Boron-Based (Nano-)Materials: Fundamentals and Applications

crystals

Editorial Boron-Based (Nano-)Materials: Fundamentals and Applications

Umit B. Demirci 1,*, Philippe Miele 1 and Pascal G. Yot 2

1 Institut Européen des Membranes (IEM), UMR 5635, Université de Montpellier, CNRS, ENSCM, Place Eugène Bataillon, CC047, F-34095 Montpellier, France; [email protected] 2 Institut Charles Gerhardt Montpellier (ICGM), UMR 5253 (UM-CNRS-ENSCM), Université de Montpellier, CC 15005, Place Eugène Bataillon, 34095 Montpellier cedex 05, France; [email protected] * Correspondence: [email protected]; Tel.: +33-(0)4-6714-9160

Academic Editor: Helmut Cölfen Received: 6 September 2016; Accepted: 13 September 2016; Published: 19 September 2016

Abstract: The boron (Z = 5) element is unique. Boron-based (nano-)materials are equally unique. Accordingly, the present special issue is dedicated to crystalline boron-based (nano-)materials and gathers a series of nine review and research articles dealing with different boron-based compounds. Boranes, borohydrides, polyhedral boranes and carboranes, boronate anions/ligands, boron nitride (hexagonal structure), and elemental boron are considered. Importantly, large sections are dedicated to fundamentals, with a special focus on crystal structures. The application potentials are widely discussed on the basis of the materials’ physical and chemical properties. It stands out that crystalline boron-based (nano-)materials have many technological opportunities in ﬁelds such as energy storage, gas sorption (depollution), medicine, and optical and electronic devices. The present special issue is further evidence of the wealth of boron science, especially in terms of crystalline (nano-)materials.

Keywords: benzoxaboronate; borane; borohydride; boron-based material; boron-treat steel; boron nitride; boronate; carborane; metallacarborane; polyborate

1. Introduction Boron (Z = 5) is one of the lightest elements of the periodic table, coming just before carbon (Z = 6). It is also certainly the least well-known element with 2p valence shell. In comparison to the very popular carbon, it has not attracted as much attention (despite more than two centuries of use), but it has fascinated and exasperated generations of scientists. We, the contributors and editors of the present special issue, are proud to belong to the current generation, and we are equally enthralled but no longer exasperated. Discovered in 1808, boron was isolated by Joseph-Louis Gay-Lussac (French chemist, 1778-1850) and Louis-Jacques Thénard (French chemist, 1777–1857), and independently by Sir Humphry Davy (English chemist, 1778–1829). The years that followed can be summarized by citing Grimes [1]. Boron “appeared normal—even boring—for a century following its isolation [ ... ]. It behaved exactly as expected, forming trivalent compounds [ ... ], and everyone [ ... ] believed that its simplest hydride had to be BH3 [ ... ]. It took the great German chemist Alfred Stock [(1876-1946)] to uncover the truth about the boron hydrides [ ... ]. Not until another half-century had passed did another towering scientist, William Nunn Lipscomb [(American chemist, 1919–2011)], ﬁnally crack the mystery of the three-center-bonded polyboranes”. Crystallinity of these polyboranes and the development of powerful characterization tools like X-ray diffraction within the 20th century, among other factors, have allowed the recent great advances in boron science. Hosmane, in the preface of the book he edited, asks the question “what is boron?” and right after gives the answer “the question itself may not seem very signiﬁcant to many people”[2]. Boron is indeed

Crystals 2016, 6, 118; doi:10.3390/cryst6090118 www.mdpi.com/journal/crystals Crystals 2016, 6, 118 2 of 7 Crystals 2016, 6, 118 2 of 7 Hosmane, in the preface of the book he edited, asks the question “what is boron?” and right after gives the answer “the question itself may not seem very significant to many people” [2]. Boron is indeed little known. It may may be be confused confused with borax, which is the the ore ore from from which boron is extracted. This This may may appear to be unfair in comparison to the reputationreputation of carbon, but obviously boron has shown to be versatile afterafter twotwo centuriescenturies ofof research research and and development. development. Let Let us us again again cite cite Hosmane Hosmane [2 ].[2]. “How “How many many of usof us know know that that a regular a regular intake intake of boron of boron can can lessen lessen the chancethe chance of prostate of prostate cancer? cancer? How How many many of us knowof us thatknow boron that playsboron aplays direct a anddirect critical and critical role in combatingrole in combating cancer through cancer athrough treatment a treatment called boron called neutron boron capture neutron therapy? capture” Medicinetherapy?” Medicine is clearly ais highlyclearly importanta highly important and crucial and area crucial of application. area of application. In fact, boron—especially In fact, boron— boron-basedespecially boron-based compounds—have compounds—have been widely been used widely in manyused in areas many of areas application, of application, including including health (e.g.,health fungicides, (e.g., fungicides, detergents, detergen soaps,ts, antiseptics, soaps, antiseptics, and cosmetics), and cosmet nutritionics), (e.g.,nutrition additives, (e.g., fertilizers),additives, electronics,fertilizers), energyelectronics, (e.g., nuclearenergy energy,(e.g., nuclear hydrogen energy, storage, hydrogen fuel cell), storage, organic andfuel inorganiccell), organic chemistry, and catalysis,inorganic metallurgy,chemistry, catalysis, enamels, metallurgy, and ceramics. enamels, and ceramics. Accordingly, the presentpresent specialspecial issueissue focusesfocuses onon fundamentalsfundamentals and applications of crystalline boron-based (nano-)materials, and and aims at illustrating the recent great advances in boron science through several examples.

2. The Journal Crystals, an Already-Identiﬁed Forum for Boron-Based (Nano-)Materials 2. The Journal Crystals, an Already-Identified Forum for Boron-Based (Nano-)Materials Boron scientists have not waited for the present special issue to report their cutting-edge research Boron scientists have not waited for the present special issue to report their cutting-edge research in this journal, Crystals. All of them are brieﬂy cited hereafter, as they also illustrate the versatility of in this journal, Crystals. All of them are briefly cited hereafter, as they also illustrate the versatility of boron and the importance of crystalline boron-based (nano-)materials in several areas of application. boron and the importance of crystalline boron-based (nano-)materials in several areas of application. 2.1. Attractive Physical Properties 2.1. Attractive Physical Properties Borophosphates are intermediate compounds made from the system MxOy−B2O3−P2O5−(H2O), Borophosphates are intermediate compounds made from the system MxOy−B2O3−P2O5−(H2O), where M is a transition metal like Fe. They are generally used as glasses, but borophosphates with where M is a transition metal like Fe. They are generally used as glasses, but borophosphates with open frameworks have been shown to be of interest as magnetic materials. This is the case, for example, open frameworks have been shown to be of interest as magnetic materials. This is the case, for for the new KFe[BP2O8(OH)] reported by Wang and Mudring [3]. It shows a three-dimensional example, for the new KFe[BP2O8(OH)] reported by Wang and Mudring [3]. It shows a three- framework crystallizing in the monoclinic structure with a space group P21/c; it is formed by dimensional framework crystallizing in the monoclinic structure with a space group P21/c; it is alternating {BO3(OH)} and {PO4} tetrahedrons. formed by alternating {BO3(OH)} and {PO4} tetrahedrons. Another class of compounds that show particular magnetic properties is constituted of substituted Another class of compounds that show particular magnetic properties is constituted of spirobiphenalenyl boron radicals (Figure1). They are paramagnetic. Another of their physical substituted spirobiphenalenyl boron radicals (Figure 1). They are paramagnetic. Another of their properties interested Pag et al. [4]. These radicals are indeed attractive molecular conductors for physical properties interested Pag et al. [4]. These radicals are indeed attractive molecular conductors optoelectronics, magnetooptics, and spintronics. for optoelectronics, magnetooptics, and spintronics.

Figure 1.1. MolecularMolecular structure structure of of substituted substituted spirobiphenalenyl spirobiphenalenyl boron boron radicals. Adapted from reference [[4].4]. Reproduced with permission fromfrom referencereference [[4],4], publishedpublished byby MDPI,MDPI, 2016.2016.

Borates are mainly used as heat-resistant glass and insulator fiberglass, ﬁberglass, as detergent and soap, and as agricultural micronutrients. Other applications are also prospected as borates have interesting optical properties. Gallium borate GaBO3 has notably been well studied with respect to luminescence

Crystals 2016, 6, 118 3 of 7 properties.Crystals 2016 This, 6, 118 was reported by Wang et al. [5], who especially worked on the single crystal structure3 of 7 (trigonal calcite type, space group R-3c) and the band gap (3.62 eV) of GaBO3. properties. This was reported by Wang et al. [5], who especially worked on the single crystal structure 2.2.(trigonal Attractive calcite Chemical type, Propertiesspace group R-3c) and the band gap (3.62 eV) of GaBO3.

2.2.Boron Attractive hydrides Chemical (e.g., Properties alkali borohydrides) are well-known as effective reducing agents. Sodium borohydride (NaBH4) is certainly the most widely used in both organic and inorganic Boron hydrides (e.g., alkali borohydrides) are well-known as effective reducing agents. Sodium chemistry. Potassium borohydride (KBH4) is an alternative. It was, for example, successfully used to borohydride (NaBH4) is certainly the most widely used in both organic and inorganic chemistry. reduce the oxide layer forming on the surface of the intermetallic material MlNi3.6Co0.85Al0.3Mn0.3 Potassium borohydride (KBH4) is an alternative. It was, for example, successfully used to reduce the used as a fuel cell anode. This work was reported by Chen et al. [6]. oxide layer forming on the surface of the intermetallic material MlNi3.6Co0.85Al0.3Mn0.3 used as a fuel Alkali borohydrides are also of interest in the ﬁeld of energy, especially as solid-state chemical cell anode. This work was reported by Chen et al. [6]. hydrogenAlkali storage borohydrides materials. are For also this of application,interest in the lithium field of borohydride energy, especially (LiBH as4) issolid-state more suitable chemical than the sodium and potassium counterparts, owing to the lightness of lithium (M = 6.94 g/mol) and hydrogen storage materials. For this application, lithium borohydride (LiBH4) isw more suitable than thethe highest sodium gravimetric and potassium hydrogen counterparts, density owing (18.5 wtto the % H).lightness Destabilization of lithium (M ofw LiBH = 6.944 hasg/mol) been and widely the investigatedhighest gravimetric with the objective hydrogen to decreasedensity the(18.5 dehydrogenation wt% H). Destabilization temperature, of LiBH and Zavorotynska4 has been widely et al.’s contributioninvestigated concerns with the the objective solid solution to decrease LiBH the4–LiCl dehydrogenation [7]. Liquid-state temperature, chemical and hydrogen Zavorotynska storage et is alsoal.’s a possible contribution application concerns for the alkali solid borohydrides, solution LiBH but4–LiCl NaBH [7].4 Liquid-stateis better than chemical the lithium hydrogen and potassium storage counterpartsis also a possible in that caseapplication [8]. Other for candidates alkali borohydrides, for chemical but hydrogen NaBH4 storageis better are than boranes, the lithium the simplest and candidatepotassium being counterparts ammonia in borane that case (NH [8].3 BHOther3). candidates Sagan et al. for [9 chemical], for their hydrogen part, considered storage are derivatives boranes, withthe the simplest following candidate molecular being structures: ammonia LiN(CHborane (NH3)2BH3BH3 3and). Sagan KN(CH et al.3) 2[9],BH for3. their part, considered derivativesThe BH3 withentity the is following not stable molecular enough tostructures: exist in anLiN(CH isolated3)2BH form.3 and KN(CH It exists3) as2BH a3. dimer, which is the diboraneThe BH molecule3 entity is B not2H 6stable. Otherwise, enough itto can exist be in stabilized an isolated by form. complexation It exists as witha dimer, a Lewis which base is the like diborane molecule B2H6. Otherwise, it can be stabilized by complexation with a Lewis base like ammonia NH3 or tetrahydrofuran (CH2)4O, leading to the formation of a Lewis adduct. Accordingly, ammonia NH3 or tetrahydrofuran (CH2)4O, leading to the formation of a Lewis adduct. Accordingly, BH3 is of key importance in molecular (organic) chemistry, as it can be used as protecting group of a potentialBH3 is of Lewiskey importance base. For in example,molecular one(organic) may chemistry, cite the work as it reportedcan be used by as Mamat protecting and group Köckerling, of a potential Lewis base. For example, one may cite the work reported by Mamat and Köckerling, who who used BH3 to protect a phosphane (Figure2)[ 10]; another work belongs to Bourque et al. [11]. used BH3 to protect a phosphane (Figure 2) [10]; another work belongs to Bourque et al. [11]. Both Both NaBH4 and a complex of BH3 were used for the synthesis of a maltolato compound; the former NaBH4 and a complex of BH3 were used for the synthesis of a maltolato compound; the former was was used as reducing agent, and the latter as Lewis acid. used as reducing agent, and the latter as Lewis acid.

FigureFigure 2. View2. View of of the the packing packing of of molecules molecules inin crystalscrystals of the phosphane borane borane adduct adduct (denoted (denoted 4) 4) alongalong the the crystallographic crystallographicb baxis, axis, and and view view ofof itsits molecularmolecular stru structure.cture. Adapted Adapted from from reference reference [10]. [10 ]. ReproducedReproduced with with permission permission from from reference reference [ 10[10],], published published byby MDPI,MDPI, 2016.

3.3. The The Journal JournalCrystals Crystals,, a a Key Key ForumForum forfor thethe Present Special Special Issue Issue TheThe present present special special issue issue is is constitutedconstituted ofof nine review and and research research papers papers ofof high high quality, quality, reportingreporting different different boron-based boron-based (nano-)materials (nano-)materials forfor differentdifferent possible areas areas of of application. application. They They are are brieﬂybriefly discussed discussed hereafter. hereafter.

3.1.3.1. Boranes Boranes and and Borohydrides Borohydrides for for Solid-State Solid-State Hydrogen Hydrogen StorageStorage

Owarzany et al. [12] present an overview on metal amidoboranes M(NH2BH3) with M = Li, Na, Owarzany et al. [12] present an overview on metal amidoboranes M(NH2BH3) with M = Li, Na, K, Rb, Cs, Mg, Ca, Sr, Y, Al, and Zn. Amidoboranes constitute a constantly growing family of NH3BH3 K, Rb, Cs, Mg, Ca, Sr, Y, Al, and Zn. Amidoboranes constitute a constantly growing family of NH3BH3 derivatives, especially investigated as solid-state chemical hydrogen storage materials. The authors

Crystals 2016, 6, 118 4 of 7 derivatives, especially investigated as solid-state chemical hydrogen storage materials. The authors propose the most exhaustive survey of the amidoborane reported so far. Their crystal structures are discussed first and in detail. Additional pieces of information are given based on nuclear magnetic resonance, infrared, and Raman spectra. The authors also scan the synthesis paths, while giving details about reactivity and selected properties, such as (among others) ionic conductivity, storage capacity, and interatomic distance. The conclusion of the article points out the issues that hinder the development and implementation of these chemical hydrogen storage materials. We [13] have also contributed to the special issue with an article dealing with the in situ Synchrotron X-ray diffraction of boranes. Pristine ammonia borane (NH3BH3), hydrazine borane (N2H4BH3), and two hydrazinidoborane derivatives (MN2H3BH3; M = Li, Na), hydrazine bisborane (BH3N2H4BH3) and sodium triborane (NaB3H8) are analyzed. All are currently considered as potential chemical hydrogen storage materials. The boranes were investigated over a wide range of temperatures (80–300 K). Differences in crystal structures, the existence of phase transition, evolutions of unit cell parameters and volumes, and variation of coefficients of thermal expansion can be observed. The main results are presented and discussed. Like Zavorotynska et al. [7] (cf. Section 2.2), Javadian et al. [14] investigated LiBH4. However, their approach is much different. They consider the destabilization of LiBH4 via the combination of two approaches; that is, chemical doping with another hydride (sodium alanate, NaAlH4) and nanoconfinement into a mesoporous carbon aerogel with high specific surface area (689 m2/g). The reciprocal is also true, because one may consider the destabilization of NaAlH4 by the presence of LiBH4. In comparison to the pure and bulk parent materials, improved dehydrogenation and rehydrogenation properties are reported, making the authors positively conclude on the viability of the proposed approach.

3.2. Polyhedral Boranes and Carboranes There are two review article contributions addressing boron-rich clusters, such as polyhedral boranes and carboranes. The ﬁrst is proposed by Planas et al. [15]. The review summarizes the synthetic routes, coordination chemistry, and properties of a series of carboranyl ligands containing N, O donors (e.g., closo-carbonylmethylalcohols with nitrogenated aromatic rings) as well as of metal (e.g., cobalt, iron, platinum, titanium) complexes of these N,O-type carborane ligands. The wealth of carborane chemistry is particularly remarked. It is of even more importance that N,O-type carborane ligand-based complexes show a variety of properties, such as those used in magnetic, chiroptical, nonlinear optical, catalytic, and biomedical applications. It is worth citing the last sentence of the authors’ conclusion, as it is true for all of the articles of the present special issue: “New developments are appearing constantly and are limited only by our imagination”. The second review by Avdeeva et al. [16] is substantial, with 85 pages about silver and copper complexes with closo-polyhedral borane (10 or 12 boron atoms), carborane (1 carbon atom with 9 or − 11 boron atoms) and metallacarborane (e.g., [3,3’-Co(1,2-C2B9H11)2] ) anions (Figure3). Each of the complexes is reported in terms of synthesis and crystal structures. They demonstrate a wide variety of structural types, relating to both the metal coordination environment and coordination modes of boron hydride anions, which is discussed in detail by the authors. This review paper is of fundamental importance, with a major part dedicated to crystal structures and coordination chemistry.

3.3. Boronate Ligands and Derivatives Sene et al.’s review [17] is about some of the key results on crystalline structures with boronate and benzoxaborolate ligands (anions) that can be used as building blocks (Figure4). The authors emphasize the fact that such ligands “had up until recently hardly been looked into as possible building blocks for materials applications”. With respect to these mentioned “materials applications”, the authors have in mind coordination networks as well as metal organic networks (MOFs). The article is structured Crystals 2016, 6, 118 5 of 7 around crystal structures involving simple boronates and benzoxaborolates, spectroscopic signatures in materials,Crystals 2016 and, 6, 118 ﬁnally emerging applications like functional coordination polymers/networks5 of 7 with temperature-dependentpolymers/networks with luminescent temperature-dependent properties or promisingluminescent magnetic properties properties or promising and benzoxaborolemagnetic drugsproperties (nanomedicine). and benzoxaborole drugs (nanomedicine). Crystals 2016, 6, 118 5 of 7

polymers/networks with temperature-dependent luminescent properties or promising magnetic properties and benzoxaborole drugs (nanomedicine).

Figure 3. Anionic polyhedral boron hydrides discussed by Avdeeva et al. in reference [16]. Reproduced Figure 3. Anionic polyhedral boron hydrides discussed by Avdeeva et al. in reference [16]. withReproduced permission with from permission reference from [16], re publishedference [16], by published MDPI, 2016. by MDPI, 2016. Figure 3. Anionic polyhedral boron hydrides discussed by Avdeeva et al. in reference [16]. 3.4. Boron3.4. Boron NitrideReproduced Nitride with permission from reference [16], published by MDPI, 2016.

Boron3.4.Boron nitrideBoron nitride Nitride is a non-oxide is a non-oxide ceramic ceramic material material of high of high interest. interest. A ﬁrst A first illustration illustration of this of isthis provided is provided by Matarin and Rimola [18]. By using computational methods (B3LYP-D2* periodic by Matarin andBoron Rimola nitride [ 18is ].a Bynon-oxide using computationalceramic material methodsof high interest. (B3LYP-D2* A first periodicillustration simulations), of this is they simulations), they investigated the adsorption of molecules like water (H2O), ammonia (NH3), formic investigatedprovided the adsorptionby Matarin and of moleculesRimola [18]. like By waterusing (Hcomputational2O), ammonia methods (NH (B3LYP-D2*3), formic acidperiodic (HCOOH), acid (HCOOH), benzene (C6H6), and methane (CH4) onto boron nitride nanotubes showing defects benzenesimulations), (C H ), and they methane investigated (CH the) adsorption onto boron of molecules nitride like nanotubes water (H2 showingO), ammonia defects (NH3), derivedformic from derived6 from6 monovacancies of 4boron and nitrogen atoms. Distinction is made between the monovacanciesacid (HCOOH), of boron benzene and (C nitrogen6H6), and methane atoms. (CH Distinction4) onto boron is nitride made nanotubes between showing the aforementioned defects aforementionedderived from polar monovacancies (H2O, NH 3,of HCOOH) boron and and nitrog non-polaren atoms. (C6 HDistinction6, CH4) molecules. is made Itbetween is shown the that polarnitrogen-rich (H2O, NH3 boron, HCOOH) nitride andnanotu non-polarbes are more (C6 Hreactive6, CH 4towards) molecules. the adsorption It is shown of polar that molecules nitrogen-rich aforementioned polar (H2O, NH3, HCOOH) and non-polar (C6H6, CH4) molecules. It is shown that boron nitride nanotubes are more reactive towards the adsorption of polar molecules (dictated by (dictatednitrogen-rich by dative boron interactions), nitride nanotu whereasbes are morethe adsorption reactive towards of non-polar the adsorption molecules of polar (physisorption molecules π dativegoverned interactions),(dictated by, by e.g., dative whereas-stacking interactions), the for adsorptionC6 Hwhereas6) does notthe of dependadsorption non-polar on ofthe molecules non-polar type of the molecules (physisorption nanotube. (physisorption It is therefore governed by, e.g., πconcluded-stackinggoverned that for by, C adsorptione.g.,6H 6π)-stacking does onto not for dependboron C6H6) doesnitride on not the na depend typenotubes of on the maythe nanotube. type be ofmodulated the Itnanotube. is therefore by theIt is presencetherefore concluded of that adsorptionsurfaceconcluded ontodefects. boron that adsorption nitride nanotubes onto boron may nitride be na modulatednotubes may by be the modulated presence by of the surface presence defects. of surface defects.

Figure 4. Representation of the structures of boronic acid/boronate (left); and FigureFigure 4. 4.Representation Representation ofof thethe structuresstructures of of boronic boronic acid/boronate acid/boronate (left); (left);and and benzoxaborole/benzoxaborolatebenzoxaborole/benzoxaborolate (right).(right). OnlyOnly the tetrahedral-borontetrahedral-boron formsforms of of benzoxaborole/benzoxaborolate (right). Only the tetrahedral-boron forms of boronate/benzoxaborolate boronate/benzoxaborolateboronate/benzoxaborolate anions anions are are shown shown here.here. AdaptedAdapted fromfrom referencereference [17]. [17]. Reproduced Reproduced with with anionspermission arepermission shown from here.from reference reference Adapted [17], [17], from published published reference by by MDPI, MDPI, [17]. 2016. Reproduced2016. with permission from reference [17], published by MDPI, 2016. HexagonalHexagonal boron boron nitride nitride (h-BN; (h-BN; Figure Figure 5) 5) isis principallyprincipally usedused as as lubricant lubricant in in domains domains ranging ranging fromHexagonalfrom metallurgy metallurgy boron to nitridecosmetics.to cosmetics. (h-BN; According According Figure to 5to )Yuan Yuan is principally etet al.al. [19], h-BN usedh-BN has ashas lubricantother other properties properties in domains that that may may ranging open open from newnew perspectives perspectives for for engineering engineering applications, applications, forfor exexample, asas aa solidsolid lubricant lubricant in in aeronautics aeronautics or asor as metallurgy to cosmetics. According to Yuan et al. [19], h-BN has other properties that may open new a perfecta perfect substrate substrate to tographene graphene for for electronic electronic dev devices.ices. SuchSuch promisingpromising developments developments require require tailored tailored perspectives for engineering applications, for example, as a solid lubricant in aeronautics or as a perfect h-BNh-BN shapes shapes displaying displaying a higha high level level of of crystallizatio crystallization,n, ensuring itsits properties properties for for the the long long term. term. This This substratearticle to graphene aims at giving for an electronic overview devices.of the strategies Such (rapid promising thermal developments annealing, Li3N as require crystallization tailored h-BN article aims at giving an overview of the strategies (rapid thermal annealing, Li3N as crystallization shapespromoter, displayingpromoter, and and aspark high spark plasma level plasma of sinter crystallization,sintering)ing) developed developed ensuring byby the its authorsauthors properties toto prepare prepare for the highly highly long crystallized term. crystallized This article aims at giving an overview of the strategies (rapid thermal annealing, Li3N as crystallization promoter, Crystals 2016, 6, 118 6 of 7

Crystals 2016, 6, 118 6 of 7 and spark plasma sintering) developed by the authors to prepare highly crystallized supported thick coatingssupported and self-standing thick coatings nanosheets. and self-standing The advantages nanosheets. and The limitations advantages of and the limitations three strategies, of the three as well as the mainstrategies, achievements, as well as are the reported main achievements, and discussed. are reported As a concluding and discussed. remark, As thea concluding authors optimistically remark, expectthe the authors development optimistically of h-BN-based expect the high developme performancent of devicesh-BN-based in the high future. performance devices Itin isthe worth future. mentioning that boron nitride is the featured molecule of a special issue of a sister It is worth mentioning that boron nitride is the featured molecule of a special issue of a sister journal; that is, Molecules MDPI [20]. The issue title is “boron nitride: synthesis and application”, journal; that is, Molecules MDPI [20]. The issue title is “boron nitride: synthesis and application”, and and is co-edited by one of the present co-editors, Philippe MIELE. This special issue is dedicated to is co-edited by one of the present co-editors, Philippe MIELE. This special issue is dedicated to the the mostmost recent recent developmentdevelopment on on the the synthesis synthesis and and applic applicationsations of boron of boron nitride, nitride, including including synthesis synthesis of of high-qualityhigh-quality crystal,crystal, nanopowders, nanopowders, nanosheets, nanosheets, nano nanostructuredstructured and porous and ceramic porous materials, ceramic high- materials, high-performanceperformance composites, composites, and and optoelectronic optoelectronic and electronic and electronic devices. devices.

Figure 5. Representation of the structure of hexagonal boron nitride, the nitrogen atoms are represented Figure 5. Representation of the structure of hexagonal boron nitride, the nitrogen atoms are in blue,represented and the boronin blue, atoms and the in boron pink. atoms in pink.

3.5. Boron-Tread3.5. Boron-Tread Steels Steels The lastThe paperlast paper of theof the series series falls falls in in a a different different fiel field.d. Through Through a research a research paper, paper, Gao Gao et al. et [21] al. give [21] give an overviewan overview of the of the use use of elementalof elemental boron boron as as anan effective alloying alloying element element employed employed to increase to increase the the hardenabilityhardenability of high of high strength strength low low alloy alloy steels. steels. TheyThey particularly particularly investigated investigated the theinfluence influence of boron of boron on initialon initial austenite austenite grain grain size size and and hot hot deformation deformation behavior of of boron boron microalloyed microalloyed steels. steels. Among Among the the various findings, the following one may be cited. Increasing the boron content of boron-tread steel various findings, the following one may be cited. Increasing the boron content of boron-tread steel increases the peak and critical strain of dynamic recrystallization, indicating that boron addition can increasesdelay the the peakonset andof dynamic critical recrystallization. strain of dynamic We recrystallization,believe that the present indicating work will that give boron the additionreader a can delaywider the onset view of of the dynamic application recrystallization. prospects of boron We and believe related that boron-based the present materials. work will give the reader a wider view of the application prospects of boron and related boron-based materials. 5. Conclusions 4. Conclusions The present special issue gives an overview of the current works dedicated to crystalline boron- Thebased present (nano-)materials. special issueThough gives the original an overview review ofand the research current articles works dedicate dedicated large sections to crystalline to boron-basedcrystallinity (nano-)materials. and crystal structures, Though the theauthors original also reviewgive details and about research various articles aspects, dedicate such as large sectionssynthesis, to crystallinity spectroscopic and signatures, crystal structures, physicochemi thecal authors properties, also giveand application details about prospects. various With aspects, suchrespect as synthesis, to this last spectroscopic aspect, advanced signatures, technologies, physicochemical such as energy properties, storage, gas and sorption application (energy prospects. and environment), biomedicine and nanomedicine, and optical and electronic devices are targeted. We With respect to this last aspect, advanced technologies, such as energy storage, gas sorption (energy and cordially invite the reader curious about science, and especially boron science, to dip into each of the environment),nine articles biomedicine of the present andspecial nanomedicine, issue. and optical and electronic devices are targeted. We cordiallyIn conclusion, invite the we reader would curious like to about thank science,all of the and authors especially who have boron contributed science, toto dipthis intospecial each of the nineissue. articles We again ofthe thank present the main/correspondence special issue. authors who accepted our invitation several months Inago. conclusion, The present we special would issue like is toan thankimportant all of scientif the authorsic contribution who have to the contributed field of boron to this science, special and issue. We againwe, the thank contributors, the main/correspondence have all been actors authors of this who successful accepted publication. our invitation “New severaldevelopments months are ago. The presentappearing special constantly issue and is are an limited important only by scientific our imagination contribution” [15]. to the field of boron science, and we, the contributors,We wish haveall of the all beenreaders actors compelling of this reading. successful publication. “New developments are appearing constantlyAcknowledgments: and are limited We only acknowledge by our imagination all of the students”[15]. (undergraduate, graduate, PhD) and post-doctorate Wefellows wish who all worked of the or readers are working compelling on boron science reading. under our supervision. Conflicts of Interest: The authors declare no conflict of interest. Acknowledgments: We acknowledge all of the students (undergraduate, graduate, PhD) and post-doctorate fellows who worked or are working on boron science under our supervision. Conflicts of Interest: The authors declare no conflict of interest. Crystals 2016, 6, 118 7 of 7

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