**3. Fabrication methods**

Artificial micro-/nanomotors could offer a significant advance in the field of miniaturized devices. However, a major challenge in artificial micro-/nanomotor field is the synthesis of such tiny devices with high quality and reliability. Relying on the intended applications, different synthesis techniques must be taken into account, since each specifically shaped micro-/nanomotor demands a specialized synthesis technique as well as defines the desired propulsion mechanism. The rapid developing of nanotechnology has resulted in various techniques and strategies for the synthesis of micro- and nanoscale motors. The purpose of this section is to present versatile schemes to the synthesis of micro-/nanomotors. The synthesis strategies and the factors that ought to be considered in the design of micro-/ nanomotors are including the shapes, compositions, and distributions of materials, and functionalization. By highlighting the progresses that have been achieved in the synthesis of artificial micro-/nanomotors over the last decades, we intend to present the challenges and opportunities facing synthesis and put forward perspectives for the upgrowth of new methods.

### **3.1 Electrochemical deposition**

Electrochemical deposition is a process that applies external electric current to grow materials and enables the growth of arbitrary three-dimensional (3D) shapes with distinct materials varying from metals to polymers, resulting in the widespread applications of this growth strategy in nanotechnology. The process can be

*Catalytic Micro/Nanomotors: Propulsion Mechanisms, Fabrication, Control, and Applications DOI: http://dx.doi.org/10.5772/intechopen.90456*

conducted without the requirements of expensive instruments and harsh working environments. Therefore, micro-/nanostructures with diverse dimensions can be grown using this method, especially the micro-/nanowire, micro-/nanorod, and tubular micro-/nanoengines.

Template-assisted electrochemical deposition, as shown in **Figure 9**, utilizes the pores of a membrane template to grow the required wires and tubes comprising of different materials [18]. Each pore of the template functions as a reactor in which the desired structure is synthesized. Membrane templates commonly used for the synthesis of micro-/nanomotors are track-etched polycarbonate (PC) membranes and porous alumina (AAO) membranes. Relying on the properties of the material and the chemistry of the pore wall, the micro-/nanomotors can be either hollow or solid. Membrane template-assisted electrodeposition provides a relatively low-cost and powerful approach for synthesizing micro-/nanowires, micro-/nanorods, and tubular micro-/nanoengines.

For membrane template-assisted electrodeposition, a layer of Au or silver (Ag) is firstly coated on one side of the membrane by physical vapor deposition (PVD) to play a role of the working electrode. Afterwards, the membrane is assembled in a Teflon plating cell with flat aluminum (Al) foil located against the metal layer to work as a conductive contact for subsequent electrodeposition. Commonly, a sacrificial layer of Ag or copper (Cu) is firstly grown, followed by sequential growth of different required metals. The Ag or Au backing and the sacrificial layer are etched away by chemical etchant or are removed by mechanical polishing. By removing the alumina (Al2O3) membrane in sodium hydroxide (NaOH) solution, the nanowires or nanotubes can be released and obtained after successive rinsing and centrifugation.

The Wang's and Pumera's research groups combined electrodeposition widely applied in the growth of nanowire-based micro-/nanomotors with the bubblepropelled tubular micro-/nanojets, as shown in **Figure 10** [19–24]. Two geometries (either cylindrical or conical) can be obtained, entirely relying on the type and geometry of porous template (either PC or AAO). In addition to such metals, the incorporation of polyethylenedioxythiophene (PEDOT), polyaniline (PANI), and polypyrrole (PPy) polymers with Pt generates catalytic microjets. On the other hand, the wall can be incorporated with molecularly imprinted polymers (MIPs) so that alternative recognition cavities can be implemented for the selective separation of biomolecules.

#### **Figure 9.**

resulted from the potential collisions between bubbles, which diminished the distance traveled by the bubbles within the tube and limits the detaching stage. They concluded that the dynamics of micro-/nanojets is influenced by their shapes, the

*Examples of bubble propelled micromotors. (A) Schematic illustration of a tubular micro-/nanomotor's movement by the bubble propulsion mechanism. (B) Janus Pt-SiO2 spheres moving in H2O2. Copyright 2014,*

Artificial micro-/nanomotors could offer a significant advance in the field of miniaturized devices. However, a major challenge in artificial micro-/nanomotor field is the synthesis of such tiny devices with high quality and reliability. Relying on the intended applications, different synthesis techniques must be taken into account, since each specifically shaped micro-/nanomotor demands a specialized synthesis technique as well as defines the desired propulsion mechanism. The rapid developing of nanotechnology has resulted in various techniques and strategies for the synthesis of micro- and nanoscale motors. The purpose of this section is to present versatile schemes to the synthesis of micro-/nanomotors. The synthesis strategies and the factors that ought to be considered in the design of micro-/ nanomotors are including the shapes, compositions, and distributions of materials, and functionalization. By highlighting the progresses that have been achieved in the synthesis of artificial micro-/nanomotors over the last decades, we intend to present the challenges and opportunities facing synthesis and put forward perspectives for

Electrochemical deposition is a process that applies external electric current to grow materials and enables the growth of arbitrary three-dimensional (3D) shapes with distinct materials varying from metals to polymers, resulting in the widespread

applications of this growth strategy in nanotechnology. The process can be

fuel composition, and the viscosity of the medium.

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

*RSC Publications. Copyright 2009, AIP Publications.*

**3. Fabrication methods**

**Figure 8.**

the upgrowth of new methods.

**3.1 Electrochemical deposition**

**174**

*Membrane template-assisted electrochemical deposition of micro-/nanomotors: (a) coating of Au or Ag backing on the membrane template, (b) electrochemical deposition of the sacrificial layer, (c) sequential electrochemical deposition of desired elements, and (d) removal of the backing and sacrificial layer and dissolution of the membrane. Copyright 2015, ACS Publications.*

#### **Figure 10.**

*Examples of micro-/nanomotors grown by template-assisted electrochemical deposition. (A) Electrodeposited Ag-Au/Pt and (B) Au/Pt-CNT nanomotors in H2O2. (C, D) Polycarbonate membrane-assisted growth and SEM images of conical PANI/Pt microtubes, respectively. (E) Growth procedures of flexible metallic nanowires with polyelectrolyte hinges after membrane template electrodeposition. (F, G) Anodized AAO membrane-assisted growth and SEM (scanning electron microscopy) images of segmented microtubes, respectively. (H) SEM image of a hinged nanowire. (I) SEM image of a Au/Agflex/Ni nanomotor with flexible central Ag segment. Copyright 2008, Wiley Online Library. Copyright 2008, ACS Publications. Copyright 2011, ACS Publications. Copyright 2007, Nature Publishing Group. Copyright 2013, RSC Publications. Copyright 2010, ACS Publications.*
