Role of Chloride Ions on Electrochemical Deposition of ZnO Nanowire

Oct 4, 2007 - Electrochemical Growth of ZnO Nanobelt-Like Structures at 0 °C: Synthesis, Characterization, and in-Situ ... The Journal of Physical Ch...
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J. Phys. Chem. C 2007, 111, 16706-16711

ARTICLES Role of Chloride Ions on Electrochemical Deposition of ZnO Nanowire Arrays from O2 Reduction Ramon Tena-Zaera,* Jamil Elias, Gillaume Wang, and Claude Le´ vy-Cle´ ment Institut de Chimie et Mate´ riaux de Paris-Est, CNRS, UMR 7182, Baˆ t. F, 2-8 rue Henri Dunant, 94320 THIAIS, France ReceiVed: May 23, 2007; In Final Form: August 7, 2007

A systematic study of the role of KCl on the electrodeposition of ZnO nanowire arrays from the reduction of oxygen in ZnCl2 solutions was performed. Besides its role as a supporting electrolyte, other major effects were found. An increase of KCl concentration ([KCl]) considerably decreased the rate of O2 reduction. The consequent decrease in OH- production rate resulted in an augmentation of the ZnO deposition efficiency, from a value around 3% for [KCl] ) 5 × 10-2 M to more than 40% for [KCl] ) 3.4 M. The increase of the deposition efficiency mainly resulted in an enhancement of the longitudinal growth rate. However, high [KCl] (>1 M) also favored the lateral growth of the ZnO nanowires, resulting in diameters as big as 300 nm (in comparison to the diameter of 80 nm obtained for [KCl] < 1 M). The observed effects were discussed in terms of Cl- ion adsorption on the cathode surface. The possible preferential adsorption of the anion on the (0001) ZnO surface was emphasized. Transmission electron microscopy revealed that the ZnO nanowires were single crystals, irrespective of [KCl] in the electrolyte. Thus, playing with the chloride content in the solution is an interesting way to obtain ZnO single-crystal nanowire arrays with tailored dimensions under controlled deposition rates. The influence of the nanowire dimensions on the optical properties was also discussed, showing the interest of this study in the frame of nanostructured solar cells.

Introduction In recent years, single-crystal ZnO nanowire arrays have emerged as promising building blocks for a new generation of devices in different technological domains such as optoelectronics,1,2 solar cells,3,4 gas sensing,5 field emission,6,7 and piezoelectrics.8 This is not only due to the high capabilities of ZnO as a multifunctional material but also due to the particular morphology of the nanowire arrays. Until now, most of the research on the deposition methods has mainly been focused on vapor-phase techniques.9 A common characteristic of these kind of techniques is the high temperature. Temperatures of 900-1100 °C are the most frequently used in vapor-liquid-solid processes,10,11 while lower temperatures of 400-500 °C are used in free-catalyst and more sophisticated techniques such as metal-organic chemical vapor deposition.12 Electrochemical deposition of ZnO nanowire arrays, from the reduction of molecular oxygen in aqueous solutions, is a convenient low-temperature (98.0%) was used as the Zn2+ precursor. KCl (Flucka, purity >99.5%) served as a supporting electrolyte, among other roles. The substrates were commercial conducting glass (10 Ω/square; SnO2:F) from Asahi Glass Company. For each working electrode, a well-determined geometric surface was masked off prior to the electrodeposition experiments. It was found that the presence of a nanocrystalline buffer layer deposited on top of the conducting glass allowed to control the diameter and density of the ZnO nanowires grown on them.27 The substrates were first coated by a nanocrystalline ZnO buffer layer galvanostatically electrodeposited (J ) 0.13 mA/cm2, Q ) 0.37 C/cm2) at room temperature in solutions of 5 × 10-3 M ZnCl2 and 0.1 M KCl. On top of the nanocrystalline ZnO buffer layer, the ZnO nanowire arrays were electrodeposited at 80 °C under a constant potential (-1 V). Two series of samples were prepared. The first series was electrodeposited from a solution of 100 mL in which [KCl] was varied from 5 × 10-2 M to saturation at room temperature (∼ 3.4 M), on a 2 cm2 geometric surface. The charge density used for this series was 20 C/cm2. This series will be referred to in the text as “constant charge density”. The second series was performed on 1 cm2, using 200 mL of a 3.4 M KCl solution, and varying the charge density from 1 to 20 C/cm2. The second series will be referred to as “constant chloride concentration”. The reproducibility of the electrodeposition experiments was checked on three or more samples for both series. Cyclic voltammograms (CV) were recorded at 80 °C. Two scans for each cathode were carried out from solution rest potential (near zero), with a cathodic sweep to -1.5 V and then an anodic sweep to +0.5 V. Scan rate was 10 mV/s. CV and electrodepositions were performed using an Autolab PGSTAT30 potentiostat. The morphology of the ZnO nanowires was analyzed using a field emission scanning electron microscope (SEM) LEO 1530. Some ZnO nanowires were scratched from the samples to analyze their structural properties by transmission electron microscopy (TEM) using a high-resolution Topcon 002B microscope operating at 200 kV. A drop of an ethanol suspension containing the nanowires was deposited on a copper grid with lacey carbon for TEM observations. Their optical

Figure 1. SEM images of sample cross-sections. ZnO nanowire arrays were obtained from the “constant charge density series” (V ) -1 V vs SCE, cathode surface ) 2 cm2, volume of solution ) 100 mL, [ZnCl2] ) 5 × 10-4 M, T ) 80 °C and Q ) 20 C/cm2) using different [KCl]: (a) 5 × 10-2 M, (b) 1 M, (c) 2 M, and (d) 3.4 M. Inset of c shows a high magnification image of ZnO nanowire tips. Note that the bar scale length is different for Figures 1a, b and c, and d.

Figure 2. Mean values of nanowire length as a function of [KCl].

transmission was measured at room temperature with a Hitachi UV-vis-NIR 4001 spectrophotometer fitted with an integrating sphere, from 300 to 1100 nm. Results and Discussion The influence of [KCl] on the ZnO nanowire dimensions is investigated. The variation of the physical properties of the ZnO nanowire arrays as a function of chloride concentration in the electrolyte is discussed. The influence of chloride in the ZnO electrodeposition mechanism is also addressed to explain the differences observed in the ZnO nanowires. 1. Nanowire Dimensions. Figure 1 shows SEM images of cross sections of ZnO nanowire arrays of the first series “constant charge density” (V ) -1 V, volume of solution ) 100 mL, cathode surface ) 2 cm2, [ZnCl2] ) 5 × 10-4 M, T ) 80 °C, and Q ) 20 C/cm2). A considerable increase of the nanowire length as a function of [KCl] is observed. The average value of the nanowire length was estimated from SEM images for each [KCl]. The obtained values are summarized in Figure 2, confirming quantitatively the increase of nanowire length with [KCl]. It is interesting to note that an increase of the electrodeposited ZnO quantity for high [KCl] values can be inferred from Figure 1. The variation of the quantity of ZnO formed suggests

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TABLE 1: Experimental Conditions and Estimated Values of Mass and Deposition Efficiency for ZnO Nanowire Arrays Electrodeposited Using Different KCl Concentrations [KCl] (M)

cathode surface (cm2)

solution volume (mL)

charge density (C/cm2)

mass (mg)

deposition efficiency (%)

0.05 0.1 1 3.4 3.4 3.4

2 2 2 2 1 1

100 100 100 100 200 200

20 20 20 20 10 20

0.5 0.9 1.6 3.0 1.8 3.3

3 5 9 18 42 39

differences in the deposition mechanism. The mechanism proposed in the literature implies the reduction of molecular oxygen that results in the generation of hydroxide ions (eq 1), which by increasing the local pH leads to the precipitation of ZnO (eq 2).16

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

(1)

Zn2+ + 2OH- f ZnO + H2O

(2)

To gain further insight into the deposition mechanism, the mass of ZnO nanowire arrays was estimated by weighing the samples before and after their electrodeposition. The deposition efficiency, defined as the ratio between the OH- that reacted with Zn2+ (yielding ZnO, eq 2) and the total amount of OHproduced from O2 electroredution (eq 1), was calculated from the estimated mass of ZnO nanowire arrays. The values are summarized in Table 1. An efficiency of approximately 3% was obtained for [KCl] ) 5 × 10-2 M. It increased as a function of [KCl], with values higher than 15% for the highest chloride concentration ([KCl] ) 3.4 M). Although the deposition efficiency can only be considered as an approximate value, a clear increasing trend was observed as a function of [KCl]. Moreover, some defects in nanowire tips were observed for [KCl] g 2 M (inset of Figure 1c and Figure 1 from Supporting Information). These defects may result from growth discontinuities because of a lack of Zn2+ in the solution (almost total consumption), revealing the stop of ZnO deposition during the last step of the electrodeposition experiment. This was confirmed by deposition efficiencies, as high as 40%, for experiments of the “constant chloride concentration series” (V ) -1 V, cathode surface ) 1 cm2, volume of solution ) 200 mL, [ZnCl2] ) 5 × 10-4 M, [KCl] ) 3.4 M, and T ) 80 °C), performed in a larger volume of solution than the first series (i.e., higher amount of Zn2+ was available). Thus, deposition efficiencies from 3 to 40% were observed as a function of [KCl], revealing its important role on ZnO deposition mechanisms. Lincot’s group28 reported a thermochemical analysis of the decomposition of ZnCl2 in 0.1 M KCl aqueous solutions. They concluded that the major soluble species at temperatures higher than 50 °C is ZnCl+, resulting in Zn2+ content lower than 20% at 90 °C. This could be one of the origins of the very low deposition efficiencies. As the increase of [KCl] (i.e., [Cl-]) may favor the formation of ZnCl+ and higher order zinc-chloride complexes, the increase of the deposition efficiency may not be due to an increase of [Zn2+] in solution but to that of [Cl-]. Current density was dynamically analyzed by chronoamperometry during the electrodeposition process for different [KCl]. A clear decrease of current density was observed when [KCl] increased (Figure 3a), resulting in longer times to reduce the same amount of O2. This indicated that the electroreduction of O2 took place more slowly. This could be due not only to differences in the mechanism of the O2 electroreduction, but

also to the modification of the electrode surface area due to the ZnO nanowire growth. To exclude the latter point, CV were performed on conducting glass substrates in deposition solutions with different [KCl]. Only the cathodic forward sweeps of the second CV are shown in Figure 3b for the sake of clarity. The second scan was chosen because ZnO deposition was expected to have covered the SnO2:F surface during the first CV. The second scan was therefore more representative of the O2 reduction process during the ZnO nanowire deposition, which was performed on a ZnO buffer layer. Figure 3b reveals that even on flat cathodes the current density decreases when [KCl] increases, confirming that [KCl] appears to be the major parameter in the current decrease. This could result from a competition of the adsorption between OH- (provided by O2 electroreduction) and Cl- on the cathode surface. Other authors have reported that the presence of Cl- modifies the oxygen reduction mechanism on metallic substrates such as silver29 and nickel,30 inducing the formation of H2O2 as an intermediate species by a two-electron reduction process (eq 3) instead of a direct four-electron process (eq 1).

O2 + 2H2O + 2e- f H2O2 + 2OH-

(3)

A similar phenomenon may also occur in our electrodeposition experiments. Lincot’s group31 has recently studied O2 electroreduction on ZnO electrodes, finding that the total number of exchanged electrons during O2 reduction is ∼ 3.4 for [KCl] ) 0.1 M. This may indicate that a four-electron process is dominant under these conditions. Nevertheless, two-electron process may become more important when [Cl-] increases as observed on metal cathodes.29,30 Besides, it is interesting to note that no significant differences are observed when NaCl is used instead of KCl (Supporting Information, Figure 2). This observation suggests that the cation (K+, Na+) does not play a major role, supporting the discussion about the role of the anion (Cl-). Though H2O2 has also been used as OH- precursor in ZnO electrodeposition,32 the ZnO surfaces seem to present poor electrocatalytic activity toward the H2O2 reduction.33 This implied that the H2O2 contribution in ZnO deposition should be lower than that of O2. The increase of [Cl-] results in slower OH- production rate that may facilitate the diffusion of Zn2+ to the electrode, enhancing the ZnO deposition efficiency. To evaluate the relevance of OH- production rate on the efficiency of the ZnO deposition, additional experiments were performed in 0.1 M KCl solutions as a function of current density. An increase in ZnO nanowire length was observed when the current density was decreased (Supporting Information, Figure 3). This confirmed that low current densities (i.e., low OH- production rate) enhanced deposition efficiency. Consequently, modifying chloride concentration appears to be an efficient way to act on the O2 reduction. Slower OH- production results in an increased efficiency of the ZnO deposition process and in longer nanowires, while generating the same amount of OH- ions. Besides the variations in the length, a decrease of the density of the nanowires in the array as a function of the [KCl] can be inferred from SEM plan view observations (Supporting Information, Figure 1), though these images only give information about the top of the arrays. A selective growth process seems to occur during the deposition, quenching the growth of the most tilted nanowires (Figure 1). This phenomenon seems to be characteristic of the present ZnO electrodeposition route, irrespective of the chloride concentration (also observed for the “constant chloride concentration series”, Figure 4), and it will be discussed in detail elsewhere.35

Electrochemical Deposition of ZnO Nanowire Arrays

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Figure 3. (a) Current density during ZnO nanowire array electrodeposition and (b) cathodic current-potential scans on SnO2:F electrode in deposition solutions (O2 saturated, [ZnCl2] ) 5 × 10-4 M and different [KCl]). The legend is the same for both figures.

Figure 4. SEM images of ZnO nanowire arrays (plan view) obtained from the “constant chloride concentration series” (V ) -1 V vs SCE, cathode surface ) 1 cm2, volume of solution ) 200 mL, [ZnCl2] ) 5 × 10-4 M, [KCl] ) 3.4 M, and T ) 80 °C) passing different charge densities: (a) 1 C/cm2, (b) 2.5 C/cm2, (c) 5 C/cm2, and (d) 10 C/cm2.

On the other hand, a large increase in nanowire diameter was observed for concentrations higher than 1 M with samples of the first series (“constant charge density” (Figure 1)). From the plan view SEM images (Supporting Information, Figure 1), a mean diameter value in the range of 70-80 nm is estimated for samples electrodeposited for [KCl] < 1 M, while samples for [KCl] > 1 M result in larger diameters up to 300 nm. This observation agrees with the role suggested by Xu et al.20 for Cl- as a stabilizer of the (0001) ZnO faces. The adsorption of negatively charged impurities on the (0001) surface was reported as a mechanism to reduce the charge density of this polar surface.34 Taking into account that ZnO nanowires grow along the [0001] direction (see section 3.2), we expect a hint of their longitudinal growth to be due to the stabilization of the (0001) ZnO face. However, we obtained the longest nanowires for the highest chloride concentration (Figure 1). We performed a further study of the chloride effects on nanowire growth mechanisms by studying the second series of samples (“constant chloride concentration”, V ) -1 V, cathode surface ) 1 cm2, volume of solution ) 200 mL, [ZnCl2] ) 5 × 10-4 M, [KCl] ) 3.4 M, and T ) 80 °C) and analyzing the evolution of the nanowire diameter as a function of the charge density (deposition time). Figure 4 shows SEM plan view images of ZnO nanowire arrays obtained from this series. Nanowires with diameters of ∼30 and 300 nm were obtained after passing 1 and 10 C/cm2, respectively. A similar trend with respect to nanowire diameters was already observed for higher charge densities, resulting in the coalescence of nanorods at 20

C/cm2. This implied that, for high [KCl], the charge increase resulted not only in longer (Supporting Information, Figure 4) but also in wider ZnO nanowires. This behavior differs strongly from that observed for low [KCl], where the diameter does not vary as a function of the charge.35 Thus, high chloride content in the electrolyte favors the lateral growth of ZnO nanowires, in accordance with the expected effects from (0001) surface stabilization. However, the most favorable ZnO deposition (due to the decreased OH- production) favored the growth along the [0001] direction as well, smearing the surface stabilization effects. Chloride concentration plays a major role on the electrodeposition of ZnO nanowire arrays because it has a significant influence on O2 reduction. At high concentration (>1 M), preferential adsorption of chloride ions on the (0001) ZnO surface favors the stabilization of the (0001) face, promoting a considerable lateral growth of the nanowires. 2. Structural Properties. The structural properties of ZnO nanowires were analyzed by TEM. Figure 5 summarizes the results obtained for nanowires scratched from arrays obtained using the lowest (5 × 10-2 M) and the highest (3.4 M) KCl concentrations of the first series (“constant charge density”). Low magnification TEM images are given for both cases in Figure 5a,b. The length of these nanowires cannot be considered as representative of the arrays because a wide distribution was observed for nanowires from the same sample, as illustrated by the two nanowires shown in Figure 5a. The length distribution was induced by the multiple fractures of nanowires resulting from the scratching process. As the fractures occurred perpendicular to the longitudinal axis of nanowires, they did not alter the nanowire diameter. Nanowires from Figure 5a (5 × 10-2 M KCl) present a diameter (∼80 nm) smaller than those of Figure 5b (∼200 nm), which were obtained in 3.4 M KCl solutions. Both values are in fairly good agreement with the mean values obtained from SEM observations. The tips of nanowires were generally thinner (∼20%) than their base. The diameter variation was gradual and can be attributed to a decrease of the Zn2+ concentration in the deposition solution due to the partial consumption of zinc by reaction 1. The effect of the concentration of the Zn2+ precursor on the ZnO nanowire diameter will be discussed elsewhere.35 Selected area electron diffraction (SAED) experiments were performed on the tip of the longest nanowire of Figure 5a and on the nanowire of Figure 5b. Same electron diffraction patterns (Figure 5c) were obtained. Both observations were performed in the [112h0 ] zone axis. The diffraction pattern reveals not only the hexagonal wurtzite structure of the ZnO nanowires, but also their growth direction along the [0001] axis. The single-crystal character of nanowires is shown in the pattern. High-resolution transmission electron microscopy (HRTEM) images (Figure

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Figure 6. (a) Optical transmittance and (b) optical reflectance of conducting glass/ZnObuffer/ZnO nanowire arrays obtained from the “constant charge density series”.

Figure 5. Low-resolution TEM images of single ZnO nanowires obtained, from the “constant charge density series”, using different [KCl] (a) 5 × 10-2 M and (b) 3.4 M. (c) SAED pattern from a single nanowire. High-resolution TEM images showing the ZnO lattice recorded on different nanowire areas (d, e, f, and g: boxes 1, 2, 3, and 4, respectively).

5d-g) confirmed not only the nanowire growth along [0001] direction (lattice spacing of ∼0.52 nm, in good agreement with the c parameter of the ZnO wurtzite phase36), but also the single crystalline character of the nanowires. The ZnO single-crystal lattice across the base of a nanowire deposited in 5 × 10-2 M KCl solution is shown in the HRTEM image (Figure 5d) recorded in the region indicated by the rectangular box 1 in Figure 5a. Figure 5e consists of the HRTEM image of the tip of the same nanowire (box 2 of Figure 5a). The consistent lattice observed between the two observation regions suggests the single crystalline character of the entire nanowire. HRTEM images of regions placed inside boxes 3 and 4 in Figure 5b are displayed in Figure 5f,g, respectively. These figures reveal wellcrystallized nanowires obtained with the highest [KCl] (3.4 M). It is worth noting that clusters of secondary phases were not observed in any of the analyzed nanowires. This was supported by the absence of spurious crystalline phases in X-ray diffraction patterns, where the analyzed region is larger (Supporting Information and Figure 5). This is especially interesting in samples deposited using high [KCl] (>1 M) where Cladsorption on the ZnO surfaces plays a major role during the growth of the nanowires. We also note that Cl- adsorption does not seem to induce additional structural defects such as dislocations. However, the presence of point defects due to atomic chlorine in the ZnO lattice must not be excluded. If point chlorine defects exist, they may play a major role on the electrical and optical properties of ZnO nanowires. The

influence of [KCl] on the electrical behavior of the ZnO nanowire arrays is now under study by electrochemical impedance spectroscopy, using a Mott-Schottky (MS) model that takes into account their particular morphology.37 3. Optical Properties. Figure 6a shows the transmission spectra of several samples from the first series (“constant charge density”). The position of the absorption onset was determined from the maximum derivative of the optical density, and it is referred to as the optical gap in the text. Optical gap values in the range from 3.32 to 3.34 eV (371-373 nm) were obtained in good agreement with the band gap of bulk ZnO.38 However, the optical transmittance for wavelengths higher than the optical band gap decrease as [KCl] increases. This is mainly due to an increase of light scattering induced in the arrays constituted by longer nanowires, as can be observed in the reflectance spectra (Figure 6b). Nevertheless, the optical absorption for wavelengths slightly higher than ZnO band gap is considerable for samples obtained in more concentrated KCl solutions ([KCl] g 2 M). This could be induced by surface defects observed in the tips of ZnO nanowires (inset of Figure 3c and Figure 1 of Supporting Information). The contribution of some energetic levels generated by chlorine impurities cannot be excluded. The increase of the nanowire length results in an augmentation of the light scattering for wavelengths higher than the ZnO band gap. This result is especially attractive in the framework of nanostructured solar cells based on very thin (∼ nanometers) layers of solar light absorber materials such as dye-sensitized solar cells (DSSC)4,39 and extremely thin absorber (ETA) solar cells.3,40,41 The use of high [Cl-] appears to be a promising way to obtain ZnO nanowire arrays with very high surface areas, one of the main limitations in the performance of DSSCs based on nanowire structures.4 Conclusions The role of [KCl] in the electrodeposition of ZnO nanowire arrays from the O2 reduction has been investigated. Until now, KCl was only considered as a supporting electrolyte. This study shows two new major roles played by KCl as a source of Cl-

Electrochemical Deposition of ZnO Nanowire Arrays ions. The first one is its effect on the O2 reduction and therefore on the OH- production. Oxygen reduction decreases as Clconcentration increases in the electrolyte favoring ZnO deposition. An important increase of the deposition efficiency has been observed, from a value around 3% for [KCl] ) 5 × 10-2 M to a value higher than 40% for [KCl] ) 3.4 M. The second role is the influence of the chloride concentration on the nanowire growth mechanism. Although all films showed arrays of onedimensional ZnO nanostructures, a chloride concentration higher than 1 M favored lateral nanowire growth, resulting in diameters as large as 300 nm. Transmission electron microscopy showed that the nanowires are single crystals, irrespectively of [KCl]. Thus, altering the Cl- content in the solution appears to be an interesting way to affect the deposition rate and dimensions of ZnO single-crystal nanowire arrays. This approach may also be extended to wet chemical methods, where strongly adsorbing ions such as chloride can also be incorporated into the synthesis. Supporting Information Available: Additional SEM images and XRD data. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Park, W. I.;Yi, G. C. AdV. Mater. 2004, 16, 87. (2) Konenkamp, R.; Word, R. C.; Godinez, M. Nano Lett. 2005, 5, 2005. (3) Le´vy-Cle´ment, C.; Tena-Zaera, R.; Ryan, M. A.; Katty, A.; Hodes, G. AdV. Mater, 2005, 17, 1512. (4) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, R. Nat. Mater. 2005, 4, 455. (5) Tien, L. C.; Sadik, P. W.; Norton, D. P.; Voss, L. F.; Pearton, S. J.; Wang, H. T.; Kang, B. S.; Ren, F.; Jun, J.; Lin, J. Appl. Phys. Lett. 2005, 87, 222106. (6) Tseng, Y. K.; Huang, C. J.; Cheng, H. M.; Lin, I. N.; Liu, K. S.; Chen, I. C. AdV. Funct. Mater. 2003, 13, 811. (7) Li, S. Y.; Lin, P.; Lee, C. Y.; Tseng, T. Y. J. Appl. Phys. 2004, 95, 3711. (8) Wang, Z. L.; Song, J. Science 2006, 312, 242. (9) Yi, C. C.; Wang, C.; Park, W. I. Semicond. Sci. Technol. 2005, 20, S22-S34 and references therein. (10) Huang, M.; Wu, Y.; Feick, H.; Tran, N.; Weber, E.; Yang, P. AdV. Mater. 2001, 13, 113. (11) Wang, X.; Summers, C. J.; Wang, Z. L. Nano Lett. 2004, 4, 423. (12) Park, W. I.; Kim, D. H.; Jung, S. W. G. C. Y Appl. Phys. Lett. 2002, 80, 4232.

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