Ag2CO3 Nanocomposites for Photocatalytic

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Article Cite This: ACS Omega 2019, 4, 2618−2629

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Synthesis of LaFeO3/Ag2CO3 Nanocomposites for Photocatalytic Degradation of Rhodamine B and p‑Chlorophenol under Natural Sunlight Bilal M. Pirzada, Pushpendra, Ravi K. Kunchala, and Boddu S. Naidu* Energy and Environment Group, Institute of Nano Science and Technology (INST), Phase-10, Sector-64, Mohali 160062, Punjab, India

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S Supporting Information *

ABSTRACT: Novel LaFeO3/Ag2CO3 nanocomposites are synthesized by co-precipitation method for photocatalytic degradation of Rhodamine B (RhB) and p-chlorophenol under visible light irradiation. Heterostructures between LaFeO3 and Ag2CO3 semiconductors are formed during the synthesis of these nanocomposites. Among the nanocomposites prepared with different ratios of LaFeO3 and Ag2CO3, 1% LaFeO3/Ag2CO3 shows the highest photocatalytic activity for the degradation of RhB. Maximum electron−hole pair decoupling efficiency is observed in 1% LaFeO3/Ag2CO3, which causes the greater activity of the heterostructure. Degradation efficiency of 99.5% for RhB and 59% for p-chlorophenol has been obtained under natural sunlight within 45 min. Interestingly, the stability of Ag2CO3 is improved dramatically after making nanocomposite, and no decomposition of the catalyst was observed even after several photocatalytic cycles. Reactive oxygen species scavenging experiments with p-benzoquinone, isopropyl alcohol, and ammonium oxalate suggest that a major degradation process is caused by holes. Degradation of RhB into small organic moieties is detected using LC−MS technique. Further, the efficient mineralization of the degradation products occurs during the catalytic process.

1. INTRODUCTION Visible-light-driven photocatalysis has achieved extensive attention since Fujishima and Honda discovered the phenomenon of photoelectrochemical reactions over titania electrode.1 Subsequently, this discovery found applications in many fields, such as photocatalytic energy generation and decontamination of air and water. Because energy and the environment have great importance for the sustenance of humankind, heterogeneous photocatalysis has garnered the attention of various researchers. So far, many technologies, including physical, chemical, and biological treatments, have been employed for the removal of organic pollutants. Among all techniques, heterogeneous photocatalysis is regarded as one of the promising technologies owing to its low cost, being ecofriendly, and sustainability.2 As a step forward, semiconductorbased photocatalysis was primarily into focus to decontaminate water by photocatalytic degradation of dyes, pesticides, and other organic effluents.3 The semiconductor photocatalyst on exposure of light generates electron−hole pairs in the conduction band (CB) and valence band (VB), respectively.4 Separation of these charge carriers produces redox centers at VB and CB, which generate reactive oxygen species (ROS) to degrade the organic pollutants. However, the simultaneous recombination of the electron−hole pairs leads to the photocorrosion of the © 2019 American Chemical Society

photocatalyst, which decreases the catalytic performance. Moreover, the large band gap semiconductors are not desirable as they show response only in the UV light, which is less abundant in the sunlight. In this respect, researchers used various methods to enhance the visible light response of semiconductor photocatalysts and also to mitigate the charge carrier recombination. The various advancements include metal and nonmetal ion doping,5−7 heterostructure formation,8,9 polymer-based nanocomposites,10 metal−organic frameworks,11 liquid metal/metal oxide frameworks,12 and introduction of other visible light responsive moieties, namely, graphene oxide,13,14 carbon nitride,15,16 melamine,17 quantum dots,18−20 PbMoO4,21 Ga2O3,22 and so forth. Ag-based photocatalysts show good photocatalytic performance because of the strong surface plasmon resonance effect of Ag nanoparticles produced on the surface.23 Silver carbonate (Ag2CO3) is a visible-light-driven photocatalyst with a band gap of ∼2.7 eV. It exhibits high photocatalytic capability for degradation of organic pollutants.24,25 However, the undesirable photocorrosion leads to poor photostability and weak photocatalytic performance. Hence, photostability is a grave Received: October 16, 2018 Accepted: December 17, 2018 Published: February 4, 2019 2618

DOI: 10.1021/acsomega.8b02829 ACS Omega 2019, 4, 2618−2629

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issue with almost all reported silver-based photocatalysts.26,27 When exposed to visible light, Ag+ would be reduced to metallic silver (Ag0) because of the strong reduction potential of photoinduced electrons.28 Hence, the photocatalytic performance of such catalysts seriously decreases on recycling because of the partial loss of the catalyst. Therefore, various strategies were employed to address this limitation. Construction of the heterostructure with two semiconductors is proven to be one of the most effective strategies to enhance the stability and subsequent photocatalytic performance. By now, a large number of composite materials are synthesized, such as Ag3PO4/BiFeO3,29 AgCl/Ag2CO3,30 Ag2CO3/Ag2O,31 Ag3PO 4/AgI,32 g-C3N4/Ag2CO3,33 BiVO 4/Ag/Ag2CO 3,34 Ag2CrO 4/LaFeO 3,35 and so forth. The heterojunction influences the charge transfer properties, such as transfer pathway, transfer direction, separation, and recombination efficiencies of photoinduced charge carriers.36,37 The abovementioned characteristics influence the photocatalytic performance and stability. In this respect, it was thought that, by making a heterostructure with a perovskite metal ferrite, the stability and activity of Ag2CO3 may be increased.38 LaFeO3 is a p-type narrow band gap semiconductor and is a potential visible-light-driven photocatalyst.39,40 So, here in this work, we have successfully synthesized novel LaFeO3/Ag2CO3 heterostructure photocatalysts for the degradation of Rhodamine B (RhB) dye and p-chlorophenol. Photocatalytic activity of these materials has been studied under both xenon lamp and sunlight irradiation. ROS responsible for the photocatalytic activity are identified. The stability and activity of these photocatalysts are studied in successive cycles of reuse.

2θ values of 22.61 (101), 32.19 (121), 39.67 (220), 46.14 (202), and 57.40° (240).39 The values matched with the JCPDS file PDF 00-037-1493. The composite samples contain peaks from both LFO and Ag2CO3. Also, the main peak of LFO at 2θ value of 32.190° (121) grows with increasing amount of LFO in the nanocomposites. The crystallite size of the pure samples and the composites was determined on the basis of Scherrer equation using the main characteristic peaks. The average crystallite sizes obtained are presented in Table 1. Table 1. Average Crystallite Size of Various Samples as Calculated from XRD Patterns Using Debye−Scherrer Equation sample

peak chosen (2θ)

crystallite size (nm)

LFO Ag2CO3 1% LFO/Ag2CO3 5% LFO/Ag2CO3 10% LFO/Ag2CO3 20% LFO/Ag2CO3

32.180 33.479 33.479 33.479 33.479 33.479

28 68 65 60 58 57

The average crystallite size of Ag2CO3 decreases with the increase in the amount of LFO. The decrease in the crystallite size can be attributed to the dissimilar boundaries provided by the LFO nanoparticles, which inhibit the crystal growth.41−43 2.2. Microscopic Studies. 2.2.1. SEM. SEM images of Ag2CO3, LFO, and 1% LFO/Ag2CO3 nanocomposite are shown in Figure 2. Uniform small rod-shaped Ag2CO3 nanoparticles are observed with an average diameter of 500 nm (Figure 2a). For LFO, small granular particles are observed (Figure 2b). In the case of the nanocomposite sample, small LFO particles are found uniformly on the surface of rod-shaped Ag2CO3 (Figure 2c,d). This indicates the proximity and nanocomposite formation between the two phases. 2.2.2. TEM. TEM micrographs of the pure Ag2CO3, LFO, and 1% LFO/Ag2CO3 samples are recorded and presented in Figure 3. The rod-shaped Ag2CO3 particles with an average diameter of ∼500nm are observed (Figure 3a). LFO particles are granular with a particle size of 40−70 nm (Figure 3b). In the case of the 1% LFO/Ag2CO3 nanocomposite sample, rough-surfaced rods of Ag2CO3 decorated with LFO particles are observed (Figure 3c,d). This indicates that the two phases are in proximity and the heterostructure is formed. The HRTEM image of the composite materials shows the reflections from both phases (Figure 3e). The interplanar spacing d = 4.32 Å corresponds to the reflection from (110) plane of Ag2CO3, whereas d = 2.77 Å corresponds to the (121) plane of LFO. The SAED pattern also exhibited the reflections for both phases (Figure 3f). The diffraction spots at interplanar spacing d = 2.74 Å correspond to the (−101) plane of Ag2CO3 phase, whereas d = 1.96 Å corresponds to the (202) plane of LFO. These results further confirm the heterostructure formation between Ag2CO3 and LFO. 2.2.3. EDS Mapping. To check the uniform distribution of the elements, EDS mapping of the nanocomposite sample was recorded and is presented in Figure 4. It can be seen from the elemental distribution that all the elements are uniformly distributed in the nanocomposite material. 2.3. Photocatalytic Studies. The photocatalytic performance of these nanomaterials for the degradation of RhB molecules in the presence of visible light is investigated, and

2. RESULTS AND DISCUSSION 2.1. XRD Studies. XRD patterns of pure LFO, Ag2CO3, and their nanocomposites are recorded and shown in Figure 1. In the case of Ag2CO3, characteristic peaks can be observed at 2θ values of 18.55 (020), 20.54 (110), 32.66 (−101), 33.67 (130), 37.12 (200), and 39.67° (031).27 It is matched with the JCPDS file PDF 00-001-1071. Similarly, LFO shows peaks at

Figure 1. XRD patterns of the pure Ag2CO3, LFO, and their nanocomposites. 2619


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Figure 2. SEM micrographs of (a) Ag2CO3, (b) LFO, and (c,d) the 1% LFO/Ag2CO3 nanocomposite.

photocatalysis, which may be due to the efficient decoupling of electron−hole pairs in these composites.44 Natural-sunlight-driven experiments are performed with the best achieved photocatalyst (1% LFO/Ag2CO3) to assure the economical and broader viability of this photocatalyst. The absorption spectra for RhB and p-chlorophenol under sunlight at different time intervals are presented in the Supporting Information (Figure S2). It is found that the nanocomposite shows excellent photocatalytic activity even under natural sunlight and RhB degradation efficiency is 99.5% (Figure 6a). In sunlight, the kapp obtained for the RhB degradation is 0.064 min−1. The overwhelming activity under the sunlight irradiation can be attributed to the small portion of the UV light in sunlight, where the heterostructured material has higher absorption. To confirm the role of UV light, RhB degradation is studied under xenon lamp without any cut on filter. It shows that there is a significant increase in the catalytic activity as can be seen in Figure 6a,b, and the kapp is found to be 0.108 min−1 with 99.8% degradation in 45 min (Table 2). This heterostucture is also used for the degradation of p-chlorophenol under natural sunlight, and it shows a very good activity with 59% degradation as can be seen in Figure

the results are shown in Figure 5a−c. The overlay absorption spectra of RhB degradation for the various samples are presented in the Supporting Information (Figure S1). The absorption spectra of RhB solution during the photocatalytic experiment at different time intervals in the presence of 1% LFO/Ag2CO3 photocatalyst are shown in Figure 5a. In the control experiments, that is, in the absence of the photocatalyst, no degradation of RhB takes place (Figure 5b). The catalytic degradation is negligible in the dark, suggesting that exclusive photocatalytic reaction mechanism has taken place. The percentage of degradation and the apparent rate constant (Kapp) for various catalysts are given in Table 2. The degradation rate of RhB in 45 min is the highest for 1% LFO/ Ag2CO3 sample (Figure 5b) and is 98.8%, which is about 4.2 and 1.15 times greater than those of LFO (23.87%) and Ag2CO3 (87.15%). Figure 5c shows that the degradation process follows the first-order kinetic equation. The 1% LFO/ Ag2CO3 photocatalyst exhibits maximum photodegradation efficiency, and the kapp of 1% LFO/Ag2CO3 is 0.062 min−1, which is 19.0 and 2.0 times higher than those of LFO (0.004 min−1) and Ag2CO3 (0.032 min−1), respectively. This suggests that the nanocomposite shows a very high synergistic effect in 2620


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6c,d. The kapp obtained for p-chlorophenol degradation is 0.033 min−1. 2.4. Recyclability and Stability. Besides the efficiency, the stability and durability of the photocatalysts are also indispensable. To evaluate the stability of the pure Ag2CO3 and 1% LFO/Ag2CO3 photocatalysts, these catalysts are subjected to recycling, and the results are shown in Figure 7. The catalytic activity of Ag2CO3 decreases with the increase in the number of cycles (Figure 7a), whereas the 1% LFO/ Ag2CO3 nanocomposite sample exhibited excellent catalytic activity even after four cycles of reuse (Figure 7b). To check the stability of the photocatalysts after repetitive use, XRD patterns of the photocatalysts are recorded and shown in Figure 8. It can be observed from the XRD patterns that there is significant accumulation of Ag0 (2θ = 38.119) in the case of pure Ag2CO3 photocatalyst on repetitive use. However, in the case of the 1% LFO/Ag2CO3 nanocomposite, there is no accumulation of Ag0, which shows that the nanocomposite photocatalyst is highly stable against photocorrosion during the photocatalytic process. 2.5. Role of ROS. The ROS scavenging experiments with different scavengers are performed to find out the most dominant species responsible for the photocatalytic degradation of RhB under the visible light irradiation over 1% LFO/ Ag2CO3 nanocomposite. ROS species, such as superoxide radical anions (O2−·), hydroxyl radicals (OH·), and the hole (h+), are known to have a role in the photocatalytic dye degradation processes. The fate and the role of ROS are investigated using radical and hole trapping experiments, and the results are presented in Figure 9. Different scavengers, namely, p-benzoquinone (BQ) as O2−· scavenger, isopropyl alcohol (IPA) as OH· scavenger, and ammonium oxalate (AO) as the hole scavenger, are used for this purpose. The degradation of RhB is decreased significantly by adding AO in the RhB solution which indicates that the holes are the most dominant species in the degradation process. The similar

Figure 3. TEM micrographs of (a) pure Ag2CO3, (b) pure LFO, and (c,d) 1% LFO/Ag2CO3 composite; (e,f) HRTEM and SAED of 1% LFO/Ag2CO3 nanocomposite.

Figure 4. SEM images showing (a) 1% LFO/Ag2CO3 and (b) mixed distribution of different elements in 1% LFO/Ag2CO3. Distribution of (c) Ag, (d) La, (e) Fe, and (f) C in the 1% LFO/Ag2CO3 composite material. 2621


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Figure 5. Kinetics of photocatalytic decolorization of RhB in the presence of pure Ag2CO3, LFO, and LFO/Ag2CO3 heterostructures. (a) Change in absorption of RhB at regular intervals of light irradiation in the presence of the 1% LFO/Ag2CO3 photocatalyst, (b) change in concentration (Ct/C0), and (c) ln(C0/Ct) versus irradiation time of RhB during its decolorization in the presence of Ag2CO3, LFO, and LFO/Ag2CO3 heterostructures.

indicates the gradual generation of OH· by 1% LFO/Ag2CO3 nanocomposite during the photocatalytic process. 2.6. EIS and Transient Photocurrent Studies. To understand the enhanced photocatalytic activity with the composite material, the decoupling efficiency of the photogenerated electron−hole pairs across the interface in the semiconductor photocatalyst needs to be addressed. For this purpose, EIS Nyquist plots are explored.46,47 EIS Nyquist plots of LFO, Ag2CO3, and 1% LFO/Ag2CO3 under the visible light irradiation are presented in Figure 10a. The arc radius of the EIS Nyquist plot of 1% LFO/Ag2CO3 was smaller than those of the pure samples. These results suggest that more efficient charge carrier separation and faster interfacial charge transfer occurs on the nanocomposite when compared to pure LFO and Ag2CO3 photocatalysts.48,49 The transient photocurrent response for the LFO, Ag2CO3, and 1%LFO/Ag2CO3 samples is recorded up to four on−off cycles with a holding time of 30 s each, under xenon lamp illumination (Figure 10b). The photocurrent density obtained in the presence of light is approximately 4.0, 6.0, and 9.5 μA cm−2 for LFO, Ag2CO3, and 1% LFO/Ag2CO3, respectively. The current drops drastically when light is off and reproduces again when the light is on. It is found that LFO is having a stable response in comparison to Ag2CO3, which exhibits a drop in the photocurrent response in the successive cycles. However, the 1% LFO/Ag2CO3 nanocomposite shows the highest photocurrent response with stability. From these results, it can be suggested that the heterostructure formation

Table 2. Percentage of Degradation and Apparent Rate Constant of the Photocatalysts under Various Conditions photocatalyst LFO Ag2CO3 1% LFO/ Ag2CO3 1% LFO/ Ag2CO3 1% LFO/ Ag2CO3 1% LFO/ Ag2CO3

% of degradation efficiency

kapp (min−1)

xenon lamp, 395 nm filter, RhB dye xenon lamp, 395 nm filter, RhB dye xenon lamp, 395 nm filter, RhB dye xenon lamp, no filter, RhB dye sunlight, no filter, RhB dye

23.8

0.004

87.1

0.032

98.8

0.062

99.8

0.108

99.5

0.064

sunlight, no filter, pchlorophenol

59.0

0.033

condition

experiments with IPA and BQ followed that the OH· radicals are less responsible than the holes for the degradation process, whereas O2−· has the least role. The generation of OH· radicals during the photocatalytic process by 1% LFO/Ag2CO3 sample is determined by the terephthalic acid oxidation method. The emission intensity of dihydroxyterephthalic acid is the direct measure of the OH· concentrations.45 The PL spectra of the terephthalic acid solution after being illuminated under the visible light for 45 min with 1% LFO/Ag2CO3 are shown in the Supporting Information (Figure S3). The substantial increase in the emission intensities with irradiation time 2622


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Figure 6. Sunlight- and xenon-lamp-driven photocatalytic degradation of RhB and p-chlorophenol in the presence of pure 1% LFO/Ag2CO3 photocatalyst. (a) Change in concentration (Ct/C0) for RhB degradation under various conditions, (b) ln(C0/Ct) versus irradiation time for RhB degradation under various conditions, (c) change in concentration (Ct/C0) of p-chlorophenol at regular intervals of light irradiation, and (d) ln(C0/Ct) versus irradiation time for p-chlorophenol degradation.

Figure 7. Catalytic activity of (a) pure Ag2CO3 and (b) 1% LFO/Ag2CO3 in successive cycles of reuse.

has led to the decrease in recombination of the charge carriers and also enhanced the charge carrier efficiency.35 2.7. Optical Properties. Light absorption properties of the pure and nanocomposite materials are studied using UV− vis DRS, and the spectra are shown in Figure 11. Figure 11a shows that the DRS spectra of all the nanocomposite samples are red-shifted because of the addition of lower band gap LFO.

Figure 11b presents the Kubelka−Munk plots of LFO, Ag2CO3, and the 1% LFO/Ag2CO3 nanocomposite. The measured band gaps are 2.45, 2.84, and 2.62 eV for pure LFO, pure Ag2CO3, and the 1% LFO/Ag2CO3 nanocomposite, respectively. The band gap of these materials is calculated using the formula50 2623


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as α is proportional to Kubelka−Munk function F(R), the equation can be modified as hv ·F(R ) = (Ahv − Eg )n /2

(2)

where F(R) is the Kubelka−Munk function, v is the light frequency, Eg is the band gap energy, and A is the proportionality constant. The value of n is determined according to the type of optical transition (for direct transition, n = 1; for indirect transition, n = 4).50 The value of n for Ag2CO3 and LFO is 4 and 1, respectively, as the former has an indirect band gap and the latter has a direct band gap. The Eg of Ag2CO3 was measured from the plot of [F(R)·hv]1/2 versus hv and is found to be 2.84 eV. Accordingly, the Eg of LFO was determined from the plot of [F(R)·hv]2 versus hv and is found to be 2.45 eV (Figure 11b). The VB edge position of 1% LFO/Ag2CO3 nanocomposite at the point of zero charge is calculated using the following empirical equation51,52 E VB = X − E c + 0.5Eg

where EVB is the VB edge potential, X is the geometric mean of the electronegativity of the constituent elements of the semiconductor, and Ec is the free electron energy on hydrogen’s scale (4.5 eV). Values of X for Ag2CO3 and LFO are ca. 6.02 and 5.70 eV, respectively.27,39 The calculated EVB of Ag2CO3 and LFO are 2.94 and 2.43 eV/NHE, respectively. The CB edge potential ECB is calculated by:

Figure 8. XRD patterns of the Ag2CO3 and the 1% LFO/Ag2CO3 nanocomposite (before the photocatalytic reaction and after the 4th cycle of the reaction).

(hv ·α) = (Ahv − Eg )n /2

(3)

(1)

Figure 9. Absorption spectra of RhB during photocatalytic reaction at different time intervals in the presence of 1% LFO/Ag2CO3 photocatalyst and (a) BQ, (b) IPA, (c) AO. (d) kapp of 1% LFO/Ag2CO3 for the degradation of RhB in the presence of various ROS scavengers. 2624


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Figure 10. (a) EIS Nyquist plots and (b) photocurrent measurements of the pure LFO, Ag2CO3, and 1% LFO/Ag2CO3 samples

Figure 11. (a) DRS spectra of LFO, Ag2CO3, and the 1% LFO/Ag2CO3 nanocomposites. (b) Band gap calculation of LFO, Ag2CO3, and the 1% LFO/Ag2CO3 nanocomposite.

Figure 12. Heterostructure formation and the Z-scheme mechanism for the generation of different ROS.

ECB = E VB − Eg

that the LFO and Ag2CO3 form a composite heterostructure, which is favorable for the charge separation. The VB and CB edge potentials of LFO and Ag2CO3 are suitable for electron− hole pair separation and their transfer across the interface of the heterostructure. The Fermi level (EF) of n-type Ag2CO3 is near the CB, and the EF of p-type LFO is close to its VB. When these two semiconductors get into contact, there is

(4)

The estimated ECB for Ag2CO3 and LFO are 0.10 and −0.03 eV/NHE, respectively. 2.8. Band Gap Structures and the Possible Degradation Mechanism. The band edge positions of the LFO and Ag2CO3 are estimated as discussed above. The results show 2625


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3. CONCLUSIONS LaFeO3/Ag2CO3 nanocomposites with different ratios of LaFeO3 and Ag2CO3 are successfully synthesized by in situ co-precipitation method. The heterostructure formation in nanocomposites leads to the improved photocatalytic properties because of the efficient decoupling of the charge carriers and increased charge transfer. Holes play the most dominant role, followed by hydroxyl radicals, in the degradation of RhB. Among all nanocomposites, 1 wt % LaFeO3/Ag2CO3 exhibits the highest photocatalytic activity with improved stability during the photocatalytic process. The composite is stable after various cycles of photocatalysis without losing the catalytic activity. This nanocomposite acts as an excellent photocatalyst for the degradation of RhB dye with 99.5% efficiency in 45 min under natural sunlight irradiation. Further, efficient mineralization of the degradation products is observed. These results give hope for future application of this material in photocatalytic degradation of various organic pollutants present in polluted water under natural sunlight. Hence, this nanocomposite may be well exploited for the remediation of the polluted water under natural sunlight on a large scale.

realignment of the Fermi levels, which leads to the heterostructure formation.53 On the basis of band positions, photocatalytic activity, stability, and other experimental results on composite materials, a possible mechanism is elucidated and shown in Figure 12. Upon shining light, electron−hole pairs are generated in both Ag2CO3 and LFO semiconductors. These photogenerated charge carriers would be separated efficiently through a Z-scheme mechanism, and this mechanism might be responsible for the increase in stability of the LFO/ Ag2CO3 nanocomposite. The holes on the VB of Ag2CO3 have sufficient potential to oxidize OH− into OH· radical. The CB potential of LFO is −0.02 eV versus NHE, which is much positive than the reduction potential of O2(aq)/O2−· (−0.33eV vs NHE). Thus, there is a least chance of O2−· generation.54 However, it has sufficient potential for the peroxide formation (O2/H2O2) (0.685 eV vs NHE) and hence generates H2O2, which eventually form HO· radical by single electron reduction. At the same time, photogenerated electrons in the CB of Ag2CO3 may cause reduction of Fe3+ to Fe2+, and these Fe2+ ions participate in the Fenton reaction.35,55 This is a cyclic process and is more favorable in generation of OH· from H2O2. Hence, the holes at the VB of Ag2CO3 and the OH· radical generated from the electrons at the CB of LFO are the most dominant species in this process of photodegradation. This is also confirmed by the ROS scavenging experiments. 2.9. LC−MS Study. LC−MS analysis was carried out to trace the degradation pathway of the RhB dye. The mass spectrograms of the degradation products at different irradiation times are shown in the Supporting Information (Figure S4). The ROS produced, namely, OH· and the hole, might attack the central carbon of RhB to degrade it via N-deethylation process. As it is evident from the mass spectra, the main intermediates have m/z values of 443, 415, and 387. These m/z values correspond to RhB (443), N,N-diethyl-N′ethylrhodamine (415), N,N-diethylrhodamine, and N-ethylN′-ethylrhodamine (387). These intermediates further undergo complete de-ethylation and degrade to the intermediate with m/z value of 331. This intermediate undergoes ring opening and subsequent hydroxylation to generate simpler compounds. The major component has m/z value of 74 and corresponds to propionic acid. These results are in agreement with previous reports on the degradation of RhB dye56−58 in the presence of various light sources. Hence, a fragmentation pathway can be proposed for photocatalytic degradation of RhB dye by the LaFeO3/Ag2CO3 photocatalysts, as shown in the Supporting Information (Scheme S1). These oxidized products eventually get mineralized into CO2, H2O, NO3−, and NH4+.59 2.10. Chemical Oxygen Demand (COD) Removal Efficiency. To investigate the mineralization of organic pollutants in the photocatalytic oxidation, the COD removal during the photocatalytic reaction is measured using the acidic dichromate method with a Bioblock COD analyzer for the 1% LFO/Ag2CO3 sample.60 The decrease in the COD value indicates the degree of mineralization of the organic species. The COD value decreases continuously as a function of irradiation time (Figure S5). After 45 min of light irradiation, the COD is reduced to 92% of the initial value. It indicates that the RhB molecules are eventually degraded into CO2 and H2O.

4. EXPERIMENTAL SECTION All the chemicals used for the synthesis are of analytical grade and are used without further processing. Lanthanum nitrate hexahydrate [La(NO3)3·6H2O, 99.9%] was purchased from Alfa Aesar. Ferric nitrate nonahydrate [Fe(NO3)3·9H2O, 98%], silver nitrate (AgNO3, 99.5%), and sodium hydrogen carbonate (NaHCO3, 99.8%) were purchased from Merck, India. 4.1. Synthesis of the Photocatalysts. 4.1.1. Synthesis of LaFeO3 Nanoparticles. Pure phase LaFeO3 was synthesized by citric acid sol−gel method.39,40 Typically, 5 mmol of La(NO3)3·6H2O and 5 mmol of Fe(NO3)3·9H2O were dissolved in 30 mL of deionized H2O in the presence of 10 mmol of citric acid as the complexing agent. The mixture was stirred at 70 °C for 24 h, and a gel was obtained. The gel was dried in an oven at 100 °C until a dry xerogel was obtained. The amorphous mass was calcined at 500 °C for 3 h and then at 700 °C for another 3 h with a heating rate of 300 °C h−1. The compound was put on natural cooling and then crushed and used for further characterization. 4.1.2. Synthesis of Ag2CO3 Nanorods. Ag2CO3 was synthesized by the co-precipitation method.27 Primarily, 2.5 mmol of NaHCO3 was dissolved in 30 mL of deionized water to obtain a clear solution. To this solution, a 20 mL of another solution was added dropwise containing 5 mmol of AgNO3. The reaction setup was put in an ice bath in the dark to yield rod-shaped Ag2CO3 structures. 4.1.3. Synthesis of LaFeO3/Ag2CO3 Nanocomposite. The LaFeO3/Ag2CO3 nanocomposites were synthesized by the in situ co-precipitation method. A definite amount of LaFeO3 nanoparticles was dispersed in 30 mL of deionized H2O under ultrasonication followed by vigorous stirring for 1 h. Then, 2.5 mmol of NaHCO3 dissolved in 10 mL of DI water was added dropwise, and stirring was continued for 2 more hours. Finally, 5 mmol of AgNO3 dissolved in 10 mL of DI water was added in the dark, and the mixture was stirred for 12 h in an ice bath to allow the gradual synthesis of the nanocomposites. The asobtained compound was centrifuged, washed, and dried in a vacuum oven at 60 °C. Different percentages of LFO were 2626


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order kinetic equation, which can be applied to explain the photocatalytic degradation, according to Langmuir−Hinshelwood kinetic model, is as follows61

chosen to obtain a series of samples. The obtained samples were designated as 0.5% LFO/Ag2CO3, 1% LFO/Ag2CO3, 5% LFO/Ag2CO3, 10% LFO/Ag2CO3, and 20% LFO/Ag2CO3. 4.2. Characterization. XRD patterns were investigated using a Bruker D8 X-ray diffractometer equipped with Cu Kα irradiation. SEM (JEOL9003) and TEM (JEOL2100) were used to obtain the morphology and particle size. HRTEM and SAED were used to confirm the formation of the heterostructure. EDS was done to confirm the elements present, and EDS mapping was done to observe the elemental distribution. UV−vis DRS (UV-2600 Spectrophotometer, Shimadzu) was used to obtain the absorption spectra of different samples and subsequent band gap calculation by using Kubelka−Munk function. Metrohm Autolab RRDE/ RDE-2 was used to obtain EIS spectra and transient photocurrent to analyze the charge transfer property and electron−hole recombination using Nyquist plots. HPLC−MS was employed to analyze the degradation products of the RhB. COD analyzer (Lovibond, RD 125) was also employed to confirm the mineralization of the degradation products. 4.3. Photocatalytic Experiments. The photocatalytic activity of the photocatalysts was assessed by the degradation of RhB and p-chlorophenol under the influence of the visible light and natural sunlight irradiation. A 450 W xenon lamp (Newport) operated at 400 W with 395 nm filter was used to carry out the visible light irradiations. A 12 cm liquid water filter was used to cut the IR light. However, the sunlight irradiations were applied at noon during the month of April at INST, Mohali, Punjab. A 10 μM solution of the RhB dye (80 mL), containing the desired quantity of the photocatalyst (1 g L−1), was taken in a Pyrex glass reactor and was stirred with a magnetic stirrer. Atmospheric oxygen was continuously passed into the solution throughout the experiment. For the first 20 min, the solution was stirred in the dark to attain adsorption− desorption equilibrium between the dye solution and the photocatalyst. Subsequently, the first sample (at 0 min) was taken out, and then, the light irradiation was started. During the irradiation, samples of 2 mL each were collected at constant time intervals. The collected samples were centrifuged, and the supernatants were subsequently analyzed using UV−vis spectroscopy. The absorbance of the dye aliquots was monitored at its λmax (554 nm) as a function of irradiation time. The dye concentrations at different time intervals of the irradiation were acquired from the standard calibration curve, which was obtained by the absorbance of the dye at various known concentrations.52 Similar procedure was applied for pchlorophenol degradation, where the aliquots were monitored at its λmax (225nm) as a function of irradiation time. The consistency and the stability of the photocatalysts were analyzed by the recycling experiments. After every cycle of the experiment, the photocatalyst was separated from the photoreactor and the aliquots by centrifugation. The photocatalyst was repeatedly washed with distilled water and ethanol. Finally, these samples were dried at 50 °C for 12 h and reused for the next cycle of the photocatalysis experiment. The degradation efficiency (%) by the photocatalyst follows the equation degradation efficiency(%) =

C0 − C × 100 C0

ij C yz lnjjj 0 zzz = kappt j Ct z k {

(6)

where C0 is the initial concentration of the dye solution and Ct is the concentration of dye at irradiation time t. kapp is the apparent first-order rate constant (min−1) for the reaction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02829. Overlay absorption spectra of RhB degradation, UV−vis absorption spectra of RhB and p-chlorophenol degradation in the presence of xenon lamp and natural sunlight, PL spectra of terephthalic acid in the presence of 1% LFO/Ag2CO3, LC−MS spectrograms of the RhB dye at different irradiation times, change in COD of the RhB solution during the visible light irradiation, and degradation pathway of the RhB dye (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. [email protected]. ORCID

Boddu S. Naidu: 0000-0002-0426-0454 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are highly thankful to DST-SERB, India for providing the National Post Doc Fellowship to B.M.P. (File Number: PDF/2016/003121) and the Early Career Research Award to B.S.N. (File number: ECR/2015/000333). The authors are also thankful to the INST, Mohali, India for providing the necessary facilities.

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REFERENCES

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(5)

where C0 is the initial concentration of the dye and C is the concentration of RhB at different irradiation times. The first2627


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