# Increase of the Photocatalytic Activity ofTiO2 by Carbon and Iron Modifications

Authors: Beata Tryba
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

Modification ofTiO2 by doping of a residue carbon and iron can give enhanced photoactivity of TiO2. Iron adsorbed on the surface of TiO2 can be an electron or hole scavenger and results in the improvement of the separation of free carriers. The presence of carbon can increase the concentration of organic pollutants on the surface of TiO2 facilitating the contact of the reactive species with the organic molecules. Carbon-doped TiO2 can extend the absorption of the light to the visible region and makes the photocatalysts active under visible-light irradiation. It was proved that TiO2 modified by carbon and iron can work in both photocatalysis and photo-Fenton processes, when H2O2 is used, enhancing markedly the rate of the organic compounds decomposition such as phenol, humic acids and dyes. The photocatalytic decomposition of organic compounds on TiO2 modified by iron and carbon is going by the complex reactions of iron with the  intermediates, what significantly accelerate the process of their decomposition. The presence of carbon in such photocatalyst retards the inconvenient reaction of OH radicals scavenging by H2O2, which occurs when Fe-TiO2 photocatalyst is used.


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## Body

## 1. Introduction

The photocatalytic process with using TiO2 photocatalyst is
very promising for application in the water purification because many organic
compounds can be
decomposed and mineralized by the proceeding oxidation and
reduction processes on TiO2 surface. The most commonly tested
compounds for decomposition through the
photocatalysis are phenols,
chlorophenols, pesticides, herbicides, benzenes, alcohols, dyes, pharmaceutics,
humic acids, organic acids, and others.TiO2 is the most commonly
used photocatalyst, because it is nontoxic, chemically stable, cheap, and very
efficient. However, it has some disadvantages: one of this is a relatively high
value of the bandgap, around 3.2 eV, which limits its using to the UV light,
high dispersion in the water which causes difficulties in sedimentation, and
sensitivity to the recombination of photoinduced electrons and holes, which
decreases its photocatalytic activity. Many efforts of researchers are focused
on the enhancing the photoactivity of TiO2 by improving the
separation between free carriers, increasing the adsorption abilities of the
photocatalyst surface, or charging one photocatalyst by another when the
mixture of two photocatalysts is used.


## 2. TiO2 Modified by Carbon

### 2.1.Carbon-Doped TiO2 Visible Ligh-Active Photocatalysts

Recently, modification of TiO2 by C
doping towards obtaining the visible light-active photocatalyst has been reported
[1–9]. The narrowing of the bandgap, the anodic shift of the quasi-Fermi
potential, the visible-light absorption, and the transfer of photoexcited
carriers to the reactive sites at the catalyst surface have been noticed as a
result of nonmetal anion doping affecting its visible-light activity.Sakthivel and Kisch [2] proved that
prepared C/TiO2 photocatalyst from TiCl4 and
tetrabutylammonium hydroxide was
active under artificial solar light and could
efficiently decompose tetrachlorophenol with the activity higher than TiO2 and TiO2 doped with nitrogen. In Figure 1, diffuse reflectance
spectra of pure and carbon-doped TiO2 are shown [2].(A) Diffuse reflectance spectra of modified and pure TiO2, (B)
plot of transformed Kubelka-Munk function versus the energy of the light absorbed; (a) TiO2, (b) TiO2-2.98%C, (c) TiO2-0.42%C, (d) TiO2-0.03%C.
(a)(b)Two absorption maxima on the UV-Vis
spectra have been observed, in the range of UV and Vis, showing that these
photocatalysts could be excited with the wavelengths in the visible region.In
Figure2, the mineralisation of the tetrachlorophenol on these samples is
presented.(A) Photomineralisation of 4-chlorophenol
with artificial visible light (λ = 455 nm; TOC0 and TOCt = total organic carbon content at times 0 and t); (B) diffuse indoor daylight
degradation of 4-chlorophenol and remazol red on pure and modified TiO2; (a) TiO2, (b) TiO2-2.98%C, (c) TiO2-0.42%C, (d) TiO2-0.03%C.
(a)(b)Shen et al. [4] prepared C/TiO2 photocatalyst through calcination of TiCl4 in air at 350°C and
proved that such prepared photocatalyst was active for decomposition of
trichloroacetic acid under visible light. Carbon-doped TiO2 can be
prepared also by hydrolysis and calcinations of TiCl4 with
tetra-n-butyl ammonium hydroxide [7], and by calcinations of TiO2 with urea and thiourea [8] or oxidation of TiC at high temperatures [9]. Choi
et al. [9] have investigated carbon-doped TiO2, and they claimed
that substitution of C for O in the TiO2 leads to a photocatalytic
decomposition of methylene blue under visible-light irradiation. Di Valentin et
al. [10] reported a theory of in-building of carbon atom in the structure of
TiO2, and they showed that carbon atom could replace oxygen atom or
titania atom depending on the concentration of oxygen in the structure of TiO2.
Replacing of carbon atom with Ti conducts to form the new states in the bandgap.
Some oxygen vacancies are formed, which could be responsible for the extending
of the photocatalytic activity of C/TiO2 to the visible range. The
model proposed by Di Valentin et al. is presented in Figure 3 [10].Partial geometry of the models for: (a) one substitutional C
atom to O (CS-O), (b) one substitutional C atom Ti (CS-Ti), (c) one interstitial C atom (CI), and (d) one interstitial C atom nearby an oxygen vacancy (CI + VO) in the anatase TiO2 supercell; the yellow spheres represent O atoms, the small brown spheres represent Ti atoms, and the black represents the carbon impurity.
(a)(b)(c)(d)Preparation of C/TiO2 photocatalyst
by heating of TiO2 with the vapours of n-hexane [11] and ethanol [12],
and by mixing of TiO2 with liquid ethanol and heating under pressure
[13] has been also reported. Enhanced photoactivity of carbon-doped TiO2 prepared under pressure has been noticed for decomposition of dyes under UV
irradiation [13].Carbon-doped TiO2 electrodes have been prepared by modified sol-gel method from
tetraisopropoxide, 2-propanol, and activated carbon at 600°C and were used for
electrochemical photooxidation of sodium oxalate; the efficiency of electrode
with 5% of doped activated carbon was two times higher than the electrode
without carbon doping [14].Effective photoresponse in the
visible-light region was observed in carbon-doped TiO2 [7, 15]. Carbon-doped
TiO2 nanotube arrays for efficient solar water splitting have been
prepared by Park et al. [16]. The synthesised TiO2-xCx nanotube
arrays showed much higher photocurrent densities and more efficient water
splitting under visible-light illumination (> 420 nm) than pure TiO2 nanotube arrays.


### 2.2.TiO2 and Activated Carbon (AC) Composites

Herrmann et al. [17], Araña et al. [18, 19], and Liu et al. [20] have observed the synergistic effect for the mixture
of TiO2 with activated carbon (AC). Improving the photocatalytic
properties of TiO2/AC composite was explained by the high adsorption
of the impurities on the surface of activated carbon and their transfer to TiO2 surface. Carbon/TiO2 microsphere has been prepared by Nagaoka et al.
[21], and
it was successfully used for decomposition of acetaldehyde through the concentration of the pollutant
around TiO2 anchored on the composite surface. In Figure 4, the enhanced
photoactivity with using TiO2/AC composite is presented under both
solar and UV irradiations, for photodecomposition of p-nitrophenol [19].Figure 4
Apparent first-order linear transform ln (n0/n) = f(t) of the p-nitrophenol concentration during its degradation; UV-light: (▴)—bare TiO2, (▪)—13% AC-TiO2; solar light: (▵)—bare TiO2, (□)—13 % AC-TiO2 .


### 2.3.Carbon-Coated TiO2

Carbon-coated TiO2 photocatalysts have been prepared by calcination of TiO2 with carbon
precursor such as (polyvinyl alcohol) (PVA), poly (terephthalate ethylene) (PET), or hydroxyl propyl
cellulose (HPC) at high temperatures, 700–900°C [22–24], or impregnation of TiO2 with sacharose 400–600°C [25]. Carbon coating TiO2 retarded the
phase transformation from anatase to rutile, which usually occurs during
heating TiO2 at 700°C and through that improved the crystallinity of
anatase phase in TiO2, which was responsible for its high
photoactivity. From the other hand, carbon coating reduced the amount of UV
radiation reaching the surface of the TiO2 particles. A balance
among different factors controlled by the carbon layer on the TiO2 particles was required to get high-photocatalytic activity. On the sample
prepared at 850°C with a carbon content of about 3.5 wt%, the highest rate constant for methylene
blue decomposition was obtained, in which the transition from anatase to rutile
was suppressed, and carbon layer was thin enough to transmit UV rays [24]. The TEM
images of carbon-coated TiO2 are presented in Figure 5 [23].TEM images of carbon-coated TiO2 prepared from
TiO2 and PVA.
(a)(b)Carbon-coated TiO2 samples showed high adsorption and high photoactivity towards methylene blue
decomposition [22–25], however in case of phenol, reactive black 5 or iminoctadine triacetate
decomposition was slower than on unmodified TiO2 [23]. In Figure 6, there
is shown a cycling decomposition of methylene blue on carbon-coated TiO2 prepared from powders of TiO2 and PVA at 900°C with ratio of TiO2/PVA
= 50/50 in weight [22].Figure 6
Color fading of MB solution under UV irradiation in the presence of carbon-coated TiO2 sample (10 wt% of carbon) with cycling to use fresh MB solution, concentration of MB = 2.94 × 10−5 M.Coating of carbon changes the nature
of TiO2 from hydrophilic to hydrophobic and results in lower
adsorption of water on the photocatalyst surface and lower formation of OH
radicals in comparison with TiO2. Probably in case of methylene blue,
decomposition is going by the direct oxidation pathway, therefore high
adsorption of methylene blue on the carbon-coated TiO2 improved its
photocatalytic activity.


### 2.4.TiO2 Loaded Carbon

TiO2 can be loaded on the activated
carbon (AC) [26–35], carbon fibers [36], carbon nanotube [37], exfoliated graphite [38, 39] with
efficient adsorption and decomposition of organic compounds such as phenol, 4-chlorophenol,
methyl orange, methylene blue, iminoctadine triacetates, oils, dichloromethane, and so forth. In
Figure 7, TiO2 loaded on different carbon structures is presented.TiO2 loaded on (a) carbon spheres [28], (b) activated carbon [28], (c) granular activated carbon [35], (d) carbon nanotubes [37], (e) carbon fibers [36], and (f) exfoliated graphite [38].
(a)(b)(c)(d)(e)(f)TiO2 loaded carbon
spheres have been prepared by hydrolysis of TiOSO4 under
hydrothermal conditions. Adsorption and decomposition of methylene blue on TiO2 loaded carbon spheres are presented in Figure 8 [28]. It can be observed
saturation with methylene blue adsorption on carbon spheres (CS), and no
change in methylene blue concentration with UV irradiation, whereas on anatase loaded carbon spheres (Ti/CS) methylene blue could be
adsorbed in the dark and decomposed with UV irradiation in the cycles.Figure 8
Changes in relative concentration of MB in the solution in which pristine carbon spheres (CS) and anatase loaded one (Ti/CS) were dispersed.Tryba et al. [29] have reported
preparation of TiO2 loaded AC from the tetraorthotitanate solution. Higher
removal of organic compounds has been achieved by combination of adsorption,
which occurred in the pores of activated carbon, with decomposition on the
anatase particles. Decreas in adsorption has been observed after TiO2 mounting, because mounted TiO2 particles blocked the entrance to the
pores of AC, however these TiO2 particles could decompose the
adsorbed organic molecules in the pores of AC, enhancing the total removal of
the pollutant [29].El-Sheikh et al. [34] have prepared
TiO2 loaded AC by different methods: chemical vapour deposition
(CVD), direct air-hydrolysis (DAH), and high-temperature impregnation (HTI), among those methods, CVD gave
the best bounding of TiO2 with the carbon surface; anatase particles
were placed in the pores of activated carbon.Tsumura et al. have mounted TiO2 on
the exfoliated graphite [38]. High sorption of oil and its decomposition under
UV irradiation have been observed.


## 2.1.Carbon-Doped TiO2 Visible Ligh-Active Photocatalysts

Recently, modification of TiO2 by C
doping towards obtaining the visible light-active photocatalyst has been reported
[1–9]. The narrowing of the bandgap, the anodic shift of the quasi-Fermi
potential, the visible-light absorption, and the transfer of photoexcited
carriers to the reactive sites at the catalyst surface have been noticed as a
result of nonmetal anion doping affecting its visible-light activity.Sakthivel and Kisch [2] proved that
prepared C/TiO2 photocatalyst from TiCl4 and
tetrabutylammonium hydroxide was
active under artificial solar light and could
efficiently decompose tetrachlorophenol with the activity higher than TiO2 and TiO2 doped with nitrogen. In Figure 1, diffuse reflectance
spectra of pure and carbon-doped TiO2 are shown [2].(A) Diffuse reflectance spectra of modified and pure TiO2, (B)
plot of transformed Kubelka-Munk function versus the energy of the light absorbed; (a) TiO2, (b) TiO2-2.98%C, (c) TiO2-0.42%C, (d) TiO2-0.03%C.
(a)(b)Two absorption maxima on the UV-Vis
spectra have been observed, in the range of UV and Vis, showing that these
photocatalysts could be excited with the wavelengths in the visible region.In
Figure2, the mineralisation of the tetrachlorophenol on these samples is
presented.(A) Photomineralisation of 4-chlorophenol
with artificial visible light (λ = 455 nm; TOC0 and TOCt = total organic carbon content at times 0 and t); (B) diffuse indoor daylight
degradation of 4-chlorophenol and remazol red on pure and modified TiO2; (a) TiO2, (b) TiO2-2.98%C, (c) TiO2-0.42%C, (d) TiO2-0.03%C.
(a)(b)Shen et al. [4] prepared C/TiO2 photocatalyst through calcination of TiCl4 in air at 350°C and
proved that such prepared photocatalyst was active for decomposition of
trichloroacetic acid under visible light. Carbon-doped TiO2 can be
prepared also by hydrolysis and calcinations of TiCl4 with
tetra-n-butyl ammonium hydroxide [7], and by calcinations of TiO2 with urea and thiourea [8] or oxidation of TiC at high temperatures [9]. Choi
et al. [9] have investigated carbon-doped TiO2, and they claimed
that substitution of C for O in the TiO2 leads to a photocatalytic
decomposition of methylene blue under visible-light irradiation. Di Valentin et
al. [10] reported a theory of in-building of carbon atom in the structure of
TiO2, and they showed that carbon atom could replace oxygen atom or
titania atom depending on the concentration of oxygen in the structure of TiO2.
Replacing of carbon atom with Ti conducts to form the new states in the bandgap.
Some oxygen vacancies are formed, which could be responsible for the extending
of the photocatalytic activity of C/TiO2 to the visible range. The
model proposed by Di Valentin et al. is presented in Figure 3 [10].Partial geometry of the models for: (a) one substitutional C
atom to O (CS-O), (b) one substitutional C atom Ti (CS-Ti), (c) one interstitial C atom (CI), and (d) one interstitial C atom nearby an oxygen vacancy (CI + VO) in the anatase TiO2 supercell; the yellow spheres represent O atoms, the small brown spheres represent Ti atoms, and the black represents the carbon impurity.
(a)(b)(c)(d)Preparation of C/TiO2 photocatalyst
by heating of TiO2 with the vapours of n-hexane [11] and ethanol [12],
and by mixing of TiO2 with liquid ethanol and heating under pressure
[13] has been also reported. Enhanced photoactivity of carbon-doped TiO2 prepared under pressure has been noticed for decomposition of dyes under UV
irradiation [13].Carbon-doped TiO2 electrodes have been prepared by modified sol-gel method from
tetraisopropoxide, 2-propanol, and activated carbon at 600°C and were used for
electrochemical photooxidation of sodium oxalate; the efficiency of electrode
with 5% of doped activated carbon was two times higher than the electrode
without carbon doping [14].Effective photoresponse in the
visible-light region was observed in carbon-doped TiO2 [7, 15]. Carbon-doped
TiO2 nanotube arrays for efficient solar water splitting have been
prepared by Park et al. [16]. The synthesised TiO2-xCx nanotube
arrays showed much higher photocurrent densities and more efficient water
splitting under visible-light illumination (> 420 nm) than pure TiO2 nanotube arrays.


## 2.2.TiO2 and Activated Carbon (AC) Composites

Herrmann et al. [17], Araña et al. [18, 19], and Liu et al. [20] have observed the synergistic effect for the mixture
of TiO2 with activated carbon (AC). Improving the photocatalytic
properties of TiO2/AC composite was explained by the high adsorption
of the impurities on the surface of activated carbon and their transfer to TiO2 surface. Carbon/TiO2 microsphere has been prepared by Nagaoka et al.
[21], and
it was successfully used for decomposition of acetaldehyde through the concentration of the pollutant
around TiO2 anchored on the composite surface. In Figure 4, the enhanced
photoactivity with using TiO2/AC composite is presented under both
solar and UV irradiations, for photodecomposition of p-nitrophenol [19].Figure 4
Apparent first-order linear transform ln (n0/n) = f(t) of the p-nitrophenol concentration during its degradation; UV-light: (▴)—bare TiO2, (▪)—13% AC-TiO2; solar light: (▵)—bare TiO2, (□)—13 % AC-TiO2 .


## 2.3.Carbon-Coated TiO2

Carbon-coated TiO2 photocatalysts have been prepared by calcination of TiO2 with carbon
precursor such as (polyvinyl alcohol) (PVA), poly (terephthalate ethylene) (PET), or hydroxyl propyl
cellulose (HPC) at high temperatures, 700–900°C [22–24], or impregnation of TiO2 with sacharose 400–600°C [25]. Carbon coating TiO2 retarded the
phase transformation from anatase to rutile, which usually occurs during
heating TiO2 at 700°C and through that improved the crystallinity of
anatase phase in TiO2, which was responsible for its high
photoactivity. From the other hand, carbon coating reduced the amount of UV
radiation reaching the surface of the TiO2 particles. A balance
among different factors controlled by the carbon layer on the TiO2 particles was required to get high-photocatalytic activity. On the sample
prepared at 850°C with a carbon content of about 3.5 wt%, the highest rate constant for methylene
blue decomposition was obtained, in which the transition from anatase to rutile
was suppressed, and carbon layer was thin enough to transmit UV rays [24]. The TEM
images of carbon-coated TiO2 are presented in Figure 5 [23].TEM images of carbon-coated TiO2 prepared from
TiO2 and PVA.
(a)(b)Carbon-coated TiO2 samples showed high adsorption and high photoactivity towards methylene blue
decomposition [22–25], however in case of phenol, reactive black 5 or iminoctadine triacetate
decomposition was slower than on unmodified TiO2 [23]. In Figure 6, there
is shown a cycling decomposition of methylene blue on carbon-coated TiO2 prepared from powders of TiO2 and PVA at 900°C with ratio of TiO2/PVA
= 50/50 in weight [22].Figure 6
Color fading of MB solution under UV irradiation in the presence of carbon-coated TiO2 sample (10 wt% of carbon) with cycling to use fresh MB solution, concentration of MB = 2.94 × 10−5 M.Coating of carbon changes the nature
of TiO2 from hydrophilic to hydrophobic and results in lower
adsorption of water on the photocatalyst surface and lower formation of OH
radicals in comparison with TiO2. Probably in case of methylene blue,
decomposition is going by the direct oxidation pathway, therefore high
adsorption of methylene blue on the carbon-coated TiO2 improved its
photocatalytic activity.


## 2.4.TiO2 Loaded Carbon

TiO2 can be loaded on the activated
carbon (AC) [26–35], carbon fibers [36], carbon nanotube [37], exfoliated graphite [38, 39] with
efficient adsorption and decomposition of organic compounds such as phenol, 4-chlorophenol,
methyl orange, methylene blue, iminoctadine triacetates, oils, dichloromethane, and so forth. In
Figure 7, TiO2 loaded on different carbon structures is presented.TiO2 loaded on (a) carbon spheres [28], (b) activated carbon [28], (c) granular activated carbon [35], (d) carbon nanotubes [37], (e) carbon fibers [36], and (f) exfoliated graphite [38].
(a)(b)(c)(d)(e)(f)TiO2 loaded carbon
spheres have been prepared by hydrolysis of TiOSO4 under
hydrothermal conditions. Adsorption and decomposition of methylene blue on TiO2 loaded carbon spheres are presented in Figure 8 [28]. It can be observed
saturation with methylene blue adsorption on carbon spheres (CS), and no
change in methylene blue concentration with UV irradiation, whereas on anatase loaded carbon spheres (Ti/CS) methylene blue could be
adsorbed in the dark and decomposed with UV irradiation in the cycles.Figure 8
Changes in relative concentration of MB in the solution in which pristine carbon spheres (CS) and anatase loaded one (Ti/CS) were dispersed.Tryba et al. [29] have reported
preparation of TiO2 loaded AC from the tetraorthotitanate solution. Higher
removal of organic compounds has been achieved by combination of adsorption,
which occurred in the pores of activated carbon, with decomposition on the
anatase particles. Decreas in adsorption has been observed after TiO2 mounting, because mounted TiO2 particles blocked the entrance to the
pores of AC, however these TiO2 particles could decompose the
adsorbed organic molecules in the pores of AC, enhancing the total removal of
the pollutant [29].El-Sheikh et al. [34] have prepared
TiO2 loaded AC by different methods: chemical vapour deposition
(CVD), direct air-hydrolysis (DAH), and high-temperature impregnation (HTI), among those methods, CVD gave
the best bounding of TiO2 with the carbon surface; anatase particles
were placed in the pores of activated carbon.Tsumura et al. have mounted TiO2 on
the exfoliated graphite [38]. High sorption of oil and its decomposition under
UV irradiation have been observed.


## 3. TiO2 Modified by Iron

### 3.1.Preparation Methods

There are few methods
of Fe doping to TiO2. The sol-gel method has been widely used for
preparation of Fe-doped  TiO2 from TiCl4 or titania alkoxide and an iron precursor like FeCl3,
Fe(NO3)3·9H2O,
or Fe(III)-acetyloacetonate [40–47]. Wang et al. [40] have reported that
preferable preparation of Fe-doped TiO2 to get the uniform
distribution of the dopant ions on TiO2 particles is hydrolysis of a
homogeneous mixture of organic titanium and organic iron precursors in
isopropyl alcohol. Preparation of Fe-doped TiO2 by hydrolysis of
TiCl4 with FeCl3 appeared to be less favourable than
using Fe(III)-acetyloacetonate as an iron precursor. Fe doping from microemulsion of Ti
tetraisopropoxide with an aqueous solution of iron and further calcinations has
been reported by Adán et al. [48], Fe-doped TiO2 can be obtained
also through the calcination of
FexTiS2 [49–51], plasma oxidative pyrolysis [52], one-step flame
spray pyrolysis (FSP) [53], coprecipitation and immersion [46], and by the wet
impregnation method from Fe(III) acetylacetonate [54] or Fe(NO3)3·9H2O [42].Navío has been reported that impregnation
with Fe(III)-acetyloacetonate gives more homogeneous distribution of iron for
each mixed oxide sample on the particle surfaces but not between particles in
comparison with impregnation with Fe(NO3)3·9H2O [54].Recently, doping of Fe to TiO2 by the ultrasonic-induced
hydrolysis reaction of tetrabutyl titanate (Ti(OC4H9)4)
in a ferric nitrate aqueous solution has been reported in [55].The mechanical alloying on the solid
state reaction of hematite,Fe2O3, and titanium has been
also sufficiently used for introducing of iron to TiO2 lattice [56].TiO2/Fe thin film has been
prepared by metal (Fe) plasma ion implantation [57, 58] and by magnetron sputtering method [59].Using a simple sol-gel method, a novel
magnetic photocatalyst was produced by immobilisation of TiO2 nanocrystal
on Fe-filled carbon nanocapsules. TiO2-coated Fe-CNC displayed good
performance in the removal of NO gas under UV exposure [60].Fe-doped TiO2 nanotubes with small diameter of 10 nm were
obtained by hydrothermal method [61].


### 3.2.Visible-Light Activity of Fe-Doped TiO2

It has been found that
the addition of transition metals to TiO2 can improve the
photocatalytic activity of the photocatalyst by UV irradiation and extend its
use in the visible region of the electromagnetic spectrum. It has been observed
the red shift in the UV-Vis spectra due to the introduction of the 3d electron
state of Fe3+, 3d5, in the conduction band of TiO2
[40, 44, 48, 49, 58, 59, 62–65].The absorbance of visible light is higher
for higher amount of doped iron, as it is presented in Figure9 [48].Figure 9
UV-Vis spectra of Fe-doped TiO2 with different iron content.This implies that iron-doped TiO2 may be photocatalytically
reactive under visible-light irradiation. Navío et al. have reported that
the photodegradation of oxalic acid under visible irradiation, not occurring
with TiO2, could be observed for Fe-doped TiO2 using 5%
Fe-containing samples [54]. Nahar et al.
reported that Fe-doped TiO2 was responsive to the visible-light
activity of phenol degradation [49]. Teoh et al. have reported that by flame spray
pyrolysis a stable Fe-TiO2 photocatalyst can be prepared and at the
ratio of Fe/Ti = 0.05 it has high activity towards oxalic acid mineralisation
under visible light [53]. Wang
et al. have reported that for Fe(III) doping in TiO2> 0.05 at.%, decomposition of
methyl orange under UV irradiation has been lower than on undoped TiO2,
but under visible-light irradiation the Fe(III)-doped TiO2 with an
intermediate iron doping concentration of ≈1 at.% had the highest photocatalytic
reactivity due to the narrowing of bandgap so that it could effectively absorb
the light with longer wavelength [52]. Chen and Peng have reported the
preparation of magnetic-nanometer titanium dioxide/ferriferous oxide (TiO2/Fe3O4)
composite photocatalyst with the particles size of 30–50 nm, which appeared to
be very active under visible light and highly effective in discoloring of
wastewater [66].It has been investigated that Fe/TiO2 particles had a higher hydrophilic property compared with TiO2 [65, 67].TiO2/Fe thin film has been
demonstrated to have antimicrobial activity after being irradiated with visible
light [68].


### 3.3.Enhancement of Photocatalytic Activity of
TiO2 by Iron Doping

It is
generally accepted that Fe(III) centres form shallow charge trapping sites
within the TiO2 matrix as well as on the particle surface through
the replacement of Ti(IV) by Fe(III) [69]. Based on the favourable energy
levels, Fe(III) centres may act either as an electron or a hole trap (see Figure 10); so that photogenerated charge carriers are temporarily separated more
effectively [40].Figure 10
Scheme of mechanism of TiO2 photocatalysis with modifications by Fe(III) ion doping (CB—conduction band, VB—valence band).Wang et al. [40] have reported that
enhancement of the quantum yield by Fe(III) doping TiO2 can be
explained by assuming that the Fe(III) centre acts predominately as a shallow
electron trap from which the electron is transferred to molecular oxygen more
rapidly than the undoped TiO2.However, it was proved that the
photoactivity of Fe/TiO2 catalyst is dependent on the way of
preparation and the amount and state of iron.Wang et al. [40] have reported that
the highest quantum yield has been obtained when Fe(III)-acetyloacetonate
precursor was used and for the optimal doping levels 0.25 and 0.5 atom %. They
have also reported that above-mentioned optimal doping level, some of the
Fe(III) dopants might act as shallow hole traps leading to an enhanced recombination
of the trapped charge carriers.Navío et al. [42] have also observed
that at the some conditions and for the certain quantity of Fe(III), the dopant
can be a center of recombination. They prepared Fe/TiO2 by an
impregnation of TiO2 with Fe(NO3)3·9H2O
and by the sol-gel method from TiCl4 and Fe(NO3)3·9H2O.
They have been reported that Fe/TiO2 prepared by the sol-gel method
was less active than TiO2 due to the fact that dopants acted more as
recombination centres than a trap sites for charge transfer and the obtained
photocatalysts had the lower amount of surface hydroxyl groups and a lower
anatase-to-rutile ratio compared with TiO2 precursor sample. They
proved that the existence of separated hematite or pseudobrookite (Fe2TiO5)
phases in samples containing more than 2% iron could decrease the activity [42].
It has been also proved that the excess of deposited iron on TiO2 can form Fe(OH)2+, which has the greater adsorption to the incidence
light than TiO2 in the range of 290–400 nm, and can cause decreasing
of the Fe/TiO2 photoactivity [69]. The low temperature of
calcinations, such as 300°C in Fe/TiO2 preparation appeared to be
favoured taking into account its photoactivity [70].Different amount of doped Fe(III) to TiO2 has been reported to affect the enhanced photoactivity of photocatalyst,
dependent on the way of preparation and degradation compound.Adán et al. [48] reported the enhancement
of photocatalytic activity of Fe-doped TiO2 for doping levels up to
ca. 1 wt%, which was attributable to the introduction of Fe3+ cations into the anatase structure. Nahar et al. [49] reported that the molar
ratio of 0.005 Fe content in Fe-doped TiO2 was the optimum for
degradation rate of phenol under both the UV and visible-light irradiations.
Hung et al. [44] also found that 0.005 mol% of iron ions can enhance the
photocatalytic activity, while too great an amount will make the iron ions
become recombination centres for the electron-hole pairs and reduce the
photocatalytic activity. Chen et al. [47] and Feng et al. [71] have found the highest
photocatalytic activity of Fe-doped TiO2 for 0.05 at.% Fe(III).
The photocatalytic decomposition of Rhodamine
B. on the nanosize Fe(III)-doped TiO2 catalysts prepared by the
hydrothermal method with TiCl4 as the precursor was higher than on TiO2, and the optimal results have been obtained for the 0.1% Fe(III)-doped TiO2 [72]. For photocatalytic oxidation of nitrite to nitrate, the most photoactive
sample was found to be TiO2 doped with 0.5 wt% of iron [73]. The mesoporous
nanocrystalline Fe-doped TiO2 samples prepared by ultrasonic method exhibited
enhanced photocatalytic activity towards oxidation of acetone in air at a small
amount of doped Fe3+ ions in TiO2 particles. The found
optimal atomic ratio of Fe to Ti was 0.25 The high activities of the Fe-doped
TiO2 powders could be attributed to the results of the synergetic
effects of Fe-doping, large BET specific surface area, and small crystallite
size [55]. Zhang et al. [74] reported that the films with low-iron
concentrations performed better photocatalytic activity than the pure TiO2 film, and the best doped iron concentration was 0.58 at.%.Wang et al. have reported that the
formation of surface defects in Fe-doped TiO2 affects the high-photocatalytic
activity of this photocatalyst [43, 75]. They reported that when the Fe content
increased in Fe-doped TiO2 prepared by the sol-gel method, the
isolated Fe2O3, Fe3O4, and FeO
species were observed and Ti-O-Fe species were formed, which increased the
surface defects of the Ti/Fe particles and led to the higher activity of the
catalyst than bare TiO2 for the degradation of tetracycline [75]. They
also reported that the concentration of titanium defects remained almost
constant below 400°C but decreased as the calcination temperature was higher than 600°C due to the decrease of
the hydroxyls in the crystalline structure. Below 400°C of calcination, all the
samples had some brookite and a majority of anatase phase. when the temperature
was 800°C ,
Fe2TiO5 was produced in the sample containing 5 wt% Fe by
a reaction between interstitial iron ions and lattice titanium ions, and in the
10 wt% Fe sample through a reaction of hematite with titania phases. [43].Egerton et al. have investigated that photoelectrocatalytic
disinfection of Escherichia coli by an
iron doped TiO2 sol-gel electrode was more efficient than
disinfection by the corresponding undoped electrode. The optimum disinfection
rate corresponded to the replacement of ≈0.1% of the Ti atoms by Fe [76].Some researchers have been observed that
doping Fe to TiO2 can be detrimental or not affect the enhancement
activity of the photocatalysts [77, 78].Fe2O3 can work as a
photocatalyst and can decompose some organic compounds like aniline [79].
However, the mixture of Fe2O3 and TiO2 has
been reported to be less active than original TiO2 [80, 81].


### 3.4.Mechanisms of Organic Compounds Decomposition by Fe-Doped TiO2

Wang et al. [82] detected by cryo-TEM that Fe(III)-doped TiO2 prepared by hydrolysis of TiCl4 with Fe(III)-acetyloacetonate as an
iron precursor forms three-dimentional networks with nanoparticles of 2–4 nm,
which act as antenna systems in photocatalysis, leading to an enhanced
photocatalytic activity of the colloidal preparation. HRTEm image of such prepared
particles is shown in Figure 11, and the scheme of this working system is
presented in Figure 12 [82].Figure 11
HRTEM image of 0.5 at % Fe(III)-doped TiO2 nanoparticles.Figure 12
The scheme of increased photocatalytic activity through energy/exciton transfer in aggregated photocatalyst particles (antenna effect). A and D represent electron and hole scavengers, respectively.Once the energy has reached the particle with the adsorbed target
molecule, the latter will act as a hole trap thus inducing the separation of the
original excitation [82].Araña et al. [83] have studied the
photocatalytic degradation of maleic acid by using Fe-doped TiO2 (0.15, 0.5, 2, and 5% w/w in Fe) catalysts. They observed that catalysts with
the lowest Fe content (0.15 and 0.5%) showed a considerably better catalytic
behaviour than nondoped TiO2 and catalysts with higher Fe contents.
Maleic acid molecules interacted with the surface of the lowest Fe-containing
catalysts and as a consequence; iron atoms were extracted from the catalyst surface
as photoactive Fe3+− maleic acid complexes. When this complex was
degraded, the resulting Fe2+ ions reacted with TiO2 holes
(h+) and the iron return to the catalyst surface as Fe2O3.
In catalysts with low-Fe content (0.15 and 0.5% w/w in Fe), this process occurs
in a fast way. On the contrary, in catalysts with high-Fe content (2 and 5% w/w
in Fe), the formation of less photoactive complexes seems to predominate [83].These authors have studied also the
photocatalytic degradation of formic acid by Fe-doped TiO2 calcined
at 773 and 1073 K [84]. For 773 K calcined catalyst, results suggested that Fe was
extracted through the formation of a [Fe-OOCH]2+ complex by which
formic acid degrades. Fe2+ ions that remain in solution after formic
acid degradation went back to the catalyst surface and were oxidised by
photogenerated holes. In this way, the catalyst was reactivated becoming ready
for a new degradation process. The formation of hydrogen-carbonates on the TiO2 catalyst through OH radicals insertion was inhibited, and the formation of
[Fe-OOCH]2+ complex was favoured. A markedly lower capacity for the
formic acid degradation has been determined for the 1073 K calcined catalysts, because
of its lower surface area and the major presence of lower-active phases such as
rutile and Fe2TiO5 [84].The formation of intermediate products
over the photocatalytic decomposition was found to be important in the further
oxidation processes.Araña et al. [85] have proved that the
mechanism of ethanol decomposition on Fe-TiO2 was gone by the
formation of ethoxides on the catalyst surface that were oxidised to acetate by
radicals O2•− and •OH. However,
the formation, an acetaldehyde as an
intermediate product, caused that their catalytic activity progressively
decreased over time, whereas on Pd-TiO2 and Cu-TiO2, formed ethyleneglycol caused faster degradation [85].It has been also reported that the formation
of a ternary compound HQ-Fe-H2O2 in phenol decomposition
or formation of an intermediate, highly-oxidised and unstable form of Fe
[Fe(IV)] in the case of chlorophenol decomposition were responsible for fast
degradation of phenol and chlorophenol, respectively [86, 87].The synergistic effect has been observed when a mixture of TiO2 and iron has been used for oxidative photodegradation of monuron
(3-(4-chlorophenyl)-1,1- dimethylurea). In a suspension of TiO2 (24 mg L-1) with addition of Fe(III) (3 × 10−4 mol L-1), the measured rate constant
was similar to that obtained in a suspension of TiO2 with a
concentration more than 20 times higher (500 mg L-1). The optimisation of the
photocatalytic systems was obtained when each photocatalyst plays a specific
role: Fe(III) as a main OH radicals source and TiO2 as an oxidizing
agent of Fe(II) to Fe(III) favouring the photocatalytic cycle Fe(III)/Fe(II).
This proposed mechanism is presented in Figure 13 [88].Figure 13
Photochemical cycle of combined system iron-TiO2.The following are photochemical reactions conducted to production of OH
radicals:(1)Fe(OH)2+⟶hvFe2++O•H,TiO2⟶hve−+h+⟶O•H,
and the reaction
occurring in the dark (Fenton reaction): (2)Fe2++H2O2⟶Fe3++OH−.It has been already established that the formation ofH2O2 is negligible in homogeneous photocatalysis with Fe(III). The reactions between
iron and TiO2 appear to be essential: (3)FeII+HO2•+H+⟶FeIII+H2O2,(4)FeII+O•H⟶FeIII+OH−.The reaction (4) is detrimental for pollutant degradation. It has been
reported that Fe(III) could easily adsorb on the surface of TiO2, and
Fe(II) cations were not adsorbed, therefore the reoxidation of Fe(II) by holes
is unlikely. In the absence of oxygen Fe(III), cations react with the electron
at the surface of TiO2, allowing the formation of the active species h+ and •OH. However,
it was proved that •OH radicals
are more efficiently formed via the photodissociation of Fe(OH)2+,
which was found to be responsible for fast degradation of monuron [88].Ranjit and Viswanathan [45] reported that the photocatalytic activity of the
iron-doped catalysts could be explained in terms of the heterojunction formed
between the Fe/TiO2 and α-Fe2O3 phases for the
sol-gel-derived catalyst.


## 3.1.Preparation Methods

There are few methods
of Fe doping to TiO2. The sol-gel method has been widely used for
preparation of Fe-doped  TiO2 from TiCl4 or titania alkoxide and an iron precursor like FeCl3,
Fe(NO3)3·9H2O,
or Fe(III)-acetyloacetonate [40–47]. Wang et al. [40] have reported that
preferable preparation of Fe-doped TiO2 to get the uniform
distribution of the dopant ions on TiO2 particles is hydrolysis of a
homogeneous mixture of organic titanium and organic iron precursors in
isopropyl alcohol. Preparation of Fe-doped TiO2 by hydrolysis of
TiCl4 with FeCl3 appeared to be less favourable than
using Fe(III)-acetyloacetonate as an iron precursor. Fe doping from microemulsion of Ti
tetraisopropoxide with an aqueous solution of iron and further calcinations has
been reported by Adán et al. [48], Fe-doped TiO2 can be obtained
also through the calcination of
FexTiS2 [49–51], plasma oxidative pyrolysis [52], one-step flame
spray pyrolysis (FSP) [53], coprecipitation and immersion [46], and by the wet
impregnation method from Fe(III) acetylacetonate [54] or Fe(NO3)3·9H2O [42].Navío has been reported that impregnation
with Fe(III)-acetyloacetonate gives more homogeneous distribution of iron for
each mixed oxide sample on the particle surfaces but not between particles in
comparison with impregnation with Fe(NO3)3·9H2O [54].Recently, doping of Fe to TiO2 by the ultrasonic-induced
hydrolysis reaction of tetrabutyl titanate (Ti(OC4H9)4)
in a ferric nitrate aqueous solution has been reported in [55].The mechanical alloying on the solid
state reaction of hematite,Fe2O3, and titanium has been
also sufficiently used for introducing of iron to TiO2 lattice [56].TiO2/Fe thin film has been
prepared by metal (Fe) plasma ion implantation [57, 58] and by magnetron sputtering method [59].Using a simple sol-gel method, a novel
magnetic photocatalyst was produced by immobilisation of TiO2 nanocrystal
on Fe-filled carbon nanocapsules. TiO2-coated Fe-CNC displayed good
performance in the removal of NO gas under UV exposure [60].Fe-doped TiO2 nanotubes with small diameter of 10 nm were
obtained by hydrothermal method [61].


## 3.2.Visible-Light Activity of Fe-Doped TiO2

It has been found that
the addition of transition metals to TiO2 can improve the
photocatalytic activity of the photocatalyst by UV irradiation and extend its
use in the visible region of the electromagnetic spectrum. It has been observed
the red shift in the UV-Vis spectra due to the introduction of the 3d electron
state of Fe3+, 3d5, in the conduction band of TiO2
[40, 44, 48, 49, 58, 59, 62–65].The absorbance of visible light is higher
for higher amount of doped iron, as it is presented in Figure9 [48].Figure 9
UV-Vis spectra of Fe-doped TiO2 with different iron content.This implies that iron-doped TiO2 may be photocatalytically
reactive under visible-light irradiation. Navío et al. have reported that
the photodegradation of oxalic acid under visible irradiation, not occurring
with TiO2, could be observed for Fe-doped TiO2 using 5%
Fe-containing samples [54]. Nahar et al.
reported that Fe-doped TiO2 was responsive to the visible-light
activity of phenol degradation [49]. Teoh et al. have reported that by flame spray
pyrolysis a stable Fe-TiO2 photocatalyst can be prepared and at the
ratio of Fe/Ti = 0.05 it has high activity towards oxalic acid mineralisation
under visible light [53]. Wang
et al. have reported that for Fe(III) doping in TiO2> 0.05 at.%, decomposition of
methyl orange under UV irradiation has been lower than on undoped TiO2,
but under visible-light irradiation the Fe(III)-doped TiO2 with an
intermediate iron doping concentration of ≈1 at.% had the highest photocatalytic
reactivity due to the narrowing of bandgap so that it could effectively absorb
the light with longer wavelength [52]. Chen and Peng have reported the
preparation of magnetic-nanometer titanium dioxide/ferriferous oxide (TiO2/Fe3O4)
composite photocatalyst with the particles size of 30–50 nm, which appeared to
be very active under visible light and highly effective in discoloring of
wastewater [66].It has been investigated that Fe/TiO2 particles had a higher hydrophilic property compared with TiO2 [65, 67].TiO2/Fe thin film has been
demonstrated to have antimicrobial activity after being irradiated with visible
light [68].


## 3.3.Enhancement of Photocatalytic Activity of
TiO2 by Iron Doping

It is
generally accepted that Fe(III) centres form shallow charge trapping sites
within the TiO2 matrix as well as on the particle surface through
the replacement of Ti(IV) by Fe(III) [69]. Based on the favourable energy
levels, Fe(III) centres may act either as an electron or a hole trap (see Figure 10); so that photogenerated charge carriers are temporarily separated more
effectively [40].Figure 10
Scheme of mechanism of TiO2 photocatalysis with modifications by Fe(III) ion doping (CB—conduction band, VB—valence band).Wang et al. [40] have reported that
enhancement of the quantum yield by Fe(III) doping TiO2 can be
explained by assuming that the Fe(III) centre acts predominately as a shallow
electron trap from which the electron is transferred to molecular oxygen more
rapidly than the undoped TiO2.However, it was proved that the
photoactivity of Fe/TiO2 catalyst is dependent on the way of
preparation and the amount and state of iron.Wang et al. [40] have reported that
the highest quantum yield has been obtained when Fe(III)-acetyloacetonate
precursor was used and for the optimal doping levels 0.25 and 0.5 atom %. They
have also reported that above-mentioned optimal doping level, some of the
Fe(III) dopants might act as shallow hole traps leading to an enhanced recombination
of the trapped charge carriers.Navío et al. [42] have also observed
that at the some conditions and for the certain quantity of Fe(III), the dopant
can be a center of recombination. They prepared Fe/TiO2 by an
impregnation of TiO2 with Fe(NO3)3·9H2O
and by the sol-gel method from TiCl4 and Fe(NO3)3·9H2O.
They have been reported that Fe/TiO2 prepared by the sol-gel method
was less active than TiO2 due to the fact that dopants acted more as
recombination centres than a trap sites for charge transfer and the obtained
photocatalysts had the lower amount of surface hydroxyl groups and a lower
anatase-to-rutile ratio compared with TiO2 precursor sample. They
proved that the existence of separated hematite or pseudobrookite (Fe2TiO5)
phases in samples containing more than 2% iron could decrease the activity [42].
It has been also proved that the excess of deposited iron on TiO2 can form Fe(OH)2+, which has the greater adsorption to the incidence
light than TiO2 in the range of 290–400 nm, and can cause decreasing
of the Fe/TiO2 photoactivity [69]. The low temperature of
calcinations, such as 300°C in Fe/TiO2 preparation appeared to be
favoured taking into account its photoactivity [70].Different amount of doped Fe(III) to TiO2 has been reported to affect the enhanced photoactivity of photocatalyst,
dependent on the way of preparation and degradation compound.Adán et al. [48] reported the enhancement
of photocatalytic activity of Fe-doped TiO2 for doping levels up to
ca. 1 wt%, which was attributable to the introduction of Fe3+ cations into the anatase structure. Nahar et al. [49] reported that the molar
ratio of 0.005 Fe content in Fe-doped TiO2 was the optimum for
degradation rate of phenol under both the UV and visible-light irradiations.
Hung et al. [44] also found that 0.005 mol% of iron ions can enhance the
photocatalytic activity, while too great an amount will make the iron ions
become recombination centres for the electron-hole pairs and reduce the
photocatalytic activity. Chen et al. [47] and Feng et al. [71] have found the highest
photocatalytic activity of Fe-doped TiO2 for 0.05 at.% Fe(III).
The photocatalytic decomposition of Rhodamine
B. on the nanosize Fe(III)-doped TiO2 catalysts prepared by the
hydrothermal method with TiCl4 as the precursor was higher than on TiO2, and the optimal results have been obtained for the 0.1% Fe(III)-doped TiO2 [72]. For photocatalytic oxidation of nitrite to nitrate, the most photoactive
sample was found to be TiO2 doped with 0.5 wt% of iron [73]. The mesoporous
nanocrystalline Fe-doped TiO2 samples prepared by ultrasonic method exhibited
enhanced photocatalytic activity towards oxidation of acetone in air at a small
amount of doped Fe3+ ions in TiO2 particles. The found
optimal atomic ratio of Fe to Ti was 0.25 The high activities of the Fe-doped
TiO2 powders could be attributed to the results of the synergetic
effects of Fe-doping, large BET specific surface area, and small crystallite
size [55]. Zhang et al. [74] reported that the films with low-iron
concentrations performed better photocatalytic activity than the pure TiO2 film, and the best doped iron concentration was 0.58 at.%.Wang et al. have reported that the
formation of surface defects in Fe-doped TiO2 affects the high-photocatalytic
activity of this photocatalyst [43, 75]. They reported that when the Fe content
increased in Fe-doped TiO2 prepared by the sol-gel method, the
isolated Fe2O3, Fe3O4, and FeO
species were observed and Ti-O-Fe species were formed, which increased the
surface defects of the Ti/Fe particles and led to the higher activity of the
catalyst than bare TiO2 for the degradation of tetracycline [75]. They
also reported that the concentration of titanium defects remained almost
constant below 400°C but decreased as the calcination temperature was higher than 600°C due to the decrease of
the hydroxyls in the crystalline structure. Below 400°C of calcination, all the
samples had some brookite and a majority of anatase phase. when the temperature
was 800°C ,
Fe2TiO5 was produced in the sample containing 5 wt% Fe by
a reaction between interstitial iron ions and lattice titanium ions, and in the
10 wt% Fe sample through a reaction of hematite with titania phases. [43].Egerton et al. have investigated that photoelectrocatalytic
disinfection of Escherichia coli by an
iron doped TiO2 sol-gel electrode was more efficient than
disinfection by the corresponding undoped electrode. The optimum disinfection
rate corresponded to the replacement of ≈0.1% of the Ti atoms by Fe [76].Some researchers have been observed that
doping Fe to TiO2 can be detrimental or not affect the enhancement
activity of the photocatalysts [77, 78].Fe2O3 can work as a
photocatalyst and can decompose some organic compounds like aniline [79].
However, the mixture of Fe2O3 and TiO2 has
been reported to be less active than original TiO2 [80, 81].


## 3.4.Mechanisms of Organic Compounds Decomposition by Fe-Doped TiO2

Wang et al. [82] detected by cryo-TEM that Fe(III)-doped TiO2 prepared by hydrolysis of TiCl4 with Fe(III)-acetyloacetonate as an
iron precursor forms three-dimentional networks with nanoparticles of 2–4 nm,
which act as antenna systems in photocatalysis, leading to an enhanced
photocatalytic activity of the colloidal preparation. HRTEm image of such prepared
particles is shown in Figure 11, and the scheme of this working system is
presented in Figure 12 [82].Figure 11
HRTEM image of 0.5 at % Fe(III)-doped TiO2 nanoparticles.Figure 12
The scheme of increased photocatalytic activity through energy/exciton transfer in aggregated photocatalyst particles (antenna effect). A and D represent electron and hole scavengers, respectively.Once the energy has reached the particle with the adsorbed target
molecule, the latter will act as a hole trap thus inducing the separation of the
original excitation [82].Araña et al. [83] have studied the
photocatalytic degradation of maleic acid by using Fe-doped TiO2 (0.15, 0.5, 2, and 5% w/w in Fe) catalysts. They observed that catalysts with
the lowest Fe content (0.15 and 0.5%) showed a considerably better catalytic
behaviour than nondoped TiO2 and catalysts with higher Fe contents.
Maleic acid molecules interacted with the surface of the lowest Fe-containing
catalysts and as a consequence; iron atoms were extracted from the catalyst surface
as photoactive Fe3+− maleic acid complexes. When this complex was
degraded, the resulting Fe2+ ions reacted with TiO2 holes
(h+) and the iron return to the catalyst surface as Fe2O3.
In catalysts with low-Fe content (0.15 and 0.5% w/w in Fe), this process occurs
in a fast way. On the contrary, in catalysts with high-Fe content (2 and 5% w/w
in Fe), the formation of less photoactive complexes seems to predominate [83].These authors have studied also the
photocatalytic degradation of formic acid by Fe-doped TiO2 calcined
at 773 and 1073 K [84]. For 773 K calcined catalyst, results suggested that Fe was
extracted through the formation of a [Fe-OOCH]2+ complex by which
formic acid degrades. Fe2+ ions that remain in solution after formic
acid degradation went back to the catalyst surface and were oxidised by
photogenerated holes. In this way, the catalyst was reactivated becoming ready
for a new degradation process. The formation of hydrogen-carbonates on the TiO2 catalyst through OH radicals insertion was inhibited, and the formation of
[Fe-OOCH]2+ complex was favoured. A markedly lower capacity for the
formic acid degradation has been determined for the 1073 K calcined catalysts, because
of its lower surface area and the major presence of lower-active phases such as
rutile and Fe2TiO5 [84].The formation of intermediate products
over the photocatalytic decomposition was found to be important in the further
oxidation processes.Araña et al. [85] have proved that the
mechanism of ethanol decomposition on Fe-TiO2 was gone by the
formation of ethoxides on the catalyst surface that were oxidised to acetate by
radicals O2•− and •OH. However,
the formation, an acetaldehyde as an
intermediate product, caused that their catalytic activity progressively
decreased over time, whereas on Pd-TiO2 and Cu-TiO2, formed ethyleneglycol caused faster degradation [85].It has been also reported that the formation
of a ternary compound HQ-Fe-H2O2 in phenol decomposition
or formation of an intermediate, highly-oxidised and unstable form of Fe
[Fe(IV)] in the case of chlorophenol decomposition were responsible for fast
degradation of phenol and chlorophenol, respectively [86, 87].The synergistic effect has been observed when a mixture of TiO2 and iron has been used for oxidative photodegradation of monuron
(3-(4-chlorophenyl)-1,1- dimethylurea). In a suspension of TiO2 (24 mg L-1) with addition of Fe(III) (3 × 10−4 mol L-1), the measured rate constant
was similar to that obtained in a suspension of TiO2 with a
concentration more than 20 times higher (500 mg L-1). The optimisation of the
photocatalytic systems was obtained when each photocatalyst plays a specific
role: Fe(III) as a main OH radicals source and TiO2 as an oxidizing
agent of Fe(II) to Fe(III) favouring the photocatalytic cycle Fe(III)/Fe(II).
This proposed mechanism is presented in Figure 13 [88].Figure 13
Photochemical cycle of combined system iron-TiO2.The following are photochemical reactions conducted to production of OH
radicals:(1)Fe(OH)2+⟶hvFe2++O•H,TiO2⟶hve−+h+⟶O•H,
and the reaction
occurring in the dark (Fenton reaction): (2)Fe2++H2O2⟶Fe3++OH−.It has been already established that the formation ofH2O2 is negligible in homogeneous photocatalysis with Fe(III). The reactions between
iron and TiO2 appear to be essential: (3)FeII+HO2•+H+⟶FeIII+H2O2,(4)FeII+O•H⟶FeIII+OH−.The reaction (4) is detrimental for pollutant degradation. It has been
reported that Fe(III) could easily adsorb on the surface of TiO2, and
Fe(II) cations were not adsorbed, therefore the reoxidation of Fe(II) by holes
is unlikely. In the absence of oxygen Fe(III), cations react with the electron
at the surface of TiO2, allowing the formation of the active species h+ and •OH. However,
it was proved that •OH radicals
are more efficiently formed via the photodissociation of Fe(OH)2+,
which was found to be responsible for fast degradation of monuron [88].Ranjit and Viswanathan [45] reported that the photocatalytic activity of the
iron-doped catalysts could be explained in terms of the heterojunction formed
between the Fe/TiO2 and α-Fe2O3 phases for the
sol-gel-derived catalyst.


## 4. TiO2 Modified by Carbon and Iron

As it was described above, carbon-coated TiO2 showed high
photoactivity towards decomposition of methylene blue but the other organic
compounds were poorly decomposed on these photocatalysts, therefore the
preparation of carbon-coated TiO2 has been modified to obtain
carbon-coated TiO2 with doped iron. For this purpose, poly(ethylene terephthalate)
(PET) was impregnated with oxalic acid solution (FeC2O4)
and then was mixed with powder TiO2 and heat treated at different
temperatures, 400–800°C, under flow of Ar [89]. The obtained samples
are consisted from anatase phase, contained carbon from 7.5 to 15.5 wt%, and an
iron from 0.34 to 0.67 wt%, mostly in the form of Fe(II) [89]. Such prepared
samples did not show the enhanced photocatalytic activity towards phenol
decomposition under UV irradiation, even though the sample prepared at 400°C exhibited high adsorption of phenol on its
surface, around 30 · 10−5 mol/g, which was much higher than on the other samples, which showed adsorption
of phenol in the range of 3–5⋅10−5 mol/g. However, high acceleration of the phenol decomposition has been observed
on the sample prepared at 400°C, when 0.03 mol/L, H2O2 was added to the reaction mixture; see Figure 14 [89]. This could be caused by
occurring photo-Fenton reactions, in which Fe2+ is oxidised to Fe3+ with H2O2 yielding in OH radicals. Increase of OH
radicals production after H2O2 addition on samples
contained iron was confirmed by OH radicals measurements [89–91].Phenol decomposition on commercial TiO2-ST-01 and modified TiO2 by carbon and iron at 400°C, (a) under UV irradiation and (b) under UV with addition of H2O2.
(a)(b)The high-decomposition rate of phenol was obtained only on the sample,
which showed high adsorption of phenol, although this sample contained the
lowest content of Fe(II) in comparison with the other samples. Increase of OH
radicals formation on samples with iron was not proportional to the
decomposition rate of phenol under UV irradiation withH2O2.
It has been proved that, in case of the sample prepared at 400°C, some complexes were formed, which were
responsible for fast degradation of phenol [91]. The FTIR studies indicated
that hydroquinone was more likely adsorbed on Fe-C-TiO2 and could
play a key role in the process of phenol decomposition, as reported
by Chen and Pignatello [86] that some hydroquinones formed in the phenol decomposition could
reduce iron and accelerate in this way the rate of phenol decomposition by the
formation of a ternary HQ-Fe-H2O2 complex [93]. Cycling
decomposition of phenol under UV and H2O2 was performed
on Fe-modified carbon-coated TiO2 photocatalyst heat treated at 400°C. For those measurements, photocatalyst was
mounted on the adhesive tape and fixed to the metal grid, which was placed
inside the reactor. For cycling performance, the metal grid with fixed
photocatalyst each time was put to the fresh phenol solution with concentration
of 2.1·10−4 mol/L; the results are presented in Figure 15.Figure 15
Cycling decomposition of phenol under UV irradiation and H2O2 on Fe-modified carbon-coated TiO2 photocatalyst heat treated at 400°C.A good performance of the phenol decomposition with cycling suggests
that an iron present in the sample could follow the processes of oxidation and
reduction, being reused in Fenton reactions and photodegradation of phenol. It
is worth to add that in this case no pH adjustment was needed as it is usually
applied in photo-Fenton process, and not any leaching of iron occurred.In the further investigations on TiO2 modified by carbon and
iron photocatalysts, the preparation method was changed, the powder of TiO2 was impregnated with FeC2O4 solution and heated at
400–800°C under flow of Ar, but not any polymer was added. The obtained Fe-C-TiO2 samples contained a residue carbon, 0.2–3.3 wt%, which came from the
carbonisation of oxalate [90]. A residue carbon did not prevent the
transformation of anatase to rutile, which occurred at 700°C, with increasing the heat treatment
temperature FeTiO3 phase (ilmenite) appeared. The highest
photoactivity for phenol decomposition under UV and H2O2 had the sample prepared at 500°C, which contained 2.4 wt% of residue carbon,
0.70 wt% of Fe(II), 0.96 wt% of Fe(III), and did not have FeTiO3 phase. The phenol decomposition on this sample was much faster than in the case
of previous experiments, in which PET was used for preparation, phenol was
decomposed after 2 hours of UV irradiation, and after 3 hours of UV irradiation
not any ring products of phenol decomposition were detected [90, 91].It was proved that in
case of TiO2 modified by FeC2O4 and heated at
550°C in air (Fe-TiO2), FeTiO3 phase is formed, which
exhibits higher photocatalytic activity than pure TiO2 for phenol decomposition
under UV irradiation, but with H2O2 addition this
activity decreases, proportional to the decreasing of OH radicals formation,
due to the scavenging effect:
(5)H2O2+O•H⟶HO2•+H2O.Formed in this reaction, HO2• radicals
are known to have lower potential of oxidation than •OH [91].
This scavenging effect has been observed to occur also on TiO2. In
Figure 16, formation of OH radicals on TiO2, Fe-TiO2, and Fe-C-TiO2 samples is presented [91].Formation of OH radicals on TiO2, Fe-TiO2,
and Fe-C-TiO2 prepared at 500°C, (a) under UV and (b) under UV with H2O2.
(a)(b)In Figure17, phenol decomposition on these
samples is presented [91].Phenol decomposition on TiO2, Fe-TiO2, and Fe-C-TiO2 prepared at 500°C, (a) under UV and (b) under UV with H2O2.
(a)(b)It can be observed that phenol decomposition is going through the
radical reaction on TiO2 and Fe-TiO2 photocatalysts,
whereas by the surface reactions on Fe-C-TiO2.The cycling decomposition of phenol on
Fe-C-TiO2 sample prepared at 500°C has been performed in the circulated flow reactor
[94]. For this purpose, the photocatalyst has been pasted on the cotton
material with a suspension of the powdered photocatalyst with aqueous solution
of sodium silicate (Na2 SiO3) and was placed inside the
reactor on the inner walls, along the UV lamp. Circulated phenol solution was irradiated with UV
for some cycles, and with UV and H2O2, as shown in Figure 18 [94].Cycling decomposition of phenol on immobilised Fe-C-TiO2 prepared at 500°C. (a) under UV, (b) under UV with H2O2.
(a)(b)Adsorption and following decomposition of phenol on immobilised Fe-C-TiO2 photocatalyst can be observed under UV irradiation, but under UV with addition
of H2O2 acceleration of phenol decomposition is observed
with stable amount of decomposed phenol, over 90% and mineralisation degree
around 50% [94].It has been proved that preparation of
Fe-C-TiO2 from the other TiO2 precursor of anatase
structure at the same conditions as described above gives similar results; the
highest photocatalytic activity under UV irradiation with H2O2 has the sample prepared at 500°C [95]. The high photoactivity of Fe-C-TiO2 sample prepared at 500°C has been found for decomposition of phenol, humic
acids, and different dyes, like acid red, methylene blue, and reactive black 5.
It has been found that FeTiO3 phase is detrimental in photocatalytic
activity of Fe-C-TiO2 photocatalysts under UV with H2O2,
and the low temperatures of preparation such as 500°C led to obtain the samples
with high amount of paramagnetic iron on the surface, not built in TiO2 lattice, which probably facilitates proceeding of the photo-Fenton reactions,
in which iron is oxidised and reduced with cycling. The EPR spectra of Fe-C-TiO2 samples prepared from anatase and FeC2O4 [95] are
presented in Figure 19.Figure 19
EPR spectra of TiO2 and Fe-C-TiO2 samples prepared from anatase and Fe-C2O4 at 500–800°C.In obtained EPR spectra, some peaks can be observed, in
anatase type TiO2 a maximum at 344 MT, g = 1.93, which is
assigned to Ti3+ ions associated to oxygen vacancies [96, 97], this
peak was also observed in Fe-C-TiO2 samples and was higher intensity in
samples heated at higher temperatures. Some peaks assigned to Fe3+ ions were observed: in Fe-C-TiO2 samples prepared from anatase at
temperature 500 and 600°C, a broad peak with maximum at around 434 MT, g = 1.53
assigned to paramagnetic Fe3+ ions, peaks with maximum at around 330
and 310 M (g≈2) assigned to Fe3+ ions in
octahedral symmetry in anatase [48, 96], and two broad peaks at the range of
133 to 272 MT assigned to Fe3+ ions substituting for Ti4+ in the TiO2 rutile lattice [96].


## 5. Summary

Modification of TiO2 by carbon can enhance its photocatalytic activity: doped C can extend the light
absorption to the visible range and give photocatalytic activity under visible
light by the narrowing of the bandgap; carbon coated TiO2 as well as
TiO2/AC composites can enhance the concentration of the organic
compounds on the surface of the photocatalyst and accelerate the process of their
decomposition through the transfer of the adsorbed molecules to the TiO2 surface; TiO2 loaded carbon can also work as a photocatalyst, on
which the molecules are adsorbed in the pores of carbon and then they undergo
the photocatalytic decomposition with UV irradiation.Doping Fe(III) to TiO2 causes formation of shallow charge trapping sites
within the TiO2 matrix and on the particle surface through the
replacement of Ti(IV) by Fe(III). Fe(III) centres may act either as an electron
or a hole trap. At high concentration of Fe(III) in Fe-doped TiO2 Fe(III), ions can act as a recombination centres. The photocatalytic activity
of Fe-doped TiO2 has been found to depend strongly on the
preparation method, iron precursor, and the amount and state of iron, generally
insignificant amount of doped iron has a positive effect in the enhancement of
the photocatalytic activity of Fe-doped TiO2, whereas amount of
doped iron > 2 wt% has been detrimental. It has been observed the red shift in the UV-Vis spectra of Fe-doped TiO2 photocatalysts due to the
introduction of the 3d electron state of Fe3+, 3d5, in
the conduction band of TiO2, what makes these photocatalysts active
under visible light.TiO2 modified by carbon and iron can work in both
photocatalysis and photo-Fenton processes, when H2O2 is
used, enhancing markedly the rate of the organic compounds decomposition. The
photocatalytic decomposition of organic compounds on TiO2 modified
by iron and carbon is going by the complex reactions of iron with the
intermediates, what significantly accelerate the process of their
decomposition. A good performance of cycling decomposition of phenol has been
obtained on immobilised Fe-C-TiO2 photocatalyst, what suggests that
an iron present in the sample could follow the processes of oxidation and
reduction, being reused in Fenton reactions and photodegradation of phenol. The
additional advantage of using this photocatalyst for the decomposition of
organic compounds is the fact that no pH adjustment is needed, and it was
observed that there was not any leaching of iron from this photocatalyst.



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*Source: 721824-2008-03-18.xml*