Liyuan Sun_Electroplating of Cr from Ionic Liquids

Characterization of Imidazolium Chloride Ionic Liquids Plus Trivalent

Chromium Chloride for Chromium Electroplating Liyuan Sun and Joan F. Brennecke

Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, USA

Results & Discussion

Figure 1. Temperature dependent density of: (A) 1:2 CrCl3·xH2O/[Ch][Cl], (B) 1:2

CrCl3·xH2O/[emim][Cl], (C) 1:2 CrCl3·xH2O/[bmim][Cl], and (D) 1:2 CrCl3·xH2O/[hmim][Cl]

IL Crmetal(at.%) CrOx(at.%)

[Ch][Cl] 70.6 29.4

[emim][Cl] 85.1 14.9

[bmim][Cl] 80.4 19.6

[hmim][Cl] 62.2 37.8

Experimental Structure of ILs used in this study

Choline chloride (ChCl) 1,3-diakyl-imidazolium chloride ionic liquid ([Cnmim]Cl)

(n = 2, 4, 6)

Experimental apparatus for characterization and electroplating

Anton Paar automated microviscometer http://www.paar.ru/files/vyazkost/vizkozimetr

Anton Paar densitometer Solartron electrochemical

impedance interface

Voltammetry and

electroplating cell

Radiometer Analytical

VoltaLab50 potentiostat http://www.radiometer-analytical.com7

Helios Nano Lab Dual Beam

(FEI) SEM/FIB workstation http://www.fei.com/products/dualbeams/helios-

660/?ind=MS

X-Ray Photoelectron

Spectrometer http://mcf.nd.edu/instruments-and-

capabilities/#XPS

Results & Discussion

Figure 2. Arrhenius plots of temperature dependent ionic conductivities

of: (A) 1:2 CrCl3·xH2O/[Ch][Cl], (B) 1:2 CrCl3·xH2O/[emim][Cl], (C)

1:2 CrCl3·xH2O/[bmim][Cl], and (D) 1:2 CrCl3·xH2O/[hmim][Cl]

Figure 3. Arrhenius plots of temperature dependent viscosity of:

(A) 1:2 CrCl3·xH2O/[Ch][Cl], (B) 1:2 CrCl3·xH2O/[emim][Cl], (C)

1:2 CrCl3·xH2O/[bmim][Cl], and (D) 1:2 CrCl3·xH2O/[hmim][Cl]

Figure 4. Walden Plots for different mixtures with same Cr: H2O ratio:

(A) 9H2O/Cr, (B) 12 H2O/Cr, (C) 15 H2O/Cr , and (D) 18 H2O/Cr

Figure 5. Cyclic Voltammograms recorded on glassy carbon electrode

in: (A) 1:2 CrCl3·xH2O/[Ch][Cl], (B) 1:2 CrCl3·xH2O/[emim][Cl], (C)

1:2 CrCl3·xH2O/[bmim][Cl], and (D) 1:2 CrCl3·xH2O/[hmim][Cl]

(υ=20 mV/s, 52 ºC)

Figure 6 (a). XPS spectra of coatings obtained on Cu substrate from

different mixtures: (A) 1:2 CrCl3 ·18H2O/[Ch][Cl], (B) 1:2

CrCl3·18H2O/[emim][Cl], (C) 1:2 CrCl3·18H2O/[bmim][Cl], and (D)

1:2 CrCl3·18H2O/[hmim][Cl] (-2.5 V vs. QRE, 52 ºC, 20 min)

Table 1. Atomic % of different chromium states in coatings

obtained from different mixtures onto Cu substrate.

Figure 6 (b). XPS spectra of coatings obtained from 1:2

CrCl3·18H2O/[bmim][Cl] onto different substrates: (E) brass, and

(F) zinc coated steel (-2.5 V vs. QRE, 52 ºC, 20 min)

lnσ = lnσ0 - EΛ/RT lnη = lnη0 + Eη/RT

Λη = constant

Conclusions

Physicochemical properties, including density, conductivity,

viscosity and ionicity, as well as the electrochemical behavior, of

a series of mixtures based on imidazolium chloride ILs and

chromium(III) chloride have been found to be influenced by the

relative amount of diluent water in the mixtures.

Coatings with comparable or even improved properties have been

successfully obtained through the series of mixtures.

The class of Cr(III)/ILs mixtures could be a promising alternative

to the conventional Cr(VI) containing aqueous baths for

chromium electroplating.

Fig 7(b). SEM image of coatings obtained from 1:2

CrCl3·18H2O/[bmim][Cl] onto : (E) brass and (F) zinc coated steel

Results & Discussion

Figure 7(a). SEM image of coatings on a Cu substrate, from: (A) 1:2

CrCl3·18H2O/[Ch][Cl], (B) 1:2 CrCl3·18H2O/[emim][Cl] (C) 1:2

CrCl3·18H2O/[bmim][Cl] and (D) 1:2 CrCl3·18H2O/[hmim][Cl]

(-2.5 V vs. QRE, 52 ºC, 20 min)

Abstract A series of mixtures consisting of the ionic liquids (ILs) 1-

ethyl-3-methylimidazolium chloride, 1-butyl-3-

methylimidazolium chloride and 1-hexyl-3-methylimidazolium

chloride ([emim][Cl], [bmim][Cl] and [hmim][Cl], respectively)

and trivalent chromium chloride have been prepared.

Physicochemical and electrochemical properties of these

mixtures have been studied and the potential applications of

these mixtures for chromium electroplating, as an alternative to

the conventional hard chromium electroplating processes using hexavalent chromium baths,

have been examined. To optimize the transport properties of the mixtures, different amounts

of ultrapure water were added to the Cr(III) salt-IL mixtures. The physicochemical and

electrochemical properties of the mixtures have been found to be dependent on the relative

water content. Preliminary electroplating results show that these types of Cr(III) salt-IL

mixtures could be promising alternatives to Cr(VI) containing baths for chromium

electroplating applications with the advantage of avoiding the use of highly toxic hexavalent

chromium.

Acknowledgements We acknowledge the National Science Foundation for financial

support for this project. We thank Dr. Aruni Desilva for synthesizing

the [bmim][Cl] and [hmim][Cl] ILs and we thank Daniel Fagnant for

helpful discussions. We thank the Materials Characterization Facilities

(MCF) at Notre Dame for the use of the X-Ray Photoelectron

Spectrometer and we thank the Notre Dame Integrated Imaging

Facility (NDIIF) for the use of the SEM/FIB Workstation. We also

thank Professor Ryan Roeder in the Department of Mechanical

Engineering at Notre Dame for free access to the Vickers hardness

tester.