Abstract

We report a facile synthesis of Titanium dioxide (TiO₂) modified compact layers on Fluorine Tin oxide (FTO) and graphene employing the Sol gel Doctor Blade technique, optimized systematically for enhanced solar energy conversion applications. UV-VIS spectrophotometer, a Varian 7000e FTIR, a Scanning Kelvin Probe Microscope, and Hall Effect setup evaluated the as deposited and films subjected to 1 step, 2°C/min and 1°C/min annealing rates. FTIR revealed considerable absorption at low frequencies (less than 798 cm^-1) in TiO₂ on graphene heterojunctions, confirming the occurrence of Ti-O and C-O-Ti bonds. The predominant anatase TiO₂ characteristic was found at 438 cm^-1. The TiO₂ on graphene film annealed at 1 °C/min exhibited the lowest porosity (46%), as well as the highest dispersion energy (11.30 eV). As the annealing rates declined, so did the surface-to-volume energy loss ratio for all the annealed films. Graphene TiO₂ annealed at 1 ^oC/min had a lower VELF/SELF than TiO₂ on FTO, implying that an electron loses less energy when passing through the TiO₂ on graphene layer than it does in TiO₂ on FTO. The light absorption coefficient α and electron diffusion coefficient D of TiO₂ on graphene improved to 4.637 x 10³ and 1.485 x 10^-4 (1 ^oC/min), respectively, whereas TiO₂ on FTO values increased to 4.221 x 10³ and 1.251 x 10^-4 (1°C/min), in that order, with decreasing annealing rates. Higher values of TiO₂ on graphene α and D indicate enhanced electron transition in the films. Hall Effect measurements on as-deposited and annealed TiO₂ on graphene films demonstrated higher conductivity as annealing rates decreased, which was attributed to film recrystallization induced by calcination. Smoluchowski smoothing model, reveal surface scan average work functions (φ) and linear profile scan average work functions (φ) ensemble variations in granular tilts and surface slopes explaining geographic variation and distribution. Local fluctuations in φ triggered by the spatially varying concentrations of electric dipole moments are intrinsic to atomic steps and influence φ. TiO₂ incorporation on graphene photoanode increased h⁺/e^-separation, electron transport, and light absorption. The continuous conduction network on compact TiO₂ nanoparticles acts as an electron leakage barrier, and the porous structure has a large specific surface area.

1. Introduction

Titanium dioxide (TiO₂) nanocomposites containing metal oxides such as fluorine tin oxide (FTO) or carbon nanostructures, particularly graphene, are currently a prominent focus of research in visible light-driven photo catalysts. In 2005, the Scotch tape method of graphene exfoliation led to exponential growth in a number of transdisciplinary activities ¹. The high carrier mobility (200,000 cm² V^-1s^-1), wide theoretical surface area (>2600 m²/g), excellent thermal conductivity (3000 - 5000 W m^-1 K^-1), and outstanding optical transparency make it an appealing alternative for use as a hybrid counterpart ^{2, 3}. When employed as photo anodes in DSSCs, tree-like TiO₂ nanostructures created directly on FTO using pulsed laser deposition showed a high light-to-electricity conversion efficiency ⁴. TiO₂ can be layered with carbon-based materials to serve as a support matrix for TiO₂ and to slow the recombination of excited electron-hole pairs ^{5, 6, 7}.

Fourier transform infrared spectroscopy (FTIR) reveals conventional graphene characteristics as overlaid strong peaks at 950-1100 cm¹. These peaks reflect C-O stretching on the graphene surface caused by the presence of a small quantity of oxygen in TiO₂ ⁸. The distinct signals at 1250, 1327, and 1385 cm¹ indicate displaced C-O-C, C-O...H, or C-O bonding, suggesting that unsaturated -C and -OH groups in alcohols are related ^{8, 9}. The distinct signals at 1250, 1327, and 1385 cm¹ indicate displaced C-O-C, C-O...H, or C-O bonding, suggesting that unsaturated -C and -OH groups in alcohols are related ¹⁰.

Hall Effect measurements, which frequently perform on structured samples like cloverleaves or small squares in van der Pauw configuration, is utilized to extract sheet carrier density and mobility ^{11, 12}. Measurements of the Hall Effect are essential for the electrical characterization of semiconductor materials and films because it reveals information about the kind of charge carriers, charge carrier density, and Hall mobility based on the Hall voltage. Although a low sheet resistance value is ideal, larger layers are required, which lowers light transmittance and efficiency ¹³. The mesoporous TiO₂ thin film contains nanocrystalline spherical particles of 15-20 nm in diameter. Due to the oxygen vacancies in the lattice, the material is an n-doped semiconductor with a carrier concentration of ND = 10^{16 14}. A distinctive electron recombination behavior is generated by the hole transport in a solid electrolyte in a solid-state DSSC, which is a hopping-like movement in the lattice sites ^{15, 16}. A distinctive electron recombination behavior is generated by the hole transport in a solid electrolyte in a solid-state DSSC, which is a hopping-like movement in the lattice sites ¹⁷.

The current study utilized TiO₂ thin films deposited on FTO and graphene substrates, and the composite evaluated holistically. The goal is to investigate the influence of different annealing rates on bonding energy, porosity, light absorption, electron transport, and conductivity of TiO₂ on FTO and graphene, along with their impact on the efficiency of dye-sensitized solar cells.

2. Experimental Procedure

Graphene monolayer was deposited on a glass substrate using roll-to-roll (R2R) chemical vapour deposition (CVD) and wet transfer synthesis. FTO SnO₂:F glass substrates were obtained from Xinyan Technology Co. Limited, China. Then, using cotton brushes and analytical acetone grade 99.5% purity, both graphene and FTO on glass substrates were carefully cleaned for about 5 minutes. They films were then immersed in ethanol for five minutes before being washed with deionized water. The samples were dried by blowing pressurized warm air over them before being stored in a desiccator.

A 1 × 1 cm² window was left after applying scotch magic tape to the substrates to adjust the thickness of the TiO₂. Using the sol-gel Doctor-blade method, T/SP, 18% wt, and 15-20 nm TiO₂ nanocrystalline were coated on the substrates ¹⁸. The drying procedure began with a 20-minute ambient temperature increase to improve film homogeneity, followed by a temperature ramp of 5 ^oC per minute up to 175 ^oC in a furnace ¹⁹. The photoanodes were sintered for 30 minutes at 450 ^oC/min before cooling in the furnace.

To improve the surface roughness and crystallinity of the films, freshly prepared samples were annealed using a Nabertherm muffle furnace at 2°C/min and 1°C/min annealing rates from 25°C to 450°C ²⁰. A few films were kept in as prepared condition, while others were treated to one-step annealing at 450 ^oC, 30 minutes of sintering, and cooling to ambient temperature.

A UV-VIS spectrophotometer (Perkin Elmer Lambda 950) was used to measure the films' transmittance and absorbance between 400 and 800 nm. SCOUT software was used to collect optical parameters, such as refractive index (n) and absorption coefficient (α), as well as determine porosity and bandgap.

Fourier Transform Infra-Red (FTIR) spectroscopy was employed to measure infrared frequencies through the films that enabled plotting of transmittance versus wavenumber spectra. Applying the Van der Pauw technique to measure charge sheet density ns and charge mobility μ by the advent of Hall Effect in conjunction with a four-point probe. Origin Pro software version 8.5 was used to process the acquired data.

3. Results and Discussion

The interaction between TiO₂ nanoparticles with graphene was studied using vibration spectra. Figure 1 shows the FTIR spectra of TiO₂ graphene sheets as deposited and annealed at 1 step, 2 ^oC/min, and 1 ^oC/min rates in the 400-4000 cm^-1 range. Low IR transmittance at high wavenumbers results in high absorption. A certain frequency without any peaks indicates the absence of a specific bond, and thus the absence of photon absorption.

Real Deal - Hardinger's Five Zone analysis was used to analyze the IR spectra ²¹. Zone 1 extends from 3700 to 3200 cm^-1, Zone 2 from 3200 to 2800 cm^-1, Zone 3 from 2800 to 2100 cm^-1, Zone 4 from 2100 to 800 cm^-1, and Zone 5 absorption peaks from 800 to 400 cm^-1. The stretching vibration modes of hydroxyl groups relate to the broadband between 2800 and 3200 cm^-1. ²². OH groups have the potential to serve as electron donors for produced H⁺, accept photo-induced holes, and generate OH radicals, all of which are very beneficial for enhancing photocatalytic activity. The stretching mode of aliphatic C-H groups is considered responsible for the absorption peaks at 2927, 2829, 3016, and 3081 cm^{-1 23}. Additionally, a slight adsorption of TiO₂ molecules on the graphene surface via the O atom of the C = O ²⁴ is suggested by a minor red shift at the C = O stretching vibration from 1464 to 1439 cm^-1 (about 5 cm^-1). The vibration bonds at 471, 693, and 789 cm^-1 correspond to the bonds Ti-O-Ti, Ti-O-O, and Ti-O, respectively ²⁵, while the LO mode of amorphous TiO₂ is at 874 cm^{-1 26}. For the samples annealed at annealing rates other than 1 ^oC / min, the 534 cm^-1 peak is very weak, indicating the film annealing at 1 ^oC / min is well-crystallized. The findings also revealed the presence of a dominating feature centered at 438 cm^-1 during annealing, which is a feature of the anatase TiO₂ ²⁷. The vibration of Ti-O-Ti and C-O-Ti was attributed to the extensive absorption at low frequency (below 798 cm^-1), which supported the development of the TiO₂ and C-O-Ti bond in graphene TiO₂ heterojunction ²⁸. Reduced annealing rates result in stronger C-O stretching, C-O-Ti bonds, and C=O carbonyl stretching, all of which could increase graphene TiO₂ electrical conductivity and boost the efficiency of photo-induced charge transfer.

The graphene TiO₂ on graphene and TiO₂ on FTO sheets optical band gap was determined by utilizing the (αhʋ)² versus hʋ curve displayed in figure 2. Equation 1 yielded the tauc bandgap ²⁹;

(1)

where E_g represents the optical band-gap, while the index r indicates the type of optical transition that occurs during photon absorption. The indices for the direct and indirect inter-band transitions are r = 1/2 and 2, respectively ³⁰ and h denotes Planck's constant. α signifies the absorption coefficient. B is a proportionality constant (band tailing parameter) with values ranging from 10⁷ to 10⁸ m^{-1 31}.

We calculated the bandgap (E_g) by fitting a linear function to spectra in the UV region and reading the abscissa intersection. Bandgap values decreased as annealing rates decreased. Graphene has no influence on TiO₂ bandgap, being a semimetal with a zero bandgap. The values for as deposited TiO₂ on graphene as was 2.89 ev, and annealed films were 1 step 3.33 ev, 2 ^oC/min 3.22 ev, and 1 ^oC/min 3.04 ev, which were quite close to the bandgap of anatase TiO₂ which is approximately 3.2 ev. The tauc bandgap of TiO₂ on FTO was found to be 3.49 eV for as deposited and in the range of (3.35 - 3.54 eV) for annealed films. The wide tin band gap isassociated to increased bandgap. of TiO₂ on FTO. It is apparent that lowering annealing rates causes a red shift in the fundamental absorption edge of TiO₂ on graphene nanocomposites. TiO₂ on graphene has lower optical band gaps than TiO₂ on FTO. Therefore, the integration of graphene into TiO₂ is expected to boost the DSSC performance by increasing the absorption intensity of light.

The average oscillator energy, E_o and dispersion energy were calculated by plotting a graph of (n²-1)^-1 verses (hv)², as shown in figure 3 for 1^oC/min films. The spectrum diminishes as (hʋ)² increases. The high specific BET area of graphene allows for a high dispersion of films within the visible region. Fitting linear at (hʋ)² = 5.1 ev² with equation 2 we get E_o and E_d values.

(2)

where E₀ denotes the energy of a single oscillator and E_d denotes the energy of dispersion. h Planck's constant, ʋ frequency, and hʋ photon energy.

Table 1 shows that average excitation energy for TiO₂ on graphene films is higher than that of TiO₂ on FTO, indicating that graphene TiO₂ films are more likely to undergo electron interband transitions than FTO-TiO₂.

Figure 4 depicts porosity against photon energy graph TiO₂ on FTO and TiO₂ on graphene annealed at 1 ^oC/min rate. The porosity (P %) (the number of pores per volume) of TiO₂ films depends on the refractive index of the films. The equation (3) presented below ^{32, 33} determines porosity of the films as a percentage.

(3)

where n_p denotes the refractivity of anatase porous sheets and n_o = 2.52 denotes the refractivity of bulk TiO₂. Porosity was seen to decrease as hv increased. Porosity values were obtained at photon energies within the visible region corresponding to 2.26ev.

Porosity was found to decrease as hv increased. Porosity values were obtained at 2.26 ev photon energies in the visible region . As shown in figure 4, porosity for TiO₂ on graphene at the specified energy is lower than that of TiO₂ on FTO, indicating that the latter gradually improved the quality of the film crystallization over the former. This is also connected with film contraction as a result of local matter migration caused by particle diffusion sintering. Light stimulated electrons (e) from the TiO₂ valence band (VB) to the conduction band (CB), leaving positively charged holes (h⁺) in the VB. In the absence of an acceptor (graphene), e^- and h⁺ would recombine instantaneously.

Figure 5 shows a plot of the band gap against porosity for as deposited and annealed films. It is seen that annealing TiO₂ produces oxygen vacancies and Ti³⁺ states, hence reducing the bandgap. Furthermore, incorporating TiO₂ into graphene results in the formation of a C-Ti bond, which improves e- transfer from TiO₂ to graphene by preventing recombination, extending the duration of separated carriers, lowering the bandgap, and achieving photocatalytic response in the visible region.

Figure 5 shows that porosity decreases with decreasing annealing rates, indicating that the film crystallization quality gradually improved. This is also connected with film shrinkage as a result of local matter migration caused by particle diffusion sintering. The Tauc bandgap values observed at the absorption edge are consistent with the Wemple Didominico (WDD) bandgap values obtained in the transparent zone. WDD bandgap were obtained relation (4) ^{34, 35}

(4)

The minimal variance was obtained by evaluating in two different spectral regions. Since graphene Dirac point resonates with the TiO₂ conduction band, TiO₂ on graphene band gap falls within the TiO₂ range. The distortion of graphene Dirac cones near the K Point of the Brillouin zone caused by a rapid increase in temperature (thermal fluctuation) at a 1-step annealing rate promotes band gap opening at this rate.

The porosity (P) of nanoporous TiO₂ influences the light absorption coefficient (α) and electron diffusion coefficient (D) ^{36, 37}. Porosity, P, with a maximum coordination number CN = 6, was established to be 0.41 (41%) for densely packed particles ^{38, 39, 40}. The light absorption coefficient (α) was calculated using equation 5 ⁴¹.

(5)

Electron transport that is dependent on the connectivity of the particles can be affected by the electrode porosity. Effect of P on the electron diffusion coefficient, D can be expressed using equation 6 ⁴².

(6)

Where a, μ and the critical porosity, P_c are 4 x 10^-4 cm²s^-1, 0.82 and 0.76 in that order. α and D for as deposited and annealed TiO₂ on FTO and TiO₂ on graphene were recorded in Table 2. Both α and D for TiO₂ on FTO and TiO₂ on graphene were found to increase with decreasing annealing rates.

The energy transferred to and from a compound semiconductor's topmost atom layer is expressed by the surface energy loss function (SELF) and volume energy loss (VELF), which are products of electron excitation in both the bulk and surface. Both SELF and VELF losses were analyzed using relation (5) and (6) respectively ⁴³.

(5)

(6)

Figure 6 shows that effective VELF/SELF decreased with increasing photon energy in the visible and ultraviolet regions of TiO₂ on graphene and TiO₂ on FTO films annealed at a rate of 1 ^oC/min. It was revealed that the effective VELF/SELF declined as photon energy increased. TiO₂ on graphene and TiO₂ on FTO were found to exhibit SELF/VELF values of 6.62 and 10.92, respectively, at hʋ = 2.26 eV. An earlier study found that when an electron is excited by plasma oscillation, it loses more energy when traveling through the bulk of a medium rather than its surface ⁴⁴.

The VELF/SELF of TiO₂ on graphene annealed at 1^oC/min was found to be lower than that of TiO₂ on FTO annealed at the same rate, suggesting that the former crystallized more readily than the latter due to enhanced ad atom surface mobility on the glass substrate's surface and a beneficial TiO₂ anchor on graphene.

Surface potential difference or contact potential difference is used to quantify the surface charge distribution. It is associated with the sample work function (Sample) and provides information about the surface charging of thin films. Thin-film work function is given by equation 7 ⁴⁵;

(7)

The Hall effect occur when a voltage differential (the hall voltage) formed across TiO₂ on graphene in response to an applied magnetic field perpendicular to the current and an electric current in the conductor. The vertical resistance (RA) and horizontal resistance (RB) are calculated using equation (8);

(8)

Van Der Pauw technique was used to calculate sheet resistance R_s for the sample utilizing the equation (9) given below

(9)

Edwin Hall, ⁴⁶ proposed equations (10), (11) and (12) for determining charge concentration ns, charge mobility μ and conductivity σ respectively.

(10)

Where B is the magnetic field, V_H is the hall voltage, I the current and q electron charge.

(11)

(12)

Figure 7 illustrates the plotting of two-dimensional (2D) plots of area scan profiles across four samples created under varying annealing rates to examine the surface contact potential on the surface of nanocomposite thin films.

The trend caused by annealing was validated by the spectra. The average work function values for as deposited, 1 step, 2 ^oC/min, and 1 ^oC/min were found to be -954.788 mV, -730.822 mV, -436.335 mV, and -189.197 mV, respectively. The measured effective work function is affected by changes in surface structure rather from being monotonic. This paper reports variation of the work function with respect to annealing rates. The enhanced crystallinity of TiO₂ is responsible for the work function's decrease with lower annealing rates.

Hall Effect measurement were recorded in Table 3.

Hall carrier concentration and mobility were seen to rise as annealing rates decreased, associated with improved crystallinity and crystallite size growth. Conductivity was found to increase when annealing rates decreased. The microstructure of the film improved as the annealing rate was decreased. This reduces potential barriers and wells (higher grain size) at grain boundaries while also reducing TiO₂/graphene agglomeration, leading in improved charge percolation and thus high conductivity.

Figure 8 shows the correlation between porosity and conductivity inas deposited and annealed TiO₂/graphene sheets. Porosity decreased as annealing rates decreased, which was attributable to local matter migration caused by particle diffusion sintering. Similarly, conductivity rose as porosity decreased. This is because as the pore size decreases, the electronic interaction between TiO₂ particles increases.

The pore size of TiO₂ on graphene annealed at 1^oC/min was 0.4612, which was found to be close to the optimum porosity reported by Meng et al., ⁴⁵, at 0.41. This provide good anchor sites for dye molecules, resulting in increased photocurrent generation in DSSCs. The film's relatively high conductivity makes it excellent for solar cell applications. Our findings revealed that when TiO₂ was deposited on graphene, the light absorption coefficient and electron transport coefficient were higher than when TiO₂ embedded on FTO, owing to TiO₂'s higher crystallinity on graphene relative to FTO. The higher light absorption coefficient is responsible for photon propagation in the scattering volume, the creation of excited electron-hole states, and the electron-hole lattice interaction associated with phonon formation. This improves the probabilities of electron diffusion coefficient, which can be attributed to higher film conductivity. High deposition temperatures (≥450^oC) diminish bond dilatation in grain boundaries, reducing charge density fluctuations explaining enhanced conductivity even at lowest annealing rates ⁴⁷. Tiny gaps and voids exist at the FTO/TiO₂ interface as a result of high porosity and the irregular microstructure of FTO glass. These voids and gaps inhibit connection and hasten the recombination process. Graphene fills the gaps because of its exceptional mobility and flexibility. Charge transfer resistance at the interface reduces when TiO₂ is compacted on graphene, improving its passivation. Oxygen vacancies and Ti³⁺ defects are created when the sub-band gap in the annealed TiO₂ on graphene composite film starts to lie deeply in the tail of the density of states. These defects speed up the interband transition while narrowing the band gap.

4. Conclusion

The study show that porosity of TiO₂ on FTO and on graphene thin films decreases with a decrease in annealing rate due to films’ densification and improved crystallinity. TiO₂ on graphene heterojunctions showed significant absorption at low frequencies (less than 798 cm^-1), as indicated by FTIR, indicating the presence of Ti-O and C-O-Ti bonds. The highest surface potential has been demonstrated for low annealing rates. The observed variations in work function are related to fluctuations in the surface dipole moment of the surface molecules and the Smoluchowski effect. Dipole moment lowers the potential barrier for charge carriers, which aids in suppressing electron-hole recombination, raising the chemical reaction's quantum yield, and boosting the catalytic reactivity of TiO₂ films. The experimental results show that films of TiO₂ on graphene and TiO₂ on FTO annealed at 1^oC per minute are ideal for use in optoelectronics due to their optimum porosity, VELF/SELF-ratio, and high light absorption coefficient, electron diffusion coefficient, conductivity.

References