Enhanced organic species identification via laser structuring of carbon monolithic surfaces

Highlights

• Developed method of laser structuring lacy fragile carbon monoliths

• Physical and chemical property determination of carbon monoliths

• Comparison presented between templated and non-templated carbon monoliths

• Significant enhancement of flow and adsorption reported for laser processed monoliths

• Elaboration of areas for further improvements carbon monolith preparation

Abstract

It is important to be able to control the physical and chemical integrity of carbon-based porous monolith structures while being tailored for targeted analytical, energy and catalytic based applications. A set up using a CO₂ laser in continuous mode (CW CO₂ laser) was implemented to cut fragile and porous carbon monolithic (CM) and nanotemplated carbon monolithic (NTCM) rods into discs with a prescribed thickness and dimensional integrity (denoted as LCM and LNTCM, or LCMs). Changes in structure, porosity and composition of LCMs were induced by the efficient thermal energy afforded by the CW CO₂ laser irradiation under tightly controlled process conditions. The main effects observed before and after laser cutting were studied in comparison with traditional scalpel blade cutting of carbon monolithic (SCM). FE-SEM images confirmed that the resulting LCMs exhibited a more open, interconnected macroporous structure and smoothed mesopores to a depth of approximately 5 μm, while the structure of the bulk section remained intact. Minimal changes in chemical compositions were confirmed by XPS. Raman spectroscopy revealed a modest increase in the graphitic content on the cross sections of LCM discs. Phenol and Bisphenol A (BPA) was used as a model analyte for demonstration of resulting discs adsorption performance.

Introduction

The development of novel porous carbon-based monolithic materials is a topical area of research within the fields of analytical chemistry and material science over the last decade. They have found diverse applications in chromatography [[1], [2], [3]], solid phase extraction (SPE) [[4], [5], [6]], energy storage [7], as catalytical supports [8] and within electrochemistry [9] owing to their potential for high specific surface area, interconnected porous structure, low flow resistance, unique adsorption properties and excellent thermal/chemical stability. Most of these applications require the carbon monolith to be of well-defined shape and chemistry. For energy related applications for example electrodes must be fabricated with exacting dimensions and carbon structure [[10], [11], [12]]. As such, carbon monoliths have been fabricated from a variety of carbon building blocks including from the carbonisaton of polymer precursors [[13], [14], [15]] and the synthesis of 3D networks of rGO, CNT, MWCNT and fullerenes [[16], [17], [18], [19]]. A review of the fabrication of tree-dimensional (3D) carbon networks (3DCNs), their properties and potential for functionalisation has recently been presented [20]. Many of the applications for carbon monoliths also require adaptation with high dimensional accuracy into a flow-through device. Though various chemical synthetic techniques were developed for tailoring microscopic properties such as pore size, pore shape, pore connectivity and pore surface reactivity, there are few studies which have been carried out on the development of a tailored monolithic material which is suitable in specific macroscopic forms such as fibers, thin films and rods, for development of end use devices and applications. In general, most carbon monoliths are cut by a scalpel or knife for their use in flow through applications. Such mechanical cutting methods are not very suitable as they tend to cause deformations or cracks within the porous structure; hence the monolithic materials subsequently lose their integrity and openness for future use. Such methods are unable to form the carbon monolith into complicated shapes and their dimensional reproducibility is not of a high standard. An alternative and superior cutting technique is needed to overcome these difficulties, to satisfy the growing interest in the use of carbon monolithic materials.

Recently, several groups have reported the synthesis of carbon monolith replicas from silica monoliths in order to achieve various macroscopic shapes with tuneable pore sizes and structures [[21], [22], [23], [24]]. The unique property of the silica monolith presents an excellent template for carbon replicas. Zhang and his co-workers developed a nano-casting method to prepare size and porosity controlled carbon replicas from hierarchical silica monoliths [24]. They molded the silica templates into the required shapes and sizes and carbonized the sucrose-filled silica monolith, and subsequently removed the silica frameworks by NaOH. The corresponding carbon monolithic replicas were in various shapes including cylinders, triangles, squares, loops, and pentagons with reverse microstructures to the parent silica monolithic templates. Such nano-casting approaches require precise control of the loading of the precursor in the mesoporous channels of the templates [25].

In the search for more efficient, flexible and reproducible fabrication strategies, laser processing is an attractive option for achieving the prescribed atomic arrangement, functionality and dimensions of monoliths for various applications [[26], [27], [28], [29]]. The increasing demand for use of laser material processing can be attributed to several unique advantages, namely high productivity, ability to automate, non-contact nature, elimination of finishing operations, reduced processing cost, improved product quality, greater material utilization and minimized heat affected zone [27]. These characteristics align well with the focus of this study, which is to use the laser beam to induce carbon phase transformation from solid to vapor to be able to shape accurately the dimensions, structure and chemistry of carbon monoliths.