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Fe (III) 2, 9, 16, 23-phthalocyanine tetracarboxylic acid [FePc(COOH)4], synthesized via the reaction of trimellitic anhydride and urea at their melting points, was used to sensitize titanium dioxide by impregnation and adsorption. Ultraviolet-visible spectroscopy showed that the catalyst could effectively degrade photosensitive organic dyes, such as methylene blue, rhodamine B, neutral red, acid red, and malachite green. With an exposure time of 100 min, the degradation rate for all the dyes was >80%. In addition, in the case of rhodamine B, the FePc(COOH)4/TiO2 system showed the best degradation efficiency at pH=6. To investigate the relationship between degradation concentration and illumination time, first-order degradation kinetics equations were derived. Upon loading TiO2 with phthalocyanine, the photoactivity of TiO2 was extended to the visible light region, potentially enabling the use of solar energy for the photodegradation of organic dyes and other pollutants in wastewater.<
Keywords: Iron phthalocyanine; TiO2; Photocatalytic degradation; Organic dye; First-order reaction kinetics
Nanometer-sized semiconductor photocatalysts use light to degrade organic matter in water and air. Because of its high photocatalytic activity, strong corrosion resistance, stability, and environmentally benign nature, TiO2 has been widely studied for use in photocatalytic degradation [1-4]. However, TiO2 has a wide bandgap (of about 3.2 eV) and can only absorb ultraviolet (UV) wavelengths 2 can be modified to extend the light adsorption to the visible region. Doping with transition metals or non-metals [5,6], surface deposition of noble metals [7,8], and dye-sensitization  are some methods used to modify TiO2. Among these methods, dye-sensitization of TiO2, which involves the adsorption of a sensitizer dye on the TiO2 surface, is a simple and effective strategy. The dye molecules upon exposure to visible light are excited, and the electrons are injected into the conduction band of TiO2. These conduction band electrons react with oxygen adsorbed on the TiO2 surface and form superoxide anion radicals (O2 -), hydroxyl anion radicals, and other active species. These reactive species, in turn, can oxidize and degrade organic pollutants [10-12].
Among various sensitizers, phthalocyanine, an aromatic conjugated complex consisting of nitrogen and carbon atoms (eight each), is widely used. Phthalocyanine shows good absorption in the visible light region, and its molecular structure can be modified as desired. However, the applications of unsubstituted phthalocyanine are limited because of its insolubility in water and organic solvents. Sulfonic acid or carboxyl functional groups can be introduced into the benzene rings of the metal phthalocyanine to greatly improve their water solubility. What’s more, phthalocyanine produces extremely reactive singlet oxygen (1O2) [13,14], which can render the photocatalytic process nonselective. Recently, metal phthalocyanine-sensitized titanium dioxides have been applied for the degradation of organic dyes and other pollutants in wastewater, because of their good photocatalytic performance [15-21].
In this study, 1, 2, 4-benzenetricarboxylic anhydride, urea, and a metal salt were used to synthesize iron phthalocyanine substituted with four carboxyl groups, by a solid catalytic method. Then, the substituted phthalocyanine (FePc(COOH)4) was used to sensitize TiO2 by dipping adsorption. The phthalocyanine and sensitized TiO2 were characterized by ultraviolet (UV)-visible and fluorescence spectroscopy, nitrogen adsorption-desorption measurements, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and a series of other characterization techniques. The photocatalytic activity of the dye sensitized TiO2 was analyzed by using it for the degradation of methylene blue (MB), rhodamine B (RB), acid red (AR), neutral red (NR), and malachite green (MG) under visible light radiation. We also investigated the influence of the pH of the solution on the degradation. Finally, we elucidated the photodegradation mechanism using photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS).
Preparation of FePc(COOH)4 and FePc(COOH)4/TiO2
The synthesis of Fe (III) 2, 9, 16, 23-phthalocyanine tetracarboxylic acid (FePc(COOH)4).FeCl3⋅6H2O, urea, and trimellitic anhydride (molar ratio=0.25:20:1) were used as the raw materials to prepare FePc(COOH)4 by a solid fusion method, and the product was purified .
The catalyst was prepared by following the procedure described by Roman et al.  An aqueous suspension of TiO2 (1 g/dm, 300 cm3) was magnetically stirred for 10 min and ultrasonicated for 20 min. Then, FePc(COOH)4 solution (10 cm3, 5 × 10-3 mol/dm) in dimethyl sulfoxide (DMSO) was added to the TiO2 aqueous suspension under stirring, and the solution was mixed overnight. The particles were collected by centrifugation; washed thoroughly with distilled water, ethanol, and acetone; and dried in a vacuum oven at 60°C. The resulting green powder was then ground in an agate mortar and stored in the dark.
A halogen lamp (300 W) equipped with a cutoff filter at 400 nm was used as the irradiation source, and a Pyrex glass vessel (120 mL) was used as the reaction vessel. Before irradiation, the aqueous suspension (100 mL) containing 0.05 g of the catalyst and 100 mL of the organic substrate were stirred for 30 min. During the reaction, 2.0 mL of the suspension was withdrawn and separated by centrifugation. The absorbance at a specific wavelength was measured at 20 min intervals with a UV-visible spectrometer. FePc(COOH)4 was dissolved in DMSO to record the UV-visible spectra. The analysis of the XPS binding energies were calibrated against the standard C 1s peak (at 285 eV).
Characterization of FePc(COOH)4 and FePc(COOH)4/TiO2
The UV-visible spectrum of tetracarboxyphthalocyanine mainly originates from the π→π* transitions of the strong Q-band (600-800 nm) and the broad B-band (300-450 nm). Quantum chemistry shows that the formation of the Q and B-bands are attributed to the a1u(π) →eg(π*) and a2u(π)→eg(π*) electronic transitions [24,25]. FePc(COOH)4 exhibited strong Q-band and B-band adsorptions at λmax=673 nm and 331 nm, respectively (Figure S1). The FT-IR spectra of TiO2 (a), FePc(COOH)4 (b), and FePc(COOH)4/TiO2 (c) samples are shown in Figure S2. The characteristic amide absorption peak occurs at around 1704 cm-1 (spectrum b) . An absorption band at 400-800 cm-1 corresponds to the Ti-O bond stretching vibration (spectrum a) and the absorption bands at 1610 and 3000 cm-1 represent the vibrations of adsorbed water . Because of the strong background absorption of TiO2 and the small amount of FePc(COOH)4, characteristic FePc(COOH)4 absorption peaks were not observed in the FT-IR spectrum of the FePc(COOH)4/TiO2 sample at 400-800 cm-1. However, characteristic amide absorption peaks at 1700 cm-1 were observed (spectrum c), indicating that the FePc(COOH)4 molecules were adsorbed on the TiO2 surface.
The N2 adsorption-desorption isotherm and pore size distribution curve of FePc(COOH)4/TiO2 are shown in Figure S3. The Brunauer- Emmett-Teller (BET) surface areas and the mean pore size of the TiO2 sample were 58.36 m2/g and 10.0 nm, respectively. After sensitizing with FePc(COOH)4, the BET surface area was 53.35 m2/g and the pore size remained nearly unchanged. This demonstrates that FePc(COOH)4 had combined with TiO2 and occupied a certain fraction of the surface area. In addition, the large BET surface area could facilitate more efficient contact of TiO2 with the organic dye, improving its photocatalytic activity.
The XRD patterns of the various samples are shown in Figure 1. FePc(COOH)4, showed a broad weak peak at ~ 20° and a sharp strong peak at ~ 27° . The XRD pattern of FePc(COOH)4/TiO2 is essentially identical to that of TiO2. Therefore, the loading of the dye did not appear to change the crystalline structure or size of TiO2. We assume that the dye molecules did not enter the crystal lattice and that the dye molecules and TiO2 interact through van der Waals forces or hydrogen bonds.
The UV-visible diffuse reflectance spectra of the pure TiO2 nanoparticles, FePc(COOH)4, and FePc(COOH)4/TiO2 are shown in Figure 2. As observed in Figure 2 (spectrum a), the spectrum of the TiO2 nanoparticles exhibited absorption only in the UV region and absorption was absent in the visible wavelengths. FePc(COOH)4/TiO2 exhibited absorption bands at 650-750 nm, which could be attributed to the Q-band of FePc(COOH)4. Absorption bands at 650-750 nm were also observed in the spectrum of pure FePc(COOH)4 (spectrum b, Figure 2). Hence, the sensitization of TiO2 with FePc(COOH)4 could extend the absorbance spectrum of TiO2 into the visible region.
The photocatalysis efficiency is determined by the balance between the charge separation and recombination processes both in the bulk and on the surface. PL spectra, which directly arise from the radiative recombination processes of the electrons and holes of two different energy states, can be used to evaluate the charge recombination rate. Therefore, the PL spectra were obtained from TiO2, FePc(COOH)4, and FePc(COOH)4/TiO2 (Figure 3). The higher the PL emission intensity, the higher is the recombination rate of the photogenerated electrons and holes. The strength of the fluorescence intensity varies in the order TiO2>FePc(COOH)4/TiO2>FePc(COOH)4. This indicates that the energy transfer between TiO2 and FePc(COOH)4 reduces the fluorescence intensity, while improving the photocatalytic efficiency.
As seen in Figure 4, the full XPS scans (Figure 4A) demonstrate the presence of Ti, O, and C in pure TiO2, while Ti, Fe, O, N, and C were present in FePc(COOH)4/TiO2. The presence of C in TiO2 can be ascribed to adventitious carbon-based contaminants. The highresolution XPS profiles corresponding to the Ti 2p region at around 460 eV, the Fe 2p region around 720 eV, and O1s region around 530 eV were analyzed. As shown in Figure 5B, two peaks were present in the Ti 2p region. The peak at 464.2 eV corresponds to the Ti 2p3/2. The difference in the Ti 2p1/2 and Ti 2p3/2 peak positions is 5.7 eV, indicating the presence of Ti4+ in the as-prepared FePc(COOH)4/TiO2 [29,30]. Further, the Ti 2p peaks of FePc(COOH)4/TiO2 showed no shift in comparison to the peak shown by pure TiO2, confirming that the structure of TiO2 remained intact after the synthesis of FePc(COOH)4/ TiO2. In the Fe 2p region (Figures 4C and D), the peaks at 710.1 and 723.5 eV were attributed to Fe 2p3/2 and Fe 2p1/2, respectively, indicating the + 3 valency of Fe in FePc(COOH)4. As seen in Figure 4E, the O 1s peak of FePc(COOH)4 split into two peaks, with one centered at 531.2 eV (corresponding to the C-OH bond in the carboxyl group) and the second peak at 532.1 eV (corresponding to the C=O bond). After the sensitization of TiO2 with FePc(COOH)4, the O 1s binding energy was reduced from 531.2 eV to 529.9 eV (Figure 4F), demonstrating electronic coupling between FePc(COOH)4 and TiO2. This effect may be caused by the hydrogen bonds formed between the -COOH and -OH groups in FePc(COOH)4 and on TiO2 surface, respectively. The carboxyl group promotes the electronic coupling between the electron donating TiO2 and electron accepting FePc(COOH)4 . The XPS signals of N 1s in FePc(COOH)4 and FePc(COOH)4/TiO2 shown in Figures 4G and H indicate the presence of at least two forms of nitrogen with different chemical properties, i.e., the nitrogen in the 4-pyrrole ring and the 4-azacycle of FePc(COOH)4, respectively.
Figure 4: (A) Full scan XPS profiles of FePc(COOH)4/TiO2 and TiO2. (B) Ti 2p XPS profiles of FePc(COOH)4/TiO2 and TiO2. Fe 2p XPS profiles of (C) FePc(COOH)4 and (D) FePc(COOH)4/TiO2. O 1s XPS profiles of (E) FePc(COOH)4 and (F) FePc(COOH)4/TiO2. N 1s XPS profiles of (G) FePc(COOH)4 and (H) FePc(COOH)4/TiO2.
Photocatalytic degration activity
The photocatalytic degradation of RB was chosen as the model reaction to evaluate the photocatalytic activities of FePc(COOH)4/ TiO2. Pure TiO2 nanoparticles and FePc(COOH)4 powder were used as reference photocatalysts. The degradation efficiency of the asprepared samples was defined in terms of the C/C0 ratio, where C and C0 represent the remnant and initial concentrations of RB, respectively. As seen in Figure 5, under visible light irradiation, FePc(COOH)4/TiO2 was completely used in the photodegradation experiment within 100 min and a maximum RB degradation of 96.4% was achieved. In the case of pure TiO2 and FePc(COOH)4 (reference catalysts), the maximum degradations were only 24.7% and 26.8%, respectively. Hence, RB could be photodegraded efficiently when visible light was used in the presence of FePc(COOH)4/TiO2.
Figure 6 shows the effect of pH on the oxidative degradation of RB under visible light using FePc(COOH)4/TiO2 as the photocatalyst. Evidently, the degradation reaction can proceed in a wide pH range (2-12). Adjusting the pH of the initial solution containing RB (6.5 mg/L) with diluted sulfuric acid led to RB conversion rates of 44.8%, 60.2%, and 96.4% at pH=2, 4, and 6, respectively, within 100 min of irradiation. When the degradation reaction was carried out in sodium hydroxide solution, the conversion rates of RB in the same irradiation time were 75.2%, 65.4%, and 36.1% at pH=7, 9, and 10, respectively. Therefore, high rates of RB photodegradation with FePc(COOH)4/ TiO2 can be achieved under approximately neutral pH conditions.
To further examine the catalytic activity of FePc(COOH)4/TiO2, the degradations of MB, NR, AR, and MG were also investigated. As shown in Figure 7, the various organic dyes were photodegraded by the catalyst within 100 min. When the reaction time was 100 min, the RB, MB, NR, AR, and MG degradation rates were 96.9%, 96.8%, 82.1%, 85.2%, and 81.9%, respectively.
From Table 1 and Figure 8, the curve fitting indicates that the photodegradation reactions follow first-order kinetics. Therefore, FePc(COOH)4-sensitized TiO2 displays good photocatalytic degradation activity on the various kinds of dyes.
Table 1: Kinetics parameters of the degradation of various dyes.
During photocatalysis, TiO2 acts as the electronic medium and FePc(COOH)4 serves as the sensitizer. When irradiated with light of wavelength >400 nm, the phthalocyanine on the TiO2 surface is excited and generates 1O2 through energy transfer  (as shown in Figure 9). The excited state charges are transferred from the excited state of phthalocyanine in FePc(COOH)4 to the TiO2 conduction band by electron transfer, resulting in the continuous accumulation of the dye radical cation (dye·+) and electrons in the TiO2 conduction band (ecb -). In addition, the oxygen molecules and singlet molecular oxygen (1O2) dissolved in the solution react with the conduction band electrons (ecb -) to form the superoxide anion radical (O2·-), which is then protonated to generate peroxide radicals (HOO·), which on further reaction generates the strongly oxidizing hydroxyl radicals (OH·) capable of decomposing organic dyes. Phthalocyanine is a solid-state p-type semiconductor with a band gap of about 2.0 eV [33-36]. Since the redox potential of the dye·+ radical is 1.2 eV (vs. normal hydrogen electrode, NHE) [23,37], it can oxidize suitable substrates (R) and reduce dyes. Furthermore, TiO2 can aggregate the excited electrons from the surface of FePc(COOH)4, which are then transferred to TiO2. These electrons can promote the generation of active oxygen species. In addition to these active oxygen species, the byproduct radical cation FePc(COOH)4·+ can react with the substrate (R) to promote its degradation.
Our studies confirmed that FePc(COOH)4/TiO2 exhibits good photocatalytic degradation effect on organic dyes, such as MB, RB, NR, AR, and MG. The dye degradation kinetics further demonstrated that the dye solutions could effectively be degraded within 100 min. The photocatalytic degradation followed first-order kinetics. Generally, the industrial wastewater generated from papermaking and textile industries contains a mixture of various complex dyes instead of just a single dye. FePc(COOH)4/TiO2, which extends the optical absorption range of TiO2 from the UV region to the visible light region, can be effectively used to degrade such industrial wastes with sunlight, enabling environmentally sustainable waste processing practices.
We gratefully acknowledge financial support from the National Natural Science Foundation of China (Grants No. 21273202 and 21473162). Y. Zhao is grateful to the Project Grants 521 Talents Cultivation of the Zhejiang Sci-Tech University. This work is also supported by the Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology.