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Article Cite This: ACS Omega 2018, 3, 17261−17275
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Systematic Investigation for the Photocatalytic Applications of Carbon Nitride/Porous Zeolite Heterojunction Abhinav Kumar, Subhajyoti Samanta, and Rajendra Srivastava* Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Punjab 140001, India
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ABSTRACT: Here, we present the integration of a commercially available titanosilicate zeolite with photocatalyst graphitic carbon nitride (g-C3N4) toward the development of an effective heterojunction photocatalyst, TCN(1-8-8). The formation of this porous heterojunction and its structural details have been confirmed by X-ray diffraction, N2 adsorption, electron microscopy, thermogravimetric analysis, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The visible light absorption and band structure have been determined from diffused reflectance ultraviolet−visible spectroscopy. For its fabrication, the contents of both the constituent materials have been optimized systematically. Its photocatalytic activity has been found to be impressive in the visible light-assisted degradation of a variety of water pollutants (dyes and antibiotics) and in the hydroxylation of phenol. Control experiments, radical scavenging/trapping experiments, influence of the reaction environment, and photoelectrochemical measurements have been carried out to establish the structure−activity relationship and the plausible reaction mechanisms. The various fragmented products, formed during the degradation of parent molecules, have been further confirmed using electrospray ionization mass spectrometry analysis. The photocatalytic degradation of 98, 96, 95, and 92%; rate constants of 0.0125, 0.01244, 0.0058, and 0.0040 min−1; and reduction of total organic concentrations of 63, 59, 57, and 55% for rhodamine B, sulforhodamine B, tetracycline, and ciprofloxacin have been achieved in 6 h, respectively. The activity of TCN(1-8-8) has been observed to be better than the state-of-the-art photocatalyst TiO2 (Degussa P25). Besides, it has also exhibited excellent degradation activity in natural solar light. The effective adsorption of pollutant molecules over the active surface, efficient charge separation at the interface, migration and retardation of charge carriers recombination process, and tailored charge-carrier dynamics in the excited state have all been identified as reasons for the higher activity. This study, therefore, provides a comprehensive and systematic grasp on the development of an economical catalyst for photocatalytic hydroxylation reaction and wastewater treatment.
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INTRODUCTION Although more than 70% of the earth’s surface is covered with aqueous bodies, the world is constantly battling toward cleaner and purified water necessary for the sustenance of every living being.1−3 Industrial evolution has, no doubt, made human life comfortable but has bequeathed the young generation with polluted water and air.4,5 The 21st century is, therefore, witnessing consistent research and developments for the removal and detection of trace organic and inorganic contaminants present in water bodies.6−9 A large number of noble/metal-derived photocatalysts and electrocatalysts are known for the detection and degradation of these organic and inorganic contaminants.10−12 The removal of organic contaminants (such as organic dyes and drug effluents) from water bodies needs a simple, cost-effective, and eco-friendly solution. The photocatalytic process of organic contaminant degradation stands out to be the most efficient and practical solution to this problem.13−15 The natural abundance of solar energy makes this process of water purification most economical and sustainable without any special method and optimization. The © 2018 American Chemical Society
next important aspect in this area is the design and development of an economical, sustainable, facile, and reproducible approach for the synthesis of a good photocatalyst at an industrial scale. The contribution made by the researchers over the last few years in the evolution of unique noble/transition metal-based solid catalysts for the photocatalytic degradation of water contaminants is indeed commendable.16−24 Literature reports reveal that the photocatalytic degradation proceeds with the assistance of reactive oxygen species (ROS) such as superoxide, hydroxide, and hydroperoxide.21 Further, the photocatalytic degradation is facilitated when the organic contaminants are efficiently adsorbed on the catalyst surface. Hence, a large surface area of the photocatalyst is desirable.22 Degussa P25 (a well-established industrial material known for UV light absorption) is not suitable for visible light Received: July 5, 2018 Accepted: December 3, 2018 Published: December 13, 2018 17261
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Scheme 1. Pictorial Presentation of the Synthesis of Photocatalyst
visible light-assisted dye degradation.38−45 Our group has reported the synthesis of metal vanadate/g-C3N4 heterojunction catalysts for various visible light-assisted chemical syntheses utilizing O2 and H2O2 as oxidizing agents.46,47 We have also reported the unique synthesis of a bifunctional gC3N4 catalyst for CO2 activation and condensation reactions.39 Although TS-1 and other zeolites have large surface area and porosity for efficient adsorption of organic contaminants, they have incapability toward visible light absorption. Again, g-C3N4 absorbs visible light, but neither possesses active sites for catalytic reaction nor exhibits good surface area and porosity. It is also incapable of curbing the electron−hole recombination phenomenon, a major drawback for efficient dye degradation. However, TS-1 and g-C3N4 can be efficiently integrated to overcome the limitations associated with each other, thereby serving as a suitable heterojunction for the visible light-assisted photodegradation of organic contaminants. Considering the existing commercial production and catalytic action of zeolite TS-1 and economical and tailormade scalable preparation of g-C3N4, this study is devoted to design a synthesis strategy for the production of TS-1/g-C3N4 nanocomposites. The utilization of TS-1/g-C3N4 heterojunctions is explored in the visible light-assisted hydroxylation of phenol. These photocatalysts are developed for the visible light-aided degradation of a range of organic dyes and antibiotics. Herein, the efficiency of these photocatalysts is demonstrated using two organic dyes, rhodamine B (RhB) and sulforhodamine B (SRB), and two antibiotics, tetracycline (TC) and ciprofloxacin (CIP), under visible light and natural sunlight exposure. The degradation activity of TS-1/g-C3N4 heterojunction photocatalyst is found to be far better than that of TS-1, g-C3N4, and state-of-the-art P25 photocatalysts. Results derived from the active species trapping experiments, control experiments, photoelectrochemical (PEC), and mass spectrometry measurements are accounted in illustrating the degradation mechanism. The developed catalyst exhibits excellent photodegradation activity under natural sunlight exposure for these model compounds.
photocatalysis until it is modified with metal and nonmetalbased visible light-absorbing materials to make heterojunction photocatalysts.23−32 Since photodegradation is mostly achieved using reactive oxygen species, one can think of the titanosilicate (TS-1) zeolite, which is another viable industrial catalyst. Titanosilicate (TS-1) is a well-known oxidation catalyst, which has the capability to produce epoxide and catalyze hydroxylation reactions using either H2O2 or tert-butyl hydroperoxide as an oxidizing agent.33 It is important to mention here that microporous zeolites (aluminosilicate) are well-known acid catalysts but do not exhibit any photocatalytic activity.34 The photocatalytic activity in the zeolites can be induced by hosting photoactive guest such as photosensitizers or by the incorporation of metal ions and metal oxides.35 Ti metal incorporation in the zeolite mordenite framework inverted (MFI) framework structure leads to the production of TS-1 catalyst.36 Ti is larger than Si, and its incorporation in the MFI framework expands unit cell volume, thereby making it difficult enough to incorporate a large amount of Ti in the MFI framework as it disrupts the framework structure beyond a certain volume expansion. Ti species present in TS-1 can increase its coordination by reacting with H2O2/H2O, etc. Although the framework itself demands four bonds in tetrahedral orientation, the resulting strain imposed on a 5-/ 6-coordinated Ti ion acts as the driving force for its high reactivity in the epoxidation and hydroxylation reactions.37 In the presence of H2O2, the light irradiation generates •OH via titanium-hydroperoxide species formed by the reaction of framework Ti atoms in TS-1 with H2O2.37 In this case, only the tetrahedral Ti atoms connected to TS-1 framework form titanohydroperoxy species. Furthermore, the large surface area of TS-1 has the capability to adsorb reactant molecules that facilitates photodegradation. TS-1 exhibits all of the properties desirable for a good industrial catalyst for photodegradation except that it is incapable of absorbing visible light. Therefore, a simple and economical visible light-absorbing material should be integrated with TS-1 to facilitate the photodegradation process. Carbon nitride (g-C3N4), a versatile visible light-absorbing material, is known for scale-up synthesis using economical precursors.38−45 In recent times, a wide range of heterojunction catalysts based on g-C3N4 have been developed for
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RESULTS AND DISCUSSION g-C3N4 is prepared by mixing equal amounts of urea (U) and thiourea (TU). The synthesis procedure is pictorially 17262
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N2 volumetric experiments suggest that TS-1 exhibits a type II isotherm with no hysteresis loop and confirms the formation of microporous zeolite-like porous structure (Figure S1). It exhibits only a marginal steady increase in the adsorption in the P/P0 range of 0.2−0.85. Above this pressure, a steep increase in the adsorption is observed. This increase corresponds to N2 adsorption in the interparticle macropores. Barrett−Joyner−Halenda (BJH) analysis shows only a weak trimodal broad pore size distribution (2−10 nm). g-C3N4 exhibits a type IV isotherm and an H3 hysteresis but with very low adsorbed volume (Figure S1). This is the reason for the low surface area of g-C3N4 compared to TS-1. A sharp BJH distribution profile with a maximum pore size of 3.8 nm is obtained. TS-1/g-C3N4 nanocomposites exhibit a type IV isotherm and an H3 hysteresis with difference in the adsorption volume (Figure S1). With increase in the g-C3N4 content in the nanocomposite, the adsorption volume decreases. This has a significant influence on the physical properties of the resultant nanocomposites. With increase in the g-C3N4 content in the nanocomposite, surface area and total pore volume decrease. Multimodal pore size distributions for various TS-1/g-C3N4 nanocomposites have been obtained, which signify the presence of both the porous structures in the resulting nanocomposite materials. The textural measurement values are summarized in Table 1.
presented in Scheme 1. Among these samples, the TS-1(1)/gC3N4 (U:TU = 8:8) (hereafter represented as TCN(1-8-8)) exhibits the best photocatalytic activity in the hydroxylation of phenol and photodegradation of all four organic contaminants. Physicochemical Characterization. Powder X-ray diffraction (XRD) was recorded to confirm the phase purity of TS-1 and g-C3N4 and the compositional behavior of TS-1/gC3N4 nanocomposites. Figure 1a shows the XRD patterns of g-
Table 1. Textural Properties of Various Materials Synthesized in This Studya
Figure 1. X-ray diffraction patterns of (a) TS-1, g-C3N4, and TCN(18-8) and (b) TCN(1-4-4), TCN(1-6-6), TCN(1-7-7), and TCN(110-10).
e. no.
sample
surface area (m2 g−1)
1. 2. 3. 4. 5. 6. 7.
TS-1 g-C3N4 TCN(1-4-4) TCN(1-6-6) TCN(1-7-7) TCN(1-8-8) TCN(1-10-10)
474 35 445 414 276 174 168
external surface area (m2 g−1)
total pore volume (cm3 g−1)
micropore volume (cm3 g−1)
68 3 78 71 64 42 37
0.352 0.258 0.450 0.476 0.416 0.354 0.327
0.209 0.016 0.204 0.199 0.175 0.138 0.112
a
SBET was calculated from the adsorption branch in the region of 0.05 < P/P0 ≤ 0.3.
Thermogravimetric analysis (TGA) was conducted in air (heating rate = 10 °C min−1). The TGA profile of TS-1 shows high stability (more than 900 °C) (Figure S2). The initial weight loss (less than 100 °C) in the TGA profile of TS-1 is due to the ejection of physisorbed water. The TGA profile of g-C3N4 displays a marginal weight loss at temperatures less than 250 °C. This weight loss corresponds to the ejection of physisorbed or weakly chemisorbed H2O molecules. The thermogram of g-C3N4 confirms that the perishing begins at 490 °C and completes at 620 °C (Figure S2). All of the nanocomposites exhibit similar TGA profiles to those of gC3N4 and provide evidence for the percentage composition of g-C3N4 and TS-1. The percentage compositions (wt %) of TS1/CN have been found to be 98.6:1.4, 84.7:15.3, 46.6:53.4, 29.8:70.2, and 24.2:75.8 for TCN(1-4-4), TCN(1-6-6), TCN(1-7-7), TCN(1-8-8), and TCN(1-10−10), respectively. TGA measurements further confirm the incorporation of different amounts of g-C3N4 in various nanocomposites of this study. The morphology and microstructure of TS-1, g-C3N4, and TCN(1-8-8) were investigated using field emission scanning
C3N4, TS-1, and TCN(1-8-8). Graphitic C3N4 exhibits two reflections at 12.8° (d = 0.691 nm) and 27.4° (d = 0.325 nm) that are indexed to (100) and (002) corresponding to the inplane structural packing and interlayer stacking of aromatic segments.40 The XRD pattern of TS-1 exhibits reflections at 7.9, 8.8, 13.2, 13.9, 14.8, 15.5, 15.9, 16.5, 17.8, 19.2, 20.3, 20.9, 22.2, 23.1, 23.8, 24.3, 26.0, 26.9, 29.2, 29.9, 30.3, 31.2, 32.1, 32.7, 33.4, 36.1, 37.5, 45.1, 45.6, 46.5, 47.4, and 48.5°, which can be indexed to (011), (020), (002), (012), (3̅01), (3̅11), (022), (2̅12), (400), (1̅32), (1̅03), (241), (340), (051), (511), (3̅13), (342), (600), (5̅32), (5̅03), (503), (2̅53), (3̅62), (063), (035), (5̅62), (0100), (1000), (3̅46), (3̅93), and (5̅36) planes. These planes are characteristics of MFI topology (Figure 1a).48 XRD patterns of TS-1/g-C3N4 nanocomposites exhibit reflections corresponding to both TS-1 and g-C3N4 (Figure 1a,b). With increase in the content of g-C3N4 in TS-1/g-C3N4 nanocomposites, reflection at 27.4° corresponding to g-C3N4 increases, which confirms the incorporation of increasing amount of g-C3N4 in the resulting nanocomposites. 17263
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clearly shows the aggregations of TS-1 and g-C3N4 in the TCN(1-8-8) nanocomposite. Transmission electron microscopy (TEM) images exhibit TS-1 and g-C3N4 domains in the nanocomposite material TCN(1-8-8). Images recorded from different regions of the specimen show a dense netlike structure corresponding to g-C3N4 and globular morphology corresponding to TS-1 (Figures 2b,c and S4). High-resolution TEM (HR-TEM) image clearly shows the formation of heterojunction between TS-1 and g-C3N4. HR-TEM image shows a sheetlike morphology for g-C3N4 (Figure 2c). HRTEM image shows that the globular morphology of TS-1 is built with highly cross-linked sheetlike morphology stacked in the vertical direction (Figure 2d). The chemical composition was surveyed using scanning transmission electron microscopy (STEM)-high-angle annular dark-field imaging (HAADF)energy-dispersive X-ray (EDX) (Figure S5). The spatial distribution of the atomic contents across the nanocomposite is evaluated using drift-corrected EDX. The STEM-HAADF image (Figure S5b) of the selected domain of specimen (Figure S5a) and the chemical maps for O, Ti, Si, C, and N are presented in Figure S5c−g. EDX confirms the heterojunction of TS-1 with g-C3N4 (Figure S5h). Si, Ti, and O elemental maps clearly show the location of TS-1 particles in the nanocomposite. The elemental mapping further shows that C and N atoms of g-C3N4 are wrapped around TS-1 and also present over the entire surface. This further confirms the formation of heterojunction nanostructure.
electron microscopy (FESEM). TS-1 exhibits a uniform globular tabletlike morphology (Figure S3a). A layered fluffy morphology is observed for g-C3N4 (Figure S3b). Figure 2a
Figure 2. (a) FESEM, (b) TEM, and (c, d) HR-TEM images of TCN(1-8-8) nanocomposite.
Figure 3. (a) DRUV−visible spectra of TS-1, g-C3N4, and other synthesized nanocomposites; (b) estimated band gap energy of TS-1, g-C3N4, and TCN(1-8-8), where n = 4 and 1 for TS-1 and g-C3N4, respectively, and TCN(1-8-8); (c) steady-state photoluminescence (PL) spectra of TS-1, gC3N4, and TCN(1-8-8) (the inset shows the photoluminescence spectra); and (d) time-resolved photoluminescence spectra of TCN(1-8-8), TS-1, and g-C3N4 (the inset shows the enlarged profile at low time domain). 17264
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Figure 4. (a) Surface survey XPS image of TCN(1-8-8) and high-resolution spectra of (b) Ti 2p, (c) Si 2p, (d) C 1s, (e) N 1s, and (f) O 1s.
depends on the type of transition involved. The value of n decides whether a transition is direct or indirect.46 Generally, if n = 1, then it is considered as a direct transition, and if n = 4, then transition becomes indirect. The best fit of the curve between (αhυ)2 and photon energy (hυ) suggests a direct allowed transition for g-C3N4, TCN(1-6-6), TCN(1-7-7), TCN(1-8-8), and TCN(1-10-10), whereas the best fit of the curve between (αhυ)1/2 and photon energy (hυ) suggests an indirect allowed transition in TS-1 and TCN(1-4-4).49 The calculated band gaps for g-C3N4, TS-1, TCN(1-4-4), TCN(16-6), TCN(1-7-7), TCN(1-8-8), and TCN(1-10-10) are 2.68, 3.16, 3.0, 2.92, 2.84, 2.77, and 2.75 eV, respectively (Figures 3b and S6). The valence band (VB) potential of parent materials has been determined by using Mulliken electronegativity theory:
DRUV−visible spectra of TS-1 and g-C3N 4 exhibit absorbance edges around 350 and 480 nm, respectively (Figure 3a). TS-1/g-C3N4 nanocomposites show absorbance edge in the range of 330−500 nm. Nanocomposite TCN(1-44) shows absorption spectrum similar to TS-1 but with higher absorbance. TCN(1-6-6) spectrum shows two resolved peaks corresponding to g-C3N4 in the range of 280−450 nm in addition to well-resolved absorption in 210−240 nm corresponding to TS-1. TCN(1-7-7) spectrum is very similar to the g-C3N4 absorption spectrum. Absorption spectra of TCN(1-8-8) and TCN(1-10-10) are similar to g-C3N4 but with higher absorbance than g-C3N4. The band gap energies of g-C3N4 and its nanocomposites are calculated using αhυ = A(hυ − Eg)n/2, where Eg is the band gap, hυ is the photon energy, A is constant, α is the Kubelka−Munk function, and n 17265
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EVB = χ − Ee + 0.5Eg, where EVB is the valence band potential, χ is the absolute electronegativity of the material, and Ee is the free electron energy in hydrogen scale.46 The computed values of EVB for TS-1 and g-C3N4 are 3.18 and 1.5 eV, respectively. The conduction band (CB) potential is calculated by ECB = EVB − Eg.50 The calculated values of ECB for TS-1 and g-C3N4 are +0.01 and −1.18 eV, respectively. The Fourier transform infrared (FT-IR) spectra of TS-1, gC3N4, and their nanocomposites are demonstrated in Figure S7. The asymmetric SiO−Si stretching of TS-1 appeared at 1100 and 1220 cm−1, and symmetric stretching/bending of SiO−Si bridge appeared at 800 cm−1. A sharp band at 550 cm−1 is attributed to the SiO−Si rocking of pentasil unit, characteristic of MFI framework structure (Figure S7).51 The band at 960 cm−1 is attributed to the stretching mode of [SiO] tetrahedral bonded with Ti atoms and finger print for the framework Ti species in TS-1. The FT-IR absorption in 3200− 3500 cm−1 of g-C3N4 is attributed to the N−H/O−H stretching vibration modes of free N−H and surface-bound hydroxyl groups (Figure S7). Absorption in the range of 800− 1700 cm−1 corresponds to the in-plane vibration, and a band at 807 cm−1 corresponds to the out-of-plane bending vibration of triazine unit in g-C3N4 (Figure S7).39 The FT-IR spectrum of TCN(1-4-4) is very similar to that of TS-1, confirming the incorporation of very low amount of g-C3 N4 in the nanocomposite. All of the other nanocomposites show FT-IR peaks similar to TS-1 and g-C3N4 bands. Furthermore, the intensity corresponding to g-C3N4 FT-IR bands increases with increase in the content of g-C3N4 in TS-1/g-C3N4 nanocomposites (Figure S7). The migration, separation, and degree of recombination of the excited-state charge carriers for a semiconductor photocatalyst can be evaluated by photoluminescence (PL) spectroscopic analysis.52 Among all of the photocatalysts, gC3N4 exhibits the highest intense PL reflection, indicating that the efficiency of charge carriers recombination is maximum for this photocatalyst (Figure 3c). But in the case of nanocomposite TCN(1-8-8), the low-intensity PL emission profile is observed, which is wider than g-C3N4, depicting the formation of heterojunction that has retarded the recombination process, leading to higher photocatalytic activity (Figure 3c). Due to the poor light absorbance capability, TS-1 induces the distinct low-intensity broad PL spectrum (Figure 3c, inset). Nanocomposite TCN(1-8-8) displays the lower intense PL spectrum than g-C3N4, implying the efficient separation and prompt migration of charge carriers immediately after flashing light on its surface (Figure 3c). Furthermore, the charge carriers dynamics at their excited states have also been analyzed with time-resolved photoluminescence spectroscopy (TRPL) for g-C3N4, TS-1, and TCN(1-8-8) photocatalysts using a 325 nm diode laser excitation source (Figure 3d). PL decay traces are fitted with a monoexponential function using software provided by the Edinburgh instruments to calculate the exciton lifetime values. The values of fitting parameter (χ2) and detailed spectroscopic results such as exciton lifetimes (τ1), preexponential factors (A1), average exciton lifetimes (⟨τ⟩), etc. are summarized in Table S1. The average lifetime values are derived from equation ⟨τa⟩ (ns) = (A1τ12)/(A1τ1).53 Here, only τ1 is caused due to free exciton’s recombination in the photocatalyst, and other exciton lifetimes such as τ2 and τ3 are not observed, which are generated due to the nonradiative recombination at the semiconductor surface. The average lifetimes of g-C3N4, TS-1, and TCN(1-8-8) have been found to
be 0.69, 0.75, and 0.52 ns, respectively (Figure 3d). The obtained results clearly indicate that the formation of heterojunction between TS-1 and g-C3N4 leads to quenching and shortening in excited-state charge carriers. Further, the shortest lifetime of nanocomposite TCN(1-8-8) further depicts that the charge carriers are effectively separated at the interface of the junction in TCN(1-8-8) (Figure 3d). The TRPL analysis confirms that the formation of heterojunction is beneficial for efficient charge-carrier migration, leading to high photocatalytic activity for nanocomposite TCN(1-8-8). The elemental constituents and their oxidation state of highly active TCN(1-8-8) were investigated by X-ray photoelectron spectroscopy (XPS). Figure 4a presents the surface survey of TCN(1-8-8), confirming the presence of Si, Ti, C, N, and O. The individual high-resolution XPS images of Si 2p, Ti 2p, C 1s, N 1s, and O 1s elements are shown in Figure 4b−f. The XPS image of Ti 2p exhibits two peaks at 459.3 and 465.4 eV, which correspond to Ti 2p3/2 and Ti 2p1/2 binding energies, for Ti4+ in TS-1 (Figure 4b).54 Only one peak is observed for Si 2p at 102.9 eV, which corresponds to the binding energy of Si 2p in TS-1 (Figure 4c).55 Peaks at 288.1 and 284.6 eV correspond to the sp2-hybridized carbon atom attached to three nitrogen atoms of g-C3N4 in TCN(1-8-8) (Figure 4d).56 N 1s exhibits three peaks with maxima at 397.7, 399, and 400.2 eV (Figure 4e). The peaks at 397.7, 399, and 400.2 eV are associated with −C−N−C−, −N(C), and (−N− H), respectively.57 The broad O 1s XPS image is deconvoluted into four bands with peak maxima at 529.9, 531, 532.1, and 533.2 eV (Figure 4f). Peaks at 529.9 and 533.2 eV correspond to the binding energies of O 1s in TS-1 (529.9 TiO and 533.1 SiO), while the remaining two peaks at 531 and 532.2 eV have been attributed to the surface oxygen species (H2O and OH−).57 The binding energy obtained during the XPS examination confirms the successful construction of TS-1 and g-C3N4 nanocomposites. Photocatalytic Activity Evaluation. Photocatalytic Hydroxylation Reaction. TS-1 is a well-known catalyst for the phenol hydroxylation using H2O2 as an oxidizing agent.58 First, TS-1 and g-C3N4 were investigated in the hydroxylation of phenol. TS-1 exhibited 23.3% phenol conversion with a product distribution of 61% hydroquinone, 33% catechol, and 6% benzoquinone (BQ) when the reaction was conducted at 65 °C for 6 h according to the reaction condition shown in Table S2. g-C3N4 exhibited no appreciable activity under this condition. This study aimed at performing the aforesaid reaction using visible light (λ > 420 nm). The same reaction condition was used to evaluate its catalytic activity under visible light at ambient temperature. No reaction took place in the dark at this condition. g-C3N4 had, therefore, been found to be inactive even under photochemical condition. TS-1 alone exhibited very low activity and produced benzoquinone as a selective product. All nanocomposites investigated in this work are active and exhibited better activity than TS-1 (Table S2). With the increase in g-C3N4 content in the nanocomposites, the conversion of phenol increased. Among the nanocomposites investigated, TCN(1-8-8) exhibited the best activity. With further increase in the g-C3N4 content, a marginal decrease in the activity was observed. Therefore, it can be concluded that TCN(1-8-8) is the best catalyst for the photocatalytic hydroxylation of phenol. If one compares the catalytic activity of TCN(1-8-8) under photocatalytic condition with the commercial catalyst TS-1 under thermal condition, the following conclusions can be made: (a) 17266
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Figure 5. UV−visible spectra of (a) RhB and (b) SRB (the insets of (a) and (b) show changes in the concentration with irradiation time); firstorder kinetics of (c) RhB and (d) SRB; comparative photocatalytic activity of all of the synthesized materials for the degradation of (e) RhB and SRB and (f) ciprofloxacin (CIP) and tetracycline (TC) (the error bars indicate SD (n = 3)).
somewhat higher phenol conversion is achieved in thermal condition over TS-1 catalyst, (b) more benzoquinone is obtained under photocatalytic condition, whereas hydroquinone is obtained in higher selectivity under thermal condition. Hydroxylation reaction is known to proceed through •OH, which is formed by the visible light-assisted photocleavage of H2O2. These results encouraged us to evaluate the performance of these catalysts in a wide range of dye degradation. Photocatalytic Degradation of Dyes (Rhodamine B (RhB) and Sulforhodamine B (SRB)). Considering the visible light absorption properties of all of the synthesized
materials, two organic dyes (rhodamine B and sulforhodamine B) have been chosen as model compounds for the photodegradation study. Since these dyes are widely used in textile industries that contaminate the groundwater, efficient and recyclable photocatalysts need to be developed. Before performing the visible light degradation experiments, both dyes were evaluated for their adsorption properties in the dark over the best catalyst TCN(1-8-8) (Figure S8a,b) for 4 h. No significant adsorption was observed after 60 min of experiment in the dark. A low adsorption of both the dyes indicates that TCN(1-8-8) is inactive toward the degradation of the two selected dyes under the dark condition. When similar 17267
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Scheme 2. Photocatalytic Degradation Products for RhB Obtained in This Study
experiments were performed under visible light illumination (λ > 420 nm) after an adsorption for 60 min in the dark, an appreciable change in their concentrations was observed during the progress of the experiments. The color of the dye solutions gradually faded, indicating that TCN(1-8-8) has effectively degraded RhB (Figure S9a). With an increase in the irradiation time, the characteristic absorption band for RhB at 553 nm was seen to decrease over TCN(1-8-8) photocatalyst, which completely vanished after 360 min (Figure 5a).59 It is important to note that an effective shift in the peak maxima wavelength to shorter wavelength is observed in the case of RhB degradation. Considering the structure of RhB, it can be concluded that the de-N-ethylation from the parent structure is mainly responsible for this effective peak shifting (Scheme 2). This observation is consistent with literature report.60 This can also be confirmed by the electrospray ionization mass spectrometric (ESI-MS) investigations during the experiment of RhB degradation (Figure S10a,b). Shift in the absorption maxima and ESI-MS spectra of the recovered liquids suggest that the stepwise removal of N-ethyl group has taken place during the light irradiation that changes its color from pink to light yellow and then to a transparent liquid (Figure S9a).
UV−visible investigation shows that after 6 h of experiments under light irradiation, 98% of RhB has been degraded by TCN(1-8-8) (Figure 5a). Further experimental observation also implies that it follows pseudo-first-order kinetics with a rate constant of 0.0125 min−1 (Figure 5c). Total organic carbon (TOC) concentration determination revealed 63% reduction in TOC after 6 h of RhB degradation by TCN(1-88). The same phenomenon was observed in the case of SRB over TCN(1-8-8) photocatalyst under similar experimental conditions (Figure 5b). After 6 h of experiments under visible light irradiation, 96% of SRB was found to be degraded by TCN(1-8-8) using UV−visible investigations (Figure 5b). TOC determination revealed 59% reduction in TOC after 6 h of SRB degradation by TCN(1-8-8). Within this period of light irradiation, the pink color of SRB faded to light yellow and finally to a transparent liquid (Figure S9b). This investigation revealed that SRB degradation also follows a first-order kinetics with a rate constant of 0.01244 min−1 (Figure 5d). ESI-MS results of the samples recovered at regular time intervals indicate the formation of a series of deethylated intermediates during photodegradation (Scheme S1 and Figure S11a,b). On 17268
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Figure 6. Control experiments for the photocatalytic degradation of (a) RhB, (b) SRB, (c) •OH trapping experiment of RhB with terephthalic acid as a probe molecule, and (d) radical scavenging experiment for the degradation of CIP and TC.
of 6 h. TOC determination revealed 55% reduction in TOC after 6 h of CIP degradation by TCN(1-8-8). These results clearly indicate that the synthesized catalyst has the ability to degrade CIP efficiently under visible light. Furthermore, the CIP photodegradation follows a first-order kinetics with a rate constant of 0.0040 min−1 (Figure S12c). The degradation products of CIP were also investigated with ESI-MS. This further supports that CIP degraded satisfactorily in the presence of light (Figure S13a,b). On the basis of the m/z obtained from the ESI-MS investigation, a plausible degradation pathway is suggested in Scheme S2. Here, it can be said that decarboxylation and dehydrogenation followed by pyrazine defragmentation have taken place, leading the reaction to the various observed degraded products. The activity for CIP photodegradation with various photocatalysts investigated in this study follows the order TCN(1-8-8) > TCN(1-10-10) > TCN(1-7-7) > TCN(1-6-6) > TCN(1-4-4) > TS-1 > g-C3N4 (Figure 5f). The results obtained in CIP photodegradation further motivated us to evaluate the same for tetracycline (TC), another common contaminant antibiotic. Unlike other dyes and CIP, the adsorption process for TC was found to continue even after 1 h of stirring at ambient temperature in the dark. The adsorbed amount of TC was found to be 14.9% after 4 h of equilibration time (Figure S8d). First, TC was allowed to physically adsorb for 4 h in the dark, and then it was subjected to photodegradation study. The characteristic absorption band at λmax = 357 nm clearly shows that TC gets degraded with the progress of illumination time as the intensity of this peak is
the basis of the ESI-MS data, degradation mechanism has been proposed, which is consistent with the literature report (Scheme S1).61 The performances of all of the photocatalytic materials investigated here were evaluated for RhB and SRB degradations, and the following trend has been observed for these catalysts: TCN(1-8-8) > TCN(1-10-10) > TCN(1-7-7) > TCN(1-6-6) > TCN(1-4-4) > TS-1 > g-C3N4 (Figure 5e). Thus, the experimental findings demonstrate that among all of the synthesized photocatalysts, TCN(1-8-8) exhibits the best dye degradation activity. This is probably due to its large surface area, suitable band edge position, and efficient chargecarrier separation. Photocatalytic Degradation of Antibiotics (Ciprofloxacin (CIP) and Tetracycline (TC)). The synthesized materials were further examined in the visible light-assisted degradation of two antibiotics, ciprofloxacin (CIP) and tetracycline (TC), which are commonly found in wastewater. Before photodegradation of both the antibiotics, their adsorption over the catalysts surface was investigated, and the results indicate that 25.4% of CIP and 5.3% of TC were physically adsorbed on the catalyst surface after 1 h in the dark. Adsorption experiments were further extended for 4 h. Results show that only a negligible change in the adsorption of CIP was observed. This signifies that the adsorption in the dark is not effective for the removal of CIP (Figure S8c). CIP was then subjected to photodegradation over TCN(1-8-8) photocatalyst. It shows that the characteristic absorption band at λmax = 274 nm for CIP decreased with the increase in the irradiation time period (Figure S12a)62 and that it degraded up to 92% in a time span 17269
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diminished with time (Figure S12b).63 The best photocatalyst TCN(1-8-8) afforded up to 95% of TC degradation under illumination for 6 h. TOC determination showed 57% reduction in TOC after 6 h of TC degradation by TCN(1-88). The degradation of TC also follows a first-order kinetics with a rate constant of 0.0058 min−1 (Figure S12d). ESI-MS study further confirms the degradation of TC in the presence of light (Figure S14a,b). The possible degraded products obtained from ESI-MS investigation are suggested in Scheme S3. The following activity trend is observed for the photodegradation of TC over various catalysts investigated in this study: TCN(1-8-8) > TCN(1-10-10) > TCN(1-7-7) > TCN(1-6-6) > TCN(1-4-4) > TS-1 > g-C3N4 (Figure 5f). In summary, it can be concluded that TCN(1-8-8) has the ability to degrade the two antibiotics effectively in the presence of light. Mechanism of Photocatalytic Degradation for Dyes and Antibiotics. Radical scavenging experiments were performed to elucidate the photodegradation mechanism of RhB under visible light. The influence of reactive oxygen species (ROS) formed during the photodegradation reaction was also investigated by scavenging studies and control experiments (Figure 6). Since the excited-state electrons in the conduction band on the catalyst surface are reactive, it therefore concurrently reacts with dissolved O2 in the aqueous dye solution, resulting in the formation of superoxide radicals (O2−.). This superoxide radical reacts with H2O in the next step to produce •OH and •OOH radicals. On the other hand, the accumulated holes in the valance band oxidize H2O to • OH. These •OH and •OOH radicals initiate the degradation of RhB and SRB in the first step by eliminating ethyl group from the parent structure. ESI-MS investigations clearly indicate that using the present catalyst system, deethylation products are formed during photodegradation of SRB and RhB. To validate the degradation mechanism, the role of dissolved molecular O2 in the aqueous dye solutions was tested by carrying out experiments under a continuous N2 flow over TCN(1-8-8) photocatalyst. Figure 6a,b presents the results obtained from this control experiment. Figure S15 shows no shift in the absorption wavelength (λmax = 554) of RhB, indicating that the catalyst was unable to degrade RhB in the nitrogen atmosphere.60 These results validate the importance of dissolved O2 for RhB/SRB degradation (Figure 6a,b). Further, both electrons and holes are capable of producing •OH radicals from the aqueous dye solution. Hence, it is necessary to measure the influence of •OH radicals in this dye degradation experiment. For this purpose, fluorescence spectra were recorded by taking terephthalic acid as a probe molecule. The free •OH radicals generated after light absorption bind to the terephthalic acid with a characteristic absorption band at λmax = 426 nm in its fluorescence emission spectra (Figure 6c).47 The gradual increase in the fluorescence intensity with irradiation time clearly indicates that •OH radicals actively participate in the photodegradation of RhB (Figure 6c). The fluorescence study validates that during photooxidation, the charge carriers (e−/h+) over the catalyst surface produce •OH radicals, which facilitate the photodegradation of RhB/SRB. When the antibiotic molecules (CIP and TC) were subjected to degradation in the presence of light, decarboxylation and deamination reactions were, respectively, observed to have taken place for CIP and TC, in their first steps.64,65 In this case also, the •OH radical acts as the initiator for the
degradation reaction. Therefore, control experiments such as degradation in N2 atmosphere and radical scavenging experiments were carried out to evaluate the mechanistic insight of the degradation pathway. When CIP and TC were degraded in the N2 atmosphere, after their respective adsorption in the dark condition for 1 and 4 h, only 30.2 and 19.4% of degradations were achieved in 6 h, respectively (Figure 6d). These results clearly indicate that the dissolved oxygen in the solution plays an important role in CIP and TC photodegradations. Furthermore, when the reactions for CIP and TC were carried out in the presence of externally supplied O2, 64 and 48% of CIP and TC, respectively, were found to be degraded in only 2 h. These results justify that the dissolved O2 effectively facilitates the antibiotics degradation. Moreover, the influence of the photogenerated •OH radicals and superoxide O2−• radicals for the antibiotics degradation were monitored by • OH and O2−• radical scavengers, tert-butyl alcohol (TBA), and benzoquinone (BQ).47 When TBA (2 mL) was added in the CIP and TC solutions during photodegradation, only 29% of CIP and 22% TC were degraded in 6 h (Figure 6d). Further, when 50 mg of BQ was added in the CIP and TC solutions, then only 41 and 34% of CIP and TC degradations, respectively, were observed after 6 h (Figure 6d). Since TBA and BQ can effectively scavenge the two main ROS, hydroxyl and superoxide radicals, respectively, a significantly lower degradation activity can be observed for CIP/TC over TCN(18-8). These results show that •OH and O2−• are the two main ROS, which initiate and accelerate the photocatalytic degradation of antibiotics. When visible light falls on the catalyst surface of TCN(1-88), electronic excitation followed by charge separation takes place. The conduction band electrons reduce the dissolved O2 resulting in the formation of superoxide radicals, and valence band holes take part in the H2O oxidation leading to the formation of •OH radicals. In the nanocomposite photocatalyst TCN(1-8-8), TS-1 has not been found so effective for light absorbance but provides high specific surface area, which eventually enables the efficient charge separation on the active light absorber g-C3N4 and facilitates the adsorption of pollutant molecule on catalyst surface. Control experiments confirm the formation of O2•−, •OH, and •OOH species, and, therefore, a plausible degradation mechanism is depicted in Scheme S4. The absorption of visible light by a photocatalyst is accompanied by the excitation of electrons from VB to CB, generating holes in VB and electron in CB. Considering the standard redox potentials, the photogenerated holes in the VB of TS-1 could only oxidize H2O to form •OH species, but the species O2•− could not be formed as the redox potential of O2/ O2•− couple (−0.16 eV vs normal hydrogen electrode) is more negative than the VB potential of TS-1 (Scheme S5).46 The VB edge potential of g-C3N4 is more negative than that of O2/ O2•− and is able to reduce O2 to O2•− species. However, gC3N4 could not oxidize H2O to •OH species due to a less positive VB edge potential. From the above discussion, it can be settled that the VB of TS-1 is involved in the formation of • OH; whereas the species O2•− is formed by the CB of g-C3N4. These ROS react with adsorbed dyes and antibiotics on the catalyst surface and result in the formation of degradation products. The proposed route followed for the degradation of RhB/SRB and CIP/TC over TCN(1-8-8) photocatalyst is given in Scheme S5. This observation is consistent with the photoelectrochemical (PEC) measurements of the nanocomposite photocatalyst. In 17270
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Figure 7. (a, b) Photoelectrochemical LSV profiles under dark and light; (c) transient photocurrent response for TCN(1-8-8), C3N4, and TCN(17-7); and (d) Nyquist plots. The inset shows enlarged profiles in low-frequency region.
lowest intense peak is obtained for TCN(1-8-8). Timeresolved photoluminescence analysis further confirmed that TCN(1-8-8) exhibits the shortest lifetime due to the efficient charge separation (0.52 ns) in its excited carriers dynamics. This also confirms that the effective separation and minimum charge-carrier recombinations over TCN(1-8-8) are mainly responsible for its high photodegradation and phenol hydroxylation performances. Besides, the catalyst was recycled to evaluate the activity of the photocatalyst during successive degradation cycles. The catalyst was centrifuged after each cycle, washed with water, and dried at 100 °C for the next cycle. Five consecutive cycling tests were carried out for the degradation of RhB over TCN(18-8) (Figure S16). No appreciable decrease in the activity was observed in the successive cycles, which suggests that the photocatalyst is stable enough and exhibits similar activity to the first even after five degradation cycles. Fresh and recovered catalysts were analyzed using CHN analyzer. C (26.8%) and N (41.7%) were obtained for the fresh TCN(1-8-8) catalyst, whereas 26.9% C and 41.6% N were obtained for the recovered catalyst, which suggests that no leaching of C3N4 took place during the RhB degradation and the catalyst was stable. Also, the photocatalytic degradation of RhB was carried out using commercially available TiO2 (Degussa P25) under UV irradiation (λ < 400 nm) for 6 h, and the obtained results indicate that TiO2 is able to degrade RhB but it is less active (68%) compared to TCN(1-8-8) (Figure S17). Most importantly, photodegradation activities of all of the dyes and antibiotics were examined under natural sunlight using TCN(1-8-8). Photographs of the reaction vessels taken during the degradation of RhB and SRB under sunlight are shown in Figure 8. The degradation profiles under sunlight were measured for all four dyes using a UV−visible spectrometer (Figure S18). Interestingly, RhB, SRB, TC, and CIP were found to degrade up to 98.4, 55.2, 76.4, and 74.9%, respectively, in 1 h, whereas RhB, SRB, TC, and CIP were
the dark, when linear sweep voltammetry (LSV) was performed at a scan rate of 20 mV s−1, a low current response was recorded for g-C3N4, TCN(1-7-7), and TCN(1-8-8) (Figure 7a). But in the presence of light, a higher current density (2-fold excess) for TCN(1-8-8) was observed. This can be attributed to the generation of electrons from the catalyst surface during light illumination (Figure 7b). TS-1 was found to be inactive in generating any significant current because of its poor visible light absorbance. Further, during the transient photocurrent (i−t) measurements, when light was shone, a sharp enhancement of current response was observed for all of the three nanocomposites, which indicates the production of current from the photogenerated electrons (Figure 7c). But when the light was switched off, the current response immediately reverted back to its initial state, indicating the fact that the photocatalyst is only active in the presence of light. Among the various nanocomposites, TCN(1-8-8) produced the highest i−t current response, which is 2-fold and 1.5-fold higher than g-C3N4 and TCN(1-7-7), respectively (Figure 7c). All of the three photocatalysts exhibited the same type of Nyquist plot in its electrochemical impedance spectroscopy (EIS) analysis, which are composed of a semicircle arc followed by a straight line. But the charge-transfer resistance (Rct) value was found to be lowest (12 Ω) for TCN(1-8-8), which indicates that it has the highest charge-carrier mobility among three catalysts (Figure 7d). This high PEC activity of TCN(1-8-8) can be attributed to its high surface area, optimum composition of TS-1/g-C3N4, efficient charge separation, reduced charge-carrier recombination, and the highest light absorption capacity. The combination of LSV, (i− t), and EIS measurements reveal that among the synthesized catalysts, TCN(1-8-8) is the best photoelectrochemically active material. The obtained activity trend of the nanocomposite is also consistent with the degradation activity of dyes/antibiotics and hydroxylation of phenol. Additionally, the photoluminescence spectroscopic analyses imply that the 17271
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impressive photocatalytic activity of this heterojunction material. The model dyes RhB and SRB were degraded up to 98% in a time span of 6 h. The antibiotics CIP and TC were also degraded up to 95% under the similar reaction condition using a xenon light source. Control experiments revealed that the dissolved O2 in the reaction mixture generated O2−., •OH, and •OOH active species with the help of light-induced electrons and holes on the catalyst surface responsible for the degradation process. Photoelectrochemical measurements and photoluminescence study further ascertained that the lightinduced charge carriers exhibited the highest lifetime and efficient separation, which facilitated the degradation process over the surface of TCN(1-8-8) photocatalyst. Further, the plausible degraded products for all of the water pollutant molecules were tracked and identified during the reaction using ESI-MS analysis. The application of this photocatalyst was also extended in the hydroxylation of phenol under the visible light illumination. The standard redox potentials and band edge potentials provided evidence that the electrons on the surface of g-C3N4 facilitated the reduction of O2 to O2−• (super oxide radical anion), whereas holes in the valence band on TS-1 assisted in the oxidation of H2O to produce •OH, which accelerated the kinetics of the overall photodegradation process. Additionally, the heterojunction photocatalyst TCN(1-8-8) exhibited better photodegradation activity than the commercially available TiO2 (Degussa P25) and TS-1 photocatalysts. TCN(1-8-8) exhibited excellent degradation activity even using natural sunlight. This study provided economical and robust photocatalytic material, which can be prepared in large scale and has the capability to degrade a wide range of dyes and antibiotic present in water effluent through a sustainable approach.
Figure 8. Changes in the color recorded during the course of the photocatalytic degradation of RhB and SRB under natural sunlight. These experiments were carried out during 25−31 April 2018 (temperature ∼30 °C) when the weather was completely sunny (latitude 30.9659° N, 76.5230° E).
degraded up to 99.1, 91.1, 89.6, and 81.3%, respectively, in 4 h. Under sunlight, the reduction in TOC contents to 79, 60, 57, and 50%, respectively, for RhB, SRB, TC, and CIP in 4 h was obtained using TCN(1-8-8). Comparative degradation activity of TCN(1-8-8) with other reported zeolite- and C3N4-based photocatalysts shows that the present catalyst exhibits better or comparable activity (Table S3). A facile, economical, and easy synthesis methodology of TCN(1-8-8) using commercially available TS-1, urea, and thiourea should be attractive from the commercial standpoint. This should win over the other reported procedure,66 also owing to its higher/comparable activity66,67 in the degradation of a wide range of dyes and antibiotics under sunlight without the application of any oxidizing agent (such as H2O2). These results strongly suggest that the presently investigated heterojunction nanocomposite photocatalyst has the potential to be an efficient material for large-scale applications in water remediation.
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EXPERIMENTAL SECTION Catalyst Preparation. Synthesis of TS-1/g-C3N4 Nanocomposites. In this study, TS-1/g-C3N4 nanocomposites were prepared by employing a solid-state synthesis procedure.39 In brief, fixed amounts of urea and thiourea were ground in a mortar and pestle with 1.0 g of TS-1 for 15 min. The resultant white powder was transferred into a cylindrical crucible (with closed lid) and heated in a programmable furnace at 550 °C for 4 h (ramp rate = 5 °C min−1). The furnace was allowed to cool to room temperature naturally. The obtained solid was collected and ground in a mortar and pestle. Materials are designated as TCN(TS-1-U-TU). For example, TCN(1-8-8) suggests that 1.0 g of TS-1 was mixed with 8 g of urea and 8 g of thiourea. Synthesis procedure of this study is described in Scheme 1. Procedure of Catalytic Reaction and Photocatalytic Degradation. Phenol Hydroxylation. The thermal and photochemical activities of various synthesized nanocomposites were tested in phenol hydroxylation. In a conventional thermal catalytic reaction, phenol (6 mmol), catalyst (50 mg), and acetonitrile (6 mL) were taken in a 25 mL round-bottom flask and heated in an oil bath at 65 °C for 6 h. During the reaction, 18 mmol H2O2 (30 wt %) was added at regular intervals (6 mmol at 0, 2, and 4 h). After completion of the reaction, the catalyst was separated by centrifugation at 8000 rpm, and the reaction mixture was analyzed by gas chromatography (GC) and products were confirmed using GC-MS. In a visible light-assisted photocatalytic reaction, the reaction mixture containing phenol (6 mmol), catalyst (50 mg),
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CONCLUSIONS In summary, visible light-active graphitic carbon nitridecoupled titanosilicate-based heterogeneous photocatalysts were developed, which provided optimum textural and photophysical properties. The appropriate amount of gC3N4-coupled TS-1, TCN(1-8-8), exhibited the best photocatalytic activity for the degradation of water pollutant dyes (RhB and SRB) and antibiotics (CIP and TC). TCN(1-8-8) also exhibited the best activity in the photocatalytic hydroxylation of phenol. Moreover, the physicochemical properties of all of the synthesized materials implied that TCN(1-8-8) has high surface area, suitable textural properties, and optimum composition of TS-1 and g-C3N4, which made it the best photocatalytic active material. Furthermore, the optoelectronic properties indicated that the high visible light absorbance capability, efficient charge carriers separation and migration at the interface, and minimization of the charge carriers recombination were some of the noteworthy characteristic features of TCN(1-8-8), which were responsible for the 17272
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through IIT Ropar. The authors are also grateful to DST-FISTfunded XPS facility from the Department of Physics at IIT Kharagpur. CRF, IIT Kharagpur, is gratefully acknowledged for providing the TEM and FESEM facilities.
acetonitrile (6 mL), and 18 mmol H2O2 (30 wt %) was irradiated with a xenon lamp for 6 h at ambient temperature (25−27 °C). A 300 W xenon arc lamp (Newport, Oriel Corporation) coupled with a UV cutoff filter (λ > 420 nm) was used as the visible light source. After completion of the reaction, the catalyst was separated by centrifugation at 8000 rpm, and the reaction mixture was analyzed by GC. Photocatalytic Degradation Study. An aqueous solution (100 mL) of rhodamine B (RhB) with a concentration of 10 mg L−1 was irradiated with a 300 W xenon arc lamp (λ > 420 nm) under continuous stirring in the presence of 150 mg of catalyst for 6 h. Prior to light irradiation, the suspension was stirred for 1 h in the dark to achieve the adsorption/desorption equilibrium between the photocatalyst and organic contaminants. Subsequently, the above suspension was irradiated under continuous stirring at ambient temperature. After an interval of 30 min, 2 mL of solution was collected and centrifuged, and the concentration of RhB present in the reaction mixture was estimated with a UV-2600 Shimadzu spectrophotometer by measuring the absorption at 553 nm. Similar experiments were carried out for the degradation of sulforhodamine B (SRB), ciprofloxacin (CIP), and tetracycline (TC). Experiment in the natural sunlight was also performed on the terrace of IIT Ropar of transit campus (latitude 30.9659° N, 76.5230° E), in which a beaker containing 100 mL of solution and 150 mg of photocatalyst was stirred for a stipulated time period. The measurements were carried out during 25−31 April 2018 (temperature ∼30 °C) when the weather was sunny. Material Characterization and Photoelectrochemical (PEC) Measurements. The details are described in the Supporting Information section.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01545.
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REFERENCES
Materials characterization, photoelectrochemical measurements, nitrogen adsorption isotherms, thermograms, TEM image, band gap energies, FT-IR spectra, UV− visible spectra, changes in the color recorded during the course of the photocatalytic degradation, UV−visible spectra of RhB degradation, changes in the color recorded during the course of the photocatalytic degradation under natural sunlight, UV−visible spectra in the presence of sunlight (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +91-1881-242175. Fax: +91-1881-223395. ORCID
Rajendra Srivastava: 0000-0003-2271-5376 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Nano Mission, DST-New Delhi (grant SR/NM/NS/10542015), is gratefully acknowledged. A.K. and S.S. acknowledge the generous fellowship support from MHRD, New Delhi, 17273
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DOI: 10.1021/acsomega.8b01545 ACS Omega 2018, 3, 17261−17275
