Metal doped titanate photo catalysts for the mineralization of congo red under visible irradiation

Abstract

Strontium titanate catalysts and titania were synthesized by the sol–gel method. 1 mol% nickel/ruthenium was doped on the strontium titanate catalyst to shift its optical response to the visible region. UV-diffuse reflectance spectral analysis confirms the red shift with the band gap of 1.9 eV and 2.13 eV for the Ni and Ru doped strontium titanate catalysts, respectively. The catalysts were characterized using various instrumental techniques such as X-ray diffraction (XRD), UV-diffuse reflectance spectroscopy (UV-DRS), BET surface area analysis, Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS), Transmission Electron Microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The photocatalytic activities of bare TiO₂, bare strontium titanate and metal doped strontium titanate catalysts were evaluated towards the decolourisation/degradation of congo red under UV, visible and solar irradiation and monitored by a UV-visible spectrophotometer and TOC analyser. Among the catalysts nickel doped strontium titanate completely decolourised CR in the shortest reaction time (300 min), and hence it was subjected to decolourise textile effluent. It was found to be very effective in the degradation of textile effluent as it showed a significant reduction in the TOC. Kinetic studies indicate that the degradation of congo red followed pseudo first order kinetics. The recycling test confirmed that the most active nickel doped strontium titanate catalyst is highly photostable.

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1. Introduction

Dyes from the textile industries pollute the environment due to their intense colour and carcinogenicity, and hence they have become an issue of worldwide concern. The release of these dye effluents also cause eutrophication and perturbs humans and other living organisms. Textile wastewater during printing and dyeing processes is hard to treat due to its higher concentration, deep chromes, great toxicity and complicated components. Azo dyes are the largest group of dyes used for dyeing cotton fabrics in the textile industry.^1–5 Using the chemical coagulation method, azo dyes cannot be removed due to their hydrophilic property. Physico chemical methods such as coagulation and flocculation produce large amounts of sludge, which creates disposal problems. Aerobic processes are not suitable to remove azo dyes due to the electron withdrawing nature of the azo bonds.^6–8 Advanced oxidation processes have been found to be the most promising technology as it overcomes the limitations of mass transfer, they have the ability to be carried out at ambient conditions and to completely degrade many organic pollutants including dyes.^9,10 Semiconductors, such as TiO₂, ZnO, CeO₂, WO₃, ZnS and CdS, are known to be good photocatalysts, among them TiO₂ has been widely studied for the degradation of a large number of pollutants, such as dyes, phenols, endocrine disruptors, and some of these semiconductors were found to be efficient for solar cell applications.^11–19 Apart from these semiconductors, some layered perovskites also have been utilized in the degradation of carcinogens.^20,21 Among the layered perovskites SrTiO₃ has been used in various fields, such as environmental clean-up and hydrogen production, due to its excellent dielectric, piezoelectric, and photoelectric properties.^22–28 Recently J. Zhang et al. reported the photo electrochemical applications of these two composite nanotubes.²⁹ However, both TiO₂ and SrTiO₃ cannot utilize visible light and make use of only the UV light (>5%) of solar beams due to its relatively large bandgap of 3.2 eV.³⁰ Therefore, recent research has been devoted to the development of visible-light active photocatalysts. Doping, loading, impregnation and sensitization of strontium titanate catalysts were mainly aimed to shift the light absorption towards the visible light and/or to increase the life time of the electron–hole pairs produced during the photo process.^31–34 One of the present authors has reported the doping/impregnation of transition metals, noble metals and nonmetals into the TiO₂ lattice.^35–37 But the improved photocatalytic activity of transition metal doped SrTiO₃ for environmental applications have been rarely reported. J. Zheng et al. reported that 94% methylene blue was degraded using Nd doped SrTiO₃ nanospheres under visible irradiation.³⁸ Irie et al. reported Pb doped SrTiO₃ resulted 0.49% quantum efficiency towards the decomposition of 2-propanol under visible irradiation.³⁹ Ag doped SrTiO₃ degraded 12% of Victoria blue dye under visible irradiation, whereas no activity was observed with Pt and Au doped SrTiO₃.⁴⁰

Congo red is a water soluble secondary diazo dye. The chemical name of congo red is, sodium salt of 3,3′-([1,1′-biphenyl]-4,4-diyl)bis (4-aminonapthalene-1-sulfonic acid). Benzidine is a toxic metabolite of congo red, which causes cancer of the bladder in humans.⁴¹ Hence, studies on the degradation of such a harmful pollutant carries significance. Basic chemical information on CR is given in Table ESI1.† Lachheb et al. studied the decolourisation of congo red using various commercially available titania catalysts.⁴² Zhaohong Zhang investigated the degradation of congo red using microwave irradiation and attained 87.7% decolourisation using activated carbon powder.⁴³ Rishi et al. studied the degradation of textile effluent using UV over TiO₂ and obtained 46% degradation within an hour.⁴⁴ LI Yun-cang et al. reported the photocatalytic degradation of textile effluent with titania pillar pellets and achieved the degradation of 39%.⁴⁵ To the best of our knowledge, no one has reported the photocatalytic degradation of congo red/original textile effluent using metal doped strontium titanate catalysts.

In this paper we report the synthesis of bare (ST) and metal doped strontium titanate viz. nickel strontium titanate (NiST) and ruthenium strontium titanate (RuST) catalysts, their characterization by various instrumental techniques such as XRD, UV-DRS, Raman, FTIR, SEM with EDX, TEM and XPS and evaluation of their photocatalytic activities towards the decolourisation/degradation of congo red under UV, visible and sunlight irradiations. The best catalyst among them was chosen and tested for the degradation of original textile effluent collected from a textile industry. The extent of degradation was determined by the total organic carbon analyser and the results are discussed in detail.

2. Experimental

2.1 Materials

Strontium chloride hexahydrate, citric acid and acetic acid from Sisco Research Laboratories Pvt. Ltd., India, titanium(IV) iso propoxide from Spectrochem Pvt. Ltd., India, nickel nitrate, congo red from Central Drug House, Delhi, and ruthenium trichloride from Aldrich were procured and used as received.

2.2 Synthesis of nano titania

The sol–gel process was adopted for the synthesis of titania. 34 mL of TIP and 80 mL of isopropanol were mixed to obtain solution A. Solution B was prepared by mixing 120 mL of isopropanol, 30 mL of glacial acetic acid and 10 mL of H₂O. Then, solution A was mixed with solution B and stirred overnight. The sol obtained was aged to get the TiO₂ gel. The gel was dried at 500 °C for 3 h and ground to get fine powders of TiO₂.⁴⁶

2.3 Synthesis of strontium titanate and metal doped strontium titanate

30 mL containing stoichiometric amounts of Sr was added to a solution containing 18.56 mL of TIP and 30 mL of glacial acetic acid and its pH was adjusted to 1.5 with 4 M citric acid. The resulting solution was stirred overnight, heated at 65 °C for 5 h in a water bath, dried at 110 °C for 12 h and again at 180 °C for 12 h. Then, it was ground, calcined at 400 °C for 12 h and again at 650 °C for another 12 h to obtain strontium titanate. NiST and RuST were synthesized following the above procedure but by adding aqueous solutions of nickel nitrate and ruthenium chloride to the precursors of Sr and Ti to obtain 1 mol% of either Ni or Ru in SrTiO₃.⁴⁷

2.4 Characterisation

A Rigaku X-ray diffractometer with Cu kα radiation (1.54 nm) was used to determine the phase and composition of the synthesized photocatalysts. The patterns were recorded over a 2θ range of 10°–70°. The bandgap measurements of the sample were carried out using a Jasco U-650 Diffuse reflectance spectrophotometer with an integrated sphere using barium sulphate as the reference. The surface area measurements were made for the degassed catalysts with nitrogen as probe molecule at liquid nitrogen temperature (77 K) using Brunauer–Emmett–Teller (BET) surface area analyzer (Micromeritics Pulse Desorb-2300 and Chemisorb-2700). FTIR spectra of all the catalysts were recorded in the range of 4000–400 cm⁻¹ using the alkali halide technique using FTIR spectrometer (Bruker Tensor-27). Raman measurements were performed at room temperature using a Laser Raman Microscope RAMAN-11. Morphology was studied using a scanning electron microscope and the elements present were determined using an HRSEM-EDAX [Quanta 200 FEG]. High resolution TEM images were obtained with a JEOL JEM 2000EX2 microscope operated at 200 kV. For the HRTEM analysis powder samples were sonicated in ethanol for 15 minutes and a drop of it was placed on a copper grid and the images were recorded. X-ray photoelectron spectroscopy (XPS) measurements were performed to determine the elements present in the catalyst on an ECALAB MKIV XPS system with an Al Kα source (1486.6 eV) and a charge neutralizer.

2.5 Photodegradation studies

2.5.1 Photodecolourisation studies of CR using a UV-visible spectrophotometer. UV photocatalytic experiments were carried out in a 100 mL quartz vessel containing CR dye solution (2.5 × 10⁻⁴ M, pH: 7.3) and 0.25 g of photocatalyst. Air was purged into the reaction vessel throughout the reaction for proper mixing of the catalyst with the dye solution. Four mercury lamps (8 W) were used as the UV light source. Before illumination the solution was stirred for 30 min in the dark to attain adsorption–desorption equilibrium between the organic substrate and the photocatalyst. The reaction was carried out for 6 1/2 h. Visible irradiation experiments were conducted in the same vessel containing the dye solution (2.5 × 10⁻⁵ M) using a 500 W, 420 nm visible lamp (Philips). Water was circulated to maintain the temperature at room temperature, as well as to protect the dye solution from evaporating due to the high temperature generated during the reaction. Sunlight irradiation was performed during the summer (month of April) between 10 : 00 am and 1 : 00 pm. Solar irradiation experiments were carried out in a glass beaker (100 mL) placed on a magnetic stirrer. Original textile effluent was collected from an industry in the Erode District, Tamil Nadu, India. After filtration the effluent was subjected to photocatalytic decolourisation. Aliquots were withdrawn from the photoreactor at regular intervals. The suspended catalyst particles were centrifuged, and filtered through a 0.20 mm filter (Millipore) to remove the fine catalyst particles and the decolourisation was monitored by a UV-visible spectrophotometer (HITACHI, U-2000). The absorbance values at the λ_max (λ_max = 497 nm) of CR were recorded to determine the concentration of dye at different intervals of time. Then, the decolourisation of CR was found out using the formula (eqn (1)),

% Decolourisation of CR = ((C₀ − C_f)/C₀) × 100 (1)

where C₀ and C_f are the initial and final concentration of CR.

2.5.2 Photo mineralisation studies of CR using HPLC and TOC analyser. To find out whether any intermediates were formed during the degradation of the dye, HPLC analysis was carried out using a C18 column. A mixture of methanol and water in the ratio of 30 : 70 was used as the mobile phase. The total organic carbon content of the dye sample was analyzed before and after decolourisation using a TOC analyser (SHIMADZU TOC-500). The sample was placed in a boat of the TOC analyser and the organic constituents were converted into CO₂. The gas was dried in the perma pure drier and passed through a particle filter to remove any foreign particles. Then, the gas was fed into the NDIR detector, in which the concentrations of CO₂ of the samples were measured. From the concentration of CO₂, the TOC was deduced.

3. Results and discussion

3.1 Characterization of synthesized catalysts

3.1.1 X-ray diffraction analysis. The XRD patterns of bare TiO₂, bare SrTiO₃, Ni and Ru doped SrTiO₃ are given in Fig. 1. Both the bare and metal doped titanates showed sharp and intense XRD peaks indicating the high degree of crystallinity. All of the X-ray diffraction patterns indicate the presence of a single oxide cubic (Pm3m) phase. The sharp peaks at 2θ = 25.3°, 48.05°, 37.8° correspond to the (1 0 1), (2 0 0) and (0 0 4) planes and 2θ = 32.5°, 46.5°, 57.8° and 40.01° corresponding to the (1 1 0), (2 0 0), (2 1 1) and (1 1 1) planes confirm the presence of the anatase phase of titania and cubic (Pm3m) phase of strontium titanates, respectively. The patterns match with the Joint Committee for Powder Diffraction Studies (JCPDS) files available in the literature (JCPDS file no. 21-1272, file no. 35-0734).^48,49 Due to the very low level doping (>1 mol%) of Ni/Ru over strontium titanate, separate peaks for their presence was not observed in the XRD patterns of the Ni and Ru doped strontium titanate catalysts. The crystallite size of the synthesized catalysts were calculated using Scherrer's formula,

[D = Kλ/β cos θ] (2)

where β is the full width at half-height, K = 0.89, θ is the diffraction angle and λ is the X-ray wavelength corresponding to the Cu Kα radiation and the results are given in Table 1.⁵⁰

Fig. 1 XRD patterns of (a) bare TiO₂ (b) ST, (c) NiST and (d) RuST.

Table 1 Crystallite size, bandgap, surface area and rate constant values in the degradation of CR

S. no.	Catalysts	Crystallite size (nm)	Bandgap (eV)	BET surface area (m^2 g^−1)	Rate constant (min^−1)
1	TiO2	19	3.17	93	9.21 × 10^−3
2	ST	43	3.17	11	7.13 × 10^−3
3	NiST	39	1.9	16	1.26 × 10^−2
4	RuST	38	2.13	19	1.08 × 10^−2

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3.1.2 UV diffuse reflectance spectroscopy. Fig. 2(A) shows the diffuse reflectance spectra of the bare and doped catalysts. The band gap of all the catalysts were determined using the formula E_g = 1240/λ, where λ is the cut-off wavelength. This cut-off wavelength is calculated by converting the absorbance into reflectance by performing the Kubelka–Munk transformation using the following equation,

K/M = (1 − R)²/2R ⇒ F(R) (3)

where K/M is reflectance transformed according to Kubelka–Munk, R is reflectance (%R), F(R) – Kubelka–Munk function.

Fig. 2 (A) UV-DRS absorption spectra of (a) bare TiO₂ (b) ST, (c) NiST and (d) RuST and (B)–(E) their corresponding Kubelka–Munk spectra.

The corresponding K/M spectra are shown in Fig. 2(B)–(E). The bandgap values were calculated from these spectra and are given in Table 1. The bandgap values of both of the doped catalysts were found to be very low and fall in the visible region when compared to their bare counterparts, which confirms the optical response of the catalyst in the visible region.⁵¹ The variation in the bandgap between the metal doped and the bare strontium titanates can be explained by understanding the band structures of the synthesized catalysts. The bandgap values (Table 1) indicate that the bare and metal doped strontium titanates assume the band structures as shown in Scheme 1. The bare SrTiO₃ has a large bandgap between the O²⁻ (2p) and Ti⁴⁺ (3d) states, and hence it cannot be easily excited under visible light irradiation.⁵² However, the introduction of metal ions into the SrTiO₃ lattice creates additional donor levels near the valence band due to the 3d orbitals of Ni²⁺ and 4d orbitals of Ru³⁺, and hence the bandgaps of these two metal doped SrTiO₃ catalysts showed the lower values of 1.9 eV and 2.13 eV. When SrTiO₃ was doped with Cr and nitrogen, a similar reduction in the bandgaps was observed by Wang et al. and Miyarchi et al.^53,54 Generally doping of metals alters oxygen vacancies and may also act as recombination centres due to which there may be a slight loss in photocatalytic activity.⁴⁷

Scheme 1 Band structures of ST, NiST and RUST.

3.1.3 BET surface area. The surface area values of the bare TiO₂, SrTiO₃ and Ni/Ru–SrTiO₃ catalysts are given in Table 1. The sol–gel method of syntheses of both titania and strontium titanate led to the formation of highly crystalline compounds. However, they differ considerably in their surface area values. The lower surface area (11–19 m² g⁻¹) obtained in the case of SrTiO₃ may be due to the high calcination temperature employed during synthesis. Not much difference in the surface area was seen between the bare and metal doped strontium titanates.

3.1.4 Raman spectroscopy. Raman spectroscopy is one of the most sensitive probes to detect the variation of local symmetry for a surface structure. Raman spectroscopic studies over the titania catalyst give information about its phase formation (Fig. 3(A)). Raman scatterings around 144 cm⁻¹ (E_g(1) the strongest), 200 cm⁻¹ (E_g(2)), 399 cm⁻¹ (B_1g(1)), 519 cm⁻¹ (B_1g(2)) and 639 cm⁻¹ (E_g(3)) indicate TiO₂ is in the anatase phase and the absence of Raman scatterings around 447 cm⁻¹ (E_g strong), 612 (A_1g strong) further confirms that the rutile phase is absent.⁵⁵ SrTiO₃ has an ideal cubic perovskite structure with the space group Pm3m with the vibration mode of 3F_1u + F_2u (Fig. 3(B)). Hence, the first order Raman modes are forbidden at room temperature. Du et al. reported that the activation of first order Raman modes are possible by strain effects, oxygen vacancies and external conditions.⁵⁶ Each of the F_1u modes splits into a doubly degenerate E mode and a nondegenerate A₁ mode, while the F_2u mode splits into E and B₁ modes. Thus, the vibration modes are 3(A₁ + E) + E + B₁. The presence of long-range electrostatic forces further splits each of the A₁ and E modes into transverse optical (TO) and longitudinal optical (LO) modes. Luo et al. observed the Raman modes at 484, 547 and 797 cm⁻¹ and assigned them as LO₃, TO₄ and LO₄.⁵⁷ We also observed the bands for the bare strontium titanate catalyst at 476 cm⁻¹, 549 cm⁻¹ and 802 cm⁻¹ due to the above mentioned modes. However, for the nickel and ruthenium doped strontium titanate catalysts these modes were obtained around 472, 514 and 740 cm⁻¹, respectively.

Fig. 3 (A) Raman spectra of bare TiO₂ (B) Raman spectra of (a) ST, (b) NiST and (c) RuST.

3.1.5 FTIR spectroscopy. The FTIR spectra of the bare and metal doped titania and titanate catalysts are shown in Fig. ESI1.† The strong peaks around 650 cm⁻¹ are due to the crystal lattice vibrations of Ti–O–Ti within the TiO₆ octahedra in both the titania and titanate catalysts, whereas the other peaks, which appear around 1400–1600 cm⁻¹ and 3400 cm⁻¹ are due to the bending and stretching vibrations of adsorbed water molecules.^58–61

3.1.6 Scanning electron microscopy with energy dispersive spectroscopy. SEM images generally give information about the shape and size and EDS tells about the composition of the synthesized catalysts. SEM images of the catalysts are shown in Fig. 4(a)–(d) and their respective EDS are shown in Fig. 4(e)–(h). Both bare and metal doped titanate catalysts show particles of different shapes and sizes. However, the shape and size of the synthesized titania were found to be almost same. All of the samples show peaks between 4.5–5 keV due to the presence of Ti in them. The presence of Sr in the strontium titanate catalysts are confirmed by the peaks between 1.5–2 keV. Nickel and ruthenium metal ions show peaks in the regions 7.5–8.4 keV and 2.3–2.9 keV, respectively.^62–65

Fig. 4 SEM images of (a) bare TiO₂, (b) ST, (c) NiST and (d) RuST; EDS spectra of (e) bare TiO₂, (f) ST, (g) NiST and (h) RuST.

3.1.7 Transmission electron microscopy. The TEM images of the titania, strontium titanate, nickel strontium titanate and ruthenium strontium titanate catalysts are shown in Fig. 5(a)–(f). The sol–gel synthesis of titania yielded particles with a mean diameter of 20 nm (Fig. 5(a)), whereas the TEM images of the metal doped strontium titanates show particles with a particle size of 5 nm (Fig. 5(c)–(f)). Fig. 5(b) and (d) show the fringe patterns of titania and ST, respectively. The inter planar distances (d) were calculated for both titania (1 0 1) and strontium titanate (1 1 0) and were found to be 0.35 and 0.28 nm, respectively. The insets in Fig. 5(b) and (d) show the SAED patterns of these two catalysts, which confirm their crystallinity. The TEM images also revealed that the incorporation of Ni and Ru into the SrTiO₃ lattice did not alter the morphology. Similar observation was made elsewhere.^66,67

Fig. 5 TEM images of (a) and (b) bare TiO₂, (c) and (d) ST, (e) NiST (f) RuST, insets show their corresponding SAED patterns.

3.1.8 X-ray photoelectron spectroscopy. The composition of the elements and their oxidation states can be confirmed by the XPS spectra. Fig. 6(a)–(c) show the overall spectra of the strontium titanate, nickel/ruthenium strontium titanate catalysts and Fig. 6(d)–(h) show the individual spectra of the elements present in the synthesized catalysts. The XPS overall spectra indicate the presence of all the components of strontium titanate (Sr, Ti, O) including the carbon from adsorbed gaseous molecules.⁶⁸ The binding energies are 458.1, 458.6, 269.1 and 357.6 eV for Ti (2p_1/2), Ti (2p_3/2), Sr (2p) and Sr (3s), respectively.⁶⁹ The binding energy at 529.6 eV is for the O²⁻ ions (Sr–O–Ti) of the SrTiO₃ frame work. The characteristic Sr 3d doublet line located around 132–134 eV confirms the presence of strontium as Sr²⁺ in the strontium titanate catalyst. The Ni (2p_3/2) peak was observed at 855.8 eV, which confirms the presence of nickel and it exists in the form of Ni₂O₃.⁷⁰ The binding energy located at 280.6 eV is due to the presence of ruthenium in Ru (3d_5/2).⁷¹

Fig. 6 Overall XPS spectra of (a) ST, (b) NiST, (c) RuST and individual XPS spectra of (d)–(h) Sr, Ni, Ru, Ti and O.

3.2 Photocatalytic studies – optimization of reaction parameters in the decolourisation of CR

The photocatalytic activity of the synthesized titania catalyst was evaluated towards the decolourisation of congo red. Studies on the effect of initial concentration, effect of catalyst dosage and effect of pH on the decolourisation of congo red were conducted to optimize the reaction parameters.

3.2.1 Effect of initial concentration on the % decolourisation of CR. The influence of the initial CR concentration on the % decolourisation was studied by varying the concentration from 1 × 10⁻⁴ M to 3 × 10⁻⁴ M at the constant catalyst weight of 250 mg at the natural pH of CR. The results obtained are shown in Fig. 7(a). It was observed that the % decolourisation increased with the increase in concentration of CR upto 2.5 × 10⁻⁴ M and on further increasing the concentration to 3 × 10⁻⁴ M, the % decolourisation decreased. The initial increase in the % decolourisation of CR with the increase in the concentration may be attributed to the availability of more OH˙ radicals on the catalyst surface than required. However, the increase in the concentration of the dye further results in a higher number of dye molecules, which outnumber the OH˙ radicals, and hence there is a reduction in the catalytic activity. In addition, the active sites of the catalyst are covered by dye molecules at the higher concentration, thus preventing the passage of light from reaching the surface of the catalyst (screening effect).⁷² Hence, the concentration at which maximum decolourisation occured was optimized at 2.5 × 10⁻⁴ M.

Fig. 7 Effect of (a) initial concentration of CR (b) catalyst weight (TiO₂) and (c) pH on % decolourisation of CR.

3.2.2 Effect of catalyst weight on % decolourisation of CR. By keeping the initial concentration of dye constant at 2.5 × 10⁻⁴ M, the photocatalytic experiments were carried out at the natural pH of the dye solution by varying the weight of the catalyst from 50 to 300 mg. The results obtained in the experiment are shown in Fig. 7(b). Congo red was decolourised to different extents (78–100%) at the end of 6 1/2 h when the catalyst weight was varied from 50 to 300 mg. The % decolourisation of congo red increased and reached 100% when the weight of the catalyst was 250 mg and on further increasing the weight to 300 mg a slight decrease in the decolourisation (93%) was observed. A large difference from 48%–85% in the % decolourisation was observed at 2 1/2 h, when the weight of the catalyst was increased from 50 to 250 mg indicating the faster decolourisation of congo red at 250 mg. This may be due to the availability of a number of active sites and ˙OH radical formation with the higher catalyst loading. However, on further increasing the catalyst weight to 300 mg, the % decolourisation decreased, which may be attributed to the increase in the opacity of the suspension leading to poor penetration of light into the solution. Venkatachalam et al. also observed a similar decrease in the % decolourisation of organic pollutant at higher catalyst loading and attributed it to both the increased turbidity and deactivation of activated species by collision with ground state species.⁷³

3.2.3 Effect of pH on percentage decolourisation. As the pH of the dye solution has a profound influence on the adsorption, dissociation of the substrate, catalyst surface charge, oxidation potential of the valence band and other physico-chemical properties of the system, the pH of the dye solution was varied from 3–11 and the experiments were conducted by keeping the initial concentration of the dye at 2.5 × 10⁻⁴ M using 250 mg of TiO₂ and the results obtained at 210 min are shown in Fig. 7(c). Varying the solution pH from 3–7, the % decolourisation was increased, reaching a maximum (85.8%) at pH 7.3 and at higher pH values the % decolourisation decreased. The behaviour of the photocatalyst at different pH can be explained on the basis of the point of zero charge (pzc). When the pH of the dye solution is less than the pH_pzc of titania, the surface of titania is enriched with positive charges and when the pH of the dye solution is higher than the pH_pzc of titania it becomes negatively charged. In this case the pH_pzc of titania is 6.8, hence, it is positively charged in acidic medium (pH < 6.8) and negatively charged in alkaline medium (pH > 7). Because CR contains two SO₃⁻ groups it strongly adsorbs on the positively charged photocatalysts in highly acidic conditions due to electrostatic attractions, thus, preventing light interacting with the catalyst surface, hence, about 10% decrease in decolourisation of congo red was observed at pH-3. However, at a higher pH, about 15% decrease in the decolourisation was observed. This may be due to the electrostatic repulsion taking place between the negatively charged catalysts and the [dye – SO₃]⁻ ions. In addition, the OH⁻ ions, which are responsible for the formation of OH˙ radicals are also repelled by the negatively charged catalysts. The pH at which the maximum decolourisation of congo red was observed was at its natural pH (7.3), hence, further experiments were carried out at pH 7.3.^35,74

3.2.4 HPLC studies on the degradation of CR. HPLC analysis was carried out to examine whether any stable intermediates are formed in the photocatalytic degradation of CR over the titania catalyst during the photocatalytic reaction. The samples collected upto 390 min were subjected to HPLC analysis and the chromatograms are shown in Fig. 8(A). From the chromatogram it is implied that congo red is eluted around 8 min. The intensity of this peak was maximum for the sample collected at 0^th min and it decreased with time on stream. This shows that CR is degraded with time. However, the peak around 8 min did not vanish for the sample collected at 390 min where decolourisation was found to be 100%. This implies that not all the dye molecules, which are decolourised are degraded. This was well supported by the TOC analyses for the sample collected at 390 min (only 32% TOC reduction). The absence of new peaks for the samples collected during the reaction indicates that no other stable intermediates are formed.

Fig. 8 High performance liquid chromatograms of CR (A) obtained at different irradiation times over TiO₂ and (B) over NiST at 240 min [reaction conditions: [CR] = 2.5 × 10⁻⁴ M, V = 100 mL and catalyst weight – (0.25 g)].

3.3 Evaluation of photocatalytic activities of bare and metal doped strontium titanate catalysts

The photocatalytic activities of ST, NiST and RuST were evaluated towards the decolourisation of congo red under optimized conditions using UV irradiation. The experimental results obtained with titanate catalysts are compared with that of titania and shown in Fig. 9(A). All the catalysts completely decolourise congo red but at different reaction times. Among the catalysts NiST showed the highest activity as it decolourises CR in the shortest reaction time (5 h). Both metal doped strontium titanate catalysts showed better catalytic activity than their bare counterparts and titania catalysts. This shows that doping of metal into the perovskite lattice has a significant effect on photocatalytic activity. The efficiency of the catalyst in terms of complete decolourisation of CR was found to be in the following order:

NiST > RuST > TiO₂ > ST

Fig. 9 (A) and (B) Decolourisation profile and kinetics of decolourisation of CR (C) degradation of CR [reaction conditions: [CR] = 2.5 × 10⁻⁴ M, V = 100 mL, catalyst weight – (0.25 g), pH-natural pH].

HPLC analysis of the dye samples collected at 240 min in the degradation of CR over NiST is shown in Fig. 8(B). Similar to titania, NiST also did not form any intermediate in the degradation of CR as no new peaks were seen in the chromatogram.

3.3.1 Kinetic studies of the strontium titanate catalysts. Kinetic plots were drawn to find the order of the reaction and rate of decolourisation (Fig. 9(B)). The plots reveal that the decolourisation of CR followed pseudo first order kinetics. The rate constant values obtained from the plots also support the earlier observation that NiST is the best of all the catalysts synthesized and were found to be in accordance with the efficiency order discussed above (Table 1).

3.3.2 Photo mineralisation of congo red. Because different catalysts completely decolourise CR at different times, the extent of mineralisation of CR was determined by a TOC analyser at the respective time of complete decolourisation for each catalyst and shown in Fig. 9(C). Both the bare and metal doped strontium titanate catalysts were found to be very effective in the mineralisation of congo red. With the metal doped strontium titanate showing higher degradation efficiency than the bare catalyst. Similar to decolourisation, NiST showed the maximum mineralisation of CR (46%) followed by RuST and ST. The % degradation values were found to be lower when compared to % decolourisation indicating that all the CR molecules, which are completely decolourised are not mineralised. This shows that photocatalysts break the chromophoric azo groups leading to decolourisation. The fragments so obtained undergo degradation to different extents.

Fig. 10 shows the spectral changes during congo red photodegradation by NiST (0.25 g/100 mL) at its natural pH at different reaction times. The absorption spectrum of the CR dye solution showed three characteristic absorption peaks at 240 nm, 338 nm and 497 nm. The main absorption peak at 497 nm belongs to the nitrogen to nitrogen double bond (–N=N–) i.e. for the azo chromophore (n–π* transition), which is responsible for the red colour of congo red. The other two peaks at 240 nm and 338 nm belong to the structure of benzene (π–π* transition) and naphthalene. The absorption peak at 497 nm rapidly decreased with the increasing reaction time, and disappeared after 300 min indicating that the azo chromophores were destroyed. The peaks at 240 nm and 338 nm disappeared with time on stream, which confirms the destruction of both the benzene and naphthalene rings. The decrease in the TOC values also supports this observation. The absence of new peaks in the decolourisation with time on stream supports the fact that the fragmented species does not contain any chromophores (Fig. 10).

Fig. 10 UV-vis spectral profiles of CR at various reaction times and the inset shows the CR samples withdrawn at different intervals of time.

3.3.3 Effluent studies. The efficiency of the best catalyst (NiST) was tested under UV irradiation for the decolourisation of textile effluent collected from an industry (dyeing and printing unit, Erode, Tamilnadu, India). As Erode (Tamilnadu, India), has a large number of dyeing industries located close to the rivers, such as Bhavani, Noyyal and Amaravathi, this area was chosen for the collection of effluent. Waste water was collected from the flow equalization tank. The colour of the effluent was deep red to black. The effluent characteristics were studied, and the intense colour of the effluent, TDS and TOC values reveal that the waste water is highly polluted and needs treatment before discharge. Hence, the photodecolourisation of the effluent (100 mL) collected from the industry was studied under UV irradiation using NiST (500 mg). The visible spectra of the effluent treated up to 510 min are shown in Fig. 11. The broadness of the visible spectra of the effluent indicates that it contains large number of unknown species including dyes. However, the synthesized catalyst (NiST) showed a positive decolourisation of the unknown dyes with time as evidenced by the decrease in the absorption values at 560 nm and 625 nm. After 510 min, the peaks at 560 and 625 nm completely disappeared, indicating the high activity of NiST towards the decolourisation of such highly concentrated dark effluent. The inset shows the decrease in the colour with time. The sample collected after 510 min, was subjected to effluent analysis. A reduction was shown in the values of TDS from 6990 to 4230 ppm, turbidity from 250 to 60 NTU and TOC from 2630 to 2177 ppm for the treated sample, which indicate the effectiveness of the photocatalyst.

Fig. 11 UV-vis spectra showing the photocatalytic decolourisation of textile effluent at various reaction times over NiST and the inset shows the effluent samples withdrawn at different intervals.

3.3.4 Decolourisation of CR under solar and visible irradiation. Solar and visible irradiation experiments were carried out under optimized reaction parameters using all of the synthesized strontium titanate catalysts and compared with that of titania (Fig. 12A and B). Because the activity under visible irradiation will be less than under sunlight (includes 4% UV) and UV irradiation the initial concentration of the dye was reduced 10 fold in the photocatalytic decolourisation of CR under visible irradiation. Here also the metal doped strontium titanate catalysts showed higher catalytic activity than the bare SrTiO₃ and TiO₂ catalysts. All of the synthesized catalysts showed significant activities both under solar and visible irradiations. However, the decolourisation of CR took a slightly longer time under solar irradiation than under UV-irradiation for all the catalysts. Among the catalysts, NiST quickly decolourised congo red (5 1/2 h) when compared to the other catalysts. Similar observation was also noticed under visible irradiation. Although a direct comparison could not be made with the time taken for complete decolourisation between the UV/solar and visible studies, the results indicate that a large difference in the % decolourisation of CR was observed between the metal doped SrTiO₃ and bare titanates. Both bare titania and SrTiO₃ are known for poor activity in the visible region due to their wide bandgaps. However doping of metals such as Ni and Ru into the lattice creates additional donor levels near the valence band, and thus reducing the bandgap between the CB and VB (1.9 eV and 2.13 eV for NiST and RuST respectively). Between the metal doped strontium titanates, the nickel doped strontium titanate showed higher catalytic activity than ruthenium doped strontium titanate. Doping of Ni into the SrTiO₃ lattice partially replaces the Sr ions, which forms the Ni_xSr_1−xTiO₃ species, whereas doping of Ru results in the formation of the SrRu_xTi_1−xO₃ species. The lower activity of RuST could be explained as follows: doping of ruthenium takes place at the photo catalytically active titanium sites, thereby reducing the number of active sites of titania distorting the lattice and creating defects. Such defects may act as recombination centres for excitons.^47,75–77

Fig. 12 (A) Decolourisation of CR under sunlight irradiation and (B) under visible irradiation [reaction conditions: [CR] = 2.5 × 10⁻⁴ M for sunlight studies, [CR] = 2.5 × 10⁻⁵ M for visible studies, V = 100 mL, catalyst weight −0.25 g, pH-natural pH].

3.3.5 Recyclability of catalysts. Generally in photocatalysis, strong chemisorption of either dye molecules or intermediates or products may result in significant decrease or even complete loss of photocatalytic activity. Hence, the most active catalyst (NiST) was subjected to a recyclability test for the photodecolourisation of CR. The catalyst was filtered after every cycle, and subjected to further decolourisation with a fresh solution of CR [2.5 × 10⁻⁴ M] and the results are shown in Fig. ESI2.† The results indicate that not much reduction in % decolourisation was observed after recycling the catalyst four times, thus showing the high photo stability of the catalyst.

4. Conclusions

The doping of nickel or ruthenium into the strontium titanate lattice shifted the optical response to the visible region. TEM measurements and EDS spectra confirmed the presence of the doped metals. Oxidation states of the elements present in the catalyst were understood by XPS. Among all the catalysts, the nickel strontium titanate catalyst showed 100% decolourisation towards CR under UV, visible and sunlight irradiation but at different reaction times. The kinetic plots indicated that the decolourisation of CR followed pseudo first order kinetics. TOC results revealed that 46% of the dye was degraded. The best catalyst tested with the original textile effluent showed considerable mineralisation.

Acknowledgements

One of the authors Amala Infant Joice Joseph, thank CSIR, New Delhi, India for the award of Senior Research Fellowship.

References

1. K. Yu, S. Yang, C. Liu, H. Chen, H. Li, C. Sun and S. A. Boyd, Environ. Sci. Technol., 2012, 46, 7318–7326.
2. P. Bansal and D. Sud, Desalination, 2011, 267, 244–249.
3. P. A. Carneiro, R. F. Pupa Nogueira and M. V. B. Zanoni, Dyes Pigm., 2007, 74, 127–132.
4. J. Amala Infant Joice, R. Ramakrishnan, G. Ramya and T. Sivakumar, Chem. Eng. J., 2012, 210, 385–397.
5. A. Baan, A. Yodeller, D. Liner, N. Kemmerer and A. Ketchup, Dyes Pigm., 2003, 58, 93–98.
6. S. Zhang, X. Liu, M. Wang, B. Pan and H. Yang, Environ. Sci. Technol. Lett., 2014, 1, 167–171.
7. A. V. Rupa, D. Divakar and T. Sivakumar, Catal. Lett., 2009, 132, 259–267.
8. G. H. Moon, D. H. Kim, H. I. Kim, A. D. Bokare and W. Choi, Environ. Sci. Technol. Lett., 2014, 1, 185–190.
9. D. D. Dionysiou, A. P. Khodadoust, A. M. Kern, M. T. Suidan, I. Baudin and J. M. Laine, Appl. Catal., B, 2000, 24, 139–155.
10. A. V. Rupa, D. Manikandan, D. Divakar and T. Sivakumar, J. Hazard. Mater., 2007, 147, 906–913.
11. N. Venkatachalam, M. Palanichamy, B. Arabindoo and V. Murugesan, J. Mol. Catal. A: Chem., 2007, 266, 158–165.
12. G. A. Suganya Josephine and A. Sivasamy, Environ. Sci. Technol. Lett., 2014, 1, 172–178.
13. Y. Chen, E. Stathatos and D. D. Dionysiou, Surf. Coat. Technol., 2008, 202, 1944–1950.
14. K. Sayama, H. Hayashi, T. Arai, M. Yanagida, T. Gunji and H. Sugihara, Appl. Catal., B, 2010, 94, 150–157.
15. L. Yue and X. M. Zhang, J. Alloys Compd., 2009, 475, 702–705.
16. X. J. Wang, F. Q. Wan, K. Han, C. X. Chai and K. Jiang, Mater. Charact., 2008, 59, 1765–1770.
17. S. A. Singh and G. Madras, Sep. Purif. Technol., 2013, 105, 79–89.
18. Z. Liu, Y. Li, C. Liu, J. Ya, W. Zhao, D. Zhao and L. An, ACS. Appl. Mater. Interfaces, 2011, 3, 1721–1725.
19. K. Lee and G. Cao, J. Phys. Chem. B, 2005, 109, 11880–11885.
20. Y. Huang, Y. Wei, S. Cheng, L. Fan, Y. Li, J. Lin and J. Wu, Sol. Energy Mater. Sol. Cells, 2010, 94, 761–766.
21. X. Zhou, J. Shi and C. Li, J. Phys. Chem. C, 2011, 115, 8305–8311.
22. D. R. Modeshia and R. I. Walton, Chem. Soc. Rev., 2010, 39, 4303–4325.
23. W. Dong, X. Li, J. Yu, W. Guo, B. Li, L. Tan, C. Li, J. Shi and G. Wang, Mater. Lett., 2012, 67, 131–134.
24. J. C. Yu, L. Zhang, Q. Li, K. W. Kwong, A. W. Xu and J. Lin, Langmuir, 2003, 19, 7673–7675.
25. W. Jiang, X. Gong, Z. Chen, Y. Hu, X. Zhang and X. Gong, Ultrason. Sonochem., 2007, 14, 208–212.
26. Y. Liu, L. Xie, Y. Li, R. Yanga, J. Qu, Y. Li and X. Li, Power Sources, 2008, 183, 701–707.
27. U. Sulaeman, S. Yin and T. Sato, Appl. Catal., B, 2011, 105, 206–210.
28. K. Guo, Z. Liu, Y. Wang, Y. Zhao, Y. Xiao, J. Han, Y. Li, B. Wang and T. Cui, Int. J. Hydrogen Energy, 2014, 39, 13408–13414.
29. J. Zhang, J. Bang, C. Tang and P. V. Kamat, ACS Nano, 2010, 4, 387–395.
30. H. F. Liu, Solid State Commun., 2012, 152, 2063–2065.
31. A. D. Paola, E. G. Lopez, G. Marci and L. Palmisan, J. Hazard. Mater., 2010, 211, 3–29.
32. J. Wang, S. Yin, Q. Zhang, F. Saito and T. Sato, J. Mater. Chem., 2003, 13, 2348–2352.
33. C.-H. Chang and Y.-H. Shen, Mater. Lett., 2006, 60, 129–132.
34. J. Wang, S. Yin, M. Komatsu and T. Sato, J. Eur. Ceram. Soc., 2005, 25, 3207–3212.
35. R. Vaithiyanathan and T. Sivakumar, Water Sci. Technol., 2011, 63, 377–384.
36. R. Ramakrishnan, S. Kalaivani, J. Amala Infant Joice and T. Sivakumar, Appl. Surf. Sci., 2012, 258, 2515–2521.
37. A. V. Rupa, R. Vaithiyanathan and T. Sivakumar, Water Sci. Technol., 2011, 64, 1040–1045.
38. J. Zheng, Y. Zhu, J. Xu, B. Lu, C. Qi, F. Chen and J. Wu, Mater. Lett., 2013, 100, 62–65.
39. H. Irie, Y. Maruyama and K. Hashimoto, J. Phys. Chem. C, 2007, 111, 1847–1852.
40. V. Subramanian, R. K. Roeder and E. E. Wolf, Ind. Eng. Chem. Res., 2006, 45, 2187–2193.
41. V. A. Sakkas, M. A. Islam, C. Stalikas and T. A. Albanis, J. Hazard. Mater., 2010, 175, 33–44.
42. H. Lachheb, E. Puzenat, A. Houas, M. Ksibi, E. Elaloui, C. Guillard and J. M. Herrmann, Appl. Catal., B, 2002, 39, 75–90.
43. Z. Zhang, Y. Shan, J. Wang, H. Ling, S. Zhang, W. Gao, Z. Zhao and H. Zhang, J. Hazard. Mater., 2007, 147, 325–333.
44. R. Ananthashankar and A. Ghaly, Am. J. Engg. Appl. Sci., 2013, 3, 252–262.
45. L. Yun-cang, Z. Lin-da and E. Hu, J. Environ. Sci., 2004, 3, 375–379.
46. E. Sanchenz and T. Lopez, Mater. Lett., 1995, 25, 271–275.
47. A. Jia, Z. Su, L. L. Lou and S. Liu, Solid State Sci., 2010, 12, 1140–1145.
48. I. K. Battisha, A. Speghini, S. Polizzi, F. Agnoli and M. Bettinelli, Mater. Lett., 2002, 57, 183–187.
49. C. N. George, J. K. Thomas, R. Jose, H. Padmakumar, M. K. Suresh, V. Ratheesh Kumar, P. R. Shobana Wariar and J. Koshy, J. Alloys Compd., 2009, 486, 711–715.
50. Y. Wu, J. Zhang, L. Xiao and F. Chen, Appl. Catal., B, 2009, 88, 525–532.
51. B. W. Faughnan, Photochromism in Transition-metal-doped SrTiO₃, Phys. Rev. B: Condens. Matter Mater. Phys., 1971, 4, 3623–3636.
52. R. Niishiro, H. Kato and A. Kudo, Phys. Chem. Chem. Phys., 2005, 7, 2241–2245.
53. D. Wang, J. Ye, T. Kako and T. Kimura, J. Phys. Chem. B, 2006, 110, 15824–15830.
54. M. Miyauchi, M. Takashio and H. Tobimatsu, Langmuir, 2004, 20, 232–236.
55. X. Chen and S. S. Mao, Chem. Rev., 2007, 107, 2891–2959.
56. Y. L. Du, G. Chen and M. S. Zhang, Solid State Commun., 2004, 130, 577–580.
57. W. Luo, Z. Li, X. Jiang, T. Yu, L. Liu, X. Chen, J. Yead and Z. Zou, Phys. Chem. Chem. Phys., 2008, 10, 6717–6723.
58. T. Bezrodna, G. Puchkovska, V. Shimanovska and J. Baran, Mol. Cryst. Liq. Cryst., 2004, 13, 71–80.
59. O. Liang, X. Shen, F. Song and M. Liu, J. Mater. Sci. Technol., 2011, 27, 996–1000.
60. T. Ohno, T. Tsubota, Y. Nakamura and K. Sayama, Appl. Catal., A, 2005, 288, 74–79.
61. A. Z. Simoes, F. Moura, T. B. Onofre, M. A. Ramirez, J. A. Varela and E. Longo, J. Alloys Compd., 2010, 508, 620–624.
62. T. Tsumura, K. Sogabe and M. Toyoda, Mater. Sci. Eng. B, 2009, 157, 113–115.
63. M. Yousefpour and A. Shokuhy, Superlattices Microstruct., 2012, 51, 842–853.
64. H. Tseng, M. C. Wei, S. Hsiung and C. Chiou, Chem. Eng. J., 2009, 150, 160–167.
65. V. Houskova, V. Stengl, S. Bakardjieva, N. Murafa and V. Tyrpekl, Appl. Catal., B, 2009, 89, 613–619.
66. D. N. Bui, J. Mu, L. Wang, S. Z. Kang and X. Li, Appl. Surf. Sci., 2013, 274, 328–333.
67. T. Setinc, M. Spreitzer, D. Vengust, I. Jerman and D. Suvorov, Ceram. Int., 2013, 39, 6727–6734.
68. H. L. Li, Z. N. Du, G. L. Wang and Y. C. Zhang, Mater. Lett., 2010, 64, 43–434.
69. B. Huang, E. Su and M. Wey, Chem. Eng. J., 2013, 223, 854–859.
70. N. S. Mclntyre and M. G. Cook, Anal. Chem., 1975, 47, 2208–2213.
71. M. A. Khan, D. H. Han and O. Yang, Appl. Surf. Sci., 2009, 255, 3687–3690.
72. L. Kumaresan, A. Prabhu, M. Planichamy and V. Murugesan, Mater. Chem. Phys., 2011, 126, 445–452.
73. N. Venkatachalam, M. Palanichamy, B. Arabindoo and V. Murugesan, J. Mol. Catal. A: Chem., 2007, 266, 158–165.
74. U. G. Akpan and B. H. Hameed, Chem. Eng. J., 2011, 169, 91–99.
75. A. Kudo, R. Niishiro, A. Iwase and H. Kato, Chem. Phys., 2007, 339, 104–110.
76. H. Li, S. Yin, Y. Wang, T. Sekino, S. W. Lee and T. Sato, J. Catal., 2013, 297, 65–69.
77. T. Ohno, F. Tanigawa, K. Fujihara, S. Izumi and M. Matsumura, J. Photochem. Photobiol., A, 1999, 127, 107–110.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra14722g

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