Malwina
Staniuk
a,
Daniel
Zindel
b,
Wouter
van Beek
c,
Ofer
Hirsch
a,
Niklaus
Kränzlin
a,
Markus
Niederberger
a and
Dorota
Koziej
*a
aLaboratory for Multifunctional Materials, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland. E-mail: dorota.koziej@mat.ethz.ch
bLaboratory of Physical Chemistry, ETH Zurich, Vladimir-Prelog-Weg 2, 8093 Zurich, Switzerland
cSwiss-Norwegian Beamlines at European Synchrotron Research Facility, 71 Avenue des Martyrs, 38043 Grenoble, France
First published on 9th June 2015
Although syntheses in organic solvents provide access to a wide range of copper-based nanoparticles, the correlation between organic reactions in solution and nucleation and growth of nanoparticles with defined properties is not well understood. Here, we utilize the Multivariate Curve Resolution-Alternative Least Squares (MCR-ALS) methodology to examine spectroscopic data recorded in situ during the synthesis of copper-based nanoparticles. While earlier studies showed that depending on the temperature copper(II) acetylacetonate reacts with benzyl alcohol and forms either copper oxides or copper nanoparticles, we link the inorganic reaction with their organic counterparts. From X-ray Absorption Near Edge Spectroscopy (XANES) and Ultraviolet-visible spectroscopy (UV-vis) data we learn that copper(I) oxide forms directly from the solution and is the final product at low temperature of 140 °C. We observe in Fourier Transformed Infrared (FTIR) spectra an increasing concentration of benzyl acetate that co-occurs with the formation of a copper enolate and evolution of benzaldehyde, which accompanies the reduction of copper ions. We also record the interaction of organic species at the Cu2O surface, which inhibits a further reduction to metallic copper. When we raise the synthesis temperature to 170 °C it turns out that the Cu2O is just an intermediate species. It subsequently transforms by solid-state reduction to metallic copper accompanied by oxidation of benzyl alcohol to benzaldehyde.
Here, our main focus is the determination of organic and inorganic species formed during the nucleation and growth of copper-based nanoparticles and their interdependence by utilizing different spectroscopic techniques. The latest development of synchrotron- and laboratory-based equipment and analysis tools allow for in situ monitoring the complex reactions in solution.7,26–36 However, the specificity of the individual methods often makes a quantitative analysis or a direct comparison of results difficult. We overcome this problem by selecting two spectroscopic methods, X-ray Absorption Near Edge Spectroscopy (XANES) and Ultraviolet–visible spectroscopy (UV-vis), which even though they probe fundamentally different phenomena, provide information on the chemical composition. In order to determine the changes of the composition of the reaction solution during nucleation and growth of Cu2O and Cu nanoparticles we apply Multivariate Curve Resolution-Alternative Least Squares (MCR-ALS) method. It has been previously successfully used to determine the concentration profile and pure XANES spectra of individual species during in situ studies of Cu-catalysts, Cu-doped V2O3 batteries and to follow the synthesis of Co, TiO2 and Ni-doped MoO2 nanoparticles.18,37–45
Additionally, we track the formation of organic compounds by Fourier Transform Infrared spectroscopy (FTIR) studies that are recorded simultaneously with UV-vis measurements. In the control experiments without copper precursor, we: (a) evaluate the decomposition of benzyl alcohol upon heating, (b) determine the lower detection limit and (c) calibrate the curve for benzaldehyde, benzyl acetate and dibenzyl ether, compounds that are considered the main organic counterparts of copper-based nanoparticle nucleation. This lays the foundation for a quantitative analysis of in situ FTIR studies. We find that only the formation of benzyl acetate correlates directly with the occurrence of copper-based nanoparticles in benzyl alcohol. At low reaction temperature, the formation of benzaldehyde as a product of Cu2O synthesis makes up approximately thirty percent of the total amount of benzaldehyde. The remaining seventy percent are a product of catalytic activity of Cu2O nanoparticles. At high reaction temperature, Cu2O nanoparticles are further reduced to Cu. Interestingly, the formation of copper nanoparticles terminates the catalytic formation of benzaldehyde, and instead triggers the catalytic formation of dibenzyl ether.
D = CST + E | (1) |
Rows of matrix D are spectra acquired during measurement, columns of matrix C and rows in matrix ST are concentration profiles and spectra of resolved components, respectively. Matrix E contains residuals not explained by the model. Eqn (1), generally valid for spectroscopic data governed by the Lambert–Beer law like XANES or FTIR, is solved by ALS algorithm, which iteratively calculates the C and ST matrices that fits best the experimental data. The optimization of C and ST is carried out for a proposed number of components. Initial estimates of C and ST are obtained by using SIMPLe-to-use Interactive Self-Modeling Algorithm (SIMPLISMA), which finds the most different spectra within the dataset that are further used as an input for ALS optimization.
The number of components was selected on the basis of Singular Value Decomposition (SVD). The initial spectra of the components were estimated with the PURE algorithm with the noise level set to 3% (XANES) and 4% (ATR-FTIR). Constraints applied for ALS calculation were as follows: non-negativity of spectra and concentration (XANES), non-negativity of concentration (XANES and ATR-FTIR), unimodality of concentration (XANES and ATR-FTIR), convergence criterion: 0.1 (XAS and ATR-FTIR). For more information please refer to ESI† Fig. S2–S4, Tables S2–S5.
We first determine the initial oxidation state of copper species upon dissolution in benzyl alcohol at 180 °C as shown in Fig. 3. Then, we utilize the MCR-ALS method to analyze the in situ XANES spectra during nucleation of copper nanoparticles. This solely variance based method allows to determine, without a priori knowledge, the number of components, their spectra and relative concentration in a solution as shown in Fig. 4.
At both reaction temperatures, as long as some of the precursor is present in the solution, the same organic compounds, benzyl acetate and benzaldehyde, form. At 140 °C the reaction terminates with the reduction of Cu(acac)2 to Cu2O. Only at that point the bands assigned to the species adsorbed at the surface of Cu2O become clearly visible as shown in ESI† Fig. S4a–b. At 170 °C the nucleation of Cu2O is only an intermediate step towards the formation of Cu nanoparticles and we do not observe adsorption of organic species at the Cu2O surface. Instead, we observe the formation of dibenzyl ether as the product of benzyl alcohol condensation, which is a result of the catalytic activity of metallic copper.
In summary, comparing the organic species from these three experiments, benzyl acetate is the only compound that exclusively forms when copper-based nanoparticles crystallize. All other organic compounds may also form as products of parallel, side reactions that are not directly related to the nucleation of the copper-based nanoparticles.
Additionally, we heat benzyl alcohol without Cu(acac)2 at 140 and 170 °C, respectively, and in both cases with FTIR we exclusively detect benzaldehyde as shown in Fig. 5 (gray curves). It is the product of the oxidation of benzyl alcohol according to the reaction shown in Scheme 1.54 The amounts of dibenzyl ether and benzyl benzoate detected by GC-MS are obviously below the detection limit of FTIR.
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Scheme 1 Oxidation of benzyl alcohol to benzaldehyde.54–57 |
Now we discuss the quantities of the two compounds benzyl acetate and benzaldehyde we measured in comparison to what would be expected from the proposed chemical reactions. The chemical reactions themselves will be further discussed in section (d).
We assume that benzyl acetate is formed by the reaction of the acetylacetonate ligand with benzyl alcohol, as shown in Scheme 2.
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Scheme 2 Formation of benzyl acetate by nucleophilic attack of benzyl alcohol on one of the carbonyl groups of the acetylacetonate ligand. |
Benzaldehyde is the oxidation product of benzyl alcohol as a result of the reduction of Cu2+ to Cu+ and finally Cu0 according to Scheme 3.
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Scheme 3 Oxidation of benzyl alcohol to benzaldehyde and reduction of Cu2+ to Cu+ (a) and Cu+ to Cu0 (b). |
Having these reactions in mind and just considering the organic species detected, we can formulate an overall chemical reaction for the formation of Cu2O, as shown in Scheme 4, and for Cu0 as displayed in Scheme 5.
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Scheme 4 Formation of Cu2O, benzyl acetate and benzaldehyde from Cu(acac)2 and benzyl alcohol at 140 °C. |
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Scheme 5 Formation of Cu, benzyl acetate and benzaldehyde from Cu(acac)2 and benzyl alcohol at 170 °C. |
From Scheme 4 it is evident that the formation of 1 mmol Cu2O produces 4 mmol of benzyl acetate and 1 mmol of benzaldehyde. The initial concentration of Cu(acac)2 in benzyl alcohol is 0.070 M, which means that 0.175 M benzyl alcohol (BnOH) are required, resulting in 0.140 M benzyl acetate, 0.035 M benzaldehyde and 0.070 M Cu+ (or 0.035 M Cu2O) as summarized in eqn (2):
0.070 M Cu2+ + 0.175 M BnOH → 0.140 M BnAc + 0.035 M BnAd + 0.070 M Cu+ | (2) |
According to Scheme 5, the formation of Cu at 170 °C follows eqn (3):
0.070 M Cu2+ + 0.210 M BnOH → 0.140 M BnAc + 0.070 BnAd + 0.070 M Cu0 | (3) |
For the reaction at 140 °C, the measured concentration of benzyl acetate matches with the nominal value of 0.137 M within the resolution limit of our method, as shown in Fig. 5a and Table 1. At the same time, the sum of the concentrations of benzaldehyde in the control experiment (0.052 M) and the nominal value based on eqn (2) equals to 0.087 M, which is much lower than the actually measured concentration of just 0.115 M. Evidently, the Cu2O nanoparticles are able to catalyze the oxidation of benzyl alcohol58,59 to increase the benzaldehyde concentration beyond the expected value. The high coverage of the surface of the Cu2O nanoparticles as mentioned before terminates the reaction at this point without further reduction and we observe a saturation of the benzaldehyde concentration as evidenced in Fig. 5a.
Compound | Lower detection limit (M) | Synthesis at 140 °C | Synthesis at 170 °C | ||
---|---|---|---|---|---|
Expected value (M) | Measured value (M) | Expected value (M) | Measured value (M) | ||
Benzyl acetate | 0.007 | 0.140 | 0.137 | 0.140 | 0.112 |
Benzaldehyde | 0.003 | 0.035 | 0.115 | 0.070 | 0.133 |
Dibenzyl ether | 0.008 | 0 | 0 | 0 | 0.047 |
Control experiment | 140 °C | 170 °C | |||
Benzaldehyde | 0.008 | — | 0.052 | — | 0.062 |
For the reaction at 170 °C, based on the shape of the concentration profile of the organic compounds, the mechanism seems to be even more complex as shown in Fig. 5b. The final concentration of benzyl acetate equals to 0.112 M, which is lower than the nominal value of 0.140 M expected from eqn (3). This can be explained by the observation of acetone and acetylacetonate in the GC-MS chromatogram. Therefore, we assume that ligand exchange reactions, as shown in Scheme 6, are competing for Cu(acac)2 with the reaction shown in Scheme 2.
In contrast to the synthesis at 140 °C, at 170 °C the measured concentration of benzaldehyde of 0.133 M agrees well with the expected value of 0.132 M, which is the sum of benzaldehyde from the control experiment (0.062 M) and from the reaction shown in eqn (3). This result is not surprising, because with the transformation of Cu2O to Cu also the catalytically active nanoparticles disappear. However, in the very short time frame between 120–150 min we observed a rapid increase of the concentration of benzaldehyde and a decrease of the precursor concentration. It is interesting to note that only at this point of the reaction, the concentration of benzaldehyde does not correlate with the concentration of benzyl acetate, which indicates that the reactions shown in Scheme 6 might become more pronounced.
Up to here, we did not discuss, where the oxygen for the formation of the copper oxide comes from. Based on the results described above, we did not find any organic compounds that are typically ascribed to condensation reactions responsible for the formation of a metal–oxygen–metal bond.15 Since the synthesis is performed under ambient conditions, we assume that water impurities in solvent can influence the formation of nanoparticles. In addition hydrogen ion formed during benzyl alcohol oxidation might react with oxygen to form water as proposed in Scheme 7a.57,61,62 Once water is present, two possible scenarios for the formation of a Cu–O bond are possible. A water molecule directly coordinates to the enolate ligand, which leads to the formation of acetone and Cu(I)–OH species as shown in Scheme 7b. Alternatively, a benzyl alcohol molecules reacts with the enolate ligand, which leads to the formation of acetone and a copper benzyl alcoholate. These scenarios are both plausible, because traces of acetone were detected by GC-MS. A consecutive reaction of the copper alkoxide with water leads to a Cu(I)–OH species, as displayed in Scheme 7c. Finally, the Cu(I)–OH species condense with each other under release of water molecules and formation of Cu–O–Cu bonds.
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Fig. 6 Overview on organic and inorganic reactions taking place during synthesis of copper-based nanoparticles. |
Footnote |
† Electronic supplementary information (ESI) available: See DOI: 10.1039/c5ce00454c |
This journal is © The Royal Society of Chemistry 2015 |