Metals 2016, 6, x doi: FOR PEER REVIEW www.mdpi.comjournal metals [626496]

Metals 2016, 6, x; doi: FOR PEER REVIEW www.mdpi.com/journal/ metals
Artic le 1
Magnesium -Gold Alloys Formation by 2
Underpotential Deposition of Magnesium onto Gold 3
from Nitrate Melts 4
Vesna S. Cvetković1*, Niko Jovi ćević2, Jasmina S. Stevanovi ć1, Miomir G. Pavlović1, Nataša M. 5
Vukićević1, Zoran Stevanovi ć3, Jovan N. Jovićević1 6
1 Institute of Chemistry, Technology and Metallurgy, Department of Electrochemistry, Unevrsity of Belgrade, 7
Njegoševa 12, 110000 Belgrade, PAK 125213, Serbia; [anonimizat] 8
2 Nissan Technical Center North America, Inc.,39001 Sunrise Drive, Farmington Hills, MI 48331 -3487, USA ; 9
niko.jovicevic@nissan -usa.com 10
3 Mining and Metallurgy Institute Bor, Zeleni bulevar 35, 19210 Bor, Serbia; [anonimizat] 11
* Correspondence : [anonimizat] ; Tel.: + 381-11-3640 -228 12
Academic E ditor: name 13
Received: date; Accepted: date; Published: date 14
Abstract: Magnesium underpotential deposition on gold electrodes from magnesium nitrate 15
-ammonium nitrate melts has been investigated. Linear sweep voltammetry and potential step 16
were used as elec trochemical techniques. Scanning electron microscop y (SEM), Energy dispersive 17
spectrometry (EDS) and X -Ray diffraction (XRD) were used for character ization of obtained 18
electrode surfaces. It was observed that reduction processes of nitrate, nitrite and traces of water 19
(when present), in the Mg underpotential range studied, proceeded simultaneously with 20
magnesium underpotential deposition. There was no clear evidence of Mg/Au alloys formation 21
induced by Mg UPD from the melt made from eutectic mixture Mg(NO 3)2 x 6H 2O + (NH 4)(NO 3) x 22
XH 2O. However, EDS and XRD analysis showed magnesium presence in the gold substrate, and 23
four different Mg/Au alloys being formed, as a result of magnesium underpotential deposition and 24
interdiffusion between Mg deposit and Au substrate from the melt made of nonaqueous Mg(NO 3)2 25
+ (NH 4)(NO 3) eutectic mixture at 460 K. 26
Keywords: magnesium/gold alloys ; underpotential deposition ; magnesium nitrate melts 27
28
1. Introduction 29
For some time now gold (Au) has roused interest in the field of solid state chemistry, materials 30
science, optics, organic light emitting diodes. In semiconductor technology also, owing to its large 31
thermal and electrical conductivities , good oxidation resi stance, good workability, gold is popular 32
material for wiring. However, in efforts to improve bonding and strength of thinner wires cost 33
reduction is one of the many challenges [1,2] . 34
There are also gold–based catalysts, which due to large sized gold clusters made of closely 35
packed gold atoms represent a bridge between atomic state and bulk mat erial, have been developed 36
[3,4] and increasingly used in many industries [5–7]. Those gold –based metal catalyst are very active 37
at low temperatures and are almost certainly more active than any other equivalent noble metal 38
catalyst [5]. 39
There has been an increase in research to develop gold -based alloys resistant to air, water, 40
organic solvents, which would stand as an alternative to pure gold and be well suited for part icular 41
technological applications [1,2,7,8] . Among those gold based alloys are Au/Mg alloys which are 42
successfully used as cathode materials in organic light -emitting diodes, in organic field effect 43
transistors, and substrates for hydrogen adsorption/desorption [1]. 44

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One of the most elegant ways to obt ain alloys is an electrochemical deposition of a metal, or 45
metals, onto another metal [9,10] . 46
It was found that Au /Mg alloys can be obtained by Mg electrodeposition onto gold from 47
Grignard solution and this studies indicated that the alloys could be used as negative electrodes in 48
rechargeable magnesium battery systems [8,11] . 49
However, magnesium onto gold substrate cannot be obtained by electrodeposition from 50
aqueous solutions because hydrogen evolution on the working cathode starts at p otentials more 51
positive than magnesium deposition . This prevents even the smallest amounts of magnesium to 52
remain as a deposit on the cathode in an aqueous solution without being dissolved. In addition, Mg 53
cannot be deposited from solutions of simple Mg sa lts, such as Mg(ClO 4)4, in conventional organic 54
solvents [8]. Most likely because the working electrode surface is covered by passivating surface 55
films whose ionic conductivity is very low [8,12] . 56
The elect rodeposition of Mg and its alloys is done from melts. For electrodeposition of Mg, or 57
other metals with very negative standard electrode potentials (Al, etc.) , the melts based on chloride 58
salts at temperatures above 900 K are usually used [9]. Some of these melts include inorganic or 59
organic chloride and fluoride salts combined with an organic or alka line/alkaline earth metal cation 60
or anion [10,13–15]. In addition, new ionic liquids proved to be suitable media for electrodeposition 61
of metals and alloys at relatively low temperatures (from 273 to 373 K) [16–18]. 62
Possible usage of melts made with nitrates of alkaline and alkaline earth metals as electrolytes 63
[19–21] became a subject of interest a few decades ago [19–23]. Among a number of nitrate melts 64
investigated electrochemically, neither magnesium nitrate, nor magnesium nitrate/ammonium 65
nitrate mixture melts were studied. To our knowledge, electrodeposit ion of magnesium, magnesium 66
underpotential deposition and alloy formation from nitrate melts onto gold has not been reported. 67
Nitrate melts are electrochemically complicated media. Oxidative characteristics of nitrates at 68
elevated temperatures are well known. Detailed investigations of the processes at potentials both 69
positive or negative to the reversible potential of magnesium, led to the recognition that a great 70
number of oxidation/reduction processes with cations and anions present in nitrate melts can take 71
place [20,21] . Any of them can hamper or even prevent the process of magnesium electro deposition 72
from nitrate melts. Difficulties in maintaining the intended melt temperature variation below ±3 K 73
arise from the large latent heats of the numerous nitrates phase transformations in the temperature 74
range from 373 K to 500 K [22]. It should be noted that the presence of traces of water in magnesium 75
melts cancels the advantages of the melts that is inherent as compared to the magnesium aqueous 76
solutions. One of the reasons is that the backbone of magnesium nitrate hexahydrate octahedral 77
complex is magnesium cation [Mg(H 2O)6]2+, which is very stable and cannot loose water thermally 78
before it transforms into MgO. 79
It is known that metals electrodeposited by underpotential deposit ion (electrodeposition of a 80
metal onto foreign substrate at potentials more positive than the equilibrium potential of the 81
depositing metal – UPD) onto a cathode of a different metal, generally, can diffuse into the substrate 82
and generate alloys [9,24–28]. Alloys obtained by electrochemical deposition (overpotential 83
deposition – OPD and UPD) can have different chemical and phase structures than the alloys of the 84
same chemical composition obtained by metallurgical (thermal) methods [9,12,13,24 –28]. 85
The goal of this study, was to establish whether there is an underpotential deposition of 86
magnesium onto gold substrate from magnesium nitrate/ammonium nitrate melts, particularly one 87
made from nonaqueous Mg(NO 3)2 +(NH 4)(NO 3) eutectic mixture and is there possibility of Mg/Au 88
formation as a result of the process. This should help us to develop one of the easier ways to 89
formation of thermally stable surface magnesium/gold alloys. 90
2. Materials and Methods 91
The electrodeposition process was carried out in a three -electrode electrochemical cell made of 92
Pyrex glass placed in a heating mantle design ed for work with melts under a purified argon 93
atmosphere ( 99.99% Ar), [28,29] with temperature controlled (electronic thermostat) between 363 94
and 463 K ± 2 K. Central neck was closed with a Teflon plug carrying the working electrode (a 95

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99.999% Au plate 0.6 cm2), left neck with a Teflon plug holding an argon glass inlet -outlet and glass 96
Luggin capillary with magnesium reference electrode (3 mm diameter 99.999% Mg wire) whose tip 97
was placed close to the working electrode. Finally, right neck with Teflon plug holding magnesium 98
anode (99.999% Mg) in the shape of a curved rectangular shovel (7.5 cm2 active surface area) and a 99
tube of thin glass with a thermocouple. In order to create a moisture free atmosphere around the cell, 100
the whole cell setup wa s placed into a transparent plastic “glove box”. Special attention was paid to 101
the chemicals for the melt preparation. The melts used in experiments were: eutectic mixture 102
Mg(NO 3)2 x 6H 2O + (NH 4)(NO 3) x XH 2O and nonaqueous Mg(NO 3)2 + (NH 4)(NO 3) eutectic m ixture. 103
Precise quantity of the mixture Mg(NO 3)2 x 6H 2O and (NH 4)(NO 3) x XH 2O were place into the cell 104
supplied with the electrodes. Then, the closed cell was placed into the heating mantel, argon supply 105
was turned on and the system was heated gradually to the wanted temperature. However, it was 106
soon established, using SEM, EDS and XRD analysis, that Mg UPD onto gold from this melt showed 107
the deposit with less Mg than expected and no clear evidence of Mg/Au alloys. Therefore, further 108
Mg UPD processes have b een carried out from the melt made of nonaqueous 109
magnesium/ammonium nitrate eutectic mixture [Mg(NO 3)2 + (NH 4)(NO 3)]. 110
To this end we have succeeded in water removal from magnesium nitrate hexahydrate and the 111
process of melt made of nonaqueous magnesium/ammonium nitrate eutectic mixture [Mg(NO 3)2 + 112
(NH 4)(NO 3)] preparation has been described in details elsewhere [28–30]. 113
Prior to the electrochemical measurement, all used electrodes were mechanically polished by 114
emery paper (FEPA P -4000) to a mirror finish and then etched. Gold cathode (99.999% Au) was 115
etched in aqua regia (1:3 volume ratio HNO 3+HCl), for several ten seconds intervals separated by 116
rinsing with deionized water. Magnesium anode and reference electrod es (99.999% Mg) were etched 117
for about 20 -50 s in the solution made of conc.HNO 3 + conc.H 2SO 4+ deionized water (78.2 cm3 + 23.4 118
cm3 +989.4 cm3, respectively). After treatment, all electrodes were rinsed with deionized water, 119
absolute ethyl alcohol, dried an d mounted into the cell. 120
Two electrochemical techniques used in the experiments were: linear sweep voltammetry (LSV) 121
and potential step. The potentials of working electrodes were measured relative to the equilibrium 122
potential of magnesium reference electro de in the melt used under given conditions [28–30]. 123
LSV experiments included the potential scanned from a starting potential, E S (usually 50 to 100 124
mV more negative than the reversible potential of Au working electrode) to a final potential, E F 125
(0.050 – 0.100 V positive to the reversible potential of Mg) , followed by the return scan. The sweep 126
rates used were between 5 mVs-1 and 100 mVs-1. System responses were recorded by 127
Potentiostat/Galvanostat Princeton Applied Research Corporation Model 273A and accompaning 128
software (Princeton Applied Research ). 129
Procedure for the potential step method included change of the working electrode potential 130
from an initial potential, E I (50 to 100 mV more negative to gold equilibrium potential in the given 131
melt) to a potential, E X (50 to 100 mV more positive to magnesium equilibrium potential in the given 132
melt). E X potential was held constant for 120 and 600 minutes, whereupon the working electrode was 133
retrieved from the cell under potential in order to preserve deposited material or possible alloys 134
formed during UPD of magnesium. 135
The deposit on the gold electrode was washed in the glove box with absolute ethyl alcohol to 136
remove the melt residue. Then, the sample was transferred out of the box and kept without exposure 137
to the atmosphere. Scanning electron microscope (SEM) with an Energy dispersive spectroscopy 138
(EDS) (SEM – “JEOL”, model JSM -5800, Japan, EDS – “Oxford INCA 3.2”, U.K.) and energy 139
dispersive X -ray spectroscopy (EDX -maping – Oxford IncaEnergy EDX) were used to examine the 140
surface and the element analysis of the electrodeposits. The crystal structures of alloys were 141
characterized by XRD using (XRD – “Enraf Nonius powder diffractometer”, Germany). 142
3. Results and Discussion 143
Potentiodyn amic polarization curves and cyclic voltammetry measurements performed on gold 144
working electrode with magnesium reference and counter electrodes have shown that magnesium 145
reversible potential in the used magnesium nitrate melts was stable. The reversible p otential of 146

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polycrystalline gold in the magnesium/ammonium nitrate eutectic mixture melt was 1.532 ±0.025 V 147
vs. Mg at temperatures between 370 -440 K and in the nonaqueous magnesium/ammonium nitrate 148
eutectic mixture melt was 1.340V ± 0.0 30 mV vs. Mg at tempe ratures between 400 -500 K. 149
Examples of linear sweep voltammograms obtained on gold electrodes using different starting 150
and final potentials and temperatures are given in Fig. 1. a) and b). Common characteristics of the 151
voltammograms studied in magnesium n itrate melts system indicated presence of several reduction 152
peaks without oxidation counterparts, Fig.1a) and b). Even when the cathodic end potential, E F, was 153
pushed into the magnesium overpotential range, 154
Figure 1. Typical cyclic voltammograms recorded on Au electrode obta ined with scan rate 10 mV/s 155
in: a) magnesium/ammonium nitrate eutectic mixture melt and b) nonaqueous 156
magnesium/ammonium nitrate eutectic mixture melt, from the initial potential a) +1.300 and b) 157
+1.000 V vs. Mg to final poten tial a) -0.700 and b) -0.500 V vs. Mg; at: a) 420 K and b) 460 K 158
respectively . 159
160
stripping peaks were not observed. In some cases, reduction peaks were spread over a wider range 161
of applied potentials without obvious steeper increase or decrease of current d ensity. Similar 162
voltammograms peaks could be obtained for the processes that start successively one after the other 163
at potentials whose values are close and proceed further simultaneously (often next process starts 164
with increasing rate at the same time tha t a previous process ends with constant or diminishing 165
rate). It should be reiterated that the obtained current peak values at each potential recorded 166
represent the sum of rates of all the processes taking part at that particular potential. 167
Peak I in the voltammograms of Fig. 1a) reflects nitrate reduction processes which start around 168
0.900 V vs. Mg when water is present in the melt [28]. From voltammogra ms recorded in 169
nonaqueous eutectic mixture melt this current wave is almost absent, Fig.1b). A broader cathodic 170
peak II, appearing in the voltammograms obtained from both nitrate melts, presents next group of 171
reactions which have very close and more negati ve nitrate reduction potentials [20,21,28] . 172
Experiments conducted in nitrate melts when water was present, Fig 1.a), showed that nitrate anions 173
reduction processes take place successively in a potential range from rather positive values to those 174
close to 0.000 V vs Mg, where cathodic peak current density decreases close to zero. At slightly more 175
negative potentials to the previous (but still in Mg UPD region) another reduction current wave 176
appears in nonaqueous eutectic mixture (peak III Fig. 1. b)) and most probably belongs to the next 177
group of the nitrate reduction processes [20,21,28] . 178
The processes that could produce reduction peaks in the magnesium underpotential region 179
investigated and melts used, apart from ones brought about by the magnesium underpotential 180
deposition itself, can be found in research reports published some time ago [20,21] . Therein, the 181
potential s of the reactions are related to Na, K or Li reference electrodes and cannot be directly used 182
for comparison with our results. However, it can be assumed with enough certainty that the 183

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sequence of reaction potentials and the differences between the potent ials of therein proposed 184
processes are preserved in our systems also, but with respect to the magnesium reference potential. 185
Of course, absolute values of the reaction potentials discussed must be different by the amounts 186
reflecting differences in referenc e potentials of Na, K and Li in their nitrate melts [20,21] and 187
magnesium reference potential in used nitrate melts. 188
As described above, the change of the gold electrode potential from positive values to the 189
magnesium reversible potential value (magnesium UPD region), induces a number of reactions 190
which include formation of different nitrogen species, very reactive oxygen anion O2-, OH- and 191
sometimes water. All produced gases were removed from the electrochemical cell by the argon 192
stream, and oxidation reaction back to initial NO 3- or H 2O could no longer be expected when the 193
gold electrode potential is reversed into positive direction. Furthermore, O2- produced in inner and 194
outer parts of the electrochemical double laye r very quickly engages in reactions with Mg2+ present 195
[12,20,21,30,31] with the result being formation of insoluble magnesium oxides in the magnesium 196
UPD region. Additionally , the increase of reduction current densities observed in the 197
voltammograms in the magnesium nitrate melts with (NH 4)(NO 3), at constant temperature in the 198
magnesium UP region investigated, suggests reduction of ammonium ion [20,21] . Under given 199
conditions this reaction is, also irreversible. As a result, when the electrode potential was reversed 200
into the positive direction, n o anodic voltammogram peaks could be recorded. Observed absence of 201
anodic counterparts to cathodic peaks was a subject of discussion in a number of works 202
[12,20,21,30,31] . The studies emphasize that changes of the gold electrode potential from anodic to 203
cathodic values (compared to the magnesium reversible potential) in nitrate melts make the working 204
electrode surface become partially, or fully, cove red with MgO layers. These layers do not dissolve 205
when the potential is returned to the starting positive value and eventually passivate the electrode. 206
Therefore, it can be concluded that anodic voltammogram peaks cannot be expected in the 207
magnesium UP reg ion examined. 208
The absence of characteristic cathodic current peak in the voltamograms on gold electrode 209
around 0.100 V vs. Mg, depicting magnesium UPD from nitrate melts used, does not mean that there 210
was no magnesium electrodeposited [12–14,28–30]. It is logical to assume that the reduction peaks 211
obtained by LSV measurements on the gold working electrode from nonaqueous 212
magnesium/ammonium nitrate melt used in the magnesium underpotential region are sums of 213
partial current densities for: Mg2+ underpotential reduction, nitrate anion reductions and ammonium 214
cation reduction. Being a sum, the recorded current waves suggest small magnesium underpotential 215
deposition partial current densities. Such sma ll current densities exhibited by the UPD 216
voltammograms from similar melts [28,32,33] were characteristic of deposited metal monolayers 217
diffusing into the substrate and forming alloys. 218
SEM photographs, EDS and EDX results showed evidence of magnesium and magnesium oxide 219
on the surface of the gold electrode from b oth kinds of melts, but the XRD analysis (Fig. 5.a)) 220
revealed no clear evidence of Mg underpotential deposition and Mg/Au alloys formation from 221
magnesium/ammonium nitrate eutectic mixture melt with water present . 222
Fig. 2, 3, 4 and Table 1 display SEM, EDS and EDX results obtained from the gold electrode 223
held on electrode potential of 0.100 V vs. Mg in the melts made of nonaqueous eutectic Mg(NO 3)+ 224
(NH 4)(NO 3) mixture and of eutectic Mg(NO 3) x 6 H 2O + (NH 4)(NO 3) x XH 2O mixture at T = 400 K 225
and 460 K. SEM photographs obtained showed cube shaped agglomerates almost uniformly 226
distributed over entire electrode surface (Fig.2 a) in the first case, and significant roughens and 227
coarse morphology with a few nodules in the second case, Fig. 2 b). 228

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Figure 2. SEM micrographs of Au substrate after Mg electrodeposition from: a) 229
magnesium/ammonium nitrate eutectic mixture melt and b) nonaqueous magnesium/ammonium 230
nitrate eutectic mixture melt; at: a) 400 K and b) 460 K by using potentiostatic techniqu es. 231
232
Figure 3. Characteristic EDS spectra of gold samples after two hours time of magnesium 233
underpotential deposition from: a) magnesium/ammonium nitrate eutectic mixture melt and b) 234
nonaqueous magnesium/ammonium nitrate eutectic mixture melt; at: 400 K and 460 K respectively. 235
236
Table 1. Results of EDS semi -quantitative analysis for the Au s ubstrate s exposed to constant 237
potential of +100 mV vs. Mg for two hours at 400 K and 460 K in used melts . 238
Magnesium/ammonium nitrate
eutectic mixture at 400 K Nonaqueous magnesium/ammonium eutectic
mixture at 460 K
Element Element (%) Atomic (%) Element (%) Atomic (%)
O K 71.52 85.26 65.06 75.52
Mg K 17.42 13.67 24.96 23.5
Au M 11.06 1.07 9.98 0.98
Total 100.0 100.0 100.0 100.0
239
240

Metals 2016, 6, x FOR PEER REVIEW 7 of 4
241
Figure 4. EDX maps of Magnesium, Oxigen and Nitrogen distribution after two hours time of 242
magnesium underpotential deposition onto gold substrate from a) eutectic and b) nonaqueous 243
magnesium/ammonium nitrate eutectic mixture at: a) 400 K and b) 460 K. 244
Typical examples o f XRD analysis results of the gold sample exposed to the underpotential of 245
0.100 V vs. Mg in the melts mentioned above are presented in Figs. 5. a) T= 400 K, b) and c) T=460 K. 246
It was reveiled that there were Mg/Au alloys formed only during magnesium UPD o nto working 247
substrate from the nonaqueous melts. 248
EDS analysis of the same samples showed presence of magnesium in the surfaces of electrodes 249
exposed to magnesium UPD from both kinds of melts used, although the presence of oxides was by 250
far higher in the c ase of melt with water present. 251
A tentative explanation can be that the reduction of water, when present, produces hydroxides 252
and active oxygen which with Mg(II) ions present in the electrode double layer feed formation of 253
porous structures of the deposi ted oxides , Fig.6 . In addition reduced oxygen species probably 254
diffuse through pores into the deposit formed and with Mg2+ or nitrate ions create different 255
compounds which crystalized on the electrode surface. The presence of the agglomerations on 256
electrod e surface can probably be blamed for decrease in the concentration of free Mg2+ ions on the 257
gold surface which could participate in Mg/Au alloys formation. It is assumed, that in 258
magnesium/ammonium nitrate eutectic mixture due to moisture magnesium hydroxi de is formed 259
which could be transformed into oxides during oxidation processes at temperatures above 570 K. 260
The process depending on the value of the thermal decomposition constant, but in nitrate melts at 261
given temperature (400 K) seems that partial decom position of Mg(OH) 2 is taking place and is 262
followed by (NH 4)(NO 3) dissociation, which appears in Mg 3(OH) 4(NO 3)2 and 263
Mg 2(OH) 3.14(NO 3)0.86(H 2O)0.19 formation (Fig.5a)). It looks like that compounds are solid at given 264
temperature (400 K) and its precipitation on the electrode surface could cause pseudo -passivation. 265
The formation of the passivating layer on the electrode surface may work as a barrier p reventing Mg 266
adatoms to diffuse into the bulk of the gold substrate. 267
The XRD obtained spectra (Fig. 5.b) and c)) indicate Mg/Au alloy formation by magnesium 268
underpotential deposition from the nonaqueous melt used. The alloys identified are listed in Table 2. 269
270

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Figure 5. Diffraction pattern of gold sample after magnesium underpotential deposition at E x = 271
+0.100 V vs. Mg in: a) magnesium/ammonium nitrate eutectic mixture at 400 K for two hours time; 272
(*) – Mg 2(OH) 3.14(NO 3)0.86(H 2O)0.19; (+) – Au 4Mg, (-Mg 3(OH) 4(NO 3)2; (o) – MgO 2; b) and c) 273
nonaqueous magnesium/ammonium eutectic mixture at 460 K for two and five hours time; b) (*) – 274
Au 4Mg; c) (AuMg 3, (●) – Mg 2Au; (*) – AuMg. 275
Table 2. The alloys identified on gold samples after magnesium deposition at different temperatures 276
and different deposition times in nonaqueous magnesium/ammonium eutectic mixture . 277
Tem.
(K) Time,
τ/ h Identified
phase Structure Approximate composition
wt.% Au References
460 K 2 Au 4Mg Orthorhombic 94-97 [34]

5
AuMg
AuMg 3
Mg 2Au Cubic
Hexagonal
Orthorhombic 85-95
73-80
73-80 [35]
[36]
[37]
278
279

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280
Figure 6. Schematic representation of the processes taking part on the surface of Au cathode during 281
magnesium underpotential deposition and Mg/Au alloys formation in the melts made of 282
nonaqueous eutectic Mg(NO 3)+ (NH 4)(NO 3) mixture and of eutectic Mg(NO 3) x 6 H 2O + (NH 4)(NO 3) 283
x XH 2O mixture. 284
Mg and Au atomic radii differ only by 10% and according to the theory, the Hume/Rothery 285
„15% rule“ [38] Mg and Au fulfill required conditions to form alloys (solid solutions). The phase 286
diagram of Au –Mg system is very complex, showing a number of phases [39]. According to the 287
diagram [40] the maximum solid solubility of Au in Mg is ~0.1 at.% Au, an d the solid solubility of 288
Mg in Au has not yet been determined. Thermal analyses showed a wide range of solid solubility of 289
Mg in Au, and at temperatures above 1080 K in the composition range of 94 to 97 mass% Au, several 290
phases of different structures hav e been found in the system: surface centered Au 4Mg, base -centered 291
orthorhombic Au 3Mg, and even hexagonal Au 41Mg 13. According to the same diagram at slightly 292
lower temperatures (above 1053 K) in the composition range of 80 to 85 mass % Au, stoichiometric 293
intermetallic compound Mg 2Au is being formed and at even lower temperatures above 1000 K, 294
within 73 -80 mass % Au hexagonal Mg 3Au is registred. It was also showed that the ordered 295
nonstoichiometric intermediate MgAu phase has a wide homogeneity range includi ng several new 296
phases in the composition range of 70 to 80 at.% Au around the MgAu 3 composition [41] like the 297
stoichiometric Mg 5Au, or Mg 22Au 78, Mg 24Au 76 and Mg 26Au 74 [41]. 298
Some of the alloys described by the above cited literature [39,40] , have been formed in studied 299
magnesium underpotentia l deposition onto gold in used nonaqueous magnesium/ammonium 300
eutectic mixture, but at temperatures which are several hundred degrees Kelvin lower (see Table 2). 301
The main phase formed by the Mg UPD onto gold electrode at 420 K was Au 4Mg. The alloys formed 302
by magnesium underpotential deposition at 460 K were AuMg 3, Mg 2Au and AuMg. As would be 303
expected, increase in the deposition temperature enlarged solid state interdiffusion rates. Prolonged 304
UPD at constant temperature resulted in Mg c ontent increase in Au -Mg alloys which is recorded by 305
increased number of alloy phases observed. 306
Which fraction of magnesium adatoms formed by underpotential deposition on gold surface 307
participated in magnesium oxide formation and which diffused into the s ubstrate and contributed to 308
magnesium/gold alloys formation could not be concluded by linear sweep voltammetry, EDS or 309
XRD results. LSV results indicated, EDS and XRD results confirmed both the magnesium oxide and 310
magnesium /gold alloys formation at the sur face of the gold electrode in nonaqueous 311
magnesium/ammonium eutectic mixture used as a result of magnesium underpotential deposition, 312
Fig.5. The process of magnesium oxide formation from nitrate melts has been described elsewhere 313
[42–44]. In addition, some novel results [13,30,31] suggest that every amount of reactive magnesium 314
on the electrode surface in the presence of O2- and OH- anions very quickly becomes MgO. 315
Therefore, the surface of the gold working electrode becomes partiall y covered with MgO even in 316
the first linear change of the potential from anodic end to cathodic end of the magnesium 317
underpotential range. This, however, did not preclude enough of magnesium adatoms from 318
participating in magnesium -gold alloy formation by i nterdiffusion. Portion of magnesium ions in 319
MgO probably diffuses through the oxide layer to the gold surface where they become discharged 320
into magnesium adatoms which are then participating in the interdiffusion processes of alloy 321
formation. Fast and unav oidable formation of insoluble MgO in the magnesium underpotential 322

Metals 2016, 6, x FOR PEER REVIEW 10 of 4
deposition range on gold from used nitrate melts explains quasi -passivation of the working 323
electrode and the lack of anodic current peaks on the voltammograms recorded. 324
5. Conclusions 325
Magnesium was successfully electrodeposited onto gold electrode from nonaqueous 326
magnesium/ammonium eutectic mixture, at potentials positive to 0.100 V vs. Mg and temperatures 327
between 400 and 500 K. However, even a small quantity of water inhibited the Mg e lectrodeposition 328
from magnesium/ammonium eutectic mixture. 329
This is the first observation of Au -Mg alloys: AuMg 3, Mg 2Au, Au 4Mg and AuMg being formed 330
as a result of magnesium underpotential deposition onto gold substrate from magnesium nitrate 331
melts used. 332
Alloys obtained were formed at temperatures several hundred degrees Kelvin lower than the 333
temperatures which are needed for their formation by thermal means. 334
It appears that underpotential deposition of metals that are unsuitable for electrodeposition 335
from aqueous electrolytes, like magnesium, can be performed from nitrate melts at low 336
temperatures and that it can lead to the formation of alloys in a very controlled manner under more 337
technologically suitable conditions. 338
Acknowledgments: This work was supported by the Ministry of Education, Science and Technology of the 339
Republic of Serbia (Grant OI 172060 and Grant OI 172046). 340
Author Contributions: Vesna Cvetković and Nataša Vukićević performed most of the experiments and 341
participated in manuscript writ ing, Niko Jovicevic provided SEM and EDS analysis and part icipated in 342
manuscript writing, Jasmina Stevanovi ć and Miomir Pavlović provided XRD analysis and helped with the 343
corrections of the manuscript , Zoran Stevanovi ć contributed materials and participate d in manuscript writing, 344
Jovan Jovićević conceived the experiments and lead the manuscript writing. 345
Conflicts of Interest: The authors dec lare no conflict of interest. 346
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