CuO Core–Shell Nanowire

Mar 15, 2014 - Department of Energy Storage and Conversion Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju. 561-756 ...
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Synthesis of Vertically Conformal ZnO/CuO Core−Shell Nanowire Arrays by Electrophoresis-Assisted Electroless Deposition Sanggon Kim,† Younghyo Lee,‡ Ayeong Gu,† Chanseok You,† Kwangjoong Oh,† Sanghyun Lee,§ and Yeonho Im*,†,‡ †

School of Semiconductor and Chemical Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju 561-756, Republic of Korea ‡ Department of Energy Storage and Conversion Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju 561-756, Republic of Korea § Korea Institute of Science and Technology, Jeonbuk 565-905, Republic of Korea S Supporting Information *

ABSTRACT: Vertically conformal ZnO/CuO core−shell nanowire (NW) arrays with high aspect ratios were synthesized by electrophoresis-assisted electroless deposition (EAELD) of CuO onto preformed ZnO NWs. Using this method, conformal CuO seeds were successfully formed on long ZnO NWs by the electrostatic attachment of colloidal Cu2O NPs under the solution and subsequent thermal oxidation. Using the CuO seeds, conformal CuO shell to core ZnO NWs were successfully fabricated by kinetic-limited deposition in the solution, which overcame the inherent diffusion limitations of conventional electroless deposition. This experimental evidence and thermodynamic modeling results are presented to validate the proposed EAELD mechanism. The vertically conformal hetero-nanostructure arrays developed in this work are a promising platform for applications such as solar cells and catalysts because of the maximized junction areas and active sites.

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INTRODUCTION Recently, one-dimensional hetero-nanostructures have attracted great interest as versatile multifunctional building blocks due to their potential applications in electronics,1 photonic devices,2,3 solar cells,4,5 environment,6 chemical and biological sensors,7,8 and novel catalysts.9 Among the promising materials in these paradigms, ZnO nanowire (NW) has been investigated as core materials due to advantages such as intrinsic n-type semiconductors with wide band gaps (3.37 eV), large exciton binding energy (60 mV), large piezoelectric constants, and welldeveloped synthesis methods.10,11 ZnO NWs with various shell materials are being designed to effectively couple different properties and lead to novel applications such as electronic devices,12 chemical and biological sensors,7,13 energy catalysis,9 and solar cells.14 The shell formation of copper oxide to vertically aligned ZnO NW arrays has been reported as an especially attractive platform for solar cells because it is a promising p-type semiconductor with narrow band gap energy and strong absorption in the solar spectrum.14−16 Furthermore, the use of copper oxide rather than other p-type semiconductor materials has merits such as nontoxicity, abundance of raw materials, and simplified fabrication processes.14,17 On the other hand, copper and its oxides (Cu, CuO, or Cu2O) can be converted easily into © 2014 American Chemical Society

each other via simple redox reactions. Therefore, ZnO NW arrays with copper-based shells can also be applied directly to wide range of applications such as electronic and magnetic effects,18 photocatalysis,19 photodetectors,20 catalysis of methanol synthesis or reformation,9 and chemical sensing.21 To date, most studies related to core−shell NWs have focused on demonstrating their feasibility for applications instead of establishing systematic and robust synthesis routes. Vertically aligned core−shell NW arrays with high aspect ratios and ultrahigh uniformity are regarded as ideal heteronanostructures for potential applications. Since cost-effective routes to create vertical ZnO NW arrays are well established,11,14 deposition of the shell material onto preformed ZnO NW cores is desirable. A number of approaches to form copper-based shells (Cu, CuO, or Cu2O) for ZnO NW arrays have been introduced using available Cu deposition processes such as physical deposition,22 electrochemical deposition (ECD),16 photochemical synthesis,23 and electroless deposition (ELD).9,24 Received: October 17, 2013 Revised: March 11, 2014 Published: March 15, 2014 7377

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aggregates. The growth solution was refreshed every 2.5 h to maintain a continuous supply of Zn ions. To control the density of the vertically grown ZnO NW arrays, the concentrations of reactants (Zn(NO3)2·6H2O and C6H12N4) varied from 10 to 25 mM while all other conditions were kept constant. The substrate was rinsed thoroughly with deionized water after ZnO NW growth was complete and then annealed at 400 °C for 30 min to remove residual organic matter from the surfaces of as-synthesized ZnO NWs. Synthesis of Vertically Aligned ZnO/Cu Core−Shell NW Arrays by a Conventional ELD Process. For the purpose of comparison, Cu was deposited using a conventional ELD process with an activation process for noncatalytic surfaces onto ZnO NWs. To activate the surfaces of ZnO NWs, annealed ZnO NW arrays were immersed in a palladium plating bath (0.1 mM PdCl2, pH 3 adjusted with HCl) at temperatures ranging from 25 to 95 °C for typical growth periods ranging from 10 to 30 min and then washed with deionized water.27 The substrate was then placed in a Cu ELD solution with a pH of 11, which consisted of 1 mM copper nitrate (Cu(NO3)2, 99.999%), 1.3 mM PSTT (potassium sodium tartrate tetrahydrate, C4H4KNaO6, 99%) as a complexing agent, 2.7 mM sodium hydroxide (NaOH, 98%, Samchun Chemical Co.) and 6.3 mM sodium carbonate (Na2CO3, 99%, Junsei Chemical Co.) as an alkaline medium, and 0.125% v/v formaldehyde (HCHO, 37%) as a reducing agent.28 The optimal ELD bath conditions for the formation of uniform Cu shells were determined by adjusting key parameters such as the density of ZnO NWs, temperature of the ELD bath (85−95 °C), and concentration of PSTT (1.3−2.6 mM). Synthesis of Vertically Aligned ZnO/CuO Core−Shell NW Arrays by the Novel EAELD Process. Vertically aligned ZnO NW arrays were placed in the same solution as that used for the conventional ELD process described above for a period of 2 h after the annealing process, except for the omission of the activation process with Pd catalyst, 2.6 mM PSTT and bath temperature of 60 °C. These were named as the EAELD bath conditions in this work. Colloidal Cu2O NPs in the solution were attached to the positive surfaces of ZnO NW arrays by electrostatic force, as discussed in the Results and Discussion section. Then, after electrophoresis of the Cu2O NPs, the substrate was rinsed thoroughly with deionized water and oxidized in air at 250 °C for 1 h for the formation of CuO seeds on the surfaces of the ZnO NW arrays. Subsequently, the oxidized substrate was reimmersed in a solution under EAELD bath conditions for growth periods ranging from 6 to 8 h to obtain the desired thicknesses of CuO shells, so that vertically aligned ZnO/CuO core−shell NW arrays could be formed. Chemical and Structural Characterization of AsSynthesized NW Arrays. The morphologies of the assynthesized samples were examined via a field-emission scanning electron microscope (FE-SEM, Horiba s-4800, 15 keV) and a field emission transmission electron microscope (FE-TEM, JEM 2200FS, 200 keV). The elemental profiles of the samples were measured via energy dispersion spectrometer analyses (EDS). The chemical compositions of the samples were analyzed using X-ray photoelectron spectroscopy (XPS, AXIS-NOVA, Kratos Inc.) with Al Kα radiation. The crystal structures of as-synthesized ZnO/CuO core−shell NWs were characterized by X-ray diffraction (XRD, X′pert Powder) and selected area electron diffraction (SAED, JEM 2200FS, 200 keV). A thermogravimetric analysis (TGA, SDT Q600) was performed to investigate the thermal decomposition of organic

Meanwhile, Cu may reasonably be considered as a starting material for the formation of copper-based shells on ZnO NW because numerous Cu deposition methods are available, and the subsequent oxidation processes can easily change deposited Cu shells into oxides. However, physical deposition methods for Cu shell such as sputtering and thermal evaporation suffer from the inherent drawback of nonconformal deposition due to the use of line-of-sight methods for high aspect ratio nanostructures, such as long NW.22 On the other hand, Cu ECD or ELD processes are well established for Cu interconnections in the semiconductor industry but are not the best alternative for the formation of conformal Cu shells on vertically long ZnO NW arrays with extremely high aspect ratios due to inherent disadvantages such as ZnO erosion in electrolyte solutions and diffusion-limited deposition.25,26 To our best knowledge, there are no systematic reports of effective synthetic approaches to address these drawbacks. Herein, we present an effective solution-based synthetic route for the formation of highly conformal CuO shells on vertically grown ZnO NW arrays, which we refer to as electrophoresis-assisted electroless deposition (EAELD). For this work, long ZnO NW arrays with high aspect ratios ranging from 100 to 200 were synthesized vertically onto ZnO seed layers by a hydrothermal method with additive components to prevent lateral growth.4 The EAELD method for forming CuO shells with high uniformity on vertically aligned ZnO NW arrays consists of the following procedures: (1) first, colloidal nanoparticles (NPs) of Cu2O are formed via homogeneous nucleation in an ELD solution; (2) second, colloidal Cu2O NPs with negative charges are adhered selectively to the positive sidewalls of ZnO NWs by electrostatic force; (3) third, the Cu2O NPs attached to ZnO NW arrays are thermally oxidized to form CuO seeds; and (4) finally, colloidal CuO NPs in EAELD solution are grown on the preformed CuO seeds of ZnO NW arrays, leading to conformal deposition under kineticlimited conditions. In this work, the superiority of the EAELD method for the formation of vertical ZnO/CuO core−shell NWs arrays is illustrated through comparisons with conventional ELD processes. Furthermore, we demonstrate that the resulting CuO shells can be changed into Cu or Cu2O shells via a simplified hydrogen reduction process. Finally, the proposed mechanisms for each step in the EAELD process are verified with the detailed experimental evidence.

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EXPERIMENTAL METHODS Chemicals. Except where noted, all chemicals were purchased from Sigma-Aldrich, Inc., and used without further purification. Synthesis of Vertically Aligned Long ZnO NW Arrays. Vertical ZnO NW arrays were prepared on the Si substrate by a hydrothermal method described previously.4 Briefly, a seed layer of ZnO with around 40 nm thickness was deposited by atomic layer deposition (ALD) on Si substrate, as described elsewhere.13 A 100 mL aqueous solution containing 10−25 mM zinc nitrate hexahydrate (Zn(NO3)2·6H2O), 10−25 mM hexamethylenetetramine (C6H12N4), and 0.05 mM polyethylenimine (PEI) branched (C2H5N) was placed on the ZnO seed layer. The substrate with a ZnO seed layer was suspended upside-down in the growth solution at 90 °C with a typical growth period of 36 h, so that vertically aligned ZnO NW arrays approximately 10 μm in length and 50−100 nm in diameter were synthesized without any formation of ZnO 7378

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Figure 1. Schematic diagram representing the growth mechanism of Cu shells on vertically aligned ZnO NW arrays by the conventional ELD method (a), typical FE-SEM images of Cu/ZnO core−shell NWs arrays for growth times of 10 min (b) and 60 min (c), and typical FE-TEM images (d) and EDX analysis (E) corresponding to the growth conditions in (b).

Figure 2. Schematic diagram to represent formation of conformal Cu shells on vertically aligned ZnO NWs by the EAELD approach (a), typical FESEM images (b, c) of ZnO/CuO core−shell NWs arrays after growth times of 6 h, a typical FE-TEM image (d) and SAED pattern (e) of single ZnO/CuO core−shell NWs (d), and EDX elemental mapping of Cu (f), O (g), and Zn (h) for the NWs represented in (d).

from the oxidation reaction of formaldehyde, used as a reducing agent on the surfaces of the catalyst. Here, a complexing agent for Cu ions (PSTT) in the ELD solution plays an important role preventing precipitation under alkaline solution conditions. In addition, the electrochemical oxidation of the reducing agent (formaldehyde) on the surfaces of Pd or Cu proceeds according to the following steps.29,30

contaminants on the surfaces of as-synthesized ZnO NWs under an air flow of 100 sccm at a heating rate of 10 °C min−1. The optical properties of the annealed ZnO NWs were investigated by photoluminescence (PL, Jasco FP-6500) measurements.

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RESULTS AND DISCUSSION Formation of Cu Shells on Vertically Aligned ZnO NW Arrays by Conventional ELD. Figure 1a is a schematic diagram representing the growth mechanism for the formation of Cu shells on vertically aligned ZnO NW arrays by the conventional ELD method. In the first step, catalytic Pd nuclei on the surface of ZnO NW arrays are generated via an electrochemical oxidation−reduction reaction. Subsequently, Cu ions in the ELD solution are reduced by electrons released

H 2CO + H 2O → H 2C(OH)2 (methylene glycol)

(1)

H 2C(OH)2 + OH− → H 2C(OH)O− + H 2O

(2)

[H 2C(OH)O−]ads_Pd + 2OH− → HCOO− + 2H 2O + 2e− (3) 7379

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Figure 3. FE-SEM image (a) and XPS analysis (b) of ZnO/CuO core−shell NW arrays after reduction under hydrogen environment at 200 °C for 1 h and typical FE-TEM images (c, d) and SAED pattern (e) for single NW represented in (a).

of low-density ZnO NW arrays. Furthermore, the manipulation of several key variables, such as elevated bath temperatures and the concentrations of the complexing agents, could not enhance the diffusion effects of Cu ions (Figure S2, Supporting Information). These results indicate that uniform Cu deposition cannot be achieved on vertical ZnO NW arrays through the conventional ELD process, due to the intrinsic limitations of the mass transfer process. Formation of Conformal CuO Shells on Vertically Aligned ZnO NW Arrays by EAELD. For the EAELD approach, CuO shells were generated uniformly on ZnO NW arrays about 10 μm in length through several sequential steps, as shown in the schematic diagram in Figure 2a. In the first step, colloidal Cu2O NPs in the solution were attached with good uniformity to ZnO NW arrays under EAELD bath conditions, and the sample was then oxidized in an elevated air environment for 1 h, leading to the formation of CuO seeds on ZnO NW arrays. CuO shells with spikelike nanostructures due to preferred growth patterns were formed at relatively slow deposition rates under EAELD bath conditions, resulting in a kinetic-limited process. Figure 2b shows a typical FE-SEM image of ZnO/CuO core−shell NW arrays grown by the EAELD approach. As shown in Figure 2c, magnified FE-SEM images for the top and bottom regions exhibit clearly conformal spikelike CuO shells. Figure 2d shows a typical FE-TEM image of an individual ZnO/CuO core−shell NW, indicating that the average thickness of the CuO shell after a growth time of 6 h was 40 nm. The corresponding SAED (Figure 2e) and XRD patterns

2[H 2C(OH)O−]ads_Cu + 2OH− → 2HCOO− + 2H 2O + 2e−

(4)

where the subscripts ads_Pd and ads_Cu denote the adsorptions of species on the surfaces of Pd and Cu, respectively. Figure 1b is a typical FE-SEM image of Cu deposition on vertically aligned ZnO NW arrays after 10 min of growth using the conventional ELD process. As shown in Figure 1d,e (typical FE-TEM image and EDX spectra for an individual NW pictured in Figure 1b, respectively), Cu growth clearly occurs on the predeposited Pd catalyst. As the growth time increases, uniformity of Cu deposition on ZnO NW between top and bottom region were getting worse as shown in a FE-SEM image of Figure 1c for growth time of 60 min. This result is due mostly to mass transfer effects, indicating that it is difficult for the Cu complex or Cu ions to penetrate into the bottom region by traveling the entire length of the NW without being reduced and deposited. As an effort to alleviate the limitations of mass transfer in the deposition process, the densities of the ZnO NW arrays were controlled by controlling the concentrations of reactants (Zn(NO3)2 and C6H12N4) and additive PEI (Figure S1, Supporting Information). However, there was no significant improvement in the uniformity of Cu deposition along the vertical ZnO NWs despite the changes in the geometric shapes of the ZnO NW arrays (Figure S1, Supporting Information). This strongly suggests that the diffusion length of the Cu complex or Cu ions in solution exceeds the inter-NW spacing 7380

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Figure 4. FE-TEM images of Cu NPs deposited on ZnO NW for 2 h (a, b) and 8 h (c, d).

EAELD Mechanism for the Formation of Conformal CuO Shell on ZnO NW Arrays Solution Thermodynamics Aspects of EAELD Solution. In the initial stage of the EAELD approach, Cu2O NPs on the surface of the ZnO NW arrays were formed mainly under EAELD conditions, as shown in FE-TEM images (Figure 4) of the samples after dipping for periods of 2 (a, b) and 8 h (c, d). Cu2O NPs with good step coverage in both the top and bottom regions of vertical ZnO NW arrays were created, leading to the formation of uniform seed layers in the next step of the process. Since no catalyst was used in this approach, the heterogeneous nucleation of Cu related species on the surface of ZnO NW was difficult without catalytic activity for the oxidation of reducing agents, indicating that the formation of Cu2O NPs on ZnO NW cannot be explained by the conventional ELD mechanism. Furthermore, there were no significant changes in the amounts of Cu2O NPs deposited as the dipping time was increased up to 8 h (Figure 4c,d). This finding will be addressed in a later section. To elucidate the phenomenon observed in Figure 4, it is necessary to understand basic chemical reactions in the EAELD solution, which is composed of a source of Cu ions, a reducing agent (HCHO), a complexant (PSTT), and a buffer (Na2CO3) in alkaline solution. The major chemical reactions of Cu ions taking place in solution are as follows:

(Figure S3, Supporting Information) indicate that the CuO shells were composed mainly of CuO (111) and CuO (002). Figures 2f−h are EDX mapping images corresponding to Cu, O, and Zn elements. Our results indicate that the EAELD method can overcome inherent diffusion limitations of the conventional ELD method. Vertically uniform ZnO/CuO core−shell NWs arrays are a promising platform to improve energy efficiency for energy applications such as solar cells because of the increased junction area afforded by the formation of conformal CuO shells on vertically aligned long ZnO NW arrays. On the other hand, Figure 3a shows a typical FE-SEM image after the reduction of as-synthesized ZnO/CuO core−shell NW arrays under a hydrogen environment at 200 °C for 1 h. The inset of Figure 3a indicates that spikelike nanostructures were changed into NPs on the surfaces of ZnO NWs after reduction. According to the FE-TEM results shown in Figure 3c,d, these NPs have average diameters of 20 nm. The corresponding SAED pattern shown in Figure 3e suggests that the CuO shells were reduced to Cu2O and Cu under these reduction conditions. Figure 3b shows XPS spectra of Cu 2p before and after the reduction of ZnO/CuO core−shell NWs, indicating that the oxidation state of Cu2+ changed to either Cu0 or Cu+ and showing good agreement with SAED patterns. Therefore, this nanostructure can be applied directly for the formation of novel catalysts in the fields of methanol synthesis and hydrogen production from the reformation of methanol.9,24 Detailed results for high performance catalysts using this core−shell NW arrays will be reported in the near future elsewhere.

Cu 2 + + xOH− → Cu(OH)x 2 − x Cu 2 + + x T2 − → CuTx 2 − 2x

(x = 1, 2, 3, 4)

(x = 1, 2)

Cu 2 + + 2T2 − + 2OH− → Cu(OH)2 T2 4 − 7381

(5) (6) (7)

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Figure 5b shows thermodynamic predictions of temperature dependence for the alkaline solution of pH 11 used in this work. Although most of the formation enthalpies ΔHr° in this work were well-known (Table S1, Supporting Information), those of the Cu−tartrate complexes (CuT, CuT22−, and Cu(OH)2T24−) have not been reported. Based on available experimental data regarding the temperature dependence of equilibrium constants,32,33 the ΔHr° (−133.7 kJ) of CuT was determined using the van’t Hoff equation. In general, ΔHr° for a metal−ligand complex depends on the metal species and the combining number of the ligand but has the same sign for the same metal regardless of the combining number.34,35 Because of absence of available information for the other Cu−tartrate complexes, we assumed that the values of ΔHr° for all of the Cu−tartrate complexes were the same as that of CuT in qualitative thermodynamic analysis in this study. As shown in Figure 5b, we found that significant amounts of Cu(OH)2T24− could be dissociated into Cu(OH)3− and Cu(OH)42− as the solution temperature increased. According to this analysis, we predicted that 50% of Cu(OH)2T24− was dissociated at the same elevated temperature of 60 °C as under EAELD conditions in this study. The coordinating binding energy of Cu(OH)2 is expected to be lower than CuT due to the low electrostatic charge of Cu(OH)2T24− as compared to Cu2+. Therefore, the thermodynamic prediction for the dissociation of Cu(OH)2T24− is overestimated due to the assumption that it has the same ΔHr° as CuT. In addition, the inset of Figure 5b shows clearly that the addition of the reducing agent (1.3 mM HCHO) to this solution at 60 °C did not lead to precipitation over time periods up to 20 h. Meanwhile, Figure 5c shows the thermodynamic results for a 2-fold concentrated solution of all chemical species to verify the degree of dissociation of Cu(OH)2T24− with variation of bath temperature. As shown in the inset of Figure 5c, the initial blue color of the EAELD solution with the addition of the reducing agent (1.3 mM HCHO) remained transparent over time periods up to 20 h at 25 °C but changed to grass green and finally brown at 60 °C. This indicates that elevated concentrations and bath temperatures can lead to the formation of precipitates due to homogeneous nucleation according to anodic oxidation of reducing agents (reactions 1−4) and cathodic reduction of Cu ions released by the dissociation of Cu(OH)2T24−. Therefore, we concluded that the qualitative thermodynamic analysis in this work is acceptable. In addition to these results, the dissociation of Cu(OH)2T24− complexes at elevated temperature can lead to homogeneous nucleations, such as Cu, CuO, and Cu2O, according to wellknown potential−pH equilibrium diagram of Cu ions and reducing agents for the EAELD conditions used in this study.36 Based on this diagram, the homogeneous nucleation of CuO and Cu2O is thermodynamically more favorable compared to that of Cu in the alkaline solution of pH 11 used in this study. Therefore, we concluded that the EAELD solution of pH 11 at 60 °C used in this study is composed of CuO, Cu2O, Cu(OH)3−, Cu(OH)42−, and Cu(OH)2T24−. Since precipitation did not occur as shown in the inset of Figure 5b, the colloidal Cu2O NPs are considered to be responsible for the formation of Cu2O on the surfaces of ZnO NWs, as shown in Figure 4. Electrophoretic Deposition of Cu2O NPs on Vertical ZnO NW. The sidewalls of hydrothermally grown ZnO NWs with typical (101̅0) planes were recently reported to be positively charged in an alkaline solution of pH 11 according to

As shown in reactions 6 and 7, three different structures of copper tartrate (T) complexes may exist depending on pH.31 To investigate their relative concentrations in EAELD solution as a function of pH, Figure 5a represents thermodynamic

Figure 5. Thermodynamic modeling of chemical compositions of the copper−tartrate system in EAELD solution. pH dependence of the EAELD solution at 25 °C (a), bath temperature effects of the EAELD solution (b), and bath temperature effects for a 2-fold concentrated EAELD solution (c). The insets of each graph are photographs of the solutions as a function of time under specific conditions.

modeling calculated using known constants for specific conditions (Tables S1 and S2, Supporting Information). In this modeling study, the same chemistries were considered as the EAELD solution except for the reducing agent at 25 °C. It was estimated that most of the Cu ions were chelated as Cu (OH)2 T24− at pHs higher than 6.5. Addition of a reducing agent (1.3 mM HCHO) to an EAELD solution of pH 11 did not cause any metal precipitation in bulk solution at 25 °C up to 20 h (inset, Figure 5a), indicating good agreement with the thermodynamic modeling study. 7382

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Figure 6. FE-SEM images of as-synthesized ZnO/CuO core−shell NWs arrays grown after annealing process on ZnO NW (a−f), TGA analysis of as-synthesized ZnO NW arrays (g), and PL spectra of as-synthesized ZnO NW arrays with the variation of annealing temperatures (h). The inset of (g) represents TGA analysis of PEI.

Meanwhile, CuO shells were not formed at annealing temperatures over 800 °C (Figure 6e,f), primarily because Cu2O seeds were not formed on the surfaces of ZnO NWs in the first step of the EAELD process after annealing the ZnO NWs. Therefore, drastic changes may have occurred on the surfaces of the annealed ZnO NWs at temperatures greater than 800 °C. Room temperature PL was conducted for annealed ZnO NWs to explore the influence of annealing, as shown in Figure 6h. Two characteristic PL peaks were found in samples of annealed ZnO NWs at temperatures up to 600 °C. One peak is the ultraviolet emission peak near 380 nm, also known as near band edge emission. The other peak, which is centered at ∼580 nm, is related to yellow emission that is typically exhibited by samples prepared by hydrothermal solution synthesis.42,43 In Figure 6h, an intense deep-level (DL) emission was detected near 515 nm from a sample annealed at 800 °C, only. Although the origin of DL emissions remains controversial, it is confirmed that there is a strong relationship between DL emissions and surface defects of ZnO.44,45 According to the previous reports with experimental results and density-functional calculations, this phenomenon can be attributed to an increase of oxygen antisite defect (OZn2−), which is an oxygen replacing Zn in a lattice site, thus forming a chemical bond (O− O) with one of the oxygen nearest neighbors.46 The oxygen antisite with high formation energy and band gap (2.38 eV) can be created under oxygen-rich conditions at high temperature and can also induce this DL emission.47 These phenomena are consistent with our results, indicating that the surface charges in annealed samples over 800 °C turned into negative direction. Therefore, we strongly suggest that the sidewall charges of ZnO NWs annealed at more than 800 °C changed to prevent the electrostatic adsorption of colloidal Cu2O NPs in EAELD

measurements by atomic force microscopy and X-ray photoelectron spectroscopy.37 Meanwhile, it was reported that colloidal Cu2O with isoelectric points of pH 5.038 had more negative charge than CuO of pH 9.539 under typical electroplating electrolyte. Therefore, it can be expected that electrostatic attachments of Cu2O NPs to ZnO NWs in ELD solution is favorable at the positively charged sidewalls of ZnO NWs. To confirm the electrostatic attachment of colloidal Cu2O NPs, the contributions of organic additives such as PEI, which are attached to the surfaces of ZnO NWs for sidewall passivation during the hydrothermal growth of vertically long ZnO NW arrays, should be investigated. In addition, PEI is a well-known chelating agent of Cu ions due to its imino groups in high-pH solutions,40,41 which leads to the formation of Cu2O seeds on the surfaces of ZnO NWs, as shown in Figure 4. To explore this issue, Figure 6a−f shows FE-SEM images of ZnO/ CuO core−shell NW arrays grown after annealing of assynthesized ZnO NWs in air and a subsequent EAELD process. For the thermal decomposition of organic additives, the assynthesized ZnO NW arrays were annealed while varying the temperature from 400 to 900 °C, with the other EAELD conditions kept fixed for the purpose of comparison. As shown in Figure 6a−d, CuO shells formed on ZnO NW arrays annealed at temperatures up to 700 °C, indicating that CuO seeds were formed by the attachments and oxidation process of Cu2O. However, the thermal decompositions of organic additives were nearly complete over 600 °C, according to TGA analyses of as-synthesized ZnO NW arrays. Furthermore, the inset of Figure 6g shows that thermal decomposition of PEI occurs mainly up to 400 °C. Therefore, it is clear that organic contaminants in the sidewalls of ZnO NWs are not responsible for the formation of CuO seeds. 7383

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solution. Finally, we concluded that the positive sidewalls of ZnO NWs are primarily responsible for the formation of Cu2O seeds observed in the EAELD approach, as shown in Figure 4a. In addition, it can be explained that the attached amounts of Cu2O NPs were determined by electrostatic forces of the positive ZnO NW sidewalls, instead of dipping time (Figure 4b). Conformal Deposition of CuO Shells on Vertical ZnO NW. Based on electrostatic interactions, the attachment of conformal Cu2O NPs to vertically aligned long ZnO NWs is a key process required to form uniform CuO seeds after thermal oxidation. After the formation of CuO seeds on the sidewalls of ZnO NWs, each sample was again dipped into EAELD solution so that CuO shells were grown continuously. According to thermodynamic modeling (Figure 5b), Cu(OH)3− and Cu(OH)42− formed from Cu ions in solutions of pH 11 that were liberated by Cu complexes at the elevated temperature of 60 °C. In EAELD solution, the following reaction can be considered.21,48

ASSOCIATED CONTENT

S Supporting Information *

FE-SEM images representing the effects of array density, bath temperature, and chemical composition on Cu deposition in the conventional ELD approach (Figures S1 and S2), XRD patterns of copper-based materials synthesized in the EAELD approach (Figure S3), absorption spectra of ZnO/CuO NW arrays (Figure S4), and thermodynamic modeling details for species distribution (Tables S1 and S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.I.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (NRF-2013R1A1A4A01012891). This study was also supported by the Ministry for Health, Welfare, and Family affairs (A101834), Republic of Korea. S. Lee thanks for the financial support from the Korea Institute of Science and Technology (KIST) Institutional Program.

Cu(OH)x 2 − x (aq) → CuO(s) + (x − 2)OH− + H 2O (x = 3, 4)

Article

(8)

Therefore, Cu(OH)3− and Cu(OH)42− play roles as precursors to CuO. Then, colloidal CuO NPs were generated in solution by homogeneous nucleation because precipitation was not observed in EAELD solution in this work. CuO NPs were supplied to form spikelike CuO shells exhibiting their own preferred growth, as shown in Figure 2a. This agrees with conventional CuO growth mechanisms proposed in previous works.21,48 In this step, a very slow kinetic rate was observed during CuO deposition compared to the conventional Cu ELD process, which made it possible to avoid mass transfer limitations for the formation of conformal ZnO/CuO core− shell NW arrays. Total deposition times greater than 6 h were required to grow CuO shells with average thickness of 40 nm. Finally, conformal CuO deposition onto long ZnO NWs was achieved due to the kinetic limited process used in this step. The optical property of ZnO/CuO core−shell NW arrays exhibited an absorption range from visible to ultraviolet, indicating a promising potential to improve solar power efficiency (Figure S4, Supporting Information). Further studies are underway to apply this hetero-nanostructure to applications such as solar cells and novel catalysts.

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REFERENCES

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CONCLUSIONS We successfully fabricated vertically conformal ZnO/CuO core−shell NW arrays with high aspect ratios using a novel EAELD method that overcomes the inherent drawback of diffusion-limited processes in the conventional ELD approach. One of the key processes in the EAELD method is the formation of uniform CuO seeds on the sidewalls of ZnO NWs by the electrophoretic attachments of colloidal Cu2O NPs and subsequent oxidation. The detailed mechanism for this process was proven by thermodynamic analyses of EAELD solution and experimental evidence. We conclude that kinetic-limited CuO deposition on initial CuO seeds may be effectively achieved during the formation of CuO shells on core ZnO NWs. Since the conformal hetero-nanostructure introduced in this work can support optimized platforms to maximize junction areas and active sites, our work offers a new way to enhance total performance in applications such as solar cells and catalysts in the near future. 7384

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