Dr. Zhengtao Zhu

Department of Chemistry and
Applied Biological Sciences
South Dakota School of Mines and Technology

Laboratory of Advanced Materials
for Flexible Electronics & Sensors

Research

Our research seeks to understand and exploit interesting properties of multi-functional hybrid materials of conjugated polymer and inorganic nanostructure. Our group focuses on developing novel synthesis and fabrication methods, understanding the structure-property relations of the hybrid materials at the nanometer scale, and exploring the applications of these materials in flexible electronics and biomedical sensors. The research scope covers the multidisciplinary fields of chemistry, materials, applied physics, and nanotechnology. Here are a few examples of our work. 
  1. Synthesis and fabrication of conducting polymer nanomaterials
  2. Dye-sensitized solar cells based on electrospun TiO2 nanofibers and organic dyes
  3. Flexible and wearable devices based on nanomaterials
1. Synthesis and fabrication of conducting polymer nanomaterials
     Conducting polymers are new class of polymers with interesting optical and electronic (semiconducting or conducting) properties. Synthesis and preparation of conducting polymers, as well as their properties, have been studied extensively. Conducting polymers are also considered to have a great technology potential in flexible and wearable electronics. In recent years, we have developed novel methods for preparation of hybrid materials of conducting polymer and nanostructure. Through these research projects, we seek to develop and understand the methods to prepare the nanomaterials of conducting polymers as low temperature, which are compatible with the flexible substrates, and further explore their applications in flexible electronic devices and sensors.
  •  “One-pot” synthesis of stable Pd/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (Pd/PEDOT:PSS) colloid. In the synthesis, the Pd/PEDOT:PSS aqueous dispersion was formed by simultaneous oxidationreduction reaction between Pd(NO3)2 and ethylenedioxythiophene (EDOT) at room temperature. The electric response of the Pd/PEDOT:PSS film to ammonia is investigated.
                    Synthetic Metals, 160, 1115 (2010).

  • Low-temperature seeding and hydrothermal growth of ZnO nanorod on poly(3,4-ethylene dioxythiophene):poly(styrene sulfonic acid). This work develops a low-temperature seeding process for hydrothermal growth of ZnO nanorods on flexible polymeric materials. The process involves decomposition of zinc (II) amine complex below 100 C to form ZnO seeds, followed by hydrothermal growth using zinc nitrate hexahydrate and hexamethylenetetramine under 90 C. The method enables the growth of ZnO nanorods on the polymeric film (e.g. PET) or the p-type conducting polymer poly(3,4ethylene dioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) film to form ZnO/PEDOT:PSS heterojunction. The electric response of the heterojunction to the UV light is explored. 
                                  Materials Letters, 183, 197 (2016).

  • Electrospun conducting polymer nanofibers. Electrospinning technique is a simple, efficient, and economic method for producing nonwovens composed of continuous and randomly overlaid fibers; these fibers have diameters typically in the range from tens of nanometers to several micrometers (commonly known as electrospun nanofibers). We have prepared the nanofibers of conducting polymers (such as Polythiophene and MEH-PPV) by electrospinning and studied the effect of nanoscale confinement and alignment on the optical properties of conducting polymers. 
                            Journal of Materials Chemistry, 21, 444 (2010).


2. Dye-sensitized solar cells based on electrospun TiO2 nanofibers and organic dyes
Materials and technologies for next-generation renewable energy are the key areas to the environmental sustainability of human development. Compared with the conventional silicon-based solar cells, dye-sensitized solar cells (DSSCs) have the advantages of simple fabrication process, low-cost, low energy input, and environmentally friendly. We have worked on understanding the effects of materials on DSSCs.
  • Charge transport in electrospun TiO2 nanostructure. We have used electrospinning technique to prepare one-dimensional TiO2 nanostructures with different morphologies including nanofibers, nanotubes, and high-surface area TiO2 nanostructures. We have further studies the charge transport of these materials as photoanode in DSSCs by IV, photocurrent transient, and photovoltage transient characterization, and discovered that the one-dimensional structure significantly improves the charge transport of the photoanode.
         Journal of Physical Chemistry C, 117, 1641 (2013)
  • Metal-free organic dye. We have also investigated four new donor-(π-spacer)-acceptor organic dyes using triarylamine or carbazole as the electron-donating group, cyanoacrylic acid as the electronwithdrawing anchoring moiety, and naphtho[2,1-b:3,4-b’]dithiophene as the linker for DSSCs. The performance of DSSCs based on novel metal-free organic dyes as sensitizers suggests that the rigid and planar structure of naphtho[2,1-b:3,4-b’]dithiophene may be a valuable spacer group to design donor-π-acceptor molecular dyes and fine tuning of the chemical structures is needed for high-efficiency DSSCs. Additionally, we have investigated the effects of surface modification on the recombination and transport of DSSC based on donor-(π-spacer)-acceptor organic dyes.
                                   ACS Applied Materials & Interfaces, 6, 1926 (2014).

  • Thermal-durable, transferable, and flexible substrate of metal oxide. We have developed a universal dual-spinneret electrospinning method to prepare a thermal-durable, transferable, and freestanding mat of metal oxides for flexible DSSCs and photosensors. By simultaneously electrospinning of polycrystalline metal oxide (e.g. TiO2) with amorphous SiO2 using dual-spinneret setup followed by pyrolysis, the hybrid mats of metal oxide nanofibers that combine functionality, flexibility and hightemperature durability can be prepared. In such materials, the polycrystalline metal oxide nanofibers are the active components with specific electronic properties, while the amorphous SiO2 nanofibers enhance the mechanical strength/flexibility. The free-standing hybrid mats can be readily integrated into devices. We have demonstrated the flexible photosensors and photoanodes of DSSCs based on the dual-spinneret electrospinning method. 
Flexible photoanodes consisting of anatase-phased TiO2 nanofibers and structurally amorphous SiO2 nanofibers are prepared by the dual-spinneret electrospinning method. The hybrid fibrous mat is then impregnated with binder-free TiO2 nanoparticles and sintered at 480 C to form a flexible composite photoanode for DSSC. The DSSC based on this composite photoanode achieves a power conversion efficiency of 6.74 ± 0.33% on FTO/glass substrate. In the composite photoanode, the TiO2 nanoparticles enhance the dye loading, the TiO2 nanofibers improve the electron transport, and the SiO2 nanofibers provide the mechanical strength/flexibility. The freestanding composite mat of TiO2 nanoparticles and electrospun TiO2 /SiO2 nanofibers, as well as the preparation methods reported herein, not only is ideal for flexible DSSCs, but also can be applied for a broad range of flexible and low-cost energy conversion devices. 


                              ACS Applied Materials & Interfaces, 6, 15925 (2014).


3. Flexible and wearable devices based on nanomaterials
Flexible electronics, i.e., electronic devices and sensors that are adaptable to a variety of substrate materials and surfaces covering large areas, will be a key enabling technology for next-generation aerospace applications, consumer electronics, and medical devices. For example, wearable systems consisted of conformable and lightweight biomedical and strain sensors, power sources, and wireless modules have great application potentials in human motion monitoring, medical, human-machine interface, safety, and soft-robotics. Development in this field is still in its infancy. The major challenge is that the conventional fabrication process of inorganic semiconductors for microelectronics is not compatible with the substrates and active materials for flexible and wearable devices. In recent years, my group has done several exploratory projects in the field of flexible and wearable electronics and sensors. Our research in this area focuses on combination of nanoscale fibers and soft materials (such as elastomers and hydrogels) to design multi-component and multi-functional composite materials as active components for wearable systems. 
  • Stretchable strain sensors. Highly stretchable and sensitive strain sensors are in great demand for human motion monitoring. This work reports a strain sensor based on electrospun carbon nanofibers (CNFs) embedded in polyurethane (PU) matrix. The piezoresistive properties and the strain sensing mechanism of the CNFs/PU sensor were investigated. The results showed that the CNFs/PU sensor had high stretchability of strain up to 300%, high sensitivity of gauge factor as large as 72, and superior stability and reproducibility during the 8000 stretch/release cycles. Furthermore, bending of finger, wrist, or elbow was recorded by the resistance change of the sensor, demonstrating that the strain sensor based on the CNFs/PU could have promising applications in flexible and wearable devices for human motion monitoring.
                               RSC Advances, 6, 79114 (2016).

  • Stretchable conductors. We have demonstrated a scalable and facile preparation of all-organic nonwoven that is mechanically stretchable and electrically conductive. Polyurethane (PU) fibrous nonwoven is prepared via the electrospinning technique; in the following step, the electrospun PU nonwoven is dip-coated with the conducting polymer poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). This simple method enables convenient preparation of PEDOT:PSS@PU nonwovens with initial sheet resistance in the range of 35∼240 Ω/ sq (i.e., the electrical conductivity in the range of 30∼200 S m−1) by varying the number of dip-coating times. The resistance change of the PEDOT:PSS@PU nonwoven under stretch is investigated. The PEDOT:PSS@PU nonwoven is first stretched and then released repeatedly under certain strain (denoted as pre-stretching strain); the resistance of PEDOT:PSS@PU nonwoven becomes constant after the irreversible change for the first 10 stretch-release cycles. Thereafter, the resistance of the nonwoven does not vary appreciably under stretch as long as the strain is within the pre-stretching strain. Therefore, the PEDOT:PSS@PU nonwoven can be used as a stretchable conductor within the pre-stretching strain. Circuits using sheet and twisted yarn of the nonwovens as electric conductors are demonstrated.
                            ACS Applied Materials & Interfaces, 9, 30014 (2017).


  • Three-dimensional and ultralight conductive sponges. We have developed a general method to prepare three-dimensional (3D), highly porous, and conductive sponge with tunable conductivity for tactile pressure sensor. The 3D conductive sponge is prepared by assembly of shortened/fragmented electrospun nanofibers of polyacrylonitrile (PAN), polyimide (PI), and PAN-based carbon. The nanofibers of PAN, PI, and carbon are dispersed in water/ethanol with polyvinyl alcohol (PVA) and then freeze dried to form a 3D conductive sponge. Subsequently, the sponge is thermally treated at 230 ◦C; and the dehydrated PVA acts as a binder to uniformly bond electrospun carbon nanofibers (CNFs) on the mechanically resilient 3D scaffold of PAN/PI. Upon varying the amounts of CNFs, the resistance of the 3D nanofibrous sponge is readily tailored from 260 kΩ to 200 Ω. The resistance change of the 3D conductive sponge under the cyclic compressive strain is investigated, and the results are correlated with the unique interconnected and hierarchically structured pores in the sponge. A tactile pressure sensor array composed of 25 devices of conductive sponges is demonstrated.
                                             Journal of Materials Chemistry C, 5, 10288 (2017).




Contact Information:

Dr. Zhengtao Zhu
Department of Chemistry and Applied Biological Sciences
South Dakota School of Mines and Technology
Rapid City, SD 57701

Email: Zhengtao.Zhu@sdsmt.edu
Phone:
605 394 2447