Hydrogen-assisted spark discharge generated metal nanoparticles to prevent oxide formation

Figures & data

Table 1. Melting temperature T_M of the produced metals and metal oxides.

Material	Bi	Bi3O4	Sn	SnO	SnO2	Co	CoO	Au
TM (°C)	272	817	234	1080	1630	1495	1993	1064

Figure 1. The SDG produces nanoparticles in a carrier gas that are neutralized by a radioactive ⁶³Ni source, and size-selected in two DMAs separated by a tube furnace. Finally, an ESP is used for particle deposition and an electrometer to measure the particle concentration. Hydrogen is supplied either at generation or between the first DMA and the tube furnace.

Figure 2. (a) Mobility diameter of 80 nm bismuth nanoparticles after sintering, for particles generated in either a hydrogen mixture or pure nitrogen. TEM images of typical nanoparticles sintered at different temperatures and generated in (b–f) nitrogen or (g–k) a hydrogen mixture.

Figure 3. (a) TEM image of a bismuth nanoparticle generated in a hydrogen mixture and compacted at 400°C, (b) with facets and a non-epitaxial shell. (c) An FFT of the particle in (a) matching the reciprocal structure of bismuth viewed along a zone axis. (d) TEM image of different particles acquired the same day and (e) 9 months later. (f) Nanoparticles generated in nitrogen and compacted at 500°C with, (g) a darker, (h) and brighter phase. (i) FFT of particle in (f) with d-spacings corresponding to Bi₂O₃ (220) in the bright phase and Bi ) for the dark phase. (j) XRD data of nanoparticles sintered at 500°C generated in the two carrier gases, matching mostly Bi for the hydrogen mixture and Bi + Bi₂O₃ for the nitrogen gas.

Figure 4. (a) Mobility diameter of 80 nm tin nanoparticles after sintering, for nanoparticles generated in nitrogen or a hydrogen mixture. TEM images of nanoparticles sintered at different temperatures: (b–e) generated in nitrogen; (f–i) generated in a hydrogen mixture.

Figure 5. (a) TEM image of a tin nanoparticle generated in a hydrogen mixture and compacted at 900°C, (b) with a single crystalline structure. (c) The FFT of the particle match the reciprocal structure of Sn viewed along a [110] zone axis. (d) XRD of tin nanoparticles generated in a hydrogen mixture and compacted at 900°C. (e) TEM image of spherical nanoparticle generated in nitrogen and compacted at 900°C, with a polycrystalline structure. (f) The FFT of the particle with two different d-spacings that match both Sn+SnO and SnO+SnO₂ planes. (g) TEM image of agglomerate generated in nitrogen and compacted at 900°C and (h) FFT with d-spacings matching SnO₂ (110).

Figure 6. (a) Mobility diameter of 80 nm cobalt nanoparticles after sintering for nanoparticles generated in a hydrogen mixture and nanoparticles generated in nitrogen. TEM images of nanoparticles sintered at different temperatures and generated in nitrogen (b–e) or a hydrogen mixture (f–i).

Figure 7. (a) TEM images of cobalt nanoparticle generated in a hydrogen mixture and sintered at 700°C. (b) High-resolution magnification of particle and (c) FFT of compared to face centered cubic Co viewed along a [110] zone axis. (d) TEM images of oxidized Co nanoparticle generated in nitrogen and sintered at 700°C. (e) High-resolution inset with d-spacings matching CoO (in yellow), and either CoO or Co₃O₄ in white. (f) FFT of the particle from (d). (g) XRD spectra of nanoparticles from (a), (d) with reference spectra for Co and CoO.

Figure 8. (a) Mobility diameter of 80 nm gold nanoparticles after sintering at different temperatures for the two carrier gases. Nanoparticles generated in nitrogen and sintered at (b) room temperature or (c) 500°C. (d) A magnification of the crystal structure with (e) the FFT matching Au viewed along [110]. Gold nanoparticles generated in hydrogen mixture and sintered at (f) room temperature or (g) 500°C. (h) Magnification of particle with (i) the FFT matching Au viewed along [100].

Table 2. Composition and identified phases of compacted nanoparticles generated in a hydrogen mixture or in nitrogen.

			Oxygen (XEDS)			Crystal structure
H2 (%)	Electrode material	Sinter temperature (°C)	(at%)	(wt%)		XRD	TEM
5	Bismuth	400	34 ± 13		04 ± 1	Bi	Bi
5	Tin	900	34 ± 11		07 ± 3	Sn	Sn
5	Cobalt	700	40 ± 16		15 ± 9	Co	Co
5	Gold	500	27 ± 20		03 ± 2	—	Au
0	Bismuth	500	60 ± 07		10 ± 3	Bi and Bi3O4	Bi and Bi3O4
0	Tin	900	72 ± 07		26 ± 7	Low signal^*	Sn and/or SnO and SnO2
0	Cobalt	700	61 ± 05		30 ± 4	Low signal^*	CoO
0	Gold	500	25 ± 10		03 ± 1	—	Au

* The particle concentration was too low for XRD.

Figure 9. The mode of nanoparticles after sintering for two series of gas modifications, for (a) bismuth, (b) tin, and (c) cobalt. Hydrogen is introduced either at generation or at sintering. The reference refers to the expected mobility diameter. TEM images of typical nanoparticles generated in nitrogen but sintered in a hydrogen mixture for (d) bismuth, (e) tin and (f) cobalt.