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Photoelectrorheological Phenomena Involving TiO2 Particle Suspensions Y. Komoda, N. Sakai, Tata N. Rao, D. A. Tryk, and A. Fujishima* Department of Applied Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received June 23, 1997. In Final Form: November 28, 1997 Photoeffects on the electrorheological (ER) properties of various types of commercial TiO2 powders dispersed in silicone oil were investigated. Two types of photoelectrorheological (PER) effects (i.e., positive and negative) were observed in these fluids, the effect being dependent on the adsorbed water content of the powder. The positive PER effect of low-water-content (3 wt %) particle suspensions is correlated with the high degree of photoelectrophoretic (PEP) oscillatory motion of particle clusters between the electrodes that was directly observed and measured using microvideo techniques under stationary conditions (i.e., no net flow). This circulation of clusters is responsible for the high photocurrents that are observed even under flowing conditions, that is, when there are few bridges. The enhancement of the PEP effect due to the presence of water is explained on the basis of (1) enhanced charge transfer from the electrode to the particles and (2) greater numbers of trapped photogenerated carriers. These carriers can be trapped via reactions involving water, and the resulting species can be either reduced at potentials less negative than the conduction band edge or oxidized at potentials less positive than the valence band edge. The large decrease in the residence time of particles at the anode due to illumination is thought to be due to a large increase in the concentration of photogenerated minority carriers (holes) in the TiO2, which is an n-type semiconductor. The enhanced PEP effect which results from increased water content is believed to be a key factor in the diminution of the ER effect in the high-water-content particle suspensions.
1. Introduction The electrorheological (ER) effect is a phenomenon in which the viscosity of a fluid, usually a suspension, increases under application of an electric field.1-20,23 Suspensions which exhibit significant ER response typically consist of metal oxide, semiconductor, or polymer particles in insulating liquids.1,2 Over the past several decades, the number of ER-related investigations has increased dramatically due to the realization that the special properties of ER fluids, for example, the continuously variable viscosity, could be used in new types of intelligent automotive mechanical devices such as valves, engine mounts, clutches, brakes, and shock absorbers.3,4 In suspensions of solid particles dispersed in an insulating liquid, an applied electric field (0.6-4 kVmm-1) causes the particles to form chainlike aggregates parallel to the electric field between the electrodes,5,6 after which the fluid flow decreases. The main reason that the particles form chains is thought to be that the electric field induces polarization at the particle/liquid interface.7 To gain additional control of the ER properties of particle suspensions, it is attractive to investigate the use of other types of external stimuli such as light. The combined use of electric fields and UV-visible illumination has been examined previously17-20 but has not yet been fully developed or explained. Ideally, two types of control can (1) Havelka, K. O.; Pialet, J. W. CHEMTECH 1996, 36. (2) Block, H.; Kelly, J. P. J. Phys. D: Appl. Phys. 1988, 21, 1661. (3) Filisko, F. E. Chem. Ind. 1992, 18, 370. (4) Brooks, D. A. In Proceedings of the International Conference on Electrorheological Fluids, Carbondale, IL, 1991; p 367. (5) Halsey, T. C. Science 1992, 258, 761. (6) Klingenberg, D. J.; Zukoski, C. F., IV. Langmuir 1990, 6, 15. (7) Block, H.; Kelly, J. P.; Qin, A.; Watson, T. Langmuir 1990, 6, 6.
be envisaged: (1) the ER effect in the dark is very small but becomes strong under illumination, and (2) the dark ER effect is strong but is almost completely canceled under illumination. The present work attempts to lay a foundation for developing such control. To incorporate the capability of controlling the ER fluid properties with light, an obvious choice for the particle material is a semiconductor. As the ER effect is sensitive to the particle permittivity6,8,9 and conductivity10-15 and the particle/ liquid interfacial conditions,16 when photoactive semiconductor particles are used in an ER fluid, the ER properties should be modified by illumination. A small number of previous reports have examined such effects.17,18 Carreira and Mihajlov have patented the use of photoelectroviscous inks, in which various types of dye particles (e.g., Monastral Green B, copper phthalocyanine) were used in ER fluids.17 They also mentioned the observation of both positive and negative photoeffects, the effects being dependent on the choice of liquid and dye combinations. Filisko also reported the photoelectrorheological (PER) effect using semiconducting phenothiazine powder-based fluids.18 However, these studies have not attempted to (8) Uejima, H. Jpn. J. Appl. Phys. 1972, 11, 319. (9) Gast, A. P.; Zukoski, C. F. Adv. Colloid Interface Sci. 1989, 30, 153. (10) Davis, L. C. J. Appl. Phys. 1992, 72, 1334. (11) Davis, L. C. J. Appl. Phys. 1993, 73, 680. (12) Tang, X.; Wu, C.; Conrad, H. J. Appl. Phys. 1995, 78, 4183. (13) Wu, C. W.; Conrad, H. J. Phys. D: Appl. Phys. 1996, 29, 3147. (14) Tang, X.; Conrad, H. J. Appl. Phys. 1996, 80, 5240. (15) Wu, C. W.; Conrad, H. J. Appl. Phys. 1997, 81, 383. (16) Otsubo, Y.; Sekine, M.; Katayama, S. J. Colloid Interface Sci. 1992, 150, 324. (17) Carreira, L.; Mihajlov, V. U.S. Patent 3553708, 1971. (18) Filisko, F. E. In Proceedings of the International Conference on Electrorheological Fluids; Carbondale, IL, 1991; p 116.
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Figure 1. Schematic diagram of the first generation viscometer (I). A blowup of the PER cell is shown in the rectangular box.
explain the mechanisms of the photoprocesses involved in such changes in the ER properties. Recently, we have reported preliminary results related to the PER effect for TiO2 particles as the dispersed phase in silicone oil.19,20 In those studies, we have used two types of TiO2 particles (Degussa P-25 and ISK ST-01), which are widely used as photocatalysts. As an example of type I control (positive PER effect), we have reported that, with Degussa P-25 TiO2 fluids, illumination causes a significant enhancement of the viscosity.19 As an example of type II control (negative PER effect), we have reported that, with ISK ST-01 TiO2 fluids, illumination leads to a significant decrease in the viscosity.20 In the former case, it was recognized that illumination leads to enhanced charge separation, giving rise to stronger interparticle forces, while, in the latter case, it was found that illumination gives rise to a strong photoelectrophoretic (PEP) effect, which lowers the effective viscosity. It was also recognized that adsorbed water plays an important role in the PEP effect. The effect of adsorbed water layers, for example, on silica and titania particles has been examined by other groups, both experimentally9,18 and theoretically,12,14 and it seems clear that the water layer, which is assumed to be conductive, enhances the dark ER effect, consistent with conductivity-type ER models.10-15 The present work has also confirmed the influence of water on the dark ER effect. However, under illumination, photocatalytic reactions involving water and dissolved O2 occur, which effectively enhance the ability of the particles to acquire excess charge. The strong PEP effect which then results has also been quantitatively examined in the present work by use of microvideo techniques. The mechanisms involved are discussed in detail. 2. Experimental Section Two types of home-built viscometers were used, depending on the experimental requirements. Due to the simplicity of the viscometer design, the simultaneous illumination and application of an electric field is straightforward. In the present work, we have been able to report the results in a form which is similar to that from the conventional rotary viscometer. A schematic diagram of the first-generation viscometer (I) is presented in Figure 1. The photoelectrorheological (PER) cell was constructed from transparent, SnO2-coated (single-sided) conducting glass (Asahi Glass Co., sheet resistance, 30 Ω/0). The SnO2 coating (19) Komoda, Y.; Rao, T. N.; Fujishima, A. Langmuir 1997, 13, 1371. (20) Rao, T. N.; Komoda, Y.; Sakai, N.; Fujishima, A. Chem. Lett. 1997, 307.
Figure 2. Schematic diagram of the second-generation viscometer (II). on the two plates was removed from both ends, leaving a central 1 cm × 1 cm area. The glass plates (SnO2-coated walls facing each other) were separated by insulating double-sided adhesive tape (thickness, 0.25 mm). The PER cell was connected to a graduated glass tube with a capacity of 5 mL. In this study, changes in viscosity under various conditions were detected by measuring the volume of fluid passing through the PER cell as a function of time. Another, second-generation viscometer (II) is shown in Figure 2. This viscometer incorporates a reservoir connected to the cell in order to maintain a constant hydrostatic pressure on the cell, facilitating the continuous monitoring of photoeffects at the same pressure. With this viscometer, the weight of the fluid passing through the PER cell was automatically measured using a balance interfaced to a computer. A dc power supply with a maximum output voltage of 1 kV was used for generation of the electric fields. A 500-W high-pressure mercury lamp was used for illumination. This lamp covers the wavelength range from 320 to 580, with peaks at 365, 405, 435, 546, and 578 nm. The influence of the wavelength on the PER and ER responses was attempted, but the low intensities (2 mW cm-2) associated with the monochromatic light were insufficient to induce a detectable response. However, photocurrents appeared only when we used the 367-nm band-pass filter, confirming the excitation of TiO2 near this wavelength. For the white light experiments reported in the present work, the light intensity at 367 nm (measured using the band-pass filter) was 26 mW cm-2. A water filter was placed in front of the PER cell to absorb IR radiation. Furthermore, a sheet of SnO2-coated glass of the same type as that used in the PER cell was placed just in front of the lamp to prevent heating of the cell. In the experiments carried out using viscometer I (Figure 1), two types of TiO2 were used, Degussa P-25 (Japan Aerosil Co., anatase, average diameter 29 nm) and ST-01 (Ishihara Sangyo Kaisha, Ltd., anatase, average diameter 20 nm). The P-25 and ST-01 powders were dried at 150 °C for 5 h under vacuum. The adsorbed water contents were determined to be 0.6 wt % (P-25) and 8 wt % (ST-01) by thermogravimetric analysis. In the experiments using viscometer II (Figure 2), twelve different types of commercial TiO2 samples, including the above two samples, were used. In this case, all of the samples were used as received except for P-25 and ST-01, which were dried as mentioned. All of the fluids were prepared by dispersing TiO2 particles at a concentration of 0.2 wt % in silicone oil (20-cP viscosity, Toshiba Co., Ltd., Japan). The particles were dispersed by sonication for 30 min at 50 kHz. However, neither clusters of primary nanoparticles (e.g., in the case of ST-01) nor larger particle agglomerates are broken up by this procedure. For example,
TiO2 Particle Suspensions
Figure 3. Dependence of the TiO2 suspension volume (A and D) flowing through the PER cell on electric field (1.6 kV mm-1) and illumination as a function of time, with corresponding flow rate (B and E) and current (C and F) for P-25 (A, B, and C), and for ST-01 (D, E, and F). In each case, curve a corresponds to the absence of both electric field and illumination, curve b to the presence of electric field and the absence of illumination, and curve c to the presence of both electric field and illumination. both P-25 and ST-01 were examined with optical microscopy, and agglomerates in the 1-2 µm range were observed. In addition to the rheological and electrical measurements, we observed the motion of the particles in the ER fluids under various electric field and illumination conditions. The cell was constructed in a fashion similar to that of the other two viscometers. The interelectrode gap was 0.5 mm. The cell was placed on the stage of a microscope equipped with a CCD camera (Hamamatsu Photonics Co., C-5810).
3. Results and Discussion 3.1. Measurement of Rheology under Dark and Illuminated Conditions. The flow behavior as a function of time for two different types of TiO2 suspensions, one exhibiting type I and the other type II behavior, is shown in parts A and D of Figure 3, respectively. The experiments were carried out using viscometer I under three different conditions: (a) in the absence of both electric field and illumination; (b) in the presence of electric field and the absence of illumination; and (c) in the presence of both electric field and illumination. The magnitude of the electric field was 1.6 kV mm-1 in these experiments. The y-axis of the plot indicates the volume of fluid passing through the cell, and hence the slope corresponds to the flow rate. The fluid in the PER cell flows due to gravitational force. Since the liquid level decreases continuously, the hydrostatic pressure and thus the flow rate decrease continuously, even in the absence of an electric field, as evidenced by the curvature of the plots seen in Figure 3A and D (curves a). For both fluids (P-25- and ST-01-containing fluids), the flow rate in the presence of electric field alone (Figure 3B and E, curves b) was less than the flow rate in the absence of both electric field and illumination (curves a), as a result of the ER effect. The flow behavior during illumination when the field was absent (not shown) was identical to that in the absence of both electric field and illumination (curve a). For the P-25-containing fluid, the flow rate in the presence of both electric field and illumination (Figure
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3B, curve c) was less than that in the presence of electric field alone (curve b). This indicates that illumination further enhances the ER effect (increasing fluid viscosity). In contrast, for the ST-01-containing fluid, the flow rate in the presence of both electric field and illumination (Figure 3E, curve c) was greater than the flow rate in the presence of the electric field alone (curve b). This indicates that illumination substantially decreases the ER effect (decreasing fluid viscosity). Thus, depending on the particle properties, either positive or negative photoeffects can be observed for TiO2 particle-containing fluids. In general, in studies of fluid rheology, the shear stress is determined as a function of shear rate using a rotary viscometer. The nature of the relationship between these two quantities serves to specify the fluid’s rheological type (e.g., Newtonian and several types of non-Newtonian behavior).21 If the relationship is linear, passing through the origin, then the fluid is said to be Newtonian. In the present work, because the viscometers used were optimized for the simultaneous application of electric field and illumination, a modified approach was used. In place of shear stress and shear rate, we examined the force being applied to the fluid in the PER cell and the resulting fluid flow rate.22 This force was simply the gravitational force23 of the residual fluid in the glass tube.24 The force plotted as a function of flow rate is shown in Figure 4 for both P-25- and ST-01-based fluids. In the absence of both electric field and illumination (curves a), the curves for both of the fluids are linear and pass through the origin, indicating Newtonian behavior. In the presence of electric field alone (curves b), the curves have positive y-intercepts, indicating that the fluids become non-Newtonian upon application of the field. Such behavior is characteristic of ER fluids.2,9 For the P-25 fluid, the value of the intercept increased in the presence of both electric field and illumination, which means that the non-Newtonian behavior increased upon illumination (Figure 4A). For the ST-01 fluid, in contrast, illumination caused the behavior of the fluid to revert back nearly to Newtonian behavior, as shown in Figure 4B. Photocurrents were observed in both fluids under illumination. The photocurrent for the P-25 fluid increased slowly as a function of time (Figure 3C). However, the photocurrent for the ST-01 fluid increased rapidly (Figure 3F). Negligibly small currents (
