Astha Singh

Rituporn Gogoi

Kajal Sharma

Najla Fourati

Chouki Zerrouki

Samy Remita

Felix Siril

Continuous Flow Synthesis of Ag-PEDOT-COF Nanocomposite for Photoreforming Plastic Waste and Chromium Remediation in Visible Light

Keywords: Continuous Flow Synthesis, Dispersibility, Photoreforming of Plastic, Photocatalytic H 2 Evolution, Photocatalytic Cr(VI) reduction

Using the freely available solar energy for photocatalytic reactions, especially environmental remediation is a highly sustainable approach. Visible light active heterogeneous photocatalysts are to be developed for driving such reactions. Heterojunction nanocomposites of conjugated polymers with metals as well as other semiconductors are highly active photocatalysts.

However, the synthesis of these materials involves multi-step reactions and harsh reaction conditions. Here, we report a single-step continuous flow synthesis of a nanocomposite (Ag@PEDOT-DAAQ-TFP Flow ), containing a conducting polymer (PEDOT), silver (Ag) nanoparticles and a conjugated covalent organic framework (COF, DAAQ-TFP). The nanocomposite could be synthesised at a lower temperature and in less time when compared to a two-step batch process. Moreover, Ag@PEDOT-DAAQ-TFP Flow exhibited higher surface area and porosity than Ag@PEDOT-DAAQ-TFP Batch . Further, the former has improved ability to absorb light in a broad range, efficiently generate charge carriers and transfer them than the latter as well as Ag@PEDOT and, DAAQ-TFP. Thus, Ag@PEDOT-DAAQ-TFP Flow could be used as an efficient photocatalyst for photoreforming plastic waste to green hydrogen (113.4 𝑚𝑚𝑜𝑙𝑔 -1 𝑐𝑎𝑡 ℎ -1 ) and other value-added products such as formate, acetate, terephthalate, and lactate. It is also one of the best visible light active photocatalysts for the reduction of toxic Cr(VI) to Cr(III). Thus, the enhanced mixing profile and better heat/mass transfer of the continuous flow synthesis process enabled us to achieve a highly active photocatalyst.

Introduction

Accumulation of plastic waste over the earth's surface has become a grave concern because of its long-term impact on the environment. [START_REF] Arp | Weathering Plastics as a Planetary Boundary Threat: Exposure, Fate, and Hazards[END_REF] Plastics are non-biodegradable and hence can remain without undergoing significant degradation for a very long period. They affect the fertility of soil by limiting the supply of oxygen. Moreover, they accumulate on the surface of oceans and adversely affect the marine ecosystem. It is expected that by 2050, there will be more plastic in ocean than fishes, if the present rate of dumping continue. Slow degradation of plastics into smaller fragments called microplastics are now entering our food chain and the long-term consequences are yet to be revealed. [START_REF] Hernandez | Plastic Teabags Release Billions of Microparticles and Nanoparticles into Tea[END_REF] Converting plastic waste into useful products through catalytic reforming is one of the potential solutions to the accumulation of plastics. Thermal reforming of plastics, pyrolysis and incineration are energy intensive (> 500 °C) and these processes release polluting gases. [START_REF] Lopez | Recent advances in the gasification of waste plastics. A critical overview[END_REF] [4] However, photoreforming (PR) of plastics to yield green hydrogen as a solar fuel and other value added products has emerged as an environment friendly, economic and sustainable alternative. [START_REF] Ashraf | Photoreforming of Waste Polymers for Sustainable Hydrogen Fuel and Chemicals Feedstock: Waste to Energy[END_REF] Driving the chemical This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d reactions using the naturally available sunlight is a dream that, if realised, could pave the way for a sustainable future.

Development of visible light active photocatalysts is the key challenge to realise this dream, as only very few such catalysts are reported so far for the PR of plastics. [START_REF] Toe | Advancing photoreforming of organics: Highlights on photocatalyst and system designs for selective oxidation reactions[END_REF][START_REF] Zhang | Photoredox-Catalyzed Plastic Waste Conversion: Nonselective Degradation versus Selective Synthesis[END_REF] Development of such a photocatalyst is a key aspect of the present paper. Photocatalysis also can be used to remediate environment of various other pollutants, apart from plastics. Cr (VI) is a highly toxic heavy metal pollutant that is being discharged to aquatic environments through anthropogenic activities. [START_REF] Saha | Sources and toxicity of hexavalent chromium[END_REF] Effluents from many industries such as tanning, dyeing, printing, metal plating and metallurgical may contain Cr(VI). [START_REF] Saha | Sources and toxicity of hexavalent chromium[END_REF] Whereas, Cr (VI) is highly water soluble and toxic, Cr (III) has hundred times lower toxicity. Cr(OH) 3 is also a green inorganic chemical having poor aqueous solubility. Thus, photo-reduction of Cr (VI) to Cr (III) using photocatalysts was proven to be an efficient solution for treating toxic Cr(VI) pollutants.

Unfortunately, single component photocatalysts lack the ability to absorb light in a wide range encompassing UV, visible and NIR regions which comprise solar light. Moreover, effective generation of charge, their stabilisation and transfer are also difficult to achieve in single component photocatalysts. [START_REF] Banerjee | Polymer photocatalysts for solar-to-chemical energy conversion[END_REF] Additionally, dispersibility/wettability of a photocatalyst in aqueous medium is another concern for photocatalysts. [START_REF] Liang | Covalent organic frameworks: Fundamentals, mechanisms, modification, and applications in photocatalysis[END_REF] Indeed, this limits the interaction between photocatalyst and pollutant, resulting in reduced photocatalytic efficiency. Formation of multi-component heterojunctions is a solution for all the above challenges. [START_REF] Wang | A covalent organic framework constructed from a donor-acceptor-donor motif monomer for photocatalytic hydrogen evolution from water[END_REF][START_REF] Yang | Covalent organic frameworks for photocatalytic applications[END_REF] However, synthesis of such multi-component nanocomposites involves several steps, which usually are energy intensive. [START_REF] Liu | Covalent-Organic-Framework-Based Composite Materials[END_REF] Interestingly, flow chemistry has recently emerged as an excellent alternative to successfully address these problems.

Synthesis of materials can be achieved in mild reaction conditions because of the effective mass and heat transfer in narrow channel flow reactors. Flow synthesis has been explored in the synthesis of semiconducting materials [START_REF] Długosz | Inorganic nanoparticle synthesis in flow reactorsapplications and future directions[END_REF], metal organic frameworks (MOF) [START_REF] Rasmussen | Scalable Continuous Flow Metal-Organic Framework (MOF) Synthesis Using Supercritical CO2[END_REF] [START_REF] Liu | Continuous flow synthesis of ordered porous materials: from zeolites to metal-organic frameworks and mesoporous silica[END_REF], covalent organic frameworks (COF) [START_REF] Singh | Continuous flow synthesis of disordered covalent organic framework for ultra-high removal of industrial pollutants in flow[END_REF], metal and metal oxide nanoparticles [START_REF] Singh | Continuous flow synthesis of Ag3PO4 nanoparticles with greater photostability and photocatalytic dye degradation efficiency[END_REF][START_REF] Prakash | Flow synthesis and in-channel photocatalysis of antimicrobially active ZnS quantum dots using an efficient planar PMMA microreactor[END_REF], and polymers [START_REF] Reis | Polymerizations in Continuous Flow: Recent Advances in the Synthesis of Diverse Polymeric Materials[END_REF]. Potential for scale-up production with uniformity in the chemical and physical properties of materials was also established. This methodology also provides additional advantages in terms of cost, time, eco friendliness and labour. Moreover, the flow synthesized materials are characterized by more interesting properties and higher efficiency in contrast to their batch counterparts. [START_REF] Martinez-Bulit | In flow-based technologies: A new paradigm for the synthesis and processing of covalent-organic frameworks[END_REF] For example, we have recently reported the continuous flow synthesis of a disordered COF for ultra-high removal of pollutants and This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d simulated textile waste in flow. The removal of pollutants in flow has resulted in better adsorption capacity, improved adsorbent regeneration, dye reclamation and recyclability. [START_REF] Singh | Continuous flow synthesis of disordered covalent organic framework for ultra-high removal of industrial pollutants in flow[END_REF] In the present work, we have reported the facile continuous flow synthesis of a visible light active photocatalyst by combining Ag nanoparticles, COF (DAAQ-TFP) and a conducting polymer (PEDOT). The prepared nanocomposites have the ability to absorb light of a broad range and efficiently generate and transfer photo-induced charge carriers in comparison with their individual components. The continuous flow synthesized nanocomposite, Ag@PEDOT-DAAQ-TFP Flow has very good visible light photocatalytic activity in the PR of waste plastics.

Moreover, the COF-PEDOT nanocomposite based photocatalysts were found to be suitable for the visible light photocatalytic environment remediation exemplified by the reduction of toxic Cr(VI) to non-toxic Cr(III).

Experimental Section

Chemicals and materials

3,4-Ethylenedioxythiophene (EDOT), p-toluene sulphonic acid (PTSA), silver nitrate (AgNO 3 ), 2,6-Diaminoanthraquinone (DAAQ), potassium hydroxide (KOH), potassium dichromate (K 2 Cr 2 O 7 ), and 1,5-diphenyl carbazide (DPC) were purchased from Sigma Aldrich. N, N-dimethylformamide (DMF), acetonitrile, ethanol, acetone, trifluoroacetic acid (TFA), hydrochloric acid (HCl), and dichloromethane (DCM) were purchased from TCI chemicals. Hexamethylenetetramine (HMTA, >99%) was procured from Alfa Aesar and phloroglucinol (98%) was from Avra. All the chemicals and solvents were used directly without any additional purification. 1,3,5-triformylphloroglucinol (TFP) was synthesized according to the reported protocol. [START_REF] Singh | Continuous flow synthesis of disordered covalent organic framework for ultra-high removal of industrial pollutants in flow[END_REF] Sylgard 184 PDMS elastomer kit was purchased from Kevin Electrochem for the fabrication of fully circular PDMS based microreactor. Tinned Cu wire (OD -750 μm) and plastic mould (ID -5 cm) were procured from the general hardware store for the same purpose. Finally, the connections in microreactors were done using silicon tubes (ID -0.8 mm) acquired from Polymax Elastomers Pvt. Ltd. Polyethylene terephthalate (PET) plastic from the drinking water bottle was used. Polylactic acid (PLA) pellets were purchased from Synbra Technology, Netherlands. Deionized (DI) water (18.2 MΩ cm) from Milli-Q systems was used according to the requirements.

Fabrication of a T-shaped fully circular PDMS microreactor

This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d T-shaped fully circular PDMS microreactor was fabricated in-house using scaffold technique, without any sophisticated instrumentation facility. [START_REF] Prakash | Flow synthesis and in-channel photocatalysis of antimicrobially active ZnS quantum dots using an efficient planar PMMA microreactor[END_REF] The sylgard PDMS elastomer kit also contains the curing agent which was mixed with the monomer in 1:10 ratio. The scaffold was fabricated by suspending tinned Cu wires (750 μm) within the circular plastic mould which was further filled with pre-prepared PDMS solution (10 mL), as shown in Figure 1a. After degassing under vacuum, the mould was placed in a hot air oven at 80 °C for 6 h. Once the mould gets cooled down to room temperature, the wires were pulled out leaving behind a cylindrical trench within the cured PDMS. Finally, the connections were made, at inlet and outlet, using silicon tubing. The microreactor was washed with DI water and ethanol for removing any debris. The fabricated PDMS microreactor has a dimensional scale of 750 μm ID and 10 cm length. It is typically a three inlet and one outlet system (Figure 1b).

Synthesis of Tri-aldehyde monomer (1, 3, 5 Triformylphlorogucinol, TFP)

The synthesis protocol of 1,3,5 Triformylphlorogucinol (TFP) has already been reported in our previous article. [START_REF] Singh | Continuous flow synthesis of disordered covalent organic framework for ultra-high removal of industrial pollutants in flow[END_REF] The synthesis procedure is detailed in SI section 1.

Synthesis of poly(3,4-ethylenedioxythiophene) in batch process

PEDOT polymer was prepared by chemical oxidative polymerization (COP) of EDOT using AgNO 3 (oxidant) in a two miscible solvent system, reported by us earlier. [START_REF] Gogoi | Revealing the unexplored effect of residual iron oxide on the photoreforming activities of polypyrrole nanostructures on plastic waste and photocatalytic pollutant degradation[END_REF][START_REF] Gogoi | Visible light enhanced photosynthesis of C-C bonds using PdO/Pd@PEDOT nanocomposite[END_REF] Typically, a

This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d fresh solution of EDOT monomer (7.95 μL, 15 mM) was prepared in acetonitrile (5 mL). To this solution, aqueous solution (5 mL) of AgNO 3 (63.70 mg, 75 mM) and PTSA (71.33 mg, 75 mM) was added dropwise with continuous stirring. PTSA was used as a catalyst for the reaction. The reaction mixture was continuously stirred at 60-70 °C for 24 h. The black precipitated solution was purified by washing and centrifugation (12000 rpm) with DI water and acetonitrile to remove the unreacted reagents. Finally, the material was washed with ethanol and dried at 60 °C in a hot air oven for 8 h.

Synthesis of DAAQ-TFP in batch process

DAAQ-TFP was synthesized in batch process using a reported Schiff base condensation reaction procedure with some specific modifications. [START_REF] Deblase | Rapid and efficient redox processes within 2D covalent organic framework thin films[END_REF] typically, the stock solutions of DAAQ (22.24 mM, 5.3 mgmL -1 ) and TFP (46.25 mM, 10 mgmL -1 ) were prepared in DMF in two separate glass vials. Diluted solutions of DAAQ (2.44 mM, 5 mL) and TFP monomers (5.08 mM, 5 mL) were prepared from the above stock solutions. Each of DAAQ and TFP stock solution (0.55 mL) was added to DMF (4.45 mL) in separate vials, making a total volume of 5 mL. TFP solution was added dropwise to the pre-heated (50 °C) DAAQ solution and the reaction was heated at 120 °C with constant stirring for 3 h. The dark reddish precipitate was filtered and washed sequentially with DMF and DI water several times till the filtrate became colourless. The material was freeze dried under vacuum for 24 h which results in a brick red powdered sample.

Synthesis of Ag@PEDOT-DAAQ-TFP in batch process

Ag@PEDOT-DAAQ-TFP Batch nanocomposite was synthesized via traditional one pot method (Figure 2a). At first, DAAQ-TFP COF was synthesized at 120 0 C, as per the protocol described above. The temperature was lowered to 60-70 °C, after 3 h. EDOT monomer (7.95 μL, 15 mM) solution in acetonitrile (5 mL) was added to the synthesized COF solution. The mixture was stirred for 5 min to attain the uniform mixing of EDOT monomer with the COF. Aqueous solution (5 mL) of the oxidant, AgNO 3 (63.70 mg, 75 mM) and PTSA (71.33 mg, 75 mM) was added slowly to the reaction mixture for the oxidative polymerization of EDOT to PEDOT.

The solution was stirred continuously (250 rpm) at 60-70 °C for 24 h. The black precipitate was filtered and washed sequentially with DMF, acetonitrile and DI water until clear filtrate was obtained. It was sequentially washed with ethanol and acetone to remove the unreacted monomers. Finally, the solvent was exchanged with DI water and the obtained black precipitate was frozen at -80 °C for 10 h followed by vacuum drying for 24 h.

Synthesis of Ag@PEDOT-DAAQ-TFP in continuous flow process

Continuous flow synthesis of Ag@PEDOT-DAAQ-TFP Flow was achieved in a fully circular 3 inlet PDMS based T-shaped microreactor (Figure 2b). Solutions of DAAQ (2.90 mg, 2.44 mM) in 5 mL DMF, TFP (5.49 mg, 5.08 mM) in DMF (5 mL), EDOT (7.95 μL, 15 mM) in acetonitrile (5 mL) and AgNO 3 (63.70 mg, 75mM) + PTSA (71.33 mg, 75 mM) in DI water (5 mL) were prepared. Two PDMS microreactors were connected via 0.5 cm long silicon tubing.

The microreactors were preheated at 60-70 °C by using hot plate for 30 min. Three Dispovan® syringes were loaded with the solutions. The DAAQ and TFP solutions were injected in the PDMS microreactor from the 1 st and 2 nd inlets respectively using syringe pumps (NE-300-ES, Infusion ONE from Darwin Microfluidics) at the flow rate of 100 μL.min -1 . The EDOT+AgNO 3 +PTSA solution was injected through the 3 rd inlet and mixed with the reaction mixture at the same flow rate. The reddish-brown suspension was collected in a glass reagent bottle and heated at 60-70 °C for the next 3 h using a hot plate. This has resulted in the formation of black precipitate. The precipitate was filtered and sequentially washed with DMF, acetonitrile and DI water for removing the unreacted reactants. The washing was performed several times until obtaining clear filtrate. Finally, the material was vacuum dried using lyophilizer for 24 h.

Characterization

This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289

P r e p r i n t n o t p e e r r e v i e w e d

Powder X-ray diffraction (pXRD) patterns were obtained using Rigaku SmartLab 9 kW rotating anode) with Ni-filtered Cu-Kα irradiation (λ = 0.1542 nm) operating at 45 kV and 100 mA. The scanning was performed in the range of 2θ = 3-80° at the rate of 2 ° min -1 (step size: 0.02°). FTIR spectra were acquired over a spectral range of 2400-400 cm -1 using L1600312 spectrum TWO LITA (ATR plate) spectrometer equipped with LaTiO 3 MIR detector.

Thermogravimetric analysis (TGA) was performed using Perkin Elmer Pyris 1 instrument. The samples were placed in a Pt crucible and heated in the range 25-800 °C at the rate of 5 °C.min -1 under nitrogen atmosphere (flow rate: 20 mL.min -1 ). X-ray photoelectron spectra (XPS) of the materials were acquired using Thermo Scientific Nexsa surface analyser attached with Al-Kα source (λ = 0.83 nm, 6.03 mA and 1200 V). The spectra were acquired with a pass energy of 50 eV at the step size of 0.1 eV. All the XPS data were deconvoluted with Thermo Scientific Avantage 5.9908 curve fitting software, using smart background (modified Shirley background with the additional constraint where the background intensity is not more than the actual data at any point in the region). All the peaks were plotted with respect to the adventitious carbon positioned at 284.8 eV. Particle morphology and elemental distribution analysis were performed using field emission scanning electron microscopy (FESEM, FEI Nova Nano SEM-450) equipped with energy dispersive X-ray spectroscopy (EDX) and backscattered imaging.

High-resolution transmission electron microscopy (HRTEM) imaging was performed using a FEI Tecnai G2 20 S-twin electron microscope equipped with selective area electron diffractometer (SAED) which was operated at 200 keV. N 2 gas adsorption-desorption isotherm was acquired using Autosorb-iQ-MP/XR (Quantachrome instruments) operated at 77 K. Using the multipoint Brunauer-Emmett-Teller (BET) method, the specific surface area and average pore size of the materials was determined. Optical absorbance of the materials was analysed in the range 200-1100 nm using UV-Vis spectrophotometer (UV 2600 SHIMADZU).

Photoluminescence (PL) spectra were obtained using the Fluorolog HORIBA scientific system using aqueous samples of all the materials. Each material (1 mg) was dispersed in DI water (1 mL) using a bath sonicator. The emission spectra were collected with an excitation wavelength of 420 nm through a 5 nm scan slit and emission range from 440 to 750 nm. The time-resolved photoluminescence (TRPL) analysis was performed using DeltaFlex TM HORIBA scientific modular fluorescence lifetime system at the excitation wavelength of 420 nm. The charge carriers were excited using a 390 nm pulsed diode LASER source and the decay time was calculated using bi-exponential decay function. For evaluating the dispersibility of synthesized nanocomposites, the Tyndall effect was studied using a 405 nm ± 10 nm blue LASER pointer.

The nanocomposite and COF, 3mg each, were sonicated for 15 min in 1 mL aqueous solution.

This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289

P r e p r i n t n o t p e e r r e v i e w e d

The solutions were immediately irradiated using blue light in a dark environment and the photographs were clicked by using a mobile camera lens.

In a typical experiment of plastic waste photoreforming, 500 μL of the reaction mixture supernatant was added to 150 μL of D 2 O for analysis of value-added products obtained from plastic reforming. Molecular structure and chemical composition of plastic decomposition products were analyzed using nuclear magnetic resonance spectra (NMR). 1 H NMR spectra were recorded on the Jeol JNM ECX 500 MHz spectrometer. Quantification of gases was performed by using a gas chromatograph (GC, Agilent 7890B) equipped with a thermal conductivity detector (TCD). Nitrogen gas was used as the carrier gas in order to detect hydrogen gas evolved in the reaction mixture. The GC was calibrated using standard gases before identifying and quantifying the unknown gases.

Photoelectrochemical Measurements

Autolab electrochemical analyzer equipped with a standard three-electrode system was used for photocurrent and electrochemical impedance spectroscopy (EIS) measurements. [START_REF] Gogoi | Revealing the unexplored effect of residual iron oxide on the photoreforming activities of polypyrrole nanostructures on plastic waste and photocatalytic pollutant degradation[END_REF] [START_REF] Sharma | Enhanced photocatalytic activity of hierarchical C/ZnO nanocomposite derived from solvothermally restructured Zn-BTC microspheres[END_REF] The synthesized materials (2 mg) were perfectly dispersed in aqueous isopropyl alcohol (1:1, 480 μL) and mixed with Nafion solution (5 wt%, 20 μL) for preparing the catalyst ink. The prepared ink (100 μL) was drop-casted over F-doped SnO 2 -coated glass (FTO glass) electrode (1.5 X 1.5 cm), with an active area of ~1.5 cm 2 and dried properly before analysis. Graphite rod (5 mm) and Ag/AgCl (saturated KCl) were used as counter and reference electrodes, respectively. A visible light source (150 W), kept at a distance of ~ 10 cm away from the working electrode was used as a light source. Aqueous solution of Na 2 SO 4 (0.1 M) was used as the electrolyte. The transient photocurrent measurement was done under an applied bias of 0.5 V. EIS measurements were carried out at the polarization potential of 1 V in the frequency range of 100 kHz to 0.1 Hz with an AC amplitude of 10 mV. The Mott-Schottky experiment was performed at the frequency of 1000 Hz.

Photocatalytic reforming of plastic waste

The photocatalytic reforming (PR) of PET and PLA waste was tested, as reported earlier. [START_REF] Gogoi | Revealing the unexplored effect of residual iron oxide on the photoreforming activities of polypyrrole nanostructures on plastic waste and photocatalytic pollutant degradation[END_REF] In a typical experiment, PET (20 mg) bottle pieces were pre-treated in aq. NaOH (5 mL, 5 M) solution for 12 h with continuous stirring at 500 rpm in a glass vial (30 mL) at room temperature. [START_REF] Du | Trash to Treasure: Photoreforming of Plastic Waste into Commodity Chemicals and Hydrogen over MoS2-Tipped CdS Nanorods[END_REF] Ag@PEDOT-DAAQ-TFP Flow catalyst (5 mg) was added to the pretreated solution of PET and sonicated for 5 min using ultrasonicator bath. The glass vial was tightly sealed with rubber septum and Teflon tape. The mixture was then purged with N 2 gas This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d for 30 min to remove the air present within the reaction vial. The solution was illuminated with visible light using four 48 W LED household lamps under continuous stirring for 24 h. The temperature inside the photoreactor was maintained by circulating air using exhaust fans. The gas produced during the photocatalytic reaction was analysed using GC. The used catalyst was separated from the reaction mixture via centrifugation at 10,000 rpm for 15 min and the supernatant was analysed using NMR for the evaluation of the value-added products formed during the process. The efficiency of this photocatalyst was investigated by performing the PR reaction in similar conditions with Ag@PEDOT, DAAQ-TFP and Ag@PEDOT-DAAQ-TFP Batch photocatalysts. An experiment was also performed for the PR of PLA with Ag@PEDOT-DAAQ-TFP Flow as the photocatalyst. Control experiments were also performed in dark condition, without catalyst and without substrate.

Photocatalytic reduction of Cr (VI)

The Cr(VI) reduction studies were performed under visible light in a homemade photoreactor equipped with one 20 W white light emitting diode (LED) and an exhaust fan. Additionally, temperature in the room as well as the photoreactor was maintained using an air conditioner.

The photocatalysts (0.33 mgmL -1 ) were added to aqueous potassium dichromate solution (166.66 ppm, 5 mL) and ethanol (1 mL) as a sacrificial agent and mixed by ultrasonication for 5 min. The pH of the solution was adjusted to 3 by adding HCl. The solution was stirred in dark for 2 h to attain adsorption-desorption equilibrium. After accomplishing the equilibrium in dark, the solution was irradiated under visible light (20 W LED) placed at a constant distance of 10 cm in all the experiments. Aliquots (100 μL) were taken from the reaction solution at different time intervals. The photoreduction of Cr (VI) to Cr (III) was analysed using 1,5diphenylcarbazide (DPC) based method in UV spectrophotometer. [START_REF] Sharma | Enhanced photocatalytic activity of hierarchical C/ZnO nanocomposite derived from solvothermally restructured Zn-BTC microspheres[END_REF][START_REF] Gogoi | Mechanically Activated Covalent Organic Framework as Metal Free Photocatalyst for Continuous Flow Fenton-like Degradation of Organic Pollutants and Hexavalent Chromium Reduction[END_REF] A complex is formed between Cr (VI) and DPC which shows a characteristic absorption peak at λ=540 nm.

The effect of hole scavenger, synthesized photocatalysts and control experiments were done under the same experimental procedure.

The used Ag@PEDOT-DAAQ-TFP Flow was separated from the reaction mixture by centrifugation at 10,000 rpm for 10 min and washed with DI water several times. Finally, the solvent was exchanged with ethanol and the catalyst was collected by centrifugation process.

It was further dried in a hot air oven at 60 °C for 6 h. The recycled photocatalyst was again used for the photocatalytic reduction of Cr (VI) using the above-mentioned protocol.

Results and Discussion

This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d Formation of Ag@PEDOT, DAAQ-TFP, Ag@PEDOT-DAAQ-TFP Flow and Ag@PEDOT-DAAQ-TFP Batch was confirmed using FTIR spectroscopy (Figure 3a). Disappearance of =Cα-H (~ 3100 cm -1 ) peak of EDOT during COP and appearance of a peak at 1430 cm -1 due to =Cα-Cα՛= indicated the formation of Ag@PEDOT. [START_REF] Gogoi | Visible light enhanced photosynthesis of C-C bonds using PdO/Pd@PEDOT nanocomposite[END_REF] Vanishing of the peaks at 3500 cm -1 due to NH 2 of DAAQ monomer, and =Cα-H (~ 3100 cm -1 ) peak of EDOT indicated the formation of the Ag@PEDOT-DAAQ-TFP nanocomposite under batch and flow synthesis conditions. Moreover, appearance of the peaks corresponding to C=O (1622 cm -1 ) / C-N (1254 cm -1 ) of DAAQ-TFP and =C-C= (1430 cm -1 ) of PEDOT also validated the formation of the composite material. [START_REF] Gogoi | Visible light enhanced photosynthesis of C-C bonds using PdO/Pd@PEDOT nanocomposite[END_REF][START_REF] Kong | Redox active covalent organic frameworkbased conductive nanofibers for flexible energy storage device[END_REF] Incorporation of Ag@PEDOT in the DAAQ-TFP framework resulted in the shifting of C-N peak slightly towards higher wavenumbers. [START_REF] Luo | A Microporous Covalent-Organic Framework with Abundant Accessible Carbonyl Groups for Lithium-Ion Batteries[END_REF] Besides, the peak at 1670 cm - 1 corresponds to the C=O functional group in the anthraquinone moiety. [START_REF] Mulzer | Superior Charge Storage and Power Density of a Conducting Polymer-Modi fi ed Covalent Organic Framework[END_REF] The pXRD pattern of DAAQ-TFP COF in Figure 3b showed peaks at 3.56°, 11.97° and 26.86° which correspond to (100), ( 220) and (001) reflection planes respectively. [START_REF] Deblase | βketoenamine-linked covalent organic frameworks capable of pseudocapacitive energy storage[END_REF] The broad peak at 26.86° was attributed to π-π stacking of the 001 planes. [START_REF] Kong | Redox active covalent organic frameworkbased conductive nanofibers for flexible energy storage device[END_REF] Noticeably, the intensity of (100) and (001) planes were reduced after the composite formation. Moreover, in Ag@PEDOT-DAAQ-TFP samples, the (001) reflection peak of DAAQ-TFP overlaps with (020) reflection peak of Ag@PEDOT at ~26°. [START_REF] Mohanadas | Facile synthesis of PEDOT-rGO/HKUST-1 for high performance symmetrical supercapacitor device[END_REF] Additionally, intense crystalline peaks were observed at 38.01° (111), 44.23° (200), 64.36° (220) and 77.30° (311) corresponding to metallic silver in Ag@PEDOT, Ag@PEDOT-DAAQ-TFP Batch and Ag@PEDOT-DAAQ-TFP Flow . These peaks were in accordance with the JCPDS card no. 04-0783. [START_REF] Pan | Synthesis of Silver Nanoparticles Embedded Electrospun PAN Nanofiber Thin-Film Composite Forward Osmosis Membrane to Enhance Performance and Antimicrobial Activity[END_REF] During the COP of EDOT, Ag + ions get reduced simultaneously into Ag 0 atoms. We thus deduce that nanocomposites made of both Ag metal and PEDOT polymer are formed. [START_REF] Gogoi | Revealing the unexplored effect of residual iron oxide on the photoreforming activities of polypyrrole nanostructures on plastic waste and photocatalytic pollutant degradation[END_REF] TGA thermal curves in the temperature range of 25-900 °C under N 2 atmosphere are depicted in Figure 3c. Initial mass loss up to 250 °C was due to the removal of water and solvent molecules. The DAAQ-TFP broke down and lost almost 100% mass above 700 °C. However, Ag@PEDOT and its nanocomposites left significant amounts of residual mass because of the presence of metallic Ag. Thus, Ag@PEDOT contained about 77 % (by mass) Ag, while Ag@PEDOT-DAAQ-TFP Batch and Ag@PEDOT-DAAQ-TFP Flow contained about 75 and 63 % (by mass) Ag, respectively. [START_REF] Gogoi | Visible light enhanced photosynthesis of C-C bonds using PdO/Pd@PEDOT nanocomposite[END_REF] XPS survey and deconvoluted narrow scans of C, N, O, S and Ag of DAAQ-TFP, Ag@PEDOT, Ag@PEDOT-DAAQ-TFP Batch and Ag@PEDOT-DAAQ-TFP Flow are shown in

Figures S1, S2, S3 and S4, respectively. The deconvoluted data clearly show the presence of

This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d respective functional groups in all materials. The N1s narrow scan of DAAQ-TFP displays representative peaks at 399.1, 400.23 and 402.8 eV conforming to C=N (imine, enol form), C-NH (enamine, keto form) and oxidized N species respectively. [START_REF] Deblase | βketoenamine-linked covalent organic frameworks capable of pseudocapacitive energy storage[END_REF][START_REF] Li | Visible-Light-Responsive Anthraquinone Functionalized Covalent Organic Frameworks for Metal-Free Selective Oxidation of Sulfides: Effects of Morphology and Structure[END_REF][START_REF] Fechler | One-pot synthesis of nitrogen-sulfur-codoped carbons with tunable composition using a simple isothiocyanate ionic liquid[END_REF] The imine bond was observed to be slightly shifted towards lower binding energy with reduced intensity (Figure 3d). Thus, keto-form was predominant in the nanocomposite. [START_REF] Kong | Redox active covalent organic frameworkbased conductive nanofibers for flexible energy storage device[END_REF] Additionally, the narrow scan of S2p in Ag@PEDOT-DAAQ-TFP Flow divulges a shift in the doublet peak of neutral S and Ag-S bond towards higher binding energy in comparison with Ag@PEDOT (Figure 3e).

This may be due to the interaction between the Ag@PEDOT and DAAQ-TFP. Morphology and elemental distribution of the synthesized materials were analysed using FESEM imaging (Figure 4a-d) and EDX mapping (Figure 4f-l). The Ag@PEDOT has spherical morphology with particle size in the sub-micrometre domain. However, the DAAQ-TFP exhibited fibrous morphology. The nanocomposites are made of particles of spherical as well as fibrous morphologies. Uniform distribution of all the elements (C, N, O, S and Ag) in the nanocomposites confirms the successful integration of Ag@PEDOT with DAAQ-TFP framework. Figure 4e shows the FESEM backscattered image of Ag@PEDOT-DAAQ-TFP Flow . The bright spots observed in Figure 4e must be attributed to silver nanoparticles, since Ag is expected to back-scatter electrons more than the other elements, due to its higher atomic number. This was further confirmed from the TEM image of Ag@PEDOT-DAAQ-TFP Flow as shown in Shape of the BET adsorption isotherms under N 2 atmosphere at 77K (Figure 5a) matched with H4 hysteresis type, which is the characteristic of microporous material containing mesopores as well. [START_REF] Cychosz | Recent advances in the textural characterization of hierarchically structured nanoporous materials[END_REF] The BET surface areas for DAAQ-TFP and Ag@PEDOT were 173 and 9 m 2 g -1 , respectively. The surface areas for Ag@PEDOT-DAAQ-TFP Batch and Ag@PEDOT-DAAQ-TFP Flow were 33 and 64 m 2 g -1 , respectively. The microporous surface area according to the V-t plot and the mesoporous surface area obtained from the BJH plot were 11 and 22 m 2 .g -1 respectively for Ag@PEDOT-DAAQ-TFP Batch . On the contrary, Ag@PEDOT-DAAQ-TFP Flow composite possess 62 m 2 g -1 of mesoporous surface area, and no surface area was contributed through micropores. The reduction in the surface area and variation in pore size distribution confirmed the successful synthesis of Ag@PEDOT-DAAQ-TFP nanocomposites.

Moreover, pore size distribution of micropores was evaluated from non-linear DFT (NLDFT) theory as shown in Figure 5b. Bare DAAQ-TFP bears the maximum micropores of size 1.4 nm. However, Ag@PEDOT-DAAQ-TFP Flow has almost no micropores with maximum This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d pores of mesoporous size, 2.8 nm. The inclusion of Ag@PEDOT within the DAAQ-TFP framework also reduces the pore volume when compared with DAAQ-TFP due to the nanocomposite formation. [START_REF] Vitaku | Phenazine-Based Covalent Organic Framework Cathode Materials with High Energy and Power Densities[END_REF] The Langmuir surface area for monolayer adsorption for both flow and batch synthesized nanocomposites was calculated to be 122 and 63 m 2 g -1 respectively. This was quite less than the monolayer adsorption capacity of DAAQ-TFP (208 m 2 g -1 ). This may be attributed to the presence of Ag@PEDOT on the surface of DAAQ-TFP in the nanocomposites. Optical and electrical properties of the materials were characterised in order to understand their suitability for photocatalytic applications. The UV-visible absorbance spectra showed that the nanocomposites display a broad absorption spectrum from NIR to UV range (1100-200 nm) with a maximum absorbance in the visible region (400 -500 nm) (Figure 6a).

The plasmonic absorbance of silver nanoparticles are expected in this range. However, it was observed that the characteristic absorption peak in nanocomposites had been shifted towards lower wavelengths in comparison with DAAQ-TFP. The peaks at 447 and 280 nm correspond to π-π*/n-π* and n-σ* transitions, respectively. [START_REF] Sagadevan | Organic Nonlinear Optical p-Toluidine p-Toluenesulfonate Single Crystal[END_REF] These transitions are due to the electronic transitions in C=C (π-π*), C=O (π-π*/ n-π*) and C-N/C-S (n-σ*) functional groups. [START_REF] Gogoi | Visible light enhanced photosynthesis of C-C bonds using PdO/Pd@PEDOT nanocomposite[END_REF] A small shoulder peak in batch synthesised materials at 374 nm corresponds to π-π* transition in C=N (imine) functional group. The extended absorbance tail in NIR range was attributed to the polaron-bipolaron transitions.

The band gap energies (E g ) of the bare materials and their nanocomposites were calculated using Tauc plot according to the equation A = (hʋ-E g ) 1/n , where n represents the nature of the transition. According to the transformed differential spectra, the value of n=1/2, This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d which implies that the materials have a direct band gap. The band gaps of corresponding bare materials, DAAQ-TFP and Ag@PEDOT were 2.05 eV and 1.62 eV, respectively (Figure S5a,b). The band gaps of Ag@PEDOT-DAAQ-TFP Batch and Ag@PEDOT-DAAQ-TFP Flow were 2.3 and 2.2 eV, respectively which is close to the band gap of DAAQ-TFP. Another transition was observed at 1.8 and 2.0 eV in flow and batch nanocomposites, respectively. This transition corresponds to the band gap of Ag@PEDOT. [START_REF] Li | Visible-Light-Responsive Anthraquinone Functionalized Covalent Organic Frameworks for Metal-Free Selective Oxidation of Sulfides: Effects of Morphology and Structure[END_REF] Thus, the synthesized nanocomposites can absorb visible light, making them viable for visible light photocatalytic applications.

The intrinsic electronic band structures of the synthesized materials were evaluated using Mott-Schottky (M-S) measurement at 1000 Hz under dark conditions and the calculations were performed according to M-S equation given below:

1 𝐶 2 = ( 2 𝑒𝜀𝜀 0 𝑁 𝑑 𝐴 2 ) {(𝑉 -𝑉 𝐹𝐵 -𝐾𝑇 𝑒 )} (1)
where, C, e, ε, ε 0 , N d , A, V FB , K, and T represent specific capacitance, electron charge, dielectric constant of semiconductor, electric permittivity of vacuum, charge-carrier density, active area of electrode, flat-band potential, Boltzmann constant, and absolute temperature respectively. As represented in Figure 6c, the flow and batch synthesized nanocomposites showcased positive slope in the linear fitted data of M-S plot (1/ C 2 Vs V vs NHE), which is consistent with the characteristics of n-type semiconductor. [START_REF] Gogoi | Metal-free Conjugated Polyphenothiazine Nanostructures as Visible Light Active Photocatalysts for the Removal of Organic Pollutants and Selective Aerobic Oxidation of Sulfides[END_REF] The flat band potential (V FB )

was considered as the conduction band potential (CB min ) which was determined from the intercept at x-axis. The resulting electronic band structure of the prepared materials are as shown in Figure 6d. Evidently, these materials could be suitable photocatalysts for the photoreduction of Cr (VI) to Cr (III) (1.33 V vs NHE) [START_REF] Nezar | Efficient reduction of Cr(VI) under visible light irradiation using CuS nanostructures[END_REF] and PR of plastic waste with parallel evolution of H 2 (0 V Vs NHE). [START_REF] Du | Trash to Treasure: Photoreforming of Plastic Waste into Commodity Chemicals and Hydrogen over MoS2-Tipped CdS Nanorods[END_REF] This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d The electron-hole charge separation ability of a material is an important eligibility criterion for being a good photocatalytic material. The Nyquist plots are represented in Figure 7a. The order in the radius of semi-circle was DAAQ-TFP > Ag@PEDOT-DAAQ-TFP Batch > Ag@PEDOT-DAAQ-TFP Flow > Ag@PEDOT. Thus, the charge transfer resistance also followed the same decreasing order. [START_REF] Liu | Tunable Covalent Organic Frameworks with Different Heterocyclic Nitrogen Locations for Efficient Cr(VI) Reduction, Escherichia coli Disinfection, and Paracetamol Degradation under Visible-Light Irradiation The two-dimensional COFs typically contain an sp[END_REF] Ag@PEDOT has the lowest charge transfer resistance due to the intrinsic conducting nature of PEDOT and to the presence of metallic Ag nanoparticles having very high conductivity. Thus, the charge transfer resistance of DAAQ-TFP could effectively be reduced when incorporating Ag@PEDOT into it. Moreover, the flow synthesized nanocomposites, Ag@PEDOT-DAAQ-TFP Flow , possess lower resistance (higher conductivity) for charge transfer as compared with Ag@PEDOT-DAAQ-TFP Batch .

Apart from a good conductivity, an excellent photocatalyst should have the ability to quickly respond to light. The photo-response of all materials to visible were studied (Figure 7b). Photo-response repeatability with multiple cycles was also studied by alternative switching ON/OFF experiments. Both DAAQ-TFP and Ag@PEDOT were photoresponsive and were able to generate photocurrent. Nevertheless, the nanocomposites showed much higher photocurrent density due to synergistic effects. In particular, Ag@PEDOT-DAAQ-TFP Flow displayed the highest values amongst all the materials.

Furthermore, the effect of light on charge transfer was also studied using PL by exciting the charge carriers at 420 nm. A significant decrement in the PL intensity of the Ag@PEDOT-This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d DAAQ-TFP nanocomposites was observed as compared to DAAQ-TFP (Figure 7c). This implies that the charge carriers were getting transferred from DAAQ-TFP to Ag@PEDOT leading to reduction in the probability of photogenerated charge carrier recombination. [START_REF] Niu | Enhanced photocatalytic reduction of Cr( vi ) to Cr( iii ) over g-C 3 N 4 catalysts with Ag nanoclusters in conjunction with Cr( iii ) quantification based on operando low-field NMR relaxometry[END_REF] The inset of Figure 7c shows the PL emission spectra of Ag@PEDOT material at 519 nm. The transient state curves obtained via TRPL analysis were well fitted with the bi-exponential decay function (Figure 7d). The decay time profiles of the photogenerated charge carriers for DAAQ-TFP andAg@PEDOT-DAAQ-TFP nanocomposites were in good resemblance with the corresponding PL spectra. The lifetime data of Ag@PEDOT was not included because its emission wavelength is different from the emission wavelength of other materials. Also, it was observed from PL spectra that the charge carriers were transferred from DAAQ-TFP to Ag@PEDOT surface. So, it is more justified to compare the charge carrier lifetime in DAAQ-TFP and Ag@PEDOT-DAAQ-TFP nanocomposites. The decay time profile of DAAQ-TFP was 0.21 ns. The lifetime of the excited electrons was reduced to 0.18 ns in batch nanocomposite and 0.09 ns in flow nanocomposite. This also confirms the effective transfer of charge carriers between DAAQ-TFP and Ag@PEDOT. The lifetime of Ag@PEDOT-DAAQ-TFP Flow nanocomposite (0.09 ns) was lower than that of Ag@PEDOT-DAAQ-TFP Batch (0.18 ns). This hints towards better charge separation and transfer within the nanocomposite synthesized via continuous flow process.

This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d

Photocatalytic Applications

Photoreforming (PR) of Plastic Waste

PR of PET under visible light irradiation in presence of the prepared catalysts yielded H 2 gas.

The amount of H 2 evolved during the PR process using all the photocatalysts is shown in

Figure 8a

. Significant amounts of H 2 gas, 113.4 𝑚𝑚𝑜𝑙.𝑔 -1 𝑐𝑎𝑡 .ℎ -1 (for Ag@PEDOT-DAAQ-TFP Flow ) and 90.3 𝑚𝑚𝑜𝑙.𝑔 -1 𝑐𝑎𝑡 .ℎ -1 (for Ag@PEDOT-DAAQ-TFP Batch ) were produced during PR of PET under visible light irradiation for 24 h. In contrast to this, Ag@PEDOT and DAAQ-TFP produced comparatively less H 2 gas. The best photocatalyst (Ag@PEDOT-DAAQ-TFP Flow ) was versatile as it could drive PR od PLA also. Significant amount of H 2 (95.56 𝑚𝑚𝑜𝑙 .𝑔 -1 𝑐𝑎𝑡 .ℎ -1 ) was generated by the PR of PLA as can be seen from Figure S6a. According to the control experiments reported in Figure S6a, hydrogen evolution did not happen in dark condition as well as without any photocatalyst. However, Ag@PEDOT-DAAQ-TFP Flow could photo-catalytically split water to produce H 2 gas in alkaline medium, even without any plastic as substrate.

This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d

In addition to green H 2 , the PR of PET as well as PLA yielded value added products to varying extents according to the photocatalytic activity of the materials used. The 1 H NMR analysis (Figure 8a) revealed that other than terephthalate, no value-added products were formed by PR of PET using DAAQ-TFP or Ag@PEDOT as photocatalysts. However, both nanocomposites could photoreform PET into small organic molecules, under visible light irradiation. Ag@PEDOT-DAAQ-TFP Flow generated highest amount of H 2 along with terephthalate (a), glycolate (b), ethylene glycol (c), methanol (d), acetate (e) and ethanol (f, g).

However, Ag@PEDOT-DAAQ-TFP Batch could generated only lactate by photoreforming apart from the hydrolyzed products (terephthalate and ethylene glycol). [START_REF] Uekert | Plastic waste as a feedstock for solar-driven H2 generation[END_REF] Additionally, CO 2 was not detected in the gas mixture, during the PR. [START_REF] Gogoi | Revealing the unexplored effect of residual iron oxide on the photoreforming activities of polypyrrole nanostructures on plastic waste and photocatalytic pollutant degradation[END_REF] The photocatalytic activities of the prepared catalysts were compared with other catalysts that are described in literature for PR of plastics as summarised in Table 1. Evidently, Ag@PEDOT-DAAQ-TFP Flow is the best photocatalysts that are reported for the PR of PET so far, in terms of the amount of green H 2 generated. Its PR activity for PLA was also better than other photocatalysts, except one (CdS/MoS 2 -800-15). The higher photocatalytic activity is mainly due to the nanocomposite formation between Ag nanoparticles, PEDOT and DAAQ-TFP COF. However, the flow synthesis not only facilitates the synthesis of the multicomponent nanocomposite by lowering the reaction temperature and time, but also enhance the photocatalytic activity.

This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d did not proceed on irradiation of Cr(VI) solution without any catalyst (Figure 9a). [START_REF] Niu | Enhanced photocatalytic reduction of Cr( vi ) to Cr( iii ) over g-C 3 N 4 catalysts with Ag nanoclusters in conjunction with Cr( iii ) quantification based on operando low-field NMR relaxometry[END_REF] 475 Ag@PEDOT-DAAQ-TFP Flow exhibited the best photocatalytic reduction efficiency of 99% in

This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d The photocatalytic efficiency of Ag@PEDOT-DAAQ-TFP Flow for Cr(VI) reduction was also compared with that of other reported catalysts in literature. For photocatalytic experiments, amount of catalyst, power of light, reaction time and pollutants concentration are some of the important factors for defining the efficiency of the catalytic material. Usually during efficiency calculations, these parameters are not considered. To evaluate the accurate efficiency of a photocatalyst, we have coined a comparison factor (CF) which includes the above-mentioned parameters. CF is defined as the efficiency of the material which is capable of handling maximum pollutant concentration with minimum amount of catalyst, light power, and time.

The CF was formulated as: 𝐶𝑜𝑚𝑝𝑎𝑟𝑖𝑠𝑜𝑛 𝐹𝑎𝑐𝑡𝑜𝑟 (𝐶𝐹) = 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 × 𝑃𝑜𝑙𝑙𝑢𝑡𝑎𝑛𝑡 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑃ℎ𝑜𝑡𝑜𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 𝑎𝑚𝑜𝑢𝑛𝑡 × 𝑡𝑖𝑚𝑒 × 𝑝𝑜𝑤𝑒𝑟 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 Accordingly, a comparative table was consolidated below for determining the capability of our synthesized photocatalyst (Table 2).

This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d

511

It is clearly evident from the Table 2 that the Ag@PEDOT-DAAQ-TFP nanocomposites are outstanding photocatalysts for the reduction of Cr(VI) to Cr(III). The CF value (3652) of the best catalyst (Ag@PEDOT-DAAQ-TFP Flow ) is more than ten folds higher than Sb 3+ -doped MOF-In 2 S 3 @FcDc-TAPT (330).

Conclusions

A single step, continuous flow method was developed for the synthesis of a multi-component nanocomposite containing Ag nanoparticles, PEDOT and DAAQ-TFP COF. The nanocomposite could be synthesised at a relatively lower temperature and in less time than the batch process. Moreover, the nanocomposite formation has resulted in improved band structure alignment allowing absobance of a wide range of light in the UV, visible and NIR regions.

Further, the generation, separation and transfer of photogenerated charge carriers was also improved by the heterojunction formation. The continuous flow synthesis has resulted in Ag@PEDOT-DAAQ-TFP Flow having enhanced surface area, porosity, charge separation and transfer in comparison with Ag@PEDOT-DAAQ-TFP Batch . This may be due to the excellent mixing and better heat/mass transfer in the microfluidic flow conditions than in the batch. The flow synthesized nanocomposite, Ag@PEDOT-DAAQ-TFP Flow , was proved to be the best visible light active photocatalyst for the photoreforming of plastics as well as pollutant (Cr(VI))

reduction. Waste plastics such as PET and PLA could be photoreformed to green hydrogen and other value-added products, without releasing any CO 2 . The present research reveals a powerful, up scalable method for the synthesis of multi-component nanocomposites having high photocatalytic efficiencies.

Figure 1 :

 1 Figure 1: (a) Fabrication steps for PDMS microreactor using scaffold technique, and (b) the fabricated PDMS microreactor having three inlets and a single outlet.

Figure 2 :

 2 Figure 2: Schematic representation of the synthesis of Ag@PEDOT-DAAQ-TFP nanocomposite using: (a) batch process, and (b) flow process

Figure 3 :

 3 Figure 3: Physico-chemical characteristics of the prepared materials: (a) FTIR spectra, (b) pXRD patterns, (c) TGA thermal curves, and high resolution XPS spectra corresponding to: (d) N1s and (e) S2p.

Figure 4m .

 4m Fibrous morphology of the COF-PEDOT hybrid was evident with moreThis preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d contrasted dark spherical Ag nanoparticles of size 10-20 nm. The interplanar spacing (d) = 0.23 nm from HRTEM (Figure4n) analysis, matched well with the (111) planes of Ag.[START_REF] Kumar | Photocatalytic Water Disinfection of CVD Grown WS 2 Monolayer Decorated with Ag Nanoparticles[END_REF] The selected area diffraction pattern on Ag nanoparticles showed that the nanoparticles are polycrystalline in nature (Figure inset of 4n). The concentric rings resemble well with the dspacing values which correspond to the individual (hkl) planes.[START_REF] Niu | Enhanced photocatalytic reduction of Cr( vi ) to Cr( iii ) over g-C 3 N 4 catalysts with Ag nanoclusters in conjunction with Cr( iii ) quantification based on operando low-field NMR relaxometry[END_REF]

Figure 4 :

 4 Figure 4: FESEM images of: (a) Ag@PEDOT (b) DAAQ-TFP (c) Ag@PEDOT-DAAQ-TFP Batch , and (d) Ag@PEDOT-DAAQ-TFP Flow . Imaging and analysis of Ag@PEDOT-DAAQ-TFP Flow : (e) FESEM back scattered image, (f-l) FESEM-EDX elemental mapping, (m) TEM image, and (n) HRTEM (inset of n) SAED.

Figure 5 :

 5 Figure 5: (a) BET adsorption isotherms of the prepared materials, and (b) NLDFT pore size distribution and pore volume of DAAQ-TFP, Ag@PEDOT-DAAQ-TFP Batch and Ag@PEDOT-DAAQ-TFP Flow

Figure 6 :

 6 Figure 6: (a) UV-visible absorbance spectra, (b) Tauc plot for band gap determination (c) Mott-schottky plots, and (d) electronic band structure alignment.

Figure 7 :

 7 Figure 7: (a) EIS spectra, (b) Photo-response, (c) Photoluminescence (PL) spectra, and (d) TRPL spectra for Ag@PEDOT, DAAQ-TFP, Ag@PEDOT-DAAQ-TFP Batch and Ag@PEDOT-DAAQ-TFP Flow .

Figure 8 :

 8 Figure 8: (a) Photocatalytic H 2 evolution using synthesized photocatalysts and (b) 1 H NMR in D 2 O for the analysis of value-added products. [photocatalyst = 5 mg, substrate = 20 mg, NaOH = 5 M, irradiation time = 24 h]

Figure 9 :

 9 Figure 9: Plots showing relative concentration of Cr(VI) with time: (a) Effect of hole scavenger, and (b) in presence of the synthesized photocatalysts. (c) Kinetic plots showing the fitting of photocatalytic reduction of Cr(VI) in Pseudo 1 st order kinetics model and (d) Recyclability study.

Table 1 :

 1 Summary of the PR activities various photocatalysts that are reported in the literature Na 2 S/0.25 M Na 2 SO 3 as hole scavengers @ mmol m -2 h -1 , # mg cm-2

				Photoreforming of Plastic
		Phtotcatalyst	Substrate	Photocatalyst	Power of light	Reaction	H 2 Yield	Reference
			Concentration	Concentration	source (W)	Time (h)	(mmol/g cat /h)
			(mg/mL)	(mg/mL)
		MoS 2 /Cd 0.5 Zn 0.5 S	PET, 25	0.166	300	24	15.9	[45]
		CN x |Ni 2 P]	PET, 25	1.6	300	20	0.026	[46]
			PLA, 25				0.047
		CN x |Ni 2 P panels	PET, 25	1.92 #	300	20	0.052 @	[47]
		CdS/MoS 2 -800-15*	PLA, 25	5	300	24	379.5	[48]
			PET, 25				60.77
		MoS 2 -tipped CdS nanorod	PLA, 25 PET, 25	1.66	300	25	6.680 3.90	[26]
			PE, 25				1.13
		CdS/CdOx QDs	PLA, 50	5	300	24	64.3	[44]
			PET, 25				12.4
			PUR, 25				3.2
		d-NiPS 3 /CdS	PLA, 10	0.1	300	11	40	[49]
			PET, 50				31
		15Ni 0⋅1 Co 0⋅9 P/rGO/C	PLA, 10	0.1	300	5	0.576	[50]
		N
		CN-CNTs-NiMo	PET, 50	1	500	4	0.09	[51]
		CPDs-CN-7	PET, 25	0.4	300	48	1.034	[52]
		nPpy@Fe 2 O 3 (1:5)	PLA, 40	10	192	48	65	[22]
		Ag@PEDOT-	PET, 4	1	192	24	90.3	This Work
		DAAQ-TFP Batch
		Ag@PEDOT-	PET, 4	1	192	24	113.4	This work
		DAAQ-TFP Flow	PLA, 4				95.96
	467 *0.35 M 468 8.2. Photocatalytic Reduction of Cr (VI)
	469	Photocatalytic reduction of Cr(VI) using heterogeneous photocatalysts proceeds
	470	through the transfer of photogenerated electrons. Hence, use of hole scavengers such as ethanol
	471	can promote the reduction reaction by minimising charge recombination. On adding ethanol in
	472	the presence of Ag@PEDOT-DAAQ-TFP Flow as a visible light photocatalyst, Cr(VI) reduction
	473	efficiency was enhanced by 30% (68% to 99%) as shown in Figure 9a. Also, reduction reaction
	474

Table 2 :

 2 Comparative table for photocatalytic Cr(VI) reduction using reported photocatalyst

			Photocatalytic Cr (VI) Reduction
	Photocatalyst	Cr (VI)	Photocatalyst	Power	Reduction	Time	Comparison	Reference
	concentration	Concentration	of light	Efficiency	(min)	Factor (CF)
		(mg/L)	(mg/mL)	(W)	(%)		x 10 4
	TPB-BT	10	1	300	99	75	4.4	[55]
	TAPT-BT				99	105	3.1
	TPB-TP				80		2.53
	COF-1	5	0.4	300	49	120	1.7	[43]
	COF-PD				66		2.2
	COF-PZ				71		2.46
	COF-PMD				99		3.43
	COF-PDZ				99		3.43
	La 3+ -doped MOF-In 2 S 3 @FcDc-TAPT	30	0.3	300	88	10	293	[56]
	Sb 3+ -doped MOF-				99		330
	In 2 S 3 @FcDc-TAPT
	PYE-COF	10	1	300	100	60	5.5	[57]
	PYS-COF				68	120	1.8
	DTB-BT-COF	10	0.5	300	99.6	15	44.2	[58]
	TPBPD-COF	0.01	1	300	80	60	0.004	[59]
	20%COF-CNNS				99		0.006
	An-TATB	9	0.33	300	97.8	120	7.41	[60]
	An-TPA				78		5.91
	JOU-6	9	0.33	300	87.8,	100	7.98	[61]
	JOU-7				95.2		8.65
	COF-H	2.5	0.3	300	98	60	4.53	[62]
	COF-Br				95		4.40
	COF-Ome				100		4.63
	TACTF	50	1	300	75	120	10.4	[63]
	TACTF@CNT				100		13.88
	TiO2@COF-4	20	6	300	99.19	30	3.67	[64]
	Py-SO1	10	0.1	300	93	90	34.4	[65]
	TzDa/Ag/AgBr	10	0.4	300	100	90	9.2592	[66]
	BiOBr/TzDa	10	0.2	300	100	90	18.5	[67]
	Ag@PEDOT-	166	0.3	20	92	75	3404	This Work
	DAAQ-TFP Batch
	Ag@PEDOT-	166	0.3	20	99	75	3652	This Work
	DAAQ-TFP Flow

This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d

This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289

Acknowledgements

The authors specially acknowledge the financial support from Scheme for Promotion of Academic and Research Collaboration (SPARC), Ministry of Education (MoE, project No. P-32); and HTRA fellowship from Ministry of Education. Advanced Material Research Centre (AMRC), IIT Mandi is thanked for providing research facilities.

wide range, enhanced charge separation and transfer. [START_REF] Yuan | Highly efficient sunlight-driven reduction of Cr(VI) by TiO2@NH2-MIL-88B(Fe) heterostructures under neutral conditions[END_REF] The photoreduction data of Cr(VI) in presence of the catalysts fitted well in the pseudofirst order kinetic model (Figure 9c). [START_REF] Gogoi | Revealing the unexplored effect of residual iron oxide on the photoreforming activities of polypyrrole nanostructures on plastic waste and photocatalytic pollutant degradation[END_REF] The rate constants followed the order: Ag@PEDOT (0.0089 min -1 ) < DAAQ-TFP (0.0222 min -1 ) < Ag@PEDOT-DAAQ-TFP Batch (0.0315 min -1 ) < Ag@PEDOT-DAAQ-TFP Flow (0.0528 min -1 ). Since Ag@PEDOT-DAAQ-TFP Flow exhibited the maximum Cr (VI) reduction efficiency, its dispersibility was compared with DAAQ-TFP using the Tyndall effect. It was possible to quickly disperse Ag@PEDOT-DAAQ-TFP Flow in aqueous medium even with sonication for 5 min, whereas complete dispersion of DAAQ-TFP could not be achieved under the same conditions. Further aqueous suspension of Ag@PEDOT-DAAQ-TFP Flow exhibited stronger Tyndall effect, than DAAQ-TFP suspension (Figure S7).

Dispersibility of a heterogeneous catalyst is an important parameter affecting its catalytic activity. [START_REF] Gogoi | Visible light enhanced photosynthesis of C-C bonds using PdO/Pd@PEDOT nanocomposite[END_REF] Further, chemical and physical stability of Ag@PEDOT-DAAQ-TFP Flow was tested over many cycles (Figure 9d). There was very little loss of activity up to 5 cycles.

Further, the used photocatalysts after 5 cycles were analyzed using pXRD, FESEM, and XPS.

Crystal structure and morphology of the photocatalyst was intact as per the pXRD and FESEM images (Figure S8a & S8b). Small amounts of Cr were found to be irreversibly adsorbed on the photocatalyst, as per the XPS data. The Cr 2p deconvoluted XPS spectra revealed that the Cr(VI) which was adsorbed on the surface of the photocatalysts was successfully reduced to Cr(III) (Figure S8c). The peaks at lower binding energies of 576.7 eV (Cr 2p 3/2 ) and 586.8 eV (Cr 2p 1/2 ) correspond to +3 oxidation state of Cr. However, the presence of Cr(VI) was also evident from the higher binding energy peaks at 578.1 eV (Cr 2p 3/ 2) and 589.03 eV (Cr 2p 1/2 ).

Nevertheless, the atomic % of Cr(III) was higher as compared to Cr(VI). [START_REF] Kaur | Photocatalytic Reduction and Recognition of Cr(VI): New Zn(II)-Based Metal-Organic Framework as Catalytic Surface[END_REF] This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4442289 P r e p r i n t n o t p e e r r e v i e w e d

Declaration of Competing Interest

The authors declare that there is no conflict of interest to influence the work reported in this paper.

CRediT authorship contribution statement