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Electrocrystallization of zinc on (111), (100) and (110) faces of copper
Etectrochintica Acta, 1972, Vol . 11, pp . 1895 to 1900. Pwgatnon Resa . Printed in Northern Ireland
ELECTROCRYSTALLIZATION OF ZINC ON (111), (100) AND (110) FACES OF COPPER* B . S . SHESHADRI and T. H . V. SETTY Department of Chemistry, Central College, Bangalore University, Bangalore-1 . India Abstract-Epitaxial growths of zinc electrodeposits deposited on copper single crystal faces have been observed when deposited from highly purified zinc sulphate and ammonium sulphate bath at eds between 2 and 20 mA/cm' . At all cds on the (I11) face hexagonal pyramids are observed, on the (100) face layers appear and on the (110) face a ridge type of deposit is found . The presence of benzaldehyde in small quantities affects the growth habits of zinc and cathodic polarization . An attempt is made to explain the mechanism of the deposition process from the overpotential/ed relationship . The changes in the growth habits of zinc are attributed to adsorption of benzaldehyde on the substrate. Resetme-On observe sat Ics faces monocritallines du cuivre des excroissances 6pitaxiales de zinc 6lectrodeposes a partir des bains de sulfate de zinc et de sulfate d'ammonium hautement purifies A des densit6s de courant comprises entre 2 et 20 mA/cm' . A toute densit6 de courant on observe des pyramides sur la face hexagonale (111) l'apparition de couches sur la face (100) et sur la face (110), un depot sur 1'ar6te . La presence d'une petite quantite de benzald6hyde affecte la croissance habituelle du zinc et la polarisation cathodique_ On essaic d'expliquer le m6canisme du processus de deposition A partir de la relation surtension/densite de courant . Les changements dans la fagon habituelle du zinc de se developper sont attribu6s A 1'adsorption de la benzald6hyde sur le substrat . Zusammertfasnmg-Man beobachtete epitaxiales Wachstum von elektrolytisch auf Kupfer-Einkristallfldchen abgeschiedenem Zink fiir Abscheidungen aus hochreinen Zinksulfat- und Ammoniumsulfatbadern bei Stromdichten zwischen 2 and 20 mA/em' . Bei alien Stromdichten beobachtete man auf den (111)-Fldchen hexagonale Py€amides, auf den (100)-Fliichen erschienen Schichten and auf der (110)-Flache fand man cine kammformige Ablagerung . Die Gegenwart von kleinen Mengen Benzaldehyd beeinflusst das Wachstumsverhalten von Zink and die kathodische Polarisation . Man unternahm einen Versuch, den Abscheidungsmechanismus mit Hilfe der Ueberspannungs/ Stromdichte-Beziehung zu erkldren . Die Unterschiede im Wachstumsverhalten des Zinks werden der Absorption von Benzaldehyd auf dem Substrat zugeschrieben . IN MOST electrocrystallization studies, the deposit and the substrate are of the same metal, usually copper . But in all technical processes the deposited metal is different from the substrate metal . There have been a few studies on the structure of electrodeposits when the deposited metal and the substrate metal are different ;' --3 these deposits grow epitaxially when the bath conditions and lattice parameters are favourable . Keen and Faro have studied the morphology of zinc on copper single crystal planes from an acid zinc sulphate bath ; they noticed that the morphology of zinc deposits are similar to the morphology of copper deposited on copper single crystal planes . Hence it is interesting to extend this study under different conditions
in detail . EXPERIMENTAL TECHNIQUE Analytical grade zinc sulphate was recrystallized twice using freshly distilled conductivity water . In each case only first crystallized zinc sulphate was used and the remaining liquid was rejected . Ammonium sulphate (Analar) was recrystallized once . A bath of composition 0-25 M ZnSO 4 ± 0 . 1 M (NHa 2 SO 4 was prepared . A desired amount of benzaldchyde was added to the highly purified zinc sulphate and ammonium sulphate bath when required . * Manuscript received 16 October 1971 . 1895
1 896
B . S . SaEsn .4Dxr and T . H. V. SE=
The single crystal copper cathodes were fixed in Tygon tubing so that only the desired face of the crystal was exposed, namely (100), (110) or (111) . The singlecrystal plane was first mechanically polished together with the Tygon tubing on emery paper, using ethyl alcohol as lubricant and finishing with 410 . The crystal plane was cleaned well with alcohol and finally washed with running distilled water for 10 min . Then it was electropolished in 50% orthophosphoric acid at 1 .2 V for 30 min with a large copper cathode in a electropolishing cell . The crystal was taken out without switching off the current and washed with 10% phosphoric acid and then with conductivity water . It was immediately transferred to the deposition cell . 5 A polycrystalline zinc foil (Analar) whose area was 40 times greater than that of cathode was used as anode . A freshly prepared zinc electrode was used as reference electrode . A battery of ca 90 V connected in series with a resistor was used as a source of constant current . The overpotential was recorded at regular intervals with an accuracy of ±5 mV using a V .T .V.M . Deposition at a particular cd was usually carried out up to a thickness equivalent to 5 C/cm 2 . The cathode was removed without switching off the current and was washed well with conductivity water and then with ethyl alcohol . The dried surface of the deposit was examined microscopically . Each experiment was repeated thrice to ensure reproducibility . RESULTS Morphology On (111) face : (a) from pure solution . Hexagonal pyramids (Fig . 1) with steps on their sides were obtained when zinc was deposited on the (111) copper face from the highly purified zinc sulphate and ammonium sulphate bath at 2 mA/cm 2 . At 5 mA/cm2 the density of hexagonal pyramids increased and they were all truncated . At 10 mA/cm2 , there was more truncation (Fig . 2) . The steps grew sideways ; at 15 and 20 mA/cm2 more pyramids got truncated and were growing sideways . The deposits at 5 mA/cm2 were studied at various thicknesses . Up to a thickness 5 C/cm2 , regular truncated hexagonal pyramids with steps were noticed . On increase of thickness to 6 C/cm2 a roof-tile type of deposit was observed (Fig . 3) . At 10 C/cm2 tiles were still found ; however their size was small . (b) from solution containing benzaldehyde . When the benzaldehyde content in the bath was 10-10 M there was no change in morphology compared to the deposit obtained from pure solution except for the truncation of hexagonal pyramids (Fig . 3) and the number of pyramids per unit area . When the concentration of benzaldehyde was increased to 10-11 M there was complete truncation of hexagonal pyramids and there was a tendency to lose their shape . At 10--8 M benzaldehyde the surface of the crystal was covered with more truncated pyramids which were not symmetrical . There was complete transformation to roof-tile structure (cf Fig . 3) at 10 -7 M benzaldehyde_ When the benzaldehyde was further increased, more of roof tile type of deposit grew, and at 10 -5 M, the size of the roof-tile decreased (Fig . 4) . At 10-4 M, the roof-tile deposit gradually broke up and the surface was covered with a black substance . Only polycrystalline deposits were found at 2 . 5 x 10-4 M benzaldehyde (Fig . 5) . Deposits from highly purified solution and at the lower concentrations of benzaldehyde were bright . The polycrystalline deposit was dull . When deposition was carried out at various cds with 10 -6 M benzaldehyde to
Ftc . 1 . Zinc deposited on copper (111) face from zinc sulphate and ammonium sulphate bath at 2 mA/cm 2 . 625 x . Ftc . 2 . Zinc deposited on copper (111) face from zinc sulphate and ammonium sulphate bath + 10 1 0 M benzaldehyde at 5 mA/cm 2 . 625 x . Ftc . 3 . Zinc deposited on copper (111) face from zinc sulphate and ammonium sulphate bath at 5 mA/cm 2 , thickness - 6 C/cm" . 625 x . Fio . 4 . Zinc deposited on copper (11 1) face from zinc sulphate and ammonium sulphate bath I 10 , M benzaldehyde at 5 mA/cm' . 625 x . Fic . 5 . Zinc deposited on copper (111) face from zinc sulphate and ammonium sulphate bath t 2 . 5 x 10 ° M benzaldehyde at 5 mA/cm= . 625 x . FIG . 6 . Zinc deposited on copper (100) face from zinc sulphate and ammonium sulphate bath at 2 mA/cms . 625 x .
1896
Fie . 7. Zinc deposited bath Fie . 8 . Zinc deposited bath Fio . 9. Zinc deposited
on copper (100) face from zinc sulphate and at 5 mA/cms, thickness - 10 Clcm° . 625 x . on copper (100) face from zinc sulphate and f 10-' M benzaldehyde at 5 mA/cmr . 625 x on copper (110) face from zinc sulphate and bath at 2 mA/cms. 625 x .
ammonium sulphate ammonium sulphate . ammonium sulphate
Electrocrystellization of zinc on (111), (100) and (110) faces of copper
1897
evaluate the Tafel slope. Roof-tile (Fig . 3) deposits were found at 2 . 5 and 10 mA/cm2 , whereas at 15 and 20 mA/cm9 only truncated hexagonal pyramids were observed . On (100) face : (a) from pure solution . Zinc was deposited on the copper (100) face from the highly purified bath at 2 mA/cm2 showed a roof-tile or small layer type of deposit (Fig . 6), aligned along [110] direction . At 5 mA/cm$ the distance between layers was larger than those obtained at 2 mA/cm 2 , and at 10, 15 and 20 mA/cms layers were also obtained with the distance between them larger (Fig . 7) . At 5 mA/cm2 in pure solution, deposits of various thickness were obtained . At 1 C/cm$ layers were observed but the edges were not well defined . At 2 C/cm2 , regular layers were observed . As the thickness was increased the distance between layers slightly increased. Even at 10 C/cm$ uniform layers (Fig . 7) with regular edges were observed. The deposits obtained at and above 10 C/cm 2 were more uniform than those obtained at 5 C/cm8 and less . (b) from solution containing benzaldehyde . Deposits obtained from the bath containing 10-10 M to 10'8 M benzaldehyde at 5 mA/cm$ were almost the same as the one obtained from pure solution . However the distance between layers decreased as the concentration of addition agent was increased ; at 10' 8 M they were almost touching each other. At 10- M, the layers broke up completely (Fig . 8) and were transforming into a polycrystalline deposit. At 5 x 10--5 M benzaldehyde only a polycrystalline deposit with a black substance covering the surface was observed (cf Fig . 5) . Deposition of zinc at various cds with 10 -8 M of benzaldehyde was carried out to evaluate the Tafel slope . Only layer-type deposits were obtained at 2, 5, 10, 15 and 20 mA/cms ; the distance between the layers increased as the cd was increased . On(110)face : (a) from pure solution . Deposits up to 5 C/cm 2 from pure solution at 2 mA/cm$ were ridge type (Fig . 9) ; the ridges were smooth, and aligned along the [100] direction . At 5 mA/cms, the ridges were thicker and were very sharp . At 10, 15 and 20 mA/cm$ the ridges were still thicker. More uniformity in the deposit was noticed at low ods than at high . Deposits at 5 mA/cm$ to various thickness showed differences . At 2 C/cm2 the ridges were thin and short ; They did not cover the whole surface but were patchy . As the thickness was increased the ridges became longer and covered the surface completely . At 10 C/cm$ they became thicker very uniform throughout the surface . (b) from solution containing benzaldehyde. There was no change in type of deposit when the benzaldehyde content was 10 -10 M . At 10'9 M the ridges were thinner, and at I0'e M the ridges were breaking up . At 10-7 M there was polycrystalline material between one set of ridges and others . A polycrystalline type of deposit with black lumps here and there was observed (cf Fig . 5) with benzaldehyde > 10-9 M . Overpotentials On (111) face . Overpotential at all cds during deposition from pure solution and from those containing benzaldehyde decreased with time and attained a steady value . The overpotential was a little less in presence of benzaldehyde than in pure solution . The steady state was reached more or less at the same thickness, 2 C/em' of the deposit . The Tafel slope in pure solution was 110 ± 5 mV for both initial and final values 10
B . S . SH,EQHADai and
1898
T. H. V. SErrr
of overpotential, with an initial exchange cd of 1-3 mA/cm 2 and a final of 1-8 mA/cm2 . Benzaldehyde had only a small influence on the Tafel slope and the exchange cd (Table 1) . TABLE I . TAFEL SLOPES AND EXCHANGE CDs (ia ) FOR DEPOSrnoN OF ZINC ON COPPER CRYSTAL PLANES Concentration Plane
of benzaldehyde
(111) (100) (110)
0 10-6 0 10 - ' 0 10 - '
io mA/cm'
Slope mV
Initial f5 mV
Steady
f5 mV
1150 109-0
110 .0 115 . 0 110 .0 1090
1146 105-0 1060 109 . 8 116-0 1140
Initial
Steady
1-3 1-6 2. 1 2.3 2-5 3-4
1 .8 2.1 1-5
1 .8 1 .9 3-0
On (100) face . Overpotential during deposition increased with time and attained a steady value for pure and benzaldehyde-containing solutions at all cds . The overpotential value little less with be nza ldehyde present . The steady state was attained at the same thickness of the deposit . The Tafel slope was 110 f 5 mV for both initial and final values of overpotential, with exchange cd initially of 2-1 mA/cms and finally 1-5 mA/cms . Benzaldehyde had little influence (Table 1) . On (110) face . Overpotential during deposition increased slightly with time and attained a steady state at all cds, with pure solution and up to 10 6 - M benzaldehyde . At 1" M benzaldehyde and upwards the overpotential was constant throughout the deposition . The Tafel slope was 110 1- 5 mV for both initial and final values of overpotential in pure solution, with exchange cd initially 2-5 mA/cm 2 and finally 1-9 mA/cm 2 . Benzaldehyde had little influence (Table 1) . DISCUSSION
Keen and Farrt studied the morphology of copper deposits on the copper (111) plane, of zinc on the zinc (0001) plane and of zinc on the copper (111) plane . They observed the same types of deposit in all the three cases . They stated that the zinc deposit grows epitaxially on the copper (100) and (110) planes . It is known that the arrangement of the surface atoms on the copper (111) plane and the zinc (0001) plane are similar . However, the crystal symmetry and lattice constants for copper and zinc are different, copper being fcc and zinc hcp . On the copper (100), (110) and (111) planes the [110] row is presents and the atomic distance along the [110] direction is 2-566 A . In the zinc (0001) plane, the atomic distance along the [1120] row is 2-644 A . The misfit between the [110] row of copper and the [1120] row of zinc is 4-22 per cent ; hence the zinc (0001) plane fits well with the copper (100) along the [1120] and [110] rows respectively . 7 .8 There is thus a possibility for zinc to grow epitaxially on the copper (111), (100) and (110) planes . During this stage of epitaxial growth, the crystallographic orientation is fixed . When once
Electrocrystalization of zinc on (111), (100) and (110) faces of copper
1899
the copper plane is covered with the zinc (0001) plane, further growth of zinc is similar to that of zinc deposition on the zinc (0001) plane . Keen and Farr4 did not notice hexagonal pyramidal growth on the copper (111) face . We have however shown above that hexagonal pyramids of zinc do grow on the copper (111) plane, supporting the above argument . When zinc is deposited on the copper (111) face the overpotential decreases to a steady value, as also observed for copper deposition on the copper (111) face,° doubtless because of the similarity in the growths . Similarly, zinc deposits on the copper (110) and (100) planes are ridges and layers respectively, and here again the same trend of overpotential with time as for copper deposition on copper is observed . The 1° values obtained from data at the initial stage of zinc deposition on copper single crystal planes are not the same as those for copper deposition on copper ; however the order is ioeiii) < loclool < locnoi as observed for copper deposition . 0 Hence the change of overpotential with time can be explained as in the copper case by Damjanovic et al° The presence of benzaldehyde had little influence on the change of polarization with time or on the Tafel slope . This indicates that the first charge transfer is the rate-determining step . Thus the morphological changes are not affected by electrodekinetics parameters . The change of morphology in presence of benzaldehyde may be attributed only to adsorption . Hexagonal pyramids of zinc deposit obtained on the copper (111) face in pure solution, truncate when benzaldehyde is present at low concentrations . Perhaps the addition agent is adsorbed on the apex of pyramids, preventing the generation of steps from the apex so that vertical growth of pyramids stops : then the pyramids can grow only sideways and by overlapping produce roof-tile deposits . At higher concentration, the surface of the copper substrate may be covered more nearly completely so that it cannot influence the growth of the zinc deposit . Zinc may now nucleate randomly and produce the polycrystalline deposit . The layer and ridge growths of zinc on the copper (100) and (110) respectively can also be explained in the same manner as in case of copper deposits .'O Zinc ions are discharged on the copper (100) plane randomly and the zinc adions diffuse to the microsteps and kinks in the copper [110] row and begin to grow by movement of microsteps . These microsteps become bunched and give rise to macrosteps on the (100) face . The layers are all aligned along the [110] direction, supporting this view . On the (110) face the zinc adions may diffuse along [110] rows and become incorporated at suitable sites, giving rise to ridge deposits . However the growth mechanism of ridges is not yet entirely clear . At higher concentration of benzaldehyde, the copper (100) and (110) surfaces may be more nearly completely covered with benzaldehyde . Hence the adions will nucleate randomly, as they are growing on an adsorbed film ofbenzaldehyde, favouring the formation of polycrystalline growth . We conclude that zinc grows epitaxially on copper crystal planes, giving the same type of deposits as does copper . Acknowledgements-The authors wish to express their grateful thanks to Dr . Ugo Bertocci, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, U .S .A. for the gift of copper single crystals . One of us (B . S . S .) expresses his thanks to Bangalore University for financial support in the form of a Research Assistantship .
1900
B. S. SHESHADru and T. H . V . SErrv
REFERENCES 1 . G. I, FINCH and D. N. LArrox, J. Electrodep . tech . Soc . 27,1 (1951) . 2. G. I. FINCH, H. WH.FtAN and L. YANG, Trans Faraday Soc. 43A 144 (1947). 3. A . GoswAtin, Ind. J. Pure appl. Phys . 7, 232 (1962) . 4. J. M. KEEN and J. P. G. FARR, J. electrochem . Soc . 109, 668 (1959). 5. S . NAGESwARA and T. H. V . SErrv, Proc. Indian Acad. Sci . No. 4, Sec. A, 178 (1968) . 6. R. Ptomnn, Conference a I' Universite Libre de Bruxelles accord culturel Italo-Belge p.15. (1966) . 7. C . S. BARRErr, Structure of Metals, p . 7. McGraw-Hill, New York (1952). 8 . C . S. BARRSrr, Structure of Metals, p . 11 . McGraw-Hill, New York (1952) . 9. A. DAwANovtc, T. H. V. Surrr and J. O'M Boccus, J. electrochem. Soc. 113, 429 (1966) . 10 . B . S. SHESHADRr and T. H. V. SE=, Indian J. Chem. 8, 162 (1970) .
ELECTROCRYSTALLIZATION OF ZINC ON (111), (100) AND (110) FACES OF COPPER* B . S . SHESHADRI and T. H . V. SETTY Department of Chemistry, Central College, Bangalore University, Bangalore-1 . India Abstract-Epitaxial growths of zinc electrodeposits deposited on copper single crystal faces have been observed when deposited from highly purified zinc sulphate and ammonium sulphate bath at eds between 2 and 20 mA/cm' . At all cds on the (I11) face hexagonal pyramids are observed, on the (100) face layers appear and on the (110) face a ridge type of deposit is found . The presence of benzaldehyde in small quantities affects the growth habits of zinc and cathodic polarization . An attempt is made to explain the mechanism of the deposition process from the overpotential/ed relationship . The changes in the growth habits of zinc are attributed to adsorption of benzaldehyde on the substrate. Resetme-On observe sat Ics faces monocritallines du cuivre des excroissances 6pitaxiales de zinc 6lectrodeposes a partir des bains de sulfate de zinc et de sulfate d'ammonium hautement purifies A des densit6s de courant comprises entre 2 et 20 mA/cm' . A toute densit6 de courant on observe des pyramides sur la face hexagonale (111) l'apparition de couches sur la face (100) et sur la face (110), un depot sur 1'ar6te . La presence d'une petite quantite de benzald6hyde affecte la croissance habituelle du zinc et la polarisation cathodique_ On essaic d'expliquer le m6canisme du processus de deposition A partir de la relation surtension/densite de courant . Les changements dans la fagon habituelle du zinc de se developper sont attribu6s A 1'adsorption de la benzald6hyde sur le substrat . Zusammertfasnmg-Man beobachtete epitaxiales Wachstum von elektrolytisch auf Kupfer-Einkristallfldchen abgeschiedenem Zink fiir Abscheidungen aus hochreinen Zinksulfat- und Ammoniumsulfatbadern bei Stromdichten zwischen 2 and 20 mA/em' . Bei alien Stromdichten beobachtete man auf den (111)-Fldchen hexagonale Py€amides, auf den (100)-Fliichen erschienen Schichten and auf der (110)-Flache fand man cine kammformige Ablagerung . Die Gegenwart von kleinen Mengen Benzaldehyd beeinflusst das Wachstumsverhalten von Zink and die kathodische Polarisation . Man unternahm einen Versuch, den Abscheidungsmechanismus mit Hilfe der Ueberspannungs/ Stromdichte-Beziehung zu erkldren . Die Unterschiede im Wachstumsverhalten des Zinks werden der Absorption von Benzaldehyd auf dem Substrat zugeschrieben . IN MOST electrocrystallization studies, the deposit and the substrate are of the same metal, usually copper . But in all technical processes the deposited metal is different from the substrate metal . There have been a few studies on the structure of electrodeposits when the deposited metal and the substrate metal are different ;' --3 these deposits grow epitaxially when the bath conditions and lattice parameters are favourable . Keen and Faro have studied the morphology of zinc on copper single crystal planes from an acid zinc sulphate bath ; they noticed that the morphology of zinc deposits are similar to the morphology of copper deposited on copper single crystal planes . Hence it is interesting to extend this study under different conditions
in detail . EXPERIMENTAL TECHNIQUE Analytical grade zinc sulphate was recrystallized twice using freshly distilled conductivity water . In each case only first crystallized zinc sulphate was used and the remaining liquid was rejected . Ammonium sulphate (Analar) was recrystallized once . A bath of composition 0-25 M ZnSO 4 ± 0 . 1 M (NHa 2 SO 4 was prepared . A desired amount of benzaldchyde was added to the highly purified zinc sulphate and ammonium sulphate bath when required . * Manuscript received 16 October 1971 . 1895
1 896
B . S . SaEsn .4Dxr and T . H. V. SE=
The single crystal copper cathodes were fixed in Tygon tubing so that only the desired face of the crystal was exposed, namely (100), (110) or (111) . The singlecrystal plane was first mechanically polished together with the Tygon tubing on emery paper, using ethyl alcohol as lubricant and finishing with 410 . The crystal plane was cleaned well with alcohol and finally washed with running distilled water for 10 min . Then it was electropolished in 50% orthophosphoric acid at 1 .2 V for 30 min with a large copper cathode in a electropolishing cell . The crystal was taken out without switching off the current and washed with 10% phosphoric acid and then with conductivity water . It was immediately transferred to the deposition cell . 5 A polycrystalline zinc foil (Analar) whose area was 40 times greater than that of cathode was used as anode . A freshly prepared zinc electrode was used as reference electrode . A battery of ca 90 V connected in series with a resistor was used as a source of constant current . The overpotential was recorded at regular intervals with an accuracy of ±5 mV using a V .T .V.M . Deposition at a particular cd was usually carried out up to a thickness equivalent to 5 C/cm 2 . The cathode was removed without switching off the current and was washed well with conductivity water and then with ethyl alcohol . The dried surface of the deposit was examined microscopically . Each experiment was repeated thrice to ensure reproducibility . RESULTS Morphology On (111) face : (a) from pure solution . Hexagonal pyramids (Fig . 1) with steps on their sides were obtained when zinc was deposited on the (111) copper face from the highly purified zinc sulphate and ammonium sulphate bath at 2 mA/cm 2 . At 5 mA/cm2 the density of hexagonal pyramids increased and they were all truncated . At 10 mA/cm2 , there was more truncation (Fig . 2) . The steps grew sideways ; at 15 and 20 mA/cm2 more pyramids got truncated and were growing sideways . The deposits at 5 mA/cm2 were studied at various thicknesses . Up to a thickness 5 C/cm2 , regular truncated hexagonal pyramids with steps were noticed . On increase of thickness to 6 C/cm2 a roof-tile type of deposit was observed (Fig . 3) . At 10 C/cm2 tiles were still found ; however their size was small . (b) from solution containing benzaldehyde . When the benzaldehyde content in the bath was 10-10 M there was no change in morphology compared to the deposit obtained from pure solution except for the truncation of hexagonal pyramids (Fig . 3) and the number of pyramids per unit area . When the concentration of benzaldehyde was increased to 10-11 M there was complete truncation of hexagonal pyramids and there was a tendency to lose their shape . At 10--8 M benzaldehyde the surface of the crystal was covered with more truncated pyramids which were not symmetrical . There was complete transformation to roof-tile structure (cf Fig . 3) at 10 -7 M benzaldehyde_ When the benzaldehyde was further increased, more of roof tile type of deposit grew, and at 10 -5 M, the size of the roof-tile decreased (Fig . 4) . At 10-4 M, the roof-tile deposit gradually broke up and the surface was covered with a black substance . Only polycrystalline deposits were found at 2 . 5 x 10-4 M benzaldehyde (Fig . 5) . Deposits from highly purified solution and at the lower concentrations of benzaldehyde were bright . The polycrystalline deposit was dull . When deposition was carried out at various cds with 10 -6 M benzaldehyde to
Ftc . 1 . Zinc deposited on copper (111) face from zinc sulphate and ammonium sulphate bath at 2 mA/cm 2 . 625 x . Ftc . 2 . Zinc deposited on copper (111) face from zinc sulphate and ammonium sulphate bath + 10 1 0 M benzaldehyde at 5 mA/cm 2 . 625 x . Ftc . 3 . Zinc deposited on copper (111) face from zinc sulphate and ammonium sulphate bath at 5 mA/cm 2 , thickness - 6 C/cm" . 625 x . Fio . 4 . Zinc deposited on copper (11 1) face from zinc sulphate and ammonium sulphate bath I 10 , M benzaldehyde at 5 mA/cm' . 625 x . Fic . 5 . Zinc deposited on copper (111) face from zinc sulphate and ammonium sulphate bath t 2 . 5 x 10 ° M benzaldehyde at 5 mA/cm= . 625 x . FIG . 6 . Zinc deposited on copper (100) face from zinc sulphate and ammonium sulphate bath at 2 mA/cms . 625 x .
1896
Fie . 7. Zinc deposited bath Fie . 8 . Zinc deposited bath Fio . 9. Zinc deposited
on copper (100) face from zinc sulphate and at 5 mA/cms, thickness - 10 Clcm° . 625 x . on copper (100) face from zinc sulphate and f 10-' M benzaldehyde at 5 mA/cmr . 625 x on copper (110) face from zinc sulphate and bath at 2 mA/cms. 625 x .
ammonium sulphate ammonium sulphate . ammonium sulphate
Electrocrystellization of zinc on (111), (100) and (110) faces of copper
1897
evaluate the Tafel slope. Roof-tile (Fig . 3) deposits were found at 2 . 5 and 10 mA/cm2 , whereas at 15 and 20 mA/cm9 only truncated hexagonal pyramids were observed . On (100) face : (a) from pure solution . Zinc was deposited on the copper (100) face from the highly purified bath at 2 mA/cm2 showed a roof-tile or small layer type of deposit (Fig . 6), aligned along [110] direction . At 5 mA/cm$ the distance between layers was larger than those obtained at 2 mA/cm 2 , and at 10, 15 and 20 mA/cms layers were also obtained with the distance between them larger (Fig . 7) . At 5 mA/cm2 in pure solution, deposits of various thickness were obtained . At 1 C/cm$ layers were observed but the edges were not well defined . At 2 C/cm2 , regular layers were observed . As the thickness was increased the distance between layers slightly increased. Even at 10 C/cm$ uniform layers (Fig . 7) with regular edges were observed. The deposits obtained at and above 10 C/cm 2 were more uniform than those obtained at 5 C/cm8 and less . (b) from solution containing benzaldehyde . Deposits obtained from the bath containing 10-10 M to 10'8 M benzaldehyde at 5 mA/cm$ were almost the same as the one obtained from pure solution . However the distance between layers decreased as the concentration of addition agent was increased ; at 10' 8 M they were almost touching each other. At 10- M, the layers broke up completely (Fig . 8) and were transforming into a polycrystalline deposit. At 5 x 10--5 M benzaldehyde only a polycrystalline deposit with a black substance covering the surface was observed (cf Fig . 5) . Deposition of zinc at various cds with 10 -8 M of benzaldehyde was carried out to evaluate the Tafel slope . Only layer-type deposits were obtained at 2, 5, 10, 15 and 20 mA/cms ; the distance between the layers increased as the cd was increased . On(110)face : (a) from pure solution . Deposits up to 5 C/cm 2 from pure solution at 2 mA/cm$ were ridge type (Fig . 9) ; the ridges were smooth, and aligned along the [100] direction . At 5 mA/cms, the ridges were thicker and were very sharp . At 10, 15 and 20 mA/cm$ the ridges were still thicker. More uniformity in the deposit was noticed at low ods than at high . Deposits at 5 mA/cm$ to various thickness showed differences . At 2 C/cm2 the ridges were thin and short ; They did not cover the whole surface but were patchy . As the thickness was increased the ridges became longer and covered the surface completely . At 10 C/cm$ they became thicker very uniform throughout the surface . (b) from solution containing benzaldehyde. There was no change in type of deposit when the benzaldehyde content was 10 -10 M . At 10'9 M the ridges were thinner, and at I0'e M the ridges were breaking up . At 10-7 M there was polycrystalline material between one set of ridges and others . A polycrystalline type of deposit with black lumps here and there was observed (cf Fig . 5) with benzaldehyde > 10-9 M . Overpotentials On (111) face . Overpotential at all cds during deposition from pure solution and from those containing benzaldehyde decreased with time and attained a steady value . The overpotential was a little less in presence of benzaldehyde than in pure solution . The steady state was reached more or less at the same thickness, 2 C/em' of the deposit . The Tafel slope in pure solution was 110 ± 5 mV for both initial and final values 10
B . S . SH,EQHADai and
1898
T. H. V. SErrr
of overpotential, with an initial exchange cd of 1-3 mA/cm 2 and a final of 1-8 mA/cm2 . Benzaldehyde had only a small influence on the Tafel slope and the exchange cd (Table 1) . TABLE I . TAFEL SLOPES AND EXCHANGE CDs (ia ) FOR DEPOSrnoN OF ZINC ON COPPER CRYSTAL PLANES Concentration Plane
of benzaldehyde
(111) (100) (110)
0 10-6 0 10 - ' 0 10 - '
io mA/cm'
Slope mV
Initial f5 mV
Steady
f5 mV
1150 109-0
110 .0 115 . 0 110 .0 1090
1146 105-0 1060 109 . 8 116-0 1140
Initial
Steady
1-3 1-6 2. 1 2.3 2-5 3-4
1 .8 2.1 1-5
1 .8 1 .9 3-0
On (100) face . Overpotential during deposition increased with time and attained a steady value for pure and benzaldehyde-containing solutions at all cds . The overpotential value little less with be nza ldehyde present . The steady state was attained at the same thickness of the deposit . The Tafel slope was 110 f 5 mV for both initial and final values of overpotential, with exchange cd initially of 2-1 mA/cms and finally 1-5 mA/cms . Benzaldehyde had little influence (Table 1) . On (110) face . Overpotential during deposition increased slightly with time and attained a steady state at all cds, with pure solution and up to 10 6 - M benzaldehyde . At 1" M benzaldehyde and upwards the overpotential was constant throughout the deposition . The Tafel slope was 110 1- 5 mV for both initial and final values of overpotential in pure solution, with exchange cd initially 2-5 mA/cm 2 and finally 1-9 mA/cm 2 . Benzaldehyde had little influence (Table 1) . DISCUSSION
Keen and Farrt studied the morphology of copper deposits on the copper (111) plane, of zinc on the zinc (0001) plane and of zinc on the copper (111) plane . They observed the same types of deposit in all the three cases . They stated that the zinc deposit grows epitaxially on the copper (100) and (110) planes . It is known that the arrangement of the surface atoms on the copper (111) plane and the zinc (0001) plane are similar . However, the crystal symmetry and lattice constants for copper and zinc are different, copper being fcc and zinc hcp . On the copper (100), (110) and (111) planes the [110] row is presents and the atomic distance along the [110] direction is 2-566 A . In the zinc (0001) plane, the atomic distance along the [1120] row is 2-644 A . The misfit between the [110] row of copper and the [1120] row of zinc is 4-22 per cent ; hence the zinc (0001) plane fits well with the copper (100) along the [1120] and [110] rows respectively . 7 .8 There is thus a possibility for zinc to grow epitaxially on the copper (111), (100) and (110) planes . During this stage of epitaxial growth, the crystallographic orientation is fixed . When once
Electrocrystalization of zinc on (111), (100) and (110) faces of copper
1899
the copper plane is covered with the zinc (0001) plane, further growth of zinc is similar to that of zinc deposition on the zinc (0001) plane . Keen and Farr4 did not notice hexagonal pyramidal growth on the copper (111) face . We have however shown above that hexagonal pyramids of zinc do grow on the copper (111) plane, supporting the above argument . When zinc is deposited on the copper (111) face the overpotential decreases to a steady value, as also observed for copper deposition on the copper (111) face,° doubtless because of the similarity in the growths . Similarly, zinc deposits on the copper (110) and (100) planes are ridges and layers respectively, and here again the same trend of overpotential with time as for copper deposition on copper is observed . The 1° values obtained from data at the initial stage of zinc deposition on copper single crystal planes are not the same as those for copper deposition on copper ; however the order is ioeiii) < loclool < locnoi as observed for copper deposition . 0 Hence the change of overpotential with time can be explained as in the copper case by Damjanovic et al° The presence of benzaldehyde had little influence on the change of polarization with time or on the Tafel slope . This indicates that the first charge transfer is the rate-determining step . Thus the morphological changes are not affected by electrodekinetics parameters . The change of morphology in presence of benzaldehyde may be attributed only to adsorption . Hexagonal pyramids of zinc deposit obtained on the copper (111) face in pure solution, truncate when benzaldehyde is present at low concentrations . Perhaps the addition agent is adsorbed on the apex of pyramids, preventing the generation of steps from the apex so that vertical growth of pyramids stops : then the pyramids can grow only sideways and by overlapping produce roof-tile deposits . At higher concentration, the surface of the copper substrate may be covered more nearly completely so that it cannot influence the growth of the zinc deposit . Zinc may now nucleate randomly and produce the polycrystalline deposit . The layer and ridge growths of zinc on the copper (100) and (110) respectively can also be explained in the same manner as in case of copper deposits .'O Zinc ions are discharged on the copper (100) plane randomly and the zinc adions diffuse to the microsteps and kinks in the copper [110] row and begin to grow by movement of microsteps . These microsteps become bunched and give rise to macrosteps on the (100) face . The layers are all aligned along the [110] direction, supporting this view . On the (110) face the zinc adions may diffuse along [110] rows and become incorporated at suitable sites, giving rise to ridge deposits . However the growth mechanism of ridges is not yet entirely clear . At higher concentration of benzaldehyde, the copper (100) and (110) surfaces may be more nearly completely covered with benzaldehyde . Hence the adions will nucleate randomly, as they are growing on an adsorbed film ofbenzaldehyde, favouring the formation of polycrystalline growth . We conclude that zinc grows epitaxially on copper crystal planes, giving the same type of deposits as does copper . Acknowledgements-The authors wish to express their grateful thanks to Dr . Ugo Bertocci, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, U .S .A. for the gift of copper single crystals . One of us (B . S . S .) expresses his thanks to Bangalore University for financial support in the form of a Research Assistantship .
1900
B. S. SHESHADru and T. H . V . SErrv
REFERENCES 1 . G. I, FINCH and D. N. LArrox, J. Electrodep . tech . Soc . 27,1 (1951) . 2. G. I. FINCH, H. WH.FtAN and L. YANG, Trans Faraday Soc. 43A 144 (1947). 3. A . GoswAtin, Ind. J. Pure appl. Phys . 7, 232 (1962) . 4. J. M. KEEN and J. P. G. FARR, J. electrochem . Soc . 109, 668 (1959). 5. S . NAGESwARA and T. H. V . SErrv, Proc. Indian Acad. Sci . No. 4, Sec. A, 178 (1968) . 6. R. Ptomnn, Conference a I' Universite Libre de Bruxelles accord culturel Italo-Belge p.15. (1966) . 7. C . S. BARRErr, Structure of Metals, p . 7. McGraw-Hill, New York (1952). 8 . C . S. BARRSrr, Structure of Metals, p . 11 . McGraw-Hill, New York (1952) . 9. A. DAwANovtc, T. H. V. Surrr and J. O'M Boccus, J. electrochem. Soc. 113, 429 (1966) . 10 . B . S. SHESHADRr and T. H. V. SE=, Indian J. Chem. 8, 162 (1970) .