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

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 MnOx, TiO2, and ZnO nanoparticles using spark ablation and calcination processes. The resulting MnOx 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.


Supplementary tables
Figure S3 Raman spectra of spark-ablated a) as-prepared and b) calcined at 350 °C, and c) sol-gel prepared manganese oxide samples using distinct power conditions of 0.1%, 1%, 5%, and 10% of the full laser power (10% caused burning of samples, thus, data are not provided).A laser power level of 1% (equivalent to 15 mW) that was employed in conjunction with a 100x magnification and an accumulation time of 60 seconds offered a minimal degree of sample degradation.

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Figure S2 Schematic representation of photocatalytic degradation setup.a) configuration of toluene introduction into the reaction system and b) phase of toluene circulation (lamp off) and circulation and reaction (lamp on).The irradiation was facilitated by using either halogen lamp emitting UV-Vis-NIR simulating solar irradiation for manganese oxide materials or black light lamp emitting an UVA spectrum for titanium dioxide and zinc oxide materials.During the use of UVA irradiation, proper precautions were implemented to ensure the safety of occupants.These measures included adequately covering the experimental setup and wearing essential protective gear, such as lab coats, gloves, and UV-absorbing goggles or face shields.These precautions were implemented to minimize the risk of skin and eye exposure.

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Figure S4 Comparison of Raman spectra at Mn-O vibration modes from 550 cm -1 to 700 cm -1 for spark-ablated a) as-prepared (MnO x Sp) and b) calcined at 350 °C (MnO x Sp 350), and c) wetchemistry prepared (MnO 2 nanoflakes) manganese oxide samples.

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Figure S6 Tauc plot of band gap for a) MnO x , c) TiO 2 , and e) ZnO samples with indirect transition energies.Band gaps E g were obtained from a), e) the extrapolation to (αhν) 2 = 0, and from c) the intersection of extrapolation to (αhν) 2 = 0 and an abscissa as the slope below the fundamental absorption, which is considered as the baseline in the sub-bandgap region of the Tauc plot.The 1 st derivative of the Tauc plot b) and d) as a more sensitive method for the determination of band gaps.

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Figure S7 SEM images of MnO x Sp samples before and after calcination.

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Figure S8 SEM images of TiO 2 Sp samples before and after calcination.

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Figure S9 XPS spectra of Mn 3s core level splitting for MnO x Sp5, MnO x Sp10, and sol-gel MnO 2 .

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Figure S11 a) Relative concentration of toluene over the degradation period under UV-Vis-NIR irradiation for spark-ablated manganese oxide (MnO x ) nanoparticles and MnO 2 nanoflakes and under UV irradiation for spark-ablated c) titanium dioxide (TiO 2 ) and e) zinc oxide (ZnO) nanoparticles.

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Figure S12 Photocatalyst stability over four consecutive toluene degradation cycles represented as relative concetration of toluene over irradiation time for a), spark-ablated and d) sol-gel manganese oxide (MnO x and MnO 2 nanoflakes), b) spark-ablated and e) commercial titanium dioxide (TiO 2 Sp5 and TiO 2 P25), and c) spark-ablated and f) commercial zinc oxide (ZnO Sp10 and ZnO NanoArc) nanoparticles.

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Figure S13 Photographs of filters with TiO 2 nanoparticles, obtained using the spark ablation method (left), and dip-coating of commercial TiO 2 nanoparticles taken after a photocatalytic stability assessment (four toluene degradation cycles).(1) shows the irradiated area with noticeable yellowing, which originated from fouling, i.e., the adsorption of intermediates.(2) shows the non-irradiated area where the nanoparticles maintained their original color, i.e., no adsorption of intermediates.(3) shows a noncoated and non-irradiated part of filter (blank).It is apparent that the fouling effect predominated on the surface of commercial TiO 2 .

Table S1
Estimation of MnO x Sp5 crystalline phase using Miller indices and calculated d-spacing

Table S2
Estimation of TiO 2 Sp5 crystalline phase using Miller indices and calculated d-

Table S3
Size of nanoparticles obtained using XRD and TEM method and anatase content estimation

Table S4
Estimation of AOS and surface defects from XPS analysis for manganese oxide samples

Surface defects or OH groups 3 MnO x 10 Sp
Table S6 Nanoparticle mass loading on filter substrates, degradation rate constant k and massnormalized degradation rate constant k norm * The mass loading was determined by the mass of a filter before and after the deposition process per unit area of the filter.

Table S7
Toluene removal efficiencies after 60 min of irradiation