ZnO/Cellulose nanocomposite: Recent Developments and Future Prospects

Biocomposites are the amalgamation of two or more biodegradable, eco-friendly materials with different chemical and physical properties. Cellulose-based biocomposites are highly in demand because of their good antibacterial activities as well as their high mechanical strength, thermal stability and flexibility properties. In this review paper, ZnO/cellulose biocomposites have been chosen to discuss in detail. ZnO is an n-type, nontoxic, low cost semiconductor with a band gap of 3.4 eV; hence, more preferable over other semiconductor materials for preparing semiconductor-cellulose biocomposites. In recent time, ZnO-cellulose biocomposites have attracted great research attention for optical (UV sensing, shielding, etc.), bioelectronics (antibacterial, drug uses, etc.), mechanical (toughness, etc.), electric (di-electric, etc.), and thermal applications. In the majority of literature, ZnO/cellulose biocomposites are either used in UV-related applications, like in cosmetic products to act as a protector or in antimicrobial activities like food packaging, fungus treatment, etc. In this paper, we have extensively included the literature reported till date on the preparation and characterization of ZnO-cellulose biocomposites. The preferable applications of the as-prepared biocomposites are also discussed in detail to find out the future scope of these biocomposites for practical applications.


Introduction
It is observed that in the last few decades, some unfriendly materials which are dangerous to our environment, habitat, and also for animals have been used in large amounts to fulfil our needs.But in recent years, environment-friendly materials have come into picture to get relief from industrial waste.Biocomposites are the best example of biodegradable, eco-friendly materials.Here two biosafe materials having different chemical and physical properties are mixed proportionately to prepare a composite material.This is done to increase the material's mechanical strength, thermal stability and durability (Hyun-U et al, 2014).Composites are used in huge amounts in different sectors, like industries, electronics, building, etc. (Minghao et al, 2019).There are a number of biocomposite materials available in the market, like rice husk, natural fibre, paper sludge, cellulose, etc.Out of the above eco-friendly materials, cellulose is considered as the most preferable for a biocomposite.Cellulose is a composite material made of hundreds or sometimes even thousands of molecules of hydrogen, oxygen, and carbon.It is a polymer material that is totally eco-friendly, biodegradable, available in a sufficient amount for use, and a bio-suitable polymer (Thennakoon M et al, 2020).It comes out to be a well-designed material named EAPap (electro-active paper).It is a thin, cellulose-coated film with metal electrodes on both sides of the film (M.S. Aida et al, 2021).Intelligent properties make it an attractive material in the family of electroactive polymers.It is used in sensors, actuators, harvesters, etc. due to its tremendous properties: low actuation power, flexibility, less power consumption, biodegradability, cheapness, light weight etc. (Sher Bahadar Khan et al, 2014).Semiconductor materials like SnO (Tin Oxide), GO (Graphene Oxide), CNT (Carbon Nanotube), TiO 2 (Titanium Oxide), ZnO (Zinc Oxide), etc., are mixed with cellulose to make multifunctional materials with enhanced chemical, antimicrobial, and mechanical properties to be used in bio, gas sensing applications., to develop flexible FETs (field effect transistors) (Diantoro M et al, 2017).Out of the above-mentioned semiconductor composites, we have chosen ZnO because ZnO is a composite of zinc (Zn) and oxygen (O 2 ).It has a band gap (E g ) of 3.4 eV and free exciton energy is 60 mV at room temperature.It has both piezoelectric and photoelectric properties (Lakshmeesha R et al, 2015).Additionally, it provides stability to the material.ZnO is considered as the most important nanomaterial in electronic as well as photonic devices.Also it is useful in the cosmetic industry, bio-electronics, and opticalelectronics due to its environment-friendly and biocompatible properties (Seongcheol M et al, 2017).ZnO forms various shapes like nanorods, nanowires, nanoribbons, nanoneedles, nanotubes and nanoplates.Based on different conditions, it has different properties, leading to a vast range of applications.Nanorods are considered the best among the various shapes above due to their most promising structure, which makes them superior to others (Amzad F et al, 2020).Many fabrication methods are available to prepare ZnO nanorods, like pulse laser, chemical vapour, metal organic chemical vapour, vapour-solid liquid epitaxial deposition etc. ZnO's ability to be easily grown on various substrates like glass, silicon, sapphire, metal, polymers and flexible plastic makes it suitable for hybrid composites (Feiya Fu et al, 2016).In this review paper, ZnO/cellulose biocomposites have been elaborated in detail in three sections.Different techniques (Sol-gel, solution casting, hydrothermal processes, etc.) to synthesise ZnO/cellulose biocomposites has been discussed in the first section.The next section deals with the different applications of biocomposites, such as its use in antibacterial activities, its optical and electrical properties etc. and finally, a short conclusion to the paper is described in the last section.

Preparation Methods of ZnO/Cellulose Composites
There are numerous methods available for ZnO/cellulose composition, out of which the solution based methods are widely chosen in the available literature in the area because precise control over a large number of synthesis parameters including chemical composition, synthesis environment (reaction temperature, humidity, pressure, air flow) and synthesis duration is possible with solution-grown methods ().Yinan li et al. have described the preparation of ZnO using zinc acetate and methanol as main sources, and methanol and KOH are used as precursors to make alkali solution.NMMO (N-methyl morpholine N-oxide) was used as a solvent to make cellulose.NMMO is known for its environmentally friendly nature.Finally, ZnO and NMMO are mixed together to form a ZnO/cellulose nanocomposite (Yinan li et al, 2022).Gil G. et al ZnO nanocomposites were prepared using the wet chemical method.In this, zinc acetate is mixed with sodium hydroxide to form ZnO nanoparticles, and to form ZnO cellulose nanocomposites, it is mixed with wood cellulose, which is supplied by Portugal (Gil G et al, 2008).Seongcheol et al. have been described for the evolution of zinc acetate, dehydration in ethanol, and using the solvent method with dried cotton pulp as cellulose.Composites are then combined to form ZnO nanocomposites using the solvent casting method (Seongcheol et al, 2017).Thennakoon M. et al ZnO NPs were prepared using the precipitation method.In this solution, zinc nitrate hexahydrate is dissolved in sodium hydroxide with the help of deionized water.Carboxymethyl cellulose (CMC) and its composites are made using the solvent casting method (Thennakoon M.et al 2020).Aida M S et al ZnO was prepared using the sol-gel technique.In this, they prepare it by dissolving zinc acetate, which dehydrates into methanol.Here, indium chloride is also used to achieve the desired ration with the help of the autoclave method (Aida M S et al, 2021).Tianyi li et al has assisted the sol-gel method and reported the formation of ZnO quantum dots by using zinc acetate, lithium hydroxide, and ethanol as a solvent.Carboxymethylcellulose (CMC) was used as cellulose.And to make a composite of both solutions, mix them together using an ultrasonic technique.The composite was formed using an environmentally friendly process (Tianyi li et al, 2018).Lakshmeesha Rao et al ZnO obtained from Sigma Aldrich is mixed with Hydroxypropylmethylcellulose (HPMC) using the solvent casting method (Lakshmeesha Rao et al, 2015).Minghao jia et al uses the solo-chemical method to synthesise ZnO.They use zinc nitrate hexahydrate as the main source and ammonium hydroxide as a precursor.They produce cellulose by dissolving MCC (microcrystalline cellulose) with sulfuric acid.And at the end, ZnO/cellulose nanocomposites are formed based on an in situ solution casting process (Minghao jia et al, 2019).Rasul Rakhshaei et al also used the same procedure to make ZnO, and further, they used AgNO 3 to make Ag/ZnO nanohybrids (Rasul Rakhshaei et al, 2016).Yiwei wang et al also uses the same method to produce ZnO; in addition, they use carboxymethyl cellulose (CMC) as a solvent (Yiwei wang et al, 2019).Thennakoon M. et al adopted the same manufacturing method using precipitation process.In this process, ZnO is formed by mixing zinc acetate dihydrate with ethanol as the main component, then dissolving it in CNC, and finally mixing it with sodium hydroxide.Here, CNC acts as a solving agent (Thennakoon M. et al, 2020).Susan Azizi et al have also done with same process.Hyun-U Ko et al ZnO nanorods are formed using the hydrothermal process.ZnO was made using zinc sulphate (ZnSO 4 ) and ammonium chloride (NH 4 Cl) (Hyun-U Ko et al, 2014).Opeyemi et al These ZnO nanoparticles were purchased from Sigma Aldrich in South Africa, whereas CNC was supplied by CSIR Durban in South Africa.And there were composites made with the help of sodium hydroxide using an ultrasonic process (Opeyemi et al, 2020).Sher Bahadar Khan et al have reported the formation of ZnO with the help of an aqueous solution of zinc nitrate and ammonium hydroxide and carbon black as a solvent.Cellulose acetate was used as a source of cellulose, and by mixing cellulose acetate and acetone solution with ZnO/ethanol solution, a composite was formed (Sher Bahadar Khan et  ZnO-CNC is prepared by using the hydrothermal process, where zinc chloride is mixed with Sodium Hydroxide and ethanol is used to stabilise the ZnO nanoparticles before they are mixed with carboxymethyl cellulose (CMC) (Somia Yassin et al, 2018).Bairui T. et al has reported the formation of ZnO using zinc nitrate hexahydrate and hexamethyltetramine (HTMA) and then mixing it with plant cellulose thin film (PCF) to form ZnO-cellulose composites using an autoclave process (Bairui T et al, 2022).In this section, solution-based preparation methods to synthesise ZnO/cellulose composite is covered extensively.Now in next section, applications of these composites are discussed in detail.Before the starting of next section, a table (table 1.1) is included to display the preparation methods and application of the composite materials reported in major literature.Nanocomposites can be used as broadband optical limiters during visible and IR region studies using laser pulses at 532 nm; they also come out to be good optical limiters, and their threshold is observed to be low; they also have a limited threshold for the development of opto-electronic devices (litty I et al, 2010).ZnO nanohybrid has the potential to stop the UV radiation as it absorbs the UV rays, preventing the negative impact on human health, human skin, and the environment.A sheet like cellulose is used to make ZnO nanohybrid, which increases its thermal stability and crystalline ability.It also shows antimicrobial (i.e., used in wound dressing) and UV shielding properties (Somia Y et al, 2018).Nanocomposites can be used as broadband optical limiters during visible and IR region studies using laser pulses at 532 nm; they also come out to be good optical limiters, and their threshold is observed to be low.They also have a limited threshold for the development of opto-electronic devices (litty I et al, 2010).Hybrid nanocellulose absorbs more UV rays than pure PVA cellulose and has thermal, mechanical, UV-vis, and morphological properties (Susan A. et al, 2014).ZnO nanocomposites possess good optical properties in the visible light region and good microbial and anti-microbiofilm activities (Magdalena et al, 2016).ZnO-CNF cellulose shows high responsiveness and sensitivity in the UV region since it is an environmentally friendly material (Hiroaki K. et al, 2022).When making several kinds of papers and fabrics using the screen printing technique, luminous CS-ZnO was also employed, and its printed layer is thin, which results its use in security issues (Saeed El et al, 2013).Optical applications such as colon metric sensors, anti-counterfeiting, and so on have been observed; additionally, it is predicted that CNC will be used more in the future due to its superior properties to other materials (Xiaoyao W et al, 2021).UV regions and under seawater biodegradable mechanism, and its UPF was increased up to 12.30, while tensile strength was increased up to 15% (Gaojun Lu et al, 2021).ZnO and cellulose can be recycled and used again with no loss of activity thanks to a solution with increased tensile strength and photocatalytic activity (Feiya Fu et al, 2016).

Bio-electronic Properties (Antibacterial and Antimicrobial)
ZnO/cellulose hybrid composites are used in food packaging as bio-active, photoluminescence antimicrobial materials.It is used to make drugs using biodegradable materials in a non-linear profile under electric field response only.Due to the formation of only one layer on the surface, it has good antibacterial activity against gamma-positive (S. aureus) bacteria but fails against gamma-negative (E.coli) bacteria (M. S.

Conclusion
On mixing ZnO nanoparticles with cellulose, a nanocomposite is formed that is used in a variety of applications like antibacterial, mechanical, optical, bioelectronics, etc.Here, ZnO is chosen because it is an easily available, wide band gap biosafe semiconductor whereas cellulose can be easily obtained naturally (like from wood, cotton, etc.) or made artificially.To design this biocomposite, different techniques are available on the market,both solid as well as solution route.However, solution-based methods are preferred in the major literature to get more control over synthesis parameters.In addition, ZnO and cellulose nanocomposites are combined together using different blending methods like mechanical, solution, or electrospinning.The final properties of the composite are the mixture of both component properties, which result in increased properties.Some of its unique properties are antimicrobial, toughness, stability, conductivity, and flexibility, which makes it useful in modern applications like the medical field, water purification, UV shielding capacity, etc., which increase its market demand.
al, 2014).Gaojun Lu et al have reported the formation of ZnO with the help of two components, i.e., ZnCl 2 as the main component and NaOH as the second component used as a precursor (Gaojun Lu et al, 2021).Diantoro M et al ZnO is formed by mixing zinc acetate dihydrate as a zinc Source, NaOH as an Oxygen Source, and Methanol as a Stabiliser, whereas cellulose Acetate is used as the Second Material to make ZnO-cellulose Nanocomposites (Diantoro M et al 2017).Yiwei W et al has reported the formation of ZnO nanoparticles by mixing zinc Nitrate Hexahydrate with sodium hydroxide and making them useful for biomedical applications (e.g., wound dressing).ZnO nanoparticles are mixed with Hydroxypropyl cellulose (Yiwei W et al, 2018).Somia Yassin et al.
Table to display ZnO/Cellulose Composition and their Applications 3. Applications of ZnO/Cellulose NanocompositesZnO nanocomposites are a highlighted and most in-demand topic in today's world.Both ZnO and cellulose have tremendous properties like optical, bio-electronic, antimicrobial, electrical, mechanical, thermal, etc.3.1.Optical Properties (UV Applications)ZnO/cellulose hybrid nanocomposites have a peak photoluminescence of 373 nm, and on increasing the intensity of UV light, the photocurrent increases linearly; cellulose releases 20% more current than ZnO.When used in wearable sensors, it easily detects the UV component of sunlight with good stability and sensitivity (Seongcheol M et al, 2017).Nanocomposites of cellulose are fabricated and utilised as a transparent and flexible organic light emitting diode (OLED) with visible light transmittance as high as 80%.It has other applications like optical transparency and thermal stability and comes out to be an environmental friendly candidate (S.Ummartyotin et al, 2011).ZnO-quantum dot cellulose composites containing 15% weight possess good optical and UV shielding properties.It can absorb wavelengths up to 375 nm (Tianyi et al, 2018).ZnO/cellulose has a loftier UV protection capacity and transparency in the visible light area, and even shows high reliability, sensitivity, etc. (Yinan li et al, 2021).As the crystalline size of nanocomposites increases in the amorphous region, it forms large crystalline structures in UV irradiation, which decreases the chances of crosslinking (lakshmeesha R et al, 2015).
(Ashavani et al, 2011) activity has been observed by other authors also (Susan A et al, 2013); (Hiroaki K. et al, 2022);(Yiwei W et al, 2022).Nanocomposites of ZnO and cellulose can be used in many fields, like food packaging, photoluminescence papers, bioactive and microbial activities, etc., which can't be achieved using pure ZnO.Its composites are used in electrical, biomedical, healthcare, antibacterial, and fluorescent properties, etc. (Si-Wei Zhao et al, 2017).Same activity has been observed by other authors also (Thennakoon M et al, 2020).CnC-ZnO nanocomposites are an efficient and environmentally friendly absorbent that is used in a variety of applications, including the rectification of water from dye pollution (Opiyemi A et al, 2022).The swelling ratio of ZnO cellulose bio-nanocomposites is undeniably higher than that of pure cellulose, despite the fact that it is used to release gastro-intestinal problems; based on these properties, the ZnO/cellulose nanocomposite is used as oral drug delivery for controlling drug addiction (Zahila Z et al, 2016).ZnO nanocomposites show high permeability, which results in their perfect use for water treatment (Sher BahadarKhan et al, 2014).Hydrogel nanocomposite acts as an anti-cancer drug, which comes out to be a better cancer therapy than hydrogel alone (Iman G et al, 2020).It can also be used in breathable wound dressings, surgical gloves, medical bags, mammary prostheses, artificial skin, penile prostheses, urethral catheters, and adhesion barriers.ashavingaveryhighpercentageyieldand aspect ratio compared to other conventional methods, high tensile strength, and a high strain to failure in nanocomposite cellulose (Bibin M. et al, 2011).ZnO UV-vis exhibits absorbance and photoluminescence; its potential use as a fluorescent enhancer in biosensors and as a gas sensor (JinxiaMa et al, 2016).Tensile strength and photocatalytic activity were observed in ZnO nanocomposites, also in multifunctional activities like biomedical, catalysts, packaging, and electronic fields (Feiya Fu et al, 2016).It represents the green packaging material of PBAT nanocomposites found under soil burial, bacterial cellulose polymer, and ZnO composite combined to form a successful wound dressing material, resulting in the best use of ZnO nanoparticles for drug delivery, skin treatment, and surgery purposes, and healing (especially in burn cases) (Sepideh H. et al, 2021).respiratoryandhumiditysensorwasmade for monitoring using GO/ZnO/cellulose, and their result defines that they exhibit good linearity, tremendous sensitivity, satisfying humidity, a short response time, and a better recovery time, whereas this sensor is used to differentiate between normal, high, and deep breathing.In short, it is useful for health monitoring and healthcare activities (Bairui T et al, 2022).ZnO/cellulose composites possess good colour stability, as well as increased mechanical strength like tensile, roughness, toughness, etc.Furthermore, it has far superior antibacterial activity when compared to pure ZnO (Minghao et al, 2019).ZnO/Cellulose's mechanical and thermal properties make it useful in energy harvesting(Ashavani et al, 2011).ZnO/cellulose composition can be formed into different shapes according to requirements because of its flexibility (Amzad F et al, 2020).Its photoluminescence property is used to make papers that are used to make dark glow pictures, photos, etc.(Gil G et al, 2008).AZC film has good mechanical and thermal properties; their main objective is to design flexible nanocomposite film that is used in various applications such as gas sensors, pressure sensors, and wearable smart fabric sensors (Feiyu Fa et al, 2018).Optical properties and thermal stability of nanocomposite are observed in this, though pure cellulose shows more thermal stability than cellulose nanocomposite, so work is going on in this area (Mozhgan B et al, 2012).