Enhanced Electrorheological Activity of Mesoporous Cr-Doped TiO2

Jun 15, 2006 - Synthesis of anatase TiO 2 with exposed (100) facets and enhanced electrorheological activity. Kai He , Qingkun Wen , Chengwei Wang ...
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J. Phys. Chem. B 2006, 110, 12916-12925

Enhanced Electrorheological Activity of Mesoporous Cr-Doped TiO2 from Activated Pore Wall and High Surface Area Jian B. Yin and Xiao P. Zhao* Institute of Electrorheological Technology, Department of Applied Physics, Northwestern Polytechnical UniVersity, Xi’an 710072, People’s Republic of China ReceiVed: September 26, 2005; In Final Form: May 20, 2006

To enhance electrorheological (ER) activity by improving interfacial polarization, we prepared a new mesoporous Cr-doped TiO2 ER material by a copolymer-templated sol-gel method. The material was characterized by differential scanning calorimeter and thermogravimetric (DSC-TG) analysis, Fourier transform infrared (FT-IR), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption, and X-ray photoelectron spectroscopy (XPS) techniques. The ER activity was studied by the rheological curve and yield stress under an electric field. The results showed that the mesoporous Cr-doped TiO2 ER material possessed a high surface area over 200 m2/g and a crystalline anatase pore wall doped by different valent Cr ions. The ER activity of mesoporous Cr-doped TiO2 was higher than that of nonporous Cr-doped TiO2. The yield stress and ER efficiency of the mesoporous Cr-doped TiO2 ER suspension was 3 times as high as that of the nonporous Cr-doped TiO2 ER suspension, 7 times as high as that of the mesoporous undoped TiO2 ER suspension, and 20 times as high as that of the nonporous pure TiO2 ER suspension. Furthermore, the ER activity of mesoporous Cr-doped TiO2 showed a dependence on surface area, and the high porosity or surface area samples showed higher ER activity. The dielectric spectra analysis showed that the mesoporous Cr-doped TiO2 ER suspension possessed a significantly larger interfacial polarizability compared with the nonporous Cr-doped TiO2 ER suspension, and the regular change of polarizability with surface area or porosity was in accordance with the change of ER activity with surface area or porosity. The improvement of dielectric properties or polarization could well explain the enhancement of the ER activity of mesoporous Cr-doped TiO2.

Introduction Electrorheological (ER) fluid is a smart material, which consists of polar particles in insulating liquid. Under an electric field, the dispersed particles are polarized and attracted to each other to form fibrouslike structures along the direction of the electric field. This anisotropic microstructure enables ER fluid to suddenly change its macroscopic behaviors, such as a viscosity increase or a liquid-solid phase translation.1-4 Especially, these changes are reversible and rapid (within several milliseconds) with the applied electric field. This simple and rapid transformation from electric to mechanical force makes ER fluid have potential uses in the active control of conventional and intelligent devices, such as dampers, clutches, and robotics.5-8 However, the practical utilizations of ER devices have not been achieved due to the lack of suitable ER materials with a sufficient ER performance. In the past years, two different formations including extrinsic (water-containing) and intrinsic (water-free) ER materials have been developed.6,9 For the extrinsic ER materials, such as starch, silica gel, and poly(lithium methacrylate), water or other polar liquids must be presented in particles in order to produce the ER effect. The function of the adsorbed water or other polar liquids is to create mobile charge carriers for particle polarization or to set up a “water bridge” between particles.2,10 Some extrinsic ER materials show an optimal yield stress; particularly, some ER materials developed according to the physical picture of water-containing ER systems have been demonstrated to show * Corresponding author. Phone: (86)29-88495950. Fax: (86) 2988491000. E-mail: [email protected].

a very high yield stress,11-13 but the adsorbed water or other polar molecules have also increased the current density of the ER fluid and limited the working temperature range. The invention of water-free ER materials, such as semiconducting polymer, aluminosilicate, and carbonaceous materials, promotes the improvement of ER performance and ER mechanisms.14-16 The ER effect of intrinsic ER material is related to its natural structure, such as polar groups17,18 and intrinsic charge carriers14-16,19,20, rather than the extrinsic promoters or additives. Thus, the intrinsic ER materials possessed a better temperature stability and lower current density compared with the extrinsic ER materials. However, the ER performance of most of the present ER materials is still insufficient to meet ER technical application due to either low yield stress or limited thermal stability or colloid instability. The ER effect is widely accepted to be originated from the polarization of dispersed particles in suspension under an electric field.1-3 The conventional polarization and conduction models are independently related to the dielectric constant and conductivity of particles and the continuous phase, respectively.1,21 These models have been used to explain the fibrous structure and the nonsquare dependence of yield stress on field strength at high electric field,1,21,22 but they cannot well interpret rheology under dynamic conditions including shear and frequency. In 1992, Anderson23 and Davis24 clarified the importance of finite conductivity in ER fluids and pointed out that the conductivity and dielectric constant took a differently dominant role on particle polarization depending on the electric field frequency. Thus, the dielectric constant and conductivity are simultaneously

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Enhanced ER Activity of Mesoporous Cr-Doped TiO2 used to treat particle polarization and the ER effect. Furthermore, Block and other researchers insisted on the importance of large interfacial polarization to the ER effect25,26 and they further proposed that a good ER effect requires ER fluids to first have a dielectric relaxation peak in ′′ (the image part of the complex permittivity, also can be expressed by σ/2πf, where σ is the conductivity and f is the frequency27) in an adequate frequency range of 102-105 Hz and then have a large ∆′ value (∆′ ) ′100Hz - ′100kHz, where ′ is the real part of the complex permittivity).14b,25,26 The peak of ′′ is related to the suitable polarization response of ER particles (denoted by τ ) 1/2πfmax, where fmax is the local frequency of the ′′ peak), which maintains the stable interaction of particles or chain structures under electric and shearing fields. ∆′ is related to the achievable polarizability. Too slow or too quick polarization responses (fmax is higher than 105 Hz or lower than 102 Hz) and small ∆′ are difficult to maintain stable and strong interaction among particles for a good ER effect. This mechanism emphasizes that not only the polarizability but also the polarization response of particles is key to the ER effect, and the interfacial polarization process has been observed in most ER active materials. TiO2 is a very interesting ER material, which has been presumed as a potential ER active substrate due to is high dielectric constant but actually shows very weak ER activity.28 This is the despair of researchers and cannot be understood by the conventional polarization mechanisms.28b,c It is noted that the weak ER activity of TiO2 mainly originates from its low active intrinsic structure that cannot provide available dielectric and conduction properties for high ER activity. In our previous studies,29,30 doping ions into TiO2 had been found to improve the ER activity of TiO2. However, doping modification mainly influenced the internal structure of TiO2, such as the defect and charge carriers state. Having considered the importance of the interfacial polarization to ER activity,26,27 designing the ER particles with a large and active interface or surface may benefit the further enhancement of ER activity based on the activated internal structure. Thus, the porous materials may be interesting candidates to develop high performance ER materials owing to the large and active interface or surface. In the recent years, the discovery of mesostrucutred and mesoporous materials has given scientists a new way to develop highly functional materials.31 The characters including high special surface area and nanosize active pore wall endue mesoporous materials with many novel or enhanced properties that exceed conventional solids.32 Choi et al. first used mesoporous silica (MCM-41) as an ER dispersal phase, but the yield stress was only 50 Pa (3 kV/mm) for a 20 wt % MCM-41/silicone oil suspension and the ER effect was influenced by water.33 It was reported that the conductor-confined mesoporous silica showed ER enhancement,34 but the contribution from mesopore itself to polarization and ER properties was not clarified. We prepared a nonsiliceous porous rare earth modified TiO2 ER system using small molecular dodecylalmine surfactant as the template,35 but the low crystalline pore wall resulted in thermal instability of the materials, which limited its application and understanding about the ER effect. Recently, we used an amphiphilic copolymer as the template to develop a mesoporous rare earth-doped TiO2 ER system.36 This material possessed good thermal stability due to its thick pore wall structure and showed high ER activity. In this study, to further enhance the ER activity of TiO2 and extend the understanding about ER enhancement from mesopores, we further prepare a new ER material of mesoporous Cr-doped TiO2 by a block-copolymer-templated sol-gel method based on the previous work30 and make a comparative inves-

J. Phys. Chem. B, Vol. 110, No. 26, 2006 12917 tigation on the structure, ER properties, and dielectric properties of mesoporous Cr-doped TiO2 with nonporous Cr-doped TiO2, mesoporous pure TiO2, and nonporous pure TiO2 ER systems. The mesoporous Cr-doped TiO2 with different surface areas is also obtained by varying the template content and its ER, dielectric properties are also studied comparatively. It is found that mesoporous Cr-doped TiO2 possesses a higher ER activity compared with nonporous Cr-doped TiO2, and the ER effect of mesoporous Cr-doped TiO2 shows a dependence on surface area. This enhancement of the ER activity of mesoporous Cr-doped TiO2 can be explained by the improvement of dielectric properties due to the presence of a high surface area mesoporous structure. Experimental Section Preparation of Mesoporous Cr-Doped TiO2. Titanium tetrabutyl titanate (TBT, Tianjin Chemical Co. China) was used as the TiO2 source, and CrCl3‚7H2O (Tianjin Chemical Co. China) was used as the dopant source. A poly(ethylene oxide)based triblock copolymer (Pluronic F-127 (PEO106PPO70PEO106), Aldrich) was used as the template to direct the formation of a mesoporous structure in materials. All chemicals were used as received. A typical synthesis went as follows: First, Pluronic F-127 was dissolved into ethanol/butanol (v/v ) 1:1) containing HCl, and then, TBT and CrCl3‚7H2O were dissolved into this mixed solution, respectively. The molar composition was x (0.00250.020) F-127/1.0 TBT/0.10 CrCl3 (here, the selected ratio of Cr/Ti is optimal for ER activity according to our previous studies30)/20 ethanol + butanol/0.20 HCl. Here, mesoporous doped TiO2 with a different surface area was obtained by varying the molar ratio, x, of the template to Ti. Second, the resulting mixed solution was aged in an open dish at 40 °C and a humidity of 50-60% for 1 week to evaporate the solvent and form a mesostructured copolymer/Ti hybrid transparent gel. Third, the gel was subjected to either a ladderlike heat treatment (60 °C/12 h, 80 °C/12 h, and 120 °C/12 h) or a microwave irradiation (200 W/5 min, 300 W/5 min, 500 W/5 min, and 700 W/5 min) in order to further consolidate the continuous phase and form a hard transparent bulk. Finally, the hard bulk was calcined with a ramp of 5 °C/min to 350 °C and remained there for 3 h and then to 450 °C and remained there for 3 h in air to remove the template and obtain crystalline mesoporous doped TiO2. The ER particles were obtain by milling and sieving to a particle size of about 0.5-5.0 µm and remained in the desiccator. To make a comparison, nonporous pure TiO2 and Cr-doped TiO2 were also prepared by the same sol-gel methods in the absence of a template. Characterization. The thermal analytical curve of the sample was obtained by DSC-TG (SH-500, NET2SCH-Gerateban Gabh Thermal Analysis) with a heating rate of 5 °C/min in the temperature range 20-800 °C. Infrared spectra were recorded on a Fourier infrared spectrometer (JASCO FT/IR-470) at a resolution of 4 cm-1. Small angle (0.5-6°, 30 kV/30 mA, 0.01°/s) and wide angle (15-70°, 40 kV/40 mA, 0.02°/s) X-ray powder diffraction (XRD) patterns were determined on a D/MAX-IIIC X-ray diffractometer with Cu KR irradiation. The crystalline size was estimated by applying the Scherrer equation to the full width at half-maximum (fwhm) of the (101) peak of anatase, with silicon as a standard of the instrumental line broadening. Scanning electron microscopy (SEM, JSM-5800) and transmission electron microscopy (TEM, JEM-2000EX) were used to observe particle and pore morphology, respectively. The nitrogen adsorption isotherms and special surface areas were obtained

12918 J. Phys. Chem. B, Vol. 110, No. 26, 2006 using a Quantachrome Nova2000e surface area and pore size analyzer. All samples were degassed at 200 °C under a vacuum for a minimum of 6 h. The special surface areas were calculated using the Brunauer-Emmett-Teller (BET) model from a linear part of the BET plot (P/P0 ) 0.10-0.30). Average pore diameters were calculated using the Barrett-Joyner-Halenda (BJH) method from the desorption branch of the isotherm. X-ray photoelectron spectroscopy (XPS) measurements were done with a PHI Quantum 2000 XPS system with a monochromatic Mg KR source and a charge neutralizer. All of the binding energies were referenced to the C1s peak at 284.6 eV of the surface adventitious carbon. Preparation of ER Suspensions. First, the ER particles were further dehydrated in a vacuum drum dryer (0.085 MPa) at 150 °C for 8 h. We had used a DHS-20 infrared moisture analyzer to measure the moisture in the powder and found that there was no detected moisture after dehydration in a vacuum for 4 h. Second, the dried samples were mixed quickly with silicone oil (η ) 25 mPa‚s, f ) 2.75-2.95, Ff ) 0.998-1.005 g/cm3 at 25 °C, also dried at 150 °C for 8 h) in a particle volume fraction of 18% to produce ER suspensions. Here, the volume fraction is defined by the ratio of the particle volume to the total volume of the ER suspension. The particle’s apparent density was determined using the pycnometer method in water. No additives were added into the suspensions. Finally, the mixed suspensions were milled for 2 h in a mortar for sufficient mixing of the particles with the oil phase and then heated at 150 °C for 2 h. The Fourier transform infrared (FT-IR) spectra of the ER suspension, by daubing it on KBr crystal, showed that no peaks at around 3400 and 1650 cm-1 from water appear. Therefore, it is considered that the amount of absorbing moisture is too small to influence the ER effect of samples. Electrorheological Measurement. The ER experiment was carried out with a THERMAL HAAKE RS600 CS electrorheolometer with a parallel plate system PP ER35, a WYZ-010 dc high voltage generator, a PC-controlled circular oil bath system for temperature control, and a PC computer. The gap of the parallel plates was 1.0 mm. The maximum voltage of the dc high voltage generator was 10 kV, and the current limitation was 2.0 mA. The rheological curves were measured by the controlled shear rate (CR) mode from 0.1 to 1000 s-1. The dynamic yield stress was measured by the controlled shear stress (CS) mode. Before reading stresses, we had initially applied the electric field on suspensions for 10 s and then sheared. Dielectric Analysis. The dielectric spectra of the ER suspension were measured by an impedance analyzer (HP4284A) within the frequency range from 20 Hz to 1 MHz using a measuring fixture (HP16452A) for liquids to investigate their interfacial polarization. The 1 V of bias electrical potential was applied to the ER fluid during measurement. It was small enough that no chain formation within the ER fluid was induced; thus, we could obtain the true behavior of the interfacial polarization between the particles and the medium and well compare the dielectric behaviors.34 Results and Discussion Structure Characteristic. The DSC-TG curve and FT-IR spectra are used to evaluate the effect of the sintering process on the material structure. The DSC-TG curve of the typical mesoporous doped TiO2 xerogel (template/Ti ) 0.010) shows three stages. At about 110 °C, there is a small endothermic peak that is attributed to the removal of residual water or solvent. Between 270 and 320 °C, there are an obvious exothermic peak and a corresponding big weight loss, which is due to the

Yin and Zhao

Figure 1. FT-IR spectrum of mesoporous Cr-doped TiO2: (A) xerogel before microwave irradiation; (B) after microwave irradiation; (C) after 200 °C calcination; (D) after 350 °C calcination; (E) after 450 °C calcination.

combustion of organic components, such as block copolymer. The third weak peak at approximately 420 °C is attributed to the phase transition of TiO2 from amorphous to anatase.29,37 This transition temperature is found to be slightly higher than that (between 380 and 400 °C) of undoped TiO2 possibly because Cr doping retards crystallization of the pore wall. This can also be revealed by the XRD result (see next section). Figure 1 shows the corresponding IR spectra at different stages. The IR spectra of as-made mesostructured Cr-doped TiO2 xerogel shows the adsorption peaks at 2960, 1520, and 1432 cm-1 corresponding to C-H vibration of copolymer. After microwave irradiation, the Ti-O vibration adsorption peak at about 520 cm-1 becomes strong due to consolidation of the gel net. After 200 °C calcination, the C-H vibration adsorption peaks do not disappear. After 350 °C calcination, lots of organic components have been combusted and only some weak peaks at around 2960 cm-1 due to C-H can be found. After 450 °C calciantion, there is one peak corresponding to Ti-O vibration at about 520 cm-1. No other adsorption peaks from organic components are detected. Therefore, the results above indicate that the copolymer template Pluronic F-127 has been completely removed from samples and the anatase framework has been formed after calcination at 450 °C. Figure 2 shows the XRD patterns of samples. The mesoporous structure is confirmed by low angle XRD (LXRD) patterns. The single LXRD diffraction peak in mesoporous samples indicates that the mesophase is wormhole-like.31b However, the peak intensity of mesoporous Cr-doped TiO2 is much larger than that of undoped mesoporous TiO2 and the full width at halfmaximum (fwhm) of mesoporous Cr-doped TiO2 is smaller than that of undoped mesoporous TiO2, indicating that Cr doping improves the order of the mesophase. No LXRD peak is detected for nonporous undoped and doped TiO2 particles. The wide angle XRD (WXRD, inset) peaks show that the framework of all calcined samples is made of nanocrystalline anatase. Especially, the clear WXRD peaks of mesoporous samples reveal the highly crystalline nature of the pore wall.38 These crystallized pore walls are helpful to ensure the produced ER materials have a high dielectric constant because the large dielectric constant mismatch between the particles and the oil phase is one of the basic factors to a good ER effect.2,3 The average crystal size, calculated from the (101) plane of anatase with the Scherrer equation, is 25.4 nm for nonporous pure TiO2, 19.7 nm for nonporous Cr-doped TiO2, 10.4 nm for mesoporous

Enhanced ER Activity of Mesoporous Cr-Doped TiO2

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Figure 2. Low angle XRD pattern of calcined samples: (A) nonporous pure TiO2; (B) nonporous Cr-doped TiO2; (C) mesoporous pure TiO2 (template/Ti ) 0.010); (D) mesoporous Cr-doped TiO2 (template/Ti ) 0.010). The inset shows the corresponding wide angle XRD pattern of calcined samples: (A) nonporous pure TiO2; (B) nonporous Crdoped TiO2; (C) mesoporous pure TiO2; (D) mesoporous Cr-doped TiO2.

undoped TiO2, and 7.2 nm for mesoporous Cr-doped TiO2. This shows that doping Cr ions into the pore wall inhibits the crystal growth of anatase, and the mesoporous samples have a lower crystal size possibly because the mesopores isolate the congregation of more crystallites. Moreover, no peaks of chromium oxide are observed in the doped samples and the anatase unit cell of doped samples is somewhat shrunk compared with the undoped one after heat treatment at 450 °C. The d(101) (the spacing value of the (101) plane of anatase) of pure TiO2, Crdoped TiO2, mesoporous pure TiO2, and mesoporous Cr-doped TiO2 is 3.542, 3.451, 3.548, and 3.465 Å, respectively. This indicates that some small radius Cr ions (the ion radius is in the 0.52-0.64 Å range for Cr3+, Cr4+, and Cr6+ that have been detected to coexistence in mesoporous doped TiO2 by XPS spectra, and the ion radius of Ti4+ is 0.68 Å) insert into the TiO2 framework and distort to some extent the anatase crystalline structure.39 The apparent density, determined by the method of a pycnometer, is 3.83 g/cm3 for nonporous pure TiO2, 3.78 g/cm3 for nonporous Cr-doped TiO2, 3.76 g/cm3 for mesoporous pure TiO2, and 3.65 g/cm3 for mesoprous Cr-doped TiO2. Figure 3 shows SEM and TEM images of the typical mesoporous Cr-doped TiO2 ER particles after calcinations at 450 °C, respectively. It is found that the size of sieved particles is in the range 500-5000 nm and the shape is irregular. Nonporous pure TiO2, mesoporous pure TiO2, and nonporous Cr-doped particles also show a similar size and shape to those of mesoporous Cr-doped TiO2 particles. From Figure 3b, the clear mesoporous structure can be found and the pore size is about 5-7 nm. No mesopores are found in single doped TiO2 particles. The electron diffraction of a selected area (see the inset of Figure 3b) shows several clear Debye-Scherrer rings corresponding to reflection of a TiO2 anatase phase. This also indicates a highly crystalline nature of the pore wall, which is in accordance with the WXRD result. Furthermore, to understand the contribution from a high surface area or porosity of the mesoporous structure to ER activity, we also prepare mesoporous Cr-doped TiO2 with different surface areas by varying the molar ratio of the template to Ti. Figure 4 shows the TEM images of mesoporous Cr-doped TiO2 ER particles with different surface areas. From Figure 4A, B, and D, it is found that the mesopores are disordered and wormhole-like when the ratio of the template to Ti is too low or too high, while the mesopore seems to increase its order when the ratio

Figure 3. (a) SEM image of typical mesoporous Cr-doped TiO2 (template/Ti ) 0.010) particles and (b) TEM image of mesoporous Cr-doped TiO2 (template/Ti ) 0.010) particles and its electron diffraction pattern characteristic of anatase (inset) (scale bar ) 50 nm).

of the template to Ti is suitable (see Figure 4C). This can also be reflected by the LXRD pattern (not shown here), in which the peak intensity of mesoporous Cr-doped TiO2 prepared with template/Ti ) 0.010 is much larger and the full wave at halfmaximum (fwhm) is smaller than those of mesoporous Cr-doped TiO2 prepared with too low or too high of a template/Ti ratio. Furthermore, a higher template/Ti ratio seems to slightly increase the pore size (see Figure 4D), which can also be indicated by the calculated BJH pore size (see Table 1) from the N2 adsorption-desorption curve. Figure 5 presents the typical N2 adsorption-desorption isotherms and pore size distributions of samples. The isotherm of mesoporous pure TiO2 and mesoporous Cr-doped TiO2 samples are identified as a type IV isotherm with a hysteresis of type H1, indicating the presence of mesopores with a relative narrow pore size distribution. However, no mesopore can be reflected according to the isotherm of nonporous pure TiO2 and Cr-doped TiO2. The BET surface area is 25 m2/g for nonporous pure TiO2, 42 m2/g for nonporous Cr-doped TiO2, 158 m2/g for mesoporous pure TiO2 (template/Ti ) 0.010), and 205 m2/g for mesoporous doped TiO2 (template/Ti ) 0.010). The BJH average pore diameter is 6.12 nm for mesoporous Cr-doped TiO2 and 6.93 nm for mesoporous pure TiO2 (see Figure 5). The pore volume is

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Figure 4. TEM images of mesoporous Cr-doped TiO2 synthesized with different template/Ti mole ratios: (a) 0.0025:1; (b) 0.005:1; (c) 0.010:1; (d) 0.020:1 (scale bar ) 70 nm).

TABLE 1: Material Properties of Mesoporous Cr-Doped TiO2 ER Materials Synthesized with Different Molar Ratios of Template to Ti sample SBETa Vb DBJHc porosityd crystal crystal (template/Ti) (m2/g) (cm3/g) (nm) (%) phase sizee (nm) 0.0025:1 0.005:1 0.010:1 0.020:1 no template

112 164 205 181 42

0.17 0.21 0.42 0.37 0.06

5.7 5.5 6.1 6.9 17.5

37.8 42.8 60.0 54.1

anatase anatase anatase anatase anatase

7.9 6.9 7.0 7.2 19.7

a BET surafce area, calculated by the linear part of the BET plot (P/P0 ) 0.1-0.3). b Total pore volume, obtained from the volume adsorbed at P/P0 ) 0.995. c Average pore diameter, determined using the BJH method from the desorption branch of the isotherm. d The porosity was estimated from the total pore volume and density of the pore wall. e The crystal size was calculated by the Scherrer equation from the (101) plane of anatase in WXRD.

0.42 cm3/g for mesoporous Cr-doped TiO2 and 0.33 cm3/g for mesoporous pure TiO2. The larger surface area (42 m2/g) of nonporous doped TiO2 compared with pure TiO2 (25 m2/g) may result from the smaller crystal size. The larger surface area and pore volume of mesoporous doped TiO2 as compared with mesoporous undoped TiO2 indicates that doping Cr ions into pore walls improves the stability of the mesoporous structure. According to WXRD, we consider this is because doping has fined crystal size and thus maintain the resulting mesoporous structure without destruction at the onset of crystal-growthinduced stress in pore walls. Table 1 lists the pore and crystal properties of samples prepared by varying the molar ratio of template to Ti. It is found that the ratio of template to Ti barely influences the crystal phase and crystallization level of framework, but it changes significantly the surface area and pore volume. When the ratio of template to Ti is about 0.010:1, the sample possesses the highest surface area and porosity. How-

Figure 5. N2 adsorption-desorption isotherms (inset) and pore diameter distribution of calcined samples: nonporous pure TiO2 (squares); nonporous Cr-doped TiO2 (circles); mesoporous pure TiO2 (up triangles, template/Ti ) 0.010); mesoporous Cr-doped TiO2 (down triangles, template/Ti ) 0.010).

ever, when the ratio of template to Ti is increased to 0.020:1, the surface area declines to 181 m2/g. This is possibly because too high of a template content in the original solution results in a lamellar mesophase, whose thermal stability is relatively poor.31c The chemical component and valence are determined by XPS spectra, and Figure 6 shows the typical full XPS spectra of mesoporous pure TiO2 and mesoporous Cr-doped TiO2. It is noted that the mesoporous Cr-doped TiO2 contains only Ti, O, Cr, and a trace amount of carbon, indicating chloride ion and the template are indeed removed by calcination. The atomic ratio, determined from the ratio of peak areas corrected with

Enhanced ER Activity of Mesoporous Cr-Doped TiO2

Figure 6. XPS survey spectrum of calcined mesoporous pure TiO2 (A) and mesoporous Cr-doped TiO2 (B) (template/Ti ) 0.010).

Figure 7. (a) Ti2p XPS spectra of (A) mesoporous pure TiO2 and (B) mesoporous Cr-doped TiO2. The inset shows the high resolution spectra of Ti2P3/2. (b) Cr2p XPS spectra of mesoporous Cr-doped TiO2. The inset shows the high resolution spectra of Cr2P3/2.

the empirical sensitivity factors, of Cr/Ti is 0.097:1, which is in good agreement with the designed atomic composition. The high resolution XPS spectra of Ti2p in Figure 7a show that the chemical state of Ti is the +4 oxidation state in mesoporous pure TiO2, while the chemical state of Ti in mesoporous Crdoped TiO2 contains two kinds of valences including Ti3+ and Ti4+ (see the high resolution XPS spectra of Ti2p3/2 in the inset of Figure 7a) and there is a weak peak shift of Ti2p toward high binding energy. This indicates that the local chemical environment of Ti ions has been significantly influenced by Cr incorporation.39-41 A similar result is also observed in nonporous Cr-doped TiO2.30 The XPS spectrum of Cr2p in Figure 7b shows that three different oxidation states of +3 (576.6 eV), +6 (578.9

J. Phys. Chem. B, Vol. 110, No. 26, 2006 12921 eV), and a small quantity of +4 (575.2 eV) coexist in mesoporous Cr-doped TiO2. This coexistence of different oxidation states may be for the charge balance in the TiO2 lattice.37 Therefore, like nonporous Cr-doped TiO2 ER materials, doping Cr ions into mesoporous TiO2 has also influenced the electric state of elements. This is important to the modification of the dielectric and conduction properties of TiO2.30 According to the characterizations above, the employment of this large molecular weight Pluronic copolymer as the template has produced a mesoporous TiO2 that possesses not only a high surface area but also a crystalline doped pore wall by Cr ions. Electrorheological Property. Figure 8 shows the flow curve of shear stress and shear viscosity as a function of shear rate measured by the CR mode for nonporous Cr-doped TiO2 and mesoporous Cr-doped TiO2 ER suspensions with a particle volume fraction of 18% under dc electric fields, respectively. Under zero electric field, both ER suspensions show a shear thinning behavior like most particle suspensions.42,43 When the electric field is applied, nonporous doped TiO2 and mesoporous doped TiO2 ER suspensions exhibit a shear stress or viscosity increase behavior, but the field-induced shear stress (defined by τE - τ0, where τE is the shear stress with an electric field and τ0 is the shear stress without an electric field) of the mesoporous doped TiO2 ER suspension is higher than that of the nonporous Cr-doped TiO2 ER suspension at the same electric field. For example, the field-induced shear stress of the mesoporous doped TiO2 ER suspension is close to 5.40 kPa at an electric field of 4.0 kV/mm and a shear rate of 100 s-1, which is higher than that (1.85 kPa) of the nonporous doped TiO2 ER suspension, and this shear stress is higher compared with mesoporous silica-based ER suspensions.33,34 The ER efficiency (defined by (τE - τ0)/τ0 or (ηE - η0)/η0, where τE and ηE are the shear stress and shear viscosity with an electric field and τ0 and η0 are the shear stress and shear viscosity without an electric field, respectively) of the mesoporous Cr-doped TiO2 ER suspension is close to 60 (4 kV/mm) at a shear rate 100 s-1 and close to 9 at 1000 s-1, which is 3 times as high as that (∼18 at 100 s-1 and 3.5 at 1000 s-1) of the nonporous Crdoped TiO2 ER suspension and 40 times as high as that (∼1.5 at 100 s-1 and 0.2 at 1000 s-1) of the nonporous pure TiO2 ER suspension. Furthermore, the mesoporous Cr-doped TiO2 ER suspension shows more stable shear stress with shear rate compared with the nonporous doped TiO2 ER suspension. From Figure 8a, for the nonporous doped TiO2 ER suspension, the shear stress as a function of shear rate initially decreases to a minimum value at a critical shear rate (this critical shear rate is different under various electric fields, and it becomes higher at larger field strengths; see the dashed line) and then increases as the shear rate increases. However, from Figure 8b, the shear stress of the mesoporous doped TiO2 ER suspension does not decline except for a slight fluctuation at 5.0 kV/mm, and the shear stress maintains a stable level for the whole shear rate regime. It is known that the rheological behavior of an ER suspension is the result of a change of fibrouslike structures. This structure change is mainly dominated by the electric-fieldinduced electrostatic interaction and the shear-field-induced hydrodynamic force.4,44 The large polarizability and enough polarization response of ER particles are important to produce stronger and faster electrostatic interaction that can maintain structures and rheological properties stable under shear flow. The flow curves with the appearance of shear stress decrease to a minimum value; for the nonporous Cr-doped TiO2 ER suspension, it may be because the destruction rate becomes faster

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Figure 8. Flow curves of shear stress and shear viscosity as a function of shear rate measured by the CR mode: (a) nonporous Cr-doped TiO2 ER suspension; (b) mesoporous Cr-doped TiO2 (template/Ti ) 0.010) ER suspension (T ) 25 °C, particle volume fraction ) 18%).

than the reformation rate of fibrouslike structures with increasing shear rate. This reveals that shear deformation gradually overcomes electrostatic interaction of particles and the polarization response of nonporous Cr-doped TiO2 particles is slow, so that more and more particle chains broken during shear do not have enough time to reform themselves by an electric field. Even after the critical shear rate, hydrodynamic force almost dominates the rheological curves due to higher shear rate. However, the stable shear stress level for the mesoporous Crdoped TiO2 ER suspension means that the electrostatic interaction and the polarization response of mesoporous Cr-doped TiO2 particles are still enough even if the shear rate is increased, and thus, particles make fibrous structures fast enough to maintain the structure and rheological properties under shear flow. These will be further discussed on the basis of the dielectric analysis in the next section. The formation of fibrouslike structures will also cause a liquid-solid phase translation of the ER suspension. When a shear stress is applied on the solidified ER suspension, the suspension can also flow. Here, the maximum stress that makes the suspension start to flow is named the yield stress, which can well characterize the stiffness of the solidified ER suspension.4,44 To well evaluate the yield stress, we measure the shear stress-shear rate relationship using the controlled shear stress mode because it is more reliable compared with an extrapolation of the curly course of the shear stress to zero shear rate measured by the controlled shear rate mode. Figure 9 is the shear stress as a function of shear rate for the mesoporous Cr-doped TiO2 ER suspension measured by the CS mode at a different electric

Figure 9. Relationship between shear stress and shear rate of typical mesoporous Cr-doped TiO2 (template/Ti ) 0.010) ER suspension measured by the CS mode (T ) 25 °C, particle volume fraction ) 18%).

field. The clear dynamic yield point shown by arrows is found. Figure 10 generalizes the dependence of dynamic yield stress on electric field strength. It is found that the yield stress of the mesoporous Cr-doped TiO2 ER suspension is about 5.8 kPa at 4 kV/mm, which is about 22 times as high as that (270 Pa) of the nonporous pure TiO2 ER suspension and 3.2 times as high as that (1.8 kPa) of the nonporous Cr-doped TiO2 ER suspension at the same Cr-doping degree. Furthermore, the yield stress of

Enhanced ER Activity of Mesoporous Cr-Doped TiO2

Figure 10. Dynamic yield stress as a function of electric field strength for the nonporous pure TiO2 ER suspension (squares), nonporous Crdoped TiO2 ER suspension (up triangles, template/Ti ) 0.010), mesoporous pure TiO2 ER suspension (circles), and mesoporous Cr-doped TiO2 ER suspension (down triangles, template/Ti ) 0.010) (T ) 25 °C, particle volume fraction ) 18%).

Figure 11. Comparison of dynamic yield stress as a function of electric field strength for mesoporous Cr-doped TiO2 ER suspensions with different surface areas (T ) 25 °C, particle volume fraction ) 18%).

the mesoporous pure TiO2 ER suspension is close to 0.85 kPa at 4 kV/mm. This value is relatively higher than that of the nonporous pure TiO2 ER suspension, but it is still much lower than that of the mesoporous Cr-doped TiO2 ER suspension. At the same time, it is found that the influence of the Cr-doping degree on the yield stress of mesoporous TiO2 is similar to the nonporous one. It indicates that doping the pore wall with Cr ions is important for activation of the ER effect of TiO2, and then, the presence of a high surface area mesoporous structure further enhances the ER activity. This phenomenon was also observed in mesoporous rare earth-doped TiO2 ER material in our previous report.36 Therefore, it is concluded that the activated pore wall due to doping and the increased interface or surface area due to mesoporous structure give a combined or synergistic contribution to large enhancement of the ER activity of TiO2. Figure 11 shows the dependence of dynamic yield stress on electric field strength for the ER suspension containing mesoporous Cr-doped TiO2 with a different surface area. It is found that the yield tress shows a dependence on surface area and the high surface area sample seems to show the higher ER activity. Because the doping degree and the crystal phase are the same for all samples and there is only a slight difference in the crystal size, this result indicates that the high surface area mesoporous structure indeed plays an important role in the enhancement of

J. Phys. Chem. B, Vol. 110, No. 26, 2006 12923

Figure 12. Dielectric spectra of the nonporous pure TiO2 ER suspension (squares), nonporous Cr-doped TiO2 ER suspension (circles), and mesoporous Cr-doped TiO2 ER fluid (triangles, template/Ti ) 0.010) (the solid symbols represent the real part ′, and the open symbols represent the image part ′ of the complex permmittivity; T ) 25 °C, particle volume fraction ) 18%).

ER activity. This will be given a preliminary discussion according to the dielectric properties. Dielectric Property. According to the proposed polarization mechanism by Block et al.,25,26 not only the polarizability but also the polarization response of particles is key to the ER effect, and a good ER effect requires that ER fluids should first have a dielectric relaxation peak in ′′ within an adequate frequency range of 102-105 Hz and then have a large dielectric constant difference, ∆′ (∆′ ) ′100Hz - ′100kHz). The dielectric relaxation peak is related to a suitable polarization response that is denoted by relaxation time, τ ) 1/2πfmax (fmax is the frequency of loss peak), and ∆′ is related to achievable polarizability. As the relaxation time within an adequate frequency range of 102-105 Hz gets smaller and higher ∆′ values within this frequency range are applied, the higher ER enhancement will be achieved. Too fast or too slow of a polarization response (fmax is higher than 105 Hz or lower than 102 Hz) and small ∆′ values are difficult to maintain stable, strong electrostatic interaction among particles for good ER effect under electric and shearing fields.25,26 Figure 12 shows the dielectric spectra of nonporous pure TiO2, nonporous Cr-doped TiO2, and mesoporous Cr-doped TiO2 ER suspensions. It is found that no relaxation peak is observed in the nonporous pure TiO2 ER suspension in the measured frequency range of 20 Hz-1 MHz. This results from the fact that the fast ionic or atomic polarization is dominant in pure TiO2, which may be the main reason for the weak ER activity of pure TiO2.45 When only introducing mesopores into pure TiO2, ∆′ is increased but the loss peak is not observed within the expected frequency range of 102-105 Hz (not shown here). Doping Cr ions increases ∆′ and induces a loss peak at about 80 Hz; this has been attributed to the enhancement of charge carriers due to defect and impurity.30 However, it is found that the mesoporous Cr-doped TiO2 ER suspension shows significantly larger interfacial polarizability, which can be reflected by the large achievable polarizability ∆′ and the loss peak within the adequate frequency range, compared with the nonporous Cr-doped TiO2 ER suspension. Therefore, according to the proposed polarization mechanism by Block et al., the large improvement in interfacial polarization is responsible for the further enhancement of the ER activity of mesoporous Crdoped TiO2. Furthermore, the rheological behavior in Figure 8 can also be explained by the dielectric properties. The stable

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TABLE 2: Dielectric Parameters of Mesoporous Cr-Doped TiO2 ER Suspensions with Different Surface Areas sample (template/Ti)

′100Hz

′105Hz

∆′ a

fmaxb (Hz)

′′ c

0.0025:1 0.005:1 0.010:1 0.020:1 no template

8.49 10.05 13.45 11.65 7.66

4.28 4.30 4.37 4.36 4.35

4.21 5.75 9.08 7.29 3.31

200 500 800 600 80

1.85 2.36 3.13 2.69 1.58

a Dielectric constant difference, calculted by ∆′ ) ′100Hz - ′105Hz. The local frequency of the peak of the dielectric loss factor ′′. c The magnitude of the dielectric loss factor at the peak frequency.

b

shear stress level in Figure 8b, for the mesoporous doped TiO2 ER suspension, may be attributed to its stronger achievable polarizability and faster polarization response within a suitable frequency range compared with the nonporous doped TiO2 ER suspension, because the large achievable polarizability induces strong electrostatic interaction among particles and the faster polarization response helps particles make fibrous structures fast enough to maintain the structure and rheological properties under shear flow. In Figure 8a, the flow curves with the appearance of shear stress decrease to a minimum value; for the nonporous Cr-doped TiO2 ER suspension, it may be because its achievable polarizability and polarization response are still insufficient to reform its fibrous structure and overcome the shear destruction when the shear rate becomes higher. For the pure TiO2 ER suspension, because the achievable polarizability is very weak, the electrostatic interaction force of particles is too low to overcome the shear destruction even if the shear rate is low. Thus, the hydrodynamic force dominates the rheological behavior of the pure TiO2 suspension for the whole shear rate region. Table 2 generalizes the dielectric properties of mesoporous doped TiO2 ER suspensions with different surface areas. It is found that ∆′ becomes larger and the loss peak shifts toward higher frequency with increasing surface area or porosity, indicating that the achievable polarization is enhanced due to the presence of mesoporous structure. This regular change of polarizability with surface area or porosity is in accordance with the change of yield stress with surface area or porosity in Figure 11. Therefore, the further improvement in dielectric or polarization properties due to the presence of a high surface area mesoporous structure is responsible for the enhancement of the ER activity of mesoporous doped TiO2. We presume that increasing the surface or interface area in mesoporous particles yields more surface sites for charge carrier (originated from the activated pore wall, such as defects and impurities due to doping ions30) accumulation,46,47 and the higher concentration of charge carriers at the interface or surface regions may have increased the interfacial polarizability of particles under high electric field and thus induced higher ER activity. Therefore, not only does doping Cr ions improve the polarization and ER properties of TiO2, but also the introduction of mesopore into Cr-doped TiO2 further enhances the polarization and ER properties. This combined contribution may be an effective approach to enhance the polarization properties for high ER activity. In addition, the mesoporous Cr-doped TiO2 ER suspension also shows a better colloid stability compared with the nonporous Cr-doped TiO2 ER suspension possibly due to a porosity effect. Its current density is about 2.0 µA/cm2 at 4 kV/mm, which is much smaller than that of microporous aluminosilicate ER materials48 and complex titanate-based ER materials.11-13 Compared with the small molecular amine-templated mesoporous rare earth-doped TiO2 ER suspension,35 this copolymertemplated mesoporous Cr-doped TiO2 ER suspension shows

more stable ER properties in repeated temperature effect tests. This can be attributed to the substantial crystalline nature of pore walls, which benefits the thermal stability of materials structure and electric properties. Conclusion Mesoporous Cr-doped TiO2 was prepared by a copolymertemplated sol-gel method for use as an ER active material. The material possessed a large surface area and a Cr-doped anatase pore wall. The electric-field-induced yield stress and ER efficiency of the mesoporous Cr-doped TiO2 ER suspension was 3 times as high as that of the nonporous doped TiO2 ER suspension, 7 times as high as that of the mesoporous pure TiO2 ER suspension, and 20 times as high as that of the nonporous pure TiO2 ER suspension. Furthermore, mesoporous Cr-doped TiO2 with different surface areas was prepared by varying the ratio of template to Ti. Interestingly, the ER activity of mesoporous Cr-doped TiO2 showed a dependence on the surface area or porosity, and a high surface area or porosity sample showed higher ER activity. By dielectric spectra analysis, it was found that the mesoporous Cr-doped TiO2 ER suspension showed a significantly larger interfacial polarizability, which was reflected by the larger achievable polarizability and loss peak within an adequate frequency range, compared with the nonporous Crdoped TiO2 ER suspension. The regular change of polarizability with surface area or porosity was in accordance with the change of ER activity with surface area or porosity. This improvement of dielectric properties could well explain the enhancement of the ER activity of mesoporous Cr-doped TiO2. Therefore, not only does doping Cr ions increase the ER activity of TiO2, but also introduction of mesoporous structure into Cr-doped TiO2 could further enhance the ER activity. This synergistic contribution further illustrated the potential of combined design in internal structure and mesoscopic structure of ER materials for achievement of high activity. Acknowledgment. We are grateful for the help from Prof. Zhen Q. Guo on the LXRD and WXRD, Associate Prof. Rui H. Zhu on TEM, and Dr. Xiao Y. Li on XPS measurements. This work was financially supported by the National Natural Science Foundation of China (nos. 50025207 and 50272054), NPU Science Foundation for Young Scholars (no. M016206), YingCai program (no. 05XE0129), and Doctoral Science Foundation (no. CX200515). References and Notes (1) Winslow, W. M. J. Appl. Phys. 1949, 20, 1137. (2) Block, H.; Kelly, J. P. J. Phys. D: Appl. Phys. 1988, 21, 1661. (3) Halsey, T. C. Science 1992, 258, 761. (4) Parthasarathy, M.; Klingenberg, D. J. Mater. Sci. Eng., R 1996, 17, 57. (5) Couter, S. P.; Weiss, K. D.; Carlson, J. D. J. Intell. Mater. Syst. Struct. 1993, 4, 248. (6) Hao, T. AdV. Mater. 2001, 13, 1847. (7) (a) Zhao, X. P.; Zhao, Q.; Gao, X. M. J. Appl. Phys. 2003, 93, 4309. (b) Tang, H.; Zhao, X. P. J. Appl. Phys. 2005, 98, 016003. (8) Wang, B. X.; Zhao, X. P. AdV. Funct. Mater. 2005, 15, 1815. (9) See, H. J. Ind. Eng. Chem. 2004, 7, 1132. (10) Stangroom, J. E. Phys. Technol. 1983, 14, 290. (11) Zhang, Y. L.; Lu, K. Q.; Rao, G. H.; Tian, Y.; Zhang, S. H.; Liang, J. K. Appl. Phys. Lett. 2002, 80, 891. (12) Yin, J. B.; Zhao, X. P. J. Colloid Interface Sci. 2003, 257, 228. (13) Wen, W. J.; Huang, X. X.; Yang, S. H.; Lu, K. Q.; Sheng, P. Nat. Mater. 2003, 2, 727. (14) (a) Block, H.; Kelly, J. P. UK Patent 217051B, 1986. (b) Block, H.; Kelly, J. P.; Qin, A.; Wastson, T. Langmuir 1990, 6, 6. (15) (a) Filisko, F. E.; Armstrong, W. E. U.S. Patent 4744914, 1988. (b) Filisko, F. E.; Radzilowski, L. H. J. Rheol. 1990, 34, 539. (16) Sakurai, R.; See, H.; Satio, T. J. Rheol. 1996, 40, 395.

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