A Facile Surface-Etching Route to Thin Films of Metal Iodides - Crystal

Environmental Science Programme, and The Center of Novel Functional Molecules, and Department of Physics, The Chinese University of Hong Kong, Sha...
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CRYSTAL GROWTH & DESIGN

A Facile Surface-Etching Route to Thin Films of Metal Iodides Xianluo Hu,† Jimmy C. Yu,*,† Jingming Gong,‡ and Quan Li‡ Department of Chemistry, EnVironmental Science Programme, and The Center of NoVel Functional Molecules, and Department of Physics, The Chinese UniVersity of Hong Kong, Shatin, New Territories, Hong Kong, China

2007 VOL. 7, NO. 2 262-267

ReceiVed May 16, 2006; ReVised Manuscript ReceiVed December 1, 2006

ABSTRACT: A facile surface-etching approach was developed for the synthesis of CuI, PbI2, and AgI thin films. Tetrahedralshaped CuI crystals were formed on a variety of copper substrates (e.g., grids, flat or porous foils, and macro- or nanowires) via an interfacial reaction between a copper substrate and iodine in water at room temperature. A possible mechanism was proposed to explain the growth of tetrahedral-shaped CuI single crystals. The resulting materials were characterized by transmission electron microscopy (TEM), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), selected area electron diffraction (SAED), energy-dispersive X-ray (EDX) spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). This preparation approach can also be used to grow PbI2 and AgI nano- and microcrystals with different morphologies on corresponding substrates. Introduction Over the past decades, considerable effort has been made to design, fabricate, and manipulate inorganic nano- and microcrystals via innovative synthetic approaches.1 Advances in physical methods have led to a molecular-level understanding of the structure-performance relationships, which are strongly related to the shape, phase, and size of materials.2 This knowledge, together with effective strategies, has inspired the design and development of novel inorganic materials for advanced applications. For instance, a large number of solutionbased techniques, such as sol-gel, hydrothermal, microemulsion, template-engaged, and biomimetic synthesis have been developed to synthesize semiconductor nano- and microcrystals.3 However, most of the materials thus obtained are in the powder form. In order to fabricate advanced electronic and optoelectronic devices, it is highly desirable to grow nano- and microcrystals directly on an appropriate substrate. The copper halides, especially cuprous iodide (CuI), have attracted intensive interest in the past decades. This is because they are prototype materials for nonlinear optical research and exhibit many unusual properties, such as large band gap, negative spin-orbit splitting, unusually large temperature dependency, and analogous diamagnetism behavior.4 CuI is a water-insoluble solid with three crystallographic phases, R, β, and γ.5 The high-temperature (above 392 °C) R-phase of cubic structure is a mixed conductor, where the charge carrier is predominantly Cu2+ ions. The hexagonal β-phase (between 392 and 350 °C) is also an ionic conductor.6 CuI normally exists in the low-temperature γ-phase (below 392 °C) of zinc-blend structure, which is one of a few kinds of p-type semiconductors with a high band gap (∼3.1 eV).7 CuI is an important additive for the production of white conducting polymeric fibers and a powerful heterogeneous catalyst in many organic reactions.8 Thin films of CuI were used as transparent conducting coating in electrophotography.9 Recent studies indicated that CuI films could act as a hole-collecting agent in dye-sensitized solid-state solar cells.10 They are also potentially applicable as building blocks for electronic/optical device fabrication.11 So far, much * Corresponding author. E-mail: [email protected]. Fax: (852) 26035057. † Department of Chemistry, Environmental Science Programme, and The Center of Novel Functional Molecules. ‡ Department of Physics.

Scheme 1.

Schematic Illustration of the Synthesis of CuI Film on Copper Substrate

effort has been made to synthesize CuI films. For example, Penner and co-workers developed a hybrid electrochemical/ chemical method for synthesizing β-CuI quantum dots on electrode surfaces.12 A gas-solid iodination route resulted in CuI particles with poorly defined shapes on a porous copper substrate.13 Recently, Gao et al. reported a hybrid CuI/βcyclodextrin film by deposition of a precursor mixture of CuI, β-cyclodextrin, acetonitrile, and dimethylformamide on glass substrates.14 In addition, pulse laser deposition, magnetron sputtering, and vacuum evaporation were utilized to fabricate CuI films.15 More recently, we have developed solution-phase approaches to grow a variety of chalcogenides on metal foils under hydrothermal conditions.16 Herein, we choose CuI as the example to demonstrate a facile and mild liquid-solid approach to grow nano- and microcrystalline metal iodides with various morphologies on substrates. Typically, continuous films of tetrahedral-shaped CuI single crystals were prepared at ambient conditions by a simple surface-etching reaction. Additionally, these CuI crystals are demonstrated to be in situ grown on a variety of copper substrates (e.g., foils, grids, and wires). The tetrahedral-shaped CuI single crystals grown on copper substrates may be explained by a kinetic control of surface reaction and mass transfer. Moreover, our facile low-temperature method can be extended to grow PbI2 and AgI nano- and microcrystals with different morphologies on corresponding metal substrates. Experimental Section Synthesis. The preparation procedure involves a slow surface-etching process and a mild water-rinsing step as shown in Scheme 1. A stock solution (SS) of I2-NaI-H2O is prepared by dissolving 60 g of NaI and 6 g of I2 in a solution containing 0.5 mL of acetic acid (5 M) and 60 mL of deionized (DI) water. The copper substrate is ultrasonically cleaned for ∼20 s first in acetone and then in a 1.5 M HCl solution,

10.1021/cg060288p CCC: $37.00 © 2007 American Chemical Society Published on Web 01/13/2007

Surface-Etching Route to Thin Films of Metal Iodides

Figure 1. SEM images of the tetrahedral CuI crystals grown on a copper grid. Inset of panel d shows a high-magnification SEM image of an individual CuI crystal (scale bar 100 nm). Solution A consisted of 1 mL of I2-NaI-H2O stock solution and 39 mL of H2O. Reaction time was 1 min. followed by repeated rinsing with DI water. After drying in a N2 gas flow, the substrate is dipped slowly into a diluted stock solution (solution A, 1 mL of SS and 39 mL of H2O) at a rate of ∼7.5 cm/min. It is allowed to react for 5 s to 30 min and is withdrawn from the solution slowly at the same rate of ∼7.5 cm/min. To remove the loosely packed CuI particles on the substrate surface, the “dipping-withdrawing” cycle is repeated by washing the product in 40 mL of H2O (solution B). The product is rinsed with ethanol and vacuum-dried for characterization. We have tested a number of copper substrates: flat foils (99.7% purity, 0.1 mm thick, Merck), TEM grids (cat. no. 01883-F, Ted Pella, Redding, CA), conventional electrical wires (0.91 mm in diameter, BDH), porous copper foils, and copper nanowires. The copper nanowires and porous nanostructures were prepared according to the previous literature report.17 Characterization. The crystal phase of the products was examined by powder X-ray diffraction (XRD) with a Bruker D8 Advance diffractometer using Cu KR1 irradiation (λ ) 1.5406 Å). The structural and compositional information of the product materials was obtained with scanning electron microscopy and energy-disperse X-ray spectroscopy (SEM/EDX, LEO 1450VP equipped with an Oxford Instrument X-ray spectrometer), transmission electron microscopy and selected area electron diffraction (TEM/SAED, Philips, CM-120), Raman spectroscopy (Renishaw RM-1000 Raman spectrometer with an excitation wavelength of 514 nm), and X-ray photoelectron spectroscopy (XPS, Physical Electronics, PHI 5600 multitechnique system with a monochromatic Al KR X-ray source). In the XPS measurements, all binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon.

Results and Discussion Different copper substrates including grids, flat or porous foils, and macro- or nanowires can be used. The surface of the substrates appears to be tarnished after reacting with the I2NaI-H2O solution at room temperature. Further observations under a scanning electron microscope indicate the formation of a great many crystallites over the entire surface of these copper substrates (Figure 1). The samples were obtained from solution A composed of 1 mL of I2-NaI-H2O stock solution and 39 mL of H2O after a reaction time of 1 min. The overall

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Figure 2. SEM images of the tetrahedral CuI crystals grown on different copper substrates: (a,b) wire; (c,d) foil. Solution A consisted of 1 mL of I2-NaI-H2O stock solution and 39 mL of H2O. Reaction time was 1 min.

Figure 3. (a) Bright-field and (b) dark-field TEM images, (c) ED pattern of an individual CuI crystal, (d) structural models of the tetrahedral and truncated tetrahedral shapes, and (e) unit cell of the CuI structure.

morphology of the product is shown in Figure 1a. It is observed that a dense and continuous film is grown on the copper grid. Figure 1b-d shows the morphology of the product at higher magnification. This film is comprised of well-defined tetrahedralshaped crystallites with smooth facets and sharp tips. The grain size of these tetrahedrons ranges from tens of nanometers to ∼0.5 µm. The inset of Figure 1d shows the high-magnification SEM image of an individual CuI crystal. It is obvious that the tetrahedral crystal is mainly bounded by four triangle facets with a slightly truncated shape. Figure 2 shows the SEM images of CuI crystals growing on a copper wire (0.91 mm diameter) and a flat foil (0.1 mm thick). The surface of the foil and the microscale wire is also completely covered by a continuous uniform film of tetrahedral-shaped CuI crystallites.

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Figure 5. XRD pattern of the resulting CuI film on a copper foil (∼2 cm2 in area).

Figure 6. EDX spectrum of the resulting CuI film on a copper foil (∼2 cm2 in area).

Figure 4. (a, b) TEM images and ED pattern taken from the same CuI single crystal by tilting θ ) ∼20°, (c) illustration of the tilting process in reconstructing the projected mass density along the electron beam, and (d) structural models of the tetrahedral and truncated tetrahedral shapes. The truncated one is bounded by eight {111} planes and six {100} planes.

The structure and morphology of these tetrahedral crystallites were further investigated by transmission electron microscopy (TEM) and electron diffraction (ED). Figure 3a shows the brightfield TEM image of an individual CuI crystal of edge ∼400 nm. Due to the high projected mass density from the largethickness crystal, the bright-field TEM image is rather dark, while the dark-field image (Figure 3b) obtained by selecting a {220}-type diffraction spot shows some changes in sample thickness from the top to the bottom. Figure 3c shows the corresponding ED pattern obtained by focusing the electron beam along the [111] direction. It can be indexed to a cubic zinc-blend phase of CuI. The crystal shape matches very well with the tetrahedron presented in Figure 3d. Figure 3e displays a unit cell for the zinc-blend phase of CuI. Further TEM and ED analysis indicates that the slightly truncated CuI tetrahedrons in our products are single crystals and are mainly enclosed by {111} facets. Figure 4a,b shows the example of the same CuI single crystal by tilting θ ) ∼20° in the TEM observation, suggesting the dark/light contrast is changed. Figure 4c,d displays the structural models of the tilting process and the tetrahedral and truncated tetrahedral shapes. The crystallinity and phase purity of the products were examined by powder X-ray diffraction (XRD). Figure 5 shows the representative XRD pattern of CuI films. All the reflections can be indexed to a cubic zinc-blend phase of CuI [space group F4h3m (No. 216)] with a calculated lattice constant a ) 6.05 Å, which is in good agreement with the literature value (JCPDS no. 6-246). Energy-dispersive X-ray (EDX) spectrometry was

Figure 7. High-resolution XPS spectra for the resulting CuI film on a copper foil (∼2 cm2 in area).

Figure 8. (a) (100) [or (200)], (b) (110), and (c) (111) planes of CuI. It shows that the surface density of atoms in the corresponding planes follows n(111) > n(200) > n(110).

also used to determine the local chemical composition of the product. A typical EDX spectrum is shown in Figure 6. Two major peaks correspond to Cu and I. We conclude that the films are composed of pure cuprous iodide based on the above XRD and EDX results. X-ray photoelectron spectroscopy (XPS) was used to determine the surface electronic states and the composition of the as-prepared products. The XPS survey spectra show strong peaks for Cu 2p and I 3d. Figure 7 shows the high-resolution XPS

Surface-Etching Route to Thin Films of Metal Iodides

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iodine is present in the products.18 The Cu 2p3/2 peak is centered at ∼932.2 eV, whereas the I 3d5/2 peak is found at ∼619.5 eV. This is characteristic of CuI.19 Different from the conventional homogeneous solution-phase synthesis of CuI (Cu2+ f Cu+), our process is an interfacial liquid-solid reaction (Cu0 f Cu+) under a slightly acidic condition:

Figure 9. (a) Typical SEM image of porous copper deposit created by electrodeposition for 10 s; inset, SEM image at a higher magnification for the 3D foam structure (scale bar 5 µm); (b-e) SEM images at different magnifications for the as-formed CuI film that was grown on the 3D copper foam.

Figure 10. (a) Typical SEM image of ultralong copper nanowires; inset, the SEM image at a higher magnification for the copper nanowires (scale bar 500 nm); (b-d) SEM images at different magnifications for the as-formed CuI film that was grown on copper nanowires.

spectra for the Cu 2p and I 3d. The I 3d5/2 peak could be fitted very well with a single peak indicating that only one type of

I2 + I- T I3-

(1)

2Cu + I3- f 2CuI + I-

(2)

where diffused Cu atoms are oxidized at the liquid-solid interface. They nucleate and grow into a water-insoluble solid phase of CuI crystals under the kinetic control of surface reaction and mass transfer. Although the exact mechanism for the growth of CuI crystals is difficult to know, we believe that this kinetic process of surface reaction and mass transfer may be vital in our approach. As shown in Figure 3e, a unit cell structure of the zinc-blend CuI is made of a face-centered cubic (fcc) Isublattice with four Cu+ in tetrahedrally coordinated sites at (1/4, 1/4, 1/4)-type positions.20 This structural configuration may be similar to the fcc lattice in nature. The structural models for the (100), (110), and (111) planes are shown in Figure 8. Evidently, the surface density of atoms in the corresponding planes follows n(111) > n(200) > n(110).21 For a fcc structured metal, the surface energy for the three lowest index planes is as follows: γ{111} < γ{200} < γ{110}.22 This rule may also hold for the zinc-blend CuI. The formation of the tetrahedral single crystals of CuI can be ascribed to the lowest energy of the {111} surfaces resulting in the slowest growth rate along 〈111〉. Of course, the detailed formation mechanism for these tetrahedralshaped CuI single crystals that were in situ grown on copper substrates needs to be further investigated. By changing the concentration of the I2-NaI-H2O precursor system and the reaction time, different grain sizes ranging from tens of nanometers to several micrometers can be obtained on various copper substrates. The CuI films may also be further extended to grow on porous, tubelike or nanostructured copper substrates. Figures 9a and 10a display the three-dimensional porous copper nanostructures and ultralong copper nanowires that were prepared according to the previous reports.17 Our investigation reveals that the continuous CuI films could be in

Figure 11. SEM images of (a, b) microprism, (c) nanosheet, and (d) oriented microprism films of PbI2 on lead foils and (e-h) AgI films on silver foils obtained after interfacial reactions for 5, 10, 60, and 120 min, respectively.

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Figure 12. (a) XRD pattern of the resulting PbI2 film on a lead foil (∼2 cm2 in area) in which all the reflections can be indexed to a hexagonal phase of PbI2 [space group: P3hm1 (No. 164)] with calculated lattice constants a ) 4.56 Å and c ) 6.88 Å, which are in good agreement with the literature value (JCPDS no. 7-235); inset, a unit cell of the hexagonal phase of PbI2, and (b) EDX spectrum of the resulting PbI2 film.

situ grown on these nanoporous and nanowire copper substrates. The corresponding SEM images are shown in Figures 9b-e (nanoporous copper as substrates) and 10b-d (copper nanowires as substrates). Moreover, our method has been applied to fabricate PbI2 and AgI nanostructured films with interesting morphologies (Figure 11). The corresponding structural model and the results of XRD, EDX, and Raman analysis are shown in Figures 12 and 13. This suggests that our low-temperature, in situ approach may be utilized to grow metal iodide films on metal substrates on a large scale without using any templates. We believe this is a much more effective approach because the conventional synthesis of metal iodide films often requires very harsh conditions (e.g., high energy input by magnetron sputtering techniques, hazardous chemical vapor deposition, or vacuum evaporation).15

Conclusions In this study, thin films of nano- and microcrystalline iodides (CuI, AgI, and PbI2) with different morphologies were successfully prepared on a variety of metal substrates (e.g., grids, flat or porous foils, and macro- or nanowires) at ambient conditions via a facile surface-etching reaction between a metal substrate and iodine in water. The resulting iodide films are potentially useful as building blocks for optical/electronic devices. They should also be ideal candidates for studying architecturedependent properties. We believe the as-synthesized products with various morphologies have useful optical properties and immense application potential. The related investigation is in progress.

Hu et al.

Figure 13. XRD pattern of the resulting AgI film on a silver foil (∼2 cm2 in area) in which all the reflections can be indexed to a hexagonal phase of AgI [space group P63mc (No. 186)], which are in good agreement with the literature value (JCPDS no. 75-1528), the peaks labeled with stars are assigned to the silver substrate, and the inset displays a unit cell of the hexagonal phase of AgI and (b) a typical Raman spectrum of the AgI film obtained at room temperature.

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