Two-dimensional (2D) transitional metal oxides (TMOs) are an attractive class of materials due to the combined advantages of high active surface area, enhanced electrochemical properties, and stability. Among the 2D TMOs, 2D tungsten oxide (WO) nanosheets possess great potential in electrochemical applications, particularly in electrochromic (EC) devices. However, feasible production of 2D WO nanosheets is challenging due to the innate 3D crystallographic structure of WO. Here we report a novel solution-phase synthesis of 2D WO nanosheets through simple oxidation from 2D tungsten disulfide (WS) nanosheets exfoliated from bulk WS powder. The complete conversion from WS into WO was confirmed through crystallographic and elemental analyses, followed by validation of the 2D WO nanosheets applied in the EC device. The EC device showed color modulation of 62.57% at 700 nm wavelength, which is 3.43 times higher than the value of the conventional device using bulk WO powder, while also showing enhancement of ∼46.62% and ∼62.71% in switching response-time (coloration and bleaching). The mechanism of enhancement was rationalized through comparative analysis based on the thickness of the WO components. In the future, 2D WO nanosheets could also be used for other promising applications such as sensors, catalysis, thermoelectric, and energy conversion.

The insufficient strategies to improve electronic transport, the poor intrinsic chemical activities, and limited active site densities are all factors inhibiting MXenes from their electrocatalytic applications in terms of hydrogen production. Herein, these limitations are overcome by tunable interfacial chemical doping with a nonmetallic electron donor, i.e., phosphorization through simple heattreatment with triphenyl phosphine (TPP) as a phosphorous source in 2D vanadium carbide MXene. Through this process, substitution, and/or doping of phosphorous occurs at the basal plane with controllable chemical compositions (3.83-4.84 at%). Density functional theory (DFT) calculations demonstrate that the PC bonding shows the lowest surface formation energy (ΔG Surf ) of 0.027 eV Å −2 and Gibbs free energy (ΔG H ) of -0.02 eV, whereas others such as P-oxide and PV (phosphide) show highly positive ΔG H . The P3-V 2 CT x treated at 500 °C shows the highest concentration of PC bonds, and exhibits the lowest onset overpotential of -28 mV, Tafel slope of 74 mV dec −1 , and the smallest overpotential of −163 mV at 10 mA cm −2 in 0.5 m H 2 SO 4 . The first strategy for electrocatalytically accelerating hydrogen evolution activity of V 2 CT x MXene by simple interfacial doping will open the possibility of manipulating the catalytic performance of various MXenes.

Identifying effective means to improve the electrocatalytic performance of transition metal dichalcogenides in alkaline electrolytes is a significant challenge. Herein, an advanced electrocatalyst possessing shells of molybdenum disulfide (MoS2) on molybdenum carbide (Mo2C) for efficient electrocatalytic activity in alkaline electrolytes is reported. The strained sheets of curved MoS2 surround the surface of Mo2C, turning the inactive basal planes of MoS2 into highly active electrocatalytic sites in the alkaline electrolyte. The van der Waals layers, which even possess van der Waals epitaxy along (100) facets of MoS2 and Mo2C, enhance the spin coupling between MoS2 and Mo2C, providing an easy electron transfer path for excellent electrocatalytic activity in alkaline electrolytes and solving the stability issue. In addition, it is found that curved MoS2 sheets on Mo2C show 3.45% tensile strain in the lattice, producing excellent catalytic activity for both oxygen reduction reaction (ORR) (with E1/2 = 0.60 V vs RHE) and oxygen evolution reaction (OER) (overpotential = 1.51 V vs RHE at 10 mA cm−2) with 60 times higher electrochemical active area than pristine MoS2. The unique structure and synthesis route outlined here provide a novel and efficient approach toward designing highly active, durable, and cost‐effective ORR and OER electrocatalysts.

Recent progress and research trends for 2D and 3D nanostructures in thermoelectric applications.

Films made from exfoliated graphene flakes have great potential in flexible thermoelectric devices, but are generally limited by the poor quality of flakes and the lack of effective n-doping strategies. The oxidative exfoliation routes typically employed to make reduced graphene oxide (rGO) reduce electrical conductivity because of defects in the basal plane, and typically creates p-type flakes because of the many remaining oxygenous groups. Here, an alternative synthesis strategy using non-oxidative intercalation and molecular adsorption is employed to create high-quality n-type graphene films. A film produced from these non-oxidized graphene flakes (NOGF) showed a Seebeck coefficient of −45.3 µV K −1 and electrical conductivity of 3280 S cm −1 at room temperature, both of which are significantly better than previously reported graphene thermoelectric films. This resulted in an extremely high power factor of 673 µW m −1 K −2 , the highest ever reported from a film made of any 2D material. The films were also shown to be extremely robust under bending conditions, with less than 3% electrical conductivity loss after 1000 bending cycles at a bending radius of 3 mm. Finally, the practicality of the films was demonstrated with a flexible thermoelectric device that generated 2.2 mV using only body heat.

In recent years, two-dimensional black phosphorus (BP) has seen a surge of research because of its unique optical, electronic, and chemical properties. BP has also received interest as a potential thermoelectric material because of its high Seebeck coefficient and excellent charge mobility, but further development is limited by the high cost and poor scalability of traditional BP synthesis techniques. In this work, high-quality BP is synthesized using a low-cost method and utilized in a PEDOT:PSS film to create the first ever BP composite thermoelectric material. The thermoelectric properties are found to be greatly enhanced after the BP addition, with the power factor of the film, with 2 wt % BP (36.2 μW m K) representing a 109% improvement over the pure PEDOT:PSS film (17.3 μW m K). A simultaneous increase of mobility and decrease of the carrier concentration is found to occur with the increasing BP wt %, which allows for both Seebeck coefficient and electrical conductivity to be increased. These results show the potential of this low-cost BP for use in energy devices.

To date, nanostructures of 3d-group transition metal (i.e., Fe, Co, Ni, etc.) derivatives show the highest electrocatalytic performance among non-noble-metal electrocatalysts for water splitting in acidic electrolyte. However, the poor electrochemical conductivity (∼10–4 S/cm) of nanostructures restricts practical application for overall electrocatalytic activity. Herein, continuously networked nanostructures of phase-tuned nickel sulfide foams for efficient water splitting electrocatalysts in both acidic and alkaline electrolytes are reported. Because continuously networked nanostructures of nickel sulfide foams possess an integral structure, they exhibit high electrochemical conductivity (∼1 S/cm), which eases adsorption/desorption of H+ and OH– ions for efficient overall water splitting. By tuning the stoichiometry of sulfur, four different phases of continuously networked nanostructures of nickel sulfides (αNiS, βNiS, Ni3S2, and Ni7S6) foams are formed by facile phase transformation of nickel. Among them, the Ni7S6 foam (Ni7S6-F) possesses superior electrocatalytic activity with extremely low overpotential of 70 mV (for hydrogen evolution reaction) and 1.37 V (for oxygen evolution reaction) at 10 mA/cm2 in acidic and alkaline medium, respectively, which is close to noble-metal-based electrocatalysts. As a result, this work demonstrates a facile synthesis route to optimize nickel sulfide electrocatalysts through phase-tuning and continuous networking for overall water splitting and would be applicable on other nanostructured electrocatalysts to improve their electrocatalytic activity for practical applications in future energy devices.

Solution-phase exfoliated graphene has always been an attractive material for flexible thermoelectric applications, but traditional oxidative routes suffer from poor flake quality and a lack of quality doping techniques to make complementary n-type and p-type films. Here, it is demonstrated that by changing the adsorbed surfactant during the intercalation-exfoliation process (polyvinylpyrrolidone for n-type, pyrenebutyric acid for p-type), both extremely high electrical conductivity (3010 and 2330 S cm −1 ) and high Seebeck coefficients (53.1 and −45.5 µV K −1 ) can be achieved. The result is that both of these films show remarkable power factors, over 600 µW m −1 K −2 at room temperature, which is over an order of magnitude better than that in previous works demonstrating complementary n-type and p-type graphene thermoelectric films. Based on these films, a full all-graphene thermoelectric device is constructed as a proof of concept, where a peak power of 5.0 nW is recorded at a temperature difference of 50 K.