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Applied Catalysis B: Environmental 55 (2005) 81–91
Photocatalytic degradation of azo dyes by organic-capped anatase
TiO2 nanocrystals immobilized onto substrates
R. Comparellia, E. Fanizzaa, M.L. Currib,*, P.D. Cozzolia, G. Mascoloc,R. Passinoc, A. Agostianoa,b
aDipartimento di Chimica, Universita di Bari, via Orabona 4, I-70126 Bari, ItalybCNR IPCF, Sez Bari c/o Dip. di Chimica, Universita di Bari,
via Orabona 4, I-70126 Bari, ItalycCNR-IRSA, Sez. Bari, Via F. De Blasio 5, I-70123 Bari, Italy
Received 18 June 2004; received in revised form 28 July 2004; accepted 30 July 2004
Available online 12 September 2004
Abstract
The UV-induced photocatalytic degradation of two azo dyes, Methyl Red and Methyl Orange, has been carried out in aqueous media in the
presence of oleic acid (OLEA)- and tri-n-octylphosphine oxide (TOPO)-capped anatase TiO2 nanocrystal powders (mean particle size: 6 nm)
deposited onto a quartz substrate. The progress of photodegradation was followed by combining UV–vis absorption measurements with
HPLC–MS analysis. The abatement efficiency for the two organic compounds was compared with that obtained with commercial TiO2 P25
Degussa by evaluating a few significant variables, such as the dye chemical structure, pH of the solution, and catalyst surface status.
Identification of several by-products by HPLC–MS analysis has allowed to propose a reasonable degradation pathway for both target
molecules. Significantly, although all titania catalysts were effective in removing both parent dyes and their related derivatives, the
degradation rate by the OLEA-capped TiO2 nanocrystals was double as that obtained with both its TOPO-capped analogous and TiO2 P25
Degussa. It is suggested that efficient catalysis strictly depends on microscopic mechanisms that occur at the catalyst surface, basically
involving specific dye adsorption and local density of terminal –OH moieties.
# 2004 Elsevier B.V. All rights reserved.
Keywords: Organic-capped nanocrystals; TiO2; UV-induced photocatalysis; Azo dyes
1. Introduction
In recent years, a new class of techniques devoted to
pollutant remediation, broadly referred to as advanced
oxidation processes (AOPs) [1,2] has emerged. AOP
methods are characterized by a common chemical action,
which basically relies on the primary reactivity of OH
radicals in driving oxidation processes, ultimately resulting
in the mineralization of a variety of pollutants.
Within AOPs, photocatalyst-based degradation methods
represent a very interesting branch of research in continuous
development. Band-gap photo-excitation of semiconductors
* Corresponding author. Tel.: +39 080 5442027; fax: +39 080 5442129.
E-mail address: m.curri@area.ba.cnr.it (M.L. Curri).
0926-3373/$ – see front matter # 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2004.07.011
generates [3–5] electron–hole pairs capable to attack the
organic matter either directly or indirectly by means of
highly reactive species deriving from their reaction with the
solvent and/or additives. In the case of TiO2, generally
recognized as one of the most efficient, non-toxic, and
inexpensive photocatalysts, most extensive studies have
been carried out by using the oxide powder suspended in
aqueous solution containing the target molecules [6]. More
recently, TiO2-based catalysts deposited onto suitable
substrates have become an attractive alternative to circum-
vent the technological difficulty as well as the high costs
related to the catalyst recovery. Unfortunately, the potential
of this approach has been limited by the inevitable reduction
of the overall surface active area associated to catalyst
immobilization, leading to significant loss of performances
R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–9182
[7,8]. In contrast to TiO2 Degussa P25, the most widely used
form of commercial titania photocatalyst, nanostructured
oxide crystals can be expected to compensate for such a
drawback due to the significant benefits associated to the
reduction of semiconductor particle size down to a few
nanometers [3–5]. First, the extremely high surface-to-
volume ratio in small particles scales with the surface
density of active sites available for substrate adsorption,
thus increasing the overall photoreaction rate. Further-
more, when the semiconductor nanocrystal size is com-
parable or smaller than the bulk exciton diameter, the band-
gap become size-dependent due to quantization effects [3],
making in principle possible to tune the electron–hole pairs
redox potentials and consequently achieve a certain
selectivity in the related photoreactions. Obviously, a
narrow size distribution is amenable in order to precisely
correlate the redox properties to the crystal size. Finally,
crystal defect concentration associated to trap states for
the photogenerated carriers may be low in high quality
nanocrystals, so that electron–hole separation can be
maximized [4,5].
For the purposes of the present work, surfactant-capped
anatase TiO2 nanocrystals with high crystallinity and with
a narrow size distribution have been used as photocatalysts.
The TiO2 nanocrystal powders were obtained by two
synthetic routes, being basically characterized by the type
of reaction, i.e. either hydrolytic or non-hydrolytic,
respectively, involved in titania nucleation and growth.
Therefore, TiO2 nanocrystals were prepared, which
essentially differed with respect to: (1) the sterical
hindrance offered by the passivating ligand, i.e. either
oleic acid or tri-n-phosphine oxide; (2) the nature of the
surfactant functional group binding to the crystal surface,
i.e. either –P=O or –CO�2 ; (3) the available surface density
of Ti–OH groups.
The obtained nanocrystals of a selected size have been
deposited onto transparent support in order to exploit them
in a photocatalytic process devoted to the photodegradation
of two azo dyes (Methyl Red and Methyl Orange). The
efficiency of the different types of nanocatalysts and that of
their equivalent commercial oxide have been compared
under the same conditions. The dye chemical structure, the
nature of catalyst surface, and pH were kept into account
in order to elucidate the catalytic process. HPLC–MS
analysis was used to identify the degradation by-product of
both target molecules and to gain an insight into the
possible decay pathways. Although the complete disap-
pearance of both parent dyes and of their related derivatives
occurred upon photocatalysis with all titania catalysts, the
oleic acid-capped TiO2 nanocrystals were found superior
catalysts than both their tri-n-phosphine oxide-capped
counterparts and TiO2 P25 Degussa. It is proposed that
efficient catalysis strictly depend on microscopic mechan-
isms that occur at the catalyst surface, basically involving
specific dye adsorption and local density of terminal –OH
moieties.
2. Experimental
2.1. Materials
Commercial TiO2 was TiO2 ‘‘Degussa P25’’ (non-porous
anatase; surface area, 50 m2 g�1; mean diameter, approxi-
mately 30 nm). Methyl Red (2-(4-dimethylamino-pheny-
lazo)-benzoic acid—C.I. 13020, MeRed), Methyl Orange
(Acid Orange 52—13025, MeOr), n-heptadecane (C7H16 or
HD, 99%), titanium tetrachloride (TiCl4, 99%), titanium
tetraisopropoxide (Ti(OPri)4 or TTIP, 98.9%), tri-n-octyl-
phosphine oxide ((C8H17)3PO or TOPO, 90%), trimethyla-
mino-N-oxide dihydrate ((CH3)3NO�2H2O or TMAO, 98%),
oleic acid (C18H33CO2H or OLEA, 90%), ethylene glycol
(C2H4(OH)2 or EG, 98%) were purchased from Aldrich.
Acetone, chloroform, methanol were of the highest purity
available and purchased from Aldrich. All aqueous solutions
were made by using twice-distilled water.
2.2. Catalyst preparation
Two different methods were applied to synthesis TiO2
nanocrystals.
2.2.1. Non-hydrolytic synthesis of TiO2 nanocrystals
All manipulation were performed using standard air-free
techniques. By this method, TOPO-capped TiO2 nanocrys-
tals were synthesised by reacting TiCl4 with TTIP at 300 8Cin a mixture of heptadecane and TOPO as surfactant, as
previously reported [9]. Briefly, two precursor stock
solutions, 1 M TiCl4 and 1 M TTIP in HD, respectively,
were prepared in a glove box. In a three-neck flask, a mixture
of TOPO and HD (6 g) was degassed at 100 8C for 1 h under
vacuum. The flask was then put under nitrogen flow and
1 mL of the TiCl4 stock solution was introduced into the
reactor. The temperature was raised to 300 8C at which 1 mL
of the TTIP solution was rapidly injected into the vessel to
nucleate titanium dioxide. The temperature was then
lowered to 250 8C and the nanocrystals growth was
completed in 10 min. The final nanocrystal size could be
tailored by varying the TOPO:Ti molar ratio in the starting
reaction mixture. TiO2 were collected by centrifugation after
adding an excess of MeOH and washed three times with
acetone to remove surfactant residuals. The obtained
nanocrystals were soluble in apolar solvents, such CHCl3or hexane, due to their non-polar surface coating (TOPO).
2.2.2. Hydrolytic synthesis of TiO2 nanocrystals
By this approach, OLEA-capped TiO2 nanocrystals were
obtained by slow hydrolysis of TTIP in technical grade
OLEA as both solvent and surfactant at low temperatures
(80–100 8C), as reported elsewhere [10]. In a typical
synthesis, 2–10 mmol of TTIP was added to 35 g of
previously degassed OLEA under nitrogen flow at 100 8Cand allowed to stir for 5 min. A 4–10 mmol of anhydrous
TMAO in 3.2–6.4 g (50–100 mmol) of EG was subsequently
R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–91 83
added. The solution was maintained in a close system and
stirred at 100 8C over 72 h. The reaction mixture appeared
clear during the whole period of the particle growth.
Hydrolysis of the titanium precursor occur upon reaction
with water released slowly upon esterification of OLEA and
EG. The final nanocrystal size could be tailored by varying
the TTIP content in the initial reaction mixture. The TiO2
nanocrystals were readily precipitated upon addition of an
excess of ethanol to the reaction mixture at room
temperature. The resulting precipitate was isolated by
centrifugation and washed three times with ethanol to
remove surfactant residuals. At this stage, the OLEA-coated
TiO2 nanoparticles were easily redispersed in non-polar
solvents, due to their non-polar surface coating (OLEA).
The TiO2 powders used for routine XRD and FT-IR
analysis were prepared by washing the precipitate repeat-
edly and evaporating the residual solvent under vacuum at
room temperature. Finally, there were subjected to the same
thermal treatments as used for the powder employed in the
photocatalytic experiments.
2.3. Catalyst characterization
FT-IR spectra of TiO2 powders were collected with a
Perkin Elmer Spectrum GX FT-IR Spectrometer with a
resolution of 4 cm�1. Measurements were performed with
pressed pellets which were made using KBr powder as
diluent.
XRD patterns were collected with a Philips PW1729
diffractometer in a conventional u–2u reflection geometry
using filtered Cu Ka radiation (l = 1.54056 A). For XRD
measurements the nanocrystal powder was placed on an Al
sample holder.
Transmission electron microscopy (TEM) images were
obtained using Philips EM 430 microscope operating at
300 kV. The samples for the analysis were prepared by
dropping dilute solutions of TiO2 nanocrystals onto 400-
mesh carbon-coated copper grids and leaving the solvent to
dry. The samples were stable under the electron beam and
did not degrade within the typical observation times.
2.4. Photocatalytic degradations
All photocatalytic experiments were assisted by catalysts
deposited onto a quartz slide by casting from either an
Table 1
Chemical structures of the dyes employed as target substrates in the TiO2-assist
Organic dye Chemical structure
Methyl Red (MeRed)
Methyl Orange (MeOr)
optically clear CHCl3 solution of organic-capped TiO2
nanocrystals, or a CHCl3 suspension of TiO2 P25 Degussa.
Typically, 0.1 mmol of TiO2 was spread over a support area
of 3.6 cm2. After deposition, the resulting films were
thermally treated at 150 8C for 20 min to improve catalyst
adhesion to the substrate. The transparent support was of
suitable shape in order to fit a 1 cm � 1 cm quartz cell and
positioned against the cuvette wall, which was further with
respect to light beam. The radiating source was a medium
pressure 200 W mercury lamp (l > 250 nm). The system
was arranged in a suitable geometry in order to monitor in
situ the reaction course by UV–vis spectroscopy. All
experiments were performed under ambient atmosphere
keeping the system under vigorous stirring.
MeRed and MeOr, two azo dyes, were chosen as target
compounds (Table 1). The initial concentration of the dyes
(whose respective pKa values are reported in Table 1) was
typically 3 � 10�5 M. The desired pH was obtained by
adding the proper amount of HCl or NaOH 0.1 M.
2.5. UV–vis and HPLC–MS analysis
Dye decoloration was monitored by using an Ocean
Optics UV–vis diode array spectrophotometer equipped
with an optical fibre and a deuterium lamp. The
determination of the concentration of the dyes and the
identification of their respective by-products were per-
formed by HPLC-UV-MS using a Varian 9012 chromato-
graphic system equipped with a Ultimate UV detector (LC-
Packing Dionex), set at 425 and 220 nm, interfaced to a
QSTAR hybrid Qq-TOF mass spectrometer (Applied
Biosystem/MSD Sciex, Canada) equipped with a turbo
ion spray interface. Samples, injected by a Gilson 234
autosampler equipped with a 9010 Rheodyne valve and a
40 ml loop, were eluted at 0.4 ml/min through a Luna
phenyl-hexyl column (3 mm, 150 mm � 3 mm) and pre-
column (Phenomenex) with the following gradient: from 5/
25/70 (ammonium acetate 50 mM in methanol/methanol/
water) to 5/75/20 in 10 min, which was then maintained for
5 min. The interface conditions were, for positive and
negative ion mode, respectively, as follows: nebulizer gas
(air) = 1.2 L/min, curtain gas (nitrogen) =1 L/min,
turboionspray gas (nitrogen at 350 8C) = 6 L/min, needle
voltage = 5000 and �4400 V, orifice declustering potential
= 40 and �40 V and focusing potential = 150 and �120 V.
ed photocatalysis
Catalyst-interacting functionality pK
–COOH 5.3
�SO�3 3.8
a
R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–9184
The flow from the HPLC-UV was split to allow 200 ml/min
to enter the turbo ion spray interface.
Fig. 2. FT-IR spectra in the 3500–400 cm�1 region of (A) OLEA-capped
and (B) TOPO-capped TiO2 nanocrystals.
3. Results and discussion
3.1. Structural and surface characterization of the
catalysts
The TiO2 nanocrystal powders employed in the present
photocatalytic experiments were obtained by two different
synthetic routes. The first approach, based on the high
temperature (300 8C) reaction of molecular organometallic
precursors (TiCl4 and Ti(OPri)4) in anhydrous heptadecane
(HD), a non-coordinating solvent, in the presence of tri-n-
phosphine oxide (TOPO), a strong surfactant and size-
regulating agent, yields TOPO-capped TiO2 nanoparticles.
Due to the non-hydrolytic reaction pathway involved in
titania nucleation, the as-prepared oxide nanocrystals
possess a negligible coverage of surface hydroxyl groups
[9]. This material will be referred to as nh-TiO2. The second
approach [10] consists of the low-temperature (100 8C)
base-catalyzed hydrolysis reaction of Ti(OPri)4 in oleic acid
(OLEA) as both solvent and surfactant. The as-obtained
OLEA-capped TiO2 nanoparticles are obviously character-
ized by a high surface density of surface hydroxyl groups,
and thus referred to as h-TiO2.
In Fig. 1 a representative X-ray diffraction (XRD) profile
of the as-synthesized nanocrystals is reported. Irrespective
of the synthetic method, the peak positions and their relative
intensities could be exclusively indexed by the known
standard TiO2 anatase pattern. Notably, the characteristic
line broadening of the diffraction peaks pointed to nanosized
crystalline domains. The average grain size, estimated to be
about 6.0 nm by applying the Debye–Sherrer formula, was
in good agreement with the mean particle size measured by
Fig. 1. XRD pattern of organic-capped 6 nm TiO2 nanocrystal powders
prepared by hydrolytic method (OLEA 35 g, TTIP 5 mmol, 2 M TMAO
5 mL). In the column diagram, the standard diffraction peaks of TiO2
anatase are reported.
transmission electron microscopy. The moderate thermal
treatment applied to improve the stability of catalyst film
induced no significant grain growth or phase transition.
TEM images (data not shown) confirmed the approximately
spherical morphology of the nanoparticles and their almost
perfect crystallinity, in agreement with literature results
[9,10].
The nature of the organic coating on the surface of the
TiO2 nanocrystals was investigated by FT-IR spectroscopy
and interpreted on the basis of pertinent studies on the
surface functionalization of titania [11,12]. In Fig. 2 the
typical IR spectra in the region 3250–400 cm�1 of the
thermally treated organic-capped TiO2 nanocrystals are
reported (Fig. 2A: OLEA-capped TiO2; Fig. 2B: TOPO-
capped TiO2). Above 2000 cm�1, both samples exhibit the
intense antisymmetric and symmetric C–H stretching
vibrations (at about 2920 and 2850 cm�1, respectively) of
the –CH2 groups in the hydrocarbon moiety. The shoulder at
2950–60 cm�1 can be associated with the asymmetric
stretching of the terminal –CH3 group of the alkyl chains.
Notably, for the OLEA-capped TiO2, a weak but definite
band at 3008 cm�1 is also present, attributable to the olefinic
C–H stretching. In addition, the C–H stretching signals in
the TiO2 sample are superimposed on an underlying surface
O–H broad stretching band centred at 3300 cm�1, being due
to titanol groups in the OLEA-capped materials, and
adsorbed H2O from the non-anhydrous solvent used in the
post-synthesis extraction procedures. Below 2000 cm�1, the
COO� antisymmetric and symmetric stretching vibrations
(two characteristic bands centred at 1520 and 1436 cm�1,
respectively) of carboxylate anions complexed with surface
Ti centers dominate in the spectra of the OLEA-capped TiO2
nanocrystals (Fig. 2A). From the value of their frequency
difference (Dn � 84 cm�1), the mode of binding of
carboxylate adsorbates onto the TiO2 surface might be
interpreted as being chelating bidentate. The lack of clear
R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–91 85
evidence for the free C=O stretching band at around 1650–
1720 cm�1 (cf. 1775 cm�1 for OLEA in the vapor phase)
seems to exclude the presence of both unionized OLEA
monomers and dimers possibly [13] having the C=O
involved in H-bonding with a Ti–OH2+ surface group.
Moreover, the expected weak contributions of the –CH2–
bending (�1450 cm�1), of the C–O–H bending
(�1410 cm�1) and of the C–OH stretching (�1280 cm�1)
bands cannot be unambiguously discerned, owing to the
possible coincidence of many signals. In the fingerprint
region (below 1300 cm�1), the broadness and the complex-
ity of the peaks make their unique interpretation quite
difficult. However, a weak P=O stretching centred at about
1240 cm�1 visible in pure TOPO is still present in the
spectrum of our TOPO-modified TiO2 sample (Fig. 2B).
Below 950 cm�1, the characteristic vibrations of the
inorganic Ti–O–Ti network in titanium dioxide can be seen.
To summarize, the titania catalysts employed in the
present photocatalytic experiments essentially differed with
respect to: (1) the chemical nature of the surface passivating
ligand, basically related to the different binding mode of the
–P=O and –CO�2 functionalities in TOPO and OLEA,
respectively; (2) the different sterical hindrance offered by
the outermost exposed alkyl chains of the surfactants on the
nanocrystal surfaces (TOPO has a rather bulky hindrance
due to the three C8-alkylchains, whereas OLEA possesses a
unique C18-chain with cis-conformation with respect to the
double bound in C8 position); (3) the initial density of
surface –OH groups, approximately corresponding to the
fraction of surface Ti centres which do not accommodate
capping surfactant molecules. As opposed to the surfactant-
capped TiO2 nanocrystal-based catalysts, the commercial
TiO2 P25 Degussa is composed of a mixture of the anatase
and the rutile phases with a considerably larger mean
particle size, a rather broad size-distribution, and organic-
free surface. The features of the employed catalysts are
summarized in Table 2.
3.2. Photocatalytic experiments
Photocatalytic degradation of dyes has been extensively
studied for several decades [3,4,15–29]. It is broadly
accepted that hydroxyl radicals (OH�) are produced in water
matrices from the direct oxidation of H2O, OH� ions or
terminal –OH groups present on the catalyst surface by the
photogenerated holes (h+). Under ambient atmosphere,
hydroxyl radicals can also arise from a series of reactions
Table 2
Characteristics of the different type of TiO2 photocatalysts
Catalyst Acronym Size Surface c
TiO2 P25 Degussa P25-TiO2 25–30 nma (polydisperse) None
Hydrolytic TiO2 h-TiO2 6 nmb (monodisperse) OLEA
Non-hydrolytic TiO2 nh-TiO2 6 nmb (monodisperse) TOPO
a [11].b From TEM and XRD.
initiated by the scavenging of the photogenerated electrons
(e�) by molecular oxygen [26,27].
3.2.1. Effect of pH
Despite of the large number of accurate studies, the effect
of solution pH during photocatalysis is still debated. The
variations of pH induce modification in the ionization state
of titania surface due to the establishment of acid–base
equilibria:
Ti�OH þ Hþ$Ti�OHþ2 pKa1 (1)
Ti�OH þ OH�$Ti�O� þ H2O pKa2 (2)
For nanosized TiO2, the pHZPC (zero point charge) is known
to be 5.1 [30], so that the surface is positively or negatively
charged at low or high pH, respectively. This behaviour can
be expected to primarily influence the adsorption of the dye
on the catalyst, thus affecting the overall photocatalytic
process.
In the case of sulfonated dyes [31–33], a model has been
proposed to account for the dramatic role played by pH in
determining the critical step of the decomposition. The
description considers the charge status of both the target
substrate and the catalyst surface by applying a simple
electrostatic reasoning. Accordingly, the increase of
bleaching recorded at acid pH was attributed to strong
dye adsorption, through the deprotonated �SO�3 moiety, on
the catalyst. This favourable interaction would enhance the
encounter probability of nascent OH radicals with the
organic dye. As opposed, at alkaline pH, the Coulombic
repulsion arising between �SO�3 and the negative oxide
surface would make the dye access to the catalyst a
diffusion-controlled process. In this case, the OH radicals
generated at the catalyst surface would more hardly attack
the target molecule. Consequently, a comparatively slower
decomposition rate was measured. The results of the present
work for MeOr are in good agreement with this model. For
all catalysts, the highest percentage of dye discoloration was
actually recorded at pH 2 following a constant illumination
period (Fig. 3B).
However, in the case of MeRed, a remarkably different
trend was recorded (Fig. 3A). For this dye, the carboxylic
group should be responsible for the absorption of the dye on
the catalyst. As pKa for MeRed is around 5.3, a Coulombic
repulsion can be expected to occur at extreme pH values.
Nevertheless, we found that the optimal condition was again
pH 2 for all catalysts. This evidence could be accounted for
apping Density of surface –OH groups Crystal phase
Higha 70% anatase, 30% rutile
High 100% anatase
Low 100% anatase
R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–9186
Fig. 3. Percentages of decoloration for: (A) MeRed and (B) MeOr obtained with different titania photocatalysts (½dyet¼0 = 3 � 10�5 M; irradiation time:
120 min) at different pH values. The values were estimated from the relative decay of the visible absorbance maximum.
by invoking other competitive factors in the mechanism of
photocatalysis. It can be suggested that the presence of
positive charge on titania surface could make the photo-
generated electrons reach more readily the catalyst surface,
thus preventing detrimental electron–hole recombinations to
a higher extent. Surprisingly, although nanosized TiO2
colloids are known to be subjected to a facile acid-catalyzed
dissolution [34], h-TiO2 encountered inherent chemical
instability problems that prevented photocatalysis at high
pH. Such drawback has been exceptionally found in
previous studies on TiO2 P25 films, however, no explanation
was provided [35,36]. As this phenomenon was exclusively
detected upon catalyst illumination, a self-oxidation process
induced by the photogenerated holes may be tentatively
proposed, in analogy with what found for other photo-
catalysts [37]. Such an UV-driven corrosion would be
possibly favoured by removal of the ligands from the surface
of titania. Previous studies on the interaction of OLEA
molecules with titania actually proved that the binding of
this surfactant is made labile at alkaline pH due to increased
solubility of deprotonated OLEA in water [11]. On the other
hand, the TOPO-capped nh-TiO2 films were inherently
stable due to the high resistance of the Ti–O–P bond toward
hydrolysis [38,39] and to the negligible solubility of TOPO
in water, being expected also to be pH-independent.
3.2.2. Effect of catalyst
To study the effect of the nature of the catalyst,
experiments were carried out under optimal pH conditions.
The efficiency of the various photocatalysts was compared
by evaluating the percentage of decoloration from the decay
of both the visible absorbance and the quasi-molecular ion
peak of the dye detected by HPLC–MS analysis at the same
irradiation time.
Controlled photocatalytic experiments carried out on a
blank sample in a scaled-up system were performed in order
to investigate the fate of the organic coating on the TiO2
surface. Significantly, the global increase of TOC value
represented only a negligible percentage of the TOC
measured in the presence of the target dyes within the
typical timescale of a photodegradation experiment (about
150 min). Such evidence allowed to exclude that the
degradation rate of each dye over the different photo-
catalysts could be significantly affected to a varying extent
by partial consumption of active oxidant species by the
residual capping ligand molecules. Moreover, for both dyes
only a negligible bleaching was observed under illumination
when the catalyst was absent or in the presence of
unirradiated catalyst. In the case of MeRed, nanostructured
h-TiO2 was found to be the best catalyst (Fig. 4A). As a
matter of fact, h-TiO2 catalyzed a 100% of dye bleaching
within 30 min of reaction, whereas discoloration remained
at 70 and 40% with P25-TiO2 and nh-TiO2, respectively. The
same trend was observed by monitoring the dye removal
decay by HPLC-MS (Fig. 4B). The slightly higher
percentage in the abatement process found can be ascribed
to the formation of by-products having absorption features
similar to parent dye (see also Section 3.3), due to minor
changes in their chemical structure. In the case of MeOr
(Fig. 5), the photocatalytic performance of the three
photocatalysts resembled the same order of efficiency:
again h-TiO2 demonstrated to be more effective than P25-
TiO2, while nh-TiO2 remained the least effective.
This behaviour could be accounted for by considering the
intrinsically different amounts of hydroxyl groups available
at the surface of the catalyst particles. As discussed above,
the presence of –OH groups is directly related to the local
production of OH radicals, while also providing acidic–
basic sites for adsorption of the substrates. As both h-TiO2
and P25-TiO2 [14] are derived from hydrolysis of Ti
molecular precursors, they should presumably have a higher
density of surface –OH moieties, as opposed to the case of
nh-TiO2, being, in fact, synthesized by using a water-free
method. The higher efficiency of h-TiO2 with respect
commercial P25-TiO2 can be justified by the increase in
surface-to-volume ratio in the nanocrystal-based oxide due
R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–91 87
Fig. 5. (A) Spectrophotometric evaluation of MeOr decoloration.
½MeOrt¼0 = 3 � 10�5 M; pH = 2. The values were estimated from the
relative decay of the absorbance maximum. (B) Percentage of target
molecule removal. The values were estimated from the relative decay of
the parent ion peak by MS.
Table 3
Percentages of dye removal measured after 30 min of illumination
Catalyst MeRed (dye removal) MeOr (dye removal)
h-TiO2 98% 55%
nh-TiO2 53% 15%
P25-TiO2 50% 20%
½Dyet¼0 = 3 � 10�5 M; pH = 2. The values were estimated from the relative
decay of the parent ion peak by MS.
Fig. 4. (A) Spectrophotometric evaluation of MeRed decoloration.
½MeRedt¼0 = 3 � 10�5 M; pH = 2. The values were estimated from the
relative decay of the absorbance maximum. (B) Percentage of target
molecule removal. The values were estimated from the relative decay of
the parent ion peak by MS.
to its considerably smaller mean particle size. This feature
would compensate for the OLEA-passivated sites (i.e. for
the surface fraction not available for catalysis), which would
not actually occupy a high fraction of surface area in h-TiO2.
Furthermore, in h-TiO2 100% of anatase phase, the most
photoactive phase for TiO2 [2,40], is present, being expected
to enhance photocatalysis with respect to P25-TiO2 which,
in fact, contains only 70%.
In the case of nh-TiO2 a lower activity than h-TiO2 was
observed, although the constituent nanocrystals possessed a
mean particle size quite close to h-TiO2. Due to the non-
hydrolytic reaction pathways involved on the synthesis of
this material, the resulting oxide can be expected to be
characterized by an almost fully TOPO-passivated surface
and, in turn, by a comparatively scarce density of free –OH
groups with respect to h-TiO2. Accordingly, as the
coordination of –P=O to titania is known to be very strong,
the ligand is even more hardly expected to be displaced in
aqueous environment (see Section 3.2.1).
3.2.3. Influence of the molecular structure of dye
Experiments taken under the best reaction conditions (pH
= 2; irradiation time: 30 min) point out that each investigated
catalyst degraded MeRed more quickly than MeOr (Table
3). This evidence suggests that a precise role must be played
by the specific functional group, which differentiates the
molecular structure of each dye. Probably, the adsorption of
the target molecule on the catalyst surface should be
regarded as a critical step toward efficient catalysis. MeRed
adsorption through the carboxylic moiety can be reasonably
expected to be weak especially at low pH, being mainly
electrostatic in nature. As opposed, due to its favourable
dimension and spatial geometry, the �SO�3 attaches to
surface TiIV centres by assuming a bidentate coordination
through the two sulfonilic oxygens. This process would be
accompanied by substitution of a surface coordinated –OH
moieties. Because of the strong overlap between the 3d
orbitals of the TiIV atoms and the 2p orbitals of oxygens, the
formation of Ti–O bonds would have a strong covalent
character [41]. As a result of MeOr complexation, a number
of surface sites would be temporarily passivated, thus
R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–9188
Table 4
Proposed chemical structures of identified by-products during MeOr degradation
By-product Chemical structure Molecular weight P25-TiO2 nh-TiO2 h-TiO2
1 291 X X X
2 277 X
3 227 X X X
4 321 X X X
5 242 X X X
6 151 X X
leading to catalyst deactivation [41] and leaving the surface
depleted in terminal –OH groups. This detrimental
mechanism can remain operative over large periods of
photocatalysis, as primary by-products of MeOr actually
keep on bearing the sulfonic groups (see Section 3.3).
3.3. By-products formation
By-products formation was also investigated by per-
forming HPLC–MS analysis of irradiated reaction mixtures.
In Tables 4 and 5 the chemical structures of several
identified by-products are reported for MeOr and MeRed
Table 5
Proposed chemical structures of identified by-products during MeRed degradatio
By-product Chemical structure Molecul
1 255
2 241
3 271
4 287
5 285
6 317
degradation, respectively. Experimental data for both dyes
are consistent with two different mechanisms involved in
the degradation of parent compound. The first one involves
the homolytic rupture of the nitrogen–carbon bond of the
amine group leading to the substitution of methyl group
with a hydrogen atom. Such a photolytic route occurs
consecutively giving rise to by-products 1 and 2 of both
dyes (Tables 4 and 5). As for the second mechanism,
experimental results showed the formation of by-products
whose molecular weights are consistent with hydroxyl
group substitution on benzene rings. These latter are by-
products 3–5 for both MeOr and MeRed. The formation of
n
ar weight P25-TiO2 nh-TiO2 h-TiO2
X X X
X X X
X X X
X
X X X
X X X
R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–91 89
Fig. 6. Time evolution of by-product (1) during MeRed degradation, by
monitoring the corresponding quasi-molecular ion by MS.
such by-products can be explained by the attack of hydroxyl
radicals to benzene rings by substitution reactions being
well-known in the literature. It is reasonable to assume that,
for both dyes, OH substitution takes first place on the
benzene ring carrying the dimethylamino group due to its
capability to stabilize the intermediate hydroxy-benzene
radical, as opposed to the other benzene ring that carries a –
COOH and –SO3H moiety in MeRed and MeOr, res-
pectively. The formation of by-products 3 and 5 from MeOr
Fig. 7. Proposed degradation pathway for MeOr during photocatalysis. The schem
demonstrates that the ipso-substitution by a hydroxy radical
can also take place at the carbon which carries the sulfonic
moiety. Furthermore, for both dyes the presence of by-
product 3, derived from both OH substitution and
photolysis, suggests that the two mechanisms are indepen-
dently active. In Fig. 6 the temporal formation-decay of by-
product 1 for MeOr is reported as a representative example
of by-product evolution, being the evolution of other by-
products similar. The trends reported in the figure show that
the production of the intermediate compound was max-
imized at reaction times between 15 and 90 min and slowly
disappeared in the late stages of irradiation. After 120 min
no by-product could be detected by HPLC–MS. Due to the
lack of standards for the identified by-products, the reported
trends must be regarded as semiquantitative only. However,
as the structures of identified by-products are similar to that
of the parent dye, it is reasonable to assume that their MS
response are similar too. By this argument, it is actually
found that the identified by-products represent just a minor
fraction (<5% as compared to initial concentration of
parent dye) as compared to other polar degradation by-
products that could not be detected by HPLC–MS. As such
polar compounds should not have the azo-bond in their
structure, the reaction mixture results in being colorless.
This behaviour suggests the occurrence of the opening of
the aromatic ring due to consecutive oxidation reactions
which ultimately yield low molecular weight compounds,
as already reported during the extensive oxidation of
organic compounds [42].
es illustrate that two mechanisms of degradation are independently active.
R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–9190
Fig. 8. Proposed degradation pathway for MeRed during photocatalysis. The schemes illustrate that two mechanisms of degradation are independently active.
On the basis of the above results, possible degradation
pathways have been proposed for MeOr and MeRed in Figs.
7 and 8, respectively. The schemes illustrate the two
mechanisms of degradation, previously suggested, based on
photolytic attack to the dimethyl-amino group and on the
attack of hydroxyl radicals to the aromatic rings.
4. Conclusions
This study demonstrates that efficient photocatalysis can
be realized with nanosized oxide catalysts supported onto
substrates. The decrease in the exposed catalyst surface area
upon immobilization can be effectively counterbalanced by
employing colloidal TiO2 nanocrystal powders of a few
nanometers in diameter, provided that the organic capping at
their surface does not prevent dye access to the catalyst and/
or limit the local density of –OH groups. In this regard,
organic-capped TiO2 nanocrystals offer the perspective of
developing a new generation of efficient photocatalysts by
tailoring the oxide particle size and its surface character-
istics.
Acknowledgment
The authors wish to acknowledge Vito Locaputo for the
assistance in carrying out the HPLC–MS analyses.
Reference
[1] R. Andreozzi, V. Caprio, A. Insola, R. Marotta, Catal. Today 53 (1999)
51.
[2] J.M. Hermann, Catal. Today 53 (1999) 115.
[3] D.W. Bahnemann, Israel. J. Chem. 33 (1992) 115.
[4] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev.
95 (1995) 69.
[5] D. Beydoun, R. Amal, G. Low, S. McEvoy, J. Nanoparticle Res. 1
(1999) 439.
[6] L. Zang, J. Chem. Soc., Faraday Trans. 91 (1995) 917, and references
therein.
[7] A. Rachel, M. Subrahmanyam, P. Boule, Appl. Catal. B: Environ. 37
(2002) 301.
[8] I.M. Arabatzis, S. Antonaraki, T. Stergiopoulos, A. Hiskia, E. Papa-
constantinou, M.C. Bernard, P. Falaras, J. Photochem. Photobiol. A:
Chem. 149 (2002) 237.
[9] T.J. Trentler, T.E. Denler, J.F. Bertone, A. Agrawal, V.L. Colvin, J.
Am. Chem. Soc. 121 (1999) 1613.
[10] P.D. Cozzoli, A. Kornowski, H. Weller, J. Am. Chem. Soc. 125 (2003)
14539.
[11] P.J. Thistlewaite, M.S. Hook, Langmuir 16 (2000) 4993.
[12] P.J. Thistlewaite, M.L. Gee, D. Wilson, Langmuir 12 (1996) 6487.
[13] M. Nara, H. Tori, M. Tasumi, J. Phys. Chem. 100 (1996) 19812.
[14] A.K. Datye, G. Riegel, J.R. Bolton, M. Huang, M.R. Prairie, J. Solid
State Chem. 115 (1995) 236.
[15] M. Muneer, R. Philip, S. Das, Res. Chem. Intermed. 23 (1997)
233.
[16] W.Z. Tang, Z. Zhang, H. An, M.O. Quintana, D.F. Torres, Environ.
Technol. 18 (1997) 112.
[17] K. Vinogdopal, P.V. Kamat, Environ. Sci. Technol 29 (1995) 841.
[18] W.R. Moser, Advanced Catalysis and Nanostructured Materials,
Academic Press, San Diego, 1990.
R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–91 91
[19] M. Schiavello, Photocatalysis and Environment, Trends and Applica-
tions, Kluwer Academic Publishers, Dordrecht, 1988.
[20] E. Pelizzetti, N. Serpone, Photocatalysis, Fundamentals and Applica-
tions, Wiley, New York, 1989.
[21] T. Wu, G. Liu, J. Zhao, H. Hidaka, N. Serpone, J. Phys. Chem. B 102
(1998) 5845.
[22] K.R. Gopidas, M. Bohorquez, P.V. Kamat, J. Phys. Chem. 94 (1990) 6435.
[23] C.S. Turchi, D.F. Ollis, J. Catal. 122 (1990) 178.
[24] B. Kraeuter, C.D. Jaeger, A. Bard, J. Am. Chem. Soc. 100 (1978) 4903.
[25] H. Gerisher, A. Heller, J. Phys. Chem. 95 (1991) 5261.
[26] A. Hoffmann, E.R. Carraway, M. Hoffman, Environ. Sci. Technol. 28
(1994) 776.
[27] F. Mahdavi, T.C. Burton, Y. Li, J. Org. Chem. 58 (1993) 744.
[28] K. Vinodgopal, I. Bedja, S. Hotchandani, P.V. Kamat, Langmuir 10
(1994) 1767.
[29] D.C. Schmelling, K.A. Gray, P.V. Kamat, Environ. Sci. Technol. 30
(1996) 2547, and references therein.
[30] P.V. Kamat, D. Meisel (Eds.), Semiconductor Nanoclusters: Physical,
Chemical and Catalytic Aspects, Elsevier, Amsterdam, 1996, p. 417.
[31] T.C.-K. Yang, S.-F. Wang, S.H.-Y. Tsai, S.-Y. Lin, Appl. Catal. B:
Environ. 30 (2001) 293.
[32] K. Tanaka, K. Padermpole, T. Hisanaga, Wat. Res. 34 (2000) 327.
[33] L.B. Reutergardh, M. Iangphasuk, Chemosphere 35 (1999) 585.
[34] C. Kormann, D.W. Bahnemann, M.R. Hoffmann, J. Phys. Chem. 92
(1988) 5196.
[35] N.A. Hamill, L.R. Weatherley, C. Hardacre, Appl. Catal. B: Environ.
30 (2001) 49.
[36] A. Mills, R.H. Davies, D. Worsley, Chem. Commun. (1994) 2677.
[37] B. Neppolian, H.C. Choi, S. Sakthivel, B. Arabindoo, V. Murugesan, J.
Hazard. Mater. B 89 (2002) 303.
[38] W. Gao, L. Dickinson, C. Grozinger, F.G. Morin, L. Reven, Langmuir
12 (1996) 6429.
[39] G. Guerrero, P.H. Mutin, A. Vioux, Chem. Mater. 13 (2001) 436.
[40] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 341.
[41] C. Bauer, P. Jacques, A. Kalt, Chem. Phys. Lett. 307 (1999) 397, and
reference therein.
[42] G. Mascolo, A. Lopez, H. James, M. Fielding, Wat. Res. 35 (2001)
1695.