Halogen lamp activated nanocomposites as nanoporous photocatalysts: Synthesis, characterization, and pollutant degradation mechanism

Article abstract of DOI:10.1016/j.molliq.2019.02.069

In this work, zeolitic imidazolate framework-8 (ZIF-8)-titania nanocomposites (ZT) with different amounts of titania nanoparticle (0.2, 0.4 and 0.6 g denoted as ZT-0.2, ZT-0.4 and ZT-0.6, respectively) as nanoporous photocatalysts were synthesized and characterized. A halogen lamp (500 W) was used as an irradiation source. The pollutant (Acid Blue 92 (AB92)) degradation mechanism was studied in details. The XRD, FTIR, SEM, TEM, BET, TGA and zeta potential were used to characterize the synthesized nanomaterials. The data showed that the ZT-0.4 nanocomposite had higher dye degradation ability due to the synergistic effect of ZIF-8 and titania. Dye degradation followed zero-order kinetics. The degradation rate of AB92 for 0.005, 0.010, 0.020 and 0.030 g of catalyst (ZT-0.4) was 0.00009, 0.00070, 0.00150 and 0.00190 mg/L min, respectively. The removal efficiency suggested that OH^? possesses a major role for photocatalytic degradation of AB92. The synthesized nanocomposite showed stability and reusability over three cycles.

Full text of DOI:10.1016/j.molliq.2019.02.069

Accepted Manuscript
Halogen lamp activated nanocomposites as nanoporous
photocatalysts: Synthesis, characterization, and pollutant
degradation mechanism
Mina Oveisi, Niyaz Mohammad Mahmoodi, Mokhtar Alinia Asli
PII:
S0167-7322(18)36398-0
DOI:
Reference:
https://doi.org/10.1016/j.molliq.2019.02.069
MOLLIQ 10470
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Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
8 December 2018
30 January 2019
13 February 2019
Please cite this article as: M. Oveisi, N.M. Mahmoodi and M.A. Asli, Halogen lamp
activated nanocomposites as nanoporous photocatalysts: Synthesis, characterization, and
pollutant degradation mechanism, Journal of Molecular Liquids, https://doi.org/10.1016/
j.molliq.2019.02.069
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ACCEPTED MANUSCRIPT
Halogen lamp activated nanocomposites as nanoporous photocatalysts: Synthesis,
characterization, and pollutant degradation mechanism
Mina Oveisi a, Niyaz Mohammad Mahmoodi b,*, Mokhtar Alinia Asli a
a Department of Chemistry, Faculty of Science, University of Zanjan, Zanjan, Iran
b Department of Environmental Research, Institute for Color Science and Technology, Tehran, Iran
*Corresponding author. Tel. +98 021 22969771, Fax. +98 021 22947537
E-mail addresses:
mahmoodi@icrc.ac.ir
nm_mahmoodi@aut.ac.ir
nm_mahmoodi@yahoo.com
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Abstract
In this work, zeolitic imidazolate framework-8 (ZIF-8)-titania nanocomposites (ZT) with
different amounts of titania nanoparticle (0.2, 0.4 and 0.6 g denoted as ZT-0.2, ZT-0.4 and
ZT-0.6, respectively) as nanoporous photocatalysts were synthesized and characterized. A
halogen lamp (500 W) was used as an irradiation source. The pollutant (Acid Blue 92
(AB92)) degradation mechanism was studied in details. The XRD, FTIR, SEM, TEM, BET,
TGA and zeta potential were used to characterize the synthesized nanomaterials. The data
showed that the ZT-0.4 nanocomposite had higher dye degradation ability due to the
synergistic effect of ZIF-8 and titania. Dye degradation followed zero-order kinetics. The
degradation rate of AB92 for 0.005, 0.010, 0.020 and 0.030 g of catalyst (ZT-0.4) was
0.00009, 0.00070, 0.00150 and 0.00190 mg/L min, respectively. The removal efficiency
suggested that OH• possesses a major role for photocatalytic degradation of AB92. The
synthesized nanocomposite showed stability and reusability over three cycles.
Keywords: Halogen lamp activated nanocomposites; Synthesis and characterization; Zeolitic
imidazolate framework; Nanoporous photocatalysts: Pollutant degradation mechanism
1. Introduction
Environmental pollution is attracted much attention due to the toxicity and mutagenicity of
contaminants. Animal models presents the effects of environmental pollutants on several
diseases such breast cancer [1]. The prevalent cancer among women is breast cancer [2,3].
The world population increases dramatically, while the natural water source is constant. The
water demand will enhance as the industries and economies of some nations expand. The
shortage and low quality of water are the main concerns in several parts of the world. The
industrial activity generates a large volume of wastewater containing different contaminants.
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Removal of anthropogenic pollutants from industrial effluents is a fascinating research field
for scientists and engineers. The pollutants are dyes, solvents, pesticides, heavy metals,
detergents, etc. which are toxic, non-biodegradable and carcinogenic. They are severe
environmental concerns due to their influence on human being and living ecosystem. Dyes as
one of the main group of pollutants are studied extensively. It is estimated 100,000
commercial dyes with 700,000 tones are produced each year in the world. These
contaminants clog the treatment channels and enhance the chemical and biochemical oxygen
demands [4-13].
Several methods are used for removing contaminants from water and wastewater such
as physical, biological and chemical processes. Each treatment method has advantages and
disadvantages. For example, physical methods such as membrane process are easy to handle
but they generate secondary pollution. Biological methods are economical techniques, but
they are time-consuming. Advanced oxidation process (AOP) as one of the chemical
treatment techniques is a promising method especially when the solar light is applied.
Photocatalysis using different catalysts as one of the AOPs and a light inducing reaction has
several advantages including (1) operation at ambient temperature, (2) complete degradation
and (3) no secondary pollution. It is considerably adopted for degrading pollutants because of
easy handling, the simplicity of design, environmentally friendly, high performance and
reproducibility. During the photocatalysis, high reactive and nonselective species such as
hydroxyl radical, superoxide, and ozonide are produced in situ for destructing organic
contaminants [14-23].
Several photocatalysts such as titania, zirconia, zinc oxide, etc. are used for
destructing pollutants [24-27]. Among the photocatalysts, titania is a promising catalyst
because of favorable properties including safety, stability, environmentally friendly,
nontoxicity, low cost, and availability [26]. However, it suffers from one limitation because
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of a large band gap. For conquering this limitation, nanocomposites with narrow band gap
must be made. Recently, novel porous composites are synthesized by titania and metal-
organic frameworks (MOFs) as a class of advanced materials. MOFs have several advantages
including high stability, discrete ordered structure, ultra-low density, high surface area
extending beyond 6000 m2/g and ease of synthesis, which distinguish from other porous
compounds such as zeolites, metal-complex hydrides, etc. Due to the unique properties of
MOFs, scientists and engineers investigate their application for various physical and chemical
processes [28-43]. MOFs are synthesized by connecting metal nodes and organic linkers
through chemical bonds. Zeolitic imidazolate frameworks (ZIFs) as one of the most
important class of MOFs have gained remarkable consideration because they can be
synthesized using a green and safe procedure at room temperature. Zeolitic imidazolate
framework-8 (ZIF-8) has high surface area and pollutant removal ability. ZIF-8 is
synthesized using 2-methylimidazole as an organic linker and zinc nitrate as a metal source.
A literature review indicated that ZIF-8 nanocomposites with different Titania
amounts were not studied for degrading dye by halogen lamp in details. In this work, ZIF-8
and its nanocomposites with different titania amounts (0.2, 0.4, and 0.6 g denoted as ZT-0.2,
ZT-0.4, and ZT-0.6, respectively) were synthesized at room temperature as a green
procedure. The synthesized nanomaterials were used to degrade organic pollutant (dye) from
colored wastewater using halogen lamp (500 W). The XRD, FTIR, SEM, TEM, BET, TGA
and zeta potential analyses were used to characterize the synthesized materials. Acid Blue 92
(AB92) was used as a model compound. The aim of present work is to study the influence of
parameters comprising the amount of titania in composite, photocatalyst dosage, initial dye
concentration, and initial pH on photocatalytic degradation of AB92. In addition, the
photocatalytic degradation kinetics, regeneration and stability of the catalysts were studied.
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2. Materials and methods
2.1. Materials
Acid blue 92 (AB92) (Fig. 1) as an anionic dye was received from Ciba Company. All other
chemicals were analytical grade and obtained from Merck. Sodium hydroxide and
hydrochloric acid solution were applied to adjust the solution pH. Anatase titania
nanoparticle was purchased from Evonik (particle size: 21 nm and surface area: 50 ± 15
m2/g).
Fig. 1. The chemical structure of Acid Blue 92.
2.2. Nanomaterials synthesis
Fig. 2 shows the synthesis procedure of ZIF-8 and ZIF-8/titania nanocomposites.
2.2.1. Synthesis of ZIF-8 nanomaterial
First, two solutions (A and B) were papered separately. Solution A was papered by dissolving
zinc nitrate (0.587 g) in methanol (40 mL). In addition, solution B was prepared by adding 2-
methylimidazole (1.298 g) into methanol (40 mL). Second, solution B was added to solution
A slightly. The mixed solution (A+B) was stirred at room temperature. Third, the mixture
was allowed stands without stirring. Then, the product was separated by centrifuge and
washed several times with methanol and deionized water. Finally, the withe nanoporous ZIF-
8 was dried in the oven [44,45].
2.2.2. Synthesis of ZIF-8/titania nanocomposites
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Three different amounts of titania nanoparticles (0.2, 0.4 and 0.6 g) were added to methanol
and sonicated for 1 h (solution A). Then, zinc nitrate (0.587 g) was added to solution A and
stirred. Solution B (1.298 g of 2-methylimidazole in 40 mL methanol) was poured to solution
A. Other steps were similar to above-mentioned procedure [46].
Fig. 2. The green synthesis procedure of the nanomaterials (ZIF-8 and ZIF-8/titania
nanocomposites).
2.3. Materials characterization
The crystalline phases of the samples were examined by X-Ray diffraction (XRD)
(PANalytical Crystal Netherlands). The FTIR spectra were recorded by Perkin-Elmer
Spectrophotometer. The SEM (LEO 1455VP) and TEM (zeiss EM900) were used to analyze
the morphology and size of the synthesized nanomaterials. The surface area, total pore
volume and pore size distribution (PSD) of the samples were obtained from N2 adsorption–
desorption isotherms at 77 K using a BELSORP-mini II, BEL, Osaka, Japan instrument.
ZETASIZER nano-NS through Malvern instruments was used to study the zeta potential of
the nanomaterials. A spectrophotometer (Perkin Elmer) was used to determine the dye
concentration. An ultrasonic bath operated at frequency of 30 kHz and power of 200W was
used.
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2.4. Photocatalytic activity
Photodegradation of AB92 by the synthesized nanomaterials was investigated in the presence
of 500 W halogen lamp (OSRAM GmbH.; Germany). The photocatalysts was added to
photoreactor. The various amounts of nanoporous photocatalysts were added to 100 mL of a
dye solution with specific concentration. After dye degradation, the catalyst particles were
separated by centrifuge and the dye concentration in solution was determined by
spectrophotometer at a maximum wavelength of AB92 (571 nm). The effect of catalyst
dosage, dye concentration and solution pH on AB92 degradation in present of halogen lamp
was investigated in details.
3. Results and discussion
3.1. Characterization
The nanoporous ZIF-8 is a MOF which is synthesized by the zinc metal node and imidazole
organic linker. It is one of the thermal and chemical stable of zeolitic imidazolate frameworks
that is synthesized by zinc ion and 2-methyl imidazole with sodalite structure. In this paper,
ZIF-8 and ZIF-8/titania nanocomposites were synthesized using coprecipitation method at
ambient temperature. Several analyses including XRD, FT-IR, SEM, TEM, BET, TGA and
zeta potential were used to characterize the synthesized nanoporous photocatalysts.
3.1.1. XRD
For identifying the crystal phase of samples, X-ray diffraction analysis is used. The XRD
patterns of ZIF-8 and the synthesized nanocomposites (ZT-0.2, ZT-0.4, and ZT-0.6) are
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shown in Fig. 3. The XRD of Titania nanoparticle is shown in Fig. 3a. The diffraction peaks
at 25° and 48° are attributed to the anatase face of Titania [47-49].
Fig. 3b presents that the synthesized nanoporous ZIF-8 with characteristic peaks at 2θ:
7.38º, 10.44º, 12.79º, 14.71º, 16.49º, and 18.05º. Fig. 3 (c, d, and e) shows XRD pattern of
ZIF-8/titania nanocomposites including ZT-0.2, ZT-0.4, and ZT-0.6, respectively. The
prominent reflections at 2θ = 7.38º, 10.44º, 12.79º, 14.71º, 16.49º, and 18.05º and 2θ = 25.38º
and 48.15º are accord with ZIF-8 and Titania, respectively. As shown in Fig. 3, no additional
obvious peaks of feasible impurities were seen. In addition, the intensity of a peak at 25º
increased by increasing the Titania amount at the composite [50].
Fig. 3. XRD patterns of the synthesized nanomaterials (a) titania, (b) ZIF-8, (c) ZT-0.2, (d)
ZT-0.4 and (e) ZT-0.6.
3.1.2. FTIR
The functional groups of the materials were studied by FTIR. Fig. 4a shows the FTIR
spectrum of the ZIF-8. The characteristic bands are at 3415, 3135, 2926, 1574, 1421 and
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1350-900 cm-1 [51,52]. These peaks are attributed to the OH starching, C-H stretching at the
aromatic ring, C-H stretching of the aliphatic chain, C=N stretching, entering stretching and
in-plane bending of H-MIM ring, respectively. In addition, the peaks at 754 and 689 cm-1 are
due to the out of plane bending of H-MIM [44,45]. FTIR spectrum of Titania nanoparticle is
indicated in Fig. 4 (b). The peak at 680 cm-1 is attributed to the Ti-O and O-Ti-O vibration.
The FTIR spectra of ZIF-8/titania nanocomposites including ZT-0.2, ZT-0.4 and ZT-0.6 are
presented in Fig. 4c, d and e), respectively. The results showed that the intensity of ZIF-8
peaks decreased by increasing the amount of Titania in the nanocomposite. It can be
attributed to the formation of ZIF-8 shell on the Titania nanoparticle core.
Fig. 4. FTIR spectra of the nanomaterials (a) ZIF-8, (b) titania, (c) ZT-0.2, (d) ZT-0.4 and (e)
ZT-0.6.
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3.1.3. SEM
The surface and morphology of nanomaterials can be investigated in details by SEM. In
addition, the size of materials can be determined [53]. In this paper, the images of ZIF-8,
Titania and ZIF-8/titania nanocomposites are presented in Fig. 5. Titania nanoparticle has
a homogenous particle size (Fig. 5a). Zeolitic imidazolate framework was a hexagonal
homogeneous with average particle size smaller than 100 nm as indicated in Fig. 5b. When
different amounts of Titania were coated with ZIF-8, there was no considerable
aggregation and change in structure (Fig. 5: c, d, and e). In addition, the size of composites
(ZT-0.2 (Fig. 5c), ZT-0.4 (Fig. 5d) and ZT-0.6 (Fig. 5e)) was smaller than the pure ZIF-8.
The SEM images show the nanoarchitecture of the composites.
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(a)
(b)
(c)
(d)
(e)
Fig. 5. SEM images of the nanomaterials (a) titania, (b) ZIF-8, (c) ZT-0.2, (d) ZT-0.4 and (e)
ZT-0.6.
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3.1.4. TEM
In the TEM analysis, a high energy electron is emitted on the thin material and electron
passes through the materials and scattered. Then, they are concentrated on the detector to give
images with high resolution [50]. The TEM images of ZIF-8/titania nanocomposite (ZT-0.4)
and ZIF-8 were obtained to further study the structure of the synthesized nanomaterials. As
indicated in Fig. 6, the nanocomposite presented hexagonal shape. The particle size was
smaller than 100 nm which is in agreement with the SEM images.
(a)
(b)
(c)
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Fig. 6. TEM images (a) ZT-0.4 (scale: 25 nm), (b) ZT-0.4 (scale: 200 nm) and (c) ZIF-8.
3.1.5. BET
Heterogeneous catalysis can be occurred at the interface among phases. Also the number of
active sites attributed to the surface area that play an important role. Eventually particle size,
particle morphology, surface pore size, pore size distribution and porosity are subset of
surface area. For the definition of surface area, the Brunauer–Emmett–Teller (BET)
procedure is applied. Nitrogen is generally used in BET analysis. After BET analysis steps
were done, the assembled data were illustrated in the BET isotherm. It plots the quantity of
gas adsorbed as a function of the relative pressure at constant temperature. Five types of
adsorption isotherms are investigated [54-56]. The N2 adsorption–desorption isotherms of
ZIF-8, Titania nanoparticle and ZIF-8/titania nanocomposites are shown in Fig. 7. In
addition, the porosity of synthesized nanomaterials are presented in Table 1. The results
indicate that the specific surface area of ZIF-8, Titania, ZT-0.2, ZT-0.4, and ZT-0.6 are 1485,
50 and 845, 607, 461 m2/g, respectively. The data present that the specific surface area of
nanocomposites are lower than the pure ZIF-8 because the cavity widow is blocked by
Titania. In addition, the total pore volume of ZIF-8, ZT-0.2, ZT-0.4, and ZT-0.6 are 1.1428,
0.7703, 0.6573 and 0.6267 cm3/g, respectively. Mean pore diameter of ZIF-8 and its
nanocomposites including ZT-0.2, ZT-0.4, and ZT-0.6 are 3.08, 3.643, 4.33, and 5.43 nm,
respectively. The adsorption behavior of as-prepared materials can be categorized as the
IUPAC typical type IV isotherms, confirming the conquering microporous feature of these
materials, and the small hysteresis loop shows the presence of mesopores [57].
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Fig. 7. (a)The BET adsorption isotherms of the synthesized nanomaterials in a nitrogen
atmosphere and (b) BJH of ZIF-8 and ZT 0.4.
Table 1. Porosity properties of the synthesized nanomaterials.
Sample
ZIF-8
SBET[m2/g]
1485.1
845
V total[cm3/g]
1.1428
Pdiam [nm]
3.08
ZT-0.2
ZT-0.4
ZT-0.6
0.7703
3.6461
4.33
607
0.6573
461
0.6267
5.4344
3.1.6. TGA
Thermogravimetric analysis (TGA) is an analytical method that does not need particular
material provision except drying. The TGA can be used to measure thermal stability and
purity of nanomaterials. In this technique, the sample is heated to high temperatures as
overseeing the mass of the material that gives the dissociation curve [58]. Thermal stability of
the synthesized nanomaterials including ZIF-8 and ZIF-8/titania nanocomposite was
investigated in details by the TGA (Fig. 8). The results show that the synthesized ZIF-8 is
stable up to 400 0C with little mass reduction. Then, abruptly decrease in mass was done from
425 to 625 0C due to the exothermic degradation of ligand.
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Fig. 8. TGA carve of the synthesized nanomaterials (ZIF-8 and ZT-0.4).
3.1.7. Zeta positional
The electrical charge generated at the interface of catalyst surface in aqueous solution is
measured by zeta positional. Generally, free ions in the solution rearrange themselves at the
interface and zero net charge density is generated. Electrical double layer (EDL) is a layer
containing charges at the interface of the liquid and solid phases and the balancing counter
ions exist in the liquid. The compact layer (CL) is a layer having counter ions next to the
charged solid surface. The counter ion in the CL is the fixed state because of electrostatic
attraction. Counter ions outside the CL (diffuse layer: DL) is moveable. The electrostatic
potential between the CL and DL is called the zeta potential [59,60]. Fig. 9 shows the zeta
potential of the nanomaterials. The results indicated that Titania, ZIF-8, ZT-0.2, ZT-0.4 and
ZT-0.6 had zeta potential values of -45.3, -21.2, -29.2, -31.2, and -33.7 eV, respectively.
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Fig. 9. Zeta potential of the nanomaterials.
3.2. Photocatalytic activity
3.2.1. Effect of different photocatalysts on degradation of AB92
The photocatalytic destruction of AB92 by nanoporous ZIF-8, Titania nanoparticle and ZIF-
8/titania nanocomposites is presented in Fig. 10. Dye degradation by only light was 1 %. The
dye adsorption was carried out without light irradiation to study the photocatalytic pollutant
degradation ability of catalysts. Dye adsorption by ZIF-8 for 180 min without illumination
(complete dark test) was done. Dye removal was 19%. In addition, dye degradation by ZIF-8
for 180 min illumination was carried out. Pollutant degradation was 45% due to its
photocatalytic activity. Also, a pure Titania nanoparticle presented 65% dye degradation. The
results showed that ZT-0.4 had the highest decolorization percentage. The dye destruction by
ZIF-8/titania nanocomposite considerably increased (96%) than that of pure Titania
nanoparticle and ZIF-8. It can be attributed to the synergistic effect of porous structure and
high surface area of ZIF-8 on Titania catalytic activity. AB92 is adsorbed on the ZIF-8/titania
by π-π interaction, which leads to further photocatalytic activity of AB92 on nanocomposite.
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Li et al. demonstrate that the photo-generated electrons can migrate from the semiconductor
to the MOF in semiconductor/MOF composite photocatalysts which helps charge separation
in the semiconductor and supplies energetic electrons to molecules adsorbed on the MOF
[57]. As well as, the porous structure and the high surface area of MOFs can too comfort the
dispersion of semiconductor NPs, leading to more reaction active centers [61,62]. Therefore,
the superior degradation of AB92 by the nanocomposite relative to that of the titania
nanoparticle and nanoporous ZIF-8 should be ascribed to the synergistic mixture of ZIF-8
and titania: (1) ZIF-8 can adsorb the dye molecule in aqueous media and (2) the charge
transfer of nanocomposite is affective. These consequences are shown that the nanocomposite
absorbs more visible light than the pure Titania nanoparticle. Thus, ZIF-8 nanocomposites
with different Titania nanoparticle amounts were used as photocatalysts for further dye
degradation.
Fig. 10. The photocatalytic activity of the synthesized nanomaterials.
3.2.2. Effect of operational parameters on degradation of AB92
Effective degradation of AB92 is feasible via ZIF-8/titania nanocomposite. Several handy
factors affect the photocatalytic activities of nanocomposite. The further comprehension of
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the photocatalytic method and the effective situations could admit good chances for its usage
to the destruction of dyes.
3.2.2.1. Effect of Titania content in nanocomposite
Adsorption and photodegradation of AB92 have been studied with an efficacy of the various
contents of Titania nanoparticle (0.2 g, 0.4 g and 0.6 g) in nanoporous ZIF-8 (denoted as ZT-
0.2, ZT-0.4 and ZT-0.6, respectively) by encapsulation for removing AB92 and the results are
shown in Fig. 11. The data present that ZT-0.4 has the highest dye degradation ability in 20
mg/L at natural pH and room temperature (96.3%) compared to other photocatalysts (ZT-0.2:
81% and ZT-0.6: 90%). The catalytic activity of nanocomposite for degrading pollutant is
considerably increased after Titania encapsulation from 0.2 g to 0.4 g. It can be attributed that
the high surface area and porosity of ZIF-8 are affective for permeating pollutant on catalyst
surface. Catalytic activity of ZT-0.6 decreased because more Titania nanoparticles
encapsulated in porous structure of nanocomposite and diminishes dye permeation. Thus, dye
degradation decreased.
Fig. 11. Effect of amount of loaded Titania (0.2, 0.4, and 0.6 g) in ZIF-8/titania
nanocomposite on dye degradation.
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3.2.2.2. Effect of photocatalyst dosage
ZIF-8/titania nanocomposite dosage can affect the pollutant degradation rate. The initial
reaction rates were found to be directly proportional to catalyst amount. Several dosages of
nanocomposites (0.005, 0.010, 0.020, and 0.030 g) were employed to study the effect of
catalyst dosage on the degradation kinetics of AB92 (Fig. 12). The increase in catalyst dosage
enhanced the degradation rate of Acid Blue 92. The number of photons absorbed via
photocatalyst increased when the amount of photocatalyst increased. By enhancing the
content of catalyst, the number of active sites and accessible adsorption sites enhanced. Thus,
percentage of photodegradation of AB92 (20 mg/L) using semiconductor/MOF enhanced
remarkably via increasing the number of catalysts until 0.030 g.
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(a)
(b)
(c)
Fig. 12. The effect of catalyst dosage on photocatalytic dye degradation by the synthesized
nanocomposites (a) ZT-0.2, (b) ZT-0.4 and (c) ZT-0.6.
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In addition, the kinetics of dye degradation by the synthesized nanocomposites at
different catalyst dosages were investigated in details. The plot between concentration (C) vs.
time (zero-order), C/C0 vs. time (first-order) and 1/C vs. time (second-order) are shown in
Table 2. The kinetic constants values are presented in Table 3. The results showed that dye
degradation by the synthesized ZIF-8/titania nanocomposites including ZT-0.2, ZT-0.4, and
ZT-0.6 followed the zero-order kinetics.
Table 2. The different kinetics of AB92 photocatalytic degradation by the synthesized ZIF-
8/titania nanocomposites
Degradation kinetics
Zero-order
First-order
Second-order
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Table 3. The kinetic constants of AB92 photocatalytic degradation by the synthesized ZIF-
8/titania nanocomposites
Zero-order
First-order
Second-order
C – C0 =-k0t
ln(C/C0) = -k1t
1/C – 1/C0 = k2t
Catalyst Dosage(g)
k0
R2
k1
R2
k2
R2
0.0050
0.00002
0.00060
0.00100
0.00150
0.00009
0.00070
0.00150
0.00190
0.00009
0.00060
0.00160
0.00190
0.8636
0.9689
0.9656
0.9798
0.8448
0.9447
0.9894
0.9597
0.8924
0.9830
0.9912
0.9660
0.00003
0.0010
0.0040
0.0070
0.0004
0.0020
0.0060
0.0140
0.0002
0.0020
0.0070
0.0110
0.8015
0.9589
0.9643
0.9775
0.8425
0.9074
0.9865
0.9376
0.8242
0.9819
0.9902
0.9564
0.00008
0.0042
0.0131
0.0428
0.0004
0.0055
0.0319
0.2571
0.0004
0.0045
0.0338
0.0920
0.8630
0.9502
0.9385
0.8729
0.8401
0.9371
0.9496
0.6610
0.8897
0.9815
0.9572
0.8826
0.0100
ZT-0.2
0.0200
0.0300
0.0050
0.0100
ZT-0.4
0.0200
0.0300
0.00500
0.0100
ZT-0.6
0.0200
0.0300
C0=initial dye concentration; C= dye concentration at any time; k0 = zero-order kinetic constant; k1
= first-order kinetic constant; k2 = second-order kinetic constant; R2= correlation coefficient
3.2.2.3. Effect of pH
The explanation of the solution pH effect on the yield of the pollutant photodegradation is a
difficult work due to the different feasible destruction mechanisms including reactive
hydroxyl radical attack, direct oxidation and reduction via the positive hole and electron in
the conducting band. ZIF-8 is stable over a wide pH ranges (4.0-12.0) [63]. In this research,
the pH of the AB92 solution was adjusted at 4, natural and 8 with HCl or NaOH to ascertain
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the efficacy of pH on the destruction of AB92 (Fig. 13). The results indicated that dye
degradation at natural pH had the highest percentage because more reactive species were
formed.
Fig. 13. The effect of solution pH on the photocatalytic dye degradation by the synthesized
nanocomposite (ZT-0.4).
3.2.2.4. Effect of dye concentration
The effect of initial concentration of AB92 on the photocatalytic activity was studied. The
different initial concentrations of dye (20, 30, 40 and 50 mg/L) with optimum photocatalyst
dosage was investigated (Fig. 14). It was found that the photodegradation percentage
decreased with increasing the initial concentration of the pollutant from 20 to 50 mg/L (20
mg/L: 96.3%; 30 mg/L: 67.7%; 40 mg/L:59.7%; and 50 mg/L:52.3%). The results are in
agreement with the previous published experimental results [64]. The degradation rate of
AB92 molecules depends on the generation of OH● radicals on the photocatalyst surface. As
the initial concentration of the dye was enhanced, pollutant molecules were adsorbed on the
surface of the photocatalyst. Then, the absorption of light by the surface of the photocatalyst
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decreases. When the active sites are surrounded by the contaminant molecules, the generation
of reactive radicals decreases [65].
Fig. 14. The effect of initial pollutant concentration on the photocatalytic dye degradation by
the synthesized nanocomposite (ZT-0.4).
3.2.3. Photocatalytic dye degradation mechanism
Fig. 15 shows that the photocatalytic degradation of dye by the catalyst is carried out by
conversion of photon energy to chemical energy. The valence band electron (e-) moves to the
conduction band and produces a valance band hole (h+). The generated electron-hole pair (e--
h+) is transferred to the surface of the catalyst and organic contaminant is destructed. Various
active species including hydroxyl radicals are produced and they destructed pollutants [66].
The metal-organic framework photocatalytic activity is due to the electron transfer from
HOMO (the highest occupied molecular orbital) to the LUMO (the lowest unoccupied
molecular orbital) by the irradiation [63,67].
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Fig. 15. The mechanism of photocatalytic degradation of AB92 by the synthesized ZIF-
8/titania nanocomposite.
3.2.4. Regeneration and stability of catalyst
Catalyst regeneration is one of the important factors for photocatalytic dye degradation
process. Herein, ethanol was used to regenerate the photocatalyst. A specific amount of
catalyst (ZT-0.4) was added to 100 mL of AB92 (20 mg/L) solution and photocatalytic dye
degradation experiments were done. After decolorization, it was rinsed with ethanol. Then,
the recovered catalyst was reused for another dye degradation in succeeding three cycles. Dye
destruction by the regenerated MOF for the first, second and third cycles was 94, 92, and
90%, respectively. The data presented that ZT-0.4 nanocomposite had suitable regeneration
ability.
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The stability of the catalyst was studied using XRD and TEM analyses. Fig. 16 shows
the XRD pattern of the catalyst (ZT-0.4) before and after dye degradation. The XRD patterns
did not show the significant change before and after photocatalytic process. It means that the
synthesized nanocomposite has high stability. Also, TEM images of nanocomposite before
and after pollutant destruction were obtained. The TEM images (Fig. 17) did not present
considerable change before and after removal process due to the catalyst stability.
Fig. 16. The XRD pattern of the catalyst (ZT-0.4) before (a) and after (b) dye degradation.
(a)
(b)
Fig. 17. The TEM image of the catalyst (ZT-0.4) before (a) and after (b) pollutant
destruction.
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Conclusion
In this paper, zeolitic imidazolate framework (ZIF-8) and its nanocomposites with different
Titania nanoparticle amounts (0.2, 0.4, and 0.6 g denoted as ZT-0.2, ZT-0.4 and ZT-0.6,
respectively) as photocatalysts for degrading dye under visible light were synthesized at room
temperature using a green synthesis procedure. Zinc nitrate and 2-methylimidazole were used
as a metal source and an organic linker, respectively. The materials were characterized by
XRD, FTIR, SEM, TEM, BET, TGA and zeta potential. Acid Blue 92 (AB92) was studied as
an organic pollutant. The results indicated that the ZT-0.4 nanocomposite was the best
photocatalyst for degrading dye because of the synergistic effect of ZIF-8 and titania. The
decolorization of AB92 followed zero-order kinetics. The removal rate for 0.005, 0.010,
0.020 and 0.030 g of catalyst (ZT-0.4) was 0.00009, 0.00070, 0.00150 and 0.00190 mg/L
min, respectively. The ZT-0.4 nanocomposite had high reusability and stability over three
cycles.
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