Zinc phthalocyanine coupled with UIO-66 (NH2) via a facile condensation process for enhanced visible-light-driven photocatalysis

Article abstract of DOI:10.1016/j.jallcom.2016.08.087

Metal-organic frameworks (MOFs) have shown a great potential for water treatment as an environmentally friendly photocatalyst. In this work, a zirconium based MOF, UIO-66 (NH₂) covalently coupled with zinc phthalocyanine (ZnTCPc) was prepared via a facile condensation process. Compared with the mixture of ZnTCPc and UIO-66 (Zr) by impregnation, ZnTCPc/UIO-66 (NH₂) presented an enhanced photocatalytic activity for the degradation of methylene blue (MB) under visible-light irradiation. The formation of strong covalent bonds and synergistic integration between ZnTCPc and UIO-66 (NH₂) was proved by characterization results such as transmission electron microscopy, X-ray photoelectron spectra and Fourier transform infrared spectroscopy. Additionally, ZnTCPc/UIO-66 (NH₂) photocatalyst retained excellent stability after five recycles, and the photocatalytic mechanism was investigated systematically. This work demonstrates a high potential of using porous MOFs-based materials not only as supports but as electron acceptors to trigger the reaction for coupling MOFs with dye, fabricating novel MOFs-dye nanocomposite systems and enhancing their photostability and photocatalytic activity.

Full text of DOI:10.1016/j.jallcom.2016.08.087

Accepted Manuscript
Zinc phthalocyanine coupled with UIO-66 (NH ) via a facile condensation process for
2
enhanced visible-light-driven photocatalysis
Qian Liang, Miao Zhang, Zhihui Zhang, Changhai Liu, Song Xu, Zhongyu Li
PII:
S0925-8388(16)32465-3
10.1016/j.jallcom.2016.08.087
JALCOM 38589
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 10 June 2016
Revised Date: 23 July 2016
Accepted Date: 11 August 2016
Please cite this article as: Q. Liang, M. Zhang, Z. Zhang, C. Liu, S. Xu, Z. Li, Zinc phthalocyanine
coupled with UIO-66 (NH ) via a facile condensation process for enhanced visible-light-driven
2
photocatalysis, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.08.087.
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ACCEPTED MANUSCRIPT
Graphical abstract
ZnTCPc was immobilized on UIO-66 (NH2) covalently and the photocatalytic degradation of
methylene blue (MB) was investigated under visible irradiation.

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Zinc phthalocyanine coupled with UIO-66 (NH2) via a facile
condensation process for enhanced visible-light-driven
photocatalysis
Qian Lianga, *, Miao Zhanga, Zhihui Zhanga, Changhai Liub, Song Xua, Zhongyu Lia, *
aJiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of
Petrochemical Engineering, Changzhou University, Changzhou 213164, PR China.
bJiangsu Collaborative Innovation Center of Photovolatic Science and Engineering,
School of Materials Science & Engineering, Changzhou University, Changzhou
213164, PR China.
* Corresponding authors: Zhongyu Li, E-mail: zhongyuli@mail.tsinghua.edu.cn;
Qian Liang, E-mail: qianliang@cczu.edu.cn, Tel.: +86 519 86334771; Fax: +86 519
86334771.

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Abstract:
Metal-organic frameworks (MOFs) have shown a great potential for water
treatment as an environmentally friendly photocatalyst. In this work, a zirconium
based MOF, UIO-66 (NH2) covalently coupled with zinc phthalocyanine (ZnTCPc)
was prepared via a facile condensation process. Compared with the mixture of
ZnTCPc and UIO-66 (Zr) by impregnation, ZnTCPc/UIO-66 (NH2) presented an
enhanced photocatalytic activity for the degradation of methylene blue (MB) under
visible-light irradiation. The formation of strong covalent bonds and synergistic
integration between ZnTCPc and UIO-66 (NH2) was proved by characterization
results such as transmission electron microscopy, X-ray photoelectron spectra and
Fourier transform infrared spectroscopy. Additionally, ZnTCPc/UIO-66 (NH2)
photocatalyst retained excellent stability after five recycles, and the photocatalytic
mechanism was investigated systematically. This work demonstrates a high potential
of using porous MOFs-based materials not only as supports but as electron acceptors
to trigger the reaction for coupling MOFs with dye, fabricating novel MOFs-dye
nanocomposite systems and enhancing their photostability and photocatalytic activity.
Keywords: Metal-organic framework; Zinc phthalocyanine; Photocatalytic
degradation
1. Introduction
With growing concerns regarding a clean environment and human health,
technologies with high efficiency and low cost to reduce the pollutant contents of
wastewater are urgently needed [1]. Various homogeneous and heterogeneous

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photocatalysts systems have been widely studied and employed in the past few
decades, including TiO2, g-C3N4, graphene, metal-organic frameworks (MOFs), etc
[2-6]. Among the reported photocatalytic materials, MOFs as a new class of porous
crystalline networks synthesized by assembling metallic ions with organic ligands
attracts more attentions in photocatalysis field [7-12]. Among these MOFs, zirconium
based MOFs (UIO-66 (Zr) and UIO-66 analogies), associated with high surface area
and superior chemical stability to water, not only possess the cage-like structure and
high surface area to accommodate the nanoparticles and disperse the entrapped
nanoparticles, but serve as the semiconductor material itself [13]. Sha et al. reported
Ag2CO3/UIO-66 (Zr) could decompose rhodamine B (RhB) under visible-light
irradiation [14]. Although UIO-66 series are potential candidates for the
photocatalyst, compared with the inorganic photocatalysts and some commercial
standards, pristine UIO-66 series failed to achieve satisfactory photocatalytic
activities because of the limited light absorption [15, 16]. Therefore, it is necessary to
develop a high-efficient visible-light driven photocatalyst to extend its spectral
response to higher absorption wavelength.
A few attempts have been made to enhance the visible light response of UIO-66
(Zr) and UIO-66 analogies, such as combination with other components, like noble
metals, semiconductors and carbon materials [17-19]. Zinc phthalocyanine (ZnPc)
and its derivatives as typical organic semiconductors possess wide visible light
response, which can be employed to sensitize other semiconductor materials for
enhancement of photocatalytic activity [20-23]. However, the traditional ZnPc suffers

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from the aggregation ascribed to its intrinsic large -conjugation [24], and therefore,
MOFs with the high surface area and desirable porosity can confine the aggregation
of ZnPc and maximize the active sites. Based on the properties of UIO-66 and ZnPc,
it is worthy to present an efficient approach to combine UIO-66 with ZnPc for the
visible-light-driven photocatalysis. To our knowledge, the photocatalytic properties of
UIO-66 incorporated with dyes have remained unexplored.
Herein we prepared composites combining zinc tetracarboxy phthalocyanine
(ZnTCPc) with UIO-66 (Zr) and UIO-66 (NH2), respectively. ZnTCPc/UIO-66 (Zr)
were obtained by a simple impregnation method, while ZnTCPc was immobilized on
UIO-66 (NH2) covalently via N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide
hydrochloride (EDC)/N-hydroxysuccinimide (NHS) activation and amide formation.
The photocatalytic performances were evaluated by the degradation of methylene
blue (MB) under visible-light irradiation, and the results show that ZnTCPc/UIO-66
(Zr) and ZnTCPc/UIO-66 (NH2) with different interactions have distinct effects on the
photocatalytic activity. Compared with ZnTCPc/UIO-66 (Zr), ZnTCPc/UIO-66 (NH2)
with stronger covalent bonds and synergistic integration exhibited higher
photocatalytic activity. Moreover, the ZnTCPc/UIO-66 (NH2) composite also presents
acceptable stability during the degradation experiments. In addition, some typical
scavengers were added to identify the active species, and the possible photocatalytic
mechanism was investigated systematically.
2. Experimental

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2.1 Synthesis of UIO-66 (Zr) and UIO-66 (NH2)
In a typical synthesis, ZrCl4 (0.2332 g, 1.0 mmol) and terephthalic acid (0.1661 g,
1.0 mmol) or 2-NH2-terephthalic acid (0.1812 g, 1.0 mmol) were dissolved in DMF
(50 mL), and then the solution was transferred to a 100 mL Teflon-lined stainless
steel autoclave [25, 26]. The autoclave was sealed and heated in an oven at 120 oC for
40 h under autogenous pressure. After being cooled naturally, the obtained sample
was purified with anhydrous ethanol several times to make sure that the DMF
molecules were eliminated, and then the off-white UIO-66 (Zr) or yellow UIO-66
(NH2) product was dried at 80 oC for 24 h.
2.2. Synthesis of ZnTCPc
ZnSO4·7H2O (0.048 mol), trimellitic anhydride (0.176 mol), ammonium chloride
(0.085 mol) and urea (1 mol) were mixed with ammonium molybdate tetrahydrate
(0.004 mol). The mixture was dissolved in 10 mL nitrobenzene and then refluxed at
185 oC during 4 h. The dark blue solid obtained was purified with 0.1 M NaOH and 1
M HCl solution, respectively.
2.3. Synthesis of ZnTCPc/UIO-66 (Zr)
ZnTCPc sensitized UIO-66 (Zr) was prepared by an impregnation method.
Typically, UIO-66 (Zr) (100 mg) was first dispersed in deionized water by ultrasonic
treatment for 30 min to give an off-white solution. ZnTCPc (10 mg) dissolved in
DMF (5 mL) was added into UIO-66 (Zr) solution drop by drop, and the mixture was
stirred for 6 h. The obtained sample was purified with deionized water and ethanol

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several times, and dried at 80 °C for 12 h. The nominal weight ratio of ZnTCPc to
UIO-66 (Zr) was 10 wt %, and the actual content of zinc was shown in Table S1.
2.4. Synthesis of ZnTCPc/UIO-66 (NH2)
Fig. 1 shows the preparation process of ZnTCPc/UIO-66 (NH2). UIO-66 (NH2)
(100 mg) was dispersed in deionized water, and was subjected to sonication (100 W,
40 kHz) at 25 °C for 3 h. ZnTCPc (10 mg) was dissolved in DMF (5 mL) by
ultrasonic treatment for 3 h to give a green solution. EDC (0.1 M) and NHS (0.1 M)
were dissolved in deionized water, and then added into ZnTCPc solution [27, 28]. The
ZnTCPc solution was added dropwise to the ultrasonic UIO-66 (NH2) dispersion.
After reacting for 6 h, the mixture was filtered and washed with deionized water and
ethanol several times. The obtained ZnTCPc/UIO-66 (NH2) was dried at 80 °C for 12
h. The nominal weight ratio of ZnTCPc to UIO-66 (NH2) was 10 wt %, and the
content of zinc was shown in Table S1.
2.5 Characterization
Powder X-ray diffraction (PXRD) patterns were recorded with a Shimadzu XRD
6000 X-ray diffractometer using Cu Kα (λ = 0.15406 nm) radiation with a nickel filter
operating at 40 kV and 10 mA. The patterns were compared with JCPDS reference
data for phase identification. The surface morphology of the catalyst was observed by
field emission scanning electron microscopy (FESEM) on a Quanta 200F instrument
using accelerating voltages of 5 kV. Samples for SEM were dusted on an adhesive
conductive carbon belt attached to a copper disk, coated with 10 nm Au prior to

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measurement. The TEM images were carried out using a JEOL JEM 2100 electron
microscope equipped with a field emission source at an accelerating voltage of 200
kV. Fourier transform infrared spectroscopy (FT-IR) spectra were obtained on an
FTS-3000 spectrophotometer manufactured by American Digilab Company. The
measured wafer was prepared as KBr pellet with the weight ratio of sample to KBr,
1/100. Raman spectra were measured at room temperature using a LabRAM XploRA
Raman spectrometer (Horiba Jobin Yvon France) with a 532 nm laser focused on a
spot about
3
nm diameter. Specific surface area was measured by
adsorption-desorption of N2 gas at 77 K with a Micromeritics ASAP 2000 gas
sorption analyzer. Before the measurements, the samples were outgassed at 160 ºC for
10 h. X-ray photoelectron spectra (XPS) were recorded on a Perkin-Elmer PHI-1600
ESCA spectrometer using Mg Kα (hν = 1253.6 eV) X-ray source. TGA data were
obtained using a Perker Elmer TGA6 instrument under N2 flow. Electron
paramagnetic resonance (EPR) measurements were carried out on a Bruker model
A300 spectrometer. UV-Vis diffused reflectance spectra (DRS) experiments were
performed on a UV-Vis spectrophotometer (Hitachi U-4100) with the integration
1
sphere diffuse reflectance attachment. H NMR spectrum was recorded on a Bruker
AV-300 spectrometer.
2.6 Evaluation of the photocatalytic activity
The photocatalytic activity was evaluated by the degradation of MB under visible
light irradiation. A 500 W Xe arc lamp with an UV cut-off filter (λ ≥ 420 nm) was
used as a visible light source. A suspension containing a powdered catalyst (20 mg)

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and a fresh aqueous solution of MB (100 mL, 15 mg/L) were magnetically stirred in
the dark for about 20 min to establish an adsorption-desorption equilibrium of MB
species. To measure the dye degradation, a 1.5 mL sample of the reaction suspension
was collected after irradiation for a certain period, and the photocatalyst was removed
by centrifugation (5000 rpm for 20 min) before the absorption spectroscopy
measurements. The filtrate was analyzed by a 759 UV-vis spectrophotometer
(Shanghai Precision & Scientific Instrument Co., Ltd.). To evaluate the photostability
of the photocatalysts, samples were separated from the suspension after photocatalytic
degradation reaction for 120 min, and then collected by centrifugation and dried at 50
oC overnight. The recovered photocatalysts were reused for subsequent cycles of
photocatalytic degradation under the same conditions. The photocatalytic mechanism
was investigated by different scavengers to compete with the potential active species
that may be involved in the degradation process of MB. Benzoquinone (BQ),
ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) and isopropyl alcohol
(IPA) were introduced as scavengers for superoxide radical anions (•O2 ), holes (h+)
−
and hydroxyl radicals (•OH), respectively.
3. Results and discussion
3.1. Material characterization
Fig. S1 shows the XRD patterns of the pure ZnTCPc, UIO-66 (Zr), UIO-66 (NH2)
and their composites. For UIO-66 (Zr) and UIO-66 (NH2) samples, all the diffraction
peaks can be well matched with the previous reports [29, 30], exhibiting the excellent
crystallinity. After the deposition of ZnTCPc, the characteristic diffraction of ZnTCPc

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at 26.3o are not detected from XRD pattern for ZnTCPc/UIO-66 (Zr) or
ZnTCPc/UIO-66 (NH2). And there is no obvious loss of crystallinity in the XRD
patterns of the above composites, indicating that the integrity of the UIO-66 (Zr) and
UIO-66 (NH2) frameworks is both maintained well during the different preparation
processes. These above results are possible due to the following reasons: (i) the
content of ZnTCPc is relatively low, and the ZnTCPc nanoparticles have small
particle size and well dispersion in the ZnTCPc/UIO-66 (Zr); (ii) as for the
ZnTCPc/UIO-66 (NH2), the covalent structure has no obvious effect on the crystal
structure of UIO-66 (NH2) [31].
Fourier transform infrared (FT-IR) and Raman spectroscopies were applied to
further characterize the hybrids, as shown in the Fig. 2 and Fig. S2. In the FT-IR
spectra of UIO-66 (Zr) and UIO-66 (NH2), the absorption peaks between 600-800
cm-1 correspond to Zr-O2 as longitudinal and transverse modes, while intense doublet
at 1433 and 1389 cm-1 (labelled with blue dot) of UIO-66 (NH2) can be assigned to
the stretching modes of the carboxylic groups in the BDC-NH2 ligands [32]. In the
spectrum of ZnTCPc, O-H stretching at 3136 cm-1, C=O stretching at 1591 cm-1, and
O-H bending at 1339 cm-1 prove the existence of -COOH. As for the ZnTCPc/UIO-66
(Zr) composite prepared by the simple impregnation, all of the IR vibration bands are
fully consistent with those of the pristine UIO-66 (Zr), suggesting the composites
have the same chemical structure as the pristine UIO-66 (Zr). Most importantly, after
the ZnTCPc was reacted with UIO-66 (NH2) covalently, the absorption peaks were
attributed to amino groups in the BDC-NH2 ligands between 1030 and 1130 cm-1

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disappear. Besides, the typical signal of CO-NH2 at 1620 cm-1 and the characteristic
absorption of the N-H stretching band at 3344 cm-1 appears in the spectra of the
ZnTCPc/UIO-66 (NH2), further confirming that the -COOH reacted with -NH2 with
EDA successfully [33, 34]. Fig. S2 shows the Raman spectra at room temperature of
the as-prepared samples. The vibrational peaks of UiO-66 (Zr) are present between
650 and 1700 cm-1, attributed to BDC organic linkers. The vibrations at 650-900 cm-1,
1140 cm-1, 1430-1625 cm-1 are assigned to C-H out of plane bending, C-H in plane
bending and C-C stretching, respectively [35]. While the Raman spectrum of UIO-66
(NH2) is not available because this material is fluorescent and the detection of its
typical spectroscopic features is not possible [36, 37]. As for ZnTCPc/UIO-66 (Zr),
the vibrational peaks of UIO-66 (Zr) disappear after the integration of ZnTCPc,
indicating that the appearance of ZnTCPc may cause some influence on the optical
property of UIO-66 (Zr).
Surface areas and pore structure of the as-prepared samples were investigated using
N2 adsorption-desorption measurements (Fig. 3). The nitrogen adsorption isotherms
of all the as-prepared samples indicate a steep nitrogen gas uptake at low relative
pressure, and belong to the typical type I, revealing microporous structure. The BET
surface areas of pure UIO-66 (Zr) and UIO-66 (NH2) are 958 and 842 m2 g-1,
respectively. The Horvath-Kawazoe (HK) micropore size distribution of UIO-66 (NH2)
shows two major pores at 0.9 and 0.7 nm. After introduction of ZnTCPc, the BET
surface areas of the composite materials (ZnTCPc/UIO-66 (Zr) and ZnTCPc/UIO-66
(NH2)) decrease to 630 and 575 m2 g-1, respectively, due to pore blockage by the

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ZnTCPc nanoparticles and ZnTCPc complex, respectively. It should be noted that
although the BET surface areas of the composites are smaller than that of pure
UIO-66 (Zr) or UIO-66 (NH2), they are still much higher than most of other ZnPc and
ZnPc-based materials. A high specific surface area of photocatalysts can supply more
surface active sites and make charge carrier transport easier, leading to an
enhancement of the photocatalytic performance [38].
The TEM and SEM images of ZnTCPc, UIO-66 (Zr), UIO-66 (NH2),
ZnTCPc/UIO-66 (Zr) and ZnTCPc/UIO-66 (NH2) samples are shown in Fig. 4 and
Fig. S3. It can be seen from the TEM image of ZnTCPc/UIO-66 (Zr) composite that
ZnTCPc nanoparticles are dispersed on the external surface of UIO-66 (Zr) compared
with the bare ZnTCPc since the UIO-66 (Zr) with high surface area can prevent the
aggregation of ZnTCPc during the preparation process. Interestingly, there are no
obvious nanoparticles in the ZnTCPc/UIO-66 (NH2) composite, different from the
composite simply mixed ZnTCPc and UIO-66 (Zr), indicating that not only the
introduction of ZnTCPc has no effect on the morphology of UIO-66(NH2) but also the
interaction between ZnTCPc and UIO-66 (NH2) could be formed by chemical bond.
The thermal stability of the samples was evaluated by thermal gravimetric analysis
(TGA, Fig. 5). UIO-66 (Zr) shows a gradual mass loss up to ~200 °C, corresponding
to the removal of adsorbed guest molecules from the porous surface. A plateau is
shown in the temperature range of 300-450 °C, indicating high thermal stability of the
sample, which is followed by the complete decomposition of the framework at around
500 °C. Compared to UIO-66 (Zr), the thermal stability of UIO-66 (NH2) slightly

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decreases, which is stable up to 330 °C and has a 43% weight loss (320-480 °C)
corresponding to the decomposition of the BDC-NH2 linkers [32, 39]. For the pure
ZnTCPc, TGA analysis shows weight loss of ZnTCPc at three different temperature
ranges and its total weight loss is up to 80%. The composites ZnTCPc/UIO-66 (Zr)
and ZnTCPc/UIO-66 (NH2) have the similar curves but higher stability properties
compared with the pristine structure (UIO-66 (Zr) and UIO-66 (NH2)), which is
ascribed to the formation of strong interaction between ZnTCPc and UIO-66.
Electron paramagnetic resonance (EPR) analysis was carried out to investigate the
electronic band structure of the samples. The EPR spectra of powdered UIO-66 (NH2),
ZnTCPc and ZnTCPc/UIO-66 (NH2) were measured at room temperature in Fig. S4.
The pure UIO-66 (NH2) shows three EPR lines at 2.036, 2.026 and 2.015, while the
EPR lines of ZnTCPc are calculated as 2.019 and 2.015. The line at 2.015 is due to
−
the defects in the Zr6O32 core and the two EPR lines at 2.036 and 2.026 are from •O2
species adsorbed on Zr3+ sites [40]. It is obvious that the intensity of the EPR lines
increases with integration of ZnTCPc in UIO-66 (NH2), which may be caused by the
−
−
•O2 species formed from the photogenerated electrons [41]. The active •O2 species
could be stabilized in the porosity structure of UIO-66 (NH2) because of its strong
conjugated interaction with ZnTCPc compound, which is consistent with the analysis
of photocatalytic mechanism. As expected, this introduction of ZnTCPc that can
induce synergetic effect sufficiently enhances the photoactivity of UIO-66 (NH2) and
ZnTCPc.

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As the photoabsorption property plays a key role in determining the photocatalytic
activity, the UV-vis diffuse reflectance spectra (DRS) of the samples have been
measured, as shown in Fig. 6. UIO-66 (Zr) spectrum characterized by an absorption
edge around 310 nm is ascribed to π-π* electronic transitions of the aromatic ring.
Although frontier orbitals of UIO-66 (NH2) show a localized electronic state, strong
photo-absorption bands from 260 nm to 430 nm in UiO-66(NH2) are observed
attributed to charge transfer process from sets of the ligand to Zr-O2 cluster.
Compared with UIO-66 (Zr), the absorption edge of UiO-66 (NH2) was red shifted,
which could be attributed to the addition of amino group with lone pair electrons,
leading to the lift of the highest occupied molecular orbital (HOMO) level [30]. As
for ZnTCPc/UIO-66 (Zr) composite, new absorption peaks at 400 nm and 600-750
nm belonging to ZnTCPc absorption band are observed, and the absorption edge of
UIO-66 (Zr) in the composite is slightly red shifted. The absorption peak of the
ZnTCPc/UIO-66 (NH2) around 600-750 nm also originates from the absorption of
ZnTCPc, but absorption peak at about 400 nm attributed to ZnTCPc is not obvious.
Besides, ZnTCPc/UIO-66 (NH2) shows much broader absorption band through the
entire visible light region, and exhibits intense and excellent visible light absorption
properties. It can be concluded that the formation of a chemical interaction between
ZnTCPc (-COOH groups) and UIO-66 (NH2) via a condensation reaction may induce
a synergetic effect to enhance the photoactivity of UIO-66 (NH2). In addition, as
shown in Fig. 7, the UV-vis spectra of ZnTCPc in DMF solution show a good
absorption on the wavelength of light about 680 nm. The absorption of

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ZnTCPc/UIO-66 (Zr) appears around 680 nm which is ascribed to the absorption of
ZnTCPc. Compared to the ZnTCPc/UIO-66 (Zr) prepared by impregnation, the
absorption of ZnTCPc/UIO-66 (NH2) reacted with EDC demonstrates about ~8 nm
red-shift, which should arise from the covalent amide structure [31].
The chemical states C, N and Zr of the as-prepared photocatalysts are probed with
XPS, as shown in Fig. 8 and Fig. S5. Fig. 8a shows the normalized C1s high
resolution XPS spectra of UIO-66-NH2, ZnTCPc and their composite. For pure
UIO-66-NH2 nanocrystals, there are three peaks at 284.5 eV, 286.2 eV and 288.5 eV,
which belong to the sp2 C, C-N and O-C-O groups, respectively [42]. The pure
ZnTCPc show two peaks at 284.5 eV and 288 eV, arising from sp2 C and C=O/C=N
groups [31]. The composite of UIO-66-NH2 and ZnTCPc has two sets of C1s XPS
peaks, which not only contain those characteristic peaks of UIO-66-NH2 and ZnTCPc,
but also contain a new peak at 287.5 eV. It demonstrates that there is amide (-CO-NH),
generating from condensation reaction between amino and carboxyl functional group.
This can be further confirmed from normalized N1s XPS spectra in Fig. 8b. The two
N1s peak at 400 eV and 401.8 eV are corresponding to -NH2 and -NH groups, while
the ratio of -NH and -NH2 in composite increases compared with that of pure
UIO-66-NH2 nanocrystals, suggesting that the successful reaction between
UIO-66-NH2 and ZnTCPc. Interestingly, there is a new doublet at 183.3 eV and 185.7
eV for Zr3d XPS spectra compared with UIO-66-NH2, as shown in Fig. S5 and Table
S2. The significant shift (∆= +1.0 eV) of the Zr 3d binding energies towards a higher
energy demonstrates that strong electron-with-drawing effect of ZnTCPc for the metal

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component [37]. So it can be concluded that the electronic structure of composite is
altered through the condensation reaction and the electron-with-drawing effect of
ZnTCPc toward metal component.
3.2. Photocatalytic activity and stability
As shown in Fig. 9, the photocatalytic activity of the as-prepared samples was
evaluated by the degradation of MB in aqueous solution under visible light irradiation
(λ ≥ 420 nm). When ZnTCPc was added alone, there was no obvious change in the
concentration of MB in the presence of ZnTCPc, suggesting that ZnTCPc had no
obvious photocatalytic activity for the degradation of MB. In the presence of pristine
UIO-66 (Zr) and UIO-66 (NH2), MB concentration was gradually decreased and the
removal rate are 58% and 56% within 120 min, respectively. It should be noted that
although the photocatalytic activity of pristine UIO-66 (Zr) is similar to that of
UIO-66 (NH2), ZnTCPc/UIO-66 (Zr) composite showed a relatively better
photocatalytic performance with a removal rate of 68%, which could be ascribed to
synergetic effect between ZnTCPc and UIO-66 (Zr) [43]. ZnTCPc/UIO-66 (NH2)
with stable covalent bonds exhibited the highest catalytic activity due to the
co-modifications of ZnTCPc and UIO-66 (NH2). The catalytic activities are in the
order of ZnTCPc < UIO-66 (NH2) < UIO-66 (Zr) < ZnTCPc/UIO-66 (Zr) <
ZnTCPc/UIO-66 (NH2). In Fig. S6, the reaction rate constant (k) described as a
pseudo-first-order reaction mode, can be calculated from the slope of the linear fit of
ln(Ct/C0) vs t. The ZnTCPc/UIO-66 (NH2) shows a k value of 0.0168 min-1 which is
1.84, 2.37 and 9.33 times higher than that of ZnTCPc/UIO-66 (Zr) (0.0091 min-1),

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bare UIO-66 (NH2) (0.0071 min-1) and ZnTCPc (0.0018 min-1), respectively. The
stability of photocatalysts is a major concern for their practical utility in real
applications. We therefore investigated the stability of ZnTCPc/UIO-66 (NH2)
composite in photocatalytic degradation of MB, as shown in Fig. 10. Although there
is a decrease in the degradation rate for each cycle, more than 70% of MB can still be
degraded in the fourth cycle, indicating its high stability in the photocatalytic reaction
under visible-light irradiation.
3.3. Photocatalytic mechanism
The photocatalytic mechanism of the ZnTCPc/UIO-66 (NH2) composite was
−
investigated through determining the potential roles of h+, •O2 and •OH during the
degradation, since they are known as the three main active species involved in the
photocatalytic process. To identify the contributions of these active species, three
different scavengers (EDTA, BQ and IPA) were separately added into the degradation
system of MB to attempt to trap h+, •O2 and •OH. As shown in Fig. 11, the addition
−
of IPA does not cause any change in the MB degradation rate, which suggests that
•OH should not be an important contributor to the photocatalytic process. However,
the degradation rate obviously decreases at the presence of EDTA significantly,
indicating that photogenerated holes should be one of the main active species
participating in the decomposition process of MB. While the addition of BQ partially
suppresses the degradation rate, and shows a significant effect on the k value which
−
decreases obviously from 0.01721 min-1 to 0.01002 min-1, showing that •O2 is also a
dominant reactive species.

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To further understand the underlying mechanism for electrical conduction
properties, the flat band position determined from the previous literature is
approximately -0.6 V, -0.8 V and -1.5 V for UIO-66 (Zr), UIO-66 (NH2) and
ZnTCPc, respectively [13, 30, 31, 44]. These data, together with the band gap energy
estimated from UV-visible spectra, allow us to present the absolute energy scheme,
including HOMO and LUMO positions. A schematic diagram is shown in Fig. S7,
which describes the band positions and band gaps of UIO-66 (Zr), UIO-66 (NH2) and
ZnTCPc samples. The photogenerated electron-hole pairs of UIO-66 (Zr), UIO-66
(NH2) and ZnTCPc simultaneously arises from visible light irradiation. Since the
LUMO of ZnTCPc is higher than that of UIO-66 (Zr)/UIO-66 (NH2), the excited
electrons of ZnTCPc could be easily injected into UIO-66 (Zr)/UIO-66 (NH2), making
electron-hole separation more efficient and reducing the probability of recombination.
Based on the above results and discussions, we propose the following mechanism
illustrated in Fig. 12: the dye molecules adsorbed on porous UIO-66 (Zr)/UIO-66
(NH2) are excited under visible light irradiation, and then the photogenerated
electrons are injected into UIO-66 (Zr)/UIO-66 (NH2). Meanwhile, the active species
holes produced from MOFs are injected into the HOMO of ZnTCPc and the
photo-generated holes are collected in the HOMO of ZnTCPc. Nevertheless,
ZnTCPc/UIO-66 (NH2) exhibited much higher photocatalytic activity than that of
ZnTCPc/UIO-66 (Zr) in our work. The main reasons can be described as follows: the
Zr-oxo clusters within UIO-66 (NH2) can behave as quantum dots surrounded by
terephthalate ligands with the NH2 groups (BDC-NH2), which can act as antennae

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absorbing light and be able to efficiently transfer the electrons to the inorganic
zirconium-oxygen clusters part [44]. Moreover, the excited electrons could be
transferred more effectively supported by the amide covalent bonds in the
ZnTCPc/UIO-66 (NH2) composite. Due to the localized electronic state in UIO-66
(NH2), the excited electrons located in the LUMO cannot diffuse into other positions
[17]. However, the electrons can be transferred to O2 molecules diffusing into pores
and adsorbing on the Zr3+ sites, beneficial for the formation of superoxide radical •O2−
and Zr4 + ion. Therefore, the chemical bonds and synergistic effect between UIO-66
(NH2) and ZnTCPc should be the intrinsic factor for its better photosensitization than
ZnTCPc/UIO-66 (Zr).
4. Conclusion
Zr-based MOF combined with ZnTCPc toward visible-light-driven photocatalytic
degradation of MB has been successfully developed. The composite combining
ZnTCPc and UIO-66 (NH2) via a condensation reaction shows the highest
photocatalytic activity in our work. The enhanced photocatalytic performance arises
from the efficient separation of photogenerated electron-hole pairs and charge
transfer in composite, because of the tight contact between ZnTCPs and UIO-66(NH2)
via a covalent amide structure, rather than the simple physical interaction from an
impregnation method. Besides, the ZnTCPc/UIO-66 (NH2) composite is able to
preserve most of its initial photocatalytic activity after four cycles of degradation
experiments. The study on the photocatalytic mechanism of the ZnTCPc/UIO-66 (NH2)
composite implies that h+ and •O2− should be the major contributors to the

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degradation process of MB. It is hoped that our work could provide a novel method
for the design and fabrication of porous MOFs-dyes composites, which may offer
more opportunities for photocatalytic applications.
Acknowledgements
This work was supported by the Natural Science Foundation of Jiangsu Province
(Nos. BK20150259, BK20160277).
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Figure captions

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Fig. 1. Synthesis process of ZnTCPc/UIO-66 (NH2).
Fig. 2. FT-IR spectra of ZnTCPc (a), UIO-66 (Zr) (b), UIO-66 (NH2) (c),
ZnTCPc/UIO-66 (Zr) (d) and ZnTCPc/UIO-66 (NH2) (e).
Fig. 3. N2 adsorption desorption isotherms of UIO-66 (Zr) (a), UIO-66 (NH2) (b),
ZnTCPc/UIO-66 (Zr) (c) and ZnTCPc/UIO-66 (NH2) (d).
Fig. 4. TEM images of UIO-66 (Zr) (a), UIO-66 (NH2) (b), ZnTCPc/UIO-66 (Zr) (c)
and ZnTCPc/UIO-66 (NH2) (d).
Fig. 5. Thermogravimetric analysis for pure ZnTCPc, UIO-66 (Zr), UIO-66 (NH2),
ZnTCPc/UIO-66 (Zr) and ZnTCPc/UIO-66 (NH2).
Fig. 6. UV-vis diffusion reflectance absorption spectra (DRS) of ZnTCPc, UIO-66
(Zr), UIO-66 (NH2), ZnTCPc/UIO-66 (Zr) and ZnTCPc/UIO-66 (NH2).
Fig. 7. UV-vis spectra of ZnTCPc, UIO-66 (Zr), UIO-66 (NH2), ZnTCPc/UIO-66 (Zr)
and ZnTCPc/UIO-66 (NH2).
Fig. 8. High resolution XPS spectra of C 1s (a) and N 1s (b) obtained from ZnTCPc,
ZnTCPc/UIO-66 (NH2) and UIO-66 (NH2).
Fig. 9. Photocatalytic degradation of MB over the as-prepared samples under
visible-light irradiation (λ ≥ 420 nm).
Fig. 10. Photocatalytic recycle degradation of MB over ZnTCPc/UIO-66 (NH2) under
visible-light irradiation (λ ≥ 420 nm).
Fig. 11. Effects of different scavengers on the degradation of MB in the presence of
ZnTCPc/UIO-66 (NH2) under visible-light irradiation.

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Fig. 12. Schematic of photocatalytic degradation of MB over the ZnTCPc/UIO-66
(NH2) under visible light irradiation.
Fig. 1. Synthesis process of ZnTCPc/UIO-66 (NH2).