High Surface Area MoS2/Graphene Hybrid Aerogel for Ultrasensitive NO2 Detection

TL;DR: In this paper, a MoS2/graphene hybrid aerogel was synthesized with two-dimensional MoS 2 sheets coating a high surface area GAs scaffold for ultra-sensitive NO2 detection.

Abstract: A MoS2/graphene hybrid aerogel synthesized with two-dimensional MoS2 sheets coating a high surface area graphene aerogel scaffold is characterized and used for ultrasensitive NO2 detection. The combination of graphene and MoS2 leads to improved sensing properties with the graphene scaffold providing high specific surface area and high electrical and thermal conductivity and the single to few-layer MoS2 sheets providing high sensitivity and selectivity to NO2. The hybrid aerogel is integrated onto a low-power microheater platform to probe the gas sensing performance. At room temperature, the sensor exhibits an ultralow detection limit of 50 ppb NO2. By heating the material to 200 °C, the response and recovery times to reach 90% of the final signal decrease to <1 min, while retaining the low detection limit. The MoS2/graphene hybrid also shows good selectivity for NO2 against H2 and CO, especially when compared to bare graphene aerogel. The unique structure of the hybrid aerogel is responsible for the ultrasensitive, selective, and fast NO2 sensing. The improved sensing performance of this hybrid aerogel also suggests the possibility of other 2D material combinations for further sensing applications.

Summary

It is well known that the resonant tori of an integrable, classical Hamil-

• Tonian system break up under a small perturbation into new tori which wind around the remaining nonresonant tori.
• (Caustic counts are difficult in any case in more than two degrees of freedom.).

Figures

Figure 1. Schematic illustration of the synthesis process for MoS 2 /graphene aerogel. a) Graphene oxide (GO) sheets are cross-linked, supercritically dried, and annealed at high temperature to form graphene aerogel (GA). b) GA is soaked in a solution of ammonium thiomolybdate (ATM) and freezedried. c) ATM-coated GA is annealed at 450 °C in 2% H 2 /Ar for 4 h. d) Two-step high temperature annealing (at 500 and 750 °C) for 1 h each in 10% H 2 /Ar with excess sulfur improves the quality of the fi nal MoS 2 /GA aerogel.

Figure 5. a) Sensor response to 0.5 ppm NO 2 at various microheater temperatures, displaying improvement in response and recovery time. b) Realtime resistance change of the sensor with the microheater temperature of 200 °C. c) Response versus NO 2 concentration for microheater temperature of 200 °C. d) Selectivity of the MoS 2 /GA sensor compared to GA alone for microheater temperature of 200 °C.

Figure 4. a) Cross-sectional schematic of the microheater sensor. b) Optical image of one microheater showing the Pt/Ti sensing electrodes above the polysilicon heater. c) Real time response of the sensor at room temperature toward different NO 2 concentrations.

Full text

UC Berkeley
UC Berkeley Previously Published Works
Title
High Surface Area MoS2/Graphene Hybrid Aerogel for Ultrasensitive NO2 Detection
Permalink
https://escholarship.org/uc/item/3x60t55q
Journal
Advanced Functional Materials, 26(28)
ISSN
1616-301X
Authors
Long, H
Harley-Trochimczyk, A
Pham, T
et al.
Publication Date
2016-07-25
DOI
10.1002/adfm.201601562
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California

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1. Introduction
2D layered materials have attracted much
attention in recent years due to their
remarkable properties and potential for
advanced electronic devices.
[ 1,2 ]
Graphene
has received much interest since the fi rst
report of its exceptional physical prop-
erties such as high carrier mobility and
excellent mechanical strength.
[ 3 ]
More
recently, layered transition-metal dichal-
cogenides, such as molybdenum disulfi de
(MoS
2
), are being explored as promising
alternatives to graphene-based systems for
many device applications.
[ 4 ]
One promising application of 2D
materials under investigation is chemical
sensing.
[ 1,2 ]
Single-layer graphene has
been shown to exhibit excellent sensi-
tivity, at the single molecule level, thanks
to its high surface to volume ratio, high
electrical conductivity, and low noise.
[ 5–7 ]
Unfortunately, graphene-based sen-
sors without proper surface modifi cation always show poor
selectivity.
[ 5–10 ]
Single or few-layer MoS
2
has recently been
explored as a potential sensing material for gases such as
nitrogen dioxide.
[ 11–21 ]
Nitrogen dioxide (NO
2
), one of the most
common and toxic air pollutants from combustion and auto-
motive emissions, can cause serious diseases such as chronic
bronchitis, emphysema, and respiratory irritation at low con-
centrations (53 ppb set by the U.S. Environmental Protection
Agency).
[ 22–25 ]
Therefore, it is crucially important to develop
high performance sensors that are capable of detecting low
concentrations of NO
2
in air accurately, reliably, and quickly
for human health protection and air-quality monitoring. The
promising features of single- and few-layer MoS
2
, including
improved selectivity compared to graphene, make this mate-
rial an exciting candidate for this application. However, both
graphene- and MoS
2
-based fi eld-effect transistor sensors have
slow response and recovery times or even no recovery when
used at room temperature, which greatly hinders their prac-
tical use.
[ 11–20 ]
Faster response and recovery can be achieved by
heating the sensing material, which increases the desorption
rate of the adsorbed species. In order to keep the sensor power
consumption low, a microfabricated heater is typically used for
the heating.
[ 10,20,26–29 ]
But integration of single-layer materials
onto a microfabricated heater platform can be diffi cult and the
surface area is limited to the heater footprint. The assembly of
High Surface Area MoS
2
/Graphene Hybrid Aerogel
for Ultrasensitive NO
2
Detection
Hu Long , Anna Harley-Trochimczyk , Thang Pham , Zirong Tang , Tielin Shi , Alex Zettl ,
Carlo Carraro , Marcus A. Worsley , and Roya Maboudian *
A MoS
2
/graphene hybrid aerogel synthesized with two-dimensional MoS
2
sheets coating a high surface area graphene aerogel scaffold is character-
ized and used for ultrasensitive NO
2
detection. The combination of graphene
and MoS
2
leads to improved sensing properties with the graphene scaffold
providing high specifi c surface area and high electrical and thermal conduc-
tivity and the single to few-layer MoS
2
sheets providing high sensitivity and
selectivity to NO
2
. The hybrid aerogel is integrated onto a low-power micro-
heater platform to probe the gas sensing performance. At room temperature,
the sensor exhibits an ultralow detection limit of 50 ppb NO
2
. By heating the
material to 200 °C, the response and recovery times to reach 90% of the fi nal
signal decrease to <1 min, while retaining the low detection limit. The MoS
2
/
graphene hybrid also shows good selectivity for NO
2
against H
2
and CO,
especially when compared to bare graphene aerogel. The unique structure
of the hybrid aerogel is responsible for the ultrasensitive, selective, and fast
NO
2
sensing. The improved sensing performance of this hybrid aerogel also
suggests the possibility of other 2D material combinations for further sensing
applications.
DOI: 10.1002/adfm.201601562
H. Long, A. Harley-Trochimczyk, Dr. C. Carraro,
Prof. R. Maboudian
Department of Chemical and Biomolecular
Engineering
Berkeley Sensor & Actuator Center
University of California, Berkeley
Berkeley , CA 94720 , USA
E-mail: maboudia@berkeley.edu
T. Pham, Prof. A. Zettl
Department of Physics
University of California, Berkeley
Materials Sciences Division
Lawrence Berkeley National Laboratory
Kavli Energy NanoSciences Institute
University of California, Berkeley and the Lawrence
Berkeley National Laboratory
Berkeley , CA 94720 , USA
H. Long, Prof. Z. Tang, Prof. T. Shi
State Key Laboratory of Digital Manufacturing Equipment
and Technology
Huazhong University of Science and Technology
Wuhan 430074 , China
Dr. M. A. Worsley
Physical and Life Science Directorate
Lawrence Livermore National Laboratory
7000 East Avenue , Livermore , CA 94550 , USA
Adv. Funct. Mater. 2016, 26, 5158–5165
www.afm-journal.de
www.MaterialsViews.com

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2D sheets into a 3D aerogel structure may open up new oppor-
tunities for enhanced sensing properties by maintaining a high
surface area in an accessible porous network.
[ 30–32 ]
Previous efforts to construct aerogels from MoS
2
sheets resulted in a fairly low specifi c surface area
(18 m
2
g
−1
),
[ 33 ]
especially compared to that of graphene aerogel
(GA; 1200 m
2
g
−1
).
[ 30–32 ]
A hybrid aerogel with a graphene scaf-
fold coated in single- to few-layer MoS
2
nanosheets leverages
the complementary properties of the two materials.
[ 33–35 ]
3D
MoS
2
/graphene hybrid structures have been investigated for
hydrogen evolution catalysis,
[ 33,35–38 ]
supercapacitors,
[ 39 ]
lithium
storage,
[ 40 ]
and DNA sensing.
[ 41 ]
To date, the use of MoS
2
/gra-
phene hybrid structure for gas sensing has not been reported.
For conductometric sensing purposes, the graphene scaffold
allows for lower noise measurements than MoS
2
alone, since
MoS
2
is much less conductive than graphene.
[ 33 ]
Furthermore,
the thermal conductivity of graphene is much higher than
MoS
2
(5000 vs 35 W m
−1
K
−1
for single layer),
[ 42,43 ]
thus the gra-
phene scaffold can effi ciently and quickly distribute heat from
the microheater platform to the MoS
2
sheets. The 2D structure
of the MoS
2
sheets on graphene not only increases the con-
tact area for effi cient charge transfer across the interface but
also shortens the charge transport time and distance, thereby
improving the device performance.
[ 4,34,35 ]
Here we report, for the fi rst time, the use of high surface
area MoS
2
/graphene hybrid aerogel (MoS
2
/GA) for the selective
detection of NO
2
at ultralow concentrations with fast response
and recovery times. Benefi ting from its large surface area,
porous structure, and high electrical conductivity, this hybrid
aerogel exhibits superior sensing performance for NO
2
detec-
tion. The NO
2
sensor is realized by integrating the novel 3D
hybrid aerogel on a low-power microheater platform. The detec-
tion limit of the sensor is below 50 ppb NO
2
at both room tem-
perature (≈0.1 mW power consumption) and 200 °C (≈4 mW
power consumption). At 200 °C, the sensor shows much-
improved response and recovery times (<1 min) compared to
room temperature, while maintaining low power consumption,
thus greatly expanding the practical application of this sensor.
2. Results and Discussion
2.1. Synthesis and Characterization of MoS
2
/Graphene
Hybrid Aerogel
Figure 1 shows a schematic of the MoS
2
/GA synthesis process.
First, an electrically conductive, highly crystalline, and mechan-
ically robust GA with ultra high surface area is prepared
according to a method reported previously.
[ 30–32 ]
Graphene
oxide sheets are cross-linked, dried using supercritical CO
2
, and
annealed at high temperature to obtain the graphene aerogel
(Figure 1 a). To create the MoS
2
/graphene hybrid aerogel, the
GA is immersed in an aqueous solution of ammonium thiomo-
lybdate (ATM), freeze-dried, and then annealed at 450 °C under
2% H
2
/Ar (Figure 1 b,c).
[ 33 ]
To ensure that the ATM precursor is
fully reduced to MoS
2
, the MoS
2
/GA sample is further treated
with a two-step annealing process in the presence of sulfur.
The morphology of the MoS
2
/GA is characterized with scan-
ning electron microscopy (SEM) and high-resolution trans-
mission electron microscopy (HRTEM). SEM images of the
as-synthesized MoS
2
/GA ( F igure 2 a,b) show that the hybrid
aerogel has the form of continuous 3D assemblies with thin
interconnected sheets. The hybrid aerogel maintains the porous
features of the bare GA, as seen in Supporting Information
Figure S1. The average pore size of the hybrid aerogel is around
6 nm.
[ 33 ]
The open porous network of the MoS
2
/GA allows the
aerogel surface area to be readily accessed and facilitates gas dif-
fusion. In the HRTEM analysis, shown in Figure 2 d,e, the gra-
phene and MoS
2
can be clearly distinguished from their lattice
spacings (0.35 nm
[ 44 ]
vs 0.65 nm,
[ 4 ]
respectively). The analysis
indicates that most of the graphene scaffold is coated on both
sides with MoS
2,
which is present in the form of one to three-
layer sheets (mainly monolayer); thus, the benefi ts of the 2D
material are preserved in this 3D structure. The selected-area
electron diffraction (SAED) pattern is shown in Figure 2 f with
several diffraction rings, which can be indexed to the planes of
hexagonal-phase MoS
2
(M) and graphene (G) sheets. The uni-
form distribution of MoS
2
on the graphene scaffold is confi rmed
Adv. Funct. Mater. 2016, 26, 5158–5165
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Figure 1. Schematic illustration of the synthesis process for MoS
2
/graphene aerogel. a) Graphene oxide (GO) sheets are cross-linked, supercritically
dried, and annealed at high temperature to form graphene aerogel (GA). b) GA is soaked in a solution of ammonium thiomolybdate (ATM) and freeze-
dried. c) ATM-coated GA is annealed at 450 °C in 2% H
2
/Ar for 4 h. d) Two-step high temperature annealing (at 500 and 750 °C) for 1 h each in 10%
H
2
/Ar with excess sulfur improves the quality of the fi nal MoS
2
/GA aerogel.

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by energy-dispersive X-ray spectroscopy (EDX) elemental maps
shown in Figure S2 (Supporting Information), where C, Mo, and
S are seen to be uniformly distributed throughout the hybrid
aerogel. Because the graphene aerogel is conformally coated
with single and few-layer MoS
2
, the hybrid structure possesses
high surface area (700 m
2
g
−1
),
[ 33 ]
an important characteristic for
sensing. For comparison, MoS
2
aerogel is also synthesized by
a similar procedure. Without the 3D graphene framework as a
scaffold, the MoS
2
aerogel has an aggregated morphology (nano-
particles) as shown in Figure S3 (Supporting Information) and a
much lower specifi c surface area (18 m
2
g
−1
).
[ 33 ]
Raman spectra, shown in F igure 3 a,b, show the characteristic
peaks of MoS
2
and graphene. The major peaks associated with
MoS
2
are at 378 and 407 cm
−1
and correspond to the in-plane
E
1
2g
and out-of-plane A
1
g
vibrational modes of hexagonal MoS
2
,
respectively, in good agreement with literature values for multi-
layer MoS
2
. Two strong peaks observed at 1357 and 1589 cm
−1
match well with the D and G bands of graphene. X-ray photo-
electron spectroscopy (XPS) is used to further study the surface
electronic state and composition of MoS
2
/GA. Sulfur, molyb-
denum, carbon, and oxygen peaks are clearly identifi ed in the
survey spectrum in Figure S4 (Supporting Information) and the
high-resolution scans in Figure 3 c–f. The S 2p region shows two
characteristic peaks located at 162.0 and 163.1 eV corresponding
to S 2p
3/2
and S 2p
1/2
, respectively, which can be indexed to
Mo
S bonding in MoS
2
. Surprisingly, the Mo peaks show two
Mo oxidation states (Mo
4+
and Mo
6+
), which can be indexed to
the Mo
S and the Mo O bonding, respectively. The calculated
S:Mo (Mo
4+
and Mo
6+
) ratio is 2.4, whereas when only the Mo
4+
is used, the S:Mo ratio is 10:1, which suggests it is unlikely that
Mo is in a separate molybdenum oxide phase. This is consistent
with the absence of molybdenum oxide peaks in Raman
spectra. Additionally, the carbon peak (Figure 3 e) can be decon-
voluted into two peaks, a large peak at 284.6 eV attributed to
C
C bonding environment associated with the graphene scaf-
fold, and a smaller peak at 286.0 eV attributed to C
O bonding
which indicates that Mo
O does not come from a MoO
3
phase,
but rather from a Mo
O C bonding environment at the inter-
face between MoS
2
and graphene. The O 1s region (Figure 3 f)
exhibits three peaks. In addition to the peak at 532.6 eV that is
commonly observed in ex situ analyzed samples, there is a peak
at 530.5 eV that can be assigned to Mo
O bonding and a peak
at 530.9 eV that can be assigned to C
O bonding, which further
corroborates the Mo
O C bonding. The formation of a Mo O
bond without Mo
S scission has been reported elsewhere,
[ 45–47 ]
and suggests strong chemical and electronic coupling between
the MoS
2
and graphene in the synthesized aerogel. This unique
Mo environment with O- and S-bonding may offer more active
defect sites as well as unusual electronic properties.
[ 45,47 ]
Based
on the morphological and compositional characterizations
described above, the MoS
2
/GA displays a high surface area with
a uniform distribution of few-layer MoS
2
sheets that are cova-
lently bonded to the graphene scaffold.
2.2. Sensor Fabrication
The synthesized MoS
2
/graphene aerogel is evaluated for NO
2
sensing by integrating it onto a low power microheater plat-
form. Figure 4 a shows a cross-sectional schematic of the
sensor. The microheater consists of a polycrystalline silicon
(Poly-Si) microheater embedded in a low-stress silicon nitride
(LSN) membrane with Pt/Ti metal contacts for the microheater
and the sensing material. Figure S5 (Supporting Information)
Adv. Funct. Mater. 2016, 26, 5158–5165
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Figure 2. Morphology of the as-prepared MoS
2
/GA. a,b) SEM images of the hybrid aerogel with different magnifi cations. c) Low magnifi cation TEM
image of the MoS
2
/GA. d) High resolution TEM image of the MoS
2
/GA, showing most of the graphene is coated by monolayer MoS
2
. e) Enlarged TEM
image demonstrating the MoS
2
coating of the few-layer graphene scaffold. f) Relevant selected area electron diffraction (SAED) pattern of the MoS
2
/GA.

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shows a real color optical image of the 3.5 × 3.5 mm
2
chip con-
taining four microheater sensors. Figure 4 b shows a zoomed-in
view of a single microheater device showing the two platinum
sensor electrodes (yellow) above the microheater (green)
embedded in the LSN membrane (purple). Full fabrication
details and heater characterization can be found elsewhere.
[ 26,27 ]
The membrane thermally isolates the heated sensing area and
minimizes heat lost through conduction to the silicon sub-
strate. With this design, the heater consumes only 15 mW to
reach 700 °C. Besides the low power consumption, the micro-
heater platform has excellent stability in the temperature range
of interest and a closed membrane confi guration to make
sensing material deposition easier. The aerogel is sonicated
into suspension in a solution of de-ionized water and isopropyl
alcohol and drop-cast onto the microheater while the heater is
powered to 3 mW (≈100 °C) to drive localized deposition.
2.3. Gas Sensing Performance
The sensor exhibits a linear current–voltage response. Figure 4 c
shows a typical gas sensor response curve at room temperature
toward different NO
2
concentrations, from 50 ppb to 5 ppm, at
a bias voltage of 0.5 V. Upon exposure to NO
2
, the sensor resist-
ance exhibits a pronounced decrease. The sensing mechanism
relies on the direct charge transfer between NO
2
and MoS
2
/
GA. Nitrogen dioxide is a known electron acceptor due to the
unpaired electron on the nitrogen atom. Upon NO
2
adsorp-
tion, since the electron extraction from MoS
2
/GA is causing a
decrease in sensor resistance, the aerogel is exhibiting a p-type
characteristic. During subsequent exposure to clean air, the
sensor resistance slowly recovers as NO
2
molecules desorb
from the surface. This behavior is consistent with the charge
transfer mechanism of single-layer graphene,
[ 7 ]
MoS
2
,
[ 15 ]
and
carbon nanotube gas sensors.
[ 22 ]
However, understanding
the complete mechanism is a complex subject in gas sensing
studies because of the combined effects of physisorption,
chemisorption, the role of defect sites, and the transduction
mechanism.
[ 17 ]
As reported for reduced graphene oxide and
few-layer MoS
2
sensors, the adsorption of gas can be divided
into two parts, adsorption on low energy binding sites (such as
sp
2
bonded carbon) and on high energy binding sites (defects
including vacancies and functional groups).
[ 11 ]
The low energy
Adv. Funct. Mater. 2016, 26, 5158–5165
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Figure 4. a) Cross-sectional schematic of the microheater sensor. b) Optical image of one microheater showing the Pt/Ti sensing electrodes above the
polysilicon heater. c) Real time response of the sensor at room temperature toward different NO
2
concentrations.
Figure 3. a,b) Raman spectra of the hybrid aerogel. X-ray photoelectron spectra of the MoS
2
/graphene hybrid aerogel: c) S 2p; d) Mo 3d and S 2s;
e) C 1s; f) O 1s.

Frequently asked questions

Q1. What are the contributions in "High surface area mos2/graphene hybrid aerogel for ultrasensitive no2 detection" ?

In this paper, a high-surface area 3D MoS 2 / graphene hybrid aerogel is synthesized and demonstrated its effectiveness as a sensing material for NO 2 utilizing a low-power microheater platform. 

Q2. What are the future works in "High surface area mos2/graphene hybrid aerogel for ultrasensitive no2 detection" ?

The MoS 2 /GA aerogel leverages the properties of the two materials to result in a high performance hybrid material for ultrasensitive and fast NO 2 sensing and suggests the possibility of other 2D material combinations for improved sensing applications. 

Q3. What is the advantage of the microheater platform?

Besides the low power consumption, the microheater platform has excellent stability in the temperature range of interest and a closed membrane confi guration to make sensing material deposition easier. 

Q4. What is the significance of the hybrid aerogel?

Because the graphene aerogel is conformally coated with single and few-layer MoS 2 , the hybrid structure possesses high surface area (700 m 2 g −1 ), [ 33 ] an important characteristic for sensing. 

Q5. What is the way to heat the sensor?

Heating the sensing material with a microfabricated heater can enhance the reversibility of the sensor and accelerate the response and recovery rates, while maintaining low power consumption. 

Q6. What is the effect of the bonding between the MoS 2 and graphene?

The bonding between the MoS 2 and graphene results in strong electronic coupling and the excess sulfur creates defects that improve the sensitivity of the sensor. 

Q7. What is the effect of graphene on the sensor?

The 2D structure of the MoS 2 sheets on graphene not only increases the contact area for effi cient charge transfer across the interface but also shortens the charge transport time and distance, thereby improving the device performance. 

Q8. What is the purpose of the graphene aerogel?

The high-quality graphene aerogel serves as the scaffold, which provides the high specifi c surface area, porosity, and high electrical and thermal conductivity. 

Q9. What is the description of the graphene hybrid aerogel?

Benefi ting from its large surface area, porous structure, and high electrical conductivity, this hybrid aerogel exhibits superior sensing performance for NO 2 detection. 

Q10. What is the reason why the GA sensor has good selectivity against CO and H 2?

In addition, the extra S in the structure may provide improved selectivity due to the possibility for increased number of bridging S atoms at the edges of MoS 2 . [ 51 ] 

Q11. What is the composition of the graphene scaffold?

The graphene scaffold is covered with single to few-layer MoS 2 sheets, which provide the sensitive and selective sensing performance. 

Q12. What is the detection limit of the graphene scaffold?

The detection limit of the sensor is below 50 ppb NO 2 at both room temperature (≈0.1 mW power consumption) and 200 °C (≈4 mW power consumption). 

Q13. What is the sensor response to NO 2?

The NO 2 desorption is enhanced at higher temperature, which speeds up the time to reach a balance between adsorption and desorption both during gas exposure and during recovery. 

Q14. What is the benefi t of the graphene scaffold?

The analysis indicates that most of the graphene scaffold is coated on both sides with MoS 2, which is present in the form of one to threelayer sheets (mainly monolayer); thus, the benefi ts of the 2D material are preserved in this 3D structure. 

Q15. How does the sensor respond to NO 2?

The sensor shows clear response to 50 ppb NO 2 with fast response and nearly complete recovery and a signal-to-noise ratio of about 11. 

Q16. What is the difference between graphene and MoS 2?

For conductometric sensing purposes, the graphene scaffold allows for lower noise measurements than MoS 2 alone, since MoS 2 is much less conductive than graphene. [ 33 ] Furthermore, the thermal conductivity of graphene is much higher than MoS 2 (5000 vs 35 W m −1 K −1 for single layer), [ 42,43 ] thus the graphene scaffold can effi ciently and quickly distribute heat from the microheater platform to the MoS 2 sheets. 

Q17. What is the name of the hybrid aerogel?

A hybrid aerogel with a graphene scaffold coated in single- to few-layer MoS 2 nanosheets leverages the complementary properties of the two materials. 

Q18. What is the XPS spectra of the graphene scaffold?

the carbon peak (Figure 3 e) can be deconvoluted into two peaks, a large peak at 284.6 eV attributed to C C bonding environment associated with the graphene scaffold, and a smaller peak at 286.0 eV attributed to C O bonding which indicates that Mo O does not come from a MoO 3 phase, but rather from a Mo O C bonding environment at the interface between MoS 2 and graphene. 

Q19. What is the pore size of the graphene aerogel?

SEM images of the as-synthesized MoS 2 /GA ( Figure 2 a,b) show that the hybrid aerogel has the form of continuous 3D assemblies with thin interconnected sheets. 

Q20. What is the sensor resistance at room temperature?

Figure 4 c shows a typical gas sensor response curve at room temperature toward different NO 2 concentrations, from 50 ppb to 5 ppm, at a bias voltage of 0.5 V. Upon exposure to NO 2 , the sensor resistance exhibits a pronounced decrease. 

Q21. What is the diffraction pattern of the graphene aerogel?

The selected-area electron diffraction (SAED) pattern is shown in Figure 2 f with several diffraction rings, which can be indexed to the planes of hexagonal-phase MoS 2 (M) and graphene (G) sheets. 

Q22. What is the simplest way to obtain graphene?

[ 30–32 ] Graphene oxide sheets are cross-linked, dried using supercritical CO 2 , and annealed at high temperature to obtain the graphene aerogel (Figure 1 a).