Light-controlled bubble propulsion of amorphous TiO₂/Au Janus micromotors

Abstract

In this work, a bubble-propelled photoactivated amorphous TiO₂/Au (Am-TiO₂/Au) Janus micromotor has been demonstrated by utilizing the efficient photocatalytic H₂O₂ decomposition over the in situ H₂O₂ sensitized Am-TiO₂ under UV irradiation. The power conversion of the micromotor experiences a process from UV light power, to chemical power and finally to mechanical power. The quantum efficiency of O₂ bubble evolution from photocatalytic decomposition of H₂O₂ reaches 28%, and the power conversion efficiency reaches 1.28 × 10⁻⁹, which enables the micromotor to generate a strong bubble thrust propelling itself forward with a maximum speed of 135 μm s⁻¹. The motion state and speed of the Am-TiO₂/Au micromotors can be reversibly, wirelessly and remotely controlled at will with an ultrafast response rate (less than 0.1 s) by regulating “on/off” switch and intensity of UV irradiation. Consequently, the as-developed Am-TiO₂/Au micromotors may promise potential challenging applications in such as “on-the-fly” adsorption and decomposition of pollutants in water.

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Introduction

Micro/nanomotors could harvest energy from the environment and transform it into autonomous motions.^1–11 Due to their fascinating capabilities to pick up, transport, and release various micro/nanocargoes in liquid media, the micro/nanomotors manifest a great potential in paramount applications in drug delivery, cell separation, microsurgeries, lithography and environmental remediation.^12–21 To meet the demands of these envisioned applications, the motions of micro/nanomotors should be regulated over both defined times and locations, including controllable activation/stop, motion direction and speed.²² Up to now, various motion controlling strategies have already been demonstrated to control the motions of the micro/nanomotors, including magnetic field,^19,23,24 light,^13,25–29 heat pulse,^30–32 ultrasound field^33,34 and electric field.³⁵ Among them, light is one of the most powerful and versatile physical triggers, and can be remotely applied with extremely high spatial and temporal precision. It exhibits outstanding advantages for controlling the micromotors.

Several pioneer works have demonstrated the light triggered “on/off” motions of micro/nanomotors by taking advantages of photothermal effect or photocatalytic reactions. But using photothermal energy to manipulate micro/nanomotors usually has a trouble of long response time, such as in activation or stop etc., because of the limited thermal conductivity of aqueous fuels (about 0.65 W m⁻¹ K⁻¹).^30,32 Photocatalytic reactions can be instantly triggered under light irradiation, and is a promising underlying propulsion mechanism for the light-controlled micromotors.^26–28 For instance, the light-controlled TiO₂ tubular microengines and TiO₂/Pt Janus micromotors have been developed by employing the photocatalytic decomposition of H₂O₂ and water respectively over the well-crystallized anatase TiO₂.^13,29 Compared with the crystalline TiO₂, amorphous TiO₂ (Am-TiO₂) has a long term disordered but short term ordered structure, and possesses obvious advantages, including the simple preparation at room-temperature and much higher surface area.³⁶ Although pure Am-TiO₂ has long been reported to be nearly inactive due to the unfavorable recombination of photogenerated electrons and holes under UV irradiation,³⁷ the H₂O₂-sensitized Am-TiO₂ recently has been reported to exhibit a superior photocatalytic activity to the crystalline TiO₂ owing to the enhanced separation of the photogenerated charge pairs by the surface peroxide complexes.^38–40 Hence, Am-TiO₂ has the potential to photocatalytically decompose H₂O₂ to generate massive O₂ bubbles due to the in situ H₂O₂ sensitization, which is promising for remote activation and propulsion of micromotors.

In this work, we for the first time demonstrate a bubble-propelled photoactivated Am-TiO₂/Au Janus micromotors by utilizing the efficient photocatalytic H₂O₂ decomposition over the in situ H₂O₂ sensitized Am-TiO₂ under UV irradiation. For the as-developed Am-TiO₂/Au micromotors, the motion state and speed can be reversibly, wirelessly and remotely controlled at will with an ultrafast response rate (less than 0.1 s) by regulating “on/off” switch and intensity of UV irradiation. In comparison with the light-controlled micro/nanomotors consisting of crystallized phases,^13,29 those with amorphous structures are expected to have a few potential advantages, including simple preparation and a much higher surface area for chemical doping or adsorption. Consequently, the as-developed Am-TiO₂/Au micromotors are expected to have potential applications in such as “on-the-fly” adsorption and decomposition of pollutants in water.

Experimental

Preparation of Am-TiO₂/Au Janus microspheres

Am-TiO₂ microspheres were firstly prepared by an O/W microemulsion method. Briefly, 2 mL caprylic acid (CA) solution with 10 wt% of tetrabutyl titanate (TBT) as an oil phase was added into 40 mL polyvinyl alcohol (2 wt%) aqueous solution. The above mixed solutions were magnetically stirred at a rate of 500 rpm for 2 min to form TBT/CA microdroplets in the aqueous solution. Then, 4.5 mL ammonia (25 wt%) solution was dropped into the O/W emulsion to trigger the hydrolysis of TBT/CA droplets, and the solution was stirred for another 5 min. After standing for 24 h, the Am-TiO₂ microspheres were formed. The Am-TiO₂ microspheres were dispersed in ethanol after centrifuged and washed with deionized water for 8 times and ethanol 3 times.

To prepare Am-TiO₂/Au Janus microspheres, 200 μL of the Am-TiO₂ microspheres suspension was dropped on a glass slide and dried at 80 °C for 5 min. Then, the exposed surfaces of the Am-TiO₂ microspheres were coated with a gold layer via ion sputtering for 240 s (about 35 nm in thickness) under a pressure of 0.6 Pa. The Am-TiO₂/Au Janus microspheres were then separated from the glass slide by an ultrasonication process.

Characterization

Scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis were obtained using a Hitachi S-4800 field-emission SEM (Japan). X-ray diffraction pattern (XRD) was obtained by using a Rigaku D/Max-RB diffractometer at a voltage of 35 kV and a current of 30 mA with Cu-Kα radiation (λ = 0.15406 nm).

Recording microscopy videos and analysis

20 μL Am-TiO₂/Au Janus microspheres aqueous suspension was added to 1 mL of the fuel solution in a Petri dish with a diameter of 35 mm. UV-LED light source was placed at 25 mm above the surface of the fuel solution, and the UV intensity on the surface of the fuel solution is 1 W cm⁻². The motions of the Am-TiO₂/Au Janus microspheres upon UV irradiation with different intensities were observed and recorded at room temperature through an optical microscope (Olympus BX60). All videos of the micromotor movement were analyzed using Video Spot Tracker V08.01 software.

Results and discussion

The Am-TiO₂/Au Janus micromotors are prepared by asymmetrically coating Au layer on the exposed surface of Am-TiO₂ microspheres (Fig. S1†) on a flat glass substrate via ion sputtering process. Fig. 1A shows that Am-TiO₂/Au Janus micromotors have an average diameters of about 15 μm. The elemental linear (Fig. 1B) confirms the binary heterostructure of the micromotor, in which about 3/4 of the surface of TiO₂ microsphere is covered by an asymmetric spherical-cap of Au layer. The broad and weak XRD peaks in Fig. 1C confirm that TiO₂ in the micromotors have an amorphous structure, while Au peaks can not be detected due to the low content of Au in the micromotors.

Fig. 1 The SEM image (A), linear EDX analysis (B) and XRD pattern of the Am-TiO₂/Au Janus microspheres.

The efficient photocatalytic H₂O₂ decomposition over the in situ H₂O₂ sensitized Am-TiO₂ under UV irradiation plays a crucial role in the light-controlled propulsion of the Janus micromotor. Am-TiO₂, which has a long term disordered but short term ordered structure, has long been reported to be nearly inactive due to the facilitated recombination of photogenerated electrons and holes under UV irradiation because of their high density of defects. However, the Am-TiO₂ sensitized with H₂O₂ exhibits an efficient photocatalytic activity even higher than the crystalline TiO₂ and commercial Degussa P25 owing to the formation of surface peroxide complexes on the amorphous TiO₂ surface and its effect for the separation photogenerated electrons and holes.^38–40 When Am-TiO₂/Au micromotors are added into the H₂O₂ fuels, the Am-TiO₂ component is in situ sensitized by H₂O₂, generating peroxide complexes on their surfaces. Under UV irradiation, the photoexcited electrons in Am-TiO₂ transfer to the surface peroxide complexes and leaves holes behind thanks to its strong affinity for electrons, prolonging the life time of the photogenerated holes and electrons to participate in the photocatalytic H₂O₂ decomposition according to eqn (1)–(3).

H₂O₂ + 2h⁺ → O₂ + 2H⁺ (2)

H₂O₂ + 2e⁻ + 2H⁺ → 2H₂O (3)

The O₂ bubbles generated from the photocatalytic decomposition of H₂O₂ could then be selectively ejected from the naked Am-TiO₂ surface to propel the micromotor. As shown in ESI-Video 1† and Fig. 2, the Am-TiO₂/Au Janus micromotor exhibits an efficient photoactivated motion in 15 wt% H₂O₂ with 5 wt% surfactant (Triton-X100). Once the UV light (wavelength λ = 368 nm) is on, a long tail of bubbles with diameter (R) of ca. 10 μm generated on one side of the Am-TiO₂/Au Janus micromotor, which engender a strong momentum that propels the micromotor forward (Fig. 2A and D) with a remarkable speed of 135 μm s⁻¹ (over 6 body lengths per second). When the UV light is off, the bubble generation and the motion of the micromotor stops immediately (Fig. 2B). The stopped micromotor can be re-activated within 0.1 s if the UV light is turned back on (Fig. 2C), reflecting the fast response of the micromotor on UV irradiation. This “on/off” propulsion of the micromotor is reversible by turning on or off the UV irradiation, as shown in ESI-Video 1.† Most of the Am-TiO₂/Au Janus micromotors exhibit anticlockwise (Fig. 2) or clockwise (Fig. S2†) rotational motions due to the rotational torque generated from the deviation of the direction of the propulsive force with the centroid of the micromotors. The Am-TiO₂/Au Janus micromotors show a limited lifetime of about 20 min based on our observation because the Am-TiO₂ could be gradually dissolved by concentrated H₂O₂ solution. However, this life time is much longer than that of the micromotors based on etching reaction of active metals.^16,41–43

Fig. 2 The lighted-controlled propulsion of a typical Am-TiO₂/Au Janus micromotor (ESI-Video 1†) in the fuel solution with 15 wt% H₂O₂ and 5 wt% Triton-X100 at a time interval of (A) 0, (B) 3, (C) 3.1, and (D) 5 s.

The intensity (I) of the UV irradiation on the micromotors is measured to be 1 W cm⁻². The photon flux (Φ) of the UV source can be calculated to be 1.85 × 10²² photons per m² per second according to eqn (4), in which h, c and λ represent Planck's constant (6.626 × 10⁻³⁴ J s), speed of light (3 × 10⁸ m s⁻¹) and wavelength (368 nm) of the UV light, respectively.

The apparent quantum efficiency (QE) of O₂ bubbles from the micromotor driven by photocatalytic decomposition of H₂O₂ can be calculated according to eqn (5).⁴⁴

here, n₀ = 2.686773 × 10²⁵ O₂ molecules per m³ is Loschmidt constant, r_o and r_m are 5 and 8.5 μm, representing the radiuses of the O₂ bubbles and micromotor show in Fig. 2, and f represents the frequency of the ejected bubbles from the micromotor (45 Hz). The QE of Am-TiO₂/Au Janus micromotor reaches 28%, indicating one O₂ molecule can be produced when about 7 photons are irradiated on the micromotor. The QE may be a key evaluation factor for the future design of the micromotors driven by photocatalysis.

The power conversion of Am-TiO₂/Au Janus micromotor experiences a process from UV light power, to chemical power and finally to mechanical power. The mechanical power of the micromotor is in form of kinetic power, which is harvested from the motion of the micromotor under the recoil force of the growth, ejection and burst of the generated O₂ bubbles.⁴⁵ The power conversion efficiency (η_c) of the UV-driven micromotor, which can be defined as the ratio of the output mechanical power (P_mecha = 6πμr_mv² = 3.15 × 10⁻¹⁵ W) into the input overall chemical power (P_chem = NΔ_rG^θ = 2.08 × 10⁻⁷ W) and UV light power (P_UV = Iπr_m² = 2.27 × 10⁻⁶ W), can be estimated from eqn (6).⁴⁶

here, v is the speed of the micromotor (135 μm s⁻¹), μ is the dynamic viscosity of 15 wt% H₂O₂ solution with 5 wt% Triton-X100 (1.08 mPa s), N (N = 4/3πr_o³f/0.0245 = 9.62 × 10⁻¹³) is the O₂ evolution rate from the micromotor in unit of mol (motor s)⁻¹, and Δ_rG^θ is the Gibbs free energy of H₂O₂ decomposition reaction (−206 kJ per mole of O₂ produced). The power conversion efficiency is calculated to be 1.28 × 10⁻⁹, and it is about 10 times higher than that of catalytic bubble propelled micromotors based on H₂O₂ decomposition on metal Pt.⁴⁶

To confirm the photocatalytic propulsion mechanism of the micromotor, the generation of O₂ bubbles on the naked Am-TiO₂ microspheres is also examined. When Am-TiO₂ microspheres are added into the H₂O₂ solution, the white microspheres turned into yellow ones, suggesting the formation of peroxide complexes on their surfaces. As shown in ESI-Video 2† and Fig. 3A and B, masses of bubbles are generated from the entire surface of naked Am-TiO₂ microspheres in the H₂O₂ fuel when UV light is turned on, while bubble generation stops when UV light is turned off, suggesting fast photocatalytic decomposition of H₂O₂ over Am-TiO₂ under UV irradiation. No directional movement can be observed for naked Am-TiO₂ microspheres in H₂O₂ fuel under UV irradiation, indicating the symmetrically released bubbles from the microspheres do not contribute to propulsion.

Fig. 3 Optical microscopies of the naked amorphous (A and B) and anatase (C and D) TiO₂ microspheres in the fuel solution under UV irradiation (1 W cm⁻²) on (A and C) and off (B and D) conditions. Scale bars: 20 μm.

It has been reported that coupling TiO₂ with Au could improve the separation of the photogenerated electrons and holes, and enhance the photocatalytic activity of the TiO₂ afterwards.⁴⁷ However, the bubble generation rate on the naked Am-TiO₂ microspheres (Fig. 3) is similar to that on the Am-TiO₂/Au Janus micromotor (Fig. 2), indicating the Am-TiO₂/Au Janus micromotor shows a negligible enhancement in the photocatalytic activity by Au coupling because it has a high density of defects, which limit the migration distances of the photogenerated electrons. Furthermore, no bubbles can be formed on the crystalline TiO₂ (anatase) microspheres (Fig. 3C and D) in the H₂O₂ fuel under UV irradiation due to the fast diffusion of O₂ molecules (diffusion coefficient in water: 2.0 × 10⁻⁹ m² s⁻¹)⁴⁸ from the convex surfaces,²⁹ indicating the H₂O₂-sensitized Am-TiO₂ has a higher photocatalytic activity for the decomposition of H₂O₂ than that of the crystalline TiO₂. Hence, the fast UV-driven motion of the Am-TiO₂/Au Janus micromotor comes from the high photocatalytic activity for H₂O₂ decomposition of Am-TiO₂ and the asymmetric structure.⁴⁹ The photocatalytic propulsion mechanism of the Am-TiO₂/Au Janus micromotor in H₂O₂ solution is shown in Scheme 1. According to the photocatalytic propulsion mechanism, adjusting UV intensity (I) could change the photon flux on the Am-TiO₂/Au Janus micromotors (eqn (4)), and thus the number of the photogenerated holes and electrons. As a result, the generation rate the O₂ bubbles and the speed of the micromotors are accordingly modulated. No bubble and motion can be observed for the micromotor when UV is off. When UV intensity increases from 0.1 to 0.8 W cm⁻², the speed of the micromotor increases linearly from 21 to 102 μm s⁻¹ with the increasing frequency of the generated bubbles, as shown in ESI-Video 3† and Fig. 4.

Scheme 1 The schematic demonstration of the UV-driven motion of the Am-TiO₂/Au Janus micromotor in the H₂O₂ solution.

Fig. 4 The propulsion of the Am-TiO₂/Au Janus micromotor in the fuel solution under UV irradiation with different output intensity: (A) 0, (B) 0.1, (C) 0.2, (D) 0.3, (E) 0.5 and (F) 0.8 W cm⁻², respectively. Scale bars: 50 μm. (G) Speed of the micromotor versus UV intensity.

The influence of the size of the Am-TiO₂/Au Janus micromotor on its light-controlled propulsion in the fuel solution is illustrated in Fig. 5 (corresponding to ESI-Video 4†). The speed of the micromotor firstly increases from 80 to 101 μm s⁻¹ when the size of micromotor increases from 7.5 to 15 μm, and then decreases to 70 and 48 μm s⁻¹ as the size further increases to 22 and 30 μm. This phenomenon could be ascribed to the contrary effect of the size increment on the propulsion of the micromotor. As the size increases, the active sites (exposed surfaces of the Am-TiO₂ microsphere) and the photon harvest of the micromotor are increased, leading to the enhanced photocatalytic O₂ bubble thrust, as evidenced by the increasing size and frequency of the released O₂ bubbles with the increasing size of micromotor from 7.5 to 30 μm (Fig. 5 and ESI-Video 4†). However, on the other hand, the increasing size of the micromotor causes the increasing viscous drag force (F_d) on the micromotor according to F_d = 6πrηv, resulting in the speed reduction. Here, r and v are the radius and speed of the micromotor. η is the dynamic viscosity of water.

Fig. 5 (A) The propulsion of the Am-TiO₂/Au Janus micromotor with different sizes under UV irradiation (0.8 W cm⁻²). Scale bars: 50 μm. (B) Speed of the micromotor versus motor size.

It has been reported that the concentration of the surfactant (C_s) could reduce the surface tension of the H₂O₂ fuel and subsequently stabilize the released bubbles and reduce their sizes,⁵⁰ and it thus has a strong impact on the motion of the photoactivated micromotors. The bubble start to grow on one end of the micromotor in the H₂O₂ fuel with no surfactant contained when the UV irradiation is on. However, the bubble gradually grows to the size of 150 μm and still can not be ejected from the micromotor to propel the micromotor. When C_s increases from 0.01 to 0.05 wt%, the size of the ejected bubbles is reduced from about 65 to 15 μm. Because of their low frequency (0.1 or 0.75 Hz for the micromotor in the H₂O₂ fuel contains 0.01 or 0.05 wt% Triton-X100, respectively) of the ejected bubbles, the micromotor still can not be propelled (Fig. 6 and ESI-Video 5†). When C_s is further increased from 0.1 to 5 wt%, the size of the ejected bubbles is further reduced to about 10 μm with a increasing ejection frequency of about 10 and 20 Hz, and these ejected bubbles propel the micromotor move forward with an average speed of 18.5 and 37 μm s⁻¹ respectively (Fig. 6 and ESI-Video 5†). The critical micelle concentration (CMC) of aqueous Triton-X100 solution is about 0.02 wt%.⁵¹ This suggests that surface tension of the fuel solution is at its lowest value (30.5 mN s⁻¹) when C_s is above 0.02 wt%. Hence, the size and frequency of the released bubbles, as well as the speed of the micromotor are supposed to have a similar value at a given H₂O₂ fuel concentration when C_s is above 0.02 wt%. However, the micromotor is not even activated at C_s of 0.05 wt%, and its speed still slightly increases from 18.5 to 37 μm s⁻¹ with increasing surfactant concentration from 0.1 to 5 wt% (Fig. 6). It can reasonably be speculated that the surfactant molecules around the micromotor are partially decomposed by the micromotor through photocatalytic reaction, hence changing the surface tension of the local environment and sequentially causing the different motion behaviors when C_s in the fuel is higher than its CMC.

Fig. 6 The average speeds of the micromotors as a function of surfactant concentration (C_s: 0.01–5 wt%) using a constant concentration of 5 wt% of H₂O₂ and UV intensity of 0.25 W cm⁻²; the insets show the bubble size and propulsion of the Am-TiO₂/Au Janus micromotor. Scale bars: 20 μm.

The Am-TiO₂/Au Janus micromotors are able to retain efficient bubble propulsion over a wide range of fuel concentrations. ESI-Video 6† and Fig. 7A–F show the propulsion of the micromotor with different fuel concentration (from 0.1 to 8 wt% H₂O₂). Fig. 7G manifests the influence of fuel concentration on the speed of the micromotors. There is no bubble generation or UV-driven motion for the micromotors when the fuel concentration is lower than 0.1 wt%. When the fuel concentration is higher than 0.5 wt%, the speed increases with the increasing fuel concentration, and exhibit a high average speed of 110 μm s⁻¹ in 8 wt% fuel. It is worthy to notice that the relationship between speed and fuel concentration follows the Michaelis–Menten law (Fig. 7G), suggesting the maximum photocatalytic decomposition rate of H₂O₂ and the maximum speed of the micromotor is limited by the total active sites of the Am-TiO₂/Au Janus microsphere.²³

Fig. 7 The propulsion of the Am-TiO₂/Au Janus micromotor in the solutions with different H₂O₂ concentration using a constant concentration of 0.1 wt% Triton-X100 and UV intensity of 1 W cm⁻²: (A) 0.1, (B) 0.5, (C) 1, (D) 3, (E) 5 and (F) 8 wt%, respectively. Scale bars: 50 μm. (G) Speed of the micromotor versus H₂O₂ concentration.

Conclusions

In this work, we have demonstrated a bubble-propelled photoactivated Am-TiO₂/Au Janus micromotor by taking advantage of the efficient photocatalytic H₂O₂ decomposition over the in situ H₂O₂ sensitized Am-TiO₂ under UV irradiation. The motion state and speed of the Am-TiO₂/Au micromotors can be reversibly, wirelessly and remotely controlled at will with an ultrafast response rate (less than 0.1 s) by regulating “on/off” switch and intensity of UV irradiation. The quantum efficiency of O₂ bubble evolution from photocatalytic decomposition of H₂O₂ reaches 28%. The power conversion efficiency reaches 1.28 × 10⁻⁹ for the photoactivated micromotor during the power conversion process from UV light power, to chemical power and finally to mechanical power. The photoactivated micromotor developed in this work may open up new possibilities for designing highly efficient swimming photocatalysts or adsorbents for water treatment due to their green room-temperature preparation and the intensive mass exchange between the surroundings and the active micromotors.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51303144, 21474078 and 51521001), the Top Talents Lead Cultivation Project and Natural Science Foundation of Hubei Province (2015CFA003 and 2012FFB05101), the Yellow Crane talents plan of Wuhan municipal government, the Fundamental Research Funds for the Central Universities (WUT: 2015-III-060 and 2014-YB-009).

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Footnote

† Electronic supplementary information (ESI) available: Supporting figures and videos. See DOI: 10.1039/c5ra26798f

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