- Email: [email protected]
Some effects of addition agents on the structure of silver deposits from the argentocyanide bath
Elecnochimica Acta, 196 ., Vol . 11, pp. 297 to 305 . Pergamon Press Ltd . Printed In Northern Ireland
SOME EFFECTS OF ADDITION AGENTS ON THE STRUCTURE OF SILVER DEPOSITS FROM THE ARGENTOCYANIDE BATH T . H . V . SETTYt and H . WILMAN Department of Chemical Engineering and Applied Chemistry, Imperial College, London, S .W .7, England Abstract-Electron-diffraction technique has been used to study the modification of surface structure of silver electrodeposits deposited from the argentocyanide bath in presence of addition agents such as carbon disulphide, thiosemicarbazide, thiourea, and urea on (110) (111) and (100) faces silver single crystals. It is found that in many of these cases there is pronounced effect on the deposit crystal habit and there is strong development of {Ill } faces on the (110) silver face . It appears that the brightening effect of these agents must be connected with their pronounced effect on the crystal habit leading to flat facets of relatively large extent and high smoothness compared with the wave length of light, rather than with any marked decrease of grain size . Optical micrographs are also shown to illustrate the deposit surface form . Rfsum€-Utilisation de la diffraction d'€lectrons en vue d'6tudier ]a modification de structure superficielle d'6lectro-dAp6ts d'Ag, 8 partir de bains Ag(CN)s en presence d'agents additionnels tels que sulfure de carbon, thio-semi-carbazide, thio-u , ur€e et sur les faces (110) (111) (100) de monocristaux Ag . Leur effet sur la forme du depot est prononca dens certains call, oil I'on observe on d6veloppement notable des faces (111) sur les faces (110) . Il apparait que de telles influences sur la forme sent en relation avec le brillant que confa rent ces agents, des facettes plates d'8tendue relativement grande presentant un haut degr6 de lissage comparativement aux longueurs d'ondes lumineuses, sans diminution sensible de 1a grosseur des grains. Des micrographies optiques en rendent compte d'autre part . Zusammenfassung-Modifikationn der Oberflachenstruktur von Silberniederschla gen aus Silbercyanid-Badern in Gegenwart von Zusatzen wie Schwefelkohlenstoff, Thiosemicarbazid, Thioharnstoff and Harnstoff auf (110), (111) and (100)-Flachen von Silber-Einkristallen warden elektronendiffraktographisch untersucht . Es wurde gefunden, dass in vielen Fallen ein starker Effekt auf die Kristallstruktur des Niederschlages auftritt and auf den (110) Oberflachen eine ausgepragte Entwicklung von [111] Flachen stattfindet. Man schliesst daraus, dass die Glanzwirkung dieser Zusatze im Zusammenhang stehen muss mit ihrer ausgepragten Wirkung auf die Kristallstruktur, indem sie die Ausbildung flacher Facetten von relativ grosser Ausdehnung and kleiner Rauhigkeit (verglichen mit der Wellenlange des Lichts) bewirken, eher als eine merkliche Kornverkleinerung. Es warden Mikroaufnahmen gezeigt, um die Oberflachenstruktur der Niederschlage zu verdeutlichen .
QUITE small
concentrations of an addition agent such as carbon disulphide, added to
an electrolytic bath, often cause large changes in deposit structure (crystal orientation, size and habit) and correspondingly affect the physical properties (hardness, brightness,
etc) of the deposit. The mode of action of addition agents in modifying the structure of electrodeposits from a given bath has not hitherto been adequately determined . The work of Setty and Wilman' correlating the structure of silver deposits on silver (110), (111) and (100) faces, has now been extended to the investigation of the modification of the silver deposit structure when deposits are prepared from a bath containing 35 g/I AgCN, 30 g/1 KCN and
38 g/1 K 2CO3,
with small amounts of typical
addition agents used in silver electroplating carbon disulphide, thiosemicarbazide, thiourea and urea . In many of these cases there was a pronounced effect on the deposit crystal habit and on the range of conditions under which a twinned structure was * Manuscript received 18 February 1965 . t Present address : Department of Chemistry, Central College, Bangalore, India . 297
298
T.
H . V. Sear and H . WILMAN
obtained . On the other hand, 1 % of potassium thiocyanate did not cause any appreciable effect. EXPERIMENTAL TECHNIQUE The details of preparation of smooth electropolished (110), (111) and (100) faces and of the silver deposits on these faces have already been described .' For each electrodeposition 200 ml of fresh argentocyanide solution was used to which had been added the desired amount of addition agent . Each experiment was repeated several times to ensure representative results . For electron-diffraction examination, 50-60 KV electrons and a camera length of 21 .5 or 48 cm . were used . A Vickers projection Microscope was used for the optical microscopic examination, using a magnification of x 600 . RESULTS In all the following results the current efficiency was found to be 100% or practically so . 1 . The effect of carbon disulphide Shaking the argentocyanide solution with a drop of carbon disulphide resulted in precipitation of a black deposit which was identified from its electron-diffraction pattern as the normal monoclinic, practically orthorhombic, Ag 2 S having a = 9 . 47, b = 6 . 92, c = 8 .28 A and /9 = 124° (Ramsdell) 2 Filtering the solution after 2 h left the filtrate clear, but again the black precipitate was formed on further standing . The standard procedure adopted was therefore to allow the solution to stand for 24 h, filter it, and then immediately use the filtrate for the electrodeposition of silver . The structure of the silver deposits investigated on Ag (110) is shown in Table 1 . Ag DEPOSITS ON Ag (110) FACE Surface orientation and habit of 10,000 A Ag deposits
TABLE 1 . EFFECT OF ADDITION AGENTS ON
Addition Agent None CS,
20'C
1 niA/cm' P, (110), T, (110)
10 mA/cm'
r[001]
60'C 20
mA/cm'
R
R
T(+ R), (110)
R
20 mA/cm'
r[001] p, • (110), r[001] p, (110),
Thiosemicarbazide 5 g)1
P, {lll)1
One-degree {21 l) Orientation
Thiourea IO and 30 gp Urea, 10 and
P •, (110)
R
weak (110) P, (110), r[001]
50 g/1 KCNS, 10 g/I
r[001]t P, (110), r[001)
T(little P) R
P, (110), r[1T0] P, (110), r[001]
T,
111 r[110] (110),
P•,
• Twinned at
up to 2000 A . facets in deposits 1000-2000 A thick. r, long ridges parallel along the stated direction . l, lamellae . t {111)
(a) Deposits on the (110) Ag face . The 10,000 A thick deposits at 20°C had single-crystal orientation parallel to the substrate lattice, with extensive {111} twinning, even at 1 mA/cm2 (the diffraction pattern was similar to Fig . 4 of the previous paper) .' The microphotographs then showed closely-spaced projections about 0-6 ,um in
Effects of addition agents on the structure of Ag deposits
2 99
diameter, with no sign of any ridges such as were developed at this deposit thickness and deposition conditions in the absence of CS 2. In the electron-diffraction patterns the spots were not much elongated ; thus the surfaces of the microscopic projections were relatively rough, with projections of the order of 100-200 A in diameter. Randomly orientated crystals first occurred on 10,000 A thick deposits at current densities of 10 mA/cm2, as compared with at 5 mA/cm 2 in the absence of CS 2. At 60°C deposits up to 10,000 A thick showed single crystal orientation parallel to the substrate lattice with, however, a moderate proportion of {l11) twinning on deposits 1000 A thick, and only mere traces of twinning at 2000 A and more (Fig . 1(a)) . Atomically smooth (110) faces parallel to the initial substrate face were indicated by
0
son/
'
0
o
ccP
aep 4 .08A
Fro, lb . Arrangement of diffraction spots due to silver (110), [170] azimuth and silver sulphide. -0- silver spots . --x-- silver sulphide spots . Fto . 1c . Lattice spacings of silver crystal and silver sulphide .
the fact that the spots in the diffraction pattern were much elongated normal to the shadow edge . Long narrow closely-spaced ridges parallel to [001] began to be noticeable, about 3-10 um long (but somewhat beaded along their length) and 0-5 pm broad, on 2000 A deposits . On 10,000 A deposits there were ridges 10-20, um long (now rather smooth along their length) and 0-5 pm broad giving microphotographs similar to Fig . 3(a) of the earlier paper .' The form of the deposits was thus closely similar to that previously described for the deposits more than 2000 A thick under the same conditions in the absence of CS, . Figure 1(a) shows many spots identified as due to Ag 2S, in addition to the spots due to the silver. Fig . 1(b) shows the centred- //2- rectangle arrangement of the diffraction spots due to the silver, and one component rectangle pattern of spots due to the silver sulphide, together with the Laue indices of these diffractions relative to the above monoclinic cell . This indexing of the Ag2S spots shows that the Ag2 S crystals were orientated with (100) planes parallel to the (110) silver surface, with [010] parallel to the silver [110] row, and perpendicular to this direction [001] Ag 2S parallel to [001] Ag . Fig. 1(c) shows the relatively close lattice spacing correspondence along [001] Ag (2a AS = 8 16 A compared with CAI, s = 8-28 A, misfit 1 ,4Y.), and the still rough correspondence along [110] Ag (T 110 = 5-76 A compared with bAS¢S = 692 A, misfit 20%) . The presence of such epitaxially orientated Ag2S on at least some parts of the silver surface thus suggests that even in the other deposits where no Ag2S was detected, minute traces of Ag2S may nevertheless have been present and
300
T. H. V.
SETrY
and H.
WILMAN
been responsible for the observed (1111 twinning of the silver at low current density at20°C . (b) Deposits on the (100) Ag face . At 20°C, deposits 10,000 A thick had, at I mA/cma, single-crystal surface structure parallel to the substrate lattice, with also a trace of {111} twinning, but at 10 mA/cma rather more {111} twinning, whereas at both these current densities parallel growth twinning was found in the absence of CS.. The microphotographs showed closely-spaced projection of the order of 0-6 ,um diameter, ie the same as in absence of CS 2. At 60°C a 10,000 A deposit showed parallel single-crystal growth with moderate {111} twinning, and again the projections were about 0-6 ,um in diameter. TABLE 2. EFPECT OF ADDmON AGENTS ON
Ag
DEPOSITS ON
Surface orientation and habit of 10,000 Addition agent None CS, Thiourea 10 g/1 Thiosemicarbazide
Ag (I11)
FACE
A Ag deposits
20°C
60'C
1 mA/cm'
10 mA/cm'
20 mA/cm'
P, (111) P, (111) P, (111)
T, (111)
T,,,, (111) and {115} T,,,, (111) {115} • R
T,,, + R R
20
mA/cm'
P, (1ll) P(+T,), (I11) and (115) R + P, (111)
* also at 30 mA/cm' . twinning on all (111 } planes, T F , twinning only on (111) parallel to surface .
T,,,,
(c) Deposits on the (111) face . The deposit orientation and habit (Table 2) were the same as in the absence of CS.' and the projections also were unchanged in size, about 0 .6 pm diameter .
2. The effect of 5 g/l of thiosemicarbazide (NHaCS-NH-NH2) About 15 min after dissolving the solid thiosemicarbazide in the argentocyanide bath, a pale pink colour became noticeable, deepening with further standing, and after 30 min a black precipitate was formed which when dried on a collodion film in vacuo yielded electron diffraction transmission patterns of sharp rings (arcs when the film was inclined), identified as corresponding to normal monoclinic Ag 2S . After standing for 24 h, the precipitate had settled to the bottom of the solution, and the deep wine-red clear solution was decanted off and used for electrodeposition, on the (110) Ag face . Deposits 10,000 A thick at 20°C and 1 mA/cma yielded Fig . 2 when the beam was inclined 2° to [110] and patterns of a similar type also at the [001] azimuth . These patterns correspond to single-crystal orientation of the deposit parallel to the substrate lattice, but there are in general pairs of spots instead of each spot of the normal pattern, the spot separation decreasing with increasing distance from a circular locus (on which single points occur) centred at the point where the [110] row met the plate, and passing through the undeflected-beam spot . From the spot separations in relation to the spot positions, calculation indicated that the spots correspond to [111]* and [111 ]* extensions from the reciprocal-lattice points meeting the Ewald sphere . This therefore shows that the silver deposits were bounded mainly by (111) and (1 T T) planes, normal to the (110) substrate surface, with no evidence of any smooth
Fto . la . Silver deposited from the argentocyanide bath + CS, at 20 mA/cm°, 60"C, on the silver (110) face . Thickness 2000 . . [1T0] azimuth .
FIG.
2 . Silver deposited from the argentocyanide bath + thiosemicarbazide at 1 MA/cm' and 20°C on the silver (110) face 2" off from [ITO] azimuth . Thickness 10,000 A .
300
Fie. 3 . Microphotograph of the silver deposited from the argentocyanide bath -, thio . urea at I n A/cm° and 20'C on the silver (110) face . Thickness 10,000 A x 600.
Fio. 4. Silver deposited from argentocyanide bath + 1 % urea at I mA/em' and 20'C on the silver (110) face . [ITO] azimuth . Thickness 2000 A .
Fin . 5 . Microphotograph of the deposit obtained from the argentocyanide bath + 3 thiourea at 1 mA/cm', 20°C on the silver (110) face. Thickness 10,000A x 600 .
Fin . 6 . Microphotograph of the silver deposited from argentocyanide bath + l % urea at I mA/cms, 20°C on the silver (110) face . Thickness 10,000 A x 600 .
Fto. 7 . Silver deposited from the argentocyanide bath + 1 % urea at 10 mA/cm 2 , 20°C on the Ag (110) face. Thickness 10,000 A [lTO] azimuth.
Effects of addition agents on the structure of Ag deposits
301
(110) faces at the tips of the projections or of the other octahedral faces (111) and (111) . The large number of strong spots, and their sharpness, indicates that the projections must have consisted essentially of relatively thin sheets having faces of large extent parallel to one or other of these two {111} faces . The microphotographs showed only closely-spaced small projections about 0-6 ,um in diameter, with no indication of any long ridges such as were found in absence of the addition agent . At 10 mA/cm2 the 10,000 A deposits at 20°C gave diffraction patterns of long sharp arcs, indicating that the surface structure of the silver deposits was polycrystalline with preferred orientation such that on average {211} planes were parallel to the (110) substrate surface, with a spread of ±15° from the average . The microphotographs again showed closely-spaced projections about 0 . 6,um in diameter . At 60°C, 10,000 A deposits were found to be in single-crystal orientation parallel to the substrate, with much {lll} twinning, giving a diffraction pattern similar to Fig. 4 of the earlier results .' Again the microphotographs showed closely-spaced projections about 0 .6 µm in diameter. 3 . The effect of 10 g/1 and 30 g/l of thiourea (CS(NH2)2)
On dissolving the solid thiourea in the argentocyanide bath no colour was observed, and no precipitation, even on standing indefinitely . (a) Deposits on the (110) Agface, from the bath containing 10 g/1 thiourea . At 20°C and 1 mA/cm2 a 1000A deposit gave a single-crystal orientation parallel to the substrate lattice but with also much {Ill} twinning . The spots in the electrondiffraction pattern including those due to the twins were much elongated normal to the shadow edge, indicating smooth (110) faces parallel to the initial (110) substrate surface . At the [0011 azimuth the spots had arrow-head form, having faint but long and narrow extensions at f 19° to the plane of incidence, indicating the presence of {120} facets in addition to the main (110) face . The microphotograph showed small closely spaced projections about 0-7 ,um in mean diameter . At 10,000 A thickness, deposits at 1 mA/cm 2 yielded microphotographs similar to that of Fig . 3, showing closely spaced ridges (about 11 µm long and 0 .6 pm broad) along [110] instead of ridges along [001] as found in absence of thiourea but otherwise the same conditions' The ridges were just beginning to be noticeable (along [110]) on deposits 5000 A thick, and correspondingly the diffraction patterns showed single-crystal orientation parallel to the substrate, with arrow-head-shaped spots indicating presence of (110) and also {111} facets, and only a few additional spots due to a trace of {111 } twinning remaining . The 10,000 A deposits, however, showed only the parallel growth with no trace of twinning remaining, Fig . 4, but the spots were of strongly arrow-head form, the elongations indicating not only smooth extensive facets parallel to the initial (110) surface, but also parallel to {l11} planes, along the sides of the ridges . Patterns obtained with the beam along [0011 also contained clear Kikuchi bands and some spots strongly elongated normal to the shadow edge due to the main (110) surface, but with no oblique extensions, thus indicating absence of any well marked {120} or other facets . At 10 mA/cm 2, 20°C, 10,000 A deposits had a randomly polycrystalline surface structure, and the microphotographs showed only closely-spaced projections about 0.6 Mm in diameter .
302
T. H. V. Serrr and H. WILMAN
At 20 mAJcm2, 60°C, deposits 1000 A thick yielded microphotographs showing closely-spaced projections about 0-6 pm in diameter . The diffractions showed single-crystal orientation parallel to the substrate lattice, with also much {111} twinning, and the spots were very strongly elongated normal to the shadow edge, indicating presence of (110) faces parallel to the initial (110) surface . At the [001] azimuth the spots had also faint but long elongations at ±19° to the plane of incidence, showing that the projections were to a less extent bounded by {120} facets . Deposits 10,000 A thick gave diffraction patterns of Kikuchi bands and also, at the [110] azimuth, strongly elongated spots, indicating as before single-crystal orientation but without any twinning at the surface of the projections . The microphotographs now showed closely-spaced long ridges (12 ,um long and 0 .6-0 •7 pm broad) not along [110] as at 20°C, but along [001] as found in the absence of thiourea . At 6000 A, where the ridges were already noticeable and about 6 pm long by 0-6 Mm broad, the parallel lattice orientation was accompanied by a trace of {111} twinning, and again (110) faces were present parallel to the initial (110) substrate, with no indication of any {120} or other facets . (b) Deposits on the (111) Ag face in the bath containing 10 g/l of thiourea . At 1 MA/ cm2, 20°C, deposits 10,000 A thick had single-crystal orientation parallel to the substrate lattice, with no sign of any twinning, and the diffraction spots were very strongly elongated normal to the substrate, showing the atomically smooth nature of the (111) surface parallel to the initial (111) substrate surface . The microphotographs showed only closely-spaced projections about 0-6 pm in diameter . This deposit orientation and form is thus the same as that in absence of thiourea .' At 10 mA/cm2, 20°C, 10,000 A deposits had parallel lattice orientation but with much twinning on all {111} planes, and also a small proportion of randomly orientated crystals at least several hundred A in diameter. The microphotographs were similar to those from 10,000 A deposits at 1 mA/cm 2. At 20 mA/cm2, 60°C, deposits 10,000 A thick had continued the single-crystal structure of the substrate, with traces of twinning on the (I11) planes which were parallel to the initial (I11) substrate face . A few faint but long narrow spot extensions showed that extensive (ll0) faces had been developed on the main parallel-growth deposit. The microphotographs again showed only projections 0.6 pm in mean diameter . (c) Deposits on the (110) Ag face, in the bath containing 30 g/1 thiourea . At 1 mA/ cm2, 20°C, 10,000 A deposits yielded microphotographs similar to Fig . 5, showing shorter extensions than with 10 g/l thiourea, about 4-2 ,um long and 2-4 pm broad, but still along [110] . The diffraction patterns were similar to Fig . 4 (for 10 g/l thiourea), showing parallel single-crystal growth with no twinning, and strong development of 1111 faces along the sides of the ridges . At 10 mA/cm2, 20°C, the surface structure of 10,000 A deposits was random polycrystalline, with again a grain size 0-6 pm diameter seen from the microphotographs . These results are thus as found with the 10 g/l thiourea . At 20 mA/cm2, 60°C, 10,000 A deposits had single-crystal orientation parallel to the substrate lattice, with no twinning, and atomically smooth (110) faces parallel to the initial substrate face . The microphotographs showed closely-spaced ridge (12 ,um long by 0-7 pm broad) along [001] . Thus also at 60°C the deposit was practically the same as in presence of 10 g/1 thiourea .
Effects of addition agents on the structure of Ag deposits 4 . The effect of 10
g/l
303
and 50 g1l of urea (Co(NH2)0
In this case also the solution was colourless, with no precipitation . On the Ag (110) face at 1 mA/cm2 , 20°C, deposits 1000 A thick had continued the single-crystal lattice of the substrate with, however, a small proportion of {lll} twinning . The diffraction pattern at the [I1OJ azimuth was similar to Fig . 4, the spots being strongly elongated normal to the shadow edge and at +45° to it, showing presence of extensive smooth (110) faces parallel to the initial substrate and also facets of {111} type . With the beam along [001] the spots had fainter extensions showing presence of well-defined but less prominent facts of {120} type . The microphotographs showed only closelyspaced projections about 0 . 6 pm in diameter. Deposits 2000 A thick prepared at 1 mA/cm 2, 20°C, gave results similar to those from 1000 A deposits except that there was no trace of {l11} twinning, but the {l11} facets were then very strongly developed . The microphotographs were also practically unchanged . Deposits 10,000 A thick prepared at 1 mA/cm2 , 20°C, gave microphotographs similar to Fig . 6, showing projections which were mostly about 0 . 5-1 . 5 ftm in diameter but often were in rows about 5 ,um . The diffraction patterns of Kikuchi bands and spots elongated normal to the shadow edge showed parallel single-crystal orientation with no twinning, and only well-marked (110) faces, parallel to the initial (110) substrate . At 10 mAJcm2 , 20°C, 10,000 A deposits yielded diffraction patterns similar to Fig . 7, when the beam along [I TO], showing by the pattern and the lack of elongation of the spots, that the surface was then rough, consisting of projections only a few hundred A in thickness. Some spots corresponding to a lattice parallel to the substrate lattice are still present though mainly weak, but the spots corresponding to the four {111} twins are strong . When the electron beam was along [0011 the patterns were similar. In this case, therefore, although no extensive crystal faces appear to have been developed, the deposition conditions had evidently favoured the outward type of growth which in turn had favoured development of the twinned lattices . At 20 mA/cm 2, 60°C, 10,000 A deposits had parallel single-crystal surface structure, with extensive facets parallel to the initial (110) substrate surface, and with no twinning . The microphotographs showed smooth ridges (7 pm long and 0 . 6 tsm broad) along [110], similar to those of Fig . 3, but these were interspersed with small projections about 0 .3-0 •6 pm in diameter . With the bath containing 50 g/l of urea the results at the above temperatures, current densities, and thicknesses, were closely similar . The main difference relative to the result for 10 g/1 was that the microphotographs from 10,000 A thick deposits at 1 mA/cm2 , 20°C, showed projections 2 . 5 µm long along [001] and 0 . 7 pm broad, ie much shorter in length . At 60°C there was no appreciable difference in size of the projections . 5 . The effect of 10 g/i potassium thiocyanate (KCNS) This caused no colour changes or precipitation in the bath . On the (110) Ag face the 1000 and 10,000 A thick deposits at 1 mA/cm2 , 20°C, and the 10,000 A thick deposits at 10 mA/cm2 , 20°C and 20 mA/cm2 , 60°C, were closely similar to those prepared in absence of the addition .'
304
T. H . V . SEn and H . WILMAN DISCUSSION
The above results show that even relatively small quantities of addition agents in some cases result in large modifications in the form of the electrodeposits obtained from a given bath at a given temperature, current density and deposit thickness . Except in the case of KCNS, where no change resulted, the addition agents investigated modified the deposits in different ways . Table 1 summarizes the orientation and habit of the 10,000 A deposits prepared on the (110) Ag face in the presence of the addition agents . In the case of the bath containing CS 2 no change in habit was caused by the CS, and at both 1 mA/cm2, 20°C, and 20 mA/cm2, 60°C, the ridges with (110) faces at their summits but no well-marked lateral faces were developed along [0011, as in absence of CS.. At 20°C . the main change was to cause the 10,000 A (or less) thick deposits to have twinned surface structure (and correspondingly no development of long ridges) even at 1 mA/cm2, although the twinning was also found up to 10 mA/cm 2 before giving place to random polycrystalline structure, as compared with only up to 5 mA/cm2 in the absence of CS.. At 20 mA/cm 2, 60°C, although the 10,000A deposits had surface twinning, appreciable {111} twinning was observed on 1000-2000 A deposits . The observation of oriented Ag 2S present at the surface of all the 2000 A deposits prepared at 20 mA/cm 2, 60°C, whereas no Ag 2S was found on all the 10,000 A deposits, suggests that the twinning in the thin deposits must have been due to inclusion of epitaxially orientated Ag 2S. Both at 20°C and 60°C the results from this bath were closely similar in type to those from the bath containing excess KCN (300 g/1) with no addition agent,' which for example gave twinned growth even at 10,000 A, 20 mA/cm2, 60°C, though no adsorbed or included material was noticed . Incidentally, addition of one drop of CS, to this bath containing 300 g/l KCN gave no precipitate or colour change even after 8 days ; no silver deposits were investigated from the resulting solution . In the case of deposits from the other baths, no adsorbed or included addition agent was observed, and although many attempts were made to find whether these materials grew in epitaxial orientation when crystallized out from aqueous solution on electropolished silver (110) and (111) faces, no orientated growth such as Bunn' found in urea crystallized on rock salt was detected either in the microscope or by electron-diffraction . It therefore seems clear that in these cases any adsorption of the addition agent or the products of its reaction with the bath constituents must have been mostly of a dynamic character, rather than such as to lead to appreciable amounts of epitaxially orientated crystalline inclusions . In these cases (thiosemicarbazide, thiourea, and urea) there was strong development of (I11) faces in deposits on the (110) silver face . This change, from what was evidently an {hko) type of main facet bounding elongated ridges along [0011 when no addition agent was present, to {111} main facets in presence of these materials, was evidently the cause of the development of long ridges instead along [110] in the case of thiourea at 20°C and urea at 60°C . A remarkable disappearance of the {111} twinning, as the deposit thickness was increased from 1000-10,000 A was observed in the case of several baths . This may indicate that the addition agent near the cathode is soon removed from solution, and not quickly enough replaced by diffusion in significant amount relative to those of the silver deposition rate . In the bath with urea, 10 mA/cm 2, 20°C, however, the virtual
Effects of addition agents on the structure of Ag deposits
305
absence of parallel growth though its {111} twins are present is evidently due to these bath conditions favouring the "outward" type of growth . The grain size of the deposits on Ag (I11) and (100) was not affected by the presence of the addition agents, but in the 10,000 A thick deposits on Ag (110) at 1 mA/cma, 20°C, and at 20 mA/cma, 60°C, the length of the ridges was reduced by half or more, except in the case of the bath with thiourea . It appears indeed that the brightening effect of these agents must be connected with their profound effect on the crystal habit, leading to flat facets of relatively large extent and high smoothness compared with the wave-length of light, rather than with any marked decrease of crystal size . This conclusion is not in conflict with other previous work, which indeed it helps to clarify . Thus, Wood' considered that the brightness of nickel deposits was due to the preferred orientation of the deposit crystals, while Hothersall and Gardam 5 and Smith, Keeler and Reads found no relationship existed between the orientation of the crystals in their nickel deposits. These two results are reconciled by the present conclusion that the habit, which with the crystal orientation relative to the deposit surface determine the surface smoothness of the deposit, is the main factor affecting the deposit brightness . The observation of Clarke and Simonson 7 that the grain size of matt, semibright and bright nickel deposits is nearly the same, was also matched by similar observations in the present experiments . Finally, the above results have brought out once again the very limited nature of the information obtained by use of the microscope as compared with those yielded by electron diffraction, though showing also that a combination of the two is desirable to show both the fine-scale and coarser details of the surface structure and form . REFERENCES 1 . T . H. V . SErrx and H. WILMAN, Trans. Faraday Soc. 51, 984 (1955). 2 . L. S. RAMSDELL, Am . Miner . 27, 401 (1943) . 3 . C . W. BONN, Proc. R . Soc ., A141, 567 (1933) . 4 . W . H. WOOD, Phil. Mag . 20, 964 (1935) . 5 . A . W . HoTHERSALt and S . E . GARDAM, J. Electrodep . tech . Soc . 15, 127 (1939) . 6 . W. SMm!, J . H. KEEEER and H . J. READ, Plating 36, 355 (1949) . 7 . G . L . CLARKE and S . H . SiMoNsoN, J. electrochem . Soc . 98, 110 (1951) .
SOME EFFECTS OF ADDITION AGENTS ON THE STRUCTURE OF SILVER DEPOSITS FROM THE ARGENTOCYANIDE BATH T . H . V . SETTYt and H . WILMAN Department of Chemical Engineering and Applied Chemistry, Imperial College, London, S .W .7, England Abstract-Electron-diffraction technique has been used to study the modification of surface structure of silver electrodeposits deposited from the argentocyanide bath in presence of addition agents such as carbon disulphide, thiosemicarbazide, thiourea, and urea on (110) (111) and (100) faces silver single crystals. It is found that in many of these cases there is pronounced effect on the deposit crystal habit and there is strong development of {Ill } faces on the (110) silver face . It appears that the brightening effect of these agents must be connected with their pronounced effect on the crystal habit leading to flat facets of relatively large extent and high smoothness compared with the wave length of light, rather than with any marked decrease of grain size . Optical micrographs are also shown to illustrate the deposit surface form . Rfsum€-Utilisation de la diffraction d'€lectrons en vue d'6tudier ]a modification de structure superficielle d'6lectro-dAp6ts d'Ag, 8 partir de bains Ag(CN)s en presence d'agents additionnels tels que sulfure de carbon, thio-semi-carbazide, thio-u , ur€e et sur les faces (110) (111) (100) de monocristaux Ag . Leur effet sur la forme du depot est prononca dens certains call, oil I'on observe on d6veloppement notable des faces (111) sur les faces (110) . Il apparait que de telles influences sur la forme sent en relation avec le brillant que confa rent ces agents, des facettes plates d'8tendue relativement grande presentant un haut degr6 de lissage comparativement aux longueurs d'ondes lumineuses, sans diminution sensible de 1a grosseur des grains. Des micrographies optiques en rendent compte d'autre part . Zusammenfassung-Modifikationn der Oberflachenstruktur von Silberniederschla gen aus Silbercyanid-Badern in Gegenwart von Zusatzen wie Schwefelkohlenstoff, Thiosemicarbazid, Thioharnstoff and Harnstoff auf (110), (111) and (100)-Flachen von Silber-Einkristallen warden elektronendiffraktographisch untersucht . Es wurde gefunden, dass in vielen Fallen ein starker Effekt auf die Kristallstruktur des Niederschlages auftritt and auf den (110) Oberflachen eine ausgepragte Entwicklung von [111] Flachen stattfindet. Man schliesst daraus, dass die Glanzwirkung dieser Zusatze im Zusammenhang stehen muss mit ihrer ausgepragten Wirkung auf die Kristallstruktur, indem sie die Ausbildung flacher Facetten von relativ grosser Ausdehnung and kleiner Rauhigkeit (verglichen mit der Wellenlange des Lichts) bewirken, eher als eine merkliche Kornverkleinerung. Es warden Mikroaufnahmen gezeigt, um die Oberflachenstruktur der Niederschlage zu verdeutlichen .
QUITE small
concentrations of an addition agent such as carbon disulphide, added to
an electrolytic bath, often cause large changes in deposit structure (crystal orientation, size and habit) and correspondingly affect the physical properties (hardness, brightness,
etc) of the deposit. The mode of action of addition agents in modifying the structure of electrodeposits from a given bath has not hitherto been adequately determined . The work of Setty and Wilman' correlating the structure of silver deposits on silver (110), (111) and (100) faces, has now been extended to the investigation of the modification of the silver deposit structure when deposits are prepared from a bath containing 35 g/I AgCN, 30 g/1 KCN and
38 g/1 K 2CO3,
with small amounts of typical
addition agents used in silver electroplating carbon disulphide, thiosemicarbazide, thiourea and urea . In many of these cases there was a pronounced effect on the deposit crystal habit and on the range of conditions under which a twinned structure was * Manuscript received 18 February 1965 . t Present address : Department of Chemistry, Central College, Bangalore, India . 297
298
T.
H . V. Sear and H . WILMAN
obtained . On the other hand, 1 % of potassium thiocyanate did not cause any appreciable effect. EXPERIMENTAL TECHNIQUE The details of preparation of smooth electropolished (110), (111) and (100) faces and of the silver deposits on these faces have already been described .' For each electrodeposition 200 ml of fresh argentocyanide solution was used to which had been added the desired amount of addition agent . Each experiment was repeated several times to ensure representative results . For electron-diffraction examination, 50-60 KV electrons and a camera length of 21 .5 or 48 cm . were used . A Vickers projection Microscope was used for the optical microscopic examination, using a magnification of x 600 . RESULTS In all the following results the current efficiency was found to be 100% or practically so . 1 . The effect of carbon disulphide Shaking the argentocyanide solution with a drop of carbon disulphide resulted in precipitation of a black deposit which was identified from its electron-diffraction pattern as the normal monoclinic, practically orthorhombic, Ag 2 S having a = 9 . 47, b = 6 . 92, c = 8 .28 A and /9 = 124° (Ramsdell) 2 Filtering the solution after 2 h left the filtrate clear, but again the black precipitate was formed on further standing . The standard procedure adopted was therefore to allow the solution to stand for 24 h, filter it, and then immediately use the filtrate for the electrodeposition of silver . The structure of the silver deposits investigated on Ag (110) is shown in Table 1 . Ag DEPOSITS ON Ag (110) FACE Surface orientation and habit of 10,000 A Ag deposits
TABLE 1 . EFFECT OF ADDITION AGENTS ON
Addition Agent None CS,
20'C
1 niA/cm' P, (110), T, (110)
10 mA/cm'
r[001]
60'C 20
mA/cm'
R
R
T(+ R), (110)
R
20 mA/cm'
r[001] p, • (110), r[001] p, (110),
Thiosemicarbazide 5 g)1
P, {lll)1
One-degree {21 l) Orientation
Thiourea IO and 30 gp Urea, 10 and
P •, (110)
R
weak (110) P, (110), r[001]
50 g/1 KCNS, 10 g/I
r[001]t P, (110), r[001)
T(little P) R
P, (110), r[1T0] P, (110), r[001]
T,
111 r[110] (110),
P•,
• Twinned at
up to 2000 A . facets in deposits 1000-2000 A thick. r, long ridges parallel along the stated direction . l, lamellae . t {111)
(a) Deposits on the (110) Ag face . The 10,000 A thick deposits at 20°C had single-crystal orientation parallel to the substrate lattice, with extensive {111} twinning, even at 1 mA/cm2 (the diffraction pattern was similar to Fig . 4 of the previous paper) .' The microphotographs then showed closely-spaced projections about 0-6 ,um in
Effects of addition agents on the structure of Ag deposits
2 99
diameter, with no sign of any ridges such as were developed at this deposit thickness and deposition conditions in the absence of CS 2. In the electron-diffraction patterns the spots were not much elongated ; thus the surfaces of the microscopic projections were relatively rough, with projections of the order of 100-200 A in diameter. Randomly orientated crystals first occurred on 10,000 A thick deposits at current densities of 10 mA/cm2, as compared with at 5 mA/cm 2 in the absence of CS 2. At 60°C deposits up to 10,000 A thick showed single crystal orientation parallel to the substrate lattice with, however, a moderate proportion of {l11) twinning on deposits 1000 A thick, and only mere traces of twinning at 2000 A and more (Fig . 1(a)) . Atomically smooth (110) faces parallel to the initial substrate face were indicated by
0
son/
'
0
o
ccP
aep 4 .08A
Fro, lb . Arrangement of diffraction spots due to silver (110), [170] azimuth and silver sulphide. -0- silver spots . --x-- silver sulphide spots . Fto . 1c . Lattice spacings of silver crystal and silver sulphide .
the fact that the spots in the diffraction pattern were much elongated normal to the shadow edge . Long narrow closely-spaced ridges parallel to [001] began to be noticeable, about 3-10 um long (but somewhat beaded along their length) and 0-5 pm broad, on 2000 A deposits . On 10,000 A deposits there were ridges 10-20, um long (now rather smooth along their length) and 0-5 pm broad giving microphotographs similar to Fig . 3(a) of the earlier paper .' The form of the deposits was thus closely similar to that previously described for the deposits more than 2000 A thick under the same conditions in the absence of CS, . Figure 1(a) shows many spots identified as due to Ag 2S, in addition to the spots due to the silver. Fig . 1(b) shows the centred- //2- rectangle arrangement of the diffraction spots due to the silver, and one component rectangle pattern of spots due to the silver sulphide, together with the Laue indices of these diffractions relative to the above monoclinic cell . This indexing of the Ag2S spots shows that the Ag2 S crystals were orientated with (100) planes parallel to the (110) silver surface, with [010] parallel to the silver [110] row, and perpendicular to this direction [001] Ag 2S parallel to [001] Ag . Fig. 1(c) shows the relatively close lattice spacing correspondence along [001] Ag (2a AS = 8 16 A compared with CAI, s = 8-28 A, misfit 1 ,4Y.), and the still rough correspondence along [110] Ag (T 110 = 5-76 A compared with bAS¢S = 692 A, misfit 20%) . The presence of such epitaxially orientated Ag2S on at least some parts of the silver surface thus suggests that even in the other deposits where no Ag2S was detected, minute traces of Ag2S may nevertheless have been present and
300
T. H. V.
SETrY
and H.
WILMAN
been responsible for the observed (1111 twinning of the silver at low current density at20°C . (b) Deposits on the (100) Ag face . At 20°C, deposits 10,000 A thick had, at I mA/cma, single-crystal surface structure parallel to the substrate lattice, with also a trace of {111} twinning, but at 10 mA/cma rather more {111} twinning, whereas at both these current densities parallel growth twinning was found in the absence of CS.. The microphotographs showed closely-spaced projection of the order of 0-6 ,um diameter, ie the same as in absence of CS 2. At 60°C a 10,000 A deposit showed parallel single-crystal growth with moderate {111} twinning, and again the projections were about 0-6 ,um in diameter. TABLE 2. EFPECT OF ADDmON AGENTS ON
Ag
DEPOSITS ON
Surface orientation and habit of 10,000 Addition agent None CS, Thiourea 10 g/1 Thiosemicarbazide
Ag (I11)
FACE
A Ag deposits
20°C
60'C
1 mA/cm'
10 mA/cm'
20 mA/cm'
P, (111) P, (111) P, (111)
T, (111)
T,,,, (111) and {115} T,,,, (111) {115} • R
T,,, + R R
20
mA/cm'
P, (1ll) P(+T,), (I11) and (115) R + P, (111)
* also at 30 mA/cm' . twinning on all (111 } planes, T F , twinning only on (111) parallel to surface .
T,,,,
(c) Deposits on the (111) face . The deposit orientation and habit (Table 2) were the same as in the absence of CS.' and the projections also were unchanged in size, about 0 .6 pm diameter .
2. The effect of 5 g/l of thiosemicarbazide (NHaCS-NH-NH2) About 15 min after dissolving the solid thiosemicarbazide in the argentocyanide bath, a pale pink colour became noticeable, deepening with further standing, and after 30 min a black precipitate was formed which when dried on a collodion film in vacuo yielded electron diffraction transmission patterns of sharp rings (arcs when the film was inclined), identified as corresponding to normal monoclinic Ag 2S . After standing for 24 h, the precipitate had settled to the bottom of the solution, and the deep wine-red clear solution was decanted off and used for electrodeposition, on the (110) Ag face . Deposits 10,000 A thick at 20°C and 1 mA/cma yielded Fig . 2 when the beam was inclined 2° to [110] and patterns of a similar type also at the [001] azimuth . These patterns correspond to single-crystal orientation of the deposit parallel to the substrate lattice, but there are in general pairs of spots instead of each spot of the normal pattern, the spot separation decreasing with increasing distance from a circular locus (on which single points occur) centred at the point where the [110] row met the plate, and passing through the undeflected-beam spot . From the spot separations in relation to the spot positions, calculation indicated that the spots correspond to [111]* and [111 ]* extensions from the reciprocal-lattice points meeting the Ewald sphere . This therefore shows that the silver deposits were bounded mainly by (111) and (1 T T) planes, normal to the (110) substrate surface, with no evidence of any smooth
Fto . la . Silver deposited from the argentocyanide bath + CS, at 20 mA/cm°, 60"C, on the silver (110) face . Thickness 2000 . . [1T0] azimuth .
FIG.
2 . Silver deposited from the argentocyanide bath + thiosemicarbazide at 1 MA/cm' and 20°C on the silver (110) face 2" off from [ITO] azimuth . Thickness 10,000 A .
300
Fie. 3 . Microphotograph of the silver deposited from the argentocyanide bath -, thio . urea at I n A/cm° and 20'C on the silver (110) face . Thickness 10,000 A x 600.
Fio. 4. Silver deposited from argentocyanide bath + 1 % urea at I mA/em' and 20'C on the silver (110) face . [ITO] azimuth . Thickness 2000 A .
Fin . 5 . Microphotograph of the deposit obtained from the argentocyanide bath + 3 thiourea at 1 mA/cm', 20°C on the silver (110) face. Thickness 10,000A x 600 .
Fin . 6 . Microphotograph of the silver deposited from argentocyanide bath + l % urea at I mA/cms, 20°C on the silver (110) face . Thickness 10,000 A x 600 .
Fto. 7 . Silver deposited from the argentocyanide bath + 1 % urea at 10 mA/cm 2 , 20°C on the Ag (110) face. Thickness 10,000 A [lTO] azimuth.
Effects of addition agents on the structure of Ag deposits
301
(110) faces at the tips of the projections or of the other octahedral faces (111) and (111) . The large number of strong spots, and their sharpness, indicates that the projections must have consisted essentially of relatively thin sheets having faces of large extent parallel to one or other of these two {111} faces . The microphotographs showed only closely-spaced small projections about 0-6 ,um in diameter, with no indication of any long ridges such as were found in absence of the addition agent . At 10 mA/cm2 the 10,000 A deposits at 20°C gave diffraction patterns of long sharp arcs, indicating that the surface structure of the silver deposits was polycrystalline with preferred orientation such that on average {211} planes were parallel to the (110) substrate surface, with a spread of ±15° from the average . The microphotographs again showed closely-spaced projections about 0 . 6,um in diameter . At 60°C, 10,000 A deposits were found to be in single-crystal orientation parallel to the substrate, with much {lll} twinning, giving a diffraction pattern similar to Fig. 4 of the earlier results .' Again the microphotographs showed closely-spaced projections about 0 .6 µm in diameter. 3 . The effect of 10 g/1 and 30 g/l of thiourea (CS(NH2)2)
On dissolving the solid thiourea in the argentocyanide bath no colour was observed, and no precipitation, even on standing indefinitely . (a) Deposits on the (110) Agface, from the bath containing 10 g/1 thiourea . At 20°C and 1 mA/cm2 a 1000A deposit gave a single-crystal orientation parallel to the substrate lattice but with also much {Ill} twinning . The spots in the electrondiffraction pattern including those due to the twins were much elongated normal to the shadow edge, indicating smooth (110) faces parallel to the initial (110) substrate surface . At the [0011 azimuth the spots had arrow-head form, having faint but long and narrow extensions at f 19° to the plane of incidence, indicating the presence of {120} facets in addition to the main (110) face . The microphotograph showed small closely spaced projections about 0-7 ,um in mean diameter . At 10,000 A thickness, deposits at 1 mA/cm 2 yielded microphotographs similar to that of Fig . 3, showing closely spaced ridges (about 11 µm long and 0 .6 pm broad) along [110] instead of ridges along [001] as found in absence of thiourea but otherwise the same conditions' The ridges were just beginning to be noticeable (along [110]) on deposits 5000 A thick, and correspondingly the diffraction patterns showed single-crystal orientation parallel to the substrate, with arrow-head-shaped spots indicating presence of (110) and also {111} facets, and only a few additional spots due to a trace of {111 } twinning remaining . The 10,000 A deposits, however, showed only the parallel growth with no trace of twinning remaining, Fig . 4, but the spots were of strongly arrow-head form, the elongations indicating not only smooth extensive facets parallel to the initial (110) surface, but also parallel to {l11} planes, along the sides of the ridges . Patterns obtained with the beam along [0011 also contained clear Kikuchi bands and some spots strongly elongated normal to the shadow edge due to the main (110) surface, but with no oblique extensions, thus indicating absence of any well marked {120} or other facets . At 10 mA/cm 2, 20°C, 10,000 A deposits had a randomly polycrystalline surface structure, and the microphotographs showed only closely-spaced projections about 0.6 Mm in diameter .
302
T. H. V. Serrr and H. WILMAN
At 20 mAJcm2, 60°C, deposits 1000 A thick yielded microphotographs showing closely-spaced projections about 0-6 pm in diameter . The diffractions showed single-crystal orientation parallel to the substrate lattice, with also much {111} twinning, and the spots were very strongly elongated normal to the shadow edge, indicating presence of (110) faces parallel to the initial (110) surface . At the [001] azimuth the spots had also faint but long elongations at ±19° to the plane of incidence, showing that the projections were to a less extent bounded by {120} facets . Deposits 10,000 A thick gave diffraction patterns of Kikuchi bands and also, at the [110] azimuth, strongly elongated spots, indicating as before single-crystal orientation but without any twinning at the surface of the projections . The microphotographs now showed closely-spaced long ridges (12 ,um long and 0 .6-0 •7 pm broad) not along [110] as at 20°C, but along [001] as found in the absence of thiourea . At 6000 A, where the ridges were already noticeable and about 6 pm long by 0-6 Mm broad, the parallel lattice orientation was accompanied by a trace of {111} twinning, and again (110) faces were present parallel to the initial (110) substrate, with no indication of any {120} or other facets . (b) Deposits on the (111) Ag face in the bath containing 10 g/l of thiourea . At 1 MA/ cm2, 20°C, deposits 10,000 A thick had single-crystal orientation parallel to the substrate lattice, with no sign of any twinning, and the diffraction spots were very strongly elongated normal to the substrate, showing the atomically smooth nature of the (111) surface parallel to the initial (111) substrate surface . The microphotographs showed only closely-spaced projections about 0-6 pm in diameter . This deposit orientation and form is thus the same as that in absence of thiourea .' At 10 mA/cm2, 20°C, 10,000 A deposits had parallel lattice orientation but with much twinning on all {111} planes, and also a small proportion of randomly orientated crystals at least several hundred A in diameter. The microphotographs were similar to those from 10,000 A deposits at 1 mA/cm 2. At 20 mA/cm2, 60°C, deposits 10,000 A thick had continued the single-crystal structure of the substrate, with traces of twinning on the (I11) planes which were parallel to the initial (I11) substrate face . A few faint but long narrow spot extensions showed that extensive (ll0) faces had been developed on the main parallel-growth deposit. The microphotographs again showed only projections 0.6 pm in mean diameter . (c) Deposits on the (110) Ag face, in the bath containing 30 g/1 thiourea . At 1 mA/ cm2, 20°C, 10,000 A deposits yielded microphotographs similar to Fig . 5, showing shorter extensions than with 10 g/l thiourea, about 4-2 ,um long and 2-4 pm broad, but still along [110] . The diffraction patterns were similar to Fig . 4 (for 10 g/l thiourea), showing parallel single-crystal growth with no twinning, and strong development of 1111 faces along the sides of the ridges . At 10 mA/cm2, 20°C, the surface structure of 10,000 A deposits was random polycrystalline, with again a grain size 0-6 pm diameter seen from the microphotographs . These results are thus as found with the 10 g/l thiourea . At 20 mA/cm2, 60°C, 10,000 A deposits had single-crystal orientation parallel to the substrate lattice, with no twinning, and atomically smooth (110) faces parallel to the initial substrate face . The microphotographs showed closely-spaced ridge (12 ,um long by 0-7 pm broad) along [001] . Thus also at 60°C the deposit was practically the same as in presence of 10 g/1 thiourea .
Effects of addition agents on the structure of Ag deposits 4 . The effect of 10
g/l
303
and 50 g1l of urea (Co(NH2)0
In this case also the solution was colourless, with no precipitation . On the Ag (110) face at 1 mA/cm2 , 20°C, deposits 1000 A thick had continued the single-crystal lattice of the substrate with, however, a small proportion of {lll} twinning . The diffraction pattern at the [I1OJ azimuth was similar to Fig . 4, the spots being strongly elongated normal to the shadow edge and at +45° to it, showing presence of extensive smooth (110) faces parallel to the initial substrate and also facets of {111} type . With the beam along [001] the spots had fainter extensions showing presence of well-defined but less prominent facts of {120} type . The microphotographs showed only closelyspaced projections about 0 . 6 pm in diameter. Deposits 2000 A thick prepared at 1 mA/cm 2, 20°C, gave results similar to those from 1000 A deposits except that there was no trace of {l11} twinning, but the {l11} facets were then very strongly developed . The microphotographs were also practically unchanged . Deposits 10,000 A thick prepared at 1 mA/cm2 , 20°C, gave microphotographs similar to Fig . 6, showing projections which were mostly about 0 . 5-1 . 5 ftm in diameter but often were in rows about 5 ,um . The diffraction patterns of Kikuchi bands and spots elongated normal to the shadow edge showed parallel single-crystal orientation with no twinning, and only well-marked (110) faces, parallel to the initial (110) substrate . At 10 mAJcm2 , 20°C, 10,000 A deposits yielded diffraction patterns similar to Fig . 7, when the beam along [I TO], showing by the pattern and the lack of elongation of the spots, that the surface was then rough, consisting of projections only a few hundred A in thickness. Some spots corresponding to a lattice parallel to the substrate lattice are still present though mainly weak, but the spots corresponding to the four {111} twins are strong . When the electron beam was along [0011 the patterns were similar. In this case, therefore, although no extensive crystal faces appear to have been developed, the deposition conditions had evidently favoured the outward type of growth which in turn had favoured development of the twinned lattices . At 20 mA/cm 2, 60°C, 10,000 A deposits had parallel single-crystal surface structure, with extensive facets parallel to the initial (110) substrate surface, and with no twinning . The microphotographs showed smooth ridges (7 pm long and 0 . 6 tsm broad) along [110], similar to those of Fig . 3, but these were interspersed with small projections about 0 .3-0 •6 pm in diameter . With the bath containing 50 g/l of urea the results at the above temperatures, current densities, and thicknesses, were closely similar . The main difference relative to the result for 10 g/1 was that the microphotographs from 10,000 A thick deposits at 1 mA/cm2 , 20°C, showed projections 2 . 5 µm long along [001] and 0 . 7 pm broad, ie much shorter in length . At 60°C there was no appreciable difference in size of the projections . 5 . The effect of 10 g/i potassium thiocyanate (KCNS) This caused no colour changes or precipitation in the bath . On the (110) Ag face the 1000 and 10,000 A thick deposits at 1 mA/cm2 , 20°C, and the 10,000 A thick deposits at 10 mA/cm2 , 20°C and 20 mA/cm2 , 60°C, were closely similar to those prepared in absence of the addition .'
304
T. H . V . SEn and H . WILMAN DISCUSSION
The above results show that even relatively small quantities of addition agents in some cases result in large modifications in the form of the electrodeposits obtained from a given bath at a given temperature, current density and deposit thickness . Except in the case of KCNS, where no change resulted, the addition agents investigated modified the deposits in different ways . Table 1 summarizes the orientation and habit of the 10,000 A deposits prepared on the (110) Ag face in the presence of the addition agents . In the case of the bath containing CS 2 no change in habit was caused by the CS, and at both 1 mA/cm2, 20°C, and 20 mA/cm2, 60°C, the ridges with (110) faces at their summits but no well-marked lateral faces were developed along [0011, as in absence of CS.. At 20°C . the main change was to cause the 10,000 A (or less) thick deposits to have twinned surface structure (and correspondingly no development of long ridges) even at 1 mA/cm2, although the twinning was also found up to 10 mA/cm 2 before giving place to random polycrystalline structure, as compared with only up to 5 mA/cm2 in the absence of CS.. At 20 mA/cm 2, 60°C, although the 10,000A deposits had surface twinning, appreciable {111} twinning was observed on 1000-2000 A deposits . The observation of oriented Ag 2S present at the surface of all the 2000 A deposits prepared at 20 mA/cm 2, 60°C, whereas no Ag 2S was found on all the 10,000 A deposits, suggests that the twinning in the thin deposits must have been due to inclusion of epitaxially orientated Ag 2S. Both at 20°C and 60°C the results from this bath were closely similar in type to those from the bath containing excess KCN (300 g/1) with no addition agent,' which for example gave twinned growth even at 10,000 A, 20 mA/cm2, 60°C, though no adsorbed or included material was noticed . Incidentally, addition of one drop of CS, to this bath containing 300 g/l KCN gave no precipitate or colour change even after 8 days ; no silver deposits were investigated from the resulting solution . In the case of deposits from the other baths, no adsorbed or included addition agent was observed, and although many attempts were made to find whether these materials grew in epitaxial orientation when crystallized out from aqueous solution on electropolished silver (110) and (111) faces, no orientated growth such as Bunn' found in urea crystallized on rock salt was detected either in the microscope or by electron-diffraction . It therefore seems clear that in these cases any adsorption of the addition agent or the products of its reaction with the bath constituents must have been mostly of a dynamic character, rather than such as to lead to appreciable amounts of epitaxially orientated crystalline inclusions . In these cases (thiosemicarbazide, thiourea, and urea) there was strong development of (I11) faces in deposits on the (110) silver face . This change, from what was evidently an {hko) type of main facet bounding elongated ridges along [0011 when no addition agent was present, to {111} main facets in presence of these materials, was evidently the cause of the development of long ridges instead along [110] in the case of thiourea at 20°C and urea at 60°C . A remarkable disappearance of the {111} twinning, as the deposit thickness was increased from 1000-10,000 A was observed in the case of several baths . This may indicate that the addition agent near the cathode is soon removed from solution, and not quickly enough replaced by diffusion in significant amount relative to those of the silver deposition rate . In the bath with urea, 10 mA/cm 2, 20°C, however, the virtual
Effects of addition agents on the structure of Ag deposits
305
absence of parallel growth though its {111} twins are present is evidently due to these bath conditions favouring the "outward" type of growth . The grain size of the deposits on Ag (I11) and (100) was not affected by the presence of the addition agents, but in the 10,000 A thick deposits on Ag (110) at 1 mA/cma, 20°C, and at 20 mA/cma, 60°C, the length of the ridges was reduced by half or more, except in the case of the bath with thiourea . It appears indeed that the brightening effect of these agents must be connected with their profound effect on the crystal habit, leading to flat facets of relatively large extent and high smoothness compared with the wave-length of light, rather than with any marked decrease of crystal size . This conclusion is not in conflict with other previous work, which indeed it helps to clarify . Thus, Wood' considered that the brightness of nickel deposits was due to the preferred orientation of the deposit crystals, while Hothersall and Gardam 5 and Smith, Keeler and Reads found no relationship existed between the orientation of the crystals in their nickel deposits. These two results are reconciled by the present conclusion that the habit, which with the crystal orientation relative to the deposit surface determine the surface smoothness of the deposit, is the main factor affecting the deposit brightness . The observation of Clarke and Simonson 7 that the grain size of matt, semibright and bright nickel deposits is nearly the same, was also matched by similar observations in the present experiments . Finally, the above results have brought out once again the very limited nature of the information obtained by use of the microscope as compared with those yielded by electron diffraction, though showing also that a combination of the two is desirable to show both the fine-scale and coarser details of the surface structure and form . REFERENCES 1 . T . H. V . SErrx and H. WILMAN, Trans. Faraday Soc. 51, 984 (1955). 2 . L. S. RAMSDELL, Am . Miner . 27, 401 (1943) . 3 . C . W. BONN, Proc. R . Soc ., A141, 567 (1933) . 4 . W . H. WOOD, Phil. Mag . 20, 964 (1935) . 5 . A . W . HoTHERSALt and S . E . GARDAM, J. Electrodep . tech . Soc . 15, 127 (1939) . 6 . W. SMm!, J . H. KEEEER and H . J. READ, Plating 36, 355 (1949) . 7 . G . L . CLARKE and S . H . SiMoNsoN, J. electrochem . Soc . 98, 110 (1951) .