Plasma-liquid interactions: Separating

electrolytic reactions from plasma/gas phase

reactions

P. Rumbach*, M. Witzke**, R.M. Sankaran**, D.B. Go*

* University of Notre Dame, Dep. of Aerospace and Mechanical Engineering

**Case Western Reserve University, Dep. of Chemical Engineering 6/12/2013

slide 2 !P. Rumbach 6/12/2013!

Plasma-Water Interactions

X. Lu et al, Plasma Sources Sci. Tech., 2012.

Plasma Medicine

the atmospheric-pressure microplasma impinges directly onthe surface of the solution, the liquid changes color withinminutes, indicating colloidal metal nanoparticle growth.

Particle growth was monitored by ultraviolet-visible!UV-vis" absorbance spectroscopy using a Shimadzu UV-1800 spectrometer. Background spectra from de-ionized wa-ter were subtracted. A plasmon band at #400 nm, character-istic of spherical Ag nanoparticles, appears and grows inintensity for increasing reaction times with Ag foil $Fig.2!a"%. The corresponding images of the solutions show thatparticles are well dispersed and do not precipitate $Fig. 2!a"inset%. The rate of reduction is indicated by the enormousabsorbance, particularly after 15 min, which shows that ahigh density of particles is rapidly synthesized. Similar time-dependent results are obtained for Au with a plasmon bandappearing at #530 nm, characteristic of spherical Au nano-particles $Fig. 2!c"%. In comparison, Au particles growslower, but also produce stable colloids $Fig. 2!d" inset%. Themorphology of as-grown colloidal metal nanoparticles wasevaluated by transmission electron microscopy !TEM". Solu-tions of metal colloids were diluted by a factor of 100 anddrop cast onto carbon-coated copper grids. Micrograph im-ages were obtained with a Philips Tecnai F30 field emissionhigh-resolution TEM operated at 300 kV. Examination of Agparticles grown from metal foil revealed nonagglomerated,uniform, spherical, and crystalline particles approximately10 nm in size $Fig. 3!a"%. The high-resolution TEM imageshows a lattice spacing of 0.20 nm which corresponds to the!200" crystalline plane of fcc Ag. A representative TEM im-age of Au particles prepared from metal foil similarly showshigh-quality particles approximately 10 nm in diameter $Fig.3!b"%.

The disparity in particle growth rates observed from UV-vis absorbance spectroscopy in metal foil experiments is re-lated to the initial step involving anodic dissolution of thebulk metal. Metal dissolution depends on several factors in-cluding the electrolyte composition, current density, appliedcell potential, and the stoichiometry of the half-cell reac-tions. Since experiments with both metals were performed atthe same cell conditions !i.e., current and voltage" we sug-gest that the anodic dissolution of Ag occurs at a higher ratethan Au because of the differing standard oxidation poten-tials !!0.80 V for Ag versus !1.52 V for Au" and number of

electrons !1 mol for Ag versus 3 mol for Au" associated withthe respective half-cell reactions. Consequently, the rate ofAg cation formation should be higher than Au cation forma-tion, leading to faster particle growth for Ag. We acknowl-edge, however, that the interaction of the metal with the elec-trolyte may still complicate this overall picture. Assuming all

FIG. 1. !Color online" Schematic of electrochemical cell with anatmospheric-pressure Ar microplasma cathode and a metal foil anode.

300 400 500 600 700 8000.0

0.5

1.0

1.5

2.0

2.5

3.0

5 min10 min15 min (x 0.5)

Absorbance

Wavelength (nm)

Increasing time

(a)

300 400 500 600 700 8000.00

0.05

0.10

0.15

0.20

0.25(b)

Absorbance

Wavelength (nm)

10 min15 min30 min

Increasing time

FIG. 2. !Color online" UV-vis absorbance spectroscopy of !a" Ag colloidsprepared from Ag foil for process intervals of 5, 10, and 15 min !inset:photo of Ag colloids for corresponding times" and !b" Au colloids preparedfrom Au foil for process intervals of 10, 15, and 30 min !inset: photo of Aunanoparticle solutions for corresponding times".

20 nm

5 nm

5 nm

5 nm

(a)

(d)(c)

(b)

FIG. 3. TEM images of !a" Ag nanoparticles synthesized from anodic dis-solution of Ag foil and microplasma reduction !process time=10 min", !b"Au nanoparticles synthesized from anodic dissolution of Au foil and micro-plasma reduction !process time=30 min", !c" Ag nanoparticles synthesizedfrom microplasma reduction of aqueous AgNO3 solution !process time=10 min", and !d" Au nanoparticles synthesized from microplasma reduc-tion of aqueous HAuCl4 !process time=10 min".

131501-2 C. Richmonds and R. M. Sankaran Appl. Phys. Lett. 93, 131501 !2008"

Downloaded 28 Oct 2012 to 129.74.250.206. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

C. Richmonds, R.M. Sankaran, Appl. Phys. Lett., 2008.

Nanomaterials Synthesis

H.S. Uhm, Y.C. Hong, Thin Solid Films, 2008.

slide 3 !P. Rumbach 6/12/2013!

Bulk Plasma Phase Reactions Products formed in the bulk plasma phase

will dissolve into solution.

Nitric acid: 3NO2 + H2O → 2HNO3 + NO

Hydrogen peroxide:

H.S. Uhm, Y.C. Hong, Thin Solid Films, 2008.

OH + OH → H2O2 HO2 + HO2 → H2O2 + O2

M.G. Kong et al., New J. Phys., 2009.

slide 4 !P. Rumbach 6/12/2013!

Electron Transfer Reactions Free electrons from the plasma will reduce

ions at the plasma-liquid interface.

Reduction of ferricyanide: Fe(CN)63- + e- → Fe(CN)6

4- C. Richmonds et al., J. Am. Chem. Soc., 2011.

Acidic water electrolysis: 2H+ + 2e- → H2(g)

200 250 300

75

150

225

300

45 min

20 min

10 min

5 min

No plasma

(v)

(iv)

(iii)(ii)

Time (s)

Inte

nsity

(a.u

.)

(i)

M. Witzke et al., J. Phys. D: Appl. Phys, 2012.

Metallic nanoparticle synthesis: Ag+ + e- → Ag C. Richmonds et al., Appl. Phys. Lett., 2008.

slide 5 !P. Rumbach 6/12/2013!

Bulk Plasma Reactions

M.G. Kong et al., New J. Phys., 2009.

OH + OH → H2O2

HO2 + HO2 → H2O2 + O2

3NO2 + H2O → 2HNO3 + NO

Ag+ + e- → Ag 2H+ + 2e- → H2(g)

Electron Transfer Reactions

slide 6 !P. Rumbach 6/12/2013!

DC Microplasma Jet Electrochemistry with a plasma cathode

Experimental setup

e- 100 µm

Plasma jet injects free electrons into and aqueous solution.

NaCl

slide 7 !P. Rumbach 6/12/2013!

Competing Processes Electrolytic reactions vs. bulk plasma reactions

3NO2 + H2O → 2HNO3 + NO Nitric acid

OH + OH → H2O2

HO2 + HO2 → H2O2 + O2

Hydrogen peroxide

These effects can be quantified with pH

measurements.

headspace filled with O2, N2, Ar, or air

2H2O + 2e- → 2OH- + H2

Water Electrolysis

2Cl- → Cl2 + 2e-

4OH– → 2H2O + 4e- + O2

Chlorine gas evolution

→ NaOH

e-

3 kV Ar

NaCl

slide 8 !P. Rumbach 6/12/2013!

0 10 203

4

5

6

7

8

traditional

pH

time, t (min)0 10 20

3

4

5

6

7

8

traditional air

pH

time, t (min)0 10 20

3

4

5

6

7

8

traditional N2

air

pH

time, t (min)

pH Measurements of Primary Solution

0 10 203

4

5

6

7

8

traditional Ar O2

N2

air

pH

time, t (min)

N2(g) + O2(g) → 2 NO(g)

2NO(g) + O2(g) → 2 NO2(g)

3NO2 + H2O → 2HNO3 + NO

Colorimetric tests indicated NO3

- present in solution for air and N2.

In air and N2 acidification due to nitric acid

Potassium iodide titrations found H2O2 ~5 ppm for all cases.

slide 9 !P. Rumbach 6/12/2013!

Isolating Gas Phase Reactions To isolate reactions, exhaust was bubbled

into an external saline solution.

slide 10 !P. Rumbach 6/12/2013!

pH Measurements of External Solution

0 10 203

4

5

6

7

Ar N2

O2

air

pH

time, t (min)

Nitric acid is no longer produced for the case of ambient N2 gas.

Colorimetric tests indicated NO3

- present in solution for air only.

slide 11 !P. Rumbach 6/12/2013!

HNO3 vs. NaOH Production Rates

0 5 10 15 200.0

2.0x10-8

4.0x10-8

6.0x10-8

8.0x10-8 conventional Ar O2

mol

es o

f NaO

H

time, t (min)

~ 5×10-11 mol/s

0 5 10 15 20

0.0

2.0x10-6

4.0x10-6

6.0x10-6

8.0x10-6

1.0x10-5

1.2x10-5

air, primaryair, bubblerN2, primary

mol

es o

f HN

O3

time, t (min)

~8.6×10-9 mol/s for air ~1.8×10-9 mol/s for N2

Faraday’s law predicts O2 gas evolution at 3.9×10-9 mol/s

HNO3 produced at 1.8×10-9 mol/s → limited by O2 production rate!

NaOH HNO3

slide 12 !P. Rumbach 6/12/2013!

Summary Final pH determined by competing reactions

(electron transfer vs. bulk plasma reactions)

1.  In mixtures containing N2/O2, significant acidification from HNO3

2.  In mixtures without N2 and O2, electrolytic production of NaOH dominates

3.  In all cases, small amounts of H2O2 (~5 ppm) produced does not significantly affect pH

slide 13 !P. Rumbach 6/12/2013!

Acknowledgements

•  Megan Witzke, Prof. Mohan Sankaran •  Air Force Office of Scientific Research

Award No. FA9550-11-1-0020