Gulnaz R. Nasretdinova

Mediated electrochemical synthesis of Pd0 nanoparticles in solution

Efficient electrosynthesis of palladium(0) nanoparticles is carried out in solution by mediated electrochemical reduction of a complex dianion [PdCl4]2– in the DMSO/0.1 M Bu4NCl medium. The mediator function was performed by methylviologen and tetraviologen calix[4]resorcines MVCA-Cn8+ with methyl (n = 1), n-pentyl (n = 5), and n-decyl (n = 10) substituents in resorcinol rings at the potentials of redox pairs MV2+/MV•+, MVCA-Cn8+/MVCA-Cn4•+. The resulting metal nanoparticles gradually aggregate to yield coarser particles. The smallest size (255 nm) have metal particles obtained in the presence of MVCA-C58+ for the rest mediators, metal particles of the micron size are formed. Sonication results in disintegration of aggregates to nanoparticles.

Electrochemical mediated synthesis of silver nanoparticles in solution

Methyl viologen MV2+ and tetraviologen calix[4]resorcinol MVCA-C58+ with n-pentyl substituents in the resorcinol rings at potentials of MV2+/MV•+ and MVCA-C58+ /MVCA-C54•+ redox couples proved to be effective mediators of the electrochemical reduction of Ag+ ions in DMF/0.1 M Bu4NPF6. The potentiostatic nondiaphragm electrolysis at controlled mediator reduction potentials at room temperature using an Ag anode as an in situ supplier of Ag+ led to the formation of metallic silver nanoparticles in solution. The only resulting effect of electrolysis was the quantitative transfer of the metal anode into the metal nanoparticles in solution. In the case of MV2+, the nanoparticles aggregated into larger particles. MVCA-CC58+ serves not only as a mediator, but also, to some extent, as a stabilizer of silver nanoparticles and can be recorded by a set of experimental methods.

Methylviologen mediated electrosynthesis of gold nanoparticles in the solution bulk

Electrosynthesis of gold nanoparticles (AuNp) was carried out by methylviologen mediated reduction of Au(I) at potentials of the MV2+/MV˙+ redox couple in water/0.1 M NaCl medium, in the absence and in the presence of stabilizers. In all the cases, AuNp are formed in the solution bulk and are not deposited on the cathode. In the absence of stabilizers, AuNp (14–100 nm) coalesce to give aggregates of various shapes that eventually form a deposit. Sonication reversibly destructs the deposit into nanoparticles. In the presence of alkylamino-modified silicate nanoparticles (SiO2–NHR, 120–160 nm), spherical AuNp (≤20 nm) are bound as inclusions in the SiO2–NHR surface layer. Polyvinylpyrrolidone (40 000 D) stabilizes spherical AuNp with a mean diameter of 5–14 nm. All the particles were characterized by electron microscopy methods (SEM, STEM) and X-ray powder diffraction (XRPD).

Methylviologen-mediated electrochemical synthesis of silver nanoparticles via the reduction of AgCl nanospheres stabilized by cetyltrimethylammonium chloride

Efficient synthesis of silver nanoparticles stabilized by cetyltrimethylammonium cations (Ag@CTA+) is carried out in aqueous medium by methylviologen-mediated electroreduction of silver chloride nanospheres stabilized by surface-active CTA+ cations (AgCl@CTA+, diameter ~330 nm), on a glassy carbon electrode at potentials of the MV2+/MV•+ redox couple. The nanospheres AgCl@CTA+ can be reduced immediately on the electrode at a low rate and the resulting metal is deposited on the electrode. In the mediated reduction, the metal is not deposited on the cathode but the quantitative reduction of AgCl to Ag@CTA+ nanoparticles proceeds completely in solution volume at the theoretical charge. In aqueous solution, the nanoparticles are positively charged (electrokinetic (zeta) potential is +74.6 mV), their characteristic absorption maximum is at 423 nm and the average hydrodynamic diameter is 77 nm. Isolated Ag@CTACl nanoparticles have the size of 39 ± 15 nm. The preferential form of metal nanoparticles is sphere with the diameter of 34 ± 24 nm; nanorods are also obtained in small amounts (4%); the average size of metal grains is 8–16 nm.

Methylviologen-mediated electrochemical synthesis of platinum nanoparticles in solution bulk

Platinum nanoparticles (PtNPs) are synthesized by methylviologen-mediated reduction of PtCl2 at the potentials of the MV2+/MV•+ redox couple in 40% aqueous DMF solution. In the absence of stabilizing agents and in the presence of a stabilizer in the form of spherical silica NPs or alkylamine-modified silica NPs (SiO2-NHR), a part of PtNPs (14–18%) are deposited on the electrode while the rest of particles remain in solution to form coarse aggregates which precipitate. In the latter case, PtNPs are also partly bound to form individual ultrafine NPs (3 ± 2 nm) on the SiO2-NHR surface. In the presence of polyvinylpyrrolidone (PVP), the generated PtNPs (18 ± 9 nm) neither aggregate nor deposit on the cathode but are completely stabilized in solution being encapsulated within the PVP matrix. The obtained PtNPs are characterized by the methods of dynamic light-scattering and electron microscopy.

Electrochemical synthesis of nanocomposite of palladium nanoparticles with polymer viologen-containing nanocapsule

An efficient mediated electrosynthesis of the spherical (85 nm) nanocomposite material Pd@p(MVCA8+-co-St) was carried out in an aqueous medium. Ultrasmall palladium nanoparticles (3—8 nm) are stabilized in nanocapsules of water-soluble nanoparticles of the copolymer p(MVCA8+-co-St) consisting of tetraviologen calix[4]resorcinol (MVCA8+) with styrene (St). The role of the mediator is played by viologen units of a polymer nanoparticle at potentials of the MV2+/MV∙+ redox couple. The high catalytic activity of the nanocomposite material in the reduction of nitrophenol with sodium borohydride is shown.

Molecular Oxygen as Mediator in the Metal Nanoparticles’ Electrosynthesis in N,N -Dimethylformamide

Ultra-fine gold (<2 nm), silver (5 ± 2 nm), and palladium (<1–2 nm) nanoparticles stabilized in polyvinylpyrrolidone shell were synthesized in N,N-dimethylformamide, using molecular oxygen dissolved in the electrolyte as mediator, by the reduction of the metals’ ions and complexes at the controlled potential of the oxygen reduction to its radical-anion. Pd-nanoparticles showed high catalytic activity in the reactions of p-nitrophenol reduction and Suzuki cross-coupling. Long-term ageing of spherical Ag-nanoparticles for 60 days in the post- electrolysis solution resulted in their consolidation (up to 17 ± 5 nm; the average size of crystallites 7.5 (3) nm). Upon similar exposure of Au-nanoparticles for 15 days, V-shaped nanoparticles were formed (length 112 ± 53 nm, width 58 ± 22 nm, crystallites 20(2)–31(1) nm); upon the isolation, dispersing into ethanol, and exposure for 48 h, hexagonal nanoparticles (105 ± 29 nm) and polygons (56 ± 25 nm, crystallites 24(2)–51(1) nm; upon dispersing into water and exposure for 8 h, spherical nanoparticles (13 ± 8 nm, crystallites 7(1)–13.4(5) nm). Thus obtained nanoparticles are characterized by methods of cyclic voltammetry, dynamic light scattering, scanning and high resolution transmission electron microscopy, and X-ray powder diffraction.

Studies of Cobalt(III) and Chromium(III) Complexes as Mediators in the Silver Nanoparticle Electrosynthesis in Aqueous Media

Metal complexes [Cr(bipy)3]3+, [Co(bipy)3]3+, and [Co(sep)]3+ in aqueous media at the potentials of M(III)/M(II) redox couple are shown playing a role of mediators in the electrosynthesis of silver nanoparticles, stabilized in a polyvinylpyrrolidone shell, by means of Ag(I) reduction. [Cr(bipy)3]3+ is consumed under the conditions of long-term preparative electrolysis, the reduction process is accompanied by cathode passivation, therefore, the Ag+ ions complete conversion to the Ag-nanoparticles is unattainable. The two other metal complexes are fully remained unimpaired; the mediated electrosynthesis of the Ag-nanoparticles is carried out well effectively: the Ag-nanoparticles are produced in the solution bulk with a nearly quantitative yield, a theoretical charge being consumed. the [Co(bipy)3]3+-mediated reduction of the Ag+ ions, generated by a silver anode in situ dissolution in the course of single compartment cell electrolysis, is accompanied by the anode metal dispersion and results in the formation of polydisperse Ag-nanoparticles. The summary Ag-nanoparticle current efficiency in the solution bulk comes to 128%. Thus formed Ag-nanoparticles are characterized by using dynamic light scattering, scanning and transmission electron microscopy, and X-ray powder diffraction. The Ag-nanoparticles are spherical, with a mean size of 83 ± 53 nm, or have a form of nanowires, with a length of l = 1216 ± 664 nm and diameter of d = 94 ± 17 nm. The [Co(sep)]3+-mediated AgCl reduction gives ellipsoidal Ag-nanoparticles sized l = 46 ± 19 nm, d = 27 ± 7 nm; the silver crystallite mean size is 20(1)–34.4(9) nm.

Electrochemical synthesis of metal nanoparticles using a polymeric mediator, whose reduced form is adsorbed (deposited) on an electrode

Efficient mediated electrosynthesis of nanocomposite Au@р(MVCA8+-co-St) (~6 nm), in which ultrasmall Au nanoparticles (Au-NP) were bound in nanocapsules of water-soluble nanoparticles of соpolymer р(MVCA8+-co-St) of tetraviologen calix[4]resorcinol (MVCA8+) with styrene (St), was accomplished by the reduction of AuI in aqueous medium. The quanti- tative reduction of AuI was carried out using the theoretically necessary amount of electricity and was not accompanied by the deposition of metal on the electrode. Radical cations of viologen units MV•+ of the molecule р(MVCA4•+-co-St) adsorbed on the electrode and π-dimers MV•+···MV•+ of π-polymers [р(MVCA4•+-co-St)]n deposited on the electrode act- ed as the reducing agents with respect to AuI. During electrolysis, the nanoparticles agglo- merated to 37—50 nm. The nanocomposite particles dispersed in ethanol had sizes of 72±16 nm and also contained Au-NP with sizes of 51±8 and 19±3 nm. The catalytic activity of the nanocomposite in the reduction of p-nitrophenol with sodium borohydride was demon- strated. A similar reduction of AgCl nanoparticles (~250 nm) led to the formation of silver nanoparticles with crystallite sizes in the range of 7—11 nm, the process was inefficient, however, even when using 250% of electricity, an incomplete reduction of AgCl was still observed.