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Sputtering Deposition of Ultra-thin α-Fe2O3 Films for Solar Water Splitting
Journal of Materials Science & Technology xxx (2015) 1e5
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Journal of Materials Science & Technology journal homepage: www.jmst.org
Sputtering Deposition of Ultra-thin a-Fe2O3 Films for Solar Water Splitting Lichao Jia*, Karsten Harbauer, Peter Bogdanoff, Kluas Ellmer, Sebastian Fiechter Helmholtz-Zentrum Berlin für Materialien und Energie, Institute for Solar Fuels, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
a r t i c l e i n f o Article history: Received 20 August 2014 Received in revised form 29 September 2014 Accepted 9 October 2014 Available online xxx Key words: Thin film Aematite Sputtering deposition Photoelectrochemical water oxidation
Ultra-thin a-Fe2O3 (hematite) films have been deposited by radio frequency (RF) sputtering technique and photoelectrochemically investigated towards their ability to oxidize water. By varying the deposition power and time as well as the sputter gas flow (argon), the microstructure and morphology of the film were optimized. It was found that the increment in the film thickness resulted in the loss of efficiency for solar water oxidation. The film with a thickness of 27 nm exhibited the best result with a maximum photocurrent of 0.25 mA cm2 at 1.23 VRHE. Addition of small amounts of O2 to the sputter gas improved the photoactivity significantly. Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
1. Introduction Photoelectrochemical (PEC) water splitting is a promising method of transforming solar energy into chemical energy stored in the form of hydrogen in an environmentally benign manner. Considerable progress is currently being made in the development of suitable semiconductor materials as photoanodes for this solar driven fuel generation[1e12]. Among different metal oxide materials, a-Fe2O3 has attracted particular interest due to its relatively small bandgap (2.1 eV), its chemical stability in aqueous alkaline solution, low toxicity, abundance in the earth's crust and scalability as thin film at low cost. Theoretical calculations suggest that this semiconductor can obtain a maximum efficiency as PEC device of 12.9%[13], however the reported water splitting efficiency for aFe2O3 is presently still far below this value[14e16]. This observation can be explained by a small optical absorption coefficiency of the oxide[17], a rapid recombination and short carrier diffusion length (2e4 nm)[18] of excited electrons (e) e holes (hþ), and slow surface reaction kinetics at the anode/electrolyte interface[19]. Much effort has been focused on studying and improving the performance of hematite photoanodes[14,20e24]. All of the results indicate that the realization a-Fe2O3 anodes consisting of nanostructures, such as nanoparticles, nanorods, nanowires, nanotubes,
* Corresponding author. Ph.D.; Tel.: þ49 30 806242442 E-mail address: [email protected] (L. Jia).
and mesoporous nanoflowers, is a route to enhance the conversion efficiency. On the one hand, those nanostructures with sizes comparable to the hole-diffusion length can essentially reduce the electronehole recombination in PEC anodes. On the other hand, nanostructured materials can reduce the carrier-scattering rate due to their large surface areas, which are favorable as well for photooxidation of water. Furthermore, it has been found that the microstructural, optical and surface properties of hematite photoanodes have a great influence on the PEC performance. Accordingly, the search for improved anodes with higher PEC activity inevitably involves fabrication and characterization of a large number of samples via different deposition methods. Synthetic techniques used so far include hydrothermal methods, spray pyrolysis, electrochemical deposition, atmospheric pressure chemical vapor deposition (APCVD), colloidal solution assembly, anodization, flame oxidation of iron foils, and atomic layer deposition (ALD), while their specific development for controlling all relevant optoelectronic properties has not been trivial and been time consuming[14,21e27]. Thus, seeking a scalable high-throughput fabrication method for high efficient electrode films is still in great need. In present research, we employ reactive magnetron sputtering (RMS) technique for the deposition of ultra-thin hematite films. This is a unique and versatile approach to study the processing parameters and microstructure-functionality relationship for thin film electrodes. For instance, when changing only one of the processing parameters while fixing the others, the films with
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Please cite this article in press as: L. Jia, et al., Journal of Materials Science & Technology (2015), http://dx.doi.org/10.1016/j.jmst.2014.10.007
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controlled thickness can be obtained, which substantially reduces the experimental cycles and thus saves time and resources. Here, we focus upon the photo-oxidation properties of RMS-derived hematite photoanodes as a function of thickness and microstructure. The key processing parameters that influence the film microstructures and the PEC properties, are identified and discussed.
mask that defines the illuminated electrode area. The potential of the hematite photoanodes was swept at a scan rate of 10 mV s1 from cathodic to anodic potentials. The potential was converted to the reversible hydrogen electrode (RHE) potential.
2. Experimental
Fig. 1(a) shows a set of current potential curves (reported with respect to the reversible hydrogen electrode (RHE), E (RHE) ¼ 0.36 V þ E (Hg/HgO) þ 0.059 pH) recorded on different thin films. It can be clearly seen that the dark response (dashed line) is negligible up to about 1.86 V vs RHE, after which direct oxidation of water sets in due to the formation of an inversion layer. As a general trend, the sputtered hematite photoanodes under illumination are characterized by an onset potential of the photocurrent in the range from 1.0 to 1.1 VRHE, which continuously increases in all films with increasing voltage before the onset of the dark current above 1.8 V occurs. However, the films deposited under different sputtering power conditions represent markedly different photoactivites. The variations can clearly be seen from Fig. 1(b), where the photocurrent density at 1.23 VRHE is plotted against the sputtering deposition power. A typical a-Fe2O3 film deposited at 50 W shows a photocurrent density of 0.13 mA cm2, which is around 3 times higher than those of films deposited at 25, 75 and 100 W. Additionally, the onset potential from a-Fe2O3 under 50 W exhibits a negative shift of around 100 mV of the photocurrent onset, which means that the oxidation kinetics at the electrode/electrolyte interface was improved and the injection barrier for minority carriers was decreased. This photoelectrochemical characterization shows the influence of the sputtering power on the hematite photoanode activity. To understand this dependence, a detailed structural and chemical characterization of the films was performed. Top views of fieldemission scanning electron microscopy (FE-SEM) are shown in Fig. 2, which illustrate that the hematite films deposited on top of the SnO2 particles have nanostructured nature. The deposited film is composed of elongated elliptic hematite particles with edge length ranging from 100 to 500 nm grown on the top of facetted FTO grains. While in the Fig. 2(a) and (b), all hematite particles are well separated from each other, and a pronounced oriented growth of the ellipsoids can be observed, which is parallel to the edges of the SnO2 crystallites. The hematite particles in Fig. 2(c) are randomly intergrown with each other forming a flat film. In Fig. 2(d), the deposited film has a higher roughness evoked by statistically intergrown particles, and the sizes of which are comparable to those shown in Fig. 2(b). Increasing the sputtering power from 25 to 50 W (see Fig. 2(a) and (b)), the average size of the
2.1. Hematite thin film preparation
a-Fe2O3 films were deposited from a pure hematite target on fluorine-doped SnO2 (FTO) substrates by radio frequency (RF) magnetron sputtering at room temperature. Before deposition, all substrates were cleaned by sonication in acetone, ethanol, and finally water and dried in an N2 gas flow afterwards. The deposition of a-Fe2O3 was performed in a homemade sputtering and evaporation system (basic pressure 4 106 Pa). The Fe2O3 sputter target (7.62 cm diameter, 99.95% purity) was pre-sputtered for 5 min before reactive sputtering onto FTO substrate was started. The distance between the sputter target and the substrate was 66 mm for all depositions. The sputtering power was varied from 25 to 100 W. The sputtering pressure was fixed at 0.5 Pa. The purity of the Ar gas was 99.99%. After deposition, the samples were annealed in air at 800 C for 10 min. 2.2. Film characterization The morphology of Fe2O3 thin films was characterized by using a high-resolution scanning electron microscope (LEO 1530 from Zeiss). The crystal structure was determined by an X-ray diffractometry using a D8-Advanced Bruker diffractometer with a CuKa Xray source in Bragg-Brentano geometry. Optical behavior was determined via a UVeVis spectrometer (Lambda 950, PerkineElmer) with a 150 mm InGaAs integrating sphere and spectrolon as standard for 100% reflectance. 2.3. Photoelectrochemical PEC testing Photocurrent measurements were performed to estimate the solar photocurrent of the photoanodes in a three-electrode compartment (EG&G, 273A). 1 mol/L KOH aqueous solution (pH 13.6), a Pt wire, and an Hg/HgO electrode were employed as electrolyte, counter electrode, and reference electrode, respectively. Intensity of the incident light was 40 mW cm2 using a tungsten halogen lamp from Xenophot. All measurements were performed illuminating the hematite/electrolyte interface through a 0.32 cm2
3. Results and Discussion
Fig. 1. (a) Current densities vs voltage curves of hematite photoanodes deposited at different sputtering deposition powers and measured in the dark (dashed lines) and under illumination (solid lines) in a 1 mol/L KOH electrolyte; (b) photocurrent densities at 1.23 VRHE as a function of deposition power.
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Fig. 2. Top view FESEM images of a-Fe2O3 films at different sputtering powers: (a) 25 W, (b) 50 W, (c) 75 W, (d) 100 W.
particles increases due to an oriented intergrowth of hematite with SnO2, while higher sputtering power of 50 W leads to a different nucleation and particle growth regime[28,29]. The improved PEC properties can be attributed to the larger, but still separated hematite particles that grow orientally along SnO2 grain edges. Higher sputtering powers lead to a change in the nucleation behavior and particle growth. At a sputtering power of 75 W, the nucleation probability increases, and a higher number of smaller individual particles are thus obtained, forming a smooth hematite film. At 100 W sputtering power the grain size increases further, but in both cases the particles intergrow randomly[28,29]. Hence, the PEC performance of these films is reduced due to the formation of an increased number of grain boundaries of randomly oriented and intergrown hematite crystallites, which presumably act as
Fig. 3. XRD patterns of samples deposited at different sputtering powers. The red bars indicate position and intensity of the standard powder diffraction pattern of hematite (JCPDS 89-0597).
recombination centers for excited electrons and holes. The grain boundaries would limit the photocurrent flow in the randomly distributed hematite particles, which are known for their anisotropic conductivity behavior, or due to increased charge transfer barriers at the anode/electrolyte interface. Fig. 3 shows X-ray diffraction (XRD) patterns of the hematite films deposited at different sputtering powers. No diffraction peaks could be observed after sputtering a layer at 25 W. When the sputtering power is increased from 25 to 50, 75 and 100 W, the XRD patterns clearly exhibit (110), (104), and (300) peaks of hematite but with decreasing full width half maxima, which indicates the increase of particle size. Further, when normalizing the peak intensity of the (110) plane to that of the (104) plane of hematite, the film deposited at 50 W shows a much higher value than those of hematite films grown at sputtering power of 75 and 100 W, which indicates a higher grain orientation in the film sputtered at 50 W. It is well known that hematite possesses a strong anisotropic conductivity, which is up to four orders of magnitude higher within the (001) basal planes (which means in the x-y planes of the hexagonal crystallites) than orthogonal to it[14]. Therefore, the observed [110] texture is expected to facilitate the charge carrier transport to back and front surface of the photoanode, improving the photooxidation properties. Varying the deposition time (at 50 W) from 10 to 90 min leads to the formation of the films with different thicknesses. Fig. 4 shows the currentepotential curves of these film anodes in the dark and under illumination. Since the deposition speed amounts to 1.35 nm min1 (checked by Dektak surface profiler), the thickness of each film can be calculated as 13.5, 27, 40.5, 81 and 121.5 nm for films deposited for 10, 20, 30, 60 and 90 min, respectively. As one can see from Fig. 4(a), thinner films show the better performance than thicker ones. The film with a thickness of 27 nm exhibits the best result with a maximum current density of 0.25 mA cm2. When the film thickness is further increased, the current density
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L. Jia et al. / Journal of Materials Science & Technology xxx (2015) 1e5
Fig. 4. (a) Current densities vs voltage curves of hematite photoanodes deposited for different times (deposition power 50 W) in the dark (dashed lines) and under illumination (solid lines); (b) photocurrent densities measured at 1.23 VRHE as a function of deposition time.
decreases, and the hematite photoanodes gradually lose efficiency (see Fig. 4(b)). It is assumed that the space charge width extends throughout the entire thickness of the films. In this case, the minority carrier diffusion length no longer limits the collection of the carriers, and charge carriers generated in the bulk can fully contribute to the photocurrent. Thinner films show the better PEC performance because the hematite grains extend from the surface of the film to the FTO/hematite interface. Once the a-Fe2O3 film thickness increases from 40 to 120 nm, while the hematite particle size essentially remains unchanged, charge carrier recombination becomes more and more probable due to an increasing number of grain boundaries of the film and the short carrier diffusion length. Additionally, the stability and reproducibility of our cells are also checked. The PEC measurement was repeated with several samples prepared under the same condition and there are no much changes in photocurrent even after three months. The efficiency of the hematite film is also likely to depend strongly on the surface states, defects and surface roughness which are thought to be a function of the hematite grain orientation at the surface. The oxygen activity during sputtering not only affects the interface trap states, but also the bulk defects (point defects) in the sample[30]. To investigate these aspects, we modified the deposition gas flow to see how this affects the PEC performance. As shown in Fig. 5, the hematite sample prepared with pure N2 gas yields a smaller photocurrent density in the investigated potential range. In contrast, when 3.5% O2 was added to the inert gas, the PEC performance of the sample shows a greatly pronounced photoactivity. It yields a photocurrent density of 0.2 mA cm2 at 1.23 VRHE, and achieves a maximum value of 1.2 mA cm2 at 1.80 VRHE (Fig. 5),
which is 2 times higher than that prepared without O2. This behavior can tentatively be explained by a lower bulk point defect density and a passivation of interface trap states, which result in an improved oxidation kinetics[31]. When we further increased the O2/ inert gas ratio during the preparation, the PEC performance of our sample did not increase, but showed a slight decrease (data not shown). These results demonstrate that an appropriate oxygen activity under magnetron sputtering conditions can obviously suppress both defect trap states in the bulk and on the surface of the sample. On the other hand, insufficient oxygen activity during growth can cause severe deviation from stoichiometry, leading to additional phases (e.g. Fe3O4); whereas excessive addition of oxygen could induces etching effects, leading to a degradation of the sputtered layer. Additionally, the PEC onset potential of a hematite film prepared in the presence of a small O2 concentration is slightly shifted to negative potentials, which could be explained by a decreased number of oxygen vacancies and surface states at the anode/electrolyte interface. To further improve PEC performance of these photoanodes, surface passivation materials or co-catalysts are thus necessary to be deposited on the electrode surface, such as RuO2, CoePi, CoOx and so on, which is still in progress. The corresponding top view SEM images are shown in Fig. 6. The hematite film deposited in an O2 containing atmosphere is characterized by the occurrence of hexagonally facetted platelets in contrast to a layer prepared without O2. Fig. 7 demonstrates the XRD pattern of these platelets, indicating the thin film is much probably amorphous due to the weak peak intensities. Since these thin hexagonal platelets are upright standing on the SnO2 grains and are separated from each other, favorable transport conditions of excited charge carriers can be expected. To further characterize the defect chemistry of the hematite particles in these films, additional structural and optoelectronic investigations are necessary. 4. Conclusion
Fig. 5. Current densities vs voltage curves of hematite photoanodes deposited with or without O2 (sputter conditions 50 W, deposition time 90 min) in the dark (dashed lines) and under illumination (solid lines).
Ultrathin a-Fe2O3 films have been deposited on FTO substrates by using magnetron sputtering. The films have been employed as photoanodes to study the structure-function relationships for lightinduced water oxidation. By varying the deposition parameters such as sputtering power, film thickness and oxygen activity, the morphology and the crystallinity of the films can be tuned. All these parameters have significant influence on the PEC performance towards the oxygen evolution reaction. By tuning the deposition powers from 25 to 100 W and the deposition time from 10 to 90 min, an optimized film with a thickness of ~27 nm (50 W, 20 min) achieves the highest photocurrent density of 0.25 mA cm2
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Fig. 6. Top view FESEM images of samples deposited without (a) and with O2 (b) added to the argon sputter gas.
Fig. 7. XRD patterns of samples deposited with and without O2. The red bars indicate position and intensity of the standard powder diffraction pattern of hematite (JCPDS 89-0597).
at 1.23 VRHE. Thinner films show better performance than thicker ones due to an improved charge carrier transport, which is caused by on the one hand the texture and orientation of the hematite grains, while on the other hand the comparable particle length with the space charge width that extends throughout the entire length of the particles. When 3.5% O2 is added to the argon sputter gas, the sample showed significantly improved photoactivity for water oxidation. The enhancement in photoactivity is due to the suppression of defect trap states in the bulk and at the surface of the anode in contact with an electrolyte. This work is demonstrating a scalable and effective method for the preparation of highly photoactive hematite thin films for PEC anodes without doping. Acknowledgement This work was supported by the German Federal Ministry of Education and Research (BMBF) under contract # 03SF0353A ‘‘H2NanoSolar’’. References
[2] R. van de Krol, Y.Q. Liang, J. Schoonman, J. Mater. Chem. 18 (2008) 2311e2320. [3] K. Maeda, K. Domen, J. Phys. Chem. Lett. 1 (2010) 2655e2661. [4] S. Han, L. Hu, Z. Liang, S. Wageh, A.A. AL-Ghamdi, Y. Chen, X. Fang, Adv. Funct. Mater. 24 (2014) 5719e5727. [5] D.A. Wheeler, G.M. Wang, Y.C. Ling, Y. Li, J.Z. Zhang, Energy Environ. Sci. 5 (2012) 6682e6702. [6] M. Roland, Adv. Funct. Mater. 24 (2014) 2421e2440. [7] A. Fujishima, K. Honda, Nature 238 (1972) 37e38. €hwald, D. Shchukin, Energy [8] H.Q. Wang, L.C. Jia, P. Bogdanoff, S. Fiechter, H. Mo Environ. Sci. 6 (2013) 799e804. [9] F.F. Abdi, L.H. Han, A.H.M. Smets, M. Zeman, B. Dam, R. van de Krol, Nat. Comm. 4 (2013) 2195. rez, J.M. Coronado, Energy Environ. [10] M.D. Hern andez-Alonso, F. Fresno, S. Sua Sci. 2 (2009) 1231e1257. [11] X. Liu, F.Y. Wang, Q. Wang, Phys. Chem. Chem. Phys. 14 (2012) 7894e7911. [12] A. Khoshakhlagh, A. Nazari, G. Khalaj, J. Mater. Sci. Technol. 28 (2012) 73e82. [13] A.B. Murphy, P.R.F. Barnes, L.K. Randeniya, I.C. Plumb, I.E. Grey, M.D. Horne, J.A. Glasscock, Int. J. Hydrogen Energy 31 (2006) 1999e2017. €tzel, J. Am. Chem. Soc. 128 (2006) 15714e15721. [14] A. Kay, I. Cesar, M. Gra [15] J.A. Glasscock, P.R.F. Barnes, I.C. Plumb, N. Savvides, J. Phys. Chem. C 111 (2007) 16477e16488. [16] L. Li, N. Koshizaki, J. Mater. Chem. 20 (2010) 2972e2978. €tzel, J. Phys. Chem. C 113 (2009) [17] I. Cesar, K. Sivula, A. Kay, R. Zboril, M. Gra 772e782. [18] K. Sivula, R. Zboril, F. Le Formal, R. Robert, A. Weidenkaff, J. Tucek, J. Frydrych, €tzel, J. Am. Chem. Soc. 132 (2010) 7436e7444. M. Gra [19] M.P. Dareedwards, J.B. Goodenough, A. Hamnett, P.R. Trevellick, J. Chem. Soc. Faraday Trans. 79 (1983) 2027e2041. [20] Y. Ling, G. Wang, D.A. Wheeler, J.Z. Zhang, Y. Li, Nano Lett. 11 (2011) 2119e2125. [21] R. Morrish, M. Rahman, J.M.D. MacElroy, C.A. Wolden, ChemSusChem 4 (2011) 474e479. [22] J.L. Liu, M. Shahid, Y.S. Ko, E. Kim, T.K. Ahn, J.H. Park, Y.U. Kwon, Phys. Chem. Chem. Phys. 15 (2013) 9775e9782. [23] X.P. Qi, G.W. She, M. Wang, L.X. Mu, W.S. Shi, Chem. Commun. 49 (2013) 5742e5744. [24] G.M. Wang, Y.C. Ling, D.A. Wheeler, K.E.N. George, K. Horsley, C. Heske, J.Z. Zhang, Y. Li, Nano Lett. 11 (2011) 3503e3509. [25] Y.Q. Liang, C.S. Enache, R. van de Krol, Int. J. Photoenergy (2008), http:// dx.doi.org/10.1155/2008/739864. [26] S.K. Mohapatra, S.E. John, S. Banerjee, M. Misra, Chem. Mater. 21 (2009) 3048e3055. [27] Y.J. Lin, Y. Xu, M.T. Mayer, Z.I. Simpson, G. McMahon, S. Zhou, D.W. Wang, J. Am. Chem. Soc. 134 (2012) 5508e5511. [28] M.T. Le, Y.U. Sohn, J.W. Lim, G.S. Choi, Mater. Trans. 51 (2010) 116e120. [29] A. Chaoumead, Y.M. Sung, D.J. Kwak, Adv. Cond. Matter Phys. (2012), http:// dx.doi.org/10.1155/2012/651587. [30] Y.C. Ling, G.M. Wang, J. Reddy, C.C. Wang, J.Z. Zhang, Y. Li, Angew. Chem. Int. Ed. 51 (2012) 4074e4079. [31] C.C. Lo, T.E. Hsieh, ECS Trans. 45 (2012) 239e243.
[1] J.W. Sun, D.K. Zhong, D.R. Gamelin, Energy Environ. Sci. 3 (2010) 1252e1261.
Please cite this article in press as: L. Jia, et al., Journal of Materials Science & Technology (2015), http://dx.doi.org/10.1016/j.jmst.2014.10.007
Contents lists available at ScienceDirect
Journal of Materials Science & Technology journal homepage: www.jmst.org
Sputtering Deposition of Ultra-thin a-Fe2O3 Films for Solar Water Splitting Lichao Jia*, Karsten Harbauer, Peter Bogdanoff, Kluas Ellmer, Sebastian Fiechter Helmholtz-Zentrum Berlin für Materialien und Energie, Institute for Solar Fuels, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
a r t i c l e i n f o Article history: Received 20 August 2014 Received in revised form 29 September 2014 Accepted 9 October 2014 Available online xxx Key words: Thin film Aematite Sputtering deposition Photoelectrochemical water oxidation
Ultra-thin a-Fe2O3 (hematite) films have been deposited by radio frequency (RF) sputtering technique and photoelectrochemically investigated towards their ability to oxidize water. By varying the deposition power and time as well as the sputter gas flow (argon), the microstructure and morphology of the film were optimized. It was found that the increment in the film thickness resulted in the loss of efficiency for solar water oxidation. The film with a thickness of 27 nm exhibited the best result with a maximum photocurrent of 0.25 mA cm2 at 1.23 VRHE. Addition of small amounts of O2 to the sputter gas improved the photoactivity significantly. Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
1. Introduction Photoelectrochemical (PEC) water splitting is a promising method of transforming solar energy into chemical energy stored in the form of hydrogen in an environmentally benign manner. Considerable progress is currently being made in the development of suitable semiconductor materials as photoanodes for this solar driven fuel generation[1e12]. Among different metal oxide materials, a-Fe2O3 has attracted particular interest due to its relatively small bandgap (2.1 eV), its chemical stability in aqueous alkaline solution, low toxicity, abundance in the earth's crust and scalability as thin film at low cost. Theoretical calculations suggest that this semiconductor can obtain a maximum efficiency as PEC device of 12.9%[13], however the reported water splitting efficiency for aFe2O3 is presently still far below this value[14e16]. This observation can be explained by a small optical absorption coefficiency of the oxide[17], a rapid recombination and short carrier diffusion length (2e4 nm)[18] of excited electrons (e) e holes (hþ), and slow surface reaction kinetics at the anode/electrolyte interface[19]. Much effort has been focused on studying and improving the performance of hematite photoanodes[14,20e24]. All of the results indicate that the realization a-Fe2O3 anodes consisting of nanostructures, such as nanoparticles, nanorods, nanowires, nanotubes,
* Corresponding author. Ph.D.; Tel.: þ49 30 806242442 E-mail address: [email protected] (L. Jia).
and mesoporous nanoflowers, is a route to enhance the conversion efficiency. On the one hand, those nanostructures with sizes comparable to the hole-diffusion length can essentially reduce the electronehole recombination in PEC anodes. On the other hand, nanostructured materials can reduce the carrier-scattering rate due to their large surface areas, which are favorable as well for photooxidation of water. Furthermore, it has been found that the microstructural, optical and surface properties of hematite photoanodes have a great influence on the PEC performance. Accordingly, the search for improved anodes with higher PEC activity inevitably involves fabrication and characterization of a large number of samples via different deposition methods. Synthetic techniques used so far include hydrothermal methods, spray pyrolysis, electrochemical deposition, atmospheric pressure chemical vapor deposition (APCVD), colloidal solution assembly, anodization, flame oxidation of iron foils, and atomic layer deposition (ALD), while their specific development for controlling all relevant optoelectronic properties has not been trivial and been time consuming[14,21e27]. Thus, seeking a scalable high-throughput fabrication method for high efficient electrode films is still in great need. In present research, we employ reactive magnetron sputtering (RMS) technique for the deposition of ultra-thin hematite films. This is a unique and versatile approach to study the processing parameters and microstructure-functionality relationship for thin film electrodes. For instance, when changing only one of the processing parameters while fixing the others, the films with
http://dx.doi.org/10.1016/j.jmst.2014.10.007 1005-0302/Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
Please cite this article in press as: L. Jia, et al., Journal of Materials Science & Technology (2015), http://dx.doi.org/10.1016/j.jmst.2014.10.007
L. Jia et al. / Journal of Materials Science & Technology xxx (2015) 1e5
2
controlled thickness can be obtained, which substantially reduces the experimental cycles and thus saves time and resources. Here, we focus upon the photo-oxidation properties of RMS-derived hematite photoanodes as a function of thickness and microstructure. The key processing parameters that influence the film microstructures and the PEC properties, are identified and discussed.
mask that defines the illuminated electrode area. The potential of the hematite photoanodes was swept at a scan rate of 10 mV s1 from cathodic to anodic potentials. The potential was converted to the reversible hydrogen electrode (RHE) potential.
2. Experimental
Fig. 1(a) shows a set of current potential curves (reported with respect to the reversible hydrogen electrode (RHE), E (RHE) ¼ 0.36 V þ E (Hg/HgO) þ 0.059 pH) recorded on different thin films. It can be clearly seen that the dark response (dashed line) is negligible up to about 1.86 V vs RHE, after which direct oxidation of water sets in due to the formation of an inversion layer. As a general trend, the sputtered hematite photoanodes under illumination are characterized by an onset potential of the photocurrent in the range from 1.0 to 1.1 VRHE, which continuously increases in all films with increasing voltage before the onset of the dark current above 1.8 V occurs. However, the films deposited under different sputtering power conditions represent markedly different photoactivites. The variations can clearly be seen from Fig. 1(b), where the photocurrent density at 1.23 VRHE is plotted against the sputtering deposition power. A typical a-Fe2O3 film deposited at 50 W shows a photocurrent density of 0.13 mA cm2, which is around 3 times higher than those of films deposited at 25, 75 and 100 W. Additionally, the onset potential from a-Fe2O3 under 50 W exhibits a negative shift of around 100 mV of the photocurrent onset, which means that the oxidation kinetics at the electrode/electrolyte interface was improved and the injection barrier for minority carriers was decreased. This photoelectrochemical characterization shows the influence of the sputtering power on the hematite photoanode activity. To understand this dependence, a detailed structural and chemical characterization of the films was performed. Top views of fieldemission scanning electron microscopy (FE-SEM) are shown in Fig. 2, which illustrate that the hematite films deposited on top of the SnO2 particles have nanostructured nature. The deposited film is composed of elongated elliptic hematite particles with edge length ranging from 100 to 500 nm grown on the top of facetted FTO grains. While in the Fig. 2(a) and (b), all hematite particles are well separated from each other, and a pronounced oriented growth of the ellipsoids can be observed, which is parallel to the edges of the SnO2 crystallites. The hematite particles in Fig. 2(c) are randomly intergrown with each other forming a flat film. In Fig. 2(d), the deposited film has a higher roughness evoked by statistically intergrown particles, and the sizes of which are comparable to those shown in Fig. 2(b). Increasing the sputtering power from 25 to 50 W (see Fig. 2(a) and (b)), the average size of the
2.1. Hematite thin film preparation
a-Fe2O3 films were deposited from a pure hematite target on fluorine-doped SnO2 (FTO) substrates by radio frequency (RF) magnetron sputtering at room temperature. Before deposition, all substrates were cleaned by sonication in acetone, ethanol, and finally water and dried in an N2 gas flow afterwards. The deposition of a-Fe2O3 was performed in a homemade sputtering and evaporation system (basic pressure 4 106 Pa). The Fe2O3 sputter target (7.62 cm diameter, 99.95% purity) was pre-sputtered for 5 min before reactive sputtering onto FTO substrate was started. The distance between the sputter target and the substrate was 66 mm for all depositions. The sputtering power was varied from 25 to 100 W. The sputtering pressure was fixed at 0.5 Pa. The purity of the Ar gas was 99.99%. After deposition, the samples were annealed in air at 800 C for 10 min. 2.2. Film characterization The morphology of Fe2O3 thin films was characterized by using a high-resolution scanning electron microscope (LEO 1530 from Zeiss). The crystal structure was determined by an X-ray diffractometry using a D8-Advanced Bruker diffractometer with a CuKa Xray source in Bragg-Brentano geometry. Optical behavior was determined via a UVeVis spectrometer (Lambda 950, PerkineElmer) with a 150 mm InGaAs integrating sphere and spectrolon as standard for 100% reflectance. 2.3. Photoelectrochemical PEC testing Photocurrent measurements were performed to estimate the solar photocurrent of the photoanodes in a three-electrode compartment (EG&G, 273A). 1 mol/L KOH aqueous solution (pH 13.6), a Pt wire, and an Hg/HgO electrode were employed as electrolyte, counter electrode, and reference electrode, respectively. Intensity of the incident light was 40 mW cm2 using a tungsten halogen lamp from Xenophot. All measurements were performed illuminating the hematite/electrolyte interface through a 0.32 cm2
3. Results and Discussion
Fig. 1. (a) Current densities vs voltage curves of hematite photoanodes deposited at different sputtering deposition powers and measured in the dark (dashed lines) and under illumination (solid lines) in a 1 mol/L KOH electrolyte; (b) photocurrent densities at 1.23 VRHE as a function of deposition power.
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Fig. 2. Top view FESEM images of a-Fe2O3 films at different sputtering powers: (a) 25 W, (b) 50 W, (c) 75 W, (d) 100 W.
particles increases due to an oriented intergrowth of hematite with SnO2, while higher sputtering power of 50 W leads to a different nucleation and particle growth regime[28,29]. The improved PEC properties can be attributed to the larger, but still separated hematite particles that grow orientally along SnO2 grain edges. Higher sputtering powers lead to a change in the nucleation behavior and particle growth. At a sputtering power of 75 W, the nucleation probability increases, and a higher number of smaller individual particles are thus obtained, forming a smooth hematite film. At 100 W sputtering power the grain size increases further, but in both cases the particles intergrow randomly[28,29]. Hence, the PEC performance of these films is reduced due to the formation of an increased number of grain boundaries of randomly oriented and intergrown hematite crystallites, which presumably act as
Fig. 3. XRD patterns of samples deposited at different sputtering powers. The red bars indicate position and intensity of the standard powder diffraction pattern of hematite (JCPDS 89-0597).
recombination centers for excited electrons and holes. The grain boundaries would limit the photocurrent flow in the randomly distributed hematite particles, which are known for their anisotropic conductivity behavior, or due to increased charge transfer barriers at the anode/electrolyte interface. Fig. 3 shows X-ray diffraction (XRD) patterns of the hematite films deposited at different sputtering powers. No diffraction peaks could be observed after sputtering a layer at 25 W. When the sputtering power is increased from 25 to 50, 75 and 100 W, the XRD patterns clearly exhibit (110), (104), and (300) peaks of hematite but with decreasing full width half maxima, which indicates the increase of particle size. Further, when normalizing the peak intensity of the (110) plane to that of the (104) plane of hematite, the film deposited at 50 W shows a much higher value than those of hematite films grown at sputtering power of 75 and 100 W, which indicates a higher grain orientation in the film sputtered at 50 W. It is well known that hematite possesses a strong anisotropic conductivity, which is up to four orders of magnitude higher within the (001) basal planes (which means in the x-y planes of the hexagonal crystallites) than orthogonal to it[14]. Therefore, the observed [110] texture is expected to facilitate the charge carrier transport to back and front surface of the photoanode, improving the photooxidation properties. Varying the deposition time (at 50 W) from 10 to 90 min leads to the formation of the films with different thicknesses. Fig. 4 shows the currentepotential curves of these film anodes in the dark and under illumination. Since the deposition speed amounts to 1.35 nm min1 (checked by Dektak surface profiler), the thickness of each film can be calculated as 13.5, 27, 40.5, 81 and 121.5 nm for films deposited for 10, 20, 30, 60 and 90 min, respectively. As one can see from Fig. 4(a), thinner films show the better performance than thicker ones. The film with a thickness of 27 nm exhibits the best result with a maximum current density of 0.25 mA cm2. When the film thickness is further increased, the current density
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Fig. 4. (a) Current densities vs voltage curves of hematite photoanodes deposited for different times (deposition power 50 W) in the dark (dashed lines) and under illumination (solid lines); (b) photocurrent densities measured at 1.23 VRHE as a function of deposition time.
decreases, and the hematite photoanodes gradually lose efficiency (see Fig. 4(b)). It is assumed that the space charge width extends throughout the entire thickness of the films. In this case, the minority carrier diffusion length no longer limits the collection of the carriers, and charge carriers generated in the bulk can fully contribute to the photocurrent. Thinner films show the better PEC performance because the hematite grains extend from the surface of the film to the FTO/hematite interface. Once the a-Fe2O3 film thickness increases from 40 to 120 nm, while the hematite particle size essentially remains unchanged, charge carrier recombination becomes more and more probable due to an increasing number of grain boundaries of the film and the short carrier diffusion length. Additionally, the stability and reproducibility of our cells are also checked. The PEC measurement was repeated with several samples prepared under the same condition and there are no much changes in photocurrent even after three months. The efficiency of the hematite film is also likely to depend strongly on the surface states, defects and surface roughness which are thought to be a function of the hematite grain orientation at the surface. The oxygen activity during sputtering not only affects the interface trap states, but also the bulk defects (point defects) in the sample[30]. To investigate these aspects, we modified the deposition gas flow to see how this affects the PEC performance. As shown in Fig. 5, the hematite sample prepared with pure N2 gas yields a smaller photocurrent density in the investigated potential range. In contrast, when 3.5% O2 was added to the inert gas, the PEC performance of the sample shows a greatly pronounced photoactivity. It yields a photocurrent density of 0.2 mA cm2 at 1.23 VRHE, and achieves a maximum value of 1.2 mA cm2 at 1.80 VRHE (Fig. 5),
which is 2 times higher than that prepared without O2. This behavior can tentatively be explained by a lower bulk point defect density and a passivation of interface trap states, which result in an improved oxidation kinetics[31]. When we further increased the O2/ inert gas ratio during the preparation, the PEC performance of our sample did not increase, but showed a slight decrease (data not shown). These results demonstrate that an appropriate oxygen activity under magnetron sputtering conditions can obviously suppress both defect trap states in the bulk and on the surface of the sample. On the other hand, insufficient oxygen activity during growth can cause severe deviation from stoichiometry, leading to additional phases (e.g. Fe3O4); whereas excessive addition of oxygen could induces etching effects, leading to a degradation of the sputtered layer. Additionally, the PEC onset potential of a hematite film prepared in the presence of a small O2 concentration is slightly shifted to negative potentials, which could be explained by a decreased number of oxygen vacancies and surface states at the anode/electrolyte interface. To further improve PEC performance of these photoanodes, surface passivation materials or co-catalysts are thus necessary to be deposited on the electrode surface, such as RuO2, CoePi, CoOx and so on, which is still in progress. The corresponding top view SEM images are shown in Fig. 6. The hematite film deposited in an O2 containing atmosphere is characterized by the occurrence of hexagonally facetted platelets in contrast to a layer prepared without O2. Fig. 7 demonstrates the XRD pattern of these platelets, indicating the thin film is much probably amorphous due to the weak peak intensities. Since these thin hexagonal platelets are upright standing on the SnO2 grains and are separated from each other, favorable transport conditions of excited charge carriers can be expected. To further characterize the defect chemistry of the hematite particles in these films, additional structural and optoelectronic investigations are necessary. 4. Conclusion
Fig. 5. Current densities vs voltage curves of hematite photoanodes deposited with or without O2 (sputter conditions 50 W, deposition time 90 min) in the dark (dashed lines) and under illumination (solid lines).
Ultrathin a-Fe2O3 films have been deposited on FTO substrates by using magnetron sputtering. The films have been employed as photoanodes to study the structure-function relationships for lightinduced water oxidation. By varying the deposition parameters such as sputtering power, film thickness and oxygen activity, the morphology and the crystallinity of the films can be tuned. All these parameters have significant influence on the PEC performance towards the oxygen evolution reaction. By tuning the deposition powers from 25 to 100 W and the deposition time from 10 to 90 min, an optimized film with a thickness of ~27 nm (50 W, 20 min) achieves the highest photocurrent density of 0.25 mA cm2
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Fig. 6. Top view FESEM images of samples deposited without (a) and with O2 (b) added to the argon sputter gas.
Fig. 7. XRD patterns of samples deposited with and without O2. The red bars indicate position and intensity of the standard powder diffraction pattern of hematite (JCPDS 89-0597).
at 1.23 VRHE. Thinner films show better performance than thicker ones due to an improved charge carrier transport, which is caused by on the one hand the texture and orientation of the hematite grains, while on the other hand the comparable particle length with the space charge width that extends throughout the entire length of the particles. When 3.5% O2 is added to the argon sputter gas, the sample showed significantly improved photoactivity for water oxidation. The enhancement in photoactivity is due to the suppression of defect trap states in the bulk and at the surface of the anode in contact with an electrolyte. This work is demonstrating a scalable and effective method for the preparation of highly photoactive hematite thin films for PEC anodes without doping. Acknowledgement This work was supported by the German Federal Ministry of Education and Research (BMBF) under contract # 03SF0353A ‘‘H2NanoSolar’’. References
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