Cyanide-free electrolyte for Au(III) and Au(I) electrodepositing using DMH as complexing agent

Abstract

A novel cyanide-free electrolyte is reported for electrodepositing bright gold layers from a solution of both Au(III) and Au(I). Gold electroplating in Au(I) electrolytes displayed instantaneous nucleation at pH = 8, 9, progressive nucleation at pH = 10. Gold electrodeposition in Au(III) electrolytes displayed progressive nucleation at pH = 8, instantaneous nucleation at pH = 10.

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Owing to their excellent physical, chemical stability and conductive properties, electrodeposited Au and Au alloys (so called hard gold) have been widely used in jewelries, artware and the connector manufacturing industry.^1,2 In addition, Au and Au alloy multilayers have also been examined as potential giant magnetoresistive materials. Valizadeh et al.^3–5 and Hütten et al.⁶ reported several Au/Co layers structures with Giant Magneto Resistive phenomenon. But most of these investigation were obtained Au layers from cyanide-based electroplating electrolytes, as one of the most toxic chemicals, cyanide brings extremely high risks to human health and the environment.⁷ To solve this problem, a number of attempts have been made in the past few decades to develop cyanide-free gold electroplating electrolytes. Numerous cyanide-free electrolytes have been developed to replace the cyanide-based electrolytes for Au electroplating. However, except for few successful cases, most of them were unsatisfactory because of the poor-quality of the plating layer or electrolytes' stability. A single Au(I) electrolyte for electrodepositing Au alloys from sulfite electrolyte were investigated,⁸ but manufacturing industry argued that the stability of the sulfite electrolyte is disappointing, owning to the reaction as follows:⁹

3Au⁺ → 2Au↓ + Au³⁺ (1)

SO₃²⁻ + H₂O → SO₂↑ + 2OH⁻ (2)

More studies on the investigation of complexing agents are needed for the development of cyanide free Au electroplating electrolytes. Hydantoin as a new metal iron complexing, with good solubility and stability in aqueous solutions has received a number of attention in gold electrodepositing.^10–16 Among a series of hydantoin derivatives, 5,5-dimethylhydantoin (DMH) was selected as a complexing agent for the cyanide-free gold electroplating electrolyte.^17–19 H NMR spectra of the gold electroplating electrolytes were employed to reveal that strong coordination bonding of DMH to Au(III) was present, and that [Au(DMH)₄] was the quantitative complexation structure.^20–22 Cyanide-free electrolyte for gold electrodepositing from either Au(III) or Au(I) solution have not been reported, although a few electrolytes containing Au(III) were investigated.^17–19

In order to investigate Au(III) and Au(I) electrodepositing from DMH electrolytes were available. In this study, a novel simple cyanide-free electrolyte for both Au(III) and Au(I) were developed for gold film using DMH as a complexing agents was reported. The Au(III) and Au(I) electrolytes compositions are given in Tables SI and SII,† respectively. Cu substrate with 2 × 2 cm² working area were used for electrodepositing. Three different pH values (8, 9, and 10) were tested. The sample number of the gold layers obtained from Au(III) and Au(I) electrolytes at different parameters are given in Table 1.

Table 1 Sample number of gold layers with different parameter

Number	1	2	3	4	5	6
Gold salt	Au(III)	Au(III)	Au(III)	Au(I)	Au(I)	Au(I)
Current densities (mA cm^−2)	8	8	8	2	2	2
pH	8	9	10	8	9	10

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The appearance of the gold electrodeposits obtained from the electrolytes are as shown in Fig. 1. It can be seen that sample 1 and sample 6 shown a little dark brown surface, oppositely, sample 2, sample 3, sample 4 and sample 5 get a more uniform and bright golden surface compared to others. The gold films obtained from various electrolytes were further studied by directly observing the surface morphology by using SEM images.

Fig. 1 Scanner images of gold electrodepositing layers.

Furthermore, SEM images shown in Fig. 2 were employed to gain insight into the micromorphology of the gold electrodeposition obtained from various electrolytes. It can be seen that the gold electrodeposits obtained from the Au(III) electrolytes had a similar roughness followed the substrate morphology and films obtained from the Au(I) electrolytes performed smooth morphology. In addition, gold film produced from Au(III) electrolytes had many pores, especially at pH = 8, 9. The pores in films obtained in Au(I) electrolytes were less than that obtained in Au(III) electrolytes. These results provide an indication that the surface quality of the gold electrodeposition changed by pH either in Au(III) electrolytes or in Au(I) electrolytes, but it is slightly better in Au(I) electrolytes than that in Au(III) electrolytes.

Fig. 2 SEM images of gold electrodeposits obtained from various electrolytes.

X-ray fluorescence was used to measure the thickness of the gold layers, and the results are shown in Table 2. The thickness were increased with the higher pH, either in Au(III) electrolytes or in Au(I) electrolytes. It can be seen that the thickness of gold electrodeposition obtained from the Au(III) electrolytes only tens of nanometers, were obviously thinner than that from Au(I) electrolytes hundreds nanometers. The thickness of electrodepositing layers have a significant relationship to current efficiency. It is obviously that a higher current efficiency at a higher pH value. Despite, the discharging metallic ion were at different valence state and electrodepositing at different current densities. The circuit must obey conservation of charge, so we can get the eqn (3) as follow:

I × t × η = (h × A × ρ)/M × z × N_A × e (3)

where I is the electrodepositing current, t is the electrodepositing time, η is the electrodepositing current efficiency, h is the electrodepositing layer thickness, A is the cathode surface area, ρ is the bulk density of the deposited material, M is the molecular weight, z is valence state of discharge metallic ion, N_A is the avogadro constant and e is the electron charge and e is elementary charge, respectively.

Table 2 The thickness of gold layers obtained from various electrolytes

Thickness	1	2	3	4	5	6
Average (nm)	52	61.3	96.3	355.7	374.7	414.3

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According to eqn (3), current efficiency η has the direct ratio relation with the value of (h × z/I). Using this relationship, it can be calculated that the gold electrodepositing current efficiency in Au(I) electrolytes were 6–8 times compared to that in Au(III) electrolytes. The EDS testing results of electrodepositing layers are shown in Table 3. The results revealed there are higher N concentration in the gold layers deposited from Au(I) electrolytes compared to Au(III) electrolytes. It had little influence to gold layers' photograph, but it has a significant effect to layers' microhardness, as shown in Table SIII and Fig. S1.† XRD patterns of the gold electrodeposits obtained in the Au(III) and Au(I) electrolytes are shown in Fig. 3. The gold layers obtained from both Au(III) and Au(I) had similar patterns to the ref. 19. In Fig. 3, peaks indexed to Au(111), (200), (220), (311), and (222) crystal faces at 2 theta value s of 38.2, 44.4, 64.6, 77.6 and 81.7 obviously discovered. The diffraction peak intensity of the (111) faces for gold electrodeposits obtained from both Au(III) and Au(I) electrolytes is higher than that of any other peak, indicating that the reduction of Au occurred preferentially on Au(111) faces,¹⁹ in Au(III) and Au(I) electrolytes. The XRD patterns dedicated that gold layers from Au(III) and Au(I) electrolytes had a typical patterns with a same preferentially on Au(111) faces.

Table 3 The EDS results of electrodeposited surfaces (wt%)

Sample	1	2	3	4	5	6
C	0.42	0.38	0.38	0.39	0.38	0.38
N	1.29	1.73	1.94	4.34	4.36	4.80
Cu	61.72	63.56	47.36	8.72	7.21	7.38
Au	36.57	34.33	50.32	86.54	88.05	87.43

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Fig. 3 XRD patterns of the gold electrodeposits obtained from DMH based electrolytes various electrolytes.

The cathodic polarization curves with Cu electrode as working electrode were obtained from Au(III) and Au(I) electrolytes at different pH are shown in Fig. 4, respectively. The scan rate for all the curves was 100 mV s⁻¹. A reduction current started at −0.75 V and a maximum value around −0.95 V at pH = 8, 9 and at pH = 10 the maximum value changed to −0.85 V, can be observed in the curves obtained from Au(III) solution. The Au limiting current density occurs in the plateau region of the curve when the cathodic potential was −0.95 V at pH = 8, 9 and −0.75 V at pH = 10. At more negative potentials, gold is electrodeposited under kinetic control. However, observing the cathodic polarization curves obtaining from Au(I) electrolytes, there appeared reduction currents at early cathodic period and a obviously increasing at around −1.15 V, then with the overpotential expand, the hydrogen evolution were occured. And an inflection point at −1.27 V at pH = 8, 9 electrolytes, and an inflection point at −1.22 V at pH = 10, can be observed. The polarization curve of Au(III) and Au(I) indicted that gold depositing shifted to more noble potential in the novel electrolyte, compared to the research reported Au(III) reduced near −0.3 V vs. Ag/AgCl²¹ and Au(I) reduced near −0.7 V vs. SCE.²³ The more negative discharge potential can explained by Au(III) and Au(I) formed stably complexes with DMH. The high discharge overpotential of Au in novel electrolyte, also result from the DMH absorption on cathode surface, as literatures demonstrate.¹⁸

Fig. 4 Polarization curves in various Au electrolytes.

Chronoamperometry was used to investigate the initial nucleation stage of the gold electrodeposition process at various addition. Potentiostatic current transients on the Cu electrode from electrolytes are presented in Fig. 4. The applied potentials were chosen to be more negative than the Au depositing potentials of Linear Sweep Voltammetry to ensure that the electrochemical reactions were under diffusion control.²³ As shown in Fig. 5, all the curves obtained from the gold electroplating electrolytes, show a current decrease at the very beginning, which results from double-layer charging. After that, the current densities increase due to simultaneous gold nuclei growth and an increasing number of nuclei. With the growth of gold nuclei, the diffusion zones around them expand and finally overlap with the diffusion zones of neighboring nuclei, while the current density reaches I_m (the maximum of the current density). The existence of a current density peak in the current transients indicates that the charge transfer process controls the reaction kinetics at early stages of gold electrodeposition and there is a nucleation stage preceding the growth of the gold deposit.

Fig. 5 Current transients on the Cu electrode, at −1 V Au(III), at −1.2 V Au(I), in electrolytes.

The SH model was employed to analyze the current transients measured in this work.

All experimental current transients were transformed into non-dimensional plots of (I/I_m)² vs. (t/t_m), and the plots were compared with non-dimensional current transients curves, as shown in Fig. 6 and 7. The nucleation mechanism models proposed by Scharifer and Hills (SH)^24–28 are famous and widely used in many reported theoretical models of nucleation mechanisms. These models provide a simple and fast way to classify experimental transients into two limiting cases, instantaneous or progressive nucleation, assuming both display 3-D growth of nuclei. In the instantaneous nucleation process, all nuclei instantaneously form on the active electrode surface, while in the progressive nucleation process the number of nuclei increases with the extension of reaction time. The SH models of 3-D instantaneous nucleation and progressive nucleation are shown in eqn (4) and (5):²⁹

(I/I_m)² = 1.9542(t/t_m)⁻¹{1 − exp[−1.2564(t/t_m)]}² (4)

(I/I_m)² = 1.2254(t/t_m)⁻¹{1 − exp[−2.3367(t/t_m)²]}² (5)

where I and t are the current and time, and I_m and t_m are the maximum current and the corresponding time, respectively. The nucleation curves of the gold electroplating in Au(III) electrolytes compared to the theoretical curves for instantaneous and progressive nucleation are displayed in Fig. 6, they indicated that the nucleation of the gold electroplating in Au(III) electrolytes displayed progressive nucleation at pH = 8, but the nucleation style changed with pH increased, displayed instantaneous nucleation at pH = 10. In contrast, the nucleation curves of the gold electroplating in Au(I) electrolytes compared to the non-dimensional curves for instantaneous and progressive nucleation are displayed in Fig. 7, they indicated that the nucleation of the gold electroplating in Au(I) electrolytes displayed instantaneous nucleation at pH = 8 and 9, but the nucleation mold also changed with pH increased, displayed progressive nucleation at pH = 10. The process of nucleation is changed by Au valence state, at the same time, the pH of electrolytes also has an obviously influence to gold nuclei model during the gold electrodepositing. Different electrodeposition parameters can give various overpotential on the cathodic surface, but the nucleation process were not changing by cathodic polarization in a range, in generally.¹⁹ Take the gold salt and pH value into consideration, diffusivity (D) of complex calculated¹⁶ in the DMH-based gold electroplating electrolytes contained Au(III) and Au(I) were different, as listed in Table 4. The diffusivity value had a significant relative with the gold salt and pH value changing. It can be explained by that Au(III) and Au(I) can form complex with four and two DMH ion, on the other hand, DMH ionizing level were controlled by electrolyte's pH value. It indicates that the ionization level of DMH and the complex condition of gold ion have an obviously influence to electrodepositing nucleation behaviour.

Fig. 6 Non-dimensional theoretical plots and experimental curves in Au(III) electrolytes.

Fig. 7 Non-dimensional theoretical plots and experimental curves in Au(I) electrolytes.

Table 4 Diffusion coefficients calculated from i–t curves

D (10^−6 cm^2 s^−1)	pH = 8	pH = 9	pH = 10
Au(III)	2.48	4.23	5.18
Au(I)	7.16	9.61	2.16

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Take the stability performance into consideration, our electrolyte has a similar stability performance with other reported DMH based electrolytes. There have not discovered any educt during electroplating process or after several weeks. Comparing to sulfite-based electrolytes, our electrolyte have a better stability performance as shown in Fig. S2.† The instability of sulfite-based electrolytes were reported in ref. 9, and it also described by eqn (1) and (2).⁹ Based on the references and our experimental, as shown in Fig. S2 (ESI†), DMH based cyanide-free electrolytes have an obvious advantage compared to sulfite based cyanide-free electrolytes.

A novel simple cyanide-free electrolyte based on DMH was reported for gold electrodepositing from both Au(III) and Au(I) valence state. In this electrolyte both gold valence state can get desirable uniformly distributed and bright golden gold layer without other brightening agents. The gold layer obtained from the electrolyte at pH = 9 were better than that from pH = 8 or 10. In addition, compared to Au(III) electrolyte, Au(I) electrolyte has a higher current efficiency and the gold layer has a higher microhardness. Cathodic polarization curves indicted that gold depositing in the bath shifted to more noble potential than literatures reported before.²⁰ The nuclei model of gold electrodepositing in different valence state are different, gold electroplating in Au(I) electrolytes displayed instantaneous nucleation at pH = 8 and 9, displayed progressive nucleation at pH = 10. Oppositely, gold electroplating in Au(III) electrolytes displayed progressive nucleation at pH = 8, displayed instantaneous nucleation at pH = 10. Lastly, all the gold electroplating electrolytes had good chemistry stability.

Acknowledgements

This study was performed with the support of the Highnic Group (China).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10418e

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