COMMUNICATION
Please do not adjust margins
Please do not adjust margins
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Mechanochemical preparation of piezoelectric nanomaterials: BN,
MoS
2
and WS
2
2D materials and their glycine-cocrystals
Viviana Jehová González,
a
Antonio M. Rodríguez,*
a
Ismael Payo,
b
Ester Vázquez*
a,c
Different 2D-layered materials of transition metal dichalcogenides
(TMDCs) such as boron nitride (BN) or molybdenum disulphide
(MoS
2
) have been theorised to have piezoelectric behaviour. Still,
the procedures to obtain these nanomaterials, with the right
quality and quantity to observe the piezoelectric performance, are
enormously expensive, halting its possible applications. Here, we
show the mechanochemical exfoliation of 2D nanomaterials (FLG,
BN, MoS
2
and WS
2
) with glycine. We have also successfully
synthesised the cocrystals for these nanomaterials, which makes it
possible to enhance their piezoelectric responses.
Piezoelectric materials have a unique property that converts
mechanical energy into electrical energy or viceversa
1
. Barium
titanate is the first piezoelectric ceramic ever discovered, but
the ceramic lead zirconate titanate, also known as PZT, is the
most commonly used material for piezoelectric harvesting.
2
Nevertheless, the extremely fragile nature of PZT ceramic and
the incorporation of lead create issues such as the reliability,
durability, and safety of this material for long-term sustainable
operation.
2D materials and the possibility to modulate their composition
in a well-controlled manner offer a platform that allows the
creation of different heterostructures for a large variety of
applications. Starting with graphene, 2D nanomaterials have
grown to include insulator (boron nitride, BN), semiconductors
(molybdenum disulphide, MoS
2
) and metals (Niobium
diselenide, NbSe
2
).
3
Together with other different properties,
the theoretical piezoelectricity of single-atomic layers of boron
nitride (BN), molybdenum disulfide (MoS
2
) and tungsten
disulfide (WS
2
) as a function of strain-induced lattice distortion
and ionic charge polarisation has been studied.
4, 5
The future
perspective of these nanomaterials have been covered in the
literature.
6
Experimentally, some applications of this
nanomaterial behaviour have been explored in energy
conversion,
7
voltage generators,
8
pressure sensors,
9
nonlinear
energy harvesters,
10
and transducers.
11
The methodologies
currently used in the production of these nanomaterials for the
nano-electromechanical applications are mainly based on
chemical vapour deposition (CVD). This technique presents
some problems, such as the high cost and necessity to deposit
on other materials, which can lead to compatibility issues.
Additionally, in real-world applications, the environmental
impact of producing any device should always be considered
beforehand, and one fundamental problem is to scale up
experiments in a safe, secure and efficient way. In that sense,
mechanochemical exfoliation of 2D materials has gained
increasing importance in the last years.
12-15
These protocols
have many advantages over their liquid-phase counterparts,
including processes with shorter reaction times, higher product
yields and the elimination of (harmful) organic solvents, which
make the approach more sustainable and cheaper. Some
examples have seen molecules such as sucrose, urea and boric
acid used as exfoliating agents.
16-18
Nowadays, there are no
examples of the application of TMDCs nanomaterials in
piezoelectric paint, coatings or adhesive matrices which could
be easily applied to heterogeneous surfaces paving the way for
applications such as sensors,
19
or power sensors
20
and
nanosystems for harvesting energy applications.
21, 22
On the other hand, in the past 60 years, piezoelectricity has
been confirmed in a variety of biological materials, such as
fibrous proteins collagen,
23
elastin,
24
bone
25
(calcified collagen),
wood,
26
and some viruses
27
exhibit relatively modest
piezoelectricity (0.1–10 pm V
−1
). Classical piezoelectric
principles have also been applied to similar uniaxially
orientated, bioactive polymers, such as poly (L-lactic acid)
(PLLA), poly (γ-benzyl glutamate) (PBG), and cellulose.
28
The
only non-chiral amino acid, glycine, has been known to
crystallize in three distinct polymorphs (α)-alpha,
29
(β)-beta,
30
and (γ)-gamma glycine
31
under ambient conditions.
32
The
a.
Instituto Regional de Investigación Científica Aplicada (IRICA), UCLM, 13071
Ciudad Real, Spain.
b.
Escuela de Ingeniería Industrial y Aeroespacial de Toledo, UCLM, Avenida Carlos
III s/n, Real Fábrica de Armas, 45071, Toledo, Spain.
c.
Facultad de Ciencias y Tecnologías Químicas, UCLM, Avda. Camilo José Cela S/N,
13071, Ciudad Real, Spain.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
COMMUNICATION Journal Name
2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins
Please do not adjust margins
crystallization of α-glycine occurs in the centrosymmetric space
group P2
1
/c, which precludes piezoelectricity. On the other
hand, β-glycine and γ-glycine belong to the non-
centrosymmetric space groups P21 and P32, respectively, and
so should exhibit a non-zero piezoelectric response. A modest
‘effective' shear and longitudinal piezoelectricity have been
measured for β-glycine (6 pm V
−1
) and γ-glycine (10 pm V
−1
),
respectively,
33, 34
using piezo response force microscopy
(PFM).
35
In previous work, we have investigated the exfoliation
procedures of graphite to graphene using ball milling
techniques in the presence of carbohydrates.
36
We could also
prepare glucose-graphene cocrystals as biocompatible systems.
In this work, we have explored the exfoliation of 2D
nanomaterials using the amino acid glycine. In a second step,
the formation of glycine-nanomaterials cocrystals has proven to
enhance the piezoelectric properties of the exfoliated material.
The relative ease of production of these materials through our
mechano-chemical process would significantly impact its
presence in future applications. In this study, we proposed a
mechanochemical exfoliation of TMDCs and other 2D
nanomaterials, such as BN and FLG, and the study of its intrinsic
piezoelectricity. Furthermore, our objective aims to integrate
the TMDCs nanomaterials in supramolecular organic matrices,
such as cocrystals that would enhance their piezoelectricity.
Based on our previous experience on mechanochemical
exfoliation of graphite, we performed the ball-milling treatment
in solvent-free conditions adding glycine as the exfoliant agent
and graphite in a 250 mL stainless-steel grinding bowl with 15
stainless steel balls (2 cm diameter each) at a 250 rpm. The
detail experimental procedure is collected in the SI. Since, no
precipitate was observed in the resulting dispersions, they were
entirely lyophilised after the dialysis. The best experimental
conditions for obtaining graphene materials of two different
sizes and the yields are represented in table S1.
Fig. S1 displays the Raman spectra of FLG1 and FLG2, showing
the different characteristic bands present in carbon
nanomaterials (D, G and 2D).
37, 38
It is possible to observe the
I
D
/I
G
value between the different peaks in sample FLG1 is 0.39
in comparison with FLG2, 1.63. This data correlates with the
minor size of FLG2 flakes (Table S1, Fig. S2) which shows a direct
relation with the time of mechanochemical treatment.
Thermogravimetrical analysis (TGA) of these materials is
collected in Fig. S3. Our analysis showed a minor presence of
nitrogen attached on the graphene layer with a minor presence
of oxygen and organic groups on the surface of graphene (2%
loss in TGA). We can draw similar conclusions regarding the TGA
loss for both FLG1 and FLG2 nanomaterials as in our previous
works.
36
Based on these good results, the high-quality exfoliation with
very high yields and the smooth, sustainable and low-cost
procedure, we decided to extrapolate these experimental
procedures to the exfoliation of other 2D-layered materials
such as BN, MoS
2
and WS
2
. The experimental conditions for the
2D nanomaterial exfoliation are collected in Table S1. Powder
X-Ray Diffraction (PXRD) of the exfoliated materials and the raw
nanomaterials are all shown in Fig. S4. In all cases, the x-ray
diffraction patterns show a clear decrease in the number of
counts on the 002-diffraction pattern. For the example of BN,
which has the lowest reduction, it is known that intensity ratio
of (BN
raw
)
002
/ (BN
exfo
)
002
of approximately 2.5 already indicates
thin BN layers and much weaker stacking at the c-direction in
the exfoliated sample.
39, 40
Fig. S5 shows the Raman spectra of the MoS
2
system, although
both WS
2
and MoS
2
have similar patterns. Both nanomaterials
possess two primary Raman modes, one in-plane mode of Mo
or W-S bond (E
2g
) another out-of-plane mode (A
1g
) at around
380 and 405 cm
-1
(MoS
2
), and 350 and 415 cm
-1
(WS
2
).
41
It is
possible to observe a blueshift and a redshift in A
1g
mode for
MoS
2
and WS
2
respectively, which corresponds to a decrease in
the number of layers (Table S2). According to the diagrams of
Terrones et al. for WS
2
nanomaterials,
42
we have a relation of
I
E2g
/I
A1g
of 0.69 which corresponds to a value of 3 layers for our
WS
2
exfoliated nanomaterial. With respect to MoS
2
,
43
according
to the distance between the bands
𝐸
"#
$
and
%𝐴
$#
,
the average
number of layers is around 3. Finally, BN exhibits a characteristic
Raman peak for E2g phonon mode (B-N vibration mode) around
1365 cm
-1
‚
44
which is analogous to E2g mode (G band) in
graphene. Moreover, a slight blue shift in the E2g peak is
consistent with the exfoliation of BN.
45
TEM images (Fig. 1)
show the exfoliated dichalcogenide with the corresponding
distribution of lateral size in table S1. As shown in Fig. S6, the
Figure 1. TEM images and distributions of sizes for the different 2D nanomaterials samples.
Figure 2. Powder X-ray diffraction results for glycine and BN–glycine cocrystals.
Journal Name COMMUNICATION
This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins
Please do not adjust margins
TGA curves of exfoliated nanomaterials show a reduced weight
loss compared with those of the pristine 2D nanomaterial
because of the small residue of exfoliant agent. Also, a wide
scan XPS spectra has been included in the SI to rule out the
presence of other impurities (Fig. S7 and Table S3). The atomic
content (in %) corresponding to C, N, H and O also correlates
with the small quantities of exfoliant agent. Raw and exfoliated
materials are not expected to contain C and O. However around
10% of the undesired C and O content probably arises from CO
and CO
2
species in air, adsorbed on the substrates.
46
Nevertheless, further analysis of the samples has shown that
the sample BN
exfo
has a residue amount of glycine around 20%,
which also corresponds to our TGA analysis (in Fig. S6).
These results are similar to those observed in the literature.
47
Once demonstrated the exfoliation of 2D nanomaterials, we
studied the formation of glycine cocrystals following a similar
procedure of lyophilization (SI). The PXRD study for all the
different materials indicated that cocrystal structures differed
wildly from the original nanomaterials or the initial glycine
crystal structure (Fig. 2 for the BN). The appearance of some
new peaks (between 20 and 65º) correspond to the different
reflections of
a
,
b
, and
g
-glycine phases in the cocrystal sample
and other new peaks in that same region, which do not
correspond to any raw material. Those new peaks can be
attributed to new cocrystal structures. Similar results are
observed for other nanomaterials (Fig. S8). It seems that the
presence of the nanomaterial in dispersion together with the
crystallization of water while freezing, “pressed out” glycine
forcing the appearance of different polymorphisms. This is a
process known in the literature,
48, 49
and it correlates well with
our understanding on the important interactions between
water molecules, exfoliating agents and 2D materials.
50
The TGA
for the 2D nanomaterial cocrystals shows a similar loss to
glycine, which might be due to the high content of such
molecule. The 3wt% of difference between glycine and the
cocrystal measurement, corresponds to the presence of the
nanomaterial in the cocrystal structure.
The structure of glycine
cocrystals has been investigated, showing the presence of γ and
β-glycine. Commercial glycine was similarly grinded as
benchmark sample, resulting in β-glycine majority and with
similar piezoelectric response to initial glycine. A comparison of
the powder X-ray diffraction results of these samples can be
found in Fig. S9. Further study of the Raman spectra of 2D
nanomaterial glycine cocrystals pointed to modifications on the
vibrational mode frequencies of the intermolecular and
intramolecular bonds in the samples. This could correspond to
the appearance of new crystal forms and it also gives
information on the quality of the exfoliated nanomaterials (Fig.
S10).
Finally, preliminary studies of the piezoelectricity of the
exfoliated nanomaterials and the glycine cocrystal forms were
performed. Fig. 3 shows the experimental setup used for
dynamic testing of the piezoelectricity. Results were amplified
with an electronic circuit as shown in Fig. S11. The
nanomaterials were placed in a simple system, sandwiched
between electrodes of area 1 cm
2
, under a force of 10 N. This
experimental setup mimics those described in the literature for
the experimental corroboration of the piezoelectricity of
different materials in powder form.
51
Table 1. Comparison of piezoelectric response raw and exfoliated 2D
nanomaterials and its cocrystals. PZT has been used as a model
piezoelectric material.
Sample
Piezoelectric response
(mV·N
-1
)
PZT
16
Polyvinylidene fluoride (PVDF)
48
Glycine (Gly)
12
Gly
grinded
15
Gly
lyophilised
36
BN
raw
8
BN
exfo
48
BN- Gly cocrystal
64
BN
exfo
+ Gly-mix
37
WS
2 raw
--
a
WS
2 exfo
--
b
WS
2 –
Gly cocrystal
88
WS
2 exfo
+ Gly-mix
60
Graphite
--
a
FLG
exfo
--
a
FLG- Gly cocrystal
95
FLG
exfo
+ Gly-mix
60
MoS
2 raw
--
b
MoS
2 exfo
--
b
MoS
2
- Gly
cocrystal
150
MoS
2 exfo
+ Gly-mix
78
a
These nanomaterials can’t be properly measured because its relatively
high conductivity, it short-circuited the electronic.
b
Given the
semiconductor behaviour of MoS
2
, the materials could be working as
super-capacitor in the measurement. Attached the figures in the
supplementary material (SI).
The piezoelectricity behaviour is better in the cocrystal form
than in the exfoliated material or with unpolarized PZT powders
(table 1, Fig. S12 and Fig S13) or other organic piezoelectric
materials. We obtained similar results for all nanomaterials in
their cocrystal form, with a maximum open-circuit voltage of
150 mV·N
-1
for the MoS
2
-Gly
cocrystal
. Similar results could be
Figure 3. a) Experimental set up for piezoelectricity measurements. b) Layer of
cocrystals on a square copper electrode (10 mm × 10 mm), insulated with paper.
c). Manual compression of a
2D nanomaterial
crystal layer. d) Piezoelectric
response of exfoliated
BN
nanomaterials. e) Measured piezoelectric response
with exfoliated
BN cocrystals.
COMMUNICATION Journal Name
4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins
Please do not adjust margins
observed with the MoS
2
and WS
2
at the same range of induced
strain (Table 1). For comparison purposes, we also mixed
thoroughly the samples of the exfoliated samples and the
glycine cocrystal separately (Table 1 samples: 2D nanomaterial
+
Gly
mix
), but it showed less piezoelectric response.
Also, both BN and WS
2
cocrystals showed outstanding
responsiveness under lower ranges of forces (around 1N) and
produced good recovery cycles and maintained in time (Fig.
S14). The remarkable piezoelectric character of these
nanomaterials has all been measured without polarisation,
while the standard procedure uses polarized materials for this
sort of measurements.
Conclusions
We describe an easy and scalable method to enhance the
piezoelectric responses of 2D nanomaterials. The process
includes the preparation of glycine- 2D cocrystals in which the
proportion of different polymorphisms of glycine is readily
changed. These powder samples, with different 2D materials,
can be used in the development of matrices with piezoelectric
character. These materials could be imbedded on paints or inks
to cover large surfaces, either in mobile devices, tablets,
keyboard, with improved piezoelectric properties. Future uses
in sensors, power sensors or in harvesting energy applications
can be predicted.
Conflicts of interest
There are no conflicts to declare.
Notes and references
1. K. Uchino, in Advanced Piezoelectric Materials, 2017, DOI:
10.1016/b978-0-08-102135-4.00001-1, pp. 1-92.
2. D. Damjanovic, Reports on Progress in Physics, 1998, 61,
1267-1324.
3. A. K. Geim and I. V. Grigorieva, Nature, 2013, 499, 419-
425.
4. K. A. N. Duerloo, M. T. Ong and E. J. Reed, J. Phys. Chem.
Lett., 2012, 3, 2871-2876.
5. A. A. M. Noor, H. J. Kim and Y. H. Shin, Phys. Chem. Chem.
Phys., 2014, 16, 6575-6582.
6. M. Samadi, N. Sarikhani, M. Zirak, H. Zhang, H.-L. Zhang
and A. Z. Moshfegh, Nanoscale Horizons, 2018, 3, 90-204.
7. W. Wu, L. Wang, Y. Li, F. Zhang, L. Lin, S. Niu, D. Chenet, X.
Zhang, Y. Hao, T. F. Heinz, J. Hone and Z. L. Wang, Nature,
2014, 514, 470-474.
8. S. Yu, K. Eshun, H. Zhu and Q. Li, Sci Rep, 2015, 5, 12854.
9. S. Wagner, C. Yim, N. McEvoy, S. Kataria, V. Yokaribas, A.
Kuc, S. Pindl, C. P. Fritzen, T. Heine, G. S. Duesberg and M.
C. Lemme, Nano Lett, 2018, 18, 3738-3745.
10. M. Lopez-Suarez, M. Pruneda, G. Abadal and R. Rurali,
Nanotechnology, 2014, 25, 175401.
11. F. Zhao, Y. Yao, X. Li, L. Lan, C. Jiang and J. Ping, Anal
Chem, 2018, 90, 11658-11664.
12. V. León, A. M. Rodríguez, P. Prieto, M. Prato and E.
Vazquez, ACS nano, 2014, 8, 563-571.
13. V. Leon, J. M. Gonzalez-Dominguez, J. L. Fierro, M. Prato
and E. Vazquez, Nanoscale, 2016, 8, 14548-14555.
14. M. Buzaglo, E. Ruse, I. Levy, R. Nadiv, G. Reuveni, M.
Shtein and O. Regev, Chemistry of Materials, 2017, 29,
9998-10006.
15. J. M. Gonzalez-Dominguez, V. Leon, M. I. Lucio, M. Prato
and E. Vazquez, Nat Protoc, 2018, 13, 495-506.
16. W. Lei, V. N. Mochalin, D. Liu, S. Qin, Y. Gogotsi and Y.
Chen, Nat Commun, 2015, 6, 8849.
17. S. Chen, R. Xu, J. Liu, X. Zou, L. Qiu, F. Kang, B. Liu and H.
M. Cheng, Adv Mater, 2019, 31, e1804810.
18. C. Cao, Y. Xue, Z. Liu, Z. Zhou, J. Ji, Q. Song, Q. Hu, Y. Fang
and C. Tang, 2D Materials, 2019, 6.
19. J. R. White, B. de Poumeyrol, J. M. Hale and R.
Stephenson, J. Mater. Science, 2004, 39, 3105-3114.
20. M. J. Ramsay and W. W. Clark, Smart Structures and
Materials 2001: Industrial and Commercial Applications of
Smart Structures Technologies, 2001, 4332, 429 – 438.
21. Z. L. Wang, Nano Res., 2008, 1, 1-8.
22. Z. L. Wang, Nano Today, 2010, 5, 512-514.
23. E. Fukada and I. Yasuda, Japan. J. Appl. Phys., 1964, 3,
117-121.
24. Y. Liu, H. L. Cai, M. Zelisko, Y. Wang, J. Sun, F. Yan, F. Ma,
P. Wang, Q. N. Chen, H. Zheng, X. Meng, P. Sharma, Y.
Zhang and J. Li, Proc Natl Acad Sci U S A, 2014, 111,
E2780-2786.
25. E. Fukada and I. Yasuda, J. Phys. Soc. Japan, 1957, 12,
1158-1162.
26. E. Fukada, J. Phys. Soc. Jap., 1955, 10, 149-154.
27. B. Y. Lee, J. Zhang, C. Zueger, W. J. Chung, S. Y. Yoo, E.
Wang, J. Meyer, R. Ramesh and S. W. Lee, Nat.
Nanotechnol., 2012, 7, 351-356.
28. E. Fukada, IEEE Transactions on Electrical Insulation, 1992,
27, 813-819.
29. G. Albrecht and R. B. Corey, J. Am. Chem. Soc., 1939, 61,
1087-1103.
30. Y. Iitaka, Nature, 1959, 183, 390-391.
31. Y. Iitaka, Acta Crystallographica, 1958, 11, 225-226.
32. A. Dawson, D. R. Allan, S. A. Belmonte, S. J. Clark, W. I. F.
David, P. A. McGregor, S. Parsons, C. R. Pulham and L.
Sawyer, Cryst. Growth Des., 2005, 5, 1415-1427.
33. R. Ashok Kumar, R. Ezhil Vizhi, N. Vijayan and D. Rajan
Babu, Physica B: Condensed Matter, 2011, 406, 2594-
2600.
34. A. Heredia, V. Meunier, I. K. Bdikin, J. Gracio, N. Balke, S.
Jesse, A. Tselev, P. K. Agarwal, B. G. Sumpter, S. V. Kalinin
and A. L. Kholkin, Adv. Funct. Mater., 2012, 22, 2996-
3003.
35. N. Balke, I. Bdikin, S. V. Kalinin and A. L. Kholkin, J. Am.
Ceramic Soc., 2009, 92, 1629-1647.
36. V. J. González, A. M. Rodríguez, V. León, J. Frontiñán-
Rubio, J. L. G. Fierro, M. Durán-Prado, A. B. Muñoz-García,
M. Pavone and E. Vázquez, Green Chemistry, 2018, 20,
3581-3592.
37. A. C. Ferrari, Solid State Commun., 2007, 143, 47-57.
38. A. C. Ferrari and D. M. Basko, Nat. Nanotechnol., 2013, 8,
235-246.
39. D. Fan, J. Feng, J. Liu, T. Gao, Z. Ye, M. Chen and X. Lv,
Ceramics International, 2016, 42, 7155-7163.
40. D. Lee, B. Lee, K. H. Park, H. J. Ryu, S. Jeon and S. H. Hong,
Nano Lett, 2015, 15, 1238-1244.
41. G. Liu and N. Komatsu, ChemNanoMat, 2016, 2, 500-503.
42. A. Berkdemir, H. R. Gutiérrez, A. R. Botello-Méndez, N.
Perea-López, A. L. Elías, C.-I. Chia, B. Wang, V. H. Crespi, F.
Journal Name COMMUNICATION
This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins
Please do not adjust margins
López-Urías, J.-C. Charlier, H. Terrones and M. Terrones,
Scientific Reports, 2013, 3.
43. R. Shahzad, T. Kim and S.-W. Kang, Thin Solid Films, 2017,
641, 79-86.
44. L. Cao, S. Emami and K. Lafdi, Materials Express, 2014, 4,
165-171.
45. R. B. Baig and R. S. Varma, Chem. Soc. Rev., 2012, 41,
1559-1584.
46. E. Er, H.-L. Hou, A. Criado, J. Langer, M. Möller, N. Erk, L.
M. Liz-Marzán and M. Prato, Chemistry of Materials,
2019, 31, 5725-5734.
47. X. Wang and P. Wu, ACS Appl Mater Interfaces, 2018, 10,
2504-2514.
48. N. V. Surovtsev, S. V. Adichtchev, V. K. Malinovsky, A. G.
Ogienko, V. A. Drebushchak, A. Y. Manakov, A. I.
Ancharov, A. S. Yunoshev and E. V. Boldyreva, J Chem
Phys, 2012, 137, 065103.
49. Z. Liu, L. Zhong, P. Ying, Z. Feng and C. Li, Biophys Chem,
2008, 132, 18-22.
50. A. M. Rodriguez, A. B. Munoz-Garcia, O. Crescenzi, E.
Vazquez and M. Pavone, Phys. Chem. Chem. Phys., 2016,
18, 22203-22209.
51. S. Guerin, A. Stapleton, D. Chovan, R. Mouras, M.
Gleeson, C. McKeown, M. R. Noor, C. Silien, F. M. F. Rhen,
A. L. Kholkin, N. Liu, T. Soulimane, S. A. M. Tofail and D.
Thompson, Nat Mater, 2018, 17, 180-186.