Fabrication of Copper Oxide Nanoboxes Containing a Platinum

Oct 5, 2009 - Mee Rahn Kim, Seol Ji Kim, and Du-Jeon Jang*. School of Chemistry, Seoul National University, NS60, Seoul 151-742, Korea. Received July ...
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DOI: 10.1021/cg900880b

Fabrication of Copper Oxide Nanoboxes Containing a Platinum Nanocluster via an Optical and Galvanic Route

2010, Vol. 10 257–261

Mee Rahn Kim, Seol Ji Kim, and Du-Jeon Jang* School of Chemistry, Seoul National University, NS60, Seoul 151-742, Korea Received July 28, 2009; Revised Manuscript Received September 17, 2009

ABSTRACT: Copper oxide hollow nanoboxes containing a platinum nanocluster have been fabricated in mild conditions at room temperature by dropping a laser-irradiated aqueous platinum colloidal solution onto a copper substrate. The irradiation of laser pulses at 1064 nm induces the ejection of photoelectrons from platinum nanoclusters suspended in water. The aqueous colloidal solution containing photooxidized platinum nanoclusters was then dropped onto a bulk copper substrate to form hollow (CuO)0.75(Cu2O)0.25 nanoboxes having an average edge size of 195 nm and an average wall thickness of 26 nm, and the resulting platinum nanoclusters, produced by galvanic reduction, remained inside the newly formed copper oxide nanoboxes.

*To whom correspondence should be addressed. E-mail: djjang@snu. ac.kr.

the excited electron-hole pairs are confined to a small volume.17,18 Although some of photoejected electrons undergo quick recombination, the rest will react with oxygen to increase the pH (see below). The electrons that do not undergo photoejection are rapidly thermalized by electron-phonon scattering.12-14 Galvanic displacement reactions that exploit differences in the reduction potentials of two or more metals can be used efficiently to prepare specific metal nanostructures.19,20 They are simple and cost-effective and can also be used to fabricate hollow nanostructures under appropriate conditions. Galvanic displacement needs a simple apparatus, generates little waste, and works at room temperature. However, it has been recognized that one of the drawbacks of galvanic displacement is the lack of control to prepare isolated metal nanostructures of various shapes and sizes. Copper oxide has been of considerable interest because it forms a basis of technologically important materials such as high-temperature superconductors.21,22 It is also a promising material for solar cells, owing to its photoconductive and photochemical properties, and lithium-ion batteries.23-27 Copper oxide nanoparticles have been prepared by a number of methods such as thermal decomposition, oxidation, reduction, and hydrolysis of metal or metal salts.28-34 Reactions involving the chemical transformation of solids are in general very slow due to high activation energies for the diffusion of atoms and ions. Thus, typical solid-phase reactions require very high temperatures or pressures and therefore would seem to be incompatible with kinetically controlled nonequilibrium nanostructures. The effective transformation of metal nanoparticles via an optical and galvanic route can be exploited to fabricate new composite nanostructures. We report herein that well-defined copper oxide nanoboxes containing a platinum nanocluster can be fabricated in mild conditions at room temperature via laser irradiation and subsequent galvanic displacement. Platinum nanoclusters suspended in water eject photoelectrons upon laser irradiation, and resulting photooxidized platinum nanoclusters placed on a copper substrate subsequently are reduced galvanically by copper to form (CuO)0.75(Cu2O)0.25 nanoboxes which trap a platinum nanocluster inside.

r 2009 American Chemical Society

Published on Web 10/05/2009

Introduction A variety of inventive techniques involving chemical, electrical, optical, and radiolytical processes have been applied to prepare nanostructured materials having functional properties.1-5 Hollow and porous structures have been attracting great attention because of their widespread applications to nanometer-sized chemical reactors, efficient catalysts, drug-delivery carriers, photonic building blocks, energystorage media, and gas sensors.6-11 Inorganic nanostructures having hollow interiors have unique physical and chemical characteristics such as high surface areas, low density, and tunable structures/compositions. The general approach for the preparation of hollow structures has involved the use of various hard and soft templates. The synthesis employing hard templates requires the elimination of cores via optical excavation, chemical etching, or calcinations in order to obtain the final products having hollow interiors.6-8 Recently, novel approaches based on the Kirkendall effect, galvanic replacement, the Ostwald ripening, and salt quasi templates have been employed.8-11 Metallic nanoparticles have also attracted enormous attention from researchers in various fields of study because of their unusual optical properties as well as their novel chemical properties. In particular, noble-metal nanoparticles, which show strong absorption bands in the visible region due to the surface plasmon oscillation modes of conduction electrons, have been recognized to promote electron-transfer processes and undergo thermal and optical shape transformation. The laser-induced size and shape changes of silver, gold, and platinum nanoparticles in water have been explained to occur via melting in general because nanoparticles show depression in melting temperature and thermal conductivity with a decrease in size.12-16 The photoemission of electrons of metallic colloidal nanoparticles may also be significantly enhanced compared with that of bulk materials because of the low density of states. Likewise, Auger scattering processes should also be enhanced in metal nanoclusters because

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Experimental Section Chloroplatinic acid hydrate (s, 99.995%), sodium borohydride (s, 99.995%), hydrazine monohydrate (l, >98%), L-ascorbic acid (s, >99.0%), and sodium citrate tribasic dihydrate (s, >99.0%) were used as purchased from Sigma-Aldrich. Platinum nanoparticles suspended in water were prepared by reducing H2PtCl6 as follows. A 34 mM sodium citrate (aq) solution of 0.2 mL and a 2 h-aged 20 mM sodium borohydride (aq) solution of 0.1 mL were added into deionized water (19.7 mL) under vigorous stirring. A 40 mM hydrazine (aq) solution of 0.9 mL, a 34 mM sodium citrate (aq) solution of 0.35 mL, and the above-prepared reducing solution of 0.1 mL were added into deionized water (90.3 mL) under vigorous stirring. The prepared reducing agent mixture of 0.34 mL and a 10 mM L-ascorbic acid (aq) solution of 0.8 mL were added into a 2 day-aged 10 mM H2PtCl6 (aq) solution of 10 mL. For the preparation of photoexcited platinum nanoclusters suspended in water, the above-prepared platinum colloidal solution of 2 mL contained in a quartz cell having a path length of 10 mm was irradiated under stirring with laser pulses of 1064 nm and 0.5 mJ from a Q-switched Nd:YAG laser of 5 ns (Quantel, Brilliant) or a modelocked Nd:YAG laser of 25 ps (Quantel, YG501) running at 10 Hz. Unless specified otherwise, samples were excited with laser pulses of 5 ns. The spot diameter of the laser beam was 8 mm at the sample. Note that samples have been irradiated at 1064 nm to excite only the surface-plasmon resonances of platinum. One can say that the excitation of platinum nanoparticles at 1064 nm is due to intraband transitions rather than surface-plasmon resonances.35 We assert that irradiation at 1064 nm is to excite the free electrons of metallic platinum exclusively. The aqueous colloidal solution of photooxidized platinum nanoclusters (0.2 mL) was dropped onto a bulk copper substrate and dried in the air. Photooxidized platinum clusters placed on the copper substrate were then galvanically reduced by substrate copper at room temperature to form (CuO)0.75(Cu2O)0.25 nanoboxes which trap a reduced platinum nanocluster inside. High-resolution transmission electron microscopy (HRTEM) images, selected-area electron diffraction (SAED) patterns, and energy-dispersive X-ray (EDX) data were measured by using a high-resolution microscope (JEOL, JEM-2100F) attached to a CCD camera as the detector. While transmission electron microscopy (TEM) images were obtained with a microscope (Hitachi, H-7600), field-emission scanning electron microscopy (FESEM) images were taken with another microscope (Hitachi, S-48000). High-resolution X-ray diffraction (HRXRD) patterns were recorded with a diffractometer (MAC Science, M18XHF-SRA) using Cu KR radiation (λ = 0.154056 nm).

Results and Discussion Figure 1 shows the well-defined morphology of copper oxide nanoboxes containing a platinum nanocluster prepared via laser irradiation and galvanic displacement. Platinum clusters suspended in water were irradiated for 30 min with laser pulses of 1064 nm having a temporal width of 5 ns and a pulse energy of 0.5 mJ, and placed subsequently on a copper substrate to prepare the nanoboxes. The nanostructures of the FESEM image in Figure 1A show the intact morphology of hexahedrons having clearly flat facets and thoroughly sharp edges. The interiors of the hexahedron structures cannot be seen in the FESEM image because all nanoboxes are well constructed without having broken facets. Although some nanoboxes are in contact with others and show twisted and disordered facets, we consider that nanostructures in Figure 1A have nanocube shapes in general. The tilted image in the inset of Figure 1A displays a three-dimensional structure of a tetragonal copper oxide hexahedron. Figure 1B shows a typical TEM image of copper oxide nanoboxes containing a platinum nanocluster. Although nanoboxes were aggregated during evaporation, their hollow morphology can be discerned easily. The 2-D projection images of hollow nanoboxes look like various

Figure 1. (A) FESEM and (B) TEM images of copper oxide nanoboxes containing a platinum nanocluster. The inset of A shows the enlarged view of a nanobox, while that of B displays the edgesize distribution of about 450 nanoboxes.

frames due to their diverse orientations in the TEM measurement. The TEM image shows contrast in brightness between the dark edges and the bright central regions, indicating that the nanoboxes have empty interiors. A close examination of the TEM image reveals that the bright central regions also contain dark spots, which are due to platinum nanoclusters trapped inside copper oxide nanoboxes. In addition, small platinum nanoparticles can also be seen around the nanoboxes. As seen in the inset of Figure 1B, the average edge size of copper oxide nanoboxes, obtained by examining the TEM images of about 450 nanoparticles, is 195 ( 21 nm, indicating that the copper oxide nanoboxes containing a platinum nanocluster are quite uniform in size. The average cavity side length of the produced hollow nanoboxes is 143 ( 9 nm. Thus, the average thickness of the walls projected into frame-like 2-D shapes is about 26 nm. The crystalline structural characteristics of copper oxide nanoboxes containing a platinum nanocluster can be observed clearly in Figure 2. The HRTEM image of Figure 2A shows that while a platinum nanocluster is trapped in each nanobox of copper oxide, many small platinum nanoclusters are present outside every nanobox. The HRTEM image of Figure 2B shows clearly the hollow structure of a copper oxide nanobox having a platinum nanocluster inside. Electron diffraction spots of square symmetry in the SAED pattern of Figure 2C can be indexed to the planes of tetragonal (CuO)0.75(Cu2O)0.25 (JCPDS Card No. 03-0879; 6CuO 3 2Cu2O), suggesting that the copper oxide nanobox has a single-crystalline well-defined structure of the tetragonal (CuO)0.75(Cu2O)0.25 crystal. Figure 2D-F shows the lattice fringes of the areas marked in Figure 2B. The HRTEM image of Figure 2D was measured at a corner of the nanobox, that of Figure 2E at a window facet of the nanobox, and that of Figure 2F at an isolated platinum nanoparticle. The

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Figure 3. EDX line-scanned elemental intensity profiles of copper (circles), oxygen (crosses), and platinum (diamonds) along the indicated dashed line of the copper oxide nanobox containing a platinum nanocluster in the inset.

Figure 2. (A) HRTEM image of copper oxide nanoboxes containing a platinum nanocluster. (B) Enlarged view of a nanobox in A. (C) SAED pattern of (CuO)0.75(Cu2O)0.25 measured at c in B. (D-F) HRTEM images measured at d-f, respectively, in B. The lattice-fringe distances of 0.202 nm in D and 0.203 nm in E correspond to the spacing lengths of the (213) planes of the tetragonal (CuO)0.75(Cu2O)0.25, while the lattice-fringe distance of 0.223 nm in F matches the spacing length of the (111) planes of platinum(s).

interplanar spaces of 0.202 nm in Figure 2D and 0.203 nm in Figure 2E agree with the spacing lengths of the (213) lattice planes of the tetragonal (CuO)0.75(Cu2O)0.25 crystal. The lattice fringes of an isolated nanoparticle in Figure 2F agree well with the (111) lattice planes of platinum(s), although the platinum nanoparticle has a polycrystalline structure having diverse domains of about 4 nm in size. The well-defined electron diffraction pattern and single-lattice fringes of the hollow copper oxide nanobox designate that the (CuO)0.75(Cu2O)0.25 nanobox of Figure 2B containing a platinum nanocluster has a single-crystalline structure, although isolated platinum nanoparticles are polycrystalline. The EDX line-scanned elemental intensity profiles of copper and oxygen in Figure 3 have two characteristic peaks showing a void structure, revealing evidence of the hollow morphology of a copper oxide nanobox containing a platinum nanocluster. Furthermore, the EDX elemental intensity profile of platinum shows a single peak concentrated at the inner part of the hollow nanobox. Consequently, the nanobox consisting of (CuO)0.75(Cu2O)0.25 has a void structure and confines a platinum nanoparticle in its interior. The profile of the wall in contact with the trapped platinum nanocluster on the right side of Figure 3 is much broader than that of the clear wall on the left because copper oxide has been produced via the galvanic reduction of photooxidized platinum by copper. Figures 1-3 have shown that copper oxide nanoboxes containing a platinum nanocluster were synthesized via a galvanic displacement reaction between photooxidized platinum nanoclusters and substrate copper. Platinum nanoclusters suspended in water eject photoelectrons upon laser irradiation, and resulting photooxidized platinum

nanoclusters placed subsequently on a copper substrate are reduced galvanically by copper to form (CuO)0.75(Cu2O)0.25 nanoboxes which trap a platinum nanocluster inside. The HRXRD patterns of Figure S1 (in the Supporting Information) reveal that laser irradiation is essential indeed for the growing of (CuO)0.75(Cu2O)0.25 nanoboxes on a copper substrate. The HRXRD pattern of a sample prepared without irradiation shows copper signals only, which arise from the copper substrate. However, the HRXRD pattern of the other sample prepared with irradiation for 120 min displays (CuO)0.75 (Cu2O)0.25 peaks as well as copper peaks. The TEM image of Figure S2 (in the Supporting Information) shows that an unirradiated aqueous platinum colloidal solution dropped onto a copper substrate produced spherical platinum nanoparticles of 91 ( 14 nm in diameter instead of tetragonal (CuO)0.75(Cu2O)0.25 hollow nanoboxes. This also suggests that laser irradiation is essential for the transformation of platinum nanoclusters into copper oxide nanoboxes trapping a platinum cluster inside. Photooxidized platinum nanoclusters35-38 only can undergo galvanic reduction by copper on a copper substrate to give birth to (CuO)0.75(Cu2O)0.25 nanoboxes. Reduced platinum nanoclusters produced by galvanic displacement remain inside newly formed copper oxide nanoboxes. Figure S3 (in the Supporting Information), showing the 2-D TEM images of nanoparticles prepared after irradiation for 15 min, does not display any bright central regions, indicating that copper oxide hexahedrons of 161 ( 14 nm in size produced with insufficient laser irradiation do not have the hollow structure of (CuO)0.75(Cu2O)0.25. This also reveals the importance of laser irradiation in the fabrication of copper oxide nanoboxes containing a platinum nanocluster. Irradiation effects on platinum nanoparticles dispersed in water were further examined by monitoring the pH of the platinum colloidal solution as a function of irradiation time (see Figure S4 in the Supporting Information). The pH increased with the irradiation time of laser pulses. We suggest that photons of 1064 nm excited the intraband trasitions of platinum nanoparticles to eject electrons.35-38 Photoejected electrons then reacted with oxygen molecules dissolved in water to increase the concentration of OH- according to the following reaction 1. ð1Þ 4e - þ2H2 O ðlÞþO2 ðaqÞ f 4OH - ðaqÞ The produced OH- reacted with H3Oþ eventually to increase pH. Thus, Figure S4 intimates that platinum nanoclusters

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Figure 4. (A) TEM and (B) HRTEM images of copper oxide nanoboxes containing a platinum nanocluster. Nanoboxes were prepared after irradiation for 30 min with laser pulses of 1064 nm having a temporal width of 25 ps and a pulse energy of 0.5 mJ.

suspended in water absorbed photons of 1064 nm to eject photoelectrons and to undergo oxidation. As the resulting photooxidized platinum nanoclusters dispersed in water were then placed on a copper substrate, they experienced galvanic reduction by copper39,40 to produce tetragonal (CuO)0.75(Cu2O)0.25 nanoboxes. It is noteworthy that the ionic compound of copper oxide often forms nanocubes or nanoboxes.24,26,31 Hollow nanoboxes instead of filled nanocubes are suggested to form as outside copper and oxygen ions migrate to photooxidized platinum nanoclusters. Because reduced platinum nanoclusters remained inside, they were trapped inside copper oxide hollow nanoboxes. This mechanism is also supported by Figure 4. Figure 4 shows TEM and HRTEM images of well-defined copper oxide nanoboxes containing a platinum nanocluster prepared after irradiation with picosecond laser pulses,41,42 instead of nanosecond laser pulses, having a very high peak power. The nanoboxes that are 178 ( 31 nm in size have very thin walls of about 10 nm in thickness and enclose an imprisoned platinum nanocluster well. The walls of copper oxide nanoboxes prepared with picosecond laser pulses are much thinner than the walls of nanoboxes prepared with nanosecond laser pulses (Figures 1 and 2). Thus, platinum nanoclusters imprisoned in copper oxide nanoboxes are easily discernible in Figure 4. There are also numerous small platinum nanoclusters outside (CuO)0.75(Cu2O)0.25 nanoboxes. Because as-prepared large platinum nanoclusters of 91 nm in size ejected photoelectrons severely upon irradiation with picosecond pulses having a high peak power, they accumulated a great number of charges on their surfaces. The resulting photooxidized platinum nanoclusters were unstable as to release many fragmented small platinum nanoparticles. Nevertheless, we consider that picosecond laser pulses having a high peak power are effective for the fabrication of tetragonal (CuO)0.75(Cu2O)0.25 nanoboxes having thin walls. Conclusions Well-defined hollow nanoboxes of tetragonal (CuO)0.75(Cu2O)0.25 containing a platinum nanocluster have been fabricated via an optical and galvanic route in mild conditions at room temperature. The irradiation of laser pulses at 1064 nm induced the ejection of photoelectrons from platinum nanoclusters suspended in water. The photoejected electrons reacted consequently with oxygen molecules dissolved in water to increase the concentration of OH-. An aqueous colloidal solution containing photooxidized platinum nanoclusters, produced after ejecting photoelectrons upon irradiation, was then dropped onto a copper substrate to undergo galvanic displacement. The photooxidized platinum

Kim et al.

nanoclusters placed on the copper substrate were reduced galvanically by substrate copper to form tetragonal (CuO)0.75(Cu2O)0.25 hollow nanoboxes having an average edge size of 195 nm and an average wall thickness of 26 nm. The resulting platinum nanoclusters produced by galvanic reduction have remained inside the newly formed copper oxide nanoboxes. Laser irradiation to an aqueous platinum colloidal solution prior to being dropped on a copper substrate has been found to be essential for the galvanic fabrication of (CuO)0.75(Cu2O)0.25 nanoboxes trapping a platinum nanocluster. Acknowledgment. This work was supported by research grants through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2009-0082846 and 2009-0071184). D.J.J. also thanks the SRC program of NRF (R11-2007-012-01002-0) while MRK acknowledges the BK21 scholarship. Supporting Information Available: HRXRD patterns of samples prepared without and with irradiation, TEM images of nanoparticles prepared without and with irradiation, and pH variation of a colloidal solution with irradiation time. This material is available free of charge via the Internet at http://pubs.acs.org.

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