Preparation, characterization and photocatalytic activity of metalloporphyrins-modified TiO2 composites

Article abstract of DOI:10.1007/s11164-012-0701-z

Three new Cu(II), Zn(II), Co(II) 5,15-di (4-hydroxyphenyl)-10, 20-diphenyl-porphyrin (CuDHPP, ZnDHPP, CoDHPP) and the corresponding metalloporphyrins-TiO₂ photocatalysts CuDHPP-TiO₂, ZnDHPP-TiO₂, CoDHPP-TiO₂ wer

Full text of DOI:10.1007/s11164-012-0701-z

Res Chem Intermed (2013) 39:1447–1457
DOI 10.1007/s11164-012-0701-z
Preparation, characterization and photocatalytic
activity of metalloporphyrins-modified TiO2 composites
•
Wanjun Sun Jun Li Xiangfei Lu¨ Fengxing Zhang
•
•
Received: 25 May 2012 / Accepted: 20 June 2012 / Published online: 8 July 2012
ꢀ Springer Science+Business Media B.V. 2012
Abstract Three new Cu(II), Zn(II), Co(II) 5,15-di (4-hydroxyphenyl)-10,
20-diphenyl-porphyrin (CuDHPP, ZnDHPP, CoDHPP) and the corresponding
metalloporphyrins–TiO2 photocatalysts CuDHPP–TiO2, ZnDHPP–TiO2, CoD-
HPP–TiO2 were synthesized and characterized by SEM, XRD, FT-IR, and DRS.
The results revealed that the metalloporphyrins impregnated onto the surface of
TiO2 did not change the phase composition and particle sizes of TiO2 samples, but
increased the photocatalytic efficiency. In addition, photoluminescence study
showed that the three photocatalysts could successfully increase the separation
efficiency of the photoinduced electron and hole. The photodegrading 4-NP
experiments indicated that the three photocatalysts greatly enhanced the photo-
catalytic activity of bare TiO2, and the photocatalytic activity of CuDHPP–TiO2
was the highest. Moreover, the possible mechanism for the photodegradation of
4-NP was also proposed.
Keywords Metalloporphyrins ꢀ TiO2 ꢀ Photocatalytic degradation ꢀ
4-Nitrophenol ꢀ Mechanism
W. Sun ꢀ J. Li (&) ꢀ X. Lu¨ ꢀ F. Zhang
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education,
College of Chemistry & Materials Science, Northwest University, Xi’an 710069,
People’s Republic of China
e-mail: junli@nwu.edu.cn
X. Lu¨
Department of Environmental Engineering, College of Resource and Environment, Shaanxi
University of Science and Technology, Xi’an 710021, People’s Republic of China
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Introduction
In recent years, there has been growing concern over many serious environmental
problems. It is, therefore, of the utmost urgency to develop ecologically clean and safe
chemical technologies, materials, and/or processes to sustain our present level of
population and economic expansion [1, 2]. 4-Nitrophenol as an important organic
intermediate is widely used in the production of pesticides, insecticides, herbicides,
and synthetic dyes, but their toxic effects on human health are well documented [3, 4].
As we all know, photocatalysis is a promising low-cost alternative method for the
treatment and purification of contaminated water compared with other technologies
[5, 6]. TiO2 is believed to be one of the most active photocatalysts. However, a critical
drawback of TiO2 is the large band gap (3.2 eV), which is too large to absorb solar
energy effectively. In addition, the high degree of recombination between photoin-
duced electrons and holes also blocks its practical application [7–9]. So a lot of effort
has been devoted to studying how to extend its light response [10–12]. Among them,
the metalloporphyrin-modified TiO2 system has been proposed for the photocatalytic
degradation of various kinds of organic pollutants in water, as porphyrin compounds
possess good chromophore activities over the solar spectrum and good electron-
donating properties due to their large p-electron systems [13–15].
Metalloporphyrins with different functional groups (–COOH, –OH, etc.) in the
periphery of the porphyrin ring have been employed as photosensitizers for
pollutant photodegradation [16–18]. It has been found that these functional groups
can interact with the surface of TiO2 and, consequently, display different
photocatalytic activity in degrading pollutants.
Motivated by the above work, we here report a novel functionalized porphyrin 5,
15-di (4-hydroxyphenyl)-10, 20-diphenyl-porphyrin (H2DHPP) and their corre-
sponding metalloporphyrins MDHPP (M=Co(II), Cu(II), Zn(II)) (Fig. 1). The
MDHPP–TiO2 photocatalysts were then synthesized and characterized by SEM,
XRD, FT–IR, and DRS. The further photocatalytic activities of the MDHPP–TiO2
samples were evaluated in the photocatalytic degradation of 4-nitrophenol in
aqueous solution under the halogen lamp irradiation. The PL spectra of the three
photocatalysts showed that they could successfully improved charge separation than
bare TiO2, which leads to a higher photocatalytic activity. Furthermore, a possible
mechanism of the photodegradation of 4-NP was also proposed.
Fig. 1 The structure of
porphyrin and their
metalloporphyrins
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Metalloporphyrins modified TiO2 composites
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Experimental
Materials
P-Hydroxybenzaldehyde, benzaldehyde, pyrrole, and dichloromethane were obtained
from Tianjin Chemical Reagents, China. All these chemicals were used without
further purification except pyrrole, which was distilled before use. The commercial
TiO2 was purchased from Acros Organics, USA (Anatase phase). The target pollutant
4-nitrophenol (4-NP) was obtained from Xi’an Chemical, China. Distilled water was
used throughout the 4-NP removal experiments.
Characterizations
The scanning electron microscopy (SEM; Hitachi-S4800) was used to characterize
the morphologies of the samples. Diffuse reflectance UV–vis spectra were recorded
on a Shimadzu UV-3100 spectrophotometer with BaSO4 as reference. The FT-IR
spectra were recorded on a BEQUZNDX-550 FT-IR spectrometer using KBr pellets
in the range 4,000–400 cm-1. The X-ray diffraction (XRD) measurement was
performed with a Bruker D8 diffractometer using graphite monochromatic copper
radiation (Cu Ka) at 40 kV, 30 mA over the 2h range 20–80ꢁ. Photoluminescence
spectra (PL) were measured at room temperature on a Hitachi FL-4500 fluorescence
spectrometer using a Xe lamp with an excitation wavelength of 330 nm.
Synthesis of porphyrin (H2DHPP) and metalloporphyrins (MDHPP)
5, 15-di (4-hydroxyphenyl)-10, 20-diphenyl-porphyrin (H2DHPP) was synthesized
according to the methods described in the literature [18].
P-Hydroxybenzaldehyde (30 mmol, 3.66 g) and benzaldehyde (30 mmol,
3.07 mL) were added to 150 mL propionic acid. Then, pyrrole (60 mmol,
4.15 mL) was added dropwise and the resulting mixture was vigorous stirred at
145 ꢁC for 50 min. After cooling and the solvent removal under vacuum, the
residue was dissolved in CH2Cl2 and purified on silica-gel with dichloromethane/
hexane as eluant. A desired purple-black solid of H2DHPP was successfully
obtained. Yield: 38 %. Mp: [250 ꢁC, Anal. Calcd. for C44H30N4O2 (Mol. wt:
646.24), %: C, 81.73 (81.71); H, 4.62 (4.68); N, 8.69 (8.66). MS: m/z
647.53([M ? H]?) amu. UV–vis (CH2Cl2): kmax/nm, 418 (Soret band), 479, 515,
549, 592 (Q bands). FT-IR: t, cm-1, 3,341, 2,963, 2,923, 2,361, 1,576, 1,418, 1,261,
1,096, 1,023, 964, 801, 697.
Copper, zinc, and cobalt porphyrins were synthesized by treating H2DHPP
(0.33 mmol) with an excess of Cu(OAc)2ꢀH2O, Zn(OAc)2ꢀ2H2O or Co(OAc)2ꢀ4H2O
in CH2Cl2 (30 mL) for 12 h at room temperature, respectively. The mixture was
monitored by TLC until the complete disappearance of the starting material
H2DHPP. The unreacted acetic acid salt was filtrate and the solvent removed under
vacuum. The crude product was purified by chromatography on a silica-gel column
with CH2Cl2 as eluant. Metalloporphyrins (CuDHPP, ZnDHPP, CoDHPP) were
obtained in nearly quantitative yields.
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W. Sun et al.
CuDHPP
Yield: 95 %. Mp:[250 ꢁC, Anal. Calcd. for C44H28N4O2Cu (Mol. wt: 707.15), %:
C, 74.59 (74.61); H, 4.03 (3.98); N, 7.89 (7.91). MS: m/z 708.02 ([M ? H]?) amu.
UV–vis (CH2Cl2): kmax/nm, 418 (Soret band), 539, 575 (Q bands). FT-IR: t, cm-1
3,423, 2,923, 2,852, 1,659, 1,606, 1,510, 1,436, 1,384, 1,343, 1,263, 1,168, 1,099,
1,068, 999, 797, 702.
,
ZnDHPP
Yield: 95 %. Mp:[250 ꢁC, Anal. Calcd. for C44H28N4O2Zn (Mol. wt: 708.15), %:
C, 74.39 (74.42); H, 3.98 (3.97); N, 7.91 (7.89). MS: m/z 709.33 ([M ? H]?) amu.
UV–vis (CH2Cl2): kmax/nm, 421 (Soret band), 560, 600 (Q bands). FT-IR: t, cm-1
,
3,424, 2,923, 1,709, 1,657, 1,607, 1,550, 1,485, 1,436, 1,384, 1,336, 1,263, 1,168,
1,099, 1,066, 997, 796, 701, 668, 583.
CoDHPP
Yield: 93 %. Mp: [250 ꢁC, Anal. Calcd. for C44H28N4O2Co (Mol. wt: 709.15),%:
C, 75.09 (75.10); H, 4.05 (4.01); N, 8.01 (7.96). MS: m/z 710.43 ([M ? H]?) amu.
UV–vis (CH2Cl2): kmax/nm, 418 (Soret band), 548, 587 (Q bands). FT-IR: t, cm-1
,
3,427, 2,925, 2,361, 1,615, 1,551, 1,453, 1,385, 1,263, 1,169, 1,056, 874, 802, 669,
615.
Preparation of MDHPP–TiO2 photocatalysts
MDHPP–TiO2 photocatalysts used as photocatalysts for the photodegradation
experiments were prepared in the following way: 6 lmol amounts of CuDHPP,
ZnDHPP, or CoDHPP were dissolved in 30 mL of CH2Cl2, respectively, and then
1 g of finely ground TiO2 was added to this solution. The resulting suspension was
magnetically stirred at room temperature for 12 h. Then, the solvent was removed
under vacuum and the photocatalyst was collected. Afterwards, the surface of white
TiO2 displayed the corresponding color of the photocatalyts, which demonstrated
that the MDHPP had been impregnated onto the surface of TiO2 nanoparticles.
Evaluation of photocatalytic activity
Photocatalytic activities of MDHPP–TiO2 composites were evaluated for the
photodegradation of 4-NP using a Model XPA-VII photocatalytic reactor (Xujiang
Electromechanical Plant, Nanjing, China). A 400-W halogen lamp with wavelength
emissions closely mimicking solar radiation was served as the light source [19]. The
distance between the light source and the reactor was 12 cm and the light intensity
was 615 mw cm-2. The system was cooled by circular water running via an
interlayer and magnetic agitation was used so that the photocatalysts can be
suspended in the solution.
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Metalloporphyrins modified TiO2 composites
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In a typical experiment, 4-NP (50 mL, 1 9 10-4 mol/L) and 10 mg of the
photocatalysts were placed in the quartz socket tube. Before irradiation, the
suspension was magnetically stirred in the dark for 30 min to ensure the establishment
of adsorption equilibrium between the photocatalysts and 4-NP. At a given time
interval, about 3 mL of the suspension was withdrawn, and the photocatalysts were
separated from the suspensions by filtration. The photodegradation process was
monitored by its characteristic absorption band of 4-NP at 317 nm using a UV–vis
spectrophotometer (UV-1800; Shimadzu). Based on the correlation between concen-
tration and absorption, C/C0 = A/A0, the decomposition efficiency was calculated.
Results and discussion
Morphology of photocatalysts
SEM observation can reveal that the samples consisted of aggregates of particles.
Figure 2 shows the representative micrographs obtained for the bare TiO2 before
and after being treated with CuDHPP, ZnDHPP, and CoDHPP, respectively. It can
be observed that the microsphere possesses a similar surface condition, indicating
that the TiO2 microsphere basically were no change before and after impregnated
with the metalloporphyrins.
Fig. 2 SEM images of bare TiO2 (a), CuDHPP–TiO2 (b), ZnDHPP–TiO2 (c), and CoDHPP–TiO2
(d) photocatalysts
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XRD characterization of photocatalysts
In order to confirm the crystal identity of TiO2 samples and the effect of MDHPP on
the crystal structure of TiO2, XRD spectra were recorded. As shown in Fig. 3, all
the characteristic peaks are well matched with that of anatase TiO2 (JCPDS file no.
12–1272). When MDHPP is impregnated onto the surface of TiO2, no detectable
other peaks are observed for the MDHPP–TiO2 photocatalysts, speculating that the
content of MDHPP are small and/or well dispersed on the surface of TiO2, which
suggests that the MDHPP impregnated onto the surface of TiO2 does not change the
bulk intrinsic property of the TiO2 photocatalyst [20].
FT-IR spectra of photocatalysts
To further investigate the interaction between MDHPP molecules and TiO2
nanoparticles, the FT-IR spectra of bare TiO2, CuDHPP–TiO2, ZnDHPP–TiO2, and
CoDHPP–TiO2 were measured. As shown in Fig. 4, for bare TiO2, the vibrations at
3,425 and 1,631 cm-1 observed correspond to the stretching vibrations and the
bending vibration of –OH on the surface of TiO2, respectively. It indicates that there
exist large amounts of hydroxy groups, which is helpful for anchoring MDHPP
molecules on the surface of TiO2 by the interaction of hydroxyl groups [21].
Furthermore, when MDHPP is impregnated onto the surface of bare TiO2, the main
characteristic vibrations attributed to the C–C at *1,400 cm-1 and C–H
*2,920 cm-1 at of MDHPP can be clearly observed, which indicate that the
TiO2 modified by MDHPP has been successfully achieved.
UV–visible diffuse reflectance spectra of photocatalysts
The optical property of a semiconductor is one of the important factors determining
its photocatalytic performance [22]. As shown in Fig. 5, the UV–visible diffuse
reflectance spectra of the bare TiO2 and MDHPP–TiO2 photocatalysts were
recorded in the range of 300–700 nm. It is obvious that there was no absorption
Fig. 3 XRD patterns of bare
TiO2, CuDHPP–TiO2,
ZnDHPP–TiO2, and
CoDHPP–TiO2 photocatalysts
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Metalloporphyrins modified TiO2 composites
1453
Fig. 4 FT-IR spectra of bare
TiO2, CuDHPP–TiO2,
ZnDHPP–TiO2, and
CoDHPP–TiO2 photocatalysts
above 400 nm for the bare TiO2, whereas the MDHPP–TiO2 photocatalysts exhibit
the feature peaks of MDHPP observed in CH2Cl2 solution (Table 1), which
indicated that MDHPP successfully loaded onto the TiO2 surface with integrated
metalloporphyrin framework, and led to broader absorption range for bare TiO2.
It was noticed that the Soret and Q bands of the MDHPP–TiO2 are all shifted and
broadened relatively to MDHPP observed in CH2Cl2 solution, implying that there
exist a weak interaction between TiO2 and metalloporphyrins molecules.
Photoluminescence (PL) spectra of photocatalysts
Photoluminescence (PL) emission spectra have been widely used to investigate the
efficiency of charge carrier trapping, immigration and transfer and to understand the
fate of electron–hole pairs in semiconductor particles [23]. As can be observed in
Fig. 6, three peaks around 400, 465, and 525 nm are observed for the bare TiO2.
Generally, the peak at 400 nm can be attributed to self-trapped excitation localized
in the TiO6 octahedron, and the peak at 465 nm to oxygen vacancies. Peak at
Fig. 5 UV–visible diffuse
reflectance spectra of bare TiO2,
CuDHPP–TiO2, ZnDHPP–TiO2,
and CoDHPP–TiO2
photocatalysts
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W. Sun et al.
Table 1 UV–visible spectra data of the MDHPP in CH2Cl2 and UV–visible diffuse reflectance spectra
data of MDHPP–TiO2 photocatalysts
Compounds
MDHPP
MDHPP–TiO2
Soret band
k max(nm)
Q bands
k max(nm)
Soret band
k max(nm)
Q bands
k max(nm)
CuDHPP
ZnDHPP
CoDHPP
418
420
418
539, 575
560, 600
548, 587
415
425
430
542, 592
562, 604
556, 595
525 nm is assigned to the oxygen vacancy with one trapped electron [24–26]. The
spectra of the three photocatalysts showed similar curve shape except for the peak
intensities. The bare TiO2 sample showed the highest PL emission intensity,
indicating that electrons and holes were more easily recombined. However, the PL
emission intensity decreased greatly in the three photocatalysts. So it indicated that
the MDHPP–TiO2 have lower recombination of electrons and holes. Furthermore, it
can be deduced that the photo-produced electrons of the excited CuDHPP–TiO2
molecules may easily transfer effectively to the conduction band (CB) of the TiO2
than ZnDHPP and CoDHPP, which reasonably leads to a higher photocatalytic
activity since the photodegradation reactions are evoked by these charge carriers
[17].
Photocatalytic activity and photocatalytic mechanism
The photocatalytic activity for the degradation of 4-NP in aqueous suspension under
the halogen lamp irradiation using MDHPP–TiO2 and CuTPP–TiO2 (as for
comparison) were tested. Obviously, as shown in Fig. 7, the blank experiment
showed that the 4-NP was hardly photodegraded in the absence of any photocatalyst.
After 60 min of irradiation, the three new photocatalysts, CuDHPP–TiO2, ZnDHPP–
TiO2, and CoDHPP–TiO2, showed much enhanced photoactivity compared with bare
TiO2, and in particular CuDHPP–TiO2 exhibited the highest efficiency of all samples,
Fig. 6 The room temperature
photoluminescence spectra with
excitation at 330 nm for bare
TiO2, CuDHPP–TiO2,
ZnDHPP–TiO2, and
CoDHPP–TiO2 photocatalysts
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Metalloporphyrins modified TiO2 composites
1455
as the following order and their corresponding photodegradation efficiency in
parentheses:
CuDHPP–TiO2 (95.8 %) [ ZnDHPP–TiO2 (85.6 %) [ CoDHPP–TiO2 (80.
8 %) [ CuTPP–TiO2 (66.8 %) [ bare TiO2.
These results indicated that metal ion plays an important role in the photore-
activity. Meanwhile, the anchoring group (–OH) of metalloporphyrin on the TiO2 is
also a main factor for the photodegradation efficiency. In addition, the photo-
electrons can be transferred more efficiently from the excited MDHPP molecules to
the conduction band of TiO2 [27, 28], which improve the separation of
photogenerated electrons and holes, resulting in the photodegradation of 4-NP
finally. Furthermore, the photocatalyst of CuDHPP–TiO2 shows the best photocat-
alytic activity due to its sharply inhibiting of the electrons and holes of combination,
which could be attributed to the lowest PL emission intensity of the three
photocatalysts.
Based on the above experimental results, we propose a mechanism accounting
for the photocatalytic degradation of 4-NP over MDHPP–TiO2 and bare TiO2 under
halogen lamp irradiation. It is demonstrated in Fig. 8. On the one hand, TiO2 itself
can be excited to create mobile electrons and then the photoinduced electrons inject
into the CB of TiO2 (Eq. 1). Thus, the holes, created in the VB of TiO2, can also
immigrate faster to the 4-NP molecules through the MDHPP, which has the
potential to photodegrade 4-NP into small molecules or CO2, H2O (Eq. 2) [29]. On
the other hand, the excitation of ground state of the [MDHPP] can be excited to its
excited state [MDHPP]*, and the excited state electrons could readily inject into the
CB of TiO2 (Eq. 3). The excited electrons in the CB of TiO2 reacted with absorbed
O2, and then O•2- or •OH were subsequently formed, which will oxidize 4-NP into
small molecules or CO2, H2O (Eqs. 4–6) [30]. In addition, according to the previous
work [31, 32], [MDHPP]? can be reduced by 4-NP to its ground state (Eq. 7).
UV
TiO2ꢁ! TiO2ðeꢁÞ þ TiO2ðhþÞ
ð1Þ
Fig. 7 Photocatalytic activity
of 4-NP as a function of
irradiation time over under
halogen lamp irradiation using
different photocatalysts
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W. Sun et al.
Fig. 8 The photocatalytic
mechanism of MDHPP–TiO2
under halogen lamp irradiation
hþ transfer
4-NP
MDHPPðhþÞ ꢁ! CO2; H2O, and other small molecules ð2Þ
Vis
TiO2ðhþÞ ꢁ!
TiO2½MDHPPꢂꢁ!TiO2½MDHPPꢂꢃ! ½MDHPPꢂþþTiO2ðeꢁÞ
ð3Þ
TiO2ðeꢁÞ þ O2 ! TiO2 þ Oꢀ2ꢁ
ð4Þ
ð5Þ
ð6Þ
ð7Þ
Oꢀ2ꢁ þ H2O ! ꢀ ꢀ ꢀ ! ꢀOH þ OHꢁ
Oꢀ2ꢁ or ꢀ OH þ 4 - NP ! CO2 þ H2O and other small molecules
½MDHPPꢂþþ4 - NP ! MDHPP þ oxidation products
Conclusions
In this work, three new metalloporphyrins MDHPP [M=Cu(II), Co(II), and Zn(II)]-
modified TiO2 composites have been synthesized and characterized. The SEM,
XRD, DRS, and FT-IR spectra demonstrated that MDHPP were successfully
impregnated onto the surface of TiO2 microsphere and did not influence the
morphology of TiO2. Furthermore, the three photocatalysts of MDHPP–TiO2 have
lower recombination of electrons and holes than bare TiO2 from the PL spectra. The
photocatalytic activity experiment indicated that the three photocatalysts greatly
enhanced the photocatalytic efficiency of bare TiO2 in photodegrading the 4-NP,
and CuDHPP–TiO2 shows the best photocatalytic activity. This can be explained by
the synergic effect between the metal ion and the –OH group improved the
separation of photogenerated electrons and holes, which reasonably leads to a higher
photocatalytic activity than bare TiO2. Furthermore, a possible photocatalytic
mechanism has also been discussed.
Acknowledgments This work was financially supported by National Nature Science Foundation of
China (No. 20971103) and the International Cooperation Project of Shaanxi province (No. 2008KW–33).
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