2.1. Low-Pressure Plasmas

Low-pressure plasmas are popular for the treatment of solid materials due to the uniformity of plasma parameters over a rather large volume. The uniformity is due to the lack of the gas-phase collisions that lead to the loss of reactive species by neutralization of charged particles, the association of neutral radicals to stable molecules and relaxation of metastables in superelastic collisions [38]. Such plasmas can be sustained by various discharges, but the most popular are electrodeless discharges. The microwave discharges are particularly useful for the synthesis of zinc oxide nanoparticles. Plasma is sustained in a dielectric tube, which is mounted into a microwave cavity. A typical configuration is shown in Figure 1a. The dielectric tube is hermetical tightly mounted between the feeding and collecting chambers. The system is pumped with a suitable vacuum pump that enables evacuation as well as the removal of gaseous reaction products. The gas flow is in the direction from the feeding to the collection chamber. The dielectric tube is mounted into a metallic cavity which is powered with a microwave source. The standing waves in the cavity ignite the discharge in the dielectric tube. As soon as the plasma is ignited, the gas within the dielectric tube becomes conductive, so the skin effect prevents propagation of the electromagnetic field inside the plasma. The field is thus concentrated to the sheath between plasma and the dielectric tube, so it has a high amplitude, which is beneficial for the acceleration of electrons to a high kinetic energy. The fast electrons accelerated within the sheath collide with slow plasma electrons and transfer their energy in elastic collisions. The electrons inside the plasma thus assume a large temperature, often a few 10,000 K. Such electrons are capable of ionizing and dissociating collisions with the precursors. The precursors and reactive gases are fed continuously into the system. The nanoparticles are formed in the plasma volume and accumulate in the collection chamber. The pumping should be configured in such a way as to prevent significant removal of as-synthesized particles.

Schematic of the low-pressure microwave plasma system for nanoparticle synthesis: (a) with organometallic gaseous or powder precursor; (b) with solid precursor placed into the plasma reactor.

The configuration in Figure 1a is useful in cases of gaseous or powder precursors fed continuously into the plasma reactor. Other authors employed precursors placed directly in the plasma. Such a configuration is presented in Figure 1b. Any metallic object placed into the microwave field will be heated due to surface currents in electrically conductive materials stimulated by the oscillating electromagnetic field, so the precursor will melt and evaporate when using the configuration of Figure 1b.

Probably the most straightforward technique for synthesizing any metal oxide nanoparticle in the gas phase is to heat precursors in an oxidative atmosphere. The precursors (often Zn(CH₃)₂) are fed into a burner along with oxygen or water vapour. However, this technique may not lead to the best quality ZnO nanoparticles. To overcome the problem of optimal oxidation of Zn(CH₃)₂ precursors, Kleinwechter et al. [23] proposed using a microwave-driven plasma reactor, as shown schematically in Figure 1a. A typical discharge power was 60 W and a typical pressure was 30 mbar, and plasma was sustained in a mixture of 20 vol% O₂ and 80 vol% Ar. The concentration of Zn(CH₃)₂ was varied between 700 and 1800 ppm. The nanoparticles were rather spherical with diameters of several nm and did not agglomerate. The particle diameter increased monotonically with the increasing concentration of the precursor. In the second experiment, the pressure was varied at a fixed precursor concentration, and the authors found an increasing particle diameter with increasing pressure. When the discharge power was varied, and other parameters were fixed, the nanoparticle dimension decreased with increasing discharge power. The authors found microwave plasma synthesis to be a suitable method to overcome the difficulties encountered in particle formation in a chemically heated rector—namely, instead of individual nanoparticles, a thin greyish film formed that strongly adhered to the substrate when the same gas mixtures were used to synthesize nanoparticles in a classical oven. According to the authors, the plasma system used by Kleinwechter et al. [23] enabled the choice between chemical and physical energy and a better understanding of particle formation processes. No details of the chemical reactions upon plasma conditions were given in [23], but there are numerous papers on microwave plasma behaviour in the range of pressures around 10 bar. A relatively complete insight into the gas-phase reactions in Ar-O₂ plasmas sustained by microwave discharges was provided by Kutasi et al. [39]. The work presents scientifically sound simulations based on experimental observations. Indeed, current characterization techniques only allow the quantitative measurement of a few types of plasma species. Kutasi et al. [39] explained different gas-phase and surface reactions and concluded that the Ar-O₂ mixtures in microwave plasma are always rich in O atoms. Dissociation of oxygen molecules occurs both at collisions of electrons from the high-energy tail of their distribution function as well as at collisions with Ar* metastables. The potential barrier for the oxidation of Zn(CH₃)₂ molecules with O atoms is much lower than for O₂. The reaction efficiency depends on the concentration of two reactants. Kleinwechter et al. [23] reported the concentration of Zn(CH₃)₂ to be around 1000 ppm, which is much less than the concentration of O atoms, which is reported to be well over 10,000 ppm. Thus, the oxidation is efficient. This is because there are numerous channels for the dissociation of Zn(CH₃)_2, and the electron energy required to subtract an H atom from the molecules is less than the dissociation energy of O₂ molecules. The Zn(CH₃)₂ molecules form different radicals, and the oligomerization probably takes place at a pressure of several 10 mbar [3,4]. The clusters are heated under plasma conditions by heterogeneous surface reactions [40] so that their temperature remains high while they are in the plasma. This high temperature favours the complete oxidation of the clusters and thus the formation of highly crystalline ZnO. The clusters assume the floating potential the same as any other object immersed in a gaseous nonequilibrium plasma. A negative charge develops on the clusters while they are in the plasma, and the retarding electrostatic force suppresses agglomeration. As disclosed by Kleinwechter et al. [23], plasma synthesis ensures the high quality of ZnO nanoparticles and prevents their agglomeration.

A schematic of plasma synthesis of ZnO nanoparticles using Zn-containing organic precursors is shown in Figure 2. The organometallic precursor forms radicals either by electron impact dissociation or quenching of Ar* metastables. The radicals are further split to form Zn atoms, which can also be excited or even ionized. Simultaneously, oxidation takes place, leading to the formation of molecules, such as OH, H₂O, and CO, which are then associated with other atoms to form stable molecules (carbon dioxide and water). Radicals tend to associate with each other and form initial clusters. This effect was elaborated for other precursors (see the recent paper [41] and references therein). The initial small clusters adsorb radicals, and zinc atoms also condense on the surface, so the clusters grow. Simultaneously, they become negatively charged due to the attachment of slow plasma electrons. The negatively charged clusters attract positively charged species, causing further growth. Hydrogen and carbon, which are present in radicals and early stage clusters, readily interact with oxygen radicals (particularly O atoms that are in abundance in Ar-O₂ plasma [39]) so almost pure ZnO is formed with a prolonged residence time. All surface reactions are exothermic. Hence, the cluster and nanoparticles’ temperature is well above the ambient temperature as long as they remain in plasma.

Schematic depiction of the complex reactions that can occur in gas mixtures of argon, oxygen and organometallic gasses under plasma conditions.

Metallic vapour is also useful for synthesizing metal oxide nanoparticles. Rapid synthesis of ZnO nanoparticles using microwave plasma was reported by Subannajui [28]. Zinc swarf was placed in the centre of an alumina container and brought near to the microwave radiation source, as shown schematically in Figure 1b. The microwaves induced an internal potential that produced a very high field at the sharp peaks and edges of the Zn swarf. As a result, dense and hot plasma formed over the swarf, causing the material to melt and vaporize. The metal vapours condensed and oxidized in the region of the less intense plasma and the material was deposited on the surfaces facing the plasma in the form of ZnO nanoparticles. Even one second of such an aggressive treatment enabled deposition of some nanomaterials, but about 4 s was found to be the optimal treatment time for the growth of nanowires with a large aspect ratio. The early stage ZnO nanowires did not appear in the form of rods but as round and irregular long-ellipsoidal nanoparticles. Plasma treatment of a few seconds allowed the synthesis of nanowires with a rather uniform diameter of about 65 nm, but longer treatment times led to the deposition of films containing different shapes of ZnO. Subannajui [28] proposed a four-step mechanism. First, Zn atoms were sputtered or evaporated out of the swarf, and then they were oxidized, deposited and agglomerated on the substrate without catalysts. In the second step, a large number of ZnO nuclei were generated, and the Zn atoms were further oxidized and developed into larger ZnO nanoparticles. In the third step, the ZnO nanoparticles were stretched and became irregular long ellipsoidal particles. These long ellipsoidal particles tended to organize into the most stable state and become ZnO nanowires in the last step. A small peak corresponding to metallic Zn in the X-ray diffraction (XRD) spectrum indicated incomplete oxidation, but the oxide was found in the wurtzite structure of the ZnO nanowires. UV–Vis absorption spectroscopy showed good absorption in the UV region with a peak at about 365 nm.

A classical plasma-driven source of metallic atoms in the gas phase is a low-pressure discharge rich in energetic ions. The ions accelerate in the potential sheath next to the negatively biased electrode and cause sputtering. The removed metal atoms (including some ions) pass into the gas phase, where they can agglomerate if the pressure is high enough. This technique was elaborated by Kylian et al. [42]. This technique allows the synthesis of nanoparticles with selected diameters and compositions, depending on the discharge parameters, as recently shown by the same group [43]. The nanoparticles are always spherical when using this technique. The production of nanoparticles by this method is limited by the sputtering rate, so it does not allow mass synthesis.

Figure 3 shows schematically that the reactions in the case zinc atoms are precursors. As mentioned above, the source of atoms can be thermal evaporation [7] or sputtering [42]. Zinc atoms are partially excited and ionized, while any metallic clusters, likely formed by strong discharges [7], assume a negative surface charge due to the attachment of slow plasma electrons. Oxidation and agglomeration occur in a similar manner as when Zn(CH₃)₂ is used as a precursor. In all cases, the synthesized ZnO nanoparticles tend to have spherical shapes, and the diameter depends on the synthesis conditions, including residence time in plasma, pressure, the concentrations of the various gases, the discharge power, and specifics of the plasma reactor.

Schematic depiction of the reactions where zinc atoms or clusters are precursors.

The application of gaseous precursors (organometallic compounds or Zn vapour) may not be economical or may not ensure the desired quantities of nanoparticles. The next option is to feed zinc powder into a plasma reactor. Hiragino et al. [26] synthesized nitrogen-doped ZnO nanoparticles using a medium-pressure gaseous plasma. The experimental system was almost identical to that presented in Figure 1a, except the microwave cavity was replaced with a coil connected to a radiofrequency (RF) generator. The discharge was concentrated to a rather small volume inside a coil which was coupled to an RF generator operating at a power of up to 30 kW and a frequency of 3.5 MHz. The gas pressure was about 200 mbar. The authors managed to keep the wall temperature of the reactor at 300 K by using water cooling instead of the forced air, as shown in Figure 1a. Zinc powder with a diameter of about 140 µm was used as the starting material. Based on the experimental details, the residence time of the powder in the hot plasma can be estimated to be about 0.1 s. Upon passing through the hot, but still nonequilibrium plasma, nanoparticles of different morphologies and a size of about 100 nm were synthesized. The aspect ratio of any nanoparticle was close to 1, and some agglomeration was observed, but the specific surface area of the synthesized ZnO materials was as large as about 20 m²/g. The materials were found to be useful for light-emitting diode (LED) applications. The schematic of the synthesis of zinc oxide nanoparticles from powder precursors is shown in Figure 4.

Schematic representation of the formation of nanoparticles with Zn powder as a precursor: (a) the formation of nanowires on the surface of metal powder; (b) the detail of the growth mechanism.

The kinetics of ZnO nanoparticle formation using Zn powder as a precursor is different from that shown in Figure 2 or Figure 3. The interaction of oxygen-containing plasma with metallic substrates was elaborated long ago, and one of the first reports on the synthesis of large aspect ratio nanowires was published as [44]. The metallic powder heats up to a high temperature when exposed to rather powerful plasma. Therefore, the powder quickly melts and sometimes even evaporates if kept in a powerful plasma for too long. The oxidation of the melted powder can lead to a variety of morphological shapes [45], ranging from nanowires with very high aspect ratios to cauliflower-like structures. The formation of nanowires on the surface of metal powder is illustrated in Figure 4a. The detail of the growth mechanism is shown in Figure 4b. The surface of a thin oxide film is never perfectly smooth but rich in humps. The plasma electrons accumulate at the top of the humps due to the Faraday effect. This creates a static electric field along the hump. The Zn⁺ ions enter the oxide layer and preferentially move in the direction of the electric field, i.e., along the hump, until they reach the tip of the hump, where they oxidize, thus lengthening the hump. A longer hump leads to further accumulation of the negatively charged electrons at the tip and thus even more extensive electro-diffusion of positive ions towards the tip. Finally, a nanowire with a large aspect ratio is formed. This mechanism, as shown in Figure 4b, works only under specific conditions, especially temperature. If the temperature is too low to enable diffusion of Zn⁺ ions through the oxide layer and on through the hump, little oxidation takes place. When the temperature is too high, thermal diffusion predominates, electro-migration is not as efficient, and structures with poor aspect ratios are formed. In one work, the range of useful parameters for the growth of silica nanoneedles was found only between 1800 and 1850 K [38].

Yet, another alternative for deposition of zinc oxide nanoparticles using a low-pressure gaseous plasma was reported by Yang et al. [25]. They used the configuration as shown in Figure 1b with some modifications to synthesize ZnO nanoparticles from ZnCl₂ powder under low-pressure conditions. Instead of using microwave heating, a furnace was placed on a glass discharge tube to melt and heat the ZnCl₂ powder to a temperature of about 350 °C, which allowed evaporation under controlled conditions. The vapour was directed toward a substrate by drifting a mixture of argon and oxygen in the direction from the precursor to the substrate. Weakly ionized gaseous plasma was generated in the discharge tube by an inductively coupled RF generator operating at a frequency of 13.56 MHz and a power of 100 W. The coil had the same function as the microwave cavity in Figure 1b. At plasma conditions, the vapour was partially atomized and the Zn atoms condensed on the substrate surface. The high concentration of oxygen atoms in the discharge tube allowed rapid oxidation of the deposited zinc. The treatment time was 15 min. The ZnO nanoparticles formed dense pillars with a typical diameter of about 0.4 µm and a length of several µm. XRD characterization confirmed the hexagonal ZnO structure with the preferential growth of ZnO was along the c-axis, which was confirmed by the transmission electron microscope (TEM) observations. Neither a cubic ZnO phase nor ZnCl₂ were found in the prepared product. Without plasma, only ZnCl₂ was found on the substrate. This work clearly demonstrates the advantages of plasma conditions. The authors suggested several mechanisms occurring in the gas phase and at the surfaces and emphasized the importance of metastable Ar in ZnCl₂ dissociation. The authors also reported that the addition of O₂ to the gaseous plasma caused an enhancement of ZnCl₂ dissociation. Optical emission spectroscopy was used to characterize the plasma; the radiative transitions of Zn atoms were significantly suppressed even with a small addition of oxygen to argon. The intensity of the Zn atomic lines decreased with increasing concentration of oxygen in the Ar-O₂ mixtures. The authors explained this by extensive oxidation of Zn in the gas phase. Therefore, the reaction mechanisms in the gas phase follow the initial case shown in Figure 3.

Sputtering or thermal evaporation using nonequilibrium low-pressure gaseous plasma may not be the most efficient atom sources due to limited power density and thus evaporation rate. Low-impedance discharges perform better as long as the evaporation intensity is the merit. Shanenkova et al. [7] reported the plasma synthesis of zinc oxide in an extremely short time process (duration less than 1 ms) using an electric discharge zinc-containing plasma jet flowing into an oxygen atmosphere. They used a home-built, high-power, arc-like plasma device operating in pulsed mode with the maximum discharge current of nearly 10⁵ A. The total energy of each pulse was nearly 30 kJ. The arc chamber was evacuated before igniting the discharge. The powered electrode was made of zinc, which vaporized upon the discharge conditions. Plasma was therefore sustained in metallic vapour. A shock wave formed during each discharge, and the plasma, rich in ionized metal vapour as well as very small Zn droplets, moved away from the main electrode at supersonic speed. The jet gradually cooled, allowing the clustering of Zn atoms in the gas phase, as shown schematically in Figure 4. The clusters oxidized in the oxygen-containing postdischarge chamber and grew by adsorption of Zn atoms. Complete oxidation of nanoparticles was observed. Various morphological shapes other than nanowires were found by TEM. Details about this unique device used for the synthesis of nanoparticles with typical dimensions of around 100 nm were reported in another work of the same group [29].