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Electrospun SiO2 “necklaces” on unglazed ceramic tiles: a planarizing strategy
Superlattices and Microstructures 81 (2015) 265–271
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Electrospun SiO2 ‘‘necklaces’’ on unglazed ceramic tiles: a planarizing strategy Alessandro Di Mauro, Maria Elena Fragalà ⇑ Dipartimento di Scienze Chimiche and INSTM UdR Catania, Università di Catania, Viale Andrea Doria, 6, 95100 Catania, Italy
a r t i c l e
i n f o
Article history: Received 24 September 2014 Received in revised form 9 December 2014 Accepted 18 January 2015 Available online 7 February 2015 Keywords: Ceramic tiles Electrospinning PVP Spin on glass Surface planarization
a b s t r a c t Silica based nanofibres have been deposited on unglazed ceramic tiles by combining electrospinning and sol–gel processes. Poly(vinyl pyrrolidone) (PVP) alcoholic solutions and commercial spin on glass (Accuglass) mixtures have been used to obtain composite fibrous non-woven mats totally converted, after thermal annealing at 600 °C, to SiO2 microsphere ‘‘necklaces’’. The possibility to get an uniform fibres coverage onto the tile surface confirms the validity of electrospinning (easily scalable to large surface samples) as coating strategy to cover the macroscopic defects typical of the polished unglazed tile surface and improve surface planarization. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, a new generation of ceramic tiles, obtained by a designed chemical–physical modulation of surface properties and functionalities, offers performances well beyond the traditional market expectation, thus making possible the idea of use them as smart architectural element or advanced coating [1]. In fact, photovoltaic cells and sensors integrated in ceramic tiles for BIPV (Building Integrated Photovoltaic) applications and self-cleaning materials having peculiar photo-catalytic properties represent an advanced class of materials in continuous development [2,3]. Nanotechnology [4] offers many insights about the potentialities of ceramic tiles as support for functional and/or hybrid materials having specific properties to address the need of the ceramic industry. Accordingly, inorganic nanostructures, like metal or semiconductor oxide nanoparticles (i.e. SiO2, TiO2) are largely
⇑ Corresponding author. Tel.: +39 095 7385095; fax: +39 095 580138. E-mail address: [email protected] (M.E. Fragalà). http://dx.doi.org/10.1016/j.spmi.2015.01.027 0749-6036/Ó 2015 Elsevier Ltd. All rights reserved.
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employed as coating material to improve the self-cleaning ability of exposed surface (i.e. glass, ceramic tiles, porcelainised stoneware tiles) or their antibacterial properties [5,6]. However, fabrication strategies and production costs must be compatible with tile manufacturing processes and, even more important, do not alter the aesthetic aspect. Spray-coating or air-brushing are examples of techniques used to deposit nanoparticle dispersions on the tile surface [7,8]. Electrospinning is a widely employed technique to obtain non- woven mats of polymers or ceramic materials, self-supported or deposited on a large variety of substrates [9]. This techniques, that relies on an electric field to draw a spinnable solution towards a collector of lower potential, represents a versatile bottom-up processing strategy to produce micro- and/or nano-fibres. Initial precursor solutions were comprised of an organic polymer that provides the backbone of long and usually entangled organic chains required to draw out fibres from a high viscosity solution. The electrostatic forces distort the solution from a spherical pendant drop to a Taylor cone. Once the electrostatic forces overcome the surface tension, a charged jet is ejected and subsequently elongated due to bending and splaying. As a consequence, a non-woven structure of continuous fibres with diameters ranging from a few nanometres to several microns is deposited on a collector plate. Ceramic precursors or particle suspensions can be combined with organic polymers to obtain, post calcination, polymer-free ceramic fibres [10,11]. Herein, we demonstrate the successful use of electrospinning to coat ceramic tiles. In particular, silica based microfibres are obtained by a combined electrospinning and sol–gel process [12] that uses, as spinning solution, poly(vinyl pyrrolidone) (PVP) and commercial spin on glass (SOG–Accuglass) alcholic solutions. Alkoxysilanes are popular single source precursors [13] for SiO2 since high quality oxides can be produced by pyrolysis at relatively low temperature. Moreover, polymer-modified silicates are very important in sol–gel chemistry and for organic–inorganic hybrid material applications [14]. Films deposited by alkoxysilanes are widely utilised as protective layer coatings, electronic sensors, chemically modified electrodes, thin film optics, and antireflection coatings [15,16]. The obtained PVP–Accuglass fibrous non-woven mats are totally converted, after thermal annealing at 600 °C, to pure SiO2 microsphere ‘‘necklaces’’. A complete surface characterisation is provided by using Field Emission Scanning Electron Microscopy (FE-SEM) and X-Rays Photoelectrons Spectroscopy (XPS). 2. Experimental 10 ml of a PVP (Mw = 143000 Dalton) ethanolic solution (v/v 10%) is mixed to 10 ml of Accuglass (Honeywell accuglassÒ T-12B spin-on glass) solution and let under continuous stirring (at T = 60 °C) for 6 hrs. This commercial methylsiloxane contains about 15% of –CH3 groups bonded to Si–O backbone and its curing is accomplished by heating the Accuglass film at a temperature of at least 400 °C and for a time period ranging from 10 min to several hours. After a waiting time of 24 hrs, the resulting solution is electrospun on substrate surfaces, using a commercial electrospinning apparatus (EC-DIG Electrospinning, IME Technologies). PVP/Accuglass composite fibres are ejected from the needle of a syringe when an electrical field, as high as several kV/cm, is applied and collected (stationary mode) on the surface of a substrate clamped on top of a conductive circular collector. Electrospinning parameters change according to the nature of the substrates and related conductivity (i.e. moving from semiconductive substrates to insulators), as shown in Table 1. Unglazed polished tile samples (1 1 cm2) have been provided by SITI-B&T (Formigine, Italy) and used as received. Silicon (1 0 0) substrates have been used as comparison: in particular, fibres surface
Table 1 Electrospinning condition to deposit SiO2 nanofibres on different substrates. Collector
Distance (cm)
Flow rate (ll/min)
Silicon Ceramic tile
14.5 18.0
11.0 9.0
Negative voltage (V) 4 4
Positive voltage (V)
Time (s)
21 19
30 30
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characterisation by X-Rays Photoelectron Spectroscopy is less influenced by charging issues if electrically non-insulating substrates are used. The obtained electrospun PVP–Accuglass fibres have been kept under ambient condition to dry and to allow the alkoxysilane precursor to completely turn into gel [16] by the moisture in the atmosphere. After annealing in a hot-wall horizontal furnace at 600 °C for 6 hrs, the complete degradation of PVP and conversion of Accuglass to SiO2 occurs, thus resulting in the formation of pure SiO2 ‘‘necklace’’ like microfibres. The morphologies of the electrospun mats have been further characterised with Leo-Supra VP 550 Field Emission scanning electron microscope (ZEISS) and NT-MDT AFM (working in contact mode). XPS data have been accumulated on PHI ESCA/SAM 5600 multy technique spectrometer equipped with a monochromatized Al Ka X-ray source. The binding energies have been calibrated by referencing C1s to 285 eV.
3. Results and discussion Fig. 1 shows the surface morphology of a ceramic tile before (Fig. 1a) and after (Fig. 1b) the deposition of PVP–Accuglass composite microfibres. SEM images reveal, after fibres deposition, a smoother and more regular tile surface than that of the untreated sample: in fact, the homogeneous and continuous fibres coverage is able to cover the surface unevenness and asperity typical of unglazed tiles. A more detailed morphological characterisation of the electrospun fibres is shown in Fig. 2, that reveals the complex microstructure of the obtained microfibres (Fig. 2a).
Fig. 1. Unglazed tile before (a) and after (b) electrospinning of PVP–Accuglass fibres (scale bar is 100 lm).
Fig. 2. SEM images (high magnification) of PVP–Accuglass fibres.
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The average fibres dimension is about 1 micron and spherical particles are included inside the polymeric ‘‘skin’’, as clearly shown in Fig. 2b. In fact, alkoxisilanes are able to form perfect spherical silica nanoparticles [16–19] by hydrolysis/condensation catalysed by PVP [18]. AFM analysis of ceramic tile surface, before and after electrospinning (Fig. 3), provides further details about the surface morphology. In particular, before electrospinning (Fig. 3a) the surface of unglazed tiles is characterised by the presence of craters having a depth of about 1 micron (as clearly indicated by the Z scale of the AFM image). After electrospinning, the fibres entanglement completely changes the surface morphology (Fig. 2b): the granular appearance is the result of the homogeneous fibre coverage, in good agreement with SEM analysis. The overall Z range, after electrospinning, is about 500 nm, lower than that recorded before treatment, thus confirming a more uniform tile surface. As to the formation of silica microspheres within the PVP fibres, despite most of the used approaches to form SiO2 nanoparticles exploit the hydrolysis and condensation in water using ammonia as catalyst, herein we achieve the formation of silica speres using a water free solvent mixture. The composition of the used commercial Accuglass is reported in Table 2 (Honeywell MSDS Number: PDIF-00464). No water is present in the mixture. It has to be reminded that a flat and uniform SiO2 film is required for microelectronics applications of spin of glass (SOG) [20] and therefore, formation of microspheres must be definitively avoided. Accordingly, the SiO2 microspheres absence during the preparation of the PVP–Accuglass precursor solution is confirmed by the aspect of the final solution that remains clear (no turbidity or colour change is observed). Therefore, we have to assume that microspheres formation is promoted by the electrospinning process [21]. Noteworthy, during electrospinning a fast solvent evaporation, accompanied by jet stretching, is responsible for the final diameter and structure of nanofibres. As we run electrospinning in atmosphere, we assume (work is on going) that humidity (RH) might be responsible for the formation of SiO2 microspheres within the PVP fibres skin [22].
Fig. 3. AFM images of (a) untreated tile surface and (b) tile surface after PVP–Accuglass electrospinning.
Table 2 Accuglass composition. Methyl siloxane Acetone Ethanol Isopropanol
8–17% 8–19% 28–42% 20–35%
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Annealing of PVP–Accuglass microfibres at 600 °C for 6 hrs leads to complete degradation of both PVP and carbonaceous components of spin on glass alkoxysilane, thus converting the polymeric composite in a ‘‘necklace’’ like arrangement of SiO2 microspheres (Fig. 4). SOG materials are conventionally used for microelectronic device planarization to fill narrow gaps in the sub-dielectric surface [20]. Unfortunately, the spin-on process cannot easily integrated in the ceramic tiles manufacturing, since the dimension of the tiles are not suitable for spin coating process. Moreover, the surface irregularities, typical of ceramic tiles, are larger and structurally different with respect to the regular (in shape and depth) trenches opened in the semiconductor interlayers. In fact, the whole surface of unglazed ceramic tile is not smooth and pores, craters, large fissures, large fractures, pits are typically observed. Accordingly, only very thick SOG layers might provide a smooth planar surface over rough topographies, thus affecting the aesthetic of the ceramic tiles. By considering all these drawbacks, electrospinning provides an alternative and more efficient solution to fill voids, pores and cracks present at the tile surfaces, by using electrospun microfibres networks that can be converted, by annealing treatments, into SiO2 microspheres arrangement. To probe the complete degradation of the polymeric component and the related compositions of obtained silica ‘‘necklace’’, stepwise XPS analyses is performed. Surface atomic composition of PVP– Accuglass electrospun fibres (collected on silicon substrates to control charging effects), before and after annealing, is reported on Table 3. Accuglass nominally contains 15 wt% CH3 (methyl) groups bonded to Si atoms in the Si–O backbone, thus explaining the carbon presence (also related to adventitious carbon) in the spin coated sample. The composition of composite PVP–Accuglass microfibres is characterised, as expected, by the presence of N1s peak, originating from amide groups of PVP. Moreover, the C content is drastically increased due to carbon based polymer chains of PVP. Finally, surface composition of annealed PVP– Accuglass provides a clear evidence of the complete degradation of PVP, as indicated by the absence of nitrogen as well as the significant reduction of carbon content and the formation of stoichiometric SiO2.
Fig. 4. Low magnification (a) and high magnification (b) SEM image of PVP–Accuglass fibres after thermal treatment.
Table 3 XPS atomic composition of electrospun fibres, before and after annealing (the composition of a spin coated Accuglass layer has been reported for comparison). Sample
N
Si
O
C
Spin coated Accuglass ES PVP–Accuglass ES PVP–Accuglass after annealing
0 5 0
29 18 32
50 34 63
21 43 5
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Fig. 5. Unglazed polished tile before (a) and after (b) electrospinning and annealing of PVP–Accuglass fibres.
The ability of electrospun PVP–Accuglass microfibres to completely cover the ceramic tile surface and to form, after annealing, silica microparticles suggest us their possible application as tile surface planarizing strategy. In fact, microfibres and microspheres are able to control the defectivity of polished tiles, related to machining procedure that induces widespread damage and uncovers the closed or bulk porosity. Fig. 5 reports the morphology of tile surface before and after electrospinning/annealing treatment. Large and deep flaws are clearly visible before coating with electrospun microfibres, as shown in Fig. 5a. After electrospinning and annealing, these defective areas are filled by silica microspheres, thus resulting in a more homogeneous surface (Fig. 5b–d). 4. Conclusions Herein, we propose electrospinning of PVP–Accuglass mixtures as an efficient approach to obtain silica (SiO2) based coating to apply on the ceramic tile surfaces. The uniform fibres coverage confirms the validity of the technique (easily scalable to large surface samples) as coating strategy to fill ceramic tiles defectivity. In fact, the capability of the microfibres to cover the microscopic defectivity, typical of the polished unglazed tile surface, offers an interesting potentiality as planarizing strategy. Particular attention has been paid to preserve the aesthetical aspect of the tiles, which remains unchanged after both electrospinning and following thermal treatments, these latter used to convert the composite PVP–Accuglass microfibres to SiO2 microsphere ‘‘necklaces’’. Acknowledgements Authors thank Ministero dell’Ambiente e della Tutela del Territorio e del Mare (MATTM) within the project Building Integrated Photovoltaics (BIPV) and Ministero dell’ Istruzione, dell’Università e della Ricerca (MIUR) within the FIRB project RENAME for financial support.
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References [1] A.M. Berto, Ceramic tiles: above and beyond traditional applications, J. Eur. Ceram. Soc. 27 (2007) 1607–1613. [2] P. São Marcos, J. Marto, T. Trindade, J.A. Labrincha, Screen-printing of TiO2 photocatalytic layers on glazed ceramic tiles, J. Photochem. Photobiol. A 197 (2008) 125–131. [3] E. Rego, J. Marto, P. São Marcos, J.A. Labrincha, Decolouration of Orange II solutions by TiO2 and ZnO active layers screenprinted on ceramic tiles under sunlight irradiation, Appl. Catal. A 355 (2009) 109–114. [4] T. Fleischer, M. Decker, U. Fiedeler, Assessing emerging technologies – methodological challenges and the case of nanotechnologies, Technol. Forecast. Soc. Chang. 72 (2005) 1112–1121. [5] L. Pinho, M.J. Mosquera, Photocatalytic activity of TiO2–SiO2 nanocomposites applied to buildings: influence of particle size and loading, Appl. Catal. B 134–135 (2013) 205–221. [6] L. Pinho, F. Elhaddad, D.S. Facio, M.J. Mosquera, A novel TiO2–SiO2 nanocomposite converts a very friable stone into a selfcleaning building material, Appl. Surf. Sci. 275 (2013) 389–396. [7] S. de Niederhãusern, M. Bondi, Self-cleaning and antibacteric ceramic tile, Surf. Int. J. Appl. Ceram. Technol. 10 (2013) 949– 956. [8] F. Bondioli, R. Taurino, A.M. Ferrari, Functionalization of ceramic tile surface by sol–gel technique, J. Colloid Interface Sci. 334 (2009) 195–201. [9] Z.-M. Huang, Y.-Z. Zhang, M. Kotakic, S. Ramakrishna, A review on polymer nanofibers by electrospinning and their applications in nanocomposites, Compos. Sci. Technol. 63 (2003) 2223–2253. [10] A. Bianco, I. Cacciotti, M.E. Fragalà, F.R. Lamastra, A. Speghini, F. Piccinelli, G. Malandrino, G. Gusmano, Eu-doped titania nanofibers: processing, thermal behaviour and luminescent properties, J. Nanosci. Nanotechnol. 10 (2010) 5183–5190. [11] M.E. Fragalà, I. Cacciotti, Y. Aleeva, R. Lo Nigro, A. Bianco, G. Malandrino, C. Spinella, G. Pezzotti, G. Gusmano, Core–shell Zndoped TiO2–ZnO nanofibers fabricated via a combination of electrospinning and metal–organic chemical vapour deposition, CrystEngComm 12 (2010) 3858–3865. [12] S.-S. Choi, S. Goo Lee, S. Soon Im, S. Hun Kim, Y.L. Joo, Silica nanofibers from electrospinning/sol–gel process, J. Mater. Sci. Lett. 22 (2003) 891–893. [13] J. Wen, G.L. Wilkes, Organic/inorganic hybrid network materials by the sol gel approach, Chem. Mater. 8 (1996) 1667– 162281. [14] G. Schottner, Hybrid sol gel-derived polymers: applications of multifunctional materials, Chem. Mater. 13 (2001) 3422– 3435. [15] G.L. Wituck, A silane primer: chemistry and applications of aikoxy silanes, J. Coat. Technol. 65 (1993) 57–60. [16] W. Stöber, A. Fink, E. Bohn, Controlled growth of monodisperse silica spheres in the micron size range, J. Colloid Interface Sci. 26 (1968) 62–69. [17] L.L. Hench, J.K. West, The sol–gel process, Chem. Rev. 90 (1990) 33–72. [18] Y. Xu, D. Wu, Y. Sun, W. Chen, H. Yuan, F. Deng, Z. Wu, Effect of polyvinylpyrrolidone on the ammonia-catalyzed sol–gel process of TEOS: Study by in situ 29Si NMR, scattering, and rheology, Colloid Surf. A: Physicochem. Eng. Aspects 305 (2007) 97–104. [19] J. Cha, P. Cui, J.-K. Lee, A simple method to synthesize multifunctional silica nanocomposites, NPs@SiO2, using polyvinylpyrrolidone (PVP) as a mediator, J. Mater. Chem. 20 (2010) 5533–5537. [20] S.K. Gupta, R.L. Chin, Characterization of Spin-On Glass Films as a Planarizing Dielectric, Microelectronics Processing: Inorganic Materials Characterization, vol. 295(22), ACS Symposium Series, 1986. pp. 349–365. [21] M.P. Dvores, G. Marom, S. Magdassi, Formation of organic nanoparticles by electrospinning of volatile microemulsions, Langmuir 28 (2012) 6978–6984. [22] S. De Vrieze, T. Van Camp, A. Nelvig, B. Hagström, P. Westbroek, K. De Clerck, The effect of temperature and humidity on electrospinning, J. Mater. Sci. 44 (2009) 1357–1362.
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Electrospun SiO2 ‘‘necklaces’’ on unglazed ceramic tiles: a planarizing strategy Alessandro Di Mauro, Maria Elena Fragalà ⇑ Dipartimento di Scienze Chimiche and INSTM UdR Catania, Università di Catania, Viale Andrea Doria, 6, 95100 Catania, Italy
a r t i c l e
i n f o
Article history: Received 24 September 2014 Received in revised form 9 December 2014 Accepted 18 January 2015 Available online 7 February 2015 Keywords: Ceramic tiles Electrospinning PVP Spin on glass Surface planarization
a b s t r a c t Silica based nanofibres have been deposited on unglazed ceramic tiles by combining electrospinning and sol–gel processes. Poly(vinyl pyrrolidone) (PVP) alcoholic solutions and commercial spin on glass (Accuglass) mixtures have been used to obtain composite fibrous non-woven mats totally converted, after thermal annealing at 600 °C, to SiO2 microsphere ‘‘necklaces’’. The possibility to get an uniform fibres coverage onto the tile surface confirms the validity of electrospinning (easily scalable to large surface samples) as coating strategy to cover the macroscopic defects typical of the polished unglazed tile surface and improve surface planarization. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, a new generation of ceramic tiles, obtained by a designed chemical–physical modulation of surface properties and functionalities, offers performances well beyond the traditional market expectation, thus making possible the idea of use them as smart architectural element or advanced coating [1]. In fact, photovoltaic cells and sensors integrated in ceramic tiles for BIPV (Building Integrated Photovoltaic) applications and self-cleaning materials having peculiar photo-catalytic properties represent an advanced class of materials in continuous development [2,3]. Nanotechnology [4] offers many insights about the potentialities of ceramic tiles as support for functional and/or hybrid materials having specific properties to address the need of the ceramic industry. Accordingly, inorganic nanostructures, like metal or semiconductor oxide nanoparticles (i.e. SiO2, TiO2) are largely
⇑ Corresponding author. Tel.: +39 095 7385095; fax: +39 095 580138. E-mail address: [email protected] (M.E. Fragalà). http://dx.doi.org/10.1016/j.spmi.2015.01.027 0749-6036/Ó 2015 Elsevier Ltd. All rights reserved.
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employed as coating material to improve the self-cleaning ability of exposed surface (i.e. glass, ceramic tiles, porcelainised stoneware tiles) or their antibacterial properties [5,6]. However, fabrication strategies and production costs must be compatible with tile manufacturing processes and, even more important, do not alter the aesthetic aspect. Spray-coating or air-brushing are examples of techniques used to deposit nanoparticle dispersions on the tile surface [7,8]. Electrospinning is a widely employed technique to obtain non- woven mats of polymers or ceramic materials, self-supported or deposited on a large variety of substrates [9]. This techniques, that relies on an electric field to draw a spinnable solution towards a collector of lower potential, represents a versatile bottom-up processing strategy to produce micro- and/or nano-fibres. Initial precursor solutions were comprised of an organic polymer that provides the backbone of long and usually entangled organic chains required to draw out fibres from a high viscosity solution. The electrostatic forces distort the solution from a spherical pendant drop to a Taylor cone. Once the electrostatic forces overcome the surface tension, a charged jet is ejected and subsequently elongated due to bending and splaying. As a consequence, a non-woven structure of continuous fibres with diameters ranging from a few nanometres to several microns is deposited on a collector plate. Ceramic precursors or particle suspensions can be combined with organic polymers to obtain, post calcination, polymer-free ceramic fibres [10,11]. Herein, we demonstrate the successful use of electrospinning to coat ceramic tiles. In particular, silica based microfibres are obtained by a combined electrospinning and sol–gel process [12] that uses, as spinning solution, poly(vinyl pyrrolidone) (PVP) and commercial spin on glass (SOG–Accuglass) alcholic solutions. Alkoxysilanes are popular single source precursors [13] for SiO2 since high quality oxides can be produced by pyrolysis at relatively low temperature. Moreover, polymer-modified silicates are very important in sol–gel chemistry and for organic–inorganic hybrid material applications [14]. Films deposited by alkoxysilanes are widely utilised as protective layer coatings, electronic sensors, chemically modified electrodes, thin film optics, and antireflection coatings [15,16]. The obtained PVP–Accuglass fibrous non-woven mats are totally converted, after thermal annealing at 600 °C, to pure SiO2 microsphere ‘‘necklaces’’. A complete surface characterisation is provided by using Field Emission Scanning Electron Microscopy (FE-SEM) and X-Rays Photoelectrons Spectroscopy (XPS). 2. Experimental 10 ml of a PVP (Mw = 143000 Dalton) ethanolic solution (v/v 10%) is mixed to 10 ml of Accuglass (Honeywell accuglassÒ T-12B spin-on glass) solution and let under continuous stirring (at T = 60 °C) for 6 hrs. This commercial methylsiloxane contains about 15% of –CH3 groups bonded to Si–O backbone and its curing is accomplished by heating the Accuglass film at a temperature of at least 400 °C and for a time period ranging from 10 min to several hours. After a waiting time of 24 hrs, the resulting solution is electrospun on substrate surfaces, using a commercial electrospinning apparatus (EC-DIG Electrospinning, IME Technologies). PVP/Accuglass composite fibres are ejected from the needle of a syringe when an electrical field, as high as several kV/cm, is applied and collected (stationary mode) on the surface of a substrate clamped on top of a conductive circular collector. Electrospinning parameters change according to the nature of the substrates and related conductivity (i.e. moving from semiconductive substrates to insulators), as shown in Table 1. Unglazed polished tile samples (1 1 cm2) have been provided by SITI-B&T (Formigine, Italy) and used as received. Silicon (1 0 0) substrates have been used as comparison: in particular, fibres surface
Table 1 Electrospinning condition to deposit SiO2 nanofibres on different substrates. Collector
Distance (cm)
Flow rate (ll/min)
Silicon Ceramic tile
14.5 18.0
11.0 9.0
Negative voltage (V) 4 4
Positive voltage (V)
Time (s)
21 19
30 30
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characterisation by X-Rays Photoelectron Spectroscopy is less influenced by charging issues if electrically non-insulating substrates are used. The obtained electrospun PVP–Accuglass fibres have been kept under ambient condition to dry and to allow the alkoxysilane precursor to completely turn into gel [16] by the moisture in the atmosphere. After annealing in a hot-wall horizontal furnace at 600 °C for 6 hrs, the complete degradation of PVP and conversion of Accuglass to SiO2 occurs, thus resulting in the formation of pure SiO2 ‘‘necklace’’ like microfibres. The morphologies of the electrospun mats have been further characterised with Leo-Supra VP 550 Field Emission scanning electron microscope (ZEISS) and NT-MDT AFM (working in contact mode). XPS data have been accumulated on PHI ESCA/SAM 5600 multy technique spectrometer equipped with a monochromatized Al Ka X-ray source. The binding energies have been calibrated by referencing C1s to 285 eV.
3. Results and discussion Fig. 1 shows the surface morphology of a ceramic tile before (Fig. 1a) and after (Fig. 1b) the deposition of PVP–Accuglass composite microfibres. SEM images reveal, after fibres deposition, a smoother and more regular tile surface than that of the untreated sample: in fact, the homogeneous and continuous fibres coverage is able to cover the surface unevenness and asperity typical of unglazed tiles. A more detailed morphological characterisation of the electrospun fibres is shown in Fig. 2, that reveals the complex microstructure of the obtained microfibres (Fig. 2a).
Fig. 1. Unglazed tile before (a) and after (b) electrospinning of PVP–Accuglass fibres (scale bar is 100 lm).
Fig. 2. SEM images (high magnification) of PVP–Accuglass fibres.
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The average fibres dimension is about 1 micron and spherical particles are included inside the polymeric ‘‘skin’’, as clearly shown in Fig. 2b. In fact, alkoxisilanes are able to form perfect spherical silica nanoparticles [16–19] by hydrolysis/condensation catalysed by PVP [18]. AFM analysis of ceramic tile surface, before and after electrospinning (Fig. 3), provides further details about the surface morphology. In particular, before electrospinning (Fig. 3a) the surface of unglazed tiles is characterised by the presence of craters having a depth of about 1 micron (as clearly indicated by the Z scale of the AFM image). After electrospinning, the fibres entanglement completely changes the surface morphology (Fig. 2b): the granular appearance is the result of the homogeneous fibre coverage, in good agreement with SEM analysis. The overall Z range, after electrospinning, is about 500 nm, lower than that recorded before treatment, thus confirming a more uniform tile surface. As to the formation of silica microspheres within the PVP fibres, despite most of the used approaches to form SiO2 nanoparticles exploit the hydrolysis and condensation in water using ammonia as catalyst, herein we achieve the formation of silica speres using a water free solvent mixture. The composition of the used commercial Accuglass is reported in Table 2 (Honeywell MSDS Number: PDIF-00464). No water is present in the mixture. It has to be reminded that a flat and uniform SiO2 film is required for microelectronics applications of spin of glass (SOG) [20] and therefore, formation of microspheres must be definitively avoided. Accordingly, the SiO2 microspheres absence during the preparation of the PVP–Accuglass precursor solution is confirmed by the aspect of the final solution that remains clear (no turbidity or colour change is observed). Therefore, we have to assume that microspheres formation is promoted by the electrospinning process [21]. Noteworthy, during electrospinning a fast solvent evaporation, accompanied by jet stretching, is responsible for the final diameter and structure of nanofibres. As we run electrospinning in atmosphere, we assume (work is on going) that humidity (RH) might be responsible for the formation of SiO2 microspheres within the PVP fibres skin [22].
Fig. 3. AFM images of (a) untreated tile surface and (b) tile surface after PVP–Accuglass electrospinning.
Table 2 Accuglass composition. Methyl siloxane Acetone Ethanol Isopropanol
8–17% 8–19% 28–42% 20–35%
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Annealing of PVP–Accuglass microfibres at 600 °C for 6 hrs leads to complete degradation of both PVP and carbonaceous components of spin on glass alkoxysilane, thus converting the polymeric composite in a ‘‘necklace’’ like arrangement of SiO2 microspheres (Fig. 4). SOG materials are conventionally used for microelectronic device planarization to fill narrow gaps in the sub-dielectric surface [20]. Unfortunately, the spin-on process cannot easily integrated in the ceramic tiles manufacturing, since the dimension of the tiles are not suitable for spin coating process. Moreover, the surface irregularities, typical of ceramic tiles, are larger and structurally different with respect to the regular (in shape and depth) trenches opened in the semiconductor interlayers. In fact, the whole surface of unglazed ceramic tile is not smooth and pores, craters, large fissures, large fractures, pits are typically observed. Accordingly, only very thick SOG layers might provide a smooth planar surface over rough topographies, thus affecting the aesthetic of the ceramic tiles. By considering all these drawbacks, electrospinning provides an alternative and more efficient solution to fill voids, pores and cracks present at the tile surfaces, by using electrospun microfibres networks that can be converted, by annealing treatments, into SiO2 microspheres arrangement. To probe the complete degradation of the polymeric component and the related compositions of obtained silica ‘‘necklace’’, stepwise XPS analyses is performed. Surface atomic composition of PVP– Accuglass electrospun fibres (collected on silicon substrates to control charging effects), before and after annealing, is reported on Table 3. Accuglass nominally contains 15 wt% CH3 (methyl) groups bonded to Si atoms in the Si–O backbone, thus explaining the carbon presence (also related to adventitious carbon) in the spin coated sample. The composition of composite PVP–Accuglass microfibres is characterised, as expected, by the presence of N1s peak, originating from amide groups of PVP. Moreover, the C content is drastically increased due to carbon based polymer chains of PVP. Finally, surface composition of annealed PVP– Accuglass provides a clear evidence of the complete degradation of PVP, as indicated by the absence of nitrogen as well as the significant reduction of carbon content and the formation of stoichiometric SiO2.
Fig. 4. Low magnification (a) and high magnification (b) SEM image of PVP–Accuglass fibres after thermal treatment.
Table 3 XPS atomic composition of electrospun fibres, before and after annealing (the composition of a spin coated Accuglass layer has been reported for comparison). Sample
N
Si
O
C
Spin coated Accuglass ES PVP–Accuglass ES PVP–Accuglass after annealing
0 5 0
29 18 32
50 34 63
21 43 5
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Fig. 5. Unglazed polished tile before (a) and after (b) electrospinning and annealing of PVP–Accuglass fibres.
The ability of electrospun PVP–Accuglass microfibres to completely cover the ceramic tile surface and to form, after annealing, silica microparticles suggest us their possible application as tile surface planarizing strategy. In fact, microfibres and microspheres are able to control the defectivity of polished tiles, related to machining procedure that induces widespread damage and uncovers the closed or bulk porosity. Fig. 5 reports the morphology of tile surface before and after electrospinning/annealing treatment. Large and deep flaws are clearly visible before coating with electrospun microfibres, as shown in Fig. 5a. After electrospinning and annealing, these defective areas are filled by silica microspheres, thus resulting in a more homogeneous surface (Fig. 5b–d). 4. Conclusions Herein, we propose electrospinning of PVP–Accuglass mixtures as an efficient approach to obtain silica (SiO2) based coating to apply on the ceramic tile surfaces. The uniform fibres coverage confirms the validity of the technique (easily scalable to large surface samples) as coating strategy to fill ceramic tiles defectivity. In fact, the capability of the microfibres to cover the microscopic defectivity, typical of the polished unglazed tile surface, offers an interesting potentiality as planarizing strategy. Particular attention has been paid to preserve the aesthetical aspect of the tiles, which remains unchanged after both electrospinning and following thermal treatments, these latter used to convert the composite PVP–Accuglass microfibres to SiO2 microsphere ‘‘necklaces’’. Acknowledgements Authors thank Ministero dell’Ambiente e della Tutela del Territorio e del Mare (MATTM) within the project Building Integrated Photovoltaics (BIPV) and Ministero dell’ Istruzione, dell’Università e della Ricerca (MIUR) within the FIRB project RENAME for financial support.
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