Dynamic Template Assisted Electrodeposition of Porous ZnO Thin

Mar 8, 2010 - The mechanism for producing the porous ZnO film can be attributed to ..... The Hong Kong Polytechnic University under project code G-YG7...
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J. Phys. Chem. C 2010, 114, 5811–5816

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Dynamic Template Assisted Electrodeposition of Porous ZnO Thin Films Using a Triangular Potential Waveform F. Hu,† K. C. Chan,*,† T. M. Yue,† and C. Surya‡ AdVanced Manufacturing Technology Research Center, Department of Industrial and Systems Engineering, The Hong Kong Polytechnic UniVersity, Hong Kong, and Department of Electronic and Information Engineering, The Hong Kong Polytechnic UniVersity, Hong Kong ReceiVed: NoVember 10, 2009; ReVised Manuscript ReceiVed: February 11, 2010

Without the use of surfactants, a uniform and porous structure zinc oxide film has been synthesized in a chloride electrolyte using a triangular potential waveform. The triangular waveform parameters have been determined based on the findings of the linear sweep voltammetry and the cyclic voltammetry in the chloride electrolyte. The mechanism for producing the porous ZnO film can be attributed to the anodic current that dissolves the metallic zinc grains to form nanosized pores and, more importantly, the Cl- ions in oxidation reactions and the ZnCl+ ions in reduction reactions that cyclically function as dynamic templates for the pinning effect and the blocking effect on the zinc oxide film, respectively. The study is of significance not only in successfully developing a promising alternative for synthesizing porous ZnO films but also in understanding the underlying mechanism by the concept of dynamic templates. The findings of the paper also lay down a good foundation for further developing this technique into practical use. 1. Introduction Thin film solar cells have attracted extensive research interest over the past decade due to their promising application potentials. Much effort has been spent to achieve high conversion efficiency by controlling the band gap, the band gap position, the surface structure, the particle size and shape, the doping density, the porosity, and the thickness of semiconductor films.1 Among these factors, the porous structure which possesses excellent charge transport path and large active surface area has been reported to lead to very high solar-to-electric energy conversion efficiency.2,3 Electrodeposition, being a low cost and low temperature deposition technology, is shown to be a promising process to fabricate semiconductor films, and it is feasible to synthesize porous structures by self-generating (creating) hydrogen bubbles functioning as molecular dynamic templates using the surfactant cetyltrimethylammonium bromide (CTAB),4,5 by using anodic alumina membranes,6 or by using other additives or surfactants.1,7,8 Much work is being done to achieve electrodeposited thin films with higher conversion efficiencies. Zinc oxide (ZnO) is one of the most attractive functional semiconductor materials due to specific optoelectronic and electrical properties, as well as excellent chemical and thermal stability. Porous ZnO films can be synthesized by the addition of surfactants. With the surfactant CTAB, the mechanism is believed to be related to the strong cleation effect between the zinc ions and the surfactant,5 and it reduces dramatically the concentration of free Zn2+ ions and the amount of carboxyl in the solution. Hydrogen bubbles arising from the electrochemical reduction of H+ function as molecular dynamic templates.4 Sodium lauryl sulfate (SDS)7 and ethylenediamine8 have also been used to generate the porous * Corresponding author: tel, +852 2766 4981; fax, +852 2362 5267; e-mail, [email protected]. † Advanced Manufacturing Technology Research Center, Department of Industrial and Systems Engineering. ‡ Department of Electronic and Information Engineering.

nature of ZnO film. They are believed to be acting as a bridge mediating for the electron transfer reactions, due to their longer molecular structures. Moreover, compounds with anionic headgroups of surfactants form effective in situ templates and produce nanostructure films.5 Although the complex shape-control reagent plays an important role in controlling the morphologies and structures, it also brings impurities into the crystal lattices and limits the deposit properties.9 Acid groups, such as -COO- and -SO3- can be adsorbed onto the growing oxide films which may form hybrid thin films such as ZnO-SDS,10 ZnO-1-pyrenebutyric acid,11 and ZnO-decanoic acid.11 As the properties of the deposited ZnO films may be affected, it is of great significance to develop other techniques to produce porous ZnO films. In cyclic voltammetry of ZnO deposition in a ZnCl2 electrolyte, it is observed that in the forward scan of a triangular potential waveform, ZnO will be deposited onto the substrate when the potential is below a critical value and above which metallic Zn will be deposited. In the reverse direction, while ZnO is relatively stable, the deposited metallic zinc is not stable and can be stripped off. It is envisaged that with the proper selection of the triangular waveform parameters for the deposition of ZnO, the dissolution of Zn may lead to a thin ZnO layer with very small pores. In this research, without using surfactants, an attempt will be made to design a triangular potential waveform suitable for synthesizing porous ZnO films and also to understand the underlying mechanisms involved. 2. Experiments An indium tin oxide (ITO) coated glass substrate with a sheet resistance of about 10 Ω/0 was used. Prior to the deposition of the ITO, the glass substrate was cleaned with detergent, ultrasonically cleaned in an acetone bath, and rinsed with distilled water. All depositions of ZnO films were

10.1021/jp910691w  2010 American Chemical Society Published on Web 03/08/2010

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Figure 1. A triangular potential waveform (ηp, peak potential; ηl, lowest potential; and T, cycle time).

carried out in a three-electrode cell by monitoring the potentiotat (EG&G, model 263A, USA), in which an ITO substrate was used as the working electrode, a platinum plate as the counter electrode, and a saturated calomel electrode (SCE) served as the reference electrode, respectively. The electrolyte contained 0.01 M zinc chloride (ZnCl2) solutions mixed with a supporting electrolyte of 0.1 M potassium chloride (KCl). For comparison purpose, electrodeposition was also conducted in a 0.01 M zinc nitrite electrolyte without chloride ions. The linear sweep voltammetry was measured at a scan rate of 10 mV s-1, and the cyclic voltammetry in ZnCl2 solution was measured at different sweep rates. The electrolyte solutions were aerated by bubbling O2 gas both before and during deposition. The electrodeposition was carried out by a potentiostatic mode and also a waveform mode at an operating temperature of 50 °C. The texture and phase composition of the films were identified by X-ray diffraction analysis using Cu KR radiation (X’Pert, Philips, 40 kV, 30 mA). The surface morphology of the films deposited under different experimental conditions was observed using field emission scanning electronic microscopy (FE-SEM, JEOL, JSM-6490, 20 kV). The porosity of the deposited ZnO thin film was determined by measuring its equivalent and the total thickness.12 While the total thickness of ZnO film is measured by SEM micrographs, its equivalent thickness is calculated by measuring weight loss after dissolving the ZnO film in saturated KCl solution. The ZnO network of the film was also observed under transmission electronic microscopy (TEM, JEOL, JEM-2010).

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O2(g) + 2H2O + 4e- f 4OH-

(1)

Zn2+ + OH- f Zn(OH)ad+

(2)

Zn(OH)ad+ + OH- f ZnO + H2O

(3)

Zn2+ + 2e- f Zn

(4)

From the reactions, it is clear that the electrodeposition behavior is induced by reducing the dissolved oxygen through continually bubbling the O2 gas into the electrolyte, and the deposition of zinc oxides is significantly affected by the oxygen concentration. Under a constant cathodic potential, the oxygen-containing electrolyte results a higher adsorption of zinc oxide, Zn(OH)ad+, and also a much higher current density can be reached as compared to the oxygen-free electrolyte. Figure 2 shows the linear sweep voltammetry results obtained in a ZnCl2 electrolyte, illustrating that due to the oxygen bubbling the current increases more rapidly when the potential is above -0.4 V. The insert diagram shows more clearly that with oxygen the current increases from -0.4 to -0.6 V and can be attributed to the reduction of the oxygen gas to hydroxide ions. Whereas, without oxygen, the change of current is not obvious under the same range of the potential. A steep increase of the cathodic current is observed at a potential of about -1.1 V for both conditions and is attributed to zinc deposition. The potential is found to be a fair match with the reported value for the reduction of Zn2+ to metallic zinc.13,14 As a result, the peak potential should be higher than -1.1 V (vs SCE). In order to determine the peak potential, ZnO thin films were electrodeposited under different potentiostatic modes of -1.0 and -1.2 V, both versus SCE. Their corresponding current densities were found to be 0.3 and 1.5 mA cm-2, as shown in Figure 3c. The thicknesses of the films are 3.5 and 10.0 µm at potentials of -1.0 and -1.2 V, respectively, and their surface morphologies are also shown in parts a and b of Figure 3, where both of the films are densely compacted. Figure 3b also reveals a variation in the crystal size. It is considered that such variation is due to the fluctuation of current density during the electrodeposition process, as shown in Figure 3c. By comparing the oxygen content of the grains by energydispersive X-ray spectroscopy, the pyramid-like (large) grains are found to be metallic Zn particles. The XRD patterns of the films shown in Figure 4 further reveal a ZnO phase obtained at a potential of -1.0 V, which can be indexed as a hexagonal wurtzite structure (hyxagonal phase). However, additional strong

3. Results and Discussion 3.1. Design of Waveform Parameters. A triangular potential waveform is designed to self-generate reduction-oxidation reactions through the potential sweeps from the cathodic current to the anodic current (see Figure 1) leading to cyclic deposition and dissolution. In order to produce porous ZnO oxide film, metallic zinc should be “planted” into the zinc oxide during deposition and should be dissolved under the anodic current, which may form the porous structure of the remaining zinc oxide films. To achieve this purpose, the following parameters of a triangular waveform have to be determined: (1) the peak potential (ηp), (2) the lowest potential (ηl), and (3) the cycle time (T). 3.2. Determination of Peak Potential (ηp). The peak potential has to be selected such that metallic zinc can be deposited into the zinc oxide grains. In the cathodic electrodeposition of ZnO thin films, the electrode reactions are

Figure 2. Linear sweep voltammetry of the cathodic process at a sweep rate of 10 mV/s: 0, without O2 bubbling; O, O2 bubbling.

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Figure 4. XRD patterns of the ZnO thin films on ITO-coated glass substrate obtained at potentiostatic modes of (a) -1.0 V and (b) -1.2 V vs SCE. The peak positions related to the ITO-coated substrate are indicated by a star (/), b, Zn; 9, ZnO.

Figure 5. The cyclic voltammograms in 0.01 M ZnCl2 + 0.1 M KCl + O2 at sweep rates of 50 mV/s.

Figure 3. Typical surface morphologies of the ZnO thin films at (a) a potentiostatic mode of -1.0 V and (b) a potentiostatic mode of -1.2 V and (c) the current desity-time curves for potentiostatic electrodeposition.

peaks at 36.32°, 39.02°, 43.28°, and 44.38° are obtained at a potential of -1.2 V, which corresponds to the hexagonal structure of metallic zinc. On the basis of the above results, a potential of -1.2 V was selected as the peak potential to ensure that Zn will be deposited into the ZnO film. 3.3. Determination of the Lowest Potential (ηl). Figure 5 shows the cyclic voltammogram for the electrodeposition of ZnO with oxygen bubbling. As in the linear voltammetry, there is no current when the potential is below -0.4 V and a steep increase in current is observed at a potential of about -1.1 V,

Figure 6. Linear sweep voltammetry of anodic process at a sweep rate of 10 mV/s.

versus SCE. In the reverse direction, the crossover at the potential of -0.5 V indicates stripping of metallic zinc. The deposited ZnO has high stability as the anodic current is still essentially zero when the stripping process is not significant. In order to better understand how the deposited Zn is dissolved into the electrolyte, a linear sweep voltammetry for reverse potentiodynamic was conducted from -1.4 to +0.6 V on a deposited Zn + ZnO film, as shown in Figure 6. At the

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Figure 8. TEM image of the porous ZnO thin films.

Figure 9. The cyclic voltammetry result in a nitrate electrolyte. The schematic diagram for the formation mechanism of porous structure ZnO thin films.

Figure 7. The typical surface morphologies of the ZnO thin films under the triangular potential waveform for (a) 5000× and (b) 20000× and (c) XRD patterns for the porous ZnO films. The peak position related to an ITO-coated substrate is indicated by a star (/).

reverse current, three peaks are observed at three cathodic potentials corresponding to the following three equations

2Zn f Zn2+ + 2e-

(3)

4OH- f 2H2O + O2 + 4e-

(4a)

4Zn(OH)ad+ f 4Zn2+ + O2 + 2H2O + 4e-

(5)

The deposited zinc is dissolved into the electrolyte according to eq 3. In order to ensure that the zinc is completely dissolved, the lowest potential is selected to be +0.4 V. 3.4. Determination of Cycle Time (T). The cycle time is designed to ensure that a significant amount of zinc grains can

be deposited into the zinc oxide grains. It is undesirable if a zinc layer is formed under a low scan rate or if only a few zinc grains are deposited under a high scan rate. The deposition mass is related to the current passing through and can be calculated using Faraday’s law. As the peak potential is selected to be -1.2 V, Zn will only be deposited between -1.1 and -1.2 V when the deposition is subject to a cathodic current. During a short cycle time (at scan rates higher than 50 mV/s), the deposition time is inadequate and will not result in a significant amount of zinc. With a much longer cycle time (at scan rates lower than 10 mV/s), a zinc layer of about 3-7 nm may be obtained. As both scan rates are not desirable, a moderate scan rate of 20 mV/s (with a cycle time of 80 s) is selected for the triangular waveform. 3.5. Electrodepostion of Porous ZnO Thin Films. On the basis of the above results, a triangular potential waveform, with the peak potential of -1.2 V, the lowest potential of +0.4 V, and the cycle time of 80 s was used to deposit ZnO film. The thickness of the film ranges from 1.55 to 1.92 µm over the film and the average film thickness is 1.7 µm. Figure 7 shows the surface morphologies of the ZnO film synthesized under the triangular potential waveform, illustrating a uniform and porous structure. It contains an extensive random growth of seemingly flexible nanosheets. They bent and connected with each other. The thickness of these nanosheets is about 10 nm. The porosity of the thin film is also found to be about 51.99%. Figure 8 further shows the TEM micrograph of the porous ZnO thin film, revealing the connectivity of the network. Although the triangular waveform is able to synthesize porous ZnO thin films, it is interesting to note that in a nitrate electrolyte the same triangular potential waveform cannot produce porous films. Nitrate ions are easily reduced to hydrogen ions, as shown in eq 6, and thus enhance the electroreduction rate of ZnO. In order to understand the mechanism in the nitrate electrolyte, cyclic voltammetry was conducted and the results are shown

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Figure 10. Schematic diagram for the formation mechanism of porous structure ZnO.

in Figure 9. In comparison to the chloride electrolyte, the cathodic current obtained in the nitrate electrolyte is much greater, as the reduction of nitrate ions forms hydrogen ions

NO3- + H2O + 2e- f NO2- + 2OH-

(6)

On the contrary, the Cl- may favor the formation of ZnCl+ close to the cathode and the OH- presents lower electroactivity. As a competitive process, the adsorption of Cl- decreases the reduction rate of oxygen. The chronoamperometry also shows that a current density decrease is observed with increasing chloride ions.15 Moreover, the anodic current can also be found to be much higher than that in the chloride electrolyte, which suggests the unstability of the ZnO film in the nitrate electrolyte, and it can be attributed to the fact that the NO2- ions can oxide to NO3- ions in an anodic current. 3.6. The Mechanism. The mechanism for the formation of porous ZnO films is summarized in the schematic diagram (Figure 10). Under the cathodic current, zinc oxide is first deposited on the substrate when the potential is below -1.1 V. When the potential sweeps from -1.1 to -1.2 V, a significant amount of zinc is embedded onto the ZnO film and a hybrid ZnO + Zn film is formed. With the switch of the current to anodic, the deposited zinc will then be dissolved into the electrolyte under the anodic current. Meanwhile, the diffusion layer close to the anode is replenished, and the chloride ions are attracted to the ZnO granular site. The chloride ions are considered to have a pinning effect on the ZnO film and are able to prevent the dissolution of the ZnO films. When the deposition changes to a cathodic current, the diffusion layer is further rereplenished and ZnCl+ ions will be absorbed onto the substrate. The cation ions are relatively stable when the potential is above -1.1 V, which provide a blocking effect that prevents zinc oxide being deposited into the pores, leading to the porous structure. In the deposition-dissolution cycles, the Cl- and ZnCl+ ions function independently as dynamic templates under

the cathodic and anodic currents, respectively, which may be regarded as the main mechanism for producing porous ZnO films. 4. Conclusions The porous structure of ZnO thin films has been synthesized by a triangular potential waveform. The linear sweep voltammetry shows that the oxygen gas gives rise to a fast electrodeposition rate. The cyclic voltammetry indicates the stability of the zinc oxide films in the chloride ions electrolyte, and the formation of the porous ZnO thin films is due to the dissolution of the metallic zinc. The chloride ions and ZnCl+ ions function as dynamic templates during the deposition-dissolution cycles. Acknowledgment. The work described in the paper was supported by the Research Committee of The Hong Kong Polytechnic University under project code G-YG77. References and Notes (1) Chen, Z.; Tang, Y.; Zhang, L.; Luo, L. Electrodeposited nanoporous ZnO films exhibiting enhanced performance in dye-sensitized solar cells. Electrochim. Acta 2006, 51, 5870. (2) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mueller, E.; Liska, P.; Vlachopoulos, N.; Gratzel, M. Conversion of light to electricity by cis-X2bis (2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium(II) charge-transfer sensitizers (X ) Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline titanium dioxide electrodes. J. Am. Chem. Soc. 1993, 115, 6382. (3) Gratzel, M. Photoelectrochemical cells. Nature 2001, 414, 338. (4) Li, Y.; Jia, W. Z.; Song, Y. Y.; Xia, X. H. Superhydrophobicity of 3D porous copper films prepared using the hydrogen bubble dynamic template. Chem. Mater. 2007, 19, 5758. (5) Li, L.; Pan, S.; Dou, X.; Zhu, Y.; Huang, X.; Yang, Y.; Li, G.; Zhang, L. Direct electrodeposition of ZnO nanotube arrays in anodic alumina membranes. J. Phys. Chem. C 2007, 111, 7288. (6) Michaelis, E.; Wohrle, D.; Rathousky, J.; Wark, M. Electrodeposition of porous zinc oxide electrodes in the presence of sodium laurylsulfate. Thin Solid Films 2006, 497, 163. (7) Xu, L. F.; Guo, Y.; Liao, Q.; Zhang, J. P.; Xu, D. S. Morphological control of ZnO nanostructures by electrodeposition. J. Phys. Chem. B 2005, 109, 13519.

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(8) Choi, K. S.; Lichtenegger, H. C.; Stucky, G. D. Electrochemical synthesis of nanostructured ZnO films utilizing self-assembly of surfactant molecules at solid-liquid interfaces. J. Am. Chem. Soc. 2002, 124, 12402. (9) Chandrasekar, M. S.; Pushpavanam, M. Pulse and pulse reverse plating-conceptual, advantages, and applications. Electrochim. Acta 2008, 53 (8), 3313. (10) Gan, X.; Gao, X.; Qiu, J.; Li, X. Growth and characterization of ZnO-SDS hybrid thin films prepared by electrochemical self-assembly method. Appl. Surf. Sci. 2008, 254, 3839. (11) Sofos, M.; Goldberger, J.; Stone, D. A.; Allen, J. E.; Ma, Q.; Herman, D. J.; Tsai, W. W.; Lauhon, L. J.; Stupp, S. I. A synergistic assembly of nanoscale lamellar photoconductor hybrids. Nat. Mater. 2009, 8, 68–75.

Hu et al. (12) Pauporte, T.; Rathousky, J. Electrodeposited mesporous ZnO thin films as efficient photocatalysts for the degradation of dye pollutants. J. Phys. Chem. C 2007, 111, 7639–7644. (13) Elias, J.; Zaera, R. T.; Clement, C. L. Effect of the chemical nature of the anions on the electrodeposition of ZnO nanowire arrays. J. Phys. Chem. C 2008, 112, 5736. (14) Mahalingam, T.; John, V. S.; Raja, M.; Su, Y. K.; Sebastian, P. J. Electrodeposition and characterization of transparent ZnO thin films. Sol. Energy Mater. Sol. Cells 2005, 88, 227. (15) Ramon, T. Z.; Elias, J.; Gillaume, W.; Claude, L. C. Role of chloride ions on electrochemical deposition of ZnO nanowire arrays from O2 reduction. J. Phys. Chem. C 2007, 111, 16706.

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