Metallic mesocrystal nanosheets of vanadium nitride for high-performance all-solid ... · 2014. 10....

Nano Res

1

Metallic mesocrystal nanosheets of vanadium nitride

for high-performance all-solid-state pseudocapacitor

Wentuan Bi1, Zhenpeng Hu

2, Xiaogang Li

1, Changzheng Wu

1(), Junchi Wu

1, Yubin Wu

1 and Yi Xie

1

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0612-y

http://www.thenanoresearch.com on October 16 2014

© Tsinghua University Press 2014

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Nano Research

DOI 10.1007/s12274-014-0612-y

TABLE OF CONTENTS (TOC)



Wentuan Bi1, Zhenpeng Hu2, Xiaogang Li1, Changzheng

Wu1*, Junchi Wu1, Yubin Wu1 and Yi Xie1

1Hefei National Laboratory for Physical Sciences at

Microscale and Collaborative Innovation Center of

Chemistry for Energy Materials, University of Science and

Technology of China, Hefei, Anhui 230026, P.R. China

2School of Physics, Nankai University, Tianjin, 300071,

P.R. China

Mesocrystal nanosheets of vanidium nitride (VN) are developed via a

confined-growth route from thermal-stable layered vanadium bronze

for the first time. VN mesocrystal nanosheets with facet-tunable

building blocks bring synergic advantages of unprecedentedly high

electrical conductivity and unique pseudocapacitive reactivity, giving a

superior specific volumetric capacitance in all-solid-state thin-film

supercapacitors.

Provide the authors’ website if possible.

http://staff.ustc.edu.cn/~czwu




2, Xiaogang Li

1, Changzheng Wu

1(), Junchi Wu

1, Yubin Wu

1 and Yi Xie

1

1 Hefei National Laboratory for Physical Sciences at Microscale and Collaborative Innovation Center of Chemistry for Energy

Materials, University of Science and Technology of China, Hefei, Anhui 230026, P.R.China 2 School of Physics, Nankai University, Tianjin, 300071, P.R.China

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

vanadium nitride,

mesocrystal nanosheets,

confined growth,

supercapacitors,

chemical and thermal

stability

ABSTRACT

Transition metal nitrides (TMNs) are of particular interest for synergic

advantages of superior electrical conductivity, excellent environmental

durability and high reactive selectivity, yet are difficult to achieve flexible

design and operation. Herein, mesocrystal nanosheets (MCNSs) of vanadium

nitride (VN) are synthesized via a confined-growth route from thermal-stable

layered vanadium bronze, representing the first two-dimensional (2D) metallic

mesocrystal in inorganic compounds. Benefiting from single-crystalline-like

long-range electronic connectivity, VN MCNSs deliver a leading electrical

conductivity of 1.44×105 S/m at room temperature among current 2D

nanosheets. Coupling with unique pseudocapaciance, VN MCNSs based

flexible supercapacitors afford a superior volumetric capacitance of 1937

mF/cm3. Nitrides MCNSs would achieve wide fascinating applications in

energy storage and conversion field when intrinsic high conductivity meets

active reactivity of inorganic lattices.

1 Introduction

Two-dimensional nanomaterials have attracted

tremendous attention for their microscopic

compressibility and macroscopic extensibility for

constructing energy storage and conversion devices

with ultrathinness [1-4]. Among them, electrically

conducting 2D nanomaterials, bringing intrinsic

capability of electron capture and migration, have

always been regarded as fundamental scientific

issues that determine the efficiency of energy storage

and conversion [5,6]. For 2D graphene with a single

layer of carbon atoms, the conduction band and

valence band cross at Dirac point, giving the

capability of symmetric conduction of electron and

hole. As a result, graphene has remarkably high

electron mobility even at room temperature and a

considerable electrical conductivity of ~104 S/m [7].

Nano Research

DOI (automatically inserted by the publisher)

Address correspondence to Changzheng Wu, [email protected]

Research Article

| www.editorialmanager.com/nare/default.asp

2 Nano Res.

However, pure carbon ingredient in graphene with

perfect honeycomb lattice makes it very difficult to

regulate intrinsic electrical properties especially for

bandgap opening or energy-band overlapping, and

also lacks in efficient reaction activity for rich

physicochemical phenomena [8]. Inorganic 2D

nanomaterials, benefiting from rich electronic

structures arising from alternative inorganic

ingredients, provide a promising platform to afford

diversified functionality while maintaining the

ultrahigh conductivity. However, available metallic

2D inorganic materials thus far remain limited to

transition metal dichalcogenides (TMDs), such as 1T

MoS2, VS2, TiS2, etc. These conducting TMDs

ultrathin nanosheets have chemically active sulfur

element and excellent mechanical flexibility, yet still

suffer from poor thermal or chemical stability under

harsh conditions of high temperature, strong acid or

base solution etc., representing a long-standing

challenge for widening applications of conducting

inorganic 2D nanomaterials [5,6,9].

Transition metal nitrides exhibit a unique

combination of high electron conductivity, excellent

mechanical property, chemical inertness and thermal

endurance, giving us the inspiration that 2D TMNs

would be a unique catalogue of conductive inorganic

nanomaterials [10-13]. However, TMNs usually

possess highly-symmetric rock-salt structure with

close-packing atoms, hampering available attempts

to access their 2D nanomaterials via either top-down

or bottom-up routes: the presence of strong covalent

or ionic bonds among every other atoms hinders the

possibility of exfoliation into 2D nanomaterials;

while highly symmetrical close-packed structure

lacks intrinsic driving forces towards 2D growth

[14-16]. To realize the anisotropic growth of TMNs,

confined growth would open a new window of

opportunity [17,18]. Whereas the reaction

temperature towards phase formation of TMNs is

usually very high (>600oC), and thus the choice of

thermal-stable template becomes the prerequisite.

Conventional layered templates such as oxides or

hydroxides always suffer from serious morphology

changes during the high-temperature annealing

process due to the emergence of melting or gas

escaping. Interestingly, with alkali metal ions

intercalated in the interlayer, layered oxysalts (AMOx,

A=Li+, Na+, K+) demonstrate superior

high-temperature durability, providing new

opportunities for the realization of 2D TMNs

nanosheets.

Herein we highlight a new structural configuration

of vanadium nitrides — mesocrystal nanosheets,

which were synthesized via a controlled

confined-growth process from thermal-stable layered

vanadium bronze. VN MCNSs composed of

facet-tunable building blocks: nanooctahedrons and

nanocubes with exposure of (111) and (100) facets

respectively, can be well controlled through

minimizing thermodynamical energy. The long-range

electronic connectivity in single-crystalline-like VN

MCNSs endows an ultrahigh electrical conductivity

of 1.44×105 S/m at room temperature. Benefiting from

synergic advantages of superior electrical

conductivity and novel pesudocapacitance activity,

VN MCNSs with unique surface reactive activity

show promising signs for applications in energy

storage field.

2 Experimental

2.1 Preparation of Na2V6O16 nanosheets

In a typical experiment, 1.5 mmol Na3VO4•12H2O

and 5 mmol NH4F were mixed together with water

(25 mL) and ethylenediamine (15 mL) for 30 min. The

resulting homogeneous solution was then transferred

to a 45 mL Teflon-lined autoclave. The autoclave was

sealed and heated at 200 °C for 12 h before it was

cooled down to room temperature. The precipitate

was finally collected by centrifugation and washed

several times with distilled water.

2.2 Preparation of VN MCNSs

In a typical procedure, the obtained Na2V6O16

nanosheets were directly heated at different

temperatures in NH3 atmosphere and then cooled

down to room temperature. The obtained powders

were washed with distilled water to remove the

soluble by-products, and then collected for further

characterization.

2.3 Fabrication and characterization of flexible

supercapacitors in all-solid-state

A typical device includes the VN MCNSs working

electrode, the solid electrolyte, and vanadium

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

3 Nano Res.

pentoxide-reduced graphene oxide (V2O5-RGO)

composites counter electrode. VN MCNSs films were

assembled on cellulose membranes via vacuum

filtration and directly served as working electrodes

without using binders or external current collectors.

PVA/LiCl gel was prepared according to the method

reported in the literature [19] and used as the solid

electrolyte and separator. V2O5-RGO composites were

prepared according to the method reported in the

literature [20] and transferred onto the Au coated

PET sheet as the counter electrode. The whole device

was carefully sealed by the transparent tape and was

dried under ambient conditions until the PVA/LiCl

gel solidified. The area and thickness of the

fabricated devices were about 1 cm2 and 0.4 mm. The

performance of the assembled supercapacitors was

evaluated by cyclic voltammetries (CV) and

galvanostatic charge/discharge in a two-electrode

configuration using the potentiostat (CHI 660D).

2.4 Characterizations

The samples were characterized by X-ray powder

diffraction (XRD) on a Philips X'Pert Pro Super

diffractometer equipped with graphite-

monochromatized Cu-Kα radiation (λ=1.54178Å).

The field emission scanning electron microscopy

(FE-SEM) images were performed on a FEI Sirion-200.

Transmission electron microscopy (TEM) images

were taken on H-7650 (Hitachi, Japan) operated at an

acceleration voltage of 100 kV. High-resolution

transmission electron microscope (HRTEM) images

were operated on JEOL-2010 at a voltage of 200 kV.

Elemental mapping was carried out on a JEOL

JEM-ARF200F atomic resolution analytical

microscope. The electrical transport property

measurements were carried out using a four-point

probe method using Quantum Design Physical

Property Measurement System (PPMS)-9. Hall

coefficient was measured using a five-probe

technique on the (PPMS)-9.

Figure 1. (a) Schematic illustration of the synthetic strategy for VN MCNSs. TEM images of VN MCNSs fabricated at (b) low

temperature and (c) high temperature, and inset are illustrations of the assemble pattern and the corresponding ED taken from an

isolated nanosheet. (d) XRD patterns of the VN MCNSs with different building blocks.


4 Nano Res.

3 Results and discussion

3.1 Synthesis strategy for VN MCNSs

The confined growth strategy for VN MCNSs is

schematically illustrated in Fig. 1(a). Thermal-stable

layered vanadium bronze nanosheets serve as ideal

confined templates, and transform to VN under NH3

vapor treatment. Thanks to the supporting effect

brought by interlayer alkali metal ions, layered

vanadium bronze Na2V6O16 (Fig. S1 in the ESM) not

only endows high-temperature durability, but also

facilitates NH3 molecule diffusion, and consequently

ensure the maintaining of two-dimensional

morphology. Of note, conventional layered vanadium

oxide templates such as V2O5 and VO2 (B) suffered

from serious morphology destruction during the

annealing process due to the emergence of melting or

phase transition (Fig. S2).

The highly ordered mesocrystal nanosheets were

revealed by a series of morphology characterizations.

Transmission electron microscopy (TEM) images in

Fig. 1(b-c) clearly demonstrated the presence of

large-area freestanding nanosheets assembled by

regular building blocks with uniform orientation.

Scanning electron microscopy (SEM) images

provided a more intuitive description of the

morphology. As is shown in Fig. S3, the products

maintained the morphology of Na2V6O16 nanosheets

with the size of several micrometers. Moreover, from

the SEM morphological analysis, one can see that VN

MCNSs obtained at different temperatures were

made up with two kinds of building blocks:

nanooctahedrons and nanocubes, respectively. To

further investigate the phase and orientation

information of the building blocks, X-ray diffraction

(XRD) measurements were carried out. All the peaks

in XRD patterns in Fig. 1(d) could be indexed to VN

(JCPDS No. 35-0768) with a cubic structure, whereas

the intensity ratio of I(200)/I(111) was distinctly deviated

from the standard ratio (3/2) and the (110) peak

almost disappeared. In detail, when the reaction was

carried out at 600°C, the intensity of (111) peak was

obviously enhanced, indicating the preferred

orientation of (111) facets, while the situation

reversed at 700°C due to the suppressed intensity of

(111) peak. The obvious discrepancy of intensity ratio

was due to the different exposing facets of the

assembly units: nanooctahedrons with exposure of

(111) facets for samples at a lower temperature and

nanocubes exposing (100) facets at a higher

temperature. Intriguingly, theoretical calculations

shown in Fig. S5 revealed that the surface energies

associated with different crystallographic planes

were in the order of (110) >(100) >(111), indicating

that (111) face was the most stable one among the

three. In this sense, the formation process of VN

MCNSs was thermodynamically controllable and

thus the facets of VN building blocks can be tuned

just by changing the reaction temperature. The

single-crystalline nature of an isolated VN nanosheet

was disclosed by electron diffraction (ED) and

high-resolution transmission electron microscopy

(HRTEM). The well-defined diffraction spots in ED

pattern shown in Fig. 1 and continuous lattice fringes

around the pores in HRTEM image (Fig. S4)

provided solid evidences for its single-crystalline

nature. Meanwhile, the geometries of the ED patterns

revealed that VN MCNSs had the [111] and [100]

preferred orientation respectively, which was

consistent with the Wulff constructions mentioned

above. Combining with the XRD, ED and HRTEM

characterizations above, VN MCNSs with

facet-tunable building blocks have been successfully

synthesized via a facile and efficient confined growth

route.

3.2 Facet-tunable electrical conductivity

Surface engineering which manipulates the

distinctive atomic and electronic structures among

different facets endows nanomaterials with unique

physicochemical properties [21-22]. In our case,

orderly stacking of facet-tunable building blocks in

VN MCNSs then provides a feasible approach to

regulate their electronic structure and electrical

property [23]. As is shown in Fig. 2(a), the

temperature-dependent electrical resistivity

increased approximately linearly with

temperature over a wide temperature range,

indicating that VN MCNSs preserved the intrinsic

metallic conductivity properties [24-27]. Particularly,

the samples with exposure of (100) face exhibited an

ultrahigh conductivity of 1.44×105 S/m (6.95×10−6 Ω·m)

at room temperature, which was superior to the

hydrogen-incorporated TiS2 films (6.76×104 S/m) [6],

as well as graphene (5.5×104 S/m) [28], even

commercial ITO (1.9×104 S/m). More importantly, the


5 Nano Res.

electrical conductivity indeed exhibited a significant

face-dependent characteristic. VN MCNSs with (100)

face exposing exhibited approximately 5-fold

increase in electrical conductivity with respect to the

samples with exposure of (111) face. Moreover, Hall

coefficient and carrier concentrations measurements

in Fig. 2(b) further confirmed this face-dependent

discrepancy. The carrier concentration of VN MCNSs

with exposure of (100) face was about one order of

magnitude higher than that of the samples

exposing (111) face. The transport measurements

above confirmed that engineering the facets of VN

building blocks provide a feasible approach to

regulate the electrical properties of VN MCNSs.

To give a more comprehensive understanding of

the facet-dependent electrical conductivity of VN

MCNSs, we carried out density functional theory

(DFT) simulations to study the conductivity of the

nano-system along [100] and [111] direction.

Conventional cell and super-thin nanowire models

are selected as two limits to disclose the discrepancy.

Based on the theory of quantum transport [29], the

conductivity (σ) is proportional to the modes for

conducting (M) and the transmission (T), σ∝M•T. In

our case, the M is relative to the number of bands

across the Fermi level. For the conventional cell limit,

there are 10 bands across the Fermi level for [100]

direction model (Fig. S6(c)), but only 3 for [111]

direction model (Fig. S6(d)). For the super-thin

nanowire limit, the numbers are still 10 for [100]

direction model (Fig. 3(c)) and 1 for [111] direction

model (Fig.3(a)). Therefore, M100/M111 should be in a

range from 10/3 to 10/1. Meanwhile, taking the

scattering of lattice into account, the possibility of

scattering of electron for the [111] direction model in

real space is as twice as that of the [100] direction

model. And the dispersion ratios of the bands

become flat in Fig. 3(a) significantly, which also

indicated the transmission for [111] direction should

be smaller than [100] direction. Therefore, T100/T111

should be 2 or more based on the scattering of lattice.

The ratio of conductivities between [100] and [111]

Figure 2. Facet tunable electrical properties of VN MCNSs.

Temperature dependent (a) electrical resistivity, (b) carrier

concentration and Hall coefficient of (111) and (100) faced

samples.

Figure 3. Band structure along Kz in first Brillouin zone for a

super-thin VN nanowire model with (a) [111] and (c) [100] as

periodic direction; blue lines mark the bands come across the

Fermi level (red dashed-line). The local density of states for the

super-thin models near the Fermi-level with (b) [111] and (d)

[100] as periodic direction.


6 Nano Res.

thus could be estimated as 111

001001

111

001

111TM

TM

, which is in

a range from 3.3 to 20, qualitatively. To have a more

direct view, LDOS for the models near the

Fermi-level are also plotted in Fig. 3 and Fig. S6. It is

clear that the LDOS of [100] nanowire model keep a

well ordered structure as well as that in the bulk VN

material, which is in good agreement with the wide

dispersion ratio of bands and higher conductivity

observed. However, the LDOS of [111] nanowire

model show a more disordered structure, which

results in a narrow dispersion ratio of bands and

lower conductivity. Theoretical simulations above

agree well with the experimental transport

measurements, confirming the strong correlation

between crystal facets and electrical conductivity.

3.3 Excellent environmental durability

Besides affording superior electrical conductivity,

transition-metal nitrides are of particular interest for

their excellent environmental durability. Here stable

two-dimensional VN MCNSs would guarantee

electron transport under harsh environment. To

demonstrate their chemical and thermal stability, we

fabricated electrodes with isolated VN nanosheet and

tested their performance under different harsh

environments. As illustrated in Fig. 4(a), the electrical

resistivity showed less than one time changes in the

temperature range from room temperature to 300°C.

In addition, the thermal gravimetric analysis (TGA)

shown in Fig. S7 confirmed this conclusion, as there

was no significant weight loss or gain both in N2 and

air atmosphere in the same temperature range.

Furthermore, the aforementioned electrode was

connected to a circuit with LED lights. After the

electrode was put into the combustion flame for 30

seconds, spatial temperature distribution of the

device was mapped by a thermal infrared camera,

which was shown in Fig. 4(d). During the whole

process, the LED lights remained work well, once

again verifying the good endurance of VN MCNSs

against high temperature. Chemical corrosion is

another inevitable issue that is detrimental to

nanodevice applications. In this regard, the resistance

of these electrodes was measured before and after

short-term exposure to acid or base solution with

different concentrations. Remarkably, these

electrodes demonstrated approximately reversible

resistance changes in the pH ranging from 3 to 11.

Without doubt, the highly conductive VN MCNSs

are insensitive to the harsh environment. As such,

they stand out as an excellent candidate for the

fabrication of technologically important electrodes

for integrated devices.

3.4 Electrochemical performance

Synergic advantages of superior electrical

conductivity and excellent environmental durability,

coupling with a series of reversible redox reactions in

the outmost few atomic layers [30-31], enable VN to

be a pseudocapacitance material with high

performance. As a proof of concept, we

demonstrated the construction of flexible

all-solid-state thin-film supercapacitors (ATFSs)

using the designed VN MCNSs electrodes. The

configuration of the as-fabricated supercapacitors

was illustrated in Fig. 5(a), in which V2O5-RGO

composites were used as the counter electrode.

Notably, highly conductive VN MCNSs films

assembled on cellulose membranes via vacuum

filtration can directly serve as electrodes without

Figure 4. Electrical measurements of the VN MCNSs electrodes

under different harsh environments. (a) Thermal stability; (b)

chemical stability, the resistance ratio of the electrode is defined

as (R-R0)/R0; (c) digital photograph of the thermal durability

testing setup and (d) spatial temperature distribution of the

electrode captured by a thermal infrared camera, (e) schematic

illustration of the device.


7 Nano Res.

using binders or external current collectors. Fig. 5

shows the electrochemical performance of our ATFSs.

As expected, cyclic voltammetries (CV) curves

exhibited obviously facet-dependent electrochemical

characteristics. VN MCNSs with exposure of (100)

facets showed a substantially larger integral area,

indicating a higher specific capacitance. The superior

performance of (100) faced VN MCNSs based ATFSs

was further confirmed by galvanostatic

charge–discharge technique. As shown in Fig. 5(d),

the specific volumetric capacitance calculated using

the galvanostatic discharge curve was 1937 mF/cm3 at

a current density of 5 A/m2, outperforming the (111)

faced VN MCNSs based ATFSs (1271 mF/cm3), as

well as the previously reported VN based ATFSs and

most of other ATFSs [19,20,32,33]. The superior

performance of (100) faced VN MCNSs based ATFSs

can be attributed to the synergistic advantages of

superior conductivity and highly open

nanostructures in (100) faced VN MCNSs, which

offer the maximal utilization of the active materials

and facilitate the electron transport and electrolyte

diffusion.

4 Conclusions In summary, VN mesocrystal nanosheets were

developed via a confined-growth route from

thermal-stable layered vanadium bronze for the first

time. Single-crystalline-like VN MCNSs with

facet-tunable building blocks accomplished an

ultrahigh electrical conductivity of 1.44×105 S/m at

room temperature. Coupling with unique

pseudocapacitive activity, conducting VN MCNSs

based all-solid-state thin-film supercapacitors

brought a superior specific volumetric capacitance of

1937 mF/cm3. We anticipate that confined growth

from thermal-stable layered bronze will be an

effective route towards 2D non-layered inorganic

solids, and definitely broaden the scope of

dimension-confined functional nanomaterials.

Acknowledgements

This work was financially supported by the National

Natural Science Foundation of China (no. 21222101,

11132009, 21331005, 11321503, J1030412), Chinese

Academy of Science (XDB01010300), the Fok

Ying-Tong Education Foundation, China (Grant

No.141042) and the Fundamental Research Funds for

the Central Universities (no. WK2060190027 and

WK2310000024).

Electronic Supplementary Material: Supplementary

material is available in the online version of this

article at http://dx.doi.org/10.1007/s12274-***-****-*

(automatically inserted by the publisher).

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Nano Res.

Electronic Supplementary Material




2, Xiaogang Li

1, Changzheng Wu

1(), Junchi Wu

1, Yubin Wu

1 and Yi Xie

1

1Hefei National Laboratory for Physical Sciences at Microscale and Collaborative Innovation Center of Chemistry for Energy Materials,

University of Science and Technology of China, Hefei, Anhui 230026, P.R. China 2School of Physics, Nankai University, Tianjin, 300071, P.R. China

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

S1. Calculation details of the surface energy

As layered template Na2V6O16 was used to guide the confined growth of two-dimension material, the growth

process of VN was in a mode that the density of V on an area was constant. In this mode, the total energy per

unit area with same amount of VN would be a good parameter for evaluating the order of stability for the low

index surfaces. In the DFT calculations, the nine-stoichiometry-layer (V9N9) slabs were used for modeling the

nano-particle with (100), (110), and (111) surface termination. And the energy per unit area of these models

were in order of (110)>(100)>(111) [-5.04>-10.19>-11.66 (eV/Å 2)].

S2. Calculation details of the electrical conductivity

The first-principles calculations were performed using density functional theory (DFT) implemented in the

code VASP package [1,2]. Projector Augmented Wave (PAW) pseudopotential [3], Perdew-Burke-Ernzerhof

(PBE) exchange-correlation functional [4] and a 400 eV energy cut-off plane wave basis set were selected to

perform the calculations. As shown in Figure S4a-b, the conventional cells with [100] and [111] as z-axis (c-axis)

were used to consider a limit of the nano-system (bulk material). For each cell, there were 24 atoms (12V and

12N). By adding vacuum in the x-y plane and elongating a-axis and b-axis to 20 Å , the other limit of

nano-system (super-thin nanowire) could be modeled. The conductivity should be in a range determined by

those two limit cases. For the real nano-system, the conductivity is dominated by the quasi-periodic direction,

which is along z-axis in the model. Therefore, the band structures along Kz in the first Brillouin zone were

calculated. Meanwhile, the local density of states (LDOS) for the models in a range from -0.05 eV to +0.05 eV

respected to the Fermi-level was plotted.

Address correspondence to Changzheng Wu, [email protected]


Nano Res.

S3. XRD pattern of Na2V6O16 nanosheets

Figure S1. XRD pattern of the Na2V6O16 nanosheets, indicates a layered product with observably preferred

orientation.

S4. Morphology evolution of different precursors before and after annealing

Figure S2. Morphology evolution of different precursors before and after annealing. (a,d) VO2(B) nanobelts;

(b,e)V2O5 nanosheets; (c,f) Na2V6O16 nanosheets, respectively.


Nano Res.

S5. SEM, TEM and HRTEM images and element mapping for VN MCNSs

Figure S3. SEM of VN MCNSs fabricated (a-b) at high temperature and (c-d) low temperature.

Figure S4. TEM of VN MCNSs and the corresponding HRTEM and element mapping.


Nano Res.

S6. Comparison of the face-dependent surface energy of VN

Figure S5. (a) Theoretical models and (b) the surface energy of the different faces of VN in the confined growth

mode.

S7. Theoretical calculations for the face-dependent electrical conductivity of VN


Nano Res.

Figure S6. Theoretical simulations for the face-dependent electrical conductivity of VN MCNSs. The VN

conventional cell with (a) [100] and (b) [111] as z-axis, red for V, and blue for N; band structure along Kz in first

Brillouin zone for VN conventional cell with (c) [100] and (d) [111] as z-axis; the local density of states near the

Fermi-level for VN conventional cell with (e) [100] and (f) [111] as z-axis.

S8. Thermal stability of the VN MCNSs

Figure S7. TGA curves of the VN MCNSs under different atmospheres

S9. Capacitance mechanism of the VN MCNSs

According to previous work by P. N. Kumta[5], the impressive specific capacitance of VN nanocrystals arises from a

combination of electric double-layer capacitance and reversible redox reactions which occur on the surface of these

partially oxidized nitrides. In the present work, an equilibrium reaction may occur on the vanadium nitride surface as

follows:

VNxOy + OH

- VNxOy‖OH

- + VNxOy-OH

-

Where VNxOy‖OH- represents the electrical double layer formed by the OH- ions adsorbed on the surface of VN

mesocrystal nanosheets. Accordingly, VNxOy-OH- represents the reversible redox reactions occuring on the surface of

these partially oxidized VN.

Reference

[1] Kresse, G.; Hafner, J. Ab initio molecular dynamics for liquid metals. Phys. Rev. B 1993, 47, 558-561.

[2] Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors

using a plane-wave basis set. Comput. Mat. Sci. 1996, 6, 15-50.

[3] Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys.

Rev. B 1999, 59, 1758-1775.

[4] Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett.

1996, 77, 3865-3868.

[5] Choi, D.; Blomgren, G. E.; Kumta, P. N. Fast and reversible surface redox reaction in nanocrystalline vanadium

nitride supercapacitors. Adv. Mater. 2006, 18, 1178-1182.

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