Improved small-angle x-ray scattering of nanoparticle self-assembly using a cell with a flat liquid surface

SAXS measurements were made of self-assembled iron oxide NP arrays at the DEG surface in sample cells fabricated with three different types of central sections (bodies). Liquids and NP colloids were injected into each cell and synchrotron SAXS measurements were made in them at the 11-BM beamline at the Brookhaven National Laboratory National Synchrotron Light Source II (NSLS-II). The x-ray windows on the entrance and exit sides of each cell body were 0.0762-mm thick, flat fluorinated ethylene propylene (FEP) because of their very small x-ray absorption. These bodies and the entrance and exit side flanges were made of the same type of material, each with O-ring grooves and screw holes. O-rings held the windows in place on either side and formed the inner seals after the bolts spanning from flange to body to flange were tightened. The body design of the sample cell used in previous work on NP self-assembly on liquid surfaces called the “cylindrical cell”, had a right circular cylinder (21.0 mm diameter, with 7.7 mm length) bored through aluminum, so the sidewalls were curved, and the x-ray windows were both vertical; small holes on top and the sides permitted fluid injection and the evaporation of any volatile solvent. Two different sample cells were manufactured using a Formlabs Form 2® printer, with a resin of methacrylic acid esters and photoinitiators to produce translucent or white poly(methyl methacrylate) (PMMA) when exposed to ultraviolet light. In the “tapered cell” the dimension of the cavity in the body along the x-ray path (the length) was 8.8 mm and the transverse dimension (the width) was 51.8 mm at the bottom. The entrance and exit walls, sealed by 57.7 mm × 22.8 mm FEP windows, and the sidewalls were all tapered outward from the bottom of the cell at an angle of 75o to the horizontal, so the length of the inner part of the body increased from 8.8 mm to 20.6 mm at the top, 22 mm above the bottom. This angle was chosen to match and thereby cancel the contact angle of DEG on FEP, as determined by dropping DEG on it and observing the angle formed between the liquid and solid surfaces; the same angle was used for the sidewalls to lessen the meniscus there. However, for the sidewalls, the 75o angle was used only half way up (11 mm height, where the width had increased from 51.8 mm to 57.7 mm) and then the walls were vertical to the top. When used for SAXS, the cell was filled with DEG to a height of 5.5 mm to form the liquid substrate, so tapering was not needed all the way to the top. The body of the second type of printed cell had vertical walls. The lengths and widths of the “reference printed cell” matched the corresponding values at the top of the liquid substrate in the tapered cell, 11.7 mm and 54.7 mm. Both printed cells had a square hole on top that permitted solvent injection and evaporation. In other applications, a solid plug or one with a small hole can be inserted in this hole to control the escape of volatile solvent. The surface tension contact angle of DEG on FEP was the same for pure DEG, and for DEG that had sat in translucent or white PMMA tapered cells for 20 h under nitrogen, so (for the purpose of these experiments) use of PMMA did not affect the DEG and the meniscus formed. Each cell was partially filled with DEG (with 1.3 mL (to 10.5 mm height) in the cylindrical cell, with 3.0 mL in the reference (4.7 mm) and tapered (5.5 mm) printed cells) to serve as a liquid substrate and then a small amount of NP colloid in the highly volatile solvent hexane was drop-cast. DEG was chosen because it satisfies the needs of drop-casting, such low volatility and insolubility with the hexane solvent, and has a relatively low, though non-negligible, x-ray absorption coefficient of 164.54/m at the 13.5 keV photon energy used (Berkeley Lab website). The formation of ordered 2D NP arrays in these cells was studied after drop-casting 20 µL of a colloid of iron oxide NPs in hexane (which contained the number of NPs needed to form one monolayer in that particular cell) on top of the DEG substrate and subsequent hexane evaporation. This corresponded to an initial hexane thickness of 0.12 mm in the cylindrical cell and 0.03 mm in the printed cells (with these thicknesses assuming flat interfaces). NPs with a diameter of 11.8 nm were synthesized using previously published methods and subsequently encapsulated by oleate ligands (Hyeon et al. 2001). After rapid hexane evaporation (<1 min in the printed cells), the structure of the NP monolayer at the liquid-gas surface was probed using SAXS. The x-ray beam size was 50 µm (vertical) × 200 µm (horizontal). The detector used for SAXS measurements was a Pilatus3 S 2M, which was set 3 m away from the sample cell. Before and after each SAXS measurement, the DEG meniscus was characterized by x-ray transmission as a function of vertical position.