Enhancement of Titania by Doping K+ for Degradation
of Methyl Orange under UV Irradiation
JING XIAO-HUI 1*, CAO HUI 1, CAI ZAI-SHENG 2
College of chemistry and chemical engineering, Nantong University, Nantong, Jiangsu 226019, China
2
College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China
1
K+-doped TiO2 nanoparticles with different doping amounts of K+ from 0 to 7.0 at.% were prepared by
modified sol-gel method. The samples were characterized by X-ray diffraction, transmission
electronmicroscopy, energy dispersive X-ray spectroscopy, and surface area measurements. The
photocatalytic degradation of methyl orange (MO) was used as a probe reaction to evaluate the photoactivities
of prepared nanoparticles. The experimental results indicate that doping K+ increased the surface area and
micropore surface area of TiO2 crystals, decreased the crystal size of TiO2, and reduced the diminishing rates
of surface area and pore volume with increasing calcination temperature, raised the temperature at which
anatase changed into rutile phase. TiO2 doped with 5% K+ and calcined at 650 °C showed much higher
photoactivity than samples with 0-7.0% doping amount of K+ and 350-850 °C calcination temperatures. The
kinetics of the MO degradation reaction fitted well to the Langmiur–Hinshelwood model.
Keywords: titanium dioxide; potassium ion;photocatalytic activity; kinetics, methyl orange
The photocatalytic decomposition of organic pollutants
has received extensive attention in recent decades [1].
Among various oxide semiconductor photocatalysts,
titanium dioxide has been proven to be the most suitable
for widespread environmental applications due to its
biological and chemical inertness, strong oxidizing power,
lower cost, and long-term stability against photocorrosion
and chemical corrosion [2]. When irradiated with UV light,
TiO 2 shows strong oxidizability and reducibility. The
photocatalytic process results from the generation of
charge-carriers, electrons (e-) in the conduction band and
holes (h + ) in the valence band. However, the
photogenerated holes recombine with conduction band
electrons easily, which can lower the efficiency of the
photocatalytic reactions [3]. Thus, preventing
recombination of electrons and holes is crucial to the
improvement of the photocatalytic activity of TiO2. Metal
ion doping is one of effective way to enhance the
photocatalytic activity of TiO 2 [4]. Titania doped with
transition metallic cations, rare metal cations, and noble
metal have been extensively investigated [5-7].
Nevertheless, doping alkali metal elements to improve its
photoactivity was seldom reported.
In the present work, K+-doped TiO2 nanoparticles were
prepared via the sol-gel procedure and their photocatalytic
activity was studied following the degradation of methyl
orange (MO) under UV irradiation. The objective is to study
the ion effect on stabilization of titania phase structure,
crystallite size, and phtoactivity in the MO decomposition.
Experimental part
Photocatalysts preparation
The K+-doped TiO2 were prepared by modified sol-gel
method. The samples with α% mole fractions of K+ and
with β°C calcination temperature were labeled as Kα-β.
Fore example, K1-650 means the doping amount of K+
was 1.0% and the calcination temperature was 650 °C.
Photocatalysts characterizations
The crystalline phases of the prepared samples were
identified by X-ray diffraction (XRD) using a D8-Advance
(German Bruker Co., Ltd). Transmission electron
microscope (TEM) were performed on aTEM-1230
transmission electron microscopy (Japan JEOL
Co.,Ltd.).Energy Dispersive Analysis of X-rays (EDAX) were
performed on EDS detector (Japan HORIBA Co.,Ltd.).The
BET surface area and pore volume of the prepared samples
were measured on a Micromeritics ASAP 2010 apparatus.
Photocatalytic activity measurement
The photocatalytic degradation was conducted in a
thermostatic cylindrical Pyrex reactor containing 200mL
of MO (with initial concentration of 10mg/L and initial pH
of 6.3) and operated at 25°C. A 300W mercury lamp
(Philips) was used as the light source. Before being
irradiated the resulting solution was stirred continuously in
the dark for 1 h to achieve the adsorption equilibrium of
MO on the catalyst. The absorbance of MO solution was
detected by a TU-1800SPC UV–vis spectrometer (Beijing
purkinje general instrument Co.,Ltd.) at 464 nm, the
maximum absorbance wavelength of MO. The efficiency
of photocatalytic process was calculated based on the
following formula:
(1)
Where φ is the efficiency of the photocatalytic process;
A0 and At stand for the absorbance of MO solution before
being irradiated and at the t moment of being irradiated,
respectively.
Characterization of K+-doped TiO2
The phase content, crystallite size and BET surface areas
of samples are summarized in table 1. The XRD patterns
of prepared samples are shown in figure 1. The average
crystallite sizes of anatase were calculated by applying
* email: jxh41@hotmail.com
REV. CHIM. (Bucharest) ♦ 62♦ No. 8 ♦ 2011
http://www.revistadechimie.ro
837
Table 1
THE PHASE CONTENT, CRYSTALLITE SIZE AND
BET SURFACE AREAS OF SAMPLES
Fig. 1. XRD patterns of
(a) K0 and (b) K5
the Scherrer’s formula, and the weight fraction of rutile
was calculated according to [8].
It is clear that the BET surface area depends on the
doping amount of K+ and the calcination temperature.The
BET surface areas are determined as K5 > K0 > K7, K5 >
K7 > K0, and K5 > K7 > K0 when the samples are calcined
at 550, 650 and 750 °C, respectively. Table 1 indicates that
the crystallite size of anatase increases with the increasing
calcination temperature. In the same calcination
temperatures, crystallite sizes of undoped TiO2 are larger
than those of K+-doped TiO2. It may be that doping K+ with
suitable doping amount inhibited the particles
agglomeration. The BET surface areas of K7 are less than
those of K5. This implies that doping K+ with excess amount
may induce the separate particles to coalesce and
decrease surface area.
Figure 1 shows the XRD patterns of K0 and K5 samples
calcined at different temperatures. For pure TiO2 samples,
the characteristic peak of rutile crystal phase appears
when increasing calcinations temperature to 650 °C and
the phase content of rutile is 27.5%. Further increasing
calcination temperature to 750 °C led to formation of 58.4%
rutile phase. For K5 samples, on the other hand, the
diffraction peaks at corresponding diffraction angles of the
anatase phase are sharpened, and the peak of the rutile
phase appears only when the calcination temperature
increases to 750 °C. The relative intensity of the 110 peak
decreases and the phase contents of rutile is about 24.4
(15.8% for K7) because of K+-doping. These results indicate
that doping K+ has a significant inhibitory effect on the
phase transformation, and that the phase transformation
temperature of anatase-to-rutile has been raised due to
K+-doping.
The TEM images of K0-550 and K5-550 samples are
shown in figure 2. All the samples are presented as
spherical nanoparticles uniformly. The average particle size
of pure TiO2 ranges from 20 to 25 nm, and K+-doped TiO2
shows a much smaller size of 10-12 nm. TEM also indicates
that doping K+ would decrease particle size of TiO2.
Figure 3 shows the EDS spectrum of K5-650. It is
confirmed that K+ was incorporated into the crystal lattice
of TiO2.
Results and discussions
Optimum doping amount of K+
The efficiency of the photocatalytic processs of MO
corresponding to TiO2 doped with different amounts of K+
and calcined at 650 °C is shown in figure 4, in which the
dosage of catalyst was 1.0g/L. The efficiency of the
photocatalytic process was improved with the increasing
of K+ doping amount when less than 5% of the K+ doping
was used. When the doping amount exceeded 5%, the
Fig. 2. TEM images of patterns of (a)K0-550, (b)K5-550
838
http://www.revistadechimie.ro
Fig.3. EDS spectrum of K5-650
REV. CHIM. (Bucharest) ♦ 62♦ No. 8 ♦ 2011
efficiency of the photocatalytic processs began to decline.
The photocatalytic activity became lower due to doping
K+ with excess amount could induce the separate particles
to coalesce and form a large unit. It is obvious from figure
4 that 5mol% K + -doped TiO 2 showed the highest
photocatalytic activity, which suggests that there existed
an optimum doping amount of potassium ions in TiO2
particles for the most efficient separation of photoinduced
electron-hole pairs.
Effect of calcination temperature
The photocatalysts calcinated at different temperatures
were used to degrade MO. The results are illustrated in
figure 5. It can be seen that for pure TiO2, the photocatalytic
activity can be increased with the increase of calcination
temperature when the thermal treatment below 550 °C.
Further increasing calcination temperature to 650 °C leads
to decrease of photocatalytic activity because phase
changes into rutile and the surface area and the surface
area decreases dramatically. So for pure TiO2, the optimal
calcination temperature was 550 °C.
maximum of 99.28% after 40 min treatment when the
catalyst amount was 1.2g/L. The enhancement of
efficiency of the photocatalytic process is due to: (i) the
increase in the amount of catalyst which increased the
number of MO molecule adsorbed and (ii) the increase in
the density of particles in the area of illumination [9].The
efficiency of the photocatalytic process, however,
decreased with the further increase of the photocatalyst
amount. This might be due to the light blocking effect,
since excessive amounts of catalyst could prevent TiO2
from illumination. In addition, the agglomeration and
sedimentation of the catalyst particles might also have a
negative effect on the efficiency of the photocatalytic
process [10].
Kinetics analysis
Generally, photocatalytic degradation of dyes follows
the Langmuir-Hinshelwood (L-H) model for most
investigated organic substrates [11]. At a low substrate
concentration, L-H model can be simplified to a pseudo
first-order equation:
(2)
Fig. 4. Effect of K+ doping amount on photocatalytic activity
Fig. 5. Effect of calcination temperature on photocatalytic activity
of K0 and K5
Where C0 is the initial concentration of the reactant (mg/
L), C is the concentration of the reactant (mg L-1) at time t
(min), kapp is the apparent rate constant( min-1). In order to
examine whether the reaction rate could be congruent
with a first-order reaction and to see how much the
efficiency of the photocatalytic process could be increased
when titania doped with the optimum amount of K+, a
ln(C/C0) versus reaction time t was plotted (fig. 7) for K0550 and K5-650 samples.
Fig.6. Effect of catalyst amount on the photocatalytic activity
For K5 samples, as mentioned previously, the rutile phase
generates and the surface area decreases dramatically only
after calcinations temperature increases to 750 °C because
of K +-doping. Consequently, the optimal calcination
temperature of K+-doped TiO2 was shifted to 650 °C. It can
be concluded that the decrease of photocatalytic activity
can be attributed to the existence of a rutile phase in the
TiO2 crystal and the dramatic decrease of the surface area.
Effect of catalyst amount
A series of experiments was conducted with various
amounts of catalyst (from 0.8 to 1.4 g/L) in order to
determine the optimal dosage of the photocatalyst. Figure
6 shows the degradation curves of MO with different
amounts of K5-650. It can be seen that, as the photocatalyst
amount increased from 0.8 to 1.2g/L, the efficiency of the
photocatalytic process of MO increased, and reached the
REV. CHIM. (Bucharest) ♦ 62♦ No. 8 ♦ 2011
Fig. 7. Relationship between ln(C/Co) and irradiation time
for K0-550 and K5-650
http://www.revistadechimie.ro
839
Figure 7 shows that a good linear relationship existed
between the ln(C/C0) versus t at reaction stage, indicating
that eq. (2) can be used to describe the photocatalytic rate
of MO. The apparent rate constants for K0-550 and K5-650
were calculated to be 0.057min-1 (R2 = 0.9949) and 0.119
min-1 (R2 = 0.9930), respectively. As shown by the reaction
rate constants, the photocatalytic activity of TiO 2 is
increased almost twice as high with suitable doping
amounts of K+.
UV–vis Spectrophotometer was used to monitor the
absorbance spectra of MO as a function of irradiation time.
Figure 8 demonstrates the temporal evolution of the
absorption spectra during photocatalytic degradation of MO
in aqueous K5-650 catalyst suspension. The absorbance
at 272 nm represented the aromatic content of MO and
the decrease of the band at this wavelength indicated
degradation of MO aromatic moiety. The absorbance at
464 nm was due to the color of MO solution and it was
used to examine the decolorization of MO solution [12].
The progressive decrease of 274 and 464 nm bands can be
seen from figure 8. It is clear that subsequent irradiation
leads to a continuous decrease in the UV and Vis bands of
MO with increasing illumination time. And there are no
new absorption bands in the UV–vis region. The absorption
peaks completely disappeared while MO had been
degraded 25 min.
Fig. 8. The UV-Vis spectrum change with time under UV irradiation
of methyl orange solution
Conclusions
The results of the present investigation conclude that
K+-doped TiO2 synthesized by modified sol-gel method is
more photoactive than pure TiO2. Doping TiO2 with K+ at
840
appropriate amounts increases surface area of TiO 2,
reduces crystallite size, raises the temperature at which
crystalling phase transforms from anatase to rutile. The
doping amount, calcination temperature, and the catalyst
dosage are the main factors that have strong influences
on the photodegradation of MO. This study reveals that the
optimal K+ doping amount, calcination temperature are
5% and 650 °C, respectively. The optimal dosage of K5-650
catalyst is 1.2 g/L. Under these optimal conditions, the
efficiency of the photocatalytic process reaches 99.28%
while MO solution with 10 mg/L initial concentration has
been irradiated for 40min. Photodegradation of MO follows
the L-H kinetics model. The results suggest that doping K+
ions can reduce the recombination of e- and h+ pair, and
significantly increases the photocatalytic activity of TiO2.
Acknowledgements: This work was financially supported by the Key
Project of Chinese Ministry of Education (109066), the Natural Science
Foundation of Jiangsu Provincial Education Commission
(06KJD530153) and Nantong Municipal Foundation (S2008007).
References
1.MURUGANANDHAM, M., SWAMINATHAN, M., Dyes Pigm., 68, nr. 2-3,
2006, p. 133
2. LIU, W., CHEN, SF., ZHAO W., et al., Desalination., 249, nr. 3, 2009,
p. 1288
3. WU, CH., Dyes Pigm., 77, nr.1, 2008, p. 31
4. LIU, GG., ZHANG, XZ., XU, YJ., et al., Chemosphere., 55, nr.9,
2004, p.1287
5. TAKAOKA, GH., NOSE, T., KAWASHITA, M., Vacuum., 83, nr.3, 2009,
p. 679
6. EL-BAHY, ZM., ISMAIL, AA., MOHAMED, RM., Hazard. Mater., 166,
nr.1, 2009, p. 138
7. WU, ZB., SHENG, ZY., LIU, Y., et al., J. Hazard. Mater., 164, nr. 2-3,
2009, p. 542
8. TIAN, BZ., LI, CZ., GU, F., et al., Chem. Eng. J.,151, nr.1-3, 2009,
p. 220
9. MURUGANANDHAM, M., SWAMINATHAN, M., J. Hazard. Mater., 135,
nr. 1-3, 2006, p. 78
10. WANG, CC., LEE, CK., LYU, MD., et al., Dyes Pigm., 76, nr. 3, 2008,
p. 817
11. TURCHI, CS., OLLIS, DF., J. Catal., 122, nr. 1, 1990, p. 178.
12. OKTE AN, YILMAZ O., Appl Catal A., 354, nr. 1-2, 2009, p. 132
Manuscript received: 27.01.2010
http://www.revistadechimie.ro
REV. CHIM. (Bucharest) ♦ 62♦ No. 8 ♦ 2011