Ag2CO3 hybrids with enhanced visible light photocatalytic activity and improved stability

Journal of Molecular Catalysis A: Chemical 395 (2014) 16–24

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In-situ anion exchange synthesis of AgBr/Ag2 CO3 hybrids with enhanced visible light photocatalytic activity and improved stability Owais Mehraj, Niyaz A. Mir † , Bilal Masood Pirzada, Suhail Sabir ∗ , M. Muneer Department of Chemistry, Aligarh Muslim University, Aligarh – 202002, India

a r t i c l e

i n f o

Article history: Received 25 March 2014 Received in revised form 14 July 2014 Accepted 23 July 2014 Available online 1 August 2014 Keywords: AgBr/Ag2 CO3 hybrids Anion-exchange method Ag◦ nanoparticles Ponceau BS Visible light photocatalysis

a b s t r a c t AgBr/Ag2 CO3 hybrids were synthesized via an in situ anion-exchange reaction between Ag2 CO3 and NaBr. The obtained hybrids were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), UV–vis diffuse reflectance spectroscopy (DRS) and terepthalic acid-photo luminescence (TA-PL) technique. The as prepared AgBr/Ag2 CO3 hybrids exhibited wide absorption in the visible light region and displayed efficient and higher photocatalytic activities towards the degradation of dye molecules (Ponceau BS) as compared to pure AgBr and Ag2 CO3 samples under visible light irradiation ( > 400 nm). The enhanced photocatalytic activity of AgBr/Ag2 CO3 was related to the efficient separation of electron–hole pairs derived from matching band potentials between AgBr and Ag2 CO3 , as well as the good electron trapping role of Ag◦ nanoparticles in situ formed on the surface of AgBr and Ag2 CO3 particles during photocatalytic oxidation process. The quenching effect of different • scavengers of reactive species suggested that OH and h+ play major role in the degradation of PBS. The enhanced stability of the hybrid was attributed to the trapping of photogenerated electrons from the surface of catalyst by Ag◦ nanoparticles which suppress the photocorrosion. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Energy conversion and environmental accountability are two major challenges to the sustainable development of human society. Over the past decades the various advancements in the field of semiconductor-based photocatalysis have received considerable and prime attention focussed on the view point of solving environment and energy related issues [1–3]. The oldest and conventional TiO2 catalyst, although still most widely used because of its easy availability, low cost and nontoxicity has relatively wide band gap (3.2 eV) which has significantly limited its application to UV light only (4% of solar spectrum only) [4–6]. To exploit the solar energy, the development of visible light responsive photocatalysts has become one of the imperative and desired topics in the photocatalytic field [7–13]. To exploit the visible light responsive photocatalysts, there are usually two ways. One way is to introduce the intermediate energy levels between valence band and conduction band by doping metal or non-metal elements such as

∗ Corresponding author. Tel.: +91 571 2700920x3366. E-mail addresses: [email protected], [email protected] (S. Sabir). † Present address: Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560 012, India. http://dx.doi.org/10.1016/j.molcata.2014.07.027 1381-1169/© 2014 Elsevier B.V. All rights reserved.

V, Cr, Fe or S, C and N so that the absorption edge shifts into the visible range [14–17]. However in terms of photocatalytic activity this method is not ideal because dopants will serve as centres for electron–hole recombination. The other strategy is to develop novel materials such as BiVO4 [18,19], Ag2 MO4 O13 [20] and Bi2 WO6 [18,21] which can effectively utilize visible light that constitutes 43% of total sunlight. Lately a feasible and highly efficient strategy of incorporating pblock elements into the narrow band gap oxides was used to design new visible light driven photocatalysts [22]. The incorporation of pblock elements such as P or C into the reported Ag2 O broadens the narrow band gap of 1.3 eV [23] and enhances the oxidative ability and photocatalytic activity of new visible light driven photocatalyst like Ag3 PO4 [22], AgSbO3 [24], Ag2 CO3 [25]. All these silver containing compounds show promising photocatalytic activity. Ag2 CO3 with band gap of 2.46 eV has been recognised as one of the most promising visible light driven photocatalyst for its excellent and efficient photo oxidative capabilities [25]. However, the recycle experiments suggested that Ag2 CO3 was not stable and displayed photocorrosion which seriously deactivates the photocatalyst [25]. It is therefore necessary to fabricate the original Ag2 CO3 for improving its stability. Dai et al. [26] employed a new approach to inhibit the photocorrosion by adding AgNO3 in the reaction system which helps in trapping the electron from conduction band of Ag2 CO3 ,

O. Mehraj et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 16–24

however this is not the intrinsic property of catalyst itself. Another strategy for improving the stability and catalytic efficiency is to couple it with another semiconductor with matching band potential to form a heterojunction at the interface which will facilitate the separation of photoinduced charge carriers. Ye and co-workers [27] reported the formation of heterocrystals of AgX/Ag3 PO4 with matching band potentials of AgX and Ag3 PO4 , which facilitated the fast transfer and separation of photoinduced carriers. Cao et al. [28] reported AgBr/Ag3 PO4 hybrid as a promising and fascinating visible light driven photocatalyst. The fabrication of hetero structured photocatalysts with matching band potentials has turned out to be a very effective strategy to enhance the photocatalytic activity of pure materials. Thus BiOI/BiOBr [29] and AgBr/Ag3 PO4 [28] proved to be better visible light driven photocatalysts than pure BiOBr and Ag3 PO4 respectively. It is a well-known fact that light irradiation of silver based compounds results in the decomposition of compound and generates Ag nanoparticles directly on the surface of the catalyst [30,31]. The presence of noble Ag nanoparticles on the surface of a catalyst enhances the absorption of visible light and it helps to trap the photoinduced electrons effectively, e.g. Ag@Agx [32–35], Ag3 VO4 /AgBr/Ag plasmonic system [36] and Ag/TiO2 plasmonic system [37]. To the best of our knowledge, no significant attention has been paid to the fabrication of Ag2 CO3 with AgX and the surface plasmon resonance of metal nanoparticles in improving photodecolourization efficiency and stability of the catalyst. Herein we report a newly constructed AgBr/Ag2 CO3 hybrid with different content of AgBr by in situ anion exchange method. The anion exchange method was chosen because it helps to grow AgBr shell more effectively with high dispersity on the surface of Ag2 CO3 . This novel composite system had some advantages over pure Ag2 CO3 . Firstly it can easily transform to plasmonic Ag@AgBr/Ag2 CO3 @Ag system in the early stages of photocatalytic reaction. Secondly AgBr and Ag2 CO3 had matching band potentials which facilitate the fast separation and transfer of photoinduced charge carriers. This composite system is expected to exhibit high visible light induced photocatalytic activity and stability. Azo dye was used as a model pollutant to evaluate the photocatalytic activity under visible light. The stability of the photocatalyst was also investigated. 2. Experimental 2.1. Chemicals and materials All reagents were of analytical grade and used without further purification. Silver nitrate (AgNO3 ), sodium bicarbonate (NaHCO3 ), sodium bromide (NaBr), Ponceau BS (PBS) (Dye content ∼60%), terepthalic acid (TA), ammonium oxalate (AO), benzoquinone (BQ), Isopropyl alcohol (IPA) and catalase (CAT) were purchased from Sigma–Aldrich India. Double distilled water was used throughout the study. 2.2. Preparation of Ag2 CO3 The Ag2 CO3 samples were synthesized by simple ion exchange reaction between AgNO3 and NaHCO3 aqueous solutions. In a typical synthetic route, AgNO3 (40 ml, 0.2 M) solution was added drop wise into NaHCO3 (40 ml, 0.1 M) solution on ice water bath under vigorous stirring. The yellow green precipitates formed were filtered and washed several times with deionized water. The products were dried in a vacuum oven at 60 ◦ C for 6 h.

17

2.3. Synthesis of AgBr/Ag2 CO3 hybrids AgBr/Ag2 CO3 hybrids were obtained through an in situ anion exchange method in dark conditions at room temperature. The obtained Ag2 CO3 (1.0 g) powder was dispersed in 50 ml of distilled water and suspension was ultra-sonicated for 20 min. Subsequently different stoichiometric amounts of NaBr solutions were added drop wise into the above Ag2 CO3 suspension with constant stirring. The obtained suspension was vigorously stirred for 4 h. The samples containing different theoretical molar percentages of added Br/original C were noted as 10% AgBr/Ag2 CO3 , 30% AgBr/Ag2 CO3 , 50% AgBr/Ag2 CO3 , and 70% AgBr/Ag2 CO3 . The precipitates were collected, washed with deionized water several times and finally dried at 60 ◦ C for 24 h. The pure AgBr samples were obtained by a simple precipitation method for comparison [38]. 2.4. Characterization of AgBr/Ag2 CO3 photocatalyst X-ray diffraction pattern of as prepared samples was carried out at room temperature with Shimadzu XRD-6100 X-ray diffractometer. The morphology was probed by scanning electron microscopy (SEM, JEOL), on which elemental analysis was also probed using EDS attachment. A UV-vis-NIR spectrophotometer (Perkin-Elmer) equipped with an integrating sphere assembly was used to obtain DRS of the samples in the region of 300–800 nm. The fluorescence emission spectra were recorded over a wavelength range of 420–650 nm on Shimadzu Spectrofluorometer 5000 using 260 nm excitation source. 2.5. Evaluation of photocatalytic activity The visible light photocatalytic activity of pure Ag2 CO3 , AgBr and the hybrids was evaluated by studying the decolourization of an azo-dye Ponceau BS. The photocatalytic tests were performed in an immersion well photoreactor (consisting of inner and outer jacket) made of Pyrex glass equipped with a magnetic bar, a water circulating jacket and an opening for molecular oxygen. Irradiations were carried out using a visible light halogen linear lamp (500 W, 9500 Lumens). The reaction temperature was kept constant at 20 ± 0.3 ◦ C using refrigerated circulating liquid bath. Prior to the illumination, 180 ml of the dye solution (0.1 mM) containing appropriate quantity of the catalyst (1 g l−1 ) was magnetically stirred, while the solution was purged continuously with atmospheric air for at least 30 min in the dark to attain adsorption–desorption equilibrium between dye and catalyst surface. Aliquotes (5 ml) were collected at 2 min interval, centrifuged and filtered through 0.22 ␮m millipore filter to remove the catalyst particles. The catalyst free dye solution was then analysed with (Shimadzu UV–vis 1601) spectrophotometer. The change in absorbance of the dye was followed at its max (505 nm) as a function of irradiation time. The observed absorbance is proportional to Beer–Lambert Law in the range of studied dye concentration. The concentration of dye was calculated by standard calibration curve obtained from the absorbance of the dye at different known concentrations. To determine the effect of reactive oxygen species, various quenchers of species were introduced in the reaction system in the manner similar to the photocatalytic experiment. The dosage of quenchers was referred to the previous studies [39,40]. To • further investigate the formation of OH on the surface of catalyst under visible light irradiation, photo luminescence (PL) technique with Terepthalic acid as a probe molecule was used. The experimental procedure was referred to the previous studies [29].

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Fig. 1. XRD patterns of (a) Ag2 CO3 (b) 10% AgBr/Ag2 CO3 (c) 30% AgBr/Ag2 CO3 (d) 50% AgBr/Ag2 CO3 (e) 70% AgBr/Ag2 CO3 (f) AgBr.

Table 1 Pseudo-first-order rate constants (kapp ) of different photocatalysts for the decolourization of PBS calculated from the plots of ln(C0 /Ct ) versus irradiation time and crystallite size of pure and hybrid composites. Sample

kapp (min−1 )

0.0095 0.0492 0.1983 0.2125 0.2620 0.2346

recycling runs displayed diffraction peaks assigned to Ag metal at 38.06◦ [28] compared with the fresh 50% AgBr/Ag2 CO3 (Fig. 2(a)). 3.2. SEM and EDS analysis

Particle size (nm) Ag2 CO3

Ag2 CO3 AgBr 10% AgBr/Ag2 CO3 30% AgBr/Ag2 CO3 50% AgBr/Ag2 CO3 70% AgBr/Ag2 CO3

Fig. 2. XRD patterns of (a) fresh 50% AgBr/Ag2 CO3 (b) used 50% AgBr/Ag2 CO3 after 1 recycling run and (c) used 50% AgBr/Ag2 CO3 after 5 recycling runs.

AgBr

49.71 47.87 47.30 42.53 48.1 0

53.08 42.26 49.31 49.61 51.71

3. Results and discussion 3.1. XRD analysis Fig. 1 presents the XRD patterns of as prepared samples. It is observed that diffraction peaks of Ag2 CO3 (Fig. 1(a)) were in good agreement with those of monoclinic structure of Ag2 CO3 . The main diffraction peak corresponding to lattice plane (1 3 0) is centred at 2 = 33.5◦ [25]. For AgBr (Fig. 1(f)) diffraction peaks with 2 values of 31.0◦ , 44.3◦ , 55.0◦ were assigned to (2 0 0), (2 2 0), (2 2 2) crystal planes of cubic structure [36]. The AgBr/Ag2 CO3 hybrids (Fig. 1(b–e)) exhibited coexistence of AgBr and Ag2 CO3 phases. With peaks at 2 values corresponding to both Ag2 CO3 and AgBr indicating the formation of AgBr crystals after reacting with Br− . Further with increasing AgBr content the intensity of diffraction peaks of AgBr increased where as those of Ag2 CO3 decreased simultaneously. The Scherrer formula L = K/ˇcos B [29] was used to calculate the crystalline sizes of Ag2 CO3 and AgBr in the Ag2 CO3 /AgBr composites, where L is taken as crystalline size, K is a constant, ˇ is the FWHM measured in radians on the 2 scale,  B is the Bragg angle for diffraction peaks. The crystalline sizes of Ag2 CO3 and AgBr/Ag2 CO3 are given in Table 1. The crystallite size of Ag2 CO3 in the heterojunctions decreased with increase in AgBr loading up to 50% suggesting high dispersity of AgBr in the hybrids. Moreover the used AgBr/Ag2 CO3 hybrids turned little dark compared to fresh hybrids and this was attributed to the formation of Ag◦ nanoparticles on the catalyst surface. The used 50% AgBr/Ag2 CO3 samples after 1 (Fig. 2(b)) and 5 (Fig. 2(c))

The SEM images of pure Ag2 CO3 , AgBr and hybrids are presented in Fig. 3 and the corresponding EDS results are shown in Fig. S1–S5 (ESI). Fig. 3(a) shows that Ag2 CO3 particles have polyhedral rod like morphology with homogenous distribution and smooth surfaces, a factor which enhances the dye adsorption and improves the electron migration. Fig. 3(b–e) presents the SEM micrographs of AgBr/Ag2 CO3 , hybrids. The formation of hybrids is obvious from these figures which show the irregular cubic shape of AgBr particles (bright spots). The particle size distribution (length (␮m) verses frequency) for the samples was calculated from their corresponding SEM images and is shown in Fig. S6. Figs. S1–S5 (ESI) show the energy dispersive X-ray spectrum (EDS) and EDS elemental mapping for pure Ag2 CO3 , and the hybrids respectively. The EDS analysis demonstrated that the pure Ag2 CO3 (Fig. S1, ESI) were composed of Ag, C and O elements whereas AgBr/Ag2 CO3 hybrids (Figs. S2–S5, ESI) were consisted of Ag, C, O and Br. Further with increase in AgBr content, the percentage of Br detected by EDS also increased. All these results clearly prove the formation of AgBr. Since EDS is used to obtain only surface elemental composition therefore actual molar percentages were determined by XRF elemental analysis and the results revealed the virtually identical or very close values. The results show that for 10, 30, 50 and 70% AgBr/AgCO3 , the actual AgBr/AgCO3 percentages are 9.19, 29.21, 48.84 and 67.93 respectively. The EDS elemental mapping clearly shows that bromine is highly dispersed in the AgBr/Ag2 CO3 hybrids indicating the high dispersity of AgBr in all hybrids. It can be seen from the figure the dispersity of Br− in hybrids increases with increase in its content. This clearly proves that AgBr did not merely happen to be on the surface of Ag2 CO3 but small AgBr nanoparticles are highly dispersed in the hybrids. TEM analysis was used to further investigate the structure and morphology of Ag2 CO3 and hybrids. The TEM image of pure Ag2 CO3 (Fig. 4(a)) displays the smooth surface where as the AgBr/Ag2 CO3 hybrids (Fig. 4(b–d)) depict the formation of small AgBr nano particles on the surface of Ag2 CO3 crystal. The EDS mapping and TEM results clearly prove the formation of small AgBr nanoparticles which are in direct contact with smooth surface of Ag2 CO3 .

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Fig. 3. SEM images of (a) Ag2 CO3 (b) 10% AgBr/Ag2 CO3 (c) 30% AgBr/Ag2 CO3 (d) 50% AgBr/Ag2 CO3 (e) 70% AgBr/Ag2 CO3 .

3.3. DRS analysis The optical absorbance of obtained samples was measured by UV–visible diffuse reflectance spectra (DRS). As shown in the Fig. 5(a), Ag2 CO3 had an absorption edge around 466 nm, while AgBr had an absorption edge around 480 nm suggesting that both have strong absorption in the visible light region. As can be seen from the Fig. 5(a), the AgBr/Ag2 CO3 hybrids exhibited mixed absorption property in between 400 and 500 nm. Fig. 5(b) shows the extended and enhanced absorption of used 50% AgBr/Ag2 CO3 after 1 and 5 recycling runs in the visible region compared to fresh 50% AgBr/Ag2 CO3 which is because of the SPR effect of Ag◦ deposited directly on the surface of AgBr [30–36] and Ag2 CO3 [26] indicating the increased amount of Ag metal as confirmed by XRD analysis of used samples also. The band gap energy of the semiconductor was determined by the following expression [41]. (h · ˛) = (Ah − Eg )

n\2

(1)

Since ˛ is proportional to kubelka–munk function F(R), the expression becomes h · F(R) = (Ah − Eg )

n\2

(2)

 is the light frequency, F(R) is the kubelka–munk function, A is the proportional constant and Eg is the Band gap energy. The value of n is determined by the type of optical transition (n = 1 for direct transition and n = 4 for indirect transition). For AgBr the value of

n is 4 [11] and for Ag2 CO3 the value of n is also 4 since it belongs to indirect band gap semiconductor [25]. The Eg of Ag2 CO3 was determined from the plot of (F(R)·h)1/2 versus h (Fig. 6) and was elicited to be 2.46 eV. Accordingly the Eg of AgBr was found to be 2.48 eV from the plot of (F(R)·h)1/2 versus h (Fig. 6). The VB and CB edge potentials of a semiconductor at the point of zero charge were calculated using the empirical formula in equation (3) [42]. EVB = X − E c + 0.5Eg

(3)

EVB is the energy of VB edge potential, X is the electronegativity of semiconductor and Ec is the energy of free electrons on hydrogen scale (4.5 eV). The top of the valence band EVB of Ag2 CO3 and AgBr were calculated to be 2.75 eV/NHE and 2.55 eV/NHE respectively. Moreover conduction band edge potential ECB can be determined by: ECB = EVB − Eg .

(4)

Thus ECB for Ag2 CO3 and AgBr were elicited as 0.29 eV/NHE and 0.07 eV/NHE, respectively. 3.4. Photocatalytic properties The photocatalytic activity and stability of the as prepared AgBr/Ag2 CO3 photocatalysts was evaluated by the degradation of Azo dye Ponceau BS under visible light irradiation. For comparison

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Fig. 4. TEM images of Ag2 CO3 (a), 50% AgBr/Ag2 CO3 (b–d).

Fig. 6. Plot of (F(R)·h)1/2 versus energy (h) for the band gap energy of AgBr and Ag2 CO3 .

Fig. 5. (a) DRS of AgBr, Ag2 CO3 , and AgBr/Ag2 CO3 hybrids. (b) DRS of Fresh 50% AgBr/Ag2 CO3 hybrid and used 50% AgBr/Ag2 CO3 after 1 and 5 recycling runs.

photocatalytic activity of pure AgBr and Ag2 CO3 samples and direct photolysis of PBS in absence of photocatalyst was also investigated. Fig. 7(a) shows the decolourization of PBS solution in the presence of pure AgBr, Ag2 CO3 and AgBr/Ag2 CO3 hybrids. The dark adsorption of PBS over each sample is insignificant and can be neglected. Fig. 7(a) shows that under visible light irradiation, all AgBr/Ag2 CO3 hybrids demonstrated efficient photocatalytic activity compared to pure AgBr and Ag2 CO3 , while PBS could hardly be decolourized in the absence of catalyst, indicating that photolysis of PBS can be neglected. Among the heterojunctions, 50% AgBr/Ag2 CO3 exhibited highest photocatalytic activity with approximately 90% decolourization of PBS compared to pure Ag2 CO3 (9%) and pure AgBr (38%) within 10 min of irradiation in visible light. It is worthwhile to

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Fig. 8. (a) Effect of scavengers on decolourization of PBS over 50% AgBr/Ag2 CO3 . (b) • PL spectral changes of OH trapping over Ag2 CO3 and AgBr/Ag2 CO3 hybrids in TA solution.

Fig. 7. a. Photocatalytic activities of Ag2 CO3 , and AgBr/Ag2 CO3 hybrids on the decolourization of PBS under visible light ( > 400 nm). (b). UV–vis spectra of PBS in aqueous suspension of 50% AgBr/Ag2 CO3 at different irradiation times. Inset: Colour change of the PBS solution at different irradiation times.

mention here that the photocatalytic activity of the hybrids is related to the content of AgBr loading and the activity sequence is in the order as follows: AgBr (50%) > AgBr (70%) > AgBr (30%) > AgBr (10%). These results suggest that the optimum content of AgBr in AgBr/Ag2 CO3 hybrid is 50% i.e. the Br/C molar ratio is 0.50. The more likely factor for the enhancement of the photocatalytic activity is the formation of heterojunction between AgBr and Ag2 CO3 which effectively separates the photoinduced charge carriers and thus enhances the photocatalytic activity. Another possibility for improved activity is the presence of Ag◦ particles in situ formed on the surface of catalyst during photocatalytic oxidation process [26,30–35]. These Ag◦ particles trap the excited electrons and hence facilitate the separation of photoinduced charge carriers [43,44]. The decrease in photocatalytic activity with increase in Br content above optimum value is attributed to decrease in heterojunction interface and increase in Ag◦ clusters. Although Ag◦ clusters can effectively trap the photo induced electrons [43–46], but with increase in the size of Ag◦ clusters the capability of accepting photoinduced holes also increases and the Ag◦ clusters become centres for recombination of electrons and holes [47,48] which counteracts the improvement in activity. It is well known that photocatalytic decolourization of most dyes follows pseudo-first-order kinetics model [49]. For our experimental conditions, data are in good

agreement with pseudo-first-order reaction as depicted by plotting ln(Co /Ct ) versus irradiation time (Figure not shown). The correlation constant for the fitted lines was calculated to be R2 ≥ 0.99 for all experiments. The values of apparent pseudo-first-order rate constant (kapp ) are given in Table 1. Fig. 7(b) shows decrease in absorption intensity of PBS at its max 505 nm as a function of irradiation time in the presence of 50% AgBr/Ag2 CO3 hybrid. The inset shows the corresponding colour changes of the PBS solution at different irradiation times. 3.5. Discussion of degradation mechanism 3.5.1. Formation of reactive species During the photocatalytic oxidation process, large number of • • reactive species including OH, h+ , H2 O2 , and O2 − are produced. To investigate the underlying mechanism of photodecolourization of PBS, various scavengers of reactive species were added to the reaction system and their effect on the overall photodecolourization efficiency was examined. Isopropyl alcohol (IPA) was added • to the system to quench OH [39,50] whereas benzoquinone (BQ) [39,51], Catalase (CAT) [39] and Ammonium oxalate (AO) [40] were • added as O2 − , H2 O2 , and h+ scavengers respectively. The results are shown in Fig. 8(a). As clear from the Fig. 8(a), BQ and catalase exhibit weaker effect on the photocatalytic decolourization efficiency indicating • that O2 − and H2 O2 are not primary reactive species in the photocatalytic oxidation process and thus played minor role. The addition of IPA or AO had a significant effect on kapp of PBS decolourization compared with no scavenger under same conditions indicating

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hybrid samples. This clearly indicates the higher electron–hole recombination rate in pure samples because of their narrow band gaps. In case of hybrid samples due to matching band potentials of Ag2 CO3 and AgBr, the photoinduced charge carriers migrate between Ag2 CO3 and AgBr and therefore reduce the electron–hole recombination. As can be seen from the Fig. 9, the fluorescence intensity of the hybrid samples decreases from 10% to 50% and then again increases in case of 70% hybrid. The lowest fluorescent emission intensity in case of 50% hybrid suggests lower electron–hole recombination and longer life time and hence more chance of these entering into the phenomenon of decolourization of substrate molecules. These results exhibit that AgBr/Ag2 CO3 heterojunction is helpful in reducing the electron–hole recombination and improving the photocatalytic activity.

Fig. 9. Fluorescence emission spectra of AgBr, Ag2 CO3 , and AgBr/Ag2 CO3 hybrids. •

the main role of OH and h+ as reactive specie in photocatalytic oxidation process. With the addition of IPA or AO, the kapp value • decreases significantly indicating that OH and h+ are primary reactive species. • To further confirm the existence of OH, terepthalic acid-photo • luminescence (TA-PL) technique was used to detect the OH on the surface of catalyst [52,53]. The PL spectral changes observed during the irradiation of AgBr/Ag2 CO3 in terepthalic acid (TA) solution (excitation 315 nm) are exhibited in Fig. 8(b). As shown in Fig. 8(b), PL signal at 425 nm is observed for all hybrids with maximum PL • intensity for 50% AgBr/Ag2 CO3 hybrid. It clearly indicates that OH is formed in the photocatalytic reaction which causes fluorescence by undergoing chemical reactions with TA [52,53]. It is important to mention here that the PL intensity decreased for 70% AgBr/Ag2 CO3 hybrid. Hence in AgBr/Ag2 CO3 hybrids the main reactive oxygen • species is OH which induces the decolourization in PBS under visible light. These results are in accordance with the fluorescence results discussed in Section 3.5.3. 3.5.2. Origin of reactive species Two possible reaction mechanisms may exist for the origin of reactive species including dye photosensitization and photocatalytic mechanism. TiO2 being visible light inactive has been used in most of the previous studies as a typical catalyst probe to study the photosensitization [54,55]. As can be seen from the Fig. 7(a) the degradation of PBS in the presence of TiO2 (Degussa P25) can be neglected indicating that photosensitization process can be ignored. It is therefore clear that the reactive species originate from the photocatalytic process of AgBr/Ag2 CO3 in which electron–hole pairs are directly produced by photocatalyst after illumination. These photoinduced charge carriers then lead to the formation of other reactive species. Further discussion regarding the formation of reactive species will be carried out in Section 3.5.4. 3.5.3. Fluorescence emission spectra: The photocatalytic activity of a photocatalyst is largely affected by the recombination of photoinduced electrons and holes [11]. The electron–hole recombination desipates energy in the form of fluorescence emission whose intensity is directly proportional to the recombination rate [56]. The fluorescence emission spectra of the pure and hybrid samples was measured using a UV light with 260 nm wavelength as excitation source and are presented in Fig. 9. As clear from the Fig. 9, the intensity of the fluorescence emission for pure Ag2 CO3 and AgBr samples is higher than those of

3.5.4. Possible activity enhancement mechanism Based on all the above discussions and DRS results, following mechanism may be proposed for the decolourization of PBS as described in the equations (5–15). Under visible light irradiation both AgBr and Ag2 CO3 are simultaneously excited and generate electron–hole pairs. The photo excited electrons on less positive conduction band of AgBr (0.07 eV) would prefer to flow down to the more positive conduction band of Ag2 CO3 (0.29 eV), while the photogenerated holes would flow from more positive valence band of Ag2 CO3 (2.75 eV) to less positive valence band of AgBr (2.55 eV). From the photo• chemistry point of view, it is not possible to reduce O2 to O2 − , • ◦ − E (O2 / O2 = −0.33 V/NHE) [57,58] through one electron reduction process because sufficient potential cannot be generated owing to the lower ECB values of Ag2 CO3 and AgBr. However the valence band edge potentials of Ag2 CO3 and AgBr are more positive • than E◦ ( OH/H2 O) = +2.27 eV [59], this demonstrated that sufficient • potential can be generated by h+ to oxidize H2 O to OH, which is a very strong oxidant for the decolourization of organic pollutants owing to its high oxidizing potential. It is important to mention here that some of the interstitial Agi ions are squeezed into the gap positions created because of the difference in the radius of Ag+ ion and Br− ions in AgBr [59]. These Agi ions are prone to reduction and get easily converted into Ag◦ particles by trapping an electron if available. Under visible light irradiation some of the photoexcited electrons on AgBr surface are trapped by these interstitial Agi ions present on the lattice gaps of AgBr. This results in the formation of Ag◦ particles on AgBr surface and eventually transform the AgBr/Ag2 CO3 into Ag@AgBr/Ag2 CO3 @Ag [28,35] in the early stages of photocatalytic reaction. These Ag◦ particle acts as excellent electron traps [41,42] which further facilitates the separation of photoinduced charge carriers and improves the stability of catalyst significantly. The Ag◦ clusters may also act as centres for two electron reduction process of O2 [27,58,60–62] (E◦ (O2 /H2 O2 ) = +0.695 V/NHE) [58] to form H2 O2 which combines • with an electron to form OH. The in situ formation of Ag◦ on the surface of catalyst during photocatalytic oxidation process is confirmed by the XRD analysis of used 50% AgBr/Ag2 CO3 after 1 and 5 recycling runs (Fig. 2(b and c)) compared to the fresh 50% AgBr/Ag2 CO3 hybrid (Fig. 2(a)). The enhanced visible light absorption of used 50% AgBr/Ag2 CO3 after 1 and 5 recycling runs (Fig. 5(b)) compared to fresh 50% AgBr/Ag2 CO3 also confirms the increased amount of Ag◦ metal. In conclusion the photogenerated electrons and holes can be effectively separated through the formation of heterojunction and efficient trapping of electrons by Ag◦ nanoparticles in situ formed on the surface of catalyst during PCO process. AgBr + h → AgBr(e− + h+ )

(5)

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Photocatalytic reaction •

HO + h+ + PBS → Products

(15)

The pictorial representation of the mechanism is given in Scheme 1.

Scheme 1. Schematic diagram of electron–hole pair separation and the possible reaction mechanism over AgBr/Ag2 CO3 photocatalyst under visible light irradiation.

Ag2 CO3 + h → Ag2 CO3 (e− + h+ )

(6)

AgBr(eCB − ) + Ag2 CO3 → Ag2 CO3 (eCB − )

(7)

+

+

Ag2 CO3 (h ) + AgBr → AgBr(h ) •

h+ + H2 O → H+ + HO

(8) (9)

Simultaneously Agi (AgBr) + e− → Ag◦

(10)

Ag2 CO3 (eCB − ) + Ag◦ → Ag◦ (e) + Ag2 CO3 −

◦

◦

AgBr(eCB ) + Ag → Ag (e) + AgBr +

−

◦

O2 + 2H + 2e (Ag ) → H2 O2 −

•

H2 O2 + e → HO + OH

−

3.5.5. Stability of the catalyst: Since Ag2 CO3 is unstable and could be easily decomposed by light irradiation which may be because of reduction of Ag+ into Ag◦ metal by electrons [26], therefore its stability is of vital consideration. However after the formation of heterojunction with AgBr the stability of the catalyst improved significantly. As shown in the Fig. 10, during the cycle experiments, the photocatalytic activity of the catalyst after 4 recycling runs is maintained except for the 6.6% decrease suggesting that the AgBr/Ag2 CO3 hybrid has high stability. Under visible light irradiation as the photocatalytic oxidation process proceeds, the AgBr/Ag2 CO3 transforms into Ag@AgBr/Ag2 CO3 @Ag plasmonic system [28,35] in the early stages of photocatalytic reaction as confirmed by XRD analysis of used 50% AgBr/Ag2 CO3 after 1 and 5 recycling runs. XPS quantitative elemental analysis of used samples after 1 and 5 recycling runs shows the amount of Ag+ reduced to Ag◦ and the data is given in Table S1 (ESI). The Ag◦ nanoparticles on the surface of catalyst act as excellent electron traps and trap the photoexcited electrons [41,42] efficiently from either of the conduction bands of heterojunction and serve as centres for two electron reduction process of O2 [27,58,60,62]. This also prevents the coupling of electrons with the interstitial Ag+ to form metallic Ag◦ and thereby preventing the photocorrosion of catalyst. Thus effective separation of electrons and holes by heterojunctions and subsequent trapping of electrons by Ag◦ nanoparticles in situ formed on the surface of catalyst helps the catalyst to retain its activity effectively after the initial reaction process.

(11) 4. Conclusion (12) (13) (14)

An efficient stable photocatalyst with double visible light active components was synthesized using an in situ anion exchange method. The AgBr/Ag2 CO3 hybrids displayed enhanced photocatalytic activity for the decolourization of Azo dye PBS compared to the single AgBr and Ag2 CO3 under visible light ( > 420 nm). Even after 4 cycling runs the photocatalytic activity did not show any significant decrease except for the 6.6%. On the basis of experimental and theoretically calculated results, the photocatalytic mechanism for the decolourization of PBS over AgBr/Ag2 CO3 under • visible light was suggested to be via OH and h+ oxidation mechanism. It was shown that the two factors i.e. heterojunction formed between AgBr and Ag2 CO3 and the efficient electron trapping by Ag◦ nanoparticles are responsible for enhanced activity and improved stability of the as obtained hybrids. Therefore this may provide promising way to construct AgX/Ag2 CO3 heterojunction photocatalysts to remove harmful pollutants and would provide another approach to understand the role of Ag◦ nanoparticles in improving stability of the catalyst.

Acknowledgements

Fig. 10. Cycling runs of 50% AgBr/Ag2 CO3 for decolourization of PBS under visible light.

The authors are highly thankful for the instrumentation facility provided by USIF AMU Aligarh, Department of Chemistry AMU Aligarh, and Department of Physics AMU Aligarh. The authors are also thankful to UGC and CSIR, New Delhi, India for providing financial assistance. The award of Senior Research Fellowship to Niyaz A. Mir from UGC New Delhi is gratefully acknowledged.

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