Layered transition metal dichalcogenide electrochemistry: journey across the periodic table.

Studies on layered transition metal dichalcogenides (TMDs), in particular for Group VIB TMDs like MoS2 and WS2, have long reached a crescendo in the realms of electrochemical applications initiated by their remarkable catalytic and electronic properties. One area that garnered considerable attention is the fervent pursuit of layered TMDs as electrocatalysts for hydrogen evolution reaction (HER), driven by global efforts towards reducing carbon footprint and attaining hydrogen economy. This Tutorial Review captures the essence of electrochemistry of different classes of layered TMDs and metal chalcogenides across the period table and showcases their tuneable electrochemical and HER catalytic attributes that are governed by the elemental composition, structure and anisotropy. Of interest to the assiduously studied Group VIB TMDs, we describe the role of elemental constituents and material purity in aspects of surface composition and structure, on their electrochemistry. Across families of layered TMDs in the periodic table, we highlight the apparent trends in their electrochemical and electrocatalytic properties through diligent comparison. Inevitably, these trends vary according to the type of chalcogen or transition metal that constitutes the eventual TMD. Beyond layered TMDs, we discuss the electrochemistry and recent progress in HER electrocatalysis of other layered metal chalcogenides that are overshadowed by the success of Group VIB TMDs. At the pinnacle of the emergent applications of layered TMDs, it is prudent to demystify the intrinsic electrochemical behaviour that originates from the participation of the elemental constitution of transition metal or chalcogen. Moreover, knowledge of the catalytic and electronic properties of the various TMD families and emerging trends across the period or down the group is of paramount importance when introducing or refining their prospective uses. The annotations in this Tutorial Review are envisioned to promote discourse into the catalytic and electrochemical trends of TMDs that is currently absent.

Nonconductive layered hexagonal boron nitride exfoliation by bipolar electrochemistry.

Boron nitride (h-BN), which is an isoelectronic analogue of graphite, has received immense attention due to its unique physical and chemical properties. Numerous methods have been developed to isolate few-layered h-BN nanosheets. These include chemical vapour deposition, solution-based exfoliation and ball-milling amongst others. The bipolar electrochemical method is one of the popular, scalable and water based exfoliation methods which has been applied to graphite, layered transition metal dichalcogenides and black phosphorus. This method was not applied to insulators as this has been assumed to be an impossible task. In this study, we report a solution-based, scalable and time efficient bipolar electrochemical method for the direct exfoliation of bulk insulator, layered h-BN into few-layered h-BN nanosheets based on bipolar electrochemistry. The electrochemical exfoliation of nonconductive materials, h-BN, opens the way to the application of this scalable method to the whole spectrum of non-conductive layered materials. This facile method offers an alternative platform for h-BN electrochemical exfoliation in wide-ranging fields encompassing electronics and biomedical science.

Tunable Pt-MoS x Hybrid Catalysts for Hydrogen Evolution.

Platinum (Pt)-based materials are inevitably among the best-performing electrocatalysts for hydrogen evolution reaction (HER). MoS2 was suggested to be a potent HER catalyst to replace Pt in this reaction by theoretical modeling; however, in practice, this dream remains elusive. Here we show a facile one-pot bottom-up synthesis of Pt-MoS x composites using electrochemical reduction in an electrolytic bath of Pt precursor and ammonium tetrathiomolybdate under ambient conditions. By modifying the millimolar concentration of Pt precursors, composites of different surface elemental composition are fabricated; specifically, Pt1.8MoS2, Pt0.1MoS2.5, Pt0.2MoS0.6, and Pt0.3MoS0.8. All electrodeposited Pt-MoS x hybrids showcase low overpotentials and small Tafel slopes that outperform MoS2 as an electrocatalyst. Tantamount to electrodeposited Pt, the rate-limiting process in the HER mechanism is determined to be the Heyrovsky desorption across Pt-MoS x hybrids and starkly swings from the rate-determining Volmer adsorption step in MoS2. The Pt-MoS x composites are equipped with catalytic performance that closely mirrors that of electrodeposited Pt, in particular the HER kinetics for Pt1.8MoS2 and Pt0.1MoS2.5. This work advocates electrosynthesis as a cost-effective method for catalyst design and fabrication of competent composite materials for water splitting applications.

Inverse Opal-like Porous MoSex Films for Hydrogen Evolution Catalysis: Overpotential-Pore Size Dependence.

Transition metal dichalcogenides (TMDs) are prized as electrocatalysts for hydrogen evolution reaction (HER). Common TMD syntheses entail conditions of high temperatures and reagents that are detrimental to the environment. The electrochemical synthesis of TMDs is advocated as a viable alternative to the conventional synthetic procedures in terms of simplicity, ecological sustainability, and versatility of deposition on various surfaces at room temperature. In this work, we demonstrate the successful fabrication of electrocatalytic inverse opal porous MoSex films, where 2 ≤ x ≤ 3, via solid template-assisted electrodeposition from the simultaneous electroreduction of molybdic acid and selenium dioxide as the respective metal and chalcogen precursors in an aqueous electrolyte. The electrosynthesized porous MoSex films contain pores with diameters of 0.1, 0.3, 0.6, or 1.0 µm, depending on the size of the polystyrene bead template used. The investigation reveals that porous MoSex films with a pore size of 0.1 µm, which prevailed over the other pore sizes, are endowed with the lowest HER overpotential of 0.57 V at -30 mA cm-2 and a Tafel slope of 118 mV dec-1, alluding to the adsorption step as rate limiting. Across all pore sizes, the Volmer adsorption step limits the HER mechanism. Nevertheless, the pore size dictates the catalytic activity of the porous MoSex films apropos of HER overpotential such that the HER performance of smaller pore sizes of 0.1 and 0.3 µm surpasses those with wider pore sizes of 0.6 and 1.0 µm. The observed trends in their HER behavior may be rationalized by the tunable surface wettability as pore sizes vary. These fundamental findings offer a glimpse into the efficacy of electrodeposited porous TMDs as electrocatalysts and exemplify the feasibility of the electrosynthesis method in altering the morphological structure of the TMDs.

Morphological Effects and Stabilization of the Metallic 1T Phase in Layered V-, Nb-, and Ta-Doped WSe2 for Electrocatalysis.

Layered transition-metal dichalcogenides (TMDs) are valued for their electrocatalytic properties toward the hydrogen-evolution reaction (HER) and oxygen-reduction reaction (ORR). One effective strategy to activate the electrocatalytic properties of TMDs is through doping. The optimistic outlook of doped-MoS2 as an electrocatalyst witnessed in previous reports spurred us to examine the effect of doping WSe2 with Group 5 transition-metal species, namely V, Nb, and Ta, in aspects of inherent electroactivities and catalysis. Apart from the mild reduction signal unique to the Group 5 transition-metal dopants, the Group 5 transition-metal-doped WSe2 materials are found to possess largely identical inherent electrochemistry to the undoped WSe2 with a characteristic anodic peak. Living up to expectations, the Group 5 transition-metal-doped WSe2 materials exhibit improved electrocatalytic HER efficiency, as evident by the lower HER overpotentials and Tafel slopes relative to undoped WSe2 . After doping with V, Nb, or Ta species, an increased number of active sites is observed given the distinct changes in morphology from thick bulky pieces in undoped WSe2 to thinner fragments in doped WSe2 . Although undoped WSe2 exists in the semiconducting 2H phase, the Group 5 transition-metal-doped WSe2 materials are dominated by the metallic 1T phase. Doping WSe2 with V, Nb, or Ta stabilizes the catalytic 1T phase and appears to induce the transition from the 2H to 1T phase. In contrast to the enhanced HER performance of WSe2 upon doping, Group 5 transition-metal dopants proved futile in activating the ORR electrocatalytic behavior of WSe2 , for which the ORR efficiency is unchanged. Therefore, these findings facilitate the understanding of the role of Group 5 transition-metal dopants in the electrochemical and catalytic properties of WSe2 relative to their morphological features and provide an evaluation of the efficacy of doping TMDs in electrocatalytic applications.

Unconventionally Layered CoTe2 and NiTe2 as Electrocatalysts for Hydrogen Evolution.

Two members of the transition metal ditelluride family, CoTe2 and NiTe2 , exist in multiple structures encompassing marcasite-, pyrite- and CdI2 related structures. The allotrope modification is influenced by weak changes in stoichiometry and synthesis. It is crucial to emphasize that the CdI2 structure type is manifested by NiTe2 while the CoTe2 adopts a related structure for a non-stoichiometric composition with ratio below 1:1.8. The obtained structure is based on LiTiS2 which is derived from CdI2 structure, however contains a polymeric cobalt network. Despite the atypical nature of their layered structure, layered phases of CoTe1.8 and NiTe2 are rarely cast into the spotlight. Here, layered CoTe1.8 and NiTe2 are investigated for their electrochemical and electrocatalytic properties. In electrocatalytic aspects, layered CoTe1.8 and NiTe2 demonstrate low overpotentials and small Tafel slopes that are quintessential features of hydrogen evolution electrocatalysts. These findings impart fundamental insights to the transition metal ditelluride family and affirm the prospective use of layered CoTe1.8 and NiTe2 in electrochemical applications.

2H → 1T Phase Change in Direct Synthesis of WS2 Nanosheets via Solution-Based Electrochemical Exfoliation and Their Catalytic Properties.

Metallic 1T-WS2 has various interesting properties such as increased density of catalytically active sites on both the basal planes and edges as well as metallic conductivity which allows it to be used in applications such as biosensing and energy devices. Hence, it is highly beneficial to develop a simple, efficient, and low-cost synthesis method of 1T-WS2 nanosheets from commercially available bulk 2H-WS2. In this study, we reported WS2 nanosheets synthesized directly from bulk WS2 via solution-based electrochemical exfoliation with bipolar electrodes and investigated their electrocatalytic performances toward hydrogen evolution and oxygen reduction reactions. We successfully synthesized WS2 nanosheets of regular hexagonal symmetry with a 2H → 1T phase transition. This represents a novel method of producing 1T-WS2 nanosheets from bulk 2H-WS2 without compromising on its electrocatalytic properties.

Layered Noble Metal Dichalcogenides: Tailoring Electrochemical and Catalytic Properties.

Owing to the anisotropic nature, layered transition metal dichalcogenides (TMDs) have captured tremendous attention for their promising uses in a plethora of applications. Currently, bulk of the research is centered on Group 6 TMDs. Layered noble metal dichalcogenides, in particular the noble metal tellurides, belong to a subset of Group 10 TMDs, wherein the transition metal is a noble metal of either palladium or platinum. We address here a lack of contemporary knowledge on these compounds by providing a comprehensive study on the electrochemistry of layered noble metal tellurides, PdTe2 and PtTe2, and their efficiency as electrocatalysts toward the hydrogen evolution reaction (HER). Observed parallels in the electrochemical peaks of the noble metal tellurides are traced to the tellurium electrochemistry. PdTe2 and PtTe2 can be discriminated by their distinct reduction peaks in the first cathodic scans. Considering the influence of the metal component, PtTe2 outperforms PdTe2 in aspects of charge transfer and electrocatalysis. The heterogeneous electron transfer (HET) rate of PtTe2 is an order of magnitude faster than PdTe2, and a lower HER overpotential of 0.54 V versus reversible hydrogen electrode (RHE) at a current density of -10 mA cm-2 is evident in PtTe2. On PdTe2 and PtTe2 surfaces, adsorption via the Volmer process has been identified as the limiting step for HER. A general phenomenon for the noble metal tellurides is that faster HET rates are observed upon electrochemical reductive pretreatment, whereas slower HET rates occur when the noble metal tellurides are oxidized during pretreatment. PtTe2 becomes successfully activated for HER when subject to oxidative treatment, whereas oxidized or reduced PdTe2 shows a deactivated HER performance. These findings provide fundamental insights that are pivotal to advancing the field of the underemphasized TMDs. Furthermore, electrochemical tuning as a means to tailor specific properties of the TMDs is advantageous for the development of their future applications.

Graphene/Group 5 Transition Metal Dichalcogenide Composites for Electrochemical Applications.

In comparison to the extensive research and great success attained by Group 6 transition metal dichalcogenides (TMDs) as hydrogen evolution reaction (HER) electrocatalysts, there is limited research focused on metallic Group 5 TMDs for use as electrocatalysts for hydrogen evolution. Density functional theory calculations have pointed out that Group 5 TMDs are highly favorable for HER, especially vanadium disulfide. In this work, nanocomposites of graphene and Group 5 TMDs were synthesized by thermal exfoliation of graphene oxide/TMD precursors in an H2 S atmosphere or in a H2 atmosphere as a control. Graphene oxide was prepared by the Hummers method while vanadium tetrachloride, niobium pentachloride, and tantalum pentachloride were utilized as TMD precursors. Then the potential of these nanocomposites as electrocatalysts towards HER was explored. Although these nanocomposites do not have comparable HER performance to Group 6 TMDs, they exhibit higher electrocatalytic activity in comparison with thermally reduced graphene oxide (TRGO) in the absence of TMD modification. In addition, the capacitive performance of these materials was also investigated in consideration of the high capacitance of graphene. It was indicated that the presence of TMDs on graphene actually suppress the capacitance performance of graphene itself.

The Origin of MoS2 Significantly Influences Its Performance for the Hydrogen Evolution Reaction due to Differences in Phase Purity.

Molybdenum disulfide (MoS2 ) is at the forefront of materials research. It shows great promise for electrochemical applications, especially for hydrogen evolution reaction (HER) catalysis. There is a significant discrepancy in the literature on the reported catalytic activity for HER catalysis on MoS2 . Here we test the electrochemical performance of MoS2 obtained from seven sources and we show that these sources provide MoS2 of various phase purity (2H and 3R, and their mixtures) and composition, which is responsible for their different electrochemical properties. The overpotentials for HER at -10 mA cm-2 for MoS2 from seven different sources range from -0.59 V to -0.78 V vs. reversible hydrogen electrode (RHE). This is of very high importance as with much interest in 2D-MoS2 , the use of the top-down approach would usually involve the application of commercially available MoS2 . These commercially available MoS2 are rarely characterized for composition and phase purity. These key parameters are responsible for large variance of reported catalytic properties of MoS2 .

Anti-MoS2 Nanostructures: Tl2S and Its Electrochemical and Electronic Properties.

Layered transition metal dichalcogenides are catalytically important compounds. Unlike the mounting interest in transition metal dichalcogenides such as MoS2 and WS2 for electrochemical applications, other metal chalcogenides with layered structure but different chemical composition have received little attention among the scientific community. One such example is represented by thallium(I) sulfide (Tl2S), a Group 13 chalcogenide, which adopts the peculiar anti-CdCl2 type structure where the chalcogen is sandwiched between the metal layers. This is the exact opposite of a number of transition metal dichalcogenides like 1T-MoS2 adopting the regular CdCl2 structure type. The electronic structure of Tl2S thus differs from MoS2. Such structure may provide a useful insight and understanding toward its electrochemical behavior in relation to the electrochemical properties of MoS2. We thus investigated the intrinsic electroactivity of Tl2S and its implications for sensing and energy generation, specifically the electrocatalytic properties toward the hydrogen evolution reaction (HER). We show that Tl2S exhibits four distinct redox signals at ca. 0.4 V, -0.5 V, -1.0 V and -1.5 V vs Ag/AgCl as a result of its inherent cathodic and anodic processes. We also demonstrate that Tl2S possesses slow electron transfer abilities with a rate (k(0)obs) as low as 6.3 × 10(-5) cm s(-1). Tl2S displays a competent performance as a HER electrocatalyst compared to a conventional glassy carbon electrode. However, the poor conductivity of Tl2S renders the HER electrocatalytic behavior second-rate compared to MoS2. Furthermore, we investigated the electronic properties of Tl2S and found that Tl2S exhibits an unusually narrow band dispersion around the Fermi level. We show here that anti-MoS2 structure of Tl2S is accompanied by highly unusual features.

Catalytic and charge transfer properties of transition metal dichalcogenides arising from electrochemical pretreatment.

Layered transition metal dichalcogenides (TMDs) have been the center of attention in the scientific community due to their properties that can be tapped on for applications in electrochemistry and hydrogen evolution reaction (HER) catalysis. We report on the effect of electrochemical treatment of exfoliated MoS2, WS2, MoSe2 and WSe2 nanosheets toward the goal of activating the electrochemical and HER catalytic properties of the TMDs. In particular, electrochemical activation of the heterogeneous electron transfer (HET) abilities of MoS2, MoSe2 and WSe2 is achieved via reductive treatments at identified reductive potentials based on their respective inherent electrochemistry. Comparing all TMDs, the charge transfer activation is most accentuated in MoSe2 and can be concluded that Mo metal and Se chalcogen type are more susceptible to electrochemical activation than W metal and S chalcogen type. With regards to the HER, we show that while MoS2 displayed enhanced performance when subjected to electrochemical reduction, WS2 fared worse upon oxidation. On the other hand, the HER performance of MoSe2 and WSe2 is independent of electrochemical redox treatment. We can conclude therefore that for the HER, S-containing TMDs are more responsive to redox treatment than compounds with the Se chalcogen. Our findings are beneficial toward understanding the electrochemistry of TMDs and the extent to which activation by electrochemical means is effective. In turn, when such knowledge is administered aptly, it will be promising for electrochemical uses.

Precise tuning of the charge transfer kinetics and catalytic properties of MoS2 materials via electrochemical methods.

MoS2 has become particularly popular for its catalytic properties towards the hydrogen evolution reaction (HER). It has been shown that the metallic 1T phase of MoS2 , obtained by chemical exfoliation after lithium intercalation, possesses enhanced catalytic activity over the semiconducting 2H phase due to the improved conductivity properties which facilitate charge-transfer kinetics. Here we demonstrate a simple electrochemical method to precisely tune the electron-transfer kinetics as well as the catalytic properties of both exfoliated and bulk MoS2 -based films. A controlled reductive or oxidative electrochemical treatment can alter the surface properties of the film with consequently improved or hampered electrochemical and catalytic properties compared to the untreated film. Density functional theory calculations were used to explain the electrochemical activation of MoS2 . The electrochemical tuning of electrocatalytic properties of MoS2 opens the doors to scalable and facile tailoring of MoS2 -based electrochemical devices.

Fluorographites (CF(x))n exhibit improved heterogeneous electron-transfer rates with increasing level of fluorination: towards the sensing of biomolecules.

Halogenated sp(2) materials are of high interest owing to their important electronic and electrochemical properties. Although methods for graphite and graphene fluorination have been extensively researched, the fundamental electrochemical properties of fluorinated graphite are not well established. In this paper, the electrochemistry of three fluorographite materials of different carbon-to-fluorine ratio were studied: (CF(0.33))n, (CF(0.47))n, and (CF(0.75))n. Our findings reveal that the carbon-to-fluorine ratio of fluorographite will impact the electrochemical performance. Faster heterogeneous electron-transfer (HET) rates and lowered oxidation potentials for ascorbic acid and uric acid are progressively obtained with increasing fluorine content. The fluorographite (CF(0.75))n was in fact found to exhibit the most improved electrochemical performances with the fastest HET rates and significantly lowered overpotentials in the oxidation of ascorbic acid. Analytical parameters such as sensitivity and linearity were subsequently investigated by applying the fluorographite (CF(0.75))n in the analysis of ascorbic acid and uric acid, which can be simultaneously detected. We determined good linear responses towards the detection of both ascorbic and uric acid. Fluorographites outperform graphites in sensing applications, which will have a profound impact on applications of fluorographites and fluorographene in sensing and biosensing.