Optical emission spectroscopy of radio frequency inductively coupled plasma for cold hydrogenation of nanoparticles

The radio frequency inductively coupled plasma (ICP) offers an alternative “cold” way to affect the size, composition, structure, and surface functionality of nanoparticles (NPs), such as metal oxide NPs, providing further adjustment of their physical and chemical properties. The ICP was monitored in-situ by optical emission spectroscopy (OES). In particular, hydrogen, oxygen, argon, and nitrogen plasma was studied. OES data show that despite the decrease of the optical emission intensity with increasing gas pressure, the concentration of atomic hydrogen increases with pressure and radio frequency power.


Introduction
Metal-oxide nanoparticles (NPs) with their high surface-to-volume ratio and related size effects are currently studied for energy conversion, photocatalytic waste water cleaning, electrochemical energy storage or sensing applications [1]. For example, zinc oxide (ZnO) is a versatile, environmentally friendly, functional semiconducting material with unique optical properties and variety of nanostructures such as nanorods, nanowires, nanobelts, nanotubes, nanoribbons or hedgehog-like NPs [2]. A low-cost technology for mass-production of ZnO NPs is a hydrothermal chemical synthesis [3]. Recently, we have shown that the UV illumination [4] and the age of precursors [5] plays a significant role in defects formation in hydrothermally grown ZnO NPs.
The interstitial hydrogen acts always as a donor in ZnO [6]. It has been shown that the high temperature annealing of ZnO in H 2 atmosphere passivates defects and acts as reducing agent creating oxygen vacancies [7]. To avoid high temperature, inductively coupled plasma (ICP) offers an alternative way of cold hydrogenation. Plasma effects the size, composition, structure, surface, and functions of NPs providing further adjustment of their physical and chemical properties [8]. We have shown that the plasma hydrogenation is an effective low temperature way to suppress defects in ZnO NPs leading to significant enhancement of an exciton-related emission band [9]. The optical emission spectroscopy (OES) provides in-situ information about chemical and physical processes that occur in

Optical emission spectroscopy
Optical emission spectra (OES) fibre coupled TE cooled CCD resistant optical fibber and quartz lens grounded stainless steel sample holder. optical emission intensity, each spectrum is averaged 10 times be comparable with other spectra. whole setup was spectrally calibrated optical diffuser placed inside the quartz chamber we investigate in this paper the effect of pressure and radio frequency (rf)

Inductively coupled plasma (ICP)
Ps is done in a prototype ICP reactor currently being with SVCS Process Innovation, s. r. o., Valašské Meziříčí, Czech Republic rf discharge power up to 300 W using argon (99.998%), and nitrogen (99.999%) process gasses. Prior any , valves, flowmeters, gauges, and all gas inlet and outlet tubes are 300 dry vacuum pump protected with the ISO Flange Vacuum Filter to residual pressure below 0.1 Pa and flushed 5 min by 50 sccm to reduce residual gas contamination. A powder sample is placed in a stirred during the plasma treatment by swinging the plasma in figure 1.
The ICP reactor: 1 -external copper wire coil, 2 -quartz chamber, grounded stainless steel sample holder ("a cradle") with a stainless steel rotating vacuum feed-through, 4 -gas inlet quartz tube, 5 -plasma shield in front of a gas outlet, 6 -gas outlet, 7 -door.

Optical emission spectroscopy (OES)
are measured in 400−1000 nm spectral range by ooled CCD spectrometer (B&W Tek BTC112E) equipped with quartz lens focused in the middle of the quartz chamber above the sample holder. The integration time varies from 10 ms to 10 ach spectrum is averaged 10 times and divided by the integration time to able with other spectra. Dark spectra are measured without plasma and spectrally calibrated with Oriel #63358 quartz tungsten halogen lamp and a white placed inside the quartz chamber. radio frequency (rf) power currently being developed in a Czech Republic. ICP operates argon (99.998%), hydrogen any plasma treatment, are evacuated with the 300 dry vacuum pump protected with the ISO Flange Vacuum Filter sccm flow of process grounded cradle-like by swinging the cradle to achieve a quartz chamber, grounded stainless steel sample holder ("a cradle") with a stainless steel plasma shield in by spectrally calibrated equipped with a solarisation in the middle of the quartz chamber above the ms to 10 s depending on and divided by the integration time to measured without plasma and subtracted. The ungsten halogen lamp and a white

Results and discussion
Each gas has a specific plasma colour identification, see figure 2. Hydrogen (H β ), and violet 434 nm (H γ ) spectral lines in the visible part of the emission spectrum are easily recognizable as Balmer series of atomic hydrogen H ionization cross section, high degree of ionization in plasma and the most intense emission the near infrared region at 764, 810, due to the lack of strong emission lines in the visible region. atomic oxygen appear in near infrared molecule is very high (9.8 eV) Nitrogen plasma has an orange colour for identification of possible plasma the reactor, which may be the case of plasma colour. Therefore, OES can be used as a Hydrogen plasma has a magenta colour. The red 656 nm (H ) spectral lines in the visible part of the emission spectrum are easily recognizable as Balmer series of atomic hydrogen H [10]. Argon has a very large electron impact on cross section, high degree of ionization in plasma and the most intense emission 764, 810, 841, 842, and 912 nm. Oxygen plasma has a pale white colour strong emission lines in the visible region. The characteristic in near infrared region at 777 and 845 nm. The dissociation eV) and therefore the dissociation ratio of N 2 is low in colour with typical diatomic emission bands [12] plasma contamination, such as leakage of N 2 from the atmosphere into tor, which may be the case of H plasma in figure 2.
OES spectra of O, N 2 , H, and Ar plasma measured at the same pressure 20 Pa and W, except of Ar were rf power had to be reduced to 50 W to avoid saturation of a fingerprint for gas The red 656 nm (H α ), blue 486 nm ) spectral lines in the visible part of the emission spectrum are easily large electron impact on cross section, high degree of ionization in plasma and the most intense emission peaks in has a pale white colour characteristic emission lines of The dissociation energy of N 2 s low in the ICP [11]. [12]. OES may be used from the atmosphere into and Ar plasma measured at the same pressure 20 Pa and to avoid saturation of  [13]. The main mechanism for dissociation of H 2 molecules into H atoms is the impact excitation due to inelastic collisions of H 2 molecules with hot electrons e, see equation (1) [8].
The addition of Ar into hydrogen plasma induces several effects related to the high degree of Ar ionization. Relatively heavy Ar + cations cause heating of the sample by ion bombardment. The ionization increases the density of plasma electrons and related electrical conductivity of plasma. On the other hand, hot electrons are cooled via inelastic collisions with heavy Ar atoms, see equation (2).
The other effect is a series of resonant energy transfer channels from excited Ar * atoms to H atoms, see equation (3) [14]. * + ↔ + * For the purpose of our OES study, the constant 2 sccm Ar flow was mixed with the H 2 flow that varied between 5−50 sccm. The total gas pressure increased from 12 Pa for 5 sccm H 2 flow to 48 Pa for 50 sccm H 2 flow. The non-linearity of total pressure with gas flow was caused by low pumping speed at low pressure that varies with species causing difficulty in estimation of partial pressures in a mixture of gases with dissimilar molecular weights. Figure 3 shows that the OES intensity increases with rf power but the increased pressure significantly reduces OES intensity of both species. The OES intensity is quenched at higher pressure due to the increased collision frequency and the non-radiative transition of electrons from excited states. Therefore, the degree of H 2 dissociation cannot be directly evaluated using H emission line intensities alone. Actinometry had been proposed as a method for evaluating the relative degree of molecule dissociation independently of optical emission quenching effects at higher pressure [15]. In our case, the relative concentration of atomic H radicals was calculated from the integral ratio of OES intensities of H 434 and Ar 751 emission lines. The lines were chosen because of the comparable excitation threshold energies of the excited states (13.06 eV for H 434 and 13.48 eV for Ar 751 ) relative to their ground state [16]. Prior the intensity integration, the broad band background emission was digitally subtracted from emission spectra. Figure 4 shows that the H 434 to Ar 751 intensity ratio and therefore the degree of H 2 dissociation increases sub-linearly at low rf power and saturates at higher rf power. These results suggest that the hydrogen molecular dissociation is more efficient at higher pressures when using higher rf power.

Conclusions
The optical emission spectroscopy (OES) provides information about chemical and physical processes occurring in plasma. OES is also useful to control of possible contamination of plasma. Actinometry is a relatively simple procedure to evaluate degree of molecular dissociation, designed to overcome the effect of plasma excitation efficiency from the OES data assuming that the optical cross sections of the inert gas and the active species are equal. OES spectra confirmed that the concentration of atomic hydrogen increases in ICP with rf power and gas pressure and saturates with a power threshold increasing with total pressure. Therefore, the ICP is an effective method of generating atomic hydrogen by dissociation of H 2 without exposing a powder sample to high temperature The plasma hydrogenation of NPs at room temperature will provide further adjustment of their properties avoiding thermally activated processes such as thermal expansion and diffusion.