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Vol. 8. Issue 5.
Pages 3995-4009 (September - October 2019)
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Vol. 8. Issue 5.
Pages 3995-4009 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.07.008
Open Access
Experimental and theoretical investigations of Mn-N-co-doped TiO2 photocatalyst for visible light induced degradation of organic pollutants
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Nidhi Sharotria,
Corresponding author
nidhisliet11@gmail.com

Corresponding author.
, Deepali Sharmab, Dhiraj Suda
a Department of Chemistry, Sant Longowal Institute of Engineering and Technology, Longowal (Deemed University), Sangrur, Punjab 148106, India
b Department of Pharmaceutical Sciences, University of KwaZulu-Natal, Durban, 4001, South Africa
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Tables (4)
Table 1. Phase compositions, lattice parameters (a, b, c, α, β and γ), lattice strain and cell volume of the synthesized Mn-N co-doped TiO2.
Table 2. Comparative studies of %age degaradtion in literature.
Table 3. Effect of pH on the % degradation of quinalphos and 2-chlorophenol (2, 4, 6, 7, 8 and 10).
Table 4. Electron distribution (HOMO-LUMO) distribution plots for interaction of (TiO2)n and (TiO2)n doped clusters with organic molecules.
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Abstract

The present paper reports the experimental and theoretical investigations of Mn-N-co-doped TiO2 photocatalyst for degradation of organic pollutants. Mn-N-co-doped TiO2 photocatalyst was synthesized by using cavitation induced technique and the incorporation of dopant (Mn2+) ion in the lattice of N-doped TiO2 and was confirmed from X-ray diffraction (XRD), Transmission electron microscopy (TEM), Energy dispersive X-ray spectrometry (EDS), Fourier transform infrared (FTIR), Raman, UV–vis and Electron paramagnetic resonance (EPR) spectroscopy. The effect of Mn2+ ion on variation in structural, morphological and optical properties of catalyst was assessed. The photocatalytic response of the Mn-N-co-doped TiO2 catalyst was investigated at different wavelength regions (490 nm, 565 nm, and 660 nm) of the solar spectrum for the degradation of organophosphate pesticides quinalphos and 2- chlorophenol. The present study has highlighted the photocatalytic efficiency of the Mn-N-co-doped TiO2 photocatalyst. To understand the photocatalytic activity at the molecular level, the adsorption energies (ΔEad) of quinalphos and 2-chlorophenol with the N-doped TiO2 and Mn-N-co-doped TiO2 photoctalysts were calculated with density functional theory (DFT) using B3LYP (Becke’s 3-parameter exchange functional with Lee-Yang-Parr correlation energy) functional and 6-311 G(d,p)/LANL2DZ basis set. The obtained ΔEad values indicated the probable positions on the surface of photocatalysts where pesticide molecules could adsorb and degrade efficiently. The computational study can be used for comprehensing the mechanism of degradation of pollutant molecules in the presence of photocatalysts.

Keywords:
Mn-N-co-doped TiO2
Density functional theory (DFT)
Photodegradation
Quinalphos
2-chlorophenol
Full Text
1Introduction

The continuous efflux of anthropogenic substances from various industrial and agricultural activities such as phenol and its derivatives, synthetic dyes, pesticides and fertilizers into aquatic streams is acquiring alarming situation and poses a serious threat to very existence of life on this planet. The resistance of these substances to natural decomposition and biodegradation and even the persistence of by-products formed after their partial degradation was reported in scientific findings [1–3]. The environmental challenges led to the emergence of finding the solutions for remediation of the anthropogenic substances from aquatic streams. Among the different types of pollutants, organophosphate pesticides are of interest because of their large scale use in agriculture. The extensive use of pesticides results in their seepage into the ground water thereby contaminating it and making unsuitable for both human and agriculatural uses [4–6]. The conventional methods such as chemical precipitation, filtration, electro-deposition, ion-exchange and adsorption are not very effective in phasing out the pesticides [7–10]. An emerging technique for the degradation of pollutants from the environmental matrix is heterogeneous photocatalysis [11,12]. Heterogeneous photocatalysis involves the use of semiconductor photocatalyst materials because of their potential to convert photon energy into chemical energy which can be employed for the degradation of synthetic substances [13–15]. Among the various semiconductor materials employed, titanium dioxide (TiO2) has been used as an efficient photocatalyst because of its interesting properties such as non-toxicity, low cost, strong oxidizing power, biological and chemical inertness [16,17]. Various physico-chemical properties like crystallinity and crystallite size, porosity, phase transformation, hydroxyl groups content (surface), etc. have shown a vital role in heterogenous photocatalysis [18,19].

TiO2 functions effectively under ultraviolet light irradiation because of its large band gap (3.25 eV). The wide band gap hinders the practical application of titanium dioxide in the visible light and also shows a higher recombination rate of photogenerated electrons and holes [20]. One of the major requirements for commercialization of the advanced oxidation process is to make it cost-effective, which may be possible by developing photocatalysts that respond to natural light instead of artificial UV light. Therefore, the main focus of the present study is to design visible light responsive TiO2 photocatalyts by anionic and cationic modifications, which can result in the shifting of the absorption wavelengths from UV to the visible region.

An intense research has been carried out in the recent years towards the modification of semiconductor TiO2 to achieve high reactivity under visible light by several methods such as metal and non-metal ion doping, surface sensitization of TiO2, coupling with narrow band gap semiconductors and introducing noble metal in TiO2 surface [21–29]. The substitutional doping of nitrogen is found to be effective in band gap lowering by mixing of p states of N with O2pz state [24].

Doping of transition metals in to TiO2 structure has resulted in its improved photocatalytic efficiency. Transition metals such as V, Cr, Fe, Mg, Co, Zn and Mo have been reported to be doped in TiO2 lattice resulting in the enhanced photocatalytic activity of transition metal doped TiO2 photocatalyst due to red shift [30–34]. The incorporation of dopants result in an increase or decrease of band gap. The red shift in as-synthesized Mn-N-co-doped TiO2 is due to the narrowing of the band gap via the formation of new energy levels in between the Ti 3d states of the conduction band and the O 2p states of the valence band [35].

Zhang et al. reported the synthesis of MnO2-doped anatase TiO2 by precipitation-hydrothermal crystallization method. The photocatalyst showed higher photocatalytic activity in comparison with Degussa P25 and anatase phase was the main phase even at 800 °C [36]. The few reported studies have shown the photocatalytic It has been observed in few reported studies on photo catalytic efficiency of Mn-N-co-doped TiO2 for the degradation of synthetic dye Rhodamine B. Li et al. synthesized a high performance visible light Mn-TiO2 nano photocatalyst with controlled size by varying the amount of manganese chloride for degradation of Rhodamine B [37]. Hu et al. discussed the facile synthesis of Mn-N-co-doped TiO2 by hydrothermal method and higher photocatalytic activity for the degradation of Rhodamine B [38]. Ashkarran et al. proved that silver and nitrogen co-doped TiO2 was an efficient visible light active photocatalyst for degradation of Rhodamine B. The double doping enhanced the photocatalytic and antibacterial activity of synthesized TiO2 as compared to single doped TiO2[39]. Mn-N-co-doped TiO2 photocatalyst synthesized by the simple sol-gel method showed enahnced photocatalytic degradation of Rhodamine B [40]. Most of the dyes have their absorption spectrum in the visible region. Thus, the efficiency of the process cannot be attributed solely to the photoefficiency of the catalyst.

The adsorption of any organic molecule on the surface of the photocatalyst can be explained through computational studies thereby describing the characteristics of the individual systems (dyes, polluthants, catalysts, semiconductors). The adsorption mechanism can be investigated by density functional theory (DFT). On the basis of DFT, it has been explained that band gap and carrier mobility have a significant role in visible light absorption. Doping of Mn in to TiO2 has been reported to show enhanced photocatalytic activity due to the formation of trap states which has been studied using DFT in Vienna Ab initio Simulation Package (VASP) [41].

Taking into consideration the current status of research on photocatalysis and in continuation with earlier reported research [25,26], Mn-N-co-doped TiO2 has been synthesized and the photocatalytic activity has been assessed by choosing organophosphate quinalphos and 2-chlorophenol as a model compounds. O,O-diethyl-O-quinoxalin-2-yl phosphorothioate (quinalphos) is one of the widely used organophosphate acaricide and insecticide in Indian agriculture. World Health Organization (WHO) and Environmental Protection Agency (EPA) have classified quinalphos in class II as moderately toxic compound. 2-chlorophenol, one of the derivatives of phenol is the most important organic contaminant found in wastewater that is released to the environment from industries, and by-products of agricultural chemicals. These compounds are also used as fungicides, bactericides, antiseptics, and disinfectants [25,26,42].

Talwar et al. studied on the biodegradation of quinalphos by Ochrobactrum sp. strain HZM. The Ochrobactrum used various organophosphate pesticides as carbon sources. After degrading with organism, quinalphos is hydrolysis to form 2-hydroxyquinoxaline and diethyl phosphate and is finallly utilized as carbon sources [43]. Gangireddygari et al., discussed the influence of various environmental factors on biodegradation of quinalphos by using Bacillus thuringiensis. The bacteria isolated from soil samples was identified as B. thuringiensis. The optimal conditions for growth of bacterium and degradation of quinalphos were recorded at pH (6.5–7.5) and 35–37 °C temperature. Addition of carbon and nitrogen sources increased the rate of degradation of quinalphos [44].

Motivated by the great interest in the exploitation of doped TiO2 photocatalysts for the degradation of dye and pollutant molecules, in the present study we report combined experimental and computational investigation for the degradation of pesticide molecules in the presence of Mn-N-codoped TiO2. A greener approach involving a cavitation process under ultrasonic irradiation is designed that has not been reported in literature to the best of our knowledge.

2Experimental2.1Chemicals

Titanium isopropoxide (98%) was purchased from Sigma Aldrich. Manganous chloride and hydroxylamine hydrochloride were used as manganese and nitrogen precursors, respectively and purchased from Himedia. Ethanol used was of analytical grade. Commercial chemicals of technical grade, quinalphos (>95%) was obtained from the Crops Chemical Limited, Punjab (India) and 2-chlorophenol from Himedia. Millipore water was used for the preparation of various solutions. The pH of the solution was adjusted with 1 M HCl or 1 M NaOH.

2.2Synthesis of Mn-N-co-doped TiO2

Mn-N-co-doped TiO2 (MnNT) photocatalyst was synthesized using cavitation induced synthetic greener methodology involving high temperature (5000 °C), high pressure (500 atm) and the heating and cooling rate greater than 109 K/s due to the formation of hotspots. The process results in less reaction time with improved product yield.

For the synthesis of Mn-N-co-doped TiO2, titanium isopropoxide (1 M) and hydroxylamine hydrochloride (0.5 M) were dissolved in 20 ml of ethanol, respectively and the mixture was stirred for one hour. Thereafter, precipitation was achieved by the addition of NH4OH solution and further stirring for half an hour. This is was followed by stepwise addition of MnCl2 solution and continued stirring for one hour. The resultant mixture was kept in ultrasonic bath and exposed to sound waves of 40 kHz for 40 min and then the mixture was kept overnight. The precipitates formed were filtered by vacuum filtration, crystallized and dried in an oven at 100 °C for 2 h. The light brown coloured precipitates were calcinated at different temperatures (350, 450, 550 and 750 °C). The synthesized Mn-N-co-doped TiO2 photocatalysts were characterized for elemental composition, morphology, crystallinity and optical properties.

2.3Characterization

The phase and structure identification of the as-synthesized Mn-N-co-doped TiO2 was carried out on X-ray diffractometer (XPERT-PRO) employing Cu K_radiation (0.15406 nm), having a scanning rate of 5°/min in the 2θ angle ranged from 20° to 80°. The IR spectrum was recorded on a fourier transform infrared (FTIR) spectrophotometer (Spectrum One, Perkin Elmer) using KBr pellets. Raman spectrum was recorded with (Make-JY HORIBA) Model-iHR Spectrograph. UV–Vis spectrophotometer (UV-1800, Shimadzu) was used to measure the absorption spectra of the synthesized catalyst and pesticide samples. The surface morphology and size of TiO2 nanoparticles were measured on a high resolution transmission electron microscope (TECNAI 200 Kv TEM), electron optics and scanning electron microscope (JSM-6610-JEOL/EO) at an accelerating voltage of 15 kV. ESR measurement was done using a 9.5 GHz JEOL spectrophotometer operated in X-band frequency.

2.4Experimental setup and procedure for photocatalytic degradation

The photocatalytic activity of the synthesized Mn-N-co-doped TiO2 was assessed by performing experiments in specially designed double-walled reaction vessels (volume 250 ml) in the photocatalytic chamber. The chamber was equipped with LED bulbs having different wavelengths covering the entire solar spectrum. The photocatalytic activity of as-synthesized Mn-N-co-doped TiO2 was investigated by irradiation of 50 ml aqueous suspension of pesticide molecules, quinalphos and 2-chlorophenol in the presence of 50 mg of synthesized Mn-N-co-doped TiO2 under visible light. The solution was stirred using a magnetic stirrer and then aeration was done by aerators. At different time intervals, the sample was withdrawn with the help of a syringe and then filtered through a Millipore syringe filter of 0.22 µm. The rate of degradation of quinalphos and 2-chlorophenol was determined spectrophotometrically, by monitoring the changes in absorption spectra recorded at 320 nm and 273 nm, respectively. The degradation efficiency (%) has been calculated as follows [45]:

where Co is initial concentration of substrate and C is concentration of substrate after irradiation.

2.5Computational studies

In the present study, by means of Gaussian 09 software package [46], density functional theory (DFT) was employed to geometry optimize the (TiO2)n where n = 38, N-doped TiO2, Mn-N-co-doped TiO2 clusters and photocatalytic degradation of model organic compounds; quinalphos and 2-chlorophenol. The structures (clusters and organic molecules) were optimized at B3LYP (Becke’s 3-parameter exchange functional with Lee-Yang-Parr correlation energy) [47–49] level with a 6–311 G(d,p) basis set for C, H, O, N, Mn atoms and the standard LANL2DZ basis set for Ti atom. GaussView 5.0.8 [50] was used to visualize the optimized structures. For the evaluation of adsorption energies of molecules and clusters, the adsorption energy (Ead) is calculated using the following equation:

Where, ΔEad is the adsorption energy of organic molecule on the cluster, Ecomplex is the total energy of the cluster and pollutant, Ecluster is the energy of (TiO2)n, N-doped TiO2, Mn-N-co-doped TiO2 clusters, and Epollutant is the energy of the isolated pollutant molecules (quinalphos and 2-chlorophenol), respectively.

3Result and discussion

In this section, different characterization techniques employed to characterize Mn-N-co-doped TiO2 have been discussed. The photocatalytic degradation of quinalphos and 2-chlorophenol along with DFT results have been analyzed.

3.1Structural properties of Mn-N-co-doped TiO2 (MnNT)

The XRD analysis of the synthesized Mn-N-co-doped TiO2 calcinated at different temperatures (350, 450, 550 and 750 °C) was performed to investigate the crystal structure and phase of the photocatalyst shown in Fig. 1(a–d). The XRD patterns of photocatalyst calcinated at various temperatures, confirmed the presence of TiO2 (anatase phase) with the same crystalline structure at 350 °C (Fig. 1a) and 450 °C (Fig. 1b). The peaks were observed at 25.4°, 37.05°, 48.15°, 53.92° and 63° corresponding to the (1 0 1), (1 1 2), (2 0 0), (2 1 1) and (2 0 4) planes of anatase phase. The peaks were similar to that reported in literature (JCPDS 21-1272) [51]. However, at 550 °C, the peaks corresponding to the anatase and rutile phases were observed that indicated the phase transformation shown in Fig. 1c. The percentage of anatase and rutile phases was found to be 21.8% and 78.2%, respectively at 550 °C. The percentage of respective peaks corresponding to different phases was calculated by using equation [26],

Fraction of rutile phase = 1/1 + 0.79(IA/IR)
where IA is the intensity of the diffraction peak of the anatase phase, IR is the intensity of the diffraction peak of the rutile phase.

Fig. 1.

(a–d) XRD pattern of Mn-N-co-doped TiO2 (MnNT) calcinated at different calcination temperatures, (a) MnNT-350, (b) MnNT-450, (c) MnNT-550, (d) MnNT-750.

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The phase transformation from anatase to rutile happened at a temperature of 550 °C in Mn-N-co-doped TiO2 as compared to N-doped TiO2 and pure TiO2. It has been reported that the transformation of phase from anatase to rutile is kinetically unfavourable and thermodynamically favourable because of the lower free energy of rutile phase [52]. Thus, the substitution of Ti4+ by low valent impurity ion (Mn2+) in the lattice may accelerate the phase transformation due to the formation of oxygen vacancies. However, on substitution with the higher valent impurities, the phase transformation rate is retarded [24]. The substitution with the Mn2+ ion having higher ionic size (0.82 Å) as compared to Ti4+ (0.61 Å) leads to the expansion of the lattice [51]. The higher ionic size of Mn2+ (0.82 Å) causes rutile nucleation i.e. it facilitates the creation of rutile nucleus within the anatase phase. Thus, rutile nucleation get initiated early which means that the rutile nucleus were formed within the anatase phase. The rate of phase transformation from anatase to rutile phase depends on nucleation and growth mechanism. The results were found to be in accordance with previously reported studies related to the influence of Mn2+ ions on the anatase–rutile phase transformation [24–26,53].

At 750 °C, a complete transformation from anatase to rutile phase occurred and only peaks corresponding to rutile phase at 27.49°, 36.1°, 39.22°, 41.26°, 44.07°, 53.87°, 56.70°, 62.63°, 69.08°, 72.61° and 76.61° were observed (Fig. 1d). The charge of the dopant and it’s position in TiO2 lattice increases or decreases the oxygen vacancy concentration in TiO2 thereby affecting the phase transformation rate [54]. The phase composition, lattice parameters (a, b, c, α, β and γ), the lattice strain crystallite size and cell volume of the synthesized Mn-N-co-doped TiO2 is given in Table 1. The crystallite size of the sample was calculated by using Scherrer equation [27],

where λ is the wavelength of the X-ray employed, β is the full width at half maximum in the radiation of the peak, θ the Bragg’s angle of the XRD peak, and K the Scherrer shape factor, K = 0.9. The average crystallite size of the synthesized Mn-N-co-doped TiO2 calcinated at different temperatures was found to be decreased from 66.9 nm (350 °C) to 33.4 nm (550 °C). The crystallite size for sample calcinated at 750 °C corresponding to the rutile phase was higher (53.1 nm).

Table 1.

Phase compositions, lattice parameters (a, b, c, α, β and γ), lattice strain and cell volume of the synthesized Mn-N co-doped TiO2.

Sample  Phase composition  Axial distance (edge length)  Axial angles  Volume of the cell  Lattice strain  Crystallite size (nm) 
Mn-N - 350  Anatase + Wurtzite  a = b = 3.7760 c = 9.4860  α = β = γ = 90°  135.25  .0031  66.9 
Mn-N - 450  Anatase + Wurtzite  a = b = 3.7760 c = 9.4860  α = β = γ = 90°  135.25  0.0022  38.3 
Mn-N - 550  Anatase + Rutile + Wurtzite  a = b = 3.7760 c = 9.4860  α = β = γ = 90°  135.25  0.0027  33.4 
Mn-N - 750  Rutile + Wurtzite  a = 3.765 b = 9.454  α = β = γ = 90°  135.25  0.0018  53.1 
3.2Morphological analysis of Mn-N-co-doped TiO2 (MnNT)

Fig. 2(a–d) showed the TEM images of the synthesized Mn-N-co-doped TiO2 nanoparticles calcinated at different temperatures 350, 450, 550 and 750 °C. Sample calcinated at 350 °C, confirmed the formation of nanoporous structures, which subsequently got crystalline as the calcination temperature increased. The morphological study established the formation of nanosized structure, having the particle size of approximately 60 nm. The EDS result of Mn-N-co-doped TiO2 nanoparticles calcinated at 550 °C was reported in Fig. 3. The EDS result confirmed the doping percentage of Mn and N as 35.70 wt.% and 26.90 wt.%, respectively in TiO2 lattice.

Fig. 2.

(a–d) TEM images of Mn-N-co-doped TiO2 calcinated at different calcination temperatures, (2a). MnNT-350, (2b) MnNT-450, (2c) MnNT-550, (2d) MnNT-750.

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Fig. 3.

EDX pattern of Mn-N co-doped TiO2 calcinated at 550 °C (MnNT-550).

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3.3Spectroscopic investigations3.3.1FTIR and Raman spectra of synthesized Mn-N-co-doped TiO2 (MnNT-550)

Vibrational studies of synthesized photocatalysts were performed using FTIR and Raman techniques in the range of 400–4000 cm−1 and 100–900 cm−1.

FTIR spectrum of synthesized MnNT-550 (Fig. 4) showed a strong absorption band at 3369.52 cm−1 and narrow band at 1629.59 cm−1 due to the stretching and bending vibrations of the hydroxyl groups present on the surface of TiO2 catalyst [16]. The FTIR peaks at 1466.62 cm−1, 1149.62 cm−1 and 1086.62 cm−1 confirmed the presence of a substituted N atom (N-H mode) in TiO2 lattice [55]. At low frequency, strong bands have been seen at 530.47 cm−1 and 629.42 cm−1 corresponding to Mn-O-Ti and O-Ti-O vibrational mode, respectively. These results were in agreement with earlier reported studies [56]. A weak peak observed at 416.63 cm−1 is assigned to the Mn-O wagging vibration mode [57].

Fig. 4.

FT-IR spectra of synthesized Mn-N-co-doped TiO2 calcinated at 550 °C (MnNT-550).

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Raman peaks are sensitive to any change occurring in the TiO2 lattice when dopant is added into the interstitial site. An insertion of Mn2+ or Mn4+ distorts the TiO2 lattice, or a strain is induced by a decrease in the number of oxygen atoms bonded with Ti. Raman spectra of synthesized MnNT-550 (Fig. 5) showed strong bands at 550 cm−1 and 620 cm−1 corresponding to Mn-O-Ti and O-Ti-O vibrational modes, respectively [56,58,59].

Fig. 5.

Raman spectra of synthesized Mn-N-co-doped TiO2 calcinated at 550 °C (MnNT-550).

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3.3.2UV–vis spectra of synthesized Mn-N-co-doped TiO2 (MnNT-550)

UV-Vis absorption spectra of synthesized MnNT-550 showed a remarkable shift in the wavelength towards the region of visible light as compared to N-doped TiO2 and pure TiO2 (Fig. 6a). The increased absorption is attributed to co-doping of Mn and N elements into the TiO2 lattice, which results in the red shift. The incorporation of Mn ion increases the adsorption capacity of the photocatalyst. In the region of 200–400 nm, an increase in the absorption capacity is related to a structural modification, which may be due to the insertion of Mn cations into the anatase TiO2 lattice [60,61].

Fig. 6.

UV–vis spectra (6a) and energy band gap graph (6b) of synthesized Mn-N-co-doped TiO2 calcinated at 550 °C (MnNT-550).

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However, increase in absorption capacity in 400–800 nm region, results from electronic transitions corresponding to different Mn states and its chemical environment [62–64]. The observed shift towards low energy suggests the decrease in band gap due to the incorporation of N as well as Mn atoms. The band gap energy of the sample has been calculated using equation [65];

(αhν) (1⁄n) =A (hν -Eg)
Where α, ν, A, Eg and n are the absorption coefficient, incident light frequency, constant, band gap, and an integer (normally equal to 1, 2, 4, or 6), respectively. The direct band gap energy calculated from a direct Tauc- plot of (αhν)1/2 vs photon energy (hν) was found to be 2.4 eV corresponding to the shift in absorption edge towards the red region (Fig. 6b).

3.3.3Electron spin resonance and paramagnetic resonance spectra of synthesized Mn-N-co-doped TiO2 (MnNT-550)

The observations from electron spin resonance (ESR) spectrum of synthesized MnNT-550 supported the incorporation of Mn-atoms in the synthesized photocatalyst (Fig. 7). The electronic ground state for Mn2+ (3d5) is 6S5/2 with nuclear spin s = 5/2 and the other possible oxidation states were Mn3+ (s = 2) and Mn4+ (s = 3/2) [55]. The EPR spectrum of synthesized photocatalyst showed a broad band between 280–350 mT with the maxima at 313 mT. The hyperfine splitting resulted in sextet observed for Mn2+ atoms present on the surface of TiO2 lattice and the relative broadening reflected that Mn2+ atom was not isolated.

Fig. 7.

EPR spectra of synthesized Mn-N-co-doped TiO2 calcinated at 550 °C (MnNT-550).

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Thus, ESR studies confirmed the Mn2+ substitution for Ti4+ in TiO2 with octahedral geometry for synthesized MnNT-550. The other prominent, sharp peak in between 125–175 mT was assigned to Mn4+. The appearance of this peak can be explained based on the migration of the isolated Mn2+ atoms to substitutional sites accompanied by oxidation to Mn4+ which is subsequently incorporated in TiO2 lattice. For Mn4+ (d3), the ground state was an orbital singlet and other states were of higher energy, therefore, the sharp signal was observed in EPR spectra. The results were in accordance with earlier studies reported for Mn-doped TiO2[66].

4Photocatalytic activity of synthesized Mn-N-co-doped TiO2 (MnNT-550)4.1Time dependent UV–vis spectra for degradation of quinalphos and 2-chlorophenol

Time dependent UV–vis spectra (Fig. 8a) has been reported for photocatalytic degradation of pesticides in the presence of MnNT-550 at 660 nm. The absorption peak at 274 nm diminishes gradually and 87.5% of the quinalphos is found to degrade within 240 min. Whereas, Fig. 8b shows the photocatalytic degradation of 2-chlorophenol, where the absorption peak diminishes gradually and 91.7% of the 2-chlorophenol was observed to degrade within 240 min. Table 2 compiles the % age degradation of Quinalphos and 2-Chlorophenol reported by researchers by using different techniques [67–69].

Fig. 8.

Time dependent UV–vis spectra of photocalalytic degradation of quinalphos (8a) and 2-chlorophenol (8b).

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Table 2.

Comparative studies of %age degaradtion in literature.

S.No  %age degaradtion  References 
1.  Quinalphos was found to be 4% (1.25 mg) after 4 days of incubation  Rose et al., [67] 
2.  97% quinalphos degraded in 24 hrs  Sidhu et al., [68] 
3.  90.5% degrdation of 2-cp at pH 6.  Mohammad et al., [69] 

The photocatalytic efficiency of the synthesized MnNT-550 for the degradation of organophosphate pesticide (quinalphos) and phenolic derivative (2-chlorophenol) is assessed. The photocatalytic activity of synthesized Mn-N-co-doped TiO2 calcinated at different temperatures viz. 350, 450, 550 and 750 °C was studied for the degradation of quinalphos and 2-chlorophenol. The calcinated photocatalysts MnNT-350, MnNT-450, MnNT-550 and MnNT-750 showed 70.2%, 70.7%, 87.5%, 75.6% and 87.5%, 89%, 91.7%, 84% degradation of quinalphos and 2-chlorophenol, respectively (Fig. 9a, b). The maximum photocatalytic activity was observed for MnNT-550 and further studies were carried out with MnNT-550.

Fig. 9.

Effect of calcination temperature (350, 450, 550 and 750 °C) on % degradation of quinalphos (9a) and 2-chlorophenol (9b).

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Experiments were performed to perceive the response of MnNT-550 towards photocatalytic degradation efficiency in different wavelength regions of the solar spectrum (490 nm, 565 nm and 660 nm). The % degradation at different wavelengths (660 nm, 565 nm and 490 nm) was found to be 87.5%, 71.6%, 64.4% and 91.7%, 74.9%, 51.3%, respectively for quinalphos and 2-chlorophenol (Fig. 10a, b). Maximum efficiency was observed in the red region (660 nm), which was selected for further investigations.

Fig. 10.

Effect of wavelength (660, 565 and 490 nm) on % degradation of quinalphos (10a) and (10b) 2-chlorophenol.

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Photocatalytic degradation of quinalphos and 2-chlorophenol was also recorded under four different conditions: (i) red light, (ii) TiO2 + dark, (iii) MnNT-550 + dark and (iv) MnNT-550 + red light. The result confirmed (Fig. 11a, b) that the maximum 87.5% of the quinalphos and 91.7% of 2-chlorophenol was degraded in the presence of MnNT-550 under red light condition (660 nm).

Fig. 11.

(a) Photocatalytic degradation of quinalphos, (b) 2-chlorophenol under four different conditions: (i) 660 nm, (ii) Dark + undoped TiO2, (iii) Dark + MnNT-550, (iv) 660 nm + MnNT-550.

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4.2Effect of pH

Experiments were performed at various pH values ranging from 2 to 10. The maximum degradation was obtained at pH 7 for the model pollutants (quinalphos and 2-chlorophenol) with concentration of 20 mg/L and photocatalyst loading of 50 mg (Table 3). The percentage degradation at pH 2, 4, 7, 8, 10 and 12 was found to be 51.8%, 54.5%, 87.5%, 79.7%, 50.9%, 36.0% and 50.7%, 64.4%, 91.7%, 71.6%, 60.2%, 40.3% for quinalphos and 2-chlorophenol, respectively.

Table 3.

Effect of pH on the % degradation of quinalphos and 2-chlorophenol (2, 4, 6, 7, 8 and 10).

S.No.  pH  % degradation of quinalphos  % degradation of 2-chlorophenol 
51.8  50.7 
54.5  64.4 
60.6  91.7 
87.5  71.6 
79.7  60.2 
10  50.9  40.3 
4.3HPLC studies

High performance liquid chromatographic (HPLC) studies were carried out to confirm the photodegradation of quinalphos (50 mg/L) in the presence of MnNT-550 under optimized conditions (catalyst dose: 1 g L−1, pH: 7). Fig. 12 shows HPLC chromatogram for photocatalytic degradation of quinalphos after 2 h and 4 h of irradiation time. The appearance of characteristic peak at retention time of 4.96 min after 2 h, confirmed the formation of intermediate. The peak intensity decreased after 4 h of irradiation which confirmed the degradation of quinalphos.

Fig. 12.

HPLC results of Quinalphos after 2 h and 4 h of irradiation time.

(0.15MB).

The HPLC chromatogram showing the photodegradation of 2-chlorophenol (50 mg/L) in the presence of MnNT-550 under optimized conditions (catalyst dose: 1 g/L, pH: 7) is presented in Fig. 13a. The characteristic peak of the compound (2-chlorophenol) is observed at retention time of 4.46 min and Fig. 13b confirms the presence of two new peaks with little shift at their retention time 3.375 min, 3.702 min. It is evident from the chromatographs that the intensity of peaks decreases after 4 h which confirms the degradation of 2-chlorophenol and the appearance of intermediates.

Fig. 13.

(a) HPLC results of 2-Chlorophenol after 2 h, (b) 4 h.

(0.71MB).

Based on experimental results, the mechanism for the photocatalytic activity has been explained. The synthesized photocatalyst MnNT-550 having band gap energy of 2.4 eV, results in the formation of electron and hole pair on the surface of the photocatalyst on excitation by the photons of light. The generated electron-hole pair participates in redox reactions themselves as well as via ˙OH free radical formed by reacting with H2O molecules. The improved photocatalytic activity of the synthesized Mn-N-co-doped TiO2 under visible light is attributed to the band gap narrowing of TiO2 by formation of new states near conduction and valence band. The co-doped Mn2+ ion is trapped with photogenerated electron which inhibits the recombination of electron-hole pair. The promising photocatalytic activity of synthesized Mn-N-co-doped TiO2 could be due to the enhanced formation of photogenerated electrons and holes under visible light irradiation by preventing the recombination rate.

5DFT studies: adsorption of pollutant onto (TiO2)n and (TiO2)n doped clusters

To explore the photocatalytic degradation mechanism of pollutant molecules in the presence of (TiO2)n and (TiO2)n doped clusters, density functional theory (DFT) was employed. The charge transfer taking place between the optimized pollutant molecules and (TiO2)n and (TiO2)n doped clusters has been explained by distribution of HOMO and LUMO orbitals. HOMO orbitals act as electron donor and LUMO orbitals act as electron acceptor [70]. The positive phase has been depicted in red colour whereas green colour represents the negative phase.

Table 4 shows the HOMO-LUMO electron distribution plots for interaction of clusters with quinalphos and 2-chlorophenol. HOMO orbitals are spread over the pollutant molecules whereas LUMO is largely spread over the clusters, thus, indicating the charge transfer from HOMO to LUMO (organic molecules to the cluster). The adsorption of organic molecules on to the surface of (TiO2)n and (TiO2)n doped clusters is an important step as it influences the photocatalytic degradation of pollutants [71]. (TiO2)n and (TiO2)n doped clusters have been selected as models for studying the photocatalytic efficiency of nanoclusters at the molecular level. The adsorption energies (ΔEad) are indicative to measure the affinity of clusters with pollutant molecules (quinalphos and 2-chlorophenol). It is evident from the reported ΔEad values in Table 4 that quinalphos is adsorbed easily as compared to 2-chlorophenol on the surface of (TiO2)n and (TiO2)n doped clusters. From the optimized structures and adsorption energy values, it was noted that 2-chlorophenol preferred to bind to the oxygen atom of (TiO2)n and N-doped (TiO2)n clusters through its hydrogen atom i.e. (OTiO2 -ClCP) whereas in the case of Mn-N-doped (TiO2)n cluster, Cl binds to the Mn atom of the cluster (MnCl bond).

Table 4.

Electron distribution (HOMO-LUMO) distribution plots for interaction of (TiO2)n and (TiO2)n doped clusters with organic molecules.

 

The ΔEad values indicate that quinalphos (−20.41 kcalmol−1) and 2-chlorophenol (−8.59 kcalmol−1) is degraded in the presence of Mn-N-doped (TiO2)n cluster (Table 4).

6Conclusion

The present study reported the cavitation induced greener synthesis of MnNT-550 using ultrasonic irradiation. Transmission electron microscopy results established the formation of nanosized structure, having the particle size of approximately 60 nm. The presence of Mn2+ ion leads to anatase to rutile phase transformation (550 °C) due to the formation of oxygen vacancies. The synthesized MnNT-550 showed a band gap of 2.4 eV and a red shift in the visible region. The maximum degradation (87.5%) of the quinalphos and (91.7%) of the 2-chlorophenol was reported at 660 nm after a time interval of 240 min at pH 7 and results were further confirmed by HPLC studies. The DFT results confirmed the degradation of quinalphos and 2-chlorophenol and suitable position could be identified on the photocatalyst where pollutant molecules can adsorb thereby leading to their degradation. The results supported that ultrasound assisted synthesis of nanophotocatalyst can be employed as a greener technique for the production of nanomaterials with improved photocatalytic properties.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The author is thankful to the authorities of Sant Longowal Institute of Engineering and Technology, Longowal for providing financial assistance. We are pleased to acknowledge the facilities provided Panjab University, Chandigarh; All India Institute of medical sciences (AIIMS), Delhi.

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