Sustainable synthesis of multifunctional nanomaterials from rice wastes: a comprehensive review

More than 60% of India’s population relies on agriculture as their primary source of income, making it the nation’s most important economic sector. Rice husk (often abbreviated as RH) is one of the most typical by-products of agricultural production. Every five tonnes of rice that is harvested results in the production of one tonne of husk. The concept of recycling and reusing waste from agricultural production has received interest from a variety of environmental and industrial perspectives. A wide variety of nanomaterials, including nano-zeolite, nanocarbon, and nano-silica, have been discovered in agro-waste. From rice cultivation to the finished product, there was a by-product consisting of husk that comprised 20% of the overall weight, or RH. The percentage of silica in RH ash ranges from 60 to 40%, with the remaining percentage consisting of various minerals. As a direct consequence of this, several distinct approaches to generating and extracting nanomaterial from rice husk have been developed. Because it contains a significant amount of cellulose and lignin, RH is an excellent and economical source of carbon precursor. The goal of this chapter is to produce carbon-based nanomaterials from RH.


Introduction
Nanotechnology is now a rising breakthrough with huge opportunities in a variety of fields, including electronics, healthcare, and the food sector. Nanofood, nanosensors, nano-packaging, nano-fertilizers, and nano-pesticides are some of the most recent nanotechnological innovations (Sahoo et al. 2021). Nowadays, numerous strategies have been developed for synthesizing various types of nanomaterials, but the Green synthesis method is one of the most effective since it uses naturally produced starting materials to provide a sustainable strategy for nanomaterial synthesis (Huston et al. 2021). Green techniques is a way of making nanomaterials that is clean, safe, cost-effective, and environmentally friendly. Nanomaterials are now made from various microorganisms, plants (Vanlalveni et al. 2021;Pal et al. 2022a, b), leaf extracts, and a variety of waste (Abdelbary and Abdelfattah 2020).
Nanomaterial has not only gained significance in the galaxy of research and development but has also caught up the eyes of industrial, environmental, and biomedical researchers worldwide (Chakroborty et al.2023;Nath et al. 2023a, b, c;Nath et al. 2023a, b, c;Pal et al. 2022a, b;Nath et al. 2023a, b, c). Carbon-based nanomaterials (CNMs) play a critical function among the various nanomaterials. In nanotechnology, carbon-based nanomaterials are particularly important (Nath et al. 2023a, b, c). Carbon nanostructures are divided into three types: three-dimensional graphite, two-dimensional graphene, one-dimensional carbon nanotubes, and zero-dimensional fullerenes (Villarreal et al. 2017;Jiang et al. 2017). The brightest stars among them are graphene, carbon nanotubes, and carbon nanofibers, which have the most promising nanotechnology applications (Siqueira and Oliveira 2017). CNMs have sparked a lot of interest in the research community because of their unusual features. Bio-imaging, tissue engineering, drug carriers, treatment of wastewater, catalysis, reduced pollution, biosensor, and energy storage system are some of the important applications of CNMs (Harrison and Atala 2007). The physical properties of synthetic nanomaterials are shown in Table 1.
Agricultural waste is the term used to describe waste produced by agricultural activities (Zhang et al. 2012). The agro-wastes are created by a variety of agricultural activities and industries, including the handling of crop waste, pesticide, insecticide, and herbicide waste, as well as the disposal of animal excreta and carcasses (Zhang et al. 2012;Ali et al. 2017b, a). However, there is a range of organic and mineral resources in agricultural wastes which is recovered and reused for a number of purposes, such as starting material for energy production and remediation of harmful pollutants (Obi et al. 2016). Due to growing agricultural productivity in recent years, agricultural wastes have expanded along with the global population. A considerable number of environmental issues are brought on by the enormous amount of agricultural waste (*998 million tonnes) (Agamuthu 2009).
Furthermore, a farm produces 5.27 kg of organic waste per 1000 kg of living weight, which accounts for 80% of all solid waste (Hadiya et al. 2018). Significant policymaker concerns for sustainable development and green agriculture have recently been muzzled by Agricultural Waste Management (AWM) (Hai and Tuyet 2010, Hegde et al. 2015, Ali et al. 2014b. Numerous procedures and factors were looked at in terms of managing waste management for prospective reuse and lowering the threat of resource pollution. The use of technology and inducements and a change in perspective and attitudes are only a few of the additional efforts needed to control AWM. There are numerous potential uses for AWM, including the application of fertilizer, anaerobic digestion, animal feed, adsorbent materials, source of heat, direct burning, and the production of nanomaterial (Obi et al. 2016). Rice is one of the world's oldest crops grown for centuries. Recently, it is grown in over 100 countries, and almost half the world's population eats it as a basic diet. Population growth and economic expansion in developing nations are expected to boost global rice consumption from 480 million tonnes (Mt) of milling rice in 2014 to roughly 550 Mt by 2030 (Rice Research Institute 2016). A huge amount of trash is produced during the production of rice-based foods. This garbage has a disposal difficulty, which has negative consequences for the environment and human health. Rice trash contains husk, straw, bran, ash, and broken rice (Moraes et al. 2014). RH is a common rice waste product, that is, used to create nanomaterials employing green technology. This chapter focussed on the synthesis of carbon-based nanomaterials using RH.

Rice husk as waste
RH is the outer coat of rice seeds that are cylindrical in shape, and its size ranges between 4 to 10 mm depending on the seed variety employed. RH is made up of 41.92% carbon, 6.34% hydrogen, 1.85% nitrogen, and 0.47% sulfur (Biswas et al. 2017). Rice husk is a global product of about 1 million tonnes per year (Liou and Yang 2011). Rice husk is mostly composed of hydrated silicon and organic compounds such as cellulose (55-60 wt%), including lignin and hemicelluloses and cellulose (22 wt%). The white ash resulting from the moderate-temperature burning of this feedstock comprises 87-97% amorphous silica and a small number of metallic contaminants ). Humans should not consume this product. Environmental concerns are created by dumping the ash and partly bored husk Jung et al. 2008, Chaudhary andJollands 2004). When 1 tonne of rice husk is burned in the field, 0.15 kg of CO 2 is released into the environment, whereas rice husks yield 90 g of methane gas when they decompose naturally in the soil (Umeda and Kondoh 2010). Many rice-producing nations burn rice husks in outdoor dumps, which can pollute the air. Some discard them in open landfills, where the rice husks degrade and eventually produce methane, a greenhouse gas contributing to global warming (Omatola and Onojah 2009). Because of the growing amount of waste, a number of research groups are looking for ways to convert it into useful products.
It was discovered that silica was substantially dispersed in the husk's outer surface using back-scattered electrons and X-ray imaging analysis of the RH, with less presence in the mid-region and inner epidermis (Stroeven et al. 1999).
RH exhibits a characteristic globular, well-organized, corrugated outer surface, as a scanning electron microscopy (SEM) study determined. SEM imaging of the side section of RH revealed an interlayer between the inner and outer surfaces. The interlayer features multiple pores with a diameter of 10 µm, is loose and honeycombed, and is constructed of interlaced plates and sheets (Jiang 2010). RH usually is 8 to 10 mm long, 2 to 3 mm wide, and 0.2 mm thick (Fang et al. 2004). Actual densities range from 670 to 740 kg/m 3 , while RH has a bulk density of between 100 and 160 kg/ m 3 . However, RH can only be compressed to 400 kg/m 3 . Approximately 80% of RH's composition is organic, and 20% is inorganic. Crude protein and fat are deficient, ranging from 2.0 to 2.8% and 0.3 to 0.8%, respectively, compared to crude fiber. It consists mostly of lignin, which ranges from 20.4 to 33.7%, hemicellulose, which ranges from 14.0 to 28.6%, and hemicellulose, which ranges from 28.6 to 41.5% (Champagne et al. 2004;Quispe et al. 2017).
RH is an excellent option for a starting material because it is a biomass, renewable material, is very inexpensive, and can be produced in massive amounts. According to a recent literature review, some research groups have synthesized silica and carbon-based nanomaterials from RH for environmental and economic reasons (Seyfferth et al. 2016;Vargas et al. 2019). As per Jonathan et al., open and closed burning of rice husk-derived silica indicated distinct phases of crystalline and amorphous silica (SiO 2 ) accordingly (Liu et al. 2013;Patil et al. 2014). RH is an essential feedstock for producing CNMs such as biochar, graphene, and graphene oxide, carbon nanotubes (CNTs) (Wang et al. 2018a, b;Guan et al. 2019;Asnawi et al. 2018). Wang et al. prepared graphene quantum dots from RH for Fe 3+ sensing (Wang et al. 2018a, b).

Rice husk ash
After being burned, RH produces rice husk ash (RHA), which weighs 17 to 20% less than the lightweight and clumsy RH. The RHA is a porous substance with a roughly 180-200 kg m −3 density. Depending on the circumstances of the combustion process, RHA is divided into two categories: black RHA and white RHA (Ugheoke & Mamat 2012). White RHA is produced by carefully controlling the pyrolysis of RH in the air. White RHA typically consists of hydrated, amorphous, pure SiO 2 (> 95%) with a high porosity, and reactive surface (Vlaev et al. 2003). In the meantime, the production of black RHA can be achieved by carefully burning RH in an inert environment such as nitrogen (Ghaly & Mansaray 1999). 1.22% K 2 O, 1.28% Fe 2 O 3 , 18.24%C, 1.20% Al 2 O 3 , 89% SiO 2 , and 1% CaO are the chemical composition of RHA (Mohamed et al. 2015). Fe, Ca, Cu, K, Mg, Mn, Na, and Zn traces are also observed (Zou and Yang 2019).
While scanning electron microscope (SEM) image of RHA ( Fig. 2) produced by the combustion of RH at 600 °C for 2 h in an electric oven, major components of RHA survived air combustion unaffected, but minor components sustained structural damage (Fig. 1A). Because both sides of RHA display a dense structure, it is indeed possible that a compacted membrane with no micropores covers both the external and interior surfaces (Fig. 1A, B) (Xu et al. 2012).
Ouyang and Chen discovered the three-layer concept while analyzing the microstructure of RHA utilizing SEM and transmission electron microscopy (TEM) studies. The interlacing of the fiber sheet creates micropores in RHA, which are reliant on the RH structure but independent of the combustion conditions, and nanopores, which are generated by nano-SiO 2 particles and are around 50 nm in size and depending on the combustion temperatures (Ouyang and Chen 2003). SiO 2 particles and nanopores at the nanoscale are the primary contributors to RHA's high surface area and activities. Along with Kim, in the diffractogram, there is a noticeable smooth bump between the angles of 15° and 35°, proving that pyrolysis changed the crystalline structure of cellulose into an amorphous, chaotic, disordered structure. Kim and other research groups reported that the crystalline structure of cellulose was altered by pyrolysis into an amorphous, chaotic, disorganized form ).

Nanomaterial synthesis from rice husk
Due to their numerous fascinating physical and chemical characteristics, such as high quantum yield, high surface area, and inventive morphological structure, nanomaterials are widely created and used for a number of applications (Naddaf et al. 2019;Emran et al. 2019Emran et al. , 2017Emran et al. 2018a, b, c, d, e, f, g;Emran et al. 2018a, b, c, d, e, f, g;Emran et al. 2018a, b, c, d, e, f, g;Gomaa et al. 2017;Thalji et al. 2019;Ali et al. 2018Ali et al. , 2013Fouad et al. 2011). They are strong contenders for a variety of applications in numerous fields of science and nanotechnology due to these intriguing characteristics (Emran et al. 2018a, b, c, d, e, f, g;Emran et al. 2018a, b, c, d, e, f, g;Emran et al. 2018a, b, c, d, e, f, g;Emran et al. 2018a, b, c, d, e, f, g;Gomaa et al. 2018;Barhoum et al. 2019;Ali et al. 2017b, a, 2014b, Abdel Ghafar et al. 2015. Removing nanomaterials from natural bio-resources, such as bacteria, plants, waste, and agricultural residues, has recently garnered much attention (Griffin et al. 2018). This strategy differs from traditional strategies in a number of ways, including flexibility, economy, a higher level of safety, and less negative environmental impact. In particular, producing nanomaterials from agricultural waste is viewed as a cost-effective alternative to industrial raw materials and a renewable source for mitigating environmental decay issues. Nanomaterials made from agricultural waste include nanosilica and nanocarbon (Mor et al. 2017).
Due to its high silica concentration, RH is currently a source for many silicon compounds, including silicon nitride, silicon tetrachloride, silicon carbide, silica, zeolite, and pure silicon. One of the many materials that are readily accessible and used in a variety of applications, including thixotropic agents, thermal insulators, and composite filler, is silica (Pek et al. 2008). The design of biosensors, drug delivery, cell labeling, imaging, and separation have all benefited from the use of silica nanoparticles (Si NPs) (Fouad et al. 2011, Ali et al. 2016, Fouad et al. 2012, Tang and Cheng 2013, Singh et al. 2017a, b, Tao 2014, Ali et al. 2020. The synthesis of Si NPs made considerable use of RH/RHA as well as other agricultural by-products and wastes (Naddaf et al. 2019;Mor et al. 2017;Pouroutzidou et al. 2019).
RH was used as a precursor to silica to make mesoporous silica NPs for drug delivery (Purwaningsih et al. 2019). SiO 2 NPs that are black were also created using RH (Almeida et al. 2019). Matsumoto et al. produced brilliant Si NPs and investigated their optical properties using Mg reduction of SiO 2 NPs extracted from RH (Matsumoto et al. 2018).
Rice husk is a valuable source of cellulose nanofiber (CNF). The CNFs produced from rice husks showed strong fluorescence emission potential with outstanding quantum yield in contrast to their thermal properties and crystalline nature (Moon et al. 2011;Leung et al. 2013). Blue fluorescence was visible when the held CNF was exposed to UV light. The mechanical treatment and acid digestion of rice husk led to highly fluorescent CNFs in two rice varieties, Ahu and Boro (Oryza sativa L. ssp. indica). These CNFs have syringyl and phenyl coumarone groups, these are what give them their fluorescence capabilities and a little amount of lignin (Kalita et al. 2015). Porous carbon nanoonions (CNO), activated using renewable RH as a carbon precursor, are made using straightforward nickel-assisted graphitization (Jin et al. 2019). On silica made from rice husks, AgNPs with a diameter of 25 nm and a surface area of 514 m 2 g −1 were synthesized and studied (Andas and Adam 2016). A different study isolated silica and magnesium oxide from RH and synthesized into 80-85 nm forsterite (Mg 2 SiO 4 ) NPs using a solid-state technique (Mathur et al. 2018). Bathla et al. produced silica nanowires with a diameter of 15 to 35 nm and a length of about 0.5 µm using rice husk ash (Bathla et al. 2018). Rice husk, for example, can be used in a microwave-assisted technique by Praneetha and Murugan to form SiO 2 (Praneetha and Murugan 2015).
There is a lot of interest in CNMs for various potential uses, including power storage, sensors, catalyst, and water treatment. The applications for the CNMs in agronomic practices and also in the environmental application are shown in Fig. 2. There are numerous types of CNMs, including nanotubes, nanospheres, fullerene, and graphene (Emran et al. 2018a, b, c, d, e, f, g;Emran et al. 2018a, b, c, d, e, f, g;Emran et al. 2018a, b, c, d, e, f, g;Emran et al. 2018a, b, c, d, e, f, g). We demonstrate the creation of carbon nanoparticles from RH in this chapter.

Synthesis of carbon-based nanomaterial from rice husk
A family of synthetic nanomaterials called agro wastederived CNMs has numerous uses because of its outstanding physical and chemical properties (Mukherjee et al. 2016).
Carbon-based materials have excellent physiochemical properties that are used for a wide range of applications (Greil 2015;Zhang et al. 2013). Carbon-based materials can often be categorized into four types based on their morphological structures: fullerenes and carbon dots are zerodimensional (0D) materials. Carbon nanotubes (CNTs) are examples of one-dimensional (1D) materials. Graphene and graphite are two-dimensional (2D) materials (Villarreal et al. 2017;Jiang et al. 2017). CNTs are a material made of carbon with remarkable qualities like great chemical and mechanical stability and interesting electrical conductivity. In addition, the 2D material graphene has remarkable features, including graphene edges and highly transmitted light (Wang et al. 2015a, b). The production and application of CNMs, such as CNTs and graphene, for items like electronic devices, sensors, composite materials, and energy storage devices have both received a great deal of attention up to this point Emran et al. 2018a, b, c, d, e, f, g;Emran et al. 2018a, b, c, d, e, f, g;Akhtar et al. 2017). The environmentally friendly synthesis of carbon-based materials utilizing waste materials has received much attention. RH is a good and plentiful source of carbon nanomaterials due to its high cellulose and lignin content (Kure et al. 2017). Table 2 lists the numerous carbon nanomaterials made from different biowaste and their synthesis methods.

Synthesis of CNTs
Since their discovery by Iijima in 1991, carbon nanotubes (CNTs) have been one of the most renowned onedimensional carbon nanomaterials (Iijima 1991). Due to their one-dimensional structure and exceptional capabilities as electrical, chemical, thermal, mechanical, optical, and electronic, CNT materials have attracted a lot of attention from researchers (de Volder et al. 2013, Chu et al. 2010, Anantram & Leonard 2006, Prasek et al. 2011, Qi et al. 2003, Zhang and Li 2009. As a result of these properties, CNTs are suitable for a variety of applications, including electron field emission, energy storage and production, hydrogen storage, nanocomposites, separation, catalyst support, and drug delivery. CNTs can be made in single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs), each with a few to several nanometers in diameter. The CVD method is often considered the most suitable commercial production method. The choice of carbon-containing feedstocks, catalyst, substrate, and the required energy consumption during the CVD process all play a role in CNT synthesis (Birὀ et al 2001). As CNT precursors, various petroleum hydrocarbons in gaseous forms, such as methane, ethylene, and acetylene, as well as liquid conditions, such as alcohols, benzene, xylene, and cyclohexane, have been widely employed (Birὀ et al 2001, Baughman et al. 2002, Jha et al. 2013, Dai 2002, Serp, & Figueiredo 2009, Yu et al. 2010, Qi et al. 2003. Much research has been done to promote green technology and increase CNT mass manufacturing in response to growing environmental concerns and the CNT market. Few studies have recently concentrated on the improvement and simplicity of CNT synthesis using agro-waste. CNTs were synthesized from waste RH powders using the microwave (MW) method. A carbon source, catalyst, and microwave oven are required to induce plasma. In the presence of ferrocene, the plasma improves and accelerates the catalytic degradation of RH (Fig. 3) (Asnawi et al. 2018). MW plasma irradiation (MWPI) method was introduced by Wang et al. for synthesizing graphene-CNTs (g-CNTs) from RH under H 2 and Ar flow (Wang et al. 2015a, b).
Zhang's research group employed a polyacrylonitrile (PAN)-aided electrospray approach to synthesize Si/nitrogen-doped C/CNT (SNCC) nano/micro-structured spheres for the first time, employing RH-derived Si NPs as a raw material. RH-derived Si NPs with diameters of 50 nm were uniformly dispersed and incorporated in an N-doped carbon matrix that was interwoven and connected by a CNT crosslinking network, resulting in microspheres having diameters of 3.2 ± 0.8 μm .
For the synthesis of N-doping of CNTs, polypyrrole (PPy) was used as the carbon precursor, while mixed salts composed of sodium chloride and zinc chloride were used as the activating agent (NCNs-A). It was determined how the physicochemical and electrochemical characteristics of NCNs-A were affected by the individual salts ZnCl 2 and Bamboo charcoals CVD (Zhu et al. 2012) NaCl, as well as the carbonization temperature and the mass ratio of PPy to the combined salts. The resultant NCNs-A exhibits good capacitive performance, thanks to their high SSA, broad pore architecture, hollow tubular shape, and high quantity of N-doping when used as electrode material for supercapacitors. Additionally, this NCNs-A has a broad pore architecture (Zong et al. 2020).

Synthesis of graphene
Graphene seems to have become a material of interest and has piqued the scientific interest of several research communities around the globe since its discovery in 2004 by Andre Geim and Novoselov at the University of Manchester (Novoselov et al. 2004). Graphene is a 2D carbon sheet with a hexagon lattice structure that is a single atom thick. The sp2 hybridized carbon forms a structure that resembles a compact honeycomb and functions as a framework for a large number of nanocarbon. Stacking results in the formation of threedimensional graphite, whereas rolling or coiling results in one-dimensional nanotubes and zero-dimensional fullerenes (Allen et al. 2010). The chemical inertness of the material, the quantum Hall effect, high carrier mobility, ambipolar field effect, and super hydrophobicity are only a few of the properties that have brought the material to the forefront of attention (Choi et al. 2010). These peculiar qualities led to the discovery of a great deal of intriguing physics and led researchers to hypothesize that graphene may be utilized in various cutting-edge electronic applications. Functionalized graphene and non-functionalized graphene are two different forms of graphene, in contrast to non-functionalized graphene, single-layer graphene, and graphene sheets, functionalized graphene oxide (GO), and reduced graphene oxide (Homaeigohar & Elbahri 2017).
Through the activation of RHA with KOH, Muramatsu et al. showed a simple, cost-effective, and scalable approach for synthesizing graphene with stable and atomically clean edges . Graphene was successfully produced using this approach by activating (RHA) using KOH at 800 °C with a 1:2 impregnation ratio, as shown in (Fig. 4) (Ismail et al. 2019). RHA-derived graphene was also synthesized by the Othman research group utilizing KOH as a dehydrating agent at 800 °C and a (1:5) impregnation ratio for CH 4 adsorption, which will be employed for natural gas storage or transit in the coming years (Che Othman et al. 2020).
Singh et al. developed a more cost-effective synthesis technique. They used RHA as a carbon source for graphene production and KOH as an activating agent in our procedure. The acquired results revealed that few-layer graphene was successfully synthesized. To test electrochemical properties for energy storage applications, cyclic voltammetry was used. RH was burned into the air to make RHA, which was then.
Moreover, 3 mg of RHA was combined with 15 gm of KOH and ground for 15 min. A mixture of rice husk and KOH was compacted in a porcelain crucible. This crucible was then wrapped in ceramic wool and placed inside a bigger graphite crucible. A sufficient amount of sacrificial RHA was applied to the top of the graphite crucible to establish a barrier preventing sample oxidation inside the porcelain crucible. The sample was annealing at 900 °C in a muffle furnace for 2 h. Following this activation process, the sample was rinsed with deionized water to eliminate excess KOH before being dried for 24 h at 100 °C (Singh et al.2017a, b). Sankar and his co-worker prepared Brown graphene nanosheets and explored their electrochemical energystorage ability from the perspective of their applicability as an electrode material. Despite its simplicity, the production Fig. 3 Conversion of RH into spherical and tubular structures of carbon CNTs (Wang et al. 2015a, b) 1 3 approach yields crystalline ultrathin graphene nanosheets with a low defect density (Mahmoud et al. 2020).
RH is used to synthesize graphene using a simple microwave technique. RH is first rinsed with distilled water by sonication for 1 h. The RH was then dried for 24 h in the open air at 40 °C. The mechanical approach is used to turn the dried RH into powder. The catalyst is ferrocene Fe(C 5 H 5 ) 2 , and the ferrocene and ethanol are combined at three different levels with a magnetic stirrer (Kumar et al. 2020). Graphene layers were produced by activating the rice husk with potassium hydroxide and then desilicating it in an alkaline solution. Rice husk samples were carbonized under the following circumstances: rice husk/KOH weightto-weight ratio was 1/4, and activation duration was 2 h at 850 °C. The NaOH desilication solution had a 1 M concentration. Raman spectroscopy was used to analyze the collected samples; the peaks indicate the presence of graphene layers. The product yielded less than 3% of its weight (Azizovna et al. 2017).
Rice husk is the feedstock used in the carbonization and KOH activation procedure to produce graphene. After that, graphene is made and investigated. It is completely even on the surface and has functional groups along its edges. In both the presence and absence of a compatibilizer, graphene is melt-mixed with polypropylene (PP) and polyamine-6 (PA6) in a ratio of 50/50 (wt/wt). The maleic anhydride (MA) grafted onto the polypropylene serves as the compatibilizer (PP-g-MA). The findings of the FTIR spectroscopy indicate that the PA6 phase and PP-g-MA react with one another. Localization of graphene occurs in the PA6 phase of a PP/ PA6 mix with a weight-to-weight ratio of 50/50 (wt/wt) and its nanocomposites in both the presence and absence of PPg-MA (Tanniru and Tambe 2022).
Rhee et al. use graphene made from rice husk to study the electrical characteristics of cement mortar (GRHs). It looks at improved varieties of agricultural waste-derived rice huskderived graphene-like materials. The control specimens in this experiment were chosen based on the size and shape of their nano component to analyze the performance of GRH within cement mortar. This set of materials included multiwalled carbon nanotubes (MWCNTs), MWCNTs decorated with COOH, and carbon nanofibers with significantly higher aspect ratios because of their 1D structures. The 2D planar or corrugated-planar xGnP M15, xGnP C650, and GRH materials made comprised the rest of the comparison group. Electrical conductivity tests have shown that 1D structured inclusions outperform 2D structured inclusions. According to measurements of the change in volume resistivity vs. stress and strain, the electrical performance of the GRH composite fell within a moderate range comparable to that of carbon fiber (Rhee et al. 2015).
Sekar and his team recently used the KOH activation method to create corrugated graphene nanosheets from rice husk biomass. The RH-CG nanosheets displayed intense Fig. 4 Experimental flowchart of synthesis graphene derived from rice husk electric conductivity and a huge surface area after they were heated to 700 °C. The outstanding HER activities with a small overpotential (9 mV at 10 mA/cm 2 ) and a small Tafel slope (31 mV/dec) were attained when the RH-CG nanosheets were used as a HER electrocatalyst in 0.5 M H 2 SO 4 . The findings provide a novel method for realizing an excellent electrocatalyst made of biomass for highly effective hydrogen production (Sekar et al. 2022).
RH was used as the primary raw material in a modified version of the Hummers' method that was used to produce GO. The homogeneity of the synthesis process was evaluated using ground pencil leads as a control powder of the initial raw material to ensure that the powder was representative of the material as a whole. The pluronic F127 solution acted as the pore template during the precipitated technique used to create the TiO 2 microspheres. Microspheres of TiO 2 were mixed with GO derived from RH (GO-RH) to develop composites with weight-to-volume ratios of 3:1, 2:2, and 1:3. According to the results of the characterization, GO-RH produced a ternary phase material that contained graphite oxide, silica, and GO (Manpetch et al. 2022).

Synthesis of carbon dots
Carbon dots (CDs) are one special type of carbon nanoparticle discovered in 2004 when extracting single-walled carbon nanotubes. These nanomaterials are fewer than 10 nm in size and feature a number of unique properties, such as strong biocompatibility, low toxicity, high water solubility, and a peculiar fluorescence property (Sharon & Mewada 2018). CDs are similar to graphene nanomaterials in that they have an amorphous structure (Fadllan et al. 2017). CDs are dispersive quasi-spherical nanoparticles made of carbon cores and shells of functional groups like the carboxyl group and the amino group and are a shining star of zero-dimensional carbon materials (Jing et al. 2019;Lim et al. 2015;Baker & Baker. 2010). The primary types of CDs are graphene quantum dots (GQDs) and carbon quantum dots (CQDs), which show significant potential in a variety of applications such as photocatalytic activity, biomedical imaging, sensing, drug delivery, and energy storage (Zhu et al. 2020). The sp 2 and sp 3 hybridized carbon atoms in these nanomaterials enable further exploration of their adjustable characteristics (Kokorina et al. 2017). CQDs can be synthesized in various ways, involving top-down and bottom-up strategies.
The CDs were produced by sonicating 10 gm of prepared dry husk in a solution of 2.5 M 50 mL HNO 3 for 15 min. After placing the mixture in an autoclave lined with Teflon, it was heated to a temperature of 200 °C for 6 h. After waiting for the dark brown liquid to reach room temperature, a solution of 10 M NaOH was used to neutralize it, and then, it was filtered. The residue was set aside and later utilized in the production of mesoporous silica. The filtrate was centrifuged for 15 min at a speed of 10,000 rpm in order to remove any large particles. After using a dialysis membrane to filter the fluid, it was then freeze-dried to produce brown solid carbon dots weighing 0.93 gm and having a concentration of 9.3%. According to the findings of DFT calculations, UV-vis testing, and electronic nose analysis, the created carbon dots are suitable for real-time sensing of alcohols and VOCs (Thongsai et al. 2019).
CQDs were produced from RH using a hydrothermal process that required varying proportions of amine and carboxyl group sources in ethylenediamine and ascorbic acid, respectively. The functionalized CQDs were evaluated by HRTEM, FTIR, UV-vis, photoluminescence PL, and XPS. To remove cadmium from an aqueous solution, functionalized CQDs with the right amount of EDA and ascorbic acid will be utilized (Abidin et al. 2020a, b). Wang et al. used RH as a precursor to synthesize high-yield CQD-grafted silica NPs (Si-C NPs). The pyrolysis of water-rinsed rice husks (5 g) inside a tubular furnace in nitrogen atmosphere at 700 °C for 2 h results in the production of RHA among both silica and carbon. To remove silica, RHA was reacted using 1.0 M NaOH at 100 °C for 2 h. Then, in two steps, RHA with solely carbon content was oxidized with H 2 SO 4 and HNO 3 ; every step was performed by sonication, producing inside a black dispersion. It is proceeded by suction filtering using a 0.22 mm microporous membrane, which is subsequently rinsed with deionized (DI) water numerous times. Before being put into a 40-mL Teflon-lined autoclave, the black substance was mixed with 30-mL DI water. The dispersion undergoes a 10-h hydrothermal treatment at 200 °C. Then, it is cooled to room temperature and filtered once more. The filtrate successfully recognized RH-generated CQDs. A solid CQDs powder is created by evaporating the filtrate at 40 °C in a vacuum oven (Fig. 5) (Wang et al. 2017).
The prior method was modified by Wongso et al. to produce CQDs from RH. Without using any functionalizing agents, CQDs were synthesized, and this process was modified by adding NaOH to adjust the pH from 0 to 14. In this mechanism, intercalation was followed by exfoliation. Intercalation begins after RHA is diffused in a solution containing H 2 SO 4 and HNO 3 . Following that, exfoliation begins by rinsing RHA in deionized water, resulting in CQDs. HR-TEM and EDX techniques were used to investigate the material. The size of CQDs and the quantity of oxygen-containing groups throughout the surface are affected by pH changes, while the sample's crystal structure is unaffected. Due to the presence of NaOH, which interacts with HNO 3 to produce NaNO 3 , different sizes appear. The excess NaOH serves as an oxidant and speeds up the intercalation process of RHA when HNO 3 is entirely treated with it (at pH neutral). Changing the pH of the synthesis helps to broaden the emission spectrum of CQDs from green to cyan-orange, according to photoluminescence spectroscopy (Wongso et al. 2019).
RH was fully rinsed using DI water, dry, and crushed into powders from a 1.0 g sample (100 mesh). Before being dried in a vacuum at room temperature for 24 h, the powder was rinsed 3 times in 0.10 M HCl and DI water. A 100 mg dried RH grains sample was mixed with 20 mL DI water in a Teflon®-walled autoclave. The hydrothermal reaction was performed at 150 °C for 5 h. The hydrothermal reaction was performed at 150 °C. The mixture was filtered after the reaction. The supernatant, consisting RH-GQDs, was obtained by centrifuging the filtrate at 28,000 g force for 15 min. The synthesized GQDs were highly sensitive to Fe 3+ ions, suggesting that they could be used for Fe 3+ sensing (Wang et al. 2018a, b).
Arivuselvi et al. used a hydrothermal technique to make the carbon quantum dot from rice husk in 4 h at 200 °C. A 5.0 g sample of rice husks was completely rinsed in deionized (DI) water before being ground to powder (100 mesh). The powder was then processed for 2 h in a tube furnace at 600 °C in an N 2 environment. Then, 0.89 g of RH ash (RHA), which contains carbon and silica, was taken. The RHA was treated with excess KOH and a magnetic stirrer for 10 min. RHA was transformed into a combination of RHC and potassium silicate throughout this procedure. The synthesized powder was then extensively rinsed with DI water and dried for 2 h at 80 °C. The resulting potassium silicate solution was collected and might be used to make various silicon-based functional materials. HNO 3 was added slowly to a sample of 6 g of RHC mixed with 20 ml of H 2 SO 4 . The solution was allowed to settle in the bottom of the flask before being carefully rinsed in DI water and filtered through Whatman filter paper. The solution was maintained at 200° C for 4 h in a Teflon-lined autoclave. The resulting solution was then allowed to cool to ambient temperature before being filtered through Whatman filter paper. Finally, anneal for 2 h at 80 °C to remove any remaining water. RH-GQDs were finally ready to be powered (Arivuselvi and Ramalingam 2017).
The synthesis of RH-derived CQDs was performed, demonstrating how functional groups influenced the RH-derived CQDs. Husks were created so the hydrothermal carbonizing RH could go more smoothly. CQDs were dialyzed for one day with deionized water using visking tubes made of cellulose 10 M to improve the level of purity achieved. The following techniques were utilized to determine CQDs' characteristics: FTIR, thermal analysis, XPS analysis, XRD, UV-Vis, and HRTEM (Abidin et al. 2020a, b).

Conclusions and future perspectives
These materials can be made in a straightforward and costeffective method. Several processes have been looked into to produce CNMs; however, greener ones with less waste material are gaining popularity. As a result, researchers are increasingly focused on building the latest techniques for manufacturing CNMs using simple, eco-friendly, cheap, and sustainable routes that use natural and abundant renewable resources, particularly agricultural waste. This chapter focuses on rice husk, an agricultural waste that can be used as a starting material to synthesize CNMs like graphene, CNTs, and CDs.
RH is a significant by-product of paddy rice milling and is produced in large quantities. The rice milling industry is facing a major challenge. When rice husk is used as a feedstock for the production of CNMs, it must be pretreated, which changes the reaction conditions and requirements. This limits the amount of rice husk that can be used to this restriction; we must create studies that do not need pre-treatment and make their application as straightforward as possible. Water pollution remediation, energy storage, catalysis, biomedical sensors, medication delivery, and bioimaging are just a few of the possibilities for rice husk CNMs.
Although several research projects on the use of RH and the result of its thermal breakdown, RHA, are currently in progress, most of these investigations have been carried out on a laboratory scale. RH/RHA is an excellent precursor for producing high-value-added CNMs that are used in practical applications since it originates from a natural, sustainable, and renewable source. RH/RHA is utilized extensively Fig. 5 RHs to RHA and RH-silica-C NPs transformation I: pyrolysis under nitrogen atmosphere; II: oxidation; III: carbon grafting, framework cutting, and oxygen-containing group reduction (Wang et al. 2017) because it makes it possible to convert easily accessible agro-waste into things with additional value while lowering the amount of pollution produced.
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