- Email: [email protected]
ZnO under visible light
Superlattices and Microstructures 62 (2013) 128–139
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Letter to the Editor
Improved photocatalytic performance over AgBr/ZnO under visible light Lei Shi b, Lin Liang a,c, Jun Ma a, Jianmin Sun a,b,⇑ a b c
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150080, China The Academy of Fundamental and Interdisciplinary Science, Harbin Institute of Technology, Harbin 150080, China School of Life Science and Technology, Harbin Institute of Technology, Harbin 150080, China
a r t i c l e
i n f o
Article history: Received 13 June 2013 Received in revised form 1 July 2013 Accepted 23 July 2013 Available online 3 August 2013 Keywords: AgBr/ZnO composite Photocatalyst Dye degradation Visible light
a b s t r a c t AgBr nanoparticles modified ZnO have been successfully synthesized by using cetyltrimethylammonium bromide as the bromide source and stabilizer. X-ray diffraction and X-ray photoelectron spectroscopy confirmed the loading of AgBr nanoparticles on ZnO support. The transmission electron microscopy showed that AgBr nanoparticles with small size were well dispersed on the surface of ZnO support. UV–visible diffuse reflectance spectra displayed the absorbance of AgBr/ZnO composite was much higher than that of pure ZnO in the visible light range. The photocatalytic activities of degradation Rhodamine B dye under visible light over AgBr/ZnO catalyst were markedly enhanced compared with pure ZnO. The improved photocatalytic activities may be attributed to the synergetic effects including enhanced visible light absorption, narrowed band gap and effective separation of photogenerated e–h+ resulted from the highly dispersed small nanosized AgBr. Moreover, the possible mechanism was tentatively proposed based on the photoluminescence spectra and surface photovoltage spectroscopy. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Widespread discharge of wastewater from the textile industries which contains large amounts of dyes has become great concern to the environment and ecosystem due to their non-biodegradability, ⇑ Corresponding author at: State Key Laboratory of Urban Water Resource and Environment, The Academy of Fundamental and Interdisciplinary Science, Harbin Institute of Technology, Harbin 150080, China. Tel.: +86 451 86403715. E-mail address: [email protected] (J. Sun). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.07.013
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toxicity, and potential carcinogenicity. Some treatments such as adsorption, membrane filtration and chemical precipitation are the effective methods for cleaning up the pollutants in wastewater [1–3]. In recent years, photocatalysis technology is considered as a cost-effective method for degrading various hazardous contaminants in water because pollutants can be oxidized quickly and non-selectivity [4,5]. Dyes can be decomposed to less organic matters and finally into H2O and CO2 by photocatalysts. Many semiconductor photocatalysts such as TiO2, ZnO, Co3O4, CdS and Ag3PO4 have been developed [6–10]. Although TiO2 is widely used as the most important photocatalyst, ZnO is also a good alternative candidate owing to its high photocatalytic activity in the ultraviolet range, low cost, thermal and chemical stability and environmentally friendly features [11]. However, the low quantum efficiency of ZnO due to the fast recombination of photogenerated holes–electrons (h+–e) pairs, and a wide band gap of 3.3 eV seriously limit its wide applications in visible light range. In order to expand the usage of ZnO photocatalyst, it is of utmost importance to prevent the h+–e pairs from recombination and enhance the absorbance of ZnO in visible light range. Hence, many efforts have been devoted to the development of ZnO-based visible light photocatalysts, such as noble metal deposition [12], non-metal doping [13], photosensitive material modification [14], coupling with semiconductor composite [15] and conjugated polymer modification [16]. The silver bromide AgBr is a well-known photosensitive material. When AgBr is well dispersed on the surface of support materials, AgBr absorbs photons in the visible light to generate h+–e pairs during the photosensitive process, and the electrons can shift to the surface of support to induce the separation of photogenerated h+–e pairs, thus improving the photocatalytic ability [17–20]. As photocatalytic reactions take place mainly on the surface of the catalyst, nanoscaled active centers are believed to exhibit better performance than its bulk counterpart due to its high surface-to-volume ratio. In addition, the probability of photogenerated h+–e pair recombination will drastically be reduced owing to their fast arrival at the surface sites. Husein reported that AgBr nanoparticles with small size and narrow size distribution were prepared via reaction of AgNO3 with counter anion of cetyltrimethylammonium bromide (CTAB) individual micelles in microemulsions, which effectively prevented the aggregation of AgBr [21]. Inspired by the roles of surfactant CTAB, in this paper, we selected CTAB as the bromide source and stabilizer to facilely synthesize AgBr modified ZnO to improve photocatalytic activities in visible light. CTAB could effectively control the small size of AgBr, leading to well-dispersed AgBr on the surface of ZnO support. The photocatalytic activities of the AgBr/ZnO composite were evaluated by the photodegradation Rhodamine B (RhB) under visible light irradiation. Moreover, the surface photovoltage spectroscopy was investigated using electrochemical method and the photocatalytic mechanism was tentatively proposed. 2. Experimental 2.1. Synthesis of AgBr/ZnO composite AgBr/ZnO composite was synthesized by the deposition–precipitation method in the presence of CTAB as surfactant and Br source. 1.0 g ZnO was added into 50 mL distilled water and stirred for 30 min at room temperature. Then 1.2 g CTAB was added with vigorous stirring for 60 min, 0.21 g AgNO3 in 1 mL NH4OH (25 wt%) was subsequently dropped into the mixture. After stirring at room temperature for 12 h, the suspensions were filtered, washed with distilled water and dried at 60 °C for 12 h. Finally, the powder was calcined at 500 °C in air for 3 h. Then yellow powder denoted as AgBr/ZnO-10% with theoretical Ag/Zn molar ratio of 1:10, was obtained. In a similar process, AgBr/ ZnO composites with different AgBr contents were obtained and denoted as AgBr/ZnO-5% and AgBr/ ZnO-20%, respectively. 2.2. Material characterizations The X-ray diffraction (XRD) measurement was carried out on Bruke D8 Advance X-ray powder diffractometer with Cu Ka radiation (40 kV, 40 mA) for phase identification. The morphology, particle
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Fig. 1. XRD patterns of (a) pure ZnO and (b) AgBr/ZnO-10% composite.
size and chemical compositions of the product were examined by transmission electron microscopy (TEM, Tecnai G2 Spirit) equipped with an energy dispersion X-ray spectrometer (EDS). X-ray photoelectron spectroscopy (XPS) measurements were recorded on a PHI 5000C ESCA system with Al Ka radiation (hm = 1486.6 eV). The UV–visible diffuse reflectance spectra (DRS) were measured by a Shimadzu UV-2500 spectrophotometer. The photoluminescence spectra (PL) were obtained by a Varian Cary Eclipse spectrometer with an excitation wavelength of 300 nm. The surface photovoltage spectroscopy (SPS) measurements were carried out with a homemade apparatus (a 500 W Xe lamp) in Ref. [22]. 2.3. Photocatalytic testing The photocatalytic performance of AgBr/ZnO composite was evaluated through degradation RhB dye under visible light. The visible light was provided by a 300 W iodine tungsten lamp (Philips Co.) with cut off filter. 0.1 g AgBr/ZnO composite was dispersed into 100 mL 5 mg L1 RhB solution for photocatalytic examinations under magnetic stirring. The dispersion was kept in dark for 30 min under magnetic stirring to reach the adsorption–desorption equilibrium before light irradiation. The distance between the surface of the dye suspension and the light source was about 50 cm. Solutions were collected every 0.5 h and centrifuged to remove the catalysts then analyzed on Shimadzu UV-2500 spectrophotometer. 3. Results and discussion 3.1. Characterizations of AgBr/ZnO composite Fig. 1 shows the XRD patterns of pure ZnO and AgBr/ZnO-10% composite. It was observed that the diffraction peaks of single hexagonal phase ZnO (JCPDS card no. 076–0704) did not obviously change after modification by AgBr nanoparticles. Additionally, some new peaks at 2h of 31.1°, 44.4°, 55.1° and 64.6°, corresponding to cubic phase AgBr (JCPDS card no. 06–0438) were observed, which confirmed the loading of AgBr on ZnO support. The synthesis procedure of AgBr/ZnO composite was shown in Scheme 1. CTAB as cation surfactant and Br source was first adsorbed on the surface of ZnO support. With the addition of Ag+, it was easily combined with CTAB to form AgBr nanoparticles, at the same time, CTAB formed individual micelles
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Scheme 1. Schematic illustration of the synthesis procedure of AgBr/ZnO composite.
that wrapped AgBr nanoparticles, preventing AgBr further agglomeration. Thus, small sized AgBr nanoparticles were attached on the surface of ZnO. The uniform distributions of AgBr nanoparticles on ZnO were confirmed by TEM images in Fig. 2. Pure ZnO displayed nearly rod-like morphology and the surface of pure ZnO was smooth. In the AgBr/ZnO composite, the small AgBr nanoparticles at the size of about 10 nm were well anchored on the surface of ZnO. EDS results revealed the presence of Ag, Br, Zn and O elements in the AgBr/ZnO composite, which were in good agreement with the XRD results. The surface structures of the pure ZnO and AgBr/ZnO-10% composite were investigated using XPS analyses in Fig. 3. In the AgBr/ZnO sample, all of the peaks ascribed to Ag, Br, O and Zn elements were observed, which were consistent with the results of EDS and XRD. Peaks at 367.4 and 373.4 eV were assigned to Ag 3d5/2 and Ag 3d3/2; the peak at 68.5 eV was ascribed to Br 3d; two symmetrical peaks centered at 531.0 and 532.5 eV were associated with the lattice oxygen of ZnO and chemisorbed oxygen caused by the surface hydroxyl, respectively [23]. No peak was present at the binding energy of about 529.0 eV attributed to O in Ag2O [24], further confirming the existence of Ag as the form of AgBr. The peak at 1022.4 eV was identified as Zn 2p3/2 of ZnO. The UV–visible diffuse reflectance spectra of pure ZnO and AgBr/ZnO-10% composite were illustrated in Fig. 4. A broad tail between 400 nm and 800 nm appeared in the AgBr/ZnO sample, indicating that it had optical capability during the visible light range. Moreover, the absorbance of the as-synthesized AgBr/ZnO composite was much higher than that of pure ZnO in the visible light range, which was due to the surface plasmon resonance effect of AgBr. And the wavelength threshold of AgBr/ZnO was estimated to 407 nm, corresponding to the band gap at 3.04 eV. The band gap of AgBr/ZnO was narrower than the 3.3 eV of pure ZnO.
3.2. Photodegradation activity of RhB Fig. 5(A) depicts the photocatalytic activity for RhB degradation over pure ZnO and a series of AgBr/ ZnO composites under visible light. The degradation RhB hardly occurred with visible light irradiation even for 3 h without any catalyst. While only 35% RhB was degraded over pure ZnO under the same conditions. Noticeably, compared with pure ZnO, AgBr/ZnO samples exhibited the enhanced photocatalytic activity. AgBr/ZnO-10% composite exhibited the optimal photocatalytic activity, and almost 95% RhB was degraded in 3 h. AgBr/ZnO composite possessed better photocatalytic activities for decomposing RhB than pure ZnO, resulting from the smaller size and good dispersibility of AgBr nanoparticles on the ZnO support. Fig. 5(B) shows the linear relationships between ln (C0/C) and the irradiation time, where C0 is the original concentration and C represents RhB concentration at certain time interval. The photocatalytic degradation curves of RhB fit well with first-order reaction. The rate constants of AgBr/ZnO-5%, AgBr/ZnO-10% and AgBr/ZnO-20% were 0.6632 h1, 1.034 h1 and 0.8238 h1,
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Fig. 2. TEM images of (a) pure ZnO, (b) AgBr/ZnO-10% composite and (c) EDS analysis for AgBr/ZnO-10% composite.
respectively. The AgBr/ZnO-10% composite exhibited the highest rate constant, which was approximately 6.5 times larger than that of ZnO (k = 0.1547 h1). The effects of pH value in the solution on the photodegradation rate were investigated in Fig. 6. At the pH 4.4, approximately 80% RhB was degraded in 3 h. However, when the solution became alkaline
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Fig. 3. XPS spectra of (a) full spectra of pure ZnO and AgBr/ZnO-10%, (b) Ag 3d spectra, (c) Br 3d spectra, (d) O 1s spectra and (e) Zn 2p spectra in the AgBr/ZnO composite.
(pH = 10), the degradation efficiency was increased to almost 100% in 2 h. At acidic conditions, the photocatalytic efficiency was decreased possibly because ZnO could react with acid. However, AgBr/ZnO
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Fig. 4. DRS spectra of (a) pure ZnO and (b) AgBr/ZnO-10% composite.
Fig. 5. Photodegradation of RhB over pure ZnO and AgBr/ZnO composite (a) blank, (b) pure ZnO, (c) AgBr/ZnO-5%, (d) AgBr/ ZnO-20% and (e) AgBr/ZnO-10%.
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surface was negatively charged by adsorbing OH ions at high pH, and OH ions in the reaction medium favored the formations of OH radicals [25], which benefited for improving the photocatalytic activity of AgBr/ZnO. Hence, pH value of the solution played an important role in the interaction of the catalyst surface and degrading activity. The stability is very important for practical application of photocatalyst. Hence, the recycling degradation experiments over AgBr/ZnO-10% composite were investigated in Fig. 7. After each run, the catalyst was collected and washed with ethanol then reused for the next run. The catalytic activities kept almost unchanged after the five cycles, indicating the good stability of the AgBr/ZnO catalyst. Fig. 8 shows the XRD patterns of AgBr/ZnO-10% composite before the photocatalytic reaction and after the catalytic reaction cycles. After three catalytic runs, the new peak attributed to Ag appeared, indicative of the instability of AgBr/ZnO composite. Ag peak intensity increased with the circulating runs increasing. The constitutions of the AgBr/ZnO sample were slightly changed after catalytic runs, but the activities did not change obviously after five recycling experiments, which have been observed in the previous studies [18,20]. 3.3. Possible photocatalytic mechanism To detect the main reactive species for the degradation RhB, the effects of radical scavengers were examined in order to discuss the reaction mechanism (Fig. 8). Here, methanol was used to quench the OH [26,27], ethylene diamine tetraacetic acid (EDTA) for h+ scavenger [27,28], p-benzoquinone for O 2 scavenger [26,27], and dimethyl sulfoxide (DMSO) for e scavenger [4]. Fig. 9 shows the effects of different scavengers on the degradations of RhB. Adding methanol, EDTA and DMSO resulted in the decrease of degradation activities, which implied that OH, h+ and e were the active reactive species. Moreover, the addition of p-benzoquinone made the degradation of RhB almost inhibit, which suggested that O 2 was the key reactive species. Since photoluminescence spectra emission arises from the recombination of free carriers, PL is widely used to investigate the mitigation, transfer and recombination processes of the photogenerated electron–hole pairs in a semiconductor. Fig. 10 shows the PL spectra of ZnO and AgBr/ZnO-10% with the excitation wavelength of 300 nm. Pure ZnO showed higher intensity of emission spectrum than AgBr/ZnO-10% composite. It is generally believed that a weaker PL intensity means a lower recombination probability of photogenetated charge carrier. Therefore, incorporation of AgBr on the surface of ZnO can improve the separation of photogenetated electrons and holes, leading to the improved photocatalytic activity. To further prove the effective separation of photogenetated electrons and holes in AgBr/ZnO system under visible light irradiation, the surface photovoltage spectroscopy (SPS) measurements were
Fig. 6. Effects of pH value of the reaction suspension on photodegradation of RhB over AgBr/ZnO-10% composite. (a) PH = 4.4, (b) PH = 7 and (c) PH = 10.
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Fig. 7. Cycling runs in the photodegradation RhB over AgBr/ZnO-10% composite.
Fig. 8. XRD patterns of AgBr/ZnO-10% composite before and after the photocatalytic reactions. (a) Fresh catalyst, (b) after 1 cycle, (c) after 3 cycles and (d) after 5 cycles.
carried out. Generally speaking, the more intense SPS signal corresponds to the better charge separation efficiency. In Fig. 11 there was a strong surface photovoltage response onset at 370 nm for the pure ZnO, related to the intrinsic transition [29]. After deposition of AgBr nanoparticles, the SPS intensity of AgBr/ZnO-10% composite was extended to the visible light (400–580 nm), and higher than that of pure ZnO in the visible range (inset). According to the DRS results, AgBr/ZnO showed high absorption under visible light, and moreover, the band gap of AgBr/ZnO was narrowed compared with bulk ZnO. Therefore, under visible light irradiation, the photogenerated e could easily transfer from AgBr to ZnO. The transfer of photogenerated e was beneficial to the effective separation of photogenerated e–h+ pairs, resulting in a superior photocatalytic activity. A possible mechanism for the degradation RhB over AgBr/ZnO was shown in Fig. 12. After AgBr/ZnO was irradiated by visible light, photogenerated e–h+ pairs were formed. Subsequently e- transferred from the conduction band (CB) of AgBr to that of ZnO, then combined with O2 to form O 2 (Eq. (2)). At the same time, h+ in the valence band (VB) of AgBr and ZnO combined with H2O to produce active OH (Eqs. (6) and (7)). The radicals of O 2 and OH were so reactive that they could efficiently degrade RhB
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Fig. 9. Effects of different radical scavengers on the degradation RhB over AgBr/ZnO-10% catalyst (a) no scavenger, (b) adding 5 mmol L1 DMSO, (c) adding 10 mmol L1 methanol, (d) adding 1 mmol L1 EDTA, and (e) adding 1 mmol L1 p-benzoquinone.
Fig. 10. Photoluminescence spectra of ZnO and AgBr/ZnO-10% composite.
into less organic matters and finally into H2O and CO2. The major reaction steps in this photocatlytic mechanism under visible light irradiation can be formulated as equations.
AgBr=ZnO þ visible light ! e þ h
þ
e þ O2 ! O2
O2
þ
ð2Þ
þ H ! OOH þ
ð1Þ
ð3Þ
OOH þ H þ e ! H2 O2
ð4Þ
H2 O2 þ e ! OH þ OH
ð5Þ
þ
ð6Þ
h þ OH ! OH
þ
ð7Þ
OH= O2 þ RhB ! Degradation products
ð8Þ
h þ H2 O ! OH þ Hþ
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Fig. 11. SPS responses of ZnO and AgBr/ZnO-10% composite (inset is the enlarged SPS spectrum in the region of 400–580 nm).
Fig. 12. Possible reaction mechanism over AgBr/ZnO under visible light irradiation.
4. Conclusions AgBr/ZnO composite was successfully synthesized using CTAB as Br source and stabilizer by deposition–precipitation method. AgBr nanoparticles with small size of about 10 nm were well dispersed on the surface of ZnO support. Furthermore, AgBr/ZnO showed strong absorption in the visible light range. The combining of AgBr with ZnO led to the great improvement in the photocatalytic efficiency for degradation RhB under visible light. The enhanced photocatalytic activity was due to the synergistic effects including effective separation of photogenerated e–h+, narrowed band gap and enhanced visible light absorption resulted from the modified AgBr with small sizes. Thus, AgBr/ZnO displayed a promising candidate with satisfying photocatalytic efficiency in the applications of pollutants abatements. Acknowledgements We sincerely acknowledge the financial supports from National Natural Science Foundation of China (21073049), New Century Excellent Talents in University (NCET-10-0064), State Key Lab of Urban Water Resource and Environment of Harbin Institute of Technology (HIT2013TS01).
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Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Letter to the Editor
Improved photocatalytic performance over AgBr/ZnO under visible light Lei Shi b, Lin Liang a,c, Jun Ma a, Jianmin Sun a,b,⇑ a b c
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150080, China The Academy of Fundamental and Interdisciplinary Science, Harbin Institute of Technology, Harbin 150080, China School of Life Science and Technology, Harbin Institute of Technology, Harbin 150080, China
a r t i c l e
i n f o
Article history: Received 13 June 2013 Received in revised form 1 July 2013 Accepted 23 July 2013 Available online 3 August 2013 Keywords: AgBr/ZnO composite Photocatalyst Dye degradation Visible light
a b s t r a c t AgBr nanoparticles modified ZnO have been successfully synthesized by using cetyltrimethylammonium bromide as the bromide source and stabilizer. X-ray diffraction and X-ray photoelectron spectroscopy confirmed the loading of AgBr nanoparticles on ZnO support. The transmission electron microscopy showed that AgBr nanoparticles with small size were well dispersed on the surface of ZnO support. UV–visible diffuse reflectance spectra displayed the absorbance of AgBr/ZnO composite was much higher than that of pure ZnO in the visible light range. The photocatalytic activities of degradation Rhodamine B dye under visible light over AgBr/ZnO catalyst were markedly enhanced compared with pure ZnO. The improved photocatalytic activities may be attributed to the synergetic effects including enhanced visible light absorption, narrowed band gap and effective separation of photogenerated e–h+ resulted from the highly dispersed small nanosized AgBr. Moreover, the possible mechanism was tentatively proposed based on the photoluminescence spectra and surface photovoltage spectroscopy. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Widespread discharge of wastewater from the textile industries which contains large amounts of dyes has become great concern to the environment and ecosystem due to their non-biodegradability, ⇑ Corresponding author at: State Key Laboratory of Urban Water Resource and Environment, The Academy of Fundamental and Interdisciplinary Science, Harbin Institute of Technology, Harbin 150080, China. Tel.: +86 451 86403715. E-mail address: [email protected] (J. Sun). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.07.013
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toxicity, and potential carcinogenicity. Some treatments such as adsorption, membrane filtration and chemical precipitation are the effective methods for cleaning up the pollutants in wastewater [1–3]. In recent years, photocatalysis technology is considered as a cost-effective method for degrading various hazardous contaminants in water because pollutants can be oxidized quickly and non-selectivity [4,5]. Dyes can be decomposed to less organic matters and finally into H2O and CO2 by photocatalysts. Many semiconductor photocatalysts such as TiO2, ZnO, Co3O4, CdS and Ag3PO4 have been developed [6–10]. Although TiO2 is widely used as the most important photocatalyst, ZnO is also a good alternative candidate owing to its high photocatalytic activity in the ultraviolet range, low cost, thermal and chemical stability and environmentally friendly features [11]. However, the low quantum efficiency of ZnO due to the fast recombination of photogenerated holes–electrons (h+–e) pairs, and a wide band gap of 3.3 eV seriously limit its wide applications in visible light range. In order to expand the usage of ZnO photocatalyst, it is of utmost importance to prevent the h+–e pairs from recombination and enhance the absorbance of ZnO in visible light range. Hence, many efforts have been devoted to the development of ZnO-based visible light photocatalysts, such as noble metal deposition [12], non-metal doping [13], photosensitive material modification [14], coupling with semiconductor composite [15] and conjugated polymer modification [16]. The silver bromide AgBr is a well-known photosensitive material. When AgBr is well dispersed on the surface of support materials, AgBr absorbs photons in the visible light to generate h+–e pairs during the photosensitive process, and the electrons can shift to the surface of support to induce the separation of photogenerated h+–e pairs, thus improving the photocatalytic ability [17–20]. As photocatalytic reactions take place mainly on the surface of the catalyst, nanoscaled active centers are believed to exhibit better performance than its bulk counterpart due to its high surface-to-volume ratio. In addition, the probability of photogenerated h+–e pair recombination will drastically be reduced owing to their fast arrival at the surface sites. Husein reported that AgBr nanoparticles with small size and narrow size distribution were prepared via reaction of AgNO3 with counter anion of cetyltrimethylammonium bromide (CTAB) individual micelles in microemulsions, which effectively prevented the aggregation of AgBr [21]. Inspired by the roles of surfactant CTAB, in this paper, we selected CTAB as the bromide source and stabilizer to facilely synthesize AgBr modified ZnO to improve photocatalytic activities in visible light. CTAB could effectively control the small size of AgBr, leading to well-dispersed AgBr on the surface of ZnO support. The photocatalytic activities of the AgBr/ZnO composite were evaluated by the photodegradation Rhodamine B (RhB) under visible light irradiation. Moreover, the surface photovoltage spectroscopy was investigated using electrochemical method and the photocatalytic mechanism was tentatively proposed. 2. Experimental 2.1. Synthesis of AgBr/ZnO composite AgBr/ZnO composite was synthesized by the deposition–precipitation method in the presence of CTAB as surfactant and Br source. 1.0 g ZnO was added into 50 mL distilled water and stirred for 30 min at room temperature. Then 1.2 g CTAB was added with vigorous stirring for 60 min, 0.21 g AgNO3 in 1 mL NH4OH (25 wt%) was subsequently dropped into the mixture. After stirring at room temperature for 12 h, the suspensions were filtered, washed with distilled water and dried at 60 °C for 12 h. Finally, the powder was calcined at 500 °C in air for 3 h. Then yellow powder denoted as AgBr/ZnO-10% with theoretical Ag/Zn molar ratio of 1:10, was obtained. In a similar process, AgBr/ ZnO composites with different AgBr contents were obtained and denoted as AgBr/ZnO-5% and AgBr/ ZnO-20%, respectively. 2.2. Material characterizations The X-ray diffraction (XRD) measurement was carried out on Bruke D8 Advance X-ray powder diffractometer with Cu Ka radiation (40 kV, 40 mA) for phase identification. The morphology, particle
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Fig. 1. XRD patterns of (a) pure ZnO and (b) AgBr/ZnO-10% composite.
size and chemical compositions of the product were examined by transmission electron microscopy (TEM, Tecnai G2 Spirit) equipped with an energy dispersion X-ray spectrometer (EDS). X-ray photoelectron spectroscopy (XPS) measurements were recorded on a PHI 5000C ESCA system with Al Ka radiation (hm = 1486.6 eV). The UV–visible diffuse reflectance spectra (DRS) were measured by a Shimadzu UV-2500 spectrophotometer. The photoluminescence spectra (PL) were obtained by a Varian Cary Eclipse spectrometer with an excitation wavelength of 300 nm. The surface photovoltage spectroscopy (SPS) measurements were carried out with a homemade apparatus (a 500 W Xe lamp) in Ref. [22]. 2.3. Photocatalytic testing The photocatalytic performance of AgBr/ZnO composite was evaluated through degradation RhB dye under visible light. The visible light was provided by a 300 W iodine tungsten lamp (Philips Co.) with cut off filter. 0.1 g AgBr/ZnO composite was dispersed into 100 mL 5 mg L1 RhB solution for photocatalytic examinations under magnetic stirring. The dispersion was kept in dark for 30 min under magnetic stirring to reach the adsorption–desorption equilibrium before light irradiation. The distance between the surface of the dye suspension and the light source was about 50 cm. Solutions were collected every 0.5 h and centrifuged to remove the catalysts then analyzed on Shimadzu UV-2500 spectrophotometer. 3. Results and discussion 3.1. Characterizations of AgBr/ZnO composite Fig. 1 shows the XRD patterns of pure ZnO and AgBr/ZnO-10% composite. It was observed that the diffraction peaks of single hexagonal phase ZnO (JCPDS card no. 076–0704) did not obviously change after modification by AgBr nanoparticles. Additionally, some new peaks at 2h of 31.1°, 44.4°, 55.1° and 64.6°, corresponding to cubic phase AgBr (JCPDS card no. 06–0438) were observed, which confirmed the loading of AgBr on ZnO support. The synthesis procedure of AgBr/ZnO composite was shown in Scheme 1. CTAB as cation surfactant and Br source was first adsorbed on the surface of ZnO support. With the addition of Ag+, it was easily combined with CTAB to form AgBr nanoparticles, at the same time, CTAB formed individual micelles
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Scheme 1. Schematic illustration of the synthesis procedure of AgBr/ZnO composite.
that wrapped AgBr nanoparticles, preventing AgBr further agglomeration. Thus, small sized AgBr nanoparticles were attached on the surface of ZnO. The uniform distributions of AgBr nanoparticles on ZnO were confirmed by TEM images in Fig. 2. Pure ZnO displayed nearly rod-like morphology and the surface of pure ZnO was smooth. In the AgBr/ZnO composite, the small AgBr nanoparticles at the size of about 10 nm were well anchored on the surface of ZnO. EDS results revealed the presence of Ag, Br, Zn and O elements in the AgBr/ZnO composite, which were in good agreement with the XRD results. The surface structures of the pure ZnO and AgBr/ZnO-10% composite were investigated using XPS analyses in Fig. 3. In the AgBr/ZnO sample, all of the peaks ascribed to Ag, Br, O and Zn elements were observed, which were consistent with the results of EDS and XRD. Peaks at 367.4 and 373.4 eV were assigned to Ag 3d5/2 and Ag 3d3/2; the peak at 68.5 eV was ascribed to Br 3d; two symmetrical peaks centered at 531.0 and 532.5 eV were associated with the lattice oxygen of ZnO and chemisorbed oxygen caused by the surface hydroxyl, respectively [23]. No peak was present at the binding energy of about 529.0 eV attributed to O in Ag2O [24], further confirming the existence of Ag as the form of AgBr. The peak at 1022.4 eV was identified as Zn 2p3/2 of ZnO. The UV–visible diffuse reflectance spectra of pure ZnO and AgBr/ZnO-10% composite were illustrated in Fig. 4. A broad tail between 400 nm and 800 nm appeared in the AgBr/ZnO sample, indicating that it had optical capability during the visible light range. Moreover, the absorbance of the as-synthesized AgBr/ZnO composite was much higher than that of pure ZnO in the visible light range, which was due to the surface plasmon resonance effect of AgBr. And the wavelength threshold of AgBr/ZnO was estimated to 407 nm, corresponding to the band gap at 3.04 eV. The band gap of AgBr/ZnO was narrower than the 3.3 eV of pure ZnO.
3.2. Photodegradation activity of RhB Fig. 5(A) depicts the photocatalytic activity for RhB degradation over pure ZnO and a series of AgBr/ ZnO composites under visible light. The degradation RhB hardly occurred with visible light irradiation even for 3 h without any catalyst. While only 35% RhB was degraded over pure ZnO under the same conditions. Noticeably, compared with pure ZnO, AgBr/ZnO samples exhibited the enhanced photocatalytic activity. AgBr/ZnO-10% composite exhibited the optimal photocatalytic activity, and almost 95% RhB was degraded in 3 h. AgBr/ZnO composite possessed better photocatalytic activities for decomposing RhB than pure ZnO, resulting from the smaller size and good dispersibility of AgBr nanoparticles on the ZnO support. Fig. 5(B) shows the linear relationships between ln (C0/C) and the irradiation time, where C0 is the original concentration and C represents RhB concentration at certain time interval. The photocatalytic degradation curves of RhB fit well with first-order reaction. The rate constants of AgBr/ZnO-5%, AgBr/ZnO-10% and AgBr/ZnO-20% were 0.6632 h1, 1.034 h1 and 0.8238 h1,
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Fig. 2. TEM images of (a) pure ZnO, (b) AgBr/ZnO-10% composite and (c) EDS analysis for AgBr/ZnO-10% composite.
respectively. The AgBr/ZnO-10% composite exhibited the highest rate constant, which was approximately 6.5 times larger than that of ZnO (k = 0.1547 h1). The effects of pH value in the solution on the photodegradation rate were investigated in Fig. 6. At the pH 4.4, approximately 80% RhB was degraded in 3 h. However, when the solution became alkaline
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Fig. 3. XPS spectra of (a) full spectra of pure ZnO and AgBr/ZnO-10%, (b) Ag 3d spectra, (c) Br 3d spectra, (d) O 1s spectra and (e) Zn 2p spectra in the AgBr/ZnO composite.
(pH = 10), the degradation efficiency was increased to almost 100% in 2 h. At acidic conditions, the photocatalytic efficiency was decreased possibly because ZnO could react with acid. However, AgBr/ZnO
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Fig. 4. DRS spectra of (a) pure ZnO and (b) AgBr/ZnO-10% composite.
Fig. 5. Photodegradation of RhB over pure ZnO and AgBr/ZnO composite (a) blank, (b) pure ZnO, (c) AgBr/ZnO-5%, (d) AgBr/ ZnO-20% and (e) AgBr/ZnO-10%.
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surface was negatively charged by adsorbing OH ions at high pH, and OH ions in the reaction medium favored the formations of OH radicals [25], which benefited for improving the photocatalytic activity of AgBr/ZnO. Hence, pH value of the solution played an important role in the interaction of the catalyst surface and degrading activity. The stability is very important for practical application of photocatalyst. Hence, the recycling degradation experiments over AgBr/ZnO-10% composite were investigated in Fig. 7. After each run, the catalyst was collected and washed with ethanol then reused for the next run. The catalytic activities kept almost unchanged after the five cycles, indicating the good stability of the AgBr/ZnO catalyst. Fig. 8 shows the XRD patterns of AgBr/ZnO-10% composite before the photocatalytic reaction and after the catalytic reaction cycles. After three catalytic runs, the new peak attributed to Ag appeared, indicative of the instability of AgBr/ZnO composite. Ag peak intensity increased with the circulating runs increasing. The constitutions of the AgBr/ZnO sample were slightly changed after catalytic runs, but the activities did not change obviously after five recycling experiments, which have been observed in the previous studies [18,20]. 3.3. Possible photocatalytic mechanism To detect the main reactive species for the degradation RhB, the effects of radical scavengers were examined in order to discuss the reaction mechanism (Fig. 8). Here, methanol was used to quench the OH [26,27], ethylene diamine tetraacetic acid (EDTA) for h+ scavenger [27,28], p-benzoquinone for O 2 scavenger [26,27], and dimethyl sulfoxide (DMSO) for e scavenger [4]. Fig. 9 shows the effects of different scavengers on the degradations of RhB. Adding methanol, EDTA and DMSO resulted in the decrease of degradation activities, which implied that OH, h+ and e were the active reactive species. Moreover, the addition of p-benzoquinone made the degradation of RhB almost inhibit, which suggested that O 2 was the key reactive species. Since photoluminescence spectra emission arises from the recombination of free carriers, PL is widely used to investigate the mitigation, transfer and recombination processes of the photogenerated electron–hole pairs in a semiconductor. Fig. 10 shows the PL spectra of ZnO and AgBr/ZnO-10% with the excitation wavelength of 300 nm. Pure ZnO showed higher intensity of emission spectrum than AgBr/ZnO-10% composite. It is generally believed that a weaker PL intensity means a lower recombination probability of photogenetated charge carrier. Therefore, incorporation of AgBr on the surface of ZnO can improve the separation of photogenetated electrons and holes, leading to the improved photocatalytic activity. To further prove the effective separation of photogenetated electrons and holes in AgBr/ZnO system under visible light irradiation, the surface photovoltage spectroscopy (SPS) measurements were
Fig. 6. Effects of pH value of the reaction suspension on photodegradation of RhB over AgBr/ZnO-10% composite. (a) PH = 4.4, (b) PH = 7 and (c) PH = 10.
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Fig. 7. Cycling runs in the photodegradation RhB over AgBr/ZnO-10% composite.
Fig. 8. XRD patterns of AgBr/ZnO-10% composite before and after the photocatalytic reactions. (a) Fresh catalyst, (b) after 1 cycle, (c) after 3 cycles and (d) after 5 cycles.
carried out. Generally speaking, the more intense SPS signal corresponds to the better charge separation efficiency. In Fig. 11 there was a strong surface photovoltage response onset at 370 nm for the pure ZnO, related to the intrinsic transition [29]. After deposition of AgBr nanoparticles, the SPS intensity of AgBr/ZnO-10% composite was extended to the visible light (400–580 nm), and higher than that of pure ZnO in the visible range (inset). According to the DRS results, AgBr/ZnO showed high absorption under visible light, and moreover, the band gap of AgBr/ZnO was narrowed compared with bulk ZnO. Therefore, under visible light irradiation, the photogenerated e could easily transfer from AgBr to ZnO. The transfer of photogenerated e was beneficial to the effective separation of photogenerated e–h+ pairs, resulting in a superior photocatalytic activity. A possible mechanism for the degradation RhB over AgBr/ZnO was shown in Fig. 12. After AgBr/ZnO was irradiated by visible light, photogenerated e–h+ pairs were formed. Subsequently e- transferred from the conduction band (CB) of AgBr to that of ZnO, then combined with O2 to form O 2 (Eq. (2)). At the same time, h+ in the valence band (VB) of AgBr and ZnO combined with H2O to produce active OH (Eqs. (6) and (7)). The radicals of O 2 and OH were so reactive that they could efficiently degrade RhB
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Fig. 9. Effects of different radical scavengers on the degradation RhB over AgBr/ZnO-10% catalyst (a) no scavenger, (b) adding 5 mmol L1 DMSO, (c) adding 10 mmol L1 methanol, (d) adding 1 mmol L1 EDTA, and (e) adding 1 mmol L1 p-benzoquinone.
Fig. 10. Photoluminescence spectra of ZnO and AgBr/ZnO-10% composite.
into less organic matters and finally into H2O and CO2. The major reaction steps in this photocatlytic mechanism under visible light irradiation can be formulated as equations.
AgBr=ZnO þ visible light ! e þ h
þ
e þ O2 ! O2
O2
þ
ð2Þ
þ H ! OOH þ
ð1Þ
ð3Þ
OOH þ H þ e ! H2 O2
ð4Þ
H2 O2 þ e ! OH þ OH
ð5Þ
þ
ð6Þ
h þ OH ! OH
þ
ð7Þ
OH= O2 þ RhB ! Degradation products
ð8Þ
h þ H2 O ! OH þ Hþ
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Fig. 11. SPS responses of ZnO and AgBr/ZnO-10% composite (inset is the enlarged SPS spectrum in the region of 400–580 nm).
Fig. 12. Possible reaction mechanism over AgBr/ZnO under visible light irradiation.
4. Conclusions AgBr/ZnO composite was successfully synthesized using CTAB as Br source and stabilizer by deposition–precipitation method. AgBr nanoparticles with small size of about 10 nm were well dispersed on the surface of ZnO support. Furthermore, AgBr/ZnO showed strong absorption in the visible light range. The combining of AgBr with ZnO led to the great improvement in the photocatalytic efficiency for degradation RhB under visible light. The enhanced photocatalytic activity was due to the synergistic effects including effective separation of photogenerated e–h+, narrowed band gap and enhanced visible light absorption resulted from the modified AgBr with small sizes. Thus, AgBr/ZnO displayed a promising candidate with satisfying photocatalytic efficiency in the applications of pollutants abatements. Acknowledgements We sincerely acknowledge the financial supports from National Natural Science Foundation of China (21073049), New Century Excellent Talents in University (NCET-10-0064), State Key Lab of Urban Water Resource and Environment of Harbin Institute of Technology (HIT2013TS01).
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