γ-Iron Phase Stabilized at Room Temperature by Thermally Processed Graphene Oxide

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Effect of the Synthetic Approach on the Formation and Magnetic Properties of Iron-Based Nanophase in Branched Polyester Polyol Matrix

This article shows the success of using the chemical reduction method, the polyol thermolytic process, the sonochemistry method, and the hybrid sonochemistry/polyol process method to design iron-based magnetically active composite nanomaterials in a hyperbranched polyester polyol matrix. Four samples were obtained and characterized by transmission and scanning electron microscopy, infrared spectroscopy and thermogravimetry. In all cases, the hyperbranched polymer is an excellent stabilizer of the iron and iron oxides nanophase. In addition, during the thermolytic process and hybrid method, the branched polyol exhibits the properties of a good reducing agent. The use of various approaches to the synthesis of iron nanoparticles in a branched polyester polyol matrix makes it possible to control the composition, geometry, dispersity, and size of the iron-based nanophase and to create new promising materials with colloidal stability, low hemolytic activity, and good magnetic properties. The NMR relaxation method proved the possibility of using the obtained composites as tomographic probes.

Thermal Evolution of C-Fe-Bi Nanocomposite System: From Nanoparticle Formation to Heterogeneous Graphitization Stage

Carbon xerogel nanocomposites with integrated Bi and Fe particles (C–Bi–Fe) represent an interesting model of carbon nanostructures decorated with multifunctional nanoparticles (NPs) with applicability for electrochemical sensors and catalysts. The present study addresses the fundamental aspects of the catalyzed growth of nano-graphites in C–Bi–Fe systems, relevant in charge transport and thermo-chemical processes. The thermal evolution of a C–Bi–Fe xerogel is investigated using different pyrolysis treatments. At lower temperatures (~750°C), hybrid bismuth iron oxide (BFO) NPs are frequently observed, while graphitization manifests under more specific conditions such as higher temperatures (~1,050°C) and reduction yields. An in situ heating TEM experiment reveals graphitization activity between 800 and 900°C. NP motion is directly correlated with textural changes of the carbon support due to the catalyzed growth of graphitic nanoshells and nanofibers as confirmed by HR-TEM and electron tomography (ET) for the graphitized sample. An exponential growth model for the catalyst dynamics enables the approximation of activation energies as 0.68 and 0.29–0.34 eV during reduction and graphitization stages. The results suggest some similarities with the tip growth mechanism, while oxygen interference and the limited catalyst–feed gas interactions are considered as the main constraints to enhanced growth.

Counterion Concentration Profiles at the Graphene Oxide/Water Interface

Despite enormous interest toward graphene oxide (GO) from the research community, surprisingly, little is known about its solutions. In particular, the questions related to the structure of the GO/liquid interface have not been yet properly addressed. In this report, we use a simple but efficient experimental approach to investigate the distribution of the four metal cations Na⁺, Cs⁺, Ni²⁺, and Gd³⁺ at the GO/water interface. We demonstrate that the concentration of the cations decreases exponentially with the distance from the GO surface. Such distribution for colloid systems was theoretically predicted and commonly accepted for a century but, to the best of our knowledge, has been never proved experimentally. We further demonstrate that the shape of the counterion distribution profiles depends on the pH of solution and on the fine chemical structure of GO. In particular, organic sulfates and vinylogous acids that are ionizable at different pH levels are responsible for the difference in the shapes of the concentration profiles. Unlike classical colloid systems, the diffuse layer in the GO solutions is rather broad (30-55 nm), and the concentration gradient is registered even at distances of >55 nm from the GO surface, which is typically considered as the bulk solution. The latter observation is explained by the immobilized character of the GO flakes in the nematic phase, impeding the flow of liquid and the migration of hydrated metal cations. This helps to establish and maintain the long-range concentration gradient in the space between the two parallel neighboring GO flakes. Based on the new findings and on the previously reported data, we formulate some basic principles of GO solutions.

Efficient oxygen reduction on sandwich-like metal@N-C composites with ultrafine Fe nanoparticles embedded in N-doped carbon nanotubes grafted on graphene sheets

In the past decade, tremendous efforts have been devoted to the search for the alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. Recently, metal-nitrogen-carbon (M-N-C) systems, especially 3d transition metals (TM) and their alloys encapsulated in nitrogen-doped carbon based materials (TM@N-C), have attracted increasing attention due to their low cost and high ORR activity. Here, a simple and novel strategy is developed to synthesize sandwich-structured TM@N-C composites, in which ultrafine Fe nanoparticles are encapsulated in nitrogen-doped carbon nanotubes (N-CNTs) grafted on both sides of reduced graphene oxide (rGO) sheets by pyrolysis of ammonium ferric citrate-functionalized zeolitic imidazolate framework-8@graphene oxide (Fe@ZIF-8@GO). The resulting Fe@N-CNTs@rGO composites naturally integrate zero-dimensional (0D) Fe nanoparticles, one-dimensional (1D) N-CNTs, and two-dimensional (2D) graphene into a three-dimensional (3D) hierarchical architecture with highly dispersed active sites, a large surface area, and abundant porosity. Because of these structural advantages, the sandwich-structured Fe@N-CNTs@rGO composites display a half-wave potential of 0.83 V in a 0.1 M KOH solution for the ORR, comparable to that of commercial Pt/C catalysts, and more excellent durability and resistance to fuel molecules. The proposed strategy paves a new way for the synthesis of non-precious high-performance electrocatalysts for energy conversion applications.

Catalytic properties of graphene oxide/palladium composites as a function of the fabrication method

In this work, we used a two-step synthetic procedure to prepare palladium nanoparticles (Pd-NP) on a graphenic support by impregnating graphene oxide (GO) with Pd(ii) ions with subsequent thermal or chemical reduction of palladium.

Growth of invar nanoparticles on a graphene oxide support

Alloyed binary invar-like FeNi nanocrystals are synthesized on the surface of graphene oxide in a two-step impregnation–annealing procedure.

Direct growth of oriented nanocrystals of gamma-iron on graphene oxide substrates. Detailed analysis of the factors affecting unexpected formation of the gamma-iron phase

Substrate-oriented nanocrystals of room-temperature-stable gamma-iron have been synthesized by a two-step impregnation/annealing method on a graphene oxide surface.