Two-dimensional Oxide Research Articles

Enhanced Ferromagnetism from Organic–Cerium Oxide Hybrid Ultrathin Nanosheets

Room-temperature ferromagnetism in two-dimensional (2D) oxide materials is an intriguing phenomenon for spintronic applications. Here, we report significantly enhanced room-temperature ferromagnetism observed from ultrathin cerium oxide nanosheets hybridized with organic surfactant molecules. The hybrid nanosheets were synthesized by ionic layer epitaxy over a large area at the water-air interface. The nanosheets exhibited a saturation magnetization of 0.149 emu/g as their thickness reduced to 0.67 nm. This value was 5 times higher than that for CeO2 thin films and more than 20 times higher than that for CeO2 nanoparticles. The magnetization was attributed to the high concentration (15.5%) of oxygen vacancies stabilized by surfactant hybridization as well as electron transfer between organic and oxide layers. This work brings an effective strategy of introducing strong ferromagnetism to functional oxide materials, which leads to a promising route toward exploring new physical properties in 2D hybrid nanomaterials.

Enhanced fatigue and durability of carbon black/natural rubber composites reinforced with graphene oxide and carbon nanotubes

Graphene oxide (GO) sheets and carbon nanotubes (CNTs) are of nanometer size and offer large shape factors which are beneficial in reducing crack propagation rates of composites when used in carbon black (CB) reinforced natural rubber (NR), thereby prolonging the service lives of rubber composites. In this research, CNT-CB/NR and GO-CB/NR composites were prepared by partially replacing CB with one-dimensional CNTs and two-dimensional flaky graphene oxide GO, respectively. The results showed that the complex filler dispersion in NR matrices was improved due to the isolation effect between the different fillers. The strain-induced crystallization (SIC) ability of CB/NR was effectively enhanced by the addition of both GO and CNT. The modulus at 100% strain and tear strength of the composites were also improved. More branching and deflections were observed at the crack tips of the composites and both effectively hindered crack propagation in the materials. Under uniaxial and multi-axial cyclic loading, the fatigue lives of CNT-CB/NR and GO-CB/NR composites greatly increased when compared with the fatigue lives of CB/NR composites. The GO-CB/NR composites exhibited evident advantages in respect of fatigue resistance and durability among the three composites.

Influence of two-dimensional oxide nanosheets seed layers on the growth of (100)BiFeO3 thin films synthesized by chemical solution deposition

Complex oxide thin films of high quality, such as the multiferroic BiFeO3 (BFO) material, are required for integrated oxides electronics. However, the properties of these materials are strongly dependent on their crystalline orientation and their optimal performances are usually obtained on expensive single-crystal substrates. A lower cost alternative approach is the use of oxide nanosheets seed layers that can be deposited on any substrate, including silicon (Si) used in CMOS technology, for the growth of complex oxides with the desired crystallographic orientation. In this work, BFO films were deposited by chemical solution deposition using polymeric precursor method. First, the optimization of synthesis parameters was performed on amorphous silica and on (100)Si substrates. The structural and microstructural characterizations revealed a polycrystalline growth of the BFO films on both substrates and showed the influence of the synthesis temperature, in particular to obtain the pure perovskite phase. The influence of a two-dimensional nanosheets seed layer was then investigated. Ca2Nb3O10− nanosheets were prepared by the exfoliation process of HCa2Nb3O10 phase that is obtained by cation exchange of the layered KCa2Nb3O10 niobate in an acidic solution. The nanosheets were deposited directly on both substrates by Langmuir–Blodgett process, followed by the subsequent deposition of BFO by chemical solution deposition. X-ray diffraction and transmission electronic microscopy experiments showed a highly oriented (100) growth of the BFO thin films confirming the great interest of two-dimensional nanosheets seed layers.

Synthesis of Rh–SnO2 nanosheets and ultra-high triethylamine sensing performance

Two-dimensional metal oxide nanosheets are widely used in many areas, particularly in gas-sensitive materials, due to their large specific surface area and ease of synthesis. In this work, Rh–SnO2 nanosheets (Rh-SNS) were obtained by a facile hydrothermal synthesis and subsequent surface impregnation precipitation and heat treatment method. The prepared SNS has a smooth and clean surface with a thickness of approximately 20–25 nm. After surface decoration, the surface becomes rough, the thickness is approximately 10–15 nm, and a porous structure appears on the SNS. The gas-sensing performance of the obtained materials towards triethylamine were investigated. At 325 °C, the response of Rh-SNS to 100 ppm TEA was 607.23, which was approximately 15 times higher than that for pure SNS. Furthermore, Rh-SNS showed excellent selectivity and short response time. The ultra-high sensitivity and excellent selectivity of Rh-SNS for TEA is attributed to the increase in the amount of adsorbed oxygen species on the Rh-SNS surface and the electronic interaction between Rh and SnO2.

A gold electrode modified with a gold-graphene oxide nanocomposite for non-enzymatic sensing of glucose at near-neutral pH values.

A nanocomposite was prepared from gold and graphene oxide via one-step electrodeposition and used to modify the surface of a gold electrode (Au-Gr/GE) that was then applied to non-enzymatic determination of glucose. The effects of deposition time and supporting substrate on the morphology, structure, and electrochemical properties of the nanocomposite were optimized. The morphologies and crystal structures were characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results indicate that gold nanoparticles grew on the surface of two-dimensional graphene oxide. The electrocatalytic activity of the modified electrode towards glucose oxidation was evaluated by cyclic voltammetry and amperometric methods at pH7.4. The Au-Gr/GE, typically operated at a potential of 0.00V (vs. Ag/AgCl), has a linear response in the 0.05-14mM and 14-42mM glucose concentration range, high sensitivity (604 and 267μAcm-2mM-1) and a low detection limit (12μM). The modified GE was applied to quantify glucose in sweat where it exhibited excellent sensitivity and accuracy. Graphical abstract The gold electrode modified with a gold-graphene (Au-Gr/GE) is prepared via a direct electrodeposition. The Au-Gr/GE is used for glucose detection in the neutral solution and it can achieve the effect of non-intrusive detection.

Interfacial two-dimensional oxide enhances photocatalytic activity of graphene/titania via electronic structure modification

The photocatalytic activity of titania nanoparticles deposited on epitaxial graphene is proven to be significantly affected by the substrate on which graphene is supported. In particular, it has been revealed that the addition of a two-dimensional TiO1.5 layer sandwiched between graphene and the supporting metal induces a p-doping of graphene itself and a consistent shift in the Ti d states. These modifications in the electronic structure are compatible with the reduction of the probability of charge carrier recombination and enhance the photocatalytic activity of the heterostructure. This is indicative of the capital role played by the interfacial thin oxide films in fine-tuning the properties of heterostructures based on graphene and pave the way to new combinations of graphene/oxides for photocatalysis-oriented applications.

Real time imaging of two-dimensional iron oxide spherulite nanostructure formation

The formation of complex hierarchical nanostructures has attracted a lot of attention from both the fundamental science and potential applications point of view. Spherulite structures with radial fibrillar branches have been found in various solids; however, their growth mechanisms remain poorly understood. Here, we report real time imaging of the formation of two-dimensional (2D) iron oxide spherulite nanostructures in a liquid cell using transmission electron microscopy (TEM). By tracking the growth trajectories, we show the characteristics of the reaction front and growth kinetics. Our observations reveal that the tip of a growing branch splits as the width exceeds certain sizes (5.5–8.5 nm). The radius of a spherulite nanostructure increases linearly with time at the early stage, transitioning to nonlinear growth at the later stage. Furthermore, a thin layer of solid is accumulated at the tip and nanoparticles from secondary nucleation also appear at the growing front which later develop into fibrillar branches. The spherulite nanostructure is polycrystalline with the co-existence of ferrihydrite and Fe3O4 through-out the growth. A growth model is further established, which provides rational explanations on the linear growth at the early stage and the nonlinearity at the later stage of growth.

Bioinspired Synthesis of Quasi-Two-Dimensional Monocrystalline Oxides

Controllable synthesis of two-dimensional (2D) monocrystalline oxide nanomaterials beyond van der Waals solids is intriguing but very challenging. Inspired by the biomineralization processes that commonly implement organic templates with both positive and negative charges for regulating the crystal nucleation and growth, we adapted mix-charges amphiphilic monolayer to the ionic layer epitaxy and enabled the growth of monocrystalline 2D nanosheets. In situ grazing incidence X-ray diffraction and ex situ crystal and elemental analyses revealed that mixing charge in the template is able to tune the 2D crystal nucleation rate and promote the growth of monocrystalline domains. Molecular dynamics simulations suggested that mixing charges could yield a stable, flatter, and more ordered monolayer template with a nonuniform distribution of charges, which are favorable for the growth of monocrystalline nanosheets. Designing the mixed amphiphilic monolayers resulted in the creation of ultrathin nanosheets from vario...

Electronic band structure of a two-dimensional oxide quasicrystal

Since the discovery of quasicrystals, the study of their electronic structure has been a challenging topic due to the absence of translational symmetry. Here, the valence bands of a novel two-dimensional ultrathin BaTiO${}_{3}$-derived oxide quasicrystal are determined by momentum-resolved photoelectron spectroscopy. The dispersion of the oxygen 2$p$ bands and the occupation of the Ti 3$d$ states are identified. They bridge the knowledge deriving from free-electron-like wave functions in bulk quasicrystals to that of the more localized electrons in confined dimensions.

Two-dimensional MnAl mixed-metal oxide nanosheets prepared via a high-shear-mixer-facilitated coprecipitation method for enhanced selective catalytic reduction of NO with NH3

Mn-based mixed metal oxides (MMOs) have been considered to be low temperature denitration catalysts for selective catalytic reduction of NO with NH3. In this work, two-dimensional (2D) MnAl mixed-metal oxide nanosheets were successfully prepared via a high-shear-mixer-facilitated coprecipitation (HSM-CP) method. The as-obtained 2D MnAl-MMO achieved high specific surface area of 180.6 m2/g, average pore size of 9.8 nm and pore volume of 0.45 cm3/g. The as-obtianed MnAl-MMO (HSM-CP) catalysts performed superior NO conversion of 96.3% and N2 selectivity of 91% at 200 °C with a gas hourly space velocity (GHSV) of 30,600 h−1, all of which are much better than those of MnAl-MMO (CP). Even at 25 °C, the MnAl-MMO (CP) catalysts nearly had catalytic activity whereas MnAl-MMO (HSM-CP) achieved a better NO conversion of 40%. Moreover, MnAl-MMO (HSM-CP) catalysts performed excellent H2O and SO2 resistance, and NO conversion is still remained 75% at 200 °C with 5% H2O and 100 ppm SO2. HSM-CP provides a new strategy for the development of MMO as efficient denitration catalysts.

Fabrication and Performance Evaluation of Six-Cell Two-Dimensional Configuration Solid Oxide Fuel Cell Stack Based on Planar 6 × 6 cm Anode-Supported Cells

Using energy efficiently and reducing environmental pollution caused by energy consumption are becoming increasingly important. In this study, a two-dimensional (2D) solid oxide fuel cell (SOFC) stack configuration was designed to be operated with six cells. This design could potentially be applied in thermal power plants in developing countries where waste heat is more plentiful; the 2D configuration six-cell stack could be an elementary module, and such modules could be more easily placed in contact with hot walls where waste heat recovery is required. In this report, the design, fabrication, and performance evaluation of the stack are described. The stack, with six 6 × 6 cm2 cells (5 × 5 cm2 effective area), is connected in series and operates successfully. The results show that the maximum potential of the six-cell stack is around 5.5 V (0.92 V per unit cell) at 700 °C. The maximum output power of the stack is 6.0 W at 700 °C, with humidified hydrogen (with 3% H2O) as the fuel. The results show that the six-cell 2D configuration SOFC stack can be innovatively constructed.

Low-Dimensional Carbon and Titania Nanotube Composites via a Solution Chemical Process and Their Nanostructural and Electrical Properties for Electrochemical Devices

We report on the synthesis of two-dimensional graphene oxide (GO) and one-dimensional titania nanotube (TNT) nanocomposites as well as one-dimensional multiwall carbon nanotube (CNT) and one-dimens...

Antilocalization in oxides: Effective spin- 32 model

Weak anti-localization offers an experimental tool to address spin--orbit coupling of two-dimensional oxide surfaces and interfaces via magneto-transport. To overcome the shortcomings of the formulation for single-band spin-1/2 electrons, we consider an effective three-band model that allows a decomposition into a pseudo-spin representation 1/2+3/2. Whereas the well-established spin-1/2 transport signature results from the singlet and triplet sectors in the Cooperon equation, a new structure originates from the quintet and septet sectors generated by the spin 3/2x3/2 representation.

Synergetic Effects of Silver Nanowires and Graphene Oxide on Thermal Conductivity of Epoxy Composites.

One-dimensional silver nanowires (AgNWs) and two-dimensional graphene oxide (GO) were combined to construct a three-dimensional network structure. The AgNWs can effectively inhibit stacking of adjacent GO sheets by occupying regions between layers of GO. Moreover, the GO sheets embedded in the gaps of the AgNWs network increase the interfacial contact area between the AgNWs and the epoxy matrix, resulting in the formation of more efficient phonon transport channels. To prepare an epoxy-based thermal conductive composite, hybrid networks were fabricated and added to epoxy resin using a solution mixing method. Significant synergistic effects were observed between the AgNWs and GO sheets. The thermal conductivity of epoxy composites filled with 10 wt.% AgNW/GO hybrids was found to be 1.2 W/mK and the impact strength was 28.85 KJ/m2, which are higher than the corresponding values of composites containing AgNWs or GO sheets alone. Thus, the thermal conductivity and impact strength of the epoxy composites were improved. The additive effects are mainly owing to the improved interfacial contact between the hybrid fillers and the epoxy resin, resulting in a more efficient phonon transport network. The use of hybrid fillers with different structures is a simple and scalable strategy for manufacturing high-performance thermally conductive materials for electronic packaging.

Simulation study on piezoelectric characteristics of two‐dimensional ZnO nanodiscs

Two-dimensional (2D) zinc oxide (ZnO) nanostructures have emerged as an attractive approach for constructing high-performance piezoelectric nanogenerators (NGs) because of their high rate of surface area to volume and good mechanical durability for applications in energy conversion devices. However, there has been to date no comprehensive study on piezoelectric characteristics of 2D ZnO nanostructures. In this paper, 2D nanodiscs have been prepared by aqueous solution method on flexible indium tin oxide coated polyethylene terephthalate substrates. Also, the piezoelectric characteristics are statistically demonstrated and analysed on the as-grown 2D ZnO nanodiscs by applying different external force with the finite element method (COMSOL Multiphysics). Besides, the size-dependent simulation analysis of piezopotentials is also conducted for further optimising output performance of NGs based on 2D ZnO nanostructures. Also, the simulation will provide a guideline for designing high-performance piezo-nanodevices.

In Situ Growing Triethanolamine-Functionalized Metal-Organic Frameworks on Two-Dimensional Carbon Nanosheets for Electrochemiluminescent Immunoassay.

A facile strategy for in situ growing triethanolamine (TEOA)-functionalized metal-organic framework (TEOA@MOF) on the two-dimensional graphene oxide (GO) or g-C3N4 nanosheets via the self-assembly technique was introduced. In this method, Zn2+ was first attached on the carbon nanosheets by electrostatic interaction; then, trimesic acid (H3btc) acted as the complex agent and TEOA as a base for the deprotonation of H3btc and a template, which leads to in situ growing the MOF on the carbon nanosheets obtaining a sandwich-like structure. Different types of surface analysis techniques were employed to characterize the GO-TEOA@MOFs and g-C3N4-TEOA@MOFs nanomaterials fabricated. The GO-TEOA@MOFs or g-C3N4-TEOA@MOFs nanomaterial-modified electrode brings out obviously enhanced electrochemiluminescence (ECL) behaviors due to numerous TEOA in the framework structures. Specifically, both TEOA and GO can serve as the co-reactants for the ECL system of Ru(bpy)32+ and have the synergic effect of enhancing the signal. Based on the GO-TEOA@MOFs modified electrodes, we developed a sensitive and rapid label-free ECL immunoassay strategy for human copeptin, and the linear range was 5 pg mL-1 to 500 ng mL-1 as well as the limit of detection was 360 fg mL-1. This work exhibits excellent specificity and good stability of the prepared immunosensor in the practical sample determination, demonstrating it can serve as a very promising method for the clinical diagnostics of acute myocardial infarction disease.

Plasmonic Nanohybrid with High Photothermal Conversion Efficiency for Simultaneously Effective Antibacterial/Anticancer Photothermal Therapy.

Plasmonic metal/semiconductor nanohybrids hold great promise in photocatalysis and biosensor development; however, their potential phototherapeutic applications are yet fully unexplored. On the other hand, the demand of high laser power density to induce antibacterial photothermal therapeutic effects greatly restricts the practical applicability of the previously developed photothermal nanoagents (PTAs) for anticancer photothermal therapy (PTT). Here, we develop a plasmonic nanohybrid by integrating plasmonic noble metal gold nanorods (AuNRs) with a two-dimensional graphene oxide (2-D GO), capable to perform photothermal ablation of both bacterial pathogens as well as tumor cells, respectively, under low power single near-infrared (NIR) laser activation. Owing to the synergistic plasmonic photothermal effect (PPTT) of dual plasmonic PTAs, the plasmonic AuNR/GO nanohybrid exhibits remarkably higher photothermal conversion efficiency (PCE, 72.59%) than either individual AuNRs or GO under low laser power density (300 mW), leading to enhanced antibacterial/anticancer PTT. In addition, the synergistic plasmonic antibacterial/anticancer PTT induced by the plasmonic nanohybrid is also far superior to individual PTAs (AuNRs or GO), whereas the flow cytometric analysis of heat shock proteins (HSP 70) clearly dictates that the substantial killing of bacterial pathogens/tumor cells is solely due to the synergistic PPTT. Thus, the plasmonic AuNR/GO nanohybrid is a standalone PTA to perform simultaneous antibacterial/anticancer PTT under low power NIR laser activation for only 5 min, without any systemic side effects. The present study provides a clear demonstration about the potential therapeutic impact of plasmonic nanohybrids and thus will surely pave the way to design other hybrid nanoagents with enhanced PCE and integrate them with chemotherapeutic agents, leading to dual-modal chemo-/photothermal antibacterial/anticancer therapy under low power single laser excitation for a short duration.

Anisotropic iron-doping patterns in two-dimensional cobalt oxide nanoislands on Au(111)

An integrated approach combining density functional theory (DFT) calculations and atomic resolution scanning tunneling microscopy (STM) is used to study well-defined iron-doped cobalt oxide nanoislands supported on Au(111). The focus is on the structure and distribution of Fe dopants within these nanoislands of CoO as a function of Fe to Co ratio. The DFT and STM results agree strongly and complement each other to allow for a more complete understanding of the dopant structure trends on the nanoscale. Using Fe as a marker, we first find that the stacking sequence of the moire structure of the host cobalt oxide nanoislands can be identified unambiguously through a combination of DFT and STM. Using the distinct contrast of the embedded Fe dopant atoms as observed with atom-resolved STM, we find correlations between Fe dopant position and the CoO/Au(111) moire pattern at varying Fe dopant densities. Formation of Fe-dopant clusters within the nanoislands is investigated in detail through DFT and found to agree with the dopant patterns observed in STM. We find that the structural effects of Fe dopants throughout the nanoislands with the basal planes and the two types of edges—the oxygen and metal edges—have different nature. Both DFT calculations and STM images show a strong preference for Fe dopants to be located directly on or near the oxygen edge of the nanoislands as opposed to being directly on or near the metal edge. Taken together, our results illustrate that Fe dopant incorporation and distribution within CoO nanoislands are highly anisotropic and governed by both the moire structure of the basal planes as well as nano-size effects present at the under-coordinated edges of different local geometry and chemistries.

Fabrication of functionalized two dimensional graphene oxide and promoted with phosphotungstic acid for reduction of organic dyes in water

In this research, a novel approach has been investigated for the fabrication of functionalized two-dimensional graphene oxide and promoted with phosphotungstic acid. First, PVP units with medium molecular weight were placed between graphene sheets for preparation of two-dimensional graphene oxide and then the surface of 2D-graphene oxide modified with (3-chloropropyl)triethoxysilane and metformin respectively. Finally, prepared support prompted with phosphotungstic acid which has the role of the active sites. This catalyst was characterized by transmission electron microscopy (TEM), X-ray powder diffraction (XRD), Scanning electron microscopy (SEM) and Fourier transforms infrared spectroscopy (FT-IR). 2D-GO-CPTES-Met/H3PW12O40 have been used for the catalytic reduction of methyl orange (MO) and methylene blue (MB) in the presence of NaBH4 as a reducing reagent. The progress of the reaction was monitor by UV–Vis spectrophotometry at different times. The obtained kinetic data for the reduction of MO and MB were fitted to first-order rate equations and the calculated rate constants for the reduction of MO and MB were as follows 0.020 and 0.102 s−1, respectively. The catalytic activity over 2D-GO-CPTES-Met/H3PW12O40 recycled catalyst remained at a satisfactory state after at least 10 runs.

Investigation of Reduced Graphene Oxide and a Nb-Doped TiO2 Nanotube Hybrid Structure To Improve the Gas-Sensing Response and Selectivity

The precise detection of flammable and explosive gases and vapors remains an important issue because of the increasing demand for renewable energy sources and safety requirements in industrial processes. Metal oxides (TiO2, SnO2, ZnO, etc.) are very attractive materials for the manufacturing of chemical gas sensors. However, their gas selectivity issues and further improvement in the sensing response remain a significant challenge. The incorporation of metal oxides with two-dimensional (2D) graphene oxide (GO) is considered to be a promising approach to obtaining hybrid structures with improved gas-sensing performance. Herein, we report the development of GO and niobium-doped titanium dioxide nanotube (NT) hybrid structures with a tunable selectivity and sensing response against hydrogen gas, achieved by properly controlling the degree of reduction and concentration of GO. The effects of these parameters are systematically studied in terms of the response amplitude and selectivity. It was found that, compared to undoped titanium dioxide nanotubes, the hybrid material with an optimal concentration of reduced-GO and the introduction of niobium shows an increase in hydrogen response of about an order of magnitude and a simultaneous reduction of the response to possible interfering compounds such as carbon monoxide and acetone, thus providing enhanced selectivity. This research may provide an efficient way to enhance the chemical sensing performance of metal oxide nanomaterials.