Femtosecond Laser-Ablated Copper Surface as a Substrate for a MoS2-Based Hydrogen Evolution Reaction Electrocatalyst

One of the methods to improve the performance of a heterogeneous electrocatalyst is the dispersion of a catalytic material on a suitable substrate. In this study, femtosecond laser ablation was used to prepare very rough but also ordered copper surfaces consisting of vertical, parallel ridges. Then, a molybdenum sulfide coating was electrochemically deposited onto these surfaces. It was observed by profilometry that the average roughness of the surface after coating with MoS₂ had decreased, but the developed surface area still remained significantly larger than the projected surface area. The electrodes were then used as an electrocatalyst for the hydrogen evolution reaction in acidic media. These were highly efficient, reaching 10 mA cm⁻² of HER current at a −181 mV overpotential and a Tafel slope of ~39 mV dec⁻¹. Additionally, scanning electrochemical microscopy was used to observe whether hydrogen evolution would preferentially occur in certain spots, for example, on the peaks, but the obtained results suggest that the entire surface is active. Finally, the electrochemical impedance spectroscopy data showed the difference in the double-layer capacitance between the ablated and non-ablated surfaces (up to five times larger) as well as the parameters that describe the improved catalytic activity of fs-Cu/MoS₂ electrodes.

Engineering Surface Architectures for Improved Durability in III-V Photocathodes

GaInP₂ has shown promise as the wide bandgap top junction in tandem absorber photoelectrochemical (PEC) water splitting devices. Among previously reported dual-junction PEC devices with a GaInP₂ top cell, those with the highest performance incorporate an AlInP₂ window layer (WL) to reduce surface recombination and a thin GaInP₂ capping layer (CL) to protect the WL from corrosion in electrolytes. However, the stability of these III-V systems is limited, and durability continues to be a major challenge broadly in the field of PEC water splitting. This work provides a systematic investigation into the durability of GaInP₂ systems, examining the impacts of the window layer and capping layer among single junction pn-GaInP₂ photocathodes coated with an MoS₂ catalytic and protective layer. The photocathode with both a CL and WL demonstrates the highest PEC performance and longest lifetime, producing a significant current for >125 h. In situ optical imaging and post-test characterization illustrate the progression of macroscopic degradation and chemical state. The surface architecture combining an MoS₂ catalyst, CL, and WL can be translated to dual-junction PEC devices with GaInP₂ or other III-V top junctions to enable more efficient and stable PEC systems.

Synergistic coupling of heterostructured porous CoP nanosheets with P doped NiO for highly efficient overall alkaline water splitting

Exploring non-noble metal materials as bifunctional catalysts for water electrolysis is of great significance for the development and utilization of hydrogen energy. Herein, a flower branch-leaf shaped phosphide/oxide heterogeneous electrocatalyst located on Ni foam (CoP/P-NiO/NF) was developed through hydrothermal and phosphorization strategy. Benefiting from the strong ability to dissociate H₂O molecules on P-NiO and the suitable adsorption of intermediate H species on CoP, the optimal CoP/P-NiO/NF exhibited outstanding performance with low overpotentials of 52 mV at current density of 10 mA cm^-2, smaller Tafel slopes of 73.6 mV dec^-1 for hydrogen evolution reaction (HER). Meanwhile, CoP/P-NiO/NF indicated 265 mV at 100 mA cm^-2 with Tafel slope of 101.8 mV dec^-1 for oxygen evolution reaction (OER) due to the optimal redistribution of electrons among Ni²⁺, Co²⁺ and Co³⁺ for favorable adsorption/desorption of oxygen-intermediates. Both HER and OER shown robust stability during 32 h without decline. The corresponding two-electrode system for overall alkaline water splitting required a low voltage of 1.6 V at 100 mA cm^-2 with long stability (20 h) which is far lower than that on RuO₂-Pt/C and many other reported non-noble metal electrocatalysts. This work demonstrates that the synergistic effect and morphology engineering play vital roles in the enhanced electrocatalytic performance.

Black Si Photocathode with a Conformal and Amorphous MoSx Catalytic Layer Grown Using Atomic Layer Deposition for Photoelectrochemical Hydrogen Evolution

We demonstrated how the photoelectrochemical (PEC) performance was enhanced by conformal deposition of an amorphous molybdenum sulfide (a-MoS_x) thin film on a nanostructured surface of black Si using atomic layer deposition (ALD). The a-MoS_x is found to predominantly consist of an octahedral structure (S-deficient metallic phase) that exhibits high electrocatalytic activity for the hydrogen evolution reaction with a Tafel slope of 41 mV/dec in an acid electrolyte. The a-MoS_x has a smaller work function (4.0 eV) than that of crystalline 2H-MoS₂ (4.5 eV), which induces larger energy band bending at the p-Si surface, thereby facilitating interface charge transfer. These features enabled us to achieve an outstanding kinetic overpotential of ∼0.2 V at 10 mA/cm² and an onset potential of 0.27 V at 1 mA/cm². Furthermore, the a-MoS_x layer provides superior protection against corrosion of the Si surface, enabling long-term PEC operation of more than 50 h while maintaining 87% or more performance. This work highlights the remarkable advantages of the ALD a-MoS_x layer and leads to a breakthrough in the architectural design of PEC cells to ensure both high performance and stability.

Integrating Amorphous Molybdenum Sulfide Nanosheets with a Co9S8@Ni3S2 Array as an Efficient Electrocatalyst for Overall Water Splitting

It is highly challenging to design low-cost, efficient electrocatalysts for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Herein, a hierarchical heterostructure was constructed on three-dimensional (3D) Ni foam, which contains Ni₃S₂ nanorods decorated with both Co₉S₈ and amorphous MoS_x nanosheets and Ni₃S₂ nanowires decorated with amorphous MoS_x nanosheets, namely, MoS_x@Co₉S₈@Ni₃S₂/NF. The synergistic effects from the strong interactions of the heterointerface and unique hierarchical heterostructure endow the MoS_x@Co₉S₈@Ni₃S₂/NF with abundant active sites and effective mass and electron transport pathways, resulting in excellent activity toward both HER and OER in 1 M KOH. It only gives a low overpotential of 76.5 mV to achieve 10 mA cm^-2 for HER and a low overpotential of 310 mV to achieve 100 mA cm^-2 for OER. Based on the superior catalytic activity of MoS_x@Co₉S₈@Ni₃S₂/NF for OER and HER, we demonstrated the activity of overall water splitting using MoS_x@Co₉S₈@Ni₃S₂/NF as both the anode and cathode. It shows a higher catalytic activity for overall water splitting with a low cell voltage of 1.52 V at 10 mA cm^-2 than commercial Pt/C/NF||IrO₂/NF (1.61 V) and superior stability. This work provides a platform for the design and preparation of efficient electrocatalysts with various hierarchical heterostructures.

Heteroatom-Doped Amorphous Cobalt-Molybdenum Oxides as a Promising Catalyst for Robust Hydrogen Evolution

The simultaneous manipulation of the catalytic activity and intrinsic electrical conductivity in a unified system is difficult yet meaningful to unravel the possible strategy that can enhance the hydrogen evolution reaction (HER) performance. Therefore, we propose a simple strategy to enhance the HER performance based on low-temperature redox reaction with ZIF-67@ZIF-8 as a sacrificial template to prepare zinc-doped amorphous CoMo₈O_x (denoted as Zn/aCMO). Benefiting from the excellent compositional- and amorphous-based structural advantages of more exposure active sites, optimized electron transfer as well as a stable frame structure, the as-prepared electrode can drive hydrogen evolution at current densities of 10, 50, and 100 mA cm^-2, which need ultralow overpotentials of 59, 138, and 189 mV, respectively, and the Tafel slope of the electrode was 66.2 mV dec^-1 (1 M KOH). Meanwhile, the intrinsic activity of the prepared low-cost electrocatalyst was also determined, and the turnover frequency was up to 1.49 s^-1 at an overpotential of 100 mV. In addition, after continuous testing for 160 h, there was a slight decay at the overpotential of 130 mV.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

MoS2 nanosheets vertically grown on CoSe2 hollow nanotube arrays as an efficient catalyst for the hydrogen evolution reaction

Although the design and synthesis of efficient electrocatalysts for the hydrogen evolution reaction (HER) are highly desirable, severe challenges still need to be addressed. Herein, ultrathin MoS₂ nanosheets were vertically grown on CoSe₂ hollow nanotube arrays via a simple three-step hydrothermal reaction by using carbon cloth (CC) as a substrate and were subsequently used as a highly efficient HER electrocatalyst (MoS₂@CoSe₂-CC hybrid). The MoS₂ nanosheets uniformly self-assembled on conductive CoSe₂ nanotube arrays exhibited a hierarchical and well-ordered structure. Such a unique structure may not only comprise more exposed active sites, but also enable fast electrolyte penetration and facilitate H⁺/electron transportation to accelerate the reduction and evolution of H₂ during the electrocatalytic process. As an HER electrocatalyst with a novel three-dimensional hierarchical structure, the MoS₂@CoSe₂-CC hybrid exhibited an outstanding catalytic HER performance with a small Tafel slope of 67 mV dec^-1 in alkaline media, while only requiring a low HER overpotential of 101 mV at 10 mA cm^-2. Notably, the MoS₂@CoSe₂-CC hybrid also demonstrated exceptional electrochemical durability and structural stability even after 1000 cycles or 48 h of continuous electrolysis. Overall, this work presents a new approach for the design and synthesis of robust, highly active, and cost-effective electrocatalysts for hydrogen generation.

Ru@Ni3S2 nanorod arrays as highly efficient electrocatalysts for the alkaline hydrogen evolution reaction

Ru-decorated Ni3S2 nanorod arrays demonstrate an superior alkaline hydrogen evolution performance. Further modification with polyaniline could significantly enhance the long-term stability for continuous hydrogen generation.

Vanadium-bearing steel slag catalysts for the selective catalytic reduction of NOx by NH3

In this paper, denitration catalysts were prepared by different modification methods using vanadium-bearing steel slag as raw material.

Agaric-like cobalt diselenide supported by carbon nanofiber as an efficient catalyst for hydrogen evolution reaction

We synthesized herein a novel 3D cathode constructed by growing cobalt diselenide in situ on the surface of carbon nanofiber for hydrogen evolution reaction. The cobalt diselenides with two typical morphologies (agaric-like and nanorod-like) were synthesized by precisely controlling reaction time and temperature in the same system. They show excellent electrocatalytic performance for hydrogen evolution reactions. Especially, the agaric-like diselenide cobalt electrode has the low overpotential (187 and 199 mV) to obtain the current density of 50 and 100 mA cm^-2 with a small Tafel slope of 37 mV dec^-1 in acidic medium. The excellent catalytic performance of the agaric-like cobalt diselenide can be attributed to its large specific surface area and fast electron transfer rate. More importantly, the agaric-like cobalt diselenide supported carbon nanofiber electrode has excellent long-term stability in electrolyte. The outstanding electrocatalytic performance and stability of agaric-like cobalt diselenide supported carbon nanofiber indicate that it is a promising electrocatalyst for hydrogen evolution reactions.

Amorphous MoSxOy/h-BNxOy Nanohybrids: Synthesis and Dye Photodegradation

Molybdenum sulfide is a very promising catalyst for the photodegradation of organic pollutants in water. Its photocatalytic activity arises from unsaturated sulfur bonds, and it increases with the introduction of structural defects and/or oxygen substitutions. Amorphous molybdenum sulfide (a-MoSxOy) with oxygen substitutions has many active sites, which create favorable conditions for enhanced catalytic activity. Here we present a new approach to the synthesis of a-MoSxOy and demonstrate its high activity in the photodegradation of the dye methylene blue (MB). The MoSxOy was deposited on hexagonal boron oxynitride (h-BNO) nanoflakes by reacting h-BNO, MoCl5, and H2S in dimethylformamide (DMF) at 250 °C. Both X-ray diffraction analysis and high-resolution TEM show the absence of crystalline order in a-MoSxOy. Based on the results of Raman and X-ray photoelectron spectroscopy, as well as analysis by the density functional theory (DFT) method, a chain structure of a-MoSxOy was proposed, consisting of MoS3 clusters with partial substitution of sulfur by oxygen. When a third of the sulfur atoms are replaced with oxygen, the band gap of a-MoSxOy is approximately 1.36 eV, and the valence and conduction bands are 0.74 eV and -0.62 eV, respectively (relative to a standard hydrogen electrode), which satisfies the conditions of photoinduced splitting of water. When illuminated with a mercury lamp, a-MoSxOy/h-BNxOy nanohybrids have a specific mass activity in MB photodegradation of approximately 5.51 mmol g-1 h-1, which is at least four times higher than so far reported values for nonmetal catalysts. The photocatalyst has been shown to be very stable and can be reused.

Sulfur dopant-enhanced neutral hydrogen evolution performance in MoO3nanosheets

Developing nonprecious-metal based catalysts with highly active and stable performance for hydrogen evolution reaction (HER) in neutral media is crucial points for realizing low-carbon economy because their practical use typically suffers from the slow kinetics. Herein, we developed S-doped MoO₃nanosheets toward neutral HER, fabricated by a versatile solvothermal and subsequently sulfuration processes. The obtained catalyst exhibits a small overpotential of 106 mV to reach 10 mA cm^-2in 1.0 M phosphate buffered saline, overwhelming most of recently reported catalysts. Meantime, it shows no notable deactivation after more than 60 h continuous electrolysis and 50 000 cycling tests. More importantly, the catalyst also can be applied in buffered seawater for electrocatalyzing HER, requiring 262 mV at 10 mA cm^-2and maintaining over 60 h. These findings open a new route for designing MoO₃-based catalysts for neutral hydrogen production.

Highly efficient sub-nanometer RuxCuyP2 nanoclusters designed for hydrogen evolution under alkaline media

Design of highly active and stable non-precious electrocatalysts towards hydrogen evolution reaction (HER) is a hot research topic in low cost, clean and sustainable hydrogen energy field, yet remaining the important challenge caused by the sluggish reaction kinetics for water-alkali electrolyzers. Herein, a robust electrocatalyst is proposed by designing a novel sub-nanometer of copper and ruthenium bimetallic phosphide nanoclusters (Ru_xCu_yP₂) supported on a graphited carbon nanofibers (CNF). Uniform Ru_xCu_yP₂ (~1.90 nm) on the surface of CNF are obtained by introducing the dispersed Ru, thereby improving the intrinsic activity for HER. On optimizing the Ru ratio, the (x = y = 1) RuCuP₂/CNF catalyst exhibits an excellent HER electroactivity with an overpotential of 10 mV in 1.0 M NaOH electrolyte to produce 10 mA cm^-2 current density, which is lower than commercial 20% Pt/C in alkaline solution. Moreover, the kinetic study demonstrated that electrochemical activation energies for HER of RuCuP₂/CNF is 20.7 kJ mol^-1 highest among different ratio bimetallic phosphide. This simple, cost-effective, and environmentally friendly methodology can pave the way for exploitation of bimetallic phosphide nanoclusters for catalyst design.

Towards Highly Efficient Chalcopyrite Photocathodes for Water Splitting: The Use of Cocatalysts beyond Pt

Solar radiation is a renewable and clean energy source used in photoelectrochemical cells (PEC) to produce hydrogen gas as a powerful alternative to carbon-based fuels. Semiconductors play a vital role in this approach, absorbing the incident solar photons and converting them into electrons and holes. The hydrogen evolution reaction (HER) occurs in the interface of the p-type semiconductor that works as a photocathode in the PEC. Cu-chalcopyrites such as Cu(In, Ga)(Se,S)₂ (CIGS) and CuIn(Se,S)₂ (CIS) present excellent semiconductor characteristics for this purpose, but drawbacks as charge recombination, deficient chemical stability, and slow charge transfer kinetics, demanding improvements like the use of n-type buffer layer, a protective layer, and a cocatalyst material. Concerning the last one, platinum (Pt) is the most efficient and stable material, but the high price due to its scarcity imposes the search for inexpensive and abundant alternative cocatalyst. The present Minireview highlighted the use of metal alloys, transition metal chalcogenides, and inorganic carbon-based nanostructures as efficient alternative cocatalysts for HER in PEC.

Prolonged-Photoresponse-Lifetime Ni2P Nanocrystalline with Highly Exposed (001) for Efficient Photoelectrocatalytic Hydrogen Evolution

Seeking highly efficient non-preference electrocatalytic materials that serve photoelectrochemical (PEC) water splitting in acidic systems is expectant in the context of environmentally friendly production. We designed Ni₂P electrocatalysts synthesized in oil phases via the hot-bubbling method with superb stability in air and sulfuric acid solution for PEC, which were found with excellent hydrogen evolution performance. A tunable particle size and highly exposed (001) planes of Ni₂P nanocrystals were achieved. The designed catalysts achieved a notable promotion in the hydrogen evolution reaction activity compared to that of Ni₂P synthesized in the water phase. More specifically, the electrode prepared by self-assembled Ni₂P nanoparticles was found to have decent over-potential of η₁₀ = 164 mV in darkness and was further decreased to 129 mV with irradiation of visible light. The cyclic stability tests manifested brilliant durability in 0.5 M H₂SO₄. Measurement of the transient photocurrent response and PEC water splitting catalytic performance indicated that the Ni₂P had high carrier concentration upon irradiation, lower carrier recombination probability, and prolonged photo-response lifetime (3.03-3.14 s).

High-Density Ruthenium Single Atoms Anchored on Oxygen-Vacancy-Rich g-C3N4-C-TiO2 Heterostructural Nanosphere for Efficient Electrocatalytic Hydrogen Evolution Reaction

Designing and constructing high-density single-atom catalysts (SACs) is vital for electrochemical hydrogen evolution to meet the demand for fundamental research and practical applications of electrocatalysis. However, it is challenging to synthesize atomically dispersed electrocatalysts with high density and high performance. Herein, an integrated g-C₃N₄-C-TiO₂ heterostructural nanosphere with oxygen-rich vacancies is constructed by a multicomponent assembly-calcination strategy. Abundant single Ru atoms (12.4 wt %) are then anchored via the occupation of partial oxygen vacancies, forming a unique Ru/g-C₃N₄-C-TiO₂ heterostructure. A reasonable configuration is developed including single Ru atoms bonded with two oxygens and two nitrogens and coupled with jacent oxygen vacancies on the g-C₃N₄-C-TiO₂ nanosphere. Density functional theory calculations reveal that the remaining oxygen vacancies are beneficial for water dissociation, while single Ru atoms facilitate hydrogen adsorption. As expected, the result exhibits high electrocatalytic activity, delivering overpotentials of 112 and 107 mV at 10 mA cm^-2, Tafel slopes of 83 and 65 mV dec^-1 in H₂SO₄ and KOH, and a turnover frequency of 0.28 H₂ s^-1 at -100 mV toward the hydrogen evolution reaction (HER). Benefiting from the outstanding electrocatalytic performance, such a unique heterostructure with dense single Ru sites and oxygen vacancies could serve as a prominent alternative HER catalyst for renewable energy applications.

Edge-Rich Bicrystalline 1T/2H-MoS2 Cocatalyst-Decorated {110} Terminated CeO2 Nanorods for Photocatalytic Hydrogen Evolution

Developing all-solid-state Z-scheme systems with highly active photocatalysts are of huge interest in realizing long-term solar-to-fuel conversion. Here we reported an innovative hybrid of {110}-oriented CeO₂ nanorods with edge-enriched bicrystalline 1T/2H-MoS₂ coupling as efficient photocatalysts for water splitting. In the composites, the metallic 1T phase acts as an excellent solid state electron mediator in the Z-scheme, while the 2H phase and CeO₂ are the adsorption sites of the photosensitizer and reactant (H₂O), respectively. Through optimal structure and phase engineering, 1T/2H-MoS₂@CeO₂ heterojunctions simultaneously achieve high charge separation efficiency, proliferated density of exposed active sites, and excellent affinity to reactant molecules, reaching a superior hydrogen evolution rate of 73.1 μmol/h with an apparent quantum yield of 8.2% at 420 nm. Furthermore, density functional theory calculations show that 1T/2H-MoS₂@CeO₂ possesses the advantages of intensive electronic interaction from the built-in electric field (negative MoS₂ and positive charged CeO₂) and reduced H₂O adsorption/dissociation energies. This work sheds light on the design of on-demand noble-metal-free Z-scheme heterostructures for solar energy conversion.

Roadmap and Direction toward High-Performance MoS2 Hydrogen Evolution Catalysts

MoS₂ intrinsically show Pt-like hydrogen evolution reaction (HER) performance. Pristine MoS₂ displayed low HER activity, which was caused by low quantities of catalytic sites and unsatisfactory conductivity. Then, phase engineering and S vacancy were developed as effective strategies to elevate the intrinsic HER performance. Heterojunctions and dopants were successful strategies to improve HER performance significantly. A couple of state-of-the-art MoS₂ catalysts showed HER performance comparable to Pt. Applying multiple strategies in the same electrocatalyst was the key to furnish Pt-like HER performance. In this review, we summarize the available strategies to fabricate superior MoS₂ HER catalysts and tag the important works. We analyze the well-defined strategies for fabricating a superior MoS₂ electrocatalyst, propose complementary strategies which could help meet practical requirements, and help people design highly efficient MoS₂ electrocatalysts. We also provide a brief perspective on assembling practical electrochemical systems by high-performance MoS₂ electrocatalysts, apply MoS₂ in other important electrocatalysis reactions, and develop high-performance two-dimensional (2D) dichalcogenide HER catalysts not limited to MoS₂. This review will help researchers to obtain a better understanding of development of superior MoS₂ HER electrocatalysts, providing directions for next-generation catalyst development.

The facile synthesis of core-shell PtCu nanoparticles with superior electrocatalytic activity and stability in the hydrogen evolution reaction

Pt is the most efficient electrocatalyst for the hydrogen evolution reaction (HER); however, it is a high cost material with scarce resources. In order to balance performance and cost in a Pt-based electrocatalyst, we prepared a series of PtCu bimetallic nanoparticles (NPs) with different Pt/Cu ratios through a facile synthetic strategy to optimize the utilization of Pt atoms. PtCu NPs demonstrate a uniform particle size distribution with exposed (111) facets that are highly active for the HER. A synergetic effect between Pt and Cu leads to electron transfer from Pt to Cu, which is favorable for the desorption of H intermediates. Therefore, the as-synthesized carbon black (CB) supported PtCu catalysts showed enhanced catalytic performance in the HER compared with a commercial Pt/C electrocatalyst. Typically, Pt₁Cu₃/CB showed excellent HER performance, with only 10 mV (acid) and 17 mV (alkaline) overpotentials required to achieve a current density of 10 mA cm^-2. This is because the Pt₁Cu₃ NPs, with a small average particle size (7.70 ± 0.04 nm) and Pt-Cu core and Pt-rich shell structure, display the highest electrochemically active surface area (24.7 m² g_Pt ^-1) out of the as-synthesized PtCu/CB samples. Furthermore, Pt₁Cu₃/CB showed good electrocatalytic stability, with current density drops of only 9.3% and 12.8% in acidic solution after 24 h and in alkaline solution after 9 h, respectively. This study may shed new light on the rational design of active and durable hydrogen evolution catalysts with low amounts of Pt.

Highly Accurate and Fast Electrochemical Detection of Scrub Typhus DNA via a Nanoflower NiFe-Based Biosensor

Owing to the lack of specific diagnostic methods, Scrub typhus can sometimes be difficult to diagnose in the Asia-Pacific region. Therefore, an efficient and rapid detection method urgently needs to be developed. Based on competitive single-stranded DNA over modified glassy carbon electrode (GCE), an electrochemical biosensor was established to detect the disease. The nano-flower NiFe layered double hydroxide (NiFe-LDH) modified GCE has a large specific surface area, which supported a large amount of gold nanoparticles, so that a wide linear detection range from 25 fM to 0.5 μM was obtained. The beacon DNA (B-DNA) with the same sequence as the Scrub typhus DNA (T-DNA), but labeled with methylene blue, was used to construct a competitive relationship. When T-DNA and B-DNA were present on the sensor simultaneously, they would hybridize with probe DNA in a strong competition, and the corresponding electrochemical response signal would be generated via testing. It contributed to reducing tedious experimental procedures and excessive response time, and achieved fast electrochemical detection of DNA. The strategy provides a worthy avenue and possesses promising applications in disease diagnosis.

Low-Temperature Direct Electrochemical Methanol Reforming Enabled by CO-Immune Mo-Based Hydrogen Evolution Catalysts

Hydrogen storage in the form of intermediate artificial fuels such as methanol is important for future chemical and energy applications, and the electrochemical regeneration of hydrogen from methanol is thermodynamically favorable compared to direct water splitting. However, CO produced from methanol oxidation can adsorb to H₂ -evolution catalysts and drastically reduce activity. In this study, we explore the origins of CO immunity in Mo-containing H₂ -evolution catalysts. Unlike conventional catalysts such as Pt or Ni, Mo-based catalysts display remarkable immunity to CO poisoning. The origin of this behavior in NiMo appears to arise from the apparent inability of CO to bind Mo under electrocatalytic conditions, with mechanistic consequences for the H₂ -evolution reaction (HER) in these systems. This specific property of Mo-based HER catalysts makes them ideal in environments where poisons might be present.

Transition Metal Chalcogenides as a Versatile and Tunable Platform for Catalytic CO2 and N2 Electroreduction

Group VI transition metal chalcogenides are the subject of increasing research interest for various electrochemical applications such as low-temperature water electrolysis, batteries, and supercapacitors due to their high activity, chemical stability, and the strong correlation between structure and electrochemical properties. Particularly appealing is their utilization as electrocatalysts for the synthesis of energy vectors and value-added chemicals such as C-based chemicals from the CO₂ reduction reaction (CO₂R) or ammonia from the nitrogen fixation reaction (NRR). This review discusses the role of structural and electronic properties of transition metal chalcogenides in enhancing selectivity and activity toward these two key reduction reactions. First, we discuss the morphological and electronic structure of these compounds, outlining design strategies to control and fine-tune them. Then, we discuss the role of the active sites and the strategies developed to enhance the activity of transition metal chalcogenide-based catalysts in the framework of CO₂R and NRR against the parasitic hydrogen evolution reaction (HER); leveraging on the design rules applied for HER applications, we discuss their future perspective for the applications in CO₂R and NRR. For these two reactions, we comprehensively review recent progress in unveiling reaction mechanisms at different sites and the most effective strategies for fabricating catalysts that, by exploiting the structural and electronic peculiarities of transition metal chalcogenides, can outperform many metallic compounds. Transition metal chalcogenides outperform state-of-the-art catalysts for CO₂ to CO reduction in ionic liquids due to the favorable CO₂ adsorption on the metal edge sites, whereas the basal sites, due to their conformation, represent an appealing design space for reduction of CO₂ to complex carbon products. For the NRR instead, the resemblance of transition metal chalcogenides to the active centers of nitrogenase enzymes represents a powerful nature-mimicking approach for the design of catalysts with enhanced performance, although strategies to hinder the HER must be integrated in the catalytic architecture.

Mechanical transfer of electrochemically grown molybdenum sulfide layers to silicon wafer

Abstract

Large area MoS2 ultra-thin film deposition is one of the big challenges in the recent years. Electrodeposition provides an opportunity to grow such ultra-thin films on large scale. However, the transfer of the electrochemically grown film is challenging. Standard transfer of those thin films is done by wet etching in which the underlying substrate is etched. In this work, the polymer coated electrodeposited MoS2 films on Au are separated mechanically from the underlying substrate by using ultra-sonication. Collapse of micron-sized bubbles produced by ultra-sonication at the interface of Au and silicon substrate provides enough energy for separation due to their weak adhesion. The Au layer is then removed by standard Au-etchant (K/KI) and the polymer coated film is transferred to a desired substrate. Ammonium tetrathiomolybdate (ATTM) has been used as precursor material for the electrodeposition of the films. Initial electrochemically grown films consist of MoS3 which is reduced to MoS2 by a post-annealing step at 450–900 °C. Obtained films are investigated by AFM, Raman, UV–Vis and XPS. Crystal quality improves by increasing the post-annealing temperature. The thickness of the thinnest film was found to be equivalent to 2 monolayers of MoS2, which is desirable for future electronics.

Graphic abstract

Amorphous inorganic semiconductors for the development of solar cell, photoelectrocatalytic and photocatalytic applications

Amorphous inorganic semiconductors have attracted growing interest due to their unique electrical and optical properties that arise from their intrinsic disordered structure and thermodynamic metastability. Recently, amorphous inorganic semiconductors have been applied in a variety of new technologies, including solar cells, photoelectrocatalysis, and photocatalysis. It has been reported that amorphous phases can improve both efficiency and stability in these applications. While these phenomena are well established, their mechanisms have long remained unclear. This review first introduces the general background of amorphous inorganic semiconductor properties and synthesis. Then, the recent successes and current challenges of amorphous inorganic semiconductor-based materials for applications in solar cells, photoelectrocatalysis, and photocatalysis are addressed. In particular, we discuss the mechanisms behind the remarkable performances of amorphous inorganic semiconductors in these fields. Finally, we provide insightful perspectives into further developments for applications of amorphous inorganic semiconductors.

Fabrication of a porous NiFeP/Ni electrode for highly efficient hydrazine oxidation boosted H2 evolution

Rational optimization of the surface electronic states and physical structures of non-noble metal nanomaterials is essential to improve their electrocatalytic performance. Herein, we report a facile dual-regulation strategy to fabricate NiFeP/Ni (P-NiFeP/Ni) porous nanoflowers, which involves Fe-doping and creating pores on nanosheets. The as-prepared P-NiFeP/Ni has a hierarchically porous surface, which exposes more electrochemically active sites and dramatically enhances the electron transfer rate. Thus, it exhibits excellent catalytic activity in both anodic hydrazine oxidation reaction (HzOR) and cathodic hydrogen evolution reaction (HER). Interestingly, the coupled electrolysis cell only offers a potential of 0.162 V at 10 mA cm^-2 to enable HzOR boosted H₂ evolution, highlighting an energy-saving hydrogen evolution strategy.

A ternary hybrid of Zn-doped MoS2-RGO for highly effective electrocatalytic hydrogen evolution

Modification of MoS₂-based catalysts is effective in solving the overdependence of hydrogen evolution reactions (HERs) on noble metal catalysts. In this work, a Zn-doped molybdenum disulfide-reduced graphene oxide (Zn-MoS₂-RGO) hybrid was synthesized in one step employing a hydrothermal method. By substituting the position of Mo, uniform doping with Zn improved the catalytic activity of MoS₂ for HER. The interlayer spacing of MoS₂ increased from 0.65 to 0.75 nm, demonstrating RGO effectively interpolate into MoS₂ nanosheets. This prevented aggregation and exposed more edge active sites of MoS₂. According to density functional theory (DFT) calculations, the layered structure of the MoS₂ nanosheets doped with Zn and intercalated with RGO promoted charge transfer and resulted in outstanding hydrogen evolution activity. Compared with MoS₂ (6.86 eV), the Zn-MoS₂-RGO hybrid (5.47 eV) with a considerably lower energy level value exhibited excellent electrocatalytic performance. Under optimal conditions, at a potential of -0.3 V vs. RHE, the current density reached -169 mA cm^-2 in a 0.5 M H₂SO₄ solution, 4.78 μmol of H₂ was produced in 6 h, and the Faraday efficiency reached 92%. The results obtained herein indicated that Zn-MoS₂-RGO was a promising candidate for application in electrocatalytic HER.

Neutron spectroscopy study of the diffusivity of hydrogen in MoS2

The diffusion of hydrogen adsorbed inside layered MoS₂ crystals has been studied by means of quasi-elastic neutron scattering, neutron spin-echo spectroscopy, nuclear reaction analysis, and X-ray photoelectron spectroscopy. The neutron time-of-flight and neutron spin-echo measurements demonstrate fast diffusion of hydrogen molecules parallel to the basal planes of the two dimensional crystal planes. At room temperature and above, this intra-layer diffusion is of a similar speed to the surface diffusion that has been observed in earlier studies for hydrogen atoms on Pt surfaces. A significantly slower hydrogen diffusion was observed perpendicular to the basal planes using nuclear reaction analysis.

Earth-Abundant Electrocatalysts for Water Splitting: Current and Future Directions

Of all the available resources given to mankind, the sunlight is perhaps the most abundant renewable energy resource, providing more than enough energy on earth to satisfy all the needs of humanity for several hundred years. Therefore, it is transient and sporadic that poses issues with how the energy can be harvested and processed when the sun does not shine. Scientists assume that electro/photoelectrochemical devices used for water splitting into hydrogen and oxygen may have one solution to solve this hindrance. Water electrolysis-generated hydrogen is an optimal energy carrier to store these forms of energy on scalable levels because the energy density is high, and no air pollution or toxic gas is released into the environment after combustion. However, in order to adopt these devices for readily use, they have to be low-cost for manufacturing and operation. It is thus crucial to develop electrocatalysts for water splitting based on low-cost and land-rich elements. In this review, I will summarize current advances in the synthesis of low-cost earth-abundant electrocatalysts for overall water splitting, with a particular focus on how to be linked with photoelectrocatalytic water splitting devices. The major obstacles that persist in designing these devices. The potential future developments in the production of efficient electrocatalysts for water electrolysis are also described.

Co-Doped Ni3N Nanosheets with Electron Redistribution as Bifunctional Electrocatalysts for Efficient Water Splitting

Preparation of high-activity and earth-abundant bifunctional catalysts for efficient electrochemical water splitting are crucial and challenging. Herein, Co-doped Ni₃N nanosheets loaded on nickel foam (Co-Ni₃N) were synthesized. The as-prepared Co-Ni₃N exhibits excellent catalytic activity toward both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in alkaline media. Density functional theory (DFT) calculation reveals that Co-Ni₃N with redistribution of electrons not only can facilitate the HER kinetics but also can regulate intermediates adsorption energies for OER. Specifically, the Co-Ni₃N exhibits high efficiency and stable catalytic activity, with an overpotential of only 30 and 270 mV at a current density of 10 mA cm^-2 for the HER and OER in 1 M KOH, respectively. This work provides strong evidence to the merit of Co doping to improve the innate electrochemical performance in bifunctional catalysts, which might have a common impact in many similar metal-metal nitride electrocatalysts.

Electrocatalytic hydrogen evolution on the noble metal-free MoS2/carbon nanotube heterostructure: a theoretical study

Molybdenum disulfide (MoS₂) is considered as a promising noble-metal-free electrocatalyst for the Hydrogen Evolution Reaction (HER). However, to effectively employ such material in the HER process, the corresponding electrocatalytic activity should be comparable or even higher than that of Pt-based materials. Thus, efforts in structural design of MoS₂ electrocatalyst should be taken to enhance the respective physico-chemical properties, particularly, the electronic properties. Indeed, no report has yet appeared about the possibility of an HER electrocatalytic association between the MoS₂ and carbon nanotubes (CNT). Hence, this paper investigates the synergistic electrocatalytic activity of MoS₂/ CNT heterostructure for HER by Density Functional Theory simulations. The characteristics of the heterostructure, including density of states, binding energies, charge transfer, bandgap structure and minimum-energy path for the HER process were discussed. It was found that regardless of its configuration, CNT is bound to MoS₂ with an atomic interlayer gap of 3.37 Å and binding energy of 0.467 eV per carbon atom, suggesting a weak interaction between CNT and MoS₂. In addition, the energy barrier of HER process was calculated lower in MoS₂/CNT, 0.024 eV, than in the MoS₂ monolayer, 0.067 eV. Thus, the study elaborately predicts that the proposed heterostructure improves the intrinsic electrocatalytic activity of MoS₂.

Molybdenum-Containing Metalloenzymes and Synthetic Catalysts for Conversion of Small Molecules

The energy deficiency and environmental problems have motivated researchers to develop energy conversion systems into a sustainable pathway, and the development of catalysts holds the center of the research endeavors. Natural catalysts such as metalloenzymes have maintained energy cycles on Earth, thus proving themselves the optimal catalysts. In the previous research results, the structural and functional analogs of enzymes and nano-sized electrocatalysts have shown promising activities in energy conversion reactions. Mo ion plays essential roles in natural and artificial catalysts, and the unique electrochemical properties render its versatile utilization as an electrocatalyst. In this review paper, we show the current understandings of the Mo-enzyme active sites and the recent advances in the synthesis of Mo-catalysts aiming for high-performing catalysts.

Fabricating MAPbI3/MoS2 Composites for Improved Photocatalytic Performance

Although lead halide perovskites are demonstrated to be promising photocatalysts for hydrogen evolution from hydrogen halide splitting, it still remains challenging to fabricate efficient and stable catalysts. Here MoS₂ nanoflowers with abundant active sites are assembled with methylammonium lead iodide (MAPbI₃) microcrystals to form a new heterostructure. Its hydrogen evolution rate can reach up to about 30 000 μmol g^-1 h^-1, which is more than 1000-fold higher than pristine MAPbI₃ under visible light irradiation (λ ≥ 420 nm). Importantly, the solar HI splitting efficiency reaches 7.35%, one of the highest efficiencies so far. The introduction of MoS₂ with proper band alignment and unsaturated species can efficiently promote the charge separation and afford more active sites for H₂ production. This finding not only provides a highly efficient and stable photocatalyst for hydrogen evolution but also offers a useful modification strategy on lead halide perovskites.

In situ total scattering experiments of nucleation and crystallisation of tantalum-based oxides: from highly dilute solutions via cluster formation to nanoparticles

The exact formation mechanism of tantalum oxides (and in general, metal/mixed metal oxides) from alkoxide precursors is still not fully understood, particularly when forming cluster-like or amorphous materials. The structural evolution of Ta-based oxides was studied in detail using X-ray total scattering experiments along with subsequent pair distribution function (PDF) analyses. Starting from a tantalum alkoxide precursor (Ta₂(OEt)₁₀), the formation of hydrolysed Ta_xO_yH_z clusters in highly diluted aqueous solution was analysed. From the PDF data, the connectivity and arrangement of Ta_xO_y octahedra in the cluster could be deduced as well as the approximate size of the clusters (<1 nm). Construction of cluster models allowed for identification of common structural motifs in the Ta_xO_yH_z clusters, ruling out the formation of chain- or ring-like clusters. More likely, bulky clusters with a high number of corner-sharing octahedra are formed. After separation of the amorphous solid from the liquid, temperature-induced crystallisation processes were monitored via in situ total scattering experiments. Between room temperature and 600 °C, only small rearrangements of the amorphous structure are observed. At about 610 °C, amorphous Ta_xO_yH_z transforms directly into crystalline orthorhombic L-Ta₂O₅ without formation of any crystalline intermediate structures.

Controllable design of 3D hierarchical Co/Ni-POM nanoflower compounds supported on Ni foam for the hydrogen evolution reaction

The outstanding HER activity of Co/Ni-POM/NF stems from the synergistic effect between the metallic elements of the Co/Ni-POM and its large specific surface area.

Self-supported wire-in-plate NiFeS/CoS nanohybrids with a hierarchical structure for efficient overall water splitting

Highly efficient low-cost electrocatalysts play a key role in overall water splitting to generate hydrogen and oxygen. Herein, a self-supported hierarchical NiFeS/CoS nanosheet/nanowire bifunctional electrocatalyst for overall water splitting supported on nickel foam is synthesized by the combined process of hydrothermal and sulfurization methods. The specific wire-in-plate micromorphology of the catalyst provides the advantages of high contact area for electrolyte penetration, extensive active surface area and plentiful accessible active sites. Moreover, the quaternary catalyst in situ grown on the substrate guarantees mechanical stability. Reasonably, the as-obtained NiFeS/CoS catalyst with a unique wire-in-plate nanostructure shows good electrocatalytic performance toward the OER, HER and efficient overall water splitting. The NiFeS/CoS catalyst delivers 50 and 150 mA cm-2 at ultralow overpotentials of 170 and 150 mV toward the OER and HER, respectively. When simultaneously used as the electrocatalyst at both the cathode and the anode of an alkaline electrolyzer, the NiFeS/CoS electrocatalyst requires a cell voltage of 1.81 V at a water-splitting current density of 100 mA cm-2 for overall water splitting. This investigation provides an effective strategy to design hierarchically multidimensional nanohybrids for bifunctional electrocatalysts by combining nanowires with nanoplates.

Direct Evidence of Electronic Interaction at the Atomic-Layer-Deposited MoS2 Monolayer/SiO2 Interface

The electronic structure of an atomic-layer-deposited MoS₂ monolayer on SiO₂ was investigated using X-ray absorption spectroscopy (XAS) and synchrotron X-ray photoelectron spectroscopy (XPS). The angle-dependent evolution of the XAS spectra and the photon-energy-dependent evolution of the XPS spectra were analyzed in detail using an ab initio electronic structure simulation. Although similar to the theoretical spectra of an ideal free-standing MoS₂ ML, the experimental spectra exhibit features that are distinct from those of an ideal ML, which can be interpreted as a consequence of S-O van der Waals (vdW) interactions. The strong consensus among the experimental and theoretical spectra suggests that the vdW interactions between MoS₂ and adjacent SiO₂ layers can influence the electronic structure of the system, manifesting a substantial electronic interaction at the MoS₂-SiO₂ interface.