Defect chemistry, surface structures, and lithium insertion in anatase TiO2

Atomistic simulation techniques are used to investigate the defect properties of anatase TiO(2) and Li(x)TiO(2) both in the bulk and at the surfaces. Interatomic potential parameters are derived that reproduce the lattice constants of anatase, and the energies of bulk defects and surface structures are calculated. Reduction of anatase involving interstitial Ti is found to be the most favorable defect reaction in the bulk, with a lower energy than either Frenkel or Schottky reactions. The binding energies of selected defect clusters are also presented: for the Ti(3+)-Li(+) defect cluster, the binding energy is found to be approximately 0.5 eV, suggesting that intercalated Li ions stabilize conduction band electrons. The Li ion migration path is found to run between octahedral sites, with an activation energy of 0.45-0.65 eV for mole fractions of lithium in Li(x)TiO(2) of x < or = 0.1. The calculated surface energies are used to predict the crystal morphology, which is found to be a truncated bipyramid in which only the (101) and (001) surfaces are expressed, in accord with the available microscopy data. Calculations of defect energies at the (101) surface suggest that single Ti(3+) defects and neutral Ti(3+)-Li(+) pairs tend to segregate to the surface.     

Quantum correction to the pair distribution function

We report a numerical technique that allows the quantum effects of zero-point motion to be incorporated into Pair Distribution Functions calculated classically for molecules using Monte Carlo or Molecular Dynamics simulations. We establish the basis for this approximation using a diatomic molecule described by a Morse potential. The correction should significantly improve the agreement between modeled and experimental data, and facilitate conclusions about inter- and intra-molecular motion and flexibility. We describe a similar approach to obtain the energy and the specific heat.     

Combining experimental and computational methods to unravel the dynamical structure of photoelectrosynthetic interfaces

At photoelectrosynthetic interfaces, an electrochemical reaction is driven by excited charge carriers from a semiconducting photoabsorber. The structure and composition of this interface determine both the electronic and electrochemical performance of devices, yet this structure is often highly dynamic both in the time-domain and upon applied potentials. We discuss the arising challenges from this dynamical nature and review recent approaches to gain an atomistic understanding of the involved processes, which increasingly involves a combination of experimental and computational methods. Bearing a similarity to solid-electrolyte interphase formation in batteries, their apprehension could help to develop functional passivation layers for high-performance photoelectrosynthetic devices.     

Structure of V(2)O(5)*nH(2)O xerogel solved by the atomic pair distribution function technique

A long-standing issue regarding the local and long-range structure of V(2)O(5)*nH(2)O xerogel has been successfully addressed. The full three-dimensional structure of the lamellar turbostratic V(2)O(5)*nH(2)O xerogel was determined by the atomic pair distribution function technique. We show that on the atomic scale the slabs of the xerogel can be described well as almost perfect pairs (i.e., bilayers) of single V(2)O(5) layers made of square pyramidal VO(5) units. These slabs are separated by water molecules and stack along the z-axis of a monoclinic unit cell (space group C2/m) with parameters a = 11.722(3) A, b = 3.570(3) A, c = 11.520(3) A, and beta = 88.65 degrees. The stacking sequence shows signatures of turbostratic disorder and a structural coherence limited to 50 A.     

Changes in electronic structure upon lithium insertion into the A-site deficient perovskite type oxides (Li,La)TiO3

Investigation on variation of the electronic structure accompanying the electrochemical lithium insertion into the perovskite type oxide, (Li,La)TiO3, has been carried out by X-ray absorption spectroscopy (XAS). During the electrochemical lithium insertion, titanium ion reduced its oxidation state from Ti4+ to Ti3+, while La3+ does not contribute to the reduction reaction resulting from Ti K-edge and La L3-edge XAS, respectively. Furthermore, O K-edge XAS showed marked spectral changes with electrochemical lithium insertion, indicating the electronic structure around oxide ion affected by lithium insertion reaction. From the XAS measurement, we have concluded the variation observed in O K-edge XAS was related to the strong interaction with inserted Li ion. To confirm this, first-principles band calculations were performed for the perovskite structure before and after electrochemical lithium insertion. The calculated results showed that the electron originated from inserted Li transferred to neighboring oxide ion locally as well as to Ti ion. This may be due to local neutralization effect of Li to reduce the electrostatic interaction in the crystal.     

Anatase TiO2 nanosheet: An ideal host structure for fast and efficient lithium insertion/extraction

Anatase TiO₂ nanosheets with largely exposed (0 0 1) facets have been synthesized by a modified method. Exploitation of these nanosheets as a host structure for reversible lithium insertion/extraction has been investigated. It is found that these TiO₂ nanosheets manifest much lower initial irreversible losses compared to other anatase TiO₂ nanostructures, and excellent cycling performance at a charge–discharge rate as high as 20 C. The superior reversible lithium storage capability can be attributed to the ultrathin nanosheet structure: a large exposed effective area and a very short diffusion path. It thus attests the promising use of these anatase TiO₂ nanosheets in high-power lithium–ion batteries.     

Nanoporous anatase TiO2 mesocrystals: additive-free synthesis, remarkable crystalline-phase stability, and improved lithium insertion behavior

Unique spindle-shaped nanoporous anatase TiO(2) mesocrystals with a single-crystal-like structure and tunable sizes were successfully fabricated on a large scale through mesoscale assembly in the tetrabutyl titanate-acetic acid system without any additives under solvothermal conditions. A complex mesoscale assembly process involving slow release of soluble species from metastable solid precursors for the continuous formation of nascent anatase nanocrystals, oriented aggregation of tiny anatase nanocrystals, and entrapment of in situ produced butyl acetate as a porogen was put forward for the formation of the anatase mesocrystals. It was revealed that the acetic acid molecules played multiple key roles during the nonhydrolytic processing of the [001]-oriented, single-crystal-like anatase mesocrystals. The obtained nanoporous anatase mesocrystals exhibited remarkable crystalline-phase stability (i.e., the pure phase of anatase can be retained after being annealed at 900 °C) and improved performance as anode materials for lithium ion batteries, which could be largely attributed to the intrinsic single-crystal-like nature as well as high porosity of the nanoporous mesocrystals.     

Spectroscopic, electrochemical, and computational aspects of the charge distribution in Ru(acac)2(R-o-benzoquinonediimine) complexes

The syntheses and properties are reported for five Ru(acac)2(R-bqdi) species where acac is acetylacetonate, and R-bqdi is the non-innocent ligand ortho-benzoquinonediimine substituted with R = H (1), 4,5-dimethyl (2), 4-Cl (3), or 4-NO2 (4), and N,N'-dimethylsulfonyl (5). Their identities and purities were confirmed by NMR, mass spectra, IR and analytical data. The large degree of metal-to-ligand pi-back-donation was analyzed by spectroscopic (UV/visible, IR, Raman) and electrochemical data, supported by molecular orbital composition computations using density functional theory (DFT), with the polarizable continuum model (PCM) to mimic the presence of solvent, and prediction of electronic spectra using time-dependent DFT methods. Extended charge decomposition analysis (ECDA) and natural population analysis (NPA) both produced a detailed picture of the bonding between the non-innocent bqdi ligand and the metal center, allowing correlations to be drawn between the nature of the R substituents and the quantitative extent of pi-back-donation and sigma-forward donation. In conclusion, the issue of whether these species are best regarded as Ru(II)(quinonediimine) or coupled Ru(III)(semiquinonediiminate) species is discussed.     

Nanoscale structural order from the atomic pair distribution function (PDF): There's plenty of room in the middle

Emerging materials of scientific and technological interest are generally complex and often nanostructured: they have atomic orderings that extend on nanometer length-scales. These can be discrete nanoparticles; bulk crystals with nanoscale chemical or displacive order within them; mesoporous materials that are bulk materials containing nanoscale holes; and nanocomposites that are intimate heterogeneous mixtures of nano-sized constituents. As always, a quantitative knowledge of the atomic structure within these materials is a prerequisite to understanding and engineering their properties. Traditional crystallographic methods for obtaining this information break down at the nanoscale, sometimes referred to as “the nanostructure problem”. We describe here some emerging methods for studying nanoscale structure. We present some examples of recent successes. Finally, we discuss future directions and opportunities and draw attention to limitations and potential problems.     

Lithium insertion into TiO2 (anatase): electrochemistry,Raman spectroscopy,and isotope labeling

This article presents a critical discussion of selected structural aspects of electrochemical Li-insertion into TiO₂ (anatase). Recent works are reviewed (almost half of the cited works is from 2010+) with a special attention to the crystal-face-specific phenomena, Raman spectroscopy, and single-crystal and nanocrystalline electrodes. The benefits of isotopic labeling are highlighted for the in-depth understanding of Raman spectra and the Raman spectroelectrochemistry of Li-insertion. The persisting open questions and contradictory issues in the field are discussed too.     

Three-dimensional structure of nanocomposites from atomic pair distribution function analysis: study of polyaniline and (polyaniline)(0.5)V(2)O(5) x 1.0 H(2)O

The three-dimensional structures of emeraldine base polyaniline (PANI) and (polyaniline)(0.5)V(2)O(5) x 1.0 H(2)O have been determined by total X-ray scattering experiments. Atomic pair distribution functions (PDF) were measured to obtain experimental observables against which structural models were tested and refined. The PDF approach is necessary because of the limited structural coherence in these nanostructured materials. Polyaniline possesses a well-defined local atomic arrangement that can be described in terms of an 84-atom orthorhombic unit cell. The nanocomposite (PANI)(0.5)V(2)O(5) x 1.0 H(2)O too is locally well ordered and may be described in terms of a small number of structure-sensible parameters. The PDF approach allows the construction of structure models of PANI and (PANI)(0.5)V(2)O(5) x 1.0 H(2)O on the basis of which important materials' properties can be explained predicted and possibly improved.     

Characterizing TiO2(110) surface states by their work function

The unreconstructed TiO(2)(110) surface is prepared in well-defined states having different characteristic stoichiometries, namely reduced (r-TiO(2), 6 to 9% surface vacancies), hydroxylated (h-TiO(2), vacancies filled with OH), oxygen covered (ox-TiO(2), oxygen adatoms on a stoichiometric surface) and quasi-stoichiometric (qs-TiO(2), a stoichiometric surface with very few defects). The electronic structure and work function of these surfaces and transition states between them are investigated by ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES). The character of the surface is associated with a specific value of the work function that varies from 4.9 eV for h-TiO(2), 5.2 eV for r-TiO(2), 5.35 eV for ox-TiO(2) to 5.5 eV for qs-TiO(2). We establish the method for an unambiguous characterization of TiO(2)(110) surface states solely based on the secondary electron emission characteristics. This is facilitated by analysing a weak electron emission below the nominal work function energy. The emission in the low energy cut-off region appears correlated with band gap emission found in UPS spectra and is attributed to localised electron emission through Ti(3+)(3d) states.     

CO2 insertion into metal-carbon bonds: a computational study of Rh(I) pincer complexes

Catalytic carboxylation reactions that use CO(2) as a C1 building block are still among the 'dream reactions' of molecular catalysis. To obtain a deeper insight into the factors that control the fundamental steps of potential catalytic cycles, we performed a detailed computational study of the insertion reaction of CO(2) into rhodium-alkyl bonds. The minima and transition-state geometries for 38 pincer-type complexes were characterized and the according energies for the C-C bond-forming step were determined. The electronic properties of the Rh-alkyl bond were found to be more important for the magnitude of the activation barrier than the interaction between rhodium and CO(2). The charge of the alkyl-chain carbon atom, as well as agostic and orbital interactions with the rhodium, exhibit the most pronounced influence on the energy of the transition states for the CO(2) insertion reaction. By varying the backbone and the donor groups of the pincer ligand those properties can be tuned over a very broad range. Thus, it is possible to match the electronic and steric properties with the fundamental requirements of the CO(2) insertion into rhodium-alkyl bonds of the ligand framework.     

Investigation of the interaction between colloidal TiO(2) and bovine hemoglobin using spectral methods

The interaction between bovine hemoglobin (BHb) and TiO(2) colloid was investigated by UV/vis absorption, UV/vis diffuse reflectance spectrum, IR, fluorescence, synchronous fluorescence, and three-dimensional fluorescence spectra techniques under physiological pH 7.40. TiO(2) effectively quenched the intrinsic fluorescence of BHb via static quenching. The process of binding TiO(2) on BHb was a spontaneous molecular interaction procedure. The thermodynamic parameters, DeltaH degrees and DeltaS degrees were estimated to be -78.07 kJ mol(-1), -110.93 J mol(-1)K(-1) according to the van' Hoff equation. This indicates that the van der Waals and hydrogen bonds interactions played a major role in stabilizing the TiO(2)-BHb complex. The effect of TiO(2) on the conformation of BHb was analyzed using synchronous fluorescence spectroscopy.     

Direct observation of a transverse vibrational mechanism for negative thermal expansion in Zn(CN)2: an atomic pair distribution function analysis

The instantaneous structure of the cyanide-bridged negative thermal expansion (NTE) material Zn(CN)(2) has been probed using atomic pair distribution function (PDF) analysis of high energy X-ray scattering data (100-400 K). The temperature dependence of the atomic separations extracted from the PDFs indicates an increase of the average transverse displacement of the cyanide bridge from the line connecting the Zn(II) centers with increasing temperature. This allows the contraction of non-nearest-neighbor Zn...Zn' and Zn...C/N distances despite the observed expansion of the individual direct Zn-C/N and C-N bonds. Thus, this analysis provides definitive structural confirmation that an increase in the average displacement of bridging atoms is the origin of the NTE behavior. The lattice parameters reveal a slight reduction in the NTE behavior at high temperature from a minimum coefficient of thermal expansion (alpha = dl/ldT) of -19.8 x 10(-6) K(-1) below 180 K, which is attributed to interaction between the doubly interpenetrated frameworks that comprise the structure.     

Changes in electronic structure upon lithium insertion into Fe2(SO4)3 and Fe2(MoO4)3 investigated by X-ray absorption spectroscopy

Changes in electronic structure upon electrochemical lithium insertion into two iron compounds, namely, rhombohedral Fe2(SO4)3 with a NASICON-type structure and monoclinic Fe2(MoO4)3, were investigated using X-ray absorption spectroscopy (XAS). Fe K-edge and L(III)- and L(II)-edge XAS revealed that the rearrangement of Fe d electrons or rehybridization of Fe d-O p bonding took place accompanied by the reduction of Fe ions upon Li insertion for both samples and that a larger change in spectra was observed in Fe2(SO4)3. In addition, the changes in the electronic structure of the polyanion units XO4(2-) (X = S or Mo) after Li insertion were also investigated by O K-edge and S K-edge or Mo L(III)-edge XAS. The results indicated that the electronic structure around oxygen markedly changed in Fe2(MoO4)3, while no significant change was observed in Fe2(SO4)3.     

Two phase morphology limits lithium diffusion in TiO(2)(anatase): a (7)Li MAS NMR study.

7Li magic angle spinning solid-state nuclear magnetic resonance is applied to investigate the lithium local environment and lithium ion mobility in tetragonal anatase TiO(2) and orthorhombic lithium titanate Li(0.6)TiO(2). Upon lithium insertion, an increasing fraction of the material changes its crystallographic structure from anatase TiO(2) to lithium titanate Li(0.6)TiO(2). Phase separation occurs, and as a result, the Li-rich lithium titanate phase is coexisting with the Li-poor TiO(2) phase containing only small Li amounts approximately equal to 0.01. In both the anatase and the lithium titanate lattice, Li is found to be hopping over the available sites with activation energies of 0.2 and 0.09 eV, respectively. This leads to rapid microscopic diffusion rates at room temperature (D(micr) = 4.7 x 10(-12) cm(2)s(-1) in anatase and D(micr) = 1.3 x 10(-11) cm(2)s(-1) in lithium titanate). However, macroscopic intercalation data show activation energies of approximately 0.5 eV and smaller diffusion coefficients. We suggest that the diffusion through the phase boundary is determining the activation energy of the overall diffusion and the overall diffusion rate itself. The chemical shift of lithium in anatase is independent of temperature up to approximately 250 K but decreases at higher temperatures, reflecting a change in the 3d conduction electron densities. The Li mobility becomes prominent from this same temperature showing that such electronic effects possibly facilitate the mobility.     

Adsorption and assembly of copper phthalocyanine on cross-linked TiO2(110)-(1 x 2) and TiO2(210)

Copper phthalocyanine (CuPc) on reconstructed rutile TiO(2) was studied with ultrahigh vacuum variable temperature scanning tunneling microscopy. On cross-linked TiO(2)(110)-(1 x 2), the CuPc molecules at low coverages sparsely lay flat at the link sites and tilted in troughs between [001] rows. Increase of the CuPc coverage led to the trapping of the CuPc molecules by the rectangular surface cells fenced by the oxygen columns along the [001] direction and the cross-link rows. Each cell could trap one CuPc molecule at intermediate coverages and two CuPc molecules at higher coverages. On TiO(2)(210), the CuPc molecules tilted in defect-free areas and lay at defect sites with their molecular planes parallel to the substrate surface. Further increase of the CuPc coverage induced the formation of one- and two-dimensional assemblies on TiO(2)(210).