Solvent-Dependent Surface Plasmon Response and Oxidation of

Jan 18, 2011 - (8) The high surface area-to-volume ratio of nanomaterials results in high sensitivity of the intensity and spectral position of the pl...
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Solvent-Dependent Surface Plasmon Response and Oxidation of Copper Nanocrystals Katherine P. Rice, Edwin J. Walker, Jr., Mark P. Stoykovich, and Aaron E. Saunders* Department of Chemical and Biological Engineering, University of Colorado at Boulder, Boulder, Colorado 80309, United States

bS Supporting Information ABSTRACT: We report the synthesis of monodisperse colloidal copper nanocrystals and subsequent solvent-dependent oxidation to form copper(I) oxide nanoparticles. The oxidation process was monitored by optical spectroscopy in the visible spectrum with the Cu nanocrystals exhibiting a surface plasmon feature that was replaced over time by an excitonic feature corresponding to the band gap of the Cu2O nanocrystals. The initial intensity of the copper plasmon was strongly dependent on the properties of the solvent used to form the nanocrystal dispersion; solvents with πbonds significantly reduced (by >3-fold) the plasmon intensity and this effect was attributed to electron sharing between the solvent and the copper surface. The damped plasmon only recovered to its solvent-independent intensity once the nanocrystal surface oxidized and eliminated the solvent-Cu surface interactions. Solvents without π-bonds induced only a very small damping of the plasmon, and at longer time scales all solvents caused similar changes in the optical properties as oxidation converted the nanocrystals from metallic copper to semiconducting copper oxide.

’ INTRODUCTION The oxidation of nanomaterials;for example, the formation of a surface oxide or the partial or complete oxidation of the material;often has a significant impact on their optical,1-5 electrical, and magnetic6 properties. Superparamagnetic iron nanoparticle fluids, for example, lose their magnetic properties upon oxidation.6 While the end result of the oxidation process is typically easily detectable by elemental analysis techniques such as X-ray photoelectron spectroscopy2 or structural analysis through X-ray diffraction or transmission electron microscopy, in situ measurements of the oxidation process of nanomaterials are typically challenging. In this article we present a system of copper/cuprous oxide (Cu/Cu2O) nanocrystals that allows the effect on the optical properties of the oxidation process, as well as the solvent used for nanocrystal dispersal, to be easily monitored in situ by optical spectroscopy. Copper nanocrystals readily oxidize to form copper(I) oxide upon exposure to oxygen;7 what makes this system amenable to study, in contrast to other metal/metal oxide pairs, is that both Cu and Cu2O have prominent and spectrally distinct features in the visible spectra, thereby allowing the oxidation reaction to be followed in real time. A prominent peak in the visible absorption spectra for Cu nanocrystals corresponds to a localized surface plasmon resonance (LSPR), a phenomenon observed in metal nanocrystals and thin films in which conduction electrons oscillate coherently in response to excitation by electromagnetic radiation.8 The high surface area-to-volume ratio of nanomaterials results in high sensitivity of the intensity and spectral position of the plasmon to the size,9 shape,10 and r 2011 American Chemical Society

local environment11 of the nanomaterial. This latter category includes the dependence of the plasmon signal from the nanomaterial on the electronic properties of the surrounding solvent/ matrix and the adsorption/reaction of chemical species at the nanostructured surface. While surface plasmons are readily observed in the visible spectrum for nanoparticles of coinage metals (i.e., copper, silver, and gold), the relative chemical inertness of gold and silver has made them the most common materials studied to date for their LSPR behavior.9,12-18 Copper nanocrystals have been previously synthesized using a variety of techniques19-26 though the reactivity and fast oxidation of Cu nanocrystals at ambient conditions20 has often limited detailed study of the Cu plasmon. Applications of such materials typically focus on the resultant oxidized Cu2O nanoparticles for catalysis19,27 or photovoltaics.28-31 The optical absorption spectra of cuprous oxide nanoparticles, in comparison, are characterized by an exciton peak at a wavelength that corresponds to the band gap of the semiconducting nanomaterials and that is distinct from the plasmon wavelength. This reactivity, and the sensitivity of the LSPR to interactions of the surface with surrounding molecules, does imply that Cu nanocrystals may be more useful than gold or silver for studying the oxidation process and such molecule-surface interactions. Here, we use UV-vis spectroscopy to probe the interaction between the Cu nanocrystals and the surrounding solvent Received: November 2, 2010 Revised: December 15, 2010 Published: January 18, 2011 1793

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Figure 1. TEM image of the as-synthesized copper oxide nanocrystals. The sample has an average core diameter of 11.7 ( 1.3 nm.

molecules and to follow the oxidation of colloidal Cu nanocrystals to Cu2O nanoparticles. While the as-synthesized colloidal Cu nanocrystals are partially coated with organic ligands in order to prevent aggregation and provide solubility in organic solvents, the metal surface may still interact strongly with solvent molecules and react with dissolved oxygen. To study these interactions, Cu nanocrystals are transferred from the air-free reaction vessel and immediately diluted in an organic solvent containing dissolved oxygen. The interaction of the metal surface, having a partially filled conduction band in the case of Cu, with solvents containing π-bonds leads to significant damping of the surface plasmon, as compared with solvents that do not contain π-bonds and that show only a modest damping of the plasmon. As the surfaces of the nanocrystals oxidize to a form Cu2O shell, the interaction with π-electrons in the solvent is screened and the surface plasmon is no longer strongly damped, resulting in a fast increase in the intensity of the plasmonic feature. As the nanocrystals continue to oxidize from metallic Cu to form semiconducting Cu2O particles, two additional optical phenomena are observed: (1) a shift in the Cu plasmon due to the changing dielectric properties of the local environment around the metal nanocrystal as an oxide shell is generated and replaces the interface between the Cu metal and the organic capping ligands/solvent and (2) the eventual attenuation of the plasmon32 and appearance of an excitonic feature that corresponds to the semiconducting Cu2O.

’ MATERIALS AND METHODS All chemicals were purchased from Sigma-Aldrich and used as received. Colloidal copper nanocrystals were synthesized in organic solvents by a previously reported method from O’Brien and co-workers.7 Briefly, 0.245 g (2 mmol) of copper(I) acetate was dissolved in 7.5 mL of trioctylamine under sonication. This mixture was transferred to a three-neck flask and 2 mL (2 mmol) of filtered oleic acid was added. The mixture was held under vacuum for 30 min to remove dissolved gases and then heated quickly under nitrogen to 180 °C and held at this temperature. After 1 h, the mixture was heated further to 270 °C and held at this elevated temperature for 1 h. At the end of the reaction, the heating mantle was removed and the reaction mixture was cooled under nitrogen to yield an organic dispersion of

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colloidal Cu nanocrystals in an inert environment within the threeneck flask. To monitor the effects of solvent interactions and oxidation on the optical properties of the Cu nanocrystals, an aliquot was removed from the inert environment and immediately ( t*), both solutions take on a light blue-green color characteristic of Cu2O nanoparticles and have similar intensities as expected for dispersions with similar particle concentrations. As mentioned earlier, the magnitude of the initial LSPR reduction correlates well with the presence (large LSPR reduction) or absence (very small LSPR reduction) of π-bonds in the solvent used for dispersing the Cu nanocrystals. The change in plasmon intensity over time is shown in Figure 5 for several different solvents and with absorption intensities normalized to the maximum (undamped) plasmon intensity achieved at t*. Solvents without π-bonds (here we tested hexanes, chloroform, methylcyclohexane, heptane, dichloromethane, and cetane) showed little variation in the plasmon intensity, while those 1795

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Figure 4. Photographs of color change during oxidation of Cu nanocrystals to Cu2O nanoparticles in (a) benzene and (b) cyclohexane as a function of time (increasing from left, t = 0, to right, t = 120 min). The fourth cuvette from the left approximately corresponds to t*, in which the maximum intensity of the LSPR is achieved and absorbance from the dispersion is relatively independent of the solvent. The nanocrystal concentration in each dispersion is equal and constant at approximately 6  1012 particles/mL.

Figure 5. Change in absorbance of the plasmon peak as a function of time for Cu nanocrystals dispersed in a series of different solvents. Open symbols correspond to the normalized LSPR peak absorbance in solvents (0 = methylcyclohexane, O = hexanes, X = chloroform) that do not contain π-bonds. Solid symbols correspond to the normalized LSPR peak absorbance in solvents containing π-bonds (1= fluorobenzene, [ = toluene, b = benzene). Green solid circles present an averaged set of normalized LSPR data for Cu nanocrystals oxidized in benzene and the error bars associated with one standard deviation between these optical measurements. The LSPR peak absorbances are normalized with respect to the average absorbance for t > t*.

solvents with π-bonds (toluene, octadecene, fluorobenzene, benzene, and dichlorobenzene) all initially showed large reductions in the plasmon intensity compared with the maximum plasmon intensity. The only exception observed in our tests of different solvent interactions with the Cu nanocrystals was for pure oleic acid; although oleic acid contains a π-bond in the middle of the alkyl chain, little increase is seen in the plasmon

intensity, which we believe may be due to the initial presence of oleic acid as a capping ligand during synthesis, which masks the effects of additional oleic acid or because the carboxylic acid group interacts more strongly with the Cu surface than the π-bond. We propose an electron sharing effect between the solvent molecules and the Cu nanocrystal surface to explain the observed reduction in plasmon signal at short times after dilution and its solvent dependence. Noble metal atoms have a high electron affinity and are known to share electrons with molecules on their surface, that is, the solvent for dispersed nanocrystals as studied here.36 The electrons in the partially filled conduction band of Cu are expected to contribute mostly to the LSPR, with the electrons in the filled valence band having a minimal contribution to the plasmon oscillations for copper.37,38 Delocalized π-bond electrons in the solvent molecules can contribute electron density to the copper surface, thus reducing the density of free electrons in the conduction band and damping the plasmon oscillations. As a thin shell of cuprous oxide begins to form at the nanocrystal surface, however, the plasmon resonance still occurs at the surface of the Cu particle that is now the buried interface between the Cu2O shell and the Cu core, and the shared electron density from the π-bonds of the solvent becomes screened. Higher densities of free electrons in the Cu conduction band therefore become available to oscillate, and the surface plasmon resonance intensity increases. Electron donation to the metal has been shown to decrease the LSPR intensity for gold nanoparticles,39 and we would expect a comparable result here due to the similar band structures and optical properties of copper and gold. This process could also be causing the modest rise in the LSPR intensity seen in the solvents that do not contain any πbonds. The oleic acid capping ligand may damp the plasmon resonance at the surface until a shell of Cu2O forms and eliminates the electron interaction between the acid ligand and the Cu nanocrystal. In this situation, copper acts as an electron acceptor and the oleic acid acts as a base to donate electron density to the surface of the nanocrystal. Oxidation of Cu Nanocrystals to Cu2O. While the initial reduction and recovery of the LSPR depends strongly on the Cu surface interactions with the solvent molecules, the change in 1796

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Figure 6. (a) Optical absorbance spectra as a function of time for Cu nanocrystals dispersed in toluene show a significant increase in plasmon absorption at early times (inset) and subsequent disappearance of the plasmon peak and appearance of the exciton peak as the nanocrystals oxidize. Some spectra have been omitted for clarity. (b) Red shift of the plasmon peak (circles) during oxidation, as well as the appearance and red shift of the copper oxide exciton peak (squares). (c) Change in intensity of the plasmon peak during the oxidation process.

optical properties during subsequent oxidation exhibits much less dependence on the choice of solvent. For example, Figure 3 panels b and d show the decrease in plasmon intensity during oxidation in benzene and cyclohexane solvents, respectively. Both solvents exhibit a decrease in the plasmon intensity as the copper domains shrink and are replaced by copper oxide, with new optical features appearing due to the formation of the semiconducting oxide. This behavior is examined more closely in Figure 6, showing the changes in optical properties as Cu nanocrystals oxidize in toluene, a π-bond-containing solvent, over a period of 10 h (Figure 6a). The spectra clearly show the disappearance of the Cu surface plasmon peak at approximately 570 nm occurring simultaneously with the appearance of an excitonic peak centered at approximately 715 nm, which corresponds to the first exciton transition for copper oxide.7,24 The width of the peak corresponding to the first exciton transition is broad and may be a result of the nanocrystal size distribution or the presence of surface defects.40 Examination of the plasmon peak from the Cu nanocrystal yields two interesting observations. First, the plasmon peak undergoes a red shift during the first 20 min of the oxidation process, as shown in Figure 6b. This result is unexpected if one considers only the change in the size of the Cu domains during the oxidation; the decrease in size of the Cu domains should cause a blue shift in the plasmon peak position, as has been observed with other metal nanocrystals.9 The origins of the red shift;approximately 30 nm;may be due to a combination of factors. One factor is the change in the dielectric constant of the material surrounding the Cu domains.41 The plasmon resonance is very sensitive to the electronic properties of the material at the metal surface and, over the course of the oxidation process, the Cu surface is initially surrounded by organic molecules with a

low dielectric of about 2.4 but eventually is surrounded with copper oxide (the dielectric constant for Cu2O is approximately 7.142,43). The increase in the local dielectric constant is expected to yield a measurable red shift, as has been observed for other metal/semiconductor hybrid nanomaterials.44 After approximately 300 min, the plasmon peak stops shifting, which may correspond to the formation of a thick copper oxide shell that effectively screens the metal domains from the lower dielectric capping ligands and solvent. An additional effect that may contribute to the observed red shift is electron transfer between the metal and metal oxide domains, as has previously been observed for Au-Fe3O4 hybrid nanocrystals.45 The appearance and subsequent red shift of a second peak around 700 nm at long oxidation times can clearly be seen in Figure 6a and is quantified in Figure 6b. This second peak, attributed to the band edge exciton absorption by Cu2O,7,24 indicates the formation and growth of a metal oxide shell. At these times, the plasmon and exciton peaks partially overlap; to determine the peak locations, the spectra were deconvoluted in the range between 550 and 900 nm. The modeled spectra were composed of two Gaussian peaks (one for the LSPR and one for the exciton) and a background contribution corresponding to the metal and semiconductor bulk absorption states. To test how sensitive the model was to the assumed shape of the background, we tested three different background functions (i.e., linear, proportional to λ3, or double exponential) and noted how the fitted peak position varied depending on the choice of background function. The error bars shown for these later times in Figure 6b are the standard deviation of the model fits by use of the three background functions; for all earlier times, the error in determining the peak location is on the order of the size of the 1797

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The Journal of Physical Chemistry C markers in the graph. At times less than 450 min, some variation in the shape of the spectra is observed that may indicate the presence of a weak exciton peak, however, the deconvolution procedure becomes much more sensitive to the shape of the background at these earlier times, and we are not able to definitively determine the location of an exciton peak.

’ CONCLUSIONS Copper nanocrystals were synthesized in an oxygen-free environment, and optical spectroscopy was used to monitor in situ and in real time their oxidation to form cuprous oxide nanoparticles under ambient conditions. Optical spectra taken during the oxidation process show an initial reduction in the plasmon intensity at short times when dispersed in some solvents (e.g., the most substantial plasmon damping occurred in solvents that contain π-bonds such as benzene, toluene, and alkenes). The origins of this reduction in plasmon intensity are attributed to the sharing of electron density between π-electrons in the solvents and the partially filled conduction bands of the copper metal surface, thereby damping the free electron oscillations that lead to the surface plasmon resonance effect. As a shell of cuprous oxide forms, the copper particles are screened from sharing electrons with the solvent, and the plasmon peak rises and redshifts due to the higher dielectric shell. As the particles continue to oxidize to become predominantly cuprous oxide, the plasmon signal steadily decreases and continues to red-shift, and excitonic optical features characteristic of semiconducting Cu2O appear. These observations by optical spectroscopy provide strong evidence of electron interactions between the solvent and metal surface, which have been observed to a lesser extent with other noble metals.39 Future studies may aim to use these optical techniques to study nanocrystal oxidation processes in detail, including for the extraction of oxidation rate parameters. ’ ASSOCIATED CONTENT

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Supporting Information. Additional text and one figure comparing experimental Cu nanocrystal absorption spectra with spectra calculated from Mie theory. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: 303-492-0722. Fax: 303-492-4341. E-mail: aaron.saunders@ colorado.edu.

’ ACKNOWLEDGMENT We thank Ian Campbell for the XRD spectra and Professors Charles Musgrave and Steven George for helpful discussions. K.P.R. thanks ConocoPhillips and the DOE GAANN program P200A060218 for partial funding support. E.J.W. was supported by the Department of Chemical and Biological Engineering REU Site Program (EEC 0851849). ’ REFERENCES (1) Inerbaev, T. M.; Masunov, A. E.; Khondaker, S. I.; Dobrinescu, A.; Plamada, A. V.; Kawazoe, Y. J. Chem. Phys. 2009, 131, 6. (2) Borchert, H.; Talapin, D. V.; Gaponik, N.; McGinley, C.; Adam, S.; Lobo, A.; Moller, T.; Weller, H. J. Phys. Chem. B 2003, 107, 9662.

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