Barium Titanium Oxynitride from Ammonia-Free Nitridation of Reduced Batio3

inorganics

Article Barium Titanium Oxynitride from Ammonia-Free Nitridation of Reduced BaTiO3

Hua Guo 1 , Aleksander Jaworski 1, Zheng Chen 2, Can Lu 2, Adam Slabon 1 and Ulrich Häussermann 1,*

1 Department of Materials and Environmental Chemistry, Stockholm University, SE-10691 Stockholm, Sweden; [email protected] (H.G.); [email protected] (A.J.); [email protected] (A.S.) 2 Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, DE-52074 Aachen, Germany; [email protected] (Z.C.); [email protected] (C.L.) * Correspondence: [email protected]; Tel.: +46-08-162390

Abstract: We investigated the nitridation of reduced BaTiO3, BaTiO2.60H0.08, corresponding to an oxyhydride with a large concentration of O defects (>10%). The material is readily nitrided under ◦ flowing N2 gas at temperatures between 400 and 450 C to yield oxynitrides BaTiO2.6Nx (x = 0.2−0.22) with a slightly tetragonally distorted perovskite structure, a ≈ 4.01 and c ≈ 4.02 Å, and Ti partially remaining in the oxidation state III. The tetragonal structure was confirmed from Raman spectroscopy. 14N MAS NMR spectroscopy shows a single resonance at 270 ppm, which is typical for perovskite transition metal oxynitrides. However, largely different signal intensity for materials with very similar N content suggests N/O/vacancy ordering when prolonging nitridation times to hours. Diffuse reflectance UV-VIS spectroscopy shows a reduction of the intrinsic band gap to 2.4–2.45 eV

compared to BaTiO3 (~3.2 eV). Mott-Schottky measurements conﬁrm n-type conductivity and reveal a slight negative shift of the conduction band edge from –0.59 V (BaTiO3) to ~–0.65 eV.

Citation: Guo, H.; Jaworski, A.; Keywords: solid-state chemistry; mixed-anion compounds; barium titanate; oxyhydride; oxynitride; Chen, Z.; Lu, C.; Slabon, A.; solid-state NMR; optical bandgap Häussermann, U. Barium Titanium Oxynitride from Ammonia-Free

Nitridation of Reduced BaTiO3. Inorganics 2021, 9, 62. https:// 1. Introduction doi.org/10.3390/inorganics9080062 In 2012 Kobayashi et al. reported that the simple perovskite BaTiO3 can be converted to BaTiO3-xHx (with H-contents up to x ≈ 0.6) at comparatively mild temperatures, around Academic Editor: Hiroshi Kageyama ◦ 600 C, by hydride reduction [1]. The paramagnetic oxyhydride BaTiO3-xHx attains a cubic structure in which O2− and H− ions commonly—and as in a solid–solution—form the Received: 28 June 2021 octahedral environment around Ti that is now in a mixed IV/III oxidation state. Shortly Accepted: 31 July 2021 after, it was shown that due to the labile nature of the hydride species BaTiO Hx repre- Published: 5 August 2021 3-x sents a versatile precursor toward new mixed-anion compounds [2]. This includes low temperature conversion to heavily nitridized BaTiO by either ammonolysis (nitridation Publisher’s Note: MDPI stays neutral 3 under a stream of NH ) but, remarkably, also nitridation with inert elemental N [2,3]. with regard to jurisdictional claims in 3 2 published maps and institutional afﬁl- Because of the ease of activation of N2, BaTiO3-xHx has been investigated as a potential iations. catalyst for ammonia synthesis [4]. Nitridized BaTiO3 has been previously obtained from the ammonolysis of mixtures ◦ of BaO (BaCO3) and TiO2 at 950 C, which produced BaTiO2.85N0.1 (0.57 wt% N) [5]. By applying an excess of BaO nitrogen contents may be increased to 0.8 wt% [6]. The low-temperature conversion from reduced BaTiO3 precursors will give even higher N Copyright: © 2021 by the authors. contents (up to 2.3 wt% as in BaTiO2.4N0.4 [3]) and provide at the same time a broader Licensee MDPI, Basel, Switzerland. range of compositions because of their variable O content, which can be—at least partially— This article is an open access article distributed under the terms and controlled through the reaction conditions in the hydride reduction. Yet there have been conditions of the Creative Commons only few investigations into the nitridation of reduced BaTiO3 and, in particular, little is Attribution (CC BY) license (https:// known with respect to the properties of nitridized BaTiO3. According to Yajima et al. [3] creativecommons.org/licenses/by/ and Masuda et al. [2] barium titanium oxynitride materials attain, depending on the N 4.0/). content, a cubic or a slightly distorted tetragonal simple perovskite structure. The tetragonal

Inorganics 2021, 9, 62. https://doi.org/10.3390/inorganics9080062 https://www.mdpi.com/journal/inorganics Inorganics 2021, 9, x FOR PEER REVIEW 2 of 14

Inorganics 2021, 9, 62 with respect to the properties of nitridized BaTiO3. According to Yajima et al. [3] and2 Ma- of 14 suda et al. [2] barium titanium oxynitride materials attain, depending on the N content, a cubic or a slightly distorted tetragonal simple perovskite structure. The tetragonal phases are non-centrosymmetric and ferroelectric. Temperature induced tetragonal-to-cubic phases are non-centrosymmetric and ferroelectric. Temperature induced tetragonal-to- transition occurs at around 120 °C, which◦ is close to that of BaTiO3. cubic transition occurs at around 120 C, which is close to that of BaTiO3. Here we re-examine the nitridation of reduced BaTiO3 with N2. We point out that Here we re-examine the nitridation of reduced BaTiO with N . We point out that 2− 3 2 reduced BaTiO3 range from oxyhydrides where O in2− the anion substructure is stoichio- reduced BaTiO3 range from oxyhydrides where O in the anion substructure is stoi- metrically replaced by H− (as in refs [1–3]) to materials with rather low contents of H and chiometrically replaced by H− (as in refs [1–3]) to materials with rather low contents of simultaneously large concentrations of O vacancies, BaTiO3-xHy◻(x-y). Hydride reduction H and simultaneously large concentrations of O vacancies, BaTiO3-xHy(x-y). Hydride of BaTiO3—involving the sintering of pelletized mixtures of BaTiO3 and metal hydride reduction of BaTiO3—involving the sintering of pelletized mixtures of BaTiO3 and metal (e.g., CaH2) over prolonged periods of time—is not well understood, but it is most likely hydride (e.g., CaH2) over prolonged periods of time—is not well understood, but it is that O-deficient forms BaTiO3-x represent intermediates in the formation of oxyhydrides. most likely that O-deficient forms BaTiO3-x represent intermediates in the formation of Reactionsoxyhydrides. conditions Reactions in conditionshydride reductions in hydride can reductions be varied can by bethe varied choice by and the choiceconcentra- and tion/activityconcentration/activity of metal hydride, of metal reaction hydride, temper reactionature, temperature, time and also time the and simultaneous also the simultane- pres- ence of a (pressurized) H2 atmosphere [7–10]. Depending on conditions, highly O-defi- ous presence of a (pressurized) H2 atmosphere [7–10]. Depending on conditions, highly cientO-deficient disordered disordered cubic phases cubic phases with x withup to x 0. up6 and to 0.6 y in and a range y in a 0.04–0.25 range 0.04–0.25 may be may obtained be ob- [10,11].tained [10We,11 show]. We that show such that O-deficient such O-deficient variants variants show showequally equally high highreactivity reactivity toward toward oxynitride formation as originally investigated BaTiO3-xHx. The sequence of materials con- oxynitride formation as originally investigated BaTiO3-xHx. The sequence of materials versionsconversions is sketched is sketched in Figure in Figure 1. 1.

FigureFigure 1. 1. SketchSketch of of material material conversions: conversions: hydride hydride reduction reduction of BaTiO of BaTiO3 leading3 leading to an to O-deficient an O-deficient oxyhydride (exemplified as H/vacancy disordered BaTiO2.5H0.125) which then is nitridized with ele- oxyhydride (exemplified as H/vacancy disordered BaTiO2.5H0.125) which then is nitridized with mental nitrogen (exemplified as BaTiO2.5N0.25 with an ordered (orthorhombic) N/vacancy arrange- elemental nitrogen (exemplified as BaTiO2.5N0.25 with an ordered (orthorhombic) N/vacancy ar- ment). Ba, O, H, N and vacancies are depicted as grey, red, green, blue, and white-blue circles, re- rangement). Ba, O, H, N and vacancies are depicted as grey, red, green, blue, and white-blue circles, spectively. respectively.

2.2. Results Results and and Discussion Discussion 2.1.2.1. BTON BTON Synthesis Synthesis and and Characterization Characterization

TheThe starting starting material material for nitridation was obtained from reacting BaTiO33 with CaH2 at ◦ 550550 °CC for 48 hh andand correspondedcorresponded toto BaTiO BaTiO2.602.60HH0.080.08◻0.320.32, ,in in the the following following abbreviated abbreviated as BTOH.BTOH. BTOH BTOH was was characterized characterized by by powder powder X-ray X-ray diffraction diffraction (PXRD), (PXRD), thermogravimetric thermogravimetric (TG)(TG) analysis, analysis, and and 11HH Magic Magic Angle Angle Spinning Spinning (MAS) (MAS) Nuclear Nuclear Magnetic Magnetic Resonance Resonance (NMR) (NMR) 1 spectroscopy.spectroscopy. The The TG TG curve is shown in Figure 22 whereas whereas thethe PXRDPXRD andand 1H-MAS NMR analysesanalyses are detailed as Supplementarysupplementary Materials,material, Figures Figures S1 S1 and and S2. S2. The The characterization characterization resultsresults are are summarized summarized in in Table Table 1.1. BT BTOHOH has has a acubic cubic lattice lattice parameter parameter of ofa = a4.022= 4.022 Å re- Å sultingresulting in ina unit a unit cell cell volume volume slightly slightly higher higher than than BaTiO BaTiO33. .From From TG TG analysis analysis the the H H content content and/orand/or O O defect defect concentration concentration of of reduced reduced BaTiO BaTiO3 cancan be be assessed assessed [1,11,12]. [1,11,12]. An An experiment experiment → underunder flowing ﬂowing air air monitors monitors the the reactions reactions BaTiO BaTiO3-3-xHxHx +x +0.75 0.75x Ox 2O →2 BaTiOBaTiO3 + 30.5+x 0.5 Hx2OH 2(1)O → and(1) andBaTiO BaTiO3-x + 3-0.5x x+ O 0.52 →x OBaTiO2 3BaTiO (2). Typically3 (2). Typically there is a there small is initial a small weight initial loss weight (between loss ◦ 0.1(between and 0.15%) 0.1 and in the 0.15%) temperature in the temperature range up to range 300 °C up which to 300 stemsC which from stems the loss from of thesurface loss of surface hydroxyl and secondary water [13,14]. The onset of oxidation is seen at around hydroxyl◦ and secondary water [13,14]. The onset of oxidation◦ is seen at around 400 °C. Oxidation400 C. Oxidation appears appearsto be completed to be completed above 650 above °C and 650 leadsC and to leads a weight to a weightincrease increase of 2.79%. of The2.79%. associated The associated x values x valuesaccording according to Equati to Equationsons (1) and (1) (2) and are (2) contained are contained in Table in Table 1. 1H-1. 1 ≈ MAS-NMRH-MAS-NMR then then revealed revealed a rather a rather small small H Hcontent content in in BTOH, BTOH, xx ≈ 0.08,0.08, and and accordingly accordingly BTOHBTOH possesses possesses a a high high concentration of O defects.

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FigureFigure 2.2. TGTG tracetrace forfor BTOHBTOH uponupon heatingheating underundera adry dryair airstream. stream.

TableTable 1. 1.Characterization Characterization summary summary for for BTOH. BTOH.

3 1 2 Sample LatticeLattice ParametersVolume (Å) Volume (Å ) XTG,H X TG,◻ XNMR,H X◻ Formula Sample X 1 X 2 X X Formula Parametersa = 3.9964(1) (Å) (Å3) TG,H TG, NMR,H BaTiO3 64.379(2) a = 3.9964(1)c = 4.0310(2) BaTiO3 64.379(2) BTOH c = 4.0310(2)4.0220(2) 65.062(1) 0.422 0.396 0.08 0.32 BaTiO2.60H0.08 1 2 BTOHXTG,H refers to 4.0220(2) the reaction BaTiO 65.062(1)3-xHx + 0.75 0.422x O2 → BaTiO 0.3963 + 0.5x H 0.082O. XTG,0.32◻ refersBaTiO to the2.60 reac-H0.08 1tion BaTiO3-x + 0.5x O2 → BaTiO3. 2 XTG,H refers to the reaction BaTiO3-xHx + 0.75x O2 → BaTiO3 + 0.5x H2O. XTG, refers to the reaction BaTiO3-x + 0.5x O2 → BaTiO3. Figure 3 shows combined TG and differential scanning calorimetry (DSC) traces of BTOHFigure when3 shows heated combined in a stream TG of and N2 to differential 500 °C (a) scanning and 600 calorimetry°C (b) at a rate (DSC) of 10 traces °C/min. of ◦ ◦ ◦ BTOHThe exothermic when heated event in at a streamaround of435 N 2°C,to concomitant 500 C (a) and with 600 a weightC (b) at increase a rate of of 10 0.99%C/min. and The1.38% exothermic respectively, event is atattributed around 435 to the◦C, concomitantformation of witha nitridated a weight phase, increase in ofthe 0.99% following and 1.38%abbreviated respectively, as BTON. is attributed The onset to of the nitridatio formationn occurs of a nitridatedat a similar phase, temperature in the following as oxida- abbreviatedtion, perhaps as relating BTON. The to the onset activation of nitridation of labile occurs H−, atbut a similarappears temperature to be a sudden as oxidation, process perhapscompared relating to more to the gradual activation oxidation. of labile Figure H−, but 3c compares appears to TG be atraces sudden of processBTOH when compared heat- toing more for a gradual prolonged oxidation. period of Figure time 3afterc compares reaching TG the traces target of temperatures BTOH when 500 heating and 600 for °C, a ◦ prolongedrespectively. period There of timewas afterno further reaching weight the target increase temperatures with time 500 in andthe 600600 °CC, respectively.experiment, ◦ Therewhich was implies no further that the weight nitridation increase reaction with time had inalready the 600 beenC experiment, completed when which reaching implies thatthis thetemperature. nitridation In reaction the 500 had°C experiment, already been continued completed weight when increase reaching was this seen temperature. for about ◦ Inadditionally the 500 C experiment,15 min. At the continued same time weight the TG increase trace wasplateaued seen for at abouta lower additionally weight gain 15 level, min. Atwhich the samesuggests time that the TGthe trace600 °C plateaued experiment at a re lowersulted weight in a product gain level, with which a slightly suggests higher that N ◦ thecontent 600 (byC experiment 15%). The N resulted content in corresponding a product with to athe slightly TG weight higher increase N content is x = (by 0.19 15%). and The0.22 Nfor content the 500 corresponding and 600 °C experiment, to the TG resp weightectively. increase The isNx content= 0.19 andwas 0.22also fordetermined the 500 ◦ andby chemical 600 C experiment, analysis for respectively.samples obtained The at N 600 content °C without was also annealing determined (BTON-600C-0 by chemical h) ◦ analysisand after for 6 h samples prolonged obtained nitridation at 600 (BTON-600C-6C without annealing h) and (BTON-600C-0found to be in good h) and agreement after 6 h prolongedwith the TG nitridation results. The (BTON-600C-6 characterization h) and results found for to BTON be in good samples agreement are summarized with the TG in results.Table 2 Theand characterizationas supplementary results material. for BTONIt is important samples to are emphasize summarized that innitridation Table2 and ex- asperiments Supplementary require Materials.strictly air-free It is importantconditions, tootherwise emphasize (partial) that nitridation oxidation will experiments occur. In requireair BTON strictly samples air-free are stable conditions, up to otherwiseabout 400–500 (partial) °C. oxidation will occur. In air BTON samples are stable up to about 400–500 ◦C.

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FigureFigure 3.3. Simultaneously measured measured TG TG and and DSC DSC trac traceses for for BTOH BTOH upon upon heating heating under under a dry a dry N2 N2 streamstream toto 500500 ◦°CC( (aa)) and and 600 600 °C◦C( (bb).). (c ()c )TG TG traces traces upon upon prolonged prolonged nitridation nitridation at at500 500 °C◦ (2C h) (2 and h) and 600600 ◦°CC (6(6 h).h).

Table 2. Characterization summary for BTON. Samples are abbreviated as BTON-xxxC-yh where Table 2. Characterization summary for BTON. Samples are abbreviated as BTON-xxxC-yh where xxx indicates the xxx indicates the nitridation temperature and y the annealing time. Note that the tetragonal distor- nitridation temperature and y the annealing time. Note that the tetragonal distortion has poor signiﬁcance since the tion has poor significance since the refinement of BTON PXRD patterns does not allow to discrimi- reﬁnement of BTON PXRD patternsnate between does notcubic allow and totetragonal discriminate phase between when c/a cubic < 1.003. and tetragonal phase when c/a < 1.003. 3 Sample Lattice ParametersSample (Å) Lattice Volume Parameters (Å ) X(Å)TG,N Volume (ÅXchem.anal.,N3) XTG,N X chem.anal.,N FormulaFormula a = 4.0086(3) a = 4.0086(3) BTON-600C-0 h BTON-600C-0 h 64.630(2) 0.2264.630(2) 0.200.22 0.20 BaTiO BaTiO2.602.60NN0.200.20◻0.200.20 c = 4.0180(5) c = 4.0180(5) a = 4.0085(3) a = 4.0085(3) BTON-600C-6 h BTON-600C-6 h 64.533(2) 0.2164.533(2) 0.180.21 0.18 BaTiO BaTiO2.602.60NN0.180.18◻0.220.22 c = 4.0184(2) c = 4.0184(2)

TheThe compositioncomposition of of nitridized nitridized BaTiO BaTiO3 3can can be be expressed expressed as as BaTiO BaTiO3-x3-NxN2x2/3x/3.. Accordingly, Accordingly, thethe inherentinherent OO defectdefect concentrationconcentration isisx x/3/3 [[3].3]. TheThe upperupper limitlimit forfor xx forfor reducedreduced BaTiOBaTiO33 appearsappears toto be bex x≈ ≈0.6 0.6 [ 1[1].]. Thus Thus the the composition composition BaTiO BaTiO2.4N2.40.4N,0.4 as, as reported reported by by Yajima Yajima et al. et [ 3al.], should[3], should represent represent the the highest highest achievable achievable N contentN content for for Ba Ba oxynitrides. oxynitrides. With With our our BTOHBTOH starting material, the composition after nitridation was expected to be BaTiO2.6N0.267. This

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Inorganics 2021, 9, 62 5 of 14 N content is clearly not achieved, which implies that Ti partially remains as Ti3+ (about 5%). Thus, our BTON materials should be paramagnetic and possibly metallic. To starting material, the composition after nitridation was expected to be BaTiO N . This accommodate for this situation, the formula for nitridized BaTiO2.6 3 0.267is rewritten as BaTiO3- N content is clearly not achieved, which implies that Ti partially remains as Ti3+ (about 5%). xN(2x/3)-y◻(x/3)+y where y = 0 refers to the stoichiometric, diamagnetic case. The application of Thus, our BTON materials should be paramagnetic and possibly metallic. To accommodate higher nitridation temperatures and/or changed heating rates did not result in fully for this situation, the formula for nitridized BaTiO3 is rewritten as BaTiO3-xN(2x/3)-y(x/3)+y nitridizedwhere y = 0 BTON refers tosamples the stoichiometric, (cf. supplementary diamagnetic material, case. The Figure application S3). of higher nitridationFigure temperatures 4 shows scanning and/or changed electron heating microsco rates didpy not(SEM) result images in fully nitridizedof the materials consti- tutingBTON samplesthe sequence (cf. Supplementary of conversions Materials, (cf. Figure Figure S3). 1). The average particle size of tetragonal Figure4 shows scanning electron microscopy (SEM) images of the materials consti- BaTiOtuting the3 used sequence for hydride of conversions reduction (cf. Figure was1 ).specified The average by particlethe manufacturer size of tetragonal as 0.5 micron. This isBaTiO confirmed3 used for in hydride Figure reduction 4a, although was speciﬁed some by particle the manufacturer size distribution as 0.5 micron. is Thisnoticeable. is The par- ticleconﬁrmed morphology in Figure4 a,of although BaTiO3 some is retained particle sizein BTOH distribution (Figure is noticeable. 4b). However, The particle upon nitridation themorphology surface oftopography BaTiO3 is retained of particles in BTOH becomes (Figure4b). noticeable However, changed upon nitridation (Figure the 4c,d). The step- surface topography of particles becomes noticeable changed (Figure4c,d). The step-like likefeatures features suggest suggest recrystallization, recrystallization, involving ion involving transport ion during transport the solid–gas during reaction. the solid–gas reac- 3 tion.Thus theThus nitridation the nitridation process of process reduced BaTiOof reduced3 may not BaTiO be truly may topochemical. not be truly topochemical.

FigureFigure 4. 4.SEM SEM images images of BaTiO of BaTiO3 employed3 employed for hydride for reduction hydride (a ),reduction of BTOH (b (),a of), BTON-600C-0of BTOH (b h), of BTON-600C- 0(c ),h and(c), BTON-600C-6and BTON-600C-6 h (d). h (d).

Figure5a depicts the changes of PXRD patterns and sample colors when going from Figure 5a depicts the changes of PXRD patterns and sample colors when going from BaTiO3 to BTOH to BTON. Detailed results of Rietveld refinements are presented as BaTiOSupplementary3 to BTOH Materials to BTON. (Figures Detailed S4–S13, results Tables S1of andRietveld S2). The refinements tetragonal structure are presented as sup- plementaryof BaTiO3 is clearlymaterial noticeable (Figures in S4 the–S13, laboratory Tables PXRD S1 and pattern, S2). Thea = 3.996,tetragonalc = 4.031 structure Å of BaTiO3 is(c/a clearly= 1.009), noticeable whereas the in patterns the laboratory for BTOH andPXRD BTON pattern, appear a cubic, = 3.996, with thec = unit4.031 cell Å (c/a = 1.009), volume of BTON slightly smaller. Compared to BTOH one could infer a slight broadening whereasof the reflections the patterns (200), (201), for (211)BTOH (in theand 2 θBTONrange 20–40appear◦), which cubic, most with clearly the wouldunit cell volume of BTONshow the slightly tetragonal smaller. split. However, Compared the refinement to BTOH of laboratoryone could data infer does a notslight allow broadening for of the reflectionsdiscrimination (200), between (201), cubic (211) and (in tetragonal the 2θ phase range (c/a 20–40°),< 1.003), which Figure5 b.most The tetragonalclearly would show the tetragonalstructure of BTONsplit. isHowever, then clearly the revealed refinement from Raman of laboratory spectroscopy, data as shown does in not Figure allow6. for discrimi- ≈ nationThus, we between chose to present cubic theand result tetragonal of the tetragonal phase refinement(c/a < 1.003), (c/a Figure1.002(2)) 5b. in The Table tetragonal2, struc- whereas the result of the cubic refinements is given in Table S2. ture of BTON is then clearly revealed from Raman spectroscopy, as shown in Figure 6. Thus, we chose to present the result of the tetragonal refinement (c/a ≈ 1.002(2)) in Table 2, whereas the result of the cubic refinements is given in Table S2.

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FigureFigure 5. 5.(a()a PXRD) PXRD patterns patterns of of BaTiO BaTiO3 3employedemployed for for hydride hydride reduction, reduction, BTOH, BTON-600C-0BTON-600C-0 h, h, and and BTON-600C-6 BTON-600C-6 h, h, with with Figure 5. (a) PXRD patterns of BaTiO3 employed for hydride reduction, BTOH, BTON-600C-0 h, and BTON-600C-6 h, with b picturespicturespictures on onon the thethe right rightright illust illustratingillustratingrating the thethe sample samplesample colors. colors.colors. ( (b(b)) )Section Section ofof RietveldRietveldRietveld plotsplotsplots forfor thethe PXRD PXRDPXRD pattern patternpattern of ofof BTON-600C-6BTON-600C-6 BTON-600C-6 hh h 22 when ﬁtting a cubic structure (a = 4.0111(2),𝜒χ 2 = 4.90, RBragg = 4.71,F R = 3.25) and tetragonal structure (a = 4.0085(3), whenwhen fitting fitting a cubica cubic structure structure (a ( a= =4.0111(2), 4.0111(2), 𝜒 = = 4.90, 4.90, R RBraggBragg == 4.71,4.71, RRF ==F 3.25)3.25) and tetragonal structurestructure ((aa == 4.0085(3), 4.0085(3), c c= = 2 𝜒 2 2 Bragg F 4.0184(2),c4.0184(2), = 4.0184(2), 𝜒= 4.25,χ= 4.25,= 4.25,R RBragg R =Bragg =5.45, 5.45,= R5.45, R =F =3.66). R3.66).F = 3.66).

FigureFigure 6.6. Compilation of Raman spectraspectra forfor BaTiOBaTiO3,, BTOH,BTOH, BTON-500C-0BTON-500C-0 h,h, BTON-500C-2BTON-500C-2 h,h, Figure 6. Compilation of Raman spectra for BaTiO33, BTOH, BTON-500C-0 h, BTON-500C-2 h, BTON-600C-0 h, and BTON-600C-6 h. BTON-600C-0BTON-600C-0 h, h, and and BTON-600C-6 BTON-600C-6 h.

TheThe Raman spectrum of of tetragonal tetragonal BaTiO BaTiO3 3isis well well understood understood [15–17]. [15–17 All]. Alltwelve twelve op- The Raman spectrum of tetragonal BaTiO3 is well understood [15–17]. All twelve op- opticaltical modes modes 3×A 31× +A B11 + 4×E B1 + are 4× RamanE are Ramanactive. Poly/nanocrystalline active. Poly/nanocrystalline samples are samples character- are tical modes 3×A1 + B1 + 4×E are Raman active. Poly/nanocrystalline samples are character- characterizedized by sharp by bands sharp at bands about at 170 about and 170 305 and cm 305−1 and cm −asymmetric1 and asymmetric broader broader bands bandsat about at ized by sharp bands at about 170 and 305 cm−1 and asymmetric broader bands at about about260, 515, 260, and 515, 720 and cm 720−1 [18,19]. cm−1 [18 Furthermore,,19]. Furthermore, a pronounced a pronounced dip at diparound at around 180 cm 180−1 is cm also−1 260, 515, and 720 cm−1 [18,19]. Furthermore, a pronounced dip at around 180 cm−1 is also ischaracteristic, also characteristic, which which has been has beenattributed attributed to interference to interference from from anharmonic anharmonic coupling coupling be- characteristic, which has been attributed to interference from anharmonic coupling be- betweentween three three A1A modes1 modes [15]. [15 Lastly,]. Lastly, the theE1 softmode E1 softmode (x,y (x,ydisplacement displacement of Ti) of expresses Ti) expresses itself tweenitselfwith withthreea sharp aA sharp 1rise modes of rise Raman [15]. of RamanLastly, scattering scatteringthe E inte1 softmodensity intensity at (x,ylowest at displacement lowest wavenumbers wavenumbers of Ti) [19]. expresses The [19 ].BTOH The itself withBTOHspectrum a sharp spectrum is featureless,rise isof featureless,Raman in agreementscattering in agreement withintensity a withcubic at a lowestperovskite cubic perovskitewavenumbers structure structure where [19]. The whereall modes BTOH all spectrummodesare Raman are is Raman featureless,inactive. inactive. The in Raman agreement The Raman spectra with spectra of ath cubice of BTON the perovskite BTON samples samples structureshow show broad where broad bands bandsall at modes 347, at − − are347,506 Raman and 506 and716 inactive. 716cm−1 cm and The1 sharpand Raman sharp bands bandsspectra at 100, at of 100,180 th and 180e BTON and302 cm 302 samples−1 cm. The1 .latter Theshow latter (assigned broad (assigned bands to the toat non- the347, −1 −1 506non-polarpolar and B 7161 mode) B cm1 mode) isand characteristic is sharp characteristic bands for at a for100, tetragonal a tetragonal180 and phase 302 phase cm [20,21]. [. 20The,21 The latter]. The same (assigned same holds holds true to truethe for non- forthe −1 polartheband band B at1 mode) 720 at 720 cm is cm− 1characteristic whichwhich would would for be a beextremely tetragonal extremely broad phase broad and [20,21]. and week week The in in samethe the cubic holds para-electricpara-electric true for the bandphase at [20]. 720 Becausecm−1 which of the would considerably be extremely lower tetragonality broad and week of the in BTON the cubic phases para-electric compared phase [20]. Because of the considerably lower tetragonality of the BTON phases compared

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phase [20]. Because of the considerably lower tetragonality of the BTON phases compared to BaTiO3, bands are generally broader and weaker. However, there are two clear major differences. The softmode in BaTiO3 appears stiffened and the corresponding band is at around 100 cm−1 in the spectra of the BTON phases. Further, the band at around 260 cm−1 (2) −1 in BaTiO3 (A1 ) is not apparent in BTON and instead a band at 347 cm is visible. The (2) A1 mode is the mutual displacement of Ba atoms and the TiO6/2 octahedron framework in the z direction [19]. It is not clear why this mode has either a very low intensity for BTON or is possibly blue-shifted by almost 90 cm−1. The Raman spectra of the four different BTON samples are very much alike (possibly −1 that the B1 peak at ~302 cm is somewhat sharper for the annealed samples, especially for BTON-600C-6 h). This is in contrast with 14N-MAS-NMR spectra, shown in Figure7. The nuclear quadrupole moment of the 14N isotope implies drastic broadening of the resonance lines unless N is in a highly symmetric environment for which the quadrupolar coupling constant is relatively small. This is the case for perovskite oxynitrides and 14N-MAS- NMR has been used previously to probe N [5,22–24]. Somewhat surprising and still not explained, 14N expresses as a single resonance at 270 ppm, regardless the kind of transition metal (Ti, Nb, Ta) or A component (Ca, Sr, Ba, La, Ce) in the perovskite oxynitride [25]. In our spectra the 270 ppm resonance is clearly there, but the signal intensity is considerably less for BTON-600-0 h compared to BTON-600C-6 h although the two materials possess virtually the same N content. We speculate that quadrupolar coupling is rather large for BTON-600C-0 h and that prolonged nitridation at 600 ◦C resulted in a more ordered N/O/vacancy structure for BTON-600C-6 h (cf. Figure1c). 1H-MAS-NMR spectra show clearly the absence of hydridic H in BTON-600-0h and BTON-600-6h. As earlier shown, hydridic H as part of the bulk structure can be discriminated from protic H (from (surface) hydroxyl and secondary water) as negative and positive chemical shift contributions, respectively [11,12]. Hydric H expresses itself as a single broad resonance peak at negative chemical shift whereas protic resonances appear at ~1 ppm (surface OH species) and in the region 6–7 ppm (secondary water). Interestingly one can recognize two H− environments for BTOH, one centered at −4 ppm and one at −55 pm. The occurrence of two hydridic environments in reduced BaTiO3 is highly unusual and perhaps relates to the large O defect concentration of BTOH (see Supplementary Materials for more details). Although the phenomenon is interesting and deserves more detailed investigations, it is not considered important for the outcome of the nitridation reactions.

2.2. Spectroscopic and Mott-Schottky Analysis

It is well known that BaTiO3 is a wide gap semiconductor with a band gap energy of 3.2–3.4 eV [26,27]. According to ﬁrst principles calculations, the band gap is indirect for the tetragonal form and direct for the cubic (high temperature) polymorph [28]. However, experimentally it appears to be difﬁcult to assign the nature of the band gap and frequently a direct band gap is also assumed for the tetragonal form. Mixed anion oxynitrides show reduced band gaps, by ca 1 eV, since the top of the valence band constitutes N-2p based states, which are at higher energy than O-2p [29]. Perovskite oxynitrides involving Ti are typically based on lanthanoid metals as the perovskite A component and ATiO2N (A = La, Nd, Ce, Pr, Eu) have been extensively investigated because of their photocatalytic activity and dielectric properties [30–32]. As outlined above, when A corresponds to an alkaline earth metal, the reduced valence has to be compensated with defects in the anion substructure, i.e., BaTiO3-xN(2x/3)-y(x/3)+y where y = 0 refers to the fully nitridized, diamagnetic case. For these materials properties are not well investigated. InorganicsInorganics 2021,, 9,, 62x FOR PEER REVIEW 8 of8 14 of 14

FigureFigure 7.7.Compilation Compilation of of MAS-NMR MAS-NMR spectra spectra for for BTON BTON and and BTON-600C-0 BTON-600C-0 h, andh, and BTON-600C-6 BTON-600C-6 h. h. 1 14 LeftLeft panel:panel:1 HH spectra.spectra. RightRight panel: panel:14 NN spectra. spectra. The The broken broken vertical vertical lines lines indicate indicate the the position position of of resonancesresonances of of hydridic hydridic H H (left (left) and) and N N (right (right). ).

2.2. SpectroscopicFigure8 compares and Mott-Schottky the UV-VIS Analysis diffuse reﬂectance spectra of the BTON-500C and

BTON-600CIt is well samples known with that BTOH BaTiO and3 is a pristine wide gap BaTiO semiconductor3. As earlier reported, with a band reduced gap BaTiOenergy3 of materials3.2–3.4 eV exhibit [26,27]. a According broad absorption to first inprinciples the visible calculations, and near-IR the region band (Figuregap is 8indirecta) [ 12]. for Thethe tetragonal actual absorption form and edge, direct corresponding for the cubic (high to interband temperature) transitions, polymorph is around [28]. 400 However, nm, whichexperimentally corresponds it appears well to theto be band difficult gap energy to assign of BaTiO the nature3. The nearof the IR-VIS band absorption gap and fre- enters a minimum upon approaching the upturn of the absorption edge, which is typical of quently a direct band gap is also assumed for the tetragonal form. Mixed anion oxyni- (intraband) free carrier absorption in heavily doped semiconductors [33]. In metallic BTOH trides show reduced band gaps, by ca 1 eV, since the top of the valence band constitutes states at the bottom of the conduction band are occupied. The absorption edge of BTON N-2p based states, which are at higher energy than O-2p [29]. Perovskite oxynitrides in- is signiﬁcantly red-shifted. In addition, the exponential onset of absorption indicates that chargevolving carriers Ti are aretypically localized based in traps on lanthanoid below the conductionmetals as the band perovskite [34]. So inA contrastcomponent with and ATiO2N (A = La, Nd, Ce, Pr, Eu) have been extensively investigated because of their pho- reduced BaTiO3 where electrons form delocalized band states [35], they appear localized in polaronstocatalytic in notactivity fully and nitridized dielectric BTON. properties [30–32]. As outlined above, when A corresponds to an alkaline earth metal, the reduced valence has to be compensated with defects in the anion substructure, i.e., BaTiO3-xN(2x/3)-y◻(x/3)+y where y = 0 refers to the fully nitridized, diamagnetic case. For these materials properties are not well investigated. Figure 8 compares the UV-VIS diffuse reflectance spectra of the BTON-500C and BTON-600C samples with BTOH and pristine BaTiO3. As earlier reported, reduced BaTiO3 materials exhibit a broad absorption in the visible and near-IR region (Figure 8a) [12]. The actual absorption edge, corresponding to interband transitions, is around 400 nm, which corresponds well to the band gap energy of BaTiO3. The near IR-VIS absorption enters a minimum upon approaching the upturn of the absorption edge, which is typical of (intraband) free carrier absorption in heavily doped semiconductors [33]. In metallic BTOH states at the bottom of the conduction band are occupied. The absorption edge of BTON is significantly red-shifted. In addition, the exponential onset of absorption indicates that

Inorganics 2021, 9, x FOR PEER REVIEW 9 of 14

Inorganics 2021, 9, 62 charge carriers are localized in traps below the conduction band [34]. So in contrast with9 of 14 reduced BaTiO3 where electrons form delocalized band states [35], they appear localized in polarons in not fully nitridized BTON.

Figure 8. (a) Diffuse reflectance UV-VIS spectra of BaTiO3, BTOH, BTON-500C-0 h, BTON-500C-2 Figure 8. (a) Diffuse reﬂectance UV-VIS spectra of BaTiO3, BTOH, BTON-500C-0 h, BTON-500C-2 h, h,BTON-600C-0 BTON-600C-0 h, h, and and BTON-600C-6 BTON-600C-6 h. h. (b (b) Normalized) Normalized Kubelka–Munk Kubelka–Munk function function of of the the spectra. spectra.

FigureFigure 8b8b shows shows the the Kubelka–Munk Kubelka–Munk (K–M) (K–M) transformed transformed spectra, spectra, which—for which—for compa- compa- rabilityrability across across the the samples—was samples—was normalized. normalized. The The K–M K–M transformed transformed spectrum spectrum of of metallic metallic BTOHBTOH is is different different from from the the BTON BTON materials materials because because of the of thefree free carrier carrier absorption. absorption. Its ab- Its sorptionabsorption edge edge is slightly is slightly (by 0.1 (by eV) 0.1 shifted eV) shifted upwards upwards compared compared to BaTiO to BaTiO3, which3, which is in line is in withline withearlier earlier observations observations [12]. [12The]. Theabsorption absorption edge edge of BTON of BTON is in is ina range a range 500–550500–550 nm. nm . TaucTauc plots plots (presented (presented as as Supplementary Supplementary Mate Materials,rial, Figure Figure S14) S14) suggest suggest band band gaps gaps in in a a rangerange 2.4–2.5 2.4–2.5 eV eV as as summarized summarized inin TableTable3 ,3, which which agrees agrees with with the the green-yellow green-yellow color color of theof thematerial. material. Mott–SchottkyMott–Schottky measurements measurements were were carried carried out out for for BTON-600-0 BTON-600-0 h h and and BTON-600-6 BTON-600-6 h h asas well well as as the the BTOH BTOH precursor precursor and and the the results results compared compared with with pristine pristine BaTiO BaTiO3 3(Figure(Figure 9).9). AllAll materials materials show show a apositive positive slope, slope, confirming confirming the the semiconducting semiconducting n-type n-type behavior. behavior. The The flatflat band band potential potential of of BaTiO BaTiO33 isis −−0.590.59 V Vvs. vs. Reversible Reversible Hydrogen Hydrogen Electrode Electrode (RHE). (RHE). In In case case ofof a an-type n-type semiconductor, semiconductor, the the flat flat band band potent potentialial is is close close to to the the bottom bottom of of the the conduction conduction bandband [36]. [36]. BTOH BTOH exhibits exhibits a a fl flatat band band potential potential at at −−0.540.54 V V vs. vs. RHE. RHE. A A slight slight positive positive shift shift ofof the the flat flat potential, potential, which which may may reach reach up up to to −−0.360.36 V V compared compared to to the the pristine pristine BaTiO BaTiO33, ,has has alsoalso been been observed observed for for other other reduced reduced materials materials BaTiO BaTiO3-xH3-xyH◻(yx-y) (withx-y) with different different x,y [12].x,y [ 12In]. contrast,In contrast, BTON BTON displays displays a negative a negative shift shift −0.65− V,0.65 without V, without any significant any significant difference difference be- tweenbetween the thedifferently differently annealed annealed samples samples (cf. Table (cf. Table 3). The3). The slopes slopes of BaTiO of BaTiO3 and3 BTOHand BTOH are veryare verysimilar, similar, although although the charge the charge carrier carrier density density of the oflatter the is latter considerably is considerably higher. higher.Since theSince slope the is slope also isdepending also depending on the ondielectric the dielectric constant, constant, this was this explained was explained with a much with a smallermuch smallerdielectric dielectric constant constant of cubic of BTOH cubic [12] BTOH. The [12 dielectric]. The dielectric constant constant of tetragonal of tetragonal BTON BTON will relate more to that of tetragonal BaTiO3 and, thus, the smaller slope compared to BaTiO3 reflects its higher charge carrier density.

Inorganics 2021, 9, x FOR PEER REVIEW 10 of 14

Inorganics 2021, 9, 62 10 of 14 will relate more to that of tetragonal BaTiO3 and, thus, the smaller slope compared to Ba- TiO3 reflects its higher charge carrier density.

Figure 9.9. Mott–Schottky plots of BaTiO33, BTOH, BTON-600C-0 h,h, andand BTON-600C-6BTON-600C-6 h.h.

TableTable 3.3. CompilationCompilation ofof bandgapsbandgaps andand ﬂatflat bandband potentials.potentials.

BandgapBandgap (eV) (eV) SampleSample FlatFlat Band Band Potential Potential (vs (vs. RHE, RHE, V)V) (Direct)(Direct) BaTiO3 3.18 −0.59 BaTiO3 3.18 −0.59 BTOHBTOH 3.33 3.33 −−0.540.54 BTON-600-0BTON-600-0 h h 2.45 2.45 −−0.640.64 BTON-600-6BTON-600-6 h h 2.46 2.46 −−0.660.66

3. Materials and Methods 3.1. Synthesis of the Materials As starting materials we used BaTiO (500 nm particle size, 99.9% purity, ABCR GmbH, As starting materials we used BaTiO3 3 (500 nm particle size, 99.9% purity, ABCR Karlsruhe, Germany) and CaH2 powder (95%, Sigma Aldrich, Darmstadt, Germany). Prior GmbH, Karlsruhe, Germany) and CaH2 powder (95%,◦ Sigma Aldrich, Darmstadt, Ger- to use, BaTiO3 was dried overnight in an oven at 200 C. All steps of sample preparation many). Prior to use, BaTiO3 was dried overnight in an oven at 200 °C. All steps of sample were performed in an Ar ﬁlled glove box. For a typical synthesis ~0.25 g (1.05 mmol) preparation were performed in an Ar filled glove box. For a typical synthesis ~0.25 g (1.05 of BaTiO3 was intimately mixed with 0.053 g (2.36 mmol) CaH2, i.e., the BaTiO3:H ratio mmol) of BaTiO3 was intimately mixed with 0.053 g (2.36 mmol) CaH2, i.e., the BaTiO3:H was about 4.5. The BaTiO3/CaH2 mixture was subsequently pressed into a pellet with ratio was about 4.5. The BaTiO3/CaH2 mixture was subsequently pressed into a pellet with a diameter of 9 mm. The pellet was then sealed inside a stainless steel ampule (with a diameter of 9 mm. The pellet was then sealed inside a stainless steel ampule (with di- dimensions 10 mm ID and 50 mm length), which—after removal from the glove box— mensions 10 mm ID and 50 mm length), which—after removal from the glove box—was was placed in a box furnace. A K-type thermocouple was inserted and located in close placed in a box furnace. A K-type thermocouple was inserted and located in close prox- proximity to the metal ampule. Ampules were heated for 48 h at 550 ◦C. After cooling to imity to the metal ampule. Ampules were heated for 48 h at 550 °C. After cooling to room room temperature, ampules were opened and the products washed with 0.1 M acetic acid temperature, ampules were opened and the products washed with 0.1 M acetic acid (HAc) (HAc) solution to remove excess CaH2 and metal oxide/hydroxide formed during hydride solution to remove excess CaH2 and metal oxide/hydroxide formed during hydride re- reduction. For washing, the pellets were crushed and sonicated with 50 mL HAc and then centrifuged.duction. For Thewashing, procedure the pellets was repeated were crus 3 times.hed and As asonicated last step, productswith 50 mL were HAc treated and withthen 20centrifuged. mL 95% ethanol The procedure and then driedwas repeated at 120 ◦C under3 times. dynamic As a last vacuum step, products (<10−5 bar were). The treated dried −5 productwith 20 mL corresponded 95% ethanol to and dark then blue drie powder.d at 120 Portions °C under of dynamic about 20 vacuum mg of the (<10 so obtainedbar). The oxyhydridedried product material corresponded were then to heateddark blue in apowder. 100 mL/min Portions nitrogen of about stream 20 mg (99.9999% of the so (6N) ob- purity)tained oxyhydride at a rate of 10 material◦C/min were to various then heated temperatures in a 100 using mL/min a Netzsch nitrogen® STA stream 449 (99.9999% F1 Jupiter ® thermal(6N) purity) analysis at a apparatusrate of 10 °C/min (Netzsch, to Selb,various Germany). temperatures The reactions using a wereNetzsch protected STA 449 by anF1 oxygenJupiter getterthermal to analysis avoid oxidation apparatus of oxyhydride(Netzsch, Selb, materials. Germany). The color The orreactions the products were afterpro- nitridationtected by an was oxygen changed getter to yellow-greento avoid oxidation (cf. Figure of oxyhydride5a). materials. The color or the products after nitridation was changed to yellow-green (cf. Figure 5a). 3.2. Powder X-ray Diffraction (PXRD) Analysis PXRD patterns were collected on a Panalytical X’Pert PRO diffractometer (Malvern Panalytical, Malvern, UK) operated with Cu Kα radiation and in θ–2θ diffraction geometry. Powder samples were mounted on a Si wafer zero-background holder and diffraction ◦ ◦ patterns measured in a 2θ range of 20–120 with 0.016 step size. The contribution of Kα2

Inorganics 2021, 9, 62 11 of 14

radiation to the PXRD patterns was removed using the Panalytical X’Pert HighScore Plus software (Malvern Panalytical, Malvern, UK). The Rietveld method as implemented in the FullProf program was used for structure and phase analysis [37]. A six-coefﬁcient polynomial function was applied for the background. The peak shape was described by a Pseudo-Voigt function. Site occupancies for the O atoms could not be reﬁned reliably and were constrained to 1.

3.3. Scanning Electron Microscopy (SEM) Investigations SEM investigations were carried out using a JEOL JSM-7000F microscope (Jeol, Tokyo, Japan) equipped with a Schottky-type ﬁeld emission gun (Oxford Instruments, Oxfordshire, UK). Images were recorded in second-electron mode with an accelerating voltage of 15 kV. Samples were prepared by ﬁrst producing a homogeneous suspension of particles in ethanol by sonication. Then droplets of the suspension were applied onto surface-polished Aluminum pin stubs and left to dry.

3.4. Raman Spectroscopy Raman spectra were measured using a Labram HR 800 spectrometer (Horiba, Kyoto, Japan) equipped with an 800 mm focal length spectrograph and an air cooled, back thinned CCD detector. Powdered sample was placed on a glass slide and excited with a double frequency Nd:YAG laser (532 nm). To avoid decomposition/oxidation the lowest possible laser power (ND ﬁlter 1%, corresponding to a power density of 5.5 × 10−6 mW·µm−2) was applied for the oxyhydride and oxynitride samples. Spectra were collected at room temperature in a range 0–1800 cm−1 and using a grating of 600 lines/mm. For oxynitride samples large exposure times (of up to 25 min) were employed.

3.5. Thermogravimetric Analysis (TGA) TGA experiments were carried out using a TA instruments Discovery system (TA Instruments, New Castle, DE, USA). The samples (~15 mg powders) were heated in a platinum crucible from room temperature to 900 ◦C with a heating rate of 10 ◦C/min. A dry air gas ﬂow of 20 mL/min was applied.

3.6. Chemical Analysis Elemental analysis of N was performed by Mikroanalytisches Labor Kolbe (www. mikro-lab.de, accessed on 12 April 2021) using a CHN-(combustion) analyzer from Ele- mentar (model “vario MICRO cube”, Elementar Analysensysteme GmbH, Langenselbold, Germany).

3.7. UV-VIS Diffuse Reflectance Spectroscopy UV-VIS diffuse reflectance measurements were performed at room temperature on finely ground samples. Spectra were recorded in the range from 200 to 800 nm with an Agilent Cary 5000 UV-VIS-NIR spectrometer (Agilent, Santa Clara, CA, USA) equipped with a diffuse reflectance accessory from Harrick (Harrick Scientific Products Inc, New York, NY, USA). A polytetrafluoroethylene (PTFE) pellet (100% reflectance) was used as the reference.

3.8. Magic Angle Spinning (MAS) NMR Spectroscopy The 1H and 14N MAS NMR experiments were performed at a magnetic ﬁeld of 14.1 T (600.1 and 43.4 MHz Larmor frequencies for 1H and 14N, respectively) and MAS frequency of 60.00 kHz on a Bruker Avance-III spectrometer (Bruker, Billerica, MA, USA) equipped with a 1.3 mm MAS HX probe. Proton spectra were acquired using a rotor-synchronized, double-adiabatic spin-echo sequence with a 90◦ excitation pulse of 1.1 µs followed by a pair of 50 µs tanh/tan short high-power adiabatic pulses (SHAPs) with 5 MHz frequency sweep [38,39]. All pulses were applied at a nutation frequency of 210 kHz. In total, 256 signal transients with a 5 s recycle delay were accumulated for each sample. The Inorganics 2021, 9, 62 12 of 14

14N MAS spectra were collected using a Hahn-echo sequence with 3.0/6.0 µs 90◦/180◦ rf pulses and 131,072 scans collected per sample using a 1 s relaxation delay. 1H shifts were referenced with respect to tetramethylsilane (TMS) at 0 ppm, whereas 14N shifts to solid NH4Cl at 0 ppm (−342.4 ppm with respect to nitromethane).

3.9. Mott–Schottky Measurements Powder samples were assembled into particle-base thin film electrodes on fluorine- doped tin oxide (FTO) glass substrate via electrophoretic deposition. The homogeneous suspension was prepared by mixing 30 mg powder samples and 10 mg iodine in 30 mL of acetone via sonication. Two pre-cleaned FTO slides were immersed in the above suspension with 1 cm distance, and a DC potential of 30 V was applied between them for 3 min. After Electrophoretic Deposition (EPD) process, the FTO slides with thin film were washed with water and dried naturally. 0.1 M NaOH (pH 13), in which oxyhydrides are stable, was used as electrolyte to perform Mott–Schottky measurements. The measurements were conducted in the conventional three-electrode system where a platinum wire, a 1 M Ag/AgCl electrode and the thin film on FTO were used as counter electrode, reference electrode and working electrode, respectively. The potentials were recorded versus 1 M Ag/AgCl and converted versus reversible hydrogen electrode (RHE) according to ERHE (V) = EAg/AgCl + (0.059 × θ pH) + E Ag/AgCl. The measurements were carried out by using the Gamry INTERFACE 1010T Potentiostat/Galvanostat/ZRA workstation (Gamry Instruments, Warminster, PA, USA) in darkness at ac amplitude of 5 mV and a frequency of 10 Hz.

4. Conclusions

It was shown that reduced BaTiO3 containing simultaneously hydride and defects in the anion substructure, BaTiO3-xHy(x-y), can be readily nitrided with elemental N2 ◦ at temperatures around 450 C to yield tetragonal phases BaTiO3-xN(2x/3)-y(x/3)+y with c/a ≈ 1.002. In contrast with the nitridation of vacancy free oxyhydride BaTiO3-xHx, oxynitrides from BaTiO3-xHy(x-y) appear not fully nitrided and titanium remains partially as Ti3+, i.e., y 6= 0. Electrons are localized in traps below the conduction band. The band gap energy of BaTiO3-xN(2x/3)-y(x/3)+y materials is in a range 2.4–2.5 eV. Since the conduction band edge lies above the reduction potential for water and optical absorption is in the visible light region, it will be interesting to investigate photocatalytic properties.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/inorganics9080062/s1, Figure S1: Rietveld fit to the PXRD data of the oxyhydride starting 1 material BaTiO2.60H0.080.32 (BTOH), Figure S2: H MAS NMR spectrum of BTOH showing the division into protic and hydridic signal intensity and explaining the quantification of hydridic H content, Figure S3: Simultaneously measured TG and DSC traces for BTOH upon heating under a dry ◦ ◦ ◦ ◦ N2 stream up to 700 C with a heating rate 10 C/min and to 600 C with a heating rate 5 C/min, Figure S4–S13: Rietveld fits to PXRD data of BTON samples, Figure S14: Tauc plots for band gap evaluation, Table S1: Refinement result summary for BTON samples using the tetragonal structure model, Table S2: Refinement result summary for BTON samples using the cubic structure model. Author Contributions: H.G. and U.H. conceived and designed the experiments; H.G., A.J., Z.C., C.L., performed the experiments; H.G., A.J., A.S., U.H. contributed to analyzing the data and writing the paper. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Swedish Research Council (VR) through grant #2016-03441. Data Availability Statement: All data presented is available in this manuscript. Acknowledgments: The authors acknowledge the financial support of the Swedish Research Council. Conflicts of Interest: The authors declare no conflict of interest. Inorganics 2021, 9, 62 13 of 14

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