From Colloidal Co/CoO Core/Shell Nanoparticles to Arrays of Metallic Nanomagnets: Surface Modification and Magnetic Properties

Plasma-based post-processing of colloidal nanocrystals for applications in heterogeneous catalysis

This review summarizes the work on the use of plasmas to post-process nanostructures, in particular colloidal nanocrystals, as promising candidates for applications of heterogeneous catalysis. Using plasma to clean or modify the surface of nanostructures is a more precisely controlled method compared to other conventional methods, which is preferable when strict requirements for nanostructure morphology or chemical composition are necessary. The ability of plasma post-processing to create mesoporous materials with high surface areas and controlled microstructure, surfaces, and interfaces has transformational potential in catalysis and other applications that leverage surface/interface processes.

Absence of a pressure gap and atomistic mechanism of the oxidation of pure Co nanoparticles

Understanding chemical reactivity and magnetism of 3d transition metal nanoparticles is of fundamental interest for applications in fields ranging from spintronics to catalysis. Here, we present an atomistic picture of the early stage of the oxidation mechanism and its impact on the magnetism of Co nanoparticles. Our experiments reveal a two-step process characterized by (i) the initial formation of small CoO crystallites across the nanoparticle surface, until their coalescence leads to structural completion of the oxide shell passivating the metallic core; (ii) progressive conversion of the CoO shell to Co3O4 and void formation due to the nanoscale Kirkendall effect. The Co nanoparticles remain highly reactive toward oxygen during phase (i), demonstrating the absence of a pressure gap whereby a low reactivity at low pressures is postulated. Our results provide an important benchmark for the development of theoretical models for the chemical reactivity in catalysis and magnetism during metal oxidation at the nanoscale.

On the kinetics of the removal of ligands from films of colloidal nanocrystals by plasmas

This paper describes the kinetic limitations of etching ligands from colloidal nanocrystal assemblies (CNAs) by plasma processing. We measured the etching kinetics of ligands from a CNA model system (spherical ZrO₂ nanocrystals, 2.5-3.5 nm diameter, capped with trioctylphosphine oxide) with inductively coupled plasmas (He and O₂ feed gases, powers ranging from 7 to 30 W, at pressures ranging from 100 to 2000 mTorr and exposure times ranging between 6 and 168 h). The etching rate slows down by about one order of magnitude in the first minutes of etching, after which the rate of carbon removal becomes proportional to the third power of the carbon concentration in the CNA. Pressure oscillations in the plasma chamber significantly accelerate the overall rate of etching. These results indicate that the rate of etching is mostly affected by two main factors: (i) the crosslinking of the ligands in the first stage of plasma exposure, and (ii) the formation of a boundary layer at the surface of the CNA. Optimized conditions of plasma processing allow for a 60-fold improvement in etching rates compared to the previous state of the art and make the timeframes of plasma processing comparable to those of calcination.

Controlling the conductivity of Ti3C2MXenes by inductively coupled oxygen and hydrogen plasma treatment and humidity

With oxygen and hydrogen plasma the resistivity of transparent MXene electrodes can be controlled.

Arrangement at the nanoscale: Effect on magnetic particle hyperthermia

In this work, we present the arrangement of Fe₃O₄ magnetic nanoparticles into 3D linear chains and its effect on magnetic particle hyperthermia efficiency. The alignment has been performed under a 40 mT magnetic field in an agarose gel matrix. Two different sizes of magnetite nanoparticles, 10 and 40 nm, have been examined, exhibiting room temperature superparamagnetic and ferromagnetic behavior, in terms of DC magnetic field, respectively. The chain formation is experimentally visualized by scanning electron microscopy images. A molecular Dynamics anisotropic diffusion model that outlines the role of intrinsic particle properties and inter-particle distances on dipolar interactions has been used to simulate the chain formation process. The anisotropic character of the aligned samples is also reflected to ferromagnetic resonance and static magnetometry measurements. Compared to the non-aligned samples, magnetically aligned ones present enhanced heating efficiency increasing specific loss power value by a factor of two. Dipolar interactions are responsible for the chain formation of controllable density and thickness inducing shape anisotropy, which in turn enhances magnetic particle hyperthermia efficiency.

Oxide nanostructures hyperbranched with thin and hollow metal shells for high-performance nanostructured battery electrodes

High-performance electrochemical energy storage (EES) devices require the ability to modify and assemble electrode materials with superior reactivity and structural stability. The fabrication of different oxide/metal core-branch nanoarrays with adjustable components and morphologies (e.g., nanowire and nanoflake) is reported on different conductive substrates. Hollow metal branches (or shells) wrapped around oxide cores are realized by electrodeposition using ZnO nanorods as a sacrificial template. In battery electrode application, the thin hollow metal branches can provide a mechanical protection of the oxide core and a highly conductive path for charges. As a demonstration, arrays of Co3O4/Ni core-branch nanowires are evaluated as the anode for lithium ion batteries. The thin metal branches evidently improve the electrochemical performance with higher specific capacity, rate capability, and capacity retention than the unmodified Co3O4 counterparts.

High-vacuum annealing reduction of Co/CoO nanoparticles

Porous films of Co/CoO magnetic nanoparticles have been obtained by inert gas condensation and partially oxidized in situ in the deposition chamber. These nanoparticle films were subjected to thermal treatments in high vacuum and the chemical and structural changes monitored by x-ray diffraction, transmission electron microscopy, transport and magnetic measurements (with a focus on the exchange-bias phenomenon), which evidence that for vacuum annealing temperatures above 360 °C, most of the CoO phase is reduced to metallic Co without requiring the presence of an external reducing agent (e.g., H₂) or a plasma. Additionally, there is a certain degree of particle coalescence resulting in the formation of greater nanoparticles as the annealing temperature increases. This yields a smaller proportion of CoO compared to metallic Co and a reduction of the Co/CoO interface density, pinpointed by a drastic decrease of the exchange-bias field. The crucial roles of the vacuum level and the surface-to-volume ratio are evidenced by magnetic measurements, highlighting the potential of magnetometry as a probe for the reduction/oxidation of composite nanostructures.

Microporous Co@CoO nanoparticles with superior microwave absorption properties

Nanoporous metal materials with many potential applications have been synthesized by a chemical dealloying approach. The fabrication of nanoporous metal nanoparticles (NPs), however, is still challenging due to the difficulties in producing suitable nanoscale precursors. Herein, nanoporous Co NPs of 31 nm have been successfully prepared by dealloying Co-Al NPs, and surprisingly they possess micropores in a range from 0.7 to 1.7 nm and a large surface area of 50 m(2) g(-1). The crystalline size of the microporous NPs is 2-5 nm. Through the passivation process, the microporous Co NPs covered with CoO (Co@CoO) are generated as a result of the surface oxidation of Co. They exhibit better microwave absorption properties than their nonporous counterpart. An enhanced reflection loss (RL) value of -90.2 dB is obtained for the microporous Co@CoO NPs with a thickness of merely 1.3 mm. The absorption bandwidth corresponding to the RL below -10 dB reaches 7.2 GHz. The microwave absorption mechanism is discussed in terms of micropore morphology, core@shell structure and nanostructure. This novel microporous material may open new routes for designing high performance microwave absorbers.

Transition-Metal Nanoparticle Oxidation in a Chemically Nonhomogenous Environment Revealed by 2p3d Resonant X-ray Emission

X-ray absorption spectroscopy (XAS) is often employed in fields such as catalysis to determine whether transition-metal nanoparticles are oxidized. Here we show 2p3/2 XAS and 2p3d resonant X-ray emission spectroscopy (RXES) data of oleate-coated cobalt nanoparticles with average diameters of 4.0, 4.2, 5.0, 8.4, and 15.2 nm. Two particle batches were exposed to air for different periods of time, whereas the others were measured as synthesized. In the colloidal nanoparticles, the cobalt sites can have different chemical environments (metallic/oxidized/surface-coordinated), and it is shown that most XAS data cannot distinguish whether the nanoparticles are oxidized or surface-coated. In contrast, the high-energy resolution RXES spectra reveal whether more than the first metal layer is oxidized based on the unique energetic separation of spectral features related to the formal metal (X-ray fluorescence) or to a metal oxide (d-d excitations). This is the first demonstration of metal 2p3d RXES as a novel surface science tool.

Hydrogen-plasma-induced magnetocrystalline anisotropy ordering in self-assembled magnetic nanoparticle monolayers

Self-assembled two-dimensional arrays of either 14 nm hcp-Co or 6 nm ε-Co particle components were treated by hydrogen plasma for various exposure times. A change of hysteretic sample behavior depending on the treatment duration is reported, which can be divided in two time scales: oxygen reduction increases the particle magnetization during the first 20 min, which is followed by an alteration of the magnetic response shape. The latter depends on the respective particle species. Based on the Landau-Lifshitz equations for a discrete set of magnetic moments, we propose a model that relates the change of the hysteresis loops to a dipole-driven ordering of the magnetocrystalline easy axes within the particle plane due to the high spatial aspect ratio of the system.

Nanoscaled alloy formation from self-assembled elemental Co nanoparticles on top of Pt films

The thermally activated formation of nanoscale CoPt alloys was investigated, after deposition of self-assembled Co nanoparticles on textured Pt(111) and epitaxial Pt(100) films on MgO(100) and SrTiO(3)(100) substrates, respectively. For this purpose, metallic Co nanoparticles (diameter 7 nm) were prepared with a spacing of 100 nm by deposition of precursor-loaded reverse micelles, subsequent plasma etching and reduction on flat Pt surfaces. The samples were then annealed at successively higher temperatures under a H(2) atmosphere, and the resulting variations of their structure, morphology and magnetic properties were characterized. We observed pronounced differences in the diffusion and alloying of Co nanoparticles on Pt films with different orientations and microstructures. On textured Pt(111) films exhibiting grain sizes (20-30 nm) smaller than the particle spacing (100 nm), the formation of local nanoalloys at the surface is strongly suppressed and Co incorporation into the film via grain boundaries is favoured. In contrast, due to the absence of grain boundaries on high quality epitaxial Pt(100) films with micron-sized grains, local alloying at the film surface was established. Signatures of alloy formation were evident from magnetic investigations. Upon annealing to temperatures up to 380 °C, we found an increase both of the coercive field and of the Co orbital magnetic moment, indicating the formation of a CoPt phase with strongly increased magnetic anisotropy compared to pure Co. At higher temperatures, however, the Co atoms diffuse into a nearby surface region where Pt-rich compounds are formed, as shown by element-specific microscopy.

Element-specific magnetic hysteresis of individual 18 nm Fe nanocubes

Correlating the electronic structure and magnetic response with the morphology and crystal structure of the same single ferromagnetic nanoparticle has been up to now an unresolved challenge. Here, we present measurements of the element-specific electronic structure and magnetic response as a function of magnetic field amplitude and orientation for chemically synthesized single Fe nanocubes with 18 nm edge length. Magnetic states and interactions of monomers, dimers, and trimers are analyzed by X-ray photoemission electron microscopy for different particle arrangements. The element-specific electronic structure can be probed and correlated with the changes of magnetic properties. This approach opens new possibilities for a deeper understanding of the collective response of magnetic nanohybrids in multifunctional materials and in nanomagnetic colloidal suspensions used in biomedical and engineering technologies.

Preparation and characterization of supported magnetic nanoparticles prepared by reverse micelles

Monatomic (Fe, Co) and bimetallic (FePt and CoPt) nanoparticles were prepared by exploiting the self-organization of precursor loaded reverse micelles. Achievements and limitations of the preparation approach are critically discussed. We show that self-assembled metallic nanoparticles can be prepared with diameters d = 2-12 nm and interparticle distances D = 20-140 nm on various substrates. Structural, electronic and magnetic properties of the particle arrays were characterized by several techniques to give a comprehensive view of the high quality of the method. For Co nanoparticles, it is demonstrated that magnetostatic interactions can be neglected for distances which are at least 6 times larger than the particle diameter. Focus is placed on FePt alloy nanoparticles which show a huge magnetic anisotropy in the L1(0) phase, however, this is still less by a factor of 3-4 when compared to the anisotropy of the bulk counterpart. A similar observation was also found for CoPt nanoparticles (NPs). These results are related to imperfect crystal structures as revealed by HRTEM as well as to compositional distributions of the prepared particles. Interestingly, the results demonstrate that the averaged effective magnetic anisotropy of FePt nanoparticles does not strongly depend on size. Consequently, magnetization stability should scale linearly with the volume of the NPs and give rise to a critical value for stability at ambient temperature. Indeed, for diameters above 6 nm such stability is observed for the current FePt and CoPt NPs. Finally, the long-term conservation of nanoparticles by Au photoseeding is presented.

Fe oxidation versus Pt segregation in FePt nanoparticles and thin films

Metallic nanoparticles containing 3d elements are generally susceptible to oxidation leading to a deterioration of desired properties. Here, the oxidation behavior of differently sized FePt nanoparticles is experimentally studied by x-ray photoelectron spectroscopy (XPS) and compared to a FePt reference film. For all as-prepared metallic samples the common features are the formation of Fe(3+), becoming detectable for exposures to pure oxygen above 10(6) langmuir whereas under identical conditions the Pt(0) signal is conserved. Most notably, these features are independent of particle size. Annealing at 650 degrees C, however, affects small and large FePt particles differently. While large particles as well as the reference film show a 100-1000 times enhanced resistance against oxidation, small FePt particles (diameter 5 nm) exhibit no such enhancement due to the thermal treatment. Additional XPS intensity analysis in combination with model calculations leads to an explanation of this observation in terms of Pt segregating to the surface. In large particles and films the thickness of the resulting Pt layer is sufficient to strongly impede oxidation, while in small particles this layer is incomplete and no longer provides protection against oxidation.

Nanocrystal plasma polymerization: from colloidal nanocrystals to inorganic architectures

Nanocrystal superstructures are increasingly becoming a subject of intense study. Such materials could constitute a new class of nanocomposites of designed structure, of homogeneous composition, and with unique properties. New phenomena are observed in these materials because of the interaction at such diminutive length scales. A common problem in the development of devices relying on colloidal nanocrystal assemblies is that the individual nanocrystal building blocks require organic molecules to control their size. These ligands are responsible for the colloidal stability of the individual nanocrystal building blocks and are thus necessary for their solution processibility. Because of the ligands' incompatibility with many solid state applications, it is important to develop post-processing techniques that mildly remove them from these nanocomposites, while maintaining the size-dependent properties of the building blocks. This Account highlights a new strategy, nanocrystal plasma polymerization (NPP), for processing colloidal nanocrystal assemblies. This technique exposes the nanocomposite to a mild air plasma and allows for the removal of the nanocrystals' capping ligands while preserving their size-dependent and material properties. As a result, the process yields a nearly all-inorganic flexible solid-state material with unprecedented characteristics. We describe early experiments, in which NPP was used to create arbitrarily complex 1D, 2D, and 3D inorganic free-standing architectures entirely composed of nanocrystals, as well as future directions and challenges. We expect this platform will be useful for the design of new materials and will be a valuable new addition to the nanoscientist's toolbox.

Signatures of clustering in superparamagnetic colloidal nanocomposites of an inorganic and hybrid nature

The individual and co-operative properties of inorganic and hybrid superparamagnetic colloidal nanocomposites that satisfy all the requirements of magnetic carriers in the biosciences and/or catalysis fields are been studied. Essential to the success of this study is the selection of suitable synthetic routes (aerosol and nanocasting) that allow the preparation of materials with different matrix characteristics (carbon, silica, and polymers with controlled porosity). These materials present magnetic properties that depend on the average particle size and the degree of polydispersity. Finally, the analysis of the co-operative behavior of samples allows for the detection of signatures of clustering, which are closely related to the textural characteristics of samples and the methodology used to produce the magnetic carriers.

Enhanced orbital magnetism in Fe(50)Pt(50) nanoparticles

X-ray absorption and magnetic circular dichroism spectra at both the Fe and Pt L(3,2) edges were measured on wet-chemically synthesized monodisperse Fe(50)Pt(50) particles with a mean diameter of 6.3 nm before and after complete removal of the organic ligands and the oxide shell covering the particles by soft hydrogen plasma resulting in a pure metallic state. After thermal treatment of the metallic particles, the coercive field increased by a factor of 6, the orbital magnetic moment at the Fe site increased by 330% and is reduced at the Pt site by 30%, while the effective spin moments did not change. A decrease of the frequency of oscillations in the extended x-ray absorption fine structure at the Pt L(3,2) edges provides evidence for crystallographic changes towards the L1(0) phase.

Synthesis and characterization of large colloidal cobalt particles

Large colloidal environmentally stable silica-coated cobalt particles were synthesized by combining the sodium borohydride reduction in aqueous solution and the Stöber method. Low size polydisperse cobalt spheres with an average size of 95 nm were synthesized by using a borohydride reduction method and were subsequently coated with a thin layer of silica by means of hydrolysis and condensation of tetraethylorothosilicate (TEOS) in an aqueous/ethanolic solution. The large uniform cobalt spheres consist of smaller metallic Co clusters, explaining the superparamagnetic behavior of the spheres. The particles were investigated by transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM).