Enhanced Photodegradation of Organic Pollutants by Carbon

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Enhanced Photodegradation of Organic Pollutants by Carbon Quantum Dot (CQD) Deposited Fe3O4@m-TiO2 Nano Kooshballs Rahul Kumar Das, Jyoti Prakash Kar, and Sasmita Mohapatra Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00792 • Publication Date (Web): 28 Apr 2016 Downloaded from http://pubs.acs.org on April 29, 2016

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Industrial & Engineering Chemistry Research

Enhanced Photodegradation of Organic Pollutants by

Carbon

Quantum

Dot

(CQD)

Deposited

Fe3O4@m-TiO2 Nano Kooshballs Rahul K. Das,† Jyoti P. Kar‡, and Sasmita Mohapatra*† †

Department of Chemistry, National Institute of Technology, Rourkela, India-769008, E-mail:

[email protected] ‡

Department of Physics and Astronomy, National Institute of Technology, Rourkela, India-

769008 KEYWORDS: mesoporous titanium dioxide, reusable, visible light photocatalyst, organic pollutants, delayed recombination.

ACS Paragon Plus Environment

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ABSTRACT: Highly water dispersible, superparamagnetic carbon quantum dot (CQD) deposited Fe3O4@m-TiO2 koosh balls have been prepared by a simple hydrothermal method. The effective deposition of carbon dots expands the light absorption of mTiO2 from UV to visible region. The results of photocurrent measurement reveal that an optimum deposition of Cdot at 20 wt % significantly increases the photocurrent density. High surface area and extended conjugation of carbon dots enhance the adsorption of organic pollutants. The active species generated in the photocatalytic system were also detected through trapping of radicals and holes in presence of various scavengers. Compared to conventional photocatalyst such as Degussa P25, the developed hetero-junction shows much higher degradation efficiency for ciprofloxacin, methylene blue, quinalphos and 4-nitrophenol under visible light. The light upconversion properties of uniformly deposited carbon quantum dots could be a reason for higher visible light catalytic activity of the synthesized magnetically recoverable hybrid photocatalyst.


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1. INTRODUCTION Development of nanostructured visible light photocatalyst is currently growing as the most exciting research area in the field of materials science because of the potential use of these catalysts in degradation of organic pollutants, water splitting and carbon dioxide sequestration.1-5 Among the photocatalysts, anatase TiO2 is a promising material because of its good oxidising power, photostability, environmental friendliness, chemical inertness and low cost.6-8 In spite of all these positive qualities TiO2 has limited applications because of 1) wide band gap (3.2 eV) instigating its absorption in UV region which is only 3% of the total sunlight, 2) the rapid recombination of photo-exciton before they reach the target site, hence resulting in low photoefficiency.9-12 To increase the photocatalytic efficiency of TiO2 two important strategies have been explored, namely (1) decreasing band gap of TiO2 by making hybrid materials so as to expand the absorption region from UV to visible and NIR range,13-15 (2) introducing conductive electron channels for separation of electrons resulting in delayed recombination of excitons.16,17 Surface heterostructures have been constructed to drain or trap photo induced electron and subsequently enhance the separation of electron hole pairs. In this regard, carbon nanomaterials such as graphene oxide as well as carbon quantum dots are the most promising one, because of their trapping ability.18,19 When combined with TiO2 nanoparticles, carbon quantum dots enhance catalytic activity to different degrees depending on their intrinsic nature of electron transportation and conducting ability.20,21 In addition to this carbon quantum dots have large number of –COOH and phenolic groups which can enhance the adsorption of cationic dyes and other organic pollutants which degrade the matter with a short degradation time. However to date, most of the papers on TiO2-C based hybrid materials reported on the post synthesis deposition of graphene oxide nanoparticles on TiO2 nanoparticle emphasizing on the excellent


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performance of the photocatalyst while little attention has been paid to the reusability of the catalyst which is crucial for their further large scale practical application.22-25 In the present work we have significantly improved the light absorption as well as photocatalytic efficiency of TiO2-C hybrid catalyst by fabricating 3 dimensional mesoporous TiO2 koosh balls. TiO2@C-dot composite has been synthesized following a bottom up approach involving in situ hydrothermal deposition of C-dot in mesoporous walls from carbohydrate precursor. Deposition of carbon dot from a bottom up method ensures the uniform distribution of C-dot on the mesoporous walls minimizing the possibility of aggregation. Furthermore, the larger interfacial contact between carbon dot and TiO2 leads to enhanced electron transfer and light absorption properties. At the same time encapsulation of magnetic nanoparticles inside mesoporous TiO2 matrix guarantees the facile recovery and reusability of the catalyst. The enhanced visible light photocatalytic activity of the composite catalyst was verified for the degradation of common pollutants like organic dyes, antibiotics, pesticides and phenolic compounds taking one molecule from each category as a model. The robust mesoporous C-dot deposited TiO2 koosh balls remains structurally unchanged even after 5 catalytic cycles. Compared to conventional TiO2 (Degussa P25) and other TiO2-graphene oxide composites our catalyst shows significantly enhanced degradation efficiency as observed in case of methylene blue.


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2. EXPERIMENTAL 2.1.

Chemicals

Analytical grade chemicals were used for the preparation of catalysts. Ferrous nitrate and Ferric chloride were used for the coprecipitation of magnetite nanoparticles. TTIP (Titanium tetraisopropoxide) was purchased from Spectrochem Ltd. D-Glucose was supplied by SD Fine Ltd and Finar chemicals. Milli-Q ultrapure water was used for synthesis and conducting photocatalytic degradation experiments. 2.2. Synthesis of Fe3O4@mTiO2 Magnetite Fe3O4 was prepared by coprecipitation method (Scheme 1). In a two necked round bottomed flask 100 mg of magnetite nanoparticles prepared by coprecipitation method were taken in a solution of CTAB (2 g) in acetonitrile and ethanol mixture. The mixture was sonicated to make uniform dispersion of NPs followed by vigorous stirring for 5 min under inert condition. 2.0 mL of TTIP was injected to the reaction mixture in an ice bath. The temperature of the system was gradually raised to 60˚C in time duration of 30 min. 2.5 mL of 25 % NH3.H2O was added for hydrolysis. Stirring was continued for further 1h and the prepared sample was magnetically recovered from the solution. CTAB was removed from the sample by washing with acidic ethanol (ethanol+1 M HCl). The sample was dried overnight at 80˚C and calcined at 400˚C for 2 h. 2.3. Synthesis of Fe3O4@mTiO2@C-dot hybrid catalyst Carbon dot was deposited by hydrothermal treatment of glucose solution on Fe3O4@mTiO2. Different amounts of glucose were added to 200 mg of Fe3O4@mTiO2 nanospheres. The above


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prepared solution was transferred to a teflon-lined stainless steel autoclave (100 mL capacity) and kept at 140˚C for 4 h. On completion and cooling, black suspension was magnetically recovered and washed with a H2O/EtOH (3:1) mixture. The sample was dried overnight and then used for photocatalysis. 2.4. Structural Characterization The Expert Pro Phillips X-ray diffractometer was used for the investigation of phase formation and crystallographic state of the materials synthesized. Morphological and microstructural analysis was conducted using a scanning electron microscope (HITACHI COM-S-4200) and a high resolution transmission electron microscope (JEOL 3010, Japan) operated at 300 kV. Quantachrome surface area analyser was utilized for the measuring the nitrogen adsorption/desorption maintained at 77 K. The Raman spectrum of as prepared samples was recorded on a Renishaw in Via Raman spectrometer (UK model). The mean zeta potential at different pH was measured by Nano ZS 90, Malvern instrument facility. Surface functional groups associated to the materials were identified by using FTIR (Thermo Nicolet, model 870). The binding energy compared with core energy levels was investigated by analysing XPS data using an AlKα excitation source in ESCA-2000 Multilab apparatus. Band gap energy and dye degradation was measured using a Shimadzu 220 V (E) UV-Vis spectrophotometer. The photoelectrochemical studies were performed using Keithley 6487 Picoammeter/ Voltage source instrument. 2.5. Photocurrent Tests For the photocurrent measurement, indium-tin oxide (ITO) coated glass slides with surface resistivity of 8-12 Ω/sq. were used. The samples for analysis were developed by drop-casting


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method (slurry of 30 mg/mL) using ethanol as solvent. Visible light photodetection properties were obtained by adopting current-voltage (I-V) and current-time (I-t) measurement. The nature of electrical contact between ITO glass and deposited nanocomposite has been observed by taking current-voltage measurement before studying the visible light detection behavior. A Philips bulb (LED lamp B22) with power of 12.5 W (1055 lumen) was used as light source. Linear enhancement of current with sweep in voltage of ±1V for all samples depicted good Ohmic nature of the electrical contact. The I~t measurement in presence of light source was recorded by maintaining a bias voltage of 0.5 V. 2.6. Photocatalytic degradation and kinetic studies The measurement of photocatalytic activity was carried out by adding 0.015g of the Fe3O4@mTiO2@ 20% C-dot of the catalyst in 20 mL solution of the organic pollutant (20 ppm) at room temperature. A mercury vapour lamp (420 nm) was used for visible light irradiation. The suspension was kept in dark and magnetically stirred for 1h to obtain adsorption equilibrium then stirred under visible light throughout. The catalyst was magnetically separated. The pollutant in the suspension was quantified by measurement of the absorbance at λmax=664 nm (methylene blue), 270 nm (ciprofloxacin), 235 nm (quinalphos), 392 nm (p-nitrophenol). 2.7. Determination of reactive species To test the active species generated in the photocatalytic system, various scavengers including isopropyl alcohol (i-PrOH), ammonium oxalate (AO) and 1,4-benzoquinone (BQ) were introduced into the solution of ciprofloxacin. i-PrOH , AO and BQ have been used as •OH, hole (h+) and superoxide radical (O•2-) scavengers respectively. The photocatalyst was suspended in aqueous solution containing 20 ppm CPN in deionized water. A batch of eight aliquots each


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containing 20 mL of CPN solution and 15 mg catalyst and 0.1 mmol ammonium oxalate (14.2 mg) were stirred for 4h under light irradiation. The experiment was repeated for Benzoquinone (11 mg) and Isopropyl alcohol (6 mg). 3.

RESULTS AND DISCUSSION

3.1. Synthesis Fe3O4@mTiO2 particles were synthesized by a sol-gel method. CTAB-stabilized nanocrystals acted as seeds for the formation of spherical mesoporous titania particles (Scheme 1). CTAB served not only as the stabilizing surfactant but also the organic template for the formation of the mesoporous TiO2 nanospheres. It was well established in reported work that sucrose can act as a precursor for the formation of carbon quantum dots.23 The hydrogen bonding interaction between the surface -OH groups and glucose facilitates the adsorption of glucose in the mesoporous channels which are converted to carbon dot upon hydrothermal treatment. Thus growth of carbon nanoparticles takes place within the porous frame work of mesoporous titania rather than outside. 3.2. Powder X-ray diffraction and Raman scattering The XRD patterns (Figure 1a) of Fe3O4@mTiO2 and Fe3O4@mTiO2@20%C-dot show characteristic peaks of inverse spinel magnetite (JCPDS 85-1436) marked by their indices (220), (511), (400), (511), (440). Similarly anatase TiO2 (JCPDS 85-1157) is clearly indicated by the typical crystallographic planes (101), (105) and (204). The broadening of the diffraction peak indicates partial crystalline nature of TiO2. No characteristic diffraction peak for carbon is observed indicating deposition of amorphous carbon dots. In contrast, the Raman spectrum of Fe3O4@mTiO2@20%C-dot nanocatalyst displays two prominent peaks at 1310 cm-1 and 1587


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cm-1 corresponding to defect band (D-band) and a graphite band (G-band) which indicates the presence of partially crystalline C-dot on titanium dioxide. The G-band is attributed to the vibration of sp2-bonded carbon atoms in a 2D hexagonal lattice whereas the D-band is associated with vibrations of carbon atoms with dangling bonds in plane terminations of the disordered graphite (Figure 1c). The ratio of intensities ID/IG for Fe3O4@mTiO2@20%C-dot 0.67 which indicates the higher level of disorderness in case of carbon dots deposited on mesoporous titanium dioxide. The low angle XRD pattern (Figure 1b) of Fe3O4@mTiO2 shows intense peak at 2θ = 1.4˚ and two low resolved peaks at 1.96˚ and 2.51˚ representing long range ordering of the mesoporous structure. The peaks have space parameter ratios √2: √6: √8 associated with cubic mesostructured TiO2 having space group Im3m. The shifting of peak position in case of Fe3O4@mTiO2@20%C-dot is attributed to the non-uniform distribution of matter in the unit cell and less structural ordering due to carbonization under hydrothermal condition.26 3.3. Morphology The FESEM image (Figure 2a) of Fe3O4@mTiO2 shows formation of spherical particles of with a uniform size of 350-400 nm and rough surface. The TiO2 spindles are uniformly distributed over a spherical surface as shown in the TEM image (Figure 2b). The TEM image of Fe3O4@mTiO2@C-dot shows spherical structures made up of self-assembled porous spindles. The increase in deposition of carbon can be easily identified by the dark areas (Figure 2b,c,d,e). The TEM image at high resolution shows the capture of crystalline Fe3O4 inside the spherical nanostructures. The lattice fringes (220), (111) corresponding to Fe3O4 and TiO2 respectively are observed in the HRTEM image (Figure 2f). Due to prolong exposure to high temperature and pressure the partially crystalline TiO2 have been transformed into nano koosh balls in accordance to the observation reported by several other research groups. The composition of the sample was


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determined from SEM-EDAX (Figure S1). The surface composition of Ti: Fe: C: O was found to be 14.69: 2.63: 46.36: 36.32 with respect their weight% distribution. 3.4. XPS analysis XPS spectrum (Figure 3) gives further insight to the composition of the Fe3O4@mTiO2@C-dot surface. The survey spectrum (Figure 3a) indicates the presence of (C1s), titanium (Ti2p), oxygen (O1s) and iron (Fe2p) in the Fe3O4@mTiO2@C-dot composite. Peak located at 459.4 eV and 464.9 eV correspond to Ti 2p3/2 and Ti 2p1/2 respectively (Figure 3b). High resolution scan of O1s (Figure 3c) region shows four peaks at 529.4, 531.3, 532.5, and 533.4 eV corresponding to Ti-O-Ti (lattice O), C=O, Ti-OH, and C-OH (and C-O-C) species. In case of Fe3O4@mTiO2@Cdot, the oxygen signals attributed to Ti-O-Ti is found to be less intense. Peak at C1s (Figure 3d) region can be fitted into three peaks at 283.9, 285.1, 287.07 eV which is attributed to sp2 carbon, C-OH and C=O respectively. Absence of any peak at 282 eV rules out the existence of Ti-C bond. In contrast to the previous literature which suggested a possible carbon doping during the hydrothermal process,28 in our case it is clear that carbon dots do not exist as a dopant, rather it is externally deposited on the mTiO2. 3.5. FTIR analysis Comparison of FTIR data (Figure S2) at various stages of synthesis i.e. Fe3O4@mTiO2 with CTAB, Fe3O4@mTiO2 after removal of CTAB and Fe3O4@mTiO2@C-dot reveals the formation of deposition of carbon dots on Fe3O4@mTiO2 nanorods. Non calcined Fe3O4@mTiO2 exhibits two peaks centred at 2922 cm-1 and 2853 cm-1 which corresponds to the asymmetric and symmetric stretching vibrations of the methylene protons of CTAB which completely vanish after calcining at 400˚C for 2h. Two intense and broad peaks appear at 1709 and 1607 cm-1


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which indicates the presence of carboxylic acids, carbonyls, epoxide, hydroxyl groups on the surface of carbon dots as reported in our previous case of hydrothermal synthesis of carbon dots. The presence of these functional groups imparts excellent hydrophilicity to the surface which ultimately results in uniform dispersion of the sample in aqueous medium. 3.6. UV-Vis Absorption study The UV-visible absorbance spectra of all photocatalyst samples prepared with different percentages of carbon dots are shown in Figure 4. Commercial P25 shows absorption in the UV region whereas mesoporous TiO2 absorbs at higher wavelength. However after deposition of carbon dots the visible light absorption is significantly enhanced. With increasing the percentage of carbon dot from 5% to 30%, the absorption in the visible region gradually increased. The band gap of the samples was estimated using Tauc plot. Kubelka-Munk function was used to process diffuse reflectance vales generated by the samples. Kubelka-Munk equation: f(R) = (1-R)2/2R where R is the absolute reflectance of the sample layer Tauc plots were generated by plotting values hʋ f(R)2 against hʋ. The Eg values were estimated by taking intercept of the extrapolation to zero absorption with photon energy (hʋ) axis. Eg values calculated for Degussa P25, mTiO2, Fe3O4@mTiO2 and Fe3O4@mTiO2@C-dot samples with variable percentage of carbon dots are summarized in Table S1. The increased absorption in the visible region Fe3O4@mTiO2@C-dot results in enhanced photocatalytic degradation under visible light.


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3.7. Photocurrent studies To determine the best catalyst for degradation, we have verified the photogenerated charge carrier by measuring transient photocurrent responses under visible light. The photocurrent responses for Fe3O4@mTiO2, and Fe3O4@mTiO2@C-dot with variable percentage of carbon dots were recorded for several on-off cycles under visible light and plotted in Figure 5. In case of Fe3O4@mTiO2, there is negligible photocurrent under visible light as reported for mTiO2 by other research groups.22 Owing to the super charge transport properties of carbon quantum dots, Fe3O4@mTiO2@C-dot composite nanoparticles are expected to have better optoelecronic properties than [email protected] With 5% deposition of C-dot the photocurrent reaches at 7µA/cm2. With gradually increasing C-dot content to 20%, the photocurrent increases to 11µA/cm2. This increase in photocurrent indicates that more electrons are transferred to the photoelectrode, which implies greater separation of photoinduced electrons and holes.25 However when the deposited carbon dots increase to 30%, there is drop in heterojunction without any direct contact with TiO2, thus generating less photocurrent as 3µA/cm2. This decrease in photocurrent is attributed to the aggregation of carbon dots at the precursor to be used to prepare the catalyst is of crucial importance. Thus, the amount of glucose is very much decisive to design an efficient catalyst. 3.8. Photoluminescence property It is already reported in literature that carbon quantum dots absorb in the visible/NIR region and then convert them into UV light due to upconversion properties.23 Photoluminescence has been utilized to verify the upconversion property as well as separation efficiency of photogenerated electron-hole pair. Figure 6 and Figure S3 represent upconverted PL spectra of


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Fe3O4@mTiO2@20%C-dot and Fe3O4@mTiO2 respectively ranging from 400 to 1050 nm. Compared to Fe3O4@mTiO2, Fe3O4@mTiO2@C-dot possesses excellent upconversion property with an emission maximum at 350 nm (λex=500 nm) which can excite mTiO2 to generate electron-hole pair. Further, the carbon quantum dots deposited on the surface of mTiO2 can drain excited electrons from conduction band of mTiO2 and increase the photocurrent.30 In addition to excitation at visible region (400 nm and 420 nm), Fe3O4@mTiO2@C-dot shows emission maximum in the UV region when excited at NIR region (840 nm, 1040 nm) too. Hence the key role of uniformly distributed C-dot on the surface of Fe3O4@mTiO2 is to absorb light in the visible and NIR region and convert it to energy of shorter wavelength thus increasing the light absorption efficiency of the composite. Hence the synthesized catalyst would be a good choice for the degradation of organic pollutants under sunlight. Further, the delayed recombination of electron-hole is well explained by comparing PL of Fe3O4@mTiO2 and Fe3O4@mTiO2@C-dot in the wavelength range 440 to 510 nm with excitation at 280 nm (Figure S4) C-dot deposited Fe3O4@mTiO2 shows decreased PL intensity compared to the intensity of Fe3O4@mTiO2. This decrease in fluorescence intensity suggests that Fe3O4@mTiO2@C-dot composite nanocatalyst possesses longer carrier life time due to the combined effect of mesoporosity and carbon dots.31 3.9. N2 adsorption-desorption To examine the porous nature of the N2 adsorption-desorption experiments were performed as shown in the Figure 7. Fe3O4@mTiO2 particles shows type IV isotherm which is a characteristic property of mesoporous materials. The average pore size is 3.86 nm (Figure S5). The BET surface area of the prepared Fe3O4@mTiO2 was found to be 489 m2/g. After deposition of carbon dot the N2 adsorption and desorption curve changes to type III. This behavior may be caused because of the existence of non-rigid aggregates of plate-like particles or assemblages of slit-


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shaped pores and in principle should not be expected to provide the reliable assessment of either the pore size distribution or the total pore volume which is indicated in the BJH pore size distribution curve of Fe3O4@mTiO2@20%C-dot. The surface area is reduced to 267.07 m2/g. The decrease in surface area after carbon dot deposition may be a consequence of carbon dot plugging the pores of the catalyst. 3.10. Photocatalytic property 3.10.1. Photocatalytic performance of Fe3O4@TiO2@C-dot The photocatalytic efficiency of Fe3O4@mTiO2@20%C-dot was investigated for model organic pollutants like ciprofloxacin, quinalphos, methylene blue and p-nitrophenol under visible light. All suspensions were first stirred in the dark for 30 min before irradiation to reach adsorption−desorption equilibrium as confirmed by the steady-state concentrations in the last 30 min of dark stirring. It was observed that our catalyst is able to degrade almost 90% of all these for contaminants in 2.5 h (Figure 8). The typical degradation by semiconductor materials can be followed as –ln(C/C0) = kt, where k is the apparent rate constant of degradation. The values of rate constant k for the degradation of ciprofloxacin, methylene blue, quinalphos and pnitrophenol are 0.015485, 0.012708, 0.015437 and 0.013935 min-1 respectively. The degradation curves of these compounds are as shown in Figure S6 to S9. It is very clear all these four pollutants do not show any photolysis in visible light up to 3h and our catalyst can efficiently decompose ciprofloxacin, methylene blue, quinalphos and p-nitrophenol. To investigate the advantages of carbon-dot deposited hybrid

catalyst, the degradation efficiency of

Fe3O4@mTiO2@20%C-dot nano-koosh balls was compared to that of Degussa P25, m-TiO2, Fe3O4@mTiO2 and H2O2 under similar experimental conditions. Figure S10 presents k values for


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degradation of pollutants using the above catalysts. It has been observed that in comparison to all other catalysts Fe3O4@mTiO2@20%C-dot nano-koosh balls show highest degradation efficiency having k value 0.015 min-1 for CPN. In addition, when this ternary catalyst is used in presence of H2O2 the degradation is further enhanced (0.019 min-1). The enhanced photocatalytic activity is attributed to the formation of nanoscale heterojunction between mTiO2 and carbon quantum dot in the composite. Moreover, the uniform distribution C-dot on the surface of mTiO2 by hydrothermal deposition plays a key role in producing maximum exciton pair hence giving optimum degradation. Furthermore, in presence of H2O2 the incorporated Fe3O4 produces the excess •OH from photo-Fenton processes, which can further degrade the adsorbed molecules on the surface of the nanocomposite (Figure S10). Therefore in all cases Fe3O4@mTiO2@20%Cdot+H2O2 shows highest k value. 3.10.2. Determination of reactive species The movement of the charge carriers such as e- and h+ is a crucial parameter to generate reactive species such as •O2-, •OH and h+.32,33 Dissociated e− and h+ pairs could be trapped at the photocatalyst surface for generating ROS that degrade various pollutants.34 In order to understand the mechanism of the improved photocatalytic activity of our C-dot deposited Fe3O4@mTiO2 nanokoosh balls the main reactive species generated in the CPN degradation process were detected through the trapping of radicals and holes in the presence of various scavengers such as i-PrOH (•OH scavenger), ammonium oxalate (h+ scavenger), and benzoquinone (•O2-scavenger). Figure 9 shows the effect of various scavengers on the degradation of CPN. Compared to the absence of any scavengers or H2O2 the addition of all the three scavengers suppresses the CPN decomposition rate, indicating that all the oxidative species contribute to the CPN degradation. The contributions of h+, •O2-, and •OH to the overall CPN


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degradation rate are thus determined to be 46.4%, 33.8%, and 19.8% respectively. These results indicate that h+ and •O2- are the main oxidative species for the CPN degradation. Although •O2- is generated in multistep, still •O2- contributes 33.8% of total degradation. This fact indicates that the electron conduction process takes place effectively due to uniform distribution of C-dot on the surface of m-TiO2 by judiciously controlling the deposited C-dot such as 20%. Relative change in the k values associated with quenching based experiments is shown in Table 1. 3.10.3. Reusability test To investigate the recyclability of the catalyst, the degradation experiment for CPN was carried out for 3 consecutive cycles. Each time the catalyst was taken out by using external magnet (Dynamag 2, Invitrogen). The magnetization curve (Ms~H) of Fe3O4@mTiO2@C-dot shows no hysteresis at room temperature indicating superparamagnetic nature of the material (Figure S11). The saturation magnetization value of Fe3O4@mTiO2@20%C-dot is 1.6 e.m.u./g. Fe3O4 particles prepared by coprecipitation method have saturation magnetization value 40 e.m.u/g. In case of Fe3O4@mTiO2@C-dot Ms is less due to presence of nonmagnetic materials. However, the particles can be magnetized in 5 s upon application of external magnet (Dynamag 2, Invitrogen) and again dispersed after withdrawing the magnet (Figure S10). The reusability of the hybrid catalysts has been evaluated for five consecutive cycles. For our hybrid catalyst the degradation efficiency was still high as 100% up to six consecutive cycles (Figure 10a). The TEM image (Figure 10b) of the six-cycled catalyst shows that there is no such change in the structure of the catalyst revealing its high structural stability.


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3.10.4. Plausible

mechanism

for

the

degradation

of

organic

pollutants

by

Fe3O4@mTiO2@20%C-dot The developed hybrid catalyst shows better degradation efficiency as compared with other graphene and carbon dot based titania hybrid catalysts reported till now (Table 2). Based on our experimental results, we propose that the higher efficiency of our photocatalytic system is based on the combined advantages of the following characteristics. 1) After carbon dots were introduced into the Fe3O4@mTiO2 composite catalyst a fraction of visible and NIR light can be converted UV light which enhance the production of electrons and holes as compared to Fe3O4@mTiO2 and P25. 2) The surface charge of Fe3O4@mTiO2@C-dot is -29 mV (Figure S12), which facilitates the higher adsorption of cationic pollutants. Apart from this the mesoporous structure along with the π-π interaction between carbon dot and aromatic pollutant molecules result in a higher adsorption followed by degradation. 3) The three dimensional kooshballs provide high surface area which provides abundant active sites.40 Moreover the defects introduced due to koosh ball architecture provide trapping sites for electrons and holes. Furthermore, carbon dots can accept the photogenerated electrons thus significantly increase the transient photocurrent and decrease luminescence intensity. The mechanism of the photodegradation has been illustrated in Figure 11. It is well known that TiO2 absorbs wave length less than or equal to 380 nm to form electrons and holes. From upconversion PL it was observed that Fe3O4@mTiO2@C-dot converts visible light to lower wavelength. Due to uniform deposition of carbon dots and close contact of C-dot and mTiO2 this converted energy is transferred to mTiO2 matrix thus result an increase in generation of electron-


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hole pairs to show enhanced photocatalytic activity. At the same time the mesoporous network and C-dots accepts photogenerated electrons and significantly reduce recombination. 4) The incorporation of magnetic core has dual benefits. It promotes facile recovery and reusability, at the same time it enhance the production of •OH through photo Fenton processes.41 4.

CONCLUSION

In summary, mTiO2 particles having magnetic nanoparticles in core have been synthesized following a sol-gel approach. Carbon quantum dots were hydrothermally grown on mTiO2 in situ from a carbohydrate precursor. The photocatalytic activity of the as prepared ternary catalyst Fe3O4@mTiO2@C-dot has been explored for the degradation of ciprofloxacin, quinalphos, methylene blue and p-nitrophenol. The results show that the catalytic activity of Fe3O4@mTiO2@C-dot > Fe3O4@mTiO2 > P25. The enhanced catalytic activity of our catalyst is due to the synergistic effect of higher light absorption and delayed recombination as a result of optimum deposition of C-dots. In addition, the catalyst reusable and shows 99% efficiency up to 5 cycles. Due to low cost and easy handling the developed catalyst may be explored for large scale degradation of organic pollutants under visible light. ASSOCIATED CONTENT EDAX spectrum of Fe3O4@mTiO2@C-dot, FTIR spectrum at various stages of synthesis, band gap energies for all catalysts, PL spectra of Fe3O4@mTiO2 and Fe3O4@mTiO2@C-dot at different excitations , PL spectra of the prepared photocatalysts at an excitation wavelength of 366 nm, pore size distribution graph for Fe3O4@mTiO2 and Fe3O4@mTiO2@C-dot, C/C0 vs time (t) data showing photodegradation of ciprofloxacin, methylene blue, quinalphos and p-


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nitrophenol under visible light, comparison of rate of degradation by different catalysts, field dependent magnetization curve of Fe3O4@mTiO2@20%C-dot and zeta potential of Fe3O4@mTiO2@20%C-dot at different pH.

ACKNOWLEDGEMENTS This work is financially supported by BRNS, DAE, Govt. of India (Ref: 2013/37C/8/BRNS/393) and DST Nanomission, Govt. of India (Ref: SR/NM/NS-27/2012). AUTHOR INFORMATION Corresponding Author Sasmita Mohapatra*, E-mail: [email protected] Fax: +91-661-2462651; Tel: +91-661-2462661 Author Contributions All authors have given approval to the final version of the manuscript.


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FIGURES CAPTIONS Figure

1.

(a)

XRD

pattern

of

(i)

Fe3O4,

(ii)

mTiO2,

(iii)

Fe3O4@mTiO2,

(iv)Fe3O4@mTiO2@20%C-dot, (b) Low angle XRD patterns and (c) Raman spectra of Fe3O4@mTiO2@20%C-dot. Figure 2. (a) FESEM image of 20wt% C-dot deposited on Fe3O4@mTiO2, TEM image of (b) 5 wt% (c) 10 wt% (d) 20 wt% and (e) 30 wt% C-dot deposited on Fe3O4@mTiO2, and (f) HRTEM image of 20wt% C-dot deposited Fe3O4@mTiO2. Figure 3. XPS spectra (a) survey spectrum and high resolution scans of (b) Ti2p, (c) O1s, (d) C1s regions of Fe3O4@mTiO2@20%C-dot nanospheres. Figure 4. UV-vis absorption spectra of Degussa P25, mTiO2, Fe3O4@mTiO2 and Fe3O4@mTiO2, with different contents of C-dot. Insets are the mTiO2 and Fe3O4@mTiO2@Cdot solution under visible (left) and UV (right) lights. Figure 5. Transient photocurrent response spectra of Fe3O4@mTiO2 and Fe3O4@mTiO2, with different contents of C-dot. Figure 6. Up-converted PL spectra of Fe3O4@mTiO2@20%C-dot with excitation of visible-near infrared wavelengths. Figure 7. N2 adsorption-desorption isotherm of Fe3O4@mTiO2 and Fe3O4@mTiO2@20%C-dot. Figure 8. Linear relationship of C/C0 vs. time of different pollutants degraded by Fe3O4@mTiO2@20%C-dot. Figure 9. Photocatalytic activity of Fe3O4@mTiO2@20%C-dot with different scavenges under visible light irradiation for CPN degradation.


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Figure 10. (a) Reusability of Fe3O4@mTiO2@20%C-dot up to 6 cycles and (b) TEM image of Fe3O4@mTiO2@20%C-dot recovered after six consecutive cycles. Figure

11.

The

proposed

mechanism

for

Fe3O4@mTiO2@20%C-dot

nanospheres

photocatalyzed dye degradation under visible light.


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Table 1. Relative change in k values with different scavenges under visible-light irradiation.

Table 2. Comparison of various Titania-carbon composites based on their photocatalytic efficiency in organic pollutant degradation.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5

Figure 6


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Figure 7

Figure 8


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Figure 9

Figure 10


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Figure 11

SCHEMES

Scheme 1 Synthesis of Fe3O4 @mTiO2@C-dot


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For Table of Contents only


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Enhanced Photodegradation of Organic Pollutants by Carbon

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