Dear Colleagues,

A recent report of the European Union is entitled "Closing the circle". In the focus of this report is the concept of circular economy. One of the EU's priorities is to promote the transition to this circular economy, where the materials and products manufactured with them are kept in the life cycle as long as possible. The change towards a circular economy model is a basic pillar in most research programs, also in the field of research and development of materials. It should be considered a basic pillar as far as it will have to protect the productive system from the shortage of raw materials, with the consequent volatility of market prices, and the damage to the planet and the development of a sustainable technological society, but also an important aspect of this new framework of action is the improvement of the processes of recycling waste from different sectors, to reintroduce them in the technological lifecycle. To accomplish this change of paradigm, new materials and approaches are needed in most of the commodity sectors. Keep a safe environment, providing clean water for all the population, while protecting the wild life, fighting against the global warming requires the development of new and more efficient photocatalytic materials, able to degrade the pollutants from air and water, but also materials with a high capacity for energy storage, or materials able to detect different deleterious substances before the non-return point is reached.

The aim of this issue is to collect a series of relevant papers on these topics to build a panorama of the current research trends in these topics from a cross-cutting perspective.

Prof. Dr. Paloma Fernández Sánchez
Dr. Ana Urbieta
Prof. Dr. María Eugenia Rabanal Jiménez
Prof. Dr. Olivera Milošević
Guest Editors

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• Sensing
• Photocatalysis
• Emergent contaminants and pollutant photo-degradation, CO₂ capture
• Recycling, and environmentally friendly synthesis methods
• Nanodevices, batteries, and supercapacitors
• Metal-oxide-based materials and devices
• Upconverting materials