Electrochimica Acta 75(2012)339–346
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Electrochimica
Acta
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c t a c t
a项目化
Voltammetric study and electrodeposition of copper in 1-butyl-3-methylimidazolium salicylate ionic liquid
Po-Yu Chen ∗,1,Yu-Ting Chang
海阔天空日语版
Department of Medicinal and Applied Chemistry,Kaohsiung Medical University,Kaohsiung 807,Taiwan
a r t i c l e i n f o Article history:
Received 12March 2012
Received in revid form 3May 2012Accepted 3May 2012
Available online 12May 2012Keywords:Copper
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Electrodeposition Ionic liquid
Voltammetric behavior Salicylate
a b s t r a c t
The voltammetric behavior of cuprous ions has been studied at disk electrodes of glassy carbon and poly-crystalline platinum in a new hydrophilic 1-butyl-3-methylimidazolium salicylate ionic liquid (BMI-SAL IL).Cuprous ions Cu(I)were introduced into the IL by the anodic dissolution of a Cu wire electrode or by the addition of CuCl.This air-and water-stable hydrophilic IL is very similar to dicyanamide (DCA)-bad ILs becau both DCA anions and salicylate (SAL)anions have very strong ligand properties;therefore,many metal salts,such as metal halides,are very soluble in this IL.However,the source of SAL anions,sodium salicylate,is considerably cheaper than sodium dicyanamide salt.The potentiostatic electrode-position of copper onto iron substrates has also been investigated in this study.The surface morphologies of the copper deposits were significantly altered by the electrodeposition potential;granular,mirror-like,and porous copper deposits could be obtained at different negative potentials.The activity of the cop-per deposit-modified iron electrodes toward the electrochemical nitrate reduction in alkaline solutions was found to significantly depend on the surfac
介意什么意思
e morphologies.The granular and the porous surfaces showed better activities.The mirror-like surface,however,had no activity on the electrochemical nitrate reduction.
©2012Elvier Ltd.All rights rerved.
1.Introduction
It has been well established that ionic liquids (ILs),especially the various air-and water-stable ionic liquids,are good electrolytes for the electrodeposition of metals,alloys,and miconductors [1,2]becau of their many special and adjustable physicochemi-cal properties,such as wide electrochemical windows,no volatility,electrochemical and thermal stability,and a wide temperature over which they are liquid.Compared to their ancient family,the metal halide-bad ILs,modern ILs are more easily handled.Zein El Abedin and Endres reported that ILs can be regarded as the miss-ing link between aqueous/organic solutions and high temperature molten salts [3].Specifically,the previous studies indicated that the co-deposition of two metals with widely parated reduction potentials is more feasible in some specific ILs than in aqueous solutions [4,5]becau the wide potential paration is obviously narrowed,or even overlapped,in tho ILs.Although ILs exhibit many advantages in electrodeposition,most ILs are still expensive,which might limit their realistic app
lications.In addition,many air-and water-stable ILs are actually not ideal solvents for common metal salts such as halides,sulfates,and nitrates becau of the
∗Corresponding author.Tel.:+88673121101x2587;fax:+88673125339.E-mail address:pyc@kmu.edu.tw (P.-Y.Chen).1
ISE member.poor solubility of the salts.The metal salts,however,are fre-quently ud for electrodeposition.For example,many metal salts are insoluble in bis(trifluoromethylsulfonyl)imide (TFSI)bad-ILs,which are perhaps the most popular ILs currently in u.Metal ions are usually introduced by the anodic dissolution of the rele-vant metal electrodes or by the addition of metal-TFSI salts (MTFSI),such as Cu(TFSI)2[6,7].Anodic dissolution can be a time-consuming process.The MTFSI salts,such as Cu(TFSI)2,are usually prepared from other compounds,and drying the MTFSI salts may some-times be a challenge.Another limiting factor for the application of TFSI-bad ILs in electrodeposition is the current price of LiTFSI salt,which is commonly ud for preparing TFSI-bad ILs.There-fore,the appearance of dicyanamide (DCA)-bad ILs [8]appears to provide another alternative for the application of ILs for elec-trodeposition.It has been demonstrated that many metal halides are very soluble in DCA-bad ILs and that electrodeposition is pos-sible [9].More importantly,the source of the DCA anions,sodium dicyanamide (NaDCA),is a commercial pr
oduct,and its price is only about one-fourth of the price of LiTFSI.However,finding low-cost ILs in which common metal salts are soluble is an important task.
Trioctylmethylammonium salicylate IL (TOMA-SAL)was first prepared by Egorov et al.[10]and has been employed for the extraction of organic compounds and metal ions with or without additional coordinating ligands [10–12].The reports indicate that the salicylate anion has very strong ligand properties.The SAL-bad ILs should therefore be promising ILs for the dissolution of
0013-4686/$–e front matter ©2012Elvier Ltd.All rights rerved./10.1016/j.electacta.2012.05.024
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Scheme1.The simplified synthetic procedures for BMI-SAL IL.
metal ions.In addition,sodium salicylate(NaSAL),which is ud to prepare the TOMA-SAL IL,is currently much cheaper than NaDCA (∼1/7of the price of NaDCA).
The electrochemistry of copper species has been studied in veral air-and water-stable ILs.CuCl,however,is insoluble in1-ethyl-3-methylimidazolium tetrafluoroborate(EMI-BF4)but became soluble when an excess of chloride ions was introduced to form the chloride-rich IL(denoted as EMI-Cl-BF4),and the electrodeposition of Cu was achieved by the reduction of Cu(I) species[13].Cu(II)species have been studied in trimethyl-n-hexylammonium TFSI IL(TMHA-TFSI)using Cu(TFSI)2as the Cu(II) source[6,7].CuCl shows very limited solubility in1-butyl-1-methylpyrrolidinium TFSI IL(BMP-TFSI),and the Cu(I)species was introduced by the anodic dissolution of a copper electrode for the electrodeposition study[14,15].
生物知识点
The electrodeposition of copper is important for electronics applications[16,17]and in the electrocatalytic reduction of nitrate ions[18,19].Although SAL-bad ILs may be a promising electrolyte for the electrodeposition of metals such as copper,the relevant study of this IL system for t
he electrodeposition of metals is very rare.Therefore,to investigate the utility of SAL-bad ILs,the new hydrophilic air-and water-stable1-butyl-3-methylimidazolium salicylate IL(BMI-SAL)(Scheme1)was prepared and ud for the voltammetric study and electrodeposition of copper.Copper halides and sulfates are soluble in this IL becau of the strong lig-and properties of the SAL anion.The dependence of the surface morphology and the activity of copper deposits toward the nitrate reduction on the electrodeposition potential were also studied. 2.Experimental
The1-butyl-3-methylimidazolium salicylate ionic liquid(BMI-SAL IL)was prepared from1-butyl-3-methylimidazolium chloride (BMI-Cl)and sodium salicylate(NaSAL)(Alfa Aesar,99%)by fol-lowing the similar metathesis reaction ud for the preparation of1-butyl-3-methylimidazolium dicyanamide IL(BMI-DCA)[20]. However,dried acetonitrile(AN)was ud as the solvent rather than acetone.The NaCl precipitates produced during the metathe-sis were removed byfiltration.The BMI-Cl was prepared using published procedures[21],but chlorobutane was ud in place of bromobutane in this study.The simplified procedures for the prepa-ration of BMI-SAL are shown in Scheme1.The as-prepared BMI-SAL was dried under vacuum at120◦C for at least1day using a diffusion pump.The water content of the IL was determined using a coulo-metric Karl–Fischer titrator(Metrohm756KF),and the value was approximately4ppm.
The voltammetry and the potentiostatic electrodeposition of copper were performed using a potentiostat/galvanostat(Princeton Applied Rearch,PAR263A)under a purified nitrogen atmosphere in a glove box(MBRAUN,UNI-LAB B),where the moisture and oxygen contents were kept below1ppm.For the electrochemical study,Cu(I)was introduced into the BMI-SAL IL by the anodic dis-solution of a copper wire electrode(Alfa Aesar,99.9%and1.0mm∅) or by the addition of CuCl(Alfa Aesar,99.999%).A traditional three-electrode electrochemical cell with one compartment was ud for all electrochemical experiments.Inside the glove box,the ref-erence electrode was Ag/AgCl in1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide IL(BMP-TFSI)dissolved with1M 1-butyl-1-methylpyrrolidinium chloride(BMP-Cl)in a glass tube with a porous Vycor tip.A platinum spiral immerd in BMP-TFSI IL and parated from the bulk solution by a porosity E glass frit was ud as a counter electrode when the electrochemistry of the neat IL was studied.For the voltammetric study,the working elec-trode was a glassy carbon disk electrode(3mm∅)or a platinum disk electrode(1.6mm∅).For the electrodeposition,a piece of iron wire (conductive area=0.079cm2)was ud as a working electrode.A copper spiral immerd directly into the solution was ud as the counter electrode for the voltammetric study and the electrode-position of copper.In an aqueous solution,Ag/AgCl(in saturated NaCl solution)and a platinum spiral were ud as the reference and counter electrodes,respectively.
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The copper-electrodeposited iron electrode(Fe/Cu)was char-acterized using a SEM(FEI Quanta400F environmental scanning electron microscope(ESEM)or Philips XL-40FEGfield-emission scanning electron microscope(FESEM)and EDS(energy dispersive spectrometer)coupled with the SEM.The crystalline structure of the electrodeposited copper was analyzed using XRD(Shimadzu XD-D1X-ray diffractometer).The catalytic activity of the Fe/Cu electrodes for the electrochemical reduction of nitrate ions was studied in a1M NaOH solution containing various concentrations of NaNO3(SHOWA,99%).
3.Results and discussion
3.1.The electrochemical window of BMI-SAL IL
Becau the BMI-SAL is a new IL,developing a detailed descrip-tion of the electrochemical window of this liquid is esntial.Fig.1 shows the cyclic voltammograms(CVs)of BMI-SAL recorded at a glassy carbon disk electrode(GC electrode)and a platinum disk electrode(Pt electrode)at70◦C.Although BMI-SAL is liquid at room temperature,a higher working temperature was ud to obtain
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Fig.1.CVs recorded at GC and Pt disk electrodes in BMI-SAL at 70◦C.Arrows indicate the initial directions of the potential scan.Scan rate:50mV s −1.
higher mass transfer efficiency.The electrochemical window is only slightly wider than 2V regardless of which electrode was ud,but the current density in the potential limits significantly depends on the electrode materials.A retardation of electron transfer was obrved at the GC electrode becau a lower current density was obrved at the potential limits.In Fig.1,some small oxidation and reduction waves are obrved within the electrochemical win-dow.The redox waves resulted from the oxidation or reduction of the species produced in the anodic or cathodic limit.The nar-rower electrochemical window of BMI-SAL compared with the 1-ethyl-3-methylimidazolium dicyanamide (EMI-DCA)[8,9]and the BMI-DCA ILs [22]should be due to the lower anodic limit of the BMI-SAL IL becau the cathodic limit is usually determined by the organic cations (EMI +or BMI +)and the anions (DCA −or SAL −)usually determine the anodic limit.Although the anodic limit of the BMI-SAL IL is lower,this IL is still good for electrodeposition becau the cathodic limit is more important for the electrodeposi-tion of metals.Note that a passivating behavior was obrved at the anodic limit.An insulating polymer might be formed on the elec-trode surface [23,24].Therefore,the counter electrode ud for the BMI-SAL voltammetric study contained BMP-TFSI rather than BMI-SAL becau the pla
tinum spiral ud as the counter electrode was easily passivated and lost conductivity when BMI-SAL was ud in the counter electrode.For the voltammetric study and electrodepo-sition of Cu,the counter electrode was a Cu spiral that was directly immerd into the IL to compensate for the concentration change of
Table 1
Results of the anodic dissolution of Cu electrode in BMI-SAL ionic liquid. m (g)105n (mol)Q theory (n =1)(C)Q exp (C)n a 0.0025 3.93 3.79 3.580.940.0026 4.09 3.95 4.01 1.020.0019
2.99
2.88
2.91
1.01
a
Average value of n is 0.99±0.04.
Cu(I).Therefore,no passivation problems were encountered during the voltammetric study and electrodeposition of Cu.
The BMP-SAL IL has also been prepared,and a higher cathodic limit was obrved.However,BMP-SAL IL is more viscous than BMI-SAL.Therefore,this study focus on the potential utility of BMI-SAL for electrodeposition.
庆祝图片3.2.Studies of cyclic voltammetry and electron absorption spectroscopy for Cu(I)
The voltammetric behavior of Cu(I)species and the electrodepo-sition of copper were studied to evaluate the potential utility of the BMI-SAL IL for electrodeposition.The Cu(I)species were introduced into the IL by the anodic dissolution of a copper wire electrode or from the addition of CuCl.
The int of Fig.2a shows the CV of a copper wire electrode in the BMI-SAL IL at 70◦C.It is obvious that the copper electrode was oxidized when the applied potential was more positive than −0.6V,indicating that the anodic dissolution of copper is possible.The small reduction wave in the reverd scan is becau the Cu species that were dissolved during the forward scan (the anodic scan)were redeposited.To determine the oxidation state of the dissolved Cu species,the changes in t
he mass of the Cu electrode were measured after each passage of a given charge,Q exp ,at the applied potential of −0.4V.The data obtained from veral of the controlled-potential coulometric experiments are prented in Table 1. m is the change in mass of the Cu electrode,n is the number of moles related to this mass change,and Q theory (n =1)is the charge bad on n for n =1(where n is the number of electrons transferred during the anodic dissolution process).The experimental results shown in Table 1indicate that Cu(I)species were produced during the anodic disso-lution of a Cu electrode in BMI-SAL IL.This result is not surprising becau Cu(I)is the common product of the anodic dissolution of copper in many ILs [1,2,14].Fig.2a and b prents the CVs recorded at the GC and Pt electrodes,respectively,in the BMI-SAL containing 50mM Cu(I)produced from the anodic dissolution of a Cu electrode.The potential was initially scanned from the open circuit poten-tial in the cathodic direction.Two redox couples (c 1/a 1and c 2/a 2)were obrved at each electrode.However,the cathodic wave,c 1,was only obrved during the backward scan when the GC elec-trode was ud,which results in an apparent current loop.This behavior implies that a kinetic retardation of the nucleation pro-cess occurred at the GC electrode.At the Pt electrode,no current loop was obrved and a typical redox couple corresponding to metal ion reduction and deposited metal reoxidation was recorded.However,an additional cathodic prewave,c 1 ,was obrved at the Pt electrode,and this prewave is attributed to the under potential deposition (UPD)of copper,which has
been reported for the Cu(I)reduction at the Pt electrode in other ILs [13].Bad on the elec-trodeposition experiment that will be discusd later and the int of Fig.2a,the redox couple,c 1/a 1,at both electrodes is assigned to the reaction of Cu(I)+e −↔Cu becau Cu was obtained if the deposition was performed at c 1,and a 1was obrved at the same potential where the copper electrode was oxidized.Another redox couple,c 2/a 2,is assigned to the redox reaction of Cu(II)+e −↔Cu(I).The oxidation of Cu(I)at the oxidative wave a 2to Cu(II)species was confirmed by an experiment of bulk electrolysis that was
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Fig.2.CVs recorded at GC and Pt disk electrodes in BMI-SAL containing50mM Cu(I)from the anodic dissolution((a)and(b))or50mM CuCl((c)and(d))at70◦C.Scan rate: 50mV s−1.Int shows the CV of a Cu wire electrode in BMI-SAL at70◦C.
performed using a Pt foil electrode in BMI-SAL containing Cu(I) from the anodic dissolution.By comparing the charge pasd during the bulk anodic electrolysis with the charge spent on the produc-tion of the Cu(I)species,the oxidation of a2was determined to be an one-electron transfer reaction.The small,broad c2wave indi-cates that the reduction of the Cu(II)species is kinetically hindered. CuCl2is easily soluble in this IL.The CV of CuCl2in the BMI-SAL has been studied(data not shown),and two redox couples,as shown in Fig.2,were obrved.A more defined c2wave was obrved, which was a very broad and low wave.Bad on the slow kinetics of the Cu(II)species,it was determined to u Cu(I)species for the electrodeposition.
Cu(I)can also be introduced into the BMI-SAL IL by the addi-tion of CuCl.In addition,many metal halides and metal sulfates show good solubility in this IL(CuSO4is soluble in this IL,and the same redox couples are obrved).Fig.2c and d prents the CVs recorded at the GC and Pt electrodes,re
spectively,in the BMI-SAL containing50mM CuCl.Two redox couples similar to tho shown in Fig.2a and b were obrved,and the UPD wave c1 also appeared at the Pt electrode.The CVs recorded in different Cu(I)solutions did not exhibit a noticeable potential shift.This phenomenon indicates that the coordinating spheres of the Cu(I)species introduced from different sources are identical in the BMI-SAL IL,implying that Cl−anions were completely substituted with SAL anions and that the Cu(I)species should be the[Cu I(SAL)x]1−x complex ion.
To further verify that the coordinating spheres of the Cu(I) species were independent of their sources,electron absorption spectroscopy measurements were performed.However,only the Cu(II)species dissolved in the BMI-SAL IL were studied becau Cu(I)species do not have a d–d transition absorption.Fig.3illus-trates the electron absorption spectra of the neat BMI-SAL IL and the ILs containing Cu(II)species introduced by the anodic dissolution or the dissolution of CuCl2.Becau of the aromatic SAL anion,the BMI-SAL IL shows strong absorption in the shorter wavelengths of the electromagnetic spectrum.The d–d transition absorption of the Cu(II)species is obrved in the region between550and800nm. There are no noticeable differences between the absorption spectra of the two Cu(II)solutions,which indicates that the coordinating spheres of the Cu(II)species in the two solutions are identical.This conclusion supports the obrvation from the cyclic voltammetric
study.
Fig.3.Electron absorption spectra of neat BMI-SAL and BMI-SAL containing Cu(II) from different sources.The concentration of Cu(II)species is5.8mM.
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Fig.4.SEM micrographs of Cu electrodeposited onto Fe wire electrodes at70◦C from BMI-SAL containing50mM Cu(I)from the anodic dissolution.The electrodeposition potential is indicated in each micrograph.
3.3.Potentiostatic electrodeposition of copper
It has been demonstrated that the cheap and commercial SAL anions have very strong ligand properties and that metal halides are soluble in the BMI-SAL IL.Compared to other air-and water-stable ILs with commercial anions,SAL-bad ILs should be more promis-ing electrolytes for the electrodeposition from the perspective of lower cost and convenience of preparation for the electrodepositing baths.However,SAL-bad ILs are more viscous than the common 1-methyl-3-alkylimidazolium-bad ILs.To evaluate the potential utility of the BMI-SAL IL for electrodeposition,the potentiostatic electrodeposition of copper was performed at the Fe wire elec-trodes in the IL containing50mM Cu(I)produced from the anodic dissolution or100mM CuCl.The CV of Cu(I)recorded at the Fe wire electrode was very similar to that obrved at the Pt elec-trode except no UPD wave was obrved.The conductive area of every Fe wire electrode was limited to0.
079cm2,and200mC of charge was accumulated during the electrodeposition.Before the electrodeposition was performed,the Fe wire was cleaned by first soaking in acetone,then concentrated hydrochloric acid,and finally doubly deionized water,and then transferred immediately into the glove box.Fe was chon as the substrate becau it does not show redox behavior in strong alkaline solutions[5],and the Cu-electrodeposited Fe wire electrodes(Fe/Cu)were ud for the nitrate reduction in NaOH solutions.
Fig.4shows the SEM micrographs of the Cu coatings that were electrodeposited at different applied potentials(as indicated in each micrograph)from the BMI-SAL IL containing50mM Cu(I)
