H2O2 evolution during the photocatalytic degradation of organic molecules on fluorinated TiO2

Article abstract of DOI:10.1039/b511320b

The effect of TiO₂ surface fluorination on the hydrogen peroxide evolution occurring in photocatalytic runs was investigated employing the azo dye Acid Red 1 (AR1) and two model organic molecules with acidic properties, i.e. formic acid (FA) and benzoic acid (BA), as substrates of oxidative degradation. While AR1 and BA photocatalytic degradation on fluorinated titanium dioxide (F-TiO₂) was markedly faster than on unmodified TiO ₂, because of enhanced hydroxyl radical formation, H ₂O₂ concentration during the photodegradation of both substrates on F-TiO₂ was lower, possibly because of the reduced rate of interfacial electron transfer. By contrast, FA underwent slower photocatalytic degradation on F-TiO₂, but, at the same time, hydrogen peroxide concentration was relatively high, while no H₂O₂ could be detected during FA photodegradation on unmodified TiO₂. Photocatalytic runs in the presence of the nitrate anion, able to react with the CO₂^?- species produced from FA oxidation, but not with conduction band electrons, demonstrated that CO₂ ^?- plays a relevant role in H₂O₂ formation during FA degradation on F-TiO₂. In fact, surface fluoride, having a shielding effect at the semiconductor-water interface, not only inhibits the photocatalytic decomposition of H₂O₂, but also favours CO₂^?- desorption and reaction with dissolved O ₂, generating H₂O₂. By contrast, CO ₂^?- mainly gives electron transfer to the conduction band of naked TiO₂ and surface reduction of the photocatalytically produced H₂O₂. the Royal Society of Chemistry the Centre National de la Recherche Scientifique 2006.

Full text of DOI:10.1039/b511320b

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PAPER
www.rsc.org/njc | New Journal of Chemistry
H2O2 evolution during the photocatalytic degradation of organic
molecules on fluorinated TiO2
Marta Mrowetz and Elena Selli*
Received (in Montpellier, France) 8th August 2005, Accepted 8th November 2005
First published as an Advance Article on the web 24th November 2005
DOI: 10.1039/b511320b
The effect of TiO2 surface fluorination on the hydrogen peroxide evolution occurring in
photocatalytic runs was investigated employing the azo dye Acid Red 1 (AR1) and two model
organic molecules with acidic properties, i.e. formic acid (FA) and benzoic acid (BA), as
substrates of oxidative degradation. While AR1 and BA photocatalytic degradation on
fluorinated titanium dioxide (F–TiO2) was markedly faster than on unmodified TiO2, because of
enhanced hydroxyl radical formation, H2O2 concentration during the photodegradation of both
substrates on F–TiO2 was lower, possibly because of the reduced rate of interfacial electron
transfer. By contrast, FA underwent slower photocatalytic degradation on F–TiO2, but, at the
same time, hydrogen peroxide concentration was relatively high, while no H2O2 could be detected
during FA photodegradation on unmodified TiO2. Photocatalytic runs in the presence of the
ꢁ
ꢀ
nitrate anion, able to react with the CO2 species produced from FA oxidation, but not with
ꢁ
ꢀ
conduction band electrons, demonstrated that CO2 plays a relevant role in H2O2 formation
during FA degradation on F–TiO2. In fact, surface fluoride, having a shielding effect at the
semiconductor–waꢁter interface, not only inhibits the photocatalytic decomposition of H2O2, but
ꢀ
also fꢁavours CO2 desorption and reaction with dissolved O2, generating H2O2. By contrast,
ꢀ
CO2 mainly gives electron transfer to the conduction band of naked TiO2 and surface reduction
of the photocatalytically produced H2O2.
To have a deeper insight into the effects of surface fluorina-
Introduction
tion on the electron transfer processes at the semiconductor–
water interface under irradiation, we focused our attention on
the main reductive counter reaction of most photocatalytic
reactions proceeding oxidatively, i.e. hydrogen peroxide evo-
lution. A preliminary note reporting a sustained production of
H2O2 on irradiated TiO2–fluoride systems appeared very
recently.16 In the present study H2O2 evolution was monitored
during the photocatalytic degradation of the azo dye Acid Red
1 (AR1, see Scheme 1), in the presence of naked or fluorinated
titanium dioxide. This dye bears two sulfonic groups, capable
of strong electrostatic interactions with the unmodified oxide
surface.17 One of the first steps in its photocatalytic degrada-
tion is the cleavage of the azo bond, responsible for its
bleaching in the visible region.18 The photocatalytic behaviour
of this rather complex aromatic molecule on F–TiO2 and the
simultaneous H2O2 evolution were compared to those of
benzoic acid (BA), chosen as a moderately acidic model
aromatic molecule, and of formic acid (FA), chosen as a
The electron transfer paths occurring at a water–semiconduc-
tor interface during photocatalytic processes1,2 undergo strong
modification upon titanium dioxide surface fluorination.3–5
Indeed, the formation of RTi–F species,6 which dominate at
acidic pH, decreases the amount of surface hydroxyl groups,
up to their almost complete displacement at pH 3.5–4.0.7
Significant variations of the semiconductor oxide photocata-
lytic behaviour have consequently been observed.8–15
An increase in the photocatalytic degradation of phenol on
fluorinated titanium dioxide (F–TiO2) led to the hypothesis
that an ꢀOH radical mediated mechanism involving the homo-
geneous aqueous phase is almost exclusively responsible for
the oxidation of this substrate in F–TiO2 systems.8,9 By
contrast, hole transfer mediated oxidations are expected to
be largely inhibited because of the hindered adsorption of
substrates on F–TiO2. At the same time, surface –F groups,
because of the strong electronegativity of the fluorine atom,
seem to act as electron-trapping sites, reducing the interfacial
electron transfer rates of conduction band electrons.13 Spin-
trapping EPR measurements provided direct experimental
evidence that a ca. ten-fold higher concentration of hydroxyl
radicals is generated on fluorinated titanium dioxide under
irradiation, demonstrating that on F–TiO2 organic molecules
15
ꢀ
mainly undergo photodegradation via OH radical attack.
`
Dipartimento di Chimica Fisica ed Elettrochimica, Universita degli
Studi di Milano, Via Golgi 19, I-20133 Milan, Italy. E-mail:
elena.selli@unimi.it; Fax: þ39 02 50314300; Tel: þ39 02 50314237
Scheme 1 Molecular structure of the azo dye Acid Red 1 (AR1).
ꢂc
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ꢁ
model aliphatic acidic compound undergoing direct minerali-
sation to CO2 and H2O.19,20
of ClO4 anions for TiO2 and their low reactivity towards
hydroxyl radicals.25 No pH change was noticed during the
runs under such conditions. By contrast, the pH value of
suspensions containing formic acid and benzoic acid increased
during the runs, from 3.5 to 5.8 and from 4.2 to ca. 6.0,
respectively, as a direct consequence of the mineralisation of
the acids to CO2 and H2O.26 With these acidic substrates, the
runs were performed under so-called natural pH conditions,
corresponding to the range of maximum fluoride adsorption
on TiO2, i.e. no buffer was added to the suspensions, to avoid
possible interference of other species (mainly anions) on the
photoredox processes at the TiO2–water interface.
The origin of the peculiarly outstanding amount of H2O2
evolved during FA photocatalytic degradation on F–TiO2 was
clarified by investigating the effects of the addition of thꢁe
ꢀ
nitrate anion, which evidenced the central role of the CO2
species, formed by photoinduced FA oxidation on the photo-
catalyst surface.
Experimental
Materials
2-mL samples were periodically withdrawn from the reactor
and analysed, after removal of TiO2 particles by centrifugation
at 3000 rpm for 30 min, employing an ALC 4225 centrifuge.21
The cleavage of the azo bond of AR1, leading to its bleaching
(also mentioned as AR1 degradation), was monitored by
spectrophotometric analysis at 531 nm (maximum AR1 ab-
sorption, e = [3.13 ꢃ 0.02] ꢄ 104 Mꢁ1 cmꢁ1) by means of a
Perkin Elmer Lambda 16 spectrophotometer.21 BA concentra-
tion during the runs was detected by HPLC analysis, employ-
ing an Agilent 1100 Series apparatus, equipped with a
mBondapack-C18 column and a UV-Vis detector. An acetoni-
trile : water 60 : 40 mobile phase was used for BA analysis,27
flowing at 1.0 mL minꢁ1. FA concentration changes were
detected using a total organic carbon (TOC) analyser in the
not purgeable organic carbon (NPOC) mode (Shimadzu In-
struments, TOC-5000A). All runs were repeated at least twice
to check their reproducibility.
Acid Red 1 (AR1), purchased from Aldrich, was purified by
repeated crystallisation from methanol. Its purity from organ-
ic contaminants was verified by NMR analysis. Degussa P25
titanium dioxide (mainly anatase) was employed as photo-
catalyst. Formic acid (FA, purity 95–97%), benzoic acid (BA,
purity >99.5%), 2-propanol (purity 99.5%), KNO3 (purity
99.5%) and NaF (purity 99.99%) were purchased from Al-
drich and employed as received. Water purified by a Milli-Q
water system (Millipore) was used throughout.
Apparatus
All degradation runs were carried out at (35 ꢃ 1) 1C under
atmospheric conditions in a magnetically stirred 400 mL
cylindrical Pyrex reactor, employing an experimental set up
similar to that already described.21 Illumination was per-
formed through the reactor Pyrex walls by means of a 250 W
iron alogenide lamp (Jelosil, model HG 200), emitting in the
315–400 nm wavelength range, with a mean emission intensity
Hydrogen peroxide concentration was monitored during the
photodegradation runs by fluorimetric analysis (lex = 316.5
nm, lem = 408.5 nm) of the fluorescent dimer formed in the
horseradish peroxidase-catalysed reaction of hydrogen perox-
ide with p-hydroxyphenylacetic acid,28,29 using a 605-10S
Perkin Elmer fluorescence spectrophotometer. H2O2 standard
solutions employed in calibration were analysed iodometri-
cally. The H2O2 concentration profiles obtained during BA
photocatalytic degradation were corrected for the relatively
small fluorescence signal originating from salicylic acid, one of
the first degradation intermediates of BA. The signal was
measured in blank photocatalytic runs under conditions iden-
tical to those employed in H2O2 analysis, but with no
p-hydroxyphenylacetic acid and enzyme addition.
on the reactor of 3.5 ꢄ 10ꢁ4 einstein Lꢁ1
s
ꢁ1, as periodically
checked by ferrioxalate actinometry.22
Procedures
The irradiated aqueous suspensions contained 0.1 g Lꢁ1 of
TiO2; the initial concentration (C0) of the substrates was 2.5 ꢄ
10ꢁ5 M for AR1, 1.0 ꢄ 10ꢁ4 M for BA and 5.0 ꢄ 10ꢁ4 M for
FA. Titanium dioxide fluorination was achieved by adding
0.01 M NaF, corresponding to 0.1 mol of Fꢁ per gram of
TiO2. Fluoride ions, able to very quickly displace the –OH
groups on the surface of titanium dioxide, were added to the
suspensions immediately before starting irradiation, to mini-
mise the coagulation of the photocatalyst.8,10 When investigat-
ing the effect of nitrate addition, the KNO3 concentration was
0.05 M, high enough to guarantee a quantitative scavenging of
the carbon dioxide radical anion.23
Adsorption studies were performed both in the presence and
in the absence of 0.01 M NaF, on suspensions containing 1.0 g
L
ꢁ1 of TiO2 and 2.5 ꢄ 10ꢁ5 M AR1, 1.0 ꢄ 10ꢁ4 M BA or 5.0 ꢄ
10ꢁ4 M FA. After continuous stirring for 24 h in the dark at
35 1C, the photocatalyst was removed and the liquid phase was
analysed for AR1, BA or FA residual content. The Fꢁ/TiO2
ratio employed in the adsorption experiments, lower with
respect to that of the photocatalytic degradation runs, guar-
anteed a similar displacement of surface –OH groups, the
maximum value of adsorbed fluoride being 2.5 ꢄ 10ꢁ4 mol gꢁ1
of TiO2.30
The pH was monitored during the runs by means of an
Amel Instruments 334-B pH-meter. A decrease in pH was
observed during AR1 photocatalytic degradation under nat-
ural pH conditions, from an initial value of 5.8 to a final value
of ca. 4.4, as a consequence of the production of stable acids,
due to the fast removal of sulfonic groups and the oxidation of
the azo double bond.24 Thus, to guarantee an efficient adsorp-
tion of fluoride anions on the surface of the oxide, during AR1
photodegradation the pH was lowered to 3.7 by adding small
amounts of HClO4, which is known to have negligible influ-
ence on the photocatalytic activity, because of the low affinity
The reduction potentials of AR1, BA and FA were deter-
mined by cyclic voltammetry measurement using a glassy
carbon working electrode vs. Ag/AgCl in 0.1 M HClO4 for
AR131 or 0.5 M NaClO4 for BA and FA solutions, with a scan
speed of 50 mV sꢁ1
.
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hydroxyl groups implies the absence of an effective trap for
photogenerated valence band holes as RTiOꢀ species, formed
through reaction (1). In fact, –F species are stable and cannot
Results and discussion
Adsorption and photocatalytic degradation of the organic
substrates
1
be oxidised by valence band holes hvb even in acidic media.
Indeed, the potential of the Fꢀ/Fꢁ couple in homogeneous
aqueous phase is 3.6 V,36 while the valence band energy is
E0vb(V) = 3.00 ꢁ 0.059 pH vs. NHE.23 Thus, valence band
holes photoproduced in F–TiO2 dirꢀectly react with water
molecules at the interface, producing OH radicals according
to reaction (2).
Preliminary adsorption studies evidenced that, while AR1 and
BA are detectably adsorbed on the unmodified TiO2 surface,
their adsorption is almost completely inhibited on F-modified
TiO2. In fact, the adsorbed fraction on naked titanium dioxide
at natural pH was 0.27 for AR1 and 0.30 for BA, but below
our detection limit (adsorbed fraction ca. 0.001) on the
fluorinated oxide. Similar results were obtained for the ad-
sorption of the azo dye Acid Orange 7 on TiO2 and on
F–TiO2.13 Thus, the displacement of –OH groups with the
formation of stable RTi–F species and the consequent nega-
tive surface charge13 induce significant alterations of the
adsorption equilibria at the water–TiO2 interface. On the other
hand, under the adopted conditions FA adsorption could not
be detected on both unmodified and F-modified TiO2. FA
adsorption on naked titanium dioxide has been recently
measured in the presence of higher oxide amounts.32 Also
the adsorption of molecular oxygen is expected to undergo
deep modification upon TiO2 fluorination, with great conse-
quences on the rates of conduction band electron scavenging
by O2.8,33
RTi–OH þ hvb - RTi–Oꢀ þ H1
(1)
(2)
1
1
1
ꢀ
RTi–F þ hvb þ H2O - RTi–F þ OH þ H
The presence of fluoride anions on the photocatalyst surꢀface,
having a shielding effect on surface reactions leading to OH
radical decomposition, favours their desorption and their
higher accumulation in the aqueous phase.15 By contrast,
ꢀOH radicals formed by water oxidation on unmodified
TiO2 are unable to leave the surface, because they rapidly
react with surface –OH groups.37
Thus, all ꢀOH radical mediated paths are favoured in
F–TiO2 suspensions and substrates that easily undergo ꢀOH
radical attack are more rapidly degraded. This is the case for
AR1 and BA, the second-order rate constants for hydroxyl
radical attack having been reported to be k = 4.3 ꢄ 109 Mꢁ1
Upon TiO2 fluorination, AR1 photocatalytic degradation at
pH 3.7, i.e. under conditions of maximum Fꢁ adsorption on
the oxide surface, was found to proceed at a more than double
rate,15 as shown by the first-order rate constants reported in
Table 1.
s
ꢁ1 for BA and k = 1.7 ꢄ 1010
M
ꢁ1 sꢁ1 for an azo dye similar
to AR1.38 On the other hand, substrates, such as FA, having a
relatively lower second-order rate constant for hydroxyl attack
(k = 1.3 ꢄ 108 Mꢁ1
s
ꢁ1),38 whose degradation is preferably
BA exhibits a behaviour similar to that of AR1, i.e. it
underwent faster reaction on fluorinated TiO2. The pseudo
first-order rate constants for BA degradation on TiO2 and
F–TiO2, reported in Table 1, also for this substrate demon-
strate an almost double photodegradation rate upon fluoride
addition under the conditions of the present study. Higher
increases in rate have been very recently reported for higher
initial BA concentrations.34
initiated by direct hole transfer, upon TiO2 fluorination under-
go slower photocatalytic degradation because of their hin-
dered adsorption (or complexation) on the F–TiO2 surface.13
Moreover, the presence of aromatic rings in AR1 and BA
can also play a role in the observed increase of photocatalytic
activity of F–TiO2. In fact, the radical or cationic species
ꢀ
formed upon reaction of aromatic rings with OH radicals or
with valence band holes are relatively long-lived, being highly
stabilised by resonance. Thus, they have a high probability to
back react with conduction band electrons, giving no net
change and enhanced recombination.39 The lower adsorption
of AR1 and BA on F–TiO2 can inhibit the detrimental back
reaction with conduction band electrons, with a consequent
increase of the overall degradation rate.
FA was found to undergo photodegradation according to a
zero order rate law on both unmodified and F-modified TiO2.
However, as evidenced by the rate constants reported in Table
1, formic acid exhibits an opposite behaviour compared to
AR1 and BA, its photocatalytic degradation on F-modified
TiO2 being slower than on naked TiO2.
At neutral pH, Fꢁ ions displace basic –OH groups (esti-
mated around 0.14 mmol gꢁ1), with an equilibrium constant35
equal to 8 ꢄ 10ꢁ7; at lower pH even acidic –OH groups are
substituted by –F atoms.6 This induces a relevant change in
the adsorption properties on the photocatalyst surface, espe-
cially for acidic molecules strongly interacting with basic sites,9
as is the case for our substrates. Moreover, the displacement of
Hydrogen peroxide evolution during photocatalytic degradation
Hydrogen peroxide, a key intermediate in photocatalytic
processes, was demonstrated to form on TiO2 exclusively via
reduction of molecular oxygen by conduction band elec-
trons.40 Fig. 1 shows the concentration profile of H2O2
evolved during AR1 photodegradation on naked TiO2 and
on F–TiO2 at pH 3.7. A lower H2O2 amount accumulated on
fluorinated TiO2 and its formation was retarded, with respect
to naked TiO2, so that it could hardly be detected on F–TiO2
during the first 30 min of irradiation. Moreover, H2O2 con-
centration always remained below 4 ꢄ 10ꢁ5 M during AR1
photocatalytic degradation on F–TiO2, i.e. lower than one
third of the maximum concentration attained in the absence of
fluoride.
Table 1 Rate constants for the photocatalytic degradation of the azo
dye Acid Red 1 (C0 = 2.5 ꢄ 10ꢁ5 M), of benzoic acid (C0 = 1.0 ꢄ
10ꢁ4 M) and of formic acid (C0 = 5.0 ꢄ 10ꢁ4 M) on TiO2 or F–TiO2
Substrate TiO2
F–TiO2
AR1
BA
FA
(3.2 ꢃ 0.4) ꢄ 10ꢁ4 sꢁ1
(1.83 ꢃ 0.05) ꢄ 10ꢁ7 M sꢁ1 (1.58 ꢃ 0.04) ꢄ 10ꢁ7 M sꢁ1
(7.34 ꢃ 0.15) ꢄ 10ꢁ4 sꢁ1
(7.84 ꢃ 0.16) ꢄ 10ꢁ4 sꢁ1
(4.65 ꢃ 0.11) ꢄ 10ꢁ4 sꢁ1
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Fig. 1 Hydrogen peroxide evolution during AR1 photodegradation
(C0 = 2.5 ꢄ 10ꢁ5 M) on naked (J) and on fluorinated TiO2 at pH 3.7
(K).
Fig. 2 Hydrogen peroxide evolution during BA photodegradation
(C0 = 1.0 ꢄ 10ꢁ4 M) on naked (J) and on fluorinated TiO2 (K).
Based on these results, AR1 photocatalytic degradation on
fluorinated TiO2 may appear in competition with molecular
oxygen reduction by conduction band electrons. This would be
compatible with our cyclic voltammetry measurements on
AR1, yielding an AR1 reduction potential value of ꢁ0.33 V
(NHE), and with a reported41 shift of the band edge potentials
of TiO2 towards more negative values upon addition of a
fluoride salt in acetonitrile. Thus, electron transfer from the
conduction band to AR1 is in principle allowed on unmodified
TiO2 [E0cb(V) = ꢁ0.13 ꢁ 0.059 pH vs. NHE]42 and even
thermodynamically more favourable on the F-modified oxide,
under the hypothesis of the above mentioned negative band
shift.41 Therefore, besides through hydroxyl radical attack or
reaction with photogenerated holes, the azo group of AR1,
which ensures extensive conjugation between its two aromatic
moieties and hence its capability of absorbing light in the
visible, might also undergo degradation through a reductive
pathway,43 directly or indirectly involving conduction band
electrons.
concentration profiles obtained during BA photodegradation
in the presence and in the absence of fluoride (Fig. 2) appear
very similar to those obtained during the degradation of the
azo dye (Fig. 1). This points to a decreased rate of interfacial
electron transfer in the presence of surface fluoride as the main
reason for the lower H2O2 evolution and also excludes any
relevant role of AR1 photocatalytic reduction and of a dye
sensitised path for H2O2 production, which are not possible
for BA.
The reactivity of the superoxide radical anion towards
aromatic radical species may be invoked to account for the
shape of the H2O2 concentration profiles obtained during AR1
and BA photocatalytic degradation on F–TiO2 (Fig. 1 and 2).
In fact, this ꢀspecies and its protonated form, the perhydroxyl
radical HO2 , are precursors of H2O2 and may easily attack
aromatic radical species.45 On fluorinated titania the active
species, i.e. the radicals generated by both reduction and
oxidation surface reactions, do not reside on the surface. This
makes their reactions in the homogeneous phase more com-
petitive, compared to further direct electron transfer at the
water–semiconductor interface. As a result, the reactions
between aromatic molecules, such as AR1 and BA, and the
superoxide radical anion are expected to be enhanced on
F–TiO2, with the consequent faster degradation of both sub-
strates and the lower production of hydrogen peroxide, as long
as the aromatic moieties are present.
Moreover, the higher H2O2 amount observed during AR1
photocatalytic degradation on naked TiO2 (Fig. 1) might
simply be a consequence of a relatively higher H2O2 produc-
tion occurring through a dye sensitised path.44 Indeed, AR1 is
able to absorb a fraction of the incident radiation and electron
transfer from the electronically excited state of AR1 to the
semiconductor conduction band is expected to be favoured by
the higher adsorption of AR1 on TiO2 with respect to F–TiO2.
However, RTi–F groups on the oxide surface have been
recently shown to act as electron-trapping sites, decreasing
interfacial electron transfer rates by tightly holding trapped
electrons, due to the strong electronegativity of fluorine.13
Thus, the slower formation of hydrogen peroxide in fluori-
nated systems might well be just the direct consequence of this
effect.
The evolution profiles of H2O2 during FA photodegrada-
tion, both in the absence and in the presence of fluoride, are
shown in Fig. 3. No hydrogen peroxide could be detected on
naked TiO2; in fact, H2O2 undergoes extremely fast decom-
position on the ‘‘clean’’ surface of titania during formic acid
photodegradation, having a high affinity for the oxide surface
and being able to complex Ti(IV) ions at the interface.16,46 By
contrast, H2O2 could be easily detected on F–TiO2. Its con-
centration increased from the beginning of the run, up to
values even higher than those attained during AR1 and BA
photodegradation, and started to decline after almost com-
plete FA degradation, i.e. when all species able to scavenge the
Hydrogen peroxide evolution during BA photodegradation
clarifies this point; in fact, for BA a reductive path is thermo-
dynamically impossible, because its reduction potential, mea-
sured by cyclic voltammetry, is ꢁ1.22 V (NHE). The H2O2
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Fig. 4 Photocatalytic degradation of hydrogen peroxide (C0 B 2 ꢄ
10ꢁ4 M) at 35 1C at pH 3.7 on naked (J) and on fluorinated TiO2
(K).
Fig. 3 Hydrogen peroxide evolution during FA photodegradation
(C0 = 5.0 ꢄ 10ꢁ4 M) on naked (J) and on fluorinated TiO2 (K).
photogenerated valence band holes had been consumed. After
this time, the electron–hole fast recombination would make
less conduction band electrons available for O2 reduction.
Fluoride anions adsorbed on the TiO2 surface clearly have a
shielding effect on the quite fast photodegradation of hydro-
gen peroxide occurring at the water–semiconductor interface.
In fact, adsorbed Fꢁ ions inhibit the formation of surface
peroxides (RTiIV–OOH),8 a preliminary key step for their
subsequent degradation by photogenerated species according
to eqn (3) and (4):40
as long as AR1 acted as a shield on the TiO2 surface, hindering
the formation of photoactive peroxo complexes.
Similar results were obtained when 2-propanol (2 ꢄ 10ꢁ3
M), a well known oxidisable scavenger with relatively low
affinity for the TiO2 surface, was added during AR1 photo-
degradation (Fig. 5). The behaviour of the two scavenger
systems is qualitatively similar, though a lower concentration
of H2O2 was observed when 2-propanol, instead of FA, was
added to the suspensions containing either naked or fluori-
nated TiO2.
The main peculiarity of FA consists in the fact that a
ꢁ
H2O2 þ 2ecb þ 2H1 - 2H2O
(3)
(4)
ꢁ
ꢀ
ꢀ
strongly reductant species, i.e. CO2 or HCO2 depending
on pH, with a pKa of 1.4, forms in its photocatalytic oxidation
on TiO2 particles,47 originating the so-called current doubling
ꢀ
1
H2O2 þ 2hvb - O2 þ 2H1
effect.48 In fact, its redox potential, i.e. E0
= ꢁ1.8
(CO2/ COꢁ2
)
The shielding effect of adsorbed fluoride anions was confirmed
by monitoring the photocatalytic degradation of H2O2 on
naked and F-modified TiO2, starting from an initial concen-
tration around 2 ꢄ 10ꢁ4 M. Both experiments were performed
at pH 3.7, i.e. under conditions of Fꢁ maximum adsorption on
TiO2. The H2O2 concentration profiles, shown in Fig. 4,
clearly demonstrate the slower photodegradation of hydrogen
peroxide on the fluorinated surface.
V,23 makes the carbon dioxide radical anion able to inject an
electron into the conduction band of titanium dioxide, see
reaction (5), and also to mediate the reduction of a wide
The addition of a rather high concentration of FA (2 ꢄ 10ꢁ3
M) during AR1 photodegradation on both naked and fluori-
nated titanium dioxide also confirmed this effect. As shown by
the H2O2 concentration profiles reported in Fig. 5, FA addi-
tion first of all caused an increase of H2O2 concentration:
molecular oxygen reduction, yielding hydrogen peroxide, is
promoted by a higher availability of conduction band elec-
trons, as a consequence of a reduced recombination rate with
photogenerated holes. This occurs when valence band holes
can directly or indirectly react with compounds, such as FA,
acting as scavengers of photoproduced oxidant species. There-
fore, the results in Fig. 5 indicate that H2O2 also forms on
F–TiO2 (as it forms on TiO2) via O2 reduction by conduction
band electrons. Moreover, in the presence of FA, H2O2
concentration continuously increased on F–TiO2, mainly
thanks to the shielding effect of surface fluorides. By contrast,
on naked TiO2 rather high amounts of H2O2 were present only
Fig. 5 Hydrogen peroxide evolution during AR1 photodegradation
at pH 3.7 on TiO2 (open symbols) and on F–TiO2 (full symbols) in the
absence of scavengers (circles) and in the presence of 2 ꢄ 10ꢁ3
M
formic acid (triangles) or 2-propanol (squares).
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ꢁ
ꢀ
The crucial role of the CO2 radical in hydrogen peroxide
formation during FA photodegradation can be appreciated
only on F–TiO2, because on naked TiO2 this highly reactive
species is adsorbed at the semiconductor–water interface and
rapidly further oxidised to CO2 according to reaction (5).
Moreover, this radical may also react with the photocatalyti-
cally formed hydrogen peroxide, also adsorbed on the TiO2
surface:
ꢁ
ꢁ
ꢀ
ꢀ
H2O2 þ CO2 - OH þ OH þ CO2
(8)
Reaction (8) provides another decomposition path for hydro-
gen peroxide, which can concur in lowering its concentration
during FA photodegradation on naked TiO2.
Finally, the difference in H2O2 concentration observed on
both TiO2 and F–TiO2 in the presence of FA compared to
2-propanol (Fig. 5) indicates that a radical with reductive
properties is generated also by the one electron oxidation of
2-propanol. Such a species is able to produce H2O2 through
reactions analogous to (5) and (6), though its lower redox
potential23 makes it less powerful in reducing molecular oxy-
gen.
Fig. 6 Photocatalytic degradation of formic acid (open symbols, left
axis) and hydrogen peroxide evolution (full symbols, right axis) on
F–TiO2 in the absence of nitrate (circles) and in the presence of 0.05
M nitrate (triangles).
variety of molecules, in particular dissolved O2, according to
reaction (6):
ꢁ
ꢁ
ꢀ
CO2 - CO2 þ ecb
(5)
(6)
Conclusions
ꢁ
ꢁ
ꢀ
ꢀ
CO2 þ O2 - CO2 þ O2
Surface TiO2 fluorination inhibits H2O2 evolution during the
photocatalytic degradation of the aromatic substrates AR1
and BA, mainly because of the reduced rate of interfacial
electron transfer through the fluorinated surface. By contrast,
H2O2 production is sustained during FA photocatalytic de-
gradation on F–TiO2: surface fluoride, having a shielding
effect at the interface, inhibits the photocatalytic decomposi-
The role of these reactions was discerned by verifying the
effects of nitrate ion addition on hydrogen peroxide formation.
In fact, nitrate anions are able to react with the carbon dioxide
radical anion according to reaction (7), but not with conduc-
tion band electrons,23 their standard redox potential being
E0
= ꢁ1.0 V vs. NHE, i.e. more negative than the
(NOꢁ3 /NO23ꢁ
)
tion of H2O2 and favours the desorption of the CO2 ꢁ radical
ꢀ
TiO2 conduction band potential (vide supra).
anion produced from FA, thus favouring its reaction with
dissolved O2, to yield H2O2. No H2O2 evolution can be
observed during FA photocatalytic degradation on naked
ꢁ
ꢁ
2ꢁ
ꢀ
NO3 þ CO2 - NO3 þ CO2
(7)
The effects of the addition of 0.05 M nitrate anions on both the
FA photocatalytic degradation rate and hydrogen peroxide
evolution on F–TiO2 are shown in Fig. 6. Of course, this type
of experiment could only be performed on F–TiO2, H2O2
evolution remaining always below the detection limit on naked
TiO2. The rate of FA photocatalytic degradation was not
modified in the presence of nitrate anions, the measured rate
constants being k = (1.58 ꢃ 0.04) ꢄ 10ꢁ7 M sꢁ1 in the absence
of nitrate and k = (1.57 ꢃ 0.12) ꢄ 10ꢁ7 M sꢁ1 in its presence.
By contrast, hydrogen peroxide formation significantly de-
creased during FA photodegradation on F–TiO2, indicating
that the carbon dioxide radical anion, which is scavenged by
nitrate, plays a major role in H2O2ꢁproduction under these
ꢁ
ꢀ
TiO2, because CO2 preferentially gives electron transfer to
the conduction band, originating the current doubling effect,
and also surface reduction of the photocatalytically produced
H2O2.
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114 | New J. Chem., 2006, 30, 108–114