YIN Xiaogang ,LIU Xinyun ,LI Hang ,WU Huimin ,FU Hai ,GONG Wei＊

(1.Key Laboratory of Functional Materials Chemistry of Gui Zhou Province, School of Chemistry and Materials Science, Gui Zhou Normal University, Guiyang 550001, China; 2.School of Pharmaceutical Sciences, Guizhou Medical University, Guiyang 550025, China)

Abstract: We investigated the removal of the organic dye rhodamine B in wastewater with recyclable AgBr/polypyrrole (PPy) nano-photocatalysts.With PPy as an active base for electron transfer,and hexadecyltrimethylammonium bromide (CTAB) as both the soft-templating agent and the bromine source,a series of AgBr/PPy nano-photocatalysts containing various proportions of silver were prepared in a convenient one-step synthesis procedure.The synthesized catalysts were characterized by TG analysis to reveal that,in comparison with pure PPy,the interaction between PPy and AgBr led to increased thermal stability.Chemical combination of PPy and AgBr was observed through XRD and XPS analyses.For the morphology study,the AgBr particles were found to be well dispersed in the PPy nanowire network from SEM results.In the photodegradation experiments,up to 92% rhodamine B was degraded by the AgBr/PPy catalysts in the period of 1 hour under 254 nm UV light.The catalysts could maintain 60% catalytic efficiency after 3 cycles in the recyclability test.

Key words: polypyrrole;AgBr;photocatalysts;rhodamine B

1 Introduction

Nowadays,organic dyes have been widely used in textile,plastic,cosmetic,food processing and drug development industry.Each year,tons of industrial effluents containing a large amount of organic dyes are discharged,which raises a substantial global problem of water pollution to both mankind and aquatic systems.Given that organic dyes are required to be highly accumulable in water as well as highly stable towards degradation,their removal from wastewater has proposed a long-standing challenge for environment science.Conventional methods in wastewater treatment,such as physical adsorption[1],ultrafiltration[2],microbial degradation[3]and electrochemical oxidation[4],are not very well suited for removing organic dyes owing to unsatisfied efficiency and the generation of toxic side products.In this regard,photocatalytic degradation has its superiority in that it rapidly oxidizes organic dyes without forming polycyclic products[5-7].TiO2,in particular,has attracted the most attentions as being a relatively cheap and stable metal oxide,which absorbs the UV region to catalyze the oxidation of organic dyes[8,9].However,the large energy band and the narrow absorption spectrum of TiO2significantly lowers its photocatalytic reactivity.Alternatively,nanoparticles of transition metals (such as silver,gold,iron and zirconium) with larger specific areas,smaller internal diffusion resistance and more binding sites[10],has emerged as novel adsorbents for organic dyes.

AgBr is a promising photocatalyst with a narrow band-gap and higher photocatalytic reactivity[11,12],however,AgBr is unstable and prone to aggregation in its crystal form[13].To overcome these drawbacks,many researchers have dispersed AgBr on support materials by forming AgBr based composites.Cao and coworkers reported the photocatalytic degration of methyl orange by AgBr/Ag3PO4hybrid,in which the use of silver nanoparticles (NPs) was confirmed to be effective for stabilizing the AgBr-based catalyst[14].To further enhance the photocatalytic reactivity of AgBr NPs,active bases with electrical conductivity have been employed to increase the electron transfer rate and dispersion[15-19].With g-C3N4as the base,Akhundi A and coworkers synthesized AgBr/Fe3O4/nano g-C3N4catalyst,and the prepared nanocomposite could maintain satisfying catalytic reactivity after multiple cycles in rhodamine B degradation[20].

Among the electrically conductive base materials,polypyrrole (PPy) is a stable and versatile polymer,which has been widely used in batteries[21,22],super capacitors[23],gas sensors[24].The morphology of PPy could be controlled via soft-templates[25,26]and hardtemplates[27,28].PPy has large specific surface area and acts as an electron conductor to accelerate electron transfer.Li and coworkers reported a TiO2/PPy catalyst that is highly efficient in degrading organic pollutant under visible light conditions[29].We envision that the abundant nitrogen atoms in PPy could limit the growth of AgBr nanoparticles,thus increasing the stability and photocatalytic activity of the AgBr/PPy composites.This study provides a simple method to prepare a series of AgBr/PPy composite catalysts for organic dye degradation with CTAB (hexadecyltrimethylammonium bromide) as both a soft-templating agent and the bromine source.The morphology studies showed that the addition of silver source did not interrupt the wire template of PPy.Meanwhile,the formation of PPy network simultaneously generated AgBr NPs.The photodegradation reactivity of the prepared AgBr/PPy composite was tested by the removal of rhodamine B in wastewater.It was found that the AgBr/PPy composite significantly improved the efficiency in degrading rhodamine B compared to un-supported AgBr and pure PPy.

2 Experimental

2.1 Materials

Silver nitrate (A.R.99.8%),Rhodamine B(RhB),Pyrrole (Py) and ethanol (＞99.5%) in these experiments were purchased from Sinopharm.Hexadecyltrimethylammonium bromide (CTAB,99%)and ammonium persulfate (APS,A.R.) were purchased from Aladdin.All the reagents were used as received without further purification.

2.2 Preparation of AgBr

CTAB(1.148 mmol,1.82 g) and deionized water(100 mL) were sequentially added to a three-necked round bottom bottle and stirred to form a homogeneous solution.In an ice bath,an aqueous solution of AgNO3(1.148 mmol,20 mL) was added.The resulted solution was allowed to react in a dark room for 24 h and then the reaction mixture was centrifuged and washed with ethanol and deionized water multiple times.The obtained pale-yellow solid was dried in a freezing dryer protected from light.

2.3 Preparation of PPy

CTAB (0.264 mmol,0.418 5 g) and deionized water (100 mL) were sequentially added to a threenecked round bottom flask and stirred to form a homogeneous solution.Pyrrole (240 uL) was added dropwise and stirred for 15 min.To the resulting transparent solution,APS (0.821 5 g,20 mL) was slowly added.The mixture was stirred for 24 h in an ice bath,filtered,washed by ethanol and deionized water to neutral.The product was dried in a freezing dryer and stored in a desiccator.

2.4 Preparation of AgBr/PPy composites

Three three-necked round bottom flasks were marked a,b and c.CTAB (0.264 mmol,0.418 5 g) and deionized water (100 mL) were sequentially added to each flask and stirred to form a homogeneous solution.Pyrrole (240 uL) was added dropwise to each flask and stirred for 15 min.The resulted transparent solution was added aqueous APS (0.821 5 g,10 mL) and AgNO3(a:0.287 1,b:0.861 3,c:1.148 4 mmol) dropwise simultaneously.The mixture was stirred for 24 h in an ice bath,filtered,washed by ethanol and deionized water.The product was dried in a freezing dryer and stored in a desiccator.The AgBr/PPy composites prepared in flask a,b and c were marked AP-1,AP-2 and AP-3.

2.5 Characterization

Fourier transform infrared spectra (FTIR) were recorded with KBr pellets on a NEXUS670 Fourier infrared spectrophotometer (Thermo Nicolet,USA).;Thermo-gravimetric (TG) analysis was used to determine the weight loss at 10 ℃/min from 25 to 700℃ in air on a Q50 DuPont,USA.The morphology of the synthesized photocatalysts were examined on a JSM-7001F scanning electron microscopy (SEM).X-Ray diffraction (XRD) analyses were performed on an XRD-6000 (Japan) under Cu Kα (1.540 6 Å).X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Kratos Analytical AXIS Ultra DLD spectrometer using a monochromatic Al Kα source.

3 Results and discussion

3.1 XRD analysis

XRD analysis was employed for the phase study of the prepared nanocomposite catalysts.Fig.1 shows the XRD results of PPy and composites of AP-1,AP-2 and AP-3.For the pure PPy,the prominent broad peak at 27° suggested PPy was in amorphous phase.Comparing the XRD results of AP-1,AP-2 and AP-3,the diffraction peaks of AgBr appeared at similar positions (2θ=26.64°,30.9°,44.21°,54.97°,64.23°,73.1°,respectively),which was consistent with the standard PDF file (PDF file No: 06-043).The diffraction peaks corresponded to (111),(200),(220),(222),(400) and (420) crystal planes of AgBr.Amplification on the broad peaks at 20°– 40° of the composites (Fig.1(b)(d)) revealed the hidden broad peaks of PPy,which indicated the existence of PPy in the composites.In comparison with Fig.1(a),the broad peaks in the composites were narrower and the absorption intensity was lower.From the analysis of the XRD results,it was concluded that in addition to the simple mechanical mixing,AgBr and PPy were chemically combined,which would be further confirmed in the XPS analysis.

Fig.1 XRD patterns of (a) PPy；(b) AP-1；(c) AP-2；(d) AP-3

3.2 IR analysis

Bonding between silver and nitrogen in the prepared AgBr/PPy composites was studied by IR analysis.Fig.2 showed that the IR spectrum of pure PPy and composites were almost similar,which confirmed the existence of PPy.In the IR of PPy,the peak at 3 450 cm-1was the N-H stretching;the peak at 2 990 cm-1was the C-H stretching of methylene groups;the peak at 1 644 cm-1was the characteristic C=C stretching on the pyrrole ring;the peaks at 1 540 and 1 455 cm-1were the characteristic peaks of the pyrrole ring;the peak at 1 300 cm-1was the C-N stretching;the peak at 1 030 cm-1was the C-H bending.Since AgBr was inactive in IR spectroscopy,there was no peak of AgBr in the spectra of the composites.Comparing with the IR spectra of PPy and composites,the peaks in the composites consistently moved to higher wavenumbers (the peaks at 1 039 and 1 542 cm-1moved to higher wavenumber by 5 and 9 cm-1).This observed blueshift effect was considered to be caused by the bonding between PPy and AgBr:The interaction between AgBr and the fully conjugated PPy was transferred via the π bonds,giving rise to the increased bond strength of PPy,which was reflected in the IR vibrational frequency.

Fig.2 FTIR spectra of PPy,AP-1,AP-2 and AP-3

3.2 TG analysis

TG analysis was used to test the content of AgBr in the composites.Fig.3(a) shows the TG and DTG results of PPy,AgBr and composites AP-1,AP-2 and AP-3,respectively.The TG test was performed under dry air at the heat rate of 10 K/min with the final temperature of 700 °C.From the TG curve,it was observed that the mass of AgBr basically maintained the same during the test;while the mass of pure PPy was almost lost under high temperature condition in air.The remaining mass percentage of composites AP-1,AP-2 and AP-3 was 45%,37% and 16%,respectively.In comparison with pure AgBr and pure PPy,the remaining mass of composites AP-1,AP-2 and AP-3 could be assigned to AgBr,which was basically consistent with the amount of AgBr added during the preparation.

Fig.3 (a) TG curves of pure PPy,AP-1,AP-2,AP-3 and AgBr;(b) DTG curves of pure PPy,AP-1,AP-2,and AP-3

Furthermore,pure PPy and the three composites all showed mass loss twice.The first mass loss occurred at 45-100 ℃,owing to the evaporation of a small amount of water.The second mass loss between 250-650 ℃ was significant,which was caused by the degradation of PPy.For the four different materials,the starting temperatures of the second mass loss were 259,258,251 and 247 ℃,respectively.However,the degradation temperatures of the four materials at 50% mass loss displayed a huge discrepancy at 457,484,536 and 536 ℃,respectively.

From the DTG of PPy and composites (Fig.3(b)),it could be seen that there were two distinguishable peaks: a peak caused by the loss of water,which was the same for all the four materials at the temperature of 75.5 ℃;the other peak corresponded to the temperature where PPy degraded the fastest.The temperature of AP-1,AP-2 and AP-3 at the maximum mass loss were 293.5,297.4 and 305.2 ℃,respectively,which were increased by 6.8,10.7 and 18.5 ℃ compared to that of PPy.These peaks occurred substantially different,with the trend that the higher content of AgBr led to larger temperature difference.In general,the temperature of the composites at the start of mass loss,50% mass loss and fastest mass loss was higher compared to PPy.Based on these observations,it was proposed that the interaction between PPy and AgBr led to more compact structure of PPy,thus increasing its thermal stability[34],which was manifested in the difference in degradation temperature.

3.4 SEM analysis

To further observe the micro-morphology and structure characteristics,the prepared composite samples were submitted to SEM analysis (Fig.4).From the SEM results,it could be seen that both PPy and the composites appeared as nanowires featuring with uniformed diameters of 40-60 nm.It was evident that the added AgNO3did not affect PPy to polymerize into nanowires in the template.With the increase of AgBr content,PPy nanowires became more dispersed and less tangled,leading to more distinguishable overall morphology.In addition,the existence of spherical AgBr was observed to be well dispersed in the active PPy nanowire base in the SEM images of composites AP-1,AP-2 and AP-3 (marked in red in Fig.4).Moreover,in composites of AP-1,AP-2 and AP-3,the increasing of AgBr contents did not cause the aggregation or the enlargement of the spherical AgBr particles;Conversely,the diameters of the spherical AgBr decreased as the particle size of AgBr in AP-1 distributed between 300-400 nm,AP-2 between 200-300 nm and AP-3 between 150-200 nm.It is postulated that this decrease is caused by the addition of silver source,thus leading AgBr to form more nuclei on PPy.Therefore,later added silver source would grow on the nuclei without increasing the particle size of the spherical AgBr.In addition,Ag-N coupling between AgBr and PPy,namely the anchoring effect of nitrogen,could possibly suppress the aggregation in the growing process of AgBr crystals,and also facilitate the dispersion of AgBr.

Fig.4 Microscopic observation for synthesized catalysts of (a) PPy nanotubes;(b) AP-1;(c) AP-2;(d) AP-3

3.5 XPS analysis

To further confirm the bonding interaction between PPy and AgBr in the AgBr/PPy composites,the composition and the chemical state of the synthesized materials were examined by X-ray photoelectron spectroscopy (XPS,Scheme1).Scheme 1 showed the high resolution XPS spectra of the N atom region.In the mechanical mixture of AgBr and PPy(Scheme 1(a)),the N 1s peak could be divided into three Gaussian peaks with binding energies of 399.29,399.81,and 402 eV,respectively,which corresponded to the neutral,imine-like structure (-C=N-),the neutral,amine-like structure (-N-H-),and positively charged nitrogen atoms (-NH+-) of the PPy nanotubes as described in the literature[30,31].The core level analysis of N in the composite AgBr/PPy (Scheme 1(b)) depicted -C=N-,-N-H-,-NH+-,respectively at 399.39,399.93,and 402.21eV.It is noteworthy that the N binding energy values were smaller in the AgBr/PPy composite compared to the mechanically mixed sample,resulting from the interaction between the nitrogen atoms in the PPy and the AgBr nanoparticles[32].To further elucidate the this interaction,high resolution core level XPS of Ag was performed (Scheme 1(c)and 1(d)) .The discrete expression of the doublet was assigned to Ag 3d 5/2 and Ag 3d 3/2 of Ag+in AgBr[33,34].In the mechanical mixture (Scheme 1(c)),the two peaks appeared at 366.67 and 372.66 eV for AgBr,respectively.However,the same doublet shifted to 366.73 and 372.76 eV in the AgBr/PPy composite(Scheme 1(d)).Such increase in the binding energy of Ag coule be attributed to the complexation of Ag+ions in the composite with PPy through nitrogen.

Scheme 1 Core level X-ray photoelectron spectra of (a) N in AgBr+PPy;(b) N in AgBr/ PPy(AP-3);(c) Ag in AgBr+PPy;(d) Ag in AgBr/PPy(AP-3)

3.6 Photodegradation test

Cycling degradation experiments were also performed to evaluate the stability of the prepared photocatalysts under the catalytic conditions.Fig.5 showed that,catalysts of AP-1,AP-2 and AP-3 still maintained photodegradation efficiency above 50% after 3 cycles.The decrease in catalytic performance was supposed to be caused by the reduction of Ag+to Ag in the photodegradation process,which was also supported by the XRD results shown in Fig.6.It is noted that AP-3 possessed the highest cycling stability among the tested catalysts,with the highest degradation rate as well as the lowest drop-off rate after 3 cycles(Fig.5(c)).The superior cycling stability of AP-3 is possibly owing to its largest content of AgBr and the smallest AgBr nanoparticle size.

Cycling degradation experiments were also performed to evaluate the stability of the prepared photocatalysts under the catalytic conditions.Fig.5 showed that,catalysts of AP-1,AP-2 and AP-3 still maintained photodegradation efficiency above 50% after 3 cycles.The decrease in catalytic performance was supposed to be caused by the reduction of Ag+to Ag in the photodegradation process,which was also supported by the XRD results shown in Fig.6.It is noted that AP-3 possessed the highest cycling stability among the tested catalysts,with the highest degradation rate as well as the lowest drop-off rate after 3 cycles(Fig.5(c)).The superior cycling stability of AP-3 is possibly owing to its largest content of AgBr and the smallest AgBr nanoparticle size.

4 Conclusions

Three AgBr/PPy photocatalysts with various AgBr contents and uniformed spherical AgBr nanoparticles were prepared from CTAB and AgNO3.The results revealed that the spherical AgBr was dispersed in the PPy nanowire network.SEM images further demonstrated that the size of AgBr was well controlled,since it did not enlarge with the increased content of AgBr.Owing to the enhanced electron transfer and dispersion in the active network PPy base,the prepared photocatalysts exhibited significantly improved activities in the photocatalytic degradation of rhodamine B compared with the pure AgBr.With PA-3 as the catalyst,the degradation of rhodamine B could reach 93% in one hour,and the catalyst remained stable after 3 catalytic cycles,which holds great promise for further development of photocatalysts for removing organic dyes in wastewater.

Conflict of interest

All authors declare that there are no competing interests.