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Synthesis, characterization and photocatalytic activity of mixed oxides derived from ZnAlTi ternary layered double hydroxides
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Synthesis, characterization and photocatalytic activity of mixed oxides derived from ZnAlTi ternary layered double hydroxides R.K. Sahu, B.S. Mohanta, N.N. Das
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S0022-3697(13)00151-0 http://dx.doi.org/10.1016/j.jpcs.2013.04.002 PCS7051
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Received date: 1 December 2012 Revised date: 28 March 2013 Accepted date: 7 April 2013 Cite this article as: R.K. Sahu, B.S. Mohanta, N.N. Das, Synthesis, characterization and photocatalytic activity of mixed oxides derived from ZnAlTi ternary layered double hydroxides, Journal of Physics and Chemistry of Solids, http://dx.doi. org/10.1016/j.jpcs.2013.04.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis, characterization and photocatalytic activity of mixed oxides derived from ZnAlTi ternary layered double hydroxides R.K. Sahu, B.S. Mohanta, N.N. Das* P. G. Department of Chemistry, North Orissa University, Baripada-757 003, Orissa, India *
Corresponding author. Tel.: +91 679 225 2088; fax: +91 679 225 3908.
Email: [email protected] Abstract A new series of Ti4+ containing ZnAl-LDHs with varying Zn:Al:Ti (~3:1:0 – 3: 0.5, 0.5) ratio were prepared by coprecipitation of homogeneous solution metal salts and characterized by various physicochemical methods. Powder XRD revealed the formation of well crystalline LDH even at highest Ti4+ content. On thermal treatment at 450 qC, the well crystalline LDH precursors yielded the mixed oxides with BET surface area in the range 92-118 m2/g. UV–vis diffuse reflection spectroscopy (DRS) showed a marginal decrease of band gap energy for calcined ZnAlTi-LDHs in comparison to either ZnO or TiO2-P25. The TEM analyses of a representative sample (as-synthesised and calcined) indicated more or less uniform distribution of titanium species. The derived mixed oxides from titanium containing LDH precursors demonstrated better activity towards photodegradation of methylene blue and rhodamine B than physical mixture of ZnO and TiO2. Moreover, the present work not only provided a first hand understanding about semiconductor properties of ZnAlTi-LDHs but also demonstrated their potential as photocatalysts for degradation of organic pollutants.
Highlights x
Synthesis and characterizations of a new series of ZnAlTi ternary layered double hydroxides.
x
Calcination of LDH precursors led to formation of mixed oxides with reduced band gap energies than ZnO or TiO2-P25.
x
The derived mixed oxides are effective photocatalysts under visible light for dyes degradation.
x
Easy separation and possibility of reuse of used catalyst.
Keywords A. inorganic compounds; A. oxides, B. chemical synthesis; C. electron microscopy; C. X-ray diffraction 1.
Introduction
Layered double hydroxides (LDHs), represent an important class of inorganic layered materials, have been a subject of numerous investigations during last three decades because of their potential applications as catalysts, catalyst supports, ion exchangers/adsorbents, layered hosts for biomolecules, precursors for composite materials etc. [1-9]. The general formula of LDHs is [M2+(1-x)M3+x(OH)2]x+ [Am-x/m ]n-.mH2O where M(II) and M(III) include a variety of bivalent (Mg2+, Zn2+, Co2+, Cu2+, Mn2+) and trivalent (Al3+, Fe3+, Cr3+, V3+, Ga3+, Ti3+) metal ions, An- is the interlayer anion that may be organic, inorganic, carboxylate, oxoanion, coordination compounds and polyoxometalates and x generally can have the values between 0.1 and 0.33 [2]. Off let efforts have been devoted to introduce a tetravalent ion (e.g. Zr4+, Sn4+ and Ti4+) in the brucite like layer as a partial replacement of M2+ or M3+ ion in order to enhance the anion exchange capacity (AEC) or to tune the acido-basicity of their resulting mixed oxides [10-18]. Although Zr4+ and Ti4+ containing LDHs have shown enhanced anion adsorption capacity [10-14] in comparison to those without tetravalent ions, the recent X-ray absorption spectroscopic studies [19,20] indicated that incorporation of tetravalent ions in octahedral sheet does not occur, but actually an amorphous M(IV) oxide is formed and impregnate the LDH crystallites. Even then the LDHs containing Zr4+ and Ti4+ could be useful as catalyst for various organic transformations requiring tailored acidobasicity and also as adsorbents with enhanced adsorption capacity for remediation of anionic pollutants from contaminated water. In recent years there has been a growing interest to develop LDH based photocatalysts as an alternative to conventional semiconductor materials like TiO2, ZnO and SnO2 for degradation of variety organic pollutant including dyes [21-26]. In particular, the mixed oxide derived from ZnAl containing LDHs, without or with Fe, Sn and Ti, are successful photocatalysts for the degradation of organic compounds like methyl orange, methylene blue and phenols in aqueous media. In contrast, the use of as synthesized LDHs as photocatalysts has been sparsely studied because of favourable capture of photoinduced 2
electron by hydroxyl group during photocatalytic process. A recent study of Silva et al. [27] has reported oxygen generation from photolysis of water using as synthesized ZnM-LDHs (M = Cr, Ti, Ce) photocatalyst under visible light and the results showed that the LDHs can be regarded as “doped semiconductor”. Keeping the above in view and as a sequel to our previous studies [12,13], we report here in the synthesis of new series of Ti containing ZnAl-LDH. The physicochemical characterizations and photocatalytic behaviours of derived mixed oxides towards two commonly occurring dyes, namely methylene blue (MB) and rhodamine B (RhB), under visible light are also reported.
2.
Experimental
2.1
Materials Zn(NO3)2.6H2O, Al(NO3)3.9H2O, TiCl4, NaOH and Na2CO3 (Merck, GR) were used
for synthesis LDH precursors without further purification. Methylene blue (Qualigens) and rhodamin B (Merck, GR) were used as received for adsorption experiments. All other chemicals used in this work were of AR/GR grades. Stock solutions of dyes were prepared by dissolving required amount of corresponding dye in double distilled water.
2.2
Preparation and calcination of LDH precursors The LDH precursors with varying Zn:Al:Ti atomic ratios were prepared by
coprecipitation of metal salts solution at constant pH ~ 10 under low supersaturating conditions. A solution containing the mixture of Zn(NO3)2, Al(NO3)3 and TiCl4 and a mixture of NaOH (2.0 M) and Na2CO3 (0.20 M) were added drop wise to a well stirred solution of Na2CO3 (100 ml 0.01M) such that the pH of the resulting slurry was maintained at ~ 10. Once the addition was completed, the resulting precipitate was aged for 18 h at room temperature, separated by centrifugation, washed thoroughly with distilled water until the precipitate was free from chloride and then dried overnight at 90 qC in air-oven. Small amounts of nitrate/chloride ions retained in the interlayer of LDH precursors were replaced by carbonate ions by suspending the dried precursors (2 g) in the solutions of Na2CO3 (100 ml, 0.20 M) and stirring for 2 h. The carbonate-exchanged solid was separated by centrifuge, washed, dried in air oven overnight at 90 oC and stored in air tight bottles for further use. The
3
samples are denoted as ZAT-0 to 4 depending on their Ti contents (Table 1). Based on the TG-DTA and FT-IR spectral analyses the dried LDH precursors were calcined in air at 450 o
C with a heating rate of 5 oC min-1 for 5 h and used for further studies.
2.3
Characterizations Zn, Al and Ti contents were determined by conventional wet chemical analyses and
also by ICP (Varian Liberty series2) (Das et al., 2010). Powder X-ray diffraction (PXRD) of carbonate exchanged LDH precursors and their calcined products were recorded on a Rigaku (Miniflex II) diffractometer at scanning speed 2q(2T) min-1 using Ni filtered CuK (30 kV, 15 mA) radiation. Thermogravimetric measurements (TG-DTA) were performed on a Shimadzu DTG 60 Thermal analyser under flowing nitrogen (40 ml min-1) at a heating rate of 10 qC min-1. The surface area of calcined samples was determined by BET method using a surface and porosity analyzer (Quantachrome, Novawin) after degassing the samples under vacuum (10-4 Pa) at 250 qC. FT-IR spectra in KBr phase were recorded on a Shimadzu IR Affinity-1 spectrophotometer averaging 45 scans with a nominal resolution of 4 cm-1 to improve signal to noise ratio. The UV-Visible diffuse reflectance spectra (UV-Vis DRS) of solid samples using BaSO4 as reference and spectral scan of the photocatalytic reaction mixture were performed with a Shimadzu UV-Visible spectrophotometer. The band gap energies were estimated from absorption edge using the relationship: E (eV) = h (c/O) nm, where h, c and O represent the Plank’s constant, velocity of light (meter/s) and is the cutoff wavelength (nm), respectively. Transmission electron micrographs were recorded using FEI Tecnai 30G2 STwin (Netherlands) operated at 300 kV.
2.2
Photocatalytic activity The photocatalytic efficiencies of calcined LDH precursors were studied taking two
model dyes viz. methylene blue and rhodamine B. All the photocatalytic experiments were performed in a 200 ml capacity double walled cylindrical quartz reactor fitted with 125 W high pressure Hg lamp ( > 420 nm) as source of visible light under magnetic stirring condition. The temperature of reaction mixture containing dye solution and photocatalyst, was maintained (30.0 r 0.2) by circulating water through outer walls from Julabo (Germany)
4
F12 water circulator. All the experiments were performed in presence of air at atmospheric pressure. The reaction was imitated by irradiating the reaction mixture containing 100-150 ml of dye solution at different concentrations (10-45 mg L-1) and amount of catalysts (1.0-5.0 g L-1) with the visible light. The initial pH of the dye solution was adjusted to ~ 6.5 which was found increase to ~ 8.2 r 0.3 at the end of photocatalytic reactions. At regular intervals (for kinetic experiments) and at the end of reaction, a definite portion of reaction mixture was withdrawn, separated the solid catalyst by centrifugation and the residual dye concentrations were computed by measuring the absorbance at 565 and 545 nm for MB and RhB, respectively. The measurements of absorbance were carried out on a Systronics 2201 UVVisible spectrophotometer using 10 mm matched quartz cell. Parallel experiments under identical conditions in presence of light without catalyst and in presence of catalyst without irradiation of light were also carried out to see the individual effect of light and catalyst on degradation/adsorption of dyes. 3.
Results and discussion
3.1
Characterization of as synthesized LDHs The composition of carbonate exchanged LDH precursors and their crystal lattice
parameters, derived from PXRD patterns, are collected in Table 1. Chemical analyses indicate that the molar ratios of Zn, Al and Ti are close to those initially taken for preparation of LDH precursors. TG-DTA plots of representative uncalcined samples are presented in Fig. 1. All the samples mainly exhibit two stage weight losses with corresponding endothermic peaks in DTA profiles. The first stage loss can be tentatively resolved in two overlapped processes and mainly attribute to the elimination of physically adsorbed and interlayer water molecules. The second loss, always higher than the first loss, is ascribed to loss of hydroxyl groups from the brucite-like layer along with interlayer carbonate ions with concomitant destruction of layered structure [2, 11, 14, 16,17]. A minor weight loss beyond 450 qC is attributed to loss of oxygen and CO2 through slow decomposition of Zn(Al/Ti) oxycarbonate which are likely to be formed after decomposition of LDH-like structure. The overall behaviour of LDH samples are in agreement with those generally reported for ZnAl-LDH samples [14,16]. The total weight losses at 600 oC ranging from 27-29 % are in good agreement with ZnAl-LDH precursors with or without Zr. Further, the temperature of weight loss is shifted to lower with
5
incorporation of Ti in the interlayer presumably due to relatively weak electrostatic interaction of interlayer anions with brucite like layer due to increased interlayer spacing as a result of higher positive charge in the brucite layer. The PXRD patterns of the dried carbonate exchanged LDHs are presented in Fig. 2. A single phase corresponding to hydrotalcite like compounds (LDH: JCPDS File No. 38-487) is observed for all the samples without any appreciable decrease of crystallinity even at higher Ti(IV) content (Al/Ti ratio ~ 1.0). No peaks from any other crystalline material could be detected either due to overlapping with the characteristic LDH peaks or formation of amorphous hydroxide phases of Ti (e.g. Ti(OH)4 or TiO2.nH2O) and/or ZnTi. Formation of amorphous TiIV oxide has also been reported in synthesis of Co2AlTi-HT [20]. Assuming a hexagonal crystal system, the lattice parameters are calculated from (110) and (003) reflections and presented in Table 1. The marginal increase of both a and c parameters for ZAT-1, 2, 3 and 4 in comparison to ZAT-0 could be, but not conclusive, due to partial incorporation of Ti4+ with relatively higher ionic radius (r = 0.072 nm) in place of Al3+ (r = 0.053 nm) in the LDH framework [12,17]. This also leads a higher amount of CO32- as compensating anion in the interlayer which in turn results an increase of c parameter. Increase of both a and c lattice parameters along with increase of carbonate content have also been observed for incorporation of Zr4+ and Ti4+ in the brucite like layer in the cases of ZnAl and MgAl-LDHs [12,13,17], respectively. FT-IR spectra of representative carbonate exchanged LDHs, shown in Fig. 3, are very similar to those generally reported for hydrotalcite like compounds. The broad and strong adsorption band centred at ~ 3480 cm-1 in case uncalcined samples is attributed to stretching vibrations of physisorbed water, structural OH group and/or hydrogen bonded hydroxyl group (OHOH) [2,28]. A weak shoulder at 3070 cm-1, causing the broadness of this band may be ascribed to the OH stretching mode of water molecule, hydrogen bonded to the interlayer carbonate anion. The band close to 1630 cm-1 is originated due to bending mode (GHOH) of interlayer water molecules. An sharp intense band observed at ~ 1370 cm-1 along with relatively less intense peak at ~ 1530 cm-1 are assigned to symmetric and antisymmetric O-C-O stretching vibrations of monodentate carbonate species. The shifts from the normal position of the free carbonate species, i.e., 1450 cm-1, and the splitting of about 130-154 cm-1 for samples ZAT-1 to ZAT-4 result from a lowering of the symmetry of the species in the
6
interlayer domain. These shifts are higher than in LDHs containing cations with large ionic sizes such as Y, V and Cr in Mg/Al/Y, Ni/V or Zn/Cr with values of 115, 116, and 126 cm-1, respectively [16]. As expected, the splitting in case of sample without Ti (ZAT-0) is about 120 cm-1 which is less than those observed for other ZAT samples. These features could account for a greater distortion of the brucite-like layers and for a heterogeneous distribution of positive charge in the brucite-like layers containing cations of different charges. On calcination at 450 qC for 5 h, the characteristic peaks due to CO32- ion are practically absent in the resulted mixed oxides. The TEM images of ZAT-2 along with EDX and SAED patterns are presented in Fig. 4. The TEM images of as-synthesised ZAT-2 in dark and bright filed indicate uniform distribution of titanium species (e.g. as TiO2.nH2O) [29.30]. EDX measurement also confirmed the presence of Zn, Al and Ti in the same proportions as that of taken for synthesis of ZAT-2. SAED pattern also reveals more or less uniform distribution of LDH particles. 3.2
Characterization of calcined LDHs The nature of crystalline phases generated after calcination of LDHs is of interest
from view points of their applications as bifunctional acid-base catalysts/catalyst support, photocatalysts, ion exchangers etc. On calcination at moderate temperature (450 qC), the LDH precursors are converted to mixed oxides (Fig. 2, inset) whose lattice ‘a’ parameters are slightly smaller than that of pure ZnO (a = 2.093 Å) indicating isomorphous substitution of Al3+ or both Al3+ and small fraction of Ti4+ for Zn2+ in the lattice. Similar observations have been reported earlier for several calcined LDH samples [2, 11, 14, 16,17]. The overlapping peaks at 2 values 36.5, 47.8, 55.6, 63.04 (Fig. 2, inset) may also be attributed to the characteristic peaks of (004), (200), (105), (201) and (204) planes of anatase phase. The surface areas of calcined Ti-containing samples exhibit lower values (Table 1) than the sample without Ti and the values are progressively decreased with increase of Ti content in the samples. A similar trend was also observed earlier for Ti and Zr containing calcined LDHs [12,16,17]. The DRS spectra of calcined LDHs along with ZnO and TiO2 (Degussa P25) are presented in Fig. 5 (inset). It is seen that spectral intensity of Ti containing calcined ZAT samples are relatively higher than that of sample without Ti (ZAT-0). There is a marginal shift in the absorption band in all ZAT samples in comparison to pure ZnO or TiO2-P25
7
presumably due to presence of intimately mixed ZnO and TiO2 in the oxide samples obtained on calcination of ZAT-precursors. The band gap energies of the photocatalysts (Fig. 5) were determined by extrapolating the linear region to the abscissa of Tauc plot, a plot of (F(R)hQ)1/2 against hQ where F(R) is the Kubelka–Munk function and the values obtained are collected in Table 1. All the ZAT samples exhibit relatively lower band gap energy than those observed in case of pure ZnO (3.29) or TiO2-P25 (3.19)/synthesized TiO2 (~3.22) [29] and are expected to show better photocatalytic activity compare to either ZnO or TiO2 under visible light. The TEM images of calcined ZAT-2 along with EDX and SAED patterns are presented in Fig. 6. As evident, the average particle size of ZAT-2 increases on calcination. The EDX analysis also shows the presence of Zn, Al and Ti more or less in the same proportion as that of uncalcined ZAT-2 sample. HR-TEM images show set of uniform lattice fringes providing further evidence in favour of crystalline nature of nanoparticles. Appearance of lattice fringes for anatase and MgAl-LDH has also been observed with dvalues 3.57 and 2.14 Å, respectively in the case of TiO2/MgAl-LDH [30]. 3.3
Photocatalytic activity The photocatalytic activity of calcined LDHs was evaluated towards degradation of
methylene blue and rhodamin B in aqueous medium. The initial pH of the dye solutions was kept constant at 6.5 r 0.2 in order to avoid any colour change of dye solutions due to acidbase equilibria with variation of pH (pKa values of MB and RhB are 10.2 and 10.5, respectively). The pH of reactant solution invariably increased to 8.2 r 0.2 during photodegradation process. The representative set of results using calcined LDHs for degradation of MB and RhB as a function of irradiation time are presented in Figs. 7 and 8, respectively. The activity of physical mixture of ZnO and TiO2-P25 with same weight percentage of ZnO and TiO2 as that of ZAT-3 is also presented in the figures for comparison. Blank experiments were also carried out without catalyst to verify the extent of decolourisation of dyes due to photocatalytic process. It is seen that in the absence of photocatalyst, about 25 % of MB is degraded in 3 h of irradiation with visible light and is very similar to that observed earlier [29]. In comparison, the photodegradation of RhB without catalyst is negligibly small even with irradiation up to 4 h with visible light. As the
8
adsorption of dyes on catalyst is believed to be the primary process in photocatalytic decolourisation/degradation, it is also essential to assess the amount of dye adsorbed on catalyst in dark to account the overall activity of ZAT samples. The calcined ZAT samples can reconstitute to its original LDH structure through rehydration and exhibit positive and negative surface charges depending on the pH (point of zero charge of calcined ZAT samples ~ 8.0). Also at working pH (initial pH ~ 6.5 and final pH ~ 8.2), MB exists in the cationic form (pKa > 12) for which the entry of MB in the interlayer is restricted and hence, the decrease of MB concentration with time in dark is primarily due to adsorption on catalyst surface. Blank experiment with ZAT-3 under identical conditions in dark shows ~ 12 % MB (Fig. 7) is adsorbed in 3 h. On the other hand ~ 7.3 % RhB, mostly exist in zwitterionic form at pH ~ 6.0 due to deprotonation of its carboxyl group (pKa =3.7), is adsorbed on ZAT-3 surface under identical conditions of photocatalytic without visible light irradiation. The repetitive spectral scan of dye solutions with irradiation time, presented in Figs. 7 and 8 (inset), show progressive decrease of absorbance in the entire range of UV-Vis spectra. It is clearly seen that the initial peaks of MB at 614 and 664 nm are merged into a relatively broad peak centred at ~ 650 nm during the reaction. The peak intensities are also reduced progressively with a shift towards higher wavelengths till the reactant solution turns colourless. On the other hand, the intensities of MB bands at 292 and 245 nm are reduced progressively without any shift in their positions. The disappearance of two major absorbance peaks of MB at 292 and 664 nm (Fig. 7, inset), due to benzene ring and heteropolyaromatic linkage, indicate their complete destruction at the end of the reaction. The decolourisation of RhB is also evident from the large decrease in absorbance change during the photocatalytic reaction without any observable shift in the peak positions (Fig. 8, inset) indicating the deolourisation of RhB is primarily due to decomposition of conjugated chromophore structure rather then de-ethylation process [31]. In order to see that effect of catalyst dose for dye degradation, the amount of most effective catalyst (ZAT-4) was varied in the range 1.0 to 5.0 g L-1 at a constant concentration of MB (10.92u10-5 M) and RhB (7.31u10-5 M), initial pH ~ 6.5 and irradiation time of 60 min. The results obtained are presented in Fig. 9. Although there is a progressive increase of degradation with increase of catalyst dose, the effect is more pronounced up to 2.0 g L-1 at least in the case of MB degradation. This increased activity with increasing catalyst dose is
9
obviously to presence more active sites which results in absorption of more number of photons for formation higher number of active species. The less pronounced activity at higher dose is presumably due to less availability of dye molecules for degradation. The concentration of initial dye concentration is another important factor towards overall degradation of dyes. In the present study, the MB and RhB concentrations were varied from (3.12-14.04)u10-5 M and (2.09-9.4)u10-5 M, respectively keeping the initial pH (~ 6.5) and catalyst dose (ZAT-4, 2.0 g L-1) fixed. As expected, the percentage of degradation decrease with increase of initial dye concentration. Further, the plots of ln(C/C0) versus irradiation time are straight line indicating the degradation of dyes follows a first order kinetics. In the above concentration range, the calculated first order rate constants are found in the range 0.012-0.067 min-1 and 0.0054-0.0191 min-1 for MB and RhB, respectively. The values of MB degradation are comparable with the reported rate constant (0.0407 min-1) for MB degradation (initial MB = 1.6 u10-5 M, catalyst dose = 2.0 g L-1) with ZnTi sample (Zn/Ti = 3:1) [29]. It is interesting to note that the activity of the physical mixture of ZnO and TiO2 is lower than all the Ti containing LDH but higher than the activity of sample without Ti (ZAT0). Further the degradation of both the dyes is found to increase rather slowly with increase of Ti content in the catalyst (Fig. 9, inset). This increased activity may be attributed to resultant effects of decreased band gap energy and BET surface area. Since the measured band gap of Zn(Al)O/TiO2 is relatively high (> 2.98 eV), the mechanism operative for dye (D) degradation by semiconductor oxide using UV light is not feasible in the present case rather a photosensitized pathways should be considered. Unlike formation of electron-hole (e-/h+) pairs due to absorption of UV light by semiconductor, light absorption in the present case mainly occurs by the dye molecule [32] adsorbed on the catalyst surface and transfer the excited electron into Zn(Al)O/TiO2 conduction band. The electrons on Zn(Al)O/TiO2 can be used further to reduce dissolved oxygen molecule in dye solution forming O2x leading to degradation of dye molecule. The overall photocatalytic process may be delineated as follows and the same is schematically presented in Fig. 10. D/ZnAl(O)/TiO2 + hQ (vis) o 1D* or 3D*/ZnAl(O)/TiO2 1
D* or 3D*/ZnAl(O)/TiO2 o D+x + ZnAl(O)/TiO2(ecb)
D+x o degradation products
10
ZnAl(O)/TiO2(ecb) + O2 o ZnAl(O)/TiO2 + O2-x O2-x + H+ o HO2x o o HOx and/or O2-x + 2H+ + e- o H2O2 D+x + O2-x o DO2 o degradation products D + HOx or H2O2 o degradation products
The practical utility of a photocatalyst lies in its long time use. A preliminary study was made using calcined ZAT-4 to see its efficiency in repeated cycle by keeping the catalyst amount (2.0 g L-1), initial dye concentrations (3.12u10-5 M for MB and 2.09u10-5 M for RhB) and initial pH (6.5 r 0.3) fixed in each cycle. For this the catalyst, separated from residual dye solutions by centrifugation after irradiation for 120 min, was treated with fresh dye solutions and irradiated again for 120 min. This was repeated for two more cycle and the degradation activity is found to decrease progressively form 100, 92 and 82 % in first, second and third cycles, respectively. The decrease in activity is primarily due to partial formation of parent ZAT-4 LDH precursor through rehydration of calcined ZAT-4 and the resulted hydroxyl groups of LDH capture some of the photoinduced electrons which in turn reduce the photocatalytic activity. The XRD pattern of calcined ZAT-4 after third cycle of photocatalytic reaction is presented in Fig. 11 along with uncalcined ZAT-4 precursor. It is evident that parent ZAT-4 is gradually formed due to memory effect which results in the decrease of activity with number of cycle. Further optimization like heating at higher calcination temperatures to avoid to the formation of parent LDH precursor through rehydration or variation Al/Ti ratio is required for long time use of LDH based catalyst in photocatalytic degradation of dyes.
4.
Conclusions Ternary LDH precursors containing Zn, Al and Ti were synthesized by
coprecipitation of metal salts solution at constant pH. PXRD confirmed the formation of single crystalline LDH like phase even with high Ti content indicating Ti4+ was mostly present as amorphous TiO2.nH2O. Incorporation of a small fraction Ti4+ in the brucite like layer could not be neglected. On calcination of LDH precursors at 450 qC, the resulted mixed oxide showed relatively lower band gap energies compare to ZnO or TiO2-P25. The Ti
11
containing calcined LDHs showed better activity for photodegration of aqueous MB and RhB than the physical mixture of ZnO-TiO2 and the activity increased with increase of Ti content in the catalyst. The photodegradation of both the dyes followed a first order kinetics and under identical conditions, the photodegradation of MB was relatively faster than RhB. Although the catalyst after reaction was separated easily, the activity of used catalyst progressively decreased in subsequent cycles. The calcined mixed oxides may be further exploited for their catalytic activity requiring tailored acid-basicity or as adsorbent for removal several anionic pollutant from aqueous medium.
Acknowledgement The financial assistances from University Grants Commission, New Delhi and Department of Science & Technology, Government of India for infrastructural facilities at Department of Chemistry, North Orissa University are gratefully acknowledged.
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Captions for Figures Fig. 1: TG-DTA profile of as synthesized LDHs. Fig. 2: Powder XRD patterns of as synthesized LDHs and their calcined products (inset). Fig. 3: FT-IR spectra of as synthesized LDH samples. Fig. 4: TEM analyses of as-synthesised ZAT-2 sample. (a) Bright field and (c) the corresponding EDX; (b) dark field images and (d) SAED pattern. Fig. 5: Band gaps from the plots of (F(R)hQ)1/2 versus hQ using UV-Vis-DR spectra (inset) of calcined LDHs along with ZnO and TiO2-P25 . Fig. 6: TEM analyses of calcined ZAT-2 sample. (a) Bright field and (c) the corresponding EDX pattern; (b) HR-TEM image and (d) SAED pattern. Fig. 7: Photodegradation of MB as a function of irradiation time under visible-light using the calcined LDHs and ZnO-P25 mixture. (catalyst, 2.0 g L-1; initial MB, 10.92u10-5 M; pH = 6.5 r 0.2). (Inset) Spectral scans of MB with irradiation time using ZAT-4 under above conditions). Fig. 8: Photodegradation of RhB as a function of irradiation time under visible-light using the calcined LDHs and ZnO-P25 mixture. (catalyst, 2.0 g L-1; initial RhB, 7.31u10-5 M; pH = 6.5 r 0.2). (Inset) Spectral scans of MB with irradiation time using ZAT-4 under above conditions.
14
Fig. 9: Effect of catalyst dose degradation of MB and RhB using calcined ZAT-4 as the catalyst (initial MB, 10.92u10-5 M; initial RhB, 7.31u10-5 M; pH = 6.5 r 0.3). (Inset) Comparative activity of different calcined LDHs for degradation of MB and RhB under visible-light irradiation for 60 min. Fig. 10: Electron transfer process from the excitation of dye in the visible region Fig. 11: Powder XRD patterns of as synthesized ZAT-4 (1) and calcined ZAT-4 after 3rd cycle of photocatalytic run (2).
15
Graphical abstract
Synthesis, characterization and photocatalytic activity of mixed oxides derived from ZnAlTi ternary layered double hydroxides R. K. Sahu, B. S. Mohanta, N. N. Das* P. G. Department of Chemistry, North Orissa University, Baripada-757 003, Orissa, India
TEM image (dark field) of as-synthesised LDH precursor (ZAT-2)
16
Fig.1
Fig.2
Fig.3
Fig.4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig.9
Fig.10
Fig. 11
Table 1
Table 1: Composition and lattice parameters of the LDH samples ______________________________________________________________________ Surface area Band gap Sample Mg : Al : Ti Lattice parameters ---------------------(m2/g)a energy (eV)a a, Å c, Å ________________________________________________________________________ ZAT-0
2.96:1.05:0
3.064
22.46
116.5
3.21
ZAT-1
2.99:0.92:0.091
3.077
22.57
109.7
3.04
ZAT-2
2.99:0.81:0.20
3.072
22.57
105.9
3.00
ZAT-3
2.99:0.70:0.31
3.072
22.77
100.3
2.98
ZAT-4
2.99:0.49:0.508
3.077
22.84
94.5
2.96
________________________________________________________________________ a
Values of calcined products.
Synthesis, characterization and photocatalytic activity of mixed oxides derived from ZnAlTi ternary layered double hydroxides R.K. Sahu, B.S. Mohanta, N.N. Das
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S0022-3697(13)00151-0 http://dx.doi.org/10.1016/j.jpcs.2013.04.002 PCS7051
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Journal of Physics and Chemistry of Solids
Received date: 1 December 2012 Revised date: 28 March 2013 Accepted date: 7 April 2013 Cite this article as: R.K. Sahu, B.S. Mohanta, N.N. Das, Synthesis, characterization and photocatalytic activity of mixed oxides derived from ZnAlTi ternary layered double hydroxides, Journal of Physics and Chemistry of Solids, http://dx.doi. org/10.1016/j.jpcs.2013.04.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis, characterization and photocatalytic activity of mixed oxides derived from ZnAlTi ternary layered double hydroxides R.K. Sahu, B.S. Mohanta, N.N. Das* P. G. Department of Chemistry, North Orissa University, Baripada-757 003, Orissa, India *
Corresponding author. Tel.: +91 679 225 2088; fax: +91 679 225 3908.
Email: [email protected] Abstract A new series of Ti4+ containing ZnAl-LDHs with varying Zn:Al:Ti (~3:1:0 – 3: 0.5, 0.5) ratio were prepared by coprecipitation of homogeneous solution metal salts and characterized by various physicochemical methods. Powder XRD revealed the formation of well crystalline LDH even at highest Ti4+ content. On thermal treatment at 450 qC, the well crystalline LDH precursors yielded the mixed oxides with BET surface area in the range 92-118 m2/g. UV–vis diffuse reflection spectroscopy (DRS) showed a marginal decrease of band gap energy for calcined ZnAlTi-LDHs in comparison to either ZnO or TiO2-P25. The TEM analyses of a representative sample (as-synthesised and calcined) indicated more or less uniform distribution of titanium species. The derived mixed oxides from titanium containing LDH precursors demonstrated better activity towards photodegradation of methylene blue and rhodamine B than physical mixture of ZnO and TiO2. Moreover, the present work not only provided a first hand understanding about semiconductor properties of ZnAlTi-LDHs but also demonstrated their potential as photocatalysts for degradation of organic pollutants.
Highlights x
Synthesis and characterizations of a new series of ZnAlTi ternary layered double hydroxides.
x
Calcination of LDH precursors led to formation of mixed oxides with reduced band gap energies than ZnO or TiO2-P25.
x
The derived mixed oxides are effective photocatalysts under visible light for dyes degradation.
x
Easy separation and possibility of reuse of used catalyst.
Keywords A. inorganic compounds; A. oxides, B. chemical synthesis; C. electron microscopy; C. X-ray diffraction 1.
Introduction
Layered double hydroxides (LDHs), represent an important class of inorganic layered materials, have been a subject of numerous investigations during last three decades because of their potential applications as catalysts, catalyst supports, ion exchangers/adsorbents, layered hosts for biomolecules, precursors for composite materials etc. [1-9]. The general formula of LDHs is [M2+(1-x)M3+x(OH)2]x+ [Am-x/m ]n-.mH2O where M(II) and M(III) include a variety of bivalent (Mg2+, Zn2+, Co2+, Cu2+, Mn2+) and trivalent (Al3+, Fe3+, Cr3+, V3+, Ga3+, Ti3+) metal ions, An- is the interlayer anion that may be organic, inorganic, carboxylate, oxoanion, coordination compounds and polyoxometalates and x generally can have the values between 0.1 and 0.33 [2]. Off let efforts have been devoted to introduce a tetravalent ion (e.g. Zr4+, Sn4+ and Ti4+) in the brucite like layer as a partial replacement of M2+ or M3+ ion in order to enhance the anion exchange capacity (AEC) or to tune the acido-basicity of their resulting mixed oxides [10-18]. Although Zr4+ and Ti4+ containing LDHs have shown enhanced anion adsorption capacity [10-14] in comparison to those without tetravalent ions, the recent X-ray absorption spectroscopic studies [19,20] indicated that incorporation of tetravalent ions in octahedral sheet does not occur, but actually an amorphous M(IV) oxide is formed and impregnate the LDH crystallites. Even then the LDHs containing Zr4+ and Ti4+ could be useful as catalyst for various organic transformations requiring tailored acidobasicity and also as adsorbents with enhanced adsorption capacity for remediation of anionic pollutants from contaminated water. In recent years there has been a growing interest to develop LDH based photocatalysts as an alternative to conventional semiconductor materials like TiO2, ZnO and SnO2 for degradation of variety organic pollutant including dyes [21-26]. In particular, the mixed oxide derived from ZnAl containing LDHs, without or with Fe, Sn and Ti, are successful photocatalysts for the degradation of organic compounds like methyl orange, methylene blue and phenols in aqueous media. In contrast, the use of as synthesized LDHs as photocatalysts has been sparsely studied because of favourable capture of photoinduced 2
electron by hydroxyl group during photocatalytic process. A recent study of Silva et al. [27] has reported oxygen generation from photolysis of water using as synthesized ZnM-LDHs (M = Cr, Ti, Ce) photocatalyst under visible light and the results showed that the LDHs can be regarded as “doped semiconductor”. Keeping the above in view and as a sequel to our previous studies [12,13], we report here in the synthesis of new series of Ti containing ZnAl-LDH. The physicochemical characterizations and photocatalytic behaviours of derived mixed oxides towards two commonly occurring dyes, namely methylene blue (MB) and rhodamine B (RhB), under visible light are also reported.
2.
Experimental
2.1
Materials Zn(NO3)2.6H2O, Al(NO3)3.9H2O, TiCl4, NaOH and Na2CO3 (Merck, GR) were used
for synthesis LDH precursors without further purification. Methylene blue (Qualigens) and rhodamin B (Merck, GR) were used as received for adsorption experiments. All other chemicals used in this work were of AR/GR grades. Stock solutions of dyes were prepared by dissolving required amount of corresponding dye in double distilled water.
2.2
Preparation and calcination of LDH precursors The LDH precursors with varying Zn:Al:Ti atomic ratios were prepared by
coprecipitation of metal salts solution at constant pH ~ 10 under low supersaturating conditions. A solution containing the mixture of Zn(NO3)2, Al(NO3)3 and TiCl4 and a mixture of NaOH (2.0 M) and Na2CO3 (0.20 M) were added drop wise to a well stirred solution of Na2CO3 (100 ml 0.01M) such that the pH of the resulting slurry was maintained at ~ 10. Once the addition was completed, the resulting precipitate was aged for 18 h at room temperature, separated by centrifugation, washed thoroughly with distilled water until the precipitate was free from chloride and then dried overnight at 90 qC in air-oven. Small amounts of nitrate/chloride ions retained in the interlayer of LDH precursors were replaced by carbonate ions by suspending the dried precursors (2 g) in the solutions of Na2CO3 (100 ml, 0.20 M) and stirring for 2 h. The carbonate-exchanged solid was separated by centrifuge, washed, dried in air oven overnight at 90 oC and stored in air tight bottles for further use. The
3
samples are denoted as ZAT-0 to 4 depending on their Ti contents (Table 1). Based on the TG-DTA and FT-IR spectral analyses the dried LDH precursors were calcined in air at 450 o
C with a heating rate of 5 oC min-1 for 5 h and used for further studies.
2.3
Characterizations Zn, Al and Ti contents were determined by conventional wet chemical analyses and
also by ICP (Varian Liberty series2) (Das et al., 2010). Powder X-ray diffraction (PXRD) of carbonate exchanged LDH precursors and their calcined products were recorded on a Rigaku (Miniflex II) diffractometer at scanning speed 2q(2T) min-1 using Ni filtered CuK (30 kV, 15 mA) radiation. Thermogravimetric measurements (TG-DTA) were performed on a Shimadzu DTG 60 Thermal analyser under flowing nitrogen (40 ml min-1) at a heating rate of 10 qC min-1. The surface area of calcined samples was determined by BET method using a surface and porosity analyzer (Quantachrome, Novawin) after degassing the samples under vacuum (10-4 Pa) at 250 qC. FT-IR spectra in KBr phase were recorded on a Shimadzu IR Affinity-1 spectrophotometer averaging 45 scans with a nominal resolution of 4 cm-1 to improve signal to noise ratio. The UV-Visible diffuse reflectance spectra (UV-Vis DRS) of solid samples using BaSO4 as reference and spectral scan of the photocatalytic reaction mixture were performed with a Shimadzu UV-Visible spectrophotometer. The band gap energies were estimated from absorption edge using the relationship: E (eV) = h (c/O) nm, where h, c and O represent the Plank’s constant, velocity of light (meter/s) and is the cutoff wavelength (nm), respectively. Transmission electron micrographs were recorded using FEI Tecnai 30G2 STwin (Netherlands) operated at 300 kV.
2.2
Photocatalytic activity The photocatalytic efficiencies of calcined LDH precursors were studied taking two
model dyes viz. methylene blue and rhodamine B. All the photocatalytic experiments were performed in a 200 ml capacity double walled cylindrical quartz reactor fitted with 125 W high pressure Hg lamp ( > 420 nm) as source of visible light under magnetic stirring condition. The temperature of reaction mixture containing dye solution and photocatalyst, was maintained (30.0 r 0.2) by circulating water through outer walls from Julabo (Germany)
4
F12 water circulator. All the experiments were performed in presence of air at atmospheric pressure. The reaction was imitated by irradiating the reaction mixture containing 100-150 ml of dye solution at different concentrations (10-45 mg L-1) and amount of catalysts (1.0-5.0 g L-1) with the visible light. The initial pH of the dye solution was adjusted to ~ 6.5 which was found increase to ~ 8.2 r 0.3 at the end of photocatalytic reactions. At regular intervals (for kinetic experiments) and at the end of reaction, a definite portion of reaction mixture was withdrawn, separated the solid catalyst by centrifugation and the residual dye concentrations were computed by measuring the absorbance at 565 and 545 nm for MB and RhB, respectively. The measurements of absorbance were carried out on a Systronics 2201 UVVisible spectrophotometer using 10 mm matched quartz cell. Parallel experiments under identical conditions in presence of light without catalyst and in presence of catalyst without irradiation of light were also carried out to see the individual effect of light and catalyst on degradation/adsorption of dyes. 3.
Results and discussion
3.1
Characterization of as synthesized LDHs The composition of carbonate exchanged LDH precursors and their crystal lattice
parameters, derived from PXRD patterns, are collected in Table 1. Chemical analyses indicate that the molar ratios of Zn, Al and Ti are close to those initially taken for preparation of LDH precursors. TG-DTA plots of representative uncalcined samples are presented in Fig. 1. All the samples mainly exhibit two stage weight losses with corresponding endothermic peaks in DTA profiles. The first stage loss can be tentatively resolved in two overlapped processes and mainly attribute to the elimination of physically adsorbed and interlayer water molecules. The second loss, always higher than the first loss, is ascribed to loss of hydroxyl groups from the brucite-like layer along with interlayer carbonate ions with concomitant destruction of layered structure [2, 11, 14, 16,17]. A minor weight loss beyond 450 qC is attributed to loss of oxygen and CO2 through slow decomposition of Zn(Al/Ti) oxycarbonate which are likely to be formed after decomposition of LDH-like structure. The overall behaviour of LDH samples are in agreement with those generally reported for ZnAl-LDH samples [14,16]. The total weight losses at 600 oC ranging from 27-29 % are in good agreement with ZnAl-LDH precursors with or without Zr. Further, the temperature of weight loss is shifted to lower with
5
incorporation of Ti in the interlayer presumably due to relatively weak electrostatic interaction of interlayer anions with brucite like layer due to increased interlayer spacing as a result of higher positive charge in the brucite layer. The PXRD patterns of the dried carbonate exchanged LDHs are presented in Fig. 2. A single phase corresponding to hydrotalcite like compounds (LDH: JCPDS File No. 38-487) is observed for all the samples without any appreciable decrease of crystallinity even at higher Ti(IV) content (Al/Ti ratio ~ 1.0). No peaks from any other crystalline material could be detected either due to overlapping with the characteristic LDH peaks or formation of amorphous hydroxide phases of Ti (e.g. Ti(OH)4 or TiO2.nH2O) and/or ZnTi. Formation of amorphous TiIV oxide has also been reported in synthesis of Co2AlTi-HT [20]. Assuming a hexagonal crystal system, the lattice parameters are calculated from (110) and (003) reflections and presented in Table 1. The marginal increase of both a and c parameters for ZAT-1, 2, 3 and 4 in comparison to ZAT-0 could be, but not conclusive, due to partial incorporation of Ti4+ with relatively higher ionic radius (r = 0.072 nm) in place of Al3+ (r = 0.053 nm) in the LDH framework [12,17]. This also leads a higher amount of CO32- as compensating anion in the interlayer which in turn results an increase of c parameter. Increase of both a and c lattice parameters along with increase of carbonate content have also been observed for incorporation of Zr4+ and Ti4+ in the brucite like layer in the cases of ZnAl and MgAl-LDHs [12,13,17], respectively. FT-IR spectra of representative carbonate exchanged LDHs, shown in Fig. 3, are very similar to those generally reported for hydrotalcite like compounds. The broad and strong adsorption band centred at ~ 3480 cm-1 in case uncalcined samples is attributed to stretching vibrations of physisorbed water, structural OH group and/or hydrogen bonded hydroxyl group (OHOH) [2,28]. A weak shoulder at 3070 cm-1, causing the broadness of this band may be ascribed to the OH stretching mode of water molecule, hydrogen bonded to the interlayer carbonate anion. The band close to 1630 cm-1 is originated due to bending mode (GHOH) of interlayer water molecules. An sharp intense band observed at ~ 1370 cm-1 along with relatively less intense peak at ~ 1530 cm-1 are assigned to symmetric and antisymmetric O-C-O stretching vibrations of monodentate carbonate species. The shifts from the normal position of the free carbonate species, i.e., 1450 cm-1, and the splitting of about 130-154 cm-1 for samples ZAT-1 to ZAT-4 result from a lowering of the symmetry of the species in the
6
interlayer domain. These shifts are higher than in LDHs containing cations with large ionic sizes such as Y, V and Cr in Mg/Al/Y, Ni/V or Zn/Cr with values of 115, 116, and 126 cm-1, respectively [16]. As expected, the splitting in case of sample without Ti (ZAT-0) is about 120 cm-1 which is less than those observed for other ZAT samples. These features could account for a greater distortion of the brucite-like layers and for a heterogeneous distribution of positive charge in the brucite-like layers containing cations of different charges. On calcination at 450 qC for 5 h, the characteristic peaks due to CO32- ion are practically absent in the resulted mixed oxides. The TEM images of ZAT-2 along with EDX and SAED patterns are presented in Fig. 4. The TEM images of as-synthesised ZAT-2 in dark and bright filed indicate uniform distribution of titanium species (e.g. as TiO2.nH2O) [29.30]. EDX measurement also confirmed the presence of Zn, Al and Ti in the same proportions as that of taken for synthesis of ZAT-2. SAED pattern also reveals more or less uniform distribution of LDH particles. 3.2
Characterization of calcined LDHs The nature of crystalline phases generated after calcination of LDHs is of interest
from view points of their applications as bifunctional acid-base catalysts/catalyst support, photocatalysts, ion exchangers etc. On calcination at moderate temperature (450 qC), the LDH precursors are converted to mixed oxides (Fig. 2, inset) whose lattice ‘a’ parameters are slightly smaller than that of pure ZnO (a = 2.093 Å) indicating isomorphous substitution of Al3+ or both Al3+ and small fraction of Ti4+ for Zn2+ in the lattice. Similar observations have been reported earlier for several calcined LDH samples [2, 11, 14, 16,17]. The overlapping peaks at 2 values 36.5, 47.8, 55.6, 63.04 (Fig. 2, inset) may also be attributed to the characteristic peaks of (004), (200), (105), (201) and (204) planes of anatase phase. The surface areas of calcined Ti-containing samples exhibit lower values (Table 1) than the sample without Ti and the values are progressively decreased with increase of Ti content in the samples. A similar trend was also observed earlier for Ti and Zr containing calcined LDHs [12,16,17]. The DRS spectra of calcined LDHs along with ZnO and TiO2 (Degussa P25) are presented in Fig. 5 (inset). It is seen that spectral intensity of Ti containing calcined ZAT samples are relatively higher than that of sample without Ti (ZAT-0). There is a marginal shift in the absorption band in all ZAT samples in comparison to pure ZnO or TiO2-P25
7
presumably due to presence of intimately mixed ZnO and TiO2 in the oxide samples obtained on calcination of ZAT-precursors. The band gap energies of the photocatalysts (Fig. 5) were determined by extrapolating the linear region to the abscissa of Tauc plot, a plot of (F(R)hQ)1/2 against hQ where F(R) is the Kubelka–Munk function and the values obtained are collected in Table 1. All the ZAT samples exhibit relatively lower band gap energy than those observed in case of pure ZnO (3.29) or TiO2-P25 (3.19)/synthesized TiO2 (~3.22) [29] and are expected to show better photocatalytic activity compare to either ZnO or TiO2 under visible light. The TEM images of calcined ZAT-2 along with EDX and SAED patterns are presented in Fig. 6. As evident, the average particle size of ZAT-2 increases on calcination. The EDX analysis also shows the presence of Zn, Al and Ti more or less in the same proportion as that of uncalcined ZAT-2 sample. HR-TEM images show set of uniform lattice fringes providing further evidence in favour of crystalline nature of nanoparticles. Appearance of lattice fringes for anatase and MgAl-LDH has also been observed with dvalues 3.57 and 2.14 Å, respectively in the case of TiO2/MgAl-LDH [30]. 3.3
Photocatalytic activity The photocatalytic activity of calcined LDHs was evaluated towards degradation of
methylene blue and rhodamin B in aqueous medium. The initial pH of the dye solutions was kept constant at 6.5 r 0.2 in order to avoid any colour change of dye solutions due to acidbase equilibria with variation of pH (pKa values of MB and RhB are 10.2 and 10.5, respectively). The pH of reactant solution invariably increased to 8.2 r 0.2 during photodegradation process. The representative set of results using calcined LDHs for degradation of MB and RhB as a function of irradiation time are presented in Figs. 7 and 8, respectively. The activity of physical mixture of ZnO and TiO2-P25 with same weight percentage of ZnO and TiO2 as that of ZAT-3 is also presented in the figures for comparison. Blank experiments were also carried out without catalyst to verify the extent of decolourisation of dyes due to photocatalytic process. It is seen that in the absence of photocatalyst, about 25 % of MB is degraded in 3 h of irradiation with visible light and is very similar to that observed earlier [29]. In comparison, the photodegradation of RhB without catalyst is negligibly small even with irradiation up to 4 h with visible light. As the
8
adsorption of dyes on catalyst is believed to be the primary process in photocatalytic decolourisation/degradation, it is also essential to assess the amount of dye adsorbed on catalyst in dark to account the overall activity of ZAT samples. The calcined ZAT samples can reconstitute to its original LDH structure through rehydration and exhibit positive and negative surface charges depending on the pH (point of zero charge of calcined ZAT samples ~ 8.0). Also at working pH (initial pH ~ 6.5 and final pH ~ 8.2), MB exists in the cationic form (pKa > 12) for which the entry of MB in the interlayer is restricted and hence, the decrease of MB concentration with time in dark is primarily due to adsorption on catalyst surface. Blank experiment with ZAT-3 under identical conditions in dark shows ~ 12 % MB (Fig. 7) is adsorbed in 3 h. On the other hand ~ 7.3 % RhB, mostly exist in zwitterionic form at pH ~ 6.0 due to deprotonation of its carboxyl group (pKa =3.7), is adsorbed on ZAT-3 surface under identical conditions of photocatalytic without visible light irradiation. The repetitive spectral scan of dye solutions with irradiation time, presented in Figs. 7 and 8 (inset), show progressive decrease of absorbance in the entire range of UV-Vis spectra. It is clearly seen that the initial peaks of MB at 614 and 664 nm are merged into a relatively broad peak centred at ~ 650 nm during the reaction. The peak intensities are also reduced progressively with a shift towards higher wavelengths till the reactant solution turns colourless. On the other hand, the intensities of MB bands at 292 and 245 nm are reduced progressively without any shift in their positions. The disappearance of two major absorbance peaks of MB at 292 and 664 nm (Fig. 7, inset), due to benzene ring and heteropolyaromatic linkage, indicate their complete destruction at the end of the reaction. The decolourisation of RhB is also evident from the large decrease in absorbance change during the photocatalytic reaction without any observable shift in the peak positions (Fig. 8, inset) indicating the deolourisation of RhB is primarily due to decomposition of conjugated chromophore structure rather then de-ethylation process [31]. In order to see that effect of catalyst dose for dye degradation, the amount of most effective catalyst (ZAT-4) was varied in the range 1.0 to 5.0 g L-1 at a constant concentration of MB (10.92u10-5 M) and RhB (7.31u10-5 M), initial pH ~ 6.5 and irradiation time of 60 min. The results obtained are presented in Fig. 9. Although there is a progressive increase of degradation with increase of catalyst dose, the effect is more pronounced up to 2.0 g L-1 at least in the case of MB degradation. This increased activity with increasing catalyst dose is
9
obviously to presence more active sites which results in absorption of more number of photons for formation higher number of active species. The less pronounced activity at higher dose is presumably due to less availability of dye molecules for degradation. The concentration of initial dye concentration is another important factor towards overall degradation of dyes. In the present study, the MB and RhB concentrations were varied from (3.12-14.04)u10-5 M and (2.09-9.4)u10-5 M, respectively keeping the initial pH (~ 6.5) and catalyst dose (ZAT-4, 2.0 g L-1) fixed. As expected, the percentage of degradation decrease with increase of initial dye concentration. Further, the plots of ln(C/C0) versus irradiation time are straight line indicating the degradation of dyes follows a first order kinetics. In the above concentration range, the calculated first order rate constants are found in the range 0.012-0.067 min-1 and 0.0054-0.0191 min-1 for MB and RhB, respectively. The values of MB degradation are comparable with the reported rate constant (0.0407 min-1) for MB degradation (initial MB = 1.6 u10-5 M, catalyst dose = 2.0 g L-1) with ZnTi sample (Zn/Ti = 3:1) [29]. It is interesting to note that the activity of the physical mixture of ZnO and TiO2 is lower than all the Ti containing LDH but higher than the activity of sample without Ti (ZAT0). Further the degradation of both the dyes is found to increase rather slowly with increase of Ti content in the catalyst (Fig. 9, inset). This increased activity may be attributed to resultant effects of decreased band gap energy and BET surface area. Since the measured band gap of Zn(Al)O/TiO2 is relatively high (> 2.98 eV), the mechanism operative for dye (D) degradation by semiconductor oxide using UV light is not feasible in the present case rather a photosensitized pathways should be considered. Unlike formation of electron-hole (e-/h+) pairs due to absorption of UV light by semiconductor, light absorption in the present case mainly occurs by the dye molecule [32] adsorbed on the catalyst surface and transfer the excited electron into Zn(Al)O/TiO2 conduction band. The electrons on Zn(Al)O/TiO2 can be used further to reduce dissolved oxygen molecule in dye solution forming O2x leading to degradation of dye molecule. The overall photocatalytic process may be delineated as follows and the same is schematically presented in Fig. 10. D/ZnAl(O)/TiO2 + hQ (vis) o 1D* or 3D*/ZnAl(O)/TiO2 1
D* or 3D*/ZnAl(O)/TiO2 o D+x + ZnAl(O)/TiO2(ecb)
D+x o degradation products
10
ZnAl(O)/TiO2(ecb) + O2 o ZnAl(O)/TiO2 + O2-x O2-x + H+ o HO2x o o HOx and/or O2-x + 2H+ + e- o H2O2 D+x + O2-x o DO2 o degradation products D + HOx or H2O2 o degradation products
The practical utility of a photocatalyst lies in its long time use. A preliminary study was made using calcined ZAT-4 to see its efficiency in repeated cycle by keeping the catalyst amount (2.0 g L-1), initial dye concentrations (3.12u10-5 M for MB and 2.09u10-5 M for RhB) and initial pH (6.5 r 0.3) fixed in each cycle. For this the catalyst, separated from residual dye solutions by centrifugation after irradiation for 120 min, was treated with fresh dye solutions and irradiated again for 120 min. This was repeated for two more cycle and the degradation activity is found to decrease progressively form 100, 92 and 82 % in first, second and third cycles, respectively. The decrease in activity is primarily due to partial formation of parent ZAT-4 LDH precursor through rehydration of calcined ZAT-4 and the resulted hydroxyl groups of LDH capture some of the photoinduced electrons which in turn reduce the photocatalytic activity. The XRD pattern of calcined ZAT-4 after third cycle of photocatalytic reaction is presented in Fig. 11 along with uncalcined ZAT-4 precursor. It is evident that parent ZAT-4 is gradually formed due to memory effect which results in the decrease of activity with number of cycle. Further optimization like heating at higher calcination temperatures to avoid to the formation of parent LDH precursor through rehydration or variation Al/Ti ratio is required for long time use of LDH based catalyst in photocatalytic degradation of dyes.
4.
Conclusions Ternary LDH precursors containing Zn, Al and Ti were synthesized by
coprecipitation of metal salts solution at constant pH. PXRD confirmed the formation of single crystalline LDH like phase even with high Ti content indicating Ti4+ was mostly present as amorphous TiO2.nH2O. Incorporation of a small fraction Ti4+ in the brucite like layer could not be neglected. On calcination of LDH precursors at 450 qC, the resulted mixed oxide showed relatively lower band gap energies compare to ZnO or TiO2-P25. The Ti
11
containing calcined LDHs showed better activity for photodegration of aqueous MB and RhB than the physical mixture of ZnO-TiO2 and the activity increased with increase of Ti content in the catalyst. The photodegradation of both the dyes followed a first order kinetics and under identical conditions, the photodegradation of MB was relatively faster than RhB. Although the catalyst after reaction was separated easily, the activity of used catalyst progressively decreased in subsequent cycles. The calcined mixed oxides may be further exploited for their catalytic activity requiring tailored acid-basicity or as adsorbent for removal several anionic pollutant from aqueous medium.
Acknowledgement The financial assistances from University Grants Commission, New Delhi and Department of Science & Technology, Government of India for infrastructural facilities at Department of Chemistry, North Orissa University are gratefully acknowledged.
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Captions for Figures Fig. 1: TG-DTA profile of as synthesized LDHs. Fig. 2: Powder XRD patterns of as synthesized LDHs and their calcined products (inset). Fig. 3: FT-IR spectra of as synthesized LDH samples. Fig. 4: TEM analyses of as-synthesised ZAT-2 sample. (a) Bright field and (c) the corresponding EDX; (b) dark field images and (d) SAED pattern. Fig. 5: Band gaps from the plots of (F(R)hQ)1/2 versus hQ using UV-Vis-DR spectra (inset) of calcined LDHs along with ZnO and TiO2-P25 . Fig. 6: TEM analyses of calcined ZAT-2 sample. (a) Bright field and (c) the corresponding EDX pattern; (b) HR-TEM image and (d) SAED pattern. Fig. 7: Photodegradation of MB as a function of irradiation time under visible-light using the calcined LDHs and ZnO-P25 mixture. (catalyst, 2.0 g L-1; initial MB, 10.92u10-5 M; pH = 6.5 r 0.2). (Inset) Spectral scans of MB with irradiation time using ZAT-4 under above conditions). Fig. 8: Photodegradation of RhB as a function of irradiation time under visible-light using the calcined LDHs and ZnO-P25 mixture. (catalyst, 2.0 g L-1; initial RhB, 7.31u10-5 M; pH = 6.5 r 0.2). (Inset) Spectral scans of MB with irradiation time using ZAT-4 under above conditions.
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Fig. 9: Effect of catalyst dose degradation of MB and RhB using calcined ZAT-4 as the catalyst (initial MB, 10.92u10-5 M; initial RhB, 7.31u10-5 M; pH = 6.5 r 0.3). (Inset) Comparative activity of different calcined LDHs for degradation of MB and RhB under visible-light irradiation for 60 min. Fig. 10: Electron transfer process from the excitation of dye in the visible region Fig. 11: Powder XRD patterns of as synthesized ZAT-4 (1) and calcined ZAT-4 after 3rd cycle of photocatalytic run (2).
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Graphical abstract
Synthesis, characterization and photocatalytic activity of mixed oxides derived from ZnAlTi ternary layered double hydroxides R. K. Sahu, B. S. Mohanta, N. N. Das* P. G. Department of Chemistry, North Orissa University, Baripada-757 003, Orissa, India
TEM image (dark field) of as-synthesised LDH precursor (ZAT-2)
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Fig.1
Fig.2
Fig.3
Fig.4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig.9
Fig.10
Fig. 11
Table 1
Table 1: Composition and lattice parameters of the LDH samples ______________________________________________________________________ Surface area Band gap Sample Mg : Al : Ti Lattice parameters ---------------------(m2/g)a energy (eV)a a, Å c, Å ________________________________________________________________________ ZAT-0
2.96:1.05:0
3.064
22.46
116.5
3.21
ZAT-1
2.99:0.92:0.091
3.077
22.57
109.7
3.04
ZAT-2
2.99:0.81:0.20
3.072
22.57
105.9
3.00
ZAT-3
2.99:0.70:0.31
3.072
22.77
100.3
2.98
ZAT-4
2.99:0.49:0.508
3.077
22.84
94.5
2.96
________________________________________________________________________ a
Values of calcined products.