1. Introduction

2. Materials and methods

3. Results and discussion

3.1. Removal of surfactant by UV-ozone treatment

As described in the experimental section, surfactant-encapsulated Pt nanoparticles were prepared using literature methods using CTAB and chloroplatinic acid as precursors (H₂PtCl₆) [36,37]. A thin film of Pt nanoparticles was prepared by the Langmuir-Blodgett (LB) method and then transferred onto a Au-coated silicon wafer. The original capping agent CTAB was exchanged to oleylamine to fabricate the LB assembly and deposition. For comparison, the surfactant-encapsulated Pt nanoparticles were first cleaned by UV-ozone treatment. X-ray photoelectron spectroscopy (XPS) was performed to track surface composition changes. The survey spectrum of the fresh sample in Figure S1 (supporting information) shows that the surface mainly contains carbon (C), gold (Au), platinum (Pt), and oxygen (O). The oxidation state of Au remains unchanged throughout all the experiments.

Since the surfactant molecules, oleylamine and CTAB, contain hydrocarbon CH_x and NH₂ groups, both C and N are detected by C 1s and N1s for the fresh sample. After UV-ozone treatment, the peak of C1s disappears, which is also reflected by the high-resolution C 1s and N 1s spectra. As shown in Figure 1, the disappearance of C 1s and N 1s peaks indicates that the surfactant molecules were removed after UV-ozone treatment. The Pt 4f spectra displayed in Figure 1 demonstrate Pt oxidation after UV-ozone treatment, as evidenced by the appearance of peaks corresponding to Pt – O species [38,39,40,41]. Table 1 summarizes the peak positions and the ratio of Pt – O/Pt⁰. Two different Pt – O species were observed and assigned to Pt(II)–O and Pt(I)–O. It shows a larger fraction of surface Pt – O(II) than Pt – O(I) after UV-ozone treatment, suggesting the significant oxidation of Pt. Therefore, when in situ IRRAS CO probe experiments were performed, CO molecules adsorb not only on metallic Pt (Pt⁰) sites, but also on Pt – O sites, as shown in Figure 2. Correspondingly, different O species, such as lattice O and adsorbed O, are observed in the O 1s spectra after UV-ozone treatment due to the reaction between active O species generated from UV-ozone and the organic surfactant [42,43].

Table 1. Pt 4f peak positions and area ratios for the different Pt species before and after UV-ozone treatment, and CO adsorption experiment.

Table 1. Pt 4f peak positions and area ratios for the different Pt species before and after UV-ozone treatment, and CO adsorption experiment.

Sample		Fresh	After UV-ozone treatment	After CO adsorption
Peak position (eV)	Pt0 4f7/2	71.17	71.16	71.04
	Pt0 4f5/2	74.52	74.51	74.39
	Pt–O(I) 4f7/2	72.45	72.70	72.18
	Pt–O(I) 4f5/2	75.80	76.05	75.53
	Pt–O(II) 4f7/2	73.79	73.98	73.57
	Pt–O(II) 4f5/2	77.14	77.33	76.92
Peak area ratio	Pt–O(I)/Pt0	0.28	0.13	0.30
	Pt–O(II)/Pt0	0.07	0.37	0.27

As shown in Figure 2, the CO vibrational frequencies at 2200 – 2100 cm^-1 in IRRAS are characteristic CO adsorption on oxidized Pt sites [44,45], while the bands in the 2100 – 1900 cm^-1 range correspond to CO molecules linearly adsorbed on Pt nanoparticles (atop) [46,47,48]. The peak areas for the CO vibrational mode corresponding to metallic Pt sites increase with increasing pressure, while the peak area corresponding to CO on Pt-O sites decreases, suggesting the reduction of Pt – O during CO exposure. As confirmed by the Pt 4f spectra (Figure 1), metallic Pt is dominant and only a small fraction of Pt – O(I) was observed after CO exposure to elevated pressures. In the presence of CO, the Pt – O(II) was first reduced to Pt – O(I), followed by the reduction to Pt⁰. Therefore, different CO vibrational frequencies were observed due to the different oxidation states of Pt in the presence of CO.

At 1×10^-5 ~ 1×10^-1 mbar CO pressure range, CO initially adsorbs on Pt – O sites with vibrational frequency of ~ 2170 and 2110 cm^-1 (See Figure S2 in the supporting information) [45]. Further increasing the CO pressure, CO adsorption on metallic Pt sites emerge. When the pressure of CO reaches 5 mbar, atop adsorption of CO on metallic Pt sites is dominant, while the CO adsorption on Pt – O sites almost disappears. This likely indicates reduction of the Pt oxides by CO. It is well known that the differences in CO stretching frequency can arise from the different sites on Pt nanoparticles. That is, the undercoordinated sites lead to stronger CO adsorption than high-coordination sites, resulting in a lower CO vibrational frequencies [44]. Consequently, the bands at ~ 2048 and ~ 1992 cm^-1 that appeared at a CO pressure of 1 mbar can be assigned to the different metallic Pt sites. Further increasing CO pressure, CO adsorption on higher coordination sites of metallic Pt (~ 2096 cm^-1) is obtained [44]. Moreover, the desorption of hydrocarbon species (as evident by the C-H stretching vibrations at 3000 – 2800 cm^-1) was observed with the increase of CO pressure, indicating the presence of hydrocarbon residuals on the Pt surface after UV-ozone cleaning. Note that these appear as peaks pointing up in the transmittance spectra in Figure 2. This result is also consistent with the decreasing peak area ratio of C 1s/Au 4f after in situ CO probe experiment (Table S1 in supporting information).

3.2. Removal of surfactant by O₂ plasma treatment

Figure 3 and Figure 4 display IRRAS and XPS measurements of surfactant-encapsulated Pt nanoparticles cleaned by in situ O₂ plasma treatment. Similar to the UV-ozone cleaning process, carbon species on the surface can be efficiently removed by O₂ plasma, while Pt was oxidized by the highly active O species generated in the plasma. As shown in Figure 3a, the bands in the 3100 – 2800 cm^-1, 2200 – 1900 cm^-1 and 1800 – 1650 cm^-1 ranges are C–H stretching of the hydrocarbons in the surfactant [49,50], CO adsorption on Pt nanoparticles [44] and C–H bending of hydrocarbons [49], respectively. After 1 min O₂ plasma treatment, the positive hydrocarbon peaks at 3100 – 2800 and 1800 – 1650 cm^-1 indicate the removal of hydrocarbon species (C–H). During plasma treatment, the active O species react with hydrocarbons, forming CO, which adsorbs on Pt nanoparticles. Note that the presence of CO is not from direct introduction to the chamber but rather the result of the oxidation of hydrocarbons under O₂ plasma. As shown in Figure 3a, the CO adsorption on Pt^δ+ sites also suggests that the surface of Pt nanoparticles is oxidized during the O₂ plasma treatment, and the fraction of Pt – O species increases, which is consistent with the Pt 4f and O 1s spectra as shown in Figure 4. Compared to the UV-ozone treatment, O₂ plasma treatment results in a smaller fraction of Pt oxide sites on the Pt nanoparticle surface (see Table 2 in the supporting information). Only adsorption of CO on metallic Pt sites is observed at CO pressures < 1×10^-1 mbar as evident by the peak originally at 2092 cm^-1 (in the background) that shifts to 2096 cm^-1 under CO pressure in Figure 3b, and only a small fraction of Pt – O species (peak at 2123 cm^-1) is evident at higher pressures. In addition, below 10 mbar of CO, the absence of CO vibrational frequency at lower wavenumber range (2050 – 1990 cm^-1) may suggest that less defective sites (low coordination) are formed after the O₂ plasma treatment in comparison to the UV-ozone process [37]. Further increase of CO pressure to 10 mbar result in additional surge in CO adsorption at lower coordination sites (1986 cm^-1), which may be due to the structural reconstruction of Pt nanoparticles caused by CO adsorption [51].

Table 2. Pt 4f peak positions and area ratios for the different Pt species before and after O₂ plasma treatment and in situ CO adsorption.

Table 2. Pt 4f peak positions and area ratios for the different Pt species before and after O₂ plasma treatment and in situ CO adsorption.

Sample		Fresh	After O2 plasma treatment	After CO adsorption
Peak position (eV)	Pt0 4f7/2	71.16	71.05	71.07
	Pt0 4f5/2	74.51	74.40	74.42
	Pt–O(I) 4f7/2	73.69	73.06	73.43
	Pt–O(I) 4f5/2	77.04	76.41	76.77
	Pt–O(II) 4f7/2	72.48	74.29	74.80
	Pt–O(II) 4f5/2	75.83	77.64	78.15
Peak area ratio	Pt–O(I)/Pt0	0.13	0.04	0.04
	Pt–O(II)/Pt0	0.07	0.17	<0.001

3.3. Removal of surfactant by H₂ plasma treatment

Altogether, both UV-ozone and O₂ plasma treatments change the oxidation states of Pt. At the same time, the surfactant is removed efficiently by forming CO₂ and CO species, with the latter (CO) adsorbing on the Pt nanoparticles. To avoid the oxidation of the Pt nanoparticles during cleaning, H₂ plasma treatment was further investigated. Figure 5 shows XP spectra of the fresh sample after H₂ plasma treatment and after CO adsorption. The data shows that the surfactant can also be removed by H₂ plasma, as demonstrated by the disappearance C 1s and N 1s peaks in XP spectra. Similar to the sample after O₂ plasma treatment, C species emerge in C 1s spectrum after CO adsorption experiment on the H₂ plasma cleaned Pt nanoparticles.

The IRRA spectra in Figure 6a shows CO adsorbed on metallic Pt during the H₂ plasma treatment. This could be due to the reaction between hydrocarbon and surface oxygen species, forming CO. As a result, Pt remains in the metallic state after H₂ plasma treatment (see Figure 5a and Table 3). Only linear adsorption of CO on Pt⁰ is observed from in situ CO probe experiments, as shown in Figure 6b. Different from the O₂-based treatment, no CO adsorption on low coordination Pt sites is detected at low CO pressures (< 1 mbar) in this case. With increasing CO pressure to 1 mbar, different CO stretching frequencies are obtained. This could be ascribed to the structural reconstruction of surface Pt atoms during CO adsorption [51]. CO initially adsorbs on those high-coordination Pt sites, resulting in CO stretching frequencies of 2069 and 2092 cm^-1. With increasing CO pressure, the coordination number of Pt atoms varies with the plasma treatment time. Therefore, CO adsorption on low-coordination Pt sites appears (1980 cm^-1). At high CO pressure, the increased CO coverage on Pt nanoparticles also results in the blue shifting of CO vibrational frequencies, which is caused by dipole-dipole interactions (from 1980 to 1994 cm^-1 and from 2092 to 2096 cm^-1). These results demonstrate that H₂ plasma treatment is an efficient strategy for removing hydrocarbon-based surfactants while preventing oxidation of Pt.

Table 3. Pt 4f peak positions and area ratios for the different Pt species before and after H₂ plasma treatment and in situ CO adsorption.

Table 3. Pt 4f peak positions and area ratios for the different Pt species before and after H₂ plasma treatment and in situ CO adsorption.

Sample		Fresh	After H2 plasma treatment	After CO adsorption
Peak position (eV)	Pt0 4f7/2	71.22	71.05	71.07
	Pt0 4f5/2	74.57	74.40	74.42
	Pt–O(I) 4f7/2	72.50	72.05	72.46
	Pt–O(I) 4f5/2	75.85	75.40	75.81
	Pt–O(II) 4f7/2	74.60	73.58	73.54
	Pt–O(II) 4f5/2	77.95	76.93	76.89
Peak area ratio	Pt–O(I)/Pt0	0.15	0.03	0.03
	Pt–O(II)/Pt0	0.08	0.02	0.02

Figure 6. (a) IRRA spectra of surfactant encapsulated Pt nanoparticles during H₂ plasma treatment at room temperature. The CO adsorption range (2200 – 2000 cm^-1) is also presented. The pressure of H₂ was 0.1 mbar, power applied for plasma treatment was 4 W; before plasma treatment, background ($\frac{p_0}{s_0}$) was collected in the presence of 0.1 mbar H₂; The transmittance signal was obtained by comparing the spectrum after plasma treatment with the background spectrum ($\frac{{p_t/s}_t}{{p_0/s}_0}$); where p_t and s_t refer to the p- and s- polarized spectra; (b) IRRAS spectra of in situ CO adsorption on Pt nanoparticles after H₂ plasma treatment. Before introducing CO, background ($\frac{p_0}{s_0}$) was collected under UHV conditions (2×10^-8 mbar); The transmittance signal was obtained by comparing the spectra under different CO pressures with the background spectrum ($\frac{{p_{CO}/s}_{CO}}{{p_0/s}_0}$); where p_CO and s_CO refer to the p- and s- polarized spectra that were collected under different CO pressures. The wavenumbers of CO vibration are presented in brackets.

Figure 6. (a) IRRA spectra of surfactant encapsulated Pt nanoparticles during H₂ plasma treatment at room temperature. The CO adsorption range (2200 – 2000 cm^-1) is also presented. The pressure of H₂ was 0.1 mbar, power applied for plasma treatment was 4 W; before plasma treatment, background ($\frac{p_0}{s_0}$) was collected in the presence of 0.1 mbar H₂; The transmittance signal was obtained by comparing the spectrum after plasma treatment with the background spectrum ($\frac{{p_t/s}_t}{{p_0/s}_0}$); where p_t and s_t refer to the p- and s- polarized spectra; (b) IRRAS spectra of in situ CO adsorption on Pt nanoparticles after H₂ plasma treatment. Before introducing CO, background ($\frac{p_0}{s_0}$) was collected under UHV conditions (2×10^-8 mbar); The transmittance signal was obtained by comparing the spectra under different CO pressures with the background spectrum ($\frac{{p_{CO}/s}_{CO}}{{p_0/s}_0}$); where p_CO and s_CO refer to the p- and s- polarized spectra that were collected under different CO pressures. The wavenumbers of CO vibration are presented in brackets.

3.4. Using H₂ plasma to reduce Pt nanoparticles treated by O₂ sources.

In addition to surface cleaning, H₂ plasma can also be used to reduce Pt oxides. The two O₂-based cleaning methods described before result in the formation of oxides. We explore here the reduction of these oxides in different ways: (1) UV-ozone treatment coupled with in situ thermal H₂ reduction, and (2) in situ O₂ plasma followed by H₂ plasma treatment. Figure 7 shows XPS measurements of surfactant encapsulated Pt nanoparticles after UV-ozone treatment followed by in situ H₂ reduction at 100 °C under 0.5 mbar H₂. Table 4 summarizes the Pt 4f peak positions and area ratios for the different Pt species before and after UV-ozone treatment, followed by in situ H₂ reduction. Figure 8 shows the surfactant encapsulated Pt nanoparticles after O₂ plasma treatment followed by H₂ plasma reduction processes at room temperature. These results suggest that oxidized Pt can be reduced via thermal or plasma treatment in H₂. Thermal treatment induces the C species emerging from bulk after O₂ plasma cleaning, while no trace amount of C was detected after H₂ plasma treatment. Figure S6 in the supporting information demonstrates that the H₂ plasma treatment after O₂ plasma further helps clean the surface. Residual hydrocarbon species desorb from the surface of Pt nanoparticles during H₂ plasma treatment.

Figure 7. XP spectra of surfactant encapsulated Pt nanoparticles after UV-ozone treatment followed by in situ H₂ reduction at 100 °C under 0.5 mbar H₂: (a) Pt 4f XP spectra; (b) C 1s XP spectra; (c) O 1s XP spectra; (d) N 1s XP spectra.

Figure 7. XP spectra of surfactant encapsulated Pt nanoparticles after UV-ozone treatment followed by in situ H₂ reduction at 100 °C under 0.5 mbar H₂: (a) Pt 4f XP spectra; (b) C 1s XP spectra; (c) O 1s XP spectra; (d) N 1s XP spectra.

Figure 8. XP spectra of surfactant encapsulated Pt nanoparticles after O₂ plasma treatment followed by H₂ plasma reduction processes at room temperature: (a) Pt 4f XP spectra; (b) C 1s XP spectra; (c) O 1s XP spectra; (d) N 1s XP spectra. The pressure of O₂ or H₂ for plasma treatment was 0.1 mbar, and the power applied for plasma treatment was 4 W.

Figure 8. XP spectra of surfactant encapsulated Pt nanoparticles after O₂ plasma treatment followed by H₂ plasma reduction processes at room temperature: (a) Pt 4f XP spectra; (b) C 1s XP spectra; (c) O 1s XP spectra; (d) N 1s XP spectra. The pressure of O₂ or H₂ for plasma treatment was 0.1 mbar, and the power applied for plasma treatment was 4 W.

Table 4. Pt 4f peak positions and area ratios for the different Pt species before and after UV-ozone treatment followed by in situ H₂ reduction.

Table 4. Pt 4f peak positions and area ratios for the different Pt species before and after UV-ozone treatment followed by in situ H₂ reduction.

Sample		Fresh	After UV-ozone treatment	After H2 reduction
Peak position (eV)	Pt0 4f7/2	71.14	71.13	71.02
	Pt0 4f5/2	74.49	74.48	74.37
	Pt–O(I) 4f7/2	72.47	72.70	72.58
	Pt–O(I) 4f5/2	75.82	76.05	75.93
	Pt–O(II) 4f7/2	73.72	73.88	73.54
	Pt–O(II) 4f5/2	77.07	77.23	76.89
Peak area ratio	Pt–O(I)/Pt0	0.15	0.07	0.02
	Pt–O(II)/Pt0	0.09	0.30	0.04

Table 5. Pt 4f peak positions and area ratios for the different Pt species before and after O₂ plasma treatment followed by H₂ plasma treatment.

Table 5. Pt 4f peak positions and area ratios for the different Pt species before and after O₂ plasma treatment followed by H₂ plasma treatment.

Sample		Fresh	After O2 plasma treatment	After H2 plasma treatment
Peak position (eV)	Pt0 4f7/2	71.07	71.08	71.03
	Pt0 4f5/2	74.42	74.43	74.38
	Pt–O(I) 4f7/2	72.31	72.15	72.10
	Pt–O(I) 4f5/2	75.66	75.50	75.45
	Pt–O(II) 4f7/2	73.56	74.19	73.52
	Pt–O(II) 4f5/2	76.91	77.54	76.87
Peak area ratio	Pt–O(I)/Pt0	0.22	0.01	0.13
	Pt–O(II)/Pt0	0.08	0.15	0.04

Figure 9 shows IRRA spectra of in situ CO adsorption on Pt nanoparticles after UV-ozone followed by thermal H₂ reduction treatment. Figure 10 shows the IRRA spectra of in situ CO adsorption on Pt nanoparticles after O2 plasma treatment followed by H₂ plasma treatment. For both samples, CO adsorption on high coordination sites dominates at low pressures (< 1×10^-1 mbar). With increase of CO pressure, lower CO vibrational frequencies appear, which is similar to the H₂ plasma treatment as discussed above (Figure 6). Since H₂ plasma helps cleaning Pt nanoparticle surface after O₂ plasma treatment, only a small fraction of hydrocarbon species at 3000 – 2800 cm^-1 desorbs from the surface at high CO pressure (5 mbar). In contrast, the thermal reduction of Pt nanoparticles by H₂ does not help the desorption of hydrocarbon species. During CO adsorption experiments, the desorption of hydrocarbon is observed at a relatively lower CO pressure (1 mbar).

Figure 9. IRRA spectra of in situ CO adsorption on Pt nanoparticles, after UV-ozone followed by thermal H₂ reduction treatment. The wavenumbers of CO vibration are presented in the brackets. Before introducing CO, background ($\frac{p_0}{s_0}$) was collected under UHV conditions (2×10^-8 mbar). The transmittance signal was obtained by comparing the spectra under different CO pressures with the background spectrum ($\frac{{p_{CO}/s}_{CO}}{{p_0/s}_0}$), where p_CO and s_CO refer to the p- and s-polarized spectra that were collected under different CO pressures.

Figure 9. IRRA spectra of in situ CO adsorption on Pt nanoparticles, after UV-ozone followed by thermal H₂ reduction treatment. The wavenumbers of CO vibration are presented in the brackets. Before introducing CO, background ($\frac{p_0}{s_0}$) was collected under UHV conditions (2×10^-8 mbar). The transmittance signal was obtained by comparing the spectra under different CO pressures with the background spectrum ($\frac{{p_{CO}/s}_{CO}}{{p_0/s}_0}$), where p_CO and s_CO refer to the p- and s-polarized spectra that were collected under different CO pressures.

Figure 10. IRRA spectra of in situ CO adsorption on Pt nanoparticles after O₂ plasma followed by H₂ plasma treatment. The wavenumbers of CO vibration are presented in brackets. The pressure of O₂ or H₂ for plasma treatment was 0.1 mbar, power applied for plasma treatment was 4 W. Before introducing CO, the background ($\frac{p_0}{s_0}$) was collected under UHV conditions (2×10^-8 mbar). The transmittance signal was obtained by comparing the spectra under different CO pressures with the background spectrum ($\frac{{p_{CO}/s}_{CO}}{{p_0/s}_0}$); where p_CO and s_CO refer to the p- and s- polarized spectra that were collected under different CO pressures. In situ tracking of structure changes by IRRAS during sequential plasma treatments are presented in the supporting information.

Figure 10. IRRA spectra of in situ CO adsorption on Pt nanoparticles after O₂ plasma followed by H₂ plasma treatment. The wavenumbers of CO vibration are presented in brackets. The pressure of O₂ or H₂ for plasma treatment was 0.1 mbar, power applied for plasma treatment was 4 W. Before introducing CO, the background ($\frac{p_0}{s_0}$) was collected under UHV conditions (2×10^-8 mbar). The transmittance signal was obtained by comparing the spectra under different CO pressures with the background spectrum ($\frac{{p_{CO}/s}_{CO}}{{p_0/s}_0}$); where p_CO and s_CO refer to the p- and s- polarized spectra that were collected under different CO pressures. In situ tracking of structure changes by IRRAS during sequential plasma treatments are presented in the supporting information.

3.5. Practical applications of plasma cleaning in advanced characterization

To further investigate the changes of particle size after various treatments, scanning electron microscopy (SEM) analysis is performed. As shown in Figure 11 and Figures S9-S13 in supporting information, the average particle size of Pt nanoparticles is ~ 7.5 nm for the fresh sample. Non-thermal treatments (UV-ozone and plasma treatments) do not influence average particle size significantly, while thermal reduction leads to the growth of average Pt nanoparticle size. As shown in Figure 11j, a broad particle size distribution of Pt is observed after thermal reduction treatment. The presence of large particles (> 12 nm) indicates the agglomeration of Pt nanoparticles during thermal treatment, which is consistent with the atomic force microscopy (AFM) result as shown in Figure S16. Compared to the thermal treatment, Pt nanoparticles show narrower size distributions after non-thermal treatments. In addition, O₂-based surface cleaning treatment (UV-ozone and O₂ plasma) results in the formation of Pt aggregates containing several small nanoparticles. On the contrary, the dispersion of Pt nanoparticles remains almost unchanged after H₂ plasma treatment. As shown in Figure S17, there is no obvious agglomeration of Pt nanoparticles after H₂ plasma treatment. These results indicate that compared to O₂-based surface cleaning methods, H₂ plasma treatment can efficiently remove hydrocarbon species without significantly changing the structure of nanoparticles.

Figure 11. SEM analysis of the changes of surfactant encapsulated Pt nanoparticles before and after treatments. (a and b) surfactant encapsulated Pt nanoparticles supported on Au film without treatment; (c and d) surfactant encapsulated Pt nanoparticles supported on Au film after UV-ozone treatment; (e and f) surfactant encapsulated Pt nanoparticles supported on Au film after O₂ plasma treatment; (g and h) surfactant encapsulated Pt nanoparticles supported on Au film after H₂ plasma treatment; (i and j) surfactant encapsulated Pt nanoparticles supported on Au film after UV-ozone treatment followed by H₂ reduction at 100 °C for 30 min; (k and l) surfactant encapsulated Pt nanoparticles supported on Au film after O₂ plasma followed by H₂ plasma treatment. The estimated particle size distributions were obtained by counting ~100 nanoparticles from the SEM images.

Figure 11. SEM analysis of the changes of surfactant encapsulated Pt nanoparticles before and after treatments. (a and b) surfactant encapsulated Pt nanoparticles supported on Au film without treatment; (c and d) surfactant encapsulated Pt nanoparticles supported on Au film after UV-ozone treatment; (e and f) surfactant encapsulated Pt nanoparticles supported on Au film after O₂ plasma treatment; (g and h) surfactant encapsulated Pt nanoparticles supported on Au film after H₂ plasma treatment; (i and j) surfactant encapsulated Pt nanoparticles supported on Au film after UV-ozone treatment followed by H₂ reduction at 100 °C for 30 min; (k and l) surfactant encapsulated Pt nanoparticles supported on Au film after O₂ plasma followed by H₂ plasma treatment. The estimated particle size distributions were obtained by counting ~100 nanoparticles from the SEM images.

Collectively, the widely used surfactant in chemical synthesis helps prepare metal nanoparticles with small particle sizes, which may exhibit promising performances due to the size effect, the formation of high density of defective sites and the different facets with atomic arrangements. However, after synthesis, the residual surfactant molecules on metal nanoparticle surface can either work cooperatively with metal nanoparticles to generate novel properties or block the surface of metal nanoparticles, sacrificing the surface metal sites. As shown in Figure S18, the presence of surfactant occupies surface Pt sites and thus the absence of CO adsorption on Pt was observed at low CO pressures. Both non-thermal UV-ozone and plasma treatment show the efficient removal of surfactant, indicating by the strong CO adsorption on Pt nanoparticles. As an example, Figure 12 compares the shape changes of Pt nanoparticles by conducting environmental transmission electron microscopy (ETEM) under different conditions to show the advantage of plasma based cleaning method in practical applications. Figure 12a and b shows that surfactant encapsulated Pt nanoparticles are stable at 127 °C. The shapes of these particles remain almost unchanged after 2 min under vacuum conditions (~1×10^-6 mbar). Further increasing the temperature to 227 °C, the small particles merge together, leading to the shape changes of these nanoparticles, as shown in Figure 12c. Movie S1 and S2 in the supporting information also show highly mobile Pt and shape changes of Pt nanoparticles at 227 °C. However, this dynamic structural reconstruction is absent at 127 °C. Figure 12d-f shows that after air plasma treatment for 1h, the shape and size of Pt nanoparticles changes in 1 min. Longer exposing time results in the further changes in shape and particle size. Movie S3 in the supporting information also indicates that the structure undergoes reconstruction at 127 °C under vacuum conditions, after plasma treatment. The utilization of air plasma is more complicated than the single gas induced plasma treatment due to the complicated gas composition. Typically, reactive oxygen and nitrogen species, such as ozone, active radicals, nitrogen oxides, etc, play a major role in air plasma [52]. These results indicate the efficient sample cleaning by non-thermal plasma treatment.

Figure 12. (a-c) In situ ETEM images showing morphological evolution of encapsulated Pt nanoparticles as a function of time and temperature; (d-f) in situ ETEM images of Pt nanoparticle after in situ air plasma treatment for 1 h at 127 °C. The shape changes of different nanoparticles are shown in movies 1-3 in the supporting information.

Figure 12. (a-c) In situ ETEM images showing morphological evolution of encapsulated Pt nanoparticles as a function of time and temperature; (d-f) in situ ETEM images of Pt nanoparticle after in situ air plasma treatment for 1 h at 127 °C. The shape changes of different nanoparticles are shown in movies 1-3 in the supporting information.

4. Conclusions

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