Investigation of the Characteristics of the Prepared TiO2/Graphene Films by Spin Coating Method

INTRODUCTION
Titanium dioxide (TiO2) is one of the transition metal oxides, which has perfect optical properties in the ultraviolet (about 3.7 eV) [1], however, the key challenge that most significant challenge that creates limitations in optical applications is the fast recombination of the photogenerated e-h. Thus, the researchers proposed several mechanisms; (i) the transition metal doping such as Ni, Co, Fe, Mo, Nb, and Ru [2]. Ions of these metals may create active traps leading to increasing the recombination that will be affecting the photocatalytic efficiency. (ii) Noble metallic nanoparticles such as gold, copper, and silver [3], in this case, the photogenerated electrons are likely to be attracted to particle surfaces which makes recombination so fast. (iii)  The mechanism with the best results was to provide a transition surface for the excited electrons. Graphene was the most potential candidate [4]. 
Graphene has unique physical properties; high thermal conductivity [5] ، high charge carrier mobility [6,7] and optical transmittance of almost 97% [5] with about 0.1% of reflectivity [6][8]. In general, for TiO2/ Graphene (TiO2/ G) composite, the transition of excited electrons depends on the presence of UV or visible light and oxygen findings that are reduced by graphene as explained in Fig. 1 [9] [10][11].
In the recent decade, TiO2/ G composite was prepared via several methods for photocatalytic degradation, photocatalytic, gas sensing, and self-cleaning applications, as explained in Table 1.
In this work, we present a discussion of the characteristics of TiO2/ G composite films that were   Prepared via the spin coating method. 

MATERIAL AND METHODS
Materials
The spherical TiO2 nanoparticles (Anatase) (TiO2, 99.5%, 10-30 nm) and ethanol (99.9%) were supplied by Sigma Aldrich. Whereas, the nanosheets graphene (G, 15 micros) with a platelet morphology were obtained from (sky spring Nanomaterials).

Synthesis of TGr composite films
Synthesis of TiO2/ G composite, a mixture of 0.02g of graphene, 1g of TiO2, and 4 ml of ethanol (99.9%) was sonicated for 20 min at room temperature. Then, the produced solution was pleased into an autoclave with adding of 50 mL of distilled water (DW). So, the reaction has been achieved under 160 °C for 72 h, then gradually cool down to room temperature, filtered, and washed with DW and ethanol several times directly. Then, dried overnight at 40 °C. TiO2/ G composite powder (0.5g) was added to 30ml ethanol on a magnetic stirrer until complete dissolution. TiO2/ G composite films were synthesized by spin coating with 2650 rpm of speed for 10 minutes, 1000 rpm/min of acceleration, and adding 500 μL of TiO2/ G composite solution (drop by drop). Then the synthesized films were treated thermally employing the thermal steps protocol for every four layers. Then, were leavened to cool gradually to room temperature throughout the night-down. Table 2 represents the thermal steps protocol.

Characterizations
TiO2/ G composite films have been characterized structurally by utilizing an X-ray diffractometer (XRD) with 4 degrees per minute of scan speed, Cu Kα1 radiation (λ=1.54060 Å), 30 kV, and 10 mA), and morphologically via studying transmission electron microscopy (TEM) and scanning electron microscopy SEM images. In addition, utilizing UV–Vis spectroscopy to determine the optical properties.

RESULTS AND DISCUSSION
X-ray Diffraction
Fig. 2 illustrates the XRD patterns of the prepared films on substrates of glass (one layer) and FTO glass (1, 2, 3, and 4) layers. However, the deposited films on the glass substrate showed a polycrystalline structure of nanocrystalline, anatase (TiO2) with a dominant phase at 2θ=25.23(011) according to ICDS 98-009-2363 cared [23] and crystalline size of 38.6 nm. In the same position, the dominant peaks emerged with a blue shift with (Δ2θ ≈1.3o) of average in all (1, 2, and 3) layers and appearing of SnO2 at 2θ ≈33.893o and 61.872o with (310) (101) respectively according to ICDD 00-041-1445 card that points out to FTO. While for four layers on FTO glass, the dominant phase emerged at 2θ = 37.89o (004) with 36.92 nm of crystalline size.  In general, the increasing of layers and the heating treatment affected the crystalline size of that deposited on FTO glass, where swing between the two values (27 and 36) nm. We believe that the annealing is responsible for the displacement of the TiO2 peaks as a result of the acquisition of high kinetic energy by both graphene and titanium oxide particles, which primarily led to the oxidation of graphene.

TEM and SEM images
TEM image of the stripped-off particles of the deposited film on the glass substrate shows the spherical clusters with 89.15±37 nm of the average size of clusters that consist of spherical particles with 20.8±6.5 nm of the average size of particles (Fig. 3a and b). The selected area electron diffraction (SAED) confirmed the crystallization indicated by XRD analysis in Fig. 2 as shown in Fig. 3c. For the deposited film on the glass substrate, the SEM image (Fig. 4) showed both shapes and types of particles formed, which it is composed of polygonal particles with (78.00±60.62) nm in a dark appearance. Conversely, the bright particles consist of needle shapes and big particles that are believed to be graphene particles.
Fig. 5 illustrates the surfaces of the deposited films of (1, 2, 3, and 4) layers on FTO glass substrates. It is clear that there is a difference between the surfaces of the films according to the number of layers that attributes to two causes; change in the roughness of the surface with the accumulation of layers, adding to the heat treatment sequence. In addition, it is noticeable that the number of layers affects the values of all the average particle size, shape, and porosity, where the average particle size increases depending on the number of layers (19±17.9, 20.55±20.8, 50.3±23.6, 63.21±28.1) nm. Nevertheless, the surface porosity fluctuated (62.78, 73.43, 48.44, 66.75) % respectively. It is worth noting also, it was observed in Fig. 5 (a and b), that there are cracks believed to be due to the difference in the tension coefficients between the coated TiO2/ G composite layer and the substrate.
The thickness was determined using the cross-section images of the deposited films. Where increase with the increasing number of layers (7.53±1.86, 14.38±1.89, 314.62±31.53, 702.78±43.48) nm for (1, 2, 3, and 4) layers, as shown in Fig. 6.

Optical properties
Fig. 7 shows the transmittance of the prepared films one layer (1L), two layers (2L), three layers (3L), and four layers (4L) respectively for (300 to 1000) nm. It is evident that the absorption coefficient (α) is greater than 104 cm-1 (as shown in the mini figure in Fig. 7), according to equation (1), which refers to having a direct optical gap [24]. 

where (A) is the absorbance and (d) is the thickness of the film. Due to the relative height optical bandgap (Eg), was calculated according to equation (2)[25].

Accordingly, that resulted to have a height transmittance. In general, the transmittance behavior was enhanced with the increase of the layers number increasing, which increased from 2.21% at 375 nm of wavelength for 1L to (27.9, 24.15, and 20.75) % for 2L, 3L, and 4L respectively. While it increased from 6.57% for 1L at 550 nm of wavelength to (55 – 62.5) % as for 3L and 2L respectively. 
There is a noticeable change in the energy gap value between it in 1L and other layers. As well, increasing in the layers and the thermal treatment did not noticeable effect on the energy gap despite the obvious fluctuation in porosity values, where, they were (2.38, 3.01, 2.99, and 3.06) eV for 1L, 2L, 3L, and 4L respectively as shown in Fig. 8. 
 
CONCLUSION
TiO2/graphene films were successfully prepared by utilizing a combination of both hydrothermal and spin coating methods, with thermal treatment for several layers. Both increases in layer number and the thermal treatment enhanced the prepared films’ optical properties, which could qualify these films as effective layers in solar cells and photovoltaic applications. 

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