Highly Efficient Light-Driven TiO2âAu Janus Micromotors - ACS Nano

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Highly Efficient Light-Driven TiO2-Au Janus Micromotors Renfeng Dong, Qilu Zhang, Wei Gao, Allen Pei, and Biye Ren ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b05940 • Publication Date (Web): 23 Nov 2015 Downloaded from http://pubs.acs.org on November 26, 2015

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Highly Efficient Light-Driven TiO2-Au Janus Micromotors Renfeng Dong,1 Qilu Zhang,1 Wei Gao,2 Allen Pei,2 Biye Ren1* 1

Research Institute of Materials Science, South China University of Technology, Guangzhou

510640, China 2

Departments of Nanoengineering, University of California San Diego, La Jolla, CA 92093,

United States KEYWORDS. Janus micormotors • self-electrophoresis • TiO2 • light driven • fuel free

ABSTRACT. A highly efficient light-driven photocatalytic TiO2-Au Janus micromotor with wireless steering and velocity control is described. Unlike chemically-propelled micromotors which commonly require the addition of surfactants or toxic chemical fuels, the fuel-free Janus micromotor (diameter ~1.0 µm) can be powered in pure water under an extremely low ultraviolet light intensity (2.5×10-3 W/cm2), and with 40×10-3 W/cm2, they can reach a high speed of 25 body length/s which is comparable to common Pt-based chemically-induced self-electrophoretic Janus micromotors. The photocatalytic propulsion can be switched on and off by incident light modulation. In addition, the speed of the photocatalytic TiO2-Au Janus micromotor can be accelerated by increasing the light intensity or by adding low concentrations of chemical fuel

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H2O2 (i.e., 0.1%). The attractive fuel-free propulsion performance, fast movement triggering response, low light energy requirement and precise motion control of the TiO2-Au Janus photocatalytic micromotor hold considerable promise for diverse practical applications.


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The autonomous motion of nano/microscale objects has stimulated considerable research efforts over past decade.

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Particular attention has been given to chemically powered

micromotors which include bimetallic catalytic nanowires, and Janus microspheres.

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microtubular microrockets,

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However, they mostly exhibit autonomous self-propulsion in the

presence of hydrogen peroxide fuel10,11,14 or hydrazine15 which greatly hinders the practical utility of chemically powered nanomotors. Therefore, efforts towards the fuel-free nanomotors powered by external stimuli such as magnetic fields, and light

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electrical fields,

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ultrasound

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are particularly exciting and considerably promising towards practical

applications. In particular, light is one of the most important and versatile physical stimuli to facilitate and regulate the propulsion of micro/nanomotors. 24, 25 However, the major challenge of light-driven micromotors is to achieve the high propulsion speed and precise motion control. 26, 27 Here we demonstrate light-driven TiO2-Au Janus micromotors (~1.0 µm diameter) based on light induced self-electrophoresis mechanism which display efficient propulsion (over 25 body length/s) under an extremely low UV light intensity (40×10-3 W/cm2) in the presence of pure water without adding any extra fuel. The light-driven Janus motors reach such fast speed which is comparable to common Pt-based chemically-induced self-electrophoretic Janus micromotors. The propulsion of Janus micromotor can be conveniently triggered even at extremely low ultraviolet (UV) light intensity (2.5×10-3 W/cm2) with high repeatability and accelerated by increasing the light intensity or by adding low concentrations of chemical fuel H2O2. As illustrated in Figure 1A, the Janus micromotors consist of plain TiO2 particles (~1.0 µm mean diameter) with one hemisphere coated with Au metal. On the one hand, Anatase TiO2 is widely studied due to its interesting properties, including photocatalytic activity, biocompatibility, high chemical stability, and low costs, 28, 29 it is commonly used for decontamination purposes due to


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the high photoactivity of TiO2. 30, 31 On the other hand, it is well-established that Au-doped TiO2 or the TiO2-Au system has a greatly enhanced catalytic performance compared to that of bulk TiO2.

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In light of these advantages, we designed catalytic TiO2-Au Janus micromotors by

coating a thin film of gold (gold film thickness: 40 nm) on TiO2 particles (Figure 1B). When the TiO2 surface is exposed to UV light, the self-electrophoretic propulsion mechanism is immediately triggered and efficient propels the micromotor in pure water.

Figure 1. (A) Schematic of catalytic TiO2-Au Janus micromotors powered by UV light in water. (B) Scanning electron microscope (SEM) image of a spherical TiO2-Au micromotor and (C-E) the corresponding Energy Dispersive X-Ray (EDX) images for Ti, Au, O, respectively. Scale bar, 0.5 µm. (F) Tracking lines (taken from Supporting Information Video S1) illustrating the distances traveled by three micromotors in pure water over 1 s. Scale bar, 10 µm.


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RESULTS AND DISCUSSION The propulsion of the TiO2-Au micromotors dominantly originates from the light induced self-electrophoresis.

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Compared to Pt-Au Janus motors which require H2O2 for self-

electrophoresis, the light-driven TiO2-Au system can generate a self-induced electric field in pure water and the motion can be controlled by light. Upon UV irradiation of the Janus micromotors, charge separation occurs within the TiO2 and electrons are injected from the TiO2 conduction band into the Au hemisphere. Protons are produced from the oxidation of water at TiO2 and the resulting electrons are consumed during the reduction of protons at Au. The resulting flux of H+ generates a fluid flow towards the Au hemisphere, generating a slip velocity and propelling the micromotors with the TiO2 hemisphere forward (Supporting Information Video S2 shows that the TiO2-Au Janus motors moving towards TiO2 side under an extremely low UV intensity, 2.5×10-3 W/cm2). In contrast, plain TiO2 microspheres without Au layer only display Brownian motion even upon exposure to 40 × 10-3 W/cm2 UV light (Supporting Information Video S1). Previous reports have demonstrated thermophoretic micromotor propulsion from infrared irradiation of Au.36 However, to validate which is the dominant mechanism resulting in the propulsion of the TiO2-Au micromotors, photo-induced selfelectrophoresis or thermophoresis, control experiments were conducted using polystyrene (PS)Au Janus particles (1.0 µm diameter). Under 40 × 10-3 W/cm2 intensity light, no obvious directional movement was observed for PS-Au Janus particles, while the TiO2-Au micromotors were moving pretty fast (Supporting Information Video S1). The control experiments of the micromotors in salt rich environment (NaCl solution) have been investigated. The speed of TiO2Au Janus motor in 1.0 mM NaCl solution is obviously smaller than that in pure water (Supporting Information Video S3). The micromotor stops moving in the presence of 10.0 mM


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salt. These results further confirm the self-electrophoresis mechanism. We also examined the mixed potentials of Au and TiO2 electrodes with and without illumination (Figure 2). It is clear from the Figure 2 that the potential difference between the Au electrode and illuminated TiO2 (∆ = 307 mV) electrode is larger than that between Au electrode and TiO2 electrode without illumination (∆ = 237 mV). As reported previously by Nakato and Tsubomura, the height of the Schottky barrier at the Au/TiO2 contact is considerably increased by illumination, and the photovoltage obtained is much larger than that expected from the barrier height in the dark.37 These findings are consistent with the obtained results. The motion of such light–driven Janus micromotor is strongly dependent on the coated layer due to the self-electrophoresis mechanism. We tried TiO2-nickel (Ni) Janus micromotors as control, as illustrated in the Supporting Information Video S4, the speed of TiO2-Ni Janus micromotor (8 µm/s under 40×10-3 W/cm2 UV light intensity) is much lower than that of TiO2-Au Janus micromotors (25 µm/s). The Tafel plot of Au, Ni, TiO2 with and without illumination (0.5×10-3 W/cm2) (shown in Figure 2) clearly indicates that the potential difference between Ni and illuminated TiO2 electrode (∆ = 150 mV) is significantly smaller than Au and illuminated TiO2 electrodes (∆ = 307 mV). This finding is consistent with the propulsion behavior of the corresponding micromotors and further confirms the selfelectrophoresis mechanism of the light-driven metal-TiO2 micromotors. Regarding the insulation layer, aluminum oxide (Al2O3)-TiO2 Janus microsphere was chosen as the control. As illustrated from Supporting Information Video S4, Al2O3-TiO2 Janus microsphere doesn’t display directional motion under 40×10-3 W/cm2 UV light intensity. Such phenomenon can be attributed to the greatly hindered electrophoresis between the insulation layer and TiO2.


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Figure 2. A Tafel plot of Au, Ni, TiO2 with and without illumination (0.5 ×10-3 W/cm2) As indicated from Figure 1A, the photoactivity of TiO2 comes from its hole–electron separation triggered by photons of energy equal to or higher than its bandgap. The conduction band of Au lies below that of TiO2, and as a result, the Au hemisphere acts as a sink for electrons, increasing electron-hole pair lifetime and recombination times which result in enhanced reactivity on the micromotors surfaces.

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Therefore, such structure makes the TiO2-

Au Janus micromotor much more efficient than the reported plain TiO2 micromotor whose average speed is around 10 µm/s under 2.5 W/cm2 UV light intensity. 27 Compositional analysis using SEM and EDX mapping indicates that the Au coating only covers half of the TiO2 spherical particle (Figure 1B, C, D, E, the relative EDX pattern in Supporting Figure 1 also illustrating the composition of the TiO2-Au Janus micromotor), generating the asymmetry


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necessary for directional motion. The micromotor tracklines in Figure 1F show that the new TiO2-Au Janus motors move immediately at a speed of around 25 µm/s in pure water upon UV light irradiation. As photocatalysis primarily drives the TiO2-Au micromotor propulsion, the motion speed of the micromotors is able to be precisely controlled by adjusting the UV irradiation intensity (I). The highly efficient TiO2-Au Janus micromotors can be propelled by super low UV light intensity. Figure 3A reveals pretty slow but directional motion when I is 2.5×10-3 W/cm2. When increasing I from 5 to 10×10-3 W/cm2, the speed of the micromotors increases from 5.6 to 11.2 µm/s (Figure 3B, 3C). The micromotors can move at a speed of over 25 µm/s corresponding to a relative speed of 25 body lengths/s under a light intensity of 40×10-3 W/cm2 (Figure 3D). Such efficient speed is almost 2 times faster than reported speed of light driven TiO2 micromotors in pure water, 27 however, over 60 times lower UV light power is needed. Also, such efficient speed is comparable to that of the catalytic micro/nanomotors, and significantly larger than the speed of previously reported photothermally-propelled motors.

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Figure 3E illustrate the diagram of the

influence of light intensity (I) on the speed of TiO2-Au Janus micromotor. To clearly indicate the directional movement of the Janus motors at extremely low UV light intensity, we experimentally estimated the enhanced diffusion coefficients of the motors. The motors (number of motors n =20) were tracked over 10 s, the mean squared displacement (MSD) was calculated at different UV light intensity (0, 2.5×10-3, 5×10-3 W/cm2) and the diffusion coefficient (D) was calculated by the equation D=MSD/ ⋅ ∆, where ∆t is the time interval. Here, for the case of two-dimensional analysis from the recorded video, i is equal to 4 (Figure 3F and G, respectively). It can be clearly observed that although the speed under 2.5×10-3 W/cm2 UV light is almost the same as that without UV light, the MSD of Janus motor under 2.5×10-3 W/cm2 UV


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light is significantly higher than that without UV light. These results demonstrated that the UV light with extremely low intensities still has obvious influence on the propulsion of the Au-TiO2 micromotors. In general, the relationship of incident light (I) and the flux (Φ, the number of incident photons per unit area per second) can be described as Equation (1). 24

= Φ

(1)

Here, h, c, and λ represent Plank’s constant (6.626×10−34 J·s), speed of light (3×108 m/s) and wavelength of the UV light (330~380 nm in this case), respectively. As a result, the flux Φ of photons of wavelength λ increases with increasing I on the micromotor, and thus the number of the photogenerated holes and electrons in TiO2 increases. Therefore, there will be a proton gradient enhancement and corresponding fluid shear velocity resulting from increased irradiation intensity. Accordingly, the speed of the micromotors can be modulated by incident light intensity.


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Figure 3. (A, B, C, D) Tracklines of micromotors (Supporting Information Video S5) with UV intensity 2.5, 5, 10, and 40×10-3 W/cm2, respectively over 1 s. Scale bar, 10 µm. (E) The influence of the UV light intensity on the speed of Janus micromotors in pure water. (F) Average MSD versus time interval (∆t) analyzed from tracking trajectories. (G) Diffusion coefficient of TiO2-Au Janus micromotor under different UV light intensity, the values determined from the MSD plots (20 micromotors were analyzed). Compared to catalytic micromotors which consume chemical fuels, the as-developed lightdriven TiO2-Au micromotors have the impressive advantages of highly repeatable simple motion control. As Figure 4 illustrates, the repeated on/off cycling of UV light illumination induces the activation and inactivation of the movement of micromotors reflecting the fast response rate of


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the micromotors upon the UV irradiation. The speed of the micromotors is around 25 µm/s under 40×10-3 W/cm2 UV light exposure, but stop moving immediately when the UV light off. This “stop/go” propulsion behavior indicates that high reversibility and controllable micromotor motion can be achieved by switching the UV irradiation on or off (as shown in Supporting Information Video S6). Even after 30 cycles of such on and off control, the micromotors still show highly repeatability.

Figure 4. Cyclic “On” and “Off” UV light activation of the TiO2-Au Janus micromotor. (A) Schematic and Time-lapse images illustration of TiO2-Au Janus micromotor moving status in pure water with UV light and without UV light, respectively. (B) Corresponding speed/time dependence illustrating the UV light triggered “On/Off” motion control of the TiO2-Au Janus micromotor in pure water without any additional chemicals. The images were taken at 1-s intervals from Video S6. Scale Bar, 10 µm. UV light intensity 40×10-3 W/cm2

Directional control of micromotor is also a critical performance for diverse practical applications. The deposition of a paramagnetic Ni layer between Au layer and TiO2 can be easily realize magnetic control of the directionality of the light-driven TiO2-Au Janus micromotor.38, 39, 40

Figure 5A illustrates the structure of such magnetic guided Janus micromotors. Using an


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external magnetic field, Au-Ni-TiO2 Janus micromotors can be precisely navigated following predetermined trajectories (Figure 5B and Supporting Information Video S7). Such directional control could offer more possibility for the light-driven micromotor applying in variety of practical applications.

Figure 5. A) Schematic of the magnetic control of multi-layer Au-Ni-TiO2 Janus micromotors. B) Time-lapse images (taken from Supporting Information Video S7) showing the magneticallyguided propulsion of an Au-Ni-TiO2 micromotor under 40×10-3 W/cm2 UV light. Scale bar, 10 µm. Interestingly, low concentrations of H2O2 can also improve the motion of the TiO2-Au micromotors. The micromotors can be accelerated gradually by increasing the concentration of H2O2 under 40×10-3 W/cm2 UV light. The TiO2-Au micromotors display Brownian motion (Figure 6A) in 0.1% H2O2 without UV light exposure, however, the speed of such light-driven Janus motor can reach 42 µm/s (Figure 6C) with additional 0.1 % H2O2, which is almost 2 times compared to the one in pure water (Figure 6B). Such fast speed of TiO2-Au Janus motors in 0.1% H2O2 is almost 20 times faster than the common Pt based Janus motors in 0.2% H2O2 at room temperature,

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which is the lowest H2O2 concentration requirement with high speed for


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Janus motors so far. The acceleration of TiO2-Au Janus motors in low H2O2 concentrations with UV light due to the enhanced self-electrophoretic effects in H2O2 compared to pure water (Figure 6D). 42,43

Figure 6. Tracklines for TiO2-Au micromotors under three conditions: (A) with H2O2 without UV, (B) without H2O2 with UV and (C) with both H2O2 and UV, respectively over 1 s.Scale bar, 10 µm (taken from Supporting Information Video S8). (D) Schematic of catalytic TiO2-Au Janus micromotors powered by UV light in low concentration of H2O2. CONCLUSIONS In conclusion, we described light-driven TiO2-Au Janus micromotors which can be powered efficiently by extremely low UV light energy in pure water. In addition, the light induced selfelectrophoresis mechanism has been demonstrated in detail also. This study showed that photocatalytic micromotors can be accelerated by increasing the light power intensity or addition of low concentrations of extra chemical fuels H2O2. Furthermore, such light driven TiO2-based


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micromotors can be driven in a precisely controlled manner, involving controllable activation, acceleration, deceleration, stop and directional control. Also the gold hemisphere in TiO2-Au Janus micromotors can be easily modified with diverse functional groups offering various possibilities to challenging complicate tasks. The fuel-free property of these light-driven micromotors combined with the decontaminative capabilities of TiO2 should naturally lead to useful applications in environmental remediation.

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These light driven, precisely controllable and

highly efficient TiO2-based phtotocatalytic Janus micromotors hold considerable promise for the design of practical light driven nanomachines toward a wide range of important future applications ranging from nanofabrication45 to environmental remediation.3 EXPERIMENTAL SECTION Synthesis of Janus micromotors.

TiO2 microspheres were prepared by the solvent

extraction/evaporation method using tetrabutyl titanate as a precursor.46 1.0 ml tetrabutyl titana (Sigma #244112) was dissolved in 40.0 mL ethanol incubate at room temperature for 3 h, then TiO2 microsphere were collected by centrifugation at 7,000 r.p.m. for 5 min and washed repeatedly with ethanol (Guangzhou Chemical Reagent Co.) and ultrapure water (18.2 MΩ•cm), three times each, then dried in air at room temperature. TiO2 (anatase) microsphere is obtained after annealed for 2 h at 400℃. The X-ray diffraction (XRD) pattern (Supporting Figure 2) reveals that the TiO2 micrspheres have a well anatase phase. For the TiO2-Au light driven Janus micromotor, using TiO2 microspheres (1.0 µm mean diameter) as the base particles. 10.0 µg of TiO2 particles were first dispersed in 150.0 µL ethanol. The sample was then spread onto glass slides and dried uniformly to form particle monolayers. The particles were sputter coated with a thin gold and nickel layer using a Quorum Q 150T ES Sputter Coater for 3 cycles with 60 s per


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cycle. The metal layer thickness was found to be 40 nm, as measured by the Veeco DEKTAK 150 Profilometer. For the TiO2-Ni-Au magnetic Janus motors, TiO2 particle monolayers were prepared as above method. A 40 nm layer of Au followed by a 10 nm layer of Ni were sequentially deposited on half of the particles by Quorum Q 150T ES Sputter Coater. The TiO2 microspheres were coated with Al2O3 layer using ultra-high Vacuum Magnetron Sputter Coater JPG 560. The micromotors were subsequently released from the glass slides via pipette pumping and dispersed into double distilled water. The Polystyrene-Au Janus microsphere as a control was fabricated with the same method using Polystyrene microsphere (Baseline #6-1-0100). Speed calibration experiments. To determine the relationship between the TiO2-Au, TiO2Ni and TiO2-Al2O3 motor speed and light intensity, the light intensity ranging from 2.5×10-3 W/cm2 to 40 × 10-3 W/cm2. 0.2 mM to 200.0 mM sodium chloride (NaCl) solutions were prepared for testing salt rich environment control experiment. 0.2 % aqueous hydrogen peroxide (Alfa Aesar #33323) were prepared and directly mixed with the motor droplets. The propulsion calibration experiments were performed by mixing 1.0 µL of the motor and hydrogen peroxide solutions each. Electrochemical Potential Measurements. Tafel plots is used to obtain the potential established at different segment of different Janus micromotors (Au, Ni, and TiO2) with and without illumination (0.5×10-3 W/cm2 UV light intensity, λ=330~380 nm) in a pure water environment. Gold, nickel and TiO2 film (all the films’ thickness: 100 nm) on ITO glass disc (diameter: 1.0 cm) were used as the working electrode in the electrochemical potential measurements, respectively. We use the CH Instrument Model CHI600C to test the potential at a


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scan rate of 5 mV/s and over a potential range of -0.2 to 0.3 V (vs. Ag/AgCl, 3 M KCl reference). Equipment. SEM and EDX pictures obtained by Zeiss VEO 18, XRD pattern obtained by X-Ray Diffractomer (Panlytical Inc. X’ Pert Pro). UV light was generated by Mercury lamp sockets, dichroic mirror DM 400 and barrier filter BA420, intensity controlled by ND filters (4×, 8×, 16×) (all from Nikon), light intensity measured by UV radiometer UV-A (Videos were captured by an inverted optical microscope (Nikon In-strument Inc. Ti-S), coupled with 40× objectives, and a Zyla scmos digital camera (ANDOR) using the NIS-Elements AR 4.3 software. ASSOCIATED CONTENT Conflict of Interest: The authors declare no competing financial interest. Supporting Information. Supporting videos description, supporting Figures. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Address correspondence to [email protected] Present Addresses Wei Gao, Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720, United States. Allen Pei, Department of Materials Science & Engineering, Stanford University, Stanford, California 94305, United States


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ACKNOWLEDGMENT The financial support from the NSFC (21274047) and the Specialized Research Fund for the Doctoral Program of the Education Ministry (20120172110005) is gratefully acknowledged. . REFERENCES 1. Wang, J. Nanomachines: Fundamentals and Applications. John Wiley & Sons: 2013. 2. Mallouk, T. E.; Sen, A. Powering Nanorobots. Sci. Am. 2009, 300, 72-77. 3. Moo, J. G.; Pumera, M. Chemical Energy Powered Nano/micro/macromotors and the Environment. Chem. Eur. J. 2015, 21, 58-72. 4. Mei, Y.; Solovev, A. A.; Sanchez, S.; Schmidt, O. G. Rolled-Up Nanotech on Polymers: from Basic Perception to Self-Propelled Catalytic Microengines. Chem. Soc. Rev. 2011, 40, 2109-19. 5. Ozin, G. A.; Manners, I.; Fournier-Bidoz, S.; Arsenault, A. Dream Nanomachines. Adv. Mater. 2005, 17, 3011-3018. 6. Purcell, E. M. Life at Low Reynolds Number. Am. J. Phys. 1977, 45, 3-11. 7. Sanchez, S.; Soler, L.; Katuri, J. Chemically Powered Micro- And Nanomotors. Angew. Chem. Int. Ed. 2015, 54, 1414-44. 8. Wang, J. Can Man-Made Nanomachines Compete with Nature Biomotors? ACS Nano 2009, 3, 4-9. 9. Guix, M.; Mayorga-Martinez, C. C.; Merkoçi, A. Nano/Micromotors in (Bio)chemical Science Applications. Chem. Rev. 2014, 114, 6285-6322. 10. Paxton, W. F.; Kistler, K. C.; Olmeda, C. C.; Sen, A.; St Angelo, S. K.; Cao, Y.; Mallouk, T. E.; Lammert, P. E.; Crespi, V. H. Catalytic Nanomotors: Autonomous Movement of Striped Nanorods. J. Am. Chem. Soc. 2004, 126, 13424-31.


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Highly Efficient Light-Driven TiO2âAu Janus Micromotors - ACS Nano

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