Boosting charge transfers in cadmium sulfide nanorods with a few layered Ni-doped MoS2 nanosheets for enhanced photocatalytic hydrogen evolution

The molybdenum sulfide (MoS₂) is a promising low-cost photocatalyst aimed at the hydrogen production reactions, however, obtaining a detailed understanding of its catalytic site has proved to be a challenging task. Several studies indicated that the active sites for catalytic reaction are mainly associated with the edge sites of 2D-layered MoS₂, and their basal plane (in-plane) displays poor activity toward catalytic reactions. Herein, we established the simple approaches to enhance the activity of MoS₂ by conversion of in-plane active sites into active surface edge sites by transition metal (Ni) doping followed by exfoliation. These activated MoS₂ was utilized for enormous upgrading of CdS photocatalytic activity for hydrogen production and is roughly 249 mmol h^?1 g^?1, which is 70 times higher than pure CdS, showed ～140 h stable H₂ production. The amended conductivity, improved surface area and huge active sites are extremely advantageous properties expanded by metal doping to MoS₂ and exfoliation. Additionally, another reason for the enhanced activity of Ni–MoS₂/CdS system was due to promotion of catalytic kinetics by Ni and Mo sits, they are admirable activity of water dissociation and higher ability of hydrogen adsorption correspondingly. These modifications made of superior photogenerated charge carriers’ separation and migration for effective utilization. As far as we know, this system demonstrates the utmost effective performance among inclusive reported MoS₂ based CdS composites. Remarkably, these outcomes will have abundant potential for the progress of immensely actual photocatalytic systems.     

Hydrogen production by water-gas shift reaction over Co-promoted MoS2/Al2O3 catalyst: The intrinsic activities of Co-promoted and unprompted sites

Co-promoted MoS₂/Al₂O₃ is the industrial-widely used catalyst for hydrogen production by water-gas shift (WGS) reaction under sulfur-containing condition. Despite of the intensive physicochemical characterizations, the intrinsic activities of Co-promoted and unprompted sites on this catalyst are still unreported, mainly owning to the lack of quantification method of catalytic sulfide sites. With low temperature CO adsorption followed by IR spectroscopy, a distinguish technique developed by our group, this short communication reports the temperature-dependent TOFs (turnover frequencies) of these two sites, and reveals that Co-promoted site is intrinsically much more active than unprompted site at low temperatures, while these two sites are catalytically comparable at higher reaction temperatures. The catalytically different performances are related to the different apparent activation energies of WGS reaction on these two sites. This work fills in the long-standing gaps in hydrogen production by WGS reaction over sulfided CoMo/Al₂O₃ catalyst.     

The thermal decomposition of hydrogen sulfide over transition metal sulfides

The decomposition of hydrogen sulfide to hydrogen and sulfur on a variety of transition metal sulfides has been studied in a flow system at 400–800°C. Hydrogen yields were measured as a function of temperature in order to compare the effectiveness of the metal sulfides in promoting the decomposition. For the series Cr₂S₃, MoS₂, WS₂ it was found that MoS₂ is the most effective catalyst above 600°C but both Cr₂S₃ and WS₂ gave higher H₂ yields than MoS₂ below 600°C. For the group of metal disulfides FeS₂, CoS₂ and NiS₂, thermal decomposition of MS₂ to non-stoichiometric metal sulfides starts at ca. 550°C. The monosulfides FeS, CoS and NiS produce high yields of hydrogen initially due to sulfidation of the solid phase by H₂S to give the same non-stoichiometric sulfides which, themselves, are not good catalysts for the thermal decomposition of H₂S. The copper sulfides Cu₂S, Cu₉S₅. CuS were not effective catalysts for the thermal decomposition of H₂S.     

Synthesis of MoS2-carbon composites with different morphologies and their application in hydrogen evolution reaction

MoS₂-carbon composites which with different morphologies were synthesized by hydrothermal method and tested with respect to their application in hydrogen evolution reaction (HER). Their performances were compared to evaluate how the morphology influence HER. The obtained results showed that the composite containing amorphous MoS₂ showed higher activity than composite which contains crystalline MoS₂. The catalytic activity of composite was highly correlated to its active surface area which was controlled by the morphology. In addition, compared with composite which contains amorphous MoS₂, the composite containing crystalline MoS₂ showed higher durability in the long-term operation. However, in acidic and alkaline environments, the stability of composite containing amorphous MoS₂ is better than which containing crystalline MoS₂. The impedance measurements suggested that the high catalytic activity of the composite stems from the synergistic effect of MoS₂ and carbon materials. The enhanced understanding of these highly active hydrogen evolution catalysts can facilitate the development of economical electrochemical hydrogen production systems.     

Investigation of the electro-oxidation of artificial Black Sea water by cyclic voltammetry on molybdenum (II)

The electro-oxidation of H₂S in Black Sea water to generate electricity was investigated. MoS₂ exhibited catalytic activity toward the oxidation of H₂S in artificial sea water. The catalytic activity of molybdenum sulfide was found to depend on the electrolyte pH and on the temperature. Cyclic voltammograms taken in the artificial sea water containing hydrogen sulfide at pH = 14 exhibited a peak at 500 mV related to the oxidation of HS⁻. Furthermore, the peak current of the MoS₂ electrode increased to 400 mA g⁻¹ from 250 mA g⁻¹ (approximately 1.6-fold) when the temperature was increased from 353 K to 363 K.     

Efficient hydrogen evolution activity of 1T-MoS2/Si-doped TiO2 nanotube hybrids

Catalytic hydrogen evolution is promising process used for production of clean fuel hydrogen and attracting many attentions. In this work, the synthesis and hydrogen evolution catalytic activity of MoS₂/TiO₂ nanotubes and MoS₂/Si-doped TiO₂ NTs hybrids were studied. The MoS₂ in the hybrids exhibited 1T structure with high conductivity and catalytic activity for hydrogen evolution reaction. MoS₂ decoration provided high light absorbance for the hybrids and highly efficient interface-induced effect between the nanotubes and MoS₂. Thus, the hybrids showed photocatalytic and electrocatalytic activities remarkably greater than the nanotubes. Moreover, the Si-doping resulted in the increase in specific surface area and hydrophilicity and so further enhanced the catalytic activity.     

MoS2||CoP heterostructure loaded on N,P-doped carbon as an efficient trifunctional catalyst for oxygen reduction,oxygen evolution,and hydrogen evolution reaction

Exploring multifunctional electrocatalysts is crucial for the development of energy conversion and storage equipments, such as fuel cells, water splitting devices and zinc-air batteries. Herein, we provide a rational design whereby the cobalt phosphide particles are introduced into molybdenum sulfide nanosheets to form a heterostructure (MoS₂||CoP) through the ultrasonic method and calcination. Subsequently, N, P-doped carbon (NPC) is obtained synchronously. The as-prepared MoS₂||CoP/NPC demonstrates highly effective multifunctional catalytic performance for oxygen evolution and hydrogen evolution reaction at lower overpotential, as well as oxygen reduction reaction at high half-wave potential. What this reveals is higher power density and superior stability in zinc-air battery. The excellent electrocatalytic activity of MoS₂||CoP/NPC may be attributed to the presence of the MoS₂||CoP heterostructure, as well as N, P-doped carbon coupled with a high percentage of pyridinic-N. This work proposes a novel and facile strategy to prepare the heterostructure compound and serves as a good reference for constructing efficient and low-cost electrocatalysts.     

Molybdenum disulfide/silver/p-silicon nanowire heterostructure with enhanced photoelectrocatalytic activity for hydrogen evolution

Suitable semiconductor and its efficient coupling with catalysts is vital to hydrogen evolution reaction (HER). Herein, Ternary heterostructured MoS₂/Ag/p-type silicon nanowires (SiNWs) array photocathode are constructed by a simple two-step method, where Ag is self-reduced on SiNWs via Galvanic Displacement method and MoS₂ is subsequently loaded by direct thermal decomposition. Ag interfacial layer is introduced between Si and MoS₂ to facilitate the charge transfer and suppress the recombination of photo-generated electron-hole pairs. MoS₂/Ag/SiNWs exhibits an onset potential of 62 mV and photocurrent density of 50 mA cm^?2 at ?1.0 V_RHE, as well as good stability. Besides, MoS₂/Ag/SiNWs is capable of generating 325.9 μL hydrogen per minute. The superior HER catalytic activity of MoS₂/Ag/SiNWs is contributed to the improved charge transport at the solid–solid interfaces by virtue of Ag layer, allowing more electrons flow from SiNWs to MoS₂ and thus effectively separating the photoelectrons and holes. This work demonstrates the potential of novel heterostructure for robust and efficient photoelectrochemical HER.     

Molybdenum disulfide decorated palm oil waste activated carbon as an efficient catalyst for hydrogen generation by sodium borohydride hydrolysis

Development of cost-effective catalyst material with enhanced activity for hydrogen generation is highly desirable for hydrogen powered portable applications. In this work, molybdenum disulfide (MoS₂) incorporated on palm oil waste activated carbon (POAC) was used as a novel catalyst for enhanced hydrogen production by sodium borohydride (NaBH₄) hydrolysis. Hydrothermally synthesized MoS₂/POAC catalyst composite was characterized by SEM, EDX, XRD, FTIR, Raman, TGA and Surface area analysis. Characterization studies revealed the uniform and complete synthesis of MoS₂ nanoparticles on the POAC surface with crystallite size of 18.2 nm. The catalyst composite showed enhancement in thermal stability and reduction in specific surface area as compared with POAC. Hydrogen generation investigations showed ideal weight ratio of composite catalyst as 10:1 (w/w of POAC: MoS₂) and optimal catalyst to feed weight ratio as 0.07. MoS₂/POAC catalyst with 10 wt% of POAC loading recorded the maximum catalytic activity of 1170.66 mL/g min with lower activation energy of 39.1 kJ/mol. The catalyst composite exhibited virtuous reusability with a 28% loss in activity for nine cycle regeneration run. Thus, MoS₂/POAC catalyst system is highly attractive for commercial applicability and is a potential candidate for enhanced hydrogen production through NaBH₄ hydrolysis.     

Role of S-S interlayer spacing on the hydrogen storage mechanism of MoS2

Hydrogen storage mechanism and hydrogen storage capacity are the great challenges for the development of hydrogen energy technology. Besides the better catalytic properties, it is crucial to search for suitable material that provides enough space to store H₂ molecule. Similar to graphene, MoS₂ with S-S layered structure opens up a new way to improve the hydrogen storage capacity. By using the first-principles calculations, in this work, we investigate the hydrogen diffusion mechanism, hydrogenation process and hydrogen storage capacity of MoS₂ with S-S interlayer. We find that hydrogen prefers to diffuse into S-S interlayer along the interstitial site (path: IT-IT). H₂ molecule is a stable in S-S interlayer because the charge interaction of H-H atoms is stronger than that of H-S atoms. Finally, we predict that MoS₂ with S-S layered-by-layered stacking can effectively improve the hydrogen storage capacity.     

Highly efficient electrocatalytic hydrogen production via MoSx/3D-graphene as hybrid electrode

Molybdenum sulfide (MoS_x) has recently emerged as a promising catalyst for the hydrogen evolution reaction (HER) in water splitting that may replace the noble metal, such as platinum, as a cost-effective and high catalytic materials. It has been reported that two-dimensional structured MoS_x exhibit significant amount of exposed S-edge, which can be an active electrocatalytic catalyst for hydrogen production. However, the current reports mainly focusing on the planar electrode, where the catalyst utilization and the number of active sites are limited due to the lower exposed specific surface area (SSA) of supporting electrodes. In this work, we utilize the freeze-drying method to produce a porous three-dimensional (3D) structure assembled by graphene flakes. The as-prepared 3D graphene scaffold shows high surface area, high porosity while low density, which makes it as an ideal conductive electrode for supporting of MoS_x catalysts. Moreover, it was found out that the crystallinity of MoS_x, controlled by thermolysis temperature of thiosalts precursor ((NH₄)₂MoS₄), shows significantly influence the performance of HER. The optimized annealing temperature for the designed hybrid electrodes (MoS_x/3D-graphene) was found to create a lot of active sites, which facilitate the electrocatalytic performance for water splitting (overpotential of 163 mV @10 mA/cm² and a Tafel slope of 41 mV/dec). The study provides a potential material, which could pave the way for future applications of hydrogen energy.     

Facile synthesis of MoS2/CuS nanoflakes as high performance electrocatalysts for hydrogen evolution reaction

The fabrication of metal sulfides heterostructure is a promising strategy for enhancing catalytic activity. Herein, the MoS₂/CuS heterostructure was successfully grown on carbon cloth (MoS₂/CuS/CC) through an efficient method. The SEM results confirmed that the fabricated MoS₂/CuS/CC composites have a flake morphology, which can not only improves the surface area but also offers ample surface catalytic active sites. Particularly, the optimized MoS₂/CuS/CC-2 electrocatalyst showed a small overpotential of 85 mV@10 mA cm^?2 and exceptional long-term cycling durability for hydrogen evolution in 1 M KOH. The outstanding catalytic activity is attributed to the fact that the combination of MoS₂ with CuS can greatly enhance the charge transport rate and improve the structural stability. These results suggest that the MoS₂/CuS/CC heterostructure is a potential electrocatalyst for hydrogen production.     

Noble metal interlayer-doping enhances the catalytic activity of 2H–MoS2 from first-principles investigations

Recent studies show that the interlayers regulatory can be used as an approach to increase catalytic activity of 2H–MoS₂ for hydrogen evolution reaction. Although the noble metals are better catalyst, the effect of noble metal interlayer doping on 2H–MoS₂ is unclear. To this end, we apply first-principles method to investigate the structural stability of the noble metal interlayer doped 2H–MoS₂. In particular, we investigate the influence of noble metal interlay doping on the catalytic hydrogen evolution activity of 2H–MoS₂. The result shows that the noble metal interlayer doping is beneficial to improve the electronic transport near Fermi level. Moreover, it is found that the Ru-doped 2H–MoS₂ has better thermodynamic stability compared to the Au-doping, Pd-doping and Pt-doping. Importantly, the noble metal interlayer doping strengthens the interaction between single layers in comparison with the weak van der Waals force between single layers of the pristine 2H–MoS₂. Finally, it is found that four noble metals interlayer doping increase the catalytic hydrogen evolution activity of 2H–MoS₂. In particular, the Ru-doped 2H–MoS₂ has better catalytic activity because the transfer of the Ru-4d state from low energy region to high energy region, which weakens the interaction between Mo and S.     

MoS2 confined on graphene by triethanolamine for enhancing electrocatalytic hydrogen evolution performance

The reduction of active sites due to reunion and slow electron transfer rates and low electronegativity greatly reduced the catalytic performance of many two-dimensional materials. In this paper, we synthesized composites for partially reducing graphene oxide and molybdenum disulfide (MoS₂@prGO) by one-step hydrothermal method. With the addition of triethanolamine, MoS₂ is highly dispersed on the prGO carrier and converted into the 1T phase MoS₂ (50.4%). Meanwhile, it helps to increase the electron transfer rate of the MoS₂@prGO composites. MoS₂@prGO composites presents a high electron cloud density due to the existence of N atoms and prGO, which promotes the occurrence of hydrogen ion conversion hydrogen reaction and decreases the electrocatalytic hydrogen evolution overpotential. MoS₂@prGO composites exhibits an overpotential of 263 mV at 10 mA/cm² and a small Tafel slope of 60 mV/dec. This work is devoted to offer a new prospect and direction for the improvement of electrochemical HER performance.     

Organic interlayers boost the activity of MoS2 toward hydrogen evolution by maintaining high 1T/2H phase ratio

1T-MoS₂ shows great promise for hydrogen production due to phenomenal performance in catalyzing hydrogen evolution reaction (HER) from water. However, this phase converts to low-active 2H–MoS₂ on superambient heating. A series of MoS₂ layered compounds (LCs) with guest organic cations was prepared to reveal the effect of cationic organics on the structure stabilization of 1T-MoS₂ and thereby on the catalytic performance of this phase in HER. The results showed that LCs provide significantly higher 1T/2H ratio after heating than non-stabilized 1T phase and some of them exhibit excellent thermal durability in the capacity of HER catalysts. The effect is most pronounced for the organics remaining tightly bound to sulfide sheets in a sulfuric acid electrolyte. Cetyltrimethylammonium and bis(dimethylamino)naphthalene perfectly matched this criterion and provided a long-term maintaining of catalyst activity. The data obtained in this study are hoped to offer new approach for rational design of the efficient non-precious 1T-MoS₂-based electrocatalysts for hydrogen production.     

The thermal decomposition of hydrogen sulfide over vanadium and molybdenum sulfides and mixed sulfide catalysts in quartz and thermal diffusion column reactors

The decomposition of hydrogen sulfide to hydrogen and sulfur on V₂S₃ has been studied in a quartz reactor using either flow or circulating systems and in thermal diffusion column reactors at 400–800°C. Hydrogen yields were measured as a function of temperature in order to determine the effectiveness of these sulfides in promoting the decomposition. The results were compared with those for MoS₂, an established catalyst for this conversion. Vanadium sulfides were shown to be effective reagents for the conversion H₂S→H₂+ S° via a two-step process. Several mixed systems (V₂S₃/FeS, V₂S₃/Cu₉S₅ and V₂S₃/ZnS) were investigated in order to determine whether H₂ yields could be improved by a cooperative effect. The mixture V₂S₃/Cu₉S₅ formed Cu₃VS₄ at elevated temperatures and exhibited higher catalytic activity than MoS₂ in a closed circulating system using a quartz reactor. By contrast, MoS₂ was found to be a better catalyst than either V₂S₃ or V₂S₃/Cu₉S₅ in a thermal diffusion column reactor, although better separation of product H₂ from unreacted H₂S was observed for the latter two systems. The use of thermal diffusion column reactors resulted in a much higher conversion of H₂S into H₂ than was attainable in the circulating system using a quartz reactor.     

Transition metal doped MoS2 nanosheets for electrocatalytic hydrogen evolution reaction

In the present work, the effect of transition metals (Ni, Fe, Co) doping on 2-dimensional (2D) molybdenum disulfide (MoS₂) nanosheets for electrocatalytic hydrogen evolution reaction (HER) was explored. A simple and cost-effective hydrothermal method was adopted to synthesis transition metals doped MoS₂ nanosheets. The morphological and spectroscopic studies evidence the formation of high-quality MoS₂ nanosheets with the randomly doped metal ions. Notably, the Ni–MoS₂ displayed superior HER performance with an overpotential of ?0.302 V vs. reversible hydrogen electrode (RHE) (to attain the current density of 10 mA cm^?2) as compared to the other transition metals doped MoS₂ (Co–MoS₂, Fe–MoS₂). From the Nyquist plot, superior charge transport from the electrocatalyst to the electrolyte in Ni–MoS₂ was realized and confirmed that Ni doping provides the necessary catalytic active sites for rapid hydrogen production. The stable performance was confirmed with the cyclic test and chronoamperometry measurement and envisaged that hydrothermally synthesized Ni–MoS₂ is a highly desirable cost-effective approach for electrocatalytic hydrogen generation.     

In situ growth of MoS2 on carbon nanofibers with enhanced electrochemical catalytic activity for the hydrogen evolution

Developing an effective and facile method to achieve mass production of MoS₂ nanostructures with abundant of edges may be the feasible way to meet the increasing demand for hydrogen evolution electrocatalysts. We developed a facile glucose-assisted hydrothermal method to in-situ grow MoS₂ nanosheets on the commercial carbon nanofibers (CNFs). The controlled growth of MoS₂ on CNFs (MoS₂@CNFs) is leveraged to reveal mass ratio- and structure-dependent catalytic activity in the hydrogen evolution reaction (HER). Due to the unique shell structure, abundant edges of the MoS₂ layer are exposed as active site, as well as the underlying CNFs effectively improves the conductivity, the resulting MoS₂@CNFs hybrid exhibited high electrocatalytic activity in HER. The catalyst demonstrated the lowest overpotential of 52 mV, the highest current density of 101.49 mA cm^?2 at ～200 mV overpotential and the smallest Tafel slope of 49 mV/decade, suggesting the Volmer–Heyrovsky mechanism for the MoS₂-catalyzed HER.     

Hydrogen absorption and desorption kinetics of MgH2 catalyzed by MoS2 and MoO2

The catalytic effect of MoS₂ and MoO₂ on the hydrogen absorption/desorption kinetics of MgH₂ has been investigated. It is shown that MoS₂ has a superior catalytic effect over MoO₂ on improving the hydrogen kinetic properties of MgH₂. DTA results indicated that the desorption temperature decreased from 662.10 K of the pure MgH₂ to 650.07 K of the MgH₂ with MoO₂ and 640.34 K of that with MoS₂. Based on the Kissinger plot, the activation energy of the hydrogen desorption process is estimated to be 101.34 ± 4.32 kJ mol⁻¹ of the MgH₂ with MoO₂ and 87.19 ± 4.48 kJ mol⁻¹ of that with MoS₂, indicating that the dehydriding process energy barrier of MgH₂ can be reduced. The enhancement of the hydriding/dehydriding kinetics of MgH₂ is attributed to the presence of MgS and Mo or MgO and Mo which catalyze the hydrogen absorption/desorption behavior of MgH₂. The detailed comparisons between MoS₂ and MoO₂ suggest that S anion has superior properties than O anion on catalyzing the hydriding/dehydriding kinetics of MgH₂.     

Low loaded MoS2/Carbon cloth as a highly efficient electrocatalyst for hydrogen evolution reaction

Active edge sites of MoS₂ nanosheets exhibit promising futures for hydrogen evolution reaction (HER), comparable with remarkable performances of highly cost platinum. However, 3D structures of MoS₂ suffer from a lack of high mobility and unexposed active sites which lower the electrocatalytic activity. In this study, we show that there is a balance between increasing the active sites on the one hand and managing the charge transfer to facilitate the reaction on the other hand, and achieving this balance increases the efficiency of the electrocatalyst tremendously. For this purpose, we directly attached exfoliated MoS₂ nanosheets onto carbon cloth (CC) substrate as a 3D network of conductive fibers via electrophoretic deposition at different applied voltages and deposition times, without adding any binder. This strategy gives rise to superior exposure of active sites while still maintaining good charge transferability over the whole 3D structure. Thus, a trace amount of loaded catalyst is enough to reach the benchmark current density of 10 mA/cm² towards H₂ production with a low overpotential of 137 mV vs. RHE. The Stability of the optimum structures under continuous operation was examined up to 50 min and further confirmed with 500 successive cyclic voltammetry (CV) sweeps. To understand the adsorption nature of hydrogen, density functional theory (DFT) was employed for the MoS₂/graphene in both cases of pristine and defected MoS₂. The results of hydrogen adsorption free energy calculations revealed that H adsorption on the S site is the most stable adsorption configuration and with increasing the MoS₂ thickness, the MoS₂/graphene activity towards hydrogen evolution decreases. A similar trend is also observed for the defected MoS₂/graphene composite up to two-layer MoS₂ and the activity remains the same for three-layer MoS₂. The experimentally observed charge transfer into the MoS₂ upon adsorption of the hydrogen atom and water molecule is also confirmed by our DFT calculations.