electrochemical synthesis of oriented cuo coatings on stainless steel substrates: solution-mediated...

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Electrochemical Synthesis of Oriented CuO Coatings onStainless Steel Substrates: Solution-Mediated Control overOrientationSumy Joseph,a P. Vishnu Kamath,a,z and Sarala Upadhyab

aDepartment of Chemistry, Central College, Bangalore University, Bangalore 560 001, IndiabDepartment of Mechanical Engineering, University Visvesvaraya College of Engineering,Bangalore 560 001, India

Anodic oxidation of an alkaline �pH � 13� copper tartrate bath results in the deposition of an adherent CuO coating. Theorientation of the coating depends on the pH of the bath and thickness of the deposit. At pH 13, deposits are unoriented, whereasat pH 13.2 the deposits are oriented along the �020� direction. At pH 13.3, the orientation switches to �110� and at pH 13.5 to �100�.The switch in orientation takes place over a very narrow range of pH values. Scanning electron micrography reveals that theparticles have an ellipsoidal morphology. This is a consequence of twinning combined with rapid crystal growth normal to the 010plane. Different orientations are obtained by the rotation of the ellipsoidal particles about the minor axis through different angles.The minor axis of the ellipsoids is parallel to the c-crystallographic axis.© 2008 The Electrochemical Society. �DOI: 10.1149/1.3005959� All rights reserved.

Manuscript submitted July 10, 2008; revised manuscript received September 18, 2008. Published November 7, 2008.

0013-4651/2008/156�1�/E18/5/$23.00 © The Electrochemical Society

There is an abiding interest in simple, inexpensive semiconduct-ing oxides for possible applications in the field of oxide electronics.1

CuO is one such nontoxic oxide. CuO has a small indirect bandgapof 1.5 eV and has been extensively studied in light of itsphotocatalytic,2 field emission,3 and photovoltaic properties.4 CuOis also of interest as, among the 3d transition metal monoxides, it isthe only one to crystallize in the monoclinic crystal system �spacegroup: C2/c, a = 4.68 Å, b = 3.42 Å, c = 5.13 Å, � = 99.57°�.5

Given its importance from both the scientific and technologicalpoints of view, there has been considerable effort in the synthesis ofCuO, both chemically and electrochemically.

Chemically CuO is prepared by heating Cu at a high temperaturein O2 atmosphere.6 Alternatively, a CuO film can be produced bychemical precipitation of Cu�OH�2 on a substrate and decomposingit to CuO.7 CuO films can also be fabricated by radio frequencymagnetron sputtering8 and pulsed laser ablation.9 These techniquesare energy and capital intensive, however, and require high vacuumgeneration besides high-power laser/ion beam sources.

Electrochemical synthesis is an ambient-temperature soft chemi-cal route to the synthesis of inorganic materials.10 It facilitates thefabrication of oxide thin/thick films and coatings, a necessary pre-requisite for many device-based applications.

One of the significant attempts to make cupric oxide films elec-trochemically was by the potentiodynamic cycling of a reactive Cumetal electrode in a baryta bath.11 Potential cycling of reactive met-als in alkaline baths leads to the formation of hydrous oxides.12 Inthe case of Cu, a Ba-containing hydrous oxide precursor was ob-tained, which on thermal treatment yielded a superconductingcoating.11

Other methods of electrochemical synthesis of oxides involveanodic oxidation of a soluble metal salt,13 cathodic reduction of ametal nitrate solution which leads to electrogeneration of base,14 oranodic deposition of an acid-insoluble oxide such as V2O5 by elec-trogeneration of acid.15 In the latter two techniques, the metal ion isnot involved in any redox reaction; rather, the redox reactions of thesolvent or the counter ion are employed to deposit the oxide.

Ogura and co-workers16,17 carried out the electrochemical basehydrolysis of Cu-amino acid complexes to deposit CuO. Poizot andco-workers18 employed yet another mechanism in which the redoxchemistry of a coordinated ligand is employed to deposit an oxide. ACu-tartrate complex was used as the precursor in a bath of high pH�pH 13�. Anodic oxidation decomposes the ligated tartrate ion, re-leasing free Cu2+. The latter undergoes hydrolysis to form a CuOcoating at the anode. A distinctive feature of this process was that

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the deposited CuO coating was oriented along the �010� direction.These textured coatings were observed on polycrystalline Pt elec-trodes, indicating that the observed texture was solution-mediated.

When the deposition was carried out on a single-crystalline sub-strate such as Au�001�19 or Cu�111�,20 epitaxial CuO films havingthe �111� texture were grown. Further, X-ray pole figure measure-ments showed that the CuO films were chiral when racemically puretartaric acid was used and achiral when a racemic mixture of tartaricacid was used as a bath stabilizing agent. These studies show thecombined effect of epitaxy as well as solution mediation in thenucleation of oriented crystals.

The interest in oriented crystallization arises from the possibilityof obtaining different k-dependent properties for differently orientedcoatings.

In this paper, we further explore the anodic deposition of CuOand report the growth of CuO along the �110� and �100� directions.We show that the direction of orientation can be switched as a func-tion of the bath pH, deposition time, and the current density. Usingthe Rietveld refinement technique, the degree of orientation is quan-tized.

Experimental

CuSO4·5H2O, disodium tartrate, and sodium hydroxide werepurchased from Merck, India. All reagents were prepared using ion-exchanged type I water �Milli-Q Academic water purification sys-tem, specific resistance 18.2 M � cm�. All syntheses were carriedout using a EG&G �PARC� Versastat model IIA scanning poten-tiostat driven by model 270 electrochemistry software. The CuOcoatings were electrodeposited galvanostatically �current density2–8 mA cm−2; time 0.25–2 h� on stainless steel �SS 304� flag elec-trodes �surface area 4.5 cm2� used as anodes in an undivided cellwith a cylindrical Pt mesh electrode �surface area, 28 cm2� ascounter. A saturated calomel electrode was used as reference to mea-sure the cell potential. The bath was prepared by mixing equal vol-umes of CuSO4 �0.15 M� and disodium tartrate �0.2 M� solutions toget a final concentration of 0.075 M in Cu2+. The pH of the bath wasraised to 12 by adding NaOH pellets with constant stirring. The finalpH adjustments to a predetermined value in the range 12–13.6 wasdone by adding a 4 M NaOH solution dropwise. The bath was al-lowed to stand overnight in order to check the stability. No chemicalprecipitation was observed when �tartrate�/�Cu2+� = 1.33. Prior todeposition the electrodes were cleaned in detergent and electro-chemically as described elsewhere.21 After the deposition the coat-ings were rinsed with ion-exchanged type I water and dried at 60°C.The coatings were uniform and black in color. All coatings werecharacterized by powder X-ray diffractometry �PXRD� by directlymounting the electrode on a Bruker model D8 Advance powder

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diffractometer operated in reflection geometry. Data were collectedwith Cu K� radiation using a continuous scan rate of 1° 2� perminute or less and then rebinned into 2� steps of 0.02°. All PXRDprofiles were fitted by the Rietveld method �FullProf suite� using thepublished structure of CuO �International Crystal Structure Databaseno. 069094�. The quality of the fit was judged both from the Rvalues and by examination of the difference profile. The observationof systematic residual intensities in the difference profiles at posi-tions corresponding to different Bragg reflections was taken to beindicative of oriented crystallization.22 In such cases, the Rietveldrefinement was repeated by inclusion of the modified March’s func-tion to account for intensity variations due to preferred orientation.The March function has two refineable parameters, G1 and G2. 0� G1 � 1 corresponds to a crystal with a platy habit. G2 = 0 cor-responds to a fully oriented coating and G2 = 1 to a film without anypreferred orientation.

Three coatings were prepared under each experimental conditionto verify the reproducibility of their structure. Scanning electronmicrographs were recorded on a JEOL model JSM 6490 LV micro-

Figure 1. PXRD patterns of CuO coatings deposited from a tartrate bath atpH 13.2. Deposition times �a� 30 and �b� 60 min. Features marked by theasterisk are due to the stainless steel substrate.

Figure 2. �a� Difference profile obtained from �b� the Rietveld fit of theobserved PXRD profile with out the use of orientation parameters. �c� Dif-ference profile obtained from �d� the Rietveld fit of the observed PXRDprofile using March function parameters to account for 020 orientation. Fea-tures due to the substrate appear in the excluded region. Open circles: ob-served intensities. Line: calculated intensities.


scope directly from the coating on the substrate by mounting smallpieces of the electrode on conducing carbon tape and sputter coatingplatinum to improve conductivity.

Results and Discussion

The Pourbaix diagram of the Cu–H2O system predicts the for-mation of CuO at anodic potentials in the pH range 5–14.23 How-ever, the solubility of Cu2+ can be shifted to still higher ranges of pHby the use of different complexing agents.23,24 A pH 13 bath wasused to deposit CuO from a Cu2+ glycinate bath.16,17 The potentialpH diagram of the Cu2+–glycine–water system shows the stability ofCuO at this pH value.25 Although the potential–pH diagram for theCu-tartaric acid–water system is not known, Poizot andco-workers,18 carried out the anodic deposition of CuO in a Cu-tartrate bath at pH �13.2.

In Fig. 1 the PXRD patterns of CuO coatings obtained by us attwo different deposition times �30 and 60 min� are shown from aCu-tartrate bath at pH 13.2 �deposition current 6 mA cm−2�. The020 reflection has an anomalously high relative intensity as opposedto the intensity expected for the CuO standard �9.7%, PDF: 80–1268�. In fact, for the thick deposit �deposition time 60 min,3.2 �m�, the 020 reflection is the most intense. Based on our previ-ous work,22,26 we attribute the anomalous increase in the intensity ofselect reflections to oriented crystallization along the respectivecrystallographic directions. The evidence for this conclusion is given

Figure 3. PXRD pattern of an unoriented CuO coating deposited at pH 13.Features marked by the asterisk are due to the stainless steel substrate.

Figure 4. PXRD patterns of CuO deposited at �a� pH 13.3, �b� pH � 13.3,and �c� pH 13.5. Features marked by the asterisk are due to the stainless steelsubstrate.




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in Fig. 2. The difference profile �Fig. 2a� observed from the Rietveldfit of the observed data �Fig. 2b� shows the presence of systematicresidual intensity at the Bragg angle corresponding to the 020 re-flection. When the Rietveld refinement was repeated by incorporat-ing the March function to account for preferred orientation along020 �Fig. 2d�, a satisfactory fit was observed with a smooth differ-ence profile �Fig. 2c�. The refined values of G1 = 0.4 and G2 = 0point to the oriented growth of crystallites with a platy habit. �SeeTable I for the refined parameters.� These results are in agreementwith those reported by Poizot et al.18 It is further observed that thedegree of orientation increases with the deposit thickness, showingthat oriented growth is solution-mediated.

Figure 5. A Rietveld fit of the PXRD profile of the CuO sample obtained bygrinding together several oriented coatings of the type shown in Fig. 4c.


Having established the earlier results of Poizot et al.,18 as a vali-dation of our own experimental procedures, we proceeded to sys-tematically vary the bath pH and the deposition current.

No CuO deposition was observed at pH � 13 under a widerange of deposition conditions �current density 1–8 mA cm−2, depo-sition time duration 0.25–2 h�. At pH 13, at low �2 mA cm−2� cur-rent densities �cell potential 0.34 V� there was no deposition ofCuO. At a moderate �4 mA cm−2� current density �cell potential0.46 V�, the deposition of unoriented CuO coatings was observed.In Fig. 3 a typical PXRD profile of CuO deposited under theseconditions is shown. The relative intensities of the different Bragg

Figure 6. PXRD patterns of CuO coatings deposited at pH 13.5. Depositiontimes �a� 15, �b� 30, and �c� 60 min. Features marked by the asterisk are dueto the stainless steel substrate.

Figure 7. SEM images of CuO coatingsoriented along �a,b� �100�, �c� �110�, and�d� �010� directions.




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reflections match with that expected of the CuO standard. At high�6 mA cm−2� current densities �cell potential 0.5 V�, the peaks dueto CuO were lower in intensity and there was an indication of theformation of Cu2O as shown by the appearance of the 111 reflectionof Cu2O at 36.4° 2� �data not shown�.

In the next set of experiments, the pH of the bath was varied. Asdescribed earlier �Fig. 1�, at pH 13.2 the CuO coatings had a 020orientation. At pH 13.3 the relative intensity of the 020 reflectioncomes down and there is growth of the 110 reflection �Fig. 4a�. AtpH � 13.3, the 200 reflection begins to grow �see Fig. 4b� and at pH13.5, a single peak due to the 200 reflection is observed �see Fig.4c�. To confirm the identity of this phase, a PXRD pattern wasrecorded by grinding several such coatings together. A Rietveld fitusing the published structure of CuO �Fig. 5� clearly indicates thatthe single-peak PXRD pattern shown in Fig. 4c corresponds to fullyoriented phase-pure CuO.

The influence of deposition time and current density on thegrowth characteristic of CuO was also examined. As the pH is in-creased from 13 to 13.5, the range of current densities that giveadherent coatings has also widened. At pH 13.5, a good adherentcoating could be obtained at 8 mA cm−2, 30 min. In Fig. 6 thePXRD patterns of CuO obtained from a bath of pH 13.5 at4 mA cm−2 at different deposition times are shown. An increase inthe relative intensity of the 200 reflection can be observed withincrease in the deposition time. This shows that the orientation issolution-mediated. A Rietveld fit of the pattern given in Fig. 6cyielded values of G1 = 0.44 and G2 = 0 for the modified March’sfunction parameters.

A switch in orientation with change in bath pH is observed in theCu2O system as well.26 However, the switch in orientation was ob-served over a much larger range of pH values �9–12�.26 The questionarises as to why in the case of CuO, the switch in orientation occursover a much narrower range �0.2 units� of pH? The answer probablylies in the Pourbaix diagrams24,25 of the Cu system. A number ofdifferent species are stabilized within a very narrow range of pH,spanning 0.3–0.5 pH units. This narrow range appears either at theupper or the lower end of the region of pH 13–14, depending on thecomplexing agent used. As the electrodeposited species is criticallydependent upon the precursor solution species, it is likely that theorientation switches as the precursor species in solution changes.

All electrodeposited CuO coatings were examined by scanningelectron microscopy �SEM�. These studies reveal the morphology ofthe electrodeposited crystallites on the micrometer scale. In Fig. 7athe micrograph of a typical coating obtained at low magnification isshown. The coating is continuous and is comprised of closelypacked, submicrometer-sized grains. Under high magnification, thegrains reveal an ellipsoidal �coffee-bean� morphology �Fig. 7b-d�.

Crystal growth takes place by accretion of atoms over the sub-nanometer to micrometer length scale.27 Growth is most aggressivenormal to the crystal face having the highest surface energy.27,28 Agood estimate of the surface energy can be obtained from the surfaceatom density; a high atom density yields a low surface energy. InCuO, the surface energies of different low-index crystal planes, aspredicted from the surface Cu atom densities, decreases in the order010 � 100 � 001.29 Consequently, the highest rate of crystalgrowth is normal to the 010 plane. Because the unique, twofold axisof symmetry is parallel to the b-crystallographic axis, crystal growthtakes place along both �010� as well as �0-10� directions, leading tothe growth of twins. The contact twins share the 001 plane as thetwinning boundary �see Fig. 8a�.30 A parallel array of twins yields an

Table I. Lattice constants and goodness of fit parameters ob-tained for the Rietveld fit of the PXRD pattern of a CuO coatingoriented along the 010 direction.

a = 4.70 Å, b = 3.42 Å, c = 5.12 Å, � = 99.80°U = 1.68, V = −0.45, W = 0.01, G1 = 0.4, G2 = 0.0RBragg = 16.4, Rf = 15.6, Rwp = 24.9, �2 = 1.47


ellipsoidal morphology with the long axis of the ellipsoid parallel tothe b-crystallographic axis �Fig. 7 and 8b�. An ellipsoidal particlecan in principle rotate about its long axis and orient different crys-tallographic axes along the normal to the substrate. When this out-of-plane orientation is along the a-crystallographic axis, the 001planes are stacked on the substrate in an end-on fashion �approxi-mately normal to the substrate, � = 99.57°� and the particles appearstriated �see Fig. 7b and 8c�. The striations correspond to the twinboundaries. At other orientations the striations are not observed �seeFig. 7c for orientation along 110�. Rotation of the ellipsoidal par-ticles about the minor axis �c-crystallographic axis� brings the b axisnormal to the substrate. The ellipsoids are seen down the long axisas conical projections from the substrate surface �see Fig. 7d�. Theparticles have a granular surface. The orientation is, however, notcomplete and a few ellipsoids are seen with their long axis parallelto the substrate.

Figure 8. Schematic of �a� a contact twin, �b� an ellipsoidal particle obtainedby an end-on array of contact twins, and �c� detail of a striated CuO particleoriented along the �100� direction.




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Conclusions

In summary, oriented coatings of CuO were deposited on stain-less steel substrates. Oriented crystallization is a result of solution-mediated kinetic control over crystal growth. The direction of ori-entation is determined by the pH of the bath. By varying the bathpH, the orientation can be switched from �010� → �110� → �100�.This switch in orientation is observed over a narrow pH range. ThePourbaix diagram shows the existence of different solution speciesover the same range of pH values, indicating the possibility of aone-to-one relationship between the different solution species andthe crystal orientation. The ellipsoidal morphology of the crystallitesis a result of twinning and growth along the �010� directions. Thedifferent orientations are obtained by the rotation of the ellipsoidalcrystallites about the minor axis. The minor axis of the ellipsoidalcrystallites is parallel to the c-crystallographic axis.

Acknowledgments

The authors thank the University Grants Commission, Govern-ment of India �GOI�, for financial support. S.J. is a Junior ResearchFellow, and P.V.K. is a Ramanna Fellow of the Department of Sci-ence and Technology, GOI.

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