Photocatalytic hydrogen evolution from water has received enormous attention due to its ability to address a number of global environmental and energy-related issues. Here, we synthesize 2D/2D Ti3C2/g-C3N4 composites by electrostatic self-assembly technique and demonstrate their use as photocatalysts for hydrogen evolution under visible light irradiation. The optimized Ti3C2/g-C3N4 composite exhibited a 10 times higher photocatalytic hydrogen evolution performance (72.3 μmol h-1 gcat-1) than that of pristine g-C3N4 (7.1 μmol h-1 gcat-1). Such enhanced photocatalytic performance was due to the formation of 2D/2D heterojunctions in the Ti3C2/g-C3N4 composites. The intimate contact between the monolayer Ti3C2 and g-C3N4 nanosheets promotes the separation of photogenerated charge carriers at the Ti3C2/g-C3N4 interface. Furthermore, the ultrahigh conductivity of Ti3C2 and the Schottky junction formed between g-C3N4/MXene interfaces facilitate the photoinduced electron transfer and suppress the recombination with photogenerated holes. This work demonstrates that the 2D/2D Ti3C2/g-C3N4 composites are promising photocatalysts thanks to the ultrathin MXenes as efficient co-catalysts for photocatalytic hydrogen production.

2D/2D heterojunction of Ti 3 C 2 /g-C 3 N 4 nanosheets for enhanced photocatalytic hydrogen evolution.

Titanium dioxide (TiO2) represents a promising candidate for hydrogen production via photocatalysis. However, its large bandgap and fast charge recombination limits its efficiency. To overcome this...

Monolayer Ti3C2Tx as an Effective Co-catalyst for Enhanced Photocatalytic Hydrogen Production over TiO2

Vanadium dioxide (VO 2 ) is a promising material for developing energy-saving “smart windows,” owing to its infrared thermochromism induced by metal-insulator transition (MIT). However, its practical application is greatly limited by its relatively high critical temperature (~68°C), low luminous transmittance ( 2 film. With a solid electrolyte layer assisting gating treatment, we modulated the insertion/extraction of hydrogen into/from the VO 2 lattice at room temperature, causing tristate phase transitions that enable control of light transmittance. The dramatic increase in visible/infrared transmittance due to the phase transition from the metallic (lightly H-doped) to the insulating (heavily H-doped) phase results in an increased solar energy regulation ability up to 26.5%, while maintaining 70.8% visible luminous transmittance. These results break all previous records and exceed the theoretical limit for traditional VO 2 smart windows, making them ready for energy-saving utilization.

Gate-controlled VO2 phase transition for high-performance smart windows

n-Type SnSe2 Oriented-Nanoplate-Based Pellets for High Thermoelectric Performance

The progress of the Internet-of-Things in the past few years has necessitated the support of high-performance sensors. Schottky-contacted nanowire sensors have attracted considerable attention owing to their high sensitivity and fast response time. Their progress is reviewed here, based on several kinds of important nanowires, for applications such as bio/chemical sensors, gas sensors, photodetectors, and strain sensors. Although Schottky-contacted nanowire sensors deliver excellent performance in these fields, they can be further improved by various methods, including defect engineering, surface modification, the piezotronic effect, and the piezophototronic effect, all of which are discussed here. With regard to practical applications, further efforts are required to address challenges such as the stability, selectivity, ultrafast response, multifunctionality, flexibility, distributed energy supply, and sustainability of Schottky-contacted nanowire sensors. Finally, future perspectives and solutions are discussed.

Schottky-Contacted Nanowire Sensors.

Sensitive H2 gas sensors are highly desirable for the prediction and early-warning of H2 leakage. Low-dimensional nanostructures of metal oxide semiconductor emerge as promising materials candidates, but it remains a challenge to preserve the nanostructures in the real sensors. In this work, we demonstrated highly sensitive H2 gas sensors based on porous network of SnO2 nanowires that exhibited ultrasmall diameter ∼ 2 nm. Colloidal SnO2 nanowires synthesized via a solvothermal process were drop-coated onto the commercial alumina substrates, followed by in-situ annealing treatment at 350 °C to remove the surface ligands. The sensors exhibited sensitive response with linear dependence on the H2 gas concentration ranging from 2 ppm to 100 ppm when operated at 250 °C. Typically, the sensor had a response of 13 toward 40 ppm of H2, with the response and recovery time being 15 s and 31 s, respectively. To further improve the sensor performance, Palladium doped SnO2 nanowires were thoroughly investigated. It’s shown that, the operating temperature of the sensor decreased from 250 °C to 150 °C after Pd doping, and the response and recovery time decreased to 6 s/3 s. The superb sensitivity was attributed to the enhanced gas reception, electron transport as well as utility factor owing to the network nanostructure of ultrathin SnO2 nanowires and catalytic activity of Pd, according to theoretical calculation and adsorption kinetics studies. Combined with the excellent solution processability, the colloidal SnO2 nanowires are potentially attractive for next-generation gas sensors with lower power consumption and integration with silicon-based substrates.

Sensitive H2 gas sensors based on SnO2 nanowires

This study has investigated the thermoelectric property of a layered metal dichalcogenide SnSe2, which is also non-toxic and earth abundant, especially with a similar composition of SnSe whose single crystals are recently revealed to possess high thermoelectricity. By investigating the electrical and thermal transport properties of Sn1−x
 Ag
 x
 Se2 (x = 0.00, 0.01, 0.03 and 0.05) in this work, it is revealed that the Ag-doped SnSe2 has the potential as a good thermoelectric material used at mid-temperature, benefiting from its relatively low thermal conductivity below 1.0 Wm−1 K−1 and moderate power factor over 350 μWm−1 K−2 at 773 K. A ZT value of ~0.4 can be finally achieved for the composition of Sn0.99Ag0.01Se2 at 773 K, which is an order of magnitude higher than the undoped ones and also larger than the reported metal dichalcogenides compounds of TiS2 and WS2. The present work also reveals that Ag-doped sample shows much better thermal stability over 700 K compared with the undoped one whose measured temperature is only limited to 573 K, even with a few mole doping content of 1%.

Ag-doped SnSe2 as a promising mid-temperature thermoelectric material

In this paper, we reported comparative study of the humidity characteristics of graphene/silver nanoparticles composite (Gr-AgNps) and graphene/silver nanoparticles/PMMA composite (Gr-AgNps-PMMA) based efficient humidity sensors. Aqueous solution of Gr-AgNps and Gr-AgNps-PMMA was drop casted over interdigitated copper electrodes with 50 μm gap embedded in the substrates in dust free environment. The band gap obtained from the UV–vis spectra for Gr-AgNps and Gr-AgNps-PMMA based humidity sensors was 4.7 and 4.1 eV respectively. The capacitive and resistive humidity response was studied using LCR meter (GW Instek817). Apparent increase in capacitance was observed (100–10,000 nF) with the increase in the humidity percentage (30–95%RH) at lower frequencies for both the sensors. Resistance of the sensors dropped to zero as the humidity level is increased from 30 to 95%RH in the chamber. The devices were tested for real time stability and for fast response/recovery time. Both the devices showed an excellent stability and response by recording their resistance and capacitance respectively. A lagging of RH decreasing response from RH increasing response was observed at 500 Hz frequency for both the sensors depicted from the hysteresis curve. The humidity response of Gr-AgNps was comparatively better than that of the Gr-AgNps-PMMA based humidity sensors.

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Capacitive and resistive response of humidity sensors based on graphene decorated by PMMA and silver nanoparticles

GaN nanowall network was epitaxially grown on Si (111) substrate by molecular beam epitaxy. GaN nanowalls overlap and interlace with one another, together with large numbers of holes, forming a continuous porous GaN nanowall network. The width of the GaN nanowall can be controlled, ranging from 30 to 200 nm by adjusting the N/Ga ratio. Characterization results of a transmission electron microscope and X-ray diffraction show that the GaN nanowall is well oriented along the C axis. Strong band edge emission centered at 363 nm is observed in the spectrum of room temperature photoluminescence, indicating that the GaN nanowall network is of high quality. The sheet resistance of the Si-doped GaN nanowall network along the lateral direction was 58 Ω/. The conductive porous nanowall network can be useful for integrated gas sensors due to the large surface area-to-volume ratio and electrical conductivity along the lateral direction by combining with Si micromachining.

/pdf/growth-of-gan-nanowall-network-on-si-111-substrate-by-55guvzmb8z.pdf

Aihua Zhong

Papers

Sensitive H2 gas sensors based on SnO2 nanowires

Ag-doped SnSe2 as a promising mid-temperature thermoelectric material

Capacitive and resistive response of humidity sensors based on graphene decorated by PMMA and silver nanoparticles

Growth of GaN nanowall network on Si (111) substrate by molecular beam epitaxy

Comparative study of Schottky diode type hydrogen sensors based on a honeycomb GaN nanonetwork and on a planar GaN film