Enhancement of Photocatalytic Degradation of Polyvinyl Chloride Plastic with Fe2O3 Modified AgNbO3 Photocatalyst under Visible-light Irradiation①

CHANG Hi-Bo LIU Jun-BoDONG Zheng WANG Dn-Dn XIN Yu JIANG Zhuo-Lin TANG Shn-Shn

(College of Resource and Enνironmental Science, Jilin Agricultural Uniνersity, Changchun 130118, China)

ABSTRACT The solid-phase photocatalytic degradation of polyvinyl chloride (PVC) plastic with AgNbO3/Fe2O3 is studied under visible-light irradiation. The PVC-(AgNbO3/Fe2O3) samples are characterized by X-ray photoelectron spectroscope (XPS), scanning electron microscope (SEM), gas chromatography (GC), and UV-vis diffusion reflectance spectra (UV-vis DRS). The photocatalytic properties of PVC-(AgNbO3/Fe2O3) samples are systematically investigated. More amounts of generated CO2, greater texture change and higher weight loss rate were exhibited in the system of PVC-(AgNbO3/Fe2O3) than pure PVC film. The weight loss rate is ten times higher than that of pure PVC film, which reaches to 46.53% with optimum amount of 0.5 wt% Fe2O3. Active radicals generate primarily on the surface of Fe2O3 particles, which cause composite plastic decomposition on the PVC-(AgNbO3/Fe2O3) interface and extend into polymer interor. The study provides a new promising way to degrade the plastic waste under visible-light.

Keywords: photocatalytic degradation, polyvinyl chloride, AgNbO3/Fe2O3;

1 INTRODUCTION

PVC plastic materials have been extensively used in various fields. They are hardly biodegraded in natural environment owing to their chemical stability. They are usually incinerated with the production of quantities of dioxins. The high and diverse toxicities cause the most concerning environment and health problems[1-3]. Actually,the field of PVC degradation has received increasing attention due to their long degradation cycle and incompleteness, which limits their application in industry and agriculture. Various methods have been made aiming at reducing the pollution. Photocatalytic technologies have been successfully utilized on developing environment friendly system by oxidizing organic pollutants. Many researchers have reported that PVC and polyethylene plastic can be degraded over photocatalysts TiO2in the open air to improve the environment[4,5]. However, a large intrinsic band gap of TiO2is 3.2 eV allowing only a small portion of solar spectrum in UV light region to be absorbed[6,7].

Silver niobate (AgNbO3) with perovskite structure is a multifunctional semiconductor material, which has been applied in microwave communication, microelectronics technology and photocatalysis[8-10]. The band gap of AgNbO3is 2.8 eV, and this higher-energy valence band derives from the filled Ag 4dorbitals, making the valence bands at more negative levels than the O 2porbitals and maintain a conduction band level high enough to reduce water and possess the ability to form H2and O2[11]. Many effective methods have been carried out to modify the AgNbO3to enhance their photocatalytic activity. Yang et al. indicated that when Ag was doped on the surface of AgNbO3, their photocatalytic activity can be remarkably promoted in comparison with that of pure AgNbO3[12]. Sun et al. studied NiO nanoparticle formed on the surface of AgNbO3, which improved the optical absorption and photocatalytic activity in the visible region[13]. Fe2O3with narrow band gap (2.2 eV) is also a semiconductor photocatalytic material. Researchers found that the Fe2O3-doped TiO2effectively enhanced the photocatalytic property[14,15]. Lin reported that hematite and maghemite were used to activate H2O2to produce hydroxyl radicals (·OH) decomposing organic pollutants to water,carbon dioxide and inorganic salts[16]. While, the photocatalytic properties of AgNbO3are mainly focused on the degradation of dye and water splitting[17-19]. The Fe2O3modified AgNbO3photocatalyst used in the PVC film is rarely reported.

In this article, a new kind of photocatalytic decomposition PVC-(AgNbO3/Fe2O3) was prepared and their photocatalytic degradation characters were investigated under visible-light irradiation. The degradation product was detected and the possible mechanism was discussed.

2 EXPERIMENTAL

2. 1 Materials

PVC, tetrahydrofuran (THF), ferric sulfate[Fe2(SO4)3·9H2O] and cetylpyridinium chloride were afforded by Sinopharm Chemical Reagent Co., Ltd. The average molecular weight of PVC was about 100,000.Deionized water was obtained from central of prepared water.

2. 2 Preparation of AgNbO3 and AgNbO3/Fe2O3 photocatalysts

In a typical procedure, 14 mL NaOH (0.6 mol·L-1) solution was slowly dropped into 3.75～26.25 mL Fe2(SO4)3(0.1 mol·L-1) solution under vigorous stirring. And then 0.20 g cetylpyridinium chloride and 0.60 g AgNbO3were added to the above solution (The synthetic procedure of original AgNbO3were described in our previous paper[20,21]). The mixtures were stirred continuously for 4 h. The final mixture was putted into a Teflon-lined stainless-steel autoclave and heated at 180 °C for 2 h under autogenous pressure. The collected crystals were centrifuged for several times with distilled water, and dried at 60 °C. The brick red AgNbO3/Fe2O3photocatalyst was obtained containing 0.1 wt%, 0.3 wt%, 0.5 wt% and 0.7wt% Fe2O3, respectively.

2. 3 Preparation of PVC-AgNbO3 and PVC-(AgNbO3/Fe2O3) composite films

1.0 g PVC particles were dissolved in 20 mL THF under stirring for 4 h and ultrasonic vibration for 1 h. And then 0.03 g AgNbO3or AgNbO3/Fe2O3was added into PVC solution with the ratio of AgNbO3or AgNbO3/Fe2O3to PVC 3 wt%.The above solution was sequentially stirred for 1 h. The films were achieved on the spin-coater machine. The viscous solution was spread on a tile surface, which was cleaned in ultrasonic deionized water and acetone bath to remove impurities. And then the films were kept in dark place for 72 h at room temperature. The pure PVC film was prepared by a similar procedure only without adding AgNbO3or AgNbO3/Fe2O3.

2. 4 Photodegradation and characterization of the PVCAgNbO3 and PVC-(AgNbO3/Fe2O3) composite films

All films were washed several times by deionized water in order to remove the traces of THF solvent. Every piece of composite film was cut into the dimensions with 4cm × 4cm.All the films were measured to be 20 μm by a micrometer.The photocatalytic degradation was performed with 350 W xenon lamp as a visible-light source, which was positioned 15 cm away from the films in the ambient air. The UV-cutofffilter was equipped providing visible light with ≥400 nm.The average light intensity striking the surface of the composite films was about 80 mW·cm-2.

The computation formula of the weight loss was as follows:

Among them,C(%) was the weight loss of PVC composite films, andm0(g) andmt(g) were the quality of PVC composite films before and after photocatalytic degradation, respectively.

The data of particles and films were obtained by the powder X-ray diffraction (XRD, Rigaku D/Max 2500 V/PC,Tokyo, Japan) using CuKaradiation (λ= 1.5418 Å) in the 2θrange of 20°～80°. Elemental composition and surface chemistry of samples were collected by X-ray photoelectron spectra (XPS, Tokyo, Japan) with a PHI 1600 spectroscope and MgKaX-ray source for excitation. The concentration of generated CO2in the process of degradation was detected by GC equipped with thermal conductivity detector (TCD) using GDX-43 steel column. The surface morphologies of composite films were performed by a scanning electronic microscope (SEM, JEOL JSM-7001F). UV-vis diffuse reflectance spectra were recorded on a UV-vis spectrophotometer (DRS, Hitachi U-4100 Japan) in order to obtain the absorption spectra of films. The 350 W xenon lampas (BL-GHX-V, Shanghai Bilang Instrument Co. Ltd.)as a visible-light source was used.

3 RESULTS AND DISCUSSION

3. 1 XRD analysis of AgNbO3/Fe2O3 samples

The XRD patterns of AgNbO3/Fe2O3are shown in Fig. 1.The peak shape of AgNbO3is sharp with high intensity,indicating its wonderful crystallinity, which is completely consistent with the standard spectrogram (JCPDS No. 52-0405) (Fig. 1a). The Fe2O3nanoparticles show sharp diffraction peak, high crystallinity and high purity as shown in Fig. 1b (JCPDS No. 33-0664). According to the following degradation experiment, the AgNbO3/Fe2O3samples prepared with 0.5 wt% Fe2O3are chosen for characterization. Fig. 1c illustrates that Fe2O3nanoparticles are successfully loaded on the surface of AgNbO3. The diffraction peaks of Fe2O3locate at 2θare 33.1°, 35.6°, 54.1° and 63.9° corresponding to crystal indices of (104), (110), (116) and (300), respectively,which are marked with * in Fig. 1c. The other diffraction peaks are assigned to AgNbO3.

Fig. 1. Patterns of AgNbO3 (a), Fe2O3 (b) and AgNbO3/Fe2O3 (c)

3. 2 XPS analysis of PVC-(AgNbO3/Fe2O3) samples

To gain an insight into the existence states of AgNbO3/Fe2O3in the film, the XPS spectra are conducted and displayed in Fig. 2. The survey spectrum indicates that the composite film includes Ag, Nb, Fe, O, C and Cl elements. The peak of C 1sat 280 eV owns to adventitious hydrocarbon from XPS instrument. Moreover, the peak at the binding energy of 199.6 eV attributes to C-Cl chemical composite of composite film (Fig. 2a). The XPS spectrum of Ag is shown in Fig. 2b. Two peaks at 373.9 and 367.8 eV are assigned to Ag 3d3/2and 3d5/2, respectively. The peaks at 367.8 and 373.9 eV are associated with the binding energies of Ag+inside AgNbO3, as reported in the earlier literatures[22].Moreover, the XPS spectrum of Fe exhibits two peaks at 711.3 and 725.1 eV related to Fe 2p3/2and 2p1/2, respectively(Fig. 2c). The result is consistent with the previous report for pure hematite power and films[23]. The energy difference is 14 eV between Fe 2p3/2and Fe 2p1/2peaks, which is characteristic of Fe3+state, demonstrating the formation ofα-Fe2O3by experimental methodology that Souza et al. have reported[24]. Meanwhile, it is clearly seen in the spectrum that the Fe3+satellite peak is centered at 718 eV, above the Fe 2p3/2peak. The doublet spin orbits of Fe 2p3/2and Fe 2p1/2indicate successful loading Fe2O3on the surface of AgNbO3.In Fig. 2d, there is one component of O1s, corresponding to 530.3 eV. The intense peak corresponds to the oxygen bonded asα-Fe2O3.

Fig. 2. Survey scan XPS spectra of PVC-(AgNbO3/Fe2O3) samples (a) and details of the Ag 3d (b), Fe 2p (c) and O 1s (d)

3. 3 Photocatalytic degradation of PVC-AgNbO3 and PVC-(AgNbO3/Fe2O3) samples

In order to reveal the photocatalytic degradation behavior and mechanism of different PVC films, the photodegradation reaction is carried out under xenon lamp at room temperature.The photocatalytic activities of these films are assessed directly by weight loss rate in Fig. 3. After irradiation for 120 min, the weight loss rate for pure PVC film is only 4.09%with pretty low photocatalytic activity, and is less than 18%for PVC-(AgNbO3). While, the weight loss rates of PVC-(AgNbO3/Fe2O3) with different amounts of Fe2O3doping are estimated to be from 20.36% to 46.53%. They are remarkably increased with raising the amount of Fe2O3. The highest value of weight loss rate is 46.53% with optimum amount of 0.5 wt% Fe2O3, displaying the Fe2O3particles play an important role in promoting the photocatalytic degradation of PVC films. The result illustrates all the active species can be introduced to polymer and exhibit an optimal surface concentration at submonolayer coverage, when both AgNbO3and Fe2O3particles enter into the surface of PVC polymer.Meanwhile, the weight loss rate decreases to 43.81% at the amount of 0.7 wt% Fe2O3due to the excess Fe2O3loaded on the active centre of AgNbO3, thus reducing the reactive group in the solution and decreasing the photocatalytic performance. The above weight loss data reveal that the photocatalytic reaction of PVC-(AgNbO3/Fe2O3) occurres and might produce a mass of volatile products, such as carbon dioxide, ethene, propane and so on.

Fig. 3. Weight loss of pure PVC, PVC-AgNbO3 and PVC-(AgNbO3/Fe2O3) samples with visible-light irradiation

The formation of CO2in weight loss of films is evaluated by GC and shown in Fig. 4. After visible-light irradiation for 100 min, the toatal amount of CO2produced during photodegradation process accounts for 90% and 95% of the weight loss of PVC-AgNbO3and PVC-(AgNbO3/Fe2O3),respectively. Pure PVC was only slightly decomposed. CO2is the major product of the photocatalytic degradation in the degradation precess of PVC plastic and it is environmentally friendly. There are two types of degradation in the whole process, which are photolytic degradation and photocatalytic degradation for PVC-AgNbO3and PVC-(AgNbO3/Fe2O3)samples. However, CO2was produced only from photolytic degradation for pure PVC film.

Fig. 4. Concentration of CO2 for pure PVC, PVC-AgNbO3 and PVC-(AgNbO3/Fe2O3) samples with visible-light irradiation

Fig. 5 shows the texture of different samples before and after visible-light irradation. As shown in Fig. 5a, the surface of pure PVC film is smooth. However, some irregular particles with heterogeneous size are stacked closely on surfaces of PVC-AgNbO3and PVC-(AgNbO3/Fe2O3),respecterly (Fig. 5c, 5e). PVC-(AgNbO3/Fe2O3) with 0.5%Fe2O3is elected as an example. After irradiation for 120 min,samples noticeably changed and cavities formed with holes.There are some holes with 1～3 μm on surface of pure PVC film (Fig. 5d). In contrast to pure PVC film, the dense network structures with the sizes of 5～10 and 20～30 μm form on the surface of PVC-AgNbO3and PVC-(AgNbO3/Fe2O3),respectively (Fig. 5f, 5e), which illustrate that the photodegradation of PVC primarily happens on film surface.The cavities are formed around AgNbO3and AgNbO3/Fe2O3particles because potocatalytic reaction firstly occures at the interface among PVC, AgNbO3and AgNbO3/Fe2O3. The photocatalytic reaction led to band scission of PVC-AgNbO3or PVC-(AgNbO3/Fe2O3).

Fig. 5. SEM images of different films before and after irradiation for 120 min. Pure PVC films before (a) and after (b) irradiation,PVC-AgNbO3 before (c) and after (d) irradiation, PVC-(AgNbO3/Fe2O3) before (e) and after (f) irradiation

The FT-IR spectra of PVC-(AgNbO3/Fe2O3) films before and after visible-light irradiationfor 120 min are shown in Fig. 6. The spectra of the composite films show the characteristic absorption of long alkyl chain in the region of 2927～2858 cm-1. The intense sharp peak at about 908 cm-1results from C-Cl stretchingvibration of PVC. After irradiationfor 120 min, a new absorption peak present at 1724 cm-1would be assigned to C=O stretching vibrations[25].Visible-light irradiation constantly induces the composites film to dehydrochlorination and form double bonds.

Fig. 6. FT-IR spectra of PVC-(AgNbO3/Fe2O3) films before (a) and after visible-light irradiation (b)

3. 4 Effect of the addition of the amount of Fe2O3

Photodegradation efficiency of PVC composite films is related to the amount of Fe2O3. The weight loss rates are strongly dependent on the mass ratio of Fe2O3from 0.1 to 0.7 wt%. Dhananjeyan et al reported that PE film was degraded via Fe2O3doped TiO2under UV irradiation. The photoresponse could be enhanced by doping Fe2O3into TiO2attributed to the separation of photogenerated charge. The result reveals that phtodegradation efficency can be promoted by increasing the amount of Fe2O3[26]. The surface morphologies of PVC-(AgNbO3/Fe2O3) samples after visiblelight irradiation for 120 min are shown in Fig. 7. All the samples are obviously decomposed. There are some cavities around the AgNbO3/Fe2O3particles in composite films. The cavity sizes are from 10 to 30 μm with changing the amount of Fe2O3from 0.1 to 0.5 wt% (Fig. 7a～7c). SEM images illustrate that the degradation of films starts from interface and leads to the formation of cavities around the photocatalyst. While, the cavity sizes diminish when the amount of Fe2O3increases to 0.7 wt% (Fig. 7d). The result corresponds with previous studies in section 3. 3 (Fig. 3),which demonstrates the Fe2O3coverage on the surface of AgNbO3would significently affect the photocatalytic activity.The Fe2O3could improve the efficiency of charge separation and inhibit the recombination of electrons and holes. But excess Fe2O3decreases the photodegridation efficiency of PVC composite films.

Fig. 7. SEM images of variations amount of Fe2O3 in PVC-(AgNbO3/Fe2O3) samples after visible-lightirradiation for 120 min. (a) 0.1 wt%, (b) 0.3 wt%, (c) 0.5 wt%, (d) 0.7 wt%

3. 5 Spectroscopic characterization

The optical properties of different photocatalysts are evaluated by UV-vis absorption spectroscopy. As displayed in Fig. 8a, two samples with AgNbO3present broad and obvious absorption in visible light region after modification.The absorption range of AgNbO3/Fe2O3is remarkably broader than that of AgNbO3sample. The red shift of absorption threshold is up to around 580 nm attributed to the effect of Fe2O3. In the diffuse reflection mode, a part of incident light is absorbed by sample, which reduces the intensity of reflection light. Therefore, the baseline absorption is enhanced[27]. In addition, the optical band gapEgof a semiconductor is deduced using the following equation: (Ahν)2=hν-Eg, where,A,h,νandEgrepresent the absorption coefficient, the Planck's constant, the incident photon frequency and the band gap, respectively. The band gap values are determined by the curves extrapolations, as shown in Fig. 8b. TheEgvalue of AgNbO3sample in this study is 2.9 eV. The direct transition of AgNbO3/Fe2O3sample is at 2.5 eV corresponding to the O2-2p→ Fe3+3dcharge transfer[28,29]. These results might be conducive to improve the photocatalytic activity of the AgNbO3/Fe2O3sample.

Fig. 8. UV-vis absorption spectra of differient samples: (a): AgNbO3, (b): AgNbO3/Fe2O3 (containing 0.5% Fe2O3)

3. 6 Photocatalytic degradation mechanism of PVC-(AgNbO3/Fe2O3) samples

The photocatalytic degradation of pure PVC film has been extensively researched. Several researchers have reported that the photodegradation process of pure PVC film included two aspects[30]. One step is the chain dehydrochlorination reaction, which could happen in both aerobic and anaerobic conditions. The C-Cl removes chlorine free radicals to form a double bond in the main chain. The PVC chain oxidation is the second step. The hydrogen atoms on the neighboring PVC polymer chains are attacked by peroxy radicals to form hydrogen peroxide. The successive reactions are carried out in the polymer chain cleavage when the carbon-cantered radicals are introduced in the polymer chains[31].

Band gap energies of AgNbO3and Fe2O3particles are 2.86 and 2.21 eV, respectively. Their light absorption threshold wavelengths are calculated as 442 and 564 nm by formulaλ=1240/Eg. The electrons are excited from the valence bands of AgNbO3and Fe2O3to their conduction bands, and holes are left in the valence band, respectively. The photoexcited electrons migrate from the conduction band of AgNbO3(CB= -0.81 eV) to the conduction band of Fe2O3(CB = 0.1 eV)due to the slightly lower level of conduction band of Fe2O3,enhancing the charge separation efficiency. The level of the conduction band of Fe2O3would not reduce the O2to ·O2-(O2/·O2-= -0.33 eV). Therefore, H2O2could be generated by direct reduction of O2by e-[32,33]. The induced electrons in the CB of Fe2O3are captured by H2O2to generate ·OH radicals,which are important active radicals for the degradation of PVC films. Meanwhile, the holes in the valence band of Fe3O2(VB = 2.31 eV) transfer to the surface of AgNbO3(VB= 2.05 eV). The transferred holes will cause the oxidation H2O or OH-to produce highly active ·OH radicals with higher redox power. Therefore, the electrons in CB and the holes with strong oxidation potential in the VB are separated effectively under visible-light irradiation. Subsequently, the PVC films could be oxidized by ·OH and generated a series of intermediates. The intermediates are oxidized to CO2and volatile substances by the aid of reactive oxygen species as Eqs. (1～10)[34,35].

In addition, some photo electrons originating from Fe2O3are immediately picked up by Fe3+, which causes the oxidation of Fe2+to Fe3+. However, Fe2+is not stable and is rapidly oxidized to Fe3+by dissolved oxygen, thus leading to inner recycles[36]. The photocatalytic reaction process of composite film under visible-light irradiation is shown in Fig.9. Appropriate amount of Fe3O2could improve the efficiency of charge separation and inhibit the recombination of electrons and holes. In the meantime, Fe3+is reduced to Fe2+during excitation. Fe2+might be reacted with H2O2to produce ·OH and separated from surface of oxide to generate a vacancy[37]. Besides, excessive amount of Fe2O3would cause the reduction efficiency of electron and holes separation.

Fig. 9. Proposed schematic diagram of electron-hole separation of AgNbO3/Fe2O3 particles under visible-light irradiation

4 CONCLUSION

In summary, PVC-(AgNbO3/Fe2O3) samples are suceccessfully prepared and the photocatalytic degradation process has been investigated under visible-light irradiation. Compared with pure PVC film, the degradation process of PVC-(AgNbO3/Fe2O3) could be controlled by changing the amount of Fe2O3particles. The weight loss rate of PVC-(AgNbO3/Fe2O3) is two times higher than that of PVCAgNbO3and ten times higher than that of pure PVC film,respectively. The PVC-(AgNbO3/Fe2O3) would become a potential photodegradable polymer material.