CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 12 2661-2666
Articles Morphology and Hydrophobicity of Micro/Nanoscaled Cuprous Iodide Crystal Xin Li and Meixiang Wan* Centre for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China ReceiVed December 3, 2005
ABSTRACT: The micro/nanoscaled γ-CuI crystal with various morphologies, such as rose-petal-like nanoflake, prism, triangle, hexagonal pyramid, and polyhedron, was successfully prepared by an in-situ etching process in an ethanol solution. In particular, the morphology and size of the crystals can be controlled by adjusting the reaction conditions. Moreover, it was found that the hydrophobicity of the crystal surface increases with the increase of “roughness” defined by a maximum difference in diameter between the maximum and the minimum diameter of the crystals measured by scanning electron microscopy; in particular, the super-hydrophobic (CA ) 151.0°) surface is observed with the coexistence of micro/nanoscaled crystals. Introduction
proposed that the super-hydrophobicity originates from the contribution of the air trapped in the interspace of rough surface, and the coexistence of a micro/nanostructure is advantageous for the formation of a super-hydrophobic surface.12,13 Here, we report the synthesis of micro/nanoscaled CuI crystals grown on copper substrate by an in-situ etching process in an ethanol solution. It was found that the morphology, size, and growth process of the CuI crystals were affected by the reactions conditions including copper substrate, iodine concentration, and etching time. Moreover, a super-hydrophobic surface (CA ) 151.0°) of the CuI crystals grown on the copper sheet was observed with the coexistence of micro/nanoscaled crystals. In particular, the CA increases with the increase of the surface roughness (R∆d) of the crystal, which is defined by a maximal difference in diameter of the crystals between the maximum and the minimum diameter measured by scanning electron microscopy (SEM), indicating the hydrophobicity of the surface would be adjusted by changing the surface micro/nanostructures.
The control of the morphology of micro/nanoscaled crystals is essential for the application of functional materials in various micro/nanodevices.1 In particular, crystals, which can carry charge and excitons efficiently, are being paid more and more attention due to their unique properties as the ideal building blocks for micro/nanoscaled electronics and optoelectronics.2 Cuprous iodide (CuI), as a hole transporting material, has attracted steadily growing interest because of its ultrafast scintillation property with a decay time of about 90 ps at room temperature.3 It has been demonstrated that electro-optic micro/ nanodevices made of cuprous iodide are affected by the morphology of the CuI crystal.4 Electrochemical processes5 and chemical6 or chemisorbed methods are used to prepare CuI crystals.7 So far, various methods have been developed for the morphology control of semiconductor micro/nano crystals,8 but few studies have examined the morphology of the CuI crystal systematically, and establishing a simple and efficient route to prepare micro/nanoscaled CuI crystals with well-controlled morphology and size is urgently important. In additional, the wettability of solid surfaces is a very important property, which depends on the surface free energy and the geometric structures of the surfaces.9 Conventionally, a surface with a water contact angle (CA) larger than 90° is called a hydrophobic surface and that higher than 150° is defined as a super-hydrophobic surface.10 Super-hydrophobic surfaces have received much attention because of their unique properties and promising applications in micro-fluidic devices and technologies.11 Recently, lotus-like morphology made in arrays of carbon nanotubes showed a super-hydrophobicity.12 It was
Materials. All reagents, such as iodine (A.R., Beijing Chemical Reagent Co.), ethanol and acetone (A.R., Beijing Chemical Plant), nitric acid (A.R., 65-68%, Tianjin Wenda Xigui Chemical Reagent Plant), were used as received without further treatment. Copper wire (0.3 mm in diameter, 99.9%, Anping Yuefa Metal Wire Mesh Co.) and copper sheet (0.2 mm in thickness, 99.9%, Beijing non-ferrous metal institution) were cleaned in nitric acid for 10 s, rinsed with distilled water and then acetone, and finally dried with dry nitrogen before using. Preparation Method. The macro/nanoscaled CuI crystals were prepared by an in-situ etching process in an ethanol solution via a reaction of copper with I2 based on the following equation:
* Corresponding author. Tel: +86-10-62565821. Fax: +86-10-62565821. E-mail:
[email protected].
I2 + 2Cu 98 2CuI
Experimental Section
10.1021/cg050644o CCC: $33.50 © 2006 American Chemical Society Published on Web 11/03/2006
ethanol
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Figure 1. XRD of CuI crystals: (a) grown on the copper wire at a concentration of 1.3 mM I2; (b) grown on the copper sheet at a concentration of 35 mM I2; and (c) XRD PDF card No. 06-0246. A typical preparation process of the crystals was to inset the copper substrate into an ethanol solution containing a calculated amount of I2 at ambient temperature. A pale yellow layer of CuI crystals immediately formed on the copper substrate. Compared with methods reported in the literature,5-7 the method used in this study is very simple and efficient for control over the morphology. To understand the growth process of the crystals, the effects of the copper substrate (e.g., copper wire and sheet), I2 concentration (0.08-160 mM), and etching time (1 s to 150 min) on the morphology and size of the crystals were investigated. Measurement. The crystal structure was characterized by X-ray diffraction on a RINT2000 Wide angle goniometer with Cu KR (λ ) 1.5406 Å). The CuI crystals used in the measurement of the XRD spectrum are powders striped off carefully from the copper substrate without scraping copper chipping. Field emitting scanning electron microscope (SEM, JSM-6700F) was used to measure the morphology of the crystals. The water CA was measured with a 1-µL water droplet at ambient temperature with an optical contact angle meter (Data physics Inc, OCA20), and the accuracy of measurements is (0.6°. The hydrophobicity measurement is going on the surface of CuI crystals grown on the copper sheet substrate.
Results and Discussion Crystal Structure. X-ray diffraction (XRD) spectra of the CuI crystals grown on copper wire at the concentration of 1.3 mM I2 and that grown on copper sheets at the concentration of 35 mM I2 were measured as shown as Figure 1. It can be seen that all the diffraction peaks match well with the data in No. 06-0246 PDF card of XRD (Figure 1c), indicating that the asgrown crystal is γ-CuI with a ) 6.051 Å and z ) 4.14 The strong diffraction intensities along the [111], [220], and [311] directions imply the preferred orientation of CuI crystal. Furthermore, no obvious impurities were detected, suggesting the CuI crystals prepared by the in-situ etching reaction of molecular iodine and copper substrate in the ethanol solution belong to γ-CuI with a high purity. Moreover, the influence of the concentration of I2 and copper substrate on the crystal structure could be negligible. Morphology. To understand the growth process of the CuI crystal, the morphology of the as-synthesized crystals at different etching times was measured. It was found that the growth process of the crystals undergo three stages including I2 monolayer formation, nucleation, and growth, which are consistent with previous results reported.7 Compared with the chemical absorption method reported by Andryushechkin et al.,7 however, the time required to form the I2 monolayer in in-situ chemical etching process is less than 1 s due to a fast reaction
Li and Wan
of copper with I2 in solution. Therefore, the morphology of CuI crystal grown at the nucleation and growth stages was studied in detail. It was found that the nucleation stage takes place in the range of 1-5 s, which is independent of the substrate. On the other hand, the concentration of I2 is a key factor significantly affecting the nucleate morphology; for instance, the critical concentration determined the crystal morphology for the nanoparticle, and the rose-petal-like nanoflake occurred at a I2 concentration of 5.0 mM. Figure 2 shows the morphology of the nucleus grown on different substrates and the concentration of I2. It shows that the nucleus grown on a copper wire or sheet at a low concentration of I2 appears to be grain-like in shape with a diameter of about 100 nm (Figure 2a,b). However, the morphology of the nucleus grown at a high concentration of I2 significantly differs from that at a low concentration. The nuclei grown on the copper wire or sheet at a high concentration of I2, for instance, are all nanoflake in shape with a length of 500-800 nm and a thickness of 50-80 nm, just like a compact piled rose petal (Figure 2c,d). But the crystal grown on copper wire has a clearer outline. Here, the concentration of I2 is crucial to the control of the nucleus morphology, and nanoparticle and nanoflake morphology of CuI crystal can be gained by controlling the concentration of I2. Various morphologies were observed during the crystal growth state as shown in Figures 3 and 4, where the crystal nucleus grew with the reaction time accompanied by a size increase and shape change. In all cases, an intermediate stage exists for crystals with a clear outline, which comes forth in the range of 1-5 min. At a low concentration of I2 for growth at 1.0 min, for example, the nanoparticle nuclei (Figure 2a,b) developed into intermediates with a shape of nanoflake of polygon from triangle to hexagon with an edge length of 100300 nm (Figure 3a,b). Nevertheless, the morphologies of crystals grown at 80 min are quite different for a copper wire and copper sheet as shown in Figure 3c,d. For a copper wire, the crystals mainly elongated in a direction perpendicular to the substrate surface to form a microprism 4-6 µm long, 800 nm to 2 µm wide, and 300-600 nm thick (Figure 3c). Here, anisotropic prisms were observed instead of the isotropic shape. But for the copper sheet, as one can see, the crystals grew up in all directions and finally appeared as polyhedron in shape with a size of 500-800 nm. As described in the experiment, the reaction between the I2 and copper substrate is a kind of in-situ etching process so that CuI crystals mainly grow on the surface of the copper substrate. Thus, the morphology of the CuI crystal on the copper substrate affected by the surface roughness and surface energy of the copper substrate is expected. Along with the increased concentration of I2, the morphology of the crystals grown on either copper wire or copper sheet is also altered. The series SEM images in Figure 4 show a change of the morphologies with an etching time from 30 s to 80 min at the high concentration of I2. From Figure 4, it can be seen that the rose-petal-like nuclei extended along the parallel direction at the very early stage (e.g., 30 s), so that the space between the “rose petal” was filled by the newborn nanochip (Figure 4a). By about 1 min, the well-formed intermediate in the shape of an equilateral triangle with an edge length of 200400 nm appeared (Figure 4b). It is clear that the triangle then grew in a layer-by-layer mode accompanied by a size increase as shown in Figure 4c. When the time increased to 80 min, on the other hand, the morphology of the crystals grown on the copper wire changed from triangle to hexagonal pyramid shape with an edge length of 800 nm to 1.5 µm (Figure 4d). As for the crystals grown on the copper sheet, the changing process
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Figure 2. Typical morphologies of CuI crystals grown on different copper substrates and concentrations of I2 during the nucleation stage (t ) 2 s): (a) copper wire and (b) copper sheet at the concentration of 1.3 mM I2; (c) copper wire and (d) copper sheet at the concentration of 35 mM I2. The multiples for (a), (b), (c), and (d) are 50, 30, 100, and 30 K, respectively.
Figure 3. Typical morphology of the crystals grown on different copper substrate at a concentration of 1.3 mM I2: (a) copper wire and (b) copper sheet for growing 1.0 min; (c) copper wire and (d) copper sheet for growing 80 min. The multiples for (a), (b), (c), and (d) are 30, 30, 10, and 12 K, respectively.
was quite similar to that on copper wire at the early stage from the beginning to about 20 min (Figure 4a-c), while the mature crystals have a different morphology from that on the copper wire (Figure 4d,e), which took on a polyhedron shape with a size of several micrometers. Nevertheless, the concentration of I2 is also an important reason inducing the formation of anisotropic shapes, such as micro/nanoprisms for copper wire. In addition, the different diameters of copper wire (Φ70-300 µm) and different thicknesses of copper sheet (50-200 µm) were also studied; the as-prepared morphologies both in the
nucleation stage and the crystal growth stage are similar to the corresponding results we have listed. Furthermore, the suitable reaction time for the final crystal morphology should be controlled at 60-90 min, and a prolonged etching time is not helpful for the well-formed morphology, since a new layer of crystal will develop by then. The over high concentration (>120 mM) is also not helpful for morphology control due to the formation of a blurry outline shape. As a result, it is proposed that the morphology of the crystals grown during the growth state is affected by the copper substrate and the concentration
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Figure 4. Typical morphology of the crystals grown on different copper substrate at the concentration of 35 mM I2: (a), (b), (c), (d) copper wire for growing at 30 s, 1.0, 20, and 80 min, respectively; (e) and (f) copper sheet for growing at 40 and 80 min, respectively. The multiples for (a), (b), (c), (d), (e) and (f) are 40, 65, 22, 30, 16, and 15 K, respectively.
Figure 5. The morphology of CuI crystals grown on copper sheet at different concentrations of I2 and the corresponding water-drop shapes, which relate to the CA value: (a) 121.1°, (b) 122.4°, (c) 124.0°, (d) 128.8°, (e) 129.7°, (f) 139.5°, (g) 148.4°, (h) 151.0°. The multiples for (a), (b), (c), (d), (e), (f), (g), and (h) are 50, 12, 10, 20, 15, 15, 5.5, and 10 K, respectively.
of I2 as well as the etching time. CuI crystal with a controllable morphology can be obtained by modulation of the above conditions. It is noticeable that the crystal on both the copper wire and the copper sheet grows in a layer-by-layer mode with its outer plane parallel to the substrate surface, which is consistent with the previous study.7 Moreover, on the surface of the hexagon (Figure 4d), there are systematically spread bright dots, and the distance between them is in the range of 40-50 nm, which is close to interatomic distances of the CuI lattice,3a,7 suggesting that the crystal grows by an atom-by-atom superimposed mode with the growth direction perpendicular to the substrate surface. The aspect ratios (R) of crystals, which are evaluated as the ratio L/S, and L and S are defined as length of the long and short axis, respectively, can be used to describe the growth process of the crystals.16 The aspect ratios calculated for the CuI crystals are small (1.0-1.83, see Table 1), further proving
that there is no special orientation during the crystal growth process except for the copper wire at the low concentration of I2. Hydrophobility. The hydrophobility of the crystals grown on the copper sheet was measured by means of the water CA. The CA value and corresponding morphology are shown in Figure 6. As one can see, the CA value is in the range of 121° and 151°, depending on the morphology of the crystals; in particular, the super-hydrophobility (CA > 150°) was observed with the coexistence of the micro- and nanoscaled crystals. This observation indicates that coexistence of micro/nano structures is an important parameter for achieving a super-hydrophobic surface.9c For a given material, CA is usually related to the surface roughness.17 Commonly, the surface roughness is expressed by different surface parameters, such as the dimensions of length18 and ratio of the area or length.19 Herein we proposed the
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Table 1. Morphologies and R Value of CuI Crystals Grown on Copper Wire and Sheet at the Growth Stage morphology I2 (mM)
35 a
growth for 1 min
copper substrate
shape
wire sheet wire sheet
polygon (n ) polygon (n ) 3-6)a triangle triangle 3-6)a
growth for 80 min R
shape
R
1.12-1.71 1.0-1.83 1.05-1.20 1.13-1.23
prism polygon (n ) 3-6)a hexagon polyhedron
1.20-1.71 1.14-1.54
n is the edge number of the polygon.
Therefore, we suppose that this method may have wide applications in exploring the crystal growth process and may provide guidance for morphology controllable synthesis. Acknowledgment. This project was supported by The National Nature Sciences Foundation of China (and 50533030). The Cooperation Lab of National Center for Nanoscience and Technology also gave the financial support. We thank Prof Lei Jiang and Jin Zhai at Institute of Chemistry, Chinese Academy of Sciences, for their help in measuring water contact angle. References
Figure 6. Relationship between the water CA and the surface roughness (R∆d, r) for the CuI crystals grown on the copper sheet at different concentrations of I2.
maximum difference in diameter (R∆d) of the crystals defined as the surface “roughness”, which is measured from SEM images by a diameter difference between the maximal and the minimum particles. According to the above proposal, it was found that CA increases with the increase of R∆d as shown in Figure 6. On the basis of Wenzel’s suggestion,18a,20 the relationship between roughness (r) and contact angle (CA) has the form:
r ) cos θw/cos θe
(1)
where θw is Wenzel’s angle (CA on a rough surface), θe is the equilibrium contact angle on an ideally smooth surface, and r is the surface roughness ratio. Therefore, r corresponding to CA for different morphologies of the crystals was calculated by taking θe as 121.1° (Figure 5a) because the crystal layer can be regarded as a flat surface. As shown in Figure 6, the R∆dCA curve is similar to the r-CA curve calculated by eq 1, suggesting the “roughness” defined by us is reasonable. Conclusions Micro/nanoscaled γ-CuI crystals (a ) 6.051 Å and z ) 4) with different morphologies was successfully prepared by an in-situ etching process in an ethanol solution, and the size and morphologies of crystals can be controlled by adjusting the reaction condition. In particular, the super-hydrophobility (CA ) 151.0°) of the CuI crystals grown on the copper sheet was observed with the coexistence of micro/nanoscaled crystals that might originate from the contribution of the air trapped in the interspace of rough surface made in micro/nanoscaled crystals. Moreover, the surface “roughness” proposed by us, which is defined as the maximum difference in diameter (R∆d) of the crystals between the maximum and the minimum particles, for explaining the dependence of CA on the morphology of the CuI crystal is consistent with Wenzel’s suggestion. Compared with methods reported in the literature, the method used in this study is very simple and efficient in controlling the morphology.
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CG050644O
