Nanostructured gold–palladium electrodeposited in dimethyl sulfoxide solutions

Materials Letters 158 (2015) 317–321

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Nanostructured gold–palladium electrodeposited in dimethyl sulfoxide solutions Oksana Dobrovetska a, Orest Kuntyi a, Ivan Saldan b,n, Sergiy Korniy c, Yevhen Okhremchuk a, Oleksandr Reshetnyak b a

Lviv Polytechnic National University, Bandery Street 12, 79013 Lviv, Ukraine Ivan Franko National University of Lviv, Kyryla and Mefodia Street 6, 79005 Lviv, Ukraine c Karpenko Physical-Mechanical Institute of the NASU, Naukova Street 5, 79060 Lviv, Ukraine b

art ic l e i nf o

a b s t r a c t

Article history: Received 4 May 2015 Accepted 11 June 2015 Available online 12 June 2015

The main purpose of the research was to obtain bimetallic nanodeposits by a simple electrochemical method. Gold and palladium were chosen as the objects for simultaneous electrodeposition. Gold–palladium deposits were obtained by pulse potentiostatic electrolysis on a glassy carbon surface in dimethyl sulfoxide solution with (0.001–0.004) M HAuCl4, 0.004 M PdCl2 and 0.05 M Bu4NClO4 at the constant temperature of 35 °C. The bimetallic nanoparticles were found in the potential range of –(0.3–1.5) V vs Ag/AgCl in saturated KCl during pulse time of 6 ms and pause of 300 ms. Increase in the potential value resulted in higher palladium content in the deposits. Depending on the electrodeposition time discrete bimetallic nanoparticles transformed into complete nanostructured films made of gold and palladium. & 2015 Elsevier B.V. All rights reserved.

Keywords: Pulse electrolysis Codeposition Gold Palladium DMSO SEM.

1. Introduction Over the last decade binary systems of palladium with d-metals like Pd–Сo [1–3], Pd–Ni [4–6], Pd–Cu [7,8] Pd–Ag [9] and Pd–Au [10–25] are the range of bimetallic nanomaterials which constitute an object of special interest in the theoretical and practical aspects. This is mainly stimulated by their high electrocatalytic activity at the electrode processes in fuel cells: (i) oxygen reduction reaction [1–3,5]; (ii) anodic oxidation of lowmolecular alcohols like methanol, ethylene glycol, glycerol [24] and, in particular, ethanol oxidation reaction [4,6,14,20,21,24]; (iii) hydrogen anodic oxidation [15] and oxidation of formic acid [8,22]. The system consists of palladium, and gold is a perspective due to its high catalytic activity and stability since it can be used in a huge variety of electrochemical processes. Out of other Pd-based binary systems Au– Pd materials are very well introduced as nanoalloys [10–12,16,18– 22,24] and core–shell segregated nanoalloys (core@shell) [13– 15,17,23]. The nanoalloys in [26] are called “mixed” or “intermixed” alloys regardless of whether the structure is ordered or random. Today there is no definite explanation of the function of gold during the synergy in the Au–Pd system. Some authors insist on higher gold electronegativity as compared to palladium that n

Corresponding author. E-mail address: [email protected] (I. Saldan).

http://dx.doi.org/10.1016/j.matlet.2015.06.041 0167-577X/& 2015 Elsevier B.V. All rights reserved.

causes the so-called electron-withdrawing effect. Therefore, in core@shell alloys the activity of Pd–shell is enhanced by Au–core as “inactive support” [15,18,26], while in “mixed” nanoalloys it is enhanced by the activity of both metals on the surface of a catalyst [16,18,20–22,26]. It is well-known that catalytic properties of nanomaterials strongly depend, besides the chemical nature of the components, on the relationship between the constituents, geometry of nanoobjects, their distribution on the surface, etc. Therefore, in a triad “synthesis–structure–properties” the first factor i.e. synthesis is the most crucial one. Indeed, the main characteristics of nanoalloys mainly depend on their method of preparation. Therefore, development of effective and controlled methods of synthesizing Au–Pd nanoalloys is a very actual issue both from the theoretical and practical points of view. Among a lot of approaches to synthesizing Au–Pd nanoalloy to be fixed on a support chemical methods are most popular [10,11,13,16,21–24] along with electrochemical ones [12,14,15,17–19]. The present experimental work is the study of controlled synthesis of Au–Pd nanoalloys by pulse electrolysis in aprotic organic solvent, i.e. non-aqueous electrolyte, simultaneously from both gold and palladium precursors. The approach of non-stationary electrodeposition is not well-developed for nanochemistry of bimetallic systems though some research works on Au–Pd [18], Pd–Ag [27] and Pt–Ru [28] confirm simplicity and efficiency of the method. As compared to usual aqueous electrolyte, aprotic organic electrolyte efficiently

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Intensity, a.u.

Аesar); hydrogen tetrachloroaurate (ІІІ) (HAuCl4
3H2O, 99.99%, Аlfa Аesar) additionally dehydrated by ethanol as in [13]; palladium chloride (PdCl2, 99.9%, Аlfa Аesar) and tetrabutylammonium

Energy, keV Fig. 1. Energy-dispersive X-ray spectroscopy of the Au–Pd deposits obtained by pulse electrolysis on GC surface in DMSO solution with 0.004 М НAuCl4, 0.004 М PdCl2 and 0.05 M Bu4NClO4 at –0.3 (a) and –1.5 (b) V vs SCE.

100

− Pd − Au

− Au − Pd − Au-Pd

3. Results and discussion 80

i, A/dm2

0.6

60 0.4 40 0.2

Using the proposed pulse electrolysis within the potential (E) range of –(0.3–1.5) V vs SCE both metallic gold and palladium can be simultaneously deposited to form the desirable Au–Pd system (Fig. 1). High [AuCl4]– complex stability (β ¼ 2.5  1029) gives us the reason to suggest that gold reduction in DMSO, as well as in aqueous solution, corresponds to the following electrochemical reaction [31]:

Content, at.%

0.8

⎡⎣AuCl4 ⎤⎦– + 3е → Au + 4Cl–

20

0.0

0 -1.6

-1.4

-1.2

-1.0

-0.8

-0.6

perchlorate (Bu4NClO4, Z99.0%, Sigma Aldric) as inert electroconductive additive were used in the electrochemical experiment. IPC-Pro potentiostate was used in all electrochemical experiments carried out as in [30]. Gold and palladium were simultaneously deposited on the clean and dried-in-argon-flow surface of glassy carbon (GC) in DMSO solution with 0.001–0.004 M HAuCl4, 0.004 M PdCl2 and 0.05 M Bu4NClO4. Electrodeposition of Au–Pd was carried out by the pulse potentiostatic method in the voltage region of –(0.3–1.5) V vs silver chloride electrode (SCE). Depending on the potential value the number of “pulse cycles” varied from 340 to 1400 as predicted for constant 0.0015 mA h amount of electricity. During all voltammetry measurements the number of the “pulse cycle” was set at 50. Experimentally obtained Au–Pd deposits were washed sequentially in DMSO and isopropanol. After that, all samples were dried in an argon flow. The morphology and chemical composition of the deposits were studied in the same manner as in [30].

-0.4

E, V vs Ag/AgCl Fig. 2. Voltammetry of GC electrode in DMSO solution with 0.004 М HAuCl4, 0.004 M PdCl2 and both 0.004 М HAuCl4 and 0.004 M PdCl2 at 35 °С. For every measurements 0.05 M Bu4NClO4 as inert electroconductive additive was used. Dependence of gold/palladium content on the applied potential is shown by dots with linear fitting in dashed lines.

increases the possibilities for influencing nanoalloys formation. Metal reduction in organic media can be carried out at high potential values without any secondary chemical/electrochemical processes [27–30] that is crucial for nanoobject formation. For example, complete absence of hydrolysis in aprotic organic electrolytes “eliminates” their pH dependence and hence makes their application easier.

2. Experimental Organic aprotic solvent – dimethyl sulfoxide (DMSO, 99%,

Аlfa

E 0 = 1.002 V

(1)

In DMSO solution PdCl2 can produce a neutral [PdCl2(DMSO)2] complex [32]. Similar to [PdCl4]2– anion [33], palladium (II) chloride can be reduced to metallic palladium on the double electric layer (DEL) that is confirmed by reaction involving only two electrons:

PdCl2 + 2е → Pd + 2Cl–

E 0 = 0.62 V

(2)

Current values of cathode (icathode) to reduce gold from [AuCl4]– during pulse electrolysis are higher than those of icathode to obtain metallic palladium from PdCl2 (Fig. 2), that is clear from reactions (1) and (2) taking into account the value of standard electrode potential (E0). Hence, deposition of gold must be facilitated in regards to that of palladium. However, in case of simultaneous gold and palladium deposition the relationship between icathode and applied E is different as compared to separate ones (Fig. 2). For equi-molar Au–Pd system the voltammetry curve has an icathode maximum at E around –1.0 V vs SCE that suggests a critical point responsible for switching to another mechanism of Au–Pd deposition. Increase in the E value corresponds to higher value of icathode that leads to fast kinetics of nucleation on the GC electrode surface and smaller Au–Pd particle size. Therefore, the value of

Table 1 The dependence of Au and Pd content in the bimetallic deposits prepared by pulse electrolysis in MDSO solutions with different molar ratios between [HAuCl4] and [PdCl2]. E, V vs Ag/AgCl

Metal content, аt% [HAuCl4]:[PdCl2] ¼ 1:1

–0.5 –1.0

[HAuCl4]:[PdCl2] ¼1:2

[HAuCl4]:[PdCl2]¼ 1:4

Au

Pd

Au

Pd

Au

Pd

71 59

29 41

69 48

31 52

57 15

43 85

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Fig. 3. SEM images of Au–Pd deposits obtained by pulse electrolysis on GC surface in DMSO solution with 0.004 М НAuCl4, 0.004 М PdCl2 and 0.05 M Bu4NClO4 at –0.5 V vs SCE for 48 (a); 123 (b); 320 (c) and 435 (d) pulse cycles. Distribution function of particle size shown in (a'–d') correspondingly.

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particle size is inversely proportional to the value of icathode and absolute number of the deposited particles similar as for monopalladium [29] and mono-gold [30]. In addition to that, in bimetallic Au–Pd system the nucleation on the nuclei of another metal [32] might cause additional increase in icathode value as it was found for Ag–Pd [27]. If we analyze the content of gold/palladium within the whole (E) range of –(0.3–1.5) V vs SCE, gold content dominates only till that critical point (Fig. 2). It is suggested that diffusion factor became more important during E increase and eventually at a certain point it can be responsible for faster reduction of kinetics of the precursor with lower steric limitations. In particular, in low concentration solution (in our case 0.001–0.004 M) and at high E values (–(0.3–1.5) V vs SCE) diffusion control makes sense. Two E values of –0.5 and –1.0 V vs SCE were chosen for detailed studies of gold and palladium content at the different molar ratio of the precursors used (HAuCl4 and PdCl2) ( Table 1). Indeed, at –0.5 V vs SCE simultaneous Au–Pd electrodeposition results in gold domination in used [HAuCl4]:[PdCl2] ratios as 1:1; 1:2 and even 1:4. However, at –1.0 V vs SCE the gold/palladium content is approximately 50 at% in 1:2 and with palladium domination in 1:4 [HAuCl4]:[PdCl2] ratio. Obviously, at –1.5 V vs SCE simultaneous Au–Pd electrodeposition results in palladium domination in used [HAuCl4]:[PdCl2] ratios as 1:1; 1:2 and 1:4. Conclusively, depending on the molar ratio between the precursors used simultaneous Au–Pd electrodeposition results in different contents of reduced gold and palladium. Metallic gold/palladium domination on the GC electrode can be easily achieved by increasing/decreasing approximately 1:2 M [HAuCl4]:[PdCl2] ratio at the most reasonable E values around –1.0 V vs SCE. Formation of Au–Pd deposits with time is similar to that of Au deposits in dimethyl formamide solution with HAuCl4 [30]. At the beginning Au–Pd deposits look as discrete nanoparticles (Fig. 3а), then through agglomeration (Fig. 3c) they transform into a nanostructured film (Fig. 3d). So, during the long pulse electrolysis 2D filling of the GC electrode surface that is possible to explain takes place. Because of high electro-donor properties of DMSO molecules they can produce surface complexes with metals as electro-acceptors. Obviously, during a pause (τoff at pulse electrolysis) CH3)2SO:-□Au and (CH3)2(O)S:-□Pd (□ is free electron orbital) can be formed on the GC electrode surface. Due to the nanosize of metallic particles, their surface energy can be high enough to make the CH3)2SO:-□Me (Me ¼ Au,Pd) quite stable. In other words, the surface of Au–Pd nucleus can be occupied by a solvent as it was discussed in [29], while the surface of the GC electrode does not initiate any visible interaction with the solvent. Hence, during pulse time (τon) at pulse electrolysis, properly speaking, the GC electrode surface is a “priority” to be the place for further Au–Pd nucleation. Indeed, during electrodeposition of 48; 123; 320 and 435 pulse cycles the surface of the GC electrode becomes more covered by Au–Pd nanoparticles with the main fraction of particle size of 44; 46; 60 and 80 nm, respectively (Fig. 3).

4. Conclusions Electrodeposition of nanostructured gold–palladium in dimethyl sulfoxide solutions was experimentally shown by pulse electrolysis. Discrete Au–Pd nanoparticles within the range of 20–120 nm transformed into complete nanostructured films during a longer time of pulse electrolysis. Increase in the potential value resulted in higher palladium content in the Au–Pd deposits. Metallic gold/palladium domination on the GC electrode can be easily achieved by increasing/decreasing approximately 1:2 M [HAuCl4]:[PdCl2] ratio at the most reasonable E values of around –1.0 V vs SCE.

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