- Email: [email protected]
Sunlight responsive new Sillén-Aurivillius A1X1 hybrid layered oxyhalides with enhanced photocatalytic activity
Solar Energy Materials & Solar Cells 161 (2017) 197–205
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
crossmark
Sunlight responsive new Sillén-Aurivillius A1X1 hybrid layered oxyhalides with enhanced photocatalytic activity Ambikeshwar Pandeya, Gollapally Naresha, Tapas Kumar Mandala,b, a b
⁎
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee 247667, India
A R T I C L E I N F O
A BS T RAC T
Keywords: Semiconductors Layered oxyhalides Sillén-Aurivillius Photocatalytic activity Dye degradation
Synthesis of new semiconductors responsive under visible light/sunlight has been considered as an elegant strategy of photocatalyst development for efficient solar light harvesting applications. In the present study, new La-substituted layered Sillén-Aurivillius oxyhalide intergrowths, Bi3LaNbO8X (X=Cl, Br), have been prepared by solid state reaction. The compounds are characterized by powder X-ray diffraction (p-XRD), field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), UV–vis diffuse reflectance spectroscopy (UV–vis DRS), electrochemical impedance spectroscopy (EIS) and photoluminescence (PL) studies. The P-XRD data revealed formation of Sillén-Aurivillius hybrid perovskites isostructural to the parent compounds, Bi4NbO8X (X = Cl, Br), and the FE-SEM-EDS confirmed the morphology to be agglomerates of submicron-sized particles with desired compositions as expected. The band gap for all the pristine oxyhalides lie in range 2.43–2.54 eV indicative of their sunlight active nature. The sunlight-driven photocatalytic activity studies through Rhodamine B (RhB) degradation unveiled excellent dye degradation efficiencies over Bi3LaNbO8Cl and Bi3LaNbO8Br in the acidic medium. The enhanced photocatalytic activities are attributed to superior charge carrier separation and dye adsorption in the La-substituted phases. Scavenger tests demonstrated the active role of hole and hydroxyl radical as key species in the RhB degradation under sunlight in presence of Bi3LaNbO8Br. The demonstration of enhanced activity under natural sunlight is significant in future development of environmental remediation technologies by harvesting solar energy.
1. Introduction
on semiconductor photocatalysis. Various oxides are explored for both UV and visible-light activity while TiO2 occupying the central position due to its stability, non-toxicity and good oxidation ability. The major drawback of TiO2 is its associated high band gap (~3.2 eV), leading to inefficient absorption of sunlight (only ~4% in the UV region). Therefore, major research efforts shifted toward developing visiblelight-responsive semiconductors for proper utilization of abundant sunlight (~43% in the visible region). In this regard, a significant advance has been made on oxide semiconductors with layered structures. It is noteworthy to mention that among several semiconductor oxides, Bi-containing layered oxides have drawn considerable attention owing to their effective visible light absorption. On the other hand, it has been reported that the band width originating from hybridization of Bi-6s with oxygen 2p in these compounds is favourable for high carrier mobility leading to enhanced photocatalytic activity [14,15]. The photocatalytic activity of Bi-based layered oxyhalides, BiOX (X=Cl, Br, I), with a ‘Sillén X2′ structure, have been reported up to a considerable extent [16–20]. The ‘Sillén X2′ structure, adopted by BiOX, is the most common one consisting of alternate [Bi2O2]2+ units
Visible-light-driven photocatalysis using semiconductor oxides has emerged as an exciting area of research to address impending issues related to clean fuel production and environmental remediation [1–5]. Production of hydrogen (a clean fuel) efficiently by water splitting reaction under sunlight requires development of novel semiconductor photocatalysts active under visible light [6–8]. Moreover, efficient photocatalysts will have wide ranging applications including, water disinfection [9,10], organic pollutant degradation [3,11], volatile organic compound (VOC) removal [12] for indoor air purification etc. In dealing with organic pollutants, especially dyes, removal or degradation may be achieved by adsorption, complete mineralization or conversion into non-toxic fragments.1 Realization of these goals by photocatalysis with efficient use of sunlight can pave the way for cleaner environments in a sustainable manner. The discovery of Fujishima and Honda in 1972 in their seminal paper on water splitting reaction for hydrogen generation using TiO2 as a semiconductor photocatalyst [13], created a flurry of research activity ⁎
Corresponding author at: Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India. E-mail address: [email protected] (T.K. Mandal).
http://dx.doi.org/10.1016/j.solmat.2016.11.040 Received 8 June 2016; Received in revised form 5 November 2016; Accepted 30 November 2016 0927-0248/ © 2016 Elsevier B.V. All rights reserved.
Solar Energy Materials & Solar Cells 161 (2017) 197–205
A. Pandey et al.
Table 1 Synthesis conditions of Bi4-yLayNbO8X (X = Cl, Br; y=0, 1). Compound
Synthesis condition
Bi4NbO8Cl Bi3LaNbO8Cl Bi4NbO8Br Bi3LaNbO8Br
700 °C/12 h+12 h 700 °C/12 h+12 h, 720 °C/12 h 700 °C/12 h+12 h 700 °C/12 h+12 h, 720 °C/12 h
2. Experimental 2.1. Synthesis All the compounds Bi4-yLayNbO8X (X=Cl, Br; y=0, 1), were prepared by solid state reactions of constituent oxides and oxyhalides. Depending upon the composition of the compounds, stoichiometric amounts of Bi2O3 (Sigma-Aldrich, ≥99%), La2O3 (Sigma-Aldrich, 99.99%), (preheated at 950 °C), Nb2O5 (Sigma-Aldrich, 99.9%), BiOCl and BiOBr (synthesized from Bi2O3 and HCl/HBr) were taken in an agate mortar and ground for 1 h. The mixtures were transferred to an alumina boat and heated under the experimental conditions as given in Table 1 with intermittent grindings. Other reagents used in the syntheses were of analytical grade. The parent Bi4NbO8Cl and Bi4NbO8Br were prepared by following the procedure reported in the literature for Bi4NbO8Cl [22]. BiOCl and BiOBr were prepared by hydrolysis method. For BiOCl, excess amount of Bi2O3 was dissolved in 3N HCl to get BiCl3-HCl system and then Millipore water was added to this solution. The pH was adjusted ~2 - 3 by using liquid NH3. The white colloid so obtained was heated at 40 °C and then washed several times with Millipore water until no traces of Cl¯ were present in the washing solution. The white precipitate so obtained after filtration was dried at 40 °C in air oven for 6 h. A similar procedure was employed for making BiOBr using HBr.
Fig. 1. Structure of Sillén-Aurivillius phase, Bi4NbO8Cl.
and two [X]− halide layers reported among other members [21]. Recently, the intergrowth compounds of Sillén and Aurivillius, namely, the Sillén-Aurivillius (SA) hybrid perovskites, have been investigated for their excellent visible-light-induced photocatalytic activity [22–26]. The characteristic feature of these layered SA hybrids is that they have the common structural motif, the [Bi2O2]2+ unit and [X]− layer, similar to that of the Sillén, sandwiched with the perovskite sheets of various thicknesses. The members of the SA series are commonly expressed by the general formula, [Bi2O2][An-1BnO3n+1][Bi2O2][Cl]m(where, n represents the thickness of the perovskite block and m indicates number of halide layers) and represented as AnXm. Fig. 1 shows the simplest member (represented by Bi4NbO8Cl) of the series containing single-layer perovskite sheet stacked between [Bi2O2]2+ layers and interleaved by single [X]− layers and the structure is commonly designated by the formula A1X1. John Ackerman [27] was the first person to describe the SA structure series correctly with the A1X1 member, Bi4NbO8Cl (n=1 and m=1), although it was reported for the first time by Aurivillius [28]. Detail structural investigations of Bi4NbO8Cl along with its Ta- analogue have been investigated later by Kusainova and co-workers [29]. Considering that these SA phases are composed of [Bi2O2]2+, [MVO4]3− and [Cl]− layers, they might possess strong internal electric fields and could lead to better electronhole separation thereby enhancing photocatalytic activity. Recently, in our effort to develop new visible-light-responsive layered perovskite photocatalysts, we have demonstrated enhanced photocatalytic activity of La-substituted four and five-layer Aurivillius bismuth-iron-titanates toward RhB degradation under sunlight [30,31]. Here, we have envisaged La-substitution in the SA hybrid layered perovskites, Bi4NbO8X (X=Cl, Br). To the best of our knowledge, we report for the first time, the solid state synthesis of new SA hybrid layered oxyhalides, Bi3LaNbO8Cl and Bi3LaNbO8Br. The photocatalytic activities of the new SA hybrids have been investigated by way of RhB degradation under sunlight-irradiation. For comparison, the parent bulk SA oxyhalides, Bi4NbO8Cl and Bi4NbO8Br, have also been synthesized and their photocatalytic activities have been compared with La-substituted compounds under similar experimental conditions.
2.2. Characterization P-XRD patterns of the as prepared samples were recorded on a Bruker AXS D8 Advance diffractometer operating at 40 kV and 30 mA, using graphite monochromatized CuKα radiation in the angular range, 5–90°. PROSZKI program was used for the refinement of the cell parameters [32]. The morphology of the powder samples were studied using a Zeiss FE-SEM Ultra plus55 operating at 20 kV. For this purpose, the samples were prepared by spreading a pinch of powder on a carbon strip pasted on an Aluminium stub. Gold sputtering was carried out over these samples for making the surface electrically conductive. Besides, EDS analysis was carried out to examine the elemental ratio and compositional homogeneity in the samples. UV–vis DRS studies of all the synthesized compounds were carried out with a Shimadzu UV-2450 UV–vis spectrophotometer in the wavelength range, 200–800 nm with BaSO4 as the reference for baseline correction. Further, with the help of the expression (1) proposed by Kubelka and Munk, the reflectance data were converted into absorption terms [33].
F (R∞) = (1–R∞)2 /2 R∞
(1)
where, R∞ is the reflectance of the sample and F(R∞) is the K-M function. Band gaps were estimated by use of the Tauc plots and the expression (2) proposed by Tauc [34].
(αhv )1/ n = A (hv –Eg )
(2)
where Eg, A, α and hν are the band gap, proportionality constant, absorption coefficient and incident light frequency, respectively. The value of n represents the nature of transition (direct or indirect), being 1/2 for direct and 2 for indirect transition [35]. The intersection point 198
Solar Energy Materials & Solar Cells 161 (2017) 197–205
A. Pandey et al.
of the extrapolated line from the linear portion of the absorption edge with the energy axes gave the band gaps. The PL studies were carried out by using Fluoromax-4 spectrofluorometer. For this purpose, 10 mg of the photocatalyst was suspended in 10 mL methanol and sonicated for 10 min. The samples were excited with 315 nm radiation to record the PL spectra. EIS of the photocatalysts were measured in a conventional three electrode system using a galvanostat (model; Versastat 3, PAR). In these study, the photocatalyst mixed with graphite was used as the working electrode (for details see supporting information), Pt wire as the counter electrode, Ag/AgCl (3 M KCl) as the reference electrode and 10 mM K3[Fe(CN)6] solution containing 0.1 M KCl as the electrolyte. The EIS measurements were carried out in the frequency range 0.01–100 kHz at open circuit potential of 0.24 V with the amplitude of the applied sinusoidal potential perturbation of ± 5 mV. To measure the flat-band potential, impedance measurements were carried out with the same electrodes in 0.1 M NaOH electrolytic solution with an Electrochemical Analyzer, CH Instruments. For this, the potential was systematically varied between −0.5 and −1.4 V with frequency of 50 Hz. The flat-band potential was calculated with the help of a Mott-Schottky plot.
Fig. 2. P-XRD patterns of Bi4-yLayNbO8X (X=Cl, Br; y=0, 1).
3. Results and discussion 3.1. P-XRD analysis
2.3. Adsorption and photocatalytic studies
The P-XRD patterns of Bi4-yLayNbO8X (X = Cl, Br; y=0, 1) are shown in Fig. 2. The P-XRD profiles for both the parent and Lasubstituted compositions reflect formation of pure SA intergrowths isostructural with the orthorhombic Bi4NbO8Cl (JCPDS PDF # 84−0843). However, small amounts of impurity (marked with asterisks) arising from either Bi3NbO7 or Bi2-xLaxO3–type phase cannot be completely avoided under the standardized experimental conditions. All other reflections (excepting these impurities) observed in the PXRD pattern for Bi4-yLayNbO8X (X = Cl, Br; y=0, 1) are indexable in the P21cn (Space group no. 33) assigned for the parent Bi4NbO8Cl. The refined cell parameters for the compounds are listed in Table 2. The least-squares refined cell parameters obtained for the parent Bi4NbO8Cl and Bi4NbO8Br are in fairly good agreement with those reported in the literature [29,38]. A decrease in the difference of the inplane ‘a’ and ‘b’ parameters (Table 2) is clearly evident in the Lasubstituted analogues in comparison to that in the parent phases and is consistent with similar reports of lattice parameter variation in the Aurivillius phases [30,31,39]. The decrease in orthorhombic distortion can probably be attributed to the reduction in lattice mismatch of the [Bi2O2]2+ layer with the perovskite block. Moreover, a distinct contraction in the c-parameter is clearly seen in the La-substituted phases.
Adsorption studies were carried out to determine the amount of dye adsorbed over the solid catalyst surfaces at different pH. For this, the dye-catalyst suspension was stirred at 340 rpm for 6 h in the dark. The UV–vis absorption data for the initial dye solution and that of the final dye suspensions were recorded on a Shimadzu 2450 UV–vis spectrophotometer. The percentage dye adsorption was calculated based on the difference between these two values. Photocatalytic dye degradation experiments for all the samples were performed on RhB at pH 2, 7 and 11 at IIT ROORKEE (29°51′ N; 77°53′ E) during the month of November 2014 having direct normal irradiance ~ 143 W/m2. For degradation studies, 75 mg of the solid catalyst was added to 100 mL of 1×10–5 M RhB solution of appropriate pH. Further, to ensure the adsorption-desorption equilibrium between the solid catalyst and RhB, the above suspension was kept on a magnetic stirrer at 340 rpm for 60 min in the dark. Then, the suspension was exposed to direct sunlight for degradation studies. For monitoring the extent of degradation, 5 mL aliquots of this suspension was withdrawn at regular intervals, centrifuged at 8200 rpm (to separate the catalyst particles from the suspension), and the absorbance of the supernatant was recorded. After recording the absorbance, the solution was transferred back to the same beaker to keep the initial dye volume nearly constant. The dye degradation efficiency of the catalyst was calculated using the following expression,
% of degradation = (1 − C / C0 ) × 100
3.2. FE-SEM analysis The FE-SEM images of Bi4-yLayNbO8X (X = Cl, Br; y=0, 1, 2) are shown in Fig. 3 (top panels). From the FE-SEM images a similar particle morphology, which are ellipsoidal or nearly spherical in shape, is evident for all the compounds Bi4-yLayNbO8X (X = Cl, Br; y=0, 1). The particle sizes range from few hundred nanometres to micrometres as inferred from the FE-SEM images. A high degree of agglomeration is also clear from these images. The corresponding EDS analysis is also shown in Fig. 3 (bottom panels). The slightly higher atomic percentage
(3)
where, C0 is the initial concentration and C being the dye concentration at any point of time (t). 2.4. Detection of reactive species To understand the role of various reactive species (h+, •OH and O2•–) on the photocatalytic RhB degradation, different scavengers were added to the RhB solution prior to the addition of the catalyst. The experimental procedure for the active species determination was similar to that described above during photodegradation studies. In our experiments, 10 mM scavenger species were added to the RhB solution. Ammonium oxalate (AO), benzoquinone (BQ) and tertiary butyl alcohol (t-BuOH) were used as h+, O2•– and •OH scavengers, respectively [36,37]. For this purpose, 0.14 g of solid AO, 0.18 g of BQ and 0.96 mL of t-BuOH were added to 100 mL dye solution to make a scavenger concentration of 10 mM in each case.
Table 2 Lattice parameters and band gap of Bi4-yLayNbO8X (X = Cl, Br; y=0, 1). Compound
Bi4NbO8Cl Bi3LaNbO8Cl Bi4NbO8Br Bi3LaNbO8Br
199
Lattice parameters (Å)
Band Gap (eV)
a
b
c
5.452(2) 5.459(2) 5.474(2) 5.467(1)
5.489(1) 5.437(4) 5.518(2) 5.466(2)
28.66(1) 28.32(2) 29.03(1) 28.68(1)
2.43 2.46 2.46 2.54
Solar Energy Materials & Solar Cells 161 (2017) 197–205
A. Pandey et al.
Fig. 3. FE-SEM images (top panels) and corresponding EDS data (bottom panels) of (a) Bi4NbO8Cl, (b) Bi3LaNbO8Cl, (c) Bi4NbO8Br and (d) Bi3LaNbO8Br.
visible region (λ > 400 nm). The band gaps for the compounds were calculated from (αhν)1/2 vs. hν plot by assuming an indirect band gap hypothesis. The estimated band gaps for the compounds lie between 2.43 and 2.54 eV without any significant change in the La-substituted phases from those of the parent. The band gap of the parent compounds are in agreement with the literature values reported earlier [22]. Since the band gap for all the compounds fall well within the visible region, they were thought to be potentially active toward
of niobium and an associated lower atomic percentage of bismuth than expected may be attributed to the overlapping X-ray peaks of niobium with that of bismuth.
3.3. UV–vis DRS analysis The UV–vis DRS data for Bi4-yLayNbO8X (X = Cl, Br; y=0, 1) is shown in Fig. 4. The absorption edges of all the compounds lie in the 200
Solar Energy Materials & Solar Cells 161 (2017) 197–205
A. Pandey et al.
Fig. 4. (a, b) UV–vis DRS of Bi4-yLayNbO8X (X=Cl, Br; y=0, 1). (c, d) Tauc plots for Bi4-yLayNbO8X (X=Cl, Br; y=0, 1).
Bi3LaNbO8Br, were found to be 757 and 370 Ω, respectively. The EIS results demonstrate that the separation of photogenerated charge carriers is more effective in the La-substituted SA phase, Bi3LaNbO8Br, as compared to that in the parent.
sunlight-driven photocatalysis. 3.4. PL analysis Fig. 5 shows the PL spectra of Bi4−yLayNbO8X (X = Cl, Br; y=0, 1). The compounds show broad emission spectra spreading across 375– 525 nm. The parent compounds show intense emissions at ~415 and ~435 nm with a relatively low intensity emission due to the band-edge recombination at higher wavelengths. In Bi3LaNbO8Cl and Bi3LaNbO8Br the primary emission peak arises due to the band-edge recombination and the intensity of lower wavelength emissions is drastically decreased as compared to that of the parents. A comparatively lower overall PL intensity in case of La-substituted analogues suggests a lower degree of electron-hole recombination [40] considering all the emission pathways. Therefore, a higher photocatalytic activity is expected in the La-substituted phases provided all other factors that play significant role in photocatalytic behaviour remain the same.
3.6. Photocatalytic activity RhB was chosen as the model dye for testing the photocatalytic activities of Bi4-yLayNbO8X (X=Cl, Br; y=0, 1) under solar-irradiation at various pH (2, 7 and 11). The photocatalytic activity was highest at pH 2 over Bi4-yLayNbO8X (X=Cl, Br; y=0, 1), while no appreciable degradation of RhB was observed at pH 7 and 11 (Fig. S1 in supplementary data), excepting for Bi4NbO8Br which showed up to 50% degradation within 30 min of sunlight-irradiation at pH 7. Fig. 7(a) shows the RhB degradation with time under natural sunlight over these catalysts at pH 2. While all the catalysts degraded RhB completely within 40 min of solar irradiation, a blank test without catalyst showed zero percent dye degradation indicating no selfdegradation of RhB under identical experimental conditions. Among all the compounds studied here, the highest photocatalytic degradation was observed for Bi3LaNbO8Br. The kinetic plots of RhB degradation by all the compounds are shown in Fig. 7(b). A comparison of photocatalytic RhB degradation by the La-substituted SA layered oxyhalides with other layered perovskites reported recently in the literature [30,31] showed the SA hybrid phases to be more active. The superior activity of the La-substituted phases can primarily be attributed to an enhanced photoexcited charge separation as evidenced by
3.5. EIS analysis EIS studies can provide information on the charge transfer resistance (RCT) and the separation efficacy of the photogenerated charge carriers [41]. As shown in Fig. 6, the diameter of the Nyquist semicircle is smaller for Bi3LaNbO8Br than that of Bi4NbO8Br. This indicates that Bi3LaNbO8Br has a lower charge transfer resistance than that of Bi4NbO8Br. The RCT for the compounds, Bi4NbO8Br and 201
Solar Energy Materials & Solar Cells 161 (2017) 197–205
A. Pandey et al.
Fig. 7. (a) Photocatalytic degradation of RhB with time by Bi4-yLayNbO8X (X=Cl, Br; y=0, 1) under sunlight-irradiation. (b) Plot of ln(C0/C) as a function of irradiation time over Bi4-yLayNbO8X (X=Cl, Br; y=0, 1).
3.7. Detection of reactive species
Fig. 5. Photoluminescence spectra of (a) Bi4-yLayNbO8Cl (y=0, 1) and (b) Bi4yLayNbO8Br (y=0, 1) at room temperature (excitation at 315 nm).
It is well known that superoxide radical anions (O2•–), holes (h+) and hydroxyl radicals (•OH) are among the main reactive species that take part in the photocatalytic dye degradation process [42]. To understand the role of various reactive species in sunlight-driven RhB degradation reported here various scavenger tests were performed over Bi3LaNbO8Br (Fig. 8). When AO (an h+ scavenger) was used as the scavenger, the photocatalytic degradation was retarded to a great
Fig. 6. Nyquest impedance plots for Bi4NbO8Br and Bi4LaNbO8Br.
decreased charge transfer resistance (see EIS data, Fig. 6) and reduced charge recombination as evidenced by a drastic decrease in the PL intensity (Fig. 5) on La-substitution in the SA hybrid phases. Moreover, our results have shown superior performance of Sillén-Aurivillius intergrowth compounds as compared to the commercial TiO2 toward RhB degradation at pH 2 under sunlight irradiation (Fig. 7(a)).
Fig. 8. Effects of different scavengers on the degradation of RhB in presence of Bi3LaNbO8Br under sunlight-irradiation.
202
Solar Energy Materials & Solar Cells 161 (2017) 197–205
A. Pandey et al.
Fig. 10. Schematic energy level diagram of Bi3LaNbO8Br with respect to potential (vs. NHE) of •OH/H2O, O2/O2•– and the HOMO–LUMO levels of RhB.
Fig. 9. Time profiles of RhB degradation for four successive cycles with Bi3LaNbO8Br.
plot (Fig. S3 in supplementary data). This corresponds to −0.31 V vs. NHE at pH 2. It is to be noted that the measured flat-band potential of Bi3LaNbO8Br is comparable to that of the recently reported SillénAurivillius intergrowth compound, Bi4NbO8Cl [43]. Moreover, the ECB and EVB of the photocatalysts were empirically calculated using the following expressions [44],
extent in the initial 20 min of solar radiation. However, when BQ (an O2•– scavenger) was used, a near complete degradation of RhB was observed within the same time interval. The situation with t-BuOH, which is a •OH scavenger, was different in a sense that intermediate retardations were observed up to 20 min of light irradiation. These observations of scavenger experiments indicated holes (h+) and •OH as the dominant species playing active role in the RhB degradation under sunlight in presence of Bi3LaNbO8Br. 3.8. Catalyst cycling and stability studies
ECB = χ (Xx Yy Zz )–1/2Eg + E0
(4)
EVB = ECB + Eg
(5)
where, Eg, χ(XxYyZz) and E0 being the band gap of semiconductor, absolute electronegativity and scale factor (assumed to be −4.5 eV), respectively. From the geometric mean of the absolute electronegativities of the constituent atoms, the absolute electronegativity of the semiconductor was calculated. As reported in the literature, the absolute electronegativities for Bi, Nb, La, O, Br and Cl were taken as 4.69, 4.0, 3.1, 7.54, 7.59 and 8.30, respectively [45]. Thus, the ECB and EVB of Bi3LaNbO8Br were estimated at 0.346 and 2.886 eV, respectively. The position of ECB for Bi3LaNbO8Br above that of Bi4NbO8Cl according to the Mott-Schottky data is consistent with the empirical calculations (calculated ECB of Bi4NbO8Cl is 0.595 eV) based on absolute electronegativities as well. The reported HOMO and LUMO levels of RhB lie at 0.95 and −1.42 eV, respectively [46]. Accordingly, the HOMO-LUMO gap is in good agreement with the absorption maxima of RhB (λmax =553 nm) as obtained in our study. The potential for •OH/H2O and O2/O2•─ were taken from the literature as 2.68 eV and 0.13 eV, respectively [42]. From the energy level diagram of Bi3LaNbO8Br, the formation and role of superoxide radical anion (O2•─) during the degradation is expected, which is inconsistent with the results of scavenger test (Fig. 8). However, from the positioning of •OH/H2O potential with respect to EVB of Bi3LaNbO8Br, the generation of •OH radical by hole transfer from the semiconductor to the water molecules is not possible. To our surprise, the scavenger test (Fig. 8) conducted with t-BuOH (a • OH scavenger) showed substantial retardation in the RhB degradation. This apparent contradiction can be explained by the reaction of the generated O2•─ with H+ to produce •OOH, which subsequently gives rise to the formation of •OH. The mechanism proposed above could possibly be argued given the acidic reaction medium (pH 2) for the RhB degradation reported here. Moreover, the formation of •OH by this mechanism is in line with the literature reports [9]. Furthermore, when AO (an h+ scavenger) was used, the RhB degradation was drastically slowed down indicating active participation of h+ in the photodegradation process. This is in agreement with the relative positioning of the EVB of Bi3LaNbO8Br and the HOMO level of RhB as shown in the energy level diagram (Fig. 10). Therefore, based on our scavenger test results, h+ and •OH are proposed as the major reactive species involved
The reusability and stability of photocatalysts are important parameters required to be evaluated for their practical use. To test the reusability, four consecutive cycles of photocatalytic RhB degradation over Bi3LaNbO8Br was carried out (Fig. 9). It has been observed that ~98% of RhB is degraded within 20 min of solar irradiation for all the four cycles, demonstrating excellent reusability of the catalyst. However, in the fourth run, a negligible loss of activity (nearly 97% degradation within 20 min of solar irradiation) is noticed attributable to the loss of some catalysts during the cycle test. The P-XRD patterns of Bi4-yLayNbO8X (X=Cl, Br; y=0, 1) recorded before and after the photocatalytic dye degradations (Fig. S2 in supplementary data) indicated retention of crystallinity and phase purity of the compounds after photocatalysis signifying very good stability for all the pristine photocatalysts. 3.9. Mechanistic insight from energy level diagram It is established in the literature that the photocatalytic degradation of dyes by a semiconductor photocatalyst involve several steps. Initially the dye gets adsorbed on the catalyst surface during the equilibration of the dye-catalyst suspension. On light irradiation, the photoexcitation of the semiconductor gives rise to the generation of e–– h+ pairs, which later migrate to the surface of the semiconductors and thereby directly react or generate reactive species, such as, O2•– and •OH. However, the generation of these species also depend on appropriate energy level ordering of the semiconductor valence and conduction band edges (EVB and ECB) with respect to the potential for •OH/H2O, O2/O2•─. To get an insight on the enhanced RhB degradation by Bi3LaNbO8Br, an energy level diagram involving the EVB and ECB of Bi3LaNbO8Br along with the HOMO - LUMO levels of RhB and potentials for •OH/H2O and O2/O2•─ is presented in Fig. 10. All the potentials are with respect to the normal hydrogen electrode (NHE). The flat-band potential of the semiconductor was approximated as the ECB and accordingly the EVB was calculated using the band gap (Eg). The flat-band potential of −1.11 V vs. Ag/AgCl at pH 12.3 for Bi3LaNbO8Br was determined from the impedance spectroscopy data with the help of a Mott-Schottky 203
Solar Energy Materials & Solar Cells 161 (2017) 197–205
A. Pandey et al.
photocatalysts where the photophysical properties may also be tuned by micro- and nanostructure control to alter the photocatalytic properties of these oxyhalides. 4. Conclusions New La-substituted Sillén-Aurivillius (SA) layered oxyhalides, namely, Bi3LaNbO8Cl and Bi3LaNbO8Br have been synthesized for the first time. The compounds are visible light absorbers having band gap ~2.43 – 2.54 eV. An enhanced sunlight-driven photodegradation of RhB is exhibited by these SA hybrid perovskites. The observed efficiencies for RhB degradation of La-substituted catalysts are quite high as compared to the parent compounds in the acidic medium. This has been attributed to lower e−- h+ recombination, thereby allowing more electron-hole pairs to participate in the reactive species generation and photodegradation process. The scavenger tests performed during RhB degradation clearly indicated the holes and hydroxyl radicals as the main reactive species that take part in the photodegradation process. Further, a close link of the enhanced RhB adsorption over Bi3LaNbO8Br with a high photodegradation rate corroborates well with h+ being a major reactive species. Owing to their excellent photocatalytic performance under sunlight-irradiation, the La-substitution has been proved to be an effective strategy for enhancing the photocatalytic activity of semiconductor catalysts.
Fig. 11. Influence of pH on RhB adsorption onto Bi4-yLayNbO8X (X = Cl, Br; y=0, 1).
in the photocatalytic RhB degradation under sunlight irradiation by Bi3LaNbO8Br.
3.10. Role of adsorption As it has already been mentioned that adsorption plays an important role in photocatalysis, the extent of dye adsorption over the catalysts was evaluated by adsorption experiments. The adsorption tests were carried out at different pH for Bi4-yLayNbO8X (X=Cl, Br; y=0, 1) and the data are shown in Fig. 11. It is apparent that the maximum dye adsorption (65%) was observed for Bi3LaNbO8Br at pH 2. The second highest adsorption (61%) was observed for Bi4NbO8Br. The fact that h+ plays the dominant role as active species; a greater extent of dye adsorption would enhance the RhB degradation efficiency. The adsorption results are in agreement with the enhanced RhB degradation (higher rate constants) with Bi4NbO8Br and Bi3LaNbO8Br at pH 2. The low adsorption percentages at pH 7 and 11 were reflected in the lower extent of photodegradation (Fig. S1 in supplementary data), which are consistent with our RhB degradation results at neutral and alkaline pH. The extent of adsorption follows the order ‘Bi3LaNbO8Br > Bi4NbO8Br > Bi3LaNbO8Cl > Bi4NbO8Cl' at pH 2. The adsorption order is consistent with the results obtained from the photocatalytic activities (Fig. 7). Moreover, a greater extent of RhB adsorption by Bi4NbO8Br at pH 7 could be corroborated to a moderately high RhB degradation (Fig. S1) in the neutral medium by Bi4NbO8Br. It is noteworthy that the La-substitution in the SA hybrid layered oxyhalides has improved the dye degradation efficiency under sunlightirradiation. Both Bi3LaNbO8Br and Bi3LaNbO8Cl showed enhanced activity (2.3 mM g−1 h−1 and 1.48 mM g−1 h−1, respectively) toward RhB degradation as compared to their parent compounds (1.64 mM g−1 h−1 and 0.93 mM g−1 h−1, respectively). This activity enhancement on La-substitution is primarily driven by suppression of electron-hole recombination as was evidenced by PL and EIS studies. With reference to literature reports [22,47], we believe that the internal static electric fields between the [Bi2O2]2+ sheets, [NbO4]3− slabs and chloride anion layers plays crucial role in efficient separation of photogenerated electron-hole pairs. Moreover, a greater extent of dye adsorption facilitates the semiconductor surface to dye h+ transfer pathway for the degradation. Based on recent reports [30,31,48] and our present investigation La-substitution may be considered as an effective strategy for enhancing the photocatalytic activity of semiconductor catalysts. Moreover, in the light of the very recent report [43] of Bi4NbO8Cl as stable and efficient water splitting photocatalyst under visible light, the La-substituted analogues may provide insight for the band level tuning in these SA hybrid layered perovskite oxyhalides. Furthermore, the SA layered hybrids may emerge as more active
Acknowledgements This work was financially supported by IIT Roorkee NO. IITR/ SRIC/259/FIG(Scheme-A) through Faculty Initiation Grant A (FIG-A). A. P. and G. N. thank the Ministry of Human Resources and Development (MHRD), Government of India for research fellowships. The authors are thankful to the Institute Instrumentation Centre, IIT Roorkee for providing the facilities. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2016.11.040. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [2] H. Zhang, G. Chen, D.W. Bahnemann, Photoelectrocatalytic materials for environmental applications, J. Mater. Chem. 19 (2009) 5089–5121. [3] D. Ravelli, D. Dondi, M. Fagnoni, A. Albini, Photocatalysis. A multi-faceted concept for green chemistry, Chem. Soc. Rev. 38 (2009) 1999–2011. [4] N. Serpone, A.V. Emeline, S. Horikoshi, V.N. Kuznetsov, V.K. Ryabchuk, On the genesis of heterogeneous photocatalysis: a brief historical perspective in the period 1910 to the mid-1980s, Photochem. Photobiol. Sci. 11 (2012) 1121–1150. [5] F. Fresno, R. Portela, S. Suárez, J.M. Coronado, Photocatalytic materials: recent achievements and near future trends, J. Mater. Chem. A 2 (2014) 2863–2884. [6] Z. Zou, J. Ye, K. Sayama, H. Arakawa, Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst, Nature 414 (2001) 625–627. [7] E. Frank, Osterloh, Inorganic materials as catalysts for photochemical splitting of water, Chem. Mater. 20 (2008) 35–54. [8] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253–278. [9] S. Dong, J. Feng, M. Fan, Y. Pi, L. Hu, X. Han, M. Liu, J. Sun, J. Sun, Recent developments in heterogeneous photocatalytic water treatment using visible lightresponsive photocatalysts: a review, RSC Adv. 5 (2015) 14610–14630. [10] D. Zhang, G. Li, J.C. Yu, Inorganic materials for photocatalytic water disinfection, J. Mater. Chem. 20 (2010) 4529–4536. [11] O. Carp, C.L. Huisman, A. Reller, Photoinduced reactivity of titanium dioxide, Prog. Solid State Chem. 32 (2004) 33–177. [12] H. Chen, C.E. Nanayakkara, V.H. Grassian, Titanium dioxide photocatalysis in atmospheric chemistry, Chem. Rev. 112 (2012) 5919–5948. [13] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [14] M. Oshikiri, M. Boero, J. Ye, Z. Zou, G. Kido, Electronic structures of promising photocatalysts InMO4 (M=V, Nb, Ta) and BiVO4 for water decomposition in the visible wavelength region, J. Chem. Phys. 117 (2002) 7313–7318.
204
Solar Energy Materials & Solar Cells 161 (2017) 197–205
A. Pandey et al.
Eng. 3 (2015) 2900–2908. [32] W. Losocha, K. Lewinski, Prozski- A system of programs for powder diffraction data analysis, J. Appl. Crystallogr. 27 (1994) 437–438. [33] P. Kubelka, F. Munk, Ein Beitrag zur Optik der Farbanstriche, Z. Tech. Phys. 12 (1931) 593–601. [34] J. Tauc, R. Grigorovic, A. Vancu, Optical properties and electronic structure of amorphous Germanium, Phys. Status Solidi 15 (1966) 627–637. [35] U.A. Joshi, P.A. Maggard, CuNb3O8: a p-type semiconducting metal oxide photoelectrode, J. Phys. Chem. Lett. 3 (2012) 1577–1581. [36] Y. Wang, K. Deng, L. Zhang, Visible light photocatalysis of BiOI and its photocatalytic activity enhancement by in situ ionic liquid modification, J. Phys. Chem. C 115 (2011) 14300–14308. [37] S. Kumar, S. Khanchandani, M. Thirumal, A.K. Ganguli, Achieving Enhanced Visible-Light-Driven Photocatalysis Using Type-II NaNbO3/CdS Core/Shell Heterostructures, ACS Appl. Mater. Interfaces 6 (2014) 13221–13233. [38] A.M. Kusainova, W. Zhou, J.T.S. Irvine, P. Lightfoot, Layered intergrowth phases Bi4MO8X (X = Cl, M = Ta and X = Br, M = Ta or Nb): structural and electrophysical characterization, J. Solid State Chem. 166 (2002) 148–157. [39] M. W. Chu, M.-T. Caldes, L. Brohan, M. Ganne, A.-M. Marie, O. Joubert, Y. Piffard, Bulk and surface structures of the Aurivillius Phases: Bi4-xLaxTi3O12 (0≤ x≤2.00), Chem. Mater. 16 (2004) 31–42. [40] K. Fujihara, S. Izuni, S. Ohno, M. Matsumura, Time-resolved photoluminescence of particulate TiO2 photocatalysts suspended in aqueous solutions, J. Photochem. Photobiol. A 132 (2000) 99–104. [41] H. Liu, S. Cheng, M. Wu, H. Wu, J. Zhang, W. Li, C. Cao, Photoelectrocatalytic degradation of sulfosalicylic acid and its electrochemical impedance spectroscopy investigation, J. Phys. Chem. A 104 (2000) 7016–7020. [42] S. Kumar, T. Surendar, A. Baruah, V. Shanker, Synthesis of a novel and stable gC3N4–Ag3PO4 hybrid nanocomposite photocatalyst and study of the photocatalytic activity under visible light irradiation, J. Mater. Chem. A 1 (2013) 5333–5340. [43] H. Fujito, H. Kunioku, D. Kato, H. Suzuki, M. Higashi, H. Kageyama, R. Abe, Layered perovskite oxychloride Bi4NbO8Cl: a stable visible light responsive photocatalyst for water splitting, J. Am. Chem. Soc. 138 (2016) 2082–2085. [44] J. Lv, T. Kako, Z. Zou, J. Ye, Band structure design and photocatalytic activity of In2O3/N–InNbO4 composite, Appl. Phys. Lett. 95 (2009) 032107–032109. [45] R.G. Pearson, Absolute electronegativity and hardness: application to inorganic chemistry, Inorg. Chem. 27 (1988) 734–740. [46] L. Pan, J. Zou, X. Liu, X. Liu, S. Wang, X. Zhang, L. Wang, Visible–light–induced photodegradation of rhodamine b over hierarchical TiO2: effects of storage period and water-mediated adsorption switch, Ind. Eng. Chem. Res. 51 (2012) 12782–12786. [47] I. Grinberg, D.V. West, M. Torres, G. Gou, D.M. Stein, L. Wu, G. Chen, E.M. Gallo, A.R. Akbashev, P.K. Davis, J.E. Spanier, A.M. Rappe, Perovskite oxides for visiblelight-absorbing ferroelectric and photovoltaic materials, Nature 503 (2013) 509–512. [48] F.N. Sayed, V. Grover, B.P. Mandal, A.K. Tyagi, Influence of La3+ substitution on electrical and photocatalytic behavior of complex Bi2Sn2O7 oxides, J. Phys. Chem. C 117 (2013) 10929–10938.
[15] J. Tang, Z. Zou, J. Ye, Efficient photocatalytic decomposition of organic contaminants over CaBi2O4 under visible-light irradiation, Angew. Chem. Int. Ed. 43 (2004) 4463–4466. [16] K.-L. Zhang, C.-M. Liu, F.-Q. Huang, C. Zheng, W.-D. Wang, Study of the electronic structure and photocatalytic activity of the BiOCl photocatalyst, Appl. Cat. B: Environ. 68 (2006) 125–129. [17] J. Zhang, F. Shi, J. Lin, D. Chen, J. Gao, Z. Huang, X. Ding, C. Tang, Self-assembled 3-D architectures of BiOBr as a visible light-driven photocatalyst, Chem. Mater. 20 (2008) 2937–2941. [18] X. Zhang, Z. Ai, F. Jia, L. Zhang, Generalized one-pot synthesis, characterization, and photocatalytic activity of hierarchical BiOX (X=Cl, Br, I) nanoplate microspheres, J. Phys. Chem. C. 112 (2008) 747–753. [19] X. Xiao, W.-D. Zhang, Facile synthesis of nanostructured BiOI microspheres with high visible light-induced photocatalytic activity, J. Mater. Chem. 20 (2010) 5866–5870. [20] L. Ye, Y. Su, X. Jin, H. Xie, C. Zhang, Recent advances in BiOX (X = Cl, Br and I) photocatalysts: synthesis, modification, facet effects and mechanisms, Environ. Sci.: Nano 1 (2014) 90–112. [21] S.M. Fray, C.J. Milne, P. Lightfoot, Synthesis and structure of CaBiO2Cl and SrBiO2Cl: new distorted variants of the Sillen X1 structure, J. Solid State Chem. 128 (1997) 115–120. [22] X. Lin, T. Huang, F. Huang, W. Wang, J. Shia, Photocatalytic activity of a Bi-based oxychloride Bi4NbO8Cl, J. Mater. Chem. 17 (2007) 2145–2150. [23] J. Fan, X. Hu, Z. Xie, K. Zhang, J. Wang, Photocatalytic degradation of azo dye by novel Bi-based photocatalyst Bi4TaO8I under visible-light irradiation, Chem. Eng. J. 179 (2012) 44–51. [24] S.S.M. Bhat, N.G. Sundaram, Efficient visible light photocatalysis of Bi4TaO8Cl nanoparticles synthesized by solution combustion technique, RSC Adv. 3 (2013) 14371–14378. [25] Y. He, Y. Zhang, H. Huang, N. Tian, Y. Guo, Y. Luo, A novel Bi-based oxybromide Bi4NbO8Br: synthesis, characterizationand visible-light-active photocatalytic activity, Colloid Surf. Physiochem. Eng. Asp. 462 (2014) 131–136. [26] S.S.M. Bhat, N. Sundaram, Photocatalysis of Bi4NbO8Cl hierarchical nanostructure for degradation of dye under solar/UV irradiation, New J. Chem. 39 (2015) 3956–3963. [27] J.F. Ackerman, The structures of Bi3PbWO8Cl and Bi4NbO8Cl and the evolution of the Bipox structure series, J. Solid State Chem. 62 (1986) 92–104. [28] B. Aurivillius, Intergrowth compounds between members of the bismuth titanate family and structures of the lithium dichlorotetraoxitribismuthate (LiBi3O4Cl2) type, Chem. Scr. 23 (1984) 143. [29] A.M. Kusainova, S. Yu Stefanovich, V.A. Dolgikh, A.V. Mosunov, C.H. Hervoches, P. Lightfoot, Dielectric properties and structure of Bi4NbO8Cl and Bi4TaO8Cl, J. Mater. Chem. 11 (2001) 1141–1145. [30] G. Naresh, T.K. Mandal, Excellent sun-light-driven photocatalytic activity by Aurivillius layered perovskites, Bi5−xLaxTi3FeO15 (x=1, 2), ACS Appl. Mater. Interfaces 6 (2014) 21000–21010. [31] G. Naresh, T.K. Mandal, Efficient COD removal coinciding with dye decoloration by five-layer Aurivillius perovskites under sunlight-irradiation, ACS Sustain. Chem.
205
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
crossmark
Sunlight responsive new Sillén-Aurivillius A1X1 hybrid layered oxyhalides with enhanced photocatalytic activity Ambikeshwar Pandeya, Gollapally Naresha, Tapas Kumar Mandala,b, a b
⁎
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee 247667, India
A R T I C L E I N F O
A BS T RAC T
Keywords: Semiconductors Layered oxyhalides Sillén-Aurivillius Photocatalytic activity Dye degradation
Synthesis of new semiconductors responsive under visible light/sunlight has been considered as an elegant strategy of photocatalyst development for efficient solar light harvesting applications. In the present study, new La-substituted layered Sillén-Aurivillius oxyhalide intergrowths, Bi3LaNbO8X (X=Cl, Br), have been prepared by solid state reaction. The compounds are characterized by powder X-ray diffraction (p-XRD), field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), UV–vis diffuse reflectance spectroscopy (UV–vis DRS), electrochemical impedance spectroscopy (EIS) and photoluminescence (PL) studies. The P-XRD data revealed formation of Sillén-Aurivillius hybrid perovskites isostructural to the parent compounds, Bi4NbO8X (X = Cl, Br), and the FE-SEM-EDS confirmed the morphology to be agglomerates of submicron-sized particles with desired compositions as expected. The band gap for all the pristine oxyhalides lie in range 2.43–2.54 eV indicative of their sunlight active nature. The sunlight-driven photocatalytic activity studies through Rhodamine B (RhB) degradation unveiled excellent dye degradation efficiencies over Bi3LaNbO8Cl and Bi3LaNbO8Br in the acidic medium. The enhanced photocatalytic activities are attributed to superior charge carrier separation and dye adsorption in the La-substituted phases. Scavenger tests demonstrated the active role of hole and hydroxyl radical as key species in the RhB degradation under sunlight in presence of Bi3LaNbO8Br. The demonstration of enhanced activity under natural sunlight is significant in future development of environmental remediation technologies by harvesting solar energy.
1. Introduction
on semiconductor photocatalysis. Various oxides are explored for both UV and visible-light activity while TiO2 occupying the central position due to its stability, non-toxicity and good oxidation ability. The major drawback of TiO2 is its associated high band gap (~3.2 eV), leading to inefficient absorption of sunlight (only ~4% in the UV region). Therefore, major research efforts shifted toward developing visiblelight-responsive semiconductors for proper utilization of abundant sunlight (~43% in the visible region). In this regard, a significant advance has been made on oxide semiconductors with layered structures. It is noteworthy to mention that among several semiconductor oxides, Bi-containing layered oxides have drawn considerable attention owing to their effective visible light absorption. On the other hand, it has been reported that the band width originating from hybridization of Bi-6s with oxygen 2p in these compounds is favourable for high carrier mobility leading to enhanced photocatalytic activity [14,15]. The photocatalytic activity of Bi-based layered oxyhalides, BiOX (X=Cl, Br, I), with a ‘Sillén X2′ structure, have been reported up to a considerable extent [16–20]. The ‘Sillén X2′ structure, adopted by BiOX, is the most common one consisting of alternate [Bi2O2]2+ units
Visible-light-driven photocatalysis using semiconductor oxides has emerged as an exciting area of research to address impending issues related to clean fuel production and environmental remediation [1–5]. Production of hydrogen (a clean fuel) efficiently by water splitting reaction under sunlight requires development of novel semiconductor photocatalysts active under visible light [6–8]. Moreover, efficient photocatalysts will have wide ranging applications including, water disinfection [9,10], organic pollutant degradation [3,11], volatile organic compound (VOC) removal [12] for indoor air purification etc. In dealing with organic pollutants, especially dyes, removal or degradation may be achieved by adsorption, complete mineralization or conversion into non-toxic fragments.1 Realization of these goals by photocatalysis with efficient use of sunlight can pave the way for cleaner environments in a sustainable manner. The discovery of Fujishima and Honda in 1972 in their seminal paper on water splitting reaction for hydrogen generation using TiO2 as a semiconductor photocatalyst [13], created a flurry of research activity ⁎
Corresponding author at: Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India. E-mail address: [email protected] (T.K. Mandal).
http://dx.doi.org/10.1016/j.solmat.2016.11.040 Received 8 June 2016; Received in revised form 5 November 2016; Accepted 30 November 2016 0927-0248/ © 2016 Elsevier B.V. All rights reserved.
Solar Energy Materials & Solar Cells 161 (2017) 197–205
A. Pandey et al.
Table 1 Synthesis conditions of Bi4-yLayNbO8X (X = Cl, Br; y=0, 1). Compound
Synthesis condition
Bi4NbO8Cl Bi3LaNbO8Cl Bi4NbO8Br Bi3LaNbO8Br
700 °C/12 h+12 h 700 °C/12 h+12 h, 720 °C/12 h 700 °C/12 h+12 h 700 °C/12 h+12 h, 720 °C/12 h
2. Experimental 2.1. Synthesis All the compounds Bi4-yLayNbO8X (X=Cl, Br; y=0, 1), were prepared by solid state reactions of constituent oxides and oxyhalides. Depending upon the composition of the compounds, stoichiometric amounts of Bi2O3 (Sigma-Aldrich, ≥99%), La2O3 (Sigma-Aldrich, 99.99%), (preheated at 950 °C), Nb2O5 (Sigma-Aldrich, 99.9%), BiOCl and BiOBr (synthesized from Bi2O3 and HCl/HBr) were taken in an agate mortar and ground for 1 h. The mixtures were transferred to an alumina boat and heated under the experimental conditions as given in Table 1 with intermittent grindings. Other reagents used in the syntheses were of analytical grade. The parent Bi4NbO8Cl and Bi4NbO8Br were prepared by following the procedure reported in the literature for Bi4NbO8Cl [22]. BiOCl and BiOBr were prepared by hydrolysis method. For BiOCl, excess amount of Bi2O3 was dissolved in 3N HCl to get BiCl3-HCl system and then Millipore water was added to this solution. The pH was adjusted ~2 - 3 by using liquid NH3. The white colloid so obtained was heated at 40 °C and then washed several times with Millipore water until no traces of Cl¯ were present in the washing solution. The white precipitate so obtained after filtration was dried at 40 °C in air oven for 6 h. A similar procedure was employed for making BiOBr using HBr.
Fig. 1. Structure of Sillén-Aurivillius phase, Bi4NbO8Cl.
and two [X]− halide layers reported among other members [21]. Recently, the intergrowth compounds of Sillén and Aurivillius, namely, the Sillén-Aurivillius (SA) hybrid perovskites, have been investigated for their excellent visible-light-induced photocatalytic activity [22–26]. The characteristic feature of these layered SA hybrids is that they have the common structural motif, the [Bi2O2]2+ unit and [X]− layer, similar to that of the Sillén, sandwiched with the perovskite sheets of various thicknesses. The members of the SA series are commonly expressed by the general formula, [Bi2O2][An-1BnO3n+1][Bi2O2][Cl]m(where, n represents the thickness of the perovskite block and m indicates number of halide layers) and represented as AnXm. Fig. 1 shows the simplest member (represented by Bi4NbO8Cl) of the series containing single-layer perovskite sheet stacked between [Bi2O2]2+ layers and interleaved by single [X]− layers and the structure is commonly designated by the formula A1X1. John Ackerman [27] was the first person to describe the SA structure series correctly with the A1X1 member, Bi4NbO8Cl (n=1 and m=1), although it was reported for the first time by Aurivillius [28]. Detail structural investigations of Bi4NbO8Cl along with its Ta- analogue have been investigated later by Kusainova and co-workers [29]. Considering that these SA phases are composed of [Bi2O2]2+, [MVO4]3− and [Cl]− layers, they might possess strong internal electric fields and could lead to better electronhole separation thereby enhancing photocatalytic activity. Recently, in our effort to develop new visible-light-responsive layered perovskite photocatalysts, we have demonstrated enhanced photocatalytic activity of La-substituted four and five-layer Aurivillius bismuth-iron-titanates toward RhB degradation under sunlight [30,31]. Here, we have envisaged La-substitution in the SA hybrid layered perovskites, Bi4NbO8X (X=Cl, Br). To the best of our knowledge, we report for the first time, the solid state synthesis of new SA hybrid layered oxyhalides, Bi3LaNbO8Cl and Bi3LaNbO8Br. The photocatalytic activities of the new SA hybrids have been investigated by way of RhB degradation under sunlight-irradiation. For comparison, the parent bulk SA oxyhalides, Bi4NbO8Cl and Bi4NbO8Br, have also been synthesized and their photocatalytic activities have been compared with La-substituted compounds under similar experimental conditions.
2.2. Characterization P-XRD patterns of the as prepared samples were recorded on a Bruker AXS D8 Advance diffractometer operating at 40 kV and 30 mA, using graphite monochromatized CuKα radiation in the angular range, 5–90°. PROSZKI program was used for the refinement of the cell parameters [32]. The morphology of the powder samples were studied using a Zeiss FE-SEM Ultra plus55 operating at 20 kV. For this purpose, the samples were prepared by spreading a pinch of powder on a carbon strip pasted on an Aluminium stub. Gold sputtering was carried out over these samples for making the surface electrically conductive. Besides, EDS analysis was carried out to examine the elemental ratio and compositional homogeneity in the samples. UV–vis DRS studies of all the synthesized compounds were carried out with a Shimadzu UV-2450 UV–vis spectrophotometer in the wavelength range, 200–800 nm with BaSO4 as the reference for baseline correction. Further, with the help of the expression (1) proposed by Kubelka and Munk, the reflectance data were converted into absorption terms [33].
F (R∞) = (1–R∞)2 /2 R∞
(1)
where, R∞ is the reflectance of the sample and F(R∞) is the K-M function. Band gaps were estimated by use of the Tauc plots and the expression (2) proposed by Tauc [34].
(αhv )1/ n = A (hv –Eg )
(2)
where Eg, A, α and hν are the band gap, proportionality constant, absorption coefficient and incident light frequency, respectively. The value of n represents the nature of transition (direct or indirect), being 1/2 for direct and 2 for indirect transition [35]. The intersection point 198
Solar Energy Materials & Solar Cells 161 (2017) 197–205
A. Pandey et al.
of the extrapolated line from the linear portion of the absorption edge with the energy axes gave the band gaps. The PL studies were carried out by using Fluoromax-4 spectrofluorometer. For this purpose, 10 mg of the photocatalyst was suspended in 10 mL methanol and sonicated for 10 min. The samples were excited with 315 nm radiation to record the PL spectra. EIS of the photocatalysts were measured in a conventional three electrode system using a galvanostat (model; Versastat 3, PAR). In these study, the photocatalyst mixed with graphite was used as the working electrode (for details see supporting information), Pt wire as the counter electrode, Ag/AgCl (3 M KCl) as the reference electrode and 10 mM K3[Fe(CN)6] solution containing 0.1 M KCl as the electrolyte. The EIS measurements were carried out in the frequency range 0.01–100 kHz at open circuit potential of 0.24 V with the amplitude of the applied sinusoidal potential perturbation of ± 5 mV. To measure the flat-band potential, impedance measurements were carried out with the same electrodes in 0.1 M NaOH electrolytic solution with an Electrochemical Analyzer, CH Instruments. For this, the potential was systematically varied between −0.5 and −1.4 V with frequency of 50 Hz. The flat-band potential was calculated with the help of a Mott-Schottky plot.
Fig. 2. P-XRD patterns of Bi4-yLayNbO8X (X=Cl, Br; y=0, 1).
3. Results and discussion 3.1. P-XRD analysis
2.3. Adsorption and photocatalytic studies
The P-XRD patterns of Bi4-yLayNbO8X (X = Cl, Br; y=0, 1) are shown in Fig. 2. The P-XRD profiles for both the parent and Lasubstituted compositions reflect formation of pure SA intergrowths isostructural with the orthorhombic Bi4NbO8Cl (JCPDS PDF # 84−0843). However, small amounts of impurity (marked with asterisks) arising from either Bi3NbO7 or Bi2-xLaxO3–type phase cannot be completely avoided under the standardized experimental conditions. All other reflections (excepting these impurities) observed in the PXRD pattern for Bi4-yLayNbO8X (X = Cl, Br; y=0, 1) are indexable in the P21cn (Space group no. 33) assigned for the parent Bi4NbO8Cl. The refined cell parameters for the compounds are listed in Table 2. The least-squares refined cell parameters obtained for the parent Bi4NbO8Cl and Bi4NbO8Br are in fairly good agreement with those reported in the literature [29,38]. A decrease in the difference of the inplane ‘a’ and ‘b’ parameters (Table 2) is clearly evident in the Lasubstituted analogues in comparison to that in the parent phases and is consistent with similar reports of lattice parameter variation in the Aurivillius phases [30,31,39]. The decrease in orthorhombic distortion can probably be attributed to the reduction in lattice mismatch of the [Bi2O2]2+ layer with the perovskite block. Moreover, a distinct contraction in the c-parameter is clearly seen in the La-substituted phases.
Adsorption studies were carried out to determine the amount of dye adsorbed over the solid catalyst surfaces at different pH. For this, the dye-catalyst suspension was stirred at 340 rpm for 6 h in the dark. The UV–vis absorption data for the initial dye solution and that of the final dye suspensions were recorded on a Shimadzu 2450 UV–vis spectrophotometer. The percentage dye adsorption was calculated based on the difference between these two values. Photocatalytic dye degradation experiments for all the samples were performed on RhB at pH 2, 7 and 11 at IIT ROORKEE (29°51′ N; 77°53′ E) during the month of November 2014 having direct normal irradiance ~ 143 W/m2. For degradation studies, 75 mg of the solid catalyst was added to 100 mL of 1×10–5 M RhB solution of appropriate pH. Further, to ensure the adsorption-desorption equilibrium between the solid catalyst and RhB, the above suspension was kept on a magnetic stirrer at 340 rpm for 60 min in the dark. Then, the suspension was exposed to direct sunlight for degradation studies. For monitoring the extent of degradation, 5 mL aliquots of this suspension was withdrawn at regular intervals, centrifuged at 8200 rpm (to separate the catalyst particles from the suspension), and the absorbance of the supernatant was recorded. After recording the absorbance, the solution was transferred back to the same beaker to keep the initial dye volume nearly constant. The dye degradation efficiency of the catalyst was calculated using the following expression,
% of degradation = (1 − C / C0 ) × 100
3.2. FE-SEM analysis The FE-SEM images of Bi4-yLayNbO8X (X = Cl, Br; y=0, 1, 2) are shown in Fig. 3 (top panels). From the FE-SEM images a similar particle morphology, which are ellipsoidal or nearly spherical in shape, is evident for all the compounds Bi4-yLayNbO8X (X = Cl, Br; y=0, 1). The particle sizes range from few hundred nanometres to micrometres as inferred from the FE-SEM images. A high degree of agglomeration is also clear from these images. The corresponding EDS analysis is also shown in Fig. 3 (bottom panels). The slightly higher atomic percentage
(3)
where, C0 is the initial concentration and C being the dye concentration at any point of time (t). 2.4. Detection of reactive species To understand the role of various reactive species (h+, •OH and O2•–) on the photocatalytic RhB degradation, different scavengers were added to the RhB solution prior to the addition of the catalyst. The experimental procedure for the active species determination was similar to that described above during photodegradation studies. In our experiments, 10 mM scavenger species were added to the RhB solution. Ammonium oxalate (AO), benzoquinone (BQ) and tertiary butyl alcohol (t-BuOH) were used as h+, O2•– and •OH scavengers, respectively [36,37]. For this purpose, 0.14 g of solid AO, 0.18 g of BQ and 0.96 mL of t-BuOH were added to 100 mL dye solution to make a scavenger concentration of 10 mM in each case.
Table 2 Lattice parameters and band gap of Bi4-yLayNbO8X (X = Cl, Br; y=0, 1). Compound
Bi4NbO8Cl Bi3LaNbO8Cl Bi4NbO8Br Bi3LaNbO8Br
199
Lattice parameters (Å)
Band Gap (eV)
a
b
c
5.452(2) 5.459(2) 5.474(2) 5.467(1)
5.489(1) 5.437(4) 5.518(2) 5.466(2)
28.66(1) 28.32(2) 29.03(1) 28.68(1)
2.43 2.46 2.46 2.54
Solar Energy Materials & Solar Cells 161 (2017) 197–205
A. Pandey et al.
Fig. 3. FE-SEM images (top panels) and corresponding EDS data (bottom panels) of (a) Bi4NbO8Cl, (b) Bi3LaNbO8Cl, (c) Bi4NbO8Br and (d) Bi3LaNbO8Br.
visible region (λ > 400 nm). The band gaps for the compounds were calculated from (αhν)1/2 vs. hν plot by assuming an indirect band gap hypothesis. The estimated band gaps for the compounds lie between 2.43 and 2.54 eV without any significant change in the La-substituted phases from those of the parent. The band gap of the parent compounds are in agreement with the literature values reported earlier [22]. Since the band gap for all the compounds fall well within the visible region, they were thought to be potentially active toward
of niobium and an associated lower atomic percentage of bismuth than expected may be attributed to the overlapping X-ray peaks of niobium with that of bismuth.
3.3. UV–vis DRS analysis The UV–vis DRS data for Bi4-yLayNbO8X (X = Cl, Br; y=0, 1) is shown in Fig. 4. The absorption edges of all the compounds lie in the 200
Solar Energy Materials & Solar Cells 161 (2017) 197–205
A. Pandey et al.
Fig. 4. (a, b) UV–vis DRS of Bi4-yLayNbO8X (X=Cl, Br; y=0, 1). (c, d) Tauc plots for Bi4-yLayNbO8X (X=Cl, Br; y=0, 1).
Bi3LaNbO8Br, were found to be 757 and 370 Ω, respectively. The EIS results demonstrate that the separation of photogenerated charge carriers is more effective in the La-substituted SA phase, Bi3LaNbO8Br, as compared to that in the parent.
sunlight-driven photocatalysis. 3.4. PL analysis Fig. 5 shows the PL spectra of Bi4−yLayNbO8X (X = Cl, Br; y=0, 1). The compounds show broad emission spectra spreading across 375– 525 nm. The parent compounds show intense emissions at ~415 and ~435 nm with a relatively low intensity emission due to the band-edge recombination at higher wavelengths. In Bi3LaNbO8Cl and Bi3LaNbO8Br the primary emission peak arises due to the band-edge recombination and the intensity of lower wavelength emissions is drastically decreased as compared to that of the parents. A comparatively lower overall PL intensity in case of La-substituted analogues suggests a lower degree of electron-hole recombination [40] considering all the emission pathways. Therefore, a higher photocatalytic activity is expected in the La-substituted phases provided all other factors that play significant role in photocatalytic behaviour remain the same.
3.6. Photocatalytic activity RhB was chosen as the model dye for testing the photocatalytic activities of Bi4-yLayNbO8X (X=Cl, Br; y=0, 1) under solar-irradiation at various pH (2, 7 and 11). The photocatalytic activity was highest at pH 2 over Bi4-yLayNbO8X (X=Cl, Br; y=0, 1), while no appreciable degradation of RhB was observed at pH 7 and 11 (Fig. S1 in supplementary data), excepting for Bi4NbO8Br which showed up to 50% degradation within 30 min of sunlight-irradiation at pH 7. Fig. 7(a) shows the RhB degradation with time under natural sunlight over these catalysts at pH 2. While all the catalysts degraded RhB completely within 40 min of solar irradiation, a blank test without catalyst showed zero percent dye degradation indicating no selfdegradation of RhB under identical experimental conditions. Among all the compounds studied here, the highest photocatalytic degradation was observed for Bi3LaNbO8Br. The kinetic plots of RhB degradation by all the compounds are shown in Fig. 7(b). A comparison of photocatalytic RhB degradation by the La-substituted SA layered oxyhalides with other layered perovskites reported recently in the literature [30,31] showed the SA hybrid phases to be more active. The superior activity of the La-substituted phases can primarily be attributed to an enhanced photoexcited charge separation as evidenced by
3.5. EIS analysis EIS studies can provide information on the charge transfer resistance (RCT) and the separation efficacy of the photogenerated charge carriers [41]. As shown in Fig. 6, the diameter of the Nyquist semicircle is smaller for Bi3LaNbO8Br than that of Bi4NbO8Br. This indicates that Bi3LaNbO8Br has a lower charge transfer resistance than that of Bi4NbO8Br. The RCT for the compounds, Bi4NbO8Br and 201
Solar Energy Materials & Solar Cells 161 (2017) 197–205
A. Pandey et al.
Fig. 7. (a) Photocatalytic degradation of RhB with time by Bi4-yLayNbO8X (X=Cl, Br; y=0, 1) under sunlight-irradiation. (b) Plot of ln(C0/C) as a function of irradiation time over Bi4-yLayNbO8X (X=Cl, Br; y=0, 1).
3.7. Detection of reactive species
Fig. 5. Photoluminescence spectra of (a) Bi4-yLayNbO8Cl (y=0, 1) and (b) Bi4yLayNbO8Br (y=0, 1) at room temperature (excitation at 315 nm).
It is well known that superoxide radical anions (O2•–), holes (h+) and hydroxyl radicals (•OH) are among the main reactive species that take part in the photocatalytic dye degradation process [42]. To understand the role of various reactive species in sunlight-driven RhB degradation reported here various scavenger tests were performed over Bi3LaNbO8Br (Fig. 8). When AO (an h+ scavenger) was used as the scavenger, the photocatalytic degradation was retarded to a great
Fig. 6. Nyquest impedance plots for Bi4NbO8Br and Bi4LaNbO8Br.
decreased charge transfer resistance (see EIS data, Fig. 6) and reduced charge recombination as evidenced by a drastic decrease in the PL intensity (Fig. 5) on La-substitution in the SA hybrid phases. Moreover, our results have shown superior performance of Sillén-Aurivillius intergrowth compounds as compared to the commercial TiO2 toward RhB degradation at pH 2 under sunlight irradiation (Fig. 7(a)).
Fig. 8. Effects of different scavengers on the degradation of RhB in presence of Bi3LaNbO8Br under sunlight-irradiation.
202
Solar Energy Materials & Solar Cells 161 (2017) 197–205
A. Pandey et al.
Fig. 10. Schematic energy level diagram of Bi3LaNbO8Br with respect to potential (vs. NHE) of •OH/H2O, O2/O2•– and the HOMO–LUMO levels of RhB.
Fig. 9. Time profiles of RhB degradation for four successive cycles with Bi3LaNbO8Br.
plot (Fig. S3 in supplementary data). This corresponds to −0.31 V vs. NHE at pH 2. It is to be noted that the measured flat-band potential of Bi3LaNbO8Br is comparable to that of the recently reported SillénAurivillius intergrowth compound, Bi4NbO8Cl [43]. Moreover, the ECB and EVB of the photocatalysts were empirically calculated using the following expressions [44],
extent in the initial 20 min of solar radiation. However, when BQ (an O2•– scavenger) was used, a near complete degradation of RhB was observed within the same time interval. The situation with t-BuOH, which is a •OH scavenger, was different in a sense that intermediate retardations were observed up to 20 min of light irradiation. These observations of scavenger experiments indicated holes (h+) and •OH as the dominant species playing active role in the RhB degradation under sunlight in presence of Bi3LaNbO8Br. 3.8. Catalyst cycling and stability studies
ECB = χ (Xx Yy Zz )–1/2Eg + E0
(4)
EVB = ECB + Eg
(5)
where, Eg, χ(XxYyZz) and E0 being the band gap of semiconductor, absolute electronegativity and scale factor (assumed to be −4.5 eV), respectively. From the geometric mean of the absolute electronegativities of the constituent atoms, the absolute electronegativity of the semiconductor was calculated. As reported in the literature, the absolute electronegativities for Bi, Nb, La, O, Br and Cl were taken as 4.69, 4.0, 3.1, 7.54, 7.59 and 8.30, respectively [45]. Thus, the ECB and EVB of Bi3LaNbO8Br were estimated at 0.346 and 2.886 eV, respectively. The position of ECB for Bi3LaNbO8Br above that of Bi4NbO8Cl according to the Mott-Schottky data is consistent with the empirical calculations (calculated ECB of Bi4NbO8Cl is 0.595 eV) based on absolute electronegativities as well. The reported HOMO and LUMO levels of RhB lie at 0.95 and −1.42 eV, respectively [46]. Accordingly, the HOMO-LUMO gap is in good agreement with the absorption maxima of RhB (λmax =553 nm) as obtained in our study. The potential for •OH/H2O and O2/O2•─ were taken from the literature as 2.68 eV and 0.13 eV, respectively [42]. From the energy level diagram of Bi3LaNbO8Br, the formation and role of superoxide radical anion (O2•─) during the degradation is expected, which is inconsistent with the results of scavenger test (Fig. 8). However, from the positioning of •OH/H2O potential with respect to EVB of Bi3LaNbO8Br, the generation of •OH radical by hole transfer from the semiconductor to the water molecules is not possible. To our surprise, the scavenger test (Fig. 8) conducted with t-BuOH (a • OH scavenger) showed substantial retardation in the RhB degradation. This apparent contradiction can be explained by the reaction of the generated O2•─ with H+ to produce •OOH, which subsequently gives rise to the formation of •OH. The mechanism proposed above could possibly be argued given the acidic reaction medium (pH 2) for the RhB degradation reported here. Moreover, the formation of •OH by this mechanism is in line with the literature reports [9]. Furthermore, when AO (an h+ scavenger) was used, the RhB degradation was drastically slowed down indicating active participation of h+ in the photodegradation process. This is in agreement with the relative positioning of the EVB of Bi3LaNbO8Br and the HOMO level of RhB as shown in the energy level diagram (Fig. 10). Therefore, based on our scavenger test results, h+ and •OH are proposed as the major reactive species involved
The reusability and stability of photocatalysts are important parameters required to be evaluated for their practical use. To test the reusability, four consecutive cycles of photocatalytic RhB degradation over Bi3LaNbO8Br was carried out (Fig. 9). It has been observed that ~98% of RhB is degraded within 20 min of solar irradiation for all the four cycles, demonstrating excellent reusability of the catalyst. However, in the fourth run, a negligible loss of activity (nearly 97% degradation within 20 min of solar irradiation) is noticed attributable to the loss of some catalysts during the cycle test. The P-XRD patterns of Bi4-yLayNbO8X (X=Cl, Br; y=0, 1) recorded before and after the photocatalytic dye degradations (Fig. S2 in supplementary data) indicated retention of crystallinity and phase purity of the compounds after photocatalysis signifying very good stability for all the pristine photocatalysts. 3.9. Mechanistic insight from energy level diagram It is established in the literature that the photocatalytic degradation of dyes by a semiconductor photocatalyst involve several steps. Initially the dye gets adsorbed on the catalyst surface during the equilibration of the dye-catalyst suspension. On light irradiation, the photoexcitation of the semiconductor gives rise to the generation of e–– h+ pairs, which later migrate to the surface of the semiconductors and thereby directly react or generate reactive species, such as, O2•– and •OH. However, the generation of these species also depend on appropriate energy level ordering of the semiconductor valence and conduction band edges (EVB and ECB) with respect to the potential for •OH/H2O, O2/O2•─. To get an insight on the enhanced RhB degradation by Bi3LaNbO8Br, an energy level diagram involving the EVB and ECB of Bi3LaNbO8Br along with the HOMO - LUMO levels of RhB and potentials for •OH/H2O and O2/O2•─ is presented in Fig. 10. All the potentials are with respect to the normal hydrogen electrode (NHE). The flat-band potential of the semiconductor was approximated as the ECB and accordingly the EVB was calculated using the band gap (Eg). The flat-band potential of −1.11 V vs. Ag/AgCl at pH 12.3 for Bi3LaNbO8Br was determined from the impedance spectroscopy data with the help of a Mott-Schottky 203
Solar Energy Materials & Solar Cells 161 (2017) 197–205
A. Pandey et al.
photocatalysts where the photophysical properties may also be tuned by micro- and nanostructure control to alter the photocatalytic properties of these oxyhalides. 4. Conclusions New La-substituted Sillén-Aurivillius (SA) layered oxyhalides, namely, Bi3LaNbO8Cl and Bi3LaNbO8Br have been synthesized for the first time. The compounds are visible light absorbers having band gap ~2.43 – 2.54 eV. An enhanced sunlight-driven photodegradation of RhB is exhibited by these SA hybrid perovskites. The observed efficiencies for RhB degradation of La-substituted catalysts are quite high as compared to the parent compounds in the acidic medium. This has been attributed to lower e−- h+ recombination, thereby allowing more electron-hole pairs to participate in the reactive species generation and photodegradation process. The scavenger tests performed during RhB degradation clearly indicated the holes and hydroxyl radicals as the main reactive species that take part in the photodegradation process. Further, a close link of the enhanced RhB adsorption over Bi3LaNbO8Br with a high photodegradation rate corroborates well with h+ being a major reactive species. Owing to their excellent photocatalytic performance under sunlight-irradiation, the La-substitution has been proved to be an effective strategy for enhancing the photocatalytic activity of semiconductor catalysts.
Fig. 11. Influence of pH on RhB adsorption onto Bi4-yLayNbO8X (X = Cl, Br; y=0, 1).
in the photocatalytic RhB degradation under sunlight irradiation by Bi3LaNbO8Br.
3.10. Role of adsorption As it has already been mentioned that adsorption plays an important role in photocatalysis, the extent of dye adsorption over the catalysts was evaluated by adsorption experiments. The adsorption tests were carried out at different pH for Bi4-yLayNbO8X (X=Cl, Br; y=0, 1) and the data are shown in Fig. 11. It is apparent that the maximum dye adsorption (65%) was observed for Bi3LaNbO8Br at pH 2. The second highest adsorption (61%) was observed for Bi4NbO8Br. The fact that h+ plays the dominant role as active species; a greater extent of dye adsorption would enhance the RhB degradation efficiency. The adsorption results are in agreement with the enhanced RhB degradation (higher rate constants) with Bi4NbO8Br and Bi3LaNbO8Br at pH 2. The low adsorption percentages at pH 7 and 11 were reflected in the lower extent of photodegradation (Fig. S1 in supplementary data), which are consistent with our RhB degradation results at neutral and alkaline pH. The extent of adsorption follows the order ‘Bi3LaNbO8Br > Bi4NbO8Br > Bi3LaNbO8Cl > Bi4NbO8Cl' at pH 2. The adsorption order is consistent with the results obtained from the photocatalytic activities (Fig. 7). Moreover, a greater extent of RhB adsorption by Bi4NbO8Br at pH 7 could be corroborated to a moderately high RhB degradation (Fig. S1) in the neutral medium by Bi4NbO8Br. It is noteworthy that the La-substitution in the SA hybrid layered oxyhalides has improved the dye degradation efficiency under sunlightirradiation. Both Bi3LaNbO8Br and Bi3LaNbO8Cl showed enhanced activity (2.3 mM g−1 h−1 and 1.48 mM g−1 h−1, respectively) toward RhB degradation as compared to their parent compounds (1.64 mM g−1 h−1 and 0.93 mM g−1 h−1, respectively). This activity enhancement on La-substitution is primarily driven by suppression of electron-hole recombination as was evidenced by PL and EIS studies. With reference to literature reports [22,47], we believe that the internal static electric fields between the [Bi2O2]2+ sheets, [NbO4]3− slabs and chloride anion layers plays crucial role in efficient separation of photogenerated electron-hole pairs. Moreover, a greater extent of dye adsorption facilitates the semiconductor surface to dye h+ transfer pathway for the degradation. Based on recent reports [30,31,48] and our present investigation La-substitution may be considered as an effective strategy for enhancing the photocatalytic activity of semiconductor catalysts. Moreover, in the light of the very recent report [43] of Bi4NbO8Cl as stable and efficient water splitting photocatalyst under visible light, the La-substituted analogues may provide insight for the band level tuning in these SA hybrid layered perovskite oxyhalides. Furthermore, the SA layered hybrids may emerge as more active
Acknowledgements This work was financially supported by IIT Roorkee NO. IITR/ SRIC/259/FIG(Scheme-A) through Faculty Initiation Grant A (FIG-A). A. P. and G. N. thank the Ministry of Human Resources and Development (MHRD), Government of India for research fellowships. The authors are thankful to the Institute Instrumentation Centre, IIT Roorkee for providing the facilities. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2016.11.040. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [2] H. Zhang, G. Chen, D.W. Bahnemann, Photoelectrocatalytic materials for environmental applications, J. Mater. Chem. 19 (2009) 5089–5121. [3] D. Ravelli, D. Dondi, M. Fagnoni, A. Albini, Photocatalysis. A multi-faceted concept for green chemistry, Chem. Soc. Rev. 38 (2009) 1999–2011. [4] N. Serpone, A.V. Emeline, S. Horikoshi, V.N. Kuznetsov, V.K. Ryabchuk, On the genesis of heterogeneous photocatalysis: a brief historical perspective in the period 1910 to the mid-1980s, Photochem. Photobiol. Sci. 11 (2012) 1121–1150. [5] F. Fresno, R. Portela, S. Suárez, J.M. Coronado, Photocatalytic materials: recent achievements and near future trends, J. Mater. Chem. A 2 (2014) 2863–2884. [6] Z. Zou, J. Ye, K. Sayama, H. Arakawa, Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst, Nature 414 (2001) 625–627. [7] E. Frank, Osterloh, Inorganic materials as catalysts for photochemical splitting of water, Chem. Mater. 20 (2008) 35–54. [8] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253–278. [9] S. Dong, J. Feng, M. Fan, Y. Pi, L. Hu, X. Han, M. Liu, J. Sun, J. Sun, Recent developments in heterogeneous photocatalytic water treatment using visible lightresponsive photocatalysts: a review, RSC Adv. 5 (2015) 14610–14630. [10] D. Zhang, G. Li, J.C. Yu, Inorganic materials for photocatalytic water disinfection, J. Mater. Chem. 20 (2010) 4529–4536. [11] O. Carp, C.L. Huisman, A. Reller, Photoinduced reactivity of titanium dioxide, Prog. Solid State Chem. 32 (2004) 33–177. [12] H. Chen, C.E. Nanayakkara, V.H. Grassian, Titanium dioxide photocatalysis in atmospheric chemistry, Chem. Rev. 112 (2012) 5919–5948. [13] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [14] M. Oshikiri, M. Boero, J. Ye, Z. Zou, G. Kido, Electronic structures of promising photocatalysts InMO4 (M=V, Nb, Ta) and BiVO4 for water decomposition in the visible wavelength region, J. Chem. Phys. 117 (2002) 7313–7318.
204
Solar Energy Materials & Solar Cells 161 (2017) 197–205
A. Pandey et al.
Eng. 3 (2015) 2900–2908. [32] W. Losocha, K. Lewinski, Prozski- A system of programs for powder diffraction data analysis, J. Appl. Crystallogr. 27 (1994) 437–438. [33] P. Kubelka, F. Munk, Ein Beitrag zur Optik der Farbanstriche, Z. Tech. Phys. 12 (1931) 593–601. [34] J. Tauc, R. Grigorovic, A. Vancu, Optical properties and electronic structure of amorphous Germanium, Phys. Status Solidi 15 (1966) 627–637. [35] U.A. Joshi, P.A. Maggard, CuNb3O8: a p-type semiconducting metal oxide photoelectrode, J. Phys. Chem. Lett. 3 (2012) 1577–1581. [36] Y. Wang, K. Deng, L. Zhang, Visible light photocatalysis of BiOI and its photocatalytic activity enhancement by in situ ionic liquid modification, J. Phys. Chem. C 115 (2011) 14300–14308. [37] S. Kumar, S. Khanchandani, M. Thirumal, A.K. Ganguli, Achieving Enhanced Visible-Light-Driven Photocatalysis Using Type-II NaNbO3/CdS Core/Shell Heterostructures, ACS Appl. Mater. Interfaces 6 (2014) 13221–13233. [38] A.M. Kusainova, W. Zhou, J.T.S. Irvine, P. Lightfoot, Layered intergrowth phases Bi4MO8X (X = Cl, M = Ta and X = Br, M = Ta or Nb): structural and electrophysical characterization, J. Solid State Chem. 166 (2002) 148–157. [39] M. W. Chu, M.-T. Caldes, L. Brohan, M. Ganne, A.-M. Marie, O. Joubert, Y. Piffard, Bulk and surface structures of the Aurivillius Phases: Bi4-xLaxTi3O12 (0≤ x≤2.00), Chem. Mater. 16 (2004) 31–42. [40] K. Fujihara, S. Izuni, S. Ohno, M. Matsumura, Time-resolved photoluminescence of particulate TiO2 photocatalysts suspended in aqueous solutions, J. Photochem. Photobiol. A 132 (2000) 99–104. [41] H. Liu, S. Cheng, M. Wu, H. Wu, J. Zhang, W. Li, C. Cao, Photoelectrocatalytic degradation of sulfosalicylic acid and its electrochemical impedance spectroscopy investigation, J. Phys. Chem. A 104 (2000) 7016–7020. [42] S. Kumar, T. Surendar, A. Baruah, V. Shanker, Synthesis of a novel and stable gC3N4–Ag3PO4 hybrid nanocomposite photocatalyst and study of the photocatalytic activity under visible light irradiation, J. Mater. Chem. A 1 (2013) 5333–5340. [43] H. Fujito, H. Kunioku, D. Kato, H. Suzuki, M. Higashi, H. Kageyama, R. Abe, Layered perovskite oxychloride Bi4NbO8Cl: a stable visible light responsive photocatalyst for water splitting, J. Am. Chem. Soc. 138 (2016) 2082–2085. [44] J. Lv, T. Kako, Z. Zou, J. Ye, Band structure design and photocatalytic activity of In2O3/N–InNbO4 composite, Appl. Phys. Lett. 95 (2009) 032107–032109. [45] R.G. Pearson, Absolute electronegativity and hardness: application to inorganic chemistry, Inorg. Chem. 27 (1988) 734–740. [46] L. Pan, J. Zou, X. Liu, X. Liu, S. Wang, X. Zhang, L. Wang, Visible–light–induced photodegradation of rhodamine b over hierarchical TiO2: effects of storage period and water-mediated adsorption switch, Ind. Eng. Chem. Res. 51 (2012) 12782–12786. [47] I. Grinberg, D.V. West, M. Torres, G. Gou, D.M. Stein, L. Wu, G. Chen, E.M. Gallo, A.R. Akbashev, P.K. Davis, J.E. Spanier, A.M. Rappe, Perovskite oxides for visiblelight-absorbing ferroelectric and photovoltaic materials, Nature 503 (2013) 509–512. [48] F.N. Sayed, V. Grover, B.P. Mandal, A.K. Tyagi, Influence of La3+ substitution on electrical and photocatalytic behavior of complex Bi2Sn2O7 oxides, J. Phys. Chem. C 117 (2013) 10929–10938.
[15] J. Tang, Z. Zou, J. Ye, Efficient photocatalytic decomposition of organic contaminants over CaBi2O4 under visible-light irradiation, Angew. Chem. Int. Ed. 43 (2004) 4463–4466. [16] K.-L. Zhang, C.-M. Liu, F.-Q. Huang, C. Zheng, W.-D. Wang, Study of the electronic structure and photocatalytic activity of the BiOCl photocatalyst, Appl. Cat. B: Environ. 68 (2006) 125–129. [17] J. Zhang, F. Shi, J. Lin, D. Chen, J. Gao, Z. Huang, X. Ding, C. Tang, Self-assembled 3-D architectures of BiOBr as a visible light-driven photocatalyst, Chem. Mater. 20 (2008) 2937–2941. [18] X. Zhang, Z. Ai, F. Jia, L. Zhang, Generalized one-pot synthesis, characterization, and photocatalytic activity of hierarchical BiOX (X=Cl, Br, I) nanoplate microspheres, J. Phys. Chem. C. 112 (2008) 747–753. [19] X. Xiao, W.-D. Zhang, Facile synthesis of nanostructured BiOI microspheres with high visible light-induced photocatalytic activity, J. Mater. Chem. 20 (2010) 5866–5870. [20] L. Ye, Y. Su, X. Jin, H. Xie, C. Zhang, Recent advances in BiOX (X = Cl, Br and I) photocatalysts: synthesis, modification, facet effects and mechanisms, Environ. Sci.: Nano 1 (2014) 90–112. [21] S.M. Fray, C.J. Milne, P. Lightfoot, Synthesis and structure of CaBiO2Cl and SrBiO2Cl: new distorted variants of the Sillen X1 structure, J. Solid State Chem. 128 (1997) 115–120. [22] X. Lin, T. Huang, F. Huang, W. Wang, J. Shia, Photocatalytic activity of a Bi-based oxychloride Bi4NbO8Cl, J. Mater. Chem. 17 (2007) 2145–2150. [23] J. Fan, X. Hu, Z. Xie, K. Zhang, J. Wang, Photocatalytic degradation of azo dye by novel Bi-based photocatalyst Bi4TaO8I under visible-light irradiation, Chem. Eng. J. 179 (2012) 44–51. [24] S.S.M. Bhat, N.G. Sundaram, Efficient visible light photocatalysis of Bi4TaO8Cl nanoparticles synthesized by solution combustion technique, RSC Adv. 3 (2013) 14371–14378. [25] Y. He, Y. Zhang, H. Huang, N. Tian, Y. Guo, Y. Luo, A novel Bi-based oxybromide Bi4NbO8Br: synthesis, characterizationand visible-light-active photocatalytic activity, Colloid Surf. Physiochem. Eng. Asp. 462 (2014) 131–136. [26] S.S.M. Bhat, N. Sundaram, Photocatalysis of Bi4NbO8Cl hierarchical nanostructure for degradation of dye under solar/UV irradiation, New J. Chem. 39 (2015) 3956–3963. [27] J.F. Ackerman, The structures of Bi3PbWO8Cl and Bi4NbO8Cl and the evolution of the Bipox structure series, J. Solid State Chem. 62 (1986) 92–104. [28] B. Aurivillius, Intergrowth compounds between members of the bismuth titanate family and structures of the lithium dichlorotetraoxitribismuthate (LiBi3O4Cl2) type, Chem. Scr. 23 (1984) 143. [29] A.M. Kusainova, S. Yu Stefanovich, V.A. Dolgikh, A.V. Mosunov, C.H. Hervoches, P. Lightfoot, Dielectric properties and structure of Bi4NbO8Cl and Bi4TaO8Cl, J. Mater. Chem. 11 (2001) 1141–1145. [30] G. Naresh, T.K. Mandal, Excellent sun-light-driven photocatalytic activity by Aurivillius layered perovskites, Bi5−xLaxTi3FeO15 (x=1, 2), ACS Appl. Mater. Interfaces 6 (2014) 21000–21010. [31] G. Naresh, T.K. Mandal, Efficient COD removal coinciding with dye decoloration by five-layer Aurivillius perovskites under sunlight-irradiation, ACS Sustain. Chem.
205