DOI: 10.1002/cplu.201300426 Dispersion Characteristics and Aggregation in Titanate Nanowire Colloids Endre Horvµth, [a] Lucie Grebikova, [b] Plinio Maroni, [b] Tamµs Szabó, [b, c] Arnaud Magrez, [a] Lµszló Forró, [a] and Istvan Szilagyi* [b] Introduction Titanate derivatives with anisotropic particle shape are attract- ing great interest owing to their growing importance in nu- merous applications, for example, in solar cells, [1–3] sensorics, [4] polymer composites, [5] as sources for the synthesis of novel materials, [6–8] and in photocatalytic water treatment. [9–13] In the latter application, their photocatalytic activity is exploited, and titanate compounds such as spherical particles, wires, platelets, and their composites with other materials are used as aqueous colloid systems. [14, 15] Major challenges in these processes in- clude the provision of a stable dispersion during the catalytic run and the post-treatment removal of the suspended catalyst from the effluent stream of purified water. The catalyst recov- ery can be achieved through aggregation and subsequent sed- imentation followed by filtration of the titanate compounds, or through their deposition on appropriate materials. [16] The type of liquid-phase aggregation of the nanoparticles has a signifi- cant effect on the textural properties such as the porosity, pore-size distribution, and mechanical stability of different so- lution casts, for example, dip-coated, screen-printed, and doctor-bladed functional coatings. [3] Importantly, the nano- and micron-scale structure of the aggregates greatly influences their level of toxicity, and hence, their potential impact on human health and the environment. [17] Therefore, detailed study of the colloidal behaviour of these catalysts and other nanomaterials suspended in liquid media is necessary. [18–20] The stability of aqueous titanate dispersions has been the focus of many research groups worldwide, with investigations on the charging, surface forces, and aggregation processes of titanate nanoparticles, [21–23] nanotubes, [24] and nanosheets. [25, 26] For instance, direct measurements of surface forces by the col- loidal probe technique based on atomic force microscopy (AFM) revealed that the interaction between titanium dioxide surfaces across electrolyte solutions can be described well with the classical theory for the stability of aqueous colloidal disper- sions developed by Derjaguin, Landau, Verwey, and Overbeek (DLVO). [27, 28] Accordingly, the dispersions are stable at low elec- trolyte concentrations at which repulsive forces originating from the overlap of the electrical double layers predominate, whereas aggregation occurs at high salt levels at which the surface charges are screened and attractive van der Waals forces drive the interactions between the particles. Although numerous titanate derivatives have been prepared and applied in aqueous systems, the aggregation processes Titanate nanowires (TiONWs) are synthesized through the hy- drothermal method and characterized in acidic aqueous dis- persions by using electrophoresis, dynamic light scattering, and atomic force microscopy. The TiONWs have a rodlike shape with an average length of about 600 nm and a thickness of 35 nm. They are positively charged under the conditions used. Their surface charge properties and aggregation are in- vestigated in the presence of oppositely charged poly(styrene sulfonate) (PSS) polyelectrolyte. Charge neutralization followed by a subsequent charge reversal process is observed, which is attributed to the adsorption of PSS. The colloids are unstable near the charge neutralization point and stable at lower and higher PSS doses, in good qualitative agreement with the theory developed by Derjaguin, Landau, Verwey, and Overbeek (DLVO). The nanowires prefer to align along the walls, leading to “spaghetti-like” oriented aggregates. The aggregation pro- cesses of bare and PSS-coated TiONWs are monitored at differ- ent concentrations of an inert electrolyte; slow aggregation is found at low salt levels, whereas aggregation is rapid beyond the critical coagulation concentration, as predicted by the DLVO theory, which describes the colloid stability of the TiONWs adequately in all the systems investigated. Coating of the nanowires with the polyelectrolyte leads to a critical coag- ulation concentration 75 times higher than that of the bare ti- tanates, indicating the enormous stabilizing effect of PSS. [a] Dr. E. Horvµth, Dr. A. Magrez, Prof. L. Forró Laboratory of Physics of Complex Matter École Polytechnique FØdØrale de Lausanne CH-1015 Lausanne (Switzerland) [b] L. Grebikova, Dr. P. Maroni, Dr. T. Szabó, Dr. I. Szilagyi Laboratory of Colloid and Surface Chemistry University of Geneva CH-1205 Geneva (Switzerland) Fax: (+ 41) 22 379 6069 E-mail : istvan.szilagyi@unige.ch [c] Dr. T. Szabó Department of Physical Chemistry and Materials Science University of Szeged 6720 Szeged (Hungary) This article is part of the “Early Career Series”. To view the complete series, visit: http://chempluschem.org/earlycareer.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPlusChem 2014, 79, 592 – 600 592 CHEMPLUSCHEM FULL PAPERS