Liquid-Crystalline Phases with Liquid Ammonia: Synthesis of Porous

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Liquid-Crystalline Phases with Liquid Ammonia: Synthesis of Porous Si3N4, TiN, VN, and H2−Sorption of Si3N4 and Pd@Si3N4 Fabian Gyger,† Pascal Bockstaller,‡ Dagmar Gerthsen,‡ and Claus Feldmann*,† †

Institut für Anorganische Chemie, Karlsruhe Institute of Technology (KIT), Engesserstrasse 15, D-76131 Karlsruhe, Germany Laboratorium für Elektronenmikroskopie, Karlsruhe Institute of Technology (KIT), Engesserstraße 7, D-76131 Karlsruhe, Germany

‡

S Supporting Information *

ABSTRACT: Liquid-crystalline phases in liquid ammonia were used to obtain meso- and microporous Si3N4, TiN, and VN. The liquid-crystalline phase was established at −50 °C with liquid ammonia as the polar phase, heptane as the nonpolar dispersant phase, dimethyldioctylammonium iodide (DDAI) as the surfactant, and heptylamine as the cosurfactant. Silicon(IV) iodide, tetrakis(dimethylamino)titan, and vanadium(IV) chloride were added and ammonolyzed in the liquid-crystalline phase. After the mixture contents were separated and washed, ammonolysis was completed by slow heating to 600 °C (TiN, VN) and 800 °C (Si3N4) in vacuum or forming gas (N2/H2). The obtained high-surface nitrides are characterized by high purity (e.g., Si3N4 with carbon content 10 wt %) that deteriorate the material properties as well as to high-temperature reactions (>1000 °C) that are counterproductive in terms of high porosity and surface area. Following the LCT approach to porous oxides, the synthesis of high-purity porous metal nitrides in liquid ammonia (lqNH3) is an alternative option. Surprisingly, knowledge on such strategy is limited by now. Lq-NH3 is yet most often used in amonothermal synthesis at elevated temperatures and pressures (up to 600 °C and 500 MPa) and typically results in crystalline bulk nitrides.33−37 Moreover, carbon composites of TiN and

INTRODUCTION Mesoporous (pore diameter 500 °C) with NH3 or related compounds (e.g., N2H4, HN3, CO(NH2)2) into meso-/microporous nitrides.1,2,8−20 Here, © XXXX American Chemical Society

Received: August 3, 2016 Revised: October 5, 2016

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DOI: 10.1021/acs.chemmater.6b03219 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

To obtain Pd@Si3N4-nanocomposites, Pd nanoparticles were synthesized in a first step by reducing PdI2 with NaBH4 in a lqNH3-in-oil microemulsion at −35 °C analogous to our previous report.40,41 Successful reduction and formation of elemental Pd can be observed by the naked eye on the basis of the immediate formation of a blackish suspension. Subsequently, the temperature was reduced to −50 °C to establish the liquid-crystalline phase. Finally, SiI4 was added and ammonolysed as described above for pure Si3N4. Electron Microscopy and Electron Spectroscopy. Different electron microscopic and electron spectroscopic techniques were applied to study the structural and chemical properties of the obtained nitrides. Overview secondary-electron images were obtained in a ZEISS SUPRA scanning electron microscope (SEM) equipped with a Schottky field emitter. The crystalline structure was analyzed by selected area electron diffraction (SAED) in a Philips CM 200 FEG/ ST transmission electron microscope at 200 keV. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were performed with an aberration-corrected FEI Titan3 80−300 microscope at 300 keV. Energy-dispersive X-ray spectroscopy (EDXS) was applied for chemical analysis. For this purpose, the FEI Titan3 80−300 was equipped with an EDAX Si(Li) detector. Quantification of the EDXS spectra was carried out with the FEI software package “TEM imaging and analysis” (TIA). Using TIA, element concentrations were calculated on the basis of a refined Kramers’ law model, which includes corrections for detector absorption and background subtraction. Standard-less quantification (i.e., by means of theoretical sensitivity factors) without thickness correction was applied. Electron-energy-loss spectroscopy (EELS) was performed with a postcolumn Tridiem 865 HR Gatan imaging filter (GIF). The energy resolution of about 0.8 eV facilitates fine structure analyses of ionization edges (ELNES: electron near-edge fine structure) in EELS spectra, which yield information on the bond characteristics. Specifically, the ELNES of the O-K- and the N-K-edges was analyzed to distinguish metal oxides and metal nitrides. EELS was performed in the STEM mode. For TEM sample preparation, diluted suspensions of the as-prepared nitrides in isopropanol were deposited on silicon wafers and evaporated on Lacey carbon films on Cu grids (Ultrathin C Film on Holey Carbon Support Film, 400 mesh, Cu Plano). It is to be noted that the as-received Lacey carbon films contain certain amount of SiO2 (cf. SI). Further details regarding chemical analysis, microscopy of liquidcrystalline phase, electron microscopy, sorption data, and characterization of Pd@Si3N4 can be found in the Supporting Information (SI).

VN were recently obtained via liquid-crystalline phases of cellulose in NH3/NH4SCN (25:75) solutions. They exhibit surface areas up to 580 m2/g.38 Due to the great amount of cellulose, however, these composites contain up to 50 wt % of carbon as well as considerable oxygen impurities, both inevitably resulting in passivation layers. Interestingly, the existence of lyotropic phases in lq-NH 3 was already demonstrated but not used for obtaining nanoparticles or porous materials.39 We could recently present microemulsions with lq-NH3 as polar micelle phase as well as the synthesis of metal and metal nitride nanoparticles herein (e.g., Bi0, Re0, Fe0, GaN, CoN).40,41 On the basis of these results, we here aim at lyotropic phases and the synthesis of high-surface, high-porosity metal nitrides such as Si3N4, TiN, and VN in lq-NH3. Moreover, uniform incorporation of Pd nanoparticles in Si3N4 via a one-pot approach as well as the hydrogen sorption of differently treated Si3N4 were studied.

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EXPERIMENTAL SECTION

General Aspects. All experiments were carried out under argon atmosphere using standard Schlenk techniques and glove boxes. This explicitly includes all centrifugation and annealing procedures. The ammonia content was determined by condensing all lq-NH3 from the liquid-crystalline phases into calibrated glass tubes. Prior to this condensation process, all gaseous NH3 remaining in the glass equipment was removed by an argon flow. Materials. Ammonia (Air Liquide N38, 99.98%) was purified by fractionated condensation. Heptane (Sigma-Aldrich, 99%) was heated over sodium under reflux for 2 days to remove water and traces of oxygen. Heptylamine (Acros, 99%) was heated twice to reflux over CaH2. Dimethyldioctylammonium iodide (DDAI) was prepared from Dimethyldioctylammonium bromide (DDAB, TCI, > 97%) by dissolving in absolute acetonitrile and addition of a 5-fold excess of NaI (Fluka, 99.9%). The precipitated NaBr was filtered off and the solvent was evaporated. To remove excess NaI, the product was dissolved in a minimal amount of absolute CHCl3 and filtered again. For further purification, DDAI was dissolved in absolute CHCl3 and dried over a molecular sieve (3 Å) for at least 7 days. Subsequently, the molecular sieve was filtered off, and chloroform was evaporated. Finally, DDAI was dried at 50 °C under reduced pressure for at least 24 h. Silicon(IV) iodide (ABCR, 99.99%), tetrakis(dimethylamino)titan (Acros Organics, 99.99%) and vanadium(IV) chloride (Acros Organics, > 99.99%) were used as received. Safety Advice. Liquid ammonia is highly volatile and reactive. Continuous cooling with a bath of dry ice and ethanol at −50 °C is required. Any respiration or contact to skin needs to be strictly avoided. As a strongly reducing and basic agent, pure ammonia can cause combustion in contact with oxidizing agents and acids. Synthesis. Ammonia-in-oil liquid-crystalline phases were established by dissolving 1 g (2.5 mmol) of DDAI as the surfactant and an 18-fold excess of heptylamine (6.7 g, 45 mmol) as the cosurfactant in 10 mL of absolute heptane as the nonpolar oil-phase. Subsequently, the resulting milky emulsion was cooled to −50 °C. Thereafter, 0.7 mL of ammonia were condensed into the emulsion resulting in the formation of a jellylike turbid phase. In a typical experiment, the metal precursors (1 g of silicon(IV)iodide, 0.8 mL of tetrakis(dimethylamino)titan, 1 mL of vanadium(IV)chloride) were dissolved in 20 mL of heptane and slowly added to the pre-established liquid-crystalline phase with rigid cooling to maintain the turbid liquid-crystalline phase and to avoid any transformation to a microemulsion system. Subsequent to completed ammonolysis, the precipitates were separated by centrifugation and annealed in vacuum for 12 h at 800 °C (Si3N4) and 8 h at 600 °C (TiN, VN) (Table 1). Subsequent to annealing, all porous nitrides were obtained as black, brittle, coarse powders with grain sizes up to a few millimeters. Si3N4 samples annealed under forming gas (N2:H2 = 90:10) were colorless.

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RESULTS AND DISCUSSION Lq-NH3-Assisted Synthesis of Meso-/Microporous Si3N4, TiN, and VN. As an alternative to our previous work addressing lq-NH3-in-oil microemulsions for obtaining metal nanoparticles,40,41 we here address the formation of birefringent phases with lq-NH3 as the polar phase (Figure 1). As expected, the temperature of the phase transition between the microemulsion system and the birefringent phase, especially, depends on the type and concentration of the solutes and the applied cosurfactants. Based on dimethyldioctylammonium iodide (DDAI) as the surfactant, heptylamine as the cosurfactant, heptane as the nonpolar dispersant phase, and lq-NH3 as the polar phase, we observed clouding points in the temperature range between −50 and −30 °C, which is significantly above the freezing point of heptane (−91 °C). Moreover, phase separation or precipitation of solid compounds were not observed. Light microscopy with polarized light clearly indicates the formation of a birefringent liquid-crystalline structure of the turbid phase system at low temperatures (−50 to −30 °C) (SI: Figures S1,S2). In contrast, the liquidB


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(Figure 2d−f; SI: Figure S4a), and VN (Figure 2g−i; SI: Figure S5a). Particles with a size of ∼10 nm and an amorphous structure are found for Si3N4 (Figure 2b,c). In contrast, lattice fringes indicate crystalline particles with sizes of 2−3 nm for TiN (Figure 2e,f) and slightly larger size (5−10 nm) for VN (Figure 2i). The observed lattice fringe distances of 2.0 Å (TiN) and 2.3 Å (VN) are well in accordance with the respective bulk phases (TiN: d200 with 2.12 Å; VN: d111 with 2.38 Å).42,43 Composition, Specific Surface Area, and Porosity of Si3N4, TiN, and VN. In addition to (HR)TEM, composition and crystallinity of the annealed Si3N4, TiN, and VN samples were validated by X-ray powder diffraction (XRD) (Figure 3), which clearly shows the characteristic Bragg peaks of TiN and VN after annealing at 600 °C (Figure 3b,c). The broadening and low intensity of the Bragg peaks can be ascribed to the small crystallite size and the associated stress-and-strain effects (Figure 2e,f,i). SAED further confirms the crystallinity of the sintered TiN and VN. The measured lattice plane distances are consistent with bulk-TiN and bulk-VN (SI: Figures S7,S8). Si3N4 crystallizes only upon annealing at 1400 °C resulting in a mixture of α- and β-Si3N4 (Figure 3a). The chemical composition and phase purity of annealed Si3N4, TiN, and VN were further investigated by energydispersive X-ray spectroscopy (EDXS) (SI: Figures S3−S5) and elemental analysis (C/H/N) (SI: Tables S1−S5). Accordingly, Si:N, Ti:N, and V:N ratios close to 3:4 and 1:1 were detected and are well in agreement with the expectation (SI: Tables S1− S5). In addition to nitrogen and the respective metal, elemental analysis does not show any remaining hydrogen indicating the completed ammonolysis after synthesis and annealing in vacuum (SI: Tables S1−S5). All nitrides contain certain amounts of carbon remaining from the surfactants (SI: Tables S1−S5). With 1.7 wt % (Si3N4), 12 wt % (TiN), and 10 wt % (VN), however, the carbon content is low in comparison to other porous nitrides reported in the literature (often 20 to 50 wt %).8−20,32−38 The carbon content can be reduced even further by annealing the porous nitrides under forming gas (N2:H2 = 90:10) resulting in 1.1 wt % (Si3N4) and 5.3 wt % (TiN) (SI: Tables S1−S4). Due to the comparably low surface area, such forming-gas treatment was less effective for VN. For Si3N4, the effect is most apparent as vacuum-sintered Si3N4 exhibits a dark grayish color, whereas the negligible carbon content of Si3N4 can be directly recognized on the basis of the white color. Despite the different color, the width and intensity of the Bragg peaks remain identical, indicating crystallinity and ammonolysis as being independent from the carbon content. The determination of the oxygen content via EDXS is generally hampered by the fact that the O-K-line is often superimposed by other lines (e.g., Lα,β-lines of V, Ti; SI: Figures S4,S5). With a typical energy resolution of about 130 eV for EDXS, moreover, precise separation of the respective lines is hardly possible. Here, electron energy loss spectroscopy (EELS) is much more powerful due to a substantially improved energy resolution (0.8 eV) and due to the fine structure of the absorption edges (ELNES: electron-loss near-edge structure) that contains information on the bonding configuration, which further facilitates the distinction between oxides and nitrides. EEL spectra show a clear separation of the N-K, O-K, and the Si-L2,3, Ti-L2,3, and V-L2,3 absorption edges in Si3N4/Pd@Si3N4, TiN, and VN, respectively (Figure 4). All EEL spectra show OK-edges with small intensities compared to the N-K-edge, which suggests low oxygen contents in the analyzed materials,

Figure 1. Scheme illustrating the synthesis of meso-/microporous Si3N4, Pd@Si3N4, TiN, and VN in liquid-crystalline phases with liquid ammonia (lq-NH3) as the polar phase.

crystalline system does not show any polarization effect under nonpolarized light. Moreover, the microemulsion system obtained at higher temperatures (above −30 °C) does also not show any polarization effect. Long-range ordering, however, is not observed for the liquid-crystalline system which can be attributed to the fact that heptane as the majority phase separates the aggregates and thus hinders long-range ordering. In view of the herein presented synthesis strategy, on the other hand, the presence of the nonpolar dispersant phase is beneficial as it can be used to dissolve the metal precursors. The focus of this work is to demonstrate the synthesis of porous, high-surface-area nitrides in liquid-crystalline phases with lq-NH3 for the first time. The absence of oxygen (e.g., in the starting materials and in the solvent) turned out as highly advantageous in view of the purity of the resulting highly porous nitrides in avoiding oxide passivation layers. In a typical experiment, the starting materials (SiI4, Ti(N(CH3)2)4, VCl4) were dissolved in heptane and slowly added to the preestablished liquid-crystalline phase at −50 °C. Temperature control is highly important to avoid any transformation of the turbid liquid-crystalline system to a transparent microemulsion. To complete the ammonolysis, the resulting solids were separated by centrifugation and annealed in vacuum or forming gas (Table 1). It is to be noted that not any NH3 is required during annealing for the completion of ammonolysis. Table 1. Conditions of Annealing of Different Si3N4, TiN, and VN Samples compound Si3N4a Si3N4a TiN TiN VN TiN-1 TiN-2 TiN-3 a

temperature (°C) 800 800 600 600 600 800 600 600

(vacuum) (forming gas) (vacuum) (forming gas) (vacuum) (vacuum) (vacuum) (vacuum)

duration (h)

heating rate (°C/h)

12 12 8 8 8 5 8 8

60 60 30 30 100 400 60 30

(Pd@Si3N4 was treated similar to Si3N4)

After synthesis and annealing, brittle and coarse powders with grain sizes up to few millimeters were obtained. Largescale high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) overview images already indicate the presence of highly porous powder samples (Figure 2). At higher magnification, transmission electron microscopy (TEM) confirms the highly porous structure of agglomerated nanoparticles of Si3N4 (Figure 2a−c; SI: Figure S3a), TiN C


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Figure 2. HAADF-STEM, TEM, and HRTEM images of (a−c) Si3N4, (d−f) TiN, and (g−i) VN at different scales of magnification.

63 m2/g for VN (Table 2; SI: Figures S10−S12). Si3N4 and TiN show a significant fraction of micropores (Si3N4: 0.28 cm3/ g and 274 m2/g; TiN: 0.1 cm3/g and 111 m2/g) contributing to the overall pore volume (Si3N4: 1.47 cm3/g; TiN: 0.31 cm3/g) and the above total specific surface area. In contrast to Si3N4 and TiN, VN does not show any microporosity and exhibits a macropore volume of 0.4 cm3/g only (Table 2). The absence of micropores can be ascribed to the comparably low thermodynamical stability of VN and certain sintering even at temperatures of 600 °C. To this concern, it is well-known in the literature that high-surface-area VN (>100 m2/g) requires high carbon contents for stabilization (typically >20%).25,5051 Sintering effects are also causal for the lower surface area of VN in comparison to Si3N4 and TiN. On the basis of BJH and tplot analysis, finally, the pore size distribution can be deduced as dominated by diameters of 1−5 nm in the case of Si3N4 and TiN as well as diameters of 10−20 nm in the case of VN. For all porous nitrides, certain heating at slow heating rates is a prerequisite for controlled decomposition of the surfactant scaffold (>600 °C) with retention of the porous matrix (10 wt %).25,36,38,52 Pd-Modification and Hydrogen Sorption of Porous Si3N4. As demonstrated by high surface areas (up to 600 m2/g), excellent phase purity with low carbon, and hydrogen contents (1.2 wt % C and 0.9 wt % H for Si3N4) as well as low oxide contamination (according to EELS), synthesis via liquidE


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Figure 4. HAADF-STEM images and EEL spectra of (a−c) Si3N4/Pd@Si3N4, (d,e) TiN, and (f,g) VN. The red crosses and red square in the HAADF-STEM images mark the regions where the EEL spectra were acquired. Pd nanoparticles show bright contrast in the HAADF-STEM image (a). The EELS spectra in (b,c) were acquired in a region without Pd.

Table 2. Surface Area and Porosity of Si3N4, TiN, and VN Including Specific Surface Area (ABET), Pore Volume (Vp), Area of Micropores (AMicro), Volume of Micropores (Vp,Micro), and Volume of Macropores (VP,Macro) compound

ABET (m2/g)

Vp (cm3/g)

AMicro (m2/g)

Vp,Micro (cm3/g)

VP,Macro (cm3/g)

Si3N4 Pd@Si3N4 VN TiN-1 TiN-2 TiN-3

610 438 63 115 125 203

1.47 1.15 0.40 0.06 0.19 0.31

274 143

0.28 0.14

114 78 111

0.05 0.05 0.10

1.19 1.01 0.40 0.01 0.14 0.21

volume of the micropores (0.3 cm3/g) of the as-obtained Si3N4, which exceed the values of many MOFs.66 Although reversible H2 adsorption and desorption as well as stability and cyclability need much more efforts, porous Si3N4 made via the lq-NH3 approach turns out as very promising.

well as Pd-modified Pd@Si3N4 exhibit lower H2 uptakes of 1.5 ± 0.3 and 0.8 ± 0.2 wt %, respectively. These findings suggest that reducing the carbon content (via forming gas posttreatment) and maximizing the surface area are key factors for the H2 uptake of porous Si3N4. Although the homogeneous incorporation of Pd nanoparticles can be very interesting for catalysis,53,64,65 according to our results, Pd is not essential for reversible H2 sorption of microporous Si3N4. Based on high purity and absence of passivating carbon and oxide contaminations, such metal cocatalyst is obviously not needed. With 2.5 ± 0.2 wt % H2 at room temperature, the uptake of the above-presented highly porous Si3N4 is similar to top-running metal organic frameworks (MOFs)66 that show 2.3 wt % (PCN-66)67 to 2.6 wt % (Be-BTB)68 at room temperature and a pressure of 100 bar. In contrast, for most carbon materials, zeolites and MOFs exhibit H2 loading capacities below 0.5 wt % at room temperature.66 This finding can be attributed to the great total pore volume (1.5 cm3/g) and, especially, to the great

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CONCLUSIONS High-surface and high-porosity metal nitrides are prepared for the first time via a nonaqueous, liquid-crystalline phase with lqNH3, which is shown for Si3N4, TiN, and VN as specific examples. Notably, additional NH3 flow in the gas phase is not required during annealing. All metal nitrides were obtained with high purity as proven by elemental analysis, EDXS, and EELS, which indicate low carbon, hydrogen, and oxygen contents. Moreover, high specific surface areas (Si3N4: 610 m2/g; TiN: 203 m2/g; VN: 63 m2/g) and total pore volumes (Si3N4: 1.5 cm3/g; TiN: 0.3 cm3/g; VN: 0.4 cm3/g) were realized. In contrast to VN showing mesoporosity only, Si3N4 and TiN F


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such as catalysis, photocatalysis, CO2 sorption and separation, luminescence materials, or as ceramics and hard materials.

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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.chemmater.6b03219. Details regarding analytical tools and techniques and chemical analysis as well as more details regarding microscopy of the liquid-crystalline phase, electron microscopic characterization, sorption data, and the characterization of Pd@Si3N4 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

Figure 5. Pd-distribution and structure of Pd@Si3N4: (a) SEM overview image; (b) HRTEM image; (c+d) HAADF-STEM images. Bright contrast in (c,d) is characteristic for the presence of Pdnanoparticles.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Deutsche Forschungsgemeinschaft (DFG) for funding (NanoMet: FE911/4-1, GE 841/22-1).

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REFERENCES

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Figure 6. Reversible hydrogen sorption of highly porous Si3N4 after postannealing in forming gas and vacuum as well as of Pdnanoparticle-modified Si3N4 (Pd@Si3N4 after postannealing in forming gas).

exhibit both high mesoporosity and microporosity. Altogether, the combination of high surface area, high pore volume, high purity, and the absence of carbon and oxide passivation layers represent the assets of the lq-NH3-based synthesis approach. As a conceptual study of the material properties, reversible hydrogen sorption of Si3N4 and Pd-nanoparticle-modified Pd@ Si3N4 was determined. Extraordinary high H2 uptakes of 2.5 and 1.5 wt % were observed at room temperature for Si3N4 annealed in forming gas and Si3N4 annealed in vacuum. Although the lq-NH3 approach is ideal for preparing Pd@Si3N4 with a uniform distribution of nanosized Pd in a porous Si3N4 matrix, the H2 uptake of Pd@Si3N4 (0.8 wt %) is significantly lower as compared to pure Si3N4 (2.5 wt %). This finding can be attributed to a blocking of micropores by the Pd nanoparticles resulting in a significantly reduced surface area and pore volume. The very high H2 uptake of lq-NH3 made porous Si3N4 can be rationalized based on the absence of carbon and oxide passivation layers. Taking the promising features of lq-NH3-made porous Si3N4, TiN, and VN and the promising H2 uptake into account, the lq-NH3-liquid-crystalline phases can become highly feasible for preparing other porous nitrides as well as for addressing further material properties G


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Liquid-Crystalline Phases with Liquid Ammonia: Synthesis of Porous

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