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J. Phys. Chem. C 2007, 111, 1638-1645
Growth of Different Nanostructures of Cu2O (Nanothreads, Nanowires, and Nanocubes) by Simple 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 EnVironmental Engineering 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 simple electrolysis process employing plain water (with ionic conductivity ∼6 µS/m) as an electrolyte. No special electrolytes, chemicals, and surfactants are needed. The method is based on anodization pursuant to the simple electrolysis of water at different voltages. Platinum was taken as cathode and copper as anode. The applied voltage 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 anode and collected from the bottom of the electrochemical cell and the other was located on the copper anode itself. The nanostructures collected from the bottom of the cell are either nanothreads embodying beads of different lengths and diameter ∼10-40 nm or nanowires (length ∼600-1000 nm and diameter ∼10-25 nm). Those present on the copper anode were nanoblocks with a preponderance of nanocubes (nanocube edge ∼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 work represents the first such attempt where Cu2O nanostructures were formed under the oxidation induced by as simple a process as electrolysis of plain water. Both anodization potential and time influence the morphology of nanostructures of Cu2O. Thus, nanothreads are formed at 6 V during 15-30 min, whereas nanowires result 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 copper anode (nanocubes), are synthesized in the same electrolysis run. The optical band gap as calculated from the UV-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 opportunities in the field of microelectronics but also provide models for studying the effect of dimensionality and size confinement on electrical, transport, and mechanical properties. These nanostructures have attracted considerable attention owing to their novel physical and chemical properties, and the potential applications in a new generation of nanodevices.1-3 The copper oxide (Cu2O) nanostructure has attracted significant attention as it is one of the first known p-type direct band gap semiconductors4 with a band gap of 2.17 eV. This makes it promising material for the conversion of solar energy into electrical or chemical energy.5 The growing interest in Cu2O nanostructures is due to several reasons. Some of these are the following: (1) Cu2O is a potential photovoltaic material that is low cost and nontoxic and can be prepared in large quantities due to natural abundance of the base material copper,6-8 (2) excitons created in Cu2O have been shown as suitable candidates for Bose Einstein Condensate because of the large exciton binding energy of 150 meV,9-11 (3) Cu2O is a basic compound * 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.
for superconducting material, (4) Cu2O nanostructures can be used as high performance gas sensors, (5) submicron Cu2O hollow spheres12,13 can be used as the negative electrode materials for lithium ion batteries,14 and (6) Cu2O has been reported to act as a stable catalyst for water splitting under visible light irradiation.15,16 The Cu2O nanostructures13,17-21 have beenpreparedbyseveraldifferentmethodssuchaselectrodeposition,22-24 the sonochemical method,25 thermal relaxation,26 liquid-phase reduction,27 the complex precursor surfactant-assisted (CPSA) route,28,29 and vacuum evaporation.30 On the basis of these approaches the synthesis of Cu2O nanostructures demands complex process control, high reaction temperatures, long reaction times, expensive chemicals, and a specific method for specific nanostructures. We have been carrying out investigations on the synthesis of nanostructures of several materials.31-33 In this paper we report a novel and simple one-step process for the synthesis of different Cu2O nanostructures based on water electrolysis in electrochemical conditions using platinum as the cathode and copper as the anode. On operating the Pt/H2O/Cu cell in the electrolysis mode under different operating voltages 2, 4, 6, 8, and 10 V, the electrolytic reactions lead to formation of Cu2O nanostructures on selective sites of the Cu-electrode. One type of Cu2O nanostructure delaminated from the Cu electrode falls into solution and can be collected from the bottom of the electrolytic
10.1021/jp0657179 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/04/2007
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Figure 1. (a) Typical transmission electron micrographs of Cu2O nanothreads embodying beads, as collected at the bottom of the cell after electrolysis at 2 V for 1 h. (b) Figure showing coalesced beads forming nanothreads (c) Selected area electron diffraction (SAED) pattern from the nanothreadlike configuration. The SAED reveals that the nanostructures are cubic Cu2O.
cell. In addition to the delaminated nanostructures, investigations of the copper anode, which were subjected to electrolysis runs, revealed the presence of another type of nanostructure of Cu2O.
employing SEM for any possible microstructural variation. The UV-visible absorption spectra were recorded on a ModelVARIAN, Cary 100, Bio UV-visible spectrophotometer.
Experimental Details
Results and Discussion
The process followed in the present investigation makes use of an inexpensive two-electrode (Pt: cathode; Cu: anode) setup with 5 mL of plain water as the electrolyte. Copper works as a sacrificial anode and forms cuprous oxide nanostructures. Copper (99.9%) sheet (2 cm × 1 cm) as the anode and platinum sheet (2 cm × 1 cm) as the cathode have been employed for the electrolysis. Before anodizing, the copper sheet was polished to a mirror finish with 0.2 µm alumina powder (to remove native oxide film) and then rinsed in distilled water. The distance between the cathode and the anode was kept at 1 cm. The electrolysis cell was made up of Perspex (4 cm × 3 cm × 3 cm) and the plain water (double distilled with very small ionic conductivity ∼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 time period of 1 h, which we determined as the optimum anodization time. During anodization some fine particulate material settled to the bottom of the cell. Structural/microstructural characterization of these fine particles has been carried out employing scanning (XL-20) and transmission electron microscopic (Philips EM, CM-12, and Technai 20G2) techniques. The fine particlelike materials, which are collected at the bottom of the cell, were taken out and dried on a Formvar coated copper grid and examined under TEM. The copper anode was also examined
The morphology and microstructures of the particles obtained at different applied voltage have been investigated employing TEM. Figure 1a is a representative TEM micrograph of the particles obtained after electrolysis at 2 V for 1 h and collected from the bottom of the cell. Similar structures were obtained for 4 V. As can be seen these represent nanothread-like structures of unspecified length comprised of nanobeads (bead size ∼10-40 nm). These structures revealed that the nanobeads coalesced together to form the nanothread as shown in Figure 1b. The TEM image indicated that the nanothreads resulted due to the agglomeration of the bead-like particles. The representative selected area electron diffraction (SAED) pattern is shown in Figure 1c. The diffraction pattern from nanothreads can be indexed to a cubic system with lattice parameter a ) 0.4269 ( 0.005 nm. This tallies quite well with the lattice parameter of Cu2O showing that the material formed under electrolysis conditions consists of cubic Cu2O lattice structure. A representative TEM micrograph of the material collected from the bottom of the cell after electrolysis at 6 V for 1 h is shown in Figure 2a. As can be seen, the Cu2O structures here consist of nanowires (length ∼600-1000 nm and diameter ∼10-25 nm). It should be mentioned that the total length of the Cu2O nanothread and nanowire was comprised of several segments. These were presumably formed due to interaction
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Singh et al. situ) varied from beads to nanowire network and the application of voltage higher than 10 V did not produce any new nanostructure other than nanowires. At higher voltage (>10 V) we are still obtaining nanowires in the electrolyte with an increased density. Panels a-d of Figure 3 are the SEM images of the copper anode after electrolysis at 2, 4, 8, and 10 V for 1 h. Figure 3a is the SEM micrograph of a copper sheet after electrolysis at 2 V for 1 h. The microstructure reveals two types of regions. One of these corresponds to etched regions devoid of material at the surface (marked by white arrows). The second is a region indicating overgrowth of material on the surface of the electrode as a result of electrolysis-induced oxidation (indicated by black arrows). As the electrolysis voltage is increased to 4 V, the second type of region, i.e., the overgrowth region, starts developing into irregular blocks (Figure 3b). As the voltage is increased further to 6 and 8 V, the block-like overgrowth region starts taking cube-like shape as shown in Figure 3c. When the applied voltage was 10 V, regular cube-like microstructures were formed. Figure 3d exhibits typical nanocubes with cube edge ∼400 nm. This type of morphological changes in Cu2O, bringing out block-like nanostructures, have also been observed through in situ thermal oxidation in TEM.34 It may be pointed out that although the cube edge is rather large, keeping in view the nomenclature used by other workers,19,35,36 these may still be called nanocubes. Tentative Mechanism for the Formation of Cu2O Nanostructures. We first propose a feasible electrochemical reaction for the synthesis of Cu2O under electrolysis conditions at the copper sacrificial anode. The electrochemical situation in the present case [Pt/water (mildly acidic)/Cu] with applied voltage always larger than ∼1.23 V corresponds to electrolysis with moderate to vigorous oxygen (and hydrogen) evolution. The operative chemical reactions are expected to be as in the following:
Figure 2. (a and b) Representative TEM micrographs of the dense Cu2O network of nanowires, obtained after electrolysis at 6 and 10 V respectively for 1 h; the inset in panel b shows the SAED pattern from these nanowires. (c) The magnified TEM micrograph of the nanowires.
between nanothreads/nanowires forming the network in which the Cu2O nanothread/nanowire configuration finally appeared. A Cu2O nanowire because of its interaction with other nanowires consists of several bent segments. When the electrolysis conditions were maintained at 10 V for 1 h, the representative TEM microstructure revealed the presence of a dense Cu2O nanowire network (length ∼1000 nm, diameter ∼10-25 nm) as shown in microstructures and the selected area electron diffraction (SAED) pattern in Figure 2b. A magnified TEM micrograph of the nanowire can be seen in the Figure 2c. It may be mentioned that with the increase in applied voltage from 2 to 10 V, the structures formed in the electrolyte solution (in
2H2O f O2(O-O)SA + 4H+ + 4e-
(1)
2Cu f 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)
where SA stands for surface adsorbed species. Under the condition as outlined by (1), a continuous layer of adsorbed oxygen on the Cu anode is expected. This would be as adsorbed oxygen [O]ads. They can combine to form O2, which evolves as oxygen. The anode (Cu) will generate Cu2+, which on reacting with the (O-O)SA will form Cu2O. Another possibility is the reaction of Cu2+ with water or OH- to form Cu(OH)2. This may lose water when it comes in contact with air. The dehydration will lead to Cu(OH)2 f Cu2O + H2O. Since Cu(OH)2 could not be detected to exist with Cu2O, it appears that in the present case Cu2O is formed due to reaction of Cu2+ with surface adsorbed oxygen (2Cu2+ + (O)SA f Cu2O). Growth of Different Nanostructures of Cu2O (Nanothreads, Nanowires, and Nanocubes). Keeping in view the observed microstructures of the Cu anode (Figure 3a), we postulate that the specific sites where oxidation takes place on the surface are of two types. One of these is small regions
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J. Phys. Chem. C, Vol. 111, No. 4, 2007 1641
Figure 3. (a and b) Time versus current plots at 2 and 6 V, respectively.
(∼10-40 nm) located near defects, surface deformation, etc., and the other corresponds to comparatively large regions (∼400 nm) corresponding to surface pits etc. The formation of Cu2O nanostructures, as is evident from Figures 1 and 2 is dependent dominantly on the applied potential (V) and hence the applied electrical field. Thus at lower applied anodizing potentials of 2 and 4 V, the nanostructures formed on small surface regions (∼20-40 nm) correspond to threads which are made of coalesced bead particles as depicted by Figure 1a,b. At a mild applied potential of 2 to 4 V oxygen evolution is expected to be mild and the migration of fresh Cu2+ ions from the interior will be slow. Therefore, the formation of local Cu2O regions in directions nearly perpendicular to the surface is expected to be slow leading to bead-like Cu2O regions adjacent to each other. These beads will then coalesce together to form threads. This is in keeping with the observations of nanothreads as shown in Figure 1. On the other hand, with higher applied anodizing potentials of 6, 8, and 10 V, oxidation initiated at small regions (∼20-40 nm) leads to formation of a well-developed Cu2O nanowire configurations, Figure 2a-c. This effect can be broadly understood based on considerations as in the following: Under comparatively mild oxygen evolution corresponding to 2 and 4 V, the oxygen evolution at the anode will be significant but not copious and the reaction zone will be confined to the surface and near-surface regions. As the anodizing voltages (fields) are increased to 6 V and higher and when anodization is carried out for adequate time (20-60 min) so that the ionic current becomes saturated, a large number of Cu2+ ions will cross the Cu/ Cu2O interface. These Cu2+ ions will react continuously with oxygen leading to formation of extended-wire-like structures protruding nearly perpendicular to the surface. This is in keeping with the observed experimental results exhibiting well-developed
Cu2O nanowires (Figure 2a-c). It may be mentioned that besides anodization at 2 and 4 V for duration of 45-60 min nanothreads can also be formed at 6 V and higher if the anodization time is small,
