Tungstophosphate Intercalated ZnAl Layered Double Hydroxide for Adsorptive and Visible Light Driven Photocatalytic Decolourisation of Organic Dyes

Bhabani Shankar Mohanta ^1,‡, Rita Das ^2,*, Nigamananda Das ¹

1. Department of Chemistry, Utkal University, Bhubaneswar – 751 004, India

2. Department of Chemistry, B.J.B. Autonomous College, Bhubaneswar 751 014, India

‡ Current Affiliation: Department of Chemistry, Maharaja Sriram Chandra Bhanja Deo University, Baripada 757 003, Odisha, India

* Correspondence: Rita Das

Academic Editor: Narendra Kumar

Received: December 21, 2022 | Accepted: September 18, 2023 | Published: October 13, 2023

Catalysis Research 2023, Volume 3, Issue 4, doi:10.21926/cr.2304025

Recommended citation: Mohanta BS, Das R, Das N. Tungstophosphate Intercalated ZnAl Layered Double Hydroxide for Adsorptive and Visible Light Driven Photocatalytic Decolourisation of Organic Dyes. Catalysis Research 2023; 3(4): 025; doi:10.21926/cr.2304025.

© 2023 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

1. Introduction

Layered double hydroxides (LDHs) having the general formula [M²⁺_1−xM³⁺_x(OH)₂]^x+ (A^m−_x/m)·nH₂O form a promising class of materials having numerous technological application competence owing to their particular lamellar structure with many possible combinations of M²⁺ and M³⁺ metal ions as host layer cations, and intercalating anions (A^m-) [1,2,3]. The host layer provides flexible confined space for accommodating a variety of catalytically active organic, polymeric, and inorganic species, resulting in numerous intercalated composite materials with improved chemical properties for the specific application. In this regard, the polyoxometalates intercalated layered double hydroxide hybrid materials have invoked much research interest in recent years due to their distinctive properties and exhibiting advantages over both parent materials, especially in catalytic applications [3,4,5,6,7].

POMs are anionic metal oxides of groups V and VI that constitute a potentially important class of inorganic materials because of their unique properties in terms of structural stability, tunable acidity, redox capability, and catalytic efficiency for various organic transformations in homo- and heterogeneous media besides application potential in other areas such as medicine, pharmaceutical, electronics, etc. [8,9,10]. The photocatalytic properties of POMs have also been explored for different reactions including the recovery of metals by reducing their ions from aqueous solutions, oxidation of water, removal of CO₂ to CH₄, etc. [11,12,13,14,15]. However, the weak hydrolytic stability and poor recyclability due to high solubility in water seriously restrict their applications for reactions carried out in an aqueous medium. To overcome these limitations, the POMs are often immobilized on appropriate supports for easy recovery from the reactant solution after the reaction. The separated catalyst can be regenerated and reused for further cycles of reaction. Relatively higher activity of immobilized POMs has often been compared to either POM or support alone due to synergistic effects between LDHs and POMs.

In recent decades, significant progress has been made in developing POMs-LDHs materials demonstrating superior catalytic performance for different reactions [16,17,18]. The photocatalytic activity of POM intercalated LDH is, however, sparsely studied [19,20,21,22], although catalytic systems like POMs/TiO₂ showed relatively higher photocatalytic degradation abilities towards aqueous organic pollutants, including dyes, than those of either neat POMs or TiO₂ alone [22,23,24,25]. More studies are needed with POM intercalated LDH to identify the effective POMs, optimization of intercalation process, and optimization of catalytic parameters for their possible utilization in the remediation process of dyes.

Herein we report the intercalation of a photoactive POM, tungstophosphate (PW₁₂O₄₀^3-), in the interlayer of ZnAl-LDH, characterization by different techniques and adsorptive/photocatalytic decolorisation of four structurally different organic dyes under visible light irradiation.

2. Materials and Methods

Analar grade zinc nitrate, Zn(NO₃)₂·6H₂O; aluminum nitrate, Al(NO₃)₃·9H₂O; sodium hydroxide (NaOH) and sodium carbonate (Na₂CO₃), as received from SRL (India), were used for the preparation of LDH precursor. Tungstophosphoric acid (H₃[PW₁₂O₄₀]) (Merck) is used for intercalating the tungstophosphate anion (PW₁₂O₄₀^3-) in the interlayer of LDH. Analar grade organic dyes viz. methyl orange (MO), Congo red (CR), methylene blue (MB), and rhodamine B (RhB) are used to evaluate the photocatalytic activity of the prepared catalyst under visible light irradiation. All other chemicals used were of analytical grades and without further purification.

ZnAl-LDH precursor in carbonate form (ZnAl-LDH-CO₃) is prepared by coprecipitation of mixed aqueous solutions of Zn(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O at ambient temperature under low supersaturating conditions (pH ~10) [1,26]. An aqueous solution of metal nitrates (Zn/Al = 3) was added dropwise to a well-stirred solution of Na₂CO₃ (100 ml 0.01 M). Simultaneously, a mixture of NaOH (2.0 M) and Na₂CO₃ (0.02 M) was added such that the pH of the resulting slurry was maintained at ~10 ± 0.2. After complete addition, the resulting slurry is aged for 12 h at room temperature, washed several times with deionized distilled water by dispersion/centrifugation cycles, and dried overnight at 90°C in an air oven. The small amount of nitrate ions retained in the interlayer of LDH precursor is 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 washed with water by dispersion/centrifugation cycles and dried overnight at 90°C. The dried LDH precursor was calcined at 450°C for 5 h to convert into mixed metal oxide, ZnAl(O) [1,26].

The tungstophosphate ion, PW₁₂O₄₀^3-(PW₁₂) was intercalated in the interlayer of LDH through rehydration of calcined ZnAl-LDH in the presence of H₃PW₁₂O₄₀ under the N₂ atmosphere to avoid the entry of CO₂ from the atmosphere. In a typical lot weighed amount of calcined ZnAl-LDH, ZnAl(O) was dispersed in 100 ml of water containing the desired amount of H₃[PW₁₂O₄₀], and the pH value was adjusted to ~4.5, which is well within the range of pH (2 to 6) where the POM anion is stable as PW₁₂O₄₀^3-, The mixture was stirred under N₂ atmosphere for 8 h. The resulting solid was separated by centrifugation, washed several times with water, and finally ethanol. The isolated solid was dried at 60°C for 8 h in a vacuum and stored in a desiccator over silica gel. The sample thus prepared was denoted as ZnAl-PW₁₂.

The Zn, Al, and W contents in the samples were determined by ICP (Varian Liberty series 2). Powder X-ray diffraction patterns were recorded in a Rigaku (Miniflex II) X-ray diffractometer using Ni-filtered CuK_α (30 kV, 15 mA) radiation source. Thermogravimetric measurements in the argon atmosphere were performed on a Shimadzu DTG 60 Thermal analyzer at a heating rate of 10°C/min. 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 the signal-to-noise ratio. The UV-visible diffuse reflectance spectra (UV-Vis DRS) were recorded on a Varian UV-visible spectrophotometer using BaSO₄ white standard. The surface areas were measured using a Smart Sorb 92/93 surface area analyzer. The samples were degassed at 100°C for 4 h before analysis.

The photocatalytic activity of the ZnAl-PW₁₂ is tested in a double-walled cylindrical glass photoreactor fitted with a 125 W high-pressure Hg lamp and a cut-off filter to eliminate light of wavelength <400 nm. In typical lots, 100 ml of different dye solutions in varying concentrations were treated with a certain amount of catalyst and irradiated with visible light under the stirring condition to initiate the photodegradation. A solution of NaNO₂ (1.0 M) was circulated through the outer jacket of the photoreactor to cut off the light with wavelengths less than 400 nm. At regular intervals, 5.0 ml of the reaction mixture was pipetted out and centrifuged to separate the catalyst and the absorbance of the supernatant solution at the pre-set wavelengths (λ_max of different dyes). The concentration of dye was calculated from the estimated absorbance value as per Eq. 1. Each catalytic run was made at least in duplicate and the average value was reported.

\[ \% \ \mathrm{of \ Dye \ degraded=[(C_0-C_\mit{t})/\mathrm{C}_0]\times100} \tag{1} \]

Where C₀ is the initial dye concentration, and C_t corresponds to the attention of dye at time t. To see the amount of paint removed from the solution due to adsorption only, batch adsorption experiments were performed by taking 100 ml of dye solution of the desired concentration with the catalyst and stirring for 4 h without the passage of visible light.

3. Results and Discussion

3.1 Characterizations of ZnAl-PW₁₂

The molar ratio of Zn:Al:W in ZnAl-PW₁₂ is found to be 0.742:0.267:0.798, indicating all the interlayer charge is not compensated by PW₁₂O₄₀^3- ion. About 80% of the interlayer amount of LDH is balanced by PW₁₂O₄₀^3- while the remaining amount is balanced by CO₃^2- or OH⁻ ions. Powder XRD patterns of ZnAl-PW₁₂, along with the ZnAl-LDH precursor used for the intercalation of PW₁₂O₄₀^3- are shown in Figure 1.

Figure 1 XRD patterns of ZnAl-LDH (1) and PW₁₂O₄₀^3- intercalated ZnAl-LDH (2).

The shifting of peaks corresponding to (003), (006), and (009) planes towards lower 2θ values in the case of ZnAl-PW₁₂ clearly indicates an expansion of the interlayer space due to intercalation of ZnW₁₂ in the interlayer [1]. Assuming the diameter of Keggin-ion is ca. 8 Å and the brucite-like layer thickness 4.8 Å, the calculated basal spacing should be 12.8 Å. Using the 2θ value of (003) reflection, the d₀₀₃ is found to be 12.73 Å, which is well within the range of values reported in the cases of POM intercalated LDHs [1,17,18,19]. The XRD pattern with the broad spread of reflections in the case of ZnAl-PW₁₂ indicates poor crystallinity due to some structural disorder with the entry of more than one intercalating species (e.g. PW₁₂ and OH^-/CO₃^2-) into the interlayer region of LDH.

The FT-IR spectra of ZnAl-PW₁₂, H₃[PW₁₂O₄₀] and Zn(Al)O precursor, are shown in Figure 2. The observed broad and intense band between 3600 and 3200 cm^-1 in all the samples is attributed to –OH stretching from the metal hydroxide and interlayer water. FT-IR of PW₁₂ displayed four characteristic vibrational bands arising from P–O bonds of the PO₄ units, W=O bonds, and two W–O–W bonds of the Keggin unit at 1080, 987, 895 and 818 cm^-1, respectively [19,27]. These bands also appeared in the case of ZnAl-PW₁₂ with reduced intensities, indicating the existence of PW₁₂ in the interlayer of LDH. The characteristic peak for CO₃^2- ion at ~1380 cm^-1 also appeared in the spectrum of ZnAl-PW₁₂, indicating that a small amount of CO₃^2- ions has entered during the intercalation process or remained in the calcined ZnAl-LDH. The appearance of low-intensity peaks in the XRD pattern of ZnAl-PW₁₂ at the same 2θ values as that of ZnAl-LDH also supported this fact.

Figure 2 FT-IR spectra of ZnAl-PW₁₂ along with pure PW₁₂O₄₀^3- and ZnAl-LDH.

The UV-Vis DR spectra of ZnAl-PW₁₂ along with PW₁₂, Zn(Al)O, and ZnAl-LDH were shown in Figure 3. The appearance of characteristic peaks of PW₁₂O₄₀^3-, at ~260 and 340 nm, in the spectra of ZnAl-PW₁₂ provided additional evidence in favor of the intercalation of PW₁₂O₄₀^3- in the interlayer of reconstructed LDH. More interestingly, the absorption of ZnAl-PW₁₂ extends beyond 400 nm, presumably due to the narrowing down of the band gap. The band gap of ZnAl-PW₁₂ (2.86 eV) is found to be marginally lower than that of either PW₁₂ (3.05 eV) or Zn(Al)O (3.00 eV). The lowering of band gap from 3.51 eV to 3.10 eV has also been reported for H₃PW₁₂O₄₀/TiO₂ composite than that of pure TiO₂ with the increase of H₃PW₁₂O₄₀ loadings from 0 to 16.7% [24].

Figure 3 UV-Vis diffuse reflectance spectra of ZnAl-PW₁₂ along with pure PW₁₂O₄₀^3-, unclaimed and calcined ZnAl-LDH [Zn(Al)O].

3.2 Photocatalytic Activity of ZnAl-PW₁₂

The photocatalytic efficiency of ZnAl-PW₁₂, along with ZnAl-LDH, was assessed for degradation of structurally different cationic and anionic dyes in aqueous solutions under visible light irradiation (λ > 400 nm). The results obtained were discussed below.

3.2.1 Decolourisation of Methyl Orange (MO)

The representative time course degradation of MO under visible light irradiation is presented in Figure 4. The Spectral scan of MO at different intervals during a photocatalytic run (Inset, Figure 4) clearly showed the decrease of absorbance in the entire spectral range without any noticeable change or shift in the peak positions. The initial concentration of MO in all cases was kept within 10⁻⁴ M (32.7 mg/L) to avoid its dimerization to obey Beer–Lambert’s law [28]. A blank test without the photocatalyst showed about 5% degradation of CR throughout 3 h. As evident from Figure 4, MO is significantly removed (~72%, 43.2 mg/g) by Zn(Al)O within 180 min, primarily due to the adsorption/ion exchange of MO. Under identical conditions, only ~12% of MO is removed through adsorption/ion exchange in the case of ZnAl-PW₁₂.

Figure 4 Comparative adsorptive/photocatalytic decolourisation of methyl orange (MO) by Zn(Al)O and ZnAl-PW₁₂; [MO] = 30 mg/L, pH = 5.5 ± 0.03, catalyst = 0.5 g/L. (Inset) successive spectral scan for photocatalytic degradation of MO using ZnAl-PW₁₂ (initial MO, 50 mg/L, catalyst dose, 0.50 g/L).

The lower percentage of MO removal, in this case, is obviously due to (i) the competition from existing interlayer anions (PW₁₂/CO₃^2-/OH⁻) for its entry in the anionic form and (ii) presence of the relatively lesser amount of charge balancing interlayer anionic sites available for MO with the same wt. (0.5 g/l) of ZnAl-PW₁₂. Hence, the overall removal of MO by Zn(Al)O is mainly due to the entry of MO in the interlayer through rehydration, while more than 98% of total MO (30 mg/l) removal within 120 min in the case of ZnAl-PW₁₂ mainly results from its photocatalytic degradation under visible light. The decolorisation data were fitted to the first-order equation, and the rate constant with 30 mg/l (initial concentration) was found to be 2.14 × 10^-2 s^-1. The results of overall MO decolorisation at varying initial MO concentrations and catalyst doses are presented in Figure S1. The overall MO decolorisation increases linearly with an increase in catalyst dose at the beginning and then nonlinearly at a higher dose of catalyst, presumably due to the turbidity of the suspended catalyst that shields the penetration of visible light. On the other hand, a progressive decrease in the overall decolorisation of MO is observed with the increase of MO concentration due to decreased active sites on the catalyst surface for MO adsorption, followed by its degradation. It may be noted that the final pH in each case is increased at the end of the photocatalytic run, and the pH values vary in the range of 6.7 to 7.5.

3.2.2 Decolourisation of Congo Red (CR)

The results of CR decolorisation, both in the presence and absence of visible light, are depicted in Figure 5. The corresponding spectral change (inset, Figure 5) over the reaction period in the presence of light shows a decrease in absorbance without any noticeable shift in the peak positions. CR, like MO, also exists in the anionic form at pH ~6.2 (pK_a of CR ~3.0-5.0) and can enter the interlayer of LDH during the photocatalytic run.

Figure 5 Comparative adsorptive/photocatalytic decolourisation of Congo red (CR) by Zn(Al)O and ZnAl-PW₁₂; (initial [CR], 50 mg/l, pH ~6.2, catalyst 0.5 g/l). (Inset) Successive UV-Vis spectra of CR during its decolourisation by ZnAl-PW₁₂ under visible light irradiation ([CR], 50 mg/l, pH ~6.2).

It is seen that ~45% of CR was removed by Zn(Al)O without light through adsorption or exchange in the interlayer of rehydrated Zn(Al)O, which is relatively less than that observed in the case of MO (72%) presumably due to bigger molecular size of CR than MO. In comparison, only 8-10% CR is removed (without light) by ZnAl-PW₁₂. The removal (decolorisation) of CR is significantly increased on light irradiation, indicating the promotional effect of PW₁₂ in the interlayer of ZnAl-PW₁₂. About 80% of CR (50 mg/L) is decolorised within 120 min under visible light irradiation with a rate constant of 1.41 × 10^-2 s^-1. The results of CR decolorisation under varying initial concentrations and catalyst doses were presented in Figure S2. As expected, the overall decolorisation of CR was found to decrease with the increase of initial CR concentration and a decrease of catalyst dose similar to that observed in the case of MO.

3.2.3 Decolourisation of Methylene Blue (MB)

Comparative efficiency of Zn(Al)O and ZnAl-PW₁₂ towards the removal of MB is presented in Figure 6, along with spectral change during a typical photocatalytic run. It is seen that the initial peaks at 614 and 664 nm are merged into a relatively broad peak, centered at ~650 nm, during the reaction, and the peak intensities are also reduced progressively with a shift towards higher wavelengths till the reactant solution turns colorless [29]. On the other hand, the powers 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, due to benzene ring and heteropolyaromatic linkage, indicate their destruction at the end of the reaction [29].

Figure 6 Comparative adsorptive/photocatalytic decolourisation of methylene blue (MB) by Zn(Al)O and ZnAl-PW₁₂; (MB, 50 mg/l, pH ~6.2, catalyst, 0.5 g/l ). (Inset) UV-Vis spectra showing progressive decolourisation of MB using ZnAl-PW₁₂ under visible light irradiation (MB, 50 mg/l, pH ~6.2, catalyst, 0.5 g/l).

In comparison to MO, the removal of MB by either Zn(Al)O or ZnAl-PW₁₂ in the dark condition is much lower as MB exists in the cationic form at pH ~6.2 (pKa > 12), and therefore, removal of MB is primarily due to its adsorption on the LDH surface by electrostatic interaction. It is seen that ZnAl-PW₁₂ shows relatively higher activity than the Zn(Al)O throughout the reaction period, indicating the promoting photocatalytic effect of intercalated POM. Like other dyes, the percentage of MB decolorisation is also found to increase with the increase of catalyst dose and a decrease of initial MB concentration (Figure S3). Complete degradation of MB (50 mg/L) is observed in 120 min of irradiation with a catalyst dose of 1.0 g/L with a rate constant of 2.25 × 10^-2 s^-1.

3.2.4 Decolourisation of Rhodamine B (RhB)

Another cationic dye, rhodamine B (RhB), was selected to assess the photocatalytic potential of ZnAl-PW₁₂ due to its absorption in the visible-light region and excellent stability over a wide range of pH. The decolorisation of RhB is evident from the significant decrease in absorbance change during the photocatalytic reaction without any significant shift in the peak positions (Figure 7, inset), indicating the decolorisation of RhB is primarily due to the decomposition of conjugated chromophore structure rather than de-ethylation process [30]. To avoid the colour change due to pH variation, the photocatalytic reaction was carried out at pH (~6.2), much above the pK_a of RhB (= 3.7), where it mainly exists in zwitterionic form due to deprotonation of the carboxyl group. Like MB, the removal of RhB in the absence of light by adsorptions over ZnAl-PW₁₂ or ZnAl(O) is relatively low, ~5.3% and 9.2%, respectively (Figure 7).

Figure 7 Comparative adsorptive/photocatalytic decolourisation of Rhodamine B (RhB) by Zn(Al)O and ZnAl-PW₁₂; (RhB, 30 mg/l, pH ~6.2, catalyst, 0.5 g/l ). (Inset) Successive spectral scan of RhB during its decolourisation by ZnAl-PW₁₂ under visible light irradiation at pH ~6.2.

The catalytic effect of PW₁₂ (in ZnAl-PW₁₂) over ZnAl(O) is also evident from the figure. Figure S4 presents the impact of catalyst dose and initial dye concentration, similar to those obtained for other dyes. In a recent study, the enhanced photocatalytic effect of H₃PW₁₂O₄₀/TiO₂ composite films with different H₃PW₁₂O₄₀ loadings (6.3, 7.7, 14.7 and 16.7%) has been seen towards degradation and mineralization of an aqueous dye RhB under solar simulating Xe lamp irradiation (320 < λ,nm < 780) [31]. Complete degradation of RhB (25 mg/L) has been observed in 120 min with a catalyst dose of only 45 mg/l. In comparison, about 86.6% of RhB (30 mg/l) is degraded in 210 min with a catalyst dose of 1.0 g/l in the present case due to different amounts of PW₁₂ loading. The decolourisation data was fitted well to the first order equation, and the rate constant with 30 mg/l (initial concentration) was found to be 8.9 × 10^-3 s^-1. The overall decolorisation of RhB is also found to decrease with the increase of initial RhB concentration and decrease of catalyst dose similar to that observed in the cases of other dyes.

3.3 Comparative Photocatalytic Activity and Probable Mechanism

The photocatalytic efficiencies of some related photocatalysts for degradation/decolourisation of different organic dyes were presented in Table 1, along with those observed in this work. It is worth noting that ZnAl-PW₁₂ efficiently decolourises several shades under visible irradiation with comparable activity as those of other photocatalysts [27,31,32,33,34,35,36].

Table 1 Comparative activity of some relevant catalysts towards decolourisation of different organic dyes.

The photocatalytic decolourisation/degradation of dyes by semiconducting oxides occurs most likely either through (i) photoexcitation of oxide leading to the electron/hole pairs on its surface followed by the formation of hydroxide radical (HO^•) by the decomposition of the water or by reaction of the holes with HO⁻ and its response with dye molecules to yield the degradation products as delineated in Eqs. (2 to 5).

\[ \mathrm{Catalyst}+\mathrm{h \upsilon }\rightarrow\mathrm{Catalyst}\left(\mathrm{e^{-}}_{\mathrm{CB}}+\mathrm{h^{+}}_{\mathrm{VB}}\right) \tag{2} \]

\[ \mathrm{H}_{2}\mathrm{O}+\mathrm{h}^{+}{}_{\mathrm{VB}}\rightarrow\mathrm{H}^{+}+\mathrm{OH}^{\bullet } \tag{3} \]

\[ \mathrm{HO}^{-}+\mathrm{h}^{+}{}_{\mathrm{VB}}\to\mathrm{HO}^{\bullet} \tag{4} \]

\[ \mathrm{HO^{\bullet}+Dye\to degradation \ products} \tag{5} \]

and/or (ii) absorption of visible light catalyst/Dye system leading to excitation of the surface-adsorbed dye molecule (Eq. 6), which affects the charge transition into the conduction band of catalyst in the excited state (Eq. 7). The electron in the conduction band (e^-_CB) is then transferred to molecular oxygen, leading to the formation of a series of radicals capable of decolorising/degrading the dye molecules.

Since the measured band gap of ZnAl-PW₁₂ is relatively high (3.02 eV), the mechanism of dye degradation based on generation (e^-/h⁺) pairs under visible light irradiation is less likely, although not completely ruled out. Alternatively, a photosensitized pathway with light absorption, mainly through dye molecule adsorbed/exchanged on the catalyst surface (ZnAl-PW₁₂/Dye), leads to excitation of the surface-adsorbed dye molecule (Eq. 6), which affects the charge transition into ZnAl-PW₁₂ conduction band (Eq. 7). The excited dye molecule undergoes degradation (Eq. 8). In addition, the electron from the conduction band (e^-_CB) is combined with the molecular oxygen yielding a series of radicals capable of decolourising/degrading of the dye molecule (Eqs. 9-14). The overall photocatalytic process may be delineated as follows:

\[ \mathrm{ZnAl}-\mathrm{PW}_{12}/\mathrm{Dye}+\mathrm{h \upsilon }\to\mathrm{ZnAl}-\mathrm{PW}_{12}/\mathrm{Dye} \ * \tag{6} \]

\[ \mathrm{ZnAl}-\mathrm{PW}_{12}/\mathrm{Dye} \ *\to\mathrm{ZnAl}-\mathrm{PW}_{12}(\mathrm{e}{}^{-}{}_{\mathrm{CB}})/\mathrm{Dye}^{+\bullet} \tag{7} \]

\[ \mathrm{Dye ^{+\bullet}\to\text{degradation products}} \tag{8} \]

\[ \mathrm{ZnAl}-\mathrm{PW}_{12}\left(\mathrm{e}^{-}{}_{\mathrm{CB}}\right)+\mathrm{O}_{2}{\rightarrow}\mathrm{ZnAl}-\mathrm{PW}_{12}+\mathrm{O}_{2}{}^{-\bullet} \tag{9} \]

\[ \mathrm{H_2O+{O}{}_2{}^{-\bullet}\to\mathrm{H}OO^{\bullet}} \tag{10} \]

\[ \mathrm{HOO}^\bullet+\mathrm{H}_2\mathrm{O}+\mathrm{e}{}^-{}_{\mathrm{CB}}\to\mathrm{H}_2\mathrm{O}_2+\mathrm{OH}^-\to\mathrm{HO}^\bullet+\mathrm{OH}^- \tag{11} \]

\[ \mathrm{H}_2\mathrm{O}_2+\mathrm{O}_2{}^{-\bullet}\to\mathrm{H}\mathrm{O}^{\bullet}+\mathrm{O}\mathrm{H}^{-}+\mathrm{O}_2 \tag{12} \]

\[ \text{Dye}^{+\bullet}+\text{O}_2{}^{-\bullet}\to\text{Dye}-\text{O}_2\to\text{degradation products} \tag{13} \]

\[ \mathrm{HO^{\bullet}/{\mathrm{O}_2}^{-\bullet}+\text{Dye}\rightarrow\text{degradation products}} \tag{14} \]

4. Conclusions

Tungstophosphate (PW₁₂O₄₀^3-) is efficiently intercalated between the layers of three-dimensionally ordered ZnAl-LDH through rehydration of calcined ZnAl-LDH (Zn/Al ~3.0) under N₂ atmosphere. The chemical analyses coupled with characterization by PXRD, FT-IR, and DR/UV-Vis spectra of the as-prepared intercalated sample (ZnAl-PW₁₂) and their comparison with pure H₃PW₁₂O₄₀ indicated about 80% of the residual positive charge in the brucite layer was balanced by PW₁₂O₄₀^3-.

The photocatalytic properties of ZnAl-PW₁₂ were evaluated using decolorisation of some model anionic (methyl orange and Congo red) and cationic (methylene blue and rhodamine B) under visible light irradiation. Decolorisation of dyes was evident from a progressive decrease of absorbance in their successive UV-Visible spectral scan during the photocatalytic runs. In all the cases, the extent of dye decolourisation increased with an increase in catalyst doses or a decrease in initial dye concentrations. Under identical conditions and without light irradiation, the adsorption/exchange of dyes on ZnAl-PW₁₂ is significantly lower than that of calcined ZnAl-LDH [ZnAl(O)]. On the other hand, in the presence of light, the adsorptive/photo-decolorisation by ZnAl-PW₁₂ was significantly higher than that shown by ZnAl(O), indicating the promoting effect of PW₁₂. In summary, the present study demonstrated that PW₁₂ intercalated LDH promises to be an excellent photocatalytic system for many practical applications. Moreover, the intercalation of PW₁₂ in the interlayer of LDH avoided the loss of costly PW12 and increased the photocatalytic activity.

Acknowledgements

The authors acknowledged the Department of Chemistry, Utkal University for extending the facilities for carrying out the work.

Author Contributions

B. S. Mohanta: Synthesis of materials, data collections, and compilation; R. Das: Experiment design, data analysis, finalization of the manuscript; N. Das: Experiment design, data analysis, rough draft, and finalization of the manuscript.

Competing Interests

The authors have declared that no competing interests exist.

Additional Materials

The following additional materials are uploaded at the page of this paper.

1. Figure S1: Effect of concentration of dye and wt. of catalyst (ZnAl-PW₁₂) towards adsorptive/photocatalytic decolourisation of methyl orange (MO).

2. Figure S2: Effect of concentration of dye and wt. of catalyst (ZnAl-PW₁₂) towards adsorptive/photocatalytic decolourisation of Congo red (CR).

3. Figure S3: Effect of concentration of dye and wt. of catalyst (ZnAl-PW₁₂) towards adsorptive/photocatalytic decolourisation of methylene blue (MB).

4. Figure S4: Effect of concentration of dye and wt. of catalyst (ZnAl-PW₁₂) towards adsorptive/photocatalytic decolourisation of Rhodamine B (RhB).