Electronic structure of nanoscale iron oxide particles measured by scanning tunneling and photoelectron spectroscopies

0.22 MB·English

by  M. Preisinger

Electronic structure of nanoscale iron oxide particles
measured by scanning tunneling and photoelectron spectroscopies
M. Preisinger,1 M. Krispin,1 T. Rudolf,1 S. Horn,1 and D. R. Strongin2
1Institute of Physics, University of Augsburg, 86135 Augsburg, Germany
2Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, USA
5 (Dated: February 2, 2008)
0
0 We have investigated the electronic structure of nano-sized iron oxide by scanning tunnelling
2 microscopy(STM)andspectroscopy(STS)aswellasbyphotoelectronspectroscopy. Nanoparticles
were produced by thermal treatment of Ferritin molecules containing a self-assembled core of iron
n
oxide. Depending on the thermal treatment we were able to prepare different phases of iron oxide
a
J nanoparticles resembling γ-Fe2O3, α-Fe2O3, and a phase which apparently contains both γ-Fe2O3
and α-Fe2O3. Changes to the electronic structure of these materials were studied under reducing
7
conditions. Weshowthatthesurfacebandgapoftheelectronicexcitationspectrumcandifferfrom
1
that of bulk material and is dominated bysurface effects.

]
i PACSnumbers: 73.22.-f,79.60.Jv,68.37.Ef,81.07.-b
c
s
-
l I. INTRODUCTION kinds of nanoparticles and nanocomposites8,9. Further-
r
t more, Ferritin molecules inherently show photocatalytic
m
activity10.
Nano-sizedtransitionmetaloxidesparticlescanbeex-
.
t
a pected to show size dependent optical, magnetic and
m chemical properties with possible applications in catal-
ysis and magnetic and optical devices. It has already II. EXPERIMENTAL
-
d been shown, that catalytic and magnetic properties of
n nanoparticles might be quite different from bulk materi- Monolayers of Ferritin molecules were prepared by
o
als. Anexamplearenano-sizedparticlesofthecompound the Langmuir-Schaefer technique11. A Ferritin solution
c
[ Fe2O3, which exhibit superparamagnetism1,2,3, and, at (0.15mg/mlhorsespleenFerritinfromSigma)in10mM
the same time, enhanced catalytic properties compared NaClwith a pH of 5.5 wasfilled in a commerciallyavail-
1 to the bulk material4. The size dependence of the cat- ableLangmuir-Trough(NimaTechnology). Afewmicro-
v
alytic properties can be associated with an influence of grams of trimethyloctadecylammonium bromide (Fluka)
7
7 size on the electronic structure of the material. This in- were solved in chloroform to form a concentration of
3 fluence can be due to the surface tension of the particle, 1 mg/ml and was then spread over the Ferritin solu-
1 resulting in an effective pressure or to changes of the tion subphase. After compression to a surface pressure
0 stoichiometry (and resulting changes of the state of oxi- of 30 mN/m and an adsorption time of a few minutes
5 dationofFe)ofsuchparticles,drivenbythelargesurface the monolayer was transferred to a freshly cleaved sur-
0
to volume ratio. faceofahighlyorientedpyrolyticgraphite(HOPG)sub-
/
at Here we investigate the electronic structure of nano- strate by the horizontal lifting method. Further details
m sized iron oxide by scanning tunnelling microscopy and of preparationof 2D arrays are described elsewhere12,13.
spectroscopy as well as by photoelectron spectroscopy Prior to the introduction into the ultra high vacuum
-
d andopticalspectroscopy. Toaddressthequestionofstoi- (UHV) chamber all samples were annealed in air at
n chiometry andoxidationstate the effect of oxidizing and different temperatures and for different time intervals
o reducing atmosphere on the electronic structure of the (∼575Kforonehour,∼675Kforonehouror∼675K
c
nanoparticlesisalsoinvestigated. Toproducenano-sized forfivehours)toeliminatetheproteinshellandtofurther
:
v iron oxide particles we utilized the ability of Ferritin to oxidizetheironcore. Afterintroductionintothechamber
Xi self-assemble and construct a core of iron oxide. Ferritin the samples were annealedin situ under a 1×10−6 mbar
is the major cellular iron-storage protein and consists of oxygen atmosphere at the respective temperature for a
r
a a sphericalhollowshell composedof 24polypeptide sub- few minutes. Utilizing this process we produced dense
units and is able to store iron as hydrated iron oxide in arraysof ironoxide nanoparticles. To vary the oxidation
the internal cavity5,6,7. The inner and outer diameters stateoftherespectivenanoparticles,wereducedthesam-
of the protein shell are about 8 and 12.5 nm, respec- plewithintheUHVsystembyexposureto1×10−6 mbar
tively. The iron core shows a structure similar to that of of an H2/Ar-atmosphere (25% H2, 75% Ar) at ∼ 675 K
the mineral ferrihydrite (5 Fe O · 9 H O). Hydrophilic for two hours.
2 3 2
andhydrophobicchannelspenetratetheproteinshelland Spectroscopic measurements were carried out in an
provide the means by which iron and other atoms can Omicron UHV system, equipped with a DAR400 X-
be accumulated within or removed from the molecules. ray radiation source (Al Kα 1486.6 eV) and an AR65
In previous work, Ferritin was used to prepare several electron analyzer for X-ray photoelectron spectroscopy
2
FIG. 1: AFM image taken on a Ferritin monolayer without FIG.2: STMimage ofan oxidized Ferritin monolayer, show-
annealing ing iron oxidenanoparticles
(XPS). Scanning tunnelling microscopy (STM) and - Depending on oxidation parameters, we obtain
spectroscopy (STS) measurements were performed us- nanoparticles of different iron oxide phases. X-ray
ing a variable temperature scanning probe microscope diffraction (XRD) performed on a thick layer of Ferritin
(VT-SPM) within the same UHV system. All STM im- annealedat∼675Kunderairforonehour(procedure1)
ages were recorded in the constant current mode, using revealsratherbroadreflectionscharacteristicofγ-Fe2O3
a tungsten tip at a sample bias of -1.5 V and a current with some contributions of α-Fe2O3. In the following
of 0.1 nA. The topographic images represent the height we will refer this mixed phase as χ-Fe2O3. Further an-
z(x,y)ofthetunnellingtipabovethesampleafteraplane nealing for five hours (procedure 2) resulted in narrow
andslopesubtractionwasperformed. TheSTSmeasure- reflections characteristic of α-Fe2O3. These results are
mentswerecarriedoutplacingthetunnellingtipabovea consistentwithtransmissionelectronmicroscopy(TEM)
singlenanoparticleandkeepingthetip-sampleseparation studies, which will be described elsewhere14. Annealing
fixed while recording the I(U) characteristic. A Bruker Ferritinmoleculesat∼575Kinair(procedure3)results
fourier-transforminfraredspectrometer(FTIR)wasused in γ-Fe2O3 nanoparticles, as confirmed by TEM.
for optical spectroscopy. STM and STS measurements A STM image of an nanoparticle monolayer after an-
weredoneonmonolayersofnanoparticles,whileXPSand nealing in air (procdure 1) is shown in Figure 2. Here
optical spectroscopymeasurements were done on thicker we note that the images of the nanoparticles of all three
samples in order to improve the signal to noise ratio. phases (not shown) look alike. The image shows densely
Such samples were prepared from a small dried drop of packed clusters of distinguishable nanoparticles. After
Ferritinsolution(1mg/ml)annealedinairattherespec- the annealing process the removal of the protein shell
tive temperatures. results in a lower coverage of the HOPG surface. The
nanoparticleclustersconsistofwelldefinedparticlesofa
size of approximately 7 - 8 nm. Such a size is expected
for the iron oxide core of Ferritin. Clusters are found
III. RESULTS
dominantlyatstepspresentontheHOPGsubstratesur-
face.
Images of monolayers of Ferritin molecules under air To obtain information concerning the iron oxidation
(Fig. 1) were taken by Tapping Mode atomic force mi- state we performed XPS on the nanoparticles. Figure 3
croscopy (TM-AFM, utilizing a Nanoscope IIIa, Digital displays the Fe 2p XPS spectra characteristic of the oxi-
Instruments). ThelayerofFerritinmoleculesisnotcom- dized nanoparticlephases and, for comparison,the spec-
pletely closed. The difference in height between the Fer- traofFe O andFeO,takenfrombulk-likematerial15,16.
2 3
ritin layer and the subjacent HOPG substrate of about The latter two spectra correspond to a Fe3+ and a Fe2+
10 nm corresponds to the thickness of a single Ferritin state, respectively. We did not observe any charging ef-
monolayer. Also the diameter of the spheres observed is fects of the nanoparticles within the instrumental width
approximately10nm,characteristicofFerritinmolecules of about0.2 eV. With respectto the Fe2+ spectrum, the
with a dehydrated protein shell caused by drying under Fe3+ spectrum is shifted to a higher binding energy by
air. 1.5 eV and shows a characteristic satellite structure at a
3

0.02
Fe 2p
3/2
Fe 2p Fe2+
1/2
FeO ref.
0.01
s]
nit
u
sity [arb. Nanoparticles I [nA] 0.00 ~ 2.0 eV
n
e
Int Fe2O3 ref. -0.01
Fe3+ NP HOPG
-0.02
-2 -1 0 1 2
-730 -725 -720 -715 -710 -705
U [V]
Binding energy [eV]
FIG. 4: STS spectra of α-Fe2O3 nanoparticles compared to
FIG. 3: XPS spectra of oxidized Ferritin molecules (Fe2O3 HOPGsubstrate
nanoparticles) and reference data of bulk-likesamples

0.02
binding energy E = 719 eV. The Fe2+ spectrum shows
b
a satellite structure at E = 716 eV. A Fe 2p peak
b 3/2
at 711.7 eV and a satellite structure at about 719 eV in 0.01
the spectrum taken from the iron oxide nanoparticles is ~ 1.3 eV
characteristic for a Fe3+-state, although the presence of
aslightweightofFe2+ cannotberuledout. Furtheroxi- A] 0.00
n
dationoftheparticleswithintheUHVsystematanoxy- I [
gen partial pressure of 1×10−6 mbar and a temperature ~ 2.0 eV
of ∼ 675 K does not result in changes of the intensity of -0.01
the characteristic satellite, nor did it shift the spectrum
to higher binding energies. We, therefore, conclude that
eitheri. theFe-atomsofthenanoparticlesarealreadyin -0.02
-2 -1 0 1 2
the highest oxidation state (Fe O ), or ii. the Fe- atoms
2 3
U [V]
are not in the highestoxidationstate, but further oxida-
tionofthenanoparticlesisnotpossibleunderthechosen
conditions. FIG. 5: STS spectra of γ-Fe2O3 nanoparticles
To investigate the electronic excitation spectrum of
single nanoparticles in the vicinity of the Fermi en-
ergyEF,STSmeasurementswereperformedonthethree few ten different nanoparticles and the gap energy was
different phases identified by XRD and TEM (α-Fe2O3, found to vary between 1.8 eV and 2.2 eV. This varia-
γ-Fe2O3, χ-Fe2O3). It is well known, that tunnelling tionmightreflectdifferencesinparticlesize,surfacecon-
spectra obtained from nanocrystals may depend on the ditions and small variations of the tunnelling junction
tunnelling parameters and tunnelling configuration due parameters for the different measurements. The gap en-
toe.g. charging17,18,renderingthedeterminedgapwidth ergy of ∆ = 2.0 eV corresponds to that measured on
unreliable. Thereforeweadjustedthetunnellingparame- a bulk α-Fe O sample (∆ = 2.2 eV)7. Figure 5 dis-
2 3
terstoperformthemeasurementsatlargetunnellingtip- playstunnellingspectratakenoffγ-Fe O nanoparticles.
2 3
nanoparticle distance. The distance was increased until Here, spectra taken from different locations of a single
a constant gap value was observed and charging effects nanoparticle, show two different values of the gap ener-
did not occur. gies,∆ =1.3eV(±0.2eV)and∆ =2.0eV(±0.2eV).
1 2
A typical tunnelling spectrum taken from a single α- Wenote,thatthegapenergyofabulksampleofγ-Fe2O3
Fe O nanoparticle (NP) is plotted in Figure 4. For showsagapof2.0eV7. Finally,tunnelling spectrataken
2 3
comparison, a spectrum taken from the HOPG sub- from χ-Fe2O3 phase nanoparticles show a band gap of
strate is also shown. The I(U) - curves taken from the 1.3 eV (± 0.2 eV) (Fig. 6), which does not change for
nanoparticleclearlyarecharacteristicforagapenergyof different locations on one nanoparticle. A gap value of
∆ = 2.0 eV. The variation of the gap energy for spec- 1.3eVdoesnotcorrespondtothegapenergyofanybulk
tra taken from the same nanoparticle was within the er- iron oxide phase7.
ror of the measurement. Spectra were taken from of a Keeping in mind that STS is a highly surface sensi-
4

0.02
Fe 2p
Fe 2p 1/2 Fe2+ 3/2
0.01 FeO ref.
s]
nit
u
b.
I [nA] 0.00 ~ 1.3 eV ensity [ar reduced NP
nt
I
-0.01 FeO NP
2 3
Fe3+
-0.02
-2 -1 0 1 2
-730 -725 -720 -715 -710 -705
U [V]
Binding energy [eV]
FIG. 6: STS spectra of χ-Fe2O3 nanoparticles
FIG. 8: XPS spectra of oxidized and reduced iron oxide
nanoparticles compared to FeO reference data
0.6
0.4
n
o
si
s
mi
s
n
a
Tr 0.2
0.0
1.0 1.5 2.0 2.5 3.0
Energy [eV]
FIG. 7: Optical transmission of χ-Fe2O3 nanoparticles
tive technique we carried out optical spectroscopy on
nanoparticles of the χ-Fe O phase, in order to obtain FIG. 9: STM image of thereduced χ-Fe2O3 nanoparticles
2 3
informationcorrespondingto”bulkproperties”. Figure7
shows the optical transmission data taken from a thick
spectrum of the reduced sample is shifted to lower bind-
layer of such particles. The spectrum displays a strong
ing energies by about 1.2 eV compared to the spectrum
decreaseoftransmissionataround2.0eV,andalesspro-
before reduction. In addition, a Fe2+ satellite structure
nounced decrease at about 1.3 eV. These energies agree
develops, marked by an arrow in Figure 8.
well with the two different gap energies inferred from
STM imaging shows (Figure 9) that shape and size
STS measurements on such particles. In a straight for-
of the nanoparticles are not changed by the reduction
ward interpretation we assign the feature at 2 eV to the
process. From STS measurements we infer, within the
bulk gap, the feature at 1.3 eV to a surface gap. This
margins of error, the same gap of 1.3 eV (± 0.2 eV) as
interpretation will be discussed in more detail below.
before the reduction (Figure 10).
Aninterestingquestionconcernsthepossibilitytoma-
nipulate the electronic properties of the nanoparticles.
To this end we exposedχ-Fe O nanoparticlesin situ at
2 3
a temperature of ∼ 675 K to a reducing H /Ar - atmo- IV. DISCUSSION
2
sphereof1×10−6 mbar(25%H ,75%Ar)fortwohours.
2
Figure 8 compares the Fe 2p spectrum of the nanopar- We firstaddressthe questionofthevalence ofthe iron
ticles before and after reduction. As is clearly seen, the ions in the nanoparticles. From the Fe 2p binding en-
5

face sensitive STS measurements detect only one gap at
0.02
1.3 eV we conclude that the 1.3 eV gap is a property of
c - FeO NP the surface of the nanoparticle, most likely due to a sur-
2 3
reduced NP face reconstruction. The bulk of the nanoparticle then
0.01
exhibits the gapexpected from bulk γ- or α-Fe O . It is
2 3
striking that a surface reconstruction would only occur
A] 0.00 in the γ-Fe2O3 and χ-Fe2O3 nanoparticles. The reason
I [n for this could be attributed to the defect rich nature of
~ 1.3 eV these phases.
The interpretation of the 1.3 eV gap as a surface ef-
-0.01
fectisfurthercorroboratedbyourreductionexperiments.
There are several studies concerning the reduction of
iron oxides. For example, α-Fe O can be reduced to
-0.02 2 3
-2 -1 0 1 2 metallic iron via a series of intermediate oxides, like
U [V] Fe O and FeO19. As shown above we achieved a sig-
3 4
nificant reduction of χ-Fe O nanoparticles by anneal-
2 3
ing in H /Ar - atmosphere, as evidenced by XPS mea-
FIG. 10: STS spectra of oxidized and reduced iron oxide 2
surements. The resulting XPS spectrum is compatible
nanoparticles
to a FeO reference spectrum taken from a macroscopic
sample. However, a stable cation-deficient phase of FeO
exists at ambient pressure only at temperatures above
ergy and the position and intensity of the correspond-
840 K. This non-stoichiometric form of FeO can be ob-
ing satellite structure as measured by XPS we conclude
tained as a metastable phase at room temperature by
that the iron valence state of the oxidized samples is
Fe3+. This is in agreement with expectations since it rapid quenching7. Although we cannot expect a stable
FeObulkphasefromtheappliedannealingprocedureun-
is well known that ferrihydrites transform into α- or γ-
Fe O at573-723Kunderair7. Furtherevidenceforan der ambient conditions, we cannot exclude that the FeO
2 3
Fe3+- state after annealing under air comes from XRD phase is stabilizedunder UHV due to its nanocrystalline
and TEM14, which show the nanoparticles to consist of form. This conclusion is supported by former studies on
epitaxial films showing that the stabilization of a FeO
different phases of Fe O , depending on the oxidation
2 3 phase depends on film thickness15,20.
procedure. Bulk Fe O is an insulator with a gap of
2 3
∆ = 2.0 eV for the γ-phase and ∆ = 2.2 eV for the α- Interestingly, a change of the iron oxidation state un-
phase. The gap energy obtained for α-phase nanoparti- derreducingconditions,asreflectedintheXPSmeasure-
clesbySTSisconsistentwiththatforbulkmaterial. The ments,doesnotaffecttheenergygapofthenanoparticles
γ-Fe O phase nanoparticles show a band gap of 1.3 eV as measured by STS. Taking into account the probing
2 3
or2.0eV,dependingonthelocationofthetunnellingtip depth of XPS at the Fe 2p binding energy we conclude
on the same particle. This suggests two different types thatabout40%ofthevolumeofaparticlecontributesto
of phases within one nanoparticle. The fact that bulk γ- the XPS spectrum, while contributions to the STS mea-
Fe O hasabandgapof2.0eV7 suggeststhatitisoneof surements originate from a surface layer of the particle,
2 3
thesephases. Fromsinglecrystalstudiesitisknownthat whichamountstoonly10%ofitsvolume. Obviously,for
γ-Fe O crystallizes in an iron deficient structure simi- nanoparticles XPS is far more bulk sensitive than STS.
2 3
lar to the inverse spinel structure of Fe O (A[BA]O , Therefore,weattributetheindependenceofthegapvalue
3 4 4
A=Fe3+, B=Fe2+), but in γ-Fe O there are randomly upon iron reduction to the fact that the gap value mea-
2 3
distributed cation vacancies on the B-site. Such a defect sured by STS is dominated by the surface structure of
richstructure might be subject to reconstructioneffects, the nanoparticle. This surface structure appears to be
especially at the surface of a nanoparticle or by impu- unaffected by reduction. This finding is consistent with
rity atoms like, e.g. carbon introduced by the original our the conclusion above that the 1.3 eV band gap of
organic shell of the particle. Such a reconstructed phase χ-Fe2O3 is due to surface effects.
can yield a different gap than the bulk. The XPS and Inconclusion,onlynanoparticlesofthethermodynam-
FTIR measurements might be affected by such surface ically stable α-Fe O phase show a band gapcompatible
2 3
effectsinamuchlesserdegreebecauseoftheirlowersur- to the bulk material in surface sensitive STS measure-
face sensitivity. This will be discussed in more detail ments. The STSmeasurementsonallothernanoparticle
below. phasesinvestigated(γ-Fe O ,χ-Fe O andFeO)demon-
2 3 2 3
The FTIR-spectrum taken from the χ-Fe O phase strate the existence of a surface band gap of 1.3 eV, al-
2 3
sheds further light on the origin of the 1.3 eV gap. Two thoughvolumesensitivemeasurementsrevealabandgap
significant features were identified in this spectrum, one compatible with the bulk material. We, therefore, con-
of which coincides with the gap of 1.3 eV found by STS. clude that the band gap does not depend on the size
Fromthe factthatthe slope ofthe absorptioncoefficient of the nanoparticles for diameters above 8 nm, but that
ismuchsmallerat1.3eVthanat2 eV,but thatthe sur- properties, like e.g. the photocatalytic activity of such
6
particles, are due to the formationof a surface band gap
different from that of the volume. The existence of such
a surface band gap appears to be triggered by defects
within the nanoparticle.
Acknowledgments
Thisworkwassupportedbythe DeutscheForschungs-
gemeinschaft through SFB 484.
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