Green synthesis of multilayer graphene/zno nanocomposite for photocatalytic application

Green nanotechnology is an innovative research field with emphasis on the development

of methods that minimize the use of health hazardous substances for environmental

remediation. This study reports on the biosynthesis of nanostructured multilayer graphene

(MLG) and zinc oxide (ZnO) from natural extracts for environmental applications. ZnO

nanoparticles are fabricated for the first-time using Ageratum Houstonianum leaf extract

as an effective chelating agent. Besides this, a green chemistry route involving the

utilization of waste biomass was used for fabrication of multilayer graphene. This route

was chosen because it offers good control of size, morphology and does not involve use of

toxic reducing agents or surfactants thus contributing towards green nanotechnology. MLG

was synthesized from corn husk via alkali-acid treatment. This entails extraction of

cellulose followed by carbonization of the nanomaterial and activation of the carbon

material. The separately synthesized nanostructures were used to synthesize MLG/ZnO

nanocomposites of different ratios of MLG/ZnO (1:1, 1:2, 1:3) through ex-situ casting of the

two materials (MLG/ZnO_1, MLG/ZnO_2, MLG/ZnO_3). The X-ray diffraction (XRD) profiles

and Raman spectra exhibited predominant features of MLG and confirmed a hexagonal

wurtzite phase of ZnO in the composite verifying the formation of MLG/ZnO

nanocomposite. The UV-Vis absorbance spectra analysis revealed that incorporation of

MLG to ZnO narrowed the band gap of ZnO nanoparticles, and consequently improved the

light absorption of the semiconductor in the visible range. From Scanning electron

microscopy (SEM) and High-Resolution TEM (HRTEM) analysis, short hexagonal nanorods

were observed for ZnO while sheet-like structures with ripples and crinkles were observed

for MLG. Energy dispersive spectroscopy (EDS) confirmed the purity of the samples and

successful incorporation of MLG and ZnO with presence of only C, O and Zn in the

composites. Brunauer-Emmett-Teller (BET) analysis revealed less surface area of 0.42 m2

/g

for bare ZnO and increased surface area of 148.74 m2

/g in the composite (MLG/ZnO_3).

Brilliant black (BB), congo red (CR) and rhodamine B (RhB) were chosen as model pollutants

in this study, because they are among the many water pollutants from textiles and

industries which are found to be stable with complex structures hence making them

environmentally problematic. The nanocomposites were initially applied for

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photodegradation of BB under direct sunlight irradiation to determine the best performing

nanocomposite. It is worth to note that MLG/ZnO_3 nanocomposite showed the best

photocatalytic performance of 93% degradation compared to pristine ZnO, MLG/ZnO_1

and MLG/ZnO_2, which showed lower photocatalytic activity. The best performing

nanocomposite was further used to degrade CR and RhB and gave degradation efficiencies

of 86 and 100%, respectively while pure ZnO showed degradations of 71% and 85% for CR

and RhB, respectively. The obtained results showed high photocatalytic activity for the

optimized MLG/ZnO nanocomposite in RhB and CR under natural sunlight irradiation. The

nanocomposite further demonstrated 95% degradation for doxycycline (DOX) under UV

light. The photodegradation mechanism was proposed and discussed in light of scavenging

experiments using the optimum composite for all the four pollutants. It was revealed that

holes play a major role in photodegradation of BB while the main reactive species in the

photodecomposition of CR, RhB and DOX were found to be superoxide radicals. This work

provides an insight for cheap, sustainable and eco-friendly methods for the fabrication of

nanomaterials for environmental remediation and better ways of recycling waste biomass

to fabricate valuable materials to solve society problems.