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16284

J. Phys. Chem. C 2007, 111, 16284-16289

Nanosized Cu2O/PEG400 Composite Hollow Spheres with Mesoporous Shells Yanyan Xu, Dairong Chen,* Xiuling Jiao,* and Keyan Xue School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, People’s Republic of China ReceiVed: July 9, 2007; In Final Form: August 15, 2007

The nanosized Cu2O/PEG400 composite hollow spheres (HSs, 50-80 nm in diameter) with mesoporous shells of ∼15-20 nm were synthesized by a poly(ethylene glycol) (PEG)-assisted wet-chemical method. In the hollow nanostructures, the polymer content was ca. 18.1 wt %, and the mean size of the component nanocrystals and the pore diameter were ca. 5 and 3.8 nm, respectively. In the fabrication process of hollow structures, poly(ethylene glycol 400) (PEG-400) molecules self-assemble to form micelles which act as templates for the formation of the hollow structures. PEG also acts as a reducing agent, solvent, and complexing agent. The formation of mesoporous structures is due to the oriented-aggregation of composite nanoparticles. The nanosized-composite HSs exhibited peculiar photoluminescence (PL) phenomenon with strong peaks at 414 and 436 nm and weak ones at 454, 570, and 637 nm. Furthermore, the HSs showed excellent adsorption ability for methyl orange (MO) because of their composite and mesoporous shell structures.

1. Introduction Hollow-spheres (HSs) with mesoporous shells have received considerable attention because of their great importance in basic research (for example investigating fundamental mechanisms of biomineralization) and potential applications in nanodevices, catalysts, carriers, and containers, etc.1 Many articles have described the interior templating of mesoporous hollow particles using hard templates,1a,2 vesicles,3 and emulsions,1c,4 with diameters from micrometers to sub-micrometers. On the other hand, nanosized HSs with solid shells have been prepared using various methods.5 However, the fabrication of mesoporous nanoHSs, i.e., the HSs with mesoporous shells and nanometer sizes, have rarely been reported.6 Particularly, mesoporous inorganic-organic (transition metal oxide-polymer) composite HSs of nanometer sizes should be explored because of their possible novel properties and applications.7 As a p-type semiconductor, cuprous oxide (Cu2O) is a promising material with potential applications in solar energy conversion, catalysis, sensing, and electrode materials in lithiumion batteries.8 Many approaches have been developed to prepare Cu2O nanocrystals with various morphologies, such as nanospheres,8a,d wires,9 cubes,10 pyramids,11 octahedrons,12 and squares.13 Wang et al. synthesized porous Cu2O single crystalline spheres by a coordination-assisted heterogeneous dissolution process,14 and Xu’s group constructed 3-D ordered macroporous Cu2O by electrochemical deposition using a polystyrene colloidal crystal as template.15 Qi and co-workers prepared octahedral Cu2O nanocages via the catalytic reduction of an alkaline copper tartrate complex with glucose followed by a catalytic oxidation process,16 and Zeng et al. fabricated Cu2O hollow nanocubes and nanospheres via reductive self-aggregation of CuO nanocrystals by a template-free solvothermal method.17 Most recently, Li and co-workers used a highly ordered mesoporous silica SBA-15 as the template to form a highly ordered mesoporous carbon CMK-3 and then obtained * Corresponding author. Telephone: 86-531-88364280. Fax: 86-53188364281. E-mail: [email protected].

ordered mesoporous Cu2O from the CMK-3 by “one-step further nanocasting”.18 Herein, the fabrication of Cu2O/PEG400 composite HSs with mesoporous shells and diameters of 50-80 nm through a poly(ethylene glycol) (PEG)-assisted wet-chemical method is first introduced. Furthermore, the structural, optical, and adsorption properties of the nanospheres are also studied. PEG has been widely used in the synthesis of nanomaterials, such as ZnO nanowires and microspheres, CeO2 nanorods, WO3 nanodisks and whiskers, TiO2 films, CuI nanosheets, and so on.19 As for Cu2O, nanocrystals with different shapes have been successfully prepared with the assistance of different molecular weight PEGs.9a,10b,13 In the Cu2O nanocrystals preparations, ascorbic acid, glucose, or hydrazine hydrate were used as the reducing agents under alkaline conditions and PEGs served as a modifier to control the crystal growth. In the present work, PEG400 was not only used as a reducing agent,20 solvent, and complexing agent,21 but also as the structure-directing agent for the formation of hollow nanospheres. 2. Experimental Section Synthesis. PEG400 (average MW 400) was of chemical grade, and other reagents were of analytical grade, used without further purification. In a typical experiment, 0.285 g of CuCl2‚ 2H2O, 0.175 g of NaNO3, and 22.0 mL of PEG400 were placed in a Teflon-lined autoclave of 30.0 mL capacity and then heated at 180 °C for 6 h in an electronic oven. After reaction, the autoclave was cooled naturally to room temperature to obtain a precursor solution without any precipitates. The precursor solution was transferred into a 500.0 mL beaker, into which 50.0 mL of water was added under stirring, and the solution turned milky white and turbid at once. After 10 min, an additional 300.0 mL of water was added into the beaker, and the color of the resultant mixture turned pale yellow within 1 min. After stirring for 30 min, the suspension was aged at room temperature for ca. 40 h to allow the solid to precipitate. Finally, the resultant precipitates were separated by decantation and washed 3 times with water and once with anhydrous ethanol.

10.1021/jp075358x CCC: $37.00 © 2007 American Chemical Society Published on Web 10/05/2007

Nanosized Cu2O/PEG400 Composite Hollow Spheres The products were dried in air for subsequent characterization and analysis. Adsorption of MO by HSs. A 50 mg amount of Cu2O/ PEG400 composite HSs was dispersed in a 100.0 mL MO solution (50 mg/L) under stirring in the dark. The dispersion was centrifuged to separate the solid particles, and the MO concentration was determined using a Lambda-35 UV-vis spectrometer. Characterization. The morphology and microstructure of the products were characterized using a transmission electron microscope (TEM, JEM 100-CX II) with an accelerating voltage of 80 kV, a high-resolution transmission electron microscope (HR-TEM, GEOL-2010) with an accelerating voltage of 200 kV, and a field-emission scanning electron microscope (FESEM, JEOL JSM-6700F). X-ray diffraction (XRD) patterns collected on a Rigaku D/Max 2200PC diffractometer with a graphite monochromator and Cu KR radiation (λ ) 0.154 18 nm) were used to analyze the crystal structure of the products. Thermal gravimetric (TG) analysis was carried out to monitor the mass loss of products at a heating rate of 10 °C‚min-1 from 25 to 800 °C under air atmosphere (Mettler Toledo, TGA/SDTA 851e). Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 5DX FT-IR spectrometer using KBr pellet technique in the range of 400-4000 cm-1. The photoluminescence (PL) spectrum of the HSs was measured by a Hitachi 850 fluorescence spectrophotometer (with an excitation wavelength of 370 nm). UV-vis absorption spectra technique (UVvis spectrometer, Lambda-35, Perkin-Elmer) was used to characterize the absorbance of the products and track the change of the precursor solution before and after heating. N2 adsorption-desorption isotherms were measured on a QuadraSorb SI apparatus at 77 K. The surface areas were calculated by the Brunauer-Emmett-Teller (BET) method, and the pore size distribution was calculated from the desorption branch using the Barett-Joyner-Halenda (BJH) theory. To characterize the product surfaces, X-ray photoelectron spectra (XPS) of Cu2p were recorded on a PHI-5300 ESCA spectrometer (PerkinElmer) with its energy analyzer working in the pass energy mode at 35.75 eV, and the Al KR line was used as the excitation source. The binding energy reference was taken at 284.8 eV for the C1s peak arising from surface hydrocarbons. 3. Results and Discussion The prepared material was identified as the cubic symmetry of cuprite (Cu2O, JCPDS no. 05-0667) by X-ray diffractometry, confirming that the inorganic component was Cu2O. The mean size of the Cu2O nanocrystals was ca.12 nm on the basis of the Debye-Schrrer equation from the 111 reflection. The corresponding low-angle XRD (LA-XRD) pattern (inset of Figure 1) gives a broad peak at 2θ ) 2.7°, which indicates that the hollow structures might be mesostructured. The TEM image shows that the products are well-defined and spherical (Figure 2a), which was also confirmed by FESEM image (Supporting information, Figure S1a). The contrast difference between the center and edge in the TEM image indicates that the products are hollow spheres. The HSs were relatively uniform in diameter, ranging from 50 to 80 nm, and the shell thickness was 15-20 nm. The corresponding selected area electron diffraction (SAED) pattern shows the typical diffraction rings of the cuprite phase, indicating its polycrystalline nature (Figure 2a inset, bottom left). The 0.248 nm spacing between two adjacent lattice planes corresponds to the separation of the {111} lattice planes of cuprite (Figure 2b). The higher magnification TEM images indicate that the HSs

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Figure 1. XRD pattern of the as-prepared Cu2O HSs. (Inset is the corresponding LA-XRD pattern.)

Figure 2. TEM (a), HR-TEM (b), and higher magnification TEM (c) images of the HSs. The inset in a is the corresponding SAED pattern.

Figure 3. IR spectrum of as-prepared HSs.

were composed of Cu2O nanoparticles of ca. 5 nm. (Figure S1b of the Supporting Information and Figure 2c, marked with black arrows) The difference between the size from the TEM observation and that from the XRD diffraction might be due to the partly ordered aggregation of the composite nanoparticles. And wormlike nanopores with sizes of ca. 4 nm can be clearly seen from Figure 2c (marked with white arrows). The HRTEM image also clearly shows that the nanosized HSs consist of a large number of nanocrystals which pack together in different orientations (Figure 2b). The FT-IR spectrum of the as-fabricated composite HSs (Figure 3) shows a peak at 626.5 cm-1 attributed to the Cu-O vibration of the Cu2O nanocrystals. The slight blue shift of the Cu-O vibration could be due to the effect of small particle size.22 The broad band at 3000-3700 cm-1 was deconvolved to make clear the existence state of PEG in the product and four peaks centered at 3251.8, 3349.8, 3442.6, and 3525.7 cm-1 appeared. The peaks at 3349.8 and 3525.7 cm-1 can be assigned to the stretching mode of hydroxyl of pure PEG400 (Supporting information, Figure S2a).23 The bands at 3442.6 and 1631.1 cm-1 correspond to the stretching and bending modes of the hydroxyls of adsorbed water.24 The bands at 3251.8 and 1096.6 cm-1 is corresponding to OH stretching band and C-O

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Figure 4. TG curve (a) and N2 adsorption-desorption isotherm (b) of as-prepared HSs. (Inset, b) The corresponding BJH pore size distribution curve. The sample for TG analysis was dried at 120 °C under vacuum for 6 h.

Figure 5. Schematic illustration of the formation process of HSs.

stretching vibration coordinating to metal cations,23a which shifts about 71 and 10 cm-1 to lower wavenumbers comparing to the IR spectrum of pure PEG400 (Supporting information, Figure S2a), suggesting the formation of chemical bonds between PEG400 and the inorganic components.23a TG curve of the nanospheres (Figure 4a) shows both weight losses and gains. The first drop in weight (up to ∼160 °C, 2.90%) is due to the desorption of absorbed water molecules on the product, and the second one (160-220 °C, 7.72%) should be attributed to the decomposition of part of PEG400, which is rapid. In the next step, a weight increase of 3.62% (220250 °C) corresponds to the oxidation of Cu2O by oxygen in air. In fact, during this period a weight loss and a weight increase happened simultaneously, but the weight increase was larger than the weight loss. After heating to 750 °C, the remaining solid product is CuO, which is 87.84% of the weight of the composite. According to the reaction: Cu2O + 1/2O2 ) 2CuO, there should be a theoretical weight increase of 8.84%. Thus, from 220 to 250 °C the net weight loss would be 5.22% (8.843.62%). In the range from 160 to 420 °C there was a total weight loss of (7.72% + 5.22% + 2.74%) ) 15.68%, corresponding to the decomposition of the entrapped PEG400. However, for pure PEG400, as the temperature rises to 422 °C, it completely decomposes (Supporting information, Figure S2b). For the composite, the last weight-loss step from 420 to 650 °C (2.43%) should be attributed to the decomposition of PEG400 molecules which are chemically bonded to Cu2O. IR results of the product calcined at 420 °C for 2 h (Supporting Information, Figure S3) still show the presence of PEG adsorptions, confirming that some of the PEG molecules are strongly bonded in the nanoparticles. However, for mesoporous oxides, the organic template is usually decomposed completely at 500 °C by TG analysis.25 The higher decomposition temperature (650 °C) observed here might be because the removal of organics during calcination was impeded by the formation of closed HSs or the formation of chemical bonds between the organic and inorganic

Figure 6. Hollow spheres with a solid core.

components prevented the organics from decomposing. Since the hydroxyl and C-O vibrations of the PEG400 molecules in the product were shifted (Figure 3) in IR analysis, we propose that the PEG400 molecules were included in the resultant composite HSs and chemically bonded to the Cu2O nanoparticles via the oxygen atoms of hydroxyl or C-O groups. So a Cu2OPEG composite is formed, with a composition of 79% Cu2O and 21% organics. Furthermore, X-ray photoelectron spectroscopy (XPS) of the composite HSs detected Cu2p3/2 and Cu2p1/2 at 932.9 and 952.6 eV, respectively (Supporting Information, Figure S4), and the peak fit revealed a main peak at 932.5 eV, corresponding to Cu(I) cations. The presence of a weak satellite feature on the higher binding energy side indicates the presence of a small amount of CuO, because the Cu(I) cations on the surface of the product are partly oxidized to Cu(II) in the drying process.26 The N2 adsorption-desorption isotherm of the HSs shown in Figure 4b exhibits a hysteresis loop in the relative pressure range of 0.4-1.0, indicating the presence of the inhomogeneous mesopores, which are formed through the aggregation of the nanocrystals and similar to our previous report.27 The corresponding pore size distribution curve calculated from the desorption branch by the BJHmethod (Figure 4b) displays a pore size distribution from 3 to 12 nm, centered at ca. 3.8 nm, which is close to the result from the TEM images (4 nm). The calculated pore volume is 0.26 cm3/g, and the specific surface area is 85.8 m2/g by the BET method. After CuCl2‚2H2O and NaNO3 were dissolved in PEG400, a yellow-green transparent solution was formed. The PEG400 molecules might complex with Cu(II) cations under the assistance of Cl(I) anions in the dissolution process.20 When the solution was heated, its color varied from yellow-green to sandy beige, and in the corresponding UV-vis spectra (Supporting information, Figure S5) the peak at 280 nm and the broad one at 700-1100 nm corresponding to Cu(II) have disappeared,28

Nanosized Cu2O/PEG400 Composite Hollow Spheres

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Figure 7. UV-vis (a) and PL (b) spectra of the as-prepared HSs.

indicating the reduction of Cu(II) to Cu(I) and the formation of a Cu(I)-PEG complex (Figure 5a).21 In this heat-treating process, NaNO3 plays an important role in the formation of clear precursor solution as described in the following part, probably through participating in the formation of the Cu(I)-PEG complex. However, the detailed effect of NaNO3 still needs further investigation. When water was added to the Cu(I)-PEG solution, it became a yellow turbid suspension, and the XRD diffraction of the precipitate indicated the formation of Cu2O phase. At the same time, the noncombining PEG400 molecules might form micellelike aggregates to reach a relatively stable state, because the polymeric chain is alternately hydrophilic and hydrophobic and has specific interactions with water leading to a helical structure.29 The aggregation process is very fast and dynamic, and the aggregates are somewhat polydispersed. Once the Cu2O nanoclusters were produced, oxygen atoms in the PEG molecular chain coordinated with the metal ions and resulted in a composite with PEG molecules chemically combined to Cu2O nanoparticles.30 Due to the relatively high PEG concentration and the strong intermolecular hydrogen bonds, one Cu2O nanoparticle might also loosely associate with several other PEG400 molecules besides the chemical combination. To decrease their high surface energy, the composite nanoparticles would aggregate.27a,31 The aggregates of PEG400 might act as soft templates around which these composite nanoparticles assemble (Figure 5b). But the PEG400 molecules adsorbed on the surface of the composite nanoparticles prevent them from aggregating compactly, so mesopores are formed from the interparticle spacing of the preformed composite nanoparticles. Unlike the ordered mesoporous structures produced from amphiphilic block copolymer templates, mesoporous structures formed by ordered aggregation of nanoparticles are usually random.27 In the whole process, PEG400 serves as solvent, complexing agent, and reducing agent to form Cu(I)-PEG complexes and also acts as a template in the formation of nanosized HSs. For the Cu(I)-PEG solution, if more than ca. 460 mL of water was added, the total concentration of PEG400 was lower than the critical aggregative concentration (CAC).29 However, the experiments showed that when 240.0-1340.0 mL of water was added to the Cu(I)-PEG solution, nanosized HSs with similar diameters were obtained. The formation of PEG400 aggregates and composite nanoparticles might be rapid processes, so an intermediate state suitable to the formation of aggregates always exists during the addition of water. Therefore, the composite nanoparticles could assemble around those temporary aggregates to form the hollow structures. Moreover, no solid spheres were formed in the present experiment. On

the other hand, when the Cu(I)-PEG solution was dropped into water under stirring, HSs with a solid core were obtained (Figure 6). This is because the PEG400 concentration was too low to form the aggregates at the early stage of this process,29 so the as-formed composite nanoparticles self-assembled into the solid spheres. With the addition of Cu(I)-PEG solution, the PEG400 concentration increased to above the CAC, and the PEG400 molecules would self-assemble to form the micelles, where the preformed solid spheres may provide an aggregating center. On the basis of the above analyses, it can be concluded that the formation of the hollow structure goes through a soft-template process. Further experiments showed that if NaNO3 was absent but other conditions were the same, the CuCl precipitates were produced directly during the solvothermal treatment at 180 °C. Replacing NaNO3 with equimolar NaCl, irregular Cu2O submicrometer particles were produced after hydrolysis in water. However, when equimolar KNO3 replaced NaNO3, the HSs were also obtained. In addition, if Cu(CH3COO)2, CuCl, Cu(NO3)2, or CuSO4 were used as the Cu sources, Cu2O or other compounds precipitates other than homogeneous Cu(I)-PEG solutions were generated, respectively. Although detailed information about the Cu(I)-PEG solution still needs further investigation, these results indicate that NO3- and Cl- anions participate in and facilitate the formation of the Cu(I)-PEG complex. The UV-visible absorption spectrum (Figure 7) of the composite HSs shows two humplike absorptions at 360 and 428 nm attributed to the band-band transition in Cu2O. The absorption shows obvious blue shift compared to the absorption at ∼570 nm of bulk Cu2O, which arises from the quantum confinement effect and is similar to that of Cu2O nanocrystals,12b,22,32 In a previous report a quantum-confinement threshold was deduced to be 14 nm for Cu2O nanocrystallites17b and the size of our HSs was 50-80 nm, so the quantumconfinement effect arose from the component Cu2O nanocrystallites. However, the corresponding photoluminescence (PL) spectrum exhibited four peaks centered at 414, 436, 570, and 637 nm, and a shoulder at 454 nm, of which the peaks at 414 and 436 nm were comparatively strong. For Cu2O nanostructures, the PL spectrum usually shows a maximum peak at around 570 nm,33 so the peculiar PL phenomenon here might be due to the hollow organic/inorganic composite nanostructures.34 The UV-vis spectra of methyl orange (MO) solutions in the presence of composite HSs were taken over time. When 0.05 g of composite HSs was added into 100.0 mL MO solution (50 mg/L), the absorption of MO was decreased to 5.8% within 10 min and 96.7% of MO was removed (Figure 8) after 30 min.

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Figure 8. Absorption spectrum of MO solution in the presence of composite HSs. The characteristic absorption of MO at 465 nm was selected for monitoring the adsorption process.

To identify the reason for the decreasing of the MO absorption, the precipitate separated after the reaction was dispersed in deionized water and dissolved by adding hydrochloric acid, and then dilute NaOH solution was added to neutralize superfluous acid. Finally, the mixture was centrifuged and the upper clear solution was characterized with UV-vis spectrometer. The results indicated that the adsorbed MO was almost the same as the original solution, so we can conclude that the declorization of the MO solution was due to the adsorption of the HSs (Supporting Information, Figure S6). Further experiments demonstrated that the maximum adsorption amount for MO was 1272 mg/g. The results indicate that the composite HSs exhibited superior adsorption abilities compared with Cu2O nanostructures,35 which must be related to their hollow organic/inorganic composite nanostructures and high specific surface area. 4. Conclusions In summary, nanosized Cu2O/PEG400 composite HSs (5080 nm in diameter) with mesoporous shells of 15-20 nm were synthesized by a PEG-assisted wet-chemical method. In the hollow nanostructures, the polymer content was ca. 18.1 wt %, and the mean size of component nanocrystals and the pore diameter were ca.5.0 and 3.8 nm, respectively. In the fabrication process of hollow structures, PEG400 molecules self-assemble to form micelles which act as templates for the formation of the hollow structures. PEG also acts as a reducing agent, solvent, and complexing agent. The formation of mesoporous structures is due to the oriented aggregation of composite nanoparticles. This method might be extended to prepare other functional compounds with similar nanostructures. The nanosized composite HSs exhibited peculiar PL phenomenon and excellent adsorption ability for MO because of their composite and mesoporous shell structures. Acknowledgment. This work is supported by the National Natural Science Foundation of China (Grant No. 20671057) and the Program for New Century Excellent Talents in the University, People’s Republic of China. Supporting Information Available: Detailed characterization, including FE-SEM and TEM images, IR spectrum and TG curve, and XPS and UV-vis spectra (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Jiang, Z.; Zuo, Y. Anal. Chem. 2001, 73, 686. (b) Dong, A.; Ren, N.; Tang, Y.; Wang, Y.; Zhang, Y.; Hua, W.; Gao, Z. J. Am. Chem.

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