Vertical graphene nanowalls supported hybrid W2C/WOx composite material as an efficient non-noble metal electrocatalyst for hydrogen evolution.

Research for the development of noble metal-free electrodes for hydrogen evolution has blossomed in recent years. Transition metal carbides compounds, such as W2C, have been considered as a promising alternative to replace Pt-family metals as electrocatalysts towards hydrogen evolution reaction (HER). Moreover, hybridization of TMCs with graphene nanostructures has emerged as a reliable strategy for the preparation of compounds with high surface to volume ratio and abundant active sites. The present study focuses in the preparation of tungsten carbide/oxide compounds deposited in a three-dimensional vertical graphene nanowalls (VGNW) substrate via chemical vapor deposition, magnetron sputtering and thermal annealing processes. Structural and chemical characterization reveals the partial carburization and oxidation of the W film sputtered on the VGNWs, due to C and O migration from VGNWs towards W during the high temperature annealing process. Electrochemical characterization shows the enhanced performance of the nanostructured hybrid W2C/WOx on VGNW compound towards HER, when compared with planar W2C/WOx films. The W2C/WOx nanoparticles on VGNWs require an overpotential of -252 mV for the generation of 10 mA cm-2. Chronoamperometry tests in high overpotentials reveal the compounds stability while sustaining high currents, in the order of hundreds of mA. Post-chronoamperometry test XPS characterization unveils the formation of a W hydroxide layer which favours hydrogen evolution in acidic electrolytes. We aspire that the presented insights can be valuable for those working on the preparation of hybrid electrodes for electrochemical processes.

Processing and Functionalization of Vertical Graphene Nanowalls by Laser Irradiation.

The processing of vertical graphene nanowalls (VGNWs) via laser irradiation is proposed as a means to modulate their physicochemical properties. The effects of the number of applied pulses and fluence of each pulse are examined. Raman spectroscopy studies the effect of irradiation on the chemical structure of the VGNWs. Results show a decrease in density of defects and number of layers, which points toward a mechanism including evaporation of amorphous or loosely bonded C from defective points and recrystallization of graphene. Moreover, the effect of laser irradiation parameters on the morphology of Mo thin films deposited on VGNWs is investigated. The received thermal dosage results in the formation of particles. In this case, the number of pulses and pulse fluence are found to affect the size and distribution of these particles. The study provides a novel approach for the functionalization of VGNWs via laser irradiation, which can be extended to other graphene-based nanostructures.

Advancements in Plasma-Enhanced Chemical Vapor Deposition for Producing Vertical Graphene Nanowalls.

In recent years, vertical graphene nanowalls (VGNWs) have gained significant attention due to their exceptional properties, including their high specific surface area, excellent electrical conductivity, scalability, and compatibility with transition metal compounds. These attributes position VGNWs as a compelling choice for various applications, such as energy storage, catalysis, and sensing, driving interest in their integration into next-generation commercial graphene-based devices. Among the diverse graphene synthesis methods, plasma-enhanced chemical vapor deposition (PECVD) stands out for its ability to create large-scale graphene films and VGNWs on diverse substrates. However, despite progress in optimizing the growth conditions to achieve micrometer-sized graphene nanowalls, a comprehensive understanding of the underlying physicochemical mechanisms that govern nanostructure formation remains elusive. Specifically, a deeper exploration of nanometric-level phenomena like nucleation, carbon precursor adsorption, and adatom surface diffusion is crucial for gaining precise control over the growth process. Hydrogen's dual role as a co-catalyst and etchant in VGNW growth requires further investigation. This review aims to fill the knowledge gaps by investigating VGNW nucleation and growth using PECVD, with a focus on the impact of the temperature on the growth ratio and nucleation density across a broad temperature range. By providing insights into the PECVD process, this review aims to optimize the growth conditions for tailoring VGNW properties, facilitating applications in the fields of energy storage, catalysis, and sensing.

Hybrid Nanostructured Compounds of Mo2C on Vertical Graphene Nanoflakes for a Highly Efficient Hydrogen Evolution Reaction.

Organizing a post-fossil fuel economy requires the development of sustainable energy carriers. Hydrogen is expected to play a significant role as an alternative fuel as it is among the most efficient energy carriers. Therefore, nowadays, the demand for hydrogen production is increasing. Green hydrogen produced by water splitting produces zero carbon emissions but requires the use of expensive catalysts. Therefore, the demand for efficient and economical catalysts is constantly growing. Transition-metal carbides, and especially Mo2C, have attracted great attention from the scientific community since they are abundantly available and hold great promises for efficient performance toward the hydrogen evolution reaction (HER). This study presents a bottom-up approach for depositing Mo carbide nanostructures on vertical graphene nanowall templates via chemical vapor deposition, magnetron sputtering, and thermal annealing processes. Electrochemical results highlight the importance of adequate loading of graphene templates with the optimum amount of Mo carbides, controlled by both deposition and annealing time, to enrich the available active sites. The resulting compounds exhibit exceptional activities toward the HER in acidic media, requiring overpotentials of 82 mV at -10 mA/cm2 and demonstrating a Tafel slope of 56 mV/dec. The high double-layer capacitance and low charge transfer resistance of these Mo2C on GNW hybrid compounds are the main causes of the enhanced HER activity. This study is expected to pave the way for the design of hybrid nanostructures based on nanocatalyst deposition on three-dimensional graphene templates.

Anisotropic Thermal Conductivity of Crystalline Layered SnSe2.

The degree of thermal anisotropy affects critically key device-relevant properties of layered two-dimensional materials. Here, we systematically study the in-plane and cross-plane thermal conductivity of crystalline SnSe2 films of varying thickness (16-190 nm) and uncover a thickness-independent thermal conductivity anisotropy ratio of about ∼8.4. Experimental data obtained using Raman thermometry and frequency domain thermoreflectance showed that the in-plane and cross-plane thermal conductivities monotonically decrease by a factor of 2.5 with decreasing film thickness compared to the bulk values. Moreover, we find that the temperature-dependence of the in-plane component gradually decreases as the film becomes thinner, and in the range from 300 to 473 K it drops by more than a factor of 2. Using the mean free path reconstruction method, we found that phonons with MFP ranging from ∼1 to 53 and from 1 to 30 nm contribute to 50% of the total in-plane and cross-plane thermal conductivity, respectively.

Ultrathin epitaxial Bi film growth on 2D HfTe2template.

Among ultrathin monoelemental two-dimensional (2D) materials, bismuthene, the single layer of heavier group-VΑ element bismuth (Bi), has been predicted to have large non trivial gap. Here, we demonstrate the growth of Bi films by molecular beam epitaxy on 2D-HfTe2template. At the initial stage of Bi deposition (1-2 bilayers, BL), both the pseudocubic Bi(110) and the hexagonal Bi(111) phases are formed. When reaching 3 BL Bi, a transformation to pure hexagonal Bi(111) occurs. The electronic band structure of 3 BL Bi(111) films was measured by angle-resolved photoemission spectroscopy showing very good matching with the density functional theory band structure calculations of 3 BL free standing Bi(111). The grown Bi(111) thin film was capped with a protective Al2O3layer and its stability under ambient conditions, necessary for practical applications and device fabrication, was confirmed by x-ray photoelectron spectroscopy and Raman spectroscopy.

Direct versus reverse vertical two-dimensional Mo2C/graphene heterostructures for enhanced hydrogen evolution reaction electrocatalysis.

Mo2C/graphene heterostructures prepared by chemical vapor deposition have demonstrated excellent electrocatalytic activity in a hydrogen evolution reaction (HER). This is attributed to the high catalytic activity of Mo2C while the high electrical conductivity of graphene facilitates charge transfer. In the as-grown direct vertical order, graphene is placed above the Mo2C film. This reduces the catalytic activity of the heterostructure, since graphene in chemically inert. Here, a simple transfer method is proposed that results in the reverse order deposition of the heterostructure on the electrode. This method places graphene at the interface between Mo2C and the electrode, enhancing charge transfer between them, which results in an overpotential of 440 mV at 10 mA cm-2 and corresponds to ∼65 mV overpotential reduction as compared to the direct heterostructure. At the same time, when a direct Cu/Mo2C/graphene junction with a Cu catalyst substrate is used as a working electrode, the improvement of the heterostructure HER activity is observed which is manifested in an overpotential of 275 mV at 10 mA cm-2 with a correspondent ∼230 mV reduction. All above performances are accompanied with excellent endurance.

Mo2C/graphene heterostructures: low temperature chemical vapor deposition on liquid bimetallic Sn-Cu and hydrogen evolution reaction electrocatalytic properties.

Thin 2D Mo2C/graphene vertical heterostructures have attracted significant attention due to their potential application as electrodes in the hydrogen evolution reaction (HER) and energy storage. A common drawback in the chemical vapor deposition synthesis of these structures is the demand for high temperature growth, which should be higher than the melting temperature of the metal catalyst. The most common metallic catalyst is Cu, which has a melting temperature of 1084 °C. Here, we report the growth of thin, ∼200 nm in thickness, semitransparent micrometer-sized Mo2C domains and Mo2C/graphene heterostructures at lower temperatures using liquid Sn-Cu alloys. No Sn-associated defects are observed, making the alloy an appealing growth substrate. Raman spectroscopy reveals the vertical interaction between graphene and Mo2C, as shown by the variation in the strain of the graphene film. The results demonstrate the capability to grow continuous nanometer-thin Mo2C films at temperatures as low as 880 °C, without sacrificing the growth rate. Mo2C films are proven to be efficient electrocatalysts for the HER. Moreover, we demonstrate the beneficial role of graphene overgrown on Mo2C in reducing the HER overpotential values, which is attributed to more efficient charge transfer kinetics, compared to pure Mo2C films.

Evaluation of Graphene/WO3 and Graphene/CeO x Structures as Electrodes for Supercapacitor Applications.

The combination of graphene with transition metal oxides can result in very promising hybrid materials for use in energy storage applications thanks to its intriguing properties, i.e., highly tunable surface area, outstanding electrical conductivity, good chemical stability, and excellent mechanical behavior. In the present work, we evaluate the performance of graphene/metal oxide (WO3 and CeO x ) layered structures as potential electrodes in supercapacitor applications. Graphene layers were grown by chemical vapor deposition (CVD) on copper substrates. Single and layer-by-layer graphene stacks were fabricated combining graphene transfer techniques and metal oxides grown by magnetron sputtering. The electrochemical properties of the samples were analyzed and the results suggest an improvement in the performance of the device with the increase in the number of graphene layers. Furthermore, deposition of transition metal oxides within the stack of graphene layers further improves the areal capacitance of the device up to 4.55 mF/cm2, for the case of a three-layer stack. Such high values are interpreted as a result of the copper oxide grown between the copper substrate and the graphene layer. The electrodes present good stability for the first 850 cycles before degradation.