Research Article | Open Access
Sunlight-Induced Photochemical Degradation of Methylene Blue by Water-Soluble Carbon Nanorods
Water-soluble graphitic hollow carbon nanorods (wsCNRs) are exploited for their light-driven photochemical activities under outdoor sunlight. wsCNRs were synthesized by a simple pyrolysis method from castor seed oil, without using any metal catalyst or template. wsCNRs exhibited the light-induced photochemical degradation of methylene blue used as a model pollutant by the generation of singlet oxygen species. Herein, we described a possible degradation mechanism of methylene blue under the irradiation of visible photons via the singlet oxygen-superoxide anion pathway.
Carbon nanorods (CNRs) [1, 2] are representing a unique class of one-dimensional carbon nanostructures, which may offer some advantageous properties in comparison with carbon nanotubes (CNTs) [3, 4] since CNRs offer straight, aligned, and hollow graphitic morphology . CNRs are more or less similar to CNTs, except their straightness. Spaghetti type arrangements of CNTs restrict their many potential applications  that can be explored via the use of these CNRs. Till now, CNRs are the least explored in comparison with other members of nanocarbon family [3, 4, 6–17]. All these allotropic nanocarbons such as multiwalled CNTs /single-walled CNTs , fullerenes , carbon nanoonions [7–9], carbon nanodiamonds , graphene , graphene quantum dots , carbon dots [13–15], carbon nanofiber , and carbon nanocubes  have attracted a great concern in the diverse fields of science and technology because of their potential applications [5, 18–21]. Based on the few published reports, CNRs exhibited impressive electrical, thermal, and mechanical properties and are promising for field emission devices , energy storage devices , composite materials , and sensing applications . The primary barrier for the successful commercialization of CNRs is its typical synthetic procedures, such as chemical vapor deposition (CVD), arc discharge methods, solvothermal synthesis, electrodeposition, catalytic copyrolysis, and soft and hard template methods . All these synthetic methods involve expensive instruments, metallic particles for growing the CNRs, high-temperature, multistep fabrication protocols, and sophisticated techniques that notably restrict their economic viability. A metal-catalyst-free synthesis of CNRs can be a significant approach.
Particularly for the biological application of CNRs , aqueous solubility is the most important parameter, which requires the surface modifications like chemical functionalization with electrophilic groups (oxygen-rich species)  and bioactive groups . Tripathi et al.  described a new viable method for the catalyst-free synthesis of multicolored emissive wsCNRs from the pyrolysis of castor seed oil. The as-synthesized CNRs after Soxhlet purification were oxidized via simple oxidative method yielded photoluminescent water-soluble form as wsCNRs. The photoluminescent properties of wsCNRs were further utilized to construct a selective, specific, and active sensor for DNA sensing based upon fluorescent “turn-off/turn-on” technique . The possible reasons for multicolored emissions from the single nanoparticle were attributed to the radiative recombination of photoinduced electrons and holes that present over the surface in the form of “surface defects” [9, 14, 20]. These surface defects can be confirmed via various microscopic and spectroscopic techniques. Recently it has been demonstrated that the shape-dependent photoluminescent CDs relate “assemblies of fluorophores”  possibly located on the outer surface of nanocarbons. Phenomenologically these are similar as found in the case of semiconductors nanocarbons.
Being concerned about the safety of ecosystem, especially the aquatic ecosystem, degradation of water-soluble contaminants in shapes of organic dyes via a simpler and viable approach is necessary and needs to be addressed with full attentions. In the present finding, wsCNRs were explored for the visible light-induced photochemical degradation of “methylene blue” in direct sunlight. For the production of many valuable products fabricated in textile, paper, and plastics industries/laboratories, we need to consume a lot of water-soluble organic dyes. These soluble dyes are further discharged with wastewater, where they are forming toxic complexes coupling with metal ions already present in aqueous system. Not only limited to the contamination of water, these are significantly hammering the photosynthetic activity of aquatic plants. Presently, various biological, chemical, and physical methods are available for the degradation of dye from sewage, like absorption, adsorption, flocculation, coagulation, ultrafiltration, and reverse osmosis, but unable to degrade properly as the transformation from one organic phase into another. To avoid this shortcoming of dye degradations, chemical oxidation, surface modifications , and advanced oxidation process (AOPs) are commonly used . Still the existing methods could not be used potentially because of their high cost and incomplete degradation.
To overcome this, photocatalysis emerged as one of the most significant and economically viable methods that only require the irradiation of light without the usage of any additional chemicals [30–32]. Till now, many nanoparticles such as anatase TiO2 , ferric tungstate , ZnS, and CdS  are in use for dye degradation. But being composed of metallic in nature, their consequent toxicity is always the serious concern. Along with metallic nanoparticles, nanocarbons  were also explored for the dye degradation purposes. For example, doped CNTs, CDs, graphene oxide (GO), graphdiyne, and graphene were used to degrade methylene blue photochemically. Doping of nanocarbons with TiO2 , Fe and Cu , N , ZnFe2O4 , ZnO [37, 38], MnO2 , and Gd  is a common practice but still fabrication is a tedious process. Herein, we reported a simple and viable method for the synthesis of undoped nanocarbons as wsCNRs for the degradation of methylene blue with more reliable and convenient route. To the best of our knowledge, the photochemical applications of wsCNRs are not investigated till now.
Castor seed (Ricinus communis) oil used here for the synthesis of wsCNRs was purchased from a local market in Jaipur, India. Methylene blue (MB) and nitro blue tetrazolium (NBT) chloride were procured from S.D. Fine-Chemicals, India. Solvents like acetone, methanol, ethanol, petroleum ether, and nitric acid are of analytical grade and procured from S.D. Fine-Chemicals, India. All chemicals were of analytical grade and used without any further purification.
The visualization of surface morphologies of wsCNRs was examined by using a field emission scanning electron microscopy (FESEM) (SUPRA 40VP, Carl Zeiss NTS GmbH, Oberkochen, Germany) operated at 10 kV. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) for the internal characterization of graphitic wsCNRs were carried out with Tecnai 20 G2 200 kV. The absorption spectra of wsCNRs were measured at room temperature by a Perkin Elmer, Lambda 35. Raman spectra of the CNRs and wsCNRs were recorded using WITEC model Raman spectrometer using Ar laser (λ = 532 nm) as an excitation source. FTIR spectra of the CNRs and their soluble version were collected on a Bruker Fourier transform infrared spectrometer (Vector 22 model). Thermogravimetric analyses (TGA) of CNRs and wsCNRs were performed with Mettler Toledo Star System under the continuous flow of argon atmosphere, with the heating rate of 10°C/min, in temperature ranges from 25°C to 1100°C. Zeta potential measurements were carried out in aqueous medium with the Beckman Coulter Delsa TM Nano. Electron paramagnetic resonance (EPR) spectra of CNRs and wsCNRs were recorded on a Bruker-EMX EPR spectrometer.
2.3. Synthesis of wsCNRs
CNRs and their water-soluble version wsCNRs were synthesized from castor oil via the traditional way of pyrolysis (similar to lighting a lamp) collected in the inverted glass beaker to avoid any metal impurities. In brief, about 5-gram soot was placed in a Whatman thimble for sequential washing with different solvents, to remove any unburnt organic contamination. For making its water-soluble version, we used the oxidative treatment of Soxhlet purified soot with concentrated nitric acid for ~12 hours in refluxing condition. Removal of the insoluble portion and excessive nitrate was done similar to the as-described method .
2.4. Photochemical Experiment
A stock solution was prepared by dissolving 100 mg of wsCNRs and CNRs in 200 mL of 20 mg/L MB solution. CNRs were dissolved in solution by sonication for 30 min and wsCNRs just by shaking with hands. The aliquots were collected at the different time and then exposed to direct sunlight for 10 min. Aliquots of MB for photochemical degradation were analyzed using UV-Vis spectrophotometer. The kinetic rate constant for the degradation of MB with wsCNRs under visible light was analyzed using the Langmuir-Hinshelwood model:where represents the apparent kinetic rate constant and and are the initial concentration and concentration at time , respectively, and is the irradiance time.
3. Result and Discussion
3.1. Microscopic Characterization
The nanostructural evaluation for the surface morphological characterization of wsCNRs had been analyzed by FESEM, TEM, and HRTEM studies. Figure 1(a) shows the FESEM image of nicely distributed wsCNRs along with their corresponding size distribution histogram in Figure 1(b). The histogram analysis showed that majority of the wsCNRs are in the range 50–100 nm (in diameter). Figures 1(c) and 1(d) are TEM and HRTEM images of wsCNRs, respectively. HRTEM illustrated in Figure 1(d) confirms the crystalline graphitic planes of wsCNRs with high-density surface defects irregularities on the outer surface and inner walls of wsCNRs (marked by black dotted bracket).
High degree surface modification/functionalization via negative oxygenous functionalities [14, 15, 19–21, 25] can impart the excellent solubility to nanocarbons even without using any sonication for its solubilization. wsCNRs are highly soluble in water and remain stable in solution for several months in aqueous solution and show high transparency in ambient light. The wsCNRs show absorbance peaks at ~210 nm, 260 nm, and 390 nm, due to transition of C=C bond and transition of C=O bonds , while the band at 390 nm is due to the bathochromic effect of OH groups attached on wsCNRs as shown in Figure 2(a). The FTIR spectra of both Soxhlet purified (solid line) and wsCNRs (dotted line) are shown in Figure 2(b), which confirms the presence of C=C, C-C, C-O, C=O, O-H, and C-H stretching and bending vibrations of sp3 and sp2 hybridized carbons. The peak at ~2919 cm−1 (strong) was attributed to the presence of C-H stretching vibrations of sp3 hybridized carbon, the peak around ~1600 cm−1 (strong) was attributed to the C=C stretching vibrations of sp2 hybridized carbons. The bending vibrations of sp3 C-H and sp2 C-H are at ~1375 cm−1 (strong) and ~579 cm−1 (medium), respectively, in FTIR spectra of Soxhlet purified CNRs, while the intense peaks at ~3431 cm−1 and ~1720 cm−1 are attributed to the presence of O-H and C=O stretching vibrations of hydroxyl and carboxylate groups, present on the oxidized form of CNR. The strong peaks around ~1615 cm−1 and ~1231 cm−1 are due to the C=C and C-O stretching vibrations. A weak stretching vibration at ~2924 cm−1 is due to the sp3 C-H stretching vibrations in the FTIR spectra of wsCNRs [2, 41].
To understand the extent of functionalization and thermal stability of CNRs and wsCNRs, a comparative thermogravimetric analysis (TGA) [2, 9, 20] of both freshly prepared Soxhlet purified CNRs (solid line, Figure 2(c)) and wsCNRs (dotted line, Figure 2(c)) was performed. TGA plots show that CNRs are steadier in contrast to wsCNRs and drop only ~13% of their weight, while wsCNRs start their weight loss progressively from 35°C and at 1100°C they lose ~46% of their total weight. The TGA of CNRs justifies the elimination of trace quantity of certain groups containing C-H and C=O vibrations; see FTIR of raw CNRs (Figure 2(b), solid line). Also, at that temperature, some elimination of fully oxidized carbon may add up some mass loss. Weight losses in wsCNRs are attributed to the removal of the surface functional groups . The Raman spectra of wsCNRs [2, 42] show two bands as the G-band and the D-band in Figure 2(d), dotted line. Here, the G-band  at ~1586 cm−1 is attributed to the graphitic carbon and the D-band at ~1332 cm−1 is attributed to the disorder in the graphitic carbon. Both bands downshifted in case of water-soluble version in comparison with CNRs where D-band and G-band are located at ~1348 cm−1 and at ~1599 cm−1, respectively, as displayed in Figure 2(d) by solid line. That is induced due to the oxidation and also because of the presence of other disorder like sp3 carbon. Solid-state EPR spectra of CNRs (blue line) and wsCNRs (black line) at room temperature are shown in Figure 3(a). The EPR of CNRs and wsCNRs virtually showed the presence of very similar carbon radical signals. The average value of 1.99 in both is due to the presence of stable carbon radicals in the singly occupied orbital of carbon dots in their ground state. Singly occupied orbital reveals the electron donor nature of both . Electron transfer properties of these CNRs and wsCNRs would be an advantage for a broad range of photochemical reactions.
3.2. Evaluation of Visible Light-Induced Photochemical Activity
The degradation of methylene blue (MB) by CNRs and wsCNRs was carried out at room temperature with irradiation of visible photons under direct sunlight. For the homogenous photochemical degradation of MB, the higher aqueous solubility of wsCNRs is an advantageous property. An aqueous solution of MB was taken in a quartz cuvette, and degradation of MB was monitored with the UV-visible spectrometer at time intervals of 10 minutes. An aqueous solution of MB was light blue in color as shown in the inset of Figure 3(b)(1). After the addition of wsCNRs (100 mg/200 mL), absorption intensity of MB (20 mg/L) solution decreases gradually with time and finally the color of solution become colorless (inset of Figure 3(b)(2)) and corresponding degradation efficiency () versus irradiance time is illustrated in Figure 3(b). Spectra were measured at a time span of 10 minutes with the irradiation of direct sunlight. After 120 minutes, blue colored solution turned colorless that confirms the maximum achieved removal of MB. wsCNRs generate ROS on exposure to aerial oxygen in aqueous solution on irradiation of visible photons in day light. The generation of ROS was confirmed by the nitro blue tetrazolium (NBT) test (Figure 3(c)) [43–45]. NBT was added in an aqueous solution of wsCNRs (inset of Figure 3(c)(1)) with few drops of CCl4 and photoirradiated with visible photons. The change in color of CCl4 layer from being colorless (inset of Figure 3(c)(2)) to being of pink color (inset of Figure 3(c)(3)) in solution indicated the reduction of NBT and hence the formation of diformazan dye.
The rate constant of MB degradation was calculated using versus time plot as demonstrated in Figure 3(d) and was found to be 0.01387 min−1. MB (control) exposed to direct sunlight without the addition of wsCNRs did not show any significant photochemical degradation under identical conditions (Figure 3(b)). We did analyze the photochemical degradation of MB exposed to irradiation of visible photons under direct sunlight in the presence of CNRs and found less degradation in comparison to wsCNRs as illustrated in Figure 3(b). Oxidation of CNRs introduces surface defects which enhance the photochemical activity of wsCNRs. The rate constant of wsCNRs is approximately two times higher in comparison to the CNRs as illustrated in Figure 3(d).
Zeta potential confirms the negative surface charge of wsCNRs (−33.4 mV) and hence facilitates the adsorption of positively charged MB over wsCNRs surface via ion-pair interaction [2, 20, 28]. Paramagnetic nature of wsCNRs (presence of high-density unpaired electrons) facilitates the electron transfer between wsCNRs and MB that would be the advantage of visible light-induced photochemical degradation of MB. Generation of singlet oxygen () occurred by energy transfer from wsCNRs excited by visible photons to triplet oxygen () at the initial stage of photoreaction. Then reactive hydroxyl radical was formed and finally leads to superoxide anion radical (). Reactive oxygen species (ROS) abstracted proton from water and react with NBT . The possible overall reactions are illustrated in Scheme 1 . wsCNRs-MB complex was isolated to analyze its interactive surface modification during the photochemical degradation process. We did FTIR analysis of wsCNR-MB composite. FTIR spectrum of the wsCNRs-MB composite is shown in Figure 4(a). The presence of characteristic absorption bands at 2880 cm−1, 2984 cm−1, and 2829 cm−1 confirmed the presence of -CH3 and terminal N(CH3)2 groups. The degradation process can easily be monitored via FESEM analysis. Figure 4(b) shows the image of MB-interacting wsCNRs that reveals the adsorption of MB on wsCNRs surface.
In summary, wsCNRs are produced by the conventional method of pyrolysis using castor oil as carbon precursor, followed by washing and oxidative treatment of as-collected soot. Because of the presence of high-density defective surfaces, carbon radicals are trapped here. Carbon radicals present over the nanocarbon surfaces are capable enough of generating ROS under irradiation of visible photons in direct sunlight via singlet oxygen-superoxide anion pathway. The wsCNRs interacting with MB lead to the decolorization of MB in the presence of light through photochemical degradation reactions. This work is expected to open a new method for the preparation of wsCNRs based nanohybrids/nanocomposites for their practical application related to solving/minimizing the various environmental issues such as wastewater treatment to remove the organic pollutants and sensing of toxic metals. Moreover, in comparison with CNTs due to their straightness, these may also be used for drug delivery purposes in future.
The authors declare that they have no competing interests.
Anshu Bhati and Anupriya Singh contributed equally to this work.
Anshu Bhati and Anupriya Singh thank MNIT Jaipur for doctoral fellowship. Kumud Malika Tripathi thanks CSIR for fellowship. Sumit Kumar Sonkar thanks DST New Delhi for funding (SB/EMEQ-383/2014) and Material Research Centre (MRC), MNIT Jaipur, for HRTEM analysis.
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