The role of hydrazine in mixed fuels (H₂O₂/N₂H₄) for Au–Fe/Ni nanomotors

Abstract

We previously introduced a novel Au–Fe/Ni alloy nanomotor that could move with a speed of up to 850 μm s⁻¹ in a mixed fuel (H₂O₂ and N₂H₄) environment. However, the propulsion mechanism and the role of hydrazine were not fully elucidated. To this end, Tafel plots, linear sweep voltammetry (LSV) and oxygen sensing were employed and integrated to explore hydrazine's function and explain the locomotive behaviour of Au–Fe/Ni nanomotors. It was found that the speed of Au–Fe/Ni nanomotors was not positively correlated with the mixed potential difference between the composite metal pair, which had not been found in similar self-electrophoresis propelled nanomotors. Moreover, the results showed that the oxidation of H₂O₂ can also be facilitated by hydrazine in mixed fuels, resulting in oxygen bubbles propelling Au–Fe/Ni nanomotors forward.

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Introduction

Synthetic, chemically powered motors, inspired by biological nanomotors, were first introduced in 2004 (Au–Pt) and 2005 (Au–Ni).^1,2 During the past decade, much effort has been devoted to redesigning novel motors, studying motion mechanisms and developing potential applications. A variety of nano- and microscale motors (rod/wire-shaped devices,^3,4 spherical vehicles,^5–7 tubular or conical microjets^8,9 etc.), have been shown to produce fast motion or swimming movements. Autonomous movement is always the most desirable property of these motors, but the mechanism that causes a directional motion in fluids has long been a subject of debate. Several propulsion mechanisms, such as diffusiophoresis, bubble propulsion, interfacial tension and self-electrophoresis, have been proposed to describe their locomotive behaviours.^10–14 In addition, ionic and electrokinetic effects^15,16 may also affect the motion of some swimmers. A fundamental understanding of the mechanisms is important for developing potential applications in bio-medicine,^17–20 self-propelled pumping,^21–23 chemical sensing,^24,25 environment remediating^26,27 etc.

Recently, Au–Fe/Ni alloy nanowire motors reported by our group not only moved with dramatic speeds in mixed fuels composed of H₂O₂ and N₂H₄, but also oriented neatly under the control of an external magnetic field.²⁸ Hydrazine, a well-known monopropellant commonly used in rocket motors, is also another fuel used to propel nano- and microscale motors. In experiments conducted by Wang's research group, it was noticed that a hydrazine additive within a hydrogen peroxide fuel results in dramatic acceleration of both catalytic nanowire motors and tubular microjets.^29,30 And they found that Iridium-based Janus mircomotors powered by ultralow hydrazine fuel alone also displayed locomotive behaviour.³¹ However, Au–Fe/Ni nanomotors was found to be immobile in either hydrogen peroxide or hydrazine solution on their own and moved only in mixed solutions.

In this paper, Tafel plots of two electrodes (Fe/Ni, Au), linear sweep voltammetry (LSV) curves of four electrodes (Fe/Ni, Au, Fe, Ni) and the concentrations of dissolved oxygen were studied to discuss the role of hydrazine played in mixed fuels and to provide more evidence for the propulsion mechanism of Au–Fe/Ni nanomotors. It was found that the relationship between the mixed potential difference and the speed in the same fluid was not consistent with the reported experimental observations.³² Furthermore, the hydrazine in mixed fuels was found to facilitate the oxidation of hydrogen peroxide to oxygen bubbles on Fe/Ni alloy electrode. Combined with visible train of bubbles moving in a direction away from the catalytic site, it was reasonable to infer that the dominant mechanism of Au–Fe/Ni nanomotors was probably by means of bubble propulsion.

Experimental section

Fabrication of Fe and Fe/Ni alloy electrodes

The Fe electrode was fabricated by electrodepositing an iron layer on the surface of an Au electrode (Φ = 3 mm) in 0.1 M FeSO₄ at −1.2 V for 90 s. The Fe/Ni electrode was obtained by electrodepositing a Fe/Ni alloy layer onto the Au electrode in the plating solution (0.1 M FeSO₄, 0.3 M NiSO₄, 0.06 M Na₃C₆H₅O₇, 0.5 M H₃BO₃) at −1.3 V for 90 s. An Ag/AgCl reference electrode and a glass carbon counter electrode were employed in the electrodeposition process. All plating solutions and conditions were kept consistent to guarantee the stability of the electrodes. After the electrodes were rinsed and ultra-sonicated in deionized water, their surfaces were checked under a microscope to ensure they were clean and smooth before use.

Electrochemical measurements

Three experimental techniques were explored and employed to study the role that hydrazine played in mixed fuels and the probable propulsion mechanism of Au–Fe/Ni nanomotors. The first technique involved the measurement of the mixed potential differences between an Au electrode and a Fe/Ni electrode in four solutions by means of Tafel plots. These measurements enabled the correlation between the mixed potential difference and velocity in the same fuel environment to be studied. The second technique involved the elucidation of the hydrazine's function in mixed fuels for the reaction that occurred on Au–Fe/Ni nanomotors by LSV. LSV curves of four different electrodes (Fe/Ni, Fe, Ni, Au) were carried out in different solutions (hydrogen peroxide, hydrazine and mixtures of these two in different ratios). The third technique involved the determination of the composition of bubble thrusts using an oxygen sensing method. A Dissolved Oxygen Meter was used to determine the concentrations of dissolved oxygen in the two solutions. A three-electrode system was introduced during these measurements, in which Fe/Ni alloy, Ag/AgCl and glass carbon were used as the working electrode, reference electrode and counter electrode, respectively. An electrochemical workstation (CHI 660d, Shanghai CH Instruments Company) was used to make the measurements. The sweep rate was set at 0.1 V s⁻¹ for the Tafel plots and LSV curves. All solutions in our electrochemical experiments were prepared in 0.1 M sodium phosphate buffer (pH = 7.0) as the supporting electrolyte.

Results and discussion

Tafel plots

According to previous reports, self-electrophoresis is always correct for bimetallic rod motors.^14,32,33 In self-electrophoresis (an electrokinetic mechanism), the directional motion of nanomotors results from the migration of charged microparticles in a self-generated electric field. Mallouk and Sen's group observed that an electrical contact between the Au and Pt ends was essential for propulsion of Au–Pt nanomotors. On this basis, they made Tafel plots of the anodic and cathodic H₂O₂ potentials at various metal electrodes to confirm the assumption of self-electrophoresis as the mechanism of various bimetallic nanomotors.³² Wang and co-workers found that the speed was proportional to the mixed potential difference (ΔE) of the corresponding segment in the fuel,^29,34 which suggested that acceleration of nanomotors was attributable to an enhanced electron transfer.

In our experiments, Tafel plots were used to obtain the mixed potential difference of the fuel at electrode materials corresponding to the individual nanomotor segment. Fig. 1 displayed Tafel plots for Au and Fe/Ni electrodes in four different solutions (0.294 M H₂O₂, 0.294 M H₂O₂ and 0.0003 M N₂H₄, 0.294 M H₂O₂ and 0.001 M N₂H₄, 0.294 M H₂O₂ and 0.002 M N₂H₄). To eliminate the influence of bubbles generated on the electrodes, the solutions used for the Tafel plots were diluted ten times. As shown in Fig. 1 and Table 1, the mixed potentials established on both the Au and Fe/Ni electrodes decreased continuously upon raising the concentration of hydrazine in the mixed fuels (0.0003 M to 0.002 M). Meanwhile, a gradual shift in potential difference from 68 to −10 mV was observed. However, such a decrease of the ΔE_Au–Fe/Ni was not positively correlated with the speed data reported by us (Table 1), which was not consistent with the relationship between the potential difference and speed of Au–Pt/CNT (carbon nanotube) and Ag/Au–Pt nanomotors propelled by self-electrophoresis. From this point of view, it seemed that the dominant propulsion mechanism of Au–Fe/Ni nanomotors might not be self-electrophoresis after all, for which further experimental evidence would be necessary to prove or disprove this inference.

Fig. 1 Tafel plots of (1) Au electrode and (2) Fe/Ni electrode in (A) 0.294 M H₂O₂, (B) 0.294 M H₂O₂ and 0.0003 M N₂H₄, (C) 0.294 M H₂O₂ and 0.001 M N₂H₄, (D) 0.294 M H₂O₂ and 0.002 M N₂H₄.

Table 1 The correlation between the mixed potential differences and speeds in different solutions

	EAu/mV	EFe/Ni/mV	ΔE/mV	ν/μm s^−1
0.294 M H2O2	241 ± 4	173 ± 1	68	0
0.294 M H2O2 and 0.0003 M N2H4	162 ± 3	106 ± 2	56	139.4 ± 9.1
0.294 M H2O2 and 0.001 M N2H4	195 ± 15	163 ± 1	27	227.6 ± 13.2
0.294 M H2O2 and 0.002 M N2H4	145 ± 11	155 ± 2	−10	368.6 ± 16.8

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LSV curves

Dramatic speeds of Au–Fe/Ni nanomotors can only be observed in mixed fuel environments (H₂O₂ and N₂H₄). A fundamental understanding of the role of hydrazine was essential for studying the propulsion mechanism of these motors. Owing to the ratio of mixed fuels used (H₂O₂ : N₂H₄ = 2940 : 20–2940 : 3, n/n), it was speculated that a small amount of hydrazine might have acted as an unconsumed co-catalyst for the oxidation of hydrogen peroxide on the Fe/Ni segment during the reduction of hydrogen peroxide and dissolved oxygen on the Au end in our previous work.²⁸ However, the role of hydrazine in mixed fuels was not fully elucidated with valid experimental evidence. In this work, LSV was first used to further discuss the function of hydrazine. LSV curves of four different electrodes (Fe, Ni, Fe/Ni, Au) were scanned in hydrogen peroxide, hydrazine and mixtures of these two in different ratios. All solutions used for scanning LSV curves were diluted ten times to eliminate the influence of bubbles generated on the electrode surfaces.

Fig. 2 showed LSV curves running in 0.001 M N₂H₄, 0.294 M H₂O₂ and 0.001 M N₂H₄ as well as curves superimposed from the curve in 0.294 M H₂O₂ and that in 0.001 M N₂H₄ on four different electrodes. There were negligible electrochemical responses on the four electrodes in 0.001 M N₂H₄ solution (curve 1). Compared with the oxidation peak currents in 0.294 M H₂O₂ solution (curve 2), that on the Fe electrode increased slightly, while those on the Ni and Au electrodes declined somewhat after adding 0.001 M N₂H₄ to 0.294 M H₂O₂. In contrast, the oxidation peak current of H₂O₂ on the Fe/Ni electrode increased (curve 3). As the oxidation peak potentials of both N₂H₄ and H₂O₂ on the Fe/Ni electrode were so close, the comparison between the oxidation peak current in the mixed solution and that superimposed from curve 1 and curve 2 was also investigated. It was obvious that the oxidation peak current of H₂O₂ on the Fe/Ni electrode in the mixed fuel was larger than that of the superimposed curve 4. These results indicated that hydrazine facilitated the oxidation of H₂O₂ on the surface of the Fe/Ni electrode.

Fig. 2 LSV curves of four electrodes (Fe, Ni, Fe/Ni, Au) in (1) 0.001 M N₂H₄; (2) 0.294 M H₂O₂; (3) mixture of 0.001 M N₂H₄ and 0.294 M H₂O₂. And (4) was superimposed from (1) and (2).

As shown in Fig. 3, the LSV curves of four different electrodes (Fe, Ni, Fe/Ni, Au) in five solutions were studied. Increasing the concentration of N₂H₄ from 0 to 0.01 M in mixed fuels (curves 1–5) had almost no effect on the oxidation of hydrogen peroxide on the Ni electrode. Meanwhile the electrochemical response decreased gradually on the Au electrode but increased continuously on the Fe and Fe/Ni electrodes. The increase in oxidation peak current on the Fe/Ni electrode suggested an enhanced oxidation rate of hydrogen peroxide on Au–Fe/Ni segment, which was consistent with an acceleration of Au–Fe/Ni nanomotors with increasing the concentration of hydrazine in mixed fuels. Thus, the results suggested that the oxidation of hydrogen peroxide facilitated by a small amount of hydrazine is probably responsible for the locomotion of Au–Fe/Ni nanomotors.

Fig. 3 LSV curves of four electrodes (Fe, Ni, Fe/Ni, Au) in (1) 0.294 M H₂O₂; (2) 0.294 M H₂O₂ and 0.001 M N₂H₄; (3) 0.294 M H₂O₂ and 0.004 M N₂H₄; (4) 0.294 M H₂O₂ and 0.007 M N₂H₄; (5) 0.294 M H₂O₂ and 0.01 M N₂H₄.

Measurement of dissolved oxygen

Au–Fe/Ni nanomotors are straight, rod-shaped, with the average length of 5.4 ± 1.2 μm and an average diameter of 280 ± 10 nm (ESI Fig. 1†). As shown in Fig. 4, there were three movement modes for Au–Fe/Ni nanomotors: rotation on its axis, flat rotation and spiral motion. No matter which mode the Au–Fe/Ni nanomotors used, obvious bubble thrusts were always observed. After checking the videos corresponding to Fig. 4A–C carefully (ESI Video 1–3†) and slowing their playback speeds (ESI Video 1′–3′†), it was clearly to see the bubbles were coming from one end of an individual nanomotor as the images shown (ESI Fig. 2A–C†) which were captured from the videos respectively. In addition, as the bubbles moved in a direction away from the catalytic site, (showed by blue arrows in the Fig. 4), it could be pointed out that the Au–Fe/Ni nanomotors were away from the site of bubble generation site.

Fig. 4 Bubble locus of Au–Fe/Ni nanowire motors: (A) rotation on its axis; (B) flat rotation; (C) spiral motion.

To identify the compostion of the bubbles, an experiment was designed to determine the concentration of dissolved oxygen by means of a three-electrode system which was placed in a closed cell. As Table 2 showed, the dissolved oxygen concentration was 0.5 mg L⁻¹ in a 0.294 M H₂O₂ solution which had oxygen removed from it by bubbling nitrogen gas through it. After adding 0.001 M N₂H₄ to 0.294 M H₂O₂, the dissolved oxygen concentration in the mixed fuel increased to 2.5 mg L⁻¹. In addition, the concentration of dissolved oxygen was also studied using chronoamperometry (CA) at 0.8 V. After the system was electrified for 500 s, the concentration of dissolved oxygen was 1.3 mg L⁻¹ in H₂O₂ solution and increased to 5.1 mg L⁻¹ with CA at +0.8 V in the mixed fuel. From the data in the table, it could be inferred that the addition of a little hydrazine facilitated the decomposition of hydrogen peroxide whether the solution was electrified or not. And the constant current polarization facilitated more hydrogen peroxide being oxidized to oxygen, which accorded with the results from the LSV curves. It was therefore reasonable to suggest that this proved the thrust was composed of a continuous stream of oxygen bubbles.

Table 2 Dissolved oxygen in hydrogen peroxide solution and the mixed fuel

	Blank/(mg L^−1)	CA (+0.8 V, 500 s)/(mg L^−1)
0.294 M H2O2	0.5	1.3
0.294 M H2O2 and 0.001 M N2H4	2.5	5.1

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Conclusions

Hydrazine plays an essential role in mixed fuels of facilitating the oxidation of hydrogen peroxide to oxygen bubbles on the Fe/Ni electrode. Combined with a directional movement away from the catalyst site and a visible train of bubbles, it seems reasonable to propose that bubble propulsion is the dominant mechanism of Au–Fe/Ni nanomotors. The clarification of the role that hydrazine plays in mixed fuels would pave the way for seeking alternative fuels with better biological compatibility to expand the applications of Au–Fe/Ni nanomotors in the near future. However, the consumption of hydrazine before and after reaction in the mixed fuels was not determined and the minimal fuel requirement of the Au–Fe/Ni nanomotors in the presence of hydrazine was not examined. These questions will be considered for future research.

Acknowledgements

This work was supported by the National Science Foundation of China (No. 21275030), the Provincial Science Foundation of Fujian (No. 2015J01322), the Fundamental Research Project for Social Commonweal Scientific Institutes in Fujian Province (No. 2014R1035-12) and the Scientific Research Project for Younger Teachers in Fujian Province (No. JB14044).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra08263c

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