Photocatalytic degradation of Amaranth and Brilliant Blue FCF dyes using in situ modified tungsten doped TiO₂ hybrid nanoparticles

Abstract

This study focuses on the process of photocatalytic degradation of popular dyes like Amaranth and Brilliant Blue, using reagent grade TiO₂ and in situ modified tungsten doped TiO₂ hybrid nanoparticles. One of the drawbacks of nanoparticles is their agglomeration and poor dispersion in the medium as well as their limited activity in the ultra violet region, which makes them less efficient. In order to overcome such drawbacks, for the first time, in situ surface modification and doping of TiO₂ nanoparticles were carried out employing n-butylamine as surface modifier and tungsten oxide as dopant. Modification was conducted under mild hydrothermal conditions (T = 150 °C, P = autogenous). Nanoparticles obtained were characterized using Powder XRD, FTIR, DLS, Zeta potential, UV-Vis spectroscopy, and TEM. The characterization indicated the desired results with respect to morphology, particle size distribution and less agglomeration. The results of the process of photodegradation of Brilliant Blue FCF and Amaranth dyes showed a higher efficiency for in situ modified tungsten doped TiO₂ hybrid nanoparticles than for reagent grade TiO₂.

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1. Introduction

Photodegradation of organic and inorganic pollutants using semiconductor metal oxides has been a very popular topic of research in recent years. Dyes form an important class of aquatic pollutants, which have begun to prove a major source of environmental contamination.¹ More than 30 000 commercial dyes based on ca. 8000 different chemical structures are widely used in the textile, food, pharmaceutical, paper and ink industries.² These dyes are usually discharged into water bodies without complete or partial treatment which can reduce the auto-purification capacity of the receiving water bodies and lead to eutrophication. The textile industry is the largest consumer of dyes (60%), using them in conjunction with a wide range of auxiliary reagents for various dyeing, printing and finishing processes. Textile industrial effluents, irrespective of dye or fiber class, are invariably deeply colored by unabsorbed dyes, the extent of which varies according to the dye-fiber system.³

Over the period of the last few decades, the increasing industrial demand for dyes has proved to possess a high pollutant potential, especially the use of azo dyes such as Amaranth and Brilliant Blue FCF. The decolouration of water is the major indicator of its quality.

In industries, the process of dyeing results in the production of large amounts of wastewater that exhibit intense colouration. The extent of colouration has to be radically reduced before the effluents are released into natural water streams. Wastewater released from dye-using industries contains different types of synthetic dyes, which are mostly toxic, mutagenic and carcinogenic. Moreover, they are very stable to light and microbial attack, which makes them recalcitrant compounds.⁴ Amaranth—an anionic dye made of tar—is a dark red to purple azo dye that is predominantly used to colour cosmetics. It could be used to colour natural and synthetic fibers, leather, paper, and phenol–formaldehyde resins. Brilliant Blue FCF is often found in ice creams, tinned processed peas, dairy products, sweets and drinks. It is also used in soaps, shampoos, and other hygiene and cosmetics applications. Table 1 shows the characteristics of these two dyes.

Table 1 Characteristics of two commercial dyes

Characteristics	Amaranth	Brilliant Blue FCF
Synonym	FD&C Red No. 2, E123, C.I. Food Red 9, Acid Red 27, Azorubin S, C.I. 16185	Acid Blue 9, Alzen Food Blue No.1, Atracid Blue FG, Erioglaucine, Eriosky blue
Color index no.	16185	42090
Chemical formula	C20H11N2Na3O10S3	C37H34N2Na2O9S3
M.W. (mol g^−1)	604.48	792.84
λ max (nm)	521	630
Structure

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An ideal waste treatment process should completely mineralize all the toxic species present in the waste stream totally removing all hazardous residues. Any efficient treatment process must be cost-effective. Most of the conventional treatments require subsequent treatments that are expensive.

To date, many kinds of semiconductor metal oxides have been studied as photocatalysts that include TiO₂, ZnO, WO₃, and so on. TiO₂ has been found to be the most widely used and most effective photocatalyst as it has a high degree of efficiency, photochemical stability, non-toxic nature and low cost. Some researchers have highlighted the performance of TiO₂ in the process of degradation of some organic compounds.^5,6

The photodegradation process, in general, occurs with the attack of organic substances activated by oxygen species. These substances include hydroxyl radicals and super oxide radicals, generated on TiO₂ particulate surface by the reduction of dissolved oxygen in solution and/or oxidation of surface hydroxyl groups using TiO₂.⁷

The photocatalytic efficiency of TiO₂ catalysts depends entirely upon the relative degree to which the reactive electron–hole pairs branch into interfacial charge transfer reactions. On absorption of a photon of energy greater than or equal to the band gap energy by TiO₂, an electron moves to a conduction band from a valence band. As a result, an electron gets distributed in the conduction band, creating an electron vacancy or “hole” that becomes distributed in the valence band. If charge separation is maintained, the electron and hole may migrate to the catalyst surface where they participate in redox reactions with adsorbed species. A hole and an electron range correspondingly and respectively at the valence band and conduction band by the irradiation of light. A hydroxyl radical (˙OH) and superoxide radical (˙O), consequently, come into being in reaction with water and oxygen present in the atmosphere. Therefore, the factor that limits the photocatalytic reaction is the process of recombination of the electron and the hole, preceding the superoxide activation step.^8–10 Thus, one of the most crucial measures lies in enhancing the interfacial charge-transfer reaction. Many research groups have recently improved this technology by implanting,¹¹ doping and depositing suitable transitional metal ions,¹² noble metals,¹³ semiconductor metallic oxides,¹⁴ and nonmetal ions¹⁵ into the TiO₂ structure.

A number of methods of TiO₂ nanoparticles preparation have been reported that range from sol–gel,¹⁶ to supercritical carbon dioxide,¹⁷ to photodeposition,¹⁸ and hydrothermal.¹⁹ The preparation of titanium dioxide nanoparticles is a challenging task as its properties greatly depend upon its size and morphology. The conventional synthesis routes yield highly agglomerated TiO₂ particles without any control over their morphology and size, and in some of the preparation processes require post-treatment in order to achieve crystalline products with the desired properties. Though hydrothermal and solvothermal techniques are more effective in obtaining high quality nanoparticles because of the highly-controlled diffusion during the synthesis process, the problem of size and morphology control cannot yet be achieved through normal routes. Hence, an appropriate capping agent or organic ligand or surfactant or chelating agent must be added to achieve control over the size and morphology of these TiO₂ particles, thereby allowing their properties to be effectively tailored.

The present paper deals with the modification of the surface of tungsten doped TiO₂ hybrid nanoparticles under mild hydrothermal conditions, using n-butylamine with different molar ratios. n-Butylamine was used as surface modifier because it has low toxicity, low density and a low melting temperature and is eco-friendly. The effect of the surfactant on the in situ modification of the doped TiO₂ hybrid nanoparticles, and the effect of doping and its percentage on the band gap energy that affects photodegradation efficiency of Amaranth and Brilliant Blue FCF, is discussed in detail in the following sections.

2. Experimental

2.1 Preparation of in situ modified tungsten doped TiO₂ hybrid nanoparticles

Tungsten doped TiO₂ hybrid nanoparticles were synthesized under mild hydrothermal conditions (T = 150 °C, P = autogeneous). 1 M of reagent grade TiO₂ (Loba Chemie, India) was taken as the starting material and the dopant (WO₃: 2 mol%; 5 mol%) was added to it. A certain amount of 1M HCl was added as a mineralizer to the precursors. At the same time different concentrations (0.8, 1.0, 1.2 and 1.4 M) of n-butylamine (Sisco Research Lab PVT, Ltd., Mumbai India, Assay (GC)) was added to the above mentioned mixture and was stirred vigorously for a few minutes. The final compound was then transferred to the Teflon liner (V_fill = 10 ml), which was later placed inside a General-Purpose autoclave. Then the assembled autoclave was kept in an oven with a temperature programmer-controller for 8–18 h. The temperature was kept at 150 °C. After the experimental run, the autoclave was cooled to room temperature. The product in the Teflon liner was transferred to a clean beaker, washed with doubly distilled water, and later the product was allowed to settle. The surplus solution was removed using a syringe and the remnants were centrifuged for 20 min at 1500 rpm. The recovered product was dried in a hot air oven at 40–50 °C for a few hours. The dried particles were subjected to a systematic characterization and photocatalytic studies.

2.2 Characterization of the in situ modified tungsten doped TiO₂ hybrid nanoparticles

The fabricated products were characterized using different analytical techniques. The Fourier Transform Infrared spectra were recorded using FTIR, JASCO-460 PLUS, Japan, at a resolution of 4 cm⁻¹. The Powder X-ray diffraction patterns were recorded using Bruker, D8 Advance, Germany, with Cu-Kα, λ = 1.542 Å radiation, voltage = 40 mV, current = 30 mA, scan speed 1.5° min⁻¹. The data were collected in the 2θ range 5–100°. The optical properties were studied by using UV-Vis spectrophotometer. Particle size and its distribution were measured using dynamic light scattering (DLS) (Horiba particle size analyzer, LB-550, Japan). Zeta potential was measured using Zetasizer 2000 instrument (Malvern instruments). TEM images of the tungsten(VI) oxide doped TiO₂ hybrid nanoparticles were recorded using JEM 2000FX II (JOEL. Ltd., Tokyo, Japan).

2.3 Photoreactor and experimental procedure

A cylindrical flow photoreactor that was designed and fabricated in our laboratory was used as shown in Fig. 1. At the center of this cylindrical vessel, a 6 W low-pressure mercury lamp placed inside a quartz sleeve with an emission peak at 264 nm (Philips, Netherlands) was positioned. This was surrounded by a circulating water jacket meant to control the temperature during reaction. In the case of visible light-irradiation, sunlight was used. The reaction suspension was prepared by adding different amounts (0.4–2 g) of photocatalyst powder per litre of dye solution. An external aerator was used as an oxygen source and air was continuously pumped into the reactor through air diffusers to fully fluidize the tungsten doped TiO₂ hybrid nanoparticles during the photoreaction. For the purpose of the application potential in practice, the initial pH was adjusted to 7.5 using 0.02 M HCl solution and 0.02 M NH₃OH solution before reaction. The tungsten doped TiO₂ hybrid nanoparticles were mixed with the Brilliant Blue and Amaranth solution respectively and were placed in darkness for 30 min to ensure balance of adsorption and desorption. The suspensions were sampled at specific intervals to monitor the changes of Brilliant Blue and Amaranth concentrations. Sampled suspensions were centrifuged at 1500 rpm for 30 min to remove the tungsten doped TiO₂ hybrid nanoparticles and then analyzed by UV-Vis spectrophotometer.

Fig. 1 Flow photoreactor.

3. Results and discussion

3.1 Characterization of in situ modified tungsten doped TiO₂ hybrid nanoparticles

The powder XRD results of the nanoparticles synthesized match well with the I4/amd space group. XRD data reveal that when compared to pure TiO₂, there is a slight change in the lattice parameters of tungsten doped TiO₂ nanoparticles (TiO₂–X, X = 2, 5 mol% WO₃) at the a-axis and c-axis. This confirms the existence of tungsten atoms in TiO₂ nanoparticles. The radii of W(VI) ion (60 pm) is bigger than that of Ti(VI) (42 pm) and is certain to make the cell parameter, on doping with W, bigger than that of pure TiO₂. The powder XRD patterns of (2 mol%; 5 mol%) tungsten doped TiO₂ hybrid nanoparticles reveal five primary peaks at 25.32°, 37.82°, 48.06°, 53.9° and 62.7° for 2 mol% dopant (Fig. 2a) and 25.36°, 37.84°, 48.08°, 55.01° and 62.72° for 5 mol% dopant (Fig. 2b). This can be attributed to different diffraction planes of TiO₂. The powder XRD data indicate that the cell volume of tungsten doped TiO₂ hybrid nanoparticles slightly increases with 2 mol% and 5 mol% tungsten doping (Table 2).²⁰

Fig. 2 Powder XRD pattern of tungsten doped TiO₂ nanoparticles modified using 1.0 M n-butylamine.

Table 2 Cell parameters of TiO₂

Catalyst	a/Å	c/Å	a : c ratio	V/Å^3	Ref.
Reagent grade TiO2	3.7845	9.5143	0.3977	136.27	20
Tungsten (2 mol%) doped TiO2	3.7859	9.5137	0.3979	136.360	Present work
Tungsten (5 mol%) doped TiO2	3.7874	9.5163	0.3980	136.506	Present work

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The functional groups present in the modified nanoparticles can be studied using FTIR spectroscopy. Fig. 3 shows the FTIR spectra of the reagent grade TiO₂ without modifier, reagent grade TiO₂ with modifier and tungsten doped TiO₂ nanoparticles modified with n-butylamine (0.8 M and 1.4 M) as a surface modifier. The intensity of the absorption peaks was stronger for 1.4 M surface modified tungsten doped TiO₂ nanoparticles compared to 0.8 M surface modified tungsten doped (2 mol% and 5 mol%) TiO₂ hybrid nanoparticles. The FTIR spectra of the modified hybrid nanoparticles show the presence of new peaks and this in turn suggests that the reagents were chemically immobilized on the surface of the nanoparticles. Thus, it can be concluded that the tungsten doped TiO₂ nanoparticles synthesized with the above mentioned modifier, have organic coverage on their surfaces, which changes the surface properties of the nanoparticles. The major vibrational modes of WO₃ are located at 808, 714 and 276 cm⁻¹ and are assigned to the W=O stretching mode, the W=O bending mode and W–O–W deformation mode, respectively, while the peak around 650 cm⁻¹ is representative of TiO₂ matrices.²¹ The peaks around 1503 and 3675 cm⁻¹ correspond to the presence of CH₃ and N–H stretching bonds. In addition, the new peaks around 2965 and 3400 cm⁻¹ correspond to O–H and NH₄⁺ stretching bond. The absorption peaks around 1638, 3600 and 3695 cm⁻¹ belong to the C=O stretching bond.²²

Fig. 3 FTIR spectra of reagent grade TiO₂ (), modified undoped TiO₂ hybrid nanoparticles (), tungsten doped TiO₂ hybrid nanoparticles modified with 0.8 M ([thick line, graph caption]), and 1.6 M n-butylamine (): (a) 2 mol% and (b) 5 mol% dopant.

Fig. 4 shows TEM images of 2 mol% and 5 mol% tungsten doped TiO₂ hybrid nanoparticles modified with (0.8 and 1.4 M) n-butylamine. It shows a thin organic coverage of the surface modifier on the synthesized nanoparticles. The agglomeration is less when a higher concentration of the surface modifier is used. The in situ surface modification leads to the controlling of growth direction and particle size, thus, preventing agglomeration. It is found that the surface modifier cannot only affect the dispensability of the synthesized tungsten doped TiO₂ hybrid nanoparticles, but also change their growth habit. The morphology attained is quite suitable for the photodegradation purposes; as the tungsten doped TiO₂ hybrid nanoparticles are rounded, they can be more active in photodegradation of the organic pollutants present in the industrial effluents.

Fig. 4 TEM image of tungsten doped TiO₂ hybrid nanoparticles: (a) 2 mol% tungsten, 0.8 M n-butylamine; (b) 5 mol% tungsten, 0.8 M n-butylamine, (c) 2 mol% tungsten, 1.4 M n-butylamine; (d) 5 mol% tungsten, 1.4 M n-butylamine.

Fig. 5 shows the particle size distribution of the tungsten doped TiO₂ hybrid nanoparticles synthesized confirming the nano-range of the particles. The range is narrower in the case of 2 mol% tungsten doped TiO₂ hybrid nanoparticles. The particle size in all cases is in the range 30–250 nm, with an average diameter of 112.9 nm in the case of 2 mol% tungsten doped TiO₂ hybrid nanoparticles modified with 1.4 M surfactant and 135.7 nm for 5 mol% tungsten doped TiO₂ hybrid nanoparticles modified with 1.4 M surfactant. These results are in agreement with powder XRD data and TEM images.

Fig. 5 Particle size distribution using DLS for (a) 2 mol% tungsten doped; and (b) 5 mol% tungsten doped TiO₂ hybrid nanoparticles modified using 1.4 M n-butylamine.

3.2 Zeta potential of in situ modified tungsten doped TiO₂ hybrid nanoparticles

Zeta (ζ) potential measurements were performed for in situ surface modified TiO₂ nanoparticles in order to characterize the surface charge of nanoparticles and Fig. 6 shows the results as a function of pH. The obtained ζ potential of the nanoparticles was found to decrease with increasing pH as is expected for a surface with acid–base groups. The iso-electric point or point of zero charge (PZC) for TiO₂ nanoparticles was found to be 4.2. The significance of the ζ potential is that its value can be related to the stability of colloidal dispersions. The ζ potential indicates the degree of repulsion between adjacent, similarly charged particles in dispersion. For small enough nanoparticles, a high ζ potential will confer stability, i.e. the solution or dispersion will resist aggregation. When the potential is low, attraction exceeds repulsion and the dispersion breaks and flocculates.^23–25 Therefore, colloids with high ζ potential (negative or positive) are electrically stabilized while colloids with low ζ potentials tend to coagulate or flocculate. As Fig. 6 indicates, ζ potential for the surface modified TiO₂ nanoparticles is −40 meV at pH 12, which indicates its stability. This property is quite suitable for photodegradation application of the synthesized nanoparticles.

Fig. 6 Zeta potential of TiO₂ nanoparticles modified with n-buytlamine as surfactant.

3.3 Band gap energy of in situ modified tungsten doped TiO₂ hybrid nanoparticles

The effect of doping was studied by UV-Vis spectroscopy in the range of 200–600 nm. Fig. 7 shows that pure TiO₂ had no absorption in the visible light region (λ > 400 nm). The in situ modified tungsten doped TiO₂ hybrid nanoparticles showed a remarkable absorption band shift toward longer wavelength region, which indicates a decrease of the band gap energy. In addition, according to the band edges of the samples determined from differentiation of the three curves, the band gap of the three samples are calculated according to the equation EG = hc/λ, where EG is the band gap energy (eV), h is Plank's constant, c is the speed of light (m s⁻¹), and λ is the wavelength (nm). The EG of pure TiO₂, 2 mol% tungsten doped TiO₂ hybrid nanoparticles and 5 mol% tungsten doped TiO₂ hybrid nanoparticles are 3.32, 3.20 and 3.03, respectively.

Fig. 7 Effect of doping on band gap energy of the modified tungsten doped TiO₂ nanoparticles: reagent grade TiO₂ (), 2 mol% tungsten doped () and 5 mol% tungsten doped () TiO₂ nanoparticles modified with n-butylamine.

3.4 Photodegradation of Brilliant Blue FCF and Amaranth dyes

The absorption of Brilliant Blue FCF and Amaranth dyes (Loba Chemie) was measured at 630 nm and 520 nm (λ_max) of the solutions after photodegradation, respectively. The absorption was converted to relative concentration of the photodegraded dyes (ln C/C₀), where C and C₀ represent concentration after reaction and initial concentration, referring to a standard curve and the displayed linear behavior between relative concentration and absorption at these wavelengths, respectively. For comparison, the photocatalysis of Brilliant Blue and Amaranth dyes on pure TiO₂ powders from Loba Chemie were conducted under the same operating conditions.

3.4.1 Effect of modified tungsten doped TiO₂ hybrid nanoparticles on photocatalytic degradation of Brilliant Blue FCF and Amaranth dyes. In a slurry photocatalytic process, catalyst dosage is an important parameter. Photocatalytic degradation of 2 mg L⁻¹ of the Brilliant Blue and Amaranth dyes was carried out with modified tungsten (2 mol%; 5 mol%) doped TiO₂ hybrid nanoparticles and reagent grade TiO₂ catalyst loading of 0–1.6 g L⁻¹ under UV and sunlight-irradiation, respectively. The process of the degradation of these dyes as a function of fabricated TiO₂ nanoparticles and pure TiO₂ dosage is presented in Fig. 8.

Fig. 8 Photodegradation of dyes using 5 mol% tungsten doped; 2 mol% tungsten doped TiO₂ nanoparticles modified with n-butylamine and [thick line, graph caption] without catalyst.

As shown in the Fig. 8, in the absence of TiO₂, the removal percentage of these dyes is almost zero. The addition of nanoparticles enhances the removal of both contaminants with different speeds. The removal rate of these contaminants increases as the concentration of the dyes increases. It should be mentioned that there was no difference in the extent of degradation based on the light source in the case of reagent grade TiO₂ but as this figure indicates, the degradation also occurred in visible light, which confirms the effect of doping on photodegradation of these dyes. An optimal result was achieved at a TiO₂ dosage of 1.4 g L⁻¹. The removal decreases with a further increase in the catalyst dosage.

In analyzing the kinetic data of photodegradiation, mediated by semiconductors or nanoparticles like TiO₂, the data are analyzed to a simple rate expression of Langmuir–Hinshelwood (L–H) form²⁶ as follows:

where C₀, K_Ad and k are the initial concentration of dye, the pseudo-first-order L–H-type adsorption coefficient and the reaction rate constant, respectively. The integration of eqn (1) yields eqn (2) as follows:The integrated form of eqn (1) for a low initial concentration of Brilliant Blue FCF and Amaranth of this study can be written as follows:Where k_app is the apparent reaction rate constant and t is the reaction time. Rate constant k_app has been chosen as the basic kinetic parameter for different systems, since it is independent of used concentration. It can be determined from the slope of the curve obtained. The rate constants are 0.075 min⁻¹ and 0.063 min⁻¹ for Brilliant Blue FCF and Amaranth, respectively.²⁷

3.4.2 Effect of light source and its duration on photocatalytic degradation of Brilliant Blue and Amaranth. Fig. 9 shows the comparison of photodegradation of Brilliant Blue FCF and Amaranth dyes at different moment in the presence of modified tungsten (2 mol%; 5 mol%) doped TiO₂ hybrid nanoparticles under sun light and UV irradiation. The photodegradation ratio in the presence of modified nanoparticles increases along with irradiation time and attains about 93.25 and 74.39%, respectively, within 3.0 h sunlight irradiation. In the case of UV irradiation, the efficiency is 70.14 and 42.90%. The results indicate the photocatalytic degradation reaction is a pseudo-first-order kinetic reaction in all cases.

Fig. 9 Effect of light source: (a) sun light and (b) UV light on photodegradation of dyes using 5 mol% tungsten doped (); 2 mol% tungsten doped () TiO₂ nanoparticles modified with n-butylamine and reagent grade TiO₂ ().

3.4.4 Effect of surfactant concentration on photocatalytic degradation. There are a variety of organic ligands, which can be used for surface modification of TiO₂, ZnO, CdS and other particles. We found that even if a ligand is non-toxic, eco-friendly, and has a suitable melting point, etc., its density and concentration plays a critical role in determining its suitability. The density of the surfactant should be less than that of water and the precursors otherwise its removal from the prepared nanoparticles will be very difficult. Fig. 10 shows the effect of surfactant concentration on the photodegradation of effluents. As it indicates the optimum concentration of the surfactant is found to be 1.0 M for in situ modification of the tungsten TiO₂ hybrid nanoparticles. A higher concentration of surfactant in the reaction mixture might give the desired morphology and particle size, but it will bring about over-coverage of the nanoparticles synthesized. Therefore, the incident light does not induce the photocatalysts due to the heavy coverage and bonding of hybrid nanoparticles. It should be mentioned that along with surfactant there are many factors, which affect the morphology and particle size of the synthesized nanoparticles.

Fig. 10 Relationship between concentrations of surfactant applied to modify surface of doped TiO₂ nanoparticles and photodegradation efficiency.

4. Conclusion

Tungsten doped TiO₂ hybrid nanoparticles were successfully modified in situ under hydrothermal conditions. n-Butylamine was used as surface modifier. Surface modification changed the morphology and size of the tungsten doped TiO₂ nanoparticles synthesized. In addition, it changed the surface charges and increased the stability of the nanoparticles, which is necessary to achieve higher photodegradation efficiency. The initial concentration and type of pollutant are also important factors, which may affect the photodegradation. Doping the TiO₂ nanoparticles with suitable metal oxide such as WO₃ can reduce the band gap energy and enhance the feasibility of the photodegradation in the visible light. Simultaneous in situ surface modification and doping TiO₂ nanoparticles not only changed the surface morphology, size and surface charges of the nanoparticles synthesized, but also significantly shifted the band gap energy to the visible region, where the photodegradation efficiency is more comparable to the UV region.

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