Oxidation States Of Transition Metals Research Articles

Electrochemical performance of overlithiated Li1+xNi0.8Co0.2O2: structural and oxidation state studies

Pure, layered compounds of overlithiated Li1+xNi0.8Co0.2O2 (x = 0.05 and 0.1) were successfully prepared by a modified combustion method. XRD studies showed that cell parameters of the material decreased with increasing the lithium content. SEM revealed that the morphology of particles changed from rounded polyhedral-like crystallites to sharp-edged polyhedral crystals with more doped lithium. EDX showed that the stoichiometries of Ni and Co agrees with calculated synthesized values. Electrochemical studies revealed the overlithiated samples have improved capacities as well as cycling behavior. The sample with x = 0.05 shows the best performance with a specific capacity of 113.29 mA·h·g−1 and the best capacity retention of 92.2% over 10 cycles. XPS results showed that the binding energy of Li 1s is decreased for the Li doped samples with the smallest value for the x = 0.05 sample, implying that Li+ ions can be extracted more easily from Li1.05Ni0.8Co0.2O2 than the other stoichiometries accounting for the improved performance of the material. Considerations of core level XPS peaks for transition metals reveal the existence in several oxidation states. However, the percentage of the +3 oxidation state of transition metals for the when x = 0.1 is the highest and the availability for charge transition from the +3 to +4 state of the transition metal during deintercalation is more readily available.

Optimizing Surface Segregation and Defect Structure of a Perovskite through Strain for Improving Oxygen Reduction and Evolution Catalysis

Renewable energy sources are intrinsically plagued by irregular power production. In order for alternative energy sources to become viable it is imperative to improve energy storage in tandem with source development. Currently oxygen evolution and oxygen reduction are two key electrochemical reactions which often limit the efficiency of energy storage devices. Identifying and developing a catalyst for these reactions would greatly benefit the development of a renewable energy economy. It has previously been demonstrated that La0.5Sr0.5Co0.5Mn0.5O3- δ 1 can accommodate both a fully oxidised phase (δ = 0) and a reduced phase (δ = 0.62), making it one of only a few reported perovskite oxides that is stable with δ > 0.5, illustrated in Figure a. The oxygen vacancies introduced as a result of this variable oxygen stoichiometry as well as the possibility of the transition metals to have a range of oxidation states make this material an ideal candidate to investigate as a catalyst for oxygen reduction and evolution reactions. Thin films of the La0.5Sr0.5Co0.5Mn0.5O3- δ perovskite have been prepared by pulsed laser deposition on a range of substrates. Thin films are an ideal system for conducting a fundamental study of catalysis as bulk effects become negligible and surface effects can be measured. Further, there is less variation of the microstructure between samples, allowing for a more controlled comparison. Initial x-ray diffraction (XRD) results indicates a change in the lattice parameter consistent with a change in oxidation states, see Figure b. This is due to the strain effects2 caused by the lattice mismatch between the perovskite and the substrate materials, strontium titanate (STO), magnesium oxide (MgO) and lanthanum aluminate (LAO). As the bulk material is able to reversibly interchange between a fully oxidised rhombohedra phase and a hypostoichiometric reduced orthorhombic phase, it is strongly suspected that changes in lattice parameter are indicative of the overall oxidation state of the thin film layer. Hence it is suggested that the number of vacancies and the metallic oxidation states can be tuned through lattice mismatch selection. X-ray photoelectron spectroscopy is used to further investigate the changes in oxidation state caused by straining the films. Atomic force microscopy has been used to identify both the surface topology as well as film thickness (10 – 300 nm). Different morphology is observed depending on growth parameters including deposition temperature and substrate. Low energy ion scattering (LEIS) and secondary ion mass spectroscopy are employed to identify the composition of surface regions. The effects of post-growth treatment, including heating and cleaning processes, on these measurements are also considered. In addition to considering the transition metal oxidation states, surface segregation studies3 suggest that perovskite materials are often A-site terminated. LEIS studies allow for investigation of the segregation that occurs at both the surface and at the sub surface levels. This is critical as segregation is one of the main causes of ceramic degradation and will greatly influence the electrocatalytic properties. Strain is therefore being investigated as a method to control the surface segregation. A systematic study of the La0.5Sr0.5Co0.5Mn0.5O3- δ thin films at room temperature delivers the groundwork characterisation that is required for further investigation of this perovskite as an oxygen reduction and oxygen evolution reaction catalyst, for future applications ranging from metal air batteries to fuel cells and electrolysers. Figure a – Thermogravimetric analysis of La0.5Sr0.5Co0.5Mn0.5O3-δ Figure b – La0.5Sr0.5Co0.5Mn0.5O3- δ lattice parameter from XRD measurements as a function of the substrate, and a comparison to lattice parameters of the bulk material determined by neutron diffraction data

Bidentate, monoanionic auxiliary-directed functionalization of carbon-hydrogen bonds.

In recent years, carbon-hydrogen bond functionalization has evolved from an organometallic curiosity to a tool used in mainstream applications in the synthesis of complex natural products and drugs. The use of C-H bonds as a transformable functional group is advantageous because these bonds are the most abundant functionality in organic molecules. One-step conversion of these bonds to the desired functionality shortens synthetic pathways, saving reagents, solvents, and labor. Less chemical waste is generated as well, showing that this chemistry is environmentally beneficial. This Account describes the development and use of bidentate, monoanionic auxiliaries for transition-metal-catalyzed C-H bond functionalization reactions. The chemistry was initially developed to overcome the limitations with palladium-catalyzed C-H bond functionalization assisted by monodentate directing groups. By the use of electron-rich bidentate directing groups, functionalization of unactivated sp(3) C-H bonds under palladium catalysis has been developed. Furthermore, a number of abundant base-metal complexes catalyze functionalization of sp(2) C-H bonds. At this point, aminoquinoline, picolinic acid, and related compounds are among the most used and versatile directing moieties in C-H bond functionalization chemistry. These groups facilitate catalytic functionalization of sp(2) and sp(3) C-H bonds by iron, cobalt, nickel, copper, ruthenium, rhodium, and palladium complexes. Exceptionally general reactivity is observed, enabling, among other transformations, direct arylation, alkylation, fluorination, sulfenylation, amination, etherification, carbonylation, and alkenylation of carbon-hydrogen bonds. The versatility of these auxilaries can be attributed to the following factors. First, they are capable of stabilizing high oxidation states of transition metals, thereby facilitating the C-H bond functionalization step. Second, the directing groups can be removed, enabling their use in synthesis and functionalization of natural products and medicinally relevant substances. While the development of these directing groups presents a significant advance, several limitations of this methodology are apparent. The use of expensive second-row transition metal catalysts is still required for efficient sp(3) C-H bond functionalization. Furthermore, the need to install and subsequently remove the relatively expensive directing group is a disadvantage.

Tailoring the surface properties of LiNi(0.4)Mn(0.4)Co(0.2)O2 by titanium substitution for improved high voltage cycling performance.

The present study aims to provide insights into the behavior of LiNi0.4Mn0.4Co0.2O2 (NMC442) and LiNi0.4Mn0.4Co0.18Ti0.02O2 (NMC442-Ti02) cathode materials under galvanostatic cycling to high potentials, in the context of previous work which predicted that Ti-substituted variants should deliver higher capacities and exhibit better cycling stability than the unsubstituted compounds. It is found that NMC cathodes containing Ti show equivalent capacity fading but greater specific capacity than those without Ti in the same potential range. When repeatedly charged to the same degree of delithiation, NMC cathodes containing Ti showed better capacity retention. Soft X-ray absorption spectroscopy (XAS) spectra for Mn and Co indicated increased reduction in these elements for NMC cathodes without Ti, indicating that the substitution of Ti for Co acts to suppress the formation of a high impedance rock salt phase at the surface of NMC cathode particles. The results of this study validate the adoption of a facile change to existing NMC chemistries to improve cathode capacity retention under high voltage cycling conditions.

The oxidation state and microstructural environment of transition metals (V, Co, and Ni) in magnetite: an XAFS study

Transition metal-substituted magnetite minerals have attracted increasing attention for their wide application in industry and environmental protection. In this study, the valence and atomic environment of some substituting metals in magnetites (Fe3−x M x O4, M = V, Co, and Ni) were investigated using X-ray absorption fine structure spectroscopy. The results deduced from X-ray absorption near-edge structure spectroscopy indicated that the valences of V, Co, and Ni in Fe3−x M x O4 were +3, +2, and +2, respectively. The valences did not change as the substitution extent increased. Extended X-ray absorption fine structure spectroscopy suggested that the substituting cations occupied octahedral sites in the magnetite structure. The M–O and M–M/Fe distances were consistent with the Feoct–O and Feoct–Fe distances, respectively, in the magnetite (Fe3O4) structure. The occupancy of the substituting cations was assessed by crystal-field theory. We also considered the relationship between the chemical environment of substituting cations and their effects on the physicochemical properties of magnetite, including thermal stability, surface properties, and catalytic reactivity.

A Comparative Study of Electron Energy-Loss Spectroscopy (EELS) and X-ray Absorption Near Edge Structure (XANES) Spectroscopy for Probing the Oxidation State of Transition Metals in Planetary Materials

A Comparative Study of Electron Energy-Loss Spectroscopy (EELS) and X-ray Absorption Near Edge Structure (XANES) Spectroscopy for Probing the Oxidation State of Transition Metals in Planetary Materials

Theoretical oxidation state analysis of Ru-(bpy)3: influence of water solvation and Hubbard correction in first-principles calculations.

Oxidation state is a powerful concept that is widely used in chemistry and materials physics, although the concept itself is arguably ill-defined quantum mechanically. In this work, we present impartial comparison of four, well-recognized theoretical approaches based on Lowdin atomic orbital projection, Bader decomposition, maximally localized Wannier function, and occupation matrix diagonalization, for assessing how well transition metal oxidation states can be characterized. Here, we study a representative molecular complex, tris(bipyridine)ruthenium. We also consider the influence of water solvation through first-principles molecular dynamics as well as the improved electronic structure description for strongly correlated d-electrons by including Hubbard correction in density functional theory calculations.

Improving the rate capability of high voltage lithium-ion battery cathode material LiNi0.5Mn1.5O4 by ruthenium doping

The citric acid-assisted sol–gel method was used to produce the high-voltage cathodes LiNi0.5Mn1.5O4 and LiNi0.4Ru0.05Mn1.5O4 at 800 °C and 1000 °C final calcination temperatures. High resolution powder diffraction using synchrotron radiation, inductively coupled plasma – optical emission spectroscopy and scanning electron microscopy analyses were carried out to characterize the structure, chemical composition and morphology. X-ray absorption spectroscopy studies were conducted to confirm Ru doping inside the spinel as well as to compare the oxidation states of transition metals. The formation of an impurity LixNi1−xO in LiNi0.5Mn1.5O4 powders annealed at high temperatures (T ≥ 800 °C) can be suppressed by partial substitution of Ni2+ by Ru4+ ion. The LiNi0.4Ru0.05Mn1.5O4 powder synthesized at 1000 °C shows the highest performance regarding the rate capability and cycling stability. It has an initial capacity of ∼139 mAh g−1 and capacity retention of 84% after 300 cycles at C/2 charging–discharging rate between 3.5 and 5.0 V. The achievable discharge capacity at 20 C for a charging rate of C/2 is ∼136 mAh g−1 (∼98% of the capacity delivered at C/2). Since the electrode preparation plays a crucial role on parameters like the rate capability, the influence of the mass loading of active materials in the cathode mixture is discussed.

Competition between covalent bonding and charge transfer at complex-oxide interfaces.

Here we study the electronic properties of cuprate-manganite interfaces. By means of atomic resolution electron microscopy and spectroscopy, we produce a subnanometer scale map of the transition metal oxidation state profile across the interface between the high Tc superconductor YBa2Cu3O7-δ and the colossal magnetoresistance compound (La,Ca)MnO3. A net transfer of electrons from manganite to cuprate with a peculiar nonmonotonic charge profile is observed. Model calculations rationalize the profile in terms of the competition between standard charge transfer tendencies (due to band mismatch), strong chemical bonding effects across the interface, and Cu substitution into the Mn lattice, with different characteristic length scales.

Topochemical reactions of layered transition-metal oxides

Low-temperature topochemical reactions performed on complex transition metal oxides allow the synthesis of novel, metastable phases often with unusual transition metal oxidation states and/or coordination geometries. Layered materials offer many opportunities to control the structural and chemical selectivity of these types of reactions to allow the preparation of desired phases in a rational manner. Using complex oxides adopting Ruddlesden–Popper type structures as examples, recent progress in the low-temperature topochemical reduction by anion de-intercalation, and oxidation by anion insertion, is reviewed.

Oxide Wizard: An EELS Application to Characterize the White Lines of Transition Metal Edges

Physicochemical properties of transition metal oxides are directly determined by the oxidation state of the metallic cations. To address the increasing need to accurately evaluate the oxidation states of transition metal oxide systems at the nanoscale, here we present "Oxide Wizard." This script for Digital Micrograph characterizes the energy-loss near-edge structure and the position of the transition metal edges in the electron energy-loss spectrum. These characteristics of the edges can be linked to the oxidation states of transition metals with high spatial resolution. The power of the script is demonstrated by mapping manganese oxidation states in Fe3O4/Mn3O4 core/shell nanoparticles with sub-nanometer resolution in real space.

The effect of XPS background removing method on the appraisal of Ti and Fe: The case of phlogopites and brookite

The determination of the oxidation state and structural role of transition metals in minerals is a crucial challenge. XPS has proven to have a great potential in probing the site distribution and chemical states of Fe and Ti transition elements, provided that the right method to process the spectra is used. XPS spectra of these elements have the 2 p core level region usually rich of features but the choice of the method for background removing can seriously affect the results of the quantitative analysis. Single crystals of brookite (TiO 2 ) and natural micas (phlogopites) are investigated to examine the effect of background subtraction on Ti2 p and Fe2 p signals. The backgrounds used are: the “Linear” background; the traditional “Shirley” background; three different Tougaard-like backgrounds; and the more recent “shape parameter, κ” method. In the case of the studied natural micas, the Fe chemical state proportion (Fe 2+ /Fe tot ) obtained with the corrected spectra varies by 10%. It is shown that TiO 2 oxides are not suitable as standard for octahedral Ti 4+ signal in the studied micas. The “shape parameter, κ” method proves to provide supplementary information useful for a full interpretation of XPS signals.

Iron oxidation state variations in zoned micro-crystals measured using micro-XANES

The determination of the oxidation state of transition metals at high spatial resolution is a crucial issue for many fields of science, including solid state physics, earth sciences, biology, bio-chemistry and catalysis. Among the other available analytical methods, micro-XANES allows to probe in situ the oxidation state with high lateral resolution, enabling an unprecedented level of description in heterogeneous samples. In geological samples the determination of the Fe3+/ΣFe ratio is of particular interest since it can be used as an indicator of the oxygen fugacity (fO2) at which a mineral formed. With this respect, we performed a micro-XANES experiment aiming to investigate the Fe-redox state variation across single-crystals of both garnet and omphacite exploiting the X-ray microprobe available at the ESRF ID22 beamline to reach a spot size of 1.7μm×5.3μm. For garnet, the absolute Fe3+ content was determined in a space-resolved way. In the case of omphacite, the analysis of the XANES data is not straightforward owing to the presence of a significant dichroism effect and to the random orientation of the different grains in the mineral assemblage. The investigated samples are highly complex materials which represent a challenge for the micro-XANES technique. These zoned micro-crystals are therefore ideal systems to develop analytical procedures which can be subsequently generalized to other relevant fields of science such as the Fe speciation in a single cell or a single grain for life science and catalysis applications, respectively.

Topochemical Fluorination of Sr3(M0.5Ru0.5)2O7 (M = Ti, Mn, Fe), n = 2, Ruddlesden–Popper Phases

Reaction of the appropriate Sr3(M(0.5)Ru(0.5))2O7 (M = Ti, Mn, Fe), n = 2, Ruddlesden-Popper oxide with CuF2 under flowing oxygen results in formation of the oxide-fluoride phases Sr3(Ti(0.5)Ru(0.5))2O7F2, Sr3(Mn(0.5)Ru(0.5))2O7F2, and Sr3(Fe(0.5)Ru(0.5))2O(5.5)F(3.5) via a topochemical anion insertion/substitution process. Analysis indicates the titanium and manganese phases have Ti(4+), Ru(6+) and Mn(4+), Ru(6+) oxidation state combinations, respectively, while Mössbauer spectra indicate an Fe(3+), Ru(5.5+) combination for the iron phase. Thus, it can be seen that the soft fluorination conditions employed lead to formation of highly oxidized Ru(6+) centers in all three oxide-fluoride phases, while oxidation states of the other transition metal M cations remain unchanged. Fluorination of Sr3(Ti(0.5)Ru(0.5))2O7 to Sr3(Ti(0.5)Ru(0.5))2O7F2 leads to suppression of magnetic order as the fluorinated material approaches metallic behavior. In contrast, fluorination of Sr3(Mn(0.5)Ru(0.5))2O7 and Sr3(Fe(0.5)Ru(0.5))2O7 lifts the magnetic frustration present in the oxide phases, resulting in observation of long-range antiferromagnetic order at low temperature in Sr3(Mn(0.5)Ru(0.5))2O7F2 and Sr3(Fe(0.5)Ru(0.5))2O(5.5)F(3.5). The influence of the topochemical fluorination on the magnetic behavior of the Sr3(M(0.5)Ru(0.5))2O(x)F(y) phases is discussed on the basis of changes to the ruthenium oxidation state and structural distortions.

Band-structure calculations for the 3dtransition metal oxides inGW

Many-body $GW$ calculations have emerged as a standard for the prediction of band gaps, band structures, and optical properties for main-group semiconductors and insulators, but it is not well established how predictive the $GW$ method is in general for transition metal (TM) compounds. Surveying the series of 3$d$ oxides within a typical $GW$ approach using the random-phase approximation reveals mixed results, including cases where the calculated band gap is either too small or too large, depending on the oxidation states of the TM (e.g., FeO/Fe${}_{2}$O${}_{3}$, Cu${}_{2}$O/CuO). The problem appears to originate mostly from a too high average $d$-orbital energy, whereas the splitting between occupied and unoccupied $d$ symmetries seems to be reasonably accurate. It is shown that augmenting the $GW$ self-energy by an attractive (negative) and occupation-independent on-site potential for the TM $d$ orbitals with a single parameter per TM cation can reconcile the band gaps for different oxide stoichiometries and TM oxidation states. In Cu${}_{2}$O, which is considered here in more detail, standard $GW$ based on wave functions from initial density or hybrid functional calculations yields an unphysical prediction with an incorrect ordering of the conduction bands, even when the magnitude of the band gap is in apparent agreement with experiment. The correct band ordering is restored either by applying the $d$-state potential or by iterating the wave functions to self-consistency, which both have the effect of lowering the Cu-$d$ orbital energy. While it remains to be determined which improvements over standard $GW$ implementations are needed to achieve an accurate ab initio description for a wide range of transition metal compounds, the application of the empirical on-site potential serves to mitigate the problems specifically related to $d$ states in $GW$ calculations.

SrFe0.5Ru0.5O2: Square-Planar Ru2+ in an Extended Oxide

Low-temperature topochemical reduction of the cation disordered perovskite phase SrFe(0.5)Ru(0.5)O(3) with CaH(2) yields the infinite layer phase SrFe(0.5)Ru(0.5)O(2). Thermogravimetric and X-ray absorption data confirm the transition metal oxidation states as SrFe(0.5)(2+)Ru(0.5)(2+)O(2); thus, the title phase is the first reported observation of Ru(2+) centers in an extended oxide phase. DFT calculations reveal that, while the square-planar Fe(2+) centers adopt a high-spin S = 2 electronic configuration, the square-planar Ru(2+) cations have an intermediate S = 1 configuration. This combination of S = 2, Fe(2+) and S = 1, Ru(2+) is consistent with the observed spin-glass magnetic behavior of SrFe(0.5)Ru(0.5)O(2).

Li3MRuO5 (M = Co, Ni), new lithium-rich layered oxides related to LiCoO2: promising electrochemical performance for possible application as cathode materials in lithium ion batteries

We describe the synthesis and crystal structure of Li3MRuO5 (M = Co and Ni), new rock salt related oxides. Both the oxides crystallize in the layered LiCoO2 (alpha-NaFeO2) structure, as revealed by powder XRD data. Magnetic susceptibility data suggest that the oxidation states of transition metals are Li3Co3+(ls)Ru4+(ls) O-5 (ls = low spin) for the M = Co compound and Li3Ni2+Ru5+O5 for the M = Ni compound. Electrochemical investigations of lithium deintercalation-intercalation behaviour reveal that both Co and Ni phases exhibit attractive specific capacities of ca. 200 mA h g(-1) at an average voltage of 4 V that has been interpreted as due to the oxidation of Co3+ and Ru4+ in Li3CoRuO5 and Ni2+ to Ni4+ in the case of Li3NiRuO5. Thus, a different role of Ru ions is played in the isostructural oxides. Finally, in both cases evidence of irreversible behaviour above 4.2 V is observed and interpreted as formation of high valent ions or alternatively oxidation of oxide ions.

A Theory for Bioinorganic Chemical Reactivity of Oxometal Complexes and Analogous Oxidants: The Exchange and Orbital-Selection Rules

Over the past decades metalloenzymes and their synthetic models have emerged as an area of increasing research interest. The metalloenzymes and their synthetic models oxidize organic molecules using oxometal complexes (OMCs), especially oxoiron(IV)-based ones. Theoretical studies have helped researchers to characterize the active species and to resolve mechanistic issues. This activity has generated massive amounts of data on the relationship between the reactivity of OMCs and the transition metal's identity, oxidation state, ligand sphere, and spin state. Theoretical studies have also produced information on transition state (TS) structures, reaction intermediates, barriers, and rate-equilibrium relationships. For example, the experimental-theoretical interplay has revealed that nonheme enzymes carry out H-abstraction from strong C-H bonds using high-spin (S = 2) oxoiron(IV) species with four unpaired electrons on the iron center. However, other reagents with higher spin states and more unpaired electrons on the metal are not as reactive. Still other reagents carry out these transformations using lower spin states with fewer unpaired electrons on the metal. The TS structures for these reactions exhibit structural selectivity depending on the reactive spin states. The barriers and thermodynamic driving forces of the reactions also depend on the spin state. H-Abstraction is preferred over the thermodynamically more favorable concerted insertion into C-H bonds. Currently, there is no unified theoretical framework that explains the totality of these fascinating trends. This Account aims to unify this rich chemistry and understand the role of unpaired electrons on chemical reactivity. We show that during an oxidative step the d-orbital block of the transition metal is enriched by one electron through proton-coupled electron transfer (PCET). That single electron elicits variable exchange interactions on the metal, which in turn depend critically on the number of unpaired electrons on the metal center. Thus, we introduce the exchange-enhanced reactivity (EER) principle, which predicts the preferred spin state during oxidation reactions, the dependence of the barrier on the number of unpaired electrons in the TS, and the dependence of the deformation energy of the reactants on the spin state. We complement EER with orbital-selection rules, which predict the structure of the preferred TS and provide a handy theory of bioinorganic oxidative reactions. These rules show how EER provides a Hund's Rule for chemical reactivity: EER controls the reactivity landscape for a great variety of transition-metal complexes and substrates. Among many reactivity patterns explained, EER rationalizes the abundance of high-spin oxoiron(IV) complexes in enzymes that carry out bond activation of the strongest bonds. The concepts used in this Account might also be applicable in other areas such as in f-block chemistry and excited-state reactivity of 4d and 5d OMCs.

Activity and stability of Pd/MMnOx (M = Co, Ni, Fe and Cu) supported on cordierite as CO oxidation catalysts

Several catalysts of the general formula, MMnOx (M=Co, Ni, Fe and Cu), were synthesised through the impregnation method; their activities were shown to be enhanced by the addition of a small amount of Pd (0.01–0.1wt%). These catalysts exhibit different activities for the catalytic oxidation of CO, due to the different valence states of various transition metal oxides. The introduction of Pd prominently enhanced both the reduction and oxidation capabilities of the catalysts. These catalysts were optimised for oxidation activities by designing orthogonal experiments. Based upon the catalysts’ properties, the stability of these samples and their ability to resist steam over Pd/CoMnOx/cordierite were investigated.

Optical conductivity due to orbital polarons in systems with orbital degeneracy

We consider the impact of orbital polarons in doped orbitally ordered systems on optical conductivity using the simplest generic model capturing the directional nature of either $t_{2g}$ (or $e_g$) orbital states in certain transition metal oxides, or $p$ orbital states of cold atoms in optical lattices. The origin of the optical transitions is analyzed in detail and we demonstrate that the optical spectra: (i) are determined by the string picture, i.e., flipped orbitals along the hole hopping path, and (ii) consist of three narrow peaks which stem from distinct excitations. They occur within the Mott-Hubbard gap similar to the superconducting cuprates but indicate hole confinement, in contrast to the spin t-J model. Finally, we point out how to use the point group symmetry to classify the optical transitions.