Mesoporous VN prepared by solid-solid phase separation

Article abstract of DOI:10.1016/j.jssc.2012.09.012

We recently reported a simple route to prepare mesoporous, conducting nitrides from Zn containing ternary transition metal oxides. Those materials result 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. In this report, we present a different route to prepare mesoporous VN from K containing vanadium oxides. In this case, ammonolysis results in a multiphase solid product that contains VN, and other water soluble compounds such as KOH or KNH₂. On removing the K containing products by washing with degassed water, only mesoporous VN remains. VN materials with different pore sizes (10 nm-20 nm) were synthesized at 600 °C by varying the reaction time, while larger pores are obtained at higher temperatures (50 nm at 800 °C).

Full text of DOI:10.1016/j.jssc.2012.09.012

Journal of Solid State Chemistry 197 (2013) 398–401
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Mesoporous VN prepared by solid–solid phase separation
n
Minghui Yang a,n, Walter T. Ralston a, Franck Tessier b, Amy J. Allen a, Francis J. DiSalvo a,
a Department of Chemistry, Cornell University, Ithaca, New York 14853-1301, USA
b UMR CNRS 6226 ‘‘Sciences Chimiques de Rennes’’, Equipe ‘‘Verres et Ce´ramiques’’, Universite´ de Rennes 1, F-35042 Rennes cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
We recently reported a simple route to prepare mesoporous, conducting nitrides from Zn containing
ternary transition metal oxides. Those materials result 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. In this report,
we present a different route to prepare mesoporous VN from K containing vanadium oxides. In this
case, ammonolysis results in a multiphase solid product that contains VN, and other water soluble
compounds such as KOH or KNH2. On removing the K containing products by washing with degassed
water, only mesoporous VN remains. VN materials with different pore sizes (10 nm–20 nm) were
synthesized at 600 1C by varying the reaction time, while larger pores are obtained at higher
temperatures (50 nm at 800 1C).
Received 13 July 2012
Received in revised form
7 September 2012
Accepted 9 September 2012
Available online 17 September 2012
Keywords:
VN
Mesoporous
Metal nitride
Ammonolysis
& 2012 Elsevier Inc. All rights reserved.
1. Introduction
vanadium nitride was synthesized by ammonolysis of potassium
containing vanadium oxides to create nano-porous VN with high
While many methods of preparing metal nitrides have been
reported [1–5], laboratory scale samples (from a few milligrams
to perhaps 10 g) are often synthesized by flowing ammonia over
the respective metal oxide or other metal precursors (both
coordination compounds and organometallics). The objective of
the above syntheses is essentially always to prepare small single
crystals or polycrystalline materials with large grains (on the
order of microns). Occasionally the object is to deposit or prepare
dense thin films [6,7]. In either case the materials generally have
few pores or voids and have a low specific surface area. There are
many advantages in preparing nano- or meso-structured materi-
als in that the relevant diffusion lengths are very short, signifi-
cantly decreasing reaction times. However, very few synthetic
approaches have been reported in nano-structured VN, including
temperature-programmed ammonia reduction of V2O5 [8] and a
two-step ammonolysis reaction of VCl4 in anhydrous chloroform
[9]. However, there are no reports of porous nitrides
with characteristic pore diameters in the mesoscale regime
(10–20 nm). Previously, we reported a synthesis route of metal
(oxy)nitride nanomaterials, based on the syntheses of VN, TiN,
CrN, Ta3N5, TaN, NbN, WN, MoN and Mo2N mesoporous materials
via ammonolysis reaction of Zn containing metal oxide (ZMO)
precursors at temperatures 4600 1C. In this investigation,
surface areas and good electrical conductivity.
2. Methods
KVO3 was prepared by a solid state reaction of the stoichio-
metric mixture of powders K2CO3 (99.6%, Fisher Scientific) and
V2O5 (99.6%, Alfa Aesar). The powders were ground together and
heated at 750 1C for 5 h. K2V8O21 was prepared by a solid state
reaction between K2CO3 (99.6% Fisher Scientific) and V2O5 (99.6%
Alfa Aesar) in a 1:4 M ratio. The powders were ground together
and heated at 450 1C for 50 h.
Few grams of these oxides were placed in an alumina boat. The
boat was then placed in a stainless steel tube with air tight
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 stainless
steel 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 1C/h. After
treatment for a specified period, the furnace power was turned
off and the product cooled to room temperature in ꢀ4 h under an
ammonia flow. Before the stainless steel tube was taken out of the
split tube furnace, argon gas was purged the tube to expel the
ammonia gas. The stainless steel tube was left in lab for 24 h with
n Corresponding authors.
E-mail addresses: m.yang@cornell.edu (M. Yang),
fjd3@cornell.edu (F.J. DiSalvo).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jssc.2012.09.012

M. Yang et al. / Journal of Solid State Chemistry 197 (2013) 398–401
399
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 CuK radiation (K ,
a1
a
˚
˚
l
¼1.5406 A and K
, l¼1.5444 A). Crystal structures of the
a2
oxides and resultant nitrides were confirmed by PXRD profiles
using the GSAS package [10]. The nitride crystalline domain size
can be estimated from Lorentzian function from the Rietveld fit as
discussed in the Zn paper [11]. Scanning electron microscopy
(SEM) and energy dispersive X-ray analysis (EDX) were per-
formed with a LEO-1550 field emission SEM (FSEM). Nitrogen
adsorption/desorption isotherms were measured at ꢁ196 1C
using a Micromeritics ASAP 2020 system. The samples were
degassed at 200 1C for 24 h on a vacuum line.
Elemental analyses of nitrogen and oxygen content of nitride
samples were determined with a LECO TC-600 analyzer using the
inert gas fusion method. Nitrogen was detected as N2 by thermal
conductivity and oxygen as CO2 by infrared detection. The
apparatus was calibrated using Lecos standard oxides, and
Si2N2O and
e-TaN as nitrogen standards [12]. A four point
measurement of conductivity of compressed powders a relatively
low pressure of 35 bar was used to estimate the conductivity.
3. Results and discussion
3.1. Ammonolysis of potassium vanadates
KVO3 and K2V8O21 were synthesized by standard ceramic
syntheses, and their purity was confirmed by powder X-ray
diffraction (PXRD) as shown in Fig. 1. The patterns matched those
expected PDF files of 00-033-1052 and 04-011-9621 for KVO3 and
K2V8O21, respectively. These precursor oxides were prepared at
high enough temperatures (750 and 450 1C) to produce large
grain sizes, mostly in the 1–20 mm range. Ammonolysis of these
oxides was carried out for a range of temperatures between 500
and 800 1C for 8 h at an ammonia flow rate of 200 cm3 minꢁ1
through a 1 in. diameter stainless steel flow tube, as summarized
in Table 1. The products were handled in an argon filled glove box,
to avoid exposure to air. The products were then studied by
powder X-ray diffraction using a mylar covered sample holder to
prevent exposure to air during that measurement. After these
initial X-ray studies, the products were then exposed to air and
further processed and characterized.
Above 500 1C and with increasing time or temperature, PXRD
showed the formation of VN with the expected rocksalt structure.
The refined cell parameters were determined by Rietveld refine-
ment of the PXRD patterns. The VN products from ammonolysis at
600–800 1C show relatively broad diffraction peaks due to the small
crystalline domain sizes or the nitride product (10–60 nm). Ammo-
nolysis of KVO3 below 700 1C produces some unknown secondary
phase(s). Ammonolysis products of K2V8O21 formed in the tem-
perature range of 600–800 1C also show similar unknown by-
product phase(s). After washing the products in water, the PXRD
shows only single phase VN as shown by the representative PXRD
pattern of the ammonolysis product of KVO3 at 600 1C in Fig. 1.
The mass loss due to the sublimation of potassium compounds
and the replacement of O by N was also determined. For example,
the ammonolysis of 0.1 g of KVO3 at 600, 700 and 800 1C for 8 h,
gives approx. a 20% weight loss. However, the expected theore-
tical mass loss, (loss¼(mass KVO3ꢁmass VN)/mass KVO3) should
be 52%. These results suggest that ammonolysis results in a
multiphase solid product that contains VN, and water soluble
potassium compounds such as KOH or KNH2. Since some mass
loss is observed, some of the by-products are apparently volatile.
Fig. 1. PXRD patterns illustrating the potassium vanadates and the ammonolysis
product of KVO3 for 8 h at 600 1C with ammonia flow (200 cm3 minꢁ1) washed
with H2O.
Table 1
Summary of ammonolysis conditions, refined lattice parameters (a) and calculated
domain size of pure VN: all reactions are 8 h at an ammonia flow rate of
200 cm3 minꢁ1. VN crystallized in space group Fm-3m.
Precursor
KVO3
T/1C
XRD phase
Domain size/nm
˚
a/A
600
700
800
VN, unknown
VN, unknown
VN, unknown
4.1231(3)
4.1262(1)
4.1341(1)
10
26
46
K2V8O21
600
700
800
VN, KVO3, other
VN, K2V8O6
4.1245(2)
4.1290(2)
4.1345(1)
14
35
52
VN, unknown
This could include the formation of some water vapor or partial
sublimation of potassium compounds. After removing the K
containing by-products by washing with degassed water, only
mesoporous VN remains.
3.2. Mesoporous VN
Fig. 2 shows the representative SEM images of the oxide
precursors and their ammonolysis products. Fig. 2a shows the
surface morphology of KVO3 prior to ammonolysis treatment. The
ammonolysis product of KVO3 at 600 1C for 8 h is shown in
Fig. 2b. The surface of the grains is not as smooth as the surface
of starting oxide (Fig. 2a), but no mesoporosity is observed.

400
M. Yang et al. / Journal of Solid State Chemistry 197 (2013) 398–401
Fig. 2. SEM images of KVO3 and its ammonolysis products from 600 to 800 1C, all reactions were 8 h at temperatures with ammonia flow (200 cm3 minꢁ1): (a) KVO3,
(b) ammonolysis product of’KVO3 at 600 1C, (c) water washed ammonolysis product of KVO3 at 800 1C and (d) water washed ammonolysis product of K2V8O21 at 800 1C.
The PXRD refinement of the product obtained without air expo-
sure shows that VN was formed and that the average nitride
crystalline domain size is 10 nm. However PXRD shows the
presence of other unidentified solid products. The PXRD refine-
ment of the 700 1C product without air exposure shows that the
average VN crystalline domain size has grown to 26 nm.
After washing the products obtained at 800 1C with water,
mesoporous surface features are clearly visible. According to the
SEM images, the size of the pores range from 30 to 50 nm. The
PXRD refinement of the 800 1C product shows that the average VN
crystalline domain size is 46 nm. Not surprisingly, larger VN
grains and pore sizes are formed at higher temperatures and/or
longer reaction times. The ammonolysis products of K2V8O21 over
the same temperature range of 600–800 1C show very similar
pore morphologies and sizes as those observed in the products
obtained from KVO3. The washed products shown in Fig. 2c and d
are also very similar to the mesoporous VN products obtained
from the ammonolysis of Zn containing oxides. [11].
temperatures the formation of KOH or KNH2 from potassium oxide
is thermodynamically possible [17]. Other compounds such as mixed
hydroxides and amides or imides may also be produced. In any case,
these by-products are either solids or liquids at the synthesis
temperature and apparently sublime slowly if at all. Thus, in the
present case, the observed mesoporosity results from phase separa-
tion of the nitride from the other by-products. Apparently, the surface
energies and diffusion constants of the relevant phases allow such
separation to proceed at the mesoscale. As the reaction temperature
increases, the scale of phase separation coarsens and the grain size of
VN grows. This mechanism is similar to that seen in the formation of
porous Vycor, where spinodal decomposition of a borosilicate glass
forms two phases that are separated on the nanoscale [18]. The mole
fraction of V in KVO3 and K2V8O21 are 20.0% and 25.8%, respectively.
By comparing the SEM images (Fig. 2c and d) of ammonolysis
products of KVO3 and K2V8O21 at 800 1C, the VN from KVO3
(Fig. 2c) shows less open porous feature than from K2V8O21
(Fig. 2d), consistent with the higher V content of the latter compound.
The above SEM images show that the pores and particles both
have rounded surfaces (see Fig. 2c and d for example). Such
rounded surfaces are also found in dealloyed Ag–Au [13], while
striking aligned faceting was found in single crystal MnO films
produced by reduction of ZnMn2O4 in forming gas [14]. The shape
of the pores is expected to be a function of the surface and bulk
diffusion rates and the surface energies of different facets of the
product crystalline structure [15] and [16].
3.4. Chemical, surface area and conductivity properties
The ammonolysis products of single phase VN show o2 wt% of
oxygen. The BET surface area of a 325 mg sample prepared from
KVO3 at 600 1C was 54.470.1 m2/g. The average pore sizes range
from 10 to 20 nm. There is some microporosity (pore diameter
r2 nm) that accounts for 3.6 m2/g and a micropore volume of
1.2 ꢂ 10ꢁ3 cm3/g. As we discussed in the report of metal (oxy)
nitrides made from Zn contained oxides, the mesoporosity obtained
is a feature of the bulk material and not just porosity induced at the
surfaces of the original crystalline oxide grains [11]. The conductivity
of compressed powders of mesoporous VN at 35 bar is 76 S/cm (VN).
This is about two orders of magnitude lower than for the bulk VN,
3.3. Formation mechanisms of mesoporous VN
In contrast to the case of Zn in which Zn2þ is reduced to elemental
Zn and subsequently sublimes [11], ammonia is not able to reduce
Kþ to K metal. Thus the mechanism of formation of mesoporous VN
in the two cases is different. Under ammonia flow at high

M. Yang et al. / Journal of Solid State Chemistry 197 (2013) 398–401
401
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This work was supported by National Science Foundation through
grant DMR-0602526.
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