Effect of Different Cobalt Concentrations on Tungsten Trioxide ... - JKU

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Jan 13, 2015 - Effect of Different Cobalt Concentrations on Tungsten Trioxide. Photoelectrodes ... is an attractive approach for conversion of solar energy into chemical ..... S. Dunn, International Journal of Hydrogen Energy, 27, 235 (2002). 3.
Journal of The Electrochemical Society, 162 (4) H187-H193 (2015)

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Effect of Different Cobalt Concentrations on Tungsten Trioxide Photoelectrodes for Use in Solar Water Oxidation Jan Philipp Kollender,a,b Andrei Ionut Mardare,a,b and Achim Walter Hassela,b,∗,z a Institute for Chemical Technologies of Inorganic Materials, Johannes Kepler University Linz, b Christian Doppler Laboratory for Combinatorial Oxide Chemisty (COMBOX) at the Institute

4040 Linz, Austria for Chemical

Technologies of Inorganic Materials, Johannes Kepler University Linz, 4040 Linz, Austria High throughput screening of photoelectrochemical activity was performed on a WO3 -CoO thin film combinatorial library using photoelectrochemical scanning droplet cell microscopy. The compositional spread was deposited using co-evaporation of WO3 and metallic Co followed by thermal oxidation in pure oxygen. The elemental compositional mapping along the library via EDX revealed a total compositional gradient of 29 at%. The microstructure and crystallographic properties were investigated via SEM and XRD. Small cubic grains of WO3 were identified on the surface for Co contents up to 13 at% while CoO remained amorphous along the entire library in mixture with both cubic and orthorhombic WO3 . A remarkable peak of photoactivity was identified in a compositional range of 7 to 15 at% Co with current density values in excess of 110 μA cm−2 for an applied potential of 1 V vs. SHE. Surface XPS investigations were used for correlating the bulk and surface composition of the library. In the compositional region corresponding to the maximum photoactivity peak, a constant W/Co atomic ratio was measured on the surface while the corresponding bulk ratio was increasing. This effect could be the result of a mixed contribution from an enhanced electrocatalytic activity for oxygen evolution on the surface and increased bulk radiation absorption. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. [DOI: 10.1149/2.0711503jes] All rights reserved. Manuscript submitted May 14, 2014; revised manuscript received December 23, 2014. Published January 13, 2015. This was Paper 1041 presented at the Orlando, Florida, Meeting of the Society, May 11–15, 2014.

Since more than 100 years fossil fuels are satisfying the vast majority of the world’s energy needs. Their current level of consumption is leading to serious environmental problems like inevitable emission of CO2 into the atmosphere. The ever increasing level of CO2 concentration in the atmosphere is leading to unfavorable changes in Earth’s climate and other environmental concerns. For decrease of consumption of fossil fuels, more and more interest is directed toward renewable energy sources.1,2 Photoelectrochemical energy conversion is an attractive approach for conversion of solar energy into chemical energy.3 It offers potentially a cost-effective way for the production of fuels and energy storage, based on renewable resources.4,5 Photoelectrochemical energy conversion based on semiconducting transition metal oxides6 like TiO2 ,7–9 Fe2 O3 10–13 and WO3 14–16 is becoming more and more important due to their high electrochemical stability, low cost and natural abundance. Unfortunately, the requirements imposed by the process of photo-induced water splitting are rather stringent. A useable material must be stable under harsh electrochemical conditions, have adequate positioning of the band edges and an appropriate bandgap, catalytic activity for the oxygen or hydrogen evolution reaction, sufficiently high charge carrier mobility and sufficient diffusion length.17 Given all these requirements, it is extremely unlikely that a single material will be used for practical applications. Most probably, complex materials containing various different elements will be used. Even for ternary or quaternary alloys, a large number of materials must be investigated to find an optimal composition. Therefore, combinatorial high-throughput methods provide an efficient way for discovery and development of new photoactive materials.18–21 Their efficiency in finding specific material compositions with enhanced properties was proven for a very large number of materials and screened properties in the last 20 years.22 Photoinduced hydrogen production was already proven in iridium complexes using parallel high throughput screening23 and thin film combinatorial libraries are already used for more than 10 years for energy applications, e.g. development of improved fuel cell anode catalysts.24 WO3 is a semiconductor with an indirect bandgap which can exist in a variety of crystallographic symmetries. It can be synthesized by various methods such as sol-gel synthesis,25 chemical vapor deposition,26 ∗ z

Electrochemical Society Active Member. E-mail: [email protected]

sputtering,27 or electrodeposition.28 Pure WO3 has poor oxygen evolution efficiency during photoelectrochemical water splitting, and rather large overpotentials are required for oxygen evolution.29 Upon doping or by adding oxygen evolution catalysts, the photocatalytic activitiy of WO3 can be drastically increased.30–33 In the present study, a WO3 -CoO thin film material library was prepared by thermal co-evaporation from two complementing sources. After heat-treatment for full oxide conversion, composition of the obtained material library was determined by scanning EDX. The photocatalytic properties were mapped along the compositional gradient with an elemental resolution of better than 1.3 at% using photoelectrochemical scanning droplet cell microscopy (PE-SDCM).34 The composition dependent microstructure of the library was investigated using SEM. Changes in the crystallographic properties along the compositional gradient were analyzed by XRD and compositions of the oxides present on the surface were investigated by scanning XPS. Experimental Thin film combinatorial library fabrication and basic characterization.— In order to study the photoelectrochemical behavior of WO3 -CoO formulations, first a WO3 -Co thin film combinatorial library was deposited on borosilicate glass substrates using a self developed state of the art thermal co-evaporator containing three individual sources. Prior to the WO3 -Co deposition, the glass substrates were covered by an ITO layer (15 /, Kintec Co.) for improving the electrical conductivity necessary for photoelectrochemical measurements at a later stage. The particularities of the co-evaporator are defined by the possibility to individually adjust the deposition distance for each of the three thermal sources while having individual in-situ readings of the evaporation rates. Both features are relevant for tuning the desired compositional spread using the cosine law of evaporation combined with the thermal evaporation process. The individual evaporation rate feedback, provided by each of the three quartz crystal microbalances (QCMs – Inficon, SQM242 acquisition board) placed above each thermal source, is used for controlling the power delivered to the sources via a software PID controller (LabView). Proper shielding of the QCMs allows minimizing down to 0.01% the evaporation rate reading errors due to cross talking of the multiple evaporation sources. This error depends on the deposition distance defined for each thermal source before the deposition. Usually, a high

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Journal of The Electrochemical Society, 162 (4) H187-H193 (2015)

compositional spread is desired when depositing a thin film library, which means minimizing the deposition distances as much as possible. In such configuration, the described QCMs errors are drastically decreased under the mentioned level due to a complete absence of line-of-sight between each of the thermal sources and both QCMs active surfaces monitoring the other two sources. Electrical power is delivered to each of the thermal sources from 3.3 kW DC power sources (N8734A – Agilent Technologies) able to provide a maximum of 165 A each. This current/power is sufficient for using either W boats or W baskets holding ceramic crucibles as evaporation sources. More details about the co-evaporation chamber can be found elsewhere.35 For the co-deposition of the WO3 -Co thin film library, the base pressure of the vacuum chamber was in the range of 10−5 Pa. The bulk materials used for the co-deposition of WO3 and Co were high purity powders (99.9%, Sigma-Aldrich and ChemPur, respectively). Since the WO3 shows good stability on the surface of its own metal during its sublimation at approximately 1070 K (at 10−4 Pa), a W boat can be used as Joule heating element for evaporation of WO3 . Alternatively, a boron nitride crucible (15 mm in diameter) heated by a W filament and containing WO3 powder was used for deposition of WO3 . As expected, no differences between WO3 films obtained using different heating elements were found. An identical boron nitride crucible was also found suitable for Co metal evaporation due to the high alloying rate of molten Co with W at its evaporation temperature of approximately 1263 K (at 10−4 Pa). A negligible condensation combined with a weak molten Co creeping was observed at the cooler rim of the crucibles, not affecting in any way the evaporation process. A compositional spread containing low amounts of Co (up to 30 at%) was desired and the suitable WO3 and Co evaporation rates used were 2 and 0.045 nm s−1 , respectively. For both thermal sources, the evaporation distance was defined at 120 mm. An average WO3 -Co thin film library thickness of 600 nm was obtained, as provided by the QCMs at the middle of the compositional spread. Due to the unavoidable direct exposure of the substrates to the thermal radiation during evaporation, the WO3 -Co thin films reached temperatures as high as 370 K at the end of the co-deposition process. For obtaining the final WO3 -CoO combinatorial library, the asdeposited WO3 -Co library was heat treated in pure oxygen. This heat-treatment ensures reoxidation of the vapor-deposited WO3 which tends to condense in slightly subvalent states due to a partial decomposition during the evaporation process leading to oxygen losses.36 Additionally, this heat-treatment ensures oxidation of Co. The heattreatment process on the as-deposited WO3 -Co material library was done in pure oxygen atmosphere for 20 h at 673 K with cooling and heating rates of 2 K min−1 . Mapping of the elemental composition of the thin film material library after heat-treatment in oxygen was done using a Zeiss 1540-XB scanning electron microscope with a built-in EDX analyzer (INCA X-sight, Oxford Instruments). The EDX-detector was carefully calibrated prior to each series of measurements using a high purity Co-standard (Micro-Analysis Consultants, United Kingdom). Processing of obtained EDX-data was done using INCA-software. Spectra were recorded using an acceleration voltage of 18 kV and an acquisition time of 180 s for each spot. The starting (zero) point for each scanned line was located at the short side of each glass/ITO slide, which was closest to the WO3 evaporation source. Along the entire thin film material library, a line of equally spaced spots was automatically measured. Mapping of the surface microstructure at relevant positions along the material library was achieved using a ZEISS 1540-XB scanning electron microscope (SEM). The crystallographic properties of the heat-treated combinatorial library were locally investigated using X-ray diffraction (Philips X’Pert Pro) and were mapped along the entire compositional gradient. Elemental composition of the surface along the entire material library was investigated using a Theta Probe X-ray photoelectron spectrometer (XPS) from Thermo Fisher with Al Kα anode at energy of 1486.7 eV and a spot size of 400 μm. Photoelectrochemical scanning droplet cell microscopy.— The photoelectrochemical properties of the WO3 -CoO thin film material

library were mapped as a function of composition. All photoelectrochemical measurements were carried out using a scanning droplet cell microscope (PE-SDCM) due to its capability to locally address a small region/spot on the surface, therefore providing a very good compositional resolution.34 Strong localization combined with automatic operation in the contact mode via XYZ translation stages allow properties mapping with high reproducibility and efficient time consumption.37 The main body of the cell was made from a plastic block (Polyoxymethylen) into which three connected channels (1.0 mm in diameter) were drilled. Each channel was sealed using plastic screws (PP) with an O-ring underneath the head of each screw. A μ-Ag/AgCl system was used as a micro reference electrode (μ-RE). Details about this reference electrode can be found elsewhere.38 The measured potential of this electrode was 0.183 V vs. standard hydrogen electrode (SHE). The counter electrode (CE) inside the PE-SDCM was made from a 100 μm in diameter Au wire (99.999 %, Wielandt Dentaltechnik, Germany). The lower part of the PE-SDCM, where the electrolyte droplet is released from, was made from a second borosilicate glass capillary with an outer diameter of 2.5 mm. It was manufactured using a capillary puller (PC-10, Narishige, Tokyo, Japan). The size of the tip was adjusted to its final diameter using a self-developed capillary polishing machine equipped with 2400 and 4000 grade SiC sandpaper (Struers A/S, Denmark). After polishing, the glass capillary was thoroughly cleaned with acetone, followed by de-ionized water and blown-dry using nitrogen. A silicone sealing was formed at the rim of the capillary by dipping it into liquid silicone (RTV, Momentive, USA) and dried under constant nitrogen flow for several hours. This silicone sealing leads to a very high reproducibility of the area addressed by the PE-SDCM due to confinement of the electrolyte droplet. Next, this capillary was installed in a screw allowing an easy way of mounting it to the main plastic block. In order to illuminate the area addressed by the PE-SDCM, a 270 μm multimode optical fiber (Thorlabs, Germany) was used. The length of the fiber, RE and CE were chosen in such a way that all three, when installed inside the tip capillary, are in close proximity to the surface. This ensures homogenous illumination of the wetted area and a minimal potential drop between RE and WE. As electrolyte inlet, a custom-made stainless steel screw equipped with a barbed fitting was used. Additional sealing was provided using PTFE-tape. This screw was used as electrolyte inlet and was connected via a Tygon tube to a precision syringe pump (World Precision Instruments, USA). The pump served as an electrolyte reservoir and was additionally used to precisely control the size of the electrolyte droplet formed at the tip of the capillary. To determine the area wetted by the PE-SDCM, a sputter deposited Nb thin film was anodized at 18 V in an aqueous 0.1 M Na2 SO4 solution. The area of the colored oxide spot was determined using an optical microscope equipped with optical pattern recognition software (NIS Elements D, Nikon, Japan). The measured area was 3.89 · 10−3 cm2 . To illuminate the spot addressed by the PE-SDCM, a 405 nm laser diode (Roithner Lasertechnik, Austria) with an optical output power of 5 mW was used. The laser beam was coupled into the optical fiber using a self-developed optical fiber port. In order to measure the optical power density directly on the wetted area, an assembled PESDCM was placed on the detector surface of an optical power meter (Coherent Lasermate Q, VL54). The measured power density was 98 mW cm−2 . All electrochemical measurements were carried out in a 0.1 M NaClO4 electrolyte.39 For positioning of the PE-SDCM along the investigated surface, a gantry robot built from three linear translation stages was used. The microscope was operated in contact mode by pressing the silicone terminated tip with a predefined force of 80 mN against the sample surface. The chosen force value ensures only elastic deformation of the silicone sealing. This leads to very high reproducibility of the wetted area for all individual measurement spots addressed during scanning of the material library. The applied force was continuously measured by using a force sensor (ME-Messsysteme, Germany) and readjusted when necessary by feedback to the z-axis. The complete

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Journal of The Electrochemical Society, 162 (4) H187-H193 (2015)

Figure 1. Compositional spread along the WO3 -CoO material library.

setup was controlled by an in-house developed LabView program. All electrochemical measurements were carried out using a CompactStat Potentiostat (Ivium Technologies, The Netherlands). Transients of photocurrents were measured at 0.7 V and 1.0 V (SHE) on each spot for 45 s. Results and Discussion Compositional and microstructural analysis of the WO3 -CoO library.— The compositional spread of the WO3 -CoO thin film library was determined using EDX-analysis after the heat-treatment in oxygen. The entire material library was automatically scanned by the EDX system in the direction of the compositional gradient. An elemental specific resolution of 0.35 at% mm−1 was achieved along the

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entire material library, as proven by measuring the elemental composition at a large number of points. The measured amounts of W and Co are shown in Fig. 1. The detected amount of O is not shown due to unavoidable partial probing of the oxygen containing ITO/glass substrate. For this reason, further references to a certain WO3 -CoO composition would be provided on the basis of the W-Co metallic composition as measured by EDX. The Co content increases from one side (left) of the library to the other side (right) according to the positions of the WO3 and Co thermal sources which defined the direction of the compositional gradient. The Co concentration increases nearly linearly from 2.8 at% at the point of origin (x = 0 mm) up to 31.9 at% at x = 84 mm. The profiles of the concentrations of W and Co along the geometrical extension of the entire material library are anticorrelating. The highest W concentration of 97.2 at% is found at x = 0. When following the compositional gradient along the material library, the W concentration continuously decreased down to 68.1 at% at the other end of the library. A visual inspection of both samples under ambient natural light conditions revealed significant differences along the compositional spread. For Co concentrations below approximately 13 at% the WO3 -CoO film has a semi-transparent milky-white appearance. When the Co concentration increases further, the film rapidly becomes completely transparent. From Co concentrations of 26 at% onwards the films are suddenly becoming dark-brown. This color change is most probably caused by the increased presence of oxidized Co in the mixed oxide film and could suggest an increased absorption of visible light. The surface microstructure of the WO3 - based thin-film material library was studied by SEM for observing the effects of varying W and Co concentrations on the surface dynamics. Surface images of different selected compositions along the entire material library are shown in Fig. 2. At Co concentrations below 6.0 at% the complete surface is covered with a dense granular film with grains up to 250 nm in diameter. The mixed oxides surface containing the lowest Co amount shows a clear fusion of these grains suggesting a smooth surface with clearly

Figure 2. SEM-images of the WO3 -CoO thin film material library at different Co concentrations. Downloaded on 2015-01-15 to IP 140.78.97.2 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

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Journal of The Electrochemical Society, 162 (4) H187-H193 (2015)

observable grain boundaries and very few intergranular voids. XPS surface analysis performed on different spots at Co concentrations below 6.0 at% revealed that all investigated surface grains contain mainly W and O at compositions of about 26 at% W and 72 at% O. The detected amounts of Co were approximately 2.5 at% for all investigated spots within this compositional range. Therefore, this may suggest that the compact granular structure consists mainly of WO3 mixed with some Co. The presence of stoichiometric WO3 was further confirmed by XRD-analysis. Above 6 at% Co, the surface of the WO3 -CoO thin film library continues to change, the intergranular distances increasing with the amount of Co and reaching up to several times the grain diameter at 8.2 at% Co. This process starts exposing a surface underlayer which becomes better visible at higher Co concentrations. A surface coverage of approximately 60% could be measured for the detected WO3 grains at 8.2 at% Co. For Co concentrations between 8.2 and 13.3 at%, the surface of the material library is covered with individual grains and their surface coverage decreases to approximately 30% for 9.4 at% Co and below 5% for 13.3 at% Co. At this last composition, different WO3 grain sizes ranging between 20 and 250 nm can be easily seen. In the same time the underlying exposed surface starts exhibiting small cracks or linear voids from 9.4 at% Co upwards. The size of the WO3 grains does not seem to change when the Co concentration increases. A detailed EDX-analysis under high magnification at various different spots within this compositional range was done. All investigated structures on the surface contained only W and O with concentrations of about 25% W and 75% O, matching the stoichiometry of pure WO3 . No Co could be found anymore in the investigated grains. Therefore, it can be concluded that the Co previously detected by EDX analysis is completely located in the underlying film. The WO3 -CoO thin film combinatorial library is decorated with WO3 grains at Co concentrations below approximately 13 at%. For Co concentrations ranging between 16.6 and 19.9 at% no more single grains could be observed on the surface. The surface morphology of the film remains constant and the previously observed cracks in the film are still present. For the highest Co concentrations (above 25.9 at%Co) a dense and compact film with a slightly rough surface is observed. At this composition, no more cracks or pinholes in the film could be observed. The crystallographic properties of the WO3 -CoO thin film material library were investigated by XRD at selected positions along the compositional spread. A collection of diffractograms are presented in Fig. 3 for various Co concentrations. Additionally, the XRD pattern of the glass/ITO substrate is presented as reference for identifying possible diffraction peaks belonging to the ITO layer. For all investigated compositions along the WO3 -CoO thin film combinatorial library, both cubic and orthorhombic phases of WO3 were identified while no peaks belonging to crystalline metallic or oxidized Co could be

Figure 3. XRD spectra measured at various concentrations along the WO3-Co material library.

found. Also, no indications of mixed W-Co oxides were found. This can suggest that independent on the oxidation number, the Co present in the library retains an amorphous state. Its proven presence by EDX and surface XPS is also indirectly observable in the XRD investigations by a disturbance of the WO3 symmetry as soon as the Co amount increases. For low Co contents up to 6 at%, a clear presence of the cubic phase is evidenced at high 2θ angles. If the cubic (422) peak overlaps with one of the ITO peaks, leading to uncertainty of its provenience, both cubic (400) and (422) peaks are easily observable for low Co concentrations. Due to the strong overlapping of cubic and orthorhombic peaks, at low angles the clear identification of both phases is difficult. Up to 6 at% Co, the most intense XRD peak is likely to result from diffraction on a combination of cubic and orthorhombic WO3 structures as indicated by the fairly broad peak centered at approximately 24◦ . This low Co compositional region coincides with the region where mainly WO3 grains were identified on the library surface. As soon as the underlying surface is exposed (in the SEM images from Fig. 2) above 8 at% Co by decreasing of the WO3 grains surface cover, the main XRD peak becomes much broader and at 10 at% Co several shoulders can already be identified as belonging to orthorhombic (001) and (200) while the central peak could be either cubic (200) or orthorhombic (020). Increasing the 2θ angle, the next three peaks observable for this composition are clearly attributed to the orthorhombic phase while the high angle cubic peaks are still visible. Combined with the previous microstructural characterization, this Co amount dependent crystallographic evolution may suggest that the densely packed WO3 grains observed at the lowest Co contents have cubic symmetry while the underlying surface contains more orthorhombic WO3 . However, this hypothesis is valid only for Co amounts up to 15 at% since after this threshold, the presence of Co is amorphising the WO3 -CoO library rather than triggering a cubic to orthorhombic transition. This in turn suggests that the underlayer exposed by recessing of the cubic WO3 grains starts to amorphize due to the increasing Co amounts present in the thin film library. The proposed behavior of the WO3 -CoO crystallinity is finally supported by the fact that at the highest amount of Co in the thin film library no clear diffraction peaks belonging to something else except the ITO can be identified in Fig. 3. PE-SDCM on the WO3 -Co material library.— Photoelectrochemical mapping of the library was performed along the compositional gradient using a PE-SDCM equipped with a 405 nm laser diode. Tungsten trioxide typically shows n-type behavior and can exist in different crystallographic modifications. It has a bandgap of about 2.9 eV,40 the exact value being dependent on the individual crystallographic particularities. Due to its rather large bandgap, WO3 can only absorb parts of the solar spectrum. Additionally, WO3 cannot be used for direct photoelectrochemical water-splitting due to the improper position of the conduction band edge relative to the reduction potential of water.41 When doping WO3 with first-row transition metals like Fe, Ni or Co the photocatalytic properties of WO3 can be drastically increased.19,42 The used 405 nm radiation generated by the laser diode corresponds to an energy of 3.1 eV which exceeds the bandgap of pure WO3 therefore leading to sufficient absorption for generation of photocurrents. The previously described PE-SDCM automatically scanned the entire compositional spread. Contact mode was chosen for operation of the PE-SDCM, ensuring high reproducibility of measured photocurrents for all addressed spots along the entire compositional gradient.34 Using PE-SDCM, the spot addressed on the surface of the thin film material library during each individual measurement is so small that the lateral composition gradient within this spot (< 0.1 at%) can be neglected and the investigated spot can be treated as homogeneous. This leads to a high compositional resolution (< 1 at%). For each individual WO3 -CoO alloy addressed by the PE-SDCM, the photoinduced currents were recorded for 45 s at two different applied potentials (0.7 and 1 V vs. SHE). The WO3 -CoO compositional spread showed only photocatalytic activity for Co concentrations below 15 at%. This matches with the slightly semi-transparent appearance of the thin film material library within this compositional range that was already

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Journal of The Electrochemical Society, 162 (4) H187-H193 (2015)

Figure 4. Selected transients of photocurrents measured along the WO3 -Co material library.

previously mentioned. Also, a lack of crystallinity of the WO3 -CoO was proven by XRD investigations (Fig. 3) above this compositional threshold. For Co-concentrations above 15 at%, the measured photocurrents were very low. For these high levels of Co, the measured currents under illumination were similar to the background current level. A few selected photocurrent transients as measured under a bias of 1.0 V in the range of low Co concentrations of the material library are shown in Fig. 4. The Co concentrations for each individual measurement are indicated in the graph. For low Co concentrations (