Synthesis of TiO2 nanorods with a microwave assisted solvothermal method and their application as dye-sensitized solar cells
Hossein Rezvani Nikabadi
1
(
Department of Basic Sciences, Imam Reza International University, Mashhad, Iran
)
Saeed Khosroababdi
2
(
Department of Electrical Engineering, Imam Reza International University, Mashhad, Iran
)
Keywords:
Abstract :
Synthesis of TiO2 nanorods with a microwave assisted solvothermal method and their application as dye-sensitized solar cells
Abstract: In this work, Titanium dioxide (TiO2) nanostructures have been synthesized via a microwave assisted solvothermal method using titanium tetraisopropoxide (TTIP), polyvinylpyrrolidone (PVP) and Ascorbic Acid (AA) in ethanol. The mole ratio of PVP/AA was found to be critical in determining the morphology and crystal phase of the final product. PVP/AA mole ratio was varied from 1 up to 15 to obtain different morphologies of TiO2. The structural analysis by XRD diffraction confirmed formation of titanium dioxide. The Williamson-Hall (W-H) analysis was used to study the individual contributions of crystallite sizes and lattice strain on the peak broadening of the TiO2 nanoparticles. FTIR spectrum was used to estimate the various functional groups present in the nanostructures. Scanning electron microscope (SEM) images demonstrated nanoparticle, short nanorod, and long nanorods for 5, 10 and 15 mole ratio of PVP/AA respectively. TiO2 nanoparticles and nanorods have been used as photoelectrode in dye-synthesized solar cell (DSSCs) fabrication. The efficiencies of solar cells were calculated 3.23% and 4.01% for nanoparticles and nanorods, respectively.
Keywords: TiO2 nanorods, Dye sensitized solar cells, Solvothermal, J-V plot.
1. Introduction
Air pollution is one of the most important issues, which scientists face with it. Hence, many works have been done to reduce this problem in our surrounding environment. Using renewable energy such as solar energy instead of fossil fuel is one solution for the mentioned problem and photovoltaic cell (PV) [1-3] is a new technology that has attracted enormous interest recently because of inexhaustible, safe and environmentally friendly [4]. It is possible to find different categories of photovoltaic cells in the literature as silicon, copper indium selenite, CdTe, perovskite solar cell [5, 6] and dye-sensitized solar cells [7-20].
Among the mentioned photovoltaic cells, Dye-sensitized solar cells (DSSCs) due to the high efficiency and low fabrication cost are more famous than that of the other ones. Moreover, two major requirements in DSSC technology are charge transport through a semiconductor and the electrolyte [4] which it can be possible to increase the electron transport and light trapping by using titanium dioxide (TiO2) nanoparticle in DSSCs [21]. TiO2 is used as an electron transport layer in PV and photoelectrochemical devices and it has been implemented as a photocatalyst [22] and electrode solar cell based on dye-sensitized photo-electrochemical [23-29]. The Various kind of TiO2 structures like TiO2 nanotubes [30] and one-dimensional TiO2 [31-33] have been used to improve the efficiency of DSSCs.
From the fabrication point of view, different methods were used to fabricate TiO2 nanostructures, which include these methods: Sol-Gel [34, 35], Micelle and Inverse Micelle, Hydrothermal, Solvothermal [36], Chemical Vapor Deposition, Electrodeposition, Sonochemical and Microwave [37-39]. Although, the Solvothermal method is more effective because, in this method, low temperature is used to format high particles crystal with high purity Using microwave helped solvothermal technique for quick heating and rapid crystallization rate [40, 41].
Titania nanotubes have been used vastly as a starting material compared to titanium dioxide since they have many hydroxyl groups and capability for ion absorption [42, 43]. Furthermore, it is possible to improve the electron transport as well as light trapping in DSSCs using TiO2 nanowires and nanorods as scattering layer [44].
In this research, microwave assisted solvothermal method is used to prepare TiO2 nanorods. We introduce PVP as an effective capping agent for nanorods formation and ascorbic acid as a mild reducing agent in this report for the first time. TiO2 nanoparticles and nanorods are used as photoelectrode in dye-synthesized solar cell (DSSCs) fabrication.
2. Experimental method
2.1 Preparing TiO2 nanostructures
A mixture of Ti-containing precursor solution based on titanium tetraisopropoxide (TTIP) and polyvinylpyrrolidone (PVP) and Ascorbic Acid (AA) was prepared in 100 ml of ethanol with different molar ratio of TTIP/PVP/AA 1:1:X (X: 1, 5, 10, 15) as mentioned in Table 1. Fig.1 shows Flowchart of synthesize process. For this solution, 5 ml of TTIP was diluted in absolute (99.99%) ethanol. Thereafter, PVP was dissolved in 50 ml of ethanol and added to the first solution. Appropriate volume of AA was also dissolved in this solution and stirred for 10 min. Then the solution was exposed to microwave irradiation for 5 min at 450 W. The microwave treated solution transferred to a Teflon sealed autoclave for solvothermal synthesis and treated at 150oC for 2 h. The obtained powder was washed and calcined at 400oC to remove residual compounds and cooled naturally to room temperature for further analysis and cell fabrication.
2.2 Characterization tools
The crystalline structure of the powders was recorded by D8-Advanced Bruker X-ray diffractometer using Cu-Kα radiation (λ = 1.54056 Å) in the range 2θ = 20 – 90 degrees. SEM images were obtained using LEO 1450VP system. FTIR data were collected using an AVATAR-370-FTIR THERMONICOLET spectrometer using two separate procedures. Sample was unpacked into a tablet shape and put onto a polished silicon wafer before analysis.
2.3 Fabrication and characterization of DSSC
The TiO2 nanostructure was made by mixing with ethyl cellulose, α-terpineol and ethanol. The solution was stirred for 30 minutes. The solution sonicated together with heat treatment at 80°C until became to a viscous paste. A few drops of acetic acid and triton-x-100 added to the solution. The paste was spread on the FTO substrate by applying doctor blade technique. This is known as photoanode. The as prepared photoanode dried at 500°C for calcinations and sintering and finally the electrode was treated in the solution of 40 mM TiCl4 for 30 min. Next, the photoanode was soaked in 0.3 mM N719 dye for 24 h. After that, the cells were filled with I−/I-3 electrolyte. The counter electrode was Pt fabricated using thermal treatment of H2PtCl4 5 mM at 400oC for 30 min. Figures 2 and 3 show schematic diagram of the cell structure.
Two main factors directly affect the photovoltaic properties of a working electrode: surface area of the TiO2 layer and TiO2 crystal characteristic. Higher surface area would allow more dye molecules to be absorbed on working electrode, hence generating more photoelectrons under the same level of excitation, while crystal property is important to electron transport. Electron transport within single crystals is faster than in a particle aggregate because the grain boundaries in the former are much less. In our case, the TiO2 nanoparticle layer had a very high surface area and dye loading. A large amount of photoelectrons were generated and injected into the nanoparticles. However, the large number of grain boundaries at the nanoparticle interfaces had caused a zigzag pathway of electron transport with ohmic loss. Charge recombination became a major obstacle in efficient energy conversion (Fig. 3a). The nanorods have higher surface area than the nanofibres but lower than the nanoparticles. However, as nanorods were in single-crystalline form, they could provide better electron pathway for electron transport than nanoparticles (Fig. 3b).
The J-V characteristic of the cells having the active area of 0.16 cm2 was measured under AM 1.5 (100 mWcm−2) illuminations using a solar simulator coupled with a Palm Sens Potentiostat for recording J-V plots. Incident photon-to-current efficiency (IPCE) was measured using a 150 W halogen lamp in combination with a grating monochromator and calibrated by a silicon photodiode.
3. Results and Discussion
Fig.4 shows that the prepared Titanium oxide nanostructures are well crystallized and composed of Anatase and Rutile phase structures. The (101)-plane for Anatase and (110)-plane for Rutile are the main diffraction planes seen in the figure. The net intensity for (101) shows that the samples are well crystallized. Increase of PVP/AA causes a considerable increase of the net intensity of the main peak of Anatase phase. This is the indication of preferential growth of Anatase phase due to the increase of PVP as capping agent. The mean crystalline size of nanoparticles is estimated using Williamson-Hall method [45, 46] and the results are plotted as Fig.5 and the calculated mean particle size and lattice strain are summed up in Table 2. The mean particle size decreases from S1 to S4 as well as the lattice strain accordingly. It is mostly stated that with the decrease of size, lattice strain increases unless the morphology of the particles changes dramatically [47, 48]. It can be stated that the increase of PVP/AA led to change of morphology of the samples.
Fig.6 shows TEM image of TiO2 nanoparticles with a narrow size of about 20 nm which was in perfect agreement with the XRD analysis results. The Energy Dispersive X-ray Spectroscopy (EDS) confirm the chemical composition of the prepared TiO2 nanoparticles (Fig.7).
Fig.8 indicates the FTIR spectra of the samples. There are absorption peaks for the wave numbers of 2500-4000 cm-1 that is confirming the presence of compounds of carbon and water. The peaks at 1640 cm−1 in the spectra are due to the stretching and bending vibration of the -OH group. The peak at 440 cm-1 is attributed to Ti-O bond for S1, which shifts to 442, 450 and 454 cm-1 for S2, S3 and S4, respectively. The shift to higher frequencies indicates the shortening of Ti-O bond and simultaneously confirms the reduction of atomic plane distances.
Fig.9 shows SEM images for the samples. Generally, they have uniform size distribution which is the advantage of this method. Nanoparticles transform to nanowire like-structure when PVP/AA mole ratio increased up to 5 (Fig.9-S2). All nanowires are roughly uniform in shape and morphology in large area scale. Fig.9-S3 showed a uniformly long and narrow rod structure with several micrometers length and 35 – 45 nm thickness. For S4 (Fig.9-S4) the colloidal solution is exposed by the excess of PVP and the morphology of nanostructures change dramatically to elongated nanorods. As the amount of PVP increased, the crystal facets of TiO2 are more influenced by PVP adherence. It means that PVP would be attached to the lateral planes with high surface energy and doesn't allow the crystal to grow in that direction. It results to the formation of nanorods and nanowires. The more PVP is used, the better these facets are covered and thus longer and thinner they become (Fig.10).
The photocurrent density-voltage (J-V) and internal photocurrent efficiency (IPCE) characteristics of DSSC are depicted in Figures 11 and 12, respectively. Samples with TiO2 nanoparticles and nanorods used as photoelectrode under simulated air mass 1.5 global (AM 1.5G) full sunlight intensity. Detailed photovoltaic parameters, namely, open-circuit voltage (VOC), short-circuit current density(JSC), fill factor (FF), and the photovoltaic power conversion efficiency (η) have been obtained and tabulated in Table 3. JSC is the parameter determined by the product of the charge carrier density under illumination, which shows the maximum number of the photo-generated carriers that can be extracted from a solar cell. The results demonstrated that the nominal values of Jsc were lower than the case of nanoparticle. The nominal efficiency of the prepared cell with nanorods was also lower than the nanoparticle case. The results indicate low photocurrent values compare to the reports for DSSCs [49-52]. IPCE values are indicating low values over visible to IR range which confirms low dye adsorption. This causes low performance of the solar cells. Although the IPCE of S4 is less than S1, the efficiency as well as JSC of S4 is higher than S1 nanoparticle samples. This may be due to better morphology and TiO2/organic interfacial interface and also could be the indication of the enhancement of electron transport rather than the particles which is confirmed also elsewhere [18]. It is also notable that high efficiency photoelectrode, in our research TiO2 nanorods sample, for DSSCs requires not only a high surface area for the loading of large amounts of dye molecules but also a closed net microstructure for light capture and facile electron transport [53].
4. Conclusions
Various TiO2 nanostructures were fabricated using a microwave assisted solvothermal method. PVP and ascorbic acid (AA) were used as surfactant and reducing agents, respectively. The results show that PVP/AA mole ratio has a crucial effect on the morphology of final powder. XRD analysis showed that both Anantase and Rutile phases are available in the powders but with the increase of PVP/AA ratio, Anatase phase is only formed. Dye sensitized solar cell fabricated using TiO2 nanorods showed an efficiency enhancement due to the enhancement of short current circuit. It indicated that nanorods enhanced electron transport due to their preferential growth morphology.
5. Acknowledgment
This research was supported by Imam Reza International University of Mashhad under Grant
No: 21703.
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Tables:
Table 1.samples with different PVP to AA molar ratios
(X= [PVP]/[AA]) | Sample name |
1 | S1 |
5 | S2 |
10 | S3 |
15 | S4 |
Table 2: Williamson-Hall data of the samples
Sample name | 0.9λ/d | Mean particle size | ε (%) (Lattice strain) |
S1 | 0.0039 | 35.53 | 0.71 |
S2 | 0.0047 | 29.48 | 0.58 |
S3 | 0.0062 | 22.35 | 0.17 |
S4 | 0.0065 | 21.32 | 0.22 |
Table 3: Cell performance parameters extracted from J-V plots of nanoparticles and nanorods used as photoanode.
Cell | Voc (v) | Jsc(mA/cm2) | Ƞ(%) | FF (%) |
S1 | 0.765 | 6.24 | 3.23 | 70.5 |
S4 | 0.765 | 7.5 | 4.01 | 72.1 |
Figures:
Fig.1: Flowchart of synthesize process.
Fig.2: Schematic diagram of the cell structure.
Fig.3: Schematic illustration of electron transportation in the working electrode made of (a) nanoparticles,
(b) nanorods.
Fig.4: XRD pattern of the samples with different PVP/AA mole ratios.
Fig.5: Williamson-Hall plot of samples for determination of lattice strains and mean particle sizes.
Fig.6: TEM image of TiO2 nanoparticles.
Fig.7: EDX spectrum image of TiO2 nanoparticles.
Fig.8: FTIR spectra of the samples.
Fig.9: SEM images of the samples.
Fig.10: Schematic mechanism of the formation of TiO2 nanorods.
Fig.11: J-V plot of the cell fabricated using TiO2 nanoparticles (S1) and nanorods (S4).
Fig.12: IPCE plot of the cell fabricated using TiO2 nanoparticles (S1) and nanorods (S4).
