Deposition of TiO2 Nanoparticles by Means of Hollow Cathode Plasma Jet

R. Perekrestov, P. Kudrna, M. Tichý Charles University in Prague, Faculty of Mathematics and Physics, Prague, Czech Republic.

Abstract. TiO2 nanoparticles have been investigated in this work. Nanoparticles were obtained in Ar plasma on monocrystaline Si(111) substrate by gas-phase deposition using hollow cathode plasma jet (HCPJ). The material of a cathode is pure titanium. Cathode’s surface has a high affinity for oxygen and thus an oxide layer rapidly forms upon exposure to the atmosphere or introducing oxygen into the main chamber. Given method is based on sputtering TiO2/Ti2O3/TiO layer of a hollow cathode. The explanation of nanoparticle growth mechanism and size distribution is given. Morphology of thin film surface was investigated by means of scanning electron microscope (SEM) and atomic force microscope (AFM). We used mass spectrometer to monitor a chemical composition of the gas inside the system during deposition. The chemical composition of the thin films was investigated by means of energy-dispersive x-ray analysis (EDX).

Introduction Synthesis of nanoparticles is a basis of nanotechnology nowadays. In a general case particles with a size

from 1 to 100 nm are called nanoparticles. The application of nanoparticles is rapidly growing in nanoscale science and engineering. Such properties of nanoparticles as surface to volume ratio, quantum confinement and surface atom arrangement are of a great importance. It means that by controlling the size of nanoparticle we can obtain new properties of material.

During the last years a lot of techniques for nanoparticles synthesis have been developed. The most widely spread techniques of gas-phase nanoparticle synthesis using evaporation of solid material are: pulsed laser ablation, spark discharge generation, inert gas condensation, ion sputtering. The means of gas-phase methods are based on achieving supersaturation necessary to start the nucleation of nanoparticles [Mark et al., 2003], [Hosokawa et al., 2007].

This article deals with ion sputtering deposition of TiO2 nanoparticles by means of hollow cathode plasma jet. TiO2 nanoparticles find their application in different fields of technology mostly because of their photocatalytic properties. They are used in environmental applications for photocatalytic treatment of wastewater and pesticide degradation. Nanoparticles of TiO2 dissociate the water molecule to produce hydrogen. In industry it is used for dye producing [Gupta et al., 2011]. As for crystalline structure of TiO2 it has three polymorphs: anatase (tetragonal), rutile (tetragonal) and brookite (orthorhombic). Experiments have shown that using TiO2 as photocatalyst is more effective in a form of nanoparticles than bulk material [Gupta et al., 2011].

The technique of HCPJ is based on sputtering of the inner part of the titanium cathode by means of glow discharge [Kudrna et al., 2010]. In comparison with more commonly used magnetron sputtering it has some significant advantages in producing of nanoparticles. A velocity of nanoparticles can be controlled by the flow rate of working gas and subsequently, we can control the sizes of nanoparticles by adjustment of the flow [Sakuma et al., 2006]. The space inside of hollow cathode serves as a place of nanoparticle nucleation because of a heightened pressure. The last important advantage is possibility of manipulation with the position of plasma jet under vacuum.

Experimental setup The scheme of HCPJ is shown in Fig. 1 [Kudrna et al., 2010]. Sputtering element (cathode) has cylindrical

shape with a hole inside through which the gas is flowing. It is powered by negative DC power supply. The common values of current and voltage are hundreds of mA and V respectively. The titanium nozzle is cooled by the water flow via the copper blocks surrounding it. The covering made from the Lava ceramic is used to isolate the cathode from the rest of the system. We usually use a pressure from 0.2 up to hundreds of Pa to sustain the discharge. Ultra high vacuum inside the chamber is achieved by the turbo-molecular pump which ultimate pressure is 10–5 Pa. Mass spectrometer (MS) is mounted to the side of main chamber to monitor the composition of gases. It is possible to adjust the position of the MS orifice to increase or decrease the signal. Load lock system provides us the manipulation with samples. It consists of the small vacuum chamber connected to the main system via two gate valves and equipped by the moveable feedthrough with “fork” at the end which holds the substrate holder and sets up the position of the substrate.

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PEREKRESTOV ET AL.: DEPOSITION OF TiO2 NANOPARTICLES BY PLASMA JET

Ar

Cooling water

Insulator

Ti nozzle

Copper block

Plasma jet

DCpower supply

DC-

+

Figure 1. The scheme of HCPJ.

Experimental results The system was pumped down to the base pressure of 10–4 Pa before start of the nanoparticle deposition

process. Each sample was prepared on the silicon (111) substrate. We cleaned the substrate with alcohol to remove all the visible impurities of the surface and then it was attached to aluminum tablet (substrate holder) and inserted inside the system by means of load lock. Load lock system provides us the manipulation with samples. The cathode was cleaned by the 60 sccm argon flow for 15 minutes to remove some condensated impurities. Then we adjusted the flow rate to a required value and ignited the discharge. In the most of experiments the flow of 30 sccm was used. We usually let the discharge run for 5–10 min to stabilize itself. The composition of gases was monitored by EQP500 mass spectrometer. After that, the substrate was introduced into a center of the discharge region to start the process of deposition. The composition of the sample was measured by means of EDX. The results of EDX measurements are present in Table 1.

Table 1. Chemical composition of the sample measured by means EDX. Element Percentage, %

O 26.91 Ti 68.99 Si 4.11

The most abundant elements are Ti, O, and Si. The main source of oxygen is passive TiO2/Ti2O3/TiO layer

on the top of the cathode. The thickness of this layer is approximately 0.1 µm taking into account porous plasma etched surface of the cathode [Gemilli et al., 2003]. Subsequently, this technique cannot be used for producing of thick nanoparticle layers. According to a percentage of O atoms we can assume that the surface of the substrate is covered by SiO2 layer. Several samples were prepared to trace the dependence of the nanoparticle growth on deposition time. The AFM scans of the films are shown in Fig. 3. From Fig. 3 it is apparent that nanoparticles increase not only their density but the dimensions. Although the process of deposition was adjusted to create the same conditions for each sample, at the initial stage we had a higher power of the discharge (power supply operated in current stabilizing mode). After 5–20 minutes it falls to a constant value. Heightened power leads to higher temperature in the discharge region. We assume it is the reason of such a difference in dimensions of nanoparticles. This question may be solved by investigating the profile of a relatively thick nanoparticle layer by means of microscope.

The results of 10 minutes deposition are shown in Fig. 5. Several huge particles are surrounded by a dense layer of smaller ones. Using the profiles of nanoparticles it is possible to measure their dimensions. The conditions and dimensions of nanoparticles are present in Table 2. The dimensions of smaller particles are in a range from 10–25 nm. It is hard to measure them more accurate because they create a porous system

The spectra of gases during the deposition are shown in Fig. 4. The small step is the start of deposition. Mass spectrometer worked in positive ion detection mode.

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Figure 2. AFM scans of deposited nanoparticles. In each experiment we used the same configuration of parameters: Pd(deposition pressure) = 0.3 Pa; fAr(argon flow) = 30 sccm; distance between HCPJ outlet and substrate = 20 mm; I(discharge current) = 300 mA; t (deposition time) = a) 15 sec, b) 30 sec, c) 1 min, d) 2 min.

00:00 00:30 01:00 01:30 02:00 02:30101

102

103

104

Ti Ar36 O2 TiN N2 TiO TiC TiO2I [

c/s]

t [min:sec] 00:00 00:30 01:00 01:30 02:00 02:30

101

102

103

104

Ti Ar36 O2 TiN N2 TiO TiC TiO2I [

c/s]

t [min:sec]

00:00 00:30 01:00 01:30 02:00 02:30 03:00 03:30101

102

103

104

Ti Ar36 O2 TiN N2 TiO TiC TiO2I [

c/s]

t [min:sec]00:00 00:30 01:00 01:30 02:00 02:30 03:00 03:30 04:00

101

102

103

104

I [c/

s]

t [min:sec]

Ti Ar36 O2 TiN N2 TiO TiC TiO2

Figure 3. The spectra of gases inside the system during a deposition. a) t = 15 sec; b) t = 30 sec; c) t = 1 min; d) t = 2 min. Each spectrum correspond to the appropriate sample in Fig. 3. The rapid fall on each graph corresponds to opening of the shutter which separates a sample from the discharge region.

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Both small and big particles have uniform size. It means that single atom attachment is dominant in the process of nanoparticle growth in comparison with the process of coagulation. The question arises, what is the reason of existing two different sorts of nanoparticles? It might be explained by a change of one or several parameters in a process of cathode sputtering. There is no visible change in the composition of gases during the deposition accordingly to measured spectra, but more accurate measurements under the higher pressures in the main chamber showed a decrease of O2 and TiO2 concentration in time. Second important parameter is the temperature inside the cathode. It decreases during the sputtering of TiO2 layer because of decrease of energy loses to sustain a constant value of the discharge current. The process of growth begins when the sputtered atoms of the cathode collide with each other to create nanoparticles. Atoms of working gas take away the extra energy of initial nanoparticles to let them continue their growth. The gradient of temperatures between the cathode and substrate plays the crucial role in a process of nanoparticle growth. In a case when the difference in temperatures is high enough to cool nanoparticles (saturation is high) they easily reach the substrate. In another case the growth of particles is hampered (saturation is weak) because the probability of nucleation is in a same order as probability of decay. These processes are studied in books [Hosokowa et al., 2007] and [Woodruff et al., 2007].

Table 2. The settings used for nanoparticles deposition and approximate nanoclusters size. Discharge current,

mA

Discharge voltage,

V

Ultimate pressure,

Pa

Deposition pressure,

Pa

Argon flow,

sccm

Nozzle- substrate distance,

mm

Deposition time,

min

Height of bigger

nanoparticles, nm

Diameter of bigger

nanoparticles, nm

300 332–320 1.6∙10–4 0.3 30 20 10 20 100

Conclusions TiO2 nanoparticles found their application in different spheres of industry and science mostly because of the

photocatalytic properties. HCPJ gives us an opportunity to obtain the nanoparticles of spherical shapes and uniform sizes.

A lot of parameters make an impact on the process of the nucleation and growth of nanoparticles. The most important are: temperature gradient between substrate and discharge region, concentration of oxygen, current of the discharge, flow of working gas. The process is complicated by the fact that varying one parameter we also change another one. Temperature gradient between substrate and discharge region plays an important role in a creation of supersaturation which is the necessary requirement for nucleation of nanoparticles in a gas phase. It depends on the distance between substrate and discharge, thickness of TiO2 layer on the cathode and discharge current. The main source of oxygen is oxidized layer on the top. It is also might be present in the main chamber in a form of residual gas. Current of the discharge affect several parameters at once. It defines the ionization of plasma, temperature and sputtering rate. The gas flow rate directly increases or decreases the time that the particle needs to reach a substrate.

Figure 4. AFM scan of nanoparticles 100 nm in width surrounded by porous structure of smaller (≈ 10–15 nm in width) particles. The profiles of the nanoparticles are shown in the right part of the figure.

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A lot of questions are still opened and need to be solved. The prediction and control of surface morphology are the main aims of our further investigation. In our plans, there is also trying to use RF and DC pulsed power supply.

Acknowledgments. The partial financial support by Czech Science Foundation, the grant No. P205/2011/0386 and by

Charles University Grant Agency, grant No. 120510 and No. 604612 is gratefully acknowledged. The work belongs to the studies performed in frame of the CEEPUS III project AT-0063.

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