Microwave-assisted modified polyimide synthesis: A facile route. to the enhancement of visible-light-induced photocatalytic

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1 Microwave-assisted modified polyimide synthesis: A facile route to the enhancement of visible-light-induced photocatalytic performance for dye degradation Jhilly Dasgupta a, Jaya Sikder a, *, Sudip Chakraborty b, Utpal Adhikari c, Venkata P. Reddy B. c, Aniruddha Mondal d, Stefano Curcio b a Department of Chemical Engineering, National Institute of Technology Durgapur, West Bengal, India b Department of Informatics, Modeling, Electronics and Systems Engineering, University of Calabria, Via P. Bucci, Cubo - 42a, Rende (CS), Italy c Department of Chemistry, National Institute of Technology Durgapur, West Bengal, India d Department of Physics, National Institute of Technology Durgapur, West Bengal, India Jaya Sikder (*Corresponding author): Department of Chemical Engineering, National Institute of Technology Durgapur, Mahatma Gandhi Avenue, Durgapur , West Bengal, India. Tel.: , ; Fax: ; umuniqueme1@gmail.com The supporting information includes Total number of pages: S1 to S25 (25 Pages) Total number of figures: S1 to S11 (11 figures) Total number of tables: S1 to S4 (4 tables) Total number of sections: S1 to S3 (3 sections) S1

2 Supplementary figure captions: Figure S1. (a) XRD patterns of different polymers samples. (b) 1 H solid-state NMR spectrum of 3.0 SWO 3 /PI. (c) ATIR-FTIR spectra of PI and thiourea/pi. (d) ATIR-FTIR spectra of PI, WO 3 /PI, and SWO 3 /PI samples. W 4f XPS spectra (e) and S 2p XPS spectra (f) of 3.0 SWO 3 /PI. (g) Proposed position of doped sulfur in 3.0 SWO 3 /PI. (h) N 1s XPS spectra of pristine polyimide (PI). (i) Positions of the N atoms in the aromatic triazine segment of PI. (j) N 1s XPS spectra of 3.0 SWO 3 /PI. (k) Positions of the N atoms in the aromatic triazine segment of 3.0 SWO 3 /PI. C 1s XPS spectra of (l) PI and (m) 3.0 SWO 3 /PI. Figure S2. FE-SEM images of (a) pristine PI (b) 3.0 SWO 3 /PI. Figure S3. TEM image(s) of (a) PI and (c, e) 3.0 SWO 3 /PI, SAED pattern of (b) PI and (d) 3.0 SWO 3 /PI. Figure S4. (a) Nitrogen adsorption-desorption isotherm plots for PI and 3.0 SWO 3 /PI. (b) EDS of 3.0 SWO 3 /PI. (c) TGA analyses of PI and SWO 3 /PI hybrids. (d) Determination of WO x content in WO 3 /PI and 3.0 SWO 3 /PI from TG thermograms. Figure S5. DFT computed HOMO and LUMO energy levels pertaining to optimized pristine PI and SWO 3 /PI (3.0 SWO 3 /PI) cluster models. Figure S6. Nyquist plots of PI and 3.0 SWO 3 /PI (using EIS under dark conditions). Figure S7. (a) Visible-light-driven photocatalytic degradation of RR 120 over the synthesized photocatalysts. (b) Structure of RR 120. (c) Kinetic plots defining the visiblelight-induced photocatalytic degradation of RR 120 over the synthesized polymers. (d) Cyclic runs apropos of visible-light-induced photocatalytic degradation of RR 120 using 3.0 SWO 3 /PI photocatalyst. Figure S8. (a) log (C 0 /C) versus t plot for each cycle. Inset: Kinetic rate constant pertaining to each cycle. (b) X-ray diffractograms of fresh and used 3.0 SWO 3 /PI. (c) TEM image of used 3.0 SWO 3 /PI, post the fifth cycle of visible-light-induced photocatalytic degradation of RR 120. Figure S9 (a-e). LC-MS spectra of RR 120 degradation products. (f) Products presumably formed during visible-light-assisted photocatalytic degradation of RR 120. S2

3 Figure S10. (a) Fluorescence spectra of the TAOH product formed by 3.0 SWO 3 /PI under visible-light (15W LED) irradiation. (b) Degradation (%) of RR 120 over 3.0 SWO 3 /PI in the presence of appropriate radical scavengers. Figure S11. VBXPS spectra of (a) thiourea/pi and (c) WO 3 /PI. Schematized band structures of (b, d) PI, (b) thiourea/pi, and (d) WO 3 /PI. Supplementary table captions: Table S1. Crystallinity (%) and average crystallite sizes of the samples. Table S2. BET surface areas of the samples. Table S3. WO x content in the SWO 3 /PI hybrids. Table S4. Kinetic rate constant values for visible-light-induced RR 120 degradation. S3

4 Section S1. Theoretical computations Geometries of organic models were optimized using gradient-corrected density functional theory (DFT) computations, which employed Becke s hybrid exchange-correlation energy functionals (B3LYP). The strategy involved fitting of three parameters by least squares method and also included the Lee Yang Parr correlation functional (LYP). The gas phase DFT computations were accomplished using the Gaussian 09 program, wherein the standard split valence (6-31G (d')) basis set was employed apropos of carbon, nitrogen, oxygen, hydrogen, and sulfur, while the computations pertaining to tungsten were carried out using relativistic effective core potential (ECP) approach involving valence double ξ (zeta) basis set (LANL2DZ-ECP). Theoretical construction of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) was carried out by means of Gview program, based on the results obtained from B3LYP/6-31G (d')-enabled optimization of organic paradigms. Section S2. Electrochemical Measurements Indium tin oxide coated glass slides (surface resistivity: 12 Ω/ sq) were employed as substrates in the fabrication of working electrodes. At the outset, 70 mg of the photocatalyst sample was mixed with 6 ml of acetone and the resulting mixture sonicated for 50 min. The slurry obtained was spread uniformly onto the conductive face (1.56 cm 2, approximately) of the ITO glass slide. The electrode was air-dried and subsequently annealed in air for 30 min at 300 C. Annealing, as such, improved the adhesion of the sample to the substrate. The electrochemical experiments pertaining to electrochemical impedance spectroscopy (EIS) were carried out using the three-electrode cell system of an electrochemical workstation (SP150, Biologic instruments, France). The working electrode along with Ag/AgCl reference electrode and platinum (Pt) counter (auxiliary) electrode was immersed in a 0.1 M sodium S4

5 sulfate (0.1 M) electrolyte solution (ph: 6.6). Herein, a perturbation signal of 10 mv was maintained, while the frequency was varied from 200 khz to 10 mhz. Section S3. Detection of reactive hydroxyl (OH ) radical species The OH radicals were detected using terephthalic acid (TA) based fluorescence technique. 50 mg of 3.0 SWO 3 /PI photocatalyst sample was dispersed in 50 ml TA solution, which was originally prepared by dissolving 0.5 mm terephthalic acid and 6 mm NaOH in deionised water. The suspension was continuously stirred under dark conditions for 10 min, and then irradiated under visible light. 4 ml aliquots were collected from the irradiated suspension at regular (2 h) time intervals and centrifuged. Under the given conditions, TA usually reacts with hydroxyl radical to produce significantly fluorescent 2-hydroxyterephthalic acid (TAOH). The fluorescent signals of TAOH, thus formed, were detected by fluorescence spectroscopy, at an excitation wavelength of 320 nm, using F-2500 fluorescence spectrophotometer. S5

6 (a) Intensity (a.u.) Diffraction peaks indexed to WO x SWO3/PI 3.0 SWO3/PI 1.0 SWO 3 /PI 0.5 SWO3/PI WO 3 /PI Thiourea/PI PI (b) theta (degree) δ (ppm) (c) (d) Thiourea/PI C=O C=O C-N C-N-C C-H/C-C 5.0 SWO 3 /PI W-O C-S W-N Transmittance (%) PI C-S C-N Transmittance (%) 3.0 SWO 3 /PI 1.0 SWO 3 /PI 0.5 SWO 3 /PI WO 3 /PI Wavenumber (cm -1 ) PI Wavenumber (cm -1 ) (e) (f) S6

7 (g) (h) (i) (j) 1 2 (k) (l) Intensity (a.u.) Experimental data Adventitious carbon and carbon in aromatic benzene-like segments Carbon in the triazine unit Carbon in carbonyl moieties Cumulative fit ΙΙ Ι C 1s ΙΙΙ Binding Energy (ev) S7

8 (m) Intensity (a.u.) Experimental data Adventitious carbon and carbon in aromatic benzene-like segments Carbon in the triazine unit Carbon in carbonyl moieties Carbon in C-S bond Cumulative fit ΙΙ Ι C 1s ΙΙΙ ΙV Binding Energy (ev) Figure S1. (a) XRD patterns of different polymers samples. (b) 1 H solid-state NMR spectrum of 3.0 SWO 3 /PI. (c) ATIR-FTIR spectra of PI and thiourea/pi. (d) ATIR-FTIR spectra of PI, WO 3 /PI, and SWO 3 /PI samples. W 4f XPS spectra (e) and S 2p XPS spectra (f) of 3.0 SWO 3 /PI. (g) Proposed position of doped sulfur in 3.0 SWO 3 /PI. (h) N 1s XPS spectra of pristine polyimide (PI). (i) Positions of the N atoms in the aromatic triazine segment of PI. (j) N 1s XPS spectra of 3.0 SWO 3 /PI. (k) Positions of the N atoms in the aromatic triazine segment of 3.0 SWO 3 /PI. C 1s XPS spectra of (l) PI and (m) 3.0 SWO 3 /PI. S8

9 (a) (b) Figure S2. FE-SEM images of (a) pristine PI (b) 3.0 SWO 3 /PI. S9

10 (a) (b) (c) (d) (e) (f) Figure S3. TEM image(s) of (a) PI and (c, e) 3.0 SWO3/PI, SAED pattern of (b) PI and (d) 3.0 SWO3/PI. S10

11 (a) (b) S11

12 (c) (d) Figure S4. (a) Nitrogen adsorption-desorption isotherm plots for PI and 3.0 SWO 3 /PI. (b) EDS of 3.0 SWO 3 /PI. (c) TGA analyses of PI and SWO 3 /PI hybrids. (d) Determination of WO x content in WO 3 /PI and 3.0 SWO 3 /PI from TG thermograms. S12

13 Figure S5. DFT computed HOMO and LUMO energy levels pertaining to optimized pristine PI and SWO 3 /PI (3.0 SWO 3 /PI) cluster models. S13

14 70 60 Z imaginary (ohm 10 4 ) PI 3.0 SWO 3 /PI Z real (ohm 10 4 ) Figure S6. Nyquist plots of PI and 3.0 SWO 3 /PI (using EIS under dark conditions). S14

15 (a) (b) (c) S15

16 (d) Figure S7. (a) Visible-light-driven photocatalytic degradation of RR 120 over the synthesized photocatalysts. (b) Structure of RR 120. (c) Kinetic plots defining the visiblelight-induced photocatalytic degradation of RR 120 over the synthesized polymers. (d) Cyclic runs apropos of visible-light-induced photocatalytic degradation of RR 120 using 3.0 SWO 3 /PI photocatalyst. (a) S16

17 (b) (c) Figure S8. (a) log (C 0 /C) versus t plot for each cycle. Inset: Kinetic rate constant pertaining to each cycle. (b) X-ray diffractograms of fresh and used 3.0 SWO 3 /PI. (c) TEM image of used 3.0 SWO 3 /PI, post the fifth cycle of visible-light-induced photocatalytic degradation of RR 120. S17

18 (a) (b) (c) (d) (e) S18

19 (f) + m/z: 179 (A1) Molecular weight (M): 224 m/z ( [M+H] + ): 225 (B1) Molecular weight (M): 337 m/z ( [M+H] + ): 338 (C1) Molecular weight (M): 338 m/z ( [M+H] + ): 339 (D1) Molecular weight (M): 225 m/z ( [M+H] + ): 226 (E1) Molecular weight (M): 276 m/z ( [M+H] + ): 277 (F1) Molecular weight (M): m/z ( [M+H] + ): (G1) Molecular weight (M): m/z ( [M+H] + ): (H1) Molecular weight (M): 168 m/z ( [M+H] + ): 169 (I1) Molecular weight (M): 365 m/z ( [M+H] + ): 366 (L1) Molecular weight (M): 366 m/z ( [M+H] + ): 367 (N1) S19 Molecular weight (M): 300 m/z ( [M+H] + ): 301 (P1)

20 Molecular weight (M): 301 m/z ( [M+H] + ): 302 (R1) Figure S9 (a-e). LC-MS spectra of RR 120 degradation products. (f) Products presumably formed during visible-light-assisted photocatalytic degradation of RR 120. S20

21 (a) Intensity (a.u.) Progressively intense fluorescence peaks centered at 424 nm 2 h 4 h 6 h Probe species: Terephthalic acid (TA) TA+OH 2-hydroxy-terephthalic acid (TAOH) (highly fluorescent) Wavelength (nm) (b) Figure S10. (a) Fluorescence spectra of the TAOH product formed by 3.0 SWO 3 /PI under visible-light (15W LED) irradiation. (b) Degradation (%) of RR 120 over 3.0 SWO 3 /PI in the presence of appropriate radical scavengers. S21

22 (a) (b) (c) (d) Figure S11. VBXPS spectra of (a) thiourea/pi and (c) WO 3 /PI. Schematized band structures of (b, d) PI, (b) thiourea/pi, and (d) WO 3 /PI. S22

23 Tables Table S1 Crystallinity (%) and average crystallite sizes of the samples. Sample Percentage Crystallinity (%) Average Crystallite size (nm) Equations Area of crystalline peak ( s ) or region Scherrer equation: Employed 100 Total area of diffractrogram Kλ Dp = B Cosθ D p : the average r crystallite size (nm), λ : the wavelength of the incident X-rays, B r : the value of Full Width at Half Maximum (FWHM) (radians), θ : the diffraction or Bragg angle, and K: Shape factor (dimensionless) (0.94, in the present case) PI Remarks The slight decrease in relative crystallinity and coherent crystal domain size, observed with regard to 5.0 SWO 3 /PI plausibly elucidated by the distorted graphite-like interlayer stacking of the conjugated aromatic blocks of PI, induced by excessive SWO 3 content Thiourea/PI WO 3 /PI SWO 3 /PI SWO 3 /PI SWO 3 /PI SWO 3 /PI S23

24 Table S2 BET surface areas of the samples. Sample BET surface area (m 2 /g) PI 4.7 Thiourea/PI 5.2 WO 3 /PI SWO 3 /PI SWO 3 /PI SWO 3 /PI SWO 3 /PI 6.4 S24

25 Table S3 WO x content in the SWO 3 /PI hybrids. Photocatalyst Quantity of WO 3 added (Wt. % calculated on the basis of the total weight of the synthesized photocatalyst (3.6 g)) WO x content obtained from TGA profiles (Wt. %) Remarks PI 0 0 A fair consistency 0.5 SWO 3 /PI SWO 3 /PI SWO 3 /PI SWO 3 /PI between the quantity of tungsten trioxide added during photocatalyst preparation and the WO x content actually present in the sample was observed. The marginal difference between the added and the calculated quantities could be attributed primarily to the slight loss of the tungsten trioxide precursor during photocatalyst synthesis. Table S4 Kinetic rate constant values for visible-light-induced RR 120 degradation. Sample Kinetic rate constant, k (min -1 ) Absence of Photocat alyst Dark and in the presence of SWO 3 /PI (3.0 SWO 3 /PI) PI 0.5 SWO 3 / PI 1.0 SWO 3 /PI 3.0 SWO 3 /PI 5.0 SWO 3 /PI WO 3 /PI Thiourea /PI S25