6.2.1 SYNTHESIS OF g-C3N4
The polymeric g-C3N4 sheets are synthesized by thermal polycondensation of urea followed by the decomposition.41 In a typical synthesis process, 2 g of urea is taken in a mortar pestle and ground it for about 10 min. Grounded urea is then put into an alumina crucible with a cover and placed in a muffle furnace for heat treatment. The furnace temperature is raised upto 500 °C at a heating ramp of 5 °C min1 and maintained at this temperature in air for 2 h. After cooling to room temperature in normal condition, the product is taken out and again ground into fine powers in the agate mortar.
6.2.2 SYNTHESIS OF ZnO NANORODS
For synthesis of ZnO NRs, 10 mmol of zinc nitrate hexahydrate (2.97 g) is dissolved in 50 mL of Milli-Q water with constant stirring. Equimolar amount of hexamine (0.7 g) is added into the reaction mixture and the solution is heated at a temperature of 90 C for 4 h with constant stirring. After cooling down to the room temperature, the resultant precipitate is centrifuged and purified by washing with water and ethanol several times. As obtained product is dried in a hot air oven at 70 C for 6 h and then calcined it at 450 C for 1 h to get final product. Scheme 6.2.1 shows the pictorial presentation of ZnO NR synthetic protocol.
Scheme 6.2.1 Pictorial representation of step-by-step synthetic protocol for ZnO Nanorods (NRs).
6.2.3 SYNTHESIS OF ZnO NANOPARTICLES
ZnO nanoparticles (NPs) are synthesized by adopting a previously reported procedure.42 In this synthetic procedure, 40 mmol of ZnCl2 (1.1 g) is dissolved in Milli-Q water (40.0 mL) and heated upto a temperature of 90 °C with constant stirring. An aqueous solution of NaOH (5 M, 3.2 mL) is added slowly into the reaction mixture with constant stirring at the same temperature for another 30 min. The reaction mixture is allowed to cool down to room temperature normally and the precipitate is separated out by discarding the supernatant liquid. For purification, this TH-1504_10612238
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precipitate is washed with distilled water several times and dispersed in iso-propanol in an ultrasonic bath for 10 min at room temperature. The solution is centrifuged and dried in a hot air oven at 70 C for 6 h. Finally, ZnO nanoparticles are obtained via calcination at 450 °C for 1 h in a muffle furnace. Detail synthetic protocol is presented schematically in scheme 6.2.2.
Scheme 6.2.2 Schematic representation of step-by-step synthetic protocol for ZnO Nanoparticles (NPs).
6.2.4 PREPARATION OF (g-C3N4ZnO NR) AND (g-C3N4ZnO NP) COMPOSITES The g-C3N4 composites of ZnO NR and ZnO NP are prepared by ultra-sonication treatment of g-C3N4 sheets and ZnO heterostructures (i.e., NR and NPs). In detail, an appropriate amount of as synthesized g-C3N4 is dispersed in ethanol under ultrasonic treatment for 30 min. As synthesized ZnO NRs or ZnO NPs are added into solution and continued the ultrasonic treatment for another 1 h. Finally, the solutions is placed inside a hot air oven for evaporation of the solvent to obtain the composite product and named as (g-C3N4ZnO NR) or (g-C3N4ZnO NP). A series of (g- C3N4ZnO NR) composites are prepared by following the same methodology with different weight ratios of g-C3N4 to ZnO NR.
6.2.5 FABRICATION OF PHOTOANODES AND DEVICES
The photoanodes with (g-C3N4ZnO NR) composites are fabricated by preparing a homogeneous paste of the composite powders. For preparation of the paste appropriate amounts of terpineol and the triblock copolymer, “PEG-PPG-PEG” are added to the powders as binder materials and mixed well in an agate mortar. This homogenous paste is applied on a pre-cleaned conductive glass substrate i.e. FTO via doctor blade technique and dried in a hot air oven at 90 °C for 3 h. After cooling down to room temperature normally, all the films are calcined at 450 C for 30 min to remove the binder. The thickness of the photoanodic films are measured by surface profilometer and they are found to be in the range of 10-12 µm. The photoanodes are sensitized with the CdS QDs via successive ionic layer adsorption and reaction (SILAR) technique. In
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sensitization process, the electrodes are first dipped into an ethanolic solution of Cd(NO3)2.6H2O (0.5 M) for 5 min, rinsed with ethanol, and dried using a hot air drier. Again they are dipped into a Na2S (0.5 M) solution in methanol for 5 min, rinsed with methanol and dried under a hot air blower. The amount of CdS grown onto the Photoanodes is controlled by the number of repeated SILAR cycles. A pictorial presentation of step by step composite preparation and photoanode fabrication processes along with expected energy transfer processes in the device is showing in the scheme 6.2.3.
Scheme 6.2.3 Step by step fabrication process of g-C3N4ZnO NR composite photoanode, CdS QD sensitization and their favorable band alignments for efficient charge transport
Similarly, (g-C3N4ZnO NP) composite based photoanodes are fabricated by following the same procedure used for (g-C3N4ZnO NR) composites. The Pt counter electrodes are prepared by spin coating a solution of chloroplatinic acid (5 mM in isopropanol) on a pre-cleaned FTO substrate followed by the calcination at 450 °C for (heating ramp of 5 °C /min) 30 min in a muffle furnace and cooled down to room temperature naturally. The photovoltaic devices are fabricated by sandwiching the photoanodes and counter electrodes. Electrolyte solution of sulfide/polysulfide (S2−/Sn2−) is added after sealing the device by using low-temperature thermoplastic sealant, (thickness ∼50 μm). Sulfide/polysulfide (S2−/Sn2−) electrolyte is prepared by dissolving 1M Sulfur powder, 1M Na2S and 0.2 KCl in Milli-Q water. The active area for all fabricated devices are fixed and it is found to be 0.5 cm2. Before the photovoltaic measurements the fabricated devices are kept under dark condition for 24 h.
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