Molybdenum disulfide (MoS_2) has been widely examined as a catalyst containing no precious metals for the hydrogen evolution reaction (HER); however, these examinations have utilized synthesized MoS_2 because the pristine MoS_2 mineral is known to be a poor catalyst. The fundamental challenge with pristine MoS_2 is the inert HER activity of the predominant (0001) basal surface plane. In order to achieve high HER performance with pristine MoS_2, it is essential to activate the basal plane. Here, we report a general thermal process in which the basal plane is texturized to increase the density of HER-active edge sites. This texturization is achieved through a simple thermal annealing procedure in a hydrogen environment, removing sulfur from the MoS_2 surface to form edge sites. As a result, the process generates high HER catalytic performance in pristine MoS_2 across various morphologies such as the bulk mineral, films composed of micron-scale flakes, and even films of a commercially available spray of nanoflake MoS_2. The lowest overpotential (η) observed for these samples was η = 170 mV to obtain 10 mA/cm_2 of HER current density

Ager, Joel W.

Gao, Wei

Hettick, Mark

Javey, Ali

Kiriya, Daisuke

Lobaccaro, Peter

Maboudian, Roya

Nyein, Hnin Yin Yin

Shiraki, Hiroshi

Sutter-Fella, Carolin M.

Taheri, Peyman

Zhao, Peida

Caltech Authors - Main

 S1 
Supporting Information of  
General Thermal Texturization Process of MoS2 for 
Efficient Electrocatalytic Hydrogen Evolution Reaction 
Daisuke Kiriya,
1,3,4‡
 Peter Lobaccaro,
2,4,5‡
  Hnin Yin Yin Nyein,
1,3,4
 Peyman Taheri,
1,4
 Mark Hettick,
1,3,4,5
 
Hiroshi Shiraki,
1,3,4
 Carolin M. Sutter-Fella,
1,3,4
 Peida Zhao,
1,3,4
 Wei Gao,
1,3,4
 Roya Maboudian,
2,4
 Joel 
W. Ager,
3,5
 and Ali Javey
1,3,4
* 
1
 Department of Electrical Engineering and Computer Sciences, University of California at Berkeley, 
Berkeley, California 94720, United States 
2
 Department of Chemical and Biomolecular Engineering, University of California at Berkeley, 
Berkeley, California 94720, United States  
3
 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, 
United States 
4 
Berkeley Sensor & Actuator Center, University of California, Berkeley, California 94720, United 
States 
5 
Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, 
California 94720, United States 
‡ These authors contributed equally for this paper.  
* Corresponding Author: 
Prof. Ali Javey 
E-mail: ajavey@berkeley.edu 
  
 S2 
Experimental Methods 
Sample preparation  
Bulk mineral MoS2 was purchased from SPI Supplies.  Micron-scale (microflake) MoS2 was purchased 
from Graphene Supermarket.  Some microflake MoS2 was further prepared by grinding it with a mortar 
and pestle in an attempt to make smaller flakes.  This material is called ground microflake.  Microflake 
and ground microflake films were prepared by the following process: 1) Disperse the flakes in DMF at a 
concentration of 30 mg/ml via sonication. 2) Cast the suspension (~100 L) on a 1 cm × 1.5 cm Mo foil 
on a hotplate at 120 °C.  After drying off the DMF solvent (about 5 min), repeat the suspension casting 
again.  All samples termed as-received MoS2 (bulk and microflake film) or as-ground (ground 
microflake film) underwent a 250 
o
C forming gas anneal for 3 hrs to clean the surface and remove any 
remaining DMF solvent.  This temperature has essentially no effect on MoS2.  High temperature 
annealed samples were annealed at 700 °C, 800 
o
C, or 900 °C in forming gas for 3 hrs.  The samples 
were allowed to cool below 50 
o
C before being taken out of the annealing chamber. 
 
HER measurement 
Bulk mineral MoS2 samples needed to be mounted before they could be used as electrodes.  Silver paste 
(CircuitWorks) was used to mount the flakes to a glass slide.  The stack was then annealed at 70 °C in 
air to cure the Ag paste.  Then Cu tape was placed on the edge of the Ag paste to make an electrical lead 
connection.  Finally, insulating, acid-resistant, polymer resin was painted over the entire area except for 
the desired MoS2 area identified for measurement.  For the microflake and ground microflake film 
samples, the same resin was used to cover the whole area of the film except for the identified 
measurement area. These samples were placed as working electrode in the 3-electrode cell as shown in 
Figure 4a.   
The HER measurements were done in 0.5 M H2SO4 (Sigma-Aldrich) with a calibrated Ag/AgCl 
reference electrode and a platinum counter electrode.  The electrolyte was initially sparged with N2 
 S3 
bubbling for more than 10 mins to remove oxygen.  N2 was then flown over the headspace of the 
electrolyte during the measurements.  The electrolyte was constantly stirred at 750 RPM with a 
magnetic, Teflon coated, stir bar.  A Biologic SP-300 potentiostat was used to make all electrochemical 
measurements.  IR drop was corrected for during the CV measurements by first running electrochemical 
impedance spectroscopy from 10
6
 to 1000 Hz to identify the resistivity.  This resistance was found to be 
typically less than 10 Ohms.  Cyclic voltammetry was used at two different scan rates to obtain the data 
for the Tafel plots (5 mV/sec) and polarization curves (50 mV s
-1
). 
 
Electrochemical Active Surface Area (ECSA) Measurement 
To identify the electrochemically active surface areas to normalize the polarization curves as shown in 
Figure 4d, ECSA measurements were used.  Bulk MoS2 (as-received and annealed samples) were 
mounted as stated to be used as the working electrode in the three electrode setup shown in Figure 4a. 
We employed electrochemical impedance spectroscopy, using a Biologic SP-300 potentiostat, at open circuit 
potential scanning frequencies from 10 Hz to ~0.05 Hz.  Then a R-C equivalent circuit was used to fit the 
data to determine the double-layer capacitance of each sample. The double layer capacitances were obtained 
as the follows: As-received, ~ 4.14 F cm-2; 700 °C annealed sample, ~ 0.47 F cm-2; 800 °C annealed 
sample, ~ 2.95 F cm-2; 900 °C annealed sample, ~ 2.83 F cm-2. The relative active surface areas are 
obtained from the relative values of the above double layer capacitances. Using these values, the currents 
(mA) were normalized as shown in Figure 4d.     
 
Characterization 
SEM images were obtained with either a FEI Quanta 200 FEG SEM with an operating voltage of 15 kV 
(Figures 2, 3, 5, 6, and S6) or with a FEI Nova NanoSEM650 with an operating voltage of 5 kV 
(Figures S3 and S4). XPS characterization was performed using a Kratos Axis Ultra DLD system using 
a monochromatic Al Kα source (hν = 1486.6 eV).  The XRD patterns were taken on a Rigaku SmartLab 
 S4 
system which is capable of measuring both grazing incidence and normal XRD patterns. TEM images 
were taken with a FEI Titan microscope with an operating voltage at 200 kV. 
 
Tafel Analysis 
To attempt to understand the HER mechanism for the samples, the Tafel plots were examined.  
The Tafel equation ( = b*log j + a, where  is overpotential, j is the current density, b is the Tafel 
slope, and a is the Tafel constant) was applied to make Tafel plots of Log(j) vs.  and the Tafel slope, b, 
was obtained. All the measurements for this analysis were carried out with 5 mV sec
-1
 scan rates. The 
obtained Tafel slopes for the samples are shown in Table S1.  
The three possible reaction mechanisms for HER in acidic media are known as the Volmer, the 
Heyrovsky, and the Tafel reactions
1,2
: 
Volmer step: H3O
+
 + e
-
 → Hads + H2O  
Heyrovsky step: Hads + H3O
+
 + e
-
 → H2 + H2O 
Tafel step: Hads + Hads → H2 
If the rate determining step in HER is one of the above reactions, the Tafel slope should be close to 
either ~120 mV decade
-1
, ~40 mV decade
-1
, or ~30 mV decade
-1
 for the Volmer, the Heyrovsky, or the 
Tafel reaction respectively. According to the above considerations, the rate determining step for the as-
received bulk MoS2 is suggested to be the Volmer reaction (Tafel slope ~150 mV decade
-1
) and the 
annealed bulk MoS2 samples could possibly be the Heyrovsky reaction (Tafel slope ~60 mV decade
-1
). 
The exchange current density (j0) is also an important parameter which indicates the inherent catalytic 
activity of the material. The exchange current density was calculated from the Tafel plot and is also 
shown in Table S1.  
 
  
 S5 
Table S1. Electrochemical parameters for bulk mineral, microflake, ground microflake and sprayed 
nanoflake films of MoS2. 
Morphology 
Annealing 
Temp. (°C) 
Overpotential (mV) 
at j = 10 mA cm
-2
 
j0 
(mA cm
-2
) 
Tafel slope 
(mV decade
-1
) 
Bulk mineral 250
*
 ‒** 0.00093 149 
Bulk mineral 700 ‒
**
 0.0011 90 
Bulk mineral 800 300 0.00069 71 
Bulk mineral 900 296 0.0012 76 
Microflake 250
*
 330 0.00012 63 
Microflake 700 227 0.0044 67 
Microflake 800 210 0.0048 63 
Microflake 900 195 0.0045 59 
Ground microflake 250
*
 342 0.00046 73 
Ground microflake 700 220 0.0093 70 
Ground microflake 800 193 0.011 64 
Ground microflake 900 174 0.019 63 
Sprayed nanoflake 250
*
 322 0.0030 92 
Sprayed nanoflake 700 215 0.0038 62 
*   
The samples are referred to as as-received or as-ground in the manuscript. Annealing at 250 °C does 
not damage MoS2 flakes.   
** 
Not observed in the measurement range.  
  
 S6 
Table S2. Comparison of electrical parameters with other reported works. Data adapted from Benck et 
al.
3
 
Catalyst Description 
Overpotential (mV) 
at j = 10 mA cm
-2
 
Reference 
Amorphous MoSx on n-doped carbon nanotubes 110 Li 2014
4
 
Li
+
 intercalated MoS2 nanoparticles on carbon fiber 110 Wang 2014
5
 
 [Mo3S13]
-2
 loaded on anodized Toray paper 149 Benck 2014
3
 
MoSx loaded on carbon fiber 152 Laursen 2013
6
 
MoS2 nanoparticles on reduced graphene oxide 154 Li 2011
1
 
Li
+
 intercalated MoS2 168 Wang 2013
7
 
100 µg/cm
2
 [Mo3S13]
-2
 loaded on Toray paper 174 Kibsgaard 2014
8
 
MoSx loaded on carbon fiber paper 194 Laursen 2013
6
 
Wet chemical synthesized amorphous MoS2 200 Benck 2012
9
 
Double gyroid morphology MoS2 206 
Kibsgaard 
2012
10
 
1-T phase MoS2 207 Voiry 2013
11
 
Electrodeposited amorphous MoS2 242 Merki 2011
12
 
MoO3 – MoS2 NWs 254 Chen 2011
13
 
Thermally texturized ground microflake MoS2 174 This work 
 
 
  
 S7 
Supporting Figures 
 
 
 
Figure S1. XPS spectra around the valence band region of the bulk mineral MoS2 of pristine (annealed 
at 250 °C for 3hrs) and annealed at 700 °C to 900 °C for 3 hrs.  
 
  
 S8 
 
Figure S2. The structure of a thermally texturized sample at 700 °C (3 hrs annealing) was investigated 
using cross-sectional TEM. (a) Low resolution TEM image of the sample.  (b,c) High-resolution TEM 
images near the surface approximately ~25 nm (b) and deeper inside the sample, approximately 500 nm 
from the surface (c). Fast Fourier transform (FFT) of these images are shown below. (d-f) D-spacings 
were obtained from the FFT patterns in Figure S2b and c and other images not shown here. (d) 
Reference powder XRD patterns of MoS2 and Mo, compared to D-spacing patterns obtained from the 
FFT analysis for the near surface region (e) and for the deeper region (f).  FFT peaks (solid line) and 
errors (dot line) are shown in the plot. 
 
 S9 
 
 
Figure S3. Mechanism of the thermal texturization process which occurs on the MoS2 surface under a 
forming gas environment (H2/N2 = 5%/95%). (a) SEM images of the bulk mineral MoS2 annealed at 
700 °C for different annealing times (1h to 5 hrs). The light gray areas are dissociatively textured by the 
thermal annealing processes. Annealing longer than 3 hrs shows clear dissociation under SEM. (b) The 
detailed surface morphology for the bulk MoS2 annealed at 700 °C for 5 hrs. The surface texturization is 
observed as either lines or circles. The circles are radially grown, indicating the dissociation occurs from 
a defect at the center. (c) A plausible mechanism for the thermal texturization process is shown. The 
texture would emanate from defects and edges on the surface.  
 
 
 
 
 
 
 S10 
 
 
Figure S4. Environmental effect of the annealing process on bulk mineral MoS2. (a) SEM images of the 
annealed bulk MoS2 in forming gas (left) and nitrogen (right) at 800 °C for 3 hrs. In the forming gas 
anneal case, the dissociation causes texturization of the surface. On the other hand, in the nitrogen 
anneal case, the surface is still smooth even after annealing at 800 °C and large cubic structures are 
observed. (b) Polarization curves for the HER activity of as-received bulk MoS2, 800 °C annealed bulk 
MoS2 under nitrogen, and 800 °C annealed bulk MoS2 under forming gas are shown.  The nitrogen 
annealed sample shows much lower HER performance than the sample annealed in forming gas.  
 
 
 
 
 
 
 
 
 
 
 S11 
 
 
Figure S5. The output characteristic curves for the MOSFETs of (a) as-received bulk MoS2, (b) 
annealed bulk MoS2 at 550 °C in forming gas, (c) annealed bulk MoS2 at 700 °C in nitrogen, and (d) 
annealed bulk MoS2 at 700 °C in forming gas.  These samples are annealed at their respective 
temperatures for 3 hrs.  MoS2 flakes (thickness of ~100 nm) were prepared by a mechanical exfoliation 
technique, then annealed at the aforementioned temperatures followed by a typical lithography process 
to make the devices. The channel length was fixed at 10 m in all cases.  Back gate potential was 
applied from -100 V to 100 V. Except for the forming gas 700 
o
C annealed sample, all samples showed 
gate dependency, indicating semiconducting n-type MoS2. The characteristics curve in Figure S5d 
shows a sample with no gate dependency, indicating metallic behavior of the MoS2 flake after the 
thermal texturization process.  
 
 S12 
 
Figure S6. (a) Macroscopic images for the as-received microflake MoS2 films (annealed at 250 °C for 3 
hrs) on Mo foil as the working electrode before (left) and after (right) HER in 0.5 M H2SO4. The area 
exposed during electrolysis was completely delaminated by the generation of hydrogen bubbles. (b) 
SEM images of Mo foil (left) and the surface of the as-received microflake MoS2 film after 
delamination (right). Small amounts of MoS2 flakes remained on the Mo foil after the delamination. (c) 
Polarization curves (50 mV s
-1
) in 0.5 M H2SO4 for the microflake MoS2 samples annealed at different 
temperatures, including Mo and Pt foils for reference. (d) Tafel plot of the samples shown in Figure S6c 
 S13 
taken at 5mV s
-1
.  (e) Chronoamperometric curve (j-t) measured for a microflake MoS2 film annealed at 
900 °C for 13hrs. 
 
 
 
 
Figure S7. (a) Polarization curves (50 mV s
-1
) in 0.5 M H2SO4 for the ground microflake MoS2 samples 
annealed at different temperatures, including Mo and Pt foils for reference.  The as-ground MoS2 film 
delaminated after evolving hydrogen similar to what is shown in Figure S6a, resulting in the observed 
activity matching closely to Mo foil. (b) Tafel plot of the samples shown in Figure S7a taken at 5 mV s
-1
.   
 
 
  
 S14 
Supporting References 
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(2)  Conway, B. E.; Tilak, B. V. Electrochim. Acta, 2002, 47, 3571–3594. 
(3)  Benck, J. D.; Hellstern, T. R.; Kibsgaard, J.; Chakthranont, P.; Jaramillo, T. F. ACS Catal. 2014, 
4, 3957–3971. 
(4)  Li, D. J.; Maiti, U. N.; Lim, J.; Choi, D. S.; Lee, W. J.; Oh, Y.; Lee, G. Y.; Kim, S. O. Nano Lett. 
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(5)  Wang, H.; Lu, Z.; Kong, D.; Sun, J.; Hymel, T. M.; Cui, Y. ACS Nano 2014, 8, 4940–4947. 
(6)  Laursen, A. B.; Vesborg, P. C. K.; Chorkendorff, I. Chem. Commun. 2013, 49, 4965. 
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Prinz, F. B.; Cui, Y. Proc. Natl. Acad. Sci. 2013, 110, 19701–19706. 
(8)  Kibsgaard, J.; Jaramillo, T. F.; Besenbacher, F. Nat. Chem. 2014, 6, 248–253. 
(9)  Benck, J. D.; Chen, Z.; Kuritzky, L. Y.; Forman, A. J.; Jaramillo, T. F. ACS Catal. 2012, 2, 
1916–1923. 
(10)  Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F. Nat. Mater. 2012, 11, 963–969. 
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General Thermal Texturization Process of MoS_2 for Efficient Electrocatalytic Hydrogen Evolution Reaction

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General Thermal Texturization Process of MoS_2 for Efficient Electrocatalytic Hydrogen Evolution Reaction

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