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Visible light photoelectrochemical responsiveness of self-organized nanoporous WO3 films
Electrochimica Acta 56 (2010) 620–625
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Visible light photoelectrochemical responsiveness of self-organized nanoporous WO3 films Wenzhang Li a , Jie Li a,∗ , Xuan Wang a,b , Sha Luo a , Juan Xiao a , Qiyuan Chen a,∗ a Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, China, College of Chemistry and Chemical Engineering, Central South University, Changsha 410083 China b Shenyang Aluminum & Magnesium Engineering & Research Institute, Shenyang 110001 China
a r t i c l e
i n f o
Article history: Received 3 April 2010 Received in revised form 7 June 2010 Accepted 8 June 2010 Available online 15 June 2010 Keywords: Tungsten trioxide Self-organized nanoporous film Voltage stepping anodization Photoelectrochemical property Electronic conductivity
a b s t r a c t Visible light-responsive WO3 nanoporous films with preferential orientation of the (0 0 2) planes were prepared by anodization in neutral F− -containing strong electrolytes. The pore diameter of the selforganized structure was estimated to be in the region of 70–90 nm. Voltages were applied by stepping, which positively influenced passivity breakdown and played a significant role in the formation of selforganized nanoporous films. Under visible light irradiation, the photocurrent density (at 1.6 V vs. Ag/AgCl) and maximum photoconversion efficiency generated by the annealed nanoporous film were 3.45 mA/cm2 and 0.91%, respectively. The annealed nanoporous WO3 films show maximum incident photon-to-current conversion efficiency of 92% at 340 nm at 1.2 V vs. Ag/AgCl. These values are higher than that of annealed compact WO3 film due to the large interfacial heterojunction area. The photoelectrochemical activities and electronic conductivities were also enhanced by annealing crystallization, which removed the recombination centers. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction The use of tungsten oxide as a stable photocatalyst for watersplitting has been attracting much attention since the 1970s [1,2]. The formation of the nanoporous structure results in a dramatic increase in specific surface area, significantly broadening the applications of tungsten oxide [3–7]. Anodization is an effective and relatively simple technique that has been widely applied to the formation of TiO2 nanotubes [8–14]. WO3 nanoporous thin films were fabricated for the first time by the process of galvanostatic anodization in oxalic acid electrolyte by Grimes et al. [15]. However, obtaining films with entirely regular and self-organized porous morphologies was difficult. Schmuki et al. [3,6,16] prepared relatively uniform self-organized nanoporous WO3 films with an average pore diameter of 70–100 nm by potentiostatic anodization in NaF and NaF/H2 SO4 electrolytes. Highly ordered and smooth TiO2 nanotubes can also be obtained in F− -containing organic electrolytes [10,12]. Rajeshwar et al. [4,7] found that anodic nanoporous WO3 could not be fabricated in F− -containing organic electrolytes. A two-step anodization method was used by Guo et al. [5] to prepare self-assembled nanoporous WO3 films, while highly ordered TiO2 nanotubes were made through a multi-step anodization [14].
Moreover, WO3 films can also be formed by sweeping the cell voltage to the desired values. In contrast, Hahn et al. [17] attempted to fabricate nanotubes in F− -free electrolytes and concluded that passivity breakdown takes place more easily with voltage stepping. More efforts need to be done to modify and control the morphology of WO3 nanoporous films. Studies focus mainly on the photoelectrochemical activity of anodic WO3 nanoporous films under UV irradiation [3,4] and rarely on the visible light response property of WO3 nanoporous films. As a stable photoanode for water splitting, WO3 has been attracting much attention due to its satisfactory photoelectron transport properties and visible light responsiveness. Thus, intensive research on the visible light response property of WO3 is necessary. In this study, the possibility of achieving self-organized nanoporous WO3 films was explored by a simple voltage stepping anodization in neutral NaF/Na2 SO4 strong electrolytes. The electronic properties and responsiveness of WO3 films under visible light were also investigated. 2. Experimental 2.1. Preparation of nanoporous and compact WO3 films
∗ Corresponding authors. Tel.: +86 731 8887 7364; fax: +86 731 8887 9616. E-mail addresses: [email protected] (J. Li), [email protected] (Q. Chen). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.06.025
A two-electrode cell consisting of a tungsten working electrode and a Pt counter electrode was used. Tungsten foils (99.95% purity, Alfa Aesar) were successively sonicated in acetone, isopropanol,
W. Li et al. / Electrochimica Acta 56 (2010) 620–625
and methanol. The tungsten foils were rinsed in deionized (DI) water, dried in a nitrogen stream, and anodized in 1 M Na2 SO4 electrolyte with 0.5 wt.% NaF for 30 min. The cell voltage was applied by a single step from the open-circuit potential to a given value and was supplied by a direct current voltage source (DH1719A5, Dahua, China). The current between the working and counter electrodes was measured by a digital multimeter (34401A, Agilent) interfaced to a personal computer. After the electrochemical treatment, the samples were again rinsed with DI water and then dried in a nitrogen stream. All solutions were prepared from reagentgrade chemicals and DI water. The as-anodized WO3 films were crystallized by annealing at 450 ◦ C in air for 3 h with a heating and cooling rate of 5 ◦ C min−1 . 2.2. Characterization of nanoporous and compact WO3 films A field-emission scanning electron microscope (FESEM, Sirion 200, FEI, Holland) was employed for the structural and morphological characterization of the as-anodized films. The crystalline structure of the samples was measured by X-ray diffraction (XRD, D/Max2250, Rigaku, Japan). The chemical compositions of the films were identified by X-ray photoelectron spectroscopy (XPS, XSAM800, Kratos, Britain). 2.3. Measurement of photoelectrochemical and electronic properties A standard three-electrode electrochemical cell was used for the photoelectrochemical and electrochemical measurements, which were performed in 0.5 M H2 SO4 (pH = 0) by an electrochemical
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workstation (Zennium, Zahner, Germany). A Pt counter electrode and an Ag|AgCl|satd. KCl reference electrode along with the working electrode completed the cell setup. The potentials were swept linearly at a scan rate of 10 mV/s, and all potentials in the photocurrent data below were quoted with respect to this reference electrode. A 500 W Xe lamp (CHF-XM35, Trusttech, Beijing) served as the visible light source with an intensity of 100 mW/cm2 . A 400nm cutoff filter was place into the path of the Xe lamp to remove the UV irradiation. Photo-action spectra were recorded using an apparatus comprising a Xe lamp source (150 W, Oriel), a monochromator with bandwith of 10 nm. A focusing lens was equipped with monochromator to enhance the incident light power on the photoelectrode. For the IPCE calculation, the absolute intensity of the incident light was measured by a model BS2281 Si detector and the photocurrent was measured at 1.2 V vs. Ag/AgCl (pH = 0). Electrochemical impedance spectroscopy was applied to explore the conductivity of the WO3 electrode over the frequency range of 0.1–10 kHz.
3. Results and discussion 3.1. Process of anodization and the morphologies of the films The current–time behavior for the anodization of tungsten foils under different conditions is shown in Fig. 1(a). In the F− -free system, the dramatic current decay indicated that a typical filedassisted oxide formation took place according to reaction (1) W + 3H2 O = WO3 + 6H+ + 6e−
(1)
Fig. 1. (a) Current transient curves during anodization under different conditions, and the resulting morphologies of as-anodized films formed at (b) 50 V in 1 M Na2 SO4 , (c) 50 V in 0.5 wt.% NaF + 1 M Na2 SO4 , and (d) 20 V in 0.5 wt.% NaF + 1 M Na2 SO4 .
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W. Li et al. / Electrochimica Acta 56 (2010) 620–625
As shown in Fig. 1(b), a very smooth and compact resulting barrier oxide layers was observed. For the F− -containing electrolytes, after an initial decrease the current rose again, this means that the barrier oxide layers were rapidly dissolved into soluble fluoride complex by F− WO3 + 6H+ + nF− = [WFn ](n−6) + 3H2 O
(2)
Finally, a quasi-steady-state current was reached. At the quasisteady-state the dissolution and formation of the oxide layers were in dynamic equilibrium and self-organized nanoporous films with a pore diameter of 70–90 nm were formed (Fig. 1(c)). As shown in Fig. 1(a), the quasi-steady-state currents were determined by the concentrations of NaF in the electrolytes. Higher current densities were observed for higher F− concentrations due to higher dissolution rate of the oxide layers. These results were coincidence with Tsuchiya et al.’s [16] research, but in our research, the current densities were significant higher due to the addition of great amount of Na2 SO4 . The initialization condition of pore formation was the passivity breakdown. Hahn et al. [17] found that stepping the voltage, which was applied in our research, is more beneficial for the establishment of breakdown conditions. Fig. 1(d) proves that the sample anodized at 20 V had a rough surface with many randomly distributed breakdown sites over the film. In literature [16], however, WO3 film prepared at 20 V by voltage sweeping exhibited a uniform surface without apparent traces of F− attack. 3.2. Compositions and crystalline structures of the films Preliminary results showed that when tungsten foil is anodized in concentrated Na2 SO4 with 0.5 wt.% NaF at 50 V for 30 min, a clearly visible color layer formed on the surface. In order to examine the composition of the layers, XPS analysis was carried out. Fig. 2 shows the survey XPS spectrum obtained from the anodized compact and nanoporous films. Neither carbon contamination nor uptake of F elements from the electrolyte could be detected. The binding energies for W4f7/2 and Wf5/2 peak were observed at 35.7 and 37.8 eV for both samples, and for O 1s at 530.9 eV. This result shows clearly that the formed layers consist essentially of WO3 , which is in a good agreement with the literature for WO3 formed by anodization [18,19]. Fig. 3 shows the X-ray diffraction (XRD) patterns of nanoporous and compact WO3 films. For the as-anodized samples, only peaks from metallic tungsten substrates were detected. After annealing at 450 ◦ C for 3 h, the amorphous WO3 clearly transformed into a crystal. As shown in Fig. 2(b), in the magnified image of Fig. 2(a) in the range of 22–25◦ , films had significant diffraction peaks representing the characteristics of monoclinic WO3 (JCPDS 83-0950). The three peaks at ∼23◦ are attributed to the (0 0 2), (0 2 0), and (2 0 0) lattice planes of WO3 . Moreover, the annealed nanoporous and compact films had different preferential orientations of crystal faces. The peak at 2 = 23.12◦ indicates a fine preferential growth of the nanoporous film in the (0 0 2) direction, which in accordance with a study by Guo et al. [5]. However, for the compact film in our case, the preferential orientation was in the (2 0 0) planes instead of the (0 2 0) planes. 3.3. Photoelectrochemical properties of the films In an electrochemical cell, the photocurrent of a photoanode indirectly indicates the photocatalytic activity [20]. As recorded in Fig. 4(a), the practical photocurrent density of the nanoporous WO3 in a 0.5 M H2 SO4 solution (pH = 0) dramatically increased with an increase of bias potential (Eb ), but the photocurrent of the compact WO3 and dark current (dark currents of all the samples were similar) were very weak. The photocurrent density of the nanoporous
Fig. 2. (a) Survey XPS spectrum obtained from the surface of the anodized compact and nanoporous films. XPS spectra of (b) for the W 4f peak and (c) for the O 1s peak from spectra (a).
WO3 films was much higher than that of the compact films. Particularly, the photocurrent density at 1.6 V vs. Ag/AgCl of the annealed nanoporous WO3 film was measured to be 3.45 mA/cm2 , which was almost four times higher than that of the annealed compact film (0.73 mA/cm2 ). In addition, Guo et al. [5] reveal that the oxidization peak in the curve at around 0.13 V vs. Ag/AgCl may be ascribed to the oxidization of the tungsten substrate. In our unpublished
W. Li et al. / Electrochimica Acta 56 (2010) 620–625
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Fig. 3. (a) Complete image and (b) the magnified image of the rectangular area of XRD patterns for WO3 : (1) as-anodized compact film, (2) as-anodized nanoporous film, (3) annealed compact film, and (4) annealed nanoporous film.
research, however, we found that the WO3 films prepared by other methods without W substrate also show this peak, and this might be caused by an electrochromic reaction due to the injection of H+ . Fig. 4(b) shows the corresponding photoconversion (light energy to chemical energy conversion) efficiencies which were calculated as follows [21]: ε(%) =
|jp | × (1.23 − |Eb |) × 100 I0
where jp is the photocurrent density (mA/cm2 ) obtained under an applied bias Eb (V), and I0 (100 mW/cm2 ) is the power density of incident light. The maximum visible spectrum efficiency of annealed nanoporous WO3 was about 0.91% which was 4.5 times the value of that of the compact WO3 films (0.20%). For comparison, the highest visible light photoconversion efficiency of TiO2 nanotubes arrays was 0.6% as far as we know [13]. Different conditions make their photoconversion efficiency uncomparable with ours, but the conclusion that WO3 film has more favorable photoelectrochemical activity than TiO2 under visible radiation could be reached to some extent. The photocurrent response of film samples as a function of wavelength of incident light was measured at a potential of 1.2 V
Fig. 4. (a) Photocurrent density and (b) photoconversion efficiency of (1) asanodized compact WO3 film, (2) as-anodized nanoporous WO3 film, (3) annealed compact WO3 film, and (4) annealed nanoporous WO3 film.
Fig. 5. Photoaction spectra (IPCE vs. wavelength) of annealed nonoporous and compact WO3 films, recorded in 0.5 M H2 SO4 at 1.2 V vs. Ag/AgCl.
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W. Li et al. / Electrochimica Acta 56 (2010) 620–625
vs. Ag/AgCl in a 0.5 M H2 SO4 solution. Photoaction spectra for the annealed nanoporous and compact WO3 films are compared in Fig. 5, where the incident photon to electron conversion efficiency (IPCE) is plotted vs. wavelength IPCE (%) =
1240jp × 100 P
where jp is the photocurrent density (mA/cm2 ), P is the incident photon flux density at the photoelectrode location (mW/cm2 ) and is wavelength (nm). With an onset of IPCE at 480 nm, the light with wavelength longer than 500 nm cannot be absorbed ascribed to intrinsic band gap energy of WO3 . In agreement with the photocurrent density curves, the annealed nanoporous film sample shows extremely high photo-conversion, giving rise to a maximum IPCE value of 92% at 340 nm, while sample of compact film shows a poorer performance, with an IPCE value of 19% at 340 nm. This is due to the larger area of semiconductor–electrolyte junction of the nanoporous film. Note that the IPCE of ∼92% obtained here for the water photooxidation is higher than the corresponding value of ∼75% reported for WO3 nanoparticulate film reported so far. Santato et al. [22] found that the high IPCE on WO3 is ascribed to its more positive flat potential compared other semiconductor such as TiO2 , which is slow down of back charge-transfer reactions
with molecular oxygen and intermediates of water photooxidation reactions. Fig. 6 shows the interfacial charge–transfer resistances of the WO3 electrode. The interfacial charge–transfer resistance of all samples without irradiation tends to infinity, and the Faraday’s currents of WO3 electrode were small. Under irradiation, photo-generated carriers were separated by photovoltage, which reduced the interfacial charge–transfer resistance, meanwhile electronic conductivities of WO3 electrode enhanced. The interfacial charge–transfer resistance of the electrode dramatically decreased following this sequence: as-anodized compact film, as-anodized nanoporous film, annealed compact film, and annealed nanoporous film. The result was in agreement with photocurrents, indicating that improve of photoinduced water splitting activity was attributed to enhance of conductivity. As shown in Fig. 6(b), the diameter of the arc radius on the EIS Nynquist plot of the annealed nanoporous film was smaller than that of annealed compact film. The smaller arc radius of the EIS Nynquist plot suggested a higher efficiency of charge separation. The conductivities of the nanoporous WO3 films were much larger than those of the compact films, which could be explained by the large interfacial heterojunction area. The larger conductivity of the film, the more photocurrent was produced. Moreover, the photocurrents increased for compact and nanoporous films after annealing. Generally, researchers regard annealing as capable of decreasing lattice imperfection, which could serve as recombination centers of photoelectrons and holes [23]. In this way, conductivity is enhanced; that is, the photocurrent can be maintained at a high level. 4. Conclusions Self-organized WO3 nanoporous films with a pore diameter of 70–90 nm were prepared in neutral NaF/Na2 SO4 strong electrolytes by voltage stepping anodization. The as-anodized films were amorphous and converted to a monoclinic phase with preferential orientation in the (0 0 2) planes after annealing. The monoclinic nanoporous WO3 exhibited much higher photocurrents (3.45 mA/cm2 ) and photoconversion efficiency (0.91%) under visible light illumination compared with that of the compact WO3 (0.73 mA/cm2 and 0.20%). The annealed nanoporous WO3 photoanode shows an IPCE of ∼92%, which is higher than that of nanoparticle electrodes reported so far. Moreover, the annealing crystallization of the film also enriched the photoelectrochemical activities of the films, making them promising materials to exploit performance in photoelectronic device applications. Acknowledgements This research was supported by a grant from the National Nature Science Foundation of China (No. 51072232). This study was also partly supported by the State Key Program of National Natural Science Foundation of China (No. 20833009). Here, we are grateful for their financial supports. References [1] [2] [3] [4] [5] [6] [7]
Fig. 6. (a) The impedance spectra of as-anodized compact WO3 film, as-anodized nanoporous WO3 film, annealed compact WO3 film, and annealed nanoporous WO3 film and (b) the magnified image of the rectangular area. For comparison the impedance spectra of samples without irradiation are included.
[8]
A. Fujishima, K. Honda, Nature 238 (1972) 37. G. Hodes, D. Cahen, J. Manassen, Nature 260 (1976) 312. S. Berger, H. Tsuchiya, A. Ghicov, P. Schmuki, Appl. Phys. Lett. 88 (2006) 203119. N.R. de Tacconi, C.R. Chenthamarakshan, G. Yogeeswaran, A. Watcharenwong, R.S. de Zoysa, N.A. Basit, K. Rajeshwar, J. Phys. Chem. B 110 (2006) 25347. Y. Guo, X. Quan, N. Lu, H. Zhao, S. Chen, Environ. Sci. Technol. 41 (2007) 4422. Y.C. Nah, A. Ghicov, D. Kim, P. Schmuki, Electrochem. Commun. 10 (2008) 1777. A. Watcharenwong, W. Chanmanee, N.R. de Tacconi, C.R. Chenthamarakshan, P. Kajitvichyanukul, K. Rajeshwar, J. Electroanal. Chem. 612 (2008) 112. W. Chanmanee, A. Watcharenwong, C.R. Chenthamarakshan, P. Kajitvichyanukul, N.R. de Tacconi, K. Rajeshwar, Electrochem. Commun. 9 (2007) 2145.
W. Li et al. / Electrochimica Acta 56 (2010) 620–625 [9] A. Watcharenwong, W. Chanmanee, N.R. de Tacconi, C.R. Chenthamarakshan, P. Kajitvichyanukul, K. Rajeshwar, J. Mater. Res. 22 (2007) 3186. [10] J.M. Macak, H. Tsuchiya, L. Taveira, S. Aldabergerova, P. Schmuki, Angew. Chem. Int. Ed. 44 (2005) 7463. [11] .K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Lett. 5 (2005) 191. [12] C. Ruan, M. Paulose, O.K. Varghese, G.K. Mor, C.A. Grimes, J. Phys. Chem. B 109 (2005) 15754. [13] M. Paulose, G.K. Mor, O.K. Varghese, K. Shankar, C.A. Grimes, J. Photochem. Photobiol. A 178 (2006) 8. [14] G. Zhang, H. Huang, Y. Zhang, H.L.W. Chan, L. Zhou, Electrochem. Commun. 9 (2007) 2854. [15] N. Mukherjee, M. Paulose, O.K. Varghese, G.K. Mor, C.A. Grimes, J. Mater. Res. 18 (2003) 2296.
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[16] H. Tsuchiya, J.M. Macak, I. Sieber, L. Taveira, A. Ghicov, K. Sirotna, P. Schmuki, Electrochem. Commun. 7 (2005) 295. [17] R. Hahn, J.M. Macak, P. Schmuki, Electrochem. Commun. 9 (2007) 947. [18] C.S. Blackman, I.P. Parkin, Chem. Mater. 17 (2005) 1583. [19] M. Yang, N.K. Shrestha, P. Schmuki, Electrochem. Commun. 11 (2009) 1908. [20] S.H. Baeck, K.S. Choi, T.F. Jaramillo, G.D. Stucky, E.W. McFarland, Adv. Mater. 15 (2003) 1269. [21] Z. Chen, T.F. Jaramillo, T.G. Deutsch, A. Kleiman-Shwarsctein, A.J. Forman, N. Gaillard, R. Garland, K. Takanabe, C. Heske, M. Sunkara, E.W. McFarland, K. Domen, E.L. Miller, J.A. Turner, H.N. Dinh, J. Mater. Res. 25 (2010) 3. [22] C. Santato, M. Ulmann, J. Augustynski, J. Phys. Chem. B 105 (2001) 936. [23] H. Tsuchiya, J.M. Macak, A. Ghicov, A.S. Räder, L. Taveira, P. Schmuki, Corros. Sci. 49 (2007) 203.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Visible light photoelectrochemical responsiveness of self-organized nanoporous WO3 films Wenzhang Li a , Jie Li a,∗ , Xuan Wang a,b , Sha Luo a , Juan Xiao a , Qiyuan Chen a,∗ a Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, China, College of Chemistry and Chemical Engineering, Central South University, Changsha 410083 China b Shenyang Aluminum & Magnesium Engineering & Research Institute, Shenyang 110001 China
a r t i c l e
i n f o
Article history: Received 3 April 2010 Received in revised form 7 June 2010 Accepted 8 June 2010 Available online 15 June 2010 Keywords: Tungsten trioxide Self-organized nanoporous film Voltage stepping anodization Photoelectrochemical property Electronic conductivity
a b s t r a c t Visible light-responsive WO3 nanoporous films with preferential orientation of the (0 0 2) planes were prepared by anodization in neutral F− -containing strong electrolytes. The pore diameter of the selforganized structure was estimated to be in the region of 70–90 nm. Voltages were applied by stepping, which positively influenced passivity breakdown and played a significant role in the formation of selforganized nanoporous films. Under visible light irradiation, the photocurrent density (at 1.6 V vs. Ag/AgCl) and maximum photoconversion efficiency generated by the annealed nanoporous film were 3.45 mA/cm2 and 0.91%, respectively. The annealed nanoporous WO3 films show maximum incident photon-to-current conversion efficiency of 92% at 340 nm at 1.2 V vs. Ag/AgCl. These values are higher than that of annealed compact WO3 film due to the large interfacial heterojunction area. The photoelectrochemical activities and electronic conductivities were also enhanced by annealing crystallization, which removed the recombination centers. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction The use of tungsten oxide as a stable photocatalyst for watersplitting has been attracting much attention since the 1970s [1,2]. The formation of the nanoporous structure results in a dramatic increase in specific surface area, significantly broadening the applications of tungsten oxide [3–7]. Anodization is an effective and relatively simple technique that has been widely applied to the formation of TiO2 nanotubes [8–14]. WO3 nanoporous thin films were fabricated for the first time by the process of galvanostatic anodization in oxalic acid electrolyte by Grimes et al. [15]. However, obtaining films with entirely regular and self-organized porous morphologies was difficult. Schmuki et al. [3,6,16] prepared relatively uniform self-organized nanoporous WO3 films with an average pore diameter of 70–100 nm by potentiostatic anodization in NaF and NaF/H2 SO4 electrolytes. Highly ordered and smooth TiO2 nanotubes can also be obtained in F− -containing organic electrolytes [10,12]. Rajeshwar et al. [4,7] found that anodic nanoporous WO3 could not be fabricated in F− -containing organic electrolytes. A two-step anodization method was used by Guo et al. [5] to prepare self-assembled nanoporous WO3 films, while highly ordered TiO2 nanotubes were made through a multi-step anodization [14].
Moreover, WO3 films can also be formed by sweeping the cell voltage to the desired values. In contrast, Hahn et al. [17] attempted to fabricate nanotubes in F− -free electrolytes and concluded that passivity breakdown takes place more easily with voltage stepping. More efforts need to be done to modify and control the morphology of WO3 nanoporous films. Studies focus mainly on the photoelectrochemical activity of anodic WO3 nanoporous films under UV irradiation [3,4] and rarely on the visible light response property of WO3 nanoporous films. As a stable photoanode for water splitting, WO3 has been attracting much attention due to its satisfactory photoelectron transport properties and visible light responsiveness. Thus, intensive research on the visible light response property of WO3 is necessary. In this study, the possibility of achieving self-organized nanoporous WO3 films was explored by a simple voltage stepping anodization in neutral NaF/Na2 SO4 strong electrolytes. The electronic properties and responsiveness of WO3 films under visible light were also investigated. 2. Experimental 2.1. Preparation of nanoporous and compact WO3 films
∗ Corresponding authors. Tel.: +86 731 8887 7364; fax: +86 731 8887 9616. E-mail addresses: [email protected] (J. Li), [email protected] (Q. Chen). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.06.025
A two-electrode cell consisting of a tungsten working electrode and a Pt counter electrode was used. Tungsten foils (99.95% purity, Alfa Aesar) were successively sonicated in acetone, isopropanol,
W. Li et al. / Electrochimica Acta 56 (2010) 620–625
and methanol. The tungsten foils were rinsed in deionized (DI) water, dried in a nitrogen stream, and anodized in 1 M Na2 SO4 electrolyte with 0.5 wt.% NaF for 30 min. The cell voltage was applied by a single step from the open-circuit potential to a given value and was supplied by a direct current voltage source (DH1719A5, Dahua, China). The current between the working and counter electrodes was measured by a digital multimeter (34401A, Agilent) interfaced to a personal computer. After the electrochemical treatment, the samples were again rinsed with DI water and then dried in a nitrogen stream. All solutions were prepared from reagentgrade chemicals and DI water. The as-anodized WO3 films were crystallized by annealing at 450 ◦ C in air for 3 h with a heating and cooling rate of 5 ◦ C min−1 . 2.2. Characterization of nanoporous and compact WO3 films A field-emission scanning electron microscope (FESEM, Sirion 200, FEI, Holland) was employed for the structural and morphological characterization of the as-anodized films. The crystalline structure of the samples was measured by X-ray diffraction (XRD, D/Max2250, Rigaku, Japan). The chemical compositions of the films were identified by X-ray photoelectron spectroscopy (XPS, XSAM800, Kratos, Britain). 2.3. Measurement of photoelectrochemical and electronic properties A standard three-electrode electrochemical cell was used for the photoelectrochemical and electrochemical measurements, which were performed in 0.5 M H2 SO4 (pH = 0) by an electrochemical
621
workstation (Zennium, Zahner, Germany). A Pt counter electrode and an Ag|AgCl|satd. KCl reference electrode along with the working electrode completed the cell setup. The potentials were swept linearly at a scan rate of 10 mV/s, and all potentials in the photocurrent data below were quoted with respect to this reference electrode. A 500 W Xe lamp (CHF-XM35, Trusttech, Beijing) served as the visible light source with an intensity of 100 mW/cm2 . A 400nm cutoff filter was place into the path of the Xe lamp to remove the UV irradiation. Photo-action spectra were recorded using an apparatus comprising a Xe lamp source (150 W, Oriel), a monochromator with bandwith of 10 nm. A focusing lens was equipped with monochromator to enhance the incident light power on the photoelectrode. For the IPCE calculation, the absolute intensity of the incident light was measured by a model BS2281 Si detector and the photocurrent was measured at 1.2 V vs. Ag/AgCl (pH = 0). Electrochemical impedance spectroscopy was applied to explore the conductivity of the WO3 electrode over the frequency range of 0.1–10 kHz.
3. Results and discussion 3.1. Process of anodization and the morphologies of the films The current–time behavior for the anodization of tungsten foils under different conditions is shown in Fig. 1(a). In the F− -free system, the dramatic current decay indicated that a typical filedassisted oxide formation took place according to reaction (1) W + 3H2 O = WO3 + 6H+ + 6e−
(1)
Fig. 1. (a) Current transient curves during anodization under different conditions, and the resulting morphologies of as-anodized films formed at (b) 50 V in 1 M Na2 SO4 , (c) 50 V in 0.5 wt.% NaF + 1 M Na2 SO4 , and (d) 20 V in 0.5 wt.% NaF + 1 M Na2 SO4 .
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As shown in Fig. 1(b), a very smooth and compact resulting barrier oxide layers was observed. For the F− -containing electrolytes, after an initial decrease the current rose again, this means that the barrier oxide layers were rapidly dissolved into soluble fluoride complex by F− WO3 + 6H+ + nF− = [WFn ](n−6) + 3H2 O
(2)
Finally, a quasi-steady-state current was reached. At the quasisteady-state the dissolution and formation of the oxide layers were in dynamic equilibrium and self-organized nanoporous films with a pore diameter of 70–90 nm were formed (Fig. 1(c)). As shown in Fig. 1(a), the quasi-steady-state currents were determined by the concentrations of NaF in the electrolytes. Higher current densities were observed for higher F− concentrations due to higher dissolution rate of the oxide layers. These results were coincidence with Tsuchiya et al.’s [16] research, but in our research, the current densities were significant higher due to the addition of great amount of Na2 SO4 . The initialization condition of pore formation was the passivity breakdown. Hahn et al. [17] found that stepping the voltage, which was applied in our research, is more beneficial for the establishment of breakdown conditions. Fig. 1(d) proves that the sample anodized at 20 V had a rough surface with many randomly distributed breakdown sites over the film. In literature [16], however, WO3 film prepared at 20 V by voltage sweeping exhibited a uniform surface without apparent traces of F− attack. 3.2. Compositions and crystalline structures of the films Preliminary results showed that when tungsten foil is anodized in concentrated Na2 SO4 with 0.5 wt.% NaF at 50 V for 30 min, a clearly visible color layer formed on the surface. In order to examine the composition of the layers, XPS analysis was carried out. Fig. 2 shows the survey XPS spectrum obtained from the anodized compact and nanoporous films. Neither carbon contamination nor uptake of F elements from the electrolyte could be detected. The binding energies for W4f7/2 and Wf5/2 peak were observed at 35.7 and 37.8 eV for both samples, and for O 1s at 530.9 eV. This result shows clearly that the formed layers consist essentially of WO3 , which is in a good agreement with the literature for WO3 formed by anodization [18,19]. Fig. 3 shows the X-ray diffraction (XRD) patterns of nanoporous and compact WO3 films. For the as-anodized samples, only peaks from metallic tungsten substrates were detected. After annealing at 450 ◦ C for 3 h, the amorphous WO3 clearly transformed into a crystal. As shown in Fig. 2(b), in the magnified image of Fig. 2(a) in the range of 22–25◦ , films had significant diffraction peaks representing the characteristics of monoclinic WO3 (JCPDS 83-0950). The three peaks at ∼23◦ are attributed to the (0 0 2), (0 2 0), and (2 0 0) lattice planes of WO3 . Moreover, the annealed nanoporous and compact films had different preferential orientations of crystal faces. The peak at 2 = 23.12◦ indicates a fine preferential growth of the nanoporous film in the (0 0 2) direction, which in accordance with a study by Guo et al. [5]. However, for the compact film in our case, the preferential orientation was in the (2 0 0) planes instead of the (0 2 0) planes. 3.3. Photoelectrochemical properties of the films In an electrochemical cell, the photocurrent of a photoanode indirectly indicates the photocatalytic activity [20]. As recorded in Fig. 4(a), the practical photocurrent density of the nanoporous WO3 in a 0.5 M H2 SO4 solution (pH = 0) dramatically increased with an increase of bias potential (Eb ), but the photocurrent of the compact WO3 and dark current (dark currents of all the samples were similar) were very weak. The photocurrent density of the nanoporous
Fig. 2. (a) Survey XPS spectrum obtained from the surface of the anodized compact and nanoporous films. XPS spectra of (b) for the W 4f peak and (c) for the O 1s peak from spectra (a).
WO3 films was much higher than that of the compact films. Particularly, the photocurrent density at 1.6 V vs. Ag/AgCl of the annealed nanoporous WO3 film was measured to be 3.45 mA/cm2 , which was almost four times higher than that of the annealed compact film (0.73 mA/cm2 ). In addition, Guo et al. [5] reveal that the oxidization peak in the curve at around 0.13 V vs. Ag/AgCl may be ascribed to the oxidization of the tungsten substrate. In our unpublished
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Fig. 3. (a) Complete image and (b) the magnified image of the rectangular area of XRD patterns for WO3 : (1) as-anodized compact film, (2) as-anodized nanoporous film, (3) annealed compact film, and (4) annealed nanoporous film.
research, however, we found that the WO3 films prepared by other methods without W substrate also show this peak, and this might be caused by an electrochromic reaction due to the injection of H+ . Fig. 4(b) shows the corresponding photoconversion (light energy to chemical energy conversion) efficiencies which were calculated as follows [21]: ε(%) =
|jp | × (1.23 − |Eb |) × 100 I0
where jp is the photocurrent density (mA/cm2 ) obtained under an applied bias Eb (V), and I0 (100 mW/cm2 ) is the power density of incident light. The maximum visible spectrum efficiency of annealed nanoporous WO3 was about 0.91% which was 4.5 times the value of that of the compact WO3 films (0.20%). For comparison, the highest visible light photoconversion efficiency of TiO2 nanotubes arrays was 0.6% as far as we know [13]. Different conditions make their photoconversion efficiency uncomparable with ours, but the conclusion that WO3 film has more favorable photoelectrochemical activity than TiO2 under visible radiation could be reached to some extent. The photocurrent response of film samples as a function of wavelength of incident light was measured at a potential of 1.2 V
Fig. 4. (a) Photocurrent density and (b) photoconversion efficiency of (1) asanodized compact WO3 film, (2) as-anodized nanoporous WO3 film, (3) annealed compact WO3 film, and (4) annealed nanoporous WO3 film.
Fig. 5. Photoaction spectra (IPCE vs. wavelength) of annealed nonoporous and compact WO3 films, recorded in 0.5 M H2 SO4 at 1.2 V vs. Ag/AgCl.
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vs. Ag/AgCl in a 0.5 M H2 SO4 solution. Photoaction spectra for the annealed nanoporous and compact WO3 films are compared in Fig. 5, where the incident photon to electron conversion efficiency (IPCE) is plotted vs. wavelength IPCE (%) =
1240jp × 100 P
where jp is the photocurrent density (mA/cm2 ), P is the incident photon flux density at the photoelectrode location (mW/cm2 ) and is wavelength (nm). With an onset of IPCE at 480 nm, the light with wavelength longer than 500 nm cannot be absorbed ascribed to intrinsic band gap energy of WO3 . In agreement with the photocurrent density curves, the annealed nanoporous film sample shows extremely high photo-conversion, giving rise to a maximum IPCE value of 92% at 340 nm, while sample of compact film shows a poorer performance, with an IPCE value of 19% at 340 nm. This is due to the larger area of semiconductor–electrolyte junction of the nanoporous film. Note that the IPCE of ∼92% obtained here for the water photooxidation is higher than the corresponding value of ∼75% reported for WO3 nanoparticulate film reported so far. Santato et al. [22] found that the high IPCE on WO3 is ascribed to its more positive flat potential compared other semiconductor such as TiO2 , which is slow down of back charge-transfer reactions
with molecular oxygen and intermediates of water photooxidation reactions. Fig. 6 shows the interfacial charge–transfer resistances of the WO3 electrode. The interfacial charge–transfer resistance of all samples without irradiation tends to infinity, and the Faraday’s currents of WO3 electrode were small. Under irradiation, photo-generated carriers were separated by photovoltage, which reduced the interfacial charge–transfer resistance, meanwhile electronic conductivities of WO3 electrode enhanced. The interfacial charge–transfer resistance of the electrode dramatically decreased following this sequence: as-anodized compact film, as-anodized nanoporous film, annealed compact film, and annealed nanoporous film. The result was in agreement with photocurrents, indicating that improve of photoinduced water splitting activity was attributed to enhance of conductivity. As shown in Fig. 6(b), the diameter of the arc radius on the EIS Nynquist plot of the annealed nanoporous film was smaller than that of annealed compact film. The smaller arc radius of the EIS Nynquist plot suggested a higher efficiency of charge separation. The conductivities of the nanoporous WO3 films were much larger than those of the compact films, which could be explained by the large interfacial heterojunction area. The larger conductivity of the film, the more photocurrent was produced. Moreover, the photocurrents increased for compact and nanoporous films after annealing. Generally, researchers regard annealing as capable of decreasing lattice imperfection, which could serve as recombination centers of photoelectrons and holes [23]. In this way, conductivity is enhanced; that is, the photocurrent can be maintained at a high level. 4. Conclusions Self-organized WO3 nanoporous films with a pore diameter of 70–90 nm were prepared in neutral NaF/Na2 SO4 strong electrolytes by voltage stepping anodization. The as-anodized films were amorphous and converted to a monoclinic phase with preferential orientation in the (0 0 2) planes after annealing. The monoclinic nanoporous WO3 exhibited much higher photocurrents (3.45 mA/cm2 ) and photoconversion efficiency (0.91%) under visible light illumination compared with that of the compact WO3 (0.73 mA/cm2 and 0.20%). The annealed nanoporous WO3 photoanode shows an IPCE of ∼92%, which is higher than that of nanoparticle electrodes reported so far. Moreover, the annealing crystallization of the film also enriched the photoelectrochemical activities of the films, making them promising materials to exploit performance in photoelectronic device applications. Acknowledgements This research was supported by a grant from the National Nature Science Foundation of China (No. 51072232). This study was also partly supported by the State Key Program of National Natural Science Foundation of China (No. 20833009). Here, we are grateful for their financial supports. References [1] [2] [3] [4] [5] [6] [7]
Fig. 6. (a) The impedance spectra of as-anodized compact WO3 film, as-anodized nanoporous WO3 film, annealed compact WO3 film, and annealed nanoporous WO3 film and (b) the magnified image of the rectangular area. For comparison the impedance spectra of samples without irradiation are included.
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