Spectroscopic investigations on the simulated solar light induced photodegradation of 4-nitrophenol by using three novel copper(II) porphyrin-TiO2 photocatalysts

Article abstract of DOI:10.1016/j.saa.2013.02.021

Three porphyrins containing different functional groups (OH, CO ₂C₂H₅, COOH), 5-(4-hydroxy) phenyl-10,15,20-triphenyl porphyrin (1a), 5-(4-ethylacetatatomethoxy) phenyl-10,15,20-triphenyl porphyrin (1b), 5-(4-carboxylatome

Full text of DOI:10.1016/j.saa.2013.02.021

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 161–168
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectroscopic investigations on the simulated solar light induced
photodegradation of 4-nitrophenol by using three novel copper(II)
porphyrin–TiO2 photocatalysts
Xiang-fei Lü a,b,1, Wan-jun Sun a,1, Jun Li a, , Wei-xia Xu a,c, Feng-xing Zhang a
⇑
a Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry & Materials Science, Northwest University, Xi’an,
Shaanxi 710069, People’s Republic of China
b Department of Environmental Engineering, College of Resource and Environment, Shaanxi University of Science and Technology, Xi’an, Shaanxi 710021, People’s Republic of China
c College of Chemistry and Chemical Engineering, Xianyang Normal University, Xianyang 712000, People’s Republic of China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
"
"
The CuPp and CuPp(2a, 2b, 2c)–TiO2
were prepared and studied by XRD,
TEM, UV–vis-DRS, FI-IR, XPS and PL
spectroscopies.
Higher photocatalytic activity of the
CuPp(2a, 2b, 2c)–TiO2 than bare TiO2
was observed under simulated solar
irradiation.
The photocatalytic activity of
CuPp(2a, 2b, 2c)–TiO2 is related
closely to the interaction between
CuPp and TiO2.
a r t i c l e i n f o
a b s t r a c t
Article history:
Three porphyrins containing different functional groups (AOH, ACO2C2H5, ACOOH), 5-(4-hydroxy) phe-
nyl-10,15,20-triphenyl porphyrin (1a), 5-(4-ethylacetatatomethoxy) phenyl-10,15,20-triphenyl porphy-
rin (1b), 5-(4-carboxylatomethoxy) phenyl-10,15,20-triphenyl porphyrin (1c), were synthesized and
characterized spectroscopically. The CuPp(2a, 2b, 2c)–TiO2 photocatalysts were then prepared and char-
acterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spec-
troscopy (XPS), UV–vis diffuse reflectance spectroscopy (UV–vis-DRS), Fourier-transform infrared
spectroscopy (FT-IR). The photocatalytic activities of the photocatalysts were investigated by carrying
out the photodegradation of 4-nitrophenol in aqueous solution under simulated solar irradiation. It
was found that the CuPp(2a, 2b, 2c)–TiO2 enhanced the photocatalytic efficiency of bare TiO2 in photo-
degrading the 4-NP due to the interaction between CuPp(2a, 2b, 2c) and TiO2, resulted in the enhance-
ment of the photogenerated electron–hole separation. The reasons of this enhanced photocatalytic
activity were also discussed. Based on the present study, it could be considered as a promising photocat-
alyst for the further industrial application.
Received 6 December 2012
Received in revised form 5 February 2013
Accepted 6 February 2013
Available online 21 February 2013
Keywords:
Porphyrin
Copper(II) porphyrin–TiO2 composites
Spectrophotometric studies
Photodegradation
Photocatalysis
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
water than other technologies [1,2]. Among various semiconductor
photocatalysts, TiO2 is believed as the most promising environ-
Photocatalysis has attracted considerable attention as a ‘‘green’’
technology for the treatment and purification of contaminated
mentally friendly photocatalyst for the treatment of water pollu-
tants, due to its superior photoreactivity, nontoxicity,
photostability and comparatively inexpensive [3,4]. Furthermore,
it is well known that TiO2 can produce strong oxidative hydroxyl
radicals which has potential to degrade lots of organics into CO2,
H2O and other small molecules [5]. Unfortunately, the bare TiO2 re-
⇑
Corresponding author. Tel.: +86 029 8830 2604; fax: +86 029 8830 3798.
E-mail address: junli@nwu.edu.cn (J. Li).
1
These authors contributed equally to this work.
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.02.021

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X.-f. Lü et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 161–168
quires UV light (k < 387.5 nm) irradiation and shows fast rates of
electron–hole recombination for the photodegradation [6,7].
To overcome this problem, different approaches have been per-
formed for the purpose of higher photocatalytic activity of TiO2
[6,8–10]. Among them, the porphyrin-sensitized TiO2 system is
one of the typical approaches to extend the spectra response of
TiO2 to the visible region owing to their high absorption coefficient
within the solar spectrum (400–450 nm, Soret band) and its good
chemical stability in comparison with other dyes [11–13]. In previ-
ous work, it has been demonstrated that copper(II) porphyrin
shows an enhanced photocatalytic efficiency in sensitizing TiO2
for the photodegradation of 4-NP [14–17]. Although some studies
concerning the effect of the distinct space tropisms of peripheral
substituents of porphyrin ring have been carried out in photodeg-
radation of 4-NP, it remains unclear how the porphyrin with differ-
ent functional groups interact with TiO2 in the process of
photodegradation of 4-NP [18–21]. In addition, phenol compounds
are toxic pollutants that can be found mainly in wastes from
petroleum manufacture, coke ovens, paint stripping operations,
and so on [22]. So it is significant and almost urgency to explore
the influential factors of the efficiency of TiO2 impregnated with
different functional group porphyrins.
For this reason, we herein reported the synthesis and
characteristics of three porphyrins, 5-(4-hydroxy) phenyl-
10,15,20-triphenyl porphyrin (1a), 5-(4-ethylacetatatomethoxy)
phenyl-10,15,20-triphenyl porphyrin (1b), 5-(4-carboxylatometh-
oxy) phenyl-10,15,20-triphenyl porphyrin (1c), and their copper(II)
porphyrins (2a, 2b, 2c), as well as the further photocatalytic
activities of three CuPp(2a, 2b, 2c)–TiO2 photocatalysts for the
photodegradation of 4-NP in aqueous solution under simulated
solar irradiation.
face of the samples. 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 20–70°. Photoluminescence spectra (PL) were mea-
sured at room temperature on a Hitachi FL-4500 fluorescence
spectrometer using a Xe lamp with an excitation wavelength of
330 nm.
Synthesis of porphyrins and copper(II) porphyrins
The synthetic routes to the three porphyrins already reported
are shown in Scheme 1 and the detailed synthetic procedures were
as follows.
General procedure for the synthesis of H2Pp (1a, 1b, 1c)
According to the method reported previously [17,24,25], the
three porphyrins (1a, 1b, 1c) were successfully achieved. 5-(4-
hydroxy) phenyl-10,15,20-triphenyl porphyrin (1a) was synthe-
sized according to the traditional Alder method. 5-(4-ethylaceta-
tatomethoxy) phenyl-10,15,20-triphenyl porphyrin (1b) was
obtained by condensation reaction of pyrrole, benzaldehyde
and (4-formylphenoxy) acetic acid ethyl ester using the Lindsey
method. 5-(4-carboxylatomethoxy) phenyl-10,15,20-triphenyl
porphyrin (1c) was synthesized by carrying out the hydrolysis
of 1b.
H2Pp (1a). Yield 11%. Mp: >250 °C. Anal. Calcd. for C44H30N4O
(Mol. Wt: 630.74), %: C, 83.83 (83.79); H, 4.82 (4.79); N, 8.83
(8.88). MS: m/z 631.51 ([M + 1]+) amu. UV–vis (CHCl3): 421 nm
(Soret band), 515 nm, 550 nm, 591 nm, 648 nm (Q bands). FT-IR
(KBr, t
/cmꢀ1): 1610 [C@C]; 1217, 1073 [CAO]; 3429, 1349 [OH];
3053, 1500, 1472, 1000, 800, 701 [porphyrin ring].
H2Pp (1b). Yield 22%. Mp: >250 °C. Anal. Calcd. for C48H36N4O3
(Mol. Wt: 716.82), %: C, 80.52 (80.43); H, 5.11 (5.06); N, 7.79
(7.82). MS: m/z 717.17 ([M + 1]+) amu. UV–vis (CHCl3): 419 nm
(Soret band), 515 nm, 551 nm, 502 nm, 666 nm (Q bands). FT-IR
Experimental
Reagents and materials
(KBr,
t
/cmꢀ1): 1598 [C@C]; 1753 [C@O]; 1249 [CAO]; 3432,
2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) was ob-
tained from Aldrich, and other reagents were obtained from Beijing
Chemical Reagents Company. They were used without further
purification except pyrrole, which was distilled before use.
(4-Formylphenoxy) acetic acid ethyl ester was prepared according
to the established procedure [23]. The commercial TiO2 was pur-
chased from Acros Organics, USA. (anatase phase). The target pol-
lutant 4-nitrophenol (4-NP) was obtained from Xi’an Chemical,
China. Distilled water was used throughout the 4-NP removal
experiments.
3056, 2923, 2855, 1500, 1468, 966, 799, 701 [porphyrin ring].
H2Pp (1c). Yield 64%. Mp: >250 °C. Anal. Calcd. for C46H32N4O3
(Mol. Wt: 688.77), %: C, 80.15 (80.21); H, 4.73 (4.68); N, 8.09
(8.13). MS: m/z 689.66 ([M + 1]+) amu. UV–vis (CHCl3): 419 nm
(Soret band), 516 nm, 551 nm, 591 nm, 645 nm (Q bands). FT-IR
(KBr,
t
/cmꢀ1): 1603 [C@C]; 1706 [C@O]; 1258, 1221 [CAO];
3423, 3056, 2921, 2853, 1504, 1464, 1097, 970, 799, 702 [porphy-
rin ring].
General procedure for the synthesis of CuPp(2a, 2b, 2c)
27.0 mg (0.15 mmol) of Cu(CH3COO)2 was added to 0.05 mmol
of the H2Pp dissolved in 20 mL CHCl3 and 5 mL C2H5OH. The mix-
ture was stirred for 24 h at room temperature and monitored by
TCL until the complete disappearance of the starting material
H2Pp. The unreacted solid salt was filtered and the solvent re-
moved under vacuum. The crude product was purified by chroma-
tography on a silica gel column with CHCl3 as eluant.
CuPp(2a)
Equipment
Elemental analysis (C, H and N) was performed by Vario EL-III
CHNOS instrument. FT-IR spectra were registered in KBr using a
BEQ UZNDX-550. UV–vis spectra were recorded by a Shimadzu
UV–vis-NIR spectrophotometer (UV-3100). Mass spectrometry
analysis were carried out on a matrix assisted laser desorption/
ionization time of flight mass spectrometer (MALDI-TOF MS, Kra-
to Analytical Company of Shimadzu Biotech, Manchester, Britain)
using a standard procedure involving 1 mL of the sample solution.
High-resolution transmission electron microscopy (TEM) was
carried out using a JEOL JEM-3010 instrument. A study of the
composition and properties of the products was performed using
X-ray photoelectron spectroscopy (XPS). The XPS spectra were
obtained by Axis Ultra, Kratos (UK) using monochromatic Al KR
radiation (150 W, 15 kV, 1486.6 eV). The vacuum in the spectrom-
eter was 10ꢀ9 Torr. Binding energies were calibrated relative to
the C 1s peak (284.8 eV) from hydrocarbons adsorbed on the sur-
Yield 93%. Mp: >250 °C, Anal. Calcd. for C44H28CuN4O (Mol. Wt:
692.27), %: C, 76.39 (76.34); H, 4.03 (4.08); N, 8.12 (8.09). MS: m/z
692.69 ([M + H]+) amu. UV–vis (CHCl3): kmax, nm, 416, (Soret
band), 540, 575 (Q bands). FT-IR (KBr,
t
/cmꢀ1): 1599 [C@C];
1283 [CAO]; 3429, 1340 [OH]; 3050, 1500, 1441, 1000, 794 [por-
phyrin ring].
CuPp(2b)
Yield 96%. Mp: >250 °C, Anal. Calcd. for C48H34CuN4O3 (Mol. Wt:
777.19), %: C, 74.09 (74.07); H, 4.36 (4.40); N, 7.19 (7.20). MS: m/z
778.11 ([M + H]+) amu. UV–vis (CHCl3): kmax, nm, 418, (Soret
band), 542, 578 (Q bands). FT-IR (KBr,
t
/cmꢀ1): 1602 [C@C];

X.-f. Lü et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 161–168
163
Scheme 1. Synthesis routes of the porphyrins H2Pp(1a, 1b, 1c) and CuPp(2a, 2b, 2c).
1733 [C@O]; 1260, 1075 [CAOAC]; 3430, 3050, 2924, 2856, 1460,
Results and discussion
1003, 802 [porphyrin ring].
CuPp(2c)
Synthesis and characterization of porphyrins
Yield 92%. Mp: >250 °C, Anal. Calcd. for C46H30CuN4O3 (Mol. Wt:
749.16), %: C, 73.67 (73.64); H, 4.06 (4.03); N, 7.51 (7.47). MS: m/z
749.75 ([M + H]+) amu. UV–vis (CHCl3): kmax, nm, 416, (Soret
band), 540, 575 (Q bands). FT-IR (KBr,
1722 [C@O]; 1262, 1072 [CAOAC]; 3426, 3049, 2922, 2853,
The structures of the six porphyrins were presented in Fig. 1.
The synthetic route of the porphyrins and corresponding copper(II)
porphyrins have been illustrated in Scheme 1. The syntheses of the
metal-free porphyrins involved three different methods to get
desired porphyrins such as the condensation of benzaldehyde,
4-hydroxybenzaldehyde with pyrrole to afford the H2Pp(1a) by
Aldler–Longo method, and the synthesis of H2Pp(1b) using the
Lindsey method. Subsequently hydrolysis of the precursor
H2Pp(1b) with 10% KOH afforded the H2Pp(1c). The insertion of
copper ion into porphyrin ring occurs by reacting the metal-free
porphyrin with excess hydrated Cu(CH3COO)2 in the mixed sol-
vents of chloroform and ethanol.
The UV–vis spectra of H2Pp(1a, 1b, 1c) show almost the same
Soret and Q bands. The insertion of Cu(II) into the porphyrin ring
caused a violet shift of 1–5 nm of the corresponding Soret bands
for UV–vis spectra as well as a decreasing number of Q bands,
which due to the symmetry of the porphyrin ring increases when
the H ions of NꢀH are replaced by Cu(II). Mass spectroscopy data
of 1a–1c and 2a–2c perfectly correspond to the expected
[M + H]+ m/z values. The main change in the FI-IR spectra of cop-
per(II) porphyrins compared with metal-free porphyrins is also
the disappearance of stretching vibration of NAH.
t
/cmꢀ1): 1599 [C@C];
1439, 1000, 798 [porphyrin ring].
Preparation of the CuPp(2a, 2b, 2c)–TiO2 photocatalyts
An amount (6 lmol) of the corresponding copper(II) porphyrin
was dissolved in 30 mL CHCl3 (2c was dissolved in ethanol) and 1 g
TiO2 was added to the solution. The suspension was stirred for 10 h
at the room temperature. Then the solvent was removed under
vacuum and the catalyst was collected. Copper(II) porphyrins were
used to impregnate TiO2, marked as CuPp(2a)–TiO2, CuPp(2b)–
TiO2, CuPp(2c)–TiO2, respectively.
Photodegradation experiments
Photocatalytic experiments were conducted using 4-NP
(1 ꢁ 10ꢀ4 mol/L) as model pollutant and 10 mg as-prepared photo-
catalysts were added and then stirred with a magnetic bar. Details
of the experimental setup have been reported in our previous work
[26]. A 350 W xenon lamp with wavelength emissions (main range
at 380–780 nm) closely simulating solar irradiation was served as
the light source. At a defined time interval, sample of 3.5 mL were
withdrawn from the suspension every sampling time during the
irradiation. The photocatalysts were separated from the solution
by centrifugation and the quantitative determination of 4-NP was
performed by measuring its absorption at 317 nm with a UV–vis
spectrophotometer (UV-1800, Shimadzu).
Characterization of bare TiO2 and CuPp(2a, 2b, 2c)–TiO2
photocatalysts
XRD analysis
Fig. 2 shows the XRD pattern of the bare TiO2 and CuPp(2a, 2b,
2c)–TiO2 photocatalysts. When CuPp is impregnanted onto the sur-
Fig. 1. The structure of porphyrins (1a, 1b, 1c) and their CuPp(2a, 2b, 2c).

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X.-f. Lü et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 161–168
found on the surface of bare TiO2 (Fig. 3A), indicating that CuPp
is well distributed on the surface of TiO2.
UV–vis-DRS analysis
The UV–vis-DRS of bare TiO2 and CuPp(2a, 2b, 2c)–TiO2 are
shown in Fig. 4 and the related maxima reflection for the Soret
and Q bands of the CuPp(2a, 2b, 2c)–TiO2 photocatalysts are also
provided in Table 1. Taking CuPp(2c)–TiO2 as an example, it was
obvious that there was no absorption above 400 nm for bare
TiO2, while CuPp(2c)–TiO2 photocatalysts exhibited the feature
peaks of CuPp(2c) observed in CHCl3 solution, indicating that
CuPp(2c) successfully impregnated onto the surface of TiO2 with
maintaining the porphyrin framework. Furthermore, it is noticed
that the corresponding peaks of CuPp(2c)–TiO2 are all red-shifted
and broadened relatively to CuPp(2c) observed in Table 1. As men-
tioned above, the other two CuPp(2a, 2b) impregnated onto the
surface of TiO2 were also observed with the similar results, which
implied that there existed an interaction between TiO2 and CuPp
molecules.
CuPp(2a)-TiO2
CuPp(2b)-TiO2
CuPp(2c)-TiO2
bare TiO2
20
30
40
50
60
70
2 Theta/degree
Fig. 2. XRD patterns of the bare TiO2 and CuPp(2a, 2b, 2c)–TiO2 photocatalysts.
face of TiO2, no detectable other peaks are observed for the CuPp–
TiO2 photocatalysts, indicating that the content of CuPp are small
and/or well dispersed on the surface of TiO2 [27], which suggests
that the CuPp impregnated onto the surface of TiO2 does not
change the bulk intrinsic property of the TiO2 photocatalyst.
FT-IR analysis
To confirm the interaction between the functionalized groups
(AOH, ACO2C2H5, ACOOH) of CuPp and TiO2, FT-IR spectra of the
photocatalysts were comparatively studied. For CuPp(2a), the
characteristic band of
t(AOH) is observed at 3429 and
TEM analysis
1340 cmꢀ1, as well as CAO stretching modes are observed at
1283 cmꢀ1. While for CuPp(2b) and CuPp(2c), the characteristic
Fig. 3 shows the TEM images of bare TiO2 and CuPp(2a, 2b, 2c)–
TiO2 composites. As expected, after modification with CuPp, the
size of TiO2 has no obvious change compared to bare TiO2, but
some nanometer CuPp particles dispersed on the surface of TiO2
can be found (Fig. 3B–D), while no any nanometer particles are
band of t
(C@O) is observed at 1733 and 1722 cmꢀ1 for the ester
and carboxylic acid group, respectively, and CAO stretching modes
are observed at 1260 and 1262 cmꢀ1 [28]. As shown in Fig. 5, when
CuPp is adsorbed onto the surface of TiO2, the IR spectra of CuP-
(A)
(B)
(C)
(D)
Fig. 3. TEM images of bare TiO2 (A) and CuPp(2a, 2b, 2c)–TiO2 (B, C and D).

X.-f. Lü et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 161–168
165
(A)
CuPp(2a)-TiO2
-OH
2855
C-H
1167
2923
bare TiO
CuPp(2b2)-TiO
CuPp(2c)-TiO2
CuPp(2a)-TiO22
bare TiO2
before irradiation
1630
after irradiation
300
400
500
600
700
Wavelength/nm
2800
2400
2000
1600
1200
Fig. 4. The UV–vis-DRS spectra of bare TiO2 and CuPp(2a, 2b, 2c)–TiO2
Wavelength/nm
photocatalysts.
(B)
CuPp(2b)-TiO2
Table 1
UV–vis data of the CuPp(2a, 2b, 2c) in CHCl3 and UV–vis-DRS data of CuPp(2a, 2b,
2c)–TiO2 photocatalysts.
Compounds
k (nm)
O-C-O
-
Soret band
Q bands
1165
CuPp(2a)
CuPp(2a)–TiO2
CuPp(2b)
CuPp(2b)–TiO2
CuPp(2c)
CuPp(2c)–TiO2
416
420
418
420
416
422
540, 575
540
542, 578
540
540, 575
541
-COOs
2853
C-H
2923
bare TiO
2
p(2a, 2b, 2c)–TiO2 showed the characteristic peaks of the corre-
sponding copper(II) porphyrins and some peaks shifted to some
extent. These information indicate that there exists some interac-
tion between the functionalized groups (AOH, ACO2C2H5, ACOOH)
of CuPp molecules and the surface TiO2.
before irradiation
1631
after irradiation
2800
2400
2000
1600
1200
Wavelength/nm
In addition, in order to confirm the photostability of TiO2
impregnated by CuPp molecules through their functionalized
groups (AOH, ACO2C2H5, ACOOH), we collected the CuPp(2a, 2b,
2c)–TiO2 photocatalysts and then characterized by FT-IR spectra
at the end of the photocatalytic process (Fig. 5). This observation
showed that the structure of the copper(II) porphyrin molecules
containing the ester or carboxyl groups adsorbed on TiO2 remained
almost unchanged before and after photocatalytic reaction, which
indicate that the mode of CuPp(2a, 2b, 2c) as well as their function-
alized groups to absorb the surface of TiO2 is maintained.
(C)
CuPp(2c)-TiO2
1273
O-C-O
1052
966
2856
C-H
1399 -
-COOs
2924
XPS analysis
The further chemical state of the bare TiO2 and CuPp(2a, 2b,
2c)–TiO2 composites in the O(1s), Ti(2p), and Cu(2p) bands was
investigated by XPS analysis, and the corresponding results are
shown in Fig. 6. From the XPS analysis of O(1s) and Ti(2p)
(Fig. 6A), it was found that there was slight shift after the impreg-
nation of CuPp(2a, 2b, 2c), which was probably due to the weak
interactions between CuPp(2a, 2b, 2c) molecules and the TiO2.
Moreover, the peaks located at ꢂ934 eV and ꢂ954 eV can be attrib-
uted to the Cu(II) (2p3/2) and Cu(II) (2p1/2) in CuPp(2a, 2b, 2c)–
TiO2 composite according to the reported binding energy [29],
indicating CuPp(2a, 2b, 2c) well impregnanted onto the surface
of TiO2 (Fig. 6A–C).
1630
bare TiO
2
before irradiation
after irradiation
2800
2400
2000
1600
1200
Wavelength/nm
Fig. 5. FT-IR spectra of bare TiO2 and CuPp(2a, 2b, 2c)–TiO2 photocatalysts.
behavior of 4-NP at their peak absorptions under simulated solar
irradiation. As shown in Fig. 7A, without any photocatalyst, there
is only 8% photolysis of 4-NP. The bare TiO2 microspheres exhibit
about 35% photodegradation of 4-NP within 300 min under the
same conditions. Interestingly, 4-NP could be photodegraded much
rapidly to almost zero within 300 min under xenon lamp
irradiation by the three new photocatalysts. It could be noted that
Photocatlytic activity
The photocatalytic performance of the three novel CuPp(2a, 2b,
2c)–TiO2 photocatalysts were evaluated by monitoring the optical

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X.-f. Lü et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 161–168
(B)
(A)
O 1s: 531.01
O 1s: 533.36
O 1s: 533.10
O 1s: 533.22
CuPp(2a)-TiO2
Cu 2p : (934.74)
3/2
Ti 2p: 460.06, 465.80
Ti 2p: 459.08, 465.31
Ti 2p: 459.32, 465.31
Ti 2p: 458.84, 465.18
Cu 2p : (954.60)
1/2
bare TiO
CuPp(2a)-TiO2
CuPp(2b)-TiO2
2
CuPp(2c)-TiO2
930
940
950
960
570
540
510
480
450
420
390
Banding Energy/eV
Banding Energy/eV
(C)
Cu 2p : (934.99)
3/2
(D)
Cu 2p : (934.51)
3/2
CuPp(2b)-TiO2
CuPp(2c)-TiO2
Cu 2p : (954.69)
1/2
Cu 2p : (954.59)
1/2
930
940
950
Banding Energy/eV
960
930
940
950
960
Banding Energy/eV
Fig. 6. XPS spectra of bare TiO2 and CuPp(2a, 2b, 2c)–TiO2 photocatalysts.
CuPp(2a, 2b, 2c)–TiO2 showed much enhanced photoactivity than
bare TiO2, and in particular CuPp(2c)–TiO2 exhibited the highest
efficiency of the three samples. The photocatalytic activity order
is as following:
TiO2 and from the CB of TiO2 to the CuPp molecule [14], and then
the oxygen molecules on the TiO2 surface obtain the electrons and
can produce reactive species OÅ2ꢀ; ÅOH as well as h+ for the photo-
degradation of 4-NP [17,31]. The photocatalytic mechanism of
the CuPp(2a, 2b, 2c)–TiO2 has been concluded in Fig. 8.
Furthermore, the larger polarity of the substituent group for the
copper(II) porphyrin is, the stronger interaction between CuPp and
TiO2 will be, and thus accelerates the transfer process of the pho-
toinduced electrons, which improve the separation of photogener-
ated electrons and holes. CuPp(2c) has the largest polarity of the
three copper(II) porphyrins, then CuPp(2a) and CuPp(2b) has the
least polarity. As a result, the CuPp(2c)–TiO2 shows the highest
photocatalytic efficiency. It can be included that the polarity of
the substituent group on the metalloporphyrins is an important
factor for the photodegradation of organic pollutants.
CuPpð2cÞ—TiO2 > CuPpð2aÞ—TiO2 > CuPpð2bÞ—TiO2 > bare TiO2
The role of Cu ion in enhancing the photocatalytic activity may
be a very quickly electron transfer between copper(II) porphyrin
and TiO2, as well as copper(II) porphyrin containing a central metal
ion with unfilled d orbitals. In addition, it has been demonstrated
that methyl orange is easier to be adsorbed on CuPp–TiO2 photo-
catalyst than bare TiO2 due to the presence of copper porphyrin
molecules and more positive charge on Cu2+ ion which calculated
by DFT [30].
The reaction kinetics of 4-NP photodegradation is also studied.
The relationship between –ln C0/C and irradiation time (t) was
shown in Fig. 7B. It can be seen that ꢀln C0/C and irradiation time
(t) was approximate straight line, indicating that the kinetics of 4-
NP photodegradation on the surface of the photocatalysts can be
described by the first-order kinetics under simulated solar light
irradiation. The rate constants (k) of CuPp(2b)–TiO2, CuPp(2a)–
TiO2 and CuPp(2c)–TiO2 are 0.01156, 0.01208 and 0.01403 minꢀ1
respectively, being 8–10 times of that over bare TiO2
(0.00135 minꢀ1). Through the comparison it is experimentally
proved that the CuPp(2a, 2b, 2c)–TiO2 photocatalysts have promis-
ing perspective in using solar energy.
To further illustrate the separation efficiency of electron–hole
pairs in the photocatalytic system, the PL emission spectra of the
bare TiO2 and three CuPp(2a, 2b, 2c)–TiO2 photocatalysts were
compared. As shown in Fig. 9, 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. This implies that the
presence of CuPp molecule reduces the possibility of the electron–
hole recombination, especially for the TiO2 modified with
CuPp(2c), which reasonably leads to a higher photocatalytic activ-
ity. Furthermore, it has been demonstrated that, in CuPp samples,
the Cu(II)/Cu(I)/Cu(II) redox loop plays a major role in the promo-
tion of the photodegradation process [32]. These results is in good
agreement with the photocatalytic activity results. In addition, the
CuPp impregnated onto the surface of TiO2 caused a slight blue-
Under simulated solar light irradiation and in presence of O2,
the possible charge-transfer processes involved a possible injection
of an electron from the excited CuPp (as sensitizers) to the CB of

X.-f. Lü et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 161–168
167
1.0
0.8
0.6
0.4
0.2
0.0
(A)
bare TiO
CuPp(2b2)-TiO
CuPp(2a)-TiO2
CuPp(2c)-TiO22
blank
bare TiO
CuPp(2b2)-TiO
CuPp(2a)-TiO2
CuPp(2c)-TiO22
350
400
450
500
550
600
0
50
100
150
200
250
300
Irradiation time/min
Wavelength/nm
Fig. 9. The room temperature photoluminescence spectra with excitation at
330 nm for bare TiO2 and CuPp(2a, 2b, 2c)–TiO2 photocatalysts.
(B)
0
Conclusion
-1
-2
-3
-4
In this paper, we have described the synthesis of three novel
metal-free and copper(II) porphyrins with three different func-
tional groups. Then these CuPp(2a, 2b, 2c) modified TiO2 compos-
ites have also been prepared and characterized. The XRD, TEM, UV–
vis-DRS, FT-IR and XPS spectra demonstrated that these CuPp(2a,
2b, 2c) were successfully impregnated onto the surface of TiO2
and did not influence the phase of TiO2. These CuPp(2a, 2b, 2c)–
TiO2 photocatalysts exhibited higher photoactivity than bare TiO2
when they were employed for the photodegradation of 4-NP under
simulated solar light irradiation. Furthermore, CuPp(2c)–TiO2
exhibited excellent photocatalytic activity for the degradation of
4-NP. This high photocatalytic efficiency can be attributed to the
lower recombination of electrons and holes observed by PL spectra.
blank(4-NP)
bare TiO2;
k = -0.00135 min-1
CuPp(2b)-TiO2; k = -0.01156 min-1
CuPp(2a)-TiO2; k = -0.01208 min-1
CuPp(2c)-TiO2; k = -0.01403 min-1
0
50
100
150
200
250
300
Irradiation time (min)
Fig. 7. (A) Performance of bare TiO2 and CuPp(2a, 2b, 2c)–TiO2 photocatalysts for
the 4-NP photodegradation and (B) their kinetic plots with first-order linearity of ln
C0/C = f(t) under simulated solar irradiation.
Acknowledgments
We are very Grateful for the financial support from National
Nature Science funds (Nos. 21271148 and 20971103).
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