Single crystals of n-type MoS_2 and n-MoSe_2 showed higher electrocatalytic activity for the evolution of H_2(g) in alkaline solutions than in acidic solutions. The overpotentials required to drive hydrogen evolution at −10 mA cm^(–2) of current density for MoS^2 samples were −0.76 ± 0.13 and −1.03 ± 0.21 V when in contact with 1.0 M NaOH(aq) and 1.0 M H_2SO_4(aq), respectively. For MoSe_2 samples, the overpotentials at −10 mA cm^(–2) were −0.652 ± 0.050 and −0.709 ± 0.073 V in contact with 1.0 M KOH(aq) and 1.0 M H_2SO_4(aq), respectively. Single crystals from two additional sources were also tested, and the absolute values of the measured overpotentials were consistently less (by 460 ± 250 mV) in alkaline solutions than in acidic solutions. When electrochemical etching was used to create edge sites on the single crystals, the kinetics improved in acid but changed little in alkaline media. The overpotentials measured for polycrystalline thin films (PTFs) and amorphous forms of MoS_2 showed less sensitivity to pH and edge density than was observed for single crystals and showed enhanced kinetics in acid when compared to alkaline solutions. These results suggest that the active sites for hydrogen evolution on MoS_2 and MoSe_2 are different in alkaline and acidic media. Thus, while edges are known to serve as active sites in acidic media, in alkaline media it is more likely that terraces function in this role

Brunschwig, Bruce S.

John, Jimmy

Lewis, Nathan S.

McDowell, Matthew T.

Pieterick, Adam P.

Sun, Ke

Torelli, Daniel A.

Velazquez, Jesus M.

Wiensch, Joshua D.

Zhao, Xinghao

Caltech Authors - Main

A Comparative Study in Acidic and Alkaline Media of the 
Effects of pH and Crystallinity on the Hydrogen-
Evolution Reaction on MoS2 and MoSe2 
Supporting Information 
 
Joshua D. Wiensch,
† 
Jimmy John,
†
 Jesus M. Velazquez,
 †,∥  Daniel A. Torelli,
† 
 Adam P. 
Pieterick,
⌂
 Matthew T. McDowell,
 †,§
 Ke Sun,
†
  Xinghao Zhao,
⌂
 Bruce S. Brunschwig,
‡ 
 Nathan 
S. Lewis*
,†,‡, a 
 
† 
Division of Chemistry and Chemical Engineering,
 ⌂
Division of Engineering and Applied 
Science, 
‡ 
Beckman Institute, and  
a
 Kavli Nanoscience Institute,
  
California Institute of 
Technology, 1200 East California Blvd, Pasadena, California 91125, United States 
 
∥ Current address: Department of Chemistry, UC Davis, CA 95616 
§ 
Current address: Woodruff School of Mechanical Engineering and School of Materials Science and 
Engineering, Georgia Institute of Technology, Atlanta, GA 30332 
 
 
Experimental 
Sample preparation 
a) Single crystals of MoS2 
Natural and synthetic crystals of MoS2 were purchased from 2D semiconductors (El Cerrito, CA, 
USA) and Structure Probe Inc. (West Chester, PA). The doping and impurities of the purchased 
crystals were not known.  To obtain crystals of uniform quality and doping, n-doped MoS2 single 
crystals were synthesized in our laboratory by a chemical-vapor transport (CVT) technique.
1
 
Briefly, stoichiometric amounts of elemental Mo (99.99% trace metal basis, Sigma Aldrich) and 
S (Puratronic, 99.9995% trace metals basis, Alfa Aesar) were sealed under vacuum in a clean 
quartz ampule and heated to ~ 900 °C for 1 day in a furnace (Carbolite, Verder Scientific, 
Watertown, Wisconsin, USA). The resulting polycrystalline powder was collected. 0.92 g of the 
polycrystalline MoS2 powder, 0.32 g of MO2Cl2 (Sigma Aldrich)[MoO2Cl2?], and excess 
elemental S (0.104 g) were loaded and vacuum-sealed in a new quartz ampule. The ampule was 
then introduced into a 3-Zone furnace (Mellen, Concord, NH, USA) with the charge zone at 
1170 °C and the growth zone at 1115 °C with a temperature gradient of ~ 2 °C  cm
-1
 across the 
ampule. The growths typically lasted for 5–7 days.  At the end of the growths, large crystals, 
typically 5-7 mm on a side, were obtained.  
b) Single crystals of MoSe2 
Single crystal samples of n-MoSe2 were grown by a chemical-vapor-transport technique.
2
 
Briefly, stoichiometric amounts of elemental Mo (99.99% trace metal basis, Sigma Aldrich) and 
Se (99.999% trace metals basis, Aldrich) were sealed under vacuum in a clean quartz ampule and 
heated to ~900 °C for 1 day in a furnace (Carbolite, Verder Scientific, Watertown, Wisconsin, 
USA). The resulting polycrystalline powder was collected. 1.00 g of the polycrystalline MoSe2 
powder and 0.400 g of SeCl4 (Sigma Aldrich) were loaded and vacuum sealed in a new quartz 
ampule. The ampule was then introduced into a 3-Zone furnace (Carbolite, Verder Scientific, 
Watertown, Wisconsin, USA) with the charge zone at 1025 °C and the growth zone at 975 °C 
with a temperature gradient of ~ 2 °C  cm
-1
 across the ampule. The growths typically lasted for 
5–7 days.  At the end of the growths, large crystals, typically 5-7 mm on a side, were obtained. 
c) Polycrystalline thin films (PTF) of MoS2 and MoSe2 
Magnetron sputtering was used to deposit 1-5 nm of Mo onto n
+
 or pn
+
 Si wafers. The sputtered 
samples were then loaded into a CVD chamber and sulphadized by flowing a mixture of H2S and 
N2 at 500 °C for 20 minutes. 
d) Polycrystalline thin films of MoSe2 
Mo was deposited by e-beam evaporation on an n
+
 Si wafer to a thickness of 5-15 nm. The 
evaporated samples were then loaded into a chemical-vapor deposition (CVD) chamber and 
selenized by flowing evaporated elemental selenium under Ar at 600°C for 20 min.  
e) Amorphous MoSx 
Amorphous MoS2 was prepared as previously documented.
3
 Briefly, 0.075 g of Na2S was 
dissolved in 12 mL of deionized water.  A solution of (NH4)6Mo7O24 was prepared by dissolving 
the salt in 12 mL of 0.2 M H2SO4(aq).  The solutions were mixed and a black precipitate formed 
immediately. The precipitate was separated by centrifuging at 12,000 RPM for 30 min at 24 °C, 
and was subsequently washed with 15 mL of isopropanol. The solid was then sonicated in 15 mL 
of isopropanol for 15-20 min, to make the suspension that was later used for preparing 
electrodes. 
f) Amorphous MoSex 
Amorphous films of MoSe2 were formed by an operando synthesis method.
4
 Ammonium 
heptamolybdate, (NH4)6Mo7O24·4H2O [0.40 g, 3.47 x 10
-4
 mol, 99.98% (Sigma-Aldrich, St. 
Louise, MO)], was dissolved in 8 mL of a 0.20 M H2SO4(aq) that was prepared by diluting 18 M 
H2SO4 with H2O.  A separate solution was prepared by dissolving 0.10 g (8 x 10
-4
 mol) of sodium 
selenide, Na2Se [99.8% (Alfa Aesar, Ward Hill, MA)], in 8 mL of H2O. The first solution of Mo 
was quickly poured into the second solution, and a black precipitate formed upon mixing. This 
mixture was agitated for 10 min with a sonicator.  The suspension was then transferred to a 
centrifuge tube and centrifuged for 30 min at ~ 4000 rpm.  A black precipitate collected at the 
bottom of the centrifuge tube.  The supernatant liquid, which still contained some black color, was 
discarded. The precipitate was rinsed with 2 10 mL aliquots of 2-propanol.  A final 20 mL of 2-
propanol was added to the centrifuge tube, and was then subjected to sonication for 30 min. This 
process resulted in suspension of the black solid in the 2-propanol.     
The choice to study MoS2 in NaOH solution and MoSe2 in KOH solution was made arbitrarily. 
Upon exploring the effect of changing the cation from Li, Na, and K in alkaline solution, we found 
a small effect of < 40 mV in the series. The pH effect on overpotential was still pronounced for 
MoS2 in LiOH and KOH in addition to the data presented on NaOH, when compared to the 
overpotential in acidic media.  
Preparation of Electrodes 
To expose fresh surfaces, the single crystals were mechanically exfoliated on both sides 
with Scotch tape.  Ohmic contact was made to the back side of the crystal using In solder. The 
crystal with the back contact was then attached to one end of a Cu wire. The Cu wire was then 
threaded through a hollow glass tube leaving the sample outside the tube on one end. The Cu 
wire acted as the electrode lead.  The area of the front face of the crystal, that was to be in 
contact with the solution, was then defined by applying epoxy (Hysol 9460, Loctite, USA) to the 
edges of the crystal. The rest of the crystal, the In back contact, and the exposed part of the Cu 
wire outside the glass tube, were all electrically isolated from the solution by applying a 
continuous film of epoxy over these materials.  The epoxy was allowed to cure overnight.  The 
exposed areas of the electrodes were imaged using an optical scanner and converted to surface 
areas using ImageJ software.  Electrode areas were typically 0.02 – 0.50 cm
2
. 
The Si/PTF-MoS2 and MoSe2 electrodes were prepared by carefully cutting the as-
synthesized wafer to appropriately sized pieces. Electrical contact to the Si was made by 
scratching In-Ga eutectic on to the back side of the Si. A Cu wire was then attached to the back 
side of the electrode using silver paint. The remainder of the electrode-making process was the 
same as that outlined for the single crystals above. Electrodes were typically of 0.5 cm
2
 in area. 
The electrodes for testing the amorphous MoSx were prepared by drop-casting the 
suspension of MoSx (prepared as described in the previous section) onto glassy-carbon rotating 
disc electrodes (RDEs) and drying the material overnight in a vacuum desiccator at 50 mbar. The 
geometric surface area of the RDE was ~0.2 cm
2
. 
To prepare electrodes from the black MoSex suspension, highly ordered pyrolytic graphite 
(HOPG) samples were cut into ~ 8mm x 8mm squares. The HOPG was polished for 10 min with 
1200 grit SiC polishing paper on a wet polishing wheel. The polished HOPG was cleaned by 
sonication in isopropanol and then acetone, each for 10 min.  Next, a 30 µL drop of the mixed 
composition suspension of Mo and Se was placed onto the polished side of a HOPG substrate.  The 
sample was dried for 18 h at an internal pressure of < 100 mTorr in a vacuum desiccator. Electrode 
areas varied from 0.4 – 0.6 cm
2
 
Electrochemistry 
H2SO4 (99.999% trace metal basis, Sigma Aldrich) and NaOH and KOH (semiconductor 
grade, 99.99% trace metal basis, Sigma Aldrich) were obtained and used without further 
purification, to prepare solutions for the electrochemical experiments. De-ionized water (ρ ≈ 18.2 
MΏ cm), supplied from a Barnstead Nanopure system (Thermo Scientific, Asheville, NC, USA) 
was used for all the experiments. The potentiostats were BioLogic SP-200 (Biologic, Grenoble, 
France), Autolab 302N (Autolab, Metrohm, Utrecht, the Netherlands) and Gamry REF 600 
(Gamry, Warminster, PA, USA). A standard three-electrode electrochemical borosilicate glass 
cell was used for the experiments.  A high-surface-area graphite electrode (99.9995% trace 
metals basis, Alfa Aesar) was used as a counter electrode, and a saturated Ag/AgCl electrode, 
obtained from either CH Instruments (Austin, TX, USA) or custom-made, housed in a micro-
agar-gel salt bridge, was used as the reference electrode. The saturated Ag/AgCl reference 
electrode in the salt bridge was calibrated against a reversible hydrogen electrode (RHE) that was 
freshly prepared before every experiment. For the purpose of presentation, the potentials are 
referenced against the relevant RHE scale using the calibration thus performed.  Typically, the 
solution was sparged with H2 before experiments, and the headspace in the cell was continuously 
purged with H2 during the experiments, to ensure a saturation concentration of H2 in the solution.  
The solution was continuously agitated using magnetic stirring or, in the case of amorphous 
MoSx on rotating-disk electrodes, by rotating the electrode at 1600 RPM.  
 No resistance compensation was applied to the measured current-density versus potential 
data. 
Microscopy 
The optical images were obtained on an Olympus BX 51 microscope.  
Mass Spectroscopy 
A Hiden HPR-20 QIC R&D equipped with an electron-impact ionization source and triple-filter 
quadrupole mass spectrometer was utilized to measure the electrocatalytic production of H2 in 
real time. Electrolyses were conducted in a 5-port flow cell at a constant N2 flow of 50 mL/min 
to maintain a positive pressure in the headspace. Both before and after electrolysis, a calibration 
gas of 500 ppm H2 (Air Liquide) in N2 was flowed through the cell at the same rate (50 mL/min). 
Once background stabilized, galvanostatic steps were applied for 6 min until signal had 
stabilized starting at -0.5 mA in -0.5 mA steps up to -2.0 mA. After each galvanostatic step, the 
electrode was allowed to return to the open-circuit potential and the mass spec signal to return to 
its background level.  
Although in principle Faradaic efficiencies for H2 production could be calculated from these 
data, the small size of the samples ~ 0.12 cm
2 
– 0.25 cm
2
 prevented accumulation of products 
sufficient for reliable quantification using this method or using other methods available in our 
laboratory such as gas chromatography or eudiometry. 
X-Ray Photoelectron Spectroscopy 
X-ray photoelectron spectroscopy (XPS) data was collected at ~5 × 10
−9
 Torr using a 
Kratos AXIS Ultra DLD with a magnetic immersion lens that consisted of a spherical mirror and 
concentric hemispherical analyzers with a delay-line detector (DLD). An Al Kα (1.486 KeV) 
monochromatic source was used for X-ray excitation. Ejected electrons were collected at a 90° 
angle from the horizontal. The CASA XPS software package v 2.3.16 was used to analyze the 
collected data.  
Tafel Analysis 
A Tafel analysis was undertaken to gain insights into the possible mechanisms of hydrogen 
evolution at MoSe2 electrodes in acidic and alkaline media. Figure S6 shows the Tafel plot of 
single-crystalline MoS2, Figure S6 shows Tafel plots for polycrystalline MoSe2, and Figure S7 
shows the Tafel plots for amorphous MoSex in 1.0 M H2SO4(aq) and 1.0 M KOH(aq). For 
single-crystalline samples, one noticeable variation between the Tafel plots in acidic and basic 
media is the presence of an inflection point for the data in acid. This inflection point is not 
present for data collected in 1.0 M KOH(aq). The Tafel data for the polycrystalline thin film 
exhibited the most similarity between acidic and alkaline solutions, with the Tafel data being 
roughly parallel over the plotted current densities. The Tafel analysis for amorphous MoSex 
showed significant variation between samples. In 1.0 M H2SO4, a linear region was observed 
between -η = 0.4 – 0.3, followed by a departure from linearity at lower –η values. In 1.0 M 
KOH(aq), the data were fit to between -η = 0.4 – 0.25, but extrapolating to predict an exchange-
current density resulted in a low value of ~ 8·10
-7 
A cm
-2
 versus an exchange-current density of 
4·10
-3 
A cm
-2
 obtained from analysis of the linear region for amorphous MoSex in H2SO4(aq). 
Supplementary Figures 
 
  
 Figure S1. Online mass spectrometry and chronopotentiometry during hydrogen evolution on n-MoS2 in 
base (top) and acid (bottom).  
 
 
Figure S2.  X-ray photoelectron spectroscopy of n-MoS2 single crystals before (left column) and 
after electrocatalysis in 1.0 M H2SO4 (center column) or 1.0 M NaOH (right column). 
 
 Figure S3. MoS2 Single crystals (synthetic, n-doped). No Ni, Fe or Co 2p signals were detected 
after electrochemistry in either acid or base. 
 
 Figure S4. ∆η for single crystal MoS2 samples from a variety of sources. Note the natural and 
undoped synthetic single crystals have larger variations in ∆η than for single crystal samples 
grown under conditions where doping is controlled. (∆η = ηAcid - ηBase  where  ηAcid is the 
overpotential required to drive H2 evolution at a current density of -10 mA cm
-2
 for samples in 
contact with 1.0 M H2SO4(aq), and ηBase is the overpotential required to drive H2 evolution at a 
current density of -10 mA cm
-2
 for samples in contact with 1.0 M NaOH(aq) or 1.0 M KOH(aq).)  
 
 Figure S5. (a) Cyclic voltammograms obtained in acid (0.10 M H2SO4(aq)) and base (1.0 M 
NaOH(aq)) before and after each electrochemical etching step of a natural single crystal MoS2. 
Scan rate: 50 mV s
-1
. Figure 2 in the main text is plotted using these data. (b) –(d) optical images 
of the same area of the electrode before and after the etching process. 
 
    
 
 
 
 
 
 
 0.0 0.2 0.4 0.6 0.8 1.0
-6
-5
-4
-3
-2
-1
0
1
2
   127
        b 
(mV/decade)
n-MoS
2
 (X'tal) /1M H
2
SO
4
L
o
g
 |
j|
η (V)
      j0 
(mA/cm2)
n-MoS2-121915-1
20 mV/s
3.8 x 10
-5
-25
-20
-15
-10
-5
0
j (m
A
/c
m
2)
 
Figure S6. Tafel plot for single-crystalline n-MoS2 in 1.0 M H2SO4(aq).  
 
 
  
Figure S7. (top) Tafel plot of single crystal MoSe2 in 1.0 M H2SO4(aq) and 1.0 M KOH(aq). 
(middle) Tafel plot of polycrystalline thin film MoSe2 in 1.0 M H2SO4(aq) and 1.0 M KOH(aq). 
(bottom) Tafel plot of amorphous MoSex in 1.0 M H2SO4(aq) and 1.0 M KOH(aq).  
 
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Chem. 1983, 49, 166-179. 
(2) (a) Kline, G.; Kam, K.; Canfield, D.; Parkinson, B. A. Efficient and Stable 
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(3) Benck, J. D.; Chen, Z.; Kuritzky, L. Y.; Forman, A. J.; Jaramillo, T. F. Amorphous 
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Comparative Study in Acidic and Alkaline Media of the Effects of pH and Crystallinity on the Hydrogen-Evolution Reaction on MoS_2 and MoSe_2

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Comparative Study in Acidic and Alkaline Media of the Effects of pH and Crystallinity on the Hydrogen-Evolution Reaction on MoS_2 and MoSe_2

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