Exploration into the Syntheses of Gallium‐ and Indiumborates under Extreme Conditions: M 5B12O25(OH): Structure, Luminescence, and Surprising Photocatalytic Properties

Abstract Explorative solid‐state chemistry led to the discovery of the two new compounds Ga5B12O25(OH) and In5B12O25(OH). Extreme synthetic conditions within the range of 12 GPa and a temperature of 1450 °C realized in a Walker‐type multianvil apparatus resulted in the formation of an unprecedented tetragonal structure with the exclusive presence of condensed BO4 tetrahedra, forming cuboctahedral cavities. Doping of these cavities with Eu3+ in In5B12O25(OH) yielded in an orange–red luminescence. Photocatalytic tests of In5B12O25(OH) revealed a hydrogen production rate comparable to TiO2 but completely co‐catalyst free.

Abstract: Explorative solid-state chemistry led to the discovery of the two new compounds Ga 5 B 12 O 25 (OH) and In 5 B 12 O 25 -(OH). Extreme synthetic conditions within the range of 12 GPaa nd at emperature of 1450 8 8Cr ealized in aW alkertype multianvil apparatus resulted in the formation of an unprecedented tetragonal structure with the exclusive presence of condensed BO 4 tetrahedra, forming cuboctahedral cavities. Doping of these cavities with Eu 3+ in In 5 B 12 O 25 (OH) yielded in an orange-red luminescence.Photocatalytic tests of In 5 B 12 O 25 -(OH) revealed ahydrogen production rate comparable to TiO 2 but completely co-catalyst free.
The transition from limited fossil fuels to renewable energy sources is one of the main challenges of humankind. In our timescale,the sun presents an eternal and plentiful source of energy.W ays of direct solar to chemical energy conversion have been investigated since the first findings of Fujishima and Honda in 1972. [1] Although hydrogen is ac onveniently usable fuel, for example,inhydrogen fuel cells,the storage of the volatile gas is an unresolved issue.Apossible solution may be the storage in form of ah ydrocarbon, for example, methanol. Hydrogen is then retrieved from the storage molecule through ar eformation process.T oe nable this hydrogen liberation, metal borates gained significance in the recent years as possible photocatalysts,aresult amongst other things of their great stability.Especially triel borates not only shifted into our research focus,b ut have been examined for their photocatalytic activity lately by other groups as well. [2] Unlike most other compounds,o ur newly discovered indium borate In 5 [5] and Ga 2 B 3 O 7 (OH) [6] the third known hydroxylated borate.
Herein we elucidate the discovery of the two new compounds M 5 B 12 O 25 (OH) (M = Ga, In) synthesized via high-pressure/high-temperature syntheses,their crystal structures,R aman and IR spectroscopic investigations,a nd the surprising results of the photocatalytic experiments with In 5 B 12 O 25 (OH). Additionally,e nergy dispersive X-ray spectroscopy (EDX) and luminescence measurements of the europium-doped indium compound are presented.  Table S2 (see Supporting Information). Interestingly.t hese borates share great structural similarities with ahigh-pressure oxonitridophosphate synthesized by Marchuk et al. in 2014. [7] Ab rief comparison of the compounds is given in the Supporting Information.
In M 5 B 12 O 25 (OH) (M = Ga, In), the metal cations constitute two crystallographically different octahedra and in accordance with these harsh high-pressure conditions all boron atoms are coordinated by four oxygen atoms each, composing large corner-sharing networks.F or the visualization of the metal-oxygen polyhedra and their correlation with the hydrogen bonds,Ga 5 B 12 O 25 (OH) was chosen, because for this compound the proton could be located via the difference Fourier syntheses.F igure 1s hows the arrangement of the GaO 6 octahedra in and throughout the unit cell. Both, the green and orange polyhedra represent the isolated double units of edge-sharing Ga1O 6 octahedra. Along the crystallographic c axis,e very second unit is displaced along b or rotated by 908 8 pertaining to the corresponding double-entity. Thed istorted Ga2O 6 octahedra with the half-occupied, deflected Ga2 positions in the center are positioned along the 4 inversion axis (light-blue in Figure 1). Each Ga2O 6 octahedron is surrounded by four Ga1 2 O 10 double-entities and connected to one of these through ah ydrogen bond. There are four possible hydrogen atom sites as depicted in Figure 2, but since the hydrogen atom has an occupancyo f aquarter,only one of these positions is occupied at atime.It seems likely that when, referring to Figure 2, one of the lower hydrogen bonds is formed, the upper Ga2 atom is occupied and vice versa. Although the hydrogen atom in In 5 B 12 O 25 -(OH) could not be located during the single-crystal structure refinement, as imilar situation is expected, as the In2octahedron shows the same distortion as Ga2. The M À O bond lengths in the octahedra of Ga2 and In2 are hence slightly longer as in the regular Ga1 and In1 octahedra, respectively.W ith average interatomic distances of 1.97 and 2.15 and individual values ranging from 1.909(2)-2.065(2) for Ga1 and 2.069(2)-2.247(2) for In1, the distances are in good agreement with those reported in the literature. [3b,4b, 6, 8] Thedistorted octahedra exhibit with 2.10 and 2.36 larger average distances and also quite uncommon maximal lengths of 2.307(2) for Ga2 and 2.748(2) for In2. All bond lengths and angles for the metal octahedra as well as for the hydrogen bonds can be found in the Tables S5-S7 in the Supporting Information. Then etwork of corner-sharing BO 4 tetrahedra in this new borate structure type is rather complicated as there are 96 BO 4 tetrahedra built up of three crystallographically different boron atoms in the unit cell. Alternately,aset of twelve corner-sharing BO 4 tetrahedra forms either ac uboctahedral cage or two six-membered curved strings that enlace the distorted In2O 6 and accordingly Ga2O 6 octahedra. Thec uboctahedral cages can be looked upon as atetrahedral arrangement of four dreier rings [9] which in doing so additionally form four sechser rings.Both of these described structural motifs and their alternative arrangement throughout the unit cell are visualized in Figure 3. Illustrations showing the displacement ellipsoids of all atoms are given in the Supporting Information ( Figures S2 and S3). All BO 4 tetrahedra show reasonable bond lengths and angles as can be checked in the Tables S5 and S6 in the Supporting Information. Thecuboctahedral cages are with adiameter of approximately 5.4 large enough to accommodate ar are earth cation like Eu 3+ ,Sm 3+ ,oreven Ce 3+ .InGa 5 B 12 O 25 (OH), these cavities are empty,w hereas in the Eu 3+ -doped Indium analogue,electron density indicating an integration of 2%Eu could be found. Hence,E u 3+ was positioned in the center of   these cuboctahedral cages with asite occupancyfactor of 0.02, which means every 50th cavity in In 5 B 12 O 25 (OH):Eu 3+ is statistically occupied with europium. Ther efinement of such as mall amount of activator ion is remarkable and was only possible because Eu 3+ is not competing with In 3+ but fills otherwise empty cavities in the structure.
To our surprise,t he compound In 5 B 12 O 25 (OH) showed extraordinary high performance for photocatalytic hydrogen production from methanol. Ther ate of hydrogen evolution was determined to be 220 AE 20 mmol h À1 g À1 (s = 11 mmolh À1 g À1 , N = 12, p = 95 %). Although the sample was not phase pure,comparative experiments with the byproduct InB 6 O 9 (OH) 3 showed no hydrogen evolution at all on the timescale of interest. [4c] Therefore,the photocatalytic activity stems from In 5 B 12 O 25 (OH) and by accounting for the inactive byproduct InB 6 O 9 (OH) 3 ,the rate can be estimated to be even higher. Figure 4s hows the hydrogen production over 12 ho f our sample as well as the background measurement of pure methanol.
Ad iscussion of the mechanism has been given for ac omparable borate structure. [6] It has been clearly shown that these borates have semiconductor properties and therefore the conduction band delivers electrons.T he photocatalytic conditions were adapted to UV light, since an irradiation above 300 nm does not contribute to the hydrogen production. Therefore,itcan be concluded that the band gap is in the region of 4.1 eV.T he semiconductor plays the combined role of light absorber and proton reduction catalyst. Methanol delivers electrons as the sacrificial donor and oxidation products thereof have been found via quadrupole mass spectrometry.T his means that the band gap and the band edges are suitable for H + reduction and methanol oxidation, thus no further component is necessary and the system is cocatalyst free.
TiO 2 as catalyst for photocatalytic hydrogen production was thoroughly studied and improved in an ongoing effort for many years.Although the comparison of literature is delicate because of different illumination setups,u pt or ecent publications the activity of In 5 B 12 O 25 (OH) is superior or on alevel with precious-metal-doped TiO 2 . [10] Lin and co-workers showed an activity of roughly 30 mmol h À1 g À1 for Pt-loaded TiO 2 . [11] Compared to other recently published borates of the 13th group of the periodic table,t he herein described In 5 B 12 O 25 (OH) shows ah igh hydrogen evolution rate even without commonly employed co-catalysts,s uch as Ni, Pt, or Ru. [2,4b, 5, 6, 12] Previously published catalysts with hydrogen evolution rates of 2.8 [4b] and 120 mmol h À1 g À1 [6] show the development of new photocatalytically active phases.The approach of explorative solid-state chemistry under extreme conditions lets us expect new and interesting catalysts to be prepared in the near future,since there is ahuge space for the catalytic finetuning of this class of compounds.P otential applications are widespread, from ap rimary energy source via artificial photosynthesis,t oh ydrogen production from room-temperature methanol reformation.
To test its luminescence properties,apowder sample containing In 5 B 12 O 25 (OH):Eu 3+ was excited using a4 60 nm laser. Ther esulting luminescence spectrum exhibits typical peaks for Eu 3+ emission as shown in Figure 5. Thep redominant intensity between 580-620 nm confirms the orange-red luminescence impression of the powder sample.B ased on literature comparisons,t he common 5 D! 7 Ft ransitions for Eu 3+ were assigned as following:T he most intense peaks at 585 and 600 nm can be attributed to 5 D 0 ! 7 F 1 transitions,the subsequent weaker peaks most likely stem from 5 D 0 ! 7 F 2 transitions,a nd the little bump at 698 nm can be explained with 5 D 0 ! 7 F 4 transitions. [13] Ther elative intensities of the 5 D 0 ! 7 F 1 transitions are significantly higher than those of the 5 D 0 ! 7 F 2 transitions,w hich is characteristic for (pseudo)centrosymmetric Eu 3+ positions in the crystal structure. [14] In contrast to the sidephase InB 6 O 9 (OH) 3 [4c] in the examined powder sample,I n 5 B 12 O 25 (OH):Eu 3+ is centrosymmetric and the Eu 3+ ion could be located at the symmetric Wyckoff position 8a. Therefore,a nd because europium could not be

Angewandte Chemie
Communications detected via EDX in as ingle-crystal of the side phase InB 6 O 9 (OH) 3 originating from the two-phase powder sample, we claim that the luminescence spectrum (see Figure 5) is representative for In 5 B 12 O 25 (OH):Eu 3+ .
To confirm the presence of europium in In 5 B 12 O 25 -(OH):Eu +3 ,E DX was performed on as ingle-crystal, which had in advance been tested on as ingle-crystal X-ray diffractometer to be the desired phase by determining its lattice parameters.T he analyzed single-crystal of In 5 B 12 O 25 -(OH):Eu 3+ clearly contained europium. However,t he semiquantitative measurement under low vacuum did not really allow to specify quantitatively the amount of Eu. Apicture of the examined single-crystal as well as the EDX spectrum and the expected and measured elemental ratios can be found in the Supporting Information ( Figure S4, S5 and Table S8).
Thes ingle-crystal IR and Raman spectra of Ga 5 B 12 O 25 -(OH) and In 5 B 12 O 25 (OH) can be found in the Supporting Information (Figures S6, S7). Ty pical vibrations of InO 6 or GaO 6 octahedra appear at the lowest wavenumbers up to about 800 cm À1 . [4b,6,15] While those peaks in the IR spectra overlap with bands of the BO 4 bending and stretching vibrations,u sually occurring at 800-1150 cm À1 ,t hey can be distinguished in the Raman spectra. [16] In both, the Raman and IR spectra, peaks at high wavenumbers confirm the presence of the protons.A ccording to Hammer et al., [17] the hydrogen bonds,w hich could be determined for Ga 5 B 12 O 25 -(OH), lie with an average D-A distance of 2.74 in the crossover between weak and strong hydrogen bonds and should therefore appear around 3200 cm À1 .
Herein, we reported on the new borate structure type M 5 B 12 O 25 (OH) (M = Ga, In), which could be synthesized under extreme high-pressure/high-temperature conditions with either Ga 3+ or In 3+ as metal cations.I ni ts large unit cell, the structure type comprises various interesting structural motifs,s uch as cuboctahedral tetrahedra-formations or isolated edge-sharing octahedra double-units.T he indiumcontaining compound could be doped with 2% Eu 3+ and showed luminescence when irradiated with as uitable laser. During the refinement, it was possible to locate the Eu 3+ ions in the otherwise empty cuboctahedral vacancies built up by the BO 4 tetrahedra cages.F urthermore,I n 5 B 12 O 25 (OH) was tested for its photocatalytic activity and indeed produced significant amounts of hydrogen without aco-catalyst without being degraded itself.Inour assessment, the research field of high-pressure indium and gallium borates has only just opened, promising not only further structure types to be discovered, but also possible applications,s uch as the demonstrated photocatalytic properties.

Experimental Section
Ga 5 B 12 O 25 (OH) and its indium isotype were both synthesized via solid state reaction in amultianvil press under extreme conditions of 11.0 GPa and 1450 8 8Cf or Ga 5 B 12 O 25 (OH) and 12.2 GPa and 1450 8 8C for In 5 B 12 O 25 (OH). Experimental details of the syntheses can be found in the Supporting Information.B oth compoundsc ould not be obtained phase-pure.T he best synthesis of Ga 5 B 12 O 25 (OH) was carried out with b-Ga 2 O 3 (Strem Chemicals, Kehl, Germany,99.9 %) and H 3 BO 3 (Carl Roth, Karlsruhe,Germany,99.5 %) in the stoichiometric ratio of Ga:B = 1:2.4 according to Equation (1). Ther eaction product additionally contained GaBO 3 [18] and an unidentified byproduct.
Concerning phase purity-them ost successful synthesiso f In 5 B 12 O 25 (OH) was achievedbyamolar ratio of In:B = 1:1.8 starting from In 2 O 3 (ChemPUR,K arlsruhe,G ermany,9 9.9 %) and H 3 BO 3 encapsulated in gold foil. As side phase,InB 6 O 9 (OH) 3 [4c] formed. The phase fractions were determinedv ia Rietveld [19] (see Supporting Information). Fort he synthesis of the europiumd oped In 5 B 12 O 25 -(OH):Eu 3+ ,a pproximately 1.5 weight %E u 2 O 3 was added to the educt mixture.D etailed initial weights can also be found in the Supporting Information. All products appeared as clean-colorless powders,the product of the Eu containing synthesis showed orangered luminescence under UV light excitation.