Polymeric materials, such as carbon nitrides, are emerging as a new-class of metal-free photocatalysts suitable for H2 evolution from water under visible light irradiation. However, to date, appreciable amount of H2 can only be produced in presence of a sacrificial electron donor and small loads of co-catalyst (Pt). Moreover, the actual H2 evolution rates are not competitive with those achieved with more conventional photocatalysts. Despite the notable synthetic efforts made to ameliorate the photoactivity of these materials, no real breakthrough has been done yet. This fact is mostly due to a lack of mechanistic insight into the H2 evolution process with polymeric photocatalysts, ranging form a classical semicondutor-based mechanism to a photochemical molecular reaction. A possible strategy for addressing, at the molecular level, the H2 evolution mechanism, would be to develop a surface science approach for building-up well-defined water/photocatalysts interfaces to be exploited in surface photochemistry experiments. To this aim, ultra-high-vacuum (UHV) sublimation methods and surface mediated mechanisms will be used for growing structurally, chemically and electronically defined thin organic films, based both on non-covalent assemblies of the carbon nitrides building blocks and more condensed phases thereof (oligoemrs and 2D polymers). The water/chromophore interface and its evolution under UV/vis irradiation will be studied both in UHV and near ambient pressure environments by mostly using X-ray photoemission spectroscopy (XPS) and scanning tunnelling microscopy (STM) techniques.
X-ray photoemission spectroscopy and other surface sensitive techniques are well established tools for the study of inorganic catalytic materials[1, 2]. Usually, such techniques operate in ultra-high-vacuum (UHV) conditions and their use from catalysis scientists is often limited to the characterization of "fresh" and "post-mortem" catalysts, providing significant insight into the active species or deactivation of a catalyst, i.e. formation of a particular chemical state or strongly bound species.
Surface scientists, on the other hand, have been exploiting UHV conditions to finely expose well-defined catalytic surfaces (i.e. single crystals) to low pressures gases (10^(-10) - 10^(-8) mbar) in order to perform model catalysis experiments[3], even at the single molecule level[4]. More realistic conditions, exposures up to few mbar, are now accessible thanks to the development of techniques like near-ambient pressure (NAP) XPS[5].
Although most heterogeneous catalytic reactions take place at the liquid/solid interface, UHV or NAP catalytic studies have been significantly contributing to improve the mechanist understanding of many catalytic processes thanks to identification of the surface active sites and the reaction intermediates. For instance, the adsorption of a few water molecules on the rutile TiO2(110) surface has shown dissociative interaction at the oxygen vacancies yielding bridging hydroxyl groups[4].
Similar approaches are still almost unexplored for the characterization of polymeric (photo)catalysts, mainly because such materials are nearly insoluble in most of the organic solvents and thus difficult to process in thin films suitable for meaningful surface science investigations. Only recently, P. Alexa et al. were able to combine STM and XPS measurements for pre- and post-reaction characterizations of a series of polymer-decorated gold surfaces suited for H2 evolution by electrochemical water-splitting[6]. In this case, monolayer porous 2D polymers were obtained by Ullman-type coupling reactions of designed organic molecules on Au(111). By combining the experimental results with theoretical simulations, it was possible to identify the N atoms of triazine- and pyrmidine-based polymers as the docking sites for both H2O and atomic H, and correlate the higher H2 evolution rate of the triazine-based polymer to an optimized balance between water and hydrogen binding energies.
This study testifies how the choice of proper synthetic and characterization tools allows to unambiguously identify the reaction intermediates, and at the same time emphasizes the role of the catalyst hydration properties. In this regard, our recent study on a monolayer of standing-up 1,3,5-triazine-2,4,6-triamine (melamine) molecules on the Cu(111) surface has shown that both N functional groups (-NH2 and triazine N) cooperatively work to H-bind the water molecules. Although the triazine-N is considered the active site of the PCET reaction, our results highlight how the co-presence of different functionalities may contribute for achieving more steady H-binding water-catalyst interfaces, and possibly a more efficient photoactivity. This would be consistent with the higher photo-activity observed for heptazine oligomers compared to melon, as they feature more abundant and better exposed dangling -NH2 and bridging -NH groups.
As deeply discussed, the TWISTHER project aims at transferring the use of surface science tools into the almost unexplored field of the polymers-based (photo)catalysis. A few available works consistent with this approach have already shown the potential knowledge deriving by this project: identification of the catalytic active sites, reactions intermediates and hydration properties. All these informations will be extremely useful to achieve a molecular-level description of the water-splitting mechanism and consequently to design more efficient polymeric photocatalysts.
[1] P. R. Davies et al., Practical guide for X-ray photoelectron spectroscopy. Applications to the study of catalysts, J. Vac. Sci. Technol. A, 2020, 38, 033204.
[2] C. Kranz et al., Characterizing photocatalysts for water splitting: from atoms to bulk and from slow to ultrafast processes; Chem. Soc. Rev., 2021, 50, 1407.
[3] A. M. Venezia, X-ray photoelectron spectroscopy (XPS) for catalysts characterization, Catal. Today, 2003, 77, 359-370;
[4] Q. Guo et al., Single Molecule photocatalysis on TiO2 Surfaces, Chem. Rev., 2019, 119, 11020-11041
[5] L. Nguyen et al., Understanding Catalyst Surfaces during catalysis through Near Ambient Pressure X-ray Photoelectron Spectroscopy, Chem Rev., 2019, 119, 6822-696905
[6] P. Alexa et al., Enhancing Hydrogen Evolution Activity of Au(111) in Alkaline Media through Molecular Engineering of a 2D Polymer, Angew. Chem. Int. Ed., 2020, 59, 8411-8415,
[7] V. Lanzilotto et al., Tailoring surface-supported water-melamine complexes by cooperative H-bonding interactions, Nanoscale Ad., 2021, 3, 2359-2365