New High-Pressure Gallium Borate Ga2B3O7(OH) with Photocatalytic

Article pubs.acs.org/IC

New High-Pressure Gallium Borate Ga2B3O7(OH) with Photocatalytic Activity Daniela Vitzthum,† Michael Schauperl,† Christof M. Strabler,† Peter Brüggeller,† Klaus R. Liedl,† Ulrich J. Griesser,‡ and Hubert Huppertz*,† †

Institute of General, Inorganic and Theoretical Chemistry, University of Innsbruck, Innrain 80−82, A-6020 Innsbruck, Austria Institute of Pharmacy, Pharmaceutical Technology, University of Innsbruck, Innrain 52c, A-6020 Innsbruck, Austria

‡

S Supporting Information *

ABSTRACT: The new high-pressure gallium borate Ga2B3O7(OH) was synthesized in a Walker-type multianvil apparatus under high-pressure/high-temperature conditions of 10.5 GPa and 700 °C. For the system Ga−B−O−H, it is only the second known compound next to Ga9B18O33(OH)15·H3B3O6·H3BO3. The crystal structure of Ga2B3O7(OH) was determined by single-crystal X-ray diffraction data collected at room temperature. Ga2B3O7(OH) crystallizes in the orthorhombic space group Cmce (Z = 8) with the lattice parameters a = 1050.7(2) pm, b = 743.6(2) pm, c = 1077.3(2) pm, and V = 0.8417(3) nm3. Vibrational spectroscopic methods (Raman and IR) were performed to confirm the presence of the hydroxyl group. Furthermore, the band gap of Ga2B3O7(OH) was estimated via quantum-mechanical density functional theory calculations. These results led to the assumption that our gallium borate could be a suitable substance to split water photocatalytically, which was tested experimentally.

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INTRODUCTION High-pressure syntheses provide the possibility of obtaining thermodynamically metastable compounds. Solids synthesized under increased pressure and temperature show specific structural features like higher coordination numbers than compounds synthesized under ambient pressure. For instance, in the substance class of borates, BO4 tetrahedra predominantly appear under high-pressure conditions, while ambient-pressure compounds mainly consist of planar BO3 groups. To date, HGa3B6O12(OH)4 was the only known compound in the system Ga−B−O−H. It was obtained via hydrothermal synthesis in 2003 by Ju et al.1 With a multianvil high-pressure device, we recently succeeded in the synthesis of a second borate in this system. This new high-pressure gallium borate Ga2B3O7(OH) is mainly built up by BO4 groups in contrast to HGa3B6O12(OH)4, which only shows 3-fold-coordinated boron atoms. In 2014, Gao et al. released a reinvestigation of the structure of HGa3B6O12(OH)4, which they afterward advanced to the sum formula Ga9B18O33(OH)15·H3B3O6·H3BO3 (in short named Ga-PKU-1). In this publication, they also reported the photocatalytic activity of gallium borates in general and especially for the compound Ga-PKU-1.2 Recently, there have also been reports of the water-splitting potential of the condensed gallium borate Ga4B2O9.3 Inspired by these results, we tested the photocatalytic hydrogen production ability of Ga2B3O7(OH) as well. In the following, the synthesis and structure of the new highpressure phase Ga2B3O7(OH) are described in detail. Moreover, this work contains theoretical calculations concerning the band gap of this gallium borate and further tests for its © XXXX American Chemical Society

photocatalytic ability. Additionally, IR and Raman vibrations of Ga2B3O7(OH) were calculated and recorded experimentally.

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EXPERIMENTAL SECTION

Synthesis. For the synthesis of Ga2B3O7(OH), we mixed β-Ga2O3 (99.998%, Strem Chemicals, Kehl, Germany) and H3BO3 (99.5%, Carl Roth, Karlsruhe, Germany) in a stoichiometric ratio of 1:3 (eq 1). After grinding the starting materials in an agate mortar, we filled the mixture into a platinum sagger (99.9%, Chempur, Karlsruhe, Germany) and encased it with a crucible and lid of α-BN (Henze Boron Nitride Products AG, Kempten, Germany). The compression and heating of the blend was performed with a multianvil device based on a Walker-type module (Voggenreiter, Mainleus, Germany) consisting of an 14/8 assembly, which was surrounded by eight tungsten carbide cubes (Hawedia, Marklkofen, Germany). A detailed description can be found in the literature. 4−6 The product Ga2B3O7(OH) formed under the conditions of 10.5 GPa and 700 °C, which were accomplished in 5 h and 10 min, respectively. The temperature was held for 20 min and afterward down-regulated to 300 °C in 50 min. Then the heating was turned off and the 15 h process of decompression started. After the synthesis, we freed the platinum capsule from its surroundings and cut it open with a scalpel. The product appeared gray and crumbly. Single crystals of Ga2B3O7(OH) were colorless, so the gray color presumably resulted from traces of elemental platinum of the platinum capsule. From our experience, it can be excluded that elementary gallium formed because, in all syntheses performed with gallium compounds as starting reactands, gallium was never reduced into its elemental form. However, when the synthesis was performed without the platinum capsule in the BN crucible, a completely different product formed. Received: September 1, 2015

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DOI: 10.1021/acs.inorgchem.5b02027 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 10.5 GPa, 700 ° C

Ga 2O3 + 3 H3BO3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ga 2B3O7 (OH) + 4 H 2O

Table 1. Crystal Data and Structure Refinement of Orthorhombic Ga2B3O7(OH)

(1)

The composition Ga2B3O7(OH) formed in pressure and temperature ranges of 7.5−12 GPa and 600−800 °C, respectively, and it also appeared using the starting materials Ga(NO3)3·8H2O (99.99%, Strem Chemicals, Kehl, Germany) and H3BO3 in molar ratios of 2:3 and 4:5. Crystal Structure Analysis. The reaction product was analyzed with a Stoe Stadi P powder diffractometer that was equipped with Ge(111)-monochromatized Mo Kα1 radiation (λ = 70.93 pm) and working in transmission geometry. Figure 1 shows a comparison of the

empirical formula molar mass, g mol−1 crystal system space group powder data powder diffractometer radiation a, pm b, pm c, pm V, nm3 single-crystal data single-crystal diffractometer radiation a, pm b, pm c, pm V, nm3 formula units per cell Z calculated density, g cm−3 crystal size, mm3 temperature, K detector distance, mm exposure time absorption coefficient, mm−1 F(000), e θ range, deg range in hkl reflections total/independent Rint reflections with I ≥ 2σ(I) Rσ data/ref. parameters absorbance correction final R1/wR2 [I ≥ 2σ(I)] final R1/wR2 (all data) GOF on Fi2 largest diff. peak/hole, e Å−3

Figure 1. (top) Powder diffraction pattern of the reaction product from 1:3 β-Ga2O3/H3BO3 (10.5 GPa and 700 °C) compared to (bottom) a simulation of a theoretical powder pattern of Ga2B3O7(OH) based on the single-crystal data. The asterisk-marked reflections refer to an unidentified byproduct. experimental powder pattern and the calculated pattern of Ga2B3O7(OH), which we obtained from single-crystal data. Obviously, Ga2B3O7(OH) represents the main phase of the reaction product, but additionally there are some minor reflections at 2θ = 8.3, 12.6, 16.1, 16.7, and 24.9° that could not yet be identified. In total, we were able to index and refine 59 reflections of the experimental powder diffractogram7 and obtain lattice parameters that fit quite well with those from the single-crystal data (Table 1). Vibrational Spectroscopy. The transmission Fourier transform infrared (FT-IR) spectra of single crystals of Ga2B3O7(OH) were measured in the spectral range of 600−4000 cm−1 with a Vertex 70 FT-IR spectrometer (spectral resolution 4 cm−1), which was equipped with a KBr beam splitter, an LN-MCT (mercury−cadmium−telluride) detector and a Hyperion 3000 microscope (Bruker, Vienna, Austria). A total of 320 scans of the sample were acquired, using a Globar (silicon carbide) rod as a mid-IR source and a 15× IR objective as the focus. During the measurement, the sample was positioned on a BaF2 window. A correction of atmospheric influences was performed with the software OPUS 6.5. Single-crystal Raman spectra were performed in the range of 100− 4000 cm−1 with a Labram-HR 800 Raman microspectrometer (Horiba Jobin Yvon, Tulln, Austria). The sample was excited using the 532 nm emission line of a frequency-doubled 100 mW Nd:YAG laser under an Olympus 50× objective lens. The size of the laser spot on the surface was approximately 1 μm in diameter. The scattered light was dispersed by an optical grating with 1800 lines mm−1 and collected by a 1024 × 256 open-electrode CCD detector. The spectra were recorded unpolarized and at ambient conditions; a correction of the background was applied.8 Photocatalytic Experiments. All photocatalytic experiments were performed in 1 mL of the irradiation mixture (different solvents) in a cuvette with a volume of 2.5 mL. The slurry was degassed twice by a freeze−pump−thaw cycle, Ga2B3O7(OH) was added, and the slurry was degassed by a freeze−pump−thaw cycle a third time. The slurry

Ga2B3O7(OH) 299.87 orthorhombic Cmce (No. 64) Stoe Stadi P Mo Kα1 (λ = 70.93 pm) 1049.08(6) 742.63(4) 1075.35(6) 0.83778(6) Enraf-Nonius Kappa CCD Mo Kα1 (λ = 71.073 pm) 1050.7(2) 743.6(2) 1077.3(2) 0.8417(3) 8 4.733 0.12 × 0.05 × 0.05 293(2) 36.0 1°/frame, 120 s/frame 12.81 1128 3.8−37.8 −18/+17; ±12; ±18 7817/1180 0.0451 1115 0.0275 63 multiscan38 0.0274/0.0766 0.0290/0.0774 1.145 1.035/−1.209

was irradiated at 23 °C by a medium-pressure mercury lamp. The produced hydrogen in the gas phase was measured by means of a micro gas chromatograph (Inficon 3000 μ-GC) that was equipped with a 5 Å molecular sieve column, followed by a thermal conductivity detector with argon as the carrier gas. More details concerning the experimental setup can be found in the Supporting Information (SI; Experimental Setup and Figures S2 and S3). Density Functional Theory (DFT) Calculations. In addition to the vibrational bands obtained by IR and Raman spectroscopy, we performed quantum-mechanical DFT calculations using the program CRYSTAL09.9,10 The crystal structure was taken as the starting geometry, and an energy minimization was performed. The vibrational frequencies of the resulting structure were analyzed at the Γ point within the harmonic approximation. For all atoms, an all-electron triple-ζ valence basis set11 was used in combination with a shrinking factor of 8. To calculate the exchange and correlation contributions, the PBEsol functional was applied, a functional using the generalized gradient approximation (GGA).12 DFT was also used to calculate the band-gap properties of the title compound. The Vienna Ab Initio Simulation Package13,14 was taken in combination with the GGA Perdew−Burke−Ernzerhof (PBE) functional15,16 to describe correlation and exchange interactions. Projectoraugmented-wave potentials 17 were used for all atoms in Ga2B3O7(OH). The size of the plane-wave basis set was set with a B

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Figure 2. Building blocks in Ga2B3O7(OH): (a) BB1 consisting of four GaO6 octahedra; (b) BB2 consisting of 12 BO4 tetrahedra. cutoff of 400 eV in combination with a 7 × 7 × 7 Monkhorst−Pack Gcentered reciprocal space mesh for geometry optimization. Band structures were calculated at discrete points along the high-symmetry lines as described by Setyawan and Curtarolo.18 Thermogravimetric Analysis (TGA). TGA was carried out with a TGA7 system (PerkinElmer, Norwalk, CT) using the Pyris 2.0 software. An amount of 1.48 mg of sample was heated in a platinum pan from 25 to 930 °C with a heating rate of 10 K min−1. The temperature calibration was performed with ferromagnetic materials (Alumel and nickel, Curie-point standards, PerkinElmer). Dry nitrogen was used as a purge gas (sample purge, 20 mL min−1; balance purge, 40 mL min−1).

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RESULTS AND DISCUSSION Crystal Structure. Ga2B3O7(OH) crystallizes in the orthorhombic space group Cmce (No. 64) with the following lattice parameters derived from single-crystal diffraction data: a = 1050.7(2) pm, b = 743.6(2) pm, c = 1077.3(2) pm, and V = 0.8417(3) nm3. Details of the crystal data and structure refinement as well as the lattice parameters that we obtained from powder diffraction data can be found in Table 1. The structure of Ga2B3O7(OH) is basically made up of two building blocks (BB1 and BB2). The building block BB1 consists of four edge-sharing GaO6 octahedra (Figure 2a), while the building block BB2 is formed by 12 BO4 tetrahedra, which compose two “Sechser” rings19 that are linked via two “Vierer” rings19 (Figure 2b). Each BB1 is fully connected with and therefore encased by two BB2s (Figure 3). Beyond the BO4 tetrahedra, further BB1s in two different orientations (perpendicular to each other) affiliate to build a layerwise network along [110] (Figure 4a). Viewed in the direction b, Ga2B3O7(OH) forms a symmetric network with boron atoms in the resulting channels (Figure 4b). The fourth ion in Ga2B3O7(OH), namely, the H+ ion, is located in the middle of a BO4 “Sechser” ring at a distance of 118 pm to O5. This H+ position was derived from geometric calculations. The oxygen atom O5 thus shows a 4-fold coordination by, on the one hand, connecting three GaO6 octahedra and, on the other hand, forming a hydroxide bond with the hydrogen atom. Furthermore, the H+ ion forms hydrogen bonds to two O4 and one O3 anions (Figure 5). We were not able to refine the hydrogen atoms into our structure model based on a difference Fourier analysis with an interpretable electron density map,20,21 but its existence in Ga2B3O7(OH) is confirmed by several reasons, which are, first

Figure 3. Two building blocks of BO4 tetrahedra surrounding a building block of GaO6 octahedra in Ga2B3O7(OH) so that each oxygen atom of the galliumoxide unit also belongs to the borate unit (except the oxygen atom O5, which connects to the hydrogen atoms).

of all, charge-neutrality considerations. Furthermore, our CHARDI22,23 and bond-length/bond-strength24,25 calculations led to worse results without the inclusion of hydrogen. The comparison of the MAPLE26−28 values of Ga2B3O7(OH) and the starting compounds β-Ga2O3,29 HP-B2O3,30 and H3BO331 showed that the presence of hydrogen atoms in our structure is clearly legitimate (see the SI, Tables S6 and S7). Additionally, Raman and IR spectra, which are described below, show that there are vibrations of a hydroxyl group. Furthermore, the presence of hydrogen atoms explains several distortions of bond lengths and angles in Ga2B3O7(OH). For example, the angles between O5−Ga2−O5 and O2−Ga2−O5 are with 75.08(9) and 77.50(7)° significantly smaller than the expected 90° because the hydrogen atom needs space and provokes the Ga5 atom to position farther away. Thus, the BB1 is not straight-lined but buckled inward with an O4−O5−O2 angle of 166.05(4)° instead of even 180°. The Ga−O distances in Ga2B3O7(OH) with a range from 190.9(2) to 211.6(2) pm and an average value of 198.6 pm accord well with data from the literature.32−34 Extended tables for interatomic distances, angles, atomic coordinates, isotropic/anisotropic displacement C

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Figure 4. Arrangement of GaO6 octahedra in Ga2B3O7(OH): (a) view along [1̅1̅0]; (b) view along [01̅0].

Figure 6. IR and Raman spectra of a Ga2B3O7(OH) single crystal.

Figure 5. Ga2B3O7(OH). The hydrogen atoms bind to the oxygen atom O5, forming additional hydrogen bonds with O4a, O4b, and O3.

at 100−1380 cm−1 gives a better view of the vibrations that occur at lower wavenumbers (see the SI, Figure S1). The two different spectra derive from two different orientations of the same Ga2B3O7(OH) crystal. For the red spectrum, the crystal was rotated about 90°. Each curve shows some modes that do not occur in the other spectrum, which indicates that our crystal shows strong orientation effects. Peaks in the range of 150−550 cm−1 can be assigned to bending vibrations of the gallium oxide octahedra. The bending vibrations of the BO4 tetrahedra as well as the bending vibrations between Ga−O−B and Ga−O−H occur in wider ranges of 290−1150, 150−1150, and 160−1050 cm−1, respectively. The stretching vibrations of the GaO 6 octahedra and of the BO 4 tetrahedra in Ga2B3O7(OH) appear at ranges of higher wavenumbers, namely, 400−650 and 800−1150 cm−1, respectively. A detailed list of all calculated vibrations and band assignments can be found in the SI (Tables S8 and S9). Photocatalytic Activity. The size of the band-gap and band-edge positions determine whether a photocatalytic material is suitable for solar water splitting.35 DFT calculations of the electronic band structures along the high-symmetry lines in the reciprocal spaces are shown in Figure 7, predicting a band gap of 4.0 eV for the title compound. The energy value of the band gap is higher as usual for a solar water-splitting material because it corresponds to ultraviolet light but is similar

parameters, Wyckoff positions, and detailed information about the hydrogen bonds can be found in the SI. Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax +49-7247-808-666; e-mail crysdata@fiz-karlsruhe.de) on quoting the deposition number CSD 430138 for Ga2B3O7(OH). Vibrational Spectroscopy. The following band assignments are based on DFT calculations. The transmission FT-IR spectrum of Ga2B3O7(OH) in the range of 400−4000 cm−1 can be seen in Figure 6. At first, the stretching vibrations of the hydroxyl group O−H at 3450 cm−1 as well as the bands from 2800 to 3000 cm−1, which come from the fat with which the crystal was fixed on a glass fiber, stand out. Mainly the bending vibrations of O−Ga−O (170−416 cm−1), O−B−O (330−1000 cm−1), and Ga−O−H (170−1030 cm−1) as well as the stretching vibrations of the gallium octahedra (400−730 cm−1) and boron tetrahedra (800−1170 cm−1) contribute to the broad absorption bands at lower wavenumbers. Figure 6 shows the single-crystal Raman spectrum in the range of 100−3600 cm−1. Similar to the IR spectrum, the O−H stretching vibrations occur at approximately 3450 cm−1. The absorption bands at 2800−3000 cm−1 come from the fat that was used to prepare the crystal. The unfolded Raman spectrum D

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Figure 7. Band structure of Ga2B3O7(OH) along the symmetry lines in the reciprocal space (left) and the corresponding density of states (right).

Figure 8. Photocatalytic hydrogen production of the semiconductor Ga2B3O7(OH) in methanol (0.21 mg of catalyst) and methanol/water (1:4; 0.18 mg of catalyst) at ambient temperature. The volume of the irradiation mixture was 1 mL. Besides methanol, no further sacrificial agent was present. As a light source, a 150 W medium-pressure mercury immersion lamp was used. Elemental platinum was the cocatalyst.

to the band gaps found for the other published gallium borates.2,3,36 The band edges can be calculated with an empirical formula based on the overall electronegativity of an atom, the size of the band gap, and the energy for free electrons on the hydrogen scale.37 According to this formula, the edge of the conduction band is found at −0.61 eV, which is more negative than the redox potential of hydrogen at pH = 7 (−0.41 eV), allowing the compound to reduce hydrogen. The edge of the valence band can be found at a potential of 3.4 eV (2.8 eV higher than the redox potential of water), allowing the compound to theoretically catalyze oxidation of water. In summary, the theoretical results lead to the assumption that this gallium borate is suitable to catalyze the overall watersplitting process. The water-splitting ability through irradiation with ultraviolet light of the presented compound was analyzed with a micro gas chromatograph. The activity of Ga2B3O7(OH) in pure water was low, yet the activity increased in a solution of methanol/ water (1:4, v/v) and in pure methanol. To prove the stability of the compound Ga2B3O7(OH), an additional powder diffraction pattern was measured after the catalysis experiment, which confirmed that there are no hydrolysis effects of water and/or detrimental heating effects. Referring to the Ga-PKU-1 system,2 we suppose that the GaO6 framework is crucial for the photocatalytic activity of Ga2B3O7(OH) because it might trap the photons to generate photoinduced holes and electrons, which are important for the generation of hydrogen. Concerning the photocatalytic mechanism, a classical triadic system consisting of the following components can be suggested: Ga2B3O7(OH) is the photosensitizer, and elemental platinum works as the electron relay in order to form hydrogen photocatalytically. If no further electrons are delivered, the catalytical cycle is stopped immediately by recombination. Therefore, the presence of a sacrificial donor is absolutely necessary. In this system, methanol is the sacrificial donor because hydrogen production increases as soon as the amount of methanol is enlarged. However, our Ga2B3O7(OH) system, with an hourly production of 25.77 μmol g−1 in methanol and 9.83 μmol g−1 in a mixture of methanol and water with a ratio of 1:4 (v/v), is much more active than Ga-PKU-1. The continuous hydrogen production is shown in Figure 8. Therefore, the surface of our gallium borate should be lower than the surface of Ga-PKU-1.

Another fact is that the array of the GaO6 octahedra in Ga2B3O7(OH) compared to the array of the GaO6 octahedra in Ga-PKU-1 is different. The GaO6 octahedra are more accessible than those in the Ga-PKU-1 system. Compared to the gallium borate high-temperature solid-state reaction Ga4B2O9,3 our system is less active. TGA. To investigate the thermal stability of our main phase Ga2B3O7(OH), TGA was performed. The resulting curve is shown in Figure 9. Below about 550 °C, the TGA curve shows

Figure 9. TGA curve between 25 and 930 °C of Ga2B3O7(OH).

a mass loss of about 1.5% with two not clearly separated steps. Above this temperature, a faster decomposition process occurs, resulting in a total mass loss of 11.3% at 930 °C. We presume that our compound is stable up to a temperature of 550 °C. The tiny weight loss (1.5%) could be attributed to small impurities (indicated by the powder diffraction pattern) evaporating from the sample. Above 550 °C, it is reasonable that our metastable high-pressure phase Ga2B3O7(OH) E

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(3) Wang, G.; Jing, Y.; Ju, J.; Yang, D.; Yang, J.; Gao, W.; Cong, R.; Yang, T. Inorg. Chem. 2015, 54, 2945−2949. (4) Huppertz, H. Z. Kristallogr. - Cryst. Mater. 2004, 219, 330−338. (5) Walker, D.; Carpenter, M. A.; Hitch, C. M. Am. Mineral. 1990, 75, 1020−1028. (6) Walker, D. Am. Mineral. 1991, 76, 1092−1100. (7) WIN XPOW INDEX, version 2.7.2; Stoe & Cie GmbH: Darmstadt, Germany, 2003. (8) LABSPEC 5; Horiba Jobin Yvon SAS: Longjumeau, Cedex, France, 2010. (9) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; ZicovichWilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; D’Arco, P.; Llunell, M. CRYSTAL09 User’s Manual; University of Torino, Torino, Italy, 2009. (10) Dovesi, R.; Orlando, R.; Civalleri, B.; Roetti, C.; Saunders, V. R.; Zicovich-Wilson, C. M. Z. Kristallogr. - Cryst. Mater. 2005, 220, 571− 573. (11) Peintinger, M. F.; Oliveira, D. V.; Bredow, T. J. Comput. Chem. 2013, 34, 451−459. (12) Perdew, J. P.; Ruzsinszky, A.; Csonka, G. I.; Vydrov, O. A.; Scuseria, G. E.; Constantin, L. A.; Zhou, X.; Burke, K. Phys. Rev. Lett. 2009, 102, 1364061−1364064. (13) Kresse, G.; Furthmüller, J. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (14) Kresse, G.; Hafner, J. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558−561. (15) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (16) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 78, 1396−1396. (17) Blöchl, P. E. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (18) Setyawan, W.; Curtarolo, S. Comput. Mater. Sci. 2010, 49, 299− 312. (19) Liebau, F. Structural Chemistry of Silicates; Springer-Verlag: Berlin, Germany, 1985. (20) Sheldrick, G. M. SHELXS-97 and SHELXL-97 Program suite for the solution and refinement of crystal structures; University of Göttingen: Göttingen, Germany, 1997. (21) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (22) Hoppe, R. Z. Kristallogr. 1979, 150, 23−52. (23) Hoppe, R.; Voigt, S.; Glaum, H.; Kissel, J.; Müller, H. P.; Bernet, K. J. Less-Common Met. 1989, 156, 105−122. (24) Brown, I. D.; Altermatt, D. Acta Crystallogr., Sect. B: Struct. Sci. 1985, 41, 244−247. (25) Brese, N. E.; O’Keeffe, M. Acta Crystallogr., Sect. B: Struct. Sci. 1991, 47, 192−197. (26) Hoppe, R. Angew. Chem. 1966, 78, 52−63. (27) Hoppe, R. Angew. Chem. 1970, 82, 7−16. (28) Hübenthal, R. MAPLE, version 4; University of Gießen: Gießen, Germany, 1993. (29) Geller, S. J. Chem. Phys. 1960, 33, 676−684. (30) Prewitt, C. T.; Shannon, R. D. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1968, 24, 869−874. (31) Zachariasen, W. H. Acta Crystallogr. 1954, 7, 305−310. (32) Åhman, J.; Svensson, G.; Albertsson, J. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1996, 52, 1336−1338. (33) Hering, S. A.; Huppertz, H. Z. Naturforsch., B: J. Chem. Sci. 2009, 64, 1032−1040. (34) Vitzthum, D.; Hering, S. A.; Perfler, L.; Huppertz, H. Z. Naturforsch., B: J. Chem. Sci. 2015, 70, 207−214. (35) Li, X.; Yu, J.; Low, J.; Fang, Y.; Xiao, J.; Chen, X. J. Mater. Chem. A 2015, 3, 2485−2534. (36) Wang, S.; Ye, N.; Poeppelmeier, K. Crystals 2015, 5, 252. (37) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Springer: New York, 1980. (38) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307− 326.

decomposes continuously into the thermodynamically stable compound GaBO3.34 Further decomposition products, e.g., HBO2, B2O3, and H2O, evaporating from the sample could explain the observed total weight loss up to a temperature of 930 °C.

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CONCLUSIONS In this work, we reported the synthesis of the new gallium borate Ga2B3O7(OH). It is the second known borate in the system Ga−B−O−H. Besides a detailed discussion of its crystal structure, further characterizations with IR and Raman spectroscopy of Ga 2 B 3 O 7 (OH) can be found in this publication. Raman and IR bands and the band gap were calculated by quantum-mechanical methods and, additionally, experiments on the photocatalytic hydrogen production ability of Ga2B3O7(OH) were performed. It is worth noting that the new gallium borate is the second compound in the System Ga−B−O−H known to date.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02027. X-ray crystallographic data in CIF format (CIF) Wyckoff positions, atomic coordinates, isotropic and anisotropic displacement parameters, interatomic distances and angles, details of the hydrogen bonds, charge distribution calculations, MAPLE values, calculated Raman and IR vibrations, and a single-crystal Raman spectrum showing orientation effects of Ga2B3O7(OH) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +43 (512) 50757099. Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Special thanks go to Dr. Klaus Wurst for the recording of the single-crystal data set, Dr. Clivia Hejni for the Raman measurements, and Johannes Prock for additional experiments concerning the catalytical activity (all three from the University of Innsbruck). We also thank Prof. Dr. Roland Stalder for giving us access to the single-crystal IR spectrometer and the VERBUND AG as well as D. Swarovski KG for the micro gas chromatograph. The computational work was supported by the Austrian Ministry of Science BMWF as part of the Konjunkturpaket II of the Focal Point Scientific Computing at the University of Innsbruck.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b02027 Inorg. Chem. XXXX, XXX, XXX−XXX