Fabrication of phthalocyanine-magnetic nanoparticles hybrid nanofibers for degradation of Orange-G

Journal of Molecular Catalysis A: Chemical 381 (2014) 132–137

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Fabrication of phthalocyanine-magnetic nanoparticles hybrid nanofibers for degradation of Orange-G Phillimon Modisha, Tebello Nyokong ∗ Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa

a r t i c l e

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Article history: Received 2 September 2013 Received in revised form 13 October 2013 Accepted 16 October 2013 Available online 25 October 2013 Keywords: Polyamide Electrospinning Zinc octacarboxy phthalocyanine Orange-G

a b s t r a c t Conjugates of zinc octacarboxy phthalocyanine (ZnOCPc) with magnetic nanoparticles (MNPs) were electrospun into fibers using polyamide-6 (PA-6) polymer. The ZnOCPc or ZnOCPc-MNPs conjugates on the fiber were employed for the photodegradation of an azo dye (Orange-G). The functionality of the ZnOCPc and ZnOCPc-MNPs was maintained within a solid fiber core. The singlet oxygen generation increases as we increase the fiber diameter by increasing the ZnOCPc concentration. The singlet oxygen quantum yield was higher for PA-6/ZnOCPc-MNPs nanofibers compared to PA-6/ZnOCPc. The rate of degradation of Orange-G increased with an increase in singlet oxygen quantum yield. Moreover, the kinetic analysis of the photodecomposition of Orange-G showed that its disappearance followed the Langmuir–Hinshelwood model. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Metallophthalocyanines (MPcs) are versatile functional molecules that have potential applications in sensors [1], nonlinear optics [2], as photosensitizers for photodynamic therapy (PDT) [3] and as photocatalysts [4]. Phthalocyanines have been shown to be highly effective photosensitizers capable of producing singlet oxygen with high quantum yields [5]. Singlet oxygen is the active species in photocatalysis. Pcs are particularly very promising sensitizers since their maximum light absorption occurs in the visible region which constitutes a large portion of the electromagnetic spectrum [6,7]. Phthalocyanines have been employed as homogeneous catalysts [8] or as heterogeneous catalysts either in the solid state or on solid support systems such as electrospun polymer nanofibers [9–12]. Immobilization of Pcs onto solid supports exhibits advantages over the corresponding homogeneous systems because of the possibility of catalyst recycling. Electrospinning is a promising technique for incorporation of functional molecules into solid polymer fiber supports. It is simple, convenient, reproducible, and a generally versatile technique for generating fibers. Magnetic nanoparticles (MNPs) have shown potential applications in various fields, especially in biomedicine and bioengineering [13–17]. MNPs increase the triplet quantum yield, consequently increasing the singlet oxygen quantum yield of phthalocyanines. This is due to enhanced intersystem crossing as a result of the heavy

∗ Corresponding author. Tel.: +27 46 603 8260; fax: +27 46 622 5109. E-mail address: [email protected] (T. Nyokong). 1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2013.10.012

atom effect [18]. Hence, in this work, the conjugate of a phthalocyanine with MNPs is employed for photocatalysis. We report on the electrospinning of the previously reported [18] conjugates of MNPs and zinc octacarboxy phthalocyanine (ZnOCPc), Fig. 1, in polyamide-6 (PA-6) polymer. The fibers containing the conjugates of ZnOCPc with MNPs are employed for the photodegradation of an azo dye (Orange-G). Experiments where ZnOCPc-MNPs conjugate was employed suspended in solution and not in electrospun fibers were also performed. In this case, the conjugates of MNPs with ZnOCPc may easily be separated with a magnet after use. Embedding ZnOCPc in fibers allows for its separation after use since it does not contain a magnet. The zinc octacarboxyphthalocyanine was chosen because it does not aggregate. Aggregates are photoinactive [19]. The photocatalytic degradation of OG has been reported in several studies [20,21], but not using the conjugate system reported in this work.

2. Experimental 2.1. Materials Polyamide-6 Ultramid® B32 grade (average molecular weight = 90,000 g/mol) was supplied by BASF. Anthracene9,10-bis-methylmalonate (ADMA), dimethylformamide (DMF), ethanol and toluene were purchased from Merck. Orange-G, acetic acid 98 vol% and formic acid 99.8 vol% were from Sigma Aldrich. Phosphate-buffered solutions (PBS) of pH 7.4 were prepared using appropriate amounts of Na2 HPO4 , KH2 PO4 and the respective chloride salts in ultra-pure water. The amino functionalized

Normalized Absorbance

P. Modisha, T. Nyokong / Journal of Molecular Catalysis A: Chemical 381 (2014) 132–137

Fig. 1. The possible structure of ZnOCPc-MNPs conjugate.

133

1.2

0.8

(iii)

0.6 0.4 0.2 0

Q-band

B-band

1

(ii) (i)

300

400

500

600

700

800

Wavelength (nm) magnetite nanoparticles (MNPs) [22] and ZnOCPc [23] were synthesized according to methods documented previously in the literature. 2.2. Equipment The morphology of the electrospun nanofibers was examined using a scanning electron microscope (JOEL JSM 840 scanning electron microscope) at an accelerating voltage of 20 kV. The average fiber diameter and their standard deviations were based on 50 measurements, using Cell D software from Olympus. Energy dispersive X-ray spectroscopy (EDX) was done on an INCA PENTA FET coupled to the VAGA TESCAM using 20 kV accelerating voltage. Irradiation for the singlet oxygen determinations was carried out using a Halogen lamp (300 W), 600 nm glass (Schott) and water filters, to filter off ultra-violet and far infrared radiation respectively. An interference filter (Intor, 670 nm with bandwidth of 40 nm) was placed in the light path just before the reaction vessel (which was a glass vial). The intensity of the light reaching the reaction vessel was measured with a power meter (POWER MAX 5100 Molelectron Detector Inc.) and found to be 1.3 × 1019 photons cm−2 s−1 . A similar photolysis set-up described above for singlet oxygen detection was used for the photodegradation experiments. The intensity of the light reaching the reaction vessel for the photodegradation studies was found to be 3.2 × 1020 photons cm−2 s−1 . A Perkin Elmer TGA 7 Thermogravimetric analyser was used to study the thermal properties of the compounds under an inert N2 atmosphere (at 20 mL min−1 ) and heating at a rate of 10 ◦ C min−1 . 2.3. Electrospinning methods A high voltage (in the range 15–25 kV) was applied to solutions of PA-6 alone or in the presence of ZnOCPc or ZnOCPc-MNPs. The solutions were held in a plastic syringe equipped with a needle. The flow rate was 0.6 mL/h for PA-6 alone, 1 mL/h for PA-6/ZnOCPc and PA-6/ZnOCPc-MNPs. Different concentrations of ZnOCPc: 0.1, 0.5 and 1 mmol were dissolved in PA-6 (B32 14 wt.%) polymer solutions in formic acid (FA)/acetic acid (AA) (75/25 vol.%). The fabrication of ZnOCPc-MNPs hybrid nanofibers was carried out by adding 30 mg of ZnOCPc-MNPs conjugate into PA-6 solution in FA/AA (50/50 vol.%), followed by electrospinning.

Fig. 2. UV–vis absorption spectra of (i) ZnOCPcs, (ii) MNPs and (iii) ZnOCPc-MNPs. Concentration ∼10−6 M.

intense and observed in the red region of the electromagnetic spectrum while the B band is weak and appears as an overlap of two bands between 300 and 400 nm [24]. The ground state electronic absorption spectra of ZnOCPc, Fig. 2(i), shows a monomeric behavior as evidenced by a single, narrow Q-band, typical of MOCPc [25]. The MNPs showed a broad absorption peak at 385 nm, Fig. 2(ii). The absorption spectrum of ZnOCPc-MNPs is broad at wavelengths less than 650 nm due to the presence of MNPs, Fig. 2(iii). 3.2. Characterization of electrospun fibers 3.2.1. SEM studies SEM image of PA-6/ZnOCPc fiber is shown in Fig. 3. A similar image was obtained for PA-6/ZnOCPc-MNPs fibers. There was an increase in fiber diameter with increased concentration of ZnOCPc, Table 1. Thus, fixing the polymer concentration and increasing the ZnOCPc concentration increases the fiber diameter. Fig. 4 shows the EDX spectrum of PA-6/ZnOCPc-MNPs nanofiber. The appearance of the elements associated with the ZnOCPc-MNPs conjugate on the EDX spectrum, confirms that the ZnOCPc-MNPs was successfully embedded in the PA-6 nanofibers. 3.2.2. Thermogravimetric analysis (TGA) The TGA thermograms in Fig. 5A show a comparison between functionalized and unfunctionalized PA-6 nanofibers. PA-6 (Fig. 5A, curve i) nanofibers decomposed completely i.e. no residuals left. Fig. 5A(ii) shows that the presence of ZnOCPc slightly increases the stability of the fiber, possibly due to hydrogen bonding between ZnOCPc and the fiber. It has been reported before that hydrogen bonding increases stability [26]. The degradation pattern for

3. Results and discussion 3.1. Characterization of ZnOCPc-MNPs conjugate The characterization of the conjugate has been described before [18]. The calculated crystal size from X-ray diffraction (XRD) was found to be 11 nm for the MNPs and did not change for ZnOCPcMNPs. As reported before the MNPs showed aggregation and this aggregation persisted upon linking to the ZnOCPc [18]. The spectra of phthalocyanines consist of the Q and B bands. The Q band is

Fig. 3. SEM images showing the nanofibers of PA-6/ZnOCPc.

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Table 1 Singlet oxygen quantum yields of the conjugates and Langmuir–Hinshelwood parameters for photocatalysis of Orange-G using unbuffered water. Catalyst

Fiber diameter

ZnOCPc-MNPs PA-6/ZnOCPc fiber (0.1 mmol)a PA-6/ZnOCPc fiber (0.5 mmol)a PA-6/ZnOCPc fiber (1 mmol)a PA-6/ZnOCPc-MNPs fiber a

111 nm 156 nm 240 nm 432 nm

k (mol L−1 min−1 )

˚

−7

3.1 × 10 4.3 × 10−6 4.9 × 10−6 4.8 × 10−6 6.8 × 10−6

0.31 0.16 0.21 0.24 0.37

KA (mol−1 L)

R2

7.30 × 10 1.10 × 104 1.10 × 104 1.39 × 104 1.12 × 105

0.9829 0.9333 0.9851 0.9874 0.9731

4

Numbers in brackets represent the concentration of ZnOCPc.

stability of PA-6/ZnOCPc-MNPs nanofibers is due to the thermally stable magnetite nanoparticles (MNPs), Fig. 5B(iii). This confirms that the thermal stability of PA-6 nanofibers can be enhanced by functionalization with the nanoparticles.

Fig. 4. EDX spectrum of PA-6/ZnOCPc-MNPs nanofibers.

PA-6/ZnOCPc is similar to that of ZnOCPc alone (Fig. 5B(i)) at temperatures greater than 450 ◦ C. The PA-6/ZnOCPc-MNPs nanofiber (iii) shows the first onset of decomposition below 300 ◦ C and the second at ∼450 ◦ C. The first degradation below 300 ◦ C was also observed for ZnOCPc-MNPs when not on the fiber, Fig. 5B(ii). The second degradation could be due to the PA-6 since the onset temperature is close to that of PA-6 alone. The increased thermal

3.2.3. Singlet oxygen generating ability of the functionalized fibers As stated already, singlet oxygen is involved in photocatalytic reactions. Hence it is important to determine the singlet oxygen generating ability of the modified fiber in the aqueous medium which is to be used for photocatalysis. The singlet oxygen quantum yield (˚ ) determinations for the ZnOCPc and ZnOCPc-MNPs in fibers were carried out in unbuffered aqueous media using ADMA as a quencher and its degradation was monitored at 380 nm, Fig. 6. In each case 15 mg of the modified fibers was suspended in an aqueous solution of ADMA and irradiated using the photolysis setup described above. The quantum yields (˚ADMA ) were calculated using Eq. (1) [27], using the molar extinction coefficient of ADMA in water, log(ε) = 4.1 [28]. (˚ADMA ) =

(C0 − Ct )VB Iabs · t

(1)

where C0 and Ct are the ADMA concentrations prior to and after irradiation, respectively; VR is the solution volume; t is the irradiation time per cycle and Iabs is defined by Eq. (2). Iabs =

˛·A·I NA

(2)

where, ˛ = 1 − 10−A() , A() is the absorbance of the sensitizer at the irradiation wavelength, A is the irradiated area (2.5 cm2 ), I is the intensity of light (1.3 × 1019 photons cm−2 s−1 ) and NA is the Avogadro’s constant. The absorbance used for Eq. (2) is that of the phthalocyanines in the fibers (not in solution) measured by placing the modified fiber directly on a glass slide. The light intensity measured refers to the light reaching the spectrophotometer cells, and

Fig. 5. TGA thermograms of (A) electrospun nanofibers of PA-6 (i), PA-6/ZnOCPc (ii) and PA-6/ZnOCPc-MNPs (iii) and (B) ZnOCPc (i), ZnOCPc-MNPs (ii) and MNPs (iii).

Fig. 6. UV–vis spectral changes observed upon photolysis of 15 mg of PA-6/ZnOCPcMNPs nanofibers in the presence of ADMA in unbuffered water for 30 min of photolysis. Starting ADMA concentration = 4.9 × 10−5 mol dm−3 , irradiation interval = 5 min.

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135

0.4

Absorbance

0 Orange-G photodegradation

0.3 0.2 0.1

30 min 0 300

400

500

600

700

800

Wavelength (nm) Fig. 7. Electronic absorption spectral changes of 0.44 × 10−4 mol L−1 Orange G during visible light photocatalysis using 15 mg of PA-6/ZnOCPc-MNPs nanofibers. The experiments were done using unbuffered water and the spectra were recorded at 5 min intervals.

it is expected that some of the light may be scattered, hence the ˚ values of the phthalocyanines in the fiber are estimates. The singlet oxygen quantum yields (˚ ) were calculated using Eq. (3) [29]. 1 1 k 1 1 = + · d · ˚ ˚ ka [ADMA] ˚ADMA

(3)

where kd is the decay constant of singlet oxygen and ka is the rate constant for the reaction of ADMA with 1 O2 (1 g ). The intercept obtained from the plot of 1/˚ADMA versus 1/[ADMA] gives 1/˚ . The singlet oxygen quantum yield (˚ ) of the PA-6 fibers containing different amounts (hence different sizes) of ZnOCPc were 0.16 for the 111 nm, 0.21 for 156 nm, 0.24 for 240 nm, Table 1. For PA-6/ZnOCPc-MNPs fibers the ˚ value is 0.37. The ˚ value for ZnOCPc alone (not linked to fiber) in water is 0.21 [30] and increases to 0.31 for ZnOCPc-MNPs in solution (not linked to fiber), Table 1.

3.3. Photodegradation of Orange G (OG) 3.3.1. Spectroscopic characterization Fig. 7 shows the absorption spectral changes observed during the photolysis of Orange-G at 5 min intervals using PA-6/ZnOCPcMNPs nanofibers as a photocatalyst. The OG absorption peak at 476 nm decreased in intensity in the presence of PA-6/ZnOCPcMNPs. Similar spectral changes were observed for PA-6/ZnOCPc without MNPs. The reaction occurring during OG photolysis includes: aryl group hydroxylation, desulphonation and oxidative cleavage of an azo group [31]. The absence of the Q-band of the phthalocyanine indicates that there was no leaching of the Pc into the water. All experiments were performed in unbuffered water. In the absence of light irradiation and oxygen (viz. nitrogen purged solutions), no degradation of Orange-G was observed. Unmodified fiber on its own showed no activity toward the degradation of OG. This also confirms that the ZnOCPc is embedded in fiber hence catalytic activity is observed. When the ZnOCPc-MNPs or ZnOCPc were employed in solution (not on a polymer fiber), a decrease in OG peak was observed, though in addition peaks due to the Pc were observed, with the B band interfering with the OG peak. In solution, the catalyst (ZnOCPc-MNPs) could be reused following recovery with a magnet and rinsing with deionized water and methanol. The spectra of the ZnOCPc-MNPs remained unchanged after use. The used fiber could simply be removed from solution following photolysis. The use of fiber is important for ZnOCPc without MNPs, and also adds an additional separation route for ZnOCPc-MNPs.

Fig. 8. First order kinetics plots for degradation of OG; (i) 1.33 × 10−4 , (ii) 2.21 × 10−4 and (iii) 2.65 × 10−4 mol L−1 using PA-6/ZnOCPc-MNPs nanofiber as a catalyst.

3.3.2. Kinetics studies for the photodegradation of Orange-G (OG) The plots of OG concentration versus irradiation time are shown in Fig. 8 and the kinetic data is listed in Table 2. The linearity of the plots obtained from ln(C0 /C) versus time indicates that this reaction follows the first order kinetics. The rate of photodegradation of OG increased with an increase in OG concentration, where the rate is faster for PA-6/ZnOCPcMNPs nanofibers compared to PA-6/ZnOCPc nanofibers, Fig. 9. This is due to higher singlet oxygen quantum yield for PA-6/ZnOCPcMNPs nanofibers, Table 1. The reduced half-lives for OG oxidation on ZnOCPc-MNPs nanofiber compared to ZnOCPc nanofiber Table 2, shows that the MNPs in the former improve the efficiency of OG degradation due to increased singlet oxygen quantum yields, Table 1. The rate for OG degradation is slower when ZnOCPc or ZnOCPc-MNPs are employed in solution (not embedded in fiber), Table 2, although the amounts of the Pc in solution and fiber are different. Langmuir–Hinshelwood rate expression has been used for heterogeneous photocatalytic degradation to determine the relationship between the initial degradation rate and the initial concentration of the organic substrate. Since the catalyst exist in a solid form, Langmuir–Hinshelwood kinetic model is employed, Eq. (4) [32]. 1 1 1 1 = + r0 kKA C0 k

(4)

where r0 is the initial photocatalytic degradation rate (mol L−1 min−1 ), C0 is the initial concentration of OG, k is the apparent reaction rate constant (mol L−1 min−1 ) and KA is the adsorption coefficient. In this reaction, 1/r0 was plotted against 1/C0 . A linear fit with a non-zero intercept was obtained showing

Fig. 9. The rate of photodegradation of OG versus initial concentration. Using PA6/ZnOCPc-MNPs (i) nanofiber and PA-6/ZnOCPc (ii) as catalysts in water.

-5

10 1/r0 (mol.-1.min-1)

17 22 30 43 50 20 24 32 46 53 22 27 35 53 61

FD 156 nm FD 111 nm

y = 0.2629x + 2.3323 R² = 0.9731

6 5

(i)

4 3

y = 0.1311x + 1.4761 R² = 0.9333

2 1 0 5

10

15

1.80 2.82 3.10 3.54 3.71 1.36 2.29 2.66 2.87 2.99 0.041 0.032 0.023 0.016 0.014

25

that the photodegradation of OG obeys the Langmuir–Hinshelwood kinetics model, Fig. 10. The Langmuir–Hinshelwood kinetics data is listed in Table 1. The value of k was obtained from the y-intercept, while KA was obtained from the slope of the line. k = 4.3 × 10−6 mol L−1 min−1 and KA = 1.10 × 104 mol−1 L were obtained for PA-6/ZnOCPc nanofiber (111 nm), Table 1. There was a general increase in k and KA with increase in size. But there is no change in KA on going from 111 nm to 156 nm or k from 156 nm to 240 nm. k and KA for PA-6/ZnOCPcMNPs nanofiber were estimated to be 6.8 × 10−6 mol L−1 min−1 and 1.12 × 105 mol−1 L. KA was estimated at 3.1 × 10−7 mol L−1 min−1 and k at 7.3 × 104 mol−1 L for ZnOCPc-MNPs in solution for the degradation of OG, Table 1, showing better activity on the fiber than in solution. KA , the adsorption coefficient is higher for PA6/ZnOCPc-MNPs nanofiber, suggesting that adsorption was more favored when PA-6/ZnOCPc-MNPs nanofiber is used. The results in Fig. 10 also show that Langmuir–Hinshelwood kinetic model is a relevant model in describing the kinetics for the photodegradation following heterogeneous catalytic system based on the fiber supported phthalocyanine complexes.

2.16 (0.23) 3.91 (0.272) 4.31 (0.279) 4.44 (0.287) 4.64 (0.291) 0.035 0.029 0.022 0.015 0.013

4. Conclusions

a

FD: fiber diameter in nanometers.

0.031 0.026 0.020 0.013 0.011 0.0490 (0.0053) 0.0444 (0.0031) 0.0324 (0.0021) 0.0201 (0.0013) 0.0175 (0.0011) 0.44 0.88 1.33 2.21 2.65

FD 156 nm

20 3

Fig. 10. Plot of the reciprocal of initial reaction rate versus the reciprocal of the initial concentration for photodegradation of OG. Using (i) PA-6/ZnOCPc-MNPs nanofiber and (ii) PA-6/ZnOCPc as catalysts in water.

1.54 2.55 2.93 3.32 3.45

FD 111 nm FD 240 nm

FD 156 nm

FD 240 nm

14 (131) 15 (224) 21 (330) 35 (553) 39 (630)

1/C0 (mol.L ) 10

FD 111 nm

PA-6/ZnOCPc fiber PA-6/ZnOCPc-MNPs fiber

(ii)

7

-1 -1

PA-6/ZnOCPc-MNPs fiber

PA-6/ZnOCPc fiber

a

Initial rate (×10−6 mol L−1 min−1 )

a

kobs (min−1 ) [OG] (×10−4 mol L−1 )

8

0

PA-6/ZnOCPc-MNPs fiber

Half life (min)

PA-6/ZnOCPc fibera

FD 240 nm

P. Modisha, T. Nyokong / Journal of Molecular Catalysis A: Chemical 381 (2014) 132–137 Table 2 The rate constant (kobs ), initial rate and half-life (t1/2 ) of various initial concentrations of OG using PA-6/ZnOCPc-MNPs or PA-6/ZnOCPc nanofibers. Values in brackets are for ZnOCPc-MNPs in solution (not embedded in fiber). All studies in water.

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The conjugates of zinc octacarboxy phthalocyanine (ZnOCPc) with magnetic nanoparticles (MNPs) were electrospun into fibers using polyamide-6 (PA-6). The functionality of the ZnOCPc and ZnOCPc-MNPs was maintained within a solid fiber core. Good singlet oxygen quantum yields were obtained within the fiber. Orange-G was easily degraded by these fabric photocatalysts and the rate was high for PA-6/ZnOCPc-MNPs fiber due to its improved singlet oxygen quantum yield. The rate also increased with the size of the ZnOCPc-MNPs fibers. The photodegradation of Orange-G is in agreement with both first order kinetics and Langmuir–Hinshewood kinetics. We have found that MNPs induces the adsorption of Orange-G onto the catalyst surface, as shown by an increased adsorption coefficient (KA ) when PA-6/ZnOCPc-MNPs fiber is used. The results in this work have also proven that, electrospun polymer fibers may be used for real life applications such as, water purification by either filtration or photocatalysis (when using functionalized fibers). Acknowledgements This work has been supported and funded by the Department of Science and Technology (DST) and National Research Foundation

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