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Nd,N,S-TiO2 decorated on reduced graphene oxide for a highly efficient visible light active photocatalyst for dye degradation: A comparison to its MWCNT/Nd,N,S-TiO2 analogue Gcina Mamba, Messai Adenew Mamo, X Mbianda, and Ajay Kumar Mishra Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie502610y • Publication Date (Web): 30 Aug 2014 Downloaded from http://pubs.acs.org on September 4, 2014

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Industrial & Engineering Chemistry Research

Nd,N,S-TiO2 decorated on reduced graphene oxide for a visible light active photocatalyst for dye degradation: A comparison to its MWCNT/Nd,N,S-TiO2 analogue. Gcina Mambaa,b, Messai A. Mamoa, Xaxier Y. Mbiandaa,b, Ajay K. Mishraa,b,* a,*

Department of Applied Chemistry, University of Johannesburg, Faculty of Science, P. O.

Box 17011, Doornfontein 2028, Johannesburg, South Africa, Telephone: +27115596180, Fax: +27115596425 Email: [email protected] b

DST-NRF Centre of Excellence in Strong Materials, School of Physics, University of the Witwatersrand, Private Bag 3, WITS 2050, Johannesburg, South Africa.

ACS Paragon Plus Environment


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Abstract Neodymium, nitrogen and sulphur tridoped titania (Nd,N,S-TiO2) was decorated on reduced graphene oxide (rGO) and multiwalled carbon nanotubes (MWCNTs) via a simple sol-gel method. The prepared photocatalysts were characterised by FE-SEM-EDX, TEM, UV-Vis, BET, FTIR, Raman and XRD. Aqueous solutions of eriochrome black T (EBT) and eosin blue shade (EBS) were used to evaluate the photocatalytic activity of the composites under simulated solar light irradiation. Degradation of the dyes was performed in single and mixed dye solutions. The reduced graphene based photocatalyst (rGO/Nd,N,S-TiO2) showed improved photocatalytic activity over the MWCNT/Nd,N,S-TiO2 composite in both single and mixed dye solutions. In the single dye solutions, a maximum degradation of 99.3% and 94.6% was achieved for EBS and EBT, respectively. Moreover, a maximum degradation efficiency of 65.7% and 58.9% was attained by rGO/Nd,N,S-TiO2 for EBS and EBT, respectively, from mixed dye solutions. These experimental results suggest the potential application of the composite photocatalysts for dye pollution remediation. Furthermore, radical scavenging experiments confirmed the superoxide and hydroxyl radicals as the active species during dye degradation. Total organic carbon analyses revealed a fairly high degree of complete mineralisation of both dyes reducing the potential formation of toxic degradation by-products.

Key words: Neodymium, MWCNTs, rGO, eosin blue shade, tridoped titania, photocatalysis.

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1.

Introduction

The large volumes and complex nature of textile effluent presents a massive environmental challenge1. Dyestuffs form a significant component of the effluent which is evident from the bright colours of the effluent. Dye colour impacts negatively on the water quality and leads to disruption of the aquatic ecosystem.1 Moreover, some dyes or their partial breakdown products have been found to have potential adverse health effects to humans due to their carcinogenic, genotoxic and mutagenic nature.2 Therefore, it is important to efficiently treat dye pollution and protect the environment. Semiconductor photocatalysis has been proposed as the potential solution to water pollution by organic compounds such as pesticides, dyes, herbicides, organic solvents and other organic industrial waste.3–5 This technology holds several advantages over conventional treatment tools for organic contamination mitigation. Some of the merits include the potential to sufficiently mineralise organic pollutants into smaller ions, carbon dioxide and water.6 This eliminates sludge formation which usually pose further disposal challenges.7 Semiconductor photocatalysis is immune to the toxicity of the organic pollutants and there is no fouling.7,8

Titania is regarded as the benchmark for semiconductor photocatalysis. This emanates from its remarkable properties which include non-toxicity, high oxidising power, biological and chemical stability, resistance to photocorrosion and accessibility in terms of cost.9 During semiconductor photocatalysis energy equal or above the band gap energy of TiO2 is required to excite electrons from the valence band to the conduction band of the semiconductor. This leaves positive holes in the valence band and creates a negative conduction band. The generated electron/hole pairs must be sufficiently separated to ensure formation of the oxidising species and minimise recombination which lowers the photocatalytic activity of the semiconductor.10 However, the large band gap of titania (3.2 eV for anatase and 3.0 eV for rutile) permits absorption in the UV region. This poses a serious energy problem since artificial UV sources are costly and it limits the potential exploitation of sunlight as a source of energy since UV light only accounts for about 4% of the solar spectrum. Moreover, titania also suffers from a high recombination rate of the electron/hole pairs which then lowers its photocatalytic activity.11

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Extensive research has been directed towards improving charge separation in titania and also extend its response into the visible light region which would allow the use of sunlight as a source of energy. Incorporation of various carbon nanomaterials such as CNTs, rGO and carbon nanospheres (CNS) into the titania matrix has been found to yield composite materials with improved optical and photocatalytic properties compared to pure titania.12,13 The strong and intimate interaction between titania and the carbon nanomaterial enables the exploitation of their cooperative effect.14 The carbon nanomaterials have been found to improve the surface area of the photocatalyst, improve its adsorption capacity for the organic pollutants and minimise aggregation of the titania nanoparticles. Furthermore, the carbon nanomaterials have been credited for improving charge separation and transportation in the photocatalyst, acting as a visible light sensitiser and also as a source of carbon doping which contributes to visible light absorption.15,16

In this publication we report the preparation of neodymium, nitrogen and sulphur tridoped titania supported on rGO to yield a composite material which seeks to exploit their combined properties. The prepared photocatalyst is anticipated to display improved visible light absorption, improved charge separation and transportation and higher photocatalytic activity. For comparison, the tridoped titania was decorated on MWCNTs to yield the corresponding composite photocatalyst. EBT and EBS were utilised to compare the performance of the prepared nanomaterials, both in single and mixed dye solutions against tridoped titania and commercial titania. The role of rGO in influencing the photocatalytic activity of the rGO/Nd,N,S-TiO2 is compared to its MWCNTs analogue. To the best of our knowledge, there is no report on the preparation of tridoped titania supported on rGO for dye degradation. Moreover, the comparison of rGO to MWCNTs in enhancing the photocatalytic properties of the tridoped titania, is seldom reported.

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2.

Experimental

2.1.

Materials

Multiwalled carbon nanotubes (purity ˃ 95.0%), neodymium nitrate hexahydrate (99.99%), thiourea (99.0%), eriochrome black T (55.0%), potassium permanganate (99.0%), graphite flakes, hydrogen peroxide solution (30%), vitamin C (98.0%), titanium (IV) butoxide (97.0%), butanol (99.4%), formic acid (99.9%) and nitric acid (55.0%) were obtained from Sigma Aldrich (South Africa) and were used as received. All other reagents were also of analytical reagent grade and were used without further purification unless stated otherwise.

2.2.

Preparation of photocatalysts

2.2.1. Preparation of MWCNT/Nd,N,S-TiO2 and rGO/Nd,N,S-TiO2 Preparation of reduced graphene proceeded through subjecting natural graphite to a strongly oxidizing environment to produce graphite oxide using a modified Hummers method.17 The obtained graphite oxide was transformed into graphene oxide via ultrasonication and then reduced according to a literature procedure.18 Raman spectroscopy was used to probe the effectiveness of the reduction process and the results are presented in the supporting information. The method for oxidation of the MWCNTs and the preparation of the nanocomposites was adapted from our previous work.19

In a typical synthesis experiment, titanium (IV) butoxide (10 mL) was dissolved in butanol (20 mL) with sonication for 30 min. In another beaker, the carbon nanomaterial (MWCNT/rGO) (0.0117 g) was dispersed in butanol (30 mL) with sonication for 30 min. The two solutions were mixed and sonicated for a further 30 min. The mixture was stirred using a magnetic stirrer at room temperature. This was followed by addition of an appropriate amount of neodymium nitrate hexahydrate solution in butanol (10 mL). A solution of thiourea (1.0 g in 15 mL formic acid) was added drop wise with vigorous stirring and stirring was continued for a further 2 hrs. The precipitate formed was dried in air at 100 oC for 12 hrs. The dried precipitate was ground into fine powder using a pestle and mortar and the powder was loaded into quartz boats and calcined at 400 oC in air for 3 hrs. This correspond to 0.6% 5



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(m/m) Nd3+ relative to TiO2 and was denoted as Carbon nanomaterial/Nd,N,S-TiO2 (carbon nanomaterial: MWCNT/rGO). The amount of the carbon nanomaterials was kept at 0.5% for all the nanocomposites fabricated. Neodymium, nitrogen and sulphur tridoped titania (Nd,N,S-TiO2) was prepared using the same method without the addition of the carbon nanomaterials and used as a control. Commercial titania (commercial TiO2) (mixture of rutile and anatase) was also used as another control.

2.3.

Materials characterisation

FE-SEM images were obtained from a Jeol JSM-7500F field emission scanning electron microscope (Japan) coupled with an INCA EDX for qualitative elemental analysis. Fourier transform infrared spectroscopy analysis was done on a PerkinElmer Spectrum 100 FT-IR spectrophotometer (USA) and samples were analysed as KBr discs. Transmission electron microscopy analysis was done on a Jeol JEM 2100 transmission electron microscope (Japan). Raman spectra were collected from a PerkinElmer RamanMicro 200 Raman microscope (UK), excited by a red diode laser at 785 nm and equipped with an Olympus BX51M microscope which was operated at a beam path magnification of 20x. UV-Vis absorption spectra of the photocatalysts were collected on a Shimadzu UV-2450 UV-Vis spectrophotometer (Japan) using barium sulphate as a reference. Total organic carbon (TOC) analysis was done in a Teledyne Tekmar TOC fusion (USA) to gain an insight into the degree of complete mineralisation of the dyes. Standard solutions of 1 ppm, 5 ppm, 10 ppm, 20 ppm and 30 ppm carbon were prepared using potassium hydrogen phthalate (KHP) and de-ionised water. The standard solutions were run prior to sample analysis for instrument calibration. TOC was measured as the difference between total carbon (TC) and total inorganic carbon (TIC). Surface area analysis was done on a Micromeritics ASAP 2020 surface and porosity analyser (USA). A Rigaku UltimaIV x-ray diffractometer (Japan) was used to obtain an insight into the crystalline phases of the photocatalysts at a diffraction angle 2θ range (20-80o). A Cuα radiation beam with an excitation wavelength of 0.15406 nm was used as an x-ray source. The average particle size of anatase TiO2 (d) was computed from Scherrer equation (Eq. 1) for a specific angle (θ) and FWHM (full width at half maximum for the (101) anatase peak, β). D=

cos

[1] 6


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2.4.

Photocatalytic activity evaluation

The fabricated nanocomposites were exploited for the degradation of EBS and EBT under simulated solar light. In all the photocatalytic degradation experiments the photocatalysts (0.10 g) was mixed with the dye solution (EBT or EBS) (20 ppm, 100 mL) and the mixture was stirred in the dark for 1 hr prior to irradiation to establish adsorption equilibrium. The photocatalytic degradation experiments were carried out for 4 hrs. A sample was drawn from the solution using a syringe fitted with a PVDF, 0.45 µm syringe membrane filter and that was taken as the initial concentration (C0). The solar simulator Oriel Newport (USA) was equipped with an Oriel 500 W xenon lamp, set at an input power of 450 W and kept at a distance of 11 cm from the sample. The solar simulator was fitted with a dichroic UV filter (λ ˃ 420 nm). Samples were drawn from the solution at 30 min intervals in the case of EBS and 40 min in the case of EBT, filtered and analysed using a Shimadzu UV-2450 UV-Vis spectrophotometer. During degradation of the dye mixture, equal volumes of EBS solution (20 ppm) and EBT solution (20 ppm) were mixed before adding the photocatalyst. UV-Vis analysis was done at the appropriate absorption wavelengths of the different dyes. A calibration curve was prepared from which the percentage EBS/EBT degraded was calculated using Eq. 2: [2]

C

% EBS/EBT degraded = C0t × 100 where C0 is the initial EBS/EBT concentration and Ct is EBS/EBT concentration after time, t. UV-Vis absorption spectra of EBS/EBT were also recorded to monitor the degradation of the dyes with increasing irradiation time. The experimental data was analysed using first order kinetics to establish the photocatalytic degradation rates according to eq. 3.

ln

C0 C

[3]

= k

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3.

Results and discussion

3.1.

Characterisation of photocatalysts

The surface morphology of the photocatalysts was investigated using FE-SEM and TEM. The FE-SEM images (Figure 1(a)) show that the MWCNTs are embedded within the titania matrix and the coverage was not complete. Figure 1(c) and 1(d) show the FE-SEM images of rGO/Nd,N,S-TiO2 from which there are no observable graphene sheets probably due to their complete coverage by titania nanoparticles. Moreover, the rGO/Nd,N,S-TiO2 appears to be a more compact unit which may suggest a strong and intimate interaction of rGO with the titania nanoparticles. The non-uniform and incomplete decoration of the titania nanoparticles on the MWCNTs was further confirmed by TEM (Figure 1(d)). There is improved coverage of the rGO by titania (Figure 1(e) and 1(f), arrows) compared to MWCNTs probably due to its planar structure which allow for better interaction with the tridoped titania. This intimate interaction between rGO and tridoped titania is beneficial for realisation of their collaborative effect during photocatalysis. Moreover, there is no observable aggregation of the carbon nanomaterials which may suggest sufficient dispersion within the titania matrix.

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Figure 1: FE-SEM images of (a) MWCNT/Nd,N,S-TiO2, (b) and (c) rGO/Nd,N,S-TiO2 and TEM images of (d) MWCNT/Nd,N,S-TiO2, (e) and (f) rGO/Nd,N,S-TiO2.

Energy dispersive x-ray analysis provided a qualitative elemental composition of the photocatalysts. Figure 2 shows the EDX spectrum of rGO/Nd,N,S-TiO2 which indicates the presence of the elements C, S, O, Nd and Ti. The C largely originates from the incorporation of rGO and to a lesser extent, from the thiourea which is used as a source of nitrogen and sulphur. The EDX spectrum of the MWCNT/Nd,N,S-TiO2 is identical to that of rGO/Nd,N,S-TiO2.

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Figure 2: EDX spectrum of rGO/Nd,N,S-TiO2.

FT-IR was employed to probe the different functional groups present in the prepared materials and the results are presented in Figure 3. For commercial titania, two prominent peaks are observed around 3428 cm-1, 1630 cm-1 related to the OH stretching and OH bending of adsorbed water molecules. Moreover, a broad peak between 500 cm-1 and 800 cm-1 also appears and is due to the Ti-O, O-Ti-O stretching vibration.20 Notably, these peaks are also present in the tridoped titania and the composite photocatalysts. However, the OH peaks are more intense compared to commercial titania which may be due to the presence of more moisture adsorbed on the surface of the photocatalyst. This may suggest improvement in the adsorption of moisture as a result of tridoping and incorporation of rGO/MWCNTs. Furthermore, the Ti-O peak is broader in the nanocomposites and tridoped titania compared to commercial titania. This may be due to the combined contribution of Ti-O, O-Ti-O, Ti-C, Ti-O-C, O-Ti-N, N-Ti-N, Ti-O-Nd and Nd-O.21,22 In addition to these three peaks observed in commercial titania, the tridoped titania and nanocomposites show peaks around 3200 cm-1 and 1398 cm-1 corresponding to deformation vibration of ammonium ions.23 Moreover, the peak at 2348 cm-1 may be attributed to N-H stretching vibration, further suggesting the incorporation of nitrogenous species in the photocatalysts.24 The presence of sulphur species is responsible for the peaks appearing at 1134 cm-1, 1050 cm-1 and 984 cm-1. These peaks are linked to the presence of bidentate SO42- coordinated to Ti4+.20,25

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70 60 50

Transmittance (%)

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40 30 20

Commercial TiO2 Nd,N,S-TiO2 MWCNT/Nd,N,S-TiO2 rGO/Nd,N,S-TiO2

10 0 -10 4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

Figure 3: FT-IR spectra of the prepared photocatalysts and commercial titania.

The optical properties of the photocatalysts were investigated using UV-Vis absorption spectroscopy. This is to determine the extent to which tridoping improves the visible light absorption of pure titania and the extent to which the incorporation of rGO/MWCNT contribute towards visible light absorption of the photocatalysts. Figure 4 shows the UV-Vis absorption spectra of the different photocatalysts. Commercial titania shows intense absorption in the UV region and no absorption in the visible light region. This is characteristic of pure titania and emanates from the wide band gap of titania, 3.2 eV for anatase titania and 3.0 eV for rutile titania.11 The tridoped titania shows absorption both in the UV and visible light region. Visible light absorption of tridoped titania is attributed to the incorporation of the three dopants (Nd, N and S) resulting in the formation of sub-band gap states within the titania band gap. Consequently, electrons are excited by visible light from the valence band of titania to some of the new sub-band gap states (Nd and S states).26,27 Alternatively, electrons may be excited by visible light from the N sub-band gap states into the conduction band of titania or the S and Nd sub-band gap states.26 The incorporation of rGO/MWCNTs significantly enhanced visible light absorption of the composite 11



photocatalysts compared to tridoped titania. This may be credited to the rGO/MWCNTs acting as visible light sensitisers and sources of carbon doping which improves visible light absorption of titania.28 Notably, the rGO incorporating photocatalyst showed better visible light absorption probably due to better interaction between rGO and tridoped titania. This may result in the formation of numerous Ti-C bonds which are essential for electron transfer between rGO and titania during visible light irradiation.29 0.30


0.25

0.20 Absorbance (a.u.)

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0.15

0.10

0.05

0.00 300

400

500

600

700

800

Wavelength (nm)

Figure 4: UV-Vis absorption spectra of the different photocatalysts prepared.

Figure 5 shows the Raman spectra of the prepared photocatalysts and commercial titania. Commercial titania and tridoped titania showed similar Raman spectra with peaks appearing at 149, 217, 391, 510 and 637 cm-1. These peaks can be linked to the anatase phase of titania and originate from the Raman active modes with symmetries Eg, Eg, B1g, A1g and Eg, respectively.30 The prepared nanocomposites also show these anatase peaks, suggesting the anatase phase as the predominant phase. In addition to the anatase peaks, the Raman spectra of the nanocomposites show peaks at 1308 and 1598 cm-1 which may be ascribed to the D-band and G-band of rGO/MWCNT.30 The appearance of these peaks in the spectra of the composite photocatalysts suggests the incorporation of rGO/MWCNTs within the titania 12


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matrix. Furthermore, the titania peaks are shifted in the composite photocatalysts compared to the tridoped and codoped titania which could be as result of the interaction of the titania and MWCNTs/rGO. 4500


Eg

4000 3500

Eg

3000

Counts (a.u.)

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B1g A1g

Eg

Gband

Dband

2500 2000 1500 1000 500 0 -500 0

400

800

1200

1600

2000

-1

Wavenumber (cm )

Figure 5: Raman spectra of the prepared photocatalyst and commercial titania.

Examining the XRD patterns of the photocatalysts (Figure 6) reveal that they are fairly crystalline and their crystallinity is not compromised by the incorporation of rGO/MWCNTs. The average crystallite size of titania estimated from Scherrer equation was found to range from 15 to 17 nm. XRD analysis confirmed the mixed phases of commercial titania(Figure 6). Peaks observed at 2θ 27.4o, 36.2o, 36.2o, 39.3o, 41.3o, 44.0o, 56.7o, and 64.1o are characteristic of the rutile phase of titania. These peaks emanate from the (110), (101), (200), (111), (210), (220), and (110) crystal planes.31 In addition to these rutile peaks, there are peaks appearing at 2θ 25.4o, 37.7o, 48.1o, 54.4o, 55.1o, 62.6o, 69.0o, 69.8o and 75.1o which are characteristic of the anatase phase of titania. These peaks correspond to the (101), (004), (200), (105), (211), (204), (116), (220) and (215) crystal planes.32,33 The same anatase peaks observed in commercial titania, are observed in the prepared photocatalysts and there are no rutile peaks, suggesting the preparation of single phase titania. There are no peaks due to 13



Nd2O3 probably due to its low content relative to titania and also probably due to its well dispersion within the titania matrix. Moreover, there is no significant difference in the XRD patterns of rGO/Nd,N,S-TiO2 and MWCNT/Nd,N,S-TiO2. In both XRD patterns there is no peak due to the MWCNT/rGO possibly due to the smaller amounts of rGO/MWCNTs (0.5%) and the overlap with the (101) peak of anatase titania.33 R

2000

1500


A A

Intensity (a.u.)

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R

1000

R AR

R

R A

A

AR

A A A

500

0 20

30

40

50

60

70

80

2 Theta (degrees)

Figure 6: XRD patterns of the different photocatalysts.

A large surface area is a desirable property for a photocatalyst to ensure adequate interaction with the pollutant species. Pollutant/photocatalyst interaction is essential since photocatalysis occurs on the surface of the photocatalyst, therefore, the pollutant species must be in contact with the surface of the photocatalyst. The inclusion of rGO/MWCNT improved the surface area of the resultant photocatalysts reaching surface areas of 128.21 m2/g and 144.87 m2/g for MWCNT/Nd,N,S-TiO2 and rGO/Nd,N,S-TiO2, respectively, compared to 78.21 m2/g for tridoped titania. The increase in surface area may be due to the incorporation of rGO/MWCNTs with large surface areas and their possible role in minimising aggregation of the titania nanoparticles.

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3.2.

Photocatalytic degradation of eriochrome black T and eosin blue shade

3.2.1. Single dye degradation The rGO/Nd,N,S-TiO2 composite photocatalyst was evaluated for the degradation of EBS and EBT from single dye solutions under simulated solar light irradiation and its performance compared to the MWCNT/Nd,N.S-TiO2 nanocomposite. Moreover, the photocatalytic activity of the nanocomposites is compared to that of tridoped titania and commercial titania in order to understand the influence of incorporating the carbon nanomaterials. Figure 7(a) and 7(b) shows the photocatalytic degradation of EBS and EBT, respectively. The prepared nanocomposites displayed improved photocatalytic activity for both EBS and EBT compared to tridoped titania and commercial titania. Degradation efficiencies of 95.7% and 99.3% for EBS and 85.8% and 94.6% for EBT, were attained using MWCNT/Nd,N,S-TiO2 and rGO/Nd,N,S-TiO2, respectively (Table 1(a) and 1(b)). These values are all higher than the degradation efficiency values obtained using the tridoped titania and commercial titania (Table 1(a) and 1(b)). The enhanced performance of the nanocomposites may be explained in terms of improved visible light absorption compared to tridoped titania and commercial titania as observed in the UV-Vis absorption spectra of the photocatalysts (Figure 4). This improvement in visible light absorption emanates from the cooperative effect of carbon nanomaterials and tridoped titania resulting in enhanced formation of the superoxide and hydroxyl radicals responsible for dye degradation.15,16 Furthermore, incorporation of the carbon nanomaterials result to improved charge separation and transportation which minimise electron/hole recombination and leads to efficient formation of the oxidising species and degradation of the dye molecules.34 The presence of Nd3+ further improves charge separation in the nanocomposites due to its electron scavenging properties.35

In addition to better visible light absorption and better charge separation, the composite photocatalysts have large surface areas compared to tridoped titania and commercial titania, owing to the inclusion of rGO/MWCNTs. This provides adequate sites for the degradation of the dye species. Moreover, rGO/MWCNTs may aid in minimising aggregation of the tridoped titania nanoparticles which would otherwise lower the photocatalytic activity of the material.36 Commercial titania displayed the lowest photocatalytic due to its poor visible light absorption and high electron/hole recombination rate. The introduction of Nd, N and S 15



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improved visible light absorption, charge separation and photocatalytic activity of the tridoped titania relative to commercial titania.37 Photocatalytic degradation kinetics of both EBS and EBT were studied using first order kinetics (Figure 7(c) and 7(d)) in order to determine the rate at which the dye molecules are degraded by the photocatalysts. Degradation rate constants of 1.4 x 10-2 min-1 and 1.9 x 10-2 min-1 for EBS and 8.0 x 10-3 min-1 and 1.1 x 10-2 min-1 for EBT were observed using MWCNT/Nd,N,S-TiO2 and rGO/Nd,N,S-TiO2, respectively. The degradation rate constants for the composite photocatalysts are higher than those obtained for tridoped and commercial titania for the same dyes (Table 1(a) and 1(b)). This is an indication of improved performance of MWCNT/Nd,N,S-TiO2 and rGO/Nd,N,S-TiO2 for the degradation of both dyes compared to both controls and further support the proposed synergy between rGO/MWCNTs and tridoped titania. Moreover, it can be observed that EBS is degraded faster than EBT, probably due to their structural differences which can offer varying degrees of stability and interaction with the photocatalysts.

Photocatalytic degradation of methylene blue (MB) by rGO/TiO2 has been studied by Min et al.15 and recorded maximum degradation rates of 2.76 x 10-2 min-1 and 7.15 x 10-3 min-1, under UV and visible light irradiation, respectively. In another work, Wang et al.38 reported the visible light photocatalytic degradation of methyl orange (MO) using rGO/TiO2 composite photocatalyst prepared from dye-sensitization induced reduction of GO and observed a maximum degradation rate of 9.5 x 10-3 min-1. In both studies, the rGO/TiO2 composite photocatalysts displayed lower degradation rates compared to the values reported in this study for the composite photocatalysts (table 1(a) and 1(b)). Moreover, Lei et al.39 reported the photocatalytic degradation of MB on CdS/MWCNT-TiO2 and recorded degradation rates of 2.4 x 10-3 min-1 and 3.3 x 10-3 min-1 under visible and UV light irradiation. Tian et al.14 prepared and evaluated TiO2/MWCNT hybrid nanostructures for the photocatalytic degradation of MB under visible light irradiation and reported a maximum degradation rate of 1.0 x 10-3 min-1. The MWCNT/Nd,N,S-TiO2 photocatalyst prepared in this work showed improved photocatalytic activity compared to previous studies.14,39 These observations highlight the significance of incorporating Nd, S and N towards improvement of the photocatalytic activity of both composite photocatalysts (rGO/Nd,N,S-TiO2 and MWCNT/Nd,N,S-TiO2). 16


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Table 1: Percentage degradation and degradation rate constants of (a) EBS and (b) EBT. (a) Sample

EBS degraded (%)

Rate constant, min-1

R2

Commercial TiO2

58.8

4.1 x 10-3

0.986

Nd,N,S-TiO2

68.0

5.2 x 10-3

0.973

MWCNT/Nd,N,S-TiO2

95.7

1.4 x 10-2

0.986

rGO/Nd,N,S-TiO2

99.3

1.9 x 10-2

0.989

(b) Sample

EBT degraded (%)

Rate constant, min-1

R2

Commercial TiO2

50.0

3.1 x 10-3

0.977

Nd,N,S-TiO2

58.8

3.8 x 10-3

0.987

MWCNT/Nd,N,S-TiO2

85.8

8.0 x 10-3

0.977

rGO/Nd,N,S-TiO2

94.6

1.1 x 10-2

0.966

UV-Vis spectra were also recorded to monitor the changes in the absorption spectra of both dyes with increasing irradiation time (Figure 8). The absorption peaks observed in both dyes (Figure 8(a) and 8(b)) decreased gradually and flattens with increasing irradiation time. Moreover, there are no new peaks observed in both spectra suggesting that there were no derivatives of the two dyes formed as a result of photocatalytic degradation. This indicates that the dyes are degraded into inorganic species or smaller molecules such as CO2, SO3, NO2, H2O, etc.40

17



1.0

(a)

0.8

C/C0

0.6

0.4

0.2


0.0

0

50

100

150

200

250

Irradiation time (minutes) 1.0

(b) 0.8

0.6

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.4


0.2

0.0 0

50

100

150

200

250

Irradiation time (minutes)

18


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(c)

(d)

Figure 7: Photocatalytic degradation of (a) EBS, (b) EBT under simulated solar light and Kinetics for degradation of (c) EBS and (d) EBT under simulated solar light.

19



0.8

(a) 0 min

0.6

Absorbance (a.u.)

30 min 60 min 90 min

0.4

120 min 150 min 180 min 0.2

210 min 240 min

0.0 400

450

500

550

600

-1

Wavelength (cm )

0.8

0 min 0.7

(b)

30 min 60 min 90 min 120 min 150 min 180 min

0.6

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.5 0.4

210 min 240 min

0.3 0.2 0.1 0.0 200

300

400

500

600

700

-1

Wavelength (cm )

Figure 8: UV-Vis absorption spectra of (a) EBS and (b) EBT during photocatalysis using rGO/Nd,N,S-TiO2.

20


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A positive effect on the photocatalytic activity of the composite photocatalysts is observed upon incorporation of rGO/MWCNTs. However, the inclusion of rGO yield better results compared to MWCNTs. This observation highlights the significant role played by rGO/MWCNT in enhancing the photocatalytic activity of tridoped titania. Most often, these carbon nanomaterials are seldom compared in terms of their capacity to enhance the photocatalytic activity of semiconductors. Consequently, each carbon material is overly praised as superior in positively influencing the photocatalytic activity titania.12,15 The incorporation of rGO in the nanocomposite resulted to improved visible light absorption compared to MWCNTs (Figure 4) which is essential for generation of the oxidising species and degradation of the dye molecules. This enhanced visible light absorption may be due to better interaction of rGO and tridoped titania leading to a strong and intimate interfacial contact which is essential for harnessing the cooperative effect of the two components of the photocatalyst. Moreover, the strong interfacial contact between tridoped titania and rGO may be linked to adequate formation of the Ti-C, Ti-O-C bonds which play a role during visible light absorption.29 The influence of rGO on the surface area of the resultant composite photocatalyst is surpass that of MWCNTs, probably due to better interaction with the tridoped titania nanoparticles yielding sufficient dispersion and less aggregation. A large surface area provides adequate contact between the photocatalyst and dye molecules and improves degradation. Moreover, rGO has higher electronic conductivity compared to MWCNTs. Therefore its incorporation in the nanocomposite may account for greater charge separation and transportation which lowers the recombination of the charge carriers.41 This would

give

rGO/Nd,N,S-TiO2

better

photocatalytic

activity

compared

to

its

MWCNT/Nd,N,S-TiO2 counterpart.

In order to ensure efficient exploitation of the combined properties of both tridoped titania and rGO, the method of preparation of the composite photocatalyst and rGO are important factors. There has to be sufficient, intimate contact between the rGO and the tridoped titania. This ensures adequate formation of the Ti-C/Ti-O-C bonds which promote visible light activity of the photocatalyst and improves electron mobility between titania and rGO.34 Huang at al.29 and Zhang at al.33 noted that complete and intimate coating of rGO/MWCNT with titania nanoparticles yielded better results than poorly coated carbon nanomaterials and a physical mixture of titania and rGO/MWCNTs. This probably explains the higher 21



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photocatalytic activity of rGO/Nd,N,S-TiO2 compared to the MWCNT/Nd,N,S-TiO2 nanocomposite which display sufficient intimate contact between rGO and the tridoped titania. This is evident from the FE-SEM and TEM images. Furthermore, the structural differences between rGO and MWCNTs renders different interactions with the titania nanoparticles, resulting in varying interfacial contact and different photocatalytic activity of the resultant photocatalyst.33

The method of preparation of rGO is also important because it determines its structure which in turn influences its properties. For an example, rGO is expected to show extremely high influence in the photocatalytic activity of tridoped titania but experimental results show a moderate difference compared to MWCNTs. Similar results have been reported elsewhere.42 An explanation for this phenomenon may lie with the method of preparation of the rGO. If the preparatory method introduced a lot of defects on the graphene framework, this will affect the electron conductivity of rGO and lowers its contribution towards charge separation and transportation in the composite photocatalyst.42 Therefore, it is beneficial to produce rGO of minimum defects in order to harness its intrinsic electronic conductivity. Moreover, a sufficient degree of reduction needs to be attained in order to transform graphene oxide (GO) from the sp3 hybridised state to rGO which is largely sp2 hybridised. This is a very critical step necessary to transform the material from being an insulator to a conducting material.43,44 The degree of reduction will have implications in the role of the rGO in electron capturing and transportation.

22


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3.2.2 Competitive photocatalytic degradation of EBS and EBT The complex nature of textile effluent is one of the major challenges during treatment. In order to evaluate the performance of the prepared composite photocatalysts for the degradation of the dye molecules under competitive conditions, degradation was performed from mixed dye solutions. Photocatalytic degradation of the mixture of EBS and EBT is shown in Figure 9(a). The rGO/Nd,N,S-TiO2 displayed better photocatalytic activity compared to the MWCNT/Nd,N,S-TiO2 for both dyes reaching maximum degradation efficiencies of 65.7% and 54.1% for EBS and EBT, respectively. These values are higher than the 58.9% and 49.0% obtained for EBS and EBT, respectively, when using the MWCNT/Nd,N,S-TiO2 as the catalyst. There is a significant decline in the degradation efficiency for both dyes from the mixed dye solution compared to single dye solution. This may be due to competition between EBS EBT for the photocatalyst surface where degradation takes place. First order kinetics (Figure 9(b)) shows that EBS is degraded faster than EBT as observed in single dye solutions. The degradation rate constants are lower than those from single dye solutions, owing to the competitive conditions. The rGO/Nd,N,S-TiO2 reached degradation rates of 5.2 x 10-3 min-1 and 3.7 x 10-3 min-1 for EBS and EBT, respectively. The obtained results suggest the application of the prepared photocatalysts as versatile dye pollution remediation tools.

(a)

1.0 0.9 0.8 0.7

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6 0.5

MWCNT/Nd,N,S-TiO2(EBT) rGO/Nd,N,S-TiO2(EBT) MWCNT/Nd,N,S-TiO2(EBS) rGO/Nd,N,S-TiO2(EBS)

0.4 0.3 0

50

100

150

200

250

Irradiation time (minutes)

23



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(b)

Figure 9: (a) Competitive photocatalytic degradation of EBS and EBT and (b) kinetics studies for the competitive degradation of EBS and EBT.

3.2.3 Effect of radical scavengers on the degradation of EBS and EBT The photocatalytic degradation of EBS and EBT by rGO/Nd,N,S-TiO2 was evaluated in the presence of radical scavengers to ascertain the role of these radicals in the degradation of the dye molecules. Benzoquinone (10 mL, 0.1 M) and 2-propanol (10 mL, 0.1 M) were used as superoxide radical scavenger and hydroxyl radical scavenger, respectively.37 Photocatalytic degradation of EBS in the presence of benzoquinone and 2-propanol reached maximum efficiencies of 59.8% and 74.0%, respectively. For EBT, maximum degradation efficiencies of 39.1% and 55.5% were observed in the presence of benzoquinone and 2-propanol, respectively. A significant decrease in the photocatalytic activity of the nanocomposite for both dyes is observed and suggests that both the superoxide and hydroxyl radicals are the active species during EBS and EBT degradation. First order kinetics (Figure 10) gave degradation rates of 4.2 x 10-3 min-1 and 3.5 x 10-3 min-1 for EBS degradation in the presence of benzoquinone and 2-propanol, respectively. Similarly, degradation rates of 3.7 x 10-3 min-1 and 2.3 x 10-3 min-1 for EBT were observed in the presence of benzoquinone and 2-propanol, 24


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respectively. The lower degradation rates in the presence of the radical scavengers further emphasise the role of the radicals in dye degradation.

Figure 10: Kinetics for EBS and EBT degradation in the presence of radical scavengers.

3.2.4 Total organic carbon analysis Total organic carbon analyses provide an insight into the amount of carbon bound to organic compounds. This may be used to determine the extent to which the dye molecules are broken down during photocatalysis. Higher TOC values suggest a higher degree of complete mineralisation of the dye molecules which is desirable during photocatalysis to minimise formation of potentially toxic by-products. Maximum TOC values of 82.0% (Figure 11(a)) and 75.6% (Figure 11(b)) were obtained from the degradation of EBS and EBT, respectively, suggesting a fairly high degree of complete mineralisation of both dyes by rGO/Nd,N,S-TiO2. It is evident from the TOC removal values that a small amount of organic degradation byproducts

remain

in

solution

after

the

4

hrs

of

irradiation.

The

higher

decolourisation/degradation efficiency compared to the TOC removal values suggest that the dye molecules are broken down into smaller colourless products first before being mineralised to carbon dioxide, water and other inorganic species. The residual organic materials may require extended irradiation times to ensure their complete mineralisation.

25



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(a)

(b)

Figure 11: Total organic carbon removal by rGO/Nd,N,S-TiO2 from (a) EBS and (b) EBT degradation.

26


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4.

Conclusion

Tridoped titania was decorated on rGO and the resultant photocatalyst displayed improved optical and photocatalytic properties compared to its MWCNT/Nd,N,S-TiO2 counterpart, tridoped titania and commercial titania. These improvements are credited to the co-operative effect of the tridoped titania and the rGO. This synergistic effect emanates from the intimate contact between the tridoped titania and rGO as evident from the FE-SEM and TEM images. Moreover, the superior electron conductivity of rGO over MWCNT is crucial in ensuring efficient electron trapping and transportation. The prepared composite photocatalysts displayed the capacity to degrade both dyes under competitive conditions which highlights the versatility of the photocatalysts and their potential application as environmental clean-up tools for dye pollution. Addition of radical scavengers in the dye solutions significantly lowers the photocatalytic degradation rates of both dyes, suggesting that the superoxide and hydroxyl radical are the active species responsible for dye degradation. A fairly high degree of complete mineralisation of both dyes was evident from TOC analyses which indicate that most of the dye molecules are broken down into simpler molecules and inorganic ions. This reduces the risk of forming toxic photocatalysis by-products.

27



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Acknowledgements Funding from the University of Johannesburg, Faculty of Science and DST-NRF Centre of Excellence in Strong Materials is highly appreciated.

Supporting Information Detailed procedure for preparing rGO and its characterisation using Raman spectroscopy. This information is available free of charge via the Internet at http://pubs.acs.org/.

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