Single-Atom Co-Decorated MoS2 Nanosheets Assembled on Metal Nitride Nanorod Arrays as an Efficient Bifunctional Electrocatalyst for pH-Universal Water Splitting

Abstract Highly efficient hydrogen evolution reactions carried out via photocatalysis using solar light remain a formidable challenge. Herein, perylenetetracarboxylic acid nanosheets with a monolayer thickness of ~1.5 nm were synthesized and shown to be active hydrogen evolution photocatalysts with production rates of 118.9 mmol g −1 h −1 . The carboxyl groups increased the intensity of the internal electric fields of perylenetetracarboxylic acid from the perylene center to the carboxyl border by 10.3 times to promote charge-carrier separation. The photogenerated electrons and holes migrated to the edge and plane, respectively, to weaken charge-carrier recombination. Moreover, the perylenetetracarboxylic acid reduction potential increases from −0.47 V to −1.13 V due to the decreased molecular conjugation and enhances the reduction ability. In addition, the carboxyl groups created hydrophilic sites. This work provides a strategy to engineer the molecular structures of future efficient photocatalysts. 

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Perylenetetracarboxylic acid nanosheets with internal electric fields and anisotropic charge migration for photocatalytic hydrogen evolution

Incorporation of conducting polymers (CPs) with TiO2 is considered a promising pathway toward the fabrication of highly efficient non-metal based photocatalysts. Herein, we report the fabrication of TiO2@polyaniline, TiO2@polypyrrole, and TiO2@poly(3,4-ethylenedioxythiophene) photocatalyst heterostructures via the facile wet incipient impregnation method. The mass ratios of CPs in the composites were optimized. The structure, morphology, optical and surface texture of the samples were characterized by XRD, TEM, TGA, DRS, and N2-physisorption techniques. The TiO2@2PEDOT, TiO2@2PPy, and TiO2@5PAn composites were found to exhibit the highest H2 evolution rate (HER) of 1.37, 2.09, and 3.1 mmol h−1 g−1, respectively. Compared to bare TiO2, the HER was significantly enhanced by 16, 24, and 36-fold, respectively. Photoelectrochemical measurements (CV, CA and EIS) were conducted, to evaluate the photoelectric properties of the synthesized composites and assist in understanding the photocatalytic mechanism. The deposition method plays a key-role in forming the photocatalyst/CP interface. This simple impregnation route was found to provide an excellent interface for charge transfer between composite components compared to chemisorption and in situ polymerization methods. This study sheds light on the promising effect of CP incorporation with semiconductor photocatalysts, as a cheap and efficient matrix, on photocatalytic performance.

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Efficient non-metal based conducting polymers for photocatalytic hydrogen production: comparative study between polyaniline, polypyrrole and PEDOT

There has been rapid progress on the chemistry of supramolecular scaffolds that harness sunlight for aqueous photocatalytic production of hydrogen. However, great efforts are still needed to develop similar photosynthetic systems for the great challenge of CO2 reduction especially if they avoid the use of nonabundant metals. This work investigates the synthesis of supramolecular polymers capable of sensitizing catalysts that require more negative potentials than proton reduction. The monomers are chromophore amphiphiles based on a diareno-fused ullazine core that undergo supramolecular polymerization in water to create entangled nanoscale fibers. Under 450 nm visible light these fibers sensitize a dinuclear cobalt catalyst for CO2 photoreduction to generate carbon monoxide and methane using a sacrificial electron donor. The supramolecular photocatalytic system can generate amounts of CH4 comparable to those obtained with a precious metal-based [Ru(phen)3](PF6)2 sensitizer and, in contrast to Ru-based catalysts, retains photocatalytic activity in all aqueous media over 6 days. The present study demonstrates the potential of tailored supramolecular polymers as renewable energy and sustainability materials.

Photocatalytic Aqueous CO2 Reduction to CO and CH4 Sensitized by Ullazine Supramolecular Polymers.

Low molecular weight gels are formed by the self-assembly of small molecules into anisotropic structures that form a network capable of immobilizing the solvent. Such gels are common, with a huge number of different examples existing, and they have many applications. However, there are still significant gaps in our understanding of these systems and challenges that need to be addressed if we are to be able to fully design such systems. Here, a number of these challenges are discussed.

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Personal Perspective on Understanding Low Molecular Weight Gels

Amino acid functionalized perylene bisimides (PBIs) form self-assembled structures in solution, the nature of which depends on the local environment. Using a high-throughput photocatalysis setup, five PBIs are studied for the hydrogen evolution reaction (HER) under a range of conditions (pH and hole scavenger concentration) across 350 experiments to explore the relationship between supramolecular structure and photocatalytic activity. Using small angle X-ray scattering (SAXS), NMR spectroscopy and ultraviolet-visible (UV-vis) absorption spectroscopy, it is shown that photocatalytic activity is determined by the nature of the self-assembled aggregate that is formed, demonstrating the potential of self-assembly to tune activity. There is a clear correlation between the presence of charged flexible cylindrical aggregates and the occurrence of photocatalytic H2 production, with UV–vis spectroscopy indicating that the most active structure type has a distinctive form of π-aggregation which is proposed to enable efficient charge separation across multiple PBI units.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/aenm.202002469

Controlling photocatalytic activity by self-assembly – tuning perylene bisimide photocatalysts for the hydrogen evolution reaction

https://pubs.rsc.org/en/content/articlepdf/2021/CC/D1CC03378F

Controlling the formation and alignment of low molecular weight gel 'noodles'.

Multicomponent supramolecular gels provide opportunities to form materials that are not accessible when using the single components alone. Different scenarios are possible when mixing multiple components, from complete co-assembly (mixing of the components within the self-assembled structures formed) to complete self-sorting such that each structure contains only one of the components. Most examples of multicomponent gels that currently exist form stable gels. Here, we show that this can be used to control the mechanical properties of the gels, but what is probably most exciting is that we show that we can use a magnetic field to control the shape of the crystals. The gelling component aligns in a magnetic field and so results in anisotropic crystals being formed.

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Exploiting and controlling gel-to-crystal transitions in multicomponent supramolecular gels

The material properties of a gel are determined by the underpinning network that immobilises the solvent. When gels are formed by the self-assembly of small molecules into a so-called low molecular weight gel, the network is the result of the molecules forming one-dimensional objects such as fibres or nanotubes which entangle or otherwise cross-link to form a three-dimensional network. Characterising the one-dimensional objects and the network is difficult. Many conventional techniques rely on drying to probe the network, which often leads to artefacts. An effective tool to probe the gel in the solvated state is small angle scattering. Both small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) can be used. Here, we discuss these approaches and provide a tutorial review to describe how these approaches work, what opportunities there are and how the data treatment should be approached. We aim to show the power of this approach and provide enabling information to make them accessible to the non-specialist.

/pdf/using-small-angle-scattering-to-understand-low-molecular-3qk0i1hy.pdf

Using small angle scattering to understand low molecular weight gels.

We investigate how apparent slight changes to the chemical structure of amino acid-functionalised perylene bisimides (PBIs) affect the self-assembled aggregates formed and their resulting physical and optical properties. PBIs functionalised with L-valine (PBI-V), L-leucine (PBI-L) and L-isoleucine (PBI-I) are investigated due to their similarly branched structure and their assemblies in water were studied using UV-vis absorption spectroscopy, cyclic voltammetry (CV), small angle X-ray scattering (SAXS) and viscosity at different pHs. It was seen that each PBI behaved differently. Each of the PBIs were then used to prepare hydrogels, and their properties again assessed, with PBI-I forming different hydrogels than the other PBIs. By understanding how slight changes in chemical structure can affect bulk properties we become a step closer to designing gels with specific physical and electrical properties.

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Daniel McDowall

Papers

Controlling photocatalytic activity by self-assembly – tuning perylene bisimide photocatalysts for the hydrogen evolution reaction

Controlling the formation and alignment of low molecular weight gel 'noodles'.

Exploiting and controlling gel-to-crystal transitions in multicomponent supramolecular gels

Using small angle scattering to understand low molecular weight gels.

Impact of subtle change in branched amino acid on the assembly and properties of perylene bisimides hydrogels