pubs.acs.org/Langmuir © 2010 American Chemical Society
A Straightforward Way To Form Close-Packed TiO2 Particle Monolayers at an Air/Water Interface Cathy E. McNamee,*,† Shinpei Yamamoto,‡ Hans-Juergen Butt,§ and Ko Higashitani^ †
Shinshu University, Tokida 3-15-1, Ueda-shi, Nagano-ken 386-8567, Japan, ‡Institute for Integrated Cell-Material Sciences, Kyoto University, Yoshida-Ushinomiyacho, Sakyo-ku, Kyoto 606-8501, Japan, § Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany, and ^Department of Chemical Engineering, Kyoto University - Katsura, Nishikyo-ku, Kyoto, Kyoto 615-8510, Japan Received July 20, 2010. Revised Manuscript Received December 6, 2010 The aim of this study was to analyze if and how monolayers of TiO2 particles could be directly formed at the air/water interface and if these monolayers could be transferred to a solid surface. TiO2 particles with diameters of 300 nm, 500 nm, 1 μm, 5 μm, 10 μm, and 20 μm formed stable monolayers at pH 2. At low surface pressures, the particles formed small two-dimensional aggregates. Particles up to a radius of 5 μm displayed close packing at increased surface pressures. Particles of 10 μm radius formed a loose network, which is attributed to the strong adhesion caused by the weightinduced lateral capillary attraction. Every monolayer of particles could be transformed to a solid surface by the Langmuir-Blodgett deposition. At pH 6 or 11, the particles did not form stable monolayers at the air/water interface. They were instead dispersed in the aqueous phase and eventually sank to the bottom of the trough. At pH 11 the monolayer could, however, be stabilized by the addition of salt (0.5 M NaCl). The results are interpreted based on a changed wettability of the particles depending on pH and salt concentration.
1. Introduction 1
2
Semiconductors and solar cells made of self-assembled nanoclusters on substrates showed the potential of giving a material whose optical and electrical properties could be controlled. TiO2 has been used for such materials due to its good physical-chemical properties such as its charge controlling ability, its photoluminescence, photocatalytic activity, and semiconductivity that may be applied for such applications and its extensive characterization.3 Substrates coated with TiO2 particles have mostly been produced via a crystal growth method,4 deposition of nanoparticles from a gas phase to a substrate and their subsequent sintering,5,6 and dipcoating of the substrate into a solution of nanoparticles.7 These methods have the disadvantage that the number of particles adsorbed to the surface and their spacing are difficult to control. Additionally, the size of the particles that can be attached to the substrates is mostly limited to nanosizes due to the attachment or growth techniques. For some applications, however, micro-sized TiO2 particles may also be required. The Langmuir trough allows monolayers of a material to be produced at an air/liquid interface. Monolayers of surfactants, polymers, and particles have been made in this manner. The packing and orientation of the materials composing the monolayer may be controlled by changing the area between the two barriers of the Langmuir trough. A Langmuir-Blodgett deposition of the monolayer to a substrate allows the transfer of the material in its *To whom correspondence should be addressed. E-mail: mcnamee@ shinshu-u.ac.jp. (1) Barbe, C. J.; Arendse, F.; Comte, P.; Jirousek, M.; Lenzmann, F.; Shklover, V.; Gratzel, M. J. Am. Chem. Soc. 1997, 80(12), 3157–3171. (2) ORegan, B.; Gr€atzel, M. Nature 1991, 353, 737–740. (3) Diebold, U. Surf. Sci. Rep. 2003, 48, 53–229. (4) Mohammadi, M. R.; Fray, D. J. Acta Mater. 2007, 55, 4455–4466. (5) Rothenberger, G.; Fitzmaurice, D.; Gr€atzel, M. J. Phys. Chem. 1992, 96(14), 5983–5986. (6) Ohtani, B.; Ogawa, Y.; Nishimoto, S. J. Phys. Chem. B 1997, 101, 3746–3752. (7) Vinodgopal, K.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1993, 97, 9040–9044.
Langmuir 2011, 27(3), 887–894
controlled state to the substrate. Monolayers of particles at an air/ liquid interface have been made using the Langmuir trough technique for polymers,8-10 synthesized Janus particles,11 modified hydrophilic particles,12,13 and small particles (nano-sized).14 If the contact angle of the water on the particle surface is larger than zero, the particles remain at the air/liquid interface due to normal capillary forces.15-17 Other forces that are known to act in such a system18 include the flotation/lateral capillary force,19,20 the force of gravity, the interparticle and particle-interface electrostatic forces, and the interparticle and particle-interface van der Waals forces. For Janus particles, the degree of amphiphilicity also affects the ability of particles to form monolayers. The size of the particle, its surface potential, its shape, and physical properties (e.g., hydrophobicity) can therefore influence the magnitude of the flotation force and gravity and therefore the ability of a particle to align at an air/liquid interface. The aim of this study was to make monolayers of TiO2 particles at an air/aqueous interface. In order to study the conditions and (8) Pan, F.; Zhang, J.; Cai, C.; Wang, T. Langmuir 2006, 22, 7101–7104. (9) Basavaraj, M. G.; Fuller, G. G.; Fransaer, J.; Vermant, J. Langmuir 2006, 22, 6605–6612. (10) Tsai, P.-S.; Yang, Y.-M.; Lee, Y.-L. Langmuir 2006, 22, 5660–5665. (11) Pradhan, S.; Brown, L. E.; Konopelski, J. P.; Chen, S. Nanopart. Res. 2009, 11, 1895–1903. (12) Jeon, H. J.; Moon, S. D.; Kang, Y. S. Colloids Surf., A 2005, 257-258 165–169. (13) Muramatsu, K.; Takahashi, M.; Tahima, K.; Kobayashi, K. J. Colloid Interface Sci. 2001, 242, 127–132. (14) Tolnai, G.; Agod, A.; Kovacs, A. L.; Ramsden, J. J.; Horv€olgyi, Z. J. Phys. Chem. B 2003, 107, 11109–11116. (15) Scheludko, A.; Toshev, B. V.; Bojadjiev, D. T. J. Chem. Soc., Faraday Trans. 1 1976, 72, 2815–2828. (16) Aveyard, R.; Clint, J. H. J. Chem. Soc., Faraday Trans. 1995, 91, 2681– 2697. (17) Preuss, M.; Butt, H.-J. Int. J. Miner. Process. 1999, 56, 99–115. (18) Bresme, F.; Oettel, M. J. Phys.: Condens. Matter 2007, 19, 413101. (19) Danov, K. D.; Kralchevsky, P. A.; Naydenov, B. N.; Brenn, G. J. Colloid Interface Sci. 2005, 287, 121–134. (20) Kralchevsky, P. A.; Nagayama, K. Adv. Colloid Interface Sci. 2000, 85, 145–192.
Published on Web 12/28/2010
DOI: 10.1021/la102893x
887
Article
McNamee et al. Table 1. Parameters of the Particles Used in This Study
particle diameter (D)
standard deviation (μm)
shape
density (g/cm3)
porosity
product no.
0.3 μm TiO2 0.5 μm TiO2 1 μm TiO2 5 μm TiO2 10 μm TiO2 20 μm TiO2 5 μm SiO2
