Facile and ultrafast green approach to synthesize bio-based

Sep 7, 2017 - The structure of RCD was characterized by UV-visible, Fourier Transform Infrared, energy dispersive X-ray and Raman spectroscopic analys...
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Research Article pubs.acs.org/journal/ascecg

Facile and Ultrafast Green Approach to Synthesize Biobased Luminescent Reduced Carbon Nanodot: An Efficient Photocatalyst Rituparna Duarah and Niranjan Karak* Advanced Polymer and Nanomaterial Laboratory, Center for Polymer Science and Technology, Department of Chemical Sciences, Tezpur University, Napaam, Tezpur 784028, Assam, India S Supporting Information *

ABSTRACT: Rising awareness pertaining to global waste management and environmental issues challenges the development of an efficient metal-free photocatalytic system for visible light-assisted degradation of organic contaminants. We herein report the synthesis of biobased luminescent reduced carbon nanodots (RCDs) (3 nm average size) by green reduction of starch-based carbon nanodots (CDs) using aqueous extracts of Calocasia esculenta leaf, Mesua ferrea Linn leaf, tea leaf, and flower bud of Syzygium aromaticum. The reduction was found to be ultrafast (3 min) under sonication using Calocasia esculenta leaf extract in the presence of Fe3+ ions at room temperature. The synthesized RCD is an efficient photocatalyst for the degradation of model dirt like methylene blue, methyl orange, and their mixture as well as toxic chemicals like bisphenol A under normal sunlight. These degradations followed the pseudo-first-order kinetics model. The catalytic efficiency of RCD was significantly higher than that of CD. The structure of RCD was characterized by UV−visible, Fourier transform infrared, energy-dispersive X-ray, and Raman spectroscopic analyses as well as X-ray diffraction and transmission electron microscopic studies. The photoluminescence characteristic of RCD was analyzed by fluorescence spectroscopy. The results showed that exploration of sustainable resource-based RCD may offer a novel scope in resolving environmental and ecological problems. KEYWORDS: Reduced carbon nanodot, Carbon nanodot, Phytoextract, Photocatalyst, Photodegradation



INTRODUCTION Chemical transformation using green solvents and nontoxic chemicals under ambient reaction conditions by harnessing the abundant energy of solar radiation has emerged as one of the major thrusts in scientific research to combat the adverse impact of industrial effluents on the environment and human health.1 Despite the significant role of chemical-based industries in society, widespread industrial and anthropogenic ventures have introduced huge amount of chemicals such as surfactants, pesticides, dyes, and so forth into the environment, which destroys the ecosystem.2 The versatility and complexity of such chemicals in use make it difficult to find a common treatment method that completely covers the efficient elimination of all categories of organic pollutants. Although certain physical techniques such as reverse osmosis, flocculation, and adsorption are not harmful, they transport the contaminants to other media, which results in secondary pollution.3 Hence, there is an imperative demand in the fields of green chemistry and materials science to develop newer methods with improved performance for the destruction of organic contaminants. The exploration of various suitable semiconductor photocatalysts like TiO2, ZnO, Fe2O3, and so forth for organic pollutant degradation is one of the major strategies for resolving environmental pollution and energy crisis.3,4 However, they © 2017 American Chemical Society

need higher energy UV and/or short wavelength visible radiation. Therefore, exploiting a photocatalyst that can be driven by solar radiation to realize the photoassisted degradation of organic compounds and reduction of inorganic ions with high efficiency in the order of days.4,5 Furthermore, sunlight-based photocatalysis is a sustainable and economical practice owing to the use of solar energy, which is a nonpolluting, inexpensive, and endlessly renewable source of clean energy.6,7 It is pertinent to mention that near-infrared (NIR) and infrared (IR) light radiation are still not completely exploited by reported photocatalysts. In this milieu, an emerging nanomaterial, carbon nanodots (CDs), has inspired intensive research owing to their excellent biocompatibility, photoluminescent, photostability, and nanoscale dispersibility in water.8,9 CD exhibits a definite “optical” energy gap that apparently depends on its surface texture, size, and shape. These guide photoinduced electron transfer capability, down- and up-converted PL, and excitation energydependent photoluminescence (PL) of CD.10 As CD normally absorbs in the UV, green, and blue regions, their utilization in Received: July 30, 2017 Revised: September 6, 2017 Published: September 7, 2017 9454

DOI: 10.1021/acssuschemeng.7b02590 ACS Sustainable Chem. Eng. 2017, 5, 9454−9466

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Therefore, we wish to report the synthesis of RCD from biobased precursors by using C. esculenta leaf extract by a green one-step approach for the first time. Most importantly, this RCD has a greater extent of sustainability in comparison to one that follows a chemical route. Further, the well-characterized sustainable resource-based luminescent RCD was attempted to be used like an effective nanophotocatalyst for degrading organic contaminants such as toxic BPA, methylene blue (MB), methyl orange (MO), and the MB/MO mixture under normal sunlight.

the region of relatively longer wavelengths is limited. Even dimensional modification, heteroatom doping, surface passivation, and so forth proved ineffective for utilizing CD in the region of red and near-infrared (NIR) light.11 In addition, the occurrence of up-conversion by the CD system is disputed in several studies. 12 However, the effective photocatalysis accomplished through down-conversion is also cited in the literature.12,13 Keeping this in mind, we tried to focus the development of reduced CD (RCD) with a strong visible to NIR light absorption capacity with enhanced fluorescence. The unique structure and PL properties of RCD may open a host of possibilities for efficient use of this photocatalyst.9,10 Literature cites the use of various conventional reducing agents like sodium borohydride, sodium citrate, ascorbic acid, and so forth to reduce CD without significant enhancement in luminescence.14−16 Further, these chemical routes are unfavorable due to environmental issues. In contrast, no report is found on the reduction of CD by using naturally renewable low cost, nontoxic reducing agent such as phytoextracts, which contain different polyphenolic compounds with high reducing capability, though the global concern with respect to the dwindling petro-based resources has instigated the utility of sustainable feedstock in the synthesis of industrially important materials.6,9,10,17 Therefore, we employed a sustainable, efficient, and harmless approach for the synthesis of biobased RCD by reducing starch-based CD using aqueous phytoextracts. Thus, the developed biobased RCD has the potential to tackle widespread challenges of environmental conservation and sustainability.6 Again, advanced oxidation processes (AOPs) including ozonation, photolysis, and photocatalysis are used to mineralize organic compounds into CO2 and H2O with light, oxidants, and semiconductors. In this process, the photocatalysts absorb photons (UV) to produce electron−hole (e−/h+) pairs where the e− and h+ react with oxygen and water molecules to give hydroxyl radical (●OH) and superoxide radical (●O2−), respectively. These active radicals subsequently oxidize pollutants to CO2 and H2O.2,7,10 Furthermore, reduction of CD may result in an optimum level of peripheral polar functional groups that in turn interact with the organic pollutants and help to adhere on the surface, which assists in better interaction with the active superoxide radicals. Therefore, RCD may photocatalytically degrade the organic dyes and other contaminants under sunlight. In this context, previous literature cites the use of reduced graphene oxide (RGO) in photocatalytic degradation of different organic compounds like rhodamine B and methylene blue.18,19 However, the method for the preparation of graphene oxide is very tedious and has its own drawbacks with respect to safety and environmental issues. Therefore, we employed abundant and cost-effective carbon resources to synthesize CD, and RCD was obtained through an environmently friendly green technique from this CD. Again, bisphenol A [2,2-bis(4-hydroxyphenyl)propane, BPA] is a nondegradable, anthropogenic chemical contaminant both in water and the soil, causing endocrine disruption.20,21 Thus, scientists have developed efficient remediation methods to degrade BPA, including ultrasonic, Fenton oxidation, H2O2 oxidation, and photocatalytic methods.22 However, there are very few reports on BPA degradation using nanophotocatalysts. These photocatalysts have very poor photocatalytic efficiencies even under UV light.20−22 In this context, the synthesized RCD with improved photocatalytic activity under sunlight may efficiently degrade BPA.



EXPERIMENTAL SECTION

Materials. Soluble starch (Sigma-Aldrich, Germany, Mn = 342.30 g mol−1), ethanol (Merck, India), and bisphenol A (BPA, Sisco Research Laboratories Pvt. Ltd., India) were used as received. Citrus limon (C. limon) fruits and S. aromaticum (Cloves) flower buds were purchased from the local market at Tezpur, India. Calocasia esculenta (C. esculenta), Mesua ferrea Linn. (M. ferrea Linn.), and tea leaves were collected from a neighboring area. Phytoextract Preparation. Approximately 2 g of washed C. esculenta, M. ferrea Linn., and tea leaves and S. aromaticum flower buds were coarsely ground and subsequently stirred for approximately 30 min in 50 mL of water at 60 °C. The aqueous extract was then filtered (Whatman no. 1 filter paper) under ambient conditions. RCD Synthesis. At first, synthesis of CD was conducted by a facile green one-step hydrothermal synthetic route with starch and C. limon extract without base as reported in our previous study.23 Briefly, starch solution (5 g in 40 mL of distilled water) with a few drops of C. limon extract and 20 mL of ethanol were taken in an autoclave and heated at 150 °C for 5 h to obtain a brown water-soluble CD with a yield of 54 mg mL−1. Subsequently, RCD was synthesized by reducing CD via a single step green facile process using various phytoextracts. In a typical process, 10 mL of aqueous extract of C. esculenta leaf and Fe3+ ions (10 mL of 0.01 M) were slowly added dropwise to 100 mg of CD solution at room temperature under continuous stirring. Other aqueous phytoextracts such as M. Ferrea Linn. leaf, tea leaf, and S. aromacticum flower bud with Fe3+ ions as well as C. esculenta with other metal ions Cu2+, Ni2+, and Cr2+ ions were also used separately for the reduction of CD under the same conditions. The change of color from light brown was taken as completion of the reduction process and was supported by UV analysis. The product was washed thrice with distilled water and separated by centrifugation for 10 min at 8000 rpm. This process removed water-soluble components, and finally, the residue was redispersed in water by ultrasonication. The preparation of RCD was also carried out under refluxed conditions using the same technique and reducing agents for comparison purposes. Characterization. The structures and chemical compositions of CD and RCD were characterized by FTIR, XRD, TEM, EDX, UV− visible spectroscopic analyses. Optical and electrical properties were analyzed by photoluminescent setup and conductivity meter. The details of these techniques are given in the Supporting Information. Photocatalytic Activity. Organic dyes such as MB, MO, MB/MO mixture, and organic contaminants (OCs) like BPA were used to study the photodegradation activity of RCD and CD. Typically, 50 mg of RCD was taken in four separate flasks containing 100 mL of aqueous solution of MO (10 mg/L), MB (10 mg/L), and MB/MO mixture (10 mg/L each, separately) and BPA (50 mg/L). The solutions were made to stir under normal sunlight (60000−80000 lx). In addition, the experiment was conducted using same amount of CD for comparison purposes. The changes in concentrations of MB, MO, and BPA were monitored from the UV absorbance intensity at wavelengths of 657, 459, and 273 nm at specific durations of time.19,20 The catalytic efficiency of RCD was calculated from the change of concentration rate of the OC. The same experiment was also conducted using same amount of CD for comparison purposes. The amount of the degraded pollutants was obtained from the equation10

degradation (%) = [(C0 − C)/C0 × 100] 9455

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ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis of RCD by C. esculenta leaf Extract in the Presence of Fe3+ Ions

where C0 = initial concentration and C = concentration after photocatalytic degradation of the OC.

biocompatibility. However, reduction of CD using renewable resources like phytochemicals results in a decrease of oxygen content in it by removing some of the oxygeneous functional groups. This optimum level of polar functionalization on RCD helps in better interaction with the polar groups of the organic pollutants and adheres on their surfaces, thus assisting in interaction with the active superoxide radicals. Upon reduction, there is also generation of a graphitized aromatic structure in RCD, which helps to delocalize the photogenerated electrons and stabilized them. This consecutively delayed the recombination of e−/h+ pairs. Further, RCD has a lower band gap than CD and thus generates more electron/hole pairs upon absorption of light energy. Therefore, we focus on reduced CD (RCD) to obtain improved photocatalytic activity of OCs under sunlight. The results showed that reduction was fastest in the case of C. esculenta leaf extract among the used phytochemicals. Further, the reduction was quicker under refluxed conditions than at room temperature in all cases because at higher temperature the activation energy for CD reduction was attained at a much faster rate. We observed that the time for reduction was unaffected by the amount of phytoextract. Further, the reduction of CD by phytochemicals with different metal ions (Fe3+, Cu2+, Ni2+, and Cr2+ ions) under ambient conditions revealed the effective



RESULTS AND DISCUSSION RCD Synthesis and Characterization. CD was synthesized by hydrothermal acid hydrolysis of the aqueous ethanolic solution of starch and citric acid as the biobased carbon precursors using the same reported method.23 The details of synthesis and most of the characterizations of CD are provided in the ESI. The times required for catalytic reduction of CD under reflux conditions as well as at room temperature are given in Table S1. We focus on reduced CD to achieve a strong visible-to-NIR light absorption capacity with enhanced fluorescence of it, which is ultimately used as an efficient solar light-based photocatalyst. There are reports on reduced graphene oxide (RGO) as a photocatalyst for degradation of various OCs like rhodamine B and MB.24 However, the method for the preparation of graphene oxide is very tedious and has its own drawbacks with respect to safety and environmental issues. Therefore, we focused on the green reduction of CD by employing easily available and cheap carbon resources through an environmentally friendly greener technique for the first time. As mentioned in the literature, CD contains a large number of polar peripheral groups, which imparts exceptional nanostate aqueous dispersibility, intriguing optical properties, and 9456

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ACS Sustainable Chemistry & Engineering Scheme 2. Plausible Electron Transfer Mechanism for the Reduction of CD

reduction with only Fe3+ ions.24 The reduction times for different phytoextracts in the presence of Fe3+ ions, with and without sonication, clearly demonstrated the efficiency of C. esculenta leaf extract (Table S2). It took 3 and 10 min with and without sonication, respectively (Table S2). This is a significant accomplishment as the reduction time was drastically reduced by this approach (Tables S1 and S2). This may be due to complex formation between Fe3+ and the polyphenol groups of the phytoextract as supported by UV−visible absorbance data.17,25 The schematic representation of the synthesis of RCD using C. esculenta leaf extract in the presence of Fe3+ ions is shown in Scheme 1. Literature cites C. esculenta leaf extract contains polyphenolic compounds like apigenin, pectins, luteolin, flavonoids, ascorbic acid, and various flavones, which showed strong potential for reduction of graphene oxide as reported earlier.17,26 The reduction potential of this phytoextract was also reported to be enhanced by addition of Fe3+ ions.27 These polyphenolic compounds contain different polar functional groups such as hydroxyl, carboxy, and so forth, which can form complexes with Fe3+ ions (Scheme 2).17,25 The absorption at 277 nm may be due to the formation of such complexes as depicted in the given reaction.24 6ArOH + FeCl3 → H3[Fe(OAr)6 ] + 3HCl iron polyphenolic complex

The reduction time was dependent on the complex formation rate, as Fe3+ ions might form a stronger and more stable complex with polyphenol compounds present in the phytoextract as compared to other studied metal ions, thereby exhibiting quicker reduction. During complexation, a large number of H+ ions were released, which brought about a change in the pH of the system.25 The pH of metal ion containing C. esculenta leaf extract and phytoextract were measured to be around 3 and 6, respectively. For better understanding of the result of pH, we also conducted the reduction by regulating the pH of the mixture at 3. However, even after 1 h, no effective reduction was achieved, which implies that pH has no influence on the course of reduction. After completion of reduction, the UV−visible spectrum of RCD shows a red shift of the characteristic peak at 269 nm, suggesting that electronic conjugation was restored.25,28 Further, the optical absorption of RCD was found to be higher than that of CD (Figure 1a). For the role of Fe3+ ions in the reduction process of CD to be clarified, an additional experiment was also conducted where we observed that CD was not reduced with only Fe3+ ions even after a prolonged time. There was no red shift of the characteristic peak of CD in the UV visible spectrum of the solution containing CD and Fe3+ as shown in Figure S1. This clearly indicates that Fe3+ is not a reductant in its bare state. However, as Fe3+ helps in complexation with the polyphenol compound and enhances the

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Figure 1. (a) UV−visible spectra of (i) CD, (ii) M. ferrea Linn.-reduced CD, (iii) tea-reduced CD, (iv) S. aromaticum-reduced CD, and (v) C. esculenta-reduced CD; (b) FTIR spectra of (i) RCD and (ii) sodium borohydride-reduced CD (used for comparison purposes); (c) XRD patterns of (i) CD and (ii) RCD; (d) Raman spectrum of RCD; and (e) EDX map of RCD.

Figure 2. (a) HRTEM image of RCD at 2 nm magnification; (b) particle distribution of RCD at 100 nm magnification with its SAED pattern as inset; FFT images of RCD phase (c) before and (d) after masking; and (e) IFFT of RCD phase showing the lattice fringes.

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Figure 3. PL spectra with variation of excitation wavelength 300−400 nm of (a) CD and (b) RCD and PL spectra with variation of concentration of (c) CD and (d) RCD.

Figure 2b). It is pertinent to mention that we also used the Bragg’s equation to calculate the interplanar distance of RCD using the equation

rate of reduction of CD, it is a promoter in this catalytic process. Further, literature supports the formation of H+ ions and faster release of electrons during complexation of C. esculenta and Fe3+ ions, which have a strong influence in this reduction process.24 From these results, it was observed that the preparation of RCD was the fastest and most effective using aqueous C. esculenta leaf extract with Fe3+ ions under normal atmospheric conditions; thus, this process was used for further study. Scheme 2 shows the plausible mechanism of electron transfer for CD reduction through electron transfer and nucleophilic attack (SN2). The FTIR absorbance bands (νmax/cm−1) like CO (1731), C−O−C (1420), C−O (1266), and epoxy group (917) in CD (Figure S2a) were diminished in RCD (Figure 1b (i)).25 The FTIR spectrum of RCD was also compared with CD reduced by sodium borohydride spectrum (Figure 1b (ii)) to understand the reduction. The XRD patterns show that the broad peak of CD near 2θ ∼ 21° disappeared, whereas the peak near 32° was slightly sharpened in RCD (Figure 1c), thus indicating the formation of a more graphitic structure.29 The characteristic bands such as G band (1590 cm−1), D band (1361 cm−1), and 2D band (2912 cm−1) in Raman spectra of RCD (Figure 1d) clearly demonstrated the increase of ID/IG as compared to CD (Figure S2b), thus indicating multilayer graphitization of RCD. This is because of restoration of sp2 carbon with a decrease in its domain size upon reduction of CD as well as owing to the unrepaired defects that remained even subsequent to the removal of oxygeneous groups.30,31 This value of ID/IG ratio is consistent with the majority of literature on the chemical reduction of RGO.24,32,33 In addition, there is an increase in the carbon-to-oxygen ratio (C/O) of RCD (11.2) in EDX analysis (Figure 1e) as compared to that of CD (1.59) (Figure S2c). HRTEM images confirmed the spherical shapes of CD (Figure S3) and RCD (Figure 2). Further, the average size of 3 nm and lattice spacing of 0.20 nm were found for RCD. The poor crystalline nature of RCD was also supported by the selected area electron diffraction (SAED) pattern (as shown inset of

d = nλ /2sin θ

(iii)

where n = integer (1) and λ = wavelength of the incident X-ray beam (0.154 nm) at a certain angle of incidence (theta, θ = 32°). Using this equation, the lattice spacing of RCD was found to be 0.15 nm for the peak at 32°, whereas from the TEM, the lattice fringe was calculated to be 0.20. Thus, the difference is not very significant. However, this little difference may be explained as follows. During TEM analysis, a very dilute solution of the sample was taken, and the lattice fringes were determined for a particular particle of RCD where the interplanar spacing between the two planes was found to be 0.20 nm. On the other hand, during XRD analysis, the sample was taken in powdered state, which causes agglomeration to take place and generates XRD peaks of many particles of different characters. Thus, the lattice fringe of RCD determined from TEM and XRD analyses may differ slightly. Optical Property. The UV−visible absorption spectrum for CD displayed absorption peaks at around 220 nm attributed to the π−π* transition band of the conjugated double bond and at around 280 nm (through a tail continuing into the visible range) credited to the band of n−π* transition of the carbonyl bond (Figure 1a (i)).9,23 However, upon reduction of CD, the band near 280 nm disappeared and a peak near 220 nm undergoes a red shift to 269 nm, which indicates the restoration of conjugation and results in the formation of an aromatized multilayered structure (Figure 1a).24 Further, RCD exhibited a broad absorption band over a wide range of wavelengths from 250 to 800 nm, indicating effective photoabsorption that would be useful for its photocatalytic activity under visible light.34 This broad absorption band over a wide range of wavelengths is due to the graphitic structure of RCD, which has long π−π conjugation. The aqueous solutions of CD and RCD were found to be brown and dark brown in color, respectively, under normal light; however, they exhibited green and blue 9459

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Figure 4. (αhν)1/2 vs hν plots of (a) CD and (b) RCD and (c) I−V curves for (i) RCD and (ii) CD.

fluorescence under UV light at various wavelengths as shown in Scheme 1. Upon reduction of CD, the quantum yield increased from 9.6 to 18.5% (excited at 340 nm, using Quinine sulfate as the reference). Size-dependent photoluminescence is the characteristic feature of the synthesized CD and RCD. The PL spectra of CD and RCD at the same concentration (10 mg/mL) shows that the intensity depends on their respective concentrations and excitation wavelengths (Figure 3).6,9 With an increase in the excitation wavelength, the emission peaks of both CD and RCD shifted to a higher wavelength, reaching fluorescence maximum while being excited at wavelength 340 nm, but subsequently decreasing upon further increasing the wavelength (Figure 3a and b). The highest PL intensity of CD and RCD was attributed to the nature of their surfaces and the fact that the maximum number of particles was excited at that particular wavelength. The polar functional groups present on the surfaces of CD and RCD might be a consequence of the emissive traps between π and π* transition of CC. The emission is dominated by a surface energy trap when CD and RCD are illuminated at a definite excitation wavelength.9 The intensity of the PL spectra increases with increasing concentrations of CD and RCD. This is because of the increase in interactions among polar surface groups at high concentration (Figure 3c and d). The difference in size of the nanoparticles leads to variation in the position of the emission peak. Similar to semiconductor quantum dots, the energy gap of CD and RCD increases with the decrease in size because of the quantum effect. Therefore, particles with larger size are excited at higher wavelengths, whereas particles with smaller size are excited at lower wavelengths.6,35 The enhanced green emission of the RCD is due to the zigzag sites of its surfaces.14,36 The following equation was used to calculate the optical band gaps of CD and RCD from the UV−visible spectra.21,24 α = C(hv − E bulk )1/2 /hv

a sustainable and green preparative route by using environmentally benign solvents, nontoxic chemicals, and renewable precursors. Compared to traditional semiconductor catalysts like TiO2, photoluminescent CD and RCD are superior in terms of functionalization, resistance to photobleaching, toxicity, and profound biocompatibility.6,16 In this regard, RCD may display efficient photocatalytic degradation of OCs under sunlight due to its strong visible light absorption band and enhanced fluorescence. Electrical Conductivity. Electrical conductivities of CD and RCD were measured from their current−voltage (I−V) characteristics as obtained by a four probe setup (Figure 4c). RCD exhibited a linear I−V relationship with the change of voltage in the range of −10 to +10 V. The slope of the I−V plot for CD was found to be almost equal to zero. This clearly indicates that, prior to reduction, CD behaved like an insulating material. This is because of the presence of large oxygeneous groups. The structure of CD is mainly amorphous owing to distortions from the high amount of sp3 carbon. The random distribution separates the sp2-hybridized aromatized rings from the sp3-hybridized rings, which lead to the insulating nature of CD. Nevertheless, the I−V slope of RCD considerably increased after reduction, indicating high electrical conductivity. Further, the linear behavior of the I−V curve of RCD relates to the Ohmic contact of the graphitic structure with the electrode. The improved electrical conductivity of RCD is attributed to the synchronized restoration of sp2 carbon networks and the elimination of oxygeneous groups upon reduction. The conductivities of CD and RCD were found to be 2.54 × 10−7 and 2.53 × 10−6 Sm−1, respectively. The Raman results and the I−V measurement data revealed that, with increasing sp2 carbon content and ID/IG values, the conductivity of CD increased. The results follow a similar trend as reported by Lopez et al. where they showed that the chemical vapor deposition graphene oxide (CVD-GO) demonstrated a linear increase in electrical conductivity with the increase in the ID/IG value.38 It is pertinent to mention that this conductivity is not due to the presence of Fe3+ ions, as no trace of iron was present in RCD, which was confirmed from EDX analysis (no peak was observed for iron at ∼6.4 keV) (Figure 1e). Further, the qualitative test of aqueous ammonium thiocyanate (NH4SCN) solution resulted in no color for aqueous dispersion of RCD, whereas it produces a blood-red color for the solution containing CD, phytoextract, and Fe3+ ions. Thermal Behavior. CD exhibited a two step thermal degradation pattern, whereas more steps were observed for RCD as shown in Figure 5. In both cases, initial (2−4)% weight

(iv)

where α = absorption coefficient, C = constant, h = Plank’s constant, ν = is the frequency, and Ebulk = “band gap”. The band gaps were calculated by drawing the tangent on the plot hv versus (αhv)2 that cut at the X-axis (Figure 4a and b). The band gaps of CD and RCD were found to be 3.64 and 3.18 eV, respectively. The result indicates that the reduction of CD results in restoration of the π-conjugated system. These band gap values are similar to the reported band gaps of the RGO and GO systems.25,37 Further, contrary to commercial TiO2, CD and RCD belong to a class of carbonaceous organic semiconductors that provide 9460

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Photocatalytic Activity. The pathways for photocatalytic degradation of organic contaminants (OCs) such as MB, MO, MB/MO mixture, and BPA were studied by CD and RCD under normal sunlight as shown in Scheme 3. Actually, when RCD/CD nanoparticles were exposed to the OC solution, they absorbed visible light along with near-infrared light, which led to the production of e−/h+ pairs in RCD/CD by excitation of valence band (VB) electrons.40 The generation of e−/h+ pairs in the RCD/CD photocatalyst is shown in eq v.41 photon activation: RCD/CD + hν → e− + h+

(v)

Consequently, the photogenerated electrons in RCD/CD easily react with the oxygen molecules (O2) present in the aqueous medium to produce superoxide radicals (●O2−), whereas the photogenerated holes in the VB react with water molecules to produce hydroxyl radicals (●OH). These reactive oxidative species, ●O2− and ●OH, degrade the organic molecules through an oxidative pathway as described in eqs vi and vii.40

Figure 5. TGA thermograms of (i) RCD and (ii) CD.

loss near (110−112) °C may be attributed to loss of entrapped water molecules between the nanoparticles due to their surface polar functional groups.25 The definite preliminary degradation for CD and RCD commences near 200 °C due to the loss of labile oxygeneous groups (hydroxyl, epoxy, etc.). RCD exhibited a steady loss of only 18 wt % up to 280 °C, which was much lower than that of CD, where loss of 32 wt % up to 250 °C was observed. These results indicate a notable decrease in the amount of oxygeneous groups in RCD. Further RCD showed minimum weight loss up to the temperature of 250 °C, and it exhibited a weight loss of 38% between 500 and 800 °C. This may be attributed to the presence of phytoextract (PE) bound to the surface of RCD. This can be confirmed from the TGA curve of pure PE, which shows the same trend.39 Therefore, RCD experiences 48−50% less weight loss as compared to CD in the temperature range of 300−800 °C, which is also an indication of elimination of oxygeneous groups by reduction and higher thermal stability of RCD compared to those of CD. The residual weights obtained for CD and RCD at 800 °C were approximately 12 and 62%, respectively.

adsorbed oxygen: (O2 )ads + e− → •O2−

(vi)

water/moisture present: H 2O + h+ → •OH + H

(vii)



Accordingly, the amounts of OH radical depend on the quantity of h+ generated in RCD/CD. Furthermore, the quantity of h+ also determines the ability of photocatalytic degradation. Subsequently, the organic pollutants are transformed into their degraded products by these active oxygen radicals into CO2 and H2O via photocatalytic pathways as shown in eqs viii and ix.40,41 R‐H + •OH → •R + H 2O •

(viii)

R + h+ → •R + → CO2 + H 2O

(ix)

It is pertinent to mention that RCD has the ability to absorb more solar light in comparison to CD, as revealed from their absorption spectra (degradation results are shown in Figures 6−8). This assists in increasing the photocatalytic efficiency of RCD under sunlight. RCD was finely ground using a mortar

Scheme 3. Proposed Photocatalytic Mechanism for Degradation of Organic Pollutants by RCD and CD

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Figure 6. Plots of UV absorbance against wavelength at different times for the degradation of MB in the presence of (a) RCD and (b) CD; plots of UV absorbance against wavelength for the degradation of MO in the presence of (c) RCD and (d) CD; plots of UV absorbance against wavelength for the degradation of MB/MO mixture in the presence of (e) RCD and (f) CD; and (g) images showing the decolorization of MB by films of (i) silica gel, (ii) silica gel with RCD, and (iii) silica gel with CD.

greater surface adsorption capacity to hydroxyl groups and lower charge carrier recombination rate as compared to those of CD. The degradations of different dyes and BPA are cited in the literature. The degradation rate of methylene blue (MB), methyl orange (MO), and MB/MO mixture with time for RCD and CD under solar light are shown in Figures 6 and 8. Wang et al. reported that 50 mg of Zn(II)-based catalyst in 10 ppm solution of MB and 10 ppm solution of MO showed effective degradation of 90.9 and 91.7%, respectively, under UV light irradiation within 120 min.42 Tang et al. showed that, using 0.3 g of CaIn2O4 photocatalyst in 100 mL aqueous solution of MB at 47.8 μmol/L concentration, nearly 80% degradation was observed under visible light within 120 min.43 Literature also reports that 0.1 g of TiO2/ZnO photocatalyst can degrade 97% MO at 20 mg/L concentration with 30 min of UV light irradiation.44 Conversely, 50 mg of RCD can degrade 100% of MB (10 ppm) within just 45 min and 96% MO (10 ppm) within 60 min under sunlight (Figure 6a and c). The dye degradation using CD was also tested to study the effect of loading and the advantage of RCD compared to CD as shown in Figure 6 b, d, and f. CD took longer for the same degradation of organic contaminants, which was completed by RCD in a shorter amount of time. This is because of their ability to act as effective solar light-assisted photocatalysts and not by the sensitization of dye molecules.

and a pestle to increase its surface area. UV absorbance was used to monitor the concentration changes of the OCs over time. Figures 6 and 7 show the plots of optical absorbance against wavelength for photodegradation of OCs at various time intervals for both RCD and CD. The photocatalytic activity of a semiconductor results from the excited e − produced under UV light in the conduction band together with the equivalent h+ in the valence band, which react with the pollutants adsorbed on the surface of the photocatalyst.10 The photocatalytic efficiency of CD is reduced by the relatively larger band gap of CD owing to low absorption capability of visible light and high recombination rate of photogenerated e−/ h+ pairs that are produced during the photocatalytic process. Because of the lower band gap of RCD (3.18 eV) than that of CD (3.64 eV), RCD has higher ability of absorbance toward sunlight than CD, and thus, the energy required for photogeneration of e−/h+ pairs is relatively low.40 Thus, the effective number of electrons generated for utilization in the degradation of OCs is higher in RCD compared to those in CD. Although recombination of e−s and h+s in RCD takes place, the neat number of e−/h+ pairs required for generating ● O2− and ●OH to degrade the OCs is higher than CD. Furthermore, RCD contains delocalized π electrons as it obtains stability through the π conjugation in its aromatized structure, which helps in reduction of e−/h+ recombination. Thus, RCD possesses higher photocatalytic efficiency due to its 9462

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Figure 7. Plots of UV absorbance against wavelength at different times for the degradation of BPA in the presence of (a) RCD and (b) CD; (c) degradation curves of aqueous solution of BPA; and (d) fitting degradation kinetic curves for BPA by CD and RCD.

Figure 8. Degradation curves of aqueous solutions of (a) MB, (b) MO, and (c) MB/MO mixture; and fitting degradation kinetic curves for (d) MB, (e) MO, and (f) MB/MO mixture using RCD and CD.

However, both CD and RCD exhibited a complete higher degradation rate for MO in comparison to MB in the MB/MO mixture (Figure 6e and f). The difference implies that RCD and CD exhibit better degradation of MO in the MB/MO mixture. The lower rate of degradation of MB in the MB/MO mixture as compared to individual MB solutions is primarily attributed to the presence of NN (which makes MO more reactive),

The photocatalytic degradation activities of CD and RCD were also studied on the MB/MO mixture. It was observed that, after 15 min of exposure under sunlight, the intensity of UV absorption peak of MO decreased sharply in the case of RCD, whereas the intensity for MB still remained high as shown in Figure 6e. After subsequent exposure under sunlight up to 90 min, MB underwent rapid degradation by RCD. 9463

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where C = concentration of the OC at specific time t and K1 = apparent rate constant. After integrating eq x, we obtain the equation

-CH3 group in MB (which makes it resistant to photodegradation), and the occurrence of competitive consumption of the oxidizing species in MO.39 Literature sites similar preferences in the photocatalytic degradation between the MB/ MO mixture for mesoporous ZnO/ZnAl2O4 powder45 and NiO-Bi2O3 nanocomposite as reported earlier.46 For the same reasons as stated above, RCD demonstrated superiority over CD when the self-cleaning property was studied by films coated with silica for photocatalytic degradation of MB as model dirt was studied by photocatalytic degradation of MB under sunlight by visual means as shown in Figure 6g. Both the nanomaterials RCD and CD decolorize the solution of MB. For an improved comparison, glass slides with silica and silica with RCD and CD films were dipped in dye solution and exposed to sunlight. The change in color of the films was recorded at different times (Figure 6g). The photographs demonstrate that RCD exhibits better decolorization of MB within 70 min in comparison to CD. Thus, RCD can be used as a material for designing self-cleaning surfaces. Among various OCs, BPA is known mostly to be difficult to degrade under visible light, and a number of reports have shown the degradation of BPA by inorganic semiconductorbased photocatalysts under UV irradiation. Bechambi et al. reported that 1 g L−1 of C-doped ZnO in 50 mg L−1 of BPA with H2O2 at pH 8 effectively attained 100% degradation of BPA after 24 h of UV light irradiaion.21 Wang et al. reported that 1% immobilized TiO2 can degrade 97% of BPA at 10 ppm concentration within 6 h upon UV radiation.2 However, only a few reports on the photodegradation of BPA are found under visible light. Qu et al. showed that 10 mg of CNT can photodegrade 74.8% of BPA at 10 ppm concentration upon exposure of solar light for 180 min.19 On the other hand, 50 mg of RCD can degrade 100% of BPA at 50 mg/L concentration within 3.5 h under sunlight (Figure 7). The present study demonstrated the degradation of MB, MO, MB/MO mixture, and BPA using RCD under solar irradiation for the first time. In all of these cases of photocatalytic degradation of OCs, the photocatalytic activity of RCD is significantly faster than that of CD. This may be due to the optimum level of polar functional groups that may interact with the OCs and help to anchor on the surfaces. This helps in better interaction of OCs with the active oxygen radicals. Thus, despite using an equal amount of nanoparticles in both cases, the same degree of degradation was not observed during the same period of exposure. Further, to clarify the doubt of self-degradation of the dye, a blank experiment was designed without using CD or RCD. No change in dye intensity even after 5 h of sunlight exposure, as measured by UV−visible spectroscopy, clearly indicates that the dye degradation is not self-degradation (Figure S4a). Further, no color change in the dye solution was observed even after exposure to 5 h of sunlight (Figure S5). In this context, another experiment of dye degradation with CD and RCD was performed under dark conditions. In this case, no change in dye intensity was again observed even after prolonged time, as measured by UV−visible spectroscopy (Figure S4b and c). Thus, this result eliminates the possibility of adsorption of the dye molecule by the nanomaterial. The photodegradation behavior of the nanomaterials was described by the pseudo-first-order kinetics model equation as (Figures 7 and 8) −dC /dt = K1t

ln(C /C0) = −K1t

(xi)

where C0 = initial concentration (at t = 0) of OC. The fitting plots of ln(C/C0) versus time are shown in Figures 7d and 8d− f, which reveal that the OC degradation is well fitted by pseudofirst-order kinetics model with fitting coefficients over 0.8, representing a regular photocatalytic degradation behavior. Although CD degrades OCs, their photocatalytic efficiencies are very poor due to their low absorbance of visible light and high recombination rate of photogenerated e−/h+ pairs. All of these results clearly indicate superior photocatalytic activity of RCD as compared to that of CD.



CONCLUSIONS Thus, from this study it can be concluded that biobased carbon nanodots (CD) can be effectively reduced through a simple, benign, one-pot, ultrafast route using aqueous phytoextract of C. esculenta in the presence of Fe3+ ions. This reduced CD (RCD) exhibited excellent excitation wavelength-dependent fluorescence. This novel zero dimensional carbon-based nanomaterial is efficiently able to degrade organic pollutants like methyl orange, methylene blue, and methyl orange/ methylene blue mixture and bisphenol A under normal solar radiation. The photocatalytic degradation efficiency of RCD is found to be higher compared to that of CD. These degradation processes follow the pseudo-first-order kinetics model. Therefore, this electron transfer mechanism of RCD may be broadened for its prospective applications in the field of solar cells and photocatalysis. In a nutshell, environmentally friendly RCD shows promise as an economical and efficient photocatalyst towards making the ecosystem safe and sustainable.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02590. Characterization techniques; synthesis and characterization of CD; FTIR spectrum, Raman spectrum, HRTEM images, and EDX map of CD; UV−vis spectra of CD, CD + Fe3+, and Fe3+; different conditions and images for dye at different times of sunlight exposure of CD; table for the reduction time of CD using different phytoextracts; and table for the reduction time of CD using different phytoextracts with Fe3+ ions with and without sonication (PDF)



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91-3712-267009. Fax: +91-3712-267006. ORCID

Niranjan Karak: 0000-0002-3402-9536 Author Contributions

R.D. and N.K. contributed equally and approved the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge SAIC, Tezpur University, Tezpur and SAIF, NEHU, Shillong, India for HRTEM analyses.



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