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Thickness effect of hematite nanostructures prepared by hydrothermal method for solar water splitting
Applied Surface Science 320 (2014) 213–217
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Thickness effect of hematite nanostructures prepared by hydrothermal method for solar water splitting Aiwu Pu, Jiujun Deng, Yuanyuan Hao, Xuhui Sun, Jun Zhong ∗ Soochow University-Western University Centre for Synchrotron Radiation Research, Institute of Functional Nano and Soft Material (FUNSOM) and Collaborative Innovation Center of Suzhou Nano Science & Technology, Soochow University, Suzhou 215123, China
a r t i c l e
i n f o
Article history: Received 17 July 2014 Received in revised form 29 August 2014 Accepted 15 September 2014 Available online 22 September 2014 Keywords: Hematite nanostructures Solar water splitting Thickness effect Hydrothermal method Layered structure
a b s t r a c t Hematite nanostructures with various thicknesses were prepared for solar water splitting by controlling the HCl concentration in hydrothermal process. Results show that when the thickness increases, hematite will form layered structure on the substrate instead of the continuing growth of nanorods. A single layer of vertical nanorods shows the best performance for solar water splitting while multi-layers of nanorods show worse performance, which can be attributed to bad conductivity between hematite layers revealed by Mott–Schottky plots. Data clearly demonstrate the thickness effect of hematite nanostructures, which can be a key role for the performance of solar water splitting. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Hematite was recently considered to be a good photocatalyst for efficient solar water splitting due to its favorable optical band gap (2.1–2.2 eV), extraordinary chemical stability in oxidative environment, abundance, and low cost [1–10]. However, the practical performance of hematite for solar water splitting is far away from their theoretical prediction. Several factors were found to limit the performance of hematite such as poor conductivity, short lifetime of the excited-state carrier (10−12 s), poor oxygen evolution reaction (OER) kinetics, short hole diffusion length (2–4 nm), and improper band position for unassisted water splitting [5–10]. To explore hematite structures with excellent performance for solar water splitting, various methods have been used to prepare hematite such as spray pyrolysis [11], sol–gel [12], hydrothermal [6], and atmosphere pressure chemical vapor deposition (APCVD) [9]. Among all these methods, hydrothermal process was reported to be an effective way to prepare highly efficient hematite nanostructures [6,8,10,13–15]. The pristine hematite nanostructures prepared by typical hydrothermal method can easily achieve a relatively high performance, which is a good start for further treatment such as doping, annealing or surface treatments to obtain better performance [6,10,15]. Recently, by coupling Pt-doping and Co–Pi catalysts to pristine hematite prepared by hydrothermal
method, a record-breaking photocurrent of 4.32 mA/cm2 at 1.23 V vs. reversible hydrogen electrode (RHE) was achieved [10]. The work also showed the important role of pristine hematite as a good starting material for further modification to achieve high performance [10]. Although hydrothermal method has been widely used to prepare hematite nanostructures, the detailed synthesis process and various factors which may affect the performance of final products in hydrothermal method are still unclear. It is thus important to investigate various influence factors in hydrothermal process as a fundamental study which will benefit the future application. Here we find that the HCl concentration may significantly affect the thickness of hematite nanostructures and then affect the performance for solar water splitting. Results show that when the thickness increases, hematite will form layered structure on the substrate instead of the continuing growth of nanorods. Interestingly, a single layer of vertical nanorods shows the best performance for solar water splitting while multi-layers of nanorods show worse performance, which can be attributed to bad conductivity between hematite layers. Data clearly reveal the thickness effect of hematite nanostructures in hydrothermal process for the performance of solar water splitting and may help to understand other solution process for the preparation of hematite nanostructures. 2. Experimental 2.1. Preparation of ˛-Fe2 O3 photoanodes
∗ Corresponding author. Tel.: +86 512 65880417. E-mail address: [email protected] (J. Zhong). http://dx.doi.org/10.1016/j.apsusc.2014.09.086 0169-4332/© 2014 Elsevier B.V. All rights reserved.
Hematite nanostructures were prepared on a fluorine-doped SnO2 (FTO, Nippon Sheet Glass, Japan, 14 ohm/sq) glass by a
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hydrothermal method [6]. A teflon-lined stainless steel autoclave was filled with 60 ml aqueous solution containing 0.15 M of ferric chloride (FeCl3 ·6H2 O, Sinopharm Chemical Reagent Co., Ltd.), 1 M sodium nitrate (NaNO3 , Sinopharm Chemical Reagent Co., Ltd.) and various amounts (0, 60, 120, 180, 240, 330 l) of HCl (45.3–45.8 wt%). The FTO glass was cleaned with acetone, ethanol and deionized water. The cleaned FTO glass slide (60 mm × 35 mm × 2 mm) was put into the autoclave and heated at 95 ◦ C for 4 h. The resulted samples were mainly yellow color film (FeOOH) (except for the sample with 330 l HCl which showed white color) formed on the FTO substrate. The samples treated with various amounts (0, 60, 120, 180, 240, 330 l) of HCl were labeled as A1 , B1 , C1 , D1 , E1 and F1 , respectively. Then the samples were washed with deionized water and cut into 17.5 mm × 10 mm × 2 mm pieces for further annealing treatment. The samples were sintered in air at 550 ◦ C for 2 h and annealed at 800 ◦ C for additional 5 min for photoelectrochemical (PEC) measurements. The final samples were labeled as A, B, C, D, E and F, respectively. 2.2. Structural characterization The scanning electron microscopy (SEM) images of hematite nanostructures were taken on a FEI-quanta 200 scanning electron microscope with acceleration voltage of 20 kV. Transmission electron microscopy (TEM) was obtained with a FEI/Philips Techai 12 BioTWIN transmission electron microscope. X-ray Diffraction (XRD, PANalytical, Zmpyrean) was also used for structural characterization. 2.3. PEC measurements Hematite photoanodes on FTO substrate were covered by nonconductive hysol epoxy except for a working area of 0.1 cm2 . All PEC measurements were carried out using CHI 660D electrochemical workstation in a three-electrode electrochemical cell with a Pt wire as a counter electrode and an Ag/AgCl electrode as a reference. The Electrolyte was an aqueous solution of NaOH with a pH of about 13.6, bubbled with N2 for 20 min before measurement. The measured voltage was converted into the potential vs. reversible hydrogen electrode (RHE). In a typical experiment, the potential was swept from 0.7 V to 1.8 V vs. RHE at a scan rate of 50 mV s−1 . Xenon High Brightness Cold Light Sources (XD-300) coupled with a filter (AM 1.5G) were used as the white light source and the light power density of 100 mW cm−2 (spectrally corrected) was measured with a power meter (Newport, 842-PE). Capacitance was derived from the electrochemical impedance obtained at each potential with 10,000 Hz frequency in the dark. Mott–Schottky plots were generated from the capacitance values. 3. Results and discussion Fig. 1 shows the PEC performance of samples A–F. PEC measurements were performed in 1 M NaOH electrolyte (pH about 13.6) using a three-electrode electrochemical cell with hematite nanostructures on FTO as the working electrode, a platinum coil as the counter electrode, and a reference of Ag/AgCl. The photocurrent density–applied potential (J–V) scans for hematite nanostructures were measured with the AM 1.5 G simulated solar light at 100 mW/cm2 . In Fig. 1 we show a comparison of the J–V scans for samples A–F prepared with various amounts of HCl. Sample A without HCl (black curve) shows a photocurrent of 0.51 mA/cm2 at 1.23 V vs. RHE. When the amount of HCl increases, hematite samples B–E show enhanced performance than that for sample A, until in sample F the amount of HCl is too much preventing the growth of
Fig. 1. J–V scans for hematite samples A–F.
hematite nanostructures. Especially, sample E shows the best performance with a photocurrent of 0.94 mA/cm2 at 1.23 V vs. RHE, which is almost two times over that for sample A without HCl. When preparing hematite nanostructures in hydrothermal process, the reaction can be simply described as below: Fe3+ + 2H2 O FeOOH + 3H+ The increase of H+ in solution may reduce the formation of FeOOH. Thus the concentration of HCl will influence the thickness of FeOOH deposited on FTO substrate, which may significantly affect the PEC performance of final hematite samples. Sample E shows the best performance indicating an optimized HCl concentration in hydrothermal process. To understand the performance evolution of hematite nanostructures prepared with various amounts of HCl, we show the SEM images of samples A, E and F in Fig. 2. The samples for FeOOH structure before annealing (A1 , E1 and F1 ) are also shown for comparison. For samples A and E, the samples before and after annealing show similar morphology but the nanostructures after annealing show slightly increased size. Both samples A and E after annealing show structures with mainly vertical nanorods deposited on the FTO substrate, which is in good agreement with previous reports using similar synthesis method [10,14]. However, sample E shows a uniform distribution of nanorods covering the FTO substrate while the nanorods in sample A tend to assemble as isolated islands. The nanorods in sample A are larger than that in sample E. The morphology difference between sample A and sample E revealed by SEM images could be related to the different PEC performance, which need to be further addressed. For sample F, SEM image shows that almost no hematite nanostructures can be observed and the surface is very similar to the FTO substrate, which is consistent with the PEC performance. XRD data for samples A, E, F and FTO substrate are also shown in Fig. 3. The XRD data for samples A and E can be indexed to the characteristic peaks of typical hematite structure (JCPDS 33-0664) after subtracting the peaks from the FTO substrate, while sample F shows similar peaks to that of FTO substrate. In Fig. 4 the SEM cross-section images of hematite photoanodes (A and E) have been shown to reveal the relationship between the concentration of HCl and the morphology of the resulted hematite nanostructures. The bottom solid film about 340 nm in both images is FTO conductive layer, while the materials on the conductive film are hematite nanorods. For sample E the hematite nanostructures are randomly deposited on the conductive film showing some vertical nanorods, which is consistent with the top view SEM image in Fig. 2. However, for sample A, a thick hematite
A. Pu et al. / Applied Surface Science 320 (2014) 213–217
215
Fig. 2. SEM images of sample A (top), sample E (middle) and sample F (bottom) before (A1 , E1 and F1 ) and after (A, E and F) annealing treatment.
film on the conductive film can be observed with the thickness of about 650 nm. Due to the low resolution of our cross-section SEM image the hematite materials seem like a thick solid film. However, it is clear that sample A shows much more thickness than that in sample E. Also compared to sample F with too much HCl almost no hematite nanostructures can be observed in sample F, the results suggest that increased concentration of HCl leads to hematite nanostructures with less thickness. The results reveal a good relationship between the concentration of HCl in hydrothermal process and the thickness of the resulted hematite. Sample E stands for the best thickness for PEC performance.
Fig. 5 shows the TEM images of sample A and sample E. The samples for TEM are scratched from the photoanodes. TEM image of sample E shows a typical nanorod shape with a length of about 300 nm. From the cross-section image the hematite nanostructures in sample E are a single layer of nanorods vertically deposited on the FTO substrate. For sample E the vertical length in Fig. 4 is consistent with the TEM image in Fig. 5. Interestingly, in Fig. 5 the TEM image of sample A also shows a nanorod shape with the length of about 300 nm. Since the cross-section image reveals that sample A has a thick hematite layer with the thickness of about 650 nm, the results show that decreased HCl concentration may increase the
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Fig. 6. Mott–Schottky plots of sample A and sample E. Fig. 3. XRD spectra of samples A, E, F and the FTO substrate.
hematite thickness but the building blocks can be similar nanorods and the thick hematite film is multi-layers of the nanorods which are randomly deposited on the substrate. Since the conductivity of hematite itself is not good and the connection between hematite layers is loose, the charge transfer for the top layer will be worse compared to that for a single layer on
the conductive FTO substrate. Thus a decreased PEC performance can be expected for the thick hematite film with multi-layers and a single layer of hematite nanostructures can be the best choice. The results are in good agreement with the PEC performance shown in Fig. 1. Mott–Schottky curves of sample A and sample E are also shown in Fig. 6 to reveal the conductivity of hematite samples with various
Fig. 4. SEM images of the cross-section of sample A (left) and sample E (right).
Fig. 5. TEM images of sample A (left) and sample E (right).
A. Pu et al. / Applied Surface Science 320 (2014) 213–217
thicknesses. The slopes from the Mott–Schottky plots are used to estimate the carrier densities using the following equation: Nd =
2 e0 εε0
d(1/C 2 ) dV
−1
where e0 is the electron charge (1.602 × 10−19 C), ε is the dielectric constant of ␣-Fe2 O3 (80) [16], ε0 is the permittivity of vacuum (8.854 × 10−14 Fm−1 ). Nd is the donor density and V is the potential applied at the electrode. Capacitances were derived from the electrochemical impedance obtained at each potential with 10,000 Hz frequency in the dark. The Mott–Schottky slopes of both sample A and sample E are positive which indicates that they are n-type semiconductors with electrons as majority carriers. The carrier densities are estimated from the Mott–Schottky plots [16]. The results show that the donor density of sample A is 1.14 × 1017 cm−3 , while the donor densities of sample E is 4.17 × 1017 cm−3 . The donor density of sample E is about 4 times over that of sample A which leads to better conductivity, in good agreement with our hypothesis that thick hematite film has worse conductivity and then shows worse PEC performance. 4. Conclusions We presented the preparation of hematite photoanodes with various thicknesses on the FTO glass by controlling the concentration of HCl in hydrothermal process. SEM and TEM results reveal that the hematite nanostructures are nanorod layers on the FTO substrate. Interestingly, thin hematite film with a single vertical layer shows the best performance for solar water splitting while thick hematite film with multi-layers shows worse performance. The performance variation can be attributed to the different conductivity of hematite films resulted from the sample thickness. Data clearly reveal that the thickness of hematite can be a key role for the performance of solar water splitting. Acknowledgements We acknowledge the National Basic Research Development Program of China (2012CB825800, 2010CB934500), the National Natural Science Foundation of China (11275137, 11179032,
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91333112) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). This is also a project supported by the Fund for Innovative Research Teams of Jiangsu Higher Education Institutions. References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] M. Grätzel, Photoelectrochemical cells, Nature 414 (2001) 338–344. [3] K. Sivula, F. Le Formal, M. Grätzel, Solar water splitting: progress using hematite (␣-Fe2 O3 ) photoelectrodes, ChemSusChem 4 (2011) 432–449. [4] A.B. Murphy, P.R.F. Barnes, L.K. Randeniya, I.C. Plumb, I.E. Grey, M.D. Horne, et al., Efficiency of solar water splitting using semiconductor electrodes, Int. J. Hydrog. Energy 31 (2006) 1999–2017. [5] D. Zhong, M. Cornuz, K. Sivula, M. Grätzel, D. Gamelin, Photo-assisted electrodeposition of cobalt–phosphate (Co–Pi) catalyst on hematite photoanodes for solar water oxidation, Energy Environ. Sci. 4 (2011) 1759–1764. [6] Y. Ling, G. Wang, D.A. Wheeler, J.Z. Zhang, Y. Li, Sn-doped hematite nanostructures for photoelectrochemical water splitting, Nano Lett. 11 (2011) 2119–2125. [7] J.J. Deng, X.X. Lv, J. Gao, A.W. Pu, M. Li, X.H. Sun, et al., Facile synthesis of carboncoated hematite nanostructures for solar water splitting, Energy Environ. Sci. 6 (2013) 1965–1970. [8] J.J. Deng, J. Zhong, A.W. Pu, D. Zhang, M. Li, X.H. Sun, et al., Ti-doped hematite nanostructures for solar water splitting with high efficiency, J. Appl. Phys. 111 (2012) 084312–84317. [9] S.D. Tilley, M. Cornuz, K. Sivula, M. Grätzel, Light-induced water splitting with hematite: improved nanoastructure and iridium oxide catalysis, Angew. Chem. Int. Ed. 49 (2010) 6405–6408. [10] Y.C. Ling, G.M. Wang, J. Reddy, C.C. Wang, J.Z. Zhang, Y. Li, The influence of oxygen content on the thermal activation of hematite nanowires, Angew. Chem. Int. Ed. 51 (2012) 4074–4079. [11] C.J. Sartoretti, B.D. Alexander, R. Solarska, I.A. Rutkowska, J. Augustynski, Photoelectrochemical oxidation of water at transparent ferric oxide film electrodes, J. Phys. Chem. B 109 (2005) 13685–13692. [12] U. Bjorksten, J. Moser, M. Grätzel, Photoelectrochemical studies on nanocrystalline hematite films, Chem. Mater. 6 (1994) 858–863. [13] L. Vayssieres, N. Beermann, S.E. Lindquist, A. Hagfeldt, Controlled aqueous chemical growth of oriented three-dimensional crystalline nanorod arrays: application to iron (III) oxides, Chem. Mater. 13 (2001) 233–235. [14] A. Pu, J. Deng, M. Li, J. Gao, H. Zhang, Y. Hao, et al., Coupling Ti-doping and oxygen vacancies in hematite nanostructures for solar water oxidation with high efficiency, J. Mater. Chem. A 2 (2014) 2491–2497. [15] J. Kim, G. Magesh, D. Youn, J. Jang, J. Kubata, K. Domen, et al., Single-crystalline, wormlike hematite photoanodes for efficient solar water splitting, Sci. Rep. 3 (2013) 2681–2688. [16] I. Cesar, K. Sivula, A. Kay, R. Zboril, M. Grätzel, Influence of feature size, film thickness, and silicon doping on the performance of nanostructured hematite photoanodes for solar water splitting, J. Phys. Chem. C 113 (2009) 772–782.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Thickness effect of hematite nanostructures prepared by hydrothermal method for solar water splitting Aiwu Pu, Jiujun Deng, Yuanyuan Hao, Xuhui Sun, Jun Zhong ∗ Soochow University-Western University Centre for Synchrotron Radiation Research, Institute of Functional Nano and Soft Material (FUNSOM) and Collaborative Innovation Center of Suzhou Nano Science & Technology, Soochow University, Suzhou 215123, China
a r t i c l e
i n f o
Article history: Received 17 July 2014 Received in revised form 29 August 2014 Accepted 15 September 2014 Available online 22 September 2014 Keywords: Hematite nanostructures Solar water splitting Thickness effect Hydrothermal method Layered structure
a b s t r a c t Hematite nanostructures with various thicknesses were prepared for solar water splitting by controlling the HCl concentration in hydrothermal process. Results show that when the thickness increases, hematite will form layered structure on the substrate instead of the continuing growth of nanorods. A single layer of vertical nanorods shows the best performance for solar water splitting while multi-layers of nanorods show worse performance, which can be attributed to bad conductivity between hematite layers revealed by Mott–Schottky plots. Data clearly demonstrate the thickness effect of hematite nanostructures, which can be a key role for the performance of solar water splitting. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Hematite was recently considered to be a good photocatalyst for efficient solar water splitting due to its favorable optical band gap (2.1–2.2 eV), extraordinary chemical stability in oxidative environment, abundance, and low cost [1–10]. However, the practical performance of hematite for solar water splitting is far away from their theoretical prediction. Several factors were found to limit the performance of hematite such as poor conductivity, short lifetime of the excited-state carrier (10−12 s), poor oxygen evolution reaction (OER) kinetics, short hole diffusion length (2–4 nm), and improper band position for unassisted water splitting [5–10]. To explore hematite structures with excellent performance for solar water splitting, various methods have been used to prepare hematite such as spray pyrolysis [11], sol–gel [12], hydrothermal [6], and atmosphere pressure chemical vapor deposition (APCVD) [9]. Among all these methods, hydrothermal process was reported to be an effective way to prepare highly efficient hematite nanostructures [6,8,10,13–15]. The pristine hematite nanostructures prepared by typical hydrothermal method can easily achieve a relatively high performance, which is a good start for further treatment such as doping, annealing or surface treatments to obtain better performance [6,10,15]. Recently, by coupling Pt-doping and Co–Pi catalysts to pristine hematite prepared by hydrothermal
method, a record-breaking photocurrent of 4.32 mA/cm2 at 1.23 V vs. reversible hydrogen electrode (RHE) was achieved [10]. The work also showed the important role of pristine hematite as a good starting material for further modification to achieve high performance [10]. Although hydrothermal method has been widely used to prepare hematite nanostructures, the detailed synthesis process and various factors which may affect the performance of final products in hydrothermal method are still unclear. It is thus important to investigate various influence factors in hydrothermal process as a fundamental study which will benefit the future application. Here we find that the HCl concentration may significantly affect the thickness of hematite nanostructures and then affect the performance for solar water splitting. Results show that when the thickness increases, hematite will form layered structure on the substrate instead of the continuing growth of nanorods. Interestingly, a single layer of vertical nanorods shows the best performance for solar water splitting while multi-layers of nanorods show worse performance, which can be attributed to bad conductivity between hematite layers. Data clearly reveal the thickness effect of hematite nanostructures in hydrothermal process for the performance of solar water splitting and may help to understand other solution process for the preparation of hematite nanostructures. 2. Experimental 2.1. Preparation of ˛-Fe2 O3 photoanodes
∗ Corresponding author. Tel.: +86 512 65880417. E-mail address: [email protected] (J. Zhong). http://dx.doi.org/10.1016/j.apsusc.2014.09.086 0169-4332/© 2014 Elsevier B.V. All rights reserved.
Hematite nanostructures were prepared on a fluorine-doped SnO2 (FTO, Nippon Sheet Glass, Japan, 14 ohm/sq) glass by a
214
A. Pu et al. / Applied Surface Science 320 (2014) 213–217
hydrothermal method [6]. A teflon-lined stainless steel autoclave was filled with 60 ml aqueous solution containing 0.15 M of ferric chloride (FeCl3 ·6H2 O, Sinopharm Chemical Reagent Co., Ltd.), 1 M sodium nitrate (NaNO3 , Sinopharm Chemical Reagent Co., Ltd.) and various amounts (0, 60, 120, 180, 240, 330 l) of HCl (45.3–45.8 wt%). The FTO glass was cleaned with acetone, ethanol and deionized water. The cleaned FTO glass slide (60 mm × 35 mm × 2 mm) was put into the autoclave and heated at 95 ◦ C for 4 h. The resulted samples were mainly yellow color film (FeOOH) (except for the sample with 330 l HCl which showed white color) formed on the FTO substrate. The samples treated with various amounts (0, 60, 120, 180, 240, 330 l) of HCl were labeled as A1 , B1 , C1 , D1 , E1 and F1 , respectively. Then the samples were washed with deionized water and cut into 17.5 mm × 10 mm × 2 mm pieces for further annealing treatment. The samples were sintered in air at 550 ◦ C for 2 h and annealed at 800 ◦ C for additional 5 min for photoelectrochemical (PEC) measurements. The final samples were labeled as A, B, C, D, E and F, respectively. 2.2. Structural characterization The scanning electron microscopy (SEM) images of hematite nanostructures were taken on a FEI-quanta 200 scanning electron microscope with acceleration voltage of 20 kV. Transmission electron microscopy (TEM) was obtained with a FEI/Philips Techai 12 BioTWIN transmission electron microscope. X-ray Diffraction (XRD, PANalytical, Zmpyrean) was also used for structural characterization. 2.3. PEC measurements Hematite photoanodes on FTO substrate were covered by nonconductive hysol epoxy except for a working area of 0.1 cm2 . All PEC measurements were carried out using CHI 660D electrochemical workstation in a three-electrode electrochemical cell with a Pt wire as a counter electrode and an Ag/AgCl electrode as a reference. The Electrolyte was an aqueous solution of NaOH with a pH of about 13.6, bubbled with N2 for 20 min before measurement. The measured voltage was converted into the potential vs. reversible hydrogen electrode (RHE). In a typical experiment, the potential was swept from 0.7 V to 1.8 V vs. RHE at a scan rate of 50 mV s−1 . Xenon High Brightness Cold Light Sources (XD-300) coupled with a filter (AM 1.5G) were used as the white light source and the light power density of 100 mW cm−2 (spectrally corrected) was measured with a power meter (Newport, 842-PE). Capacitance was derived from the electrochemical impedance obtained at each potential with 10,000 Hz frequency in the dark. Mott–Schottky plots were generated from the capacitance values. 3. Results and discussion Fig. 1 shows the PEC performance of samples A–F. PEC measurements were performed in 1 M NaOH electrolyte (pH about 13.6) using a three-electrode electrochemical cell with hematite nanostructures on FTO as the working electrode, a platinum coil as the counter electrode, and a reference of Ag/AgCl. The photocurrent density–applied potential (J–V) scans for hematite nanostructures were measured with the AM 1.5 G simulated solar light at 100 mW/cm2 . In Fig. 1 we show a comparison of the J–V scans for samples A–F prepared with various amounts of HCl. Sample A without HCl (black curve) shows a photocurrent of 0.51 mA/cm2 at 1.23 V vs. RHE. When the amount of HCl increases, hematite samples B–E show enhanced performance than that for sample A, until in sample F the amount of HCl is too much preventing the growth of
Fig. 1. J–V scans for hematite samples A–F.
hematite nanostructures. Especially, sample E shows the best performance with a photocurrent of 0.94 mA/cm2 at 1.23 V vs. RHE, which is almost two times over that for sample A without HCl. When preparing hematite nanostructures in hydrothermal process, the reaction can be simply described as below: Fe3+ + 2H2 O FeOOH + 3H+ The increase of H+ in solution may reduce the formation of FeOOH. Thus the concentration of HCl will influence the thickness of FeOOH deposited on FTO substrate, which may significantly affect the PEC performance of final hematite samples. Sample E shows the best performance indicating an optimized HCl concentration in hydrothermal process. To understand the performance evolution of hematite nanostructures prepared with various amounts of HCl, we show the SEM images of samples A, E and F in Fig. 2. The samples for FeOOH structure before annealing (A1 , E1 and F1 ) are also shown for comparison. For samples A and E, the samples before and after annealing show similar morphology but the nanostructures after annealing show slightly increased size. Both samples A and E after annealing show structures with mainly vertical nanorods deposited on the FTO substrate, which is in good agreement with previous reports using similar synthesis method [10,14]. However, sample E shows a uniform distribution of nanorods covering the FTO substrate while the nanorods in sample A tend to assemble as isolated islands. The nanorods in sample A are larger than that in sample E. The morphology difference between sample A and sample E revealed by SEM images could be related to the different PEC performance, which need to be further addressed. For sample F, SEM image shows that almost no hematite nanostructures can be observed and the surface is very similar to the FTO substrate, which is consistent with the PEC performance. XRD data for samples A, E, F and FTO substrate are also shown in Fig. 3. The XRD data for samples A and E can be indexed to the characteristic peaks of typical hematite structure (JCPDS 33-0664) after subtracting the peaks from the FTO substrate, while sample F shows similar peaks to that of FTO substrate. In Fig. 4 the SEM cross-section images of hematite photoanodes (A and E) have been shown to reveal the relationship between the concentration of HCl and the morphology of the resulted hematite nanostructures. The bottom solid film about 340 nm in both images is FTO conductive layer, while the materials on the conductive film are hematite nanorods. For sample E the hematite nanostructures are randomly deposited on the conductive film showing some vertical nanorods, which is consistent with the top view SEM image in Fig. 2. However, for sample A, a thick hematite
A. Pu et al. / Applied Surface Science 320 (2014) 213–217
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Fig. 2. SEM images of sample A (top), sample E (middle) and sample F (bottom) before (A1 , E1 and F1 ) and after (A, E and F) annealing treatment.
film on the conductive film can be observed with the thickness of about 650 nm. Due to the low resolution of our cross-section SEM image the hematite materials seem like a thick solid film. However, it is clear that sample A shows much more thickness than that in sample E. Also compared to sample F with too much HCl almost no hematite nanostructures can be observed in sample F, the results suggest that increased concentration of HCl leads to hematite nanostructures with less thickness. The results reveal a good relationship between the concentration of HCl in hydrothermal process and the thickness of the resulted hematite. Sample E stands for the best thickness for PEC performance.
Fig. 5 shows the TEM images of sample A and sample E. The samples for TEM are scratched from the photoanodes. TEM image of sample E shows a typical nanorod shape with a length of about 300 nm. From the cross-section image the hematite nanostructures in sample E are a single layer of nanorods vertically deposited on the FTO substrate. For sample E the vertical length in Fig. 4 is consistent with the TEM image in Fig. 5. Interestingly, in Fig. 5 the TEM image of sample A also shows a nanorod shape with the length of about 300 nm. Since the cross-section image reveals that sample A has a thick hematite layer with the thickness of about 650 nm, the results show that decreased HCl concentration may increase the
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Fig. 6. Mott–Schottky plots of sample A and sample E. Fig. 3. XRD spectra of samples A, E, F and the FTO substrate.
hematite thickness but the building blocks can be similar nanorods and the thick hematite film is multi-layers of the nanorods which are randomly deposited on the substrate. Since the conductivity of hematite itself is not good and the connection between hematite layers is loose, the charge transfer for the top layer will be worse compared to that for a single layer on
the conductive FTO substrate. Thus a decreased PEC performance can be expected for the thick hematite film with multi-layers and a single layer of hematite nanostructures can be the best choice. The results are in good agreement with the PEC performance shown in Fig. 1. Mott–Schottky curves of sample A and sample E are also shown in Fig. 6 to reveal the conductivity of hematite samples with various
Fig. 4. SEM images of the cross-section of sample A (left) and sample E (right).
Fig. 5. TEM images of sample A (left) and sample E (right).
A. Pu et al. / Applied Surface Science 320 (2014) 213–217
thicknesses. The slopes from the Mott–Schottky plots are used to estimate the carrier densities using the following equation: Nd =
2 e0 εε0
d(1/C 2 ) dV
−1
where e0 is the electron charge (1.602 × 10−19 C), ε is the dielectric constant of ␣-Fe2 O3 (80) [16], ε0 is the permittivity of vacuum (8.854 × 10−14 Fm−1 ). Nd is the donor density and V is the potential applied at the electrode. Capacitances were derived from the electrochemical impedance obtained at each potential with 10,000 Hz frequency in the dark. The Mott–Schottky slopes of both sample A and sample E are positive which indicates that they are n-type semiconductors with electrons as majority carriers. The carrier densities are estimated from the Mott–Schottky plots [16]. The results show that the donor density of sample A is 1.14 × 1017 cm−3 , while the donor densities of sample E is 4.17 × 1017 cm−3 . The donor density of sample E is about 4 times over that of sample A which leads to better conductivity, in good agreement with our hypothesis that thick hematite film has worse conductivity and then shows worse PEC performance. 4. Conclusions We presented the preparation of hematite photoanodes with various thicknesses on the FTO glass by controlling the concentration of HCl in hydrothermal process. SEM and TEM results reveal that the hematite nanostructures are nanorod layers on the FTO substrate. Interestingly, thin hematite film with a single vertical layer shows the best performance for solar water splitting while thick hematite film with multi-layers shows worse performance. The performance variation can be attributed to the different conductivity of hematite films resulted from the sample thickness. Data clearly reveal that the thickness of hematite can be a key role for the performance of solar water splitting. Acknowledgements We acknowledge the National Basic Research Development Program of China (2012CB825800, 2010CB934500), the National Natural Science Foundation of China (11275137, 11179032,
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91333112) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). This is also a project supported by the Fund for Innovative Research Teams of Jiangsu Higher Education Institutions. References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] M. Grätzel, Photoelectrochemical cells, Nature 414 (2001) 338–344. [3] K. Sivula, F. Le Formal, M. Grätzel, Solar water splitting: progress using hematite (␣-Fe2 O3 ) photoelectrodes, ChemSusChem 4 (2011) 432–449. [4] A.B. Murphy, P.R.F. Barnes, L.K. Randeniya, I.C. Plumb, I.E. Grey, M.D. Horne, et al., Efficiency of solar water splitting using semiconductor electrodes, Int. J. Hydrog. Energy 31 (2006) 1999–2017. [5] D. Zhong, M. Cornuz, K. Sivula, M. Grätzel, D. Gamelin, Photo-assisted electrodeposition of cobalt–phosphate (Co–Pi) catalyst on hematite photoanodes for solar water oxidation, Energy Environ. Sci. 4 (2011) 1759–1764. [6] Y. Ling, G. Wang, D.A. Wheeler, J.Z. Zhang, Y. Li, Sn-doped hematite nanostructures for photoelectrochemical water splitting, Nano Lett. 11 (2011) 2119–2125. [7] J.J. Deng, X.X. Lv, J. Gao, A.W. Pu, M. Li, X.H. Sun, et al., Facile synthesis of carboncoated hematite nanostructures for solar water splitting, Energy Environ. Sci. 6 (2013) 1965–1970. [8] J.J. Deng, J. Zhong, A.W. Pu, D. Zhang, M. Li, X.H. Sun, et al., Ti-doped hematite nanostructures for solar water splitting with high efficiency, J. Appl. Phys. 111 (2012) 084312–84317. [9] S.D. Tilley, M. Cornuz, K. Sivula, M. Grätzel, Light-induced water splitting with hematite: improved nanoastructure and iridium oxide catalysis, Angew. Chem. Int. Ed. 49 (2010) 6405–6408. [10] Y.C. Ling, G.M. Wang, J. Reddy, C.C. Wang, J.Z. Zhang, Y. Li, The influence of oxygen content on the thermal activation of hematite nanowires, Angew. Chem. Int. Ed. 51 (2012) 4074–4079. [11] C.J. Sartoretti, B.D. Alexander, R. Solarska, I.A. Rutkowska, J. Augustynski, Photoelectrochemical oxidation of water at transparent ferric oxide film electrodes, J. Phys. Chem. B 109 (2005) 13685–13692. [12] U. Bjorksten, J. Moser, M. Grätzel, Photoelectrochemical studies on nanocrystalline hematite films, Chem. Mater. 6 (1994) 858–863. [13] L. Vayssieres, N. Beermann, S.E. Lindquist, A. Hagfeldt, Controlled aqueous chemical growth of oriented three-dimensional crystalline nanorod arrays: application to iron (III) oxides, Chem. Mater. 13 (2001) 233–235. [14] A. Pu, J. Deng, M. Li, J. Gao, H. Zhang, Y. Hao, et al., Coupling Ti-doping and oxygen vacancies in hematite nanostructures for solar water oxidation with high efficiency, J. Mater. Chem. A 2 (2014) 2491–2497. [15] J. Kim, G. Magesh, D. Youn, J. Jang, J. Kubata, K. Domen, et al., Single-crystalline, wormlike hematite photoanodes for efficient solar water splitting, Sci. Rep. 3 (2013) 2681–2688. [16] I. Cesar, K. Sivula, A. Kay, R. Zboril, M. Grätzel, Influence of feature size, film thickness, and silicon doping on the performance of nanostructured hematite photoanodes for solar water splitting, J. Phys. Chem. C 113 (2009) 772–782.