Template-Free Synthesis of Mesoporous Transition Metal Nitride

Article pubs.acs.org/cm

Template-Free Synthesis of Mesoporous Transition Metal Nitride Materials from Ternary Cadmium Transition Metal Oxides Minghui Yang and Francis J. DiSalvo* Department of Chemistry, Cornell University, Ithaca, New York 14853-1301, United States ABSTRACT: A simple process for preparing mesoporous transition metal nitrides by the ammonolysis of bulk ternary oxides that contain cadmium is reported. Mesoporous NbN, VN, Ta3N5, and TiN have been obtained. The products were characterized by Rietveld refinement of powder X-ray diffraction patterns, scanning electron microscopy (SEM), and nitrogen adsorption/ desorption analysis. Pore sizes ranging from 10 to 40 nm are easily accessible.

KEYWORDS: mesoporous, transition metal nitride, synthesis

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temperatures as low as 450 °C. The TMNs were analyzed by X-ray diffraction (PXRD), scanning electron microscopy (SEM) and nitrogen absorption/desorption analysis.

INTRODUCTION Transition metal nitrides (TMNs) have long been studied and their properties as metallic ceramic materials have been used in a wide variety of applications, such as fuel cells, optical coatings, electrical contacts, and catalysts.1,2 The diversity of TMN applications stems largely from their unique and varied properties. Like their parent metals, TMNs are electrically conductive. However, their extremely high melting point, hardness and corrosion resistance resemble that of ceramic materials. TMNs are also more catalytically active and selective for some reactions such as alkylation, hydroprocessing and hydrotreating than pure metals3−6 and show great electrochemical stability in harsh conditions, including high temperature and acidic conditions.7 TMNs have been synthesized in a variety of ways including reacting metals with gas-phase reagents (N2, NH3, hydrazine, urea, etc.), liquid phase methods, high pressure, etc.8−10 Most traditional high-temperature synthesis methods produce products with low surface areas because significant sintering occurs at high temperatures. Low-temperature syntheses are therefore favored for applications where obtaining higher surface areas is a consideration.11 This report extends our recent work on the nitriding of Zn containing transition metal oxides in order to produce smaller pores and lower processing temperatures. In the Zn case, ammonolysis at temperatures near 700 °C and above produces mesoporous nitrides resulting from the condensation of atomic scale voids created by the loss of Zn by evaporation, the replacement of 3 oxygen anions by 2 nitrogen anions, and in most cases the loss of oxygen to form water on the reduction of the transition metal. Since Cd is more easily reduced than Zn, and since Cd has a higher vapor pressure than Zn at a given temperature, we hypothesized that mesoporous nitrides with smaller pores could be obtained from Cd oxides at lower processing temperatures than from Zn oxides. Indeed, this report confirms that mesoporous NbN, VN, TiN, and Ta3N5 with much smaller pores (below 10 nm) can be obtained by the ammonolysis of cadmium containing oxide precursors at © 2012 American Chemical Society

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EXPERIMENTAL PROCEDURES

Cd metal oxides were prepared by solid-state reactions between CdO and Nb2O5, V2O5, TiO2, or Ta2O5 in a stoichiometric ratio. All chemicals used are commercially purchased and with highest possible purities (≥99.99%). Cd4V2O9 and CdTiO3 were prepared at 600 °C for 20 h. Cd2Nb2O7 and Cd2Ta2O7 were prepared at 1000 °C for 10 h in a platinum crucible. These oxides were placed in an alumina boat. The boat was then placed in a silica tube with airtight stainless steel end-caps that had welded valves and connections to input and output gas lines. All gases were purified to remove trace amounts of oxygen or water using pellet copper, nickel, palladium and platinum with zeolites as support. The silica tube was then placed in a split tube furnace and the appropriate connections to gas sources made. Argon gas was passed over the sample for 15 min to expel air before establishing a flow of ammonia gas (Anhydrous, Air Gas). The sample was heated to the above reaction temperatures at 150 °C/h. After treatment for the specified period, the furnace power was turned off and the product cooled to room temperature in ∼4 h under an ammonia flow. Before the silica tube was taken out of the split tube furnace, argon gas was flowed through the silica tube to expel the ammonia gas. The silica tube was left in lab for 24 h with one valve open in order to expose the ammonolysis product to air slowly. This latter procedure resulted in the formation of only a very thin oxide on the nitride surface. Finely ground powders were examined with a Rigaku Ultima VI powder X-ray diffractometer (PXRD) with CuKα radiation (Kα1, λ = 1.5406 Å and Kα2, λ = 1.5444 Å). Crystal structures of the oxides and resultant nitrides were confirmed by PXRD profiles using the GSAS package.12 Scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX) were performed with a LEO-1550 field emission SEM (FSEM). Nitrogen adsorption/desorption isotherms were measured at −196 °C using a Micromeritics ASAP 2020 system. The samples were degassed at 200 °C for 24 h on a vacuum line. The Received: August 22, 2012 Revised: October 26, 2012 Published: November 1, 2012 4406

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nitride crystalline domain size can be estimated from a Rietveld fit of the broadened Lorentzian X-ray line shape in GSAS.

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RESULTS AND DISCUSSION 1. Mesoporous TMN from Zn and Cd Containing Oxide Precursors. Cd sublimes at a lower temperature than Zn, because Zn has a higher boiling point. Cd is also easier to reduce than Zn, since Zn is more electropositive. Therefore, mesoporous TMN products might be expected to form at lower temperatures from CdTMO precursors than from ZnTMO precursors (cadmium and zinc transition metal oxide, respectively). Because the diffusion rates in the product nitride will be lower at lower temperatures, we would expect that smaller pore sizes in Cd-derived TMN can be obtained. However, lower temperatures also result in slower ammonolysis rates; the net balance of the reaction rates need to be determined by experiment and might be expected to be different for different transition metals. Indeed, mesoporous NbN with 10 − 15 nm mean pore size can be prepared at 600 °C from CdNb2O6 after 8 h of ammonolysis, but NbN can only be made from ZnNb2O6 after 8 h of ammonolysis at 700 °C, which results in pore sizes of 25−30 nm, as shown in Figure 1a,b. Similarly, mesoporous VN with pore diameters near 10 Figure 2. SEM images of mesoporous TMN from ammonolysis of CdTMO: (a) Cd2Nb2O7 at 600 °C, (b) CdTiO3 at 700 °C, (c) Cd4V2O9 at 500 °C, (d) Cd2Ta2O7 at 800 °C, (e) Cd4V2O9 at 450 °C, and (f) CdV2O6 at 500 °C.

min−1 through a 1 in. diameter silica flow tube. Typically a few hundred milligrams of oxide were used in each ammonolysis reaction. Above 500 °C and with increasing time or temperature, we generally observed the formation of metal (oxy)nitrides (MNaOb, where a + b ≈ 1) that adopted the rocksalt structure (Pm3̅m). The refined cell parameters determined by PXRD are summarized in Table 1. Under the reaction conditions employed here, the N content in the product was substantially larger than the residual oxygen content. For simplicity we refer to these products as “TMN”, without explicitly referring to any residual oxygen content. We have discussed the oxygen content in our recent publication.13 These MNaOb products show relatively broad diffraction peaks due to the small crystalline domain sizes of the refractory nitrides (calculated domain sizes of 10−50 nm) as shown by the representative PXRD patterns from ammonolyses of different CdTMOs in Figure 3a. During ammonolysis, a bright silver powder was observed to deposit on the cooler parts of the silica flow tube downstream from the sample. This deposit formed only when MOaNb was formed in the product and was determined to be Cd metal by PXRD as shown in Figure 3. Under the conditions reported, the expected mass loss due to the sublimation of Cd and the replacement of N by O was experimentally measured and confirmed. For example, the ammonolysis of 0.2107 g of Cd4V2O9 at 800 °C for 8 h yields 0.0401 g of VN, which is the expected mass after the reduction and sublimation of Cd from and the replacement of nine O in the precursor oxide by two N. 3. Mesoporous TMNs. Figure 2 shows the SEM images of representative ammonolysis products from different CdTMO precursors. These show a variety of mesoporous structures that depend on the Cd mole fraction and the identity of the TM in the precursor. In all samples, the general shape of the starting oxide crystallites remains after ammonolysis; the reactions are topotatic. The ammonolysis product of Cd2Nb2O7 at 600 °C

Figure 1. SEM images of mesoporous TMN obtained from the ammonolysis of ZnTMOs and CdTMOs at the lowest temperature that produces a single phase product in 8 h: (a) CdNb2O6 at 600 °C, (b) ZnNb2O6 at 700 °C, (c) Cd2V2O7 at 500 °C, and (d) Zn2V2O7 at 600 °C. The scale bars are all 100 nm.

nm can be prepared by ammonolysis Cd2V2O7 after 8 h at 500 °C. In contrast, the ammonolysis of Zn2V2O7 does not proceed to completion in 8 h at temperatures below 600 °C (Figures 1c,d) and results in much larger pores. The morphology of the pores depends on the starting composition of the oxide (see Figure 2). 2. Ammonolysis of CdTMO. In this investigation, we chose to study CdTMO precursors in which TM = Nb, V, Ta, Ti, since all readily form metal (oxy)nitrides by ammonolysis. We first synthesized Cd-containing ternary oxides as precursors by standard ceramic syntheses. The phase purity of the ternary oxide precursors was confirmed by powder X-ray diffraction (PXRD). These precursor oxides were prepared at high enough temperatures to produce large grain sizes, mostly in the 1 to 20 μm range. Ammonolysis of these oxides was carried out for a range of temperatures between 450 to 800 °C and for a range of time from 8 to 30 h at an ammonia flow rate of 200 cm3 4407

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VN and Ta3N5, the ammonolysis products of Cd4V2O9 at 500 °C for 8 h and Cd2Ta2O7 at 800 °C for 8 h, respectively. Both ammonolysis products show mesoporous structures again retaining the starting oxide crystallite morphology. The pore size for both TMNs is 20−30 nm, with a domain size of 6 nm for VN and a domain size of 36 nm for Ta3N5. The above SEM images show that in some cases the pores and particles both have rounded surfaces (see images a and b in Figure 3, for example). This type of pore has been observed in other systems. For example, rounded pores are found in dealloyed Ag−Au,14 whereas striking aligned faceting was found in single-crystal MnO films produced by reduction of ZnMn2O4 in forming gas.15 The shape of the pores is expected to be a function of the surface diffusion rates and the difference of those rates on different facets of the product crystalline structure.16,17 4. Analysis of the Mesoporous Structure. To show that this bulk ammonolysis product is also fully mesoporous, we carried out N2 BET measurements of these TMNs. BET surface areas of approximately 500 mg of each sample was studied. There is some microporosity (pore diameter ≤ 2 nm) of TMNs that accounts for micropore area of up to 5.6 m2/g and micropore volume of up to 2.3 × 10−3 cm3/g. As given in Table 1, the surface areas of all samples, as determined by BET, were in the range of 15−60 m2/g. This surface area is very large in comparison to TMNs prepared by traditional high temperature synthesis. These latter synthesizes generally produce TMNs with surface areas less than 1 m2/g.11 VN had the highest surface area and Ta3N5 had the lowest surface area at 59.1 and 15.1 m2/g, respectively. The pore area and volume were also obtained by BET. TiN exhibited the highest micropore area with an area of 5.6 m2/g. VN had the lowest micropore areas of 1.3 m2/g. Figure 4 shows the representative adsorption and desorption isotherm of mesoporous TMNs. At high relative pressures, the curve shows an increased uptake of absorbate as the pores become filled.18 5. Conductivity of Mesoporous TMN. Bulk transition metal nitrides are almost all good electrical conductors: exceptions include the semiconductors Ta3N519 and Cu3N.16 A few reported values for rock salt nitrides are: 1.28 × 104 S/ cm (TiN),17 1.23 × 104 S/cm (VN),20 0.59 × 104 S/cm (TaN),20 whereas a lower conductivity is found for WN (3 × 102 S/cm).21 A simple apparatus that allows the four point measurement of conductivity of compressed powders as a function of applied pressure was used to estimate the conductivity of the compacted mesoporous powders. As expected, the conductivity increased as a function of pressure. At a relatively low pressure of 35 bar, we obtained low conductivities of these mesoporous nitrides as summarized in Table 1. These are about 2 orders of magnitude lower than for the respective bulk materials, presumably because of both the porosity and weak particle−particle contacts at low pressure. 6. Formation Process of Mesoporous Metal (Oxy)nitrides. At relatively low temperatures under flowing ammonia, mesoporous structures of MOnNm begin to form on the surface of the reactant oxides as described previously.13 At higher temperature (500 °C for the case of Cd4V2O9), the product crystallites grow in size (as determined by PXRD) and the pores consolidate to become fewer and larger as shown in Figure 2c. As expected, a similar behavior is observed in the case of ammonolysis of Cd2Nb2O7, CdTiO3, or Cd2Ta2O7 in panels a, b, and d in Figure 2, respectively.

Table 1. Summary of Ammonolysis Conditions, Refined Lattice Parameters, and Calculated Domain Size of Pure Transition Metal (oxy)nitrides (TMN)a TMN precursor condition a (Å) domain size (nm) electrical conductivity (S/ cm) at 35 bar pressure BET surface area (m2/g) C value Langmuir surface area (m2/g) micropore volume (cm3/g) micropore volume (cm3/metal) micropore area (m2/g)

NbN

TiN

VN

Ta3N5

Cd2Nb2O7 600 °C, 24 h 4.3188(1) 11 102

CdTiO3 700 °C, 24 h 4.2189(1) 23 383

Cd4V2O9 500 °C, 8 h 4.0892(1) 6 89

Cd2Ta2O7 800 °C, 8 h 3.8898(1) 36 ---

26.6 ± 0.1

27.3 ± 0.1

59.1 ± 0.1

15.1 ± 0.1

154.3 33.2 ± 0.7

197.6 34.3 ± 0.6

102.7 72.8 ± 0.9

154.4 18.9 ± 0.3

1.6 × 10−3

2.3 × 10−3

0.1 × 10−3

0.9 × 10−3

0.17

0.14

0.01

0.18

4.1

5.6

1.3

2.2

All reactions are at an ammonia flow rate of 200 cm3 min−1. Ta3N5 crystallized in space group Cmcm, all others MN (M = Nb, Ti, and V) crystallized in space group Fm3̅m. Units for temperature and reaction time are degree Celsius (C) and hour (h), respectively; Lattice parameters b and c for Ta3N5 are 10.2207(1) and 10.2733(1) Å. Electrical conductivities were measured at pressure of 35 bar. Ta3N5 is a semiconductor. a

Figure 3. (a) PXRD patterns illustrating the mesoporous TMNs as shown in Table 1 from ammonolysis of Cd containing transition metal oxides, (b) BET surface area plots of mesoporous TMN from ammonolysis of CdTMO: Cd4V2O9 at 500 °C and Cd2Ta2O7 at 800 °C.

for 8 h is shown in Figure 2a. At this temperature, it is clear that a mesoporous structure has formed, while the overall gross morphology of the starting oxide crystallites is maintained. According to the SEM images, the size of both the pores and TMN “struts” are 10−40 nm. The PXRD refinement of the 8 h product shows single-phase NbN was formed and that the average nitride crystalline domain size is 11 nm. Figure 2b shows the ammonolysis product of CdTiO3 obtained at 700 °C after 8 h; mesoporous TiN clearly forms while the general shape of the starting oxide crystallites is maintained. The domain size of this ammonolysis product was 23 nm and the pore sizes are 20−30 nm. Figure 2c, d show the SEM images of 4408

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(8) Marchand, R.; Laurent, Y.; Guyader, J.; L’Haridon, P.; Verdier, P. J. Eur. Ceram. Soc. 1991, 8, 197−213. (9) DiSalvo, F. J.; Clarke, S. J. Curr. Opin. Solid State Mater. Sci. 1996, 1, 241−249. (10) Fuertes, A. Dalton Trans. 2010, 39, 5942−5948. (11) Jaggers, C. H.; Michaels, J. N.; Stacy, A. M. Chem. Mater. 1990, 2, 150−157. (12) Larson, A. C.& Von Dreele, R. B. Los Alamos National Laboratory Report LAUR 86−748: General Structure Analysis System (GSAS); Los Alamos National Laboratory: Los Alamos, NM, 1994. (13) Yang, M.; MacLeod, M. J.; Tessier, F; DiSalvo, F. J. J. Am. Ceram. Soc. 2012, 1551. (14) Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450−453. (15) Toberer, E. S.; Grossman, M.; Schladt, T.; Lange, F. F.; Seshadri, R. Chem. Mater. 2007, 19, 4833−4838. (16) Du, Y.; Ji, A. L.; Ma, L. B.; Wang, Y. Q.; Cao, Z. J. Cryst. Growth 2005, 280, 490−494. (17) Petrykina, R. Y.; Shvedova, L. K. Powder Metall. Met. Ceram. 1972, 11, 276−279. (18) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309−319. (19) Swisher, J. H.; Read, M. H., Metall. Trans. 1972, 3, 489-&. (20) Brese, N. E.; Okeeffe, M.; Rauch, P.; Disalvo, F. J. Acta Crystallogr., Sect. C 1991, 47, 2291−2294. (21) Becker, J. S.; Gordon, R. G. Appl. Phys. Lett. 2003, 82, 2239− 2241. (22) Pierson, H. O. Handbook of Refractory Carbides and Nitrides: Properties, Characteristics, Processing, and Applications; Noyes Publications: Westwood, NJ, 1996. (23) Pugh, D. V.; Dursun, A.; Corcoran, S. G. J. Electrochem. Soc. 2005, 152, B455−B459. (24) Stratmann, M.; Rohwerder, M. Nature 2001, 410, 420−423. (25) Rugolo, J.; Erlebacher, J.; Sieradzki, K. Nat. Mater. 2006, 5, 946−949.

Several factors are expected to control the mesoporous morphology of the products obtained: the first is the temperature at which the rate of reaction is sufficiently fast to remove all of the Cd and most of the O from the large grained reactant oxides; the others are the bulk and surface diffusion rates of the cations and anions in any intermediates and products at the reaction temperature. Diffusion constants are generally found to be controlled by a number of factors, including crystal structure and bond strengths. The latter is also related to physical properties such as the melting point (especially if congruently melting), hardness, or Debye temperature. Indeed, the size of the pores obtained in the rocksalt structure products roughly follows the reported Debye temperatures at room temperature: NbN (307 K), TiN (579 K), and VN (420 K).22 As the reaction temperature increases, the pores coarsen and the TMN grain size grows. Eventually with increasing temperature and time, the pores are completely eliminated. This description is consistent with that proposed for mesoporous materials obtained by dealloying.23−25

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CONCLUSIONS A novel synthesis method for metal (oxy)nitride nanomaterials with small pores has been developed using relatively low temperature ammonolysis of Cd containing oxides. As expected, smaller pores can be obtained than for the equivalent Zn oxide precursors, since lower processing temperatures are able to produce single phase products in the Cd case. NbN, VN, Ta3N5 and TiN were investigated. The general shape of the starting oxide crystallites was maintained after ammonolysis; the reaction is topotatic. Depending upon the starting composition and the ammonolysis temperature, the TMNs had pore sizes from 10−40 nm and domain sizes of 5−40 nm. For a given composition, the pore sizes grow with increasing temperature and process time. It was also found the mesoporous TMN were conductive; however, because these powders are only compressed at low pressures, their conductivities were about 2 orders of magnitude less than that of the bulk materials. TMNs synthesized by this method may have applications in a variety of electronic devices where the high surface area and good conductivity are important.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.J.D.); [email protected] (M.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Science Foundation through Grant DMR-0602526.

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REFERENCES

(1) Williams, W. S. JOM−J. Miner. Met. Mater. Soc. 1998, 50, 62−66. (2) Oyama, S. T. The Chemistry of Transition Metal Carbides and Nitrides; Blackie Academic & Professional: Glasgow, U.K., 1996. (3) Ramanathan, S.; Oyama, S. T. J. Phys. Chem. 1995, 99, 16365− 16372. (4) Nagai, M. Appl. Catal., A 2007, 322, 178−190. (5) Oyama, S. T. Catal. Today 1992, 15, 179−200. (6) Chen, J. G. G. Chem. Rev. 1996, 96, 1477−1498. (7) Ham, D.; Lee, J. Energies 2009, 2, 873−899. 4409

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