The Titania, existing in rutile or anatase phase, is considered an important material for different applications because of its characteristic of photoexcitation phenomenon. Among the applications, the efficiency of photovoltaic solar cells and photocatalytic devices could be improved if the band gap of titanium oxide layers could be reduced   . Titanium oxide (TiO2) has excellent chemical stability, mechanical hardness and optical transmittance with high refractive index. It has attracted great attention these years and it is widely used in solar cells  and other areas.
Originally, the Titania has a band gap more than 3 eV. The lowering of the band gap is essential for optimal use of sunlight and therefore for improving the efficiency of the devices thus produced. A Graphene oxide (GO) is explored for its important effect on the band gap of titanium thin layers. In this context, the composites of GO and TiO2 are highly promising. There are many reports on the preparation of graphene-TiO2 composite  and its application in the dye-sensitized solar cell  . In fact, several studies have been reported in the literature    in order to reduce the band gap of TiO2.
TiO2 thin films can be prepared by different techniques such as sputter depositions  , sol-gel process  , chemical vapour deposition (CVD)  , and ion beam-assisted processes  . In this investigation spin coating technique was used for the deposition of TiO2 thin films. TiO2-Graphene oxide composite was successfully prepared via a simple coating approach. Structural, morphological, and optical properties of obtained layers were studied and well discussed in this work. The optical properties give information regarding the band gap Eg, the refractive index n and the extinction coefficient k.
The Titanium (IV) Isopropoxide (TIP) (99.999%), the graphene oxide in powder form (4% - 10% edge-oxidized) and other chemical reagent were purchased from Sigma-Aldrich. All chemicals were of analytical grade and used as-received.
2.2. Preparation of TiO2-Graphene Oxide
At room temperature the Titanium isopropoxide (TIP) (precursor) was mixed with acetic acid (stabilizer) and ethanol (solvent) in the molar ratio TIP: ethanol: acetic acid = 1:0.1:9. Then, the resulting solution form was stirred during 2 h. The reaction mechanism for TiO2 formation is:
On the other hand the Graphene oxide solution was prepared separately by dispersing graphene oxide (sheets) in powder form in 5 ml of ethanol. After that it is sonicated for 20 minutes. The mass of GO dispersed and mixed with TiO2 solution is 15 mg. The mixture of TiO2 and Graphene oxide solutions (5:1) were used and stirred for 30 minutes. Finally, the solution containing TiO2-Graphene oxide composite is obtained. A spin coating (Model WS-650MZ-23NPPB) was performed in air by flooding the substrate surface (glass substrate) with spinning at 3500 r/min for 30 s. Thin homogenous layers of TiO2-GO with 250 nm thickness were obtained.
2.3. Characterization Methods
A shimadzu type XRD-6000 X-ray diffractometer with a Cu-Kα radiation (λ = 1.5418 Å) were used for X-ray diffraction analysis. The morphologies of the samples were examined by scanning electron microscopy (SEM, Shimadzu Supers can SSX-550). A spectrophotometer with a resolution of 0.1 nm (Shimadzu 3150 UV-VIS-NIR) were used for the measurement of the transmittance spectra T (λ) and the reflectance R (λ) of the layers under a normal incidence at room temperature in the spectral range (200 - 1800 nm). The thickness layer was determined by a stylus displacement of a Veeco Dektak 150 profilometer.
3. Results and Discussions
3.1. XRD Analysis
The crystal structure of TiO2 and TiO2-GO samples obtained by XRD is shown in Figure 1. The XRD pattern for the samples showed no detectable peaks, indicating that the samples were amorphous. The possible reason is either to organic solvents which inhibited the formation of the crystalline structure of TiO2  , or to the low calcination temperature given during the growth of TiO2 thin layers   . Complete transformation of brookite to anatase was achieved at a calcination temperature of 400˚C  . No characteristic diffraction peaks for Graphene oxide are observed because it’s relatively low diffraction intensity and the low amount used.
3.2. SEM Analysis
Figure 2 shows the images of as-prepared TiO2 and TiO2-GO samples obtained with scanning electron microscope and scanned with 10,000 magnifications. The titanium oxide product film has homogeneity of the nanoparticles to appreciable dimensions. Agglomerated nanoparticles and pinholes have also been observed on the film. The film also shows a basic morphology of semi-spherical grains with random formation in the shape of a flower. The surface morphology of the
Figure 1. X ray diffraction patterns of TiO2 and TiGO samples synthesized by spin coating.
Figure 2. SEM patterns of TiO2 and TiO2-GO samples synthesized by spin coating.
TiO2-GO composite shows a spherical/rod-like distribution of TiGO and partially agglomerated at the surface. Thus, the TiO2 spheres were practically supported on the surface of the graphene oxide sheets and combined with each other.
3.3. Optical Measurements
Figure 3(a) depicts the transmission T and the reflection R spectra as a function of wavelength for TiO2 and TiO2-GO samples. These layers exhibited good transparency in the infrared region. A decrease in both R and T values which attributed to GO particles is distributed in the total matrix volume  is noted. Figure 3(b) describes the variation between (αhν)2 and photon energy (eV), which shows that the band gap of pure TiO2 is about 3.67 eV; value in accordance to the literature  , whereas the band gap obtained of the TiO2-GO composite has been reduced to 2.0 eV. This value is interesting for large spectra photon absorption  . This, shows that the addition of graphene oxide particles effectively increases the visible light absorption capacity of the titanium dioxide thin layer  . This can be explained by the presence of acids and oxidizing agents in the synthesis solution. Many groups containing oxygen, such as hydroxyl (C-OH), carboxyl (C-OH) and epoxy (C-O-C) groups become covalently bonded to surfaces of Graphene oxide sheets. So, in the presence of some functional group such as (-OH) and (-COOH) Graphene oxide reacted. Some
Figure 3. (a) Transmission and reflection of samples; (b) Band gap energy (hν) of TiO2 and TiO2-GO composites; and (c) Refractive index n and Extinction coefficient k spectra of TiO2 and TiO2-GO samples.
unpaired Π-electrons bonded with the free electrons on the surface of TiO2 forms a Ti-O-C structure, which shifted up the valence band edge and reduce the band gap. In addition, we can suppose that formed C-Ti bond can also participate in the reduction of the band gap of TiO2.
Figure 3(c) grouped the variation of extinction coefficient and refractive index of samples. The refractive index n, was determined using the reflection, R, values from equation above according to:
Here, R is the reflectance value, (k = αλ/4π) is the extinction coefficient.
The values of the refractive index n, rise in the Ultraviolet spectral range for samples. The n values shifted to longer UV wavelengths when GO is added toTiO2. This could be attributed to the fact of the GO that introduced in the solution which may increase density, leading to an increase of n values. The extinction coefficient, k, indicates that high values are indicated in the zone of the strong absorption or the extrinsic absorption, and decreases in the zone of the weak absorption.
The mechanism diagram showing this process is given in Figure 4. Under light irradiation, electrons were excited from the valence band to the conduction band of TiO2, leaving positively charged holes in the valence band. In the absence of acceptor, the recombination of electrons and positive charges would take place. A C-Ti bond will be formed participating in the reduce of the TiO2 band gap.
Figure 4. Schematic illustrating the charge transfer in the TiO2-GO composite under visible light irradiation with a new energy level EFX.
In this study, TiO2-GO composite thin layers were synthesized by a spin coating
ethod. The dispersion of graphene oxide nanosheets in TiO2 thin layers, crystallinity, morphology and band gap was evaluated. It was revealed by scanning electron microscopy that the deposited layers are detected some aggregates. The band gap obtained of TiO2-GO composite thin layer was 2 eV as compared to pure titania thin film. This result is significant and encouraging for various applications in different fields, such as dye-sensitive solar cells.
The author would like to thank King Abdulaziz City of Science and Technology (KACST) for the financial support (Project number: 125-37).
The author also thanks Prof. S. N. Alamri from the Taibah University, Madinah, KSA for samples characterizations.
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