Spinel ferrites MFe2O4 (M = Co, Cu, Zn) for photocatalysis: theoretical and experimental insights
Abstract
We are grateful to the UK Materials and Molecular Modelling Hub for computational resources, which is partially funded by EPSRC (EP/T022213/1, EP/W032260/1 and EP/P020194/1). C. A. H. acknowledges nancial support for her PhD studies by Diamond Light Source (DLS) and the University of Reading. Collaboration with the Utrecht group was funded by COST Action CA18234 (CompNanoEnergy), supported by COST (European Cooperation in Science and Technology) https:// www.cost.eu. Authors thank D. L. S. for beamtime on B07-B under proposal SI33639 and SI34919 and on I10 under proposal MM36558; ESFR for beamtime on BM25 under proposal A25-2-1023; ISIS Neutron and Muon Source, for access to the facilities at the Materials Characterisation Laboratory. We also thank the Research Complex and the UK Catalysis Hub at Harwell Campus. J. R. Z. acknowledges the Spanish Ministerio de Ciencia, Innovaciónn y Universidades and Consejo Superior de Investigaciones Científicas for financial support and for provision of synchrotron radiation facilities at BM25-SpLine(PIE 2010 6 0E 013 and PIE 2021 60 E 030).
Full text
Spinel e i es MFe
2
O
4
(M =Co, Cu, Zn) o
pho oca alysis: heo e ical and expe imen al
insigh s†
Cha lo e A. Hall,
ab
Pila Fe e , *
b
Da id C. G in e ,
b
San osh Kuma ,
b
I an da
Sil a,
c
Juan Rubio-Zuazo,
de
Pe e Bencok,
b
F ank de G oo ,
Geo g Held
b
and Rica do G au-C espo *
a
Spinel e i es exhibi significan p omise in pho oca alysis and o he applica ions due o hei composi ional
di e si y and a ou able elec onic s uc u e, magne ism, and pa ially uneable ca ion dis ibu ion.
Howe e , hei complex p ope ies, o example, he diffe en beha iou o bulk and nanos uc u ed
ma e ials, a e no well unde s ood. He e, we combine ad anced compu a ional and expe imen al
me hods wi h eac i i y measu emen s o explo e he in e sion deg ees, elec onic s uc u es, and
pho oca aly ic ac i i ies o MFe
2
O
4
spinels (M =Co, Cu, Zn). X- ay diff ac ion and anomalous X- ay
sca e ing measu emen s de e mined bulk in e sion deg ees o 0.81, 0.91, and 0.26 o CoFe
2
O
4
,
CuFe
2
O
4
, and ZnFe
2
O
4
, espec i ely. Pho oca aly ic es s showed ha only ZnFe
2
O
4
is ac i e in he
oxygen e olu ion eac ion (OER), which co ela es wi h i s a ou able band alignmen , as de e mined
h ough elec onic s uc u e simula ions. Su ace-sensi i e X- ay Abso p ion Spec oscopy (XAS)
measu emen s p o ided insigh s in o he ca ion dis ibu ions a he su aces, showing significan
de ia ions om bulk p ope ies, pa icula ly in ZnFe
2
O
4
in which 52% o he nea -su ace e ahed al
si es a e occupied by Fe ca ions, compa ed o 26% in he bulk. DFT simula ions o ZnFe
2
O
4
illus a ed
how he su ace e mina ions can al e he he modynamic p e e ence o ca ion dis ibu ion in
compa ison wi h he bulk. Ou findings illus a e he complex in e play be ween su ace and bulk
p ope ies in spinel e i es.
1. In oduc ion
Spinel e i es cons i u e a e sa ile amily o ma e ials wi h
impo an applica ions in pho oca alysis,
1,2
wa e pu ica ion,
3
biomedicine,
4
and o he elds.
5
They a e me al oxides wi h
composi ion MFe
2
O
4
, whe e M ep esen s a di alen me al
ca ion (e.g. Co
2+
,Cu
2+
,Zn
2+
) and i on is p esen in i alen
o m, Fe
3+
.Ina“no mal”spinel he M
2+
and Fe
3+
ca ions occupy
he e ahed al (Td) and oc ahed al (Oh) posi ions, espec i ely,
o he spinel s uc u e (Fig. 1). Bu o some composi ions, he
ca ions a e edis ibu ed ac oss he Td and Oh si es, he e o e
he o mula can be w i en as (M
1−x
Fe
x
)[M
x
Fe
2−x
]O
4
whe e ()
ep esen s he Td si es and [] ep esen s he Oh si es. The
deg ee o in e sion (0 #x#1) is dened he e as he ac ion o
Fe
3+
ca ions occupying he Td si e.
Spinel e i es a e a ac i e ma e ials o pho oca aly ic
applica ions o se e al easons. Fi s , hei elec onic s uc u e
and op ical p ope ies can be uned ia hei composi ion and/
o ca ion dis ibu ion, which allows op imising ligh abso p ion
o a ge ing specic band alignmen s.
6–8
Second, hey end o be
chemically s able unde a wide ange o empe a u es and pH
le els.
9
Thi d, hey a e magne ic, which allows o easy eco e y
and euse o he pho oca alys om he eac ion mix u e,
educing was e and imp o ing p ocess efficiency.
5
Spinel
e i es a e also ela i ely cheap ca alys s and can be made up o
na u ally abundan me als.
10,11
One po en ial applica ion o
spinel e i e pho oca alys s is wa e spli ing o p oduce
enewable hyd ogen gas, H
2
.
12
Spinels such as CoFe
2
O
4
and
CuFe
2
O
4
ha e been epo ed as po en ial pho oca alys s o
wa e spli ing; howe e , hey a e ypically used as pa o
composi e pho oca alys s.
10,13
D awbacks o cobal and coppe
e i es include poo conduc i i y, insufficien ac i e si es, apid
cha ge ca ie ecombina ion, and i egula mo phology.
14,15
In
a
Depa men o Chemis y, Uni e si y o Reading, Whi eknigh s, Reading RG6 6DX, UK.
E-mail: .g au-c esp[email p o ec ed]
b
Diamond Ligh Sou ce, Ha well Science and Inno a ion Campus, Didco OX11 0DE,
UK. E-mail: pila . e e -esco i[email p o ec ed]
c
ISIS Neu on and Muon Sou ce, STFC, Ru he o d Apple on Labo a o y, Didco , OX11
0QX, UK
d
BM25-SpLine, Eu opean Synch o on Radia ion Facili y, CS40220, F-38043 G enoble
Cedex 9, F ance
e
ICMM-CSIC, So Juana In´
es de la C uz 3, Can oblanco, Mad id, Spain
Debye Ins i u e o Nanoma e ials Science, U ech Uni e si y, 3584 CA U ech , The
Ne he lands
†Elec onic supplemen a y in o ma ion (ESI) a ailable. See DOI:
h ps://doi.o g/10.1039/d4 a04941a
Ci e his: J. Ma e . Chem. A,2024,12,
29645
Recei ed 16 h July 2024
Accep ed 30 h Sep embe 2024
DOI: 10.1039/d4 a04941a
sc.li/ma e ials-a
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many s udies whe e ac i i y has been obse ed, a co-ca alys o
sac icial agen has been used alongside he spinel-based
ca alys ; he use o diffe en sac icial agen s, such as
oxala es
16,17
o me hanol,
14,18
can ha e a signican effec on he
efficiency o he ca alys . Ano he common s a egy o
imp o ing he ac i i y o CoFe
2
O
4
o CuFe
2
O
4
is cons uc ing
he e ojunc ions wi h o he wide band gap ma e ials, such as
Fe
2
O
3
( e . 19 and 20) o C
3
N
4
.
21,22
Compa ed o cobal and
coppe e i es, zinc e i e, ZnFe
2
O
4
, is a mo e efficien pho-
oca alys .
23
In a s udy by Rod ´
ıguez e al.,
24
mo e han wice he
amoun o H
2
was p oduced by ZnFe
2
O
4
compa ed o CoFe
2
O
4
o e 8 hou s wi h a me hanol sac icial agen . Howe e ,
ZnFe
2
O
4
is also s ill oen used in conjunc ion wi h a co-ca alys
o as a he e o-junc ion.
23,25,26
In insically al e ing he spinel by
ca ion subs i u ion, such as Ga in place o Fe, has p o ed
effec i e in imp o ing zinc e i es pe o mance as a pho o-
ca alys .
27
To o e come he limi a ions o spinel e i es o
wa e spli ing, a undamen al unde s anding o hei elec onic
and pho oca aly ic p ope ies is equi ed.
P e ious wo k has sugges ed subs an ial diffe ences in he
p ope ies o spinel e i es be ween bulk c ys als and nano-
pa icles.
28,29
Fo example, he ca ion dis ibu ion in nano-
pa icles can be signican ly diffe en om ha in he bulk, and
is hea ily inuenced by ac o s such as p epa a ion me hod
30
and/o he mal ea men .
31
CoFe
2
O
4
is known o ha e ully
in e se ca ion dis ibu ion (xz1) in he bulk,
32,33
whe eas in
nanopa icles lowe deg ees o in e sion in he ange o x=
0.66–0.68 a e obse ed.
28,34
CuFe
2
O
4
also displays a high bulk
in e sion deg ee; Siddique e al.
29
epo x=0.88 in he bulk
compa ed o x=0.80 in nanopa icle o m. Howe e , in e sion
deg ees as low as x=0.57 ha e been obse ed in coppe e i e
samples wi h a pa icle size o less han 10 nm.
35
In con as ,
bulk ZnFe
2
O
4
has a e y low in e sion deg ee o xz0,
36
whe eas
ZnFe
2
O
4
nanopa icles display a highe deg ee o in e sion o
up o x=0.4, depending on pa icle size and he mal
his o y.
30,37
Unde s anding he p ope ies ha a e inhe en o
he bulk ma e ials and diffe en ia ing hem om he effec s o
he su ace is impo an in applica ions including pho o-
ca alysis, because bo h he bulk and he su ace pa icipa e wi h
diffe en oles in he pho oca aly ic p ocess.
In his s udy we ha e used a combina ion o compu a ional
simula ions, X- ay abso p ion spec oscopy, X- ay diff ac ion,
and pho oca aly ic ac i i y measu emen s o in es iga e bulk
and su aces p ope ies o MFe
2
O
4
(M =Co, Cu o Zn) in an
effo o a ionalise he bulk/su ace beha iou o hese me al
e i es nanopa icles (pa icle sizes < 35 nm). In addi ion o he
cha ac e isa ion o s uc u al, pho oca aly ic and elec onic
p ope ies, ou modelling offe s insigh s on he depa u es
om bulk beha iou seen in small nanopa icles due o
diffe en beha iou o bulk and su aces in e ms o deg ee o
in e sion.
2. Me hods
2.1 Ab ini io simula ions o bulk and su ace models
The calcula ions we e pe o med using densi y unc ional
heo y (DFT) as implemen ed in he VASP code.
38,39
Geome y
op imisa ions we e pe o med using he gene alised g adien
app oxima ion (GGA) wi h he Pe dew–Bu ke–E nze ho (PBE)
exchange–co ela ion unc ional.
40
Hubba d (GGA + U) co ec-
ions wi h U
eff
alues o 3.3 eV and 4.0 eV we e applied o he Co
and Cu/Fe d o bi als, espec i ely; hese alues we e ob ained by
Wang e al.
41
ia ing o he expe imen al oxida ion en halpies
o he co esponding bina y me al oxides, and ha e been ound
o ans e well o he s udy o mo e complex oxides (e.g.
FeSbO
4
,
42
LaCoO
3
and LaFeO
3
,
43
YBa
2
Fe
3−x
Co
x
O
8
,
44
and
BiFe
1−x
Co
x
O
3
( e . 45)). The in e ac ion be ween he alence and
co e elec ons was desc ibed wi h he p ojec o augmen ed
wa e (PAW) me hod.
46
An ene gy cu -offo 520 eV, 30% abo e
he ecommended alue o he PAW po en ials, was used o all
geome y op imisa ions in ol ing cell olume cha ges, o
dec ease he Pulay e o s.
Fo he bulk calcula ions, p imi i e uni cells o MFe
2
O
4
(M
=Co, Cu, Zn) con aining wo o mula uni s we e modelled wi h
in e sion deg ees o x=0, 0.5 and 1. Fo each in e sion deg ee
he e is only one symme ically diffe en congu a ion in he
p imi i e cell, he e o e allowing us o calcula e he in e sion
ene gy (congu a ional con ibu ion only) as DE
con
(x)=E(x)−
E(0). The elec onic s uc u e calcula ions o de e mine he
band gap and alignmen o he mos s able congu a ions we e
Fig. 1 C ys al s uc u e o MFe
2
O
4
spinels ep esen ed by (a) he con en ional cubic cell and (b) he p imi i e uni cell. Colou scheme: e a-
hed al (Td) si es =sil e ; oc ahed al (Oh) si es =gold; oxygen = ed.
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comple ed using he hyb id unc ional by Heyd, Scuse ia and
E nze ho (HSE06),
47
which includes 25% o Ha ee–Fock
exchange as well as ange-sepa a ed sc eening wi h an a enu-
a ion pa ame e o 0.2 Å
−1
. The HSE06 calcula ions used he
geome ies op imised a GGA + U le el, i.e. we did no e-
op imise he s uc u es a HSE06 le el (bu we checked, using
ZnFe
2
O
4
as a es , ha his is an accep able app oxima ion,
leading o bo h cell pa ame e s and band gaps e y close, wi hin
0.5%, o hose ob ained wi h he ull HSE06 app oach). The
band alignmen was calcula ed using he op o he alence
band and he bo om o he conduc ion band in he HSE06
band s uc u e.
In all calcula ions, he magne ic momen o Fe
3+
(and o
Co
2+
in he case o CoFe
2
O
4
) was ini ialised in high-spin
s a e.
48,49
Fo CoFe
2
O
4
he e is an addi ional deg ee o
eedom: bo h cobal and i on ca ions can exis in +2 o +3
oxida ion s a es; he e o e, cha ge ans e could occu esul -
ing in Co
3+
and Fe
2+
being p esen . Tes calcula ions we e
comple ed in which cha ge ans e and diffe en spin s a es o
he ca ions we e conside ed. In all cases, he s uc u es we e
ei he highe in ene gy o con e ged back o he mo e s able
Co
2+
/Fe
3+
high-spin congu a ion. Magne ic momen o ien a-
ions we e ini ialised a he magne ic g ound s a e which was
ound by conside ing all he possible o ien a ions o he
momen s. Fo ZnFe
2
O
4
wi h no mal dis ibu ion, an an i e o-
magne ic congu a ion o he Fe
3+
in Oh si es had he lowes
ene gy. Howe e , a e omagne ic o ien a ion o Fe
3+
in Oh si es
was mos s able o no mally dis ibu ed CoFe
2
O
4
and CuFe
2
O
4
.
Fo he g ound s a e o all h ee spinels s uc u es, when
pa ially o ully in e se, he Fe
3+
in Td si es had magne ic
momen s opposi e o he momen s o he Fe
3+
in he Oh si es.
To simula e he (100) and (111) ZnFe
2
O
4
e mina ed
su aces, ou pe iodic slab models o ZnFe
2
O
4
wi h diffe en
e mina ions sepa a ed by a acuum gap o 10 Å we e op imised
using he same pa ame e s ou lined o he bulk GGA + U
calcula ions. These su aces o he spinel a e Taske ype-III
su aces, in which he e is a dipole momen pe pendicula o
he su ace which can only be elimina ed by su ace econ-
s uc ion.
50
The e o e, we need o modi y he slabs o build
s oichiome ic, non-pola su ace models, which a e gene ally
expec ed o be s able unde neu al (no e y educing o e y
oxidising) condi ions. Ha ing s oichiome ic and non-pola
slabs is also impo an in ou wo k because hey a e used o
es ima e he offse o he mac oscopic elec os a ic po en ial
be ween he bulk and he acuum le el. Howe e , i is gene ally
possible o s udy de ia ions in s oichiome y, as done elsewhe e
o spinel oxides,
51–55
o unde s and he a ia ion in su ace
s oichiome y wi h ex e nal condi ions, such as he oxygen
pa ial p essu e; we ha e no conduc ed such analysis he e. The
de ails o econs uc ions o he su aces o ou s udy a e
shown in he ESI Fig. 5,†and he no a ion o he s oichiome ic
non-pola su aces ollows he one used in e . 52. As seen in
Fig. 2, econs uc ions A and B o he (100) su ace e mina e on
Zn and Fe/O espec i ely. Building he (111) p esen ed a g ea e
challenge, as he uni cell needed o be expanded in bo h la e al
di ec ions. The (111) su aces a e Fe- e mina ed, bu he (111)
B
econs uc ion also has Zn exposed a he su ace, om he
second a omic laye . An in e sion on he su ace was modelled
by swi ching one Fe a om in an Oh si e wi h one Zn a om in a Td
si e on ei he side o he slabs o main ain he s oichiome y
and symme y. Fo he mos s able su ace e mina ion u he
in e sions we e c ea ed p opaga ing in o he su ace.
Su ace ene gies (g) o he diffe en su ace e mina ions
we e ob ained om he equa ion:
g¼Eslab Ebulk
2A;
whe e E
slab
is he o al ene gy o he elaxed slab, E
bulk
is he
ene gy o he bulk wi h he same numbe o o mula uni s as he
espec i e slab and Ais he su ace a ea o one side o he slab.
In he calcula ion bo h sides o he symme ic slab a e allowed
o elax and he e o e bo h mus be conside ed in he su ace
ene gy calcula ion.
2.2 Semi-empi ical simula ions o co e-le el spec a
The semi-empi ical quan um many-body p og am QUANTY,
56–58
wi hin he g aphical use in e ace CTM4XAS,
59
was used o
simula e he Fe L
2,3
edges. This semi-empi ical app oach
conside s Coulomb in e ac ion, spin–o bi coupling and
c ys al-eld spli ing a ound a gi en species, wi hou conside -
a ion o he c ys alline s uc u e. Independen calcula ions we e
comple ed o he h ee Fe species obse ed in he spinels: Fe
3+
in Oh and Td symme y and Fe
2+
in Oh symme y. Based on
expe ience in modelling simila sys ems, he Coulomb in e -
ac ion was scaled o 94% and 88% o he Ha ee–Fock alues o
he Sla e in eg als, whe eas he spin–o bi coupling pa ame e
we e kep a 1.0 (no sc eening) o bo h co e and alence le els.
59
A b oadening o 0.1 eV was used o Gaussian unc ions and
b oadenings o 0.2–0.4 eV we e used o Lo en zian unc ions in
bo h he Nea -Edge X- ay Abso p ion Fine S uc u e (NEXAFS)
Fig. 2 Su ace s uc u es o ZnFe
2
O
4
(100) and (111) su ace econ-
s uc ions leading o s oichiome ic and non-pola e mina ions. Only
one side o he slab is shown bu he o he side o he slab is equi alen
by symme y. Colou scheme: Zn =sil e ; Fe =gold; oxygen = ed.
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and X-Ray Magne ic Ci cula Dich oism (XMCD) simula ions.
The b oadening alues o each Fe species ha e been ou lined in
Table 1, alongside he c ys al eld pa ame e s. The in eg a ed
Fe
2+
and Fe
3+
spec a o he we e no malised o he co e-
sponding numbe s o d elec ons (6 and 5, espec i ely). The
ene gies o he simula ed spec al shapes we e aligned by ing
o he expe imen al XMCD signals.
2.3 X- ay diff ac ion
Powde X- ay diff ac ion (PXRD) pa e ns om CuFe
2
O
4
and
ZnFe
2
O
4
samples we e collec ed on a Rigaku Sma Lab diff ac-
ome e , using CuKa
1
adia ion, in eec ion mode and a oom
empe a u e, o e a 2q ange o 10–80°.
In he case o he CoFe
2
O
4
spinel, he PXRD echnique is
limi ed because Co and Fe ha e simila a omic numbe s (27 and
26, espec i ely), leading o simila sca e ing ac o s, and
making i difficul o dis inguish be ween hese ca ions when
hey sha e a gi en spinel si e. Fo his sample, an Anomalous X-
Ray Sca e ing (AXRS) expe imen was ca ied ou on he
mul ipu pose six-ci cle geome y diff ac ome e o SpLine
BM25 Beamline a he Eu opean Synch o on Radia ion Facili y
(G enoble, F ance). The CoFe
2
O
4
sample was loaded in
a 0.5 mm diame e bo osilica e capilla y, and he eco ded
diff ac ion pa e ns we e collec ed o sample CoFe
2
O
4
: one
using a beam ene gy o 20 000 eV (6–60° 2q ange), and ano he
one using an ene gy o 7097 eV (15–67° 2q ange), ha is 15 eV
below he Fe K-edge abso p ion edge a 7112 eV.
The Rie eld me hod was used o ing he powde diff ac-
ion pa e ns and c ys al s uc u e enemen s, by means o he
Topas Academic 6 sowa e. The s uc u al s a ing model used
o he enemen s was he no mal spinel s uc u e and, o he
case o AXRS da a, he diff ac ion pa e ns a bo h inciden
ene gies we e ed simul aneously using he same s uc u al
pa ame e s o CoFe
2
O
4
. In all cases, he A and B ca ion occu-
pancy a bo h Td and Oh c ys allog aphic posi ions we e ened,
applying he cons ain ha bo h si es should be ully occupied,
and ha he nal calcula ed o mula should be AB
2
O
4
.
Ins umen peak p ole pa ame e s, which we e calcula ed
om a Silicon NIST-640C s anda d e e ence sample measu ed
a he same condi ions as he h ee diffe en samples, we e used
o calcula e b oadening effec s, due o c ys alline size, and
pe o m pa icle size analysis.
2.4 Ca aly ic es ing
The ca aly ic es ing was unde aken a he Ca alysis Hub based
a he Resea ch Complex a Ha well. The pho oca aly ic oxygen
e olu ion was measu ed a oom empe a u e in a gas- igh
50 mL qua z pho o eac o . The ligh in ensi y was adjus ed o
1 sun (100 mW cm
−2
) using an AM 1.5G mass l e ed 300 W Xe
sou ce. 25 mg o sample was used o each spinel (MFe
2
O
4
,M=
Co, Cu, Zn) measu emen in a 0.5 M AgNO
3
medium o ac as
he hole sca enge . The sys em was pu ged o 1 hou wi h 1.5
ba A gas. The gas composi ion was moni o ed by gas ch o-
ma og aphy wi h a ba ie ionisa ion discha ge (BID) de ec o
(GC, Shimadzu GC-2010 Plus). The oxygen e olu ion was
measu ed o e 5 hou s. Re e ence measu emen s o each
medium we e aken o no malise he oxygen e olu ion da a o
he spinel samples; u he de ails o he con ol measu emen s
can be seen in ESI Fig. 3.†
2.5 Nea -edge X- ay abso p ion ne s uc u e
Nea -edge X- ay abso p ion ne s uc u e (NEXAFS) measu e-
men s we e ca ied ou on b anch B o he B07 (Ve SoX)
beamline a Diamond Ligh Sou ce (DLS) using he o al elec-
on yield (TEY) mode collec ed unde 1 mba helium and co -
ec ed o he beamline ansmission.
60,61
2.6 X- ay magne ic ci cula dich oism
X- ay magne ic ci cula dich oism (XMCD) measu emen s we e
ca ied ou on he I10 beamline a DLS on he elec omagne
end s a ion in TEY mode. The measu emen s we e done a
no mal incidence wi h a posi i e helici y a oom empe a u e.
Va ied ex e nal magne ic elds o ±1.5 o ±1.9 T we e applied o
he spinels.
62
3. Resul s
3.1 Bulk s uc u e: DFT simula ions and X- ay diff ac ion
We s discuss he he modynamics o ca ion dis ibu ion in
he h ee e na y oxides CoFe
2
O
4
, CuFe
2
O
4
, and ZnFe
2
O
4
.To
app oxima e he ee ene gy o in e sion as a unc ion o
in e sion deg ee and empe a u e, we in e pola e he in e sion
ene gies ob ained om he h ee DFT calcula ions a x=0, 0.5
and 1 using a quad a ic dependence, which was o iginally
p oposed by O'Neill and Na o sky
63
and has subsequen ly been
used in se e al in es iga ions o he he modynamics o in e -
sion in spinels.
64–66
The in e pola ed in e sion ene gy unc ions,
DE
con
, a e shown in Fig. 3a. They only depend on he congu-
a ion and a e he e o e independen o he empe a u e.
F om he calcula ed in e sion ene gies, he congu a ional
ee ene gy o in e sion, DF
con
, can be es ima ed as:
DF
con
=DE
con
−TDS
con
,
whe e DS
con
is he ideal congu a ional en opy o
in e sion:
67,68
DScon ¼Rhxln xþð1xÞlnð1xÞþxln x
2
þð2xÞln1x
2i:
Table 1 C ys al field pa ame e s o independen Fe species o he
CTM4XAS inpu
Fe species 10 Dq/eV
Exchange eld/meV
XAS XMCD
Fe
3+
(Td) −0.5 0 −90
Fe
3+
(Oh) 1.6 0 90
Fe
2+
(Oh) 1.2 0 90
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The e migh be o he (excess) con ibu ions o he in e sion
en opy, a ising om ene gy diffe ences be ween congu a ions
a a gi en in e sion deg ee, o om ib a ional con ibu ions.
Howe e , p e ious wo k
65
showed ha hese con ibu ions a e
ela i ely small and can be igno ed in a s app oxima ion.
The dominan effec is he in e sion ene gy, whe eas he
en opic e m plays a ela i ely small ole. Bo h CoFe
2
O
4
and
CuFe
2
O
4
each he minimum in e sion ene gy a x=1,
implying a p e e ence o be ully in e se. In con as , he
in e sion ene gy o ZnFe
2
O
4
is posi i e ac oss he ull ange o x,
wi h he mos s able congu a ion being no mal (x=0). These
esul s can be a ionalised based on simple physical a gumen s.
In A
2+
B
23+
O
42−
spinels, he la ice (Madelung) ene gy sligh ly
a ou s he no mal ca ion dis ibu ion. Thus, in he absence o
c ys al eld s abilisa ion ene gy (CFSE) effec s, he no mal
dis ibu ion is p e e ed, as obse ed o ZnFe
2
O
4
(d
10
ca ions
like Zn
2+
and d
5
ca ions like Fe
3+
do no ha e CFSE). In bo h
CoFe
2
O
4
and CuFe
2
O
4
he di alen ca ion is a ansi ion me al
wi h highe CFSE in he oc ahed al han in he e ahed al si e
(excess oc ahed al s abilisa ion ene gy is 30.9 kJ mol
−1
o Co
2+
and 63.5 kJ mol
−1
o Cu
2+
e . 69); he e o e, hese spinels
a ou he in e se dis ibu ion.
The inclusion o congu a ional en opy effec s pe mi s he
conside a ion o ni e empe a u es wi hin his simple model,
bu i does no change he pic u e conside ably. The ee ene gy
o in e sion a ia ion wi h xa diffe en empe a u es is shown
in Fig. 3b. In he cases o CoFe
2
O
4
and CuFe
2
O
4
, he ee ene gy
o in e sion minima occu s a x=1 e en a high empe a u es
up o 900 K. On he o he hand, he in e sion ee ene gy
minima o ZnFe
2
O
4
a e close o he no mal end, anging om x
=0 a oom empe a u e up o x=0.2 a 900 K.
We now conside how hese heo e ical bulk alues o he
in e sion deg ee compa e wi h he obse ed alues o small
nanopa icles. The nanopa icle size, in e sion deg ee (x), cell
pa ame e (a) and he oxygen coo dina e pa ame e (u) o he
spinel samples, as de e mined by AXRS o XRD wi h he Rie -
eld me hod, a e lis ed in Table 2. The co esponding XRD and
AXRS pa e ns can be seen in ESI Fig. 1 and 2.†The pa icle sizes
o he cobal , coppe and zinc e i es a e 35, 24, and 22 nm
espec i ely. CoFe
2
O
4
and CuFe
2
O
4
display high in e sion
deg ees (x) o 0.81 and 0.91 espec i ely, which a e sligh ly
below he DFT-p edic ed alue o 1 ( ully in e se) o bo h
sys ems. In con as , he deg ee o in e sion o ZnFe
2
O
4
(0.26),
al hough much lowe compa ed o he cobal and coppe
e i es, is mo e in e se han he DFT p edic ed no mal s uc-
u e. The in e sion ene gy cu e, howe e , shows a e y small x-
dependence, he e o e small addi ional en opy con ibu ion
can al e he posi ion o he minimum signican ly. O e all, he
gene al ends o xag ee wi h ha o DFT p edic ions and wi h
p e ious li e a u e epo s.
29,30,32
The absolu e alues o he cell and oxygen pa ame e s o he
nanopa icle samples a e gene ally close o hose calcula ed by
DFT, wi h he la ges pe cen age diffe ences be ween he expe i-
men al and calcula ed aand u(2% and 3% espec i ely) being
obse ed in he case o CuFe
2
O
4
. The end in a alues obse ed in
he expe imen al da a (Cu < Co < Zn) is also seen in he simula-
ion esul s. Fac o s effec ing he accu acy o he simula ion
p edic ion include he app oxima ions made in he densi y
unc ional ( he gene alised g adien app oxima ion), he absence
o nanos uc u ing effec s (calcula ions a e done o he inni e
bulk c ys al), and he empe a u e diffe ence (0 K in DFT, oom
empe a u e o expe imen ). Also, a di ec compa ison be ween
he pa ame e s is difficul as he simula ed spinels ha e ex eme
in e sion deg ees (i.e. x =0 o 1), om which he nanopa icles
de ia e.
Fig. 3 (a) In e sion ene gies pe o mula uni (DE
con
, configu a ional
con ibu ions only) ob ained by DFT, and (b) configu a ional ee
ene gies (DF
con
) a 300 K (solid line), 600 K (do ed line) and 900 K
(dashed line) o CoFe
2
O
4
(blue ci cles), CuFe
2
O
4
(g een diamonds) and
ZnFe
2
O
4
(pink iangles).
Table 2 Summa y o expe imen ally de e mined (AXRS da a o
CoFe
2
O
4
and XRD da a o CuFe
2
O
4
and ZnFe
2
O
4
) and DFT-calcula ed
s uc u al pa ame e s x(in e sion deg ee) and a(cell pa ame e ). The u
pa ame e de e mines he O a oms coo dina es (u,u,u) in he s anda d
se ing o he Fd
3mspace g oup
Sample
Expe imen al (nanopa icles a
oom empe a u e) DFT (bulk a 0 K)
Size/nm xa/Å uxa/Å u
CoFe
2
O
4
35 0.81 8.398 0.246 1 8.423 0.245
CuFe
2
O
4
24 0.91 8.371 0.249 1 8.409 0.242
ZnFe
2
O
4
22 0.26 8.439 0.243 0 8.481 0.239
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3.2 Elec onic s uc u e, band alignmen , and pho oca alysis
The wa e spli ing eac ion can be sepa a ed in o hal -
eac ions, he hyd ogen e olu ion eac ion (HER), 2H
+
+2e
−
/H
2
, and he oxygen e olu ion eac ion (OER), 2H
2
O/4H
+
+
4e
−
+O
2
. The kine ics o he OER a e subs an ially slowe han
hose o he HER, as e idenced by a la ge o e po en ial, making
his he a e-limi ing s ep.
12,70
The pho oca aly ic OER using Ag
+
ions as an elec on sac i-
cial agen on all he samples was ca ied ou unde simula ed
sola ligh (1 sun). As seen in Fig. 4, CoFe
2
O
4
and CuFe
2
O
4
exhibi e y li le pho oca aly ic oxygen e olu ion ac oss 5 hou s
o i adia ion (<3 mmol g
−1
o O
2
). In con as , ZnFe
2
O
4
p oduced app oxima ely 65 mmol g
−1
o O
2
ae 5 hou s.
We now a emp o a ionalise he pho oca aly ic beha iou
o he samples in e ms o he elec onic s uc u es and band
alignmen s. The densi y o s a es (DOS), including he pa ial
DOS con ibu ions om he ions, o he spinels can be seen in
Fig. 5. In each case, he Fe 3d le els a e he main con ibu ion o
he conduc ion band (CB). On he o he hand, he cha ac e o
he alence band (VB) diffe s among he spinels: he high-lying
lled Co 3d le els make he main con ibu ion o he VB o
CoFe
2
O
4
, leading o he na owes gap in he se ies; whe eas he
absence o d le el con ibu ions o he VB o ZnFe
2
O
4
leads o
he wides gap among he h ee spinels. The calcula ed band
gaps o CoFe
2
O
4
, CuFe
2
O
4
and ZnFe
2
O
4
a e 1.96, 2.17 and
2.84 eV espec i ely, which a e simila o hose obse ed in
espec i e nanopa icle samples in he li e a u e.
71,72
These
band gap alues a e all sufficien in p inciple o pho oca alysis
o he o e all wa e spli ing eac ion, o which a minimum
he modynamic po en ial o 1.23 eV is equi ed.
12,70
In addi ion o ha ing a sui able band gap, a semiconduc o
mus also ha e CB and VB posi ions s addling he HER and OER
le els in o de o be a good wa e spli ing pho oca alys (in
a single-semiconduc o congu a ion).
12
F om he bulk simula-
ions, he CB minimum and VB maximum a e calcula ed wi h
espec o he a e age elec on po en ial in he solid. To compa e
hese po en ials wi h espec o he HER and OER po en ials he
elec onic s uc u e needs o be aligned ela i e o he acuum
le el. To do so, a slab calcula ion can be used o de e mine he
po en ial diffe ence (DV) be ween he pseudo-bulk a e age and
he acuum po en ial. Fig. 6 shows a s oichiome ic slab, wi h
a symme ic (100) e mina ed su ace and acuum le el.
The calcula ed band alignmen o he spinels wi h espec o
acuum scale a e shown in Fig. 7, compa ed wi h po en ials o
he wa e spli ing hal - eac ions. The po en ials o he HER and
OER in he acuum scale a pH =0 a e −4.44 and −5.67 eV
espec i ely. These ene gy le els a e shied up wi h a pH > 0 a
empe a u e Tby k
B
T×pH ×ln 10.
45,73
The e o e, a oom
empe a u e and pH =7 he HER and OER po en ials a e −4.03
and −5.25 eV espec i ely, co esponding o hose seen in Fig. 7.
Despi e all h ee spinels ha ing a sui able band gap o ca alyse
he o e all wa e spli ing p ocess, hei band alignmen s do no
he he modynamic equi emen s, due hei high-lying CB
minima wi h espec o he HER po en ial (−4.03 eV). The lack
o oxygen e olu ion displayed by CoFe
2
O
4
and CuFe
2
O
4
(Fig. 4)
could be explained by he VB maxima posi ions, which lie abo e
he OER po en ial (−5.25 eV). In con as , he band alignmen o
ZnFe
2
O
4
mee s he he modynamic equi emen s o he OER,
wi h a VB po en ial −5.90 eV.
3.3 Su ace effec s: NEXAFS and XMCD expe imen s and
slab calcula ions
While he p io discussion is ocused on bulk p ope ies, he
beha iou a he oxide su aces, whe e he ac ual ca aly ic
eac ions ake place, migh depa conside ably om he bulk
beha iou . Unde s anding he su ace p ope ies o hese
complex oxides is challenging, bu some insigh s can be ob-
ained om using su ace-sensi i e echniques, such as o al
elec on yield (TEY) NEXAFS and XMCD. These echniques
p obe app oxima ely 2 nm in o he sample su ace. The Fe L
2,3
edges measu ed in TEY mode by NEXAFS o CoFe
2
O
4
, CuFe
2
O
4
,
and ZnFe
2
O
4
a e shown in Fig. 8a. The spec al ea u es a e
dependen on he ela i e quan i ies o each i on species, as
calcula ed in Fig. 8b. The in ensi y o he ea u e be ween he
p e- and main edges a 708 eV (indica ed by he ed a ow in
Fig. 8a) can be ela ed o he p esence o absence o e ahed al
(Td) Fe
3+
. The wid h o he main edge a 709 eV is also affec ed
by he ca ion dis ibu ion. In ZnFe
2
O
4
, he in ensi y o his
ea u e is lowe compa ed o CoFe
2
O
4
and CuFe
2
O
4
, indica ing
ha he e is less Fe
3+
(Td) (i.e. less in e sion) in he zinc e i e
su ace compa ed o he su ace o he o he wo spinels.
The e o e, he same end in ca ion dis ibu ion is obse ed in
bo h he su ace and bulk o hese ma e ials. Howe e , quan-
i i ely de e mining he ela i e amoun s o he diffe en Fe
species is difficul o do by jus ing NEXAFS spec a; mo e
spec al ea u es o in o ma ion is equi ed, which can be ob-
ained om XMCD.
The Fe L
3
edges measu ed by XMCD seen in Fig. 9a–c (also
collec ed in TEY mode, and he e o e su ace-sensi i e) display
spec al ea u es ha a e mo e dis inguishable be ween he
diffe en Fe species, compa ed o NEXAFS. The ea u es a
707.5 eV, 708.7 eV and 709.4 eV can be a ibu ed mainly o
con ibu ions om Fe
2+
(Oh), Fe
3+
(Td) and Fe
3+
(Oh), espec-
i ely, wi h he Oh and Td ions displaying opposi e dich oism.
Fig. 4 Pho oca aly ic oxygen e olu ion o MFe
2
O
4
(M =Co, Cu o Zn)
o e a ime o 5 hou s unde simula ed sun ligh using AgNO
3
as an
elec on sac ificial agen .
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The diffe ence o a ound 2 eV obse ed in ou spinel XMCD
spec a be ween he Fe
2+
and Fe
3+
peak maxima in Oh coo di-
na ion is smalle han ha epo ed o magne i e (Fe
3
O
4
),
a ully in e se spinel (2.5 eV).
74
Howe e , he ela i e shis
be ween he i on species peak maxima obse ed in Fig. 9a–c a e
compa able wi h XMCD shis epo ed o spinel e i e e-
po ed in he li e a u e.
75,76
The ela i e quan i ies o he Fe
species a he su ace can be es ima ed by ing a combina ion
o he calcula ed species-specic spec a (Fig. 9d) o he expe -
imen al spec a. Since he XMCD measu emen s we e also
collec ed in TEY mode, he Fe dis ibu ion a he nea -su ace
can be de e mined. The pe cen ages o Fe
2+
/Fe
3+
in Oh/Td
si es, as de i ed om he XMCD , a e lis ed in he ESI Table
1.†The o he CoFe
2
O
4
signal showed 76% pe cen o nea -
su ace Td si es a e occupied by Fe ca ions, in con as o 81%
o Fe occupied Td si es in he bulk. A simila diffe ence was
obse ed in CuFe
2
O
4
wi h 74% o he nea -su ace Td si es
being occupied by Fe ca ions, compa ed o 91% in he bulk.
ZnFe
2
O
4
showed a signican con as om 26% o Td occupied
by Fe in he bulk o 52% a he su ace. The XMCD signals also
indica e ha a ound 20% o he Fe in Oh was Fe
2+
in all h ee o
he samples, indica ing some le el o su ace educ ion which
could be explained by he o ma ion o oxygen acancies o
o he su ace de ec s.
To illus a e how he p esence o he su ace can al e he
p e e ed ca ion dis ibu ion obse ed in he bulk, we con-
duc ed DFT simula ions in ZnFe
2
O
4
slabs wi h diffe en su ace
e mina ions (Fig. 2) and ca ion dis ibu ions (swapping Zn and
Fe ca ions a he op laye ). Only ZnFe
2
O
4
was conside ed o he
su ace calcula ions, since o his composi ion nanopa icles
Fig. 5 Densi y o s a es (DOS) a HSE06 le el o (a) CoFe
2
O
4
, (b) CuFe
2
O
4
and (c) ZnFe
2
O
4
and pa ial DOS con ibu ions om Co, Cu, Zn and Fe
d o bi als and O p o bi als.
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ha e a signican diffe ence in he in e sion deg ee obse ed in
he su ace compa ed o he bulk. Also, unlike CoFe
2
O
4
and
CuFe
2
O
4
, zinc e i e was he only sample ha demons a ed
any ca aly ic ac i i y o he OER.
The elaxa ion o he diffe en e mina ions can be seen in
ESI Fig. 6 and 7.†In he (100)
A
and (111)
A
e mina ed su aces,
he su ace ca ions shi owa ds he bulk by up o 1.2 Å, which
gene a es mino dis o ion o shiing owa ds su ace wi hin
hei sub-su ace laye s. The (100)
B
sub-su ace laye s shi
owa ds bulk, howe e he op ca ions emain ela i ely xed in
he squa e s uc u e. Minimal ca ion shiing occu s in he
su ace and sub-su ace laye s in (111)
B
s uc u es du ing
elaxa ion.
The calcula ed su ace ene gies a e summa ised in Table 3.
The mos s able su ace is he Zn- e mina ed (100)
A
su ace,
whe eas he Fe/O- e mina ed (100)
B
su ace is much less s able.
This esul aligns wi h a p e ious ab ini io s udy ha ound ha
he (100) su ace o zinc e i e is always mo e s able when Zn-
e mina ed, ega dless o he p esence o O- ich o O-poo
condi ions.
54
We also nd ha he (111) su ace is less s able
han he (100)
A
e mina ion. This con as s wi h he conclusion
om a heo e ical s udy by Guo e al.
53
ha ound ha he (111)
su ace is he mos s able unde he ange o chemical po en-
ials a which bulk ZnFe
2
O
4
is s able. Since we do no pe o m
an analysis he e as a unc ion o chemical po en ials, i is
difficul o compa e wi h he esul s o e . 53. Howe e , o he
pu pose o his wo k, we a e less in e es ed in he ela i e
s abili ies o he su aces, and mo e ocused on he effec o
changes in he ca ion dis ibu ion a he su ace wi h espec o
ha o he bulk.
The compa ison o he no mal s. in e ed dis ibu ion o
ca ions shows ha in all cases he su ace becomes mo e s able
ae he ca ion in e sion a he su ace. The (100)
A
su ace has
he smalles diffe ence in su ace ene gy be ween he no mal
and in e ed su ace (0.02 J m
−2
). In con as , he (100)
B
e mina ion was he leas s able no mal su ace, bu showed he
mos s abilisa ion wi h he in e sion. The (111)
B
e mina ion is
mo e s able han he (111)
A
e mina ion wi h no in e sion;
howe e , when in e ed he e is only a 0.01 J m
−2
diffe ence in
su ace ene gy be ween he A and B e mina ions.
The s abilisa ion o in e sion a he ZnFe
2
O
4
su aces illus-
a es how su ace e mina ions, which imply a change in ca ion
coo dina ion, can al e he he modynamic p e e ences
obse ed in he bulk, which we e d i en by c ys al eld effec s.
Fig. 6 (a) ZnFe
2
O
4
slab wi h a (100) e mina ion and (b) he plana -
a e ages o he elec os a ic po en ial.
Fig. 7 Calcula ed CB and VB posi ions and band gaps o MFe
2
O
4
(M =
Co, Cu o Zn). Hal - eac ion po en ials o wa e spli ing a e ep e-
sen ed by do ed lines.
Fig. 8 (a) Fe L
2,3
edge NEXAFS spec a o MFe
2
O
4
(M =Co, Cu o Zn)
and (b) calcula ed spec a o Fe
3+
(Td), Fe
3+
(Oh) and Fe
2+
(Oh) by
CTM4XAS.
29652 |J. Ma e . Chem. A,2024,12, 29645–29656 This jou nal is © The Royal Socie y o Chemis y 2024
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To in es iga e how deep his effec can p opaga e om he oxide
in he op laye , a second ca ion pai in he sub su ace was
in e ed in he mos s able su ace e mina ion, (100)
A
. The
elaxa ion o his su ace can be seen in ESI Fig. 8.†A simila
shiand dis o ion in he su ace and sub-su ace laye s is
obse ed in bo h he (100)
A
su ace wi h one and wo in e ed
ca ion pai s. The calcula ed in e sion ene gies (DE) o he
no mal and in e ed (100)
A
su aces a e shown in Table 4. A
nega i e in e sion ene gy o −10.6 kJ mol
−1
is obse ed when
one in e sion is c ea ed on he su ace, he e o e inc easing he
s abili y as seen in he su ace ene gies (Table 3). When c ea ing
a second in e sion in he sub-su ace laye he in e sion ene gy
is e en lowe a −16.9 kJ mol
−1
. This demons a es ha i is
he modynamically a ou able o in e sion o be p opaga ed
deepe in o op laye o ZnFe
2
O
4
. In con as , c ea ing an
in e sion in he bulk is an un a ou able p ocess, wi h a calcu-
la ed in e sion ene gy o 10.6 kJ mol
−1
.
Gi en he small size and high specic su ace o he oxide
nanopa icles, hese su ace effec s can ha e a signican
impac on he o e all ca ion dis ibu ions in he nanopa icles.
I is indeed epo ed ha small nanopa icles o ZnFe
2
O
4
end
o ha e highe deg ee o in e sion compa ed o bulk ma e-
ial.
30,77,78
Due o he na u e o he simula ed su aces, an
ex ensi e s udy o he su ace effec s, including educ ion and
oxygen acancies, ha e no ye been in es iga ed. Howe e ,
gi en he impo an ole o nanos uc u ing (and su aces) in
pho oca alysis, hese effec s dese e u he esea ch a en ion.
4. Conclusions
Ou comp ehensi e s udy on MFe
2
O
4
(M =Co, Cu, Zn) spinel
e i es, using a combina ion o heo e ical and expe imen al
echniques, p o ides insigh s in o hei s uc u e, elec onic
p ope ies, and pho oca aly ic beha iou s. The esul s om
DFT simula ions align well wi h expe imen al ndings,
e ealing dis inc in e sion deg ees and pho oca aly ic ac i i ies
ac oss he spinels. The DFT-p edic ed p e e ence o in e se
congu a ions in CoFe
2
O
4
and CuFe
2
O
4
and a no mal cong-
u a ion in ZnFe2O
4
we e con med by X- ay diff ac ion and
AXRS measu emen s which showed in e sion deg ees o 0.81,
0.91, and 0.26, espec i ely.
Among he h ee spinel composi ions, only ZnFe
2
O
4
demons a ed pho oca aly ic ac i i y o he oxygen e olu ion
eac ion (OER), gene a ing 65 mmol g
−1
o oxygen o e 5 hou s
unde UV i adia ion. This can be a ibu ed o i s a ou able
band alignmen , as demons a ed h ough ou elec onic
s uc u e simula ions. CoFe
2
O
4
and CuFe
2
O
4
do no exhibi
Fig. 9 Fe L
3
edge XMCD spec a o (a) CoFe
2
O
4
, (b) CuFe
2
O
4
and (c)
ZnFe
2
O
4
in which he ci cles a e he expe imen al da a and he solid
line a e he calcula ed spec a. (d) he calcula ed Fe L
3
edge o Fe
3+
(Td), Fe
3+
(Oh) and Fe
2+
(Oh) by CTM4XAS.
Table 3 Calcula ed su ace ene gies (g) o he elaxed e mina ions o
(100) and (111) su aces o ZnFe
2
O
4
Su ace Te mina ion
Ca ion dis ibu ion
a su ace g/J m
−2
(100) A No mal 1.28
In e ed 1.26
B No mal 2.91
In e ed 1.91
(111) A No mal 2.32
In e ed 1.62
B No mal 1.75
In e ed 1.61
Table 4 Calcula ed in e sion ene gies (DE) o he elaxed e mina ions
o he (100)
A
ZnFe
2
O
4
su aces wi h 0, 1 and 2 in e ed ca ion pai s
Su ace
Numbe o ca ion
pai in e sions on su ace DE/kJ mol
−1
(100)
A
0 0.0
1−10.6
2−16.9
This jou nal is © The Royal Socie y o Chemis y 2024 J. Ma e . Chem. A,2024,12, 29645–29656 | 29653
Pape Jou nal o Ma e ials Chemis y A
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