Advances and Challenges in Developing Efficient Graphene Oxide-Based ZnO Photocatalysts for Dye Photo-Oxidation

Abstract

The efficient remediation of organic dyes from wastewater is increasingly valuable in water treatment technology, largely owing to the tons of hazardous chemicals currently and constantly released into rivers and seas from various industries, including the paper, pharmaceutical, textile, and dye production industries. Using solar energy as an inexhaustible source, photocatalysis ranks among the most promising wastewater treatment techniques for eliminating persistent organic pollutants and new emerging contaminants. In that context, developing efficient photocatalysts using sunlight irradiation and effectively integrating them into reactors, however, pose major challenges in the technologically relevant application of photocatalysts. As a potential solution, graphene oxide (GO)-based zinc oxide (ZnO) nanocomposites may be used together with different components (i.e., ZnO and GO-based materials) to overcome the drawbacks of ZnO photocatalysts. Indeed, mounting evidence suggests that using GO-based ZnO nanocomposites can promote light absorption, charge separation, charge transportation, and photo-oxidation of dyes. Despite such advances, viable, low-cost GO-based ZnO nanocomposite photocatalysts with sufficient efficiency, stability, and photostability remain to be developed, especially ones that can be integrated into photocatalytic reactors. This article offers a concise overview of state-of-the-art GO-based ZnO nanocomposites and the principal challenges in developing them.

Keywords: dye photodegradation; graphene oxide; photocatalysis; wastewater treatment; zinc oxide.

Figure 1

Photocatalytic mechanism of ZnO in wastewater.

Figure 2

Structure of (a) graphene, (b) graphene oxide (GO), and (c) reduced GO (rGO), along with (d) the route of synthesizing rGO by reducing GO.

Figure 3

(a) Illustration of the sol–gel method, (b) transmission electron microscopy images of ZnO–GO nanocomposites (reproduced with permission from ref. [50], with permission from American Institute of Physics, 2014), and (c) scanning electron microscopy of rGO sheets (left) and rGO-ZnO core@shell structures (right) (reproduced with permission from ref. [53], with permission from Elsevier, 2020).

Figure 4

Illustration of the (a) hydrothermal and (b) solvothermal synthesis of ZnO/GO nanocomposites; reproduced with permission from ref. [60] (with permission from Elsevier, 2016) and [56] (with permission from Springer, 2019), respectively.

Figure 5

(a) Synthesis based on electrospinning for Au- or Pd-functionalized rGO-loaded ZnO nanofibers (reproduced with permission from ref. [66], with permission from Elsevier, 2018), (b) illustration of the chemical deposition of ZnO on rGO (reproduced with permission from ref. [62], with permission from RSC Publishing, 2016), and (c) illustration of the in situ growth of ZnO on the surface of GO.

Figure 6

Graphic illustration of the photomineralization of organic dyes by using ZnO–GO or ZnO–rGO nanocomposites as photocatalysts [67].

Figure 7

(a) Scanning electron microscopy images of ZnO–rGO nanocomposites, showing the uniform distribution of ZnO nanoparticles (left) and the time-dependent, normalized concentration of methylene blue under visible light irradiation for rGO (TRG), ZnO nanoparticles (ZnO) and ZnO–rGO nanocomposites (TRGZ_ST and TRGZb) different ZnO loadings (right). Adapted with permission from ref. [90], with permission from Elsevier, 2019. (b) Schematic representation of the synthesis and scanning electron microscopy images of ZnO–GO nanocomposites (left) and proposed photocatalytic mechanism for the photodegradation of methylene blue dye under UV-light irradiation using the prepared ZnO–GO nanocomposites. Adapted with permission from ref. [60], with permission from Elsevier, 2016.

Figure 8

(a) Scanning electron microscopy images of ZnO microspheres (top), and ZnO microspheres-rGO composites (bottom). Adapted with permission from ref. [93], with permission from Elsevier, 2017. (b) Schematic illustration of the synthesis of rGO/ZnO nanocomposites (left), scanning electron microscopy (left) and transmission electron microscopy (right) images of rGO/ZnO nanocomposites. Adapted with permission from ref. [114], with permission from Elsevier, 2012.