MATERIALS CHEMISTRY FRONTIERS This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online RESEARCH ARTICLE View Journal | View Issue Oscillatory patterns in redox gradient materials through wireless bipolar electrochemistry. The Cite this: Mater. Chem. Front., dynamic wave-like case of copper bipolar Open Access Article. Published on 14 July 2022. Downloaded on 11/2/2022 9:51:14 AM. 2022, 6, 2284 oxidation† L. Fuentes-Rodrı́guez, a E. Pujades,a J. Fraxedas, b A. Crespi,a K. Xu, a L. Abadc and N. Casañ-Pastor *a Bipolar electrochemistry allows the development of processes in a wireless manner, with reactions occurring at the induced anodes and cathodes of an immersed conducting material in the electrolyte. As a result, a gradient oxidation state may appear along the main axis field on the surface or bulk of the material depending on the type of reaction available at each induced potential. Redox intercalation gradients have been observed, metal anodization, or deposition, and also reactions at the electrolyte in the nearby environment of the poles induced. The complex oxidation of copper and interconversion between phases formed yields in this work an oscillating redox gradient, thanks to the great resistance change when the oxidized phases are formed. Parallel stripes containing mainly Cu2O, CuO, and Cu(OH)2 with large resistance are formed perpendicular to the electric field, forming a sequence of secondary dipoles in intermediate Cu stripes, that depends on the external voltage applied, and that Received 23rd May 2022, oscillates in time at the same spatial coordinates. With longer times, copper solubilizes at the larger Accepted 14th July 2022 induced potential zones, probably as Cu(OH)42. A simple finite element electrostatic model defines the DOI: 10.1039/d2qm00482h complex potential waves induced in the piece. The resulting dynamics offer an example of the complexity of order in unwired conducting materials in wet media, either in catalysis, bioelectrodes, rsc.li/frontiers-materials electronics, photovoltaics, or energy storage. Introduction reactions involving intercalation, deposition or anodization, how- ever, create redox gradients that are hard to characterize.8–10 Redox gradient materials are not usual in materials chemistry Electrochemical energy storage electrodes, mixed conductors, but are found in nature due to the natural geochemical processes, and photochemical systems are also relevant examples, where pressure, and time evolution or atmosphere exposition.1–5 It is ionic and redox gradients and their dynamics define the final thought that such natural redox gradients are at the origin of the performance of devices.9 original amino acid synthesis.6 On the other hand, the existence In parallel, synthetic wireless electrochemical processes are of redox gradients is at the root of charge transport in many known to be possible through dipole induction in immersed biological processes,7 regulates homeostasis,5 and is desirable in conducting materials, in what is usually called bipolar photosystems for efficient charge transport cascades.3 On the electrochemistry.11–13 When a conducting material is placed other hand, the materials science field prioritizes the search of along the externally imposed electric field lines, it polarizes in homogenous phases, required in many characterization techni- an opposite direction to the external field, and an induced ques and purity control. Significant processes like electrode anode and cathode appear at opposite borders, which induce electrochemical reactions and may create a compositional a Institut of Materials Science of Barcelona, CSIC, Campus UAB, 08193 Bellaterra, gradient along the sample. This effect occurs for any geome- Barcelona, Spain. E-mail: nieves@icmab.es tries and size of the immersed material and of the electric field b Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, imposed, and how the field lines are intercepted, with different Campus UAB, Bellaterra, 08193 Barcelona, Spain spatial effects and has rather significant applications in pat- c Institut of Microelectronics of Barcelona-Center National of Microelectronics, CSIC, terning, polymer motion, synthesis of anisotropic particles or Campus UAB, 08193 Bellaterra, Barcelona, Spain † Electronic supplementary information (ESI) available: Supplementary Material even influencing neural cell behavior.11–23 available containing XPS survey and deconvolutions, X-ray diffraction data and The horizontal geometry simplified, in particular, yields to Comsol variables. See DOI: https://doi.org/10.1039/d2qm00482h an oxidation at the surface of the induced anode, and a 2284 | Mater. Chem. Front., 2022, 6, 2284–2296 This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2022 View Article Online Research Article Materials Chemistry Frontiers corresponding reduction at the induced cathode in two clearly differentiated poles on the same surface. Those reactions may be of species in the electrolyte nearby or correspond to the material itself, depending on the conditions and materials This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. chosen. Thus, intercalation of ions has been reported in induced cathodes in iridium oxide or conducting polymers,13,24 asym- metric electrodeposition of metals, and solvent oxidation and reduction (forming O2 and H2 in the case of water, with corres- ponding pH changes). Thus, electron transfer occurs also at the induced poles of what can be described as a bipolar electrode, and impedance changes are observed in the cell. Actual electron Open Access Article. Published on 14 July 2022. Downloaded on 11/2/2022 9:51:14 AM. conduction among the poles eventually cancels the charge dif- ference, in some cases with ionic re-equilibration.24 In some of Fig. 1 (a) Cell geometry for a centred piece immersed between two those cases, a redox relaxation behavior after bipolar electro- parallel Pt electrodes. (b) Scheme of voltage profiles for the electrolyte chemistry has been clear. Also, along with the formation of new (black), and for a cell with a conducting immersed copper piece under an species and/or stoichiometries, the conductivity of the material external applied field. (c) Scheme of voltage profiles for the electrolyte (black), and for a cell with an insulating immersed piece under an external may also be changing, which in turn may modify the point of zero applied field. D values express the deviation from the electrolyte drop and charge, initially present at the center of the conducting material, contribute to the induced dipole. or modifying the dipole initially formed. It is also relevant that, in particular, copper applications are wide and of great interest, either in biosystems, electronics, phosphate electrolytes, had also been tested in prior work, and photovoltaics, catalysis, or CO2 reduction.25–29 Many of those resulted in copper dissolution and/or gel formation of Cu+2 cases present redox gradients during use or require the exis- species, and are not shown here. tence of such gradient. In an attempt to elucidate the dynamics The voltage was applied either with a Biologic VSP potentio- that correlate the redox gradients with the eventual changes in stat/galvanostat or with a VWR Perfect BlueTM 300 V power physical properties, our study has chosen an initially simple supply. Both differ in the time of reaching the set potential and case, the oxidation of copper metal in wireless bipolar condi- create small differences among the samples, for the same tions in alkaline media. As this work describes, the formation voltage. Final stripes at the samples however coincide in the of a gradient is nothing but simple in this case, since the characterization tests. formation of insulating phases creates new zones and sub- Cyclic voltammetry of connected copper pieces in anodic dipoles with their respective induced anodes and cathodes, and cathodic starting runs was made in 1 M KOH in order to resulting in a wave–like oscillation of the oxidation, along the identify the copper oxidation processes in this media, using the electric field. This new type of nano- and micro-structuring is same potentiostat described above. The working electrode was relevant from the basic point of view but also in any of the a Cu wire, 0.5 mm in diameter, 99.9% Advent. An Ag/AgCl processes described above and illustrates new possibilities in double bridge reference electrode (using as external solution a materials preparation and interfaces design. 3 M LiCl aqueous solution), and a Pt sheet as counterelectrode were used. All characterization carried out on the set of stripes on Experimental samples was done ex situ and with spatial resolution. The nomenclature of different zones is homogenous for all char- Copper metallic sheet cut in specific square dimensions (Alfa acterization techniques and is based on the colour observed. Aesar 99.95%), and previously cleaned in 1 M HNO3, was used Zones of the same colour are compared in different samples for in all cases as immersed metal in 1 M KOH electrolytes. Electric reproducibility. Two batches of the same Alfa Aesar copper fields were applied using Pt sheets (rectangular 15  35 mm2, described above yield different greenish zone amounts. The use immersed 6 mm in the electrolyte) as anode and cathode, of a power source or a potentiostat, as described above, also placed in parallel at distances of 25 mm in a Petri dish with induces small differences. an electrolyte height of 6 mm. See the scheme in Fig. 1a. Cell SEM images were obtained in a SEM QUANTA FEI 200 FEG- voltages of 6 V to 12 V (corresponding to 2.4–4.8 V cm1) were ESEM electron microscope. The beam position was achieved applied, corresponding to previously established safe potentials through the Motorstage EC, 50  50 BDM Spares controller. in water and where the effect is observed for this geometry. The EDX was performed at specific zones using visual inspection of copper piece (10  10 mm2) was always centred among the the low magnification images. platinum electrodes. The expected initial voltage drop across Ex situ XPS experiments were performed using a SPECS the cell including the conducting copper piece is schematically PHOIBOS150 hemispherical analyser with a monochromatic shown in Fig. 1b. Sample cleaning after bipolar treatment was X-ray source (1486.6 eV) operated at 300 W in a base pressure of performed with deionized water, soaking three times, and drying 5  1010 mbar. Binding energies are referred to the 284.8 eV under N2 atmosphere. Additional media, based on chloride and C1s line to correct for eventual surface charging even if the bulk This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2022 Mater. Chem. Front., 2022, 6, 2284–2296 | 2285 View Article Online Materials Chemistry Frontiers Research Article material (copper) is metallic. Since the dimensions of the X-ray Also, in order to emphasize the dipoles observed (each spot is of about 1 mm  3 mm, the samples have been fixed to individual charge separation) a profile of Vcell–Velectrolyte for the sample holders so that the stripes are parallel to the long each position in copper along the field axis was made, in a wide dimension of the spot. The different regions are visually span of conductivities of the less conductive phases and for This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. identified through a small aperture along the analyser cone various conductive phases. Each of the calculations corre- axis pointing perpendicularly to the samples surface and sponds to a specific snapshot related with the possible dipoles selected with the help of a xyz-manipulator. However, since observed as the induced oxidation occurs in the immersed the short dimensions of the stripes are in the order of 0.5 mm copper. In the finite element model, the grid was optimized for and below, each spectrum may contain contributions from the the electrolyte interface and also for the different phases that neighbouring regions. Despite that, relative changes result appear on the copper surface, and the fitting parameters obvious. Deconvolutions were performed with the CasaXPS compared to reach final convergence. Finally, a comparison Open Access Article. Published on 14 July 2022. Downloaded on 11/2/2022 9:51:14 AM. software,30 using Gaussian–Lorentzian line shapes and Shirley of the potential drop across the cell was made for various background subtraction. conductivities of the less conductive phase formed vs. a con- X-Ray diffraction was performed using a Bruker-AXS D8 ductive piece (copper piece) and for various conductive phases Advance diffractometer (Cu Ka1,2) equipped with a Vantec-500 vs. an insulating piece. The conductivities of electrolytes and 2D detector. An omega-scan with a 15a frame width during copper were obtained from literature32,33 while the experi- 900 seconds is used. With a two-dimensional detector, the mental measurements were used for conductivities for appear- diffraction is no longer limited to the diffractometer plane ing phases. A summary of other significant parameters of the and a large portion of the diffraction rings can be measured calculation is shown in Table S1 in ESI.† simultaneously. When integrating, an area detector frame in the 2y–w directions a standard intensity versus 2y diagram pattern is obtained. Since anodization occurs at the surface and causes Results and discussion, experimental growth of new phases, the basal plane is not homogeneous in texture and causes some shifts among peaks when the total area When a conducting material is immersed in an electrolyte, is integrated. Thus, Cu metal peaks have been used as reference. upon application of an external applied field, an induced dipole The incident beam is defined to go along each stripe and position is created among its borders that opposes the external field. was achieved using an xy table. Spectra shown are extracted from Thus, an anode and cathode appear at opposite poles, where the bidimensional detector signal to facilitate representation. oxidation and reduction reactions respectively may occur, in RAMAN spectra were obtained at each specific zone using a what is known as bipolar electrochemistry. Thus, copper oxida- Horiba–Jobin Yvon Labram HR800 spectrometer with Open- tion is expected to take place at the induced anode at the Electrode TE-cooled CCD detector, using a 514.5 nm laser and copper surface, at sufficiently high driving external potential, 600 l mm1 grating. (Fig. 1b). Polarization then would shift towards the opposite Resistance measurements were carried out using a two term- side where the metal remains unoxidized. In contrast, if the inal Keithley multimeter (model 2470), using round shape pins. immersed piece is insulating, only a weak perturbation of the A simple electrostatic model was built using COMSOL Multi- field is observed (Fig. 1c). physics software (version 6.0)31 using the Electrostatics module, As observed in Fig. 2, bipolar oxidation of copper occurs at in order to explain the experimental wave-like oscillation in the the induced anode. However, it does not happen in a linear oxidation of the copper sheet. We implement the simulation by form, according to the induced initial potential. Significantly, applying an external voltage through the driving electrodes and in the configuration described in Fig. 1b, oxidation in form of accounting for the insulating boundary conditions in the stripes perpendicular to the field direction appear. Propagation geometry of experimental bipolar cell used in this work. In of the oxidation front occurs from the induced anode towards such module classical equations from electromagnetic theory the centre of the sample. This perpendicular orientation occurs are used, with finite boundary conditions, and no electrochem- always independently of the shape of the immersed copper istry is defined. Thus, it corresponds to a stationary initial state. (ex. square or circular) on the surface aligned with the field. In The conductivities of electrolytes and copper were obtained this work, in order to maximize the spatial dimensions of the from literature32,33 while the experimental measurements were stripes observed, an orientation along one of the diagonals of a used for the conductivity values of the new appearing phases square has been chosen (Fig. 2). (Table S1, ESI†). In the finite element model, the grid was Fig. 2 shows the different evolution of wireless bipolar optimized for the electrolyte interface and also for the different anodization on the copper sheet, depending on the external phases that appear on the copper surface (see the ESI† such as voltage applied between the external driving Pt electrodes, mesh parameters (Table S1) and a geometric illustration with during the first 6 minutes. In all cases a brown-black zone is the mesh used across the bipolar cell (Fig. S4)). Finally, the formed, suggesting CuO formation with small particle sizes. fitting parameters were compared to reach final convergence. The formation of the dark stripe occurs after a time that is Each of the calculations showed in this work corresponds to a inversely proportional to the applied external potential, and specific snapshot related with the possible internal dipoles reaches a larger spatial extension within the positive pole (left observed as the induced oxidation occurs as stripes. of the copper piece). Comparing the 8 V and 10 V cases, the 2286 | Mater. Chem. Front., 2022, 6, 2284–2296 This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2022 View Article Online Research Article Materials Chemistry Frontiers This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 14 July 2022. Downloaded on 11/2/2022 9:51:14 AM. Fig. 2 Time evolution (6 min) of copper oxidation induced at 6, 8 and 10 V between Pt driving electrodes with left Pt being negative and the Pt at right being positive. The initial picture in each file corresponds to time zero and no applied potential. The last picture of each series corresponds to the piece once extracted from the solution. final width of such black zone is larger when the applied through the inner copper metal. However, equivalent experi- potential is lower, but at 6 V the voltage seems to be too low ments with thin layer copper deposited on TEM grids also show for such propagation. At first sight, a longer time at 8 V bipolar the same effect. treatment reaches almost the same than shorter times at larger Finally, when the process stops, up to four different zones voltage, 10 V, once such threshold is achieved. But further with a darker colour than the original in copper and apparently observations show that it is not exactly the case. Larger voltages empty clean Cu stripes separating them, are found, at different reduce the width of the black stripes, while enhancing the positions along the axis of the sample aligned with the field. dissolution of the border induced anode which has larger A bluish green colour also appears in some zones especially induced voltages. Thus, the oscillating behaviour observed once the sample is exposed to the atmosphere, evidencing the visually depends greatly on the external applied voltage. formation of copper hydroxides or hydroxicarbonates. Experi- A second stripe appears simultaneously to the right, with a mentally, as potentials imposed are lowered (see 6 V vs. 8 V red colour suggesting the formation of Cu2O, but separated by a external voltage), the stripes get wider evidencing an influence copper colour zone, apparently unreacted. As the potential is of the growth rate of each insulating phase (Fig. 2). expected to be lower moving away from the vortex, the sequence There is some degree of variability in the position of the seems logical at first sight with Cu+2 near the positive border stripes among different copper batches used with the same and Cu+ farther away from it. But the evolution is not so commercial reference, but the results of the techniques that obvious. Although the black zone appears initially at the vortex, follow have shown reproducible results for all the zones of the where a larger potential is expected, it seems to move across the same colour. The data, in each case, is accompanied by an sample with time. Time evolution shows that the vortex phase image for easy identification, with all zones named equally dissolves, probably due to the formation of another soluble across samples. Small changes in curvature of the stripes are species. At larger potentials, 10 V, the vortex shows directly a also observed and correlated with the bending during the cut in clean copper colour, as if such soluble species is formed. Thus, square pieces. the final sample also contains a larger clean vortex after the SEM images (Fig. 3) evidence the original microstructure of process, and the black zone has advanced more along the main the Cu sheet used and the growth of bundled fibres in the Z4 axis of the electric field (copper diagonal). In most cases, a zone, corresponding to the black stripe. Also, the Z3 zone shows stripe with a bluish colour of different intensity observed by the appearance of microcrystals with smaller size than those of reflection is observed, mixed within the reddish-brown stripe, Z4. Inner stripes are also found within Z4 zone (Fig. 3b) suggesting the formation of hydroxides or hydroxicarbonates A first evaluation of the chemical changes due to bipolar along with oxides. It is also important to remark that the charge treatment comes from the ex situ study of the relative oxygen difference induced in these experiments has a discharge path content, through EDX, along the main axis of the sample This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2022 Mater. Chem. Front., 2022, 6, 2284–2296 | 2287 View Article Online Materials Chemistry Frontiers Research Article This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 14 July 2022. Downloaded on 11/2/2022 9:51:14 AM. Fig. 3 (a) SEM images with spatial resolution corresponding to the different zones marked in the optical image for a copper sample treated using 8 V external voltage. (Please note that roughness exists in the original as purchased Cu foil.) (b) Detail SEM images with lower resolution showing internal stripes at zone Z4. (c) EDX relative change in O, K and Cu content. (Please note that the signal from underlying metallic Cu shifts the global copper %, and therefore the y axis is arbitrary. Also note that the K constant small value suggests a small degree of contamination from the solvent.) (Fig. 3c). In it, it is obvious that regions Z3 and Z4 have the largest O/Cu ratios, although a small increase in oxygen is also observed for Z2, Z5 and Z6. Surprisingly oxygen content at Z2 is smaller than at Z3 and Z4. No stoichiometry is extracted from such values since the underlying copper sheet is contributing to the signal and O sensitivity is low. Thus, the original visualization of this process including the obvious variation of oxygen stoichiometry seen in EDX, sug- gests that the degree of copper oxidation follows an unusual pattern in space, and contains several redox processes. In order to Fig. 4 (a) Copper CV started in both oxidation direction (black) and reduction direction in the available water range from the open circuit evaluate the number of processes, a CV on a directly connected potential (0.5 V vs. Ag/AgCl) using scan speed of 20 mV s1; (b) sequential Cu piece was run. Two different modes were explored, directly CV in the smaller range not reaching water splitting waves. Scans start at on the pristine sample towards anodic processes, or initiating open circuit potential and are run towards oxidation, using scan speed of the CV with a reduction that could eliminate the Cu2O present 20 mV s1. always at the Cu surfaces (see Fig. 4). Fig. 4a shows that if the scan starts with reduction, two small almost non observable waves, appear at E 0.7 V and reversibility exists for the large waves at 0.2 and 0.1 V vs. 1.15 V vs. Ag/AgCl, which may correspond to the reduction of Ag/AgCl. While the smaller waves in the initial scans are surface Cu2O to metallic Cu. The small intensity in such waves probably related to the reduction of Cu2O or its formation from is thought to be related to the small amount of Cu2O endemi- Cu, we observed several processes at larger overpotentials that cally present at the surface of Cu, (see inset in Fig. 4a). The may involve in principle larger oxidation states and not a single corresponding return oxidation shows a small wave at 0.4 V one corresponding to CuO. and two larger waves at 0.2 V and 0.1 V vs. Ag/AgCl. An Also, the existence of a wave overlapped with water oxidation additional wave is observed at 0.9 V vs. Ag/AgCl overlapped with suggests a larger oxidation state for copper, possible irreversi- water oxidation. bility, and a change in pH derived from O2 formation. Thus, If the scan starts directly towards oxidation, similar waves Fig. 4b shows a scan within a more limited voltage range, and are obtained, but the first oxidation near 0.4 V vs. Ag/AgCl is with several scans that may determine which processes are not shifted and has lower intensity, while the larger waves at 0.2, reversible. The anodic peak at 0.4 V vs. Ag/AgCl assumed to be and 0.1 and 0.9 V vs. Ag/AgCl having larger intensity, as also the formation of Cu2O, appears only in the first scan. Then, the occurring for water oxidation. The cathodic return has two two large waves at 0.2 and 0.1 V vs. Ag/AgCl are shown to be small waves at 10.5 and 0.7 V vs. Ag/AgCl, and show that no irreversible, and only the small reduction wave, previously seen 2288 | Mater. Chem. Front., 2022, 6, 2284–2296 This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2022 View Article Online Research Article Materials Chemistry Frontiers at 0.5 V, is seen. Intrinsically, it is clear that the oxidation of acting. Still, the information from the voltammetry is a good copper beyond Cu2O is irreversible, even if the potential of 0.9 V hint of the possible processes. We consider in this work that the near H2O oxidation is not reached, probably because the phases same type of processes should be possible, but, as an induced formed include Cu2O, CuO, Cu(OH)2 and are much more bipolar system is set, the copper surface is subject to a potential This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. insulating or because some phases detach or solubilize. Of gradient, and therefore the induced anode surface is initially interest is to observe how the two large waves coalesce in the subject to each of the oxidizing potentials. Despite this simul- second scan while several waves coexist at similar potentials in taneous presence of several potentials in the bipolar electrode, consecutive scans. Despite those observations, the main CV we may consider that the potentials observed in CV are reached waves do not shift, and therefore, no change in pH is detected. for each of the processes present. Once more, the oxidation of copper seems to be complex, even The characterization by XPS and X-ray diffraction of the on the same connected surface. stripes formed on Cu may offer additional information on the Open Access Article. Published on 14 July 2022. Downloaded on 11/2/2022 9:51:14 AM. Previous reports from literature34–45 for direct contact elec- oxidation states achieved at each position in the main field axis. trochemical oxidation of copper electrodes in alkaline aqueous In the initial conditions, the larger potential at the induced solutions are similar, although some contradictions also exist. anode will be at the vortex closer to the negative Pt driving In general, up to four oxidation waves and two reduction waves electrode, (see Fig. 1 and 2), and should be decreasing if are observed, with complex mechanisms and interconversion moving away from that position towards the centre of the Cu among several species, including formation of Cu(OH)2 at lower piece. Therefore, the phase formed at the corner should require potentials than Cu2O, and the Cu(OH)42 adsorbed soluble a larger potential, while the phases formed at longer distances species being proposed along with simultaneous reduction of from the pole require a smaller potential, and would have lower CuO and Cu2O to Cu, and dehydroxilation of the tetrahydroxide oxidation states expected. to oxides. A simple scheme, in increasing potential, may be Fig. S1 (ESI†) shows the survey XPS spectra of the analysed summarized as follows: regions, namely Z1, Z2, Z4, Z5, Z6, and Z8, as well as that of a Anodic waves (positive current) reference clean, (Ar sputtered), metallic Cu(111) single crystal surface measured in the same XPS system. The main XPS and 2Cu + OH - Cu2O + H+ + 2e E 0.4 V vs. Ag/AgCl Auger lines are identified. Apart from the lines associated to (1) copper, we observe the presence of carbon, oxygen and potas- Cu + OH - CuO + H + 2e (or CuOx) E 0.2 V vs. Ag/AgCl sium in all regions as well as negligibly small traces of silicon (2) and nitrogen. The presence of K in the general spectra sug- gested occluded traces of the solvent. However, additional Cu2O + 2OH - CuO + Cu(OH)2 + 2e E 0.18, 0.0 vs. Ag/AgCl soaking in water induced additional carbonation. The compar- (3) ison with the clean Cu(111) references helps to visualize the CuO + OH + 2H2O - Cu(OH)42 + H+ E 0.05 vs. Ag/AgCl signatures from carbon and oxygen species. From the figure we (4) can conclude that the surfaces are clean after the electro– oxidation process in water and unavoidable exposure to the Cathodic waves (negative current) atmosphere, apart from endemic carbonate (resulting from CO2 adsorption) or remnant potassium. Dehydrox. 3Cu(OH)42 + 6H+ - 3CuO + 9H2O Fig. 5a–d show high-resolution XPS spectra of the C1s, O1s, (5) Cu2p and CuLMM lines, respectively, for the same regions 2Cu(OH)42 + 2e - Cu2O + 4H2O (6) discussed in the XPS survey (Fig. S1, ESI†). At first sight, the zone Z4 is clearly distinguishable from the rest in the Cu 2p XPS 2CuO + H - Cu2O + OH E 0.52 vs. Ag/AgCl +  spectra. The Cu2p line of Z4 is clearly differentiated from the (7) rest with an extra line centred around 935 eV and a satellite at Cu2O + H2O + 2e - 2Cu + 2OH E 0.7 vs. Ag/AgCl higher energy that may correspond mainly to Cu(II) hydroxides (8) and to a lesser extent to carbonates,46–48 since it correlates with an O 1s signal typical of OH and CO32 groups. The rest of Also, in our opinion, non-stoichiometric oxides cannot be regions exhibit a dominance of Cu2O at the surface (with discarded and other mixed valence phases such as CuOx, various thicknesses of Cu2O possible) with a binding energy KxCuO2 and Cu4O3, with mixed oxidation states, or copper of the main peak, located at 932.2 eV48 an energy sufficiently hydroxicarbonates due to possible absorption of CO2, may be separated from the 932.6 eV (metallic copper) and from formed and are not discarded in this study. 933.8 eV (CuO). Fig. 5d further confirms the dominance of It is worth remarking here that cyclic voltammetry on a Cu2O at the surface (main feature at 916.9 eV kinetic energy) connected copper piece does not represent the actual bipolar with some contribution from the subsurface metallic copper potential distribution. All processes in a CV involve a change on foil (see feature at 918.6 eV). Contributions from hydroxides the surface influencing the next process, while in a bipolar and carbonates are expected around 916 eV, thus at lower configuration, high potentials are available without the previous energy than that corresponding to Cu2O, and indeed, for region scan at lower potentials, modifying the possible mechanism Z4 a shoulder appears in that energy range. The main feature This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2022 Mater. Chem. Front., 2022, 6, 2284–2296 | 2289 View Article Online Materials Chemistry Frontiers Research Article This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 14 July 2022. Downloaded on 11/2/2022 9:51:14 AM. Fig. 5 High resolution (a) C1s, (b) O1s, (c) Cu 2p and (d) CuLMM spectra for the various zones described in image. Sample shown has been treated using 8 V external field. Other voltages reproduce results. Please note that zones 2 and 4 may contain zones 4 and 2 respectively since the incident beam (0.8–1 mm) is wider than the stripes (near o smaller than 5 mm). Z2 0 and Z4 0 correspond to an additional sample. associated to CuO should be centred at 918 eV but no clear specific absorptions, and thus black represents an evidence of feature is observed at this energy. From the Auger lines, we non-stoichiometry or of CuO, but may also hinder visualization cannot exclude the CuO phase in Z4, but if present, O1s shows of less coloured phases. Thickness, microstructure and particle that it would be in small amount and accompany a copper size are also significant in that sense. Thus, despite the colour hydroxide species, in agreement with the Cu2p spectra from differences, Z5, Z6 and Z8 are similar at first sight in XPS, Fig. 5c. evidencing the existence of Cu2O probably in different amounts, The colour of each stripe is indeed a valuable complemen- but Z6 also shows, when dried in atmosphere, a different degree tary evidence, since stochiometric phases are known to have of bluish green colour and brilliant effect that may agree with 2290 | Mater. Chem. Front., 2022, 6, 2284–2296 This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2022 View Article Online Research Article Materials Chemistry Frontiers the simultaneous formation of copper hydroxides, hydroxicar- bonates or extraction of chromium as oxide to the surface. The potential presence of some impurities has been checked but has not been observed. Thus, for example, Cr would be masked by This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. the intense Cu L3M45M45 Auger line at about 570 eV in Fig. S1 (ESI†). XPS is a helpful tool in that sense, evidencing hydroxide species as described below, and also the endemic presence of carbonates in the sample. It also evidences the existence of Cu2O species on spatial coordinates to the left and right of the main Cu+2 species, Cu(OH)2 and possibly (see X-ray data), CuO. At the same time, the border where the initial induced Open Access Article. Published on 14 July 2022. Downloaded on 11/2/2022 9:51:14 AM. anode exists, a high potential zone, is black in initial stages but ends up showing only the Cu metal signals for 8 and 10 V external applied voltages, possibly involving an oxidation that yields to the copper tetrahydroxide anion species, as described in literature.41 Deconvolution, considering other reports,40–52 as summar- ized in Fig. S2, (ESI†) confirms that the most relevant changes correspond to region Z4, with Cu(OH)2 formation, while Cu2O or Cu (intermediate clean stripes) predominate in other zones, with different thicknesses. CuO cannot be discarded from the XPS data given the surface sensitivity of the technique. Carbo- nates and hydroxicarbonates are probably present in some extent due to CO2 exposure. X-Ray diffraction patterns obtained with a linear GADD detector result in complementary results. Fig. S3 (ESI†) (global diffraction pattern) and Fig. 6 (magnified peaks) show the crystalline phases that can be observed in each of the regions, with standard incidence angles, and aligning the incident beam along the stripe. Since anodization occurs at the surface and causes growth of new phases, the basal plane is not homogeneous and causes some shifts among peaks. Along with the omnipresent Cu diffraction lines several indexed phases appear at each zone. Cu, Cu2O, CuO and Cu(OH)2 are indicated by the several coloured lines.53 When a greenish phase appears, no differences are observed. While endemic Cu2O is present even in pristine Cu zones, the Z4 zone is the one where Fig. 6 (a) X-ray diffraction patterns extracted from GADDs diffractometer in more changes are observed, as XPS data also had shown. CuO and each of the lines, and indexation. Spectra correspond to a sample treated at 8 V Cu(OH)2, easily interconverted, are present in Z4, while it is the external voltage. Other voltages reproduce results. The global diffraction pattern is shown in Fig. S3 (ESI†). (CuO: PDF 00-001-1117; Cu2O: PDF 01-077-0199; only zone where the Cu(OH)2 phase is seen. Indexing of additional Cu: PDF 00-004-0836; Cu(OH)2: PDF 00-35-0505, ICDD PDF4+ 2022 crystalline phases as Cu4O3 and hydroxicarbonates has been database53). (b) Raman spectra for different zones. attempted with no success, but their existence or other amorphous phases cannot be discarded.53 Oxidation of impurities present in Copper (50 ppm maximum on Cr for example) that could get hydroxides at the copper surface in alternating oxidation states, in extracted at the surface, have also been investigated but no a complex behaviour that occurs sequentially in direct contact evidence of them exists in X-ray diffraction. electrodes, but simultaneously in bipolar electrochemistry config- Local resolution Raman spectra (Fig. 6b) confirms the urations. As expected from the chemical identity of the phases, a existence of Cu (300 cm1), Cu2O, (peaks at 644, 216 and simple two contact measurement, shows that all are more insulating 148 cm1) and Cu(OH)2 (peaks at 288, 450, 489 cm1) and than the starting copper metal, with Cu, and oxidized brown and CuO (peaks at 288 cm1) although some of the peaks are black zones yielding values in the ranges of 101 O, 104 O and 106 O common to the last two phases.54,55 (It is remarkable the large respectively. These values are in agreement with the data span crystallinity of the peak at 490 cm1 corresponding to Cu(OH)2. usually reported for copper oxides56,57 and with the presence It is also evident the sequence of oxidation states for Cu across of copper hydroxide plus the microstructure of CuO electro- the different zones. chemically grown. Thus, chemical identification is in agreement with the formation From a purely electrostatic approach, the appearance of new of insulating or large band gap semiconductor oxides and phases with larger resistivity modifies the piece conductivity This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2022 Mater. Chem. Front., 2022, 6, 2284–2296 | 2291 View Article Online Materials Chemistry Frontiers Research Article and also the local field distribution, generating changes in the stripe could appear with a positive and larger potential that the induced dipoles. In a simple case of a metal forming a single potential at each border of the copper piece. Furthermore, an insulating oxide, the anode part becomes insulating, and only inversion in the sign of the anode pole voltage may occur. the remaining conducting surface gets polarized, thus shifting Experimental data compared with that simulated screening of This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. the point of zero charge towards the cathode. That occurs conductivities suggests that for insulating phases with conduc- despite the underlying conducting material.24 However, the tance between 105 and 1010 S m1, a dipole change is expected existence of several insulating phases formed in copper may to be substantial, since electrolyte conductivity is 17.8 S m1 at yield to a different case, as in this case. Thus, given the change 20 1C,32,33 a resistance smaller in two orders of magnitude than the in conductivity of the surface observed, an evaluation of the insulating phases (see Fig. 7a, for the smaller conductivity case). critical parameters that may modify the original dipole induced On the basis of this calculation and the patterns observed is of interest. That has been done simulating, through a experimentally, a simulation based on each snapshot of the Open Access Article. Published on 14 July 2022. Downloaded on 11/2/2022 9:51:14 AM. COMSOL finite element approach, the voltage profile on copper formation of stripes observed during oxidation has been done for various unknown phases of different conductivity being using COMSOL single element electrostatic model. Each stripe formed during oxidation. taken into account in the model has the resistance that was As described in Fig. 7a, for the same conductivity of the measured experimentally. Fig. 8 shows the scheme of stripes electrolyte (1 M KOH, 17.8 S m1,32,33) different conductivities (line 1), and the simulated isopotential curves (line 2), charge of the new phases formed generates induced anodes with distribution (line 3) and voltage profiles (line 4) for the evolu- different potentials, with significant changes appearing at the tion patterns observed visually. induced anode. A critical change is observed when the differ- As observed in Fig. 8, COMSOL simulations may explain the ence between the electrolyte conductivity and the material existence of secondary dipoles within the Cu oxidized surface. conductivity reaches a difference of two orders of magnitude. As an insulating zone appears by oxidation, a possible barrier is Thus, a threshold exists where the resulting potential changes suspected and a new dipole appears behind. After a first sign from the original positive found for copper conductivities insulating phase appears on the copper surface, a new created to a negative value, expected for insulating materials (see dipole appears behind and the b situation appears. The Fig. 1b and c). induced secondary dipoles in the new zone are not, however, Since the actual potential of the induced cathode and anode of the same intensity, since the distance to the driving Pt is related with the deviation of the potential with respect to the electrode has increased and the screening of previous insulat- electrolyte voltage drop as shown in Fig. 1b, the plots on Fig. 7 ing zones has appeared, but it retains the same direction than are given as the difference between the calculated voltage and the original one, opposing the external field. Thus, a positive the electrolyte voltage at that point. For easy comparison, also induced potential (secondary anode) is lowered in the second modifications of the conductivity at the induced cathode are conducting zone. It is relevant that also the negative reduction shown in Fig. 7b, although all evidence suggests only metallic pole on the opposite border of the new sub-dipole is quantita- copper is present at that spatial coordinate. As the conductivity tively smaller (see part 4b in Fig. 8). As other new stripes of the material at the induced cathode is decreased, also appear, up to three voltage maxima appear, all of smaller decreases the intensity of the induced potential. Thus, the intensity than the original calculated for metallic copper, combination of both calculations suggest already that a central explaining the appearance of consecutive parallel stripes. Fig. 7 (a) Simulation of Potential profiles (Induced voltage-electrolyte voltage) across the cell (sample shaded at the center) for various conductivities of the oxidized phases formed as compared with the original dipole formed (black maximum potential difference). For easy comparison (b) shows the voltage profile for the existence of conducting parts at the induced cathode, with several conductivities. 2292 | Mater. Chem. Front., 2022, 6, 2284–2296 This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2022 View Article Online Research Article Materials Chemistry Frontiers This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 14 July 2022. Downloaded on 11/2/2022 9:51:14 AM. Fig. 8 COMSOL finite element modelling of each of the zones detected, assuming insulating phases being formed in different stages. Rows correspond to the (1) visual scheme, (2) isopotential lines, (3) charge distribution and (4) linear voltage profile along the main axis of the electric field for each time. Thus, COMSOL finite element simulations allows us to give a simple model to understand the dynamic evolution of waves or oscillations as new oxidation zones appear on the copper surface with high resistance. As observed in Fig. 7 and 8, the alternance of positive and negative zones induced after insula- tors appear, shifts the maximum induced potential towards the center of the sample, yielding smaller positive potential, but also induces negative zones where originally a positive potential was appearing, which may induce the reduction of previously oxidized copper. Thus, we observe metallic copper stripes between oxidized copper stripes, as observed visually and in XPS and X-ray diffraction. In our experiments up to four zones are detected but also the starting of a new patterns of secondary dipoles in each zone, as stripes of insulating phases appear in a singular agreement with finite element electrostatic modelling (see Fig. 9). Such simulations, based on the experimental observations, Fig. 9 Time and space evolution of the simulated potential profile at the copper surface, as oxidized insulating phases are formed. Each time step, may also be visualized as the interference between the voltage t0 to t5, relate to the images placed on the left side. Please note that sample drop across the insulating stripes and the dipoles induced at spatial dimensions are delimited by t0 potential maxima. the conducting stripes since the insulating phases align with the external field and conducting parts oppose to the external field. The alternance of polarization and charge distribution to the initial setting yielding to an interference pattern in yields to a dynamic sequence, of what may be simplified as potential and a final oscillation in the copper oxidation state. resulting dipoles, all with lower magnitude than the one found At longer times, the pattern seems to reproduce again along the at initial t0, and to a time evolution switching between positive spatial axis within the field, and stops due to the existence of an and negative induced charges (Fig. 9). Such space and time induced cathode and the finite dimensions of the copper piece. oscillation induces oxidation and consecutive reduction at the The appearance of such significant oscillations with a same spatial coordinates, yielding a space and time oscillating simple imposed external field is relevant to many cases where behaviour. gradients are involved as part of the process. From electrodes in As a summary, the formation of insulating phases at differ- electrochemical energy storage systems to biological redox ent potentials, yields, even at the starting point, different processes or catalysis, a possibility arises on the possible border lines that create new secondary dipoles of different mechanisms acting, and the patterns that may induced specific intensity within the surface dimensions, shifting the front of redox reactions. The same simple nature that modulates such the bipolar effect. At some space coordinates, the sign is opposite behaviour is an awakening scientific reminder of the need to This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2022 Mater. Chem. Front., 2022, 6, 2284–2296 | 2293 View Article Online Materials Chemistry Frontiers Research Article account for oscillation in synthesis, characterization, device 3 M. J. Russell, L. M. Barge and R. 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