Chapter 2: Experimental Conditions
7.1. Overview
Nanostructured semiconductor materials are widely used for photocatalytic degradation of organic pollutants because they are highly efficient at mineralization of various types of organic materials at normal ambient conditions. This is of great importance in many commercial applications, such as water purification and self-cleaning [335-339]. Nanostructured ZnO semiconductors with a high exciton binding energy of ~ 60 meV exhibit excellent optoelectronic characteristics [323], which make them a popular material for energy conversion applications such as photovoltaics, photo-electrocatalytic water oxidation, and photocatalytic degradation of organic dyes [265, 266]. ZnO-based photocatalysts have exhibited optimal photocatalytic performance in neutral media, in contrast to TiO2-based photocatalysts that usually perform better in acidic media. This makes nanostructured ZnO-based materials and their nanocomposites (NCs) widely used for photocatalytic degradation of various types of organic dyes, such as phenol [340], 2-phenylphenol [341], 4-nitrophenol [342], methyl orange [343-345], and methylene blue (MB) [346, 347]. However, ZnO-based material has exhibited low photocatalytic activity due to the high optical bandgap in the UV region [278] as well as the high recombination of photogenerated electron-hole pairs [256].
The formation of nanocomposites between nanostructured semiconductors and graphene species is widely used for photon energy conversion applications. This arises from the overall improvement in the charge transfer kinetics at the semiconductor/solution interface [348-352]. The formation of nanocomposites is accompanied by tuning the interfacial surface states between the nanostructured grain boundaries. Hence, the space-charged layer of the nanosized grains would strongly affect the photogenerated carrier concentration at the semiconductor/electrolyte interface. The tunability of nanocomposite interfacial states makes it possible to improve their performance for energy conversion and storage applications [270-273].
Graphene-based NCs are widely used in wastewater treatment from the contaminating organic dyes through the photocatalytic degradation process. Vinothkannan et al., [353] synthesized reduced graphene oxide (rGO)/Fe3O4 hybrid NCs, and the nanosized rGO was prepared by the Hammer process. Besides, chemical precipitation was used for preparing Fe3O4 nanoparticles (NPs). The synthesized NCs exhibited high adsorption and photocatalytic degradation efficiency for MB dye molecules under visible light irradiation. However, the fast recombination process of photogenerated electron-hole pairs is the main drawback of Fe3O4-based materials. This results in an overall reduction of the solar energy
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photoconversion efficiency. On the other hand, ZnO-based materials and NCs are distinguished by long minority carrier lifetime (i.e., holes), and this makes them promising materials for solar energy conversion applications. However, the wide bandgap of ZnO in the UV region restricts the usage of the pure ZnO nanostructure in visible light harvesting and conversion applications [354]. This limitation can be overcome by the formation of hybrid NCs with either lower bandgap materials or graphene nanosheets.
Jenita Rani et al., [207] have reported the preparation of rGO/zinc ferrite NCs using a two-step process involving the Hammer technique for reducing graphite micro-sized powder to rGO and solvothermal technique for heterostructure formation with nanosized zinc ferrite and rGO nanosheets. The designed rGO/zinc ferrite NCs photocatalyst with narrow band gap of 1.86 eV exhibited high photocatalytic degradation efficiency toward MB degradation in the presence of H2O2 oxidative agent due to the -
interactions and hydrogen bonding between the rGO and MB dye species. Hsieh et al., [355] prepared Cu- doped ZnO-graphene NCs using a combination of Hammer process and microwave techniques. The incorporated Cu ions in the prepared NCs resulted in the enhancement of visible light harvesting and MB photocatalytic degradation efficiency in H2O. Ahmad et al., [356] synthesized ZnO-graphene NCs using a combination of the common Hammer technique for graphene preparation and solvothermal technique for hybridization of graphene and ZnO NPs. The prepared NCs exhibited a variety of optical band gap extending from 2.9 to 3.3 eV that depends on the mixing ratio between ZnO and rGO species. The incorporation of graphene nanosheets in ZnO resulted in the improvement of visible light harvesting as well as the enhancement of MB photocatalytic degradation efficiency. Atchudan et al. [357] reported the preparation of rGO decorated with ZnO NPs using several consecutive steps for the incorporation of ZnO species involving thermal refluxing at 65 oC for 24 h that followed by thermal oxidation in the oven at 100 for 5 h. Xue et al. [358] prepared ZnO-rGO NCs using Hummer and Offeman technique to reduce the micro-sized graphite powder to nanostructure rGO and photochemical precipitation technique for ZnO NPs. The synthesized heterostructure ZnO-rGO NCs exhibited high photocatalytic degradation efficiency of MB under UV and visible light irradiation.
The photocatalytic degradation of organic pollutants using nanostructured semiconductor materials has mainly two forms, the dispersed particulate, and the thin film. Nalajala et al. [359] demonstrated that the thin film form is more efficient since 1 mg of nanostructured thin film photocatalysts has the same photocatalytic activity as 25 mg of sample powder photocatalysts. Besides, the removal of the dispersed nanoparticles from the solution after the organic material degradation is very difficult and needs a centrifuge with a very high rotation speed (> 6,000 rpm) to guarantee that all the dispersed powder is
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collected. This increases the cost of organic dye removal using a nano-semiconductor powder; hence, thin- film is more suitable for most practical applications.
The widespread of nanosized graphene in various commercial applications concerned with energy conversion and storage has driven the need to find a suitable technology that takes into consideration mass production with high cost-efficiency. There are multiple techniques for graphite-to-graphene transformation, including top-down techniques such as sonochemical-assisted graphite exfoliation [91], shear exfoliation [94], as well as dry- and wet-ball milling [49, 54, 95, 97]. However, these techniques have some restrictions, such as a slow rate of graphite-to-graphene transformation, low production yield, involvement of waste products, and high production cost [259].
In contrast to the above-mentioned techniques, the fabrication of hybrid NCs using the direct deposition by the nanoparticle deposition system (NPDS), which is a vacuum kinetic spray process [101, 163, 164]. This technique takes into consideration the production cost efficiency, which is of great importance for commercial applications related to wastewater treatment because it provides a large deposition area in a short time with a reasonable production cost. Besides, no chemical treatment was involved that make our technique eco-friendly [99, 262]. Here we report on the direct transformation of graphite into graphene nanosheets in a one-step process at room temperature with a very short preparation time, using the NPDS. The instantaneous fragmentation of the 2D layered structure of bulk graphite to graphene nanosheets is usually accompanied by a sharp increase in the localized temperature. This provides a suitable environment for strong bonding between the grains and hybridization between the nanostructured transition metal oxide (TMO) and the formed graphene species in the fabricated nanocomposites thin films. NPDS has been used by our research group to fabricate various functional nanostructured thin films from TMO hybrids with 2D material, such as graphene and MoS2 nanosheets.
Such optimized nanostructured thin films have been utilized in many electrochemical energy conversion and storage applications, such as supercapacitors [260] and electrocatalytic water splitting [261, 262], as well as for non-enzymatic H2O2 detection in an alkaline medium [258, 264].
In the present study, we provided an effective and economically viable technique for the fast fabrication of nanosized hybrid photocatalysts using a one-step kinetic spray deposition at room temperature by the NPDS technique. Herein, we directly deposited ZnO-graphene nanocomposites (NCs) hybrid photocatalysts with various graphite contents (25, 50, and 75 wt.%) on nickel foam (NF) porous substrate without any chemical treatment from ZnO and graphite microparticles. Also, we investigated the
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dependence of the graphite-to-graphene transformation in the deposited ZnO-graphene NCs/NF heterostructured thin films on the incorporated graphite ratio using various analytical techniques. We also evaluated the effect of graphite content variation on the photocatalytic activity of ZnO-graphene NCs/NF hybrid photocatalysts using the photocatalytic degradation of MB under visible light illumination.