Longlu Wang

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.

Facile fabrication of mediator-free Z-scheme photocatalyst of phosphorous-doped ultrathin graphitic carbon nitride nanosheets and bismuth vanadate composites with enhanced tetracycline degradation under visible light

To realize the sustainable employment of solar energy in contaminant degradation and environmental recovery, design and development of an efficient photocatalyst is urgently needed. Herein, a novel direct Z-scheme composite photocatalysts consist of phosphorous-doped ultrathin g-C3N4 nanosheets (PCNS) and bismuth vanadate (BiVO4) are developed via a one-pot impregnated precipitation method. The as-prepared hybrid nanocomposite was utilized for the degradation tetracycline (TC) under visible light irradiation. Among the composites with various PCNS/BiVO4 ratios, the prepared PCNS/BVO-400 photocatalyst presents the best performance, showing a TC (10mg/L) removal efficiency of 96.95% within 60min, more than double that of pristine BiVO4 (41.45%) and higher than that of pure PCNS (71.78%) under the same conditions. The effects of initial TC concentration, catalyst dosage, pH value and different water sources have been studied in detail. The improved photocatalytic performance of the as-prepared PCNS/BiVO4 nanocomposites could be attributed to the promoted separation efficiency of the photogenerated electrons and the enhanced charge carrier lifetime (1.65ns) owing to the synergistic effect between the PCNS and BiVO4. The degradation intermediates and decomposition pathway of TC were also analyzed and proposed. Additionally, radical trapping experiments and ESR measurement indicated that the photogenerated holes (h+), superoxide radical (O2-) and hydroxyl radical (OH) all participated in the TC removal procedure in the reaction system. The high performance of PCNS/BVO-400 in real wastewater indicated the potential of the prepared composite in practical application. This work provides an efficient and promising approach for the formation of high performance Z-scheme photocatalyst and study the possibility for real wastewater treatment.

Cracked monolayer 1T MoS2 with abundant active sites for enhanced electrocatalytic hydrogen evolution

Molybdenum disulfide (MoS2) is a promising non-precious-metal catalyst, but its performance is limited by its density of active sites and poor electrical transport. Here, we report the design and preparation of cracked monolayer 1T MoS2 with a porous structure through the ultrasonication enhanced lithium intercalation of hydrothermally synthesized MoS2 nanosheets. The unique resulting catalyst can have more active sites introduced via the formation of porosity within the monolayer nanosheet, and the electrical transport ability can be increased through the change in electronic states from semiconducting in the 2H phase to metallic in the 1T phase. As is expected, the cracked monolayer 1T MoS2 exhibited good durability and an excellent hydrogen evolution reaction performance with a low overpotential (at 10 mA cm−2) of 156 mV (V vs. RHE) in acid media and a small Tafel slope of 42.7 mV dec−1. This work will provide an intriguing and effective approach to designing electrocatalysts based on MoS2 or other layered materials with enhanced HER performance.

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.

Hollow Microsphere TiO2/ZnO p–n Heterojuction with High Photocatalytic Performance for 2,4-Dinitropheno Mineralization

A unique hollow microsphere TiO2/ZnO p–n heterojuction was successfully fabricated via a one-step hydrothermal method for degradation of 2,4-Dinitrophenol. The enhancement of mineralization of 2,4-dinitrophenol (2,4-DNP) via photocatalytic degradation of TiO2/ZnO p–n heterojuction was investigated. The 2,4-DNP was degraded completely by TiO2/ZnO, and 78% of the total organic carbon (TOC) was removed which is greatly superior to that on the controlled TiO2, ZnO with 53%, 45%, respectively. A schematic diagram of photocatalytic oxidation mechanism of 2,4-DNP was also presented by ⋅OH radical detection. The oxidation of 2,4-DNP and the intermediate productions was based on the hydroxyl (OH) with a high oxidation potential of 2.8V. Moreover, the excellent stability and reliability of the TiO2/ZnO composite hollow microsphere photocatalyst demonstrated its promising application for removal of organic pollutant from water.

Semimetallic vanadium molybdenum sulfide for high-performance battery electrodes

The ultrathin thickness and lateral morphology of a two dimensional (2D) MoS2 nanosheet contribute to its high surface-to-volume ratio and short diffusion path, rendering it a brilliant electrode material for lithium-ion batteries (LIBs). However, the low conductivity and easy restacking character of the pure MoS2 nanosheet during extended cycling result in severe capacity fading and poor cycling performance. In this work, we developed an attractive strategy by using a metal-doping method to engineer chemical, physical and electronic properties of MoS2, achieving an outstanding performance in LIBs. The computational results show that V–Mo–S has semimetallic properties. Semimetallic vanadium molybdenum sulfide nanoarrays (V–Mo–S NAs) were prepared to overcome the low conductivity of semiconducting MoS2 and thus further optimize its performance in LIBs. A reversible capacity as high as 1047 mA h g−1 was achieved at 1000 mA g−1. It also displayed an excellent stability even after 700 cycles. This fascinating study may pave a way for utilizing semimetallic material-based nanomaterials for batteries.

Cu-Doped Fe@Fe2O3 core–shell nanoparticle shifted oxygen reduction pathway for high-efficiency arsenic removal in smelting wastewater

Studies on the removal of As(III) by Fe-based materials have been carried out for decades, but the time-consuming process and low removal capacity are obstacles for large-scale practical applications. Here, a rapid and efficient technique was proposed for the removal of As(III) using Cu-doped Fe@Fe2O3 core–shell nanoparticles (CFF) synthesized by a facile two-step reduction method and aging process, which realized a thorough removal of As(III) from smelting wastewater at neutral pH within 30 min. The copper doped in CFF not only provided two extra oxygen reduction pathways to enhance the molecular oxygen activation, but also improved the electron transfer ability and removal efficiency of As(III). The existence of copper contributed to the rapid oxidization and adsorption of As(III), and the removal rate increased nearly 10-times in the aerobic system. Meanwhile, the proposed Cu-doped Fe@Fe2O3 core–shell nanoparticles and shifted oxygen reduction pathway could be easily scaled up for other transition metals, such as Ni. Molecular dynamics (MD) simulations based on the large-scale atomic/molecular massively parallel simulator (LAMMPS) were also employed to investigate the formation process of CFF. Furthermore, the removal efficiency of arsenic in smelting wastewater remained to be 90% after 6 times of cycling. Therefore, the distinctive oxidation activities of CFF hold great promise for applications in arsenic removal.