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.

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.

Method for fabricating self-powered cement sensors based on gold nanoparticles.

Nowadays, cement industry researchers are working hard to develop cement sensors based on nanocomposites because they can be used to develop intelligent and sustainable civil structures, self-powered, self-healing, or self-monitoring. In this light, this paper shows a methodology to obtain piezoelectric cement sensors, which produce enough energy not to require an external power source in sensing-strain applications. Mainly, two proposed experimental procedures increased the piezoelectric properties of these cement-based composites: add gold nanoparticles in the proper concentrations and apply a constant electric field during the curing stage. Firstly, the gold nanoparticles were obtained through a pulsed laser ablation system, and their particle size distribution was measured with a particle analyzer Litesizer 500 from Anton Paar, and their morphology was corroborated using a scanning electron microscope. Two concentrations (442 ppm and 658 ppm) of gold nanoparticles were obtained by changing the total ablation time. Next, we fabricated the cement sensors as described by ASTM standards C39-C39M. Hence, the cement was hand mixed with a water-to-cement ratio (w/c) of 0.47 for then poured on cylindrical molds saving the proportions recommended by the ASTM standard; in this stage, the gold nanoparticles were already part of the water ratio. Then, the cement sensors were cured under an external electric field and dried for 24 hours more in an oven to be finally ready for electromechanical characterization. Meanwhile, the electric response in altern current and the piezoelectric behavior were corroborated through electrical impedance spectroscopy and open circuit potential measurements, respectively. The piezoelectric behavior was obtained when a compressive strength was applied to the sensor, and the generated voltage was simultaneously measured. Finally, the electrical and mechanical characterization measurements were processed and analyzed using Python scripts.•The particle size and the families amount of Au NPs are affected by the ablation time.•The correct proportion of Au NPs increases the inherent piezoelectricity of cement paste.•The piezoelectric response can be addressed by coupling electric and mechanical tests.

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.

Synthesis of Oxide Iron Nanoparticles Using Laser Ablation for Possible Hyperthermia Applications.

In this work, iron oxide nanoparticles produced using the laser ablation technique were studied in order to determine the characteristics of these nanoparticles as a function of the laser energy for the possible application in magnetic hyperthermia. Nanoparticles were obtained by varying the power of the laser considering values of 90, 173, 279 and 370 mJ. The morphology of these nanoparticles was determined using the dynamic light scattering (DLS) and scattering transmission electron microscopy (STEM) techniques, confirming that the size of the particles was in the order of nanometers. A great influence of the laser power on the particle size was also observed, caused by the competition between the energy and the temperature. The composition was determined by X-ray diffraction and Raman spectroscopy, showing the presence of magnetite, maghemite and hematite. The hyperthermia measurements showed that the temperature rise of the iron oxide nanoparticles was not greatly influenced by the energy change, the heating capacity of magnetic NPs is quantified by the specific absorption rate (SAR), that tends to decrease with increasing energy, which indicates a dependence of these values on the nanoparticles concentration.

Time-Stability Dispersion of MWCNTs for the Improvement of Mechanical Properties of Portland Cement Specimens.

This study shows the energy optimization and stabilization in the time of solutions composed of H2O + TX-100 + Multi-Wall Carbon Nanotubes (MWCNTs), used to improve the mechanical properties of Portland cement pastes. For developing this research, sonication energies at 90, 190, 290, 340, 390, 440, 490 and 590 J/g are applied to a colloidal substance (MWCNTs/TX-100 + H2O) with a molarity of 10 mM. Raman spectroscopy analyses showed that, for energies greater than 440 J/g, there are ruptures and fragmentation of the MWCNTs; meanwhile at energies below 390 J/g, better dispersions are obtained. The stability of the dispersion over time was evaluated over 13 weeks using UV-vis spectroscopy and Zeta Potential. With the most relevant data collected, sonication energies of 190, 390 and 490 J/g, at 10 mM were selected at the first and the fourth week of storage to obtain Portland cement specimens. Finally, we found an improvement of the mechanical properties of the samples built with Portland cement and solutions stored for one and four weeks; it can be concluded that the MWCNTs improved the hydration period.

Unravelling the Photocatalytic Behavior of All-Inorganic Mixed Halide Perovskites: The Role of Surface Chemical States.

Within the most mesmerizing materials in the world of optoelectronics, mixed halide perovskites (MHPs) have been distinguished because of the tunability of their optoelectronic properties, balancing both the light-harvesting efficiency and the charge extraction into highly efficient solar devices. This feature has drawn the attention of analogous hot topics as photocatalysis for carrying out more efficiently the degradation of organic compounds. However, the photo-oxidation ability of perovskite depends not only on its excellent light-harvesting properties but also on the surface chemical environment provided during its synthesis. Accordingly, we studied the role of surface chemical states of MHP-based nanocrystals (NCs) synthesized by hot-injection (H-I) and anion-exchange (A-E) approaches on their photocatalytic (PC) activity for the oxidation of ß-naphthol as a model system. We concluded that iodide vacancies are the main surface chemical states that facilitate the formation of superoxide ions, O2●-, which are responsible for the PC activity in A-E-MHP. Conversely, the PC performance of H-I-MHP is related to the appropriate balance between band gap and a highly oxidizing valence band. This work offers new insights on the surface properties of MHP related to their catalytic activity in photochemical applications.

Growth of crystalline hydroxyapatite thin films at room temperature by tuning the energy of the RF-magnetron sputtering plasma.

Right angle radio frequency magnetron sputtering technique (RAMS) was redesigned to favor the production of high-quality hydroxyapatite (HA) thin coatings for biomedical applications. Stoichiometric HA films with controlled crystallinity, thickness varying from 254 to 540 nm, crystallite mean size of 73 nm, and RMS roughness of 1.7 ± 0.9 nm, were obtained at room temperature by tuning the thermodynamic properties of the plasma sheath energy. The plasma energies were adjusted by using a suitable high magnetic field confinement of 143 mT (1430 G) and a substrate floating potential of 2 V at the substrate-to-magnetron distance of Z = 10 mm and by varying the sputtering geometry, substrate-to-magnetron distance from Z = 5 mm to Z = 18 mm, forwarded RF power and reactive gas pressure. Measurements that were taken with a Langmuir probe showed that the adjusted RAMS geometry generated a plasma with an adequate effective temperature of Teff ≈ 11.8 eV and electron density of 2.0 × 10(15) m(-3) to nucleate nanoclusters and to further crystallize the nanodomains of stoichiometric HA. The deposition mechanism in the RAMS geometry was described by the formation of building units of amorphous calcium phosphate clusters (ACP), the conversion into HA nanodomains and the crystallization of the grain domains with a preferential orientation along the HA [002] direction.