The relationship between solution pH and effects of applied voltage in layer-by-layer (LbL) self-assembly was investigated, using weakly charged polyelectrolytes and nanoparticles. Poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA) films and PAH/SiO 2 nanoparticle films were fabricated at several solution pH conditions, by LbL self-assembly under an applied voltage. Adsorption behavior and surface morphology of films fabricated under applied voltages depended on the solution pH. A textured structure was formed by applied voltage at a specific pH for PAH/PAA films. The effect of applied voltage on film deposition was related to the electrical charge density and polymer chain conformation. The effects of applied voltage were higher, when the polymer electrical charge density was higher, that is, when the polymer chain exhibited a stretched structure. It was found that the amount of adsorption increased at all pH conditions under the applied voltage.
■ INTRODUCTIONLayer-by-layer (LbL) self-assembly is a promising technique for fabricating uniform thin films composed of polyelectrolytes and nanoparticles. LbL films can be fabricated through the alternating adsorption of cationic and anionic solutions, with intermediary aqueous rinsing. LbL films are suitable for use as optical films, 1−3 antithrombotic films, 4 superhydrophobic films, 5,6 separation films, 7,8 biosensors, 9,10 and chemical sensors. 11 The eco-friendly method uses aqueous solutions to control the film thickness on the nanometer scale, under room temperature and ambient pressure. 12 Despite the LbL methods advantages, adsorption takes a long time, because it involves the electrostatic interaction of oppositely charged materials. 13 To improve the practical use of LbL self-assembly, it is necessary to reduce the adsorption duration. Fabricating LbL films rapidly has been attempted. 14 A spray LbL method has been developed, which significantly reduces the adsorption duration, and involves solutions sprayed directly onto substrates. 15−17 Applying external forces to the LbL process has also been studied to enhance adsorption, and applied magnetic fields and dynamic pressures reportedly accelerate LbL adsorption. 18−20 LbL methods under applied voltages have attracted recent attention. 21−32 A counter electrode is arranged parallel to the substrate, and voltage is applied between the substrate and counter electrode during adsorption. Applied voltages influence the film deposition of polyelectrolytes, nanocrystals, nanoparticles, enzyme, and nanotubes. Sun and co-workers reported that adsorbed volumes increased upon applying voltage, and dense poly(diallyldimethylammonium chloride (PDDA)/CdTe nanocrystal films could be fabricated. 25 Zhang and co-workers showed that PDDA/poly(sodium 4-styrenesulfonate) (PSS) and PDDA/PSS-ZrO2 film thickness and surface roughness increased upon applying a voltage. 21 Ko and co-workers fabricated linear poly(ethylene imine) (LPEI)/poly(acrylic acid) (PAA) films by LbL self-assembly under an applied voltage, and i...