Two-dimensional (2D) transition metal carbides and nitrides (MXenes) have shown outstanding performances in electrochemical energy storage and many other applications. Delamination of MXene flakes in water produces colloidal solutions that are used to manufacture all kinds of products (thin films, coatings, and electrodes, etc.). However, the stability of MXene colloidal solutions, which is of critical importance to their application, remains largely unexplored. Here we report on the degradation of delaminated-Ti3C2Tx colloidal solutions (T represents the surface functionalities) and outline protocols to improve their stability. Ti3C2Tx MXene solutions in open vials degraded by 42%, 85%, and 100% after 5, 10, and 15 days, respectively, leading to the formation of cloudy-white colloidal solutionss containing primarily anatase (TiO2). On the other hand, the solution could be well-preserved when Ti3C2Tx MXene colloidal solutionss were stored in hermetic Ar-filled bottles at 5 °C, because dissolved oxygen, the main oxidant of the MXene flakes, was eliminated. Under such a recipe, the time constant of the solution was dramatically increased. We have found that the degradation starts at the edges and its kinetics follows the single-exponential decay quite well. Moreover, we performed size selection of the MXene solution via a cascade technique and showed that the degradation process is also size-dependent, with the small flakes being the least stable. Furthermore, a dependence between the degradation time constants and the flake size allows us to determine the size of the nanosheets in situ from UV–vis spectra and vice versa. Finally, the proposed method of storing the MXene colloidal solution in Ar-filled vials was applied to Ti2CTx to improve its stability and time constant, demonstrating the validity of this protocol in improving the lifetime of different MXene solutions.

2D transition-metal carbides and nitrides, known as MXenes, have displayed promising properties in numerous applications, such as energy storage, electromagnetic interference shielding, and catalysis. Titanium carbide MXene (Ti3C2Tx), in particular, has shown significant energy-storage capability. However, previously, only micrometer-thick, nontransparent films were studied. Here, highly transparent and conductive Ti3C2Tx films and their application as transparent, solid-state supercapacitors are reported. Transparent films are fabricated via spin-casting of Ti3C2Tx nanosheet colloidal solutions, followed by vacuum annealing at 200 °C. Films with transmittance of 93% (≈4 nm) and 29% (≈88 nm) demonstrate DC conductivity of ≈5736 and ≈9880 S cm−1, respectively. Such highly transparent, conductive Ti3C2Tx films display impressive volumetric capacitance (676 F cm−3) combined with fast response. Transparent solid-state, asymmetric supercapacitors (72% transmittance) based on Ti3C2Tx and single-walled carbon nanotube (SWCNT) films are also fabricated. These electrodes exhibit high capacitance (1.6 mF cm−2) and energy density (0.05 µW h cm−2), and long lifetime (no capacitance decay over 20 000 cycles), exceeding that of graphene or SWCNT-based transparent supercapacitor devices. Collectively, the Ti3C2Tx films are among the state-of-the-art for future transparent, conductive, capacitive electrodes, and translate into technologically viable devices for next-generation wearable, portable electronics.

•Large quantities of high quality, two-dimensional (2D) vanadium pentoxide (V2O5) nanosheet (NS) have been exfoliated in environmental friendly solvents.•Large-area, flexible and binder-free V2O5 NS/SWCNT hybrid electrodes have been efficiently fabricated through programmed aerosol printing technique.•The interlayer spacing of 2D orthorhombic V2O5 NS can be well manipulated, ranging from 4.4Å to 11.5 Å in ethanol and water-based colloidal solutions, respectively.•The SWCNT further improves the reversible phase transition reactions of the V2O5 NS and maintains the structural integrity of the electrodes.•The flexible printed V2O5 NS/SWCNT electrode has demonstrated a high discharge capacity of 370 mA h g−1 at 0.05 C, high energy density (1110 W h kg−1) and power density (4350 W kg−1), etc.With a layered crystal structure and good Li+ storage performance, vanadium pentoxide (V2O5) is potentially a high-energy and cost-effective cathode material for Li-ion batteries (LiBs). Networks of two-dimensional V2O5 nanosheets (2D V2O5 NS), with large interlayer distance, are ideal for enhancing the Li+ diffusion kinetics and thus for building high power LiBs. However, the lack of a simple, scalable and environmentally friendly route to nanosheet production still hinders the development of V2O5 applications. Here we demonstrate, liquid-phase exfoliation (LPE) of commercial V2O5 powder in environmental friendly solvents (water and ethanol) to achieve large quantities of 2D V2O5 NS dispersions. The V2O5 NS are of high-quality whose interlayer spacing can be well manipulated, ranging from 4.4Å to 11.5 Å in ethanol and water (forming NS xerogel), respectively. Ultrasonic aerosol printing of V2O5 NS xerogel/single-wall carbon nanotube (SWCNT) blended dispersions resulted in large-area, flexible, and binder-free hybrid electrodes, which showcase a high discharge capacity of 370 mA h g−1 at 0.05 C, high energy density (555 W h kg−1) and power density (2175 W kg−1), etc. These properties can be attributed to the synergistic effects between the expanded hydrated NS and the conductive SWCNT matrix; the latter improves the reversible phase transition reactions of the NS, enhances the ion diffusion kinetics, maintains the electrode's mechanical integrity and provides electron transport pathways. The Li+ storage mechanism was investigated, suggesting the capacity was majorly contributed by the non-diffusion controlled process (pseudocapacitive). We believe the LPE/aerosol printing approach is environmentally green, general and scalable, and could be extended to other layered transitional metal oxides or dichalcogenides for fabrication of corresponding flexible, binder-free, conductive composites for energy storage systems.Large quantities of two-dimensional V2O5 orthorhombic nanosheets have been exfoliated in environmental friendly solvents. The interlayer distance of the nanosheets can be well manipulated. The large-area, binder-free, flexible V2O5/SWCNT composite, which can be efficiently fabricated through an aerosol printing technique, well explores the synergistic effects of broadened interlayer distance of V2O5 and electrical, mechanical properties of SWCNT. As a result, high capacity and excellent energy and power density Li-ion batteries have been achieved.Download high-res image (470KB)Download full-size image

We describe the soft chemistry synthesis of amine-templated gallium chalcogenide nanotubes through the reaction of gallium(III) acetylacetonate and the chalcogen (sulfur, selenium) using a mixture of long-chain amines (hexadecylamine and dodecylamine) as a solvent. Beyond their role as solvent, the amines also act as a template, directing the growth of discrete units with a one-dimensional multilayer tubular nanostructure. These new materials, which broaden the family of amine-stabilized gallium chalcogenides, can be tentatively classified as direct large band gap semiconductors. Their preliminary performance as active material for electrodes in lithium ion batteries has also been tested, demonstrating great potential in energy storage field even without optimization.

MoSe2 nanoplatelets were synthesised by liquid phase exfoliation. MoSe2 nanoplatelets and MoSe2 nanoplatelets/SWCNTs electrodes were manufactured using a spray-deposition method and their charge storage properties were investigated in a 1 M LiPF6/EC/DMC electrolyte in a three-electrode cell and a 3-0 V electrochemical window. MoSe2 nanoplatelets electrodes showed charge storage activity without the use of an electrically conductive additive. However, their rate performance and cycling stability was poor, which was improved by adding SWCNTs. The MoSe2 nanoplatelets/SWCNTs electrodes showed a capacity of 798 mAh g−1 at a 0.05C rate (cycle 8), much higher than the capacity of a bulk MoSe2/carbon black electrodes of 557.0 mAh g−1 at 0.05C (cycle 1). An improved rate performance, respect the bulk counterpart, from 0.1C (569 mAh g−1) to 5C (153 mAh g−1) rates, and a capacity retention of 58 % over 100 cycles at a 0.5C rate were achieved. The improved performance of the MoSe2 nanoplatelets/SWCNTs electrodes was attributed to the nanosize of the MoSe2 nanoplatelets and the combination with SWCNTs that provided mechanical stability.

This work describes silicon nanoparticle-based lithium-ion battery negative electrodes where multiple nonactive electrode additives (usually carbon black and an inert polymer binder) are replaced with a single conductive binder, in this case, the conducting polymer PEDOT:PSS. While enabling the production of well-mixed slurry-cast electrodes with high silicon content (up to 95 wt %), this combination eliminates the well-known occurrence of capacity losses due to physical separation of the silicon and traditional inorganic conductive additives during repeated lithiation/delithiation processes. Using an in situ secondary doping treatment of the PEDOT:PSS with small quantities of formic acid, electrodes containing 80 wt % SiNPs can be prepared with electrical conductivity as high as 4.2 S/cm. Even at the relatively high areal loading of 1 mg/cm2, this system demonstrated a first cycle lithiation capacity of 3685 mA·h/g (based on the SiNP mass) and a first cycle efficiency of ∼78%. After 100 repeated cycles at 1 A/g this electrode was still able to store an impressive 1950 mA·h/g normalized to Si mass (∼75% capacity retention), corresponding to 1542 mA·h/g when the capacity is normalized by the total electrode mass. At the maximum electrode thickness studied (∼1.5 mg/cm2), a high areal capacity of 3 mA·h/cm2 was achieved. Importantly, these electrodes are based on commercially available components and are produced by the standard slurry coating methods required for large-scale electrode production. Hence, the results presented here are highly relevant for the realization of commercial LiB negative electrodes that surpass the performance of current graphite-based negative electrode systems.Keywords: anode; battery; binder; conducting additive; conducting polymer; negative electrode; PEDOT:PSS; silicon

•Highly transparent ruthenium oxide/poly(3,4-ethylenedioxythiophene): poly(styrene-4-sulfonate), (RuO2/PEDOT:PSS) hybrid thin films have been successfully fabricated.•The hybrid thin film shows remarkably high transparency (93%), high conductivity (σDC =279 S/cm), excellent volumetric capacitance (CV =190 F/cm3) and areal capacitance of C/A=1.2 mF/cm2.•Transparent, flexible devices have been fabricated with excellent electrochemical performances, such as C/A=0.84 mF/cm2 and 100% capacitance retention over 10,000 charge/discharge cycles.•Large-area transparent supercapacitor device has been built.Transparent, conductive electrodes are important in many applications such as touch screens, displays and solar cells. Transparent energy storage systems will require materials that can simultaneously act as current collectors and active storage media. This is challenging as it means improving the energy storage capability of conducting materials while retaining transparency. Here, we have used aerosol-jet spraying strategy to prepare transparent supercapacitor electrodes from ruthenium oxide/poly(3,4-ethylenedioxythiophene): poly(styrene-4-sulfonate), (RuO2/PEDOT: PSS) hybrid thin films. These films combine excellent transparency with reasonably high conductivity (DC conductivity =279 S/cm) and excellent volumetric capacitance (CV =190 F/cm3). We demonstrate electrodes with historical high transparency of 93% which display an areal capacitance of C/AC/A=1.2 mF/cm2, significantly higher than the rest reported electrodes with comparable transparency. We have assembled flexible, transparent, solid-state symmetric devices which exhibit T  =80% and C/AC/A=0.84 mF/cm2 and are stable over 10,000 charge/discharge cycles. Asymmetric solid-state device with RuO2/PEDOT: PSS and PEDOT: PSS thin films as positive and negative electrodes, respectively, display an areal capacitances of 1.06 mF/cm2, a maximum power density (P/A)(P/A) of 147 μW/cm2 and an energy density (E/A)(E/A) of 0.053 μWh/cm2. Furthermore, large area transparent solid-state supercapacitor device has been built. We believe the solution-processed transparent films could be easily scaled-up to meet the industrial demands.The aerosol-jet spraying of blended dispersion results in RuO2/PEDOT: PSS hybrid thin film, which explores the synergistic effects of high DC conductivity and low optical conductivity of PEDOT: PSS, coupled with high pseudocapacitance from RuO2 nanoparticles. Consequently, the hybrid thin films demonstrate excellent transparency and areal capacitance in transparent, flexible, solid-state supercapacitors.

Liquid-phase exfoliation of layered materials offers a large-scale approach toward the synthesis of 2D nanostructures. Structural properties of materials can however change during transition from bulk to the 2D state. Any such changes must be examined and understood for successful implementation of 2D nanostructures. In this work, we demonstrate nonbulk stacking sequences in the few-layer MoS2 and WS2 nanoflakes produced by liquid-phase exfoliation. Our analysis shows that nonbulk stacking sequences can be derived from its bulk counterparts by translational shifts of the layers. No structural changes within the layers were observed. Twenty-seven MoS2 and five WS2 nanoflakes were imaged and analyzed. Nine MoS2 and four WS2 nanoflakes displayed nonbulk stacking. Such dominance of the nonbulk stacking suggests high possibility of unusual stacking sequences in other 2D nanostructures. Notably, the electronic structure of some non bulk stacked bilayers presents characteristics which are uncommon to either the bulk phase or the single monolayer, for instance, a spin-split conduction band bottom. Our main characterization technique was annular dark-field scanning transmission electron microscopy, which offers direct and reliable imaging of atomic columns. The stacking characterization approach employed here can be readily applied toward other few-layer transition metal chalcogenides and oxides.Keywords: aberration-corrected scanning transmission electron microscopy; MoS2; transition metal dichalcogenides; two-dimensional nanomaterials; WS2

Structural characterization of 2D nanomaterials is an important step towards their future applications. In this work we carried out imaging and structural analysis of 2D h-BN produced by chemical-exfoliation, emphasizing the stacking order in few-layer sheets. Our analysis, for the first time has shown conclusively that non-bulk stacking can exist in 2D h-BN.

Versatile routes to functionalise few-layer graphene and hBN with nickel phthalocyanine (Ni-Pc) were achieved using liquid phase exfoliation and sonication methods. EDX performed on the graphene//Ni-Pc specimen showed nickel on the flake surface whilst Raman spectroscopy revealed a prominent D peak indicating the presence of basal plane defects. New Raman active modes were also found in the hBN//Ni-Pc complex. X-ray photoelectron spectroscopy showed a charge transfer for both graphene and hBN confirming that the flake edges and basal-planes were covalently functionalised by the phthalocyanine molecules. Transmission electron microscopy confirmed the only presence of single and few-layer flakes of hBN and graphene in solution, demonstrating that the exfoliation yield was not affected by the functionalisation step. We therefore proved that tuning of the electronic and optical properties of graphene and hBN nanosheets is indeed conceivable. We used the Z-scan technique to prove the nonlinear optical (NLO) behaviour of the functionalised graphene sheets. Based on such optical response, we demonstrate an optical limiting effect for nanosecond laser pulses at 532 nm, proving these materials to be a suitable candidate for photonic and optoelectronic applications.

Versatile routes to functionalise few-layer graphene and hBN with nickel phthalocyanine (Ni-Pc) were achieved using liquid phase exfoliation and sonication methods. EDX performed on the graphene//Ni-Pc specimen showed nickel on the flake surface whilst Raman spectroscopy revealed a prominent D peak indicating the presence of basal plane defects. New Raman active modes were also found in the hBN//Ni-Pc complex. X-ray photoelectron spectroscopy showed a charge transfer for both graphene and hBN confirming that the flake edges and basal-planes were covalently functionalised by the phthalocyanine molecules. Transmission electron microscopy confirmed the only presence of single and few-layer flakes of hBN and graphene in solution, demonstrating that the exfoliation yield was not affected by the functionalisation step. We therefore proved that tuning of the electronic and optical properties of graphene and hBN nanosheets is indeed conceivable. We used the Z-scan technique to prove the nonlinear optical (NLO) behaviour of the functionalised graphene sheets. Based on such optical response, we demonstrate an optical limiting effect for nanosecond laser pulses at 532 nm, proving these materials to be a suitable candidate for photonic and optoelectronic applications.