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2017, 19 , 19407

Surface chemistry and stability of metastable
corundum-type In
2
O
3
†
Eva-Maria Ko
¨ ck, ‡
a
Michaela Kogler , ‡
a
Chen Zhuo,
a
Lukas Schlicker,
b
Maged F. B ek heet,
b
Andr ew Dora n,
c
Alek sa nder Gu rlo
b
an d S imon Pe nner *
a
To account for the explanation of an eventual sensing and catalytic behavior of rhombohedral In
2
O
3
(rh-In
2
O
3
) and the dependence of the metastability of the latter on gas atmospheres, in situ electrochemical
impedance spectroscopic (EIS), Fourier-transform infrared spectrosco pic (FT-IR), in situ X-ray diffraction
and in situ thermogravimetric analyses in inert (helium) and reactive gases (hydrogen, carbon monoxide
an d carb on diox ide) h ave bee n cond uct ed to link th e gas- dep en dent el ectr ical con du cti vity fe atu res and
th e surf ace ch emi cal pro pert ies to its me tast abil ity to war ds cub ic In
2
O
3
. In parti cu lar, for h ighly re duci ble
oxides such as In
2
O
3
, for which not only the formation of oxygen vacancies, but deep reduction to the
metallic state ( i.e. metallic indium) also has to be taken into account, this approach is imperative.
Temperature-depen dent impedance features are strongly dependent on the respective gas composition
and are assigned to distinct changes in either surface adsorbates or free charge carrier absorbance,
allowing for differentiating and distinguishing between bulk reduction-related features from those
directly arising from surface chemical alterations. For the measurements in an inert gas atmosphere, this
analysis specifically also included monitoring the fate of differently bonded, and hence, differently
reactive, hydroxyl groups. Reduction of rh-In
2
O
3
proceeds to a large extent indirec tly via rh-In
2
O
3
-
c-In
2
O
3
- In metal. As dedu ced f rom the CO and C O
2
adsorption exper iments, rhombohedr al I n
2
O
3
exhibits
predom inantly Lewis acidic surfac e sites. The basic character is less pronoun ced, directly explaining the
previously obser ved high (inverse) water–gas shift activity and the low CO
2
selectivity in methanol steam
reforming.
1. Introduction
Corundum-type rhombohedral In
2
O
3
(rh-In
2
O
3
hereafter) is
the only metastable indium oxide modification accessible for
physico-chemical studies.
1,2
The latter, conducted for a direct
comparison to the stable cubic bixbyite-type polymorph,
3
is of
vital importance for the fundamental understanding of the
structure–property relationships of In
2
O
3
-based materials.
Considerable effort has therefore been made to establish con-
trollable synthesis routines to various morphologies, such as
na no-fib re s, na no-s heet s or (hol low) nano -s pher es, to pote ntia lly
circumvent high-temperature and high-pressure synthesis
pathways.
1,4,5
Most focus was placed on introducing new ambi-
ent pressure sol–gel synthesis routines to reproducibly and
more easily obtain larger quantities of materials for subsequent
physico-chemical characterization.
6,7
Recently, a new water-free
solvothermal route was successfully established, allowing for
synthesizing large quantities of rh-In
2
O
3
.
8
Despite these efforts,
on ly limi ted info rmat ion abo ut the phy sico -che mica l prop er ties
of me tasta ble In
2
O
3
po ly morphs is cu rren tly ava ila ble. Ch ar ac-
te rizati on is bas ical ly rest rict ed to lite rall y si ngl e F T-I R or Ram an
spec tr a or, in th e case of st ruct ura l char acte riza tion , el ectr on
microscopy imaging. A systematic characterization approach
highlighting the intrinsic properties of rh-In
2
O
3
is almost
mis s ing , but g iv en th e pot ent i al te chn ol ogi ca l app lic ati ons , ev en
mor e i mpe rat ive . Be si des t he ex per im ent al e le ctr on ic st ru ctu re o f
rh -I n
2
O
3
,
9,1 0
on ly t he ch arg e- car ri er co nc ent rat ion s an d mob il i-
tie s a re ava il abl e fr om H all -e ffe ct s tu di es, p rov id ing s om e ins ig ht
into the gas-sensi ng properties of corundum-type In
2
O
3
, as ses sed
previously.
11
Recently, the catalytic properties, alongside the
stability limits upon reduction, have been successfully elucidated.
12
Also missing are studies on the surface chemistry of rh-In
2
O
3
,
which directly controls its adsorption properties and steers its
a
Institut fu
¨ r Physikalische Chemie, Univers ita
¨ t Innsbruck, Innrain 52c,
A- 60 20 Inn s bru ck, Au st ria . E-m ail : s imon . pe nn er@ uib k. ac. at ; T el: + 43 512 507 58003
b
Fachgebiet Keramisch e Werkstoffe/Chair of Advanced Ceram ic Materials,
Institut fu
¨ r Werkstoffwissenschaft en und-technologien,
Technische Universita
¨ t Berlin, Hardenbergstr. 40, D-10623 Berlin, Germany
c
Advanced Light Source, Lawrence Berkeley Natio nal Laboratory, Berkeley,
California 94720, USA
† Electronic supplemen tary information (ESI) available: Additional EIS and FT-IR
experiments; collected FT-IR spectra, activation energies and EIS tables. See
DOI: 10.1039/c7cp03632a
‡ These authors contributed equally.
Received 30th May 2017 ,
Accepted 3rd July 2017
DOI: 10.1039/c7cp03632a
rsc.li/pccp
PCCP
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(surface) reactivity. The studies are also partially fueled by the
outstanding reactivity properties of its cubic In
2
O
3
counterpart
(c-In
2
O
3
hereafter), which exhibits extraordinarily high CO
2
sel ecti viti es in meth anol st eam refo rmin g (esse nt ial ly refe renc ed
to th e sim ulta neou s larg ely su ppre sse d inve rse wa ter– gas sh ift
reactivity
13,14
), as wel l as the acti vity in ca taly tic dehy dr ogen ation
of ligh t hydr ocar bons
15
and in ph otoc atal ytic CO
2
red ucti on.
16 ,17
St udie s of th e sur face ch emis tr y of In
2
O
3
po ly morphs ar e als o
ne eded fo r und ersta ndin g the mech ani sm of gas d etec ti on wi th
in dium ox ide a ppli ed as a ch emir esis tive ga s sen sor.
18 –21
As fo r
th e l atte r, many ox ides ex hibi t excel lent se nsin g prop erti es for
various gases. To explain the sensing behavior, a profound
knowledge about the interplay between chemistry and conduc-
tion properties is required. This is especially true if the con-
duction is confined to the surface-near regions. In this case,
only a concerted approach of operando su rf ac e c hemi ca l c har -
ac teri zati on ( e. g. by FT -IR spec tros copy ) and ass essm ent of the
co mp leme nt ary sur face co nduct ion pr oper ties ( e. g. by impe -
da nce spec tros copy ) yie lds va lua ble re sult s. Incr easi ngl y, als o
me tasta ble ox ide po lymorp hs ( e. g. rhom bohe dr al In
2
O
3
)
18–2 3
ar e
appl ie d as such s ens ing mate ri als. In this sp ecia l ca se, the sit ua-
ti on is fu rt her comp licat ed by the poss ib le tr ansfo rma tion to
th ermo dyn amic all y more st abl e modif ica tion s duri ng sen sor
operation. As this of course is entangled with the surface
chemistry/conduction behavior, a thorough knowledge about
the interplay between the latter two and the overall surface and
bulk structure stability in various gas atmospheres is necessary.
Despite the fact that many such sensing studies are reported,
information on this correlation with stability issues, especially
as a function of gas composition, is still scarce. This is a
particular pity, as the inherent structural metastability will
certainly alter the sensing behavior. Exactly such a behavior
has been recently monitored in the ZrO
2
polymorphic system,
where the surface chemistry and the correspond ing conduction
properties have been a strong function of the metastability of
tetragonal ZrO
2
and the associated transformation to mono-
clinic ZrO
2
in various gas atmospheres.
The present contribution exemplifies these studies on an
ar che ty pic al , rec en tly m uch s tu die d met as ta ble o xid e po lym orp h,
na mel y rh -I n
2
O
3
. As it will turn out, the coincidence and correla-
tion be tween surface chemical and conducti on property changes
an d sta b il it y are o uts ta nd ing i n va ri ous g as a tmo sp here s , enc om-
passing reductive (H
2
, CO), reactive (CO
2
) and basically inert
surroundings (He). Hence, the present study is essentially
focused on studying the interaction of H
2
, CO, He and CO
2
with rh-In
2
O
3
in the temperature region up to 673 K, which is of
interest for catalytic and gas-sensing studies, and to link the
surface chemistry to physical properties such as electrical con-
ductivity. This approach yields not only a particular convenient
basis to explore also other (metastable) polymorphic systems,
which are yet to be studied, but also data on the catalytically
relevant acidity and reactivity of the rh-In
2
O
3
surfaces and,
therefore, allows highlighting the similarities to and differences
from thermodynamically stable c-In
2
O
3
. Particular emphasis is
also given to the structural and chemical stability of rh-In
2
O
3
during these treatments.
2. Experimental
2.1. Synthesis of the materials and general
experimental features
The synthesis of the materials has been outlined in detail in a
recent publication.
8
High-purity He, H
2
,O
2
, CO and CO gases
were provided by Messer-Griesheim (Germany). Condensable
impurities were removed from the gases by cooling traps. For
He and H
2
, liquid N
2
at 77 K was used; O
2
, CO and CO
2
were
purified using ethanol/liquid N
2
mixtures at 153 K (O
2
and CO)
and 223 K (CO
2
). Gas treatments were carried out under flowing
conditions ( B 1m Ls
 1
) up to 700 K to avoid triggering the
phase transformation to c-In
2
O
3
to a large amount.
2.2. In situ electrochemical impedance spectroscopy (EIS)
Ah e a t a b l e in situ impedance c ell designed f or gas treatme nts
up to 1273 K, situated in a tubular furnace (Linn, Germany) and
controlled using a t hermocoup le located 5 mm downstream of the
sample and a Mi cromega PID temp erature controller, was used
for analysis. Temperature-depe ndent impedance measurements
were carried out using an IM6e impedance spectrometer with the
upper limit of 6 G O (Zahner Elektrik, Germany). For all m easure-
ments, an excitation frequency of 1 Hz and an amplitude of 20 mV
of the superimposed modulation voltage signal at an overall
DC potential of 0 V were applied to two circular Pt electrodes
(dia me ter 5 mm) in mech ani cal ly en forced co nt act wi th the
sa mple ( corr espo ndin g to a forc e of 2 N) . The la tter is a pel leti zed
po wder ma teri al (s ever al hund red mill igra ms of the powd er ar e
pr esse d by 2 t in to a di sk of 5 mm diame ter and B 0. 1 m m
th ickn ess) . The im peda nce of th e pell et is th eref ore eff ecti ve ly
me asur ed in an elec troc hemi call y unp olar ized st ate . In all
te mper ature -dep end ent ex peri ment s the im peda nce mo dulus
val ue | Z | will be fu rthe r refe rred to as ‘‘i mped anc e’’. To coll ect
th e freq uenc y-dep end ent da ta, th e fr equen cy wa s va ried be tween
100 mH z and 1 MH z at th e same a mplit ude of th e sup erim po se d
sinusoidal voltage signal also use d for the temperature-dependent
impedance m easurements. To fit the fre q uency-de pendent data
(Nyquist plots), a fit model consis ti ng of grain int erior (bulk) and
grain boundary contributions of resistances and con stant phase
elements has been exploited (discussed in deta il in Section 3.3).
To account for the intr insic properties of the porous sam ple,
constant ph ase elements instead of capaciti ve elements have been
used. The intertransition of the latter two is given by the formula:
C ¼ R 1  a  Q

1
a with Q constant phase element = O  1 s a
with a = 1, the phase element behaves like an ideal capacitor
and with a = 0, it represents an ideal resistor.
The activation energies ( E
A
’s) of the conductivities fo r selected
temperature regions were calculated from the resistance values
obtained from the fit of the Nyquist plots, as well as from the
temperature-dependent EIS measurements.
2.3. In situ FT-infrared spectroscopy (FT-IR)
All FT-IR spectra were recorded using a Cary 660 spectro-
meter (Agilent, Germany) and a home-built high-temperature,
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high-pressure in situ quartz reactor cell, capable of collecting
spectra at temperatures up to 1273 K and pressures up to 1 bar
in reactive gases.
24
Its dedicated construction allows the use of
ultra-dry conditions with a water partial pressure p o 1.0 
10
 7
mbar. For analysis, the powders are pressed into thin
pellets using a hydraulic press at B 2 t, yielding pellets with a
diameter of B 1.0 cm and a thickness of B 0.1 mm. Treatments
under flowing (exclusively performed in here) and static condi-
tions are equally possible. Both for FT-IR and EIS experiments,
the experiments have been repeatedly conducted using several
pellets to exclude influences of a different degree of pelletizing.
No changes have been observed by repeating the experiments
using different pellets.
2.4. In situ X-ray diffraction
The time-resolved in situ synchrotron X-ray powder diffraction
(XRPD) studies (angle-dispersive transmission mode, mono-
ch ro mati c 25 keV fo cuss ed beam , l =0 . 4 9 5 9Å .3 0 m ms p o ts i z e )
ar e co nduc ted a t the Ad vanc ed Ligh t Sou rce Bea mlin e 12.2. 2,
Lawrence Berkeley National Labs (California). The rh-In
2
O
3
samples in 700 m m quartz capillaries are heated to 700 K with
10 K min
 1
hea ting ra te in an IR ir radia ted tu be fu rna ce
25
unde r
co nt inuo us air, He and pu re H
2
fl ow inj ecte d by an open -e nded
300 m m capi llar y. A Perk in Elme r flat pa nel dete ctor (X RD 162 1,
dark image and strain correction) is used to record the XRPD
patterns every 25 seconds.
2.5. In situ thermogravimetry – mass spectrometry
Simultaneous Thermal Analysis (STA) is conducted in a STA 409
PC LUXX (Netzsch, Germany) device under Air, He and 5% H
2
in Ar using 100 mg sample powder placed in alumina crucibles.
The released gaseous species ( m / z = 2, 16, 17, 18, 44) are
simultanously analysed in an OMNi Star GSD 320 mass spectro-
meter (Pfeiffer Vacuum, Germany).
3. Results and discussion
3.1. Summary of ex situ determined phase stability and
reduction behavior
To s im pl ify th e co rr ela te d dis cus sio n be tw ee n su rfa ce c he mi stry
an d su rfa ce c ond uc tio n pr ope rt ies , we w ill pr ovi de a very brie f
ac co unt o f the pre vi ous f ind in gs o f th e red uc ti on be ha vio r an d
associated phase stability i n H
2
a nd CO. In b ot h re duc ing ag en ts,
a dra st ic impe d an ce dec rea s e, lea di ng to me ta lli c co ndu ct ivi ty
( B 10 O )a b o v e B 673 K, ha s b een ob se rved d uri ng hea ti ng of
rh -I n
2
O
3
. Be lo w 673 K , th is d ecre a se c an be a t lea s t in p art c orr e-
la te d w ith t he fo rm ati on of m od est a mo unt s of cu bi c In
2
O
3
.A t
tem p era tu re s at an d es pec ia lly a bov e 67 3 K, t he fo rma ti on of
di ffe rent a mo unts o f meta l lic In se ts in , c aus ing m eta ll ic con-
du ct ivi ty. A nne al ing to sim i lar tem p era tu re s i n i nert h el ium do es
no t tri gg er th is ph ase t ran sfo rm atio n .
12
Th e su ppr es sio n of tra ns -
formation o f large amounts o f rh-In
2
O
3
in to i ts th erm od yna mi-
cally stable cubic counte rpart, be ing a nece ssary pre -requisite f or
st ud yin g t he int ri nsic p hys ico -c hem ic al pro pe rti es of rh- In
2
O
3
,
c a nb ea tl e a s ti np a r tc o n t r o l l e db yt h ec h o i c eo ft h er e d u c i n g
ag ent , an nea lin g tem p era ture an d ti me. At te mper a tu re s b el ow
B 623 K, rh-In
2
O
3
is s ta bl e for d ay s, wi tho ut tr an sfo rm ing in to
c-In
2
O
3
or b ein g red u ced to me t all ic In . In c ont ra st, t rea tm en t a t
92 3 K le aves onl y 30 m in ute s fo r su cc ess ful ch ara cte ri za ti on
before the transfo rmation sets in ref. 8 and 12.
3.2. In situ determined phase stability
To understand the relationship between the structure and
surface chemistry of In
2
O
3
, the phase stability of rh-In
2
O
3
was
examined in He, CO
2
and H
2
atmospheres between room
temperature and 700 K by time-resolved in situ synchrotron
XRPD experiments. The contour plots in Fig. 1A–C (left panels)
show the in situ XRD patterns collected during the heating of
rh-In
2
O
3
in He, CO
2
and H
2
atmospheres, respectively. The
in situ XRPD data reveal that rh-In
2
O
3
remains stable upon
heating to 700 K in He and CO
2
, while it starts to transform into
c-In
2
O
3
at 640 K in the H
2
atmosphere. The intensities of
c-In
2
O
3
reflections increase with temperature suggesting the
development of the c-In
2
O
3
phase. The Rietveld refinement of
the sample ex situ heated up to 673 K in H
2
atmosphere indi-
cates the formation of 90 wt% c-In
2
O
3
and 10 wt% metallic In.
12
However, no additional reflections related to metallic indium
have been observed in the three atmospheres up to 700 K under
the chosen experimental conditions, in contrast to the ex situ
measurements.
To confirm the phase stability of rh-In
2
O
3
, as well as to
obtain more precise information about the eventual phase transi-
tion observed in XRPD experiments, simultaneous thermal analysis
(STA) was performed under He, CO
2
, and 5% H
2
/Ar (100% H
2
was technically not possible). Fig. 1 (right panels) shows the
TG, DTG, and DTA curves of rh-In
2
O
3
and the mass spectra
collected on-line for H
2
( m / z = 2), O
2
( m / z = 16), H
2
O( m / z =1 7
and 18) and CO
2
( m / z = 44). The sample exhibits a weight loss of
B 0.5% at 370 K that can be ascribed to the release of some H
2
O
( m / z = 17 and 18). The exothermic peak observed in the DTA
curve at 625 K, which only appears in the H
2
atmosphere, is
accompanied by a weight loss of B 0.5%, the release of H
2
O
(and, most probably, surface-bound CO
2
) and the consumption
of H
2
gas ( m / z = 2). The in situ XRPD data (Fig. 1C) display
the phase transition of rh-In
2
O
3
to c-In
2
O
3
in the same tem-
perature range. Hence, rh-In
2
O
3
becomes destabilised due to
the reaction with H
2
, which in turn causes the transformation
to c-In
2
O
3
. The release of H
2
O as well as the consumption of H
2
gas ( m / z = 2) during the phase transition suggests the reaction
of H
2
being due to the phase transition of rh-In
2
O
3
to c-In
2
O
3
.
This exothermic peak was accompanied by a weight loss of
B 0.5% that is due to the release of some H
2
O, and CO
2
gases,
as revealed by the MS analysis. The mechanism of the appear-
ance of CO
2
species will be studied in more detail in sub-
sequent work. In He and CO
2
atmospheres rh-In
2
O
3
does not
display any reaction with the gaseous species and remains
stable up to 700 K.
3.3. Surface chemistry in an inert gas atmosphere
To provide a reference measurement to those in reactive gas
atmospheres, Fig. 2A and B discuss the state and changes of the
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surface chemistry of rh-In
2
O
3
as determined by in situ electro-
chemical impedance and FT-IR spectroscopy. This will in turn
help separate direct and indirect (chemical) temperature-
induced effects. As such, Fig. 2A shows the impedance change
during a heating–cooling cycle from room temperature to 700 K
and back. An upper temperature limit (as determined by
previous experiments
8,12
) has been pre-set to avoid triggering
unwanted phase transformations to a large extent. As a general
feature, it is noted that heating rh-In
2
O
3
slightly above room
temperature causes a drastic decrease in the impedance by
mo re th an one orde r of ma gn itud e, be ing tent ativ ely ass igne d to
ef fect iv e de- hydr ox yl atio n of the sur face (s imil ar to c-In
2
O
3 26
).
The impedance re-rises between 300 and 330 K, before it
smoothly decreases up to 700 K, finally reaching an impedance
value of about 1.5  10
4
O . In the intermediate temperature
region, rh-In
2
O
3
shows semiconductive behavior, with a more
or less pronounced change in the impedance-temperature slope
at around 500 K (note that this is exactly the point between two
Fig. 1 In situ collected XRD patterns on rh-In
2
O
3
during heating in He (A), CO
2
(B) and H
2
(C) up to 700 K. Reference data are shown as ticks below the
patterns. The respective right panels show the associated in situ STA (TG, DTA and MS) profiles under comparable experimental conditions.
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temperature regions in the Arrhenius plot with different activa-
tion energies, see also Table S1, ESI † ). In fact, the slope in the
temperature region 330–500 K is much smaller compared to
between 500 K and 700 K, pointing to a more or less pro-
nounced change in the surface chemistr y in that temperature
regime. This will subsequently be directly proven by the analy-
sis of the FT-IR spectra shown in Fig. 2B and Fig. S1 (ESI † ). The
cooling behavior between 700 and 600 K very much resembles
the heating routine in this temperature region, with notable
differences between 600 K and room temperature, where the
surface does not get replenished with hydroxyl groups. Hence,
higher final impedance values at room temperature (above the
G O detection limit of the spectrometer) result. Fig. 2B at the
same time reveals the changes in surface chemistry as moni-
tored by in situ FT-IR spectroscopy. Following the results of a
recent work,
12
this figure shows the development of particularly
water-associated species. In addition, the changes of the total
transmittance, being an indication of conduction band electron
abs orpt ion,
26
de term in ed at a spec ific wa venu mber (2 000 cm
 1
),
wher e no ov erl ap with an y ads orbed spec ie s oc curs , are s hown .
As a general phenomenon, the removal of OH groups starts
continuously already at around 330 K up to the highest tem-
peratures and proceeds during the isothermal periods. During
cooling, the surface remains essentially in a de-hydroxylated
state, corroborating the impedance measurements. More
specifically, in the temperature region between 330 and 573 K
a continuous removal of specific OH groups at 3640 cm
 1
(sharp peak, surface OH groups) takes place, which to some
extent between 330 and 410 K proceeds in parallel with the easy
removal of bulk OH groups (broad negative feature, incorpo-
rated water). Between 410 and 673 K, removal of the latter is
more effective. The characteristic change of impedance vs.
temperature slope at B 500 K can be directly related to signi-
ficant changes in the total transmittance. The latter shows a
pronounced maximum exactly at 500 K, which indicates that
the electronic state of rh-In
2
O
3
changes significantly. Correlat-
ing this phenomenon directly with the impedance data, we may
assume that thermal removal of reactive lattice oxygen starts
with the associated formation of oxygen vacancies, which is
also confirmed by previous volumetric adsorption and oxygen
re-titration measurements.
26
rh-In
2
O
3
differs significantly from
c-In
2
O
3
, for which the bulk remains stable and unreactive until
B 770 K.
26
As will be shown below, these features are much
more pronounced if reducing agents like H
2
or CO are used.
The FT-IR data strongly suggest that this particular feature is
not related to the removal of surface OH groups, but rather to a
bulk-restricted phenomenon, as the maximum of absorbance
can be directly correlated with the effective loss of intercon-
nected OH groups. Selected FT-IR spectra during annealing in
He are shown in the ESI, † Fig. S1.
To obtain more information on the different surface- and
bulk-restricted conductivity contributions, we performed asso-
ciated frequency-dependent impedance measurements and the
corresponding Arrhenius activation analysis. Fig. 3 and 4 show
these experiments at selected temperatures between 423 K and
673 K. These temperatures were chosen as the first semicircles
appear at 423 K, and 673 K was deliberately set as the highest
temperature to suppress the eventual phase transformation.
From the data it is obvious that for He especially the grain
boundary contributions appear separated into two temperature
regimes (423–523 K and 473–623 K). These are also the tem-
peratures, where significant changes in the FT-IR experiment
were apparent. The inset in Fig. 3 highlights the fit model used
to evaluate the frequency-dependent data. Tables S2 and S3
(ESI † ) feature the equivalent circuit parameters used to model
the impedance spectra. They consist of two grain-related con-
tributions, i.e. grain interior and grain boundary, both of them
contributing resistance and constant phase element parts. The
general appearance of all data is the presence of at least two
depressed semicircles, being directly related to the aforementioned
individual contributions. In general, the first semicircle is always
fully present, whereas the second one is only partially represented
in the chosen frequency range. As for a typical semiconductor,
the resistances (especially the grain interior contribution)
Fig. 2 Panel A: in situ impedance spectra on rh -In
2
O
3
in flowing He (1 mL s
 1
).
Panel B: in situ FT-I R data of rh -In
2
O
3
in flow ing He ( 1 mL s
 1
). Heating–coo ling
cycles from room temperature (RT) to 7 00 K, as well as heating and cooling
ra tes (10 K mi n
 1
), ar e ide nti cal i n both ex peri ment s . h = he ati ng, c = co ol ing ,
i.p . = iso th erma l per iod.
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decrease as the temperature is increased. In line with that, we
assume that the conductivity vs. temperature behaviour of
rh-In
2
O
3
can be either addressed to semiconductive properties
(in He) or is (especially in reducing gases, as will be shown
below) in some way connected to the formation of oxygen
vacancies. An exception concerns the measurement at 673 K,
where at low frequencies an additional process is visible, over-
lapping with the grain boundary contribution. One might also
include the possibility of a beginning phase transformat ion
into the cubic polymorph. According to the Arrhenius analysis
(Fig. 4), the apparent activation energy of the grain interior
contribution is about 82 kJ mol
 1
(0.82 eV), which is compar-
able to YSZ ( B 1 eV) for bulk O
2 
activation, and is significantly
smaller than the activation energy (1.5 eV) attributed to the
electronic conductivity (above 973 K) associated with the forma-
ti on of ox ygen va canc ies in c-In
2
O
3
.
26
In esse nce, th is once ag ain
und erl ines th at oxy gen va canc ies mu st pla y a role in ex pla inin g
th e condu ctiv ity of rh -In
2
O
3
. The act iva ti on ener gy valu es of the
gra in bo undar y part , how ever , are sp lit in to two pr oces se s: a
hi gh-t empe ratu re and low -tem pera ture pr oces s. At high er te m-
pe ratu re s (523 –62 3 K) an act ivati on ener gy of abo ut 90 kJ mol
 1
(0.9 4 eV) and a t lowe r temp erat ure s (423 –52 3 K) an activ ati on
en ergy of 16 kJ mol
 1
(0.1 6 eV) are det ermi ned . Th is indi cate s
th at th e latt er is es sent ially a sur face -res tric ted ph enom enon ,
domi na ting at lo wer te mper atu res, bu t becom ing in crea sing ly
ov erwhel med by th e oxyg en vaca ncy- domi nate d cond ucti vity
co nt ribu tion , once the te mper atur es start s to exce ed 523 K. Note
th at the d ark gr een sq uare in Fig. 4 wa s purp osel y not ch osen fo r
th e Arrh eniu s fit si nce it is pa rt of th e lo w-te mper atur e proc ess
an d wou ld, he nce, fa lsif y th e dete rmin atio n of the a ctiv ati on
en ergy fo r the high -t empera tu re pr oc ess .
3.4. Surface and bulk reduction in H
2
In contrast to the experiments in an inert atmosphere, a direct
influence of the reducing agent on the surface chemistry and
(surface) conduction properties can be expected. The associated
results are highlighted in Fig. 5 and Fig. S2 (ESI † ). Note that
to provide insight into the kinetics of reduction, different
maximum temperatures as well as repeated experiments to
identical maximum temperatures have been conducted. For
the sake of clarity, in Fig. 5A the measurement in He is
Fig. 3 Nyquist plots (data poin ts) and simulated spectra (continuous lines) of rh-In
2
O
3
treated in flowing He (1 mL s
 1
) at various temperatures between
423 K and 523 K (A) and between 473 K and 673 K (B). The lowest frequency of 100 mHz is at the right side and the highest one of 1 MHz is at the left side
of the x -axis. The equivalent circuit model used for fitting the frequency -dependent impedance data of the sampl es with contributions from the grain
interiors (gi) and grain boundaries (gb) is shown as the inset.
Fig. 4 Arrhenius analysis of the grain interior and grain boundary con-
tribution of a rh-In
2
O
3
powder pellet sample treated in dry He and CO
2
.
Based on the data of Fig. 3A (blue traces), Fig. 3B (green traces) and 11
(purple traces). gi contr. = grain interior contribution, gb contr. = grain
boundary contribution.
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also included. As a general remarkable feature, which is
observed in almost all experiments displayed in Fig. 5A, the
impedance vs. temperature course exhibits two more or less
pronounced plateau-like features, where the impedance is
almost independent of the annealing temperature. One of these
plateaus is observed at lower temperatures (between 310 and
430 K), and the other one between 480 and 550 K. While the
latter is present in all experiments, the former is missing for
the measurement up to 673 K. However, a slower decrease in
the impedance is visible. The reason for this is not entirely clear
yet, but given the tremendous influence of the kinetics on the
extent of reduction, slight variations in e.g. the heating routine
might limit the access to this specific surface reduction state.
At temperatures below 310 K ( i.e. below the first plateau) and
between 430 and 500 K ( i.e. between the two plateaus) semi-
conductive behavior is apparent ( cf. Table S1, ESI † for E
A
’s in
these regions for different measurements). A second general
feature is the final constant impedance value above B 570 K
( B 15 O ), which is invariant upon re-cooling. The kinetics of the
reduction process is also directly apparent if the two experi-
ments up to a maximum temperature of 523 K (Fig. S2, ESI † )
are compared: while the final impedance value reached upon
entering the second plateau ( B 40 O ) and the onset temperature
of the plateau ( B 310 K) appear unaltered, the extent of the
plateau in both cases differs by 20 K. Most importantly, these
differences can be directly correlated with associated changes
in the reduction degree/the amount of states in the conduction
band (determined by the transmittance change at 2000 cm
 1
),
but less so with direct changes of the surface chemistry. As
shown in Fig. 5B, the OH groups (most likely bulk-related) get
only lost rapidly between 593 and 633 K, i.e. at temperatures
where the impedance already reached its final metallic value of
15 O . In contrast, the relative total intensity at 2000 cm
 1
, being
a direct indicator of the extent of reduction, exactly follows the
trends of the impedance vs. temperature course and, also irres-
pective of the final maximum temperature, exactly copies the
plateau-like temperature-invariant impedance features. Also
the FT-IR experiments reveal that the extent of reduction is
not reversed substantially upon re-cooling, as the relative total
intensity hardly changes. One can now directly deduce that all
the particular impedance features up to about 600 K without
exception arise from a different extent of reduction. This also
fits perfectly to previously reported temperature-programmed
reduction and -desorption results, which show an increased
formation of oxygen vacancies in this temperature region (up to
600 K). More specifically, by correlating the course of the impe-
dance with the volumetric H
2
uptake,
12
it is clear that the first
impedance plateau must be associated with predominant sur-
face and the second one with bulk reduction. This follows from
a quantification of the oxygen vacancies by oxygen re-titration
measurements ( cf. Table 2, ref. 12), which shows only modest
amounts after reduction at B 414 K, but a more or less drastic
increase particularly at temperatures T 4 550 K, i.e. especially
in the high-temperature part of the second plateau. Selected
FT-IR spectra during annealing in H
2
are shown in the ESI, †
Fig. S3. Rietveld analysis of ex situ collected X-ray diffraction
data at selected temperatures during the impedance measure-
ments indicated the formation of not more than 10–15 wt%
c-In
2
O
3
at temperatures of about 473 K, i.e. just before entering
the second plateau. This ratio between rh-In
2
O
3
and c-In
2
O
3
does not change upon heating to B 570 K, i.e. after the second
plateau and when the impedance stays constant even after
heating to higher temperatures. Hence, we conclude that the
impedance course is not entirely affected by the phase trans-
formation, but reflects the true intrinsic reduction – conduc-
tion properties of rh-In
2
O
3
.
Frequency-dependent measurements in H
2
also reveal the
influence of the reduction agent (Fig. 6). While the experiment
at RT exhibits the well-known behavior of grain interior and
grain boundary contributions, the Nyquist plots at higher tem-
peratures are clearly different from those in He (fit parameters
summarized in Table S4, E SI † ). This special fre quency-de pendent
impedance course indicates that the sample changes during
the measurement (which is on the time scale of B 5 minutes for
a full measurement cycle). Given the previous experiments on
Fig. 5 Panel A: in situ electrochemical impedance measurem ent on
rh-In
2
O
3
in flowing H
2
(1 mL s
 1
). Panel B: in situ FT-IR data o f rh-In
2
O
3
in flowing H
2
(1 mL s
 1
). Heating–cooling cycles from room temperature
(RT) to 700 K, as well as heating and cooling rates (10 K min
 1
), are identical
in both experiments. h = heating, c = cooling, i.p. = isothermal period.
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the dependence of the phase transformation on the reduction
in the same temperature region, the peculiar impedance course
between 323 K and 373 K is related to the phase transformation
between rh- and c-In
2
O
3
and maybe to some extent also metallic
In, as deduced from correlation with previous data. At 523 K,
the imaginary part of the impedance is basically zero and a
purely ohmic resistance results (for a detailed representation of
this NP, see Fig. S4, ESI † ). According to previous experiments,
this is right on the edge of te mperatures, where metallic
In appears in the product mixture
12,27
–h e n c e , t h eo h m i c
resistance essentially arise sf r o mt h em o d e s ta m o u n t so f
me ta lli c In .
3.5. Surface and bulk reduction in CO
To directly connect to the experiments in flowing H
2
and to
previous experiments on the reducibility in CO,
12
we aim in the
following at linking the observed impedance behavior to the
associated changes in surface chemistry. This is anticipated to
be much more complex than in He or H
2
due to the potential
formation of surface formates and, via formed and re-adsorbed
CO
2
, also carbonate species. From previous experiments it is
already known that CO at a similar temperature to H
2
causes
drastic reduction of rh-In
2
O
3
, leading to CO
2
formation by the
reaction of CO with lattice oxygen.
12
Its effect on surface chemistry
is, however, t o date unknown.
Fig. 7 now provides the direct link between surface and bulk
conduction properties upon annealing in flowing CO (Fig. 7A)
and the corresponding FT-IR fingerprint of surface-adsorbed
species (Fig. 7B). In Panel A, the inert He measurement is also
included. The impedance vs. temperature course in CO shows a
very steep impedance decrease between RT and approximately
410 K (depending on the measurement) with a corresponding
activation energy of 136 kJ mol
 1
( cf. Table S1, ESI † ). After this
steep decrease a plateau, where the impedance value changes
only slightly, is apparent exhibiting a value of B 60 O (light
green trace in Fig. 7A) and B 20 O (light purple trace in Fig. 7A),
after which the impedance decr eases again to adopt a metallic-
like value ( B 12 O ) at temperatures above B 550 K ( E
A
:7 3k Jm o l
 1
between 512 and 559 K). The impedance hysteresis between
he atin g and co olin g is eq uall y pron ounc ed in CO . Kine tic lim ita-
tions between two successive experiments (light green/dark
green and light purple/dark purple t r aces) still prevail and manifest
themselves as differently extended temperature-independent
impedance plateaus. Notable differences from the H
2
measure-
ment concern (i) the missing low-temperature plateaus and
(ii) the generally earlier decrease of the impedance (starting
already at room temperature) and rapidly accelerating up to
B 410 K. Fig. 7B directly reveals the complex interplay between
co nd ucti o n pro pert i es, r ed ucti o n and s urf ac e ch em is try . Be tw ee n
300 K and 500 K, the relativ e total absorbance increases drasti-
cally, with at the same time complete absence of any changes in
surface-adsorbed species. This indicates that during the impe-
dance decrease between 300 K and 410 K, but also within the
plateau region, the simple reduction of rh-In
2
O
3
by CO is the
predominant reaction pathway, but without the pronounced
Fig. 6 Nyquist plots (data points) and simulated spectra (continuous
lines) of rh-In
2
O
3
treated in dry H
2
at selected temperatures betw een RT
and 523 K.
Fig. 7 Panel A: in situ electrochem ical impedance measur ement on
rh-In
2
O
3
in flowing CO (1 mL s
 1
). Panel B: in situ FT-IR data of rh-In
2
O
3
in flowing CO (1 mL s
 1
). Heating–cooling cycles from room tempera ture
to different maximum temperatures, as well as applied heating and cooling
rates (10 K min
 1
), are identical in all expe riments. h = heating, c = cool ing,
i.p. = isotherma l period.
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formation of CO
2
by the reaction of CO with lattice oxygen.
Surface chemistry only starts to play a role at the end of the
plateau region, where the impedance suffers a final drop towards
metallic conductivity behavior. In due course, the relative total
absorbance significantly dec reases between 500 K and 600 K and
formate signals (as determined from the asymmetric stretching
vibra tion of sur face -bound for mate s pecie s ( cf. Fig. S5, ESI † )) start
to increase in the same temperature region up to 610 K. The surface
of rh-In
2
O
3
suffers a reversible poisoning by CO until adsorbed CO
predominantly forms formate spec ies, which reverses the reduction
to some extent. Formate decomposition is then observed in the
temperature region between 633 K and 673 K, which goes along
with further massive reduction o f surface and probably also bulk
regions, finally also yielding gas-phase C O
2
.A l t h o u g hi ta p p e a r s
that CO
2
is mostly formed by the direct reaction of CO and lattice
oxygen (resembling a Mars–van-Krevelen-type reaction mecha-
nism), the decomposition of bulk-like OH groups at and above
650 K (proceeding in parallel with formate decomposition)
suggests that in part CO
2
is also formed via the reaction of
the latter two. Deep reduction of rh-In
2
O
3
beyond 650–670 K is
necessarily accompanied by increased formation of In metal.
12
Upon cooling, the state o f the surface remains basically unchanged,
with the notable exception of CO
2
production upon cooling up to
600 K. As for the a lready discussed experiments in H
2
,a ni n f l u e n c e
of the phase transformation into c-In
2
O
3
on the surface conduction
and chemistry properties cannot be ruled out, but appears unlikely,
as the ratio between rh-In
2
O
3
and c-In
2
O
3
in the relevant tem-
perature region (before and during the plateau region) remains
unchanged ( B 90 weight% rh-In
2
O
3
, 10 weight% c-In
2
O
3
,n o
In metal
12
).
The frequency-dependent experiments in Fig. 8 at low
temperatures (especially at room temperature) resemble that
of H
2
at room temperature in terms of the general appearance
of the two depressed semicircles (fit parameters Table S5, ESI † ).
Also at 323 K semicircles are present, albeit with a decreased grain
boundary resistance in comparis o n to room tempe rature. Af ter
heating to 473 K, exclusive ohmic resistance is observed without
the imaginary cont ribution, indicating the formation of metallic
In. This temperature- wise matches n ot only the observ ations in
H
2
(purely ohmic resistance starting at 523 K), but also previous
temperature-de pendent conductivity measurements in CO.
12
3.6. Surface chemistry in CO
2
The reason for including adsorption studies of CO
2
is mainly
fueled by its catalytic relevance in terms of explaining the
high CO
2
selectivity of its cubic counterpart in methanol steam
reforming
13
and by the fact that it necessarily is a part of the
reaction mixture if rh-In
2
O
3
is reduced in CO, at least a t elevated
temperatures. Fig. 9A (EIS) and 9B (FT-IR) reveal that CO
2
only has
Fig. 8 Nyquist plots (data points) and simulated spectra (continuous lines)
of rh-In
2
O
3
treated in dry CO at various temperatures between RT and
473 K. The lowest frequency of 100 mHz is at the right side and the highest
one of 1 MHz is at the left side of the x -axis.
Fig. 9 Panel A: in situ electrochem ical impedance measurement on
rh-In
2
O
3
in flowing CO
2
(1 mL s
 1
). Panel B: in situ FT-IR s pectra of rh-In
2
O
3
in flowing CO
2
(1 mL s
 1
). Heating–cooling cycles from room tempera ture
to different maximum temperatures, as well as applied heating and cooling
rates (10 K min
 1
), are identical in both experiments. h = heating,
c = cooling, i.p. = isothermal period.
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a comparably small effect on the properties of In
2
O
3
. The impe-
dance vs. temperature course in certain temperature regions
resembles that in He. However, differences in the activation
energies are apparent for He and CO
2
( cf. Table S1, ESI † ).
Expectedly, no metallic con ductivity is reached even at the highest
temperatures. This blocking effect is also directly reflected in
the FT-IR data (Fig. 9B). Most likely bridged carbonate species
are predominantly formed upon CO
2
adsorption, but no trans-
formation or even decomposition during the heating–cooling
cycle has been observed. This indicates that the surface is
essentially blocked for further adsorption. Quantitative analysis
of the OH region is severely hampered in the present case due
to overlap of the CO
2
overtones, but irreversible removal of bulk
OH groups upon heating is observed and directly proven by the
vacuum spectra (black trace Fig. 9B) and by the lower impe-
dance values between RT and 310 K (light green trace Fig. 9A).
Fig. S6 further elaborates on the development of the discussed
carbonate species and proves by plotting the relative absor-
bance vs. the annealing temperature, that reduction of the
surface is essentially absent.
The general appearance of the Nyquist plots in flowing CO
2
at different temperatures is very similar to the ones obtained in
He and H
2
and in CO at lower temperatures pointing out at
grain interior and boundary contributions (fit parameters see
Table S6, ESI † ). At the highest temperatures (673 K), as in He,
the potential sample–electrode interaction is visible as a third
process (linear increase at the low frequency end, Fig. 10).
As also the apparent activation energy of the grain interior
contribution at B 0.86 eV (473–673 K) is very similar to the one
obtained for He (0.85 eV between 423–523 K and 0.83 eV
between 423–623 K) as shown in Fig. 4.
He nc e, on e mig ht co nc lud e th at CO
2
in fact block s th e surface
of the sam ple and the interaction wit h the rh-In
2
O
3
su rfa ce is
es se nti all y sup pre ss ed . T he ap pare n t a ctiv at io n ene rg y de ri ved
fro m the g rai n bo un dary re si st anc es (0 .3 e V), ho we ver, i s di ffe ren t
than in He (0.17 eV between 423–523 K and 0.94 eV between
423–623 K), indicating that the contribution of the bulk ion
conduction is higher. Table S7 (ESI † ) summarizes the calcu-
lated activation energies determined in all gases.
3.7. Comparative summary about the influence of different
gas atmospheres on surface and bulk chemistry of rh-In
2
O
3
Fig. 11 summarizes the change of the total absorbance at
2000 cm
 1
– which is a good measure for the concentration
of charge carriers – for all gas treatments in a comparative view.
In He, H
2
and CO, the absorbance increase is comparable, but
more pronounced in the latter two gases. Above 500 K, the
surface chemistry, but also the phase transformation, has a
significant influence on the absorbance. In CO
2
, the changes
are not as pronounced, since the surface is effectively blocked
by carbonate species. The observed maximum in the relative
absorbance at around 500 K in hydrogen and CO (coinciding
with the end of the plateau regions in the impedance graphs),
Fig. 10 Nyquist plots (data poin ts) and simulated spectra (continuous
lines) of rh-In
2
O
3
treated in dry CO
2
at selected temperatures between
473 and 673 K. The lowest frequency of 100 mHz is at the right side and
the highest one of 1 MHz is at the left side of the x -axis.
Fig. 11 Panel A: temperature-dependent impedance traces compara-
tively shown for all studied gases. Panel B: compa rison of the change of
the total absorbance at 2000 cm
 1
for all gas treatments.
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th us, repr esen ts the in trin sic co ndu ction pr oper ties of rh -In
2
O
3
,
whic h migh t be dire ctl y ass ocia te d with th e exte nt of ox ygen
vaca nc ies form ed by redu ction . The effe ct of CO can be und er-
stoo d in term s of re duc tion an d form atio n of oxyg en vaca ncie s.
Ho weve r, the in crea se in th e cond ucti vity of rh- In
2
O
3
in H
2
at
523 an d 673 K co uld no t be dire ct ly co rrel ated wi th the in crea se
in th e to tal abs orba nce ass oc iate d wi th th e cond ucti on ba nd
elec tr ons. Th is indi cate s that mo re comp lex pr oces ses ta ke pla ce
on th e surf ace/ bul k of rh-I n
2
O
3
in hy drog en (se e als o the ST A
an alysis in Fi g. 1).
Directly connected to the vacancy-mediated mechanism is
the answer to the question how the reduction of rh-In
2
O
3
takes
place, i.e. if rh-In
2
O
3
is transformed to c-In
2
O
3
before metallic
In is formed, or if a direct reduction to metallic In takes
place. In view of the presented results, a substantial contribu-
tion of the latter appears unlikely and the pathway to a large
extent most likely is: rh-In
2
O
3
- c-In
2
O
3
- In metal. The latter
would, however, be addressed in more detail in subsequent
work.
3.8. Comparative discussion of the surface chemistry of
rh-In
2
O
3
and c-In
2
O
3
The differences in the surface chemistry between the two most
abundant In
2
O
3
polymorphs, rh-In
2
O
3
and c-In
2
O
3
, are best
visualized if discussed in the context of the catalytic applica-
tions, as detailed earlier.
12,13
Two different pathways of argu-
mentation are worthwhile in this respect. On the one hand, the
FT-IR studies now directly prove the gas–surface interaction of
the relevant probe molecules, on the other hand the combi-
nation with the impedance data allows some conclusions on
the mechanism of the methanol steam reforming reaction on
both modifications. Previously only anticipated and indirectly
deduced from the catalytic profile s, we are now able to give definite
answers to the questions: (1) h ow does the surface chemistry
influence th e catalytic propertie s and (2) does the reaction me cha-
nism of methanol steam reform ing take different reaction routes
on c-In
2
O
3
and rh-In
2
O
3
.
To introduce question (1), c-In
2
O
3
is an outstandingly
CO
2
-selective methanol steam reforming catalyst,
13
which can
be argued directly on the basis of the acid–base properties of
the catalytically relevant oxide surface sites. On the basis of a
concept by Tatibouet to account for the selectivity pattern in
methanol synthesis,
28
also the adsorption and transformation
of the intermediates in the reverse reaction, that is, methanol
steam reforming, can be directly rationalized. Following disso-
ciative adsorption of methanol, the further fate of the adsorbed
methoxy group strongly depends on the acidic strength of the
adsorption site of the methoxy group and on the chemical
nature of the sites close by. A further reaction of the methoxy
grou p requ ires C– H bond br eaki ng, wh ich, in th e case of he tero -
ly tic bo nd sc issio n, st rong ly dep end s on th e bas ic ( i.e. nucl eo-
philic) character of the oxygen species. Strong basic sites will
help in effectively abstracting the H atoms from the adsorbed
methoxy group, thus, favoring the fast onward reaction of the
adsorbed formaldehyde intermediate, which is formed by
increasing H abstractio n from the methoxy group. Weaker basic
sites favor the pro longed life time of th e adsorbed formaldehyd e,
eventually giving rise to its desorption. In contrast, if the acidic
sites are strong, the life time of the adsorbe d methoxy group is
long enough to eventually form condensed products like dimethyl-
ether, dioxomethylene or methylal species. Re-addressing the
surface chemistry of the In
2
O
3
polymorphs under question, the
extraordinarily high CO
2
selectivity of c-In
2
O
3
directly proves
the presence of strong basic, but rather weak acidic sites.
Therefore, only CO
2
, but no formaldehyde or other condensed
reaction products have been observed. In contrast, the inter-
mediate formation of formaldehyde in methanol steam reform-
ing on rh-In
2
O
3 12
strongly suggests that neither the basic nor
the acidic character of the relevant surface sites is too pro-
nounced. The CO
2
selectivity is accordingly low, but in-parallel
also no condensed products are observed.
As for the answer to question (2), we note that the steam
reforming reaction can in principle take two different pathways,
depending on the partial pressure and temperature: a more
oxygen vacancy-dominated route and a more surface-restricted
formate-related pathway.
13
The former is very much pro-
nounced on c-In
2
O
3
, due to the easy removal of reactive lattice
oxygen and at the same time, the associated formate-related
mechanism is essentially suppressed. In contrast – although
the reduction behavior on rh-In
2
O
3
is comparably pronounced
(especially in CO) – the formate mechanism does obviously
contribute significantly more on rh-In
2
O
3
. This follows from the
correlated impedance and FT-IR data upon treatment in CO. A
vi tal chem istr y of form ates , espe cial ly in the cat alyt ica lly rel evant
te mper ature re gime of 500– 650 K, is evid ent. Su peri mp osed on
th is form ate chem istr y is th e si gnif icant c ontr ibut ion of the
vaca nc y reac tivi ty, as re fl ec ted in th e chan ge of th e re lati ve tota l
tra nsmi ttan ce.
4. Conclusions
 Particularly important for sensing mechanisms involving
redox cycles, by performing reduction experiments in hydrogen
and carbon monoxide under identical experimental conditions,
it was possible to relate temperature-dependent conductivity
features to pronounced changes in surface chemistry, allowing
for disentangling purely reduction-related features from those
directly arising from surface-chemical alterations. For highly
reducible oxides such as rh- or c-In
2
O
3
, for which not only the
formation of oxygen vacancies, but deep reduction to a metallic
valence state In
0
also has to be take n into account , this approac h
is imperative.
 In the reference inert gas atmosphere, the analysis also
included monitoring differently reactive hydroxyl groups (note
that the formation of reactant water on particle coarsening or
morphology needs to be separately assessed since the latter is
continuously removed by performing flowing experiments in
the present case).
 Important for catalysis, as deduced from carbon monoxide
and carbon dioxid e adsorption, rh-In
2
O
3
exhibi ts predominan tly
Lewis acidic surface sites. The bas i cc h a r a c t e ri sl e s sp r o n o u n c e d ,
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directly explaining the previously observed high (inverse)
water–gas shift activity and the low CO
2
selectivity in methanol
steam reforming.
 Necessary for the judgment of the influence of the phase
transformation on eventual sensing properties in reductive
atmospheres, reduction of rh-In
2
O
3
proceeds to a large extent
indirectly via rh-In
2
O
3
- c-In
2
O
3
- In metal and in both CO
and H
2
is essentially dominated by the reactivity of the formed
oxygen vacancies.
It is worth noting that only via the excellent correlation
between in situ – determined electrochemical information and
surface chemistry by combining spectroscopic characterization
methods performed under identical experimental conditions
ar e suc h conc lus ions po ssib le. In du e cou rse, it all ows no t only
to ga in acce ss to the inh eren tly hard -to -obt ain ph ysic o-ch emic al
pr oper ti es of meta sta ble ox ide mod ific ati ons, s uch as th e truly
technologically or catalytically important surface reactivity. By
performing highly correlated measurements, this information
can be related to the surface and bulk conduction properties
and to build a bridge to material’s-oriented properties, such as
the phase stability. The comparison of those features using the
In–O, and specifically, the In
2
O
3
polymorphic system appears
particularly worthwhile given the outstanding technological
and catalytic fingerprints of the cubic and rhombohedral
modifications. Given the extraordinary coincidence of in situ
obtained electrochemical impedance and FT-IR spectra in the
present case, we anticipate the extension of these experiments
not only to the remaining polymorphs in the In
2
O
3
system –
provided synthesis routines to access substantial amounts of
the material exist – but envision also the establishment of state-
of-the-art correlated in situ characterization for oxide or related
materials research.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
We thank the FWF (Austrian Science Foundation) for financial
support under the SFB project FOXSI F45-03. This work was
performed within the framework of the platform ‘‘Materials
and Nanoscience’’ at the University of Innsbruck. We thank
the Advanced Light Source (which is supported by the
Director, Office of Science, Office of Basic Energy Sciences, of
the U.S. Department of Energy under Contract No. DE-AC02-
05CH11231) where in situ PXRD measurements were conducted
at Beamline 12.2.2 within the framework of the AP program
ALS-08865. LS appreciates the ALS for supporting his work with
a doctoral fellowship. L. S. and A. G. also acknowledge the DFG
support (grant GU 992/12-1) within the framework of the
priority programme 1415 ‘‘Crystalline non-equilibrium phases’’
(‘‘Kristalline Nichtgle ichgewich tsphasen – Pra
¨ paration, Cha rakter-
isierung und in situ Untersuchung der Bildungsmechanismen’’).
This work was also a part of the Cluster of Excellence
‘‘Unifying Concepts in Catalysis’’ coordinated by the Tech-
nische Universita
¨ t Berlin and supported by the Deutsche
Forschungsgemeinschaft.
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