Electrochemical Water Oxidation of Ultrathin Cobalt Oxide-Based

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Electrochemical Water Oxidation of Ultra-thin Cobalt OxideBased Catalyst Supported onto Aligned ZnO Nanorods Nandanapalli Koteeswara Reddy, Stefanie Winkler, Norbert Koch, and Nicola Pinna ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10858 • Publication Date (Web): 19 Jan 2016 Downloaded from http://pubs.acs.org on January 21, 2016

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ACS Applied Materials & Interfaces

Electrochemical Water Oxidation of Ultra-Thin Cobalt Oxide-Based Catalyst Supported onto Aligned ZnO Nanorods

Nandanapalli Koteeswara Reddy1,*,#, Stefanie Winkler2, Norbert Koch2, Nicola Pinna1,*

1 - Humboldt-Universität zu Berlin, Institut für Chemie, Brook-Taylor-Str. 2, 12489 Berlin, Germany 2 - Institut für Physik & IRIS Adlershof, Humboldt-Universität zu Berlin, Brook-TaylorStrasse 6, 12489 Berlin, Germany

Corresponding Author Email: [email protected], [email protected] #Present Address: Department of Physics, School of Engineering and Technology, BML Munjal University, Sidhrawali, Gurgaon-122413, India

Keywords: Water splitting, zinc oxide nanorods, cobalt oxide, oxygen evolution reaction, electrolysis of water.

Abstract: A stable and durable electrochemical water oxidation catalyst based on CoO functionalized ZnO nanorods (NRs) is introduced. ZnO NRs were grown on FTO by using a low-temperature chemical solution method and were functionalized with cobalt oxide by electrochemical deposition. The electrochemical water oxidation performance of cobalt oxide functionalized ZnO NRs was studied under alkaline (pH=10) conditions. From these studies it is noticed that cobalt oxide functionalized ZnO NRs show electrocatalytic activity towards water oxidation with current density in the order of several mA cm-2. Further, 30 s CoO deposited ZnO nanorods exhibited excellent galvanostatic stability at a current density of 1 mA cm-2 and potentiostatic stability at 1.25 V vs. Ag/AgCl over an electrolysis period of 1 h. 1 ACS Paragon Plus Environment


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Introduction: Development of fuels by water splitting with the help of semiconductors and sun light has received great attention since the discovery of photocatalysis in titanium dioxide (TiO2) by Honda and Fujishima.1 From the last few decades, a wide range of semiconductors has been tested as photoelectrodes for the production of hydrogen fuels.2 In this direction, various p-type semiconductors including GaAs, Si, and Cu2O have been used as photocathodes, whereas n-type semiconductors like TiO2, SnO2, and ZnS were adopted as photoanodes.3-5 Among the various n-type semiconductors, zinc oxide (ZnO) has attracted more attention as photoanode for efficient electrochemical water oxidation due to its physical and electrical properties.6-9 From recent investigations on ZnO material, it was noticed that highly porous ZnO-based nanostructures exhibit enhanced performance in energy applications than bulk and thin film counterparts due to the larger semiconductor-electrolyte interface area.10-12 In this view, ZnO nanostructures have been synthesized with different morphologies including nanorods, nanosheets, nanowires, nanoparticles, and nanoporous structures using different synthetic methods and tested as photoanodes.13,14 Among these ZnO nanostructures, vertically aligned

nanorods

(NRs)

structures

offer

an

excellent

surface

morphology

for

photoelectrochemical water splitting due to their lower carrier recombination loss and axial light absorption.15 According to the photodegradation principles of semiconductors,16 in all electrolyte solutions, ZnO is most stable against photodecomposition by electrons since the cathodic decomposition potential of ZnO is above the redox potential of water.17 On the other hand, the oxidation potential of water lies slightly below the anodic decomposition potential of ZnO. Thus, the photogenerated holes destabilize ZnO and as a result, ZnO decomposes into Zn2+ ions rather than water oxidation. However, the anodic decomposition of ZnO can be overcome by modifying the pH of the electrolyte between 9.5 and 13,17 and/or adoption of a catalyst as second order structure.18 The integration of a water oxidation catalyst allows the 2 ACS Paragon Plus Environment

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accumulated holes to react more efficiently with surface absorbed species and enhances the durability of the semiconductor. ZnO nanostructures functionalized with different metals (Au, Ni, Co) or semiconductor materials (TiO2, CdTe) have been successfully tested as photoanodes.19-23 A few groups also studied pure or doped ZnO material as photoanode for water oxidation process.17,24-26 The factors that should be considered in developing catalyst film include cost-effective, chemically stable, electronically conductive, and should not affect the optical properties of the core semiconductor.27 In this direction, we made an attempt to test cobalt oxide as catalyst onto ZnO NRs since cobalt species can catalyze water oxidation process by collecting accumulated holes, and thereby it can protect the ZnO NRs from anodic decomposition.28 As compared to other transition metal catalysts, predicted as desirable candidates for large scale applications, cobalt oxide is a low-cost, non-toxic, and an efficient catalyst material. Moreover, cobalt oxides have already been used as active catalyst for water oxidation on various photoanode materials.29 For example, Kent et al. observed excellent water oxidation performance from cobalt deposited onto nanostructured FTO and also noticed sustained water oxidation catalysis at a current density of 0.16 mA cm-2 (with Ewe = 1.61 V vs. NHE in pH 7.2) over an electrolysis period of 2 h.30 Pijpers et al. have shown an efficient photoelectrochemical water splitting from crystalline Si photovoltaic solar cells integrated with Co-Pi (cobalt-phosphate) catalyst at neutral pH.31 Steinmiller et al. have studied the anodic photocurrent performance of photoand electrochemically cobalt deposited (10-30 nm) ZnO structures.32 Noticeably, while depositing cobalt nanostructures by anodic electrochemical deposition in pH 7 buffer-solution at a potential of 1.2 V vs Ag/AgCl, a severe dissolution of ZnO was observed. On the other hand, the photoconversion efficiency of ZnO NRs has been greatly enhanced with the functionalization of Ag@Ag3(PO4)1-x NPs. This coating strongly enhances the photogenerated charge contribution to photocatalysis.33 Copper phthalocyanine deposited onto ZnO nanowires exhibits excellent photoresponse sensitivity and faster response time due 3 ACS Paragon Plus Environment


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to the passivation of surface states on ZnO.34 Liu et al. developed 3D ZnO structures by chemical vapor deposition and noticed a current density of 0.18 mA cm-2 at a bias voltage of 1.21 V vs. RHE.35 Albadi et al. have fabricated gold NPs decorated CuO-ZnO nanostructures and have applied them to the aerobic oxidation of alcohols as an efficient and recyclable heterogeneous catalyst.36 A cobalt-based phosphate functionalized ZnO strongly enhanced the photocatalytic water oxidation performance in the aqueous solution containing AgNO3 as sacrificial reagent.37 In this work, we have developed aligned ZnO NRs on FTO substrates by using atomic layer deposition and chemical bath deposition methods. The as-grown ZnO NRs were functionalized with CoO using cathodic electrochemical deposition. Then, the impact of CoO film thickness on water oxidation performance as well as protection from chemical corrosion of ZnO was studied in alkaline electrolyte solution (pH=10). From these studies, it is noticed that ZnO NRs functionalized with CoO exhibit stable water oxidation performance and excellent protection from anodic decomposition. Finally, our approach for the development of catalysts and protection layer on ZnO nanostructures is simple, cost-effective, scalable, and uses inexpensive analytical grade chemicals.

Experimental procedure: Synthesis of ZnO NRs: Fluorine-doped tin oxide (FTO) coated glass substrates (size ~ 2x1 cm2) were cleaned in acetone, methanol and isopropanol for 5 min. Then drying in an argon stream at room temperature.38 A thin film of ZnO, acting as seeding layer for the hydrothermal growth of ZnO nanorods, was deposited by ALD at 175 °C and 2x10-1 Torr.39, 40 Deionized water and diethyl zinc were sequential injected into the reaction chamber with a steady nitrogen stream (20 sccm). The pulse, exposure, and purge time for each precursor were maintained as 20 ms, 15 s and 12 s, respectively and the deposition was carried out for 300 ALD cycles. ZnO nanorods were grown from ZnO seed layer deposited on FTO by 4 ACS Paragon Plus Environment

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chemical bath deposition (CBD) at a growth temperature of 80 °C for 6-7 h.41, 42 A 10 mM aqueous solution containing zinc nitrate and hexamethylenetetramine was used as bath solution. After the growth, ZnO NRs were repeatedly rinsed in deionized water and dried under Argon.

Functionalization of ZnO NR with CoO: Cobalt oxide was deposited on ZnO NRs/FTO by electrochemical deposition (ECD) at three deposition times of 30, 60 and 180 s. All the depositions were carried out in an aqueous 0.1 M cobalt (II) nitrate hexahydrate electrolyte solution at a constant cathodic potential of 1.5 V.43 In order to estimate the thickness of CoO film deposited on ZnO NRs, CoO was also deposited on cleaned Si substrates under the same conditions. Schematic representation of ZnO NRs growth and functionalization with CoO is shown in Figure 1a.

Characterization: The thickness of CoO films deposited on Si substrates was estimated by Ellipsometry (Model: Sentech spectroscopic ellipsometer SENpro). Pure and CoO functionalized ZnO NRs were analyzed by field emission scanning electron microscopy (FESEM, Hitachi S-4000 FEG microscope operating at a 2 kV), energy dispersive X-ray spectroscopy (EDS), X-ray diffractometry (XRD, STOE MP Diffractometer), transmission electron microscopy (TEM, Philips CM 200 microscope at 200 kV), and UV-VIS-NIR spectroscopy (PerkinElmer, Lambda 950). X-ray photoelectron spectroscopy (XPS) was performed using Mg/Kα radiation (1253.6 eV photon energy) to examine the states of the cobalt oxide films onto the ZnO nanostructures. The spectra were recorded at room temperature and normal emissions using a hemispherical Omicron energy analyzer. The binding energies for the as synthesized sample were calibrated to the Co 2p3/2 peak, which was determined for the sample after electrolysis at 780.0 eV to compensate for charging effects. Electrochemical water oxidation studies were carried out using a three-electrode setup. A small volume cell (20 mL) with a fixed distance between all electrodes (1.5 cm) was 5 ACS Paragon Plus Environment


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connected to a multi-channel potentiostat (BioLogic VMP3). All the measurements were performed under continuous flow of Argon in the headspace of the solution. NRs grown onto FTO, Pt wire, and Ag/AgCl have been used as working, counter, and reference electrodes, respectively. The reference electrode was connected to the cell via a Luggin tube. A 30 mM borate buffer (pH=10) solution was used as electrolyte. After measuring the open circuit voltage (OCV), a cyclic voltammetry (CV) scan was performed with a scan rate of 20 mV s-1 in the potential range, -0.3 to 1.2 V (vs. Ag/AgCl). The electrode was activated at electrode potentials of -0.5 and 1.5 (V vs. Ag/AgCl) to reduce and oxidize the surface species. Then, linear sweep voltammetry (LSV) measurements were performed with a scan rate of 5 mV s-1 in the anodic potentials range, 0-2.0 V (vs. Ag/AgCl).

Results and discussion: The linear increase of CoO film thickness (estimated from ellipsometry data) with deposition time is shown in Figure 1b. By increasing the deposition time (from 15 to 300 s), the thickness of the CoO film increased from 0.5 to 25 nm, and the overall growth rate is found to be 0.08 nm s-1. The surface morphology of pure and CoO deposited ZnO NRs was studied with FESEM and the results are shown in Figure 2. The as-grown ZnO NRs are slantly aligned on FTO substrates with lengths up to 1 µm and the diameter of NRs varied between 50 and 150 nm, Figure 2(a, b). FESEM images of 30 s CoO deposited ZnO NRs reveal that CoO is deposited on ZnO NRs as thin film (Figure 2 c,d and S1). Upon increase of the deposition time from 30 to 180 s, the thickness of the CoO film gradually increases and the morphology changed to a cone-like morphology finally forming a uniform thick film (Figure S2).

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Figure 1: (a) Schematic representation of the synthesis procedure.Step-1: Deposition of ZnO film onto FTO substrates by ALD, Step-2: Growth of ZnO NRs by CBD and Step-3: Deposition of CoO film on ZnO NRs by ECD. (b) Variation of CoO film thickness as a function of deposition time (CoO deposited on Si substrates measured by ellipsometry) and linear fit (growth rate = 0.08nm s-1).

Figure 2: High and low magnification FESEM images of (a, b) pure, (c, d) 30 s, and (e, f) 1 min CoO deposited ZnO NRs.

In order to quantify the amount of CoO deposited on ZnO NRs, the structures were analyzed by energy dispersive spectroscopy under a scanning electron microscopy, and the obtained spectra are shown in Figures S3-S6. The amount of CoO on ZnO NRs gradually 7 ACS Paragon Plus Environment


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increased with the increase of deposition time, and the Zn/Co peak ratio decreased with the increase of deposition time (Figure S7). The crystal structure of ZnO NRs was studied by XRD (Figure 3). Pure ZnO and CoO-modified ZnO NRs exhibit one strong reflection at 2θ = 34.65 and a few weak reflections at 26.64, 33.85, 36.45, 37.85, 47.73, 51.63, 54.67, 61.62, 63.1, 65.62, and 68°. The reflections at 34.65, 36.45, 47.73, 63.1 and 68 are assigned to hexagonal wurtzite structure of ZnO (PDF: 36-1451). Compared to tabulated data, the (002) reflection has a higher relative intensity indicating a preferential growth along the [001] direction, i.e. along c-axis. The remaining XRD reflections are due to the FTO substrate (cf. labels on Figure 3). Upon deposition of CoO no changes in the diffraction patterns are detected due to the amorphous character of the CoO film (see below).

Figure 3: XRD patterns of pure and CoO-functionalized ZnO NRs at different deposition times (S1 - pure, S2 - 30 s, S3 - 1 min, and S4 - 3 min CoO deposited ZnO NRs).


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The morphology and crystallinity of pure and 30 s CoO deposited ZnO NRs was further examined by transmission electron microscopy (Figure 4). For these studies, the nanostructures were detached from the FTO substrate by sonication in ethanol and then, dropcasted on carbon coated copper grids. Figure 4a shows the TEM image of tip of an as-grown ZnO NR. The high resolution TEM confirms that the NRs are single crystalline and their preferential growth direction is parallel to [001] (Figure 4b). Figure 4c shows the bright field TEM image of a 30 s CoO deposited ZnO NR. The presence of a thin film on the surface of NRs indicates that the deposited CoO film is loosely attached to ZnO NRs surface and upon sonication these films very probably detach from ZnO NRs surface. HRTEM analysis of CoO on the surface of ZnO NR (Figure 4d) indicates that the as-deposited CoO is amorphous (Figure S8). Further, power spectra (PS) taken on the center of ZnO/CoO NR and the interface of ZnO and CoO (inset of Figure 4e-g) show reflections corresponding to a dspacing of 0.265 nm, which belongs to (002) family of planes of the wurtzite ZnO. On the other hand, the PS took exclusively on the CoO region shows diffuse rings confirming that the electrodeposited CoO is amorphous.

Figure 4: (a, b) TEM and HRTEM images of pure ZnO NRs, (c, d) TEM and HRTEM images of 30 s CoO deposited ZnO NRs and (e-g) FFT taken at center, interface, and CoO regions of ZnO/CoO structures. 9 ACS Paragon Plus Environment


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UV-Vis-NIR transmittance (%T) spectra (Figure S9) show that pure and 30 s CoO deposited ZnO NRs display very similar optical properties. These results clearly emphasize that the deposition of CoO for short times (30 s) have a negligible impact on the optical transmittance of ZnO NRs. On the other hand, an increase of CoO thickness leads to a decrease of the transmittance in the visible and near infrared region. This is attributed to the absorption of the CoO.

After cyclic voltammetry (CV) measurements (Figure 10), electrocatalytic oxygen evolution reaction (OER) activity of pure, 30, 60 and 180 s CoO functionalized ZnO NRs measured by linear sweep voltammetry (LSV) with a scan rate of 20 mV s-1 at an external potential (E) of 0-2 (V vs. Ag/AgCl) under dark conditions are shown in Figure 5a. Pure ZnO NRs exhibit no OER activity. In contrast, 30 s CoO functionalized ZnO NRs showed a current density of 2.6 mA cm-2 at 2.8 (V vs. RHE) with an onset potential (Eon) and series resistance (Rs) (here Rs is calculated from the slope of current density vs. E plot at above Eon) of 1.74 (V vs. RHE) and 391 Ω (Figure S11). While increasing the CoO deposition time from 30 s to 3 min, the current density gradually increases, whereas Eon and Rs significantly decrease (see Table 1). The energy level diagram of nanostructured CoO reveals that the valance band edge of CoO is pinned below the reduction potential of water (Figure 6).44 Therefore, the accumulated charge carriers or trapped states at the interfaces of ZnO NRs and electrolyte are significantly reduced and as a result, the overall current density of ZnO NRs functionalized with CoO greatly enhanced. It implies that the current density of CoO functionalized ZnO NRs is strongly dependent on the electrodeposited CoO thickness.


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Figure 5: (a) Linear sweep cyclic voltammetry plots of pure and CoO deposited ZnO NRs, (b) variation of the over potential, estimated at 2 mA cm-2, with CoO deposition time, (c) Tafel plots and (d) variation of Tafel slops, measured at lower and higher potential (LP and HP) regions, as a function of CoO deposition time.

Table 1: Current density of pristine and CoO deposited ZnO NRs Current density (mA cm-2) at 2.8 V vs. RHE Pristine 0.03 30 s CoO 2.58 1 min CoO 5.25 3 min CoO 7.44 Sample

The overpotential (η) of anodic oxygen evolution reaction was calculated using the following equation, where EA is applied potential and the off-set potential of Ag/AgCl is ~0.195 V.



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ηV E V 1.23 0.195 0.059 pHV

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(1)

The variation of overpotential determined at J=2 mA cm-2 as a function of the CoO deposition time is shown in Figure 5b. By increasing CoO thickness on ZnO NRs, the overpotential decreases. Further, in order to understand the electrochemical kinetics of the device, the J-η data of CoO functionalized ZnO NRs was fitted to the following Tafel equation: ln(J/Jo) ~ αnFη⁄RT

(2)

where Jo is the exchange current density, α is charge transfer coefficient, n is number of electrons transferred, F is Faraday’s constant, R is gas constant, and T is an absolute temperature.45 Tafel plots (ln(J) vs. η) of pure (Figure S12) and CoO functionalized ZnO NRs are shown in Figure 5c. Intercept (=ln(Jo)) and slope (= αnF/RT) of the Tafel plots were derived at small and large overpotentials, 0.3-0.7 and 0.9-1.5 V (R-I and R-II regions). Variation of Tafel slope with the CoO deposition time is shown in Figure 5d.

Figure 6: Schematic diagram of ZnO NRs functionalized with CoO and their energy band diagram with redox potentials of water

From Tafel and LSV plots it can be seen that the catalytic current density of pure ZnO NRs based OER devices is very low (5 µA cm-2) even at higher overpotentials. However, 12 ACS Paragon Plus Environment

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after CoO functionalization, the devices exhibited significant catalytic current density of 20 µA cm-2 beginning at η=200 mV. With the increase of CoO thickness, the OER catalytic performance of the device is further improved. Upon functionalization of ZnO NRs with CoO, the exchange current density is greatly enhanced, Jo=22 µA cm-2. By increasing the CoO thickness the exchange current density of the device is slightly increased and a maximum value for 3 min CoO deposition is found to be 80 µA cm-2. The change in Tafel slope with increasing CoO thickness is marginal. Noticeably, the slope of the Tafel plots, in both regions, slightly decreased with the increase of CoO deposition time, as shown in Figure 5d. These observations indicate that the functionalization of ZnO NRs with CoO promotes the anodic reaction since the current density of the device effectively depends on active reaction sites of the electrode.46

Figure 7: (a) Galvanostatic at J=1 mA cm-2 and (b) potentiostatic (at 0.5 and 1 V vs. Ag/AgCl) measurements of 30 s CoO deposited ZnO NRs for 1 h.

The long-term stability for OER catalytic activity of the nanostructures has also been assessed. Galvanostatic (Chronopotentiometry at J = 1 mA cm-2) and potentiostatic (triggered between 0.5 and 1 V vs. Ag/AgCl) measurements were carried out for 1 h each. The variation 13 ACS Paragon Plus Environment


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of the electrode potential and the current density of ZnO/CoO heterostructure device with time are shown in Figure 7. The overall change in the performance of the device is negligible, despite some small oscillations due to desorption of oxygen bubbles from the surface. However, a transient current can be noticed when the external potential of the device is triggered (Figure 7b).23 The overall galvanostatic as well as potentiostatic OER electrocatalytic activity of functionalized ZnO NRs with CoO is very stable over two hours. After one hour measurement, the color of the CoO deposited ZnO NRs anodes changes from light-white to black (see inset of Figure 7a).

In order to assess the chemical properties of the cobalt oxide-based catalyst, the nanostructures were examined with XPS. A comparison of cobalt 2p regions for the functionalized ZnO nanostructures before and after the electrolysis is depicted in Figure 8. Before electrolysis the peaks are found at 780.0 eV and 795.9 eV binding energies (BE) for the 2p3/2 and 2p1/2 levels, respectively, associated with a peak-splitting of 15.9 eV. After electrolysis this splitting is decreased to 15.1 eV. The intense 2p satellite features at 785.8 eV and 802.0 BE, which are attributed to CoO,47, 48 are found to be significantly shifted to higher BE (790.0 eV and 805.4 eV) and reduced in intensity upon electrolysis. The weak satellite structures are characteristic for spinel structures with octahedral 3+ cations and tetrahedral 2+ cations. This assignment is further supported by inspecting the peak-narrowing of the 2p3/2 line upon electrolysis. This is attributed to the conversion of the predominant CoO phase, where the unpaired nature of half-filled Eg band of the Co2+ cation results in strong electron correlation and substantial broadening upon photoemission, to the predominant Co3O4 phase. In the latter phase, the low-spin diamagnetic nature of Co3+ and the weaker crystal field of tetrahedral Co2+ result in similar photoemission binding energies, which then results in a sharper 2p3/2 feature than the one that related to the strong crystal field in CoO.49 These results strongly suggest that cobalt was electrochemically deposited as CoO with cobalt in the 14 ACS Paragon Plus Environment

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predominant oxidation state (II), whereas it was oxidized to a mixture of Co2+ and Co3+ during electrolysis, which corresponds most likely to the Co3O4 spinel structure.

795.1 eV 790.0 eV 805.4 eV intensity [a.u.]

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after ECP

before ECP

795.9 eV

780.0 eV 785.8 eV 775

780

785

790

802.0 eV 795

800

805

810

binding energy [eV] Figure 8: Co 2p x-ray photoelectron spectra of 30 s CoO deposited ZnO NRs before and after electrolysis.

Finally, the morphological properties of the cobalt oxide coated ZnO NRs after one hour durability measurement were examined by XRD, FESEM and TEM (Figure 9). The XRD profile shows only reflections from the ZnO nanorods and the substrate, similarly to the pristine sample (Figure 9a). The FESEM image (Figure 9b) shows that the surface morphology is mostly unaffected by the OER test, which is also in agreement with TEM studies (Figure 9c).



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Figure 9: (a) XRD profiles of as-grown and OER performed 30 s CoO deposited ZnO NRs, (b) FESEM and (c) TEM images of 1 h OER performed heterostructures.

Conclusions: Thin CoO films deposited onto ZnO NRs grown on FTO substrates were synthesized in three-steps. ALD was used for the deposition of a ZnO seed layer, chemical bath deposition for the growth of ZnO NRs and electrochemical deposition for CoO functionalization. ZnO NRs functionalized with CoO exhibited interesting water oxidation performances with a current density of 2 mA cm-2 at 2 V (vs. Ag/AgCl). ZnO NRs functionalized with short deposition time (30 s) exhibited good stability in alkaline electrolyte solution (pH=10) under an external potential of 1.25 V (vs. Ag/AgCl) and current density 1 mA cm-2 with good durability. XPS studies demonstrate that during electrolysis the amorphous CoO film is oxidized to Co3O4. All in all, the ZnO NRs functionalized with CoO can be considered as a promising anode material for electrochemical water oxidation.

Acknowledgments: N. K. Reddy gratefully acknowledges the support of European Community under Marie Curie International Incoming Fellowship within the 7th European Community 16 ACS Paragon Plus Environment

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Framework Programme (call identifier FP7-PEOPLE-2012-IIF and Proposal Nr. 331003). Reddy also acknowledge the support of Dr. Gisela Weinberg, Fritz-Haber-Institute der MaxPlanck-Gesellschaft for FESEM & EDS measurements, Sebastian Wahl for corrections & electrochemical measurements, Dr. Gianvito for TEM studies, Dr. Donato Ercole Conte & Dr. Matthias Karg for XRD measurements and Prof. Klaus Rademann for SEM measurements.

Supporting Information: FESEM images, Growth mechanism, EDS results, HRTEM, Optical properties, and electrochemical catalysis of ZnO NRs and ZnO/CoO nanorods structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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Table of Content: ZnO/CoO nanorods structures developed by ALD, chemical bath deposition and electrochemial deposition exhibit good electrochemcial catalytic performance and stability for the oxygen evolution reaction.


Electrochemical Water Oxidation of Ultrathin Cobalt Oxide-Based

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