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J. Phys. Chem. B 2000, 104, 12012-12020
Surfactant-Templated TiO2 (Anatase): Characteristic Features of Lithium Insertion Electrochemistry in Organized Nanostructures Ladislav Kavan,*,† Jirˇ´ı Rathousky´ ,† Michael Gra1 tzel,‡ Valery Shklover,§ and Arnosˇt Zukal† J. HeyroVsky´ Institute of Physical Chemistry, DolejskoVa 3, CZ-182 23 Prague 8, Czech Republic, Laboratory of Photonics and Interfaces, Swiss Federal Institute of Technology, EPFL, Ecublens, CH-1015, Lausanne, Switzerland, and Laboratory of Crystallography, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zu¨ rich, Switzerland ReceiVed: October 3, 2000
Thin layer electrodes (0.2-0.5 µm) of highly organized nanotextured anatase were prepared by hydrolysis of TiCl4 in the presence of poly(alkylene oxide) block copolymer, Pluronic P-123, acting as the structure-directing agent. Electrochemical properties of these layers were studied in KCF3SO3 + propylene carbonate and in LiN(CF3SO2)2 + ethylene carbonate + dimethoxyethane. The electrodes showed unusually fast capacitive and Li-insertion charging. A most striking effect was the occurrence of two new pairs of peaks in cyclic voltammograms in Li+ containing electrolyte solutions. These, so-called S-peaks, appear in addition to the “ordinary” peaks of Li insertion into anatase. The S-peaks can act as indicators of mesoscopic ordering of the skeleton as they diminish after mechanical or thermal destruction of the organized nanotexture. We suggest that the occurrence of S-peaks is connected to the presence of amorphous TiO2 in the organized skeleton.
Introduction electrochemistry1-10
In recent years, the nonaqueous and photoelectrochemistry11-14 of mesoscopic anatase have attracted considerable academic and practical interest. Among the latter, applications in lithium batteries,5 electrochromic device,s15,16 and solar cells12,13 are foreseen. The dark electrochemistry of anatase is concentrated mainly on the problem of Li insertion, while fundamental theoretical8 and experimental1-10 issues have been addressed. Investigations on single crystal1,4 and polycrystalline4 electrodes have shown that the insertion capacity, Coulombic efficiency, reversibility, and stability depend significantly on the electrode morphology. However, previous reports have mostly dealt with statistically sintered nanocrystals without any mesoscopic ordering and controlled porosity.4-10 Template-synthesized materials with organized mesopores find interesting applications in electrochemistry,17 but the data about TiO2 are still scarce.2 The first surfactant-templated synthesis of mesoporous titania was announced in 1995, but this work was later questioned.18 A key problem is easy crystallization of TiO2 during calcination, which causes the collapse of organized nanotexture at detemplating.18,19 Ordered TiO2 (anatase) was prepared by templating with latex speheres20,21 or with laser-patterned photoresist,22 but the ordering lengths were outside the mesoscopic scale, typically, 0.1-1 µm. Mesoscopic ordering of TiO2 (anatase) was achieved via selforganization of nanocrystals23 and surfactant templating with zirconia-stabilization (PNNL-1).24 These materials also exhibited characteristic Li-insertion electrochemistry as compared to that of “ordinary” nanocrystalline electrodes.2,25 The synthesis of ordered mesoporous metal oxides was recently addressed by Stucky et al.26-28 Their synthetic protocol employs amphiphilic poly(alkylene oxide) triblock copolymer, which acts as a structure-directing agent in ethanolic solution †
J. Heyrovsky´ Institute of Physical Chemistry. Laboratory of Photonics and Interfaces. § Laboratory of Crystallography. ‡
of the corresponding metal chloride (e.g., TiCl4). The organized mesoscopic oxide is formed through a mechanism involving block-copolymer self-assembly and complexation of the metal atom during restrained hydrolysis of the metal chloride. The prepared thermally stable mesoporous oxides exhibit a robust inorganic framework with thick channel walls. In the case of TiO2, the walls are from amorphous TiO2, with embedded nanocrystalline anatase.26,27 It is interesting to note that the hydrolysis of TiCl4 in the absence of any templates gives only the rutile phase of TiO2.14 This paper upgrades our previous effort2,25 pointing at differences between organized and nonorganized mesoscopic anatase used as the Li-insertion host. The cited works2,25 have confirmed that the Li-insertion electrochemistry provides a sensitive tool to investigate subtle changes in the individual nanoarchitectures. This paper aims at a first electrochemical investigation of mesoporous anatase templated by poly(alkylene oxide) block copolymers. Thin layer electrodes were prepared by a modified procedure of Stucky et al.26,27 Experimental Section I. Materials. The block copolymer surfactant Pluronic P-123 was purchased from BASF, its formula (calculated from the composition given by the producer) is HO(CH2CH2O)20[CH2CH(CH3)O]70(CH2CH2O)20H. Alternatively, the material Synperonic P-123 from Fluka was also used. Both surfactants showed virtually identical properties, and the formed TiO2 products were also comparable. Titanium-tetrachloride (99.995+ %), ethanol (spectroscopic grade), trifluoromethylsulfonic acid, ethylene carbonate (EC), 1,2-dimethoxyethane (DME) and propylene carbonate (PC) were from Aldrich. LiN(CF3SO2)2 (Fluorad HQ 115) was from 3M. The SnO2(F) coated glass from Nippon Sheet Glass, 10 Ω/square, served as a support for the electrode preparation. II. Preparation of Electrodes. The stock solution for the film deposition was prepared by dissolving 0.9 g of the P-123
10.1021/jp003609v CCC: $19.00 © 2000 American Chemical Society Published on Web 11/28/2000
Surfactant-Templated TiO2 surfactant in 11 mL of ethanol. To this solution, 1 mL of titanium tetrachloride was added under vigorous stirring. The mixture was maintained in an open beaker at 40 °C for 5 days, the evaporated ethanol being filled up every 12 h. The clear yellowish solution can be stored at room temperature for several weeks without apparent changes. S1: Small amount of the stock solution was spread on the SnO2(F) support, and the liquid layer was subsequently gelled in air at 40 °C for 2 days. The as-made film was then calcined at 400 °C for 5 h in air. The projected electrode area was adjusted by scraping the film’s edges to be 1 × 1 cm2; the free SnO2(F) surface served further for making the electrical contact. The prepared layer is further referred to as S1. S2: Alternatively, the stock solution was deposited onto a rotating support (2000-4000 rpm, 1 min). The spin-coated SnO2(F) was immediately (by omitting the gelation step) calcined at 400 °C for 5 h in air. The projected electrode area was adjusted by cutting the glass supports and scraping the film’s edges to be 1 × 1 cm2; the free SnO2(F) surface served for making the electrical contact. The prepared layer is further referred to as S2. S3: A large-area film was prepared on a glass sheet following the procedure described for S1 (see above). After calcination, the film was scraped from the glass and ground in an agate mortar. The powder was further dispersed under continuous mortaring with small amount of Triton X (Fluka) until a consistence of viscous paste was achieved. The dispersion was promoted by adding particle stabilizers, 0.1 M HNO3 or acetylacetone as described in ref 4. The formed paste was homogenized by stirring and deposited onto SnO2(F). Scotch tape at the edge of the support defined the film’s thickness and left part of the support uncovered.4 The film was finally calcined for 5 h in air at 400 °C. Several electrodes were fabricated from one layer by cutting the glass supports and scraping the film’s edges to be 0.5 × 0.5 cm2; the free SnO2(F) surface served for making the electrical contact. S1-HT: The electrode S1 was calcined in air for 40 h at 430450 °C. For comparison, also TiO2 electrodes made by anodic oxidative hydrolysis of TiCl3 were prepared. The TiO2 layers were deposited at 0 V vs SCE on SnO2(F); the electrolyte solution was 50 mM TiCl3, pH 2.5, the total anodic charge was 90 mC/cm2, the corresponding mass of electrodeposited TiO2 was 37 µg/cm2. Further preparative details are given elsewhere.29 The electrodeposited layer was cleaned by rinsing with 1 mM HCl, water, and ethanol and dried at 100 °C (this electrode is further referred to as “non-calcined electrode”). The calcined electrode was prepared by heat treatment in air at 450 °C for 30 min. III. Characterization of Electrodes. BET surface areas of the prepared materials were determined from nitrogen adsorption isotherms at 77 K (Accusorb 2100E, Micromeritics). The sample for adsorption measurements (equivalent to S1) was prepared in a Petri dish as a thick film. Such a film can be easily scraped from the support and studied in the form of flakes. The S3 and S1-HT materials were prepared from S1 as described above. Alternatively, also the BET areas of the actual thin film electrodes (TiO2 supported on conducting glass) were measured by using Kr-adsorption isotherms at 77 K. Experimental details of adsorption studies of thin films are described in ref 4. The thickness of TiO2 films was measured with an Alpha-step profilometer (Tencor Instruments). SEM images were obtained by a Hitachi S-900 apparatus. Raman spectra were measured using a T64000 spectrometer (Instruments, SA, France) equipped
J. Phys. Chem. B, Vol. 104, No. 50, 2000 12013 with an Olympus BH2 microscope. The spectra were excited in a 180° backscattering geometry by Ar+ laser (Innova 305, Coherent, USA), λ ) 514 nm. The submicron thin layers (S1, S2) were illuminated by the focused laser beam perpendicular to the cross-sectional area of the film. (The vertical excitation of the film’s surface gave too weak a Raman signal). ESCA analysis was performed with a Scienta 310 instrument (Gammadata AB). IV. Electrochemistry. LiN(CF3SO2)2 was dried at 130 °C/1 mPa. KCF3SO3 was prepared by neutralization of trifluoromethylsulfonic acid, recrystallized from aqueous solution, and dried at 120 °C/1 mPa. EC, DME, and PC were dried over the 4A molecular sieve (Union Carbide). The electrolyte solutions, 1 M KCF3SO3 + PC or 1 M LiN(CF3SO2)2 + EC/DME (1/1 by mass) contained typically
