Growth of Different Nanostructures of Cu2O (Nanothreads, Nanowires, and Nanocubes) bySimple Electrolysis Based Oxidation of Copper

Dinesh Pratap Singh,† Nageswara Rao Neti,‡ A. S. K. Sinha,§ and Onkar Nath Srivastava*,†

Department of Physics, Banaras Hindu UniVersity, Varanasi-221005, India, National EnVironmentalEngineering Research Institute, Nehru Marg, Nagpur-440020, India, and Department of Chemical Engineering,Banaras Hindu UniVersity, Varanasi-221005, India

ReceiVed: September 3, 2006; In Final Form: NoVember 7, 2006

Cuprous oxide (Cu2O) nanostructures have been synthesized by anodic oxidation of copper through a simpleelectrolysis process employing plain water (with ionic conductivity∼6 µS/m) as an electrolyte. No specialelectrolytes, chemicals, and surfactants are needed. The method is based on anodization pursuant to the simpleelectrolysis of water at different voltages. Platinum was taken as cathode and copper as anode. The appliedvoltage was varied from 2 to10 V. The optimum anodization time of about 1 h was employed for each case.Two different types of Cu2O nanostructures have been found. One type was delaminated from copper anodeand collected from the bottom of the electrochemical cell and the other was located on the copper anodeitself. The nanostructures collected from the bottom of the cell are either nanothreads embodying beads ofdifferent lengths and diameter∼10-40 nm or nanowires (length∼600-1000 nm and diameter∼10-25nm). Those present on the copper anode were nanoblocks with a preponderance of nanocubes (nanocubeedge∼400 nm). The copper electrode served as a sacrificial anode for the synthesis of different nanostructures.A tentative mechanism for the formation of Cu2O nanostructures has been suggested. The present workrepresents the first such attempt where Cu2O nanostructures were formed under the oxidation induced by assimple a process as electrolysis of plain water. Both anodization potential and time influence the morphologyof nanostructures of Cu2O. Thus, nanothreads are formed at 6 V during 15-30 min, whereas nanowiresresult when anodization time is extended to 45-60 min. Also two different types of Cu2O nanostructures,one which is present in the solution (nanothreads and nanowires) and the other which is located on the copperanode (nanocubes), are synthesized in the same electrolysis run. The optical band gap as calculated from theUV-visible absorption spectra of the nanothreads and nanowires corresponds to 2.61 and 2.69 eV, respectively,which is larger than the known band gap (2.17 eV) of bulk Cu2O.

Introduction

One-dimensional nanostructures not only open new op-portunities in the field of microelectronics but also providemodels for studying the effect of dimensionality and sizeconfinement on electrical, transport, and mechanical properties.These nanostructures have attracted considerable attention owingto their novel physical and chemical properties, and the potentialapplications in a new generation of nanodevices.1-3 The copperoxide (Cu2O) nanostructure has attracted significant attentionas it is one of the first known p-type direct band gapsemiconductors4 with a band gap of 2.17 eV. This makes itpromising material for the conversion of solar energy intoelectrical or chemical energy.5 The growing interest in Cu2Onanostructures is due to several reasons. Some of these are thefollowing: (1) Cu2O is a potential photovoltaic material that islow cost and nontoxic and can be prepared in large quantitiesdue to natural abundance of the base material copper,6-8 (2)excitons created in Cu2O have been shown as suitable candidatesfor Bose Einstein Condensate because of the large excitonbinding energy of 150 meV,9-11 (3) Cu2O is a basic compound

for superconducting material, (4) Cu2O nanostructures can beused as high performance gas sensors, (5) submicron Cu2Ohollow spheres12,13 can be used as the negative electrodematerials for lithium ion batteries,14 and (6) Cu2O has beenreported to act as a stable catalyst for water splitting undervisible light irradiation.15,16The Cu2O nanostructures13,17-21 havebeenpreparedbyseveraldifferentmethodssuchaselectrodeposition,22-24

the sonochemical method,25 thermal relaxation,26 liquid-phasereduction,27 the complex precursor surfactant-assisted (CPSA)route,28,29 and vacuum evaporation.30 On the basis of theseapproaches the synthesis of Cu2O nanostructures demandscomplex process control, high reaction temperatures, longreaction times, expensive chemicals, and a specific method forspecific nanostructures. We have been carrying out investiga-tions on the synthesis of nanostructures of several materials.31-33

In this paper we report a novel and simple one-step process forthe synthesis of different Cu2O nanostructures based on waterelectrolysis in electrochemical conditions using platinum as thecathode and copper as the anode.

On operating the Pt/H2O/Cu cell in the electrolysis modeunder different operating voltages 2, 4, 6, 8, and 10 V, theelectrolytic reactions lead to formation of Cu2O nanostructureson selective sites of the Cu-electrode. One type of Cu2Onanostructure delaminated from the Cu electrode falls intosolution and can be collected from the bottom of the electrolytic

* To whom correspondence should be addressed. E-mail: hepons@yahoo.com. Fax:+91-542-2368468. Phone:+91-542-2368468.

† Department of Physics, Banaras Hindu University.‡ National Environmental Engineering Research Institute.§ Department of Chemical Engineering, Banaras Hindu University.

1638 J. Phys. Chem. C2007,111,1638-1645

10.1021/jp0657179 CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 01/04/2007

cell. In addition to the delaminated nanostructures, investigationsof the copper anode, which were subjected to electrolysis runs,revealed the presence of another type of nanostructure of Cu2O.

Experimental Details

The process followed in the present investigation makes useof an inexpensive two-electrode (Pt: cathode; Cu: anode) setupwith 5 mL of plain water as the electrolyte. Copper works as asacrificial anode and forms cuprous oxide nanostructures.Copper (99.9%) sheet (2 cm× 1 cm) as the anode and platinumsheet (2 cm× 1 cm) as the cathode have been employed forthe electrolysis. Before anodizing, the copper sheet was polishedto a mirror finish with 0.2µm alumina powder (to remove nativeoxide film) and then rinsed in distilled water. The distancebetween the cathode and the anode was kept at 1 cm. Theelectrolysis cell was made up of Perspex (4 cm× 3 cm × 3cm) and the plain water (double distilled with very small ionicconductivity ∼6 µS/m and pH 6.7) served as the electrolyte.The electrolysis was performed at different applied voltages (2,4, 6, 8, and 10 V) without any potentiostatic control, for a timeperiod of 1 h, which we determined as the optimum anodizationtime. During anodization some fine particulate material settledto the bottom of the cell. Structural/microstructural characteriza-tion of these fine particles has been carried out employingscanning (XL-20) and transmission electron microscopic (PhilipsEM, CM-12, and Technai 20G2) techniques. The fine particle-like materials, which are collected at the bottom of the cell,were taken out and dried on a Formvar coated copper grid andexamined under TEM. The copper anode was also examined

employing SEM for any possible microstructural variation. TheUV-visible absorption spectra were recorded on a Model-VARIAN, Cary 100, Bio UV-visible spectrophotometer.

Results and Discussion

The morphology and microstructures of the particles obtainedat different applied voltage have been investigated employingTEM. Figure 1a is a representative TEM micrograph of theparticles obtained after electrolysis at 2 V for 1 h and collectedfrom the bottom of the cell. Similar structures were obtainedfor 4 V. As can be seen these represent nanothread-likestructures of unspecified length comprised of nanobeads (beadsize∼10-40 nm). These structures revealed that the nanobeadscoalesced together to form the nanothread as shown in Figure1b. The TEM image indicated that the nanothreads resulted dueto the agglomeration of the bead-like particles. The representa-tive selected area electron diffraction (SAED) pattern is shownin Figure 1c. The diffraction pattern from nanothreads can beindexed to a cubic system with lattice parametera ) 0.4269(0.005 nm. This tallies quite well with the lattice parameter ofCu2O showing that the material formed under electrolysisconditions consists of cubic Cu2O lattice structure.

A representative TEM micrograph of the material collectedfrom the bottom of the cell after electrolysis at 6 V for 1 h isshown in Figure 2a. As can be seen, the Cu2O structures hereconsist of nanowires (length∼600-1000 nm and diameter∼10-25 nm). It should be mentioned that the total length ofthe Cu2O nanothread and nanowire was comprised of severalsegments. These were presumably formed due to interaction

Figure 1. (a) Typical transmission electron micrographs of Cu2O nanothreads embodying beads, as collected at the bottom of the cell after electrolysisat 2 V for 1 h. (b) Figure showing coalesced beads forming nanothreads (c) Selected area electron diffraction (SAED) pattern from the nanothread-like configuration. The SAED reveals that the nanostructures are cubic Cu2O.

Growth of Different Nanostructures of Cu2O J. Phys. Chem. C, Vol. 111, No. 4, 20071639

between nanothreads/nanowires forming the network in whichthe Cu2O nanothread/nanowire configuration finally appeared.A Cu2O nanowire because of its interaction with other nanowiresconsists of several bent segments. When the electrolysisconditions were maintained at 10 V for 1 h, the representativeTEM microstructure revealed the presence of a dense Cu2Onanowire network (length∼1000 nm, diameter∼10-25 nm)as shown in microstructures and the selected area electrondiffraction (SAED) pattern in Figure 2b. A magnified TEMmicrograph of the nanowire can be seen in the Figure 2c. Itmay be mentioned that with the increase in applied voltage from2 to 10 V, the structures formed in the electrolyte solution (in

situ) varied from beads to nanowire network and the applicationof voltage higher than 10 V did not produce any newnanostructure other than nanowires. At higher voltage (>10 V)we are still obtaining nanowires in the electrolyte with anincreased density.

Panels a-d of Figure 3 are the SEM images of the copperanode after electrolysis at 2, 4, 8, and 10 V for 1 h. Figure 3ais the SEM micrograph of a copper sheet after electrolysis at2 V for 1 h. The microstructure reveals two types of regions.One of these corresponds to etched regions devoid of materialat the surface (marked by white arrows). The second is a regionindicating overgrowth of material on the surface of the electrodeas a result of electrolysis-induced oxidation (indicated by blackarrows). As the electrolysis voltage is increased to 4 V, thesecond type of region, i.e., the overgrowth region, startsdeveloping into irregular blocks (Figure 3b). As the voltage isincreased further to 6 and 8 V, the block-like overgrowth regionstarts taking cube-like shape as shown in Figure 3c. When theapplied voltage was 10 V, regular cube-like microstructures wereformed. Figure 3d exhibits typical nanocubes with cube edge∼400 nm. This type of morphological changes in Cu2O, bringingout block-like nanostructures, have also been observed throughin situ thermal oxidation in TEM.34 It may be pointed out thatalthough the cube edge is rather large, keeping in view thenomenclature used by other workers,19,35,36these may still becalled nanocubes.

Tentative Mechanism for the Formation of Cu2O Nano-structures. We first propose a feasible electrochemical reactionfor the synthesis of Cu2O under electrolysis conditions at thecopper sacrificial anode. The electrochemical situation in thepresent case [Pt/water (mildly acidic)/Cu] with applied voltagealways larger than∼1.23 V corresponds to electrolysis withmoderate to vigorous oxygen (and hydrogen) evolution. Theoperative chemical reactions are expected to be as in thefollowing:

where SA stands for surface adsorbed species. Under thecondition as outlined by (1), a continuous layer of adsorbedoxygen on the Cu anode is expected. This would be asadsorbed oxygen [O]ads. They can combine to form O2, whichevolves as oxygen. The anode (Cu) will generate Cu2+, whichon reacting with the (O-O)SA will form Cu2O. Anotherpossibility is the reaction of Cu2+ with water or OH- to formCu(OH)2. This may lose water when it comes in contact withair. The dehydration will lead to Cu(OH)2 f Cu2O + H2O.Since Cu(OH)2 could not be detected to exist with Cu2O, itappears that in the present case Cu2O is formed due toreaction of Cu2+ with surface adsorbed oxygen (2Cu2+ +(O)SA f Cu2O).

Growth of Different Nanostructures of Cu2O (Nano-threads, Nanowires, and Nanocubes).Keeping in view theobserved microstructures of the Cu anode (Figure 3a), wepostulate that the specific sites where oxidation takes place onthe surface are of two types. One of these is small regions

Figure 2. (a and b) Representative TEM micrographs of the denseCu2O network of nanowires, obtained after electrolysis at 6 and 10 Vrespectively for 1 h; the inset in panel b shows the SAED pattern fromthese nanowires. (c) The magnified TEM micrograph of the nanowires.

2H2O f O2(O-O)SA + 4H+ + 4e- (1)

2Cuf 2Cu2+ + 4e- (2)

4H+ + 4e- f 2H2 (3)

2Cu2+ + (O-O)SA + 4e- f Cu2O + (O)SA (4)

2Cu+ 2H2O f 2H2 + Cu2O + [O]SA (5)

1640 J. Phys. Chem. C, Vol. 111, No. 4, 2007 Singh et al.

(∼10-40 nm) located near defects, surface deformation, etc.,and the other corresponds to comparatively large regions (∼400nm) corresponding to surface pits etc. The formation of Cu2Onanostructures, as is evident from Figures 1 and 2 is dependentdominantly on the applied potential (V) and hence the appliedelectrical field. Thus at lower applied anodizing potentials of 2and 4 V, the nanostructures formed on small surface regions(∼20-40 nm) correspond to threads which are made ofcoalesced bead particles as depicted by Figure 1a,b. At a mildapplied potential of 2 to 4 V oxygen evolution is expected tobe mild and the migration of fresh Cu2+ ions from the interiorwill be slow. Therefore, the formation of local Cu2O regions indirections nearly perpendicular to the surface is expected to beslow leading to bead-like Cu2O regions adjacent to each other.These beads will then coalesce together to form threads. Thisis in keeping with the observations of nanothreads as shown inFigure 1. On the other hand, with higher applied anodizingpotentials of 6, 8, and 10 V, oxidation initiated at small regions(∼20-40 nm) leads to formation of a well-developed Cu2Onanowire configurations, Figure 2a-c. This effect can bebroadly understood based on considerations as in the following:

Under comparatively mild oxygen evolution correspondingto 2 and 4 V, the oxygen evolution at the anode will besignificant but not copious and the reaction zone will be confinedto the surface and near-surface regions. As the anodizingvoltages (fields) are increased to 6 V and higher and whenanodization is carried out for adequate time (20-60 min) sothat the ionic current becomes saturated, a large number of Cu2+

ions will cross the Cu/ Cu2O interface. These Cu2+ ionswill react continuously with oxygen leading to formationof extended-wire-like structures protruding nearly per-pendicular to the surface. This is in keeping with theobserved experimental results exhibiting well-developed

Cu2O nanowires (Figure 2a-c). It may be mentioned thatbesides anodization at 2 and 4 V for duration of 45-60 minnanothreads can also be formed at 6 V and higher if theanodization time is small,<45 min. In this case insufficientoxygen evolution on the anode surface and only nanothreadsare expected to result. This was verified experimentally asevidenced by Figure 5.

Dependence of Morphology of Cu2O Nanostructure onAnodization Conditions: Current versus Time Plots (I vst). To obtain better insight of the Cu2O nanostructures formation,we proceeded to monitor the current as a function of anodizationtime. Panels a and b of Figure 4 depict the trend of variation ofcell current with electrolysis time at applied potentials of 2 and6 V, respectively. Initially during the first 10 min, there is asteep increase in current. The charging rate in this region inboth cases is comparable (approximately 0.092µAs-1). Thecurrent becomes saturated gradually beyond 30 min giving riseto a maximum saturation current of 77µA at 2 V and 120µAat 6 V. This corresponds to a current density of 38.5 and60 µA cm-2 (electrode area 2 cm2). It may be inferred thatprimary electrode reactions such as copper dissolution andoxygen evolution commence in the initial steep region ofI vst plots, while a steady formation of nanostructures takes placein the saturated current region.

Effect of Increasing Anodization Time. It is expected fromthe above that anodization time at a particular current wouldalso affect the morphology of nanostructures. The effect ofincreasing anodization time at a fixed potential (e.g., 6 V, 60µA cm-2) has been investigated. We interrupted anodization at5, 15, 30, 45, and 60 min and characterized the as-formedstructures in the electrolyte.

Morphology at 5 min.At 5 min, we were not able to obtainany structure repeatedly. In one experiment, we obtained a few

Figure 3. (a and b) Time versus current plots at 2 and 6 V, respectively.

Growth of Different Nanostructures of Cu2O J. Phys. Chem. C, Vol. 111, No. 4, 20071641

isolated nanoparticle-like structures in very less quantity. Thedensity of these structures was very low in the electrolyte. Asa result, more frequently no structures were present on aTEM grid. It may be noted that this anodization time was withinthe steep region ofI vs t plots (Figure 4b). Here, Cu2O formationwould not have been adequate to develop nano-structures.

Morphology at 15 and 30 min.We found the nanothread-like structures at 15 (40µA cm-2) and 30 min (50µA cm-2)as shown in Figure 5a,b. These nanothreads were made up offine nanoparticles (dimensions 5-20 nm).

Morphology at 45 and 60 min.The structures obtained afteranodization for 45 min, 60 min, and higher (∼60 µA cm-2)showed nanowire-like structures (Figure 5c). The TEM micro-graphs of 45 and 60 min sample revealed the dense nanowiresof diameter ranged between 10 and 25 nm, while their lengthswere typically more than 500 nm. The nanowire structures looklike exfoliated tree branches. Thus it is confirmed that theanodization time at any particular applied potential influencesthe morphology of the nanostructure. We obtain nanothreadsof Cu2O by anodizing Cu as described in this study at 6 V for15-30 min, while by anodizing for 45-60 min, nanowires canbe obtained. It appears that at longer anodization times andhigher current density the nanothread-like structure transformsinto nanowire-like structures. It is pertinent to mention here that

only nanothread-like structures were obtained even after 1 h ofanodization at lower applied potentials, viz., 2 and 4 V. This israther expected since at 2 and 4 V even in the saturation regionof the I-V curve, the current density is only 38.5µA cm-2. Asdiscussed above for the nanowire formation a current densityof ∼60 µA cm-2 is required. The observed nanostructures(found in electrolyte) may be assumed to have formed on theCu anode. If nanothreads/nanowires were formed in the solutionsubsequent to settling of Cu2O at the bottom, these structuresshould have been found in all specimens independent ofanodization time (15, 30, 45, and 60 min). Microstructuralevidence for the formation of nanothreads and nanowires onthe electrode surface has also been obtained.

As regards the formation of nanocubes on the surface of thecopper anode, we propose that these are formed when copiousoxygen produced through electrolysis attacks comparativelylarge regions/blocks of the copper anode (∼400 nm). Theseregions may be pits or surface inhomogeneities etc. Here, similarto the formation of nanowires as described earlier, Cu2O blocknanostructures as overgrowth on copper anode will result. Wepropose that the higher applied voltage (e.g., 8 or 10V) forelectrolysis represents the optimum conditions for the formationof nanocubes (Figures 3c and d). These nanocubes reflect thebasic cubic unit cell of Cu2O.

Delamination of Cu2O Nanowire and Stabilization ofCu2O Nanocubes.The observed nanostructueres (formed in theelectrolyte) may be assumed to have formed on the copperanode. If nanothreads/nanowires were formed in thesolution subsequent to settling of Cu2O at the bottom, thesestructures should have been formed in all specimens independentof anodization time (15, 30, 45, and 60 min). As regards thedelamination of nanostructures, Cu2O nanowire and similarconfiguration nanothread from the copper electrode, thismay be as hypothesized by others37 due to vigorous oxygenevolution near the copper oxide/copper electrode surface.However, in the present case, the Cu2O nanothreads andnanowires become delaminated, whereas the nanocubes did notshow this characteristic. Figure 6 illustrates the schematic ofthe torque as applicable to nanowires and nanocubes. Inview of this we propose that delamination takes place due tothe self-weight induced effect of the nanostructures manifestedby torque per unit area. The following gives the estimates ofthis torque.

(a) Cu2O nanowires (lengthlnw ≈ 1000 nm, diameterd ) 20nm, Cu2O density,F ) 6.49× 103 kg/m3, A ) contact area ofthe nanostructures)

(b) Cu2O nanocubes (cube edge:anc ≈ 400 nm)

Figure 4. (a-c) TEM micrographs of the synthesized nanostructuresfor 15, 30, and 45 min at 6 V.

torque/ area) τnw/Anw ) Wnw × r/π(d/2)2

(whereWnw is the weight of the nanowire)

) lnw × F × g × r (wherer ) lnw/2)

) 32.45× 10-9 N/m if lnw ≈ 1000 nm

) 11.68× 10-9 N/m if lnw ≈ 600 nm

torque/area) τnc/Anc ) Wnc × r/anc2-

(whereWnc is the weight of the nanocube)

) anc × F × g × r (wherer ) anc/2)

) 5.19× 10-9 N/m

1642 J. Phys. Chem. C, Vol. 111, No. 4, 2007 Singh et al.

The above calculations show that the self-weight inducedtorque per unit area for Cu2O nanowires is greater (more thantwo times) as compared to that of Cu2O nanocubes. Therefore,the Cu2O nanowires would delaminate whereas the Cu2Onanocubes remain on the Cu anode. We believe this to be thereason for the delamination of Cu2O nanowires and stabilizationof Cu2O nanocubes.

Optical Properties. We recorded the UV-visible absorptionspectra of the nanothreads and nanowire-like structures syn-thesized at 2 and 6 V. The samples were well dispersedin distilled water to form a homogeneous suspension.Panels a-c of Figure 7 show the absorption spectra andcorresponding band gap calculations of the as-synthesizedsamples at 2 and 6 V, which show broad absorption peaks at365 and 361 nm, respectively. An estimate of the optical bandgap is obtained by using the following equation for a semicon-ductor:38

whereA is the absorbance,K is a constant, andm equals 1 fordirect transition and 2 for indirect transition. Panels b and c ofFigure 7 show a plot of (Ahν)2 versusEphot (for nanothreadsand nanowires) for a direct transition, and the value ofEphot

extrapolation toA ) 0 gives an absorption edge energy thatcorresponds to band gapEg.39 The band gap of as-prepared Cu2Onanothreads and nanowires is estimated to be 2.61 and 2.69 eVfrom the UV-visible absorption spectrum, which is larger thanthe direct band gap (2.17 eV) of bulk Cu2O.40 The higher bandgap can be attributed to the size effect of the present nano-structures. Thus the increase of band gap as compared to thebulk can be understood on the basis of quantum size effect,which arises due to very small size of nanothreads andnanowires in one dimension.

Figure 5. (a-d) SEM micrographs of the copper electrode after electrolysis at different voltage (2, 4, 8, and 10 V, respectively).

Figure 6. Schematic presentation showing the delamination of Cu2O nanowires and stabilization of Cu2O nanocubes.

A )K(hν - Eg)

m/2

hν

Growth of Different Nanostructures of Cu2O J. Phys. Chem. C, Vol. 111, No. 4, 20071643

Conclusion

On the basis of the results of present investigation it can beconcluded that we are able to synthesize different possiblenanostructures (nanothreads, nanowires and nanocubes) of Cu2O

by a very simple and inexpensive method of water electroly-sisinduced by anodization of a copper electrode. The presentmethod, in contrast to other known processes, of the formationof Cu2O nanostructures does not require special electrolytes,chemicals, or surfactants. Also this method leads to theformation of different Cu2O nanostructures (nanothreads, nanow-ires, and nanocubes) through the same process. Feasiblemechanisms for the formation of these nanostructures and alsodelamination of Cu2O nanowires and stabilization of nanocubeshave been proposed. The calculated optical band gap of the as-prepared Cu2O nanostructures (nanothread and nanowires) at 2and 6 V from the UV-visible absorption spectra correspondsto 2.61 and 2.69 eV, which are larger than the direct band gap(2.17 eV) of bulk Cu2O.

Acknowledgment. The authors are extremely grateful toDr. A. R. Verma, Dr. C. N. R Rao, Dr. S. K. Joshi, and Dr. A.K. Ray Chaudhary for their encouragement and fruitful discus-sions. The authors acknowledge with gratitude the financialsupport from DST: UNANST, Council of Scientific andIndustrial Research (CSIR) and Ministry of Non-conventionalEnergy Sources (MNES), New Delhi, India. D.P.S. is gratefulto the University Grant Commission (UGC) for the financialsupport.

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Figure 7. (a) UV-visible absorption spectrum of the nanothreads andnanowires like structures synthesized at 2 and 6 V. (b and c) Band gapcalculation of the nanothreads and nanowires like structures from theUV-visible absorption spectrum.

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