Synthesis of Ag nanoparticles prepared by a solution

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Aug 26, 2017 - Naoto Yamamotob, Tsuyoshi Mizutanib, Muneaki Yamamotob,. Satoshi Ogawab, Shinya Yagic, Hirofumi Namekid, Hisao Yoshidae,f a Osaka ...
Catalysis Today 303 (2018) 320–326

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Synthesis of Ag nanoparticles prepared by a solution plasma method and application as a cocatalyst for photocatalytic reduction of carbon dioxide with water

T



Tomoko Yoshidaa, , Naoto Yamamotob, Tsuyoshi Mizutanib, Muneaki Yamamotob, Satoshi Ogawab, Shinya Yagic, Hirofumi Namekid, Hisao Yoshidae,f a

Osaka City University, Advanced Research Institute for Natural Science, Osaka 558-8585, Japan Nagoya University, Graduate School of Engineering, Nagoya 464-8603, Japan Nagoya University, Institute of Materials and Systems for Sustainability, Nagoya 464-8603, Japan d Aichi Center for Industry and Science Technology, Kariya, 448-0013, Japan e Kyoto University, Graduate School of Human and Environmental Studies, Kyoto 606-8501, Japan f Kyoto University, Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto 615-8520, Japan b c

A R T I C L E I N F O

A B S T R A C T

Keywords: Solution plasma method Ag loaded Ga2O3 photocatalyst CO2 reduction

Silver nanoparticles (Ag NPs) were synthesized by a solution plasma method (SPM) in an aqueous solution of ammonia. Optical emission spectra of the plasma revealed that Ag NPs are fabricated with the sputtering of Ag rods as electrode by the produced energetic plasma particles such as H, OH and O radicals. In-situ optical absorption measurements of the solution during the discharge directly presented the concerted formation and aggregation processes of the Ag NPs, which controlled the size of Ag NPs. The synthesized Ag NPs were loaded on gallium oxide (Ga2O3) photocatalyst, and the photocatalytic activities of the obtained Ag loaded Ga2O3 (Ag/ Ga2O3) samples were evaluated. Although the photocatalytic reaction proceeded over all the samples to produce CO, the CO production rates decreased with the reaction time. Measurements of DR UV–vis spectra and TEM images revealed that a part of the Ag NPs migrated and aggregated on the photocatalyst surface to become larger particles during the photocatalytic reaction, which would be related to the decrease of the photocatalytic activity. It was also found that the photoirradiation treatment on the prepared Ag/Ga2O3 sample before the use for the photocatalytic reaction improves the photocatalytic performance.

1. Introduction Solution plasma method (SPM) is a new useful and simple preparation method of metal nanoparticles (NPs), because this nonequilibrium plasma can provide rapid reactions due to the reactive chemical species, i.e., radicals. Actually, various kinds of metal NPs have been synthesized by the glow discharge between metal rods with electrolytes in an aqueous solution without any dispersants [1–7]. In addition, the SPM has the advantage of controlling the size of the metal NPs by changing the amount of the electrolytes [1–3] and the space between the metal rods. The metallic NPs have been widely employed as a cocatalyst for heterogeneous photocatalyst [8] and the suitable size and clean surface of the NPs are believed to be important for the high photocatalytic properties [9,10]. Reduction of carbon dioxide (CO2) with photocatalysts, an artificial photosynthesis, has been widely studied from the viewpoints of



contribution to the energy, environmental and carbon resource issues [11–17]. The photocatalytic reduction of CO2 with water is one of the most challenging catalytic reactions, because the reduction ability of water is much lower as compared to other reduction reagents such as hydrogen (H2) [18]. Recently, silver nanoparticles (Ag NPs) have been attractive as a cocatalyst for heterogeneous photocatalysts such as TiO2 [19], BaLa4Ti4O15 [20], Ga2O3 [21–23], LDH (layered double hydroxides) [24], Zn-Ga2O3 [25], KCaSrTa5O15 [26], La2Ti2O7 [27] and CaTiO3 [28,29]. For example, the photocatalytic activity for the reduction of CO2 with water was increased by loading Ag NPs on Ga2O3 (Ag/Ga2O3) [21–23,25]. The photocatalytic activity is likely to correlate with the chemical state, shape and size of the Ag NPs which depend on the preparation method of the photocatalysts. In the present study, we tried to synthesize Ag NPs by a SPM, loaded them on a Ga2O3 photocatalyst and evaluated the photocatalytic activity for the reduction of CO2 with water. Probably, this is the first

Corresponding author at: Advanced Research Institute for Natural Science, Osaka City University, 3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan. E-mail address: [email protected] (T. Yoshida).

http://dx.doi.org/10.1016/j.cattod.2017.08.047 Received 5 June 2017; Received in revised form 2 August 2017; Accepted 22 August 2017 Available online 26 August 2017 0920-5861/ © 2017 Elsevier B.V. All rights reserved.

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Fig. 1. Schematic of the set up for the Ag NPs synthesis and in-situ optical absorption measurement during the solution plasma discharge.

as 0.02–0.24 wt% with inductively coupled plasma atomic emission spectrometry (ICP-AES, iCAP6500 duo, Thermo Scientific). The size and the oxidation state of the Ag particles in the Ag/Ga2O3 samples were investigated by transmission electron microscopy (TEM), diffuse reflectance ultraviolet visible spectroscopy (DR UV–vis) and x-ray absorption near-edge structure (XANES). TEM images of the samples were obtained by an electron microscope (JEOL JEM-2100M and Hitachi H800) with high voltage of 200 kV. DR UV–vis spectra were measured by a spectrophotometer (JASCO V-670). Ag L3-edge XANES measurements were carried out at the beam line 6N1 at Aichi Synchrotron Radiation Center using a two-crystal Ge(111) monochromator. The spectra were measured at room temperature in an atmospheric chamber filled with helium gas. The data were recorded in the fluorescent X-ray yield mode with a silicon drift detector (Vortex Electronics).

study that Ag NPs prepared by SPM was used as a co-catalyst for photocatalytic CO2 reduction. In order to improve the photocatalytic performance of the prepared Ag/Ga2O3 sample, we clarified some important aspects for these processes to optimize the fabrication of Ag NPs by the SPM as well as the stabilization of the Ag NPs on Ga2O3 by a post treatment. 2. Experimental 2.1. Synthesis of Ag NPs by SPM and preparation of Ag/Ga2O3 samples Ag NPs were synthesized by a solution plasma method (SPM) as mentioned below. Two Ag rods (diameter: φ1.0 mm, Nilaco Co.) were placed between two sides of a reactor cell and the space between them was 0.3 mm (Fig. 1). Additionally, distilled water (180 mL) and 28 mass % ammonia aqueous solution (0.1 mL) were poured into the reactor cell. The glow discharge was generated with high pulse voltage (voltage: 2.7 kV, current: 2.0 A, repetition frequency: 20 kHz, discharge times: 20 min, Kurita Co.). By using the synthesized aqueous solution containing the Ag NPs, some Ag/Ga2O3 samples were prepared as follows: Ga2O3 powder (2.0 g, Kojundo Chemical Labo. Co.) was put into the solution containing the synthesised Ag NPs and mixed thoroughly. Subsequently, the powder was separated from the solution by filtration, washed with distilled water and dried in air at 293 K overnight. Loading amounts of the Ag NPs were controlled by the ratio of the solution and the Ga2O3 powder. As a reference sample, another Ag/Ga2O3 sample containing 0.06 wt% of Ag was prepared by a conventional impregnation method from the Ga2O3 powder and aqueous solution of silver (I) nitrate (AgNO3) through mixing, drying up and calcination at 673 K for 2 h, which was prepared in the previous study [23] and here referred to as Ag/Ga2O3(imp).

2.3. Photocatalytic reaction tests The photocatalytic reaction was conducted as follows: the Ag/ Ga2O3 sample (0.2 g) was put into a fixed-bed flow reactor cell under CO2 gas with a flow rate of 3 mL/min [22]. This reactor cell was exposed under UV light irradiation by using a 300 W xenon lamp and a UV light reflecting mirror (light intensity measured in the range of 254 ± 10 nm was 25 mW/cm2) for 1 h, and an aqueous solution of sodium hydrogencarbonate (H2O 10 mL, NaHCO3 0.92 g) was introduced to this reactor cell in the dark. After 1 h, background measurement was conducted with an online TCD gas chromatograph (GC8APT, Shimadzu Co.). Successively, photocatalytic reduction of CO2 under UV light irradiation was started and production rates of CO, H2 and O2 were measured every hour up to five repetitions. 3. Results and discussion 3.1. The synthesis of Ag NPs by the solution plasma method

2.2. Characterizations

The optical emission spectrum was measured during the Ag NPs’ synthesis in the ammonia aqueous solution to obtain information about the products in the discharge. As shown in Fig. 2, several emissions were detected from the hydroxyl radical at 309 nm, the atomic hydrogen at 486 and 656 nm, and atomic oxygen at 777 and 844 nm [1]. The emissions due to excited states of atomic Ag were also observed at 328, 338, 520 and 546 nm, demonstrating that the sputtering of the Ag rods took place in the discharge [30]. It is speculated that the Ag electrodes would be continuously bombarded by the produced energetic plasma particles such as H, OH and O radicals and the Ag atoms were ejected from the solid electrodes.

During the synthesis of Ag NPs in the solution, optical emission spectra of the plasma and absorption spectra of the solutions were measured. The emission spectra of the plasma were collected with a CCD detector (PMA-12, HAMAMATSU) from 190 to 900 nm. For the measurements of the optical absorption spectra, the light from a xenon lamp was introduced through an optical fibre to the solution and the absorption bands due to the Ag NPs were recorded with the CCD detector. The amounts of Ag loading in the Ag/Ga2O3 samples were estimated 321

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Fig. 2. Optical emission spectrum from the ammonia aqueous solution during the discharge.

Fig. 4. TEM images and size distribution histograms of Ag NPs obtained after 75 s (upper) and 600 s (lower).

Fig. 3. Optical absorption spectra of the ammonia aqueous solution during the discharge.

desired size. This is an important point clarified in the present study.

As the plasma sputtering proceeded in the aqueous solution of ammonia, the colour of the solution changed from colourless to light yellow, which would be characteristic of Ag NPs. Then, we performed in-situ UV–vis absorption measurements for the solution during the Ag NPs’ synthesis. In the UV–vis spectra (Fig. 3), two absorption bands around 250 nm and 390 nm were observed, and they were respectively assignable to small (Ag)n+ clusters and the surface plasmon resonance (SPR) due to Ag metal NPs [31]. The intensities of these bands increased with the discharge time. In particular, the band at 390 nm rapidly grew with the peak shift from 390 to 395 nm and slightly broadened to the longer wavelength, suggesting the concerted formation and aggregation processes of Ag metal NPs. The formation of the larger Ag particles would be explained by Ostwald ripening, characterized by the growth of the large particles that could receive the atoms from the smaller dissolving particles and/or (Ag)n+ clusters [32]. The size and morphology of the Ag NPs were determined using TEM. Fig. 4 shows TEM images and the size distribution histograms of the Ag NPs obtained after the different discharge times. In the solution after discharge for 75 s, spherical NPs of ca. 10 nm were predominantly obtained. After discharge for 600 s, the particle size distribution was widespread to the larger side. These TEM results were consistent with those of the in-situ optical absorption measurements, indicating that Ag NPs could be formed rapidly by the present solution plasma method. In addition, it was confirmed that the size of the Ag NPs could be determined by monitoring the optical absorption measurements under the plasma discharge, although the size could be much affected by the discharge time. Here, it must be mentioned that the precise control of the plasma state was actually difficult in the present condition. Since the small differences in the state of the plasma much affected the resulting size of the Ag NPs, precise control of the Ag NPs size according to the discharge time only was also difficult in the present solution plasma method. Thus, it is suggested that in-situ monitoring during the preparation would be necessary in order to obtain the Ag NPs with the

3.2. Loading the Ag NPs on Ga2O3 The synthesized Ag NPs under a certain condition were loaded on Ga2O3 powder. The size distribution histograms of the Ag NPs before and after the loading were compared as shown in Fig. 5. These histograms are almost the same. It was thus clarified that the Ag NPs could be loaded on Ga2O3 as synthesized, i.e., no remarkable aggregation or dispersion of the Ag NPs would occur by the loading in the present method. Fig. 6a shows DR UV–vis spectrum of the 0.06 wt% Ag/Ga2O3 sample. As well as the large absorption band at less than 320 nm in wavelength due to the band gap transition of the Ga2O3, the SPR band around 450 nm due to the metallic Ag NPs was clearly observed. It is notable that the band due to the SPR for the loaded Ag NPs was at a much longer wavelength than that for the Ag NPs synthesized in the solution (see Fig. 3), even though the particle size was not affected by the loading. This would arise from the difference in the permittivity of materials, i.e., Ga2O3 and water, surrounding the Ag NPs. 3.3. Photocatalytic reduction of CO2 with water The photocatalytic reduction of CO2 with water proceeded over all the prepared Ag/Ga2O3 samples to produce CO, H2 and O2, where the CO was the product from CO2. Fig. 7 shows time courses of the production rates in the photocatalytic reduction of CO2 with water over the 0.06 wt% Ag/Ga2O3 sample and a bare Ga2O3 sample. The CO production rate over the Ag/Ga2O3 sample was much higher than that over a bare Ga2O3 sample, while the H2 production rate was slightly higher for the Ag/Ga2O3 sample. Thus, the photocatalytic activity and selectivity for CO production was significantly enhanced by the loading of the synthesized Ag NPs on Ga2O3. The production rates of H2 and O2 322

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Fig. 7. Time dependences of production rates of CO, H2 and O2 for the 0.06 wt% Ag/ Ga2O3 sample prepared by SPM (a) and a bare Ga2O3 sample (b).

Fig. 5. Size distribution histograms of Ag NPs before (upper) and after (lower) the loading on Ga2O3 powder.

impregnation method. As shown in Fig. 8Ba, in the initial reaction stage after 1 h, the CO production rate was 0.60 μmol/h for the Ag/Ga2O3 sample prepared by SPM. However, it decreased to 0.38 μmol/h after the reaction for 5 h. Over the Ag/Ga2O3(IMP) sample with the same loading amount of 0.06 wt%, the CO production rates were 0.46 and 0.37 μmol/h after the reaction for 1 h and 5 h, respectively (Fig. 8Bb). Thus, the present 0.06 wt% Ag/Ga2O3 sample prepared by SPM showed similar photocatalytic activity to the corresponding 0.06 wt% Ag/ Ga2O3(IMP) sample prepared by a conventional method, and at least initially, the former sample could promote CO production more than the latter sample. However, the problem is that they exhibited a pronounced tendency to reduce their photocatalytic activities during the photocatalytic reaction and this tendency was significant for the present sample prepared by SPM. 3.4. Characterization of the Ag NPs loaded on Ga2O3 Fig. 6. DR UV–vis spectra of the 0.06 wt% Ag/Ga2O3 sample of as-prepared (a) and after use for the photocatalytic reaction (b).

To elucidate the reason why the CO production rate decreased during the photocatalytic reaction test, we compared DR UV–vis spectra (Fig. 6) and TEM images (Fig. 9) of the present 0.06 wt% Ag/Ga2O3 sample before and after the use for the photocatalytic reaction. In Fig. 6, the SPR band around 450 nm due to metallic Ag NPs was clearly observed for both spectra before and after the reaction. After the reaction for 5 h (Fig. 6b), the longer wavelength region of the wide bands in particular became large, which continued to the near-infrared region, indicating the aggregation of the metallic Ag NPs during the photocatalytic reaction. As shown in Fig. 9a, Ag NPs less than 10 nm in diameter were predominantly observed in the as-prepared sample before the reaction. After the photocatalytic reaction, however, some of these NPs aggregated and became larger particles with a size of more or less than 20 nm, and the particle size distribution became widespread. In addition, the small hemispherical NPs observed in the as-prepared samples varied to small and large particles with various shapes after the reaction

gradually increased and that of CO decreased for 5 h. Since the Ag NPs could be considered to contribute especially for the formation of CO as reported in the previous studies [20–29], the activity of Ag NPs in the photocatalytic activity will be discussed according to the CO production rate. Fig. 8A shows the CO production rates evaluated at 1 h after the start of the photocatalytic reaction test over the Ag/Ga2O3 samples with various Ag loading amount. As shown, the CO production rates over all the Ag/Ga2O3 samples were faster than that over the bare Ga2O3 sample. Among the samples, the 0.06 wt% Ag/Ga2O3 sample showed the highest CO production rate. Fig. 8B shows time courses of the CO production rates in the photocatalytic reduction of CO2 with water over the 0.06 wt% Ag/Ga2O3 samples prepared in the present method and in the conventional 323

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for 5 h. The variation in the size and shape of the Ag NPs suggests that during the photocatalytic reaction the Ag atoms in the Ag NPs would be oxidised by the photoformed holes and dissolved as Ag+ cation, followed by re-photodeposition on the surface of the Ag NPs and the Ga2O3 photocatalyst with the photoexcited electron, repeatedly. Such aggregation of Ag NPs would suppress the CO production rate. However, we could not discuss which size of Ag nanoparticle was effective for the CO production, since the figures showed a wide size distribution of Ag NPs. In our future work, we will further optimize the condition of SPM to investigate the dependence of the photocatalytic activity on the size of Ag NPs. In order to obtain Ag NPs with a narrow size distribution, the monitoring by in-situ absorption spectra during the synthesis of the Ag NPs will be very useful.

3.5. Improvement of the stability of the Ag NPs on Ga2O3 As mentioned above, we found that the photocatalytic activities of the Ag/Ga2O3 samples decreased gradually with the reaction time. In the present method, the metallic Ag NPs synthesized by SPM were directly loaded on the Ga2O3 only by filtration. It was considered that the Ag NPs loaded on the Ga2O3 would be almost physically adsorbed on the surface and not well stabilized so that they could migrate and aggregate easily. To improve the photocatalytic activity and the stability by the enhancement of the interaction between the Ag NPs and the surface of the Ga2O3 photocatalyst, the Ag/Ga2O3 sample was subjected to post-prepared treatments (as pre-treatment before the photocatalytic reaction) such as UV light irradiation and calcination. As for the former treatment, the as-prepared Ag/Ga2O3 sample in the reaction cell was irradiated with UV light of less than 420 nm in wavelength by using a UV reflectance mirror in air at room temperature for 2 h before the introduction of the aqueous NaHCO3 solution. This UV irradiation could activate the Ga2O3 photocatalyst. As for the latter treatment, the as-prepared Ag/Ga2O3 sample was heated in air at 673 K for 2 h. Fig. 10 shows Ag L3-edge XANES spectra of three Ag/Ga2O3 samples, i.e., as prepared (Ag/Ga2O3), after UV light irradiation (Ag/ Ga2O3(irrad.)) and after calcination (Ag/Ga2O3(cal.)), together with

Fig. 8. (A) Variation of CO production rate with the amount of Ag loading in the Ag/ Ga2O3 samples prepared by SPM. (B) Time dependences of CO production rate for the 0.06 wt% Ag/Ga2O3 samples prepared by SPM (a) and an impregnation method (b).

Fig. 9. TEM images and Ag particle size distribution histogram of the 0.06 wt% Ag/Ga2O3 samples before and after reaction. Fig. 10. Ag L3-edge XANES spectra of the Ag/Ga2O3 samples as prepared (Ag/Ga2O3) (a), after UV light irradiation (Ag/Ga2O3 (irrad.)) (b), after calcination at 673 K for 2 h (Ag/ Ga2O3 (cal.)) (c), and those of Ag foil (d) and Ag2O powder (e).

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4. Conclusions In the present study, in order to improve the photocatalytic performance of the silver-loaded gallium oxide (Ag/Ga2O3) photocatalysts, each step for the preparation of the Ag/Ga2O3 photocatalysts was examined, such as the synthesis of the Ag NPs by the solution plasma method (SPM), the loading of the Ag NPs on the Ga2O3 photocatalyst and the stabilisation of the Ag NPs on the Ga2O3 surface. Then, the following was clarified: (1) The size of the Ag NPs is influenced by the discharge time in the present SPM, and probably also by other indiscernible conditions such as the state of the nonequilibrium plasma. The Ag NPs can be rapidly fabricated with the sputtering of the Ag rods by the produced highly energetic plasma particles, where the generation and the reaction of the radical species might be unsteady and much affected by the condition. Therefore, in order to obtain Ag NPs with a desired size distribution, the monitoring by in-situ absorption spectra during the synthesis of the Ag NPs would be required. In other words, it was clarified that, if it was monitored, the size of the Ag NPs could be well controlled with a narrow size distribution. (2) The Ag NPs suspended in the aqueous solution can be simply loaded on the Ga2O3 photocatalyst by the adsorption and filtration method. The size of the Ag NPs was not changed when they were loaded on the surface of Ga2O3 powder by the present filtration method, although the SPR band was much shifted by loading on the surface of the Ga2O3 photocatalyst. It was also found that the UV irradiation as a post treatment before the use for the photocatalytic reaction can improve the photocatalytic performance.

Fig. 11. Time dependence of the CO production rates for Ag/Ga2O3 samples as prepared (Ag/Ga2O3), after UV light irradiation (Ag/Ga2O3 (irrad.)), after calcination at 673 K for 2 h (Ag/Ga2O3 (cal.)) and Ga2O3.

those of Ag and Ag2O reference samples. In the XANES spectra of the asprepared Ag/Ga2O3 sample and the Ag/Ga2O3(irrad.) sample, the fine resonance peaks were found around 3370, 3378 and 3399 eV, which were characteristic of the metallic Ag. This suggests that metallic Ag NPs exist as a major component in these samples. On the other hand, XANES spectrum of the Ag/Ga2O3(cal.) sample exhibited a very small absorption peak at 3353 eV due to the oxidized Ag and an indistinct broad feature except for the small peak. This spectrum could not be reproduced by simple imposing the spectra of the Ag foil and the Ag2O reference samples. This result suggests that Ag species in this sample would be not a simple mixture of metallic particles and oxidized particles but probably partially oxidized particles or well-dispersed species. In our previous study, we measured similar XANES features for Ag/Ga2O3 samples prepared by impregnation method and concluded that small metallic Ag particles are partially oxidized and highly dispersed by connecting directly to the surface of Ga2O3 by HAADF STEM observation and EXAFS analysis [33,34]. Note that the absorbance just above the edge around 3350–3365 eV for the Ag/Ga2O3(cal.) sample was lower than that for the bulk Ag metal. Similar XANES features were reported by Bzowski et al. for the XANES spectra of Au-Ag alloys and they concluded that the electron density in the d-orbital of the Ag atom increased by alloying with Au [35]. Therefore, in the present Ag/Ga2O3 (cal.) sample, small Ag species would be highly dispersed and they probably accept more electrons in the d-orbitals as the result of interaction with the Ga2O3 surface. On the other hand, the Ag NPs on the Ag/Ga2O3(irrad.) sample would be usual metallic Ag nanoparticles similar to those on the as-prepared sample. Here, we compared their CO production rates in the photocatalytic reduction of CO2 with water over these Ag/Ga2O3 samples, i.e., the Ag/ Ga2O3(irrad.) sample and the Ag/Ga2O3(cal.) sample. As is evident in Fig. 11, the CO production rate over the Ag/Ga2O3(irrad.) sample was higher than the others even after 5 h passing although the Ag NPs on the Ag/Ga2O3(irrad.) sample were characterised to be metallic Ag nanoparticles similar to those on the as-prepared sample as mentioned above. The UV irradiation under air in the absence of the aqueous NaHCO3 solution would improve the connection between the Ag NPs and the surface of the Ga2O3 photocatalyst due to the photoexcitation of the Ga2O3 photocatalyst and the electron transfer from the photocatalyst to the Ag cocatalyst would become efficient. On the other hand, it was demonstrated that the calcination of the Ag/Ga2O3 sample before the use could not improve the photocatalytic performance in the present case. The Ag NPs might be stabilized on the Ga2O3 surface by calcination but the partially oxidised state might decrease the photocatalytic activity instead. However, the decreasing photocatalytic activities of the Ag/Ga2O3 samples with the reaction time was still observed in these cases, which should be further studied in the next study.

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