Morphology Control of Electrodeposited Cu2O Crystals in Aqueous Solutions Using Room Temperature Hydrophilic Ionic Liquids He
Li,†
Run
Liu,*,†
Rongxiang
Zhao,†
Yifang
Zheng,‡
Weixiang
Chen,†
and Zhude
Xu*,†
Department of Chemistry, Zhejiang UniVersity, Hangzhou, 200237, China, and Breeding Base State Key Laboratory of Green Chemical Synthesis Technology, Zhejiang UniVersity of Technology, Hangzhou 310014, P. R. China
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 12 2795-2798
ReceiVed June 27, 2006; ReVised Manuscript ReceiVed September 29, 2006
ABSTRACT: Room temperature hydrophilic ionic liquids, 1-methyl-3-ethylimidazolium salts containing ethyl-sulfate anions, have dramatic effects on the morphology changes of electrochemically grown Cu2O crystals at room temperature in aqueous solutions. The shape of Cu2O crystals evolves from cubic to octahedral and spherical shape only by adding a varied small amount of the ionic liquids in the deposited solutions. The possible mechanism has been explored, and the ethyl-sulfate anion is believed to play a key role in the morphology control. 1. Introduction The ability to tune the shape of inorganic crystals is of extraordinary importance because their electronic structure, bonding, surface energy, and chemical reactivities are directly related to their surface morphology. Isolated microcrystals or nanocrystals with monodispersed size and varied shapes obtained from solutions by the use of surfactants and by the precise control of the growth conditions have been well documented.1-5 Electrodeposition also has been shown to be well-suited to grow thin films composed of crystals with controlllable morphology.6 A unique feature of electrodeposition is the ability to tune the orientation, morphology, and chirality of electrodeposited films by controlling the electrochemical solution conditions, complexed agents, or dyes.7-12 These degrees of freedom are not available in vapor deposition. Room temperature ionic liquids (RTILs) have attracted great interest in recent years because of their unique chemical and physical properties, such as high chemical and thermal stability, negligible vapor pressure, high conductivity, wide electrochemical window, and the ability to dissolve a large variety of organic and inorganic compounds.13-15 RTILs have been used in various fields including organic synthesis, catalysis, and electrochemistry.16-19 Recently, RTILs also have been used in inorganic synthesis, including hollow TiO2 microspheres,20 lamellar silicas,21 metal nanostructures,22 ZnO, Te and Bi2S3 nanowires.23-26 Electrodeposition of nanocrystalline metals such as Al, Fe, Ag, and Al-Fe alloys and semiconductor Ge and Si in RTILs also has been reported.27-33 So far, most of the RTILs used on inorganic material synthesis were used as solvents; however, very few works on the additive effects of RTILs for inorganic material synthesis, especially for electrodeposited metal oxide thin films, have been reported. Electrodeposition of Cu2O from aqueous solutions with tunable morphology depends on the solution pH8,9 and organic additives10 have been reported. However, to the best of our knowledge, electrodeposition of Cu2O crystals with tunable morphology using RTILs has not been reported in the literature. Even though RTILs are normally expensive, relatively inexpensive RTILs used as additives (normally less than 1% in the solution) for morphology * To whom correspondence should be addressed. E-mail: runliu@ zju.edu.cn (R.L.);
[email protected] (Z.X.); tel: 86-571-87953390. † Zhejiang University. ‡ Zhejiang University of Technology.
control would not cost too much. Besides that, the cation and anion of RTILs may have synergic effects on the morphology changes of the deposits. Endres et al. have demonstrated that the cation of RTILs has effects on the morphology of Al metal electrodeposition.34 The RTILs might provide a new class of additives for the morphology control for electrodeposited materials. In this work, we demonstrate that truncated octahedral, octrohedral, and spherical shapes of electrodeposited Cu2O crystals can be obtained by adding a small amount of hydrophilic RTILs, 1-methyl-3-ethylimidazolium ethyl-sulfate ([MEIM]+[ES]-), in the aqueous deposition solution. The surface areas of spherical Cu2O crystals are expected to be larger than those of other shape crystals and were reported to have higher sensitivity and better selectivity to some flammable gases.35 Scheme 1. Molecular Structure of 1-Methyl-3-ethylimidazolium Ethyl-sulfate ([MEIM]+[ES]-)
2. Experimental Procedures The structure of the 1-methyl-3-ethylimidazolium ethyl-sulfate ([MEIM]+[ES]-) is shown in Scheme 1. The [MEIM]+[ES]- was synthesized using the method developed by Holbrey et al.36 Electrochemical experiments were carried out using a CHI 660B potentiostat/galvanostat. The deposition was performed in a threeelectrode cell with a platinum wire as counter electrode and a saturated calomel electrode (SCE) as reference electrode. Indium doped tin oxide (ITO) glass slide (≈20 Ω/cm) was used as a working electrode. Prior to electrodeposition, the ITO substrate was rinsed with acetone and distilled water and then cleaned with distilled water in an ultrasonic set for about 15 min. After that, the ITO substrate was dipped in nitric acid (10 wt %) for acid activation and followed by rinsing with distilled water. The deposition solutions were 0.02 M Cu(NO3)2 with different amounts of 1-methyl-3-ethylimidazolium ethyl-sulfate, ranging from 0 to 0.2% volume ratio. The deionized water was used to prepare all solutions. The pH of the solutions was adjusted to 5.5 in all the cases initially. The deposition temperature was 25 °C. The deposition potential of -0.25 V vs SCE was applied during all the depositions. X-ray diffraction spectra of the films were carried out in a θ-2θ and parallel mode [ω ) 1°, 2θ varied from 20° to 80° Cu KR1 radiation (λ ) 1.54056 Å)] using a Rigaku D/max 2550PC X-ray diffracometer with a thin film optic. Data were collected at a scan step of 0.02° and
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Li et al.
at conditions of 40 kV and 100 mA. Hitachi S-4700 II field emission scanning electron microscopy was used for observation of the microstructures of the films. TEM images’ selected area electron diffraction and high resolution transmission electron microscopy (HRTEM) were taken with a JEOL JEM-2010 high-resolution transmission electron microscope.
3. Results and Discussion Scanning electron microscopy (SEM) images of Cu2O grown on ITO in 0.02 M Cu(NO3)2 without and with adding 0.2% volume ratio [MEIM]+[ES]- are shown in Figure 1. The morphology of the crystals has fairly dramatic differences. Figure 1a shows a top view of Cu2O crystals grown without [MEIM]+[ES]-; cubic crystals composed of only {001} faces appeared on top of the ITO. When [MEIM]+[ES]- was added to the electrolyte, however, the crystal shape of Cu2O was dramatically changed from cubic to almost spherical shape (Figure 1b). The TEM results (Figure 1c) show that the spherical shaped Cu2O crystals actually have a flat side. The flat side was attributed to the connection between the Cu2O crystal and the substrate. The inset is the electron diffraction of the semispherical crystal and shows that the semispherical Cu2O crystal actually is single-crystal like. The high-resolution TEM result (Figure 1d) shows that the interatom distance is d ) 0.25 nm along a certain direction and is closed to the interplane distance of Cu2O crystal along the [111] direction. To determine the crystal structures of the deposits, the X-ray diffraction spectroscopy of the deposits was carried out in a θ-2θ and parallel mode [ω ) 1°, 2θ varied from 20° to 80° with CuKR1 radiation (λ ) 1.54056 Å)] using a Rigaku D/max 2550PC X-ray diffractometer with a thin film optic. The X-ray Bragg-Brentano scans of the deposited films obtained from 0.02 M Cu(NO3)2 without and with adding 0.2% [MEIM]+[ES]are shown in Figure 2, panels a and b, respectively. There are seven characteristic peaks that appeared in both XRD patterns at 29.6°, 36.4°, 42.3°, 52.5o, 61.6o, and 73.6°, respectively. They could be correspondingly indexed as (110), (111), (200), (211), (220), and (311) of the cubic Cu2O with space group of Pn3hm (a ) 4.267 Å), respectively. Peaks labeled with a star symbol belong to ITO substrates. There are no preferred orientations for both films. The results are consistent with the above TEM results, and the deposits are Cu2O crystals. We further studied the shape evolution of the Cu2O crystals by changing the amount of [MEIM]+[ES]- dissolved in 0.02 M Cu(NO3)2 aqueous solution while other solution conditions were the same. The constant potential -0.25 V vs SCE was applied in all the cases, and the deposition time was 2 ks and charges were about 0.25 C/cm2. In all cases, the electrodeposited materials were examined by XRD prior to SEM imaging and were found to belong to Cu2O. The SEM images of the deposits are shown in Figure 3a-f. Figure 3a-f shows that the shape of Cu2O crystals with perfect cubic, truncated octahedral, perfect octahedral, split octahedral, and spherical shape were obatined when the amounts of the [MEIM]+[ES]- in deposited solutions were 0, 0.02, 0.04, 0.06, 0.08, and 0.2% volume ratio, respectively. In our experiments, when the amount of the [MEIM]+[ES]- was above 0.08% in solutions, the shape of Cu2O crystals became spherical. The deposited durable time effects were also studied and are shown in Figure 4. From the SEM images, it can be seen that the deposited durable time only affects the grown crystal size not the morphology. The size of the Cu2O spherical crystals was changed from about 100 nm in diameter for a 50 s deposition time to 800 nm in diameter for a 1000 s desposition
Figure 1. SEM images of Cu2O electrodeposited from 0.02 M Cu(NO3)2 solutions (a) without [MEIM]+[ES]-, (b) with 0.2% [MEIM]+[ES]-. (c) TEM image of Cu2O electrodeposited from 0.02 M Cu(NO3)2 solutions with 0.2% [MEIM]+[ES]-. (d) HRTEM of Cu2O semisphere electrodeposited from 0.02 M Cu(NO3)2 solutions with 0.2% [MEIM]+[ES]-. The applied potentials were -0.25 V vs SCE and deposition time was 1 ks and the applied charges were 0.15 C/cm2.
Figure 2. X-ay diffraction results of electrodeposited Cu2O from 0.02 M Cu(NO3)2 solutions (a) without [MEIM]+[ES]- and (b) with 0.2% [MEIM]+[ES]-. The applied potentials were -0.25 V vs SCE and deposition charges were 0.15 C/cm2.
time when the amount of the RTILs was 0.2% volume ratio in the solutions. The exact mechanism for the change of morrphology of Cu2O grown with and without [MEIM]+[ES]- is still unclear. It might be explained in terms of the kinetics of the growth process. The growth rates vary along the different crystallographic directions. The lowest growth direction will determine the final morphology of the crystals.37 It is well-known that relative order of growth rates along the different crystallographic directions can be modified when organic or inorganic additives are added during the crystal growth process.7-12,37 Because of anisotropy in adsorption stability, the additives are adsorbed onto a certain crystallographic plane more strongly than others. This preferential adsorption lowers the surface energy of the bound plane and hinders the crystal growth perpendicular to this plane, resulting in a change in the final morphology. The [MEIM]+[ES]-
Morphology Control of Electrodeposited Cu2O Crystalsin
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Figure 5. Possible mechanism scheme of morphology control of electrodeposited Cu2O crystals achieved by preferential adsorption of [ES]- during the crystal growth process.
Figure 6. SEM images of Cu2O electrodeposited under -0.25 V vs SCE: (a) 0.02 M Cu(NO3)2 solutions with 0.2% [MEIM]+[ES]-, (b) 0.02 M Cu(NO3)2 solutions with 0.2% diethyl sulfate. The deposition time was 2 ks, and the charges were about 0.25 C/cm2.
Figure 3. SEM images of Cu2O electrodeposited under -0.25 V vs SCE from 0.02 M Cu(NO3)2 solutions with [MEIM]+[ES]- at different concentration. (a) 0, (b) 0.02%, (c) 0.04%, (d) 0.06%, (e) 0.08%, (f) 0.2%. The deposition time was 2 ks and the charges were about 0.25 C/cm2, respectively.
Figure 4. SEM images of Cu2O electrodeposited under -0.25 V vs SCE from 0.02 M Cu(NO3)2 solutions with 0.2% [MEIM]+[ES]- at different times: (a) 50 s, (b) 100 s, (c) 500 s, and (d) 1000 s.
is known to be hydrophilic ionic liquid and is miscible with water.36 In the aqueous solution, the [MEIM]+[ES]- could be dissociated into cations, [MEIM]+, and anions, [ES]-. The [ES]might be selectively adsorbed on {111} faces of Cu2O crystals and slow growth rates along the 〈111〉 directions. This causes the final morphology to appear as octahedral with {111} plane faces under a certain amount of [ES]- and changed to spherical shape when the [ES]- concentrations were above 0.08%. The possible mechanism scheme is shown in Figure 5. When there are no RTILs in solution, the electrodeposited Cu2O crystals keep their cubic structure. While there are small amounts of RTILs in solution, the morphology of electrodeposited Cu2O
crystals changes to octahedral shapes. Furthermore, when a larger amount of RTILs are added in the solution, spherical Cu2O crystals are achieved. [ES]- effects on the morphology of electrodeposited Cu2O were further studied by changing the cations. The diethyl sulfate (DES) was added in the deposition solution instead of [MEIM]+[ES]-, and the amount was 0.2% volume ratio. To compare, the same amount of [MEIM]+[ES]- was added in the different solutions. The solution and electrodeposition conditions were the same in both solutions. The SEM results are shown in Figure 6. The morphology also became spherical when the DES was added in the aqueous solutions. The difference between them was that the spherical crystals caused by [MEIM]+[ES]were much smoother. This may due to the higher solubility of [MEIM]+[ES]- in water or the effect of [MEIM]+ ions in the solutions. The effect of cations on the morphology change of Al electrodeposition was observed by Endres et al.34 The above results indicate that [ES]- did play a key role in the morphology control of the electrodeposited Cu2O crystals. The detail and concrete mechanism have also been investigated by IR spectra and electrochemical techniques (such as CV and potential step experiments); however, so far, we could not get convincing information. Further experiments on mechanism are still under study. In conclusion, we have demonstrated that RTILs, [MEIM]+[ES]-, have dramatical effects on the morphology changes of electrodeposited Cu2O crystals. The shape of the Cu2O crystals evolves from cubic to octahedral and spherical shape by adding a varied small amount of the ionic liquids in the deposited aqueous solutions. The possible formation mechanism has been discussed. This may provide a new and facile way to control the morphology of other electrodeposited semiconductor materials on conducting substrates using RTILs. These systems will be useful for a broad range of applications such as catalysis, sensors, and optoelectronics where their properties depend on different crysallographic planes of the crystals.37-41 Acknowledgment. This work was financially supported by the Zhejiang Provincial Natural Science Foundation of China (Y405131), Scientific Research Foundation for Returned Oversea Chinese Scholar of State Education Ministry, Scientific
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