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Pulse Electrodeposition of Palladium Nanoparticles onto Silicon in DMSO.

1. Introduction

The deposition of metal nanoparticles (MNPs) on a silicon surface is one of its modifying methods to obtain (i) silicon nanowires (SiNWs) and nanomatrices [1-3], (ii) elements of highly sensitive sensors [4-6], (iii) water-splitting photoelectrodes for hydrogen generation [7, 8], and so on. Electrolysis is an effective way to form the MNPs/Si surface systems [9, 10] due to the relative simplicity of hardware and technology. The most studied is the electrodeposition of noble metals that is caused due to their high stability and high values of the standard electrode potential ([mathematical expression not reproducible]. The latter is especially important for providing a high value of [mathematical expression not reproducible], which is identical to the high etching rate of silicon during the formation of the nanoporous surface [1-3].

By varying the electrolyte composition, the values of the cathodic potential or current density, and the methods of current supply (stationary, cyclic, and pulsed), it is possible to obtain MNPs of various shapes and sizes on the silicon surface. Most of the studies on electrochemical production of MNPs/Si surface systems have been performed in aqueous solutions (see Table 1) [9-28]. However, the water factor as a medium does not always provide the ability to control the geometry of MNPs at the main stages of their formation: cathodic reduction (1), nucleation, i.e., formation of metal nanoclusters (MNCs) (2), and nanoparticles (3). After all, at high values of cathodic potential ([E.sub.cathode]) and low concentrations of reducible metal ions which is a prerequisite for the deposition of MNPs, parallel processes take place, e.g., the cathode reaction (4) and the interaction of silicon with water (5).

[M.sup.n+] + ne [right arrow] [M.sup.0] (1)

m[M.sup.0] [right arrow] MNCs (2)

MNCs + p[M.sup.0] [right arrow] MNPs (3)

2[H.sub.2]O + 2e [right arrow] [H.sub.2] + 2O[H.sup.-] (4)

Si + 2[H.sub.2]O [right arrow] Si[O.sub.2] + 4[H.sup.+] (5)

Reaction (4) due to gas evolution causes the local turbulence of electrolyte and, accordingly, the instability of the mass transfer of metal ions into the pre-electrode layer. Reaction (5) results in the passivation of the silicon surface. To prevent such undesirable processes, a nonaqueous medium maybe used as it was shown previously in [29-32], where the deposition of metals on silicon was performed in organic solvents. However, these papers report about the deposition of noble metals via the galvanic replacement method. Electrodeposition on the silicon surface in organic solvents has not been practically studied wide. Moreover, the formation of a nanostructured Ru/Si surface at high cathodic potentials from an ionic liquid [25] indicates the prospect of such a direction of silicon surface modification. The purpose of this work is to establish the conditions of deposition of PdNPs on a silicon surface in dimethyl sulfoxide (DMSO) solutions under a pulsed electrolysis regime. Application of pulse electrodeposition and the use of organic aprotic solvents allow ensuring the concentration stability of reducible metal ions in the pre-electrode layer at high [E.sub.cathode] values. As shown in [33, 34], this provides conditions for the controlled deposition of MNPs of a certain geometry.

2. Experimental

Crysteco n-Si (100) wafers with resistivity 4.5 [ohms] x cm were used for the experiments. Wafers were cut into equal samples of 1 x 1 [cm.sup.2]. Palladium deposition on a silicon surface was carried out by electrolysis from solutions of 1, 2, 4, and 6mM Pd[(N[O.sub.3]).sub.2] + 50 mM [Bu.sub.4]NCl[O.sub.4] in DMSO solvent. The working electrode was the n-type Si (100) cremation plate, the anomalous platinum plate (S = 2.7 [cm.sup.2]). A silver chloride electrode manufactured as Ag/AgCl in saturated KCl solution was used as the reference electrode. Before electrolysis, the silicon samples were etched for 10 s in HF solution to remove the oxide film from the substrate surface. After that, the surface of the working electrode was washed with DMSO solvent. Subsequently, the three electrodes were connected to an IPC-Pro 200 potentiostat and immersed into a glass cell filled with ~50 [cm.sup.3] of the electrolyte at 25[degrees]C. Palladium was precipitated under pulsed electrolysis mode: pulse time ([[tau].sub.on]) 6 ms and pauses ([[tau].sub.off]) 300 ms at E = -1.8, -2.0, and -2.2 V for 25, 50, 100, and 400 cycles. After the process is completed, the palladium-precipitated samples were washed with ethanol and air-dried.

The morphology and composition of sediment on the silicon surface were studied using a ZEISS EVO 40XVP scanning electron microscope (SEM) and a Solver P47-PRO atomic force microscope (AFM). The images of the modified surface were obtained by recording secondary electrons by scanning an electron beam of 15 kV energy. The chemical composition of the resulting precipitates was characterized using energy-dispersive X-ray spectroscopy (EDX).

The statistical histograms were obtained using Origin software pack with its standard deviation values of nanoparticle size. Additionally, NPs size and density were determined by using the public domain Java image processing program ImageJ2 [35].

3. Results and Discussion

Electrochemical deposition of metals on the silicon surface differs significantly from deposition on a metal substrate. This is due to the electrical conductivity of n-Si or p-Si substrate which is much lower than metallic one, which causes poor cathodic metal reduction on a semiconductor surface. As a result, this leads to the priority 3D island growth, which is caused by the prevailing reaction rate (1) at the metal surface, resulting in the formation of nucleation centers [8]. Therefore, in order to ensure a constant nucleation on the doped Si surface during the electrodeposition, it must be periodically excited. This function is performed by the pulse mode of electrolysis at high cathode potentials ([E.sub.cathode]). To determine the dependence of the size of electrodeposited Pd particles and their distribution on the silicon surface from the electrolysis conditions, we studied the impact of the following main factors: (i) the value of the cathode potential, (ii) the concentration of Pd ions in solution, and (iii) the number of pulse-pause cycles.

In a wide range of the [E.sub.cathode] values (Figure 1), the concentration of palladium ions (Figure 2) and regardless of the number of pulse-pause cycles (Figure 3), the formation of spherical metal particles is observed. This is explained by the influence of DMSO donor molecules (L), notably the formation of surface complexes due to the donor-acceptor interaction. These complexes are formed primarily on the ledges, which block them and prevent dendritic formation.

3.1. Impact of the Cathode Potential. With the increase of [E.sub.cathode], there is an upward trend in the density of the silicon surface filling with Pd particles and their size increases (Figure 1). Therefore, we can assume that the pulse regime of electrodeposition leads simultaneously to the nucleation and growth of PdNPs on the semiconductor substrate. It is expected that higher [E.sub.cathode] values, as an energy factor, contribute to this. The -1.8 ... -2.2 V range determines the maximum values of the cathode currents. The latter provides a high deposition rate of palladium nanoparticles (PdNPs), their small size, and uniform distribution across the substrate surface. Thus, for [E.sub.cathode] = - 1.8 V (Figure 1(a)), -2.0 V (Figure 1(b)), and -2.2 V (Figure 1(c)), the number of particles, respectively, is 8,14, and 18 per 1 x 1 [micro][m.sup.2]. So, when [E.sub.cathode] increases, the tendency for agglomeration of PdNPs is observed (Figure 1(c)).

3.2. Impact of the Concentration of Pd Ions. Pulse-mode electrolysis at [[tau].sub.on] = 6 ms and [[tau].sub.off] = 300 ms provides diffusion of solvated ions of [[Pd(DMSO)n].sup.2+] to the cathode surface during the pause. Therefore, for each pulse-pause cycle, the reduction of Pd during the pulse occurs at a stable concentration of Pd(II). This contributes to the relatively uniform distribution of PdNPs over the substrate surface (Figures 2(a)-2(d)) and their small size variation (Figure 2(e)). With increasing concentration of ions of [[Pd(DMSO)n].sup.2+] which is caused by high mass transfer, the average particle size also increases significantly. Thus, discrete PdNPs with an average size of 75, 95, and 115 [+ or -] 10 nm are precipitated from solutions containing 1, 2, and 4 mM of Pd[(N[O.sub.3]).sub.2], respectively. In a solution of 6mM Pd[(N[O.sub.3]).sub.2], agglomeration of PdNPs is observed with the formation of nanofilm fragments. Consequently, the size of the precipitated PdNPs is very sensitive to the concentration of Pd(II), which can be considered as one of the main factors influencing the morphology of the metal precipitate and the geometry of its structural particles. As we can see in Figure 2, the amount of Pd[(N[O.sub.3]).sub.2] in the solution did not influence the increase in the concentration of Pd precipitates.

3.3. Impact of the Number of Pulse-Pause Cycles. According to the SEM images, an increase in the pulse-pause cycles tends to fill the silicon surface with PdNPs. The PdNPs with an average diameter of 55, 65, and 75 [+ or -] 10 nm are precipitated from 25, 50, and 400 cycles, respectively (Figure 3). Therefore, during deposition under pulsed electrolysis mode, the nucleation and growth of PdNPs occur simultaneously.

The AFM results (Figure 4) are shown to demonstrate the growth of nanoparticles in height with increasing electrodeposition duration (number of cycles) at E = -2.0 V. As shown in Figure 4, when the number of cycles increases, the height of PdNPs changes slightly. Thus, after 25 cycles, the average height of PdNPs is about 9 nm (Figure 4(b)) and after 50 and 400 cycles, the average height of PdNPs is about 14 nm (Figure 4(c)) and 17 nm (Figure 4(f)), respectively. The same trend was observed for other values of [E.sub.cathode].

The effect of suppression of the growth of NPs in height can be attributed to electrochemical Ostwald ripening [36]. In fact, the adhesion of MNPs to the surface of semiconductors is lower than that of metals [37]. So, this facilitates the migration of small nanoparticles across the substrate surface and their absorption by larger 2D nanoparticles.

4. Conclusions

Palladium nanoparticles with an average size of 40-160 nm are deposited on the silicon surface from solutions containing 1-6 mM Pd[(N[O.sub.3]).sub.2] and DMSO by the pulsed electrolysis method at [E.sub.cathode] = - 1.8 ... -2.2 V. The organic aprotic solvent medium prevents the side cathodic processes to occur during electrochemical reduction of Pd(II) at high cathode potentials. The latter contributes to the energy excitation of the doped semiconductor surface, which causes the formation of discrete PdNPs uniformly distributed on the substrate surface. The electron donor nature of DMSO molecules and the pulsed electrolysis mode induce the deposition of spherical nanoparticles and the 2D filling of the silicon surface during electrolysis. The main factors influencing the size of PdNPs are the value of the cathode potential, the concentration of palladium ions in solution, and the number of pulse-pause cycles. As these values increase, the nanoparticle size and the density of the substrate surface filling increase.

https://doi.org/10.1155/2019/5859204

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

This work was financially supported by the Ministry of Education and Science of Ukraine under the project "Controlled electrochemical synthesis of metal nanoparticles and nanostructured materials" (State registration number: 0118U000268).

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Orest Kuntyi, (1) Mariana Shepida [ID], (1) Oksana Dobrovetska, (1) Stepan Nichkalo [ID], (1) Sergiy Korniy, (2) and Yuriy Eliyashevskyy (3)

(1) Lviv Polytechnic National University, Lviv 79013, Ukraine

(2) Karpenko Physico-Mechanical Institute of the NAS of Ukraine, Lviv 79060, Ukraine

(3) Ivan Franko National University of Lviv, Lviv 79005, Ukraine

Correspondence should be addressed to Mariana Shepida; maryana_shepida@ukr.net

Received 8 August 2019; Accepted 11 October 2019; Published 30 October 2019

Academic Editor: Sedat Yurdakal

Caption: Figure 1: SEM images of PdNPs on the silicon surface obtained by electrolysis in a solution of 4 mM Pd[(N[O.sub.3]).sub.2] + 50 mM [Bu.sub.4]NCl[O.sub.4] mixed with DMSO for 400 cycles at [E.sub.cathode] (V): -1.8 (a), -2.0 (b), -2.2 (c), and EDX-spectrum (d).

Caption: Figure 2: SEM images of PdNPs on the silicon surface obtained by electrolysis in a solution containing 1 mM (a), 2 mM (b), 4 mM (c), and 6 mM (d) of Pd[(N[O.sub.3]).sub.2,] 50 mM [Bu.sub.4]N[Cl.sub.O]4, and DMSO for 400 cycles at [E.sub.cathode] = -2.0 V and the size distribution histograms of PdNPs (e).

Caption: Figure 3: SEM images of PdNPs on the silicon surface obtained by electrolysis in a solution containing 4 mM of Pd[(N[O.sub.3]).sub.2], 50 mM [Bu.sub.4]NCl[O.sub.4], and DMSO at [E.sub.cathode] = -2.0 V for 25 (a), 50 (b), and 400 (c) cycles and the size distribution histograms of PdNPs (d).

Caption: Figure 4: AFM images of PdNPs obtained by electrolysis in a solution containing 4 mM of Pd[(N[O.sub.3]).sub.2], 50 mM [Bu.sub.4]NCl[O.sub.4]> and DMSO at [E.sub.cathode] = -2.0 V after 25 (a), 50 (c), and 400 (e) cycles and the height distribution histograms of PdNPs (b, d, and f).
Table 1: Conditions for the formation of noble metal nanoparticles
on the silicon surface by electrodeposition method.

Metal   Type of silicon         The composition of the solution

           n-Si(643)        Commercial cyanide-based silver plating
Ag                                      solution (KROHN)
           n-Si(111)            5 mM AgCN, 0.2 M KCN, 0.5 M KOH

         Microporous         0.1 M AgN[O.sub.3] + 0.5 M KN[O.sub.3]
           p-Si(100)
           n-Si(643)         0.1 mM Hau[C1.sub.4], 1 mM KC1, 1 mM
                                  [H.sub.2]S[O.sub.4], 100 mM
                                      [K.sub.2]S[O.sub.4]
          p-type Si       (0.1 or 0.01M)HAuC14 in 0.5 M NaCl or 0.5 M
          wafers(100)                 [Na.sub.2]S[O.sub.4]
           n-Si(100)      (2, 50 mM) KAu[(CN).sub.2] + (0.2, 1.0 M)KCN

Au         n-Si(100)        Neutronex[R] 309 Au solution, a sulfite-
                            based Au chemistry + thallium(I) acetate
           H-Si(lll)            (1) [10.sup.-4]M HAuC14 + 0.1 M
                             [K.sub.2]S[O.sub.4]; (2) [10.sup.-4]M
                          Kau[(CN).sub.2] + 2 x [10.sup.-4] M NaCN + 2
                                             M NaOH
           n-Si(100)        Sulphite/thiosulphate electrolytic baths
           p-Si(100)             1 mM KAu[(CN).sub.2] + 0.25 M
                                      [K.sub.2]C[O.sub.3]
           n-Si(643)      0.5 M NaCl, 3mM [K.sub.2]Pt[Cl.sub.4]; pH=4
           p-Si(100)      (0.1 or 0.01 M)[H.sub.2]Pt[Cl.sub.6] in 0.5
                              M NaCl or 0.5 M [Na.sub.2]S[O.sub.4]
         Microporous           0.1 M K2Pt[Cl.sub.4] + 0.5 M NaCl
           p-Si(100)
           p-Si(100)      [10.sup.-2]M [Na.sub.2]Pt[Cl.sub.6] + 0.5 M
Pt                                    [H.sub.2]S[O.sub.4]
         n-Si(1 1 1):H        1 mM [H.sub.2]Pt[Cl.sub.6] + 0.1 M
                                      [K.sub.2]S[O.sub.4]
          n-Si(100)       (0.03 ... 2.5)mM Pt[Cl.sub.4.sup.2-] + 2M HF
           p-Si(100)
        Pt/SiNWs array     1 g/L [H.sub.2]Pt[Cl.sub.6], pH = 2.0 (by
                                              HC1)
          p-Si (100)       (0.1 or 0.01 M)Pd[Cl.sub.2] + 0.2 M HC1 +
Pd                          1.0 M NaCl or 0.5 M [Na.sup.2]S[O.sub.4]
         Microporous       0.1 M Pd[Cl.sub.2] + 0.2 M HC1 + 0.5M NaCl
          p-Si (100)
         Scratches on          0.01 g/1 Pd[Cl.sub.2] + 0.1 M HC1
          p-Si (100)
Ru         n-Si(100)           4mM Ru[C1.sub.3] in BMIP[F.sub.6]
         p-Si (100),         0.16 mM [K.sub.2]Ru[C1.sub.5] + 0.18 M
         p-Si (111),                  [H.sub.2]S[O.sub.4]
          n-Si (100)
Ir         n-Si(111)          10 mM Ir[Cl.sub.3] + 0.5M KCl + 5%

                                     isopropanol. pH = 2.2
Re         p-Si(100)          10 mM N[H.sub.4]Re[O.sub.4], pH = 2
                                     ([H.sub.2]S[O.sub.4])

                                  Condition of deposition

Metal   Type of silicon      [i.sub.cathode]               E

           n-Si(643)        10 mA/[cm.sup.2]               --
Ag
           n-Si(111)               --              -0.80 ... -0.90V
                                                        vs. SHE
         Microporous          6.4 [micro]A/                --
           p-Si(100)           [cm.sup.2]
           n-Si(643)               --              -1.9V vs. Ag/AgCl

          p-type Si                --               0 V vs. Ag/AgCl
          wafers(100)
           n-Si(100)               --             (-1.2 ... -1, 7) V
                                                      vs. Ag/AgCl
Au         n-Si(100)          1, 5, 10 mA/           -1.1 V vs. SCE
                               [cm.sup.2]
           H-Si(111)               --                    -2.0 V

           n-Si(100)       Constant potential, constant current, cyclic
                            voltammetry and pulsating overpotential
           p-Si(100)               --             -1.0 ... -1.4 V vs.
                                                        Ag/AgCl
           n-Si(643)               --             -0.58 V vs. Ag/AgCl
           p-Si(100)               --              -0.4 or -0.8 V vs.
                                                        Ag/AgCl
         Microporous          6.4 [micro]A/                --
           p-Si(100)           [cm.sup.2]
           p-Si(100)               --             CVA between -300 and
Pt                                                       100 mV
         n-Si(1 1 1):H             --             -0.35 V vs. Ag/AgCl

          n-Si(100)       (0.035 ... 0.15) mA/             --
           p-Si(100)           [cm.sup.2]
        Pt/SiNWs array      Pulse electrodeposition (-0.35 V vs. Ag/
                           AgCl): [[tau].sub.off]/[[tau].sub.off] = 25/
                                         100 ms
          p-Si (100)                               -0.4 or -0.8 V vs.
Pd                                                      Ag/AgCl
         Microporous        10 mA/[cm.sup.2]       -0.4 V vs. Ag/AgCl
          p-Si (100)
         Scratches on              --             0.0, 0.2, 0.4 V vs.
          p-Si (100)                                    Ag/AgCl
Ru         n-Si(100)               --              -1.4 ... - 2.0 V
                                                     vs. Pt/Pt(II)
         p-Si (100),          1 ... 20 mA/            vs. Ag/AgCl
         p-Si (111),           [cm.sup.2]
          n-Si (100)
Ir         n-Si(111)               --                  -0.9 V SCE
Re         p-Si(100)             CV (between 0.50 and 0.65 V)

Metal   Type of silicon             Metal sediments              Ref.

           n-Si(643)                 Epitaxial film              [9]
Ag
           n-Si(111)            100 ... 300 nm particles         [11]
         Microporous            Deposit in silicon pores         [12]
           p-Si(100)
           n-Si(643)                 Epitaxial film              [9]
          p-type Si             Deposit in silicon pores         [13]
          wafers(100)
           n-Si(100)             Hemispherical clusters          [14]
Au         n-Si(100)      Particles [empty set] 30 ... 200 nm    [15]
           H-Si(111)              Islands 4 ... 30 nm            [16]
           n-Si(100)                 Bulk deposits               [17]
           p-Si(100)               Particles 0 <100nm            [18]

           n-Si(643)                 Epitaxial film              [9]
           p-Si(100)            Deposit in silicon pores         [13]
         Microporous            Deposit in silicon pores         [12]
           p-Si(100)
           p-Si(100)            100 ... 300 nm particles         [19]
Pt
         n-Si(1 1 1):H          Nanofilms, nanoparticles         [20]
          n-Si(100)                  Islands <100nm              [21]
           p-Si(100)
        Pt/SiNWs array        Flower-like Pt-nanoclusters        [22]
          p-Si (100)            Deposit in silicon pores         [13]
Pd
         Microporous            Deposit in silicon pores         [23]
          p-Si (100)
         Scratches on               Sub-100 nm lines             [24]
          p-Si (100)
Ru         n-Si(100)                 -100 nm films               [25]
         p-Si (100),            0 20-30 nm hemispherical         [26]
         p-Si (111),                     shape
          n-Si (100)
Ir         n-Si(111)              5-10 nm nanoislands            [27]
Re         p-Si(100)                    Nanofilm                 [28]
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Title Annotation:Research Article
Author:Kuntyi, Orest; Shepida, Mariana; Dobrovetska, Oksana; Nichkalo, Stepan; Korniy, Sergiy; Eliyashevsky
Publication:Journal of Chemistry
Date:Nov 1, 2019
Words:4487
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