Synthesis of anatase TiO2 with exposed (100) facets and enhanced electrorheological activity.

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Soft Matter View Article Online

DOI: 10.1039/C7SM01422H

Synthesis of Anatase TiO2 with exposure of (100) facets and its enhanced electrorheological activity Shoushan Yu, Chuncheng Hao and Kezheng Chen* College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China

ABSTRACT: A simple hydrothermal method was employed to synthesize the anatase TiO2 dominated with (100) facets using titanate nanofibers derived from alkali treatment as precursor. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) were carried out to confirm the surface morphology and phase structure of TiO2 product. The formation mechanism of TiO2 enclosed by (100) and (101) facets was deduced to be the selective adsorption of OH- on the (100) facets of anatase TiO2. Electroheological(ER) properties test indicted that the tetragonal-facets-rod anatase TiO2 with (100) facets exposure exhibited an excellent ER performance with a high ER efficiency up to 52.5, which is may result from the anisotropy of the special morphology. In addition, the shape effect on the dielectric property was investigated using a broadband dielectric spectroscopy equipment. Key words: Electrorheological fluid; Solvothermal method; Hydrothermal method; Nanorod

*To whom correspondence should be addressed. Tel: 86-532-84022509. Fax: 86-532-84022509. E-Mail: [email protected]; [email protected]

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Published on 28 September 2017. Downloaded by University of Newcastle on 29/09/2017 10:25:36.

Kai He, Qingkun Wen, Chengwei Wang, Baoxiang Wang*,

Soft Matter

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DOI: 10.1039/C7SM01422H

1. Introduction Inorganic nanoparticles with tailored shape and controllable size have received intensive attention in recent years

[1]

. Titanium dioxide, as one of the most important

photocatalyst [2], semiconductor [3], sensors as well as electrorheology [4-14]. As a kind of smart materials, electrorheological fluid (ERF) cans response to an external stimulation within several hundredth milliseconds. Normally, the ERF is comprised with an electro-responsive soft material dispersed in insulting oil, whose rheological properties can be easily controlled under an applied electric field reversibly. For instance, the shear stress and shear viscosity will be enhanced dramatically accompanying with a formation of chain-like or column-like structure between polarized particles along the direction of external electric field. For the advantage of controllable rheological properties, ER materials have promised wide applications in clutches, brakes and shock absorbers

[15-18]

. So far, researchers have used various

materials to serve as ERF dispersed phase to enhance the electrorheological efficiency ranging from organic polymer to inorganic materials to meet the industrial demand [19-21]

. Among them, TiO2 is considered as a suitable candidate for its low cost,

non-toxic, chemical stability. Particularly, the electroheological performance greatly depends on the morphology and structure of TiO2 particles. Therefore, a facial strategy to control the morphology of TiO2 nanoparticles is in great demands. Apart from the morphology, the crystalline structure is also an important factor influencing the TiO2 properties. There are three different crystalline forms of TiO2 in nature, known as: anatase, rutile, and brookite. Compared with two other forms, anatase has been widely investigated owing to its better behavior in various fields

[22-24]

. Since the crystal structure plays an important

role in electrorheological properties, the design for single crystal anatase exposed different crystalline facets with well-defined shape is necessary. Based on the experimental and computational result suggestion, the high surface energy planes tend to diminish rapidly to minimize the total surface energy during the nucleation

2



inorganic metal oxide, has been most studied due to its promising potential in

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progress. In the case of anatase TiO2, the shape preferred octahedron dominated by eight relatively thermal-dynamically stable facets (101) according to the Wuff construction, which is in good agreement with the order of anatase surface energy

facet 0.44 J/m2, (001) facet 0.90 J/m2, (100) and (010) facet 0.54 J/m2, respectively. However, the anatase TiO2 crystal grows into tetragonal-facet-rod shape constructed with eight (101) facets and four (100) facets or truncated octahedron comprised by eight (101) facets on the sides and two (001) facets on the top and bottom under non-equilibrium condition. As a consequence, the anatase exposed more high-index facets exhibited superior performance in various fields, such as photovoltaic cells, dye-sensitive solar cells, gas sensing device and smart surface coating [27-30]. Since the single crystal anatase TiO2 constructed with a high percent high energy facet (001) was firstly reported by Yang[31], an increasing interest was attracted to design the strategies and routes to obtain anatase TiO2 exposed more relatively active facets. Following Yang’s first achievement of anatase TiO2 dominated with 47% (001) facets by introducing HF as a capping agent to stabilize the growth of (001) facet, various capping agents, especially F-containing species, were used as structure-directing agent to control the growth ratio of different plane by the effect of the selective absorption

[32,33]

. Moreover, EDTA

[34]

, PVA

[35]

and CTAB

[36]

were successfully

introduced to fabricate anatase TiO2 exposed (001) facet. However, although the F- ion is effective to control the growth ratio of different facets and expose more (001) facets, the capping agents are difficult to remove and HF is extremely corrosive and toxic. From the point of green synthesis, it is desirable to find an approach without involving F-containing species. As mentioned above, the anatase TiO2 (100) facets are more active than (101) facets but paid less attention, which is with 100% 5-coordinated Ti atoms as same as (001) facets. Interestingly, a new approach to create anatase TiO2 exposed (100) facet was reported by Xu[37] using a sodium titanium nanotube as precursor under a mild hydrothermal condition. It should be worth to be noted that the anatase TiO2 exposed different facets and served as electrorheological material is rarely reported. 3



reported [25,26]. For anatase TiO2, in general, the surface energy is calculated to be (101)

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In this work, we present a strategy choosing titanium nanofibers derived from an alkaline hydrothermal condition as precursor to fabricate anatase TiO2 nanoparticles dominated with (100) facets. Following a subsequent hydrothermal treatment, the

transformed into anatase TiO2. Lastly, the electrorheological performance of obtained anatase TiO2 powders was investigated. The TiO2 nanocrystal exposed (100) facets exhibit an excellent ER effect.

2. Experimental Section 2.1 Preparation of titanium nanofibers Titanium nanofibers were synthesized under an alkaline solution hydrothermal condition. Typically, 2g commercial TiO2 powder was added into a 10M concentrated NaOH solution following a continuous stirring until a homogeneous milk white solution was formed. Then the white solution was transferred to a tightly sealed Teflon-lined autoclave and reacted at 150 °C for 24h. After cooling down to room temperature naturally, the white precipitate was harvested by centrifuging and washed with deionized water for several times to remove the residual NaOH, denoted as Na-titanium precursor. Subsequently, the alkali-metal ion was exchanged by dispersing the as-obtained Na-titanium white powder in a dilute HCl aqueous solution and agitating for several hours. Then the white precipitate was collected by centrifugation and repeatedly wash with deionized water, denoted as H-titanium precursor. 2.2 Preparation of anatase TiO2 crystal dominated with (100) and (101) facets respectively The TiO2 nanocrystal dominated with (100) facets was prepared as following: The 0.3g as-prepared Na--titanium precursor were added into 70 ml deionized water under ultrasonication. Then the solution transferred to a Teflon-lined autoclave and heated to 200 °C for 24 h. After the hydrothermal treatment, the anatase TiO2 products were collected and dried at 70 °C for overnight, denoted as TiO2-(100). The Na--titanium precursor was replaced by H-titanium precursor to prepare TiO2 nanocrystal 4



titanium nanofibers went through a dissolution and recrystallization process and

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dominated with (101) facets, while other experimental condition remained the same, and the obtained TiO2 product was denoted as TiO2-(101). 2.3 Preparation of ER fluids

1.5 mL silicon oil (Polydimethylsiloxane, viscosity = 100 cSt, dielectric constant = 2.73, specific density = 0.973 g/cm3 ) at 10 wt. % and mixed until the uniform ER suspension was formed. 2.4 Characteristic Transmission electron microscopy (TEM, JEOL-2100) and field-emission scanning electron microscopy (FE-SEM, JEOL-6700) equipped with an energy-dispersive spectrometer were used to observe the surface morphology of obtained TiO2 samples. More detailed information of surface morphology was revealed using a High-resolution transmission electron microscopy (HR-TEM). The XRD patterns of anatsase TiO2 were recorded with a X-ray diffraction (Rigako, D/max-2500PC).The permittivity and dielectric loss factor of TiO2 based ER fluid were measured using a broadband dielectric spectroscopy equipment (Novocontrol Concept 40). The nitrogen adsorption measurements (ASAP 2020, Micromeritics Instrument, USA) were implemented to determine the specific surface area and pore size distribution of TiO2 nanoparticles. 2.5 Electrorheological properties measurements The rotational rheometer (HAAKE Rheo Stress 6000, Thermo Scientific, Germany) equipped with a parallel-plate system (PPER35) and a WYZ-020 DC high-voltage generator (voltage: 0 – 5 kV, current: 0 – 1 mA) was carried out to investigate ER properties. The gap between two parallel plates was set at 1 mm and the measurement temperature was maintained at 25 °C.

3 Result and Discussion The crystalline structures of as-obtained powders were characterized by XRD, as showed in Fig. 1. Both the Na-titanium precursor derived from alkali treatment and H-titanium precursor after ion exchanging by proton show the well-crystallized

5



Both the 0.15 g TiO2-(100) and 0.15 g TiO2-(101) powders separately dispersed into

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properties. For Na-titanium precursor, the typical diffraction peaks locate in 9.7°, 24.8°, 27.2° and 48° are ascribed to the planes of (200), (110), (210) and (002) (JCPDS 47-0124). On the other hand, the H-titanium precursor has similar diffraction

treatment of the precursors, both the samples transfer into anatase phase TiO2 completely. The location of diffraction peaks corresponds to (101), (004), (200), (105), (211), (204) and (215) facets (JCPDS 21-1272). From the intensity of the anatase peaks, we can see that the samples are well crystallized and no peaks ascribed to rutile or brookite are detected. Previous obtained TiO2-(101) sample contain an impurity peak around 30 degree. So, the pure anatase-(101) sample was re-prepared to use as a control sample for a comparision with anatase-(100) sample. Fig.1 a) shows the XRD patterns of the as-synthesized pure anatase-(101) sample and anatase-(100) sample. Fig.1 b) shows that previous as-obtained anatse-(101) sample contain an impurity peak. Both obtained samples were studied to investigate the influence on electrorhological effect. Fig. 2 shows the typical images of Na-titanium precursor and H-titanium precursor. As showed in fig. 2a, the as-prepared Na-titanium precursor is composed with hundreds of nanofibers with 20-30 nanometers in width and a length ranging up to several micrometers. Moreover, the surface of nanofibers is rather clean with no particles attached. The inset is EDS analysis of Na-titanium nanofibers precursor. Besides Na, Ti and O element, no other impure element was detected. Through the analysis of EDS combined with XRD patterns, the Na-titanium precursor is confirmed to be Na2Ti3O7. On the other hand, the H-titanium precursor has the similar morphology to Na-titanium precursor. The elements analysis for H-titanium is presented in inset.b, it is clearly observed that sodium ion is completely exchanged by washing with dilute HCl, and Si element is resulted from the Si substrate. Fig. 3 shows the representative SEM and TEM images of anatase TiO2-(100) prepared by subsequent hydrothermal treatment in pure aqueous solution. As showed in fig. 3a, the tetragonal-facets-rod TiO2 nanoparticles are observed with a size about 50 nm in width and 200 nm in height in a high yield. The TEM and HR-TEM images were used 6



peaks to Na2Ti3O7 crystalline structure after the ion exchanging. After hydrothermal

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to further reveal the detailed surface morphology of obtained TiO2 product. Fig. 3b displayed the typical TEM picture of tetragonal-facets-rod TiO2 nanoparticles. The TEM picture shows the similar shape with SEM result. Fig. 3c and fig. 3d are shown

rang round with red circles in fig. 3b, respectively. The corresponding lattices of 0.35 nm and 0.48 nm in fig. 3c can be ascribed to (101) and (002) facets, while the lattice of 0.35 nm in fig. 3d can be assigned to (101) facet. On the basis of all above observation and analysis of the as-obtained anatase TiO2 surface structure and characterizations, we can confirm that the tetragonal-facets-rod anatase TiO2-(100) nanoparticle has four (100) facets exposed on the sides and eight (101) facets on the top and bottom, as the three-dimensional model shown in inset fig. 3b. The exposed ratio of (100) facets were carefully calculated using an equation: e=a/(a+b), where a and b denote the length of red line and green line labeled in the three-dimensional model of TiO2-(100) 38. The calculated percentage of exposed (100) facets was 81 %. On the other hand, the SEM, TEM and HR-TEM pictures of TiO2-(101) obtained using H-titanium as precursor presented in fig. 4. Firstly, the SEM image, as showed in fig. 4(a), demonstrate the bipyramid-shape of TiO2 product with 50 nm in width and 150 nm in length. Secondly, the morphology of bipyramid-shape TiO2 was also further investigated by TEM and HR-TEM. The lattice of 0.35nm and 0.24 nm in HR-TEM is indexed to (101) and (001) facet of anatase TiO2. Through observation of TEM and analysis of HR-TEM, the bipyramid-shape TiO2 is indicated to be enclosed by (101) facets with a three-dimensional model was shown in fig. 4(c). The nitrogen adsorption measurements (ASAP 2020, Micromeritics Instrument, USA) were implemented to determine the specific surface area and pore size distribution of TiO2 nanoparticles, as shown in fig.5. The N2 adsorption-desorption isotherms of TiO2 crystals can be identified as type Ⅳ isotherm with a typically mesoporous hysteresis loop. The specific surface area of TiO2-(100) and TiO2-(101) were calculated as 33 and 15 m2/g using BET equation, respectively. Whereas, the distinct pore size peak is not observed from the corresponding pore size distribution curves, indicating that the TiO2-(100) and TiO2-(101) particles do not exhibit the intrinsic porosity. 7



the corresponding HR-TEM images of side area and angle area of TiO2 nanoparticle

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The possible transformation process from Na-titanium nanofibers and H-titanium nanofibers to TiO2-(100) crystals and TiO2-(101) is depicted in Fig.6. For the

experience a dissolution and nucleation process. Under the hydrothermal treatment, the precursor H-titanium nanofibers start to destroy firstly and gradually decomposes into Ti(OH)4 fragments. As the decomposition process further conducts, the Ti(OH)4 fragments dehydro between Ti-OH and HO-Ti in an edge-shear manner, resulting the formation of anatase TiO2 crystal with a zigzag structure. In the case of TiO2-(100), similarly, the Na-titanium nanofibers decomposed to Ti(OH)4 fragments at the beginning of the transformation process, unlike H-titanium nanofibers, besides Ti(OH)4 fragments Na-titanium nanofibers decomposed and generated hydroxyl and sodium ions. Then the Ti(OH)4 fragments produced from precursor dehydroed to form anatase TiO2 crystals. As mentioned above, the anisotropic growth is ascribed to the capping agents preferential absorption of different facets, the growth rate of absorbed facets is limited at a great extent and the absorbed facets exposed eventually. In this system, the hydroxyl ion released from Na-titanium precursor absorbs on the anatase (100) facet selectively, then surface energy of anatase (100) facets are lowered and the growth rates of (100)-axis and (010)-axis are hindered, resulting the exposure of (100) facets. The transformation evolution can be expressed as following equations:

1)

Based on above-mentioned, the exposure of (100) facets is ascribed to the selective absorption of hydroxyl ion released from Na-titanium precursor on the anatase (100) facet. Therefore, the control experiments were carried out using less volume of 0.1 M HCl to partly exchange the Na+ ion to lower the concentration of hydroxyl ion released from Na-titanium. The TiO2 tetragonal-facets-rod with different percentage exposure of (100) facets were characterized by SEM and TEM, as shown in fig. 7. 8



bipyramid anatase TiO2 crystals dominated (101) facets, the H-titanium nanofibers

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The volume of 0.1 M HCl added in solution and the exposed ratio of (100) facets of TiO2-(100) and control samples were summarized in table1. The ER suspensions consist polarized particles dispersing in a non-conducting oil

TiO2-(100) and TiO2-(101) powders in silicon oil at 10%wt. The flow curves of TiO2 nanoparticles based ER fluids were measured using a controlled shear rate model under an external electric fields ranging from 0 to 3 kV/mm. According to the ER curves in fig. 8, it can be seen that the slope of flow curves in the absence of electric field is lower than 1 when for shear rate was less than 10 (1/s). After 10 (1/s), the flow curves tend to be stable with a slope as 1 in a log-log plot. When an external electric field was applied, the shear stress enhanced drastically at low shear rate region as the result of formation of chain-like or column-like structure along the adjacent polarized particles. As the electric field strength increases, the enhancement of shear stress is becoming significant. In addition, the shear stress shows a wide plateau region within the low shear rate range[39-42]. Normally, the chain-like structures experience a broken and re-formation process under mechanical shearing, which is depends on the competition between hydrodynamic force generated from the fluid flow and electrostatic force along polarized particles induced by electric field. Within the low shear rate range, the electrostatic force is predominant, therefore the shear stress decreased slightly with the increasing of shear rate. When the competition approached to a dynamic balance the shear stress would be maintained at a constant, resulting the appearance of plateau area in shear stress curves

[43-45]

. The plateau area would be

maintained until the shear rate reaches a critical value. On the other hand, in the high shear rate range, which is higher than the critical value, the hydrodynamic force is predominant. The shear stress increases with the increasing of shear rate as soon as the chain-like structures were destroyed. As the applied electric field strength increases, the plateau area is becoming broader and the enhancement of shear stress at a certain shear rate is remarkable. The pure anatase-(101) sample was prepared to use as a control sample and its ER performance was investigated, as shown in fig. 8 (e, f). As a result, the flow curve of control sample TiO2-(101) based ER fluid was similar to 9



normally. In this paper, the ER suspensions were prepared by dispersing both

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those ER fluids. In the absence of electric field, the shear stress increases with the increasing of shear rate linearly, which is a typical Newtonian fluid behavior. When an external electric field was applied, the shear stress enhanced drastically at low shear

addition, the ER efficiency is a critical evaluation of ER performance. The ER efficiency of pure anatase-(101) sample was calculated to 21.9, which is a higher value compared with the impure anatase-(101) sample, indicating that the pure phase and composition of TiO2 nanocrystal make a difference in the ER activity indeed. However, in comparison with the TiO2-(100), the ER efficiency of pure anatase-(101) sample is much lower, which is may be ascribed to the stronger anisotropy and larger specific surface area of TiO2-(100). The Bingham fluid model is generally adopted to examine the flow curves of ER fluids, as described as follows: ⋅

τ=τ0+η · γ

τ≥τ0

⋅

γ =0

τ＜τ0

(2) ⋅

Where τ is the shear stress, τ0 is the yield stress, γ is the shear rate, and η is the shear viscosity. The dynamic yield stress can be obtained by extrapolating the shear stress to a zero limit. However, the unusual decreasing trend is more often occurred. Thus, a recently reported Cho–Choi–Jhon (CCJ) model can be used to fit these flow curves well. The CCJ model is described by the equation as below:

τ=

το .

1 + (t1 γ )

+ η ∞ (1 + α

1

.

)γ

.

(t 2 γ )

(3)

β

Here, τ0 is the dynamic yield stress; η∞ is the viscosity at infinite shear rate; t1 and t2 are time constants used to describe the variation in the shear stress; the exponent α is related to the decrease in the shear stress at the low shear rate region, while β in the range of 0–1 is for the high shear rate region. It can be seen from the flow curves that the Bingham model fits well only when for the electric field was 3 kV/mm. Compared with the Bingham model, it is obvious that the CCJ model can fit the curves better especially for the flow curve at a low shear rate region when for the electric field 10



rate region and shows a wide plateau region within the low shear rate range. In

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between 1 kV/mm to 2.5 kV/mm as shown in figure 8 a and b. The optimal parameters values are shown in Table 2 and table 3. It is generally accepted that the absolute value of shear stress at a certain shear rate

where the ER efficiency is defined as: I=(τE-τ0)/τ0 (where the τ0 is the shear stress in the absence of electric field, and the τE is the shear stress under the corresponding electric field strength). Fig. 9 displays the corresponding shear stress absolute value at 1s-1 and ER efficiency for both TiO2-(100) and TiO2-(101) based ERF. As the result shows, both the shear stress and ER efficiency of TiO2-(100) based ERF are higher than those of TiO2-(101) TiO2 based ERF, which means the ER activities of TiO2-(101) is more superior. As the BET results suggest, the TiO2-(100) particles exhibit an obvious hysteresis and a larger specific surface area. According to the reported literatures, apart from the anisotropy, the specific surface area is also an important factor influencing TiO2 ER effect

[46-48]

. The large specific surface area can induce a

strong interfacial polarization and a suitable dielectric relaxation, which can be demonstrated from the dielectric spectrum of TiO2-(100) and TiO2-(101). Therefore, it can be speculated that the large specific surface area plays an important role in inducing the ER effect of TiO2 particles. As reported in previous lectures, the anisotropic particles exhibited an outstanding ER performance compared with spherical or regular shape particles. Jang’group investigated the morphology effect of mesoporous silica particles with different aspect ratios, the better ER activity was obtained for long-rod particles with a high aspect ratio[49]. In addiction, PANI nanotube was successfully prepared by Choi’group and showed an improved ER performance due to the contribution of robust chain structure formed with long nanotube[50]. Therefore, the anisotropy plays an important role in inducing a stronger ER behavior. In this work, the TiO2-(100) exhibits a higher anisotropy compared with TiO2-(101) due to more exposure of (100) facets. When the mechanical shearing was performed, the TiO2-(100) particles showed stronger flow resistance and mechanical stability, which was ascribed to densely packed particles and the formation of more robust chain structure. Moreover, the anisotropy of TiO2 11



and ER efficiency are two important parameters to estimate a ER fluid performance,

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nanoparticles depends to the exposed ratio of (100) facets. The more exposure of (100) facets the TiO2 nanoparticles exhibit, the stronger anisotropy they possess. For the particles with a strong anisotropy, they link more strongly together and form robust

investigated the relation between exposed ratio of (100) facets and the electrorheological activity. As a consequence, the ER efficiency of TiO2 exposed different ratio of (100) facets were calculated as shown in fig 9 b), proving the larger exposed ratio of (100) facets has a positive effect on ER performance. The SEM observation was carried out to investigate the microstructure of chains formed by TiO2 particles under electric field. In the SEM observation, the TiO2 particles were suspended in a drop of ethanol and a planar electric field was applied across the droplet. The droplet was then allowed to evaporate while the electric field was maintained. This allows for the direct examination of on-state structure. The SEM images describe the behavior of TiO2 particles when an electric field is applied. Figure 9c) shows the alignment of TiO2 nanoparticles after the presence of electric field. As it can be seen from the figure 9 c), the adjacent particles begin to assemble to form a chain structure, the orientation of TiO2 nanoparticles along the direction of electric field is clear shown. However the aligned TiO2 particles were aggregated due to their nanoscaled size. In order to gain more insight of ER properties the yield stress as a function as power of electric field was obtained through shear stress extrapolation (fixing the shear rate at 0.1s-1) as shown in figure 10. The power-law equation is widely used to describe the relationship between yield stress and electric field strength, expressed as:

τy ∝ E α

2)

The conduction model is applied when the exponent α value is 1.5. In another case, the exponent α value is 2, the electrostatic polarization mechanism is responsible for the ER response. As plotted in fig. 8, for both types of ER fluids, the yield stress is in direct proportion of E2 at low electric field strength (below 1.5kV/mm), while the α approaches 1.5 at high electric field strength (beyond 1.5kV/mm).

12



chain structure. Therefore, the ER control experiments were conducted to further

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Additionally, the dielectric properties were measured to further indict the anisotropy effect on the ER performance. The dielectric properties, including dielectric constant and dielectric loss, of the TiO2-(100) and TiO2-(101) particles based ER fluids as a

the dielectric properties are strongly associated with polarization behavior, which influences the ER performance greatly. To further demonstrate the dielectric properties of TiO2 nanoparticles based ER fluids, the Cole-Cole plots were created in fig. 11 b) using an equation [51]:

∆ε 1-α （1 + iωλ）

ε ∗ (ω ) = ε '+iε ' ' = ε ∞ +

(1)

where ε' is dielectric constant, ε" is dielectric loss, the difference in dielectric constant ∆ε=ε0-ε∞ represents the strength of polarization and interaction among particles of ER fluid under electric filed (ε0 and ε∞ are the value of dielectric constant at the frequency closed to 0 and high limit, respectively), ω is angular frequency, the relaxation time λ=1/2πfmax (fmax is the frequency location of dielectric loss factor peak) reflects the

rate of particles polarization under an external electric field. The exponent 1-α indicates the distribution of relaxation time over the entire frequency range. According to the results fitted to the equation, the detailed dielectric properties of TiO2 nanoparticles based ER fluids were summarized in table4. The difference of dielectric constant ∆ε (∆ε= ε'1Hz- ε'106Hz) is associated with the strength of interfacial polarization induced by electrostatic interaction between adjacent particles. The relaxation time (λ=1/2πfmax), where the fmax defined as the maximum frequency of dielectric loss peak, represents the rate of interfacial polarization when the external electric field is applied. Generally, a good ER material candidate requires a large ∆ε and a short λ[52-56]. According to the dielectric spectrum, the ∆ε value of the TiO2-(100) and TiO2-(101) particles based ER fluids are determined to be 0.63 and 0.45. On the other hand, the relaxation time is calculated as 2.83 ms and 6.71 ms. Based on the result of dielectric properties, the TiO2-(100) particles based ER fluids show a strong interfacial polarization and a rapid interfacial polarization rate. Considering the analysis of the dielectric spectrum, the anisotropy of 13



function of electric field frequency is plotted in fig. 11. It is generally accepted that

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particles plays an important role to obtain a positive ER effect.

4 Conclusions A facial hydrothermal method was employed to synthesize the TiO2 enclosed with

nanofibers as precursors, respectively. The crystalline structure of precursors and TiO2 was indicted using a XRD analysis, and the SEM, TEM and HR-TEM were used to reveal the morphologies information. The selective adsorption of OH- released from the deposition of Na-titanium is considered as the reason of the (100) facets exposure. The TiO2-(100) showed a considerably higher ER efficiency compared with TiO2-(101) due to the anisotropy growth. Subsequently, the anisotropic effect on dielectric properties was confirmed by using a broadband dielectric spectroscopy equipment, the large ∆ε and short λ were obtained with the increasing of anisotropy, which accorded with the result of ER performance. It suggests that the anisotropy plays an important role to obtain a positive ER effect, which provides an effective method to fabricate ER materials.

Acknowledgment This work was supported by the National Natural Science Foundation (NSFC 51472133), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, PR China. Reference [1]. Dina F R, Adriana Z, Thomas B, Three-dimension titanium dioxide nanomaterials, Chem. Rev., 2014, 114, 9487-9558. [2]. Pan J H, Zhang X W, Du J H, Sun D D, James O L, Self-etching reconstruction of hierarchically mesoporous F-TiO2 hollow microspherical photocatalyst for concurrent membrane water purification, J.Am.Chem.Soc., 2008, 130 (34), 11256-11257. [3]. Ren S T, Liu W J, One-step photochemical deposition of PbAu alloyed nanoparticles on TiO2 nanowires for ultra-sensitive H2 detection, J.Mater.Chem.A, 2016, 4, 2236-2245. [4]. Tao R. J., Tang H., Tawhid-Al-Islam K., Du E. P., and Kim J. Electrorheology leads to healthier and tastier chocolate PNAS 2016, 113 ( 27)，7399–7402. [5]. Choi, H. J.; Jhon, M. S. Electrorheology of polymers and nanocomposites. Soft Matter 2009, 5, 1562-1567. [6]. Wen, W. J.; Huang, X. X.; Yang, S. H.; Lu, K. Q.; Sheng, P. The giant electrorheolegical effect in suspensions of nanoparticles. Nat. Mater. 2003, 2(11), 727-730. [7]. Shen, R.; Wang, X. Z.; Lu, Y.; Wang, D.; Sun, G.; Cao, Z. X.; Lu, K. Q.

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(100) facets and (101) facets utilizing Na-titanium nanofibers and H-titanium

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DOI: 10.1039/C7SM01422H

The list of figure captions Fig.1 a) XRD patterns of Na-titanium, H-titanium precursors, TiO2-(100) and TiO2-(101); b) XRD patterns of TiO2-(101) containg impurity peak.

the EDS analysis for Na-titanium nanofibers and H-titanium nanofibers) Fig.3 The typical SEM image (a), TEM image (b) (inset: the three-dimensional model) of anatase TiO2-(100), corresponding HR-TEM pictures for side area (c) and angle area (d) Fig.4 The typical SEM image (a), TEM image (b,c) (inset: the three-dimensional model) of anatase TiO2-(101), corresponding HR-TEM pictures (d) Fig.5 N2 adsorption-desorption isotherms of TiO2-(100) and TiO2-(101), (b) the corresponding pore size distributions Fig.6 Scheme for growth mechanism of TiO2-(100) and TiO2-(101) Fig.7 SEM and TEM images of control samples TiO2-(100): a) c) sample 1; b) d) sample 2 Fig.8 Shear stress and shear viscosity curves of TiO2-(100) (a,c) and TiO2-(101) (b,d) (e, f) pure anatase TiO2-(101) based ERF as a function of shear rate under an increasing electric field strength; In fig8. a and b, the dashed lines are fitted by a conventional Bingham model; the solid lines are fitted by a CCJ model. The optimal parameter values are in Table 2 and 3. Fig.9 a) The shear stress absolute value at 1s-1 and ER efficiency for both TiO2-(100) and TiO2-(101) based ERF, b). The exposed ratio of (100) facets dependence of ER efficiency for TiO2-(100) samples, c) the SEM image of aligned TiO2 nanoparticles after the presence of electric field strength E=1kV/mm. Fig.10 Yield stress as a function as electric field strength for TiO2-(100) and TiO2-(101) ERFs Fig. 11 (a) Dielectric constant and loss factor as a function of the frequency and (b) Cole–Cole plot of TiO2-(100) and TiO2-(101)-based ER fluids. 18



Fig.2 SEM images of Na-titanium nanofibers (a) and H-titanium nanofibers (b) (inset:

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DOI: 10.1039/C7SM01422H

Table1. The detailed experimental conditions in synthesis of TiO2-(100) Table 2. The optimal parameters in the Bingham and CCJ models obtained from the flow curves of TiO2-(100) based ER fluids. Table 3. The optimal parameters in the Bingham and CCJ models obtained from the flow curves of TiO2-(101) based ER fluids. Table 4. Dielectric parameters of the TiO2-(100) and TiO2-(101) based ER fluids

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The list of tables

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Graphical Abstract Figure

The tetragonal-facets-rod anatase TiO2 with (100) facets were synthesized via a solvothermal method, which can exibit smart electrorheological behavior under external electric field.



DOI: 10.1039/C7SM01422H

Enhanced dye-sensitized solar cells performance using anatase TiO2 mesocrystals with the Wulff construction of nearly 100% exposed {101} facets as effective light scattering layer.

Anatase TiO2 Nanoparticles with Exposed {001} Facets for Efficient Dye-Sensitized Solar Cells.

Synthesis of the double-shell anatase-rutile TiO2 hollow spheres with enhanced photocatalytic activity.

graphene nanosheets for enhanced visible-light photocatalytic activity.

Green synthesis of anatase TiO(2) nanocrystals with diverse shapes and their exposed facets-dependent photoredox activity.

Highly efficient low-temperature plasma-assisted modification of TiO2 nanosheets with exposed {001} facets for enhanced visible-light photocatalytic activity.

Au photosensitized TiO2 ultrathin nanosheets with {001} exposed facets.

Enhanced photocatalytic CO₂-reduction activity of anatase TiO₂ by coexposed {001} and {101} facets.

Direct Observation of Charge Separation on Anatase TiO2 Crystals with Selectively Etched {001} Facets.

Seed-Mediated Growth of Anatase TiO2 Nanocrystals with Core-Antenna Structures for Enhanced Photocatalytic Activity.

Low-temperature synthesis of anatase TiO2 nanoparticles with tunable surface charges for enhancing photocatalytic activity.

Hybrids of Two-Dimensional Ti3C2 and TiO2 Exposing {001} Facets toward Enhanced Photocatalytic Activity.

Highly exposed surface area of {001} facets dominated BiOBr nanosheets with enhanced visible light photocatalytic activity.

Synthesis of concave gold nanocuboids with high-index facets and their enhanced catalytic activity.

TiO2(B) nanotube heterojunction.

Simple sol-gel process to obtain silica-coated anatase particles with enhanced TiO2-SiO2 interfacial area.

Anatase TiO2 nanotube powder film with high crystallinity for enhanced photocatalytic performance.

mesoporous crystalline anatase TiO2 ceramic with enhanced photocatalytic performance for recalcitrant wastewater degradation.

Electrochemical degradation of refractory pollutants using TiO2 single crystals exposed by high-energy {001} facets.

Porous single-crystalline palladium nanoflowers with enriched {100} facets for highly enhanced ethanol oxidation.

Reduced TiO2 rutile nanorods with well-defined facets and their visible-light photocatalytic activity.

Controllable defect redistribution of ZnO nanopyramids with exposed {101̅1} facets for enhanced gas sensing performance.

Palladium nanoparticles anchored to anatase TiO2 for enhanced surface plasmon resonance-stimulated, visible-light-driven photocatalytic activity.

TiO2 anatase with a bandgap in the visible region.