AVS2014 Session TF-MoA: Self-Assembled Monolayers, Layer-by-Layer Assemblies, and Hydrophobic/Amphiphobic Thin Films

Self-assembled monolayers (SAMs) based on C-Au bonding and prepared by reaction of terminal n-alkynes (HCΞC(CH₂)_nCH₃, n = 5, 7, 9, and 11) with Au(111) at elevated temperatures (60 ^oC), were characterized using scanning tunneling microscopy (STM), infra-red reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS) and contact angle of water.¹ In contrast to previous spectroscopic studies^2-4 of this type of SAM, these combined microscopic and spectroscopic experiments confirm the formation of highly-ordered SAMs having packing densities and molecular chain orientations very similar to those of alkanethiols on Au(111). Physical properties—hydrophobicity, high surface order, and packing density—also suggest that SAMs of alkynes are similar to SAMs of alkanethiols. The preparation of high-quality SAMs from alkynes requires careful preparation and manipulation of the reactants in a rigorously oxygen-free environment: trace quantities of oxygen lead to oxidized contaminants and disordered surface films. The influence of oxygen on the quality of the SAM is apparently not related to reaction of the Au−C bonds in a SAM with oxygen as suggested earlier,³ but instead, suggests gold-catalyzed oxidation of the terminal acetylene in solution before incorporation into the SAM. Importantly, once clean alkyne based SAM is formed it becomes resistant to further oxidation in ambient conditions. This stability, together with high structural order, provides the basis for potential applications of this new type of SAM.

References

(1) Zaba, T.; Noworolska, A.; Bowers, C. M.; Breiten, B.; Whitesides, G. M.; Cyganik, P. submitted, 2014

(2) Zhang, S.; Chandra, K. L.; Gorman, C. B. J. Am. Chem. Soc. 2007, 129, 4876.

(3) McDonagh, A. M.; Zareie, H. M.; Ford, M. J.; Barton, C. S.; Ginic-Markovic, M.; Matisons, J. G. J. Am. Chem. Soc. 2007, 129, 3533.

(4) Scholz, F.; Kaletova, E.; Stensrud, E. S.; Ford, W. E.; Kohutova, A.; Mucha, M.; Stibor, I.; Michl, J.; Wrochem, F. , J. Phys. Chem. Lett. 2013, 4, 2624.

Ultra thin polyelectrolyte layers have been constructed by sequential adsorption of PAH (polyallylamine hydrochloride) as cationic polyelectrolyte and PSS (polystyrene sulfonate) as anionic polyelectrolyte on silicon substrates using layer-by-layer assembly. Negatively capped AgCu nano-sized particles are incorporated as the outermost layer on the ultra thin film. The sequential order of adsorbed layers and nanoparticles are monitored by relative depth profile of the sample with respect to the intensity changes of corresponding photoelectron peaks, via Angle Resolved XPS analysis at different take off angles. Dynamic XPS analysis is performed under the application of external voltage bias with a known amplitude and frequency to control and probe the charging shifts, as well as polarity dependent intensity changes. The shifts in the binding energy position and the intensity fluctuations of photoelectron peaks reveal information about the relative position of the adsorbed polyelectrolyte layers, as well as proximity of atomic constituents of the nanoparticles.

In recent years, production of materials with tunable wetting properties is of immense interest. The extreme water repellent property of superhydrophibic surfaces and complete water spreading on superhydrophilic surface allow them to have considerable technical potential for various applications. The wetting properties of the materials can be explained by their surface chemistry and topographical structures.

In this report, we demonstrate that the growth ambient induced drastic change in wetting properties of indium oxide (IO) nanowires. The IO nanowires were synthesized by using chemical vapor deposition method where Ar gas (200 sccm) was used as carrier gas. The deposition parameters were calibrated in such a way to obtain nanowire morphology. Three different ambient conditions were used for growth of IO nanowires; (a) Ar gas mixed with water vapors, (b) only Ar gas and (c) Ar gas mixed with hydrogen gas (50 sccm) and keeping other deposition parameters constant. The Scanning electron microscope (SEM) images confirm that all the three samples have nanowires like morphology. The diameter and length of nanowires ranges from 50 - 120 nm and 6 - 15 µm. The contact angle measurements were done on all the three samples. It is found that the nanowires prepared in presence of Ar gas mixed with water vapors (oxidising ambient) are superhydrophilic in nature with contact angle of 8 ̊ ± 5 ̊. The IO nanowires synthesized in presence of only Ar gas are hydrophobic in nature with contact angle of 144 ̊ ± 4 ̊ whereas the IO nanowires synthesized in presence of Ar gas mixed with H₂ gas (reducing ambient) are superhydrophobic in nature with contact angle of 168 ̊ ± 2 ̊ and water droplet rolling downward with a roll-off angle of 3 ̊ over the superhydrophobic surface. The mechanism behind the drastic change in contact angle on IO nanowires prepared in different growth ambient is examined by using photoluminescence (PL) and electron paramagnetic resonance (EPR) measurements. The mechanism of change in wetting properties of IO nanowires has been proposed and can be attributed as on the reduced surface where oxygen vacancy are more, only molecular water is stable and absorb weekly. This is most likely because of the lack of surface oxygen that could accept hydrogen from dissociated water or decrease water dissociation probability and hence the surface is superhydrophobic. Whereas the sample prepared in oxidizing ambient have more surface oxygen and hence both molecular and dissociative adsorption of water is possible results in superhydrophilic surface. The photoinduced reversible wetting properties of IO nanowires (sample b) are also studied.

Proton conductive copolymers of perfluorodecylacrylate (PFDA) and methacrylic acid (MAA) are synthesized by initiated Chemical Vapor Deposition (iCVD). The MAA provides the –COOH groups useful to conduct protons, while the PFDA is responsible for creating the hydrophobic backbone to stabilize the structure during tests in water. The ultimate goal is to use these copolymers as proton exchange membranes in fuel cells. Preliminary experiments have shown that proton conductivities in the range of 70 mS/cm can be reached with these copolymers.¹ The aim of the new research is to study the effect of preferred crystallographic orientation on the proton conductivity and water / temperature stability of the copolymers. Preferred crystallographic orientation (texture) in thin films frequently has a strong effect on the properties of the materials and it is important for stable surface properties. Poly-PFDA has a high tendency to give organized molecular films. Crystalline poly-PFDA have been fully obtained also by iCVD.² The degree of crystallinity and the preferred orientation of the perfluoro side chains, either parallel or perpendicular to the surface, can be controlled by tuning the CVD process parameters (i.e. initiator to monomer flow rate ratio, filament temperature, and substrate temperature). Super-hydrophobicity (advancing water contact angle, WCA, of 160°, low hysteresis of 5°), and oleophobicity (advancing CA with mineral oil of 120°) were achieved.³ Low water contact angle hysteresis was obtained with high crystallinity, particularly when the orientation of the crystallites resulted in the perfluoro side groups being oriented parallel to the surface. The latter texture resulted in smoother film (RMS roughness < 30 nm) than the texture with the chains oriented perpendicularly to the surface. This can be very advantageous for our application that require smooth but still crystalline films. When the PFDA is copolymerized with MAA, the degree of crystallinity decreases and therefore also the stability in water, but the proton conductivity increases due to the higher number of acid groups embedded in the structure. A good trade-off has been obtained when using 20% of MAA in the gas feed.

¹ A.M. Coclite et al., Polymer, 2013, 54, 24-30

² A. M. Coclite et al., Adv. Funct. Mater. 2012, 22, 2167–2176

³ A. M. Coclite et al., Adv. Mater. 2012, 24, 4534–4539