Understanding properties of engineered catalyst supports using contact angle measurements and X-Ray reflectivity

Abstract

There is significant interest in broadening the type of catalyst substrates that support the growth of high-quality carbon nanotube (CNT) carpets. In this study, ion beam bombardment has been utilized to modify catalyst substrates for CNT carpet growth. Using a combination of contact angle measurements (CAMs) and X-ray reflectivity (XRR) for the first time, new correlations between the physicochemical properties of pristine and engineered catalyst substrates and CNT growth behavior have been established. The engineered surfaces obtained after exposure to different degrees of ion beam damage have distinct physicochemical properties (porosity, layer thickness, and acid-base properties). The CAM data were analyzed using the van Oss-Chaudhury-Good model, enabling the determination of the acid-base properties of the substrate surfaces. For the XRR data, a Fourier analysis of the interference patterns enabled extraction of layer thickness, while the atomic density and interfacial roughness were extracted by analyzing the amplitude of the interference oscillations. The dramatic transformation of the substrate from "inactive" to "active" is attributed to a combined effect of substrate porosity or damage depth and Lewis basicity. The results reveal that the efficiency of catalyst substrates can be further improved by increasing the substrate basicity, if the minimum surface porosity is established. This study advances the use of a non-thermochemical approach for catalyst substrate engineering, as well as demonstrates the combined utility of CAM and XRR as a powerful, nondestructive, and reliable tool for rational catalyst design.

Rational Modification of a Metallic Substrate for CVD Growth of Carbon Nanotubes

“…Note that for the same Al x O y thickness, CNTs grown on damaged substrates have higher coverage and I G /I D , which demonstrates that ion beam bombardment enhances CNT growth efficiency from Fe catalysts. We observed similar improvement in growth efficiency on c-cut sapphire substrates after ion beam bombardment, which was attributed to changes in Lewis basicity, porosity, and damage depth of substrates 28 . Note that unlike c-cut sapphire, in the present study, ion beam damage alone does not transform SS to an “active” catalyst support for CNT growth, as evidenced by the lack of growth on Damaged-0.…”

“…From Fig. 2 , coverage of CNTs on pristine and damaged SS increases with barrier-layer thickness, which based on previous studies correlates with increased surface porosity and the basic environment provided by the Al x O y barrier layer 28 – 30 , 32 . Also, from the Raman data in Fig.…”

“…2 ), this suggests that modification via ion beam bombardment alone under the conditions used in this study is incapable of transforming SS from an “inactive” to an “active” substrate. In the case of sapphire 28 , 30 , it was possible to achieve complete transformation after ion beam damage alone, because the process created both surface porosity and increased basicity. Creation of cationic vacancies during ion beam damage, which was evidenced by an increased O/Al ratio, accounted for increased basicity.…”

“…The modification approach implemented in this study results in growth of CNT arrays on 316 SS. The approach is inspired by previous investigations 28 – 32 on the role of Al x O y as a catalyst support in CNT carpet growth, whereby the high growth efficiency observed for a supported Fe catalyst was attributed to the combined effect of surface porosity and Lewis basicity of Al x O y . In particular, our results show that ion beam bombardment of 316 SS decreases the film thickness of Al x O y required for CNT growth to 5 nm Al x O y , a thickness that does not impede electronic transport across the barrier 17 , 33 .…”

A Forest of Sub-1.5-nm-wide Single-Walled Carbon Nanotubes over an Engineered Alumina Support

“…Amorphous alumina having high porosity has been preferred and able to result in good yield and fast growth, while single crystalline sapphire has been recognized as an “inactive” support. In light of modifying the alumina property, energetic ions bombarding onto a single-crystalline sapphire layer has been shown to turn the exposed surface into a porous layer, usable as an active catalyst support for the VA-CNT growth3334. Bearing a potential practicability issue for large scale production, the effect of this method on the resultant nanotube diameters remains to be elucidated.…”