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BY-NC-ND 3.0 license Open Access Published by De Gruyter November 13, 2015

Synthesis of Ag2O nano-catalyst in the spherical polyelectrolyte brushes and its application in visible photo driven degradation of dye

Ayyaz Ahmad, Jianjia Liu, Xiaochi Liu, Li Li, Yisheng Xu and Xuhong Guo
From the journal e-Polymers

Abstract

Ag2O nanoparticles (NPs) were synthesized using colloidal solution of spherical polyelectrolyte brushes (SPB) as nano-reactors. In this work, the average diameter of Ag2O NPs was around 10 nm as determined by transmission electron microscopy (TEM). The composite NPs of Ag2O immobilized in SPB (Ag2O-SPB) showed significant absorption in the visible light region as confirmed by UV-Vis diffuse reflectance spectra (DRS), and their photoluminescence (PL) exhibited emission peak in the visible range. Ag2O-SPB has shown outstanding photocatalytic activity during degradation of methyl blue (MB) in the visible light. This work will open up a new way to prepare ideal Ag2O nano-catalyst for the remediation of wastewater using visible light.

1 Introduction

In the last century, industries have been grown rapidly. This rapid industralization, however, has generated large amount of pollutants which has created alarming situations for the environment and human health (1, 2). Dealing with these pollutants is becoming a big problem with the passage of time. It is well known that photocatalyst has been extensively used in the successful degradation of different kinds of pollutants present in the wastewater (3, 4). In order to make photocatalyst cost-effective and efficient, reseachers have exploited the visible light of the sun because the visible light consists of about 45% of solar light spectrum while UV light only has around 4% (5, 6). TiO2 is the most extensively used photocatalyst for the treatment of organic waste but it can be activated in UV light due to its wide band gap of 3.2 eV (4, 7–10). Besides TiO2, other photcatalysts include, for instance, ZnO, SnO2, and ZrO2, but they are also active in UV light due to their wide band gap (11–19). Several efforts have been made to utilize wide-band gap photocatalyst that could also be effective under visible light by doping it with some metal/non-metal or coupling with some narrow-band gap semi-conductors (4, 15, 20–23). Methylene blue is an organic dye which is used in leather, cotton fiber and as a biological stain. It is dangerous if consumed. It effects the humans by causing respiratory tract, skin and eye irritation (24).

Silver oxide (Ag2O) with a narrow-band gap of about 1.2 eV has been employed as a highly efficient and stable photocatalyst. The Ag2O photocatalyst is a promising candidate for the removal of hazardous organic materials from wastewater (25–27). Ag2O has been employed in many industrial applications such as anti-bacterial activity, electrode material, and as cleaning agents (28–30). Due to its high catalytic activity and selectivity, Ag2O has attracted the attention of many researchers (31, 32). As reported by Yu et al. and Wang et al., Ag2O nanoparticles (NPs) as a visible light driven photocatalysts had exhibited self-stability during the photo-degradation process (26, 33).

On the other hand, NPs are required to be dispersed in the liquid phase. For that purpose, stabilizers are used to keep them dispersed stably in the liquid phase (34). However, NPs tend to aggregate in the absence of a stable system. Polymers have been widely used as a stabilizer in NP stabilization (35). Spherical polyelectrolyte brushes (SPB) consist of a spherical polymeric core, where linear polyelectrolyte (PE) chains are attached either by covalent bond or by physical adsorption (36, 37). The counterion concentration inside the SPB always remains constant because of the Donnan effect (38). Due to the confinement of these counter ions within brushes, SPB have proven to be well-defined and effective nanoreactors. If metal ions replace the counter ions confined within SPB, NPs can be generated by the reduction of these metal ions. NPs will be stabilized by SPB without addition of other stabilizers (35, 39–41).

Herein, we report the facile and simple synthesis of Ag2O NPs in SPB nanoreactors. First, we synthesized the poly(acrylic acid) (PAA) brushes, having carboxylic groups, attached to the polystyrene (PS) core. Then, H+ ions of carboxylic group were exchanged with silver ions. Finally, Ag2O NPs formed after the addition of NaOH in the mixture, as shown in the schematic diagram (Figure 1). The Ag2O NPs remains very stable in the dispersion of SPB for a long time. The composite NPs of Ag2O immobilized in SPB (Ag2O-SPB) showed outstanding photocatalytic activity during degradation of methyl blue (MB) in the visible light. This study opens up a new approach to prepare invisible photo-droven Ag2O nano-catalyst for dye wastewater treatment.

Figure 1: Schematic representation of Ag2O synthesis on SPB (A) SPB (B) Ag+ ions replacing the counter ions by the addition of AgNO3, (C) Ag2O NPs synthesis by adding the NaOH.

Figure 1:

Schematic representation of Ag2O synthesis on SPB (A) SPB (B) Ag+ ions replacing the counter ions by the addition of AgNO3, (C) Ag2O NPs synthesis by adding the NaOH.

2 Experimental section

2.1 Materials

Potassium peroxodisulfate (KPS), styrene, sodium hydroxide and acrylic acid were obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. Sodium dodecylsulfate (SDS) was obtained from Shanghai Reagent Company (Shanghai, China). The initiator KPS was purified by re-crystallization in water prior to use and stored in refrigerator. SDS and the monomers were purified by reduced pressure distillation to remove inhibitor. Silver nitrate (AgNO3) was purchased from Sinopharm Chemical Research Co., Ltd. (Beijing, China). The water used in the experiment was purified with the help of a Millipore Milli-Q system. All reagents have been purchased from different companies (names are given in Materials section) within China.

2.2 Synthesis of spherical polyelectrolyte brushes

The 2-[p-(2-hydroxy-2-methylpropiophenone)]-ethyleneglycol-methacrylate (HMEM), photo initiator was prepared according to method reported in our previous paper (36). The PS core was synthesized by conventional radical polymerization. Briefly, 2.0 g of styrene was mixed with 0.05 g of SDS and 0.12 g of KPS. Before the start of reaction, the whole reactor was degassed by repeated evacuation and subsequent injection of nitrogen. The mixture was then heated to 80°C with vigorous stirring for 2 h. Then, 0.02 g of photo initiator dissolved in 1.8 g acetone was added under starved conditions (0.05 ml/min). A thin shell of photo initiator layer was generated on the surface of PS core. The latex was cooled to room temperature and purified by serum replacement against pure water until the conductivity of eluate keeps constant.

PS core latex (1 wt%) was mixed with an appropriate amount of acrylic acid (100%) in a homemade UV photo reactor. The reactor was degassed by repeated evacuation and subsequent injection of N2. The photo-emulsion polymerization was carried out under the radiation of UV light (150 W) at room temperature. The obtained SPB colloid was purified by dialysis against the water.

2.3 Synthesis of Ag2O NPs in SPB

For the synthesis of Ag2O NPs, 5 ml of SPB (1 wt%) was dispersed in 40 ml water in three round neck bottom flask. Two milliliter of a 62.5 mm solution of AgNO3 was added at the controlled rate by micro syringe to the SPB dispersion to make the total concentration of AgNO3 at 2.6 mm in the solution and stirred for 2 h to exchange H+ with Ag+ under a nitrogen environment. Then 2 ml of equimolar aqueous sodium hydroxide solution was added to the above mixture dropwise at the feeding rate of 0.1 ml/min at 50°C. The reaction was carried out for further 2 h. The SPB/Ag2O composite was then cooled down at room temperature and purified afterwards by dialysis against the pure water.

2.4 Characterization

Using the particle sizing system of NICOMP 380 ZLS at a fixed scattering angle of 90°, dynamic light scattering (DLS) was used to determine the hydrodynamic size of particles. The transmission electron microscopy (TEM) was performed using a JEOL-2100 electron microscope operating at 200 kV. To study the interaction between the Ag2O and SPB, the Fourier transmission infrared (FTIR) spectra was performed using a 6700 Fourier transform spectrometer (Thermo Nicolet Corp.). The UV-Vis diffuse reflectance spectra (UV-Vis DRS) for the dry-pressed disk catalyst samples were obtained using a scan UV-Vis spectrophotometer (Varian, Cary 500) fitted with an integrating sphere assembly employing BaSO4 as the reference sample. The photolominance (PL) spectra were collected with a spectrofluorophotometer (Shimadzu, RF-5301) using a Xe lamp as an excitation source with the excitation wavelength of 325 nm at room temperature. Before UV-Vis DRS, and FTIR measurements, the samples were freeze-dried in lyophilizerfor 24 h.

2.5 Photocatalytic activity of Ag2O and Ag2O-SPB

The photocatalytic activity of Ag2O was measured in terms of the degradation of MB under visible-irradiation. The visible light was provided by a 300 W Xenon (Perfect Light, China) with a UV-cut filter (λ≥400 nm). All the experiments were conducted at ambient pressure and temperature. The distance between the lamp and the reactor was 10 cm for each experiment. For a typical run, 1 ml Ag2O composite particle dispersion (0.1 wt% solid content) and 20 ml of 10 mg/l aqueous solution of MB were mixed together in a quartz glass reactor with stirring. This dispersion was stored in the dark for ca. 30 min prior to irradiation to establish the adsorption/desorption equilibrium of the dye on the catalyst surface. After a given time interval, the change in concentration was recorded as C/C0 where C0 was the initial concentration of MB aqueous solution, and C was the concentration at any given time (t).

3 Results and discussion

3.1 Synthesis of Ag2O on the SPB

Narrow size distributed SPB was synthesized by photo-emulsion polymerization as confirmed by DLS. The mean diameter of PS core was about 80 nm while it was around 405 nm for SPB (Figure 2). For the preparation of Ag2O-SPB through the chemical precipitation method, NaOH was added slowly so that well-dispersed particles were obtained. PAA brushes attached onto the PS core are annealed brushes. It means that they are responsive to the ionic strength. It can be seen from Figure 2 that the chains of the PAA brushes had changed with different concentrations of silver salt. As we increased the concentration of salt, the chain length of brushes decreased substantially. This study helped in choosing the optimum concentration for silver salt.

Figure 2: (A) Size distribution of PS core and PS-PAA SPB as measured by dynamic light scattering (DLS). (B) Effect of silver salt concentration on the chain length of polyelectrolyte brushes.

Figure 2:

(A) Size distribution of PS core and PS-PAA SPB as measured by dynamic light scattering (DLS). (B) Effect of silver salt concentration on the chain length of polyelectrolyte brushes.

TEM was used to observe the morphology of the pure SPB and Ag2O immobilized in SPB. Figure 3A and B showed the TEM images of Ag2O-SPB. It was observed from the TEM that Ag2O NPs were successfully immobilized on the SPB after the precipitation reaction between AgNO3 and NaOH. The average size of Ag2O NPs was around 10 nm. Figure 3C showed that both the size and size distribution of Ag2O-SPB increased compared with SPB as observed by DLS. It reflected that the counter ions of Ag+ in SPB were reduced due to the reaction. The reduction of screening effect by counter ions will lead to the expanding of SPB, while slight aggregation of SPB may result during the reaction.

Figure 3: TEM images of (A & B) Ag2O-SPB composite, and (C) DLS data of pure SPB and Ag2O-SPB composite.

Figure 3:

TEM images of (A & B) Ag2O-SPB composite, and (C) DLS data of pure SPB and Ag2O-SPB composite.

In order to investigate how Ag2O NPs were immobilized on the SPB, one possible reason for this may be the preferential association of Ag2O with the carboxylic groups (COO-) present on the polyelectrolyte brush chains (35). The Ag2O precursor was chemically co-precipitated directly in the brush layer. The generated silver oxide NPs within 10 nm were bound by the carboxyl groups and stabilized by the polymer chains in SPB.

It has been assumed that the COO-group of PAA on SPB has good co-ordination with silver NPs and that is why they are stable SPB (35). The coordination of functional group of PAA and Ag2O might provide the good stability of the formed NPs. FTIR spectra of SPB without Ag2O loading and with Ag2O NPs immobilization on the SPB are shown in Figure 4. A strong absorption at 1710 cm-1 was observed which was derived from the C=O stretch in carboxyl groups in PAA brush. When Ag2O NPs were formed, absorption of C=O stretch at 1710 cm-1 decreased, the absorption of symmetric (COO-) stretch was enhanced at 1405 cm-1, and a new absorption appeared at 1550 cm-1 from the asymmetric (COO-) stretch (42). Free carboxyl groups (1710 cm-1) still existed after the loading of Ag2O NPs. These changes in FTIR spectra may reflect in part the coordination of carboxyl groups in PAA the Ag2O.

Figure 4: FTIR spectra of pure SPB, and Ag2O-SPB composite.

Figure 4:

FTIR spectra of pure SPB, and Ag2O-SPB composite.

The optical absorption properties of SPB and Ag2O-SPB were investigated by UV-visible diffuse reflectance spectra as shown in Figure 5. Compared to SPB, Ag2O-SPB showed much higher absorption in the visible range, which should be significantly activated in the visible light.

Figure 5: UV-visible diffuse reflectance spectra of Pure SPB, and Ag2O-SPB composite.

Figure 5:

UV-visible diffuse reflectance spectra of Pure SPB, and Ag2O-SPB composite.

The PL spectra of SPB and the Ag2O-SPB with an excitation wavelength of 325 nm are shown in Figure 6. There was no significant peak of pure SPB in the visible region, which is due to its non-conductive behavior. The wide peak of Ag2O-SPB in the visible range implied that Ag2O was present in SPB and its photoluminescence at around 450 nm.

Figure 6: Photoluminescence spectra of as prepared SPB, and Ag2O-SPB composite.

Figure 6:

Photoluminescence spectra of as prepared SPB, and Ag2O-SPB composite.

3.2 Photocatalytic activity of Ag2O-SPB composite

Photocatalytic activity and reaction kinetics of Ag2O-SPB were observed during the degradation of MB using visible light as shown in Figure 7. We had determined photocatalytic activity of composite in dark and in visible light for the comparison. After a specific time interval under visible light, the intensity of the peak at 600 nm, which is related to the characteristic UV-Vis absorption of the MB molecule, had decreased stepwise and disappeared after around 1.5 h which is shown in Figure 7A. We observed from Figure 7B that there was almost negligible degradation of MB in the dark because Ag2O catalyst was only active in the visible range. Around 70% degradation of MB was achieved within 1 h of the photocatalytic reaction. The kinetics of MB degradation under visible light can be described by pseudo first order reaction

Figure 7: (A) UV-Vis spectra of the degradation of MB in the presence of Ag2O-SPB composite, (B) photocatalytic degradation of methyl blue and (C) kinetics of as prepared Ag2O-SPB composite.

Figure 7:

(A) UV-Vis spectra of the degradation of MB in the presence of Ag2O-SPB composite, (B) photocatalytic degradation of methyl blue and (C) kinetics of as prepared Ag2O-SPB composite.

ln(C0C)=kt

where k is the rate constant, C0 and C are initial and concentration at given degradation time t, respectively. We plotted the graph of ln(C0/C) as a function of t (Figure 7C). The graph for Ag2O-SPB fitted linearly where the correlation coefficient value of R2 was 0.9607. The calculated value of rate constant k was 0.021 min-1, which showed the high catalytic activity of Ag2O-SPB.

When Ag2O-SPB composites were irradiated in the presence of visible light, electrons were produced in the conduction band while holes were produced in valence bands (Figure 8). During the irradiation, Ag might be formed by partial decomposition of Ag2O attached on to SPB (26). Formation of Ag by in-situ photo generation made Ag2O more stable (43).

Figure 8: Proposed mechanism for degradation of MB.

Figure 8:

Proposed mechanism for degradation of MB.

4 Conclusions

We have demonstrated the successful synthesis of the visible light photocatalyst Ag2O with the help of SPB as ideal nano-reactors. As observed by TEM, the Ag2O NPs showed well-defined spherical structures with a size around 10 nm. UV-Vis diffuse reflectance and photoluminescence spectra were used to confirm the optical property of Ag2O-SPB, which showed absorption and emission peaks in the visible range, respectively. The composite NPs of Ag2O-SPB showed high photocatalytic activity in the degradation of dye MB under the visible light. This stable dispersion of Ag2O-SPB is envisioned to be used in the applications of dye wastewater treatment under visible photcatalysis.


Corresponding authors: Ayyaz Ahmad and Xuhong Guo, State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China, Phone/Fax: +86 21 6425 3491, e-mail: (A. Ahmad); (X. Guo)

Acknowledgments

This work was supported by the NSFC Grants (51273063 and 21476143), the Fundamental Research Funds for the Central Universities, and 111 Project Grant (B08021). The author (Ayyaz Ahmad) is thankful to Higher Education Commission (HEC) Pakistan for providing PhD scholarship under UESTP programme.

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Received: 2015-8-10
Accepted: 2015-9-28
Published Online: 2015-11-13
Published in Print: 2016-1-1

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