Characteristics and Synthesis of TiO2 and B-TiO2 by Solvothermal Method

INTRODUCTION
Nanotechnology is commonly used by emerging technologies. Nanoparticle are small particles with average diameters of less than 100 nanometers. As the size of these materials approache the nanoscale, their properties alter. When compared to bulk materials, nanoparticles feature show a number of unique qualities. One of the most essential properties of these nanoparticles is their ability to recognize steric stabilization, which leads to an increase in particle stability in biological systems. This opens up the possibility of employing nanoparticles in drug delivery systems (DDS)[1,2]. TiO2 has been extensively explored as a popular n-type semiconductor photocatalyst due to its chemical, thermal, nontoxic properties as well as its inexpensive cost [3]. However, because of huge band gap values and high recombination rate of photogenerated charge carriers, it has low photocatalytic efficiency; which limits its large-scale uses in photocatalysis[4,5]. To improve photocatalytic activity, producing and developing doped TiO2 materials to promote charge separation efficiency and improve light absorption has been recommended. To improve the photocatalytic efficiency of TiO2, numerous modification procedures have recently been used, such as doping with ions (Cu, Ni, B) [6,7,8]. In order to ensure effective electron extraction and thus balanced carrier transport, the optimal state in TiO2 electron transport layer it is used  should have a high electron concentration as well as a high electron mobility; this according to the theoretical model of electrical conductivity.
Boron (B) doping has previously been shown to increase electron injection from excited dye molecules to the TiO2 photo anode, resulting in improved dye-sensitized solar cell power conversion efficiency. It was recently found that a small amount of boron doping could improve TiO2 carrier mobility and electrical conductivity [9]. As a result, B-doped TiO2 (abbreviated as B-TiO2) is known to be a suitable electron transport layer for Perovskite solar cells in order to concurrently improve power conversion efficiency and reduce hysteresis [10]. Due to high pressure/temperature inside the closed environment of the autoclave, solvothermal approach is a relatively simple and straightforward way to modify the morphology of nanoparticles [11-15]. Through our research, we expect to reduce the energy gap of titanium dioxide as well as improve the photoelectric and optical properties through boron doping in a simple and inexpensive way.

MATERIALS AND METHODS
Titanium oxide nanoparticles were synthesized by modified solvothermal methodat at different temperatures of  The solution was prepared by adding 500mg TiO2 bulk (99%, M.Wt=79.87g/mol, HIMEDIA) into 50 ml of 10 M sodium hydroxid (NaOH, M.Wt.=40 g/mol, 98%, ALPHA) aqueous solution. The suspension system was stirred for 1hour at room temperature to form a homogeneous solution. When doping with Boric acid () in the ratio of 5 wt%, the same method was used with addition of 25 mg of boric acid. Then, the mixture was transferred to the autoclave was lined with Teflon and then well-sealed with a stainless steel top with a stainless steel temperature probe. A thermocouple is put in a heating coil attached to a temperature regulating board in this probe. The autoclave was heated for 10 hours at temperatures of 70,100,130,160, and 190 °C (heating rate=4°C/min). The cell was then cooled in air to reach the room temperature. The solution was separated by centrifuge with a speed of 4000 rpm for 10 to 15 min, and then, the precipitates were collected. The collected powders (white to pale yellow and light gray) were washed with deionized water, filtered and washed repeatedly until reached the pH of (6.8-7.2). The products were then immersed in 300 ml of 0.1M Hydrochloric acid (HCl, 35.4%, M.Wt=36.46g/mol,CDH) solution for one hour, followed by filtering and repeatedly washed by deionized water until reached the pH of (6.8-7.2).  The washed samples were dried in oven at 80 °C for 24 hours. Finally, the powders were sintered in furnace for 4 hours at 400 °C. All samples were saved in a storage.

RESULTS AND DISCUSSION
UV-vis measurements
UV-vis measurements gave us an idea about the size of the particles, as well as the energy gap values, and thus gave us a preliminary idea of the optical and photoelectric properties of these materials.
According to  Tauc plot:  (αhv)n = K(hv-Eg)
In this equation, α is the absorption coefficient, hv is incident photon energy, K is energy independent constant, and E g is the optical band gap energy of nanomaterials, the exponent n indicates the essence of a transition [16].
At 70 ℃, the value of energy gap for  was 3.23 eV, while the value of energy gap for  was 2.94 eV. We noticed a clear decrease in the value of the energy gap in the presence of boron for the same temperatures. While at temperature of 100 ℃, the energy gap was 3.21 eV for , and 2.50 eV for .Again, we noticed a clear decrease in the value of the energy gap in the presence of boron for the same temperature, and the value of the decrease depends on the type of doping. At 130 ℃ the value of energy gap for was 3.11 eV, while the value of energy gap for  was 2.70 eV. At temperature of 160 ℃, the value of energy gap was 3.04 eV for , and the value of energy gap was 2.40 eV for . Finally, at temperature of 190 ℃, the value of energy gap was 3.01 eV for , while the value of energy gap was 3.04 eV for .. In general, the arising order of the energy gap values was We believe the reason for the apparent discrepancy in the energy gap values for B-TiO2 is due to several reasons: first, the metal oxides that have been doped with have energy gap values that are much lower than the energy gap value for titanium dioxide, therefore, the energy gap value is expected to decrease and the value of the decrease depends on the type of doped metal and the percentage of doping, secondly, one of the most important influencing factors is the metallurgy of each material or the crystal structure of the material, which depends mainly on temperature and pressure formation, as well as heating speed and cooling speed. 

XRD Crystallography
XRD crystallography gave us a lot of information about the nature of crystals, the most important of which are crystal size and shape, as well as distortions that may occur in the crystal structure.
From XRD pattern of B-TiO2 in Fig. 5, we noticed a large convergence at 70 ℃ with pure titanium dioxide at same temperature as shown in Fig. 4 i.e. convergence in values ​​of angles unit shoulders. This means that at low temperature, doping by boron with such a large difference percent not occur on the crystal shape or size, and through Scherrer equation calculations, it was found that the crystal thickness at 70 ℃ is equal to 15.575 nm, which means less than the thickness of the prepared crystals are at the same temperature in the pure state as shown in Table 2.
As increasing temperature and when it reaches 130 ℃, we noticed an increase in the values of the angles and a small change in the positions with an increase in width of the shoulders and a large decrease in the absorbance. This means a significant decrease in the crystal size, which could be reflected on the optical properties of TiO2 as a semiconductor. The size of crystals reached 6.887nm, which means smaller than the thickness of the crystals prepared at the same temperature at the pure state, see Table 2.At temperature of 190 ℃, it is noticed that the shoulders are sharper and the intensities are higher than 130 °C, which means that the crystals sizes increased, but it remains less than those prepared at 70 ℃. When compared with the same temperature in the pure state, the intensities are more intense and the shoulders are sharper. This gives an idea that the crystal size is larger. From Scherrer equation, the thickness of B-TiO2 crystals at 190 ℃ equals 9.79nm, which is greater than those prepared at 130 ℃ andat190 ℃ in the pure state[17,18]. , see Table 2.

Field Emission Scanning Electron Microscopes (FE-SEM)
The importance of FE-SEM images is to display three-dimensional images of the samples, and this gives us an idea of the shape and dimensions of the nanoparticles.
From Fig. 6, we noticed that at temperature of  the sizes of the nanoparticles of B-TiO2 are smaller than the sizes of the pure state, as shown in the table 3, and the shapes of the particles are a nanodots for both of them.
From Fig. 7, we noticed that at temperature of  the size of the nanoparticles of B-TiO2 are bigger than the sizes of the pure state, as shown in the table 3, and the shape of the particles are nanorods for both of them.
While from Figure 8, we noticed that at temperature of  the size of the nanoparticles of B-TiO2 are bigger than the sizes of the pure state, as shown in the table 3, and the shapes of TiO2 particles are nanorods and B-TiO2 are nano flowers and nanotubes.
According to Table 3, the highest particulate thickness was recorded at  for B-TiO2, while the lowest nanoparticles size was recorded at  in B-TiO2, But the order of the sizes was generally from top to bottom B-TiO2>TiO2 [19]. 

CONCLUSION
The properties of titanium dioxide were found to be improved with increasing temperature the energy gap values increased as the size decreases, which means an increase in the surface area.  Also, the crystal size decreases in the presence of boron, and the shape of the nanoparticle’s changes depending on temperature, pressure, type and percentage of doping and the period for manufacturing. We cannot say which sample is better because it depends on the need of the users and the applications.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.