ABSTRACT
Two-dimensional (2D) van der Waals (vdW) materials with tunable heterostructures and superior optoelectronic properties have opened a new platform for various applications, e.g., field-effect transistors, ultrasensitive photodetectors and photocatalysts. In this work, an InSe/InSe(Ge) (germanium doped InSe) vdW heterostructure is designed to improve the photoresponse performance of sole InSe in a photoelectrochemical (PEC)-type photodetector. Photoelectrochemical measurements demonstrated that this heterostructure has excellent photoresponse characteristics, including a photocurrent density of 9.8 µA cm-2, a photo-responsivity of 64 µA W-1, and a response time/recovery time of 0.128 s/0.1 s. Moreover, the measurements also revealed the self-powering capability and long-term cycling stability of this heterostructure. The electronic properties of the prepared pure and Ge-doped single crystals unveiled a negative and temperature-independent thermoelectric power and temperature-activated resistivity. The negative character of dominating charge carriers was confirmed by Hall measurements, which corroborated by electrical resistivity revealed a carrier concentration below â¼1015 cm-3 and an electron mobility of â¼500 cm2 V-1 s-1 in Ge-doped crystals. Additionally, the Mott-Schottky model explored the mechanism of charge transfer and enhanced PEC performance. Band bending at the InSe/InSe(Ge)-electrolyte interface benefits the separation and transformation of photogenerated carriers from the heterostructure to electrolyte due to the tunable energy band alignment. These results indicate that the InSe/InSe(Ge) vdW heterostructure is promising for PEC-type photodetectors, which provide a novel way to utilize 2D vdW heterostructures in optoelectronics.
ABSTRACT
Transition metal dichalcogenides (TMDs) are an intriguing family of materials with large application potential in a variety of scientific fields ranging from electronics to electrocatalysis. Within this group of materials, MoS2 has been attracting a lot of scientific attention due to its chemical and physical properties. In this report, we studied the exfoliation of the largely unexplored 3R MoS2 polytype prepared by high-temperature, high-pressure synthesis. Bulk as well as sodium naphthalenide exfoliated materials were studied in terms of their quality and performance for the hydrogen evolution reaction (HER). The HER performance was benchmarked versus the commonly available 2H polytype. The reported results show that the 3R polytype is more suitable for the conversion of MoS2 into the metallic 1T phase, which was attributed to surface oxidation occurring in the 2H polytype. Higher content of the 1T phase then resulted in an overall lower overpotential of -0.25 V vs. RHE for the 3R polytype compared with the overpotential of -0.30 V for the 2H polytype. These results show that the 3R polytype might serve as a better starting point for the synthesis of highly active chemically exfoliated MoS2 catalysts for hydrogen evolution.
ABSTRACT
The intercalation of cations into layered-structure electrode materials has long been studied in depth for energy storage applications. In particular, Li+ -, Na+ -, and K+ -based cation transport in energy storage devices such as batteries and electrochemical capacitors is closely related to the capacitance behavior. We have exploited different sizes of cations from aqueous salt electrolytes intercalating into a layered Nb2 CTx electrode in a supercapacitor for the first time. As a result, we have demonstrated that capacitive performance was dependent on cation intercalation behavior. The interlayer spacing expansion of the electrode material can be observed in Li2 SO4 , Na2 SO4 , and K2 SO4 electrolytes with d-spacing. Additionally, our results showed that the Nb2 CTx electrode exhibited higher electrochemical performance in the presence of Li2 SO4 than in that of Na2 SO4 and K2 SO4 . This is partly because the smaller-sized Li+ transports quickly and intercalates between the layers of Nb2 CTx easily. Poor ion transport in the Na2 SO4 electrolyte limited the electrode capacitance and presented the lowest electrochemical performance, although the cation radius follows Li+ >Na+ >K+ . Our experimental studies provide direct evidence for the intercalation mechanism of Li+ , Na+ , and K+ on the 2D layered Nb2 CTx electrode, which provides a new path for exploring the relationship between intercalated cations and other MXene electrodes.
