Low temperature plasmas (LTP) are a unique class of open-driven systems in which chemical reactions are unpredictable using established concepts. The terminal state of chemical reactions in LTP, termed the superlocal equilibrium state, is hypothesized to be defined by a proposed set of state variables. Using a LTP reactor wherein the state variables have been measured, it is shown that CO 2 spontaneously splits and the effluent speciation is independent of the influent speciation if the state variables are held constant and the residence time is long. CO 2 conversion at long residence times, which is expected to be nominally zero from equilibrium thermodynamics, can be as high as 70% in the LTP. The employed low pressure plasma reactor (P = 10 mbar) had a similar volume, productivity, and energy efficiency compared to an atmospheric pressure dielectric barrier discharge reactor, thanks to reaction rates that were three orders of magnitude faster. K E Y W O R D SCO 2 splitting, low-pressure reactor, low-temperature plasma, plasma chemistry, superlocal equilibrium 1 | INTRODUCTION A major challenge currently facing chemical engineering is the development of processes to transform CO 2 into platform chemicals from which valuable materials can be synthesized, with the goal of removing the greenhouse gas from the atmosphere. CO 2 is a relatively inert species that must be activated before it can react. Activation can be accomplished by photocatalysis, 1-3 electrochemistry, 4-7 raising the gas to high temperature, 8 or using low temperature plasma, [9][10][11] which is a partially ionized gas. Of these activation methods, plasma has a combination of desirable traits that stands out from the others: high-energy efficiency 12 (up to 90%), high reaction rates at the laboratory scale (approximately 1 t m −3 hr −1 ), and the possibility of low background gas temperature for compatibility with temperature-sensitive molecules. The synthesis of platform chemicals from CO 2 will likely require sophisticated reaction engineering. A major challenge preventing engineering of reactions between CO 2 and other molecules such as CH 4 or H 2 O in low temperature plasma is that one cannot make the local equilibrium assumption, 13 and consequently, practical methods to predict the direction and maximum extent of chemical reactions have not been forthcoming.The concept of thermodynamic equilibrium is generally used to predict the direction of chemical reactions and theoretical maxima for conversion and yield. In flow systems, which are intrinsically not at equilibrium, 14 the local equilibrium assumption 15 is often used for process modeling. One aspect of the local equilibrium assumption is that at given location in space, one can describe a single temperature, which is defined as T = ∂U ∂S À Á x , where U is internal energy, S is entropy, and the partial derivative is taken at constant work displacement x. Nonequilibrium systems can then be modeled using temperature gradients and the associated heat fluxes. 16 Such approaches for modeli...

High quality gallium nitride (GaN) nanocrystals (NCs) are promising materials in a wide range of applications including optoelectronics, photonics and biomedical devices. Unlike II–VI semiconductors, the synthesis of free-standing GaN NCs is not well-established, and there is a need for a synthesis platform that can provide GaN NCs with tunable size and photonic properties. In this work, we present a flexible gas-phase synthesis method that can deliver crystalline, free-standing, pure GaN NCs with controlled size and narrow size distributions. The method, termed nonequilibrium plasma aerotaxy (NPA), employs an aerosol of Ga and gaseous N2 as the precursors. The term aerotaxy means growth on an unsupported surface, in this case promoted by a nonequilibrium plasma. The key to narrow size distributions is that the NPA mechanism is based upon surface growth, as opposed to coagulation mechanisms that result in broad size distributions. The NPA process converts the Ga aerosol into GaN NCs within 10–100 ms of residence time. The mechanism involves non-thermal vaporization of the source Ga aerosol, which is followed by nucleation and reaction with the excited N2 species in the plasma. Particles can be made to be either hollow or solid. Solid NCs were found to be photoluminescent. Large NCs emitted photons at a peak wavelength near the bulk band-gap transition. Tuning the size to be smaller than 7 nm average diameter led to a blue-shifted photoluminescence. Inline processing of these bare GaN NCs into porous films by supersonic impact deposition is demonstrated. Moving beyond the specific example of GaN, the NPA mechanism is general and can be extended to many other binary, ternary or doped semiconductors.

Low temperature plasma (LTP) is a highly nonequilibrium substance capable of increasing the specific free energy of mass that flows through it. Despite this attractive feature, there are few examples of the transformation of solid material with an equilibrium atomic structure into a material with a nonequilibrium atomic structure. As a proposed example of such a transformation, in this work, it is argued that the transformation of crystalline metal nanoparticles into amorphous metal nanoparticles is feasible using LTP. To inform the feasibility calculations, detailed characterization was performed to determine the electron temperature, ion density, and background gas temperature as a function of axial position in a typical flow-through, radiofrequency, capacitively-coupled plasma reactor. Measurements revealed the existence of an intense zone with sharply elevated ion density and gas temperature in the vicinity of the powered electrode. The high intensity zone, amidst an otherwise low-intensity plasma, provides a means by which to transform the atomic structure of nanoparticles while maintaining unipolar negative charge to suppress coagulation. Theory suggests that such an intense zone would provide intense heating of nanoparticles, and subsequent rapid cooling. Calculations for copper-zirconium (CuZr) alloy show that the temperature history of a nanoparticle depends primarily on the intensity of the zone in the vicinity of the powered electrode, and on particle size. If one considers melting CuZr nanoparticles in the intense zone and then rapidly cooling them in the low-intensity plasma downstream, then the quenching rates are found to be high, on the order of 10 5 K s −1 . Since quenching rates of this magnitude are sufficient to arrest an amorphous atomic structure, LTP reactors can be used to transform crystalline metal nanoparticles into amorphous metal nanoparticles via a highly nonequilibrium quenching process.

Thermodynamics dictate the direction of all chemical and physical processes. In the case of aerosols, maximization of entropy leads to a broadening of the size distribution as the system proceeds toward equilibrium. The expectation is that as an aerosol ages, the size distribution will broaden. Contrary to this expectation, in this work we demonstrate that the unique nonequilibrium environment in a low temperature plasma can modify particulate materials to make the size distribution narrower. Submicrometer aerosols composed of bismuth particles with a polydispersed size distribution were prepared and passed through a low temperature argon plasma. For lower powers at which the plasma operated near room temperature, the incoming polydispersed aerosol was converted into a monodispersed aerosol of geometric standard deviation approximately 1.1 with 65% mass yield. The mechanism by which the process took place involved the particles vaporizing in the plasma operating at near room temperature, which resulted in very large supersaturation of metal vapor. Particle heating and sputtering by ion bombardment are discussed as possible mechanisms leading to vaporization that causes the change in the size distribution to make it narrower.

A recently introduced gas-phase method for the synthesis of III–V semiconductor nanocrystals (NCs), termed nonequilibrium plasma aerotaxy, is extended to the synthesis of indium nitride. The InN NCs were made to be free-standing, stoichiometric, and crystalline in the hexagonal phase. NCs were degenerate, and they displayed localized surface plasmon resonance in the mid-infrared, which was tunable between 0.22 and 0.34 eV (3650–5600 nm). Free electron densities span across (1.6–6.0) × 1020 cm–3, as estimated from the Drude–Lorentz theory. Absorption edges shifted by the Burstein–Moss effect were calculated to be between 1.34 and 2.04 eV (∼600–925 nm). Transient absorption (TA) measurements, which were conducted in visible and near-infrared range across a time scale spanning from picoseconds to microseconds, verified the high degeneracy of the NCs. Measurements showed that in these NCs excited carriers relax and recombine extremely fast. The recombination time was found to be ∼2 ps, which is similar to the values measured on InN nanowires and silicon-doped InN thin films. TA signals present in the nanosecond and microsecond ranges were attributed to temperature transients.