**About the Editors**

#### **Protima Rauwel**

Protima Rauwel is the Chair Professor of Energy Applications at the Institute of Technology of the Estonian University of Life Sciences. She obtained her PhD in 2005 from the University of Caen, France, in condensed matter and materials science with a special focus on nitride thin films with quantum dots and quantum wells. She has post-doctoral experience from the University of Aveiro, Portugal, and the University of Oslo, Norway, where she continued her research on thin films and freestanding nanomaterials. Her research focus is currently on the synthesis and characterization of nanomaterials for water remediation, optoelectronic, electrochemical, and anti-microbial applications. She has co-authored more than 90 publications, 3 books, and 5 book chapters. She is a co-inventor of two patents and has a Scopus H-index of 30.

#### **Erwan Rauwel**

Erwan Rauwel received his PhD in Materials Science from the University of Caen in 2003. He then began his postdoctoral studies in collaboration with STMicroelectronics at Minatec, Grenoble, France, and at the University of Aveiro (Marie Curie IE Fellowship), Portugal. He worked as a senior researcher at the University of Aveiro, in collaboration with Statoil, and then joined Taltech, Estonia, as a professor for five years. He is currently a professor at the Estonian University of Life Science, Estonia, where his group investigates the properties of nanoparticles best used for targeted applications that include water purification (heavy metal ions extraction, organic pollutant removal), the development of biocidal coating to fight AMR and COVID-19, and the development of nanocomposites for photocurrent generation and energy harvesting (with Minatec, Grenoble). His group is also investigating new possible treatments for cancer (with the LBN, Montpellier,). He has more than 83 peer-reviewed publications with an H-index of 25, 5 book chapters, and 6 patents. He is the chief scientist of his start-up company, which specializes in nanomaterials (PRO-1 NANOSolutions).

### *Editorial* **Functional Nanomaterials for Optoelectronics and Photocatalysis**

**Protima Rauwel 1,\* and Erwan Rauwel 1,2**


The present energy crisis has encouraged the use of energy-efficient devices and green energy sources. In addition to their energy-efficient operation, it is now essential that the production of these devices is cost-effective. Devices requiring energy-efficient operation and production include light emitting diodes (LEDs) applied to general lighting systems or for specific applications in electronic devices. With regard to the production of energy, cost-effective and new materials are being intensively investigated. The new generation of devices consists of hybrid materials and nanomaterials, involving polymers coupled with inorganic counterparts. The advantages of these hybrid materials include lower production costs, an overall weight reduction in the device and easier recyclability. In this regard, functional nanomaterials appear to be the most suitable choice of materials for these applications. In electronic devices, they allow miniaturization, while in energy-harvesting applications, i.e., photovoltaics and photocatalysis, they allow for more efficient energy conversion owing to the higher surface-to-volume ratio. Since the active sites for energy conversion in these nanomaterials are localized on the surface, the volume of the device is therefore reduced. Hence, the energy produced per unit mass is higher, as a lower amount of material is required in the device. Along with cost-effective production techniques, the overall device costs are therefore lowered.

The present Special Issue focuses on functional nanomaterials applied to optoelectronics and photocatalysis with several common nanomaterials to both fields, in particular ZnO. The compilation is clearly divided into three categories: (i) optoelectronics, (ii) photovoltaics and (iii) photocatalysis. In the optoelectronics section, the publication of Kabongo et al. describes the synthesis of ZnO doped with Ho, exhibiting ferromagnetic properties under microwave excitation [1]. The second publication, by Rauwel et al., reports on the combination of ZnO nanoparticles with CNT and Ag nanoparticles, and emphasizes the plasmonic effect of Ag nanoparticles in the enhancement of UV emission owing to the Burstein–Moss effect [2]. The plasmonic effect of metal nanoparticles has also been theoretically studied by Shivangi et al. in the enhancement of an SPR-based sensor device of BlueP/WS2-covered Al2O3-nickel nanofilms [3]. Another article, by Nagpal et al., describes the enhancement and suppression of the visible light emission of ZnO nanostructures with the addition of carbon nanotubes (CNTs) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), respectively [4]. Polymers, such as Poly(methyl methacrylate) (PMMA), are also used as electron-blocking layers in a II–VIsemiconductor QLED, as described by Zvaigzne et al. [5]. Similar II-VI semiconductor materials were also grown by Hou et al. via a phosphine-free method, and their photovoltaic properties were evaluated, which marks the second topic of this Special Issue [6]. The third topic, i.e., photocatalysis, is composed of four publications. This topic can be divided into two subgroups: (i) H2 production and (ii) dye degradation. For H2 production, Xia et al. report on a heterostructure of an organic/inorganic interface of g-C3N4/LDH that can be activated under visible light

**Citation:** Rauwel, P.; Rauwel, E. Functional Nanomaterials for Optoelectronics and Photocatalysis. *Nanomaterials* **2023**, *13*, 2694. https://doi.org/10.3390/ nano13192694

Received: 22 September 2023 Accepted: 26 September 2023 Published: 3 October 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

radiation [7]. This Special Issue contains three publications on the study of dye degradation using nanomaterials: The first publication is a review article by Paredes et al. that surveys the Cu3N nanomaterials used to date [8]. The second publication by Paredes et al. describes the one-step synthesis of nanoparticle mixtures of Cu-Cu3N-Cu2O and their potential in the sunlight-driven photocatalytic degradation of azo dyes [9]. The last publication, by Hendrix et al., reports on the degradation of azo dyes using ZnO nanomaterials, and investigates, for the first time, the influence of their morphology and defect states under both UV and sunlight [10].

We wish you a pleasant read and hope that this Special Issue on "Functional Nanomaterials for Optoelectronics and Photocatalysis" will serve as a valuable resource for researchers and PhD students in the field.

**Acknowledgments:** The Guest Editors would like to thank the Editor-in-Chief and the Editorial Assistants for their contribution in making the guest editing process smooth and efficient. We also acknowledge the authors for submitting their valuable work to this Special Issue, as without it, the successful completion of this Special Issue would not have been possible. Finally, a special thank you goes to all of the reviewers who participated in the peer review process of the submitted manuscripts.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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