J. Phys. Chem. 1994, 98, 40834086
4083
Generation of Mo2N Nanoparticles from Topotactic Mo2N Crystallites K. L. Roberts and E. J. Markel' Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208 Received: October 27, 1993'
MozN nanoparticles (4-1 5 nm) have been synthesized by ultrasonic dispersion of topotactic MozN crystallites in solution. Nanostructure is developed during topotactic conversion of Moo3 crystallites to porous MozN crystallites. These materials do not sinter during temperature-programmed syntheses up to 700 OC. The characteristic blue color of the dispersed MozN in aqueous solution is attributed to the surface oxide/hydroxide phase. Samples were characterized using TEM, X-ray diffraction, scanning tunneling microscopy, and BET surface area measurements.
Introduction The optical, magnetic, electronic, and structural properties of nanoparticles and aggregates of nanoparticles can be quite unlike those of their bulk Interest in these materials has led to syntheses for nanoparticulate oxides, semiconductors, metals, and biological compounds. Metal syntheses have been hampered by the insolubility of metals, by the tendency of metal particles to sinter, and by the high surface energies of metal nanoparticles which make them air-sensitive and susceptible to complete oxidation. Successful methods include metal ion reduction on surfaces,3-12 surfactant vesicle encapsulation,1>21 decomposition-polymerization of organometalliccompounds,Zr26 reduction of spray-dried metal salts,1V2evaporation of metals,Z7-"J deposition of metals in fullerene template^,^'.^^ and sol-gel While these methods involve the assembly of proces~es.3~.~* molecular building blocks to form nanoparticles, this report addresses a new method for the synthesis of nanoparticles in which solid crystallites are shattered to form nanoparticles.
Review Slow temperature-programmed reaction of NH3 and N2/H2 gas mixtures with polycrystalline Moo3 powders converts the solid to nanoporous Mo2N crystallites (typically 15-pm diameter) in which the average size of the internal solid domains is as small as -3 nm.35 The high surface area of these powders has generated interest in their application as heterogeneous catalysts.3Ul However, little is known about the pore structure or solid nanostructure in these materials. The generation of high surface area r-Mo2N requires a reaction temperature ramping rate of 5 K/min or less to a final temperature of 980 K. Very slow heating rates (0.6K/min to a final temperature of 980 K) are required to produce the most finely divided Mo2N.35 The evolution of surface area and pore volume during reduction/ nitridation of Moo3 with NH3 have been studied. The nitridation/reduction reaction is topotactic because the crystalline solid undergoes chemical changes but retains elements of its original crystallographic structure and symmetry. Specifically, oxygen atoms diffuse out of the Moo3 crystal as nitrogen atoms diffuse in, while the lattice of molybdenum atoms remains ordered. Selected-area electron diffraction data indicate that the [OlO] planes of Moo3 are parallel to the [loo] planes of product Y - M O ~ NIn . ~this ~ way, the characteristic flat platelet morphology of the Moo3 crystallite does not change as it is converted to Mo2N. During the topotactic crystalliteconversion, the lattice fractures in a poorly understood process to produce pores. The fracturing of the crystal lattice may be related to the
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* To whom correspondence should be addressed.
Abstract published in Aduance ACS Abstracts, March IS, 1994.
0022-365419412098-4083304.50/0
shrinking of the crystal unit cell or the presence of interstitial vacancies in the lattice which can generate internal stresses in the crystal. Electron diffraction data, X-ray diffraction line width data, and surface area measurements indicate that each high surface area M02N platelet is a porous aggregate of 3-4-nm solid domains in crystallographic alignment.3s The freshly prepared Mo2N will oxidize vigorously if exposed directly to air. Air-stable samplescan be produced by passivating the surface by slow, controlled oxidation. Effective passivation is accomplished by flowing 1% oxygen in helium over the sample at room temperature or by allowing air to slowly diffuse to the sample from a distant Surface area is lost during the passivation procedure: in the syntheses yielding highest surface area, specific surface area falls from 220 to 170 m2/g during passivation. We have previously shown4*that passivated r-Mo2N is stabilized by an amorphous oxide surface layer with an infrared spectrum like that of Mo02. While the thin platelet morphology of the high surface area Mo2N is evident,little is known of the important solid fine structure or the structure of pores. Postulated structures fall between two extrema: highly regular porous crystal structures similar to zeolites are possible, owing to the topotactic processes producing surface area, but irregular structures are also postulated. TEM images generally exhibit an irregular granular appearance on a scale of several nanometers, but the overlap of several layers of material in these porous aggregates renders the transmission images inconclusive. Scanningtunneling microscopy of topotactic Mo2N has been hampered to date by oxidation of the surface (which interferes with the electron tunneling process) and by difficulties in handling powdered samples. We have overcome these obstacles by using oxygen exclusion methods and by using large single crystals of topotactic Mo2N.43
Experimental Section Synthesis Reactions. High surface area Mo2N powders were synthesized using a 4 mm i.d. X 8 mm 0.d. X 1 m quartz tubular flow reactor fitted with a porous quartz frit. The reactor was loaded with 0.1 g of Moo3 (Johnson Matthey, 99.99%) and connected to a N2/H2 feed gas manifold (Matheson, 99.9995%). Flow was metered using electronicmass flow controllersupstream of the reactant bed to produce a bed space velocity of 100 000 h-l. A 100-cm Lindberg furnace equipped with a programmable temperature controller was used for all reactions. Each synthesis reaction began with a brief period for stabilization of the gas flow, followed by initiation of the temperature ramping program (heating from 35 to 660 OC at 0.6 OC/min followed by 2 h a t 660 OC). When the temperature program was complete, gas flow was maintained as the tubular reactor was removed from the furnace. The high surface area materials were 0 1994 American Chemical Society
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The Journal of Physical Chemistry, Vol. 98, No. 15, 1994
air sensitiveand oxidize completely if exposed to air immediately following reaction. Air-stable samples were formed using a passivation procedure36 in which air is allowed to slowly diffuse though 0.5 m X 1 cm i.d. tubing to the sample over a period of 24 h. This procedure was selected to minimize overoxidation of the sample: assumingmass transfer of air occurs only by diffusion and that any oxygen present reacts immediately with the solid surface, only 2-3 monolayers of oxygen can form on the surface during the passivation procedure, depending on the type of metal oxide surface structure assumed. Other reported passivation methods35*3740involve flowing inert gases containing 1% oxygen-much higher oxygen levels than used in our procedure. Regardless of the method used, it is clear when a sample is not properly passivated: the sample burns brightly when exposed to air. Characterization. A Hitachi Model 8000 microscope operating at 200 keV was used for TEM analyses. Nanoparticles were mounted on carbon-coated TEM grids for analysis by evaporating a droplet containinga very dilute aqueous suspensionof dispersed nanoparticles. Nanoparticle concentrations over several orders of magnitude were tested to optimize particle density on the grid. Undispersed Mo2N crystallite suspensionswere also mounted on TEM grids for analysis. Scanning tunneling microscope images were obtained using a Nanoscope I1 instrument equipped with a 1 pm X 1 pm X 0.3 pm scan head and a platinum scan tip. Samples were immersed in an oil droplet during analysis to exclude air. X-ray diffraction analyses were performed using a Rigaku D-max B diffractometer equipped with a Cu source and a grating monochromator system for rejection of spurious lines. Samples were ground as required and mounted on backless aluminum sample holders using an amorphousadhesive tape. All data were compared to JCPDS catalogue values44for identification. X-ray diffraction peak widths were used to determine the size of coherently diffracting crystalline domains in solid products using the Scherrer equati0n:~5
where b is the corrected peak width at half-maximum in terms of goniometer angle, 28, X is the incident radiation wavelength, 6 is the angle of diffraction, dhk, is the dimension of coherently reflecting domains in the ( h k l )direction, and Kis taken as unity. The corrected value b should be distinguished from B, the measured angular width at half-maximum (fwhm). The value of b is obtained using Warren’s formula of Gaussian type curves, b2 = B2 - bo2,where bo is the fwhm interpolated to the angle of interest of KBr or a similar material with particle dimensions in excess of 3000 A.45 It should also be mentioned that the value of dhk, is used as a relative measure of lattice extent in the ( h k l ) direction only. If d is defined as the cube root of volume, the value of K would be a function of the crystallite morphology and orientation. The solid particle diameter may also be estimated from surface area using the relation Sg= 6/pdp,35 BET surface area analysis and pore volume measurements were performed using a Micromeritics 2700 dynamic adsorption analyzer. The N2/He mixtures used in adsorptionmeasurements were prepared in the laboratory by mixing commercial gases (Matheson N2, 99.9995%, and He, 99.9995%) in a Sierra electronic dual-channel mass flow controller. An integrating thermal conductivity detector was used to determine test gas composition during adsorption/desorption. Adsorption was carried out at 77 K. Results X-ray diffraction analysis confirm the passivated product of temperature-programmedreaction is yMo2N with no crystalline impuritiesobserved (Figure 1). The X-ray lines are quite broad:
Roberts and Markel
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20 Figure 1. X-ray diffraction analyses of high surface area topotactic y-Mo2N and Mo2N nanoparticles produced by ultrasonic dispersion of toptactic Mo2N.
Figure 2. Scanning tunneling micrograph of topotactic y-Mo2N crystallite.43
TABLE 1. Physical Properties of Nanoparticles and Starting Materials sample
Moo3 (starting material)
surface area (Sg,m2/g)
2.9
topotactic Mo2N crystallites
105
Mo2N nanoparticles
121
dhkl (nm) (from X-ray line width)
dp(nm) (from S,)
>300 1.3 [200]
>200 6.0
8.4 [ l l l ] 8.1 [200] 8.7 [ l l l ]
5.2
Scherrer analysis (Table 1) of the X-ray reflections found at 43.4O [200] and 37.4O [ 1111 find the average solid domain in the porous solid is slightly larger than 7 nm in the [200] direction and smaller than 9 nm in the [ 1111direction, as observed previously in similar ntenarations.35136 The fresh. nassivated Mo9N samde exhibiteda BET specificsurface area of 105m2/g, corresponding to an average particle diameter of 6.0 nm. The particle sizes obtained from surface area data and X-rav line width data do not match precisely because of assumptions made in the calculations: values will match if the sample is composed of a narrow size distribution of smooth, crystalline spheres. STM analysis of the undispersed, high surface area Mo2N (Figure 2) shows an irregular, granular morphology. The sample is an aggregate of roundish, sometimes oblong particles with a range of sizes, but most are between 7 and 15 nm in diameter. No striking pattern of particle facetting or spatial ordering of the particles is observed. This result is remarkable, given the topotactic reaction mechanism. In isolated instances, the particles group in lines and are oriented in one direction. We were not able to establish the orientation of this preferred axis relative to the crystallographic axis of the Mo2N or the parent MoO3.
Generation of Mo2N Nanoparticles
The Journal of Physical Chemistry, Vol. 98,No. 15, 1994 4085
Figure 3. Transmission electron micrograph of y-Mo2N nanoparticles.
The freshly passivated Mo2N sample was placed in deionized water and ultrasonically dispersed for 15 min. The black solids dispersed to form an opaque black suspensionwhich did not settle over a period of days. In a period of a week, the suspension turned a deep blue, with some undispersed material remaining at the bottom. A portion of the blue solution was removed after 168 h of settling and dried for analysis by X-ray diffraction. Only peaks due to Mo2N were observed (Figure 2) and Scherrer analysis of peak widths indicates little change in the size of the crystalline domains: the nanoparticles extend 8 nm in the [200] direction and 9 nm in the [ 1111 direction (Table 1). The specific surface area of the dried suspension was 121 m2/g, somewhat higher than that of the porous starting material. TEM analysis of th dried blue solution (Figure 3) shows the presence of fine particles ranging in size from 4 to 15 nm. The particles are rounded, like those observed by STM in the porous, topotacticMo2N starting material. Small clumpsof nanoparticles are also observed. It is not apparent from the TEM data if these consist of free nanoparticles bound by weak electrostatic forces or if these are fragments of the parent porous crystal consisting of solid nanoparticlesbound in crystallographicalignment. Lattic fringes are observed in one particle in Figure 3 which indicate the relatively large particle is polycrystalline. Fringes are not observed in the smaller particles. TEM analysis of undipsersed topotacticMo2N (Figure 4) shows the sharp edges and preferential cleavage planes of the parent Moo3 crystal as well as granularity on a scale of several nanometers. The zigzag structure of the solid is also interesting: similar structures were reported in early Moo3 reduction work and were described as “herringbone”or “parquet floor” patterns which nucleate and grow during reduction and can increase the porosity and surface area of the solid.46-49 However, the very high surfaceareas of topotactic Mo2N require much finer features (from S, = 6/pd,,) on the scale of nanometers.
Discussion Ultrasonic dispersionof high surface area topotactic materials is a convenient route for the large-scale synthesis of metallic nanoparticles. The topotactic Mo2N starting material used in this work has been synthesized on the 100-g scale by reaction of N2/H2 mixtures and powdered M 0 0 3 . ~Large-scale ~ reactions employing NH3 as the nitriding/reducing agent were not successful due to endothermicNH3 decomposition reactionswhich produce bed temperature gradients and interfere with the temperature program.50 Other workers have used temperatureprogrammed reaction methods to synthesize topotactic molybdenum carbides, borides, and In addition, all the topotactic tungsten analogs of the molybdenum compounds as well as some topotactic vanadium analog compounds have been
Figure 4. Transmission electron micrograph of undispersed topotactic y-MqN crystalliteshowing fine granularcontrast effects and preferential cleavage planes in the crystallite.
reported.51 Building on these known syntheses, the range of potential nanoparticle chemistries which can be accessed using topotactic reaction/ultrasonic dispersion methods is large. Particle sizes observed by STM, TEM, X-ray diffraction, and surfacearea measurementsvaried. STM is the most direct method to measure the particle size and shape in the starting material and indicates the crystallites are composed of rounded, interconnected particles ranging from 7 to 15 nm in diameter. Yet, X-ray diffraction line widths indicate the average crystal diameter in the starting material is between 7 and 9 nm while surface area measurements predict even smaller average particle sizes. The smallvalue predicted from X-ray line widths is due to the presence of the amorphous oxide surface phase which decreases the size of the coherently diffractingcrystallinedomains as well as possible near-surface disorder in the Mo2N nanocrystallite core. The small particle size predicted from surface area measurements is at least partially due to the rough, nonspherical nature of the particles (the equation to predict particle size assumesthe particles are smooth spheres). It is also possible that multilayer adsorption in the narrow regions between particles may result in higher than expected surface areas. It is also possible that the assumed absolute surface area for adsorption of a nitrogen molecule (16.2 A2) is incorrect. Dispersed nanoparticle sizes determined by TEM were smaller than the diameters measured in the crystallite by STM. This observation is corroborated by the increased specificsurface area of the nanoparticles relative to the topotactic crystallite. The increasein specificsurface area is partly due to exposureof surface during the dispersion process. It is likely that some increase is also the result of the removal of large, low surface area impurities during solution settling. Both of these effects are expected to contributeto the increasein specificsurfacearea during dispersion. The deep-blue solution produced by aging the nanoparticle solution for a period of a week is probably due to the oxide layer coating the suspended Mo2N nanoparticles in solution, although additional analyses are required to establish the composition of all molybdenum compounds in solution. X-ray diffraction and TEM analysesclearly show the presenceof Mo2N nanocrystallites in solution. It is also possible that Mo2N nanoparticles dissolve or corrode in solution to form molybdateions. PreviousworkersS2 observed the formation of deep-blue solutions by the reduction of molybdate(V1) ions (clear solutions) or Moo3in solution. The reduction product was reported to be a mixtureof oxide/hydroxide species of mixed valence. On the basis of the available evidence,
4086 The Journal of Physical Chemistry, Vol. 98, No. 15, 1994 we conclude that our blue solutions consist of MozN crystallites in aqueous suspension (as shown by XRD and TEM) coated by a reduced surface molybdenum oxide or hydroxide phase. The particle sizeof the topotacticstarting material is determined by a number of factors including temperature ramping rate, gas spacevelocity,and the presence of gas-phase H20.These factors can be used to control the size of the nanoparticle product. Undispersed Mo2N particle sizes from 3 to 100 nm have been reported.36 We have shown the particle size is affected by the ultrasonic disperion process. Particle size reduction procedures based on ultrasonic milling or chemical corrosion are envisioned. We have recently synthesized4'topotactic MozN in macroscopic single-crystal form in addition to the powder and nanoparticle form. Like the powder, this material is an aggregate of interconnected Mo2N nanocrystallites in crystallographic alignment separated by internal pores. The crystals are 1-2 cm long and can be produced routinely in 30-g batches. Thus, Mo2N nanophase materials have been synthesized in sizes ranging from
