Fabrication of carbon and sulphur-doped nanocomposites with seaweed polymer carrageenan as an efficient catalyst for rapid degradation of dye pollutants using a solar concentrator

Abstract

Here we demonstrate direct use of sulphate rich seaweed polysaccharides, carrageenans, namely kappa (κ-), iota (ι-) and lambda (λ-) as source of sulphur and carbon doping in TiO₂ photocatalysts. During TiO₂ synthesis in the presence of sulphate rich carrageenan leaves, mainly residual carbon and sulphur doping resulted in a highly active photocatalyst. Evaluation of the dye degradation pattern shows rapid degradation of reactive black-5, methylene blue & methyl orange using modified TiO₂ nanocomposites in different light sources. Robust dye degradation was achieved between 1 and 4 h under daylight whereas, the use of a solar concentrator reduced the degradation time of MB and RB-5 to <5 min and MO solution was turned colourless within 20 min. The present study elaborates the effect of seaweed carrageenans in inducing heteroatoms like sulphur and residual carbon for the photodegradation of industrially important dyes.

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Introduction

At present biomass based materials are receiving great importance from both academic and industrial sectors due to their sustainability and green features. Among several biomaterial sources, marine chemicals and seaweeds possess versatile characteristics due to their inherent saline environmental origin and growth. Further, seaweed cultivation offers both social and economic benefits. Therefore, for commercial aspects seaweeds fulfill the prerequisite of being low cost and easy to use. Carrageenans are one such red seaweed polysaccharide family available in 3 different sulphated forms and have been used in number of applications from food ingredients to water treatment agents. Most importantly, the number of sulphate groups on kappa (κ-, one sulphate), iota (ι-, two sulphates) and lambda (λ-, three sulphates) forms of carrageenans can be utilized effectively for in situ or post-modification of industrially important products.¹

On the other hand, many researchers have explored the unique properties of several photocatalysts via non-metal doping. There have been several reports in the literature describing the properties and activities of several catalysts being influenced by non-metal dopants like carbon (C-) and sulphur (S-).^2,3 It is more evident from the photocatalytic degradation of dyes using TiO₂.⁴ Several combinations of metal and non-metal dopants have been successfully used for doping TiO₂ targeting photodegradation applications. In several cases, a textile dye degradation study has been the common objective because textile industries consume large quantities of water and colouring agents in their manufacturing processes.^5,6 Modern day textile industries are vibrant and serve as essential part of our day-to-day lives. Despite stringent environmental regulations, industries still lack comprehensive wastewater treatment methods. Annually >500 tons of non-degradable textile colour wastes are being disposed of in natural streams without adequate treatments.⁷

It is estimated nearly 60 to 70% of hydrolyzed dyes remain in wastewater at the end of the dying process. Therefore, textile wastewater generally characterized by high concentrations of redundant dyes. It is also important to note that modern dye or colour pigments are designed to sustain harsh washing conditions in the consumer products like textile fabrics.⁸ Among several textile dyes, reactive dyes are being used largely for dyeing cellulosic, wool and nylon fabrics. Hence, reactive dyes make up a large portion of the colouring agents used in the textile industry, some of them are toxic, even suspected to be carcinogenic in nature.⁹ Once the dye contaminated wastewater is disposed of in large bodies of water, the presence of high concentrations of dyes in wastewater causes a drastic rise in chemical oxygen demand and biological oxygen demand (COD and BOD).⁸ Ozone oxidation is one of the prominent methods used for breaking double bonded azo-dyes in treatment processes.^10,11 Even though, conventionally, adsorption, oxidation, membrane filtration and multi-step biological treatments are being used for the treatment of coloured wastewater, most of these methods are relatively costly. Also, it is well known that methods like membrane treatment, adsorption or coagulation merely concentrate the contaminants instead of completely breaking down the dye molecule chemically to non-toxic constituents. Carrageenan based nanocomposite hydrogels as dye adsorbents have also been reported in literature.^12–15 In contrast, catalytic dye degradation is one such method where toxic and hazardous dyes can be degraded to yield non-toxic products.

Titanium dioxide (TiO₂) has been one such conventional photocatalyst widely used for photocatalytic degradation of dyes.^16–21 TiO₂ attracts enormous interest due to its tunable photocatalytic activity²² and it is capable of breaking down the organic species leaving behind colourless solutions.²³ Accordingly, TiO₂ instigates the degradation process under the influence of ‘light’, generally in the UV-Visible region of the electromagnetic spectrum without undergoing chemical changes during the catalytic process. Interestingly, non-metal dopants like carbon, nitrogen and sulphur have shown remarkable influence on enhancing the photocatalytic activity of TiO₂.^{9,18,19,24–29} Sulfur doping induces strong Brønsted acidic sites on the surface of TiO₂ exhibiting a high photocatalytic activity.^19,27,28,30 Therefore, it is important to prepare novel nanostructured materials from sustainable resources which can directly induce heteroatom doping to perform efficient photocatalysis in daylight.

The present study utilizes sustainable material derived from seaweeds as a template in combination with a titanium precursor in a one step calcination process resulting in carbon and sulphur-doped TiO₂ nanoparticles. Further, we discuss the rapid and efficient daylight photodegradation phenomenon of industrially important dyes like methyl orange, methylene blue and reactive black-5 using the modified photocatalyst.

Experimental section

Chemicals and materials

Titanium isopropoxide was supplied by Sigma Aldrich. Ammonia solution (25%) was purchased from SRL Pvt. Ltd, India. The commercial TiO₂ (CASR no-13463-67-7) used in this study was purchased from LABORT, Surat, India. All chemicals were of analytical grade and were used as received. κ-Carrageenan (CAS no.: 11114-20-8), ι-carrageenan (CAS no.: 9062-07-1) and λ-carrageenan (CAS no.: 9064-57-7) were purchased from Tokyo Chemical Industry Co., Ltd, Japan, while reactive black-5, methylene blue & methyl orange (Fig. S1†) were purchased from Sigma Aldrich. These chemicals were used without further purification. Milli Q water was used throughout the process. Fig. S1† shows the molecular structure of the chemicals used in the present study.

Properties of carrageenans

The carrageenans, a family of high molecular weight sulphated polysaccharides (average molecular weight ranging from 100 to 1000 kDa) are obtained from certain species of red seaweeds. They consist of galactose and anhydrogalactose units linked through glycosidic bonds. κ-Carrageenan is known to form strong gels in the presence of potassium ions and ι-carrageenan forms soft gels in the presence of calcium ions, while λ-carrageenan is non-gelling in nature.¹ The moisture content 10–12%, viscosity 5–10 cPs and ash content 30–40% have been mentioned in the TCI product (carrageenans) catalogue. The carrageenans contain high levels of carbon and sulphur in their matrix along with metal ions including calcium and sodium.³¹

Synthesis of TiO₂ nanoparticles

1% w/v of carrageenan aqueous solution (1 g in 100 ml Milli-Q water) was prepared by autoclaving it at 120 °C for 15 min. To the above solution, 14% v/v of titanium isopropoxide (14 mL) was added with continuous stirring and the pH of the reaction mixture was raised to 10 by the drop wise addition of aqueous ammonia (25%) solution at room temperature. The obtained white coloured reaction mixture was placed at 60 °C for 24 h in a hot oven for aging.³² Then the TiO₂–carrageenan blend was collected by centrifugation followed by Milli-Q water washing. The washed TiO₂–carrageenan blend was dried in an oven at 80 °C overnight to remove moisture and then calcination of each sample was performed at 600 °C in a muffle furnace for 4 h. TiO₂ nanocomposites thus prepared by using κ-carrageenan, ι-carrageenan, λ-carrageenan as a green template were designated as κ-S–TiO₂, ι-S–TiO₂, λ-S–TiO₂ respectively. Further, in all photodegradation experiments, commercial TiO₂ was used as a reference sample.

Instruments and measurements

Fourier transform infra-red (FTIR) spectra of samples were recorded using a Perkin-Elmer Spectrum GX (FT-IR System, USA) instrument. The UV-Vis absorption spectra of standard and degraded dyes were recorded with a Varian CARY 500 UV-VIS-NIR Spectrophotometer. Nitrogen sorption isotherms were obtained using a Beckmann Coulter model SA3100 surface area analyzer at 77 K. Prior to the measurement, the samples were degassed at 393 K for 6 h. Thermogravimetric analysis (TGA) was carried out using a Mettler Toledo Thermal Analyzer, (TGA/SDTA 851e, Switzerland) with a heating rate of 5 °C min⁻¹ under nitrogen atmosphere. The band-gap energy of the nanocrystalline TiO₂ was determined using DRS (Shimadzu UV-3101PC) equipped with an integrating sphere; BaSO₄ was used as a reference sample. The sulphate content was analysed by (ICP) with a Perkin-Elmer ICP-OES Optima 2000DV machine, and carbon was analysed using a Perkin-Elmer-2400, CHNS/O analyser.

The morphology of the TiO₂ nanocomposites was determined using a scanning electron microscope (Carl-Zeiss), model LEO 1430 VP, Germany, at an accelerating voltage of 20 kV. A requisite amount of sample was mounted on an aluminum stub and placed in the vacuum chamber. All images were recorded at the same magnification. Transmission electron microscope (TEM) images of the TiO₂ nanocomposites were dispersed in acetone and images were recorded with a JEOL HR-TEM (JEOL JEM 2100, Japan) instrument operated under an accelerating voltage of 200 kV. Powder X-ray diffraction (XRD) measurements of the TiO₂ samples were recorded using an X’pert MPD powder X-ray diffractometer using Cu-K (λ = 0.15406 nm) radiation operating at 40 kV and 30 mA. Data were collected in the range of 20 to 70° 2θ angle at a scan rate of 0.1° s⁻¹. XPS analysis was carried out using Multi-technique X-ray Photoelectron Spectroscopy (XPS) with a monochromic AlKα-ray source.

Photocatalytic activities

The photocatalytic activities of seaweed carrageenan modified TiO₂ were measured in three different experimental conditions by adding 30 mg of each TiO₂ catalyst into 20 mL of each dye solution (25 ppm) in a glass vessel. (i) First, dye degradation studies were performed under a UV lamp with wavelength 254 nm and power 25 W cm⁻² at room (30 °C) temperature, (ii) second, experiments were performed under maximum daylight intensity conditions (during 12–2 pm) to evaluate the feasibility of the photocatalysts, and (iii) third, the feasibility of these photocatalysts were further evaluated using a solar concentrator box under maximum daylight intensity. In all experiments 3 commercially important dye samples were exposed with the carrageenan modified photocatalyst and commercial TiO₂ was used as the reference photocatalyst.

Results and discussion

We used seaweed polysaccharides namely carrageenans as a template due to their high sulphate and high molecular weight with retainable carbon content. In addition, carrageenans contain traces of metal ions including sodium, potassium and calcium.³¹ To prepare carbon and sulphur rich TiO₂, 3-different types of carrageenans namely, kappa, iota & lambda (κ-, ι-, and λ-) were used as initial template precursors as shown in Fig. 1(a). The final mass of the composite TiO₂ product resulted under the influence of a hydrothermal calcination procedure in presence of ammonia and was characterized using wide angle X-ray diffraction (XRD). The powder XRD spectra of all the samples analysed are shown in Fig. 1(b), and were in close agreement with the JCPDS 21-1272 standard patterns.^17,27 All the prepared TiO₂ nanocomposites exhibited major peaks at 2θ angle values of 25.31°, 37.81°, 48.01°, 53.8°, 54.98° and 62.64° corresponding to the (101, d-spacing = 0.3512 nm), (004, d-spacing = 0.2375), (200, d-spacing = 0.1893), (105, d-spacing = 0.1695), (211, d-spacing = 0.1667), (204, d-spacing = 0.1480) of anatase crystal planes. This result revealed the formation of the anatase form of TiO₂ in the presence of carrageenans as a template.

Fig. 1 (a) Scheme of the synthesis of a non-metal doped TiO₂ nanoparticle composite using different carrageenans (κ-, ι- and λ-), (b) XRD pattern of prepared TiO₂ using three different carrageenans (c–f) SEM images showing the control commercial TiO₂, kappa-C TiO₂, iota-C TiO₂ and lambda-C TiO₂ with close magnifications, respectively.

Further, the morphologies of the TiO₂ composites were recorded using a scanning electron microscope (SEM). SEM analysis exhibited a homogeneous mass of the non-metal atom doped TiO₂ composite as shown in Fig. 1(c)–(f). However, there is no clear shaped morphology identified even at close magnifications.

The transmission electron microscope (TEM) images in Fig. 2 give a close view of the composite. Fig. 2(a)–(c) shows no significant difference in the morphology which is almost spherical in shape, obtained with 3 different carrageenan sources. However, there is a notable difference in particle size for all 3 cases, ca. 20.10 nm ± 1.1 nm, ca. 15.43 nm ± 1.1 nm and ca. 5.93 nm ± 1.1 nm for the κ-, ι- and λ-carrageenan precursors, respectively (Fig. 2 & S2†). This finding indicates that different sizes of nanocomposites might be obtained by using carrageenans of different sulphate level. Further, the inset image of the selected area electron diffraction (SAED) pattern in Fig. 2(e) confirms the TiO₂ nanoparticles as crystalline anatase phase, which further supports the XRD result.

Fig. 2 TEM images of composite TiO₂ prepared from (a) kappa (b) iota and (c) lambda carrageenans referred as κ-S–TiO₂, ι-S–TiO₂ and λ-S–TiO₂, respectively. (d–f) Images show a close view of the TEM morphology of TiO₂ composite with (g) λ-diffraction pattern inserted.

The thermal stability of all three TiO₂ nanocomposites was tested by employing thermogravimetric analysis (TGA) (Fig. S3†). The most notable feature is that almost similar trends in mass losses were obtained up to 800 °C for all the TiO₂ nanocomposites prepared from different carrageenan templates. Further, the pattern of mass loss was almost comparable to C–TiO₂ used as a reference sample in this study. In all cases the mass loss was obtained in the range of 2 to 4% up to 800 °C and may be attributed to traces of impurities in the form of moisture and some organic species.³³ This result indicates that the thermal stability of the resulting TiO₂ nanocomposites was comparable to the commercially available TiO₂ used in this study as a reference sample. The negligible difference in thermal stability may be due to the availability of varying sulphur groups in the carrageenan templates, which can act as a bridge between the nano-TiO₂ particles and the residual graphitic carbon.

The X-ray photospectroscopic (XPS) analysis was performed for the characterization of the carbon and sulphur doped TiO₂ nanocomposite. Fig. 3(a) gives the XPS spectra for κ-TiO₂, λ-S–TiO₂ and the control commercial (C–TiO₂) sample. With a survey of these spectra which show intensity as a function of binding energy (BE), we can identify the presence of C1s, O1s, S2p and Ti2p_3/2, Ti2p_1/2 spin-orbital splitting photoelectrons by peaks corresponding to their binding energies of 286.0, 532.6, 168, 457.31 & 463.5 eV respectively. It can be seen that all the samples contain Ti, O, S and C except C–TiO₂ which is devoid of sulphur. The appearance of a peak at 284.7 eV in Fig. 3(b) corresponds to the binding energy of C1s. The Ti2p spectra (Fig. 3(c)) exhibit two peaks corresponding to binding energies of Ti³⁺ 2p_3/2 at 457.31 eV and Ti⁴⁺ 2p_1/2 at 463.5 eV. For the S-doped TiO₂ nanocomposite, the slight shift of the Ti2p_3/2 peak with respect to that of Ti⁴⁺ in pure anatase TiO₂ (458.5–459.7 eV) indicates the presence of Ti³⁺. The peak observed at 532.6 eV in Fig. 3(d) clearly indicates presence of O1s. Fig. 3(e) clearly shows the S2p level centered at 168 eV which corresponds to sulfur bonded to oxygen, confirming the presence of sulphate groups. It is also important to note that due to the absence of peaks in the region 160–163 eV, it can be inferred that no new Ti–S bond is forming at the expense of Ti–O bonds.³⁴ Also, sulphur is present in the form of metal sulphate which is confirmed by the presence of a peak at 168 eV.

Fig. 3 X-ray photospectrometric analysis of S-doped TiO₂ nanoparticles. (a) XPS survey spectra, (b) C 1s (c) Ti 2p (d) O 1s and (e) S 2p region spectra of C–TiO₂ (blue), κ-S–TiO₂ (black) and λ-S–TiO₂ (red). (f) FTIR spectra to identify the sulphate content of TiO₂ composites produced from three different carrageenan precursors.

In Fig. 3(e) the S_2p spectrum indicates that sulphur is present in different oxidation states as two isolated peaks at binding energies of 168 and 164.4 eV correspond to the chemical states of S⁴⁺ and S⁰. Higher calcination temperatures (600 °C) lead to the disappearance of the S⁰ peak (164.4 eV) and an intense peak at ∼168 eV is observed. This is an indication of the conversion of S⁰ to a higher oxidation state at elevated temperatures. These sulphate ions can also form O–S–O bonds on the TiO₂ surface, creating an unbalanced charge on the Ti and vacancies/defects on the surface of the TiO₂ nanoparticles. The C1s peak is centered at 287.4 eV and is graphitic in nature.³⁵

The FTIR spectra of the nanocomposites (Fig. 3(f)) and TEM-EDX (Fig. S4†) further confirm the presence of the sulphur group in the carrageenan templated TiO₂ composites. The sulphate presence is evident from the peak at 1125 cm⁻¹ which is due to the asymmetric stretching frequency of S–O bonds in the SO₄²⁻ groups while the presence of peaks below 1000 cm⁻¹ is due to TiO₂ crystal lattice vibrations.²⁸ It must be noted that in the case of λ-S–TiO₂, a peak at 1100 cm⁻¹ can be observed which is due to the symmetric stretching of the S–O bonds. For ι-S–TiO₂ and κ-S–TiO₂ intensity of this peak is so low that it merges with the peak due to asymmetric stretching (1125 cm⁻¹). This is probably due to the presence of more SO₄²⁻ in λ-S–TiO₂ as compared to the other 2 samples. Also, the intensity of the S–O bond asymmetric stretching band increases from κ-S–TiO₂ to λ-S–TiO₂ which may be due to the same reason. The presence of the broad adsorption bands that appear in the range 500–700 cm⁻¹ are characteristic of a Ti–O–Ti symmetric stretching vibration mode.³⁶ Therefore, the presence of a less electronegative S having high energy level molecular orbitals compared to O effectively reduces the band gap in the composite TiO₂ compared to commercial TiO₂. Further, ICP analysis confirms that λ-TiO₂ contains highest sulphate content (0.649% in the form SO₄²⁻). Surface area and pore size measurements using N₂ adsorption–desorption isotherms showed an increase in the trend from κ-TiO₂ to λ-TiO₂ as seen in Table 1.

Table 1 Surface area, pore size and chemical characteristics of κ-S–TiO₂, ι-S–TiO₂ and λ-S–TiO₂

Sample	BET-SA (m^2 g^−1)	Langmuir SA (m^2 g^−1)	BJH-SA (cm^3 g^−1)	BJHD-SA (cm^3 g^−1)	Pore volume (cm^3 g^−1)	Pore size (nm)	% carbon by elemental analysis	% SO4^2− by ICP
κ-S–TiO2	25.72	39.07	24.83	31.07	0.141	21.93	0.10	0.144
ι-S–TiO2	36.49	56.50	33.11	43.72	0.167	18.36	0.17	0.248
λ-S–TiO2	40.35	50.63	44.74	46.41	0.184	28.55	0.25	0.649
C–TiO2	5.26	7.21	6.16	7.48	0.02	15.61	NA	NA

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Evaluation of dye degradation

Before using direct sunlight or daylight for direct dye degradation, the UV intensity percentage in sunlight was measured. The intensity of sunlight and the UV percentage varies with place and the time of the day. To reduce the degradation time and the influence of intensity, maximum intensity timings were measured for both UV and total daylight as shown in Fig. 4(a). The intensity of the average daylight in the month of July was recorded in two fragments (i) total intensity and (ii) UV intensity fraction. It was measured that in a maximum (>1000 W m⁻²) total daylight intensity UV light intensity was ∼30 W m⁻² (3% of total daylight). Firstly, the random dye degradation pattern for RB-5 was analysed by exposing the vial containing dye solution and photocatalyst (λ-S–TiO₂) to direct daylight. At three different time periods of the day from 0900 hours to 1700 hours, the colour change of the solution in the vial was monitored at different intervals as shown in Fig. 4(b). Remarkably, >99% RB-5 degradation was achieved in <2 h between 1200 hours to 1400 hours. Further, controlled dye degradation experiments were carried out to infer the effect of UV radiation on RB-5, MB and MO. The 3 dyes were subjected to a degradation process using λ-TiO₂ under controlled UV light exposure using a standard UV lamp of wavelength 254 nm and power 25 W cm⁻² at room temperature. The photographs in Fig. 4(c)–(e) give the degradation pattern which lasted between 4 and 8 h under UV radiation. On the other hand, similar experiments were performed under direct daylight exposure with sample vials containing the above 3 dye solutions and the photocatalyst which resulted in rapid colour change. Photographs Fig. 4(f)–(h) showed TiO₂ (κ-S–TiO₂) activity at its peak between 1200 hours and 1400 hours and a drastic colour change was noticed for RB-5 and MB within ∼1 h. However, MO colour remains intense even after 2 h under daylight exposure. Even though MO retained some colour during the UV lamp experiment, substantial decolouring was noticed in vial. It is also important to note that among the three composites, κ-S–TiO₂, ι-S–TiO₂ & λ-S–TiO₂ derived from different carrageenans (kappa, iota and lamda), λ-S–TiO₂ exhibited the fastest dye degradation activity. A detailed study as shown in Fig. 5(a) and (b) reveal that λ-S–TiO₂ activity was rapid and instant in comparison to κ-S–TiO₂ and ι-S–TiO₂ in degrading MB and RB-5 which took <1 h to get clear water.

Fig. 4 (a) Intensity profile of total daylight and UV fraction measured during an average July month, and (b) degradation of RB-5 at different daylight times using κ-S–TiO₂. (c–e) Degradation profile of three different dyes under controlled UV light exposure and (f–h) under direct exposure of daylight using κ-S–TiO₂ as the photocatalyst.

Fig. 5 (a) Degradation of RB-5, MB and MO using κ-S–TiO₂ under a standard UV lamp (b) degrading concentration profiles of RB-5, MB and MO in daylight between 12 and 2 pm using photocatalysts κ-S–TiO₂, ι-S–TiO₂ and λ-S–TiO₂, tested following similar sample conditions as under the UV lamp.

The rapid degradation achieved through carbon and sulphur doped TiO₂ follows the trend; λ > ι > κ precursors. These trends may be due to number of sulphate groups inducing an increasing order of sulphur content in the final TiO₂ composite from the parent κ-carrageenan to λ-carrageenan. To best of our knowledge, solar concentrators have been utilized for the first time to induce high intensity daylight, thereby accelerating the dye degradation process. A portable solar concentrator box with a flexible lenses panel was used to focus the sunrays to a sample chamber as shown in Fig. 6(a). In the set-up, the sample solution temperature variation due to sunray concentration was measured to be 55–60 °C for a period of 20 min, whereas the temperature outside the concentrator was recorded to be 42 ± 2 °C for the same time period. Therefore, the concentrator box attains an elevated ∼15 °C due to concentrated intensity of daylight. Once the solar concentrator box was optimized, dye samples with 3 different S–TiO₂, commercial TiO₂ and a control dye sample without TiO₂ were placed inside the chamber.

Fig. 6 (a) Photograph of solar concentrator unit used in this study, (b) degradation profile of MB, (c) degradation profile of RB-5, (d) degradation profile of MO using all three (κ-, ι- and λ-) carrageenan templated carbon and sulphur doped TiO₂ photocatalysts in comparison to commercial TiO₂ and a control vial, and (e) comparative degradation profile of RB-5, MB, MO.

Fig. 6(b)–(d) shows the dye degradation pattern for the first 20 min in the chamber. It is noted that in all cases, the control dye sample without any TiO₂ and with commercial TiO₂ showed no significant colour change. Whereas, MB and RB-5 become completely colourless in <5 min and MO took ∼20 min to undergo complete degradation. Reported literature on the synthesis of metal and non-metal doped TiO₂ & their photocatalytic performance has been summarised in Table 2.

Table 2 Comparative literature reports containing metal and non-metal doped TiO₂ and their photocatalytic performance

Doped element	Dopant source	Light source	Dye used	Degradation time (min)	Reference
C	Activated carbon	UV	MO	140 min	2
B, C, N, P, S, Fe, Zn, Zr, Sb, Ce	Boric acid, CTAB, NH3, H3PO4, ethylene glycol, citric acid, non metal salts	Sunlight	MB	>60 min	18
S	Sodium thiosulphate, sodium and dodecylbenzene sulphonate	Sunlight	MB	330 min	19
Ce, N, S	Ammonium cerium sulphate, urea	Fluorescent lamp	MO	90 min	26
C, S	Sulfur hydrosol, tetrabutyl orthotitanate,	Visible light	MB	20 min	28
C, S	κ-, ι-, & λ-carrageenans	Solar concentrator	RB-5	<5 min	Present study
			MB	<5 min
			MO	<20 min

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Fig. 6(e) shows comparative analysis by using only λ-S–TiO₂ as an efficient photocatalyst for the degradation of dyes in a solar concentrator. In the presence of commercial TiO₂, MB, RB-5 and MO have shown a colour change (C/C_o) < 5% in the solar concentrator box whereas all three dyes under the influence of κ-S–TiO₂, ι-S–TiO₂ and λ-S–TiO₂ have undergone rapid and robust degradation to yield colourless solutions.

Recovery and reuse of photocatalysts

The stability, recovery and sustainability of any catalyst is very important to make the product & process economically feasible. The reusability of the TiO₂ samples was tested by recovering samples from each dye solution after achieving complete degradation of the dye and colourless water (Fig. 7(a)). For recovery of the catalysts, samples were centrifuged and catalyst was separated as residue. Subsequently the recovered residue was washed with milli-Q water and acetone 3–4 times followed by oven drying to obtain the pure catalyst. The recovered catalyst was characterized for structure and integrity using FTIR and XRD prior to dye degradation experiments (Fig. 7(b)–(e)). The catalytic efficiency was tested up to 6 cycles and it was found that the degradation efficiency was maintained at >97% (Fig. 7(a)). This result makes the nanocomposite more cost-effective as well as eco-friendly for industrial applications.

Fig. 7 (a) Recyclability of photocatalyst for RB-5 dye using λ-S–TiO₂, (b–e) catalyst recovery and characterization after each dye degradation.

Degradation mechanism, effect of residual carbon and doped sulphur on catalytic activity

The degradation mechanisms of photocatalysis exhibited by pristine TiO₂ (rutile or anatase) as well as transition metal or main group element doped TiO₂ are well known and documented.^23,37–39 Anatase TiO₂ absorbs electromagnetic radiation in the near-UV region, which is about 3% of the total daylight (Fig. 4(a)). Due to this intrinsic constrain on TiO₂ nanoparticles, direct utilization of solar energy is inefficient for photocatalysis. To broaden the photoresponsive region of TiO₂ nanoparticles, non-metals are being used as dopants.

Here, we have doped or induced graphitic carbon in TiO₂ which was confirmed by XPS and SEM images. The TiO₂ nanoparticles are embedded in these graphitic layers as can be seen in the HRTEM images (Fig. 2) which increases the available surface area for photocatalysis. This residual carbon can be thought of as a few layers of graphite which acts as a support and an interconnecting media for the TiO₂ nanoparticles.⁴⁰ There is a synergistic effect between the nanoparticles and carbon which results in the exfoliation of the graphitic sheets due to intercalation of the TiO₂ nanoparticles and formation of a graphitic matrix which binds the TiO₂ nanoparticles together.

The role sulphur plays here is less straightforward. It must be emphasised that the role of sulphur and carbon or for that matter any other non-metal dopant depends completely on whether the visible radiation photoresponse of doped titania outweighs a modest drop in the efficiency of the undoped catalyst using UV radiation. In 2001, Asahi and co-workers proposed delocalised mixing of the O 2p and N 2p orbitals resulting in the rise of the valence band level as compared to undoped TiO₂.⁴¹ Current research supports an alternative explanation in which dopant atom orbitals generate a mid-gap level above the valence band.^37,42–45 As the bulk valence band energy level rises, the oxidising power of the photochemically generated holes decreases which in turn limits the performance of the doped TiO₂. In other words, there will be certain reactions in which only the pristine TiO₂ photocatalyst could perform.

We propose a mechanism for dye degradation in typical anatase TiO₂, in which the O2p and Ti3d orbitals contribute to a valence band (VB) and a conduction band (CB), respectively. In pristine TiO₂ daylight absorption has a negligible effect on the photocatalytic activity (Fig. 6(b)–(d)). On the other hand, sulphur doped TiO₂ results in indirect moderation of the band gap by forming S-centers from which electrons may fall. This will lower the oxidizing power of the holes; relative to undoped TiO₂ assuming charge migration is faster compared to reaction with the dye molecule. However, the consequences of this theory are still a matter of discussion.⁴⁰ From the DRS study it was found that the band gap energy is lowest for λ-S–TiO₂ and highest for κ-S–TiO₂ (Fig. S5†). The presence of graphitic carbon induces a larger surface area which enhances the adsorption and light absorption capacity of the S–TiO₂ nanocomposites. Continued exposure to daylight effectively separates holes with the strong oxidation power in VB and electrons in CB. Therefore, MB and RB-5 get completely reduced to colourless solutions in ∼5 min though, MO underwent slow degradation reaching a completely colourless state at around ∼20 min.

Conclusion

In summary, seaweed heteroatom doped TiO₂ nanoparticles were successfully designed to perform photodegradation on frequently used dyes. The residual carbon and S-doping from the seaweed carrageenan precursor in the TiO₂ composite meant that the composite exhibited a larger surface area, which helped in robust dye degradation in <20 min in daylight. The λ-S–TiO₂ has a precursor of lambda carrageenan with 3-suphlate groups and from the parent compound showed relatively higher surface area and faster photodegradation activity in daylight and with a solar concentrator. The enhanced daylight activity of λ-S–TiO₂ could be explained on the basis of the large surface area induced by the graphite like carbon residue, higher sulphur content and good anatase crystallinity. Therefore, the present study illustrates the effective use of low-cost, sustainable marine resources as doping agents in addressing environmental issues related to coloured wastewaters.

Acknowledgements

SKN and AM gratefully acknowledge DST, Government of India for the DST-INSPIRE Fellowship and Research Grant (IFA12-CH-84). RM and JPC are thankful to DST, New Delhi, Government of India (SB/EMEQ-052/2013) and CSIR, New Delhi, India (CSC0130) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10317k

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