Chemical Exfoliation Process Research Articles

Scientific Machine Learning of 2D Perovskite Nanosheet Formation.

We apply a scientific machine learning (ML) framework to aid the prediction and understanding of nanomaterial formation processes via a joint spectral-kinetic model. We apply this framework to study the nucleation and growth of two-dimensional (2D) perovskite nanosheets. Colloidal nanomaterials have size-dependent optical properties and can be observed in situ, all of which make them a good model for understanding the complex processes of nucleation, growth, and phase transformation of 2D perovskites. Our results demonstrate that this model nanomaterial can form through two processes at the nanoscale: either via a layer-by-layer chemical exfoliation process from lead bromide nanocrystals or via direct nucleation from precursors. We utilize a phenomenological kinetic analysis to study the exfoliation process and scientific machine learning to study the direct nucleation and growth and discuss the circumstances under which it is more appropriate to use phenomenological or more complex machine learning models. Data for both analysis techniques are collected through in situ spectroscopy in a stopped flow chamber, incorporating over 500,000 spectra taken under more than 100 different conditions. More broadly, our research shows that the ability to utilize and integrate traditional kinetics and machine learning methods will greatly assist in the understanding of complex chemical systems.

Green and facile production of high-quality graphene from graphite by the intercalation and decomposition of oxalic acid

• A green and environmentally friendly chemical exfoliation method was developed. • Oxalic acid is used for the first time alone for chemical intercalation exfoliation of natural flake graphite. • No water-washing and effluent-treatment are involved during the whole chemical exfoliation process. • The 9-μm-thick-graphene film exhibits excellent electrical conductivity (5.05 × 10 5 S/m) and EMI shielding property (53.9 dB). The environmentally friendly preparation of graphene remains a great challenge. Herein, a novel and green preparation method for graphene has been developed. Graphene sheets were successfully exfoliated from graphite by intercalation and decomposition of oxalic acid for the first time. The whole chemical exfoliation process does not involve any waste liquid disposal, and, more importantly, the decomposed products by oxalic acid are CO 2 and H 2 O, respectively, demonstrating that the exfoliation method is environmentally friendly. The obtained graphene sheets exhibit high-quality and large-size with a yield of about ∼ 100%. The prepared graphene film exhibits excellent electrical conductivity (5.05 × 10 5 S/m) and EMI shielding property (53.9 dB with a thickness of 9.0 μm), suggesting a great prospect for miniaturized electronic devices and flexible wearable applications.

Chemically exfoliated inorganic nanosheets for nanoelectronics

Two-dimensional (2D) nanomaterials constitute one of the most advanced research targets in materials science and engineering in this century. Among various methods for the synthesis of 2D nanomaterials, including top-down exfoliation and bottom-up crystal growth, chemical exfoliation has been widely used to yield monolayers of various layered compounds, such as clay minerals, transition metal chalcogenides (TMDCs), and oxides, long before the discovery of graphene. Soft chemical exfoliation is a technique to weaken the layer-to-layer interaction in layered compounds by chemical modification of interlayer galleries, which promotes monolayer exfoliation. The chemical exfoliation process using organic substances, typically amines, has been applied to a range of layered metal oxides and hydroxides for two decades, establishing high-yield exfoliation into their highly crystalline monolayers and colloidal integration processes have been developed to assemble the resultant 2D nanomaterials into well-organized nanoscale devices. Recently, such a strategy was found to be effective for TMDC and MXene nanosheets, expanding the lineup of functionalities of solution-processed 2D nanomaterial devices from dielectrics, optics, magnetics, and semiconductors to superconductors. Throughout this review, we share the historical research flow, recent progress, and prospects in the development of soft-chemical exfoliation, colloidal integration, and thin film applications of oxides, TMDC, and MXene nanosheets.

Understanding the structural properties of feasible chemically reduced graphene

<abstract> <p>The production of pristine graphene materials for industrialization, often limited by the complicated synthesis route, has introduced other graphene derivatives with a workable and facile synthesis route, especially for mass production. For the chemical exfoliation process, the synthesis involves oxidants and reducing agents to exfoliate the graphene layer from the 3D graphite and remove excess oxygen-containing functional groups yielding graphene-like materials known as reduced graphene oxide (rGO). This work feasibly produces rGO with nanoplatelet morphology through the green solution-processable method. Upon reduction, the crystallite size for the a-axis (<italic>L<sub>a</sub></italic>) is more prominent (22.50 Å) than the crystallite size for the c-axis (<italic>L<sub>c</sub></italic>) (11.50 Å), suggesting the nanoplatelets structure of the end product, which is also confirmed by the morphology. The integrated intensity (<italic>I</italic><sub>D</sub>/<italic>I</italic><sub>G</sub>) ratio and average defect density (<italic>n<sub>D</sub></italic>) of as-prepared rGO confirmed the sp<sup>2</sup> restoration in the graphitic structure. Overall, the Raman and X-ray diffraction (XRD) characterization parameters validate the production of rGO nanoplatelets, especially with four graphene layers per domain, suggesting that high-quality rGO are achievable and ready to be implemented for the large-scale production.</p> </abstract>

Ultrathin 2D-oxides: A perspective on fabrication, structure, defect, transport, electron, and phonon properties

In the field of atomically thin 2D materials, oxides are relatively unexplored in spite of the large number of layered oxide structures amenable to exfoliation. There is an increasing interest in ultrathin film oxide nanostructures from applied points of view. In this Perspective paper, recent progress in understanding the fundamental properties of 2D oxides is discussed. Two families of 2D oxides are considered: (1) van der Waals bonded layered materials in which the transition metal is in its highest valence state (represented by V2O5 and MoO3) and (2) layered materials with ionic bonding between positive alkali cation layers and negatively charged transition metal oxide layers (LiCoO2). The chemical exfoliation process and its combination with mechanical exfoliation are presented for the latter. Structural phase stability of the resulting nanoflakes, the role of cation size, and the importance of defects in oxides are discussed. Effects of two-dimensionality on phonons, electronic band structures, and electronic screening are placed in the context of what is known on other 2D materials, such as transition metal dichalcogenides. The electronic structure is discussed at the level of many-body-perturbation theory using the quasiparticle self-consistent GW method, the accuracy of which is critically evaluated including effects of electron–hole interactions on screening and electron–phonon coupling. The predicted occurrence of a two-dimensional electron gas on Li-covered surfaces of LiCoO2 and its relation to topological aspects of the band structure and bonding is presented as an example of the essential role of the surface in ultrathin materials. Finally, some case studies of the electronic transport and the use of these oxides in nanoscale field-effect transistors are presented.

Study on the Preparation of GO and RGO by Chemical and Mechanical Exfoliation of Natural Graphite for the Aluminum Industry

Aluminum is produced in chemical electrolysis by the Hall–Heroult process using carbon-based materials in electrodes (cathode and anode). The amorphous calcined petroleum coke is normally used as the carbon material. During the electrolysis of alumina, a voltage drop of about 0.15–0.25 V occurs due to the formation of gas bubbles at the anode which therefore reduces the electrolysis reaction kinetics. To improve the electrolysis efficiency, graphite material is partly used in both the cathode and anode. To further improve the efficiency of the electrolysis, the graphite may be replaced by graphene oxide (GO) and reduced graphene oxide (RGO) because of their outstanding electrical, mechanical and chemical properties. As the demand for graphene oxide and graphene is increasing daily, large-scale and economic processes should be developed to meet this challenge. This study was carried out to produce GO and RGO from natural high-purity graphite (HPG) by applying oxidation and reduction processes through chemical and mechanical exfoliation processes. In the chemical exfoliation process, multi-metal alloys of zinc, iron, aluminum and silicon are used as the reductant instead of using hazardous chemical materials. In the mechanical exfoliation process, a planetary ball mill is used to exfoliate the natural graphite by purging gaseous oxidants and reductants such as O2 and H2 for the oxidation of GO and the reduction of RGO, respectively. The morphology, structure and composition of HPG, GO and RGO have been characterized using different analytical techniques, such as X-ray diffraction, scanning electron microscopy, and transmission electron microscopy with selected area electron diffraction, micro-Raman spectroscopy and Fourier-transform infrared spectroscopy. The number of layers of the GO and RGO samples has been determined based on the characterization data and found to be four to five layers.

Graphene Reinforced Composites as Sensing Elements

The present study deals with the optimal design of a Graphene reinforced composite. The Graphene was prepared by chemical exfoliation process and was chemically blended with matrix material in acetone. Further chemically mixed solution was exposed to air for acetone vaporization. Next, this Graphene composite was extruded through twin screw extrusion (TSE) for preparation of feedstock filament with 1.75±0.05mm diameter via fused deposition modelling (FDM). The presented results suggest that statistically controlled Graphene reinforced functional prototypes can be usefully employed as sensors for bio-medical and engineering applications.

Mechanical Exfoliation of Reduced Graphene Oxide From Old Coconut Shell as Radar Absorber in X-Band

The purpose of this study is to determine the effect of heating temperature and the chemical exfoliation process on the reflection loss of r-GO synthesized from coconut shell. The heating process is carried out at temperature of 400°C and 700°C. The chemical exfoliation process is carried out by adding 1M H2SO4 solution in a ratio of 1: 1, 1: 5, and 1:10. Then, the process of washing is done using an ultrasonic cleaner. The XRD pattern indicates that coconut shell charcoal has formed the r-GO phase. In the chemical exfoliation process with the addition of 1M H2SO4 solution in a ratio of 1: 10 at a temperature of 400°C, it shows that the maximum reflection loss is -7.186 dB at 10.48 GHz with an electrical conductivity of 1.075 x 10-3 S/cm.

New Insights into the Triton X‐100 Induced Chemical Exfoliation of MoS2 to Derive Highly Luminescent Nanosheets

AbstractThe exfoliation of two dimensional (2D) transition metal dichalcogenides (TMDs) into mono‐ or few‐layers without compromising their semiconductor properties has momentous interest for both point of view; fundamental studies and further implementation in practical applications. Herein, we reported a novel and inexpensive approach for high yield nanosheets from bulk MoS2 to few layers of strong luminescent MoS2 nanosheets using Triton X‐100 as a surfactant with tailoring the bulk band gap 1.2 eV to 1.79 eV of few layers of nanosheets after chemical exfoliation process, which can be easily scaled‐up in large quantity. The microstructural results reveal that the exfoliated nanosheets have thickness in the range of few layers and lateral dimension in the range of few hundred nanometers. Our findings may offer a new innovative one setup chemical exfoliation process to design a few layer of MoS2 nanosheets without suppressing luminescent properties, which is highly desirable for the next generation optoelectronic devices.

Conductive polymer based high-performance hybrid thermoelectrics: Polyaniline/tin(II) sulfide nanosheet composites

Composites of tin sulfide (SnS) nanosheets with conductive polyaniline (PANI) are fabricated to achieve high-performance hybrid thermoelectric materials. SnS nanosheets with lateral sizes from 200 to 500 nm are prepared from a SnS ingot by applying a Li + -intercalation and chemical exfoliation process and are incorporated with conductive PANI. The thermoelectric power factor of the SnS nanosheet/PANI composites is optimized by manipulating the content of SnS nanosheets in the composite, and the changes in the thermoelectric properties are examined and discussed with respect to the carrier transport properties. The maximum ZT of 0.29 × 10−2 is obtained with the SnS nanosheet/PANI composite containing 20 wt% of SnS nanosheets, attributed to the dominance of the improvement in the Seebeck coefficient over the reduction in the electrical conductivity. The incorporation of conductive PANI and SnS nanosheets is a promising strategy for the actualization of high-performance thermoelectric applications.

Molten-salt chemical exfoliation process for preparing two-dimensional mesoporous Si nanosheets as high-rate Li-storage anode

Two-dimensional (2D) materials have attracted enormous attention due to their functional applications in energy storage. In this work, a low-temperature molten-salt chemical exfoliation methodology is developed for producing free-standing 2D mesoporous Si through deintercalation of CaSi2 in excess molten AlCl3 at 195 °C. The average dimension of these sheets is 1.5 μm, and the thickness of a single sheet is approximately 10 nm. The as-prepared 2D Si has a Brunauer–Emmett–Teller surface area of 154 m2·g−1 and an average pore size of 5.87 nm. With this unique structure, the 2D Si exhibits superior Li-storage performance, including a reversible capacity of 2,974 mA·h·g−1 at 0.2 C, reversible capacities of 2,162, 1,947, and 1,527 mA·h·g−1 at 0.8, 2, and 5 C after 200 cycles, and a capacity retention of 357 mA·h·g−1 even at 30 C (90 A·g−1).

Influence of chemical exfoliation process on the activity of NiCl2-FeCl3-PdCl2-graphite intercalation compound towards methanol electrooxidation

In the present work the influence of chemical exfoliation process of quaternary graphite intercalation compound (NiCl2-FeCl3-PdCl2-GIC) on its catalytic activity in the methanol electrooxidation reaction was examined. NiCl2-FeCl3-PdCl2-GIC was synthesized by a molten salts method using purified flaky graphite and anhydrous metal chlorides as reactants. The process of intercalation was carried out at 450°C for 72h. To obtain exfoliated graphite intercalation compound (NiCl2-FeCl3-PdCl2-EGIC), a part of gathered product underwent chemical exfoliation in hydrogen peroxide solution. Crystalline structure of the obtained GIC and product of its exfoliation was characterized by XRD while the their morphology by SEM and TEM techniques. Development of the surface area was calculated by BET method. Electrochemical activity of both materials were examined in the model process of methanol electrooxidation by cyclic (CV) and linear sweep (LSV) voltammetry as well as potentiostatic method.The XRD and SEM analysis have showed that the chemical exfoliation process contributes to the changes in structure and morphology of the examined material.From the electrochemical results it is revealed that prepared NiCl2-FeCl3-PdCl2-GIC exhibits activity towards electrochemical process of methanol oxidation. Furthermore is proved that electroactivity of the examined NiCl2-FeCl3-PdCl2-GIC significantly increases due its chemical exfoliation.

Controlled Intercalation and Chemical Exfoliation of Layered Metal-Organic Frameworks Using a Chemically Labile Intercalating Agent.

Creating ordered two-dimensional (2D) metal-organic framework (MOF) nanosheets has attracted extensive interest. However, it still remains a great challenge to synthesize ultrathin 2D MOF nanosheets with controlled thickness in high yields. In this work, we demonstrate a novel intercalation and chemical exfoliation approach to obtain MOF nanosheets from intrinsically layered MOF crystals. This approach involves two steps: first, layered porphyrinic MOF crystals are intercalated with 4,4'-dipyridyl disulfide through coordination bonding with the metal nodes; subsequently, selective cleavage of the disulfide bond induces exfoliation of the intercalated MOF crystals, leading to individual freestanding MOF nanosheets. This chemical exfoliation process can proceed efficiently at room temperature to produce ultrathin (∼1 nm) 2D MOF nanosheets in ∼57% overall yield. The obtained ultrathin nanosheets exhibit efficient and far superior heterogeneous photocatalysis performance compared with the corresponding bulk MOF.

Tunable Volatile-Organic-Compound Sensor by Using Au Nanoparticle Incorporation on MoS2.

Controlling the charge concentrations of two-dimensional (2D) materials is a critical requirement for realizing versatility and potential application of these materials in high-performance electronics and sensors. In order to exploit the novel chemical-sensing characteristics of 2D materials for sensitive and selective sensors, various functionalization methods are needed to ensure efficient doping of channels based on 2D materials. In the present study, the gas-sensing performance of MoS2 has been significantly enhanced by controlled Au nanoparticle functionalization. By using the difference in reduction potential between the Au precursor and MoS2 work functions, MoS2 prepared by chemical exfoliation process was decorated with nanoparticles with sizes of tens of nanometers. The n-doping effect of Au nanoparticles was observed, that is, these particles were found to have facilitated in electron charge transfer from Au to MoS2. The controlled n-doping effect enables the tuning of the sensing of hydrocarbon-based volatile organic compounds (VOCs) and oxygen-functionalized compounds by MoS2. A significant step has therefore been made with this study toward solving the limitations imposed by previous MoS2-based sensors, which mostly produce a single response to various VOC analytes. This controllable chemical doping process for tuning the VOC-sensing performance of MoS2 can eventually be used in early detection using multichannel sensing systems that have different responses and recognize patterns for target analytes.

Interfacial Transformation of an Amorphous Carbon Nanofilm upon Fe@Ag@Si Nanoparticle Landing and its Colloidal Nanoscrolls: Enhanced Nanocompositing-Based Performance for Bioapplications.

We report a novel method for generating magneto-plasmonic carbon nanofilms and nanoscrolls using a combination of two gas-phase synthetic techniques. Ternary Fe@Ag@Si "onion-like" nanoparticles (NPs) are produced by a magnetron sputtering inert gas condensation source and are in situ landed onto the surface of carbon nanofilms, which were previously deposited by a DC arc discharge technique. Subsequently, a polyethylenimine-mediated chemical exfoliation process is performed to obtain carbon nanoscrolls (CNS) with embedded NPs (CNS-NPs). Of note, the carbon nanofilms undergo an interfacial transition upon addition of NPs and become rich in the sp2 phase. This transformation endows and enhances multiple functions, such as thermal conductivity and the plasmonic properties of the nanocomposites. The obtained two-dimentional (2D) nanocomposites not only exhibit a highly efficient surface-enhanced Raman scattering property, allowing sensitive detection of malachite green isothiocyanate (MGIT) and adenosine-triphosphate (ATP) molecules at concentrations as low as 1 × 10-10 M, but also show enhanced near-infrared-responsive photothermal activity when forming stable colloidal 1D CNS-NPs. In addition, the CNS-NPs present an enhanced single- and two-photon fluorescence in comparison with pristine CNS and NPs. These results make them suitable for the rational fabrication of "all-in-one" multifunctional nanocomposites with tubular structures toward a wide range of biomedical solutions.

Switchable polarization in an unzipped graphene oxide monolayer.

Ferroelectricity in low-dimensional oxide materials is generally suppressed at the scale of a few nanometers, and has attracted considerable attention from both fundamental and technological aspects. Graphene is one of the thinnest materials (one atom thick). Therefore, engineering switchable polarization in non-polar pristine graphene could potentially lead to two-dimensional (2D) ferroelectric materials. In the present study, based on density functional theory, we show that an unzipped graphene oxide (UGO) monolayer can exhibit switchable polarization due to its foldable bonds between the oxygen atom and two carbon atoms underneath the oxygen. We find that a free standing UGO monolayer exhibits antiferroelectric switchable polarization. A UGO monolayer can be obtained as an intermediate product during the chemical exfoliation process of graphene. Interestingly, despite its dimensionality, our estimated polarization in a UGO monolayer is comparable to that in bulk ferroelectric materials (e.g., ferroelectric polymers). Our calculations could help realize antiferroelectric switchable polarization in 2D materials, which could find various potential applications in nanoscale devices such as sensors, actuators, and capacitors with high energy-storage density.

Chemically exfoliated single-layerMoS2: Stability, lattice dynamics, and catalytic adsorption from first principles

Chemically and mechanically exfoliated MoS$_2$ single-layer samples have substantially different properties. While mechanically exfoliated single-layers are mono-phase (1H polytype with Mo in trigonal prismatic coordination), the chemically exfoliated samples show coexistence of three different phases, 1H, 1T (Mo in octahedral coordination) and 1T$^{'}$ (a distorted $2\times 1$ 1T-superstructure). By using first-principles calculations, we investigate the energetics and the dynamical stability of the three phases. We show that the 1H phase is the most stable one, while the metallic 1T phase, strongly unstable, undergoes a phase transition towards a metastable and insulating 1T$^{'}$ structure composed of separated zig-zag chains. We calculate electronic structure, phonon dispersion, Raman frequencies and intensities for the 1T$^{'}$ structure. We provide a microscopical description of the J$_1$, J$_2$ and J$_3$ Raman features first detected more then $20$ years ago, but unexplained up to now. Finally, we show that H adsorbates, that are naturally present at the end of the chemical exfoliation process, stabilize the 1T$^{\prime}$ over the 1H one.

Incorporation of Graphenes in Nanostructured TiO2 Films via Molecular Grafting for Dye-Sensitized Solar Cell Application

This paper presents a systematic investigation on the incorporation of chemical exfoliation graphene sheets (GS) in TiO(2) nanoparticle films via a molecular grafting method for dye-sensitized solar cells (DSSCs). By controlling the oxidation time in the chemical exfoliation process, both high conductivity of reduced GS and good attachment of TiO(2) nanoparticles on the GS were achieved. Uniform GS/TiO(2) composite films with large areas on conductive glass were prepared by electrophoretic deposition, and the incorporation of GS significantly improved the conductivity of the TiO(2) nanoparticle film by more than 2 orders of magnitude. Moreover, the power conversion efficiency for DSSC based on GS/TiO(2) composite films is more than 5 times higher than that based on TiO(2) alone, indicating that the incorporation of GS is an efficient means for enhancing the photovoltaic (PV) performance. The better PV performance of GS/TiO(2) DSSC is also attributed to the better dye loading of GS/TiO(2) film than that of TiO(2) film. The effect of GS content on the PV performances was also investigated. It was found that the power conversion efficiency increased first and then decreased with the increasing of GS concentration due to the decrease in the transmittance at high GS content. Further improvements can be expected by fully optimizing fabrication conditions and device configuration, such as increasing dye loading via thicker films. The present synthetic strategy is expected to lead to a family of composites with designed properties.