Syn hesis and applica ion o laye ed i ana es in he pho oca aly ic
deg ada ion o phenol
S e lana I ano a*,Anna Penko a, Ma ía del Ca men Hidalgo, José An onio Na ío,
F ancisca Rome o-Sa ia, Miguel Ángel Cen eno, José An onio Od iozola
Depa amen o de Química Ino gánica e Ins i u o de Ciencia de Ma e iales de Se illa
(ICMS), Cen o mix o CSIC-Uni e sidad de Se illa, A da. Ame ico Vespucio 49,
41092 Se illa, Spain
E-mail: [email p o ec ed]
Abs ac
This s udy p oposes a di ec syn he ic ou e o single i ana e shee s h ough he
mild and e sa ile condi ions o he “chimie douce”. The s ages o he p oduc ion
include he complexa ion o he i anium alkoxide p ecu so by benzoic acid, he
o ma ion o i anium oxo-clus e s and hei con olled ans o ma ion in o single shee
i ana es du ing he hyd olysis s age. The esul ed ma e ial appea s o be an excellen
p ecu so o sel -o ganized TiO2 nano ubes o ma ion which p esen s an excellen
ac i i y as pho oca alys in he pho o-deg ada ion o phenol.
Keywo ds: i ana es, sel o ganized TiO2 nano ube laye s, pho oca aly ic deg ada ion o
phenol
In oduc ion
O e he pas decades, he TiO2 de i ed nanosized ma e ials ha e been widely
in es iga ed o as applica ions, including gas senso s, pho oca alys s, sola cells and
biomedical applica ions [1-5]. Mo e p ecisely, he TiO2 based nano ubes (TNT) ha e
been ex ensi ely s udied since hei peculia p ope ies, like high speci ic a ea, ion-
exchange abili y and pho oca aly ic ac i i y [6]. Cu en ly de eloped me hods o TNT
p oduc ion include he assis ed- empla e me hod [7,8], he elec ochemical anodic
oxida ion [9,10] and hyd o he mal ea men [11-14]. Howe e , all hese me hods su e
some d awbacks, like complica ed ab ica ion p ocess in he case o empla e me hod,
limi a ion o mass p oduc ion and high ab ica ion cos s o he elec ochemical me hod
and long eac ion du a ion and di icul ies in achie ing he uni o m size nano ubes
h ough he hyd o he mal ea men . Nowadays, he mos popula me hod emains he
hyd o he mal ea men o TiO2 ana ase in NaOH solu ions, i s ly epo ed by Kasuga
e al. [11].
The laye ed i ana es ha e ecen ly ecei ed a g ea a en ion due o hei
in e es ing in e laye chemis y, high abili y o ion exchange/in e cala ion eac ions and
po en ial applica ions in he syn hesis o new nanoma e ials. Mo eo e , hey can
unde go ex olia ion/delamina ion upon in e cala ion o he bulky o ganic molecules,
p oducing single shee s wi h dis inc i e 2D mo phology, which a e e y a ac i e
building blocks o p oducing a i icial a chi ec u es, hyb id mul ilaye s o mic opo ous
ma e ials [15].
The only known me hod o he syn hesis o laye ed i ana es is he solid-solid
eac ion a high empe a u e be ween TiO2 and an alkali me al ca bona e, ollowed by
acid exchange eac ion o p oduce he p o ona ed o m HxTi2-x/4□x/4O4.H2O[16,17].
On he o he hand, he e ogeneous pho oca alysis has ound inc easing in e es as
a p omising echnique o wa e and ai emedia ion being an en i onmen al iendly
and sus ainable echnology [18]. So a , TiO2 in i s o m ana ase o ana ase- u ile
mix u es has been he mos widely s udied ma e ial o be used as pho oca alys .
Howe e , his ma e ial s ill p esen s some d awbacks, such as he high deg ee o
ecombina ion o pho ogene a ed cha ges which limi he e iciency o he
pho oca aly ic p ocesses. Thus, one in e es ing challenge nowadays is he design and
de elopmen o ailo ed nanos uc u ed ma e ials o o e come hese limi a ions. In his
con ex , TiO2 nano ubes due o hei 1D geome y may allow a as e and sho e cha ge
anspo o he su ace which would lead o less ecombina ion a e and consequen ly o
he imp o ing in he o e all e iciency o he ma e ial [19].
This s udy p oposes a di ec syn he ic ou e o ex olia ed laye ed i ana es and
hei con e sion in o sel -o ganized TiO2 nano ubes ei he by a simple con ac wi h
sodium hyd oxide solu ion o by a hyd o he mal syn hesis. The pho oca aly ic ac i i y
o he p oduced ma e ials is es ed by ollowing he pho oca aly ic deg ada ion o
phenol, chosen as model eac ion.
Expe imen al
Modi ied sol-gel syn hesis
In a ypical syn hesis, 1 mol o i anium (IV) bu oxide (Sigma-Ald ich ASC
97%), and 4 mol o benzoic acid (Sigma-Ald ich ASC 99.5%), we e added o 5 mol o
absolu e e hanol (P olabo, 100%) and s i ed unde hea ing up o 92ºC un il he
homogeniza ion o he solu ion. A e ha , 8 mol o dis illed wa e o he hyd olysis
s ep was added d opwise and main ain unde igo ous s i ing o 3 h. The ob ained
slu y was d ied o e nigh a 80ºC. The excess o o ganic compounds was emo ed
om he esul ing solid by Soxle ex ac ion du ing 24h using e hanol as sol en . The
inal p oduc was hen d ied a oom empe a u e. The sample will be labelled as LT.
A po ion o he esul ing solids was imme sed in a 0.1 mol L-1 HCl solu ion o
24 hou s in o de o assu e he p o onic exchange, washed abundan ly wi h dis illed
wa e o neu al pH and inally d ied a oom empe a u e. The sample is labelled as H-
LT.
Ti-nano ubes syn hesis
The p oduc , ob ained a e he modi ied sol-gel me hod (LT), was dispe sed in
an aqueous solu ion o NaOH (7 mol L-1) and s i ed o 24 hou s, washed wi h dis illed
wa e and d ied a 80ºC. The as p epa ed sample is called Na-TNT (S).
In a simila way, suspension o he same sol gel p oduc (LT) and 7 mol L-1
NaOH was submi ed o au ogeneous p essu e a 130ºC o 24h, and a e , washed wi h
dis illed wa e and d ied a 80ºC. The as p epa ed sample is called Na-TNT (HT).
Finally Na-TNT(S) and Na-TNT(HT) solids we e imme sed in a 0.1 mol L-1 HCl
solu ion o 24 hou s, washed abundan ly o neu al pH and inally d ied a oom
empe a u e. The samples p oduced a e labelled espec i ely H-TNT (S) and H-TNT
(HT).
Cha ac e iza ions
X- ay di ac ion (XRD) analysis was pe o med on an X’Pe P o PANaly ical.
Di ac ion pa e ns we e eco ded wi h Cu K adia ion (40 mA, 45 kV) o e a 2Θ-
ange o 3 o 60º and a posi ion-sensi i e de ec o using a s ep size o 0.05º and a s ep
ime o 80 s.
X- ay di ac ion (XRD) analysis a high empe a u e was pe o med in a high
empe a u e came a An on Paa HTK 1200 coupled wi h an X'Pe P o Philips
di ac ome e , equipped wi h X'Cele a o de ec o wi h an opening o 2.18°, a s ep o
0.05° and an equi alen ime acquisi ion o 30 s. The di ac og ams we e aken e e y
50 °C in he 50 o 600º C empe a u e ange, o e a 2 - ange o 3 o 60º in low o
syn he ic ai .
The Raman spec a we e eco ded on a dispe si e Ho iba Jobin Y on LabRam
HR800 mic oscope wi h a 20 mW He-Ne g een lase (532.1 nm), wi hou il e and wi h
a 600 g mm-1 g a ing. The mic oscope used a 50x objec i e wi h a con ocal pinhole o
1000 μm.
BET su ace a ea and po osi y measu emen s we e ca ied ou by N2 adso p ion
a 77 K using a Mic ome i ics ASAP 2010 ins umen .
T ansmission elec on mic oscopy (TEM) obse a ions we e ca ied ou in a
Hi achi H 800 mic oscope ope a ing a 200 kV. The samples we e dispe sed in e hanol
by sonica ion and d opped on a coppe g id coa ed wi h a ca bon film.
Pho oca aly ic expe imen s
The e alua ion o he pho oca aly ic ac i i y was pe o med by using he
pho ooxida ion o phenol as model eac ion.
Suspensions o he samples (1 g.L-1) in phenol solu ion (25 ppm) we e placed in a
200 ml py ex discon inuous ba ch eac o en eloped by an aluminum oil and
illumina ed h ough a UV- anspa en Plexiglas® op window ( h eshold abso p ion a
250 nm) by an Os am Ul a-Vi alux lamp (300 W) wi h sun-like adia ion spec um and
a main line in he UVA ange a 365 nm. The in ensi y o he inciden UVA ligh on he

solu ion was de e mined wi h a PMA 2200 UVA pho ome e (Sola Ligh Co.) being
ca. 140 W. m-2.
Magne ic s i ing and a cons an oxygen low as oxida i e agen we e used o
p oduce a homogeneous suspension o he ca alys in he solu ion. P io illumina ion,
ca alys -subs a e equilib a ion was ensu ed by s i ing he suspension 20 minu es in he
da k. Phenol concen a ion was ollowed by HPLC echnique (Agilen Technologies
1200) equipped wi h UV-Vis de ec o using Elipse XDB-C18 column (5 µm, 4.6 x 150
mm). Mobile phase was wa e /me hanol (65:35) a a low a e o 0.8 ml.min-1.
Blank expe imen s we e pe o med in he da k as well as wi h illumina ion and no
ca alys , wi hou obse able change in he ini ial concen a ion o phenol in bo h cases.
Resul s and discussion
Be o e s a ing he discussion o he esul s, one p ecision should be made. In
gene al, he subsequen ea men o all he samples by acid solu ions does no p o oke
any changes in mo phology, shape and/o c ys allini y. Tha is why only he
cha ac e iza ion o he p o ona ed o ms is p esen ed h ough he whole manusc ip . The
only di e ence esul s in he speci ic su ace a ea and po e olume as p esen ed in
Table 1.
The esul s o he speci ic su ace a ea measu emen s show ha he ea men o
he sol-gel p oduc sample (LT) wi h sodium hyd oxide solu ion does no change
excessi ely he ini ial su ace a ea. Howe e , he subsequen acid ea men and mo e
p ecisely he abundan washing ill neu al pH p o oke a signi ican inc ease o he
su ace a ea caused by po e libe a ion. Ne e heless he applied Soxle ex ac ion in
e hanol o he o ganic phase, he p esence o Na-benzoa es and/o benzoic acid es s
could be imagined as esponsible o po e illing in o he non-acid ea ed samples. The
con ac wi h acid solu ions p oduce mesopo ous ma e ials wi h a he high speci ic
su ace a ea neighbou ing 200 m2 g-1.
The XRD pa e n o he syn hesized p o ona ed sol-gel p oduc (H-LT) is
p esen ed in Figu e 1.
The ob ained e lec ions indica e he p oduc ion o lamella s uc u e which can
be indexed as (0k0) body cen e ed o ho hombic i ana e lepidoc oci e like s uc u e. In
addi ion, he asymme ic line shape wi h a ail owa d highe angle di ac ions, in he
010 e lec ion peak, was epo ed by Sasaki e al. [20] as ypical o a wo-dimensional
la ice, poin ing o he p oduc ion o 2 D ne wo k o he laye ed i ana es.
A mo e de ailed s udy o he H-LT pa e n indica es he p esence o an
amo phous phase, which could be in e p e ed ei he as a combina ion o sca e ing
be ween wa e and/o o ganic molecules (benzoa es) in be ween he i ana e shee s o by
he p esence o low c ys alline i ania ana ase phase which majo di ac ion appea s a
2 ~ 25 (indica ed wi h he c oss symbol in Figu e 1). Fu he mo e, he di ac ion lines
a a ound 2 ~ 8 and 17º ( iangles) speci ied also he p esence o an addi ional phase
iden i ied as c ys alline benzoic acid, sugges ing ha some o ganic le o e s (mo e
likely benzoa es) s ill exis a e he Soxle ex ac ion and ha he acid ea men o he
LT sample esul s, among o he s, in he con e sion o he benzoa es o benzoic acid and
i s c ys alliza ion.
On he basis o XRD da a, he in e laye dis ance o 1.6 nm is measu ed. This
alue is signi ican ly highe han he alue o 0.93 nm p e iously epo ed o
p o ona ed i ana es [21], indica ing he benzoic acid in e cala ion be ween he i ana e
laye s. Using he model p oposed by Sasaki e al. [20], and aking in o accoun he size
o he benzoic acid molecule, he calcula ed dis ance be ween he i ania shee s anges
be ween 1.3 and 1.8 nm depending on he p esence o absence o wa e molecules
in e cala ion. I is impo an o men ion, ha he p esence o la ge excess o benzoic
acid, as in ou case, sugges s s ongly an agg ega ion o he a oma ic ings h ough π- π
s acking, which is also p omo ed by he p esence o unc ional g oups, like ca boxylic
one. This phenomenon could explain he inc ease o he in e laye dis ance, hos ing
mo e han one laye o benzoic acid. The desc ibed abo e sugges s a i s place he
p esence o benzoic acid and also con i ms he s abiliza ion o he 2D la ice h ough he
in e laye benzoic acid/es e s in e cala ion (Figu e 2).
As o he TNT samples, bo h pa e ns ea u e he XR Di ac ion peak a 2 ~
10, which is a diagnos ic o he galle y spacing o he laye ed i ana es (Figu e 3). The
di ac ions o he bo h samples can be assigned as e lec ions o he basal in e -laye
(0k0) se ies and in-laye (200) o he lepidoc oci e ype i ana e [22, 23]. The c ys al
s uc u e o i ania nano ubes is s ill unde deba e, al hough mos s udies ag eed on he
chemical composi ion (NaxH2-xTi3O7 and NaxH2-xTi2O4(OH)) a ibu ing he i ania
nano ubes o he laye ed i ana es g oup [6].
The sample p epa ed by hyd o he mal ea men (H-TNT (HT)) appea s o
p esen highe c ys allini y han he co esponding sample p epa ed by s i ing (H-
TNT(S)) ega ding o he sha pness o he di ac ion peaks. Fo all p o ona ed samples,
a XRD s udy as a unc ion o he empe a u e in he p esence o s a ic ai was ca ied
ou in o de o ollow he modi ica ion and he phase ansi ions. As expec ed, he
laye ed i ana es (H-LT) con e s apidly a empe a u e as low as 200 ºC o he
he modynamically s able ana ase o m o i ania and main ains such phase in he whole
ange o s udied empe a u es (up o 600 ºC, di ac og ams no shown). Howe e , mo e
signi ican changes a e obse ed in he nano ubes p esen ing samples. The esul s a e
p esen ed in Figu e 4 and Figu e 5 o H-TNT(S) and H-TNT(HT), espec i ely.
The di ac og ams o he sample ob ained by s i ing, H-TNT(S), oge he wi h
he p esence o low c ys alline TNT, show he exis ence o a phase simila o he
NaTiO3 one (JCPDS 00-033-1295). I should be men ioned howe e , ha i s p o ona ed
analogue has no been ound in he da abase, bu should gi e a simila pa e n as he
sodium o m. This i ana e phase becomes he majo one a highe empe a u es,
indica ing he con e sion o he non-s acked lamella shee s in o i ana e. The mos
impo an esul is he he e ogenei y o his sample composed by a mix u e o TNT and
ee i ana e nanoshee s which ha e been unable o es ack inco po a ing he Na+, in he
condi ion o simple s i ing. In addi ion, he ana ase phase (JCPDS 00-021-1272) was
obse ed a empe a u es highe han 500 ºC. The di ac ion peak a a ound 2 49º,
a ibu ed o pu e TNT, does no show any change ill he inal empe a u e o he
ea men , when disappea s. Howe e , he di ac ion peak a a ound 2 ~ 10º shi s o
highes alues.
The nano ubes ob ained hyd o he mally, H-TNT(HT), do no p esen signi ican
phase ansi ion in he whole ange o s udied empe a u es. Howe e , as o he H-TNT
(S), he di ac ion peak a a ound 2 ~ 10º shi s o highe alues, sugges ing he
sho ening o he in e -shee spacing. This can be due, ei he o he con inuous
es acking o he i ana e nanoshee s, o mo e likely o he loss o he in e -shee wa e
con en [24], as he change occu s a low empe a u e (below 150ºC).
The lamina s uc u es (LT sample) a e clea ly isible by ansmission elec on
mic oscopy (Figu e 6 A,) and allow he measu e o he in e laye dis ance, ound o be
be ween 1.5 and 3 nm. The dis ance o 3 nm sugges s ha he sample consis s o
indi idual nanoshee s wi h no in e ac ion be ween hem, which con i ms he abili y o
he modi ied sol-gel me hod o p oduce single i ana e shee s.
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