Experimental and theoretical studies of DMH as a complexing agent for a cyanide-free gold electroplating electrolyte

Abstract

In this study, a cyanide-free gold electroplating electrolyte using 5,5-dimethylhydantoin (DMH) as a complexing agent was introduced. A golden bright gold electrodeposit with smooth and compact surface was obtained from the introduced cyanide-free gold electroplating electrolyte. The results of scanning electron microscopy (SEM) measurements confirmed that the golden bright gold electrodeposit possesses an excellent leveling capability as well as smooth and compact morphology. The crystalline structure of the gold electrodeposits was characterized by X-ray diffraction (XRD) analysis. Computational chemistry was employed to provide an insight view of the reason for selecting DMH among the various hydantoin derivatives as the complexing agent for the introduced cyanide-free gold electroplating electrolyte. Quantum chemical calculations were employed to study the electronic properties and orbital information of the investigated complexing agents. The adsorption interactions between these complexing agents and the metal surfaces were investigated by molecular dynamic (MD) simulations. Consequently, the results of these theoretical studies revealed that DMH was selected among the various hydantoin derivatives as the complexing agent for the introduced cyanide-free gold electroplating electrolyte due to its strong electron donating abilities and high adsorption energies on the metal surfaces.

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1. Introduction

Owing to their excellent physical, chemical, and conductive properties, gold electrodeposits obtained from cyanide-based gold electroplating electrolytes have been widely used in jewelries, artware, electronics, and aerospace industries for more than 100 years.^1–4 Unfortunately, as one of the most toxic chemicals, cyanide brings extremely high risks to human health and the environment.^5–7 Numerous cyanide-free gold electroplating electrolytes have been developed to replace the cyanide-based electrolytes for gold electroplating.^8–12 However, except for few successful cases, most of them were unsatisfactory because of the poor-quality of the electroplating electrolytes or gold electrodeposits.¹³ More studies on the investigation of complexing agents are needed for the development of cyanide-free gold electroplating electrolytes.

Hydantoin is a low cost and commercially available heterocyclic organic compound with good solubility and stability in aqueous solutions over a large temperature range.¹⁴ Its derivatives are promising and stable complexing agents for several metal ions.^15,16 Among a series of hydantoin derivatives, 5,5-dimethylhydantoin (DMH) was selected as a complexing agent for the cyanide-free gold electroplating electrolyte in our study. A golden bright gold electrodeposit was obtained with the addition of some additives in the investigated cyanide-free gold electroplating electrolyte. Besides the application of DMH in the introduced cyanide-free gold electroplating electrolyte, DMH has been employed as a complexing agent for silver and copper electroplating in our previous studies.^17–20 It is of significant importance to obtain an insight view of the reasons to select DMH among the various hydantoin derivatives as a complexing agent for the introduced cyanide-free gold electroplating electrolyte and other metal electroplating electrolytes.

Studying the mechanism of the complexing agents for cyanide-free gold electroplating electrolytes through experimentation is extravagant both in time and resources, especially for the consumption of the extremely precious gold. Time-saving techniques, such as quantum chemical calculations^21–24 and molecular dynamic (MD) simulations,²⁵ are useful and urgently needed to study the organometallic compound or to select complexing agents for a cyanide-free metal electroplating electrolyte. To investigate their behavior in small chemical systems at the molecular level, quantum chemical calculations and MD simulations are emerging in all research areas of chemistry.^26–34 In the coordinate systems, bonding interactions between metal ions and complexing agents can be predicted and investigated by quantum chemical calculations.^35–38 Quantum chemical calculations and MD simulations are convenient methods to study the properties of complexing agents,^39–41 additives,^42,43 inhibitors^44–51 and other molecules,^52–58 as well as their adsorption behavior on metal surfaces.

In this study, the electronic properties and orbital information of hydantoin and its derivatives were demonstrated by quantum chemical calculations. MD simulations were employed to reveal the adsorption interactions between all the organic compounds and the metal surfaces studied. The results of these theoretical studies revealed that due to its strong electron donating abilities and high adsorption energies on metal surfaces, DMH was the optimal choice among the various hydantoin derivatives as the complexing agent of the introduced cyanide-free gold electroplating electrolyte.

2. Experimental

2.1. Measurements and apparatus

Gold electroplating electrolyte was prepared using 0.025 mol L⁻¹ HAuCl₄, 0.3 mol L⁻¹ DMH, 0.36 mol L⁻¹ K₂CO₃, and 30 mg L⁻¹ PEI in deionized water. All the reagents used in this study were of analytical grade. The pH of all the electrolytes was maintained at pH = 10. Gold electrodeposits used for performance measurements were prepared under galvanostatic conditions (8 mA cm⁻²) with mild agitation at 318 K in a cell employing an insoluble anode and a copper substrate coated with nickel electrodeposit. Field emission scanning electron microscopy (FE-SEM, Hitachi SU8000) was used to study the surface morphologies and composition of the gold electrodeposits. The crystalline structure of the gold electrodeposits was characterized by XRD analysis with a D/max-3C X-ray diffractometer at a scanning rate of 0.02° s⁻¹ with Cu Kα radiation.

2.2. Quantum chemical calculations and MD simulations

All the quantum chemical calculations were carried out by density functional theory (DFT) methods using the B3LYP exchange–correlation functional method. In all these quantum chemical calculations, 6-311G** basis set was used for hydrogen, carbon, nitrogen, oxygen, chlorine, and bromine atoms in the studied systems. All the calculations on the systems under investigation were performed using the Gaussian 09 program software package at 298 K with water as the solvent in the IEFPCM theoretical model.

Simulations of the adsorption interactions between the studied organic compounds and the nickel and gold surfaces were carried out in a simulation box with periodic boundary conditions using Materials Studio (from Accelrys Inc.). The box consisted of a nickel or gold surface (cleaved along the (111) plane, with a volume of 2.88 nm × 2.88 nm × 1.18 nm for Au and 2.49 nm × 2.49 nm × 1.02 nm for Ni), a liquid phase, and a vacuum layer of 1 nm height. The liquid phase was water molecules with a density of 1 g cm⁻³ containing the 3 organic compounds studied. The MD simulations were performed at 298 K, utilizing an NVT ensemble and the ab initio polymer consistent force field (PCFF) with a time step of 1 fs and simulation time of 500 ps.

The interaction energy between the metal surface and organic molecules was calculated using eqn (1).

E_Interaction = E_Total − E_Metal − E_Agents (1)

where E_Total is the total energy of the gold or nickel crystal together with the adsorbed organics and E_Metal and E_Agents are the total energy of the gold or nickel crystal and free agents, respectively.

3. Results and discussion

3.1. Function of DMH in gold electroplating

Using DMH as the complexing agent, a golden bright gold electrodeposit was obtained from the introduced gold electroplating electrolyte in the presence of a suitable additive. Fig. 1 displays the macroscopic images of the gold electrodeposits obtained from the introduced cyanide-free gold electroplating electrolyte with and without additive, as well as the HAuCl₄ electrolyte without DMH.

Fig. 1 Macroscopic images of the gold electrodeposits obtained from the introduced cyanide-free gold electroplating electrolyte: (a) with additive, (b) without additive, and (c) from the HAuCl₄ electrolyte without DMH.

It can be clearly seen that the gold electrodeposit, as shown in Fig. 1(a), is golden bright with a smooth and compact surface. This indicates that the introduced cyanide-free electrolyte is effective for producing excellent gold electrodeposit, which meets the requirements of its applications for decorative purposes. As displayed in Fig. 1(b) and (c), the colour of both the gold electrodeposits obtained from the electrolyte without additive and the HAuCl₄ electrolyte without DMH was brown. However, the gold electrodeposits obtained from the electrolyte with DMH and without additive had a relatively more uniform surface.

The surface morphology of the gold electrodeposits obtained from the introduced cyanide-free gold electroplating electrolyte with and without additive, as well as the HAuCl₄ electrolyte without DMH was characterized by SEM measurements. Top-view SEM images of the gold electrodeposits obtained from the introduced cyanide-free gold electroplating electrolyte with and without additive, as well as the HAuCl₄ electrolyte without DMH are displayed in Fig. 2.

Fig. 2 SEM images of the top views of the gold electrodeposits obtained from the introduced cyanide-free gold electroplating electrolyte: (a) and (b) with additive, (c) and (d) without additive, and (e) and (f) from the HAuCl₄ electrolyte without DMH.

It can be seen from Fig. 2(e) and (f) that the gold electrodeposits obtained from the HAuCl₄ electrolyte without DMH possess rough and loose morphologies with large crystal grains. Fig. 2(c) and (d) show that the crystal grains of the gold electrodeposit obtained from the DMH based gold electroplating electrolyte without additive were smaller than that obtained from the HAuCl₄ electrolyte without DMH, and its surface was more smooth and compact. These observations most likely originate when DMH was used as the complexing agent, which causes an increase in cathodic polarization and results in the formation of smaller grains in the gold electrodeposit. As displayed in Fig. S1 (ESI†), after immersing the copper substrate coated with nickel electrodeposit into the introduced cyanide-free gold electroplating electrolyte with additive and without additive, no difference was observed on the surface after the immersion. Thus, when DMH was employed as a complexing agent in the electrolyte to coordinate with HAuCl₄ to form [Au(DMH)]⁻, the stability of HAuCl₄ was significantly improved and no strike-plating process was necessary to obtain efficient adherent gold deposits on the substrate when the introduced gold-plating electrolyte containing DMH was used as the complexing agent.

However, the gold electrodeposit obtained from the cyanide-free gold electroplating electrolyte with additive is more smooth and compact with smaller crystal grains than that obtained from the electrolyte without additive, as shown in Fig. 2(a) and (b). These results provide an indication that the surface quality of the gold electrodeposit could be further improved upon the addition of a suitable additive. To distinguish the role of DMH and additive in the gold electroplating electrolyte, the additive was added into the HAuCl₄ electrolyte without DMH to obtain a gold electrodeposit from HAuCl₄ electrolyte without DMH. Unfortunately, when the additive was added to the HAuCl₄ electrolyte without DMH, the HAuCl₄ electrolyte became turbid and a precipitate was formed, as displayed in Fig. S2(d) (ESI†). However, Fig. S2(c)† shows that no changes to the electrolyte were detected after it was used for many times for gold electroplating, indicating that the gold electroplating electrolyte possesses good stability. DMH was employed as a complexing agent in the introduced electrolyte to coordinate with HAuCl₄ to form [Au(DMH)₄]⁻, and the stability of HAuCl₄ was significantly improved. In addition, the influence of additive on the electrochemical behavior and deposition of gold for the introduced cyanide-free electrolyte were investigated using electrochemical measurements and material characterization.⁵⁹

The phase structures of the gold electrodeposits that were obtained from the introduced cyanide-free gold electroplating electrolyte with and without additive, as well as those of the electrodeposits obtained from the HAuCl₄ electrolyte without DMH were confirmed using EDS and XRD. All the electrodeposits obtained from the introduced cyanide-free gold electroplating electrolyte with and without additive, as well as from the HAuCl₄ electrolyte without DMH, were pure gold and the purity of the gold deposit was very high, as shown in the EDS patterns (Fig. S3 in ESI†). The XRD patterns of the gold electrodeposits obtained from the introduced cyanide-free gold electroplating electrolyte with and without additive, as well as from the HAuCl₄ electrolyte without DMH, are displayed in Fig. 3.

Fig. 3 The XRD patterns of (a) Au (JCPDS file: 04-0784) and gold electrodeposits obtained from the introduced cyanide-free gold electroplating electrolyte, (b) with additive, (c) without additive, and (d) from the HAuCl₄ electrolyte without DMH.

As displayed in Fig. 3, all the peaks can be indexed to the Au (111), (200), (220), (311), (222) and (400) crystal face without any additional peak observed when the gold electrodeposits were obtained from the introduced cyanide-free gold electroplating electrolyte with additive and without additive. However, a peak corresponding to Cu (220) was detected on the pattern for the gold electrodeposits obtained from HAuCl₄ electrolyte without DMH. This indicates that the Cu substrate was partially exposed after the gold electroplating in the HAuCl₄ electrolyte without DMH, which can be confirmed by the replacement reaction of a copper substrate with nickel electrodeposit coated in the HAuCl₄ electrolyte without DMH, as shown in Fig. S1.† As seen in the surface morphology of the gold electrodeposits displayed in Fig. 2(e) and (f), the gold electrodeposits obtained from the HAuCl₄ electrolyte without DMH possess rough and loose morphologies with big crystal grains. The rough and loose gold electrodeposits make the exposure of copper substrate possible.

3.2. Quantum chemical calculations

The molecular structures of hydantoin and its usual derivative molecules, including aminohydantoin (AHD), 1-bromo-3-chloro-5,5-dimethylhydantoin (BCDMH), 1,3-dibromo-5,5-dimethylhydantoin (DBDMH), 1,3-dichloro-5,5-dimethylhydantoin (DCDMH), dimethylol dimethylhydantoin (DMDMH), 5,5-dimethylhydantoin (DMH), and 1,5,5-trimethylhydantoin (TMH), are shown in Fig. S4 (ESI†). It is of significant importance to obtain an insight view of the reasons for selecting DMH from the hydantoin derivatives as the complexing agent for the introduced cyanide-free gold electroplating electrolyte.

As displayed in Fig. S4,† all the organic compounds studied have the same nitrogen-containing five-membered ring with same C=O bond, but different substituents, which might generate diverse influences on the electronic properties and frontier molecular orbital information of all the organic compounds studied. This indicated that these organic molecules show different abilities to form gold-complex coordinated bonds and adsorb on the metal surfaces due to their different electronic properties and frontier molecular orbital information. Quantum chemical calculations were conducted to reveal the electronic properties and frontier molecular orbital information of all the organic compounds studied to verify the prediction.

Fig. S5 (ESI†) exhibits the distribution of the HOMO and electron cloud densities of the hydantoin studied and its usual derivatives. The distribution of the LUMO is displayed in Fig. S6 (ESI†).

The presence of nitrogen and oxygen atoms exhibited significant contributions to the HOMO, as displayed in Fig. S5.† This was due to the electron donating properties of nitrogen and oxygen atoms, providing the ability to form gold-complex coordinated bonds. With the replacement of hydrogen atoms with chlorine atoms, bromine atoms or methylol, BCDMH, DBDMH, DCDMH, and DMDMH manifested more complicated localization of HOMO than DMH, as displayed in Fig. S5(b)–(f),† respectively.

Molecules (b)–(d) shown in Fig. S6† show a similar localization of the LUMO and high electron cloud densities, indicating similar electronic properties and strong electron accepting abilities. On the other hand, the molecule (a) possessed similar electronic properties and electron-accepting abilities with molecules (e)–(h).

In addition to the localization of the molecular orbitals, E_HOMO, E_LUMO, and their difference (ΔE) are useful tools to characterize the electronic properties and adsorption behavior of each molecule. According to the frontier molecular orbital theory, the energy of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are often associated with the electron-donating ability and electron-accepting ability of the organic molecules. Higher values of E_HOMO (energy of HOMO) and lower values of E_LUMO (energy of LUMO) indicate the tendency of a molecule to donate and accept electrons, respectively. The E_HOMO and E_LUMO, along with ΔE are important molecular electronic properties that relate to the behavior and adsorption properties of the organic compounds studied. A schematic depiction of these values based on frontier molecular orbitals is presented in Fig. 4.

Fig. 4 Schematic diagrams of the frontier molecular orbitals of (a) AHD, (b) BCDMH, (c) DBDMH, (d) DCDMH, (e) DMDMH, (f) DMH, (g) hydantoin, and (h) TMH.

As displayed in Fig. 4, structures (d)–(g) have similar relatively high E_HOMO values of −7.393 eV, −7.343 eV, −7.404 eV, and −7.444 eV, respectively. They are likely to be good complexing agents owing to their propensity to donate electrons to gold ions and form coordinated gold-complex structures. The difference between the frontier molecular orbitals of these structures was mainly in the E_LUMO values. The relatively high E_LUMO values of −0.427 eV for structure (f) and −0.392 eV for structure (h) indicate that these two organics have a similar weak ability to form back-donation bonds when accepting electrons from the anti-bonding orbitals within gold ions. On the other hand, the higher E_LUMO values of DMH imply that the acceptance of electrons from the gold ions and cathodic metal atoms are more difficult relative to that from other structures due to the formation of an anti-bond with the cathodic metals and ions. Thus, the coordinated gold-complex of DMH and gold ions may be the most stable structures among all the coordinated gold-complexes of all the organic compounds and gold ions studied.

ΔE values lead to different stabilities for the adsorption layers formed by the complexing agent on the metal surface.^60,61 The smallest ΔE calculated from structures (d)–(g) suggests that DCDMH has a more effective adsorption on the metal surfaces. On the other hand, the higher E_LUMO value of structure (d) implies that the acceptance of electrons from the cathodic metal atoms or ions is easier for forming an anti-bond with the cathodic metals or ions. Thus, the stability of the coordinated gold-complex or the adsorption of structure (d) on the metal surface may be decreased.

To investigate the ability of the complexing agents studied to form the gold-complex coordinate bonds, quantum chemical calculations were employed to study the charge distributions of all the complexing agents studied. The charge distributions of the organic molecules studied are shown in Fig. 5.

Fig. 5 Charge distributions of (a) AHD, (b) BCDMH, (c) DBDMH, (d) DCDMH, (e) DMDMH, (f) DMH, (g) hydantoin, and (h) TMH (unit of e).

As seen in the charge distributions shown in Fig. 5, with low electronegativity to act as an electron donor, nitrogen and oxygen atoms in all the organic compounds studied, as well as the chlorine and bromine atoms in structures (b)–(d), might be the possible atoms that form coordinate bonds with gold ions. Combined with the frontier molecular orbital information and the charge distributions shown in Fig. 5, structures (e)–(g) are comparatively better complexing agents owing to their propensity to donate electrons to gold ions and form gold-complex coordinated bonds.

3.3. MD simulations

The adsorption behaviors of all the complexing agents studied on the Ni (111) and Au (111) surfaces were investigated using MD simulations in this study. The results of AHD simulations are shown in this manuscript and remaining results are provided in the ESI† due to space limitations. The adsorption behaviors of AHD on Au (111) and Ni (111) surfaces are shown in Fig. 6 and S7,† respectively. Fig. 6(a) reveals the initial configuration of MD simulation boxes. Fig. 6(b) displays the final equilibrium configuration of MD simulation boxes at 298 K with a time step of 1 fs and simulation time of 500 ps. Fig. 6(c) shows the top view of the final equilibrium configuration of the simulation boxes. Fig. 6(d) and (e) show the energy and temperature fluctuation curves of the MD simulations. These plots indicate that the systems were already at equilibrium at the completion of the simulation.

Fig. 6 Adsorption behaviors of AHD on the Au surface: (a) initial configuration of the simulation box (AHD visualized by balls and sticks and water molecule visualized by lines); (b) final equilibrium configuration of the MD simulation box; (c) top view of the final equilibrium configuration of the simulation box; (d) energy fluctuation curves for the MD simulation; and (e) temperature fluctuation curve for the MD simulation.

The adsorption energies and MD simulation boxes of all the other complexing agents studied are summarized in Table 1 and Fig. S7–S21 (ESI†).

Table 1 Adsorption energies for all the studied complexing agents on the Ni (111) and Au (111) surface (unit = kJ mol⁻¹)

		Ni (111)	Au (111)
a	Aminohydantoin (AHD)	244.760	314.881
b	BCDMH	244.312	337.349
c	DBDMH	290.857	288.399
d	DCDMH	201.312	266.578
e	DMDMH	297.777	305.429
f	DMH	248.861	289.469
g	Hydantoin	259.131	217.384
h	TMH	298.177	339.674

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The main conclusions drawn from these simulations illustrate that all the complexing agents studied can adsorb on the metal surfaces with high energies. Moreover, as shown in Fig. 6(c) and S7–S21 (ESI†), the heterocyclic rings in the studied organic compounds were virtually parallel to the nickel and gold surfaces, suggesting effective adsorption on the metal surfaces. The results of the MD simulations manifested that all the investigated hydantoin derivatives can strongly adsorb on the nickel and gold surfaces, leading to a higher inhibition effect for gold electrodeposition on the nickel and gold surfaces.

To summarize the results of the DFT calculations and MD simulations, with a relatively high E_HOMO value and effective adsorption on the nickel, gold, silver, copper, and iron surfaces, DMH, with environmental compatibility, low cost, good solubility, and superior stability in alkaline solution over a large temperature range, was the optimal choice among various hydantoin derivatives as the complexing agent for the cyanide-free gold electroplating electrolyte and other metal electroplating electrolytes due to its strong electron donating abilities and high adsorption energies on metal surfaces.

4. Conclusions

In conclusion, we have reported a cyanide-free gold electroplating electrolyte using 5,5-dimethylhydantoin (DMH) as the complexing agent. The cyanide-free electrolyte is stable and environmentally friendly with low toxicity. With the addition of a suitable additive into the investigated cyanide-free gold electroplating electrolyte, a golden bright gold electrodeposit with a smooth and compact surface was obtained. SEM images confirm that the gold electrodeposit has an excellent leveling capability as well as smooth and compact morphology. This indicates that the introduced gold electroplating electrolyte is a promising candidate to replace the conventional cyanide-based gold electroplating electrolyte. Computational chemistry was employed to obtain an insight view of the reasons for selecting DMH among the hydantoin derivatives as the complexing agent for the introduced cyanide-free gold electroplating electrolyte and other metal electroplating electrolytes. Quantum chemical calculations were employed to study the electronic properties and orbital information of the complexing agents investigated. The adsorption interactions between these complexing agents and the metal surfaces were investigated by molecular dynamic (MD) simulations. Consequently, the results of the DFT calculations and MD simulations revealed that due to its strong electron-donating abilities and high adsorption energies on metal surfaces, DMH was the optimal choice from the hydantoin derivatives as the complexing agent for the introduced cyanide-free gold electroplating electrolyte. This efficient and versatile method used to study the mechanism of complexing agent selection, thus opens a new window to obtain insight into the coordination interaction and adsorption behaviour of complexing agents for metal ions and metal surfaces, respectively, during metal electroplating and will vigorously promote the level of research in this area.

Acknowledgements

Financial support from the State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (2015DX09) for this study is gratefully acknowledged.

References

1. J. W. Ko, H. C. Koo, D. W. Kim, S. M. Seo, T. J. Kang, Y. Kwon, J. L. Yoon, J. H. Cheon, Y. H. Kim, J. J. Kim and Y. J. Park, J. Electrochem. Soc., 2010, 157, D46–D49.
2. A. He, Q. Liu and D. G. Ivey, J. Mater. Sci.: Mater. Electron., 2009, 20, 543–550.
3. Z. Liu and A. C. West, Electrochim. Acta, 2011, 56, 3328–3333.
4. A. I. de Sa, S. Eugenio, S. Quaresma, C. M. Rangel and R. Vilar, Thin Solid Films, 2011, 519, 6278–6283.
5. M. A. Barakat, Y. T. Chen and C. P. Huang, Appl. Catal., B, 2004, 53, 13–20.
6. M. Larsen, A. S. Ucisik and S. Trapp, Environ. Sci. Technol., 2005, 39, 2135–2142.
7. J. Doménech and J. Peral, Sol. Energy, 1988, 41, 55–59.
8. T. A. Green and S. Roy, J. Electrochem. Soc., 2006, 153, C157–C163.
9. J. S. Chen, Y. M. Fang, Q. Y. Qiu, L. X. You, J. Song, G. M. Zhang, G. N. Chen and J. J. Sun, Green Chem., 2011, 13, 2339–2343.
10. H. Honma and Y. Kagaya, J. Electrochem. Soc., 1993, 140, L135–L137.
11. X. Wang, N. Issaev and J. G. Osteryoung, J. Electrochem. Soc., 1998, 145, 974–981.
12. H. Watanabe, S. Hayashi and H. Honma, J. Electrochem. Soc., 1999, 146, 574–579.
13. S. Dimitrijevic, M. Rajcic-Vujasinovic and V. Trujic, Int. J. Electrochem. Sci., 2013, 8, 6620–6646.
14. M. Meusel and M. Gütschow, Org. Prep. Proced. Int., 2004, 36, 391–443.
15. G. Z. Pavlovich and R. G. Luthy, Water Res., 1988, 22, 327–336.
16. M. Puszyńska-Tuszkanow, T. Grabowski, M. Daszkiewicz, J. Wietrzyk, B. Filip, G. Maciejewska and M. Cieślak-Golonka, J. Inorg. Biochem., 2011, 105, 17–22.
17. A. Liu, X. Ren, M. An, J. Zhang, P. Yang, B. Wang, Y. Zhu and C. Wang, Sci. Rep., 2014, 4, 3837.
18. A. Liu, X. Ren, B. Wang, J. Zhang, P. Yang, J. Zhang and M. An, RSC Adv., 2014, 4, 40930–40940.
19. A. Liu, X. Ren, J. Zhang, G. Yuan, P. Yang, J. Zhang and M. An, New J. Chem., 2015, 39, 2409–2412.
20. J. Zhang, A. Liu, X. Ren, J. Zhang, P. Yang and M. An, RSC Adv., 2014, 4, 38012–38026.
21. M. K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru and M. Grätzel, J. Am. Chem. Soc., 2005, 127, 16835–16847.
22. H. Li, Y. Li and M. Chen, RSC Adv., 2013, 3, 12133–12139.
23. D. V. Deubel and T. Ziegler, Organometallics, 2002, 21, 1603–1611.
24. S. Liang and A. E. Roitberg, J. Chem. Theory Comput., 2013, 9, 4470–4480.
25. W. Plazinski and M. Drach, Appl. Surf. Sci., 2012, 262, 153–155.
26. F. Rico, L. Gonzalez, I. Casuso, M. Puig-Vidal and S. Scheuring, Science, 2013, 342, 741–743.
27. S. J. Zhou, D. L. Preston, P. S. Lomdahl and D. M. Beazley, Science, 1998, 279, 1525–1527.
28. B. Saha, S. Shindo, S. Irle and K. Morokuma, ACS Nano, 2009, 3, 2241–2257.
29. K. Honkala, A. Hellman, I. N. Remediakis, A. Logadottir, A. Carlsson, S. Dahl, C. H. Christensen and J. K. Norskov, Science, 2005, 307, 555–558.
30. M. Altarawneh, M. W. Radny, P. V. Smith, J. C. Mackie, E. M. Kennedy and B. Z. Dlugogorski, Appl. Surf. Sci., 2008, 254, 4218–4224.
31. M. E. Tuckerman, D. Marx, M. L. Klein and M. Parrinello, Science, 1997, 275, 817–820.
32. G. Agalya, C. Lv, X. Wang, M. Koyama, M. Kubo and A. Miyamoto, Appl. Surf. Sci., 2005, 244, 195–198.
33. B.-Y. Tang, M.-D. Chen, K.-L. Han and J. Z. H. Zhang, J. Phys. Chem. A, 2001, 105, 8629–8634.
34. R. Chen, M. Liang, J. Luo, H. Lei, D. Guo and X. Hu, Appl. Surf. Sci., 2011, 258, 1756–1761.
35. H. Chermette, Coord. Chem. Rev., 1998, 178–180, 699–721.
36. G. Harrach, Z. Valicsek and O. Horváth, Inorg. Chem. Commun., 2011, 14, 1756–1761.
37. C. I. Maxwell, A. A. Neverov, N. J. Mosey and R. Stan Brown, J. Phys. Org. Chem., 2014, 27, 419–429.
38. H.-T. Liu, X.-G. Xiong, P. D. Dau, Y.-L. Wang, J. Li and L.-S. Wang, Chem. Sci., 2011, 2, 2101–2108.
39. S. L. Daifuku, M. H. Al-Afyouni, B. E. R. Snyder, J. L. Kneebone and M. L. Neidig, J. Am. Chem. Soc., 2014, 136, 9132–9143.
40. D. R. Armstrong, S. E. Baillie, V. L. Blair, N. G. Chabloz, J. Diez, J. Garcia-Alvarez, A. R. Kennedy, S. D. Robertson and E. Hevia, Chem. Sci., 2013, 4, 4259–4266.
41. T. Wang, Y. Liang and Z. X. Yu, J. Am. Chem. Soc., 2011, 133, 9343–9353.
42. C. Wang, M. An, P. Yang and J. Zhang, Electrochem. Commun., 2012, 18, 104–107.
43. C. Wang, J. Zhang, P. Yang and M. An, Electrochim. Acta, 2013, 92, 356–364.
44. A. Kokalj, S. Peljhan, M. Finšgar and I. Milošev, J. Am. Chem. Soc., 2010, 132, 16657–16668.
45. J. Zhang, G. Qiao, S. Hu, Y. Yan, Z. Ren and L. Yu, Corros. Sci., 2011, 53, 147–152.
46. R. Hasanov, M. Sadıkoğlu and S. Bilgiç, Appl. Surf. Sci., 2007, 253, 3913–3921.
47. S. Xia, M. Qiu, L. Yu, F. Liu and H. Zhao, Corros. Sci., 2008, 50, 2021–2029.
48. R. M. Issa, M. K. Awad and F. M. Atlam, Appl. Surf. Sci., 2008, 255, 2433–2441.
49. A. Liu, X. Ren, J. Zhang, C. Wang, P. Yang, J. Zhang, M. An, D. Higgins, Q. Li and G. Wu, RSC Adv., 2014, 4, 40606–40616.
50. N. Kovačević and A. Kokalj, J. Phys. Chem. C, 2011, 115, 24189–24197.
51. S. Issaadi, T. Douadi and S. Chafaa, Appl. Surf. Sci., 2014, 316, 582–589.
52. Z. Rinkevicius, X. Li, J. A. R. Sandberg, K. V. Mikkelsen and H. Ågren, J. Chem. Theory Comput., 2014, 10, 989–1003.
53. W. T. Cahyanto, G. Shukri, M. K. Agusta and H. Kasai, Appl. Surf. Sci., 2013, 266, 405–409.
54. A. Kokalj, P. Gava, S. de Gironcoli and S. Baroni, J. Phys. Chem. C, 2008, 112, 1019–1027.
55. N. K. Das and T. Shoji, Appl. Surf. Sci., 2011, 258, 442–447.
56. F. Y. Guo, C. G. Long, J. Zhang, Z. Zhang, C. H. Liu and K. Yu, Appl. Surf. Sci., 2015, 324, 584–589.
57. S. Peljhan and A. Kokalj, J. Phys. Chem. C, 2009, 113, 14363–14376.
58. M. Zhang, K. Yang and Y. Yu, Appl. Surf. Sci., 2015, 328, 583–590.
59. X. Ren, Y. Song, A. Liu, J. Zhang, P. Yang, J. Zhang, G. Yuan, M. An, H. Osgood and G. Wu, RSC Adv., 2015 10.1039/c5ra12217a.
60. S. El Issami, L. Bazzi, M. Mihit, B. Hammouti, S. Kertit, E. A. Addi and R. Salghi, Pigm. Resin Technol., 2007, 36, 161–168.
61. K. Barouni, L. Bazzi, R. Salghi, M. Mihit, B. Hammouti, A. Albourine and S. El Issami, Mater. Lett., 2008, 62, 3325–3327.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra13140e

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