A New Sol-Gel Route Towards Nd3+-doped SiO2-LaF3 Glass-Ceramics for Photonic Applications

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Ci e his: Ma e . Ad ., 2020,
1,3589
A new sol–gel ou e owa ds Nd
3+
-doped
SiO
2
–LaF
3
glass-ce amics o pho onic applica ions
Ma ı
´a Eugenia C uz, *
a
Alicia Du a
´n,
a
Rolindes Balda,
bc
Joaquı
´n Fe na
´ndez,
d
Glenn C. Ma he
a
and Yolanda Cas o
a
Glass-ce amic ma e ials wi h composi ion 0.9Nd
3+
–80SiO
2
–20LaF
3
we e success ully ob ained and u he
hea - ea ed a 450 1C o 6 h. S able and homogeneous LaF
3
nanopa icle suspensions wi h and wi hou
Nd
3+
we e i s p epa ed by a chemical ou e, inco po a ing poly inylpy olidone (PVP) as dispe san . The
suspensions we e hen concen a ed and cha ac e ised by XRD, HRTEM and ze a po en ial, con i ming ha
LaF
3
c ys allises as he only phase, wi h pa icle size a ound 16 nm. The suspensions we e inco po a ed in a
silica sol o ob ain a 0.9Nd
3+
–20LaF
3
–80SiO
2
pa icula e sol, xe ogel and glass-ce amic. HRTEM con i med
he homogeneous inco po a ion o he doped nanoc ys als in o he SiO
2
ma ix wi hou modi ica ion o he
nanopa icle s uc u e. Rie eld e inemen was used o de e mine he c ys allini y and quan i y o LaF
3
nano-
pa icles p esen in he glass-ce amic a e ea men o he pa icula e sol a 450 1C o 6 h. Luminescence
measu emen s o nea in a ed Nd
3+
ion emissions in he lan hanum luo ide nanopa icles and SiO
2
–LaF
3
glass-ce amic showed well-s uc u ed emission spec a wi h li e imes simila o hose o heo e ical Nd
+3
in
LaF
3
c ys als.
In oduc ion
Glass is a key ma e ial wi h a wide ange o applica ions,
anging om adi ional glasswa e p oduc s o ad anced
op ical ma e ials. Op ical anspa ency in an ex ended spec-
um, chemical du abili y, and manu ac u abili y ende glass
op imal o p oducing ma e ials o diffe en shapes. In addi-
ion, signi ican imp o emen s in ce ain p ope ies may be
achie ed on p epa a ion o glass ce amics (GCs) h ough
con olled c ys allisa ion.
1
S. D. S ookey i s de eloped a glass-ce amic named Fo oce am
in 1957, la e called Py oce am,
2
coining he e m glass-ce amic
and de ining i as: ‘‘...made by i s mel ing and o ming special
glasses con aining nuclea ing agen s and hen causing con olled
c ys alliza ion o he glass pa icles’’.
2
Fo many yea s, his de ini ion
only included ma e ials con aining mo e han 50% o c ys als.
Howe e , GCs wi h lowe c ys al ac ion ha e also been de eloped
in he las 60 yea s. The de ini ion was upda ed a e he ‘‘12 h
In e na ional Symposium on C ys alliza ion in Glasses and Liquids’’,
o ganized by TC07 o he In e na ional Commission on Glass (ICG),
o include diffe en echnologies and p ocessing me hods.
‘‘Glass-ce amics a e ino ganic, non-me allic ma e ials p epa ed
by con olled c ys alliza ion o glasses ia diffe en p ocessing
me hods. They con ain a leas one ype o unc ional c ys alline
phase and a esidual glass. The olume ac ion c ys allized may
a y om ppm o almos 100%’’.
3
Pa icula a en ion is cu en ly de o ed o he p epa a ion
o oxide glasses con aining luo ide nanoc ys als (NCs). Such
ma e ials, known as oxy luo ide glass-ce amics (OxGCs), we e
ob ained o he i s ime in 1998 by Dejneka (1998). Fluo ide
nanoc ys als add unique p ope ies o oxide glasses, making
hem a ac i e o pho onic applica ions. They combine he
e y low phonon ene gy (300–450 cm
1
) o luo ide nanoc ys-
als wi h he high chemical, mechanical and he mal s abili y
o he glass ma ices, he eby inc easing he luminescence
e iciency.
4–6
The esul ing oxy luo ide glass-ce amics ha e
no able ad an ages compa ed wi h pu e glass o ce amic
ma e ials. The LaF
3
–SiO
2
sys em has been in es iga ed due o
he beha iou o he LaF
3
c ys als as a e-ea h (RE) hos s. LaF
3
has supe io solid solubili y compa ed wi h o he luo ide
c ys al phases.
7,8
Among he diffe en echniques employed o he p epa a-
ion o OxGCs, glass mel ing-quenching (MQ) is he mos
ele an . Howe e , high mel ing empe a u es (1400–1700 1C)
cause signi ican luo ine loss, limi ing he inal luo ine
con en o he c ys al phase and esul ing in uncon ollable
composi ions wi h espec o luo ine and lan hanide (Ln
3+
)
ions. Many esea che s ha e p oposed al e na i e p ocessing
me hods o o e come hese limi a ions
9
such as Spa k Plasma
a
Ins i u o de Ce a
´mica y Vid io (CSIC), Campus de Can oblanco, 28049 Mad id,
Spain. E-mail: [email p o ec ed]
b
Dep . Fı
´sica Aplicada I, Escuela Supe io de Ingenie ı
´a, Uni e sidad del Paı
´s Vasco
(UPV-EHU), 48013 Bilbao, Spain
c
Cen o de Fı
´sica de Ma e iales, (UPV/EHU-CSIC), 20018 San Sebas ian, Spain
d
Donos ia In e na ional Physics Cen e DIPC, 20018 San Sebas ian, Spain
Recei ed 16 h Sep embe 2020,
Accep ed 19 h No embe 2020
DOI: 10.1039/d0ma00708k
sc.li/ma e ials-ad ances
Ma e ials
Ad ances
PAPER
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Sin e ing (SPS). The p epa a ion o xZ O
2
–(100–x)SiO
2
glass-
ce amics by SPS wi h a high con en o c ys alline phase (highe
han 45 mol%)
10
has been epo ed, bu LaF
3
–SiO
2
ma e ials
ha e no ye been epo ed. Mo eo e , he SPS p ocess is a he
expensi eand hesizeandshapeo samplesa equi elimi ed.
10
On he o he hand, he sol–gel p ocess is a p omising ou e
o p oduc ion o glass-ce amic ma e ials, which a oids he
d awbacks o mel ing-quenching, wi h he e sa ili y o allow
di e en ly shaped p oduc s such as hin ilms, powde s, and
bulk ma e ials ob ained a low empe a u es. Fu he ad an-
ages o he me hod include homogenei y and high pu i y o
he esul an ma e ials. Howe e , sol–gel was no widely
applied o glass-ce amic p ocessing un il signi ican imp o e-
men s in s uc u al and op ical p ope ies appea ed in he la e
nine ies. P ecise con ol o he syn hesis and c ys allisa ion
p ocess is equi ed o ob ain anspa en oxy luo ide glass
ce amics, which a oids bo h uncon olled pa icle g ow h and
he o ma ion o undesi able phases. Ca e ul p ocess design is,
he e o e, c i ical o ob aining well-o de ed, anspa en , and
e icien op ical glass-ce amics.
The mos s udied oxy luo ide composi ions p epa ed by sol–gel
a e LnF
3
(Ln=La,Y,Gd)andRLnF
4
(R=Na,K,Li;Ln=Gd,Y),
doped wi h di e en a e-ea h (RE
3+
)ions(E
3+
,Eu
3+
,Nd
3+
,e c.).
11,12
The i s pape s epo ing he p ocessing o pu e LaF
3
on
silica glass subs a es by sol–gel we e published in 1998 by
Fuhija a and Tada (1999), who showed ha con ol o he
syn hesis pa ame e s and hea - ea men we e essen ial o
ob ain anspa en LaF
3
ma e ials a oiding he p ecipi a ion
o o he phases such as LaOF.
13,14
Up o 2018, he li e a u e ypically epo ed he p epa a ion
o oxy luo ide glass-ce amics wi h nominal con en s o 5–10 mol%
o ac i e c ys al phases, wi h li le inno a ion ela ed o syn h-
esis and p ocessing condi ions.
13,15
Ne e heless, in he pas
decade, he GlaSS G oup o he Ins i u o de Ce a
´mica y Vid io
(CSIC) has op imised he sol–gel p ocess o ob ain oxy luo ide
glass-ce amics wi h high ac i e-phase con en s (up o 20 mol%)
by sol–gel. Quan i a i e Rie eld e inemen o 80SiO
2
–20LaF
3
bulk samples doped wi h 0.5E
3+
and ea ed a 550 1C o 1 min
indica ed a c ys al ac ion B18 w %,
13
he highes epo ed
in he li e a u e o anspa en oxy luo ide glass-ce amics
p epa ed by sol–gel and he highes compa ed o any glass-
ce amics p epa ed by mel ing. Howe e , pho oluminescence
emission and exci a ion spec a o SiO
2
:LaF
3
samples exhibi ed
only a ew, b oad s uc u ed bands due o he small size o he
nanoc ys als (B2 nm o hin ilms and 8 nm o bulk samples).
Go ni e al.
15
p oposed ha he weak luminescence p ope ies
a e due o uncon olled nuclea ion and g ow h o he LaF
3
c ys als in he sol–gel ma ix, because, a high empe a u es,
he c ys als ends dissol e and each chemical-po en ial equili-
b ium. I is essen ial, he e o e, o imp o e he op ical p ope -
ies by ob aining la ge nanoc ys al sizes, which is di icul
using cu en syn hesis me hods based on a wo-s ep chemical
p ocess.
An al e na i e, ecen ly p oposed p ocedu e o ob ain glass-
ce amics is he p oduc ion o nano-c ys alline powde s
ollowed by hei dispe sion in a sol–gel ma ix. Al hough his
me hod is p omising, he luminescence s udies a e no
conclusi e.
16
The aim o he cu en wo k is he p epa a ion o Nd
3+
doped-LaF
3
nanopa icle suspensions and hei inco po a ion
in o silica (SiO
2
) sols p epa ed by acid ca alysis. The dispe sion
o he nanopa icles and s abili y o he LaF
3
and LaF
3
–SiO
2
suspensions we e s udied, con i ming ha LaF
3
appea s as he
only c ys alline phase. Xe ogels and glass-ce amic powde s
we e p oduced by he mal ea men and cha ac e ized by
di e en echniques wi h p omising luminescence esul s.
Expe imen al
Syn hesis o LaF
3
and Nd
3+
:LaF
3
nanopa icle suspensions
Lan hanum chlo ide (LaCl
3
, Al a Aesa ) and ammonium luo -
ide (NH
4
F, Me ck) we e used as eagen s wi hou u he
pu i ica ion and mixed wi h deionized wa e o a La
3+
concen-
a ion o 0.04 M.
17
The solu ion was main ained a 75 1C o
2 h wi h con inuous s i ing o ob ain he LaF
3
–0.04 M nano-
pa icle suspension wi h a inal pH = 7. Following he same
p ocess, neodymium ace a e (Nd(CH
3
CO
2
)
3
, Al a Aesa ) was
added o NH
4
F/LaCl
3
/H
2
O solu ion in a mola a io Nd
3+
/
LaCl
3
= 0.9. The solu ion was main ained a 75 1C o 2 h unde
con inuous s i ing o ob ain a 0.9Nd
3+
–LaF
3
–0.04 M nano-
pa icle suspension.
Poli inylpi olidone (PVP) dispe san was added o LaF
3
–
0.04 M and 0.9Nd
3+
–LaF
3
–0.04 M nanopa icle suspensions
wi h concen a ions o LaF
3
nanopa icles in he ange 1–10 w %.
Subsequen ly, he LaF
3
–0.04 M and 0.9Nd
3+
–LaF
3
–0.04 M sus-
pensions we e sonica ed using an ul asound p obe (Ul aso-
nica ion P obe, GM 2200, Bandelin elec onic, Ge many) o 2
min. The esul ing suspensions we e labelled as LaF
3
–0.04
M–PVP(X) o 0.9Nd
3+
–LaF
3
–0.04 M–PVP(X), whe e Xindica es
he w % o added PVP.
All he suspensions we e concen a ed using a o a y e a-
po a o (R-210 wi h acuum pump V-700, Buchi) o each a
concen a ion o 0.25 M, and labelled as LaF
3
–0.25 M–PVP(X)
and 0.9Nd
3+
–LaF
3
–0.25 M–PVP(X).
Cha ac e isa ion o Nd
3+
–LaF
3
nanopa icle suspensions
The dispe sibili y and s abili y o a LaF
3
–0.04 M nanopa icle
suspension was s udied h ough he a ia ion o ze a po en ial
as a unc ion o pH using a Ze asize Nano ZS ins umen
(Mal e n S, UK). Suspensions o di e en pH in he ange 3–12
we e p epa ed on addi ion o ni ic acid (HNO
3
, Sigma Ald ich)
and e ame hylammonium hyd oxide (TMHA, Me ck) hen
s abilised o 12 h.
Pa icle size was also measu ed as a unc ion o w % PVP
o LaF
3
–0.04 M–PVP(X) nanopa icle suspensions (X=1,3,5,7,
9 and 10) using he Ze asize Nano ZS ins umen .
LaF
3
–0.04 M and 0.9Nd
3+
–LaF
3
–0.25 M–PVP(X) suspensions
we e cen i uged a 6000 pm o 5 min, and he esul ing
powde s insed wi h deionized wa e ; he p ocess was epea ed
h ee imes o emo e all he o ganic componen s. Powde s
we e u he d ied a 80 1C o 12 h and cha ac e ized by X- ay
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di ac ion (XRD). Di ac og ams we e acqui ed in he ange
20–7012ywi h a s ep size o 0.021and 1 second o in eg a ion
ime using a D8 Ad ance di ac ome e (B uke , MA). The
c ys al size was calcula ed by applying Sche e ’s equa ion,
T¼Kl
BbðÞcos y
whe e lis he wa eleng h o Cu Ka adia ion (l= 0.15405 nm),
B he ull wid h hal maximum o he LaF
3
peak a 2y=441,
y he co esponding di ac ion angle and b he co ec ion o
he ins umen ; he cons an ac o , K, co esponds o sphe ical
c ys als and a alue o 0.94 is commonly adop ed.
High Resolu ion T ansmission Elec on Mic oscopy
(HRTEM) and Ene gy-dispe si e X- ay spec oscopy (EDX)
we e used o cha ac e ize 0.9Nd
3+
–LaF
3
–0.04 M and 0.9Nd
3+
–
LaF
3
–0.25 M–PVP(X) nanopa icles d ied a 80 1C o 12 h and
edispe sed in e hanol employing a JEOL 2100F mic oscope.
The samples we e p epa ed by d opping he suspensions on o a
ca bon-coa ed coppe g id (Lacey Ca bon, LC-200-Cu 25/pk).
HRTEM images we e p ocessed using ImageJ
s
so wa e. The
la ice pa ame e s o LaF
3
nanopa icles in 0.9Nd
3+
–LaF
3
–0.25
M–PVP(10) we e de e mined om he co esponding elec on-
di ac ion pa e n.
The c ys al ac ion o 0.9Nd
3+
–LaF
3
–0.25 M–PVP(10) nano-
pa icles in a glass ce amic ea ed a 450 1C o 6 h was
es ima ed by he Rie eld me hod as desc ibed p e iously
4
using he FULLPROF p og am,
18
wi h NaF as in e nal weigh
s anda d in an app op ia e quan i y (5 w %). The XRD da a
we e collec ed in he ange 201 2y 1201in a s ep wid h o
0.01671employing a PHILIPS X-PERT PRO y/2ydi ac ome e
ope a ing a 45 kV and 40 mA.
Syn hesis o 0.9Nd
3+
–20LaF
3
:80SiO
2
pa icula e sols
20LaF
3
–80SiO
2
pa icula e sols (weigh ela ion 80/20 be ween
SiO
2
and LaF
3
) wi h and wi hou Nd
3+
doping we e p epa ed
om 14.5 ml o e ae hyl o hosilica e (TEOS, Sigma Ald ich)
and 13 ml o me hyl- ie hoxysilane (MTES, ABCR) mixed wi h
con inuous s i ing unde ambien condi ions. Subsequen ly,
28 g o a concen a ed suspension o 0.9Nd
3+
–LaF
3
–0.25
M–PVP(X) nanopa icles was inco po a ed and concen a ed;
hyd ochlo ic acid (HCl, Sigma Ald ich) was hen added unde
igo ous s i ing o p oduce he hyd olysis and condensa ion
eac ions,
17
and achie e 0.9Nd
3+
–80SiO
2
:20LaF
3
–PVP(X) pa i-
cula e sols on apid cooling in ice ba hs o one minu e; a
inal concen a ion o 171 g l
1
SiO
2
was a ained on dilu ing
wi h 11 ml o absolu e e hanol.
Two u he sols o composi ion 0.9Nd
3+
–5LaF
3
:95SiO
2
and
0.9Nd
3+
–10LaF
3
–90SiO
2
we e also syn hesised ollowing he
same p ocess, inco po a ing he co esponding mola a ios
o 0.9Nd
3+
–LaF
3
–0.25 M–PVP(10) nanopa icle suspensions o
TEOS and MTES solu ions.
Cha ac e isa ion o 0.9Nd
3+
–20LaF
3
:80SiO
2
ma e ials
The s abili y o he 0.9Nd
3+
–80SiO
2
:20LaF
3
pa icula e sols was
de e mined using an AND Vib o Viscome e on measu ing he
iscosi y once a day o i e days.
Diffe en ial he mal analysis (DTA) and he mog a ime ic
analysis (TGA) we e pe o med wi h a SDT Q600 ins umen (TA
Ins umen s, New Cas le, DE, USA). App oxima ely 15 g o
0.9Nd
3+
–20LaF
3
:80SiO
2
xe ogel, d ied a 80 1C o 12 h, was
measu ed in ai om oom empe a u e o 800 1C wi h a
hea ing a e o 10 1C min
1
.
The composi ional and mo phological cha ac e iza ion o
he xe ogels and glass-ce amics ob ained om pa icula e sols
was pe o med by HRTEM and EDX spec oscopy (JEOL 2100F).
The 0.9Nd
3+
–20LaF
3
:80SiO
2
pa icula e sol was d ied and ea-
ed a 450 1C o 6 h. The powde was e-dispe sed in e hanol
and a d op o he suspension deposi ed on o ca bon-coa ed
coppe g ids (Lacey Ca bon, LC-200-Cu 25/pk); he size dis i-
bu ion o he pa icles was de e mined using ImageJ
s
so wa e.
The quan i a i e c ys alline ac ion o 0.9Nd
3+
–20LaF
3
:80-
SiO
2
samples ea ed a 450 1C o 6 h was de e mined by he
Rie eld me hod using he pa ame e s and in e nal weigh
s anda d indica ed ea lie .
Op ical cha ac e isa ion o LaF
3
nanopa icles and Nd
3+
–LaF
3
–
SiO
2
glass-ce amics
0.9Nd
3+
–LaF
3
–0.25 M–PVP(10) nanopa icle suspensions wi h-
ou any addi i e we e d ied a 80 1C and ea ed a 450 1C be o e
p essing in a uniaxial p ess o 3 min a 1000 MP o lumines-
cence analysis. The inal compac samples we e exci ed wi h a
uneable Ti-sapphi e ing lase (0.4 cm
1
linewid h) in he 770–
920 nm spec al ange. The emi ed ligh was analysed wi h a
single g a ing monoch oma o ( ocal leng h 0.25 m), de ec ed
by an ex ended IR Hamama su H10330A-75 pho omul iplie ,
and ampli ied by a s anda d lock-in echnique. Luminescence
decay cu es we e ob ained by exci ing he samples wi h a Ti-
sapphi e lase pumped by a pulsed equency-doubled Nd:YAG
lase (9 ns pulse wid h), and de ec ing he emission wi h a
Hamama su H10330A-75 pho omul iplie . All measu emen s
we e pe o med a oom empe a u e.
Op ical cha ac e isa ion o he glass-ce amics was pe -
o med by luminescence measu emen s on samples o
0.9Nd
3+
–5LaF
3
:95SiO
2
, 0.9Nd
3+
–10LaF
3
:90SiO
2
and 0.9Nd
3+
–
20LaF
3
–80SiO
2
powde s ea ed o 6 h a 450 1C and hen
p essed a 1000 MP o 3 min. The s eady-s a e emission spec a
and luminescence decay cu es we e collec ed as
desc ibed abo e.
Resul s and discussion
Cha ac e isa ion o LaF
3
and Nd
3+
:LaF
3
nanopa icle
suspensions
Fig. 1a shows he a ia ion o ze a po en ial as a unc ion o pH
o he LaF
3
–0.04 M nanopa icle suspension om an ini ial pH
o 7, co esponding wi h a ze a po en ial o 22 mV. The
addi ion o HNO
3
modi ied he ze a po en ial o mo e posi i e
alues, om 22 mV (pH 7) o 10 mV (pH 3), eaching he
isoelec ic poin (x-po en ial = 0) a a pH o 4.8. On he o he
hand, he ze a po en ial emains p ac ically cons an , B25 mV,
wi h he addi ion o TMAH. The a ia ion o he ze a po en ial
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p o ides in o ma ion on he s abili y o he nanopa icle suspension,
whichg ea lydependson hepH.Ahigh ze a po en ial, in absolu e
alues, o en indica es mo e s able suspensions, hence a pH o 7 was
selec ed o p epa e he LaF
3
–0.04 M nanopa icle suspension,
co esponding wi h a ze a po en ial o 22 mV.
Ze a po en ial and pa icle size o LaF
3
–0.04 M nanopa icle
suspensions a pH 7 we e measu ed as a unc ion o PVP w %,
Fig. 1b. Al hough a sligh dec ease o he ze a po en ial is
obse ed up o 3 w % PVP, i emains cons an , a ound
35 mV, wi h he inc emen o PVP. Howe e , as obse ed in
Fig. 1b, he hyd odynamic pa icle size o he nanopa icles
dec eases wi h he addi ion o PVP. In gene al, he addi ion o
PVP o LaF
3
–0.04 M–PVP(X) nanopa icle suspensions p oduces
a signi ican inc ease in i s s abili y, ela ed o he epulsi e o ces
om he hyd ophobic ca bon chains o he PVP molecules.
19
In compa ison wi h p e ious wo ks o G. Go ni e al.,
16
s a ing
om a wo-s ep p ocess wi h u he p ecipi a ion o NP,
la ge pa icle sizes ha e been ob ained using his new me hod.
No limi a ion o c ys al g ow h occu s because c ys alliza ion
occu sin heliquidphaseand heion mobili y acili a es he
di usion and g ow h o NPs. A maximum hyd odynamic size o
27 nm was ob ained, which is an o de o magni ude g ea e han
ha epo ed by G. Go ni.
22
Concen a ions o 0 and 10 w % PVP
we e selec ed o p epa e he co esponding suspensions ha we e
la e added o he SiO
2
sol.
Fig. 2 shows he XRD pa e ns o LaF
3
nano powde s
ob ained a e d ying he LaF
3
–0.04 M and 0.9Nd
3+
–LaF
3
–0.25
M–PVP(10) suspensions a 80 1C. The XRD analysis e ealed
c ys allisa ion o LaF
3
wi h hexagonal symme y (JCPDS 03-1013);
no o he c ys alline phases we e de ec ed. The desi ed c ys al-
lisa ion was achie ed o he LaF
3
–0.04 M suspension as well as
o he doped and concen a ed suspensions. This con i ms
ha nanoc ys al g ow h is a ec ed nei he by he inco po a ion
o he a e-ea h ace a es no by he dispe san . Mo eo e , he
nanoc ys als emain s able in size a e he concen a ion o he
suspensions om 0.04 o 0.25 M in he o a apo .
The pa icle size (T) o he powde s de e mined using he
Debye–Sche e equa ion, was 12 and 15 nm espec i ely, much
highe han pa icle sizes epo ed in he li e a u e.
20–22
0.9Nd
3+
–LaF
3
–0.04 M and 0.9Nd
3+
–LaF
3
–0.25 M–PVP(10) NP
suspensions we e d ied a 80 1C and analysed by HRTEM o
comple e cha ac e isa ion o he mo phology. Fig. 3a shows
he p esence o agg ega ed LaF
3
nanopa icles wi h a ubula
o m and a e age size, B18 nm. In he case o 0.9Nd
3+
–LaF
3
–
0.25 M–PVP(10), he inc ease in concen a ion om 0.04 o
0.25 M p oduces g ea e agg ega ion o he LaF
3
nanopa icles
(Fig. 3b), al hough he mo phology and he pa icle size
emained cons an . The c ys al phase was analysed wi h he
ImageJ
s
so wa e (Fig. 3c), gi ing a plana dis ance (dspacing)
o 0.325nm, which co esponds o he la ice dis ance be ween
he (101) planes o he LaF
3
hexagonal phase. A Fas Fou ie
T ans o m (FFT) also pe o med wi h ImageJ
s
, Fig. 3(d), shows
he co esponding di ac ion ings and whi e spo s in he
elec on-di ac ion pa e n whe e (101) is iden i ied a 2y= 27.421.
Fig. 3e shows he EDX analysis o he 0.9Nd
3+
–LaF
3
–0.25 M–
PVP(10) sample co esponding o he a ea shown in Fig. 3c. The
chemical analysis e eals he p esence o F, La and Nd, p o id-
ing clea e idence o Nd
3+
inco po a ion in he nanopa icles.
Fig. 1 (a) Va ia ion o he ZP (M ) o LaF
3
–0.04 M nanopa icle suspensions
as a unc ion o pH and (b) a ia ion o he ze a po en ial and pa icle size o
LaF
3
–0.04 M nanopa icle suspensions a pH = 7 as a unc ion o PVP w %.
Fig. 2 XRD pa e ns o LaF
3
–0.04 and Nd
3+
–LaF
3
–0.25 M–PVP(10) nano-
pa icle suspensions d ied a 80 1C o 12 h.
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Quan i a i e Rie eld e inemen o LaF
3
(no shown) was
pe o med o 0.9Nd
3+
–LaF
3
–0.25 M–PVP(10) nanopa icles
ea ed a 450 1C o 6 h. A c ys allised ac ion o 13 w %
was de e mined, below ha expec ed and indica ing ha he
LaF
3
nanopa icles a e highly amo phous. I should also be
conside ed ha a lowe limi o de ec ion occu s when he
peaks become so b oad due o he small pa icle size ha hey
disappea in o he backg ound adia ion. Al hough he e is no
exac limi o de ine when his occu s, pa icles o low size a e
likely o appea amo phous and special condi ions such as a
slow s em scan a e equi ed.
Cha ac e isa ion o 0.9Nd
3+
–20LaF
3
:80SiO
2
pa icula e sol
The s abili y o he sol wi h ime was s udied h ough he
e olu ion o iscosi y. Fig. 4 shows he a ia ion o he iscosi y
wi h aging ime o he 0.9Nd
3+
–20LaF
3
:80SiO
2
pa icula e sol up
o i s geli ica ion. The ini ial iscosi y, measu ed wo hou s
a e he sol syn hesis, was 4.1 mPa s. inc easing o 4.2 mPa s.
a e 42 hou s. Du ing he ollowing days, he iscosi y was
measu ed once pe day achie ing a alue o 8.5 mPa s on he
i h day, be o e gela ion occu ed he ollowing day. Al hough
hese alues a e high o deposi ing hin ilms, hey a e sui able
o p oducing xe ogels in a sho e ime om s able sols.
Fig. 5 shows he DTA and TGA analysis o he 0.9Nd
3+
–
20LaF
3
:80SiO
2
xe ogel measu ed in ai . Weigh loss occu s
in ou di e en s eps. The i s s ep, a ound 25–100 1C, is
associa ed wi h an endo he mic p ocess a 76.7 1C, co es-
ponding o he elimina ion o wa e and adso bed e hanol,
con ined in la ge po es. The OH g oups o he silica ne wo k
gene a e hyd ogen b idges, inc easing he ene gy equi ed
o deso b he e hanol and wa e .
23
The second s ep, be ween
100–200 1C, is usually assigned o he e apo a ion o sol en s
abso bed in small po es. The combus ion o emaining o ganic
ma e ial is esponsible o he hi d s ep, occu ing in he ange
250–500 1C, along wi h inal elimina ion o apped sol en s
and wa e molecules in nanopo es up o 500 1C.
24
A sha p
exo he mic peak a 607 1C appea s associa ed wi h he inal
weigh loss e en , co esponding o he c ys allisa ion o LaOF,
as con i med by XRD (no shown).
The s uc u al cha ac e isa ion o LaF
3
nanopa icles
inco po a ed in he 0.9Nd
3+
–20LaF
3
:80SiO
2
pa icula e sol was
analysed by HRTEM. Fig. 6a shows he HRTEM image o
he 0.9Nd
3+
–20LaF
3
:80SiO
2
sample ea ed a 450 1C o 6 h,
showing well-dispe sed nanopa icles. The a e age pa icle size
is B16 nm acco ding o he size dis ibu ion gi en in Fig. 6b.
This pa icle size is consis en wi h ha de e mined by XRD
and HRTEM (Fig. 2 and 3). Fu he mo e, he LaF
3
nanopa icles
main ained hei o al shape a e inco po a ion in o he silica
Fig. 3 HRTEM images o (a) Nd
3+
–LaF
3
–0.04 M powde and (b) Nd
3+
–
LaF
3
–PVP(10)–0.25 M powde (c) single nanoc ys al showing a la ice
spacing o 0.325 nm o sample 0.9Nd
3+
–LaF
3
–0.25 M–PVP(10) (d) FFT
pa e n co esponding o (c) and (e) EDX analysis o he egion.
Fig. 4 Va ia ion o he iscosi y o he 0.9Nd
3+
–80SiO
2
:20LaF
3
pa icu-
la e sol as a unc ion o aging ime up o he geli ica ion poin .
Fig. 5 TGA and DTA analysis o he 80 1C–12 h d ied 80SiO
2
:20LaF
3
pa icula e sol.
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sol. Fig. 6c shows a plana dis ance o 0.32 nm co esponding
o he (101) plane o he nanoc ys alline pa icles, simila o
ha o he 0.9Nd
3+
–LaF
3
nanopa icles be o e hei inco po a-
ion in o he silica sol. Fig. 6d con i ms he p esence o Nd
accompanying La and F, measu ed by STEM analysis.
I is con i med ha no new c ys allisa ion p ocess occu s,
and ha he shape and size o he LaF
3
nanopa icles a e no
affec ed ei he by inco po a ion in o he silica sol o by he hea
ea men a 450 1C o 6 h. Inco po a ion in he silica ma ix does
no p oduce agglome a ions o g ow h o he nanopa icles.
Quan i a i e Rie eld e inemen o he 0.9Nd
3+
–20LaF
3
:80-
SiO
2
glass-ce amic ea ed a 450 1C o 6 h p o ided a weigh
ac ion o 5.2 w % o c ys alline phase (Fig. 7). Conside ing he
low c ys alline ac ion ob ained o he nanopowde s o LaF
3
(13%), i is likely ha inco po a ion in he silica sol and u he
he mal ea men inc eases he a e age pa icle size and
imp o es c ys allisa ion o he NPs, leading o a highe ela i e
c ys alline ac ion.
Op ical cha ac e isa ion o Nd
3+
:LaF
3
nanopa icles be o e and
a e hei inco po a ion in a silica ma ix
The nea -in a ed emission spec a co esponding o he
4F
3/2
-4I
11/2
lase ansi ion we e ob ained o all samples
a oom empe a u e by exci ing a 786 nm in he 4I
9/2
-4F
5/2
abso p ion band. Fig. 8a shows he emission spec um o he
0.9Nd
3+
–LaF
3
nanopa icle suspension ea ed a 80 1C, oge he
wi h he expe imen al decay o he 4F
3/2
le el. The emission
spec um shows simila spec al ea u es o he Nd
3+
emission o
he LaF
3
c ys al. Howe e , he expe imen al decay o he 4F
3/2
le el
ob ained unde exci a ion a 786 nm wi h collec ion
o luminescence a 1064 nm shows a di e en beha iou .
The decay de ia es om a single exponen ial unc ion and he
li e ime is unexpec edly sho . The a e age li e ime.
hi
¼Ð1
0 I ðÞd
Ð1
0I ðÞd
whe e I( ) ep esen s he luminescence in ensi y a ime
co ec ed o he backg ound, is educed o 61 ms.
On he o he hand, he nea -in a ed emission spec um o
0.9Nd
3+
–LaF
3
nanopa icles ea ed a 450 1C o 6 h (Fig. 8b)
shows a se ies o peaks wi h emissions o compa able in ensi y,
a ound 1047 and 1064 nm. The spec al ea u es co espond
o he emission o Nd
3+
ions in LaF
3
,
25
which con i ms he
inco po a ion o he a e-ea h ion in he LaF
3
NPs. Mo eo e ,
he luo escence decay cu e o he 4F
3/2
le el ob ained unde
Fig. 6 (a) HRTEM image o 0.9Nd
3+
–LaF
3
nanopa icles in he 0.9Nd
3+
–
80SiO
2
:20LaF
3
sol. (b) Dis ibu ion o nanoc ys al size o he nanopa icles
in he sol. (e) EDX o he co esponding 0.9Nd
3+
–LaF
3
nanopa icle and i s
scanning lines.
Fig. 7 Obse ed ( ed ci cles), calcula ed (con inuous black line) and
diffe ence (con inuous blue line) X- ay diff ac ion p o iles o 0.9Nd
3+
–
80SiO
2
:20LaF
3
pa icula e sol powde s ea ed a 450 1C o 6 h wi h 5%
NaF as in e nal s anda d. The B agg peaks o NaF and LaF
3
a e indica ed by
op and bo om e ical ba s, espec i ely.
Fig. 8 (a) Emission spec um o 0.9Nd
3+
–LaF
3
nanopa icles ea ed a
80 1C and co esponding semi-loga i hmic plo o he expe imen al decay
unde exci a ion a 786 nm collec ing he luminescence a 1064 nm. (b)
Emission spec um o 0.9Nd
3+
–LaF
3
nanopa icles ea ed a 450 1C o
6 h ob ained unde exci a ion a 786 nm and co esponding semi-
loga i hmic plo o he expe imen al decay ob ained a e exci a ion a
786 nm collec ing he luminescence a 1064 nm.
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exci a ion a 786 nm on collec ing he luminescence a 1064 nm
can be desc ibed o a good app oxima ion by a single exponen-
ial unc ion wi h a li e ime o 528 ms, which is close o he
li e ime epo ed o Nd
3+
in a LaF
3
c ys al (522 ms). These
esul s unambiguously con i m he inco po a ion o Nd
3+
ions
in he LaF
3
NPs.
The li e ime o NPs ea ed a 80 1C is nea ly one o de o
magni ude sho e han ha obse ed in he NPs hea ea ed
o 6 h a 450 1C. This s ong quenching o he li e ime could
be ela ed o he p esence o agg ega ed LaF
3
nanopa icles;
howe e , his is no app ecia ed in he HRTM mic og aphs.
The mos p obable explana ion is ha NPs ea ed a 80 1C
s ill con ain a high pe cen age o PVP, which is elimina ed on
ea men a 450 1C.
Fo he 0.9Nd
3+
–20LaF
3
:80SiO
2
sample ea ed a 450 1C o
6 h, bo h he emission spec um and he li e ime (Fig. 9)
co espond o hose ob ained o LaF
3
NPs wi h he same
ea men . This esul ag ees wi h he s uc u al cha ac e isa-
ion, which indica es ha LaF
3
nanopa icles a e no a ec ed
by hei inco po a ion in he silica sol, main aining hei high
e iciency as ac i e phase.
On he o he hand, emission measu emen s pe o med in
0.9Nd
3+
–LaF
3
:SiO
2
glass-ce amics wi h h ee di e en LaF
3
concen a ions (5, 10 and 20%), ea ed a 450 1C o 6 h,
con i med ha he inc ease in he c ys alline ac ion esul s in
a nea ly linea inc ease o he emission in ensi y (Fig. 10).
This p omising inno a i e p ocess which p oduces op ically
ac i e oxy luo ide glass-ce amics is unde u he in es iga ion
o p oduce op ically efficien anspa en glass-ce amics coa -
ings wi h wide and di e se applica ions.
Conclusions
S able Nd
3+
-doped LaF
3
nanopa icle suspensions wi h pa icle
size B16 nm, we e ob ained using a a he simple p ecipi a-
ion p ocess. The inco po a ion o PVP as dispe san inc eases
he s abili y o he LaF
3
NP suspensions.
The inco po a ion o Nd
3+
ions in o he LaF
3
nanopa icles,
as well as in SiO
2
–LaF
3
powde s, was con i med by HRTEM,
STEM and op ical cha ac e isa ion. The luminescence spec a
o Nd
3+
-doped LaF
3
nanopa icles ea ed a 450 1C o 6 h
con i ms he inco po a ion o Nd
3+
ions in he nanopa icles
and he comple e elimina ion o PVP a e sui able hea ea -
men . A li e ime o 528 ms was de e mined, simila o ha o
Nd
3+
in pu e LaF
3
c ys als.
Mo eo e , s able SiO
2
–LaF
3
sols wi h and wi hou Nd
3+
we e
p epa ed by mixing he LaF
3
NP suspension wi h a silica sol, up
o a maximum mola a io o 20% LaF
3
NPs. To he bes o ou
knowledge, his is he i s ime ha Nd
3+
NPs inco po a ed
in o a glass-ce amic has been p oduced by his ou e.
Rie eld e inemen indica ed ha LaF
3
NPs p esen a
c ys alline ac ion o 13%, likely ela ed o a high pe cen age
o amo phous ac ion. Howe e , he ela i e c ys allised ac-
ion in he silica ma ix wi h 20% o NP de e mined by he same
me hod inc eases o B5%.
The Nd
3+
–LaF
3
nanopa icles a e no a ec ed by hei inco -
po a ion in o he silica sol, main aining hei composi ion,
shape and size. The luminescence esponse o 0.9Nd
3+
–
20LaF
3
:80SiO
2
glass-ce amic ea ed a 450 1C o 6 h con i ms
he e iciency o he ac i e phase.
Con lic s o in e es
The e a e no con lic s o decla e.
Acknowledgemen s
The au ho s acknowledge he inancial suppo om MINECO
unde p ojec s MAT2017-87035-C2-1-P/-2-P (AEI/FEDER, UE),
Basque Coun y Uni e si y GIU17/014 and Basque Go e nmen
PIBA2018-24. This a icle is a pa o he dissemina ion ac i -
i ies o he p ojec FunGlass, which has ecei ed unding om
he Eu opean Union’s Ho izon 2020 esea ch and inno a ion
p og am unde g an ag eemen no. 739566. The au ho s also
acknowledge he suppo o he T ansmission Elec on Mic o-
scope (TEM) se ice o he Ins i u e o Ca alysis and Pe ochem-
is y (CSIC).
No es and e e ences
1 G. Go ni, M. J. Pascual and A. Caballe o, Effec o p ocessing
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Fig. 9 Emission spec um o 450 1C–6 h 0.9Nd
3+
–80SiO
2
:20LaF
3
sol and
i s co esponding semi-loga i hmic plo o he expe imen al decay unde
exci a ion a 786 nm collec ing he luminescence a 1064 nm.
Fig. 10 Emission spec um o 450 1C–6 h 0.9Nd
3+
–SiO
2
:LaF
3
GCs wi h
h ee di e en c ys alline ac ions 5% (black), 10% ( ed) and 20% (blue)
ob ained unde 786 nm exci a ion.
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