“Green” Synthesis of Metallic Nanoparticles by Sonoelectrochemical and Sonogalvanic Replacement Methods

The main features of the “green” synthesis of metallic nanoparticles (MNPs) by the sonoelectrochemical methods are manufacturability, environmental friendliness, and the possibility of controlling the geometry of the forming particles. The electrochemical reduction technique allows efficiently designing the metal nanoparticles and provides the control of the content of components of bimetallic nanoparticles, as well as minimizing the number of precursors in working solutions. Due to the generation of turbulence, microjets, and shock waves, ultrasound increases mass transfer and formation of radicals in aqueous solutions and, accordingly, accelerates the processes of nucleation and growth of MNPs. Therefore, this hybrid method, which combines electrolysis and ultrasound, has attracted the interest of researchers in the last two decades as one of the most promising techniques. The present work presents a short analysis of the reference literature on sonoelectrochemical synthesis of metallic and bimetallic nanoparticles. The main factors influencing the geometry of nanoparticles and their size distribution are analyzed. The use of pulsed ultrasound and pulsed current supply during sonoelectrochemical synthesis is especially effective in designing MNPs. Emphasis is placed on the role of surfactants in the formation of MNPs and sacrificial anodes in providing the algorithm: “anodic dissolution-electrochemical reduction of metal-nucleation and formation of МNPs.” It is noted that ultrasound allows synthesizing the MNPs and M1M2NPs during the galvanic replacement, and an analogy of the formation of nanoparticles by sonogalvanic replacement and sonoelectrochemical method is shown.


Introduction
Metal nanoparticles are widely used in biomedicine [1][2][3][4][5], biosensors [4,6,7], catalysis [1,8], and other fields. e nanoparticles of noble metals are the most studied in terms of the synthesis methods, as well as physicochemical and functional properties. AgNPs have a special place among all noble metals because they are characterized by high antimicrobial and anti-inflammatory activity and antitumor action. is gives the possibility of effectively applying them in the pharmaceutical and cosmetic industry; antibacterial, antiviral, and cancer therapy; tissue engineering; and other areas of biomedicine ( Figure 1). Using the AuNPs [3] and PtNCs [4] for the treatment of cancer and bacterial diseases gave encouraging results in the last decade. In addition, AuNPs and PtNCs are considered as a new effective class of chemotherapeutics in the treatment of hematopoietic, lung, and hepatocellular malignant tumors.
Nanoparticles and nanoclusters of noble metals are effective in detecting the biological agents and therefore play a very important role in biomedicine. us, in [6], the advantages of plasmonic AgNPs, AuNPs, and PtNPs for the colorimetric sensing applications in drugs assays in pharmaceutical and biological samples are described. e use of gold nanoparticles in the chemiluminescence technique showed a high sensitivity for the detection of biological agents [7].
Functional properties of MNPs, especially their biological activity, depend on both the nature of the metal and the geometry of the particles, that is, shape (sphere, rod, nanoshell, nanostar, etc.) and size (from 1 to 100-150 nm) [1][2][3][4][5][6][7]. e geometry of MNPs determines their excellent biocompatibility and optical (caused by the surface plasmon resonance phenomenon) and catalytic properties ( Figure 2). e different nanoeffects of the size and shape of metal nanoparticles mentioned above give the possibility of effectively using the MNPs in the biosensors.
Taking into account the requirement in such materials, as well as the dependence of their functional properties on the geometry of the particles and their composition, the choice of the correct method of MNPs' synthesis is very important. After all, in the triad, the method of synthesis ⟶ geometry MNPs (and composition for M 1 M 2 NPs) ⟶ functional properties, the first component is decisive. e processes of obtaining of MNPs and M 1 M 2 NPs should correspond to the following basic modern criteria: (1) the control of the geometry and the composition (for binary) of nanoparticles; (2) "green" synthesis; and (3) manufacturability. Regardless of the method of synthesis (chemical [9,10], electrochemical [11,12], or physicochemical [13]), the controlled synthesis is the main condition for its effectiveness. "Green" synthesis is based on the use of nontoxic precursors, primarily reducing agents and surfactants of natural origin, such as plant extracts [9] and microorganisms [10]. "Green" synthesis also includes the electrochemical methods due to the cathodic reduction of metal ions and the use of nontoxic surfactants [11][12][13].
Manufacturability of synthesis of MNPs and M 1 M 2 NPs means, first of all, the maintenance of a high rate of processes. is is provided by electrochemical microplasma [13] and ultrasound [14][15][16][17][18][19][20][21]. Herein, we will consider ultrasound as one of the effective factors of MNPs engineering intensification.
In nanomaterial science, galvanic replacement is used for obtaining the metal nanostructures on the surface of the sacrificial substrate [69][70][71][72][73] and submicron metal powders [74]. Sacrificial nanoscale metal is required for the synthesis of MNPs [75]. In [76][77][78][79], MNPs were synthesized using the sacrificial metal in the form of sheets, plates, or foils via the combining method of galvanic replacement and ultrasound in solutions containing the surfactants. Ultrasound is effective in the synthesis of M 1 M 2 NPs by galvanic replacement [80][81][82][83][84][85][86], and the sizes of obtained nanoparticles are in the range from a few nanometers up to 100 nm (Table 3). Sonogalvanic replacement also can be attributed to the sonoelectrochemical technique because the GR process proceeds via an electrochemical mechanism [69][70][71][72]. On the surface of the sacrificial metal, there are anode and cathode areas, where the electrons are "generated" due to electrochemical dissolution with the subsequent reduction of the metal ions and the MNPs formation. erefore, the effect of the ultrasonic field on the electrochemical processes during galvanic replacement is similar to its effect during electrolysis.
An effect of "cavitation" is the main factor of ultrasound influencing the processes which occur (1) in the bulk of the electrolyte, (2) in the double electric layer, and (3) on the surface of the electrodes. All of these processes take place during the sonoelectrochemical synthesis of MNPs. e main factors contributing to the effective synthesis of MNPs are the following:   4 Bioinorganic Chemistry and Applications  [59] Bioinorganic Chemistry and Applications 6 Bioinorganic Chemistry and Applications (1). e destruction of cavitation bubbles causes high local temperatures and pressures. As a result, water is sonolized [14] and the radicals H * and * OH (equation (1)) as well as the products of their interaction (equation (2)) are formed. In the presence of surfactants in the solution (which are commonly used as stabilizers of MNCs and MNPs), organic radicals R· (equation (3)) are also formed. Such radicals and products reduce the metal ions (equations (4)-(6)), primarily noble metals, which are characterized by high values of standard electrode potentials. Atoms M 0 combine into nanoclusters and nanoparticles (equation (7)). us, a typical sonochemical synthesis of MNPs takes place in the bulk of electrolyte: (2) In the ultrasound field, a thinning of the electrode diffusion layer thickness occurs. It causes a great enhancement in mass transport near the electrode, thereby accelerating the rate of the electrochemical reactions. At the same time, the values of currents of anodic dissolution of sacrificial metal [41,42,45,49,54] and cathode currents of metal ion reduction [41] increase significantly. Such a phenomenon leads to the increase in the rate of nucleation and, respectively, the formation of the smaller particles in comparison with the synthesis without US (Figure 3). Ultrasound periodically performs the function of "shaking" of MNPs from the cathode surface after their deposition during the current pulse. Moreover, the ultrasonic horn is periodically used as the vibrating working electrode, that is, as "sonoelectrode" in a three-electrode setup [17,34]. During the pause of electrolysis (τ off ), in addition to the "shaking" of MNPs, the diffusion of metal ions to the cathode occurs ( Figure 5). It was noted that the time of the ultrasound (τ US ) is a period of peculiar ablation of metallic nuclei from the cathodic surface [53].
Sonoelectrochemical synthesis of metallic nanoparticles as a hybrid technique significantly predominates the sonochemical and electrochemical methods in terms of the rate of the process. In addition, it is controllable and allows obtaining mono-and bimetallic nanoparticles of a wide range of sizes from 2 to 100 nm (Tables 1 and 2). However, the techniques described in the references differ in the constructions of sonoelectrochemical cells, power, and frequency of ultrasound applied and the constructions of current supply and modes of electrolysis. erefore, the purpose of this review is to offer a systematic analysis of the references on sonoelectrochemical synthesis of metallic nanoparticles over the past two decades and provide the guidelines of operating conditions to maximize the beneficial effects. Bioinorganic Chemistry and Applications 7

Sonoelectrochemical Synthesis of Metallic Nanoparticles
e vast majority of reported methods of sonoelectrochemical synthesis of MNPs and M 1 M 2 NPs are based on the use of corresponding simple or complex salts as the precursors of metal ions (Tables 1 and 2). Only in some references, the source of metal ions is sacrificial anodes; for example, silver electrodes were applied in solutions of HCl [38] and NaPA [41] for the synthesis of AgNPs. Most of the sonoelectrochemical syntheses of metallic nanoparticles are based on the use of surfactants to stabilize of MNPs and M 1 M 2 NPs. Usually the polymeric substances containing different functional groups are used as the surfactants, for example, PVP [32,33,37,39,43,52], PVA [31], NaPA [41,62], and rhamnolipid [42]. In some cases, the role of the surfactant can play the molecules of the solvent, for example, THF [54,58,59].   [33] or Ag (I) ions form complexes due to electron-donor atoms of the oxygen and nitrogen of the pyrrolidone fragment of the polymer molecule of PVP, which leads to the formation of a bond via the donor-acceptor mechanism: O: ⟶ □Ag(+) and (or) N: ⟶ □Ag(+). erefore, in PVP solution, the complexes of Ag(_) localized in the polymer chain with a bidentate ligand are formed (Figure 6(a)). Complexes formed in NaPA solutions are presented in Figure 6(b).

CuNPs.
e main difficulty of sonoelectrochemical synthesis of CuNPs is the tendency for easy oxidation of copper in aqueous solutions. Even an inert atmosphere does not prevent the formation of CuO because, in aqueous solutions hydroxyl radicals (equation (1)), H 2 O 2 (equation (2)) and O 2 (equation (6)) are formed due to sonolysis and oxidize CuNPs. erefore surfactants (PVP [30,32,33] or PVA [31]) are used as a capping agent. In addition, flectron-donor atoms of the oxygen and nitrogen of the pyrrolidone fragment of the polymer molecule of PVP form bonds with Cu 2+ ions via donor-acceptor mechanism: O: ⟶ Cu 2+ and (or) N: ⟶ Cu 2+ [33]. erefore, PVP-Cu 2+ complexes are formed, which are transformed into an intermediate neutral PVP−Cu 0 complex, and the surface PVP−CuNPs complex is formed after the cathodic reduction of Cu 2+ to Cu 0 . Synthesis of CuNPs is performed by well-known pulsed sonoelectrochemical method in accordance with the following protocol: electrolysis (τ on ) ⟶ ultrasound (τ US ) ⟶ rause of electrolysis and ultrasound (τ off ). e size of nanoparticles depends on the following main factors: sonication power, current density, temperature, experiment duration. In [30], an optimal range of values for the sonication power was shown. us, at 20 W, complete separation of precipitate from the sonoelectrode is not provided, but ultrasonic power more than 90 W causes a significant increase in the size of CuNPs. Such phenomenon is due to overheating of the solution, but the increase in temperature is undesirable for obtaining small nanoparticles. For example, at sonoelectrochemical synthesis at 15°S, CuNPs with an average size of 17 nm are formed, while at 50°S, the size of particles is increased to 62 nm [33]. erefore, ultrasonic power should be sufficient to ensure complete removal of nanoparticle sediment from the sonoelectrode but should not be too high to cause the overheating of solution. e influence of current density is also significant. A decrease in the size of CuNPs with decreasing the value of i sathode was noted; for example, the mean diameter of CuNPs reduced from ∼800 nm to ∼45 nm with decreasing i sathode from 760 mA·cm −2 to 240 mA·cm −2 [30]. e tendency to decrease the size of nanoparticles with increasing i sathode is also shown by [33]: at 55, 70, and 100 mA·cm −2 , CuNPs with mean diameters of 29, 24, and 10 nm, respectively, were obtained.
A study of sonoelectrochemical synthesis of AuNPs without a stabilizer [44] demonstrated that the current density in a wide range of values has little effect on the UV-Vis spectra of colloid solutions and the geometry of nanoparticles. However, the effect of the current pulse duration is significant, with the increase in which there is a tendency to increase the size of nanoparticles (Figure 8).
In another study [47], a method of sonoelectrochemical synthesis of AuNPs by the dissolution of Au substrates via electrochemical treatments of "oxidation-reduction" cycles with subsequent sonoelectrochemical reductions of Au-ions to synthesize Ch/Au nanocomposite was proposed. Such Ch/ Au nanocomposite can be destroyed in the ultrasonic field, and AuNPs are formed. erefore, chitosan was included as a polymeric surfactant performing the function of a matrix during synthesis. e overall process can be represented by the algorithm: "gold polycrystalline + Cl − + Ch ORC ⟶ Ch− [AuCl 4 ] − sonoelectrochem ⟶ Ch−AuNPs sonification ⟶ AuNPs," which makes it possible to obtain the nanoparticles with the size of ∼12 nm and narrow size distribution. So, ultrasound performs a dual function in the proposed method of synthesis: the first is the promotion of the AuNPs formation during the electrochemical reduction of Ch− [AuCl 4 ] − and the second is the destruction of the formed Ch−AuNPs nanocomposites.

PdNPs and PtNPs.
In [50], it was shown that controlled sonoelectrochemical synthesis of PdNPs is affected by the following parameters, namely, current density, power of ultrasound, and surfactant concentration. It was shown that, with increasing the current density, the size of nanoparticles is decreased. Specifically, at і cathode equal to 8 and 13 mA·sm −2 , the diameters of formed PdNPs were equal to 10 and 5 nm, respectively. e optimal range of ultrasound power also has been determined. Under intensities lower than 20 W·cm −2 , the particles were irregularly shaped and ere agglomerated due to insufficient ultrasound energy for effective ablation of PdNPs from the cathode surface. Hence, some of them continued to grow in the next cycle of cathodic reduction. e power range from 20 to 80 W·cm −2 was effective for obtaining PdNPs with a diameter from 5 to 10 nm. Using the intensities above 120 W·cm −2 was also problematic due to the local overheating of the cathode surface (up to 90°C), which causes the agglomeration of nanoparticles. Also, it has been shown that a surfactant should be present to prevent the agglomeration of nanoparticles. Another study [51] also revealed a similar trend to PdNPs size dependence on the value of current density.
A significant feature of sonoelectrochemical synthesis [52] is the formation of 3D dendritic structures of platinum with a diameter of about 30 nm. e authors showed the possibility of controlled influence of synthesis parameters on the size and morphology of nanoparticles. As specific example, it was reported that, at 5 mA·sm −2 , monodispersed PtDPNs with size of 6 nm were formed; at 20 mA·sm −2 , heterogeneous PtDPNs with broad variation in size were formed; and finally at 40 mA·sm −2 , agglomeration of DPNs was observed. Another study [53] emphasized the importance of optimization of the duration of the electrochemical pulse for obtaining the desired size of PtPNs. It was shown that an increase in τ on leads to an increase in the size of nanoparticles and also causes difficulties in PtPNs ablation from the surface of the sonoelectrode.

FeNPs.
In [54], the synthesis of FeNPs in tetrahydrofuran and atmosphere of argon was described, and the molecules of solvent [(C 4 H 9 ) 4 N] play the role of the nanoparticles stabilizer. e size of FeNPs was regulated by the frequency of ultrasound during the electrochemical synthesis. It was found that, under the action of ultrasonic of 200 kHz and 20 kHz and simultaneous action of 200 and 20 kHz, the average sizes of the obtained iron particle were 29, 18, and 7 nm, respectively. e authors explain this as in the case of only low-frequency ultrasound at 20 kHz, an area of intense cavitation in the interelectrode space is formed. e cavitation effect is continued after the shaking of FeNPs from the surface of the sonocathode also in the volume of the solution, and this promotes the formation of smaller nanoparticles. At high frequencies (200 kHz), the cavitation effect occurs mainly on the cathode surface. Moreover, simultaneous use of low-and high-frequency ultrasonic fields can, in addition to cavitation, create both microflows of significant intensity and large-scale ultrasonic flows. is simultaneous action of low and high frequency ultrasonic fields increases the efficiency of the ultrasonic action in the liquid and leads to decreasing of the size of formed FeNPs.
In aqueous solutions, the sonoelectrochemical synthesis of FeNPs is based on the simultaneous cathodic reduction of Fe(+2) and H(+), where the formed H 2 bubbles, in addition to providing a reducing atmosphere, affect the formation of nanoparticles [55].
2.1.6. WNPs. Tungsten cannot be reduced in aqueous solutions, and hence the method of pulsed sonoelectrochemical coprecipitation of Fe and W can be used [57]. e process occurs due to the catalysis of reduction of tungsten (equation (11)) by the formed iron (equation (10)) with the formation of nanoparticles of FeWNPs. After ultrasonic ablation of particles from the cathode surface, FeWNPs in acidic solution are converted into WNPs due to the dissolution of iron: 2.1.7. MgNPs and AlNPs. Magnesium and aluminum are metals that can be reduced only in a nonaqueous medium due to their standard electrode potentials being equal to −2.356 V and −1.66 V, respectively. erefore, the synthesis of MgNPs [58] and AlNPs [59] can be performed by the sonoelectrochemical method in an organic aprotic solvent (THF), which, in addition to the nonaqueous medium, acts as a stabilizer. Due to the donor-acceptor binding (CH 2 ) 4 O: ⟶ Mg(Al), molecules of THF form surface complexes. is provides "encapsulation" of MNPs, preventing their agglomeration.
e main function of the pulsating ultrasound was to shake the metal nanoparticles deposited on the cathode. Taking into account the requirement in MgNPs and AlNPs in the production of hydride materials based on these metals, the proposed techniques are considered suitable for use on a technological scale [58,59].

Sonoelectrochemical Synthesis of M1M2NPs.
In the case of composites, the coreduction of M 1 and M 2 is performed from solutions containing ions of two corresponding metals (  [61], FeCo (1)(2) NPs [62], and FeCo (1)(2)(3)(4) NPs [62]. In the case of metals with large differences in the values of ΔE 0 (Cu and Ni [64], Cu and Pt [65], Pd and Fe [66]), an advanced reduction of more electrodegradable metal is observed. For example, during the sonoelectrochemical synthesis of Cu-Pt nanopowders [65], part of the reduced copper acts as a sacrificial metal to reduce platinum (equation (12)). is allows synthesizing the Cu@ PtNPs. In addition (equation (12)), the process of leaching copper from Cu-Pt nanopowders by reaction (13) is used to increase the platinum content. e combination of concentrations of precursors and processes (equations (12) and (13)) allows controlling the composition of particles: Cu 55 Pt 45 , Cu 25 Pt 75 , and Cu 42 Pt 58 : Cu Similarly, WCoNPs can be obtained using the electrochemical reduction of tungsten from the tungstate ion WO 2− 4 , which occurs due to catalysis by the citrate complex [Co(II)(C 6 H 5 O 7 )WO 2 ] − (ads) formed during the sonoelectrolysis [68].
In the sonoelectrochemical syntheses of M 1 M 2 NPs described in [60,61,63,[65][66][67][68], preferably the sizes of several nanometers are obtained. is is facilitated by the factor of alternate reduction of metals M 1 and M 2 , that is, the deposition of each of them on a foreign surface. e synthesis is carried out mainly in solutions without surfactants (

Synthesis of Metallic Nanoparticles by Galvanic Replacement in Ultrasound Field
e process of galvanic replacement (according to the generalized equation (14)) takes place via the electrochemical mechanism [69][70][71][72], where ionization of the sacrificial metal M 1 with "generation" of electrons (15) takes place at the anode areas and the reduction of M n+ 2 ions (16) occurs at the cathode sites: mM n+ 2 + nM 1 ⟶ nM 2 + nM m+ 1 (14) anode: Depending on the nature of the sacrificial metal pair "sacrificial (M 1 ) -reduced (M 2 )," the formation of sediments of the following three types is possible: the islands, the dendritic, and the film. e first two types are of the greatest interest for obtaining metal nanoparticles. e formation of "island" sediment occurs with a significant difference between the crystal lattices M 1 and M 2 and is realized according to the Volmer-Veber mechanism [75]. For example, the galvanic replacement of metals on the surfaces of magnesium [72] and silicon [71,73] can be considered. e formation of dendritic sediment occurs at a high rate of the process (equation (14)), which is primarily due to the significant difference between the values of standard electrode potentials between M 1 and M 2 [72][73][74][75]. However, galvanic replacement is mainly used to extract metals from leaching solutions of ore and secondary raw materials. In the last decade, the galvanic replacement method has been intensively studied as effective for surface modification by metal nanoparticles and nanostructures [69][70][71][72][73]. e use of ultrasound significantly accelerates the rate of galvanic replacement, primarily due to the acceleration of the electron-generating reaction (equation (15)) because of the intensive renewal of the anode areas and the prevention of their passivation. erefore, at the corresponding ultrasound parameters, as well as in solutions containing the surfactants, MNPs and M 1 M 2 NPs are obtained via the galvanic replacement technique (Table 3). In addition, ultrasound, by analogy with sonoelectrochemical syntheses, performs the function of "shaking" of MNPs from the cathode regions of the sacrificial metal. Surfactants, due to adsorption and formation of surface complexes with MNPs, inhibit their growth during the galvanic replacement and prevent the agglomeration of particles in solutions ( Figure 9). erefore, it is possible to obtain the MNPs with a small mean size (up to 10 nm) and narrow size distribution. Specifically, the example of galvanic replacement of gold and platinum [78] showed that acoustic cavitation causes rapid nucleation, which contributes to the formation of a large number of MNPs with a uniform size distribution over a short period of time (usually several minutes). Simultaneously with ultrasound galvanic substitution, the formation of MNPs can also occur using the sonochemical reactions (4-6), since cavitation induces the formation of H· and HO· and other secondary radicals that reduce metal ions in solutions. However, such parallel processes are not described in the literature. Using ultrasound galvanic replacement, stabilized MNPs were synthesized (M � Fe, Co, Sn, Cu, Ag, Au, Ru, Pt) in PVP solutions [76,78,79]. Compact metal samples, among which the active ones such as magnesium, aluminum [79], as well as iron [78], copper [76,78], and silver [77], were used as sacrificial templates for the synthesis of nanoparticles.
In the literature, the ultrasound galvanic replacement techniques have been demonstrated for obtaining of core@ shell nanoparticles M 1 @N 2 NPs [82,84,86] and nanoalloys M 1 N 2 NPs [80,81,83,85]. In [86], a hybrid method of Pd@ Pt core-shell nanoparticles synthesis was proposed. In accordance with this method, the ultrasound is used for the realization of two successive processes: first is the sonochemical synthesis of PdNPs in PdCl2 solutions followed by the second one which is the galvanic substitution of platinum on the surface of palladium nanoparticles (Figure 10).

Conclusions
e combination of electrochemical reduction and ultrasonic field in aqueous solutions allows carrying out the controlled synthesis of metal nanoparticles with given geometry and bimetallic nanoparticles with controlled composition. Sonoelectrochemical synthesis, as a hybrid technique, is characterized by a high rate of nucleation and formation of MNPs and M 1 M 2 NPs with the minimum number of precursors and controllability. erefore, it can be attributed to the "green" and promising technologies of nanomaterials production. e use of sacrificial anodes provides an algorithm "anodic dissolution-electrochemical  reducing of metal−nucleation and growth of NNPs" which allows the design of nanoparticles.
Galvanic replacement under ultrasonic field in solutions containing the surfactants allows synthesizing stabilized MNPs. Sonogalvanic replacement is an effective method for obtaining nanoalloys (M 1 M 2 NPs) and core@shell nanoparticles (M 1 @M 2 ). e processes at the anode and cathode areas of sacrificial metals in ultrasound are similar to the processes occurring at the sonoelectrochemical synthesis of MNPs due to the electrochemical nature of galvanic replacement.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.