Influence of Sputtering Temperature of TiO₂ Deposited onto Reduced Graphene Oxide Nanosheet as Efficient Photoanodes in Dye-Sensitized Solar Cells

Abstract

Renewable solar energy is the key target to reduce fossil fuel consumption, minimize global warming issues, and indirectly minimizes erratic weather patterns. Herein, the authors synthesized an ultrathin reduced graphene oxide (rGO) nanosheet with ~47 nm via an improved Hummer’s method. The TiO₂ was deposited by RF sputtering onto an rGO nanosheet with a variation of temperature to enhance the photogenerated electron or charge carrier mobility transport for the photoanode component. The morphology, topologies, element composition, crystallinity as well as dye-sensitized solar cells’ (DSSCs) performance were determined accordingly. Based on the results, FTIR spectra revealed presence of Ti-O-C bonds in every rGO-TiO₂ nanocomposite samples at 800 cm^–1. Besides, XRD revealed that a broad peak of anatase TiO₂ was detected at ~25.4° after incorporation with the rGO. Furthermore, it was discovered that sputtering temperature of 120 °C created a desired power conversion energy (PCE) of 7.27% based on the J-V plot. Further increase of the sputtering temperature to 160 °C and 200 °C led to excessive TiO₂ growth on the rGO nanosheet, thus resulting in undesirable charge recombination formed at the photoanode in the DSSC device.

1. Introduction

The demand of global energy usage has increased tremendously by 0.9%, equivalent to a 120 million tonnes of oil (Mtoe) in 2019 as compared to 2018 [1]. The consumption of fossil fuel (i.e., oil, coal, gas) is expected to keep rising due to economic growth and increasing population around the world. Further emission of fossil fuels produces carbon monoxide (CO) gas, a driver of the greenhouse effect. This continuous reliance on conventional energy resources will lead to a negative impact on the global warming crisis [2]. These climate change issues will result in erratic patterns like ice melt, sea levels and ocean acidification, plants and animals, and also social effects [3].

In this context, transitioning away from fossil fuels by executing alternative research for renewable energy with low-carbon sources is mandatory. Solar energy is an obvious choice towards a clean energy source, which is free, abundant, and everlasting source that could be provided in a pollution free manner. Nowadays, photovoltaic (PV) technologies have received great attention from researchers due to its ability in generating electricity that is clean, inexpensive, and sustainable, from sunlight [4,5,6]. To date, these technologies are achievable for the optimization of crystalline silicon solar cells at a power conversion energy (PCE) of about 27.6% [7]. Further generation in thin film solar cells involving CIGS, CdTe, and amorphous silicon, could achieve as high as 23.4% in 2019 [7,8,9]. However, these technologies have high cost production and mass scale panels [10].

Emerging PV technology cells of dye-sensitized solar cells (DSSCs), relatively low-cost, and ease for fabrication, have obtained an ideal PCE of 12.6% [11]. Practically, the PCE performance of DSSCs usually depends on the materials used in the photoanode part. Thus, the photoanode is the crucial element, which is applicable for absorbing the incoming light and allowing it to pass into the dyes for photoelectrochemical process [12]. Commonly, titanium dioxide (TiO₂) is utilized for photoanodes due to its high thin film transparency and good photocatalytic characteristics [13,14]. However, TiO₂ has some drawbacks such as recombination and the potential of causing undesirable effects for the excited photogenerated electrons in the interfacial transfer and leads to low PCE performance [15].

Recently, a two-dimensional (2D) carbon nanomaterial, graphene, has attracted interest with several outstanding properties that fit the DSSCs and its mechanism features in photoanode [16,17,18]. Furthermore, graphene exhibits efficient charge carrier transport, which will probably facilitate the excited electrons’ flow towards the outer circuit and improve the overall PCE performance of DSSCs [19]. Moreover, graphene has excellent optical transparency properties with good absorption rate that could efficiently allow the illumination light into the dye molecules. However, graphene without functionalized or further incorporation with other metal oxide is insufficient to be applied as a photoanode [20]. Besides, it also suffers from lattice defects and this leads to low PCE for DSSCs [21].

Researchers have attempted to improve the PCE performance of DSSCs by incorporating the TiO₂ with reduced graphene oxide (rGO) as reported elsewhere [22,23]. Recently, it has been discovered that hydrothermal deposition of rGO could be deposited onto TiO₂ with various concentrations of GO for photocatalytic degradation of RhB dye [24]. Later, Sayali et al. and their group found that the rGO-TiO₂ nanocomposite preparation via ultrasound assisted/sonochemical method could obtain good Ti-O-C bonding [25]. Some recent updates about rGO-TiO₂ formation via different techniques are shown in

Table 1. Summary of rGO-TiO₂ nanocomposites by different techniques.

Composite Formation	Optimized Concentration	Dopants	Method	Findings	Ref.
rGO/TiO2	3 wt%	GO	solvothermal	ACT degradation and mineralization on photocatalytic	[26]
TiO2-rGO	0.5 wt%	GO	hydrothermal	FM photodegradation	[27]
TiO2-rGO	0.4 wt%	GO	hydrothermal	DSSCs	[28]
rGO-TiO2	0.5mg	rGO	hydrothermal	DSSCs	[29]
Ag/rGO/TiO2	-	GO	solvothermal	Plasmonic DSSCs	[30]

. However, these techniques are emphasized on surface deposition/coating and there is a lack of accurate bonding onto the material lattice and inadequate concentration formation by the dopant.

In this paper, the preparation rGO-TiO₂ nanocomposite as photoanode for DSSCs is reported via an RF sputtering technique approach. Specifically, an optimization of sputtering temperature of the TiO₂ target and direct penetration of the rGO nanosheet could suppress the recombination while improving the photoinduced charge carrier transport. Furthermore, the sputtering technique is promising to maximize the opportunity to fill the oxygen vacancy to reduce the intrinsic defect of rGO in oxides lattice with TiO₂. Herein, RF sputtering is a better approach comparable to other physical coating or depositing for exterior dopants. In fact, this technique is associated with a better adhesion and uniform distribution onto rGO nanosheet with efficient atom bombardment. Until now, detailed studies of rGO decorated with TiO₂ with various sputtering temperatures onto rGO nanosheet for DSSCs performance are still lacking. Yet, the influence of sputtering temperature of TiO₂ onto rGO nanosheet, reaction mechanism, and their physical/chemical characteristics as photoanode remains unclear. Henceforth, comprehensive work is conducted to optimize the rGO-TiO₂ nanocomposite as photoanode element for DSSCs and to be tested under 100 W solar illumination power.

2. Experimental Details

2.1. Materials

Graphite powder (<20 µm; 99.99%); potassium permanganate, KMnO₄ (≥99.0%); hydrazine solution (35 wt% in H₂O); fluorine doped tin oxide coated glass slide, FTO coated glass (surface resistivity: ~7 Ω/sq), Di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) (ruthenium dye), platinum, Pt (≥99.9% trace metals basis), and silver conductive paste were purchased from Sigma Aldrich, Malaysia. Sulfuric acid, H₂SO₄ (95–97%); ortho-phosphoric acid, H₃PO₄ (85%); hydrogen peroxide, H₂O₂ (30%); hydrochloric acid fuming, HCl (37.0%); absolute ethanol, C₂H₅OH (≥99.5%), acetonitrile, C₂H₃N (41.05 g/mol), and potassium iodide electrolyte, KI (≥99.0%) were purchased from Merck, Malaysia. Titanium target for sputtering (99.99% purity, diameter in 50,800 µm with thickness of 6350 µm) was purchased from ULVAC Inc. The deposition process of TiO₂ onto rGO nanosheet was conducted using an RF sputtering machine at SIRIM Berhad, Malaysia.

2.2. GO and rGO Preparation

Ideal GO and rGO nanosheets were synthesized via improved Hummer’s method and chemical reduction technique as reported in our previous work [31,32,33]. The overall reaction is illustrated in Figure 1a whereas the chemical structure of graphite, GO, and rGO are shown in Figure 1b–d, respectively. Comprehensively, GO was prepared from graphite powder as the precursor material via improved Hummer’s method. A total of 1.5 g of graphite powder was poured into an acid ratio of 9:1 (H₂SO₄:H₃PO₄) [34]. Next, 9.0 g of oxidizing agent, KMnO₄, was then slowly poured into the mixture under ice bath condition (<20 °C). The solvent color changed from dark purplish green to dark brown, indicating that the oxidizing process was taking place. After 24 h, the solvent mixture was slowly transferred into ~200 mL ice solution and the overall reaction was conducted under ice bath condition. The oxidization process was terminated by adding 3 mL of H₂O₂ dropwise into the mixture and turned the color from dark brown to light brownish, indicating that a high oxidation level of graphite was well formed [34]. The suspension was centrifuged and washed with diluted HCl and DI water until pH7 was achieved. The sol-gel GO byproduct was formed after being dried for 24 h in a dry oven. Furthermore, 1.26 g of fine GO was produced from graphite powder. For rGO synthesis, it was well prepared via a chemically reduction process. Additionally, 300 mg of GO flakes were added into 100 mL distilled water while 100 µL of hydrazine solvent was immediately dropped into the mixture. The overall reaction was heated under oil bath conditions and maintained at ~80 °C [35]. The mixture was centrifuged and the supernatant was decanted away. Lastly, approximately 0.84 g of rGO samples were formed after being dried in a dry oven for 24 h. Henceforth, the yield production of synthesized rGO from GO went up to 67%.

2.3. rGO-TiO₂ Nanocomposite Formation

The rGO-TiO₂ nanocomposite via sputtering technique was prepared as an efficient photoanode for DSSCs devices as depicted in Figure 2. Firstly, the rGO nanosheet layer was deposited onto FTO glass via an electrodeposition technique as reported in our finding [33]. Size of the entire DSSCs device had been fixed with 2 cm × 2 cm area for both the anode and the cathode. The rGO was deposited on FTO glass for the anode part with an active area of 0.67 cm². From our understanding, the sputtering method is one of the effective routes to produce photoanode to achieve an ideal PCE of DSSC performance [36]. In other words, the sputtering technique has the potential to allow more dopant atom particles to penetrate onto the rGO nanosheet under high acceleration and are well formed within a second [37]. Thus, it would enhance the properties of rGO-TiO₂ nanocomposites in terms of charge carrier transport rate, resulting in high PCE of DSSC performance. In this typical procedure, several FTO with coated rGO were placed for RF sputtering with different sputtering temperatures of 40, 80, 120, 160, and 200 °C. The utilized titanium dioxide target was placed in a chamber with the optimization of being placed with distance of 10 cm apart [33]. For the uniformity of dopant onto the rGO nanosheet, the sputtering duration of TiO₂ and input power were maintained at 60 s and 150 W, respectively. The flow rate of Argon, Ar, gas was 15 mL/min, pressure at 266.64 mPa with base pressure of 0.67 mPa. Finally, the rGO-TiO₂ nanocomposite was successfully formed for the photoanode element.

2.4. DSSCs Fabrication

Theoretically, a working DSSCs device is integrated in a sandwich configuration, which consists of TCO/photoanodes/dye/electrolyte/counter electrode/TCO as shown in Figure 3. Practically, our study aims at modification of rGO photoanodes (conventional in TiO₂ material), which is the core element for the incoming light absorption ability. The main role of the photoanode is used to allow the excited photo-electron from dye molecules into the conduction band of TiO₂ under the illumination process. In addition, the incorporation of rGO with TiO₂ is applied to lift-down the incoming electrons at TiO₂ since it is a wide band gap metal oxide semiconductor material (3.2 eV). The role of rGO also helps in the internal movement of exciton electrons from the valence band of TiO₂ into the conduction band of TiO₂, where it is possesses high carrier mobility with almost zero band gap characteristics [16]. In this way, the rGO-TiO₂ nanocomposite could efficiently transfer the excited photo-electrons by minimizing the charge recombination.

For details of DSSCs fabrication process, photoanode contained rGO-TiO₂ was soaked into a solvent containing 0.5 mL N719 dye (0.3 mM) and C₂H₅OH for 24 h. The photoanode was rinsed with C₂H₃N and post baked for 10 min. Then, the counter electrode was coated with Pt with an active area of 0.67 cm² via spin coating method. Both of the electrodes were sandwiched and 0.5 M KI electrolyte were dropped on the gap. The overall device was sealed by silver paste.

2.5. Characterization

The surface morphologies of graphite, GO, and rGO were observed using field emission scanning electron microscopy (FESEM, FEI Quanta 200 FEG) with attachment of energy dispersive X-ray analysis (EDX), 5 kV. For surface morphologies of TiO₂, it was viewed under scanning electron microscopy TM3030 tabletop microscope at a working distance of approximately 2.0 mm at high vacuum mode with 5.0 kV. Besides, the rGO-TiO₂ nanocomposite was monitored under HITACHI UHR Cold-Emission FE-SEM SU 8000. The lattice of rGO-TiO₂ nanocomposite was examined under high-resolution transmission electron microscopy (HRTEM), JEM 2100F with an accelerating voltage of 200 kV. The topologies of the rGO-TiO₂ nanocomposite were measured using atomic force microscope controller—AFM5000II with 3D rotation. The purity phases of TiO₂, crystalline of rGO, and rGO-TiO₂ nanocomposite were determined using X-ray diffraction (XRD), D8 Advance X-ray diffractometer-Bruker AXS, the spectra were measured from 10° to 70° with scanning rate of 0.033 deg/s under CuKα radiation (λ = 1.5418 Å). The structural characterization of rGO, TiO₂, and rGO-TiO₂ nanocomposite were recognized by the Raman analysis, Renishaw inVia microscope with applied HeCd laser source with λ = 514.0 nm at room temperature. Furthermore, its functional groups of rGO, TiO₂, and rGO-TiO₂ nanocomposite were identified under the Fourier transform infrared (FTIR) spectroscopy, Bruker-IFS 66/S along 500–4000 cm⁻¹ wavelength by the KBr pellet method. The J-V curves of DSSCs were obtained from Autolab PGSTAT204 with solar irradiation (mercury xenon lamp) under 100 W input power.

3. Results and Discussion

3.1. Morphology

The FESEM images of graphite, graphene oxide (GO), and rGO are shown in Figure 4a–c, respectively. There are thick massive graphite flakes with nonuniform graphitic sheets distributed along the sample (Figure 4a). Figure 4b shows thin layers of GO after oxidation and exfoliation. On the other hand, Figure 4d shows the TiO₂ nanoparticles, which are sputtered on the surface of rGO to form the rGO-TiO₂ nanocomposite (Figure 4e) with average nanoparticles of ~30 nm (inserted in Figure 4e). Furthermore, the EDX results revealed that the atomic, at.%, content of carbon, C, element in the rGO-TiO₂ nanocomposite have been recorded at 37.29%, which is almost three times more than the titanium, Ti, element with 12.61%. However, the overall oxygen, O, remained the most contained element due to its contribution from the oxygenated group of graphene and also oxygen from TiO₂. The broad peak detected at 4.5 eV with high Ti content, is mainly due to the huge amount of Ti material sputtered and mixed along the rGO surface.

3.2. HRTEM

Further insight into the detailed microstructure of HRTEM and the typical image under 2 nm magnification determined that the TiO₂ is homogenously well anchored with rGO and formed rGO-TiO₂ nanocomposite as shown in Figure 5. Generally, the brighter color (0.336 nm) represented rGO nanomaterial; darker color (0.349 nm) those composed of TiO₂ nanoparticles, while grey color (0.399 nm) denoted the rGO-TiO₂ nanocomposite [33]. These phenomena were in agreement as Ti-O-C bonding, which was present and proven in Figure 8. The lattice fringes of the rGO (3.36°) and TiO₂ (3.49°) correspond to the rGO (002) plane and TiO₂ (101) plane, respectively [31].

3.3. AFM

The topologies and cross-section of rGO-TiO₂ nanocomposite were analyzed by atomic force microscopy (AFM) as shown in Figure 6. Scanning areas for the surface were up to 300 nm × 300 nm whereas Figure 6a shows the 3D images with the highest depth of ~25 nm. In addition, the entire thickness with roughness of rGO-TiO₂ nanocomposite was ~75 nm, which the highest with brightest color denoted as TiO₂. Besides, the 2D image was focused under 250 nm scan area for a better view on the surface roughness (refer Figure 6b). It is clearly shown that there are two different formation colors, whereby the bottom with darker color classified as TiO₂ was fully sputtered onto the rGO nanosheet and formed rGO-TiO₂ nanocomposite whereas the standalone brighter color represented sputtered TiO₂ that covered the top of the nanocomposite. Moreover, the cross-section of rGO-TiO₂ nanocomposite along Figure 6b revealed that the surface thickness was ~35 nm, which was in agreement with Figure 6a.

3.4. FTIR

The functional groups of graphene oxide (GO) and rGO were completely analyzed and identified as shown in Figure 7. Several intense peaks appeared in the GO sample, indicating oxygen containing groups that successfully formed from graphite after oxidation (Figure 7a). The absorption peaks including aromatic C-H deformation at 670 cm⁻¹, C-O stretching at 1052 cm⁻¹, phenolic C-OH stretching at 1200 cm⁻¹, C-OH at 1361 cm⁻¹, hydroxyl groups of molecular water and C=C at 1625 cm⁻¹, C=O stretching at 1729 cm⁻¹, and a broad peak assigned as O-H stretching vibrations of C-OH groups at 3400 cm⁻¹ [31]. Definitely, the broad band of O-H stretching at 3400 cm⁻¹ is significantly reduced and also the presence of C-O at 1052 cm⁻¹ and C-OH at 1361 cm⁻¹ in the rGO pattern. These phenomena clearly indicate that the GO has been reduced and the oxygenated group is eliminated [38].

The FTIR transmission spectrum of rGO is placed in Figure 8 for further investigation between the TiO₂ and rGO-TiO₂ nanocomposite, which is formed via different sputtering temperatures. The FTIR spectrum of TiO₂ was also been identified and depicted the peaks as high purity TiO₂, which corresponded to TiO₂. From the TiO₂ spectrum, several peaks at 467 cm⁻¹, 1345 cm⁻¹, 1629 cm⁻¹, and 3396 cm⁻¹ can be observed. To the best of our understanding, the broad peak in the range of 500–1000 cm⁻¹ region is ascribed to the Ti-O and Ti-O-Ti bridging stretching modes while the peak is denoted as anatase titania [39]. In the rGO-TiO₂ samples, most of the rGO peaks did not appear in nanocomposite samples except 80 °C and 200 °C in the range between 1600–1750 cm⁻¹, which indicated high C=C content. Interestingly, the intense peak absorption appeared for each rGO-TiO₂ nanocomposite sample in the range of 550–900 cm⁻¹ that was designated as Ti-O-C or Ti-O-Ti linkage bonds formed. This shows that these nanocomposite samples were well formed and established agreement for rGO-TiO₂ via sputtering method [40].

3.5. XRD

The XRD pattern was utilized to analyze the crystallinity of introduced TiO₂ that sputtered onto the rGO nanostructure. Figure 9 shows the XRD spectra for synthesized rGO, TiO₂, and rGO-TiO₂ nanocomposites deposited at various sputtering temperatures. The XRD of rGO had a sharp peak presence at 25.2°, 43.8°, and 45.6° as shown in Figure 9a. These peaks correspond to (002), (001), and (001) diffraction planes while the 25.2° peak indicated that the reduction process from graphene oxide (GO) was successfully obtained [41,42]. Moreover, less intense peaks at 43.8° and 45.6° indicated highly disordered carbon material [32]. On the other hand, Figure 9b shows XRD patterns of high crystallinity TiO₂ as raw nanoparticle recorded in the range from 15° to 65°. The sharp Bragg peaks indicate that the highly crystalline TiO₂ nanomaterials are well-formed. The presence of the broad peaks and Bragg diffraction peaks indexed along 25.4°, 28.1°, 41.0°, and 54.6° with (101), (112), (211), and (204) orientations, respectively, corresponded to anatase phase TiO₂ (JCPDS card no: 21-1272) [43]. The XRD patterns from (c) to (g) were detected for rGO-TiO₂ nanocomposite in variations of sputtering temperature and well aligned along at 25.2°, which were in good agreement with the obtained unique properties along the (101) orientation.

3.6. Raman

The Raman spectra of rGO, anatase TiO₂, and rGO-TiO₂ nanocomposites deposited at temperatures of 40 °C, 80 °C, 120 °C, 160 °C, and 200 °C, respectively, are shown in Figure 10. The appearance of rGO peaks in the Raman spectrum of D band and G band at 1348.20 cm⁻¹ and 1592.84 cm⁻¹, respectively, as analyzed in our previous work, confirmed successful reduction of GO to rGO [31,44]. Besides, the significance peaks for the sputtered TiO₂ have been identified as anatase phase TiO₂ due to the aligned Raman frequencies at 148.24 cm⁻¹ (E_g1), 391.37 cm⁻¹ (B_1g), 508.64 cm⁻¹ (A_1g), and 629.65 cm⁻¹ (E_g), which correspond to the literature [45]. Meanwhile, the Raman spectrum of rGO-TiO₂ nanocomposites with different sputtered temperatures have recognized entire material characteristics as every essential peak for particular anatase TiO₂ and rGO clearly appeared in the composite. Furthermore, the I_D/I_G ratios of rGO and rGO-TiO₂ nanocomposites were calculated and displayed in Figure 10. The I_D/I_G ratio could determine the defects of carbon nanomaterial based on the intensity of D band and G band. Among these rGO-TiO₂ nanocomposites, sputtered temperature condition at 80 °C was the highest defect credited to its I_D/I_G ratio. In contrast, the sputtered temperature at 200 °C with lowest I_D/I_G ratio indicated that the ideal sp² C-C network was well formed. Based on our understanding, D band-mode represented disordered structure of graphene material (sp³-bonded) whereas G band arose from C-C bond stretching in graphitic material or known as more relevant to sp²-bonded carbon atoms [46].

3.7. DSSCs

The DSSCs performance of sputtered raw TiO₂ and rGO-TiO₂ nanocomposite with different sputtering temperature onto rGO nanosheets were tabulated in Figure 11. The values of

Table 2. Summary of photovoltaic characteristics of DSSCs via different sputtering temperatures of TiO₂.

Sputtering Temperature, °C	Short Circuit Current, Jsc	Open Circuit Current, Voc	Maximum Power Current, Jmp	Maximum Power Voltage, Vmp	Fill Factor, FF	Efficiency, η
TiO2	3.98	0.36	2.82	0.28	0.55	0.79
40	5.75	0.35	5.07	0.25	0.63	1.27
80	19.18	0.39	12.58	0.30	0.50	3.74
120	15.74	0.70	12.16	0.60	0.66	7.27
160	11.31	0.60	9.69	0.30	0.43	2.92
200	16.46	0.38	14.85	0.10	0.24	1.50

are calculated based on the results in Figure 11 by reference from the equations below:

$$FF=\frac{V_{mp}{J}_{mp}}{J_{sc}{V}_{oc}}$$

(1)

$$\eta =\frac{J_{sc}{V}_{oc}FF}{P_{in}}$$

(2)

where Jsc = short circuit current; Voc = open circuit voltage; Jmp = maximum current; Vmp = maximum voltage; FF = fill factor; Pin = input power; and η = efficiency.

The PCE performance (η) of the DSSCs based on sputtering temperature studies of rGO-TiO₂ were determined accordingly by the details of photovoltaic characteristics such of DSSCs as short circuit current (Jsc), open circuit voltage (Voc), maximum power current (Jmp), maximum power voltage (Vmp), and fill factor (FF) (Table 2). It was revealed that 120 °C rGO-TiO₂ nanocomposites obtained an ideal PCE result of 7.27% with Jsc of 15.74 A/cm², Voc of 0.70 V, Jmp of 12.16 A/cm², Vmp of 0.60, and FF of 0.66. Among these rGO-TiO₂ nanocomposite samples, 120 °C also achieved the highest Voc, which indicated the shifting energy band of sputtered TiO₂ with effective transferring of the photoinjected electrons from excited electron into the conduction band [47]. Furthermore, this impact would definitely benefit the 120 °C rGO-TiO₂ nanocomposite with the efficient electron lifetime and obtained the highest value of FF.

From Table 2, a sputtered raw anatase TiO₂ was measured and attained the lowest PCE with 0.79% while 40 °C sputtering temperature of TiO₂ has the lowest PCE of 1.27% due to its small amounts of TiO₂ content reacts at photoanode element and difficult to absorb visible light from solar simulator [48]. The PCE is significantly increased from 40 °C with 1.27% to 80 °C and 120 °C with 3.74% and 7.27%, respectively. There was an estimate that improved by double according to the increases of sputtering temperature. In contrast, the PCE dramatically dropped from 7.27% to 2.92% (160 °C) and lastly to 1.50% (200 °C). This occurrence might be due to the excessive amount of TiO₂ which act as recombination centers that lead to high resistance of photo-induced charge carriers flow through the outer circuit [49].

4. Conclusions

This work discussed the effects of sputtering temperature of TiO₂ introduced onto rGO nanosheet and photoanode film for DSSCs PCE performance was accomplished. The sputtering route for rGO-TiO₂ nanocomposite formation is an impressive and effective approach. In this study, the rGO nanosheet was applied to facilitate photoinduced charge carrier electron transport while reducing electron-hole recombination pairs, resulting in better PCE performance of doped TiO₂. The uniform distribution of TiO₂ was found at 120 °C sputtering temperature on the rGO nanosheet as demonstrated by FESEM images where the lattice of rGO, TiO₂, and rGO-TiO₂ nanocomposites were presented. Surface roughness of rGO-TiO₂ nanocomposite was measured at ~35 nm. Crystallinity of TiO₂ onto rGO nanosheets was analyzed and this confirmed that a mixture of TiO₂ anatase phase was sputtered. Both symmetry modes of rGO and anatase TiO₂ were presented on rGO-TiO₂ nanocomposite samples for various sputtering temperatures. The presence of Ti-O-C bonds was confirmed by FTIR spectra, associated with the oxygenated functional groups as shown in GO and rGO, respectively. It was found that 120 °C sputtering temperature eventually enhanced the overall mobility of electron transport to the outer circuit. The TiO₂ sputtered at 120 °C possessed the ideal PCE of 7.27%, five times better PCE than the 40 °C sputtering temperature. The results indicated that precise charge carrier loading concentration of TiO₂ is applied to achieve great absorptivity of dyes and charge separation, thus it improves the overall transportation properties. In contrast, the formation of rGO-TiO₂ nanocomposite at highest sputtering temperature with 200 °C acquired the lowest PCE performance of 1.50%, which is attributed to its excessive amounts of TiO₂ that penetrated and performed higher electron-hole pair recombination centers.