Photoactivated micromachines are at the forefront of the micro- and nanomotors field, as light is the main power source of many biological systems. Currently, this rapidly developing field is based ...on metal-containing segments, typically TiO2 and precious metals. Herein, we present metal-free tubular micromotors solely based on graphitic carbon nitride, as highly scalable and low-cost micromachines that can be actuated by turning on/off the light source. These micromotors are able to move by a photocatalytic-induced bubble-propelled mechanism under visible light irradiation, without any metal-containing part or biochemical molecule on their structure. Furthermore, they exhibit interesting properties, such as a translucent tubular structure that allows the optical visualization of the O2 bubble formation and migration inside the microtubes, as well as inherent fluorescence and adsorptive capability. Such properties were exploited for the removal of a heavy metal from contaminated water with the concomitant optical monitoring of its adsorption by fluorescence quenching. This multifunctional approach contributes to the development of metal-free bubble-propelled tubular micromotors actuated under visible light irradiation for environmental applications.
Research on graphene materials has refocused on graphite oxides (GOs) in recent years. The fabrication of GO is commonly accomplished by using concentrated sulfuric acid in conjunction with: a) ...fuming nitric acid and KClO3 oxidant (Staudenmaier); b) concentrated nitric acid and KClO3 oxidant (Hofmann); c) sodium nitrate for in situ production of nitric acid in the presence of KMnO4 (Hummers); or d) concentrated phosphoric acid with KMnO4 (Tour). These methods have been used interchangeably in the graphene community, since the properties of GOs produced by these different methods were assumed as almost similar. In light of the wide applicability of GOs in nanotechnology applications, in which presence of certain oxygen functional groups are specifically important, the qualities and functionalities of the GOs produced by using these four different methods, side‐by‐side, was investigated. The structural characterizations of the GOs would be probed by using high resolution X‐ray photoelectron spectroscopy, nuclear magnetic resonance, Fourier transform infrared spectroscopy, and Raman spectroscopy. Further electrochemical applicability would be evaluated by using electrochemical impedance spectroscopy and cyclic voltammetry techniques. Our analyses highlighted that the oxidation methods based on permanganate oxidant (Hummers and Tour methods) gave GOs with lower heterogeneous electron‐transfer rates and a higher amount of carbonyl and carboxyl functionalities compared with when using chlorate oxidant (Staudenmaier and Hofmann methods). These observations indicated large disparities between the GOs obtained from different oxidation methods. Such insights would provide fundamental knowledge for fine tuning GO for future applications.
Diverse methods: The structural and electrochemical properties of the different graphite oxides (GOs) prepared by using various oxidation methods are described. The GO obtained by using permanganate as oxidant introduced a higher amount of carbonyl and carboxyl functional groups compared with the chlorate oxidant. The electrochemical characterizations highlighted a higher heterogeneous electron‐transfer rate for GO obtained by using the chlorate oxidant.
In the face of rising energy demand and depleting energy resources, energy sustainability has proven to be a formidable environmental and economic challenge, and the search for environmentally ...friendly energy sources is exigent. To this end, much research has been focused on improving the efficiency of the hydrogen evolution reaction (HER), which generates hydrogen gas, a renewable source of energy carrier. To resolve the steep cost issue of employing Pt, transition metal dichalcogenides (TMDs) have been proposed as emerging HER electrocatalysts, among other electrochemical applications. As the exfoliated TMD exhibits properties distinct from the bulk material, it is prudent to investigate the exfoliation procedure with specific regard to the employed intercalant. Here, we prepared exfoliated MoS2, MoSe2, WS2, and WSe2 from their bulk counterparts using aromatic intercalants, i.e. phenyllithium, sodium naphthalenide, and sodium anthracenide, and characterized them using scanning electron microscopy (SEM), Raman spectroscopy, methylene blue surface area measurement, and X-ray photoelectron spectroscopy (XPS). Voltammetric investigations were performed to examine the effects of different intercalants on the electrochemical sensing capability and HER catalytic efficiency of these TMDs. Our findings establish both advantageous and detrimental impacts of altering the intercalant on the electrochemical performances of TMDs upon exfoliation, depending on the intended application of their electrochemistry.
Graphene and its derivatives have been reported in many articles as “metal-free” carbon electrocatalytic materials. Its synthesis procedures are generally based on the chemical oxidation of graphite ...and subsequent thermal or chemical reduction. Because graphene oxide has a large surface area and typically contains a variety of oxygen functionalities, metallic ions (impurities) from reaction mixtures can be adsorbed on its surface. These impurities can significantly enhance the electrocatalytic activity and thus lead to data misinterpretation; such impure samples are referred to as “metal-free” catalysts. In this paper, we report the synthesis of impurity-free graphene, which is compared with graphene prepared by standard methods based on the thermal and chemical reduction of two graphene oxides. Detailed analysis of graphene prepared by standard methods shows a direct relation between metallic impurities and the electrocatalytic activity of graphene. In contrast, impurity-free graphene exhibits poor electrocatalytic activity.
Arsenene, as an exotic representative of two-dimensional (2D) materials, has received great interest, yet the interest is mainly based on theoretical study. The reason for this is a restricted ...ability to operate the material from its synthesis to implementation. Beginning with the production, electrochemical exfoliation has been found as an extremely effective method for the preparation of 2D materials from bulk materials. Here, for the first time, we demonstrate the electrochemical exfoliation of bulk black arsenic in the anhydrous electrolyte medium. Spectro- and microscopic analyses evidence micrometer lateral size few-layer arsenene in a netlike porous shape formed of 2D flakes. We demonstrate that the surfactant-free exfoliation successfully resulted in a stable dispersion for which only washing with the corresponding solvent was sufficient. This electrochemistry route for the black arsenic exfoliation toward few-layer arsenene will broaden the materials’ scope applications in new-generation devices.
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.
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.
MoS₂ belong to a class of inorganic 2D nanomaterials known as transition metal dichalcogenides (TMDs) which have recently attracted a renewed and growing interest due to their interesting electronic ...and catalytic properties when scaled down to single or few layer sheets. Although exfoliated MoS₂ nanosheets have been proposed for numerous energy-related and biosensing applications, little is known about the toxicological impacts of using MoS₂ nanosheets. Here, we report about the in vitro toxicity of MoS₂ nanosheets that have been chemically exfoliated with different lithium intercalating agents and compared their respective cytotoxic influence. Methyllithium (Me-Li), n-butyllithium (n-Bu-Li) and tert-butyllithium (t-Bu-Li) were used for the exfoliation of bulk MoS₂ and we found the t-Bu-Li and n-Bu-Li exfoliated MoS₂ nanosheets to be more cytotoxic than MoS₂ exfoliated by Me-Li. t-Bu-Li and n-Bu-Li provide more efficient exfoliation over Me-Li, and we establish that the extent of exfoliation that MoS₂ undergo is a factor influencing their toxicity. Specifically, the more exfoliated the MoS₂ nanosheets, the stronger its cytotoxic influence, which may be due to an increase in surface area and active edge sites. The potential toxicity of these MoS₂ nanosheets should be taken into account before their employment in real world applications and we have shown the effect the amount of exfoliation can have on the toxicity of MoS₂ nanosheets, representing the first step towards a better understanding of their toxicological properties.
Chiral Au nanorods (c-Au NRs) with diverse architectures constitute an interesting nanospecies in the field of chiral nanophotonics. The numerous possible plasmonic behaviors of Au NRs can be coupled ...with chirality to initiate, tune, and amplify their chiroptical response. Interdisciplinary technologies have boosted the development of fabrication and applications of c-Au NRs. Herein, we have focused on the role of chirality in c-Au NRs which helps to manipulate the light–matter interaction in nontraditional ways. A broad overview on the chirality origin, chirality transfer, chiroptical activities, artificially synthetic methodologies, and circularly polarized applications of c-Au NRs will be summarized and discussed. A deeper understanding of light–matter interaction in c-Au NRs will help to manipulate the chirality at the nanoscale, reveal the natural evolution process taking place, and set up a series of circularly polarized applications.
Graphene oxide is an oxidized form of graphene containing a large variety of oxygen groups. Although past models have suggested carboxylic acids to be present in significant amounts, recent evidence ...has shown otherwise. Toward the production of carboxyl-graphene, a synthetic method is presented herein based on the Kolbe–Schmitt process. A modified procedure of heating graphite oxide in the presence of a KOH/CaO mixture results in up to 11 atom % of carboxylic groups. The graphite oxide starting material and reaction temperature were investigated as two important factors, where a crumpled morphology of graphite oxide flakes and a lower 220 °C temperature preferentially led to greater carboxyl functionalization. Successful carboxylation caused a band gap opening of ∼2.5 eV in the smallest carboxyl–graphene particles, which also demonstrated a yellow fluorescence under UV light unseen in its counterpart produced at 500 °C. These results are in good agreement with theoretical calculations showing band gap opening and spin polarization of impurity states. This demonstrates the current synthetic process as yet another approach toward tuning the physical properties of graphene.
