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Nanoporous carbons are capable of efficiently absorbing photons, which can then be converted into photochemical reactions in confined nanopore spaces. We spoke to Dr Conchi Ania about the work of the PHOROSOL project in investigating how metal-free nanoporous carbons can be modified to harvest light, which could open up wider possibilities in various fields.

Since the ability of some metallic oxides to absorb light and decompose water was first demonstrated, a lot of attention in research has been focused on modifying and tailoring the characteristics of different materials to improve their light absorption features. In particular, scientists are exploring how to achieve higher efficiencies and improve the stability of materials through tuning their composition, size, or structure. Based at the French National Centre for Scientific Research (CNRS), Dr Conchi Ania is Principal Investigator of the PHOROSOL project, an ERC-backed initiative investigating the potential use of light responsive nanoporous carbons. “We want to explore the ability of these materials to absorb light and to integrate them in various applications,” she outlines.

The disruptive approach of the project is based on exploiting the potentialities of earth-abundant and metal-free carbons with well-defined nanopores for photochemical applications. “We have to address two main challenges; the first is that most of the photochemistry is based on non-porous materials, whereas we investigate the fate of the light/solid interactions upon nanoconfinement. The second is that carbon materials are strong absorbers themselves, thus it is a challenge to reach efficiencies comparable to those of common materials,” says Dr Ania.

The ambition of PHOROSOL is to develop more efficient materials to boost technologies for light energy utilization, profiting from the dual nature of nanopore carbons as strong light absorbing materials with unique electronic features and nanoporosity. They are also commercially available at a relatively low cost in comparison to most competitors.

PHOROSOL project Porous carbons are some of the oldest materials known to human beings with a long history of applications going back to BC times (inks in cave paintings, Chinese ink, water purification, medicine). Their fabrication has ancient origins, and current manufacturing processes are well known and based on work carried out more than a century ago. However, these conventional methods do not allow perfect control of the pores in the full nanometric scale; this has triggered intensive research efforts, with the aim of developing new methods to obtain porous carbons with perfectly defined pore architectures. “Designing the perfect material for a given application is complex, as the properties must be tuned/tailored to a certain extent to fulfill the requirements of the specific application; and for this, understanding of the fundamentals of the technological process itself becomes crucial,” explains Dr Ania.

Researchers are investigating novel approaches to control and tune carbon materials with ad-hoc nanopore voids (nanopores are very small in size, below 100 nanometres in length), while adjusting other properties, such as structure, chemical composition and morphology, that are essential to improving photochemical performance. An abundantly available set of materials is being considered, which includes gels and grapheneoxide frameworks. when photochemical reactions occur in the confined space of the nanopores,” says Dr Ania. “We are not only handling these pores, we are also handling the materials. A large amount of light is absorbed directly by the matrix itself, and is not directly used in the photochemical reaction.”

Another difficulty is the lack of consensus and collective understanding of how to evaluate photonic efficiencies in all type of materials. This is a central part of the project’s research agenda, along with modifying these materials and tailoring their properties. “In order to understand the interactions of light in the confined space, we have to modify the average nanopore size in these materials,” continues Dr Ania. Understanding the properties of these nanoporous carbons and how their characteristics can be modified is an important step towards the objective of

We have seen that we can tune the amount of light absorbed and the type (in terms of energy or wavelength) by modifying the composition. This is important to control the amount of energy that can be harvested and that will define the photochemical reaction where the material can be applied.

It is the porosity at the nanoscale level that confers extremely interesting properties to porous carbons, as it is their photochemical activity that is being explored in this project. Such photochemical features have been reported for graphene, whereas the behaviour of nanoporous carbons (which are disordered and defective graphene layers) has not been studied. Dr Ania’s work has shown that it is possible to harvest light in these materials and to promote photoinduced reactions inside the pores. This has opened up new perspectives for the old, black nanoporous carbons (very often considered the ‘ugly duckling’ of the carbon family).

Evaluating the performance of these materials using standard methods is complex. “The difficult part is determining and quantifying how light interacts in the confined space of the nanopores,” explains Dr Ania. This is mainly due to the difficulties associated with measuring the photons that have been absorbed and discriminating the fraction of light reaching the carbon surface and the pores, from that absorbed by the carbon matrix itself. “One of the big goals in the project is to understand what happens bringing them to wider application. “In some contexts there are a few mechanisms that are well understood, but the overall picture is highly complex,” explains Dr Ania.

By building a deeper understanding of the effect of pore size and chemical composition on photoactivity, researchers hope to lay the foundations for a more structured approach in future. “We hope to develop a portfolio of carbon materials with known photoactivity, defining as many physico-chemical features as possible,” says Dr Ania. While a great deal of progress has been made in this respect over the course of the project so far, there is still significant scope to further improve efficiency, which will remain an important topic in Dr Ania’s research.

The composition of these materials is another major factor in terms of how effectively and efficiently the carbon nanopores absorb light. “We have seen that we can tune both the amount of light absorbed and the type (in terms of energy or wavelength) by modifying the composition,” outlines Dr Ania. This is important in terms of controlling the amount of energy that can be harvested, which will define the photochemical

Reprinted from Adv. Sci. 2018, 180029

reaction (the application) where the material can be applied. “We want to understand which surface groups are more effective in increasing the overall efficiency of converting light into chemical reactions,” says Dr Ania.

The first step here is to functionalise the materials. “We are working with the O-, N-, and Ssurface groups,” outlines Dr Ania. The significance of the various physical and chemical defects and the role of the moieties that are incorporated has become the focus of intense research.

Applications Part of the project team is working on investigating the origin of the interactions of light within nanopores, and studying the nature of the ensuing reactions, while researchers are also looking towards the possible applications of these materials. “One part of the team is looking towards integrating the materials in certain reactions, to see if they can work effectively with - or in competition - currently available materials for these applications,” continues Dr Ania. “They could potentially work in combination with other materials, but it’s not yet clear how versatile they will prove to be.”

A number of possible applications, coupling their photochemical performance with nanoconfinement effects, have been put forward, including the removal of pollutants in certain environments, synthesis of fuels and chemicals with added value, nanoenergetic materials and organic synthesis, among others. An exciting potential application of photoresponsive nanoporous carbons is in transforming CO 2 and H 2 O into sustainable fuels, which is a major global research priority. “In a world that urgently needs to redress the balance of energy derived from fossil fuels, efficient conversion of sunlight into energy is key to producing carbon-neutral fuels,” points out Dr Ania.

This challenge calls for the development of more efficient materials, a deep understanding of the nano-microscale phenomena, and a clear approach to the design of devices to integrate such materials. “Carbon catalysts have been shown to be among the most interesting candidates to replace metallic catalysts in the production of CO 2 -derived solar fuels,” says Dr Ania. The potential of noble metal-free carbon catalysts has attracted a lot of attention across the wider scientific community, and Dr Ania plans to continue her research in this area in the future. “We have gained some promising results, combining functionality and porosity; we are now looking at how this can be optimized to synthesize long fuel molecules”, she continues.

The team has recently been able to measure light emission features in some of these porous carbons, which could have a significant impact on the development of sensors with improved sensitivity and selectivity. The team is now working to control the emission/absorption wavelength, and the stability in different environments, as these may be the key to broadening the portfolio of applications beyond those initially forecast in the project. As an example, a tight nanopore confinement of oxidizers in porous carbons has been revealed to be critical to the performance of the composite in terms of releasing thermal energy; some mixtures are capable of competing with the best performing nanoenergetic materials in the market for civil and military applications, with better safety performance. While a great deal of progress has been made in this respect over the course of the project so far, there is still significant scope to further improve efficiency, which will remain an important topic in Dr Ania’s research.

PHOROSOL Integrating photochemistry in nanoconfined carbon-based porous materials in technological processes Project Objectives The main goal of the research project is to exploit the potentialities of coupling the nanoconfinement and photochemical ability of metal-free carbon materials reach important breakthrough in interdisciplinary fields of applied photochemistry for gas sensing, energy conversion and environmental protection. The challenge stands from the understanding of the fundamentals of the role of confinement in the light/solid/molecule interactions. An example would be the design of spatially organized carbons with high electron mobility and multimodal pore systems. These systems are expected to show enhanced diffusion and photoactivity, with a great potential in applications where a fast, sensitivity and selective response is needed. Project Funding This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No 648161, PHOROSOL). Contact Details Dr Conchi Ania Conditions Extremes Materiaux: Haute Temperature et Irradiation (CEMHTI) CNRS (UPR 3079), Site Haute Température 1D Av. de la Recherche Scientifique CS 90055 45071 - Orléans Cedex 2, France T: +33 (0) 238 25 55 13 E: conchi.ania@cnrs-orleans.fr W: https://www.cemhti.cnrs-orleans.fr

Dr Conchi Ania

Dr Conchi Ania is the head of POR2E group at CEMHTI-CNRS (Orléans, France). She has a long-standing interest on developing nanoporous materials with tailored surface chemistry and architectures as structural and functional solids for high-tech applications, covering energy storage and conversion and environmental protection. Current research interests focus on water treatment, photocatalysis, electrocatalysis, gas adsorption and separation, and solar energy conversion.