Precursor- and waste-free synthesis of spark-ablated nanoparticles with enhanced photocatalytic activity and stability towards airborne organic pollutant degradation

Abstract

Photocatalyst synthesis typically involves multiple steps, expensive precursors, and solvents. In contrast, spark ablation offers a simple process of electrical discharges in a gap between two electrodes made from a desirable material. This enables a precursor- and waste-free generation of pure metal oxide nanoparticles or mixtures of various compositions. This study presents a two-step method for the production of photocatalytic filters with deposited airborne MnO_x, TiO₂, and ZnO nanoparticles using spark ablation and calcination processes. The resulting MnO_x and TiO₂ filters demonstrated almost twice the activity with outstanding performance stability, as compared to sol-gel MnO₂ and commercial TiO₂. The introduced method is not only simple, precursor- and waste-free, and leads to superior performance for the case studied, but it also has future potential due to its versatility. It can easily produce mixed and doped materials with further improved properties, making it an interesting avenue for future research.

Fig. 1. Illustration of the nanoparticle spark generation process followed by particle collection onto filter media (step 1). Step 2 represents the calcination of coated filters in an air atmosphere at 350 °C for MnO_x Sp, and at 450 °C for TiO₂ Sp and ZnO Sp. Step 3 illustrates the application of coated filters for the photocatalytic degradation of toluene.

Fig. 2. a) XRD diffraction patterns of as-prepared spark-ablated manganese oxide (MnO_x Sp5), calcined at 350 °C (MnO_x Sp5 350 °C) and sol–gel prepared MnO₂ nanoflakes. b) Raman spectra of as-prepared (MnO_x Sp5) and calcined (MnO_x Sp5 350 °C) spark-ablated samples. c) Raman spectrum of the sol–gel prepared MnO₂ nanoflake sample. d) HRTEM images of MnO_x Sp showing hausmannite, ramsdellite and pyrolusite crystalline structures; e) measured planar spacing with the determination of miller indices (hkl). f) Selected area electron diffraction (SAED) image of heterogeneous MnO_x Sp5.

Fig. 3. XRD diffraction patterns of a) titanium dioxide (TiO₂) and b) zinc oxide (ZnO) produced by spark ablation in comparison with calcined and commercial samples. c) HRTEM image of TiO₂ Sp5 displaying the anatase crystalline structure and its corresponding selected area electron diffraction (SAED) image. d) HRTEM image of ZnO Sp nanoparticles.

Fig. 4. TEM images of a) manganese oxide (MnO_x) Sp10, b) MnO_x Sp5, and c) MnO_x Sp1 spark-ablated nanoparticles using 10 l min⁻¹, 5 l min⁻¹, and 1 l min⁻¹ flow rates, respectively, for nanoparticle generation and deposition; d)–f) the corresponding primary particle size distributions.

Fig. 5. TEM images of a) titanium dioxide (TiO₂) Sp10, b) TiO₂ Sp5, and c) zinc oxide (ZnO) Sp10 spark-ablated nanoparticles using 10 l min⁻¹ (Sp10) and 5 l min⁻¹ (Sp5) flow rates, for nanoparticle generation and deposition; d)–f) the corresponding particle size distributions.

Fig. 6. SEM images of spark-ablated nanoparticles and nanoparticles prepared by the sol–gel method for a) and b) manganese oxide (MnO_x), c) and d) titanium dioxide (TiO₂), and e) and f) zinc oxide (ZnO), respectively.

Fig. 7. SEM images of manganese oxide spark-ablated (MnO_x Sp) nanoparticles with carrier gas flow rates of a) 10 l min⁻¹, b) 5 l min⁻¹, and c) 1 l min⁻¹. The red circles highlight different pore sizes originating from different flow rates.

Fig. 8. Absorption properties of spark-ablated particles for a) manganese oxide (MnO_x) prepared under different conditions and b) the estimation of indirect optical band gap using the 1st derivative of the Tauc plot for MnO_x Sp5 and Sp1 samples with the fits of three levels of transitional energy.

Fig. 9. a) Absorption properties and indirect optical band energy obtained using b) the 1st derivative of the Tauc plot with the fits of optical band transition energy levels for titanium dioxide (TiO₂) anatase and rutile phase Sp10 and Sp5 samples. c) Absorption and d) the 1st derivative of the Tauc plot for zinc oxide (ZnO) spark-ablated and commercial nanoparticles.

Fig. 10. High-resolution a) Mn 2p, b) Ti 2p, and c) Zn 2p XPS spectra of spark-ablated manganese oxide (MnO_x), titanium dioxide (TiO₂), and zinc oxide (ZnO) nanoparticles, respectively, and the comparison with the XPS spectra of d) sol–gel prepared MnO₂ and commercial e) TiO₂ P25 and f) ZnO NanoArc. Peak deconvolution indicates the oxidation states of the material.

Fig. 11. High-resolution O 1s XPS spectra of spark-ablated a) manganese oxide (MnO_x), b) titanium dioxide (TiO₂), and c) zinc oxide (ZnO) nanoparticles, respectively, and the comparison with O 1s XPS spectra of d) sol–gel prepared MnO₂ and commercial e) TiO₂ P25 and f) ZnO NanoArc. Peak deconvolution indicates the presence of lattice oxygen, non-lattice oxygen and/or adsorbed OH groups, and adsorbed water.

Fig. 12. Normalized 1st order reaction rate constant k for a) spark-ablated manganese oxide (MnO_x Sp) nanoparticles and MnO₂ nanoflakes, b) spark-ablated and commercial titanium dioxide (TiO₂ Sp and TiO₂ P25), and c) spark-ablated and commercial zinc oxide (ZnO Sp10 and ZnO NanoArc) nanoparticles. Photocatalyst stability over four consecutive toluene degradation cycles represented as toluene removal efficiency after 60 min of irradiation for d) MnO_x Sp and MnO₂ nanoflakes, e) TiO₂ Sp5 and TiO₂ P25, and f) ZnO Sp10 and ZnO NanoArc nanoparticles.