Liquid-phase Exfoliation Process Research Articles

Self-powered photodetector based on liquid phase exfoliated Cu–WSe2 nanosheets

According to its tuneable optoelectronic capabilities that enhanced the application of the photo-sensing device, the 2D-TMDC (transition metal dichalcogenides) deposited thin films have been the subject of additional research. Herein, we describe the high-quality crystalline behaviour of 10% incorporation of Cu in the pristine WSe2 nanosheets using two step liquid phase exfoliation process and its use as a broad range photodetector. The vapour phase technique is used to grown compounds. The elemental mapping and purity were analysed by EDAX. Powder Xray diffraction shows that all exfoliated nanosheets have a hexagonal lattice structure. The structural morphology was investigated by SEM and HR-TEM. The Raman Spectra reveal that the exfoliated nanosheets have in plane E2g vibration mode. The effect of alloy engineering on photo responsiveness has been investigated. The maximum photodetector parameters are examined for the ITO based Cu (10%) WSe2 photodetector under various wavelength dependent monochromatic light sources for 40 mW/cm2 power intensity.

Improved bioceramic coatings reinforced by nanostructured talc

Nano-talc was successfully incorporated in the hydroxyapatite matrix via pulsed electrodeposition after being obtained using an eco-friendly liquid-phase exfoliation process. Scanning electron microscopy, atomic force microscopy, X-ray spectroscopy, Raman spectroscopy, corrosion and wear resistance, and cytocompatibility tests were used to characterize the biocomposite ceramics. Talc significantly improves the nanomechanical and wear properties of bioceramics (i.e., higher stiffness, reduced friction coefficient, and lower wear damage) as well as corrosion resistance. Talc does not induce cytotoxic activity in in vitro cells and may induce bone maturation as per biocompatibility tests.

Highly sensitive and rapidly responding room-temperature NH3 gas sensor that is based on exfoliated black phosphorus

Black phosphorus (BP) has recently attracted significant interest in gas sensing because of its outstanding electronic and optical properties. In this paper, we demonstrated a series of gas sensors for detecting NH3 at room temperature that were based on BP nanosheets that were exfoliated for various durations. The BP nanosheet gas sensors showed excellent sensitivity for the detection of NH3 down to 100 ppb (~32%) under ambient conditions compared to previous reports. The BP nanosheets that were exfoliated for 60 min showed the highest response to 100 ppm NH3, namely, 121%, which was 2.5 times greater than the response of ground BP. The corresponding sensor also exhibited a relatively stable response in humidity between 22% RH and 35% RH. Moreover, the prepared NH3 gas sensor showed high selectivity, fast response and recovery, and excellent repeatability. According to various experimental and theoretical studies, the response and binding energy of BP towards NOx were higher than those towards NH3. However, very interestingly, the response of the exfoliated BP nanosheets to NH3 was higher than that to NOx in our work. The selectivity transformation from NOx to NH3 was mainly caused by the different layer spacing of the ground BP and the change in spacing between the BP nanosheet layers with different exfoliation times, increased oxygen incorporation, phosphate ions and defects that is induced by the liquid-phase exfoliation process, which led to the variations in its band gap and surface states.

Reducing noise current in exfoliated WS2 nanosheets using an ultra-nanocrystalline diamond substrate and their enhanced NIR photodetection properties

Herein, we report the fabrication of a near-infrared (NIR) WS2 nanosheet/ultra-nanocrystalline diamond (UNCD) heterojunction photodetector through a liquid phase exfoliation process and microwave plasma-enhanced chemical vapor deposition (MPECVD) method, for the first time.

High-Yield Production of Selected 2D Materials by Understanding Their Sonication-Assisted Liquid-Phase Exfoliation.

Liquid-phase exfoliation (LPE) is a widely used and promising method for the production of 2D nanomaterials because it can be scaled up relatively easily. Nevertheless, the yields achieved by this process are still low, ranging between 2% and 5%, which makes the large-scale production of these materials difficult. In this report, we investigate the cause of these low yields by examining the sonication-assisted LPE of graphene, boron nitride nanosheets (BNNSs), and molybdenum disulfide nanosheets (MoS2 NS). Our results show that the low yields are caused by an equilibrium that is formed between the exfoliated nanosheets and the flocculated ones during the sonication process. This study provides an understanding of this behaviour, which prevents further exfoliation of nanosheets. By avoiding this equilibrium, we were able to increase the total yields of graphene, BNNSs, and MoS2 NS up to 14%, 44%, and 29%, respectively. Here, we demonstrate a modified LPE process that leads to the high-yield production of 2D nanomaterials.

2D layered Mn and Ru oxide nanosheets for real-time breath humidity monitoring

Collecting real-time breath humidity data is important for calibrating gas sensors from interfering signals in breath components, ultimately for accurately monitoring a patient’s physiological information for applications in non-invasive and point-of-care diagnostics. In this work, atomically thin 2D metal oxide nanosheets (NSs) were synthesized by a liquid phase exfoliation process and their humidity sensing properties were investigated. Interestingly, Opposite humidity sensing responses (RD/RH) were observed between semiconducting oxide and metallic oxide NSs. For the semiconducting manganese (Mn) oxide NSs, decreasing resistance transitions were obtained with the response of 24.01 at 44.5% RH at low humidity levels (i.e., 6.1–45% RH), which was governed by proton (H+) conduction. On the other hand, the metallic ruthenium (Ru) oxide NSs exhibited increasing resistance transitions with the response of 0.28 at 96.3% RH at a high humidity range (i.e., 50–99.9% RH) as a result of proton trapping by accepting electrons upon the exposure to excess water molecules. Ru oxide NSs exhibited the response and recovery times of 68 sec and 8 sec, respectively, at 96.3% RH. Real-time breath humidity monitoring is demonstrated by integrating Ru oxide NSs with a wristband-type wireless sensing module, which can transmit the sensing data to a mobile device.

Role of Nanodiamond Grains in the Exfoliation of WS2 Nanosheets and Their Enhanced Hydrogen-Sensing Properties.

Herein, for the first time, a combination of detonation nanodiamond (DND)-tungsten disulfide (WS2) was devised and studied for its selective H2-sensing properties at room temperature. DND-WS2 samples were prepared by a sonication-assisted (van der Waals interaction) liquid-phase exfoliation process in low-boiling solvents with DND as a surfactant. The samples were further hydrothermally treated in an autoclave under high pressure and temperature. The as-prepared samples were separated as two parts named DND-WS2 BH (before hydrothermal) and DND-WS2 AH (after hydrothermal). The exfoliated bilayer to few-layer DND-doped WS2 nanosheets were confirmed by ultraviolet-visible spectra, atomic force microscopy, and transmission electron microscopy studies. It was observed that the DND powder not only acted as a surfactant but also doped and expanded on WS2 nanosheets. The difference between samples BH and AH treatment was further investigated using Raman spectroscopy. The WS2 and DND-WS2 samples on SiO2/Si were fabricated using a sputtered Pd/Ag interdigitated electrode and utilized for H2 gas-sensing measurements. Surprisingly, the DND-WS2 exhibits an ultrahigh sensor response of 72.8% to H2 at 500 ppm when compared to only 9.9% for WS2 alone. Also, the DND-WS2 shows a fast response/recovery time, high selectivity, and stability toward H2 gas. It can be attributed to the correlation of the intergrain phase of DND nanoparticles and WS2 nanosheets, which contributes to the easy transportation of charge carriers when exposed to the air and H2 gas atmosphere. Moreover, it is believed that DND-induced WS2 exfoliation might inspire future synthesis of transition metal dichalcogenides induced by DND in green solvents.

Effect of Co-Solvent Percentages on the Exfoliation Rate of NiTe2 Thin Film for Transparent Electrodes

We attempted to maximize the transmittance of 2D NiTe2 thin film using the liquid-phase exfoliation (LPE) process to confirm the applicability of NiTe2 as a transparent electrode. The LPE process, using a co-solvent of organic solvent and water, is a stable and efficient method of increasing transmittance at low cost. In this report, the effect of 12 different co-solvents, mixtures of acetone, ethanol, isopropyl alcohol, and water, on exfoliation rate was studied. NiTe2 thin film with a thickness of 6.3 nm prepared by sputtering, and exhibited a highest transmittance of 68% and a lowest resistivity of 291 μΩ·cm after 12 hrs sonication in ethanol/water co-solvent (ethanol : water = 60 : 40 vol. %). Three physical properties, polarization and dispersion ratio (p/d ratio), boiling point, and water contents, were compared to determine which property was the main control factor for the LPE process. Unlike previous LPE processes for powders of 2D materials, it was revealed that the improvement in the transmittance of the NiTe2 thin film was more strongly dependent on both of vol.% of water and boiling point of the solvents. This was because the transmittance improved after removing the NiTe2 thin film from the substrate, rather than layer by layer exfoliation. We believe that NiTe2 thin film prepared by sputtering followed by exfoliation process can be one of the potential candidates for transparent electrode.

Improved morphology and excitonic emission of 2D MoS2 by incorporating mechanical grinding in the liquid phase exfoliation synthesis process

Abstract MoS2 nanosheets are synthesized by incorporating a manual grinding of the bulk powder prior to the sonication in the standard liquid phase exfoliation process. The light emission properties of MoS2 is thereby found to be significantly improved. Transmission electron microscopy shows continuous ultrathin MoS2 nanosheets of larger dimensions in the grinding-assisted synthesis. Raman spectroscopy measurements show that MoS2 is exfoliated down to 3–4 layers by an inclusion of the grinding. UV–visible and photoluminescence spectroscopy were used to analyze the changes in the optical properties. Grinding helps to reduce the influence of the degradation of the NMP solvent on the emission characteristics of the exfoliated material. The strong visible photoluminescence emission due to sonochemical degradation of NMP is suppressed and the excitonic emission due to the A and B excitons relative to NMP emission is significantly enhanced. The results suggest an easy and reliable approach to enhance the optical emission of two-dimensional MoS2 appreciably.

A green approach to water‐based graphene ink with reverse coffee ring effect

In printing electronics, graphene is used as a functional material due to its excellent physical properties. Therefore, to prepare a conductive graphene ink that contains non-toxic and environment-friendly solvent is very important. Direct preparation of graphene from graphite using liquid-phase exfoliation process is beneficial since it is simple and easy to use. Here in this work, we have synthesized the graphene dispersion using ethanol and water by liquid-phase exfoliation method and prepared a conductive graphene ink that shows an absence of coffee ring formation during the evaporation of solvents. A film has been prepared using the prepared ink which shows the electrical resistivity 1.06 Ω-cm at temperature 300 °C.

The effect of equal-channel angular pressing (ECAP) on the properties of graphene reinforced aluminium matrix composites

In the present study, graphene nanosheets were synthesised by liquid phase exfoliation process, and the obtained graphene was reinforced in the rates of 0.1, 0.2, 0.3 and 0.6 wt.%. After the composites were characterised, they were exposed to Equal Channel Angular Pressing (ECAP) process. While 0.1 wt.% and 0.2 wt.% graphene reinforced composite samples successfully completed the ECAP process, 0.1 wt.% and 0.2 wt.% graphene reinforced composite samples were broken during the ECAP process. Electrical, thermal, and mechanical properties of the composite increased with the increased amount of graphene. The mechanical properties of ECAP-processed samples showed significant increases compared to non-ECAP processed samples. To figure out the effect of the ECAP process, moulds with different channel angles were used, the ECAP temperature was changed, and different passes were performed and the angles of 120° and 90° were used. ECAP-processed samples in both mould angles showed similar mechanical properties. With the increasing ECAP temperature, the mechanical properties of the sample decreased, but its electrical conductivity increased. As the number of passes increased, mechanical properties increased and crack formation in material increased. In addition, it was not possible to successfully remove the matrix composites containing more than 0.3 wt.% graphene from the ECAP process. Especially in the sample containing 0.6 wt.% graphene, brittle fractures were seen during the ECAP process and the sample was divided into many parts. The results showed that the composite responded better to the ECAP process when low amounts of graphene were reinforced in the al matrix. Significant improvements were observed in the properties of these composites after the ECAP process. In this study, the properties of composites with and without ECAP process were extensively investigated. The results were compared in detail with the previous studies. The graphene was produced using a simple method and it was reinforced with the Al matrix with the easiest possible method.

The production of graphene by direct liquid phase exfoliation of graphite at moderate sonication power by using low boiling liquid media: The effect of liquid media on yield and optimization

There are many production methods for graphene production. One of the most successful production methods in terms of scale-up is liquid phase exfoliation (LPE) method. In this study, after optimizing the sonication time and sonication power that are effective in the production to increase the efficiency of the LPE method, the experiments were continued at moderate sonication powers and the effect of liquid media, another important parameter, was examined. In this study, hexagonal graphite powders were subjected to direct liquid phase exfoliation process by using different liquid media (N-Methyl-2-Pyrrolidone (NMP), N,N-Dimethylformamide (DMF), ethanol, 2-propanol, acetone, methanesulfonic acid, ethanol + Sodium dodecyl sulphate (SDS), 2-propanol + SDS) and the obtained samples were characterized. As a result of the characterization, the liquid media with the highest efficiency was determined to be NMP. Besides, it was observed that a new hybrid solvent obtained by adding a surfactant such as SDS into low-cost and environmentally friendly liquid media such as ethanol and 2-propanol showed a higher efficiency than NMP. It was observed that 3–5 layer graphene ranged between 15 and 19 wt% in either graphene produced by using hybrid solvents containing NMP or SDS. The amount of double or single-layer graphene was negligibly small. The production with the lowest efficiency was observed in the use of acetone. The produced graphenes were dispersed in the liquid and their sedimentation time was observed. The sedimentation time of graphenes, produced using NMP and SDS-doped hybrid solvents, in liquid was more than 120 days. The sedimentation time of graphene produced using acetone was less than 20 days. In the use of graphene as a reinforcing material or adsorbent, large amounts of graphene are needed. In such applications, mass production of graphene is important rather than the number or quality of sheets of graphene. Therefore, optimization of production parameters of LPE method, which is suitable for mass production, is significance.

Zwitterion-assisted transition metal dichalcogenide nanosheets for scalable and biocompatible inkjet printing

Inkjet printing of two-dimensional (2D) transition metal dichalcogenide (TMD) nanosheets fabricated by liquid-phase exfoliation (LPE) allows simple, mass-producible, and low-cost photo-electronic devices. Many LPE processes involve toxic and environmentally hazardous solvents; however, dispersants have restricted the extent of applications of 2D-TMD inks. Herein, various 2D-TMD nanosheets, including MoS2, MoSe2, WS2, and WSe2, in addition to few-layered graphene, are inkjet-printed using a LPE process based on zwitterionic dispersants in water. Zwitterions with cationic and anionic species are water-soluble, while alkyl chain moieties associated with two ionic species adhere universally on the surface of TMD nanosheets, resulting in high throughput liquid exfoliation of the nanosheets. The zwitterion-assisted TMD nanosheets in water are successfully employed as an ink without the need for additives to adjust the viscosity and surface tension of the ink for use in an office inkjet printer; this gives rise to A4 scale, large-area inkjet-printed images on diverse substrates, such as metals, oxides, and polymer substrates patchable onto human skin. Combination with conductive graphene nanosheet inks allowed the development of mechanically flexible, biocompatible-printed arrays of photodetectors with pixelated MoSe2 channels on a paper exhibiting a photocurrent ON/OFF ratio of approximately 103.8 and photocurrent switching of 500 ms.

Three-dimensional lion's mane like AlV3O9 deposited on graphene surface for supercapacitors with a promising electrochemical performance

3D lion's mane like AlV3O9 microspheres deposited on the well-exfoliated graphene were successfully synthesised through a facile solvothermal method where the graphene sheet was prepared solely via the liquid phase exfoliation process. The highly electrical conductive graphene was introduced as a conductive substrate or a backbone to support the diffusion of the electrolyte ions (OH−). In order to achieve the optimum electrochemical performance, graphene/AlV3O9 nanocomposites with different weight ratios were fabricated wherein G-5AlV possessed an exceptional charge storage ability which surpasses that of other nanocomposites. The G-5AlV sample was found to possess a magnificent specific capacitance of 551.72 F∙g−1 at 0.5 A∙g−1 and achieve a remaining capacitance of 468.97 F∙g−1 as the current density was increased. After 5000 continuous intercalation/de-intercalation processes, G-5AlV delivered a specific capacitance of 441.38 F∙g−1 and a coulombic efficiency of 95%, confirming its remarkable cycling stability and coulombic efficiency. More importantly, it exhibits good energy and power densities as a symmetric supercapacitor, further suggesting itself as a potential candidate for use in high-performance charge storage devices.

Poly(methyl methacrylate)-Assisted Exfoliation of Graphite and Its Use in Acrylonitrile-Butadiene-Styrene Composites.

One of the applications of graphene in which its scalable production is of utmost importance is the development of polymer composites. Among the techniques used to produce graphene flakes, the liquid-phase exfoliation (LPE) of graphite stands out due to its versatility and scalability. However, solvents suitable for the LPE process are generally toxic and have a high boiling point, making the processing challenging. The use of low boiling point solvents could be convenient for the processing, due to the easiness of their removal. In this study, the use of poly(methyl methacrylate) (PMMA) as a stabilizing agent is proposed for the production of graphene flakes in a low boiling point solvent, that is, acetone. The graphene dispersions produced in the mixture acetone-PMMA have higher concentration, +175 %, and contain a higher percentage of few-layer graphene flakes (<5 layers), that is, +60 %, compared to the dispersions prepared in acetone. The as-produced graphene dispersions are used to develop graphene/acrylonitrile-butadiene-styrene composites. The mechanical properties of the pristine polymer are improved, that is, +22 % in the Young's modulus, by adding 0.01 wt. % of graphene flakes. Moreover, a decrease of ≈20 % in the oxygen permeability is obtained by using 0.1 wt. % of graphene flakes filler, compared to the unloaded matrix.

Higher thermal conductivity and mechanical enhancements in hybrid 2D polymer nanocomposites

Nanocomposites based on graphene oxide (GO), hexagonal boron nitride (h-BN), and their hybrid GO/h-BN as nanofillers in polyurethane (PU) were prepared, and their structures and morphologies were studied. The efficient production of a few layers of nanofillers was completed using the direct liquid-phase exfoliation process. Mechanical and thermal tests were carried out to study the effect of the addition of GO, h-BN, and GO/h-BN at different wt% in the PU system. The tensile strength and Young's modulus showed an increase of up to 85% and 140%, respectively, for the composite containing 0.5 wt% of the hybrid GO/h-BN mixture. An impressive improvement of up to ~1450% in thermal conductivity was observed for the same sample when compared to neat polymer. In order to gain further insights regarding the mechanism behind the nanofiller dispersion and adhesion processes in PU, fully atomistic classical molecular dynamics simulations were carried out. Based on these simulations, it was possible to build structural models that help to explain the thermal and mechanical improvements in the hybrid GO/h-BN composites, with a focus on its stabilization energy.

Exfoliated graphene oxide-based nanofluids with enhanced thermal and optical properties for solar collectors in concentrating solar power

Abstract Nanofluids are considered a promising alternative to the classic fluids used in heat transfer processes. One interesting application of nanofluids is their use as heat transfer fluid in thermosolar plants, such as those using concentrating solar power (CSP) technology. In turn, graphene oxide is an interesting nanomaterial for preparing nanofluids for solar thermal applications due to its appealing properties, such as high thermal conductivity, easy exfoliation and high black coloration. Therefore, this study presents the preparation by means of a liquid phase exfoliation (LPE) process of nanofluids based on graphene oxide in a typical fluid used in CSP plants. The efficiency of the LPE process and the stability of the nanofluids were analysed using UV–Vis spectroscopy, particle size measurements and transmission electron microscopy. Thermal properties were also measured, improvements of up to 6.6% and 45.5% being found for isobaric specific heat and thermal conductivity, respectively. Finally, strong-colored nanofluids were obtained and their optical properties were therefore characterized. Due to the strong coloration of the nanofluids, they can be used for designing Direct Absorption Solar Collectors (DASC).

Ultrathin exfoliated WS2 nanosheets in low-boiling-point solvents for high-efficiency hydrogen evolution reaction

Two-dimensional mono-or few-layer WS2, as a typical transition metal dichalcogenide, usually exhibits excellent electrocatalytic hydrogen evolution reaction (HER) activity. In this work, low-boiling-point solvents are adopted for the controllable preparation of ultra-thin WS2 nanosheets by an improved liquid phase exfoliation process directly from the raw WS2 powder. The thickness of as-obtained WS2 nanosheets is about ∼1.5 nm with lateral size of ∼1.2 μm. Their HER performance was investigated by loading them onto a glass carbon electrode in 0.5M H2SO4 solution. The resulting polarization and Tafel curves indicate that the as-obtained WS2 nanosheets are effective electrocatalysts for HER with a low onset potential of 65mV and a Tafel slope of 55mV/dec. The proposed liquid phase exfoliation technique is lucrative for exfoliating transition metal disulfide materials in various applications.

Few-layer and large flake size borophene: preparation with solvothermal-assisted liquid phase exfoliation.

The preparation of two-dimensional boron (B) nanosheets, especially for borophene, is still a challenge because of its unique structure and complex B–B bonds in bulk boron. In the present work, a novel preparation technology for borophene with only a few layers and large flake sizes is developed by a solvothermal-assisted liquid phase exfoliation process, consisting of ball milling-thinning, solvothermal swelling, and probe ultrasonic delamination. The exfoliation effect of the bulk B precursors is related to the surface tension and Hildebrand parameter of the selected solvents such as acetone, N,N-dimethyl formamide (DMF), acetonitrile, ethanol, and N-methyl pyrrolidone (NMP), and a relative small surface tension when using solvents is favorable for the exfoliation of bulk B. Four-layer thick borophene and an average lateral size of 5.05 μm can be obtained in acetone as the exfoliating solvent. The surface composition of the exfoliated few-layer borophene with large flake size hardly changes, while the chemical state of B changes to some extent because they are partly oxidized on the surface by contaminates before and after exfoliation. This acetone solvothermal-assisted liquid phase exfoliation technique can be used to prepare high quality borophene with large horizontal sizes, and it will provide the basis to study few-layer borophene with large sizes further.

Exfoliation Behavior of Large Anionic Graphite Flakes in Liquid Produced by Salt-Assisted Ball Milling

Functionalization of graphite is crucial for efficient and effective exfoliation to graphene. When negative charges are fixed to the edges of natural graphite, the resulting anionic graphite shows negative charging in a polar solvent. This enhanced negative charging is assumed to contribute the exfoliation of graphite during liquid-phase exfoliation (LPE). In this study, we prepared large anionic graphite flakes (~10 μm) by salt-assisted ball milling, as well as natural graphite flakes of the same size for comparison. During the LPE process, centrifugation speed and solvent type have dominant effects on graphene concentration and quality (e.g., size and thickness), so we investigated these factors for anionic graphite flakes in detail. The anionic graphite showed higher exfoliation efficiency in every type of solvent (isopropanol, methyl ethyl ketone, acetone, and water-based cosolvent) compared with the natural graphite. Monolayer graphene, with an average size of 80–200 nm, was obtained with relatively high yield (&gt;10%) at only 3 min of sonication. The small size of graphene was due to edge fragmentation during the LPE process. The recyclability of the sediment and the characterization of the exfoliated powders for anionic graphene were also investigated.