In Situ and Ex Situ X-ray Diffraction and Small-Angle X-ray Scattering Investigations of the Sol-Gel Synthesis of Fe3N and Fe3C

Iron nitride (Fe₃N) and iron carbide (Fe₃C) nanoparticles can be prepared via sol-gel synthesis. While sol-gel methods are simple, it can be difficult to control the crystalline composition, i.e., to achieve a Rietveld-pure product. In a previous in situ synchrotron study of the sol-gel synthesis of Fe₃N/Fe₃C, we showed that the reaction proceeds as follows: Fe₃O₄ → FeO_x → Fe₃N → Fe₃C. There was considerable overlap between the different phases, but we were unable to ascertain whether this was due to the experimental setup (side-on heating of a quartz capillary which could lead to thermal gradients) or whether individual particle reactions proceed at different rates. In this paper, we use in situ wide- and small-angle X-ray scattering (wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS)) to demonstrate that the overlapping phases are indeed due to variable reaction rates. While the initial oxide nanoparticles have a small range of diameters, the size range expands considerably and very rapidly during the oxide-nitride transition. This has implications for the isolation of Rietveld-pure Fe₃N, and in an extensive laboratory study, we were indeed unable to isolate phase-pure Fe₃N. However, we made the surprising discovery that Rietveld-pure Fe₃C nanoparticles can be produced at 500 °C with a sufficient furnace dwell time. This is considerably lower than the previous reports of the sol-gel synthesis of Fe₃C nanoparticles.

Milling as a route to porous graphitic carbons from biomass

This paper reports a simple way to produce porous graphitic carbons from a wide range of lignocellulosic biomass sources, including nut shells, softwood sawdust, seed husks and bamboo. Biomass precursors are milled and sieved to produce fine powders and are then converted to porous graphitic carbons by iron-catalysed graphitization. Graphitizing the raw (unmilled) biomass creates carbons that are diverse in their porosity and adsorption properties. This is due to the inability of the iron catalyst precursor to penetrate the structure of dense biomass material. Milling enables much more efficient impregnation of the biomass and produces carbons with homogeneous properties. Lignocellulosic biomass (particularly waste biomass) is an attractive precursor to technologically important porous graphitic carbons as it is abundant and renewable. This simple method for preparing the biomass enables a wide range of biomass sources to be used to produce carbons with homogeneous properties. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 2)'.

Evolution of the Local Structure in the Sol-Gel Synthesis of Fe3C Nanostructures

The sol–gel synthesis of iron carbide (Fe₃C) nanoparticles proceeds through multiple intermediate crystalline phases, including iron oxide (FeO_x) and iron nitride (Fe₃N). The control of particle size is challenging, and most methods produce polydisperse Fe₃C nanoparticles of 20–100 nm in diameter. Given the wide range of applications of Fe₃C nanoparticles, it is essential that we understand the evolution of the system during the synthesis. Here, we report an in situ synchrotron total scattering study of the formation of Fe₃C from gelatin and iron nitrate sol–gel precursors. A pair distribution function analysis reveals a dramatic increase in local ordering between 300 and 350 °C, indicating rapid nucleation and growth of iron oxide nanoparticles. The oxide intermediate remains stable until the emergence of Fe₃N at 600 °C. Structural refinement of the high-temperature data revealed local distortion of the NFe₆ octahedra, resulting in a change in the twist angle suggestive of a carbonitride intermediate. This work demonstrates the importance of intermediate phases in controlling the particle size of a sol–gel product. It is also, to the best of our knowledge, the first example of in situ total scattering analysis of a sol–gel system.

In situ total scattering analysis has uncovered the importance of iron oxide as an intermediate phase in the synthesis of Fe₃C nanoparticles from sol−gel precursors. A sudden increase in order is observed between 300 and 350 °C, associated with the crystallization of iron oxide nanoparticles. The local structure of the Fe₃N intermediate is also investigated.

The aggregation of an alkyl-C60 derivative as a function of concentration, temperature and solvent type

Contrast-variation small-angle neutron scattering (CV-SANS), small-angle X-ray scattering (SAXS), nuclear magnetic resonance (NMR) measurements of diffusion and isothermal titration calorimetry (ITC) are used to gain insight into the aggregation of an alkyl-C₆₀ derivative, molecule 1, in n-hexane, n-decane and toluene as a function of concentration and temperature. Results point to an associative mechanism of aggregation similar to other commonly associating molecules, including non-ionic surfactants or asphaltenes in non-aqueous solvents. Little aggregation is detected in toluene, but small micelle-like structures form in n-alkane solvents, which have a C₆₀-rich core and alkyl-rich shell. The greatest aggregation extent is found in n-hexane, and at 0.1 M the micelles of 1 comprise around 6 molecules at 25 °C. These micelles become smaller when the concentration is lowered, or if the solvent is changed to n-decane. The solution structure is also affected by temperature, with a slightly larger aggregation extent at 10 °C than at 25 °C. At higher concentrations, for example in solutions of 1 above 0.3 M in n-decane, a bicontinuous network becomes apparent. Overall, these findings aid our understanding of the factors driving the assembly of alkyl-π-conjugated hydrophobic amphiphiles such as 1 in solution and thereby represent a step towards the ultimate goal of exploiting this phenomenon to form materials with well-defined order.

Fluorescent liquid pyrene derivative-in-water microemulsions

Using a liquid pyrene derivative as the oil, stable oil-in-water microemuslions are prepared, with tunable fluorescence emission via droplet size.

In situ TEM observation of a microcrucible mechanism of nanowire growth

The growth of metal oxide nanowires can proceed via a number of mechanisms such as screw dislocation, vapor-liquid-solid process, or seeded growth. Transmission electron microscopy (TEM) can resolve nanowires but invariably lacks the facility for direct observation of how nanowires form. We used a transmission electron microscope equipped with an in situ heating stage to follow the growth of quaternary metal oxide nanowires. Video-rate imaging revealed barium carbonate nanoparticles diffusing through a porous matrix containing copper and yttrium oxides to subsequently act as catalytic sites for the outgrowth of Y₂BaCuO₅ nanowires on reaching the surface. The results suggest that sites on the rough surface of the porous matrix act as microcrucibles and thus provide insights into the mechanisms that drive metal oxide nanowire growth at high temperatures.