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Ag@ZnO CoreShell Nanoparticles Formed by the Timely Reduction of Ag+ Ions and Zinc Acetate Hydrolysis in N,N-Dimethylformamide: Mechanism of Growth and Photocatalytic Properties Matías E. Aguirre,† Hernan B. Rodríguez,‡ Enrique San Roman,‡ Armin Feldhoff,§ and María A. Grela*,† †
Departamento de Química, Universidad Nacional de Mar del Plata, Funes 3350, B7602AYL Mar del Plata, Argentina INQUIMAE/DQIAyQF, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellon 2, 1428 Buenos Aires, Argentina § Institut f€ur Physikalische Chemie und Elektrochemie, Leibniz Universit€at Hannover, Callinstraße 3-3, D-30167 Hannover, Germany ‡
bS Supporting Information ABSTRACT: A new synthetic route for the preparation of AgZnO hybrid nanostructures under well-defined conditions using silver nitrate and zinc acetate in N,N-dimethylformamide as starting materials is presented. The solvent simultaneously behaves as a reducing agent for Ag+ ions and provides the basic medium for zinc acetate hydrolysis at room temperature, without making resource of stabilizers and any kind of additives. As determined by electron microscopy studies, the prepared nanostructures have well-defined coreshell architectures, with a cover layer of ZnO protecting the Ag center from oxidation by external agents. Intermediates and final products were further characterized by FTIR, XRD, ICP-OES, and UVvis absorption and luminescence spectroscopy. The change in the emission properties of ZnO as a result of the assemblage proves the strong interaction existing between the semiconductor and the metallic nucleus and points to an efficient electron transfer from ZnO to Ag. Accordingly, comparative photocatalytic experiments of ZnO and Ag@ZnO nanostructures loaded with the chelating xanthene dye, 9-phenyl-2,3,7-trihidroxy-6-fluorone, confirm that the presence of Ag nanoparticles in the hybrid nanostructures serves to slow down charge recombination both under UV and visible light excitation.
’ INTRODUCTION The mechanism of formation of composite materials is one of the most active areas of nanoparticle research.14 The interest in hybrid nanostructures is aimed at the exploitation and tuning of the properties of the individual components and the development of novel functionalities. Nanocomposite materials involving metals and semiconductors as building blocks are especially attractive, as they provide the opportunity to modulate optical phenomena including harvesting, emission, and concentration of electromagnetic radiation.57 These features are of practical relevance for the optimization of photovoltaic and photocatalytic systems and for the development of optoelectronic and sensing devices.811 ZnO is an interesting wide band gap semiconductor which shows high stability and low toxicity in neutral and alkaline media.12 In addition, its direct character makes it an appealing material to replace CdSe in cell labeling applications13,14 and for the development of practical hybrid metalsemiconductor devices based on the emission properties of zinc oxide.11,15,16 The efficiency of harvesting, emission, and concentration of the electromagnetic radiation within the nanocomposite critically depends on how the plasmonic and semiconductor units are integrated.57,17 The comparative benefits of the core@shell arrangement over other structures in which the metal nanoparticles are deposited on the semiconductor surface as isolated nanoislands have been discussed recently.18 To summarize, coreshell r 2011 American Chemical Society
architecture maximizes the interaction between the building blocks, avoiding metal exposure to the solvent and favoring the semiconductor interaction with dissolved acceptors and donors. Although several successful synthetic routes exist for M@TiO2,2,9,10,18 the production of M@ZnO nanocomposites is still challenging:19 most of the reported procedures have focused on the synthesis of decorated nanorods, with the metal nanocrystals located at the tip or along the surface of the semiconductor units,21,20 or yielded heterodimers21 showing a dumbbell-like morphology.16,22 In this work, we take advantage of the properties of N,Ndimethylformamide both as a reductant of Ag+ ions and as a convenient basic solvent to induce zinc acetate hydrolysis and design a simple one-pot room-temperature strategy to obtain ZnO-covered silver nanostructures with interesting plasmonic features.17
’ EXPERIMENTAL SECTION Materials. Zinc acetate dihydrate (Fluka), 9-phenyl-2,3,7trihidroxy-6-fluorone, PF (Aldrich), sodium acetate (Cicarelli), ammonium acetate (Cicarelli), silver nitrate (Cicarelli), and Received: September 21, 2011 Revised: November 10, 2011 Published: November 10, 2011 24967
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The Journal of Physical Chemistry C potassium bromide (Cicarelli) were of the highest available purity and used as received. Absolute ethanol, methanol, 2-propanol (spectrophotometric grade), and N,N-dimethylformamide, DMF, (chromatographic grade) were purchased from Sintorgan and also used without further purification. Synthesis of Nanocomposites. An aqueous solution of AgNO3 with concentrations ranging between 3 and 15 mM and an 8.3 mM solution of zinc acetate (ZnAc2 3 2H2O) in DMF were used to prepare the reaction mixture. In a typical procedure, 0.5 mL of the aqueous solution of Ag+ was added to 9 mL of the Zn2+ solution, and then 5.5 mL of DMF was immediately added under vigorous shaking. The mixture was allowed to react at room temperature, (25 ( 2) °C. The nominal molar ratio of the precursors in the reaction mixture, defined as RN = (nAg+/nZn+2) 100, was varied between 5 and 40, keeping the concentration of zinc acetate and water fixed at 5 mM and 1.85 M, respectively. Glassware was cleaned with sulfochromic acid, washed with a nonionic detergent, and wiped away with ultrapure water. Characterization. Optical absorption spectra of the Ag/ZnO nanocomposites were recorded on a Shimadzu UV-2101 PC dual beam spectrophotometer. The excitation and emission spectra were measured using a PTI Model QM-1 spectrofluorometer. Measurements were performed using a quartz cell having a path length of 10 mm. To block excitation from reaching the detector at the different wavelengths explored in the UV (270, 300, 320, 325, 350 nm) and visible range (430450 nm), an appropriate optical filter (Schott WG290, WG330, WG340, WG360, GG385, or GG475) was installed in the emission light path. Emission and excitation spectra were corrected for changes in the detector responsivity and filter transmittance with wavelength and considering the spectrum of the excitation lamp. The crystalline properties of ZnO and Ag/ZnO nanostructures were examined via powder X-ray diffraction (XRD). The Pert PRO (PANalytical) powder analysis was carried out on a X X-ray diffractometer, with Cu Kα (1.54 A°) as the incident radiation, and operated at an accelerating voltage of 40 kV with a current intensity of 40 mA. Samples were prepared by ultracentrifugation of the corresponding colloids. The solid was repeatedly washed with small amounts of ethanol and finally dried under a nitrogen stream and then heated at 120 °C. The infrared measurements were performed on a PerkinElmer Fourier transform spectrophotometer model Spectrum BX equipped with a DTGS detector. Measurements were performed on pressed pellets, using potassium bromide as diluent. Spectra are the average of 10 scans taken at 4 cm1 resolution vs the appropriate single-beam background spectrum. A Perkin-Elmer (Norwalk, CT, USA) ICP Optima 3100 XL (axial view) simultaneous inductively coupled Ar plasma optical emission spectrometer (ICP-OES) provided with a model AS 90 autosampler was used to determine the actual ratio Ag/Zn in the final nanostructures. Ag/ZnO nanocomposites were also investigated by transmission electron microscopy (TEM) using a JEOL JEM-2100F fieldemission instrument, equipped with an energy-dispersive X-ray spectrometer (EDXS) Oxford Instruments INCA-200 TEM with ultrathin window. An ca. 10 μL drop of colloidal suspension was put on a copper-supported carbon foil (TEM grid) and dried under a red light lamp. Photocatalytic Studies. To analyze the photocatalytic activity of the samples, 12 mg of powdered ZnO or Ag@ZnO samples was dispersed in 250 mL of 1 mM NaOH aqueous solution, and
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Figure 1. XRD pattern of powders obtained by ultracentrifugation of the as-prepared ZnO sol after prolonged (10 days) aging. The solid was washed with ethanol and dried at 120 °C under N2.
then 2 mL of a 1.1 mM PF solution in methanol was added under vigorous stirring, rendering a 8.7 μM final PF concentration. The mixture was allowed to equilibrate 120 min in dark prior to the irradiation experiments. Under these conditions, the amount of chemisorbed dye in both the ZnO and Ag@ZnO dispersion was higher than 97% as estimated by measuring the amount of dye in the supernatant solution after equilibration and ultracentrifugation. Steady-state photolysis was carried out in 3 mL of solution placed in a 1 cm optical cell at the selected wavelengths (303 or 536 nm) using a 1000 W HgXe lamp coupled to a Kratos Schoeffel monochromator.
’ RESULTS AND DISCUSSION To assess the mechanism of formation of Ag/ZnO nanocomposites, the limiting cases corresponding to RN = 0 (pure ZnO) and RN = ∞ (pure Ag) were first tested. ZnO Synthesis in Dimethylformamide, RN = 0. The use of DMF:water mixtures as a convenient reaction media for the arrested basic hydrolysis of zinc salts has recently been pursued in the literature.23 At variance with the usual synthesis of ZnO in alcoholic solutions, in this approach, the source of hydroxyl species is controlled by DMF hydrolysis in the presence of small amounts of water. Another benefit of this synthetic procedure is that zinc salts show a much higher solubility in DMF than in alcohol, avoiding the need of heating.24 Thus, we decided to carry out the hydrolysis reaction at room temperature, as a strategy to further control the kinetics of the ZnO growth. The hydrolysis of 0.5 mM zinc acetate in DMF in the absence of Ag (RN = 0) readily leads to a clear suspension of ZnO nanoparticles which remains stable for days. The formation of hexagonal-wurzite nanocrystal particles could be confirmed by XRD analysis after precipitation and washing with ethanol. The peaks indicated between brackets in Figure 1 correspond to the reflection from the different crystal planes of ZnO and were taken from the Joint Committee on Powder Diffraction Standards card (JCPDS 0361451).25 The spectra also reveal the presence of peaks at smaller scattering angles marked with an asterisk (at ca. 13.5°, 21.2°, and 24968
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Figure 2. FTIR spectrum of powders obtained by ultracentrifugation of ZnO sols after prolonged (10 days) aging. The solid was washed with ethanol and dried at 120 °C under N2 and then dispersed with KBr.
Figure 3. Absorbance evolution at the maximum of the ZnO exciton band in a 3% V/V water:dimethylformamide solvent, using different zinc acetate concentrations: (9) 0.5, (b) 2.5, and (2) 5.0 mM.
24.0°). This feature is indicative of incomplete hydrolysis of residual precursors and disappears after annealing at 400 °C.26 On the other hand, the FTIR spectrum of the reaction products, shown in Figure 2, exhibits a broad band at 450 cm1 corresponding to the ZnO stretching frequency and the presence of the symmetric and asymmetric CO2 stretching modes (at 1405 and 1587 cm1, respectively).27 The last two modes are indicative of acetate chemisorption,28,29 which probably accounts for colloid stabilization.27,30 The kinetics of ZnO formation is determined by the reaction temperature, the acetate concentration, and the hydrolysis ratio defined as H = [H2O]/[ Zn2+]. All our experiments were performed at room temperature and fixed water content ([H2O] = 1.85 M); however, different acetate concentrations ranging from 0.5 to 10 mM were explored. We found that higher precursor concentrations (i.e., lower H values) progressively retard the
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Figure 4. Comparison between the absorption spectra of (a) pure ZnO and (b) Ag sols with (c) the spectrum obtained for the coreshell nanostructure, RN = 6.7.
appearance of the band of ZnO at 360 nm and result in a sigmoidal growth of the absorbance at this wavelength, as shown in Figure 3. As we will discuss later, this fact can be exploited to obtain a fine control of the nanocomposite synthesis. The existence of an induction period, which depends inversely on the ratio between water and the oxide precursor, has also been reported in other solgel processes31,32 and discussed within the framework of LaMer's model33 or more elaborate analyses.32 A plausible explanation to account for the induction period is that lower H values favor condensation over hydrolysis reactions. However, after the clusters reach a critical size, the slow replacement of acetate by hydroxyl groups lowers the degree of cross-linking and facilitates the subsequent entrance of water, thus explaining the observed sudden change in the rate of oxide formation. Ag+ Reduction by Dimethylformamide, RN = ∞. Addition of AgNO3 to the DMF/water mixture results in the rapid reduction of the metal ions.34 However, the nascent yellow colloid readily turns gray-brown and fades.35 X-ray diffraction was used to analyze the chemical nature of the precipitate, i.e., to determine the possible formation of Ag2O or AgO. These oxides are normally found during the reduction of Ag+ ions in basic wateralcohol mixtures.26,36 However, the XRD pattern, included as Supporting Information, unambiguously indicates that the destabilized colloid consists of pure metallic Ag particles. Probably, the replacement of the alcohol by DMF prevents the formation of silver oxides due to the change in the solvent properties from dehydrating to more reducing. On the basis of the known capping effect of citric acid through the CO(OH) groups, we made an attempt to stabilize the nascent Ag nanoparticles with acetate anions.37,38 To this purpose, sodium or ammonium acetate salts were added to the reaction mixture. However, in the conditions explored in this work ([H2O] = 1.85 M, 0.5 e [CH3COO]/mM e 10), no stable Ag dispersions could be obtained by reduction of the silver salt in DMF. Stabilized Ag/ZnO Nanocomposites, 5 e RN e 10. The nanocomposites were prepared as described in the Experimental Section, and the reaction progress was monitored by UVvis absorption spectroscopy as indicated in the following section. The final spectrum of the reaction mixture obtained for RN = 6.7 ascribed to the Ag/ZnO nanostructure, curve (c) in Figure 4, is 24969
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Figure 5. STEM dark-field images of the assemblies formed by simultaneous Ag+ reduction and hydrolysis of zinc acetate, RN = 6.7, in DMF.
compared with the spectra of pure ZnO and Ag in DMF, curves (a) and (b), respectively. The plasmon peak is known to be sensitive to the size and shape of the metal nanoparticles and to the refractive index of the medium in which they are dispersed. Since, as mentioned above, no stable Ag sol could be obtained by Ag+ reduction in DMF, spectrum b in Figure 4 was obtained by appropriate redispersion of the metal nanoparticles synthesized by the Turkevich method, i.e., by reducing silver ions with citrate.38 According to literature reports, this approach yields relatively large spherical nanoparticles (∼20 nm) very similar to what we observe in the nanocomposites. Thus, we considered that silver nanoparticles synthesized with citrate and dispersed in DMF may represent an adequate reference to evaluate the changes in the band occurring upon formation of the ZnO shell. For a coreshell nanostructure, the peak position of the plasmon band may be accounted by eq 19 λ ¼ λp ½ε∞ þ 2n2DMF þ 2gðn2ZnO n2DMF Þ=31=2
Figure 6. Absorption spectra of a 1:3 dilution of the reaction mixture ([Zn2Ac 3 2H2O]0 = 5.0 mM, RN = 6.7), taken at different time intervals.
ð1Þ
In the above expression, n is the refractive index of the surrounding medium (nDMF = 1.42689 and nZnO = 1.92); ε∞ = 5.9 is the high-frequency dielectric constant of Ag; λp = 136.3 nm is the bulk plasma wavelength of Ag; and g is the volume fraction of the shell layer. The use of g = 0 and 1 in eq 1, corresponding to the case of naked Ag nanoparticles and a perfect coreshell (Ag@ZnO) structure, leads to λ = 430.8 and 453.8 nm, respectively. These figures are in close agreement with the results observed for the Turkevich Ag particles (λ = 432.5) and the sintesized Ag@ZnO nanocomposites (λ = 456 nm). TEM analysis of the aged colloid confirmed the presence of coreshell arrangements as shown in Figure 5, involving relatively large (ca. 2030 nm) Ag particles surrounded by smaller (ca. 10 nm) ZnO particles. Distribution of phases was confirmed by EDXS and EELS (see Supporting Information). Nanocomposites prepared using RN values between 5 and 10 lead to optically clear sols. Moreover, in these conditions, the spectra of the samples are nearly identical to that shown in Figure 4; i.e., the ratio between the absorbance of the excitonic and plasmonic bands at 360 and 456 nm are nearly constant, A(360)/A(456) = 1.2 ( 0.1. The actual ratio, RF, between Ag and ZnO in the nanostructures was investigated by ICP-OES analysis after acidic dissolution of the precipitated nanocomposites. A constant value, RF = 3.5 ( 0.2, was obtained for all the syntheses involving a different initial ratio of precursors, within
Figure 7. Comparison between the absorbance evolution at the maximum of: (a) the Ag plasmon band and (b) the ZnO exciton band. Values for plot (b) were calculated as the difference between the total experimental absorbance and the contribution of Ag at the corresponding wavelength as estimated by a Lorentzian fit of the plasmon band. Conditions as in Figure 6.
the range 5 e RN e 10. However, a preliminary analysis of the TEM results shows that the mean diameter, dAg, and the size distribution of the silver nanoparticles are affected by RN. In particular, high RN values tend to favor bigger metal particles. For example, using RN = 6.7 and RN = 10, the estimated diameters of the metal nanoparticles are dAg = 18.5 ( 5.4 and 26.8 ( 8.0 nm, respectively. By increasing RN above 10, we observe a progressive enhancement of the light scattering due to the destabilization of Ag nanoparticles, and beyond RN ∼ 40 no formation of the nanocomposite could be achieved. Mechanism of Growth and Stabilization of Ag@ZnO Nanocomposites. Figure 6 shows the absorption spectra of a reaction mixture with RN = 6.7 at different time intervals. 24970
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Figure 8. XRD pattern of powders obtained by ultracentrifugation of (a) the reaction mixture, [ZnAc2 3 2H2O] = 5 mM, RN = 6.7, after 10 h of reaction, and (b) the final Ag/ZnO nanostructures after prolonged (10 days) aging. The solid was washed with ethanol and dried at 120 °C under N2. The peaks corresponding to the reflection from the crystal planes of Ag and ZnO (#) are indicated between brackets.
Figure 9. Comparison between the FTIR spectra of powders obtained by ultracentrifugation of (a) the reaction mixture, [ZnAc2 3 2H2O] = 5 mM, RN = 6.7, after 10 h of reaction, and (b) the final Ag/ZnO nanostructures after prolonged (10 days) aging. The solid was washed with ethanol and dried at 120 °C under N2 and then dispersed with KBr.
The reduction of the Ag+ ion is apparent from the beginning of the reaction as accounted from the gradual increase in the absorbance at about 425 nm. Conversely, the appearance of the excitonic band at 360 nm shows a prolonged induction period, in agreement with the results shown in Figure 3. The time evolution of the Ag plasmon band determined at its absorption maximum wavelength and that evaluated for the ZnO exciton band are plotted in Figure 7. The absorbance due to the exciton band was calculated as the difference between the experimental absorbance at 360 nm and the contribution of Ag at this wavelength as estimated by a Lorentzian fit of the plasmon band. After the induction period, the nascent ZnO nanoparticles readily organize around the larger Ag units as revealed by the concomitant shift in the Ag-plasmon wavelength (see Figure 6). TEM analysis supports this conclusion. Notice that the presence of Zn2+ species is critical for the stabilization of Ag nanoparticles since, as already mentioned in the above section, Ag nanoparticles grown in the presence of other acetate salts precipitate. On the basis of the known ability of acetate ions to form mixed cation complexes, we envisage the possible formation of an intermediate precursor consisting of Ag+ and Zn2+ species.39,40 To test this assumption, in a separate experiment, the reaction course was interrupted within the induction period, and the products were analyzed by XRD, FTIR, and ICP-OES spectroscopy. For this purpose, after 10 h of incubation the mixture was subjected to intense ultracentrifugation (3 15 min at 15000g), the supernatant removed, and the solid repeatedly washed with ethanol. The XRD pattern (curve a, Figure 8) shows, in addition to the incipient peaks corresponding to Ag at ca 38.1° and 44.3° (JCPDS 87-0597), the presence of intense signals at small scattering angles at ca. 12.41° and 18.75°. These signals differ from those observed in the synthesis of pure ZnO and may be tentatively assigned to a basic complex comprising hydroxyl, acetate, Zn2+, and Ag+ ions.39 The involvement of acetate is necessary to account for the FTIR spectrum of the intermediate presented in Figure 9. On the other hand, quantitative ICP-OES analysis allowed us to confirm the presence of Ag+ and Zn2+ in the
acidic dissolution of the precipitated intermediate and also guided us to the conclusion that hydroxyl groups must be present in the complex to satisfy electroneutrality and mass conservation principles. On the basis of the above evidence, we envisage that the growth and stabilization of the nanostructures involve the steps briefly outlined below. Protected Ag nanoparticles steadily grow by association during the induction period and facilitate the subsequent nucleation and growth of ZnO precursors on Ag clusters. Ulterior hydrolysis and condensation reactions lead to the formation of ZnO nanoparticles which constitute the shell of the final hybrid nanostructure. The room-temperature XRD and FTIR analyses of the final nanostructures are also included in Figures 8 and 9 for comparison. The relative heights of the peaks ascribed to the semiconductor in the XRD pattern of pure ZnO and the nanocomposites differ slightly; however, they showed nearly the same full width at its half-maximum intensity and were observed at the same angles, within experimental confidence (0.03°). Considering the much larger ionic radius of Ag+ (1.22 Å) compared with Zn2+ (0.72 Å),26 and the clear discernible shifts in the X-ray peak position observed in previous studies on Ag-doped ZnO nanocrystals at very low percentages,41 the above findings seem to indicate that Ag ions are not included in the ZnO lattice. It is interesting to remark that, at variance with the results observed for pure ZnO in Figure 1, XRD analysis of the nanocomposites indicates that the precursors have been completely converted into the desired products. Accordingly, all the diffraction peaks shown in plot (b) of Figure 8 correspond to pure ZnO and Ag, without traces of organometallic species. Also, the absence of the >CdO stretching bands in the FTIR spectrum presented in Figure 9 indicates that acetate ions have been extensively removed by hydrolysis from the semiconductor particle surface. This feature may be of interest for further utilization of the nanostructures as building blocks of more complex architectures through ZnOsurface derivatization. It is also worthwhile to notice that the resulting nanocomposites can be readily dispersed in water at basic pH, a fact that expands the range of application of the synthesized materials. 24971
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Figure 10. Emission spectra for ZnO and Ag@ZnO nanocomposites prepared with different molar precursor ratios. Excitation wavelength: 300 nm.
It should be stressed that attempts to reduce the reaction time by working isothermically at higher temperatures, i.e., at 60 °C, do not yield the desired nanocomposites but instead conduce to Ag precipitation and aggregated ZnO nanoparticles. We also tested the possibility of heating the reaction mixture after the induction time; however, the results were not completely reproducible. It seems that the synthesis of coreshell arrangements requires the adequate timing of two processes (the synthesis of the core and the growth of the shell over it) that must occur sequentially and that this condition is ensured at room temperature. AgZnO Interactions: Emission Properties of the Hybrid Nanostructures. The interaction between individual nanoparticles in the metalsemiconductor nanostructures may be monitored by determining the changes in the emission properties of the fluorescent component as a consequence of the assemblage process. Enhancement of emission may occur as a result of amplification of the electric field by plasmon resonance, whereas emission quenching may be indicative of energy or charge transfer from the semiconductor to metal nanoparticles. To test the behavior of the synthesized nanostructures, emission spectra were obtained upon UV excitation (λEXC: 300 nm) for different RN values, including the case RN = 0. The results are summarized in Figure 10. It should be stressed that in these measurements the absorbance of all samples at the excitation wavelength and plasmon maximum was fixed near 0.1 units to ensure minimal (if any) reabsorption of the emission by Ag particles and that emission spectra were normalized for absorption at the excitation wavelength to make them comparable in intensity. In the absence of Ag nanoparticles, the spectrum shows a broad band between 400 and 800 nm and a weak near-band-edge band in the UV region. In comparison, pure ZnO nanoparticles synthesized in DMF, hereafter named as ZnO (DMF), are considerably less fluorescent than those obtained by alkaline hydrolysis in 2-propanol, referred to as ZnO (ROH). It is also worthwhile to mention that ZnO (ROH) and ZnO (DMF) nanoparticles dispersed in a 50% V/V 2-propanolDMF mixture are clearly differentiated (see Supporting Information).
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Figure 11. Comparison between the absorption spectra of a 8.5 μM PF (a) and a 0.45 mM sol of ZnO nanoparticles before (b) and after derivatization with 8.5 μM PF (c), all at pH 11.
These results indicate that (the reductive) reaction conditions induce irreversible modifications in the surface properties of ZnO which cannot be reversed by changing the solvent environment, once the reaction has been completed. The broad visible band overlies contributions from multiple transitions,42,43 making their definitive assignment problematical and still controversial.4447 Current knowledge indicates that the broad visible band is the superposition of blue, green, and orangered bands,48,49 which arise from the radiative recombination of different carriers: the green band is supposed to be associated with oxygen vacancies, while the blue and orange-red bands are proposed to be related to Zn vacancy defects.45 In the presence of Ag, the visible emission becomes progressively quenched (notice that reabsorption of the emission by the plasmon band is negligible as stated above). It is also apparent from the results in Figure 10 that not only the intensity but also the shape of the emission spectrum is affected by the presence of silver nanoparticles. Excitation spectra at 360, 450, 560, and 650 nm, as representative wavelengths of the bands commonly referred to in the literature as excitonic, blue, green, and orange-red emissions, respectively, were determined for RN = 10 and found to be almost coincident with that obtained for pure ZnO and to the absorption spectrum of ZnO nanoparticles. This result, not shown for simplicity, confirms that Ag does not contribute to the observed luminescence. Although, as already stated, the nanocomposites prepared with RN values between 5 and 10 have almost the same Ag/ZnO ratio, they could be differentiated by their emission properties, probably as a result of the above-mentioned difference observed in the size and distribution of silver nanoparticles. The quenching of the emission observed in the nanocomposites is ascribed to an efficient interfacial charge transfer from ZnO to Ag nanoparticles, which act as electron sinks and hamper the recombination of photoinduced carriers.26 At RN ∼ 10 the samples show a slight increase of the UV and blue emission bands. This effect becomes progressively more pronounced for RN > 10, probably indicating that the mechanism of electric field amplification by the plasmon resonance prevails over charge transfer, as recently reported.5052 24972
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room temperature using DMF, zinc acetate, and a silver salt. In comparison with other approaches based on the use of stronger reducing agents such as NaBH4 the method appears as an environmental-friendly alternative, which in addition does not require the use of stabilizers. The obtained nanocomposites consist of relative large Ag nanoparticles (2030 nm) surrounded by smaller ZnO units (10 nm). The synthetic procedure allows its facile dispersion in water at basic pH and offers the possibility of derivatizing the ZnO shell by dye attachment. Accordingly, the chemisorption of PF to ZnO and Ag@ZnO particles proves that charge separation is enhanced by a 2.5 and 1.3 factor under UV and visible excitation, respectively.10 Besides, the coreshell arrangement makes Ag@ZnO nanocomposites suitable for the development of plasmonic dye sensitized solar cells due to the Ag protection by ZnO. Further work on this issue is currently underway.
’ ASSOCIATED CONTENT Figure 12. Normalized decay of PF determined at 517 nm as a function of light exposure at 303 ( 10 nm for ZnO (b) and Ag@ZnO nanocomposites (9). The inset shows the UVvis absorption spectra of Ag@ZnO nanocomposites, RN = 6.7, modified with PF at different irradiation times.
Comparative Photocatalytic Degradation of PF Anchored to ZnO and Ag@ZnO. The photocatalytic degradation of PF was
investigated in ZnO and Ag@ZnO nanocomposites, suspended in alkaline aqueous media. PF is an interesting xanthene dye which has been used to sensitize TiO2 nanoparticles for the development of dye-sensitized solar cells and as a model molecule to investigate the dynamics of electron injection under strong coupling conditions.53 The interaction with the semiconductor nanoparticles is facilitated through the adjacent OH and CdO groups or though the enediol functionality of PF. In effect, the chemisorption of the dye to the ZnO surface becomes apparent as the visible band of the ligand broadens and slightly red shifts upon the addition of ZnO (see Figure 11). Irradiation at 303 nm results in a rapid depletion of PF which was monitored by measuring the time evolution of the absorbance at 517 nm for both ZnO and Ag@ZnO systems. The normalized changes in the absorbance at 517 nm for both systems are compared in Figure 11. Analysis of the data indicates that the presence of Ag introduces a 2.5 enhancement factor, confirming that metal nanoparticles effectively improve charge separation. The time course of PF modified Ag@ZnO nanoparticles during irradiation is shown in the inset of Figure 12. It is apparent that Ag@ZnO remains stable after complete oxidation of PF. Control experiments comprising a mixture of ZnO and Turkevich Ag nanoparticles suspended in DMF reveal that the presence of segregated metal nanoparticles does not alter the rate of PF decay, as expected. Irradiation at visible wavelengths also results in PF degradation. In this case, the decay is accompanied by a decrease in the rate of photon absorption of the dye, leading to a slow down of the oxidation process. However, considering the initial stages of the irradiation processes, the relative rates of PF depletion were still about a factor 1.3 higher for the nanocomposite system.
’ CONCLUSIONS Our studies have shown that core@shell AgZnO nanostructures can be successfully obtained by one-pot synthesis at
bS
Supporting Information. We provide additional material showing: (a) XRD pattern of the destabilized reaction product obtained by Ag+ reaction in DMF; (b) STEM (scanning transmission electron microscopy) dark-field images of the samples and elemental distribution by EDXS and EELS analysis of the assemblies formed by simultaneous Ag+ reduction and hydrolysis of zinc acetate, RN = 6.7, in DMF; (c) the effect of DMF on the emission yield of ZnO (ROH) nanoparticles, and (d) a comparison between the emission characteristics of ZnO (ROH) and ZnO (DMF) under the same solvent composition. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thanks Dr. Pablo Botta (Intema, CONICET) for technical assistance in the DRX analysis. This work was financially supported by the University of Mar del Plata and the National Research Council of Argentina (CONICET), project PIP 319. MAG and ESR are members of the research staff of CONICET. MEA and HBR thank CONICET for a doctoral and a postdoctoral fellowship, respectively. ’ REFERENCES (1) Liz-Marzan, L. M.; Mulvaney, P. J. Phys. Chem. B 2003, 107, 7312–7326. (2) Tom, R. T.; Nair, A. S.; Singh, N.; Aslam, M.; Nagendra, C. L.; Philip, R.; Vijayamohanan, K.; Pradeep, T. Langmuir 2003, 19, 3439–3445. (3) Barreca, D.; Gasparotto, A.; Tondello, E. J. Mater. Chem. 2011, 21, 1648–1654. (4) Costi, R.; Saunders, A. E.; Banin, U. Angew. Chem., Int. Ed. 2010, 49, 4878–4897. (5) Achermann, M. J. Phys. Chem. Lett. 2010, 1, 2837–2843. (6) Fedutik, Y.; Temnov, V.; Woggon, U.; Ustinovich, E.; Artemyev, M. J. Am. Chem. Soc. 2007, 129, 14939–14945. (7) Govorov, A. O.; Bryant, G. W.; Zhang, W.; Skeini, T.; Lee, J.; Kotov, N. A.; Slocik, J. M.; Naik, R. R. Nano Lett. 2006, 6, 984–994. 24973
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