Yunxiong Zeng

Monolayer MoS2 with S vacancies from interlayer spacing expanded counterparts for highly efficient electrochemical hydrogen production

It is challenging to prepare monolayer MoS2 with activated basal planes in a simple and efficient way. In this study, an interlayer spacing expanded counterpart, ammonia-intercalated MoS2, was obtained by a simple hydrothermal reaction of ammonium molybdate and elemental sulfur in hydrazine monohydrate solution. Then, the ammonia-intercalated MoS2 could be easily exfoliated by ultrasonication to get monolayer MoS2. Importantly, this monolayer MoS2 possessed rich S vacancies. The produced MoS2 demonstrated a proliferated active site density as well as low-loss electrical transport for efficient electrochemical hydrogen production from water. As expected, the monolayer MoS2 with S vacancies exhibited an excellent electrocatalytic hydrogen evolution reaction performance with a low overpotential (at 10 mA cm−2) of 160 mV (V vs. RHE) in acid media and a small Tafel slope of 54.9 mV dec−1. Furthermore, the catalyst displayed a good long-term stability and chemical stability during the electrochemical hydrogen production process. Computational studies prove that the S vacancies enabled the inert basal planes by introducing localized donor states into the bandgap and lowered the hydrogen adsorption free energy. This study could open new opportunities for the rational design and a better understanding of structure–property relationships of MoS2-based catalysts for water splitting or other applications.

A three-dimensional graphitic carbon nitride belt network for enhanced visible light photocatalytic hydrogen evolution

Three-dimensional (3D) network-like graphitic carbon nitride nanobelts (g-C3N4 NBs) were facilely achieved by the hydrothermal treatment of bulk g-C3N4 in a medium strong oxalic acid solution (1 M, pH 0.89). The positions of the conduction band (CB) and valence band (VB) were upraised from −0.90 and +1.86 eV for bulk g-C3N4 to −0.92 and +1.92 eV for g-C3N4 NB networks with enhanced redox ability, respectively. With an optimized Pt loading of 3%, the g-C3N4 NB networks showed excellent visible-light photocatalytic H2 production activity (1360 μmol g−1 h−1), which was 10.9 times higher than that of optimized 2% Pt@bulk g-C3N4 (124.7 μmol g−1 h−1) using triethanolamine as a sacrificial agent. Furthermore, Pt@g-C3N4 NBs exhibited a considerable rate of H2 evolution of 33.3 μmol g−1 h−1, much higher than 1.79 μmol g−1 h−1 for Pt@bulk g-C3N4 in distilled water without any sacrificial agents, revealing a great potential for photocatalytic overall water splitting. This outstanding performance not only originates from its unique 3D nanostructure and prolonged electron lifetime, but also from the electronic structure modulation and improved redox capacities of the CB and VB. The pH effect of hydrothermal conditions on the g-C3N4 molecular structure, chemical elements, optical properties and catalytic performance is also expounded. This study demonstrates a facile and environmentally friendly strategy to design highly efficient g-C3N4 catalysts for potential applications in solar energy driven photocatalytic water splitting.

A bamboo-inspired hierarchical nanoarchitecture of Ag/CuO/TiO2 nanotube array for highly photocatalytic degradation of 2,4-dinitrophenol

The optimized geometrical configuration of muitiple active materials into hierarchical nanoarchitecture is essential for the creation of photocatalytic degradation system that can mimic natural photosynthesis. A bamboo-like architecture, CuO nanosheets and Ag nanoparticles co-decorated TiO2 nanotube arrays (Ag/CuO/TiO2), was fabricated by using simple solution-immersion and electrodeposition process. Under simulated solar light irradiation, the 2,4-dinitrophenol (2,4-DNP) photocatalytic degradation rate over Ag/CuO/TiO2 was about 2.0, 1.5 and 1.2 times that over TiO2 nanotubes, CuO/TiO2 and Ag/TiO2, respectively. The enhanced photocatalytic activity of ternary Ag/CuO/TiO2 photocatalyst was ascribed to improved light absorption, reduced carrier recombination and more exposed active sites. Moreover, the excellent stability and reliability of the Ag/CuO/TiO2 photocatalyst demonstrated a promising application for organic pollutant removal from water.

Fe2P/reduced graphene oxide/Fe2P sandwich-structured nanowall arrays: a high-performance non-noble-metal electrocatalyst for hydrogen evolution

Transition metal phosphides (TMPs) have been one of the ideal candidates as low-cost and high-efficiency catalysts for hydrogen evolution reactions (HERs). We report herein a novel TMP architecture, Fe2P nanoparticles/reduced graphene oxide (rGO) nanosheets/Fe2P nanoparticles (Fe2P@rGO) sandwich-structured (Fe2P@rGO) nanowall arrays on a Ti plate. This nanostructure was easily prepared via one-step electrodeposition followed by a low-temperature phosphidation reaction. The Fe2P@rGO nanowall array film is featured with maximally exposed catalytic sites, fast electron and mass transport, and robust structure stability, and therefore it behaves as an excellent HER electrocatalyst. The Fe2P@rGO shows a low overpotential of 101 mV at a current density of 10 mA cm−2 and a small Tafel slope of 55.2 mV dec−1 with a large exchange current density of 0.146 mA cm−2. Furthermore, the catalyst exhibits superior durability evidenced by about 87% catalytic activity retention against about 55% for the commercial Pt/C catalyst after a 12 h test. The study presents a new nanoengineering strategy for high-performance TMP-based HER catalysts.

Porous nitrogen-rich carbon materials from carbon self-repairing g-C3N4 assembled with graphene for high-performance supercapacitor

Nitrogen-rich carbon material derived from carbon self-repairing g-C3N4 is self-assembled with graphene oxide (GO) to form a porous structure. Different from the pristine g-C3N4, which has scarcely been employed in supercapacitors because of its low charge mobility, the carbon self-repairing g-C3N4 (C-C3N4) shows an improved electrochemical activity. After carbon-repairing, a delocalized big π-bond can be formed by the homogeneous C-substitution for N atoms or the formation of new interstitial C–N bond. The extending π-conjugation planar layer of C-C3N4 possesses a closer contact with GO to form a three-dimensional (3D) pore structure, which ensures good mobility for electrons and quick access for electrolytes. Under the optimum C-repairing content of 5.99 at%, the C-C3N4@rGO exhibited high specific capacity of 379.7 F g−1 and energy density of 52.7 W h kg−1 at a current density of 0.25 A g−1. Moreover, the electrode kept 85% capacity retention after 10 000 cycles at a high constant current density of 10 A g−1. The active sites of pseudocapacitance can be confirmed in the oxygen-containing groups and the carbon atoms close to the nitrogen by the XPS results.

MoS2 Quantum Dot Growth Induced by S Vacancies in a ZnIn2S4 Monolayer: Atomic-Level Heterostructure for Photocatalytic Hydrogen Production

It is highly demanded to steer the charge flow in photocatalysts for efficient photocatalytic hydrogen reactions (PHRs). In this study, we developed a smart strategy to position MoS2 quantum dots (QDs) at the S vacancies on a Zn facet in monolayered ZnIn2S4 (Vs-M-ZnIn2S4) to craft a two-dimensional (2D) atomic-level heterostructure (MoS2QDs@Vs-M-ZnIn2S4). The electronic structure calculations indicated that the positive charge density of the Zn atom around the sulfur vacancy (Vs) was more intensive than other Zn atoms. The Vs confined in monolayered ZnIn2S4 established an important link between the electronic manipulation and activities of ZnIn2S4. The Vs acted as electron traps, prevented vertical transmission of electrons, and enriched electrons onto the Zn facet. The Vs-induced atomic-level heterostructure sewed up vacancy structures of Vs-M-ZnIn2S4, resulting in a highly efficient interface with low edge contact resistance. Photogenerated electrons could quickly migrate to MoS2QDs through the intimate Zn-S bond interfaces. As a result, MoS2QDs@Vs-M-ZnIn2S4 showed a high PHR activity of 6.884 mmol g-1 h-1, which was 11 times higher than 0.623 mmol g-1 h-1 for bulk ZnIn2S4, and the apparent quantum efficiency reached as high as 63.87% (420 nm). This work provides a prototype material for looking into the role of vacancies between electronic structures and activities in 2D photocatalytic materials and gives insights into PHR systems at the atomic level.

CdS‐Nanoparticles‐Decorated Perpendicular Hybrid of MoS2 and N‐Doped Graphene Nanosheets for Omnidirectional Enhancement of Photocatalytic Hydrogen Evolution

A new hierarchical nanoarchitecture with integration of multiple active materials has been developed for highly efficient hydrogen evolution reaction (HER), by decorating a perpendicular hybrid of MoS2 and N‐doped graphene nanosheets with CdS nanoparticles. The unique architecture promoted light trapping and absorption for highly efficient light harvesting and photocarrier generation, and offered an unblocked electron transport pathway for rapid charge separation/transport to suppress charge recombination. Its high surface area and high density of active sites result in highly efficient utilization of photogenerated carriers for productive HER. Significantly, without using noble metals as co‐catalysts, the photocatalysts demonstrated rapid HER rates as high as 5.01 mmol h−1 g−1 under visible‐light irradiation, which was approximately 25 times that of pure CdS. The hydrogen production remained stable after a continued test for 30 h, showing an exceedingly high performance and superior stability.