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Graphene Jet Nanomotors in Remote Controllable Self-Propulsion Swimmers in Pure Water Omid Akhavan, Maryam Saadati, and Marziyeh Jannesari Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02175 • Publication Date (Web): 02 Aug 2016 Downloaded from http://pubs.acs.org on August 4, 2016
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Graphene Jet Nanomotors in Remote Controllable Self-Propulsion Swimmers in Pure Water
Omid Akhavan1,2*, Maryam Saadati1, Marziyeh Jannesari2
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Department of Physics, Sharif University of Technology, P.O. Box 11155-9161, Tehran, Iran
Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O. Box 1458889694, Tehran, Iran
__________________________________ * Corresponding author. Tel.: +98-21-66164566 Fax.: +98-21-66022711 E-mail address:
[email protected] (O. Akhavan).
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ABSTRACT A remote controllable graphite nanostructured swimmer working based on a graphene jet nanomotor has been demonstrated for the first time. Graphite particles with pyramidal-like morphologies were fabricated by creation suitable defects in high purity wide graphite flakes followed by a severe sonication. The particles were able to be self-exfoliated in water after Na intercalation between the graphene constituents. The self-exfoliation resulted in jet ejection of graphene flakes from the end of the swimmers (with speeds as high as ~7000 m/s), producing a driving force (at least ~0.7 L (pN) where L(μm) is swimmer size), and consequently motion of the swimmer (with average speed of ~17–40 μm/s). The jet ejection of the graphene flakes was assigned to explosion of H2 nanobubbles produced between the Na intercalated flakes. The direction of motion of the swimmers equipped with TiO2 nanoparticles (NPs) can be controlled by applying a magnetic field in the presence of UV irradiation (higher UV intensity, lower radius of rotation). In fact, the negative surface charge of the graphene flakes of the swimmers increased by UV irradiation, due to transferring the photoexcited electrons of TiO2 NPs into the flakes. Because of higher production of H2 nanobubbles under UV irradiation, the speed of swimmers exposed to UV light significantly increased. In contrast, UV irradiation with various intensities could not affect total distance traversed by the self-exfoliated swimmers having the same initial sizes. These confirmed the mass ejection mechanism for motion of the swimmers. The selfexfoliation of swimmers (and so their motion) was occurred only in water (and not, e.g., in organic solutions). Such swimmers promise designing remote controllable nanovehicles, with capability of initiating and/or improving their operations in response to environmental changes, in order to realize broad ranges of versatile and fantastic nanotechnology-based applications.
KEYWORDS: graphene, nanomachine, nanobubbles, self-exfoliation, fuel-free swimmers
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Artificial micro/nanoscale swimmers equipped with nanomotors are known as one of the most fascinating research fields in nanoscience and nanotechnology. 1 0F
– 10 22F33F44F5 1F
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In recent years, some different
kinds of micro/nanoscale swimmers propelled by external stimulations and/or chemical reactions have been developed in order to overcome the propulsion challenges raised at low-Reynolds-number regime and by Brownian motion. 11 Among them, the swimmers working by chemically-powered catalytic 10F
micro/nanomotors such as, spherical Janus micromotors, 12 – 16 tubular microengines, 17 – 22 and 1F
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bimetallic nanowires, 23 – 28 have attracted increasing attentions. But, these kinds of swimmers can 2F
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present their autonomous self-propulsion only in the presence of hydrogen peroxide fuel, which prevent many desired applications. Hence, researches have been directed to investigate fuel-free swimmers and/or other possible chemical fuels. The fuel-free nanomotors (often designed using multisegment nanowires) usually work based on magnetically29– 31 and/or electrically32, 33 propulsions. However, they always require an external 28F
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stimulator agent, and so, cannot present an autonomous self-propulsion which is highly desirable in various applications dealing with inaccessible areas. To overcome this problem, Gao et al. 34 proposed a 3F
flexible multisegment Pt-Au-Agflex-Ni hybrid nanomotor with capability of combination of chemically powered propulsion (still using H2O2) and magnetically driven locomotion. Concerning substitution of H2O2 by other chemicals, Gao et al. 35 recently proposed Ir-based 34F
Janus micromotors which work in ultra-low concentrations of hydrazine solutions. They showed ppb– ppm concentrations of hydrazine fuel can efficiently power Ir-based Janus micromotors.35 Meanwhile, such hydrazine concentrations may induce negligible toxic effects on humans. 36 Liu et al. 37 reported 35F
autonomous
Cu-Pt
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solutions.
The
micro/nanomotors which work based on magnesium-water reaction in chloride-rich water were also recently proposed and potentially utilized in some biological and environmental applications. For example, Mou et al. 38 studied self-propulsion micromotors driven by the Mg-water reaction and their 37F
hemolytic properties. Gao et al. 39 proposed the Mg-based Janus micromotors for environmental 38F
remediation of seawater. The research group of Gao et al. 40 also proposed the zinc-based microrockets 39F
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which work based on hydrogen bubble propulsion mechanism in strongly acidic media, for the first time. As one of the first steps toward in vivo use of synthetic motors, they also used such artificial microrockets in the mouse’s stomach having a suitable acidic condition for the motion of the micromotors. 41 Although based on these investigations the possibility of removing toxic chemicals 40F
(including H2O2) from the solutions has been provided, still nearly all of the chemically-powered micro/nanomotors must work in solutions having chemical additive(s) as fuel(s). To overcome this problem, Dong et al. 42 tried to remove hydrazine from the solution of Ir-based Janus micromotors by 41F
proposing hydrazine vapor-powered propulsion of Ir-Au catalytic micromotors. Although in this method the direct addition of hydrazine into the solution is not necessary, hydrazine vapor is still required (which does not seem desirable, especially in biological and environmental applications). In fact, no investigation has been reported on designing a fuel-free autonomous self-propulsion micro/nanomotor which can work in pure water. Meanwhile, there has been no report on application of graphene as the main component in micro/nanomotors. Of course, there are some few investigations on application of graphene (only as template) in micromotors. For example, Yao et al. 43 reported application of graphene 42F
oxide (GO) in scrolling the Pt layer in GO/Ti/Pt multilayered structure in which the interior Pt layer can act as a catalyst for self-propulsion of the scrolled layers through O2 bubbling in the presence of H2O2. Recently, Feng et al. 44 also reported application of graphene as a template for alveolate surface of MnO2 43F
which can involve in catalytic self-propulsion in the presence of H2O2. In this work, we have presented the first report on fuel-free graphene-based nanomotors which work based on mass (graphene flakes) ejection-based propulsion mechanism in pure water. The highspeed ejection of graphene flakes (graphene jets) from the end of Na-intercalated graphite swimmers was occurred due to Na-water reaction between the graphene flake constituents, resulting in local H2 gas production, accumulation and explosion, without requirement to adding any chemical into water as fuel. The kinematics and dynamics of the swimmers have been investigated for various swimmer sizes. The direction of motion of the swimmers decorated by TiO2 nanoparticle (NP) photocatalysts was controllable by applying a magnetic field in the presence of UV irradiation. The effects of UV ACS Paragon Plus Environment
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irradiation on the total distance and forward speed of the graphite-titania swimmers have been studied. The selective sensitivity of the self-propulsion of the swimmers to water has also been investigated by comparing the sensitivity to some organic solutions before and after addition of water.
RESULTS AND DISCUSSION Figure 1 shows material characterizations (including SEM, AFM, XPS and Raman spectroscopy) of the swimmers. The SEM images of two kind of swimmers are presented in Figure 1a. The SEM images show a pyramidal-like shape for the swimmers (also similar to reversed ships). The typical schematic structures of the swimmers are presented as the insets of Figure 1a. The pyramidal-like particles showed a layered structure, as expected for graphene-based materials. Figure 1b shows the graphene-based sheet components of the swimmers using SEM at higher magnifications. It was found that the graphene-based sheets of some swimmers (~70 %) were mainly parallel to surface of the base of the pyramidal-like particles (called swimmers with horizontally graphene sheets (H-swimmers)). On the other hand, the graphene-based sheets of other swimmers were found parallel to the backside of the swimmers (nearly perpendicular to the surface of the base of pyramids). This kind of swimmers (with abundance of ~30%) was called swimmers with vertically graphene-based sheets (V-swimmers). The inset of Figure 1Bb shows SEM image of the backside of the V-swimmer in a close-up window. It shows a nearly uniform surface with significant different morphology from the cross sectional images taken from the front sides of the swimmers (H-swimmer and V-swimmer which show horizontal (Figure 1Ab) and vertical (Figure 1Bb) graphene-based sheets, respectively). It should be noted that the morphology of the backside of the H-swimmers was very similar to the morphology of their front sides (i.e., layered morphology). Figure S1A also shows SEM image of the cross section of a graphite-titania swimmer (the V-swimmer). Some particle-like features are observable on surface of the graphene component sheets. Meantime, Figure S1B presents XPS survey spectra of the graphite-titania particles. It was found that Na/Ti atomic ratio of the samples obtained by energy dispersive X-ray (EDX) was higher than the ratio obtained by XPS.
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This implied the effective presence of Na in depth of the samples (rather than on the surface which is observable by XPS), i.e., the effective Na intercalation among the graphene sheets components. Hence, the surface particle like features can be assigned to the TiO2 NPs (the decorated NPs). The inset of Figure 1Ab shows high-resolution XPS of C(1s) core level of graphite-titania swimmers and GO sheets obtained through self-exfoliation of the swimmers in water. The binding energy of the main peak of the graphite-titania swimmers (located at 285.0 eV) was assigned to the C─C and C=C bonds. Since binding energies of the C─O, C─O─C, C=O, and then O=C─O oxygencontaining functional groups are typically at higher binding energies (from ~286.6 to 289.4 eV), 45– 47 the 4646F 45F
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appearance of a shoulder at higher energies can be assigned to the formation of such oxygen-containing groups (especially the hydroxyl one, due to hydration) on surface of the self-exfoliated GO. The well dispersion of the self-exfoliated GO sheets in water was also consistent with the presence of hydroxyl groups. The O/C ratio of the self-exfoliated GO sheets was found ~0.4, indicating partial oxidizing the exfoliated sheets in water (the typical O/C ratios of the GO and hydrazine-reduced GO sheets were reported ~0.51 and 0.16, respectively). 48– 50 47F
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AFM images of the GO sheets produced by self-exfoliation of the H and V swimmers in water are shown in Figure 1Ac and Figure 1Bc, respectively. The overlapped sheets are clearly distinguishable in the images. The height profile distributions of the sheets are presented in Figure 1Ad and Figure 1Bd. Since the first peak in the height profile distributions presents surface roughness fluctuations of the Si substrates, the position of the second peak at ~1 nm was assigned to the thickness of single-layer GO sheets deposited on the substrate (the typical thickness of single-layer GO sheets is ~0.8 nm). 51– 53 The 51F5252F
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location of the second peak at ~1.5 nm indicated the overlapping of the sheets during their deposition. The AFM images clearly show that the exfoliated GO sheets possessed a triangle-like morphology. For the GO sheets obtained from the H-swimmers, the triangles are more elongated than the sheets obtained from the V-swimmers. This is completely consistent with the layered structure of the H- and Vswimmers observed by the SEM images. The average lateral size distributions of the GO sheets deposited on the Si substrate were also presented in Figure 1e, as also previously used in [ 54]. The 53F
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average lateral dimension of the GO sheets produced by the self-exfoliation of the H-swimmers in water was found ~350 nm, while it was found ~130 nm for the GO sheets obtained from the V-swimmers, consistent with their layered structures shown in the SEM images. It should be noted that the overlapping of the sheets on the substrate (causing to formation of further boundaries) resulted in reduction of the average lateral dimensions of the sheets, as compared to their real average sizes. The higher value for the average lateral dimension of the H-swimmers (as compared to the V-swimmers) can be attributed to their elongated triangle-like morphology. The horizontally or vertically layered structure of the swimmers can also be examined by Raman spectroscopy, as presented in Figure 1f. The famous D and G bands of carbon materials were found at ~1350 and 1580 cm–1, respectively. The D/G peak ratio (as a parameter used to evaluate the defects of carbon materials) was found ~0.94 and 1.41 for the H- and V-swimmers, respectively. This indicated the presence of higher defects in the V-swimmers, as expected from the layered structures observed by the SEM images. Consistently, the G/D peak ratio of the GO sheets (as a parameter applied in evaluation of the mean sp2 domain size of the sheets having sp3 and sp2 bonds) was found ~0.86 and 0.61 for the sheets produced by the self-exfoliation of the H- and V-swimmers in water, respectively. The higher defect in the V-swimmers is consistent with the lower lateral dimension of the sheets produced by the self-exfoliation of the V-swimmers (observed by the AFM analysis). In Raman spectroscopy, the 2D band of graphene materials is high sensitive to the single- and multi-layer characteristics of the materials. 55 For example, the 2D/G peak ratio of single-, double-, triple- and multi(> 4)-layer graphene 54F
would be typically >1.6, ~0.8, ~0.30 and ~0.07, respectively (see, e.g., [ 56– 58]). Based on Figure 1f, 5F
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the 2D/G peak ratios of the GO sheets obtained from the self-exfoliation of the both swimmers were found ~0.31, indicating the presence of single- and multi-layer sheets (with abundance of triple-layers) after completing the exfoliation of the swimmers in water. This result also suggests that this method can be extended to obtain single- and multi-layer GO sheets using self-exfoliation of graphite (rather than graphite oxide) in water.
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Figure 1. SEM images of a) graphite-titania swimmers (H-swimmer in (A) and V-swimmer in (B)) and b) the cross sections of the graphite sheet components of the swimmers, c) AFM images of GO sheets produced by self-exfoliation of the swimmers in water, d) height profile histogram of GO sheets shown in AFM images, and e) lateral size distribution of the GO sheets (for the H-swimmers in (A) and Vswimmers in (B)). The typical schematic structures of the H-swimmer and V-swimmer are also presented in (a) as the insets. The C(1s) core levels of XPS of i) graphite-titania swimmers and ii) GO sheets obtained by self-exfoliation of the swimmers in water are presented as the inset of (Ab). Oxidation of the exfoliated sheets (ii) is observed comparing to the starting material (i). The nearly uniform surface of the backside of the V-swimmers is presented as the inset of (Bb). f) Raman spectra of i) H-swimmers and ii) V-swimmers, and GO sheets produced through the self-exfoliation of iii) Hswimmers and iv) V-swimmers in water.
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Track of the swimmers are presented in Figure 2A. No net progressive motion was observed for the H-swimmers. In fact, they often showed a rotational motion, as illustrated in Figure 2Aa. The instantaneous speeds of the H-swimmers are shown in Figure 2Ba. Although some nonzero instantaneous speeds and consequently some traveled distances were recorded for some H-swimmers (depending to their randomly layered structures resulting to the presence of some layers with vertical components), the average speed of the H-swimmers was obtained effectively zero. In contrast, the Vswimmers presented a progressive motion with of course some slight and random deviations from their straight line, as can be seen from the track of one of the V-swimmers in Figure 2Ab. The instantaneous speeds of the V-swimmers are presented in Figure 2Bb. The average speed of the V-swimmers was found ~345 μm/s in typical ranges of ~60 μm (the speeds as high as ~800 μm/s were also found, depending on morphology of the swimmers). It was found that the direction of motion of the Vswimmers can be changed by applying a magnetic field perpendicular to the surface of motion (typically surface of water) under UV irradiation (see Figure 2Bc for a V-swimmer exposed to the magnetic field of ~0.1 T and UV irradiation of ~10 mW/cm2). By inverting the direction of the applied magnetic field, the direction of rotational motion was reversed. In the absence of magnetic field, no net deviation from the straight line motion was observed. Based on the well-known relation of r = mv/qB, the inset of Figure 2Bc shows the number of photoexcited electrons (Ne) accumulated on the swimmers by the
various intensities of UV irradiation. It is seen that by increasing the UV intensity, the Ne linearly increased, indicating the linear effects of UV irradiation on the q parameter (the electric charge accumulated on a swimmer). On the other hand, Figure 2Bc indicates a dependence of r ∝ (IUV)-0.64 for fitting the radius data as a function of the UV intensity. Therefore, radius is not exactly proportional to the inverse of the charge accumulated on the swimmers. This implies that the UV intensity can also change the instantaneous speed of the swimmers, as investigated in some details in the following. Meantime, we also checked whether it is possible to observe a complete circular motion for the Vswimmers. The main problem was exiting the majority of the swimmers from the microscopic frame (this became worse by increasing the size of the swimmers). In addition, for the seldom captured ACS Paragon Plus Environment
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microscale swimmers showing a nearly circular motion (~5% of tests), we rarely found a complete circular motion (only
