Intercalating Oxygen Research Articles

The fate of graphene on copper: Intercalation / de-intercalation processes and the role of silicon

Intercalation and de-intercalation processes below graphene (g) grown by chemical vapor deposition (CVD) on Cu foils (g/Cu) were investigated in a combined low-energy electron microscopy and X-ray photoemission electron microscopy study. Exposure of g/Cu to air induces oxygen and water intercalation which can be removed by annealing in vacuum leading to clean and well-ordered graphene on Cu. However, prolonged air exposure leads to intercalation of large amounts of oxygen, most likely inducing the formation of copper oxides. If sufficient intercalated oxygen remains at the interface when exceeding 320 °C, graphene is oxidized and burned off. Cu foils can be loaded with silicon on purpose during foil pre-treatment or accidently during long growth time when applying high temperatures at elevated H2 pressure inducing the reactive removal of Si species from the quartz reactor wall. Due to the dissolution of Si in the Cu bulk, the Si surface concentration remains below detection limit and graphene of equal crystalline quality is grown as if the Cu foil was silicon-free. However, oxygen intercalation underneath graphene on Si-containing Cu foils can induce Si segregation towards the surface and formation of intercalated silica without attacking the covering graphene. Even at high temperatures, segregating Si acts as oxygen scavenger so that graphene resists oxidation. The observed effect explains the usefulness of certain synthesis protocols and paves the way towards large-scale fabrication of electronically decoupled graphene. The effect can be used to immobilize adsorbing oxygen at the interface and image the initial steps of intercalation below graphene in-situ.

Eco-friendly@one-pot-simultaneous oxygen and chlorine intercalation on graphite surface at low-cost

The production of in-situ chlorinated graphene oxide was accomplished in a single step without the use of hazardous reagent conditions. This eco-friendly method of producing chlorinated graphene oxide was created by combining the potent oxidizer H2SO5 (Piranha) in place of H2SO4 with NaClO4 and m-CPBA. These techniques provide the major advantage of simultaneously intercalating chlorine and oxygen onto the surface of graphite by a one-step synthesis. The oxygen and chloride functionality were confirmed by spectral techniques (P-XRD, HR-TEM, XPS, Raman Spectra, FT-IR, and TGA analysis). This is a one-pot synthesis method for generating Cl-GO materials for use in many industries. It is cost-effective, eco-friendly, simple and high scalable. The focus of our present study is on determining the way Cl-GOperforms in supercapacitor applications.

Heteroepitaxy of Scandium Delafossite on ZnO

Heteroepitaxy of Scandium Delafossite on ZnO

Oxygen-Induced In Situ Manipulation of the Interlayer Coupling and Exciton Recombination in Bi2Se3/MoS2 2D Heterostructures.

Two-dimensional (2D) heterostructures are more than a sum of the parent 2D materials, but are also a product of the interlayer coupling, which can induce new properties. In this paper, we present a method to tune the interlayer coupling in Bi2Se3/MoS2 2D heterostructures by regulating the oxygen presence in the atmosphere, while applying laser or thermal energy. Our data suggest that the interlayer coupling is tuned through the diffusive intercalation and deintercalation of oxygen molecules. When one layer of Bi2Se3 is grown on monolayer MoS2, an influential interlayer coupling is formed, which quenches the signature photoluminescence (PL) peaks. However, thermally treating in the presence of oxygen disrupts the interlayer coupling, facilitating the emergence of the MoS2 PL peak. Our density functional theory calculations predict that intercalated oxygen increases the interlayer separation ∼17%, disrupting the interlayer coupling and inducing the layers to behave more electronically independent. The interlayer coupling can then be restored by thermally treating in N2 or Ar, where the peaks will requench. Hence, this is an interesting oxygen-induced switching between "non-radiative" and "radiative" exciton recombination. This switching can also be accomplished locally, controllably, and reversibly using a low-power focused laser, while changing the environment from pure N2 to air. This allows for the interlayer coupling to be precisely manipulated with submicron spatial resolution, facilitating site-programmable 2D light-emitting pixels whose emission intensity could be precisely varied by a factor exceeding 200×. Our results show that these atomically thin 2D heterostructures may be excellent candidates for oxygen sensing.

Long‐Term Structural Evolution of an Intercalated Layered Semiconductor

Intercalated molecules can dramatically modify the electronic band structure of layered semiconductors, significantly altering the optical properties of the material. In the layered monochalcogenide Gallium Telluride (GaTe), exposure to air induces a nearly 1 eV reduction of its band gap due to the interlayer diffusion and chemisorption of oxygen. The effect of oxygen chemisorption at the Te‐terminated surfaces on the structure of GaTe, however, is much less known. To better understand the structure–property relationship of intercalated GaTe, a systematic, long‐term, X‐ray diffraction study has been performed on GaTe exfoliated crystals exposed to ambient conditions. It is observed that the structural changes are not limited to a previously observed short‐term increase in lattice expansion. Over the course of months and even years after exfoliation, the oxygen adsorbates continue to modify the structure of GaTe, inducing significant disorder and grain reorientation. It is estimated that approximately one out of every two grains is slightly displaced by the intercalating oxygen, demonstrating a significant increase in grain mosaicity, while still maintaining the original {−2 0 1} out‐of‐plane texture. Correlating these structural transformations to observed changes in electrical and optical properties will enable capitalization of the use of adsorbates to engineer novel properties in these layered materials.

Chemical gating of epitaxial graphene through ultrathin oxide layers.

We achieved a controllable chemical gating of epitaxial graphene grown on metal substrates by exploiting the electrostatic polarization of ultrathin SiO2 layers synthesized below it. Intercalated oxygen diffusing through the SiO2 layer modifies the metal-oxide work function and hole dopes graphene. The graphene/oxide/metal heterostructure behaves as a gated plane capacitor with the in situ grown SiO2 layer acting as a homogeneous dielectric spacer, whose high capacity allows the Fermi level of graphene to be shifted by a few hundreds of meV when the oxygen coverage at the metal substrate is of the order of 0.5 monolayers. The hole doping can be finely tuned by controlling the amount of interfacial oxygen, as well as by adjusting the thickness of the oxide layer. After complete thermal desorption of oxygen the intrinsic doping of SiO2 supported graphene is evaluated in the absence of contaminants and adventitious adsorbates. The demonstration that the charge state of graphene can be changed by chemically modifying the buried oxide/metal interface hints at the possibility of tuning the level and sign of doping by the use of other intercalants capable of diffusing through the ultrathin porous dielectric and reach the interface with the metal.

The influence of intercalated oxygen on the properties of graphene on polycrystalline Cu under various environmental conditions.

Intercalation of oxygen at the interface of graphene grown by chemical vapour deposition and its polycrystalline copper catalyst can have a strong impact on the electronic, chemical and structural properties of both the graphene and the Cu. This can affect the oxidation resistance of the metal as well as subsequent graphene transfer. Here, we show, using near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), X-ray absorption near edge spectroscopy (XANES), energy dispersive X-ray spectroscopy (EDX) and (environmental) scanning electron microscopy (ESEM) that both the oxygen intercalation and de-intercalation are kinetically driven and can be clearly distinguished from carbon etching. The obtained results reveal that a charge transfer between as grown graphene and Cu can be annulled by intercalating oxygen creating quasi-free-standing graphene. This effect is found to be reversible on vacuum annealing proceeding via graphene grain boundaries and defects within the graphene but not without loss of graphene by oxidative etching for repeated (de-)intercalation cycles.

Thermal conductivity measurements on individual vapor-grown carbon nanofibers and graphene nanoplatelets

The thermal flash technique was utilized for measuring the thermal conductivity of vapor-grown carbon nanofibers and graphene nanoplatelets. The vapor-grown carbon nanofibers with stacked-cone morphology and heat treated to 1100 °C and 3000 °C were measured to have thermal conductivities of 1130 W/m K and 1715 W/m K, respectively. The physical dimensions of the constitutive cones determining the mean free path due to static phonon scattering were estimated to be ∼128 nm and ∼176 nm for the low and high heat treatment temperatures, respectively. Static scattering lengths shorter than the Umklapp scattering length indicate ballistic transport within individual cones and limit the thermal conductivities of the nanofibers. Additionally, nanoplatelets of few-layer oxygen intercalated graphene and multi-layer reduced graphene exhibited thermal conductivities of 776 W/m K and 2275 W/m K, respectively. The lower thermal conductivity of few-layer (∼3 layers) graphene is attributed to the presence of intercalating oxygen atoms which introduce covalent character to the interlayer interactions, acting as phonon scattering centers and hence reducing the phonon mean free path. The thermal conductivity measured for multi-layer graphene with ∼30–45 layers lies within range of the thermal conductivities previously reported for bulk graphite.

Studies on room temperature electrochemical oxidation and its effect on the transportproperties of TBCCO films

A novel room temperature electrochemical process for the synthesis of single-phaseTl2Ba2Ca2Cu3O10 (TBCCO/Tl-2223) superconducting films has been developed. Electrochemicalparameters were optimized by studying linear sweep voltammetry (LSV), cyclicvoltammetry (CV) and chronoamperometry (CA) for the deposition of Tl–Ba–Ca–Cualloy at room temperature. The superconducting films of the TBCCO wereobtained by two oxidation techniques. In the first technique, the electrodepositedTl–Ba–Ca–Cu alloyed films were oxidized at various temperatures in flowing oxygenatmosphere. In the second technique, stoichiometric electrocrystallization to getTl2Ba2Ca2Cu3O10 (Tl-2223) was completed by electrochemically intercalating oxygen species intoTl–Ba–Ca–Cu alloy at room temperature for various lengths of time. The oxygen contentin the samples was varied by varying the electrochemical oxidation period, andthe changes in the crystal structure, superconducting transition temperature(Tc) and criticalcurrent density (Jc) were recorded. The high temperature furnace oxidation technique was replaced by theroom temperature electrochemical oxidation technique. The dependence of superconductingparameters on oxygen content is correlated with structure–property relations.

Room temperature electrochemical synthesis of Hg-1212 superconducting thin films

In the present investigation, the novel two-step electrochemical process of room temperature synthesis of Hg-1212 superconducting films has been developed and reported first time. Electrochemical parameters were optimized by studying cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometry (CA) for the deposition of Hg-Ba-Ca-Cu alloy at room temperature. Current time transient showed progressive growth with hemispheriodal granules, which were then revealed by scanning electron microscopy (SEM). Stoichiometric electrocrystallization to get Hg1Ba2Ca1Cu2O6+x (Hg-1212) was completed by electrochemically intercalating oxygen species into Hg-Ba-Ca-Cu alloy at room temperature. The oxygen content in the samples was varied by varying the electrochemical oxidation period and the changes in the crystal structure, microstructure, and superconducting transition temperature (Tc) and critical current density (Jc) were recorded. The films oxidized for 28 min showed Tc = 104.7 K with Jc = 1.437 x 103 A/cm2. The dependence of superconducting parameters on oxygen content is correlated with structure property relations and reported in this paper.

First-principles study of the atomic oxygen adsorption on the (0 0 0 1) graphite surface and dissolution

In the framework of the density-functional theory we studied the atomic oxygen adsorption on the basal graphite surface at three imposed oxygen coverages 50, 25, 5.5% for three different adsorption sites, and also the dissolution of the atomic oxygen in graphite at one fixed coverage. Independently of the concentration, the most stable position of oxygen atoms is directly above a carbon–carbon bond (bridge), followed by the directly above a carbon atom position (top), and the directly above a hexagonal hollow (hex) site. Intercalating oxygen just below the surface layer, shows that dissoluted oxygen is more stable than adsorbed one.

New synthetic routes for preparing perovskites: Electrochemical oxidation and oxidation by NO 2

Two new synthetic low temperature routes for preparing perovskite-related materials, especially metastable phases are depicted. The first one, the electrochemical oxidation, uses the electric field as the driving force for intercalating oxygen atoms within parent oxide networks. The reaction is achieved under anodic potential, in alkaline solution (1 M KOH or NaOH), at room temperature, in air. This process has been used for preparing various perovskite compounds such as AMO 3 (A=Sr, La; M=Fe 1− x Co x ) or A 2MO 4+ δ (A=La, Nd, Sr). The most relevant results are reported. Potentiostatic and galvanostatic experiments have shown that the amount of intercalated oxygen can be controlled and that the process is reversible. Structural as well as electronic aspects of the oxygen intercalation are discussed and a reaction mechanism is proposed. The second route is based on the exothermic reaction of nitrogen dioxide NO 2 with NH 4 + ions at moderate temperatures (typically T<300°C), which allowed the destruction of NH 4 + cations in situ. Topotactic reactions are described for preparing new hexagonal forms of WO 3 or MoO 3. The reaction process is discussed.

New “Chimie Douce” Oxidizing Processes: Electrochemical Oxidation and in situ Oxidation by NO2

Two new oxidation processes are described : the electrochemical oxidation in alkaline solution uses the electrical potential as the driving force for intercalating oxygen, at room temperature, into oxide networks such as perovskites or Ln2MO4 (M ˭Ni, Cu) compounds. The second one is based on the in situ destruction of ammonium ions in oxide lattices by interaction with NO2 gas, at moderate temperatures. Pictures of the reaction mechanisms are also proposed.

Electrochemical oxygen insertion into La 2CuO 4-related compounds

We proposed in 1990 a new preparative method in Solid State Chemistry, the electrochemical oxidation in alkaline solution, for intercalating oxygen atoms at room temperature in ceramic materials, thin films or crystals of La 2CuO 4 (or substituted compounds), “214” structure; this process was used by several groups and a review of the main results is given. The physical properties of these materials show that at least two and likely three superconducting phases are formed with different compositions in oxygen overstoichiometry (δ) and T C. The surprising insertion of these additional oxygen atoms may result from the formation of holes in both CuO 2 planes and La 2O 2 layers. In addition, electrochemical studies have shown that the electroactive species formed during the oxidation process would be O - with a rather high diffusion coefficient. We assume the existence of a mixed valence of oxygen in these materials and propose that the O - species would be the mobile species thanks to a coupling with the conduction electrons via the electron transfer Oxygen → Copper.

Electrochemical and high pressure superoxygenation of YCuO2+x and LaCuO2+x delafossites

Electrochemical and high pressure oxygenation experiments are described which extend the range of intercalated oxygen content beyond that accessible in 1 atm O2 treatments for the triangular planar cuprate delafossites YCuO2+x and LaCuO2+x, with the intention of increasing the hole doping, and filling vacancies in the intercalated oxygen lattice to decrease electrical resistivities. Stabilization of the 2H polytype of YCuO2 through 1% Ca substitution for Y and oxidation conditions for obtaining the pure orthorhombic intercalated oxygen superlattice form of YCuO2.50 are also described. Temperature dependent resistivities for some of the superoxygenated compositions are reported; metallic behavior was not observed.

Oxygen ordering, peroxide-ion formation, and polarization fluctuations in YBa 2Cu 3O 7− δ

It is argued that (1) an internal electric field parallel to the c-axis confines the redox reactions associated with oxygen intercalation/disintercalation to the intercalation layer, (2) coulombic interactions between intercalated oxygen introduce oxide-ion ordering within/between b-axis chains as well as ordering onto the b-axis, (3) peroxide-ion formation occurs between copper-bridging oxygen in contact with one another if the oxidation exceeds 7− δ = 6.5, and (4) an overlap of partially filled σ ∗ and π ∗ bands introduces polarization fluctuations that enhance the superconducting transition temperature.

Determination of germanium by graphite-furnace atomic-absorption spectrometry

The premature loss of germanium as volatile GeO results in low sensitivity and poor reproducibility in the determination of germanium by graphite-furnace atomic-absorption spectrometry. This interference can be eliminated by suppressing the premature reduction of GeO 2 to GeO during the ashing step, and dissociating the germanium oxides into the atoms simultaneously with their vaporization during the atomization step. The premature reduction of GeO 2 to GeO has been successfully prevented by several approaches: (1) diminishing the reducing activity of the graphite furnace by ( a) oxidizing the graphite surface and intercalating oxygen into the graphite lattice with oxidizing acids, such as nitric or perchloric, in the sample solution, or ( b) using a tantalum-treated graphite furnace; (2) keeping the analyte as germanium (IV) by addition of sodium or potassium hydroxide to the sample solutions.

Model studies of oxygen-intercalated graphite

The possibility of intercalating oxygen to reduce the conductivity of graphite has been investigated by modified intermediate neglect of differential overlap 3 and tight-binding methods. The cluster calculations suggest that the most stable position for the oxygen atom is 1.25 \AA{} above a carbon---carbon bond. The tight-binding band calculation predicts the stage-1 intercalated graphite to be a zero-indirect-gap semiconductor. Higher-stage intercalated graphite is expected to have a finite insulating gap whose value is governed by the carbon-oxygen interaction.