Atomic Layer Deposition of Pt Nanoparticles for Microengine with Promoted Catalytic Motion
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Nanoparticle-decorated tubular microengines were synthesized by a combination of rolled-up nanotechnology and atomic layer deposition. The presence of Pt nanoparticles with different sizes and distributions on the walls of microengines fabricated from bilayer nanomembranes with different materials results in promoted catalytic reaction efficiency, which leads to an ultrafast speed (the highest speed 3200 μm/s). The motion speed of the decorated microengines fits the theoretical model very well, suggesting that the larger surface area is mainly responsible for the acceleration of the motion speed. The high-speed nanoparticle-decorated microengines hold considerable promise for a variety of applications.
KeywordsMicroengine Atomic layer deposition Pt nanoparticles Surface area
atomic layer deposition
scanning electron microscopy
The syntheses of micro-/nano-engines that are able to perform various tasks have attracted great attention with the development of nanotechnology. Among these artificial engines, catalytic micro-/nano-engines with different shapes of rod , sphere , helical , and tubes , mimicking their counterparts in the nature , are capable of moving autonomously in the presence of corresponding fuels or powered by various external stimuli such as light , magnetic , or ultrasound fields . Particularly, bubble-propelled tubular microengines have become highly attractive due to their impressive features including high-power output, ultrafast movement speed, and independence of motion on ionic strength in liquid media . In order to fabricate microtubular structures with catalytic inner surfaces, different methods have been employed, including template electrodeposition methods using porous membranes [10, 11] and roll-up technology . Rolled-up technology have a few advantages like wide range of materials engaged and easy tuning of length and diameter , and the fabricated microengines have been applied to cargo-towing , tissue-drilling , dynamic assembly , and so on. With further development of micro-/nano-electromechanical system, powerful micro-/nano-engines with high speed and large driving force are demanded to accomplish complex tasks by overcoming the viscous force at low Reynolds number , and various measures have been applied to improve the performance of the catalytic microengines. For instance, graphene , carbon nanotube , and nanoparticles  have been used to promote catalytic reactions, and the hierarchical nanoporous microtubular engines  have been reported to improve fuel refilling. Although these methods can improve the performance of microengines and the motion speeds to some extent, the preparation process is relatively complicated and the poor utilization of the expensive Pt material is also an obvious drawback. There exists a need for scalable synthetic methods to coat the surface of the microengines with precise control of the catalyst distribution. Most importantly, the size distribution of nanoparticle and efficient loading of the noble-metal catalyst should be of great importance to improve the performance of microengines.
We consider that a convenient method to commendably satisfy the requirements may be the combination of rolled-up nanotechnology and atomic layer deposition (ALD). ALD has emerged as an important technique of depositing thin films for a variety of applications . Sequential self-limiting surface reaction steps enable excellent thickness control, conformal coating on highly complex nanostructures, and good uniformity over a large area . The ALD of noble metals such as Pt has been shown to generate well-dispersed nanoparticles during the initial stages of growth [23, 24, 25, 26]. This feature could be meaningful for catalytic engines since the nanoparticle array with large surface area and high surface-area-to-volume ratio can effectively improve the utilization efficiency of catalyst .
Here, we demonstrate a simplified approach using ALD of fabricating Pt nanoparticles for the mass production of highly efficient microtubular engines. The presence of Pt nanoparticles with different sizes and distributions on the walls of microengines results in promoted catalytic reaction efficiency. Correspondingly, the Pt nanoparticle-decorated microengines exhibit significant speed acceleration compare to the theoretical speed of smooth microengines with the same diameter and length. The high performance of current Pt nanoparticle-decorated microengines offers a great opportunity for designing and producing ultrapowerful micro-/nanomachines for practical applications like cargo and drug delivery.
Results and Discussion
Fabrication of Pt Nanoparticle-Decorated Tubular Microengine
Figure 1b displays bird-view scanning electron microscopy (SEM) image of a typical 50-μm-long Pt nanoparticle-decorated SiO/SiO2 microtube under low magnification. A close examination of such tubular structure (Fig. 1c) reveals that, unlike common rolled-up microtube with a Pt smooth surface , the current microtube is covered by nanoparticles with average diameters of ~10 nm. As will be illustrated below, such Pt nanostructure leads to a dramatically increased catalytic surface area  and corresponding improved propulsion efficiency .
The Motion of Pt Nanoparticle-Decorated Tubular Microengine
The Experimental Results and Theoretical Model
The above equation suggests that the average speed of a microengine is mainly determined by the geometrical parameter X under the certain H2O2 concentration, as plotted by the red dashed curve in Fig. 4b. One can see that the Pt nanoparticle-decorated SiO/SiO2, Ti/SiO2, and SiO2/Ti microengines exhibit higher speeds compared with the theoretical prediction (1.38, 2, and 1.18 times, respectively), mainly due to the increase in the surface areas. If the surface areas are normalized (red, blue, and black squares in Fig. 4b), the experimental results can fit theoretical prediction very well if one notices that the surface areas were calculated by a simple approach (Additional file 1: Figure S3). This further proves that the larger surface area of the Pt nanoparticles (as calculated before) is mainly responsible for the highly efficient propulsion behavior of microengines, although the nanoparticle geometry may also affect the catalytic activity [32, 33]. Whereas in the case of Pt nanoparticle-decorated Ti/Co microengine (green square in Fig. 4b), the motion speed is slower than the theoretical prediction if the surface area is normalized. The surface area increased 1.42 times, but the speed increased only 1.26 times compared to the theoretical calculation. We assign this deviation to different surface morphology: the surface of the Ti/Co is unflat compared to other three samples, especially those with pure oxide bilayer nanomembrane, as we have mentioned above (see Fig. 2g). This may significantly influence the nucleation of gaseous microbubbles in the tubular chamber during catalytic motion and may also influence the dynamics of the microengine when it moves with high speed at low Reynolds number. In addition, we cannot rule out the possibility of the existence of electrochemical process in the O2 production. The Ti/Co microengine is the only one in the current four samples with conductive tube wall. Although this needs further investigation, we consider that the electrochemical process therein may be one of the possible reasons leading to smaller O2 productivity and corresponding slow motion speed.
We have demonstrated a convenient method to produce modified microtubular structures for high-speed microengines by employing ALD of Pt nanoparticles. Experimental results demonstrated that Pt nanoparticles coated on the walls of microtubes enabled a dramatic enhancement of the catalytic reaction and correspondingly acceleration of motion speed due to increased surface area. The efficient propulsion performance of microengines holds considerable promise for catalysis support, drug/gene delivery, and medical imaging/diagnostics.
Fabrication of Microtubular Structures
Rolled-up microtubes consisting of different bilayer nanomembranes were prepared on polymer sacrificial layers. The 3510 T photoresist was spun coated on silicon substrate for 9 s at 600 rpm and 30 s at 3000 rpm then baked at 100 °C for 1 min, plus 10 min cooling down in the air. The resist got exposed in the mask-aligner for 10 s after the photomask has been aligned, and then resist was developed for 30~60 s. Ti/Co, SiO/SiO2, SiO2/Ti, and Ti/SiO2 bilayers of 10/10, 5/20, 10/20, and 20/10 nm, respectively, were then deposited on photolithographically patterned circles and squares via e-beam evaporation. The samples were deposited with different rates (i.e., 1/1, 5/0.5, 1/2, and 2/1 Å s−1, respectively) to build a strain gradient in nanomembrane under a high vacuum of 3.0 × 10−4 Pa. The samples were put in different angles inclined relatively to the horizontal to open an etching window at the far end of patterns. The intrinsic strain gradients in the bilayers after removing sacrificial photoresist layer by acetone made the bilayers roll into microtubular structures. To avoid collapse caused by the surface tension of the etchants, the samples were then dried in a critical point dryer (Leica CPD 030) using liquid CO2 as the intermedium.
Pt Nanoparticle Deposition
Seventy cycles of Pt were deposited on the inner and outer surfaces of the prepared microtubes by ALD in a fluidized bed reactor. During the ALD process, (MeCp)Pt(Me)3 and oxygen were used as precursors. Herein, the precursors (MeCp)Pt(Me)3 and O2 were pulsed into the reaction chamber by the carrier gas argon, and the temperature was kept at 70 °C. During the ALD process, the working pressure in the chamber was maintained at 5 mbar.
H2O2 solutions with different concentrations as fuel sources were added to activate the microengines at room temperature. An optical microscope (Olympus BX51) with an integrated camera was adopted to observe movement and locomotion of the microengines at a rate of 30 frame s−1. With the assistance of Image J, a detailed investigation of trajectories and speed was carried out.
This work was supported by the Natural Science Foundation of China (Nos. 5132220 and 51475093), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20120071110025), and Science and Technology Commission of Shanghai Municipality (No. 14JC1400200).
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