Electrochemical assembly of ZnO architectures via deformation and coalescence of soft colloidal templates in reverse microemulsion

Abstract

Electrodeposition of ZnO in a novel electrolyte, namely reverse micelles containing zinc nitrate and dissolved oxygen, has been successfully performed. By simply altering the applied current densities or using different substrates, various morphologies are obtained, from 2D ultrathin nanosheets to 3D hierarchical flower-like morphologies, which are quite different from those obtained in conventional electrolytes. Importantly, the behaviors of the soft colloidal templates have a great influence on the morphologies of the ZnO nanostructures. It can be inferred that the versatile deformation, coalescence and rearrangement of the soft colloidal templates also contribute to the specific surface morphologies. The soft colloidal templates are inclined to coalesce and assemble into larger templates on ITO, whereas deformation into different shapes is observed on carbon paper. In this way, an overall new idea to control the shapes and sizes of the template via electrochemistry is proposed.

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1. Introduction

In the past decade, emulsion synthesis has been demonstrated to provide efficient soft colloidal templates for preparing material architectures, with feasible and precise control of size, shape and crystallinity.^1–10 Typically, emulsions can be divided into macroemulsions and microemulsions according to their size features and thermodynamic stability. Macroemulsions are thermodynamically unstable solutions and their size features are varied, in a wide distribution and with poor uniformity. To overcome the instability, surfactants are usually employed to act as stabilizing agents in formation of thermodynamically stable microemulsions.¹¹ Surfactants are molecules with a polar hydrophilic head and a hydrophobic hydrocarbon tail, which are inclined to self-assemble into colloidal templates when exceeding a critical concentration. Various well-defined shapes can be obtained by using surfactants with different functions and geometry, as well as by tuning the ratio of surfactant in surfactant (sometimes with cosurfactants)–water–oil systems.^12–15 Based on their solvents, microemulsions can be classified as either direct micelles (oil in water) or reverse micelles (water in oil). As most soft colloidal template syntheses of nanoparticles are confined to the water phase, water-in-oil, or reverse micelles or microemulsions, have attracted much more attention.¹¹ Reverse micelle solutions are homogeneous/isotropic systems, consisting of three different phases in thermodynamic equilibrium, namely water droplets stabilized in a continuous oil phase with the assistance of a surrounding monolayer of preferentially aligned surfactants and cosurfactants.^16–18

Traditionally, for the soft-colloidal template synthesis of target materials, two different micelles or microemulsion droplets containing two types of reaction precursors are prepared.^19–25 The reaction is enabled when the two different droplets fluctuate and collide with each other, and subsequent treatments comprise the separation of products, which is fairly complicated. The aim of this study is therefore to extend this type of electrolyte to an electrolyte in which the soft template is directly adhered to an electrode, offering simple and precise control of the dimensions of the as-synthesized structures.²⁶ Our previous work involved the electrochemical synthesis of a conducting polymer and a transition metal oxide in water-in-oil microemulsion, namely reverse micelles and their application in supercapacitors.^27–29 The results indicated that a dispersed soft colloidal template in this kind of novel electrolyte solution contributed to the uniform size distribution of the above materials. Meanwhile, it is clear that in addition to the soft template, the current density or potential applied to the electrode and the composition of solutions also influence the size, shape and composition of the materials. Therefore, further research is needed on the growth mechanism of materials in this kind of electrolyte.

ZnO, a typical II–VI compound semiconducting material with a wide band gap of 3.37 eV and a large exciton binding energy, has attracted much attention as a result of its potential applications in various fields such as UV detector, electro-optical switches, chemical sensors, photo-catalysts, and dye-sensitized solar cells.^30–35 To date, a series of works concerning cathodic electrodeposition in aqueous solution or organic electrolyte have been reported and a diversity of ZnO nanostructures, including nanorods, nanowires, nanotubes, nanosheets, nanobelts, nanoplates and nanocombs have been obtained.^36–43 It has been found that overpotential, temperature and composition of the electrolyte have significant influence on the morphology and composition of the nanostructure.^44–46 However, to the best of our knowledge, no work on the electrodeposition of ZnO and its mechanism in this unconventional electrolyte has been reported. As stated above, it is of great significance to schematically evaluate the morphologies and compositions of the ZnO precursors obtained via electrochemical assemblies in this novel electrolyte.

In this contribution, a simple method is demonstrated for obtaining ZnO nanostructures with different morphologies (from 2D ultrathin nanosheet to 3D nanorods) by changing current densities, without tuning the ratio of surfactant in the reverse micelle. A comprehensive investigation is carried out on the effect of preparatory conditions on the morphologies and compositions of zinc oxides, including the current densities and substrates. It is the electric field at the interface of electrode/electrolyte that mainly controls the nucleation, growth and behaviors of soft templates and afterwards influences the morphologies. A probable mechanism is then proposed, indicating a new method for controlling the shapes and sizes of the soft template. This work not only lays a good foundation in fundamental electrochemistry and materials science but also shows diverse prospects and potentially valuable applications.

2. Experimental

Materials

The nonionic surfactant p-octyl polyethylene glycol phenyl ether (Triton X-100), cosurfactant n-hexanol and n-hexane, zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), sodium nitrate (NaNO₃) and nitric acid (HNO₃) were used. All chemical reagents used were analytical grade without further purification. All solutions were prepared with deionized water.

Preparation of reverse micelle

An electrochemical bath of a water-in-oil microemulsion (W/O) was prepared by dispersing water droplets with reactant into continuous oil phase, in accordance with the previous work.^27–29 The oil phase contained 60 mL of n-hexane, 60 mL of n-hexanol and 32.1 g of Triton X-100, and the water droplets were obtained by dissolving Zn(NO₃)₂·6H₂O (0.1 mol L⁻¹) and 0.1 mol L⁻¹ NaNO₃ into 0.8 mol L⁻¹ nitric acid aqueous solution.

Electrochemical assembly of ZnO architecture

ZnO precursors of different morphologies were electrochemically grown on carbon paper or ITO substrates in a three-electrode configuration with Ag/AgCl (with saturated KCl) as reference electrode and a graphite rod of 12 cm in diameter being a counter electrode at room temperature. Electrodeposition was performed with a CHI 1140A electrochemical workstation. Before electrodeposition, the carbon paper and ITO substrates were first washed with deionized water and then ethanol by ultrasonication. Ranges of current densities and growth time were applied in electrodeposition. Afterwards, the assembled films were rinsed thoroughly in deionized water and ethanol, and then dried at 120 °C in a vacuum drying oven.

Characterizations and electrochemical analyses

The morphologies and microstructures of the assembled films were observed by X-ray diffraction (XRD, Rigaku, D/max-RB), field emission scanning electron microscopy (FESEM, JEOL, JSM-6701F), and transmission electron microscopy and high resolution TEM patterns (TEM, JEOL, JEM-2010). Crystal structures were analyzed by X-ray diffraction (XRD, Rigaku, D/max-RB) in the 2θ range of 30–80° at a rate of 5° s⁻¹ while the compositions of the surface were characterized by X-ray photoelectron spectroscopy (XPS), performed on a Kratos AXIS Ultra DLD with monochromatic Al Kα (hν = 1486.6 eV). All XPS spectra were corrected according to the C 1s spectra at 284.8 eV. Curve fitting and background subtraction were done using XPSPEAK software.

3. Results and discussion

The FESEM photographs of the ZnO fabricated on ITO substrates at different current densities in the water-in-oil microemulsion are shown in Fig. 1. All other preparatory conditions are the same in all conditions while the charge loadings for all samples are 1000 mC. Clearly, the morphologies are influenced by the applied current densities. As revealed in Fig. 1a, the substrate is covered by nanoplates with a thickness of 100 nm when the applied current density is 1 mA cm⁻². The nanoplates have a tendency of linking and rolling together. When a larger constant current of 3 mA cm⁻² is applied, the nanoplates tend to self-assemble into flower-like nanostructures with a diameter of about 2 μm and the petals stretch parallel to the substrate, as shown in Fig. 1b. This phenomenon is ascribed to the slow diffusion rate of reactant which hinders the growth of [002] direction and the soft colloidal template plays a key role in restriction of the size of the flower-like morphologies. However, the petals tends to grow almost perpendicularly to the substrate when the current density is increased to 5 mA cm⁻², indicating preferential growth on (002) facet, shown in Fig. 1c. A probable reason for this will be discussed later. In Fig. 1d, a rough and porous surface with particles growing together is obtained at 8 mA cm⁻². When 10 mA cm⁻² constant current is employed, a faster nucleation rate results in formation of octahedral nanostructures with a size of 500 nm. Interestingly, the octahedral particles are made up of clusters of about 2 μm in size, which can be seen in Fig. 1e. As shown in Fig. 1f, with a further increase of current density to 15 mA cm⁻², it was found that the hexahedral rods self-assembled into a hierarchical flower-like morphology with a size of 1 μm.

Fig. 1 FESEM of photographs of the as-prepared ZnO thin film electrodeposited on ITO substrates at different current densities. (a) 1 mA cm⁻² (b) 3 mA cm⁻² (c) 5 mA cm⁻² (d) 8 mA cm⁻² (e) 10 mA cm⁻² (f) 15 mA cm⁻².

To investigate the effect of substrates on the surface morphologies, carbon paper was employed for electrochemical assembly of ZnO architectures for comparison. The FESEM and the corresponding TEM images of ZnO electrodeposited onto carbon paper substrate are shown in Fig. 2. The morphologies of the ZnO obtained on carbon paper are quite different from those on ITO substrates. Fig. 2a shows a thinner nanosheet covering the substrate at 1 mA cm⁻², which resembles graphene. The corresponding TEM shows the obtaining of an ultrathin nanosheet. With increase of current density, the ultrathin nanosheet turns into nanoplates assembled into different shapes. The sizes of the hierarchical aggregates are much smaller than those assembled on ITO substrates, which may be because of the fast nucleation rates in all directions. This phenomenon can be ascribed to deformation rather than coalescence of the soft templates at the interface of carbon paper/solution.

Fig. 2 FESEM photographs of the as-prepared ZnO thin film electrodeposited on carbon paper substrates at different current densities. (a) 1 mA cm⁻² (b) 3 mA cm⁻² (c) 5 mA cm⁻² (d) 8 mA cm⁻² (e) 10 mA cm⁻² (f) 15 mA cm⁻².

The mechanism of ZnO deposition by electrochemical methods in both aqueous and organic solutions has been widely studied. This process is based on electroreduction of dissolved oxygen and/or NO₃⁻ ions to generate OH⁻ ions at the surface of a working electrode. The generated OH⁻ reacts with Zn²⁺ ions instantaneously to form zinc hydroxide, which is then dehydrated to produce the more stable zinc oxide. The general scheme for the electrodeposition of ZnO from aqueous zinc nitrate solution is thought to be as follows:^47–49

NO₃⁻ + H₂O + 2e → NO₂⁻ + 2OH⁻ (E^θ = −0.240 V vs. SCE) (1)

NO₃⁻ + 6H₂O + 8e → NH₃ + 9OH⁻ (E^θ = −0.373 V vs. SCE) (2)

O₂ + 2H₂O + 4e → 4OH⁻ (E^θ = 0.151 V vs. SCE) (3)

Zn²⁺ + 2OH⁻ → Zn(OH)₂ → ZnO + H₂O. (4)

However, it has been found that the potential for the reduction of nitrate shifts positively in the presence of Zn²⁺, as Zn²⁺ ion can act as a catalyst in reduction of nitrate. Moreover, the reduction potential of Zn²⁺ ion to its corresponding metal also shifts positively. So it can be inferred that the electrodeposition of ZnO in the present system is expected to proceed as follows:⁴⁸

Zn²⁺ + NO₃⁻ + 2e → NO₂⁻ + ZnO (E^θ = 0.246 V vs. SCE) (5)

5Zn²⁺ + NO₃⁻ + 2H₂O + 8e → ZnO + NH₄⁺ (E^θ = 0.213 V vs. SCE). (6)

The probable mechanism for the behaviors of soft templates under an electric field is therefore proposed as in Scheme 1.

Scheme 1 The behaviors of the soft colloidal templates at the interface of ITO/solution, (a) with no current being applied; (b) with current being applied; (c) with different current densities.

It can be observed that the water phases constrained by the soft colloidal membrane are evenly dispersed in the continuous oil phase, as shown in Scheme 1a. When a current is applied, the droplets fluctuate and collide under the effect of the electric field, and the anions and cations inside the cores transfer between the adjacent polar aggregates. Thus, the deformation and coalescence of the soft template lead to formation of ion channels (Scheme 1b).^{50, 51, 53} The reactions at the interface of electrode/reverse micelle comprise three steps: first, the water phases electromigrate to the electrode; second, the anions and cations inside the cores diffuse to the surface of the electrode through the interface membrane; third, nucleation is enabled by the redox reaction of relevant ions. This process is remarkably different from that in the traditional electrolyte as the transporting behaviors at the interface membrane are more complicated and the interactive thermodynamic and kinetic properties are elusive. So it can be inferred that the nucleation of ZnO is controlled by the electric field at the interface of electrode/solution, while the growth is governed by the soft colloidal template. It is known that the ultrahigh field intensity near the electrode causes deformation, coalescence and rearrangement of the soft colloidal templates at the interface, as shown in Scheme 1c.^{52, 53} That is to say, it is the electric field at the interface of electrode/solution that finally influences nucleation and growth of ZnO. As shown in Scheme 1c, at different current densities, the soft colloidal template fluctuates, collides and self-assembles into different shapes. So the mechanism can be summarized: at 1 mA cm⁻² constant current, the growth of ZnO has a height advantage on nucleation rate which leads to the large nanoplate morphology, and the rearrangement of soft colloidal templates at the surface of electrode shown in Scheme 1c decides the continuous plane. With increasing current density, the growth rates decrease while the nucleation rates increase. So the morphologies within the soft template are quite different, from nanoplates to octahedral nanostructures and nanorods. Reassembly of the soft colloidal templates occurs because of the destruction of balance caused by the electric field. The dispersed droplets tend to deform and coalesce under the effect of the electric field and will separate when a critical current density is reached. So soft templates with different shapes can be presented. However, the nucleation, growth and behaviors of soft colloidal templates at the carbon substrate are very different. The nanoplates tend to aggregate into hierarchical morphologies with different shapes and sizes. This is because of the faster nucleation rate resulting in formation of smaller particles and because the template tends to deform rather than coalesce.

Cyclic voltammograms (CVs) and potential–time curves are demonstrated for further illustration of the different morphologies obtained on ITO and carbon paper substrate. Fig. 3a shows the CVs of the ITO and carbon paper in the W/O microemulsion without Zn²⁺ ions; the polarization current for ITO substrates is higher than that of carbon paper, which indicates a faster depolarization ability. The nucleation and growth rates for ITO substrate are higher. With Zn²⁺ ions added in the solution, the CVs are clearly different. As shown in Fig. 3b, the reduction of nitrate becomes easier and the potential shifts positively in the presence of Zn²⁺, which is in accordance with the above-stated mechanism. Potential–time curves obtained with the constant current density of 1 mA cm⁻² are shown in Fig. 3c (without Zn²⁺) and Fig. 3d (with Zn²⁺). The polarization potential for the ITO substrate is lower than that of carbon paper, see Fig. 3c. In the presence of Zn²⁺, the reduction potential of nitrate shifted positively.

Fig. 3 CVs for the ITO and carbon paper in the W/O microemulsion containing no Zn²⁺ ions (a), and with Zn²⁺ (b). Potential–time figures for the ITO and carbon paper in the W/O microemulsion containing no Zn²⁺ ions (c), and with Zn²⁺ (d).

The structure and crystal parameters of ZnO assembled onto carbon paper with different current densities are characterized by XRD spectra, shown in Fig. 4. The sharp diffraction peaks (Fig. 4a) are assigned to carbon substrates. The other weak diffraction patterns can be indexed as hexagonal wurtzite structures (hexagonal phase, space group P63mc) according to JCPDS card (no. 36-1451), which can be clearly observed in Fig. 4b. The four major peaks arising from the (100), (002), (101) and (102) planes of hexagonal structure indicate that they are polycrystalline. It is noted that the relative peak intensities for the ZnO film are different from the standard power sample, indicating differences in their crystallographic orientation. Clearly, there is preferential growth orientation on the (002) plane. To analyze the detailed change of crystal orientation, orientation indices of the three major peaks were calculated for each XRD pattern using the following method. An intensity factor (IF_hkl) is defined as the ratio of the peak intensity for the diffraction peak of interest, relative to the sum of the intensities of the diffraction peaks.^47,48 For example, the calculation of the (100) plane is presented as follows:

Fig. 4 X-ray diffraction patterns of the as-prepared ZnO films on carbon paper with different current densities. (a) 1 mA cm⁻²; (b) 5 mA cm⁻²; (c) 10 mA cm⁻²; (d) 15 mA cm⁻².

The I_(hkl) refers to the diffraction peak intensity in XRD. The standard intensity factors IFS_(hkl) were obtained from the data of a standard powder sample (JCPDS). Then the intensity factor (IF_hkl) was compared with standard intensity factors (IFS_(hkl)) using the defined formula:

Any crystal faces for which these values exceed 1 are preferentially oriented. The specific values can be seen in Table 1. It is found that all the ZnO films on carbon paper substrates with different current densities are oriented with its (100) and (002) planes. But the (100) preferential orientation is less significant compared with the preferential growth on the (002) plane. The OIS₍₁₀₁₎ value becomes significantly smaller than 1. The results clearly demonstrate the preferential growth parallel rather than perpendicular to the substrate, which is consistent with the flower-like morphology observed for these films in Fig. 2.

Table 1 Intensities of the diffraction peaks (I_(hkl)) of the ZnO films, intensity factors ((IF_(hkl)) = I_(hkl)/ΣI_(hkl)) and orientation indices relative to the standard powder sample ((OIS_(hkl)) = IF_(hkl)/IFS_(hkl))

Sample	hkl	I(hkl)	IF(hkl)	OIS
Standard card	(100)	57	0.284
	(002)	44	0.219
	(101)	100	0.498
a	(100)	77.78	0.297	1.046
	(002)	84.03	0.321	1.466
	(101)	100	0.382	0.767
b	(100)	89	0.317	1.116
	(002)	92.1	0.327	1.493
	(101)	100	0.356	0.715
c	(100)	97.54	0.335	1.180
	(002)	93.44	0.321	1.466
	(101)	100	0.344	0.691
d	(100)	83.4	0.302	1.063
	(002)	92.89	0.336	1.534
	(101)	100	0.362	0.727

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To gain further insights into the structural features and crystallography of the nanosheet of the hierarchical aggregates, TEM and HRTEM techniques were employed to investigate the sample under the condition of 8 mA cm⁻². The flower-like morphology is composed of many ultrathin sheets, see Fig. 5. In the high-magnification image of the ultrathin deposit, a uniform lattice image can be observed with inter-space calculated to be 2.6 Å over the entire sheet, corresponding to the inter-space of the (002) plane. The results again confirmed the preferential growth of ZnO with the (002) plane on carbon paper substrates.

Fig. 5 TEM and HRTEM of the ZnO film electrochemical assembled at 8 mA cm⁻².

The surface compositions of the ZnO architectures were analyzed by X-ray photoelectron spectroscopy (XPS). A wide survey spectrum with the applied current density of 8 mA cm⁻² is shown in Fig. 6a, with the observed peaks corresponding to the elements Zn, O and C.^54,55 The Zn 2p spectrum in Fig. 6b shows a doublet whose binding energies are 1021.4 and 1044.4 eV, attributed to Zn 2p_3/2 and Zn 2p_1/2, respectively, indicating the presence of a single Zn²⁺ divalent state, corresponding to both Zn(OH)₂ and ZnO. The spin–orbit splitting of 23.0 eV (±0.1 eV) is consistent with the literature value of 22.97 eV.⁵² Fig. 6c shows the N 1s narrow scan spectrum: the two peaks refer to ammonium (NH₄⁺) and nitrate (NO₃⁻) with binding energies of 399.8 and 406.7 eV, respectively. The result confirms the proposed mechanism. The O 1s core-level spectrum of ZnO is fitted with three Gaussian peaks. The lowest binding energy of 530.17 eV is assigned to the O²⁻ ions in Zn–O bonds of the hexagonal wurtzite structure of ZnO. The other two peaks located at 531.38 and 532.70 eV are related to the absorbed OH⁻ and NO₃⁻ on the surface with the ZnO. So it is concluded that the probable reaction at the interface of electrode/solution is eqn (6).

Fig. 6 XPS spectra of the ZnO film electrochemical assembled at 8 mA cm⁻²: (a) a wide survey, (b) Zn 2p, (c) N 1s, (d) O 1s.

4. Conclusions

In summary, a novel and interesting method for electrochemical assembly of ZnO with different nanostructures is demonstrated by simply altering the current densities and working substrates. It is found that the nucleation, growth and behaviors of soft colloidal templates on carbon paper and ITO substrate are quite different. The nucleation rate on the carbon paper substrate was much faster than on the ITO substrate at the same current density, which resulted in formation of smaller size ZnO. But the mechanisms for nucleation and growth of ZnO in the non-traditional electrolyte are more complicated for the remarkably special transfer of reactant through interfaces of different liquid phases because of the elusive interactive thermodynamic and kinetic parameters. It is inferred that the versatile deformation, coalescence and rearrangement of the soft colloidal templates under the electric field are influenced by current density and substrates, which in turn influence the surface morphologies. This work is of great significance overall for the new idea of controlling shapes and sizes of soft templates using electrochemical methods rather than chemical strategies, which broadens the applications of fundamental electrochemistry.

Acknowledgements

This work was supported by the National Science Foundation of China (no. 51322402, 81301345), the Program for New Century Excellent Talents in University (NCET-2011-0577), Ministry of Education of China, the Fundamental Research Funds for the Central Universities (FRF-TP-12-002B), and the National Key Fundamental Research (973) Program of China (no. 2011CB922204).

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