of 25
1
Supporting
Information for: Demonstration of a Sensitive and Stable Chemical Gas Sensor
Based on Covalently Functionalized MoS
2
Jake M. Evans,* Kyra S. Lee,* Ellen X. Yan,* Annelise C. Thompson, Maureen B. Morla,
Madeline
C. Meier, Zachary P. Ifkovits,
Azhar I. Carim, and Nathan S. Lewis
* these authors contributed equally to this work
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
CA, 91125, United States
Table of Contents
Experimental Methods
Supplemental Data Tables and Figures
o
Abbreviations and Structures for Functional Groups
o
Sensitivities (
S
R
) and Calculated Error of Bare MoS
2
for Polar Analytes
o
Diagram of sensor design
o
R
max
/R
b
vs
P/P
0
plots used for calculation of
S
R
o
SEM of functionalized MoS
2
sensors
o
Water contact angle measurements on functionalized MoS
2
sensors
o
Sensitivity and resistivity of functionalized MoS
2
sensors to polar analytes
o
Resistance vs time plot of sensor over multiple analyte exposures
o
XPS of functionalized MoS
2
sensors
2
Experimental Methods
Materials
All solvents including n
-butyllithium (1.6 M in hexanes) were from VWR and Sigma Aldrich, all
of which was used as needed without need for further purification. Molybdenum disulfide powder
(99%), iodopropane, 2-
methyl
-1- iodopropane, iodoacetonitrile, iodoacetamide, and
trifluoromethyl benzyl bromide were obtained from Sigma Aldrich. All chemicals listed were
stored in
Argon in a glove box (<1 ppm O2). Nanopure water (resistivity > 18.2 MΩ∙cm) was
obtained from Nanopure E
-Pure system.
Synthesis of chemically exfoliated 1T MoS
2
400 mg of MoS
2
(99%) was heated at 98 ºC with 4 mL of n-
butyllithium for 46 h in a sealed glass
tube. Afterward the MoS
2
was filtered and washed with 20 mL of anhydrous hexanes. The MoS
2
was sonicated in 180 mL nanopure water for 1 h (Bandelin, Sonorex Digital 10P, DK 255 P, 640
W), then centrifuged at 2000 rpm for 5 min to remove unexfoliated mater
ial. The supernatant was
collected, washed repeatedly with H
2
O and then washed with anhydrous dimethylformamide
(DMF) until clear. The final product was resuspended in 2:1 water/isopropanol or DMF, at a
concentration of 2 mg/mL. Samples were characterized by x
-ray photoelectron spectroscopy
(XPS)
.
X-ray photoelectron spectroscopy
XPS data were collected using a Kratos AXIS Ultra spectrometer. Samples were excited with a
monochromatic Al Kα x
-ray source (1486.6 eV) at 150 W at pressures < 1 × 10
-9
Torr. The analyzer
pass energy was set to 10 eV and all spectra were calibrated to adventitious C at 284.8 eV. Data
3
were analyzed using CasaXPS software.
All peaks used a Voigt GL(30) function with 70%
Gaussian and 30% Lorentzian character.
The following constraints were placed on the Mo 3d
3/2
peak relative to the Mo3d
5/2
peak: area
(3d
5/2
) × 0.67, FWHM (3d
5/2
) ± 0.2, and position (3d
5/2
) + 3.13 eV. The S 2p
1/2
peaks were fit with
the following constraints: area (2p
1/2
) = area
(2p
3/2
) × 0.5, FWHM equal to that of the 2p
3/2
peak,
and position
(2p
3/2
) + 1.18 eV.
Additionally, the S 2p
3/2
peak
was constrained to a BE of 161.9 ±
0.2 eV
for 1T′ MoS
2
and
162.5 ± 0.2 eV for 2H MoS
2
to agree with literature precedent as well as
with
standards
determined
in the present work
(Figure S
18b, d)
.
1–4
The S-
C bond S 2p
3/2
peak was
constrained to 163.0 ± 0.3 eV again to align with literature precedent and to account for the fact
that C is more electronegative than Mo and therefore a higher BE is expected for a S bound to a
C.
1–4
The S* S 2p
3/2
peak due to defects produced during exfoliation was constrained to 161.2 ±
0.2 eV.
1–3
Similar cons
traints were placed on the Mo 3d spectra.
The Mo 3d
5/2
emission for
1T′
MoS
2
was constrained to
229.0 ± 0.2 eV
whereas
for 2H MoS
2
it was constrained to 229.7 ± 0.2
eV
, in accord with
literature precedent a
s well as with
standards
determined
in the present work
(Figure S
18a, c
).
1–4
Mo 3d
5/2
peaks for MoO
2
were constrained to 229.3 ± 0.2 eV and
for
MoO
3
at
232.5 ± 0.2 eV.
5
Scanning Electron Microscopy
Scanning electron microscopy (SEM) was performed with a FEI Nova NanoSEM 450 at an
accelerating voltage of 5.00 kV with a working distance of 5 mm and an in-
lens secondary electron
detector.
Functionalization of IT’
-MoS2
4
The chemically exfoliated MoS
2
was functionalized in DMF where the alkyl halides were added
10-
fold and stirred for 42hrs, completely covered in Al foil. The reaction was than centri
fuged and
washed at 6000 rpm for 30 min. The precipitate was collected, resuspended and rewashed 3x. The
final product was washed with isopropanol, methanol, and nanopure water. The final product was
characterized by XPS and the solvent was removed in vacu
um, obtaining the final dry powder.
1
For reductant
-activated functionalization,
MoS
2
was suspended in DMF
(10 mL)
and the alkyl
halide was added (10 eq.).
Cobaltocene was then added in an Ar
-purged glovebox. The solution
was covered with aluminum foil and stirred
for 66 h, then purified by c
entrifugation at 6500 rpm
(5820
rcf (×g))
in 10 min rounds, resuspending the precipitate by sonication in between rounds.
The product was washed with DMF until the
cobaltocene
color was not visible (typically 3
-4
rounds ×12 mL), then with isopropanol (2 ×12 mL), and methanol (2 ×12 mL).
Sensor fabrication
Gold interdigitated electrodes and contacts were prepared on glass slides. Glass slides were
cleaned with acetone and isopropanol, and then baked at 170 ºC to remove any residual solvent.
Microposit S1813
photoresist (MicroChem) was spun onto the cleaned slide at 500 rpm for 30 s
and then 4000 rpm for 60 s. The coated slides were exposed to a 425 nm lamp for 10 s underneath
a mask in a contact mask aligner (Suss MicroTech MA6/BA6). The pattern was developed
in MF
-
319 developer (MicroChem) for 90 s. Contacts were formed by sequentially evaporating 5 nm Ti
and then 90 nm Au onto the masked slides. Lift off was completed by sonicating slides at 60 ºC in
Remover PG (MicroChem) for 45 min.
MoS
2
samples were redispersed for electrode placement
. Electrode construction
was conducted
using 1.3-
1.5 mg of respective material immersed in 2.6 -
3.0 mL of the respective solvent to bring
5
the nanomaterial concentration to 0.5 g/L. The dispersed samples wer
e sonicated for 20 min before
being drop casted onto gold interdigitated electrodes. Sensors were placed in the gas
-tight vapor
testing chamber and baseline resistance was measured.
Vapor testing
Sensors were tested using a custom setup that has been described previously.
1,6
–9
N
2
(g) was used
as a carrier gas at a flow rate of 3000 mL/min. Organic vapors were generated by sparging N
2
through 45 cm tall bubblers filled with the appropriate solvents. The analyte concentration was
controlled by adjusting the volumetric mixing ratio of the saturated analyte stream to the
background N
2
stream. The flow rates of the background and analyte gases were regulated using
mass flow controllers. Each run started with a 700 s background collection. Each analyt
e exposure
consisted of 300 s of pure background gas, 80 s of diluted analyte, and then 300 s of background
gas to purge the system. The sensors were loaded into a rectangular, 16-
slot chamber connected
by Teflon tubing to the gas delivery system. For each
sensor type, 4 identical sensors would be
loaded. The resistance of each of the sensors in the array was measured by a Keysight technologies
34970A data acquisition/switch unit with Keysight 34903A 20 Channel Actuator. The
measurement electronics were int
erfaced with a computer via a GPIB connection and were
controlled with LabVIEW software.
Data processing
All data processing was conducted through custom
-routines in Matlab, where the sensor response
was expressed as
R
max
/R
b
, where
R
max
is the baseline-
corrected maximum resistance change of the
sensor, and where R
b
is the baseline resistance under inert N
2
. A spline was best
-fit and the values
of
R
max
/R
b
were determined by subtracting the values of the spline over the deduced exposure ti
me
6
with its observed resistance during the length of exposure. The sensitivity (
S
R
) of the sensors
(a
dimensionless quantity)
was quantified as the slope of the linear least
-squares fit of
R
max
/R
b
vs the
P/P
0
where
P
is the partial pressure of the analyte in the gas stream and
P
0
is the vapor pressure of
the analyte.
6,8,9
(1)
푆푆
푅푅
=
(
푝푝
푖푖
−푝푝
̅
)(
푅푅
푖푖
−푅푅
)
푛푛
푖푖=1
(
푝푝
푖푖
−푝푝
̅
)
2
푛푛
푖푖=1
Where
푝푝̅
is the mean of exposure partial pressures relative to the vapor pressure
(P/P
0
),
푅푅
is the
mean of the
R
max
/R
b
values,
p
i
is the value of
P/P
0
on the i
th
exposure and
R
i
is the
R
max
/R
b
value at
the respective
p
i
value.
Error
was estimated using standard error. This quantity allows for direct
comparison between dissimilar sensors and analyte concentrations, assuming linearity in the
response over the partial pressure regime.
Two different linear fits were performed
, one
in whi
ch
the intercept was unrestricted, and another where the intercept was forced through
zero
. Results
from these two fits are reported in Tables S2 and S3
and are depicted
in Figures S4-10.
In the
following cases, the calculated
S
R
values were within error of one another:
ethanol and
ethyl acetate
for both 2H and 1T′ controls
, heptane
for
all functional groups except Ace
, chloroform
for
Ace,
MeCN, and TFBz, THF for Pr,
and toluene for MeCN. The values reported
in the main text used
an unrestricted intercept
as these fits produced higher R
2
values.
Table S1
. Abbreviations for Functional Groups
Functional Group
Abbreviation
Methyl cyanide
MeCN
Acetamidyl
Ace
Propyl
Pr
7
2
-
methyl propyl
2MePr
Trifluoromethyl benzyl
TFBz
Figure S1
. Molecular structures of functional groups
Table S2
. Sensitivities (
S
R
) and Calculated Error of Bare MoS
2
for Polar Analytes
with Unrestricted
Intercept and
with Intercept Set to Origin
Phase/
Analyte
Unrestricted Intercept
Intercept at
origin
2H
i
sopropanol
ethanol
ethyl a
cetate
THF
3.3
±
2.8
12.0 ± 1.6
8.9 ± 1.4
7.0
±
1.0
9.5
±
1.9
12.0 ± 0.4
10.1 ± 0.5
12.0
±
1.
5
1T′
i
sopropanol
ethanol
ethyl a
cetate
THF
4.9
±
1.2
17.6 ± 6.1
12.8 ± 2.6
9.0
±
1.4
15.2
±
3.0
23.4 ± 2.3
12.9 ± 0.7
15.6
±
1.
9
N
H
2
N
O
F
F
F
propyl (Pr)
2-methyl-1-propyl (2MePr)
methyl cyanide (MeCN)
acetamidyl (Ace)
4-trifluoromethyl benzyl (TFBz)
8
Table S3
.
Sensitivity (
S
R
) of Functionalized MoS
2
for Analytes Studied
with Unrestricted Intercept
and with Intercept Set to Origin
Functional Group
/
Analyte
Unrestricted Intercept
Intercept at origin
2MePr
i
sopropanol
ethanol
ethyl a
cetate
THF
toluene
heptane
chloroform
2.8
±
0.2
4.8 ±
0.3
3.4 ±
0.2
4.0 ±
0.4
1.7 ±
0.1
1.4 ±
0.2
1.2
±
0.1
5.2
±
0.
7
8.3 ±
1.1
6.8 ±
1.1
6.7 ±
0.5
2.8 ±
0.3
1.4 ±
0.1
2.0
±
0.
3
Pr
i
sopropanol
ethanol
ethyl a
cetate
THF
toluene
heptane
chloroform
2.3
±
0.1
3.5 ±
0.2
4.0 ±
0.2
4.0 ±
0.4
3.0 ±
0.1
3.5 ±
0.5
2.4
±
0.1
3.5
±
0.
4
5.4 ±
0.6
6.8 ±
0.9
4.0 ±
0.4
6.0 ±
1.0
3.7 ±
0.2
4.6
±
0.
2
Ace
isopropanol
ethanol
ethyl a
cetate
THF
toluene
heptane
chloroform
3.3 ±
0.3
5.8 ±
0.1
3.3 ±
0.2
3.7 ±
0.4
4.3 ±
0.3
3.1 ±
0.1
1.
1
±
0.
3
6.5 ±
1.0
8.7 ±
0.9
6.3 ±
0.9
6.2 ±
0.8
3.5 ±
0.3
2.3 ±
0.2
0.9
±
0.1
MeCN
isopropanol
ethanol
ethyl a
cetate
THF
toluene
heptane
chloroform
2.0 ±
0.2
7.0 ±
0.4
3.2 ±
0.3
3.1 ±
0.5
2.6 ±
0.1
2.0 ±
0.2
0.9
±
0.
3
5.2 ±
1.0
11 ±
1.2
6.8 ±
1.2
6.8 ±
1.2
2.7 ±
0.1
1.8 ±
0.1
1.2
±
0.1
TFBz
isopropanol
ethanol
ethyl a
cetate
THF
toluene
heptane
chloroform
9.0 ±
0.6
10 ±
0.4
24 ±
2.1
21 ±
2.1
14 ±
1.7
15 ±
3.1
9.1
±
0.
3
20 ±
3.5
21 ±
3.5
47 ±
7.1
43 ±
6.8
21 ±
2.2
15 ±
1.2
9.2
±
0.1
9
Table S4
. Hammett Parameters for Functional Groups
Functional Group
Hammett Parameter
σ
p
MeCN
0.18
Ace
0.07
Pr
-
0.13
2MePr
-
0.12
TFBz
0.54
Values derived from literature
10
Figure S2
.
Diagram
of interdigitated Au electrode with drop
-cast MoS
2
sensing material.
Contacts
are made to either side of the electrode and the resistance across the contacts is measured
.
10
Figure S3
. SEM of MoS
2
drop-
cast on Au interdigitated electrode.
11
Figure S4
. R
max
/R
b
vs
P
0
/P
data for MeCN
-functionalized MoS
2
. Points are raw data, dashed line
is a linear fit with unrestricted intercept, and dotted line is
a linear fit with the intercept set at the
origin.
Each R
max
/R
b
value is the average of four sensors
with error bars repres
enting the standard
deviation.
All sensors were exposed to analytes at a range of concentrations (0.1% ≤
P
/
P
0
≤ 0.5%)
under a flow rate of 3000 mL min
-1
of N
2
.
12
Figure S5
.
R
max
/R
b
vs
P
0
/P
data for Ace
-functionalized MoS
2
. Points are raw data, dashed line is
a linear fit with unrestricted intercept, and dotted line is
a linear fit with the intercept set at the
origin.
Each R
max
/R
b
value is the average of four sensors
with error bars representing the standard
deviation.
All s
ensors were exposed to analytes at a range of concentrations (0.1% ≤
P
/
P
0
≤ 0.5%)
under a flow rate of 3000 mL min
-1
of N
2
.
13
Figure S6
.
R
max
/R
b
vs
P
0
/P
data for TFBz
-functionalized MoS
2
. Points are raw data, dashed line
is a linear fit with unrestricted intercept, and dotted line is
a linear fit with the intercept set at the
origin.
Each R
max
/R
b
value is the average of four sensors
with error bars representing the standard
deviation.
All s
ensors were exposed to analytes at a range of concentrations (0.1% ≤
P
/
P
0
≤ 0.5%)
under a flow rate of 3000 mL min
-1
of N
2
.
14
Figure S7
.
R
max
/R
b
vs
P
0
/P
data for Pr-
functionalized MoS
2
. Points are raw data, dashed line is
a
linear fit with unrestricted intercept, and dotted line is a
linear fit with the intercept set at the origin.
Each R
max
/R
b
value is the average of four sensors
with error bars representing the standard
deviation.
All sensors were exposed to analytes at a range of concentrations (0.1% ≤
P
/
P
0
≤ 0.5%)
under a flow rate of 3000 mL min
-1
of N
2
.
15
Figure S8
. R
max
/R
b
vs
P
0
/P
data for 2MePr
-functionalized MoS
2
. Points are raw data, dashed line
is a linear fit with unrestricted intercept, and dotted line is
a linear fit with the intercept set at the
origin.
Each R
max
/R
b
value is the average of four sensors
with error bars repre
senting the standard
deviation.
All sensors were exposed to analytes at a range of concentrations (0.1% ≤
P
/
P
0
≤ 0.5%)
under a flow rate of 3000 mL min
-1
of N
2
.
16
Figure S9
.
R
max
/R
b
vs
P
0
/P
data for 2H MoS
2
. Points are raw data, dashed line is
a linear fit with
unrestricted intercept, and dotted line is
a linear fit with the intercept set at the origin.
Each R
max
/R
b
value is the average of four sensors
with error bars representing the standard deviation
. All sensors
were exposed to analytes at a
range of concentrations (0.1% ≤
P
/
P
0
≤ 0.3%) under a flow rate of
3000 mL min
-1
of N
2
.
17
Figure S10
.
R
max
/R
b
vs
P
0
/P
data for 1T′ MoS
2
. Points are raw data, dashed line is
a linear fit with
unrestricted intercept, and dotted line is
a linear fit with the intercept set at the origin.
Each R
max
/R
b
value is the average of four sensors
with error bars representing the standard deviation
. All sensors
were exposed to analytes at a
range of concentrations (0.1% ≤
P
/
P
0
≤ 0.3%) under a flow rate of
3000 mL min
-1
of N
2
.
s
Figure S11
. Sensitivity of covalently functionalized MoS
2
films to (a) polar and (b) nonpolar
VOCs. All sensors were exposed to analytes at a range of concentrations (0.1% ≤
P
/
P
0
≤ 0.5%)
under a flow rate of 3000 mL min
-1
of N
2
. Each value is the average of four sensors per sensor
18
type. Error bars represent calc
ulated standard error in slope of
R
max
/R
b
vs
P/P
0
plot used for
calculation of
S
R
.
Figure S12
. Water contact angle as a function of surface functionalization
for MoS
2
drop cast
on
Si. Each data point is the average of 3 measurements
on a sample
. Error bars quantifying standard
error of 3 measurements are included, but on the order of ±0.1 degree and thus appear as lines.
19
Figure S13
. Sensitivity
of Pr
-functionalized MoS
2
to a variety of different analytes
. All sensors
were exposed to analytes at a range of concentrations (0.1% ≤
P
/
P
0
≤ 0.5%) under a flow rate of
3000 mL min
-1
of N
2
. Each value is the average of four sensors per sensor type. Pr
functionalization
percent was carried out by varying the equivalents of alkyl halides present in the functionalization
step and through the addition of
cobaltocene
(cc)
as a reducing agent.
Error bars represent
calculated standard error in slope of
R
max
/R
b
vs
P/P
0
plot used for calculation of
S
R
.
20
Figure S14
. Co 2p XP spectrum for
reductant
-activated Pr functionalized MoS
2
. Presence of large
satellite
features at
binding energies of
~787 and ~803 eV indicate CoO or CoOH rather than
Co
3
O
4
.
Figure S15.
Resistance vs time measurements for
10
repeated exposures of TFBz
-functionalized
MoS
2
to
P/P
0
= 0.5% ethyl acetate. Analyte was pulsed for 1 second, then 3 seconds
of background
N
2
w as flowed
to recover baseline resistance of sensor
. Flow rate was 3000 mL min
-1
.
21
Figure S16
. Mo 3d XP spectra
for functionalized MoS
2
.
22
Figure S17
. S 2p XP spectra
for functionalized MoS
2
. Fitting accomplished as in previous work
on functionalized MoS
2
.
1
Slightly oxidized S species indicate
S-C bonds forming.
23
Figure S18
. C 1s XP spectra for functionalized MoS
2
.
24
Figure S1
9
. (a) Mo 3d and (b) S 2p XP spectra for 2H
MoS
2
and (c) Mo 3d and (d) S 2p XP spectra
for chemically exfoliated MoS
2
used for functionalization
.