Remarkable Decrease in the Oxidation Rate of Cu Nanocrystals

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On the Remarkable Decrease in the Oxidation Rate of Cu Nanocrystals Controlled by Alkylamine Ligands Jérémy Cure, Arnaud Glaria, Vincent Collière, Pier-Francesco Fazzini, Adnen Mlayah, Bruno Chaudret, and Pierre Fau J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12877 • Publication Date (Web): 17 Feb 2017 Downloaded from http://pubs.acs.org on February 20, 2017

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The Journal of Physical Chemistry

On the Remarkable Decrease in the Oxidation Rate of Cu Nanocrystals Controlled by Alkylamine Ligands Jérémy Cure,a,b Arnaud Glaria,b Vincent Collière,b, c Pier-Francesco Fazzini,e,c Adnen Mlayah,c, d Bruno Chaudret,e,c Pierre Fau*b, c

a b

STMicroelectronics SAS Tours, 10 impasse Thales de Millet, 37070 Tours, France. Laboratoire de Chimie de Coordination, CNRS, BP 44099, 205 Route de Narbonne, 31077 Toulouse Cedex 4, France.

c

Université Fédérale de Toulouse Midi-Pyrénées, UT III Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex 9, France.

d

Centre d’Elaboration de Matériaux et d’Etudes Structurales, CNRS, 29 Rue Jeanne Marvig, 31055

Toulouse, France. e

Laboratoire de Physique et de Chimie des Nano-Objets, INSA, UPS, CNRS, 135 avenue de Rangueil,

31077 Toulouse, France.

Supporting

Information

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ACS Paragon Plus Environment

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ABSTRACT: Colloidal solutions of copper nanoparticles (7.2 ± 1.1 nm diameter), stabilized by alkylamine ligands, show a remarkably long persistence (several months) of the localized surface plasmon resonance (LSPR) when exposed to ambient air at room temperature. The oxidation kinetics of these nanoparticles have been investigated by optical spectroscopy and modeled using numerical simulations. Three distinct oxidation regimes are evidenced: i) A fast regime in which oxygen is adsorbed and dissociated on the nanoparticle to form pre-oxide islands ii) A slower regime where the coalescence of the oxide islands takes place up to the formation of a complete Cu2O shell iii) and finally an extremely slow oxidation of the residual copper core and eventually the formation of hollow Cu2O nanoparticles. The adsorption rate of oxygen on copper nanoparticles is controlled by the amount of alkylamine ligands in solution (from 0.1 to 2 molar equivalents).

I. INTRODUCTION

Coinage metal nanoparticles (Cu, Ag, Au), are particularly attractive due to their physical properties such as high electrical conductivity, strong localized surface plasmon resonance (LSPR) in the visible range,1-4 and good efficiency in catalytic reactions.5-7 Among these metals, copper is the most prone to oxidize, but it is nonetheless used in many applications such as microelectronics, biology, optics and catalysis. Indeed, copper based nanomaterials present a high reactivity which is of strong interest in catalysis.8,9 As an example, CuZn alloys are used for methanol synthesis from syngas mixture.10-11 Usually, in ambient air, colloidal solutions of metallic copper nanoparticles (NPs) rapidly evolve towards the formation of metastable copper oxide Cu2O.12 In some cases, where metallic copper is required, it can be useful to block or to limit the reactivity of copper against ambient oxygen. The oxidation mechanism of nanostructured copper is still controversial in the literature.13-16 In order to slow down, or even stop, the oxidation of colloidal copper nanoparticles, one solution is to add a strong reducing agent as hydrazine (N2H4) in the medium.17-20 In this case, metallic nanocrystals are stable in water-based solutions. Another method is to add various protective agent, like aminoclays, complementary to strong reducing agents.21 Alternatively, copper nanoparticles have been protected by an O2-proof graphene layer in order to preserve a sufficiently high conductivity level of copper nanocrystals for electronics applications.22 On the other hand, a careful choice of the capping ligands is an alternative way to slow down the 2


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oxidation process of copper nanoparticles in organic solvents. For instance, aliphatic chains bearing a functional amine group are efficient for decreasing the rate of copper oxidation. On the contrary, carboxylic or phosphonic acid functions are known to favor the growth of copper oxide.23 In the present work we report and analyze a remarkably slow oxidation kinetics of 7.2 ± 1.1 nm copper nanocrystals stabilized by alkylamines. The use of amine ligands, which have a good affinity with the copper NPs surface, allows to reduce the oxidation rate of copper by a factor of more than 30 owing to a competitive adsorption mechanism. The copper nanoparticles have been characterized by transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Quantitative analysis of the UV-vis spectroscopy data, supported by numerical simulations, has allowed us to distinguish three sequential oxidation regimes: i) An initial adsorption and dissociation of dioxygen on the nanocrystal to form pre-oxide islands, ii) the formation of Cu2O islands and their coalescence and iii) the slow copper oxidation triggered by the formation of a complete Cu2O shell. Our results suggest the long time persistence of sub-monolayer levels of Cu2O on copper nanocrystals. The controlled amount of alkylamine ligands in solution is the key experimental parameter which governs the adsorption rate of oxygen on the copper nanoparticles.

II. RESULTS AND DISCUSSION:

Colloidal copper nanoparticles have been prepared according to a previously reported synthetic procedure.23 The hydrogenation of a copper amidinate precursor in toluene with alkylamine ligands (0.1 molar equivalent (eq.) per copper atom) yields 7.2 ± 1.1 nm copper nanoparticles with a narrow size dispersion (Figure 1).

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Figure 1: TEM image and size distribution (insert) of Cu NPs stabilized by 0.1 eq HDA.

Recently, in-situ solution NMR characterizations of the colloidal copper solution revealed the dynamic exchange of amine ligands between the copper nanocrystal surface and the solvent.24 When the solution was exposed to ambient air, two main effects were evidenced: i) A stronger bonding of the amine ligand on the oxidized copper, and ii) Oxygen atom insertion into the copper network, which destroys the organization of the alkylamine chains. In the present study, we have performed HRTEM analysis of the copper NPs exposed to air for increasing duration. We have correlated these results with a UV-vis analysis supported by numerical simulations, in order to achieve a precise tracking of the oxide growth process, including at its very early stage.

High Resolution TEM analysis:

In Figure 2 are presented HRTEM analyses of Cu nanocrystals, stabilized by 0.1 eq of HDA in solution, and exposed to air for 4, 15 and 50 days. First, the HRTEM reveal the polycrystalline nature of the nanocrystals; some of them display well defined pentagonal geometry (Figure 2a). The facets of the nanocrystals consist of dense (111) crystalline planes of the face centered cubic system (fcc). Other crystalline planes, (200) and (220), are evidenced by Fourier Transform (FT) analysis of the whole nanocrystal, thus confirming its polycrystalline nature. The nanocrystals can be described as penta-twinned nanostructures. Analysis of the collection of NPs showed that it is composed of both decahedra and icosahedra. The HRTEM images of such penta-twinned structures may 4


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show distinctive and characteristic contrasts related to the geometry of the objects. In the icosahedron case, due to the numerous (111) planes, the overlap of these planes produces some interference patterns in the HRTEM image. By contrast, there is no overlap of (111) planes in the case of decahedral structures, thus leading to HRTEM images showing perfectly aligned atomic columns. Therefore, we deduce that the object shown in Figure 2a is a decahedral nanoparticle. It is noteworthy that the apex of the pentagonal pyramid is truncated, and lines of stacking defaults are visible in each row between two adjacent faces (Figure 2a). These defaults reflect the relaxation of the internal stresses of the nanocrystal, which is necessary to accommodate the twinned relationships between the (111) plans.25

Figure 2: HRTEM images of Cu NPs stabilized by 0.1 eq. of HDA and their corresponding Fourier transform indexed with Cu (white) and Cu2O (yellow) (hkl) planes. a) before air exposure, b) after 4 days exposure to air, c) 15 days, and d) 50 days (dotted circles show the oxide shell).

Since not all the observed crystals present such a perfect atomic stacking, one cannot rule out the presence of icosahedral nanoparticles. The decahedral nanostructures have been chosen to experimentally track the growth of Cu2O shell by HRTEM analysis, during the exposure of the colloidal solution to air.

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Second, After 4 days of exposure to ambient atmosphere, a nanometer-sized oxide shell appears on the copper NPs (Figure 2b). The Fourier transform (FT) of the complete image reveals a majority of metallic copper atomic planes, and possibly the presence of a single (200) plane of the Cu2O structure. After 15 days, an oxide shell is clearly visible (Figure 2c). The particles mean size is 9.4 ± 2 nm with an oxide shell of 2.0 ± 0.4 nm. This oxide layer is well crystallized and displays (111) plans. The copper core retains its initial decahedron shape, but the oxide shell shows a corrugated structure, a characteristic of the copper oxide growth process.26 Finally, after 50 days, some of the particles are pure Cu2O oxide (particles mean size 8.9 ± 2.7 nm), as revealed by the FT analysis of their crystalline planes (Figure 2d and Figure S1), and consist of hollow structures due to the so-called nanoscale Kirkendall effect.27 Since its description by Yin et al.,28 this effect has been observed in various type of metal oxide nanoparticles. The driving force for such an effect is the outward and unbalanced diffusion of copper atoms during a slow oxidation process, which leaves copper vacancies in the core lattice. Further coalescence of the vacancies eventually forms a void in the center of the particle.29

Optical spectroscopy of the surface plasmon resonance:

The UV-visible spectrum (Figure 3) of the copper colloidal solution prepared with 0.1 eq. HDA shows a LSPR peak located at around 565 nm as expected for nanosized copper particles.23 The UV-vis spectra of colloidal copper solutions exposed to ambient air show very distinct features depending on the exposure duration (Figure 3). Immediately after exposure and up to 8 min, the LSPR absorption peak is red-shifted from 565 nm to 580 nm, and both the baseline level and the surface plasmon absorptions display a significant intensity increase. As shown by the numerical simulations (see below), the LSPR red-shift is due to the formation of a copper oxide shell surrounding the metallic core. The increase of the LSPR peak absorption observed at the very first stage of the reaction may have different origins: (i) A commonly accepted explanation for the LSPR rise is the increase of the dielectric contrast between the metallic core and the surrounding medium.30 (ii) In the case of copper, the presence of an oxide shell red-shifts the surface plasmon resonance with respect to the d-to-s interband transitions thus reducing the LSPR damping.31-32 (iii) K.P. Rice et al. proposed a LSPR damping due to solvent π-bonds with the metallic copper surface.33 Besides the LSPR 6


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peak increase, the baseline absorption at short wavelength (400-550 nm) also increases (Figure 3). This effect is not well understood and several mechanisms could be involved. One of the reasons could be the very first oxidized copper islands which may contribute to the optical absorbance in this wavelength region. As a matter of fact, the band-gap of bulk Cu2O is around 590 nm,34 and because of quantum size effects, Cu2O clusters may absorb light at shorter wavelengths and in a broad spectral range.

Figure 3: UV-vis spectra of Cu NPs stabilized by 0.1 eq. HDA and exposed to air during increasing time: a) between t0 and 8 min, b) between 11 min and 31 days.

After 8 min, the LSPR peak continues to red-shift and both the baseline level and the LSPR peak intensity start to decrease (Figure 3 b). This is associated with the formation of a copper oxide shell and the consumption of the metallic copper core.35 The oxidation rate can be extracted by using the change of the LSPR wavelength and absorption as a function of time. To do so, we have measured the integrated absorption (IA) in the 400 to 900 nm range; the soobtained value was normalized by the integrated absorption (IAn) before oxidation. The oxidation rate is defined as:

Rate = 100x ((IAn,(δt) – IAn,0)/ IAn,0) / δt

(1)

where IAn,0 , IAn (δt) are the normalized integrated intensity, respectively, before and after air exposure. δt (in min) being the exposure time. The plot of the normalized integrated absorption IAn as a function of δt exhibits three regimes separated by distinct slopes (Figure 4). First, as discussed previously, just after exposure to 7



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air, and during approximately 6 minutes, a very fast oxidation mechanism takes place, where IAn is strongly enhanced: the oxidation rate 12.9 %.min-1 is positive (oxidation regime (1)). Then appears a second oxidation regime characterized by a weak decrease of IAn between 8 min and 100 min, with a rate of -1.4.10-2 %.min-1 (oxidation regime (2)). Finally, after approximately 2 hours, and up to more than 40 days, a regular and very slow oxidation regime takes place. The latter is characterized by a drop of the oxidation rate by one order of magnitude (-1.10-3 %.min-1, oxidation regime (3)).

Figure 4: Evolution of the normalized integrated absorption (400-900 nm) of copper colloidal solution (0.1 eq. HDA), obtained from the experimental UV-vis spectra recorded at the different exposure duration (δt).

The change of the LSPR integrated absorption with time is very sensitive to the oxidation of the copper nanoparticles and points out to three distinct oxidation regimes. Hence, it can be used to probe the oxidation process and particularly the dependence of the oxidation rate on the nature and concentration of the capping ligands in the solution.

Effect of HDA against oxidation: To investigate the role of the amine ligands, an increasing amount of HDA has been added to the initial colloidal solution prepared with 0.1 eq. HDA. The same oxidation protocol has been applied and the evolution of the LSPR with time has been used to monitor the oxidation process (Figure 5).

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Figure 5: UV-Vis spectra recorded at different exposure duration (δt) of colloidal copper solutions containing: a) 0.5 eq. HDA; b) 1 eq. HDA; c) 2 eq. HDA. Left side graphs correspond to oxidation regime (1), and right side graphs correspond to oxidation regimes (2) and (3).

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With increasing HDA amount (0.5, 1 and 2 eq.), the evolution of IAn as a function of oxidation time again shows out the three distinct oxidation regimes already observed for 0.1 eq. HDA (Figure S2). The oxidation rates of each regime have been extracted from these data and quoted in Table 1 and plotted in Figure S3. An important result is the slowdown of the oxidation rates of each of the three regimes with increasing HDA content. A spectacular reduction of regime (1) oxidation rate by a factor of 30 is observed when the HDA content is increased from 0.1 to 2 eq. Similarly, a reduction of the oxidation rate by two orders of magnitudes is observed for regime (2), and by one order of magnitude for regime (3). We also notice that the oxidation rates of the three regimes are quite stable for HDA content larger than 1 eq. HDA. It is known that, whatever the ligands concentration, only few of them (0.1 to 0.2 eq.) form the first coordination sphere in dynamic exchange with the nanocrystal surface.36 Therefore, this suggests that beyond 0.5 eq., the extra HDA ligands form a second coordination sphere, dense enough, which controls the exchanges at the copper surface.

Table 1: Effect of the HDA content on the oxidation rate of copper NPs during the three regime. HDA content (eq.)

regime (1) rate (%/min)



0.1

12.9

-1.4E-2

-1.0E-3

0.5

4.4

-2.8E-3

-1.0E-3

1

0.8

-2E-4

-8.0E-5

2

0.4

-3.8E-4

-1.2E-4

Interestingly, the end of the second oxidation regime always corresponds to a LSPR wavelength close to 600 nm according to the HDA content (0.1, 0.5, 1 and 2 eq., Table S1). We will show by numerical simulation (vide infra) the importance of this wavelength value which corresponds to the formation of the first complete Cu2O monolayer over the copper nanocrystal. The offset of the LSPR with time may also bring some information on the oxidation rate of the nanoparticles. We found that the red-shift of the LSPR peak is well described by a phenomenological power law: 10


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Λ(δt) = Λ0 + A δtα (3)

Where Λ(δt) is the LSPR wavelength at the time δt, Λ0 is the LSPR of the non-oxidized particles, A and α are fitting parameters which determine the amplitude of the red-shift and its evolution with time, respectively. In Figure 6 is presented a log-log plot of the LSPR shift Λ(t) - Λ0 as a function of δt and for increasing amounts of HDA.

Figure 6: Shift of the LSPR peak with oxidation duration, for increasing HDA content: 0.1 eq., 0.5 eq., 1 eq. and 2 eq. HDA.

The power law (Eq. 3) fits rather well the experimental points over several decades of oxidation duration and for all investigated HDA contents. Interestingly, the slope of each linear curve, in Figure 6, is defined by the α coefficient (quoted in table 2). We found that α decreases with increasing HDA content, which is interpreted as the slowing down of the oxidation rate of the Cu NPs.

Table 2: coefficient α extracted by fitting Eq. 3 to the measured LSPR shift with oxidation duration for increasing HDA content HDA (molar eq.)

0.1

0.5

1

2

α

0.17

0.11

0.10

0.09

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Unlike IAn, the plot of the LSPR peak wavelength as a function of the oxidation duration doesn’t allow to distinguish between the three oxidation regimes previously mentioned. In that sense, the shift of the LSPR wavelength is less sensitive to change in oxidation rate because of its monotonic increase with oxide layer thickness until it reaches saturation when the whole plasmonic near-field is within the oxide shell. Nevertheless, the slowdown of the oxidation process with increasing HDA content is clearly evidenced in Figure 6 (and Table 2).

Numerical Simulations of the optical response of Cu NPs:

In order to get a deeper insight into the effect of the oxidation on the LSPR, numerical simulations based on the discrete dipolar approximation (DDA) model have been performed assuming spherical copper nanoparticles.37 The spherical shape gives a good approximation of the simulated LSPR of icosahedral NPs, which are the majority of the NPs present in the colloidal solution.38 The oxidation of the nanoparticle surface is modeled by the growth of a Cu2O monolayer to the detriment of the copper core. The initial particle size is 7.2 nm (average size in Figure 1) and an oxide thickness of 0.427 nm (lattice parameter of bulk Cu2O) is defined as a monolayer (i.e. a complete oxide shell, 1 ML). A thicker oxide shell is implemented by adding supplementary Cu2O monolayers. An incomplete oxide shell is defined as half a monolayer. The optical index of the HDA surrounding the NP as well as the size-reduction induced damping of the LSPR were taken into account in the calculations (SI). The simulated absorption spectra are presented in Figure 7:

Figure 7: DDA simulations of the optical absorption spectra of 7.2 nm diameter Cu NPs with increasing Cu2O shell thickness (0 to 2.13 nm, corresponding to 0 to 5 ML). The absorption spectrum of a 7.2 nm Cu2O spherical nanoparticle is shown for comparison. 12


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The modeling of Cu2O growth at the Cu NP surface reveals the following features: i) the LSPR peak of the 7.2 nm Cu NPs is initially located at 573 nm, ii) the LSPR red shifts when the Cu2O shell is formed, iii) the maximum absorption of the LSPR increases up to the formation of 1.5ML, and then decreases, and iv) the LSPR peak located at 600 nm corresponds to 1 ML oxide. With increasing Cu2O shell thickness, the LSPR peak continues to red-shift (around 605 nm for 2 ML) and its maximum absorption vanishes due to the consumption of the metallic Cu core. For a 6 ML Cu2O shell (around 2.6 nm), the Cu core diameter is around 2.1 nm. The latter corresponds to the critical size below which the surface plasmon resonance disappears.30 Experimentally, the onset of the final and slowest oxidation regime (3) corresponds to a LSPR peak close to 600 nm, whatever the HDA content (table S1). This LSPR value is associated to the formation of a complete Cu2O monolayer according to the numerical simulations (Figure 7). It can also be inferred from the previous findings that an experimental LSPR peak below 600 nm corresponds to a sub-monolayer of Cu2O, which is consistent with the formation of isolated seeds of copper oxide distributed on the nanoparticle surface.39

Oxidation process: scheme and evolution

Based on the above presented experimental and modeling results, we propose a scheme for the oxidation process of our HDA capped Cu NPs. In the initial and rapid oxidation regime (1), the adsorption and dissociation of dioxygen on copper (111)-oriented fcc structure is thermodynamically favorable and should proceed immediately.40 This first step is described in the literature as an oxygen-deficient induction layer, or a “pre-oxide layer” formed before the growth of Cu2O islands.41-44 According to Wiame et al.,45 this defective oxide layer presents a band-gap of around 1.5 eV, i.e., between the bulk band-gap of CuO (1.4 eV) and Cu2O (2.1 eV). Our oxidation process is characterized by the strong increase of the UV-vis absorption in the 400-700nm region corresponding to the oxidation regime 1 (Figure 4 and Figure S2). This initial oxidation step cannot be reflected by the DDA simulations since the optical properties of bulk Cu2O layer have been used in the calculations, thus explaining the difference between the baselines of the simulated and the experimental absorption spectra

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in oxidation regime (1). As a matter of fact, the optical properties of the pre-oxide layer are certainly different from those of bulk Cu2O. During the second oxidation regime, the pre-oxide layer is transformed into Cu2O islands. At ambient temperature, it is known that the growth of Cu2O islands is driven by the migration of metal cations due to the electric field between the interfacial Cu+ species and the adsorbed O2- [26]. As the oxide layer covers the surface, this driving force weakens and the oxidation rate slows down. At the end of this oxidation regime (LSPR peak located around 600 nm as defined by DDA simulation and experimental UV-vis spectra), all the Cu2O islands have coalesced and the Cu NPs are covered by a first complete Cu2O monolayer. The onset of the third and final oxidation regime corresponds to the lowest oxidation rate (Figure 1 and Figure S2) since a complete oxide layer covers the copper surface forming a barrier; hence, a slower oxidation regime limited by two main factors takes place: i) a weaker electric driving force for copper migration towards oxide sites, ii) formation of a copper oxide barrier that prevents further dissociation of O2.41 Moreover, the alkylamine ligands play a major role in the oxidation process of the three oxidation regimes. Indeed, HDA and dioxygen in solution are competing for adsorption on the Cu sites at the NP surface.

Figure 8: Schematic of a (111) oriented Cu NPs surface: a) before exposure to air, b) during oxidation regime 1 with adsorption/ dissociation of O2, c) formation of Cu2O islands (regime 2), and d) diffusion of O across a complete Cu2O layer (regime 3).

The ligands coordinated at the nanocrystal surface are in dynamic equilibrium with the free ligands in the solution (Figure 8a). Due to the steric hindrance of the alkylamine moiety, only 14


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a fraction of the surface copper atoms are coordinated with the first coordination sphere of amine ligand. Besides, O2 presents a very thermodynamically favorable dissociation path on copper atoms, and will irreversibly adsorb on copper (Figure 8b). Once the oxidation starts, most of the surface copper atoms are not directly protected by coordinated amine ligands, and are possibly transformed into CuI species. This step corresponds to oxidation regime (1). But this rapid copper oxidation contains the germs of its own counter-effect since CuI is more favorable to the coordination of the electron doublet of amine ligands. In addition, the adsorption of O2 on oxidized copper sites also slows down. Indeed, it has been shown that above a certain oxygen coverage (corresponding to 0.25 mono layer), the binding of oxygen species drops, as a result of repulsive interactions between surface adsorbates.43 By increasing the HDA content in the medium, the probability for an adsorption site (either Cu or CuI) to interact with an amine, rather than with an O2, also increases, and eventually the nanocrystal is durably protected against oxygen adsorption and subsequent oxidation (Figure 8c). Moreover, with HDA content increases, the second coordination sphere of ligands gets denser and offers a better protection against oxygen diffusion. The second oxidation regime, where Cu2O islands are formed and coalesce on the copper nanocrystal, is also controlled by the amount of HDA in the medium (Figure S3). Finally, when the complete oxide layer is formed (LSPR peak located at 600 nm) and the oxidation regime (3) starts, there is still a strong modulation of the oxidation rate driven by the HDA content and the competitive adsorption of amine ligands versus O2 on the Cu2O shell (Figure 8d). The role of the HDA in the oxidation process can be envisaged as a “freezing” agent which considerably reduces the adsorption rate of O2 and accordingly the oxidation speed of Cu NPs in solution.

III. CONCLUSION

The air stability of the colloidal solution of ca. 7.2 ± 1.1 nm Cu-HDA capped NPs has been analyzed by coupling numerical simulations of the optical properties and experimental UV-vis spectroscopy. From the simulations, we found that a LSPR located at 600 nm corresponds to the formation of a complete Cu2O monolayer on the copper nanocrystal. This is confirmed by the experimental finding of different oxidation regimes evidenced by the analysis of the UVvis integrated absorption with time. We showed that the oxide growth on copper nanocrystals 15



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is strongly modulated by the amount of alkylamine ligands in the medium. This slowing down of the oxidation processes allowed us to clearly point out three oxidation regimes. The first and very rapid regime corresponds to the O2 adsorption and dissociation on the copper surface. It is followed by a second regime in which Cu2O clusters are formed, leading to a complete Cu2O monolayer (LSPR at around 600 nm). The build-up of a complete Cu2O shell is shifted from 1 hour to 20 days by increasing the HDA content from 0.1 to 2 equivalents. The final and third oxidation regime is the slowest one due to the complete oxide shell forming a barrier. The competitive adsorption between O2 and HDA on copper sites has a considerable impact on the oxidation rate of Cu NPs. The retarded oxidation controlled by alkylamine ligands could suitably be employed in several applications (catalysis, microelectronics) in which the metallic nature of copper nanocrystals has to be preserved in ambient air conditions.

AUTHOR INFORMATION Corresponding author: *E-mail: [email protected] Present Adress: Laboratoire de Chimie de Coordination, CNRS, BP 44099, 205 Route de Narbonne, 31077 Toulouse Cedex 4, France. Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information - Experimental details (materials and reagents, synthesis and characterization tools, DDA simulations). - Dark field TEM image of hollow Cu2O particles. - Representation of Ian as a function of air exposure duration for HDA contents from 0.5 to 2 eq. - Evolution of the oxidation rate extracted from IAn as a function of HDA content. - Table of the HDA content on the duration of the oxidation regime (1) and (2), and LSPR wavelength at the end of regime (2).

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ACKNOWLEDGEMENTS This work was supported by the HPC facilities of CALMIP center at Paul Sabatier University of Toulouse. The authors are grateful to STMicroelectronics and ANRT for funding, CNRS and Université Fédérale de Toulouse UFT, UT3 Paul Sabatier for their support.

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TOC Graphic

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Remarkable Decrease in the Oxidation Rate of Cu Nanocrystals

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