Functional Materials, Holstenhofweg 85, Hamburg, Germany b Helmholtz Centre Geesthacht, Institute for Materials Research, Max-Planck-Straße 1,

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1 / ecst The Electrochemical Society Cold Gas Sprayed TiO 2 -based Electrodes for the Photo-induced Water Oxidation I. Herrmann-Geppert a,b, P. Bogdanoff c, H. Gutzmann a, T. Dittrich d, T. Emmler b, R. Just b, M. Schieda b, F. Gärtner a and T. Klassen a,b a Helmut-Schmidt University, University of Armed Forces, Mechanical Engineering, Functional Materials, Holstenhofweg 85, Hamburg, Germany b Helmholtz Centre Geesthacht, Institute for Materials Research, Max-Planck-Straße 1, Geesthacht, Germany c Helmholtz-Zentrum Berlin für Materialien und Energie, Institute for Solar Fuels, Hahn- Meitner-Platz 1, Berlin, Germany d Helmholtz-Zentrum Berlin für Materialien und Energie, Institute for Heterogeneous Materials, Hahn-Meitner-Platz 1, Berlin, Germany Cold gas spraying (CGS) is presented as an innovative approach to deposit semiconductor particles onto substrates in order to form photoelectrodes for electrochemical applications. Thin layers of TiO 2 (P25-20 by Evonik Industries) are deposited onto titanium substrates (TiO 2 -CGS films) at different temperatures of the gas carrier within the CGS process ( C). Structural characterization reveals unchanged bulk properties of the TiO 2 nanoparticles. Clearly, the short duration time of the CGS process hinders crystalline bulk changes of the TiO 2 particles in the hot gas stream. However, surface photovoltage measurements indicate that the CGS process modified defect states at the surface when exposed to different gas temperature. In photoelectrochemical measurements TiO 2 -CGS films yield seven times higher photocurrents and IPCE values than comparable films prepared by the well-established doctor blade technique. The increased efficiency might be due to an enhanced particle to substrate bonding caused by particles welding to the metallic substrate during the cold gas spray process. Introduction Photo-induced water splitting is seen as a clean and renewable energy resource to produce hydrogen as a fuel. Thus, photoelectrochemical research is focused on the development of devices for the solar fuel generation. In such a system, a photoactive material generates charge carriers by light absorption, which can participate in the redox reactions of water splitting via the catalytic surface. Several metal oxides are potential candidates for the photo-induced oxygen evolution reaction (OER) due to the position of the positive valence band with respect to the redox potential of OER and the suitable band gap for efficient solar light absorption. In the last decades, wet-chemical methods were developed to prepare optimized nano-sized particles. Coating techniques such as sol-gel, screen-printing and inkjet printing were performed to deposit nanopowders onto a photoelectrode. Nevertheless, stabilizing agents are required in these coating techniques to enable the formation of homogeneously distributed films 21

2 on the substrate. Therefore an additional heat treatment step is involved in which organicbased agents are removed. On the one hand, the particle to particle as well as the particle to substrate bonding is enhanced due to sintering processes. On the other hand, complete removal of the agents and their decomposition products cannot be ensured and the heat treatment bears this risk of undesired crystal phase transitions. In contrast to the wet chemical deposition, the unique technique of cold gas spraying enables the coating of substrates with photoactive particles without any heat treatment steps. The particles are added as a powder to a compressed and heated gas stream and are accelerated in a De Laval nozzle to a velocity up to 1200 m/s. This particle/gas stream is directed towards the substrate on which a thin layer of the material is formed. During this process the high kinetic energy of the particles leads to a plastic deformation of the substrate at the spot of the impact. Thereby, the kinetic energy is dominantly transferred to thermal energy so that the particles bond to the substrate by welding processes. Thus, an efficient electric contact to the substrate is formed, which is necessary for photoelectrochemical reactions (1). In our recent contribution (2), we reported about the successful probing of this technique for the preparation of TiO 2 photoelectrodes on titanium substrates. Seven times higher photocurrents are yielded compared to reference samples prepared by the established doctor blade technique. In the following contribution analysis by surfacephoto-voltage measurement is applied in order to discuss the surface modification of the TiO 2 particles in the CGS process and its effect on the catalytic activity in the photoinduced water reaction. Cold gas spraying (CGS) Experimental methods The preparation of the films is described in detail in Ref. (2). In contrast to other thermal spray processes, in cold gas spraying, the particles are not molten and impact in the solid state on the substrate surface (3). The impact velocity is in the range between 200 and 1200 m/s, depending on spray conditions. The impact velocity depends on the nozzle type, particle shape, size, density, carrier gas type and its pressure and temperature. The bonding between particles and substrate depends on the impact velocity, the material properties, particle size and temperature. For our experiments, titanium substrates are employed. The powder used in this study is the VP Aeroperl P25-20 (TiO 2 ) from Evonik Industries. Therefore, P25 particles with an original particle size of ca 25 nm have been agglomerated to 5 20 µm sized spheres by spray drying. The P25-20 has a fraction of approximately 10 % rutile and 90 % anatase, as evaluated by XRD measurements (not shown here). This TiO 2 powder is chosen as feedstock material due to its suitable particle size and flowability needed for cold gas spraying. The TiO 2 is sprayed with nitrogen as carrier gas (4 MPa gas pressure) at a spray distance of 60 mm. Samples prepared at varied gas temperatures ( C) are considered for this study. Electrochemical characterization The photoelectrochemical characterization of the samples has been performed in a typical three-electrode configuration using a Pt plate counter electrode, an Ag/AgCl 22

3 reference electrode (0.2 V(NHE)), and 0.5 M H 2 SO 4 electrolyte. The area of the working electrode is 0.28 cm 2. Incident-photon-to-current efficiency (IPCE) has been measured using a 300 W xenon light source and a monochromator (set-up is described in reference (4)). Cyclic voltammetry has been conducted at a scan rate of 20 mvs 1. The samples, connected as working electrode, have been illuminated under AM 1.5G light (100 mwcm -2 ) provided by a 150 W xenon lamp (with AM 1.5G filter) solar simulator. Structural characterization UV/Vis spectroscopy (UV/Vis) UV/Vis measurements have been performed in a spectrometer by a Lambda 950 spectrometer by Perkin-Ellmer equipped with an integrated sphere in order to measure reflection and absorbance. Spectral dependent surface photovoltage (SPV) measurements SPV measurements have been performed in the fixed capacitor arrangement at modulation frequency of 8 Hz. The measurements have been carried out at 0.02 mbar. A halogen lamp with a quartz prism monochromator has been used for illumination. SPV signals have been detected with a high impedance buffer (Elektronik Manufaktur Mahlsdorf, RC time constant larger than 1 s) and a double phase lock-in amplifier. The phase of the lock-in amplifier has been adjusted with a Si photodiode, which response time is much faster than the measurement period. The SPV amplitude results from the square root of the sum of the squared in-phase and by 90 phase-shifted SPV signals. The tangent of the so-called phase angle is the ratio between the 90 phase-shifted and in-phase SPV signals. A phase angle of 0 corresponds to very fast charge separation and relaxation in comparison to the modulation period while the electrons are separated preferentially towards the internal surface. A change of the phase angle (values < 90 ) corresponds to a change of the retardation of charge separation and relaxation. Results and Discussion Figure 1 shows the photoelectrochemical characteristics of cold gas sprayed TiO 2 samples at different gas temperatures (300 and 800 C) determined by CV measurements under dark and AM1.5 illumination. The cold gas sprayed P25-20 TiO 2 film deposited at 800 C onto a titanium substrate yields a seven times higher photocurrent at 1.23 V(NHE) than a comparable doctor blade reference sample, also deposited on a titanium substrate. An enhancement in photocurrent is observed when increasing the temperature of the gas, from 300 C to 800 C as shown in figure 1. 23

4 Current density [ma/cm 2 ] 0,40 0,35 0,30 0,25 0,20 0,15 0,10 0,05 0,00 CGS at T gas = 800 C under illumination CGS at T gas = 300 C under illumination (doctor blade sample) dark 0,0 0,5 1,0 1,5 Potential E(NHE) [V] Figure 1. CV measurements of TiO 2 cold gas sprayed samples under AM1.5G at 1.23 V(NHE) in 0.5 M H 2 SO 4. (solid line under illumination, dotted line dark, blue T gas = 800 C, green T gas = 300 C, red doctor blade sample In order to compare all prepared samples in figure 2, the photocurrent density obtained at 1.23 V(NHE) is plotted against the applied gas temperature. The values are corrected by the corresponding dark currents. photocurrent density at 1.23V(NHE) [ma/cm 2 ] 0,8 0,6 0,4 0,2 doctor blade reference 0, T gas gas temperature [ C] Figure 2. Comparison of TiO 2 deposited by CGS (blue) and doctor blade (red) photocurrent density (corrected) versus gas temperature under AM1.5G at 1.23 V(NHE) in 0.5 M H 2 SO 4 In general, it becomes apparent that at all temperatures higher photocurrents are yielded for the CGS samples compared to the doctor blade reference sample. The latter one has been prepared by heating the TiO 2 layer at 450 C for 30 min. However, we have to underline that a direct comparison of the preparation temperature between doctor blade and CGS technique is not possible because the TiO 2 particles are exposed to the hot gas of the CGS process only on the ms-time scale. 24

5 Furthermore, it is observed from figure 2 that the photocurrent is continuously enhanced with increasing temperature of the CGS carrier gas. In contrast to that the onset potentials of the photocurrent, which is found at about 0.05 to 0.1 V(NHE) (exemplary in figure 1 for 300 C and 800 C) are nearly unchanged within the range of the applied gas temperature. We assume that due to accumulation of charge carriers the band bending is relatively small under AM 1.5G illumination so that the onset potential is representative for the flat band potential of these samples. Indeed the values are in good accordance to the Fermi level of anatase at ca. 0 V(NHE). From the constant onset potentials (or Fermi-level positions) of the photocurrents it is concluded that no significant phase transitions occur during the cold gas spray process at temperatures up to 1000 C. This is in accordance to the structural characterization by XRD which also reveals unchanged bulk properties compared to the precursor powders (already published in reference (2)). In UV-Vis measurements (see figure 3), the CGS-TiO 2 films show a well-defined absorption edge at about 400 nm in accordance to the band gap of P25 particles. No significant shift of the absorption edge is observed with increasing gas temperature, which reveals no phase transition of the sprayed material compared to the TiO 2 precursor powder. Nevertheless, small variations of the absorption edge are observed. They might be reasoned in the different roughness of the samples, which leads to different reflectance behavior (reported in reference (2)). 1,5 Absorbance [a.u.] 1,0 800 C 600 C 400 C 0, Wavelength [nm] Figure 3. UV/Vis spectroscopy of TiO 2 cold gas sprayed samples Finally, it can be concluded that the bulk of the TiO 2 particles is unaffected by the CGS process. Although anatase powder is completely transformed to rutile at temperature of 1000 C in the sintering process (5), the anatase phase of the sprayed TiO 2 particles can be retained during deposition by CGS. This is reasoned due to the short duration time of the particles in the hot gas stream. This fact presents one of the unique advantages of 25

6 the cold gas spraying for the preparation of photoelectrodes in contrast to established procedures, which are based on sintering treatments like the doctor blade technique. However, we have to admit that the particle surface seems to be modified as it is discussed later by surface-voltage measurements. Anyway, the higher current densities of the CGS-samples compared to the doctor blade reference sample seem to be due to a better coupling of the particles to the metallic titanium substrate. During the impact process, the particles have a high kinetic energy, so that the agglomerates are allowed to penetrate the always existing thin titanium dioxide layer of the titanium substrate. In contrast to the coating via the doctor blade technique, thereby the particles are directly bonded to the conducting metallic substrate so that less electron transport barriers are provided. Incident-photon-to-current efficiency (IPCE) measurements of the cold gas sprayed TiO 2 films (prepared with gas temperatures from 300 to 800 C) reveal IPCE values between % in the wavelength range from 370 to 270 nm. In contrast to that, the doctor blade sample only reaches max 2 % at 325 nm. This is in agreement with the photocurrent measurements under AM 1.5 of the CGS samples and the doctor blade reference (fig. 1) and could support the assumption of a better contact to the substrate. 20 IPCE [%] C 900 C 800 C 700 C 600 C 500 C 400 C 300 C 5 doctor blade P25/20 TiO 2 on Ti Wavelength (nm) Figure 4. IPCE measurements on TiO 2 cold gas sprayed samples under AM1.5G at 1.23 V(NHE) in 0.5 M H 2 SO 4. 26

7 20 15 IPCE at 300nm [%] Gas temperature T gas [ C] Figure 5. IPCE at 300 nm of the TiO 2 cold gas sprayed samples under AM1.5G at 1.23 V(NHE) in 0.5 M H 2 SO 4. (The dashed line is just to guide the eye) Interestingly, CGS samples prepared with gas temperatures higher than 900 C show a significant increase of the IPCE up to 19 % at 1000 C (see figure 5). At the first view, this seems to be in contradiction to the photocurrent measurements shown in figure 2, where no extraordinary enhancement of the current is observed. This could be explained by the different light power density used for the CV and the IPCE measurements. While the CV measurements are peformed under illumination with 100 mwcm -2, lower light intensity of 100 µwcm -2 is used in the IPCE measurements. It is known that recombination processes are enforced at high light power density due to the high concentration of generated charge carriers. Accumulation of these charge carriers in the space charge region leads to a back-bending of the bands and thereby to a worse charge carrier separation and a higher recombination rate. This effect seems to be pronounced for the cold gas sprayed samples prepared at 900 and 1000 C. Apparently, the surfaces of these deposited TiO 2 particles provide surface states in the band, which can act as recombination centers under illumination with high light power density. For more detailed information about the charge separation and recombination processes, surface photovoltage measurements (SPV) were performed varying the incident light wavelength. In figure 6 the SPV signals of samples which have been prepared at 300, 400 and 500 C gas temperature are shown. Modulated SPV signals depend on the separation of photo-generated charge carriers and on their relaxation, i.e. processes which are much slower than the period of the modulated light are not detected. However, one should take into account that electronic states with long trapping and/or detrapping times are occupied by photo-generated charge carriers during modulated SPV measurements. Furthermore, in-phase signals correspond to the part of the SPV signals, which follows directly the modulated light whereas the phase-shifted by 90 signal can be related to processes, which are retarded in relation to switching the light on and off. Therefore, some conclusions can be drawn about the kinetics of charge separation and relaxation in relation to excitation from defect states and to excitation by fundamental 27

8 absorption with photon energies above the band gap. Thereby, the in-phase values in figure 6 are attributed to fast processes in comparison to the modulation period (8 Hz) whereas the out-phase data correspond to slower separation and relaxation kinetics. In general, the SPV spectra in figure 6 can be divided into two different domains. At light energies higher than 3.1 ev a significant out-phase SPV signal is detected that is attributed to the band-band excitation of anatase. As the SPV signal is only rarely observed in the in-phase time domain, we conclude that the charge separation and relaxation processes associated to the band-band exitation are comparatively slow C 400 C 500 C d mod = 8 Hz filled symbols: in-phase open symbols: phase-shifted by 90 Photovoltage (µv) ,0 1,5 2,0 2,5 3,0 3,5 Photon energy (ev) Figure 6. Surface photovoltage spectra of the in-phase (filled symbols) and phaseshifted by 90 (open symbols) signals of P25-20 TiO 2 cold gas sprayed samples prepared at different gas temperatures (turquoise circles 300 C, blue triangles 400 C, olivecolored squares 500 C) In addition in-phase SPV signals were observed at photon energies lower than 3.1 ev, which reveal the presence of electronic states within the band gap. From the two maxima at 2 ev and 2.6 ev, the position of at least two states can be roughly estimated to 1.1 ev and 0.5 ev above the valence band edge of TiO 2. As these SPV signals are mainly detected as an in-phase signal, we conclude that the corresponding charge carrier separation and recombination processes are comparatively fast. The band-band signal as well as the interband state signals shows a dependency on the temperature of the applied carrier gas during the CGS-deposition process. In figure 7, both the in-phase and the out-phase SPV signals at 3.1 ev (less the signal at 3.0 ev as background) and at 2 ev are plotted versus the gas temperature for all investigated samples. As already described, we obtained no hints for a significant change of the crystalline bulk properties. Hence, we have to assume that the observed changes of the SPV signals with increasing gas temperature are due to surface modifications. As a first simple interpretation, we argue that surface states within the band gap are formed during the CGS process, which serve as deep traps for photogenerated holes. Because the kinetics of the SPV signal, which is assigned to the band to-band excitation (3.1 ev) is comparatively slow, also the electrons are assumed to be trapped most 28

9 probably in interband states of the crystalline bulk. These trapping processes lead to a slow recombination rate and thereby to a slow kinetic of the SPV signal. Interestingly, significant faster kinetics is observed for excitations from the surface states. Apparently, the lower energy gap between the surface state and the conduction band (2 ev) compared to the band-band gap (3.1eV) accelerates the recombination rate. Thereby, electrons which are generated at the surface by illumination of surface states can recombine to some extent faster than they are separated by the electric field of the space charge region. 6 S(3.4 ev) - S(3.0 ev) S(2.0 ev) 3 PV signal (µv) filled symbols: in-phase open symbols: phase-shifted by Temperature ( C) Figure 7. In-phase and the out-phase SPV signals taken at 3.1 ev (less the signal at 3.0 ev as background) and at 2 ev are plotted versus the CGS-gas temperature Anyway, the results from the SPV measurements reveal interband states, which exist at the surface. Their properties vary with the applied gas temperature in the cold gas spray process. Figure 7 reveals significant changes of the SPV spectra (in-phase, blue) in the temperature from 300 to 600 C. It is assumed that the surface modification obtained in this temperature range is accompanied with the formation and decomposition of OH groups of the surface. Similar conclusions are drawn from experiments of heat treated P25 particles. In FT-IR measurements, the authors observed the increase of the signal at 1650 and 3400 cm -1 for heat treated P25 particles at 400 and 500 C, which is attributed to the formation of surface adsorbed hydroxyl groups. At higher temperatures, however, these signals are diminished again (5). Noticeably, pronounced increase of the SPV signals of band-to-band excitations is observed for the CGS samples prepared at 900 and 1000 C. Apparently, the surface is drastically changed so that enhanced charge carrier separation and charge carrier kinetics of the processes is obtained. Interestingly, an increase of the IPCE efficiencies is also observed for the samples prepared with T gas = 900 and 1000 C. Nevertheless, the results from the SPV and IPCE measurements cannot be directly correlated. In the IPCE measurements, the interface is in contact with the aqueous electrolyte in contrast to the ex-situ conditions in the SPV measurements. Furthermore, charge carriers (holes) are transferred across the interface in the electrochemical measurements, which are no longer available for recombination processes as in SPV measurements. However, the correlation 29

10 between the increase of the IPCE and the SPV signals of the band-to-band excitation at higher temperatures could be a hint that an improved charge carrier dynamic is responsible for the improved photocurrents. Charge carriers produced by the excitation of surface states might not be sufficient to maintain current density in the ma-range. Nonetheless, they can act as recombination states and therefore influence the overall obtained photovoltage and photocurrent. Summarized, the CGS process does not affect the bulk properties of the TiO 2 particles but results in the generation of surface states. The consequence for the resulting higher photocurrent and the IPCE is not fully understood. Additionally, the enhanced coupling of the TiO 2 particles to the titanium substrate seems to make a significant contribution to the improved photoelectrochemical properties compared to the doctor blade reference. Conductivity measurements and surface analysis by XPS are therefore in preparation. Conclusion Cold gas spraying (CGS) was successfully probed to couple agglomerated P25 particles on titanium substrates and presents an alternative technique to transfer semiconductor powders to photoelectrodes. The photoelectrochemical activity for the photo-induced water oxidation of the CGS TiO 2 films is meaningful higher than for doctor blade references. Although no changes of the crystalline bulk characteristics could be observed, it was found that the surface of the particles is modified by the CGS process in dependence of the applied temperature of the carrier gas. Surface photovoltage measurements reveal the presence of surface states within the band gap which influences the photogenerated charge carrier dynamics. Acknowledgments The authors would like to thank Dr. A. Ramírez Caro (HZB) for careful reading of the manuscript. We are also thankful for funding by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) under grant No. KL 1945/1-2. References 1. T. Klassen and F. Gärtner, Materialwissenschaft und Werkstofftechnik / Materials Science and Engineering Technology, 41, p (2010), 2. T. Emmler and I. Herrmann-Geppert, Proc. SPIE 8822, Solar Hydrogen and Nanotechnology VIII, 88220C (2013) 3. H. Assadi and F. Gärtner, Acta Materialia, 51, p (2003) 4. R. van de Krol and M. Grätzel, Photoelectrochemical Hydrogen Production, Springer US, New York page 121 (2012) 5. Wang and Xu, International Journal of Photoenergy, Volume 2012, Article ID , 9 pages, doi: /2012/