Plasma Treatment of Metal Oxide Nanoparticles: Development of Core

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Plasma Treatment of Metal Oxide Nanoparticles: Development of Core-shell Structures for a Better and Similar Dispersibility Stella Mathioudaki, Bastien Barthélémy, Simon Detriche, Cédric Vandenabeele, Joseph Delhalle, Zineb Mekhalif, and Stephane Lucas ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00645 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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ACS Applied Nano Materials

Plasma Treatment of Metal Oxide Nanoparticles: Development of Core-shell Structures for a Better and Similar Dispersibility Stella Mathioudaki†,*, Bastien Barthélémy†, Simon Detriche‡, Cédric Vandenabeele†, Joseph Delhalle‡, Zineb Mekhalif‡ and Stéphane Lucas† †

Laboratory of Analysis by Nuclear Reaction, Namur Institute of Structured Matter, University

of Namur, Rue de Bruxelles 61, B-5000, Namur, Belgium ‡

Laboratory of Chemistry and Electrochemistry of Surfaces, University of Namur, Rue de

Bruxelles 61, B-5000, Namur, Belgium KEYWORDS: Plasma polymerization, Cyclopropylamine, Metal oxide nanoparticles, Hansen solubility parameters, Dispersibility

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ABSTRACT

Low pressure plasma polymerization of cyclopropylamine was employed for the surface functionalization of commercial ZnO, Al2O3 and ZrO2 nanoparticles in a homemade hollow cathode plasma reactor. The analysis of the modified nanoparticles by X-ray Photoelectron Spectroscopy (XPS) revealed the incorporation of reactive functional groups such as primary and secondary amines, which was confirmed by Fourier Transform Infrared Spectroscopy (FTIR). The raw and the plasma functionalized nanoparticles were evaluated in terms of dispersibility. Application of Hansen Solubility Parameters (HSP) theory showed that the efficient plasma polymerization that led to the deposition of an approximately 5 nm thick plasma polymer film, as determined by Transmission Electron Microscopy (TEM), causes a similar shift toward the Hansen solubility space for the functionalized nanoparticles and changes their physicochemical affinity within selected solvents, regardless the kind of nanoparticles used. Hence, a combined exploitation of nanoparticles having different cores is feasible in applications such as nanocomposites and bio-applications having certain reactivity after grafting an amine based plasma polymer film that allows achieving a similar dispersibility.

1. INTRODUCTION Metal oxide nanoparticles such as ZnO, ZrO2 and Al2O3 have been widely used during the past decades in a variety of fields due to their outstanding properties. In particular, ZnO is a wide band gap semiconductor, UV-absorber, possesses high thermal and mechanical stability at room temperature and owns anti-bacterial properties combined with low toxicity.1–5 These properties has rendered ZnO a good candidate for a broad range of applications, including electrical devices, optoelectronics, cosmetics and nanocomposites.2,6–10 ZrO2 exhibits unique dielectric 2 ACS Paragon Plus Environment

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properties, chemical and thermal stability, antimicrobial activity as well as low thermal conductivity and its exploitation in nanocomposites, catalysis, fuels cells, and medical prosthetics is already a fact.11–15 Al2O3 also belongs in the class of materials that have received much interest thanks to its hardness, stability as well as its good mechanical properties and it has already been used in applications as composite materials, ceramics and protective coatings.16,17 Despite the significant properties that ZnO, ZrO2 and Al2O3 nanoparticles maintain, their surfaces lack from functional groups and thus do not allow strong covalent bonds when dispersed in a medium. Moreover, due to their high surface area to volume ratio, as in a variety of nanomaterials, agglomeration occurs.1,18 Thus, surface functionalization of nanoparticles is considered essential when homogenous dispersion is required in applications such as nanocomposites. In this work, low pressure plasma polymerization was employed to functionalize ZnO, ZrO2 and Al2O3 nanoparticles in a homemade plasma reactor described previously.19 Plasma polymerization has already been efficiently used for the modification of nanomaterials, such as carbon nanotubes, nanoparticles and nanofibers.19–24 The dry, eco-friendly approach combined with the ability to deposit polymer films of organic compounds with controllable thickness has rendered plasma polymerization a remarkable method to improve surface properties and reactivity.22,23,25,26 Thanks to its potential to produce highly cross-linked and pinhole free films on almost any kind of substrates, plasma polymerization has replaced “wet” chemical techniques and it has contributed drastically to fields such as microelectronics, composites and bio-applications.27–31 In this study, low pressure plasma was used for two main objectives. Initially, oxygen plasma treatment was used to remove carbon contamination from the nanoparticles surface. The conversion of hydrocarbons into volatile compounds (CO, CO2 and H2O) makes possible their



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evacuation by the pumping system and establishes the appropriate conditions for a good adhesion between the oncoming polymer film and the nanoparticles surface.32 Subsequently, plasma polymerization of cyclopropylamine (CPA) was performed in order to produce plasma polymer films (PPFs) that contain high concentration of amino functional groups. The selection of the monomer was done due to earlier published results, which have shown great stability and reactivity of the coatings produced by cyclopropylamine plasma.33–36 Next, the influence of the PPFs deposited onto the nanoparticles surface on dispersibility was investigated by the Hansen Solubility Parameters theory (HSP). HSP theory is a useful tool for a number of applications, where compatibility and colloidal stability is required. In this approach, which has already been used for nanomaterials,37–40 the evaluation of the dispersibility of the studied material is done through its interaction with a number of organic solvents and leads to the determination of the three solubility parameters, named dispersion (δD), polarity (δP) and hydrogen bonding (δH). These parameters describe the non-polar atomic interactions, the permanent dipole-permanent dipole interactions and the hydrogen bonding interactions, between the studied material and the selected solvent, respectively. The solubility parameter δ of the studied material is given by = + + and is represented by a sphere in a three coordinates system with δD, δP and δH as axes. The solubility sphere indicates whether there is physicochemical affinity between the studied material and a solvent or not, and differentiates the compatible solvents that are located within the sphere from the non-compatible ones that are located outside of the sphere.41 Herein, raw and plasma functionalized nanoparticles were tested in 48 of organic solvents for the determination of their HSPs and the impact of the deposited PPFs on nanoparticles dispersibility was examined. As it will be shown, the homogeneous surface functionalization of


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nanoparticles is feasible, so that their intrinsic properties are maintained and their interaction with selected organic solvents becomes similar, independently if ZnO, ZrO2 or Al2O3 nanoparticles are used.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Cyclopropylamine (98% purity) was purchased from Acros Organics and used for the plasma polymerization. All other chemicals, which were used for the Hansen Solubility Parameters experiments, were provided by Sigma Aldrich (>98% purity). ZnO nanoparticles were kindly supplied from Umicore Zinc Chemicals (Belgium) as Zano 20 (mean particle size ~ 20 nm). ZrO2 (mean particle size ~ 30 nm) and Al2O3 (mean particle size ~ 30 nm) nanoparticles were provided from Plasma & Ceramic Technologies Ltd (Latvia). All chemicals and nanoparticles were used as received without any further purification.

2.2. Plasma set-up. Plasma functionalization of ZnO, ZrO2 and Al2O3 nanoparticles was carried out utilizing a homemade plasma reactor described in detail previously.19 In summary, the reactor is a cylindrical hollow cathode discharge with a fixed magnetron; it is able to rotate (25 rpm), mix the nanoparticles and expose their surface to the discharge in order to effectively functionalize them. The whole set-up is placed in a vacuum chamber of 65 L volume. 0.25 g of nanoparticles were introduced into the reactor, which was pumped down to 6 × 10–4 Pa by using a turbomolecular (Turbovac) and a rotary pump connected in series. The experimental procedure consisted of two steps. Oxygen plasma treatment lasted five minutes and subsequently plasma polymerization of cyclopropylamine (CPA) was employed with a duration of twenty minutes. In each step, the flow of oxygen and of cyclopropylamine was regulated at 5



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standard cubic centimeters per minute (sccm) utilizing mass flowmeters (Bronkhorst), while the operating pressure was maintained at 10 Pa by using a throttle valve connected to a baratron gauge. For the plasma excitation, the applied power (Ppeak) was set to 84 W and the pulsing unit was configured in bipolar mode with pulse on-time (ton) and pulse off time (toff) of 10 µs and 25 µs, respectively, which led to a mean power (Pmean) of 24 W delivered to the discharge.

2.3. Characterization techniques. XPS analysis was carried out by using a ThermoFisher KAlpha photoelectron spectrometer. The nanoparticles were deposited onto double side carbon tape and at least 10 points were analyzed. The photon source was the monochromatized Al Kα line (1486.6 eV). Survey and high resolution spectra of N 1s and C 1s were recorded at pass energies of 200 eV and 30 eV, respectively, using a 250 µm diameter X-ray spot. The data were analyzed with the Thermo Avantage software (Version 5.945). N 1s and C 1s core level spectra were fitted with Lorentzian-Gaussian peaks (L/G = 30%) with a full width at half maximum (FWHM) between 1.8 and 2.0 eV. The correspondence of the chemical shifts to functional groups was done according to the available literature.42,43 FTIR was performed utilizing a Perkin Elmer 65 spectrometer (spectral resolution of 4 cm–1). The spectra were recorded after fabricating pressed KBr circular pellets that contained 2 mg of each nanoparticle powder, in the wavenumber region from 500 to 4000 cm–1. The peaks were assigned based on the literature.44–46 The morphology of the raw and the plasma functionalized nanoparticles was examined by using Transmission Electron Microscopy (TEM). The TEM images were acquired with a Philips Tecnai 10 transmission electron microscope operating at 80 keV in the bright-field mode. The


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samples for TEM analysis were obtained by diluting the nanoparticles in pure ethanol (1 mg/mL) and by depositing a droplet of 150 µL onto carbon-coated Cu grids. In addition, HRTEM was performed by using a TALOS F200A microscope at 200 keV. X-ray Diffraction (XRD) measurements were performed by using an X’Pert PRO diffractometer (Panalytical) with Cu Kα radiation (1.5406 Å). The patterns were analyzed according Joint Committee of Powder Diffraction Standards (JCPDS) database. 2.4. Application of Hansen Solubility Parameters. The determination of Hansen Solubility Parameters (HSP) of raw and plasma functionalized nanoparticles was done by evaluating their degree of dispersibility in the 48 selected solvents, based on other works described in the literature.37,47 The experimental method was constituted of three steps. First, 1 mg of nanoparticles powder was added in 1 mL of each solvent; then the solutions were sonicated for 5 seconds (Hielscher UP400S apparatus, 70 % of amplitude) and were subsequently centrifuged for 5 minutes (Piccolo centrifugator). The physicochemical affinity between the tested nanoparticles and the solvents was estimated visually. Colloidal stability indicated a strong interaction between the nanoparticles and the solvent and consequently the solvent was considered as compatible, while incompatibility and sedimentation of the nanoparticles indicated lack of interaction and the solvent was considered as non-compatible. The classification of the solvents as “compatible” or “non-compatible” was carried out in order to use the HSPiP software48 and obtain the HSPs of the tested nanoparticles. Score of “compatible” and “noncompatible” solvents was set to 1 and 6, respectively, while the intermediate behavior of median dispersibility was set to 3. UV-Visible measurements were performed in order to evaluate the degree of the nanoparticles dispersibility in selected solvents. The experiments recorded the absorbance of the nanoparticles



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in a specific wavelength (375 nm) versus time at a concentration of 0.2 mg/mL. The apparatus used was an Analytik Jena Specord 205 spectrometer.

3. RESULTS AND DISCUSSION 3.1. Surface chemical analysis (XPS). The elemental atomic composition of the ZnO, Al2O3 and ZrO2 nanoparticles before and after the oxygen plasma is summarized in Table 1, as calculated from the survey spectra. The oxygen treated nanoparticles are labeled as ZnO-T, Al2O3-T and ZrO2-T. The intensity of the carbon peak, C 1s, decreases up to 70% and the plasma treatment efficiently removes the organic contaminants. The remaining contamination is probably due to the nanoparticle’s transfer from vacuum to XPS and/or not all nanoparticles were exposed to oxygen plasma during the treatment. Oxygen plasma treatment mechanism acts in two ways. Briefly, the oxygen energetic ions bombard and break the bonds of the surface molecules (sputtering) and/or the atomic oxygen chemically reacts with the contaminants and converts them into volatile compounds.49,50 In each case, the contaminants are pumped down and a stronger adhesion of the polymer film can be achieved.51

Table 1. Comparison of the surface elemental composition measured by XPS for the raw ZnO, Al2O3 and ZrO2 nanoparticles, the oxygen plasma treated ZnO-T, Al2O3-T and ZrO2-T nanoparticles and the CPA functionalized ZnO-F, A Al2O3-F and ZrO2-F nanoparticles. M* corresponds to Zn, Al and Zr in ZnO, Al2O3 and ZrO2 nanoparticles, respectively.

Sample

C (at. %)

O (at. %)

M* (at. %)

N (at. %) 8

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ZnO

14.1±0.8

46.5±0.3

39.4±0.4

ZnO-T

5.8±0.5

46.5±0.6

47.7±0.2

ZnO-F

32.7±1.4

35.9±0.9

23.6±0.8

Al2O3

28±1.4

43.9±4.5

28.1±2.9

Al2O3-T

8.0±0.6

56.4±0.4

35.6±0.9

Al2O3-F

29.3±2.3

38.6±1.9

24.1±1.7

ZrO2

51.6±1.9

35.7±1.4

12.7±0.5

ZrO2-T

15.6±1.0

59.5±0.9

24.9±0.2

ZrO2-F

37.0±0.6

39.4±0.4

14.7±0.9

7.8±0.4

8.0±1.2

8.9±0.1

Cyclopropylamine plasma functionalization of the oxygen treated nanoparticles followed subsequently. Table 1 shows the relative atomic composition of the CPA functionalized nanoparticles, marked as ZnO-F, Al2O3-F and ZrO2-F, as computed from survey spectra. The elemental composition of the CPA functionalized nanoparticles revealed the incorporation of carbon and nitrogen. Additionally, the presence of Zn, Al and Zr for the ZnO, Al2O3 and ZrO2 nanoparticles, respectively, indicated that the deposited film does not exceed 10 nm (analysis depth of XPS) and/or not all the nanoparticles are coated uniformly. Another observation is that the functionalized nanoparticles contain high content of oxygen. Two explanations can be provided at this point. First, this content of oxygen can be attributed to the prior oxidation (induced by the oxygen plasma) and to the detection of the core of the metal oxide nanoparticles and second it can be attributed to post-reactions with atmospheric ambient contaminants,43 since cyclopropylamine does not contain any oxygen in its initial structure. In order to investigate and quantify the chemical functions grafted onto the nanoparticles surface, the C 1s and N 1s core level spectra were analyzed. Figure 1 depicts the fitting of the carbon (a, b and c) and nitrogen (d, e and f) peaks, for the ZnO-F, Al2O3-F and ZrO2-F,



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respectively. The carbon peak was fitted by a combination of four components indicating the different chemical environments. Binding energy of 285.0 eV corresponds to C–C and C–H bonds, the peak at ~286.5 eV is assigned to amines C–N, imines C=N, nitriles C≡N and C–O (carbon single bonded to oxygen), the peak at ~288 eV includes C=O bond (carbon double bonded to oxygen) and possibly amides N–C=O and the peak at binding energy of ~289.5 eV corresponds to COOH (carboxyl groups). The nitrogen peak was fitted by a combination of three components. The peak at ~398 eV corresponds to imine functions N=C and possibly conjugated imine N=C=N groups, the peak at ~399.2 eV is assigned to amino groups NHx (x = 1, 2) and the peak at ~400.5 eV includes the nitrile groups, N≡C, and the amides, N–C=O, which are formed due to post-plasma oxidation of amines.52


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Figure 1. Fitting of the XPS core level spectra of C 1s and N 1s peaks of ZnO-F (a and d), Al2O3-F (b and e) and ZrO2-F (c and f), respectively. The deconvolution of the C 1s and N 1s high resolution peaks revealed similar chemistry of the coatings deposited on the different kinds of nanoparticles. The contribution of the functional groups is very close, whatever the initial substrate used. According to the fitting of the C 1s peak, the contribution of the first peak (C-C, C-H) at binding energy of 285 eV ranges from 60% to 66%, the peak at ~286.5 eV that contains mostly the amine and imine chemical environments is from 20% to 25% and the two peaks at higher binding energies, attributed mostly to the post-



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oxidation of the amine layers, have a contribution that ranges from 12% to 15% of the total carbon environment. Regarding the analysis of the N 1s peak, the amino selectivity (NHx/N) ranges between 51% and 57% and in all cases the primary and secondary amino groups consist half of the functional groups, with the rest being a mixture of imine, nitrile and amide functions. The peak associated with nitriles and amides is the one with the lowest contribution, leading us to the conclusion that the formation of nitriles is low in the plasma polymerization of CPA, as was also observed previously.33 The degree of amino selectivity is in the same range with results obtained in previous studies for PPFs deposited from a mixture of CPA and argon plasma. Indeed, in the case of PPFs deposition onto polycaprolactone nanofibers the amino selectivity was found equal to 60%,33 while for deposition onto quartz crystal microbalance and onto silicon wafers was about 65%.35,53 Hence, a high degree of amino functionalization was achieved here that is comparable with the amino functionalization reported previously, regardless the fact that the initial substrates used in this study are nanoparticles.

3.2. Molecular bonding identification (FTIR). Figure 2 illustrates the FTIR spectra of the raw and plasma functionalized nanoparticles. In addition, to facilitate the recognition of the new bands that appear after the plasma modification, a spectrum of a coated KBr pellet at the same conditions is also plotted in figure 2(d). The main band in the spectra of the raw nanoparticles is located in the region of 3490-3440 cm–1 and is assigned to stretch vibration of O-H group. Additionally, contributions of hydrocarbons are observed in the region of 2960-2850 cm–1 and at 1380 cm–1, associated with asymmetric stretch vibrations of CH3 (2960 cm–1) and CH2 (2925 cm– 1

), symmetric stretch of CH2 (2850 cm–1) and symmetric deformation vibration of CH3 at 1380


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cm–1. Stretch vibration of C=O appears at 1620 cm–1. These bands are attributed mostly to the surface contamination. All spectra of CPA functionalized nanoparticles exhibit a broadening in the already existing peak of O-H group that is associated with the presence of amino functional groups (NH stretch, NH2 asym. and sym. stretch) in the range 3360–3220 cm–1, although the peaks cannot be clearly distinguished due to overlapping with the broad OH band. The additional peak at 1650 cm–1 (NH2 deformation vibration) along with the broadening of the peak around 3300 cm–1 is a strong indication that primary amino groups are also present in the PPFs deposited onto the nanoparticles surface. CH3 and CH2 stretch vibrations are observed in all spectra of the functionalized nanoparticles, as well as an intense and broad band at 1620 cm–1, which corresponds to C=O stretch vibration. Band around 2180 cm–1 is associated with nitriles, isonitriles (C≡N–) and/or conjugated imine (–N=C=N–) stretch vibrations. Infrared spectra of the plasma functionalized nanoparticles verified the incorporation of amino functions onto the nanoparticles surface and confirmed our XPS results. Moreover, as can be seen, the intensity of these peaks is similar for the different kinds of nanoparticles revealing the reproducibility of the plasma deposition in the rotary reactor, regardless the initial nanostructure introduced.



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Figure 2. FTIR spectra of the raw and the CPA plasma functionalized nanoparticles: (a) ZnO, (b) Al2O3, (c) ZrO2 and (d) spectrum of KBr pellet coated with CPA. * corresponds to the wavenumber of 2925 cm–1.

3.3. Morphology and structure of nanoparticles. Bright-field TEM images of the raw and the plasma functionalized nanoparticles are illustrated in figure 3. The raw nanoparticles show aggregates without any coating on their surface, while the plasma functionalized nanoparticles show a core-shell structure and are coated with PPFs of approximately 5 nm thickness. The thickness of the polymer film, which can be easily controlled by tailoring the experimental parameters such as power or treatment time, is approximately the same for the different sizes of nanoparticle. Earlier published results showed that the thickness of the polymer layer resulting


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from plasma polymerization on nanoparticles can be approximately the same54 or not.23 In our investigation, we found that the majority of the nanoparticles and clusters have an organic layer that is uniformly deposited, although few parts of the nanoparticles and /or some nanoparticles remained uncoated. This observation can have two possible explanations. First, the initial state of the nanoparticles was such that did not allow the deposition due the high degree of agglomeration or second the nanoparticles were not well mixed during the plasma polymerization. Nevertheless, the surface modification by plasma polymerization of CPA led to grafting of amino functional groups on the majority of the nanoparticles.

Figure 3. TEM micrographs of the raw (a) ZnO, (b) Al2O3 and (c) ZrO2 nanoparticles and the CPA plasma functionalized (d) ZnO-F, (e) Al2O3-F, (f) ZrO2-F nanoparticles.



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Figure 4 shows the X-ray diffraction patterns of the nanoparticles before and after the plasma deposition and a High Resolution Transmission Electron Microscopy (HRTEM) image of Al2O3F plasma functionalized nanoparticles. The X-ray diffraction patterns reveal that in each case the peaks correspond to the ZnO, Al2O3 and ZrO2 nanoparticles and no other peaks are observed. More specifically, the pattern of ZnO nanoparticles is in accord with the reported JCPDS data (No. 36-1451) and shows hexagonal wurtzite structure. In the case of Al2O3 nanoparticles, two different phases are observed that correspond to α-Al2O3 at rhombohedral system (No. 10-173) and to γ-Al2O3 at cubic system (No. 10-425). The broadening of the peaks of γ-Al2O3 indicates that the particles are smaller in comparison to α-Al2O3 nanoparticles. ZrO2 nanoparticles have single tetragonal phase (No. 80-0965), also without any additional peaks. The comparison between the X-ray diffraction patterns of the raw and the plasma functionalized nanoparticles indicates two different features. First, we can observe that the nanoparticles keep their crystalline structure after the plasma deposition, and second that the film deposited is an amorphous layer, since no additional peaks are present after plasma. The latter is also evidenced by the HRTEM image (Fig 4 (b)), where is clearly seen the crystalline structure of Al2O3-F nanoparticles and the amorphous polymer film around them.


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Figure 4. (a) X-ray diffraction patterns of raw and functionalized nanoparticles. In the case of Al2O3 nanoparticles patterns, the “orange color” labelling corresponds to the diffractions of αAl2O3 and the “green color” labelling to the diffractions of γ-Al2O3. (b) HRTEM image of Al2O3F nanoparticles.



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3.4. Effect of CPA plasma polymerization on HSPs. The impact of the PPFs was examined by the Hansen solubility parameters theory. Hansen Solubility Parameters and sphere radius are listed in Table 2, as obtained by using the HSPiP software.48 Table 2. Hansen solubility parameters and sphere’s radius of the raw and the CPA plasma functionalized nanoparticles.

Nanoparticles

δD (MPa1/2)

δP (MPa1/2)

δH (MPa1/2)

Radius (MPa1/2)

ZnO

17.81

10.60

10.79

6.6

ZnO-F

19.47

9.39

12.72

8.2

Al2O3

17.52

11.29

10.6

6.4

Al2O3-F

19.47

8.75

12.91

8.4

ZrO2

19.08

11.40

8.65

6.6

ZrO2-F

19.46

9.22

12.86

8.3

Prior the plasma polymerization, the nanoparticles have different solubility parameters, indicating the diverse physico-chemical affinities between the different cores of the nanoparticles and the tested solvents. The interactions between the nanoparticles and the tested solvents depend on the nature of the materials surface and thus, it is expected that ZnO, Al2O3 and ZrO2 nanoparticles will behave differently when immersed. In contrary, after plasma functionalization, the solubility spheres appear approximately at the same coordinates in the solubility space, as can be seen in Table 2. While initially, the raw nanoparticles are more compatible in polar solvents due to their oxide layer (δP parameter), after the plasma deposition, the additional functional groups onto the nanoparticles surface lead to changes in compatibility and dispersibility in the tested solvents and therefore, affect the HSPs values.


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More specifically, the δD parameter accounts for the dispersion forces. These forces are strongly related to the atoms polarizability. The more the atoms are polarizable (their electronic cloud can be deformed by the presence of an external electric dipole) the more the atoms can interact with their surroundings by strong dispersion forces. The polarizability increases when the molar mass increases in the same chemical family and thus the δD parameter increases (e.g.: the δD value for methyl chloride (CH3Cl) with molar mass of 50.49 g/mol is 15.3 MPa1/2, while for methyl iodide (CH3I) with molar mass of 141.94 g/mol, the δD value is 17.5 MPa1/2). In contrary, the polarizability decreases when electronegativity increases,55 and as a consequence, the dispersion parameter δD also decreases. After the plasma deposition, the values of the δD parameters of the ZnO-F, ZrO2-F and Al2O3-F are similar and show an increase. This is explained by the fact that the external chemical functions that interact with the solvents are carbon-nitrogen-based functions instead of oxygencontaining ones and are thus are more polarizable (since nitrogen is less electronegative than oxygen). Moreover, the contribution of amino and nitrile groups is considered significant since their polarizability is high in comparison with oxygen and hydroxyl groups.56,57 Contrary to the dispersion parameter δD, the polarity parameter δP decreases for the functionalized nanoparticles. The parameter δP is related to the dipole-dipole interactions between permanent dipoles. These permanent dipoles are generated by the partial charges created by the electronegativity difference between two atoms sharing a chemical bond. In the three functionalized samples, the decrease of the δP component can be understood as the consequence of the presence of weaker permanent dipoles in the surface functions after the functionalization. Once again, this is compatible with nanoparticles bearing nitrogen-containing functions on their surface instead of oxygen-containing ones.



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The parameter δH is related to the hydrogen bonding interactions. Their formation needs the presence of a strongly electronegative atom with a free electronic pair and the presence of a hydrogen bonded to a strongly electronegative atom such as fluorine, oxygen and nitrogen. In Table 2, we can observe that before plasma functionalization the δH parameter for ZnO, Al2O3 and ZrO2 nanoparticles has lower value than after the deposition of the plasma polymer film. Initially, the ability to form hydrogen bonds is attributed to the hydroxyl groups located on the surface of the nanoparticles, since oxygen can act both as a donor and as an acceptor. After the functionalization, the presence of nitrogen atoms is responsible for the higher values of δH. Despite the fact that oxygen is more electronegative than nitrogen, the lower value of δH prior functionalization is an indication that the density of hydroxyl groups is low and leads to a weak ability to form hydrogen bonds. After the plasma modification, the polymer film surrounds the nanoparticles, as is ideally illustrated in figure 5, and the nitrogen-containing chemical functions (amine, imine and nitrile) are in contact with the solvents when the nanoparticles are immersed, with the nitrogen atoms being capable to form hydrogen bonds.


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Figure 5. Representation of a ZrO2 nanoparticle (a) raw, (b) ideally coated after plasma deposition and (c) UV-Visible measurements for ZrO2 nanoparticles at 375 nm in 1-methoxy-2propanol.



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As a conclusion, the analysis of the Hansen Solubility Parameters allows us to describe the consequence of the plasma functionalization as a transition from a surface with weakly polarizable atoms with low hydrogen bond ability and strong electronegativity to a surface with more polarizable atoms with higher hydrogen bond ability and weaker electronegativity. This is consistent with the modification of the surface from a metal oxide (with occasional hydroxyl groups) to a mixture of carbon and nitrogen bonds with a high percentage of amine groups able to form hydrogen bonds. This transformation of the nanoparticles can strongly affect and improve the dispersibility in specific solvents, as can be seen in Fig. 5(c). Here, ZrO2 nanoparticles are dispersed in 1-methoxy-2 propanol (δD=15.6 MPa1/2, δP =6.3 MPa1/2, δH=11.6 MPa1/2) and the time needed for sedimentation strongly increases when the nanoparticles are treated. This is a consequence of the modification that resulted to a higher hydrogen bonding parameter for the ZrO2 and enabled a better stability in a solvent that has close Hansen solubility parameters with the functionalized nanoparticles. Along with the estimation of the Hansen Solubility Parameters, the sphere radius was also calculated. The sphere in the solubility space encompasses the compatible solvents and excludes the non-compatible solvents. Prior the modification, the nanoparticles show solubility spheres of lower radius in comparison with the plasma functionalized nanoparticles. An increase of the sphere radius can have two distinct origins. The first comes from the signification of this radius. The sphere radius describes the allowed difference between the Hansen Solubility Parameters of the solvent and the solute (the nanoparticles in this study) in order to achieve good dispersibility. From a thermodynamic point of view, the mixing is allowed (formation of a solution between a solvent and a solute) if the variation of the Gibbs free energy of mixing (∆GM) takes a negative value (or zero). The free


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energy of mixing is equal to = − ∙ , where corresponds to the heat of mixing, is the absolute temperature and is the entropy change in the mixing process.41 The heat of mixing per unit volume is described by the equation ⁄ = − , where and are the solubility parameters of the solvent and the solute and and are the volume fractions of the solvent and the solute, respectively. A negative , and thus a spontaneous mixing, is obtained when the parameter is increased in order to counterbalance the parameter. The lighter (in terms of molecular mass) the molecules of the solute, the higher will be the increase of entropy, for a given mass of solute. In conclusion, a low molecular weight solute will allow higher tolerance (for a good dispersion) in terms of difference between the Hansen Solubility Parameters of the solute and the solvent.41 An increase of the sphere radius, from this thermodynamic point of view, signifies a decrease in the molecular mass of the solute (of the nanoparticles) and this is not what is observed in this case. Instead, an increase of the nanoparticles size, and thus of their molecular weight, is observed. The second possible origin that could be considered in this case is the inhomogeneity of the chemical functions grafted after plasma on the nanoparticles surface. This could have as consequence the existence of several overlapping small spheres with close Hansen Solubility Parameters, giving the appearance of a broader solubility sphere with greater radius. Indeed, XPS and FTIR analyses showed that different chemical functional groups including amines, imines and nitriles are present and each one of them contributes differently to the dispersibility of the nanoparticles. For instance, primary and secondary amino groups have the ability to form hydrogen bonds with each other and with other molecules contrary to nitrile groups that can only generate hydrogen bonds with other molecules and not with each other, due to the fact that no hydrogen atom is bonded to the electronegative nitrogen atom. Hence, if the nanoparticles were



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ideally coated only with amino groups, a higher δH parameter would be expected. This assumption is more reinforced if we consider two molecules with similar number of atoms but with different terminal functional groups, namely butylamine and butyronitrile, which are depicted in figure 6 along with their HSPs.41 Here the δH parameter is increased in the case of butylamine (8 MPa1/2) that has as terminal group the primary amine in comparison with the butyronitrile (5.1 MPa1/2) that has nitrile as terminal functional group. Regarding the δD parameter, the two molecules have similar values and the small difference is attributed to their different geometry and to the fact that butyronitrile has two less hydrogen atoms. The polarity parameter δP, is significantly different for the two molecules, with butyronitrile having higher value (12.4 MPa1/2) in comparison with butylamine (4.5 MPa1/2). This difference is attributed to the triple bond of the nitrile function that can generate more partial charges on the N and C atoms and as this is increased, the dipole moment is also increased in comparison with the one of the single bond in the amine function. Concerning the imine functional groups that are also present after plasma functionalization, one can expect intermediate values for the δP and δH parameters. The imine functions can generate hydrogen bonds with each other and other molecules, or only with other molecules, depending on if they are terminal imines or not (in the terminal imines a hydrogen is bonded to the nitrogen atom). Thus, in comparison with the amines and nitriles, we can assume that the δH parameter would show an intermediate value. The same assumption can be done for the value of the δP parameter as well. Here, the polarity would decrease due to the decrease of the triple bond to double compared with the nitrile function but it would increase compared with the single bond of amines.


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As a conclusion, we can say that the existence of several overlapping spheres with close Hansen solubility parameters is the explanation for the greater radius of the Hansen solubility spheres of the functionalized nanoparticles, due to the different chemical functional groups that are present onto the nanoparticles surface.

Figure 6. Representation of butylamine (left) and butyronitrile (right) molecules and their Hansen solubility parameters.

4. CONCLUSIONS Low pressure plasma polymerization of cyclopropylamine was successfully used to functionalize ZnO, Al2O3 and ZrO2 nanoparticles. XPS and FTIR analyses revealed the incorporation of reactive functional groups such as amines and imines after the plasma functionalization of the nanoparticles. The amino selectivity NHx/N was estimated to be 55%, and thus regardless the fact that nanoparticles were used as substrates, high incorporation of amino groups was achieved. TEM micrographs showed that a plasma polymer film of approximately 5 nm was deposited onto the nanoparticles surface, but some parts of the nanoparticles remained uncoated. The XRD analysis indicated that the structure of the polymer film deposited is amorphous and that the plasma treatment does not affect the crystallinity of the samples. Furthermore, HSPs were assigned to the raw and the plasma functionalized 25 ACS Paragon Plus Environment


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nanoparticles. While initially the HSPs of the raw nanoparticles differed, after plasma functionalization, the new functional groups grafted led to similar surface chemistry and close HSPs for the different kinds of nanoparticles regardless the core, revealing the reproducibility of the plasma deposition. Moreover, the consequence of the plasma functionalization in the dispersibility of the nanoparticles was examined and results indicated a transition from a surface with weakly polarizable atoms with low hydrogen bond ability and strong electronegativity to a surface with more polarizable atoms with higher hydrogen bond ability and weaker electronegativity after the plasma polymerization of CPA. Hence, the capability to change the surface chemical state of nanostructures via plasma by developing a core-shell structure and achieve similar Hansen solubility parameters can act as a tool-box for further application where the treated nanoparticles can be simultaneously exploited, since the dispersibility will be kept the same.

ASSOCIATED CONTENT Supporting information. The supporting information is available free of charge at http://pubs.acs.org and includes the following figures: The XPS survey spectra of the raw and after oxygen plasma treatment nanoparticles, the XPS survey spectra of the cyclopropylamine plasma functionalized nanoparticles and the Hansen solubility spheres before and after plasma functionalization. In addition, the table with the quantification of functional groups after polymerization, deduced from the fitting of the core level spectra of C 1s and N 1s peaks, the table with the main band assignments for the plasma functionalized nanoparticles in the infrared spectra and the table with the distinction between the “compatible” and the “non-compatible” solvents used for the Hansen solubility parameters experiments are included.


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AUTHOR INFORMATION Corresponding author *E-mail: [email protected]

ORCID Stella Mathioudaki: 0000-0002-8199-8014

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the Walloon region under the Flycoat (N° 1318147) and the Nanosol (N° 6933) projects.

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Table of Contents:



Fitting of the XPS core level spectra of C 1s and N 1s peaks of ZnO-F (a and d), Al2O3-F (b and e) and ZrO2-F (c and f), respectively. 143x120mm (300 x 300 DPI)


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FTIR spectra of the raw and the CPA plasma functionalized nanoparticles: (a) ZnO, (b) Al2O3, (c) ZrO2 and (d) spectrum of KBr pellet coated with CPA. * corresponds to the wavenumber of 2925 cm–1. 220x286mm (300 x 300 DPI)



TEM micrographs of the raw (a) ZnO, (b) Al2O3 and (c) ZrO2 nanoparticles and the CPA plasma functionalized (d) ZnO-F, (e) Al2O3-F, (f) ZrO2-F nanoparticles. 165x182mm (300 x 300 DPI)


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(a) X-ray diffraction patterns of raw and functionalized nanoparticles. In the case of Al2O3 nanoparticles patterns, the “orange color” labelling corresponds to the diffractions of α-Al2O3 and the “green color” labelling to the diffractions of γ-Al2O3. (b) HRTEM image of Al2O3-F nanoparticles. 178x381mm (300 x 300 DPI)



Representation of a ZrO2 nanoparticle (a) raw, (b) ideally coated after plasma deposition and (c) UV-Visible measurements for ZrO2 nanoparticles at 375 nm in 1-methoxy-2-propanol. 183x403mm (300 x 300 DPI)


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Representation of butylamine (left) and butyronitrile (right) molecules and their Hansen solubility parameters. 33x13mm (300 x 300 DPI)


Plasma Treatment of Metal Oxide Nanoparticles: Development of Core

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