Applied Ma e ials Today 34 (2023) 101881
A ailable online 29 July 2023
2352-9407/© 2023 The Au ho s. Published by Else ie L d. This is an open access a icle unde he CC BY license (h p://c ea i ecommons.o g/licenses/by/4.0/).
Hema ene: A sus ainable 2D conduc i e pla o m o isible-ligh -d i en
pho oca aly ic ammonia decomposi ion
Jana Dzíbelo ´
a
a
,
b
,
1
, S.M. Hossein Hejazi
a
,
c
,
1
, Ve onika ˇ
Sedajo ´
a
a
, Da id Pan´
aˇ
cek
a
,
Pe Jakubec
a
,
*
, Zdenˇ
ek Ba
ď
u a
a
,
b
, Ondˇ
ej Malina
a
, Jose Kaˇ
slík
a
, Jan Filip
a
, ˇ
S ˇ
ep´
an Kmen
a
,
c
,
Michal O yepka
a
,
d
,
*
, Radek Zboˇ
il
a
,
c
,
*
a
Regional Cen e o Ad anced Technologies and Ma e ials, Czech Ad anced Technology and Resea ch Ins i u e (CATRIN), Palacký Uni e si y Olomouc, ˇ
Slech i elů 241/
27, Olomouc 779 00, Czech Republic
b
Depa men o Expe imen al Physics, Facul y o Science, Palacký Uni e si y Olomouc, 17. lis opadu 1192/12, Olomouc 779 00, Czech Republic
c
Nano echnology Cen e, Cen e o Ene gy and En i onmen al Technologies, Vˇ
SB–Technical Uni e si y o Os a a, 17. lis opadu 2172/15, Po uba, Os a a 708 00,
Czech Republic
d
IT4Inno a ions, Vˇ
SB–Technical Uni e si y o Os a a, 17. lis opadu 2172/15-Po uba, Os a a 708 00, Czech Republic
ARTICLE INFO
Keywo ds:
Hema ene
Ammonia
Hyd ogen P oduc ion
Pho oca aly ic decomposi ion
ABSTRACT
The eme ging class o 2D non- an de Waals (n- dW) ma e ials, including 2D i on oxides, possesses unique
p ope ies and high applicabili y, making hem a ac i e o a ious echnological applica ions. Howe e , he
syn hesis o hese ma e ials h ough a scalable and eco- iendly me hod emains a challenge, as mos known
chemical ex olia ion p ocesses equi e oxic o ganic sol en s. In his s udy, we epo a g een syn hesis o 2D
hema ene (
α
-Fe
2
O
3
) using an ul asound-suppo ed ex olia ion me hod o ea h-abundan i on oxide o e in a pu e
aqueous solu ion. The esul ing hema ene shee s, only a ew nanome e s hick, exhibi supe io elec ochemical
pe o mance in e ms o cha ge ans e p ocesses, making hem ideal o pho oca aly ic applica ions. By doping
a conduc i e hema ene subs a e wi h u henium, we demons a e a syne gis ic e ec o gene a ing elec ons
and holes unde isible ligh i adia ion. Using his app oach, we success ully decomposed ammonia in o
hyd ogen and ni ogen, highligh ing he po en ial o his no el class o en i onmen ally- iendly pho oca alys s
o clean ene gy p oduc ion. O e all, ou wa e -assis ed scalable syn hesis o hema ene o e s a p omising
s a egy o p oducing e icien and sus ainable pho oca alys s.
1. In oduc ion
The disco e y o wo-dimensional (2D) ma e ials has kindled
immense in e es among scien is s as hey exhibi unique chemical,
physical, and elec onic p ope ies, compa ed o hei 3D coun e pa s
[1]. Thei p ope ies can be exploi ed in a ious applica ions, including
elec onics [2,3], senso s [4–6], ene gy s o age [7–9], and spin onics
[10,11]. Owing o hei bonding na u e, 2D ma e ials can be so ed in o
wo main classes: i) an-de Waals ( dW) 2D ma e ials, such as g a-
phene, black phospho us (BP), ansi ion me al dichalcogenides,
me al-o ganic amewo ks (MOFs), o MXenes, whe e he s ong
chemical bonding is loca ed in-plane and he weak dW bonding
ou -o -plane; and ii) non- an-de Waals (n- dW) 2D ma e ials like me al
chalcogenides, oxides, ni ides, and phosphides, which exhibi s ong
chemical bonding in all h ee di ec ions [12]. The n- dW ma e ials ha e
a laye ed s uc u e wi h an o e all hickness o a ound 10 nm and
encompass high ene gy su aces [13].
Me hodologies used o he syn hesis o n- dW ma e ials include a
sel -assembly app oach [14–16], ex olia ion [17], lamella in e media e
ex olia ion [18,19], and opochemical ans o ma ion [12,20,21]. The
ex olia ion p ocess has been adop ed as he mos con enien s a egy o
he p epa a ion o i on oxide de i ed n- dW 2D ma e ials such as
hema ene (2D
α
-Fe
2
O
3
), magne ene (2D Fe
3
O
4
), ilmenene (2D FeTiO
3
),
and ch omi een (2D FeC
2
O
4
) [22]. All hese ma e ials we e p epa ed
om na u al o es in o ganic sol en s ( ypically DMF; N,
N-dime hyl o mamide), deploying ul asonica ion, wi h ha being he
* Co esponding au ho s a : Regional Cen e o Ad anced Technologies and Ma e ials, Czech Ad anced Technology and Resea ch Ins i u e (CATRIN), Palacký
Uni e si y Olomouc, ˇ
Slech i elů 241/27, Olomouc 779 00, Czech Republic.
E-mail add esses: [email p o ec ed] (P. Jakubec), [email p o ec ed] (M. O yepka), [email p o ec ed] (R. Zboˇ
il).
1
These au ho s con ibu ed equally.
Con en s lis s a ailable a ScienceDi ec
Applied Ma e ials Today
jou nal homepage: www.else ie .com/loca e/apm
h ps://doi.o g/10.1016/j.apm .2023.101881
Recei ed 31 Ma ch 2023; Recei ed in e ised o m 30 June 2023; Accep ed 13 July 2023
Applied Ma e ials Today 34 (2023) 101881
2
main d awback o hese ex olia ion p ocesses due o he inhe en
oxici y o DMF and unwan ed su ace unc ionaliza ion [23]. Because o
he DMF oxici y [24,25], he Eu opean Union is adop ing egula ions
es ic ing i s use om Decembe 2023 onwa ds [26]; hence he need o
he de elopmen o al e na i e ex olia ion p ocesses in non oxic
sol en s.
Since he amoun o ossil uels is limi ed, alongside he ac ha
global ene gy consump ion is es ima ed o inc ease a leas wice by he
midcen u y compa ed o he p esen ime [27], he exploi a ion o
enewable and sus ainable ene gy sou ces ep esen s one o he mos
impo an asks. P e iously published epo s show ha n- dW ma e-
ials such as hema ene (2D
α
-Fe
2
O
3
), magne ene (2D Fe
3
O
4
), ilmenene
(2D FeTiO
3
), and ch omi een (2D FeC
2
O
4
) ha e s ong po en ial in he
ield o pho oelec ochemis y, whe e he combina ion o hese allo-
opes wi h Ti nano ubes shows excellen esul s [28,29]. N- dW ma-
e ials also hold g ea po en ial in a ious elec oca aly ic applica ions
a ge ing he ene gy con e sion p ocesses, including wa e spli ing
(HER and OER eac ions), CO
2
educ ion eac ion (CRR), ni ogen
educ ion and oxida ion eac ions (NRR and NOR), NO
x
educ ion and
NH
3
oxida ion [12]. Up o now, he highes elec oca aly ic ac i i y is
achie ed wi h noble me als, which ep esen s he main d awback o he
scalable applica ion o hese elec oca alys s on he comme cial le el,
mainly due o hei p ice and a ailabili y. Thus, he de elopmen o
no el ma e ials wi h enhanced elec oca aly ic and/o pho oca aly ic
ac i i y owa ds sus ainable ene gy con e sion is highly desi ed.
An ammonia decomposi ion eac ion is a p omising al e na i e o
ossil uels ha leads o he elease o hyd ogen and ni ogen, hus
o e coming he challenges o hyd ogen s o age [30,31]. Ammonia is an
ideal hyd ogen ca ie since i has a high g a ime ic (17.8 w . % H
2
)
and olume ic (121 kg H
2
m
−3
in he liquid o m) H
2
densi y and
p oduces a high amoun o CO
x
ee hyd ogen a e decomposi ion [30,
32]. The e o e, a clean p ocess based on enewable ene gy sou ces o
con e NH
3
o H
2
(as a uel) and N
2
(as a ha mless gas) is an impo an
scien i ic challenge. The aqueous ammonia decomposi ion (1) is an
uphill eac ion and he equi ed eac ion ene gy can be p o ided by he
sun ligh pho ons using pho oca alys ma e ials [33].
NH3(aq.)→3/2H2(g) + 1/2N2(g)(1)
ΔG298K =27 kJ mol−1
Cu en ly, only a ew pho oac i e ma e ials a e ound o be able o
decompose he aqueous ammonia solu ion, including TiO
2
[34–38], ZnO
[39,40], ZnS [33], C
3
N
4
[41], g aphene, and o he ca bon based ma e-
ials [42]. Howe e , all hese ma e ials su e om se e al d awbacks
limi ing hei wide applica ion in p ac ice. TiO
2
is a e y popula pho-
oac i e ma e ial ha has ela i ely low cos , high chemical s abili y,
and good ligh con e sion e iciency. Howe e , he e iciency is s ill
ela i ely low because o he as ecombina ion o he pho oinduced
cha ge ans e . Ano he handicap is ha TiO
2
, o ins ance, wo ks only
unde he UV illumina ion [34,39,40]. The complica ed mul iple-s ep
syn hesis o he pho oca alys s using a ious oxic sol en s (e.g., DMF)
[40,42] limi s he p ac ical applicabili y o o he pho oca alys s.
The e o e, he p epa a ion o pho oca alys s by g een chemis y p in-
ciples while elying on cheap and eco- iendly ansi ion me al oxides as
success ul N
2
ac i a o s emains one o he majo challenges in he ield
o pho oca aly ic decomposi ion o ammonia.
In his wo k, we show ha he wo-dimensional hema ene can be
syn hesized by a g een en i onmen ally iendly scalable me hod om
an ea h abundan aw ma e ial—i on oxide o e specula i e. The hem-
a ene shee s we e a ew nanome e s in hickness and ga e a sa is ac o y
elec ochemical pe o mance in e ms o he cha ge ans e limi ing
p ocesses. Re lec ing hese p ope ies, we exploi ed he combina ion o a
conduc i e 2D hema ene subs a e wi h he ca aly ic su ace o RuO
2
nanopa icles o es hem as a pho oca aly ical pla o m sui able o he
p oduc ion o hyd ogen unde he isible ligh i adia ion. The ob ained
esul s clea ly p o ed ha his nanocomposi e exhibi ed a syne gic
e ec in e ms o he gene a ion o elec ons and holes. In his case, he
holes oxidized he al eady abso bed ammonia h ough he quasi-
me allic u henium oxide and le he elec ons in hema ene o he
educ ion o ammonia unde he isible ligh i adia ion. The excellen
pho oca aly ic pe o mance o he u henium oxide-loaded hema ene
suppo ed by he ac ha he en i e pho oca alys can be p epa ed by
en i onmen ally iendly and scalable ways indica es ha he de eloped
s a egy is iable o he ab ica ion o a no el class o pho oca alys s
sui able o he p oduc ion o hyd ogen.
2. Expe imen al
2.1. Common eagen s
Po assium e icyanide (p.a.) was pu chased om Lachema (B no,
Czech Republic) and po assium chlo ide (p.a.) was ob ained om Pen a.
I on oxide o e specula i e was ob ained om he Mo a ian Museum
(B no, Czech Republic). The RuCl
3
.xH
2
O (con en o Ru: 38% min) and
ammonium hyd oxide (28 % NH
3
) we e pu chased om Al a Aesa . The
magnesium oxide powde was pu chased om Sigma Ald ich. Deionized
wa e (DI) was used o he p epa a ion o all aqueous solu ions and
dispe sions.
2.2. Syn hesis o hema ene and loading wi h u henium
The hema i e o e specula i e was g ound in o a ine powde using a
Re sch Plane a y Ball Mill PM 100. The bulk sample was ball milled o 5
minu es, allowed o cool down o a oid excessi e hea ing ha migh
ha e led o undesi able e ec s, e.g., sin e ing, and hen ball milled again
o ano he 5 minu es. The ensued ine hema i e powde (2 g) was
dispe sed in 180 mL o DI wa e , mixed ho oughly wi h a e ical o-
a o (Heidolph) a 45 pm o 96 h, and sonica ed in a ba h sonica o
(B anson; powe ou pu : 130 W, equency: 40 kHz) o 48 h. The
ex olia ed hema ene shee s we e sepa a ed by sedimen a ion om he
unex olia ed specula i e and u he p ocessed o he desi ed concen-
a ion using cen i uga ion (cen i uge Sigma 4–16 K, Sigma Labo -
zen i ugen GmbH, Os e ode am Ha z, Ge many) a 21 000 c . The
unex olia ed powde o hema i e was collec ed and used again o he
p epa a ion o hema ene shee s.
The u henium loading was done ia he imp egna ion me hod. Fi s ,
he hema ene powde s we e sepa a ed om he solu ion by cen i u-
ga ion and d ied a 80 ◦C o 1 h. A e p epa ing he p ede ined con-
cen a ion o RuCl
3
in DI wa e , he measu ed amoun o d ied hema ene
was added o he solu ion and s i ed o 3 hou s. Then he emulsion was
cen i uged o sepa a e he solid p ecipices om he supe na an . The
cen i uga ion was epea ed wice while i was washed wi h DI wa e o
emo e he ex a amoun o RuCl
3
. Then, he powde s we e d ied a
80 ◦C o 1 h and calcined a 300 ◦C o 3 h in ai a mosphe e.
2.3. Mic oscopic echniques
Scanning elec on mic oscopy (SEM) and a omic o ce mic oscopy
(AFM) analyses we e pe o med wi h a FIB-SEM ins umen SCIOS 2
(The mo Fishe ) equipped wi h an in eg a ed AFM Li escope (NenoVi-
sion). The sample was analyzed on a Si wa e . The co ela i e p obe and
elec on mic oscopy (CPEM) was used o he su ace analysis, allowing
simul aneous SEM and AFM da a acquisi ion a he same place in he
same coo dina e sys em. Fo SEM imaging, an accele a ing ol age o
2 kV, a beam cu en o 25 pA, and an ETD de ec o was deployed. The
sel -sensing Akiyama p obe in he apping mode was used o he AFM
measu emen . High- esolu ion TEM (HRTEM) images including STEM-
HAADF (high-angle annula da k- ield imaging) analyses o elemen al
mapping o he p oduc s we e collec ed wi h an FEI Ti an HRTEM mi-
c oscope using an ope a ing ol age o 80 kV. Fo hese analyses, a
d ople o dispe sion o he ma e ial in ul apu e H
2
O a a concen a ion
o ~0.1 mg mL
−1
was deposi ed on o a ca bon-coa ed coppe g id and
J. Dzíbelo ´
a e al.
Applied Ma e ials Today 34 (2023) 101881
3
d ied.
2.4. Spec oscopic measu emen s
The X- ay pho oelec on spec oscopy (XPS) measu emen s we e
ca ied ou by a PHI Ve saP obe II p obe (Physical Elec onics) wi h an
Al Ka sou ce (15 kV, 50 W). The measu ed spec a we e decon olu ed
using he Mul iPak so wa e (Ul ac-PHI, Inc.). The Raman spec a o all
he samples we e eco ded on a DXR Raman mic oscope using a diode
lase wi h an exci a ion line o 633 nm. The EPR spec a we e collec ed
on a X-band (~9.14–9.17 GHz) spec ome e JEOL JES-X-320 equipped
wi h a a iable He empe a u e se -up ES-CT470 appa a us. The
expe imen al empe a u e was se o 80 K. The quali y ac o (Q) was
kep abo e 6000 o all measu emen s o make he spec a compa able.
High pu i y qua z ubes (Sup asil, Wilmad, ≤0.5 OD) we e used as a
sample holde . The accu acy o he g- alues was de e mined by
compa ing hem wi h a Mn
2+
/MgO s anda d (JEOL s anda d). The mi-
c owa e powe was se o 1.0 mW o a oid any powe sa u a ion e ec s.
A modula ion wid h o 1 mT and a modula ion equency o 100 kHz
we e used. All he EPR spec a we e collec ed wi h a ime cons an o 30
ms and a sweep ime o 4 min.
2.5. S uc u al and phase analysis
X- ay di ac ion (XRD) pa e ns o bulk i on o e and ex olia ed
hema ene samples we e collec ed on a PANaly ical X’Pe PRO di ac-
ome e (i on- il e ed Co K
α
adia ion: λ =0.178901 nm, 40 kV and 30
mA) in a B agg-B en ano geome y equipped wi h an X’Cele a o de-
ec o , p og ammable di e gence, and di ac ion beam an i-sca e ing
sli s. Each sample was placed on a ze o-backg ound Si slide, gen ly
p essed, and scanned in he 2θ ange om 5◦ o 105◦The phase iden-
i ica ion was pe o med using PANaly ical HighSco e Plus so wa e
wi h PDF-4+and ICSD da abases.
2.6. Elec ochemical measu emen s
All he elec ochemical measu emen s in a h ee-elec ode se -up
we e pe o med a ambien oom empe a u e (22 ±2 ◦C) using he
Me ohm Au olab PGSTAT128N po en ios a (Me ohmAu olab B.V.,
Ne he lands). The ob ained elec ochemical da a se s we e e alua ed by
he NOVA so wa e package ( e sion 1.11.2). The sys em comp ised a
glassy ca bon elec ode (GCE) se ing as he wo king elec ode, a pla -
inum wi e elec ode used as he coun e elec ode and an Ag/AgCl (3 M
KCl) elec ode as he e e ence elec ode. The GCE was modi ied as
ollows: a 10 µL d op o a sample dispe sion in DI wa e (concen a ion
o ≈1 mg mL
−1
) was coa ed on o he su ace o he GCE elec ode and
allowed o d y a ambien empe a u e o o m a hin ilm. Po assium
chlo ide (c =1 mol L
–1
) con aining po assium e icyanide (c =5 mmol
L
–1
) as a edox p obe se ed as a suppo ing elec oly e o impedance
measu emen s. Indi idual se ings o impedance spec oscopy mea-
su emen s (EIS) a e gi en in he Figu e legends.
2.7. Pho oca alys cha ac e iza ion and pho odecomposi ion o ammonia
The ul a iole - isible di use e lec ance spec a (UV–Vis DRS) o
he syn hesized samples we e ob ained by a Speco d 250 plus (Analy ik
Jena, Jena, Ge many) spec opho ome e . An in eg a ing sphe e was
used o collec he spec um. The magnesium oxide powde se ed as a
backg ound e e ence sample.
The pho odecomposi ion o NH
3
was conduc ed in a 17.5 mL qua z
eac o . A e sono-dispe sion o he pho oca alys in 10 mL o ammo-
nium hyd oxide, he ec o was igh ly sealed wi h a ubbe sep um.
Since dissol ed oxygen in a solu ion could ac as an elec on sca enge ,
i was bubbled wi h a gon o 15 min o emo e he oxygen. Then, he
sample was illumina ed unde LED (Solis® High-Powe LED om
Tho labs) wi h he powe o 3 W⋅m
−2
and a wa eleng h o 400‒700 nm.
To a oid he he mal decomposi ion o ammonia, he eac ion was
conduc ed in a wa e ba h o keep he empe a u e o he solu ion a
24 ◦C du ing he eac ion. The pho oca aly ically e ol ed hyd ogen was
de ec ed wi h a gas ch oma og aph GCMS-QP2010 SE (Shimadzu,
Kyo o, Japan) and a TCD (The mal conduc i i y de ec o ) using A as
ca ie gas.
3. Resul s and discussion
3.1. S uc u al, mo phological, and (elec o)chemical cha ac e iza ion o
hema ene
Mul i-laye ed hema ene shee s we e p epa ed ia an ex olia ion
p ocess om an (ul a)pu e wa e dispe sion o specula i e, as discussed
ea lie in he expe imen al sec ion. The mo phology o he hema ene
sample was e alua ed by a CPEM echnique (combina ion o SEM and
AFM analysis) and a HRTEM analysis, as shown in Fig. 1a– and
Fig. S1a–c. I is e iden ha he ex olia ed sample was o a 2D-like na-
u e wi h he la e al sizes in uni s o mic ome e s (Fig. 1a–c). Homog-
enous dis ibu ion o he indi idual elemen s such as i on o oxygen
(Fig. S1b) implied high pu i y o he p epa ed hema ene sample, which
was a i med by an EDAX analysis (Fig. S1c), ul ima ely con i ming he
dominan p esence o hese elemen s; he appea ance o he coppe
elemen ela ed o he g id used o he HRTEM analysis. The AFM
analysis (Fig. 1b,c) and he ela ed heigh p o ile (Fig. 1d) e ealed ha
he hickness o he ep esen a i e shee was o a ound 5 nm. The
hickness o he molecula dynamic simula ed hema ene (001) and (010)
planes was es ima ed as 3.98 Å and 3.2 Å, espec i ely, co obo a ing a
p e iously published epo [28]. Thus, he hickness o 5 nm ep e-
sen ed ≈12 hema ene laye s in he inal p oduc . A highe numbe o
laye s alida es he heo y ha he selec ion o a chemical sol en
decidedly a ec s he possibili y o ex olia ing he sample by ul asonic
i adia ion up o indi idual laye s [43]. As expec ed, he sonochemical
p epa a ion s a egy o hema ene om (ul a)pu e wa e did no enable
o p o ide a mono o wo-laye ed sys em. To p o e he size and hick-
ness o he hema ene ma e ial, CPEM images wi h ela ed heigh p o iles
we e collec ed om di e en shee s, and a e depic ed in Fig. S2. The 2D
s uc u e o he hema ene sample was also con i med om magne ic
da a measu emen s. De ails o hose expe imen s and ela ed discussion
a e p o ided in he ESI. The composi ion o he p oduc was also sup-
po ed by XPS, as shown in he box (Fig. 1e, ). The ob ained HR-XPS
spec um o he Fe 2p egion (Fig. 1e) e ealed wo peaks a a ound
712 and 725 eV, co esponding o Fe 2p
3/2
and Fe 2p
1/2
, espec i ely
[28,44]. The sa elli e peak loca ed a a ound 719 eV u he indica ed
he Fe
3+
oxida ion s a e [44]. As e iden om he decon olu ion, bo h
spin-o bi componen s we e i ed wi h wo peaks. These i ed peaks
se ed mo e as a e e ence, indica ing a end based on al eady pub-
lished s udies [45,46]; he ansi ion me als in hei 2p s a es should be
i ed by mul iple componen s, ep esen ing he mul iple spli ing. The
HR-XPS spec um o he O 1 s egion (Fig. 1 ) was i ed in h ee com-
ponen s, based on p e ious epo s [47,48], co esponding o he la ice
inco po a ed oxygen a highe binding ene gies and he su ace p esen
oxygen (in a de ec s a e and as su ace oxida ion [28,47,48]) a lowe
binding ene gies, espec i ely.
The na u e o he hema ene sample was also e alua ed by means o
Raman spec oscopy. Fig. 2a shows he Raman spec a o o iginal
specula i e o e in compa ison wi h he hema ene sample. The posi ions
o he mos impo an bands we e ound a a ound 224 and 495 cm
–1
(A
1g
modes) and 249, 290, 409, 611 cm
–1
(E
g
modes). The b oad in ense
band isible a a ound 1318 cm
–1
was connec ed wi h he sca e ing o
wo magnons in he s uc u e o he hema i e o e [49]. As expec ed, he
hema ene showed he posi ion o he mos impo an bands, simila ly o
specula i e. Ne e heless, he addi ional band was obse ed a 663 cm
–1
,
which can be assigned o he s uc u al diso de s inside he c ys al la -
ice (E
u
modes) ela ed o he 2D c ys al s uc u e [28,48,50,51]. The
c ys alline s uc u e and chemical phase pu i y o bo h he o iginal
J. Dzíbelo ´
a e al.
Applied Ma e ials Today 34 (2023) 101881
4
specula i e and he ex olia ed hema ene we e addi ionally e alua ed by
XRD measu emen s (Fig. 2b). The collec ed pa e ns illus a e ha bo h
samples con ained only he
α
-Fe
2
O
3
phase o he co undum c ys al
s uc u e (JCPDS ca d 01–089–0599, hombohed al s uc u e, space
g oup R-3c), which ules ou he p esence o any o he c ys alline pha-
ses. The di ac ion pa e n o hema ene (la ice pa ame e s: a =5.035 Å
and c =13.745 Å) showed ha he di ac ion peaks we e sligh ly
b oadened and sligh ly shi ed in hei posi ion, compa ed o he o iginal
specula i e (la ice pa ame e s: a =5.034 Å and c =13.742 Å). These
e ec s a e isible in Fig. 2c, p o iding a de ail abou he posi ions o he
wo di ac ion peaks, whe e he di ac ion on he (104) plane is ypi-
cally dominan o he
α
-Fe
2
O
3
s uc u e. Such obse a ion is consis en
wi h he ex olia ion o hema i e in p e ious s udies [28,52], whe e he
b oadening o he lines is ela ed o he educ ion in c ys alli e size [17,
53].
P io o he modi ica ion o he hema ene wi h u henium, he
elec ochemical pe o mance o specula i e and i s 2D analogue was
e alua ed by means o impedance spec oscopy (EIS). Fig. 3a illus a es
he EIS spec a as he Nyquis plo o a ba e glassy ca bon elec ode
(GCE) and GCE modi ied wi h he specula i e p ecu so and he
hema ene sample. As isible in Fig. 3b, all EIS spec a should be i ed
wi h a simple Randles ci cui , which illus a es he non-complica ed
elec ochemical beha io o all he es ed ma e ials. In gene al, esis-
ance R
S
loca ed a poin A (see Fig. 3b) was assigned o he solu ion
esis ance o he so-called ESR esis ance (known also as he in e nal
esis ance), whe eas he diame e o he semici cle R
AB
=R
B
– R
A
ep-
esen ed he sum o he elec ode and con ac esis ance and he cha ge
ans e esis ance (R
c
) [54,55]. A magni ied e sion o he Nyquis plo
(inse o Fig. 3a) e ealed ha he cha ge ans e esis ance (R
c
)
inc eased signi ican ly when he wo king elec ode (R
c
=40.4 Ω) was
modi ied ei he wi h he specula i e (R
c
=222 Ω) o he hema ene (R
c
=173 Ω). Such beha io sugges ed he success ul immobiliza ion o
bo h ma e ials on he su ace o he wo king elec ode. I is pe ec ly
clea ha he diame e o he semici cle was smalle in a o o he
hema ene (inse o Fig. 3a), which e lec ed a be e con ac o he
hema ene ilm wi h he su ace o he wo king elec ode and hus a
lowe impac on he cha ge ans e limi ing p ocesses. The e o e, i can
be expec ed ha he elec on ans e was enhanced in ha case.
Fig. 1. (a–c) CPEM images o hema ene shee and ela ed heigh p o ile (d); (e) HR-XPS spec um o Fe 2p and ( ) O 1 s egion.
Fig. 2. (a) Raman spec a o specula i e and hema ene; (b) XRD pa e ns o specula i e and hema ene; (c) de ail o XRD pa e ns showing he b oadening and shi o
hema ene di ac ion peaks compa ed o pa en specula i e.
J. Dzíbelo ´
a e al.
Applied Ma e ials Today 34 (2023) 101881
5
3.2. Pho oca alys cha ac e iza ion
Fig. 4 shows he HRTEM images o he hema ene and he Ru-
hema ene oge he wi h he elemen al analysis. The dominan c ys al
plane dis ance in he hema ene was 2.7 Å, which was ela ed o he (104)
plane, while he p esence o u henium oxide was con i med by i s
dominan c ys al plane dis ance 3.2 Å ela ed o he (110) plane. Bo h
s uc u es in Ru-hema ene we e also iden i ied by XRD (Fig. S6).
The EDS elemen al analysis o he Ru-hema ene showed a well
dispe sed u henium oxide pa icles wi hou any sign o agglome a ion
a he su ace o he hema ene s uc u e as ca alys si es o ammonia
oxida ion. This uni o m dispe sion o u henium oxide helped o in-
c ease he cha ge sepa a ion and he e o e i s pho oac i i y.
The XPS echnique was employed o de e mine he oxida ion s a es
o he u henium in he Ru-hema ene sample. As shown in Fig. 5a, he
HR-XPS Ru 3d egion was analyzed and decon olu ed o e eal wo
oxida ion s a es o u henium. The Ru
4+
peak a a ound 280.5 eV u he
p o ed he p esence o RuO
2
species [56–58], which was in ag eemen
wi h he EPR obse a ions (Fig. 5d), and was easonable o expec due o
he hea ea men o he sample a 300 ◦C. The adjacen Ru
3+
band
below 282 eV indica ed he exis ence o he Ru(III) [56,57,59] a oms,
which can be u he explained by he co-p esence o he chlo ine in his
sample (1.2 a .% coming om he p ecu so ) (Fig. S5). The ca bon
con en (C 1 s peak) is in insic o he hema ene i sel and was no ully
elimina ed du ing he hea ing p ocess. These XPS obse a ions
con i med he p e ious cha ac e iza ion echniques as well as he pho-
oca aly ic ac i i y [58–62]. Fig. 5b shows he alence band XPS
(VB-XPS) o he hema ene and he Ru-hema ene. The posi ion o he
alence band edge o he hema ene did no change a e loading wi h
u henium (+1.1 eV), while he densi y o s a es (DOS) showed an in-
c ease a a ound +0.5 eV below he Fe mi le el wi hin he band gap.
This ail was possibly due o he p esence o he RuO
2
species, c ea ing
he e ojunc ion a he su ace o hema ene, which p o ided a mid-band
gap ene gy s a e le els a ound +0.5 eV below he hema ene Fe mi
le el (Fig. 5c). These ene gy s a e le els we e able o acili a e he
pho ogene a ed hole anspo a ion om he hema ene o RuO
2
and
oxidize ammonia o N
2
by p o iding o bi al o e laps [63]. On he o he
hand, he pho ogene a ed elec ons wen h ough he hema ene and
educed he ammonia o hyd ogen (Fig. 5c). This spa ial cha ge sepa-
a ion in he place o RuO
2
and he hema ene he e ojunc ion esul ed in
highe pho oac i i y o his pho oca alys in compa ison wi h he ba e
hema ene.
To ob ain mo e de ailed in o ma ion abou he s uc u e and he
na u e o he Ru coupling mechanism on hema ene, we ca ied ou an
elec on pa amagne ic esonance (EPR) s udy a low empe a u e (T =
80 K). In a nea hema ene, in spi e o assuming he p esence o a la ge
numbe o spin-con aining de ec s loca ed on he edges o indi idual
lakes, he eco ded spec um in a ozen wa e ma ix was indeed EPR
Fig. 3. (a) Nyquis plo o ba e GCE and GCE modi ied wi h specula i e and hema ene; pa ame e s o EIS: equency ange om 100 kHz o 0.1 Hz, he ampli ude o
10 mV and hal -wa e po en ial o 0.24 V; (b) isualiza ion o Nyquis plo and ela ed equi alen ci cui used o he da a e alua ion.
Fig. 4. HRTEM image (le ), STEM-HAADF images and elemen al EDS mapping ( igh ) o (a) hema ene and (b) Ru-hema ene. The inse s show he FFT pa e ns o he
shown a ea by dashed ed ec angle.
J. Dzíbelo ´
a e al.

Applied Ma e ials Today 34 (2023) 101881
6
silen (g een line in Fig. 5d). Since, his ma e ial exhibi ed an i e o-
magne ic o de ing a T below 250 K, his phenomenon was no unex-
pec ed. Ne e heless, a s a is ical pe cen age o an e ec i e spin
momen (d
χ
II
/dB ∕= 0) should be le a 80 K, which aises a ques ion why
hese e ec i e spins canno be obse ed a X-band equency. The
easoning can be back aced in he as spin-la ice elaxa ion o he
Fe
3+
cen e s unde an exchange coupled egime, and in he la ge ze o-
ield spli ing e m a ising om he coupling scheme, which ende s
he sys em o be mo e likely a non-K ame mul iple .
The esul ing EPR spec um o RuCl
3
as he me al-sal sou ce o he
deco a ion o he hema ene lakes is shown o compa ison in Fig. 5d
(blue line). The hombic esonan line o Ru ca ions in +3 oxida ion s a e
showed small g- enso aniso opy, wi h componen s a g
x
=2.220, g
y
=
2.094 and g
z
=1.802. These g- alues a e ypical o Ru
3+
ions, howe e ,
he e, hey we e sligh ly smalle han in he Ru
3+
complexes coo dina ed
o he o ganic amewo ks [64] because o he absence o a s ong ligand
ield. In Fig. 5d ( ed spec um), he EPR en elope o Ru-hema ene
showed signi ican di e ences compa ed wi h he EPR spec a o he
wo p e ious e e ence samples. In he EPR spec um o he composi e
Ru-hema ene ma e ial, a e y b oad and asymme ic esonan line
became dominan . This line exhibi ed an a e age g- alue o abou ~ 2.7,
which was ully consis en wi h clus e ed Fe
3+
ca ions on he laye ed
su ace ha became pe u bed by he Ru ca ions in a way ha hei
an i e omagne ically coupling in e ac ion, as seen in he nea hema-
ene, was s ongly weakened and he e o e EPR de ec able. We assume
ha he Ru ions eac ed wi h oxygen on he op o he hema ene su ace
o o m small RuO
2
nanopa icles, and by doing so, hey pa ially al e ed
he ac ion o he Fe-O-Fe supe exchange pa h. The e o e, he obse ed
e ec on he esonance spec um, combined wi h clea asymme y in he
b oad esonance, sugges s his was a su ace localiza ion o hese de-
ec s, which is in ull ag eemen wi h he 2D s uc u e o he hema ene
lakes and he TEM analysis (Fig. S1a). Fu he mo e, in he a ea high-
ligh ed in blue in he EPR spec um o he Ru-hema ene (Fig. 5d), a weak
modula ion o an Fe
3+
en elope a ound 310 mT was obse ed. I was
posi ioned a g ~ 2.09, a alue ha is in line wi h he signal o Ru
3+
, and
p obably loca ed on he su ace o he RuO
2
nanopa icles, as usually
obse ed [65]. Since he Ru loading was e y low, and we could de ec
only he Ru
3+
species ia EPR, he signal o Ru was obse ed as weak
modula ion in he b oad esonan line o Fe
3+
. Howe e , pa o he
Ru
3+
agmen s in he EPR spec um can also be an indica o o a small
amoun o esidual RuCl
3
sal in he esul ing ma e ial, which was also
obse ed in XPS.
3.3. Pho oca aly ic decomposi ion o HN
3
ia u henium loaded hema ene
P io o pho oca aly ical expe imen s, bo h p is ine and Ru-
hema ene we e e alua ed by means o UV–VIS spec oscopy o assess
he sui abili y o bo h ma e ials o pho odecomposi ion o ammonia.
The band edge abso p ion o bo h samples was a ound 729 nm, as
shown in Fig. 6a. The e o e, hese ma e ials can abso b bo h UV and a
isible pa o he ligh spec um. The eason o he da k ed colo in he
samples is also ela ed o his wide abso p ion egion. The amoun o
Fig. 5. (a) Decon olu ion o HR-XPS o Ru 3d o bi al o Ru-hema ene pho oca alys ; (b) Valence band XPS (VB-XPS) o hema ene and Ru-hema ene; (c) Schema ic
densi y o s a es (DOS) and band edge posi ions o he main elec onic le els o Ru-hema ene ela i e o he ene gy le els o he edox couples o aqueous ammonia
(le ) and he p oposed mechanism o ammonia pho odecomposi ion by Ru-hema ene pho oca alys ( igh ). (d) X-band CW EPR spec a o nea hema ene (g een
line), RuCl
3
(blue spec um) and EPR en elope o hema ene deco a ed by RuO
2
nanopa icles ( ed cu e), dispe sed in DI H
2
O and eco ded in he ozen solu ion (T
=80 K).
J. Dzíbelo ´
a e al.
Applied Ma e ials Today 34 (2023) 101881
7
ligh abso p ion (especially in he isible egion) was highly inc eased
a e adding u henium. This huge inc ease in isible ligh abso p ion
ul ills one o he main condi ions o an ideal pho oca alys o pho o-
ca aly ic eac ions. To highligh his unique p ope y o cu en pho o-
ca alys s, all o he pho o eac ions in his wo k we e conduc ed unde
he isible pa o he ligh (wa eleng h 400–700 nm). The op ical band
gap ene gy o he hema ene and he Ru-hema ene calcula ed by he Tauc
plo [66] based on he Kubelka-Munk [67] heo y was 1.70 and 1.71 eV,
espec i ely (Fig. 6b). I should be no ed ha in spi e o he huge di -
e ence in abso bed ligh , he band gap o he pho oca alys s was almos
he same.
As discussed in de ail, he p esence o RuO
2
was p o ed by he su -
ace XPS analysis (Fig. 5a). This oxida ion occu ed p obably du ing he
calcina ion s ep a 300 ◦C. Nagaoka and cowo ke s [68] epo ed ha
RuO
2
p oduces a ca aly ic su ace o exo he mic adso p ion o NH
3
.
This chemiso p ion o ammonia dec eased he o e all he modynamic
ene gy needed o i s decomposi ion. Then, he subsequen pho o ab-
so p ion by hema ene esul ed in he gene a ion o elec ons and holes.
The holes we e able o oxidize he al eady abso bed NH
3
h ough he
quasi-me allic u henium oxide and le he elec ons in he hema ene
o he educ ion o he ammonia.
The con ol expe imen s show ha he ca aly ic ac i i y o hema ene
and u henium loaded hema ene o decomposi ion o ammonia unde
da k condi ion is almos ze o. Fu he mo e, he pho odecomposi ion o
ammonia as ammonium hyd oxide wi hou he p esence o pho o-
ca alys a 24 ◦C is ze o.
A e he u henium oxide was con i med as a p ope co-ca alys o
hema ene o decompose he ammonia, he loading op imiza ion o Ru
was pe o med. Fig. 6c shows ha loading he hema ene wi h he op-
imum amoun o Ru—0.5 w . %—led o a con inuously inc eased
amoun o H
2
. In lowe amoun s o he Ru loading, he cha ge sepa a ion
was no e ec i e enough o p oduce hyd ogen, while a highe amoun o
Ru was able o p oduce cha ge ecombina ion cen e s a he su ace o
he hema ene [68]. All he cha ac e iza ion was done while loading he
hema ene wi h 0.5 w .% o Ru, labelled as Ru-hema ene.
To ha e a ai and eliable compa ison be ween he pho oac i i y o
he hema ene and he Ru-hema ene, he mass o he pho oca alys had o
be op imized agains he pho oac i i y (Fig. 6d and Fig. S4). The eason
is ha he amoun o he H
2
p oduc ion du ing he pho oca aly ic e-
ac ion is no necessa ily p opo ional o he mass o he pho oca alys
due o he limi a ion o ligh abso p ion and di usion condi ions [69,
70]. I is ob ious om Fig. 6d ha he loading o 2 mg o he pho o-
ca alys in bo h cases was able o p oduce he op imum amoun o he
p oduc . This plo shows ha he Ru-hema ene pho oca alys p oduced
2.5 imes mo e H
2
han he p is ine hema ene unde op imum
condi ions.
The Ru-hema ene showed an 11% dec ease in ac i i y a e i e
pho oca aly ic uns o he o al o 120 h (Fig. 6e). A cons an dec ease in
ac i i y a e each un could be co ela ed o a loss in he pho oca alys
caused by he a achmen o he eac o walls and e en o he magne
s i e . Addi ionally, as he ca alys was no washed a e each un o he
pho o eac ion, he su ace o he ca alys could ha e been passi a ed by
eac an s, p oduc s o in e media e species.
4. Conclusions
A non- an de Waals 2D ma e ial, hema ene, was simply p epa ed
ia he ex olia ion o i on oxide o e specula i e, deploying an (ul a)pu e
wa e solu ion. Using his s a egy, he p epa ed ma e ial exhibi ed
a o able elec ochemical p ope ies in e ms o cha ge ans e and
di usion limi ing p ocesses. As a p oo -o -concep , we ha e amply
demons a ed ha his pu e n- dW ma e ial can be easily modi ied wi h
u henium oxide nanopa icles and can se e as a ca aly ic eco- iendly
pla o m o he pho oca aly ic decomposi ion o an aqueous solu ion o
ammonia in o de o p oduce hyd ogen ia he isible ligh i adia ion.
The ob ained esul s p o ed ha he combina ion o RuO
2
ca aly ic
su ace wi h he conduc i e 2D hema ene subs a e exhibi ed a syne gic
e ec in e ms o he gene a ion o elec ons and holes. Based on he
esul s, he holes we e able o oxidize he al eady abso bed ammonia
h ough he quasi-me allic u henium oxide and le he elec ons in he
Fig. 6. (a) UV–VIS DRS o hema ene and Ru-hema ene and (b) co esponding Tauc plo based on Kubelka-Munk model o calcula e he op ical band gap o hema ene
and Ru-hema ene; (c) Op imizing he loading amoun o Ru co-ca alys o H
2
e olu ion o ammonia pho odecomposi ion. The eac o s we e loaded wi h 8 mg o
pho oca alys ; (d) The amoun o e ol ed H
2
om pho odecomposi ion o ammonia wi h di e en loading amoun o p is ine hema ene and Ru-hema ene. The
eac o s we e unde illumina ion o 24 hou s; (e) Reusabili y o 2 mg o hema ene and Ru-hema ene o H
2
e olu ion om ammonia pho o decomposi ion. A e
each cycle, he solu ion was sonica ed and bubbled wi h A o 15 min. In all cases, he eac o was unde isible LED illumina ion (wa eleng h om 400 o 700 nm,
powe 3 W⋅m
−2
).
J. Dzíbelo ´
a e al.
Applied Ma e ials Today 34 (2023) 101881
8
hema ene o he educ ion o ammonia. Expe imen al da a indica e ha
he op imum dosage o u henium was a ound 0.5 w %, yielding he
maximum amoun o hyd ogen a e 24 hou s. Wi h espec o he
p is ine hema ene, a co-doped ma e ial p o ided a leas 2.5 imes be e
pho oca aly ic esponse owa ds hyd ogen e olu ion. Wi hou any
cleaning s eps, he Ru-hema ene pho oca alys exhibi ed only 11 % o
he pho oca aly ic eac ion dec ease a e i e success ul pho oca aly ic
uns, p edisposing i o p ac ical applica ion. The disco e y opens up
possibili ies o c ea ing obus and e ec i e pho oca alys s o a wide
ange o applica ions using non- an de Waals 2D ma e ials and me al
oxide nanopa icles.
CRediT au ho ship con ibu ion s a emen
Jana Dzíbelo ´
a: In es iga ion, W i ing – o iginal d a , W i ing –
e iew & edi ing. S.M. Hossein Hejazi: Concep ualiza ion, In es iga-
ion, W i ing – o iginal d a , W i ing – e iew & edi ing. Ve onika
ˇ
Sedajo ´
a: In es iga ion, W i ing – o iginal d a . Da id Pan´
aˇ
cek:
In es iga ion, W i ing – o iginal d a . Pe Jakubec: Concep ualiza-
ion, W i ing – o iginal d a , W i ing – e iew & edi ing. Zdenˇ
ek
Ba
ď
u a: In es iga ion, W i ing – o iginal d a . Ondˇ
ej Malina: In es-
iga ion, W i ing – o iginal d a , W i ing – e iew & edi ing. Jose
Kaˇ
slík: In es iga ion. Jan Filip: Concep ualiza ion. ˇ
S ˇ
ep´
an Kmen :
Concep ualiza ion. Michal O yepka: Concep ualiza ion, W i ing – e-
iew & edi ing. Radek Zboˇ
il: Concep ualiza ion, W i ing – o iginal
d a , W i ing – e iew & edi ing.
Decla a ion o Compe ing In e es
The au ho s decla e ha hey ha e no known compe ing inancial
in e es s o pe sonal ela ionships ha could ha e appea ed o in luence
he wo k epo ed in his pape .
Da a a ailabili y
Da a will be made a ailable on eques .
Acknowledgmen s
R.Z. acknowledges he suppo om he Czech Science Founda ion,
p ojec No. 19–27454X. P.J. acknowledges he Resea ch In as uc u e
NanoEn iCz, suppo ed by he Minis y o Educa ion, You h and Spo s
o he Czech Republic unde P ojec No. LM2018124. We also
acknowledge he suppo om ERDF/ESF “Nano4Fu u e” (No.
CZ.02.1.01/0.0/0.0/16_019/0000754) and he Technology Agency o
he Czech Republic, P og am TREND (GEFSEM, FW01010183). The
au ho s g a e ully hank Ondˇ
ej Tomanec (HR-TEM); Jiˇ
í Hoˇ
sek (CPEM);
Ka eˇ
ina Roh´
aˇ
co ´
a (Raman); and Jan Pauswang (pa o elec ochemical
es ing).
Supplemen a y ma e ials
Supplemen a y ma e ial associa ed wi h his a icle can be ound, in
he online e sion, a doi:10.1016/j.apm .2023.101881.
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