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Photoelectrochemical performance of ZnO thin film anodes prepared by solution method
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 0 7 6 4 e2 0 7 7 0
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Photoelectrochemical performance of ZnO thin film anodes prepared by solution method Dang-Thanh Nguyen a, Eui-Chol Shin a, Dong-Chun Cho a, Ki-Woong Chae b, Jong-Sook Lee a,* a b
School of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea Department of Materials Engineering, Hoseo University, Asan 336-795, Republic of Korea
article info
abstract
Article history:
ZnO films on ITO substrates were grown in the nitrate solution at 90 C upon the seed layer
Available online 23 July 2014
prepared by bias-assisted deposition and thermal annealing at 500 C. High concentration of the nitrate solution of 0.5 M compared with 0.1 M results in the formation of three-
Keywords:
dimensional skeleton of the seed layer which coarsened into the brush-like ZnO nano-
ZnO
structure. The films, however, exhibit large leakage currents, which is firstly ascribed to the
Solution method
incomplete coverage of the ITO substrate. The formation of dense sublayer formation e.g.
Water splitting
by increased electrochemical deposition time and the nanostructured overlayer can result
MotteSchottky analysis
in high performance ZnO photoanode. For the frequency-independent estimation of the
Photoelectrochemical
flat-band potential of the non-ideal Schottky barrier behavior of the ZnO photoanodes,
Capacitance spectroscopy
capacitance spectroscopy is applied for the potential range corresponding to the reversebias regime of the Schottky barrier. The shift in the flat-band potential suggests the different ZnO interface characteristics resulted from the preparation conditions. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Zinc oxide has been widely investigated due to its properties of piezoelectricity, conductivity, optical absorption and emission, and catalytic activity. ZnO has been also investigated as photoanodes for the water splitting. Compared to other common n-type oxides for the photoelectrochemical (PEC) applications such as TiO2 and Fe2O3, much higher electronic mobility in ZnO reduces the recombination loss [1]. Moreover, nanostructured ZnO materials can be easily prepared and widely applied [2]. Although ZnO was the model PEC system already fifty years ago [3], and a variety of nanostructured ZnO can be
and has been prepared, not so many reports on the photoelectrochemical water splitting performance of nanostructured films can be found. This may be due to the chemical instability of ZnO in aqueous solution compared to TiO2 or Fe2O3, and probably because the PEC performance is not as good as expected without sufficient clarifications. The nanostructured ZnO films for PEC performance have been prepared by physical deposition methods such as sputtering [4e6] and PLD [7] or hydrothermal method [8]. It is also notable the reported IeV curves of nanostructured ZnO under illumination do not show the plateau behavior of the reverse-biased regime of an ideal Schottky barrier at the ZnO/electrolyte interface. Although ZnO
* Corresponding author. Tel.: þ82 62 530 1701; fax: þ82 62 530 1699. E-mail address: [email protected] (J.-S. Lee). http://dx.doi.org/10.1016/j.ijhydene.2014.07.010 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 0 7 6 4 e2 0 7 7 0
nanorods can be easily grown in aqueous solution in ambient condition [9,10], PEC performance has not been reported. In this work, we prepared porous but well-connected ZnO films by two-step solution method. The method is similar as previously reported [9,10]: First the seed layer is grown by electrochemical deposition and the further growth is performed in warm solution (90 C) without bias. By varying the experimental parameters, different morphologies of porous ZnO films were obtained. Photoelectrochemical performances of ZnO films are compared.
Experimental ZnO films on indium doped tin oxide (ITO) substrates were prepared in an aqueous solution similarly as in the previous report [9,10]. Electrochemical deposition in zinc nitrate was used to seed the ZnO template on ITO substrates at room temperature. An electric potential of 2.5 V was applied to the ITO substrate as cathode and a platinum plate was used as the anode in zinc nitrate and hexamine solution. After heat treated at 500 C for 30 min ZnO films were grown again in on 0.1 M zinc nitrate and hexamine at 90 C for 3 h. Two different types of films were prepared using 0.5 M and 0.1 M Zn(NO3)2 solutions, respectively.
Fig. 1 e SEM images of top (a) and cross-section (b) of ZnO seed layer by electrochemical deposition at 2.5 V bias in 0.5 M Zn(NO3)2 solution.
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The morphological characterization of the ZnO films was carried out by FE-SEM (S-4700 Hitachi, Japan). The phase analysis was performed by powder XRD (Rigaku-D/MaX2200 V). Photoelectrochemical properties of ZnO photoanodes were studied in a lab-made PEC test station. A miniature PEC cell was designed using a quartz optical cuvette with a drilled hole of the diameter 3 mm for the semiconductor electrodes fixed using O-ring from outside. Ag/AgCl reference electrode and Pt mesh counter electrode were used for a three-electrode configuration. Aqueous solution with 0.5 M NaClO4 (pH ¼ 6.8) were used as electrolyte. The potential was measured against an Ag/AgCl reference electrode. Electrochemical measurements was performed by a potentio/galvanostat with FRA (Autolab 302N, The Netherlands). Current-voltage characteristics were measured during the light-off and light-on cycles from 0.3 to 2.1 V (Ag/AgCl). The light intensity was set at 200 mW/cm2. Electrochemical impedance spectroscopy (EIS) was performed as a function of anodic potentials over the same range as IeV measurements at 0.1 V intervals. The incident-photon-to-current-conversion-efficiency (IPCE) measurement was carried out under monochromatic irradiation (Spectro, Korea) and a 150 W Xenon lamp light source (LS-150, ABET Technologies Inc., USA) for the wavelength range from 300 nm to 600 nm. Light intensity was calibrated using a silicon reference cell (BS-500 S/N 017, Bunkoukeiki Co., Ltd, Japan).
Fig. 2 e SEM images of top (a) and cross-section (b) of ZnO films after secondary growth of the seed layer in 0.5 M Zn(NO3)2 solution at 90 C for 3 h.
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Results and discussion Growth characteristics of ZnO films Formation of ZnO nanocrystalline seeds on ITO substrates by electrochemical deposition method has been previously reported [9,11]. Morphology of the ZnO films depends on the seed layer formation stage as well as heat-treatments and the secondary growth process. Fig. 1(a) shows characteristic morphology of the ZnO seed layer prepared in 0.5 M nitrate solution by applying bias of 2.5 V. Randomly oriented flakes are homogeneously developed and each flake is constituted by nanocrystallites of about 30 nm. The thickness of the film is about 2 mm. Further growth in the solution at 90 C results in the coarsening of the flakes as shown in Fig. 2. A close comparison of the seed layer and the film further grown in warm solution shows that the framework of the flakes of the seed layer is roughly maintained. The film thickness also roughly maintained. The flakes are coarsened as the nanocrystallites grew to the hexagonal rods or truncated hexagonal pyramids, which are connected with each other in the tetrahedral geometry. Although more irregular and in larger scale, the morphological feature is similar to the recently reported brush-like ZnO nanostructures [12,13]. The growth direction should be 〈001〉 cþ direction Flattened hexagonal rods as
Fig. 3 e SEM images of ZnO seed layer by electrochemical deposition at 2.5 V bias in 0.1 M Zn(NO3)2 (a) and after secondary growth (b).
Fig. 4 e Schematic illustration for the formation of threedimensional skeleton structure assisted by the electric field in the electrochemical deposition.
indicated in Fig. 2(a) can be ascribed to the geometrical constriction in the growth process. Lower concentration of 0.1 M nitrate solution for the preparation of the seed layer as well as for the secondary growth resulted in the different morphology as shown in Fig. 3. More conventional island-like clusters are grown into the nodular microstructure. Regular hexagonal rods of similar size about 80e100 nm are developed. Cross section in Fig. 3 indicates also the presence of a dense layer with columnar structures, which should be aligned ZnO rods, of thickness around 1 mm. The growth of caxis oriented ZnO nanorods is supposed to be further promoted by the hexamine additive [14]. They are shown to be preferentially attached on the non-polar surfaces, thus directing the kinetic shape of ZnO crystals. Although the final thicknesses of the films are similar, ~2 mm, the films in 0.1 M solution were increased in thickness with growth time from the thin, insufficiently covered seed layer. That is, the growth can be said roughly two-dimensional
Fig. 5 e XRD patterns of ITO after heat-treatment at 500 C (a) and ZnO films prepared by the electrochemical deposition in 0.5 M Zn(NO3)2 and heat-treated at 500 C with the microstructure shown in Fig. 1 (b) and further grown in 90 C solution, Fig. 2 (c), and grown using 0.1 M solution, Fig. 3 (d).
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in microscale. However, the flake-like structure already formed the framework of the thickness of the final films structure. The mechanism is schematically represented in Fig. 4. The three-dimensional skeleton structure is suggested to be promoted by the DC bias. Protruding regions, indicated by the atoms numbered 1, are under a higher effective field than the valley region of the ITO/electrolyte interface, indicated by atoms numbered 2. Further growth results in the three-dimensional flake-like porous structure as shown in Fig. 1. The process may be compared to the well-known electrically driven dendritic growth. The difference between 0.1 M solution and 0.5 M solution may be also ascribed to the effective field strength in the solution due to the difference in the conductivity of the electrolyte, which affect the electric field strength, as well as the difference in the availability of the Zn ion sources. Not only the difference in the porosity of ZnO films, the state of the interface between ZnO and ITO substrate should be noted. The ITO surface seems not fully covered in the film grown in 0.5 M solution, as schematically illustrated in Fig. 4. As well as in the growth process, in the photoelectrochemical experiments the ITO substrate will be then directly in contact with the 0.5 M NaClO4 solution. This is shown to unfavorably affect the photoelectrochemical performance as described in the next section. Fig. 5 shows the XRD patterns of ITO substrates and different ZnO films shown in Figs. 1e3. The peaks of annnealed ITO film are indexed according to JCPDS, No. 06-0416, In2O3 crystal. The diffraction patterns of ZnO films can be completely indexed by ITO and ZnO peaks. The XRD also semi-quantitatively explains the microstructural aspects of ZnO films. The low intensity of ZnO peaks of the film as shown the micrographs in Fig. 1 compared to the films grown further (c,d) reflects the low amount of ZnO material. As the ZnO films are thicker and denser upon secondary growth, the ITO peaks becomes weaker. It should be noted that the preferential orientation of the film grown in 0.1 M solution in Fig. 3 shown in the dense layers of the columnar structure is also indicated by the strong (002) peak in (d). On the other hand, the flake-like and brush-like structure with randomly oriented ZnO nanorods in Figs. 1 and 2 shows very weak (002) reflections and very strong (100) and (101) reflections. Since the PEC performance was found to be critically affected by the coverage of the substrate as will be shown in the next section, electrochemical deposition time was increased to 10 min from 5 min with dc bias of 2.5 V in 0.1 M nitrate solution as for the sample shown in Fig. 3. Fig. 6 shows the SEM images of the sample. The samples are inhomogeneous in the macroscopic scale with the mogule-like aggregates (a). The high magnification figures in b, c, and d, can be compared with the features of the samples shown in Fig. 3, prepared using the shorter seed deposition period of 5 min with other conditions the same. The film with the longer seed deposition time indicates the presence of the dense sublayer of aligned ZnO nanorods. The ITO substrate is supposed to be more completely covered. This aspect is related with the PEC performance. Fig. 6 e SEM images of the ZnO film after secondary growth of the seed layer deposition at 2.5 V in 0.1 M solution as for
the sample in Fig. 3 but with increased seed deposition time of 10 min in different magnifications.
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Photoelectrochemical characteristics Fig. 7 shows the IeV characteristics of ZnO films as photoanodes. The three-dimensional skeleton structure and high surface area prepared in 0.5 M solution in (a) exhibited rather a poor electrochemical performance compared with the film grown in 0.1 M solution (b). From the breakdown behavior
Fig. 7 e The IeV curves (left axis) and MotteSchottky plots (right axis) of ZnO nanorods prepared in 0.5 M (a) and in 0.1 M nitrate solution (b) and with extended seed layer formation time of 10 min (from 5 min).
above 0.6 V and high leakage currents the poor performance is firstly ascribed to the incomplete coverage of ITO substrates. Single crystal samples as well as dense nanorods films showed negligible dark currents and the flat photocurrent curves [15]. This aspect is also demonstrated by the sample prepared with extended seed layer deposition time of 10 min shown in Fig. 7(c). Indeed the sample exhibits much reduced leakage current under dark condition and flatter photocurrent of the ideal Schottky barrier. The photocurrents are not however substantially increased but more growth of the ZnO materials on top of the dense sublayer is expected to the increase photocurrents. It should be mentioned that the sloped photocurrent-potential appears to reflect the leakage current in dark condition as shown in Fig. 7(a,b). In the literature the photocurrent-potential curves of ZnO nanostructures exhibit the sloped behavior, although the dark current does not show the appreciable leakage [4e8]. The magnification of the scale or the logarithmic presentation may indicate the similarly leaky behavior of the dark IeV curves. The impedance spectra in Fig. 8 shows the typical behavior near the flat-band potential or cathodic region (a), reversebiased or anodic region (b,c) and the pre-breakdown region (d). The numbers are logarithmic frequency values. Very high capacitance effects associated with the response near flatband potential are indicated by the frequency values. The capacitive effects can be explained by the chemical capacitance of the electronic charge carriers [16]. Therefore a reliable evaluation of the Schottky barriers at the ZnO-electrolyte interface should be done for the voltage range 0.2e0.6 V. Even for the range the measured AC response cannot be assumed as an ideal Schottky barrier. Conventional monofrequency MotteSchottky (MS) analysis thus results in the different capacitanceevoltage (CeV) characteristics depend on the frequency values. This makes difficult to discuss or to compare the flat-band potential or donor density evaluated by MS analysis. The impedance modeling for the spectra as shown in Fig. 8 often employs a constant phase element (CPE) with complex capacitance QðjuÞa1 for the non-ideal capacitance behavior. The capacitance values depend on the frequencies for as1 and the parameter Q could not be taken as the representative capacitance. A capacitance spectroscopy as shown in Fig. 9 is thus suggested for the evaluation of the frequency-independent Schottky capacitance. Similarly as the evaluation of the resistance components in the impedance spectra shown in Fig. 8, the magnitude of the complex capacitance arcs can be evaluated to determine parametric capacitance values. The values may also vary depend on the modeling, but for the present samples well-defined values can be obtained from the real capacitance values at the cusps. The capacitance values were similarly evaluated for the flat-band/ accumulation region and the pre-breakdown region. The characteristic capacitance behavior in the different regimes is clearly shown in the MotteSchottky plots in Fig. 7. The evaluation using the reverse-biased region as indicated gives the flat-band potential of ZnO film prepared in 0.5 M and 0.1 M solution þ0.276(þ0.0724) V, 0.270(0.552) V, and 0.009(0.213) V respectively, vs. Ag/AgCl (SHE). The apparent donor density ND from the slope assuming the nominal electrode area of p(3/2)2 mm2 are 1.73 1015 cm3,
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Fig. 8 e Impedance spectra at different bias voltages of the ZnO film prepared in 0.5 M Zn(NO3)2 solution. The numbers are logarithmic frequency values.
6.61 1014 cm3, and 3.41 1015 cm3, respectively. Since the effective semiconductor/electrolyte interface area is not known, the significance of donor concentration for the nanostructure or porous structure is difficult to quantitatively discuss. The film of flake-like skeleton with the brush-like ZnO nanorods prepared in higher concentration of 0.5 M exhibits the higher flatband potential than the samples prepared in 0.1 M. For the water splitting without external bias the high flatband potential is desirable. The origin of the different flatband potentials should be further investigated. It should be emphasized again that the issue of the reliable estimation of the flatband potential needs to be first addressed. The IPCE values measured at 0 V vs. Ag/AgCl also indicate the higher performance of the dense film grown in 0.1 M solution near the band gap wavelength (see Fig. 10). It should
Fig. 9 e Complex plane representation of the capacitance of the AC response shown in Fig. 8 of the ZnO film grown in 0.5 M nitrate solution.
be noted that the measurement potential is located negative to the flat-band potential, in the accumulated regime of the semiconductor/electrolyte interface. The performance may be then comparable to the report where IPCE on ZnO nanostructure was also taken near the transition voltage [7]. Since IPCE corresponds to the difference between the current under illumination and in dark condition for the monochromatic light, IPCE performance at the single potential value may not properly represent the overall photoelectrochemical performance. A much higher efficiency reported [8] for the ZnO nanostructure with IeV performance comparable to the present work may be mainly attributed to the potential range for the high photocurrents. An enhanced visible absorption may occur for the porous brush-like structure as indicated in the wavelength near 400 nm. Possibility of the high performance of the brush-like structure in view of the large surface area still exists.
Fig. 10 e IPCE measured at 0 V vs. Ag/AgCl SCE in 0.5 M NaClO4.
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Conclusion A three-dimensional skeleton of the seed layer which coarsens into the brush-like nanostructured ZnO film can be prepared by applying high DC bias in high concentration of nitrate solution in the electrochemical deposition. The photoelectrochemical performance of the ZnO film was however limited due to the insufficient coverage of the ITO substrate. Formation of dense sublayer by increasing electrochemical deposition time reduced the leakage substantially. In contrast to the conventional monofrequency MotteSchottky analysis, complex plane representation of the capacitance is suggested for the evaluation of the frequency-independent Schottky capacitance parameters. The AC characteristics in the near flat-band potential and in the pre-breakdown region should be distinguished from the Schottky barrier behavior. Thus obtained well-defined capacitance values can be used to compare the flat-band potential values which may be related with the characteristics of the semiconductoreelectrolyte interface depending on the preparation condition.
Acknowledgment This work was financially supported from World Class University (WCU) program (R32-2009-000-20074-0) and Priority Research Centers Program (2010-0029626) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) of the Korean government.
references
[1] Fortunato EM, Barquinha PM, Pimentel AC, Gonc¸alves AM, Marques AJ, Martins RF, et al. Wide-bandgap high-mobility ZnO thin-film transistors produced at room temperature. Appl Phys Lett 2004;85(13):2541e3. [2] Djurivsic AB, Chen X, Leung YH, Ng AMC. ZnO nanostructures: growth, properties and applications. J Mater Chem 2012;22(14):6526e35.
[3] Gerischer H. In: Delahay P, editor, editors. Advances in electrochemistry and electrochemical engineering. New York: Interscience; 1961. [4] Ahn KS, Shet S, Deutsch T, Jiang CS, Yan Y, Al-Jassim M, et al. Enhancement of photoelectrochemical response by aligned nanorods in ZnO thin films. J Power Sources 2008;176(1):387e92. [5] Shet S, Wang H, Deutsch T, Ravindra N, Yan Y, Turner J, et al. Synthesis of ZnO nanostructures and their influence on photoelectrochemical response for solar driven water splitting to produce hydrogen. Ceram Trans 2010;229:143e53. [6] Shet S. Zinc oxide (ZnO) nanostructures for photoelectrochemical water splitting application. ECS Trans 2011;33(38):15e25. [7] Wolcott A, Smith WA, Kuykendall TR, Zhao Y, Zhang JZ. Photoelectrochemical study of nanostructured ZnO thin films for hydrogen generation from water splitting. Adv Funct Mater 2009;19(12):1849e56. [8] Yang X, Wolcott A, Wang G, Sobo A, Fitzmorris RC, Qian F, et al. Nitrogen-doped ZnO nanowire arrays for photoelectrochemical water splitting. Nano Lett 2009;9(6):2331e6. [9] Cao B, Cai W, Duan G, Li Y, Zhao Q, Yu D. A template-free electrochemical deposition route to ZnO nanoneedle arrays and their optical and field emission properties. Nanotechnology 2005;16(11):2567. [10] Chae KW, Zhang Q, Kim JS, Jeong YH, Cao G. Lowtemperature solution growth of ZnO nanotube arrays. Beilstein J Nanotech 2010;1(1):128e34. [11] Pauporte T, Lincot D. Heteroepitaxial electrodeposition of zinc oxide films on gallium nitride. Appl Phys Lett 1999;75(24):3817e9. [12] Wang J, Zhuang H, Li J, Xu P. Synthesis, morphology and growth mechanism of brush-like ZnO nanostructures. Appl Surf Sci 2011;257(6):2097e101. [13] Xiang B, Wang P, Zhang X, Dayeh SA, Aplin DP, Soci C, et al. Rational synthesis of p-type zinc oxide nanowire arrays using simple chemical vapor deposition. Nano Lett 2007;7(2):323e8. [14] Sugunan A, Warad HC, Boman M, Dutta J. Zinc oxide nanowires in chemical bath on seeded substrates: role of hexamine. J Sol-Gel Sci Technol 2006;39(1):49e56. [15] E.-C. Shin, D.T. Nguyen, D.-C. Cho, J.-S. Lee, [In preparation]. I, [16] Fabregat-Santiago F, Garcia-Belmonte G, Mora-Sero Bisquert J. Characterization of nanostructured hybrid and organic solar cells by impedance spectroscopy. Phys Chem Chem Phys 2011;13(20):9083e118.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Photoelectrochemical performance of ZnO thin film anodes prepared by solution method Dang-Thanh Nguyen a, Eui-Chol Shin a, Dong-Chun Cho a, Ki-Woong Chae b, Jong-Sook Lee a,* a b
School of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea Department of Materials Engineering, Hoseo University, Asan 336-795, Republic of Korea
article info
abstract
Article history:
ZnO films on ITO substrates were grown in the nitrate solution at 90 C upon the seed layer
Available online 23 July 2014
prepared by bias-assisted deposition and thermal annealing at 500 C. High concentration of the nitrate solution of 0.5 M compared with 0.1 M results in the formation of three-
Keywords:
dimensional skeleton of the seed layer which coarsened into the brush-like ZnO nano-
ZnO
structure. The films, however, exhibit large leakage currents, which is firstly ascribed to the
Solution method
incomplete coverage of the ITO substrate. The formation of dense sublayer formation e.g.
Water splitting
by increased electrochemical deposition time and the nanostructured overlayer can result
MotteSchottky analysis
in high performance ZnO photoanode. For the frequency-independent estimation of the
Photoelectrochemical
flat-band potential of the non-ideal Schottky barrier behavior of the ZnO photoanodes,
Capacitance spectroscopy
capacitance spectroscopy is applied for the potential range corresponding to the reversebias regime of the Schottky barrier. The shift in the flat-band potential suggests the different ZnO interface characteristics resulted from the preparation conditions. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Zinc oxide has been widely investigated due to its properties of piezoelectricity, conductivity, optical absorption and emission, and catalytic activity. ZnO has been also investigated as photoanodes for the water splitting. Compared to other common n-type oxides for the photoelectrochemical (PEC) applications such as TiO2 and Fe2O3, much higher electronic mobility in ZnO reduces the recombination loss [1]. Moreover, nanostructured ZnO materials can be easily prepared and widely applied [2]. Although ZnO was the model PEC system already fifty years ago [3], and a variety of nanostructured ZnO can be
and has been prepared, not so many reports on the photoelectrochemical water splitting performance of nanostructured films can be found. This may be due to the chemical instability of ZnO in aqueous solution compared to TiO2 or Fe2O3, and probably because the PEC performance is not as good as expected without sufficient clarifications. The nanostructured ZnO films for PEC performance have been prepared by physical deposition methods such as sputtering [4e6] and PLD [7] or hydrothermal method [8]. It is also notable the reported IeV curves of nanostructured ZnO under illumination do not show the plateau behavior of the reverse-biased regime of an ideal Schottky barrier at the ZnO/electrolyte interface. Although ZnO
* Corresponding author. Tel.: þ82 62 530 1701; fax: þ82 62 530 1699. E-mail address: [email protected] (J.-S. Lee). http://dx.doi.org/10.1016/j.ijhydene.2014.07.010 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 0 7 6 4 e2 0 7 7 0
nanorods can be easily grown in aqueous solution in ambient condition [9,10], PEC performance has not been reported. In this work, we prepared porous but well-connected ZnO films by two-step solution method. The method is similar as previously reported [9,10]: First the seed layer is grown by electrochemical deposition and the further growth is performed in warm solution (90 C) without bias. By varying the experimental parameters, different morphologies of porous ZnO films were obtained. Photoelectrochemical performances of ZnO films are compared.
Experimental ZnO films on indium doped tin oxide (ITO) substrates were prepared in an aqueous solution similarly as in the previous report [9,10]. Electrochemical deposition in zinc nitrate was used to seed the ZnO template on ITO substrates at room temperature. An electric potential of 2.5 V was applied to the ITO substrate as cathode and a platinum plate was used as the anode in zinc nitrate and hexamine solution. After heat treated at 500 C for 30 min ZnO films were grown again in on 0.1 M zinc nitrate and hexamine at 90 C for 3 h. Two different types of films were prepared using 0.5 M and 0.1 M Zn(NO3)2 solutions, respectively.
Fig. 1 e SEM images of top (a) and cross-section (b) of ZnO seed layer by electrochemical deposition at 2.5 V bias in 0.5 M Zn(NO3)2 solution.
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The morphological characterization of the ZnO films was carried out by FE-SEM (S-4700 Hitachi, Japan). The phase analysis was performed by powder XRD (Rigaku-D/MaX2200 V). Photoelectrochemical properties of ZnO photoanodes were studied in a lab-made PEC test station. A miniature PEC cell was designed using a quartz optical cuvette with a drilled hole of the diameter 3 mm for the semiconductor electrodes fixed using O-ring from outside. Ag/AgCl reference electrode and Pt mesh counter electrode were used for a three-electrode configuration. Aqueous solution with 0.5 M NaClO4 (pH ¼ 6.8) were used as electrolyte. The potential was measured against an Ag/AgCl reference electrode. Electrochemical measurements was performed by a potentio/galvanostat with FRA (Autolab 302N, The Netherlands). Current-voltage characteristics were measured during the light-off and light-on cycles from 0.3 to 2.1 V (Ag/AgCl). The light intensity was set at 200 mW/cm2. Electrochemical impedance spectroscopy (EIS) was performed as a function of anodic potentials over the same range as IeV measurements at 0.1 V intervals. The incident-photon-to-current-conversion-efficiency (IPCE) measurement was carried out under monochromatic irradiation (Spectro, Korea) and a 150 W Xenon lamp light source (LS-150, ABET Technologies Inc., USA) for the wavelength range from 300 nm to 600 nm. Light intensity was calibrated using a silicon reference cell (BS-500 S/N 017, Bunkoukeiki Co., Ltd, Japan).
Fig. 2 e SEM images of top (a) and cross-section (b) of ZnO films after secondary growth of the seed layer in 0.5 M Zn(NO3)2 solution at 90 C for 3 h.
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Results and discussion Growth characteristics of ZnO films Formation of ZnO nanocrystalline seeds on ITO substrates by electrochemical deposition method has been previously reported [9,11]. Morphology of the ZnO films depends on the seed layer formation stage as well as heat-treatments and the secondary growth process. Fig. 1(a) shows characteristic morphology of the ZnO seed layer prepared in 0.5 M nitrate solution by applying bias of 2.5 V. Randomly oriented flakes are homogeneously developed and each flake is constituted by nanocrystallites of about 30 nm. The thickness of the film is about 2 mm. Further growth in the solution at 90 C results in the coarsening of the flakes as shown in Fig. 2. A close comparison of the seed layer and the film further grown in warm solution shows that the framework of the flakes of the seed layer is roughly maintained. The film thickness also roughly maintained. The flakes are coarsened as the nanocrystallites grew to the hexagonal rods or truncated hexagonal pyramids, which are connected with each other in the tetrahedral geometry. Although more irregular and in larger scale, the morphological feature is similar to the recently reported brush-like ZnO nanostructures [12,13]. The growth direction should be 〈001〉 cþ direction Flattened hexagonal rods as
Fig. 3 e SEM images of ZnO seed layer by electrochemical deposition at 2.5 V bias in 0.1 M Zn(NO3)2 (a) and after secondary growth (b).
Fig. 4 e Schematic illustration for the formation of threedimensional skeleton structure assisted by the electric field in the electrochemical deposition.
indicated in Fig. 2(a) can be ascribed to the geometrical constriction in the growth process. Lower concentration of 0.1 M nitrate solution for the preparation of the seed layer as well as for the secondary growth resulted in the different morphology as shown in Fig. 3. More conventional island-like clusters are grown into the nodular microstructure. Regular hexagonal rods of similar size about 80e100 nm are developed. Cross section in Fig. 3 indicates also the presence of a dense layer with columnar structures, which should be aligned ZnO rods, of thickness around 1 mm. The growth of caxis oriented ZnO nanorods is supposed to be further promoted by the hexamine additive [14]. They are shown to be preferentially attached on the non-polar surfaces, thus directing the kinetic shape of ZnO crystals. Although the final thicknesses of the films are similar, ~2 mm, the films in 0.1 M solution were increased in thickness with growth time from the thin, insufficiently covered seed layer. That is, the growth can be said roughly two-dimensional
Fig. 5 e XRD patterns of ITO after heat-treatment at 500 C (a) and ZnO films prepared by the electrochemical deposition in 0.5 M Zn(NO3)2 and heat-treated at 500 C with the microstructure shown in Fig. 1 (b) and further grown in 90 C solution, Fig. 2 (c), and grown using 0.1 M solution, Fig. 3 (d).
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in microscale. However, the flake-like structure already formed the framework of the thickness of the final films structure. The mechanism is schematically represented in Fig. 4. The three-dimensional skeleton structure is suggested to be promoted by the DC bias. Protruding regions, indicated by the atoms numbered 1, are under a higher effective field than the valley region of the ITO/electrolyte interface, indicated by atoms numbered 2. Further growth results in the three-dimensional flake-like porous structure as shown in Fig. 1. The process may be compared to the well-known electrically driven dendritic growth. The difference between 0.1 M solution and 0.5 M solution may be also ascribed to the effective field strength in the solution due to the difference in the conductivity of the electrolyte, which affect the electric field strength, as well as the difference in the availability of the Zn ion sources. Not only the difference in the porosity of ZnO films, the state of the interface between ZnO and ITO substrate should be noted. The ITO surface seems not fully covered in the film grown in 0.5 M solution, as schematically illustrated in Fig. 4. As well as in the growth process, in the photoelectrochemical experiments the ITO substrate will be then directly in contact with the 0.5 M NaClO4 solution. This is shown to unfavorably affect the photoelectrochemical performance as described in the next section. Fig. 5 shows the XRD patterns of ITO substrates and different ZnO films shown in Figs. 1e3. The peaks of annnealed ITO film are indexed according to JCPDS, No. 06-0416, In2O3 crystal. The diffraction patterns of ZnO films can be completely indexed by ITO and ZnO peaks. The XRD also semi-quantitatively explains the microstructural aspects of ZnO films. The low intensity of ZnO peaks of the film as shown the micrographs in Fig. 1 compared to the films grown further (c,d) reflects the low amount of ZnO material. As the ZnO films are thicker and denser upon secondary growth, the ITO peaks becomes weaker. It should be noted that the preferential orientation of the film grown in 0.1 M solution in Fig. 3 shown in the dense layers of the columnar structure is also indicated by the strong (002) peak in (d). On the other hand, the flake-like and brush-like structure with randomly oriented ZnO nanorods in Figs. 1 and 2 shows very weak (002) reflections and very strong (100) and (101) reflections. Since the PEC performance was found to be critically affected by the coverage of the substrate as will be shown in the next section, electrochemical deposition time was increased to 10 min from 5 min with dc bias of 2.5 V in 0.1 M nitrate solution as for the sample shown in Fig. 3. Fig. 6 shows the SEM images of the sample. The samples are inhomogeneous in the macroscopic scale with the mogule-like aggregates (a). The high magnification figures in b, c, and d, can be compared with the features of the samples shown in Fig. 3, prepared using the shorter seed deposition period of 5 min with other conditions the same. The film with the longer seed deposition time indicates the presence of the dense sublayer of aligned ZnO nanorods. The ITO substrate is supposed to be more completely covered. This aspect is related with the PEC performance. Fig. 6 e SEM images of the ZnO film after secondary growth of the seed layer deposition at 2.5 V in 0.1 M solution as for
the sample in Fig. 3 but with increased seed deposition time of 10 min in different magnifications.
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Photoelectrochemical characteristics Fig. 7 shows the IeV characteristics of ZnO films as photoanodes. The three-dimensional skeleton structure and high surface area prepared in 0.5 M solution in (a) exhibited rather a poor electrochemical performance compared with the film grown in 0.1 M solution (b). From the breakdown behavior
Fig. 7 e The IeV curves (left axis) and MotteSchottky plots (right axis) of ZnO nanorods prepared in 0.5 M (a) and in 0.1 M nitrate solution (b) and with extended seed layer formation time of 10 min (from 5 min).
above 0.6 V and high leakage currents the poor performance is firstly ascribed to the incomplete coverage of ITO substrates. Single crystal samples as well as dense nanorods films showed negligible dark currents and the flat photocurrent curves [15]. This aspect is also demonstrated by the sample prepared with extended seed layer deposition time of 10 min shown in Fig. 7(c). Indeed the sample exhibits much reduced leakage current under dark condition and flatter photocurrent of the ideal Schottky barrier. The photocurrents are not however substantially increased but more growth of the ZnO materials on top of the dense sublayer is expected to the increase photocurrents. It should be mentioned that the sloped photocurrent-potential appears to reflect the leakage current in dark condition as shown in Fig. 7(a,b). In the literature the photocurrent-potential curves of ZnO nanostructures exhibit the sloped behavior, although the dark current does not show the appreciable leakage [4e8]. The magnification of the scale or the logarithmic presentation may indicate the similarly leaky behavior of the dark IeV curves. The impedance spectra in Fig. 8 shows the typical behavior near the flat-band potential or cathodic region (a), reversebiased or anodic region (b,c) and the pre-breakdown region (d). The numbers are logarithmic frequency values. Very high capacitance effects associated with the response near flatband potential are indicated by the frequency values. The capacitive effects can be explained by the chemical capacitance of the electronic charge carriers [16]. Therefore a reliable evaluation of the Schottky barriers at the ZnO-electrolyte interface should be done for the voltage range 0.2e0.6 V. Even for the range the measured AC response cannot be assumed as an ideal Schottky barrier. Conventional monofrequency MotteSchottky (MS) analysis thus results in the different capacitanceevoltage (CeV) characteristics depend on the frequency values. This makes difficult to discuss or to compare the flat-band potential or donor density evaluated by MS analysis. The impedance modeling for the spectra as shown in Fig. 8 often employs a constant phase element (CPE) with complex capacitance QðjuÞa1 for the non-ideal capacitance behavior. The capacitance values depend on the frequencies for as1 and the parameter Q could not be taken as the representative capacitance. A capacitance spectroscopy as shown in Fig. 9 is thus suggested for the evaluation of the frequency-independent Schottky capacitance. Similarly as the evaluation of the resistance components in the impedance spectra shown in Fig. 8, the magnitude of the complex capacitance arcs can be evaluated to determine parametric capacitance values. The values may also vary depend on the modeling, but for the present samples well-defined values can be obtained from the real capacitance values at the cusps. The capacitance values were similarly evaluated for the flat-band/ accumulation region and the pre-breakdown region. The characteristic capacitance behavior in the different regimes is clearly shown in the MotteSchottky plots in Fig. 7. The evaluation using the reverse-biased region as indicated gives the flat-band potential of ZnO film prepared in 0.5 M and 0.1 M solution þ0.276(þ0.0724) V, 0.270(0.552) V, and 0.009(0.213) V respectively, vs. Ag/AgCl (SHE). The apparent donor density ND from the slope assuming the nominal electrode area of p(3/2)2 mm2 are 1.73 1015 cm3,
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Fig. 8 e Impedance spectra at different bias voltages of the ZnO film prepared in 0.5 M Zn(NO3)2 solution. The numbers are logarithmic frequency values.
6.61 1014 cm3, and 3.41 1015 cm3, respectively. Since the effective semiconductor/electrolyte interface area is not known, the significance of donor concentration for the nanostructure or porous structure is difficult to quantitatively discuss. The film of flake-like skeleton with the brush-like ZnO nanorods prepared in higher concentration of 0.5 M exhibits the higher flatband potential than the samples prepared in 0.1 M. For the water splitting without external bias the high flatband potential is desirable. The origin of the different flatband potentials should be further investigated. It should be emphasized again that the issue of the reliable estimation of the flatband potential needs to be first addressed. The IPCE values measured at 0 V vs. Ag/AgCl also indicate the higher performance of the dense film grown in 0.1 M solution near the band gap wavelength (see Fig. 10). It should
Fig. 9 e Complex plane representation of the capacitance of the AC response shown in Fig. 8 of the ZnO film grown in 0.5 M nitrate solution.
be noted that the measurement potential is located negative to the flat-band potential, in the accumulated regime of the semiconductor/electrolyte interface. The performance may be then comparable to the report where IPCE on ZnO nanostructure was also taken near the transition voltage [7]. Since IPCE corresponds to the difference between the current under illumination and in dark condition for the monochromatic light, IPCE performance at the single potential value may not properly represent the overall photoelectrochemical performance. A much higher efficiency reported [8] for the ZnO nanostructure with IeV performance comparable to the present work may be mainly attributed to the potential range for the high photocurrents. An enhanced visible absorption may occur for the porous brush-like structure as indicated in the wavelength near 400 nm. Possibility of the high performance of the brush-like structure in view of the large surface area still exists.
Fig. 10 e IPCE measured at 0 V vs. Ag/AgCl SCE in 0.5 M NaClO4.
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Conclusion A three-dimensional skeleton of the seed layer which coarsens into the brush-like nanostructured ZnO film can be prepared by applying high DC bias in high concentration of nitrate solution in the electrochemical deposition. The photoelectrochemical performance of the ZnO film was however limited due to the insufficient coverage of the ITO substrate. Formation of dense sublayer by increasing electrochemical deposition time reduced the leakage substantially. In contrast to the conventional monofrequency MotteSchottky analysis, complex plane representation of the capacitance is suggested for the evaluation of the frequency-independent Schottky capacitance parameters. The AC characteristics in the near flat-band potential and in the pre-breakdown region should be distinguished from the Schottky barrier behavior. Thus obtained well-defined capacitance values can be used to compare the flat-band potential values which may be related with the characteristics of the semiconductoreelectrolyte interface depending on the preparation condition.
Acknowledgment This work was financially supported from World Class University (WCU) program (R32-2009-000-20074-0) and Priority Research Centers Program (2010-0029626) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) of the Korean government.
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