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Building organosilica hybrid nanohemispheres via thiol-ene click reaction on alumina thin films deposited by atomic layer deposition (ALD)
Journal Pre-proofs Building organosilica hybrid nanohemispheres via thiol-ene click reaction on alumina thin films deposited by atomic layer deposition (ALD) Gabriela Ambrožić, Maria Kolympadi Markovic, Robert Peter, Ivna Kavre Piltaver, Ivana Jelovica Badovinac, Duško Čakara, Dean Marković, Mato Knez PII: DOI: Reference:
S0021-9797(19)31261-5 https://doi.org/10.1016/j.jcis.2019.10.074 YJCIS 25568
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
31 July 2019 14 October 2019 19 October 2019
Please cite this article as: G. Ambrožić, M. Kolympadi Markovic, R. Peter, I. Kavre Piltaver, I. Jelovica Badovinac, D. Čakara, D. Marković, M. Knez, Building organosilica hybrid nanohemispheres via thiol-ene click reaction on alumina thin films deposited by atomic layer deposition (ALD), Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.10.074
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Building organosilica hybrid nanohemispheres via thiol-ene click reaction on alumina thin films deposited by atomic layer deposition (ALD)
Gabriela Ambrožić,a,b* Maria Kolympadi Markovic,a,b Robert Peter,a,b Ivna Kavre Piltaver,a,b Ivana Jelovica Badovinac,a,b Duško Čakara,b,c Dean Marković,c Mato Knez d,e aDepartment bCentre
of Physics, University of Rijeka, Radmile Matejčić 2, 51000 Rijeka, Croatia
for Micro- and Nanosciences and Technologies, University of Rijeka, Radmile
Matejčić 2, 51000 Rijeka, Croatia cDepartment
of Biotechnology, University of Rijeka, Radmile Matejčić 2, 51000 Rijeka,
Croatia dCIC
nanoGUNE, 20018 San Sebastian, Spain
eIKERBASQUE,
Basque Foundation for Science, 48013 Bilbao, Spain
*E-mail: [email protected] Phone: (+385)51-584-632
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Abstract The present work shows a surface-induced preparation of sub-100 nm organosilica nanohemispheres on atomic layer deposited (ALD) Al2O3 thin films, which was achieved by cooperative condensation/hydrolysis and thiol-ene click chemical reactions. The two-step synthetic approach consists of an initial silanization of the Al2O3 film with vinyltrimethoxysilane (VTMS), followed by a photo-promoted growth of surface-bound nanoparticles
in
the
presence
of
(3-mercaptopropyl)trimethoxysilane
(MPTMS).
Characterization by means of FE-SEM, XPS and EDS points towards the growth of the nanohemispherical structures being governed by an initial nucleation of thiolated organosilica seeds in solution as a result of self-condensation of MPTMS and oxidation of thiols to disulfides. Once bound to the vinyl terminated Al2O3 via photo-assisted thiol-ene coupling, these seeds promote area-selective growth of the nanoparticles through binding of further MPTMS from the solution. After an additional ALD deposition of ZnO, the resulting thin hybrid film exhibits enhanced hydrophobicity when compared to ZnO films deposited directly on Al2O3 under the same processing conditions. Keywords: organosilica hybrids, nanohemispheres, thiol-ene reaction, thin films, atomic layer deposition, Al2O3, ZnO.
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1. Introduction Functionalization of substrates with hybrid nanoscale films is a rapidly growing research area in materials chemistry. Such films allow altering the functionalities of a material without considerable modification of the substrate itself, thus expanding the functionalities of the substrate. Such functional films are currently investigated for applications in optics [1], catalysis [2], energy storage [3], conversion and saving [4], as well as in environmental research and biotechnology [5]. The chemical or physical properties of nanoscaled hybrid materials often result from synergies between the constituent organic and inorganic parts. The compatibility and interplay of the two involved phases are tailored by the nature of the interface and the applied synthetic methodology, both of which define the structural and chemical composition as well as the properties of the resulting hybrid. Understanding the molecular interactions, the structures of the organic and inorganic components, and their degrees of organization are crucial for the development of high-performance materials. Silanes are among the most commonly used reagents for surface modification. Their grafting on a solid substrate typically relies on the formation of covalent bonds between the silane and terminal OH functionalities of a substrate [6]. The chemical versatility of available silanes results in a great diversity of surface properties that hybrid thin films can exhibit. However, using silanes with more than one leaving group can easily result in loss of the control over the reaction course, since secondary reactions such as crosslinking of the silanes upon hydrolysis and condensation may occur either on the surface or in solution, or both. The reaction conditions, i.e. the solvent viscosity and polarity, the reaction temperature, the amount of water present in the reaction mixture, the presence of acid or base catalysts and surfactants, and the silane concentration, are of crucial importance for tailoring both the surface morphology and structure. Consequently, a surface modification is not restricted to the formation of a monolayer only, but may also involve formation of multilayered structures, or deposition of oligomer/polymer films at the inorganic surface [6]. Heterofunctional silanes, containing additional reactive end-groups, have been used as versatile anchors for organic post-functionalization by linking active molecules, polymers or catalysts to solid matrices [7]. In that regard, click chemistry has become one of the most elegant chemical pathways for imparting new functionalities on a pre-existing hybrid support, as the reactions are highly selective, simple to perform and greatly variable. The thiol-ene free-radical reaction has all the desirable features of a click reaction, i.e., high efficiency, high reaction rates,
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mild conditions, oxygen and water insensitivity, etc., making it a powerful and versatile methodology for surface modification and patterning [8]. Besides carefully controlling the chemical conditions in solution, the composition and surface properties of the inorganic substrate as constituent part of the hybrid material have also to be considered. As the properties of nanoscaled metal oxide films strongly depend on their purity, crystallinity and thickness, it is crucial that the substrates to be chemically modified are prepared in a reproducible way. Among the variety of available vapor phase deposition techniques, atomic layer deposition (ALD) is the method-of-choice for depositing conformal nanoscaled metal oxide thin films with controlled thickness, crystallinity and composition. This is attributed to the self-saturating nature of ALD processes based on a sequential exposure of a substrate to organometallic precursors and counter-precursors. Here, we report on the first catalyst-free formation of organosilica hybrid nanohemispheres with terminal mercaptopropyl functional groups, grown on ALD-deposited Al2O3
thin
films.
The
nanohemispheres
were
synthesized
via
cooperative
condensation/hydrolysis and thiol-ene reactions of heterobifunctional silanes. The process is based on a two-step approach consisting of an initial silanization of a 50 nm thick ALDdeposited Al2O3 film with vinyltrimethoxysilane (VTMS), followed by a photo-promoted thiolene click reaction with (3-mercaptopropyl)trimethoxysilane (MPTMS). As thiol groups have strong ability to bind to many transition metals, an ALD deposition of ZnO was subsequently applied and the surface-chemical properties and wetting behavior of the obtained hybrid films were investigated. The fabricated materials offer a great potential for the development of hierarchically structured silanized thin-films for bioimaging and biosensing of thiol-carrying biomolecules through the formation of disulfide bridges [9].
2. Experimental section 2.1. Materials (3-Mercaptopropyl)trimethoxysilane (MPTMS, 95 %), vinyltrimethoxysilane (VTMS, 98 %) 2,2-dimethoxy-2-phenyl-acetophenon (DMPA, 95 %) were purchased from Aldrich, toluene (puriss p.a.), was obtained from Sigma-Aldrich, while methanol (99.8 %, for HPLC) was purchased from Acros Organics. Double-side polished silicon wafers from Semiconductor Wafer, Inc. (SWI) were used as substrates for the ALD deposition of Al2O3. The ALD precursors diethylzinc (DEZ, ≥ 95 %) and trimethylaluminum (TMA, ≥ 98 %) were purchased from Strem Chemicals. All chemicals were used as received. 4
2.2. Synthesis of materials 2.2.1. ALD processing Atomic layer deposition was carried out using a Beneq TFS-200 reactor with nitrogen (purity 6.0) as carrier and purging gas. For the deposition of 50 nm Al2O3 film on Si substrates (0.70 cm x 0.70 cm), 425 ALD cycles were applied at 200 ºC using TMA and purified water as precursors. One ALD cycle consisted of 180 ms TMA pulsing, 1 s N2 purging, 180 ms water pulsing and 1 s N2 purging. The thickness of the alumina film was verified by SIMS and from FE-SEM images taken from cross-sectioned samples (see Fig. S1. in the Supplementary material). After ALD processing, the substrates were immediately transferred and stored in a desiccator under vacuum before use for further processing. Deposition of ZnO on the hybrid thin film substrates (30 or 200 ALD cycles) was performed at 50 ºC using DEZ and purified water as precursors. For complete hydrolysis of potentially remaining alkoxy silane groups, an initial water pulse was applied for 5 s, followed by 25 s of N2 purge. One ALD cycle consisted of 200 ms DEZ pulsing, 5 s N2 purging, 180 ms water pulsing and 25 s N2 purging. The comparatively long purging times were needed to ensure an efficient removal of potentially physisorbed precursors and reaction by-products from the substrate at the applied reaction temperature. After ALD processing, the substrates were immediately transferred and stored in a desiccator under vacuum. 2.2.2. Functionalization of ALD-deposited alumina thin films with VTMS Five Si wafer pieces (0.70 cm x 0.70 cm), coated with 50 nm of Al2O3, were immersed in solutions of VTMS in toluene with following concentrations: 0.24 M (sample: 0.24 M VTMS), 0.36 M (sample: 0.36 M VTMS) and 0.48 M (sample: 0.48 M VTMS), respectively, and then heated for five hours while gently stirring under reflux. After cooling to room temperature, the wafers were thoroughly washed with toluene and ethanol, dried in an oven at 100 °C for 10 minutes and stored in a desiccator. 2.2.3. Functionalization of pre-prepared VTMS-modified alumina with MPTMS The modified wafers, prepared according to 2.2.2., were immersed in a quartz vial containing a solution of 80 μl MPTMS in 10 ml of methanol. As photoinitiator DMPA (0.1 % by weight) 5
was used. The photochemical reaction was carried out in a UV photoreactor (Intelliray 600, Uvitron) equipped with a metal halide type arc lamp and a filter glass (for wavelengths < 315 nm), operating at 35 % intensity for 600 seconds. The obtained samples (0.24 M VTMS + MPTMS, 0.36 M VTMS + MPTMS and 0.48 M VTMS + MPTMS) were thoroughly washed with methanol, dried in an oven at 70 ºC for 10 minutes and stored in a desiccator. 2.3. Characterization Photoemission spectra of all samples were measured with a SPECS XPS spectrometer equipped with a monochromatized source of Al Kα X-rays of 1486.74 eV and a hemispherical electron analyzer (Phoibos MCD 100). Spectra around the C 1s state were recorded with a pass energy of 10 eV, and spectra around the S and Si 2p core-levels were recorded with a pass energy of 20 eV. The typical pressure in the UHV chamber during the XPS analyses was in the 10-7 Pa range. The spectra were analyzed by the Unifit software [10] and simulated with several sets of mixed Gaussian-Lorentzian functions, while the background was subtracted according to Shirley’s method [11]. The position of the C 1s peak, adjusted to the binding energy of 284.5 eV, was used for the energy calibration. Field Emission Scanning Electron Microscopy (FE-SEM) was performed with a Jeol JSM7800F SEM. The surface morphology was analyzed applying a gentle beam mode with an electron beam acceleration voltage of 0.7 kV and at a working distance of 2 mm. Cross-sectional SEM images were recorded at an acceleration voltage of 10 kV to enhance the contrast of the backscattered electrons. The element distributions on the surface was determined by Energy Dispersive X-Ray Spectroscopy (EDS). The point&id spectra were recorded at a working distance of 10 mm with an electron beam acceleration voltage of either 3 kV or 5 kV. Lower electron beam acceleration voltage was used to minimize scattered signal stemming from the substrate. Contact angles (CA) were measured with a Krüss GmbH contact angle goniometer (DSA30S) using water droplets of 5 μl volume under ambient conditions. The CA values were obtained on as-prepared films from measurements on more than three different positions of the same sample.
3. Results and discussion As a substrate for the two-step silanization process we have chosen ALD-deposited Al2O3 because its growth is highly reproducible, the films are compact with a well-defined 6
composition and thickness [12], and alumina has a high affinity to silanes [6]. A further important criterion is the lack of photo-reactivity, which guarantees that the substrate will not interfere in the UV-initiated photochemical reactions. The characterization of the amorphous 50 nm Al2O3 film on a Si substrate was done by SIMS, FE-SEM, and XPS and is shown in Fig. S1 in the Supplementary material for this manuscript. Our wet chemical modification of the alumina surface consisted of: a) silanization of ALD-deposited alumina with VTMS to functionalize the substrate with photo-reactive vinyl groups, and b) photochemical coupling of MPTMS by inducing a thiol-ene reaction of the surface-bound vinyl groups with the thiol functionalities. For being suitable for the second, photo-promoted reaction step, the substrate should satisfy some important prerequisites that encompass the homogeneous distribution and a quantitative surface coverage of the alumina with VTMS. Therefore, we initially investigated the properties of VTMS-modified Al2O3 prepared from three different concentrations of VTMS (0.24 M, 0.36 M and 0.48 M) while keeping all other reaction parameters constant (5 hours of silanization in refluxing toluene). In all three cases, XPS spectroscopy confirmed a successful silanization of the surfaces. Fig. 1 shows the XPS spectra of samples prepared with 0.24, 0.36 and 0.48 M VTMS (samples 0.24 M VTMS, 0.36 M VTMS and 0.48 M VTMS, correspondingly). In the high-energy resolution spectrum of the Si 2p region a new peak at the binding energy around 103.0 eV appears (Fig. 1a). Its intensity varies with the applied initial concentrations of VTMS, showing the highest surface coverage in the case of 0.48 M VTMS. An intensity trend similar to the Si 2p peaks is also seen in the C 1s region at 285 eV, representing both C=C and C-Si bonds, as shown in Fig. 1b for 0.36 M VTMS and 0.48 M VTMS. A deconvolution shows two additional components that can be assigned to C-O and C=O bonds at binding energies of 286.5 and of 289.0 eV, respectively. Unfortunately, a quantification of the amount of C-O carbons, representing unhydrolyzed Si-OCH3 bonds, and the surface silanization yield, would not be accurate, as the spectra contain contributions of C=O, C-O and C-C, stemming from unavoidable adventitious carbon contamination [13].
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Fig. 1. (a) Si 2p and (b) C 1s XPS core-level spectra of VTMS-modified Al2O3 thin films (0.24 M VTMS, 0.36 M VTMS and 0.48 M VTMS) prepared from the indicated VTMS concentrations. FE-SEM micrographs of VTMS-modified alumina thin films are shown in Fig. 2. The silanized surfaces prepared from the two lower concentrations of VTMS, i.e. 0.24 M VTMS and 0.36 M VTMS, are smooth and do not exhibit apparent inhomogeneities, which otherwise would result from an attachment of polysiloxane structures to the ceramic surface (see Fig. 2a for 0.36 M VTMS). However, the samples prepared from the highest initial concentration of VTMS (0.48 M VTMS) show many randomly distributed agglomerates with sizes ranging from several tens to several hundreds of nanometers (Fig. 2b). In accordance with the literature [14, 15], these irregular morphologies result from uncontrolled silane polymerization and formation of polysiloxane clusters already in solution at higher VTMS concentrations, which, once formed, bind to –OH groups on the Al2O3 surface. The XPS results in Fig. 1 are in good agreement with the morphology analyses as the higher amounts of carbon and silicon reflect thicker deposits in form of polymerized islands. The very heterogeneous surface morphology and thus randomly distributed functionalities make this particular sample not suitable for a subsequent photo-assisted thiol-ene reaction with MPTMS.
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Fig. 2. FE-SEM micrographs of VTMS-modified Al2O3 thin films prepared from 0.36 M (a) and 0.48 M (b) solutions of VTMS (0.36 M VTMS and 0.48 M VTMS, respectively). The inset in (a) shows the FE-SEM image of the corresponding sample in a bird's-eye view. Consequently, the samples 0.24 M VTMS and 0.36 M VTMS were further subjected to photo-promoted coupling of the terminal vinyl groups with MPTMS in presence of DMPA as a photoinitiator. The S 2p XPS spectral region on both of the obtained hybrid films (0.24 M VTMS + MPTMS and 0.36 M VTMS + MPTMS) shows newly appearing peaks, confirming the grafting of MPTMS to the silanized alumina (Fig. 3a). The peak intensity was higher in the case of 0.36 M VTMS + MPTMS, resulting from the larger amount of surface-bound vinyl groups. The characteristic S 2p doublet, peak centered at the binding energy of 163.6 eV, can be assigned to the sulfur atoms in the thioether (-C-S-C-), formed upon the thiol-ene click reaction, free mercapto groups (S-H) and/or sulfur atoms in disulfide bonds (-C-S-S-C-), which may be formed by photoinitiated recombination of two thiyl radicals (R-S˙). A further, lowintensity signal at 168.5 eV shows the presence of oxidized sulfur groups (SOx) resulting from air-promoted oxidation of the thiols and/or irradiative damage caused by the X-rays during the XPS measurements [16-18]. The intensities of the Si 2p core-level peaks (Fig. 3b) indicate a higher surface coverage with silane species after the second modification step in both samples. Moreover, the C 1s spectral deconvolution shows the presence of an additional component at around 285.5 eV, which can be ascribed to C-S bonds (Fig. 3c).
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Fig. 3. (a) S 2p, (b) Si 2p and (c) C 1s XPS core-level spectra of the samples 0.24 M VTMS + MPTMS and 0.36 M VTMS + MPTMS. The substrates were VTMS-modified Al2O3 thin films silanized with the indicated VTMS concentrations. FE-SEM micrographs of the sample 0.36 M VTMS + MPTMS (Fig 4a) exhibit densely distributed surface structures with rather narrow size distribution and an average particle size of cca. 70 nm, some of which are merged (the size distribution is shown in the Supplementary material Fig. S2). The bird’s-eye view (inset in Fig. 4a) reveals that these particles are nearly hemispherical, indicating their growth on the surface (“grafting from”), rather than in solution (“grafting in”), since attachment of already formed nanoparticles to the inorganic substrate would result in spherical shapes at the interfaces between the particles and the substrate. To the best of our knowledge, the formation of hemispherical nanoscaled organosilica structures grown on the surface of inorganic oxides has not been reported yet. In comparison, the sample prepared from a lower initial concentration of VTMS, i.e. 0.24 M VTMS + MPTMS, exhibits similar structures, but with significantly lower density and 10
higher polydispersity of the hemispheres (Fig. 4b). According to XPS measurements shown in Fig. 1 and Fig. 3, this particular sample exhibits a lower and likely incomplete coverage of the alumina surface with vinyl functionalities, indirectly showing a key contribution of the thiolene coupling with -SH groups of MPTMS.
Fig. 4. FE-SEM images of 0.36 M VTMS + MPTMS (a) and 0.24 M VTMS + MPTMS (b), and EDS elemental microanalysis from the area marked with a rectangle shown in the corresponding FE-SEM images for 0.36 M VTMS + MPTMS (c) obtained at 3 kV electron beam acceleration voltage. The insets in (a) and (b) show magnified images of the corresponding samples (upper insets in a bird’s-eye view).
Sample 0.36 M VTMS + MPTMS (Fig. 4a) was chosen for further processing and evaluation of surface chemical properties, because of the highest density and uniformity of nanohemispherical surface structures. An EDS analysis of the surface was performed at a lower electron beam acceleration voltage (3 kV) in order to gain an insight into the composition of the material (Fig. 4c). The elemental analysis showed that Si, C and S, representing the organosilica species, are detected prevalently on the hemispherical particles, suggesting the 11
formation of a much higher amount of hybrid silica species after the chemical surface modification of Al2O3 under the applied two-step reaction. Therefore, we can assume that the interparticle regions on the alumina surface consist mostly of silane monolayers and/or shortchain species, while the nanostructures are mainly composed of polymerized MPTMS moieties. This suggests that besides the thiol-ene coupling reaction between the immobilized vinyl and MPTMS thiol groups, polycondensation reactions, involving methoxysilane groups in neat MPTMS (formation of siloxane -Si-O-Si- bonds), also occur, leaving numerous SH groups intact. The growth of hemispherically shaped particles on the alumina surface is most probably resulting from the nature of MPTMS itself. Namely, under certain reaction conditions, MPTMS has been shown to be an excellent precursor for fabricating thiolated nanoparticles in solution [19-21]. The reported studies on the synthesis of organosilicates in sub-100 nm regimes investigated the growth upon application of both protic (water, water-ethanol) and aprotic solvents (DMSO) in the presence of bases as catalysts. The formation of spherical nanostructures in solution was ascribed to simultaneous disulfide bridging, caused by partial oxidation of the thiol groups, which was induced by oxygen while bubbling with air [19, 20]. In our case, the particles develop on a solid support, without any acid/base catalyst, oxidant or surfactant involved, confirming the high tendency of MPTMS to attach to the vinyl-modified alumina surface. It should be noted that under the applied reaction conditions the supernatant above the modified alumina substrate remained transparent after the reaction with MPTMS. This suggests that dispersed colloidal particles do not form as their presence would result in an increase in turbidity of the solution. Since the surface –OH groups of amorphous Al2O3 are not regioselective for the attachment of silanes, a regioselectivity of the reaction of MPTMS with the functionalized surface in the second silanization step is also not expected. Therefore, the structure directors for the sphere growth must derive from the solution. By combining the results from XPS, SEM and EDS we can postulate that the initial nucleation of oligomeric siloxane-based primary particles from MPTMS occurs in methanol, in which the hydrolysis and self-condensation of silanes generally proceed with a high reaction rate [22]. As thiol-ene reactions between MPTMS and alkenyl groups in the presence of DMPA as photoinitiator are highly efficient and fast [23, 24], the colloidal primary particles diffuse and subsequently stabilize by coupling with the photoreactive vinyl anchoring groups on the alumina surface via a thiol-ene click reaction.
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This assumption is supported by a control experiment in which as-deposited ALD alumina was treated with identical photochemical reaction conditions (80 μl MPTMS in 10 ml of methanol, DMPA, 10 min reaction time). No specific surface features on the alumina substrate were observed, providing further evidence that the thiol-ene reaction between surfacegrafted VTMS and MPTMS plays a key role in the growth of the nanostructures. After grafting to the solid support, the MPTMS oligomer species can eventually act as directors for regioselective growth of nanoparticles through hydrolysis/condensation reactions between free Si-OCH3/Si-OH groups of the grafted MPTMS and Si-OCH3/Si-OH groups of MPTMS in solution. However, as the mechanism of the thiol-ene reaction involves an initial photochemical formation of thiyl radicals (R-S˙), their recombination to disulfide bonds can also occur. The resulting -S-S- bridges could participate in the organization of self-condensing MPTMS into nanohemispherical surface particles, as previously reported in the cases of solution-made thiolated nanoparticles from MPTMS [19-21]. Moreover, individual MPTMS molecules can also diffuse to surface vinyl groups forming simple „clicked“ bilayers with pending Si-OCH3/Si-OH terminal groups, represented by the smooth interparticle regions on the surface of the solid Al2O3 support. The proposed structure of the resulting modified Al2O3 is schematically depicted in Scheme 1.
Scheme 1. Schematic illustration of the proposed surface structure obtained after a two-step approach: silanization of alumina with VTMS, followed by a thiol-ene click reaction with MPTMS. While due to the high reactivity of TMA Al2O3 can be deposited by ALD on almost every inorganic or organic substrate even at low temperatures, DEZ, a precursor used for ALD deposition of ZnO, exhibits higher selectivity towards nucleophilic functional groups on a substrate [25, 26]. Furthermore, zinc ions have a high affinity towards mercapto groups and 13
form zinc-thiolate complexes. This affinity is beneficial for our work as it enables us to evaluate the composition of the nanoparticulate surface. We performed a low-temperature ZnO ALD process onto our hybrid films. In order to assure a complete hydrolysis of potentially remaining unhydrolyzed Si-OCH3 bonds, the hybrid films were initially exposed to a 5 seconds long water vapor pulse, followed by 30 or 200 ALD DEZ/water vapor pulsing cycles. The lower number of pulsing cycles was applied for evaluating the composition of the organic phase by XPS spectroscopy, since a thick ZnO film would prohibit the measurement. The XPS spectrum of the ZnO-coated samples showed the appearance of new peaks at 1022.5 eV, representing photoemission from the Zn 2p3/2 core-level (Fig. 5a). The stronger intensity of the Zn 2p signal of ZnO deposited on 0.36 M VTMS + MPTMS is directly correlated with a higher surface coverage of samples with nanohemispherical structures. When compared to the XPS spectrum of the hybrid sample before ZnO deposition (Fig. 3), numerical fits of the S 2p peaks following the ZnO deposition reveal that, besides the thioether (-CH2-SCH2-) and disulfide (-CH2-S-S-CH2-) S 2p signals at 163.6 eV, a new peak appears at 162.0 eV (Fig. 5b). This peak stems from the zinc thiolate bond (Zn-S) formed after complexation of the mercapto group with zinc. Its intensity is directly correlated with the larger amount of free –SH groups observed in the case of 0.36 M VTMS + MPTMS. Comparing to the samples prior to ZnO deposition, the signal intensity of oxidized sulfur at 168.5 eV has increased, most probably as a consequence of the catalytic activity of ZnO for oxidizing thiol groups [27]. Thus, the XPS measurements confirm that after the two-step chemical modification of Al2O3 the samples contain free nucleophilic -SH groups which react with DEZ to form Zn-S bonds. The presence of free mercapto groups evidently shows that nanohemispherical structures consist prevalently of MPTMS entities polymerized through self-condensation. In Fig. 6, the FE-SEM bird´s-eye view of the ZnO-coated 0.36 M VTMS + MPTMS shows that ZnO is uniformly distributed on the surface with the nanohemispherical structures being preserved (insert in Fig. 6b). No obvious formation of ZnO agglomerates are observed. For comparison, the surface of Al2O3 coated with ZnO under the same ALD processing conditions is shown in Fig. 6a. Further inspection of the FE-SEM features shows an increase of the average particle size on the 0.36 M VTMS + MPTMS sample to approximately 90 nm, which is a consequence of an additional layer of ZnO deposited with 200 ALD cycles (see Fig. S2 in Supplementary material).
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Fig. 5. XPS spectra of Zn 2p3/2 (a), and S 2p core-levels (b) of 0.24 M VTMS + MPTMS and 0.36 M VTMS + MPTMS measured after ALD deposition of ZnO (30 ALD cycles). The EDS analysis provides important data concerning the zinc distribution after the ALD deposition of ZnO on 0.36 M VTMS + MPTMS. The weight percentage of Zn is higher on the surface of the nanoparticles than on the remainder of the surface (Fig. 6c). The surface-exposed functional groups on the nanohemispheres comprise, on the one hand, free –SH functionalities with a high tendency to chelate Zn2+, and on the other hand, free hydroxyl groups in residual silanol moieties (Si-OH), prepared through hydrolysis of Si-OCH3 bonds upon initial pulsing of the water precursor. Moreover, the deposition of ZnO in the interparticle region shows an absence of vinyl groups, since the surface would otherwise be inert towards ZnO growth. This further confirms their consumption during the thiol-ene reaction. The nucleophilic sites on these surface domains most probably derive from hydroxyl groups in Si-OH moieties pending from “clicked“ VTMS-MPTMS bilayers, as proposed in Scheme 1.
15
c
d
Fig. 6. FE-SEM images of the ZnO thin film deposited after 200 ALD DEZ/water pulsing cycles on (a) the neat 50 nm alumina film, and (b) 0.36 M VTMS + MPTMS. The inset in (b) shows a magnified image of the corresponding sample in a bird’s-eye view. (c) EDS elemental microanalysis of the areas marked with a rectangle shown in the corresponding FE-SEM image of 0.36 M VTMS + MPTMS, obtained at 5 kV electron beam acceleration voltage. Due to the presence of hydroxyl groups on its surface, zinc oxide is intrinsically hydrophilic. However, ZnO can be used to prepare hydrophobic surfaces by creating hierarchical nano/micro morphologies. Because the wettability of a surface strongly depends on its functional groups and its surface roughness [28], we performed water contact angle measurements (CA) before and after the growth of ZnO. The as-prepared alumina substrate was used as reference and exhibited a moderately hydrophilic surface with a CA of 50°, typical of ALD grown Al2O3 films (Fig. 7a) [11]. This is directly correlated with the amount of hydroxyl groups present on the surface of the metal oxide, 16
which amount to approx. 15 % (as estimated by XPS, see Supplementary material). However, after the ALD deposition of ZnO on Al2O3, the surface roughness increases on the nanometer scale. Consequently, the CA increases to 53° showing a slight decrease in water wettability as can be seen in Fig. 7b [29]. After treatment with VTMS, the alumina surface becomes significantly more hydrophobic, exhibiting a CA of 84°, which is due to the terminal vinyl groups (Fig. 7c). This result is consistent with the reported literature data of smooth Si substrates modified with VTMS from the gas phase (CA 86o) [30]. With a CA of 62° the hybrid sample after the twostep silanization process is more hydrophilic than the VTMS-modified alumina as a consequence of the larger number of –OH groups originating from the bound silanol species (Fig. 7d). Ultimately, after the deposition of ZnO, the CA increases significantly to 98° (Fig. 7e), highlighting the role of the surface roughness of ZnO in a substrate’s wetting behavior. Namely, a roughening on two hierarchical scales, i.e. hemispheres of several tens of nanometers and the corrugation on the nanometer scale with ZnO, significantly increases the hydrophobic character of the surface [31, 32].
Fig. 7. Water contact angle measurements (CA) of neat 50 nm Al2O3 film (a), ZnO thin film deposited after 200 ALD DEZ/water pulsing cycles on Al2O3 (b), 0.36 M VTMS (c), 0.36 M VTMS + MPTMS before (d) and after coating with ZnO, deposited after 200 ALD DEZ /water pulsing cycles (e).
17
Conclusions In summary, this article reports on the formation of novel thiolated hemispherical nanoscaled organosilica structures grown on ALD-deposited Al2O3 thin films. The synthesis is based on a two-step approach consisting of initial silanization of Al2O3 with vinyltrimethoxysilane (VTMS), followed by a photo-promoted thiol-ene click reaction with (3mercaptopropyl)trimethoxysilane (MPTMS). When compared to the existing literature on the wet chemical synthesis of nanoparticles from MPTMS [19-21], the developed method exploits the high tendency of MPTMS to bind to the vinyl-modified substrate without an additional acid/base catalyst, oxidant or surfactant involved. Characterization by XPS and FE-SEM confirms that the formation of nanohemispheres at the surface depends on the initial concentration of VTMS applied in the first modification step, which determines the density and dispersion of the grafted vinyl groups. Elemental analysis by EDS showed that the presence of sulfur and silicon moieties prevails on the hemispheres, suggesting that the interparticle regions on alumina contain silane monolayers and/or short-chain species, while the nanospherical structures mostly consist of polymerized MPTMS moieties. The formation of zinc-thiolate (Zn-S) bonds after the subsequent ALD deposition of ZnO confirms a substantial amount of free thiol groups on the surface of the nanohemispheres. Moreover, the ZnO-coated nanoscaled hierarchical structures exhibit higher hydrophobicity when compared to ZnO films deposited directly on Al2O3 under the same ALD processing conditions. We hypothesize that during the initial (nucleation) stage of the second modification reaction with MPTMS two consecutive reactions occur: the formation of oligomeric siloxanebased seeds by rapid self-condensation reactions in solution, and photo-promoted oxidation of MPTMS thiols into disulfides. The further evolution of surface-bound nanohemispheres is ascribed to an initial attachment of MPTMS seeds to the vinyl-modified surface via photoassisted thiol-ene chemistry, followed by a “classical” growth through hydrolysis/condensation reactions between free Si-OCH3/Si-OH groups of grafted MPTMS, and Si-OCH3/Si-OH groups of MPTMS molecules diffusing from solution. The silanized thin-films with spatially-distributed free thiol groups opt for applications in bioimaging or biosensing of thiol-carrying biomolecules through selective and localized formation of disulfide bridges, which will be investigated in forthcoming research.
18
Acknowledgements This work has been mainly supported by Croatian Science Foundation under the project IP2016-06-3568, and in part by the University of Rijeka under the project numbers 16.12.2.1.01 and 12.12.1.1.01. The characterization instruments applied in this work were acquired through the European Fund for Regional Development and Ministry of Science, Education and Sports of the Republic of Croatia under the project Research Infrastructure for Campus-based Laboratories at the University of Rijeka (grant number RC.2.2.06-0001). Mato Knez achnowledges support by the Spanish Ministry of Economy and Competitiveness (MINECO) [GA No. MAT2016-77393-R], including FEDER funds.“
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Figure captions: Fig. 1. (a) Si 2p and (b) C 1s XPS core-level spectra of VTMS-modified Al2O3 thin films (0.24 M VTMS, 0.36 M VTMS and 0.48 M VTMS) prepared from the indicated VTMS concentrations.
Fig. 2. FE-SEM micrographs of VTMS-modified Al2O3 thin films prepared from 0.36 M (a) and 0.48 M (b) solutions of VTMS (0.36 M VTMS and 0.48 M VTMS, respectively). The inset in (a) shows the FE-SEM image of the corresponding sample in a bird's-eye view.
Fig. 3. (a) S 2p, (b) Si 2p and (c) C 1s XPS core-level spectra of the samples 0.24 M VTMS + MPTMS and 0.36 M VTMS + MPTMS. The substrates were VTMS-modified Al2O3 thin films silanized with the indicated VTMS concentrations.
Fig. 4. FE-SEM images of 0.36 M VTMS + MPTMS (a) and 0.24 M VTMS + MPTMS (b), and EDS elemental microanalysis from the area marked with a rectangle shown in the corresponding FE-SEM images for 0.36 M VTMS + MPTMS (c) obtained at 3 kV electron beam acceleration voltage. The insets in (a) and (b) show magnified images of the corresponding samples (upper insets in a bird’s-eye view).
Scheme 1. Schematic illustration of the proposed surface structure obtained after a two-step approach: silanization of alumina with VTMS, followed by a thiol-ene click reaction with MPTMS.
Fig. 5. XPS spectra of Zn 2p3/2 (a), and S 2p core-levels (b) of 0.24 M VTMS + MPTMS and 0.36 M VTMS + MPTMS measured after ALD deposition of ZnO (30 ALD cycles). Fig. 6. FE-SEM images of the ZnO thin film deposited after 200 ALD DEZ/water pulsing cycles on (a) the neat 50 nm alumina film, and (b) 0.36 M VTMS + MPTMS. The inset in (b) shows a magnified image of the corresponding sample in a bird’s-eye view.
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(c) EDS elemental microanalysis of the areas marked with a rectangle shown in the corresponding FE-SEM image of 0.36 M VTMS + MPTMS, obtained at 5 kV electron beam acceleration voltage.
Fig. 7. Water contact angle measurements (CA) of neat 50 nm Al2O3 film (a), ZnO thin film deposited after 200 ALD DEZ/water pulsing cycles on Al2O3 (b), 0.36 M VTMS (c), 0.36 M VTMS + MPTMS before (d) and after coating with ZnO, deposited after 200 ALD DEZ /water pulsing cycles (e).
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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S0021-9797(19)31261-5 https://doi.org/10.1016/j.jcis.2019.10.074 YJCIS 25568
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
31 July 2019 14 October 2019 19 October 2019
Please cite this article as: G. Ambrožić, M. Kolympadi Markovic, R. Peter, I. Kavre Piltaver, I. Jelovica Badovinac, D. Čakara, D. Marković, M. Knez, Building organosilica hybrid nanohemispheres via thiol-ene click reaction on alumina thin films deposited by atomic layer deposition (ALD), Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.10.074
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Building organosilica hybrid nanohemispheres via thiol-ene click reaction on alumina thin films deposited by atomic layer deposition (ALD)
Gabriela Ambrožić,a,b* Maria Kolympadi Markovic,a,b Robert Peter,a,b Ivna Kavre Piltaver,a,b Ivana Jelovica Badovinac,a,b Duško Čakara,b,c Dean Marković,c Mato Knez d,e aDepartment bCentre
of Physics, University of Rijeka, Radmile Matejčić 2, 51000 Rijeka, Croatia
for Micro- and Nanosciences and Technologies, University of Rijeka, Radmile
Matejčić 2, 51000 Rijeka, Croatia cDepartment
of Biotechnology, University of Rijeka, Radmile Matejčić 2, 51000 Rijeka,
Croatia dCIC
nanoGUNE, 20018 San Sebastian, Spain
eIKERBASQUE,
Basque Foundation for Science, 48013 Bilbao, Spain
*E-mail: [email protected] Phone: (+385)51-584-632
1
Abstract The present work shows a surface-induced preparation of sub-100 nm organosilica nanohemispheres on atomic layer deposited (ALD) Al2O3 thin films, which was achieved by cooperative condensation/hydrolysis and thiol-ene click chemical reactions. The two-step synthetic approach consists of an initial silanization of the Al2O3 film with vinyltrimethoxysilane (VTMS), followed by a photo-promoted growth of surface-bound nanoparticles
in
the
presence
of
(3-mercaptopropyl)trimethoxysilane
(MPTMS).
Characterization by means of FE-SEM, XPS and EDS points towards the growth of the nanohemispherical structures being governed by an initial nucleation of thiolated organosilica seeds in solution as a result of self-condensation of MPTMS and oxidation of thiols to disulfides. Once bound to the vinyl terminated Al2O3 via photo-assisted thiol-ene coupling, these seeds promote area-selective growth of the nanoparticles through binding of further MPTMS from the solution. After an additional ALD deposition of ZnO, the resulting thin hybrid film exhibits enhanced hydrophobicity when compared to ZnO films deposited directly on Al2O3 under the same processing conditions. Keywords: organosilica hybrids, nanohemispheres, thiol-ene reaction, thin films, atomic layer deposition, Al2O3, ZnO.
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1. Introduction Functionalization of substrates with hybrid nanoscale films is a rapidly growing research area in materials chemistry. Such films allow altering the functionalities of a material without considerable modification of the substrate itself, thus expanding the functionalities of the substrate. Such functional films are currently investigated for applications in optics [1], catalysis [2], energy storage [3], conversion and saving [4], as well as in environmental research and biotechnology [5]. The chemical or physical properties of nanoscaled hybrid materials often result from synergies between the constituent organic and inorganic parts. The compatibility and interplay of the two involved phases are tailored by the nature of the interface and the applied synthetic methodology, both of which define the structural and chemical composition as well as the properties of the resulting hybrid. Understanding the molecular interactions, the structures of the organic and inorganic components, and their degrees of organization are crucial for the development of high-performance materials. Silanes are among the most commonly used reagents for surface modification. Their grafting on a solid substrate typically relies on the formation of covalent bonds between the silane and terminal OH functionalities of a substrate [6]. The chemical versatility of available silanes results in a great diversity of surface properties that hybrid thin films can exhibit. However, using silanes with more than one leaving group can easily result in loss of the control over the reaction course, since secondary reactions such as crosslinking of the silanes upon hydrolysis and condensation may occur either on the surface or in solution, or both. The reaction conditions, i.e. the solvent viscosity and polarity, the reaction temperature, the amount of water present in the reaction mixture, the presence of acid or base catalysts and surfactants, and the silane concentration, are of crucial importance for tailoring both the surface morphology and structure. Consequently, a surface modification is not restricted to the formation of a monolayer only, but may also involve formation of multilayered structures, or deposition of oligomer/polymer films at the inorganic surface [6]. Heterofunctional silanes, containing additional reactive end-groups, have been used as versatile anchors for organic post-functionalization by linking active molecules, polymers or catalysts to solid matrices [7]. In that regard, click chemistry has become one of the most elegant chemical pathways for imparting new functionalities on a pre-existing hybrid support, as the reactions are highly selective, simple to perform and greatly variable. The thiol-ene free-radical reaction has all the desirable features of a click reaction, i.e., high efficiency, high reaction rates,
3
mild conditions, oxygen and water insensitivity, etc., making it a powerful and versatile methodology for surface modification and patterning [8]. Besides carefully controlling the chemical conditions in solution, the composition and surface properties of the inorganic substrate as constituent part of the hybrid material have also to be considered. As the properties of nanoscaled metal oxide films strongly depend on their purity, crystallinity and thickness, it is crucial that the substrates to be chemically modified are prepared in a reproducible way. Among the variety of available vapor phase deposition techniques, atomic layer deposition (ALD) is the method-of-choice for depositing conformal nanoscaled metal oxide thin films with controlled thickness, crystallinity and composition. This is attributed to the self-saturating nature of ALD processes based on a sequential exposure of a substrate to organometallic precursors and counter-precursors. Here, we report on the first catalyst-free formation of organosilica hybrid nanohemispheres with terminal mercaptopropyl functional groups, grown on ALD-deposited Al2O3
thin
films.
The
nanohemispheres
were
synthesized
via
cooperative
condensation/hydrolysis and thiol-ene reactions of heterobifunctional silanes. The process is based on a two-step approach consisting of an initial silanization of a 50 nm thick ALDdeposited Al2O3 film with vinyltrimethoxysilane (VTMS), followed by a photo-promoted thiolene click reaction with (3-mercaptopropyl)trimethoxysilane (MPTMS). As thiol groups have strong ability to bind to many transition metals, an ALD deposition of ZnO was subsequently applied and the surface-chemical properties and wetting behavior of the obtained hybrid films were investigated. The fabricated materials offer a great potential for the development of hierarchically structured silanized thin-films for bioimaging and biosensing of thiol-carrying biomolecules through the formation of disulfide bridges [9].
2. Experimental section 2.1. Materials (3-Mercaptopropyl)trimethoxysilane (MPTMS, 95 %), vinyltrimethoxysilane (VTMS, 98 %) 2,2-dimethoxy-2-phenyl-acetophenon (DMPA, 95 %) were purchased from Aldrich, toluene (puriss p.a.), was obtained from Sigma-Aldrich, while methanol (99.8 %, for HPLC) was purchased from Acros Organics. Double-side polished silicon wafers from Semiconductor Wafer, Inc. (SWI) were used as substrates for the ALD deposition of Al2O3. The ALD precursors diethylzinc (DEZ, ≥ 95 %) and trimethylaluminum (TMA, ≥ 98 %) were purchased from Strem Chemicals. All chemicals were used as received. 4
2.2. Synthesis of materials 2.2.1. ALD processing Atomic layer deposition was carried out using a Beneq TFS-200 reactor with nitrogen (purity 6.0) as carrier and purging gas. For the deposition of 50 nm Al2O3 film on Si substrates (0.70 cm x 0.70 cm), 425 ALD cycles were applied at 200 ºC using TMA and purified water as precursors. One ALD cycle consisted of 180 ms TMA pulsing, 1 s N2 purging, 180 ms water pulsing and 1 s N2 purging. The thickness of the alumina film was verified by SIMS and from FE-SEM images taken from cross-sectioned samples (see Fig. S1. in the Supplementary material). After ALD processing, the substrates were immediately transferred and stored in a desiccator under vacuum before use for further processing. Deposition of ZnO on the hybrid thin film substrates (30 or 200 ALD cycles) was performed at 50 ºC using DEZ and purified water as precursors. For complete hydrolysis of potentially remaining alkoxy silane groups, an initial water pulse was applied for 5 s, followed by 25 s of N2 purge. One ALD cycle consisted of 200 ms DEZ pulsing, 5 s N2 purging, 180 ms water pulsing and 25 s N2 purging. The comparatively long purging times were needed to ensure an efficient removal of potentially physisorbed precursors and reaction by-products from the substrate at the applied reaction temperature. After ALD processing, the substrates were immediately transferred and stored in a desiccator under vacuum. 2.2.2. Functionalization of ALD-deposited alumina thin films with VTMS Five Si wafer pieces (0.70 cm x 0.70 cm), coated with 50 nm of Al2O3, were immersed in solutions of VTMS in toluene with following concentrations: 0.24 M (sample: 0.24 M VTMS), 0.36 M (sample: 0.36 M VTMS) and 0.48 M (sample: 0.48 M VTMS), respectively, and then heated for five hours while gently stirring under reflux. After cooling to room temperature, the wafers were thoroughly washed with toluene and ethanol, dried in an oven at 100 °C for 10 minutes and stored in a desiccator. 2.2.3. Functionalization of pre-prepared VTMS-modified alumina with MPTMS The modified wafers, prepared according to 2.2.2., were immersed in a quartz vial containing a solution of 80 μl MPTMS in 10 ml of methanol. As photoinitiator DMPA (0.1 % by weight) 5
was used. The photochemical reaction was carried out in a UV photoreactor (Intelliray 600, Uvitron) equipped with a metal halide type arc lamp and a filter glass (for wavelengths < 315 nm), operating at 35 % intensity for 600 seconds. The obtained samples (0.24 M VTMS + MPTMS, 0.36 M VTMS + MPTMS and 0.48 M VTMS + MPTMS) were thoroughly washed with methanol, dried in an oven at 70 ºC for 10 minutes and stored in a desiccator. 2.3. Characterization Photoemission spectra of all samples were measured with a SPECS XPS spectrometer equipped with a monochromatized source of Al Kα X-rays of 1486.74 eV and a hemispherical electron analyzer (Phoibos MCD 100). Spectra around the C 1s state were recorded with a pass energy of 10 eV, and spectra around the S and Si 2p core-levels were recorded with a pass energy of 20 eV. The typical pressure in the UHV chamber during the XPS analyses was in the 10-7 Pa range. The spectra were analyzed by the Unifit software [10] and simulated with several sets of mixed Gaussian-Lorentzian functions, while the background was subtracted according to Shirley’s method [11]. The position of the C 1s peak, adjusted to the binding energy of 284.5 eV, was used for the energy calibration. Field Emission Scanning Electron Microscopy (FE-SEM) was performed with a Jeol JSM7800F SEM. The surface morphology was analyzed applying a gentle beam mode with an electron beam acceleration voltage of 0.7 kV and at a working distance of 2 mm. Cross-sectional SEM images were recorded at an acceleration voltage of 10 kV to enhance the contrast of the backscattered electrons. The element distributions on the surface was determined by Energy Dispersive X-Ray Spectroscopy (EDS). The point&id spectra were recorded at a working distance of 10 mm with an electron beam acceleration voltage of either 3 kV or 5 kV. Lower electron beam acceleration voltage was used to minimize scattered signal stemming from the substrate. Contact angles (CA) were measured with a Krüss GmbH contact angle goniometer (DSA30S) using water droplets of 5 μl volume under ambient conditions. The CA values were obtained on as-prepared films from measurements on more than three different positions of the same sample.
3. Results and discussion As a substrate for the two-step silanization process we have chosen ALD-deposited Al2O3 because its growth is highly reproducible, the films are compact with a well-defined 6
composition and thickness [12], and alumina has a high affinity to silanes [6]. A further important criterion is the lack of photo-reactivity, which guarantees that the substrate will not interfere in the UV-initiated photochemical reactions. The characterization of the amorphous 50 nm Al2O3 film on a Si substrate was done by SIMS, FE-SEM, and XPS and is shown in Fig. S1 in the Supplementary material for this manuscript. Our wet chemical modification of the alumina surface consisted of: a) silanization of ALD-deposited alumina with VTMS to functionalize the substrate with photo-reactive vinyl groups, and b) photochemical coupling of MPTMS by inducing a thiol-ene reaction of the surface-bound vinyl groups with the thiol functionalities. For being suitable for the second, photo-promoted reaction step, the substrate should satisfy some important prerequisites that encompass the homogeneous distribution and a quantitative surface coverage of the alumina with VTMS. Therefore, we initially investigated the properties of VTMS-modified Al2O3 prepared from three different concentrations of VTMS (0.24 M, 0.36 M and 0.48 M) while keeping all other reaction parameters constant (5 hours of silanization in refluxing toluene). In all three cases, XPS spectroscopy confirmed a successful silanization of the surfaces. Fig. 1 shows the XPS spectra of samples prepared with 0.24, 0.36 and 0.48 M VTMS (samples 0.24 M VTMS, 0.36 M VTMS and 0.48 M VTMS, correspondingly). In the high-energy resolution spectrum of the Si 2p region a new peak at the binding energy around 103.0 eV appears (Fig. 1a). Its intensity varies with the applied initial concentrations of VTMS, showing the highest surface coverage in the case of 0.48 M VTMS. An intensity trend similar to the Si 2p peaks is also seen in the C 1s region at 285 eV, representing both C=C and C-Si bonds, as shown in Fig. 1b for 0.36 M VTMS and 0.48 M VTMS. A deconvolution shows two additional components that can be assigned to C-O and C=O bonds at binding energies of 286.5 and of 289.0 eV, respectively. Unfortunately, a quantification of the amount of C-O carbons, representing unhydrolyzed Si-OCH3 bonds, and the surface silanization yield, would not be accurate, as the spectra contain contributions of C=O, C-O and C-C, stemming from unavoidable adventitious carbon contamination [13].
7
Fig. 1. (a) Si 2p and (b) C 1s XPS core-level spectra of VTMS-modified Al2O3 thin films (0.24 M VTMS, 0.36 M VTMS and 0.48 M VTMS) prepared from the indicated VTMS concentrations. FE-SEM micrographs of VTMS-modified alumina thin films are shown in Fig. 2. The silanized surfaces prepared from the two lower concentrations of VTMS, i.e. 0.24 M VTMS and 0.36 M VTMS, are smooth and do not exhibit apparent inhomogeneities, which otherwise would result from an attachment of polysiloxane structures to the ceramic surface (see Fig. 2a for 0.36 M VTMS). However, the samples prepared from the highest initial concentration of VTMS (0.48 M VTMS) show many randomly distributed agglomerates with sizes ranging from several tens to several hundreds of nanometers (Fig. 2b). In accordance with the literature [14, 15], these irregular morphologies result from uncontrolled silane polymerization and formation of polysiloxane clusters already in solution at higher VTMS concentrations, which, once formed, bind to –OH groups on the Al2O3 surface. The XPS results in Fig. 1 are in good agreement with the morphology analyses as the higher amounts of carbon and silicon reflect thicker deposits in form of polymerized islands. The very heterogeneous surface morphology and thus randomly distributed functionalities make this particular sample not suitable for a subsequent photo-assisted thiol-ene reaction with MPTMS.
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Fig. 2. FE-SEM micrographs of VTMS-modified Al2O3 thin films prepared from 0.36 M (a) and 0.48 M (b) solutions of VTMS (0.36 M VTMS and 0.48 M VTMS, respectively). The inset in (a) shows the FE-SEM image of the corresponding sample in a bird's-eye view. Consequently, the samples 0.24 M VTMS and 0.36 M VTMS were further subjected to photo-promoted coupling of the terminal vinyl groups with MPTMS in presence of DMPA as a photoinitiator. The S 2p XPS spectral region on both of the obtained hybrid films (0.24 M VTMS + MPTMS and 0.36 M VTMS + MPTMS) shows newly appearing peaks, confirming the grafting of MPTMS to the silanized alumina (Fig. 3a). The peak intensity was higher in the case of 0.36 M VTMS + MPTMS, resulting from the larger amount of surface-bound vinyl groups. The characteristic S 2p doublet, peak centered at the binding energy of 163.6 eV, can be assigned to the sulfur atoms in the thioether (-C-S-C-), formed upon the thiol-ene click reaction, free mercapto groups (S-H) and/or sulfur atoms in disulfide bonds (-C-S-S-C-), which may be formed by photoinitiated recombination of two thiyl radicals (R-S˙). A further, lowintensity signal at 168.5 eV shows the presence of oxidized sulfur groups (SOx) resulting from air-promoted oxidation of the thiols and/or irradiative damage caused by the X-rays during the XPS measurements [16-18]. The intensities of the Si 2p core-level peaks (Fig. 3b) indicate a higher surface coverage with silane species after the second modification step in both samples. Moreover, the C 1s spectral deconvolution shows the presence of an additional component at around 285.5 eV, which can be ascribed to C-S bonds (Fig. 3c).
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Fig. 3. (a) S 2p, (b) Si 2p and (c) C 1s XPS core-level spectra of the samples 0.24 M VTMS + MPTMS and 0.36 M VTMS + MPTMS. The substrates were VTMS-modified Al2O3 thin films silanized with the indicated VTMS concentrations. FE-SEM micrographs of the sample 0.36 M VTMS + MPTMS (Fig 4a) exhibit densely distributed surface structures with rather narrow size distribution and an average particle size of cca. 70 nm, some of which are merged (the size distribution is shown in the Supplementary material Fig. S2). The bird’s-eye view (inset in Fig. 4a) reveals that these particles are nearly hemispherical, indicating their growth on the surface (“grafting from”), rather than in solution (“grafting in”), since attachment of already formed nanoparticles to the inorganic substrate would result in spherical shapes at the interfaces between the particles and the substrate. To the best of our knowledge, the formation of hemispherical nanoscaled organosilica structures grown on the surface of inorganic oxides has not been reported yet. In comparison, the sample prepared from a lower initial concentration of VTMS, i.e. 0.24 M VTMS + MPTMS, exhibits similar structures, but with significantly lower density and 10
higher polydispersity of the hemispheres (Fig. 4b). According to XPS measurements shown in Fig. 1 and Fig. 3, this particular sample exhibits a lower and likely incomplete coverage of the alumina surface with vinyl functionalities, indirectly showing a key contribution of the thiolene coupling with -SH groups of MPTMS.
Fig. 4. FE-SEM images of 0.36 M VTMS + MPTMS (a) and 0.24 M VTMS + MPTMS (b), and EDS elemental microanalysis from the area marked with a rectangle shown in the corresponding FE-SEM images for 0.36 M VTMS + MPTMS (c) obtained at 3 kV electron beam acceleration voltage. The insets in (a) and (b) show magnified images of the corresponding samples (upper insets in a bird’s-eye view).
Sample 0.36 M VTMS + MPTMS (Fig. 4a) was chosen for further processing and evaluation of surface chemical properties, because of the highest density and uniformity of nanohemispherical surface structures. An EDS analysis of the surface was performed at a lower electron beam acceleration voltage (3 kV) in order to gain an insight into the composition of the material (Fig. 4c). The elemental analysis showed that Si, C and S, representing the organosilica species, are detected prevalently on the hemispherical particles, suggesting the 11
formation of a much higher amount of hybrid silica species after the chemical surface modification of Al2O3 under the applied two-step reaction. Therefore, we can assume that the interparticle regions on the alumina surface consist mostly of silane monolayers and/or shortchain species, while the nanostructures are mainly composed of polymerized MPTMS moieties. This suggests that besides the thiol-ene coupling reaction between the immobilized vinyl and MPTMS thiol groups, polycondensation reactions, involving methoxysilane groups in neat MPTMS (formation of siloxane -Si-O-Si- bonds), also occur, leaving numerous SH groups intact. The growth of hemispherically shaped particles on the alumina surface is most probably resulting from the nature of MPTMS itself. Namely, under certain reaction conditions, MPTMS has been shown to be an excellent precursor for fabricating thiolated nanoparticles in solution [19-21]. The reported studies on the synthesis of organosilicates in sub-100 nm regimes investigated the growth upon application of both protic (water, water-ethanol) and aprotic solvents (DMSO) in the presence of bases as catalysts. The formation of spherical nanostructures in solution was ascribed to simultaneous disulfide bridging, caused by partial oxidation of the thiol groups, which was induced by oxygen while bubbling with air [19, 20]. In our case, the particles develop on a solid support, without any acid/base catalyst, oxidant or surfactant involved, confirming the high tendency of MPTMS to attach to the vinyl-modified alumina surface. It should be noted that under the applied reaction conditions the supernatant above the modified alumina substrate remained transparent after the reaction with MPTMS. This suggests that dispersed colloidal particles do not form as their presence would result in an increase in turbidity of the solution. Since the surface –OH groups of amorphous Al2O3 are not regioselective for the attachment of silanes, a regioselectivity of the reaction of MPTMS with the functionalized surface in the second silanization step is also not expected. Therefore, the structure directors for the sphere growth must derive from the solution. By combining the results from XPS, SEM and EDS we can postulate that the initial nucleation of oligomeric siloxane-based primary particles from MPTMS occurs in methanol, in which the hydrolysis and self-condensation of silanes generally proceed with a high reaction rate [22]. As thiol-ene reactions between MPTMS and alkenyl groups in the presence of DMPA as photoinitiator are highly efficient and fast [23, 24], the colloidal primary particles diffuse and subsequently stabilize by coupling with the photoreactive vinyl anchoring groups on the alumina surface via a thiol-ene click reaction.
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This assumption is supported by a control experiment in which as-deposited ALD alumina was treated with identical photochemical reaction conditions (80 μl MPTMS in 10 ml of methanol, DMPA, 10 min reaction time). No specific surface features on the alumina substrate were observed, providing further evidence that the thiol-ene reaction between surfacegrafted VTMS and MPTMS plays a key role in the growth of the nanostructures. After grafting to the solid support, the MPTMS oligomer species can eventually act as directors for regioselective growth of nanoparticles through hydrolysis/condensation reactions between free Si-OCH3/Si-OH groups of the grafted MPTMS and Si-OCH3/Si-OH groups of MPTMS in solution. However, as the mechanism of the thiol-ene reaction involves an initial photochemical formation of thiyl radicals (R-S˙), their recombination to disulfide bonds can also occur. The resulting -S-S- bridges could participate in the organization of self-condensing MPTMS into nanohemispherical surface particles, as previously reported in the cases of solution-made thiolated nanoparticles from MPTMS [19-21]. Moreover, individual MPTMS molecules can also diffuse to surface vinyl groups forming simple „clicked“ bilayers with pending Si-OCH3/Si-OH terminal groups, represented by the smooth interparticle regions on the surface of the solid Al2O3 support. The proposed structure of the resulting modified Al2O3 is schematically depicted in Scheme 1.
Scheme 1. Schematic illustration of the proposed surface structure obtained after a two-step approach: silanization of alumina with VTMS, followed by a thiol-ene click reaction with MPTMS. While due to the high reactivity of TMA Al2O3 can be deposited by ALD on almost every inorganic or organic substrate even at low temperatures, DEZ, a precursor used for ALD deposition of ZnO, exhibits higher selectivity towards nucleophilic functional groups on a substrate [25, 26]. Furthermore, zinc ions have a high affinity towards mercapto groups and 13
form zinc-thiolate complexes. This affinity is beneficial for our work as it enables us to evaluate the composition of the nanoparticulate surface. We performed a low-temperature ZnO ALD process onto our hybrid films. In order to assure a complete hydrolysis of potentially remaining unhydrolyzed Si-OCH3 bonds, the hybrid films were initially exposed to a 5 seconds long water vapor pulse, followed by 30 or 200 ALD DEZ/water vapor pulsing cycles. The lower number of pulsing cycles was applied for evaluating the composition of the organic phase by XPS spectroscopy, since a thick ZnO film would prohibit the measurement. The XPS spectrum of the ZnO-coated samples showed the appearance of new peaks at 1022.5 eV, representing photoemission from the Zn 2p3/2 core-level (Fig. 5a). The stronger intensity of the Zn 2p signal of ZnO deposited on 0.36 M VTMS + MPTMS is directly correlated with a higher surface coverage of samples with nanohemispherical structures. When compared to the XPS spectrum of the hybrid sample before ZnO deposition (Fig. 3), numerical fits of the S 2p peaks following the ZnO deposition reveal that, besides the thioether (-CH2-SCH2-) and disulfide (-CH2-S-S-CH2-) S 2p signals at 163.6 eV, a new peak appears at 162.0 eV (Fig. 5b). This peak stems from the zinc thiolate bond (Zn-S) formed after complexation of the mercapto group with zinc. Its intensity is directly correlated with the larger amount of free –SH groups observed in the case of 0.36 M VTMS + MPTMS. Comparing to the samples prior to ZnO deposition, the signal intensity of oxidized sulfur at 168.5 eV has increased, most probably as a consequence of the catalytic activity of ZnO for oxidizing thiol groups [27]. Thus, the XPS measurements confirm that after the two-step chemical modification of Al2O3 the samples contain free nucleophilic -SH groups which react with DEZ to form Zn-S bonds. The presence of free mercapto groups evidently shows that nanohemispherical structures consist prevalently of MPTMS entities polymerized through self-condensation. In Fig. 6, the FE-SEM bird´s-eye view of the ZnO-coated 0.36 M VTMS + MPTMS shows that ZnO is uniformly distributed on the surface with the nanohemispherical structures being preserved (insert in Fig. 6b). No obvious formation of ZnO agglomerates are observed. For comparison, the surface of Al2O3 coated with ZnO under the same ALD processing conditions is shown in Fig. 6a. Further inspection of the FE-SEM features shows an increase of the average particle size on the 0.36 M VTMS + MPTMS sample to approximately 90 nm, which is a consequence of an additional layer of ZnO deposited with 200 ALD cycles (see Fig. S2 in Supplementary material).
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Fig. 5. XPS spectra of Zn 2p3/2 (a), and S 2p core-levels (b) of 0.24 M VTMS + MPTMS and 0.36 M VTMS + MPTMS measured after ALD deposition of ZnO (30 ALD cycles). The EDS analysis provides important data concerning the zinc distribution after the ALD deposition of ZnO on 0.36 M VTMS + MPTMS. The weight percentage of Zn is higher on the surface of the nanoparticles than on the remainder of the surface (Fig. 6c). The surface-exposed functional groups on the nanohemispheres comprise, on the one hand, free –SH functionalities with a high tendency to chelate Zn2+, and on the other hand, free hydroxyl groups in residual silanol moieties (Si-OH), prepared through hydrolysis of Si-OCH3 bonds upon initial pulsing of the water precursor. Moreover, the deposition of ZnO in the interparticle region shows an absence of vinyl groups, since the surface would otherwise be inert towards ZnO growth. This further confirms their consumption during the thiol-ene reaction. The nucleophilic sites on these surface domains most probably derive from hydroxyl groups in Si-OH moieties pending from “clicked“ VTMS-MPTMS bilayers, as proposed in Scheme 1.
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c
d
Fig. 6. FE-SEM images of the ZnO thin film deposited after 200 ALD DEZ/water pulsing cycles on (a) the neat 50 nm alumina film, and (b) 0.36 M VTMS + MPTMS. The inset in (b) shows a magnified image of the corresponding sample in a bird’s-eye view. (c) EDS elemental microanalysis of the areas marked with a rectangle shown in the corresponding FE-SEM image of 0.36 M VTMS + MPTMS, obtained at 5 kV electron beam acceleration voltage. Due to the presence of hydroxyl groups on its surface, zinc oxide is intrinsically hydrophilic. However, ZnO can be used to prepare hydrophobic surfaces by creating hierarchical nano/micro morphologies. Because the wettability of a surface strongly depends on its functional groups and its surface roughness [28], we performed water contact angle measurements (CA) before and after the growth of ZnO. The as-prepared alumina substrate was used as reference and exhibited a moderately hydrophilic surface with a CA of 50°, typical of ALD grown Al2O3 films (Fig. 7a) [11]. This is directly correlated with the amount of hydroxyl groups present on the surface of the metal oxide, 16
which amount to approx. 15 % (as estimated by XPS, see Supplementary material). However, after the ALD deposition of ZnO on Al2O3, the surface roughness increases on the nanometer scale. Consequently, the CA increases to 53° showing a slight decrease in water wettability as can be seen in Fig. 7b [29]. After treatment with VTMS, the alumina surface becomes significantly more hydrophobic, exhibiting a CA of 84°, which is due to the terminal vinyl groups (Fig. 7c). This result is consistent with the reported literature data of smooth Si substrates modified with VTMS from the gas phase (CA 86o) [30]. With a CA of 62° the hybrid sample after the twostep silanization process is more hydrophilic than the VTMS-modified alumina as a consequence of the larger number of –OH groups originating from the bound silanol species (Fig. 7d). Ultimately, after the deposition of ZnO, the CA increases significantly to 98° (Fig. 7e), highlighting the role of the surface roughness of ZnO in a substrate’s wetting behavior. Namely, a roughening on two hierarchical scales, i.e. hemispheres of several tens of nanometers and the corrugation on the nanometer scale with ZnO, significantly increases the hydrophobic character of the surface [31, 32].
Fig. 7. Water contact angle measurements (CA) of neat 50 nm Al2O3 film (a), ZnO thin film deposited after 200 ALD DEZ/water pulsing cycles on Al2O3 (b), 0.36 M VTMS (c), 0.36 M VTMS + MPTMS before (d) and after coating with ZnO, deposited after 200 ALD DEZ /water pulsing cycles (e).
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Conclusions In summary, this article reports on the formation of novel thiolated hemispherical nanoscaled organosilica structures grown on ALD-deposited Al2O3 thin films. The synthesis is based on a two-step approach consisting of initial silanization of Al2O3 with vinyltrimethoxysilane (VTMS), followed by a photo-promoted thiol-ene click reaction with (3mercaptopropyl)trimethoxysilane (MPTMS). When compared to the existing literature on the wet chemical synthesis of nanoparticles from MPTMS [19-21], the developed method exploits the high tendency of MPTMS to bind to the vinyl-modified substrate without an additional acid/base catalyst, oxidant or surfactant involved. Characterization by XPS and FE-SEM confirms that the formation of nanohemispheres at the surface depends on the initial concentration of VTMS applied in the first modification step, which determines the density and dispersion of the grafted vinyl groups. Elemental analysis by EDS showed that the presence of sulfur and silicon moieties prevails on the hemispheres, suggesting that the interparticle regions on alumina contain silane monolayers and/or short-chain species, while the nanospherical structures mostly consist of polymerized MPTMS moieties. The formation of zinc-thiolate (Zn-S) bonds after the subsequent ALD deposition of ZnO confirms a substantial amount of free thiol groups on the surface of the nanohemispheres. Moreover, the ZnO-coated nanoscaled hierarchical structures exhibit higher hydrophobicity when compared to ZnO films deposited directly on Al2O3 under the same ALD processing conditions. We hypothesize that during the initial (nucleation) stage of the second modification reaction with MPTMS two consecutive reactions occur: the formation of oligomeric siloxanebased seeds by rapid self-condensation reactions in solution, and photo-promoted oxidation of MPTMS thiols into disulfides. The further evolution of surface-bound nanohemispheres is ascribed to an initial attachment of MPTMS seeds to the vinyl-modified surface via photoassisted thiol-ene chemistry, followed by a “classical” growth through hydrolysis/condensation reactions between free Si-OCH3/Si-OH groups of grafted MPTMS, and Si-OCH3/Si-OH groups of MPTMS molecules diffusing from solution. The silanized thin-films with spatially-distributed free thiol groups opt for applications in bioimaging or biosensing of thiol-carrying biomolecules through selective and localized formation of disulfide bridges, which will be investigated in forthcoming research.
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Acknowledgements This work has been mainly supported by Croatian Science Foundation under the project IP2016-06-3568, and in part by the University of Rijeka under the project numbers 16.12.2.1.01 and 12.12.1.1.01. The characterization instruments applied in this work were acquired through the European Fund for Regional Development and Ministry of Science, Education and Sports of the Republic of Croatia under the project Research Infrastructure for Campus-based Laboratories at the University of Rijeka (grant number RC.2.2.06-0001). Mato Knez achnowledges support by the Spanish Ministry of Economy and Competitiveness (MINECO) [GA No. MAT2016-77393-R], including FEDER funds.“
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Figure captions: Fig. 1. (a) Si 2p and (b) C 1s XPS core-level spectra of VTMS-modified Al2O3 thin films (0.24 M VTMS, 0.36 M VTMS and 0.48 M VTMS) prepared from the indicated VTMS concentrations.
Fig. 2. FE-SEM micrographs of VTMS-modified Al2O3 thin films prepared from 0.36 M (a) and 0.48 M (b) solutions of VTMS (0.36 M VTMS and 0.48 M VTMS, respectively). The inset in (a) shows the FE-SEM image of the corresponding sample in a bird's-eye view.
Fig. 3. (a) S 2p, (b) Si 2p and (c) C 1s XPS core-level spectra of the samples 0.24 M VTMS + MPTMS and 0.36 M VTMS + MPTMS. The substrates were VTMS-modified Al2O3 thin films silanized with the indicated VTMS concentrations.
Fig. 4. FE-SEM images of 0.36 M VTMS + MPTMS (a) and 0.24 M VTMS + MPTMS (b), and EDS elemental microanalysis from the area marked with a rectangle shown in the corresponding FE-SEM images for 0.36 M VTMS + MPTMS (c) obtained at 3 kV electron beam acceleration voltage. The insets in (a) and (b) show magnified images of the corresponding samples (upper insets in a bird’s-eye view).
Scheme 1. Schematic illustration of the proposed surface structure obtained after a two-step approach: silanization of alumina with VTMS, followed by a thiol-ene click reaction with MPTMS.
Fig. 5. XPS spectra of Zn 2p3/2 (a), and S 2p core-levels (b) of 0.24 M VTMS + MPTMS and 0.36 M VTMS + MPTMS measured after ALD deposition of ZnO (30 ALD cycles). Fig. 6. FE-SEM images of the ZnO thin film deposited after 200 ALD DEZ/water pulsing cycles on (a) the neat 50 nm alumina film, and (b) 0.36 M VTMS + MPTMS. The inset in (b) shows a magnified image of the corresponding sample in a bird’s-eye view.
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(c) EDS elemental microanalysis of the areas marked with a rectangle shown in the corresponding FE-SEM image of 0.36 M VTMS + MPTMS, obtained at 5 kV electron beam acceleration voltage.
Fig. 7. Water contact angle measurements (CA) of neat 50 nm Al2O3 film (a), ZnO thin film deposited after 200 ALD DEZ/water pulsing cycles on Al2O3 (b), 0.36 M VTMS (c), 0.36 M VTMS + MPTMS before (d) and after coating with ZnO, deposited after 200 ALD DEZ /water pulsing cycles (e).
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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