Magnetic Propulsion of Recyclable Catalytic Nanocleaners for

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Magnetic Propulsion of Recyclable Catalytic Nanocleaners for Pollutant Degradation José García-Torres,†,‡,§ Albert Serrà,†,‡ Pietro Tierno,‡,§ Xavier Alcobé,⊥ and Elisa Vallés*,†,‡ †

Grup d’Electrodeposició de Capes Primes i Nanoestructures (GE-CPN), Departament de Ciència de Materials i Química Física, Institute of Nanoscience and Nanotechnology (IN2UB), and §Departament de Física de la Matèria Condensada, Universitat de Barcelona, E-08028 Barcelona, Catalonia, Spain ⊥ Unitat de Difracció de Raigs X, Centres Científics i Tecnològics de la Universitat de Barcelona (CCiTUB), E-08028 Barcelona, Catalonia, Spain ‡

S Supporting Information *

ABSTRACT: Electrochemically fabricated magnetic mesoporous CoNi@Pt nanorods are excellent nanomotors with controlled magnetic propulsion and excellent catalytic properties. The core− shell structure allows a double functionality: (i) controlled motion of the nanorods by applying rotating magnetic fields at different frequencies and field strengths and (ii) effective catalytic activity of the platinum shell for reactions involving sodium borohydride. The structure and magnetic properties of the CoNi core are not modified by the presence of the Pt shell. Nanorods were propelled via a tumbling-like dynamic by a rotating magnetic field. While in absence of NaBH4, nanorods move at constant speed showing a linear path; in the presence of NaBH4, they showed an intermittent trajectory. These catalytic nanorods can be used as nanocleaners with controlled directionality for pollutants degradation in the presence of borohydride. Their magnetic character allows control of the velocity and the direction throughout the contaminated solution by degrading the different pollutants in their path. The magnetic character of nanorods also allows their easy recycling. KEYWORDS: mesoporous magnetic nanorods, nanocleaners, magnetically propelled catalysis, pollutant degradation, magnetic actuation, hydrogen production, electrodeposition

1. INTRODUCTION Research in nanomotors and nanomachines has attracted increasing interest during the last years due to the applicability of these kinds of structures to different fields such as biomedicine, energy, catalysis, and nanoengineering.1−4 Nanomotors can be externally actuated to perform the desired function. Different actuation schemes can be used to induce nanomotor’s propulsion: electrical, ultrasound, chemical, and magnetic fields.5−10 Understanding the dynamics of these nanostructures under their specific actuation force is essential for their correct guidance and applicability. The movement of these nanodevices will be conditioned by their size and aspect ratio, physical and chemical properties, medium in which they will be dispersed, and the propulsion mechanism. Among the different external forces, magnetic actuation is one of the most preferred ways because it allows a remote control of nanomotor’s movement and does not alter the dispersing medium. Moreover, no additional apparatus having physical contact with the actuator is required. Thus, magnetic nanorods (NRs) have been proposed as nanomotors because they can be displaced and directed in their movement by the application of magnetic fields of different magnitudes and directions.11−14 Moreover, the incorporation of new functionalities by designing different materials that compose their © 2017 American Chemical Society

structure allows expansion of their applicability as nanocarriers for drug delivery,2,15 nanomotors for cell separation,16,17 or catalysts for decontamination or energy generation.18−20 In previous studies performed in our laboratory, a novel methodology was developed to grow magnetic mesoporous NRs. These NRs have been tested for different applications depending on their nature: catalysts for methanol oxidation in fuel cells or nanocarriers for drug delivery inside HeLa cells.2,19,21−23 The objective of the present work is to study the capability of mesoporous NRs as magnetically propelled catalyst with a controlled pollutant degradation path. The control of the speed and direction of the NRs allows a localized degradation reaction to their way throughout the contaminated solution. To attain this objective, CoNi@Pt mesoporous NRs were selected. First, these NRs can be magnetically actuated due to the ferromagnetic character of the CoNi core. Second, the Pt shell catalyzes pollutants destruction in the presence of borohydride. Third, as Pt shell also favors H2 production by decomposition of the borohydride, the movement of the NRs Received: May 26, 2017 Accepted: June 29, 2017 Published: June 29, 2017 23859

DOI: 10.1021/acsami.7b07480 ACS Appl. Mater. Interfaces 2017, 9, 23859−23868

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ACS Applied Materials & Interfaces

Figure 1. Schematic representation of the proposed work scheme. All solutions were prepared using deionized water (Millipore QSystem) with a resistivity of 18.2 MΩ cm. Polycarbonate templates were coated with a gold layer (100 nm thick) on one side to act as working electrodes prior to NRs electrodeposition. The electrochemical deposition was carried out using a microcomputer-controlled potentiostat/galvanostat Autolab with PGSTAT30 equipment, GPES software, and a three-electrode electrolytic cell at room temperature, in which the work electrode is a 100 nm Au-PC template, the reference electrode is an Ag/AgCl/KCl (3 M) electrode, and the counter-electrode is a platinum wire. After the NRs were synthesized, the Au layer was removed using a saturated solution of I2/I−. Then polycarbonate membranes were dissolved with chloroform (5 times) and washed with chloroform (10 times), ethanol (5 times), 0.1 M NaOH (3 times), and deionized water (5 times). Finally, mesoporous CoNi@Pt NRs were prepared by immersing CoNi NRs in 20 mM Na2PtCl6 solution, where a nanometric platinum shell was obtained by galvanic displacement. 2.2. Materials Characterization. A high-resolution (HR) TEM Jeol JEM 2100 equipped with a LaB6 source operated at 200 kV was used for the morphological study of all synthesized mesoporous NRs (images were recorded with Digital Micrograph v.1.82.80 software). Before HR-TEM observation, NRs were diluted in ethanol, and then a droplet of the suspension was poured in holey carbon covered copper TEM grids (300 Mesh Cu, Agar Scientific). Chemical composition was explored by Energy-dispersive X-ray spectroscopy (EDX) analysis. The electrochemically active Pt surface area (ECSA) of CoNi@Pt mesoporous NRs was obtained from the hydrogen adsorption/ desorption currents extracted from cyclic voltammograms recorded in H2SO4 (0.5 M) solution between appropriate potential limits. The samples were prepared by NRs dilution in ethanol, and then 5 μL of NRs dispersion was dropped on the surface of polished glassy carbon electrode (working electrode). X-ray powder diffraction (XRPD) measurements were acquired with a PANalytical X’Pert Pro MPD diffractometer in the Bragg− Brentano reflection θ/θ geometry, Cu Kα radiation (λ = 1.5418 Å), at

could be modified. Finally, the mesoporous nature of the NRs allows a more effective pollutant destruction because of the high area/volume ratio. Characterization of the NRs allowed detection of their core− shell structure, essential to perform the double functionality, because they were designed for magnetic actuation and catalytic degradation. The magnetic actuation of the NRs was performed to study the influence of a time dependent magnetic field not only on the velocity and direction of the NRs movement, but also on the mechanism of motion. We have observed that the presence of sodium borohydride modifies the mechanism of motion at certain magnetic fields due to the generated H2 bubbles onto the Pt surface. Finally and for comparison purposes, CoNi NRs were also studied.

2. MATERIALS AND METHODS 2.1. Electrochemical Fabrication. The mesoporous NRs were electrosynthesized according to a previously published procedure.2,19,23 Briefly, an ionic liquid-in-water (IL/W) microemulsion was used as a pore director (soft-template), and commercially available nanoporous polycarbonate membrane (20 μm thick, 100 nm pore diameter; Millipore IsoporeTM membrane filters) was used as a NR shape director (hard-template). IL/W microemulsions were prepared by mixing stirring the adequate proportion of each component for 5 min (300 rpm) and argon bubbling, which led to a transparent, isotropic, and thermodynamically stable electrochemical medium: • 83.8 wt % of W − aqueous solution: 0.2 M Co (II) chloride + 0.9 Ni(II) chloride + 30 g dm−3 boric acid + 4.5 mM saccharin at a pH adjusted to 4.5. • 15.1 wt % of S − surfactant, p-octyl poly(ethylene glycol) phenyl ether a.k.a. Triton X-100 [Acros Organics, 98%]. • wt % of IL − ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate a.k.a. bmimPF6 [Solvionic, 99%]. 23860


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Figure 2. TEM images of the surface of (a) CoNi and (b) CoNi@Pt mesoporous NRs with the corresponding elemental mapping using an EDSTEM. Scale bar: 20 nm. Pore size distributions of (c) CoNi and (d) CoNi@Pt NRs. 45 kV and 40 mA, selected by means of a flat graphite crystal secondary monochromator, and a 1D Silicon strip X’Celeretor detector (active length 2.122°). Samples were prepared by drop casting and evaporation of the solvent over a monocrystalline Si zero background sample holder. Sample surfaces of about 20−30 mm2 and thickness of about 0.1 mm were obtained. XRPD θ/2θ scans were measured in automatic divergence slit mode, from 20−120° 2θ (CoNi NRs sample) or to 150° 2θ (CoNi@Pt NRs sample), with a step size of 0.0172°, a measuring time of 200 s per step, and by repeating the measurement six times to acquire sufficient statistics. Magnetic properties of the prepared structures were characterized using a SQUID magnetometer at 300 K in helium atmosphere. 2.3. Magnetic Actuation Experiments. The external magnetic field was provided by using three custom-made coils oriented along three perpendicular directions. Each coil was made by ∼1100 turns with 4 mm diameter wire and had 5 and 2 cm of outer and inner diameter, respectively. The rotating magnetic field was achieved by

connecting two of the three coils to a waveform generator (TTi TGA1244) commanded by a current amplifier (IMG STA-800). Different frequencies (1−400 Hz) and magnetic field strengths (360− 4800 A m−1) were applied to study their influence on the net NR movement. The NRs were imaged with a 100× oil immersion objective mounted on a light microscope (Nikon). Videos for image analysis were taken at 75 fps by using a charge-coupled device color camera (Basler A311F). 2.4. Magnetically Assisted Catalysis Experiments. The hydrogenation of 4-nitropnehol was chosen as a model reaction to test the magnetically assisted catalysis of mesoporous NRs. The reactions were conducted by mixing 300 μL of 4-nitrophenol (0.7 mM solution) with 2 mL of freshly prepared NaBH4 (25 mM) and 20 μL of NRs dispersion (1 mg mL−1). The reaction was also subjected to an external rotating magnetic field in the x−z plane (frequency 220 Hz, amplitude 4800 A/m) at room temperature. After 8 min, the optical absorption spectrum of 4-nitrophenol was recorded for all conditions 23861


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ACS Applied Materials & Interfaces tested using a UV-1800 Schimadzu UV−vis spectrophotometer in a quartz cuvette with an optical length of 1 cm. After the viability and the effectiveness of the NRs were analyzed for magnetically assisted catalysis, we evaluated their capability for directional pollutants’ degradation. Thus, we designed a simple and a complex 3D-printed circuit (Figure 1) where we added three contaminants (4-nitrophenol, methylene blue, and rhodamine B). First, three nonconnected channels were designed to study the degradation of each pollutant separately and in a single direction. Each channel contained a mixture of 100 μL of x mM of the contaminant (x = 0.1 mM of 4-nitrophenol, 0.06 mM of methylene blue, or 0.05 mM of rhodamine B), 25 mM of NaBH4, and 1 μL of catalyst dispersion (1 mg mL−1). An x−z rotating magnetic field (220 Hz, 4800 A/m) was applied to propel the NRs from one side to the other. After that, the three pollutants were mixed in a stair-shape channel to evaluate degradation in a complex system and forcing NRs to move in two directions. The mixture contained 200 μL of 0.1 mM of 4-nitrophenol, 0.06 mM of methylene blue, and 0.05 mM of rhodamine B, 25 mM NaBH4, and 2 μL of catalyst dispersion (1 mg mL−1). The external magnetic field was rotating in the (x,z) plane to move the NRs along the x-direction or in the (y,z) plane to propel them along the ydirection.

nickel were simultaneously deposited at the selected conditions. Similar percentages of the two metals were desired to have significant magnetization of saturation of the NRs, but with enhanced stability due to the presence of nickel in the alloy. Modified CoNi mesoporous NRs were also obtained by immersing the as-deposited CoNi NRs in a Na2PtCl6 solution. Galvanic displacement led to a core−shell CoNi@Pt NRs (Figure 2) in which the mesoporous morphology remained. The compositional analysis of the samples showed the platinum presence (20 at. % Pt, 34 at. % Co, 46 at. % Ni) and the formation of a shell on the NRs surface (Figure 2b). A platinum shell was formed to both make the NRs more catalytic and confer chemical stability to the CoNi core against oxidation. In fact, the CoNi NRs were easily dissolved in an aggressive H2SO4 solution (0.5 M), whereas the CoNi@Pt remained totally unaltered in the same solution after a long immersion period (1200 s). The voltammetric determination (Figure 3) of the electrochemically active surface area (ECSA) of the NRs showed a

3. RESULTS AND DISCUSSION As the goal of the work is to show the utility of NRs for directional catalytic degradation of pollutants by means of magnetic actuation, we have followed work scheme in Figure 1: (i) Preparation of two types of NRs (CoNi and CoNi@Pt) by electrodeposition. (ii) Confirmation of their mesoporousity and evaluation of the effective area, to be used as catalysts. (iii) Identification of the platinum shell to enhance the NRs catalytic properties. (iv) Measurement of NRs magnetic properties to evaluate their soft or hard character. (v) Evaluation of the capability of the NRs to induce borohydride decomposition with hydrogen production. (vi) Study of NRs velocity and mechanism of motion under variable rotating magnetic fields, in the presence or in absence of borohydride and, therefore, of hydrogen bubbles formation. (vii) Test of NRs as effective catalysts for pollutant destruction in the presence of borohydride and magnetic actuation by UV−vis adsorption. (viii) Evaluation of the directional catalytic degradation of pollutants using 3D-printed circuits. 3.1. Electrochemical Fabrication of CoNi and CoNi@Pt NRs. CoNi NRs were grown inside gold-coated polycarbonate membranes, in which the membrane channels define the shape and diameter of the NRs and the deposition charge allows controlling the rods length. Potentiostatic electrodeposition at −1100 mV was used to prepare the NRs. The potential was selected from a previous voltammetric study of the electrolyte.2 A significant deposition rate (16 nm s−1) was obtained at the selected potential. 3.2. TEM Observation and Electrochemically Active Surface Area (ECSA) Determination. TEM observation allowed us to determine a good definition and homogeneity of the cylindrical rods, with an average length of 6.5 μm; NRs showed clear mesoporous morphology (Figure 2) (pore’s diameter of 5 ± 2 nm), which demonstrated that the drops of ionic liquid distributed into the aqueous electrolytic solution permit definition of the nanopores of the deposit. NRs’ composition (55 at. % Co, 45 at. % Ni) revealed that cobalt and

Figure 3. Cyclic voltammetry (15th cycle) in H2SO4 0.5 M solutions at 25 °C with a scan rate of 20 mV s−1 for (i) CoNi@Pt and (ii) CoNi mesoporous NRs.

highly effective area (236 m2 g−1) due to their mesoporous nature. Therefore, the very high area/volume ratio of the NRs makes them very suitable as potential catalysts. Moreover, after the core@shell formation, the characterized high ECSA values also verified that the interconnected mesoporous structure is maintained because it corresponds to 12 times as high as the corresponding value for compact core@shell NRs. The platinum atoms are covering all surfaces of all pores outside and inside the NRs and, therefore, forming a thin layer that could enhance catalytic efficacy, as the Pt lattice is more compressed than that in the common surface Pt shell from the pure Pt nanostructures. 3.3. Structural Characterization of CoNi and CoNi@Pt NRs. The five most intense peaks observed in the XRPD pattern of the CoNi NRs sample have been indexed to a fcc phase with cell parameter around 2.53 Å. The sample contains only a major phase: a cubic fcc CoNi alloy. The XRPD pattern of the CoNi@Pt NRs sample is compatible with a mixture of two fcc phases: a Pt rich alloy with cell parameter around 3.90 Å and the CoNi alloy with cell parameter around 2.53 Å. Additionally, the two patterns indicate the presence of a minor phase in both samples, also fcc, with cell parameter 4.076 Å, that corresponds to Au of the rests of the seed layer used during the NRs preparation. Figure 4 depicts the indexed CoNi and 23862


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Figure 4. (a) Observed XRPD patterns of CoNi and CoNi@Pt NRs samples. Indexation of the main phases: CoNi fcc phase in both patterns and Pt fcc in CoNi@Pt NRs. All the expected hkl reflections are observed. The main peaks of the minor Au fcc phase, 111, 200, and 311 at 38.2, 64.6, and 77.6°2θ (220 at 44.4°2θ completely overlapped) are also observed. (b) CoNi Rietveld plot. Red point curve, observed profile; black solid curve, calculated profile; blue solid curve, difference profile; first line of tick’s position of hkl reflections of Co0.55Ni0.45; second line of ticks’ position of hkl reflections of Au. (c) CoNi@Pt Rietveld plot. Red point curve, observed profile; black solid curve, calculated profile; blue solid curve, difference profile; first line of tick’s position of hkl reflections of Pt0.925Co0.075; second line of ticks’ position of hkl reflections of Co0.55Ni0.45; third line of ticks’ position of hkl reflections of Au.

3.4. Magnetic Characterization. Magnetization versus magnetic field curves were recorded to analyze the magnetic properties of the as-fabricated CoNi and CoNi@Pt NRs. Figure 5 shows that normalized curves were very similar in both cases,

CoNi@Pt patterns. All the expected reflections are observed and indexed. To confirm the observed structural types and to evaluate the microstructure of the observed phases, Rietveld full profile analyses have been performed by means of the FullProf software.24,25 In the case of the CoNi NRs pattern, fcc Co0.55Ni0.45 (according the measured chemical composition) and the Au fcc structures have been considered. Clear hkldependent shifts are observed in the Co0.55Ni0.45 phase, due very probably to defects or residual stresses, and simulated using a model implemented in the FullProf software. Additionally, anisotropic size broadening has been observed and considered (spherical harmonics model). The semiquantitative phase analysis resulted in a 0.9% weight of Au. The obtained average crystallite size for the Co0.55Ni0.45 is 4.7 nm. Figure 4b shows the final Rietveld plot showing a very reasonable agreement.26 Nevertheless, some possible wide peaks around 42 and 47.5° 2θ are not explained. Probably another minor nanocrystalline phase is present. A distorted hcp Co-like phase is a possibility. Attempts to introduce this kind of phase in the Rietveld procedure are no determinant. For the Rietveld refinement of the CoNi@Pt pattern, a Pt0.925Co0.075 fcc structure (it has been considered that Co entering the structure of Pt produces the observed small cell parameter variation; the amount of Co, 7.5%, was obtained by applying the Vegard’s law between pure Pt and Co structures), a Co0.4Ni0.6 fcc (estimated composition), and the Au fcc structures were considered. The same models for hkl-shifts and anisotropic size broadening than in the case of the CoNi pattern were applied. For the Pt0.925Co0.075, only the anisotropic size broadening model was applied. The semiquantitative analysis resulted in weights of 1.3% of Au, 84.7% of Pt0.925Co0.075, and 14.0% of Co0.4Ni0.6. The average crystallite sizes are 2.3 nm for Pt0.925Co 0.075 and 5.0 nm for Co0.4Ni0.6. Figure 4c shows the final Rietveld plot showing a very reasonable agreement. The quantitative amount of Pt (total relative amount of atomic Pt of 61%) does not agree with the measured chemical composition. Some kind of differential absorption or microabsorption effects of the shell Pt material could, at least partially, explain the overestimated amount of Pt obtained from the XRPD analysis.

Figure 5. Room temperature (25 °C) hysteresis loop of the (i) CoNi@Pt and (ii) CoNi mesoporous NRs.

indicating that the magnetic properties were unaltered by the presence of the Pt shell. Typical curves of ferromagnetic material were recorded with moderate coercivity values (450 Oe). The curves’ shape justifies the relatively fast response of the two types of rods when they were manipulated with a magnet to sediment the rods in the different steps of the cleaning procedure. The magnetic behavior of the NRs allows us to expect a feasible magnetic actuation by means of not very high magnetic fields. 3.5. Catalysis of Borohydride Decomposition; Hydrogen Generation. Different authors demonstrate that several metallic structures favor borohydride hydrolysis (eq 1), leading to hydrogen production:18,27−30 NaBH4 + 2H 2O → NaBO2 + 4H 2

(1)

We tested if the fabricated NRs induce hydrogen generation from borohydride in alkaline media (pH = 13), in which the self-hydrolysis of borohydride is minimized. Twenty microliters 23863


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ACS Applied Materials & Interfaces of catalyst dispersion (1 mg mL−1) was added to 2 mL of freshly prepared NaBH4 (25 mM), observed after a few second the formation of hydrogen. Significant hydrogen production was observed, even at the tested pH value, on the CoNi@Pt NRs, due to the presence of highly active superficial atoms of platinum in the shell and the extraordinary effective area due to the accessible interconnected mesoporous structure. From these results, we can expect NaBH4 decomposition on CoNi@ Pt NRs at the conditions tested for pollutants degradation test. However, CoNi NRs show low catalytic activity toward borohydride decomposition. 3.6. Magnetic Actuation. We next study the propulsion of the synthesized NRs when subjected to an external rotating magnetic field circularly polarized in a plane perpendicular to the substrate. As shown in Figure 6, to transport the NR along

amplitude, this angular velocity depends on the driving frequency f. For low values of f, the NR rotation is synchronous with the magnetic field such that Ω = f and the phase-lag angle θ is constant. Beyond a critical frequency, the NR can no longer follow the fast dynamics of the field, and its motion becomes asynchronous. In this regime, the NR displays a characteristic “back-and-forth” motion during each field cycle, and Ω decreases as f increases.33 We now describe how this rotational dynamic is rectified into a net translational motion due to the close proximity of a solid wall. If the NR were in the bulk of the fluid rather than floating close to a plane, its rotational motion will be reciprocal,34 as it will not generate any asymmetry in the friction and thus net displacement, during each field cycle. In contrast, the presence of the surface is able to provide such asymmetry, since when the tip of the rod passes close to the substrate it experiences a higher friction than when it displaces far from it. Thus, the periodic rotation of the NR is rectified into a net translational motion via tumbling-like dynamics, as schematically illustrated in Figure 6 (Supporting Information, Video S1). The transport of rotating micro-object close to a surface has been the subject of different works.35−38 In particular, for the case of a magnetic NR, it was shown that its trajectory close to a surface is a prolate cycloid, with a small backward motion during each cycle that depends on the elevation of the NR from the surface.39 We find that this propulsion mechanism represents an efficient way to transport NRs in water, as it allows to reach maximum velocity around 165 μm s−1. When compared to other driven microsystems,40,41 our NRs show a similar dynamics in the dispersing medium that result from a balance between magnetic and viscous torques. However, since the actuating field is not linearly oscillating but it has a defined chirality, the NRs did not show dispersion in the average transport velocities. Furthermore, the possibility to easily steer the NR in any place of the experimental platform by simply varying the direction or the sense of rotation of the applied field, allows fastening and optimizing the NR transport both key features for pollutant degradation purposes. In Figure 7, we investigate the transport properties of two types of NRs, the CoNi and the CoNi@Pt, by measuring their average speed v as a function of the driving frequency and at

Figure 6. Schematic showing the tumbling-like motion of a NR in water and above a solid substrate. The NR has a permanent moment m, and it is subjected to an external magnetic field H rotating in the (y,z) plane. The dashed line in the schematic demotes the cycloid-like trajectory of the center of mass of the NR during propulsion.

the y-direction, the field is rotated in the (x,z) plane as H⃗ = H0(0, cos(2πf t), − sin(2πf t)), being H0 the amplitude and f the driving frequency. The anisotropic shape of the ferromagnetic NR favors the spin alignment along the NR long axis, which gives rise to a net permanent moment m.31,32 Thus, the applied field tries to align the NR and exerts a net magnetic torque τ⃗m. Here τ⃗m = μwm⃗ × H⃗ = μwmH sin θŷ being μw the permeability of the medium and θ the phase angle between the NR long axis and the applied field. This torque is balanced by the viscous drag τ⃗d ≈ Ωŷ arising from the motion in water and, as a result, the NR rotates at an angular velocity Ω around its short axis. At parity of field

Figure 7. Average speed v versus driving frequency f for different field amplitudes H0 for (a) CoNi and (b) CoNi@Pt NRs. 23864


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Figure 8. (a) Rescaled position versus time of two CoNi@Pt NRs in a solution with NaBH4 (black line) and without NaBH4 (blue line) and for a rotating magnetic field with amplitude H0 = 3300 A m−1 and frequency f = 12 Hz. (b) Log−log plots of the mean squared displacement calculated from the particle trajectories of two CoNi@Pt NRs in a solution with NaBH4 (black line) and without NaBH4 (blue line). The squares (disks) refer to the MDS along the direction of motion (y-axis) or in the perpendicular one (x-axis). The parameter α denotes the expected exponent of the MSD for diffusive (α = 1) and ballistic (α = 2) regimes.

different amplitude of the applied field. In both cases, we find a similar behavior, with v first increasing linearly with f in a synchronous regime until reaching a maximum frequency where the rotational motion becomes asynchronous and v decreases. The amplitude of the applied field controls the magnetic torque τ⃗m and thus the critical frequency that bridges the synchronous from the asynchronous dynamics. Therefore, increasing H0 raises the transition frequency, which allows the transport of the NRs at much higher speed. Further, we find that the presence of the Pt layer in the CoNi@Pt composite does not affect significantly its magnetic properties and the corresponding dynamics. We then analyze the influence of the sodium borohydride (NaBH4) present in the dispersing medium on the NRs propulsion. The catalytic decomposition of NaBH4 due the Pt layer generates hydrogen bubbles (H2) in water, and this could eventually induce a net motion powered by the chemical reaction, as previously observed in other Pt coated nanorods42 or microspheres.43 While this situation could work for a Janus -like morphology, where the Pt covers only one side of the NR, here our NRs are symmetrically surrounded by the Pt, and thus no drift motion was observed. However, we find that the decomposition of NaBH4 affects the dynamics of the propelling NRs when transported by the rotating field. In Figure 8, we compared the rescaled trajectories of two CoNi@Pt NRs with and without NaBH4 when driven by a rotating field such that the NRs are in the synchronous regime. While in absence of sodium borohydride, the NR moves at constant speed showing a linear path (blue curve, Figure 8a), the chemical reaction significantly affects the motion, and the NR shows an intermittent trajectory composed by stationary periods with no net transport, black curve in Figure 8a (Supporting Information, Video S2). This type of motion was more evident for the CoNi@Pt NRs than for the CoNi due to the higher NaBH 4 catalytic decomposition by the formers. The intermittent dynamics arises because of the H2 bubbles generated in the NR pores. The produced H2 bubbles are symmetrically expelled from the NRs inner, increasing temporarily the distance of the NR from the surface, and this effect reduces the rectification of the rotational motion due to the hydrodynamic interaction with the plane. However, once the H2 bubbles completely saturate the NR pores, the reaction reduces and gravity forces to NR to come closer to the surface, recovering the tumbling dynamics. We also characterize the transport of the NR by calculating the mean squared

displacement (MSD) from the experimental trajectories, as shown in Figure 8b. The MSD can be used to characterize diffusion and transport of a colloidal particle,44 and it is given by MSD = ⟨x(τ) − x(t + τ)⟩ ≈ τα, α being the exponent of the power law. Diffusive dynamics is characterized by α = 1, while superdiffusive has α > 1 and ballistic α = 2, where the latter case corresponds to a moving particle with a constant speed. For the calculated MSD, we find that along the transport direction (yaxis), the MSDs in both cases (with and without NaBH4) tend to a ballistic transport mode that results from the direct particle movement. However, fluctuations in the perpendicular directions increases for the case of NaBH4, as the propelling nanorod approaches a higher value of the MSD that corresponds to an enhanced diffusive dynamics, see Figure 8b. We also observe that this type of intermittent dynamics become less important for large magnetic fields, as shown in Figure 9. In this graph, we show measurements of the propulsion velocity versus frequency for the CoNi@Pt NR without and with NaBH4 but at two different field strengths. In both cases, the curves of data display the same trends, with synchronous and asynchronous regimes separated by a critical frequency. However, for low field amplitudes we find that the critical frequency is lower for the NaBH4-containing solution, and thus more evident the influence of the NaBH4 on the velocity (Figure 9a). In contrast, at higher magnetic fields (Figure 9b) the intermittent dynamics reduces, and both curves with and without NaBH4 become similar (Supporting Information, Video S3). 3.7. Catalysis of the Reaction Test. Borohydride degradation was studied by recording the decrease of the UV−vis absorption peak of 4-nitrophenol in solution. The degradation was carried out under the presence of either CoNi or CoNi@Pt and with or without the rotating magnetic field. However, it is well-known that mesoporous materials have strong physical adsorption for organic pollutants such as 4nitrophenol, which could drastically affect the degradation process. Therefore, blank experiments (i.e., 4-nitrophenol with NRs, without NaBH4) were carried out; these confirmed that no reaction after 24 and 48 h takes place in the absence of borohydride. As seen in Figure 10, magnetically propelled NRs show a higher reduction in adsorption peak than under silent conditions. On the other hand, the nature of the NR is also decisive. Thus, CoNi@Pt leads to a stronger decrease in the adsorption peak than CoNi NRs because of the higher catalytic 23865


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favors the recovery and separation of them from the reaction medium, as the manner that they can be reused more than eight times. Moreover, durability of the NRs is really demonstrated as a function of their catalytic activity, because after they have used and manipulated several times, their catalytic activity maintains with the same efficiency. 3.8. Proof of Concept: Directional Decomposition of Pollutants. Finally, the effectiveness of CoNi@Pt mesoporous NRs for directional degradation of three different pollutants, 4nitrophenol (colored yellow), rhodamine B (colored pink), and methylene blue (colored blue), was analyzed. A simple and a more complex circuit were designed and fabricated by 3D impression to test the directional decomposition of pollutants. In the first test, solutions of the three pollutants were separately introduced into the channels of the circuit. After that, 1 μL of the CoNi@Pt suspension was added in the left end of the channel and immediately subjected to a rotating magnetic field (220 Hz, 4800 A m−1) to move the NRs from left to right inside the channels. The pollutants solutions gradually decolourated during the controlled motion of the NRs along the channel, thereby demonstrating their excellent effectiveness in degrading pollutants in their path (Figure 11 and Videos S4 and S5). Significant bubbles formation was detected during the entire experiment. Moreover, the movement of black particles constituted by aggregates of NRs jumping up and down during the experiments was clearly observed. After 10 min, the three solutions became clearly decolorated, although the fading speed was dependent on each pollutant. To definitively demonstrate the control of the movement of NRs, a more complex circuit was used: a stair-shape circuit. The channel of the circuit was filled with a mixture of the three pollutants (0.1 mM 4-nitrophenol, 0.06 mM methylene blue, 0.05 mM rhodamine B, and 25 mM of NaBH4; the mixture became lilac in color (Figure 11 and Video S6)). After the addition of NRs suspension (2 μL) at one end of the circuit, the application of rotating magnetic fields (220 Hz, 4800 A m−1) in either x−z or y−z planes allowed controlling NRs motion in x or y directions, respectively. Consequently, gradual decolouration of the solution was observed with NRs passage, indicating that the degradation of the pollutants mixture with NRs passage was taking place. At the end of the experiment, the entire solution was colorless.

Figure 9. (a, b) Average speed v versus driving frequency f for CoNi@ Pt NRs with NaBH4 (black circles) and without NaBH4 (blue circles) and at two different amplitudes of the rotating field: (a) H0 = 3300 A m−1 and (b) H0 = 4800 A m−1.

4. CONCLUSIONS Electrochemically fabricated CoNi@Pt mesoporous magnetic NRs have been demonstrated as excellent motorized catalysts. The core−shell structure allows a double functionality. Meanwhile, the magnetic CoNi core allows controlling NRs movement with rotating magnetic fields, the platinum shell effectively catalyzes the reaction between borohydride and pollutants to degradate them. The extraordinarily high mesoporosity of the NRs leads to an effective surface area of 236 m2 g−1, which reveals the accessibility of the reagents to the interior of the interconnected pores therefore making pollutant degradation faster. The presence of the Pt shell does not modify the magnetic properties and structure of the CoNi core actuating as nanomotors, whereas the Pt shell, with a distorted crystal structure, confers the catalytic behavior, and therefore, the fabricated mesoporous CoNi@Pt NRs can be proposed as motorized nanocatalysts. The analysis of the NRs motion allowed detecting a slightly different mechanism if NaBH4 was present in solution. While a

Figure 10. UV−visible spectra of 4-nitrophenol catalyzed reduction at time zero (i) and after 8 min (ii−v) under (ii and iii) silent and (iv and v) magnetic actuation of (ii and iv) CoNi and (iii and v) CoNi@Pt mesoporous NRs. Magnetic field strength and frequency are 4800 A m−1 and 220 Hz, respectively.

activity of the Pt shell. Therefore, these results demonstrate that CoNi@Pt mesoporous NRs are adequate and effective catalysts for both borohydride hydrolysis and pollutants reduction with borohydride. While the platinum shells catalyzes the 4nitrophenol degradation, accompanied by hydrogen generation, the CoNi core allows motion by the applied magnetic field favoring pollutant decomposition. Therefore, we propose the CoNi@Pt mesoporous NRs as motorized catalysts with controlled movement. The magnetic character of the NRs 23866


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Figure 11. Schematic representation of 3D-printed circuits and motorized catalysis degradation of 4-nitrophenol (colored yellow), rhodamine B (colored pink), and methylene blue (colored blue) and solutions of the three pollutants (colored lilac). Magnetic field strength and frequency are 4800 A m−1 and 220 Hz, respectively.

ORCID

continuous rolling motion was detected without borohydride, a step mechanism was observed with it. The reason could be related to the “symmetric” ejection of H2 bubbles from the pores, which make the movement under the magnetic field more difficult. However, this step motion was practically irrelevant at high magnetic fields. The direction and speed of NRs can be controlled by means of rotating magnetic fields of different frequencies and amplitudes. This implies that CoNi@ Pt NRs can be used as catalytic nanomotors of several reactions, including pollutant degradation under the presence of borohydride, in which the catalyst induces a localized and effective reaction in their path.

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José García-Torres: 0000-0002-3996-0274 Albert Serrà: 0000-0003-0147-3400 Elisa Vallés: 0000-0001-7176-4102 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the EU ERDF (FEDER) funds and the Spanish Government Grant No. TEC2014-51940-C22-R from Ministerio de Economiá y Competitividad (MINECO). Authors thank the CCiT-UB for the use of their equipment. J.G.-T. and P.T. acknowledge support from Abiomater project (ID: 665440).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07480. Actuation experiments (PDF) Actuation of CoNi@Pt nanorod (AVI) Actuation of a CoNi@Pt nanorod (AVI) Actuation of a CoNi@Pt nanorod (AVI) Comparison of the catalytic performances of CoNi@Pt nanorods (AVI) Comparison of the catalytic performances of CoNi@Pt nanorods (AVI) Catalytic performance of CoNi@Pt nanorods (AVI)

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REFERENCES

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Corresponding Author

*E-mail: [email protected]. 23867


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