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Integrating photochemistry in nanoconfined carbon-based porous materials in technological processes

Periodic Reporting for period 3 - PHOROSOL (Integrating photochemistry in nanoconfined carbon-based porous materials in technological processes)

Reporting period: 2018-07-01 to 2019-12-31

PHOROSOL is based on 3 interconnected axis: (i) the development of nanoporous multifunctional carbon-based materials at a nanometric and macrometric scale, and their characterization combining different techniques for (ii) unraveling the origin of the light-carbon interactions and (iii) enable the exploitation of the potentialities of confined pore spaces in technological processes related to gas sensing, energy conversion and environmental protection.

Context, Background and Impacts
Photochemical reactions are those triggered by the absorption of UV, visible of infrared radiation. Most of them proceed through the formation of thermodynamically instable compounds (e.g. radicals) that allow a high reactivity during short periods, otherwise inaccessible by conventional methods. Many photochemical reactions take place in nature. Examples are photosynthesis, human skin production of vitamin D, ozone formation/dissociation in the atmosphere, or photochemical smog.
The use of light energy (particularly sunlight) has been long searched in various fields (other than biological photochemical) such as synthetic chemistry (chemical bond breaking), cleaning surfaces, solar energy storage and conversion (e.g. water decomposition for hydrogen generation, carbon dioxide photoconversion into fuels), and environmental remediation (water and air purification). Many molecules and materials are capable of absorbing light and undergo a chemical or physical change in response to the illumination. The most common photoactive materials are semiconductors having well-defined absorption features in the UV-vis range. Examples are inorganic oxides or salts (for instance TiO2, ZnO, CdS, WO3, Fe2O3) and organic semiconductors (phthalocyanine, polytiophenes), among others. In these materials, the mechanism of light absorption is based on the interband electron transitions that occur when photons of adequate energy are adsorbed by a photocatalyst. This generates excitons (electron−hole pairs) which can recombine emitting light or heat, migrate to the material’s surface, or react with electron donor or acceptors. Major drawbacks of semiconductors are related to their low efficiency under visible light, the instability upon long irradiation times, and the high recombination rate of photogenerated electron−hole pairs.
Even though carbon materials are strong light absorbing solids, the use of different forms of carbons as supports and additives have been extensively studied as an alternative approach to improve the photoactivity of semiconductors. It is generally accepted that the role of a carbon material as an additive to a semiconductor depends on the structural features of the carbon itself. For instance, for carbons with a high electron mobility (e.g. carbon nanotubes, graphene), the enhanced photoactivity of semiconductor/carbon composites is attributed to strong interfacial effects between the carbon phase and the semiconductor, that favor the separation of the photogenerated charge carriers, delaying their recombination. In the case of porous carbons, the enhanced mass transfer and the nanoconfinement of the adsorbed species also play important roles. Additionally, monolayer or double layer graphene is considered as gapless semiconductor, and aqueous suspensions of carbon nanotubes are known to be able to generate reactive oxygen species upon sunlight illumination.
Despite the structural and chemical similarities between amorphous carbons and graphene, there has been a dearth in exploring the self-photochemical activity of these forms of carbon. Here it is important to mention that in fact the addition of an amorphous carbon phase to a semiconductor for increasing the photoactivity was initiated well before (ca. 90.s) the graphene discovery (2004). In this regard, our pioneering works (2010-2013) have reported the ability of aqueous suspensions of amorphous nanoporous carbons (metal-free and semiconductor-free) to generate ROS when exposed to UV-VIS light, highlighting the important effect of nanoconfinement in the pores for achieving high photochemical quantum yields in solution.
Amorphous carbons can exist in a 3-D structure reaching surface areas over 1000 m2 g-1, and a hierarchical pore structure. It is also known that their pore walls are built of thin distorted graphene layers containing sp2 domains, defects/heteroatoms and sp3 configurations. Even though their electrical conductivity is lower than that of graphene or carbon nanotubes, it is still much advantageous than that of inorganic semiconductors. Such features are considered favorable in photocatalysis for promoting the electron/hole transfers or radical reactions in the nanopore space, where the strong adsorption potential can contribute to bond splitting. This has allowed to settle the basis for a future implementation in carbon-based advanced oxidation processes for water treatment.
Owing to the versatility and availability of nanoporous carbons with varied pore architectures and composition, as well as the abundance of precursors, it is highly feasible to push the nanoporous carbons as additives to semiconductors and as sustainable metal-free photocatalysts for various emerging applications in the fields of gas sensing, energy conversion and environmental protection. An adequate choice of the carbon additive may overcome the low photonic yield of certain semiconductors under sunlight, as well as increasing the overall efficiency. Nanoporous carbon photocatalysts may also offer the advantage of coupling high adsorption features to tunable photoactivity; the challenge is in further enhancing the photochemical activity by balancing the surface composition, porosity, and change-carrier mobility. Even though the porous/nanoporous carbons are old materials of ancient origin and application, their ""new life"" is still in front of them and being of low costs and environmentally benign are certainly pluses from economic and environmental conservation viewpoints."
Although the application of nanoporous carbons themselves as photocatalysts or photosensitizers is still in its infancy, there is a strong indication that visible light activates their surface centers, generates charge carriers, which either directly or indirectly (via radical formation) increase the efficiency not only of chemical reactions but also contribute to other charge transfer reactions (e.g. applications in charge storage in supercapacitors, gas sensing, etc.).
The objectives of PHOROSOL focus in exploring the role of amorphous carbon surface features in the photoactive/photosensitizing behavior and to indicate the advantage of their textural properties for these applications. Another objective is to identify specific directions of surface modifications, which are beneficial for visible light harvesting and utilization of this group of materials. To carry out these fundamental goals, efforts have been directed towards various interconnected axis: (a) the development of nanoporous multifunctional carbon-based materials; (b) the characterization of the pore architectures and chemical composition combining different techniques; (c) the understanding of the carbon-ligh interactions and (d) the exploitation of the potentialities of the prepared materials in gas sensing, energy conversion and environmental protection.
The tasks focused on the synthesis nanoporous carbons with varied controlled features been developed as planned, towards the use of light-assisted procedures for the synthesis and the preparation of 3D architectures with enhanced conductivity with graphene-like units linked to an amorphous porous carbon phase. The materials are being characterized by a number of techniques available in the HI and through international collaborators (Prof. Bandosz USA; Prof. Maurino, Italy; Prof. Rodriguez-Castellon Spain). The activities related to the understanding the carbon/light interactions by using various light sources and materials with tailor-made features at a various levels (porosity and surface chemistry) have also been initiated, using new spectroscopic and electrochemical techniques in collaboration with Prof. Lima (Univ. Nova de Lisboa, Portugal), Prof. Iniesta (Univ. Alicante, Spain) and Dr. Balan (Univ. Haute Alsace, France). Particularly, we have concentrated on the understanding of the carbon/light interactions by using monochromatic light and photoactive carbon materials with tailor-made features at two levels (porosity and surface chemistry). The outcome of these results has provided useful information on the understanding of the confinement effects and the light/solid/molecule interactions, and has become a key point for the research activities to be developed in the upcoming months.
Regarding the integration of the materials in new technological processes, studies have been initiated for the photoelectrochemical conversion of water, the photoconversion of CO2 into fuels, the upgrade of chemicals using photochemical reactions, formulation of novel self-cleaning paints based on carbon/semiconductor components, gas sensing and environmental protection. Ongoing work on the screening of the best performing materials for sensing applications, is been carried out collaboration with a technological partner (CEA-LETI, France), aiming at the final integration of the materials prepared in various sensing prototypes.
The results obtained so far during have been published in 12 scientific papers in high impact peer-reviewed open access journals -complying with H2020 policy-, as well as in 2 book chapters (Ania CO, Raymundo-Pinero E “Synthesis of nanoporous carbons for high-pressure energy storage”, in Energy Storage in Nanoporous Solids. (Kaneko, Rodriguez-Reinoso, Eds.), Wiley-VHC (2018) ; Gomis-Berenguer A, Ania CO, “Metal-free nanoporous carbons in photocatalysis”, in Carbon-Based Metal-Free Catalysts. Design and Applications (Dai, Edt.) Wiley-VHC (2018)). Other manuscripts are under review and in preparation.
The results have also been disseminated in several international conferences and workshops of relevance for the communities of adsorption, carbon-based and semiconductors material’s research, and solar photocatalysis and advanced oxidation processes, among others. The results were disseminated as plenary (4), keynote (4), invited speaker (6), oral (34) and poster (18) communications.

Various Scientific Progress Meetings and Workshop have been celebrated. A first one (Sept 2017) was hosted at Orleans with collaborators of other institutions (Belgium, France, Spain) and researchers participating in the Action, to discuss progress of the work and opportunities for new collaborative projects in the near future. In Feb 2018, a second meeting was organized at Univ. Nova de Lisboa with the Photochemical Dept. head by Prof. Lima, for organizing the progress of the tasks related to unraveling the origin and fate of carbon-light interactions and their wavelength dependence.

More recently (May 2018), the outcome of the results related with the understanding of the light/carbon/molecule interactions published recently in C-journal (2018) has received the Award of the French Carbon Society (SFEC) for young researchers, being Alicia Gomis-Berenguer (postdoctoral researcher in this action) the awardee.
Owing to their unique properties, nanoporous carbons deserve another look and reaching beyond adsorption in their applications. Perhaps the future of photovoltaics, smart-self-cleaning surfaces and photocorrosion protection, or solar energy conversion and CO2 mitigation is in exploring the use of nanoporous carbons. The challenge is of course in their complexity, which is not only in their amorphous nature but also in the difficulty for separating the effect of the surface chemistry, microstructure and nanopore confinement. Thus the photoactivity is controlled by the ratio of sp2 to sp3 configurations, the abundance of surface defects, and the presence of S-, N- O-, P- containing moieties, which can act as chromophores or even as pseudo light sensitive dyes attached to the conductive matrix.
Our investigations in this domain are expected to contribute to unravel the role of the nanoporous carbons features in their photochemical performance, and will contribute to stimulate research leading to better understanding their exciting behavior, which has long been neglected and not considered as having any physical bases.
In many photoactivity studies of nanoporous carbon materials, inspiration is in their semiconducting nature and in similarity to graphene, whose distorted and full of defects units are in fact the building blocks of their pore walls. Due to their high 3D nanoporosity, graphene and carbon nanotubes cannot compete with nanoporous carbons in the applications herein envisaged, and only MOFs, zeolites or COFs could be considered as being competitive (and indeed evidence of photoactivity has been reported for some of them). Despite the photoactivity of nanoporous is still in its infancy, they are very advantageous in advanced oxidation processes, separation processes, photocatalysis or even DSSC applications.

Regarding the expected outcome of the project and progress beyond the state of the art, based on our ongoing results:
a) in advanced oxidation processes based on nanoporous carbon photocatalysts for the degradation of pollutants, progress relies on simultaneously promoting the in-situ photodegradation of pollutants and regeneration of the exhausted carbon bed.
b) the formulation of novel self-cleaning paints based on carbon/semiconductor components will allow the degradation of indoor (where UV light is cut-off) and outdoor environments where current photocatalysts based on inorganic semiconductors fail.
c) more selective and sensitive gas sensing devices are expected to be built upon the combination of nanoporous carbons of controlled pore architectures and enhanced conductivity provided by the graphene-like units. Ongoing work on the screening best performing materials for sensing applications is been carried out collaboration with a technological partner (CEA-LETI, France) expert in sensor platforms for the construction of a demonstrator that will integrate aptamer for biological recognition of explosives, volatile organic compounds and other gases of interest in various sensing prototypes.
d) carbon-based photoelectrodes for phot electrochemical reactions, enabling high
e) conversion of CO2 for the production of fuels combining bioelectrochemical approaches with regeneration of cofactors needed to obtain high quantum yield
f) upgrading of chemicals (biomass-derivatives) using low-cost and sustainable photocatalysts incorporating nanoporous carbons.