Structural and Surface Study of Praseodymium-Doped
SnO2Nanoparticles Prepared by the Polymeric Precursor MethodFermin
H.Arago n,*,Ismael Gonzalez,Jose .A.H.Coaquira,Pilar Hidalgo,Hermi
F.Brito,Jose .D.Ardisson,Waldemar A.A.Macedo,and Paulo
C.Morais,Laborato rio de Fsica Aplicada,Centro de Desenvolvimento
da Tecnologia Nuclear,Belo Horizonte MG 31270-901,BrazilNu cleo de
Fsica Aplicada,Instituto de Fsica,Universidade de Braslia,Braslia
DF 70910-900,BrazilFaculdade Gama-FGA,Setor Central
Gama,Universidade de Braslia,Braslia DF 72405-610,BrazilInstituto
de Qumica,Universidade de Sa o Paulo,Sa o Paulo SP
05508-000,BrazilSchool of Automation,Huazhong University of Science
and Technology,Wuhan 430074,ChinaABSTRACT: In the present study, we
report on the successful synthesisof Pr-doped SnO2 (SnO2:Pr)
nanoparticles using a polymeric precursormethod while setting the
Pr-content in the range from 0 to 10.0 mol %.The as-prepared
samples were characterized in regard to their
structural,morphological andsurfaceproperties.
X-raydiraction(XRD)patternsrecordedfromall
samplesrevealedthetetragonal rutile-typestructurewith a systematic
average size reduction (in the range from 11 to 4 nm)while
enhancing the residual strain (in the range of 0.186 to 0.480%)
asthe Pr-content was increased. From the Rietveld renement analysis
wefound that the lattice parameters (a, c, u, and V) showed a
linearbehavior, indicating a solid solution regimen for the
Pr-doping.Transmissionelectronmicrographsprovidedmeanparticlesizesof
8.7 0.5 nm, for 2.5 mol % Pr-content, and 5.2 0.5 nm,for 10.0 mol %
Pr-content, which are in very good agreement with values obtained
from the XRD data analysis: 7.4 1.0 nm and 4.0 1.0 nm,respectively.
FromX-rayphotoelectronspectroscopy(XPS)measurements[O]/[Sn]=1.44ratiohasbeenestimatedatthesurface
of the undoped SnO2 nanoparticles, which is below the expected
value for bulk compound ([O]/[Sn] = 2), suggestingthat the system
is strongly nonstoichiometric at the nanoparticle surface.
Actually, we found the [O]/[Sn] ratio value
increasingmonotonicallyasthePr-content wasincreased,
whichwasinterpretedasduetotheeliminationof
thesurfacechemisorbedoxygen and/or oxygen-related vacancies.
Moreover, a redshift of the Sn(3d) XPS peaks has been determined as
the Pr-contentwas increased, evidencing the change of the oxidation
state of tin ions from Sn4+to Sn2+. Our analyzes of the Pr(3d) XPS
peaksindicatedthepreferenceof thePr-ions for thePr3+oxidationstate,
althoughsmall amountsof thePr4+-ions couldnot becompletely ruled
out,particularly for the lower Pr-doping samples.1. INTRODUCTIONIn
recent years, the growing interest in nanomaterials based onmetal
oxide (SnO2, CeO2, Al2O3, andWO3among others)nanoparticles,
nanotubes, nanospheres, nanodisks, nanolamel-lae, and nanobers1,2is
mainly due to the diversity ofapplications in dierent areas.3More
specically,
thesenanomaterialsareattractivecandidatesforgassensorsduetotheir
high surface area. In this regard, tin dioxide (SnO2)compound is
one of the n-type semiconductors commonly usedanditsnatural
nonstoichiometry, lowcostandhighchemicalstability are features to
be emphasized.4Moreover,the surfaceof theSnO2 nanoparticlesexhibits
goodabsorptionpropertiesandreactivity due tothe presence of free
electrons intheconduction band plus the presence of surface and
bulk oxygenvacancies and active chemisorbed oxygen.5A main drawback
of the SnO2-based chemical sensors is thelow chemical selectivity,
which does not allow one to separatethe contribution made by a
particular type of molecule withinthe gas phase to the total
electric signal. In order to improve theselectivity for specic
gases, oxides nanoparticles are dopedwith dierent metal
ions.6,7However, the doping approach induces strong eects on
thestructural,8electronic,9catalytic, and
magnetic7,10,11properties.Regarding the magnetic properties many
oxides such as CeO2,ZnO, SnO2, etc.1215exhibit room temperature
ferromagnetism(RTFM) when doped with a small amount of metals. The
mostused ions to produce the diluted magnetic oxides (DMO)
arethetransitionmetals(TM), meanwhilefewreportsarefoundregarding
rare earth (RE) ions. The introduction of the RE ionscanprovide
desirable optoelectronic properties for the hostmatrix, which can
be used for the development of new devicessuch as lasers,LEDs,and
optical ampliers.Received: January 24,2015Revised: March
28,2015Published: March 30,2015Articlepubs.acs.org/JPCC 2015
American Chemical Society 8711 DOI: 10.1021/acs.jpcc.5b00761J.
Phys. Chem. C 2015,119, 87118717Ontheotherhand,
divalentearthrareionssuchasCe3+,4+,Pr3+,4+, and Tb3+,4+are
interesting fromthe standpoint ofDMO, due to the possibility of
doping with mixed valence stateof RE ions, which can favor a
charge-transfer mechanism.Therefore, thesystematicstudyof
thestructural andsurfaceproperties of those systems can contribute
to the betterunderstandingof theDMO. Inthis respect, Paunovicet
al.7showthatthesuppressionoftheferromagnetismobservedinPr-doped
CeO2 nanocrystals can be well explained in terms ofdierent dopant
valence state and the presence of Pr3+-ions onthesurface.
Ontheotherhand,
theentranceofamultivalentionsasPr(3+,4+)substitutingSn4+ionsassolidsolutioninthePr-dopedSnO2systemcangenerateoxygenvacanciesduetothe
charge imbalance, providing free electrons to theconductionband,
whichcanbe strongly dependent onthedopant content, however if this
ions will located on the surfacelayer there may be a annihilation
of defects as oxygen vacancies.As exposed above, in the present
work we reports thesuccessful Pr-doping of SnO2 nanoparticles with
Pr-content upto 10.0 mol % and synthesized via a polymer precursor
method.The structural, morphological and surface properties of the
as-produced undoped SnO2and Pr-doped SnO2nanoparticleswere assessed
by X-ray diraction (XRD) measurements,transmission electron
microscopy (TEM), high-resolutionTEM (HRTEM), Energy-dispersive
X-ray spectroscopy(EDS), andX-rayphotoelectronspectroscopy(XPS),
respec-tively.2. EXPERIMENTAL DETAILSUndoped and the Pr-doped SnO2
nanoparticles were producedfollowingPechinis
method,16inwhichtheundopedsamplewas obtained from the SnCl22H2O
precursor whereas the Pr-doped samples used Pr(NO3)36H2O as the
source for the Pr-ions. Following the mentionedprotocol the SnO2:Pr
nano-particles with Pr-content in the range from 0.0 to 10.0 mol
%havebeenobtained. Details of
thesamplepreparationweredescribedelsewhereintheliterature.6Thecrystallinequality,structural
parameters, crystallitesize, andresidual strainwereobtained from
the XRD technique using a commercialdiractometer(Bruker,
D8Advanced)equippedwithCuKradiation.
TheparticlesizedistributionwasassessedfromtheTEMmicrographs by
counting 1368 (for 10.0 mol %Pr-doped) and 974 (for 2.5 mol %
Pr-doped) particles and usingthe Sturges approach.17In order to
conrmthe nominal doping concentrationelemental
analysiswascarriedoutusingtheEDSoptionof
ascanningelectronmicroscope(FESEM, model SIGMAVP).The XPS analysis
was performed on a SPECS surface analysissystem equippedwith
thePhoibos150 electronanalyzer.
ThemonochromatizedAluminumradiation(1486.6eV)withtheoutput power
set at 400W was used for the XPS analyses of allsamples.
TheC(1s)signal (284.6eV)was employedas thereferencefor
thecalibrationof thebindingenergies(BE)ofdierent elements in order
to correct for the charge eect. TheCasaXPS software was used to
analyze all XPS data. The
surfaceatomicconcentrationinthesampleswasestimatedusingtheinstrument
sensitivity factors to scale for the calculatedphotoelectron peak
areas.3. RESULTS AND DISCUSSIONIn order to determine experimentally
the praseodymiumcontent in the SnO2:Pr nanoparticles EDS
measurementswere carried out. Figure 1a shows the EDS spectrum of
the 10mol % Pr-doped sample. The peaks located at around 0.9,
5.0,5.5, and 5.9 keV are assigned to the characteristic
X-rayemissions of Pr-ions. In order to quantify the Pr-content
severalmeasurements
havebeenperformedbyconsideringdierentregionsofthesamples.
Withintheexperimental uncertainties,results
showninTable1conrmthenominal values of Pr-content in the SnO2:Pr
samples. The XRDdata analysisindicatedthe formationof pure
tetragonal rutile-type SnO2phase(JCPDScard, 41-1445; spacegroup,
P42/mnm)inallsamples, with no evidence of any other crystalline
oramorphousphase, ascanbeseeninFigure1b. Likewise,
wefoundthattheline width (fullwidth athalf-maximum)oftheXRDpeaks
tends to increase as the nominal Pr-contentincreases. This
ndingcanbecorrelatedwiththedecreaseofthe particle size and/or to
changes in the residual strain extent.In order to obtain further
information the XRD patterns wereanalyzed using the Rietveld
renement method using theThompson-Cox-Hastings pseudo-Voigt
function (TCH-pV) ofthe DBWS program, given by18TCH pV = L + (1
)G,where L and G represent the Lorentzian (L) and the Gaussian(G)
functions whereas is a mixing parameter.
AtypicalRietveldrenementpatternisshowninFigure1cforthe5.0mol
%Pr-dopedsample. Thelinebroadeningrelatedtotheinstrumental
contribution was corrected by subtracting the linewidth of a
standard sample (SiO2 single crystal) from the linewidth of the
studied samples. The deconvolution of the
TCH-pVfunctionprovidesthelinewidthsoftheGaussianandtheLorentzian
components given by HG = (U tan2 + V tan + W+ Z/cos2)1/2and HL = X
tan + Y/cos , where U, V, W, Z, X,and Y are the rened parameters
and can be used to determinethe mean crystallite size (D) and the
residual strain (). Detailsregarding the calculationof Dand canbe
found intheliterature.19The list of the structural parameters
obtained fromthe Rietveldrenements is collectedinTable 1. Figure
2ashows the expected decrease of the mean crystallite size (D)
asthenominal Pr-contentincreases, whichisabehavioralreadyFigure1.
(a)EDSspectrumobtainedfor the10.0mol %SnO2:Prnanoparticles. (b) XRD
patterns of the SnO2and SnO2:Prnanoparticles. (c)Rietveldrenementof
aXRDpatternforthe5.0mol % SnO2:Pr sample. The experimental and the
calculated data arerepresentedbysymbolsandsolidlines, respectively.
Thegreensolidline represents the dierence between the experimental
and thecalculated data.The Journal of Physical Chemistry C
ArticleDOI: 10.1021/acs.jpcc.5b00761J. Phys. Chem. C 2015,119,
871187178712reported in the literature regarding the TM- and
RE-doping ofSnO2nanoparticles.6,8,20The entry of the Pr-ion
originatesdistortions in the crystal lattice evidenced by the
increase of thelatticestrain(). Theunitcell
volumebecomeslargerasthenominal Pr-contentincreases.
Thesametendencyisobservedfor both lattice constants (a and c), as
can be seen in Figure 2,parts b and c. Whereas the c/a ratio shows
an increasingtendency as the nominal Pr-content increases the
internalparameteroftherutilestructure(u)tendstodecrease. Thesetwo
opposite trends provide key structural changes to theintrinsically
attenedoctahedronof oxygenions surroundingthetinions. Thealmost
linear increaseof thec/aratiobyincreasingof thenominal Pr-content
indicatesananisotropicexpansionof theunitcell
alongthec-axisinducedbythePr-doping.Parts a and b of Figure 3 show
the TEM micrographs of the10.0and2.5mol
%SnO2:Pralongwiththeircorrespondingparticle size histograms, which
are well modeled by a log-normal distributionfunction.
Themeanparticlesizecanbeestimated by using the relation: D = D0
exp(2/2), where D0is the median value and is the polydispersion
parameter. Thevalues found for D from thets (see the red solid line
in theinsets of Figure 3, parts a and b, were 5.2 0.5 and 8.7
0.5nmfor the 10.0 and 2.5 mol % SnO2:Pr nanoparticles,respectively.
Theparticlesize determinedfrom theTEMdataare invery goodagreement
withthe meancrystalline
sizesvaluesdeterminedfromtheXRDdataanalysis(seeTable1).As presented
in Figure 3c the HRTEM image obtained for theTable 1. List of
Parameters Obtained from the XRD data Rietveld Renement of the SnO2
and SnO2:Pr NanoparticlesaPr (x, mol %) (nominal) Pr (x,mol %)
(EDS) mean size (D) (nm) strain () (%) a () c () c/a () u () V (3)
S ()0 11.0 1.0 0.1864 4.7334 3.1842 0.6727 0.3004 71.34 1.561.0 0.9
0.1 9.1 1.0 0.1787 4.7361 3.1863 0.6728 0.2990 71.47 1.482.5 2.3
0.1 7.4 1.0 0.2996 4.7370 3.1878 0.6730 0.2980 71.53 1.945.0 4.5
0.2 5.2 1.0 0.3724 4.7397 3.1919 0.6734 0.2959 71.71 1.417.5 6.9
0.3 4.9 1.0 0.4715 4.7413 3.1941 0.6737 0.2933 71.80 1.3610.0 8.9
0.5 4.0 1.0 0.4796 4.7447 3.1999 0.6744 0.2890 72.04 1.23aS
(R-wp/R-expected)values represent the goodness of thet.Figure 2.
(a) Mean particle size (D) as a function of the Pr-content. (band
c) Pr-content dependence of the lattice parameters (a and c).
Thedashed lines are drawn only to guide the eyes.Figure 3. TEM
images of the SnO2:Pr nanoparticles doped with (a) 10.0 and (b) 2.5
mol % with their corresponding particle size histograms (thered
solid lines represent the log-normal function). (c) HRTEM image of
2.5 mol % SnO2:Pr sample. (d) Selected area electron diraction
(SAED)pattern of the 2.5 mol % SnO2:Pr nanoparticles.The Journal of
Physical Chemistry C ArticleDOI: 10.1021/acs.jpcc.5b00761J. Phys.
Chem. C 2015,119, 8711871787132.5 mol % SnO2:Pr sample shows two
interplanar distances (d).Byusingtherelation:1/d2=(h2+k2)/a2+l2/c2,
validforatetragonalstructure, whereh, k,
laretheMillerindicesandaand c are the lattice constants, the d1 =
0.34 nm and d2 = 0.26nm distances (see Figure 3c) have been
assigned to the (110)and (101) diraction planes of the rutile type
structure,respectively. It is worth mentioning that these diraction
planesare commonly found in nanosized SnO2 systems.21The selected
area electron diraction (SAED) patternrecordedfromaset of
nanoparticles is showninFigure3d,revealing three bright rings
corresponding to d-values of 0.337,0.266, and 0.177 nm expected for
the (110), (101), and (211)diraction planes of the rutile type SnO2
structure.The XPS spectrumof the undoped
SnO2nanoparticlespresentedinFigure4showspeaksoriginatedfromSnandOplusaweakfeatureC(1s)locatedat284.6eVduetocarboncontamination.
TheenergydierencebetweentheSn(3d5/2)and the O(1s) XPS peaks was
found to be 43.89 (1) eV.
ThisdierencecanbeusedastheindexoftheSnoxidationstate.Thus, for the
stoichiometric SnO2 the expected index would be43.90 eV, which is
0.08 eVlower than the index for thestoichiometric SnO.22This nding
conrms that the Sn4+species are the dominant one in the
SnO2nanoparticles.AfterthepraseodymiumdopingXPSpeakscorrespondingtoPr(3d)emerge,
evidencingthesuccessful doping. Inordertoshow the chemical
composition evolution of the as-synthesizedsamples due to the
Pr-doping we recorded high resolution XPSspectrafor
boththeSnO2andSnO2:Pr nanoparticles inthetypical Sn, Pr,and O
binding energy ranges.Figure5 showstheSn(3d5/2) andtheSn(3d3/2)XPS
peakswhich were curve-tted using Gaussian-like functions,
providingfortheSnO2nanoparticlespeakpositionsat486.6and495.1eV,
respectively. The Sn(3d5/2) XPS peak position observed inFigure 5
conrms that the oxidation state of the tin ions in theSnO2
nanoparticle is 4+. Actually, the binding energies for
theSn(3d5/2)associatedwiththeSn2+andSn4+areexpectedat485.9and486.6eV,
respectively.23ForthePr-dopedsampleswefounda systematic shift of
theSn(3d5/2) andSn(3d5/2)peaks to lower energy values as the
nominal Pr-contentincreases (seetheinset of Figure5for
theSn(3d5/2)peak).Thisnding is clear evidence that the oxidation
state of the tinions changes from Sn4+to Sn2+as the Pr-content
increases.Figure 6 shows the XPS spectra of the 2.5, 5.0, and 10.0
mol%SnO2:Pr nanoparticles in the binding energy region ofPr(3d)
ions. As indicated inFigure. 6 the two XPS peakslocated at around
933.6 and 954.0 eV have been assigned to
thePr(3d5/2)andPr(3d3/2)levels, respectively.
Thedierenceof20.4eViscommonlyreportedintheliteratureforthespinorbitsplittingofthe3d5/2and3d3/2levels.24Thepresenceofboth
peaks, namely Pr(3d5/2) and Pr(3d3/2) does not permit toexclude one
of the two possible oxidation states of Pr-ions
(Pr3+andPr4+)intheas-synthesizedsamples, sincethe3d5/2and3d3/2peaks
have been observed in both Pr2O3and PrO2compounds.24Furthermore,
ashoulderataround967eVwasalso observed for samples with lower
Pr-content (see the peakdenoted by two asterisks in Figure 6). A
satellite peak centeredat967eVin theXPS spectrumofPrO2 hasbeen
reportedasbeingexclusivelyrelatedtothetetravalent stateof Pr
inthePrO2compound.24,25Unfortunately,
thisXPSsatellitepeakisoverlappedwiththeoxygenAugerpeak(OKLL),
makingitsexact determination dicult.On the otherhand, as can beseen
in theinset ofFigure 6,the position of the binding energy of the
3d5/2 peak is locatedbetween933.2and933.9eV. AccordingtoMolderetal.
thispeakpositionisexpectedfor thePr2O3compoundandit isquitedierent
fromthepeakpositionexpectedforthePrO2compound whose 3d5/2 XPS peak
should appear between 935and 936 eV.26This analysis indicates that
most of thepraseodymiumions intheas-synthesizedsamples
areinthePr3+oxidation state, in agreement with the
preferentiallytrivalent state like most of the RE-ions.27However,
smallamounts of the Pr4+ion cannot be completely ruled
out,especiallyinthelower Pr-content region, wherethebindingenergy
is shifting towardhigher values as the Pr-content isFigure 4. XPS
spectrum of the SnO2 nanoparticles,showing the Sn-,O-, and
C-related peaks.Figure 5. Sn 3d XPS spectra of the SnO2:Pr
nanoparticles. The insetshows the experimental evolutionof the
Sn(3d5/2) peak withthenominal Pr-content.Figure 6. Pr 3d XPS
spectra of the SnO2:Pr nanoparticles for 2.5, 5.0,and 10.0 mol %
Pr-doping.The Journal of Physical Chemistry C ArticleDOI:
10.1021/acs.jpcc.5b00761J. Phys. Chem. C 2015,119,
871187178714reduced (see the inset of Figure 6). Finally, the 3d5/2
and 3d3/2XPS components also showsatellites at the lower
bindingenergy end (represent by the asterisks in Figure 6)
attributed tothe well-screened 4f3nal state.28In order to estimate
the relative contents of Pr and Sn on thenanoparticles surface the
total areas of the Pr(3d) and Sn(3d)peaks were divided and
corrected to the atomic
sensitivityfactortakenfromtheusedsoftware(seeTable2). Ascanbeseen
from data collected in Table 2 the [Pr]/[Sn] ratio shows amonotonic
increase as the nominal Pr-content increases.Moreover, after
thesputteringtreatment onthe10.0mol %Pr-doped sample we found a
reduction of the [Pr]/[Sn] ratio ofabout 16%, which shows that the
Pr-ions are segregated on thenanoparticles surface. This results is
in agreement with reportsfound in the literature and it can explain
the observed crystallitesize reduction as the Pr-content increases
(see Table 1).16On the other hand, a [O]/[Sn] ratio of 1.44 has
beendetermined for the undoped SnO2 nanoparticles, which is
wellbelowtheexpectedvalueforthebulksystem.2Nevertheless,[O]/[Sn]ratiovaluesbelow2havebeenalreadyreportedintheliterature,
whichincreases while exposingthesampletomolecular
oxygenO2.23,29,30The low [O]/[Sn] ratio found inthe undoped
SnO2nanoparticles conrms the nonstoichio-metric character at the
nanoparticles surface. For the SnO2:Prnanoparticles the [O]/[Sn+Pr]
ratio increase as the praseody-miumcontent increases (see data
inTable 2). It is worthmentioning that the oxygen concentration
([O]) is given by thesumofthestructural oxygen(OStru),
thechemisorbedoxygen(OChem), andtheoxygen-relatedvacancies(OVac).
Withinthisviewpoint,the increase of the [O]/[Sn + Pr] ratio shows
thatthe Pr-doping takes place while removing oxygen-relatedvacancy
and/or chemisorbed oxygen.Figure 7a and 7b show the O(1s) region of
the XPS spectrafor both the SnO2 and the 10.0 mol % SnO2:Pr
nanoparticles.The spectrum for the 10.0 mol % SnO2:Pr sample
submitted tothe sputtering process is also shown. After a simple
visualinspection one can observe that the O(1s) feature is wide
andasymmetric, which can be deconvoluted into three
well-denedGaussian-likepeaks, revealingthepresenceof threetypes
ofoxygen-related species. In agreement with the literature
thersttwocomponents shouldcorrespondtothestructural oxygen(OStru)
with chemical states Sn2+(SnO) and Sn4+(SnO2)located at 529.8 and
530.5 eV, respectively.29The origin of thethirdcomponent locatedat
around531.4eViscontroversialandit has beenassignedeither
tothechemisorbedoxygen-relatedspecies, suchas thehydroxyl
group(OH)or otherradicals(CO,
CO2)atthesamplessurface,3133oritcanbeassociatedwiththepresenceof
oxygenvacancies (OVac).34,35We believe that any of these sources
cannot be excluded sincethedopingwithPr-ions substitutingSn4+-ions
must developOVacto compensate charge.Additionally, the OChemis
present in small amount asindicated by the presence of the XPS
feature around 288 eV intheC(1s)region,
whichiscommonlyattributedtotheCOxspecies (see Figure 4) and
therefore conrming the presence ofthis type of oxygenspecies inthe
O(1s) region. After thesputtering treatment the XPSspectrumof the
10.0 mol %SnO2:Prsamplerevealedalmost acompletereductionof
thecarbonsignal (centeredat 288eV)whilereducingtheXPSpeak located
at around531.4 eV(the third component toO(1s) peak) inabout 46%.
TheremainingXPSsignal (seeFigure 7c) was attributed to the
oxygen-related vacancies(OVac).34,35This result can be interpreted
as the elimination ofthe chemisorbed oxygen and/or oxygen-related
vacancy.On the other hand, the [OSn4+]/[OSn2+] ratio decreases as
thePr-content increases as can observed in Table 2,
thussupportingtheassumptionthattheoxidationstateoftinionsgraduallychanges
fromSn4+toSn2+, inagreement withtheresults obtained from the
Sn(3d5/2) binding energy. AsobservedinTable2the[OChem,
OVac]/[OSn4++OSn2+]ratioshows a slight increase while going from
the undoped to the 1.0mol % Pr-doped SnO2 nanoparticle, decreasing
afterward as thePr-content increases up to 10.0 mol %. This
behavior evidencethat the surface segregation of the Pr-ions leads
to thereduction of OChemand/or OVac.4. CONCLUSIONSSnO2:Pr
nanoparticles with rutile-type structure have
beensuccessfullysynthesizedbythepolymeric precursor
method.Theestimatedcrystallitesizemonotonicallydecreases(belowTable
2. [Pr]/[Sn] and [O]/[Sn+Pr] ratio determined for the SnO2 and
SnO2:Pr nanoparticles form XPS measurementsnominal Pr-content (x
mol %) [Pr]/[Sn] [O]/[Sn + Pr] [OSn4+]/[OSn2+] [OChem,OVac]/[OSn4++
OSn2+]0 1.44 4.04 0.441.0 0.012 1.45 4.07 0.482.5 0.029 1.53 1.13
0.475.0 0.046 1.52 1.06 0.417.5 0.065 1.55 0.62 0.2710.0 0.070 1.61
0.73 0.1310.0 sputtering 0.059 1.19 0.64 0.06Figure 7. O (1s) XPS
spectra of the (a) undoped SnO2 nanoparticles,(b) 10.0 mol %
SnO2:Pr nanoparticles before sputtering, and (c) 10.0mol % SnO2:Pr
nanoparticles after sputtering. The black symbols arethe
experimental data whereas the redsolidlines are the best
tconsidering three components (brown,blue,and green solid
lines).The Journal of Physical Chemistry C ArticleDOI:
10.1021/acs.jpcc.5b00761J. Phys. Chem. C 2015,119,
87118717871510nm)upontheincreaseof thePr-doping. FromtheXRDanalysis
we determined that the Pr-ions enter the
rutilestructuremainlysubstitutingtinions. TheXPSdataanalysisreveals
the presence of mainly Pr3+-ions and a
continuouschangeoftheoxidationstateoftinionsfromtheSn4+totheSn2+as
the Pr-content is increased. Evidence of a surfacesegregation of
Pr-ions has been determined for the 10.0 mol %SnO2:Pr sample. The
surface enrichment seems tobecomeenhanced as the Pr-content is
increased and leads to thereductionof thechemisorbedoxygenand/or
oxygen-relatedvacancies at the nanoparticles surface.AUTHOR
INFORMATIONCorresponding Author*(F.H.A.) Telephone +55 31
30693210.E-mail: [email protected] authors declare no
competingnancial interest.ACKNOWLEDGMENTSThisworkwas
nanciallysupportedbytheBrazilianagenciesCNPq, CAPES,and
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