could be found in the literature prior to 2001. The standard 13C spectra of Nafion and Teflon are broadened due to the large 19F chemical-shift anisotropy, which prevents on-resonance 19F decoupling. We have obtained the first high-resolution 13C NMR spectra of solid perfluorinated polymers by combining 28-kHz magic-angle spinning (MAS) with rotation-synchronized 19F 180°-pulses. The small line width shows that most Nafion backbone segments are helical and conformationally ordered, even though Nafion is a random copolymer. Conformational disorder is concentrated at the branch points. [Macromolecules 37, 5995-6003 (2004)] Furthermore, motional narrowing of 13C-19F dipolar splittings proved that most chain segments between branch points rotate by more than 150o around their helix axes. This rigidity of the backbone excludes many models of Nafion that are based on the assumption of random coiling. [Macromol. Chem. Phys. 208, 2189-2203 (2007)] Nevertheless, the helices do not pack into well-ordered bundles, according to orientational correlation data from 19F CODEX (centerband-only detection of exchange) NMR.

Quantitative SAXS simulations of the nanostructure of Nafion. Important aspects of the long elusive nanometer-scale structure that underlies many of these outstanding properties of Nafion have now been conclusively determined by a quantitative analysis of small-angle scattering data, using a novel approach based on numerical Fourier transformation (see above). The characteristic "ionomer peak", see inset in Figure 2d, arises from long parallel but otherwise randomly packed water channels surrounded by partially hydrophilic sidebranches, forming inverted-micelle cylinders. They are stabilized by the rigid polymer backbones proven by NMR. At 20 vol% water, the water channels have diameters between 1.8 and 3.5 nm, with a 2.4-nm average. Nafion crystallites (~10 vol%), which form physical crosslinks crucial for the mechanical properties of Nafion films, are elongated and parallel to the water channels, with cross sections of ~(5 nm)2. Simulations for various other models of Nafion, including Gierke's cluster and the polymer-bundle model, do not match the scattering data. The new model can explain important features of Nafion, including fast diffusion of water and protons through Nafion and its persistence at low temperatures. It also provides a valid target for the design of other, cheaper ionic polymers that could replace Nafion. [Nature Mater. 7, 75-83 (2008)]

Figure 2. Parallel water-channel (inverted-micelle cylinder) model of Nafion. a, Two views of an inverted micelle cylinder, with the polymer backbones on the outside and the ionic sidegroups lining the water channel. Shading is used to distinguish chains in front and in the back. b, Schematic of the approximately hexagonal packing of several inverted-micelle cylinders. c, Cross sections through the cylindrical water channels (white) and the Nafion crystallites (black) in the noncrystalline Nafion matrix (dark gray), as used in the simulation of the small-angle scattering curves in d. d, Small-angle scattering data (circles) of Rubatat et al. in a log(I) vs. log(q) plot for Nafion at 20 vol% of H2O, and our simulated curve from the model shown in c. The inset shows the ionomer peak in a linear plot of I(q). Simulated scattering curves from the water channels and the crystallites by themselves (in a structureless matrix) are shown dashed and dotted, respectively.

Straightness of water channels in Nafion. The SAXS data have provided detailed information on the lateral packing of the channels, but knowledge on the behavior of the channels in the third dimension is limited. Specifically, the persistence length of the water channels, i.e. the length scale on which they are essentially straight, has not been determined reliably. An analysis of the narrowing of the 2H NMR spectrum of D2O in Nafion can contribute here. After initial narrowing due to exchange between free D2O in the interior of the channels and the D2O at the channel wall, the 2H quadrupolar coupling is averaged down to ~ 1 kHz for 10 wt% D2O, as observed in drawn Nafion samples (provided by Dr. Robert B. Moore, Virginia Tech) with almost straight channels. The coupling shows quantitatively the expected decrease with increasing dilution of bound water by free water.

If D2O diffuses along a curved channel on the millisecond time-scale, it experiences varying 2H quadrupolar couplings due the orientation dependence of the residual coupling, resulting in further motional narrowing. Such an additional reduction in line width by a factor > 10 is indeed observed in undrawn commercial Nafion membranes, see Figure 3(a) indicating that the channels are relatively tortuous on the micrometer scale probed by the diffusion with D ~ 1 µm2/ms. Treating a water channel as a chain of many short straight segments with different orientations, the diffusion of D2O can be numerically simulated as a multi-site exchange process. The preliminary simulations in Figure 3(b) show the expected dramatic line narrowing as the channels become more tortuous. The most stringent upper limit on the persistence length is provided by the 2H T2 relaxation time, which is also calculated in the simulations. For simulations of channels with short persistence lengths, a large number of segments must be used; the simulation time can be kept manageable by lumping several original segments together into an average-coupling segment.

3) Structure of Natural Organic Matter

Knowledge of the chemical structure of soil organic matter and other polymers in the environment is rather limited, but would be required for a true understanding of soil formation, diagenesis, or retention of water and contaminants. Solid-state NMR provides unique opportunities for analyzing these otherwise nearly intractable systems.

We have been pursuing a worldwide unique program of developing solid-state NMR methods for characterizing the composition of such complex organic solids. By a suite of new NMR techniques, we can identify more than 36 functional groups (see Fig. 3), and determine their concentrations based on quantitative direct-polarization NMR. Information on the nanometer-scale structure is obtained by various spin-diffusion and relaxation experiments. The focus of these investigations is natural organic matter (NOM), as found in soils, swamps, rivers, oceans, or sediments, but the NMR methods are similarly applicable to many other organic solids. On the basis of greatly increased knowledge on the structure of soil organic matter, we are addressing questions of soil fertility, formation of natural organic matter, and the sorption of nonpolar contaminants in soil. We have used advanced NMR to study:

  • Humic acids [PNAS, 2004; Geoderma, 2007]
  • Contaminant sorption in humic substances [Environ. Sci. Tech., 2002]
  • Untreated mineral soil
  • Fulvic acid from a lake in Antarctica [Org. Geochem., 2007]
  • Natural organic matter in water [Geochim. Cosmochim. Acta, 2007]
  • Kerogen
  • Whole wood [J. Agri. Food. Chem., 2006]
  • Biochar [Environm. Progress, 2009]
  • Melanoidins formed in the Maillard reaction

Figure 3. Top: Typical 13C MAS NMR spectrum of a humic acid, and the limited traditional peak assignments, with significant uncertainties due to overlap with other functional groups (dashed lines). Bottom: More than 30 functional groups that we can identify by advanced solid-state NMR techniques. Ovals refer to identification based on the 13C chemical-shift-anisotropy, dashed circles to dipolar dephasing.

Identification of functional groups. We have developed a set of spectral editing techniques for 13C NMR spectra of complex organic matter. With detection efficiencies sufficient for complex natural organic matter, we can selectively observe 13C NMR signals of:

  • CH2 groups (by three-spin coherence selection) [J. Magn. Reson. (2005)]
  • CH (methine) groups, in particular OCH and NCH (by dipolar DEPT) [JACS, 2002]
  • Nonprotonated carbons and CH3 groups (quantitatively at high magic-angle spinning frequencies) [Environ. Sci. Technol. (2004)]
  • Carbons in the interior of fused aromatic rings, as found in charcoal (by recoupled long-range C-H dephasing) [J. Magn. Reson. 162, 217-227 (2003)]
  • Nitrogen-bonded C=O, CH, and aromatic carbons (by 13C{14N} SPIDER NMR) [Chem. Phys. Lett. (2002); PNAS (2004)]
  • NCH groups in pyridine rings (by CHn selection)
  • Alkyl O-CHR-O and O-CRR-O (e.g. anomeric carbons in sugar rings), without overlap from aromatic carbons (by chemical-shift anisotropy filtering) [Solid State NMR (2004)

In addition, we can select signals of CH vs. OH and NH protons (by 1H CSA recoupling). Pulse programs for these methods can be found at http://www.public.iastate.edu/~nmrksr/. Much of this work was supported by the National Science Foundation.

In combination with the 13C isotropic chemical shift, these advanced spectral-editing filters often enable unique identification of functional groups, see Fig. 3. Most of these signals can also be quantified, for instance by means of optimized direct-polarization experiments. Furthermore, we can identify esters, COOH groups, phenols, and aromatic ethers by means of various two-dimensional techniques. Various applications of these methods are briefly discussed in the following.

A likely molecular origin of a rice-yield decline. Intensive cropping of irrigated lowland rice for 20-30 years has led to significant declines in grain yield for high-yielding field trials. This yield decline has been attributed to decreased availability of soil nitrogen (N), which is mostly held in the soil organic matter. In collaboration with Dr. Dan Olk of the National Soil Tilth Laboratory in Ames, and formerly of the International Rice Research Institute (IRRI) in the Philippines, we have performed a detailed structural analysis of a humic acid fraction extracted from a continually submerged, triple-cropped rice soil, and a corresponding humic acid fraction extracted from an aerobic single-cropped rice soil for reference. In particular, we applied our recently developed SPIDER method for selecting the signals of carbons bonded to nitrogen. We have found significant amounts of N directly bonded to aromatic rings in the intensively cropped soil. Since N bonded to aromatics is not readily plant available, this observation may help explain the observed yield decline.

Our quantitative 13C NMR combined with spectral editing have shown that the triple-cropped soil humic acid is rich in lignin derivatives (>45% of all carbon). The chemical shift of the N-bonded aromatic carbons and the peak intensities indicate that these signals are due to amide groups bonded to lignin aromatic rings. In contrast, the single-cropped humic acid has a lower lignin content and showed peak intensities more characteristic of easily degradable peptides, and less N bonded to aromatic carbons. [Proc. Nat. Acad. Sci. 101, 6351-6354 (2004)]

Nearly quantitative 13C NMR of untreated mineral soil. Do the amounts of recalcitrant components of soil organic matter (SOM) vary across the landscape position? To address this question, Dr. Michael Thompson in the Agronomy department at ISU selected four Mollisols in central Iowa for study by NMR. Spin counting by correlation of the integral NMR intensity with the C concentration by elemental analysis showed that our NMR spectra were ≥ 85% quantitative for the majority of the samples studied. For untreated whole-soil samples with <2.5 wt% C, which is considerably less than in most previous quantitative NMR analyses of SOM, useful spectra that reflected ≥65% of all C were obtained. The NMR analyses allowed us to conclude (1) that the HF treatment (with or without heat) had low impact on the organic C composition in the samples, except for protonating carboxylate anions to carboxylic acids, (2) that most organic C was observable by NMR even in untreated soil materials, (3) that esters were likely to compose only a minor fraction of SOM in these Mollisols, and (4) that the aromatic components of SOM were enriched to ~53% in the poorly drained soils, compared with ~48% in the well-drained soils; in plant tissue and particulate organic matter (POM) the aromaticities were ~18% and ~ 32%, respectively. Nonpolar, non-protonated aromatic C, interpreted as a proxy for charcoal C, dominated the aromatic C in all soil samples, composing 69 - 78% of aromatic C and 27 - 36% of total organic C in the whole soil and clay-fraction samples. [Geochim, Cosmochim. Acta, accepted for publication]

Biochar. Pyrolysis or gasification of biomass may economically produce energy and chemicals from a wide range of biorenewable resources. These processes yield some amount of char, typically between 5 and 20% of feedstock mass, which may be used as biochars, i.e. applied as a soil amendment and/or a carbon sequestration agent, as in the dark, fertile terra preta soils in the Amazon, which have been shown to contain man-made charcoal functioning as soil organic matter. The link between char properties and their efficacy in soils, however, is not well understood, much less how to engineer the process conditions to produce desired biochar properties. This is especially true for chars from gasification and fast pyrolysis.

A key aspect of determining char quality for biochar and other applications is the ability to quantitatively characterize the forms of carbon present. In particular, concern has been expressed about "incompletely" pyrolyzed biomass as it may provide too much bio-available carbon to the soil without enough simultaneous nitrogen, resulting in nitrogen immobilization and therefore, negative short-term effects on plant yield. In collaboration with Catherine Brewer and Dr. Robert Brown on campus, we are studying biochars by NMR. Direct-polarization magic-angle spinning is the only available method for reliably determining the aromaticity and composition of chars. Further, it can be combined with dipolar dephasing to quantify the fraction of non-protonated aromatic C. The size of clusters of fused aromatic rings in chars can also be determined based on complementary NMR methods of spectral analysis and 1H-13C dipolar distance estimates. [Environ. Progr. Sustain. Energy 28, 386, 2009]

Products of the Maillard reaction. The Maillard reaction between reducing sugars and amino acids is important in food science (like baking cookies: reaction of sugar and egg protein) and has also been considered as a potential humification pathway in soils. In the melanoidin polymers formed, the sugar is completely transformed into complex structures resembling humic acids. The exciting aspect about this "artificial" complex organic matter is the possibility of introducing specific 13C and 15N isotopic label, which enable NMR structural studies in unprecedented detail. In particular, the fate of the amino acid in the model Maillard reaction between glucose and glycine in a 1:1 molar ratio has been investigated by applying advanced 13C and 15N solid-state nuclear magnetic resonance (NMR) techniques to 13C- and 15N-labeled melanoidins formed in dry and solution reactions. Quantitative 13C NMR shows that ~23% of carbon is from glycine; the ~2% loss compared to the 25% glycine C in the reactants is due to the COO moiety being liberated as CO2 (Strecker degradation). 13C J-modulation experiments on melanoidins made from doubly 13C-labeled glycine show that the C-C backbone bond of ~2/3 of the incorporated amino acid stays intact, and about half of all glycine is incorporated as N-CH2-COO without fragmentation. Degradation processes without CO2 loss affect ~1/8 of glycine in dry reaction, and ~1/4 in solution. These results indicate that Strecker degradation affects ~1/4 (dry reaction) to 1/3 (in solution) of all glycine but is not the main pathway of glycine incorporation. Spectra of Strecker degradation products show that C2 of glycine reacts to form N-CH3, C-CHn-C, or aromatic units. The gycine-C1 carbon incorporated into the melanoidins remains ≥95% part of COO moieties; ~5% of amides are also detected. The C2-N bond stays intact for ~70% of the incorporated glycine. The 15N spectra show many peaks, over a 200-ppm range, documenting a multitude of different chemical environments of nitrogen. The majority (>78 %) of nitrogen, in particular most pyrrolic N, is not protonated. Since N-H predominates in amino acids and proteins, nonprotonated nitrogen may be a characteristic marker of Maillard reaction products. [J. Agri. Food Chem., accepted for publication]

4) High-Resolution NMR of Thermoelectric Tellurides

The increase in demand for energy world-wide combined with concerns regarding burning of fossil fuels has propelled the need for new forms of energy generation and increased efficiencies in existing systems as one of the paramount research challenges for the 21st century. Thermoelectric materials, which generate electricity when their two ends are exposed to different temperatures, present a promising technology for increasing energy efficiency since they can convert waste heat directly into electricity, or in turn, provide vibration- and coolant-free refrigeration when a voltage is applied. The thermoelectric performance of a material is characterized by figure of merit ZT = S2σT/κ, where S is the Seebeck coefficient, σ the electrical conductivity, κ the thermal conductivity, and T the absolute temperature. Historically, the upper limit for ZT has been near 1.5. The development of economically viable thermoelectric power generation and refrigeration devices will require greatly improved thermoelectric materials, with ZT in excess of 4. This would dramatically increase their utilization for converting waste-heat to electricity, improving the efficiency of power plants, internal combustion engines and even solar energy systems or on the other hand, provide for more efficient solid-state refrigerators. Even an incremental increase in ZT will have an immediate impact by improving the efficiency of existing commercial devices based on Seebeck and Peltier effects.

It has been difficult to find semiconductor materials that would yield ZT greater than 1.5. In tellurides, which have been identified as particularly promis ing thermoelectrics, nanostructuring has been pursued to reduce the (lattice contribution to the) thermal conductivity σ by phonon scattering while retaining a high electrical conductivity κ and to modify the density of states near the Fermi energy to maximize S. For characterizing nanoclusters of dopants, electronic inhomogeneity, lattice strain, abundant defects, and other kinds of disorder in high-performance thermoelectrics, a reliable, high-resolution local probe of the chemi cal and electronic structure is needed. Advanced high-resolution 125Te nuclear magnetic resonance (NMR) is emerging as a method with unique capabilities to fill this need. It can determine (i) the bulk composition, (i) the presence of inclusions (in particular those on the nanoscale), (ii) deviations from local cubic symmetry, (iv) presence of defects, (v) the charge-carrier con centra tion, and (vi) its inhomogeneity (if present). Many of these observables, in particular the reduced local symmetry and charge-carrier inhomogeneity, cannot be measured by any other method. For obtaining a comprehensive structural picture, NMR must be applied in combination with reliable measurements of electric and transport properties using the same carefully prepared samples.

Composition and electronic inhomogeneity of Sb- and Ag-doped PbTe. In collaboration with Drs. Bruce Cook (Ames Laboratory) and Mercouri Kanatzidis (Northwestern University), we have studied high-performance thermoelectrics Ag1-yPb18Sb1+zTe20 ("LAST-18") by magic-angle spinning 125Te and 207Pb NMR. We found that there are two phases of 10-fold different free-electron concentration, n, proven by pairs of Knight-shifted NMR peaks and biexponential relaxation. The ratio of the phases is typically 2:1, with n ≈ 2x1019 cm-3 and 0.2x1019 cm-3, respectively, determined from the spin-lattice relaxation times. 125Te NMR spectra show that both phases contain similar concentrations of Sb. The low-n component is assigned to Ag-rich regions with Ag-Sb pairing (but not AgSbTe2, whose signal is not observed in the 125Te spectra), the dominant high-n component to PbTe:Sb, resulting from the Ag:Sb imbalance. The observed electronic inhomogeneity must be taken into account in any analysis of the excellent thermoelectric properties of these materials. [Phys. Rev. B 80, 115211-1 to -6 (2009)]

Broadband "infinite-speed" magic-angle spinning NMR. High-resolution magic-angle spinning NMR of high-Z spin-1/2 nuclei such as 125Te, 207Pb, 119Sn, 113Cd, and 195Pt is often hampered by large (>1,000 ppm) chemical-shift anisotropies, which result in strong spinning sidebands that can obscure the centerbands of interest. In various tellurides with applications as thermoelectrics and as phase-change materials for data storage, even 22-kHz magic-angle spinning cannot resolve the center- and sidebands broadened by chemical-shift dispersion, which precludes peak identification or quantification. For sideband suppression over the necessary wide spectral range (up to 200 kHz), radio-frequency pulse sequences with few, short pulses are required. We have identified Gan's two-dimensional magic-angle-turning (MAT) experiment with five 90° pulses as a promising broadband technique for obtaining spectra without sidebands. We have adapted it to broad spectra and fast magic-angle spinning by accounting for long pulses (comparable to the dwell time in t1) and short rotation periods. Spectral distortions are small and residual sidebands negligible even for spectra with signals covering a range of 1.5 γB1, due to a favorable disposition of the narrow ranges containing the signals of interest in the spectral plane. The method performed successfully for various technologically interesting tellurides with spectra spanning up to 170 kHz, at 22 kHz MAS. [J. Am. Chem. Soc. 131, 8390 (2009)]

5) NMR Technique Development

We continue to work on developing radio-frequency pulse sequences for extending the capabilities of solid-state NMR. In addition to the six spectral editing methods listed above, we have introduced the following techniques:

  • SUPER for measuring chem.-shift anisotropy powder patterns [J. Magn. Reson., 2002]
  • C-HN REDOR for measuring distances between 13C and amide H [JACS, 2003]
  • MAD for measuring 13C-13C 2D exchange with 1H spin diffusion [Macromolecules, 2004]
  • 1H probehead background suppression [Solid State NMR, 2004]
  • Local measurement of the 1H spin diffusion coefficient [Solid State NMR, 2006]
  • HARDSHIP for measuring thickness in nanocomposites, see above [J. Chem. Phys., 2007]
  • Effect of L-spin T1Q relaxation in S{L} recoupling [J. Magn. Reson., 2009]
  • Fast magic-angle turning for broadband "infinite-speed" MAS NMR, s. above [JACS, 2009]