pubs.acs.org/Langmuir © 2011 American Chemical Society
Template-Assisted Electrostatic Spray Deposition as a New Route to Mesoporous, Macroporous, and Hierarchically Porous Oxide Films S. Sokolov,† B. Paul,‡ E. Ortel,‡ A. Fischer,‡ and R. Kraehnert*,‡ †
Leibniz-Institute for Catalysis at the University of Rostock, Albert-Einstein Str. 29a, D-18059 Rostock, Germany, and ‡Department of Chemistry, Technical University of Berlin, Strasse des 17. Juni 124, D-10623 Berlin, Germany Received October 25, 2010. Revised Manuscript Received December 10, 2010
A novel film coating technique, template-assisted electrostatic spray deposition (TAESD), was developed for the synthesis of porous metal oxide films and tested on TiO2. Organic templates are codeposited with the titania precursor by electrostatic spray deposition and then removed during calcination. Resultant films are highly porous with pores casted by uniformly sized templates, which introduced a new level of control over the pore morphology for the ESD method. Employing the amphiphilic block copolymer Pluronic P123, PMMA latex spheres, or a combination of the two, mesoporous, macroporous, and hierarchically porous TiO2 films are obtained. Decoupled from other coating parameters, film thickness can be controlled by deposition time or depositing multiple layers while maintaining the coating’s structure and integrity.
1. Introduction Titanium dioxide films find commercial applications in different areas including water and air purification,1,2 gas sensing,3,4 and photovoltaics.5,6 Most of these applications benefit from high accessible surface areas and controlled pore sizes. Porous TiO2 films can be prepared by particle-assisted chemical vapor deposition,7 hydrothermal methods,8 or, more commonly, solgel-based techniques.9-11 The advantage of sol-gel-based approaches is that they allow the introduction of templates as structure directing agents, enabling strict control over the pore size and morphology in the meso- and macropore regime. In particular, crystalline TiO2 films with ordered mesostructure were prepared by template-assisted dip-coating where pore order was derived from evaporation-induced self-assembly (EISA) of amphiphilic block copolymer micelles formed in titania precursor solution.12-14 The TiO2 film thickness typically achieved in a single coating step is in the range of about 100-400 nm depending on coating *Corresponding author. E-mail:
[email protected]. (1) Butterfield, I. M.; Christensen, P. A.; Curtis, T. P.; Gunlazuardi, J. Water Res. 1997, 31(3), 675–677. (2) Wang, T.; Wang, H.; Xu, P.; Zhao, X.; Liu, Y.; Chao, S. Thin Solid Films 1998, 334(1-2), 103–108. (3) Demarne, V.; Balkanova, S.; Grisel, A.; Rosenfeld, D.; Levy, F. Sens. Actuators, B 1993, 14(1-3), 497–498. (4) Lu, C.; Chen, Z. Sens. Actuators, B 2009, 140(1), 109–115. (5) Li, Y.; Hagen, J.; Schaffrath, W.; Otschik, P.; Haarer, D. Sol. Energy Mater. Sol. Cells 1998, 56(2), 167–174. (6) Kang, M. G.; Park, N.-G.; Ryu, K. S.; Chang, S. H.; Kim, K.-J. Sol. Energy Mater. Sol. Cells 2006, 90(5), 574–581. (7) Backman, U.; Auvinen, A.; Jokiniemi, J. K. Surf. Coat. Technol. 2005, 192 (1), 81–87. (8) Zhao, X.; Liu, M. H.; Zhu, Y. F. Thin Solid Films 2007, 515(18), 7127–7134. (9) Negishi, N.; Takeuchi, K.; Ibusuki, T. Appl. Surf. Sci. 1997, 121-122, 417– 420. (10) Kajihara, K.; Yao, T. J. Sol-Gel Sci. Technol. 2000, 17(3), 239–245. (11) Arconada, N.; Duran, A.; Suarez, S.; Portela, R.; Coronado, J. M.; Sanchez, B.; Castro, Y. Appl. Catal., B 2009, 86(1-2), 1–7. (12) Crepaldi, E. L.; Soler-Illia, G.; Grosso, D.; Cagnol, F.; Ribot, F.; Sanchez, C. J. Am. Chem. Soc. 2003, 125(32), 9770–9786. (13) Fattakhova-Rohlfing, D.; Wark, M.; Brezesinski, T.; Smarsly, B. M.; Rathousky, J. Adv. Funct. Mater. 2007, 17(1), 123–132. (14) Bosc, F.; Ayral, A.; Albouy, P. A.; Guizard, C. Chem. Mater. 2003, 15(12), 2463–2468.
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conditions, whereas a film thickness exceeding 1 μm can be obtained only by repeated coating/calcination cycles (see e.g. ref 15). However, dip- or spin-coating preparative techniques typically used for such films impose certain limitations on size and geometry of the coated substrates. One can overcome these limitations by applying a more flexible film deposition technique known as electrostatic spray deposition (ESD). In this method, high electric potential applied between the solution feed (nozzle) and the substrate atomizes liquid and transports droplets to the substrate surface. Upon landing, droplets dry leaving a film of solid precursor, which is converted into metal oxide during calcination. The morphology of the sprayed films can be controlled by adjusting deposition parameters such as flow rate, applied potential, nozzle geometry, substrate temperature, and precursor solution composition.16,17 Porous films can also be prepared by ESD. Spraying high-boiling solution onto a substrate heated above the solutions boiling point results in films with open reticular structure formed by gas bubbles originated within the droplets upon boiling. Numerous metal oxides with such morphology were prepared by ESD,16,18,19 including TiO2.20 In the latter study, a series of titania films were produced by spraying differently aged titania sols onto hot steel substrates. By combining aged and freshly prepared sols, the authors obtained macroporous films with good integrity and interconnected pores of ca. 5 μm in size. However, control over pore morphology and size in conventional ESD is difficult: pores produced by solvent boiling show irregular shapes and a wide size distribution. Moreover, only macropores can be formed due to the nature of the pore forming process, (15) Prochazka, J.; Kavan, L.; Zukalova, M.; Frank, O.; Kalbac, M.; Zukal, A.; Klementova, M.; Carbone, D.; Graetzel, M. Chem. Mater. 2009, 21(8), 1457–1464. (16) Chen, C. H.; Emond, M. H. J.; Kelder, E. M.; Meester, B.; Schoonman, J. J. Aerosol Sci. 1999, 30(7), 959–967. (17) Jaworek, A.; Sobczyk, A. T. J. Electrost. 2008, 66(3-4), 197–219. (18) Chen, C. H.; Buysman, A. A. J.; Kelder, E. M.; Schoonman, J. Solid State Ionics 1995, 80(1-2), 1–4. (19) Neagu, R.; Djurado, E.; Ortega, L.; Pagnier, T. Solid State Ionics 2006, 177 (17-18), 1443–1449. (20) Nomura, M.; Meester, B.; Schoonman, J.; Kapteijn, F.; Moulijn, J. A. Chem. Mater. 2003, 15(6), 1283–1288.
Published on Web 01/10/2011
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Table 1. Composition of Solutions Employed for the Synthesis of Titania Films solution solution “RDC” (reference dip-coating) stock TTIP/P123 stock PMMA ESD solution 1 ESD solution 2 ESD solution 3 ESD solution 4
TTIP, mmol/L
P123, g/L
280
36.4
50
7.1
5 50 5 5
0.71 7.1
PMMA, g/L
6.2
0.71
3.1 3.1
mainly gas templating. To gain more control over the pore size and pore shape in the ESD process, template-assisted electrostatic spray deposition (TAESD) was developed in this work. Templates such as amphiphilic block copolymers (Pluronic P123) and/or PMMA spheres could be successfully integrated, individually or jointly, into the ESD process, giving rise to well-defined mesoporous, macroporous, or hierarchically porous structures. Merging the two film deposition techniques combines the benefits of nanocasting and spray deposition, namely control over the pore morphology with flexibility in substrate choice and film thickness.
2. Experimental Section Formation of a stable cone jet is an essential requirement for obtaining films with controlled thickness and morphology via ESD. In a series of preliminary experiments, typical ethanol-based dip-coating solutions containing Pluronic F127 as a template and titanium tetrachloride as titanium oxide precursor failed to form a cone jet in the 2-10 kV range. However, solutions of Pluronic P123 and titanium(IV) isopropoxide in dry 1-butanol gave a stable cone jet in a reasonable voltage window. Hence, butanol was selected as a solvent for further spraying experiments. For TAESD of TiO2 films, stock solutions of 50 mmol/L titanium(IV) isopropoxide (TTIP, Fluka Chemie) and 7.1 g/L (∼1.2 mmol/L) Pluronic P123 (BASF) in 1-butanol (Roth) were prepared. PMMA spheres employed as macropore template were synthesized by surfactant-free emulsion polymerization as reported elsewhere.21 The average sphere size obtained from scanning electron micrographs was 408 ( 12 nm. Stock solutions of PMMA were prepared by mixing 1.24 mL of 48 wt % PMMA aqueous suspension with 100 mL of 1-butanol, which resulted in the final PMMA concentration of 6.2 g/L. The solutions for ESD tests were obtained by mixing the stock solutions followed by dilution with 1-butanol. Compositions of all solutions used in this study are given in Table 1. Titania coatings were prepared by electrostatic spray deposition in a cone-jet mode16,22 using a vertical setup similar to that described in the work of Chen et al.23 Si wafers and steel plates (grade 1.4517) were used as substrates. The nozzle-to-substrate distance was varied for different experiments between 12 and 20 mm. Voltages in the 3.0-4.0 kV range were required to establish a cone-jet spraying mode. Spray solutions were fed by a syringe pump at 0.5 or 1 mL/h. The substrate temperature was kept between 80 and 120 °C, and deposition times were varied from 6 to 60 min. Substrates with a double layer of titania were thermally treated between successive depositions (300 °C for 30 min; heating ramp 1 K/min) remaining mounted on the heater. All coated substrates were calcined for 30 min at 500 °C (heating rate 1 K/ min) in air flow, except the samples used for TiO2 phase transformation study, which were calcined at 600, 700, and 800 °C. (21) Munro, D.; Goodall, A. R.; Wilkinson, M. C.; Randle, K.; Hearn, J. J. Colloid Interface Sci. 1979, 68(1), 1–13. (22) Taylor, G. Proc. R. Soc. London, A 1964, 280(138), 383. (23) Chen, C. H.; Kelder, E. M.; vanderPut, P.; Schoonman, J. J. Mater. Chem. 1996, 6(5), 765–771.
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Figure 1. SEM images of calcined TiO2 films on Si substrates prepared by (a) dip-coating “solution RDC”, (b) TAESD of “solution 1” for 6 min, and (c) TAESD of “solution 2” for 6 min. All films were calcined at 500 °C. Reference samples were prepared by dip-coating Si substrates in a butanol solution of TTIP and P123 (Table 1) with the concentration of P123 being close to its solubility limit. Dip-coating was performed in a controlled atmosphere at 35 °C, 20% relative humidity, and a withdrawal rate of 60 mm/min. The as-prepared films were calcined in flowing air at 500 °C for 30 min with a heating rate of 1 K/min. Integrity and morphology of the calcined films were investigated by scanning electron microscopy (SEM) on JEOL 7401F operated between 4.0 and 8.0 kV. Pore size and pore size distribution were determined by measuring the diameter of at least 100 pores using ImageJ software (National Institutes of Health). The mesostructure of the film fragments removed from the substrate was further investigated by transmission electron microscopy (TEM) measurements on a Tecnai G220S-Twin operated at 200 kV as well as a Zeiss EM Omega 912X at an acceleration voltage of 120 kV. X-ray diffraction (XRD) of the films was performed on a URD 6 diffractometer in BraggBrentano configuration using Cu KR radiation (λ = 0.1546 nm) with a position-sensitive detector (PSD-50M, Hecus, Graz). The total BET surface area of the calcined samples, normalized to the substrates geometrical dimensions (m2 coating surface area per m2 substrate), was measured via Kr physisorption at 77 K on a Micromeritics ASAP 2010 instrument. DOI: 10.1021/la104272h
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Figure 2. XRD patterns of TiO2 films prepared by TAESD from “solution 1” and calcined at temperatures of 500, 600, 700, and 800 °C. Reflections indexed on a diffractogram of 800 °C sample correspond to anatase.
3. Results and Discussion 3.1. Dip-Coated Mesoporous Reference TiO2 Coatings. Prior to the TAESD experiments, feasibility of templating mesopores with micelles of Pluronic P123 in water-free butanol solutions had to be verified. In the earlier work Prochazka et al. reported the synthesis of mesoporous TiO2 films via dip-coating from solutions containing butanol and water in ca. 1:8.4 weight ratio,15 corresponding to a reverse micellar phase in the P123butanol-water phase diagram.24 However, at zero water content, the reverse micellar phase is formed only at low P123 concentrations (e.g., less than 5 wt %). This could be the upper concentration limit for this particular template in water-free butanol solutions, above which the templated structure is lost. Indeed, as solvent evaporates during film drying, polymer concentration reaches the solubility limit beyond which phase separation, i.e., disintegration of micelles, occurs. On the other hand, condensing inorganic precursor can trap the micelles before they collapse, thus “freezing” the mesophase. To study which process dominates, the reference samples were prepared by dip-coating using dry n-butanol solution of TTIP and P123 (solution “RDC” reference dip-coating, Table 1). Figure 1a shows an SEM image of an as-prepared TiO2 film after template removal at 500 °C. The film exhibits disordered mesopores with ca. 5 nm openings with pore walls somewhat sintered during calcination. Templated mesostructure of the film verifies that P123 micelles survive in nonaqueous butanol solution under current coating conditions. This finding leads us to propose that similar mesophase can be obtained in the films coated by ESD where heated substrates afford similar drying conditions. 3.2. Templated Mesoporous TiO2 Films by TAESD. Mesoporous TiO2 films were prepared by TAESD under identical spraying conditions and deposition time (6 min) from precursor solutions with different TTIP/P123 concentrations (see Table 1) on 1.4571 steel substrates heated to 80 °C. Figure 1b displays SEM micrographs of the film obtained by spraying the “solution 1” followed by calcination at 500 °C. The film showed good substrate coverage (Figure 1b, left) and was penetrated by a network of disordered yet interconnected pores with an average opening size of 5.4 ( 0.8 nm (Figure 1b, right). Ten times more (24) Holmqvist, P.; Alexandridis, P.; Lindman, B. J. Phys. Chem. B 1998, 102(7), 1149–1158.
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Figure 3. TEM images with electron diffraction (insets) of TiO2
films prepared by TAESD from “solution 1” and calcined at 500 °C (a) and at 800 °C (b).
concentrated solution 2 produced strongly fragmented film (Figure 1c, left), which did not appear to have templated mesostructure. Examined at higher magnification (Figure 1c, right), the film fragments exhibited some porosity; however, the pores were not visibly connected and had substantially smaller openings than in the film formed from the solution of lower precursors’ concentration. Loss of mesostructure indicates that micelles collapsed before TTIP condensation occurred, which could be a result of greater film thickness and consequently slower drying. The above findings demonstrate that ESD is a nanocasting compatible process for deposition of mesoporous metal oxide films. Solution 1 was assumed for further spraying experiments to ensure formation of the mesostructure and homogeneous surface coverage. To analyze crystallization behavior of TiO2 films formed by ESD, films calcined at different temperatures were analyzed by X-ray diffraction (Figure 2). Films calcined at 500 and 600 °C exhibit no signs of crystallinity. At 700 °C, the TiO2 starts to crystallize as evidenced by a single broad reflection at 2θ = 25.3°, which might correspond to the most intense peak (101) of the anatase phase. Further heating to 800 °C considerably improved crystallinity of the sample, and the XRD pattern could be indexed as anatase (25.3° (101), 37.8° (004), 48.1° (200), and 53.9° (105)). The absence of crystallinity observed in the diffractograms for the ESD-derived samples calcined at 500 and 600 °C cannot be Langmuir 2011, 27(5), 1972–1977
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Figure 4. SEM cross section (a) and planar (b) views of calcined TiO2 films prepared from “solution 1” by ESD on a steel plate heated to 120 °C in the attempt to cast macropores by solvent boiling. Film thickness is ca. 3 μm.
attributed to a lack of sensitivity of the employed XRD setup, since films of similar thickness that were prepared via dip-coating from TiCl4 solutions, calcined by the same calcination protocol, and then analyzed with the same setup provide clear evidence of crystalline TiO2 (see e.g. ref 25). Hence, formation of the anatase phase is shifted toward higher calcination temperatures than commonly observed for titania. To further analyze the films crystallinity evolution, electron diffraction (ED) was performed as a part of TEM investigation. TEM images of the films prepared by TAESD and calcined at 500 and 800 °C are shown in parts a and b of Figure 3, respectively. The image of the sample calcined at 500 °C reveals the mesoporous structure originating from the micelle template extending through the entire thickness of the film fragment. Examined by the electron diffraction, the pore walls showed no signs of crystallinity as evidenced by absence of the diffraction rings (inset in Figure 3a), which is in agreement with XRD measurement. Calcined at 800 °C, the TiO2 films turned crystalline as evidenced by the diffraction rings on ED pattern (inset in Figure 3b). However, the templated pore structure was partially lost due to crystallites sintering as seen on the TEM image in Figure 3b. The observations made on TAESD-derived TiO2 films indicate clearly the influence of the metal precursor and pore template on the metal oxide crystallization temperature and mesostructure thermal stability. For instance, the crystallization behavior described above differs from that of the films prepared by dipcoating from the KLE block copolymer-TiCl4 ethanolic solution, which were completely crystalline when treated at 500 °C.26 (25) Ortel, E.; Sokolov, S.; Kraehnert, R. Microporous Mesoporous Mater. 2009, 127(1-2), 17–24. (26) Rathousky, J.; Rohlfing, D. F.; Wark, M.; Brezesinski, T.; Smarsly, B. Thin Solid Films 2007, 515(16), 6541–6543.
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Figure 5. SEM images of calcined TiO2 films on Si substrate prepared by TAESD with PMMA latex templates from “solution 3”. Film thickness is ca. 1.7 μm.
Lower crystallization temperature of those TiCl4-derived samples most likely arises from a partial condensation of titania precursor in the acidic solution of TiCl4. On the other hand, TTIP in butanol remains in molecular solution through ESD and starts to condense during drying and thermal treatment. It is also anticipated that bigger pores will be separated by thicker walls which should improve mechanical and thermal stability of the structure.26 Pore size is a function of template, since employing larger diblock copolymers (“KLE”) allowed coating TiO2 films with ordered 10 nm pores separated by 9-10 nm walls.26 In such films the mesoporous architecture remained intact when calcined at temperatures up to 700 °C, while with the smaller P123 template the pores started to collapse at 600 °C.13 Since the primary goal of the present study was to test the potential of the template-assisted ESD, all further samples were calcined at 500 °C to avoid the pore structure collapse. 3.3. Spray-Coating Templated Macroporous and Hierarchically Porous Films. By analogy with template-free ESD preparation procedures reported in the literature for metal oxide films with open reticular structure molded by evaporated solvent bubbles,16,17 the solution 1 used for mesoporous films coating was sprayed on a steel substrate heated to 120 °C (1-butanol bp 117 °C) followed by calcination at 500 °C. SEM images of the obtained film (Figure 4) show abundant macropores replicated by solvent bubbles. However, the pores appeared disconnected, varied in size greatly, and were isolated from the film surface by a layer of lower porosity (Figure 4a). Moreover, the integrity of the film significantly deteriorated compared to the coatings made at 80 °C: the film deposited at 120 °C bore numerous intersecting fractures of different length and width which radiated from the areas with corrugated or missing coating fragments (Figure 4b). In contrast, employing hard organic porogens such as latex spheres should enable a firmer control over the macropore size DOI: 10.1021/la104272h
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Figure 7. SEM images of calcined TiO2 films coated from solution 1 on Si substrates for 24 min (a, planar view; b cross section) and of a double-layer film 2 24 min (c, planar view; d, cross section).
Figure 6. SEM images of hierarchically porous calcined TiO2 films deposited by TAESD from “solution 4” containing P123 and PMMA templates: (a) 1000, (b) 10 000, and (c) 200 000. Film thickness is ca. 1.8 μm.
and morphology. PMMA latex has been previously successfully used as a pore template for macroporous metal oxide powders and films,27,28 yet as far as we know never in ESD process. We thus tested feasibility of integrating hard templates into ESD by spraying a colloidal solution of PMMA latex in TTIP/butanol (solution 3) on a Si substrate heated to only 80 °C to avoid bubble formation. After calcination at 500 °C, the film was extensively porous with macropores interconnected and clearly molded by PMMA spheres (Figure 5a). It is noteworthy that the macropore walls had no discernible mesostructure as can be seen on a higher magnification micrograph in Figure 5b. Comparing macropores produced by boiling solvent and by PMMA templates, one can observe significant improvements in film porosity, pore connectivity, and size control rendered by hard templates. Once the procedures for depositing meso- and macroporous films were worked out, the two templates were combined in order (27) Stein, A.; Li, F.; Denny, N. R. Chem. Mater. 2007, 20(3), 649–666. (28) Kuang, D.; Brezesinski, T.; Smarsly, B. J. Am. Chem. Soc. 2004, 126(34), 10534–10535.
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to create films with hierarchical pore architecture. Solution 4 containing PMMA latex spheres and Pluronic P123 was electrosprayed on Si substrate under the same conditions as for purely macroporous films. The top-view SEM image of the calcined film shown in Figure 6a reveals titania domains densely covering the substrate interspersed with occasional glades of Si showing through the coating. Loss of the film integrity likely occurred during templates burning and TTIP decomposition into TiO2 accompanied by volume reduction. A micrograph of a single domain displayed in Figure 6b shows the resultant open macroporous structure practically indistinguishable from the one observed on purely macroporous PMMA-templated films. However, close inspection of the coating revealed a network of mesopores extensively penetrating macropore walls (Figure 6c). The morphology of the pores appears very similar to that found in purely mesoporous films discussed earlier (Figure 1), which clearly points at the template origins of the macropore walls’ mesostructure. Remarkably, such hierarchical pore architecture spanned over the entire coated substrate area, with no phase separation between the templates, which would result in purely mesoporous or macroporous inclusions. 3.5. Extended Deposition Time and Multilayer Synthesis To Increase the Coatings Surface Area. Increasing film thickness creates more active surface available per planar area of a substrate, a characteristic desirable for many applications. However, thicker films are prone to fracturing, which can be a disadvantage for certain applications. In this work, we demonstrate that it is possible to increase the film thickness with the current coating method while preserving the mesostructure and maintaining integrity by depositing a double layer. Solution 2 was deposited on Si substrates for 24 min for a single layer film and in two 24 min stages with intermediate thermal stabilization of the first stage at 300 °C for a double layer. The overview SEM images of these samples presented in Figure 7a (24 min) and Figure 7c (2 24 min) demonstrate good coating integrity with no visible film fracturing on a millimeter scale. The thickness measured in the center of the deposition spot on the film cross sections increased Langmuir 2011, 27(5), 1972–1977
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from 145 nm after 24 min (Figure 7b) to 285 nm for a double-layer film (Figure 7d). Moreover, also the surface area scaled respectively from 88 to 165 m2 coating surface area per m2 substrate. It is noteworthy that both single- and double-layer films were porous through their entire thickness as seen on the cross-section images.
4. Conclusion Template-assisted electrostatic spray deposition (TAESD) was developed and tested in coating titania films with controlled porosity and thickness. Pores were formed by nanocasting techniques employing organic templates. Amphiphilic block copolymer Pluronic P123 as well as PMMA latex spheres were successfully employed in water-free butanol as meso- and macropore templates, respectively. Films with controlled mesoporous, macroporous and hierarchical pore structure were prepared by using either or a combination of the two templates. Pores were randomly oriented in meso- and macroregimes, but formed highly open interconnected networks including hierarchically porous films, where macropore walls
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were threaded with P123-templated mesopores. Feasibility of forming thicker mesoporous coatings with preserved integrity was demonstrated by depositing a double-layer film. The surface area of the film per planar area of the substrate was nearly doubled by coating the second layer. In summary, the presented work demonstrates that the template-assisted synthesis of porous metal oxides can be integrated into electrostatic spray deposition, which was illustrated here for TiO2 films. Independent choice of templates allows for a strict control over the pore size and morphology in both ranges while film thickness can be adjusted via deposition time. The ability to structure films on macro- and mesoscale realized through the use of organic templates extends the versatility of the ESD and places it among other coating techniques with structure directing function. Acknowledgment. Generous funding from BMBF within the frame of the NanoFutur program (FKZ 03X5517A) is gratefully acknowledged.
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