Sustainable hydrothermal and solvothermal synthetic ap-2 proaches for advanced carbon materials in multidimensional 3 applications : A review 4

There is great importance and need of improving existing carbon materials fabrication 15 methods. As such, this work proposes to offer deeper discussions of a selected and promising liter16 ature. This review will interrogate, viable hydrothermal, solvothermal, and other advanced carbon 17 materials synthetic methods while making several other viable options. The advanced carbon ma18 terials to be interrogated will include the synthesis of carbon dots, carbon nanotubes, nitrogen/tita19 nia-doped carbons, graphene quantum dots, and their nanocomposites with solid/polymeric/metal 20 oxide supports. This will be done with special mind to microwave-assisted solvothermal and hy21 drothermal synthesis due to their favourable properties such as rapidity, low cost, and green/envi22 ronmentally-friendliness. Thus, these methods are important during the current and future synthe23 sis and modification of advanced carbon materials for application in energy, gas separation, sens24 ing, and water treatment. Simultaneously, the work will pay special cognizance to methods reduc25 ing the fabrication costs and environmental impact while enhancing the properties as a direct result 26 of the synthesis methods. As a direct result, the expectation is to impart a significant contribution 27 to the scientific body of work regarding the improvement of the said fabrication methods. 28


33
Carbon materials are extremely versatile in terms of their applications, and as such, 34 these materials have high and growing demand worldwide. This demand, in turn, neces- 35 sitates the development of rapid, cost-friendly, scalable, and safe synthetic methods. 36 These synthetic methods include hydrothermal, solvothermal, ionothermal, chemical va-37 pour deposition (CVD), hard/soft templating, among other approaches [1][2][3][4]. The course, 38 however, is changing to the direction of more favourable methods of synthesis because 39 most of the above-mentioned methods possess several drawbacks such as contamination, 40 lengthy reactions, the use of catalysts and toxic chemicals, among others [5]. To achieve 41 sustainable carbon materials with enhanced properties for wide applicability, several 42 modifications and changes need to be carried out and the [6]. The properties allowing 43 this are solely dependent on the reaction conditions such as the precursor concentrations, 44 There has been great interest generated for the development of carbon materials in 47 the recent past. In description of the methods mentioned above, the first being the sol-gel 48 method of synthesis [8]. This method usually involves the preparation of catalysts to serve 49 the carbonisation (thermal treatment) of different precursors for the carbon nanomaterials 50 to be utilised in various applications such as capacitive deionization (desalination) of sa-51 line water [9]. As such, many exciting morphologies, and chemical compositions of ad- 52 vanced carbon materials (and their modified counterparts) have been prepared [10]. 53 Solvothermal synthesis employs solvents and high temperatures in the preparation 54 of the carbon materials. In one study, a facile solvothermal carbonization of organic com- 55 ponents for the synthesis of Fe3O4@C as an anode material for energy applications (lithium 56 ion batteries) was presented [11]. Of the number of challenges faced by solvothermal syn-57 thesis include the high demand of energy because of the high temperatures required to 58 satisfy the carbonisation. Hydrothermal synthesis of materials will have the reactants or 59 carbon source dispersed in water prior to the carbonisation under high temperatures and 60 pressures. The benefits of this method include ease of synthesis, ease of composition con- 61 trol, and thus controlling the resulting particle properties such as high surface area, size, 62 shape, and dispersion [12,13]. The advantages of these high temperature and high pres-63 sure (HTHP) methods of synthesis, also include thermal and chemical stability, and thus, 64 mitigate the metal dissolution/leaching when metals are used as dopants to carbon [14]. 65 As previously mentioned, one of the caveats of these methods lie in energy demand due 66 the high reaction temperatures. As such, other methods have been developed to reduce 67 energy demand. These methods are low temperature hydro-or solvothermal approaches. 68 Low temperature solvothermal synthesis (LHS) is not a really new method because 69 as far back as 2004, Kuang et al (2004) presented the preparation of graphene nanosheets 70 using this approach [15]. Recently, Manohara et al (2019) prepared an aluminium-func-71 tionalised carbon-based nanocomposite using LHS for a carbon membrane, successfully 72 removing several environmental insults in water [16]. To further address problems such 73 as complicated procedures/equipment and environmental threats as confronted by 74 HTHP, an even simpler method to prepare graphene nanosheets was presented by Ye et 75 al (2020). This method used involved a one pot reaction without the use of harsh chemicals 76 [17]. 77 As such, this section of the book chapter aims to address the recent developments in 78 the synthetic methods in terms of favourable, rapid, and environmentally friendly ap- 79 proaches for the fabrication and carbon materials. These are especially critical in the de-80 velopment of advanced carbon materials that will be used for energy, water treatment, 81 sensing, and gas separation nanocomposite technologies. Because of the importance of the 82 cost considerations of HTHP processes in this regard, low temperature and high/low- 83 pressure synthetic methods will be interrogated with specific mind to microwave chem- 84 istry approaches. The use of sustainable carbon sources such as biomass and other carbon 85 precursors shall also be looked at in this book chapter. 86 2. Short summary of the development of advanced carbon materials synthesis meth-87 ods and applications 88 The development and growth of the solvothermal/hydrothermal methods are di-89 rectly-related to the creation of nanomaterials [1]. The first study on the solvothermal pro- 90 cess can be dated back to the middle of the nineteenth century, when micrometer-to-na- 91 nometer-sized quartz particles were used to prepare them [2]. The analysis and introduc- 92 tion of the hydrothermal method in material synthesis, however, lagged between the 93 1840s and the early 1990s because nanoscale product characterization techniques were not 94 available and to some degree, because knowledge of hydrothermal solution chemistry 95 was inadequate to effectively monitor crystal growth [3]. In the 1990s, hydrothermal meth-96 ods were resurrected along with the revolution in nanoscale materials and the advent of 97 high-resolution microscopes from the 1980s [3]. At the same time, significant progress was 98 made in understanding the chemical and physical features of hydrothermal processes, 99 leading to the introduction of the solvothermal method in which organics were intro-100 duced as solvents in the manufacture of well-controlled nanomaterials [4]. Because of 101 these outstanding advantages, including low process temperature, reaction efficiency in 102 liquid environments, low energy consumption, and environmental benignity, the hydro-103 thermal/solvothermal processes have achieved considerable success in the twenty-first 104 century in developing nanomaterials with crystallinity, crystal phase, morphology, and 105 size control [5]. 106 Ever since the discovery of fullerene in 1985 by Kroto and co-workers, just one year 107 before the discovery of carbon soot, which was synthesized for the first time in an inert 108 atmosphere from graphite by a resistive heating process [18]. Many new allotropes of car-109 bon were discovered. Among these are carbon nanotubes (CNTs), first discovered in the 110 year 1991 by Iijima [19] and graphene, which is a single layer of carbon atoms arranged in 111 a honey comb lattice, discovered in the year 2004 by Novoselov and workers [20]. Gra-112 phene is formed out of flat monolayer of carbon atoms which are densely packed in a 113 honeycomb lattice in 2-dimensions. The carbon atoms are sp 2 hybridized; hence, they form 114 strong intra layer bonding within the hexagonal carbon-rings, which makes graphene one 115 of the strongest materials to be prepared [21]. This results in carbon materials finding a 116 myriad of applications where developments have led to the discovery of a multitude of 117 uses of carbon. Its allotropes and many structures/modified permutations have been ap-118 plied in water treatment, gas separation, energy, photovoltaics, and the other above-said 119 applications [22,23]. These include the use of carbon nanomaterials in the catalytic detec-120 tion, removal, and degradation of organic compounds in water through photocatalysis, 121 electrocatalysis, sonocatalysis and all their respective amalgamations [1,24]. Furthermore, 122 these approaches have been included in the development of mixed matrix membranes for 123 water treatment from microfiltration to reverse osmosis applications. In terms of the syn-124 thetic and modification methods, major approaches include hydrothermal and solvother-125 mal techniques, with the green/er being among those gaining the recent greater interest. 126 The use of toxic and expensive solvents is losing favour because these are difficult and 127 costly to remove. Among such new approaches are hydrothermal and solid phase thermal 128 methods. 129 130 In the present section we would like to give a general overview of the different syn-131 thetic methods of carbon and carbon-based materials. The important role of the synthesis 132 parameters that were demonstrated to be key factors during the growth process is also 133 outlined. The results summarized in this work are essentially the studies from the last 134 decade 135 3.1. Arc discharge method 136 The arc discharge method was first used by Krastchmer and Hoffman in 1990 to syn-137 thesize fullerene (C60) [25]. This technique generally involves the use of two high-purity 138 graphite electrodes i.e., anode and cathode. The electrodes are then shortly brought into 139 contact and an arc is struck. Li et al. (2010a) synthesized N-doped multi-layered graphene 140 in the mixing atmosphere of He and NH3; this method involved the use of an electric arc 141 oven that mainly comprises two electrodes and a steel chamber cooled by water [26]. The 142 cathode and anode are both pure graphite rods and the current in the discharge process 143 is maintained at 100-150 A [27]. From the TEM image in Figure 1 it is evident that they 144 obtained large area of high purity multi-layer graphene with the size of 100~200 nm. The 145 graphene as synthesized through this method possessed distinct characteristics and can 146 be utilized in different applications such as electronics, gas sensors, energy storage, wa-147 ter treatment etc [28]. 148 Figure 1. A TEM image of multi-layered graphene produced by arc-discharge method. (Li et al. 150 Reproduced by permission of Elsevier Limited) [29]. 151 In 2015, Sharma and co-workers synthesised multi-walled carbon nanotubes 152 (MWCNTs) using arc discharge method, where an arc was produced in between the elec-153 trodes by a D.C. power supply capable to provide 100-200 Amps current voltage range of 154 20-30 V. The reaction environment was provided by an open vessel containing de-ionized 155 water, carbon nanotubes (CNTs) having lesser number of layers and diameter in the range 156 15-150 nm was obtained. Arc synthesized MWCT's were treated with 8M Nitric acid and 157 8M sulfuric acid solution and it was revealed by the X-ray diffraction (XRD) that CNTs 158 synthesised by arc discharge has good crystallinity with less impurity contents. The 159 MWCNTs grown by this method are short, thick and curved as seen in Figure 2 and also 160 that they gave a good yield and these MWCNTs were reported to have been used in phar-161 maceutical applications [27].  This method was also used to dope graphene with nitrogen (N-graphene) where rel-171 atively smaller layers were obtained as compared to pristine graphene. N-graphene has 172 been applied in different fields but the most favoured being in photocatalysis. Sanchez et 173 al. showed that heteroatom doped graphene give high-performance photocatalysts for the 174 H2 evolution with an enhanced activity in the visible range [31]. In the recent years, great progress has been made in the preparation of carbon and 178 carbon-based materials. Depending on the carbon source used, the synthesis methods can 179 generally be divided into top-down and bottom-up approaches [32]. Recently, Hlongwa 180 and co-workers (2020) synthesised graphene by the reduction Hummer's method, to 181 achieve thin, aggregated, and wrinkled nanosheets. They further used the wet chemical 182 method to synthesise Graphene/Ni-doped LiMnPO4 (G-LMNP), the results showed out-183 standing peak current intensity from electrochemical characterisation. This observation 184 suggested an improved electrochemical reversibility; this can be ascribed to the conduct-185 ing graphene layers, which provided new pathways for electron transfer thus facilitating 186 the redox reaction [33]. The below graph ( Figure 3) suggests the suitability of the G-LMNP 187 for high power energy storage applications.   In 2013 Jumeri et al. synthesised the ZnFe2O4-reduced graphene oxide nano-compo-203 site was synthesised using a microwave synthesis technique where 50ml of deionised wa-204 ter was used to dissolve 0.2g of NaOH [34]. They used the method by Kumar et al. to treat 205 the wastewater samples that were evaluated by measuring the degradation of methyl blue 206 (MB). From Figure 4 it is evident that the nanocomposite exhibited excellent adsorption 207 and photodegradation in the first cycle of wastewater treatment although, the perfor-208 mance was slightly reduced in subsequent cycles, the performance still showed promising 209 results for future use, which suggests that magnetic ZnFe2O4-graphene nanocomposites 210 have potential as an alternative to existing water purification processes. The understand-211 ing of low-dimension carbon material family (CNTs, fullerenes, graphenes, and QCDs) 212 arrived, and indicated extensively remarkable properties for various applications [21]. 213 Hoang et al. synthesised graphene quantum dots (GQDs) by mixing 5 ml of graphene 214 oxide, 10 ml of distilled water and 2 ml of ammonia solution together with a magnetic 215 stirrer and sent into a Teflon container. The container was kept in a protective box and 216 heated in a microwave oven at 700 Watts in 10 minutes [35]. The researchers further in-217 vestigated performance of GQDs as an effective hole transport material in which ITO/PE-218 DOT:PSS/P3HT:PCBM:GQDs/Al was fabricated. GQDs assisted to lower potential differ-219 ence between active layers and electrodes, this increased the short-circuit current density 220 (JSC) from 4.11 mA/cm 2 (no GQD doping) to 6.31 mA/cm 2 (2 mg GQD doping) [35,36]. 221 Chae et al (2017) pyrolyzed the AB2 type lysine utilizing MWAS-induced thermal 222 polyamidation and carbonization and obtained the water-soluble CQDs with QY of 23.3% 223 and the synthesis process took 5 min to be completed. They fabricated the CQDs with 224 chitin nanofibers using microwave assisted hydrothermal method, which took approxi-225 mately 3 min. The nanocomposite was used for drug sensing based on quenching effect. 226 The fabricated CQDs exhibited high stability and sensitive fluorescent to D-penicillamine 227 as evidenced on Figure 4 [37]. were optimized by modifying the modulator quantity (benzoic acid, BenAc and hydro-235 chloric acid, HCl), reaction time and temperature. It was observed that an improvement 236 in modulator quantity improved UiO-67's real surface area and pore volume due to the 237 promotion of linker deficiency; and (ii) the involvement of modulators impaired the 238 numbness of the surface area and pore volume. For synthesizing UiO-67 under micro-239 wave irradiation, optimum quantities of BenAc and HCl were calculated as 40 mol equiv-240 alent and 185 mol equivalent (to Zr salt), respectively. Microwave methods have encour-241 aged quicker synthesis with a reaction time of 2-2.5 h (at equivalent temperatures of 120 242 °C and 80 °C for BenAc and HCl, respectively) relative to traditional solvothermal synthe-243 sis, which usually takes 24 h. In microwave-assisted synthesis, the thermal influence of 244 the microwave is assumed to lead to the rapid synthesis of UiO-67. The efficiency of reac-245 tion mass and space-time yield indicate that the simple but highly effective preparation of 246 Zr-based MOFs was promoted by microwave heating. In addition, UiO-67 MOFs were 247 tested using single component (CO2 and CH4) adsorption from various synthesis methods 248 i.e. microwave-assisted and solvothermal methods), showing similar gas uptakes [38] . In 249 closed batch processes, Glover and Mu (2018) synthesized metal-organic frameworks 250 (MOFs), which is not favorable for large scale production. Here, based on a continuous 251 flow tubular reactor fitted with microwave volumetric heating, we report a scalable MOF 252 synthesis path. Under relatively mild conditions (temperature: 100-110 °C and time: 50 253 min), the device allowed continuous crystallization of MIL-100 (Fe) with a high space time 254 yield of ~771.6 kg/m 3 /day. The MIL-100(Fe) was eventually used as a support for the prep-255 aration of Cu(I)-modified π complexation adsorbents. The adsorbents exhibited favored 256 CO adsorption over CO2, and the efficiency of adsorption (CO adsorption capability and 257 CO/CO2 selectivity) was comparable to or even greater than most Cu(I)-modified π com-258 plexation adsorbents previously mentioned [39]. 259 4. Hydrothermal methods of preparation and modification of carbon materials. 260 As previously mentioned, the disadvantages of HTHP synthesis include the use of 261 toxic reactants and the high costs accrued through the energy intensive synthetic condi-262 tions. With this knowledge, LTHP processes have generated great interest due to lower 263 energy demands and more environmentally friendly processes. Thus, the result is wider 264 applicability of the synthetic routes to develop versatile materials for large scale industrial 265 use. 266 The LTHP approaches for carbon materials synthesis are termed hydrothermal car-267 bonization (HTC). As a method, HTC lowers working temperatures to 150 -350°C from 268 the ~500°C of HTHP synthesis. The result of this can also be reduced operating pressures 269 because these are generated by the reaction itself (as low as ~1MPa). The synthetic method 270 to result from such low working pressures is then referred to as low temperature and low-271 pressure hydrothermal synthesis (LTLP). The added advantage of LTHP and LTLP are in 272 that methods are simple, green, provide great added-value, easy to control, and are CO2-273 negative [40]. It is the same study carried out just recently, presenting the facile in-situ 274 synthesis of multi-layered graphitic carbon nanosheets while using Cu as both template 275 and catalyst via HTC at temperatures below 300°C. Another advantage as presented by 276 this study was the use of natural biomass, plant leaves of Parchira aquatica Aubl as the 277 carbon source. 278 In another study, Chai et al (2019) used this method to produce graphene quantum 279 dots (GQDs) from sugarcane bagasse through a process called hot water pre-treatment 280 (HWP). This process mostly extracts cellulosic materials without the use of toxic or corro-281 sive media such as organic solvents or alkalis, respectively. In this specific study, low tem-282 peratures (170°C) were used to obtain GQDs of small size at 2.26 nm. Several sugars were 283 obtained as value-added by-products in addition to the GQDs which can be used in a 284 number of applications such as solar cells, supercapacitors, the removal and sensing of 285 toxic metal ions in water, and drug delivery, among others [41]. Fluorescent/photolumi-286 nescent carbon dots (CDs) have also been synthesized using a similar method for the ap-287 plications in imaging, sensing, and biolabeling. This was achieved through the hydrother-288 mal treatment of waste wheat straw where it was autoclaved at 250°C in a short duration 289 of ten (10) hours [42]. These methods, however, are still slow even though they are envi-290 ronmentally friendly and cheap. This then requires the development of more rapid meth-291 ods that also possess the low-cost and green factors to develop carbon materials with the 292 required properties. These methods include, to be more specific, microwave-assisted hy-293 drothermal/solid-state carbonization [43].  As mentioned, microwave assisted hydrothermal carbonization is a fast method as 297 opposed to conventional HTHP and LTLP. This is credited to the penetrative nature of 298 microwave radiation where these waves can interact with the internal molecular structure 299 of compatible materials. These materials do also include carbon sources such as sugars 300 and biomass/biowastes, which are polar in nature; and thus, microwave-active [43]. These 301 biowastes may encompass celluloses that can be derived from sugar bagasse and other 302 plant wastes [41]. Other usable material are dairy manure as reusable wastes in the 303 production of graphene-like lamellar hydrochars as synthesized by Gao et al (2018) [44]. 304 The said study reported on the shortened reaction times as required to prepare these hy-305 drochars at the operating temperatures of 240°C with water as the media. The value-306 added liquid by-products (light oils) were then extracted using diethylether. Another 307 study reported on a facile and green HTC synthesis of a hydrochar through the carboni-308 zation a sugar (glucose). This reaction took place at 200°C in deionized water over varied 309 reaction times between 5 -60 minutes, resulting in largely spherical microparticles after 310 45 minutes. These reaction times are a great improvement from the conventional heating 311 methods where this product would be obtained after hours and hours of reaction periods. 312 The envisaged applications of such materials include electrodes, adsorbents, and catalyst 313 supports, among others [45]. 314 In another interesting study, Adolfsson and co-workers presented an HTC carboni-315 zation and post-modification of solid phase polypropylene in its upcycling as a waste ma-316 terial. The method, as described, was one of the few in literature designed to repurpose 317 waste plastics via microwave-assisted HTC. Furthermore, this method yielded high car-318 bonization in water while it was incomplete in air. In addition to this, the successful mod-319 ification with SiC was also caried at relatively low temperatures. These achievements are 320 credited to the in-vessel and self-generated pressure forming favourable subcritical con-321 ditions in water [46].  (2016) reported on 100% yields of 326 highly ordered, single-layered rGO from a MWA-HTC [47]. Herein, the approach was a 327 simple one in that the GO obtained using the Modified Hummers' method, was coagu-328 lated by a stream of a CaCl2 solution into the reaction vessel in the presence of argon. Short 329 bursts (1 -2s) of MW irradiation were then introduced, leading to the reduction of GO; 330 the formation and annealing of rGO because of the high in-situ temperatures (arcing) gen-331 erated. The intended application of this annealed rGO was towards oxygen evolution re-332 actions, however, we have already established earlier in this chapter the multitude of ap-333 plications of such advanced carbon materials. Thermal annealing using microwave irra-334 diation as a reduction approach has been reviewed by Lyu et al (2018) as requiring tem-335 perature bursts ca. 2000°C to occur [48]. Jakhar et al (2020) suggested in his review that 336 the microwave-thermal annealing method provides superior exfoliation of GO than MWA 337 HTC [49]. 338 Another study recently presented a two-step preparation of N-and B-doped C-com-339 posites prepared from fir bark with ammonium tetraborate as the dopant source. The re-340 sult of the doping yielded greatly changed surface morphologies and consequently, high 341 surface areas (955 m 2 .g −1 ) and enhanced capacitance retention (above 90%). These proper-342 ties can be credited to the favourable synthetic method which resulted in the synergistic 343 improvement of the said properties [50]. 344 In another study, the microwave plasma approach to produce large-scale N-doped 345 graphene (N-G) from simple ammonia and ethanol as the precursors. The advantage of 346 this method was that it required no toxic solvents; it is a one-step approach, and as such, 347 it is both cost and environmentally friendly. Furthermore, the reaction takes place at am-348 bient conditions and results in a product possessing high electrical conductivities due to 349 the high purity of the N-G prepared [5]. However, further developments have recently 350 shown that the use of susceptors (supports possessing higher microwave conductivities 351 than the sample material) can significantly enhance the exfoliation as compared to the 352 above studies [49]. However, at this point in science, the superiority of MWA reduction 353 and annealing of GO seems to largely apply to the exfoliation and inclusion of heteroa-354 toms into the aromatic structure of graphenes. Thus, this development does not negate 355 the importance of MWA HTC in terms of the synthesis and modification of GO, rGO, and 356 hydrochars. Water is a scarce and precious resource the world over, and as such, it is critical to 362 develop methods and materials to harness, protect, and recover it however possible. Nev-363 ertheless, climate change and environmental (air/water/soil) pollution remain an unfor-364 giving threat to water sustainability and air quality. Current and conventional methods 365 of addressing these challenges are not sufficient. This is where carbon-based membrane 366 nanotechnology has become much more important in the recent years [43,45]. With this 367 knowledge, sustainable, fast, and green methods are needed to this effect to develop suit-368 able membranes and membrane materials. Microwave-assisted carbonization towards the 369 synthesis of these materials are a greatly promising and exciting technologies [51]. 370 Other than carbonization, MWAS was recently used by Ashfaq et al (2020) to hydro-371 thermally form a polyacrylic acid (PAA) film on a GO-modified commercial TFC mem-372 brane. This reaction was also achieved in short periods of 40s, with the result being an 373 antibiofouling and anti-scaling membrane against H. aquamarina (97%) and metal salts 374 (CaCl2 and Na2SO4), respectively. The membrane hydrophilicity and salt rejections were 375 also enhanced with a decrease in permeability due to the grafting which narrowed the 376 pore size [52]. An alternative to this method, however, would be conduct the graft 377 polymerization of the AA to GO in-situ with the RO membrane within the MW reactor 378 cell. With the control of parameters such as the monomer concentration and GO loading, 379 a membrane with enhanced bioactive/antibiofouling is attainable due to the increased 380 availability of crosslinked GO towards the active layer. 381 Kovtun et al presented a scalable microwave-assisted synthesis (MWAS) method of 382 attaching layered GO to waste polysulfone (PSU) plastics and prepared a mixed matrix 383 membrane (MMM) from the resulting PSU-GO-MW nanocomposite. The PSU-GO syn-384 thesis method was a simple (100°C), fast (45 min), green, and cheap one as no solvents 385 were required; and only an ethanol-water solution was used for washing the product. For 386 comparison, the same product was prepared by conventional oven heating at 200°C (PSU-387 GO-OV). Chemical analysis by X-ray photoelectron spectroscopy (XPS) indicated a certain 388 degree of collapse to the molecular structure of the GO nanosheets. The PSU-GO-MW 389 permutation, as such, presented higher Rhodamine B (RhB) and ofloxacin adsorption ca-390 pacities than both the PSU-GO-OV and pristine PES membranes [53]. This indicates that 391 the microwave chemistry approaches for advanced carbon materials yield MMMs with 392 superior properties as compared to conventional heating. 393 The important discovery of MWAS-HTC has led to critical contributions towards the 394 development of environmentally friendly synthesis methods. The research on the prepa-395 ration of hydrochars has resulted in significant interest within the research fraternity of 396 water treatment and membrane nanotechnology. One of such studies to this effect was 397 recently carried out by Hossain et al (2020) [54]. In this work, rice husk, one of the waste 398 biomass that continue to generate increasing research interest, was used to prepare a hy-399 drochar as a sustainable which can be included in the fabrication of MMMs for water 400 treatment. The authors synthesized this biochar using a temperature of 70°C over a reac-401 tion period of 20 minutes at 70 bars working pressure. As such, these reaction parameters 402 improved adsorption capacities and catalytic efficiencies. 403 404 Coal gas coming directly from the bench was historically a noxious chemical soup 405 and it was necessary to eliminate the most deleterious fractions, to increase the 406 consistency of the gas, to avoid exposure to machinery or premises [55]. As such, the elim-407 ination of hydrogen sulfide was assigned the highest priority level at the gas works. To 408 remove the sulfuret of hydrogen, known as the purifier, there was a special building [56]. 409 The purifier, if the retort-bench itself is not used, was arguably the most significant facility 410 in the gas-works [57]. Originally, purifiers were simple lime-water containers, also re-411 ferred to as lime cream or milk, where the raw gas from the retort bench was bubbled out 412 to extract hydrogen sulfide [58]. This initial purification method was known as the process 413 of "wet lime". One of the first real wet lime" was the lime residue left over from the "toxic 414 wastes" process, a substance called blue billy" [59]. 415 Recently, researchers have begun to work on redesigning synthetic methods using 416 MW chemistry for many smart materials, have been found to be quite useful in the fields 417 of power generation, biomaterials, nanoelectronics, and nanomedicine, among others. Re-418 searchers, scientists, and industrialists are following environmentally benign synthetic 419 routes for the fabrication of desired goods in the age of new science and technology. Of 420 the twelve (12) principles of green chemistry, two major criteria for MW-assisted synthetic 421 chemistry are choice of green solvents and energy efficiency. In many systems, MW heat-422 ing is seen as a more effective way to manage heating because it is less energy-intensive 423 than traditional methods [60]. The 'Green Chemistry Monograph of the American Chem-424 ical Society' proposed the use of catalysts or MW irradiation to decrease the energy re-425 quirements for the synthesis process [61]. 426 Membrane nanotechnology is emerging to be a critical approach in not only improv-427 ing selectivity and affordability of gas sensing, fuel cells, but also gas purification [62]. 428 Membrane nanotechnology is critical in this regard to assist in the efforts towards the 429 mitigation of the effects of air pollution (industrial greenhouse emissions), global warm-430 ing, and climate change. Carbon-based nanocomposite membranes have incorporated 431 GO, carbon nanotubes (CNTs), and graphenes as nanofillers towards gas purification ap-432 plications such as separating mixtures of CO2/CH4, and the difficult removal of H2S from 433 biogas [63]. Furthermore, they can be used in the separation/capture/adsorption of other 434 gases mixtures of CO2/H2, H2/CH4, and N2/H2, among others as reviewed by Sazali (2020) 435 [64]. In this work, the author favours the use of carbon membranes over their polymeric 436 counterparts, citing limitations including operating temperatures, poor selectivity in 437 terms of solubility and diffusivity for H2 and CO2 gases. As such, these challenges influ-438 ence the design, improvement, and fabrication of such membranes by modifying them 439 with carbon materials. 440 There are further synthetic methods that have been developed to this effect regarding 441 the green hydrothermal approaches of these carbon materials towards the development 442 of these nanocomposite membranes. One work demonstrated the nanoindentation and 443 the further modification of vertical array C nanotubes (VACNT) via the in-situ polymeri-444 zation of aniline. The facile method yielded nanocomposite membrane with a seamless 445 deposition of polyaniline (PANi) onto the said nanotubes, achieving a nanocomposite 446 with enhanced gas transport properties [65]. Figure 4 illustrates the synthesis route and 447 the transmission electron microscopy (TEM) images for the resulting nanocomposite as 448 presented in Figure 5. In the envisaged satisfaction of the rising global energy demands and to simultane-458 ously combat the environmental impacts such as global greenhouse gas emissions, it is 459 critical to search for potential energy alternatives [66]. Natural gas is one of such vital 460 components of the world's supply of energy that has fulfilled the previously mentioned 461 requirements. Natural gas is a cleaner energy source compared to other fossil-based en-462 ergy sources owing to its low emissions [67]. However, natural gas contains acidic gases 463 (including CO2, N2, Hg, He, and H2S), which may cause equipment corrosion and envi-464 ronmental damage [68]. To date, Kazmi et al., 2019, researched on amine-based absorption 465 techniques that have been used to remove acidic gases from natural gas to reach regulated 466 concentration limits. However, a tremendous amount of heating is required to regenerate 467 amine-based solvents, which remains a major issue with traditional absorption-based acid 468 gas removal units [69]. 469 Twenty years after the first production of solvothermal reactions, it seems important 470 to trace future developments, taking into account their potential and the various economic 471 constraints, through the current research activities [70]. Solvothermal reactions have been 472 used primarily in the processing of micro-or nanoparticles of various morphologies over 473 the last twenty (20) years. Such a presentation would only concentrate on the potential of 474 solvothermal reactions in material synthesis, considering the importance of disposing of 475 new materials towards the production of either fundamental science or industrial appli-476 cations [71]. Researchers have investigated future materials that will be prepared by sol-477 vothermal methods, an example of this was illustrated in the work by Pahinkar and 478 Garimella (2018) [72]. The authors of the said work discovered a novel TSA-based gas 479 separation cycle using a microchannel monolith coated along the inner walls of each mi-480 crochannel with a hollow polymer-adsorbent matrix was investigated. By passing impure 481 feed gas through the microchannels, CO2 is eliminated from CH4, followed by a concur-482 rent flow through the same microchannels of desorbing hot liquid, cooling liquid, and 483 purging gas. Owing to the intimate interaction with the transport fluid and the adsorbent 484 sheet, this configuration is supposed to improve the heat and mass transfer to the adsor-485 bent, and thus decrease the total device size because of the flow into the same microchan-486 nels of the operating and coupling fluids. Computational models are built in this part one 487 of a two-part analysis to study the related fluid dynamics, heat transfer and mass transfer 488 in each phase. Parametric experiments are done to determine the optimal geometry of the 489 microchannel and components for adsorbent and heat transfer fluid (HTF). In contrast 490 with bed-based designs, the process is supposed to purify up to two orders of magnitude 491 higher gas throughput. A detailed process efficiency chart and optimization of energy 492 needs for the process are addressed in the accompanying report. 493 Zhu et al., 2019 studied the environmental purification and engineering because of 494 the importance of effective yield of reactive-oxygen species (ROS). In their study, the per-495 fected p-conjugated g-C3N4 (PNa-g-C3N4) photocatalysts were constructed at the vacancy 496 structure of tri-s-triazine polymer for ROS evolution and HCHO and NO removal by co-497 ordination between 3p orbits of Na and N 2p lone electron. The structured p-conjugated 498 structure increases the ability to absorb visible light, enriches active O2 activation sites, 499 and promotes the directional charge transfer from N 2p of C3-N to Na and C. Superior 500 operations, including the evolution of O2 (35 mmol.L -1 ) and H2O2 (517 mmol.L -1 ) over the 501 photocatalyst PNa-g-C3N4, have therefore been accomplished. Consequently, PNa-g-C3N4 502 photocatalysts display high performance removal efficiency of NO (53% over 6 min con-503 tact time) and HCHO (almost 100% over 55 min). The findings may provide a promising 504 strategy for developing an effective photocatalytic device to produce ROS for environ-505 mental purification [73]. Wiheeb et al (2013) investigated hydrogen sulfide (H2S). Hydro-506 gen sulfide includes biogas, natural gas, and synthesis gas from coal gasification, which 507 is extremely poisonous to humans and corrosive to devices. Prior to utilization, H2S must 508 be separated from fuel gases. The goal of Wiheeb et al., 2013 was to equate the use of 509 commercial and alkaline impregnated activated carbons to the adsorption of H2S. The 510 commercial and alkaline impregnated activated carbons were tested for adsorption of H2S 511 at 30 o C and 550 o C by the temperature program. Alkaline activated carbons adsorbed H2S 512 much higher than commercial activated carbon at elevated adsorption temperatures (3-29 513 times higher depending on the process of modification). In addition, the concentration of 514 H2S in the outlet gas after the KOH and Na2CO3 impregnated activated carbons were 515 treated was less than 30 ppm, which was safe for mechanical and power engine use [74]. 516 Solvothermal carbonization (STC), as previously mentioned, require the use of sol-517 vents during the carbonization of carbon precursors. The solvents commonly used in in 518 this approach are usually ionic (and toxic) in nature, with some examples being p-toluene 519 sulfonic acid, o-dichlorobenzene, and 1,2,4,5-tetraaminobenzene, dimethylformamide 520 (DMF), among others [17,75]. In one study, using DMF as solvent in a solvothermal 521 method, a graphene/metal-organic framework was prepared and it yielded high adsorp-522 tion capacities for nitrogen gas and benzene [76]. Furthermore, this approach is will often 523 require the use of high reaction temperatures, another unwanted reaction parameter as it 524 contributes to high costs. However, there has been several developments to mitigate or 525 eradicate the use of high temperatures and toxic solvents. These solvents can also be 526 expensive and as such, their negative impacts on the environment need to be reduced. 527 Nevertheless, many works have reported on the use of ethanol a green as solvent, and the 528 carbon source being biowastes [77]. An example of such an investigation was conducted 529 by Jin et al (2017). In this work, the authors synthesized a magnetic nanocomposite mate-530 rial (Fe3O4/C) from keratin-rich chicken feathers as beginning biowaste in the presence of 531 ethanol [78]. 532 Habartová et al., 2013 worked on chloroform (CHCl3) and water vapor gas-phase 533 treatments that have been used to extract metal impurities (MIs) from as-prepared multi-534 walled carbon nanotubes (AP-MWCNTs), which are a non-destructive means of pro-535 cessing highly pure CNTs with exceptionally low MI content and CNT property preser-536 vation. The MIs in the AP-MWCNTs decreased dramatically to 12 ppm after purification 537 using CHCl3, reflecting a purification efficiency of 99.8%. This method of purification, ap-538 plicable with various AP-MWCNTs with a high content and different MI compositions, is 539 more efficient than acid-based liquid-phase purification and does not affect the structure 540 and morphology of the CNT. A mixture of factors is responsible for this mild purification. 541 Chlorine radicals and hydrochloric acid from the thermal decomposition of CHCl3 react 542 to form metal chlorides that sublimate at high temperatures with MIs encased by graphitic 543 carbon layers. During the forming of the metal chlorides, hydrogen, oxygen, and water 544 vapor created etch graphitic carbon layers and lead to the formation of new pores and 545 cracks that provide quick reaction and diffusion pathways. The MWCNT surface becomes 546 lightly covered with chlorine or chlorinated carbon during metal purification and is ex-547 tracted using water vapor from the surface post-treatment [79]. 548 Choudhury et al., 2017 investigated one of the pollutants to be nitric oxide (NO from 549 anthropogenic pollution, which creates multiple environmental issues. An alternative 550 technology for reducing NO is provided by microbial fuel cells (MFCs) with a gas diffu-551 sion cathode. In this work, pure NO was confirmed as the single electron acceptor of gas 552 diffusion cathode (NO-MFC) MFCs. The overall power density of the NO−MFCs was 489 553 ± 50 mW/m 2 . The columbic performance improved from 23.2 % ± 4.3 % (Air−MFCs) to 55.7 554 % ± 4.6 % (NO−MFCs) relative to MFCs using O2 in air as an electron acceptor (Air−MFCs). 555 The rate of NO elimination was 12.33 ± 0.14 mg/L/h and the principal reduction product 556 was N2. The dominant route of NO conversion in NO-MFCs, including abiotic electro-557 chemical reduction and microbial denitrification process, was cathode reduction. In the 558 anodic microbial culture, the prevalent genera modified from exoelectrogenic bacteria in 559 Air-MFCs to denitrifying bacteria in NO−MFCs and effected the power generation [73]. 560 Because of the problems associated with the release of CO2 to the environment, Liu 561 et al (2017) presented a solvothermal method for the preparation of boron and nitrogen-562 doped 3D graphene aerogels for carbon capture. This synthesis was carried out at working 563 temperatures of 180°C over a reaction time of 6h [80]. This section further discusses carbon nanodots and their synthetic routes as per ini-568 tially introduced in the previous sections. This is because of their rising importance over 569 the years due to their versatility. Carbon nanodots (CDs) refer to a class of zero-dimension 570 carbon nanoparticles such as carbon quantum dots (CQDs), graphene quantum dots 571 (GQDs), carbonized polymer dots (CPDs), carbonized polymer dots (CPDs) and various 572 carbogenic nanodots (CNDs) [81,82]. Their carbon cores is usually sp 2 hybridized with 573 different functional groups such as amino group, epoxy, ether, carbonyl, aldehyde, hy-574 droxyl, and carboxylic acid functional groups on the surface [81,83]. They are quasi-spher-575 ical, amorphous to crystalline in nature with size less than 10 nm as shown in Figure 5 576 [81]. These possess unique optical properties (photoluminescence) and differing physico-577 chemical properties [81].

MWAS of carbon-based nanocomposites for gas purification
These nanomaterials and their nanocomposites are 578 characterized by facile synthetic methods, water solubility, low cost, biocompatibility, low 579 toxicity, and chemical inertness. Furthermore, CDs are abundant because of the use of 580 inexpensive precursors resulting in sustainable synthesis [84,85]. As a direct result of this, 581 materials have found widespread applications in anti-counterfeiting, sensing, bioimaging, 582 optoelectronic, energy-related fields, and even wastewater treatment [84][85][86]. 583 Various synthesis methods have been developed for fabrication of CDs where these 584 include electrochemical exfoliation of a graphitic source [82]; the incomplete combustion 585 of carbon soot [84]; the carbonization of polymerized resoles on silica spheres [87]; the 586 thermal oxidation of suitable molecular precursors [88]; and also the dehydration of car-587 bohydrates [89]. Most of these methods often require complex synthetic controls which 588 may cause adverse degradation of the required CD properties such as poor crystallinity, 589 introduction of impurities, extensive post-treatments techniques, complicated purifica-590 tion and separation procedures [90]. Previous reviews have detailed synthesis strategies 591 and different applications of CDs in bioanalysis, bioimaging, and energy conversion. Such 592 collated knowledge greatly improved our level of understanding and promoted the de-593 velopment of CDs in works that followed [90]. Nevertheless, to obtain CDs with high pu-594 rity and better physico-chemical properties is still faces high challenges due to the com-595 plicated purification and separation procedures [90]. 596

Synthesis of CDs through solvothermal synthesis 597
Commonly, the synthesis method is categorized as "top-down" and "bottom-up" as 598 per Figure 6. In the top-down approach, large carbon structures are converted into quan-599 tum-sized and fluorescent carbon quantum dots [85]. In the bottom-up method, the CDs 600 are obtained from carbonizing small molecules as precursors [85,91]. These include elec-601 trochemical exfoliation of a graphitic source [82], incomplete combustion of carbon soot 602 [84], carbonizing polymerized resoles on silica spheres [87], thermal oxidation of suitable 603 molecular precursors [88], and the dehydration of carbohydrates as previously noted [89]. 604 Studies of these synthetic methods indicate that the obtained CD structure and composi-605 tions is closely related to the synthetic methods and the nature of carbon sources or the 606 molecular precursors [83]. 607 608 Figure 6. Different Synthesis methods of CDs courtesy of Long et al [86]. 609 In the past few years, there have been several studies and reviews on the develop-610 ment of facile synthetic methods that generate enhanced physicochemical properties and 611 surface functionalization for various applications [85]. From these methods hydrother-612 mal/solvothermal synthesis have continued to gain popularity [90]. These methods in-613 volve the use of inexpensive precursors, simple synthetic processes, environmentally 614 friendly approaches, large instruments, and non-toxic routes [85]. Since most of the 615 prepared CD-based materials already possess a mass of hydrophilic functional groups, no 616 additional modification treatment is required to impart hydrophilic properties and reac-617 tivity to the CDs [85]. An example of this was presented by Zhu et al. [92] where they 618 developed a simple microwave-assisted method to synthesize new types of CDs. In that 619 work, a carbohydrate was used as a carbon source and polyethylene glycol (PEG200) em-620 ployed as both solvent and coating agent. The reaction gradually changed from colorless 621 to dark brown solution when under 500W microwave irradiation for a duration ranging 622 2∼10min. The product was diluted with water to attain fluorescent CDs. The obtained 623 particle size and the quantum yields of fluorescent CDs were observed to depend on the 624 reaction time [92]. 625 Recent studies indicate that short external heat pulses can contribute to the chemical 626 oxidation and carbonization of organics and turn them into CDs. Microwaves with fre-627 quencies ranging from 300 MHz to 300 GHz provide sufficient energy to break the chem-628 ical bonds in the raw materials [85]. The microwave-assisted hydrothermal/solvothermal 629 synthesis provide uniform heating effectively reducing the reaction time, so that the par-630 ticle size distribution of CDs is small. Additionally, MWAS does not require hydrothermal 631 reactor for the sealing and reaction of the organic precursor at high pressure, high tem-632 peratures, and long reaction times compared with conventional synthesis [93]. 633 634 Although CDs have various advantages, they also suffer from problems associated 635 with aggregation of pristine CDs in solid state. This often leads to property change such 636 as luminescence quenching, surface defects, and decreasing surface areas [81]. To mitigate 637 these unwanted phenomena, and to further optimize their optical and electrical proper-638 ties, various solid supports such as polymer matrices, inorganic salts and porous materials 639 (PMs), etc., have been used to support CDs [94,95]. To prepare such nanocomposites, sol-640 vothermal and hydrothermal synthesis techniques are in the forefront in establishing var-641 ious modified CDs. Typically, for CD nanocomposites, two main synthetic approaches 642 have been proposed, i.e., a one-step method and a two-step method [83]. 643 The one-step method means the simultaneous generation of the CDs into the sup-644 porting matrix in the same reaction is carried out. The two-step method means that the 645 CDs are first prepared using the top-down/down-top route and then embedded in the 646 host matrices via chemical or physical methods [83]. A good example of the latter was 647 demonstrated by Ming et al [90] where a novel photocatalyst (TiO2/CD) by combining CD 648 with TiO2 through an easy hydrothermal method. The obtained TiO2/CDs exhibited ex-649 cellent visible-light photocatalytic activity. The CDs where initially fabricated through 650 electrochemical exfoliation of a graphitic source as follows; dropwise addition (1 mL 651 min−1) of Ti-[OCH(CH3)2]4 (1 mL, dissolved in absolute alcohol with a ratio of 1:19) into 652 CDs solution (55 mL, 0.1 mg mL−1). After continuous stirring of the mixture for 4 h, a 653 colloidal solution of TiO2/CDs nanohybrids then formed. The resulting solution was 654 sealed into a Teflon-lined autoclave, followed by hydrothermal treatment at 180 °C for 48 655 h. The obtained gray solid TiO2/CDs was washed with water and ethanol, and dried in a 656 vacuum oven at 80 °C. 657 The direct in situ incorporation of CDs into the host matrix is somewhat more diffi-658 cult with very few literature examples [83]. This is mostly due to difficulties in controlling 659 the simultaneous generation of CDs in one system and the requirement of a strong driving 660 force to co-assemble the guest CDs into the host matrix [96]. Nevertheless, Wang et al [97] 661 recently synthesized luminescent CDs@zeolite and CDs@AlPO nanocomposites through 662 direct in-situ incorporation of luminescent CDs into the host matrix using the solvother-663 mal approach. In this method, triethylamine was utilized as the template and triethylene 664 glycol as the solvent to form uniformly embedded CDs (≈3.7 nm) in AlPO-5 crystals as 665 shown in Figure 7.  [81]. 669 The successful one-step synthesis of CDs@zeolite composites was credited to the fac-670 ile solvothermal synthetic method. The end-result was the highly dispersed CDs due to 671 the well-confined nanospaces and high stability of zeolites. In this case, the organic species 672 (i.e., templates and solvents) used in the synthesis of zeolites provide the source materials 673 for CDs and the simultaneous formation of CDs and zeolites can further be achieved by 674 varying certain synthetic conditions [83]. The use of one-step reaction processes not only 675 simplify the preparation procedure compared with the multistep preparation method, the 676 prepared CDs also indicate enhanced fluorescence, crystallinity, and uniform size [98]. 677 Different CDs with varying properties can be achieved by choosing different solvents (car-678 bon source) [98]. 679 So far, a variety of PMs (zeolites, MOFs, mesoporous materials, and other disordered 680 porous nano-carries) have been used as host matrix for CDs [81]. Given the diversity and 681 inherent features of PMs and CDs, and the capability of solvothermal and hydrothermal 682 synthesis. A synergistic approach using a mixed synthesis approach of solvothermal syn-683 thesis is therefore possible [81]. In another work, Liu et al [99] used a multi-step synthetic 684 approach to synthesize CD-zeolite by carbonizing CDs in-situ, encapsulating the CDs 685 into zeolite crystals produced by hydrothermal crystallization (Figure 8). This strategy 686 allowed CDs to be tightly confined in the interrupted zeolite framework and form abun-687 dant H-bonds. This resulted in the restricted vibration/rotation of functional groups of 688 CDs, and thus protected the triplet excitations. The produced composite indicated unique 689 thermally activated delayed fluorescence (TADF) emissions. In contrast, no TADF was 690 observed for pure CDs isolated from the synthetic mother liquid [81].  [81]. 695 Pyrolyzing organic solvents or guest molecules confined in MOFs (CDs@MOF nano-696 composites) can also be feasibly prepared. Unlike zeolites, the carbonization of MOFs is 697 usually conducted below 200°C due to their inferior thermal stability. In addition, the one-698 step synthetic method used for synthesis of CDs@zeolites is generally not possible. This is 699 because the precursors of CDs influence the crystallization of MOFs and the synthetic 700 temperatures required for the preparation of MOFs is also much lower than the carboni-701 zation temperature of CDs [81,100]. This means that the most suitable synthetic routes for 702 carbon-modified MOFs are the LTHP or LTLP approaches as discussed in subsection 4.1. 703

704
The importance of the synthetic methods as applied in the preparation of advanced 705 carbon materials was discussed in this book chapter. These synthesis methods were dis-706 seminated in terms of their developments in terms of the current developments from con-707 ventional to sustainable materials and environmentally friendly hydrothermal and sol-708 vothermal carbonization approaches. As has been discussed, the use of green solvents and 709 sustainable carbon sources has drawn great attention. Water and ethanol are favored as 710 green solvents where solvents, biomass or biowastes have proven to be sustainable car-711 bon-negative materials as carbon precursors. Special attention was also paid to the use of 712 microwave-assisted hydrothermal and solvothermal carbonization (MWAS HTC and 713 STC). The studies, as discussed, show that the latter methods can be applied during the 714 synthesis of advanced carbon materials, yielding different morphologies and physico-715 chemical properties. These properties allow for the (future) application of these materials 716 in the fabrication of nanocomposites in energy, sensing, photocatalysis, electrocatalysis, 717 and water treatment. These can be further be indiscriminately subdivided to fuel cells, 718 photovoltaics, the degradation/adsorption of in-/organic environmental insults; and in 719 membrane nanotechnology for gas separation/storage/adsorption; and water purification.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the 734 design of the study; in the collection, analyses, or interpretation of data; in the writing of the manu-735 script, or in the decision to publish this work. 736