**Contents**


#### **Tamara Tsebriienko and Anatoli I. Popov**


## **About the Editors**

## **Aleksej Zarkov**

Dr. Aleksej Zarkov is a Chief Researcher at the Institute of Chemistry of Vilnius University, Lithuania. He obtained a PhD in 2016 at Vilnius University, where he conducted research on the development of synthetic approaches for the preparation of oxide-based solid electrolytes. His research interests encompass synthesis and characterization of functional inorganic materials in the form of bulk materials, nanoparticles and thin films. Recently, A. Zarkov has focused on the synthesis and investigation of calcium phosphates for medical, optical and environmental applications. He is a co-author of 75 articles and numerous conference announcements and presentations. During his scientific career, A. Zarkov held long-term internships at various prestigious scientific institutions, including Osaka University (Osaka, Japan), Georgetown University (Washington D.C., USA), University of Cologne (Cologne, Germany), University of Aveiro (Aveiro, Portugal) and others. For his scientific achievements, A. Zarkov was awarded with several awards and scholarships from Vilnius University Rector and the Lithuanian Academy of Sciences.

## **Aivaras Kareiva**

Prof. Aivaras Kareiva was promoted to the position of Full Professor at the Department of Inorganic Chemistry of Vilnius University in 1998. He is an expert in the sol–gel synthesis of different oxide materials. Since then, the "sol–gel chemistry" group has "prepared" 40 PhDs and 6 Postdocs who have used the sol–gel method to synthesize superconductors, optical and magnetic materials, bioceramic materials, and nanostructured materials. The sol–gel method has also been used successfully to preserve cultural heritage. He cooperated for many years with scientists from Stockholm University, the University of Tuebingen, Masaryk University Brno, Muenster University of Applied Sciences, the University of Cologne, Kyushu University, the University of Venice, the University of Strasbourg, the National Taipei University of Technology, and others. Together with co-authors, he has published more than 350 scientific articles and has participated in many research projects, grants, and contracts. In addition to scientific work, he also had some administrative positions. He was elected for the Dean position of Faculty of Chemistry (2006–2016) at Vilnius University, and from 2017 to 2022, he was also Director of Institute of Chemistry, and in 2022, he was elected as Dean of Faculty of Chemistry and Geosciences at Vilnius University. He is a Member of the Lithuanian Academy of Sciences, received five Vilnius University Rector's Research Awards (2003, 2007, 2011, 2016, 2020), two Lithuanian Republic Research Awards (2004, 2020), Lithuanian State Last Degree Fellowship (2007–2008), and Lithuanian Academy of Sciences' Juozas Matulis Award (2009). He is laureate of the Medal SAPIENTI SAT of the Association and Chapter of A. Sniadecki, K. Olszewski and Z. Wrublewski (Poland) (2016) and was awarded the Medal of the Lithuanian Academy of Sciences (2021) and Theodor von Grotthuss Medal.

## **Loreta Tamasauskaite-Tamasiunaite**

Dr. Loreta Tamasauskait ˇ e-Tama ˙ siˇ unait ¯ e. is currently working as a chief research associate at ˙ the Department of Catalysis at the Center for Physical Sciences and Technology (FTMC) in Vilnius (Lithuania). She has interests ranging from physical chemistry, catalysis, and electrochemistry to developing catalysts for low-temperature polymer membrane fuel cells, water splitting, and hydrogen generation from aqueous hydride solutions. She has published 124 scientific articles and is a co-author of 19 patents (https://orcid.org/0000-0001-7555-4399).

## *Editorial* **Advances in Functional Inorganic Materials Prepared by Wet Chemical Methods**

**Aleksej Zarkov 1,\*, Aivaras Kareiva 1 and Loreta Tamasauskaite-Tamasiunaite 2**


Functional inorganic materials are an indispensable part of innovative technologies, which are essential to the development of many fields of industry. The use of new materials, nanostructures, or multicomponent composites with specific chemical or physical properties promotes technological progress in electronics, optoelectronics, catalysis, biomedicine, and many other areas that are concerned with plenty of aspects of human life. Due to the broad and diverse range of the potential applications of functional inorganic materials, the development of superior synthesis pathways, reliable characterization, and a deep understanding of the structure–property relationships in materials, are rightfully considered to be fundamentally important scientific issues. Only synergetic efforts of scientists dealing with the synthesis, functionalization, and characterization of materials will lead to the development of future technologies. The Special Issue on "Advances in Functional Inorganic Materials Prepared by Wet Chemical Methods" covers a broad range of preparation routes, characterization, and the application of functional inorganic materials, as well as hybrid materials that are important in the fields of electronics, optics, biomedicine, and others.

The sol-gel method is a simple, time- and cost-effective synthetic approach providing high homogeneity and stoichiometry control of the products. These reasons make it highly suitable for the preparation of mixed-metal oxide materials and different solid solutions. For instance, Grazenaite et al. [1] employed an aqueous sol-gel method for the preparation of Ga-substituted cobalt–chromium spinels as ceramic pigments. The ion substitution resulted in significant color tuning. This demonstrated that the full substitution of Cr3+ by Ga3+ ions led to the formation of light blue powders, which yielded a violet blue color for the corresponding ceramic glaze. Karoblis et al. [2] utilized the sol-gel method for the synthesis of (1−x)SrTiO3-xBiMnO3 solid solutions in order to obtain new multiferroic material. The results indicated that single-phase perovskites with a cubic structure can only be synthesized up to x = 0.3. A higher BiMnO3 content led to the formation of a negligible amount of the neighboring Mn3O4 phase. Nevertheless, some compositional trends were observed in this range. The grain size increased drastically with an increase of BiMnO3, moreover, the gradual increase of the BiMnO3 content resulted in noticeably higher magnetization values. Finally, using the sol-gel method, Inkrataite et al. [3] prepared cerium and chromium co-doped gadolinium–aluminum–gallium garne<sup>t</sup> (GAGG, Gd3Al2Ga3O12). The remarkable feature of this study is that an exceptionally intense emission in the near-infrared region (NIR) was achieved with an extremely low doping level. The chromium content in the obtained materials reached only 15 ppm. Another example of optical materials was provided by Buyer et al. [4]. The authors were able to synthesize Eu3+-substituted NaY[SO4]2 H2O under hydrothermal conditions. The anhydrated version of this material (NaY[SO4]2) was obtained by the post-annealing of as-prepared species. The structural and thermal properties of the synthesized compound were investigated in detail. Both compounds exhibited a strong emission in red region.

**Citation:** Zarkov, A.; Kareiva, A.; Tamasauskaite-Tamasiunaite, L. Advances in Functional Inorganic Materials Prepared by Wet Chemical Methods. *Crystals* **2021**, *11*, 943. https://doi.org/10.3390/cryst11080943

Received: 4 August 2021 Accepted: 12 August 2021 Published: 13 August 2021

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Wet chemical methods allow not only crystalline to synthesize, but also amorphous materials, which can be very attractive for medical applications. A hydrothermal approach was employed by Anh Tuan et al. [5] for the preparation of bioactive glass 58SiO2-33CaO-9P2O5. The proposed synthetic approach avoided the use of harmful acid catalysts and was confirmed as one of the ideal methods for the preparation of ternary bioactive glass. For specific medical applications more complex hybrid organic–inorganic structures can be used. For example, the successful preparation of the biocompatible nanohybrid of ciprofloxacin-Ag/TiO2/chitosan was demonstrated by Zafar et al. [6]. The obtained nanaheterostructures were shown to be effective against mastitis causing *E. coli*.

Titanium dioxide, besides being used in heterostructures for biomedical applications, is a very technologically important material, which can be utilized for a broad range of applications. In this light, simple and cost-effective methods for the synthesis of TiO2 are highly desirable. Serga et al. [7] demonstrated for the first time the possibilities of the extraction–pyrolytic method (EPM) for the production of nanocrystalline TiO2 powders. It was observed that the EPM permitted the production of both monophase (anatase or rutile polymorph) and biphase (mixed anatase–rutile polymorphs) nanocrystalline TiO2. The influence of poly(titanium oxide) obtained using the sol-gel method on the viscoelastic and thermophysical properties of interpenetrating polymer networks (IPNs) based on crosslinked polyurethane (PU) and poly(hydroxyethyl methacrylate) (PHEMA), was studied by Tsebriienko and Popov [8]. It was found that an increase in poly(titanium oxide) content led to a decrease in the intensity of the relaxation maximum for PHEMA phase and an increase in the effective crosslinking density due to the partial grafting of the inorganic component to acrylate.

Carbon-supported MnO2 nanocomposites for the application of supercapacitors were fabricated by Jablonskiene et al. [9] using the microwave-assisted heating method. For comparison, the nanocomposites were synthesized by one-step and two-step approaches. The high specific capacitance of 980.7 F g<sup>−</sup><sup>1</sup> was achieved from cyclic voltammetry measurements, whereas the specific capacitance of 949.3 F g<sup>−</sup><sup>1</sup> at 1 A g<sup>−</sup><sup>1</sup> was obtained from a galvanostatic charge/discharge test. The specific capacitance retention was 93% after 100 cycles at 20 A g<sup>−</sup>1, indicating good electrochemical stability.

Zeng et al. [10] utilized hydrothermally synthesized ZnO nanowires for the fabrication of 2D Bi2Se3/ZnO heterojunction, which was employed for the broadband photodetection. The fabricated heterojunction device demonstrated not only an enhanced photoresponsivity of 0.15 A/W at 377 nm, which was three times higher than that of bare ZnO nanowire (0.046 A/W), but also achieved a broadband photoresponse from UV to near-infrared region was achieved.

The present Special Issue on "Advances in Functional Inorganic Materials Prepared by Wet Chemical Methods" demonstrates the versatility of wet chemical methods for the preparation of functional materials for a broad range of applications. It can be considered as a status report reviewing the progress that has been achieved in the processing of inorganic materials.

**Author Contributions:** Conceptualization, A.Z., A.K. and L.T.-T.; writing—original draft preparation, A.Z., A.K. and L.T.-T.; writing—review and editing, A.Z., A.K. and L.T.-T.; funding acquisition, A.Z. and A.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project has received funding from European Social Fund (project No. 09.3.3-LMT-K-712-19-0069) under gran<sup>t</sup> agreemen<sup>t</sup> with the Research Council of Lithuania (LMTLT).

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