Generation of Transparent Oxygen Evolution Electrode Consisting of

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Generation of transparent oxygen evolution electrode consisting of regularlyordered nanoparticles from self-assembly cobalt phthalocyanine as a template Ahmed Ziani, Tatsuya Shinagawa, Liga Stegenburga, and Kazuhiro Takanabe ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12006 • Publication Date (Web): 04 Nov 2016 Downloaded from http://pubs.acs.org on November 6, 2016

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ACS Applied Materials & Interfaces

Generation of transparent oxygen evolution electrode consisting of regularly-ordered nanoparticles from self-assembly cobalt phthalocyanine as a template Ahmed Ziani, Tatsuya Shinagawa, Liga Stegenburga and Kazuhiro Takanabe* King Abdullah University of Science and Technology (KAUST), KAUST Catalysis Center (KCC) and Physical Sciences and Engineering Division (PSE), Thuwal, 23955-6900, Saudi Arabia.

ABSTRACT: The decoration of (photo-) electrodes for efficient photoresponse requires the use of electrocatalysts with good dispersion and high transparency for efficient light absorption by the photoelectrode. As a result of the ease of thermal evaporation and particulate self-assembly growth, the phthalocyanine molecular species can be uniformly deposited layer-by-layer on the surface of substrates. This structure can be used as a template to achieve a tunable amount of catalysts, high dispersion of the nanoparticles and transparency of the catalysts. In this study, we present a systematic study of the structural and optical properties, surface morphologies, and electrochemical oxygen evolution reaction (OER) performance of cobalt oxide prepared from a phthalocyanine metal precursor. Cobalt phthalocyanine (CoPc) films with different thicknesses

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were deposited by thermal evaporation on different substrates. The films were annealed at 400 °C in air to form a material with the cobalt oxide phase. The final Co oxide catalysts exhibit high transparency after thermal treatment. Their OER measurements demonstrate well expected mass activity for OER. Thermally evaporated and treated transition metal oxide nanoparticles are attractive for the functionalization of (photo-) anodes for water oxidation.

KEYWORDS: Cobalt phthalocyanine, evaporation deposition, thermal treatment, transparent electrode, Oxygen evolution reaction

INTRODUCTION Photoelectrochemical (PEC) splitting of water is considered to be one of the most promising technologies to generate solar fuel.1-9 Water splitting comprises the hydrogen evolution reaction (HER) on the cathode side and the oxygen evolution reaction (OER) at the anode. When HER or OER electrocatalysts are directly decorated on a photon absorber in a single device, the catalyst layer requires not only active but also transparent, which is essential to promote the absorption of the irradiated photon by the photoelectrode.10 In particular, the OER requires a larger overpotential than the HER,11 which is a bottleneck to the widespread use of the water electrolysis process. The OER proceeds via sequential four electron transfer process, which corresponds to the presence of proposed four different surface species that participate in the reaction scheme.12-14 It is inevitable to find active OER catalysts to establish efficient PEC system.15,16 IrOx and RuOx electrocatalysts are known to exhibit small overpotentials, which results in high OER rates compared to other typical metal oxides.11-17 However, Ir and Ru are scarce and expensive elements, which hampers their widespread use for practical applications.


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Many chemical methods have been developed to synthesize transparent OER catalysts on the surface of photoanodes with cost-effective materials.18-24 It has been demonstrated that the decoration of photon absorbers with transparent and active OER catalysts results in improved performance; e.g., a transparent NiFe oxide on a hematite photoanode,18 Fe-treated NiO on top of p-type Si photocathode,19 and Ni oxide, NiFe oxide, Co or Cu nanowire-based OER catalysts.2023

More recently, robust sub-monolayers of Co3O4 nano-islands have been synthesized by flame

spray pyrolysis of solutions containing cobalt acetylacetonate, which exhibited high transparency as well as high OER performance.24 Other strategies have involved using molecular cobalt catalysts to functionalize the surface of the (photo-) electrodes.25,26 Several cobalt molecules, such as Co porphyrins,27,28 cobaloximes,29 Co-Salen complexes30 and cobalt phthalocyanine (CoPc),31,32 have been reported as active OER catalysts because they are not limited by the accessibility of the active sites.31 Furthermore, molecular catalysts can be easy tailored to fit the required structures and morphologies of the electrodes. Although the reported chemical approaches to functionalize the surface with molecular catalysts are promising, the instability of the ligands in the catalytic solution presents a challenge to this approach.33 In reality, the molecular catalyst is a starting catalyst that is eventually transformed into another form of metal oxide or hydroxide catalyst,33-35 and the dispersion of the final catalyst is random and difficult to control. Therefore, the development of an alternative controllable approach to deposit the final catalysts derived from molecular species on the substrate is highly desired. Metal-organic frameworks (MOFs) have appeared to be beneficial for this purpose.36-39 The strategy involves growing a well-ordered MOF structure followed by thermal treatment to form well-dispersed particles on the surface of the substrate.40-42 The metal-metal distance is spatially isolated, thanks to the presence of ligand/node in the MOF structure, resulting in the density


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control of the resultant metal oxide after the thermal treatment. Because sophisticated MOFs can involve a complex synthesis and relatively long processes, herein we report a simple physical deposition method to obtain a layered CoPc molecular structure that can provide highly ordered molecular assembly without complicated synthesis protocol. In addition to an appropriate subsequent thermal treatment, this method will result in the deposition of amount-tunable, ultrafine, regularly ordered and transparent OER catalysts on the surface of the substrate. Moreover, the formation of these nanoparticle catalysts is independent of the nature of the substrate, i.e., a conductor, semiconductor or insulator. CoPc was chosen as a model material to be deposited on various substrates by the thermal evaporation method. CoPc is a planar molecule with the cobalt metal located at the center, and it is a chemically and thermally stable organometallic semiconductor.43 CoPc is commonly used in gas sensors, color dyes or electrocatalysts for PEC cells.43-45 In particular, CoPc can be easily deposited by thermal evaporation due to its thermal robustness.46-48 The thermal treatment is optimized in this process, and the structures, morphologies, optical properties and OER performance are presented in this study. The deposited catalysts show stable and comparable OER performance with the reported conventional Co-based materials.

EXPERIMENTAL SECTION CoPc deposition: Thin films of cobalt phthalocyanine (CoPc) were deposited by thermal evaporation at room temperature of a commercial CoPc powder purchased from Aldrich Chemicals with a purity of 99.99%. The product was used without purification. The films were deposited on fused glass, Au-coated glass, FTO and Si substrates.


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Structural characterization: The characterization of the crystal structural of the CoPc thin films was performed by recording X-ray diffraction patterns from a Bruker Discover diffractometer, Cu-Kα radiation with λ = 0.154 nm. The diffraction patterns were recorded from 10° to 80° with a step size of 0.02° at 2° min−1. Scanning electron microscopy images were collected on a Magellan scanning electron microscope (SEM) using a TLD detector at an accelerating voltage of 5 kV. Transmission electron microscope (TEM) images and FFT diffraction measurements were taken on a FEI Titan 80-300 equipped with a field emission gun operating at 300 kV. Samples were prepared by direct evaporation of CoPc material on the copper grid. The same grid was used for thermal treatments. The sample was mounted on a double-tilt holder and transferred to the microscope. Atomic force microscopy (AFM) measurements for CoPc on Si substrates were recorded using an Agilent 5500 SPM microscope under ambient conditions in air. The acoustic AC (AAC) mode was used with a cantilever resonant frequency of approximately 330 kHz and a spring constant of 40 N/m. AFM topographic images were analyzed with Gwyddion 2.45 software. Contact angles were measured using a contact angle goniometer (KRUSS EasyDrop Standard) at ambient pressure and temperature; static angle measurements were made for 1 µl drops of deionized water on the surface of the samples. Raman spectra were recorded on a Micro Lab Raman Aramis Spectrometer (Horiba Jobin Yvon) equipped with an Olympus BXFM-ILHS microscope with a 100x long-distance objective; 1800 l/mm gratings were used. A HeNe laser was used for excitation at 633 nm. Scans from 500 to 2000 cm−1 were recorded with a 20 sec acquisition time. Optical characterization: The transmittance spectra of the CoPc samples were collected at room temperature using a spectrometer (JASCO V-670). Signals were differentiated by using two channels. One monitored the CoPc film on the substrate (TF,S) and the second monitored a


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blank substrate (TS). The film transmittance contribution is obtained by examining the TF,S/TS value. Chemical analysis: The inductively coupled plasma mass spectrometry (ICP-MS) measurements were performed using an ICP-OES Varian 72 ES. For sample preparation, 1 cm2 of the original sample was dissolved in 1 ml aqua regia for 12 h and then diluted in 14 ml of MilliQ-water. Electrochemical characterizations: The prepared CoPc and NiFePc films were tested for the electrochemical oxygen evolution reaction in 1.0 M KOH electrolyte solution, in a threeelectrode system connected to a Bio-Logic VMP3 electrochemical work station. The geometric surface area of the electrode immersed in the cell was 1× 1 cm2. In all measurements, a Pt wire and Hg/HgO (1.0 M NaOH) were used as the counter and reference electrodes, respectively. Prior to each measurement, the cell, the counter electrode and the reference electrode were washed with nitric acid (~10 %, diluted with water). Cyclic voltammograms (CVs) were recorded at a scan rate of 10 mV s−1. Chronoamperometry (CA) was carried out at five different potentials (between 1.6 - 2.1 V on the reversible hydrogen electrode potential scale) for 1 h. Oxygen (99.999 %) was supplied to the cell before and during all the measurements. All currentpotential relationships in this study were already iR-corrected with the values measured by impedance spectroscopy (100 kHz, 10 mV amplitude), unless otherwise specified. All the measurements for CoPc were performed at least two times, and the averaged values are presented in this study. BiVO4/WO3 photoanode preparation First, tungsten trioxide (WO3) films were grown by direct current (DC) magnetron sputtering using a pure tungsten (W) target. Deposition was performed at room temperature on FTO


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substrates. A WO3 phase was obtained by the adding reactive oxygen gas to the argon in the sputtering gas. The Ar and O2 flows were fixed at 7 and 13 standard cubic centimeters per minute (sccm), respectively. The total pressure and DC power density for the target were fixed at 1.33 Pa and 7.3 W cm−2, respectively. Post-annealing treatments were carried out for 1 h under air at a temperature of 400 °C. BiVO4 was deposited on the surface of WO3 by drop casting the precursor according to the literature.49 We dissolved 20 mmol of Bi(NO3)3 and NH4VO3 at an equal molar ratio in 10 mL of aqueous nitric acid solution (2 M). A yellow precursor solution was obtained. We drop cast the precursor solution on the surface of WO3/FTO samples. Then, the samples were covered with the yellow liquid of the precursor solution. Calcination of these films was then performed at 550 °C for 2 h in air.

RESULTS AND DISCUSSIONS Self-assembly CoPc thin films were deposited by thermal evaporation of a commercial CoPc powder at room temperature. Different amount of deposited CoPc was controlled by simply varying the amount of CoPc precursor in a heating boat. The films were deposited on fused glass, Au-coated glass, fluorine-doped tin oxide (FTO) and silicon (Si) substrates. Films deposited on fused glass were used for UV-Vis transmittance measurements, films deposited on FTO were used for OER electrochemical measurement and those deposited on the Au-coated glass substrate and the smooth Si surface were used for morphological characterization. Then, they were subjected to heat treatment at 300, 350, 400 or 500 °C for 30 min under air in a static furnace. In Figure 1, the transmittance values of the representative films on fused glass (CoPc amount: 12.4


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nmol cm−2) are presented. The as-grown films (without heat treatment) show Q-band absorption at 615 and 690 nm, which is characteristic for CoPc compounds.31,50 When treated below 300 °C, the transmittance was almost unaffected by the annealing treatment. The transmittance of the film was improved as the annealing temperature increased and reached a maximum at 400 °C. The film annealed above 400 °C presented a transmittance of approximately 95 % at a wavelength of 400 nm and a transmittance of approximately 98 % at 600 nm. After heating was conducted at 500 °C, the film showed a slight decrease of the transmittance, which confirms that the optimum transparency was obtained at 400 °C.

Figure 1. UV-Vis transmittance of evaporated CoPc deposited on fused glass substrate and annealed at different temperatures (CoPc amount: 12.4 nmol cm−2).


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We first characterize the CoPc powder as purchased and the product obtained from heat treatment of CoPc bulk powder at 500 °C for 1 h using a ceramic boat. In Figure 2A, we presented X-ray diffraction (XRD) patterns of pristine and annealed CoPc aggregates. The pristine CoPc aggregate presents intense characteristic diffraction peaks at (001) and (20-1), consistent with literature for crystalline CoPc.46 After annealing, a phase transformation of the material from CoPc to the Co3O4 crystal structure occurred, which was clearly indexed in accordance with the JCPDS card 00-042-1467. The complete peak indexation is shown in Figure 2A. The CoPc complex was decomposed by the annealing and the resulting product is a cobalt oxide Co3O4 spinel. Next, CoPc films on fused glass (the same sample in Figure 1) were characterized by XRD. Figure 2B shows magnified XRD patterns of CoPc films treated at different annealing temperatures (see Figure S1 for the full XRD patterns). Only one peak for the as-grown film was observed at 2θ = 6.74°, corresponding to the d-spacing of 12.64 Å. This is attributed to the (001) facet of α-phase CoPc crystal.46 This result indicates well-ordered arrangement of 2D CoPc planer structure which stacks by a layer-by-layer growth. The film annealed at 300 °C presents a diffraction peak ascribed to the (001) plane and the intensity of the peak was higher than that for the as-grown film, which indicates an improved organization of stacking of the film. When the sample was annealed at 350 °C, the (001) peak was shifted to 6.97°, which corresponds to a contraction of the d-spacing associated with transformation from α-phase to β-phase (12.64 Å to 12.53 Å).46 Further increasing the annealing temperature to 400 °C resulted in a reduction of the (001) peak intensity, coinciding with the enhanced optical transparency of the material (Figure 1). The XRD patterns did not show any diffraction peaks from the cobalt oxide phase most likely because of the small amount of the material present on top of the surface (CoPc amount: 12.4 nmol cm−2).


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The Raman spectra of the CoPc films deposited on Au-coated glass substrates (Co amount: 10.7 nmol cm−2) are shown in Figure S2. The as-grown film has characteristic peaks associated with Pc ligand framework.31 After annealing at 400 °C, these peaks were diminished, confirming the destruction of ligands and the introduction of a new chemical structure. The amount of Co remains unchanged after annealing at 400°C (Co amount: 10.2 nmol cm−2). These results confirm that the thermal treatment decomposed the ligand structures of CoPc and forms the Co oxide material.


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Figure 2. (A) XRD diagrams of CoPc powder before and after annealing at 500 °C for 1 h (The inset shows a high magnification of the XRD diagram of the CoPc powder), and (B) XRD diagrams of 100 nm CoPc films evaporated on fused glass and annealed at different temperatures. Figure 3 presents the contact angle measurements of the CoPc films deposited on the FTO substrate (Co amount: 10.7 nmol cm−2). Figure 3A shows the response of the substrate without coatings; the substrate has a contact angle of 52°. After coating the FTO with CoPc, the hydrophobic nature of the surface was introduced by the CoPc layer and a contact angle became 87° (Figure 3B). After thermal treatment, the surface became hydrophilic and a contact angle of less than 10° is detected (Figure 3C). Similar results were found for CoPc films deposited on Au-coated glass substrates on which the similar amount of CoPc was deposited (Figure S3; the same films as Figure S2). The hydrophilic nature indicates the transformation of the coating from CoPc to Co oxide. High hydrophilicity of the electrode was reported to expedite the oxidation of water into molecular oxygen.51,52


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Figure 3. Contact angle measurements of evaporated CoPc on bare FTO (A), as-grown CoPc on FTO (B), and the 400 °C annealed CoPc on FTO (C).

We also characterized various samples using microscopic techniques to visualize the CoPc transformation. Figure 4A and B show scanning electron microscopy (SEM) images for the asgrown CoPc film on FTO and the annealed films (Co amount: 2.7 nmol cm−2). As can be seen in Figure 4A, FTO surface contains rough surfaces and the CoPc was homogeneously deposited on top of the FTO surface before annealing. In contrast, after annealing, the formation of regular and uniformly dispersed nanoparticles with 10 nm size was observed on the FTO surface (Figure 4B). To confirm this transformation from thin films to nanoparticles, CoPc was directly evaporated on the Cu grid that is used for transmission electron microscopy (TEM) investigations. Figure 4C and D show a homogenous CoPc film deposited on the top of the Cu grid, and we observe the transformation of this layer to a region with uniform particles of approximately 10 nm that are dispersed on the Cu grid (the high-magnification image is shown in Figure S4). The same information can be observed by atomic force microscopy (AFM). Figure 4E and F present the 2D and 3D images, respectively, of the as-grown CoPc film on the Si substrate. We observe densely-packed particles, the size of which is similar to what is observed via SEM images (~30-40 nm). Figure 4G and H show the 2D and 3D images of the


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films after annealing. Here, the formation of small well-dispersed Co3O4 particles (5-20 nm) with decent interparticle distance in contrast to opaque dense CoPc film.

Figure 4. SEM images of the CoPc thin film deposited on FTO before (A) and after annealing at 400 °C (B), TEM images of the CoPc thin film deposited directly on the Cu grid before (C) and after annealing at 400 °C (D), 2D-AFM mapping of the CoPc thin film deposited on Si before (E) and after annealing at 400 °C (G), and 3D-AFM mapping of the CoPc thin film deposited on Si before (F) and after annealing at 400 °C (H).

We prepared a series of samples with different amounts of cobalt (different thicknesses) on FTO and Si substrates; the cobalt amount was varied from 0.3 to approximately 120 nmol cm−2. In Figure 5, transmittance spectra of CoPc are presented. These values are obtained by subtracting the contribution of a blank FTO substrate from the total contribution of the CoPc film and FTO. The transmittance decreases as the cobalt amount on the surface of the substrate increases. For the smallest amount of the Co species (0.3 nmol cm−2), the transmittance is approximately 98 % at a wavelength of 400 nm and almost 100 % at 600 nm; the films are almost entirely


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transparent. When the amount of cobalt increased, the films become opaque, and a brown color that is characteristic of the Co3O4 material is observed. For the CoPc film with a Co amount of 109 nmol cm−2, the transmittance decreases to approximately 50 % at 400 nm and approximately 80 % at 600 nm.

Figure 5. UV-Vis Transmittance of CoPc annealed films with different amounts of Co on FTO measured by ICP-MS (a: 0.32, b: 2.7, c: 12.4, d: 19.2, e: 25.4, f: 57.2 and g: 109 nmol cm−2). The inset presents images of the CoPc samples before and after annealing.

Figure 6 presents the SEM images of CoPc samples with different Co amounts deposited on FTO after annealing at 400 °C. For a small amount loading of Co species, a good dispersion of 10 nm Co oxide particles was observed on the FTO surface. From 20 nmol cm−2, as the amount of Co increases, the particles move closer to each other and start to agglomerate. For higher


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amounts, the particles form a thin porous layer of agglomerated Co oxide. A similar behavior is observed for the CoPc films deposited on Si (Figure S5).

Figure 6. SEM images of CoPc films deposited on FTO and annealed at 400 °C with different amounts of Co measured by ICP-MS (A: 0.32, B: 2.7, C: 12,4, D: 19.2, E: 57.2 and F: 109 nmol cm−2)

The electrochemical OER performance of the prepared CoPc samples was examined. Chronoamperometry (CA) at varied applied potentials was adopted in this study to explicitly examine the steady-state OER performance in 1.0 M KOH solution while O2 was bubbled. The representative CA profiles obtained for CoPc are presented in Figure S6. As a reference, cyclic voltammetry (CV) was also carried out for the samples. Figure S7 compares the currentpotential relationships obtained from both CA and CV measurements for the CoPc films on FTO.


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In all cases, the anodic currents monotonically increased as the overpotential increased. With increasing the amount of Co, the currents also improved proportionally, consistent with the proposed mechanism that not only the surface but also bulk Co is responsible for the OER performance.53-54 Generally, the following relation between the electrode potential E and the current density j has been experimentally validated for electrocatalytic reactions; this is known as the Tafel equation:

E = b log ( j ) + a

(1)

where b and a are constants.16,55 In the current experiments, CoPc indeed exhibited clearly linear relations, as observed in Figures S7, and the Tafel slope b extracted is compiled in Figure 7. From these results, iR-corrected overpotentials (at 10 mA for a 1 cm × 1 cm geometric surface area) were calculated, and they are also presented in Figure 7 as a function of the Co amount for CoPc samples. The samples show decreasing overpotentials as the Co amount increases. When the OER activity-Co amount relationship for the CoPc samples was compared with electrochemically deposited Co electrodes, no substantial difference was observed (Figure S8). The observation indicates that the total amount of deposited Co was most likely the primary descriptor for the electrocatalytic performance. On the other hand, the transparency of the CoPc catalyst was found to monotonically decrease with Co amount (Figure 7). Overall, for photoelectrochemical applications, an optimum point for the electrocatalysis and transparency should be selected, which requires additional detailed examination depending on the identity of the semiconductor electrodes. Additionally, the Tafel analysis indicated a similarity between the reaction mechanism and the rate-determining step (70−80 mV dec−1). In particular, although the pretreatment was not optimized for different Co loading amounts (all the CoPc samples were pretreated at 400 °C for


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30 min), the pretreatment did not significantly influence the steady-state OER performance. Figure S9 compares the current-potential relationship for the CoPc sample (19 nmol cm−2) with and without the annealing treatment. As is clearly observed, long-term CA led to almost identical performance as CoPc film treated at 400 °C, suggesting that CA generates similar active site for OER. Consistent with this observation, anodic electropolymerization of Pc species was reported to form electronically conductive thin film, which is otherwise semiconducting.50 However, with regards to the material transparency and nanoparticle dispersion, the thermally treated sample present improved transmittance values and uniform particle arrangement (Figure S10). Under CV treatment, the CoPc semiconducting film became conductive to form active phase without totally removing organic ligand moiety from the film.33-35


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Figure 7. Tafel slope, the transmittance at 400 nm, and overpotential measured from the CAs of the CoPc films deposited on FTO and annealed at 400 °C with different amounts of Co (measured by ICP-MS).

To summarize the synthesis mechanism of the CoPc (Co oxide) catalyst, the scheme presented in Figure 8 shows the CoPc layer-by-layer structure with uniformly dispersed Co species on top of the substrate. After the annealing treatment, this disposition promotes the formation of small Co


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oxide particles that are spatially separated from each other with approximate size of 10 nm when the loading was less than 10 nmol cm−2. The phthalocyanine structure acts as a template for the organization of the cobalt oxide catalyst. By considering that a monolayer of CoPc material is deposited on a 1 cm2 surface of the substrate and estimating that the geometric area of a molecule of CoPc coordinated in thin films is approximately 3 nm2 (the area for a single molecule is approximately 1 nm2),56 we estimate that the number of cobalt atoms on the top of the 1 cm2 substrate is approximately 3.33×1013 Co. For a 92 nm CoPc film (measured by AFM), the estimated number of CoPc layers is 73 (with a d-spacing of 1.26 nm obtained from the XRD diagram for CoPc with a (001) orientation). The total number of Co atoms is 2.4×1015 Co, which corresponds to 4.0 nmol cm−2. We measured a value of 2.7 nmol cm−2 using ICP-MS measurements, which is reasonable given the accuracy of the estimation. By controlling the thickness of the CoPc film, we are also able to control the Co catalyst amount. We observe that thin films promote the formation of small nanoparticles that are well dispersed on the substrate surface. Increasing the thickness of the CoPc films results in agglomeration of the nanoparticles. In addition, the morphology becomes denser, which causes the reduction in the transparency of the films. This methodology can be extended to other metal oxides and can be used to functionalize various photoelectrode or photocatalyst systems.


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Figure 8. Scheme of the transformation of the layered structure of the CoPc thin films to the particulate structure with well-dispersed Co3O4 particles on the surface of the substrate.

To demonstrate the potential application of this CoPc catalyst for photoanode water splitting applications, we decorated CoPc particles on top of a BiVO4/WO3 photoanode (synthesis details of BiVO4/WO3 are given in supplementary information). We evaporated approximately 100 nm of CoPc films on the photoanode and treated the surface at 400 °C for a maximum of 30 min to obtain a well-dispersed and transparent cobalt oxide layer. To investigate the PEC properties, the CoPc/BiVO4/WO3 and BiVO4/WO3 photoanodes were tested and compared. Current-potential curves were obtained from linear voltammetry sweeps (LSV) from 0 to +2.5 V vs. RHE at a scan rate of 10 mV s-1 in an aqueous 0.1 M Na2SO4 solution (pH 6) under intermittent AM 1.5 G irradiation using a solar simulator, as shown in Figure 9. An anodic photocurrent was observed for the BiVO4/WO3 photoanode. The onset potential is at +0.5 V vs. RHE and photocurrent


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density at 1.8 V vs. RHE is 1.1 mA cm−2. After CoPc functionalization, the onset potential shifts to 0.1 V vs. RHE, which represents a noticeable shift of 400 mV. The photocurrent of 2.4 mA cm−2 at 1.8 V vs. RHE improves more than twofold. This indicates that the transparent CoPc catalyst can act as an efficient cocatalyst on top of the photoanode for water oxidation with mostly preventing parasitic light absorption. Nevertheless, it should be noted that the photoelectrochemical performance can be further improved by extended optimization in terms of not only electrocatalysis and transparency but also the construction of effective charge transfer at the interface.

Figure 9. Linear sweep voltammogram (anodic direction) of BiVO4/WO3 photoanode with and without CoPc cocatalyst under AM 1.5 G solar light irradiation in a 0.1 M Na2SO4 electrolyte solution (pH 6) at 10 mV sec−1.


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Overall, the presented method in this study demonstrates the accurate control of the amount of Co, even for small amounts, without a loss in the intrinsic catalytic performance of the CoPc films. This accurate control is of particular significance in solar fuel production systems, such as photoelectrochemical and photocatalytic water splitting systems,4,9,57 since the absorption of light by the surface electrocatalyst leads to a loss of the incident photons.18 Importantly, this method can be readily applied to other materials, such as Ni and Fe. Figure S11 presents CV and CA profiles obtained using a prepared NiFe mixed oxide (a NiPc-FePc sample with a 1/1 ratio of Ni/Fe). The figure demonstrates that a value of approximately 1.56 V on the RHE scale was sufficient to achieve 10 mA, which agrees with the reported performance of the NiFe electrode.58-61 This result confirms the universality of the developed method for various transparent film electrodes originating from organometallic phthalocyanine precursors via thermal evaporation.

CONCLUSION In summary, CoPc molecules were used as a template to promote good arrangement of cobalt on the surface of the (photo-)anode. CoPc was deposited on various substrates by thermal evaporation. After heat treatment, the ligands on CoPc were removed, as confirmed by XRD and Raman spectroscopy. Microscopic images showed that the particle size of the resultant Co species was approximately 10 nm. In addition, the prepared sample showed a tunable transmittance, which may be higher than 95 % for low loading of Co species. ICP analysis clearly indicated that the amount of Co deposited on the substrates was precisely controlled through this method. The electrochemical study demonstrated that the CoPc-derived Co oxide


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exhibited a similar performance compared with other Co-based OER catalysts. These observations indicate that the proposed thermal evaporation method allows for accurate control of the Co amount deposited on the substrate without a loss of the intrinsic properties of Co. Additionally, a CoPc OER catalyst was used to functionalize the surface of a BiVO4/WO3 photoanode. This resulted in an improvement in the onset potential and photocurrent density. Finally, we presented an extension of the strategy to other phthalocyanine molecules (a combination of FePc and NiPc). While this study is of an applicative nature, the obtained results are intriguing in the design and the development of catalyst-decorated (photo-)electrodes for solar water splitting applications.

SUPPORTING INFORMATION XRD diagrams of CoPc films evaporated on FG and annealed at different temperatures in extended range; Raman spectra of the evaporated CoPc on Au-coated glass substrates at two different positions; Contact angle measurements of evaporated and annealed CoPc on Au-coated glass substrates; TEM images of the CoPc thin film deposited directly on the Cu grid before and after annealing at large scale; SEM images of annealed CoPc films deposited on Si substarte with different amounts of Co; supplementary details of the electrochemical measurements; Representative chronoamperometric profiles for CoPc samples with different Co amount recorded in 1.0 M KOH; Representative current-potential relationship for CoPc samples with different Co amount obtained from CVs and CAs in 1.0 M KOH; Table of the calculated TOF for CoPc samples at 1.63 V vs. RHE; Current-potential relationship for CoPc (19 nmol cm−2) with and without pre-treatment recorded in 1.0 M KOH; SEM images of annealed and CV scan CoPc (12.4 nmol cm−2) films and their corresponding transmittance spectra; Description of the


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preparation of BiVO4/WO3 photoanodes; Linear sweep voltammogram of BiVO4/WO3 photoanode with and without CoPc cocatalyst; Current-potential relationship and original Ca profiles for the NiFePc electrode in 1.0 M KOH.

Corresponding Author *Email: [email protected]

ACKNOWLEDGMENT The research reported in this publication was supported by funding from King Abdullah University of Science and Technology (KAUST). The authors thank Dr. Natalia Morlanes, Amal Baqais and Dattatray Dhawale at KAUST for help in performing contact angle, ICP-MS measurements and TEM images.

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TOC Graphic 221x121mm (300 x 300 DPI)


Generation of Transparent Oxygen Evolution Electrode Consisting of

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