J Sol-Gel Sci Technol (2014) 72:273–281 DOI 10.1007/s10971-014-3341-5
ORIGINAL PAPER
TiO2 and TiO2/SiO2 nanoparticles obtained by sol–gel method and applied on dye sensitized solar cells Marina Teixeira Laranjo • Natalia Carminati Ricardi • Leliz Ticona Arenas • Edilson Valmir Benvenutti • Matheus Costa de Oliveira • Marcos Jose´ Leite Santos Tania Maria Haas Costa
•
Received: 15 November 2013 / Accepted: 25 March 2014 / Published online: 2 April 2014 Ó Springer Science+Business Media New York 2014
Abstract In this work we show the synthesis and characterization of TiO2 and TiO2/SiO2 nanoparticles synthesized by sol–gel method using HF and HCl as catalysts. The obtained nanoparticles were analyzed by N2 adsorption– desorption isotherms, transmission electronic microscopy, Ultraviolet–visible spectroscopy and X-ray diffractometry. Mesoporous, homogeneously polycondensed TiO2/SiO2 materials, containing nanocrystalline anatase phase with band gap similar to pure titania were obtained. Films of the powdered oxides were applied to assemble dye sensitized solar cells that presented electrical parameters, Fill Factor and efficiencies similar to devices obtained by only TiO2. The sol–gel route arises as an alternative way to prepare TiO2/SiO2 materials for solar cells. Keywords Silica xerogel Silica/titania xerogel Solar cell devices Silica–titania photoelectrodes
1 Introduction Since the work published by Graetzel et al. in 1991 [1] Dye sensitized solar cells (DSSCs) have been widely studied and have become a broad research field involving
M. T. Laranjo N. C. Ricardi L. T. Arenas E. V. Benvenutti T. M. H. Costa (&) Laborato´rio de So´lidos e Superfı´cies, Instituto de Quı´mica, Universidade Federal do Rio Grande do Sul, CP 15003, Porto Alegre, RS 91501-970, Brazil e-mail:
[email protected] M. C. de Oliveira M. J. L. Santos Laborato´rio de Materiais Aplicados e Interfaces, Instituto de Quı´mica, Universidade Federal do Rio Grande do Sul, CP 15003, Porto Alegre, RS 91501-970, Brazil
semiconductors, organic dyes and inorganic complexes [2– 4]. Due to the better understanding of the conversion mechanism and the synthesis of new materials, devices presenting power conversion efficiencies of 9–12 % have been reported [5, 6]. Films of mesoporous oxide such as TiO2, ZnO, SnO2 and Nb2O5 have been widely studied [7, 8], once the nanoparticulated structure of the oxide network allows for a huge surface area for dye adsorption [9–11]. Among all the semiconductors, TiO2 is the most studied, however due to its large band gap (3–3.2 eV), it can absorbs only the ultraviolet part of the solar emission and so has low conversion efficiencies. In order to obtain high efficiency solar cells, it is required large absorption of incident light and therefore large amounts of sensitized must be adsorbed on the surface of semiconductors. To address this problem many research groups have devoted their efforts towards the development of semiconductors, presenting huge surface areas [1]. The semiconductors powders are usually assembled in a 10 lm thick film, presenting significant porosity and surface area, allowing large amounts of dye adsorption. It is important to have large porosity, which allows for the electrolyte to permeate through the whole system, making electrical contact from the anode to the cathode [9–11]. Recent works in the literature have been describing systems using silica particles to improve the efficiency of the solar cells by increasing porosity, enhancing the electronic conduction pathway and the scattering optical properties of the TiO2 photoelectrodes [12–14]. Although, the majority of papers use amorphous or crystalline commercial silica nanoparticles recovered with TiO2, sol–gel comes to light as a very interesting alternative to prepare the nanostructured silica/titania mixed oxides. By controlling the synthesis condition, this method allows for systems with controlled textural and morphological properties in a
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relatively simple and low cost way [15]. One of the most important parameters during the synthesis is the use of catalyst, as it affects the morphological and textural properties of the xerogel, mainly the pore size and hence the surface area [16]. Among all the acid catalysts HCl and HF have been widely used to obtain silica micro and mesoporous materials [17], respectively. Although HF has been previously used in the preparation of silica xerogels, there is few reports in the literature showing its application in titania sol–gel synthesis [15, 18]. In this work, TiO2 and TiO2/SiO2 xerogels were obtained by sol–gel method using HF and HCl as catalyst, the obtained materials were calcined to promote the crystallization of anatase phase. The aim is the formation of mesoporous materials presenting large surface area and with adequate characteristics allowing photoelectrode preparation. The materials were characterized by X-ray diffraction (XRD), N2 adsorption/desorption isotherms, transmission electron microscopy, UV–Vis spectroscopy and further applied in DSSCs.
10 mL of ethanol, the mixture was stirred for 30 min at room temperature, after that it was added 43 mmol (13.0 mL) of TIPO under mixing. The first sample was obtained by adding the 0.1 mL of HF in 2 mL of ethanol solution to the former TEOS solution. The second sample was obtained by adding the 0.1 mL of HCl in 2 mL of ethanol solution in the former TEOS solution. The two resulting sols were stored during 15 days until the xerogel formation. The syntheses were performed in triplicate in lower amounts, making that the total synthesis volume was not larger than 26 mL and maintaining a constant volume for the gelation process. These precautions ensure that the textural and morphological properties were not changed and also that it was obtained appropriate product quantity, enough to perform all experiments and characterization. The TiO2/SiO2 xerogel prepared using HF as catalyst was called Ti/Si-HF and the sample prepared using HCl was called Ti/Si-HCl. These xerogels were ground in an agate mortar and part of the samples was submitted to thermal treatment at 500 °C during 4 h and they were called Ti/SiHF-C and Ti/Si-HCl-C respectively.
2 Experimental
2.4 Characterization of the TiO2 and TiO2/SiO2 xerogels
2.1 Materials Titanium (IV) isopropoxide 97 % (Aldrich) (TIPO), tetraethylorthosilicate (TEOS) (Reagent Grade 98 %), ethanol absolute Merck, HCl 37 % (Merck), HF 40 % (Merck), acetone p.a. (Merck), ruthenium dye complex of cisbis(isothiocyanato)bis(2,20 -bipyridyl-4,40 -dicarboxylato)ruthenium(II),hexachloroplatinic acid and meltonix were obtained from Solaronix. 2.2 Preparation of the TiO2 xerogel TiO2 xerogel was synthesized according to the following procedure: 10 mmol of TIPO was dissolved in 20 mL of ethanol under vigorous stirring for 1 h at room temperature. After that, a solution containing 40 mmol of water and 0.1 mL of HF was added. The mixture was vigorous stirred for 15 min in a glass beaker at room temperature and left to gelation and drying during 20 days. The resulting xerogel was ground in an agate mortar and this sample was called TiO2. Aiming to obtain the TiO2 anatase, part of the material was thermally treated at 500 °C during 4 h and this sample was called TiO2C. 2.3 Preparation of the TiO2/SiO2 xerogels Two different TiO2/SiO2 samples were prepared by the sol–gel method according to the following procedure: Firstly, 5 mmol (1.1 mL) of TEOS, was dissolved in
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The N2 adsorption–desorption isotherms of the samples were determined at liquid nitrogen boiling point, using a Tristar Kr 3020 Micromeritics equipment, equipped with krypton accessory. The samples were previously degassed at 140 °C under vacuum, for 4 h. The specific surface areas were determined by BET (Brunauer, Emmett and Teller) multipoint technique, and the pore size distribution was obtained by using the BJH (Barret, Joyner and Halenda) method. Ultraviolet and visible spectra were obtained using a Shimadzu spectrophotometer model UV-2450, with integrating sphere ISR-2200, that allows the diffuse reflectance measurements. In order to determine the phases of the xerogels, XRD studies were carried out using Siemens D-500 powder diffractometer equipped with Soller slits and a graphite monochromator in the secondary beam. Data were collected with Cu Ka radiation, with a wavelength of 0.15418 nm, in the angular range from 10° to 80° (2h). The materials were analyzed by scanning electron microscopy (SEM) in Jeol equipment, model JEOL JSM6060, with 20 kV and magnification of 10,0009. The images of the samples were obtained in a transmission electron microscope JEOL operating at 200 keV– JEM 2010. The powder suspensions were obtained in acetone and further dispersed using an ultrasonic bath, for 5 min. The samples were deposited over a copper grid covered with carbon.
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2.5 Dye sensitized solar cells assembly Three kinds of pastes have been prepared, the first containing anatase TiO2 nanoparticles (TiO2C), the second and the third containing titania/silica Ti/Si-HF-C and Ti/SiHCl-C. The pastes were prepared according to a procedure previously reported in the literature [19].The paste was screen printed on the transparent conductive substrate (fluorine-doped tin oxide—FTO) previously soaked in 40 mM TiCl4 aqueous solution at 60 °C for 30 min, forming after calcination a TiO2 layer, in order to improve bonding strength between the FTO and the porous TiO2 layer in addition to block charge recombination at the interface between the FTO and redox pair [20, 21]. The paste was heated on a hot plate at 125 °C for 20 min and afterwards heated to 500 °C and then sensitized in ruthenium dye complex of cis-bis(isothiocyanato)bis(2,20 bipyridyl-4,40 -dicarboxylato)-ruthenium(II), used as received. The counter-electrodes were prepared by coating the FTO surface with a 30 lL of hexachloroplatinic acid 1 mM and heated at 500 °C. The dye sensitized TiO2 electrode and Pt counter electrode were assembled into a sandwich-type cell and sealed with a hot-melt gasket of 25 lm thickness (Meltonix) on a heating stage. A drop of the electrolyte was injected through a hole in the back of the counter electrode. The electrolyte solution was composed of 0.5 mmol L-1 of dimethyl hexyl imidazolium, 20 mmol L-1 of iodine (I2), 40 mmol L-1 of lithium iodide (LiI), and 500 mmol L-1 of tert-butyl pyridine dissolved in acetonitrile [22]. 2.6 Characterization of the solar cells The performance of the DSSCs was evaluated by current versus potential measurements and incident photon to current efficiency (IPCE). The measurements were carried out using a 300 W Xenon arc lamp, using AM1.5 filter, with light intensity calibrated to 100 mW cm-2 and recorded by a picoamperimeter Keithley, model 2400.
3 Results and discussions 3.1 Characterization of the TiO2 and TiO2/SiO2 xerogels Titania and titania/silica prepared by sol–gel method resulted in amorphous porous materials. According to the results of surface area and pore volume for all samples (Table 1) it can be observed that pure TiO2 presents smaller surface area than all titania/silica samples. In addition, the Ti/Si-HF presents higher surface area than Ti/ Si-HCl. After calcinations a significant decrease in surface
275 Table 1 Specific surface area and pore volume of the samples Sample
Specific surface area BET (m2g-1)
Pore volume (cm3g-1)
TiO2
257 ± 6
0.14 ± 0.01
Ti/Si-HF
532 ± 13
0.14 ± 0.01
Ti/Si-HCl
488 ± 12
0.08 ± 0.01
TiO2C
33 ± 2
0.07 ± 0.01
Ti/Si-HF-C
143 ± 5
0.30 ± 0.01
Ti/Si-HCl-C
161 ± 5
0.20 ± 0.01
area of all samples was observed and an interesting result is that Ti/Si-HCl-C presents slightly higher surface area than Ti/Si-HF-C, however the pore volume of both samples increased significantly. Figure 1a, b shows the N2 adsorption–desorption isotherms of all samples before and after calcination, respectively. According to the results presented in Fig. 1a, all samples present isotherms indicating the predominance of micropores. After calcinations (Fig. 1b) it is observed a decrease of the adsorbed nitrogen for TiO2C, indicating a larger decrease in the porosity of this material. The Ti/SiHF-C and Ti/Si-HCl-C samples present type IV isotherms with a hysteresis loop, characteristic of mesoporous material [23]. The pore size distribution curves obtained by BJH method are displayed in Figs. 2 and 3. Before calcination, one can observe that all samples present pores with pore diameter smaller than 4 nm (Fig. 2). However, after calcination (Fig. 3) the results clearly indicate that although the TiO2 have the porosity decreased the pore size distribution curve present pores with ca. of 13 nm in diameter (Inset Fig. 3). For the Ti/Si-HF sample it was observed a distribution of mesopores with a maximum at near 7.5 nm, which is larger than for the Ti/Si-HCl, as observed in the distribution curve with maximum about 4 nm of diameter. These results corroborates with the larger pore volume observed for the Ti/Si samples (Table 1). The decrease in surface area and the porosity observed for the titania after the calcination, can be ascribed to the crystallization process from amorphous to anatase that takes place at ca. 400 °C [24]. In other hand, the crystallization of silica only takes place at a temperature way higher (ca. 1,000 °C) than the one used to calcinate the samples [25].Therefore the differences observed in textural properties, like the presence of mesopores in the titania/ silica samples can be ascribed to the presence of the amorphous silica phase, being the pore volume and the average pore size higher for the Ti/Si-HF-C sample. Figure 4 shows the micrographs obtained by SEM. One can observe that the xerogel particles are of micrometer size and after calcination, the particles of both Ti/Si samples were sintered together, increasing the particle size and
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Fig. 2 Pore size distribution curve of sample TiO2, Ti/Si-HF and Ti/ Si-HCl
Fig. 1 N2 adsorption–desorption isotherms of samples a TiO2, Ti/SiHF and Ti/Si-HCl and b calcined samples TiO2-C, Ti/Si-HF-C and Ti/Si-HCl-C
changing the morphology mainly for the Si/Ti-HF-C that presented spherical particles. Therefore the decreasing surface area is a result of two processes; the first is closing of the nanopores, in the titania phase resulting from crystallization of anatase, and the second is the sintering process, leading to the formation of large agglomerates, as observed by SEM images. Figure 5 shows the TEM images of all calcinated samples. Figure 5a shows that TiO2C is mostly crystalline meanwhile the Ti/Si samples present both crystalline and a small extent of amorphous domains with sizes near 10 nm according to visual observation of images. These regions are characteristic of anatase titania and amorphous silica respectively. In the Fig. 5 were measured, using the Quantikov software [26], the interplanar distances observed in the titania phase. The values found were compatible to values found in XRD analysis for the titania
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Fig. 3 Pore size distribution curve of sample TiO2C, Ti/Si-HF-C and Ti/Si-HCl-C. Inset Figure expanded detail of TiO2C curve
˚ phase. In the Fig. 5a, b were measured the distances 3.5 A corresponding to 101 plane and for the Fig. 5c it was ˚ corresponding to 004 plane. measured the distance 2.4 A Figure 6 shows the diffractograms of all calcinated samples. Peaks characteristic of anatase phase are found at the angles (2h): 25.3°, 37.9°, 48.0°, 53.9° and 54.7° for (a) TiO2C, (b) Ti/Si-HF-C and (c) Ti/Si-HCl-C. The interplanar distances were calculated using Bragg’s law, as ˚ respectively. These data 3.53, 2.37, 1.89, 1.70 and 1.68 A are in accordance to the JCPDS 21-1272 card. For the titania/silica samples it is possible to assert that the silicon atoms are homogeneously distributed in titania network because the characteristic hallo of amorphous silica was not observed near 22°. In addition the crystallization
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Fig. 4 SEM images for a Ti/Si-HF, b Ti/Si-HF-C, c Ti/Si-HCl and d Ti/Si-HCl-C
process of titania was disturbed possibly by the presence of disperse silicon atoms, as observed by lower intensity and broader diffraction peaks of the titania phase. Crystallite sizes of 15 nm for TiO2C, 8 nm for Ti/Si-HF-C and Ti/SiHCl-C were calculated by using the Scherrer equation [24] This behavior corroborates previous study in the literature showing similar decrease in crystallite size with the presence of silica phase [13]. Considering all this points, we cannot discard also the presence of small quantity of amorphous silica nanodomains, showed by the TEM images. When the domains are lower than 10 nm and in reduced concentration they were not detected by XRD. All calcinated samples show typical absorption spectra of titanium oxide. According to the literature TiO2 present direct and indirect band gaps [27–29]. The band gap energies were calculated by plotting (ahm)1/m as a function of the incident radiation energy (hm) (Fig. 7) [27–29]. Where a is the absorption coefficient, m is equal to 1/2 for a direct gap and 2 for an indirect gap. The band gap values were determined by extrapolating values of a to zero. The indirect band gap transition of pure TiO2 was found at ca. 3.00 eV and is not affected by SiO2 in the Ti/ Si-HF-C samples, however a very small blue-shift was observed for Ti/Si-HCl-C presenting indirect electronic transition at ca. 2.94 eV as showed in Fig. 7a. The direct band gaps of all samples were found at ca. 3.30 eV as showed in Fig. 7b, therefore the presence of silica did not
affect the optical absorption properties of TiO2 in the present work. This result is already expected once the addition of silicon to the TiO2 network should not affect its electronic levels nor create new intermediary energy levels, because the band gap of silica is near 9 eV, very higher than the titania band gap which is near 3.2 eV, evolving very different spectrum regions turning this observation impossible. These observations are valid to mono crystalline systems however in the nanostructured systems other factors can promote small changes in the band gap value. Changes in the band gap of silica–titania materials were found in literature being that in these systems the silica grade is larger reaching in some cases 80 %. [30] The change in the band gap was also observed by other authors being ascribed to the difference in preparation methods, the existence of defects and impurities, the average crystal size, by the different methods of calculating the band gap values, as well as the morphological differences like thickness of the phases in core shell systems and nanoparticle size [31, 32]. In the present work the band gap did not change with the presence of silica because the samples present low grade of silica, lower than 10 %, the method used in the preparation of the three samples was very similar. The sol–gel method allows a high degree of dispersion of silica in the titania matrix and also allowing the formation of small size silica domains maintaining the optical properties.
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Fig. 6 X-ray diffraction of samples a TiO2C, b Ti/Si-HF-C and c Ti/ Si-HCl-C
3.2 Cell measurements
Fig. 5 TEM images for a TiO2C, b Ti/Si-HF-C and c Ti/Si-HCl-C
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Figure 8 shows the current versus potential curves of the solar cells assembled with TiO2C, Ti/Si-HF-C and Ti/SiHCl-C. From this measurement, we have obtained the following parameters: short-circuit current (Isc), open circuit voltage (Voc), Fill Factor (FF) and efficiency (g) that are described in Table 2. According to the results, the FF of the device assembled with TiO2C is slightly better than for both Ti/Si samples. The presence of SiO2 at the surface of TiO2 nanoparticles result in higher sintering temperatures [22], at 500 °C some particles in the film may not be sintered, they are just embedded and trapped by other nanoparticles that sintered around them. Therefore, the FF observed from the TiO2C cell can be related to an improved bonding strength at the FTO/TiO2 interface when compared to the bonding between the FTO and the Ti/Si layers, in addition to a more efficient bonding between the TiO2 nanoparticles, resulting in improved electrical contact and charge transport [20, 21]. In addition, the Voc and the Isc for TiO2C are higher than for Ti/Si samples, and the Isc of Ti/Si-HCl-C is slight higher than for Ti/Si-HF-C. These results are quite interesting, although the Ti/Si samples present a five times fold enhancement in surface area when compared to TiO2C (Table 1), this behavior did not result in larger photocurrent or efficiency. The obtained power conversion efficiencies (at AM 1.5G illumination) were 3.7, 2.4, and 2.1 %, for the devices based on TiO2C, Ti/Si-HCl-C and Ti/Si-HF-C,
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279 10 TiO2 Ti/Si-HCl-C Ti/Si-HF-C
-2
j (mA.cm )
8
6
4
2
0 0.0
0.2
0.4 0.6 Applied Potential / V
0.8
Fig. 8 Current versus potential curves of the solar cells assembled with TiO2C, Ti/Si-HCl-C and Ti/Si-HF-C
Table 2 Electric parameters obtained for the solar cells Sample
Jsc (mA cm-2)
Voc (V)
FF (%)
g (%)
TiO2C
8.8
0.79
54
3.7
Ti/Si-HCl-C
6.5
0.74
50
2.4
Ti/Si-HF-C
5.5
0.74
51
2.1
TiO2 Ti/Si-HCl-C Ti/Si-HF-C
25 20
respectively, which are comparable to results earlier described in the literature [21, 33, 34] although no improvement was obtained from SiO2. According to the literature, TiO2/SiO2 nanoparticles present larger ability to scatter light than bare TiO2, the scattering increases the path length of the photons inside the dye sensitized semiconductor, resulting in larger photocurrent [22, 33]. In the case of TiO2/SiO2 core–shell structures, the performance of the DSSC is highly dependent on the thickness of the silica layer, it is expected that an ultrathin layer on titania modify the density and the activity of surface states [35], resulting in more efficient systems. Considering previous studies and our results, one can observe an interesting relation between optical and electrical contribution of SiO2 to the mesoporous film. Although SiO2 improves light scattering, surface area,
15 IPCE / %
Fig. 7 Absorbance spectra for indirect (a) and direct (b) electronic transition ((ahm)1/m vs. hm) for estimating the band gap energy values of the materials
10 5 0 400
450
500
550 600 650 Wavelength / nm
700
750
Fig. 9 IPCE action spectra of TiO2C, Ti/Si-HCl-C and Ti/Si-HF-C
and suppress charge recombination preventing the electron leakage from the semiconductor to the electrolyte, it also decreases the charge transport through the mesoporous film towards the FTO [35, 36]. The IPCE action spectra of assembled devices (Fig. 9) present approximately the same behavior, generating
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photocurrent at the same spectral range. Corroborating with the results obtained from the IV curves, the device assemble with TiO2C present higher efficiency of conversion in all wavelengths. By comparing these results to the literature using titania/ silica particles, it is observed that Ti/Si xerogel is in fact a very interesting and promising route to obtain materials for application on solar cells [12, 13, 37]. In the approach used in this work, the silica was not simply deposited on the mesoporous titania, instead it is embedded into the TiO2 matrix, which certainly resulted in very high surface areas (Table 1), enabling larger amounts of dye to be adsorbed and therefore in efficiencies similar to obtained from TiO2C.
4 Conclusion In conclusion, we have presented a simple and cost effective method, to prepare titania/silica materials using sol–gel reactions. This method open perspectives to prepare materials where the silica is well distributed in the titania network maintaining the positive properties like band gap and reducing nanocrystallite size. On the other hand improves the textural properties for application in DSSCs allowing efficiency similar to pure titania materials. Acknowledgments This work was supported by CNPq (process 490221/2012-2 and 477599/2013-3), and FAPERGS (process 11/0848-4). The students thank CNPq, CAPES and FAPERGS for the scholarships. We would like to thank also the Centre of Electron Microscopy CME UFRGS.
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