Insigh in o Mo
2
C Nanopla es G ow h by Means
o Scanning Elec ochemical Cell Mic oscopy o
High-Resolu ion Elec oca aly ic Hyd ogen E olu ion
Ac i i y Mapping
Geo ane A uda de Oli ei a, Da id E. Sanchez, Alexande J. S edenschek, And es Fes ,
Moonjoo Kim, Ca la San ana San os, Mau icio Te ones, Wol gang Schuhmann,*
and Daniel G asseschi*
1. In oduc ion
T ansi ion me al ca bides (TMCs) a e a
g oup o in e s i ial compounds o med
be ween g oup IV–VI ansi ion me als
and ca bon. TMCs exhibi a combina ion
o physical p ope ies ypically ound in
me als and ce amics, such as high elec ical
conduc i i y, mel ing poin , and ha d-
ness.
[1]
Molybdenum ca bides (Mo
2
C) ha e
ga ne ed significan a en ion due o hei
ema kable physicochemical p ope ies,
[2]
making hem sui able o a ious applica-
ions, including ene gy s o age,
[3]
sens-
ing,
[4]
and ca alysis.
[5–14]
Mo
2
C is a p omising elec oca alys o
he hyd ogen e olu ion eac ion (HER)
due o i s high ca aly ic ac i i y and s abili y
in bo h acidic and alkaline en i on-
men s.
[5,6,8,15]
Va ious s udies ha e shown
ha Mo
2
C nanoma e ials, especially hose
wi h enginee ed su aces and s uc u es,
exhibi enhanced HER pe o mance. Fo example, Bang e al.
demons a ed ha a s uc u e wi h mul iple s ep-edges o
G. A. de Oli ei a, M. Kim, C. San ana San os, W. Schuhmann,
D. G asseschi
Analy ical Chemis y-Cen e o Elec ochemical Sciences (CES)
Facul y o Chemis y and Biochemis y
Ruh Uni e si y Bochum
Uni e si ¨
a s . 150, 44780 Bochum, Ge many
E-mail: [email p o ec ed]; dg asses[email p o ec ed]
D. E. Sanchez, A. Fes , M. Te ones
Depa men o Ma e ials Science and Enginee ing
The Pennsyl ania S a e Uni e si y
Uni e si y Pa k, PA 16802, USA
The ORCID iden ifica ion numbe (s) o he au ho (s) o his a icle
can be ound unde h ps://doi.o g/10.1002/smsc.202500220.
© 2025 The Au ho (s). Small Science published by Wiley-VCH GmbH.
This is an open access a icle unde he e ms o he C ea i e
Commons A ibu ion License, which pe mi s use, dis ibu ion and
ep oduc ion in any medium, p o ided he o iginal wo k is p ope ly ci ed.
DOI: 10.1002/smsc.202500220
A. J. S edenschek, M. Te ones
Depa men o Physics
The Pennsyl ania S a e Uni e si y
Uni e si y Pa k, PA 16802, USA
M. Te ones
Cen e o 2-Dimensional and Laye ed Ma e ials
The Pennsyl ania S a e Uni e si y
Uni e si y Pa k, PA 16802, USA
M. Te ones
Depa men o Chemis y
The Pennsyl ania S a e Uni e si y
Uni e si y Pa k, PA 16802, USA
D. G asseschi
Depa men o Ino ganic Chemis y
Chemis y Ins i u e
Fede al Uni e si y o Rio de Janei o
A . Ped o Calmon, 550, Cidade Uni e si á ia, Rio de Janei o
21941-901, B azil
Molybdenum ca bides ha e eme ged as p omising ca alys s o he hyd ogen
e olu ion eac ion (HER). While nume ous s udies ha e in es iga ed syn hesis
me hods, s uc u al p ope ies, and hei applica ion, he unde s anding o hei
local elec ochemical beha io and he co ela ion be ween pa icle size and
ac i i y emains elusi e. This s udy add esses his gap by ca ying ou a com-
p ehensi e in es iga ion o he HER ac i i y o well-defined mo phologies and
sizes o α-Mo
2
C nanopla es, g own ia chemical apo deposi ion. Scanning
elec ochemical cell mic oscopy (SECCM) is employed o high- esolu ion HER
mapping on flakes wi h dimensions anging om 1 μm o40μm in la e al size,
using SECCM capilla ies wi h 130 nm ip diame e . Ou findings e eal a
significan a iabili y in he HER ac i i y a he subpa icle le el, sugges ing ha
he he e ogeneous ac i i y obse ed in p is ine flakes la ge han 10 μm is due o
an addi ion o e ec s caused by he long- e m g ow h, such as s ep-edge o -
ma ion, Mo
2
C oxida ion, and he p esence o esidual g aphene. This s udy
unde sco es he impo ance o local cha ac e iza ion o indi idual Mo
2
C nano-
pla es, shedding ligh on he impac o size-dependence on he HER ac i i y.
RESEARCH ARTICLE
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α-Mo
2
C nanoshee s achie ed e ficien HER ac i i y ac oss a wide
pH ange.
[9]
In addi ion o HER, Mo
2
C is also epo ed as a ca a-
lys o he oxygen educ ion eac ion (ORR).
[10–12]
Zhang e al.
poin ed o he po en ial o quasi-pai ed P a omic si es on Mo
2
C
o unning ORR wi h high selec i i y o he ou -elec on ans-
e eac ion o wa e ,
[10]
wi h aluable pe o mance o uel cell
applica ions. Fu he mo e, Mo
2
C has also been explo ed o elec-
ochemical CO
2
educ ion eac ion.
[13,14]
Liki h also discussed
he inc eased he modynamic s abili y o o ho hombic Mo
2
C
due o he o ma ion o molybdenum oxyca bides du ing he
CO
2
educ ion eac ion.
[14]
These s udies ha e shed ligh on
he explo a ion o Mo
2
C as a p omising ca alys o ene gy con-
e sion eac ions.
S uc u ally, he s abili y and pe o mance o Mo
2
C can be
enhanced h ough p ecise con ol o he ma e ial size and
phase composi ion.
[16–18]
Howe e , he e is a lack o a deepe
unde s anding o he ela ionship be ween ma e ial size and
elec ochemical ac i i y. Gene ally, hinne and la ge 2D
Mo
2
C nanopla es exhibi a highe su ace a ea- o- olume a io,
leading o a highe numbe o ac i e si es on he basal plane.
Recen e o s ha e ocused on implemen ing a bo om-up o
op-down app oach o syn hesize 2D Mo
2
C. The o me a e
known as nonlaye ed ul a hin TMCs (UThTMCs) and he la e
a e known as laye ed MXenes. In his con ex , Wu e al. imple-
men ed a bo om-up sal -assis ed, me hane ca bu iza ion ou e o
syn hesize ul a hin Mo
2
C.
[7]
Mic ocell de ices p obed selec ed
egions o he Mo
2
Cflakes and a high HER pe o mance was
measu ed on he edge and on hin flakes (0.36 nm).
[7]
Xu
e al. de eloped a bo om-up, liquid-me al-assis ed chemical
apo deposi ion (LMCVD) p ocess o syn hesize ace ed, highly
c ys alline, UThTMC nanopla es wi h a hickness down o a ew
nanome e s (3.4 nm) and ens o mic ome e s la e al a ea.
[2]
Geng e al. g ew ul a hin Mo
2
C, on coppe -molybdenum oils
using LMCVD, wi h good HER pe o mance.
[19]
While he HER pe o mance o 2D Mo
2
C has been p e iously
s udied, he la e al size-ac i i y ela ionship has no been consid-
e ed since con en ional elec ochemical measu emen s ypically
in ol e measu ing a ela i ely la ge sample a ea, whe e a wide
size dis ibu ion o ma e ials is conside ed. The e o e, we
employed scanning elec ochemical cell mic oscopy (SECCM)
o locally cha ac e ize indi idually he elec ochemical ac i i y
o di e se sizes o Mo
2
C. SECCM has eme ged as a pi o al ech-
nique o in es iga ing local elec ochemical p ope ies a he
nanoscale
[20–23]
mainly applied o acqui ing elec oca aly ic
ac i i y mapping o nanopa icles
[23–25]
and 2D ma e ials
[26–32]
such as g aphene
[26]
and ansi ion me al dichalcogenides.
[33–35]
SECCM can e eal di e ences in ac i i y a he nanoscale and
wi h subpa icle measu emen s, being capable o esol ing he
elec ochemical beha io wi hin indi idual sec ions inside a
single nanopa icle.
[25,36]
This echnique enables he co ela ion
o physical and chemical p ope ies wi h ca aly ic pe o mance,
which is essen ial o op imizing ca alys s a he nanoscale.
Ad anced mic oscopy echniques such as SECCM a e c ucial
o gaining deepe insigh s in o nanoscale elec ochemical
p ocesses, guiding he op imiza ion o syn hesis me hods o
enhanced ca aly ic pe o mance.
Recen ly, SECCM was applied o explo e he elec ochemical
beha io o a MXene.
[37]
This s udy employed cyclic ol amme y
(CV) and elec ochemical impedance spec oscopy (EIS) o
in es iga e he p ope ies o a Ti
3
C
2
T
x
MXene. The findings indi-
ca ed ha he pseudocapaci i e beha io o Ti
3
C
2
T
x
is p ima ily
a ibu ed o i s unique su ace chemis y and s uc u e, which
acili a e as and e e sible edox eac ions. The esul s demon-
s a ed a significan inc ease in capaci ance wi h highe scan
a es, highligh ing he dominance o su ace-con olled p o-
cesses. Addi ionally, he EIS da a e ealed low cha ge ans e
esis ance and high capaci ance, u he confi ming he e ficien
pseudocapaci i e pe o mance o Ti
3
C
2
T
x
. To he bes o ou
knowledge, no s udies ha e used SECCM o unde s and he local
elec ochemical p ope ies o Mo
2
C UThTMC nanopla es o
di e en sizes.
The e o e, SECCM was employed sys ema ically o p o ide
nanoscale mappings o he HER ac i i y o Mo
2
C UThTMC
nanopla es in acidic media. A comp ehensi e s udy o he
HER ac i i y o single hexagonal Mo
2
C nanopla es wi h sizes
anging om 2 o 50 μm in la e al size was pe o med using
nanoscale SECCM p obes wi h 130 nm ip diame e . Ou find-
ings e eal significan a iabili y in HER ac i i y a he subpa -
icle le el, wi h nanopla es smalle han 10 μm exhibi ing
highe HER ac i i y han la ge nanopla es (>10 μm). We a ib-
u ed his beha io o a con olu ion o e ec s: size-dependen o -
ma ion o s ep-edges and na i e Mo oxide du ing he CVD
g ow h, and he p esence o esidual g aphene. The co ela ion
be ween he elec ochemical ac i i y o Mo
2
C wi h he ma e ial
size was suppo ed by a omic o ce mic oscopy (AFM), ansmis-
sion elec on mic oscopy (TEM), ene gy-dispe si e X- ay spec-
oscopy (EDX), Raman spec oscopy, X- ay di ac ion (XRD),
and X- ay pho oelec on spec oscopy (XPS).
2. Resul s and Discussion
The size dependence o he elec ochemical ac i i y o Mo
2
C
nanopla es a he subpa icle le el was sys ema ically in es iga ed
by employing a high- esolu ion SECCM o map he local elec o-
chemical ac i i y owa ds HER. Mo
2
C nanopla es we e g own ia
a con ollable a mosphe ic p essu e liquid me al chemical apo
deposi ion p ocess,
[2]
as de ailed in he suppo ing in o ma ion.
As depic ed in Figu e S1, Suppo ing In o ma ion, Mo a oms di -
use h ough he liquid coppe oil and eac wi h me hane on he
Cu su ace. The coppe oil hickness, me hane-hyd ogen con-
cen a ion, and g ow h du a ion se e as he basis o con ol
he g ow h o specific la e al sizes, nuclea ion densi y, and hick-
ness o he Mo
2
C nanopla es.
[38]
Op ical mic oscopy and scanning elec on mic oscopy (SEM),
we e employed o e alua e he e ec o coppe hickness, me h-
ane concen a ion and g ow h ime, and une he syn hesis
pa ame e s o achie e he desi ed nanopla es’la e al size and
hickness. Op ical mic oscopy and SEM imaging p o ided a dis-
ibu ion o la e al leng hs o he nanopla es, as illus a ed in
Figu e 1 and S2, Suppo ing In o ma ion. The p edominan
mo phology o Mo
2
C appea s o be hexagonal p isms wi h a la -
e al side leng h a ying om 2 μm o50μm. Two samples we e
selec ed o he elec ochemical s udy and desc ibed in he sup-
po ing in o ma ion. Summa izing, Sample I was p epa ed wi h
pa ame e s o induce flakes wi h small size, esul ing in Mo
2
C
nanopla es wi h la e al size dis ibu ion o 5.3 2.4 μm
(Figu e 1A,B). While Sample II was g own by main aining
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pa ame e s o achie e la ge la e al-sized flakes. Sample II Mo
2
C
flakes p esen ed a size dis ibu ion o 14.0 12.8 μm (Figu e 1C,D).
No e ha Sample II g ow h p o ocol p o ided a b oad size
dis ibu ion wi h he p esence o small (la e al sizes <10 μm)
and la ge flakes (>10 μm). In e es ingly, he o ma ion o
s ep-edges in Sample II was obse ed equen ly, which was
p obably p o oked due o an excess o molybdenum on he
su ace due o he long g ow h ime.
[39]
The hickness o he wo samples o Mo
2
C nanopla es was
de e mined by a omic o ce mic oscopy (AFM) on nanopla es
ans e ed o he mally oxidized silicon (SiO
2
/Si) subs a es,
u ilizing he we chemical ans e me hod (as ou lined in he
Suppo ing In o ma ion). The nanopla e hickness anged om
40 nm o 150 nm, demons a ing he o ma ion o ul a hin
nanopla es (Figu e S3, Suppo ing In o ma ion), depending
on he p ocessing condi ions. AFM da a e ealed a co ela ion
be ween he la e al size and he hickness o he nanopla es,
whe e he hickness was in e sely p opo ional o he nanopla e’s
size. Specifically, he Mo
2
Cflakes on Sample II, wi h la e al size
>10 μm, we e obse ed o be hinne han he nanopla es in
Sample I, wi h la e al size <10 μm. This could be ela ed o
he e ical and la e al g ow h modes s udied by Buke e al. wi h
a ying he coppe hicknesses and g ow h ime.
[38]
We no e,
howe e , ha since he coppe oils a e s acked on o he molyb-
denum oil, he e is a challenge in main aining a uni o m flux o
molybdenum ac oss he en i e subs a e which could lead o a
dis ibu ion o nanopla e hicknesses.
2.1. Co ela i e Mapping o Topog aphic and Elec ochemical
P ope ies o he Sample I and Sample II Flakes
The elec ochemical ac i i y SECCM mapping was pe o med by
eco ding ol ammog ams o ollow he local HER. The SECCM
ip was filled wi h 10 mmol L
1
HClO
4
, and he map was pe -
o med on flakes o he Sample I (Figu e 2) and Sample II
(Figu e 3). The HER was selec ed as an inne -sphe e elec on
ans e eac ion, since i is highly dependen on he physico-
chemical p ope ies o he ca alys su ace, making i a aluable
p obe o s udying s uc u e-ac i i y ela ionships.
High- esolu ion HER cu en maps o single Mo
2
Cflakes
we e ob ained using nanoscale SECCM p obes wi h 130 nm
ip diame e , as seen in Figu e S4, Suppo ing In o ma ion,
acco ding o he me hodology de ailed in he suppo ing in o -
ma ion. AFM opog aphy image pe o med a e he SECCM
measu emen s e eals ha he SECCM d ople size had an
e ec i e spo diame e o 214 1 nm (Figu e S5, Suppo ing
In o ma ion), confi ming ha he SECCM esolu ion co e ed
subpa icle esolu ion.
SECCM can also p o ide a opog aphical image o he sample
du ing he acquisi ion o elec ochemical ac i i y maps. This ea-
u e allowed us o co ela e opog aphical SECCM, mo phologi-
cal (SEM), opog aphical AFM da a, and elec ochemical
(SECCM) in o ma ion, espec i ely, as shown in Figu e 2A–D.
As depic ed in Figu e 2A–C by mo phological and opog aphical
imaging, wo ou o he h ee Mo
2
C nanopla es o Sample I
Figu e 1. Analysis pe o med on A,B) Sample I and C,D) Sample II. A,C) Mo phology is seen om op ical mic oscopy and B,D) dis ibu ion o Mo
2
C
la e al size.
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Figu e 2. Mo phological and local elec ochemical cha ac e iza ion o Mo
2
C nanopla es g own on sample I. A) SECCM opog aphic mapping cons uc ed
om he z-piezo ex ension. B) SEM image using seconda y elec ons and elec on accele a ion ol age o 5 kV. C) AFM opog aphy image acqui ed in he
noncon ac mode. D) SECCM mapping o HER cu en a 0.6 V e sus RHE. E) Ta el slope mapping calcula ed be ween 0.40 and 0.55 V s RHE.
F) AFM heigh p ofile ex ac ed om lines 1 and 2 in (C). Rep esen a i e LSV cu es o G) flake 1, H) flake 2, and I) flake 3 a e numbe ed in (D). The (x,y)
coo dina es om whe e he LSV cu es we e ex ac ed a e shown in he legend. LSV cu es we e ob ained in 10 mmol L
1
HClO
4
wi h scan a e o 2 V s
1
SECCM image was acqui ed wi h a hopping dis ance o 500 nm and SECCM ip o 130 nm diame e .
Figu e 3. SECCM mappings o Mo
2
C nanopla es in Sample II showing A) he HER cu en a 0.6 V e sus RHE and C) he Ta el slopes calcula ed
be ween 0.4 and 0.55 V e sus RHE acqui ed wi h a hopping dis ance o 1 μm. B) Cu en and D) Ta el slope his og ams we e ex ac ed o he big
flake (flake 4, o ange ba s) and small flake (flake 5, blue ba s), espec i ely. The inse in (A) shows he op ical mic oscopy image o he measu ed flakes.
HR-TEM o α-Mo
2
C nanopla es a E,F) he basal plane and G,H) he c oss-sec ion plane.
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displayed a hexagonal shape, while one appea ed ec angula .
These flakes we e also isualized in he cen al bo om pa
o Figu e 1A. Figu e 2D,E demons a es he capabili y o
SECCM o locally measu ing he HER elec ochemical ac i i y
co e ing nanopla es and subpa icle in o ma ion, while di e en-
ia ing he ac i i y o he basal plane and he edges.
Rega dless o he nanopla e geome y, all h ee Mo
2
Cflakes
measu ed in Sample I ha e la e al sizes below 10 μm and exhib-
i ed smoo h basal planes wi h low su ace oughness wi hou
s ep-edges. This obse a ion was confi med by SEM and AFM
analyses (Figu e 2B,C, espec i ely), e ealing fla su aces wi h
edges “bu ied”in he Cu subs a e. The AFM heigh p ofile
(Figu e 2F) illus a es ha he edges o bo h hexagonal nano-
pla es (depic ed by he ed line) a e no ably mo e p ominen
han he basal plane, e iden om he inc eased heigh a he
edges, also isualized in he SECCM opog aphical mapping
(Figu e 2A). Addi ionally, he hexagonal Mo
2
C nanopla es’basal
plane su ace lies below he Cu su ace because he g ow h
in ol es Mo di usion h ough Cu. In con as , he beha io o
he ec angula nanopla e is dis inc , g owing “ou ”o he coppe
su ace, as shown by he black line in Figu e 2F. AFM and SEM
e ealed pa icula e ea u es a he nanopla e edges a ibu ed o
coppe oxide pa icles. Raman spec oscopy (Figu e S6,
Suppo ing In o ma ion) shows he p esence o pa icles on
he edges o Mo
2
Cflakes which ma ch he Raman signa u e
o coppe oxide. STEM-EDS mapping o ans e ed and isola ed
flakes in Figu e S7, Suppo ing In o ma ion, shows coppe oxide
pa icles p esen on he basal plane which we a ibu e o he
incomple ely e ched coppe subs a e. The absence o coppe
oxide pa icles on he edges o he Mo
2
Cflakes is consis en wi h
he in e p e a ion ha hese pa icles o igina e om he oxidized
coppe subs a e and a e no in insic ea u es o he Mo
2
C
flakes. The AFM and SEM images align wi h he opog aphical
SECCM image in Figu e 2A.
The HER ac i i y o Mo
2
C nanopla es in Sample I (Figu e 2D),
appea ed ela i ely uni o m, sugges ing homogenei y ac oss he
basal plane. The cu en ange o all h ee nanopla es was abou
800 pA a 0.6 V s RHE, indica ing ha di e ences in flake
mo phology do no significan ly influence elec ochemical ac i -
i y. The sho g ow h ime used in Sample I also esul ed also in a
homogeneous coppe su ace, obse ed in he ac i i y SECCM
mapping (yellow colo —cu en ange <50 pA), indica ing
uni o m bu ela i ely poo HER ac i i y, accen ua ing he con-
as compa ed o he ac i e small Mo
2
C nanopla es.
Rep esen a i e linea sweep ol amme y cu es (LSVs) o
each flake we e selec ed and plo ed in Figu e 2G–I, o del e
deepe in o he local elec ochemical beha io . Figu e 2G–I illus-
a es findings o he ec angula -shaped Mo
2
C nanopla e
(flake 1), he cen al hexagonal Mo
2
C nanopla e (flake 2), and
he igh hexagonal Mo
2
C nanopla e (flake 3), espec i ely.
Fo all h ee flakes, a consis en pa e n eme ged: Bo h he
edge and basal plane o he flakes demons a ed significan ly
enhanced pe o mance, cha ac e ized by ele a ed cu en s and
imp o ed HER ac i i y when compa ed wi h he Cu su ace.
As p e iously discussed, a he nanopla es’edges, he e is an
in e ace be ween he Mo
2
C nanopla es and he Cu subs a e ha
exhibi s a dis inc opog aphical p ofile. The LSV cu es ex ac ed
om he edges exhibi lowe HER ac i i y (Figu e 2G–I), p oba-
bly esul ing om SECCM ip landing on he he e ogenei y. This
educed ac i i y is p ima ily a ibu ed o he g ow h mechanism,
whe e Mo di uses h ough he Cu oil du ing syn hesis.
I is impo an o no e ha he coppe oxide pa icles in
he nanopla es’edge do no domina e he HER ac i i y
(Figu e 2D). The ec angula Mo
2
Cflake p esen ed in Figu e 2
o e s addi ional e idence ha esidual coppe does no block he
ca aly ically ac i e si es a he edges. In his case, he flake clea ly
g ows ou o he Cu subs a e, wi h mo e exposed edges, as
confi med by he co esponding AFM image in Figu e 2B,C,
which shows inc eased flake heigh and minimal pa icle co e -
age a he pe iphe y. No ably, he elec ochemical ac i i y o his
ec angula flake is compa able o ha o neighbo ing hexagonal
flakes, as shown in he SECCM cu en and Ta el slope maps
(Figu e 2D,E), sugges ing ha he edge ac i i y is no signifi-
can ly impeded by coppe oxide clus e s.
Addi ionally, Ta el analysis conduc ed be ween 400 and
550 mV e sus RHE (Figu e 2E) e ealed a Ta el slope close
o 70 mV dec
1
o all h ee analyzed flakes, hus confi ming uni-
o mi y i espec i e o geome y. No ably, he di e ence be ween
some spo s a he edge became mo e appa en in he Ta el plo ,
displaying highe and andom Ta el slope alues in Figu e 2E. All
Ta el plo s and hei espec i e egions can be ound in Figu e
S8, Suppo ing In o ma ion. I is impo an o no e ha Ta el
analysis was u ilized o enhance image con as and flake isual-
iza ion, acili a ing he e alua ion o he size-ac i i y ela ionship,
a he han o mechanis ic in es iga ion o quan i a i e kine ics
analysis. This conside a ion a ises because isualizing nano-
pla es smalle han 10 μm in la e al size poses challenges, pa ic-
ula ly when displaying opog aphical/ac i i y mapping.
Figu e S9–S11, Suppo ing In o ma ion, p esen SECCM
mappings o he HER ac i i y o se e al Mo
2
Cflakes o
Sample I exhibi ing simila beha io as discussed in Figu e 2:
namely, displaying a homogenous elec ochemical ac i i y o
he basal plane wi h consis en and simila cu en alues o indi-
idual flakes. These esul s indica e ha he di e ence be ween
flakes’mo phology has no significan influence on he elec o-
chemical ac i i y o Sample I.
Subsequen ly, LSV cu es we e conduc ed a di e en scan
a es (1, 2, and 4 Vs
1
) in a 10 mmol L
1
HClO
4
solu ion (see
Figu e S8I–K, Suppo ing In o ma ion). As expec ed, he limi -
ing HER cu en emained a he same alues, and a ia ions in
he scan a e did no a ec he o e po en ial o di e en selec ed
egions o he flake. Using he mass anspo limi ed-cu en
and he we ed oo p in o he SECCM meniscus (Figu e S5,
Suppo ing In o ma ion) he expec ed cu en densi y is a ound
4.8 A cm
2
. Hence, ou localized cu en s (800 pA) a e indeed
consis en wi h A cm
2
magni ude. This ea fi ms ha ou local-
ized cu en s eside in he A cm
2
ange, compa able o cu en
densi ies in mac oscopic HER benchma ks,
[5]
confi ming he
capabili y o SECCM o s udy HER wi h high cu en densi y
due o high mass anspo . Con e sely, when he elec oly e
concen a ion was inc eased om 10 o 100 mmol L
1
HClO
4
,
esul ing in a dec ease in pH, an inc ease in cu en was
obse ed (see Figu e S8L, Suppo ing In o ma ion). This inc e-
men was also no ed o he Cu su ace, which was a ibu ed o
he a ailable H
þ
concen a ion. These esul s indica e ha no
o he p ocess han HER occu s on he su ace o p is ine small
Mo
2
C nanopla es.
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I is no ewo hy ha commonly 0.5 mol L
1
H
2
SO
4
is
employed as elec oly e solu ion o e alua e he HER ac i i y
o Mo
2
C nanopla es, wi h epo ed high cu en densi ies and
o e po en ials anging be ween 200 and 400 mV e sus
RHE.
[5,8]
Howe e , we delibe a ely chose o wo k wi h a dilu ed
HClO
4
solu ion o dec ease he ma e ial’s ac i i y, accen ua ing
he local ac i i y di e ences in he basal plane and be ween flakes
o di e en sizes. The adop ed condi ion also imp o ed he s a-
bili y o SECCM measu emen s by mi iga ing e apo a ion and
ionic s eng h e ec s.
[40]
To in es iga e he e ec o size on he elec ochemical ac i i y
o Mo
2
C nanopla es, SECCM measu emen s we e conduc ed on
Sample II (Figu e 3). A con as ing end eme ged in SECCM
expe imen s on Sample II compa ed o Sample I. Figu e 3
depic s wo dis inc -sized flakes in he same SECCM ac i i y
mapping: flake 4 on he le side o Figu e 3A wi h 45 μm wid h
and flake 5 ( igh side o Figu e 3A) wi h 15 μm size. The
SECCM map e ealed ha flake 4 exhibi ed he e ogeneous
and lowe HER ac i i y han he smalle flake 5.
The basal plane o flake 4, which exhibi s non-uni o m ac i i y
as illus a ed in Figu e 3A, can be segmen ed in o h ee dis inc
egions based on he obse ed cu en acco ding o he his o-
g am in Figu e 3B. The yellow egions ( egion 1— ed squa e
in Figu e 3A), sca e ed andomly ac oss he basal plane, e idenc-
ing he ac i i y he e ogenei y, wi h cu en s <|50|pA. Analyzing
he beha io o he ligh and da k blue egions o he basal plane,
ep esen ing egions 2 and 3 espec i ely, a ansi ion om a
non-uni o m o a mo e uni o m cu en ange is obse ed.
Specifically, egion 3 demons a es a mo e uni o m cu en a
200 pA a 0.6 V e sus RHE. This is an inc ease o a ound
170 pA a 0.6 V e sus RHE compa ed o he yellow a eas in
egion 1. The ep esen a i e LSV cu es o each egion a e
depic ed in Figu e S12, Suppo ing In o ma ion.
Ta el analysis was employed o enhance comp ehension and
isualiza ion o he Mo
2
C nanopla es ac i i y. The hexagonal
s uc u e o he nanopla es is eadily disce nible o bo h flakes
4 and 5 (Figu e 3C). Howe e , flake 4 exhibi ed non-uni o m
Ta el slopes spanning a wide ange om 50 o 150 mV dec
1
(Figu e 3D), indica ing he he e ogenei y o he local ac i i y.
I is impo an o no e ha o flake 5 in Figu e 3, he ac i i y
exhibi ed highe le els o inhomogenei y compa ed o flakes
o simila la e al size in Sample I (Figu e 2 and S8–S11,
Suppo ing In o ma ion). This obse a ion was subsequen ly
confi med by SECCM mapping and co obo a ed by he Ta el
slope mapping in Figu e 3A,C, espec i ely. Specifically, he
ac i i y o flake 5 demons a ed a Ta el slope o 80 mV dec
1
o he basal plane. This finding unde sco es he significan
influence o g ow h condi ions on he Mo
2
C su ace
cha ac e is ics.
I is wo h no ing ha he coppe subs a e in Sample II seems
o exhibi a non-uni o m and ough su ace, as e idenced by wo
colo ed egions wi h dis inc HER ac i i y (Figu e 3A). Howe e ,
he scale ba in Figu e 3A indica es a simila cu en ange o he
coppe su ace ob ained in Figu e 2.
The low HER ac i i y o he Mo
2
C nanopla es in Sample II, he
high subpa icle he e ogenei y, and he high oughness o he
coppe su ace hampe s he clea isualiza ion o Mo
2
C nano-
pla es o Sample II in he SECCM elec ochemical mapping.
Mo eo e , as showed in he AFM analysis in Figu e S3,
Suppo ing In o ma ion, la ge nanoflakes esul ed in a educed
hickness, making i di ficul o isualize he nanopla es in he
opog aphic SECCM image.
Figu e S13, Suppo ing In o ma ion shows AFM opog aphy
and heigh p ofile o Mo
2
Cflakes o Sample II, e ealing he
p esence o s ep-edges, which a y in heigh be ween 15 nm
and 25 nm. In e es ingly, educed HER ac i i y was disce nible
a he cen e o flake 4, indica ed by he ed a ow in Figu e 3A.
Ta el plo imaging explica ed he well-shaped-low ac i i y egion
(see Figu e 3C), indica ing he p esence o he s ep-edge on he
cen e o he flake, as seen in he op ical mic oscopy image in he
inse o Figu e 3A. When he SECCM ip landed on he s ep-
edge, he majo i y o he SECCM meniscus possibly con ac ed
ei he he basal plane o he e ace and bo om laye , as well
as he c ys alline plane o he s ep-edge. In o he wo ds,
SECCM mapping o flake 4 has a highe he e ogenei y, p obably
due o he p esence o he s ep-edges ha expose di e en planes.
Plana HR-TEM images in Figu e 3E,F e eal he Mo
2
C (021)
planes wi h an in e plana dis ance o 0.257 nm, which a e pe -
pendicula o he (200) planes, as shown in Figu e 3F. This obse -
a ion confi ms he p esence o he (200) planes on he Mo
2
C
basal su ace. Addi ionally, c oss-sec ional TEM images in
Figu e 3G,H show he (200) planes wi h an in e plana dis ance
o 0.242 nm, demons a ing ha he (001) planes, which a e pe -
pendicula o he (200) planes, a e exposed la e ally in he nano-
pla e. Figu e S14, Suppo ing In o ma ion, e eals ha he
obse ed coppe oxide pa icles we e emo ed du ing he sample
ans e o he TEM g id. Consequen ly, he edge s uc u es
obse ed in TEM ep esen he clean, in insic mo phology o
he highly c ys alline Mo
2
C nanopla es.
The e o e, he HER ac i i y a he s ep-edges eflec s he a e -
age esponse o bo h (200) and (001) planes. The (001) su ace
exhibi s Mo e mina ions wi h he C a oms in a sublaye , while
he (200) su ace ea u es mixed Mo and C e mina ions. Wang
e al. calcula ed a mo e nega i e hyd ogen adso p ion ene gy o
he (001) planes a low H co e age compa ed o he (200) planes
using densi y unc ional heo y.
[15]
Con e sely, a high H co e -
age, bo h su aces showed nea ly he moneu al H adso p ion.
[15]
Hence, we an icipa e ha a low H
þ
concen a ions, he (001) and
(200) planes would exhibi dis inc HER ac i i y, wi h he (200)
being mo e ac i e han he (001), hus elucida ing he low ac i i y
si es a he s ep-edges obse ed in he SECCM ac i i y mapping
in Figu e 3A. Fu he mo e, since bo h flake edges and s ep-edge
ea u es expose he (001) c ys allog aphic planes, i is easonable
o conclude ha he flake edges and he s ep edges would exhibi
simila in insic HER ac i i y, u he suppo ing ou in e p e a-
ion ha coppe esidues do no enhance o domina e he
obse ed ca aly ic beha io o he flake’s edges.
Howe e , he p esence o only one s ep edge in flake 4
depic ed in Figu e 3A, along wi h he obse a ion ha nanopla es
in Sample II wi hou s ep edges also exhibi a he e ogeneous
HER esponse ( e e o Figu e S15, Suppo ing In o ma ion),
sugges s ha he dis inc local elec ochemical beha io o flakes
syn hesized wi h longe g ow h imes (Sample II) canno solely
be a ibu ed o he p esence o s ep edges.
Figu e 4A p esen s an AFM opog aphic image, acqui ed on
Sample II, showcasing h ee di e en Mo
2
C nanopla es, indi-
ca ed as flakes 6, 7, and 8. Figu e 4D displays he heigh p ofile
ex ac ed om he AFM opog aphy image in Figu e 4A o he
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h ee flakes. Flake 6 in Figu e 4D has a la e al size o 20 μm and
deepe g ow h in o he coppe su ace, e iden om a significan
alley in he AFM heigh p ofile. In con as , flake 7 exhibi s a
la e al size o 8μm and edges “bu ied”in he Cu su ace, like
he nanopla es in Sample I. In e es ingly, flake 8 in Figu e 4D
e eals he p esence o e ically aligned nanopla es, wi h heigh s
be ween 400 nm and 600 nm abo e he coppe su ace, and a
clea ele a ion o he Cu su ace. The opog aphical SECCM
image in Figu e S16, Suppo ing In o ma ion was aligned wi h
he AFM analysis, e ealing a highe Z ex ension on he e ical
flake.
As an icipa ed, he 20 μmMo
2
C nanopla es (flake 6) exhibi ed
poo and non-uni o m ac i i y, almos indis inguishable om
he Cu su ace (Figu e 4B), wi h HER cu en s smalle han
70 pA (Figu e 4F). The opposi e end was obse ed o flake 7,
which displayed cu en s highe han 300 pA a 0.7 V e sus
RHE (Figu e 4F). Con e sely, flake 8, he e ical one, showed
an in e media e end wi h HER cu en s a ound 200 pA a
0.7 V e sus RHE (Figu e 4B,F). The Ta el slope image in
Figu e 4C shows ha he h ee flakes can be dis inguished om
he Cu su ace due o di e ence on Ta el slope e sus he Cu
su ace.
As discussed in Figu e 3 and confi med in Figu e 4, Sample II
exhibi ed a b oad size dis ibu ion o Mo
2
C nanoflakes, wi h high
he e ogenei y in he elec ochemical ac i i y owa d HER o he
big flakes han o he small ones. Mo eo e , he small flake 7
p esen ed a la ge elec ochemical ac i i y han he big flake 6.
The e ical flake (flake 8) displayed he e ogeneous ac i i y, wi h
egions showing a simila o e po en ial as flake 7 (g een cu e in
Figu e 4E), and o he s wi h an o e po en ial o 550 mV e sus
RHE (ligh blue cu e in Figu e 4E). The e ical flake is expec ed
o expose he (001) plane a he su ace; hus, a simila ac i i y as
he s ep-edges obse ed in he ho izon al la ge flakes is
an icipa ed. Figu e S17–S19, Suppo ing In o ma ion, p esen
SECCM HER mapping o e ically aligned Mo
2
Cflakes,
showing a simila beha io . In hese measu emen s, no Cu oxide
nanopa icles a e obse ed in ei he he AFM image (Figu e 4A)
o he co esponding SEM images (Figu e S17 and S20,
Suppo ing In o ma ion), allowing a clean assessmen o he
in insic ac i i y o he exposed c ys al planes. These measu e-
men s consis en ly demons a e ha he (001) planes exposed
a he flake edges exhibi lowe HER ac i i y compa ed o he
(200) basal planes, as e ealed by he SECCM cu en and
Ta el slope maps. This ein o ces ou in e p e a ion ha he
dec eased edge ac i i y is no an a i ac o su ace con amina-
ion, bu a he eflec s he in insic elec oca aly ic p ope ies
o he Mo
2
C c ys al ace s. Howe e , we encoun e ed challenges
in mapping he elec ochemical ac i i y o he e ical flakes,
such as: 1) Ve ical nanopla es had small wid hs (500 nm—
Figu e S20, Suppo ing In o ma ion), making i easy o acciden-
ally skip o e hem wi h a hopping dis ance o 500 nm o less,
esul ing in he loss o impo an in o ma ion abou hei ac i i y,
and 2) isualiza ion o e ical nanopla es was complica ed due o
mo e he e ogeneous flakes o Sample II and coppe su ace
(Figu e S19, Suppo ing In o ma ion).
Based on he SECCM measu emen s a he subpa icle le el in
Sample II, i can be concluded ha nanopla es la ge han 20 μm
in la e al size exhibi a he e ogeneous and low HER ac i i y, wi h
cu en s lowe han 70 pA (Figu e 3 and 4). The beha io o
flakes wi h sizes anging be ween 10 and 20 μm in la e al size
di e s om la ge flakes, showing cu en alues a ound
200 pA ac oss he en i e basal plane, as depic ed in he his o-
g am in Figu e 3B. This beha io is dis inc om ha o Sample
I, whe e he sho e g ow h ime led o flakes wi h sizes smalle
han 10 μm and exhibi ing cu en s a ound 1000 pA a 0.6 V
e sus RHE (see Figu e 2). These findings unde sco e a clea
size-elec ochemical ac i i y ela ionship o Mo
2
C nanopla es
owa ds HER.
Bulk elec ochemical measu emen s pe o med in samples
ans e ed o glassy ca bon (GC) su ace (Figu e S21,
Figu e 4. A) AFM opog aphy image, B) SECCM HER cu en , and C) Ta el slope mapping, acqui ed wi h a hopping dis ance o 1 μm, o Mo
2
C
nanopla es in Sample II. D) Heigh p ofile o flakes 6, 7, and 8 ex ac ed om he lines indica ed in (A). E) Rep esen a i e LSV cu es o flakes 6, 7,
and 7 ex ac ed om he (x,y) coo dina es o he cu en mapping in (B). F) Cu en his og ams o flakes 6, 7, and 8 ex ac ed om (B), only he pixels
inside he flakes we e conside ed.
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Suppo ing In o ma ion) e eal no clea di e ence be ween he
HER ac i i y o di e en Mo
2
C samples, which does no eflec
he size-dependen he e ogenei y ha is clea ly esol ed by he
SECCM maps. This esul is expec ed, as he low su ace co e -
age o Mo
2
C and he ensemble a e aging inhe en o bulk meas-
u emen s obscu e he local a ia ions in ac i i y. Mo e impo an ,
he LSV cu es using s anda d me hods did no achie e mass
anspo limi ed cu en , which is shown in he SECCM LSV
cu es (Figu e 2G–I).
2.2. Comp ehensi e S uc u al Cha ac e iza ion o Mo
2
C Flakes
The co ela i e AFM/SECCM mappings demons a ed a dis inc
co ela ion be ween he nanoflake size and he elec ochemical
ac i i y o Mo
2
C. Fu he mo e, he obse ed he e ogenei y in
elec ochemical ac i i y o flakes syn hesized wi h longe g ow h
imes (Sample II) could no be solely a ibu ed o he p esence o
s ep edges. To elucida e he unde lying di e ences in chemical
composi ion be ween Sample I and Sample II, a comp ehensi e
cha ac e iza ion was conduc ed, p o iding deepe insigh in o
hei espec i e physicochemical p ope ies.
XRD analysis was conduc ed in he B agg–B en ano geome y
on he as-g own Sample I and Sample II. The XRD di ac o-
g ams we e plo ed in loga i hmic in ensi y scales o enhance
he isualiza ion o low-in ensi y peaks, as depic ed in
Figu e 5A,B and S2C, Suppo ing In o ma ion.
Ou analysis e ealed ha he flakes consis o α-Mo
2
C (PDF
01-071-0242) wi h an o ho hombic uni cell, belonging o space
g oup Pbcn, wi h a (200) su ace o ien a ion. This o ien a ion is
e idenced by he p ominen peaks nea 2θ=38.04° and 81.37°
and 2θ=37.99° and 81.23° in Sample I and II, espec i ely.
Addi ionally, flakes o ien ed along a ious planes we e ob ained,
such as he (021), (121), (221), (023), and (321) planes, whe e he
(200) plane is o ien ed pe pendicula o he subs a e su ace.
This o ien a ion is acili a ed by ex u ing be ween he (111) cop-
pe subs a e and he e ically o ien ed flakes, enabling he
obse a ion o planes pe pendicula o he (200) basal su ace.
In Sample II, he p esence o an uniden ified XRD peak a
2θ=37° was no ed and addi ionally he p esence o i s
second-o de di ac ion peak a 2θ=78.8° (Figu e 5B and S2,
Suppo ing In o ma ion). We hypo hesized ha he unknown
XRD peaks co espond o c ys alline (020) and (040) MoO
2
(PDF 01-073-1249) wi h a monoclinic uni cell belonging o space
g oup P2
1
/c. The obse a ion o MoO
2
om XRD indica es ha i
is likely ex u ed along he (200) basal su ace o α-Mo
2
C.
No ably, he p esence and in ensi y o MoO
2
exhibi ed a clea
dependence on he syn hesis pa ame e s. Sample I, wi h Mo
2
C
flakes wi h la e al sizes smalle han 10 μm lacked he a ibu ed
MoO
2
peak, as illus a ed in Figu e 5A. Con e sely, Sample II,
wi h nanopla es la e al sizes anging om 2 μm o40μm,
consis en ly exhibi ed MoO
2
peaks (Figu e S2, Suppo ing
In o ma ion). XRD di ac og ams on a linea in ensi y scale
in Figu e S2, Suppo ing In o ma ion, e idenced he hypo he-
sized MoO
2
peaks a e o low in ensi y and low quan i y, making
i a challenge o de ec addi ional XRD peaks co esponding o
MoO
2
. Addi ionally, he linea scale highligh s he sha pness
o he Mo
2
C (200) peak mo e clea ly, confi ming he high c ys-
allini y o he samples. Al hough he peaks a 2θ=37° and 78.8°
Figu e 5. XRD on as-g own subs a es in log in ensi y scales o flakes in A) Sample I and B) Sample II. Chemical and s uc u al cha ac e iza ion o Mo
2
C
om C–F) Sample I wi h 9 μm la e al size and G–J) Sample II wi h 27 μm la e al wid h. HAADF-STEM and he co esponding EDS maps showing he
dis ibu ion o D,H) molybdenum and E,I) ca bon h oughou he flakes. F,J) SAED acqui ed along he ex u ed [200] o Mo
2
C.
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ma ch wi h monoclinic MoO
2
peaks, we obse ed significan
b oadening, which could be ela ed o he p esence o molybde-
num oxyca bides (MoO
x
C
y
). To check he p esence o a mix u e
o MoO
x
C
γ
, we simula ed powde XRD pa e ns wi h x and y
being 1:4 and 1:8 a ios be ween oxygen and ca bon.
The simula ed di ac og ams e ealed ha subs i u ing
oxygen in o he Mo
2
C la ice inc eases he (200) and (400) in e -
plana dis ances, shi ing he di ac ion peaks o smalle angles,
owa ds he ex a peaks a 37° and 78.8° (Figu e S22, Suppo ing
In o ma ion). Howe e , a high concen a ion o oxygen (1:4)
esul s in in e plana dis ances la ge han hose associa ed wi h
he unknown XRD peaks. This indica es ha specific, low con-
cen a ions and a o able oxygen configu a ions wi hin he Mo
2
C
la ice a e necessa y o ep oduce he expe imen ally obse ed
XRD pa e n. Based on bo h expe imen al and heo e ical
XRD analysis, Sample I likely con ains a lowe oxygen con en ,
insu ficien o p oduce a p onounced (010) MoO
2
peak. In con-
as , Sample II exhibi s a highe oxygen con en , leading o he
o ma ion o a well-defined MoO
2
and low in ensi y peak in he
XRD pa e n.
Mo
2
Cflakes we e ans e ed on o a TEM g id u ilizing he
we chemical ans e me hod (as ou lined in he Suppo ing
In o ma ion). Examina ion ia low-magnifica ion high-angle
annula da k-field scanning ansmission elec on mic oscopy
(HAADF-STEM), as depic ed in Figu e 5C,G, e eals he hexag-
onal nanopla e mo phology o flakes wi h 9 μm (Sample I) and
27 μm (Sample II) in la e al leng h, espec i ely. Selec ed a ea
elec on di ac ion (SAED) om bo h flakes, shown in
Figu e 5F,J, unequi ocally confi ms he high c ys allini y o
he syn hesized α-Mo
2
C nanopla es, wi h he [200] o ien a ion
and he (200) plane p ominen ly exposed a he basal su ace.
No ably, he SAED pa e n e eals dis inc s ong and weak spo s,
bo h co esponding o he molybdenum subla ice, wi h he weak
spo s indica i e o long- ange pe iodic molybdenum la ice dis-
o ions induced by o de ed ca bon acancies wi hin α-Mo
2
C
(Figu e S23, Suppo ing In o ma ion). Fu he mo e, TEM anal-
ysis showed ha he hexagonal nanopla e comp ises six g ains
wi h ace s co esponding o (001) planes, pe pendicula o
[001], o he zigzag plane o he c ys al, as schema ically depic ed
in Figu e S23, Suppo ing In o ma ion.
[41,42]
Upon close inspec-
ion o SAED, we obse ed a no able di e ence in he spo pa -
e n. In Sample II, ex a spo s we e obse ed ha could no be
indexed o he o bidden eflec ions in α-Mo
2
C (Figu e S24,
Suppo ing In o ma ion). Fu he mo e, hese ex a spo s appea
o o m a hexagonal pa e n along wi h he p ima y spo s. We
hypo hesize ha hese ex a spo s occu due o double di ac ion
and he p esence o s acking aul s wi hin he s uc u e.
XPS was conduc ed di ec ly on he as-g own samples. The su -
ey scan e eals ha Sample II con ains molybdenum, ca bon,
and oxygen, as seen in Figu e S25A, Suppo ing In o ma ion.
The high- esolu ion Mo 3d spec um, shown in Figu e S25B,
Suppo ing In o ma ion, consis s p ima ily o an asymme ic
double wi h Mo3d
5/2
a 228.13 eV, which co esponds o
Mo
2
C.
[13]
A second con ibu ion, wi h Mo3d
5/2
a 229.2 eV, is
a ibu ed o Mo
4þ
p esen ed as MoO
2[43]
and was modeled
conside ing he cha ac e is ic sa elli e ea u es o MoO
2
.
[43,44]
The high- esolu ion C1s spec um, shown in Figu e S25C,
Suppo ing In o ma ion, was decon olu ed in o ou compo-
nen s: 1) a peak a 284.7 eV, a ibu ed o sp
3
ca bon; 2) a peak
a 284.4 eV, which co esponds o sp
2
hyb idized ca bon, o igi-
na ing om g aphene p esen in he as-g own sample; 3) a peak
a 283.3 eV, which is a ibu ed o ca bon in Mo
2
C;
[13]
and
4) a small peak a 288.3 eV, which co esponds o ca bonyl
g oups. The o al ca bon/molybdenum a io (C/Mo) was de e -
mined o be 10.8, due o he dominan p esence o g aphene.
Impo an ly, when conside ing only he componen s assigned
o Mo
2
C (Figu e S25, Suppo ing In o ma ion) on each o he
high- esolu ion egions, he C/Mo a io is 0.57, in close ag ee-
men wi h he s oichiome y o Mo
2
C. We poin ed ou ha p io
o he SECCM measu emen s, he g aphene o med on he
Mo
2
C nanopla es was emo ed ia mechanical ex olia ion using
a Kap on ape, as desc ibed in he suppo ing in o ma ion
(Figu e S26, Suppo ing In o ma ion). Howe e , ce ain flakes
wi h la ge la e al size could exhibi i egula su aces wi h esid-
ual g aphene, which may influence he elec ochemical pe o -
mance o hese flakes (Figu e S27, Suppo ing In o ma ion).
To e alua e he possible composi ion a ia ion wi h he la e al
size, EDS mappings we e acqui ed bo h in HAADF-STEM and
SEM modes. STEM-EDS mapping shown in Figu e 5 and
S28–S29, Suppo ing In o ma ion e eals a uni o m dis ibu ion
o molybdenum and ca bon in he nanopla es. In he SEM EDS
mapping, due o low sensi i i y and he e ec o Cu subs a e,
jus Mo and C we e de ec ed in he nanopla es mapped in he
as-g own samples ega dless o hei sizes (Figu e S30 and S31,
Suppo ing In o ma ion). STEM-EDS p ofiles in Figu e S29,
Suppo ing In o ma ion, e eal a s onge oxygen signal nea
he edges o he nanopla es in Sample I, whe eas in Sample II,
he oxygen signal appea s uni o mly dis ibu ed ac oss he
basal plane. AFM da a indica ed a co ela ion be ween he la e al
size and hickness o he nanopla es (Figu e S3, Suppo ing
In o ma ion). Specifically, o Sample I, g own wi h low me hane
concen a ions and sho g ow h imes, he nanopla es ha e
la e al sizes below 10 μm and hicknesses anging om 80 o
150 nm, wi h oxide o ma ion localized a he edges. Con-
e sely, o Sample II, p epa ed wi h highe me hane concen a-
ions and longe g ow h imes, he nanopla es exhibi a b oade
la e al size dis ibu ion (5–40 μm) bu a e hinne , wi h hick-
nesses below 50 nm, and oxide o ma ion dis ibu ed uni o mly
ac oss he basal plane.
These findings sugges ha na i e MoO
2
ends o o m on he
slowe -g owing su aces. In sho -g ow h- ime samples, he
g ow h p edominan ly occu s h ough he basal plane, esul ing
in smalle nanopla es wi h oxide localized a he edges. In con-
as , longe g ow h imes p omo e la e al g ow h, p oducing
la ge bu hinne flakes wi h ex ensi e oxide co e age ac oss
he basal plane.
Spa ially esol ed Raman mapping on Sample II was con-
duc ed o di ec ly co ela e he dimensions and densi y o na i e
oxide si es (Figu e S32, Suppo ing In o ma ion). Cha ac e is ic
Raman modes o o ho hombic Mo
2
C
[45]
we e obse ed in
Sample II. Howe e , no Raman peaks ela ed o MoO
2
in ei he
he small o la ge la e al dimension nanoflakes we e obse ed.
Significan a ia ions in he Mo
2
C peak in ensi y, posi ion,
and wid h we e obse ed o he la ge flakes, indica ing a he e o-
geneous c ys alline s uc u e. These a ia ions we e especially
obse ed in he 143 cm
1
(B
3g
,A
g
) Raman peak likely caused
by s uc u al de ec s o oxygen impu i ies, which can induce local
s ain in he c ys alline s uc u e. De ec s such as s acking aul s
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