Microplasma: A New Generation of Technology for Functional Nanomaterial Synthesis

Plasma technology has been widely applied in the ozone production, material modification, gas/water cleaning, etc. Various nanomaterials were produced by thermal plasma technology. However, the high temperature process and low uniformity products limit their application for the high value added chemicals synthesis, for example the functional materials or the temperature sensitive materials. Microplasma has attracted significant attentions from various fields owing to its unique characteristics, like the high-pressure operation, non-equilibrium chemistry, continuous-flow, microscale geometry and self-organization phenomenon. Its application on the functional nanomaterial synthesis was elaborately discussed in this review paper. Firstly, the main physical parameters were reviewed, which include the electron temperature, electron energy distribution function, electron density and the gas temperature. Then four representative microplasma configurations were categorized, and the proper selection of configuration was summarized in light of different conditions. Finally the synthesis, mechanism and application of some typical nanomaterials were introduced.

In order to get insights into the intricate features of microplasma, systematic diagnostic 120 studies of different types of microplasma have already been carried out. The parameters 121 characterizing microplasma feature such as the electron temperature T e , electron energy 122 distribution functions (EEDFs), the electron density n e and the gas temperature T g have 123 been measured and calculated by various established techniques.  As to the micro dimension, a typical example is using an Argon plasma sustained 162 between a capillary exit and a substrate (the distance can be changed) to study the rela-163 tionship between the electron/gas temperatures and the micro dimension [82]. The results 164 showed that micro discharges exhibited the non-equilibrium characteristics. And reducing 165 the plasma size always leads to the increase of the electron temperature. Since the plasma 166 volume is reduced, a better power coupling from the electrical fields to the electrons could 167 be obtained, resulting in the increase of electron temperature. Electron temperature as high 168 as 14 eV could be achieved when the distance between the substrate and the capillary exit 169 was reduced to 0.2 mm. 170 In different microplasma conditions and configurations, the electron temperature varies 171 significantly from each other, so does the electron energy distribution. Although there are 172 no specific results on the relationship between EEDFs and other parameters, one common 173 result is that the electron energy distribution in microplasma is not following Maxwellian 174 distribution, which has already been verified by experimental studies [ A typical experiment was conducted between two parallel plate electrodes separated by 177 200 lm, which reflecting the electron and ion kinetics in atmospheric pressure He DC 178 microplasma [92]. In their experiments, various locations with different distances to the 179 cathode were investigated, and the EEDFs were tested. The results indicated that three 180 groups of electrons can be identified according to their energy: low energy electrons 181 (\1 eV), mild energy electrons (1-20 eV) and high energy electrons ([20 eV). Reflected 182 by the significant high energy tail attributed to the acceleration of electrons near the 183 cathode, the high energy electrons are abundant in this regime. In the bulk region, the high 184 energy tail disappears because the electric field is much lower than the sheath area. The 185 density of mild energy electrons also changes drastically with their energy, but is less than 186 the low energy electrons. This result proves again that the electron energy distribution 187 functions are not following the Maxwellian distribution.
188 Electron Density 189 Another important parameter to characterize microplasma is the electron density, n e , which 190 varies significantly with the electrodes distance, pressure, power, gas component and so on. 191 Compared with conventional plasmas, microplasma can be operated at a higher pressure 192 due to the possible breakdown of ''pressure times discharge distance'' scaling, indicated by 193 the Paschen's law. As a result, the higher density energetic species such as electrons or 194 other charged particles can be obtained in microplasma. In nanofabrication process, the 195 high electron density can enhance the collision rates of precursors to produce high con-196 centration of radical moieties, resulting in more efficient processes for the particle for-197 mation and growth, especially highly favorable for the nanomaterial synthesis. In this 198 section the electron density measurement is introduced, along with some typical examples 199 of experimental parameters and the corresponding electron densities. 200 The electron density varies in different conditions, and it could be measured by the 201 Stark broadening technique or by the optical emission spectroscopy (OES) line-ratio 202 method. Experiment diagnostics demonstrate that the Stark broadening technique has a 203 fundamental limitation when the electron density is in the range between 10 13 and 10 16 204 cm -3 . It is because the presence of resonance known as the van der Waals and the Doppler 205 broadenings, where the sum of low electron density may dominate the line broadening. The 206 OES line-ratio method is a good option when measuring the electron density in such a 207 range. While in the range of 10 16 -10 18 cm -3 , the Stark broadening technique is a better 208 alternative [93]. 209 Many studies have been carried out to characterize the electron density of microplasma. 210 The results show that it differs largely in various setups, for example, the electron density 211 of glow discharge under atmospheric pressure was found in the region between 10 9 and 212 10 13 cm -3 , in cases such as non-uniform discharges like atmospheric pressure micro-213 plasma jet, this value could be between 10 14 and 10 15 cm -3 [81]. For the microwave 214 excited microplasma in argon near atmospheric pressure, the value could be more than 215 1 9 10 14 cm -3 . The other factors, such as pressure, power, plasma dimension and oper-216 ating gas also have an impact on the electron density. 217 Some general features have been observed in many unbounded microdischarges at 218 atmospheric pressure. A higher electron density will be obtained by operating at a narrower 219 discharge gap with a constant power. A higher power input will not change the electron 220 density but only lead to an expansion of the discharge volume [94,95]. This is because the 221 small spatial size of plasma always has a relative large surface-to-volume ratio, resulting in 222 a relative high ionization degree, which is benefit for obtaining a higher electron density. , where the electron density increased 231 from 0.35 9 10 15 to about 1.29 9 10 15 cm -3 by increasing the power from 5 W to 50 W.
232 Gas Temperature 233 The gas temperature is another important parameter that must be considered in the process 234 of nanomaterial synthesis. A high gas temperature can result in coagulation and 235 spheroidization effects, leading to a significant thermal damage to the temperature sensi-236 tive precursors, products or substrates involved, sometimes even cause the evaporation of 237 solid objects. This problem is more severe in microelectronic manufacturing industry 238 where the high gas temperature always easily exceed the melting points of some metallic 239 interconnects [1]. Microplasma is termed as the non-equilibrium low-temperature plasma 240 because the electron temperature is much higher than the neutral and ion temperature. The 241 non-equilibrium feature makes it a good choice for nanomaterial synthesis, especially for 242 temperature sensitive materials. In this section some interesting examples are provided, 243 along with the introduction of the gas temperature measurement. 244 There are normally two methods to measure the gas temperature in plasma. One is by 245 measuring the Doppler profile of emission or absorption lines, which is due to the velocity 246 distribution of atoms. Through the degree of Doppler broadening the translational tem-247 perature can be determined [98], [99]. The N 2 s positive system can also be adopted to 248 estimate the gas temperature, according to the study of M. Bazavan et al. [100], the 249 electron temperature is generally much greater than the gas temperature, while the rota-250 tional temperature is equal with the gas temperature. Therefore, by modeling the N 2 251 emission spectra of the molecular rotational bands belong to the N 2s positive system, the 252 rotational temperature in the microplasma can be obtained [100]. 253 The gas temperature is in close connection with the discharge current and the gas 254 composition. Generally it is higher for molecular gases and lower for rare gases. The 255 pressure also has a significant impact on the temperature [101]. By observing the rotational 256 profile of the band of the second positive N 2 system in atmospheric pressure air micro-257 plasma, the gas temperature was measured to range from 1700 to 2000 K when regulating 258 the discharge current from 4 to 12 mA [102, 103]. As to a microplasma generated in a 259 hollow cathode metal tube, the value was approximately 1200 K for pure Ar and kept 260 constant at different discharge currents. When nickelocene or copper acetylacetonate was 261 introduced to the plasma, the value was estimated to be 1200 and 1500 K at the current of 4 262 and 8 mA respectively [104]. Also the gas temperature depends strongly on pressure [105]. 263 It increases from about 380 K at 50 mbar to around 1100 K at 400 mbar in the Ar 264 microplasma. 265 As can be seen from above, compared with conventional plasmas, microplasma has a 266 relatively higher electron temperature as well as a larger electron density, also its uniquely 267 featured non-equilibrium thermodynamics makes it particularly suitable for nanomaterial 268 synthesis. Besides, more potential applications in various fields are being studied and 269 investigated currently.  Table 3 274 shows a comparison of some features valued by nanofabrication process between micro-275 plasma and conventional plasma. A more detailed elaboration about the microplasma 276 application will be discussed in ''Microplasma Configurations for Nanomaterial Fabrica-277 tion'' and ''Nanomaterials Fabricated by Microplasma and Their Applications'' sections. 278 Thanks to the high reactivity and relatively low operation cost, microplasmas have been 279 studied for the environmental application, especially for the destruction of the volatile 280 organic compounds (VOCs) such as ethylene, benzene, toluene and octane. Becker [109] used capillary plasma electrode reactors in different 296 gas mixtures such as pure He, He/air, He/N 2 and air to study the inactivation of Bacillus 297 subtilis spores and Bacillus stearothermophilus spores. The results showed that the decimal 298 reduction factor (D-value, which is used to describe the time for reducing a specific active 299 microorganism concentration by one order of magnitude) could be achieved in less than 300 2 min. 301 Another interesting application is as an ultraviolet radiation source. Since the presence 302 of a high density of energy electrons in microplasma, it enables the excimer formation for 303 gases. Many researchers studied extensively on rare gases such as He [110], Ne [111], Ar Microplasma has also been intensively studied for its application in surface treatment of 311 glasses or polymers [115,116]. In industries such as the display manufacturing, eye glasses 312 and automobile side-view mirrors, hydrophilic property is required to make electrical 313 connections between flat-panel surfaces, or to keep the glass surface transparent under mist 314 conditions. As mentioned above, there are abundant active species and radicals in the 315 microplasma environment, which can activate the molecules or atoms on the surface of 316 glasses or polymers. With such interactions the surface will be modified and a hydrophobic 317 layer could be formed to improve their hydrophilic property [117]. 318 Other applications like biomedical diagnostics [118], spectroscopic analysis [119] and 319 medical treatment of human skin [120] were also reported and under development.    Figure 2 shows several 329 microplasma systems with different electrode geometries.
330 Hollow-Electrode Microcharges 331 Hollow-electrode microcharges are relatively simple structures used for nanomaterial 332 fabrication and can be operated stably at atmospheric pressure and room temperature. 333 Generally there are two hollow metal capillary tubes separated by 1-2 mm, both are 334 connected to a DC power supply and act as the cathode and the anode respectively. 335 Meanwhile, they also function as the precursor transporters, in which precursor vapors are 336 introduced by a flow of inert gas such as Ar or He, and dissociated in the plasma area 337 between two electrodes. The formed aerosol particles can be collected by an electrostatic 338 precipitator or by a filter installed after the reactor. In an emblematical hollow-electrode 339 microcharge, the typical voltage and current used to prepare nanomaterials are at the level 340 of hundred V and several mA, which are too small to ionize electrodes. Therefore, elec-341 trodes won't take part in the reactions. 342 In a representative structure developed by Chiang and Sankaran [128], microplasma was 343 formed between two electrodes which were separated by 2 mm. The cathode was a 344 stainless steel capillary tube with an inner diameter of 180 lm, the anode was a stainless 345 steel tube/mesh, and both were sealed inside a quartz tube to keep stable plasma operation. 346 During the discharge process, the size and distribution of the generated nanoparticles could 347 be measured by the followed aerosol size classification. In addition, they used the produced 348 nanoparticles as catalysts for carbon nanotubes (CNTs) growth in a tube furnace, and 349 studied catalytic properties of various compositionally-tuned Ni x Fe 1-x nanoparticles. The 350 two-stage microplasma system is schematically illustrated in Fig. 3. 351 A similar microplasma reactor as Fig. 3 was used to produce multimetallic nanoparti-352 cles by the dissociation of organmetallic vapors [129]. The organmetallic compounds such 353 as Ni(Cp) 2 , Fe(Cp) 2 , Cu(acac) 2 , Pt(acac) 2 were used, and a series of mono-, bi-, and tri-354 metallic nanoparticles with various compositions were synthesized by varying the flow rate 355 of precursors. Ultra-fine (less than 5 nm in diameter) nanoparticles with narrow size dis-356 tribution could be fabricated because of the extremely short residence time (about 1 ms) in 357 such micro-structure. 358 In addition to preparing metal nanoparticles and CNTs, hollow-electrode microcharges 359 were also used to prepare Si nanowire [104] or nanodiamonds [130]. The results showed 360 that this novel approach could produce ultrafine nanoparticles at atmospheric pressure and 361 room temperature.

Microplasma Jets
363 Currently various configurations of microplasma jets were proposed and used as nano-364 material fabrication tools, with different types of power supplies (dc, rf or microwave) to 365 ignite and sustain the plasma. Gas jets with external electrodes, like wire electrode or tube 366 electrode, were built. Belmonte et al. [81] pointed out that the following three items should 367 be addressed concerning microplasma jets: (1) Controlling the location of the deposition 368 area by regulating the precursors' flow -currently the most common way is locating 369 capillaries where the precursors flow through. (2) Managing consumable wires that were 370 used as nanomaterial sources -most of the microplasma jets use the consumable wires as 371 precursors of the to-be-built nanostructures; the wire is consumed as the reaction goes on. 372 (3) Coupling the power supply (dc, rf or microwave) to the system to form plasma -the Generally the microplasma jets using consumable wires as electrodes were quite similar 383 in their configurations. Usually a metal wire was used as the solid precursors and inserted 384 inside a capillary tube. Various plasma gases such as Ar, He, N 2 , H 2 , O 2 or their mixtures 385 flew through the capillary tube to form the plasma. According to the different ways of 386 coupling the power supply, microplasma could be formed inside or outside the tube. A part 387 of the metal wire resided within the microplasma volume, and reactions would take place 388 on its surface. In such a configuration, different nanomaterial structures could be obtained 389 on the wire surface or on the substrate below the plasma jet [136]. During the deposition 390 process, process parameters such as the gas flow rate, gas composition, substrate distance 391 and input power can be varied to produce a wide range of nanomaterial structures. The 392 results showed that this approach is a cost-effective and versatile way to produce metal/ 393 metal-oxide nanomaterials compared with other methods. 394 A representative configuration is shown in the Fig. 4c [133]. A Mo wire (100 lm 395 diameter) was used as the precursor as well as the electrode and connected to a high 396 voltage power supply (5 kV). A Si substrate was placed above the copper electrode and 397 used for the collection of nanomaterials. Ar and O 2 mixtures were introduced during the 398 deposition process. Ignited by a high voltage pulse, microplasma was sustained between 399 the Mo wire tip and the Si substrate by an ultrahigh frequency (450 MHz) power supply, 400 finally the MoO x nanomaterials could be obtained. Compared with the microplamas using consumable wires as electrodes, this type of 408 microplasma jet has a higher degree of flexibility in configuration. Since there are more 409 available ways to couple the power supply to the system, a wider range of process 410 parameters could be set, like precursors ratio, power coupling mode, residence time and so 411 on. Furthermore, in addition to metal/metal-oxide nanomaterials, this method can also 412 produce other nanomaterials by dissociating corresponding precursors, making it possible 413 to prepare more products with different structures. 414 One typical example of the microplasma jet with tube electrodes is shown in Fig. 5  415 [143]. A hollow WC tube was used as the cathode electrode and Ar carrier, and connected 416 to a RF (13.56 MHz) power supply via a matching circuit. During the experiments, a 417 microplasma jet was formed at the tip of the WC tube and contacted directly with a Fe-418 coated Si substrate. CH 4 was supplied to the microplasma jet through a separate gas line. In 419 this research, variables such as the plasma exposure time, plasma power, gas flow rate and 420 composition were studied. By plasma heating, a mixture of CNTs, Si nanowires and Si 421 nanocones were produced via the combination processes of FeSi x catalytic growth, Si 422 diffusion and oxidation. Another example was using a hollow microneedle as the electrode 423 by connecting it to an AC power supply [137]. After introducing He and the precursor -gas A representative example of microplasma jet with external electrodes is a similar 429 structure as the Fig. 4a [144]. A tungsten wire was inserted in a quartz tube and connected 430 to a high DC power supply. A copper coil surrounding the quartz tube acted as the external 431 electrode. Microplasma was ignited by a DC power supply (15 kV) and sustained at 432 atmospheric pressure with the external electrode. CH 4 and Ar were used as the precursor 433 and the carrier gas to form carbon materials without extra heating. Another similar 434 microplasma jet with external electrodes was applied to deposit TiC, TiO 2 and TiN 435 coatings [140]. Plasma was generated in a SiO 2 nozzle, which was wounded by a copper 436 wire as the external electrode. By using titanium tetraisopropoxide as the precursor, tita-437 nium-based nanomaterials were deposited on stainless steel rods to improve their 438 performance. 439 Considering the scaling-up of microplasma jet system, Cao et al.
[145] studied a ten-jet 440 microplasma array (Fig. 6) for its electrical and optical characteristics, and found that it 441 had achieved excellent uniformity jet-to-jet both in time-and space-wise. If microplasma 442 arrays could be properly designed and used in the process of nanomaterial synthesis, it may 443 allow to increase nanomaterials throughput drastically and to enable the deposition of thin 444 films in a relatively large area.  Fig. 7 [81]. It is worthwhile to mention that they are contamination free since 454 they could be formed without any internal electrodes. 455 When the plasma torch is confined in a relative small spatial zone, it becomes ''micro-456 torch''. Seriously speaking, there is no clear boundary between plasma torch and micro-457 plasma torch. However, compared with microplasma jets, the plasma formed in micro-458 torch commonly has a larger zone and a higher gas temperature, and the produced 459 nanomaterials always have larger sizes and a wider size distribution. 460 Precursors fed to microplasma torches are mainly solid or liquid. Clogging in the small 461 discharge space should be avoided. For DC microplasma torches there are usually two 462 approaches to introduce the precursors, as illustrated in Fig. 8 [81]. One approach is 463 feeding precursors perpendicularly to the microplasma torch but not in contact with any 464 electrode, as shown in Fig. 8a. In the other case, precursors could be feed axially flowing 465 through inner electrodes, as shown in Fig. 8b. When a microplasma torch is applied for the 466 deposition of various performance coatings, the coatings properties are mainly influenced 467 by two factors-the particle velocity and the particle temperature [146]. To improve the 468 quality of microplasma sprayed coatings, one effective way is to inject precursors axially 469 along the plasma torch, allowing a longer dwelling time for precursors in the plasma zone. 470 However, the sprayed particles may adhere to electrodes in such case, limiting the practical 471 application of DC microplasma torches. Since microwave torches do not have internal 472 electrodes, the blocking problem can be avoided. 473 As mentioned above, temperatures in microplasma torches are usually very high, par-474 ticularly suitable for producing refractory material coatings, or handling refractory pre-475 cursors. However, they are unfriendly for high quality nanoparticle synthesis, especially 476 the heat-sensitive materials. And the particles produced by microplasma torch always have 477 a relatively broader size distribution and are easier to agglomerate compared with the other 478 microplasma methods. In order to prevent particle agglomeration and to improve the 479 product quality, a quenching section becomes necessary, which helps to obtain particles 480 with reduced size distribution. Other methods such as adding extra electrical field,  One typical example of producing refractory nanomaterial coatings is using a hollow 484 cathode microplasma torch to deposit Al 2 O 3 coating [146], in which commercially 485 available Al 2 O 3 powder with a size distribution from 10 to 20 lm was used as the pre-486 cursor. Argon acted as both the operating gas and the carrier gas during the deposition 487 process. Different plasma powers were used to deposit Al 2 O 3 films, which were obtained at 488 a 10 mm spray distance on a steel plate used as the substrate. The results revealed that 489 almost the Al 2 O 3 particles in the coating were well-flattened, and the particle thickness 490 would decrease with the increase of plasma power. . In the former one, plasma is formed in the 497 gas phase that above the solution surface, while in the latter case plasma is formed inside 498 the solution. Anyhow, the plasma and the liquid interface is a ighly complex area where 499 multiple phases (gas, liquid and vapor) exist. Moreover, charged or highly reactive species 500 such as gaseous and solution ions, electrons and radicals make the liquid phase micro-501 plasma of promising applications in various fields. One particularly interesting application 502 is for nanomaterial synthesis [148]. 503 It's a new and attractive way to produce colloidal nanoparticles in an aqueous solution, 504 where the microplasma is spatially located, and the bulk liquid still remains at the ambient 505 condition. As we know, the liquid density is much larger than the gas density. Moving the 506 plasma to the liquid phase allows the increasing of pressure more significantly than the gas 507 phase plasma, resulting in an additional confinement of the plasma [80]. The highly 508 confined and localized discharges may offer potential routes for preparing nanomaterials 509 directly and efficiently. Furthermore, a lot of active chemical species can be generated in 510 aqueous phase, including OHÁ, OÁ, HÁ, H 2 O 2 and O 3 , which are beneficial for nanofabri-511 cation. There are also studies showed that the water may contribute to the non-equilibrium 512 state of microplasma. As well known, a fraction of the energy in plasma that coupling to 513 the electrons could heat up the bulk gas. In liquid plasma, water acts as a heat sink, the heat 514 can be quickly dissipated, thus it could prevent the gas temperature from drastic increase 515 and preserve a non-equilibrium state [149]. 516 For nanomaterials synthesized by the GDE method, one example was demonstrated by 517 Huang et al. [150], in which they produced Ag nanoparticles via a microplasma-assisted 518 electrochemistry process (Fig. 9a). A capillary SS tube acted as the cathode and was placed Helium was coupled as the operating gas and 520 formed the plasma between the cathode and the electrolyte surface. A Pt foil was used as 521 the anode and placed 3 cm away from the cathode, a DC power supply with a high voltage 522 around 2 kV was applied to ignite the plasma. They synthesized Ag nanoparticles with 523 various sizes and dispersions by controlling different process parameters. After that, they 524 prepared Au nanoparticles in HAuCl 4 solution by a similar setup, which is schematic 525 shown in Fig. 9b [151]. A SS capillary and a Pt foil acted as the cathode and the anode 526 respectively, with He as the plasma gas. Au NPs were synthesized at the interfacial region 527 where the formed plasma interacted with the solution. They found the size of Au NPs was a  For nanomaterials prepared by the CGDE method, one typical example was demon-532 strated in Fig. 10, in which Au nanoparticles of different shapes were fabricated in aqueous 533 solutions [154]. Tungsten electrodes were separated by 0.3 mm and put in a vessel filled 534 with HAuCl 4 solution, while the temperature was maintained at 25°C by a cooling system. 535 The plasma was ignited by a pulsed DC power supply with a frequency of 15 kHz. During 536 the discharge process, applied voltages of 1600 and 3200 V were used. The solution was 537 stirred by a magnetic stirrer, with sodium dodecyl sulfonate as a stabilizer. By this 538 approach, Au nanoparticles around 20 nm in diameter with exotic shapes such as trian-539 gular, pentagonal or hexagonal were obtained. 540 A novel technique was used to prepare water-soluble CNTs by a setup based on the 541 CGDE technology (Fig. 11) [155]. The microplasma was generated between two elec-542 trodes by applying high pulsed voltage. O 2 , Ar and N 2 were used as the bubbling gases to 543 enhance the discharges between electrodes. Commercial CNTs were added in deionized 544 water, and the formed suspensions were treated by the microplasma. After one hour's 545 treatment in a stirred tank, high water-soluble CNTs were obtained. The study showed that 546 the high energy electrons of the microplasma could enhance the excitation and ionization 547 of H 2 O molecules and lead to the generation of radicals such as OÁ and HÁ. Then the 548 strongly oxidative OÁ reacted with HÁ to form OHÁ groups at the CNT surface, and the 549 introduction of hydrophilic OHÁ groups to the CNT surface caused a higher solubility of 550 CNTs in water. 551 There is a general agreement that the liquid phase microplasma would provide a 552 potential guidance for various applications, such as the nanoparticle synthesis [ The selection of microplasma configuration mainly depends on the precursors and the 563 desired products. One benefit of microplasma is its micro-geometry. Therefore, the 564 required quantity of precursors is quite small compared with other methods, which 565 inevitably associates with low product output. It should be pointed out that the application 566 of microplasma on nanomaterial synthesis may only be cost-effective for producing high 567 value-added products or products that can't be produced by other means. 568 For the hollow-electrode microcharges, the electrodes also function as the ''precursor 569 carriers''. Owing to their small inner diameters, the solid or liquid precursors are not 570 allowed to be used directly in such configurations, because they can easily block the tubes. 571 Therefore, nanomaterials can only be synthesized by the gas-phase nucleation from their 572 precursor vapors. If a precursor is in solid or liquid state, the most common way is using 573 another gas (often plasma gas, such as Ar, He and N 2 ) to carry its vapor to the reaction 574 zone. If a precursor has a low vapor pressure at room temperature, the heating tape or oven 575 can be adopted to get the desired value. 576 For microplasma jets and microtorches, the option for precursors has more flexibility. In 577 principle, liquid, gas or solid can be used as precursors, although in most cases gas or solid 578 state precursors are chosen as they are easy to handle in such configuration. There is even a 579 report about using supercritical CO 2 as the precursor to produce carbon materials by a 580 DBD microplasma jet [162], which shows that supercritical fluid could also serve as an 582 Another attractive concept to supply precursor is evaporating or sputtering a sacrificial 583 metal electrode, correspondingly metal nanostructures or metal oxides can be prepared, 584 which can't be approached by other microplasma configurations. However, as mentioned, 585 the temperatures of microtorches are usually very high, which may lead to a thermal 586 damage to precursors that are temperature sensitive. In such cases, it's better to choose 587 microplasma jets rather than microtorches. On the other hand, if the spherical nanoparticles 588 are to be prepared, the microtorches are best suited, because the droplet-like nanostructures 589 are expected to be the prominent shape in high temperatures [1]. 590 The precursors of liquid phase microplasma are quite different from that of other kinds 591 of plasma, and almost are electrolytes containing metal ions of the desired nanomaterials. 592 By such device, the salts with low vapor pressures are especially suited as precursors. In 593 addition to the metal salts, sacrificial metal electrodes are also used as precursors, which 594 are oxidized to form metal ions in the solution and then reduced to metal nanoparticles by 595 the impact of plasma. Compared with traditional electrochemical methods which need 596 organic stabilizer to prevent the agglomeration or the deposition of nanoparticles on 597 electrodes, this approach is stabilizer-free because of the free-contact of electrolytes with 598 electrodes. 599 Above all, Table 6 shows a brief summary of precursors used and products obtained by 600 the above mentioned microplasma configurations. It can be used as a guideline for process 601 design or for choosing an appropriate type of plasma in nanomaterial synthesis.

Nanomaterials Fabricated by Microplasma and Their Applications
603 Since nanomaterials exhibit unique properties that are attractive to various high perfor-604 mance applications, currently the nanomaterial fabrication is considered as one of the most 605 promising research fields. Microplasma is particularly suitable for nanomaterial fabrication 606 thanks to its unique characteristics, such as the non-equilibrium state, stable operation at 607 atmospheric pressure and room temperature, high radical densities. It can dissociate pre-608 cursors efficiently and nucleate nanoparticles from atomic level, allowing preparing  Fig. 6 [145] ), which can increase material output easily via 804 arranging series of microplasmas together and operating them simultaneously. Moreover, 805 microplasma arrays also have the potential to deposit more than one layer coatings with 806 different performance when using a two-dimensional design, which is depicted in Fig. 16  807 [80]. Operated with different precursors and conditions in each array, multilayer functional 808 coatings can be obtained by scanning microplasma arrays in one direction. Therefore, if 809 microplasma arrays were well designed, with appropriate precursors and processes, 810 functional metal oxides coatings could be prepared in large scales by this approach. One smart microplasma system for producing nanodiamonds at atmospheric pressure 818 and room temperature is using the hollow-electrode microcharge configuration (Fig. 3) 819 [130]. Ethanol vapor was chosen as the carbon source and continuously introduced to the 820 microplasma by Ar and H 2 mixture. A glass filter was installed at the exit of aerosol flow to 821 collect nanoparticles. The results confirmed that high-purity nanodiamonds could be pre-822 pared at relatively neutral conditions by this route. Although the detailed mechanism has 823 not been discussed yet, it was confirmed that the existence of C2 and CHÁ species during 824 the dissociation process would contribute to the nucleation of solid carbon clusters and 825 nanodiamonds respectively. The introduction of H 2 was affecting the diamond growth and 826 the non-diamond carbon etching, resulting in the stabilization of diamond phase carbon and  827 the selective removal of non-diamond phase carbon. Another example about the deposition 828 of nanodiamond crystals was achieved by a microplasma array [189], in which four jets 829 were operated in parallel in the capillary tubes. The deposition process was conducted at a 830 sub-atmospheric pressure (200 Torr) chamber, with H 2 and CH 4 as precursors. Molyb-831 denum foils were adopted as substrates and kept at 800°C during the process. The results 832 showed that the high quality diamond nanocrystals could be prepared at various CH 4 833 concentrations (Fig. 17). 834 A pin-electrode microplasma configuration was used to prepare carbon nanomaterials in 835 a SEM chamber, where a Pd needle was used as the anode and a Si wafer coated by Pt films 836 was used as the cathode [190]. CH 4 was adopted as carbon source and a pulsed high 837 voltage power supply operated at 10 Hz frequency was used to sustain the plasma. Various 838 carbon nanomaterials such as the CNTs, carbon nanoparticles or carbon nanosticks were 839 obtained at different deposition times in this process. 840 The mechanisms are quite different in the processes of carbon nanomaterials synthesis, 841 which are closely associated with deposition parameters such as the input power, gas 842 composition, substrate temperature, catalyst, gas flow rate, etc. Generally they undergo a 843 similar rule, the chemical bonds of carbon sources are broken by the impact of energetic 844 electrons in microplasma. The precursors are dissociated into various radicals. These high 845 reactive radicals collide with each other to nucleate small clusters and form nanomaterials 846 after a series of complex processes. 847 One interesting phenomenon observed during carbon nanomaterial synthesis by 848 microplasma is the self-organization. Nanomaterials prepared in plasma are always 849 charged and interact with each other under Coulombic forces and van der Waals forces, 850 allowing the formation of well ordered, large scale nanostructures [80]. An example 851 system was using a stainless steel pipe as the electrode and a Ni wire as the substrate, in 852 which vertically aligned CNTs were deposited on the Ni wire surface when a negative bias 853 voltage was applied on it [188]. The studies showed that a large quantity of species such as 854 CH 3 Á, CH 2 Á and CHÁ could be obtained in the plasma. With the negative bias voltage, a 855 perpendicular static electric field was formed on the Ni substrate, resulting in the formation 856 of vertically aligned CNTs bundles. Another example is the formation of self-organized 857 carbon nano-connections between the catalyst particles when exposed to a microplasma jet 858 [191]. In the experiment, CH 4 and Ar were introduced to form the plasma which was 859 sustained by a UHF (450 MHz) power supply. A Si substrate coated by Ag or Fe was 860 treated by this plasma. After the depositing process for a certain time, self-organized 861 carbon connections were produced. The results suggested that the electric field determined 862 the growth direction of carbon nano-connections. In this case the catalyst particles were 863 dispersed over the substrate surface and created a two dimensional electric field. Between 864 these two adjacent catalytic nanoparticles the electric field gradient was maximum along 865 the straight line, which significantly affected the carbon surface diffusion and growth 866 process. Therefore, compared with vertically aligned CNTs formed due to the electric field 867 perpendicular to substrate, carbon nano-connections grew along the surface in virtue of the 868 electric field along the substrate surface.
869 Applications 870 The carbon nanomaterials have attracted enormous interest for many potential applications 871 due to their extraordinary properties. For example, nanodiamond has superior hardness, 872 high thermal conductivity and chemical stability, so it is used as an excellent composites 873 for filler materials [192]. With the addition of nanodiamonds to the polymers, substantial