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Catalytic platinum layers for dye solar cells: A comparative study
Thin Solid Films 511 – 512 (2006) 342 – 348 www.elsevier.com/locate/tsf
Catalytic platinum layers for dye solar cells: A comparative study Guram Khelashvili a,b,*, Silke Behrens a,T, Claudia Weidenthaler c, Carmen Vetter d, Andreas Hinsch d, Rainer Kern d, Krzysztof Skupien e, Eckhard Dinjus a,b, Helmut Bo¨nnemann a,c a
c
Forschungszentrum Karlsruhe, ITC-CPV, Post Box 3640, D-76021 Karlsruhe, Germany b University of Heidelberg, D-69117 Heidelberg, Germany Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mu¨lheim, Germany d Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2. 79110 Freiburg, Germany e Cracow University of Technology, Al. Jana Pawla II 37, 31-864 Krakow, Poland Available online 19 January 2006
Abstract Comparative investigations concerning the preparation and characterization of zerovalent platinum nanoparticles to be used as precursors for dye-sensitized solar cells (DSSCs) have been carried out. Pt nanopowders were prepared via triorganohydroborate reduction, polyol method, hydrogen reduction, and thermal decomposition of complex compounds. The powders resulting from the various sources were then immobilized on support. Subsequently, the size and crystalline structure of the particles were examined using Transmission Electron Microscopy (TEM), XRay diffraction (XRD). The metallic state of the platinum surface- and core-atoms were studied by X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Absorption Near Edge Structure (XANES) analysis. The platinum nanoparticles resulting from the various preparation pathways were each incorporated in a printable platinum paste and then printed on transparent conductive (TCO) glass. After thermal treatment (at 630 -C) the electrochemical performance of the different platinum layers obtained in this study was compared applying impedance spectroscopy. The charge transfer resistance was best observed for the catalyst prepared by the hydrogen reduction method. D 2005 Elsevier B.V. All rights reserved. Keywords: Dye solar cell; Platinized counter electrode; Platinum nanoparticles; Platinum-tin dioxide film
1. Introduction Since the late 1980s, when Gra¨tzel and co-workers [1–3] introduced high-surface-area nanocrystalline semiconductors for dye-sensitized solar cells (DSSC), DSSC have attracted an increasing interest because of their low cost, easy production and relatively high efficiency (8–10%). In general, DSSC consist of a thin layer (<10 Am) of titanium dioxide particles (¨20 nm) treated with a ruthenium dye. The dye absorbs a photon energy of the light and generates electrons. The oxidized dye is reduced by I ions in the electrolyte (3I Y I3 + 2e ). The TiO2/dye layer is attached to a front electrode, which is a transparent conducting oxide (TCO) glass. The counter electrode is also TCO glass covered with a small amount of platinum (5–10 Ag/cm2). The
* Corresponding authors. Forschungszentrum Karlsruhe, ITC-CPV, Post Box 3640, D-76021 Karlsruhe, Germany. E-mail addresses: [email protected] (G. Khelashvili), [email protected] (S. Behrens). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.059
platinum is responsible for catalytic cathodic reduction of triiode to iodide (I3 + 2e Y I ). As an alternative to the platinum catalyst, the counter electrode can be prepared by the carbon coating [4,5]. But platinum is an advantageous catalytic material rather than carbon because of its excellent catalytic properties. Besides, platinum helps to reduce the overvoltage for the triiodide– iodide reduction [5]. Hauch and Georg [6] made a comparative electrochemical impedance spectroscopy study on the electrolyte/platinum interface. They prepared platinized electrodes by electronbeam evaporation, sputtering and thermal decomposition of H2PtCl6. Papageorgiou et al. [7,8] prepared platinized electrodes by electrochemical deposition and thermal treatment. They reported high catalytic activity of platinum on the electrode which contained 5 Ag platinum per cm2. Lin et al. [9] made XPS analytical investigations of the stability of the thermally prepared platinized electrode. In Ref. [10] the same research group studied electrochemical performance of the platinized counter electrode prepared by employing techniques of colloid preparation, Langmuir-
G. Khelashvili et al. / Thin Solid Films 511 – 512 (2006) 342 – 348
Blodgett film and self-assembly. Ma et al. [11] studied how the thickness of the Pt film affects the performance of the counter electrode. Platinum complexes like CODPtMe2 (COD = 1,5-cyclooctadiene) and MeCpPtMe3 (Cp = cyclopentadienyl) were used as a catalytic precursor in Ref. [12]. Selfassembly of Pt nanoparticles was used by Wang et al. [13] to fabricate counter electrodes for dye-sensitized solar cells. The late developments in the production of dye solar cells are based on screen-printing and sealing technologies [14 – 16]. During the manufacturing of cells titanium dioxide, zirconium dioxide layers are screen-printed on the front TCO glass plate, the platinum layer is screen-printed on the back TCO glass plate. The main goal of our research is the preparation and comparison of zerovalent platinum nanoparticles on a support (carbon or tin dioxide), which will be later mixed with screen-printing media and printed as an effective conductive catalyst layer on the counter electrode. Another key goal of catalyst preparation is to increase the catalytic active surface area by reducing the size of the platinum particles. In this paper we report about the comparative study concerning the particle size and the metallic state of platinum nanoparticles prepared via triorganohydroborate reduction, polyol method, hydrogen reduction and thermal decomposition.
2.3. Hydrogen reduction For the hydrogen reduction 0.0192 g of H2PtCl6IxH2O (Pt content ¨38 – 40%) was dissolved in 3 ml water and added to 1.0000 g of antimony doped tin dioxide (Zelec ECP 3010-XC). Then the solution was removed and the impregnated platinum precursor was reduced in a preheated hydrogen flow at 150 -C during 1 h. 2.4. Thermal decomposition 0.5000 g of H2PtCl6IxH2O and 0.41g antimony doped tin dioxide, (polymeric precursor, Alfa Aesar) were dispersed in a 20g mixture of ethylcellulose –terpineol screen-printing media. The mixture was treated in the oven first at 400 -C for 15 min, then at 630 -C for 30 min. As a supporting material XC-72R carbon black (Cabot Corp.) and antimony doped tin dioxide (SnO2:Sb) (Milliken Chemical, Zelec ECP 3010-XC, Lot number Q1010) were tested. 2.5. Preparation of samples for electrochemical impedance spectroscopy Three samples of antimony doped tin dioxide were impregnated by 0.0192 g of H2PtCl6IxH2O (Pt content ¨38– 40%). Platinum loading on SnO2ISb was 0.7% by mass. This concentration gives 5 Ag platinum for 1 Am (thickness) 1 cm2 volume in the dried tin dioxide layer. Subsequently, catalyst 1 underwent hydrogen reduction. Catalyst 2 was prepared via polyol reduction pathway. In order to remove organic residues and accomplish platinum reduction the sample was then treated with hydrogen and argon at 100 -C. Catalyst 3 consisted of H2PtCl6/SnO2:Sb and was applied without any additional treatment.
2. Experimental 2.1. Triorganohydroborate reduction This method requires inert atmosphere with the rigorous exclusion of air and water. All solvents have to be dried and stored under argon atmosphere. Tetraalkylammonium stabilized platinum nanoparticles were prepared according to Ref. [17]. TEM, XRD, XPS and XANES analysis also were performed under inert atmosphere. All experimental manipulations and measurements concerning polyol reduction, hydrogen reduction and thermal decomposition were performed in air.
2.6. Cell preparation The measurements of the charge transfer resistances at the electrolyte-Pt-contact were performed on so-called electrolyte cells. Each cell is composed of a front and a counter electrode. The front-as well as the counter electrode are platinum coated TCO glasses [6]. The platinum layer was applied by screen printing of the different kinds of Pt-pastes. A screen printed silver stripe on both sides of each cell ensured proper conductance. Glass frit as cell sealant was also applied by screen printing. After sintering the front and counter electrode
+
+
N
N – – Cl + Cl
N
Ethylene glycol (EG), which was applied as a reducing agent, was used without further purification. 0.1 g (0.1931 mmol) of H2PtCl6IxH2O (Pt content ¨38– 40%) was dissolved in 5 ml EG and 10 ml distilled water and maintained at 120 -C for two hours. The resulting black powder was washed with water and dried in air. Yield 94%.
–
Cl
N +
Pt
–
N
N
+
+
–
Cl
–
Cl
R = Alkyl, C6-C8 Scheme 1.
Cl–
+ N
2.2. Polyol method
PtCl2 + 2 R4N(BEt3H)
343
+ – N Cl
Cl
+ R4NCl + 2 BEt3 + H2
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Fig. 1. TEM image of tetraoctylammonium stabilized platinum nanoparticles and corresponding particle size distribution.
were brought on top of each other in a fusing step at a temperature above 600 -C. The distance between the electrodes was approximately 40 Am. Afterwards the cells were filled with an electrolyte solution composed of lithiumiodide, iodine, tertbutylpyridine (TBP) in acetonitrile. The electrochemical impedance spectroscopy (EIS) measurements were carried out with the Impedance Measuring Unit (IM 6) from Zahner. All impedance measurements were performed in the dark. TEM measurements were performed with a Hitachi HF 2000 and Philips Tecnai F20 TEM (for Pt particles prepared via thermal decomposition) with a field emission gun at 200 kV. The XRD patterns were collected on a Stoe STADI P transmission diffractometer with a primary monochromator and a linear position sensitive detector (CuKa radiation). The data were collected in the range between 15- and 120- 2u with a step width of 0.01- 2u. The XPS measurements were performed with a Kratos HSi spectrometer with a hemispherical analyzer. The monochromatized Al Ka X-Ray source (E = 1486.6 eV) was operated at 15 kV and 15 mA. For the narrow scans, an analyzer pass energy of 40 eV was applied. The binding energy values are
referenced to C 1s at 284.5 eV. XANES measurements were performed at beam line BN3 at the electron stretcher and accelerator ring (ELSA) at the University of Bonn, operating in 2.3 GeV mode. 3. Results and discussion 3.1. Triorganohydroborate reduction In general, this method results in monodisperse metal nanoparticles dispersible in organic media with an extremely clean metal surface [18 – 20]. Normally, metal nanopowders prepared by the triorganohydroborate reduction are pyrophoric as compared to air-dried metal powders, which have a similar particle size. The reduction of the platinum chloride with tetraalkylammonium triethylborhydride results in highly concentrated platinum zero colloids. Both the stabilizing agent (NR4+group) and the reducing hydro group (BEt3H ) are coupled in the same reagent (Scheme 1).
(200)
Intensity
(222)
(220)
(111)
Intensity
(311)
Pt 4f5/2 74.5
Pt 4f7/2 71.2 eV
e 85 40.0
50.0
60.0
70.0
80.0
80
75
70
65
Binding Energy [eV]
2Theta/˚ Fig. 2. XRD plot of the tetraoctylammonium stabilized platinum particles.
Fig. 3. The Pt(4f) XPS spectra of a PtIN(n-Octyl)4Cl colloidal powder. The curve (e) represents the experimental data, others are fitting curves.
G. Khelashvili et al. / Thin Solid Films 511 – 512 (2006) 342 – 348
345
Pt 4f7/2 species 1
Intensity
Pt 4f5/2 species 1
Pt 4f7/2 species 3
Pt 4f5/2 species 2 Pt 4f5/2 species 3
Pt 4f7/2 species 2
e 90
85
80
75
70
65
60
Binding Energy [eV]
Fig. 4. TEM image showing platinum particles prepared by polyol reduction.
The long chain tetraalkylammonium salts are attached to the Pt(0) surface, and they act as very effective protecting agents to prevent particles from agglomeration. The resulting colloids isolable as a redispersible dry powder, which contains up to 85% platinum particles. The TEM micrograph and corresponding histogram (Fig. 1) indicate that the mean particle size is around 2.2 nm (T 0.5 nm), and the particle size distribution is quite narrow. XRD analysis of the nanoparticles revealed a crystallized material with a crystallite size of ~2 nm indicated by the broad reflections (Fig. 2). The XPS spectra of the PtIN(n-Octyl)4Cl colloidal powder (Fig. 3) shows that the particles’ surface atoms remain in zero valent state. The XPS plot is represented by one species of platinum with binding energies Pt (4f7/2) at 71.15 eV and Pt (4f5/2) at 74.50 eV and nicely fits with curves of Pt(0) standard. The metallic state of the particles was confirmed also by XANES measurement, where the Pt colloid spectrum was identical to the Pt foil. Regardless the excellent metallic characteristics of the particles, this method has some disadvantages, which restricts its application for the DSSCs: (i) tetraalkylammonium stabilized platinum colloids do not tolerate tin dioxide as a carrier; (ii) contamination by boron residues and (iii) the
protective shell. DSSC is based on n-type semiconductors. Therefore, the presence of boron as a p-type semiconductor on the counter-electrode may cause malfunction. It is possible to get rid off boron along with the protective shell by using a conditioning set-up [21], but this step makes fabrication of cells complicated.
Pt 4f7/2 71.1 eV
Intensity
Pt 4f5/2 74.4 eV
Fig. 6. XPS spectra of Pt powder after thermal treatment at 380 -C for 15 min in the presence of ethyl-cellulose/terpineol mixture. Species 1, 2 and 3 correspond to Pt0, Pt+2 and Pt+4, respectively. The curve (e) represents the experimental data.
e 85
80
75
70
65
Binding Energy [eV] Fig. 5. XPS spectra of a Pt powder prepared via polyol method. The curve (e) represents the experimental data, others are fitting curves.
Fig. 7. TEM images of Pt nanoparticles obtained by thermal treatment of H2PtCl6 at 380 -C (a) and 630 -C (b).
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In order to accomplish reduction and remove organic impurities, which normally accompany the polyol process [24], it is necessary to apply argon and hydrogen. This step is even required if polyol is substituted by other organic reducing agents e.g., alcohols, like methanol or ethanol [25], and aldehydes [26] and etc.
3.2. Polyol method For our purpose, we apply polyol reduction without stabilizers. Absence of the surfactant results in a broad particle size distribution. As the TEM image shows (Fig. 4) the particle sizes range between 2 and 20 nm. The average crystallite size of the platinum powder is 2 nm [22]. According to our XPS studies platinum mainly corresponds to the metallic state (Fig. 5). Nevertheless, the differences between fitted and measured curve indicate that a small amount of platinum is not reduced completely and remains in a higher oxidation state. The XANES measurement also confirm that the Pt mainly remains in zero valent state. Polyol method can be used as an electroless deposition method of the platinum films onto counter electrodes [23]. Though, in this case it seems to be complicated to control properly the amount of platinum deposited onto the substrate.
3.3. Hydrogen reduction The conditioning is an essential step for the preparation of zerovalent nanoparticles. Therefore, the hydrogen reduction method seems to be the most promising preparative method. The preheated hydrogen gas allows to reduce the impregnated platinum precursor to the zerovalent state even at relatively low temperatures [21] and the carrier restricts the growth of the particles.
a -80
-60
Phase Thermal decomposition Hydrogen reduction Polyol reduction Zelec ECP 3010 XC
10
-40
-20
Phase / Grad
| Impedance| / Ohm
|Impedance| Thermal decomposition Hydrogen reduction Polyol reduction Zelec ECP 3010 XC
0
1 300m
1
10
1k
100
10k
100k 300k
Frequency / Hz
b 77.2
Rct (Ohm cm2)
27.4
5.4
1.2
t3
ys
l ta
Ca
t2
Ca
t1
ys
ys
l ta
Ca
l ta
P EC C l 0X Ze 01 3 ec
Fig. 8. a. Impedance spectra measured on electrolyte cells with different catalytic layers. The catalytic layers consists of 0,7% Pt loaded SnO2/Sb prepared by hydrogen reduction (catalyst 1), polyol reduction (catalyst 2), thermal decomposition (catalyst 3), and Zelec ECP 3010 XC SnO2/Sb. Of each catalytic layer 5 identical cells were prepared and measured. Only one representative spectrum of each layer is shown. b. Charge transfer resistance_s obtained from the impedance spectra. In each case the shown result is the average value of 5 identical cells.
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347
3.4. Thermal decomposition
4. Conclusions
The preparation of platinum catalyst for DSSC via thermal decomposition of H2PtCl6 is the most disseminated preparative pathway. The platinum precursor is solved in dry isopropanol and tempered at 380 – 390 -C [6,7,9]. Our thermogravimetric (TG) study show that full decomposition of pure (without any organic additives) H2PtCl6 occurs above 580 -C. At 380 -C the mixture of PtCl2 and PtCl4 is formed. Only in the presence of isopropanol or screen-printing media occurs partial reduction of Pt(VI) to Pt(0). XPS analysis (Fig. 6) revealed three species for Pt 4f at these temperatures. Most part (¨76%) of the platinum precursor is reduced to zero valent state (species 1) and the rest remains in oxidation state +2 (¨12%) (species 2) and + 4 (12%) (species 3). Our results are in good agreement with Lin et al. [9]. After sealing the printable solar cells at 630 -C, hexachloroplatinic acid decomposes completely and results in zerovalent platinum. On the other hand the high temperatures lead to highly agglomerated particles. As the TEM images show the particle size distribution for the platinum particles after 400 -C (Fig. 7a) is very broad and ranges between 4 and 18 nm. After 630 -C the particle size increases further more and ranges between 5 and 30 nm (Fig. 7b).
Our comparative detailed study of preparation pathways revealed that hydrogen reduction seems to be the most convenient and promising preparative method for producing catalytic layers for DSSC. The XPS and XANES analysis show that hydroborate, polyol and hydrogen reduction result in Pt(0). Thermal decomposition of H2PtCl6 precursor to Pt(0) occurs only above 580 -C. The XRD and TEM analysis indicated that the mean particle sizes remain in nanosize range. The best electrochemical performance showed the Pt catalyst prepared by hydrogen reduction.
3.5. Immobilization of nanoparticles on support
[1] B. O’Regan, M. Gra¨tzel, Nature 353 (1991) 737. [2] M.K. Nazeerudin, A. Kay, I. Rodicio, R. Humphry-Backer, E. Mu¨ller, P. Liska, N. Vlachopoulos, M. Gra¨tzel, J. Am. Chem. Soc 115 (1993) 6382. [3] B. O’Regan, J. Moser, M. Anderson, M. Gra¨tzel, J. Phys. Chem. 94 (1990) 8720. [4] G.P. Smestad, M. Gra¨tzel, J. Chem. Educ. 75 (1998) 752. [5] A. Kay, M. Gra¨tzel, Sol. Energy Mater. Sol. Cells 44 (1996) 99. [6] A. Hauch, A. Georg, Electrochim. Acta 46 (2001) 3457. [7] N. Papageorgiou, W.F. Maier, M. Gra¨tzel, J. Electrochem. Soc. 144 (1997) 876. [8] N. Papageorgiou, Coord. Chem. Rev. 248 (2004) 1421. [9] G. Wang, Y. Lin, X.R. Xiao, X.P. Li, W. Wang, Surf. Interface Anal. 36 (2004) 1437. [10] J.M. Chen, Y.T. Ma, G.Q Wang, Z.P Wang, X.W Zhou, Y. Lin, X.P. Li, X.R. Xiao, Chin. Sci. Bull. 50 (2005) 11. [11] X.M. Fang, T.L. Ma, G.Q. Guan, M. Akiyama, T. Kida, E. Abe, J. Electroanal. Chem 570 (2004) 257. [12] L.N. Lewis, K.H. Janora, J. Liu, S. Jie, E.P Gasaway, GE Global Research, Niskayuna, NY, USA, Proceedings of SPIE-The International Society for Optical Engineering, Organic Photovoltaics V, vol. 5520, 2004, p. 244. [13] W.B. Wang, B. Wei, Z. Luo, X.R. Xiao, Y. Lin, Chin. J. Chem. 22 (2004) 256. [14] S. Burnside, S. Winkel, K. Brooks, V. Shklover, M. Gra¨tzel, A. Hinsch, R. Kinderman, C. Bradbury, A. Hagfeldt, H. Pettersson, J. Mater. Sci., Mater. Electron. 11 (2000) 355. [15] M. Spa¨th, P.M. Sommeling, J.A.M. van Roosmalen, H.J.P. Smit, N.P.G. van der Burg, D.R. Mahieu, N.J. Bakker, J.M. Kroon, Prog. Photovolt. 11 (2003) 207. [16] R. Kern, R. Sastrawan, J. Ferber, R. Stangl, J. Luther, Electrochim. Acta 47 (2002) 4213. [17] H. Bo¨nnemann, R. Richards, in: W.A. Herrmann (Ed.), Synthetic Methods of Organometallic and Inorganic Chemistry (Herrmann/Brauer), Catalysis, vol. 10, Georg Thieme Verlag, Stuttgart-New York, 2002, p. 211. [18] S. Kinge, H. B—nnemann, Appl. Organomet. Chem. 19 (2005) 750. [19] H. Bo¨nnemann, W. Brijoux, R. Brinkmann, R. Fretzen, T. Joussen, R. Ko¨ppler, P. Neiteler, J. Richter, J. J. Mol. Catal. 86 (1994) 129.
The specific temperature regime is necessary for the manufacturing of DSSC. Therefore, it is impossible to avoid the agglomeration of particles completely. The only way to restrict the particles from the sintering is the immobilization of the particles on support. Another problem is a good adhesion of the Pt particles on support [6]. If the Pt does not stick to substrate properly, particles begin moving through the cell and catalyze dark currents at the TiO2/electrolyte interface. Among the most widely reported supports we tested carbon black and antimony doped tin dioxide because of their very good conductive performance. Carbon black has some benefits as compared with tin dioxide: (i) it can bare any colloid, (ii) higher BET surface area (264 m2/g vs. 51 m2/g) and (iii) cost factor. But the main disadvantage of carbon black is a sensitivity against the temperature. Our TG observations show that carbon black ignites in air at 500 -C. For that reason SnO2:Sb seems to be favorable for application in DSSC. 3.6. EIS measurement In Fig. 8a impedance measurements carried out on electrolyte cells are shown. From the impedance spectra the charge-transfer resistance Rct was evaluated [6]. The Rct’s presented in Fig. 8b are in each case the average values of five identical cells. The best catalytic performance was achieved by the catalyst prepared by the hydrogen reduction (catalyst 1). Relatively high Rct values of catalysts as compared with Ref. [7] could be explained by the agglomeration of Pt nanoparticles treated at higher temperature.
Acknowledgment The authors thank A. Dreier from MPI Kohlenforschung for TEM performance, Dr H. Modrow and N. Palina (Physical department of the University of Bonn) for XANES measurements and Dr. R. Stahl at the Forschungszentrum Karlsruhe, ITC-CPV for the TG analysis. This work is supported by the German Federal Ministry of Education and Research (BMBF) (grant # 01 SF 0304). References
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[20] H. Bo¨nnemann, G. Braun, W. Brijoux, R. Brinkmann, A.S. Tilling, K. Seevogel, K.J. Siepen, Organometal. Chem. 520 (1996) 143. [21] H. Bo¨nnemann, U. Endruschat, J. Hormes, G. Ko¨hl, S. Kruse, H. Modrow, R. Mo¨rtel, K.S. Nagabhushana, Fuel Cells 4 (2004) 297. [22] G. M. Chow, P.E. Shoen, L.K. Kurihara, U.S. patent No 5759230, 2 Jun. 1998 [23] L.K. Kurihara, P.E. Shoen, G.M. Chow, Nanostruct. Mater. 5 (1995) 607.
[24] F. Fievet, in: T. Sugiomoto (Ed.), Fine Particles, Chapter 9.2, Marcel Dekker, New York, 2000, p. 460. [25] H. Haray, H. Chawanya, N. Toshima, React. Polym. 3 (1985) 127. [26] Z.H. Zhou, W.J. Zhou, S.L. Wang, G.X. Wang, L.H. Jiang, H.Q. Li, G.Q. Sun, Q. Xin, Catal. Today 93 – 95 (2004) 523.
Catalytic platinum layers for dye solar cells: A comparative study Guram Khelashvili a,b,*, Silke Behrens a,T, Claudia Weidenthaler c, Carmen Vetter d, Andreas Hinsch d, Rainer Kern d, Krzysztof Skupien e, Eckhard Dinjus a,b, Helmut Bo¨nnemann a,c a
c
Forschungszentrum Karlsruhe, ITC-CPV, Post Box 3640, D-76021 Karlsruhe, Germany b University of Heidelberg, D-69117 Heidelberg, Germany Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mu¨lheim, Germany d Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2. 79110 Freiburg, Germany e Cracow University of Technology, Al. Jana Pawla II 37, 31-864 Krakow, Poland Available online 19 January 2006
Abstract Comparative investigations concerning the preparation and characterization of zerovalent platinum nanoparticles to be used as precursors for dye-sensitized solar cells (DSSCs) have been carried out. Pt nanopowders were prepared via triorganohydroborate reduction, polyol method, hydrogen reduction, and thermal decomposition of complex compounds. The powders resulting from the various sources were then immobilized on support. Subsequently, the size and crystalline structure of the particles were examined using Transmission Electron Microscopy (TEM), XRay diffraction (XRD). The metallic state of the platinum surface- and core-atoms were studied by X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Absorption Near Edge Structure (XANES) analysis. The platinum nanoparticles resulting from the various preparation pathways were each incorporated in a printable platinum paste and then printed on transparent conductive (TCO) glass. After thermal treatment (at 630 -C) the electrochemical performance of the different platinum layers obtained in this study was compared applying impedance spectroscopy. The charge transfer resistance was best observed for the catalyst prepared by the hydrogen reduction method. D 2005 Elsevier B.V. All rights reserved. Keywords: Dye solar cell; Platinized counter electrode; Platinum nanoparticles; Platinum-tin dioxide film
1. Introduction Since the late 1980s, when Gra¨tzel and co-workers [1–3] introduced high-surface-area nanocrystalline semiconductors for dye-sensitized solar cells (DSSC), DSSC have attracted an increasing interest because of their low cost, easy production and relatively high efficiency (8–10%). In general, DSSC consist of a thin layer (<10 Am) of titanium dioxide particles (¨20 nm) treated with a ruthenium dye. The dye absorbs a photon energy of the light and generates electrons. The oxidized dye is reduced by I ions in the electrolyte (3I Y I3 + 2e ). The TiO2/dye layer is attached to a front electrode, which is a transparent conducting oxide (TCO) glass. The counter electrode is also TCO glass covered with a small amount of platinum (5–10 Ag/cm2). The
* Corresponding authors. Forschungszentrum Karlsruhe, ITC-CPV, Post Box 3640, D-76021 Karlsruhe, Germany. E-mail addresses: [email protected] (G. Khelashvili), [email protected] (S. Behrens). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.059
platinum is responsible for catalytic cathodic reduction of triiode to iodide (I3 + 2e Y I ). As an alternative to the platinum catalyst, the counter electrode can be prepared by the carbon coating [4,5]. But platinum is an advantageous catalytic material rather than carbon because of its excellent catalytic properties. Besides, platinum helps to reduce the overvoltage for the triiodide– iodide reduction [5]. Hauch and Georg [6] made a comparative electrochemical impedance spectroscopy study on the electrolyte/platinum interface. They prepared platinized electrodes by electronbeam evaporation, sputtering and thermal decomposition of H2PtCl6. Papageorgiou et al. [7,8] prepared platinized electrodes by electrochemical deposition and thermal treatment. They reported high catalytic activity of platinum on the electrode which contained 5 Ag platinum per cm2. Lin et al. [9] made XPS analytical investigations of the stability of the thermally prepared platinized electrode. In Ref. [10] the same research group studied electrochemical performance of the platinized counter electrode prepared by employing techniques of colloid preparation, Langmuir-
G. Khelashvili et al. / Thin Solid Films 511 – 512 (2006) 342 – 348
Blodgett film and self-assembly. Ma et al. [11] studied how the thickness of the Pt film affects the performance of the counter electrode. Platinum complexes like CODPtMe2 (COD = 1,5-cyclooctadiene) and MeCpPtMe3 (Cp = cyclopentadienyl) were used as a catalytic precursor in Ref. [12]. Selfassembly of Pt nanoparticles was used by Wang et al. [13] to fabricate counter electrodes for dye-sensitized solar cells. The late developments in the production of dye solar cells are based on screen-printing and sealing technologies [14 – 16]. During the manufacturing of cells titanium dioxide, zirconium dioxide layers are screen-printed on the front TCO glass plate, the platinum layer is screen-printed on the back TCO glass plate. The main goal of our research is the preparation and comparison of zerovalent platinum nanoparticles on a support (carbon or tin dioxide), which will be later mixed with screen-printing media and printed as an effective conductive catalyst layer on the counter electrode. Another key goal of catalyst preparation is to increase the catalytic active surface area by reducing the size of the platinum particles. In this paper we report about the comparative study concerning the particle size and the metallic state of platinum nanoparticles prepared via triorganohydroborate reduction, polyol method, hydrogen reduction and thermal decomposition.
2.3. Hydrogen reduction For the hydrogen reduction 0.0192 g of H2PtCl6IxH2O (Pt content ¨38 – 40%) was dissolved in 3 ml water and added to 1.0000 g of antimony doped tin dioxide (Zelec ECP 3010-XC). Then the solution was removed and the impregnated platinum precursor was reduced in a preheated hydrogen flow at 150 -C during 1 h. 2.4. Thermal decomposition 0.5000 g of H2PtCl6IxH2O and 0.41g antimony doped tin dioxide, (polymeric precursor, Alfa Aesar) were dispersed in a 20g mixture of ethylcellulose –terpineol screen-printing media. The mixture was treated in the oven first at 400 -C for 15 min, then at 630 -C for 30 min. As a supporting material XC-72R carbon black (Cabot Corp.) and antimony doped tin dioxide (SnO2:Sb) (Milliken Chemical, Zelec ECP 3010-XC, Lot number Q1010) were tested. 2.5. Preparation of samples for electrochemical impedance spectroscopy Three samples of antimony doped tin dioxide were impregnated by 0.0192 g of H2PtCl6IxH2O (Pt content ¨38– 40%). Platinum loading on SnO2ISb was 0.7% by mass. This concentration gives 5 Ag platinum for 1 Am (thickness) 1 cm2 volume in the dried tin dioxide layer. Subsequently, catalyst 1 underwent hydrogen reduction. Catalyst 2 was prepared via polyol reduction pathway. In order to remove organic residues and accomplish platinum reduction the sample was then treated with hydrogen and argon at 100 -C. Catalyst 3 consisted of H2PtCl6/SnO2:Sb and was applied without any additional treatment.
2. Experimental 2.1. Triorganohydroborate reduction This method requires inert atmosphere with the rigorous exclusion of air and water. All solvents have to be dried and stored under argon atmosphere. Tetraalkylammonium stabilized platinum nanoparticles were prepared according to Ref. [17]. TEM, XRD, XPS and XANES analysis also were performed under inert atmosphere. All experimental manipulations and measurements concerning polyol reduction, hydrogen reduction and thermal decomposition were performed in air.
2.6. Cell preparation The measurements of the charge transfer resistances at the electrolyte-Pt-contact were performed on so-called electrolyte cells. Each cell is composed of a front and a counter electrode. The front-as well as the counter electrode are platinum coated TCO glasses [6]. The platinum layer was applied by screen printing of the different kinds of Pt-pastes. A screen printed silver stripe on both sides of each cell ensured proper conductance. Glass frit as cell sealant was also applied by screen printing. After sintering the front and counter electrode
+
+
N
N – – Cl + Cl
N
Ethylene glycol (EG), which was applied as a reducing agent, was used without further purification. 0.1 g (0.1931 mmol) of H2PtCl6IxH2O (Pt content ¨38– 40%) was dissolved in 5 ml EG and 10 ml distilled water and maintained at 120 -C for two hours. The resulting black powder was washed with water and dried in air. Yield 94%.
–
Cl
N +
Pt
–
N
N
+
+
–
Cl
–
Cl
R = Alkyl, C6-C8 Scheme 1.
Cl–
+ N
2.2. Polyol method
PtCl2 + 2 R4N(BEt3H)
343
+ – N Cl
Cl
+ R4NCl + 2 BEt3 + H2
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Fig. 1. TEM image of tetraoctylammonium stabilized platinum nanoparticles and corresponding particle size distribution.
were brought on top of each other in a fusing step at a temperature above 600 -C. The distance between the electrodes was approximately 40 Am. Afterwards the cells were filled with an electrolyte solution composed of lithiumiodide, iodine, tertbutylpyridine (TBP) in acetonitrile. The electrochemical impedance spectroscopy (EIS) measurements were carried out with the Impedance Measuring Unit (IM 6) from Zahner. All impedance measurements were performed in the dark. TEM measurements were performed with a Hitachi HF 2000 and Philips Tecnai F20 TEM (for Pt particles prepared via thermal decomposition) with a field emission gun at 200 kV. The XRD patterns were collected on a Stoe STADI P transmission diffractometer with a primary monochromator and a linear position sensitive detector (CuKa radiation). The data were collected in the range between 15- and 120- 2u with a step width of 0.01- 2u. The XPS measurements were performed with a Kratos HSi spectrometer with a hemispherical analyzer. The monochromatized Al Ka X-Ray source (E = 1486.6 eV) was operated at 15 kV and 15 mA. For the narrow scans, an analyzer pass energy of 40 eV was applied. The binding energy values are
referenced to C 1s at 284.5 eV. XANES measurements were performed at beam line BN3 at the electron stretcher and accelerator ring (ELSA) at the University of Bonn, operating in 2.3 GeV mode. 3. Results and discussion 3.1. Triorganohydroborate reduction In general, this method results in monodisperse metal nanoparticles dispersible in organic media with an extremely clean metal surface [18 – 20]. Normally, metal nanopowders prepared by the triorganohydroborate reduction are pyrophoric as compared to air-dried metal powders, which have a similar particle size. The reduction of the platinum chloride with tetraalkylammonium triethylborhydride results in highly concentrated platinum zero colloids. Both the stabilizing agent (NR4+group) and the reducing hydro group (BEt3H ) are coupled in the same reagent (Scheme 1).
(200)
Intensity
(222)
(220)
(111)
Intensity
(311)
Pt 4f5/2 74.5
Pt 4f7/2 71.2 eV
e 85 40.0
50.0
60.0
70.0
80.0
80
75
70
65
Binding Energy [eV]
2Theta/˚ Fig. 2. XRD plot of the tetraoctylammonium stabilized platinum particles.
Fig. 3. The Pt(4f) XPS spectra of a PtIN(n-Octyl)4Cl colloidal powder. The curve (e) represents the experimental data, others are fitting curves.
G. Khelashvili et al. / Thin Solid Films 511 – 512 (2006) 342 – 348
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Pt 4f7/2 species 1
Intensity
Pt 4f5/2 species 1
Pt 4f7/2 species 3
Pt 4f5/2 species 2 Pt 4f5/2 species 3
Pt 4f7/2 species 2
e 90
85
80
75
70
65
60
Binding Energy [eV]
Fig. 4. TEM image showing platinum particles prepared by polyol reduction.
The long chain tetraalkylammonium salts are attached to the Pt(0) surface, and they act as very effective protecting agents to prevent particles from agglomeration. The resulting colloids isolable as a redispersible dry powder, which contains up to 85% platinum particles. The TEM micrograph and corresponding histogram (Fig. 1) indicate that the mean particle size is around 2.2 nm (T 0.5 nm), and the particle size distribution is quite narrow. XRD analysis of the nanoparticles revealed a crystallized material with a crystallite size of ~2 nm indicated by the broad reflections (Fig. 2). The XPS spectra of the PtIN(n-Octyl)4Cl colloidal powder (Fig. 3) shows that the particles’ surface atoms remain in zero valent state. The XPS plot is represented by one species of platinum with binding energies Pt (4f7/2) at 71.15 eV and Pt (4f5/2) at 74.50 eV and nicely fits with curves of Pt(0) standard. The metallic state of the particles was confirmed also by XANES measurement, where the Pt colloid spectrum was identical to the Pt foil. Regardless the excellent metallic characteristics of the particles, this method has some disadvantages, which restricts its application for the DSSCs: (i) tetraalkylammonium stabilized platinum colloids do not tolerate tin dioxide as a carrier; (ii) contamination by boron residues and (iii) the
protective shell. DSSC is based on n-type semiconductors. Therefore, the presence of boron as a p-type semiconductor on the counter-electrode may cause malfunction. It is possible to get rid off boron along with the protective shell by using a conditioning set-up [21], but this step makes fabrication of cells complicated.
Pt 4f7/2 71.1 eV
Intensity
Pt 4f5/2 74.4 eV
Fig. 6. XPS spectra of Pt powder after thermal treatment at 380 -C for 15 min in the presence of ethyl-cellulose/terpineol mixture. Species 1, 2 and 3 correspond to Pt0, Pt+2 and Pt+4, respectively. The curve (e) represents the experimental data.
e 85
80
75
70
65
Binding Energy [eV] Fig. 5. XPS spectra of a Pt powder prepared via polyol method. The curve (e) represents the experimental data, others are fitting curves.
Fig. 7. TEM images of Pt nanoparticles obtained by thermal treatment of H2PtCl6 at 380 -C (a) and 630 -C (b).
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In order to accomplish reduction and remove organic impurities, which normally accompany the polyol process [24], it is necessary to apply argon and hydrogen. This step is even required if polyol is substituted by other organic reducing agents e.g., alcohols, like methanol or ethanol [25], and aldehydes [26] and etc.
3.2. Polyol method For our purpose, we apply polyol reduction without stabilizers. Absence of the surfactant results in a broad particle size distribution. As the TEM image shows (Fig. 4) the particle sizes range between 2 and 20 nm. The average crystallite size of the platinum powder is 2 nm [22]. According to our XPS studies platinum mainly corresponds to the metallic state (Fig. 5). Nevertheless, the differences between fitted and measured curve indicate that a small amount of platinum is not reduced completely and remains in a higher oxidation state. The XANES measurement also confirm that the Pt mainly remains in zero valent state. Polyol method can be used as an electroless deposition method of the platinum films onto counter electrodes [23]. Though, in this case it seems to be complicated to control properly the amount of platinum deposited onto the substrate.
3.3. Hydrogen reduction The conditioning is an essential step for the preparation of zerovalent nanoparticles. Therefore, the hydrogen reduction method seems to be the most promising preparative method. The preheated hydrogen gas allows to reduce the impregnated platinum precursor to the zerovalent state even at relatively low temperatures [21] and the carrier restricts the growth of the particles.
a -80
-60
Phase Thermal decomposition Hydrogen reduction Polyol reduction Zelec ECP 3010 XC
10
-40
-20
Phase / Grad
| Impedance| / Ohm
|Impedance| Thermal decomposition Hydrogen reduction Polyol reduction Zelec ECP 3010 XC
0
1 300m
1
10
1k
100
10k
100k 300k
Frequency / Hz
b 77.2
Rct (Ohm cm2)
27.4
5.4
1.2
t3
ys
l ta
Ca
t2
Ca
t1
ys
ys
l ta
Ca
l ta
P EC C l 0X Ze 01 3 ec
Fig. 8. a. Impedance spectra measured on electrolyte cells with different catalytic layers. The catalytic layers consists of 0,7% Pt loaded SnO2/Sb prepared by hydrogen reduction (catalyst 1), polyol reduction (catalyst 2), thermal decomposition (catalyst 3), and Zelec ECP 3010 XC SnO2/Sb. Of each catalytic layer 5 identical cells were prepared and measured. Only one representative spectrum of each layer is shown. b. Charge transfer resistance_s obtained from the impedance spectra. In each case the shown result is the average value of 5 identical cells.
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3.4. Thermal decomposition
4. Conclusions
The preparation of platinum catalyst for DSSC via thermal decomposition of H2PtCl6 is the most disseminated preparative pathway. The platinum precursor is solved in dry isopropanol and tempered at 380 – 390 -C [6,7,9]. Our thermogravimetric (TG) study show that full decomposition of pure (without any organic additives) H2PtCl6 occurs above 580 -C. At 380 -C the mixture of PtCl2 and PtCl4 is formed. Only in the presence of isopropanol or screen-printing media occurs partial reduction of Pt(VI) to Pt(0). XPS analysis (Fig. 6) revealed three species for Pt 4f at these temperatures. Most part (¨76%) of the platinum precursor is reduced to zero valent state (species 1) and the rest remains in oxidation state +2 (¨12%) (species 2) and + 4 (12%) (species 3). Our results are in good agreement with Lin et al. [9]. After sealing the printable solar cells at 630 -C, hexachloroplatinic acid decomposes completely and results in zerovalent platinum. On the other hand the high temperatures lead to highly agglomerated particles. As the TEM images show the particle size distribution for the platinum particles after 400 -C (Fig. 7a) is very broad and ranges between 4 and 18 nm. After 630 -C the particle size increases further more and ranges between 5 and 30 nm (Fig. 7b).
Our comparative detailed study of preparation pathways revealed that hydrogen reduction seems to be the most convenient and promising preparative method for producing catalytic layers for DSSC. The XPS and XANES analysis show that hydroborate, polyol and hydrogen reduction result in Pt(0). Thermal decomposition of H2PtCl6 precursor to Pt(0) occurs only above 580 -C. The XRD and TEM analysis indicated that the mean particle sizes remain in nanosize range. The best electrochemical performance showed the Pt catalyst prepared by hydrogen reduction.
3.5. Immobilization of nanoparticles on support
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The specific temperature regime is necessary for the manufacturing of DSSC. Therefore, it is impossible to avoid the agglomeration of particles completely. The only way to restrict the particles from the sintering is the immobilization of the particles on support. Another problem is a good adhesion of the Pt particles on support [6]. If the Pt does not stick to substrate properly, particles begin moving through the cell and catalyze dark currents at the TiO2/electrolyte interface. Among the most widely reported supports we tested carbon black and antimony doped tin dioxide because of their very good conductive performance. Carbon black has some benefits as compared with tin dioxide: (i) it can bare any colloid, (ii) higher BET surface area (264 m2/g vs. 51 m2/g) and (iii) cost factor. But the main disadvantage of carbon black is a sensitivity against the temperature. Our TG observations show that carbon black ignites in air at 500 -C. For that reason SnO2:Sb seems to be favorable for application in DSSC. 3.6. EIS measurement In Fig. 8a impedance measurements carried out on electrolyte cells are shown. From the impedance spectra the charge-transfer resistance Rct was evaluated [6]. The Rct’s presented in Fig. 8b are in each case the average values of five identical cells. The best catalytic performance was achieved by the catalyst prepared by the hydrogen reduction (catalyst 1). Relatively high Rct values of catalysts as compared with Ref. [7] could be explained by the agglomeration of Pt nanoparticles treated at higher temperature.
Acknowledgment The authors thank A. Dreier from MPI Kohlenforschung for TEM performance, Dr H. Modrow and N. Palina (Physical department of the University of Bonn) for XANES measurements and Dr. R. Stahl at the Forschungszentrum Karlsruhe, ITC-CPV for the TG analysis. This work is supported by the German Federal Ministry of Education and Research (BMBF) (grant # 01 SF 0304). References
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