# Preparation, Characterisation, and Photocatalytic

Authors: Rossano Amadelli; Luca Samiolo; Andrea Maldotti; Alessandra Molinari; Mario Valigi; Delia Gazzoli
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

The preparation of cobalt-modifiedTiO2 (Co-TiO2) was carried out by the incipient impregnation method starting from commercial TiO2 (Degussa, P-25) and cobalt acetate. XPS data show that cobalt is incorporated  as divalent ion, and it is likely present within few subsurface layers. No appreciable change in structural-morphologic properties, such as surface area and anatase/rutile phase ratio, was observed. Conversely, Co addition brings about conspicuous changes in the point of zero charge and in surface polarity. Diffuse reflectance spectra feature a red shift in light absorption that is dependent on the amount of cobalt.
The influence of cobalt addition on the performance of TiO2 as a photocatalyst in the degradation of 4-chlorophenol and Bisphenol A is investigated. The results show that the modified oxide presents a higher photoactivity both for illumination with
UV-visible (λ>360 nm) and visible light (λ>420 nm; λ>450 nm), and that this enhancement depends on the amount of the added species and on the final thermal treatment in the preparation step. We also show that Co-TiO2 is a more active catalyst than pure TiO2 for the reduction of O2 in the dark, which is an important reaction in the overall photocatalytic processes.


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## Body

## 1. Introduction

Recent research in semiconductors photocatalysis is
strongly oriented toward the preparation of functional materials that are able
to meet specific requirements  such as high efficiency and visible light
response. These are important issues in the case, for example, of TiO2 which is an attractive photocatalyst under many aspects but whose use is
severely limited by the fact that it absorbs only a relatively small fraction
of solar light. On the other hand, from the point of view of photocatalysis for
fine chemicals production, illuminated TiO2 is a too oxidizing
system to expect selectivity. In this case, surface tailoring by molecular
modification appears to be a viable direction [1].Different investigations show that doping of TiO2 by low amounts of foreign elements (both metals and nonmetals)
sometimes leads to enhanced
photocatalytic activity and to an increase of visible light absorption
[2–6].Metal-doped oxides are also extensively studied for the
development of semiconductor nanomaterials with magnetic properties (diluted
magnetic semiconductors). This field of research is only broadly related to
application of semiconductors in heterogeneous photocatalysis. However, the two
disciplines share the necessity of in-depth materials characterisation by a
number of physical methods whose results are generally valid independently of
the final application.Among these studies, Co-doped  anatase, in particular, is the object of a
very large number of publications and review papers [7–14]. Although there
seems to be no general consensus on results and interpretation, most authors
agree on the fact that (i) Co is present as the divalent form; (ii) Co(II) states are located within the band gap; (iii) cobalt can be located in interstitial positions of the host oxide.In this paper, we report on the characterisation and
photocatalytic behaviour of Co-TiO2 obtained by the impregnation
method, using commercially available TiO2 and cobalt acetate as the
precursor salt. We discuss, in particular, structural and interfacial
properties such as the site occupation and valence state of the metal inside
the host oxide, the degree of surface dispersion, changes in the polarity, and
acid-basic properties of the surface.We use the photodegradation of 4-chlorophenol and
Bisphenol A to assess the photocatalytic activity of the Co-TiO2 and
TiO2 and show that the activity of the former is higher, both under
UV and visible light irradiation, depending on the degree of doping.In previous literature work, authors mainly discuss Co-TiO2 photocatalysts in the context of a more general examination of the effect of
different metal dopants [15–20], and the doping degree is generally fixed at
one value. Results on the influence of cobalt are contradictory: some authors
report an altogether detrimental [15, 16] effect, while, in some cases, cobalt
seems to improve photodegradation slightly [17] or selectively for some organic
compounds as found by Di Paola et al. [18].The valence state of Co is also a matter of debate. Thus Wang et al. [19] reach the conclusion that
Co3+ is more active in bringing about a photoactivity enhancement,
in contrast to  the works of Choi et al. [15, 16] and of Iwasaki et al. [21]. The paper
of the latter authors, on the photodegradation of acetaldehyde, is an
interesting work that also examines the effect of the cobalt doping degree.
However, neither that paper nor the other cited articles report on the problem
of O2 reduction activation which we deal with in the present
publication and to which we attach a considerable importance for an evaluation
of the overall activity of photocatalytic processes.


## 2. Experimental

### 2.1. Materials

Titanium dioxide P-25 from Degussa was employed in all
experiments.  The precursor salt used for
TiO2 modification was cobalt acetate obtained from Carlo Erba. The
Reichardt reagent (used for measurements of the surface polarity) and all other
chemicals were Aldrich pure reagents.


### 2.2. Apparatus and Methods

X-ray diffraction (XRD) patterns in the 2θ angular range
from 5° to 60° were obtained by a Philips PW 1729 diffractometer using Cu Kα (Ni-filtered)
radiation. UV-visible spectra of solutions were recorded on a
Kontron spectrophotometer (Uvikon 943), while diffuse reflectance UV-visible
spectra (DRS) were recorded in absorbance mode at room temperature in the range
200 nm–2500 nm on a Cary
5E spectrophotometer by Varian Win UV software. Halon was used as reference.
The concentration of cobalt in the solution phase was determined by atomic
spectroscopy using a Perkin-Elmer, Optica 3100 XL model, instrument.XPS measurements
were carried out with a Leybold-Heraeus LHS10 spectrometer using the Al Kα (1486.6 eV) radiation on samples deposited on
a gold sheet at a residual pressure in the chamber of 10−9 Torr.
Differential thermal analysis (DTA) data were obtained on a Perkin Elmer
instrument, in the temperature interval from 30° to 800°C at a heating rate of
20°C/min in O2 flow (50 mL/min), using α-Al2O3 as a reference.Electrochemical
experiments were carried out using an EG&G model 273A
potentiostat-galvanostat using EG&G software. A conventional
three-compartment glass cell was employed where the working electrode
compartment had an optical window for illumination. The counter electrode was a
Pt gauze and the reference electrode was SCE in all cases.Photocatalytic
experiments have been conducted on suspensions (2 g L−1) or
films of the semiconductor whose preparation is described in the following
sections. A Helios
Italquartz Q400 medium-pressure mercury lamp (15 mW cm−2), equipped with suitable cutoff
filters, has been employed, and all experiments have been
carried out at room temperature (298±1 K) under an atmosphere of 101.3 kPa of O2.


### 2.3. Preparation of Co-TiO2

Cobalt-modified TiO2 has been prepared by an
incipient wetness impregnation method analogous to that described in the
literature [22]. In a typical experiment, 0.5 g of TiO2 (Degussa P-25) was
dispersed in 10 mL H2O that contained the necessary amount of cobalt
acetate to obtain the desired percent of doping. The suspension is first
sonicated for about 10 minutes to help particles dispersion, then stirred for 2 hours
and finally dried in an oven at 100°C for 18 hours. The resulting powder was finely
ground in a mortar and calcinated at different temperatures between 100° and
500°C for different times. For comparison, TiO2 was also treated in
the same way but in the absence of added cobalt. The modified oxide is
henceforth written as Co-TiO2 or as Co(x%)-TiO2 when the
percent (w/w Co/TiO2) of the added metal needs to be specified. The
final powder is greenish while, for comparison, Co-TiO2 obtained by
the hydrolysis of titanium tetraisopropoxide in the presence of cobalt has a
yellow/orange colour.


### 2.4. Preparation of TiO2 Films

Films were prepared from the TiO2 powders as
described in the literature [23]. Pyrex glass was used as the support in
photocatalytic experiments. Electrodes were obtained in the same way except
that a titanium substrate was used as a support. This was pretreated in hot
oxalic acid for 20 minutes before deposition of the TiO2 films.


## 2.1. Materials

Titanium dioxide P-25 from Degussa was employed in all
experiments.  The precursor salt used for
TiO2 modification was cobalt acetate obtained from Carlo Erba. The
Reichardt reagent (used for measurements of the surface polarity) and all other
chemicals were Aldrich pure reagents.


## 2.2. Apparatus and Methods

X-ray diffraction (XRD) patterns in the 2θ angular range
from 5° to 60° were obtained by a Philips PW 1729 diffractometer using Cu Kα (Ni-filtered)
radiation. UV-visible spectra of solutions were recorded on a
Kontron spectrophotometer (Uvikon 943), while diffuse reflectance UV-visible
spectra (DRS) were recorded in absorbance mode at room temperature in the range
200 nm–2500 nm on a Cary
5E spectrophotometer by Varian Win UV software. Halon was used as reference.
The concentration of cobalt in the solution phase was determined by atomic
spectroscopy using a Perkin-Elmer, Optica 3100 XL model, instrument.XPS measurements
were carried out with a Leybold-Heraeus LHS10 spectrometer using the Al Kα (1486.6 eV) radiation on samples deposited on
a gold sheet at a residual pressure in the chamber of 10−9 Torr.
Differential thermal analysis (DTA) data were obtained on a Perkin Elmer
instrument, in the temperature interval from 30° to 800°C at a heating rate of
20°C/min in O2 flow (50 mL/min), using α-Al2O3 as a reference.Electrochemical
experiments were carried out using an EG&G model 273A
potentiostat-galvanostat using EG&G software. A conventional
three-compartment glass cell was employed where the working electrode
compartment had an optical window for illumination. The counter electrode was a
Pt gauze and the reference electrode was SCE in all cases.Photocatalytic
experiments have been conducted on suspensions (2 g L−1) or
films of the semiconductor whose preparation is described in the following
sections. A Helios
Italquartz Q400 medium-pressure mercury lamp (15 mW cm−2), equipped with suitable cutoff
filters, has been employed, and all experiments have been
carried out at room temperature (298±1 K) under an atmosphere of 101.3 kPa of O2.


## 2.3. Preparation of Co-TiO2

Cobalt-modified TiO2 has been prepared by an
incipient wetness impregnation method analogous to that described in the
literature [22]. In a typical experiment, 0.5 g of TiO2 (Degussa P-25) was
dispersed in 10 mL H2O that contained the necessary amount of cobalt
acetate to obtain the desired percent of doping. The suspension is first
sonicated for about 10 minutes to help particles dispersion, then stirred for 2 hours
and finally dried in an oven at 100°C for 18 hours. The resulting powder was finely
ground in a mortar and calcinated at different temperatures between 100° and
500°C for different times. For comparison, TiO2 was also treated in
the same way but in the absence of added cobalt. The modified oxide is
henceforth written as Co-TiO2 or as Co(x%)-TiO2 when the
percent (w/w Co/TiO2) of the added metal needs to be specified. The
final powder is greenish while, for comparison, Co-TiO2 obtained by
the hydrolysis of titanium tetraisopropoxide in the presence of cobalt has a
yellow/orange colour.


## 2.4. Preparation of TiO2 Films

Films were prepared from the TiO2 powders as
described in the literature [23]. Pyrex glass was used as the support in
photocatalytic experiments. Electrodes were obtained in the same way except
that a titanium substrate was used as a support. This was pretreated in hot
oxalic acid for 20 minutes before deposition of the TiO2 films.


## 3. Results and Discussion

### 3.1. Catalyst Characterisation

#### 3.1.1. General Characterisation

Surface microstructure is highly dependent on the
preparation conditions and on posttreatments. Small additions of various metal
oxides can also greatly influence (alter) the crystalline morphology as well as
the conductivity of the host matrix.A general investigation of the materials characteristics
includes measurements of surface area (BET method), X-ray diffraction (XRD),
and differential thermal analysis (DTA). Surface area measurements were carried
out on TiO2 samples doped with 0.2, 0.5, 1, and 2% w/w and the values
obtained ranged from 47 to 49 m2g−1, indicating that the
adopted doping procedure does not bring about any surface area change with
respect to the pristine oxide (∼50 m2g−1). Likewise,
XRD spectra of samples with 0.2, 0.5, and 1% Co, collected in the 2θ range 5–60°, showed only peaks due to anatase
and rutile, which is expected since the commercial TiO2 used is well
known to be a mixture of the two polymorphs. There is neither an evident change
due to Co in the initial anatase/rutile phase distribution nor shift of the
diffraction peaks, which can be due to the low resolution in the 5–60 angular
range and to the low cobalt content.DTA
experiments gave no indication of phenomena associated with the combustion of
organic species (Co acetate has been used as the precursor salt). On the other
hand, an exothermic peak, attributable to anatase-rutile phase transformation,
was evident in the range from 750° to 800°C.Invariance of specific surface
area facilitates a comparison of the photocatalytic properties of the different
materials. More detailed analyses were carried out by XPS and
DRS and will be discussed in some detail in the following section.


#### 3.1.2. XPS and DRS Measurements

The XPS investigation has provided us with
information  on both the valence state and the degree of dispersion of cobalt.
The Co binding energy (BE) was determined with reference to the Ti2p(3/2) peak
at 458.5 eV.In presence of Co(II), the Co2p region feature two
main peaks, Co2p(3/2)-Co2p(1/2), separated by 15.8 eV, and a Co2p(3/2)/Co2p(1/2) intensity ratio of 2. The peaks have shake-up satellites located at∼5.8 eV  with higher BE than the main peaks which present a satellite-main peak ratio
of about 0.8. Because of covalence and final state effects, the BE of Co(III) is
lower than that of Co(II). The magnitude of the chemical shift between the two
states is  1.0 eV at most [24]. Owing to the small BE difference and band broadening
caused by “multiplet splitting,” the presence of Co(II) or Co(III), or both,
cannot be established from the position of the Co2p components alone. A
spectral feature that will distinguish between Co(II) and Co(III) is a decrease
in the spin-orbit separation and the marked decrease of the satellites, because
the main Co(II) peaks present a satellite, whereas the Co(III) peaks do not. As shown in Figure 1(a), in both the Co(1%)-TiO2 and Co(2%)-TiO2 samples, the Co2p(3/2) peak is accompanied by the
shake-up signal characteristic of Co(II). In addition, for Co(1%)-TiO2 (curve a), Co2p(3/2) is at 780.8 eV with a shake-up peak separated by 5.7 eV
and a satellite-main peak ratio of about 0.8 which indicates the presence of
divalent cobalt. For Co(2%)-TiO2 (curve b) the Co2p(3/2) component
is at 780.4 eV with a shake-up separation of 5.2 eV and a satellite-main peak
ratio of about 0.45, indicating the presence of Co(II) and Co(III). It can be
inferred that in the Co(1%)-TiO2 sample cobalt is present as Co(II),
whereas at increasing cobalt content both Co(II) and Co(III) states are present.  The conclusion that Co is present in the
divalent form is in accord with the above-cited literature [9].(a) XPS spectra in the Co2p(3/2) binding energy region for
Co(1%)-TiO2 (curve a) and Co(2%)-TiO2 (curve b); (b) ratio
of XPS peaks intensity, I(Co2p)/I(Ti2p), as a function of the surface
concentration of cobalt for Co(x%)-TiO2 with (1) x = 0.2%; (2) x =
0.5%; (3) x = 1%; (4) x = 2%. Full circles are the experimental data and the
dashed line refers to calculated values (see text). The final calcination
temperature for all samples was 400°C.
(a)(b)The dispersion degree of cobalt on TiO2 was determined from the (Co2p/Ti2p) intensity ratios. The plot of the intensity ratio as a function of the
surface concentration of cobalt (see Figure 1(b)) shows a deviation from linearity as
the content of cobalt increases. For a quantitative evaluation of the
surface morphology, the experimental intensity ratios ICo2p/ITi2p were
compared with the values predicted by two models: the “nonattenuating  overlayer over a semiinfinite support” and the “solid solution” models [25]. In the first model, the Co-TiO2 system was represented as consisting of a nonattenuating dispersed layer of
cobalt species on TiO2 support with  semiinfinite thickness:
(1)ICoITi=DCoDTiσCoσTisCoρTiλTi.
In the second model   the Co
species are considered homogenously dispersed in the TiO2 lattice:(2)ICoITi=DCoDTinConTiσCoσTiλCoλTi
(ρTi indicates the Ti atomic density in
the support (ρTi=0.29⁢1023 atoms cm−3); D is the instrumental
efficiency factor; σCo, σTi are the cross-sections (σCo,
19.16; σTi,
7.91); λ refers to the escape depth of electron (λTi,
1.88 nm; λCo, 1.44 nm); sCo is the Co atomic surface concentration  (atoms nm−2); and
nCo/nTi is the atomic concentration ratio).The excellent agreement between
experimental and calculated values in the Co concentration range 0.2%–1%,
as in Figure1(b), clearly indicates that cobalt is fully dispersed on the surface and
within few subsurface layers. Conversely, the intensity ratios calculated by
the solid solution model and the experimental ratios disagree (the calculated
values are 3-4 times lower than the experimental ones).The behaviour observed for the Co(2%)
sample suggests an aggregation of the cobalt species, likely forming islands or small clusters of Co3O4. This is clearly evident in the diffuse reflectance data (see below).The salient feature of the diffuse reflectance spectra
(DRS) displayed in Figure2 is that Co doping causes a shift of absorption to
higher wavelengths (curves a–c). A more in-depth DRS analysis shows interesting
details concerning absorption in the visible region (inset in Figure 2) that
are useful in establishing the nature of sites occupied by cobalt within the
TiO2 host. For percentages of added Co from 0.2 to 1%, in the
wavelength range from 500 to 1000 nm, one can identify three relatively broad
peaks close to each other at approximately 530–550 nm, 600–620 nm, and 660–680 nm. For Co(2%)-TiO2 the spectra
(not shown) feature peaks, among others, at 450 nm and 730 nm which are typical of
octahedrically coordinated Co(III) [26] indicating a separate phase formation
at these degrees of doping, in keeping with the above discussed XPS results.Figure 2
Diffuse reflectance spectra of  TiO2 (a);
Co(0.2%)-TiO2 (b); Co(0.5%)-TiO2 (c). The inset shows
measurements on an expanded scale. The final calcination temperature for all
samples was 400°C.The pattern, shown in the inset of Figure2, strongly
suggests tetrahedrically coordinated Co and, in fact, the data are comparable with the diffuse reflectance spectra of several Co-modified inorganic materials [27, 28] and of Co-ZnO [29], for which it is
well established that the doping metal is divalent and substitutionally
occupies Zn tetrahedral sites. However, it is not possible to state that all Co
are present in tetrahedral sites since the intensity of transitions attributed to
octahedrically coordinated Co is reported to be about two orders of magnitude
lower than that for the tetrahedral coordination [29].The fact that at least a significant amount of Co has a
tetrahedral coordination means that the added species which entered the lattice, for
a few layers, are occupying interstitial positions. This is probably not very
surprising considering that our experimental conditions do not involve
preparation of Co-TiO2 by coprecipitation or calcination at high
temperatures, which should favour the formation of a homogeneous solid solution
with Co in octahedral sites. In this connection, it is interesting to note
that among the vast literature on the structure and magnetic properties of
Co-TiO2 prepared by different methods, some studies [10] consider the presence of Co in interstitial tetrahedral sites as well as in octahedral
ones plausible; in both cases, the dopant introduces states located about
midway within the TiO2 band gap and causes a raise in the Fermi
level [10].


#### 3.1.3. Point of Zero Charge (PZC) and Surface Polarity

Oxide surfaces can become charged as a consequence of the
ability of coordinated OH groups to undergo hydrolysis. Thus negatively and
positively charged surfaces are obtained by the dissociation of acid groups and
protonation of basic groups, respectively. The point of zero charge (PZC),
corresponding to the condition where no net charge is present on the surface,
is an important factor that controls the adsorption of solution species and
photocatalytic activity [18].In the present work, the PZC has been determined by the
powder titration method at various values of the solution ionic strength [30].
For undoped TiO2 (P-25), the value of the PZC is 5.8 as often
reported in the literature. In the case of the Co-doped oxide, the PZC is
shifted to higher pH values: a value of 7.2 was obtained for TiO2 doped with 1% Co, as reported earlier [18]. Thus the PZC
seems to approach that of bulk Co3O4 [30], and one would be led to consider this as a possible cause; it can be ruled out,
however, since our DRS and XPS measurements do not indicate the presence
of  Co3+ and of any separate
phase formation for Co amounts  ≤1%.Quite obviously, the phenomenon we observed originates
from changes in the surface properties that can be even rather subtle, as
pointed out in an illuminating work by Contescu et al. on heterogeneity of
hydroxyl groups on TiO2 polymorphs [31]. Among
other factors, differences in coordination of –O- and –OH sites to
cations, a different proportion of proton binding groups (oxo and hydroxo),
variation of the proton-metal distances, and the contribution of different
crystal planes (on different planes, terminal oxygen has different acidity) can
bring about observable changes in the surface acid/basic behaviour. In contrast
to relatively abundant information available for pure TiO2,
investigations on doped TiO2 appear scanty [18].From our X-ray data, we have neither evidence of changes in
the crystallographic planes intensity upon Co incorporation nor evidence
of changes in the TiO2 unit cell parameters. In this regard, it is
worth stressing that Geng and Kim [10] showed that the introduction of interstitial Co into Ti16O32 model clusters leads to volume expansion,
and an increased Ti–O bond length can then possibly lead to a more
basic behaviour of the surface as discussed by Contescu et al. [31].In the present work, we also undertook the evaluation of
surface polarity, which is an important parameter since changes in polarity
(hydrophilicity), along with PZC, can strongly influence the
adsorption/desorption equilibria of several compounds and can, consequently,
affect the photocatalytic activity.Experimentally, the quantification of surface polarity is
based on suitable indicators which are employed to construct polarity scales.
Among these,  theETN(30) scale [32], based on the use of
Reichardt’s dye, is very well established and can be used to evaluate the
polarity of surfaces [33].Elaboration of our measurements are based on the
dimensionlessETN(30) scale [34] which ranges from 0 to 1
for nonpolar and very polar surfaces, respectively. The results yielded ETN(30)
values of 1 for undoped TiO2 and 0.79 for the Co(0.5%)-doped oxide,
clearly indicating a sensible decrease in the surface polarity.TheET(30) parameter is correlated to the more
accurate Kamlet-Taft scale [35, 36]:
(3)ET(30)=301.1+12.99π*−2.74δ+14.45α+2.13β,
where α and β describe the hydrogen-bond acidity and the
hydrogen-bond basicity, respectively; δ is a correction factor and    π* is a
polarizability/polarity factor. The latter is clearly an important parameter
whose evaluation needs separate experiments; however, based on the simple
examination of the α parameter, the above-reported decreased acidity of Co-TiO2 can provide one explanation of the decrease in the ET(30) or ETN(30)
compared to undoped TiO2.


#### 3.1.4. Photoelectrochemical Measurements

Oxygen is the common electron acceptor in semiconductor
photocatalysis and the study of its reduction bears a high relevance to the
optimisation and elucidation of photocatalytic processes [37].Our electrochemical measurements, in the pH range from 2
to 13, showed that cobalt brings about a shift of the onset of the O2 reduction
process (see Figure 3) toward more positive potentials, which is ascribed to an
electrocatalytic effect. On this basis, one can expect an improvement in the
overall photocatalytic processes and, indeed, there is ample literature
evidence confirming that coordinated cobalt is an efficient catalyst for oxygen
and peroxides activation [38, 39], through pathways involving Co(II) and/or
Co(I) species [40].Figure 3
Cyclic voltammetry curves for O2 reduction on TiO2 and Co(0.5%)-TiO2 in the dark. Sweep
rate: 20 mV s−1. Electrolyte: 0.1 M NaOH. Electrodes geometric  area: 2 cm2.Our experimental evidence on the effect of added cobalt on
peroxide formation during photooxidation processes will be discussed in a
separate communication. The results of preliminary investigations show that
during photocatalytic experiments, a relatively pronounced accumulation of
peroxides in the solution phase is observed with Co-TiO2 compared to
the undoped TiO2, and this can possibly correlate with the above-reported lower polarity of the former material.In Figure4, the photocurrent action spectra
plots for TiO2 and Co-TiO2,  are shown as IPCE (%) [41];
(4)IPCE(%)=1240*iph(mA/cm2)λ(nm)*I(mW/cm2)*100,
where
iph is the photocurrent and I is the intensity of the
light source. The data, obtained from photocurrent-time transients, show a
clear response to visible light, in agreement with the DRS data of Figure 2.
The lower UV response of Co-TiO2 compared with TiO2 (see Figure 4) has been frequently reported in the literature for doped TiO2 [41, 42] and attributed to the presence of
states in the band gap.Figure 4
Photoaction spectra recorded from photocurrent-time transients
using TiO2 and Co(0.5%)-TiO2 electrodes in 1 M NaClO4 solutions. Electrodes geometric area: 2 cm2.The possible charge transfer (CT) processes that occur upon visible
light absorption include the dopant centred d-d (MMCT) electronic transitions.
In principle, however, metal to ligand (MLCT) and ligand to metal (LMCT) excitation
[43, 44] are possible. In our specific case, they would imply electron
transfer from the TiO2 valence band to Co(II) (LVBMCT)
and from Co(II) to the conduction band (MLCBCT), respectively.There are investigations on the optical absorption by Co-TiO2 [11]; however, we want to single out the related, elegant study by Liu et
al. [45] because we think it is particularly detailed and informative,
considering also that the band gap and band edge positions for ZnO and TiO2 are similar. Two subbandgap transitions can lead to charge separation: a
relatively weak MLCBCT at ∼715 nm corresponding to Co3+ formation and a more intense LVBMCT at higher energy corresponding to Co+ formation. According to Schwarz and Gamelin, the
Co+ state is located in the proximity of the ZnO CB [46].


### 3.2. Photocatalytic Activity

An important goal of the present investigation is the
assessment of the effect of cobalt addition on the photocatalytic activity of
TiO2 in the degradation of model organic pollutants. In the previous
paragraph, we have drawn attention to the fact that the better catalytic
activity of Co-TiO2 for O2 reduction, compared with the
undoped oxide, offers one good reason to expect improvements in the efficiency
of photocatalytic processes. In the following, we examine the photocatalytic
activity, for irradiation with both UV and visible light, as a function of
additional parameters in the Co-TiO2 preparation method such as the
amount of cobalt and the calcination temperature.


#### 3.2.1. Optimisation of the Photocatalytic Activity of Co-TiO2

We have discussed so far the effect of the degree of
cobalt doping on the physical characteristics of TiO2. In this
section, we report on the influence of the amount of doping on the
photoactivity of Co-TiO2. Since the final treatment in the
preparation of the photocatalysts is calcination, we also carried out a series
of experiments aiming to determine the optimal calcination temperature. For
this series of correlated experiments, we used 4-chlorophenol as a model
substrate because its photodegradation has been extensively investigated and
the reaction mechanism is well established [47].The plots of Figure5 illustrate the effect of calcination
temperature on the time required for mineralization of 4-chlorophenol in suspensions
of Co(0.2%)-TiO2 and of TiO2 for comparison. Examination
of the results reveals that Co-TiO2 is always more active than the
original TiO2 that was subjected to the same thermal treatment. The
best result is obtained for heat treatment at 400°C for 30 minutes: prolonged
heating (>2 hours) at this temperature did not have appreciable effects.Figure 5
Effects of the calcination time and temperature on the time
required for the photodegradation of 1 mM 4-chlorophenol in aqueous suspensions
of  TiO2 and Co(x%)-TiO2, where the degree
of doping (x%) is 0.2.It is important to
point out, in this respect, that at this temperature no significant change in
the morphology and phase composition is observed for the pure TiO2 (P-25, Degussa). It is also noteworthy that a low-temperature treatment
(100°C), following TiO2 impregnation with the cobalt salt, has a
negligible influence on the photocatalytic activity (see Figure 5). Moreover, this
temperature is not sufficiently high to drive the metal into the lattice, and
hence it can be concluded that adsorbed Co(II) does not provide active sites
for improving the photocatalytic efficiency. This lack of effect is confirmed
by photocatalytic experiments carried out in the presence of Co(NO3)2 added to the solution.Experiments for the determination of the photodegradation
efficiency of 4-chlorophenol in Co-TiO2 suspensions were performed
as a function of the amount of doping. The plots in Figure 6 show the effect of
a different content in cobalt on the efficiency of 4-chlorophenol
photodegradation in Co-TiO2 suspensions. It is evident that the best
results are obtained for a cobalt doping degree of 0.2%.Figure 6
Time required for the photodegradation of 1 mM 4-chlorophenol
in aqueous suspensions of  Co(x%)-TiO2 as a function of the amount of cobalt (x%). The final calcination temperature
was 400°C for 30 minutes.From the applied point of view, it is often more practical
to use photocatalysts in the form of films supported on different substrates
such as glass. This is mainly because the use of films avoids the separation of
the solid by filtration at the end of experiments, which is a necessary
procedure when one uses suspensions. For this reason, we made experiments using
glass supported films of TiO2 and Co-TiO2 with a
different percent of the doping element, again using the photodegradation of
4-chlorophenol as a model reaction. The results showed that in agreement with
the reported data for suspensions, Co-TiO2 is more active than the pristine oxide. Also in accordance with the results
of Figure 6, there is an optimum degree of doping for which the photocatalytic
activity reaches a maximum that, in this case, turns out to be around 0.5%.Leaching of cobalt
during photocatalytic measurements deserves to be mentioned. For the
experimental conditions of Figures5 and 6, that is, irradiation for ∼6 hours at a pH
close to neutrality, the loss of cobalt as soluble species from Co(0.5%)-TiO2 amounts to 1.44 mg L−1 which corresponds to 2.8% of the cobalt
concentration that is impregnated into TiO2. Measurements at
different pH values gave a leaching of 38% and 1.25% at pH=2 and 10,
respectively. It is evident that Co-TiO2 is not a recommended
photocatalyst for applications requiring acidic conditions. For a comparison
with literature data, Tayade et al. [17] report a cobalt leaching of 4.6% at
pH=5.


#### 3.2.2. Visible Light Photoactivity

Both the DRS data of Figure2 and the photoaction spectra
of Figure 4 show that Co-doping causes a red shift in the absorption of light,
and in the discussion above we speculated on possible causes (vide supra).
We thought it was meaningful to carry on experiments in which irradiation is
restricted to the visible region (λ>420
or 450 nm).The photodegradation of 4-chlorophenol in Co-TiO2 suspensions, illuminated with light of wavelength λ>420 nm, confirmed that the doped
photocatalyst responds to visible light in a way that depends on the amount of
cobalt; the optimal doping degree was found to be 0.5%. The photocatalytic
activity of Co-TiO2 is still good at longer wavelengths (λ>450 nm), but it is interesting to note
that undoped TiO2 still shows residual, albeit low, activity even at
these wavelengths (see Figure 7).Figure 7
Visible light (λ>450 nm) photodegradation of 1 mM
4-chlorophenol as a function of time in aqueous suspensions of  TiO2 and Co(x%)-TiO2, where the degree
of doping (x%) is 0.5. The final calcination temperature was 400°C for 30 minutes.We extended our studies on the visible light response
of Co-TiO2 to the photodegradation of 2,2′-bis(4′-hydroxyphenyl)
propane (Bisphenol A), a more complex chemical substrate than 4-chlorophenol,
that is used for epoxy and polycarbonate resins and is well known to be an endocrine
disrupting chemical [48, 49]. Typical results, obtained on irradiation of TiO2 and Co(0.5%)-TiO2 suspensions at λ>420 nm, are shown in Figure 8. The data
confirm the higher photocatalytic efficiency of Co-TiO2, especially
at short irradiation times. As noted for the case of 4-chlorophenol, undoped
TiO2 has an appreciable activity that, in our opinion, is not fully
explained. Possible reasons are discussed in a recent paper by Kuznetsov and
Serpone [50].Figure 8
Visible light (λ>420 nm) photodegradation of 0.8 mM
2,2′-bis(4′-hydroxyphenyl) propane (Bisphenol A), as a function of time in
aqueous suspensions of  TiO2 and Co(x%)-TiO2, where the degree
of doping x% is 0.5. The final calcination temperature was 400°C for 30 minutes.


## 3.1. Catalyst Characterisation

### 3.1.1. General Characterisation

Surface microstructure is highly dependent on the
preparation conditions and on posttreatments. Small additions of various metal
oxides can also greatly influence (alter) the crystalline morphology as well as
the conductivity of the host matrix.A general investigation of the materials characteristics
includes measurements of surface area (BET method), X-ray diffraction (XRD),
and differential thermal analysis (DTA). Surface area measurements were carried
out on TiO2 samples doped with 0.2, 0.5, 1, and 2% w/w and the values
obtained ranged from 47 to 49 m2g−1, indicating that the
adopted doping procedure does not bring about any surface area change with
respect to the pristine oxide (∼50 m2g−1). Likewise,
XRD spectra of samples with 0.2, 0.5, and 1% Co, collected in the 2θ range 5–60°, showed only peaks due to anatase
and rutile, which is expected since the commercial TiO2 used is well
known to be a mixture of the two polymorphs. There is neither an evident change
due to Co in the initial anatase/rutile phase distribution nor shift of the
diffraction peaks, which can be due to the low resolution in the 5–60 angular
range and to the low cobalt content.DTA
experiments gave no indication of phenomena associated with the combustion of
organic species (Co acetate has been used as the precursor salt). On the other
hand, an exothermic peak, attributable to anatase-rutile phase transformation,
was evident in the range from 750° to 800°C.Invariance of specific surface
area facilitates a comparison of the photocatalytic properties of the different
materials. More detailed analyses were carried out by XPS and
DRS and will be discussed in some detail in the following section.


### 3.1.2. XPS and DRS Measurements

The XPS investigation has provided us with
information  on both the valence state and the degree of dispersion of cobalt.
The Co binding energy (BE) was determined with reference to the Ti2p(3/2) peak
at 458.5 eV.In presence of Co(II), the Co2p region feature two
main peaks, Co2p(3/2)-Co2p(1/2), separated by 15.8 eV, and a Co2p(3/2)/Co2p(1/2) intensity ratio of 2. The peaks have shake-up satellites located at∼5.8 eV  with higher BE than the main peaks which present a satellite-main peak ratio
of about 0.8. Because of covalence and final state effects, the BE of Co(III) is
lower than that of Co(II). The magnitude of the chemical shift between the two
states is  1.0 eV at most [24]. Owing to the small BE difference and band broadening
caused by “multiplet splitting,” the presence of Co(II) or Co(III), or both,
cannot be established from the position of the Co2p components alone. A
spectral feature that will distinguish between Co(II) and Co(III) is a decrease
in the spin-orbit separation and the marked decrease of the satellites, because
the main Co(II) peaks present a satellite, whereas the Co(III) peaks do not. As shown in Figure 1(a), in both the Co(1%)-TiO2 and Co(2%)-TiO2 samples, the Co2p(3/2) peak is accompanied by the
shake-up signal characteristic of Co(II). In addition, for Co(1%)-TiO2 (curve a), Co2p(3/2) is at 780.8 eV with a shake-up peak separated by 5.7 eV
and a satellite-main peak ratio of about 0.8 which indicates the presence of
divalent cobalt. For Co(2%)-TiO2 (curve b) the Co2p(3/2) component
is at 780.4 eV with a shake-up separation of 5.2 eV and a satellite-main peak
ratio of about 0.45, indicating the presence of Co(II) and Co(III). It can be
inferred that in the Co(1%)-TiO2 sample cobalt is present as Co(II),
whereas at increasing cobalt content both Co(II) and Co(III) states are present.  The conclusion that Co is present in the
divalent form is in accord with the above-cited literature [9].(a) XPS spectra in the Co2p(3/2) binding energy region for
Co(1%)-TiO2 (curve a) and Co(2%)-TiO2 (curve b); (b) ratio
of XPS peaks intensity, I(Co2p)/I(Ti2p), as a function of the surface
concentration of cobalt for Co(x%)-TiO2 with (1) x = 0.2%; (2) x =
0.5%; (3) x = 1%; (4) x = 2%. Full circles are the experimental data and the
dashed line refers to calculated values (see text). The final calcination
temperature for all samples was 400°C.
(a)(b)The dispersion degree of cobalt on TiO2 was determined from the (Co2p/Ti2p) intensity ratios. The plot of the intensity ratio as a function of the
surface concentration of cobalt (see Figure 1(b)) shows a deviation from linearity as
the content of cobalt increases. For a quantitative evaluation of the
surface morphology, the experimental intensity ratios ICo2p/ITi2p were
compared with the values predicted by two models: the “nonattenuating  overlayer over a semiinfinite support” and the “solid solution” models [25]. In the first model, the Co-TiO2 system was represented as consisting of a nonattenuating dispersed layer of
cobalt species on TiO2 support with  semiinfinite thickness:
(1)ICoITi=DCoDTiσCoσTisCoρTiλTi.
In the second model   the Co
species are considered homogenously dispersed in the TiO2 lattice:(2)ICoITi=DCoDTinConTiσCoσTiλCoλTi
(ρTi indicates the Ti atomic density in
the support (ρTi=0.29⁢1023 atoms cm−3); D is the instrumental
efficiency factor; σCo, σTi are the cross-sections (σCo,
19.16; σTi,
7.91); λ refers to the escape depth of electron (λTi,
1.88 nm; λCo, 1.44 nm); sCo is the Co atomic surface concentration  (atoms nm−2); and
nCo/nTi is the atomic concentration ratio).The excellent agreement between
experimental and calculated values in the Co concentration range 0.2%–1%,
as in Figure1(b), clearly indicates that cobalt is fully dispersed on the surface and
within few subsurface layers. Conversely, the intensity ratios calculated by
the solid solution model and the experimental ratios disagree (the calculated
values are 3-4 times lower than the experimental ones).The behaviour observed for the Co(2%)
sample suggests an aggregation of the cobalt species, likely forming islands or small clusters of Co3O4. This is clearly evident in the diffuse reflectance data (see below).The salient feature of the diffuse reflectance spectra
(DRS) displayed in Figure2 is that Co doping causes a shift of absorption to
higher wavelengths (curves a–c). A more in-depth DRS analysis shows interesting
details concerning absorption in the visible region (inset in Figure 2) that
are useful in establishing the nature of sites occupied by cobalt within the
TiO2 host. For percentages of added Co from 0.2 to 1%, in the
wavelength range from 500 to 1000 nm, one can identify three relatively broad
peaks close to each other at approximately 530–550 nm, 600–620 nm, and 660–680 nm. For Co(2%)-TiO2 the spectra
(not shown) feature peaks, among others, at 450 nm and 730 nm which are typical of
octahedrically coordinated Co(III) [26] indicating a separate phase formation
at these degrees of doping, in keeping with the above discussed XPS results.Figure 2
Diffuse reflectance spectra of  TiO2 (a);
Co(0.2%)-TiO2 (b); Co(0.5%)-TiO2 (c). The inset shows
measurements on an expanded scale. The final calcination temperature for all
samples was 400°C.The pattern, shown in the inset of Figure2, strongly
suggests tetrahedrically coordinated Co and, in fact, the data are comparable with the diffuse reflectance spectra of several Co-modified inorganic materials [27, 28] and of Co-ZnO [29], for which it is
well established that the doping metal is divalent and substitutionally
occupies Zn tetrahedral sites. However, it is not possible to state that all Co
are present in tetrahedral sites since the intensity of transitions attributed to
octahedrically coordinated Co is reported to be about two orders of magnitude
lower than that for the tetrahedral coordination [29].The fact that at least a significant amount of Co has a
tetrahedral coordination means that the added species which entered the lattice, for
a few layers, are occupying interstitial positions. This is probably not very
surprising considering that our experimental conditions do not involve
preparation of Co-TiO2 by coprecipitation or calcination at high
temperatures, which should favour the formation of a homogeneous solid solution
with Co in octahedral sites. In this connection, it is interesting to note
that among the vast literature on the structure and magnetic properties of
Co-TiO2 prepared by different methods, some studies [10] consider the presence of Co in interstitial tetrahedral sites as well as in octahedral
ones plausible; in both cases, the dopant introduces states located about
midway within the TiO2 band gap and causes a raise in the Fermi
level [10].


### 3.1.3. Point of Zero Charge (PZC) and Surface Polarity

Oxide surfaces can become charged as a consequence of the
ability of coordinated OH groups to undergo hydrolysis. Thus negatively and
positively charged surfaces are obtained by the dissociation of acid groups and
protonation of basic groups, respectively. The point of zero charge (PZC),
corresponding to the condition where no net charge is present on the surface,
is an important factor that controls the adsorption of solution species and
photocatalytic activity [18].In the present work, the PZC has been determined by the
powder titration method at various values of the solution ionic strength [30].
For undoped TiO2 (P-25), the value of the PZC is 5.8 as often
reported in the literature. In the case of the Co-doped oxide, the PZC is
shifted to higher pH values: a value of 7.2 was obtained for TiO2 doped with 1% Co, as reported earlier [18]. Thus the PZC
seems to approach that of bulk Co3O4 [30], and one would be led to consider this as a possible cause; it can be ruled out,
however, since our DRS and XPS measurements do not indicate the presence
of  Co3+ and of any separate
phase formation for Co amounts  ≤1%.Quite obviously, the phenomenon we observed originates
from changes in the surface properties that can be even rather subtle, as
pointed out in an illuminating work by Contescu et al. on heterogeneity of
hydroxyl groups on TiO2 polymorphs [31]. Among
other factors, differences in coordination of –O- and –OH sites to
cations, a different proportion of proton binding groups (oxo and hydroxo),
variation of the proton-metal distances, and the contribution of different
crystal planes (on different planes, terminal oxygen has different acidity) can
bring about observable changes in the surface acid/basic behaviour. In contrast
to relatively abundant information available for pure TiO2,
investigations on doped TiO2 appear scanty [18].From our X-ray data, we have neither evidence of changes in
the crystallographic planes intensity upon Co incorporation nor evidence
of changes in the TiO2 unit cell parameters. In this regard, it is
worth stressing that Geng and Kim [10] showed that the introduction of interstitial Co into Ti16O32 model clusters leads to volume expansion,
and an increased Ti–O bond length can then possibly lead to a more
basic behaviour of the surface as discussed by Contescu et al. [31].In the present work, we also undertook the evaluation of
surface polarity, which is an important parameter since changes in polarity
(hydrophilicity), along with PZC, can strongly influence the
adsorption/desorption equilibria of several compounds and can, consequently,
affect the photocatalytic activity.Experimentally, the quantification of surface polarity is
based on suitable indicators which are employed to construct polarity scales.
Among these,  theETN(30) scale [32], based on the use of
Reichardt’s dye, is very well established and can be used to evaluate the
polarity of surfaces [33].Elaboration of our measurements are based on the
dimensionlessETN(30) scale [34] which ranges from 0 to 1
for nonpolar and very polar surfaces, respectively. The results yielded ETN(30)
values of 1 for undoped TiO2 and 0.79 for the Co(0.5%)-doped oxide,
clearly indicating a sensible decrease in the surface polarity.TheET(30) parameter is correlated to the more
accurate Kamlet-Taft scale [35, 36]:
(3)ET(30)=301.1+12.99π*−2.74δ+14.45α+2.13β,
where α and β describe the hydrogen-bond acidity and the
hydrogen-bond basicity, respectively; δ is a correction factor and    π* is a
polarizability/polarity factor. The latter is clearly an important parameter
whose evaluation needs separate experiments; however, based on the simple
examination of the α parameter, the above-reported decreased acidity of Co-TiO2 can provide one explanation of the decrease in the ET(30) or ETN(30)
compared to undoped TiO2.


### 3.1.4. Photoelectrochemical Measurements

Oxygen is the common electron acceptor in semiconductor
photocatalysis and the study of its reduction bears a high relevance to the
optimisation and elucidation of photocatalytic processes [37].Our electrochemical measurements, in the pH range from 2
to 13, showed that cobalt brings about a shift of the onset of the O2 reduction
process (see Figure 3) toward more positive potentials, which is ascribed to an
electrocatalytic effect. On this basis, one can expect an improvement in the
overall photocatalytic processes and, indeed, there is ample literature
evidence confirming that coordinated cobalt is an efficient catalyst for oxygen
and peroxides activation [38, 39], through pathways involving Co(II) and/or
Co(I) species [40].Figure 3
Cyclic voltammetry curves for O2 reduction on TiO2 and Co(0.5%)-TiO2 in the dark. Sweep
rate: 20 mV s−1. Electrolyte: 0.1 M NaOH. Electrodes geometric  area: 2 cm2.Our experimental evidence on the effect of added cobalt on
peroxide formation during photooxidation processes will be discussed in a
separate communication. The results of preliminary investigations show that
during photocatalytic experiments, a relatively pronounced accumulation of
peroxides in the solution phase is observed with Co-TiO2 compared to
the undoped TiO2, and this can possibly correlate with the above-reported lower polarity of the former material.In Figure4, the photocurrent action spectra
plots for TiO2 and Co-TiO2,  are shown as IPCE (%) [41];
(4)IPCE(%)=1240*iph(mA/cm2)λ(nm)*I(mW/cm2)*100,
where
iph is the photocurrent and I is the intensity of the
light source. The data, obtained from photocurrent-time transients, show a
clear response to visible light, in agreement with the DRS data of Figure 2.
The lower UV response of Co-TiO2 compared with TiO2 (see Figure 4) has been frequently reported in the literature for doped TiO2 [41, 42] and attributed to the presence of
states in the band gap.Figure 4
Photoaction spectra recorded from photocurrent-time transients
using TiO2 and Co(0.5%)-TiO2 electrodes in 1 M NaClO4 solutions. Electrodes geometric area: 2 cm2.The possible charge transfer (CT) processes that occur upon visible
light absorption include the dopant centred d-d (MMCT) electronic transitions.
In principle, however, metal to ligand (MLCT) and ligand to metal (LMCT) excitation
[43, 44] are possible. In our specific case, they would imply electron
transfer from the TiO2 valence band to Co(II) (LVBMCT)
and from Co(II) to the conduction band (MLCBCT), respectively.There are investigations on the optical absorption by Co-TiO2 [11]; however, we want to single out the related, elegant study by Liu et
al. [45] because we think it is particularly detailed and informative,
considering also that the band gap and band edge positions for ZnO and TiO2 are similar. Two subbandgap transitions can lead to charge separation: a
relatively weak MLCBCT at ∼715 nm corresponding to Co3+ formation and a more intense LVBMCT at higher energy corresponding to Co+ formation. According to Schwarz and Gamelin, the
Co+ state is located in the proximity of the ZnO CB [46].


## 3.1.1. General Characterisation

Surface microstructure is highly dependent on the
preparation conditions and on posttreatments. Small additions of various metal
oxides can also greatly influence (alter) the crystalline morphology as well as
the conductivity of the host matrix.A general investigation of the materials characteristics
includes measurements of surface area (BET method), X-ray diffraction (XRD),
and differential thermal analysis (DTA). Surface area measurements were carried
out on TiO2 samples doped with 0.2, 0.5, 1, and 2% w/w and the values
obtained ranged from 47 to 49 m2g−1, indicating that the
adopted doping procedure does not bring about any surface area change with
respect to the pristine oxide (∼50 m2g−1). Likewise,
XRD spectra of samples with 0.2, 0.5, and 1% Co, collected in the 2θ range 5–60°, showed only peaks due to anatase
and rutile, which is expected since the commercial TiO2 used is well
known to be a mixture of the two polymorphs. There is neither an evident change
due to Co in the initial anatase/rutile phase distribution nor shift of the
diffraction peaks, which can be due to the low resolution in the 5–60 angular
range and to the low cobalt content.DTA
experiments gave no indication of phenomena associated with the combustion of
organic species (Co acetate has been used as the precursor salt). On the other
hand, an exothermic peak, attributable to anatase-rutile phase transformation,
was evident in the range from 750° to 800°C.Invariance of specific surface
area facilitates a comparison of the photocatalytic properties of the different
materials. More detailed analyses were carried out by XPS and
DRS and will be discussed in some detail in the following section.


## 3.1.2. XPS and DRS Measurements

The XPS investigation has provided us with
information  on both the valence state and the degree of dispersion of cobalt.
The Co binding energy (BE) was determined with reference to the Ti2p(3/2) peak
at 458.5 eV.In presence of Co(II), the Co2p region feature two
main peaks, Co2p(3/2)-Co2p(1/2), separated by 15.8 eV, and a Co2p(3/2)/Co2p(1/2) intensity ratio of 2. The peaks have shake-up satellites located at∼5.8 eV  with higher BE than the main peaks which present a satellite-main peak ratio
of about 0.8. Because of covalence and final state effects, the BE of Co(III) is
lower than that of Co(II). The magnitude of the chemical shift between the two
states is  1.0 eV at most [24]. Owing to the small BE difference and band broadening
caused by “multiplet splitting,” the presence of Co(II) or Co(III), or both,
cannot be established from the position of the Co2p components alone. A
spectral feature that will distinguish between Co(II) and Co(III) is a decrease
in the spin-orbit separation and the marked decrease of the satellites, because
the main Co(II) peaks present a satellite, whereas the Co(III) peaks do not. As shown in Figure 1(a), in both the Co(1%)-TiO2 and Co(2%)-TiO2 samples, the Co2p(3/2) peak is accompanied by the
shake-up signal characteristic of Co(II). In addition, for Co(1%)-TiO2 (curve a), Co2p(3/2) is at 780.8 eV with a shake-up peak separated by 5.7 eV
and a satellite-main peak ratio of about 0.8 which indicates the presence of
divalent cobalt. For Co(2%)-TiO2 (curve b) the Co2p(3/2) component
is at 780.4 eV with a shake-up separation of 5.2 eV and a satellite-main peak
ratio of about 0.45, indicating the presence of Co(II) and Co(III). It can be
inferred that in the Co(1%)-TiO2 sample cobalt is present as Co(II),
whereas at increasing cobalt content both Co(II) and Co(III) states are present.  The conclusion that Co is present in the
divalent form is in accord with the above-cited literature [9].(a) XPS spectra in the Co2p(3/2) binding energy region for
Co(1%)-TiO2 (curve a) and Co(2%)-TiO2 (curve b); (b) ratio
of XPS peaks intensity, I(Co2p)/I(Ti2p), as a function of the surface
concentration of cobalt for Co(x%)-TiO2 with (1) x = 0.2%; (2) x =
0.5%; (3) x = 1%; (4) x = 2%. Full circles are the experimental data and the
dashed line refers to calculated values (see text). The final calcination
temperature for all samples was 400°C.
(a)(b)The dispersion degree of cobalt on TiO2 was determined from the (Co2p/Ti2p) intensity ratios. The plot of the intensity ratio as a function of the
surface concentration of cobalt (see Figure 1(b)) shows a deviation from linearity as
the content of cobalt increases. For a quantitative evaluation of the
surface morphology, the experimental intensity ratios ICo2p/ITi2p were
compared with the values predicted by two models: the “nonattenuating  overlayer over a semiinfinite support” and the “solid solution” models [25]. In the first model, the Co-TiO2 system was represented as consisting of a nonattenuating dispersed layer of
cobalt species on TiO2 support with  semiinfinite thickness:
(1)ICoITi=DCoDTiσCoσTisCoρTiλTi.
In the second model   the Co
species are considered homogenously dispersed in the TiO2 lattice:(2)ICoITi=DCoDTinConTiσCoσTiλCoλTi
(ρTi indicates the Ti atomic density in
the support (ρTi=0.29⁢1023 atoms cm−3); D is the instrumental
efficiency factor; σCo, σTi are the cross-sections (σCo,
19.16; σTi,
7.91); λ refers to the escape depth of electron (λTi,
1.88 nm; λCo, 1.44 nm); sCo is the Co atomic surface concentration  (atoms nm−2); and
nCo/nTi is the atomic concentration ratio).The excellent agreement between
experimental and calculated values in the Co concentration range 0.2%–1%,
as in Figure1(b), clearly indicates that cobalt is fully dispersed on the surface and
within few subsurface layers. Conversely, the intensity ratios calculated by
the solid solution model and the experimental ratios disagree (the calculated
values are 3-4 times lower than the experimental ones).The behaviour observed for the Co(2%)
sample suggests an aggregation of the cobalt species, likely forming islands or small clusters of Co3O4. This is clearly evident in the diffuse reflectance data (see below).The salient feature of the diffuse reflectance spectra
(DRS) displayed in Figure2 is that Co doping causes a shift of absorption to
higher wavelengths (curves a–c). A more in-depth DRS analysis shows interesting
details concerning absorption in the visible region (inset in Figure 2) that
are useful in establishing the nature of sites occupied by cobalt within the
TiO2 host. For percentages of added Co from 0.2 to 1%, in the
wavelength range from 500 to 1000 nm, one can identify three relatively broad
peaks close to each other at approximately 530–550 nm, 600–620 nm, and 660–680 nm. For Co(2%)-TiO2 the spectra
(not shown) feature peaks, among others, at 450 nm and 730 nm which are typical of
octahedrically coordinated Co(III) [26] indicating a separate phase formation
at these degrees of doping, in keeping with the above discussed XPS results.Figure 2
Diffuse reflectance spectra of  TiO2 (a);
Co(0.2%)-TiO2 (b); Co(0.5%)-TiO2 (c). The inset shows
measurements on an expanded scale. The final calcination temperature for all
samples was 400°C.The pattern, shown in the inset of Figure2, strongly
suggests tetrahedrically coordinated Co and, in fact, the data are comparable with the diffuse reflectance spectra of several Co-modified inorganic materials [27, 28] and of Co-ZnO [29], for which it is
well established that the doping metal is divalent and substitutionally
occupies Zn tetrahedral sites. However, it is not possible to state that all Co
are present in tetrahedral sites since the intensity of transitions attributed to
octahedrically coordinated Co is reported to be about two orders of magnitude
lower than that for the tetrahedral coordination [29].The fact that at least a significant amount of Co has a
tetrahedral coordination means that the added species which entered the lattice, for
a few layers, are occupying interstitial positions. This is probably not very
surprising considering that our experimental conditions do not involve
preparation of Co-TiO2 by coprecipitation or calcination at high
temperatures, which should favour the formation of a homogeneous solid solution
with Co in octahedral sites. In this connection, it is interesting to note
that among the vast literature on the structure and magnetic properties of
Co-TiO2 prepared by different methods, some studies [10] consider the presence of Co in interstitial tetrahedral sites as well as in octahedral
ones plausible; in both cases, the dopant introduces states located about
midway within the TiO2 band gap and causes a raise in the Fermi
level [10].


## 3.1.3. Point of Zero Charge (PZC) and Surface Polarity

Oxide surfaces can become charged as a consequence of the
ability of coordinated OH groups to undergo hydrolysis. Thus negatively and
positively charged surfaces are obtained by the dissociation of acid groups and
protonation of basic groups, respectively. The point of zero charge (PZC),
corresponding to the condition where no net charge is present on the surface,
is an important factor that controls the adsorption of solution species and
photocatalytic activity [18].In the present work, the PZC has been determined by the
powder titration method at various values of the solution ionic strength [30].
For undoped TiO2 (P-25), the value of the PZC is 5.8 as often
reported in the literature. In the case of the Co-doped oxide, the PZC is
shifted to higher pH values: a value of 7.2 was obtained for TiO2 doped with 1% Co, as reported earlier [18]. Thus the PZC
seems to approach that of bulk Co3O4 [30], and one would be led to consider this as a possible cause; it can be ruled out,
however, since our DRS and XPS measurements do not indicate the presence
of  Co3+ and of any separate
phase formation for Co amounts  ≤1%.Quite obviously, the phenomenon we observed originates
from changes in the surface properties that can be even rather subtle, as
pointed out in an illuminating work by Contescu et al. on heterogeneity of
hydroxyl groups on TiO2 polymorphs [31]. Among
other factors, differences in coordination of –O- and –OH sites to
cations, a different proportion of proton binding groups (oxo and hydroxo),
variation of the proton-metal distances, and the contribution of different
crystal planes (on different planes, terminal oxygen has different acidity) can
bring about observable changes in the surface acid/basic behaviour. In contrast
to relatively abundant information available for pure TiO2,
investigations on doped TiO2 appear scanty [18].From our X-ray data, we have neither evidence of changes in
the crystallographic planes intensity upon Co incorporation nor evidence
of changes in the TiO2 unit cell parameters. In this regard, it is
worth stressing that Geng and Kim [10] showed that the introduction of interstitial Co into Ti16O32 model clusters leads to volume expansion,
and an increased Ti–O bond length can then possibly lead to a more
basic behaviour of the surface as discussed by Contescu et al. [31].In the present work, we also undertook the evaluation of
surface polarity, which is an important parameter since changes in polarity
(hydrophilicity), along with PZC, can strongly influence the
adsorption/desorption equilibria of several compounds and can, consequently,
affect the photocatalytic activity.Experimentally, the quantification of surface polarity is
based on suitable indicators which are employed to construct polarity scales.
Among these,  theETN(30) scale [32], based on the use of
Reichardt’s dye, is very well established and can be used to evaluate the
polarity of surfaces [33].Elaboration of our measurements are based on the
dimensionlessETN(30) scale [34] which ranges from 0 to 1
for nonpolar and very polar surfaces, respectively. The results yielded ETN(30)
values of 1 for undoped TiO2 and 0.79 for the Co(0.5%)-doped oxide,
clearly indicating a sensible decrease in the surface polarity.TheET(30) parameter is correlated to the more
accurate Kamlet-Taft scale [35, 36]:
(3)ET(30)=301.1+12.99π*−2.74δ+14.45α+2.13β,
where α and β describe the hydrogen-bond acidity and the
hydrogen-bond basicity, respectively; δ is a correction factor and    π* is a
polarizability/polarity factor. The latter is clearly an important parameter
whose evaluation needs separate experiments; however, based on the simple
examination of the α parameter, the above-reported decreased acidity of Co-TiO2 can provide one explanation of the decrease in the ET(30) or ETN(30)
compared to undoped TiO2.


## 3.1.4. Photoelectrochemical Measurements

Oxygen is the common electron acceptor in semiconductor
photocatalysis and the study of its reduction bears a high relevance to the
optimisation and elucidation of photocatalytic processes [37].Our electrochemical measurements, in the pH range from 2
to 13, showed that cobalt brings about a shift of the onset of the O2 reduction
process (see Figure 3) toward more positive potentials, which is ascribed to an
electrocatalytic effect. On this basis, one can expect an improvement in the
overall photocatalytic processes and, indeed, there is ample literature
evidence confirming that coordinated cobalt is an efficient catalyst for oxygen
and peroxides activation [38, 39], through pathways involving Co(II) and/or
Co(I) species [40].Figure 3
Cyclic voltammetry curves for O2 reduction on TiO2 and Co(0.5%)-TiO2 in the dark. Sweep
rate: 20 mV s−1. Electrolyte: 0.1 M NaOH. Electrodes geometric  area: 2 cm2.Our experimental evidence on the effect of added cobalt on
peroxide formation during photooxidation processes will be discussed in a
separate communication. The results of preliminary investigations show that
during photocatalytic experiments, a relatively pronounced accumulation of
peroxides in the solution phase is observed with Co-TiO2 compared to
the undoped TiO2, and this can possibly correlate with the above-reported lower polarity of the former material.In Figure4, the photocurrent action spectra
plots for TiO2 and Co-TiO2,  are shown as IPCE (%) [41];
(4)IPCE(%)=1240*iph(mA/cm2)λ(nm)*I(mW/cm2)*100,
where
iph is the photocurrent and I is the intensity of the
light source. The data, obtained from photocurrent-time transients, show a
clear response to visible light, in agreement with the DRS data of Figure 2.
The lower UV response of Co-TiO2 compared with TiO2 (see Figure 4) has been frequently reported in the literature for doped TiO2 [41, 42] and attributed to the presence of
states in the band gap.Figure 4
Photoaction spectra recorded from photocurrent-time transients
using TiO2 and Co(0.5%)-TiO2 electrodes in 1 M NaClO4 solutions. Electrodes geometric area: 2 cm2.The possible charge transfer (CT) processes that occur upon visible
light absorption include the dopant centred d-d (MMCT) electronic transitions.
In principle, however, metal to ligand (MLCT) and ligand to metal (LMCT) excitation
[43, 44] are possible. In our specific case, they would imply electron
transfer from the TiO2 valence band to Co(II) (LVBMCT)
and from Co(II) to the conduction band (MLCBCT), respectively.There are investigations on the optical absorption by Co-TiO2 [11]; however, we want to single out the related, elegant study by Liu et
al. [45] because we think it is particularly detailed and informative,
considering also that the band gap and band edge positions for ZnO and TiO2 are similar. Two subbandgap transitions can lead to charge separation: a
relatively weak MLCBCT at ∼715 nm corresponding to Co3+ formation and a more intense LVBMCT at higher energy corresponding to Co+ formation. According to Schwarz and Gamelin, the
Co+ state is located in the proximity of the ZnO CB [46].


## 3.2. Photocatalytic Activity

An important goal of the present investigation is the
assessment of the effect of cobalt addition on the photocatalytic activity of
TiO2 in the degradation of model organic pollutants. In the previous
paragraph, we have drawn attention to the fact that the better catalytic
activity of Co-TiO2 for O2 reduction, compared with the
undoped oxide, offers one good reason to expect improvements in the efficiency
of photocatalytic processes. In the following, we examine the photocatalytic
activity, for irradiation with both UV and visible light, as a function of
additional parameters in the Co-TiO2 preparation method such as the
amount of cobalt and the calcination temperature.


### 3.2.1. Optimisation of the Photocatalytic Activity of Co-TiO2

We have discussed so far the effect of the degree of
cobalt doping on the physical characteristics of TiO2. In this
section, we report on the influence of the amount of doping on the
photoactivity of Co-TiO2. Since the final treatment in the
preparation of the photocatalysts is calcination, we also carried out a series
of experiments aiming to determine the optimal calcination temperature. For
this series of correlated experiments, we used 4-chlorophenol as a model
substrate because its photodegradation has been extensively investigated and
the reaction mechanism is well established [47].The plots of Figure5 illustrate the effect of calcination
temperature on the time required for mineralization of 4-chlorophenol in suspensions
of Co(0.2%)-TiO2 and of TiO2 for comparison. Examination
of the results reveals that Co-TiO2 is always more active than the
original TiO2 that was subjected to the same thermal treatment. The
best result is obtained for heat treatment at 400°C for 30 minutes: prolonged
heating (>2 hours) at this temperature did not have appreciable effects.Figure 5
Effects of the calcination time and temperature on the time
required for the photodegradation of 1 mM 4-chlorophenol in aqueous suspensions
of  TiO2 and Co(x%)-TiO2, where the degree
of doping (x%) is 0.2.It is important to
point out, in this respect, that at this temperature no significant change in
the morphology and phase composition is observed for the pure TiO2 (P-25, Degussa). It is also noteworthy that a low-temperature treatment
(100°C), following TiO2 impregnation with the cobalt salt, has a
negligible influence on the photocatalytic activity (see Figure 5). Moreover, this
temperature is not sufficiently high to drive the metal into the lattice, and
hence it can be concluded that adsorbed Co(II) does not provide active sites
for improving the photocatalytic efficiency. This lack of effect is confirmed
by photocatalytic experiments carried out in the presence of Co(NO3)2 added to the solution.Experiments for the determination of the photodegradation
efficiency of 4-chlorophenol in Co-TiO2 suspensions were performed
as a function of the amount of doping. The plots in Figure 6 show the effect of
a different content in cobalt on the efficiency of 4-chlorophenol
photodegradation in Co-TiO2 suspensions. It is evident that the best
results are obtained for a cobalt doping degree of 0.2%.Figure 6
Time required for the photodegradation of 1 mM 4-chlorophenol
in aqueous suspensions of  Co(x%)-TiO2 as a function of the amount of cobalt (x%). The final calcination temperature
was 400°C for 30 minutes.From the applied point of view, it is often more practical
to use photocatalysts in the form of films supported on different substrates
such as glass. This is mainly because the use of films avoids the separation of
the solid by filtration at the end of experiments, which is a necessary
procedure when one uses suspensions. For this reason, we made experiments using
glass supported films of TiO2 and Co-TiO2 with a
different percent of the doping element, again using the photodegradation of
4-chlorophenol as a model reaction. The results showed that in agreement with
the reported data for suspensions, Co-TiO2 is more active than the pristine oxide. Also in accordance with the results
of Figure 6, there is an optimum degree of doping for which the photocatalytic
activity reaches a maximum that, in this case, turns out to be around 0.5%.Leaching of cobalt
during photocatalytic measurements deserves to be mentioned. For the
experimental conditions of Figures5 and 6, that is, irradiation for ∼6 hours at a pH
close to neutrality, the loss of cobalt as soluble species from Co(0.5%)-TiO2 amounts to 1.44 mg L−1 which corresponds to 2.8% of the cobalt
concentration that is impregnated into TiO2. Measurements at
different pH values gave a leaching of 38% and 1.25% at pH=2 and 10,
respectively. It is evident that Co-TiO2 is not a recommended
photocatalyst for applications requiring acidic conditions. For a comparison
with literature data, Tayade et al. [17] report a cobalt leaching of 4.6% at
pH=5.


### 3.2.2. Visible Light Photoactivity

Both the DRS data of Figure2 and the photoaction spectra
of Figure 4 show that Co-doping causes a red shift in the absorption of light,
and in the discussion above we speculated on possible causes (vide supra).
We thought it was meaningful to carry on experiments in which irradiation is
restricted to the visible region (λ>420
or 450 nm).The photodegradation of 4-chlorophenol in Co-TiO2 suspensions, illuminated with light of wavelength λ>420 nm, confirmed that the doped
photocatalyst responds to visible light in a way that depends on the amount of
cobalt; the optimal doping degree was found to be 0.5%. The photocatalytic
activity of Co-TiO2 is still good at longer wavelengths (λ>450 nm), but it is interesting to note
that undoped TiO2 still shows residual, albeit low, activity even at
these wavelengths (see Figure 7).Figure 7
Visible light (λ>450 nm) photodegradation of 1 mM
4-chlorophenol as a function of time in aqueous suspensions of  TiO2 and Co(x%)-TiO2, where the degree
of doping (x%) is 0.5. The final calcination temperature was 400°C for 30 minutes.We extended our studies on the visible light response
of Co-TiO2 to the photodegradation of 2,2′-bis(4′-hydroxyphenyl)
propane (Bisphenol A), a more complex chemical substrate than 4-chlorophenol,
that is used for epoxy and polycarbonate resins and is well known to be an endocrine
disrupting chemical [48, 49]. Typical results, obtained on irradiation of TiO2 and Co(0.5%)-TiO2 suspensions at λ>420 nm, are shown in Figure 8. The data
confirm the higher photocatalytic efficiency of Co-TiO2, especially
at short irradiation times. As noted for the case of 4-chlorophenol, undoped
TiO2 has an appreciable activity that, in our opinion, is not fully
explained. Possible reasons are discussed in a recent paper by Kuznetsov and
Serpone [50].Figure 8
Visible light (λ>420 nm) photodegradation of 0.8 mM
2,2′-bis(4′-hydroxyphenyl) propane (Bisphenol A), as a function of time in
aqueous suspensions of  TiO2 and Co(x%)-TiO2, where the degree
of doping x% is 0.5. The final calcination temperature was 400°C for 30 minutes.


## 3.2.1. Optimisation of the Photocatalytic Activity of Co-TiO2

We have discussed so far the effect of the degree of
cobalt doping on the physical characteristics of TiO2. In this
section, we report on the influence of the amount of doping on the
photoactivity of Co-TiO2. Since the final treatment in the
preparation of the photocatalysts is calcination, we also carried out a series
of experiments aiming to determine the optimal calcination temperature. For
this series of correlated experiments, we used 4-chlorophenol as a model
substrate because its photodegradation has been extensively investigated and
the reaction mechanism is well established [47].The plots of Figure5 illustrate the effect of calcination
temperature on the time required for mineralization of 4-chlorophenol in suspensions
of Co(0.2%)-TiO2 and of TiO2 for comparison. Examination
of the results reveals that Co-TiO2 is always more active than the
original TiO2 that was subjected to the same thermal treatment. The
best result is obtained for heat treatment at 400°C for 30 minutes: prolonged
heating (>2 hours) at this temperature did not have appreciable effects.Figure 5
Effects of the calcination time and temperature on the time
required for the photodegradation of 1 mM 4-chlorophenol in aqueous suspensions
of  TiO2 and Co(x%)-TiO2, where the degree
of doping (x%) is 0.2.It is important to
point out, in this respect, that at this temperature no significant change in
the morphology and phase composition is observed for the pure TiO2 (P-25, Degussa). It is also noteworthy that a low-temperature treatment
(100°C), following TiO2 impregnation with the cobalt salt, has a
negligible influence on the photocatalytic activity (see Figure 5). Moreover, this
temperature is not sufficiently high to drive the metal into the lattice, and
hence it can be concluded that adsorbed Co(II) does not provide active sites
for improving the photocatalytic efficiency. This lack of effect is confirmed
by photocatalytic experiments carried out in the presence of Co(NO3)2 added to the solution.Experiments for the determination of the photodegradation
efficiency of 4-chlorophenol in Co-TiO2 suspensions were performed
as a function of the amount of doping. The plots in Figure 6 show the effect of
a different content in cobalt on the efficiency of 4-chlorophenol
photodegradation in Co-TiO2 suspensions. It is evident that the best
results are obtained for a cobalt doping degree of 0.2%.Figure 6
Time required for the photodegradation of 1 mM 4-chlorophenol
in aqueous suspensions of  Co(x%)-TiO2 as a function of the amount of cobalt (x%). The final calcination temperature
was 400°C for 30 minutes.From the applied point of view, it is often more practical
to use photocatalysts in the form of films supported on different substrates
such as glass. This is mainly because the use of films avoids the separation of
the solid by filtration at the end of experiments, which is a necessary
procedure when one uses suspensions. For this reason, we made experiments using
glass supported films of TiO2 and Co-TiO2 with a
different percent of the doping element, again using the photodegradation of
4-chlorophenol as a model reaction. The results showed that in agreement with
the reported data for suspensions, Co-TiO2 is more active than the pristine oxide. Also in accordance with the results
of Figure 6, there is an optimum degree of doping for which the photocatalytic
activity reaches a maximum that, in this case, turns out to be around 0.5%.Leaching of cobalt
during photocatalytic measurements deserves to be mentioned. For the
experimental conditions of Figures5 and 6, that is, irradiation for ∼6 hours at a pH
close to neutrality, the loss of cobalt as soluble species from Co(0.5%)-TiO2 amounts to 1.44 mg L−1 which corresponds to 2.8% of the cobalt
concentration that is impregnated into TiO2. Measurements at
different pH values gave a leaching of 38% and 1.25% at pH=2 and 10,
respectively. It is evident that Co-TiO2 is not a recommended
photocatalyst for applications requiring acidic conditions. For a comparison
with literature data, Tayade et al. [17] report a cobalt leaching of 4.6% at
pH=5.


## 3.2.2. Visible Light Photoactivity

Both the DRS data of Figure2 and the photoaction spectra
of Figure 4 show that Co-doping causes a red shift in the absorption of light,
and in the discussion above we speculated on possible causes (vide supra).
We thought it was meaningful to carry on experiments in which irradiation is
restricted to the visible region (λ>420
or 450 nm).The photodegradation of 4-chlorophenol in Co-TiO2 suspensions, illuminated with light of wavelength λ>420 nm, confirmed that the doped
photocatalyst responds to visible light in a way that depends on the amount of
cobalt; the optimal doping degree was found to be 0.5%. The photocatalytic
activity of Co-TiO2 is still good at longer wavelengths (λ>450 nm), but it is interesting to note
that undoped TiO2 still shows residual, albeit low, activity even at
these wavelengths (see Figure 7).Figure 7
Visible light (λ>450 nm) photodegradation of 1 mM
4-chlorophenol as a function of time in aqueous suspensions of  TiO2 and Co(x%)-TiO2, where the degree
of doping (x%) is 0.5. The final calcination temperature was 400°C for 30 minutes.We extended our studies on the visible light response
of Co-TiO2 to the photodegradation of 2,2′-bis(4′-hydroxyphenyl)
propane (Bisphenol A), a more complex chemical substrate than 4-chlorophenol,
that is used for epoxy and polycarbonate resins and is well known to be an endocrine
disrupting chemical [48, 49]. Typical results, obtained on irradiation of TiO2 and Co(0.5%)-TiO2 suspensions at λ>420 nm, are shown in Figure 8. The data
confirm the higher photocatalytic efficiency of Co-TiO2, especially
at short irradiation times. As noted for the case of 4-chlorophenol, undoped
TiO2 has an appreciable activity that, in our opinion, is not fully
explained. Possible reasons are discussed in a recent paper by Kuznetsov and
Serpone [50].Figure 8
Visible light (λ>420 nm) photodegradation of 0.8 mM
2,2′-bis(4′-hydroxyphenyl) propane (Bisphenol A), as a function of time in
aqueous suspensions of  TiO2 and Co(x%)-TiO2, where the degree
of doping x% is 0.5. The final calcination temperature was 400°C for 30 minutes.


## 4. Conclusions

Cobalt-modified TiO2 (Co-TiO2) is able to bring about the photocatalytic degradation of
pollutants such as 4-chlorophenol and Bisphenol A under both UV and visible
light irradiation. The best photoactivity was observed for an amount of cobalt
between 0.2% and 0.5% Co/TiO2 w/w.Diffuse reflectance measurements indicate that
a substantial amount of incorporated Co is present in interstitial positions.
XPS measurements show that Co enters the TiO2 lattice as the
divalent species, that it is well dispersed, and that there are no separate
cobalt phase formation for percents of added Co<1%. In this case, the
foreign species are likely localized at the surface and within relatively few
subsurface layers.In comparison to
pristine TiO2, Co-TiO2 features change in the point of
zero charge (PZC) toward more basic values and a substantial decrease in the
surface polarity. We also show that Co-TiO2 is a more active
catalyst than pure TiO2 for the reduction of O2 in the
dark.



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*Source: 853753-2008-01-17.xml*