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Method Article

Metal Oxide Heterojunction for Photocatalytic Activities

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DOI:

10.3791/64077

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July 20th, 2022

 ,  ,  ,  , 
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

In This Article

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.

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Protocol

1. Nanomaterial synthesis

  1. ZnO-Mn2O3 nanocomposite synthesis
    1. 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.
    2. 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.
    3. Pour the salt precursor solutions, zinc nitrate hexahydrate at a concentration of 90% v/v, and manganese sulfate at a concentration of 10%v/v into the above-dissolved PVA solution with continuous stirring for about 10 min and decrease the temperature to 70 °C.
      NOTE: Salt precursors were mixed simultaneously to balance the nanocomposite precursor reactivity to follow the nucleation-doping approach16,24. The temperature was reduced to 70 °C to control the nanoparticles' accelerated growth and aggregation, following the La Mar model25,26.
    4. Age the metal hydroxide's developed sol (colloidal particles) by keeping it in a closed and dark area for 2 days. Then, dehydrate the solution by heating at 110 °C (in the air) to form a gel.
      NOTE: The PVA polymer acts as an architecture, directing templates and complexing agents, which assist in the homogeneous dispersion of metal cations, initiating the combustion process and preventing aggregation/agglomeration properties.
    5. Subject the gel to combustion in air by heating the oven to an ignition temperature of ~150-250 °C (approximate temperature checked using a simple thermometer). The ignition temperature is the minimum temperature required to start combustion. During combustion, use hoods to collect all the toxic gas by-products that affect human health.
      NOTE: The combustion process was activated by forming complexes between PVA polymer and nitrate precursors, which act as fuel to facilitate the combustion process.
    6. Calcine the combusted materials for 3 h at 500 °C in a muffle furnace, optimized using the differential thermogravimetry (DTG) analytical technique. DTG decomposes the non-burnt impurities and improves the crystallinity of the materials27.
  2. Bare ZnO and Mn2O3 NPs synthesis
    1. Synthesize bare metal oxides using the sol-gel approach. Synthesize bare ZnO and Mn2O3 without PVA using all steps mentioned earlier, steps 1.1.1.-1.1.6., except for step1.1.2. Due to the absence of the metal nitrate and PVA polymer complexes, no self-propagation process occurs during the final drying step.

2. NP characterization

  1. Determine the thermogravimetry ratio, specifically the thermal thermogravimetric/differential thermal (DT/DTA analysis), in a nitrogen atmosphere at a 20.0 mL/min flow rate and a 50 °C/min ramp time to study the thermal stability and degradation behavior of the NPs and NCs.
  2. Perform the Fourier transform-infrared spectroscopy (FTIR) using KBr pellets in the range of 400-4000 cm−1 to study the surface functional group behavior of NPs and NCs.
  3. Perform X-ray diffraction (XRD) to study the crystallographic structure of PVA, NPs, and NCs.
  4. Use the Brunauer-Emmett-Teller (BET; N2 adsorption-desorption isotherms) method to calculate the specific surface area of the samples in the relative pressure (P/Po) range of 0.05-0.35. Determine the samples' pore-size distributions using the Barrett-Joyner-Halenda (BJH) method. Lastly, measure the N2 sorption of all the NPs and NCs at −196.15 °C.
  5. Use scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) and high-resolution transmission electron microscopy (HRTEM) to study the morphologies and perform compositional studies of the NPs and NCs.
  6. Perform an X-ray photoelectron spectroscopy (XPS) analysis on a system integrated with a Kratos patented magnetic immersion lens, charge neutralization system, and spherical mirror analyzer. Calibrate the peak energies based on the energy of external carbon.
    NOTE: The researcher adopted all standard procedures and protocols during the characterization process.

3. Batch degradation studies

  1. Perform the photocatalytic experiment by dissolving 20 ppm of AO8 dye in 250 mL of aqueous solution (water solvent) with 0.06 g of ZnO NPs and NCs photocatalysts.
  2. Use the degradation experiment as a conductor in a 176.6 cm2 circular glass reactor. For this experiment, use a medium-pressure mercury vapor lamp (Hg lamp) as a light source (λmax = 365 nm, 125 W)28. Before illumination, stir the reaction suspension continuously in the dark for 30 min to create the adsorption/desorption equilibrium of AO8/CR on the NPs/NCs.
  3. Irradiate the samples directly by focusing light on the reaction mixture from a distance of 20 cm. Use a magnetic stirrer at 110 rpm to continuously mix the solution. Control the temperature of the overall reactor during the experiment using water circulation.
  4. Draw 5 mL of the dye solution every 15 min to measure their concentrations at time t by UV-vis spectrophotometer. Calculate the percentage of photocatalytic degradation efficiency using the equation:
    Percent efficiency formula, %η=(Co−Ct)/Co×100, mathematical equation for experimental analysis.
    where Co and Ct are the initial and after time t irradiation concentrations, respectively, of the AO8 and CR dyes solution; and η is the photo decolorization efficiency,
  5. Use the pseudo-first-order kinetic equation to study reaction dynamics:
    Pseudo - first - order kinetics: First-order kinetics equation, log(Ct/Co)=-kt, chemical reaction rate analysis.
    where Co and Ct are the initial and equilibrium concentrations of AO8 dye (mg/L), respectively, k is the rate constant, and t is the time in minutes.

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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.

...

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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...

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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.

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Acid orange 8Sigma-Aldrich65%,
ChlorineSigma-Aldrich7782-50-5
DithienogermoleSigma-Aldrich773881-43-9
HClSigma-Aldrich7647-01-0
Manganese nitrate (10%) saltSigma-Aldrich15710-66-410%
Manganese sulfate monohydrateSigma-AldrichDensity: 2.95 g/cm³; solubility in water: 70 g/100 mL (70 °C); 99.95%,  MnSO4.H2O
Poly (vinyl alcohol)Sigma-Aldrich9002-89-5Density: 1.19–1.31 g/cm³ @20 °C, soluble in water only @ > 80 °C
Zinc nitrate hexahydrate (90%)Sigma-Aldrich10196-18-698%; Density: 2.065 g/cm³ @20 °C; solubility in water: 184.3 g/100 mL @20 °C
Instruments used
Materials nameModelAnalysis
BET (N2 adsorption-desorption isotherms)Quanta chrome instrument.Textural properties
DT/DTAShimadzu DTG-60HMeasure thermal stability
FTIRPerkin Elmer FT-IR, Spectrum 65Chemical bonding information
HRTEMJEOL TEM 2100 HRTEMMorphological, size, and composition analysis
SEM-EDXSEM-EDX-EVO 18 with low vacuum facility and ALTO 1000 cryo attachmentMorphological analysis
XPSAXIS ULTRA from AXIS 165
XRDShimadzu, XRD-7000Crystallinity, structure, and approximate average crystallite size
Common software used
NameCompanyUse
MendeleyMendeley-Desktop-1.19.8-win32For citing references
OriginOriginPro 8XRD, BET, UV-vis-DRS data analysis

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Sol-Gel SynthesisSolution CombustionPorous MaterialsCharge TransferDifferential ThermogravimetryX-Ray DiffractionHigh-Resolution TEMAcid Orange Degradation

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