Solar Energy Materials & Solar Cells 95 (2011) 423–431 Contents lists available at ScienceDirect Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat Polyhedral oligomeric silsesquioxane trisilanols as pigment surface modifiers for fluoropolymer based Thickness Sensitive Spectrally Selective (TSSS) paint coatings I. Jerman a, M. Mihelčič a, D. Verhovšek b, J. Kovač c, B. Orel a,n a National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia Cinkarna—METALURŠKO KEMIČNA INDUSTRIJA CELJE, d.d. Kidričeva 26, 3001 Celje, Slovenia c Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia b a r t i c l e in fo abstract Article history: Received 22 April 2010 Accepted 4 August 2010 Available online 12 October 2010 Thickness Insensitive Spectrally Selective (TISS) paint coatings based on black pigment (PK 3060, Ferro Company) dispersed in a fluoropolymeric resin binder (Lumiflon, Asahi Company, Japan) have recently been made without added aluminium flakes and their properties have been reported for the first time. In this study we investigated in more detail the effect of trisilanol isobutyl (IB7 T7(OH)3) polyhedral oligomeric silsesquioxane (trisilanol POSS) on the surface modification of PK 3060 pigment. Infrared spectral analysis of the surface modified pigment particles provided firm evidence for the formation of a POSS layer on the surface of the pigment particles, substantiated by the corresponding TEM and Energy Dispersive X-ray Spectroscopy (EDXS) measurements of functionalized and as-received pigments. SEM micrographs of the diluted dispersions in fluoropolymeric resin binder revealed uniform distribution of pigment particles with an average size of  300 nm and the beneficial effect of the pigment functionalization was assessed from the measured spectral selectivity of coatings of various thicknesses. & 2010 Elsevier B.V. All rights reserved. Keywords: Pigment surface modification Open POSS Selective paint 1. Introduction High-quality dispersion of pigments in organic phases is prerequisite in the fabrication of homogeneous hybrid composite materials used as plastics, adhesives, restorative biomaterials, electronic packages [1,2] and solar absorber paints used for making spectrally selective paint coatings for solar absorbers in solar thermal collectors [3]. Only finely grained pigment particles that are uniformly dispersed in the polymer resin binder can provide spectrally selective paint coatings exhibiting high solar absorption (as) and low thermal emittance (eT) [4], because such pigment dispersions enable the formation of coating with closely packed assembly of particles bounded with the smallest amount of polymer resin binder. In order to overcome the problem of incompatibility of organic or polymeric phases with high-surfacearea nanosized mineral fillers or commercial pigments, the surface of the particles must first be modified, which means altered from hydrophilic to organophilic. Many organic systems have been investigated for surface modification. Functionalization is usually achieved by intense n Corresponding author. E-mail address: boris.orel@ki.s (B. Orel). 0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.08.005 mixing or ball milling of the pigment in the presence of a surfactant or macro-molecules [5], which become attached to the pigment surface by preferential adsorption of the polar groups via electrostatic interactions. Alternatively, the pigment surface can be modified by chemical reactions relying on chemical interactions between the modifiers and the pigment surface. In this case, modifiers are usually bifunctional alkoxysilanes (X-Si(OR)n) with hydrolysable (–Si(OR)n– usually alkoxy) and organo-functional ends (X-functional head group) [6] used since 1966 [7]. The nonhydrolysable group, X, can be used to tune the chemical and physical properties of the layers, while the hydrolysable alkoxy groups in the presence of acid catalysts give reactive silanol (Si–OH) groups, enabling the condensation of hydrolyzed silanes to gels by the establishment of siloxane (Si–O–Si) bonds and ensure adequate interactions with the other metal centres on the surface of the pigments by forming covalent M–O–Si bonds (M¼ Al, Sn,y). The functionalization of nanosilicas [8], magnetic nanoparticles [9], TiO2 nanoparticles [10], WS2 fullerene-like particles [11] and composite of antimony doped tin oxide conductive nanoparticles and acrylate by grafting 3-methacryloxypropyltrimethoxysilane [12] are typical examples of using silanes for modification of the particle surface in order to provide stable and non-agglomerated dispersion of nanoparticles and for tailoring their properties, such as viscosity [13], and for better 424 I. Jerman et al. / Solar Energy Materials & Solar Cells 95 (2011) 423–431 understanding the behavior of various products [14]. It is clear that the covalent modification of the pigment surfaces with alkoxysilanes in many respects follows the formation of ultrathin organic or alkoxysilane self-assembled (SAMs) coatings [15,16], enabling control of wetting, adhesion, lubrication [17–19] and corrosion on the surfaces and interfaces [20–22]. Organic dispersants have been used exclusively in the past for the production of commercial Thickness Sensitive Spectrally Selective (TSSS) and Thickness Sensitive Spectrally Selective (TSSS) paints [(Solarect-Z24s is manufactured by Color d.d. (SI)], as described in patents [23–25], but for TISS paint coatings [3] trisilanol isobutyl (IB7 T7(OH)3) polyhedral oligomeric silsesquioxane (POSS; trisilanol POSS, for short) has been used recently, with good success, providing TISS paint coatings with @s 0.94 and eT ¼  0.23. Accordingly, one of the aims of this study was to demonstrate whether the trisilanol POSS dispersant could provide PK 3060 pigment dispersions suitable for making fluropolymer based TSSS paint coatings. Trisilanol POSS (Fig. 1) is a novel dispersant with organic– inorganic (hybrid) structure prepared via the sol–gel chemistry routes from bifunctional alkoxysilanes. In general, the hydrolytic condensation of alkyltrialkoxysilanes leads under appropriate conditions to organic–inorganic hybrid nanocomposites in basically three different forms: randomly connected siloxane networks (T-resin) (i), ladder polysilsesquioxanes (ii) and polyhedral oligomeric silsesquioxanes (POSS) (iii), the latter representing the most ordered product of condensation. POSS consists of silica core –(SiO3/2) and organic corona. The variety of organic groups that are located at the corners of the silsesquioxane polyhedra gives an enormous number of heteroleptic POSS with multifunctional properties. The most common are octasilsesquioxanes (T8), discovered in 1946, when the molecule was first produced from the co-hydrolysis of methyltrichlorosilane with dimethylchlorosilane [26–28]. The corresponding open-cage-like POSS structure, shown in Fig. 1, has been used as surface modifier for pigments that served for making TSSS paint coatings [3]. In view of its open-cage-like structure (Fig. 1) with reactive silanol groups, trisilanol POSS is expected to possess several distinct advantages over traditional silane surface modifiers. Since it possesses three stable silanol moieties, they cannot react with one another by forming siloxane bonds (Si–O–Si) as in the case of ordinary trialkoxysilanes, with which interactions between the silanol groups can compete with the pigment surface/silanol interactions. In the case of alkyltrialkoxysilanes, the surface monolayers are difficult to make in a well controlled manner. Understanding the formation and structure of the layer around the pigment particle is of general importance for obtaining stable pigment dispersions, which require well balanced pigment surface/polymer binder interactions. Since a single POSS molecule has three reactive silanol groups per molecule, a robust pigment/ POSS interfacial bonding forms. In view of the structure of trisilanol POSS molecules, consisting of reactive silanols at one end and bulky isobutyl groups around the cage, the POSS Fig. 1. Structure of heptaisobutyltrisilanol silsesquioxane (trisilanol POSS). molecules are expected preferentially to interact with the pigment surface rather than among themselves. Because of this, trisilanol POSS molecules can gradually occupy available active sites on the pigment surface and the corresponding surface coverage depends on concentration of the POSS molecules equilibrated with the pigment. Furthermore trisilanol POSS molecules, which can potentially pile up on the pigment surface, are considerably less strongly bonded and can be removed by washing. Accordingly, we focused in this study on establishing the amount of trisilanol POSS molecules attached to the pigment surface and how they bind to the pigment surface. Infrared spectra and TEM combined with EDAX measurements were exploited for this purpose. Only a few reports exist on trisilanol POSS molecules as dispersants for pigments in polymeric resins. Trisilanol POSS (Fig. 1) has been already tested and used in the modification of nanosized TiO2 particles, which were then incorporated in polypropylene (PP) polymer [29]. Chemically similar trisilanolphenyl POSS has also been used so far for the incorporation of TiO2 particles in PMMA with good success [30] and blended into polycarbonate (PC) but, conversely, trisilanolisooctyl POSS makes bulk PC opaque, indicating poor dispersion of the POSS [31]. In our recent studies of the TISS paint coatings, it was demonstrated with thermogravimetric (TG) and infrared spectra studies that the interactions between IB7 T7(OH)3 POSS dispersant and the pigment surface are sufficiently strong to provide the formation of highly concentrated fluoropolymer binder/pigment dispersions [3], but direct evidence of strong, probably covalent coupling of the trisilanol POSS dispersant with the surface of the PK 3060 pigment has not yet been established. Infrared spectroscopy has been extensively used for recording the effect of surface modification. For thin molecular layers on smooth metal surfaces, near-grazing incidence reflection–absorption (NGIRA) spectra are used [20,32]. For pigments, the attenuated total reflection (ATR) spectroscopy developed by Harrick [33] and Fahrenforth [34] is a more appropriate technique, enabling the study of adsorption and reactions at liquid/solid interfaces. Such studies have been carried out in a variety of research and technological areas, including biomembranes [35], biofilms [36], thin film structure and reactivity [37,38] and electrochemistry [39], for the identification of the interactions of silane modifiers for textile fabrics [40]. Despite the advantages of the ATR surface sensitive technique, in this study we used infrared transmission spectra measurements because they provide clear assignment of the bands of the pigment. Namely, in the ATR spectra, the shape of the strong bands of the pigments is distorted and their frequencies are shifted to lower frequencies with respect to those observed in the transmission infrared spectra [41]. In this study, a subtracting procedure based on the measured transmission spectra was used for detecting the thin layer of trisilanol POSS on the surface of the PK 3060 pigment. TEM measurements were chosen because – as has been recently demonstrated by Siddiquey et al. [10] and also Bruce and Sen [9] – they represent an elegant means of detecting the thin layer of the coupling agent that forms on the surface of the nanoparticles. For example, in high resolution TEM micrographs silica layer just a few nanometers thick formed after the application of polydimethoxysiloxane on the spindle-like TiO2 nanocrystals [10], and aminoprolyltrimethoxysilane on the surface of magnetic nanoparticles [9] has also been identified. Even though TEM combined with EDAX measurements is an efficient method for establishing the surface layer on pigment particles, it does not provide direct evidence about chemical bonding. The same holds for the Langmuir adsorption isotherm, which gives a clear indication of the amount of adsorbed layer on the surface of a pigment but the nature of chemical bonding I. Jerman et al. / Solar Energy Materials & Solar Cells 95 (2011) 423–431 425 between the adsorbed layer and the pigment surface cannot be assessed. Infrared spectra are capable of providing such information through frequency changes or, more often, the complete disappearance of vibrational bands attributed to molecular groups involved in surface modifier/pigment interactions. Accordingly, in order to corroborate the results of the SEM and EDAX measurements and to obtain evidence of the trisilanol POSS/pigment surface interactions, infrared spectra of the functionalized PK 3060 pigment were performed. The as-received pigment was equilibrated in solutions having increasing amounts of trisilanol POSS, which was not done in our previous study [3]. The surplus trisilanol POSS was then washed off and the concentration of the POSS that remained attached to the pigment surface was established from the measured intensity of the vibrational bands. These measurements thus provided information about the builtup POSS layer formed as a function of the POSS concentration solution equilibrated with the pigment. Moreover, the infrared spectra also gave an indication that the silanol groups of POSS participated in the established bonding. Stability of pigment dispersion is usually controlled by diluting the pigment dispersion in a suitable solvent and determining the corresponding sedimentation times. However, this approach is not quantitative and SEM micrographs of pigment dispersion, as demonstrated in this study, proved to be more illustrative of the effect of functionalization, revealing the distribution of the particles and their size. Finally, properties of the novel fluoropolymer based Thickness Sensitive Spectrally Selective (TSSS) paint coating is briefly discussed. preparation of pigment dispersions inorganic pigment (i.e. asreceived Mn–Fe spinel) PK 3060 (Ferro Company) was used. Trisilanol POSS (20 g) was dissolved in hexane (600 mL) to which the black pigment PK 3060 (200 g) was added and mixed with a high speed dissolver (Dispermat CN F2 (VMA-GETZMAN GMBH, D-51580 Reichshof) to prepare good dispersion, which was then displaced to ball mill (glass balls ø 3 mm, 4000 rpm) for 2 h. We repeat the same procedure also for other concentrations of trisilanol POSS. For IR spectroscopy samples were washed three times with THF and dried in vacuum before preparation of KBr pellets. Open trisilanol POSS with attached titanium or silicon atoms was obtained by the corner-capping methodology [42,43]. The resultant dry functionalized (10% w/w trisilanol POSS) PK 3060 pigment was used for the preparation of paints. The concentration of the pigment in the resin binder was 53%. The pigment paste was spread on the silicon wafer and used for making SEM micrographs. Perfluoropolymer resin binder (Lumiflon 200) was obtained from Asahi Company, Japan], while 1,6-diisocyanatohexane (DICH) hardener and some other hightemperature cross-linkers were purchased from Bayer, while Cymel hardener was obtained from Cytec Industries Inc. Paint coatings were made as reported in [23,24,44] by omitting the incorporation of aluminium flakes in the paint dispersions. In addition to Lumiflon TSSS paints cured at ambient conditions, TSSS coatings cured at high temperatures (up to 200 1C) were also made and tested. The latter TSSS paint coatings are important for practical applications because addition of high-temperature curing agents enables the manufacturing of absorbers via the continuous coil-coating application techniques. 2. Experimental 3. Results and discussion 2.1. Instrumental 3.1. Functionalization of pigment Infrared transmission spectra of functionalized pigment PK 3060 were recorded on a Bruker IFS 66/S spectrophotometer with a resolution of 4 cm  1 and 64 scans for each sample. For measuring the infrared spectra the pigment was finely triturated with KBr and pressed into pellets. SEM micrographs were obtained on a FE-SEM Supra 35 VP electron scanning microscope. Transmission electron microscopy (TEM) micrographs were obtained on a JEOL JEM-2100 high resolution transmission electron microscope (HR-TEM) operating at 200 keV. Energy dispersive X-ray spectroscopy (EDXS) measurements were performed on a JEOL JED-2300T EDS system with high energy resolution and high sensitivity. X-ray photoelectron spectroscopic (XPS or ESCA) analyses were carried out on a PHI-TFA XPS spectrometer produced by Physical Electronics Inc. The pigments were pressed into compact and homogeneous pellets; the analyzed area was 0.4 mm in diameter and the depth analyzed was about 1–3 nm. The sample surfaces were excited by X-ray radiation from an Al source. Quantification of surface composition was performed from the XPS peak intensities by taking into account the relative sensitivity factors provided by the instrument manufacturer. Sample charging during XPS analysis was compensated by a lowenergy electron gun neutralizer. 3.1.1. Infrared spectra measurements The adsorption of silanes is a complex phenomenon because many different bonds can form: hydrogen bonds, acid–base bonds (Lewis or Broensted) and siloxane bonds. In the case of adsorption from an organic solvent, the situation is complicated by the presence of physisorbed water on the pigment surface, which influences hydrolysis of the silane molecules [45–48]. Most of the alkoxysilane hydrolyze and quickly condense, forming not just silane/pigment bonding but also interacting among themselves, making control of the modifier monolayers difficult. Building up the modifier layer around the pigment particles can be followed either by inspection of the spectra of the pigment [13,49] or it can be inferred from the vibrational bands of the surface modifier, the intensities of which depend on the number of the pigment surface sites occupied by the modifier (see below). The alkoxysilane modifier–pigment interactions can be ascertained from the disappearance of the silanol (Si–OH) bands and the concurrent presence of the Si–O substrate [9] and the siloxane (Si–O–Si) modes. However, the latter modes complicate the situation, since they signal condensation reactions and the consequent formation of the silsesquioxane layer around the pigment particles. In this regard, silanol groups of open-cage POSS may be an exception because they do not spontaneously lead to condensation as silanols of other simple alkoxysilanes but preferentially interact with active sites on the surface of the pigment, as demonstrated below. In this case, a build-up of monolayers around the pigment particles is more likely to be established as shown below. In the first step, infrared signature of various silanol POSS molecules was established from measured transmission spectra [3,50]. The most typical vibrations are attributed to the presence 2.2. Preparation of materials Trisilanol POSS dispersant was synthesized as already reported [3] and the description will not be repeated here. For the 426 I. Jerman et al. / Solar Energy Materials & Solar Cells 95 (2011) 423–431 of the silanol moieties at 3250 and 893 cm  1 (Fig. 2A). A prominent band of the trisilanol silsesquioxane cage appeared at 1118 cm  1 [3] and was accompanied by CH2 (2927 and 2903 cm  1) and CH3 (2954, 28701 and 2817 cm  1) stretching [51] and corresponding deformational modes (1461, 1350, 1228 and 839 cm  1) [52]. The infrared spectra of as-received and heat-treated PK 3060 pigment (Fig. 3) were measured next. The broad shoulder bands at 1070 and  940 cm  1 also observed in the spectra of heattreated pigment correspond to the Si–O linkages inherent to the as-received spinel pigment (see EDAX bellow), showing the most prominent bands of the spinel lattice at 465 and 580 cm  1, which match exactly the bands observed in the spectra of thin polycrystalline spinel films made via the sol–gel process reported previously [53,54]. Even though the infrared spectra of PK 3060 pigment are relatively simple to interpret, they did not provide information about the surface hydroxyl groups prerequisite for the establishment of the trisilanol POSS/pigment interactions but merely revealed a broad HOH vibrational band of hydrated oxide (Fig. 3). Inorganic pigments (titania, ZnO, rare-earth metal oxides and other Mo- and V-containing oxide systems) mostly have hydroxyl–hydrate coverage and the corresponding surface hydroxyl vibrational modes can be found in the infrared spectra in the spectral region well above (3700–3400 cm  1) the spectral region, where there is a broad vibrational band of adsorbed water molecules (3300–3200 cm  1; Fig. 3) [55]. Spinels are no exception, showing 3–4 sharp vibrational bands in the spectral region from 3520 to 3675 cm  1. The hydroxyls appear because of the presence of the coordinatively unsaturated sites usually present around crystallite sharp corners and cusps (see Section 3.1.3, Fig. 8), creating sites onto which H2O molecules become attached. Recording hydroxyl modes in infrared spectra requires the vacuum treatment of powders at high temperatures ( 4250 1C), which was outside the scope of this study. Previous studies of the functionalization of various nanoparticles [8–11,14] confirmed that silanes tethered to defects on the imperfect surface sites of the nanoparticles, replacing water molecules, and become attached to the surface hydroxyl groups. It is highly probable that the same mechanism also exists for the functionalization of PK 3060 pigment with trisilanol POSS molecules, as demonstrated below. The pigment was equilibrated in trisilanol POSS/hexane mixtures containing 0.6%, 1.25%, 2.5%, 5% and 10% of trisilanol POSS modifier, washed 3 times with THF, dried and the spectra were then recorded (Figs. 4 and 5). Even though the bands attributed to the trisilanol POSS were weak, closer inspection of the peak intensity of the band at 2954 cm  1 and the gradually increased area of the band at 1228 cm  1 indicated the progressive occupation of the accessible sites on the pigment surface (Fig. 6). Because the modified pigment was thoroughly washed before spectra measurements, it can be imagined that, at lower concentrations, the trisilanol POSS molecules formed a discontinuous layer on the pigment surface. Above the saturation limit (  2–3% concentration of POSS), complete coverage of the pigment was achieved and a monolayer of trisilanol POSS was formed on the pigment surface. Comparison of the infrared spectra of the trisilanol POSS (Fig. 4h) and modified PK 3060 pigment (Figs. 4 and 5) revealed Fig. 2. Infrared transmission spectra of open IB7T7(OH)3 POSS: (A) metalsilsesquioxane (trisilanol POSS with attached titanium atoms), (B) the synthesis product of open POSS with attached iodopropyltrimethoxysilane (IPTMS) and (C) made by corner-capping reactions. Fig. 3. Infrared spectra of as-received Mn–Fe spinel pigment, pigment heattreated at 400 1C (24 h). Inset: detailed view of the bands in the 1200–800 cm  1. Fig. 4. IR transmission spectra of PK 3060 pigment equilibrated in trisilanol POSS/ hexane mixture containing different concentrations of trisilanol POSS and after ball milling for 2 h afterwards washing 3 times with THF before the spectra were recorded. a—as received, b—0.6%, c—1.25%, d—2.5%, e—5%, f—10% w/w, g—unwashed pigment in 10% w/w of modifier in hexane and trisilanol POSS and h—IB7 T7(OH)3 POSS. I. Jerman et al. / Solar Energy Materials & Solar Cells 95 (2011) 423–431 427 Fig. 5. Intensity variations of bands in the spectral regions 2600–3100 and 1700–700 cm  1 (a—as-received, b—0.6%, c—1.25%, d—2.5%, e—5% and f—10% w/w and g—unwashed pigment in 10% w/w of modifier in hexane). Fig. 6. Variations of the peak area (band at 1228 cm  1) and peak height (2954 cm  1) of the trisilanol POSS equilibrated in trisilanol POSS/hexane mixtures. POSS, which gave the octameric IPIB7 T8 POSS (Fig. 2C), while the reaction of titanium isopropoxyde led to the formation of closed cages with titanium atoms attached to one corner of the closed silsesquioxane cage (Fig. 2B). Inspection of the IR spectra (Fig. 2) revealed the presence of the characteristic bands attributed to established Si–O–Si and Ti–O–Si linkages [57]. Vibrational bands originating from the established covalent bonding of trisilanol POSS with the PK 3060 pigment (Si–O–M, M ¼Mn, Fe) obviously could not be identified from the infrared spectra, since they appeared outside the measuring range of our spectrometer ( o400 cm  1). However, as regards the dispersion study results demonstrated that trisilanol POSS gradually took a position on the pigment surface by forming discontinuously at the beginning but at saturating conditions the monolayer coverage was obtained. Higher trisilanol POSS coverage was possible but in this case the surplus layer of the modifier was easily washed off, leaving just the monolayer firmly attached to the pigment surface. that the silanol band at 893 cm  1 (Fig. 2) was not present in the spectra of the modified pigment but could still be seen in the spectra of the unwashed pigment (Fig. 5g). This band has comparable intensity with the band at 1228 cm  1 (  CH3 deformational mode) and its absence from the spectra of all but the washed modified pigments confirmed without doubt the establishment of strong interactions between the trisilanol POSS and the surface of the pigment. The corresponding interactions are very probably covalent in nature due to reactivity of the silanols of simple silanes [20] and POSS trisilanols [22] and their tendency for condensation and to bond via the establishment of Si–O–M (M ¼Al, Cu) [56]. The existence of covalent bonding with the surface of the PK 3060 pigment was also expected due to the ability of IB7 T7(OH)3 POSS to form via corner-capping [42,43] closed octameric T8 silsesquioxanes. For example, we reacted simple IPTMS with trisilanol 3.1.2. XPS spectra of pigments In order to obtain information about the surface of the pigments, XPS depth profiles of variously treated pigments were measured. The XPS depth profiles were first determined for asreceived commercial PK 3060 pigment, followed by measurements of the same pigment washed 3 times with THF, while the third sample corresponded to pigment modified with trisilanol POSS (see Experimental). The last-named sample was then put in an oven and heat treated at 400 1C for 24 h. The XPS depth profiles are shown schematically in Fig. 7. As expected, the as-received PK 3060 pigment showed the composition of Mn–Fe spinel, with a small amount of silicon, suggesting the presence of inorganic silicate in the spinel structure or merely on the pigment surface. This problem was beyond the scope of this study. After washing, a small part of oxygen was removed and, consequently, carbon and silicon concentrations increased. Functionalization of the PK 3060 pigment with trisilanol POSS increased considerably the concentration of silicon from 0.8 to 428 I. Jerman et al. / Solar Energy Materials & Solar Cells 95 (2011) 423–431 Fig. 7. Schematic presentation of XPS depths profiles analysis (at%) obtained for the as-received and washed PK 3060 pigment and washed (3  ) functionalized pigment and the same pigment after heat-treatment at 400 1C (24 h). Fig. 9. (A) TEM micrographs of analyzed areas marked with circles and (B) corresponding EDSX spectra of the functionalized pigment PK 3060. Fig. 8. TEM micrographs of the surface modified pigment grains at different magnifications (A) at 100 nm and (B) at 20 nm. The blurred contour at the edge of the grain marked with a broken line represents a layer of trisilanol POSS modifier. 2.15 at% and increased further to 3.7 at% after heat treatment. These changes were accompanied with a decrease in the concentration of carbon, in line with the thermal decomposition of the surface modifier. The results of the XPS spectra were quite conclusive and fully supported the results obtained from analysis of the infrared spectra of PK 3060 pigment (Fig. 7). 3.1.3. TEM and EDAX measurements of Mn–Fe spinel pigment TEM micrographs (Fig. 8A and B) of the functionalized pigment showed agglomerates of the pigment particles, with dimensions up to a few hundreds of nanometers (Fig. 8A). At higher magnifications, a thin contour was noted at the edge of the agglomerates and on the surface of the grains of approximately the same size as that of the POSS molecules (i.e. 1.8–2.4 nm; Fig. 8B). We attributed these contours to the POSS layer. In order to confirm that the observed contours could be attributed to the POSS attached to the surface of the grains, EDXS analysis of the treated pigment was performed on selected sites, as shown in Fig. 9A and B. Examination of the EDXS results (Fig. 9B) revealed silicon and carbon signals together with other signals attributed to the spinel pigment. The existence of all signals is clearly consistent with the presence of the pigment modified with POSS, agreeing with the XPS depths profiles (Fig. 7) and infrared spectra analysis (Figs. 3–5). As expected the EDAX spectra of as-received PK 3060 also showed the presence of silicon in the samples. 3.1.4. SEM micrographs The effect of the functionalization of the PK 3060 pigment particles with trisilanol POSS can be inferred from the SEM micrograph shown in Fig. 10, revealing well separated and evenly distributed pigment particles with a particle size of around 300 nm. The empty space between the particles was filled with resin binder. It should be noted that despite the high dilution of I. Jerman et al. / Solar Energy Materials & Solar Cells 95 (2011) 423–431 the paint with the solvent a homogeneous layer formed without any signs of flocculated pigment particles. 3.2. Spectral selectivity of TSSS paint coatings Paint coatings for solar absorbers have attracted interest since the introduction of the concept of spectral selectivity [58–60]. According to this concept, the most viable solar absorber coatings for solar collectors are based on the absorber–reflector tandem technique. In this technique, selectivity is achieved through successive deposition of highly reflecting and solar absorbing materials. Because bulk aluminium or copper sheets are excellent low-emitting substrates, many tandem coatings have been developed in the past [61,62]. The absorber–reflector tandem concept is directly applicable for paints [63,64], because it provides TSSS paint coatings using the controlled deposition (spraying, brushing and coil coating) of suitably diluted pigment dispersions (i.e., paints) onto a lowemitting metal substrate [61]. It is therefore not surprising that the first formulations of paints and the corresponding selective coatings were reported nearly 40–50 years ago [65,66]. The paints have been made of a silicone resin binder with dispersed carbon soot 429 [67] or inorganic pigment (Mn–Fe spinel black) [68,69], the latter providing TSSS paint coatings with solar absorptance (as)  0.90 and thermal emittance (eT) from 0.18 up to 0.25 (depending on thickness) and adequate long-term stability [70,71]. For the absorber–reflector tandem, thickness of a paint coating is one of the most crucial parameters, having the greatest influence on its spectral selectivity. In general, as curves show a relatively steep increase at low coating thicknesses and leveling at higher coating thicknesses, while the eT curve inclines regularly with coating thickness [72]. The fast increase in as values at small coating thicknesses indicates that it is difficult to dilute the paint in order to control thickness, due to flocculation of the pigment particles at high dilutions (i.e. small g/m2) but pigment agglomeration when coating thickness increases. The leveling of the as curve observed at higher coating thicknesses usually results in binding of a large amount of the resin binder, which causes an unwanted increase in eT values. In the limiting case, spectrally non-selective coatings with as 40.92 and eT  0.90–0.92 form. In order to provide evidence supporting the beneficial effect of POSS dispersant on spectral selectivity, the as and eT curves (Fig. 11) as a function of paint thickness (in g/m2) were measured. Inspection of spectral selectivity variation as a function of coating thickness revealed smooth increase in as and eT values in the whole range of coating thicknesses, confirming high stability of the pigment dispersion against dilution and absence of the pigment flocculation. Acceptable values (as  0.90 and eT  0.20) of the spectral selectivity of TSSS paint coatings can be obtained for fluoropolymer based paints at a coating thickness of around 1.5–2.0 g/m2. The fact that relatively thick coatings also provided spectral selectivity has a far reaching effect on the possibility of applying these coatings using a coil-coating process. Obviously, the key factor for selectivity was high pigment loadings achieved with trisilanol POSS dispersant as discussed in more detail above. 4. Conclusions Fig. 10. SEM micrograph of cured TISS coating surface. The results of this study revealed that Lumiflon 200 fluoropolymer based paint coatings can be made from a pigment dispersion containing trisilanol POSS modified PK 3060 pigment particles. Infrared spectra analysis confirmed that the trisilanol POSS bound covalently onto the pigment surface by gradually building up a monomolecular monolayer, which was also inferred from the measured TEM micrographs and the corresponding EDAX Fig. 11. Variation of thermal absorbance as (dotted line) and emittance eT (solid line) as a function of thickness of paint (g/m2) made with modified pigment (10% w/w). 430 I. Jerman et al. / Solar Energy Materials & Solar Cells 95 (2011) 423–431 measurements. TEM and SEM micrographs revealed uniformly dispersed pigment particle with an average particle size of around 300 nm. The functionalized pigment particles showed extremely good compatibility with the resin binder, deriving from their easy embedment in high concentrations (450%) into the resin binder matrix without pigment particle flocculation being observed, resulting in solar absorptance above 0.90 and thermal emittance of around 0.20. UV stability and high corrosion resistance of the coatings ranked fluoropolymer based TSSS paint coating among the most promising selective paint coatings for solar absorbers made to date. The TSSS paint coatings also exhibited inherent water repellent properties expressed by water contact angles around 1101. The hydrophobicity of the coating stemmed directly from the pigment particles functionalized with the hydrophobic trisilano isobutyl POSS surface modifier. 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