Titanium dioxide (TiO2) paste was prepared by sol-gel and hydrothermal method with various precursors. Nanostructured mesoporous TiO2 thin-film back electrode was fabricated from the nanoparticle colloidal paste, and its performance was compared with that made of commercial P25 TiO2. The best performance was demonstrated by the DSSC having a 16
Titanium dioxide (TiO2) has attracted tremendous attention from researchers worldwide due to its potential applications in environmental protection and energy generation [
The TiO2 film working electrode is an important part of DSSC. It can be prepared by various methods such as sol-gel [
The addition of some acidic compounds, such as HCl, HNO3, and CH3COOH in Si(OR)4, may be used to control the speed of hydrolysis [
The dye adsorption and charge transport of TiO2 nanoparticle thin film coated with a substrate (e.g., ITO-or FTO-coated glass) have been studied extensively in recent years. The transport of photogenerated electrons or efficiency of device will be dependent on the adhesion of TiO2 thin film. In this study, pencil hardness technique was adopted to test the adhesion of TiO2 film prepared. Furthermore, nanocrystallites will beneficially influence the photocatalytic properties of TiO2 film by increasing the number of active sites, which reduces the risks of charge carrier recombination. In order to efficiently separate and collect photogenerated electrons and holes, TiO2 thin film must offer an environment that has high electron-transferring rate and less carrier traps. According to the results of recent studies on dye-sensitized solar cell, metal oxide is the main constituent for making the thin film. Among the various metal oxides, titanium dioxide is most often selected [
Here, three types of precursors were used in the sol-gel method to prepare TiO2 working photoelectrode. The purpose is to control the film thickness and produce a mesoporous TiO2 film. The previously reported mesoporous films were only few micrometers thick. Such thin films provide insufficient surface areas for dye adsorption. On the other hand, a thick film with large BET surface areas will harvest more light, which translates to higher efficiency. The TiO2 film prepared in this study was sensitized by commercial N3 dye and was applied as the photoelectrode in dye-sensitized solar cell.
P-25 TiO2 (70% anatase, 30% rutile, primary particle size 30 nm, Degussa) powder purchased from Aldrich was used for comparison purpose. Titanium(IV) isopropoxide (TTIP, 97%, Aldrich), titanium(IV) ethoxide (TTIE, ACROS), and titanium tetrachloride (TiCl4, Merck) were precursors for preparing titanium dioxide by sol-gel method. The adjustment of pH was done by adding reagent grade NaOH (Merck). Triton X-100 (Merck) and polyethylene glycol (PEG M.W = 20,000, Fluka) were used as binders, and N3 dye (Ruthenium 533 bis-TBA, Solaronix) was used as the sensitizer. The R150 redox electrolyte was purchased from the Solaronix Commercials. Acetic acid and ethanol were purchased from Merck. All other solvents and reagents were analytical-grade quality, purchased commercially, and used without any further purification.
ITO-conducting glass (20~30 Ω/cm2, Merck, Co., ltd.) was selected as the substrate for TiO2 film. Wolff-wilborn hardness pencil test was adopted for the TiO2 film adhesion test. The crystalline property of TiO2 film was modified using the hydrothermal method with a bomb-type autoclave. In order to avoid the contamination of colloidal paste and protect the autoclave, a teflon beaker was lined in the stainless steel bomb.
A series of TiO2 nanoparticles were prepared by sol-gel method using acetic acid and nitric acid as the catalytic agent. The sol-gel was prepared from three precursors including titanium(IV) isopropoxide (TTIP), titanium(IV) ethoxide (TTIE), and titanium tetrachloride (TiCl4). The solvent used was ethanol to give a solvent/precursor molar ratio of 1/1. High-purity helium (99.99%) was flowed through the reactor. This solution was added dropwise to the mixture containing 5.2 moles of acetic acid and 50 moles of DI water (deionized water) cooled at 5°C under helium gas purging and vigorous stirring. In order to increase the stability of TTIP and control the particle size [
Comparison of different preparation methods for titanium dioxide nanoparticle colloidal paste from TiCl4.
Sample | Hardness | BET surface area (m2/g) | Pore diameter (Å) | Pore volume (cm3/g) | ||
---|---|---|---|---|---|---|
P25 | HB~F | 51.69 | 104.71 | 0.17 | 29.1 | — |
ETIP-TiO2 | 6H | 60.54 | 129.29 | 0.19 | 24.8 | 28 |
H-ETIP-TiO2 | HB | 52.78 | 74.32 | 0.13 | 28.4 | 45 |
TTIP-TiO2 | H | 71.40 | 136.92 | 0.21 | 21.0 | 30 |
H-TTIP-TiO2 | F | 72.05 | 232.01 | 0.13 | 20.8 | 30 |
TiCl4-TiO2 | HB | 83.10 | 125.49 | 0.26 | — | 105 |
The mesoporous TiO2 thin film was characterized by nitrogen sorption, Nano-ZS, XRD, SEM, and UV-vis. The efficiencies of photoelectrodes fabricated by these TiO2 thin films were also tested with a solar simulator.
Particle size distribution of the colloidal paste was measured by a nanoparticle analyzer (Malvern zetasizer Nano-ZS).
Powder X-ray diffraction (XRD) measurements were taken using a Bruker-D8-ADVANCE powder diffractometer with Cu-K
BET surface areas were obtained by physisorption of nitrogen at −197°C using a micromeritics ASAP-2020 instrument. Prior to measurement, the samples were degassed to 0.1 Pa at 100°C. The surface areas were calculated in a relative pressure range
Scanning electron microscopy (SEM) images were obtained with a Hitachi 4800 field emission microscope using an acceleration voltage of 20 kV. Samples were placed on a stage especially made for SEM. They were coated with Pt prior to analysis and imaged directly. SEM images were recorded at magnification that ranged from 50000 X to 110000 X. The magnification was calibrated in pixel/nm on the camera. The chemical composition of the sample was determined by scanning electron microscopy-X-ray energy-dispersive spectrum (SEM-EDS) with accelerating voltage of 20 kV.
The diffuse reflectance UV-vis spectra were measured with a UV 3101PC UV-visible spectrophotometer. Powder samples were loaded in a quartz cell with suprasil windows, and spectra were collected in the range from 300 nm to 800 nm against quartz standard.
Dye-sensitized solar cell prepared was consisted of a TiO2 working electrode coated with ITO conducting glass, a counter electrode, and electrolyte dispersed in between. TiO2 film working electrodes prepared from different titanium precursors (e.g., TTIP-TiO2, TTIE-TiO2, and TiCl4-TiO2) were coated on ITO glass (20 mm × 10 mm) by doctor-blade method [
Counter electrode was made by sputtering a layer of platinum on ITO glass. The photovoltaic property of the cell was measured by solar simulator (i.e., AM 1.5, 100 mW/cm2, YAMASHITA YSS-80A). The light intensity of solar simulator was calibrated by standard silicon solar cell (223 mV).
For dye-sensitized solar cell (DSSC), the adhesion of titania film on ITO glass is an important criterion that will impact the cell performance. This is because cracking of titania film tends to influence the interfacial transfer of charge carriers [
Figure
Effect of surfactant addition on the P25 thin film hardness.
Furthermore, adding surfactant has the effect of increasing the thickness of TiO2 film. This will prevent film cracking since thin film tends to crack more easily due to the shrinkage effect, that is, the change of TiO2 volume due to evaporation and decomposition of organic substances, which induces considerable stress on the film [
Since the alkoxide titanium will quickly react with water to generate titanium hydroxide, the reaction was kept at 5°C during hydrolysis and followed by the acidification reaction. The acidification reaction can enhance the crystalline property of the nanoparticle. The mixture at the moment was not considered a paste since the particle size and solvent constituent must be further conditioned by the hydrothermal treatment at a temperature of 190°C. Dominant factors that will influence the characteristics of the thin film include particle size, particle morphology, and solvent constituent of the paste [
Colloidal nanoparticle size distribution before hydrothermal treatment.
The SEM images in Figures
TTIE-TiO2 thin film morphology after annealing at 500°C.
TTIP-TiO2 thin film morphology after annealing at 500°C.
TiCI4-TiO2 thin film morphology after annealing at 500°C.
H-TTIP-TiO2 thin film morphology after annealing at 500°C.
H-TTIE-TiO2 thin film morphology after annealing at 500°C.
The differences in morphology and particle size among the prepared TiO2 paste were observed by SEM. Nanoparticles of both TTIE-TiO2 and TTIP-TiO2 displayed spherical shape with apparent boundary. In contrast, nanoparticles of TiCl4-TiO2 had a rod-like shape. Obviously, differences in morphology or shape of the TiO2 nanoparticles prepared with different precursors and acids were observed. The porous structure of TiO2 film was clearly observed from the SEM images.
As shown in Table
All TiO2 films prepared in this study have nanosize particles. Figure
XRD patterns of TiO2 synthesized hydrothermally at 190°C for 12 h and calcinated at 500°C for 0.5 h (A: TTIP (190), B: TTIE (190), C: H-TTIP (190), D: TTIP (500), E: TTIE (500), F: H-TTIP (500), G: H-TTIE (500), H: TiCl4 (500), I: P25).
The XRD patterns of different samples prepared under temperature of 190°C are shown in Figure
The XRD results show that TTIP-TiO2 (500) and TTIE-TiO2 (500) were composed of both anatase and rutile phases. The rutile phase was formed during high-temperature calcination at 500°C. The main peaks of TTIP-TiO2 and TTIE-TiO2 became sharper as the calcination temperature was increased, indicating an increase in its crystallinity. The XRD patterns of TiCl4-TiO2 (500) show both rutile and anatase phases; however, no change in peak sharpness has been observed. Nevertheless, the XRD patterns show that these TiO2 have the structure with short-range mesophase order, which is a typical characteristic of TiO2 [
The photoconversion efficiency of solar cells fabricated from various TiO2 working electrodes were investigated in this study. The thickness of TiO2 film after calcination was estimated to be 8
The amount of chemisorbed dye on the different thicknesses.
The amount of dye chemisorbed on H-TTIP-TiO2 film electrode was tested. Despite the fact that H-TTIP-TiO2 exhibited high surface area, its dye adsorption behavior was very poor. This can be explained by the small pore volume of H-TTIP-TiO2 (0.13 cm3/g) as shown in Table
The H-TTIE-TiO2 film prepared was composed mostly of spherical primary particles and some ellipse-like secondary particles. The use of TTIP precursor to react with different solvent and reactant will change the rate of hydrolysis and primary particle growth [
Figure
Performances of various TiO2 thin film electrodes for DSSC.
Sample | Thickness ( | Jsc (mA/cm2) | Voc (V) | Fill factor | |
---|---|---|---|---|---|
P25 | 10-11 ± 0.5 | 6.25 | 0.64 | 52.64 | 2.19 |
TTIE-TiO2 | 8-9 ± 0.5 | 5.63 | 0.70 | 62.18 | 2.45 |
H-TTIE-TiO2 | 8-9 ± 0.5 | 4.25 | 0.69 | 68.45 | 1.90 |
TTIP-TiO2 | 8-9 ± 0.5 | 13.13 | 0.76 | 44.96 | 4.45 |
H-TTIP-TiO2 | 8-9 ± 0.5 | 5.13 | 0.65 | 68.15 | 2.25 |
TiCl4-TiO2 | 8-9 ± 0.5 | 2.13 | 0.64 | 74.09 | 0.99 |
TTIP/TTIP* | 15-16 ± 0.5 | 15.50 | 0.67 | 57.89 | 6.03 |
*Double layer of TTIP-TiO2 for efficiency test.
Photocurrent voltage characteristics of nanocrystalline TiO2 films prepared with different precursors.
High photocurrent could be generally related to high surface area, which results in an increase in the amount of dye adsorbed if the film thickness and light irradiation intensity were kept constant. It might also be due to the presence of more anatase TiO2, which facilitates the electron transport [
TiCl4-TiO2 shows the highest surface area; however, its solar energy conversion efficiency is only 0.99% with low Jsc of 2.13 mA/cm2. It is attributed to the poor adhesion of TiO2 on ITO glass observed. The pH of TiCl4-TiO2 paste is around 1
In this study, various TiO2 pastes were prepared with different precursors and acids. There are many factors that will influence the performance of DSSC, for instance, the nature of dye, semiconductor electrode, and activity of redox electrolyte, and so forth. In DSSC, TiO2 is the key component in determining the device efficiency. TiO2 prepared by sol-gel and hydrothermal method has the advantage of tuning its composition, particle size, and pore size distribution. The criteria of making an effective TiO2 working electrode include high surface area, appropriate band gap, and good adhesion [