Conjugated Microspheres FeTCPP–TDI–TiO2 with Enhanced Photocatalytic Performance for Antibiotics Degradation Under Visible Light Irradiation

Article abstract of DOI:10.1007/s10562-016-1888-1

Abstract: Toluene disocyanate (TDI) was used as a bridging molecule, a bridge bonding conjugated microsphere (FeTCPP–TDI–TiO₂) was successfully prepared by grafting tetra-(carboxyphenyl) porphyrin iron (FeTCPP) on the surface of TiO₂ microspheres. The FT-IR spectra revealed that the hydroxyl group (–OH) of TiO₂ microspheres surface and the carboxyl group (–COOH) of FeTCPP reacted respectively with the active isocyanato groups (–NCO) of TDI to form a surface conjugated microsphere FeTCPP–TDI–TiO₂. The UV–vis DRS analysis demonstrated that the formation of FeTCPP–TDI–TiO₂ extended remarkably the photoresponse of as-prepared samples to visible light region. The photocatalytic activity of FeTCPP–TDI–TiO₂ was evaluated using the photocatalytic degradation of norfloxacin (NFC), tetracycline (TC) and sulfapyridine (SPY) antibiotics in aqueous solution under visible-light irradiation. The results showed that, TDI, as a bond unit, was used to form a steady chemical bridging bond linking between FeTCPP and the surface of TiO₂ microspheres, and the prepared catalyst exhibited higher photocatalytic activity under visible-light irradiation for antibiotics degradation in comparison with P25. The degradation of antibiotics all followed the pseudo first-order reaction model under visible light irradiation, and the degradation mechanisms of NFC, TC and SPY were also proposed. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1888-1

Catal Lett (2016) 146:2543–2554
DOI 10.1007/s10562-016-1888-1
Conjugated Microspheres FeTCPP–TDI–TiO2 with Enhanced
Photocatalytic Performance for Antibiotics Degradation Under
Visible Light Irradiation
Binghua Yao1 · Chao Peng1,3 · Yangqing He1 · Wen Zhang2 · Qinku Zhang1 ·
Ting Zhang1
Received: 22 July 2016 / Accepted: 7 October 2016 / Published online: 28 October 2016
© Springer Science+Business Media New York 2016
Abstract Toluene disocyanate (TDI) was used as a bridg-
ing molecule, a bridge bonding conjugated microsphere
(FeTCPP–TDI–TiO2) was successfully prepared by graft-
ing tetra-(carboxyphenyl) porphyrin iron (FeTCPP) on the
surface of TiO2 microspheres. The FT-IR spectra revealed
that the hydroxyl group (–OH) of TiO2 microspheres sur-
face and the carboxyl group (–COOH) of FeTCPP reacted
respectively with the active isocyanato groups (–NCO) of
TDI to form a surface conjugated microsphere FeTCPP–
TDI–TiO2. The UV–vis DRS analysis demonstrated that
the formation of FeTCPP–TDI–TiO2 extended remarkably
the photoresponse of as-prepared samples to visible light
region. The photocatalytic activity of FeTCPP–TDI–TiO2
was evaluated using the photocatalytic degradation of nor-
loxacin (NFC), tetracycline (TC) and sulfapyridine (SPY)
antibiotics in aqueous solution under visible-light irradia-
tion. The results showed that, TDI, as a bond unit, was used
to form a steady chemical bridging bond linking between
FeTCPP and the surface of TiO2 microspheres, and the
prepared catalyst exhibited higher photocatalytic activity
under visible-light irradiation for antibiotics degradation
in comparison with P25. The degradation of antibiotics all
followed the pseudo irst-order reaction model under vis-
ible light irradiation, and the degradation mechanisms of
NFC, TC and SPY were also proposed.
* Binghua Yao
bhyao@xaut.edu.cn
* Wen Zhang
wenzhang@uark.edu
1
Department of Applied Chemistry, Xi’an University
of Technology, Xi’an 710048, China
2
Department of Civil Engineering, University of Arkansas,
Fayetteville 72701, USA
3
School of Materials Science and Engineering, Xi’an
University of Technology, Xi’an 710048, China
1 3

2544
B. Yao et al.
Graphical Abstract
photocatalytic performance [19, 20]. For overcoming this
drawback, much efort has been contributed to reinforce the
interaction between metalloporphyrins and TiO2 surface
instead of physical adsorption [21–23]. So far, numerous
articles reported that anchoring groups (such as sulfonic
or carboxylic acid) of metalloporphryins sensitized on the
TiO2 surface is expectant to solve the desorption problem
[24–26]. Unfortunately, it is rather diicult to synthesize
metalloporphyrin containing anchoring groups, and time
consuming [27, 28]. On the other hand, the formation of a
steady chemical bond between metalloporphyrin and TiO2
surface is hardly achieved due to the limited amounts of
hydroxyl groups on TiO2 surface [29, 30]. The introduc-
tion of bridging molecule, tolylene diisocyanate (TDI) con-
taining bifunctional groups, was an excellent strategy for
anchoring of metalloporphyrins on the TiO2 surface [31–
34]. Because of its bifunctional groups, the –NCO groups
of TDI molecules could react with the surface hydroxyl
of TiO2 and anchoring group of porphyrin respectively to
form a steady bridge bond. Based on these published arti-
cles, the aim of the present work is to report a novel bridge
bonding conjugated microspheres FeTCPP–TDI–TiO2 and
evaluate the potential utilization of this microsphere for
photocatalytic degradation of antibiotics. Because antibi-
otics as a emerging environmental pollutant, has attracted
wide attention in recent years, and the removal of antibiot-
ics by the conventional water treatment technologies, such
as biological treatment and adsorption methods, are not as
eicient as anticipated [35–37]. Therefore, it is very impor-
tant and urgent to develop eicient treatment methods to
control the concentration level of antibiotics in aquatic
environments. In this work, three representative antibiotics,
norloxacin (NFC), tetracyclines (TC) and sulfapyridine
(SPY), were chosen as target degradation compounds, the
visible light photocatalytic activity of FeTCPP–TDI–TiO2
Keywords FeTCPP–TDI–TiO2 complex · Bridge-
bonding conjugated microsphere · Surface modiication ·
TiO2 · Visible light photocatalyst · Antibiotic
1 Introduction
Photocatalysis has attracted increasing attention as an envi-
ronment friendly technology for the puriication of waste-
water compared to other conventional treatments [1–4].
Among various semiconductor photocatalysts, TiO2 micro-
spheres have been recognized as the most promising pho-
tocatalyst due to its unique photoactivity, nontoxicity, pho-
tostability and low cost [5–7]. However, the wide-band gap
of TiO2 (Eg =3.2 eV for anatase) means that TiO2 can only
absorb the UV light that wavelength is less than 387 nm,
which seriously restricts its practical application. In addi-
tion, the high recombination rate of photoinduced electrons
and holes is also a disadvantage of its application. So, much
work has been devoted to extending the photoresponse of
TiO2 to the visible light region and improving its photo-
catalytic performance via element doping, semiconductor
coupling, dye sensitization and exposure of reactive face
[8–14].
Among the sensitized agents, metalloporphyrins have
been recognized as the most superior photo-sensitiz-
ers because of their large π-electron systems and strong
absorption in visible light region [15–18]. Moreover, this
metalloporphyrin-TiO2 system can also improve the inter-
facial charge transfer and lower the electron–hole recom-
bination rate. Although the above mentioned outstanding
properties, metalloporphyrins adhered on the surface of
TiO2 by physical adsorption tend to occur partial desorp-
tion, thereby severely reducing eiciency of sensitiza-
tion, lowering quantum yield and leading to the decline of
1 3

Conjugated Microspheres FeTCPP–TDI–TiO2 with Enhanced Photocatalytic Performance for Antibiotics…
2545
was investigated, and their degradation mechanisms were
also proposed.
was dispersed into acetone by ultrasonic technology in a
three-neck lask to form a suspension. The suspension was
stirred and heated slowly, while TDI (molar ratio of TiO2
to TDI=1:0.5, this optimum ratio of TiO2 to TDI has been
quantify with previous work [34]) was dripped into the
lask. And then the mixture solution was reluxed at 50°C
for 30 min. The mixture solution was iltered and washed
with acetone 3–4 times, and dried at room temperature
to obtain TDI cross-linked TiO2 (TDI–TiO2). Finally, the
appropriate amount of TDI–TiO2 was added into FeTCPP/
acetone (20 mg/100 mL) solution under ultrasonic disper-
sion for 30 min (FeTCPP/TiO2 0.1 wt%). After the mixed
solution was transferred into a 250 mL three-necked lask,
and then reluxed at 50°C for 120 min in oil bath. The
resulted product was washed three times with acetone, and
then dried under vacuum to obtain the FeTCPP-conjugated
microspheres FeTCPP–TDI–TiO2.
2 Materials and Methods
2.1 Reagents
All reagents were of analytical grade and used without fur-
ther puriication. Toluene diisocyanate (TDI, AR, Xi’an
Chemical Reagent Factory), acetone (AR, Tianjin Fuyu
Fine Chemical Co. Ltd.). Degussa P25-TiO2 nanoparti-
cles with an average particle size of 21 nm were purchased
from Degussa Corporation, which consists of a mixture of
anatase (70%) and rutile (30%) mineral phases (Germany).
Tetracycline (TC, 98%) and sulfapyridine (SPY, 98%) were
supplied by Wolsen and Dr. Ehrenstofer Gmbh respec-
tively. Norloxacin (NFC, 99%) was obtained from Shanxi
Tanyuan pharmaceutical Co. Ltd. The mobile phase solvent
(acetonitrile) is of HPLC grade from Sigma-Aldric Inc.,
USA. Tetra-(carboxyphenyl) porphyrin iron (FeTCPP) was
obtained according to our previous paper [34]. Ultrapure
deionized water was used throughout all experiments.
2.3 Characterization of Samples
The FT-IR spectra were obtained using a Shimadzu FT-IR
8900 (Japan) with the reference of KBr. The morphology of
samples was observed by Tescan VEGA 3 SBH with tube
current of 30 μA, and the tube voltage of 20 kV (Czech).
The crystalline phase of samples was characterized by a
powder X-ray difractometer (Shimadzu XRD-7000 S,
Japan) at tube current of 30 mA, tube voltage of 40 kV, and
scanning speed of 10°/min. The UV–vis difuse relectance
2.2 Preparation of FeTCPP–TDI–TiO2 Microspheres
The preparation of FeTCPP–TDI–TiO2 microspheres
was shown in Scheme 1. TiO2 powder (Degussa P25)
CH3
NCO
H
C
H
O
3
H
O
H
O
H
O
NCO
NH
O C
Acetone
50 oC, reflux 120 min
Ti
O2
H
H
O
H
OOC
H
COO
O
H
O
O
H
O
H
O
H
NC
O
O
H
O
Ti
O2
H
N
Fe
N
H
O
Ti
O2
TDI
O
H
O
N
N
H
O
TDI-TiO2
H
COO
HOOC
C
H
COO
O
H
O
HN
N
H
O
H
O
O
H
N
Fe
Ti
C
O
O2
H
H
3C
O
N
N
H
O
H
O
N
H
O
H
COO
H
OOC
FeTCPP-TDI-TiO2
FeTCPP
Scheme 1 Schematic illustration of the preparation of FeTCPP–TDI–TiO2
1 3

2546
B. Yao et al.
spectra (UV–vis DRS) were obtained by a double-beam
UV–vis difuse relectance spectrophotometer (TU-1901,
China) with BaSO4 as reference.
several additional accessories. In a typical process, 50 mg
of photocatalysts and 50 mL of antibiotic solution (NFC:
25 mg/L, TC: 25 mg/L, SPY: 50 mg/L) were added to sam-
ple tube respectively. The air tube was inserted into the bot-
tom of tube, maintaining a controlled air low at 3 L/min to
achieve the suspended catalyst in the degradation solution.
The antibiotic solution was kept in dark for 30 min to reach
the adsorption equilibrium in the photocatalytic system
(preliminary results indicated that adsorption equilibrium
could be quickly reached less than 30 min), and then sam-
ples were withdrawn at diferent intervals (20 min). The
source of visible light was a Xe lamp, and a NaNO2 solu-
tion(25 g/L) was circulated through the cooling jacket to
ilter out the UV emission of the lamp below 400 nm. The
absorbance of the supernatant from high-speed centrifuga-
tion was measured by high performance liquid chromatog-
raphy (HPLC, Shimadzu LC-20 A, Japan) system was with
UV absorbance detector adjusted at 276 nm, 358 nm and
261 nm for detections of NFC, TC and SPY respectively.
A reverse phase column (Shimadzu VP-ODS 5012094)
with 150 nm length and 4.6 mm internal diameter was
used. The mobile phase was composed of acetonitrile and
ultrapure deionized water. The volume ratio CH3CN/H2O
was 75/25 and the low rates were 0.5 mL/min and 0.2 mL/
min for NFC and SPY separation, respectively. The volume
ratio CH3CN/H2O was 25/75 and the low rate was 0.3 mL/
min for TC separation. A UV-Spectrophotometer (UNICO,
UV-2102 PC, American) was used to measure absorbance
of antibiotics solutions. And according to the relationship
between the absorbance and the concentration of antibiotic
solution, the degradation rate D was calculated using the
equation D=(A0 −At)/A0 ×100%. Where A0 is the initial
absorbance of antibiotic solution, At is the absorbance of
antibiotic solution at diferent degradation times, D is used
to evaluate the photocatalytic activity of samples.
2.4 Electrochemical Measurements
Electrochemical properties of the FeTCPP–TDI–TiO2
were investigated by an electrochemical workstation (CHI
660b, Shanghai, China) in the three electrodes system,
which consisted of indium-tin oxide (ITO) glass working
electrode coating with the dispersed sample powders, a
platinum wire as auxiliary electrode and saturated calomel
electrode (SCE) as the reference electrode. The electrodes
joined a cell of 5-mL volume through holes in its Telon
cover. 0.1 mol/L Na2SO4 solution was used as the electro-
lyte solution to carry out the electrochemical experiments
at room temperature. The preparation procedure of the ilm
ITO working electrode was as follows: 2 mg FeTCPP–
TDI–TiO2 was added to 2 mL ethanol under ultrasonic for
30 min to form a suspension. Consequently, the ITO glass
is coated with suspension and dried in the oven at 100°C
for 2 h. Under the same conditions, FeTCPP–TiO2, TDI–
TiO2 and TiO2 ilm electrode were prepared. A 150 W Xe
lamp was utilized as the light source on the measurement.
The electrochemical impedance spectra Nyquist plots were
recorded from 100 kHz to 0.1 Hz frequency.
2.5 Photocatalytic Degradation of Antibiotics
Self-made photocatalytic reaction device (as shown in
Scheme 2) was used for the photocatalytic degradation
of antibiotics. The device was consisted of a light source
(Xe-lamp, 150 W), the sample tube (100 mL quartz
tube; length 22.0 cm, diameter 2.0 cm, 10 cm away from
the light source), a cold trap, an air bubbler, as well as
3 Results and Discussion
3.1 XRD Analysis
The phases and crystallinity of samples were characterized
by XRD. As shown in Fig. 1, the TiO2 component being
the well-known commercial Degussa P25, there is no sur-
prises regarding the crystal phase composition (70 wt%
anatase/JCPDS No.21-1272, 30 wt% rutile/JCPDS No.21-
1276) [38]. The characteristic difraction peaks of TDI–
TiO2, FeTCPP–TiO2 and FeTCPP–TDI–TiO2 in XRD pat-
terns do not exhibit relocation or any change in the peak
shape and the anatase/rutile ratio. There results indicated
that the modiication of TDI and sensitization of FeTCPP
which chemical bonded on the surface of TiO2 microsphere
had no efect on the crystal structure of TiO2 P25 powders.
Scheme 2 Schematic illustration of self-made photocatalytic reaction
device
1 3

Conjugated Microspheres FeTCPP–TDI–TiO2 with Enhanced Photocatalytic Performance for Antibiotics…
2547
3.3 FT-IR Analysis
FT-IR is a reliable technique for the monitoring of the por-
phyrin structure variations and obtaining further informa-
tion about the interaction between FeTCPP and modiied
TiO2 microspheres. Figure 3 shows the FT-IR spectra of
TCPP and FeTCPP. From the FT-IR spectrum of TCPP, the
peak at 3307 cm−1 was a result of stretching vibration of
the two N–H bonds at the center of porphyrin ring. Com-
pared to TCPP, FeTCPP spectrum has peak at 1000 cm−1
[39], which was attributed to the bond stretching/bending
vibration between Fe3+ and TCPP. Meanwhile, N-H vibra-
tion absorption peak disappeared, which caused by the for-
mation of metal ligand. The results suggested that Fe3+ has
been coordinated to TCPP.
Fig. 1 XRD patterns of TiO2 and photocatalysts
Figure 4a, b show the FT-IR spectra of TDI, TiO2 and
prepared samples respectively. The chemical bridging bond
linking of FeTCPP to TDI–TiO2 surface was conirmed by
FT-IR spectra. In the FT-IR spectrum of TDI, the absorp-
tion peak at 2268 cm−1 was due to isocyanate (–NCO) [31],
which indicating that there were still residual isocyanate
groups on the surface of TiO2, further conirmed the suc-
cessful functionalization of TiO2 surface. Simultaneously,
the new peaks were observed at 1653 and 1228 cm−1 due
to the asymmetric stretching vibration and the symmet-
ric stretching vibration of –NHCOOTi [32]. Additionally,
the FT-IR spectrum of FeTCPP–TDI–TiO2 showed that
the peak at 2268 cm−1 became weakened compared to the
characteristic absorption peaks of –NCO for TDI–TiO2. It
must be noted that the reactions between some amounts of
–NCO on the surface of TDI–TiO2 and –COOH groups of
FeTCPP led to the weakened peaks at 2268 cm−1. Mean-
while, the newly formed absorption peaks at 3323, 1630,
1535, 1420, 1381 and 1301 cm−1 corresponding to the for-
mation of –NHCO– are assigned as follows: two intense
3.2 SEM Analysis
Figure 2 shows the SEM image of FeTCPP–TDI–TiO2
sample. It was found that the FeTCPP–TDI–TiO2 parti-
cles was consisted of microspheres with 30–40 nm and
a uniform distribution, while no any other nanometer
microsphere found, indicating that FeTCPP and bridg-
ing bond molecules were well distributed on the surface
of TiO2. It is worthy to note that the surface of conju-
gated microspheres is much rougher than naked TiO2. In
addition, as compared to the well mono-dispersed TiO2,
the agglomeration of irregular particles with larger size
(1–2 μm) than microspheres (20–30 nm) is observed from
the image, which also clearly demonstrated the excellent
adhesion and stronger interfacial bridging bond of conju-
gated microspheres.
Fig. 2 SEM image of FeTCPP–TDI–TiO2
Fig. 3 FT-IR spectra of TCPP and FeTCPP
1 3

2548
B. Yao et al.
Fig. 4 FT-IR spectra of TDI, TiO2 and photocatalysts (a) and their expended spectra between the range of 1100–1900 cm−1 (b)
Fig. 6 EIS Nyquist plots of TiO2 and photocatalysts
Fig. 5 UV–vis DRS of TiO2 and photocatalysts
peaks at 3323 and 1535 cm−1 can be assigned to the
antisymmetric and symmetric N–H [40], the C=O vibra-
tion at 1630 and 1381 cm−1 [41, 42], the peak at 1420 cm−1
could be assigned to the esteriication resulted from the
reaction of –NCO (isocyanate of TDI) and –COOH (car-
boxyl of FeTCPP) [43] and the peak at 1301 cm−1 can be
assigned to the N–C vibration [44]. The FT-IR spectra
analyses indicated that the FeTCPP was ixed irmly on
the surface of TiO2 via TDI linking. However, after only
sensitized by FeTCPP, there was no new peak formed in
the FT-IR spectrum of FeTCPP–TiO2, which revealed that
FeTCPP only could be physical absorbed on TiO2 surface.
bridging bonded, the formation of TDI–TiO2 complex
resulted in a red-shift of the absorption threshold towards
to the visible region (up to 600 nm) and enhanced the uti-
lization of the visible-light. An obvious red shift of the
absorption edge toward to the visible region is observed
for FeTCPP–TDI–TiO2, which could arise from the tran-
*
*
sitions of a1u(π)−eg (π) and a2u(π)−eg (π) attributed to
FeTCPP molecule [45]. Furthermore, FeTCPP–TDI–TiO2
signiicantly showed almost feature peaks of FeTCPP and
Soret band at 420 nm, which was indicative of successful
grafting FeTCPP on the TiO2 surface with bridging bond
molecules. In addition, it can also demonstrate from DRS
data that the FeTCPP–TDI–TiO2 has the visible absorption,
which may use of visible light except UV light.
3.4 UV–vis DRS Analysis
Optical property of a semiconductor is one of the impor-
tant factors determining its photocatalytic performance.
Figure 5 illustrates the comparison of UV–vis difuse
relectance spectra (DRS) of samples. It can be seen that
TiO2 did not absorb any light above 400 nm. After surface
3.5 EIS Analysis
The electrical conductivity and electrons transfer of the
samples were studied by analyzing the electrochemical
impedance spectroscopy (EIS). Figure 6 shows the typical
1 3

Conjugated Microspheres FeTCPP–TDI–TiO2 with Enhanced Photocatalytic Performance for Antibiotics…
2549
Nyquist plots of FeTCPP–TDI–TiO2, FeTCPP–TiO2, TDI–
TiO2 and TiO2. The diameter of the semicircle in the
Nyquist plots represents the electron-transfer resistance
(R) of the samples [46, 47]. Noticeably, the Nyquist plot
of the FeTCPP–TDI–TiO2 displays the minimum semicir-
cle diameter, indicating the lowest R and the best electrical
conductivity. On the contrary, TiO2 shows the largest semi-
circle diameter among all the samples. Furthermore, TiO2
shows a value of R equal to 585 Ω which sharply reduces
to 134 Ω for FeTCPP–TDI–TiO2, indicating an easy charge
transfer at the sample surface of FeTCPP–TDI–TiO2. This
demonstrates that the recombination of photo-induced elec-
trons and holes in FeTCPP–TDI–TiO2 is more efectively
inhibited, which is consistent with the UV–vis DRS analy-
sis and photocatalytic results.
applying the Langmuir–Hinshelwood irst-order model as
expressed by Eq. (1):
(
)
ct
ln
= −kt
(1)
c0
This model is generally used for the photocatalytic deg-
radation process, where c0 and ct are the concentration of
initial and t time, rate constant (k) can be obtained from
the slope of the curve of ln(c0/ct) versus t. From the Fig. 8,
the data from the experimental results it well with irst-
order kinetic model, the kFeTCPP–TD–TiO (0.0342 min−1) is
2
the 11.2 times higher than kTiO2 (0.00304 min−1). Moreo-
ver the half-life time t1/2 was reduced from 227 min of
naked TiO2 to 20.2 min of FeTCPP–TDI–TiO2 (Table 1).
This result suggests that FeTCPP–TDI–TiO2 has the high-
est photocatalytic activity for the degradation of NFC. The
HPLC chromatograms (inset in Fig. 8) obtained from NFC
aqueous solution in presence of FeTCPP–TDI–TiO2 exhib-
ited that the intensity of the NFC peak (retention time at
3.74 min) decreased during photocatalytic process. At the
same time the new peak (retention time, 13.6 min, the inter-
mediate may be attributed to the attack of hydroxyl radicals
occurs at the piperazine ring and the desethylene of NFC
3.6 Evaluation of Photocatalytic Activity for Three
Antibiotics
3.6.1 Degradation of Norloxacin
Norloxacin is the major human-use luoroquinolone anti-
biotic, which has been caused wide attention as an organic
pollutant of wastewater [48]. Figure 7 shows the degrada-
tion rates (D%) of NFC in the presence of diferent cata-
lysts. The degradation rate of NFC with FeTCPP–TDI–
TiO2 is much faster and higher (the degradation rate is up
to 99.2%) than others. The insert of Fig. 7 (The UV–vis
absorption spectrum of the photodegradation of NFC solu-
tion by FeTCPP–TDI–TiO2) shows that the characteris-
tic absorption peak of NFC at 276 nm decreased sharply
and disappeared completely after 120 min [49]. In order to
better compare the efect of the diferent catalysts on the
NFC degradation reaction, the kinetic models of antibiotic
aqueous solution degradation was quantitatively studied by
Fig. 8 First-order kinetic for the degradation of NFC without cata-
lysts, in the presence of diferent photocatalysts. The inset is the
HPLC chromatograms of NFC in the degradation by FeTCPP–TDI–
TiO2
Table 1 Degradation kinetics parameters for NFC using the irst-
order kinetic model
Catalyst
k/10−2 min−1
t1/2/min
R2
D/%
Blank
0.105
0.304
0.780
0.640
3.42
660
227
88.9
108
20.2
0.992
0.991
0.989
0.989
0.997
11.4
28.5
64.1
57.2
99.2
TiO2
TDI–TiO2
FeTCPP–TiO2
FeTCPP–TDI–TiO2
Fig. 7 Degradation rates of NFC without catalyst, in the presence of
diferent photocatalysts. The inset is the UV–vis absorption spectra of
NFC solution in the presence of FeTCPP–TDI–TiO2
1 3

2550
B. Yao et al.
O
N
O
O
N
O
O
O
F
F
F
H
O
H
O
H
O
-
N
H5N
C2
H2N
Piperazine ring
cleavage
HN
N
HN
H2N
NFC
1
2
Fig. 9 Suggested pathway of the degradation of NFC by FeTCPP–TDI–TiO2
Fig. 10 Degradation rates of TC without catalyst, in the presence of
diferent photocatalysts. The inset is the UV–vis absorption spectra of
TC solution in the presence of FeTCPP–TDI–TiO2
Fig. 11 First-order kinetic for the degradation of TC without cata-
lyst, in the presence of diferent photocatalysts. The inset is the HPLC
chromatograms of TC in the degradation by FeTCPP–TDI–TiO2
molecule) was detected and its intensity increased with
the reaction time. The suggested pathway of the degrada-
tion of NFC by FeTCPP–TDI–TiO2 was depicted in Fig. 9.
The degradation of NFC occurs by the attack of hydroxyl
radicals on the piperazine. Then NFC undergoes piperazine
side chain cleavage to form the compound 1. Then com-
pound 1 further loses a C2H5N group to produce compound
2 (intermediate) [50]. Finally above results and analyses
indicated that NFC molecules were totally mineralized.
the increase of irradiation time (insert of Fig. 10), and
99.7% of TC was degraded after 120 min, which was 4.4
times higher than that of naked TiO2 (22.3%) [53]. As can
be seen from Fig. 11, the lnc0/ct values of TC degradation
are very good linear with respect to the degradation times
t for all catalysts. This shows that the degradation of TC
follows the irst order kinetics model. From Table 2, the
kFeTCPP–TDI–TiO of TC is 0.029 min−1, which is signii-
2
cantly higher than that of other catalysts. The greater activ-
ity of FeTCPP–TDI–TiO2 is particularly notable taking
into account that bridging bond and conjugated structure.
The insert of Fig. 11 illustrates the HPLC chromatograms
of TC degraded by FeTCPP–TDI–TiO2, the peak at reten-
tion time of 3.28 min gradually disappeared, meanwhile the
two new peaks (retention time at 3.03 and 4.10 min) were
detected and their intensities decreased as the reaction time
increased, indicating the presence of two intermediates and
inal mineralization [54]. The degradation pathway of TC
was shown in Fig. 12. The intermediates were observed in
3.03 and 4.10 min, resulted from the cleavage of double-
bond in aromatic ring b sites and formation of the com-
pounds 4 and 3. The peak observed in 3.03 min that the
retention time is slightly shorted than that of tetracycline
indicated that the more polar intermediate compound 4 was
3.6.2 Degradation of Tetracycline Hydrochloride
As an antibiotic, tetracycline hydrochloride (TC) is often
detected in the inal eluents from wastewater treat-
ment plants [35, 51]. When the value of solution pH lies
between 3.3 and 7.7, TC appears in form of non-biodeg-
radation (TCH2) [52]. Therefore, the degradation eicient
of diferent catalysts was evaluated by degradation of TC.
As can be observed in Fig. 10, the degradation rate of TC
without catalyst under direct visible light illumination
was only 7.64%. However, FeTCPP–TDI–TiO2 exhib-
ited a higher degradation rate than that of other catalysts
under identical conditions. Speciically, absorbance of TC
at 358 nm decreased by FeTCPP–TDI–TiO2 sharply with
1 3

Conjugated Microspheres FeTCPP–TDI–TiO2 with Enhanced Photocatalytic Performance for Antibiotics…
2551
Table 2 Degradation kinetics parameters for TC using the irst-order
kinetic model
Catalyst
k/10−2 min−1
t1/2/min
R2
D/%
Blank
0.075
0.234
0.611
0.785
2.97
913
296
113
88.3
23.2
0.996
0.984
0.994
0.990
0.997
7.64
22.3
53.8
64.2
99.7
TiO2
TDI–TiO2
FeTCPP–TiO2
FeTCPP–TDI–TiO2
formed. After 120 min, most of those intermediates would
be degraded and evolved to CO2, H2O and inorganic ions
etc.
Fig. 13 Degradation rates of SPY without catalyst, in the presence of
diferent photocatalysts. The inset is the UV–vis absorption spectra of
SPY solution in the presence of FeTCPP–TDI–TiO2
3.6.3 Degradation of Sulfapyridine
Sulfapyridine (SPY), a sulfonamide antibiotic, may enter
food chain and provoke acute and chronic adverse efects
in the environment. In comparison to conventional water
treatments, photocatalytic degradation has been proved to
be a successful alternative to eliminate non-biodegradable
compounds [55, 56]. Figure 13 displays the time course
evolution of SPY degradation as function of time in the
case of catalysts. The naked TiO2 exhibited about 17.5%
degradation rate of SPY after 120 min. On the contrary, the
degradation rate of FeTCPP–TDI–TiO2 (99.1%) displayed
5.6 times higher than that of naked TiO2 (17.5%) under
the same time. The UV–vis absorption spectra of SPY at
the varied irradiation times using FeTCPP–TDI–TiO2
were shown in the insert of Fig. 13. It is obvious that the
characteristic absorption peak of SPY at 261 nm gradually
becomes weak with the increase of reaction time, and the
degradation process of SPY followed a irst-order kinetic
model (Table 3). As shown in Fig. 14, the t1/2 of SPY deg-
radation with FeTCPP–TDI–TiO2 was 21.9 min, which
was about 18.7 times faster compared to that observed for
naked TiO2 (t1/2 =410 min). In addition, the kFeTCPP–TDI–TiO
2
(0.03 min−1) is higher than the kFeTCPP–TiO (0.002 min−1),
2
kTDI–TiO (0.003 min−1), kTiO2 (0.001 min−1) and kBlank
2
(0.0004 min−1). This comparison indicated that grafting
sensitizer on the TiO2 surface with bridging bond molecule
played an important role in the improvement of semicon-
ductor photocatalytic performance. These results are in
good agreement with the degradation of NFC and TC, and
H
H
O
O
c
O
O
a
O
H
H
O
O
O
O
O
O
H
O
H
O
NH2
NH2
d
b
H
O
H
H
C
H
H
H
O
H
H
H
O
3C
H
3C
N(CH3)2
O
T
H
O
O
H
O
O
H
O
H
O
O
O
H
O
H
O
H
H
O
H
H
O
H
O
O
O
H
H
O
H
H
O
H
H
H
3C
H
3C
H
O
O
3
4
Fig. 12 Suggested pathway of the degradation of TC by FeTCPP–TDI–TiO2
1 3

2552
B. Yao et al.
Table 3 Degradation kinetics parameters for SPY using the irst-
order kinetic model
Catalyst
k/10−2 min−1
t1/2/min
R2
D/%
Blank
0.0456
0.169
0.334
0.201
3.16
1520
410
296
344
21.9
0.987
0.980
0.997
0.989
0.990
4.52
17.5
23.8
19.8
99.1
TiO2
TDI–TiO2
FeTCPP–TiO2
FeTCPP–TDI–TiO2
Fig. 16 Photodegradation rates of NFC, TC and SPY in the presence
of FeTCPP–TDI–TiO2 for ive cycles
3.6.4 Stability of FeTCPP–TDI–TiO2 Catalyst
To evaluate the reusability of FeTCPP–TDI–TiO2 catalyst,
the degradation tests of three antibiotics by the recycled
catalyst were carried out 5 times under identical conditions.
50 mg of FeTCPP–TDI–TiO2 was dispersed into 50 mL of
antibiotic solution. After 120 min of photocatalytic reac-
tion, the FeTCPP–TDI–TiO2 was separated and washed
with deionized water for several times, then dried in an
oven at 80°C. As shown in Fig. 16, no obvious decrease of
the degradation eiciency was observed by using the same
Fig. 14 First-order kinetic for the degradation of SPY without cata-
lyst, in the presence of diferent photocatalysts. The inset is the HPLC
chromatograms of SPY in the degradation by FeTCPP–TDI–TiO2
O
O
N
H
N
H2N
H2N
H
O
S
S
Cleavage of
the S-N bond
O
SPY
O
5
Fig. 15 Suggested pathway of the degradation of SPY by FeTCPP–TDI–TiO2
furthermore evidence that FeTCPP–TDI–TiO2 possesses
the best photocatalytic performance. The HPLC chroma-
tograms of SPY solution degraded by FeTCPP–TDI–TiO2
illustrates in the insert of Fig. 14, the chromatographic
peak intensity of SPY at tR =18.9 min was reduced with a
simultaneous formation of few intermediate peaks at lower
retention times. The major intermediate peak observed
at tR =7.84 min is ascribed to the sulfanilic acid, which
resulted from the cleavage of the bond between the aniline
ring and the sulfonic group followed by an ·OH attack to
the S-N bond [55]. Moreover, their retention times were
shorter than that of SPY. It indicated that the major more
polar intermediate was sulfanilic acid (compound 5). The
proposed intermediates and degradation pathway of SPY
were depicted in Fig. 15.
batch of catalyst. This demonstrates that the stability of the
synthesized FeTCPP–TDI–TiO2 is very excellent under the
cyclic tests, and has good potential to be used in wastewater
treatment.
4 Conclusion
With TDI as a bridging bond molecule, the conjugated
microspheres FeTCPP–TDI–TiO2 were successfully
obtained by grafting FeTCPP on the surface of TiO2. The
characterization results showed that the FeTCPP–TDI–
TiO2 particles were of microsphere structure (a mixture of
anatase (70%) and rutile (30%) phases) with sizes ranged
1 3

Conjugated Microspheres FeTCPP–TDI–TiO2 with Enhanced Photocatalytic Performance for Antibiotics…
2553
18. Niu JF, Yao BH, Chen YQ, Peng C, Yu XJ, Zhang J, Bai GH
(2013) Appl Surf Sci 271:39
between 20 and 30 nm, and there exists obvious aggrega-
tion in the powder. As expected, the chemical interaction
occurred between the FeTCPP and TiO2 surface rather than
physical adsorption. The FeTCPP–TDI–TiO2 microspheres
exhibited an excellent photocatalytic performance toward
the degradation of antibiotics NFC (99.2%), TC (99.7%)
and SPY (99.1%) within 120 min under visible light irra-
diation. According to HPLC experimental results, the pos-
sible photocatalytic mechanism of FeTCPP–TDI–TiO2 for
antibiotics was also discussed. Additionally, the FeTCPP–
TDI–TiO2 showed the long-term stability and reusability,
up to 5 times without signiicant deactivation. Our work
is not only useful in the synthesis of bridge-bond complex
between sensitizer and TiO2 but also provide some insight
into the design of tailoring the mult-composite materials
with high photocatalytic performance, which may open
new ield of synthesizing photocatalytic materials.
19. Wang C, Yang GM, Li J, Mele G, Slota R, Broda MA, Duan
MY, Vasapollo G, Zhang XF, Zhang FX (2009) Dyes Pigm
80:321
20. Chang MY, Hsieh YH, Cheng TC, Yao KS, Wei MC, Chang CY
(2009) Thin Solid Films 517:3888
21. Cai JH, Huang JW, Yu HC, Ji LN (2012) J Taiwan Inst Chem E
43:958
22. Gao BJ, Fang L, Men JY, Lei QJ (2012) Mater. Chem Phys
134:1049
23. Murphy S, Saurel C, Morrissey A, Tobin J, Oelgemöller M,
Nolan K (2012) Appl Catal B 119:156
24. Daphnomili D, Sharma GD, Biswas S, Thomas KRJ, Coutsole-
los AG (2013) J Photochem Photobiol A 253:88
25. Kathiravan A, Renganathan R (2009) J Colloid and Interface Sci
331:401
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P, Fei JJ, Tan ST (2011) Dyes Pigm 88:75
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Acknowledgments This work was supported by the Naional Natural
Science Foundation of China (No. 21276208), the International Sci-
ence Technology Cooperation Program of China (2015DFR50350),
the Research Fund of Shaanxi Key Laboratory of Comprehensive
Utilization of Tailings Resources (No. 2014SKY-WK003), the Spe-
cial Research Fund of Shaanxi Provincial Department of Education
of China (No. 15JK1862), and the Research Fund for Innovation Doc-
toral Thesis of Xi’an University of Technology (No. 310-11202J304,
310-252071508).
29. Mallakpour S, Aalizadeh R (2013) Prog Org Coat 76:648
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264:670
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181:593
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Chem 180:1787
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174:77
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Phys 27:200
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