Metal Oxide Heterojunction for Photocatalytic Activities

Published: July 20, 2022

doi:

Buzuayehu Abebe, H. C. Ananda Murthy, Abdulraheem S. A. Almalki, A. Alhadhrami, Abul Kalam Azad

¹Department of Applied Chemistry,Adama Science and Technology University, ²Department of Chemistry, Faculty of Science,Taif University, ³Pharmaceutical Technology Unit, Faculty of Pharmacy,AIMST University

Summary

The development of a heterojunction boosts the photocatalytic activities of solution combustion synthesis, which is a time-/energy-efficient process. Advanced analytical characterization techniques were used in this protocol to evaluate the materials' characteristics, and nanocomposites demonstrated improved acid orange-8 dye degradation.

Abstract

There is a significant global demand for improvements in synthesis techniques and their optimal characteristics, especially for industrial-scale applications. Sol-gel-based solution combustion synthesis (SG-SCS) is a simple method to produce ordered porous materials. In this regard, Pearson's hard and soft acids and bases theory assists in selecting host-dopant reactivity to form a proper heterojunction.

The formation of a heterojunction also changes the essential properties of the materials, improving photocatalysis via charge transfer or synergistic activities. A calcination temperature of 500 °C is ideal for this process based on the results of the stability assessment via a differential thermogravimetry ratio analysis (DTG).

The nanoscale dimensions of the nanoparticles (NPs) and nanocomposites (NCs) generated were validated using X-ray diffraction and high-resolution transmission electron microscopy (HRTEM). Furthermore, the scanning electron microscopy micrographs and BET analyses confirmed the porosity nature of the materials. HRTEM, X-ray photoelectron spectroscopy, and energy-dispersive X-ray investigations established the materials composition. The study found that NCs degraded the acid orange 8 (AO8) color more efficiently than bare ZnO.

Introduction

Environmental protection has become a major concern with the rapid rise in companies worldwide. Consequently, nanotechnology-based nanomaterials (NMs) and their synthesis have attracted researchers' attention over bulk materials in the modern scientific world¹. Several physicochemical approaches have been adapted to treat organic and inorganic contaminants²^,³. In this regard, due to its simplicity and capability of dissolving toxins without creating secondary contamination, heterogeneous photocatalysis is regarded as an adaptive remediation technique⁴. Studies have designed a heterojunction or doping between suitable bandgap semiconductors, which helps to reduce the constituent's electron-hole recombination, surface area, and volume. This condition subsequently increased the photocatalytic degradation of dyes⁵^,⁶^,⁷. Recent works have also reported a synergistic and charger transfer improvement role through heterojunctions/hybrids⁸^,⁹, and semiconductor metal oxides demonstrate unique physical and chemical properties for multifunctional applications¹⁰. As a result, TiO₂ and zinc oxide NPs (ZnO NPs) have received significant attention¹¹^,¹² among researchers.

Compared to single materials, the formation of a heterojunction has become one of the unique preferences for increasing the surface area and volume ratio of materials and improving a material's photocatalytic and antibacterial performance. Furthermore, the synergistic impact of binary heterojunctions improves the separation of photogenerated electron/hole pairs compared to binary heterojunctions¹³^,¹⁴. Studies have shown that a heterojunction between Mn₂O₃ and ZnO NPs¹⁵ improves the stability and substrate adsorption capacity and reduces charge transfer resistance in synthesized NPs. Moreover, several studies have used host-dopant reactivity based on Pearson's hard and soft acids and bases (HSAB) theory to test heterojunction or dopant formation. It was found that hard Lewis acids (such as Mn(III)) cannot diffuse into the borderline of the Zn (II) host lattice in the presence of a hard base solvent like water¹⁶^,¹⁷. They are adsorbed onto the host surface and oxidized to form a hybrid upon calcination.

Due to its potential, the present global focus for industrially scalable applications of material synthesis is on improving the approach and its critical outlooks¹³. Solution combustion synthesis (SCS) is a simple, time-/energy-efficient method to create regularly ordered porous materials¹⁸, which play a significant role in the ion-/mass-transport phenomenon¹⁹. SCS comprises a decent dopant-host distribution or heterojunction based on Pearson's hard and soft acids and bases (HSAB) theory. The doping/heterojunction can tune the materials' optical, magnetic, and electrical properties, subsequently boosting the application of materials through effective charge transfer and/or synergistic roles²⁰. The architecture-directing agent (ADA)-assisted SCS can also produce ordered colloidal nanocrystal frameworks (CNFs) used for mass-/ion-transport in energy-converting devices²¹^,²².

This study produced a poly-vinyl alcohol (PVA) surfactant and complexing agent to synthesize ZnO NPs and ZnO-based binary nanocomposite (NCs) heterojunction through an environmentally friendly SG-SCS approach. The heterojunction between the oxides, which plays a vital role in charge transfer was estimated based on the HSAB theory. Characterization techniques were utilized to understand the materials' structural, optical, and morphological properties. The material's degradation efficiency was tested on both stable and toxic AO8 dyes.

Protocol

1. Nanomaterial synthesis ZnO-Mn2O3 nanocomposite synthesis Synthesize nanocomposites using poly-vinyl alcohol as surfactant and a complexing agent-assisted SG-SCS approach. For a graphical illustration of the SG-SCS approach, see Supplementary Figure S1. Dissolve 1.5 g of PVA polymer in 100 mL of distilled water with continuous stirring on a magnetic stirrer for about 15 min at 115 °C23. Pou…

Representative Results

Figure 1A depicts the thermal stabilities of binary NCs before a DTG instrument analyzes calcination in the N2 atmosphere. A sequence of vaporization of adsorbed H2O molecules, intramolecular decay, metal hydroxides or/and PVA side-chain decomposition, intermolecular/PVA main chain decomposition, and finally, the crystalline part took place to give carbon, hydrocarbons, and ash29,30. …

Discussion

The present protocol describes the synthesis of nanocrystals using a bottom-up strategy with precise shape, size, and structure. The study observed that the nucleation and growth of nanocrystals were significant before forming the nanocrystals. Here, the ZnO and manganese oxides were synthesized based on LaMer's group theory²⁵, which postulates the process of nanocrystal formation after reducing precursors into atoms and nuclei, leading to seed formation to produce nanocrystals. In this regard…

Disclosures

The authors have nothing to disclose.

Acknowledgements

We would like to acknowledge Adama Science and Technology University for their support in this work. Funding was provided from Taif University Researchers Supporting Project number (TURSP-2020/44), Taif University, Taif, Saudi Arabia.

Materials

Acid orange 8	Sigma-Aldrich		65%,
Chlorine	Sigma-Aldrich	7782-50-5
Dithienogermole	Sigma-Aldrich	773881-43-9
HCl	Sigma-Aldrich	7647-01-0
Manganese nitrate (10%) salt	Sigma-Aldrich	15710-66-4	10%
Manganese sulfate monohydrate	Sigma-Aldrich		Density: 2.95 g/cm³; solubility in water: 70 g/100 mL (70 °C); 99.95%, MnSO4.H2O
Poly (vinyl alcohol)	Sigma-Aldrich	9002-89-5	Density: 1.19–1.31 g/cm³ @20 °C, soluble in water only @ > 80 °C
Zinc nitrate hexahydrate (90%)	Sigma-Aldrich	10196-18-6	98%; Density: 2.065 g/cm³ @20 °C; solubility in water: 184.3 g/100 mL @20 °C
Instruments used
Materials name	Model	Analysis
BET (N₂ adsorption-desorption isotherms)	Quanta chrome instrument.	Textural properties
DT/DTA	Shimadzu DTG-60H	Measure thermal stability
FTIR	Perkin Elmer FT-IR, Spectrum 65	Chemical bonding information
HRTEM	JEOL TEM 2100 HRTEM	Morphological, size, and composition analysis
SEM-EDX	SEM-EDX-EVO 18 with low vacuum facility and ALTO 1000 cryo attachment	Morphological analysis
XPS	AXIS ULTRA from AXIS 165
XRD	Shimadzu, XRD-7000	Crystallinity, structure, and approximate average crystallite size
Common software used
Name	Company	Use
Mendeley	Mendeley-Desktop-1.19.8-win32	For citing references
Origin	OriginPro 8	XRD, BET, UV-vis-DRS data analysis

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