Application of nanocellulose ink in printed electronic devices

Nanocellulose (NCC) is a type of natural polymer that can be obtained from nature, such as from wood, cotton, flax, grain and rice straw and sugarcane bagasse.NCC is a fiber or crystal with a diameter of no more than 100nm and a length of microns. Because of its biodegradable and lightweight properties, its strength can even be higher than that of ordinary metal products.NCC can be divided into three types: cellulose nanofibrils (CNF), cellulose nanocrystals (CNC), and bacterial cellulose (BC).As a nanoscale unit of cellulose, NCC has better performance than traditional cellulose, with strong flexibility, good thermal stability, good optical properties and mechanical properties, and can be used in flexographic printing and large-scale printing. As an excellent insulating material for the next generation of 'green' electronic devices, it has broad application prospects in the field of printed electronics.
Preparation of NCC
At present, there are three main methods for preparing NCC, namely physical and mechanical methods, chemical methods and biological enzymatic hydrolysis methods.
1. Physical and mechanical methods
Physical and mechanical methods refer to the use of external forces generated by high-pressure homogenization, grinding, freeze crushing, high-intensity ultrasonic crushing and other methods to destroy the tight structure inside cellulose and obtain nanoscale cellulose fibers.NCC produced by mechanical methods can maintain its original chemical structure without producing any charged groups. NCC prepared by this method has a large surface area and contains a large number of hydrogen bonds. It has a complete crystal structure and is suitable for Industrial production.In addition, the physical method does not require the addition of too many chemical reagents and causes less environmental pollution.
2. Chemical method
Chemical methods are often used to prepare and modify NCC. Currently, there are two main methods: acid hydrolysis and TEMPO oxidation.
(1) Acid hydrolysis method
The acid hydrolysis method refers to the hydrolysis of cellulose in acid to remove the disordered parts of the cellulose macromolecules. The crystalline part remains intact due to its acid resistance, thereby converting cellulose into NCC crystals.The hydrolysis method was first proposed by Nickerson and Habrle in 1947. They used a mixed solution of hydrochloric acid and sulfuric acid to hydrolyze cellulose to prepare cellulose nanocrystals.The properties of NCC obtained by hydrolysis with different acids are also different, and different acid hydrolysis can be used to prepare NCC that meets the needs.However, at present, acid hydrolysis to prepare NCC mainly uses inorganic strong acids, such as hydrochloric acid and sulfuric acid, which are highly corrosive and are not conducive to the long-term use and maintenance of equipment, and are also destructive to the environment.Therefore, attention should be paid to the handling of such inorganic strong acid catalysts during acid hydrolysis.In recent years, the acid hydrolysis method will also be combined with microwave and ultrasonic methods to enhance the penetration of acid into cellulose and improve the preparation efficiency of NCC.
(2) TEMPO (2,2,6,6-tetramethylpiperidine oxide) oxidation method
The process of preparing NCC by oxidation method is shown in Figure 1.As a mild oxidant, TEMPO can selectively oxidize the primary hydroxyl group (-OH) at the C6 position of cellulose into a carboxyl group (-COOH), causing repulsion between cellulose fibrils and thus reducing the hydrogen bonding between fibrils. Force, further mechanical force can be applied to achieve separation from each other.During the TEMPO oxidation of cellulose, the interaction between cellulose microfibers weakens. This feature can be used to achieve efficient extraction of NCC.The oxidation conditions of the TEMPO oxidation method are mild and cause less environmental pollution. The NCC produced by this method has uniform size and good dispersion, allowing the cellulose nanofibers to be further functionalized.However, the cost of this method is high, and the performance of the NCC produced is unstable.NCC prepared by TEMPO oxidation method is widely used in electronic products, packaging, papermaking and other fields. At present, domestic research on NCC prepared by TEMPO is not yet mature.

3. Enzymatic hydrolysis
Enzymatic hydrolysis is a more environmentally friendly method. The cellulase system consists of endo-β-glucosidase, exo-β-glucosidase and β-glucosidase. NCC can be prepared through the interaction between the three.The above three have different functions in different positions, and all three are indispensable.However, the enzymatic hydrolysis reaction process has very high requirements on solvents, pH values, temperature conditions, etc., and the preparation time is long, so the activity of the enzyme must be ensured.In recent years, biological enzyme treatment methods are often used in conjunction with other chemical methods and physical and mechanical methods to prepare NCC. The combination of physical, chemical and biological methods can not only save energy, but also improve the preparation efficiency of NCC.Enzymatic hydrolysis is environmentally friendly and the required materials are renewable, playing an important role in the contemporary pursuit of sustainable and green development.
Based on the various advantages of NCC, we can add NCC materials to the original printing ink or mix other materials with NCC materials to make NCC ink. Electronic devices with better performance can be obtained through different printing technologies.
Printed electronics
Printed electronic devices are electronic devices made by printing different functional inks on different printing materials according to needs through 'additive manufacturing and layer-by-layer printing', and then drying and sintering them.Because circuits and components are printed directly on the substrate, the structure is complete and integrated, with good connectivity.At the same time, electronic devices based on flexible materials are thin and bendable, adaptable to various shape requirements, have good portability, and have good application prospects.
NCC, as a colloidal active material that has developed rapidly in recent years, has been widely used in conductive materials and power generation because of its characteristics such as conductivity, high chemical activity and optical activity, light weight, low price, high strength and environmental friendliness. Devices and key materials such as machines, sensors, solar cells and light-emitting diodes are playing an increasingly important role.As technologies such as screen printing and 3D printing mature, their applications in the preparation of microelectronic devices are becoming more and more widespread.
1.Generator
In 2012, Professor Wang Zhonglin and his team from Georgia Institute of Technology first reported triboelectric nanogenerators (TENG).Triboelectric nanogenerators can convert the mechanical energy generated by devices during friction into electrical energy and collect it.Direct writing and printing (DIW) is a type of 3D printing that can print the required materials layer by layer based on the files passed into the device, which can improve the accuracy of manufacturing devices and thereby improve the performance of nanogenerators.The ink required for DIW can be prepared by adding other reagents to eucalyptus bleached sulfate pulp through the oxidation-high-pressure homogenization method under certain conditions. This ink can be used to print the friction layer of the triboelectric nanogenerator.Using DIW technology to prepare CNF cathodes (as shown in Figure 2), the resulting CNF cathode not only has a patterned structure on the surface, but also has many lamellar porous structures inside, which greatly improves the friction effectiveness of the resulting device.Introducing CNF composite films prepared from conductive and ferroelectric nanomaterials as triboelectrically charged materials for triboelectric nanogenerators can significantly improve the performance of the generator.

2. Light emitting diode (LED)
LEDs are made of compounds containing gallium, arsenic, phosphorus, nitrogen, etc.NCC can be applied to LEDs, which not only effectively expands the application range of NCC, but also allows the preparation of nanopaper with optical control functions (as shown in Figure 3).Because the size of nanopaper is similar to the wavelength of light, it has optical properties that ordinary paper does not have. NCC paper can be used as a masking board to meet the requirements for regulating the optical properties of LEDs.

3. Flexible wearable sensing devices
Wearable sensing devices can be directly attached to the human skin or integrated into the user's clothing, and can continuously and closely monitor changes in the user's physiological information, which is of great significance in disease diagnosis and treatment.Wearable sensing devices are mainly composed of three parts: a flexible substrate, a sensing element and a signal output element, among which the sensing element is particularly important.NCC has a high specific surface area, which can effectively increase the contact area between the sensor and target molecules, thereby improving the sensitivity of sensor detection and playing a positive role in wearable sensing devices.In addition, cellulose materials can also improve the folding resistance and tensile properties of the sensor, and expand the strain range of the sensor. For example, a multi-modal sweat sensor using cellulose materials (as shown in Figure 4) can be directly fixed on human skin. , achieving simultaneous detection of data such as pH value, lactic acid, glucose and chloride.

4. Solar cells
The haze value of CNF nanopaper is related to distance.The reason why CNF nanopaper looks completely transparent or blurry is because the size of CNF is smaller than the wavelength of visible light and the scattering factor is small, making it almost impossible for light to scatter.In this case, the transparency of CNF is very high. By adjusting the size of the fiber or the internal holes, the scattering factor can be adjusted, thereby affecting the transparency of CNF.Therefore, CNF nanopaper can be used as a cover with high total light transmittance/high haze value (as shown in Figure 5) to improve the utilization rate of sunlight by solar cells, while high transparency and low haze paper is more suitable for applications in on the monitor.

5. Supercapacitors (SCs)
As energy shortages and ecological problems become increasingly serious, people are beginning to turn their attention to green renewable energy.As a new type of energy storage device, supercapacitors have attracted much attention due to their high power density, long cycle life, fast charge and discharge speed, wide operating temperature range and simple preparation principle.More and more cellulose-based materials are being used as alternatives to materials such as aluminum, iron, gallium and indium in traditional energy materials.
Electrodes are one of the key materials for preparing supercapacitors and have a great impact on their performance. Research on electrode materials with excellent performance has always attracted industry attention.Thanks to its large specific surface area and excellent pore structure, NCC can better adsorb electrolyte ions, and combining it with conductive materials can prepare electrode materials with excellent electrochemical properties.It can be used as a flexible skeleton to be combined with conductive substances with excellent electrochemical properties, or it can be directly carbonized as an electrode material for supercapacitors, thereby obtaining electrode materials for flexible energy storage devices with high specific capacitance and high electrochemical stability.Introducing NCC into electrodes can reduce manufacturing costs and optimize performance.
Flexible supercapacitors have higher power density, longer cycle life, faster charge and discharge rates than traditional batteries, are easy to assemble and are lightweight. They not only meet the requirements of wearable energy storage electronic devices, but also conform to the development trend of new environmentally friendly materials. , has great application potential.
As a new type of material that can be obtained directly from nature, NCC has the advantages of high specific surface area, light weight, high strength, green and degradable, etc., which meets the development requirements of green and environmental protection. It is widely used in flexible electronic devices, diodes, supercapacitors, etc. Composite functional materials and other fields have broad application prospects.Relevant studies have shown that modified NCC is a green material that can adsorb heavy metal ions in water and can be used for filtering adsorption.Because of its excellent mechanical properties, unique optical properties and good biocompatibility, NCC can also be used as an anti-counterfeiting material.
However, NCC materials still have many shortcomings. For example, they need to be compounded with other conductive polymers, carbon materials, metal compounds and other conductive substances before they can be further used in flexible energy storage devices; NCC is prone to deformation or even breakage when exposed to external forces. , its mechanical strength needs to be further improved; at the same time, NCC has excellent hydrophilic properties due to the large number of hydroxyl groups exposed on its surface, which will affect its mechanical strength to a certain extent and seriously restrict its application scope and application effect.Therefore, when researching and utilizing NCC materials, it is necessary to strengthen the interconnection between NCC-related industries and fully consider whether the materials used will reduce the weather resistance of energy devices and how to solve these problems.