**3. Role of radicals, ions and surrounding chemistry in nanomaterials synthesis by SPP**

In metal NPs synthesis, SPP is used without any chemical reduction agent, since hydro‐ gen radical produced in plasma gas phase is transferred in liquid phase where the metal ion is reduced to the neutral form. The role of hydrogen radical in the reduction process of Au ions from HAuCl4 to Au neutral atoms, which nucleate to produce gold NPs, is illustrated in Figure 6.

In this experiment, solutions with different ion concentrations and conductivities were used to synthesize gold NPs. From the optical emission spectra, we can observe that in a solution with high concentration of ions (40 mM LiCl), the relative number density of hydrogen radical was smallerthan in the solution with 4 mM LiCl concentration (Figure 6(a)). Therefore in these conditions, a smaller number of Au ions were reduced to the neutral form and the relative number density of gold NPs was less, as can be observed from the UV‐vis spectra (Figure 6 (b)) and photos (Figure 6 (c)) [10].

> more detected than PBN‐OH adduct. This indicated that H radicals are supplied from plasma to the solution. Moreover, the formation of gold nanoparticles was not observed in the solution containing PBN since the H radicals produced in the solution which represent the reducing

> **Figure 7.** Dependence of chemical surrounding on Au NPs prepared in solutions with different pH values. (a) ToF-SIMS mass spectra of the negative ions fragments Au, AuOH and CNAu, Au2OH, and Au2Cl3. (b) HRTEM images of Au NPs

Synthesis of Nanomaterials by Solution Plasma Processing 115

Size‐controlled gold NPs have been synthesized using SPP. The gold NPs exhibit sizes from 1‐2 nm to 10 nm when the solution pH was adjusted in the range from 12 to 3, respective‐ ly. The chemical environment surrounding the gold NPs depends on the preparation conditions and determines the electrostatic interaction among the nanoparticles, which alters their final size. Information obtained from XPS analysis, ToF‐SIMS mass spectra, and UV‐visible absorption spectroscopy were consistent and demonstrate that the gold NPs are partially oxidized on the surface, when synthesized in a pH 12 solution, and remain surrounded by gold chloride compounds when synthesized in a pH 3 solution. Plasma diagnostics shows that a high electron density contributes to generate a larger number of hydrogen radicals, which represent the main component in the reduction process of the

Figure 7 shows the dependence of chemical surrounding on Au NPs prepared in solutions with different pH values. The ToF‐SIMS mass spectra of the negative ions fragments were Au, AuOH and CNAu, Au2OH, and Au2Cl3. The chemical surrounding of gold NPs synthesized in pH 3 solution is mainly formed by AuCl species, while the chemical surrounding of gold NPs produced in pH 12 solution is composed by oxidized form of AuO. Figure 7 (b) displays the HRTEM images of Au NPs synthesized in solutions with pH 3 and 12. We can clearly

The main role of SPP during the synthesis of the gold NPs consists in the production of the H radicals in the plasma gas phase which are necessary to reduce the gold ion Au3+ to atomic

observe that small size gold NPs were synthesized in solution with pH 12.

agent for the gold ions, were trapped by PBN molecules [11].

gold ion into the neutral form [18].

synthesized in solutions with pH 3 and 12.

, in the liquid phase.

Au0

**Figure 6.** Role of hydrogen radical in the reduction reaction of Au ion to neutral form, in the synthesis process of gold NPs. (a) Optical emission spectra of SP generated in LiCl solution with different molar concentrations. (b) Time evolu‐ tion of the UV-vis spectra of solutions resulted after SPP, showing gold NPs formation. (c) Photos of the solutions con‐ taining gold NPs.

Plasma has the role to provide the reactive species, especially the H radicals necessarily for the reduction process. In order to confirm more the role of H radical, we attempted to synthesize the gold nanoparticles from the solution containing PBN (n‐tert‐buthyl‐α‐phenylnitrone), which works as a spin trap agent.

When PBN reacts with H and OH, PBN‐H and PBN‐OH adducts are produced, respectively. In an ESR measurement, these adducts can be detected. During SPP, the PBN‐H adduct was

**3. Role of radicals, ions and surrounding chemistry in nanomaterials**

In metal NPs synthesis, SPP is used without any chemical reduction agent, since hydro‐ gen radical produced in plasma gas phase is transferred in liquid phase where the metal ion is reduced to the neutral form. The role of hydrogen radical in the reduction process of Au ions from HAuCl4 to Au neutral atoms, which nucleate to produce gold NPs, is

Micro-Nano Mechatronics — New Trends in Material, Measurement, Control, Manufacturing and Their Applications in

In this experiment, solutions with different ion concentrations and conductivities were used to synthesize gold NPs. From the optical emission spectra, we can observe that in a solution with high concentration of ions (40 mM LiCl), the relative number density of hydrogen radical was smallerthan in the solution with 4 mM LiCl concentration (Figure 6(a)). Therefore in these conditions, a smaller number of Au ions were reduced to the neutral form and the relative number density of gold NPs was less, as can be observed from the UV‐vis spectra (Figure 6

**Figure 6.** Role of hydrogen radical in the reduction reaction of Au ion to neutral form, in the synthesis process of gold NPs. (a) Optical emission spectra of SP generated in LiCl solution with different molar concentrations. (b) Time evolu‐ tion of the UV-vis spectra of solutions resulted after SPP, showing gold NPs formation. (c) Photos of the solutions con‐

Plasma has the role to provide the reactive species, especially the H radicals necessarily for the reduction process. In order to confirm more the role of H radical, we attempted to synthesize the gold nanoparticles from the solution containing PBN (n‐tert‐buthyl‐α‐phenylnitrone),

When PBN reacts with H and OH, PBN‐H and PBN‐OH adducts are produced, respectively. In an ESR measurement, these adducts can be detected. During SPP, the PBN‐H adduct was

**synthesis by SPP**

Biomedical Engineering

114

illustrated in Figure 6.

taining gold NPs.

which works as a spin trap agent.

(b)) and photos (Figure 6 (c)) [10].

**Figure 7.** Dependence of chemical surrounding on Au NPs prepared in solutions with different pH values. (a) ToF-SIMS mass spectra of the negative ions fragments Au, AuOH and CNAu, Au2OH, and Au2Cl3. (b) HRTEM images of Au NPs synthesized in solutions with pH 3 and 12.

more detected than PBN‐OH adduct. This indicated that H radicals are supplied from plasma to the solution. Moreover, the formation of gold nanoparticles was not observed in the solution containing PBN since the H radicals produced in the solution which represent the reducing agent for the gold ions, were trapped by PBN molecules [11].

Size‐controlled gold NPs have been synthesized using SPP. The gold NPs exhibit sizes from 1‐2 nm to 10 nm when the solution pH was adjusted in the range from 12 to 3, respective‐ ly. The chemical environment surrounding the gold NPs depends on the preparation conditions and determines the electrostatic interaction among the nanoparticles, which alters their final size. Information obtained from XPS analysis, ToF‐SIMS mass spectra, and UV‐visible absorption spectroscopy were consistent and demonstrate that the gold NPs are partially oxidized on the surface, when synthesized in a pH 12 solution, and remain surrounded by gold chloride compounds when synthesized in a pH 3 solution. Plasma diagnostics shows that a high electron density contributes to generate a larger number of hydrogen radicals, which represent the main component in the reduction process of the gold ion into the neutral form [18].

Figure 7 shows the dependence of chemical surrounding on Au NPs prepared in solutions with different pH values. The ToF‐SIMS mass spectra of the negative ions fragments were Au, AuOH and CNAu, Au2OH, and Au2Cl3. The chemical surrounding of gold NPs synthesized in pH 3 solution is mainly formed by AuCl species, while the chemical surrounding of gold NPs produced in pH 12 solution is composed by oxidized form of AuO. Figure 7 (b) displays the HRTEM images of Au NPs synthesized in solutions with pH 3 and 12. We can clearly observe that small size gold NPs were synthesized in solution with pH 12.

The main role of SPP during the synthesis of the gold NPs consists in the production of the H radicals in the plasma gas phase which are necessary to reduce the gold ion Au3+ to atomic Au0 , in the liquid phase.

Before plasma, in solution, the hydrolysis of HAuCl4 occurs:

$$\mathrm{AuCl\_4^{+} + jOH^{-} \leftrightarrow AuCl\_{4-} (OH)\_{j}^{\cdot} + jCl^{\cdot}} \tag{2}$$

where 0 < j < 4 and the replacement of Cl‐ by OH‐ depends on solution pH, as was found in the UV‐visible results.

In the liquid phase of plasma, the reduction of gold ions occurs in different ways:

$$\text{AuCl}\_4^\cdot + 3\text{H} \cdot \text{D} \rightarrow 3\text{HCl} + \text{Au}^0 + \text{Cl}^\cdot \tag{3}$$

$$\text{Au(OH)}\_{4}^{\cdot} + 3\text{H} \cdot \text{I} \rightarrow 3\text{H}\_{2}\text{O} + \text{Au}^{0} + \text{OH}^{\cdot} \tag{4}$$

structures similar to those nanoparticles obtained by the SPP method. However, from the detailed HRTEM analysis, the relative amount of the Au MTPs and incomplete MTPs to the total amount of the Au NPs synthesized by the SPP method was observed to be around 94 %, whereas the relative amount of these kinds of crystal structures fabricated by the CRS method was about 63 %. It is most likely that the enhanced formation of the Au MTPs is due to the fact that the SPP method generates highly reaction‐activated species under low environmental temperature conditions. Figure 9 shows microstructural characteristics of gold NPs synthe‐

**Figure 8.** Synthesis of gold NPs in reverse micelle solutions in SPP. (a) Mechanism of reverse micelle solution with dodec‐ ane as solvent and sodium bis(2-ethylhexyl)sulfosuccinate (AOT), as surfactant. (b) Photos of solutions obtained by SPP, with different processing times. (c) TEM images of gold NPs synthesized in reverse micelle solutions for two ratios W.

Synthesis of Nanomaterials by Solution Plasma Processing 117

**Figure 9.** Microstructural characteristics of gold NPs synthesized in SPP. The synthesized Au NPs, with an average size of 6.3 ± 1.4 nm, have different crystal characteristics; fcc single-crystalline particles, multiply-twinned particles (MTPs),

and incomplete MTPs (single-nanotwinned fcc configuration).

sized in SPP.

where the hydrogen radicals (H•) are provided by the gas phase plasma and the reaction (3) and (4) takes place in the pH 3 and pH 12 solution, respectively. The pH 6 solution is an intermediate case to the othertwo solutions, where the hydroxo‐complexes as AuCl3(OH)‐ and AuCl2(OH)2‐ represent a molar fraction of about 0.6 in the solution [17].

By changing the solution pH in the preparation condition, the size of the gold NPs can be controlled. The connection between the gold NPs size and the solution pH can be understood by: (i) the effect of the redox standard potential in the reactions (3) and (4), (ii) the electrostatic repulsion force between AuO‐ ions, and (iii) the protective layer of the surfactant (Figure 7).

We also attempted to regulate the size of the gold NPs by a method based on discharge in reverse micelle solutions [12] (Figure 8). The reactive species generated by the discharge reduced[AuCl4] ‐ only inside waterdroplets in the reverse micelle solutions.Atthe lower values of water to surfactant ratio (W), the average diameter is smaller and the size distribution is narrower.

The size of gold nanoparticles varied from 4.0 to 11.4 nm. Size of gold NPs formed inside the water droplets was regulated by the size of reverse micelles. This suggests that SPP in glow discharge regime in reverse micelle solutions can be applied as a plasma nanoreactor for nanomaterial fabrication.

We analyzed the gold NPs synthesized by chemical reduction process and SPP and we investigate the microstructural characteristics of these in SPP [24]. Microstructural character‐ istics of gold nanoparticles (Au NPs) fabricated by SPP in reverse micelle solutions have been studied by high‐resolution transmission electron microscopy (HRTEM).

The synthesized Au NPs, with an average size of 6.3 ± 1.4 nm, have different crystal charac‐ teristics: fcc single‐crystalline particles, multiply‐twinned particles (MTPs), and incomplete MTPs (single‐nanotwinned fcc configuration). The crystal structure characteristics of the Au NPs synthesized by SPP method were analyzed and compared with similar‐size Au NPs obtained by the conventional chemical reduction synthesis (CRS) method. The TEM analysis results show that the Au NPs synthesized by the CRS method have shapes and crystal

Before plasma, in solution, the hydrolysis of HAuCl4 occurs:

AuCl4

Au(OH) 4

AuCl2(OH)2‐ represent a molar fraction of about 0.6 in the solution [17].

studied by high‐resolution transmission electron microscopy (HRTEM).

‐ <sup>+</sup> *<sup>j</sup>*OH‐ <sup>↔</sup> AuCl4‐ *<sup>j</sup>*

In the liquid phase of plasma, the reduction of gold ions occurs in different ways:

(OH) *<sup>j</sup>*

‐ + 3H∙ → 3HCl + Au<sup>0</sup> + Cl‐ (3)

‐ + 3H∙ <sup>→</sup> 3H2O + Au<sup>0</sup> + OH‐ (4)

where 0 < j < 4 and the replacement of Cl‐ by OH‐ depends on solution pH, as was found in the

Micro-Nano Mechatronics — New Trends in Material, Measurement, Control, Manufacturing and Their Applications in

where the hydrogen radicals (H•) are provided by the gas phase plasma and the reaction (3) and (4) takes place in the pH 3 and pH 12 solution, respectively. The pH 6 solution is an intermediate case to the othertwo solutions, where the hydroxo‐complexes as AuCl3(OH)‐ and

By changing the solution pH in the preparation condition, the size of the gold NPs can be controlled. The connection between the gold NPs size and the solution pH can be understood by: (i) the effect of the redox standard potential in the reactions (3) and (4), (ii) the electrostatic repulsion force between AuO‐ ions, and (iii) the protective layer of the surfactant (Figure 7).

We also attempted to regulate the size of the gold NPs by a method based on discharge in reverse micelle solutions [12] (Figure 8). The reactive species generated by the discharge

of water to surfactant ratio (W), the average diameter is smaller and the size distribution is

The size of gold nanoparticles varied from 4.0 to 11.4 nm. Size of gold NPs formed inside the water droplets was regulated by the size of reverse micelles. This suggests that SPP in glow discharge regime in reverse micelle solutions can be applied as a plasma nanoreactor for

We analyzed the gold NPs synthesized by chemical reduction process and SPP and we investigate the microstructural characteristics of these in SPP [24]. Microstructural character‐ istics of gold nanoparticles (Au NPs) fabricated by SPP in reverse micelle solutions have been

The synthesized Au NPs, with an average size of 6.3 ± 1.4 nm, have different crystal charac‐ teristics: fcc single‐crystalline particles, multiply‐twinned particles (MTPs), and incomplete MTPs (single‐nanotwinned fcc configuration). The crystal structure characteristics of the Au NPs synthesized by SPP method were analyzed and compared with similar‐size Au NPs obtained by the conventional chemical reduction synthesis (CRS) method. The TEM analysis results show that the Au NPs synthesized by the CRS method have shapes and crystal

‐ only inside waterdroplets in the reverse micelle solutions.Atthe lower values

‐ + *j*Cl‐ (2)

AuCl4

UV‐visible results.

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116

reduced[AuCl4]

nanomaterial fabrication.

narrower.

**Figure 8.** Synthesis of gold NPs in reverse micelle solutions in SPP. (a) Mechanism of reverse micelle solution with dodec‐ ane as solvent and sodium bis(2-ethylhexyl)sulfosuccinate (AOT), as surfactant. (b) Photos of solutions obtained by SPP, with different processing times. (c) TEM images of gold NPs synthesized in reverse micelle solutions for two ratios W.

structures similar to those nanoparticles obtained by the SPP method. However, from the detailed HRTEM analysis, the relative amount of the Au MTPs and incomplete MTPs to the total amount of the Au NPs synthesized by the SPP method was observed to be around 94 %, whereas the relative amount of these kinds of crystal structures fabricated by the CRS method was about 63 %. It is most likely that the enhanced formation of the Au MTPs is due to the fact that the SPP method generates highly reaction‐activated species under low environmental temperature conditions. Figure 9 shows microstructural characteristics of gold NPs synthe‐ sized in SPP.

**Figure 9.** Microstructural characteristics of gold NPs synthesized in SPP. The synthesized Au NPs, with an average size of 6.3 ± 1.4 nm, have different crystal characteristics; fcc single-crystalline particles, multiply-twinned particles (MTPs), and incomplete MTPs (single-nanotwinned fcc configuration).
