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2007, 111, 6-9 Published on Web 12/07/2006
PEG-Assisted Fabrication of Single-Crystalline CuI Nanosheets: A General Route to Two-Dimensional Nanostructured Materials Yanyan Xu,† Dairong Chen,*,† Xiuling Jiao,*,† and Long Ba‡ Department of Chemistry, Shandong UniVersity, Jinan, 250100 P. R. China, and Laboratory of Molecular and Biomolecular Electronics, Southeast UniVersity, Nanjing, 210096 P. R. China ReceiVed: October 10, 2006; In Final Form: NoVember 23, 2006
CuI single-crystalline nanosheets have been prepared for the first time via a PEG-assisted aqueous solution route at room temperature. The thickness and in-plane size of the nanosheets were ca. 60-80 nm and several micrometers, respectively. The two basal surfaces of these nanosheets were (111) planes. The phase transformation temperature and the melting point decreased 8 and 12 °C compared with those of the bulk CuI, respectively. The resistance of a single CuI nanosheet was measured by using a conductive AFM tip method, and a high conductivity of 1.996 × 10-2 Ω·cm and a photoconduction phenomenon were observed. It is found that the coexistence of PEG600 and SDBS was vital to the formation of nanosheets. This simple route could be employed to synthesize more 2D nanostructures as a general process.
Introduction In recent years, the research into two-dimensional (2D) nanomaterials is a hot subject in the progress of nanoscience, nanotechnology, and nanodevices for their special electronic, magnetic, optical, and catalytic properties.1-2 Various chemical methods have so far been developed, among which polymeror surfactant-assisted solution-based methods have aided greatly to the development of 2D nanostructures.3-7 However, this method to obtain nanosheets still remains a great challenge due to the complexity of solution chemistry and interaction manner between materials and the structure-directing agents as well as the lack of generality. To date, many nanosheets with different shapes and thickness have been prepared. However, this work is mainly limited to the noble metals3-6 and oxides or hydroxides,7 and there are very few reports on the synthesis of planar I-VII semiconductors nanostructures except for CuCl nanopleplates8 and AgI nanoplates.9 As an extensively used hole conductor in solid-state dyesensitized solar cells10 and catalyst for many organic coupling reactions,11 I-VII semiconducting CuI has received considerable attention. However, the investigations on the properties of CuI nanostructures are significantly delayed because of the limited studies on their synthesis.12-14 In the present work, we report the synthesis, formation process, and properties measurements of single-crystalline CuI nanosheets. This is likely to be the first report on the synthesis of CuI nanosheets and conductivity measurement on an individual CuI nanosheet. Experimental Section In a typical synthesis, 0.380 g of KI and 0.360 g of sodium dodecyl benzenesulfonate (SDBS) were added into 26.5 g of * To whom correspondence should be addressed. Fax: +86-53188364281. Tel: +86-531-88364280. E-mail:
[email protected]. † Shandong University. ‡ Southeast University.
10.1021/jp066649t CCC: $37.00
PEG600 under stirring to give a clear solution and 0.180 g of CuCl2‚2H2O was dissolved in another 26.5 g of PEG600. The two solutions were mixed together to give a clear amaranth solution which was used as the precursor solution. Then 12.7 g of the precursor solution was added into 20.0 mL of 0.05 M NaNO3 solution drop by drop with a burette under stirring to generate CuI precipitate at room temperature (25 °C). Finally, the resultant precipitate separated from the mother solution by centrifugation and decantation was washed with deionized water and anhydrous ethanol several times and dried at 70 °C for several hours for the consequent characterization and analysis. The morphology of the nanosheets was characterized with a Hitachi S-520, JXA-840 scanning electron microscope (SEM) and a JEOL JSM-6700F field-emission scanning electron microscope (FE-SEM). The crystal structure and microstructure of the products were examined using an X-ray diffractometer (XRD, Rigaku D/Max 2200PC diffractometer with Cu KR radiation) and a high-resolution transmission electron microscope (HR-TEM, GEOL-2010), respectively. The Raman spectra were recorded on a Jasco Ventuno21 Micro Raman spectrophotometer. The samples were excited with a 532 nm laser excitation source. UV-vis absorption and photoluminescence (PL) spectra were taken on a Lambda-35 UV-vis spectrometer and a Hitachi 850 Fluorescence spectrometer with a Xe lamp as the excitation light source at room temperature, respectively. The excitation wavelength of the sample was 397 nm. Thermal gravimetric (TG) analysis and differential scanning calorimetric (DSC) measurements were carried out on a Mettler Toledo SDTA851e thermogravimetric analysis (TGA) under a N2 flow in the temperature range from 25 to 650 °C, simultaneously. For the conductivity measurements on single nanosheet, a nanoscope atomic force microscope (AFM, Veeco/Digital Instruments, Santa Barbara, CA) was used. CuI nanosheets were ultrasonically treated and dispersed on an Au-coated Si wafer serving as the bottom electrode. A silicon MDT AFM tip with curvature radius of 25 nm was used as the top electrode. © 2007 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 111, No. 1, 2007 7
Figure 1. SEM images of CuI nanosheets with different amounts of SDBS (a) 0 g, (b) and (c) 0.360 g, and (d) 0.720 g.
Figure 3. UV-vis absorption (solid line) and PL (dashed line) spectra (a) and TG and DSC curves (b) of the as-prepared CuI nanosheets.
Figure 2. XRD pattern (a), TEM image/SAED pattern/and HR-TEM and (b) image of CuI nanosheets prepared with 0.360 g SDBS.
Results and Discussion As shown in Figure 1, without the addition of SDBS, the product was irregular nanosheets with an in-plane size smaller than 2 µm. When 0.360 g of SDBS was added, the SEM images show that the products were the well-defined sheets with a thickness of ca. 60-80 nm and in-plane sizes of several micrometers. The nanosheets exhibited a glazed and flat surface and irregular shapes. Even so, the thickness of an overall nanosheet was almost the same even on the edges. In addition, a very small amount of irregular nanosheets of 300-500 nm is found for the fragments of nanosheets. When the amount of SDBS was further increased to 0.720 g, the in-plane size increased up to larger than 6 µm (Figure 1d) and some hexagonal-ring like nanosheets appeared as shown in the inset of Figure 1d. However, many nanoparticles appeared. So when 0.360 g of SDBS was used, a better morphology was obtained, and this sample was chosen for further characterization. Figure 2a shows the XRD pattern of this product; all diffraction peaks can be indexed as the facial cubic cell of pure marshite CuI (γ-CuI) with lattice constant a ) 6.051 Å (JCPDS no. 06-0246; Figure 2a). The intensity ratio between the (220) and (111) diffraction peaks is much lower than the literature’s value (0.127 vs 0.55), which implies that the basal planes of the nanosheets should be mainly dominated by {111} facets, and therefore, their {111} planes tend to be pre-ferentially oriented parallel to the surface of the substrate in the XRD experiment. Figure 2b shows the representative TEM image of a single nanosheet. The flower patterns over the sheet are bending contours because of the strain resulting from the slight bend of thin nanosheets. As an electron diffraction (ED) phenomenon, it is frequently observed in thin nanosheets for slight deformation and bending.15 The well-defined ED spots
shown in the selected-area electron diffraction (SAED) pattern (Figure 2b inset down-right) correspond to the [111] zone axis diffraction pattern of the CuI crystal, indicating the singlecrystalline structure and the preferred {111} plane orientation of the nanosheet. A total of 19 SAED patterns are recorded on different nanosheets, and all of the SAED patterns correspond to the CuI crystal viewed along [111] direction, among which nine patterns show only three sets of regular Bragg diffraction spots of {220}, {422}, and {440} type (Figure 2b inset downright). In the left 10 SAED patterns, kinematically fractional (forbidden) {422} spots appear with different intensities (Figure S1a and inset, Supporting Information). The appearance of the fractional {422} reflections might be the result of double diffraction of electrons, or they may result from the existence of a unique (111) stacking fault parallel to the (111) surface, which have been also observed in other nanoplates or nanofilms.16 Figure 2b inset top-left is a representative HR-TEM image of a [111] oriented single-crystalline CuI nanosheet. The two-dimensional lattice distances are both calculated to be 0.213 nm, corresponding to {202h} lattice plane spacing, respectively, further confirming the preferred {111} surface orientation of the nanosheets. Few nanosheets are found to orientate along other direction through HRTEM observation and calculation, which might be derived from disintegration of CuI nanosheets under long time electron beam irradiation (Figure S1b and c, Supporting Information). UV-vis absorption and PL spectra of the as-prepared CuI nanosheets was shown in Figure 3a. The hump at 339 nm is attributed to the excitation of electrons from sub bands in the valance to the conduction band, and the peak at 410 nm originates from the excitation of electrons from valance band to the conduction band. The onset of the absorption spectrum is not pronounced, probably because of the wide size distribution. PL spectrum of CuI nanosheets presents a characteristic emission peak at the position of 419 nm, which may due to the surface trapping sites (∼0.2 eV higher than the valence band edge of CuI) created by adsorbed iodine because CuI always
8 J. Phys. Chem. C, Vol. 111, No. 1, 2007 contains a stoichiometric excess of iodine.17 Figure 3b shows TG and DSC curves of the as-prepared CuI nanosheets. The DSC peak at 374 °C is the phase transformation temperature from γ-CuI to β-CuI, which has little change compared with those of the bulk CuI. However, the phase transformation temperature from β-CuI to R-CuI (397 °C) and the melting point (593 °C) reported here decreased by 8 and 12 °C for their nanoscale size and surface effect (Figure 3b), respectively.14 The room-temperature electroresistivity of a single CuI nanosheet is measured and estimated to be 1.996 × 10-2 Ω·cm by using a conductive AFM tip method, and its photoresistivity is 7.688 × 10-3 Ω·cm when the nanosheets are exposed to an ordinary electric lamp. Compared with products prepared with different techniques reported in the literature, the electroresistivity of asprepared CuI nanosheets reached the lowest value.18-19 Diffusion and liberation of iodine atoms from the unit cell might be a reason for the high conductivity of the as-prepared CuI nanosheets.18 The low electroresistivity and the photoresistivity of the CuI nanosheets may find practical use in electronic devices and photoelectrochemical or photosensing apparatus. It is found that the Cu(I)-I chemical bond could be formed in the mixture of CuCl2/PEG600 solution and KI/SDBS/PEG600 solution (precursor solution) by Raman technique (Figure S2, Supporting Information). Thus, PEG600 may serve as a particle stabilizer to prevent conglomeration of CuI nanoparticles or a complex agent to prevent the formation of CuI. Further TEM observation on the precursor solution confirmed that no CuI nanoparticles formed, which means that in the precursor solution a I-Cu(I)-PEG complex rather than CuI nanoparticles formed. More experiments have shown that with only PEG600 or only SDBS as additive the products were also the nanosheets, but their sizes were smaller than 2 µm (Figure 1a). So the concurrence of an appropriate amount of PEG600 and SDBS is crucial for the formation of the well-defined CuI nanosheets with an in-plane size of several micrometers. As reported previously, the atom O in the PEG molecular chain exhibited coordination abilities with metal ions,20 and PEG was easily adsorbed on the surface of the metal oxide colloid.21 So concerning the use of PEG600 in our synthetic process, two major roles could be identified. First, it could provide a neutral complex media to stabilize Cu(I) and prevent the precipitation of CuI nanoparticles in the precursor solution. Second, PEG600 might efficiently be adsorbed on the surfaces of CuI colloids during the hydrolysis reaction and direct the formation nanoplates. For the SDBS molecules, there is no evidence confirming them participating in the chemical reaction in the precursor reaction. However, as anionic surfactant, SDBS might be selectively adsorbed on various crystallographic facets through interaction with copper atoms.22 The roles of SDBS on the formation of CuI nanosheets were similar to those of CTAB on the formation of Ag and Au nanosheets.6,23 Therefore, PEG and SDBS molecules were both selectively adsorbed on certain crystallographic facets during grain growth. Such selective adsorption prevented the growth of these facets and resulted in the formation of plate-like microcrystals. The adsorption of PEG and SDBS must own similar selectivity on different crystallographic facets of CuI. Also as reported previously, ionic surfactants and uncharged polymers had a certain interaction in aqueous solution.24 In our system, the interaction manner between SDBS and PEG600 was unknown but a certain point was that their interaction promoted their adsorptions on certain CuI facets. Therefore, coexistence of PEG600 and SDBS in the precursor solution benefited larger CuI nanosheets. This process preferentially took place on {111} facets, which could be
Letters described as follows. When the precursor solution was dropped into water under mild magnetic stirring, the complex hydrolyzed and CuI nanoparticles formed. PEG and SDBS first absorbed on the surface of CuI colloids preventing randomly aggregation and then lead to the growth of CuI crystals along certain directions. Further experiments found that stirring speed during the hydrolyzation had an important effect on the morphology of the products. When the precursor solution was added into water without stirring, the CuI microprisms rather than the nanosheets were produced, but fierce stirring resulted in smaller nanosheets. It was interesting that, when the CuI powders (microparticles) were dissolved into PEG600 to form a solution and the solution was dropped into water under stirring, the purephase γ-CuI nanosheets with a thickness of 60-90 nm and inplane sizes of 200-500 nm could be obtained (Supporting Information 3 and Figure S3). The transformation of CuI nanosheets from their irregular microparticles through a simple dissolution and reprecipitation process further confirms the structure-direct effect of PEG600. Furthermore, this PEG600assisted process can also be applied to prepare nanosheets of other inorganic compounds such as Ag and BiOI (Supporting Information 4 and Figure S4). Conclusions In summary, single crystalline CuI nanosheets were prepared for the first time via a simple room-temperature solution route. Such nanosheets have a thickness and in-plane size of ca. 6080 nm and several micrometers, respectively. During the synthetic process, the low molecular polymeric PEG600 and SDBS acted as structure-directing agents to direct the formation of nanosheets, and the cooperation of PEG600 and SDBS play key role in the formation of nanosheets with large in-plane size. The conductivity of a single CuI nanosheet was estimated to be 1.996 × 10-2 Ω·cm at room temperature, which shows a drastic conductivity enhancement compared with its bulk materials. A photoconduction phenomenon was observed, which may find possible applications in photoelectrochemical cells and photosensing apparatus. We further extended this strategy to prepare nanosheets of Ag and BiOI. As a general process, it is anticipated to be employed to synthesize more 2-D nanostructures. Acknowledgment. The financial support of Program for New Century Excellent Talents in University, P. R. China is gratefully acknowleged. Supporting Information Available: TEM/HRTEM images of CuI nanosheets, Raman spectra of precursor solution, control experiments and corresponding XRD data/ SEM images/TEM image, preparation process and SEM images/XRD data of nanosheets of other inorganic compounds. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Tian, Z.; Voigt, J.; Liu, J.; Mckenzie, B.; Mcdermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. Nat. Mater. 2003, 2, 821. (2) Chen, S. J.; Liu, Y. C.; Shao, C. G.; Mu, R.; Lu, Y. M.; Zhang, J. Y.; Shen, D. Z.; Fan, X. W. AdV. Mater. 2005, 17, 586. (3) Kim, J. U.; Cha, S. H.; Shin, K.; Jho, J. Y.; Lee, J. C. AdV. Mater. 2004, 16, 459. (4) Li, C.; Cai, W.; Cao, B.; Sun, F.; Li, Y.; Kan, C.; Zhang, L. AdV. Funct. Mater. 2006, 16, 83. (5) Pastoriza-Santos, I.; Liz-Marza´n, L. M. Nano Lett. 2002, 2, 903. (6) Chen, S.; Carroll, D. L. J. Phys. Chem. B 2004, 108, 5500. (7) Hou, Y. L.; Kondoh, H.; Shimojo, M.; Kogure, T.; Ohta, T. J. Phys. Chem. B 2005, 109, 19094.
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J. Phys. Chem. C, Vol. 111, No. 1, 2007 9 (17) Perera, V. P. S.; Tennakone, K. Sol. Energy Matter. Sol. Cells 2003, 79, 249. (18) Sirimanne, P. M.; Rusop, M.; Shirata, T.; Soga, T.; Jimbo, T. Mater. Chem. Phys. 2003, 80, 461. (19) Tanaka, T.; Kawabata, K.; Hirose, M. Thin Solid Films 1996, 281282, 179. (20) Kerker, M. J. Colloid Interface Sci. 1985, 105, 297. (21) Dobryszycki, J.; Biallozor, S. Corros. Sci. 2001, 43, 1309; Liu, X. H.; Yang, J.; Wang, L.; Yang, X. J.; Lu, L. D.; Wang, X. Mater. Sci. Eng. A 2000, 289, 241. (22) Liu, Z. P.; Yang, Y.; Liang, J. B.; Hu, Z. K.; Li, S.; Peng, S.; Qian, Y. J. Phys. Chem. B 2003, 107, 12658. (23) Wang, L. Y.; Chen, X.; Zhan, J.; Chai, Y. C.; Yang, C. J.; Xu, L. M.; Zhuang, W. C.; Jing, B. J. Phys. Chem. B 2005, 109, 3189. (24) Wang, G.; Olofsson, G. J. Phys. Chem. B 1998, 102, 9276.
