gels Review Review on Sol-Gel Synthesis of Perovskite and Oxide Nanomaterials Daniel Navas 1 , Sandra Fuentes 2,3, *, Alejandro Castro-Alvarez 4 and Emigdio Chavez-Angel 5, * 1 Departamento de Química, Facultad de Ciencias Naturales, Matemática y del Medio Ambiente, Universidad Tecnológica Metropolitana, Las Palmeras 3360, Ñuñoa, Santiago 7800003, Chile; daniel.navas@utem.cl 2 Departamento de Ciencias Farmaceúticas, Facultad de Ciencias, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta 1270709, Chile 3 Center for the Development of Nanoscience and Nanotechnology, CEDENNA, Av. Libertador Bernardo O’Higgins 3363, Santiago 9160000, Chile 4 Laboratorio de Bioproductos Farmacéuticos y Cosméticos, Centro de Excelencia en Medicina Traslacional, Facultad de Medicina, Universidad de La Frontera, Av. Francisco Salazar 01145, Temuco 4780000, Chile; alejandro.castro.a@ufrontera.cl 5 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain * Correspondence: sfuentes@ucn.cl (S.F.); emigdio.chavez@icn2.cat (E.C.-A.) Abstract: Sol-Gel is a low cost, well-established and flexible synthetic route to produce a wide range of micro- and nanostructures. Small variations in pH, temperature, precursors, time, pressure, atmosphere, among others, can lead to a wide family of compounds that share the same molecular structures. In this work, we present a general review of the synthesis of LaMnO3 , SrTiO3 , BaTiO3 perovskites and zinc vanadium oxides nanostructures based on Sol-Gel method. We discuss how small changes in the parameters of the synthesis can modify the morphology, shape, size, homogene-   ity, aggregation, among others, of the products. We also discuss the different precursors, solvents, Citation: Navas, D.; Fuentes, S.; working temperature, reaction times used throughout the synthesis. In the last section, we present Castro-Alvarez, A.; Chavez-Angel, E. novel uses of Sol-Gel with organic materials with emphasis on carbon-based compounds. All with a Review on Sol-Gel Synthesis of perspective to improve the method for future applications in different technological fields. Perovskite and Oxide Nanomaterials. Gels 2021, 7, 275. https://doi.org/ Keywords: Sol-Gel; nanoparticles; perovskites; metal oxides 10.3390/gels7040275 Academic Editor: Andrei Jitianu 1. Introduction Received: 3 November 2021 Sol-Gel method is one of the simplest techniques to synthesize high-quality nano and Accepted: 15 December 2021 microstructures. This method provides several advantages over other synthesis routes such Published: 18 December 2021 as control over the texture, size and surface properties of the materials, easy to implement, low cost, high quality, and production of materials with large surface areas [1]. This Publisher’s Note: MDPI stays neutral flexibility and simplicity make it very popular in the production of nanoscale powders [2–4] with regard to jurisdictional claims in and its wide used as a coating method [5–7] (see Figure 1). published maps and institutional affil- iations. The first works on the Sol-Gel synthesis date from the mid-1800s with the pioneer studies of Ebelman [8] and Graham [9] on the fabrication of silica gels. The interest in gels grew very fast and notable researchers added their contributions to the field. Ostwald [10] and Lord Rayleigh [11] study the formation of Liesegang rings and the growth of crystals in gel potassium dichromate [12]. In the 1950s, Roy et al. extended its use and a large variety Copyright: © 2021 by the authors. of ceramics were fabricated [13,14]. Years later, in the 1970s, Dislich presented new routes of Licensee MDPI, Basel, Switzerland. synthesis of multicomponent oxide glasses [15] and the Sol-Gel method became a popular This article is an open access article route to fabricate bulk glasses and ceramics. Since then, a number of researchers have distributed under the terms and conditions of the Creative Commons contributed to the development of the Sol-Gel technology. A deep historical background of Attribution (CC BY) license (https:// the Sol-Gel process and its evolution is presented by Sakka [16]. creativecommons.org/licenses/by/ 4.0/). Gels 2021, 7, 275. https://doi.org/10.3390/gels7040275 https://www.mdpi.com/journal/gels Gels2021, Gels 2021,7,7,275 275 2 2ofof1920 Figure1.1.Top: Figure Top:Number Numberof ofpublications publicationsin inthe thelast last25 25years yearsincluding includingthe thekeywords keywords“Sol-Gel” “Sol-Gel”(green), (green), “Sol-Gel” + “oxide” (yellow), “Sol-Gel” + “coating” (grey), “Sol-Gel” + “nano” “Sol-Gel” + “oxide” (yellow), “Sol-Gel” + “coating” (grey), “Sol-Gel” + “nano” (red) (red) and “Sol-Gel” and + “Sol-Gel” “perovskite” (blue). + “perovskite” Bottom: (blue). table Bottom: of number table of number of publication of publicationin the with in the the the with number of publication number in of publication in 2020–2010 2020–2010 period. period. All All the the datadata waswas extracted extracted from from webweb of science of science [17].[17]. The TheSol-Gel Sol-Gelmethod methodinvolves involvesthetheuse useof ofaacolloidal colloidalsolution solution(Sol) (Sol)that thatevolves evolvesinto intoaa gel-like gel-likenetwork networkincluding includingboth bothaaliquid liquidand andaasolid solidphase phaseasasaaresult resultofofseveral severalchemical chemical reactions. reactions. ItIt can can be bedivided dividedinto intotwo twotypes: types:aqueous aqueousor orhydrolytic hydrolyticandandnon-aqueous non-aqueousor or non-hydrolytic. non-hydrolytic. The The aqueous aqueous Sol-Gel Sol-Gel process process can canbebedescribed describedin infive fivesteps: steps:hydrolysis, hydrolysis, condensation, condensation,aging,aging,drying dryingand andcrystallization crystallization[18] [18](see (seeFigure Figure2). 2). Gels 2021,Gels 2021, 7, 275 7, 275 3 of 20 3 of 19 Figure 2.Figure 2. Schematic Schematic representation representation of step-by-step of step-by-step Sol-Gel method. Sol-Gel method. In the first step (hydrolysis) the metal alkoxide (MA) precursors and solvents are In the first step (hydrolysis) the metal alkoxide (MA) precursors and solvents are mixed. In this step, a nucleophilic attack takes place on MA by the oxygen from the mixed. In this step, a nucleophilic attack takes place on MA by the oxygen from the water. water. As a result, simultaneous removal of alkoxide functional group and formation of As a result, simultaneous removal of alkoxide functional group and formation of alcohol alcohol functional group (M − OH) occurs. The hydrolysis and condensation reactions are functional group (M − OH) occurs. The hydrolysis and condensation reactions are strongly strongly affected by process parameters such as the nature of the R-group, alkoxy functional affected by process parameters such as the nature of the R-group, alkoxy functional group group steric hindrance, temperature, pH, use of aqueous or non-aqueous media or organic steric hindrance, temperature, pH, use of aqueous or non-aqueous media or organic sol- solvents like alcohols, the ratio of solvent to MA or acetylacetonates as precursors and the vents like usealcohols, the ratio of and concentration of solvent catalyststo(acid MAor oraacetylacetonates base). All of theseasvariables precursors canand make the synthetic use andcontrol concentration of catalysts (acid or a base). All of these variables can difficult through this route. The structure of the resulting gel is significantlymake syn- thetic control difficult different through depending onthis the route. TheThis catalyst. structure is due of to the the resulting gel isofsignificantly relative rates the hydrolysis and differentcondensation depending on the catalyst. This is due to the relative rates of the hydrolysis reactions. In general, the hydrolysis step gets progressively slower and under condensation reactions. In general, the hydrolysis step acidic conditions and faster under basic conditions [19]. gets progressively slower under acidic conditions and faster under The condensation basic conditions step or polycondensation [19].is associated to the formation of a polymer Thenetwork condensation made of metal oxides linkages as aassociated or polycondensation step is result of the to elimination the formation of of a poly- solvent (water or mer network made of metal oxides linkages as a result of the elimination alcohol) to form a sol. This process increases the viscosity of the solution as of solvent (water the polymer or alcohol) to form network a sol. This grows, process forming increases a porous the viscosity structure withinofthe theliquid solution as the phase polymer (Gel). The general networkchemical grows, forming a porous reaction for structureand the hydrolysis within the liquidprocess condensation phase is (Gel). givenThe general below: chemical reaction for the hydrolysis and condensation process is given below: M − OR + H2 O → MOH + ROH (hydrolysis) M-OR + H O → MOH + ROH (hydrolysis) MOH + XO − M → MOM + XOH (condensation) MOH + XO-M → MOM + XOH (condensation) where M = metal, X = H or alkyl Group (Cn H2n+1 ). During the aging step, polycondensation where Mcontinues = metal, X = H orthe within alkyl Groupsolution localized (CnH2n+1).along Duringwiththe aging step, polycondensation precipitation of the gel network, which continues within the localized solution along with precipitation decreases porosity and increases thickness between colloidal of theparticles. gel network, Anotherwhich important decreases porosity and increases thickness between colloidal particles. Another important factor is physical treatment of the sol or gel. Something as simple as evaporation rate during factor isgelation physicalcantreatment of the sol orimpact have a substantial gel. Something as simple on gel structure. as evaporation When rate the liquid is dur- from removed ing gelation canmajor the gel, have changes a substantial to theimpact network on structure gel structure. may When occur. the liquid If the is removed structure is maintained, from theangel, majorischanges aerogel formed—on to thethe network contrary structure may occur. if the structure If the astructure collapses, is formed. xerogel is main- If the tained, liquid an aerogel is formed—on the contrary if the structure collapses, is removed at low temperatures the term cryogels is coined. The aerogel shows a xerogel is high formed.pore If thevolume liquid isandremoved at low temperatures the term cryogels is coined. surface area and xerogel results in low surface area and pore volume. The aer- ogel shows Heat high pore volume treatment is alsoand surface area important and xerogel for drying gels as results well asin low surfacesurface removing area and hydroxyl pore volume. groups, Heat treatment densifying theismaterial also important to producefor adrying ceramic gels as well or monolith as converting removing surface to a crystalline hydroxyl groups,[1]. material densifying the material to produce a ceramic monolith or converting to a crystalline material Compared [1]. with other methods, the Sol-Gel method has many unique advantages, e.g.,: Compared 1. with other methods,special Possibility of obtaining the Sol-Gel method products suchhas many unique as powders, filmsadvantages, or coatings, micro- e.g.,: spheres, fibers; 1. 2. Obtaining Possibility new solids of obtaining specialwith improved products suchproperties; as powders, films or coatings, micro- spheres, fibers; Gels 2021, 7, 275 4 of 19 3. High purity and homogeneity of the materials obtained; 4. Saving energy during the process; 5. Full control over the particle size and morphology; 6. The solution and reaction step allows to incorporate easily, uniformly and quantita- tively some trace elements, achieving a uniform doping at the molecular level. 7. Compared with the solid-phase reaction, the chemical reaction is simpler and only requires a lower synthesis temperature (<220 ◦ C). But also some disadvantages such as, e.g.,: 1. Very sensible to moisture; 2. Difficult to scale up; 3. Can include several steps and is a time-consuming process; 4. Dimension and volume changes during different steps. 2. Synthesis of Perovskites Alkoxide, alkoxide-salt, and Pechini methods are the most popular Sol–Gel-based techniques used in the synthesis of perovskites. The Sol-Gel Pechini method (chelate poly esterification) became the most used due to its versatility in preparing perovskite membranes, depositing dielectric films for the production of capacitors and multicom- ponent oxide materials [20]. One of the most significant advantages of this process is its simplicity and the low-temperature precursor handling. This results in the fabrication of nanopowders with excellent purity and uniformity, as well as precise control of the final composition of the material. Pechini method is also a popular choice for the synthesis of various mixed oxides due to its capacity to combine chemicals (such as lactic, glycolic, citric, and EDTA acids) leading to the creation of polybasic acid chelates with dissolved cations [21]. Chelating agents are employed to prevent partial metal segregation in the final compound, which might occur as a result of various interactions between metal ions in the solution [22]. The polyesterification of chelates occurs when a polyhydroxy alcohol is added to the solution and heated, resulting in a cross-linked chain of metal atoms connected to organic radicals [14]. When two chelating agents work together to complex all metal ions, a precursor solution is formed with all metal ions entirely bound. As a result, a more stable chelate complex system develops and the polymerization is aided by the following addition of ethylene glycol, which enhances uniformity. This method tends to reduce metal segregation after decomposition under heat treatment settings. In the synthesis process and subsequent heat treatment, the ratio of metal precursors to chelating agents is critical. In theory, this ratio should be high enough to ensure that all metal ions are tightly bound to their structures and prevents precipitates in the solution. The Sol-Gel Pechini method offers an excellent control over perovskite structure producing very homogeneous solutions. At temperatures near 1000 ◦ C, this approach results in the creation of pure crystal perovskite structure. Using this method, Shlapa et al. [23] synthesized very homogenous La1-x Srx MnO3 (with 0.23 < x < 0.25) nanoparticles with a very narrow particle sizes distribution between 30–35 nm. They reported that the precursor arises during the reaction between metal ions and organic compounds (citric acid and ethylene glycol) and pyrolysis of the resulting gel. The crystalline perovskite structure emerged in one step at around 600 ◦ C, and the level of crystallinity increased as the treatment temperature was increased. Zhang et al. [24] studied a large variety of novel rare earth-doped (Dy3+ , Eu3+ , Sm3+ , Tb , Yb3+ /Er3+ , Yb3+ /Tm3+ , and Yb3+ /Ho3+ ) La4 Ti3 O12 perovskite-like structures. The 3+ as-synthesized phosphors exhibit characteristic down-conversion (DC) or up-conversion (UC) emissions corresponding to the rare earth activator ions upon UV or NIR excitation. Lima et al. [25] investigated the preparation of red-emitting LaAlO3 :Eu3+ and green- emitting LaAlO3 :Tb3+ phosphors using a modified Pechini’s synthesis and sorbitol as polyalcohol chelating agents. The authors showed that the use of sorbitol as polyalcohol instead of ethylene glycol increases luminescence. A comparison of the Sol-Gel and Sol-Gel Pechini method is given in Table 1. yalcohol chelating agents. The authors showed that the use of sorbitol as polyalcohol in- stead of ethylene glycol increases luminescence. A comparison of the Sol-Gel and Sol-Gel Pechini method is given in Table 1 Table 1. Main characteristic of methods for synthesis by Sol-Gel. Gels 2021, 7, 275 5 of 19 Synthesis Particle Calcination Agglomeration Purity Precursors Observations Methods Size Temperature Table 1. Main characteristic of methods for synthesis by Sol-Gel. Obtaining of uniform Alkoxide or Sol-Gel >10 nm Moderate Excellent 800 °C and small sized Acetylacetonates Synthesis Particle Calcination powders Agglomeration Purity Precursors Observations Methods Size Temperature Accurate control of the Sol-Gel Alkoxide or Obtaining of uniform and Sol-Gel >10 nm >10 nm Moderate Moderate Excellent Excellent Nitrates 800–1000 800 C◦ °C final material compo- Pechini Acetylacetonates small sized powders sition Sol-Gel Accurate control of the >10 nm Moderate Excellent Nitrates 800–1000 ◦ C Pechini Recently, Osman et al. [26] reported the final preparationmaterial of composition Y3+ doped BaCe0.54Zr0.36Y0.10O2.95 (BCZY) powders with various chelating agents as: citric acid, tartaric acid, glycolic Recently,acid, Osmannitriloacetic et al. [26] reported the preparation of Y3+ doped acid, ethylenediaminetetraacetic acidBaCe and0.54 triethylenetetra- Zr0.36 Y0.10 O2.95 mine (TETA). The influence of the molecular weight of the chelating (BCZY) powders with various chelating agents as: citric acid, tartaric acid, agent and functional glycolic acid, group on thermal nitriloacetic acid, decomposition, phase formation ethylenediaminetetraacetic acid and morphology of the samples triethylenetetramine (TETA). were The studied. influence The different of the typesweight molecular of chelating of the agents significantly chelating agent and affected functional thegroup elemental com- on thermal position, especially decomposition, on formation phase B site of ABO perovskite structure and3morphology of BCZY. of the samples wereThe prepared studied. The pow- differ- ent types ders usingof chelating chelating agents agents significantly from carboxylicaffected thepolyamino acid and elemental carboxylic composition, especially groups formed on B site of ABO perovskite structure of BCZY. The prepared powders using Zr-cluster and3 Ce-cluster respectively, giving rise to the fluctuation in the calculated ele-chelating agents from carboxylic mental acid and mole fractions. Thepolyamino use of TETAcarboxylic in thegroups synthesisformed Zr-cluster minimized theand Ce-cluster formation of respectively, BaCO 3, leadinggiving to therise to the fluctuation formation in the calculated of high percentage of BCZYelemental perovskitemolephase fractions. and homo- The use of TETA geneous in thecomposition. elemental synthesis minimized A summarythe formation of PechiniofandBaCO 3 , leading Sol-Gel conventional to the formation method isofshown high percentage of BCZY in the Figure 3. perovskite phase and homogeneous elemental composition. A summary of Pechini and conventional Sol-Gel method is shown in the Figure 3. Figure 3. Summary of steps for each perovskite synthesis method discussed. Figure 3. Summary of steps for each perovskite synthesis method discussed. 2.1. 2.1.Sol-Gel Sol-GelSynthesis SynthesisofofStrontium Strontiumand andBarium BariumTitanates TitanatesandandItsItsDerivate Derivate Strontium Strontiumand andbarium bariumtitanates (SrTiO33and titanates(SrTiO andBaTiO BaTiO33, ,respectively) respectively)arearewidely widelyusedusedin in the theindustry industry and and in medical areas. areas. BaTiO BaTiO33 isisaalow-cost low-costferroelectric ferroelectricmaterial material with with a posi- a positive tive temperature temperature coefficient, coefficient, highhigh dielectric dielectric constant constant andand non-linear non-linear optical. optical. It also It is is also used used as as catalyst,microwave catalyst, microwaveabsorber absorberand anddielectric dielectriccomponent componentof of class class II II ceramic [27]. On On the the other otherhand, hand, SrTiO SrTiO33 isis a well-known well-known perovskite perovskitematerial materialforforquantum quantumparaelectricity paraelectricity with with an an unusually unusually highhigh dielectric dielectric constant constant [28].[28]. SrTiO SrTiO 3 can3 can be also be also foundfound as a as a base base material material in in high- high-voltage capacitors, voltage capacitors, voltage-dependent voltage-dependent resistors resistors and and substrates substrates for superconductors for superconductors [29]. Despite [29]. the fact Despite the that SrTiO fact that 3 does SrTiO not show 3 does ferroelectric not show properties ferroelectric at room properties temperature, at room tempera-it is possible ture, to induce it is possible ferroelectric to induce behavior ferroelectric by doping, behavior substitution, by doping, amongamong substitution, other factors. other SrTiO3 SrTiO factors. and BaTiO and their 3 and3BaTiO 3 and derivates are veryare their derivates promising for gas sensing very promising applications for gas sensing [30] applica- and as n-type thermoelectric materials [31]. tions [30] and as n-type thermoelectric materials [31]. Sizeeffects Size effectsininnanostructured nanostructured materials are of great great importance importancefrom fromboth bothfundamental fundamen- considerations tal considerations andandpractical practicalapplications. applications.Initial Initial research research on on size effects in in ferroelectric ferroelectric materials has been mainly concentrated on BaTiO3 (BT). It is well-known that ferroelectricity properties strongly depend on its microstructure (crystallinity and grain size) and decreases with decreasing particle (grain) size, and disappears below a certain critical dimensions [32]. Then, the preparation methods play a critical role to define the desired properties. The traditional preparation methods of perovskite-type powders are performed by solid state reactions at high temperatures, usually higher than 1000 ◦ C, and under specific pressure and pH conditions. This method involves several stages, such as the mixing of the raw material, calcinations at high temperatures, washing, milling, classification, filtering, drying Gels 2021, 7, 275 6 of 19 and sifting. However, it is difficult to control the morphology and size of particles of these materials causing a diminution or destruction of the ferroelectric behavior [33,34]. Another approach is the synthesis in solution, where the titanates are prepared through coprecipitation and pyrolysis of metallo-organic precursors or through Sol-Gel processes. In general, the process uses titanium alkoxide and Ba (or Sr) salts, with and without occurrence of organic species. The reaction modifies and stabilize cations agents preventing the accumulation of the particles [35,36]. The main advantages of this method are: low working temperatures, large control of the particle size (from 10 to 100 nm) and the possibility of synthesize mixed oxide networks. However, starting products are expensive and the synthesis can be challenging. The hydrothermal synthesis is another popular option, this method uses alkoxide or oxidizes as titanium source in presence of a salt from the alkaline-earth metal ion (halide, acetate, nitrate or hydroxide). The reaction occurs in alkaline conditions (pH = 13–14) in a temperature range from 90 ◦ C to 200 ◦ C. The main advantage of this method is that the experimental procedure is less complex and uses cheaper starting products than the Sol-Gel method. The main disadvantage of the hydrothermal method is that the titanates synthesized crystallize, mostly, in the metastable cubical phase which does not exhibit ferroelectric properties [37–40]. On the other hand, the use of water as reaction medium can also induce the incorporation of OH species to the BaTiO3 structure, which in turn leads to defects in the network [32,41]. These defects also modified the chemical stoichiometry and reduce the quality of the final products. A popular way to produce these oxide titanates is by reaction of titanium n-butoxide (Ti(OC4 H9 )4 ), tetrachloride (TiCl4 ) or TiOSO4 with metal-nitrate M(NO3 )2 [42], -chloride MCl2 [43] or -salts M(OH)2 [44] solutions (with M = Mg, Ca, Sr, Ba, Mn, etc.) through a Sol-Gel synthesis. In general oxides titanates are synthesized using Sol-Gel method to yield a crystalline material at much lower temperatures than usually required for solid-state reactions [45]. This method involves the use of hydrolysis to form gels from metal oxides before subjecting them to post processing to obtain high purity titanates. Fuentes et al. [43] synthesized high-purity BaTiO3 (BT) using a Sol-Gel-hydrothermal reaction of TiCl4 and a BaCl2 solution in an oxygen atmosphere to obtain pseudo-tetragonal BT and particle sizes < 100 nm. The authors studied the effect of temperature (70–220 ◦ C) and the incorporation of an oxygen partial pressure (60 bar) on the transition of BT from cu- bic to a pseudo-tetragonal phase. The results demonstrated that the oxygen partial pressure plays a crucial role in the BT nanoparticle formation under hydrothermal conditions, ad- vantaging the increase of the ferroelectric exchange interactions, which are mediated by the oxygen ions, influencing of this mode, in the ferroelectric properties of BT. Using the same method, Fuentes et al. also synthesized SrTi(1-x) O3 :Fex powders with x = 0, 1, 3 and 5 mol% Fe, Ba1−x Srx TiO3 (x = 0–1) and BaTiO3 -ZnO:Eu heterostructures [46–48]. The results in- dicated increasing substitutional doping of Fe in SrTiO3 and that Sr ions were in the 2+ oxidation state in the perovskite structure, Fe participate as a mixture of Fe3+ and Fe4+ oxidation states, increasing the formation of oxygen vacancies as the Fe content increased in the sample. On the other hand, the reactant type used: BaCl2 and SrCl2 or Ba(OH)2 and Sr(OH)2 salts, directly influences the chemical and structural properties of Ba1−x Srx TiO (BST) nanoparticles grown. The presence of Cl− ions resulted in a better Sr incorporation into the network of BST when the Ba:Sr mole ratio used in the starting reactants was less than one and the presence of OH− ions led to BST nanoparticles with less structural defects and oxygen vacancies. 2.2. Synthesis of LaMnO3 Nanoparticles LaMnO3 (LMO) is a perovskite with the general formula ABO3 . Each B atom occupies the vertex of the perovskite lattice and six oxygen atoms surround it to form an octahedron. The A element occupies the center of eight octahedrons. The oxidation state of B cation can be modulated by varying preparation steps during the perovskite synthesis, such as temperature [21], non-stoichiometry of cations A or B [12,14] or substituting La3+ by Gels 2021, 7, 275 7 of 19 lower oxidation state cations, such as Ca2+ , Ba2+ or Sr2+ , or even higher oxidation state cation such as Ce4+ . Sol-Gel technique is one of the easiest and most efficient methods to synthesize different LaMnO3 NPs. Several groups have reported the formation of LaMnO3 nanostructures via Sol-Gel method [49–51]. The typical studies focus on the optimization of the synthesis conditions during the Sol-Gel process to improve the oxidation capacity (Mn3+ /Mn4+ mixed oxidation state), including complexant agent to metal nitrate ratio, pH of the gel and calcination proto- col to get pure perovskites with no phase segregation and enhanced textural properties. Xue et al. [52] obtained nanocrystalline LMO powders by Sol-Gel combustion method. They reported the effect of citric acid (CA) to metal nitrates (MN) molar ratio of the precur- sor solution on the structure of gels and the powders. The crystallite size of the as-formed LMO powders increases with increasing the CA/MN molar ratio from 30 at 100 nm. In a similar study, Miao et al. [53] prepared modified LMO perovskites with porous tubular skeleton structure. They integrated this NPs into a double network hydrogel and dispersed in polyacrylamide (PAM)/polyvinyl alcohol (PVA) matrixes to prepare a flexible superca- pacitor. The LMO-PAM/PVA hydrogel electrodes showed a significant improvement in mechanical properties and charge-discharge cycle stability. These encouraging results were attributed to the double network structure of the hydrogel electrodes and the introduction of LMO nanoparticles. Liu et al. [54] produced LMO of high surface area using CA and EDTA as chelating agents. They found that EDTA-citrate produced smaller NPs than the single citrate acid. The samples exhibited a high specific surface area showing excellent oxygen reduction and evolution reaction catalytic performance. In another work, Shaterian et al. [55] prepared LMO nanoparticles via Sol-Gel process using stearic acid as complexing reagent. The as-synthesized nanoparticles showed pure perovskite structure which presented a rhombohedral structure with average particle sizes of 20–30 nm. More recently, Onrubia-Calvo et al. [56] prepared a series of Sr-doped La1−x Srx MnO3 perovskites (x = 0.1, 0.2, 0.3, 0.4 and 0.5) by varying citrate to nitrate molar ratio in the starting solution, pH and calcination protocol. Conversely, Chaozhu Shu et al. [57] used a facile nonstoichiometric strategy to in- troduce A-site cationic vacancies in LaMnO3 , La0.9 MnO3 , La0.7 MnO3 , perovskite oxides were synthesized by combining Sol-Gel method and thermal treatment process in air. This procedure provides the advantage of great specific surface area and abundant cationic La vacancies on the surface, La0.7 MnO3− δ (L0.7 MO) can provide effective active sites for oxygen electrode reactions and a large storage space for Li2 O2 accommodation. 3. Synthesis of Vanadium Oxides Vanadium oxides display a wide range of oxides states ranging from V2+ to V5+ , leading to different valence and mix valence states. V2 O5 and their variations V2 O5 ·nH2 O (xerogel), NH4 VO3 , VO(OR)3 and VOCl3 are well-known as intercalation host lattices. Many guest molecules can be reversibly intercalated in the oxide layers under soft chemical reactions [58]. Which, in turn, induces structural changes on the vanadium oxides and different shapes and morphologies can be obtained [59]. Under hydrothermal treatment, novel structures have been reported. These structures feature a mix oxidation state ra- tio (V3+ to V5+ ), however, the relation between morphology and oxidation state is still under debate. For the Sol-Gel synthesis an alkoxide is used as precursor which, after hydrolysis and condensation reactions, can lead to different vanadium morphologies. Normally this process can be tuned with hydrothermal treatment varying the reaction times and the temperature. A simple route to obtain xerogel is described as follows [60]: Reflux V2 O5 (s) + 6 (CH3 )3 COH(l) → 2 VO(OC(CH3 )3 )3 (l) + 3 H2 O(l) (V3+ to V5+), however, the relation between morphology and oxidation state is still under debate. For the Sol-Gel synthesis an alkoxide is used as precursor which, after hydrolysis and condensation reactions, can lead to different vanadium morphologies. Normally this pro- Gels 2021, 7, 275 cess can be tuned with hydrothermal treatment varying the reaction times and the8tem- of 19 perature. A simple route to obtain xerogel is described as follows [60]: V O ( ) + 6 (CH ) COH( ) ⎯⎯⎯⎯⎯ 2 VO(OC(CH ) ) ( ) + 3 H O( ) By adding water, the hydrolysis reaction takes place By adding water, the hydrolysis reaction takes place VO(OC(CH3 )3 )3 (l) + H2 O(l) ↔ VO(OH)(OC(CH3 )3 )2 (l) + (CH3 )3 COH(l) VO(OC(CH ) ) ( ) + H O( ) ⎯⎯ VO(OH)(OC(CH ) ) ( ) + (CH ) COH( ) The The condensation condensation reaction reaction immediately immediately runs runs forming forming the the layered layered xerogel xerogel (CH3 )3 )3 )(l))+( VO VO(OC(CH VO(OC OH)(OC(CH3 )3))2) (l() )↔⎯⎯((CH ) + (VO(OH)(OC(CH ((CH ) CO) OV − O − VO(OC(CH ) ) + (CH ) COH( ) 3 )3 CO)3 OV − O − VO(OC(CH3 )3 )2( ()l) + (CH3 )3 COH (l) VO(OH )(OC(CH3 )3 )2)(l)) +( )VO VO(OH)(OC(CH (OH)(OC(CH3 ))3 ))2 ((l)) ↔⎯⎯((CH + VO(OH)(OC(CH ((CH ) CO) OV − O − VO(OC(CH ) ) + H O ) 3 )3 CO)2 OV − O − VO(OC(CH3 )3 )2( )(l) + H2(O (l) Xerogel is Xerogel is used used to to develop develop many many vanadium vanadium oxides oxides nanostructures, nanostructures, alkylamines alkylamines andand thiols (usually thiols (usually 1-octadecanethiol) 1-octadecanethiol) have have been been successfully successfully intercalated intercalated between between the the layers. layers. Once thiols formed the layered nanocomposite with xerogel, the compound Once thiols formed the layered nanocomposite with xerogel, the compound is submitted is submitted to hydrothermal to hydrothermal treatment treatment atat 180 180 ◦°C during seven C during seven days. days. This This process process induces induces six six folds folds symmetric cogs [61] and the star fruit morphology [62] (see Figure symmetric cogs [61] and the star fruit morphology [62] (see Figure 4). 4). (a) (b) Figure 4. Figure 4. SEM SEMMicrographs MicrographsofofVanadium Vanadiumoxides: oxides:(a)(a) High yield High synthesis yield of vanadium synthesis oxides of vanadium nanocogs. oxides (b) V(b) nanocogs. 6O11V archi- 6 O11 tecture rotationally architecture six folds rotationally layered six folds material. layered material. Othermorphologies Other morphologies thatbe can that can be synthetized synthetized viananotubes via Sol-Gel are Sol-Gel and arenanourchins nanotubes[63,64]. and nanourchins The first stage[63,64]. The firststarts of the synthesis stagewithof the thealkoxyde synthesis starts with3 )2the (VO(OCH(CH alkoxyde )3 ) precursor (VO(OCH(CH in )2)3) precursor presence of 3alkylamine in presence ofCHalkylamine (e.g., hexadecylamine 3 (CH2 )14 CH2(e.g., NH2 )).hexadecylamine The water and CH3(CHturns ethanol 2)14CH 2NH into an2)). The water alkylamine–V intercalated and ethanol turns O 2 5 into layeredan intercalated nanocomposite. alkylamine–V Subsequently, 2O5 layered the reactionnanocomposite. is subjected toSubsequently, the reaction hydrothermal treatment is subjected for seven days at to ◦ C. The composites 180hydrothermal treat- mentup rolls fororiginating seven daysvanadium at 180 °C.oxide The composites nanotubes or rolls up originating nanourchins vanadium [65] (high densityoxide nano- vanadium tubes or oxide nanourchins[65] nanotubes arrangement(highspherical density vanadium oxide nanotubes clusters) which only differsarrangement spherical on the stoichiometry molar clusters)proportion which only (seediffers Figureon 5).the stoichiometry molar proportion (see Figure 5). Hydrolysis stage: VO(OCH(CH ) ()+ VO)(OCH H O ( ) ⎯⎯ VO(OH)(OCH(CH ) ) ( ) + (CH ) CHOH( ()CH ) CHOH (CH 3 )2 )3 (l) + H2 O(l) ↔ VO(OH)(OCH(CH3 )2 )2 (l) + 3 2 (ac) Condensation stages Condensation stages VO(OCH(CH3 )2 )3 (l) + VO(OH)(OCH(CH3 )2 )2 (l) ↔ ((CH3 )2 HCO)2 OV − O − VO(OCH(CH3 )2 )2 (l) + (CH3 )2 CHOH(ac) VO(OH)(OCH(CH3 )2 )2 (l) + VO(OH)(OCH(CH3 )2 )2 (l) ↔ ((CH3 )2 HCO)2 OV − O − VO(OCH(CH3 )2 )2 (l) + H2 O(l) Gels 2021, 7, 275 9 of 20 VO(OCH(CH ) ) () + VO(OH)(OCH(CH ) ) () ⎯⎯⎯ ((CH ) HCO) OV − O − VO(OCH(CH ) ) () + (CH ) CHOH( ) Gels 2021, 7, 275 9 of 19 VO(OH)(OCH(CH ) ) () + VO(OH)(OCH(CH ) ) () ⎯⎯⎯ ((CH ) HCO) OV − O − VO(OCH(CH ) ) () + H O( ) (a) (b) Figure Figure 5. 5. SEM SEM Micrographs Micrographs vanadium vanadium oxides: oxides: (a) (a) nanotubes, nanotubes, (b) (b) center center hollowed hollowed vanadium vanadium oxide oxide nanourchin. nanourchin. Another Another interesting interestingmicrostructure microstructureare are(enH(enH 2V27V O716O ) 16 and (NH(NH ) and 4)2V4 7O V7layered-com- )216 O16 layered- pounds[66,67]. compounds [66,67]. These compounds These compounds are synthesized are synthesized using V2OV52precursor using O5 precursor with ethylenedi- with ethylene- amine diamine (en) suspended (en) suspended in ainmixture a mixtureof water-ethanol. of water-ethanol. Then the mixture Then is treated the mixture withwith is treated ace- tic acidacid acetic or NH VO43VO or 4NH with NaBH 3 with 4 in water. NaBH The synthesis 4 in water. The synthesisis aided with hydrothermal is aided with hydrothermal treat- treatment. ment. Another Another procedure procedure to obtain to obtain (NH(NH4)2V74O)216Vwas 7 O16performed was performed by Navas by Navas et al. they et al.[68], [68], they treated treated the NH the4VO NH VO 3 precursor 4 3 precursor with with acetic acetic acid in acid presencein presence of of 1–hexadecylamine 1–hexadecylamine dissolved dissolved in ethanol.inThen, ethanol. Then, the vanadium the vanadium alkoxyde alkoxyde was obtained was obtained by decreasing by decreasing the pH ofthe thepH of sys- the system. Once water is added, an intercalation compound is formed tem. Once water is added, an intercalation compound is formed under the Sol-Gel process. under the Sol-Gel process. A hydrothermal treatment ◦ C yields squared layered A hydrothermal treatment for at least for 24 hatatleast 180 °C 24 yields h at 180 squared layered structures of structures different of different sizes (see Figure sizes (see Figure 6). This morphology6). Thiscanmorphology be modified can into be VOmodified 2 crosses byinto VO2 chang- crosses ing by changing the reaction timethe andreaction working time and working temperature (10temperature days and 200 (10°C). daysThe and 200 ◦process whole C). The whole can be process described canasbe described as follows: follows: Gels 2021, 7, 275 10 of 20 NH VO ( + 3CH )NH COOH ) ⎯⎯⎯ 4 VO3 (ac) +( 3CH VO(OOCCH ) ) + NH 3 3 COOH(ac) → VO( (OOCCH OH)3 ((ac)) + +HNH O( 4)OH(ac) + H2 O(l) Hydrolysis stage VO(OOCCH ) ( ) + H O( ) ⎯⎯ VO(OH)(OOCCH ) ( ) + CH COOH( ) Condensation stages VO(OOCCH ) ( ) + VO(OH)(OOCCH ) ( ) ⎯⎯ (OOCCH ) OV − O − VO(OOCCH ) ( ) + CH COOH( ) VO(OH)(OOCCH ) ( ) +VO(OH)(OOCCH ) ( ) ⎯⎯ (OOCCH ) OV − O − VO(OOCCH ) ( ) + H O( ) (a) (b) Figure 6. Figure 6. SEM SEM Micrographs Micrographs of: of: (a) (a) (NH (NH44))22V V77O O1616microsquares, microsquares,and and(b) (b)Isolated IsolatedVO VO2 2cross. cross. 4. Synthesis of Zinc Hydrolysis Oxides stage Throughout the years, ZnO nanostructures and nanoparticles (ZnO NPs) have been VO(OOCCH )3 (ac) + H2 O(l) ↔ VO(OH)(OOCCH3 )2 (ac) + CH3 COOH(ac) a subject of research3for promising applications in many technological, chemical and bio- logical fields [69–75]. The different chemical routes to produce ZnO have been incorpo- rated to produce 1D (nanotubes, nanoparticles, nanowires, nanorods) [3,76–80], 2D (nanosheets, nanoplates and nanopellets) and many 3D self-assemble with novel mor- phologies flowers, urchins, and dandelions [75,81–83]. Even though, these fascinating Gels 2021, 7, 275 10 of 19 Condensation stages VO(OOCCH3 )3 (ac) + VO(OH)(OOCCH3 )2 (ac) ↔ (OOCCH3 )2 OV − O − VO(OOCCH3 )2 (ac) + CH3 COOH(ac) VO(OH)(OOCCH3 )2 (ac) + VO(OH)(OOCCH3 )2 (ac) ↔ (OOCCH3 )2 OV − O − VO(OOCCH3 )2 (ac) + H2 O(l) 4. Synthesis of Zinc Oxides Throughout the years, ZnO nanostructures and nanoparticles (ZnO NPs) have been a subject of research for promising applications in many technological, chemical and biologi- cal fields [69–75]. The different chemical routes to produce ZnO have been incorporated to produce 1D (nanotubes, nanoparticles, nanowires, nanorods) [3,76–80], 2D (nanosheets, nanoplates and nanopellets) and many 3D self-assemble with novel morphologies flowers, urchins, and dandelions [75,81–83]. Even though, these fascinating nanostructures can offer novel properties to ZnO, some defects are often found in the shape, size distribution and homogeneity. In general, these defects have a correlation with the chemical synthetic approach, starting from the Zn2+ salts precursors, pH, solvents, reaction time and tem- perature, additives like surfactants and other organic molecules, etc. [84–86]. One of the main challenges with these synthetic procedures is to control the size distributions, which can be vary from a few nanometers to micrometers. In terms of synthesis, the precursor Zn(CH3 COO)2 ·2H2 O has been widely used because it provides a moderate hydrolysis step, which can be controlled using organic molecules (e.g., surfactants or glycols), nitrogenated amines (used as the precipitating agent) or ethanol. This process originates ZnO NPs with a narrow size distribution ranging from 14 to 70 nm [87–89]. A short and simple Sol-Gel route to obtain ZnO nanostructures was reported by Yung– Kuan Tseng et al. [90] The authors described a hydrolysis and condensation processes from zinc acetate dihydrate with different types of glycols: ethylene glycol (EG), glycerol (GLY) and diethylene glycol (DEG) performed under reflux for one hour and increasing the temperature up to 160 ◦ C (heating rate used 1 ◦ C/minute). The final products were treated under two different heating processes, dripped onto silicon wafers at 160 ◦ C and calcined at 500 ◦ C. Under these conditions they generated three different morphologies with each solvent: fibers, flakes and spheres. The fabrication steps are given by: Step 1 Reflux Zn(CH3 COO)2 + HO − CH2 CH2 − OH → Zn2+ ·− OCH2 CH2 O− + 2CH3 COO− + 2H+ Step 2 3 Reflux 3 2+ − Zn(CH3 COO)2 + HO − CH2 CHOHCH2 − OH → Zn · OCH2 CHOHCH2 O− + 3CH3 COO− + 3H+ 2 2 Reflux Zn(CH3 COO)2 + HO − CH2 CH2 OCH2 CH2 − OH → Zn2+ ·− OCH2 CH2 OCH2 CH2 O− + 2CH3 COO− + 2 H+ The fibers are formed by the assembly of several unit structures of Zn2+ and the solvents (EG, GLY and DEG). For EG-based structures, Zn2+ −− OCH2 CH3 O− terminal oxygen ions are formed, depending on temperature used at the final stage different shapes can be obtained. Samples dried at 160 ◦ C reveal smooth surface, whereas the samples annealed at 500 ◦ C in air results into small ZnO particles (see Figure 7a1,a2). TEA as a complexating agent (Lewis base role, electron pair donor) in combination with a strong base NaOH and Zn(CH3COO)2·2H2O as Zn2+ source. Zn2+ and TEA reagents were mixed and stirred at 80°C bath. NaOH was added drop by drop until pH reaches 11. A white precipitate was washed up several times and dried at 60 °C. The TEA was used to Gels 2021, 7, 275 11 of 19 obtain wurtzite structure. The precipitation was tuned by changing the temperature, pH and synthetic procedures. Gels 2021, 7, 275 12 of 20 Figure Figure7.7.SEM SEMimages imagesofofdifferent differentZnO ZnOnanstructures nanstructuresobtained obtainedwith withdifferent differentglycol glycolsolvents solventsand and heated heatedatat160 ◦ C(1) 160°C (1)and andcalcined calcinedatat500 ◦ C(2): 500°C (2):(a1,a2)ethylene (a1,a2) ethyleneglycol, glycol,(b1,b2), (b1,b2),glycerol glyceroland and(c1,c2) (c1,c2) diethylene diethyleneglycol glycolsolvents. solvents.Adapted Adaptedfrom from[90]. [90]. For this method Another two popular mechanisms synthesis is thehave been proposed: precipitation method using a titration to ensure a Mechanism controlled 1 reaction. Md Jahidul Haque et al. [91] reported a synthesis route using Sol-Gel Zn(CH3 COO)2 ·2H2 O as precursor mixed with NaOH dissolution at 35 ◦ C. Once mixtures Zn(CH COO) × 2 H O ) + 2 TEA( ) ⎯⎯⎯ Zn[TEA]( ) + 2 CH COO( ) turn homogeneous, methanol was added( dropwise under vigorous stirring for 90 min until a white gel was formed. The gel was dried overnight at 80 ◦ C and calcinated at 250 ◦ C Zn[TEA]( ) ⎯⎯⎯ Zn( ) + TEA( ) during four hours. The reaction is given by: Zn(CH3 COO)2 × 2 H2 O(2aqNaOH ) ⎯⎯⎯⎯ 2CH Na3 OH + 2 OH ) + 2 (NaOH (aq) → ( ) ZnO(s) (+ )2 CH3 COONa(aq) + H2 O(l) Zn( ) + 2 using This synthesis was performed OH( ) a⎯⎯⎯ Zn(OH) strong base which ( ) has the ability to ionize completely in water. Shokuhfar et al. [92], on the other hand, performed the Sol-Gel pro- cesses using a weak base which ∆ Zn(OH)usually ( ) ⎯⎯⎯ developed ZnO( ) +a H partial O( ) ionization (%I < 5%). They used triethanolamine (N(CH3 CH2 OH)3 , TEA) as surfactant, with methanolic solutions of Mechanism 2 Zn(CH COO) × 2 H O( ) + 2 TEA( ) ⎯⎯⎯ Zn[TEA]( ) + 2 CH COO( ) Zn[TEA]( ) ⎯⎯⎯ Zn( ) + TEA( ) Gels 2021, 7, 275 12 of 19 Zn(CH3 COO)2 ·2 H2 O at different ratios. The resultant solution was kept at 50 ◦ C all night allowing the completion of hydrolysis and condensation reactions. The final product was calcinated at 500 ◦ C and ZnO Nps were obtained. The increase of the surfactant concentra- tion led to a smaller particle size with narrow size distribution and enhanced homogeneity. Azimul Haque and Mahalakshmi [93] performed the precipitation processes avoiding the Sol-Gel step. The authors used the same weak base with a partial ionization (<5%), TEA as a complexating agent (Lewis base role, electron pair donor) in combination with a strong base NaOH and Zn(CH3 COO)2 ·2H2 O as Zn2+ source. Zn2+ and TEA reagents were mixed and stirred at 80 ◦ C bath. NaOH was added drop by drop until pH reaches 11. A white precipitate was washed up several times and dried at 60 ◦ C. The TEA was used to obtain wurtzite structure. The precipitation was tuned by changing the temperature, pH and synthetic procedures. For this method two mechanisms have been proposed: Mechanism 1 Zn(CH3 COO)2 × 2 H2 O(aq) + 2 TEA(aq) → Zn[TEA](aq) + 2 CH3 COO− (ac) + Zn[TEA](aq) → Zn2(ac ) + TEA(aq) H O 2 2 NaOH(s) → 2 Na+ (ac) + 2 OH− (ac) + Zn2(ac ) + 2 OH− (ac) → Zn(OH)2 (s) ∆ Zn(OH)2 (s) → ZnO(s) + H2 O(g) Mechanism 2 Zn(CH3 COO)2 × 2 H2 O(aq) + 2 TEA(aq) → Zn[TEA](aq) + 2 CH3 COO− (ac) + Zn[TEA](aq) → Zn2(ac ) + TEA(aq) H O 2 2 NaOH(s) → 2 Na+ (ac) + 2 OH− (ac) + Zn2(ac ) + 2 OH− (ac) → Zn(OH)2 (s) Zn(OH)2 (s) + 2 OH− (ac) ↔ Zn(OH)24−(aq) Zn(OH)24−(aq) → ZnO(s) + H2 O(g) + 2 OH− (ac) Even though both mechanisms are quite similar, the addition of TEA plays an outstand- ing role in the morphology of ZnO NPs enhancing the conversion from Zn(OH)2 to ZnO. 5. Bioorganic Material in Sol-Gel Preparation The Pechini synthesis ensures that metal ions are combined and stabilized homoge- neously within a covalent matrix or ‘gel’ by synthesizing a polymer in situ. Consequently, it seems natural that new variants consider the direct synthesis of Sol-Gel precursors from natural or synthetic polymers. The polymers create well-defined structures in solution, and metal ions have strong interactions with them. The ability of some polymers to create ordered superstructures can be used to control the morphology of the desired material. Authors such as Liu et al. [94], Shao et al. [95] and Yong-Jin Han et al. [96] have worked with organic compounds and zinc or vanadium oxide [97,98] obtain new functional materials such as supercapacitors [99,100], taking advantage of the benefits offered by organic materials in relation to the natural and structured arrangement provided by the different natural sources from which they are obtained. Yoseph Bar-Cohen coined the statement that “ . . . through evolution, nature has ‘experimented’ with various solutions to its challenges and has improved the successful ones” [101]. Materials as versatile as activated carbon [102–104], porous carbon [105], graphene [106,107] and carbon nanotubes (CNT) [108–110] have been considered as a prospective type of electrode materials for supercapacitors due to their economic acquisi- tion, high surface area, excellent electrical conductivity and outstanding electrochemical stability [111,112]. Each of these materials, however, has drawbacks. Commercial activated the different natural sources from which they are obtained. Yoseph Bar-Cohen coined the statement that “…through evolution, nature has ‘ex- perimented’ with various solutions to its challenges and has improved the successful ones” [101]. Materials as versatile as activated carbon [102–104], porous carbon [105], gra- phene [106,107] and carbon nanotubes (CNT) [108–110] have been considered as a pro- Gels 2021, 7, 275 13 of 19 spective type of electrode materials for supercapacitors due to their economic acquisition, high surface area, excellent electrical conductivity and outstanding electrochemical stabil- ity [111,112]. Each of these materials, however, has drawbacks. Commercial activated car- bon, for example, carbon, for example,has ahaspoor specific a poor capacitance specific due to capacitance itstosmall due surface its small area [113]. surface On area [113]. the Onother hand,hand, the other CNTsCNTs and graphene have have and graphene a large specific a large surface specific area and surface areaexcellent elec- and excellent trical conductivity, electrical but their conductivity, manufacturing but their process manufacturing is complex process and of is complex high and of economic cost high economic [114]. Furthermore, cost [114]. graphene Furthermore, spontaneously graphene agglomerates spontaneously and stacks agglomerates during and stacksthe produc- during the tion process, process, production limiting limiting its synthesis and large-scale its synthesis application and large-scale [115,116]. application [115,116]. Therefore, Therefore,mostmoststudies studieshave have focused focused onon obtaining obtaining porous porous carbon carbonmaterials duedue materials to their versatility to their at the versatility attime of fabrication the time and the of fabrication and physicochemical the physicochemical advantages over other advantages over types otheroftypes carbon-based materials. of carbon-based AuthorsAuthors materials. such as such Hao et asal. obtained Hao hierarchical et al. obtained porous hierarchical porous(HPC) carbon carbonaerogel (HPC) fromaerogel from sugarcane sugarcane waste products waste products containingcontaining mainly cellulose mainly cellulose deriv- derivatives, atives, bagasse.bagasse. The porous The porous material material waswas obtained obtained by by carbonization carbonization ofof thefreeze-dried the freeze-dried bagasseaerogel bagasse aerogel[117] [117](see (seeFigure Figure8). 8).The Theauthors’ authors’preparation preparationmethod, method,by bySol-Gel Sol-Gelreaction, reaction, results in carbon aerogels with hierarchical macro-, meso- and microporous results in carbon aerogels with hierarchical macro-, meso- and microporous structures and structures and possessing possessing a highaspecific high specific surfacesurface area m area 1892.4 1892.4 m2 g−1specific 2 g−1, whose , whosecapacity specific was capacity 142 Fwas g−1 − 1 − 1 at142 0.5FAg g .at 0.5 A g . −1 Gels 2021, 7, 275 14 of 20 Figure Figure8.8.Schematic Schematicrepresentation representationofofthe thefabrication fabricationofofhighly highlyporous porousbagasse-derived bagasse-derivedcarbon carbonaerogels. aerogels. A lot of effort has gone towards designing porous carbon spheres (PCSs) using hy- A lot of effort has gone towards designing porous carbon spheres (PCSs) using hy- drothermal carbonization [118,119]. Liu et al., for example, used Pluronic F127 as a soft drothermal carbonization [118,119]. Liu et al., for example, used Pluronic F127 as a soft template to generate carbon spheres with organized mesopores and a large surface area template 2to −1generate carbon spheres with organized mesopores and a large surface area (1100 m g ) during hydrothermal treatment of D-fructose. In a three-electrode configu- (1100 m2 g−1 ) during hydrothermal treatment of D-fructose. In−1a three-electrode config- ration, the produced spheres had a high capacitance of 350 F g at −11.0 A g−1, and −1 ,aand capac- uration, the produced spheres had a high capacitance of 350 F g at 1.0 A g a itance of 190 F g−1 at 2.0 − 1 A g−1 in a−two-electrode 1 configuration [120]. A year later, Wang’s capacitance of 190 F g at 2.0 A g in a two-electrode configuration [120]. A year later, group used hydrothermal treatment of glucose and sodium molybdate (Na2MoO4 as a Wang’s group used hydrothermal treatment of glucose and sodium molybdate (Na2 MoO4 asporogen agent a porogen to create agent porous to create carbon porous spheres) carbon to create spheres) PCSs with to create PCSsa with specific capacitance a specific ca- of 260 F of pacitance −1 outstanding g 260 high-speed F g−1 outstanding performance, high-speed and strong performance, andlong-term cycle stability. strong long-term cycle Alginic Alginic stability. acid is acid carbonized hydrothermally is carbonized and then hydrothermally and activated with with then activated KOHKOH[121]. A brief [121]. A summary of porous carbon spheres is shown in Figure brief summary of porous carbon spheres is shown in Figure 9. 9. Figure9.9.Schematic Figure Schematicrepresentation representationofofthe thefabrication fabricationofofthe theporous porouscarbon carbonspheres (PCS). spheres (PCS). Hydrothermal carbonization is a strong, cost-effective, and ecologically acceptable method for converting organic molecules into usable carbon materials. Under mild hy- drothermal conditions, a variety of organic compounds such as glucose, sucrose, starch, fructose, alginic acid, among others, can be transformed into carbonaceous spheres [122]. Pan et al. obtained a porous carbon from asphalt, whose properties highlight its high −1 −1 Gels 2021, 7, 275 14 of 19 Hydrothermal carbonization is a strong, cost-effective, and ecologically acceptable method for converting organic molecules into usable carbon materials. Under mild hy- drothermal conditions, a variety of organic compounds such as glucose, sucrose, starch, fructose, alginic acid, among others, can be transformed into carbonaceous spheres [122]. Pan et al. obtained a porous carbon from asphalt, whose properties highlight its high specific capacity of 277 F g−1 at a current density of 0.05 A g−1 . According to the authors, the ultrathin and porous structure reduces resistance by minimizing the diffusion distance of electrolyte ions and aids the plentiful porosity channels as active locations for ion transport and storage [123]. These precedents utilized organic wastes to make porous carbon with good capaci- tance performance, opening a novel way to transform organic wastes into supercapacitor material and lowering carbon material costs. The specific capacitance of these carbon materials, on the other hand, has yet to increase their scalability. Therefore, it is important to prepare carbon materials with high specific capacity from low-cost biomass feedstock. These precedents utilized organic wastes to make porous carbon with good capaci- tance performance, opening a novel way to transform organic wastes into supercapacitor material and lowering carbon material costs. The specific capacitance of these carbon materials, on the other hand, has yet to increase their scalability. Therefore, it is important to prepare carbon materials with high specific capacity from low-cost biomass feedstocks. In this sense, Kong et al. employed maize starch as a carbon precursor, which they synthesized by dissolving corn starch in KOH solution and fabricating a starch-based gel after the water was removed. Finally, 3D-reticular porous carbon (3D-RPC) was created by carbonizing a starch gel and activating KOH in situ (a process like those described by Hao et al.). The advantages of 3D-RPC include a wide surface area, high porosity and an optimum pore size distribution. The macropores formed with this new material (3D- RPC) are primarily from the intermediate space between the three-dimensional starch gel networks, whereas the KOH activation process produced mesopores and micropores. The material’s porosity displayed exceptional supercapacitive performance with a 372 F g−1 specific capacitance (0.5 A g−1 ). The electrochemical performance of 3D-RPC outperforms that of most previously characterized carbon materials, opening a new path for large-scale production of a low-cost, high-purity porous carbon for supercapacitors [124]. 6. Conclusions Sol-Gel method offers a unique synthesis route to produce novel and tailor-made nanomaterials with full control of morphology, size, composition, crystallinity, porosity enhanced by combustion or hydrothermal/solvothermal treatment. Nevertheless, some precautions must be taken in order to obtain the best results. The nanoparticle composition must be analyzed before running the synthesis (pH, working temperature, reaction time, metal cation precursors mixtures). The morphology and size must be also taken into account, e.g., organic surfactants or molecules are required throughout the process. The velocity of both hydrolysis and condensations reactions should be investigated extensively, this is the key stage that leads into the final product and also confers the distinctive feature in every synthesis. Sol-Gel also offers the possibility to produce in situ doped nanomaterial and mixed lattices. However, a precise control of doping requires full-knowledge of the different equilibriums that take place during hydrolysis and condensation reactions. When doping cation precursors are added, soft chemistry must also be used to avoid fast alterations from the final product. The size distribution and aspect ratio might be another challenging task to improve. Although the use of organic template molecules as structure directors in many syntheses generates great results, most of the time, secondary phases are originated that affect the performance in different applications. Sol-Gel processes in the future could be assisted with photocatalysts during synthesis. This could induce the formation of desirable doping processes or could reduce the forma- Gels 2021, 7, 275 15 of 19 tion of secondary phases. The use of biomaterials in the synthesis of Sol-Gel, especially organic waste, offers a novel low-cost and ecological platform for the manufacture of functional materials. Author Contributions: Conceptualization, E.C.-A. and S.F.; resources S.F.; writing—original draft preparation, D.N., S.F., A.C.-A. and E.C.-A.; writing—review and editing, D.N. and E.C.-A.; supervi- sion, S.F. and E.C.-A.; funding acquisition, S.F. All authors have read and agreed to the published version of the manuscript. Funding: This work has been partially financed by P. Fondecyt 1190721 and Basal Financing Program CONICYT, AFB180001 (CEDENNA). ICN2 acknowledges support from Severo Ochoa program, the Spanish Research Agency (AEI, Grant SEV-2017-0706), and the CERCA Programme/Generalitat de Catalunya. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. References 1. Danks, A.E.; Hall, S.R.; Schnepp, Z. The evolution of ‘sol–gel’ chemistry as a technique for materials synthesis. Mater. Horizons 2016, 3, 91–112. [CrossRef] 2. Ye, C.-Q. Sol-Gel Processes of Functional Powders and Films. In Chemical Reactions in Inorganic Chemistry; IntechOpen: London, UK, 2018. 3. Purcar, V.; Şomoghi, R.; Niţu, S.G.; Nicolae, C.-A.; Alexandrescu, E.; Gîfu, I.C.; Gabor, A.R.; Stroescu, H.; Ianchiş, R.; Căprărescu, S.; et al. 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