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Electrochemical Synthesis of Very Stable Photoluminescent Copper Clusters† Noelia Vilar-Vidal,* M. Carmen Blanco, M. Arturo Lo´pez-Quintela,* Jose´ Rivas, and Carmen Serra‡ Laboratory of Magnetism and Nanotechnology, Technological Research Institute, UniVersity of Santiago de Compostela, E-15782 Spain ReceiVed: NoVember 30, 2009; ReVised Manuscript ReceiVed: July 6, 2010
Small fluorescent atomic copper clusters, stabilized by tetrabutylammonium nitrate, have been synthesized by a simple electrochemical technique. These small clusters (CuN, N < ≈14) show photoluminescence in the visible range with unusual high quantum yields (13%) and were characterized by UV-vis and fluorescence spectroscopies, LDI-TOF (laser desorption/ionization time-of-flight) mass spectroscopy, XPS (X-ray photoelectron spectroscopy), and TEM/HRTEM (high-resolution/transmission electron microscopy). Cu clusters are very stable and can be dispersed in both apolar and polar solvents, which makes them useful as very small building blocks, bringing new possibilities to construct novel nano/microstructures, with potential applications in fields such as biosensors, biomedicine, etc. Introduction
Experimental Section
Clusters are groups of a small number of atoms with sizes below ≈1 to 2 nm, which were mostly studied in detail in the gas phase since the beginning of the 1980s;1-8 they establish a bridge between atoms and nanoparticles and exhibit a transition to a moleculelike behavior,9 being highly stable due to their unique electronic and geometrical properties.10-22 Clusters show very different properties compared with nanoparticles, such as size-dependent photoluminiscence,23 discrete size-dependent electronic states,24 different magnetic behavior,25 electrocatalytic26 and catalytic27-34 properties, and cytoprotection35 among others, being very attractive for their potential applications in different areas, such as biology, catalysis, sensors, etc. Characteristic absorption UV-vis bands appear at other wavelengths than the plasmon band for nanoparticles due to the fact that they are too small to have a continuous density of states necessary to support this plasmon band.36,37 Although, in the last years, different chemical methods have been developed11,15-20,25,26,36,38-45 for cluster synthesis, there is still a long road ahead until a good control of the cluster size and their properties will be achieved. Electrochemical synthesis offers several advantages over chemical synthesis, for example, size particle control, which can be easily achieved by adjusting the current density.46,47 This method, first introduced by Reetz48 in 1994, is very simple49 and has been used to synthesize different kind of nanoparticles of different shapes50-52 and sizes down to the cluster regime.26 Here, we report a simple electrochemical synthesis of small CuN (N < ≈14) clusters in aqueous solution based on a modification of Reetz’s method. Clusters prepared in this way are photoluminiscent with very large band gaps; they can be solubilized in different solvents and are very stable over years. Different techniques (UV-vis and fluorescence spectroscopies, TEM, XPS, LDI-TOF) were used to characterize the synthesized clusters.
Electrochemical Synthesis of Copper Clusters. Copper clusters were synthesized by an electrochemical method in galvanostatic conditions. The synthesis procedure53 is based on the reduction of copper ions produced during the electrolysis from the soluble Cu anode, at 25 mA/cm2, carried out with an Autolab PGSTAT 20 potenciostat. The experiment was performed in a thermostatted three-electrode conventional electrochemical cell. Tetrabutylammonium nitrate (TBAN, Fluka, 97%) aqueous solution (0.1M) was used as a supporting electrolyte, showing good stability under the synthesis conditions, which never exceeded -1.8 V (vs Ag/AgCl). Stability of TBA up to this potential was previously tested in our group54 and also by other authors.55 A copper sheet (GoodFellow, 99%) was used as a anode, the same size platinum sheet (GoodFellow, 99.95%) as a cathode, and Ag/AgCl as a reference electrode. Both the working electrode and the counter electrode were carefully cleaned before synthesis; the Pt electrode was cleaned by polishing to mirror-like with aluminum oxide (Al2O3, Alfa Aesar, 99.99%) and the copper electrode with sandpaper. Both were then washed with water in an ultrasonic bath. Further electrochemical polishing was carried out on the Pt electrode by repeated cyclic voltammetries in 1 M sulfuric acid (H2SO4, 1M, Merck, 97%). The cell was maintained under a nitrogen atmosphere (to remove the oxygen and avoid the oxidation) and magnetic stirring, keeping the temperature at 25.0 ( 0.1 °C along the whole process. The obtained suspensions are composed of a yellow solid and a colorless supernatant. Purification and Solubilization of Copper Clusters. The suspensions were purified by centrifugation (8000 rpm, 10 min) in order to separate the solid from the supernatant. The supernatant was removed, and the solid was centrifuged again after dispersing in water to eliminate the tetrabutylammonium nitrate excess and copper ions generated in the last stage of the synthesis. This process was repeated three times. With this process, copper ions are totally eliminated but not the tetrabutylammonium salt. Finally, clusters were redispersed in the selected solvents (water, pentane, chloroform, and ethanol). Characterization Methods. UV-vis absorbance and fluorescence spectroscopies were measured at room temperature.
* To whom correspondence should be addressed. E-mail: malopez.
[email protected] (M.A.L.-Q.),
[email protected] (N.V.V.). † Part of the “Protected Metallic Clusters, Quantum Wells and Metallic Nanocrystal Molecules”. ‡ Present address: C.A.C.T.I., Universidad de Vigo, Lagoas-Marcosende, 36200 Vigo, Spain.
10.1021/jp911380s 2010 American Chemical Society Published on Web 07/26/2010
Electrochemical Synthesis of Photoluminescent CuCLs Absorption spectra (190-800 nm) were performed using a Hewlett-Packard HP8452A spectrophotometer in a 1 mm × 1 cm × 3 cm quartz cuvette. Photoluminescence spectra (200-600 nm) were carried out in a Cary Eclipse Varian spectrophotometer. Low-resolution TEM images were obtained on a Philips CM20 electron microscope operating at an acceleration voltage of 120 kV, whereas HRTEM images were performed on a JEOL JEM-2010F electron microscope operating with an acceleration voltage of 200 kV. Samples were prepared on carbon-coated TEM grids by adding a drop onto the grid and evaporating the solvent in ambient conditions. The measured lattice separations were compared against the standards for Cu, Cu2O, and CuO. Mass spectrometry was performed by LDI-TOF (laser desorption/ionization time-of-flight) using a Bruker Autoflex MALDITOF instrument with a pulsed nitrogen laser of 337 nm. Mass spectra were acquired in the reflectron mode in the absence of any matrix because of the usual complications arising from its large fragmentation, especially in the low m/z range used here for the characterization of the synthesized clusters. Experimental conditions for the mass spectra acquisition were optimized for two different m/z ranges: low mass range (m/z ) 0-800 amu) and high m/z range (m/z ) 750-2000 amu). Spectra were collected in negative ion mode and averaged from 300 spectra measured in each range. Samples of copper clusters in water were spotted on the target plate directly and left to dry in ambient conditions. The assignment of the clusters had been done comparing their experimental and theoretical isotopic patterns. Photoelectron spectra were recorded using a VG Escalab 250iXL spectrometer (VG Scientific) equipped with a hemispherical analyzer and Al KR X-ray monochromated source. An X-ray spot of 500 µm was used to generate photoelectrons, which were collected from a take-off angle of 90°. The argon partial pressure in the analysis chamber was maintained at 3 × 10-8 mbar during data acquisition by turbomolecular differential pumping. The measurement was done in a constant analyzer energy mode (CAE) with a 100 eV pass energy for survey spectra and 20 eV pass energy for highresolution spectra. The intensities were estimated by calculating the area under each peak after subtraction of the S-shaped background. Binding energies (BEs) of Cu 2p could be determined by referencing to the adventitious C 1s peak at 285.0 eV. Atomic ratios were computed from peak intensity ratios and Schofield atomic sensitivity factors. Samples for XPS analysis were prepared, leaving a drop of solution containing CuCLs in water on a piece of silicon wafer mirror finish polished, evaporating quickly the solvent, and immediately introducing the sample into the XPS prechamber under highvacuum conditions. Ar+ ions (1.5 keV, 60 s) were employed to remove adventitious carbon, which comes from the surface sample exposure to the atmosphere, and to remove also possible oxides at the sample surface. The amount of material removed is a function of the incident energy and sputtering time. Samples were kept rotating in order to avoid a shadow effect. The efficiency of the sputtering process was checked by the disappearance of the Si oxide native layer from the silicon wafer substrate. Bombardment was performed using an EXO5 ion gun incorporated into the equipment, provided with a scanning unit to raster the ion beam, operating at a voltage of 1.5 kV and a scan size of 2 mm, producing a sample current of 0.3 µA during 60 s of bombardment. The higher oxidation state of copper(II) can be identified by a shakeup satellite at ∼10 eV from the Cu 2p3/2 main peak. This satellite peak is not present for other chemical species of copper, such as Cu(0) or Cu(I).
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Figure 1. UV-visible and fluorescence spectra of copper clusters in aqueous solution (black line, absorption spectrum; black dashed line, excitation spectrum, λemission ) 410 nm; red dashed line, emission spectrum, λexcitation ) 300 nm) (A). Inset: blue emission observed for an aqueous solution of CuCLs irradiating at λexcitation ) 296 nm (B).
Results and Discussion UV-vis Spectra. Figure 1A shows room-temperature UV-vis spectra of CuCLs with three absorption bands located at 5.85 eV (212 nm), 5.36 eV (231 nm), and 4.19 eV (296 nm). Absorption of copper clusters in the gas phase have been reported by Smalley et al.56 and Jarrold and Creegan,57,58 showing that, in the gas phase, clusters have strong absorption bands through the whole UV-vis region, from 370 to 710 nm. Cu clusters synthesized in solution and stabilized with different ligands or matrices have also been reported. For example, Gurin et al.59 synthesized copper clusters in zeolites and other porous matrices by hydrogen reduction at high temperatures. Besides the surface plasmon resonance of copper nanoparticles, observed at around 2.21 eV (560 nm),60 they also found absorption bands at λ < 400 nm, which they related to the presence of copper clusters. Ershov61 et al. also found relatively stable copper clusters in solution synthesized by pulse radiolysis with absorption in the UV region. Balogh et al.62 reported the synthesis of copper clusters in PAMAM dendrimers. Stable Cu clusters showing three absorption bands at 250, 310, and 350 nm were obtained with G5 dendrimers. Also, we have reported45 very recently the synthesis of Cu clusters in microemulsions displaying similar optical characteristics (UV absorption and emission; see below) to the ones reported here. Band-Gap Determination. It is known that small metal clusters display a semiconductor-like behavior because of the appearance of a band gap at the Fermi level.63,64 For this reason, we have tried to determine the band gap by the traditional Tauc approach used for semiconductors, using the equation (Rhν) ) A(hν - Eg)n, where hν is the photoenergy (h is Plank constant, and ν is the frequency radiation), R is the absorption coefficient, A is a constant, Eg is the band-gap value, and n equals 1/2 for a direct transition and 2 for an indirect transition. One can see (Figure SI-1, Supporting Information) that it is difficult to fit the spectral data by this equation, which indicates that clusters do not display a classical semiconductor behavior. Nevertheless, the approximate band gaps, which can be deduced by the Tauc approach (Figure SI-1, Supporting Information), 3.7 eV for a direct transition and 3.25 eV for an indirect transition, are larger than the direct band gap of bulk Cu2O (2.17 eV) and CuO (1.4 eV)65 and also larger than other reported copper oxide nanostructures, showing values in the range of 2.24-2.75 eV.66-68 Photoluminescent Properties. Excitation and emission spectra of the samples (Figure 1A) were recorded under ambient
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Figure 2. Excitation (dashed line)-emission (line) spectra (maximum intensity) of copper clusters in different solvents. Pentane: line, λexc ) 355 nm; dashed line, λemi ) 406 nm. Chloroform: line, λexc ) 350 nm; dashed line, λemi ) 418 nm. Ethanol: line, λexc ) 350 nm; dashed line, λemi ) 420 nm. Water: line, λexc ) 297 nm; dashed line, λemi ) 406 nm.
conditions showing excitation and emission bands at 4.13 eV (300 nm) and 3.02 eV (410 nm). Sample solutions are colorless under daylight but are highly blue when irradiated with UV light (300 nm), as it is shown in the inset of Figure 1 (panel B). The quantum yield of the samples is ≈13% (calculated over the entire emission wavelength range and relative to a quinine sulfate standard). Mooradian69 and Darugar et al.70 studied the fluorescence in copper films and copper nanoparticles, respectively, getting emission bands at around 2.06 eV (600-610 nm) with excitations at 2.54 eV (488 nm) and 3.68 eV (337 nm), respectively, but with extremely low quantum yields (10-10 and 10-5, respectively). Very recently, we have also reported45 fluorescence in small CuN clusters (N approximately
