Hydrothermal Process to Prepare Novel Phase Titanium Sub-Oxide Ti 6 O 11 from Nano Rutile Titanium Dioxide Particles with Different Autodave Reactors

Hydrothermal process method using different dimensions reactors with volume 100 ml (homemade) was employed to prepare titanium sub-oxide Ti 6 O 11 , where one gram of TiO 2 nanoparticles 30-50 nm and 3M (20 ml) of NaOH as suspension was used . The samples are characterized using X-ray diffraction, Raman spectroscopy, and Field Emission Scanning Electron Microscopy (FE-SEM). X-ray diffraction revealed the formation of sub-oxide titanium Ti 6 O 11 of triclinic structure with Magneli phase, when the temperature applied was 363K for 9h.While FE-SEM showed uniform hierarchical structures with planar grass-like shapes. A novel phase has been found from rutile titanium.


Introduction
The preparation of partially oxidized titanium dioxide by hydrothermal method is an important research aspect in recent times. Because of its nanostructures diverse and peculiar properties, it is used in many applications.
Crystal phase, particle size, and particle shape all influence the physical and chemical characteristics of TiO 2 [1,2]. Rutile, brookite, and anatase are the three crystalline phases of TiO 2. Thermodynamically, the rutile phase is stable, but brookite and anatase are metastable [3,4]. The TiO 2 anatase phase is widely considered to be the active phase in photochemical applications [5]. According to several observations in the literature, rutile TiO 2 is also more active in photochemical applications than anatase TiO 2 [6,7].
Magneli phases are titanium sub oxides with the general formula Ti n O 2n-1 (n is an integer between4-10) [8,9]. Ti n O 2n-1 is a highly conductive oxide with distinct physical, chemical, and optical properties that could be employed in photo catalysis, photovoltaics, fuel cells, data storage, and energy conversion devices because of its narrow band gap [10,11].
Magnéli [12,13] solved the structure of Ti n O 2n-1 in the 1950s. Which is identical to the edge shared TiO 6 octahedra chain structure, with the exception that sharing occurs in each octahedron due to Ti reduction and the consequent oxygen vacancies in Ti n O 2n-1 [12,10]. Due to their outstanding conductivity and extraordinary absorptivity of visible light, the titanium-oxygen system plays a vital role in the research of a material's non-stoichiometry and is employed in a variety of applications [14]. Magneli phase has been utilized as a counter electrode in solar cells and has shown to have a greater short-circuit current density than Pt [15,16]. Magneli phase is also corrosion resistant.
Controlling the crystallization, size, and shape of nanostructured TiO 2 is crucial for fabricating materials with the desired properties, and it is a hot topic of research. [17,18]. Although a direct correlation between surface and physicochemical qualities is not always possible, it has been shown that TiO 2 activity is influenced by crystal structure, surface condition, and size distribution [19,20].
Because differences in Raman spectra with the decrease of particles size may easily be identified, Raman spectroscopy is considered as a valuable tool for investigating the structural features of nanoparticles [21,22]. This technique provides valuable information on the surface and the effects of nanoparticles' finite size as new bands can appear or vanish, as particle size decreases due to an increase in the surface-to-volume ratio, or a broadening band can appear. Campbell and Fauchet reported that grain size effects produce large shifts and broadening of the Raman spectrum [23].

Materiel and Methods
Three stainless steel reactors (A, B and C) of 100ml capacity with dimensions [diameter: height]: A (55mm: 130mm), B (65mm: 100mm) and C (75mm: 80mm) (as shown in Figure 1) were designed and fabricated in our laboratory. Each reactor is composed of an inner Teflon autoclave positioned inside a stainless steel cylinder that can be tightly closed with a screwed cover. The cylinder acts as a furnace as it is surrounded externally by a heater of 300W power, and is supplied with a thermocouple for temperature control.    One gram of TiO 2 nanoparticles rutile (Purchased from Hongwu international Group) was added to 20ml of 3M NaOH at room temperature with continuous stirring until its colour changed to white, a process that required 20 minutes at the most. 20ml of the white suspension was put inside each autoclave reactor and heated to a temperature of 363k for 9 hours to prepare nano titanium sub oxide with Magneli phase. The product of the reaction in the autoclaves reactors was washed several times with ethanol and distilled water, dried for 30 min and filtered by Büchner funnel using filter paper (of pore size=200nm). Furthermore, the product was heated at 523k for 1 h for homogeneity and to remove any residual organic materials.
Structural properties of the prepared samples were studied by X-ray diffraction (XRD) analysis (Analytical X' Pert Pro/ United Kingdom, Raman spectroscopy (HORIBA XploRA PLUS/ Japan). In addition, the surface morphology of the prepared samples was studied with field emission scanning electron microscope (EBSD Instrument: ZEISS SIGMA VP./ Germany).

Results and discussion
Titanium sub oxide prepared from Rutile titanium dioxide by hydrothermal method was characterized by XRD. Figure 3 shows the XRD patterns of the Ti sub oxide prepared in the three reactors (A, B and C). The crystalline planes were found to be: for reactor A at ( , ( - It can be noticed that the XRD pattern of Ti sub oxide prepared in reactor C revealed the disappearance of the crystal plane (0 0 2). Scherrer equation was used to calculate the average crystalline size (C.S) of Ti sub oxide which where (16.7nm, 16.06nm and 19.7 nm), from reactors A, B and C, respectively (as shown in Figure 4 and the values are tabulated in Table1). The difference in the surface area of each reactor causes a difference in the crystal size of the prepared sample, as it is believed that the difference in the surface area is the cause of a difference in the crystal growth of the resulting sample due to the increase in the speed of movement of the suspended particles during the reaction inside the reactor by increasing the surface area of the liquid.
Applying heat and pressure to titanium suspension in the reactors caused the formation of titanium and oxygen of imbalance ratios. The production of sub-oxide titanium with the Magneli phase was specific to autoclave reactors A and B. This was confirmed by Raman spectroscopy.
Stabilized Magneli phase were materials characterized by high corrosion resistance in acidic and basic solutions, electrical conductivity and electrochemical stability are both high [8]. Comparing the Raman spectra of rutile titanium dioxide with those of the samples from the three reactors autoclaves, as illustrated in Figure 5, revealed that the product of the reaction of reactor B shows two prominent Raman lines at 413 and 575cm -1 comparable to the rutile Raman lines at 446.6 and 6 9.8 cm −1 . The shift of the position of Raman lines is due to a decrease in the ratio of oxygen indicating the presence of titanium sub-oxide. Hence, these results confirmed the formation of titanium sub-oxide with Magneli phase (Ti 6 O 11 triclinic). Therefore, the dimensions of autoclave reactor B (65mm: 100mm) are the best for the formation of Magneli phase Ti 6 O 11 triclinic.
The absence of these two Raman lines in the spectra related to autoclave reactor C and reactor A indicates a decrease in the ratios of oxygen bonds with titanium, which could not lead to the formation of the Magneli phase. These results agree with those of Parker and Siegel [24]. JCPDS card no:96-100-8196  The products (Ti 6 O 11 ) with hierarchical nanostructures have many important applications such as in support of the Pt catalyst in solar cells, organic degradation, coatings and supports in various electronic and optoelectronic devices. Titanium sub-oxides are expected to become important materials in the future [14] because of its excellent conductivity and excellent absorption of visible light [26].And the hierarchical nanostructures Ti 6 O 11 are also very important in various applications, because of its huge surface area to volume ratio [25].

Conclusions
The change in the dimensions of the reactors of the same capacity has caused variable forms of the reaction due to the different heat distribution and the change in reactor pressure. This resulted in the formation of type Ti 6 O 11 sub oxide titanium with Magneli phase. Because of the re-formation of oxygen bonds (Ti-O) in titanium for rutile TiO 2 in the reactor, type Ti 6 O 11 sub oxide titanium with Magneli phase of hierarchical nanostructures was produced.