Rational additive and linker selection for enhanced fabrication and photocatalytic H2 evolution efficiency of liquid phase exfoliated 2D-MoS2

Abstract

As a consequence of the present global urgency to resolutely reduce carbon dioxide emissions, the search for renewable, non-fossil fuel based energy sources and efficient energy storage concepts has become more important than ever. Over the past years, the rapid progress in modern battery technology has enabled a wide-spread implementation of effective power packs, forming the basis for the distribution of sustainable electricity in a variety of mobile applications. However, due to the comparably low specific energies of batteries, fields like heavy-duty traffic and aviation are mainly inaccessible to date, paving the way for hydrogen (H2) as a flexible, light-weight and alternative energy carrier. In regards to green H2 production, the direct utilization of sunlight via a process known as artificial photosynthesis (often referred to as photocatalysis, respectively) thereby marks a particularly attractive option, considering its inexhaustibility. Unfortunately, though, common benchmark photocatalyst materials like noble metal endowed TiO2 (e.g. TiO2/Pt) predominantly lack to provide a reasonable combination of high efficiency, non-toxicity, straightforward synthesis and low costs, impeding an industrial breakthrough of this approach. In light of its cost advantage compared to noble metals, two-dimensional molybdenum disulfide (2D-MoS2) revealed to be an excellent co-catalyst for photocatalytic water splitting, capable of dramatically improving the H2 evolution efficiency of e.g. pristine TiO2 once combined. Besides its own direct band gap semiconductor characteristics in case of monolayers, the beneficial effect of 2D-MoS2 here primarily stems from the great number of active sites for reversible hydrogen adsorption provided at its sheet edges. However, taking full advantage of these outstanding properties requires intimate contact between the hybrid components, in order to allow for efficient charge separation and transfer. As the basal planes of 2D-MoS2 are known to be rather inert though, practicable “mix & match” methods for the fabrication of 2D-MoS2/TiO2 hybrids may fall short of ensuring these defined, stable interfaces. Focussing on the facile fabrication of 2D-MoS2 by liquid phase exfoliation (LPE), another restriction is faced, since this process is currently only feasible for a selected number of pure solvents – most of them moreover characterized by rather high boiling points. As a remedy, additives like sodium cholate are commonly used to enhance exfoliation yields, yet provide no further function apart from nanosheet stabilization, and possibly even hinder subsequent processing. In the course of this work, a rationally selected molecular compound is investigated upon its potential to overcome both of the limitations stated. Its qualification as an LPE additive to increase the product concentrations of 2D-MoS2 in a row of low boiling point solvents including methanol, ethanol, isopropyl alcohol, ethyl acetate and acetonitrile, is tested. Furthermore, UV/Vis spectroscopy, DLS measurements and TEM imaging are utilized to analyze the impacts of this species on the geometry of the yielded nanosheets. Beyond that, its function as a possible linking agent to improve the interface properties between 2D-MoS2 and TiO2 is examined using three different pathways of integrating the molecule to the hybrid 2D-MoS2/TiO2 photocatalysts. To determine the H2 evolution efficiencies of the prepared materials, several series of UV light driven HER experiments are performed and evaluated via GC analysis. As it is shown, the application of the selected additive opens up a new, promising approach for drastically enhancing the 2D-MoS2 yields in a row of simple organic solvents, along with the possibility for one-step nanosheet exfoliation and functionalization. This concept may represent a versatile future strategy for the implementation of 2D-MoS2 to a broad variety of fields, since the functional groups of the investigated molecule allow for numerous options of further processing. Moreover, it is demonstrated that the linkage of 2D-MoS2 and TiO2 via the chosen species largely results in increased H2 evolution efficiencies compared to the corresponding conventionally hybridized materials. However, the achievable improvements are strongly dependent upon the exact method of introducing the linker to the 2D-MoS2/TiO2 system.