Two-dimensional Oxide Research Articles

Highly light-tunable memristors in solution-processed 2D materials/metal composites

Memristors—competitive microelectronic elements which bring together the electronic sensing and memory effects—potentially are able to respond against physical and chemical effects that influence their sensing capability and memory behavior. However, this young topic is still under debate and needs further attention to be highly responding to or remaining intact against physical effects, e.g., light illumination. To contribute to this scenario, using a composite of two-dimensional graphene or MoS2 doped with meso-structures of metal/metal-oxides of Ag, Cu and Fe family, we presented scalable and printable memristors. The memristive behavior shows strong dependency upon light illumination with a high record of 105 ON/OFF ratio observed so far in 2-terminal systems based on two-dimensional materials or metal oxide structures. Moreover, we found that the memristors can remain stable without illumination, providing a novel approach to use these composites for developing neuromorphic computing circuits. The sensing and memristive mechanisms are explained based on the electronic properties of the materials. Our introduced materials used in the memristor devices can open new routes to achieve high sensing capability and improve memristance of the future microelectronic elements.

Au Nanorods on Carbon-Based Nanomaterials as Nanohybrid Substrates for High-Efficiency Dynamic Surface-Enhanced Raman Scattering.

Gold nanorods (AuNRs) with different aspect ratios were prepared by the seed-mediated growth method and combined with three carbon-based nanomaterials of multiple dimensions (i.e., zero-dimensional (0D) carbon black (CB), one-dimensional (1D) carbon nanotubes (CNTs), and two-dimensional (2D) graphene oxide (GO)). The AuNR/carbon-based nanomaterial hybrids were utilized in dynamic surface-enhanced Raman scattering (D-SERS). First, cetyltrimethylammonium bromide (CTAB) was used to stabilize and coat the AuNRs, enabling them to be dispersed in water and conferring a positive charge to the surface. AuNR/carbon-based nanomaterial hybrids were then formed via electrostatic attraction with the negatively charged carbon-based nanomaterials. Subsequently, the AuNR/carbon-based nanomaterial hybrids were utilized as large-area and highly sensitive Raman spectroscopy substrates. The AuNR/GO hybrids afforded the best signal enhancement because the thickness of GO was less than 5 nm, which enabled the AuNRs adsorbed on GO to produce a good three-dimensional hotspot effect. The enhancement factor (EF) of the AuNR/GO hybrids for the dye molecule Rhodamine 6G (R6G) reached 1 × 107, where the limit of detection (LOD) was 10-8 M. The hybrids were further applied in D-SERS (detecting samples transitioning from the wet state to the dry state). During solvent evaporation, the system spontaneously formed many hotspots, which greatly enhanced the SERS signal. The final experimental results demonstrated that the AuNR/GO hybrids afforded the best D-SERS signal enhancement. The EF value for R6G reached 1.1 × 108 after 27 min, with a limit of detection of 10-9 M at 27 min. Therefore, the AuNR/GO nanohybrids have extremely high sensitivity as molecular sensing elements for SERS and are also very suitable for the rapid detection of single molecules in water quality and environmental management.

Controllably grown single-crystal films as flexoelectric nanogenerators for continuous direct current output

Until now, conventional nanogenerators could only produce electric pulses with relatively low-power densities. Herein, we invent a novel controllable growth technique for two-dimensional (2-D) cuprous oxide (p-Cu2O) single-crystal films, and on this basis, a new concept of 2-D single-crystal film flexoelectric nanogenerators (FENGs) are rationally designed and constructed for the first time, which has the characteristics of long-range order lattice, few grain boundaries and defects. More importantly, the accumulated built-in polarization potential in the bent 2-D p-Cu2O single-crystal film FENGs is in the same orientation as the output electricity, resulting in the first nanogenerator that can output continuous and stable electric signals with high voltage (Voc of 2.8 V), current (Jsc of 11.5 μA·cm−2) and power density (14.4 μW·cm−2), exhibiting great practical application potential for power generation and motion capture. This research breaks new ground and establishes a precedent for high-performance and continuous-output nanogenerators, as well as smart wearable sensors.

Concurrent detection of low levels of two important neurotransmitters in real physiological samples by a nano-needle metal oxide hybridized with graphene oxide

Acetylcholine and dopamine are the most important neurotransmitters in the nervous system. Lack of the balance in their activity can cause serious mental disorders. So, it is important to detect the trace levels of these compounds by rapid and simple methods. Herein, nanosized needle-shaped WO3 as a two-dimensional metal oxide was synthesized and hybridized with graphene oxide and applied for modification of a graphite pencil electrode (WO3/GO-pencil). The synthetic material was characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images and applied for fabrication of an electrochemical electrode. The as-prepared sensor was used successfully for simultaneous detection of acetylcholine and dopamine in real body fluids via electrocatalytic oxidation of the compounds. Dopamine and acetylcholine were detected concurrently by the stable and repeatable WO3/GO-pencil in the very low levels (2.5 and 3.7 nM, respectively). Also, the fabricated sensor showed a good stability with a significant RSD% about 2.67 % for reproducibility. The sensor performance was Eventually validated by the high performance liquid chromatography (HPLC) as a reliable method.

Two-dimensional molybdenum disulfide oxide (O-MoS2) enhanced tight ultrafiltration membrane with improved molecular separation performance and antifouling properties

Tight ultrafiltration (TUF) membranes have shown potential for separating small molecules such as dyes and natural organic matters. However, low permeability and membrane fouling are critical challenges for TUF membranes. In this study, we reported a feasible method to fabricate a TUF membrane by blending polyethersulfone (PES) with novel two-dimensional (2D) molybdenum disulfide oxide (O-MoS2) nanosheets via the phase inversion method. The results indicated that the O-MoS2 could tune the properties and morphologies of TUF membranes. It was found that the addition of 2.0 wt% of O-MoS2 into the PES membrane could reduce the molecular weight cut-off (MWCO) of the O-MoS2/PES membrane from 15322 Da to 8404 Da. As a result, the optimized O-MoS2/PES membrane showed excellent rejections to PEG 10 kDa (92.67 ± 0.12%) and Rose bengal (99.9%). Moreover, The optimized O-MoS2/PES membrane exhibited a high pure water flux (75.24 ± 4.29 L m−2h−1bar-1). By using bovine serum albumin (BSA) as the simulated foulants, the antifouling performance of as-prepared membranes was investigated. The result indicated that the optimized O-MoS2/PES membrane had 88.15% water recovery rate after a 420-min continuous filtration test, which was higher than 80.53% of the pristine membrane. This study demonstrated O-MoS2 could be used as a novel additive to prepare a TUF membrane with high filtration performance.

Solar driven photocatalytic dye degradation through the novel Ti2C– ZnCo2O4MXenes nanocomposite

Two-dimensional (2D) zinc cobalt oxide (ZnCo2O4) has considerable promise material for efficient dye photocatalytic degradation. A novel family of two-dimensional anisotropic carbides and nitrides from early transition metals (MXenes) is particularly fascinating among 2D materials, such as graphene, due to their potential uses in electronics, medicine, and photo-catalysis. The extended Ti2C phases were created by synthesizing them from their parent Ti2AlC MAX phases under the same circumstances as the traditional acidic aluminum extraction procedure. However, due to its limited specific surface area and high carrier recombination rate, photo-catalytic performance is usually inadequate. Preparation, morpho-structural characterization, band gap measurement, and congo red photo-degradation potential of Ti2C MXene and three nano-composites made up of MXene enhanced by ZnCo2O4 are presented in this study. SEM images revealed hill like agglomerated rough microstructure with round shape many particles morphology. The experimentally obtained band gaps using Tauc's approach are in the 2.60–2.84 eV range. In recent years, the approach of photo-catalytic breakdown of contaminants employing semiconductor photo-catalysts with UV–visible light has grown in importance. The addition of ZnCo2O4 boosted the establishment of strong interfacial coupling between Ti2C and ZnCo2O4, shortening the route of carrier transfer and promoting the direct transfer of photo-generated electrons from Ti2C's valence band to ZnCo2O4.The specific surface area of composites increased significantly at the same time, which was beneficial for improving the UV-absorption capability of Ti2C composites. This research proposes a practical design method for increasing the degrading performance of 2D photo-catalysts, which may serve as a technical suggestion for the modification design of additional photo-catalysts. The MXene based nano-composites revealed high activity in the congo-red photo-degradation reaction (56%–78% of degraded congo-red after 270 min). Ti2C(97%)/ZnCo2O4(3%) composite showed an enhanced efficiency for degradation of congo-red.

Coupling of nanocrystal hexagonal array and two-dimensional metastable substrate boosts H2-production

Designing well-ordered nanocrystal arrays with subnanometre distances can provide promising materials for future nanoscale applications. However, the fabrication of aligned arrays with controllable accuracy in the subnanometre range with conventional lithography, template or self-assembly strategies faces many challenges. Here, we report a two-dimensional layered metastable oxide, trigonal phase rhodium oxide (space group, P-3m1 (164)), which provides a platform from which to construct well-ordered face-centred cubic rhodium nanocrystal arrays in a hexagonal pattern with an intersurface distance of only 0.5 nm. The coupling of the well-ordered rhodium array and metastable substrate in this catalyst triggers and improves hydrogen spillover, enhancing the acidic hydrogen evolution for H2 production, which is essential for various clean energy-related devices. The catalyst achieves a low overpotential of only 9.8 mV at a current density of −10 mA cm−2, a low Tafel slope of 24.0 mV dec−1, and high stability under a high potential (vs. RHE) of −0.4 V (current density of ~750 mA cm−2). This work highlights the important role of metastable materials in the design of advanced materials to achieve high-performance catalysis.

Bimetallic CuPd alloy nanoparticles decorated ZnO nanosheets with enhanced photocatalytic degradation of methyl orange dye

Photocatalytic technology is widely explored as a promising alternative for water treatments. However, low photocatalytic efficiency and selectivity usually limit its practical application. Herein, we develop the synthesis of two-dimensional zinc oxide (ZnO) nanosheets decorated with copper (Cu)-palladium (Pd) bimetallic nanoparticles (NPs) for the degradation of organic dyes in an aqueous solution. Compared to pristine ZnO nanosheets, the prepared CuPd/ZnO composites exhibited superior performance for the photocatalytic degradation of organic dyes under visible-light irradiation. The remarkable improvement of degradation activity was attributable to the enhanced separation and transfer efficiency of photogenerated charge carriers. The highest catalytic efficiency of CuPd/ZnO nanocomposite with the CuPd content of 0.5 wt% exhibited 95.3% removal of methyl orange (MO) (40 mg/L) within 45 min. From the experimental data, we believe this study provides a new avenue for the design and fabrication of high-performance photocatalysts capable of water treatments.

HMTA-assisted synthesis of praseodymium oxide nanostructures integrated multiwalled carbon nanotubes for efficient levofloxacin electrochemical sensing

In this study, two-dimensional (2D) flake-like praseodymium oxide (Pr6O11) nanostructures were synthesized via a hexamethylenetetramine (HMTA)-assisted hydrothermal method. The sonochemical technique is used to prepare the Pr6O11 integrated multiwalled carbon nanotubes (MWCNTs) for the detection of levofloxacin (LFN). The physical properties of the Pr6O11/MWCNT were examined by X-ray diffractometer (XRD), X-ray photoelectron microscope (XPS), Raman, Fourier-transform infrared (FT-IR) spectroscopy, and field-emission scanning electron microscope. The glassy carbon electrode (GCE) surface was modified using the flake-like Pr6O11 and non-covalently integrated MWCNTs. Significantly, the fabricated electrode shows good electrocatalytic activity, more active sites, a large specific surface area, and excellent conductivity. The studied electro-oxidation mechanism of LFN at the Pr6O11/MWCNT modified electrode/electrolyte interface is two electrons and two proton transfer process. Owing to the synergistic effect, the Pr6O11/MWCNT modified electrode exhibits a long linear concentration range of 0.499–684 μM, appreciable detection limit (LOD) of 5.3 nM (3 × S/b), and high sensitivity of 7.97 μA/μM/cm2 toward LFN detection. Besides, the Pr6O11/MWCNT-modified electrode shows the good anti-interfering property, reproducibility, repeatability, and storage stability. Satisfactory recovery results were calculated for the detection of LFN in human urine, serum, and river water samples.

Growth and Characterization of Ultrathin Vanadium Oxide Films on HOPG.

Integration of graphene into various electronic devices requires an ultrathin oxide layer on top of graphene. However, direct thin film growth of oxide on graphene is not evident because of the low surface energy of graphene promoting three-dimensional island growth. In this study, we demonstrate the growth of ultrathin vanadium oxide films on a highly oriented pyrolytic graphite (HOPG) surface, which mimics the graphene surface, using (oxygen-assisted) molecular beam epitaxy, followed by a post-annealing. The structural properties, surface morphology, and chemical composition of the films have been systematically investigated by in situ reflection high-energy electron diffraction during the growth and by ex situ techniques, such as atomic force microscopy, scanning tunneling microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS). Crystalline monolayer vanadium oxide can be achieved on HOPG by systematically tuning the deposition time of V atoms and by subsequent annealing at 450 °C in controlled atmospheres. Increasing the partial pressure of O2 during the deposition seems to decrease the mobility of V atoms on the graphitic surface of HOPG and promote the formation of a two-dimensional (2D) vanadium oxide. The obtained oxide layers are found to be polycrystalline with an average grain size of 15 nm and to have a mixed-valence state with mainly V5+ and V4+. Moreover, XPS valence band measurements indicate that the vanadium oxide is insulating. These results demonstrate that a 2D insulating vanadium oxide can be grown directly on HOPG and suggest vanadium oxide as a promising candidate for graphene/oxide heterostructures.

Dodecahydrododecaborate anion cluster-reduced nano-Au-decorated graphene oxide for 100% hydrogenation of nitroaromatics

The reduction of nitroaromatics is important in industrial wastewater treatment. However, the activity of an external catalyst is critical for the reaction. In this study, Au nanocrystals were anchored in situ on two-dimensional graphene oxide (GO), which had oxygen-rich groups on its surface. Oxygen-containing groups combined to form functional boron clusters through hydrogen bond interactions (BGO). The mild reducibility of a dodecahydrododecaborate anion cluster (closo-[B12H12]2−) made ultrafine Au nanocrystals uniformly disperse on GO (Au/BGO). 4-Nitrophenol and other nitro-aromatic hydrocarbons could be rapidly hydrogenated into aminoaromatic hydrocarbons by Au/BGO, and the hydrogenation efficiency was approximately 100%. It retained high catalytic activity after 16 catalytic cycles. Therefore, the proposed innovative preparation strategy and high catalytic activity indicate the advancement of closo-[B12H12]2− as a nucleation target for nanometals. This study provides a stable and efficient catalyst for the hydrogenation of nitroaromatics and an alternative approach for the preparation of supported nano-metals.

Cross-linked laminar graphene oxide membranes for wastewater treatment and desalination: A review.

Two-dimensional (2D) lamellar graphene oxide (GO) membranes are emerging as attractive materials for molecular separation in water treatment because of their single atomic thickness, excellent hydrophilicity, large specific surface areas, and controllable properties. To yet, commercialization of GO laminar membranes has been hindered by their propensity to swell in hydrated conditions. Thus, chemical crosslinking of GO sheets with the polymer matrix is used to improve GO membrane hydration stability. This review focuses on pertinent themes such as how chemical crosslinking improves the hydration stability, separation performance, and antifouling properties of GO membranes.

RETRACTED ARTICLE: Polydopamine-modified MWCNT/graphene oxide hybrid 3D carbon nano-structure for flexible symmetric supercapacitor electrodes

The nanocarbon-based three-dimensional carbon nano-structure was prepared from the combination of one-dimensional multi-walled carbon nanotubes (MWCNTs) and two-dimensional graphene oxide by a simple hydrothermal method. The MWCNT/graphene hybrid hydrogel (CGOH) was used as electrode, and it exhibited higher specific capacitance (193.3 F g−1) compared to graphene oxide hydrogel (62.5 F g−1) due to the increase of electrical conductivity. Further enhancement of the capacitance was studied in this work by utilizing polydopamine (PDA) coating on the CGOH surface and heat treatment at 600 °C under argon gas condition for 2 h (PDA-CGOH-600). The specific capacitance of the PDA-CGOH-600 (217.4 F g−1) was enhanced by 12.5% compared to the CGOH. Using these hybrids, the flexible symmetric supercapacitor (FSS) devices were prepared with a solid polymer electrolyte (polyvinyl alcohol/sulfuric acid). The charge/discharge tests of FSS device confirmed that the highest capacitance of the FSS based on PDA-CGOH-600 because of the nitrogen-doping effect and the graphitic carbon formation. Furthermore, FSS device can be applied in the wire-shaped supercapacitor and good parameters of the energy storage device were achieved, such as specific capacitance of 50 mF cm−2, stability during 3000 times of charge/discharge, as well as stretchability and bending ability.

Enhanced CO hydrogenation performance via two-dimensional NiAl-layered double oxide decorated by SiO2 nanoparticles

The CO methanation reaction has been widely used in the fields of synthetic natural gas and ammonia (NH3). This study improves the CO methanation performance using a two-dimensional NiAl-layered double oxide (2D NiAl-LDO) decorated by SiO2 nanoparticles and reduced under hydrogen atmosphere. The as-obtained H–NiAl-LDO/SiO2 exhibited a high specific surface area of 240.5 m2/g and high surface-adsorbed oxygen of 20.77%. Furthermore, it had an excellent CO conversion of 100% at 300 °C and 96.87% at 250 °C, which was much better than those of H–NiAl-LDO (84.03% at 300 °C and 0% at 250 °C). We believe that it provides an additional strategy to easily and effectively improve CO methanation performance and shows potential for the application of similar catalysts.

Two-dimensional oxygen vacancy-doped tungsten oxide hydrate nanosheets for high-performance electrochromic device

Facile fabrication of advanced tungsten oxide (WO3) or WO3 hydrate materials with superior electrochemical and electrochromic properties is of great value in energy-related applications. Self-doping (oxygen vacancy doping) and nanostructure regulation are considered efficient strategies to enhance the performance of bulk WO3 materials. Herein, novel WO3-x hydrate nanosheets were prepared by a one-pot template/surfactant-free solvothermal method employing Na2WO4·2H2O as precursors and ethanol as solvent. The morphology, structure, and composition of the prepared WO3-x hydrate nanosheets were demonstrated by TEM, AFM, XRD, Raman spectrum, and XPS. Furthermore, WO3-x hydrate nanosheets films with mesostructured architecture were formed on FTO glass substrates by the dip-coating process, which exhibited excellent electrochemical and electrochromic performance. As a proof-of-concept example, an analog smart window device (ASWD) was assembled by using the WO3-x hydrate nanosheet film on FTO glass as the working electrode, an FTO glass as the counter electrode, and LiClO4 in propylene carbonate as the electrolyte. The fabricated ASWD presented fascinating electrochromic performance with a wide optical modulation range (82.1%) at the wavelength of 670 nm, moderate switching time (9.6/10.4 s for bleaching/coloration), high coloration efficiency (111.8 cm2/C) and good cyclic stability (maintains 71.6% of the maximum dynamic modulation range after 1000 cycles). Ascribed to the easy preparation process and outstanding electrochromic properties, this kind of WO3-x hydrate nanosheet and its corresponding electrochromic devices are believed to have great potential applications in energy-saving engineering.

Probing the Function of a Li-CO2 Battery with a MXene/Graphene Oxide Composite Cathode Electrocatalyst.

We systematically diagnose here the various phases formed at the electrodes in a Li-CO2 battery. The CO2 cathode comprises a mixture of two-dimensional electrocatalysts, MXene and graphene oxide (MXene/GO), configured on Ni foam. The observed overpotential for MXene/GO (2.4 V) is lower than that for GO (2.8 V). MXene/GO also outperforms GO in terms of battery stability and performance. The overall battery reaction (Li2CO3 ↔ Li + CO2) is more efficient in the case of MXene/GO than in the case of GO. This is convincingly demonstrated using ex situ high-resolution synchrotron X-ray diffraction and Raman scattering spectroscopy, which strongly indicates that the MXene/GO composite is more capable than GO in converting Li2CO3 to Li and CO2. When the Li anode is probed, CO2 crossover is evident via the observation of the formation of LiOH/Li2CO3 phases, the proportions of which change during successive cycles.

Magnetic proximity effect at the interface of two-dimensional materials and magnetic oxide insulators

Two-dimensional (2D) materials provide a platform for developing novel spintronic devices and circuits for low-power electronics. In particular, inducing magnetism and injecting spins in graphene have promised the emerging field of graphene spintronics. This review focuses on the magnetic proximity effect at the interface of 2D materials and magnetic oxide insulators. We highlight the unique spin-related phenomena arising from magnetic exchange interaction and spin-orbital coupling in 2D materials coupled with magnetic oxides. We also describe the fabrication of multifunctional hybrid devices based on spin transport. We conclude with a perspective of the field and highlight challenges for the design and fabrication of 2D spintronic devices and their potential applications in information storage and logic devices.

Reveal of Magnetic Domains and Tunable Supercell Structures in Two-dimensional Layered Oxide Thin Film via Differential Phase Contrast Imaging and Atomic-resolution STEM

Reveal of Magnetic Domains and Tunable Supercell Structures in Two-dimensional Layered Oxide Thin Film via Differential Phase Contrast Imaging and Atomic-resolution STEM

Fast room-temperature hydrogenation of nitroaromatics on Pd nanocrystal-boron cluster/graphene oxide nanosheets

The reduction of nitroaromatics to aminoaromatics is essential for fine chemical production and effective sewage treatment. However, the activity of an external catalyst is essential for the reaction. In this study, Pd nanocrystals were anchored in situ on two-dimensional graphene oxide (GO), which acted as a catalyst support with high specific surface area. The oxygen-containing groups on the surface of GO bonded to the functionally rich boron clusters through hydrogen bonding interactions. A mildly reducible closed-dodecahydrododecaboric acid anion cluster (closo‑[B12H12]2–) was employed as the target site. The mild reducibility of closo‑[B12H12]2– resulted in a wide dispersion of ultrafine Pd nanocrystals on GO. Under ambient conditions, Pd/BGO rapidly hydrogenated nitroaromatics, such as 4-nitrophenol, to aminoaromatics with approximately 100% efficiency. Moreover, Pd/BGO retained its high catalytic activity for the hydrogenation/reduction of 4-nitrophenol after five catalytic cycles. Therefore, Pd/BGO could be a promising and economically viable candidate for various practical applications. The proposed innovative preparation strategy and highly efficient catalytic activity suggested the effective performance of closo‑[B12H12]2– as nanometal nucleation target sites. In addition to providing an alternate route for preparing supported nanometals, this study presents a stable and efficient catalyst for the hydrogenation of nitroaromatics.

Photo-Thermal Tuning of Graphene Oxide Coated Integrated Optical Waveguides.

We experimentally investigate power-sensitive photo-thermal tuning (PTT) of two-dimensional (2D) graphene oxide (GO) films coated on integrated optical waveguides. We measure the light power thresholds for reversible and permanent GO reduction in silicon nitride (SiN) waveguides integrated with one and two layers of GO. For the device with one layer of GO, the power threshold for reversible and permanent GO reduction are ~20 and ~22 dBm, respectively. For the device with two layers of GO, the corresponding results are ~13 and ~18 dBm, respectively. Raman spectra at different positions of a hybrid waveguide with permanently reduced GO are characterized, verifying the inhomogeneous GO reduction along the direction of light propagation through the waveguide. The differences between the PTT induced by a continuous-wave laser and a pulsed laser are also compared, confirming that the PTT mainly depend on the average input power. These results reveal interesting features for 2D GO films coated on integrated optical waveguides, which are of fundamental importance for the control and engineering of GO’s properties in hybrid integrated photonic devices.