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 ica 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 dened 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 specic 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 icial agen has been used alongside he spinel-based
ca alys ; he use o diffe en sac icial agen s, such as
oxala es
16,17
o me hanol,
14,18
can ha e a signican 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 icial agen . Howe e ,
ZnFe
2
O
4
is also s ill oen 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 signican ly diffe en om ha in he bulk, and
is hea ily inuenced 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 congu a ion in he
p imi i e cell, he e o e allowing us o calcula e he in e sion
ene gy (congu 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 congu 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 congu 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 congu 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 eec 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 enemen s, by means o he
Topas Academic 6 sowa e. The s uc u al s a ing model used
o he enemen 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 ened,
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 ole 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 congu-
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 congu 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 congu a ional en opy o
in e sion:
67,68
DScon ¼Rhxln xþð1xÞlnð1xÞþxln x
2
þð2xÞln1x
2i:
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 congu 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 congu 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 congu 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 signican 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 inni 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
ae 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 congu 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 shied 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 shis
be ween he i on species peak maxima obse ed in Fig. 9a–c a e
compa able wi h XMCD shis 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-specic 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 signican 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 signican 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 shiing 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 shiing 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
ae 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
shiand 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 specic su ace o he oxide
nanopa icles, hese su ace effec s can ha e a signican
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
congu a ions in CoFe
2
O
4
and CuFe
2
O
4
and a no mal cong-
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
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