Defective ZnIn2S4 Nanosheets for Visible-Light and Sacrificial-Agent-Free H2O2 Photosynthesis via O2/H2O Redox.

H2O2 photosynthesis has attracted great interest in harvesting and converting solar energy to chemical energy. Nevertheless, the high-efficiency process of H2O2 photosynthesis is driven by the low H2O2 productivity due to the recombination of photogenerated electron-hole pairs, especially in the absence of a sacrificial agent. In this work, we demonstrate that ultrathin ZnIn2S4 nanosheets with S vacancies (Sv-ZIS) can serve as highly efficient catalysts for H2O2 photosynthesis via O2/H2O redox. Mechanism studies confirm that Sv in ZIS can extend the lifetimes of photogenerated carriers and suppress their recombination, which triggers the O2 reduction and H2O oxidation to H2O2 through radical initiation. Theoretical calculations suggest that the formation of Sv can strongly change the coordination structure of ZIS, modulating the adsorption abilities to intermediates and avoiding the overoxidation of H2O to O2 during O2/H2O redox, synergistically promoting 2e- O2 reduction and 2e- H2O oxidation for ultrahigh H2O2 productivity. The optimal catalyst displays a H2O2 productivity of 1706.4 µmol g-1 h-1 under visible-light irradiation without a sacrificial agent, which is ∼29 times higher than that of pristine ZIS (59.4 µmol g-1 h-1) and even much higher than those of reported photocatalysts. Impressively, the apparent quantum efficiency is up to 9.9% at 420 nm, and the solar-to-chemical conversion efficiency reaches ∼0.81%, significantly higher than the value for natural synthetic plants (∼0.10%). This work provides a facile strategy to separate the photogenerated electron-hole pairs of ZIS for H2O2 photosynthesis, which may promote fundamental research on solar energy harvest and conversion.

Self-Supporting Multicomponent Hierarchical Network Aerogel as Sulfur Anchoring-Catalytic Medium for Highly Stable Lithium-Sulfur Battery.

The low utilization rate of active materials, shuttle effect of lithium polysulfides (LiPSs), and slow reaction kinetics lead to the extremely low efficiency and poor high current cycle stability of lithium sulfur batteries (Li-S batteries). In this paper, a self-supporting multicomponent hierarchical network aerogel is proposed as the modified cathode (S/GO@MX@VS4 ). It consists of graphene (GO) and MXene nanosheets (MX) loaded with VS4 nanoparticles. The experimental results and first-principles calculations show that the GO@MX@VS4 aerogel has strong adsorption and reversible conversion effects on LiPSs. It can not only inhibit the shuttle effect and improve the utilization rate of active substances by keeping the chain crystal structure of VS4 , but also promote the reversibility and kinetics of the reaction by accelerating the liquid-solid transformation in the reduction process and the decomposition of insoluble Li2 S in the oxidation process. The GO@MX@VS4 aerogel modified cathode with a multicomponent synergy exhibits the capacity ratios (Q1 /Q2 ) at different discharge stages is close to the theoretical value (1:2.8), and the capacity decay per cycle is 0.019% in 1200 cycles at 5C. Also, a high areal capacity of 6.90 mAh cm-2 is provided even at high sulfur loading (7.39 mg cm-2 ) and low electrolyte/sulfur ratio (E/S, 8.0 µL mg-1 ).

Strongly Coupled g-C3 N4 Nanosheets-Co3 O4 Quantum Dots as 2D/0D Heterostructure Composite for Peroxymonosulfate Activation.

The development of effective approaches for the preparation of 0D quantum dots (QDs)/2D nanosheets (NSs) heterostructures, which have been proven to be favorable for heterogeneous catalysis, is highly desirable but remains a great challenge. Herein, 0D metal oxide nanocrystals-2D ultrathin g-C3 N4 nanosheets (Co3 O4 /CNNS) heterostructures are synthesized via a facile chemical reaction, followed by annealing in air. Ultrafine Co3 O4 QDs (≈2.2-3.2 nm) are uniformly and tightly attached on the surface of g-C3 N4 nanosheets. Detailed characterization reveals that the specially designed unique 0D/2D structure is critical to the high photocatalytic performance for the degradation of tetracycline (TC) via peroxymonosulfate (PMS) activation. The optimal catalyst, namely, Co3 O4 /CNNS-1100, exhibits excellent performance and ≈98.7% TC can be degraded under visible light irradiation. Moreover, TC degradation is almost completely insusceptible to several real water samples. Meanwhile, other dye pollutants can also be efficiently degraded by the Co3 O4 /CNNS-1100/PMS/vis system. The quenching tests display that that the h+ , ∙OH, O2∙- , and SO4∙- are responsible for TC removal. The improved photocatalytic performance can be attributed to the synergistic effect of the photocatalytic- and chemical-processes in the PMS activation. This work gives an insight into the development of multifunctional 0D/2D nanocomposites for further potential applications which are not limited to environmental purification.

Highly Reversible Lithium-Ion Battery with Excellent Rate Performance and Cycle Stability Based on a Ti3C2/CoS2 Composite Anode.

Transition metal sulfide (TMS) CoS2 is considered an ideal anode material for new-generation lithium-ion batteries (LIBs) because of its high specific capacity, high electrochemical activity, and low cost. However, CoS2 is prone to volume expansion and structural collapse when it participates in the internal conversion reaction of the battery, which limits its practical application. After analyzing the failure mechanism of CoS2 as the anode material of LIBs, the concept of nanoengineered materials is introduced here. CoS2 particles are nanosized and stabilized by constructing a composite structure on an alkali-treated two-dimensional Ti3C2 Mxene conductive network. Both experiments and theoretical calculations show that special Ti-O-Co bonds are formed at the interface of the Ti3C2/CoS2 composite through oxygen-containing functional groups. Ti-O-Co bonding with adjustable electronic characteristics can effectively promote the utilization rate of anode materials, electronic conductivity, and ionic diffusivity and thus enhance the redox reaction kinetics of the device. When the Ti3C2/CoS2 composite is used as the anode material for LIBs, it still provides a high specific capacity of 405.8 mAh g-1 after 100 cycles at 0.1 A g-1. After running for 1000 cycles at a high current of 1 A g-1, the capacity retention is still close to 100%. Also, high cycle stability under the condition of highly active material loading (10.58 mg cm-2) and low electrolyte/active material ratio (10 µL mg-1) is achieved. This work provides a new idea for the development of commercial LIBs as anode materials.

Electrochemical production of hydrogen peroxide by non-noble metal-doped g-C3N4 under a neutral electrolyte.

Electrochemical oxygen reduction (ORR) for the production of clean hydrogen peroxide (H2O2) is an effective alternative to industrial anthraquinone methods. The development of highly active, stable, and 2e- ORR oxygen reduction electrocatalysts while suppressing the competing 4e- ORR pathway is currently the main challenge. Herein, bimetallic doping was successfully achieved based on graphitic carbon nitride (g-C3N4) with the simultaneous introduction of K and Co, whereby 2D porous K-Co/CNNs nanosheets were obtained. The introduction of Co promoted the selectivity for H2O2, while the introduction of K not only promoted the formation of 2D nanosheets of g-C3N4, but also inhibited the ablation of H2O2 by K-Co/CNNs. Electrochemical studies showed that the selectivity of H2O2 in K-Co/CNNs under neutral electrolyte was as high as 97%. After 24 h, the H2O2 accumulation of K-Co/CNNs was as high as 31.7 g L-1. K-Co/CNNs improved the stability of H2O2 by inhibiting the ablation of H2O2, making it a good 2e- ORR catalyst and providing a new research idea for the subsequent preparation of H2O2.

Transition metal single atom-optimized g-C3N4 for the highly selective electrosynthesis of H2O2 under neutral electrolytes.

Neutral electrosynthesis of H2O2via the 2e- ORR is attractive for numerous applications, but the low activity and high cost of electrocatalysts have become important constraints. Therefore, the development of cheap and efficient electrocatalysts for the 2e- ORR is necessary. Herein, we report the embedding of transition metal single atoms (TM SAs) in g-C3N4 nanosheets (CNNS). The introduction of TM SAs increases the N-CN content and reduces the C-C/CC content in CNNS, which contributes to the increased selectivity of TM SA/CNNS for the 2e- ORR. TM SA is the main reason for the enhanced activity of the 2e- ORR. Based on the results obtained by replacing a series of TM SA, the Ni0.10 SA/CNNS with optimal N-CN content exhibited the best selectivity (∼98%) and highest yield of H2O2 (∼503 mmol gcat-1 h-1), which is ∼14.6 times higher than that of CNNS (∼34.4 mmol gcat-1 h-1). Other TM SA/CNNS also exhibited high activity and selectivity. This study demonstrates the ability of TM SA to modulate the selectivity and activity of CNNS, making it a promising candidate for the 2e- ORR and providing more reference ideas for the preparation of H2O2.

Interfacial Synergy between the Cu Atomic Layer and CeO2 Promotes CO Electrocoupling to Acetate.

Cu is considered to be an effective electrocatalyst in CO/CO2 reduction reactions (CORR/CO2RR) because of its C-C coupling into C2+ products, but it still remains a formidable challenge to rationally design Cu-based catalysts for highly selective CO/CO2 reduction to C2+ liquid products such as acetate. We here demonstrate that spraying atomically layered Cu atoms onto CeO2 nanorods (Cu-CeO2) can lead to a catalyst with an enhanced acetate selectivity in CORR. Owing to the existence of oxygen vacancies (Ov) in CeO2, the layer of Cu atoms at interface coordinates with Ce atoms in the form of Cu-Ce (Ov), as a result of strong interfacial synergy. The Cu-Ce (Ov) significantly promotes the adsorption and dissociation of H2O, which further couples with CO to selectively produce acetate as the dominant liquid product. In the current density range of 50-150 mA cm-2, the Faradaic efficiencies (FEs) of acetate are over 50% with a maximum value of 62.4%. In particular, the turnover frequency of Cu-CeO2 reaches 1477 h-1, surpassing that of Cu nanoparticle-decorated CeO2 nanorods, bare CeO2 nanorods, as well as other existing Cu-based catalysts. This work advances the rational design of high-performance catalysts for CORR to highly value-added products, which may attract great interests in diverse fields including materials science, chemistry, and catalysis.

Sulfur-Bridged Bonds Boost the Conversion Reaction of the Flexible Self-Supporting MnS@MXene@CNF Anode for High-Rate and Long-Life Lithium-Ion Batteries.

Manganese sulfide (MnS) has been found to be a suitable electrode material for lithium-ion batteries (LIBs) owing to its considerable theoretical capacity, high electrochemical activity, and low discharge voltage platform, while its poor electrical conductivity and severe pulverization caused by volume expansion of the material limit its practical application. To improve the rate performance and cycle stability of MnS in LIBs, the structure-control strategy has been used to design and fabricate new anode materials. Herein, the MnS@MXene@CNF (MMC, CNFs means carbon nanofibers) electrode has been prepared by electrospinning and a subsequent high-temperature annealing process. The MMC electrode exhibits excellent cyclic stability with a capacity retention rate close to 100% after 1000 cycles at 1000 mA/g and an improved rate performance with a specific capacity up to 500 mAh/g at a high current density of 5000 mA/g, much higher than the 308 mAh/g of the MnS@CNF (MC) electrode. The elevated electrochemical performance of the MMC electrode not only benefits from the unique structure of MnS nanoparticles evenly dispersed in the well-designed flexible self-supporting three-dimensional (3D) CNF network but, more importantly, also benefits from the formation of sulfur-bridged Mn-S-C bonds at the MnS/MXene interface. The newly formed bonds between MnS and MXene nanosheets can stabilize the structure of MnS near the interfaces and provide a channel for fast charge transfer, which notably increase both the reversibility and the rate of the conversion reaction during the charge/discharge process. This work may pave a new path for designing stable and self-supporting anodes for high-performance LIBs.

Multi-Dimensional Composite Frame as Bifunctional Catalytic Medium for Ultra-Fast Charging Lithium-Sulfur Battery.

The shuttle effect of soluble lithium polysulfides (LiPSs) between electrodes and slow reaction kinetics lead to extreme inefficiency and poor high current cycling stability, which limits the commercial application of Li-S batteries. Herein, the multi-dimensional composite frame has been proposed as the modified separator (MCCoS/PP) of Li-S battery, which is composed of CoS2 nanoparticles on alkali-treated MXene nanosheets and carbon nanotubes. Both experiments and theoretical calculations show that bifunctional catalytic activity can be achieved on the MCCoS/PP separator. It can not only promote the liquid-solid conversion in the reduction process, but also accelerate the decomposition of insoluble Li2S in the oxidation process. In addition, LiPSs shuttle effect has been inhibited without a decrease in lithium-ion transference numbers. Simultaneously, the MCCoS/PP separator with good LiPSs adsorption capability arouses redistribution and fixing of active substances, which is also beneficial to the rate performance and cycling stability. The Li-S batteries with the MCCoS/PP separator have a specific capacity of 368.6 mAh g-1 at 20C, and the capacity decay per cycle is only 0.033% in 1000 cycles at 7C. Also, high area capacity (6.34 mAh cm-2) with a high sulfur loading (7.7 mg cm-2) and a low electrolyte/sulfur ratio (7.5 µL mg-1) is achieved.

Three-Dimensional Hierarchical g-C3N4 Architectures Assembled by Ultrathin Self-Doped Nanosheets: Extremely Facile Hexamethylenetetramine Activation and Superior Photocatalytic Hydrogen Evolution.

Photocatalytic hydrogen evolution has broad prospects as a clean solution for the energy crisis. However, the rational design of catalyst complex, the H2 evolution efficiency, and the yield are great challenge. Herein, three-dimensional hierarchical g-C3N4 architectures assembled by ultrathin carbon-rich nanosheets (3D CCNS) were prepared via an extremely facile hexamethylenetetramine activation approach at the bulk scale, indicating the validation of scale-up production process. The two-dimensional ultrathin carbon-rich nanosheets were several hundred nanometers in width but only 5-6 nm in thickness and gave rise to a unique 3D interconnected network. The unique composition and structure of the nanosheets endow them with a remarkable light absorption spectrum with the tunable band gap, high electrical conductivity, fast charge separation, and large surface areas with abundant reaction active sites, and thus significantly improved H2 production performance. As high as ∼7.8%, quantum efficiency can be achieved by irradiating 3D CCNS at 420 nm with a H2 evolution rate >2.7 × 104 µmol/g/h, which is ∼31.3 times higher than that of the pristine g-C3N4. Our work introduces an extremely facile route for mass production of doping modified 3D g-C3N4-based photocatalyst with excellent H2 evolution performances.

Constructing highly dispersed 0D Co3S4 quantum dots/2D g-C3N4 nanosheets nanocomposites for excellent photocatalytic performance.

The development of noble-metal-free catalysts with high efficiency photocatalytic properties is critical to the heterogeneous catalysis. Herein, zero-dimensional (0D) metal sulfide quantum dots/two-dimensional (2D) g-C3N4 nanosheets (Co3S4/CNNS) nanocomposites are synthesized by a two-step method, including the ways of in-situ deposition and water bath. The highly dispersed Co3S4 quantum dots (particle size is 2-4 nm) are evenly and tightly fixed on CNNS, which can be used as co-catalyst to effectively replace noble metals to improve the photocatalytic properties of CNNS. Co3S4/CNNS-900 has the apparent quantum efficiency, which is up to 7.85% at 400 nm. At the same time, the H2 evolution rate of Co3S4/CNNS-900 is 20,536.4 µmol g-1 h-1, which is 555 times than CNNS. The excellent photocatalytic performance is due to the highly dispersed Co3S4 quantum dots on 2D CNNS, which facilitate the formation of more active sites, Co3S4/CNNS promotes the separation and migration of photogenerated carriers, shortens the migration distance of photogenerated carriers, and eventually leads to an increase of the photocatalytic performance.

Metal-Free Graphitic Carbon Nitride Photocatalyst Goes Into Two-Dimensional Time.

Graphitic carbon nitride (g-C3N4) is always a research hotspot as a metal-free visible-light-responsive photocatalyst, in the field of solar energy conversion (hydrogen-production by water splitting). This critical review summarizes the recent progress in the design and syntheses of two-dimensional (2D) g-C3N4 and g-C3N4-based nanocomposites, covering (1) the modifications of organic carbon nitrogen precursors, such as by heat treatment, metal or metal-free atoms doping, and modifications with organic functional groups, (2) the influencing factors for the formation of 2D g-C3N4 process, including the calcination temperature and protective atmosphere, etc. (3) newly 2D g-C3N4 nanosheets prepared from pristine raw materials and bulk g-C3N4, and the combination of 2D g-C3N4 with other 2D semiconductors or metal atoms as a cocatalyst, and (4) the structures and characteristics of each type of 2D g-C3N4 systems, together with their optical absorption band structures and interfacial charge transfers. In addition, the first-principles density functional theory (DFT) calculation of the g-C3N4 system has been summarized, and this review provides an insightful outlook on the development of 2D g-C3N4 photocatalysts. The comprehensive review is concluded with a summary and future perspective. Moreover, some exciting viewpoints on the challenges, and future directions of 2D g-C3N4 photocatalysts are discussed and highlighted in this review. This review can open a new research avenue for the preparation of 2D g-C3N4 photocatalysts with good performances.

Fabrication of Hierarchical ZnO@NiO Core-Shell Heterostructures for Improved Photocatalytic Performance.

ZnO@NiO core-shell heterostructures with high photocatalytic efficiency and reusability were prepared via electrochemical deposition on carbon fiber cloth substrates. Their photocatalytic properties were investigated by measuring the degradation of rhodamine B and methyl orange (MO) under ultraviolet light irradiation. The photodegradation efficiency of the ZnO@NiO heterostructures toward both dyes was better than those of the pure ZnO nanorods and NiO nanosheets. The higher performance could be attributed to the formation of p-n heterojunction between ZnO and NiO. Especially, the ZnO@NiO heterostructure formed upon deposition of NiO for 10 min degraded 95% of MO under ultraviolet light irradiation for 180 min. The high photodegradation efficiency of the ZnO@NiO heterostructures was also attributed to the high separation efficiency of photogenerated carriers, as confirmed by the higher photocurrent of the ZnO@NiO heterostructures (eightfold) when compared with that of the pure ZnO nanorods. Moreover, the high photodegradation efficiency of the ZnO@NiO heterostructures was maintained over three successive degradation experiments and decreased to 90% after the third cycle.

Hedgehog-inspired nanostructures for hydrogel-based all-solid-state hybrid supercapacitors with excellent flexibility and electrochemical performance.

High-security deformable energy-storage devices that are mechanically robust, with considerable energy and power densities are becoming desirable for smart wearable electronics. Here, a highly flexible hydrogel-based all-solid-state hybrid supercapacitor was rationally designed and assembled, with unique NiCo2O4@NixCoyMoO4 (x : y = 3 : 1) nanostructures as the electrode, which was bio-inspired by the curling up and relaxation of hedgehogs. The hybrid supercapacitor shows no obvious decay in capacitance during bending to different states, indicating its outstanding flexibility and mechanical stability. The capacitance was still maintained at 92.0% of the initial value, even after continuous bending for 3000 cycles. The highly monodisperse NiCo2O4@NixCoyMoO4 nanostructures releasing stress during bending is responsible for the favorable stability and flexibility. Furthermore, the hybrid supercapacitor displayed outstanding electrochemical performance, with a high specific capacitance of 207 F g-1 at 1 A g-1, a high energy density of 64.7 W h kg-1 at 749.6 W kg-1, and favorable cycling stability (nearly 100% after 10 000 cycles). The flexible hybrid supercapacitor could be charged with a solar cell and served as the power source to light up LEDs. This simple and reliable hybrid supercapacitor, with extraordinary mechanical stability and electrochemical performance, is a promising power source for smart wearable electronics.

One-pot Synthesis of CdS Irregular Nanospheres Hybridized with Oxygen-Incorporated Defect-Rich MoS2 Ultrathin Nanosheets for Efficient Photocatalytic Hydrogen Evolution.

Robust and highly active photocatalysts, CdS@MoS2, for hydrogen evolution were successfully fabricated by one-step growth of oxygen-incorporated defect-rich MoS2 ultrathin nanosheets on the surfaces of CdS with irregular fissures. Under optimized experimental conditions, the CdS@MoS2 displayed a quantum yield of ∼24.2% at 420 nm and the maximum H2 generation rate of ∼17203.7 umol/g/h using Na2S-Na2SO3 as sacrificial agents (λ ≥ 420 nm), which is ∼47.3 and 14.7 times higher than CdS (∼363.8 µmol/g/h) and 3 wt % Pt/CdS (∼1173.2 µmol/g/h), respectively, and far exceeds all previous hydrogen evolution reaction photocatalysts with MoS2 as co-catalysts using Na2S-Na2SO3 as sacrificial agents. Large volumes of hydrogen bubbles were generated within only 2 s as the photocatalysis started, as demonstrated by the photocatalytic video. The high hydrogen evolution activity is attributed to several merits: (1) the intimate heterojunctions formed between the MoS2 and CdS can effectively enhance the charge transfer ability and retard the recombination of electron-hole pairs; and (2) the defects in the MoS2 provide additional active S atoms on the exposed edge sites, and the incorporation of O reduces the energy barrier for H2 evolution and increases the electric conductivity of the MoS2. Considering its low cost and high efficiency, this highly efficient hybrid photocatalysts would have great potential in energy-generation and environment-restoration fields.

Constructing the novel ultrafine amorphous iron oxyhydroxide/g-C3N4 nanosheets heterojunctions for highly improved photocatalytic performance.

Ultrafine particles, more heterojunction interfaces and amorphous materials can effectively enhance the photocatalytic activity of photocatalysts. In this work, a facile in-situ precipitation method was developed to prepare ultrafine amorphous iron oxyhydroxide/ultrathin g-C3N4 nanosheets heterojunction composites. The amorphous iron oxyhydroxide possessed an ultrafine particle size and a wide range of visible light absorption. In this process, the ultrafine particles not only shortened the diffusion distance of photogenerated carriers, but also facilitated the formation of more heterojunctions with ultrathin g-C3N4 nanosheets. The photocatalytic activities were evaluated using rhodamine B, methylene blue, and methyl orange as pollution models under visible light irradiation. Notably, the optimal photocatalytic activity of a-FeOOH/CNNS-800 composite is ~17.8 times higher than that of CNNS towards the degradation of rhodamine B under visible light. The outstanding photocatalytic activities were ascribed to the narrower band gap, the enhanced visible light absorbance, abundant heterojunction interfaces, and the effective separation of the photogenerated charges driven by the matched band edge in the heterostructures. We trusted that the facile and easy-to-extend synthesis method can be further expanded to synthesize other ultrafine semiconductors coupled with g-C3N4 for enhancing the photocatalytic activities.

In-situ synthesis of amorphous silver silicate/carbonate composites for selective visible-light photocatalytic decomposition.

Coupling two different semiconductors to form composite photocatalysts is an extremely significant technique for environmental remediation. Here, a one-step in-situ precipitation method has been developed to prepare amorphous silver silicate/carbonate (AgSiO/Ag2CO3) nanoparticles (NPs) composites, which are well dispersed sphere-like particles with the sizes of around ~50-100 nm. The high-efficiency photocatalytic activities under visible light (VL) have been carefully evaluated, and the AgSiO/Ag2CO3 NPs composites exhibit selective photocatalytic degradations on Methylene Blue (MB) and Rhodamine B (RhB). The maximum degradation rate for MB can reach ~99.1% within ~40 min under VL irradiation, much higher than that of RhB (~12%) in the same condition, which can be ascribed to (I) the smaller molecule size of MB than that of RhB, (II) the fast charge separation between AgSiO NPs and Ag2CO3 NPs, abundant heterojunction interfaces as well as fully exposed reactive sites. These composites are proposed to be an example for the preparation of other silicate composite photocatalysts for practical applications in environmental remediation.

One-pot hydrothermal synthesis of CdS decorated CuS microflower-like structures for enhanced photocatalytic properties.

CdS decorated CuS structures have been controllably synthesized through a one-pot hydrothermal method. The morphologies and compositions of the as-prepared samples could be concurrently well controlled by simply tuning the amount of CdCl2 and thiourea. Using this strategy, the morphology of the products experienced from messy to flower-like morphologies with multiple porous densities, together with the phase evolution from pure CuS to the CdS/CuS composites. Serving as a photocatalyst, the samples synthesized with the addition of 1 mmol cadmium chloride and 3 mmol thiourea during synthetic process, showed the best photocatalytic activity, which could reach a maximum photocatalytic efficiency of 93% for methyl orange (MO) photodegradation after 150 min. The possible mechanism for the high photocatalytic efficiency of the sample was proposed by investigating the composition, surface area, structure, and morphology before and after photocatalytic reaction.