Photoinduced hydroxyl radical and photocatalytic activity
of samarium-doped TiO2nanocrystalline
Qi Xiao∗, Zhichun Si, Jiang Zhang, Chong Xiao, Xiaoke Tan
School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China
Sm3+-doped TiO2 nanocrystalline has been prepared by sol–gel auto-combustion technique and characterized by X-ray diffraction (XRD),
Brunauer–Emmett–Teller (BET) method, and also UV–vis diffuse reflectance spectroscopy (DRS). These Sm3+-doped TiO2samples were tested
for methylene blue (MB) decomposition and•OH radical formation. The analysis of•OH radical formation on the sample surface under UV
irradiation was performed by fluorescence technique with using terephthalic acid, which readily reacted with•OH radical to produce highly
activity for MB degradation under UV light irradiation because both the larger specific surface area and the greater the formation rate of•OH
radical were simultaneously obtained for Sm3+-doped TiO2nanocrystalline. The adsorption experimental demonstrated that Sm3+-TiO2had a
higher MB adsorption capacity than undoped TiO2and the adsorption capacity of MB increased with the increase of samarium ion content. The
results also indicated that the greater the formation rate of•OH radical was, the higher photocatalytic activity was achieved. In this study, the
optimum amount of Sm3+doping was 0.5mol%, at which the recombination of photo-induced electrons and holes could be effectively inhibited,
the highest formation rate of•OH radicals was, and thereby the highest photocatalytic activity was achieved.
Keywords: Sm3+-doped TiO2nanocrystalline; Hydroxyl radical; Photocatalytic activity
TiO2photocatalyst has attracted a great deal of attention in
environmental wastewater treatment in the past decade, because
it generates highly oxidative hydroxyl free radicals (•OH),
which can degrade many toxic and non-biodegradable organics
[1–3]. Previous studies indicated that the photocatalytic activity
of TiO2 catalysts depended strongly on two factors: adsorp-
tion behavior and the separation efficiency of electron–hole
pairs [1,2]. The adsorption capacity can be generally improved
by increasing the specific surface area of catalysts. On the
other hand, in order to eliminate the recombination rate of the
electron–hole pairs, several approaches have been proposed,
including transition metals doping [3,4], coupling with other
ions doping [8–11]. Especially, the photocatalytic activity of
∗Corresponding author. Tel.: +86 731 8830543; fax: +86 731 8879815.
E-mail address: firstname.lastname@example.org (Q. Xiao).
ions with 4f configurations because lanthanide ions could form
complexes with various Lewis bases including organic acids,
amines, aldehydes, alcohols, and thiols in the interaction of
the functional groups with their f-orbital [12–14]. Xu et al.
 reported that doping with La3+, Ce3+, Er3+, Pr3+, Gd3+,
Nd3+, or Sm3+was beneficial to NO2adsorption. Ranjit et al.
[13,14] reported that doping with Eu3+, Pr3+, or Yb3+increased
the adsorption capacity and also adsorption rate of TiO2cata-
lysts simultaneously in aqueous salicylic acid, t-cinnamic acid,
and pchlorophenoxy–acetic acid solutions. Thus, doping with
lanthanide ions could provide a means to concentrate on the
organic pollutant at the semiconductor TiO2surface and there-
fore enhance the photoactivity of titania [15,16]. On the other
hand, doping with lanthanide ions with 4f electron configura-
tions also could significantly improve the separation rate of
enhance the photocatalytic activity of TiO2[8–11]. Wang and
was more efficient in the lanthanide ion-doped TiO2including
Li et al.  reported that the introduction of Ce 4f level led to
eliminate the recombination of electron–hole pairs and enhance
the photocatalytic activity.
•OH radicals have been proposed to be the responsible
for many oxidation pathways of chemical compounds initiated
through heterogeneous photocatalytic processes . Accord-
ingly,•OH radicals as much as possible are required to be
generated on the TiO2surface in order to increase the reactivity.
The yield of•OH radicals depends on the competition between
bination. Therefore, the measurement of the formation rate of
the hydroxyl radical is considered to give more important infor-
mation to help understand the role of lanthanide ions doping
in increase of photocatalytic activity. However, there are few
papers about the effect of lanthanide ions doped TiO2on the
amount of the•OH radicals formation.
which was often used as a standard target compound in a test of
photocatalysts. Moreover, a detailed photocatalytic degradation
pathway of MB has been determined by a careful identification
of intermediate products, and the photocatalytic reaction was
found to proceed preferably through the oxidation of MB by
the generated•OH radicals . Fluorescence technique was
applied to the detection of•OH radicals formed on a photo-
illuminated TiO2surface using terephthalic acid which readily
reacted with to produce highly fluorescent products. Previous
studies  have demonstrated the possibility of using this
method to quantify the complete hydroxyl radical production
from the photogenerated holes at the semiconductor surface.
In addition, the Sm3+-doped TiO2samples were characterized
by X-ray diffraction (XRD), UV–vis diffuse reflectance spec-
troscopy. The aim of this study was to examine and correlate the
photocatalytic degradation of MB with•OH radicals formed
on Sm3+-doped TiO2 samples, and disclose the mechanisms
of photocatalytic activity enhancement due to samarium ion
doping by investigating the effects of samarium ion doping
on the separation of electron–hole pairs under either UV light
2.1. Synthesis of Sm3+-doped TiO2nanocrystalline
Sm3+-doped TiO2 nanocrystalline was synthesized by a
sol–gel auto-combustion method. The detailed process can be
(Ti (OPri)4), Sm (NO3)3,C2H6O2(ethylene glycol, abbrevi-
ated as EG), C6H8O7(citric acid, abbreviated as CA), ammonia
(25%) and nitride acid (65%–68%) were used as raw materi-
als. Appropriate amount of Ti (OPri)4and Sm (NO3)3were
added to CA and EG mixture under constant stirring condition.
The amounts of doped Sm3+were 0.5–1.5mol%. The molar
ratios of CA/Ti and CA/EG were kept constant at 2:1 and 1:1,
respectively. After adjusting the pH value with ammonia to 6–7,
the mixture solution was evaporated at 90◦C to gradually form
a clear precursor gel. The precursor gel was baked at 150◦C
in muffle furnace and expanded, then was auto-ignited at about
for 2h in air.
2.2. Characterization of Sm3+-doped TiO2nanocrystalline
The crystalline structure of the samples was determined by a
D/max-?A diffractometer (Cu K? radiation, λ=0.154056nm)
studies. The averaged grain sizes D were determined from the
XRD pattern according to the Scherrer equation D=Kλ/βcosθ,
where k is a constant (shape factor, about 0.9), λ is the X-ray
wavelength (0.15418nm), β the full width at half maximum
(FWHM) of the diffraction line, and θ the diffraction angle.
The values of β and θ of anatase and rutile were taken from
anatase (101) and rutile (110) diffraction line, respectively.
The amount of rutile in the samples was calculated using the
following equation : XR=(1+0.8IA/IR)−1, where XRis the
mass fraction of rutile in the samples, IAand IRwere the X-
ray integrated intensities of (101) reflection of the anatase and
(110) reflection of rutile, respectively.
The specific surface area of the powders was measured by
the dynamic Brunauer–Emmett–Teller (BET) method, in which
a N2gas was adsorbed at 77K using a Micromeritics ASAP
2000 system. The diffuse reflectance spectra (DRS) of the
photocatalyst sample in the wavelength range of 200–800nm
were obtained using a UV–vis scanning spectrophotometer
(Shimadzu UV-3101) and were converted from reflectance to
absorbance by the Kubelka–Munk method.
2.3. Determination of•OH radicals
The analysis of•OH radical’s formation on the sample
surface under UV irradiation was performed by fluorescence
technique with using terephthalic acid, which readily reacted
2-hydroxyterephthalic acid . The intensity of the peak
attributed to 2-hydroxyterephtalic acid was known to be propor-
tional to the amount of•OH radicals formed . The selected
concentration of terephthalic acid solution was 5×10−4M
in a diluted NaOH aqueous solution with a concentration
of 2×10−3M. It has been proved that under these exper-
imental conditions (low concentration of terephthalic acid,
less than 10−3M, room temperature), the hydroxylation reac-
tion of terephthalic acid proceeds mainly by•OH radicals
Two hundreds milligrams of the prepared Sm3+-doped TiO2
samples was added to 200mL of the 5×10−4M terephthalic
acid solution in 2×10−3M NaOH, and then UV irradiation
of the solution was started. For UV irradiation, a 160W high-
above the surface solution was used as UV light source. Sam-
pling was performed in every 15min. Solution after filtration
through 0.45?m membrane filter was analyzed on a Hitachi
F-4500 fluorescence spectrophotometer. The product of tereph-
thalic acid hydroxylation, 2-hydroxyterephthalic acid, gave a
peak at the wavelength of about 425nm by the excitation with
the wavelength of 315nm.
•OH radicals to produce highly fluorescent product,
2.4. MB adsorption experiment
To determine the adsorption behavior of Sm3+ion doped and
pure TiO2catalyst, a set of adsorption isotherm tests in the dark
was performed. In each test, 0.02g of catalyst was added to
and put in the dark for 24h at 298±1K. The MB concentra-
tion in the suspension before and after the adsorption tests was
analyzed and the adsorbed amount of MB on the catalysts was
calculated based on a mass balance.
2.5. Photocatalytic degradation of MB
In order to evaluate photocatalytic activity of the prepared
samples, photocatalytic degradation of MB was performed. For
a typical photocatalytic experiment, 200mg of the prepared
Sm3+-doped TiO2nanocrystalline was added to 200mL of the
100ppm MB aqueous solution. The prepared Sm3+-doped TiO2
nanocrystalline was dispersed under ultrasonic vibration for
10min. After keeping at least 20min, MB concentration in
the solution was found to be constant on all samples prepared.
Therefore, the solution in which the prepared Sm3+-doped TiO2
nanocrystalline was dispersed was kept in the dark for 30min
diation, a 160W high-pressure mercury lamp (GYZ-160) fixed
at a distance of 150mm above the surface solution was used as
UV light source. The light absorption of solution after filtration
for MB) at a set time. The decolorization of MB was calculated
by formula: Decolorization=(C0−C)/C0, where C0and C is
and (C0−C) is the concentration of the decomposed MB. The
absorbance of the MB solution was measured with a UV–vis
spectrophotometer (Shimadzu UV-3101).
3. Results and discussion
3.1. XRD analysis
XRD patterns of Sm3+-doped TiO2nanocrystalline with var-
ious samarium content calcined at 600◦C for 2h were shown
in Fig. 1. From these XRD results, it was shown that the X-ray
diffraction peak at 25.5◦corresponded to characteristic peak
of crystal plane (101) of anatase, and the peak at 27.6◦corre-
sponded to characteristic peak of crystal plane (110) of rutile.
In undoped titania sample calcined at 600◦C for 2h, rutile was
the dominant crystallized phase, and the sample contains 97.5%
of rutile phase, while Sm3+-doped TiO2samples showed a mix-
ture phase of anatase and rutile, and the relative ratio of rutile
to anatase was reduced with the increase of samarium con-
greatly inhibited by samarium ion doping. Lin et al.[23,24] also
found that other rare earth ions (La, Y, and Ce) could inhibited
the anatase-to-rutile phase transformation during the thermal
treatment. The inhibition of the phase transition was ascribed
to the stabilization of the anatase phase by the surrounding
rare earth ions through the formation of Ti–O–rare-earth ele-
Fig. 1. XRD patterns of TiO2with various amounts of samarium calcined at
600◦C for 2h.
ment bonds . In the present system, I thought that the likely
formation of Sm–O–Ti interaction took place and inhibited the
transition of the anatase phase. But the mechanism dominating
in the anatase-to-rutile phase transformation process was com-
plex and not fully understood yet. Therefore, the more detailed
study should be done in the future in order to extensively under-
stand the effect of samarium ion doping on the anatase-to-rutile
3.2. Diffuse reflectance spectra
To investigate the optical absorption properties of catalysts,
the diffuse reflectance spectra (DRS) of TiO2and Sm3+-doped
TiO2in the range of 220–850nm was examined and the results
were shown in Fig. 2. It was shown that while TiO2had no
absorption in the visible region (>400nm), Sm3+-doped TiO2
had significant absorption between 400 and 500nm, which
increased with the increase of samarium ion content. In addi-
tion, the optical absorption in the UV region was also enhanced.
Li et al.  reported that the band gap of TiO2nanoparticles
Fig. 2. UV–vis absorption spectra of pure and doped TiO2calcined at 600◦C
The characteristics of Sm3+-doped samples containing different samarium content calcined at 600◦C
Samarium content %AnataseRutile Specific surface
MB at 120min (%)
XA(%) Crystal size
was reduced by Nd3+doping and the band gap narrowing was
primarily attributed to the substitution Nd3+ions which intro-
duced electron states into the band gap of TiO2to form the new
lowest unoccupied molecular orbital. In order to understand the
functional theory calculations were in going.
3.3. Adsorption behavior of methylene blue
The BET results (shown in Table 1) showed that the spe-
cific surface areas of the catalysts significantly increased from
24.52m2g−1for TiO2to 82.94m2g−1for 1.5% Sm3+–TiO2.
The larger specific surface area of Sm3+-doped TiO2nanocrys-
aqueous suspension. The MB adsorption isotherms were shown
in Fig. 3. It was shown that the Sm3+-doped TiO2nanocrys-
onto the Sm3+-doped TiO2nanocrystalline increased with the
3.4. Formation of•OH radicals
The fluorescence emission spectrum (excitation at 315nm)
of terephthalic acid solution was measured every 15min during
illumination. Fig. 4 showed the induction of fluorescence from
5×10−4M terephthalic acid solution in 2×10−3M NaOH.
Fig. 3. The MB adsorption isotherms on the pure and doped TiO2.
As shown in the figures, gradual increase in the fluorescence
intensity at about 425nm was observed with increasing illumi-
nation time. Based on the reports in radiation chemistry 
and sonochemistry [28–30], it was reasonable to assume that
photogenerated O2−, HO2•and H2O2did not interfere with
the reaction between•OH and terephthalic acid. Moreover,
the generated spectrum had the identical shape and maxi-
results suggested that fluorescent products formed during 0.5%
Sm3+–TiO2 photocatalysis were due to the specific reaction
between•OH radicals and terephthalic acid.
Fig. 5 showed the plots of increase in fluorescence intensity
against illumination time at 425nm. The fluorescence intensity
almost linearly against time. Consequently, we can conclude
that•OH radicals formed at the TiO2interface are in propor-
tional to the light illumination time obeying zero-order reaction
rate kinetics. The formation rate of the•OH radicals could be
expressed by the slop of these lines shown in Fig. 5. Accord-
ing to Fig. 5, the order of the formation rate of•OH radicals
formed on the Sm3+-doped TiO2nanocrystalline was as fol-
doping enhanced the formation rate of•OH radicals and there
was an optimum doping content of Sm3+ions in TiO2particles.
valence band hole, according to reaction (1). It was commonly
Fig. 4. Fluorescence spectral changes observed during illumination of 0.5%
Sm3+–doped TiO2in 4×10−4M NaOH solution of terephthalic acid (excita-
tion at 315nm). Each fluorescence spectrum was recorded every 15min of UV
Fig. 5. Plots of the induced fluorescence intensity at 426nm against irradiation
time for terephthalic acid on samarium doped TiO2.
accepted that the hole was quickly converted to the hydroxyl
radical upon oxidation of surface water, according to reaction
(2), and that the hydroxyl radical was the major reactant, which
•OH radicals depended on the competition between oxidation
of surface water by the holes (reaction (2)) and electron–hole
recombination according to reaction (3). Therefore, the greater
the formation rate of
ration efficiency of electron–hole pairs was achieved. All
above-mentioned results demonstrated that Sm3+doping could
effectively enhance the separation efficiency of electron–hole
pairs, and in this study the optimum doping content of Sm3+
in the titania was found be 0.5mol%, at which the recombi-
nation of photo-induced electrons and holes could be the most
doping content of rare earth ions in TiO2particles for the most
efficient separation of photo-induced electron–hole pairs.
•OH radicals was the higher sepa-
−→eTiO2− + h+
h++ eTiO2− → TiO2
3.5. Photocatalytic activity
The photocatalytic degradation of MB over Sm3+-doped
TiO2 samples calcined at 600◦C with different content of
Sm3+ion was evaluated and the results were shown in Fig. 6.
According to Fig. 6, the order of photocatalytic activity of
0.5>1.0>1.5>0mol%, which suggested that the Sm3+doping
enhanced the photocatalytic activity of TiO2and there was an
optimum doping content of Sm3+ions in TiO2particles.
The photocatalytic activity of TiO2 catalysts depended
strongly on two factors: adsorption behavior and the separation
efficiency of electron–hole pairs [1,31,32].
Fig. 6. Photocatalytic decomposition profiles of methylene blue over different
samarium doped TiO2nanoparticle calcined at 600◦C for 2h.
The photo-induced electron transferred to adsorbed organic
species resulted from migration of electrons and holes to the
semiconductor surface. The electron transfer process was more
efficient if the species were pre-adsorbed on the surface .
According to Figs. 3 and 6, the photocatalytic reactivity of
Sm3+–TiO2was higher than that of undoped TiO2, which was
consistent with the higher adsorption capacity of Sm3+–TiO2
than undoped TiO2. However, it was noticeable that a higher
adsorption capacity with a higher samarium ion dosage did not
lead to a higher photocatalytic activity, which might be limited
by lower separation efficiency of electron–hole pairs.
On the other hand, according to Figs. 5 and 6, it was found
that the order of photocatalytic activity was the same as that
of the formation rate of•OH radicals, namely, the greater the
formation rate of•OH radicals was, the higher photocatalytic
activity was achieved, indicating that the photocatalytic activity
was positive correlation to the formation rate of•OH radicals
on the catalysts. During the photocatalytic process, the absorp-
tion of photons by the photocatalysts led to the excitation of
electrons from the valence band to the conduction band, thus
generating electron–hole pairs. Oxygen molecules dissolved in
the suspension captured the electron in the conduction band,
and the hole in the valence band was captured by H2O species
adsorbed on the surface of the catalysts, to produce the•OH
radicals, which subsequently oxidized an adsorbed pollutants.
According to Houas et al., firstly the•OH radical generated
then•OH radical oxidized the adsorbed MB molecules. For that
reason, the greater the formation rate of•OH radicals was, the
higher photocatalytic activity was achieved. Therefore, in this
study 0.5mol% was the optimum content of Sm3+in the tita-
nia, at which the recombination of photo-induced electrons and
•OH radicals arrived at, and thereby the highest photocatalytic
activity was achieved.
It was observed that the presence of Sm3+ion as a dopant
dationunderUVlightirradiationbecauseboththelargerspecific Download full-text
surface area and the greater the formation rate of•OH radical
were simultaneously obtained for Sm3+-doped TiO2nanocrys-
talline. Sm3+–TiO2had a higher MB adsorption capacity than
undoped TiO2. The photocatalytic activity was positive correla-
tion to the formation rate of•OH radicals, namely, the greater
the formation rate of•OH radicals was, the higher photocat-
alytic activity was achieved. In this study, the optimum amount
of Sm3+doping was 0.5mol%, at which the recombination of
the highest formation rate of•OH radicals was, and thereby the
highest photocatalytic activity was achieved.
This work was supported by the Provincial Excellent PhD
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