THE PHYSICO-CHEMICAL PROPERTIES OF Ті-CONTAINING STAINLESS STEEL COMPOSITES AND ITS PHOTOACTIVITY

The present study aims to obtain the supported Ti-containing catalyst on a surface of stainless steel foil by low temperature ionic implantation method. The geometrical sizes of the implantation chamber allow one to synthesize composites with the maximum size of 30×30 cm. The shape and size of the catalysts provide an opportunity to use obtained samples for removal of harmful substances from both aqueous solutions and gas mixtures. A quantitative estimation of bond strength of the surface layer of the prepared composites was realized by the use of sclerometric method. It is shown that implantation of titanium ions on stainless steel foil provide significant increase in surface layer mechanical strength. The heat treatment of the sample leads to a decrease in this strength, but its value rests higher than that in initial support (stainless steel). The composition of samples surface and the effect of calcination temperature were determined by XRD, SAXS, SEM, AFM, and XPS. It is shown that after ionic implantation nanoscale layer of the implant on the surface of stainless steel was formed. Presence of titanium oxide, nitride and oxynitride was determined by XPS method. The high photocatalytic activity of this catalyst in the process of neutralizing benzene in aqueous solutions under irradiation with visible light, which significantly exceeds its activity under UV-radiation was shown. Increasing the calcination temperature leads to decreasing the samples activity and can be explained by the influence of the ratio between the nitride, oxynitride, and oxide phases of titanium. Exactly the presence of those phases on the surface explains its high activity in the degradation of benzene in aqueous solution under visible light irradiation. Thus, using of the obtained samples in the neutralization of aqueous benzene solutions under visible light irradiation is perspective from ecological point of view.


INTRODUCTION
Worldwide industrial revolution in the 21st century brought a wide spectrum of problems, mainly contamination of the water with harmful and waste materials, leading to significant adverse effect on the environment and wildlife. Direct disposal of industrial compounds into water makes its unsuitable for drinking and for other purposes. Particularly, nonbiodegradable and undesirable chemicals have negative consequences on health of human and aquatic life [1]. Among those chemicals aromatic hydrocarbons (phenol and benzene, particularly) are the most prevailed contamination entering on the environmental with wastewater of paint and varnish, refinery and chemical-recovery industries [2]. Also its can be generated during the decomposition of pesticides, fungicides, and herbicides. Chlorination is the one of the most used method on water-treatment plant. But this stage leads to formation cancerogenic chlorinated substances [3]. The new law about reception of wastewater to drainage system was implemented since January 2018. Rigid requirements to aromatic hydrocarbonic contents were accepted [4]. The known methods for benzene recovery provide its precipitation or sorption removal by solid sorbents. This operations lead to an increase of the cost of purification treatment [5]. The aromatics photodestruction with the use of heterogeneous catalysts can be the alternative method of benzene removal from water. It is known that titanium dioxide is one of the most studied and active photocatalyst [6]. The main disadvantage of this catalyst is absorption only UV-irradiation (i.e., wavelength < 388 nm) [7]. For the purpose of activity shift to the visible radiation range the intensive researches on TiO 2 doping by various elements including nitrogen are carried out [8][9][10][11]. On the other hand, the use of ТіО 2 in the form of fine powder is a technological drawback. The use of such form of ТіО 2 leads to a problem of spent catalyst removal from the reaction aqueous mixture [12].
The solve of this problem may be related to the use of various supported TiO 2 -containing catalysts [13,14]. The known supported catalysts have a number of disadvantages the causes of which are also primarily complicated methods of their synthesis and difficulties with preparation of the durable and stable TiO 2 coatings. Therefore, the photocatalysts deposited on supports are promising and for the preparation of the supported samples both mechanically strong granules and hard flat profile composites can be used. In the paper the results of the ionic implantation method use for the preparation of TiO 2 -containing photocatalyst on the base of stainless steel foil as a support are reported.

MATERIALS AND METHODS
Synthesis of Ti-containing composites supported on stainless steel (SS) AISI 321H (thickness 80 μm) was carried out by ionic implantation method described in [15]. Metallic Ti was used as an implant. The cathode sputtering of the target (Ti) was carried out by N 2 ions. The energy of implantation was 20 keV at a fluence of 5×10 17 ions/cm 2 . The camera for implantation allows one receiving samples with the maximum linear sizes 30×30 cm. The prepared Ti-containing foils (Ti/SS) were calcinated in air at the temperature range of 200-600 °С and determined as Ti/SS/T °C. X-ray diffraction (XRD) patterns were obtained with a Philips PW 1830 diffractometer (monochromatic CuK α -radiation). Phase composition of the surface layer was studied by the method of low-angle X-ray scattering (SAXS) with a 2D X-ray Rigaku (Dmax rapid). The surface morphology of the samples was studied by atomic force microscopy (AFM) with a Nanoscope Multi Mode V and by canning electronic microscopy with a «Hitachi S-400» (SEM). The surface composition of the samples was obtained by X-ray photoelectron spectroscopy (XPS) with a SES R 4000 (Gammadata Scienta) instrument. Distribution of elements through the depth of composite was carried out by the XPS method (a VG Scientific ESCA-3) by step-by-step removal of surface layers (argon bombing).
The quantitative estimation of the bond strength of the surface layer of the prepared composites was realized by sclerometric method [16].
The activity of synthesized samples was determined in the reaction of photodegradation of benzene in aqueous solution (50 threshold limit values (TLV)). The study was carried out in a cylindrical reactor (diameter was equal to 9 cm) with wall-placed catalyst (synthesized composite) which has height equal to 10 cm (implantation of titanium was effected on both sides of the foil). The thermostatically controlled radiation source was immersed into the reactor. The design of the reactor, the shape and size of the catalysts provide an opportunity to use received samples for removal of harmful substances from both aqueous solutions and gas mixtures ( Fig. 1). The high pressure mercury or sodium lamps were used as a source of radiation. The reaction products were analyzed with a SelmiChrom-2 gas chromatograph equipped with a FID on a stainless steel column (length 1 m, diameter 3 mm) filled with Porapak Q.

RESULTS AND DISCUSSION
The mechanical strength of the active components surface layer and the composite is one of important properties of catalysts used in aqueous solution for their utilization on practice. The data presented in Table 1 show that implantation of titanium ions into stainless steel foil provide a significant increase in surface layer mechanical strength. The heat treatment of the sample leads to a decrease in this strength, but its value rests higher than that in initial support (stainless steel). Thus it is possible to claim that the ionic implantation method allows receiving mechanically strong layer of active component on the support surface with strength which is practically not influenced by heat treatment and mechanical deformation. It actually permits us to use prepared composites in reactors of various shapes and size.
An analysis of synthesized samples by XRD method (Fig. 2) shows that they contain only the reflexes of the planes (111), (200), and (220) of the austenite phases, which can be referred to the stainless steel [17]. The heat treatment of the samples (up to 600 °C) does not change the structural state of the surface.  The absence of the reflexes of implanted titanium or its compounds may indicate their amorphous state or a low concentration in the surface layer of the composite. Therefore, the samples were studied by SAXS method (Fig. 3).
An analysis of obtained results shows that in this case there are only austenite phase reflexes on XRD and SAXS patterns. Nevertheless, it should be noted that implantation of titanium ions leads to low-angle shift of all reflexes of austenite that can be seen from the given example of a reflex from plane (111). This fact can indicate the implant ions incorporation into a support lattice and its corresponding broadening.
The obtained results demonstrate that after ionic implantation on a support surface titanium amorphous structure are formed or concentration of its crystal particles is so small that is out of detection limits by these research techniques.
According to the AFM data titanium implantation leads to smoothing of a relief of initial support surface, but at the same time new narrower surface defects (depth of 200-400 nm) are formed that causes its larger inhomogeneity (Fig. 4). The head treatment of the sample is followed by new peaks formation in the existing hollows. As a result, an insignificant change of surface irregularities with its rather big average roughness is observed.
The SEM data (Fig. 5) demonstrate that in samples calcinated at 200 and 300 °C the surface irregularities, characteristic for initial support, are observed. It may indicate formation of a nanoscale layer of implanted titanium on the support surface.
It is also possible to note an emergence of new surface defects that has good accordance with the results obtained by AFM data. An increase of the treatment temperature leads to the subsequent agglomeration of the part of supported titanium layer with formation of new spherical particles.
Results of the XPS characterization of the prepared composites are summarized in Table 2. The obtained data show that the ionic implantation of titanium leads to formation of titanium nitride (the peaks of electrons with binding energies: Ti2p 3/2 -460.3 eV, N1s -396.1 eV) and titanium oxynitride (the peaks of electrons with binding energies: Ti2p 3/2 -457.9 eV, N1s -400.0 eV, O1s -527.6 eV) on the sample surface. After sample heat treatment the peaks which can indicate the existence in surface of the composite both oxynitride of the titanium (BE Ti2p 3/2 = 458.1, N1s = 400.4, O1s = 528.0 eV) and its oxide (BE Ti2p 3/2 = 459.1, O1s = 529.7 eV) are identified. These results permit us to assume that heat treatment leads to oxidation of titanium nitride to its oxynitride, and titanium oxynitride to TiO 2 .  The XPS method with the removal of surface layers (surface bombing by argon) was used for the study of implant incorporation depth in the support and the determination of its surface layer thickness (Fig. 6). The obtained results show that thickness of the implant layer on support surface is about 80 nm and depth of titanium intrusion in foil near 30 nm. It permits to assume that the formation of an intermediate layer with implant introduced in stainless steel determines the high mechanical strength of the composite.
The activity of the synthesized samples was investigated in reaction of photodegradation of benzene in aqueous solution (Fig. 7). The presented results show that the activity of practically all synthesized samples at visible light irradiation is significantly higher than at UV irradiation (conversion up to 25 and 7 %, respectively). In the case of UV-irradiation the most active catalyst is the initial implanted sample Ti/SS. The heat treatment reduces the catalyst photoactivity. This fact can be related to the existence of surface initial implanted sample of the titanium oxynitrude and its nitride. It is known [19][20][21] that titanium oxynitride demonstrates high activity in photocatalytic processes. The heat treatment leads to TiO 2 phase formation and it has negative effect on samples properties.  It is possible to propose a hypothesis that the high activity of the synthesized samples is caused by presence of titanium nitride and oxynitride on the support surface. A hypothesis of active phases on the surface of support, which ensures its high activity in the reaction of photodegradation of benzene in aqueous solution under visible light irradiation is proposed.
A perspective is shown of practice use of the obtained samples in the process of removal of benzene from its aqueous solutions with visible light irradiation, which today is very important from the ecological point of view.